Vibrational properties of sodosilicate glasses from first-principles calculations

The vibrational properties of three sodosilicate glasses have been investigated in the framework of Density Functional Theory. The pure vibrational density of states has been calculated for all systems and the different vibrational modes have been assigned to specific atoms or structural units. It is shown that the Na content affects several vibrational features as the position and intensity of the $R$ band or the mixing of the rocking and bending atomic motions of the Si-O-Si bridges. The calculated Raman spectra have been found to agree with experimental observations and their decomposition indicated the dominant character of the non-bridging oxygen contribution on the spectra, in particular for the high-frequency band, above 800~cm$^{-1}$. The decomposition of the high-frequency Raman feature into vibrations of the depolymerized tetrahedra (i.e. $Q_n$-units) has revealed spectral shapes of the partial contributions that cannot be accounted for by simple gaussians as frequently assumed in the treatment of experimentally obtained Raman spectra.

I. INTRODUCTION
Raman and infra-red (IR) spectroscopies are well-established characterization tools of the silicate glasses and melts in both research laboratories and industries. Although the Raman and IR spectra are easy to collect, the spectral responses are complex, combining broad and overlapping peaks due to the inherent structural disorder of these materials. Consequently, the decomposition and extraction of structural information from Raman and IR data are often based on parallelisms to well-known crystalline analogues, and result in a phenomenological and mostly qualitative approach 1,2 . A good and quantitative understanding of the structural significance of the features of the Raman spectra, as well as that of the two other vibrational spectra (IR and inelastic neutron scattering spectroscopies) can be achieved theoretically using atomistic calculations, in particular ab initio ones. In this context, the major aim of the present study is to use atomistic simulation approaches in order to improve our ability to connect measured vibrational properties of binary silicates to their structural features.
The great advantage of the atomistic modelling is that the positions of the atoms in the system under study are continuously monitored and the structure is exactly known. Therefore, correlations can be extracted between the atomic structure and the vibrational properties in order to arrive to a more quantitative interpretation of the vibrational properties and their dependance on the chemical composition, pressure, quench rates etc. . .. The latter is not directly accessible in experiments, but derived through functions describing how the modes with a given frequency are coupled to the probing radiation (photons, neutrons, etc.) [3][4][5] .
The vibrational density of states (VDOS) can be in practice accessed either through the diagonalization of the dynamical matrix of the glass structure (in a local potential minimum), or the Fourier transform of the velocity autocorrelation function. Within a classical approach, most of the empirical potentials existing in the literature are quite reliable from the point of view of the structure but they fail to reproduce the experimental VDOS, even for systems as simple as pure silica or germania 6,7 . Ab-initio methods are much more accurate in the prediction of the vibrational properties of materials but they have been scarcely used in this scope, due to the high computational cost associated with the treatment of glass systems 4,6,[8][9][10][11][12][13][14][15][16][17][18] . More specifically, the fact that one has to model a system which lacks symmetry makes the calculations more time-consuming and finally results to systems that at best contain a few hundreds of atoms.
The calculations of the Raman spectra of a glass have in general three stages: (i) generation of a structural model; (ii) the calculation of its vibrational properties, and (iii) the calculation of derivatives of the polarizability tensor with respect to atomic displacements (i.e. the Raman susceptibilities). For the latter, two frameworks can be considered, either a first principles one 19,20 or an empirical one, called bond polarizability model 21,22 . Within the bond polarizability approximation, calculations for silica and binary sodo-silicate glasses have achieved satisfying agreement with experimental data [23][24][25][26] . We note equally that Raman intensities can be equally computed from the dynamical autocorrelation functions of the dielectric tensor in a molecular dynamics (MD) simulation 5,27 approach which can be applied to liquids as well as to anharmonic solids, but is very demanding, requiring the calculation of the dielectric tensor at each MD step.
Concerning the vibrational spectra of silicate glasses, most work carried out within an abinitio approach has focused on the simplest case of vitreous SiO 2 . The early work of Sarnthein et al. elucidated the nature of the high-frequency modes 8 , while more recent works further elaborated on the VDOS of silica, as well as its IR and Raman spectra 4,9,10,18 . For more complex cases, theoretical calculations and analysis of the vibrational spectra have been carried out for sodosilicates 25 , borosilicates 17 , aluminosilicates 13,28 , phosphosilicates 14,29,30 and magnesium-silicates 16 .
In this work, we report on the vibrational properties of three binary sodosilicate glasses whose structures have been prepared by combining classical and ab-initio MD simulations.
We present their VDOS and its decompositions according to types of vibrations and contributions from specific structural units. At the same time we report results on the total and partial Raman spectra, aiming to help resolve the origins of different vibrational modes and better interpret experimental results. As Raman spectroscopy probes the short and medium range order in glasses, this modelling approach may supply quantitative criteria for the spectral analysis.

II. SIMULATION METHODS
The selected sodo-silicate glasses of this study contain 20.0%, 25.0% and 33.3% Na 2 O, hereafter called NS4, NS3 and NS2, respectively. Two samples per composition have been prepared, containing 180, 204 and 207 atoms for NS4, NS3 and NS2, respectively. In order to circumvent the costly preparation within an ab initio molecular dynamics (MD) simulation, we have generated our structural models by employing classical MD approach during the melt-and-quench cycle followed by a rather short ab initio MD run at room temperature. As it has been shown in past studies conducted using this combined approach 4,13,31-34 the use of the ab-initio approach mostly serves as a way to refine the short-range structure, while the medium range remains practically unchanged. The switch from a classical to an ab initio approach can have a dramatic effect on the calculated vibrational properties 6 . Similarly, Giacomazzi et al. extracted the vibrational properties of silica glass prepared with both methods with the results suggesting that they generally produce similar results, with some discrepancies arising due to the slightly different Si-O-Si angle distributions 4 .
The initial preparation of the NS2 and NS3 glass structures, as well as that of one NS4 structure, has been performed using classical MD by employing the Guillot-Sator potential, with a cutoff set at 6Å 35 , starting from a random atomic distribution in a cubic box reproducing the experimental density 36 . The systems were then heated to 4000 K and equilibrated for 20 ps using the constant volume-constant temperature (NVT) ensemble, before being quenched stepwise to 300 K with a rate of 10 11 K/s using NVT. Due to an inaccurate description of the medium-range order in the case of the NS2, a third system has been prepared this time using a 10 10 K/s quench rate. Two final equilibrations of 20 ps and 5 ps were finally performed, using the constant pressure-constant temperature (NPT) ensemble and the constant volume-constant energy (NVE) ensemble, respectively. Due to slight contractions of the glasses during the NPT phase, some of the final structures were rescaled in order to match the experimental density values, and were then re-equilibrated for 5 ps using the NVE ensemble. The classical MD simulations were carried out with Hoover thermostats and barostats, using an integration step equal to 1 fs. They were carried out using the DL POLY 4.01 software 37 . As a second structure for the NS4 composition, we have used one previously generated, similarly through combined classical and ab initio MD simulations, albeit with a different classical potential 38 .
In a second step, we used the above classical structural models as input configurations for ab initio MD simulations carried out using the Vienna ab initio package (VASP) 39,40 . The electronic degrees of freedom were treated in the framework of density functional theory (DFT), using the general gradient approximation (GGA) and the PBEsol functional 41,42 .
We started with an NVT run at room temperature for 2 ps, followed by an NVE run of same length. Complete computational details can be found in previous publications 17 . For the second NS4 structure, the ab initio MD simulations have been perfomed using Car-Parrinello MD as implemented in CPMD code (see simulation details in Ref. 38 ).
Further, in a third step, we have calculated the vibrational frequencies and eigenmodes, as well as the Raman tensors, using a DFT scheme as provided in the Quantum-Espresso (QE) package (version 5.4) 43 . The atomic positions have been initially relaxed using the BFGS minimizer with a fixed simulation box size setting the convergence criteria for the total energy and forces at 4 × 10 −8 Ry and 10 −5 Ry/a.u., respectively. In case of a pressure higher than 1 GPa after the relaxation, the box was slightly suitably rescaled, keeping the final densities within 1.5% of the experimental value. The calculations were carried out using the GGA-PBE exchange-correlation functional 41 with Troullier-Martins norm-conserving pseudopotentials 44 . The performance of the pseudopotentials was assessed by investigating the structure and vibrational properties of two crystalline phyllosilicates, the sodium metasilicate (Na 2 SiO 3 ) and natrosilite (α−Na 2 Si 2 O 5 ). Calculations on the crystalline compounds were carried out using a kinetic energy cutoff of 100 Ry and Monkhorst-Pack k-point grids with a 4 × 4 × 4 and 3 × 4 × 3 resolution for the metasilicate and natrosilite, respectively.
For the glasses, the same kinetic energy cutoff was used and calculations were carried out at the Γ-point.
The vibrational properties (eigenfrequencies and eigenmodes) were calculated using the density-functional perturbation theory method (DFPT) as implemented in QE code 43,45 . All spectra for the NS3 and NS4 systems presented in the Sec. III have been averaged over the two models for the corresponding composition, after broadening the discret spectra using gaussians with a 20 cm −1 FWHM, unless explicitly stated. In the case of NS2, the results only concern the system prepared with the slower quench rate (i.e. 10 10 K/s). In Na 2 SiO 3 and α−Na 2 Si 2 O 5 the 4-fold coordinated Si atoms are connected to two and three bridging (i.e. double-coordinated) oxygens, respectively 48,49 , usually called Q 2 and Q 3 units (We recall that Q n species are tetrahedral environments with n = 4, 3, 2, 1, 0 BOs).
Unit cells containing 12 and 36 atoms for Na 2 SiO 3 and α−Na 2 Si 2 O 5 were relaxed, with the results presented in Tab. I. The cell constants are predicted within 5% compared to experiment 50,51 . The intra-tetrahedral structural parameters are more accurately predicted than the inter-tetrahedral ones, whereas the largest deviations from the experimental values are observed in the case of Na-O bond lengths. The overestimation of these bond lengths is probably connected to the use of GGA/PBE approximation, as already reported in the literature 52 .
The Raman spectra for the two systems are given in Fig. 1. As there is a constant underestimation of the frequencies of the calculated spectra, the presented values have been multiplied by a factor of 1.04. For both crystal structures, we observe a good agreement of the  In Tab. II we summarize some structural parameters of the three glass models used for the vibrational calculations (i.e. relaxed to 0 K). The amount of NBO in the structures was found to be in very close agreement to available NMR results [55][56][57] . The percentages of the NBOs are accurately reproduced in all glasses, while the agreement of the Q n species with experimental results is better for the well-polymerized NS4 glass than for NS2. Overall one can state that the short range structural features of our models compare reasonably well with experiments, but deviations exist in terms of distribution of the Q n species, and thus for the medium-range structure. This discrepancy can be attributed to the choice of the empirical potential during the initial preparation of the glass models, since the ab-initio equilibration at room temperature and the subsequent energy minimization produced negligible changes in the medium-range structure. It can be also linked to the employed high quench rate from the liquid.
It is also interesting to analyze the intertetrahedral Si-O-Si angles with respect to the Q n speciation of the two silicons. In this scope, we have averaged the corresponding values over all five glass models, wherever sufficient statistics were available, and found the largest     17,49 . Of course, this implies that more than one alkali atom is found in the vicinity of each NBO, therefore deviating from the traditional theoretical model for the structure of sodosilicates, which assumes a one-to-one correspondence. where N is the number of atoms in the system. The total VDOS has been then computed as: The knowledge of the eigenfrequencies and eigenvectors makes possible further calculations, starting with the so-called partial VDOS corresponding to the contributions of different groups of atoms (as for example the species): where N α is the number of atoms in the group labelled α and e I,k (ω p ) are the 3-components of the eigenvector e(ω p ) with the displacement of the particle I.
The total VDOSs of NS4, NS3 and NS2 glasses are shown in Fig. 4  heat-capacity calculations and concern the lower end of the spectra 64 , and show an intensity increase with increasing sodium content. For the color version, the reader is referred to the web version of the article.
The change in the Na amount also gives rise to modifications of the total VDOSs for the frequency ranges above 400 cm −1 . We notice an intensity decrease with increasing Na concentration in the range 400 − 500 cm −1 (see Fig. 4a), a feature that originates from the changes presented by both oxygen and silicon partial VDOSs (see Figs. 4b and c) and can be correlated to changes in the glass polymerization due to the modifier atoms. Further on, there is a peak pointing at ≈ 740 cm −1 which is strongly affected by the Na content: its intensity decreases with increasing Na amount, alongside a slight shift to lower frequencies.
Again, we can link these variations to changes shown by both Si and O partial VDOS, and especially the ones of the silicons with a decrease of the maximum intensity of 40% between NS4 and NS2 samples, as shown in Fig. 4b. After obtaining a qualitative overview of the vibrational modes of the NSX glasses, we can proceed to a more quantitative analysis. As mentioned earlier, the most intense contribution to the VDOS stems from the O atoms. In Fig. 6, we present the decomposition of the O  Fig. 7 show a mix of rocking and bending motions in the low-and mid-frequency range, with the former being more intense in the region between 100-400 cm −1 , whereas the latter are relatively more pronounced between 500-800 cm −1 .
On the contrary, the high frequency band contains essentially stretching modes of BO.
Regarding the effect of the glass composition, the increase of Na amount when going from NS4 to NS2 leads to an increase of the contributions from rocking and bending modes at the lower part of the VDOS (below 700 cm −1 ). At the same time, their intensities for the band around 750 cm −1 decrease, and also shift to lower frequencies (the effect being more pronounced for the relative contributions, as shown in Fig. 7d-e). The increase of Na has also a limited effect on the intensity of the high-frequency band which only shifts to lower frequencies (see Fig. 7e). Similar decompositions of the oxygen motions have been previously carried out for pure silica described using an effective pair potential 26,67 or within an ab initio framework 10,68 , as well as for the NS4 glass using a harmonic potential 23 . These studies have revealed the same strong dependence of BO motions with respect to the frequency, and a qualitative agreement with respect to the relative ratios between the three contributions. stretching in Q n species since, as will be discussed in Section III B, smaller Si-O-Si angles are expected as the amount of NBOs on the Si atoms increases.
Finally, we have investigated the contribution from the NBOs by decomposing it with respect to the type of Q n species that they are linked to. Figure 9 shows the contributions to the VDOS from NBOs on Q 2 and Q 3 entities. We remark that NBOs on Q 2 demonstrate a significantly higher intensity, compared to those on Q 3 for all three glasses. However, the most important difference between the two cases can be identified in the high-frequency band, where in all systems we observe a bimodal contribution for NBOs on Q 2 species and an overlapping peak for Q 3 . In the former case, the peak centers were found to be at the same frequencies, 920±10 cm −1 and 1020±10 cm −1 , whereas the maximum of the Q 3 contribution shifts from 1060 cm −1 to 1010 cm −1 when going from NS4 to NS2. We should also note that we do not comment on the contribution to the VDOS from NBOs on Q 1 entities, since only one exists in the NS2 system and therefore good statistics are not ensured.

D. Raman spectra
The Raman spectra have been obtained by applying uniform electric fields and calculating the second order derivatives of the electron density matrix 19 . The Raman susceptibility for each mode p is given by: where V is the system volume, i, j, k = 1, 2, 3, M I the atom I mass, while e I,k (ω p ) is the 3-components vector defined in Eq. (2). In Eq. (3), the Raman tensor ∂χ ij ∂R I,k is given by: with F Iγ is the force exerted on atom I and E the applied electric field. For the polarized Raman spectra (VV), the intensity for each mode p is given by: where α p and τ p are the trace and anisotropy of the susceptibility: The depolarized Raman spectra (VH) have been calculated by multiplying the VV spectra by the depolarization ratio ρ p dep : In order to decompose the Raman spectra into partial contributions of a specific group of atoms, we have written the intensity of mode p as a sum of three terms: where only the atoms of type α are considered in Eq. (3) prior to the calculation of the partial intensities I p V V,α using Eqs. (5)-(7) (i.e. α = Si, O, Na or α = BO, NBO). The second and third terms are respectively the contributions from the remaining atoms in the system (i.e. the ones of Si and Na if α = BO, NBO) and the overlapping (or interference) terms, due to the squared terms in Eqs. (5)- (7). As already pointed out by Umari and Pasquarello for the case of pure SiO 2 68 , even if the sum of the partial Raman spectra does not recover the total spectrum, as in the case of the VDOS, it is reasonable to assume that these partial spectra give a measure of the relative ones.
The VV and VH calculated Raman spectra of the glasses under study are given in Fig. 10 and compared to recent experimental results 69 Fig. S1), we also provide a comparison between the calculated spectra and the experimental ones, using an alternate definition for the reduced intensities. In that case, the latter have been multiplied by 1/[ω(n(ω) + 1)], which is equivalent to dividing the calculated spectra in Fig. 10 by ω 2 .
For either VV and VH spectra, the positions of the main bands are well reproduced by the calculated spectra but there exist discrepancies in terms of band intensities. If we consider the band between 500-600 cm −1 in the VV spectra, its form and intensity is well reproduced for the highly polymerized (high silica content) NS4 composition. As it will be discussed later, this band contains the signal from Si-O-Si vibrations, i.e. the BO atoms. The calculated band is broader with respect to the experimental one and it is not narrowing with increasing sodium content. This indicates that there are differences in the local and medium range structures present in our glass models and those in the real glasses.
These differences are likely due to the small size of our glass models, which in the case of NS2 contains considerably less BOs and therefore poorer statistics compared to NS4.
Further, the discrepancies for the high-frequency band (900-1200 cm −1 ), corresponding to the response from the Q n species are due to the fact that observed Q n distributions are not exactly reproduced in our models. For the VH spectra, we see a broad band below 600 cm −1 (see Fig. S1 in Suppl. Mat.), which was assigned to two types of alkali motion 69 , one at ≈175 cm −1 and the second one at ≈350 cm −1 . In spite of the small size of our models, we can identify in the calculated VH spectra the presence of small peak at ≈350 cm −1 showing an increasing intensity with increasing sodium content.
As for the pure VDOS, we have initially decomposed the VV spectra with respect to the contributions of the three species, Si, O, and Na and present the results in Fig. 11. The partial contributions stemming from the O atoms are by far the most significant, as it was already noticed for pure silica 68 , as well as in the partial VDOS of our three sodosilicates (see Fig. 4). This trend is far more substantial in the polarized Raman spectra. Consequently, there is strong similarity between the form of the oxygen contributions plotted in Fig. 11b and the total VV Raman spectra shown in Fig. 10. With increasing sodium content, it is the oxygen contribution exhibiting the most noticeable changes, namely a shift to the right of the band between 400-700 cm −1 and a small shift to the left of the high-frequency band.
Due to the prominent contribution of oxygen vibrations to the Raman spectra, we have further considered three decompositions: according to their species (BO or NBO), the bond angle of the Si-O-Si bridgings, and the contribution of the NBOs to the high frequency band.
The decomposition according to the contributions from BOs and NBOs is given in Fig. 12.
We initially point out the strong contribution of the NBOs to the high-frequency, or Q n , band. This happens in spite of their smaller concentration with respect to the BOs in any of the three compositions, suggesting that they are the most Raman active in this band. The intensitiy of these modes largely dominates the one of the oxygens in Si-O-Si bridges, that one finds in the pure silica glass. The vibrations of NBOs then define the shape and intensity of this Raman band, and we expect that they are predominantly of stretching type 66 . For the rest of the spectra (below 800 cm −1 ) the NBOs present a distinct signal whose intensity increases with increasing sodium content, and surpasses the one of BOs in the case of NS2. 140 • -160 • is rather asymmetric and seems to become stronger with increasing Na content for the highest frequencies, with respect to the one of BOs with angles smaller than 140 • .
Finally we note that the contribution of the very large angles, above 160 • , is almost negligible below 850 cm −1 , and seems to slightly increase with increasing frequency in the Q n -band.
An investigation of the per-atom NBO contributions in the high-frequency band, given in Fig. 14 It could be argued that the bimodal distribution of the Q 2 contribution to the Raman spectra results from the relatively low total number of these units which lead to poor statistics. In order to clarify this point, we present in Fig. 15  Inset: Total contribution from NBO's on Q 2 and Q 3 species for the three models.

IV. CONCLUSION
We have presented in this work the investigation of the vibrational spectra, VDOS and Raman, for three sodosilicate glasses. The visualization of the vibrational modes has been used to make an initial analysis in order to show that the experimentally predicted modes are reproduced in the calculations. The per-species decomposition of the VDOS revealed the relatively high contribution from O atoms in the whole frequency range, and the contribution of the Na atoms at low frequencies, typically below 400 cm −1 . The mode analysis of the Si-O-Si bridges has shown that the vibrations up to 800 cm −1 are of rocking and bending type, whereas above that frequency they are essentially stretchings. The average contributions of the bridgings to the VDOS seems to be affected by the angle formed by the three atoms, with a slight upshift around 500 cm −1 and a downshift at the high frequency band as the angle becomes smaller. Concerning the high-frequency band, the results reveal important overlaps between NBO contributions in Q 2 and Q 3 species, as well as with those of the BOs.
The Raman spectra reproduce well the experimental band positions, whereas the calculated intensities show some deviations. We have shown that this is due to the limited system size and the prediction of the Q n species distribution by the classical potential which was used to quench the glasses from the melt. The calculation of the Raman response by Si-O-Si bridges has shown that large angles have a particularly flat contribution to the lower and middle part of the spectra while, similarly to the VDOS, relatively small angles exhibit an upshift around 500 cm −1 . The calculation of the BO and NBO partials to the Raman spectra clearly shows that the form of the Q n band is defined by the latter. The further analysis of the NBOs in terms of their local environment shows a bimodal contribution with well-defined maxima for those connected to Q 2 and a highly overlapping Q 3 peak. The fact that the BO partial is also overlapping and spans the entire width of the Q n band indicates that one has to be particularly prudent when deconvoluting experimental spectra with the use of simple functions.


extfieldsofstudy: Medicine Chemistry

text:
Crystal structures of 2-aminopyridine citric acid salts: C5H7N2 +·C6H7O7 − and 3C5H7N2 +·C6H5O7 3−

2-Aminopyridine and citric acid mixed in 1:1 and 3:1 ratios in ethanol yielded crystals of two 2-aminopyridine citric acid salts, viz. C5H7N2 +·C6H7O7 − (I) and 3C5H7N2 +·C6H5O7 3− (II). Salt I is formed by the protonation of the pyridine N atom and deprotonation of the central carboxylic group of the acid, while in II all three carboxylic groups of the acid are deprotonated and the charges are compensated for by three 2-aminopyridinium cations.

2-Aminopyridine and citric acid mixed in 1:1 and 3:1 ratios in ethanol yielded crystals of two 2-aminopyridinium citrate salts, viz. C 5 H 7 N 2 + ÁC 6 H 7 O 7 À (I) (systematic name: 2-aminopyridin-1-ium 3-carboxy-2-carboxymethyl-2-hydroxypropanoate), and 3C 5 H 7 N 2 + ÁC 6 H 5 O 7 3À (II) [systematic name: tris(2aminopyridin-1-ium) 2-hydroxypropane-1,2,3-tricarboxylate]. The supramolecular synthons present are analysed and their effect upon the crystal packing is presented in the context of crystal engineering. Salt I is formed by the protonation of the pyridine N atom and deprotonation of the central carboxylic group of citric acid, while in II all three carboxylic groups of the acid are deprotonated and the charges are compensated for by three 2-aminopyridinium cations. In both structures, a complex supramolecular three-dimensional architecture is formed. In I, the supramolecular aggregation results from N amino -HÁ Á ÁO acid , O acid Á Á ÁH-O acid , O alcohol -HÁ Á ÁO acid , N amino -HÁ Á ÁO alcohol , N py -HÁ Á ÁO alcohol and C ar -HÁ Á ÁO acid interactions. The molecular conformation of the citrate ion (CA 3À ) in II is stabilized by an intramolecular O alcohol -HÁ Á ÁO acid hydrogen bond that encloses an S(6) ring motif. The complex threedimensional structure of II features N amino -HÁ Á ÁO acid , N py -HÁ Á ÁO acid and several C ar -HÁ Á ÁO acid hydrogen bonds. In the crystal of I, the common chargeassisted 2-aminopyridinium-carboxylate heterosynthon exhibited in many 2-aminopyridinium carboxylates is not observed, instead chains of N-HÁ Á ÁO hydrogen bonds and hetero O-HÁ Á ÁO dimers are formed. In the crystal of II, the 2-aminopyridinium-carboxylate heterosynthon is sustained, while hetero O-HÁ Á ÁO dimers are not observed. The crystal structures of both salts display a variety of hydrogen bonds as almost all of the hydrogen-bond donors and acceptors present are involved in hydrogen bonding.

Chemical context
Systematic structural and statistical analysis focusing on the identification of robust supramolecular synthons or patterns are essential for crystal engineering and the design of new solid-state structures with desired properties. Organic crystals, especially salts, are now considered as potential materials for optical applications because of their flexibility in molecular design (Jayanalina et al., 2015a), thermal stability and delocalized clouds of electrons (Jayanalina et al., 2015b). An analysis of the Cambridge Structural Database (Groom et al., 2016) by Bis & Zaworotko (2005) revealed that 77% of compounds that contain both the 2-aminopyridine and carb-oxylic acid moieties generate 2-aminopyridine-carboxylic acid supramolecular heterosynthons rather than carboxylic acid or 2-aminopyridine supramolecular homosynthons. In the absence of other competing functionalities, the occurrence of heterosynthons increased to 97%. Several salts and co-crystals containing 2-aminopyridine or 2-acetaminopyridine and a carboxylic acid moiety have been reported (Jayanalina et al., 2015a,b;Bis & Zaworotko, 2005;Aakerö y et al., 2006;Jasmine et al., 2015;Jin et al., 2001). In all of these reported structures, the charge-assisted 2-aminopyridinium-carboxylate or neutral 2-acetaminopyridine-carboxylic heterosynthon is observed, as suggested by statistical analysis. Keeping this in mind, the crystal structure analyses of two 2-aminopyridinium citrate salts, C 5 H 7 N 2 + ÁC 6 H 7 O 7 À (I) and 3C 5 H 7 N 2 + ÁC 6 H 5 O 7 3À (II), were undertaken in order to study the packing patterns and identify the supramolecular synthons present in each salt.
In the crystal of II, all of the strong hydrogen-bond donors and acceptors are utilized in a supramolecular association. Full details of the hydrogen-bonding interactions are given in Table 2, and illustrated in Figs. 2, 5 and 6. A number of the C ar -H groups are also involved in C-HÁ Á ÁO hydrogen bonds (Table 2) (Table 1). Table 2 Hydrogen-bond geometry (Å , ) for II.

Figure 4
A partial view along the b axis of the crystal packing of salt I, illustrating the layer-like structure. Red and blue dashed lines denote the various intermolecular interactions (Table 1).

Database survey
A survey of the Cambridge Structural Database (CSD, Version 5.39, last update May 2018; Groom et al., 2016) revealed 80 organic structures involving a citric acid moiety in the form of solvates/hydrates, salts/salt hydrates and co-crystals. 25 structures among these are salts/salt hydrates of citric acid (deprotonated to different extents) with various organic cations. It is observed that most of the organic citrates appear as their hydrates, with the exception of a few (including I and II). The most common hydrogen bonds observed in these hydrated salts are N amine -HÁ Á ÁO citric , N amine -HÁ Á ÁO water and O water -HÁ Á ÁO citric , forming different supramolecular architectures. In the absence of a water molecule, the most common hydrogen bonds are N amine -HÁ Á ÁO citric and O citric -HÁ Á ÁO citric . However, the nature of these supramolecular synthons varies from one structure to another, depending on the nature of the organic cations.
The crystal structure of 2-amino 5-chloropyridinium-ltartarate (Jayanalina et al., 2015b) shows that despite of the presence of other competing functionalities on the carboxylic acid (two alcoholic OH groups in tartaric acid), the most frequent 2-aminopyridinium-carboxylate heterosynthon is still observed. However, the presence of the alcoholic OH group in citric acid has resulted in a deviation from the regular trend as the charge-assisted 2-aminopyridinium-carboxylate heterosynthon is not observed in I; instead chains of N-HÁ Á ÁO hydrogen bonds and hetero O-HÁ Á ÁO dimers are observed. The 2-aminopyridinium-carboxylate heterosynthon is sustained in the crystal structure of II because of the nonavailability of the alcoholic OH group for intermolecular hydrogen bonding.
Hence, the study of the crystal structure of 2-aminopyridinium citrate, mixed in a 2:1 ratio, would be highly significant in understanding the packing-pattern trends observed in this family of salts. Unfortunately, despite a number of attempts, we have not been able to obtain goodquality single crystals of this salt.

Refinement details
Crystal data, data collection and structure refinement details are summarized in For both structures, data collection: APEX2 (Bruker, 2009); cell refinement: APEX2 and SAINT-Plus (Bruker, 2009); data reduction: SAINT-Plus and XPREP (Bruker, 2009); program(s) used to solve structure: SHELXT2016 (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2016 (Sheldrick, 2015b); molecular graphics: Mercury (Macrae et al., 2008); software used to prepare material for publication: SHELXL2016 (Sheldrick, 2015b). where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.29 e Å −3 Δρ min = −0.30 e Å −3 Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.


Separation of Binary Solutions on the Basis of Zeolites Separation of Binary Solutions on the Basis of Zeolites

In this chapter, the author analyzed binary systems, ethanol + water, methanol + water and benzene + water, and an original mathematical model allowing the determination of the complete adsorption of binary systems on KaA, CaA, CaX, NaA and NaX zeo- lites using the Gibbs adsorption theory is proposed. The Gibbs equation and the Gibbs-Duhem equation have a number of limitations and do not take into account the properties of the investigated zeolites. Therefore, it is necessary to use the equations obtained by the author as a result of laboratory research, for the theoretical calculation and development of dehydration and concentration systems for alcohols.

Introduction
The study of the processes occurring at the phase interface attracts many researchers from all over the world, and they have enormous practical and theoretical importance. A detailed description of the processes occurring at the phase interface is given in [1].
Considering these processes can be argued, the issues of adsorption of gas on adsorbents have been studied quite deeply and have an extensive theoretical and experimental basis. so small that the properties of the liquid are largely determined by the intrinsic volume of the molecules and by the mutual attraction between them, while in gases under ordinary conditions the influence of these factors is negligible and can be neglected. Small distances in fluid molecules impose certain limitations on the mathematical models being developed and also impose serious limitations that require taking into account the polar properties and geometric parameters of the molecules [2].
The properties of polar liquids depend on the interaction of the molecule with the molecule, but also on the interaction between individual parts of different molecules [2].
The first attempts to create a theory of fluid and to develop a mathematical model for analyzing the behavior of liquid molecules were based on a comparison of the liquid with mathematical models of the behavior of molecules in gas and did not yield any practical results, since they did not reflect the complexity of the interaction between molecules in the liquid [2].
The verification and application of new methods for analyzing the internal structure of liquids made it possible to establish the polarity of the molecules studied and the dielectric properties. And the use of nuclear magnetic resonance has led to the development of models for describing the behavior of molecules in a liquid [2].
It is established that the molecules of the liquid have a polarity, in addition, the attraction between them, inherent to nonpolar molecules, which manifests itself in a weak interaction between different parts carrying an electric charge. The total effect of mutual attraction of molecules is often described as the internal pressure of a liquid. For liquids other than electrolytes or weak electrolytes, the internal pressure varies from 3000 to 6000 bar under standard conditions [2]. At strong electrolytes, it can reach 10,000 bar. A large internal pressure, which is inherent in electrolyte liquids, characterizes the rest of their properties, such as considerable absorption of heat during evaporation and low compressibility [2].

Drying of liquid media
The moisture content of organic liquids significantly changes the properties of the materials. In this regard, their dehydration is of great importance.
Dewatering freon refrigeration oils: The reliability and durability of airtight refrigeration machines largely depends on the purity of the refrigerants and lubricating oils. Up to 80% of the contaminants generated in freon refrigerating machines, which cause corrosion of the system, and ultimately, the combustion of the built-in electric motors is associated with the presence of moisture [3].
When drying oil with zeolites without a binder (NaA), the degree of drying, the time of the protective effect of the layer and the dynamic activity increase significantly [3].
Dehydration of transformer oil: Methods of dehydrating transformer oil with zeolites and oil degassing are developed on the basis of mass transfer processes. In the adsorption method, contacting the oil with zeolites is carried out at ordinary temperatures and, as a result, does not cause oxidative processes that occur during heating. The adsorption process is widely used for the recovery of waste transformer oils, for decreasing the dielectric losses of fresh transformer oils, for drying oils with zeolites, in filters for continuous regeneration of transformer oil, and so on [3].
Removal of radionuclides from liquid wastes of nuclear power plants: Synthesis of ceramic matrices by the method of sorption of radionuclides on zeolites and their subsequent conversion to feldspar allows using radionuclides to remove radionuclides from liquid wastes using a simple process scheme [3]. This method is based on the ability of synthetic zeolites with high selectivity to react with respect to Sr and Cs. Zeolites are completely iso-chemical to feldspars; moreover, the ion-exchange sorption process makes it possible to obtain zeolites of a given composition, and this process is relatively easy to control. Ion exchange on zeolites is technologically well developed and is widely used in the industry for purification of liquid waste [3].

The mechanism of the penetration of atoms and molecules through the windows of molecular sieves
The mechanism of the passage of molecules through the windows connecting the cavities of zeolites is complex because here are faced with the peculiarities of the forces of attraction and repulsion between individual molecules and atoms, as well as the structure of molecules and the structure of zeolites [4].
Numerous studies have shown that molecular sieves with a small size of the connecting windows (e.g., 5 Å) adsorb paraffin hydrocarbons of normal structure, but they do not adsorb the isomers of these hydrocarbons, since they have a branched structure and cannot pass through these channels. Figure 1 shows that isooctane cannot pass through a window of molecular sieves with a diameter of 4.9 Å, while normal octane passes freely [4].
Adsorption of molecules, whose size is close to the diameter of the entrance windows of the zeolite, proceeds with the expenditure of additional energy. The possibility of deforming molecules containing two or more atoms within a small range makes it possible to adsorb molecules even within a larger critical diameter, through the window size [4].  For example, through the windows NaA penetrates into the internal structure of ethane molecules having a critical diameter of 4 Å; penetration facilitates the thermal pulsation of the crystallite lattice. However, at some temperature characteristic of the sorbed material, the kinetic energy reserve of the molecule becomes insufficient to overcome the energy barrier; below this temperature, the substance is not sorbed by the zeolite of this type [4].
In Figure 2 (according to R. Barreru) are curves showing how the adsorption of some gases changes with NaA-type zeolites, but as the temperature decreases. At 0°C, oxygen is only slightly adsorbed by zeolites, but at a temperature of liquid nitrogen, that is, at −196° C, approximately 12 molecules of oxygen are adsorbed in each cavity. For 1 g of zeolite, this will be about 130 cm 3 of oxygen (when reduced to normal conditions). Thus, there is a process of adsorption of oxygen, on a zeolite accompanied by an increase in the adsorption capacity in oxygen with a decrease in temperature [4].
However, the adsorption of other gases (nitrogen and argon) occurs differently. As the temperature decreases, initially the adsorption of these gases increases, goes also as oxygen, but then, having reached a certain maximum, sharply decreases. The maximum adsorption of nitrogen is achieved at −120°C and argon at −150°C. At very low temperatures (−190°C and below), the adsorption of nitrogen and argon becomes quite insignificant compared to its maximum value [4]. Even at a temperature of −196°C, the windows between the cavities do not contract so much that the oxygen molecules cannot shine through them. At the same time, larger molecules of nitrogen and argon do not pass through the windows at very low temperatures, as a result of a decrease in their cross section.
Studies have shown that even relatively small changes in the cross section of the windows connecting the surfaces lead to significant changes in the nature of the adsorption of the individual components. Both shabasite-and CaA-type screens adsorb normal hydrocarbons, but they do not adsorb isomers of hydrocarbons having a branched structure that cannot pass through the windows of these zeolites [4]. However, in crystals of both types, a certain difference in adsorption is observed, and shabasite adsorbs different normal hydrocarbons unequally [4]. The greater the molecular weight of the hydrocarbon, the slower it adsorbs the shabasite. For example, u-heptane is not adsorbed insignificantly [4]. At the same time, synthetic molecular sieves of the CaA type adsorb even a normal hydrocarbon such as tetradecane (n-C 14 H 30 ), the hydrocarbon chain of which is twice as long as n-heptane [4].
Different types of natural and synthetic zeolites have windows of unequal dimensions [4].
The penetration of molecules through these windows depends on the properties of those ions that are located at the edges of the window. A cation that compensates for the charges of the Si─O or Al─O complex is often located at the edge of the inlet (window) and prevents the penetration of molecules whose dimensions exceed the critical diameter, that is, the diameter that allows the molecule to penetrate through the narrowed window due to the presence of the cation [4].

Effect of water on zeolites
The addition of water to the dehydrated zeolites results in a pronounced change in the electrical conductivity. With an increase in the water content at a temperature of 25°C, the electrical conductivity of zeolite X increases by a factor of 10 4 . In the case of type A zeolite, the electrical conductivity increases nonlinearly, which indicates preferential hydration of cations of the same type.
In the case of type A zeolite, electrical conductivity increases with hydration until the water content is about five molecules per unit cell. This is equivalent to the hydration of four mobile sodium ions, in the vicinity of the eight-membered oxygen rings forming the entrance windows. Apparently, these sites have the highest adsorption energy. Complexes of water-sodium ions, localized in eight-membered rings, very effectively block the entrances and prevent the penetration of other molecules there.
Thus, it can be concluded that the circumstances confirm the results of the determination of physical adsorption, showing that in the presence of even traces of water adsorption of gases of the oxygen type does not occur.
Hydration of water molecules is accompanied by the formation of an unstable bond with sodium ions, and the localization of ions, and there is also an insignificant increase in the electrical conductivity of the sample. After the number of water molecules in each unit cell exceeds 16, H 2 O molecules occupy places with the lowest adsorption energy, forming hydrogen bonds with the anionic surface of the zeolite. As the crystals are saturated with water, the electrical conductivity of the zeolites increases; therefore, the system of channels is filled with water molecules. Some of the sodium ions remain sufficiently free to chaotically move through the channels of the zeolites. These conclusions are in agreement with the data of IR (adsorption of water) and NMR spectra.

Hypothetical properties of zeolites
The first works of prediction of skeletons were carried out in the 1960s of the past century [5]. Most of these pioneering studies were performed manually. The development of computer technology and new algorithms allowed the generation of millions of hypothetical structures of zeolites [6,7]. At present, there are two main directions for forecasting zeolite frames. One of the directions is based on the creation of hypothetical zeolites with given structural features. These hypothetical scaffolds are of great importance for functionally oriented synthesis [8,9]. Another way of predicting zeolite frames is to create as many hypothetical structures as possible and enumerate all possible three-dimensional grids under certain topological and geometric constraints [10,11]. Thus, in [12,13], in order to enumerate all possible four-connected grids with a given number of unique T-atoms for all spatial groups, generation of hypothetical structures was carried out, consisting of the following basic procedures: • Crystallographically unique T-atoms were successively placed in different positions (private and general) of the unit cell and by means of symmetry operations were generated on the whole cell.
• Of all the possible distributions of T-atoms in the cell, those that allow tetrahedral coordination of atoms are selected.
• When the parameters of the unit cell were varied, optimization was performed to obtain acceptable values for the distances T-T and the angles T-T-T. At the last step, oxygen atoms were added between the bound T atoms, and the whole cell was optimized. As a result, several million hypothetical four-connected zeolite frameworks were obtained with a number of unique T-atoms ≤7, which are now presented in an online database [12,13].

Zeolite Molecular Sieve
Zeolites are molecular sieves [4,14]. Their wide application that they can be used for the separation of substances, not only on the basis of selectivity of adsorption, but also on the basis of the difference in size and shape of the molecules to be absorbed. In order to penetrate the adsorption cavity, the critical diameter of the adsorbate molecules must be smaller than the size of the entrance window [15].
The main factor determining the molecular sieve properties is the size of the entrance windows of zeolites, which depends on the location of the oxygen rings of the zeolite and on the number of oxygen atoms in the ring. The size of the input window is also affected by the size of the cation entering the zeolite. Cations located close to the window block the entrance for molecules. For example, in cation exchange, in which two sodium cations are replaced by a single calcium cation, the input window expands; As a result, the Na zeolite has an inlet window size of 4Å, and the CaA zeolite has a size of 5Å [15]. A similar exchange in a zeolite of type X leads to a certain narrowing of the window. Considering the properties of the KA zeolite, it can be explained that, at ordinary temperatures, this kind of zeolite sorbs water very well. This property has prevented its use for the drying of unstable substances prone to polymerization reactions.
If we consider the property of NaA zeolite, which is capable of sorbing most of the components of industrial gases, the critical size of the following molecules does not exceed 4Å: hydrogen sulfide, carbon disulfide, carbon dioxide, ammonia, lower diene and acetylene hydrocarbons, ethane, ethylene, propylene, organic compounds with one methyl group in molecule, as well as methane, neon, argon, krypton, xenon, oxygen, nitrogen and carbon monoxide [15]. The latter group of substances is absorbed in considerable quantities only at low temperatures.
Propane and organic compounds with more than three carbon atoms in the molecule are not adsorbed by the zeolite and thus do not suppress the adsorption of the above impurities during drying and purification.
CaA zeolites adsorb hydrocarbons and alcohols only of a normal structure, and therefore, it is widely used in the processes of separation of multicomponent organic substances on a molecular sieve basis. Moreover, zeolite CaA is absorbed by methyl and ethyl mercaptans, organic compounds with the number of carbon atoms in molecule 2 (ethyl alcohol, ethylamine), diborane, and so on. Among the general-purpose zeolites of CaA type is increased resistance in a weakly acid medium, and therefore, it is used in desulfurization processes and decarbonization of gases.
Zeolites of type X have rather wide entrance windows and adsorb the vast majority of the components of complex mixtures: all types of hydrocarbons are organic sulfur, nitrogen and oxygen compounds (mercaptans, thiophene, furan, quinoline, pyridine, dioxane, etc.), halogenated hydrocarbons (chloroform, carbon tetrachloride and freons), pentaborane and decaborane [15]. The use of zeolites of СаХ and NaX is based on the selectivity of adsorption and not on molecular sieve properties. With the complete replacement of the sodium cation for calcium, the zeolite of СаХ, unlike the NaX zeolite, does not adsorb aromatic hydrocarbons or their derivatives with branched radicals, e.g., 1, 3, 5β-triethylbenzene and metadichlorobenzene. This method is based on the method for identifying the zeolites of these two types and establishing the completeness of the ion exchange in the preparation of the zeolite of СаХ.
In the case when the critical diameter of the molecule is close to the diameter of the input window, the adsorption process occurs with a high activation energy and the ad-molecule must have a certain kinetic energy reserve to overcome the energy barrier. The kinetic energy of molecules rises with increasing temperature [15]. At the same time, an increase in temperature leads to an increase in the thermal pulsation of the zeolite lattice, which facilitates the penetration of the molecule into the adsorption cavity. Thus, by changing the temperature regime, it is possible to reach a point at which the adsorbent molecules begin to be absorbed by the zeolite.
Localization of cations in the zeolite structure: The localization of cations in the structure depends on the degree of hydration of the zeolites. Dehydration promotes the migration of cations in its structure, and it is necessary to take into account the degree of hydration of the zeolite under given conditions. In addition, protons from 100°C also show increased mobility. The location of the cation is determined not only by the degree of hydration of the zeolite, but also by the nature of the reacting substances. Thus, it was shown that in zeolites of type Y a significant number of Ni 2 + and Cu 2 + ions again move from small cavities to large ones when the latter are filled with olefins, ammonia, pyridine or NO molecules [16].
In NaA zeolite, the possible sites for cation localization are four-, six-and eight-membered rings and large cavities. Cations in the eight-membered rings react to the pore size due to the partial blocking of the entrance windows of the cavities [16]. In the faujasite structure, the cations are localized apparently at the centers of hexagonal prisms and on six-and fourmembered rings.
The catalytic properties of zeolites, which are studied using X-ray diffraction analysis, should be carefully considered; in fact, the localization of cautions largely depends on the conditions for pretreatment of the zeolite: the heating rate and the final temperature of the zeolite, the concentrations and the parameters for calcinations, the thickness of the layer and the possibility of reducing the cautions by hydrocarbons, and also from the possibility of separation from the vacuum lubricant used in installation.
As a rule, when analyzing the cation distribution in faujasite, the following factors are taken into account [16]: 1. the need for optimal coordination; in hexagonal prisms, cations are easily coordinated with six skeleton oxygen atoms, whereas in the sodalite cavities tetrahedral coordination with three skeleton oxygen atoms and one residual water molecule is possible; 2. the difficulty of local charge compensation with the help of multiply charged ions; 3. the need to minimize the electrostatic energy of the system (direct repulsion of the cationcation should be excluded);

stabilization by a crystalline field or a field of ligands;
5. covalence and the existence of directed bonds.

Adsorption properties of zeolites
Considering the term adsorption can be understood this term as the ability of solids to absorb certain substances. This ability is closely related to the special properties of the surface of solids. Molecules of a gaseous or liquid substance are in contact with a solid, adsorbed by the surface of solids. The larger the surface of a solid, the more the amount of gas or liquid a given body can hold [4].
If the adsorbed substance is located for a long time on the surface of the adsorbent, the process of diffusion of adsorbed molecules inside [4] of the solid begins. Molecules of gaseous and liquid substances adsorbed on the surface of a solid. They have a certain mobility, due to which they gradually penetrate into the solid body.
At low pressures, adsorption increases in proportion to the increase in pressure. However, as the pressure rises, the linear relationship is broken; the amount of adsorbed gas decreases, and then, at a certain pressure, saturation appears; with further increase, the pressure of adsorption of substances does not increase [4].
In a number of cases, the relationship between the gas pressure and the amount of gas absorbed by the adsorbent is more complex; after reaching a certain pressure, the amount of adsorbed gas begins to increase sharply. It was found that the nature of this dependence is related to the shape and size of the adsorbent pores [4].
In the adsorbent pores, at a certain relative pressure, the vapor becomes a liquid state and the entire internal structure begins to be filled with condensing vapor, as a result of which the amount of absorbed substance sharply increases. This phenomenon is called capillary condensation. In the fine pores of molecular sieves, capillary condensation does not occur; it can be noted only in relatively large pores formed by a binder during granulation-in a secondary porous structure [4].
Theories were advanced to explain the features of adsorption processes, and equations describing the dependence of the amount of adsorbed matter on pressure, temperature, and other conditions were derived.
The theory developed by Langmuir proceeds from the concept of the formation of a monomolecular layer of an adsorbed substance due to the action of active sites of the adsorbent.
In the potential theory advanced by Polanyi, it is assumed that the scope of the attractive forces during adsorption extends not only to the nanomolecular layer of matter.
Proceeding from all of the above, the main ways to regulate the selective adsorption capacity of zeolites are as follows.

1.
Change in composition in the process of crystallization. From the same starting materials, it is possible to obtain aluminosilicate porous crystals having different properties. For example, type A zeolite is formed from mixtures rich in alkalis and poor in silica, and type Y zeolite crystallizes in the region with the lowest alkalinity of the medium and the highest concentration of silica [17].

2.
Method of ion exchange. Ionic exchange can be regulated by molecular sieve properties, especially type A. Knowing the dimensions of the adsorbed molecules and zeolite windows, it is possible to select a particular cation exchange form of the zeolite to separate any mixture of gases or dissolved gases. For example, a zeolite, a spacecraft with a window size of about 3 Å, adsorbs water well, but water does not adsorb molecules of methanol, carbon dioxide, whose critical molecular diameter is greater than 3 </s>. Zeolite NaA in which the window size is 4 ÅA 0 adsorbs methanol and carbon dioxide and does not adsorb the molecules of propane, hexane and other molecules with a critical diameter greater than 4 </s>. On a CaX zeolite with a window size of 8 Å, 1,3,5-triethylbenzene molecules are not adsorbed, and on NaX zeolite (the size of windows 9 Å) they are well adsorbed [17].
Zeolites can manifest themselves as ions in many processes. For example, sodium in zeolite type X cannot be exchanged for alkyl ammonium cations because of the large size of the latter. Cation sieve effects in zeolites can also be caused by the fact that due to too large a size the cation cannot penetrate into small channels and cavities in the zeolite framework, or exchange cations during the synthesis of some zeolites are localized in inaccessible areas and therefore not are replaced [17].
Adsorption on the surface of zeolite of polar substances, around the entrance windows, prevents the diffusion of adsorbate molecules, and prevents their movement. Thus, for example, the pre-sorption of small amounts of water vapor on zeolite type A sharply reduces the adsorption of oxygen. Effective window diameters can be adjusted to form organometallic complexes. Thus, when pyridine is treated with a copper form of zeolite X, a very strong pyridine cation complex is formed. The adsorption of molecules of gases and vapors on such a zeolite indicates a significant decrease in pore sizes due to their blocking by organometallic complexes [17].
A special place among the cation-substituted zeolites is occupied by hydrogen, or decationized, forms of zeolites. Replacement of cations of zeolite with hydrogen is one of the ways of modifying porous crystals. The hydrogen form of zeolites, unlike other forms, cannot be obtained by simple treatment of the zeolite with acids, since the latter destroy the crystal lattice, especially low-silica zeolites. Therefore, in the beginning, sodium ions are replaced by ammonium ions, then the latter is thermally decomposed, ammonia is released and a proton is formed, which ensures the neutrality of the zeolite lattice [17].
One of the methods for modifying zeolites is dealumination. Treatment of zeolite with acids leads to the dissolution of tetrahedral aluminum in the lattice. As a result, the adsorption capacity of the zeolite increases. Dealumination can also be carried out by treating the zeolite with substances that form complex compounds with aluminum ions or by treating the zeolite layer with water vapor at elevated temperatures [18]. Dealumination allows, within certain limits, to vary the ratio of silicon and aluminum-oxygen tetrahedra in the zeolite without changing its crystal lattice.
Selective adsorption on zeolites is also possible when the molecules of all components of the mixture are sufficiently small and freely penetrate into the adsorption space. Other things being equal, the exchange cations are adsorption centers and determine the specificity of the interaction during adsorption on the zeolites of molecules of different structure and electronic structure [18]. By changing the nature and size of the exchange cation, it is possible to enhance or weaken the contribution of a specific interaction to the adsorption energy. In addition to interacting with the positive charge of cations, the adsorbate molecule undergoes strong dispersion effects from other atoms forming the walls of the zeolite channels. One of the important issues of adsorption interaction on zeolites is the elucidation of the nature of active centers [18].

A mathematical model for determining the total adsorption of solutions on zeolites
Considering the process of adsorption from solutions should be considered as a process of concentration on the surface of one of the two adsorbed components. Therefore, to determine the adsorption energy of solutions, we use the data on the surface tension of the pure components and try to obtain the adsorption through Gibbs energy.
Considering the fundamental Gibbs law, we get where G 1 and G 2 are the values of the Gibbs adsorption of components; μ 1 and μ 2 are the chemical potential of the components in order to relate Gibbs adsorption to the concentration of the component; it is necessary to use the Gibbs-Duhem Eq. [19] where x 1 and x 2 are the mole fraction of the components. Then, We substitute it into the Gibbs Eq. (3) and obtain Surface properties of solutions are considered from the surface properties of pure liquids and strongly depend on the composition of the surface layer [20]. When a substance having a lower surface tension than a pure solvent dissolves, the surface tension of the solution decreases spontaneously, since the free energy of the system decreases. The concentration of solute in the surface layer as compared to its concentration in the solution volume increases [20]. Substances that increase the surface tension of the solution, on the contrary, are contained in the surface layer in a lower concentration than those in the volume [20].
Often substances that increase the surface tension of the solvent themselves have a higher surface tension in their pure form, while lowering substances have a lower surface tension than the solvent. A large surface tension means greater energy unsaturation of molecules on the surface [20]. Such molecules tend to leave the surface, since to reduce free surface energy it is more advantageous to have molecules with low energy unsaturation. Naturally, the complete separation of molecules is impeded by the loss of entropy of solution formation [20]. As a result of the action of these two factors, a composition change occurs on the surface of the solution as compared with the volume, that is, adsorption occurs [21]. For all liquid organic substances, the surface tension is less than the surface tension of water. Assuming that the total number of moles before and after adsorption remains unchanged, we find that when one component is adsorbed, the amount of the other is increased evenly. Then, G 1 = G 2 , x 2 + x 1 = 1. From this, it turns out: Thus, the most important feature of the Gibbs method is that it allows us to give some general characteristic of the region of inhomogeneity without knowing the true course of the concentration profile.
To solve Eq. (6), it is necessary to find the value of the surface tension for a binary solution.
Determination of the surface tension of a binary solution is where N/m is the surface tension of water; c 1 is the initial concentration of the solution, % vol.; c 2 is the final concentration of the solution, % vol.
Since Gibbs adsorption does not allow comparison of the results with the results of the experiment, it is necessary to use the equation to determine the total adsorption. Let us single out that to determine the total adsorption (absolute adsorption), it is necessary to relate Gibbs adsorption to the experimental results [21][22][23][24].
where W is the volume of the space in which adsorption occurs, cm 3 /g; a is the complete adsorption of g /g; ρ is the density of the equilibrium phase, g/cm 3 . Then, Thus, Gibbs adsorption does not reflect the properties of zeolite adsorbents; therefore, it is necessary to combine the formulas (6)-(8) into a single equation for the theoretical calculation of the total adsorption of binary solutions on zeolites. With the application of the method of complete content, from the concept of the region of inhomogeneity at the interphase interface as a separate real phase of finite volume [22][23][24].

Experimental part
As a result of experimental research in the laboratory, the following hydrometer Hydrometer AHON-1 (measuring range 700-1840 kg/m 3 ), laboratory scales Leki 5002, calipers digital 31C628 (error) and SORBTOMETR-M were used.
The following characteristics of the investigated adsorbents are established ( Table 1). The volume of the solution is 200 ml, and the mass of adsorbent for adsorption is 100 g.
To check the theoretical relationships and compare them with the experimental data, it is necessary to analyze a number of binary solutions, such as ethanol + water, methanol + water, and benzene + water; we distinguish that in Table 2 the characteristics of the pure components of organic solutions for theoretical calculations are listed.
For a simplified calculation, let us set the condition that adsorption in the systems under consideration occurs only for one of the components; therefore, the concentration change will be monitored by the change in the percentage of water in the solutions in question. To do this, Adsorbent Diameter of grain, mm а, total adsorption, g/g W , the volume of the space in which adsorption takes place, cm 3  when testing the mathematical model for calculating the total adsorption, we take the initial water concentration of the binary systems under consideration. The results of the calculation using formula (9) are given in Table 3.

Conclusion
The data presented show that the mathematical model developed by the author allows us to reliably estimate the total adsorption of various binary systems. When comparing theoretical values with experimental ones, it can be asserted that the values obtained fully satisfy the requirements of theoretical calculations and can be used in the design of drying apparatuses and concentrating alcoholic solutions. Let us emphasize that the accumulation of data on Gibbs adsorption will contribute to the refinement and verification of models taking into account bulk effects in adsorption solutions and will allow theoretical calculations to be carried out with high accuracy [26,27].
Thus, the use of the approximate method of calculating absolute adsorption makes it possible to obtain more realistic information about the properties of adsorption solutions, which indicates significant structural changes in them, depending on the concentration.


extfieldsofstudy:
Physics
Materials Science

text:
Estimation of the Gibbs Free Energy for Calcium Aluminate Sulphate Hydrates Using Iterative Least Squares Regression Method

The iterative least squares method was used for the estimation of the Gibbs free energy △G of calcium aluminate sulphate hydrates the lime desulfurization product in alumina production. By means of the tree diagram theory, the optimal selection of exponential power Xi in the regression equation is realized. The recursive least square method was used to solve the regression equation, and a regression method for estimating the Gibbs free energy of calcium aluminate sulphate hydrates was obtained. The Gibbs free energies of two kinds of calcium aluminate sulphate hydrates at 343K were estimated by this method. The experimental results were consistent with the estimated Gibbs free energies. It shows that the estimated method is correct and the result is reliable. The Gibbs free energy of calcium aluminate sulphate hydrates at any temperature can be estimated by this means. The study will provide theoretical and experimental basis for the lime desulfurization and synthesis of calcium aluminate sulphate hydrates by process of alumina production.

Introduction
Calcium aluminate sulphate hydrates is an important compound in the production of alumina.
In the process of alumina production by sintering process, desilication is an essential process in the process of production. Improving the desilication depth of sodium aluminate solution plays a key role in improving the quality of alumina products and the decomposition rate of sodium aluminate solution. Calcium aluminate sulphate hydrates is the most effective desilication agent for calcium compounds. Under the optimum conditions, the desilication of sodium aluminate solution by sintering method was carried out, and the silicon content index of the refining solution was up to 12500. The synthetic conditions and properties of calcium aluminate sulphate hydrates are helpful to the study of desilication mechanism and the improvement of desilication efficiency, thus affecting the quality and yield of alumina produced by sintering process [1][2][3][4][5]. Therefore, it is important to study and optimize the synthesis thermodynamics of calcium aluminate sulphate hydrates for alumina production by sintering process.
The Bayer process of producing alumina, Sulfur pollution is very harmful Sulfur in bauxite mainly exists in the form of pyrite (in China). The pyrite reacts with sodium aluminate and caustic into the solution in the forms of sulfide, thiosulfate and sulfite, and finally transforms into sodium sulphate (Na 2 SO 4 ) [6][7][8].so that the alkali consumption is increased; the accumulation of sulfur in the production process reduces the decomposition rate of seed. Sulfide and thiosulfate aggravate the corrosion of steel equipment, resulting in higher concentrations of soluble iron in solution and polluting aluminum hydroxide and so on, therefore, it is important to remove sulfur compounds from the process to control Na 2 SO 4 content in a harmless range. It is of great significance to reduce the 2 1234567890''""
It was noted that the desulfurization efficiency is three times higher when calcium aluminate trisulfate hydrate is generated that than when calcium aluminate monosulfate hydrate is produced, leading to a reduction in the loss of Al 2 O 3 .
In order to study and optimize the synthesis of calcium aluminate sulphate hydrates it is necessary to know the Gibbs free energy generated by calcium aluminate sulphate hydrates However, the thermodynamic data such as Gibbs free energy of calcium aluminate sulphate hydrates are rarely reported in the literature, which makes it difficult to further study the related problems.
For this reason, we will study the thermodynamic conditions for the formation of calcium aluminate sulphate hydrates. A method for estimating the free energy of calcium aluminate sulphate hydrates Gibbs is proposed. The free energy of Gibbs and the free energy of reaction are calculated to predict the reaction results. The experimental results show that the calculated results are in agreement with the experimental results, The existence and characterization of calcium aluminate trisulfate hydrates 3Ca·Al 2 O 3 ·3CaSO 4 ·nH 2 O (Ca 6 Al 2 SO 18 ·nH 2 O) were determined, which provided the basis for the thermodynamic study of the synthesis and desulfurization of calcium aluminate sulphate hydrates.

Estimation of gibbs free energy using iterative least squares regression method
For the determination of Gibbs free energy of Complex oxysalt, the commonly used methods are experimental determination methods (such as calorimetry, hydrofluoric acid dissolution method, etc.) and empirical theory calculation method For the complex compounds such as calcium aluminate sulphate hydrates, the determination of Gibbs free energy is very difficult, complex and timeconsuming.
Karpov et al., [16] Nriagu, [17] Chen [18] and Tardyand et al. [19] Proposed empirical formulas for estimating the thermodynamic data of some minerals. It is generally believed that the "regression method", which is first applied to silicate minerals by karpov et al., and further improved by Chen, is a widely used and accurate method.
With the help of regression equation such as Chen estimated standard formation Gibbs free energy of 20 kinds of silicate minerals by using complex multiple regression extrapolations to the limit, and the value is compared with the experiment, the relative error is 1%. However so far, all literatures using iterative least squares regression method are those to estimate the standard Gibbs free energy of formation of silicate mineral. Due to the complex silicate is various oxide compound [20,21] [22], the Gibbs free energy of calcium aluminate sulfate hydrates is estimated by iterative least squares method.

IWMSE2018
IOP Publishing IOP Conf. Series: Materials Science and Engineering 381 (2018) 012099 doi:10.1088/1757-899X/381/1/012099 343K(70°C), but they failed to obtain an exact evidence of generation of calcium aluminate trisulfate hydrates (3CaO·Al 2 O 3 ·3CaSO 4 ·nH 2 O), and did not give the thermodynamic basis of both existing. So, this paper will use the above method to estimate the Gibbs free energies of the two calcium aluminate sulphate hydrates at the temperature, in order to determine whether they can generate and react in sodium aluminate solution. Because of lack of the corresponding thermodynamic data published as reference. The validity and reliability of the estimation will be experimentally tested here. G o value of each oxide under standard condition (298K, 25°C) can be found from the thermodynamic data manual. From this, G o of the calcium aluminate sulphate hydrates under standard condition can be calculated. For the estimation of G values of calcium aluminate sulphate hydrates under other temperatures, changing G o value of each oxide under standard condition into the value under the temperature. The thermodynamic data under certain temperature can be found from the thermodynamic data manual or got from calculation by the classical thermodynamic formula, Table 1 lists G o of thermodynamic data of each oxide under standard conditions [23 ] and the thermodynamic data of corresponding substance under 343K (70°C) [24].  The above method, for each ∑ΔG fi , selected an integer value x i , the x i is called the exponent of ∑ΔG fi . In this way, a form such as a list of the following functions is constituted: The y i =∑ΔG fi , noting y i as observation value of ∑ΔG fi , then fitting curve for the listed functions according to exponential equation, thus a best fitting curve can be obtained. The equation is: Where: a, b and c are for the undetermined coefficients; c stands for the ΔG f of the mineral.

The selection of the exponent xi
In equation (2), exponent x i corresponding ∑ΔG fi is selected artificially. How to select x i is the key of the regression method. In order to eliminate arbitrariness, according to the nature of exponential curve, the selection of x i should combine with tree diagram theory [25] . Figure 1 is that when the mineral combination form number p=4, the combination tree diagram of the corresponding exponent x i . In the figure from the straight branch from top 0 down along multi groups of x i value can be obtained. It can be known from figure 1 that the combination forms of a group of ∑ΔG fi corresponding x i are 0012, 0013, 0023, 0024, 0123, 0124, … Figure 1. When p=4, a tree diagram of possible combination form corresponding x.

The iterative least squares method to solve regression equation
In order to determine the values of a, b and c in the best fitting curve equation (2), firstly it was assumed that a0, b0 and c0 were the initial values, a1, b1 and c1 were the correction values of a0, b0 and c0 respectively, that is, a =a0 +a1, b =b0 +b1 and c =c0 +b1. Thus, The difference between the observation value and the best fitting curve value a exp(b x i )+c was R, there were Substituting equation (3)  can be calculated. According to this method, the values of the least squares fitting curve and its corresponding can be calculated for each x combination form. Then selecting the combination of minimum value in as a best combination, and its corresponding fitting curve is just the best fitting curve. The c value is the Gibbs free energy △G of the compound.
This paper adopts MATLAB programming to calculate [27] (Program omitting). The computational procedure is as Figure 2

Estimation of the gibbs free energies for two kinds of calcium aluminate sulphate hydrates
The values of the Gibbs free energies calculated by the tree diagram corresponding x i combination for the fitted two compounds, the results are in table 4.    (10) Using the above estimated Gibbs free energies, the directions of reactions (8) -(10) at 343K (70°C) were determined. When calculating the reaction equations (8) and (9), the thermodynamic data for each substance 343K (70°C) are shown in table 5.   (7), let n=31 calculate G 7 , and substitute G 7 into reaction (10), △G 10 was calculated .result also is -95.54 kJ/mol All the Gibbs free energies calculated for the desulfurization of a sodium aluminate solution were negative (△G 8 =-104.96 kJ/mol, △G 9,10 =-95.54 kJ/mol). These results show that the reactions to form the calcium aluminate monosulfate and trisulfate hydrates are favorable, i.e., two types of calcium aluminate sulfate hydrate should be produced in the sodium aluminate solution. It is impossible to compare and judge whether the thermodynamic calculation mentioned above is reliable because there are no related reports. We use experiments to verify it.

Experimental
A sodium aluminate solution was prepared in a high-pressure steel bomb using puriss sodium hydroxide and aluminum hydroxide. Puriss calcium hydroxide was placed in a crucible and calcined at 7 1234567890''""
The sodium aluminate solution was prepared with a molar ratio of 3:1 and then diluted with 4 times as much distilled water. The diluted sodium aluminate solution (500 mL) was placed in a 1 L stainless steel cup, and the Na 2 SO 4 reagent was then added to it. The experiments were performed in a constant temperature water bath (the temperature was automatically controlled with precision of ±1°C under , stirring by an agitator at a constant speed. A certain amount of calcium oxide was dissolved in water to make lime milk, which was slowly added to the sodium aluminate solution (24 g/L Na 2 SO 4 ) under stirring for 50 or 120 min at 70°C. The solution was decompressed and filtered after the reaction, and the filter cake quality was determined. The cakes were washed repeatedly with water and dried with cold wind. The reaction products were analyzed by chemical methods, X-ray diffraction and infrared spectroscopy.

Results and Discussion
As shown in the XRD spectrum of the products in Figure 3, both the calcium aluminate monosulfate and trisulfate Hydrates (Ca 4 Al 2 SO 10 ·12H 2 O and Ca 6 Al 2 (SO 4 ) 3 (OH) 12 respectively) were formed when the lime content was 17 g/L, the reaction temperature was 70°C, and the reaction time was 50 min. Notably, the content of these two compounds reached 96%.
The diffraction peaks of calcium aluminate monosulfate hydrate (3CaO·Al 2 O 3 ·CaSO 4 ·12H 2 O) were observed in the XRD pattern of the desulfurization product ( Figure 3) and had characteristic d spacings (characteristic values) of 0.890, 0.445, 0.399, 0.287, 0.242, and 0.223 nm. The diffraction peaks of calcium aluminate trisulfate hydrate [Ca 6 Al 2 (SO 4 ) 3 (OH) 12 ] were also observed and had d spacings of 0.980, 0.561, 0.554, 0.491, 0.388, 0.279, and 0.256 nm. The diffraction pattern was important for identifying the compound products, especially calcium aluminate trisulfate hydrate, which was not identified in previous work. These experimental results are consistent with the fact that the calculated △G o values were significantly less than 0, meaning that the formation of both calcium aluminate sulfate hydrates is favorable and they both should be produced in the sodium aluminate solution. Thus, the estimated Gibbs free energies of the calcium aluminate sulfate hydrates is consistent with the experimental results.
The absence of calcium aluminate trisulfate hydrate (Ca 6 Al 2 (SO 4 ) 3 (OH) 12 ) in previous experiments performed under the same conditions can be explained by the △G calculations and the experimental results presented here. Figure 4 shows the results of an experiment performed under the same conditions as the experiment in Figure 3, except the reaction time was 120 min. When this longer reaction time was employed, the reaction products were only calcium aluminate monosulfate hydrate and Ca 3 Al 2 (OH) 12 ; CaSO 4 and calcium aluminate trisulfate hydrate (Ca 6 Al 2 (SO 4 ) 3 (OH) 12  Subtracting reaction (9) from reaction (8) (7) and (8) The results demonstrating this reaction process are shown in Figures 3 and 4. The presence of calcium aluminate (Ca 3 Al 2 (OH) 12 ) was observed in Figure 4 because calcium aluminate monosulfate hydrate (3CaO·Al 2 O 3 ·CaSO 4 ·12H 2 O) is also a metastable material and decomposes into calcium aluminate and calcium sulfate after prolonged heating [8][9][10][11], shown as by reaction (12) 12 , respectively were formed.
3) Adding lime to a sodium aluminate solution to remove sulfur can lead to the formation of calcium aluminate trisulfate hydrate (3CaO·Al 2 O 3 ·3CaSO 4 ·6H 2 O). The diffraction peaks in the XRD pattern of calcium aluminate trisulfate hydrate were also observed and had d spacings of 0.980, 0.561, 0.554, 0.491, 0.388, 0.279, and 0.256 nm. This is an important characterization.

Acknowledgement
This research was financially supported by the National Science Foundation of China (Ggrant No. 21366009)


extfieldsofstudy: Chemistry Medicine

text:
A novel and highly efficient esterification process using triphenylphosphine oxide with oxalyl chloride

Triphenylphosphine oxide (TPPO) and oxalyl chloride ((COCl)2) are used as novel and high-efficiency coupling reagents for the esterification of alcohols with carboxylic acids via the TPPO/(COCl)2 system at room temperature for 1 h. The reaction represents the first TPPO-promoted esterification under mild and neutral conditions with excellent yields. Furthermore, we proposed a plausible mechanism with the help of 31P NMR spectroscopy.

Introduction
As is well known, carboxylic esters are fundamental organic compounds in organic synthesis and have been widely used in chemical and pharmaceutical industries, such as spices, daily chemical industries, foods, medicines, rubbers, coating materials and so on [1]. Owing to the importance of esters, numerous chemical methods have been reported to accomplish this basic transformation [2,3]. Esters are primarily prepared from the condensation of carboxylic acids with alcohols; generally, the most common methods for the preparation of ester proceed via carboxyl group activation and subsequent reaction with a suitable alcohol [4]. Among them, acid halides were recognized as powerful esterifying agents because of their complete conversion and high yields; however, to the best of our knowledge, acid halides always generate highly acidic by-products such as hydrochloric acid, which could result in decomposition of the initial materials; this method has almost no application in the synthesis of a natural product because of greater possibility of reaction with some acidsensitive functional groups [2][3][4]. Moreover, acid chlorides are prone to hydrolysis under basic conditions through the standard ketene intermediate (scheme 1) [4]. Therefore, it is crucial to find a mild coupling system for the further development of chemistry. ignored with reference to the listed advantages compared with the classical Mitsunobu reaction. This neutral method is applicable for natural product synthesis. Moreover, there is great potential in the chemical industry to resolve the waste due to the by-products of TPPO. Fortunately, there is no report describing the utilization of TPPO with (COCl) 2 for the esterification system thus far.

Preparation of esters by using TPPO/(COCl) 2
To a cold solution of TPPO (1.4 g, 5 mmol) in acetonitrile (CH 3 CN, 5 ml), (COCl) 2 (0.55 ml, 6.5 mmol) was added slowly in drops under magnetic stirring. After 10 min, carboxylic acid (1 equiv, 5 mmol) was added and stirred for 10 min. Then, alcohol (1.3 equiv, 6.5 mmol) and Et 3 N (0.67 ml, 5 mmol) were added in sequence. The reaction was carried out under the protection of nitrogen gas, and the reaction temperature was room temperature. Stirring was continued for 1 h. The progress of reaction was followed by thin layer chromatography (TLC). After the reaction, mixture was evaporated in vacuo and the final product was purified by column chromatography with petroleum ether/ethyl acetate (8 : 1) as the eluent. All esters presented in table 3 are previously known and reported compounds. (table 1) TPPO (1.4 g, 5 mmol), CH 3 CN (5 ml), (COCl) 2 (0.55 ml, 6.5 mmol), PhCOOH (0.67 g, 5 mmol), PhCH 2 OH (0.67 ml, 6.5 mmol) and Et 3 N (0.67 ml, 5 mmol) were added as in the previous procedure. The reaction was carried out under different reaction temperatures and times. In the end, the yield was measured by TLC (table 1).

Procedure for recycling TPPO
At the end of the reaction, the mixture was evaporated in vacuo and corresponding esters with TPPO were separated by column chromatography. The residue was purified with an eluent of 8 : 1 petroleum ether/ethyl acetate and then the polarity of the eluent was changed to 2 : 1, and TPPO was obtained. The crude product was evaporated and dried in vacuo. The white solid obtained was the reusable TPPO.

Results and discussion
As mentioned above, in order to optimize the reaction conditions, we firstly chose benzoic acid (1 equiv) and benzyl alcohol (1.3 equiv) as reaction substrates, which were stirred in CH 3 CN for 1 h at room temperature under the protection of nitrogen gas and then was added TPPO (1 equiv)/(COCl) 2 (1.3 equiv); the reaction mixture was neutralized by triethylamine (Et 3 N), and the desired ester, benzyl benzoate, was obtained in a 90% yield (table 1, entry 1). Initially, we tested the influence of reaction time from 1 to 4 h and different organic solvents including CH 3 CN, PhMe, CH 2 Cl 2 and C 2 H 4 Cl 2 at different temperatures to find the most suitable conditions for the reaction; the best results are summarized in table 1.
According to the results obtained in different organic solvents (table 1, entries 4-6), the esterification yields had no obvious difference. As is well known, TPPO has a better solubility in CH 2 Cl 2 or C 2 H 4 Cl 2 than CH 3 CN and PhMe, and during the experimental phenomena, we found that the solid       in CH 3 CN was dissolved rapidly after the addition of (COCl) 2 , and there were no obvious changes in the phenomena in other solvents; hence we chose CH 3 CN as the reaction solvent. Moreover, by monitoring the reaction using TLC, we found that the reactant was consumed within 1 h and that extended reaction time cannot improve product yields appreciably. By contrast, as the reaction progressed, some generated esters were decomposed because of the reversibility of the reaction (table 1, entries 1-3). Generally, temperature is an important factor for various reactions. However, it had no obvious influence in this system as seen in table 1, entries 6-8.
We also examined the effect of different ratios of TPPO/(COCl) 2   As the esterification reaction of benzoic acid to benzyl benzoate gave a 90% yield (table 3, entry 1), we first applied our reaction conditions to the carboxylic acids carrying both electron-donating groups (-OMe and -Me) (table 3, entries 3 and 4) and electron-withdrawing groups (-NO 2 and -Cl) (table 3, entries 2, 5 and 17), and these carboxylic acids gave their corresponding esters in good yields. However, there was a little difference between electron-donating groups and electrophilic groups. The position of substituent groups could affect the esterification yields (table 3, entries 2 and 17); it was easy to find that m-nitrobenzoic acid has lower conversion (90%) compared to p-nitrobenzoic acid (94%). By now, we clearly realized that the double bond had a crucial role in the esterification (table 3, entry 6); it improved the yield (95%). Of course, steric effects via acids were examined. It was evident that the steric hindrance of acids is an influencing factor for the esterification yield (table 3, entries 7, 8 and 16), which reduces the conversion appreciably.
In comparison, aliphatic alcohols had lower reactivity than aromatic alcohols (table 3, entries 1 and 9, 1 and 10). However, as is to be expected, this impact could be ignored when employing electron-deficient carboxylic acids (table 3, entries 6 and 11, entries 2 and 14). Moreover, the aromatic acid was more reactive than the aliphatic acid (table 3, entries 1 and 15) just as we expected. Tertiary alcohols because of their steric hindrance were tested to react with benzoic acid; as we expected, there was little corresponding ester generated (table 3, entry 12); besides, secondary alcohols gave lower yields for the same reason (table 3, entries 6 and 13).
To explore the effect between substrates and corresponding yields accurately, we have provided a summary about the times of reaction for different substrates in table 4. The progress of the reaction was followed by TLC, and the final product was purified by column chromatography.
As shown in table 4, some cases indicated that the reactants were consumed at 1 h, and most substrates proved that increased reaction time will not change yields greatly, with the exception of table 4, entry 7 by TLC. In conclusion, esterification was difficult to be carried out for substrates with bulky chemical constitution.  165.60, 139.53, 135.81, 131.13, 128.76, 128.67, 128.60

Conclusion
In conclusion, we developed a new and efficient method for the synthesis of esters with excellent yields by the TPPO/(COCl) 2 system. In comparison with the previous methods for the esterification between carboxylic acids and alcohols, this system offered several advantages. Firstly, this system reduced the side reactions that occurred during the classical Mitsunobu reaction, improved the atom efficiency and reduced the reaction cost, because the raw material TPPO is an industrial by-product of the production of various chemicals and has the characteristic of being widely available, and at the end of this reaction, TPPO could be recycled and only CO, CO 2 and HCl are wasted. Secondly, the corresponding esters could be generated with excellent yields in mild and neutral conditions, and can be applied to some substrates bearing sensitive groups in contrast to esterification via the formation of acid chloride. Finally, this system has simple experimental operation at the end of the reaction and the reaction liquid was purified by column chromatography directly. Moreover, we also proposed a plausible mechanism according to 31 P NMR spectroscopy. In our laboratory, we will conduct further investigation of the modified reaction system to extend the application of TPPO.