Assembling of tungstovanadogermanic heteropoly acid into mesoporous molecular sieve SBA-15
Graphical abstract
Introduction
Heteropoly acids (HPAs) are widely used as catalysts because of their strong Brønsted acidity, high charge and the ability of being reversibly reduced [1], [2], [3]. However their extremely small surface area limited their utility to applications [4], [5]. For them to be effective as catalysts they should be supported on carrier with a large area. Previously, acidic or neutral substances such as silica, alumina, active carbon, acidic ion-exchange resin, clays and zeolites were used as supports [6], [7], [8], [9]. Most of these supports are of relatively small surface area and nonuniform pore size, because of which grafting of large HPAs anions is limited.
Mesoporous molecular sieves (MMS) [10], which have significantly large surface area , big pore size and uniform pore distribution, were shown to be excellent supports for HPAs. HPAs can disperse well inside the mesopore due to the unique properties of the MMS. These HPA supported MMS materials could be useful as heterogeneous catalysis for bulkier molecules where diffusion of reactant molecules could be facilitated. There were two ways that HPAs could be supported on the surface of MMS. In one of these, HPAs were introduced onto the silica by incipient wetness method [11], [12], [13], [14], [15], [16]. However, the dispersion and stability of the acidic phase were not perfectly controlled. For these materials, leaching of HPAs could also be observed when they were applied to polar solvent. A second way was an indirect method compared with the first one. The silica surfaces were first functionalized with an amine ligand and then treated with HPAs [17], [18], [19], [20]. The amine ligand reacted with HPA to result salt (NH3⋅HPA) whose bond is far strong than hydrogen bond between silanol group and HPA, thus the HPA was held firmly to the silica surface.
In most of these cases, the researchers focused on the HPAs/M41s materials. Although newly synthesized MMS SBA-15 has high surface area, large pore volume and satisfied stability compared with M41s materials [21], there were only a few studies on the SBA-15/HPAs materials. Herein we describe a system of modifying amine-functionalized SBA-15 support with HPA (HPA/AMINO/SBA-15) through a two-step synthesis. The structure of this material was studied. The results show a new kind of HPA has been attached successfully on the modified SBA-15.
Section snippets
Synthesis
Preparation of H5GeW11VO40 was adapted from literature methods [22]. FT-IR of the polyhedral crystal: 973, 885, 813, 769, 463 cm−1 was in agreement with the literature.
SBA-15 was synthesized following the method of Zhao et al. [21]. 2 g of EO20PO70EO20 (P123) was dissolved in 60 mL of HCl (2 M) which added with another 15 mL of H2O, followed by 4.25 g of TEOS. The mixture was stirred at 35 °C for 5 h, transferred into an autoclave for additional reaction at 100 °C for 24 h, collected by
Results and discussion
The two-step synthesis of HPA/AMINO/SBA-15 was presented in Scheme 1.
The SBA-15 support was treated with (EtO)3SiCH2CH2CH2NH2 in refluxing chloroform, by this condensation react with silanol group, amino group can be introduced [23]. Fig. 1 shows solid-state 29Si MAS NMR patterns of SBA-15, AMINO/SBA-15 and HPAs/AMINO/SBA-15. For SBA-15, only one wide strong peak is observed in the pattern, corresponding to a mixed band of Si(OH)(OSi)3 and Si(OSi)4 shape. The NMR
Conclusions
A two-step synthesis has been employed to synthesize a new type of heteropoly acid on amino functionalized SBA-15. Evidence for attachment of HPA clusters to the SBA-15 surface was provided by several spectroscopic techniques. The amino group anchored on the SBA-15 surface and HPA clusters dispersed well on the amino-functionalized SBA-15 surface through strong bond. The major advantage of the supported HPA cluster over pure HPA was not their reactivity but the ease in separation and recovery
Acknowledgments
The financial support from the National Natural Science Foundation of China under Grant No. 20271045 and the Foundation of State Key Laboratory of Inorganic Synthesis and Preparative Chemistry of Jilin University for this work is greatly appreciated.
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2014, Journal of Molecular Catalysis A: ChemicalCharacterization of H <inf>3+x</inf>PMo <inf>12-</inf><inf>x</inf>V <inf>x</inf>O <inf>40</inf> heteropolyacids supported on HMS mesoporous molecular sieve and their catalytic performance in propene oxidation
2012, Microporous and Mesoporous MaterialsCitation Excerpt :This result is consistent with that of the textural study that shows the destruction of the structure of the HMS when the acid percentage is high (>30%). The decrease in intensity of the diffraction peaks attributed to the mesostructure was already observed on MCM-41 [39,40], SBA-15 [19] and HMS [31,41] in the presence of heteropolyacids. Moreover, no patterns of the V2 crystal phase are observed even at high HPA loading (40–50 wt.
Immobilization of 12-tungstophosphoric acid on LaSBA-15 and its catalytic activity for alkylation of o-xylene with styrene
2012, Chemical Engineering JournalCitation Excerpt :This would make the material as a suitable host for acid catalysts. There are some reports available on the modification of SBA-15 with heteropoly acids and its applications [14–22]. However, weak interaction between the HPW and the support resulted in its leaching in polar media [23].