Gaseous cyclohexanone catalytic oxidation by a self-assembled Pt/γ-Al2O3 catalyst: process optimization, mechanistic study, and kinetic analysis

γ-Al2O3 nanocatalysts with a Pt loading of 0.6–1.0% were prepared successfully via a self-assembly method to be used in the catalytic oxidation of cyclohexanone in a fixed-bed reactor. The nanocatalysts were characterized by the Brunauer–Emmett–Teller method, X-ray diffraction, transmission electron microscopy, Fourier-transform infrared spectroscopy, and temperature programmed reduction-H2 to correlate their activity with their physiochemical properties. Based on these analyses, the catalytic oxidation efficiency for cyclohexanone and the optimal catalytic temperature of 1.0% Pt/γ-Al2O3 were the best among all the nanocatalysts. A novel response surface methodology (RSM) method was employed to evaluate the interactions of the gaseous cyclohexanone concentration (500–4000 mg m−3), the gas hourly space velocity (GHSV, 5000–20 000 h−1), and relative humidity (RH 10–70%) on the catalytic oxidation under normal atmospheric pressure and a catalytic temperature of 235 °C. When the initial catalyst concentration, GSHV, and RH were 2000 mg m−3, 12 000 h−1, and 50%, respectively, the efficiency was 86.5%, nearly equal to that (89.6%) predicted by the RSM model. Intermediate generation, CO2 production, and variations in the cyclohexanone oxidation route were evaluated under the optimal process parameters. During catalytic oxidation, nearly all the cyclohexanone was converted to CO2 and H2O, and the mineralization rate was 80% under a RH of 50%. A kinetic model was proposed to describe the interaction effects between the partial pressure and catalytic temperature on the conversion of cyclohexanone. It was found that the predicted values by the model fit well with the experimental values, with an R2 of 0.99.

Introduction
Atmospheric pollution has become a serious problem in recent years because of the rapid development of the chemical industry and increased human activities. 1 As some of the main air pollutants, volatile organic compounds (VOCs) are directly released without any treatments, which not only increases the degree of atmospheric pollution, but also leads to harmful effects on human health. 2 Recent studies showed that the increase of ozone in the troposphere is related directly to the emission of VOCs. 3,4 Therefore, efficient and economic methods for reducing pollutants like VOCs are required.
Among the various current technologies for VOC abatement, catalytic oxidation has been considered to be one of the most effective and promising technologies because of its low operation temperature, high purication efficiency, recoverable heat, and lack of secondary pollution. 5,6 The performances of different methods for VOC removal are affected by several factors, such as the types of VOCs, the types of catalysts, the process parameters, and purication requirements. 7,8 O'Malley and Hodentt 9 showed that the reactivity of VOCs, in terms of total oxidation, with different functional groups, such as alcohols, aromatics, ketones, carboxylic acids, and alkanes, varies. Three types of catalysts are used widely for VOC oxidation, namely precious metals, transition metal oxides, and zeolites, and most studies have focused on the relationship between their structural characteristics and performances. [10][11][12][13] Process parameters, such as target concentration, space velocity, and temperature, are mostly considered for the evaluation of the prepared catalysts. According to the previous studies, more attention should be paid to aromatics, chlorinated VOCs, and alkanes because aromatics (like benzene and toluene) are emitted mostly by industry, and chlorinated VOCs are toxic to catalysts (mainly because of the formation of highly chlorinated by-products), which affects its usage time, while alkanes are not oxidized efficiently by other methods (such as bio-oxidation and photo-oxidation). 14 As an important industrial material, ketones are also used widely and emitted not only from the centralized emission (usually chimney) but also decentralized emission. 15 Cyclohexanone is an important chemical material in the manufacture of nylon, caprolactam, adipic acid, and resin. Because of its low boiling point, cyclohexanone is highly volatile, and, thus, it is a typical VOC, which constitutes a serious threat to human health and the environment. Long-term exposure to low concentrations of cyclohexanone may result in serious diseases, e.g., neurasthenia and leukemia, while inhaling high levels of cyclohexanone in a short time causes headaches and unconsciousness, and it can anesthetize the central nervous system. Cyclohexanone and other VOCs transform into the precursor of ozone in the atmosphere, and they react with other precursors of ozone (like NO x ) under light to form the most serious form of air pollution. 16 Therefore, there is a need to control the production of cyclohexanone.
The essence of VOC oxidation is the breaking of chemical bonds. Once the rst bond is broken, sequential reactions to form CO 2 and H 2 O are accomplished more easily. 17 Among the factors affecting VOC destruction (promoters, surface areas or sizes of the catalysts, and catalyst loading), the active component is the most crucial factor. The noble metals Pt and Pd in g-Al 2 O 3 catalysts have been used widely and efficiently for the oxidation of VOCs. 18 Pd exhibits higher activity toward shortchain hydrocarbons, while Pt is more active for the oxidation of long-chain hydrocarbons and aromatics. Pt/g-Al 2 O 3 is widely known as one of the most effective commercial catalyst for the oxidation of hydrocarbon molecules such as benzene. 18,19 As a result, Pt/g-Al 2 O 3 is usually added to a non-thermal plasma reactor for VOC removal. It has been shown that the incorporation of this catalyst might improve energy efficiency, maximize product yield, and minimize byproducts. 20,21 Compared with other VOCs, there are few studies of the purication of cyclohexanone and its oxidation using different catalysts. Although Pt/g-Al 2 O 3 is the most widely applied catalyst, 22 its use for cyclohexanone treatment has not been reported, and the oxidation mechanism of cyclohexanone has not been revealed. In addition, most previous studies only focused on the catalytic oxidation effect caused by a single factor, without investigating the interaction effects of different factors. As is well known, several process parameters collectively determine the VOC removal performance, and consideration of their combined effect is much more important. 23 Such knowledge (like the optimal operational parameters) is useful for industrial applications. Based on these reasons, studies of the catalytic oxidation of cyclohexanone by Pt/g-Al 2 O 3 are needed not only to hasten its possible industrial application, but also to determine its oxidation mechanism.
In the present study, we used the Brunauer-Emmett-Teller method, transmission electron microscopy (TEM), X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), and temperature programmed reduction-H 2 (H 2 -TPR) to evaluate the performance of a new, self-assembled Pt/g-Al 2 O 3 catalyst containing 1.0 wt% Pt in the catalytic oxidation of cyclohexanone and to conduct a structural characterization of the catalyst. The activity of the prepared catalyst was tested under several reaction conditions, and the optimal conditions (including the concentration of cyclohexanone, the relative humidity [RH], and the space velocity) were determined by the response surface methodology method, thus providing a quantitative understanding of the interaction effects and determining the relationship between the variables and the cyclohexanone removal efficiency. The product distribution and formation pathway of by-products are discussed to explore the oxidation mechanism of cyclohexanone. Additionally, a kinetic analysis of the catalytic process was also studied, and the obtained kinetic model was in good agreement with the experimental results. The fundamental information derived from this study is important for the application of Pt/g-Al 2 O 3 and other similar catalysts in cyclohexanone and other VOC abatement.

Catalyst preparation
The Pt/g-Al 2 O 3 catalyst was prepared according to a previously reported procedure, with a few modications. 24,25 K 2 PtCl 4 (1.4 g) was dissolved in 12 mL of distilled water, followed by the addition of an ethanol solution containing 2.36 g of bis-dba (dba ¼ dibenzylideneacetone) and 2.8 g of sodium acetate at 50 C. Then, the mixed solution was reuxed for 1 h at 90 C, and nally it changed from an initial pale-yellow liquid into a dark purple precipitation. Then, the precipitate was ltered aer 12 h, washed three times with 20 mL of n-pentane, and dried overnight under vacuum; the wash and dry cycle was conducted for twice. Then, Pt 2 (dba) 3 was prepared for further use.
A certain amount of Pt 2 (dba) 3 was added to a propylene carbonate solution, placed in a stainless-steel cylinder, and mixed evenly. Then, hydrogen was used to replace the air for the balance of the partial pressure (3 MPa). The solution was stirred at room temperature for nearly 2 h until a brown colloid was produced, and then g-Al 2 O 3 was added with stirring for 24 h. The solid catalyst was separated by ltration and dried at 80 C. Only trace levels of Pt were observed in the clear ltrate, indicating a complete adsorption of the Pt on the carrier. Different mass fraction Pt-nano particles (0.6-1.0%) of g-Al 2 O 3 were prepared by the assembled method. The packing density of the prepared catalyst was 0.826 kg cm À3 and the particle size was 0.5-3.0 mm. For comparison, a blank g-Al 2 O 3 catalyst with no Pt loading was prepared.

Catalyst characterization
TEM images were acquired using a Tecnai G2 F30 S-Twin (Philips-FEI, Amsterdam, the Netherlands) instrument at an acceleration voltage of 200 kV. The structures of the catalysts were assessed using an XRD analyzer (PANalytical, Almelo, the Netherlands). The conditions of analysis were as follows: Cu-Ka radiation (l ¼ 1.54178Å); scanning rate, 0.03 s À1 ; and scanning range (2q), 20-80 C.
The BET surface area of the catalyst was measured by N 2 adsorption at a liquid-nitrogen temperature of 196 C, using a surface area analyzer (ASAP 2010; Micromeritics, Norcross, GA, USA). To assess surface functional groups, FTIR (Nicolet 6700; Thermo Scientic, Waltham, MA, USA) was conducted at wave numbers of 400-4000 cm À1 . TPR-H 2 measurements were conducted on a Micromeritics Pulse ChemiSorb 2705 to address the reducibility of nanocatalysts. Approximately 50 mg of the sample was loaded into a reactor and pretreated at 200 C in an Ar stream for 1 h to remove the adsorbed carbonates and hydrates. Aer cooling to 50 C, a ow of 10% H 2 -90% Ar was passed through the samples, and the temperature was increased at a rate of 10 C min À1 up to 1000 C, while a thermal conductivity detector (TCD) was used.

Experimental setup
The measurement of the catalytic activity of the oxidation of cyclohexanone was performed at atmospheric pressure in a conventional uidized bed reactor reaction system, and the schematic diagram is shown in Fig. S1. † Air was supplied by an air-compressor and divided into three streams, all of which were controlled by a mass ow controller. One was saturated with cyclohexanone by injecting it into a bottle of cyclohexanone liquid, followed by mixing with a second air stream to dilute the gaseous cyclohexanone to obtain a certain concentration. The third one was injected into distill water to produce a certain humidity of gas. Finally, all these streams went into the mixed bottle, and the reactant waste gas was obtained.

RSM design
RSM (Design Expert Version 8.0) was adopted in this study, and central composite design (CCD) is one of the most popular methods for the experimental design. Three factors were considered with two levels. The a (a ¼ AE1.2) distance was designed according to the need of the experiments, and six central points were generated by the automatic system, as shown in Table 1.
Aer analyzing the responses according to the designed experiments, a mathematical model with interactions was obtained by the following equation: where y is the expressed response; X represents the independent variables; b 0 indicates the intercept constant; b i shows the coefficients of linear terms; b ii represents the coefficients of quadratic terms; b ij shows the coefficients of interaction terms; x indicates the experimental error; and k represents the number of independent variables.

Analytical methods
Inlet gas and outlet gas were analyzed by a gas chromatograph equipped with a ame ionization detector (GC-FID, Agilent GC 7890N; Santa Clara, CA, USA). The analytical column was a capillary column (Agilent 19091N-213 HP-INNOWax polyethylene glycol 32 m Â 320 mm Â 0.50 mm) with a temperature of 180 C, while the detector temperature was 200 C. The sample volume was 1000 mL, and each experiment was repeated three times under the same conditions. CO 2 accumulation was tested with an HP-Plot-Q column (30 m Â 0.32 mm Â 20 mm) and a TCD. Helium at a ow rate of 5 mL min À1 was used as the carrier gas. The column and detector temperatures were 313 K and 473 K, respectively.
The off-gas (containing various gaseous products) was captured using an adsorption tube at a set of temperatures for 1 h. Then, the absorbed compounds were released into a thermal desorption instrument, which were injected into a GC-mass spectrometry (GC-MS) system (Agilent 7890 A GC equipped with an Agilent 5975C Mass Selective Detector) with an HP-INNOWax capillary column (60 m Â 0.32 mm Â 0.5 mm). The temperature was maintained at 36 C for 2 min, and then it was increased to 250 C at a rate of 10 C min À1 , at which point it was maintained for 6 min.
For other small organic intermediates, the catalysts were analyzed by FTIR (Nicolet 6700, Thermo) in the wave number range of 400-4000 cm À1 . The differences in the peaks between the fresh and used catalyst might be some intermediates that are generated by the catalytic process.

Catalyst characterization
g-Al 2 O 3 catalysts with three different Pt contents (0.6%, 0.8%, and 1.0%, wt%) were prepared, and their structural characteristics were analyzed by BET, TEM, XRD, FTIR, and H 2 -TPR. The optimal catalyst was determined by both the structure and conversion of cyclohexanone.
3.1.1 Specic surface area and porous volume by BET measurements. The BET surface area and the distribution of the pore volume are shown in Table 2. It was observed that the addition of Pt had little effect on the surface area of g-Al 2 O 3 . For 0.6% Pt/g-Al 2 O 3 , the surface area and the pore volume were 179 m 2 g À1 and 0.94 cm 3 g À1 , respectively, and adding Pt to 0.8%, and 1.0% wt% decreased both the surface area and pore volume to 168 and 150 m 2 g À1 and 0.88 and 0.83 cm 3 g À1 , respectively. Such results suggested that increasing the Pt content leads to covering the surface of the aluminum oxide, which decreased the overall surface area. Adsorption is one of the important steps of the catalytic reaction, and the surface area of the catalyst is a VOC-adsorption site. 14 In addition, some mesopores of the g-Al 2 O 3 support might be blocked by Pt species during the preparation. However, Table 2 shows that the total pore volume did not vary much, which indicated that the high dispersion of the Pt nanoparticles did not block the pores of g-Al 2 O 3 . The larger surface area and pore volume could be a result of suitable preparation, similar to the phenomenon reported by Abbasi et al., 17 who showed that a high surface area could be a result of nanometric particles and a suitable calcination temperature. It is noted that the percentage of mesoporous (2-50 nm) particles increased slightly with the loading amount of Pt, which suggested that the prepared catalyst would have better activity. 26 Such irregular pores would create numerous active sites, and this special structure would facilitate the adsorption and diffusion of organic molecules or the produced compounds (CO 2 and H 2 O), thus avoiding the limitations of interphase mass transfer and promoting their catalytic activities.
3.1.2 TEM. The morphology and microstructure of the synthesized catalysts were investigated by TEM. Fig. 1 shows the TEM images of g-Al 2 O 3 and Pt (0.6-1.0%)/g-Al 2 O 3 . The results showed that the active phase (Pt) was well dispersed with a particle size of 2-10 nm. Pt nanoparticles (black dots) were distributed uniformly aer being loaded onto the g-Al 2 O 3 support, and the average diameters of the Pt changed little, without obvious aggregation, indicating that the preparation had no effect on the size of the Pt nanoparticles. Nobel metals promote the migration/exchange of oxygen species, and their presence provides catalysts with many more reactive sites. [27][28][29] With increasing amounts of active Pt, the nanoparticles on the surface increased gradually and the pore structure decreased gradually, which is consistent with the BET surface area and pore size distribution of the corresponding pre-and post-Pt loaded g-Al 2 O 3 .
3.1.3 XRD and FTIR analysis. The XRD patterns of the selfassembled Pt/g-Al 2 O 3 catalyst with different Pt mass fractions are shown in Fig. 2a. The diffraction peak at 2q ¼ 66.64 (440), 45.92 (400), and 37.40 (331) corresponded to the planes of g-Al 2 O 3 . 30 However, there were no Pt peaks in the XRD spectra at 2q near 40.0 (111), 46.5 (200), and 67.4 (220), unless the Pt content increased to 1.0%. These results indicated that the dispersion of Pt on the g-Al 2 O 3 support was very good, and no large crystallites formed. 31 With the increase in the amount of loaded Pt, the intensity of the peak at 2q ¼ 14.5 increased. This suggested that the interactions between Pt and g-Al 2 O 3 were strengthened, which would lead to the production of more free radicals for VOC oxidation. 32,33 Fig. 2b describes the FTIR spectra of the Pt-1.0%/g-Al 2 O 3 catalysts and g-Al 2 O 3 . Stretching vibrations for structural -OH and adsorbed H 2 O for all the samples were observed at wave numbers of approximately 3430, 1640, and 1410 cm À1 , and the peak intensity of the Pt-1.0%/g-Al 2 O 3 catalyst was much stronger than that of unloaded g-Al 2 O 3 . This difference suggested that more hydrogen bonds formed on the catalyst, which would lead to the production of more hydroxyl radicals, thereby beneting the catalytic oxidation of VOCs. 30 A characteristic vibrational band next to 588 cm À1 for Al-O-Al has been reported, and its intensity was much stronger aer Pt loading. A new vibration telescopic peak at 1474 cm À1 appeared for the prepared Pt-1.0%/g-Al 2 O 3 catalyst. This peak could be a Pt-Al-O bond that was formed by the strong interaction between Pt and g-Al 2 O 3 during the preparation. Such bond formation could enhance the catalytic activity. 24 3.1.4 TPR-H 2 . To investigate the reduction behavior of the catalysts, H 2 -TPR studies were performed for g-Al 2 O 3 and Pt-(0.6-1.0%)/g-Al 2 O 3 catalysts as shown in Fig. 3. There was no obvious hydrogen consumption in the TPR prole of pure Al 2 O 3 . For different amounts of Pt loaded-g-Al 2 O 3 , reduction peaks were found at a temperature range of approximately 200-400 C. By comparing the reduction peak position and its intensity, we found that the peak position shied to lower values (from 375 C to 216 C) and the intensity increased (approximately 10fold) as the Pt-loading amount increased from 0.6% to 1.0%. The peaks observed below 216 C were mostly attributed to the well-dispersed state of Pt on the Al 2 O 3 support, as well as the reduction of surface oxygen species adsorbed to oxygen vacancies or surface lattice oxygen. 34 In addition, based on the reduction mechanism, the reducing temperature was highly associated with the activity of oxygen species on the catalysts, indicating that the catalysts with low-temperature reducibility would have more active oxygen species. 35 Such results suggested that more active oxygen species could be produced by introducing Pt decoration, which would be benecial for heterogeneous catalytic oxidation reaction, especially at lower temperatures.
In addition, according to the H 2 -TPR spectra of Pt-1.0%/g-Al 2 O 3 before and aer the purication of gaseous cyclohexanone, we found that the reduction peak shied to a much higher temperature of 412 C, which suggested that the active center around Pt was lost aer utilization. 31 3.2 Catalytic activity and stability during cyclohexanone oxidation    temperature, respectively. Fig. 4a shows that the activity of the synthesized catalysts increased with increasing amounts of Pt loading. We also found an abrupt change in the catalytic activity at 190 C for larger amounts of Pt-loading, which was much lower than that observed for Pt-0.6%/g-Al 2 O 3 . Based on these results, proper Pt amounts (0.8-1.0%) might induce higher catalytic activity, but higher Pt loading only increased the catalytic activity over a limited temperature range. Therefore, considering the handling cost and the catalyst's characteristics, Pt-1.0%/g-Al 2 O 3 could be considered as the most suitable catalyst for the catalytic oxidation of cyclohexanone. Catalytic stability is critical for the application of catalytic oxidation. To investigate the long-term activity of the Pt-1.0%/g-Al 2 O 3 catalyst, experiments were performed for up to 250 h, and the results are shown in Fig. 4b. A decrease in cyclohexanone conversion from 100% to less than 80% was observed at 225 h. A deactivation of noble metal catalysts during VOC catalytic oxidation is a common phenomenon. Most frequently, deactivation of noble metal-loaded catalysts is related to metal sintering, transformation of the active metallic oxide to a metal element at higher temperature, or occlusion by the support. 36,37 In this case, to clarify the reasons for the deactivation of the 1.0% Pt-based catalyst, two views were taken into consideration: a change of the BET surface and a temperature shi for the H 2reduction peak. A small difference was observed in the BET surface areas of the catalyst samples before and aer the catalytic tests. For fresh Pt-1.0%/g-Al 2 O 3 , the BET surface area was 150 m 2 g À1 , while for the aged catalyst, the BET surface area decreased to nearly 122 m 2 g À1 . Additionally, the H 2 -TPR data indicated that the aged catalyst had a much higher temperature for the reduction peak. Other studies also found these mostly similar phenomena when they studied catalyst stability. 38,39 Fig. S2 † shows the morphology images of the catalyst before and aer use. The color changed from brown to yellow, which suggested that catalytic intermediates accumulated at the surface of catalysts and catalytically active elements might be gradually lost during the utilization. This phenomenon is similar to that reported in other studies, which showed that coking deactivated catalysts aer long periods of use. 40,41

Optimization of the process parameters by RSM
Several process parameters have a combined effect on catalytic oxidation, and using a single factor may not accurately describe the actual situation. 23 In a xed-bed catalytic-oxidation reactor, the reaction processes are very complex; however, most studies usually focused on the inuence of single process parameter variables on catalytic degradation, and they rarely considered the combined effect by jointly examining the effect of several parameters. In the present study, CCD, 42 the most popular experimental method, was used to obtain the optimal operational conditions for cyclohexanone catalytic oxidation.
Three main parameters, the initial catalyst concentration, the RH, and the GHSV, were chosen. A series of experiments were conducted, and the results of the CCD are shown in Table 3. Through the interactive factors analysis by analysis of variance, we found that the initial catalyst concentration and the GHSV had obvious effects on the catalytic oxidation of cyclohexanone, as did the RH. Additionally, the combined effect of every combination of two factors was obvious, indicating that the interactions between two different parameters were very strong, for example, between the initial catalyst concentration and the GHSV, and between the RH and the GHSV. Not only in  terms of single factors, but also in combination, both the GHSV and RH were more important parameters. The removal efficiency of cyclohexanone decreased with increasing GHSV. A much larger GHSV will remove a large amount of heat, and thus affect the catalytic oxidation efficiency. Meanwhile, a larger RH also had a negative effect on the removal efficiency, since there was competition between the much greater number of H 2 O molecules and the target pollutants in the surface-active sites. All these phenomena could be obtained from the response surface graph and a contour plot between two process parameters, which are supplied in Fig. S3. † A predicted model was obtained from the CCD and RSM analysis and used to demonstrate the relationships among the initial catalyst concentration, the GSHV, and the RH within the range of the tested values. The mathematical model is given below: where A is the initial catalyst concentration (mg m À3 ); B is the GHSV (h À1 ); and C is the RH (%). According to analysis from the RSM model, the maximum conversion efficiency was predicted to be 89.6%, when the initial catalyst concentration, the GSHV, and the RH were 1992.26 mg m À3 , 11 788.84 h À1 , and 47.02%, respectively. A conrmatory test was conducted using an initial concentration of 2000 mg m À3 , a GSHV of 12 000 h À1 , and an RH of 50%, and the actual efficiency was 86.5%, suggesting that the predicted result was in good agreement with the experimental results.

Intermediate distribution and mechanistic analysis
Under the optimal reaction conditions, the oxidation products of cyclohexanone in the effluent were analyzed by GC-MS, and the results are shown in Fig. 5. The results showed that several intermediates, such as benzene, toluene, and cyclopentanone, formed during the catalytic process.
Although the concentrations of some intermediates in the outlet were too low to enable their detection by GC-MS, they might have accumulated on the surface of the catalyst. Therefore, Pt/g-Al 2 O 3 (before and aer use) was analyzed by FTIR. Its spectrum of intermediates is shown in Fig. 5, in which some differences between the curves were observed. A larger peak at a wave number of 3400 cm À1 was observed for the used catalyst, and this peak was identied as OHc. The intensity of a peak at approximately 2900-3100 cm À1 (mainly belonging to CH 3 c or CH 2 c) decreased, suggesting that it might have been oxidized during the reaction. Additionally, some new peaks that were observed at approximately 2400-2300 cm À1 and 700-500 cm À1 might be CO 2 . Peaks at approximately 1900-1000 cm À1 indicated that cyclohexanone derivatives accumulated. Some small aldehydes and carboxylic acids also formed during the catalytic oxidation of cyclohexanone, which might be expressed as the peaks at approximately 1600-1500 cm À1 and 1300-1000 cm À1 , respectively. Based on these FTIR analyses, the main intermediates were alcohols, small-molecule carboxylic acids, aldehydes, carbon dioxide, and cyclohexanone ring derivatives.
Nobel metals for catalytic oxidation may follow two classic mechanisms: the Langmuir-Hinshelwood model (in which all reactants are adsorbed prior to the redox process) and the Eley-Rideal model (in which gaseous hydrocarbon is oxidized directly by the free radical Oc). 17,43 The general mechanism of oxidation using noble metals is considered to involve the dissociative adsorption of oxygen with the formation of a "surface site". Then, the gaseous organic reactant can react directly at these surface sites. However, sometimes the reactant is also weakly adsorbed initially and then oxidized. It has been proposed that catalytic oxidation oen occurs by a parallelseries mechanism. Lattice oxygen, as well as gas-phase oxygen, participates in the catalytic oxidation process, in which [Pt 4+ ] acting as an active redox center plays an important role during the redox cycle. [44][45][46] Based on the detected intermediates and the mechanism of catalytic oxidation, the main degradation pathways of cyclohexanone were proposed, and they are shown in Fig. 6. The removals of cyclohexanone can occur in two possible ways, in which both the dissociative adsorption of oxygen and gas-phase radicals (such as Oc, HO 2 c, and OHc) have the largest effects. Near the surface of the catalyst where oxygen is absent, H-abstraction always played a role during the reaction process.
In the rst way, cyclohexanone is converted via H 2 -abstraction reactions to yield the cyclohexanone radicals R1, R2, and R3. However, these radicals are unstable and might change through a series of rapid reactions, forming 2-cyclohexene-1ketone (for R1, R2, and R3) and C 3 H 5 COC 2 H 5 (for R2 and R3). 47 2-cyclohexene-1-ketone can be attacked directly by the hydroxyl radical, oxygen free radical, and other free radicals, nally forming complete mineralized products (e.g., H 2 O and CO 2 ). The catalytic oxidation of C 3 H 5 COC 2 H 5 is relatively complex. Through ring regeneration, C 3 H 5 COC 2 H 5 is converted to methyl cyclopentyl ketone, and then it is attacked by radicals to form cyclopentanone via a missing methyl radical. Finally, cyclopentanone undergoes ring opening and is further oxidized to form CO 2 and H 2 O. In this study, the intermediates of 2-cyclohexene-1-ketone were not detected, and this pathway was hypothesized based on previous studies. 46,47 In the second way (shown at the bottom of Fig. 6), reduced intermediates (such as benzene, toluene) appeared. It might be that the distribution of oxygen was inhomogeneous in the reaction atmosphere; some places nearly the surface of the catalyst had a lower oxygen concentration (<2%) or no oxygen. Therefore, when both free oxygen and oxygen adsorbed by the catalyst surface were consumed, cyclohexanone underwent a reforming reaction (H-abstraction) that was induced by the active component of catalyst (mainly Pt 4+ ), which produced some reduced intermediates like benzene and toluene. 48 Hydrogen on the benzene ring could be replaced by CH 3 c under special conditions; as a result, benzene was converted to toluene. When toluene and benzene le the surfaces, both these compounds could be attacked by free radicals (such as oxygen free radical), thereby resulting in the formation of benzyl alcohol. Then this compound underwent a series of hydroxylation and oxidation reactions of its aromatic ring, resulting in its conversion to benzaldehyde and benzoic acid. All these intermediates might be easily oxidized to the nal products (CO 2 and H 2 O).
To evaluate the prepared catalyst for the complete removal of cyclohexanone, the amount of CO 2 was measured by a gas chromatograph equipped with a TCD detector. The initial concentrations of cyclohexanone and the GSHV were the same as the obtained optimal values, while RHs were taken at three different ranges. Fig. S4 † shows that both the catalytic oxidation efficiency and mineralized efficiency increased with increasing reaction temperatures. We found that higher RHs (65-75%) had a more negative effect on the mineralization, which indicated that more H 2 O could compete with the reactants and intermediates for the active site on the surface of the catalyst. According to our calculations, the maximum mineralization rate was nearly 80% under a RH of 50%.

Kinetic analysis
To study the kinetics of cyclohexanone catalytic oxidation using Pt/g-Al 2 O 3 , a more sophisticated kinetic model was used as follows: 49,50 where P cyclohexanone and P O 2 are the partial pressure of cyclohexanone and oxygen, respectively; a and b are the reaction orders; k 0 is the kinetic constant; E refers to the activation energy of the reaction; and R is the molar gas constant (equal to 8.314 J mol À1 K À1 ).
To obtain a and b, a series of experiments were conducted under different partial pressures of cyclohexanone and oxygen. The calculated reaction rates are shown in Table 4. Table 4 shows that the partial pressure of cyclohexanone had a decisive inuence on the reaction rate under both high and low temperatures. When the temperature was kept constant, the reaction rate increased with increasing partial pressures, and the increase of the reaction rate at high temperatures was much larger than the one obtained at low temperatures. The effects of oxygen were not greater than 2%. When the catalytic reaction occurred at low temperatures, the oxygen taking part in the reaction might be the adsorbed oxygen on the surface of the active catalyst, and the oxygen coverage would increase slightly with increasing partial pressures; at the same time, the absorbed oxygen would also gradually leave the surface. As a result, the oxygen coverage decreased because the absorbed rate was less than the desorbed one, nally arriving at a lower value when the adsorption and desorption were equal. Therefore, the effect of the partial pressure of oxygen can be ignored, which is to say that b ¼ 0; our results are similar to those of Artizzu-Duart et al. 51 The reaction rate can be written as follows: Using the logarithm method, some parallel lines between ln P cyclohexanone and ln r cyclohexanone were obtained under different temperatures (Fig. 7a). The slopes of these lines were the value of a, 0.31.
To obtain the values of E and k 0 , the logarithm method was also used. Some parallel lines were obtained under different process parameters, with ln r cyclohexanone and ln P cyclohexanone , and ln r cyclohexanone and ln(1/RT) (Fig. 7). Based on these lines, E and k 0 were 52.516 kJ mol À1 and 1.10 Â 10 6 , respectively. These results are very promising because they imply that the modication of g-Al 2 O 3 yielded a very effective synthetic effect, which induced E values that are comparable to those of some noble metal or dual-metal catalysts in the VOC oxidation process. Finally, the reaction rate was obtained as follows: Àr cyclohexanone ¼ 1:10 Â 10 6 exp À6316:58 To determine the accuracy of this model, experiments were conducted, and the results were compared with the values predicted by the model. Fig. 7b shows that the theoretical model was in good agreement with the experimental results, and the relative deviation was less than 3%, with an R 2 of 0.9934. Therefore, the obtained kinetics model for cyclohexanone catalytic oxidation was accurate and credible compared to the actual experimental values.

Conclusions
A platinum-doped g-Al 2 O 3 catalyst was prepared and used for the complete oxidation of cyclohexanone in the present study. A series of characterizations, including TEM, H 2 -TPR, FTIR, and BET, were used to identify the structure and properties of the prepared catalysts. The results showed that nano Pt or Pt oxides formed as the crystalline phase on the Al 2 O 3 support, with a larger surface area for catalytic application. Other experiments revealed that 1.0% Pt-g-Al 2 O 3 had a higher activity for the abatement of cyclohexanone. Through an RSM analysis, the optimal process parameters were obtained, and it was found that the GHSV and RH had much greater interaction effects on the conversion of cyclohexanone. According to the model based on the RSM, the predicted catalytic oxidation efficiency was similar to the experimental value under the optimal operation conditions. A possible conversion pathway was constructed based on the detected intermediates, and the results showed that both the dissociative adsorption of oxygen and gas-phase radicals (such as Oc, HO 2 c, OHc) had effects.
Based on the reaction rates under different partial pressures of oxygen and cyclohexanone, a kinetic model was created with the obtained tting constants, and it accurately predicted the rates of catalytic oxidation. This study provides a possibility for the catalytic combustion of cyclohexanone in industrial applications.

Conflicts of interest
There are no conicts of interest to declare.


A mini-review on organic synthesis in water

Organic synthesis in water has attracted the attention of chemists for many years. Water is the nature’s solvent and possesses distinguished physical and chemical properties. It exhibits powerful hydrogen bonding and wide temperature range to remain in liquid state. In recent years, many organic transformations have been carried out in water.1,2 Many organic solvents like benzene, methanol, toluene are carcinogenic and can be toxic to human health and cause an environmental menace by polluting the atmosphere. The replacement of volatile organic solvents in organic reaction is an important aim of green chemistry.3,4 Earliest known examples of organic synthesis in water include Wohler’s urea synthesis and Baeyer and Drewsen’s indigo synthesis.5–7

Introduction
Organic synthesis in water has attracted the attention of chemists for many years. Water is the nature's solvent and possesses distinguished physical and chemical properties. It exhibits powerful hydrogen bonding and wide temperature range to remain in liquid state. In recent years, many organic transformations have been carried out in water. 1,2 Many organic solvents like benzene, methanol, toluene are carcinogenic and can be toxic to human health and cause an environmental menace by polluting the atmosphere. The replacement of volatile organic solvents in organic reaction is an important aim of green chemistry. 3,4 Earliest known examples of organic synthesis in water include Wohler's urea synthesis and Baeyer and Drewsen's indigo synthesis. [5][6][7] In recent years, several N, O, and S containing heterocyclic compounds are synthesized employing green synthetic protocols. It is of considerable interest to learn more about varied green technology platforms, which have been utilized for the synthesis of important heterocyclic scaffold. Being an important and growing area of research, these green protocols have been regularly reviewed. The present attempt is to review briefly water mediated organic reactions, resulting in the synthesis of various heterocyclic frame works and to highlight the significance as well as utility of water as a green solvent. 8 Zarei M et al. 8 reported the synthesis of pure azo Schiff bases (2) in high yields by mixing of the reagents either as aqueous slurry or by grinding at room temperature. Azoaldehyde (1) was prepared from p-anisidine and 2-hydroxy-3-methoxybenzaldehyde (o-vanillin) in aqueous medium at 0-5°C and then 1 was allowed to react with amines in a small amount of water at room temperature to produce azo Schiff bases (2) in excellent yields ( Figure 1).

Synthesis of pyrano[2,3-c]pyrazoles 9
Silica-water reaction medium was applied by Pravin et al. 9 for the synthesis of various pyrano [2,3-c]pyrazole derivatives based on the adsorptive nature of silica. The reaction was carried out under mild and neutral conditions accepting several functional groups present in the molecules thus reducing the possibility of many unwanted side reactions. Present method offers marked improvements with regard to product yield, reaction time, and greenness of procedure, avoiding hazardous organic solvents/toxic catalysts and provides a better, clean and practical alternative to the existing protocols ( Figure 2).

Synthesis of aryldipyrromethanes 10
Taoufik et al. 10 have reported the oxidation of aryldipyrromethanes with p-chloranil (in dichloromethane), or DDQ (2,3-dichloro-5,6-dicyano-1,4-benzoquinone) at room temperature to obtain the corresponding dipyrrins in good yields (50-78%) ( Figure 3). has been developed by Kidwai et al. 11 without using any catalyst and hazardous organic solvents and corrosive acids or bases. Waterinsoluble solid products obtained in short time are found to be essentially pure and in very high yield ( Figure 4).

Synthesis of 2-aminothiazoles 12
A highly efficient and facile method has been described by Taterao et al. 12 for the synthesis of substituted 2-aminothiazoles in water without any added catalyst or co-organic solvent. The reaction was carried out at ambient temperature and the products were obtained in excellent isolated yields. The developed protocol is successfully applied for the preparation of an anti-inflammatory drug, fanetizole ( Figure 5).

Synthesis of alkyl or aryl-14H-dibenzo [a, j]xanthenes 13
Alkyl-or aryl-14H-dibenzo [a,j]xanthene derivatives were synthesized efficiently by the reaction of beta-naphthol and aliphatic and aromatic aldehydes in the presence of KAl(SO 4 ) 2 x12H 2 O (alum) under aqueous condition at 100°C by Dabiri et al. 13 Different types of aromatic and aliphatic aldehydes were used in the reaction and in all cases the products synthesized successfully. Several solvents were examined for this reaction; however, in terms of reaction yield and time, water was found to be the optimum solvent ( Figure 6). Syntheses of N-azacycloalkanes, pyrazole, pyrazolidine, and phthalazine derivatives 14 Ju et al. 14 have reported the synthesis of nitrogen-containing heterocycles from alkyl dihalides (ditosylates) and primary amines and hydrazines via a simple and efficient cyclocondensation in an alkaline aqueous medium that occurs under microwave irradiation. This improved greener synthetic methodology provides a simple and straightforward one-pot approach to the synthesis of a variety of heterocycles, such as substituted azetidines, pyrrolidines, piperidines, azepanes, N-substituted 2,3-dihydro-1H-isoindoles, 4,5-dihydropyrazoles, pyrazolidines, and 1,2-dihydrophthalazines ( Figure 7).  15 Polshettiwar et al. 15 have reported an expeditious room temperature synthesis of pyrazoles and diazepines by condensation of hydrazines/ hydrazides and diamines with various 1, 3-diketones. This greener protocol was catalyzed by polystyrene supported sulfonic acid (PSSA) and preceded efficiently in water in the absence of any organic solvent within 1-2 min (Figure 8).

Synthesis of Substituted 3,4-dihydropyrimidin-2(1H)--ones 16
An environmentally benign aqueous Biginelli protocol for the synthesis of substituted 3,4-dihydropyrimidin-2(1H)-ones using polystyrenesulfonic acid (PSSA) as a catalyst has been achieved by Polshettiwar et al. 16 These microwave-assisted reactions proceed efficiently in water in the absence of organic solvent, with simple filtration as the product isolation step (Figure 9).

Synthesis of 1,3 dioxanes 17
A novel tandem bis-aldol reaction of ketone with paraformaldehyde catalyzed by polystyrenesulfonic acid is reported by Polshettiwar et al. 17 in aqueous medium to yield 1,3-dioxanes. This one-pot, operationally simple microwave-assisted synthetic protocol proceeds efficiently in water in the absence of any organic solvent, with excellent yield (Figure 10).

Synthesis of coumarins 19
A simple and convenient method for the efficient synthesis of coumarins in water catalyzed by K 2 CO 3 is developed by Rehanaanjum. 19 Optimal result was obtained using 20mol% of K 2 CO 3 . The experiment was conducted with phenolic substrate and β-keto ester in the presence of catalytic amount of K 2 CO 3 (20mol%) in water. The reaction proceeded spontaneously at ambient temperature and was completed within 1 hr. The precipitated solid was filtered, dried and washed with 20% ethyl acetate in petroleum ether to afford coumarins ( Figure 12). 20 Kumaravel et al. 20 have developed a catalyst-free combinatorial library of novel 2-amino-4-(5-hydroxy-3-methyl-1H-pyrazol-4-yl)-4H-chromene-3-carbonitrile derivatives by the reaction of hydrazine hydrate, ethyl acetoacetate, 2-hydroxybenzaldehydes and malononitrile in water at ambient temperature ( Figure 13).

Synthesis of 3, 4-dihydroquinoxalin-2-amine derivatives 24
A simple and efficient one-pot synthetic approach for the preparation of biologically interesting 3,4-dihydroquinoxalin-2amine derivatives using EDTA-catalyzed three component reactions of o-phenylenediamines, carbonyl compounds, and isocyanides in an aqueous medium is described by Kolla et al. 24 This method is of great value because of its environmentally benign character, high yields, and ease of handling ( Figure 17).

Synthesis of substituted fused thiazole derivatives 25
Alison et al. 25  This stands as a significant advance over the previous methods using Pt, Cu or Ag salts paired with ligands in organic solvents such as benzene, acetonitrile or methylene chloride ( Figure 18).

Figure 18
Synthesis of substituted fused thiazole derivatives. 25 Gawande 26 provided a brief overview of some selective catalystfree reactions in aqueous media. 26 Further, many organic molecules are partially soluble in water and reactions that appear as heterogeneous mixtures and suspensions may involve on-water and in-water reaction modes occurring simultaneously. Organic synthesis reactions on water at the organic-liquid water interface were studied by Butler et al. 27 García-García et al. 28 have reported the preparation of a mesoscopic metal-organic structure based on single-layer assembly of aluminium chains and organic alkylaryl spacers. The material markedly accelerates condensation reactions in water in the absence of acid or base catalyst, as well as organocatalytic Michael-type reactions that also show superior enantioselectivity when comparing with the host-free transformation. 28 Alaoui et al. 29 have reported the ultrasound assisted facile one pot synthesis of sulfonamide-isoxazoles using cerium (IV) ammonium nitrate (CAN) as an efficient oxidant in aqueous medium. 29 Dilauro et al. 30 have reported the nucleophilic additions of highly polar organometallic compounds to imines and nitriles using water as a reaction medium. 30 They reported that the addition of highly polar organometallic compounds to non-activated imines and nitriles proceeds quickly, efficiently, and chemoselectively with a broad range of substrates at room temperature and under air with water as the only reaction medium.

Conclusion
In this review article, we have focused on some aqua mediated organic transformations which will surely help scientists and researchers across the globe to develop new inexpensive, clean and environmentally benign strategies to carry out organic synthesis.


extfieldsofstudy: Chemistry Medicine

text:
Crystal structure of (–)-methyl (R,E)-4-[(2R,4R)-2-amino-2-trichloromethyl-1,3-dioxolan-4-yl]-4-hydroxy-2-methylbut-2-enoate

In the title compound, the 1,3-dioxane ring has an envelope conformation. In the crystal, classical O—H⋯O and N—H⋯O hydrogen bonds link molecules into a sheet structure, and a weak intermolecular C—H⋯Cl interaction extends the sheet structure into a three-dimensional network.

Chemical context
Cyclic compounds often play a significant role, not only in controlling stereochemistry due to their conformational rigidity, but also as protecting groups in organic synthesis. On the basis of this concept, we have explored the utilization of cyclic orthoamides, prepared from allylic diol and triol with known conditions (Overman, 1974;1976), and have developed a new strategy for the total synthesis of a certain natural product (Nakayama, et al., 2013). The title compound is a structural isomer of a recently reported compound (Oishi et al., 2016).

Figure 3
The crystal packing of the title compound, viewed along the c axis, showing the sheet structure parallel to (001). The helical chain running along the b-axis direction is drawn as overlapped molecules. Yellow lines indicate the intermolecular N-HÁ Á ÁO hydrogen bonds. Only H atoms involved in the hydrogen bonds are shown for clarity. [Symmetry code: (ii) x -1, y, z.]

Figure 1
The molecular structure of the title compound, with the atom labelling. Displacement ellipsoids are drawn at the 50% probability level. The purple dotted line indicates the short intramolecular N-HÁ Á ÁCl contact (see Table 1). Only H atoms connected to N, O and chiral C atoms are shown for clarity.
For the cyclic orthoamide core with a trichloromethyl group on the central carbon atom, four structures are registered in the CSD. These are two derivatives (WEKWOY: Haeckel et al., 1994;and LAGMAK: Oishi et al., 2016) of 1,3-dioxolane (b), one derivative (WAXBEE: Metwally, 2011) of 1,3-oxathiolane (c), and one derivative (LIBHIO: Rondot et al., 2007) of 1,3-dioxane (d). The amino H atoms were refined as adopting an sp 2 configuration for WEKWOY and WAXBEE, while they were refined assuming an sp 3 configuration of the N atom for LIBHIO and LAGMAK, as in the present study. Each N-H bond of the amino group in LIBHIO is mostly eclipsed by the neighbouring C-Cl bonds of the trichloromethyl group, whereas those in the title compound are slightly tilted (Fig. 6). There is an intramolecular N-HÁ Á ÁCl interaction [H6AÁ Á ÁCl1 = 2.66 (4) Å ; N6-H6AÁ Á ÁCl1 = 115 (3) ] in the title compound (Table 1), while the corresponding geometries are 2.76 Å and 109 in LIBHIO. These amino groups may be oriented to avoid intramolecular non-bonding short contacts as well as to form classical intermolecular hydrogen bonds. The amino H atoms in LAGMAK are disordered according to the possible intramolecular N-HÁ Á ÁO and NÁ Á ÁH-O hydrogen bonds with the hydroxy group (Oishi et al., 2016).

Synthesis and crystallization
The title compound was afforded from l-threose, which can be prepared according to the reported procedure (Smith et al., 1992) from d-galactose (Kidena et al., 2017). Purification was carried out by silica gel column chromatography, and colourless crystals were obtained from a benzene solution under a hexane-saturated atmosphere, by slow evaporation at ambient temperature (m.p. 358-359 K

Figure 6
A projected diagram looking through the N atom of the amino group onto the C atom of the trichloromethyl group.

Special details
Experimental. IR (film): 3393, 3325, 2953, 1714, 1438, 1239, 1093, 1035, 825, 803, 749 cm -1  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. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > 2σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq


Heterobimetallic 3 d-4 f ( Zn / La ) { 6 , 6 ′-Dimethoxy-2 , 2 ′-[ naphthalene-2 , 3-diylbis ( nitrilomethylidyne ) ] diphenolato }-pyridylzinc ( II )-tris ( nitrato-O , O ′ ) lanthanum ( III ) Monohydrate

Herein, we report the synthesis and structural characterization of a complex with extended aromaticity based on a naphthalene-bridged Schiff base complex with two metal binding pockets. In a first step, the Zn(II) complex (ZnL), {6,6′-dimethoxy-2,2′-[naphthalene-2,3diylbis(nitrilomethylidyne)]diphenolato}a-pyridylzinc(II) was isolated followed by refluxing with lanthanum(III) nitrate to produce the heterobimetallic complex {ZnL[La(NO3)3·H2O]}. Crystals suitable for X-ray diffraction were grown by vapor diffusion of THF into a pyridine solution of the complex. The monoclinic unit cell (P21/n) is commensurately modulated along the c-axis direction and contains four crystallographically independent molecules with pseudo-translational symmetry being broken by slightly differing orientations of included pyridine and THF solvate molecules. The zinc atom adopts a distorted square pyramidal geometry and sits slightly above the ligand plane, whereas the lanthanum atom adopts an all-face capped trigonal prismatic geometry and sits below the ligand plane. Structural characterization of this diamagnetic complex lays the groundwork for applying these synthetic techniques to the near-infrared emitting lanthanides for practical application.

Introduction
The salen ligand (N,N -bis(salicylidene)ethylenediamine) and associated metal-salen complexes have long been used in homogeneous catalysis [1], with a particular emphasis on their ability to perform enantioselective transformations [2][3][4]. The attention this ligand system has garnered is due, in part, to its facile synthesis through the condensation of an aldehyde and an amine. More recently, salen type ligands, produced by the condensation of a diamine with o-vanillin, have been used to form heterobimetallic complexes. In these systems, a first-row transition metal is bound traditionally in the Schiff base/phenolate (κ 4 -N 2 O 2 ) pocket and a second metal occupies a peripheral (κ 4 -O 2 O 2 ) binding site. These complexes have recently been made with divalent s-block metals in the second position [5], but far more examples exist in the literature for heterobimetallic 3d-4f complexes of this type. Choice of lanthanide ion determines applicability; the Gd 3+ ion produces interesting magnetic materials [6], and Eu 3+ produces functional luminescent materials [7]. The luminescent ions Yb 3+ and Nd 3+ have been well studied with this ligand system for near-infrared luminescence applications [8][9][10][11][12][13]. Near-infrared luminescence in the region of maximum tissue penetration has potential for fluorescence imaging applications in biology, but most lanthanide complexes of this type require high-energy UV excitation. Extension of the aromatic system of these ligands has the potential to shift this excitation further towards the visible region, which is more amenable to any future biological applications [14,15]. The naphthalene bridged salen ligand (1, Scheme 1) provides this opportunity; metal complexes of this ligand have been previously studied for their anti-cancer properties [16,17], and structural information is available for a related Co 3+ complex [18]. Described herein is the synthesis and structural characterization of a heterobimetallic Zn 2+ /La 3+ complex of this naphthalene bridged salen ligand.

Synthetic Details
The naphthalene-bridged Schiff base ligand (1) was produced by a modified procedure from the literature in 75% yield (Scheme 1) [17]. The 1 H-NMR (400 MHz, DMSO-d6) displayed characteristic signals at δ = 9.05 ppm and δ = 12.94, for imino and phenolic protons, respectively. Reaction of the ligand with one equivalent of zinc(II) nitrate resulted in the formation of Zn 2+ complex (2) in 67% isolated yield. Upon addition of zinc(II), the 1 H-NMR resonance for the imino proton shifted to δ = 9.15 ppm and the phenolic resonance disappeared. Further refluxing the Zn 2+ complex with a slight excess of lanthanum(III) nitrate resulted in a yellow solid that was identified as the heterobimetallic Zn 2+ /La 3+ complex (3). Scheme 1. Synthesis of heterobimetallic Zn 2+ /La 3+ Schiff base complex. (3) Crystals suitable for X-ray crystallography were grown by vapor diffusion of THF into a pyridine solution of 3 ( Figure 1). Each complex contains one Zn(II) ion in the κ 4 -N2O2 pocket, sitting 0.52 Å above the plane of the ligand, with an axially bound pyridine molecule, thereby adopting a distorted square pyramidal geometry. The bound La(III) ion sits on the opposite side of the ligand plane (0.86 Å below) in the κ 4 -O2O2 pocket; open coordination sites are occupied by one aqua ligand and three nitrates for charge balance. The interatomic distance between the two metals is 3.626 Å [19].

X-Ray Structure of Zn 2+ /La 2+ Complex
Selected bond distances are found in Table 1, reported here for each crystallographically independent molecule in the unit cell (A-D) and as the average of the four. The Zn-N (imine) bonds are equal and slightly longer than those previously reported using an analogous phenylene bridged ligand (2.05/2.06 vs. 2.03 Å) [13], presumably due to the increased electron donation from the naphthalene bridge reported here. The Zn-O (phenolate) bonds are shorter; the relative bond distances for this binding pocket are consistent with literature values. Scheme 1. Synthesis of heterobimetallic Zn 2+ /La 3+ Schiff base complex.

Synthetic Details
The naphthalene-bridged Schiff base ligand (1) was produced by a modified procedure from the literature in 75% yield (Scheme 1) [17]. The 1 H-NMR (400 MHz, DMSO-d 6 ) displayed characteristic signals at δ = 9.05 ppm and δ = 12.94, for imino and phenolic protons, respectively. Reaction of the ligand with one equivalent of zinc(II) nitrate resulted in the formation of Zn 2+ complex (2) in 67% isolated yield. Upon addition of zinc(II), the 1 H-NMR resonance for the imino proton shifted to δ = 9.15 ppm and the phenolic resonance disappeared. Further refluxing the Zn 2+ complex with a slight excess of lanthanum(III) nitrate resulted in a yellow solid that was identified as the heterobimetallic Zn 2+ /La 3+ complex (3).

X-ray Structure of Zn 2+ /La 2+ Complex (3)
Crystals suitable for X-ray crystallography were grown by vapor diffusion of THF into a pyridine solution of 3 ( Figure 1). Each complex contains one Zn(II) ion in the κ 4 -N 2 O 2 pocket, sitting 0.52 Å above the plane of the ligand, with an axially bound pyridine molecule, thereby adopting a distorted square pyramidal geometry. The bound La(III) ion sits on the opposite side of the ligand plane (0.86 Å below) in the κ 4 -O 2 O 2 pocket; open coordination sites are occupied by one aqua ligand and three nitrates for charge balance. The interatomic distance between the two metals is 3.626 Å [19].
Selected bond distances are found in Table 1, reported here for each crystallographically independent molecule in the unit cell (A-D) and as the average of the four. The Zn-N (imine) bonds are equal and slightly longer than those previously reported using an analogous phenylene bridged ligand (2.05/2.06 vs. 2.03 Å) [13], presumably due to the increased electron donation from the naphthalene bridge reported here. The Zn-O (phenolate) bonds are shorter; the relative bond distances for this binding pocket are consistent with literature values.   (7) 2.525 (6) 2.504 (7) 2.499 (6) 2.508 1 Average bond distance across four independent molecules in unit cell.
The unit cell was found to be monoclinic in the P21/n space group, and contains four molecules lined up along the c-axis (Figure 2). The molecules are close to being perfectly translated and related by a pseudo two-fold screw axis incompatible with unit cell shape. Exact translation and rotational symmetry is broken by modulations or disorder of solvate pyridine and THF molecules that are Hbonded to the aqua ligand (See Supporting Information). Layers of metal complex are formed along the a-and b-axes with pyridyl zinc moieties pointing toward the void space between layers; the remaining void space is filled with aforementioned solvent molecules.    (7) 2.525 (6) 2.504 (7) 2.499 (6) 2.508 1 Average bond distance across four independent molecules in unit cell.
The unit cell was found to be monoclinic in the P2 1 /n space group, and contains four molecules lined up along the c-axis (Figure 2). The molecules are close to being perfectly translated and related by a pseudo two-fold screw axis incompatible with unit cell shape. Exact translation and rotational symmetry is broken by modulations or disorder of solvate pyridine and THF molecules that are H-bonded to the aqua ligand (See Supporting Information). Layers of metal complex are formed along the aand b-axes with pyridyl zinc moieties pointing toward the void space between layers; the remaining void space is filled with aforementioned solvent molecules.  The unit cell was found to be monoclinic in the P21/n space group, and contains four molecules lined up along the c-axis (Figure 2). The molecules are close to being perfectly translated and related by a pseudo two-fold screw axis incompatible with unit cell shape. Exact translation and rotational symmetry is broken by modulations or disorder of solvate pyridine and THF molecules that are Hbonded to the aqua ligand (See Supporting Information). Layers of metal complex are formed along the a-and b-axes with pyridyl zinc moieties pointing toward the void space between layers; the remaining void space is filled with aforementioned solvent molecules.

General Considerations
Unless otherwise noted, all chemicals and reagents were purchased from Sigma Aldrich (St. Louis, MO, USA). 1 H-and 13 C-NMR spectra were recorded on an Avance 400 MHz spectrometer (Bruker, Billerica, MA, USA) and high-resolution ESI mass spectrometry was performed on a 6545 Q-TOF LCMS (Agilent, Santa Clara, CA, USA). Zn 2+ /La 3+ complex (3, ZnL(La))-To a solution of (2) (0.06 g, 0.13 mmol) in absolute ethanol (5 mL) was added lanthanum(III) nitrate hexahydrate (0.09 g, 0.12 mmol). The resulting solution was refluxed at 78 • C for 24 h, after which time a bright yellow precipitate formed. The solid was collected by vacuum filtration, washed with cold ethanol (100 mL), and redissolved in pyridine for crystallization by vapor diffusion of THF. Over the course of two weeks, yellow block crystals were grown and collected by vacuum filtration (0.07 g, 48%).

X-ray Diffraction Studies
Single crystals of 3 were coated with a trace of mineral oil and quickly transferred to the goniometer head of a Bruker Quest diffractometer (Billerica, MA, USA) with a fixed chi angle, a sealed tube fine focus X-ray tube, single crystal curved graphite incident beam monochromator, a Photon100 CMOS area detector and an Oxford Cryosystems low temperature device (Oxford, UK). Examination and data collection were performed with Mo Kα radiation (λ = 0.71073 Å) at 150 K. Data were collected, reflections were indexed and processed, and the files scaled and corrected for absorption using APEX3 [20]. Crystals of 3 were found to be four-fold commensurately modulated along the c-axis direction, and to be slightly non-merohedrally twinned. The orientation matrices for the two components were identified using the program Cell_Now, with the two components being related by a 180-degree rotation around the real a-axis. Integration using SAINT proved problematic due to the excessive multiple overlapping of reflections, resulting in large numbers of rejected reflections. Attempts were made to adjust integration parameters to avoid excessive rejections (through adjustments to integration queue size, blending of profiles, integration box slicing and twin overlap parameters), which led to fewer, but still substantial numbers of, rejected reflections. With no complete data set obtainable through simultaneous integration of both twin domains, the data were instead integrated in SAINT as if not twinned, with only the major domain integrated, and converted into an hklf 5 type format hkl file after integration using the "Make HKLF5 File" routine as implemented in WinGX. The twin law matrix was used as obtained from SAINT, see above. The Overlap R1 and R2 values used were 0.45, i.e., reflections with a discriminator function less or equal to an overlap radius of 0.45 were counted as overlapped, all others as single. The discriminator function used was the "delta function on index non-integrality". No reflections were omitted. The transformation matrix used was 1.000 0.000 0.000, 0.000 −1.000 0.000, −0.150 0.000 −1.000.
The space group was assigned and the structure was solved by direct methods using XPREP within the SHELXTL suite of programs [21,22] with the hklf 4 type file, and refined by full-matrix least-squares against F 2 with all reflections using Shelxl2016 [23,24] with the graphical interface Shelxle [25], and using the hklf 5 type file, which resulted in a BASF value of 0.0336 (4). No R int value or number of independent reflections were obtainable for the hklf 5 type file using the WinGX routine [26]. The values from the HKLF 4 type refinement are given instead.
The structure is characterized by a commensurate modulation along the c-axis. Four molecules are lined up along this axis, with molecules A and C, and B and D being close to perfectly translated, and molecules A and B, and C and D being related by a pseudo two-fold screw axis incompatible with the unit cell shape. Exact translation and rotational symmetry is broken by modulations and/or disorder of solvate pyridine and THF molecules, and slight modulations of the coordinated nitrate anions. For entity B, both the coordinated, as well as the non-coordinated pyridine molecules were refined as disordered by a rotation around the molecule axis through nitrogen.
For entities A, B and C, the THF molecules H bonded to the metal-coordinated water molecules, while the interstitial THF molecules were refined as disordered over two orientations. For entity B, the two THF molecules were also disordered, with a water molecule H bonded to the more prevalent of the two THF moieties.
All disordered entities were restrained to have a geometry similar to that of at least one other not-disordered entity of the same kind (SAME commands in Shexl). U ij components of ADPs of disordered atoms were restrained to be similar for atoms closer to each other than 1.7 Å. THF O atoms O15 a and O15e were constrained to have identical positions and thermal parameters. H atoms attached to carbon atoms were positioned geometrically, and constrained to ride on their parent atoms, with carbon hydrogen bond distances of 0.95 Å for and aromatic C-H, 0.99 and 0.98 Å for aliphatic CH 2 and CH 3 moieties, respectively. Methyl H atoms were allowed to rotate, but not to tip to best fit the experimental electron density. Water H atoms were restrained to have O-H distances of 0.84(2) Å. For the partially occupied water molecule H atom positions were restrained based on H-bonding considerations. U iso (H) values were set to a multiple of U eq (C/O) with 1.5 for OH and CH 3 , and 1.2 for C-H and CH 2 units, respectively. Subject to these conditions, the occupancy rates refined to values between 0.719 (14) and 0.281 (14). See atom tables in the cif for all values.
Crystal Complete crystallographic data, in CIF format, have been deposited with the Cambridge Crystallographic Data Centre. CCDC 1577726 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.