Elsevier

Solid State Sciences

Volume 7, Issue 3, March 2005, Pages 333-337
Solid State Sciences

Assembling of tungstovanadogermanic heteropoly acid into mesoporous molecular sieve SBA-15

https://doi.org/10.1016/j.solidstatesciences.2004.10.035Get rights and content

Abstract

Heteropoly acid of Keggin structure tungstovanadogermanic (H5GeW11VO40) has been chemically anchored to the modified SBA-15 surface. The material was characterized by solid-state 29Si MAS NMR, FT-IR spectroscopy, XRD, TG-DTA and N2 adsorption experiments. The preservation of the structure of the HPA and dispersion of the cluster on the support were verified. On the bases of the spectroscopic evidences, HPA clusters were attached firmly to the surface.

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 (<10m2g−1) 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 (10Å) is limited.

Mesoporous molecular sieves (MMS) [10], which have significantly large surface area (1000m2g−1), 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 (Q3100ppm) and Si(OSi)4 (Q4110ppm) 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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