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Polyoxometalate-Based Metal-Organic Frameworks as Catalysts for the Selective Oxidation of Alcohols in Micellar Systems [ChemPlusChem](ChemPlusChem Via Acquire Media NewsEdge) A series of nanosized metal-organic frameworks (MOFs) encapsulating different polyoxometalates (POMs) including H3 PW4 O12, H5 PMo12 O40, H5 PVMo10 O40, H5 PV2 Mo10 O40, and H5 PV3 Mo10 O40 was synthesized and used in the selective oxidation of alcohols. The catalyst with a uniform size and morphology offered easy accessibility between substrates and catalyst. At the same time, the MOF ensured that the POM was encapsulated, which could dramatically prevent the assembly of the catalyst. Furthermore, the catalyst showed clear chemoselectivity, which was related to the size or accessibility of the substrates for surface pores. With cetyltrimethyl ammonium bromide aqueous solution as solvent, both improved reaction efficiency and simple recycling of the catalytic system were achieved to afford a green oxidation process. Keywords: alcohols · metal-organic frameworks · micelles · oxidation · polyoxometalates Introduction Selective oxidation of primary and secondary alcohols into the corresponding aldehydes or ketones is undoubtedly one of the most important and challenging transformations in organic chemistry.[1] Many catalysts have been developed to accom- plish the reaction including (2,2,6,6-tetramethylpiperidin-1- yl)oxyl (TEMPO),[2] metal oxides,[3] hypervalent iodine,[4] and so on. Another common and efficient route uses polyoxometa- lates (POMs) together with hydrogen peroxide (H2O2) as termi- nal oxidant, which has commonly been considered a "green oxidant" because of its high content of active oxygen species and coproduction of only water. Research published since the 1980s has established the high potential of POMs for activation of H2O2, thanks to the nanoarrays of d0 transition-metal ions such as WVI,MoVI, and VV exposed on their surfaces.[5] No matter which POM is used, however, separation and re- covery are usually key problems to be solved owing to the sol- ubility of POMs. Much effort has been devoted to the field of immobilizing POMs, with organic hybrids of POMs as an attrac- tive possibility for various oxidation reactions. Examples in- clude surfactant-POM combinations,[6] coordination poly- mers,[7] polymer-POM conjugates,[8] and so on. However, the difficulty in controlling the separation, the aggregation of POMs in the preparation of the catalyst, and the limited varia- tion in chemical formulation and functionality often limit their applications. Lately, the use of transition metals, organic ligands, and POMs to assemble POM-based metal-organic frameworks has emerged as one of the approaches to solve the problems.[9] A stable host HKUST-1,[10] as a well-known MOF, was reported to be able to encapsulate various Keggin-type POMs. Wee and co-authors reported on a microporous cubic material with strictly repetitive 5 nm-wide mesopores and the stability was enhanced by Keggin-type phosphotungstate (HPW) systemati- cally occluded in the cavities constituting the walls between the mesopores.[11] Although the approach successfully solved the problem of POM aggregation in catalytic processes and offers great potential for chemical and structural diversity, in- vestigation of the catalytic properties of these kinds of materi- als is rather limited.[12] The use of water as a medium for promoting organic reac- tions is very important and has received much attention in recent years from the point of view of green and economical chemistry. To perform oxidation reactions in water, the solubili- ty problem of substrates or catalysts must be overcome, be- cause of the limited mutual solubility between water and or- ganic agents. Micelles, which are dynamic clusters of surfactant molecules that possess both hydrophilic and hydrophobic structures, may associate in aqueous media to circumvent the problem. Micelles can concentrate the reactants within their small volumes; stabilize substrates, intermediates or products ; and orient substrates. Thus, they can alter the reaction rate, mechanism, and regio- and stereochemistry.[13] In this study, selective oxidations were performed in micellar systems with MOF-encapsulated POMs as catalyst. A serious of nanosized metal-organic frameworks were synthesized encap- sulating different POMs including H3PW4O12,H5PMo12O40, H5PVMo10O40,H5PV2Mo10O40, and H5PV3Mo10O40 (noted as MOF- HPW, MOF-HPMo, MOF-HPMoV, MOF-HPMoV2, and MOF- HPMoV3, respectively), which all offered organized multiple porosity and high surface area. Their catalytic performance for the selective oxidation of various alcohols was tested and com- pared in a micellar system. Both improved reaction efficiency and simple recycling of the catalytic system were achieved in cetyltrimethyl ammonium bromide (CTAB) micellar solutions. Thus a green oxidation process was achieved. Results and Discussion The concept of the catalysts The catalysts were easily prepared from a synthesis mixture containing the molar composition of 18Cu/10 BTC/POM/CTAB/ 170 EtOH/2000 H2O (BTC =1,3,5-benzenetricarboxylate). Ther- mogravimetric analysis (TGA) was applied to the catalysts as shown in Figure S1 in the Supporting Information. From the TGA data, weight contents of POMs were established to be 0.506, 0.429, and 0.330 for MOF-HPW, MOF-HPMo, and MOF- HPMoV2, respectively, which matched well with their corre- sponding theoretical values of 0.485, 0.374, and 0.362, respec- tively. Chui et al. revealed that the polymer framework of HKUST- 1 is composed of dimeric cupric tetracarboxylate units. Twelve carboxylate oxygen atoms from the two BTC ligands bind to four coordination sites for each of the three Cu2 + ions of the formula unit. For most POM-bearing MOFs, the POM is added to the hydrothermal synthesis and found in the cavities after- wards. In the case of the HKUST-1, however, strong interaction between CuII and Keggin-type ions is discovered, which leads to spontaneous self-assembly of microporous MOF-POM.[14] The Keggin ion acts as a templat- ing species during synthesis and stabilizes the microporous struc- ture by means of the synergism between the metal and Keggin ions.[15] ACu2 + /POM interaction directs Cu2 + ions to preferential- ly reside close to the positions needed to build the MOF frame- work. In such a scenario, the role of the organic linker is simply to connect the already structured Cu2+ /POM units through com- plexation. Namely, the MOF as- sembly results in strictly system- atic encapsulation of POM mole- cules. In addition, the nanosize and high surface area of Cu-MOF make it an excellent choice for POM anchoring. MOFs and zeo- lites are very similar in terms of their porous structures, but quite different in the situation of immobilizing POMs. There- fore it would be expected that firstly, the MOF could be used as a catalyst for reactions and the accessibility between sub- strates and the catalyst could be maintained ; and secondly, dif- ferent from a normal support in which the active site is anch- ored on its surface, the MOFs ensured that the POM was en- capsulated, which could dramatically prevent the assembly of the catalyst. Figure 1 shows the IR spectra of MOF-HPW, HKUST-1, and H3PW4O12 raw material. It can be seen that some characteristic peaks belonging to W^O vibrations around 1000 cm^1 are ex- hibited, which clearly differs from that of bare HKUST-1 (curve b). Also, when comparing the IR spectrum of the MOF with its relative POM (curve a with c), curves a and b show detectable changes around 1500 cm^1 that are characteristic of C^H, C^C, and C=O vibrations, which suggests the formation of the framework, and which are absent from the spectrum of the POM alone. IR spectra of MOF-PMo, MOF-HPMoV1, MOF- HPMoV2, and MOF-HPMoV3 are shown in the Supporting In- formation. To further confirm the structure of the MOF-POMs, X-ray dif- fraction (XRD) studies were also performed. Figure 2 shows the XRD pattern of MOF-HPW and MOF-HPMo, and the results for MOF-HPMoV, MOF-HPMoV2, MOF-HPMoV3, and POM raw ma- terials are listed in the Supporting Information. From Figure 2, a high degree of mesoscopic ordering is confirmed by the sharp reflections appearing at low angles. TEM and SEM images of MOF-HPW and MOF-HPMo (Figure 3 ; images of MOF-HPMoV are shown in the Supporting Information) indicate that the catalyst was a framework struc- ture with uniform size and morphology. Viewed from the bright field, highly ordered mesostructures with long-range or- dering could be observed, which testified that Keggin ions were contained in the microporous wall structures, next to the less dense mesopores in the structure. Parallel lines with a dis- tance of 3.6 nm could be observed in Figure 3c, similar to those previously reported in reference [11] , which means that the catalyst we prepared follows an analogous structure. For MOF-HPMo and MOF-HPMoV, the size of the framework changed greatly, but the mesostructures were still clearly visi- ble, which also confirmed that the Keggin ion acted as the templating species during the synthesis of MOF. Exploration of the reaction conditions First, MOF-HPW was taken as a model catalyst to show the im- portant effect of micellar media upon the reactions. Oxidations of benzyl alcohol were studied in different systems and the comparative results are demonstrated in Figure 4. Three common surfactants were chosen to generate micellar systems including cationic surfactant ce- tyltrimethyl ammonium bromide (CTAB), anionic surfactant sodium dodecyl sulfate (SDS), and neutral surfactant TritonX- 100 (TX-10). For all the surfac- tants, experiments were per- formed at their tenfold critical micellar concentration (10 cmc) in the presence of MOF-HPW. In addition, for comparison with micellar systems, acetonitrile, which is usually used in POM- catalyzed oxidations, was chosen as an organic solvent. As can be seen in Figure 4, the CTAB micel- lar system gave the best result, followed by acetonitrile, TX-10, and SDS, respectively. The yield of benzaldehyde was able to reach 98% after 3 h in CTAB aqueous solution, which was even faster than that in an or- ganic solvent (acetonitrile solu- tion). So using water as the reac- tion media was a good choice in view of the reaction efficiency as well as green and economical chemistry. The fact that the reac- tion could be facilitated by use of a cationic surfactant also testi- fied to the nature of micellar re- actions. The reactant benzyl al- cohol accumulated not in the surrounding water phase, but in the micelle through interactions with the micelle surface or through insertion into the mi- celle itself, as was the catalyst. Since the oxidant H2O2 was in the aqueous phase,[16] the oxida- tion process took place at the surface of the micelles. As a result of the attraction of the POM anion to the cationic sur- factant, the reaction rate was accelerated greatly in CTAB mi- cellar solution. To further optimize the reaction conditions, preliminary ex- periments were performed with benzyl alcohol as the sub- strate and H2O2 as the oxidant in CTAB micellar solution. The comparable results are summarized in Table 1. It was found that almost no product was obtained in aqueous solution without any catalyst (entry 1) or with HKUST-1 without POMs (entry 2). Also, if no surfactant was added, the reaction turned out to be less efficient in two phases (entry 4). For both MOF- HPW and MOF-HPMo, a catalytic amount of catalyst was enough for the reaction to proceed and reaction termination was observed on its removal. A further increase of catalyst amount would be meaningless. The reaction was initiated at about 40 8C and high temperature was clearly beneficial for the reaction. But at the same time, a high temperature acceler- ates the decomposition of H2O2 so more H2O2 was required to complete the reaction. The reaction conditions were finally op- timized to be 80 8C with 4 mmol H2O2. With the optimized reaction conditions, the catalytic per- formance of MOFs with different kinds of POMs was tested and compared. As shown in Figure 5, similar results could be obtained with MOF-HPW and MOF-HPMo as catalysts. Howev- er, when the MOFs of vanadium-containing POMs were used as catalyst, notable improvement was observed in the oxida- tion efficiency of benzyl alcohol. Also, with an increase of the vanadium content in the POMs, the conversion of benzyl alco- hol showed a rising trend. However, further oxidation reactions generated benzoic acid in a proportion that could not be ne- glected. The results showed excellent catalytic oxidizability of these vanadium-containing POMs, but at the same time, selec- tivity of the oxidation tended to be less controllable. The mechanism of the reaction occurs in two steps including the oxidation of organic substrate catalyzed by POMs and the re- generation of reduced POMs by H2O2 with the formation of water. A combined influence of framework Mo and V was sup- posed to be responsible for the activity difference between single POM-functionalized MOFs and vanadium-containing ones.[17] The distribution of products was further defined with MOF- HPW and MOF-HPMoV3 as catalyst. As shown in Figure 6, ben- zaldehyde was almost the only product in the MOF-HPW-cata- lyzed system. Whereas for MOF-HPMoV3, the yield of benzalde- hyde decreased after a maximum at 1 h. At the same time, benzoic acid was generated gradually and showed an increas- ing yield in the following reaction period. The total conversion of benzyl alcohol followed a similar trend to that of MOF-HPW. Overall, excellent selectivity could be obtained through con- trolling the reaction time for MOF-HPW and MOF-HPMo, whereas for vanadium-containing POMs, improvement in oxi- dation efficiency could be obtained but the selectivity of the oxidation tended to be less controllable. All these facts indicated that it was still Keggin anions that served as the active species for oxidation reactions. The encap- sulation process did not change the reaction mechanism and an activity decrease was not obvious either. The good catalytic performance of this system probably resulted from two as- pects. First, the nanosized metal-organic framework containing the active sites provided a large surface area for the ready ac- cessibility of substrate molecules. Second, a pseudo-homoge- neous process was obtained owing to the assistance of an emulsion system, which also greatly increased the chances for the contact between substrate and catalyst. Catalytic performance of MOF-HPW To examine the utility and generality of this methodology for oxidation of alcohols, we applied the present system to a varie- ty of substrates with MOF-HPW as the model catalyst. As shown in Table 2, the substrate scope could be extended to benzylic, allylic, heterocyclic, alicyclic, and aliphatic alcohols. In all of the cases tested, ketones or aldehydes were the only de- tected products. Clearly, all primary benzylic alcohols were con- verted into their corresponding aldehydes in high yields. Sec- ondary alcohols such as benzhydrol and 2-phenylethanol also gave moderate yields of 59 and 66 %, respectively. Benzyl alco- hol showed the highest yield among the substrates tested, and the introduction of another group (electron-withdrawing or electron-donating group) decreased the results more or less. When the volume of substituted groups did not differ much, substrates with electron-withdrawing groups performed better than the those with electron-donating groups. Further- more, oxidations of some aliphatic alcohols were also tested as listed in Table 3. With double the amount of the catalyst, the activities exhibited by secondary alcohols were also not so bad in the system. Good to excellent conversions were observed for allyl alcohol and cinnamyl alcohol. More importantly, it was found that the oxidative efficiency was not affected by the ex- istence of a double bond and the double bond remained stable during the oxidation process. Isooctyl alcohol performed somewhat better than 1-C8H17OH, but unfortunately both re- sults were unsatisfactory even after lengthening the reaction time. Based on the results in Table 2, 4-methylbenzyl alcohol and 4-methoxybenzyl alcohol, which were thought to perform simi- lar to or even better than benzyl alcohol, gave lower yields in- stead. The MOF catalyst seemed to be chemoselective. So, to probe the chemselectivity, substrates with increasing dimen- sions were tested. The volume and dimension of the substrates were calculated by using Gaussian 03 at the B3LYP/6-31G ++ (d,p) level.[18] As illustrated in Figure 7, the conversion of benzyl alcohol with a dimension of 89.35 cm3 mol^1 reached 98 % after 3 h. In contrast, the conversion of 4-methylbenzyl al- cohol and 4-methoxybenzyl alcohol with dimensions of 101.27 and 107.57 cm3 mol^1, respectively, decreased to 95 and 92 % under similar conditions. 4-tert-Butylbenzyl alcohol, a larger al- cohol with molecular dimension of 142.72 cm3 mol^1 gave a conversion of about 63 % after 12 h. Although the 4-tert- butyl group activated the benzene ring the most (better than the methyl and methoxy group), the conversion of 4-tert-butyl- benzyl alcohol was the lowest. The catalytic activity and selec- tivity of the MOF seemed to depend on the size of the sub- strates and their accessibility to surface pores. It is thereby rea- sonable that the reactants with smaller sizes such as benzyl al- cohol could diffuse swiftly through the pores. In contrast, the substrates with larger sizes were not readily diffused through the pores, but they may adsorb onto the surface pores con- taining Keggin complexes, and the polyanions catalyzed the reaction at the surface of the pores. In many systems, the ali- phatic alcohols with longer carbon chains performed better than the shorter ones.[2, 7] The same trend was observed in this system when comparing the reaction efficiency of nC8H17OH and nC12H25OH. However, nC12H25OH also showed a higher con- version than nC16H31OH, which was assumed to be due to this chemoselectivity. Overall, the MOF-HPW catalyst demonstrated great catalytic activity. For a series of alcohols including benzylic, allylic, heter- ocyclic, and alicyclic alcohols as substrates, good to excellent yields were obtained with ketones or aldehydes as the only de- tected products. Furthermore, the catalyst showed clear che- moselectivity, which was related to the size of the substrates or their accessibility to surface pores. The advantage of the catalytic system lies in not only the high catalytic activity in aqueous solvent with H2O2 as oxidant, but also the easy recovery of both catalyst and solvent. Since both the catalyst and solvent were immiscible with diethyl ether, the catalytic system could be recovered after extraction, to the maximum amount. After completion of the oxidation re- action, the mixture was allowed to cool to room temperature and was extracted with diethyl ether. The aqueous layer, con- taining the catalyst, was able to be separated and reused with the addition of substrate without any treatment. The organic layer, containing the products, was analyzed after drying with anhydrous sodium sulfate. When benzyl alcohol was used as a model substrate, the procedure was successfully repeated five times without any great loss of catalytic activity (Figure 8). Conclusion In conclusion, we have synthesized a series of POM-based metal-organic frameworks in this study. The catalysts were easily prepared and exhibited an ordered size and morphology. The nanosized MOF-POMs, which offered organized multiple porosity and high surface area, were proven to be an efficient catalyst for various alcohol oxidations in CTAB micellar solu- tion. With MOF-HPW as model catalyst, a wide set of aliphatic, allylic, heterocyclic, and benzylic alcohols were oxidized into their corresponding carbonyl compounds with good to excel- lent yields. The reaction conditions, generality of the method- ology, selectivity, and reusability of MOF-HPW were further studied in detail. The catalytic activity and selectivity of the catalyst seem to depend on the size of the substrates and their accessibility to surface pores. The reactants with smaller sizes such as benzyl alcohol could diffuse swiftly through the pores thus a higher efficiency was obtained. Also, good recy- clability was exhibited in the CTAB micellar system. After com- pletion of the oxidation reaction, the mixture was allowed to cool to room temperature and was extracted with diethyl ether. The catalyst remained in the aqueous layer for use in the next run. With benzyl alcohol as substrate, the procedure was successfully repeated five times without any great loss of catalytic activity. Experimental Section Materials and methods All the solvents and reagents were purchased from Sinopharm Chemical Reagent Co. Ltd and were used without further purifica- tion. IR spectra were recorded on a NICOLET NEXUS870 instrument. Products were identified by using a 6820 gas chromatograph (GC) with an Agilent Technologies HP-Innowax (30 m ^0.32 mm^ 0.5 mm). XRD data were collected with CuKa radiation on Bruker C8 ADVANCE spectrometer. TEM images were recorded on a JEM-2100 instrument. TGA was performed on a TGA/SDTA851e instrument under a N2 atmosphere from 50 to 700 8C, with a heating rate of 108Cmin^1 and N2 flowing rate of 30 mL min^1. Calculation of the volume was performed by using Gaussian 03 at the B3LYP/6-31G + + (d,p) level. Catalyst preparation Preparation of HKUST-1: HKUST-1 was prepared according to Ref- erence [10b] . In a typical procedure, a slurry of Cu(OH)2 (0.98 g, 0.01 mol) in water (18 mL) was added to a solution containing tri- mesic acid (TA, 1,3,5-benzenetricarboxylic acid ; 2.1 g, 0.01 mol), di- methyl formamide (DMF, 10 mL), and ethyl alcohol (46.6 mL) under moderate stirring. The molar composition of the resulting mixture was 1 Cu/1TA/12.9 DMF/80EtOH/100 H2O. The crystallization was carried out at room temperature under moderate stirring. After- wards, the product was recovered by filtration and washed with ethanol, and finally dried at 65 8C. Preparation of MOF-POM : MOF-POM was prepared according to Reference [11] with some expansion. In a typical synthesis, copper(- II) nitrate trihydrate (1.45 g, 6 mmol) and POMs (0.33 mmol) were dissolved in distilled water (12 mL). Another solution containing BTC (98 %, 0.70 g, 2.8 mmol) and CTAB (0.12 g, 0.33 mmol) dis- solved in absolute ethanol (16 mL) was prepared and mixed with the solution mentioned above under vigorous stirring. The mixture was kept stirring for approximately 30 min at room temperature and then aged without stirring for a further 5 days. After filtration, the solid product was washed with water before it was dried in air at 60 8C for 24 h. To remove CTAB, Soxhlet extraction with ethanol was performed for 48 h at 105 8C. Finally, the product was dried in air at 60 8C. Oxidation process of alcohols General procedure : In a typical process, alcohol (2 mmol) and CTAB solution (10 mL, 10 cmc) were added to a 50 mL round-bot- tomed flask. The mixture was stirred at room temperature for 5 min and catalyst MOF-POM (50 mg) and H2O2 (4 mmol) were then added. The temperature of the mixture was raised to 60 8Cto initiate the reaction. The progress of the reaction was monitored by GC with samples taken periodically. The samples were cooled to room temperature, extracted with Et2O three times, and dried over anhydrous Na2SO4 before being analyzed by GC. Acknowledgements We thank the Scientific Innovation Program of Jiangsu Porvince (no : CXLX13_198) for support of this research. [1] a) D. Romano, R. Villa, F. Molinari, ChemCatChem 2012, 4, 739 - 749 ; b) S. Caron, R. W. Dugger, S. G. Ruggeri, J. A. Ragan, D. H. Brown Ripin, Chem. Rev. 2006, 106, 2943 - 2989 ; c) R. W. Dugger, J. A. Ragan, D. H. Brown Ri- pin, Org. Process Res. Dev. 2005, 9, 253 - 258. [2] a) Z. 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Received : January 7, 2014 Published online on June 4, 2014 Jie Zhu, Meng-nan Shen, Xue-jing Zhao, Peng-cheng Wang, and Ming Lu*[a] [a] J. Zhu, M.-n. Shen, X.-j. Zhao, P.-c. Wang, Prof. M. Lu School of Chemical Engineering Nanjing University of Science and Technology Xiaolingwei 200, Nanjing, Jiangsu (P. R. China) E-mail : [email protected] * Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cplu.201400009. (c) 2014 Blackwell Publishing Ltd. |