Changes in Surface Acidity of Silica-Supported Dodecatungstosilicic Acid in Relation to the Loading Amount

Abstract

Changes in surface acidity of H4SiW12O40/SiO2 in relation to the loading amount were investigated by using temperature-programmed desorption (TPD) of benzonitrile (BN), BN infrared (BN-IR) spectroscopy, and test reactions, including skeletal isomerization of n-butane and double-bond shift of 1-butene. From BN-TPD profiles and BN-IR spectra, H4SiW12O40/SiO2 had medium strength Brønsted (Mid-B site) and strong Brønsted acid sites (Strong-B site) on its outermost surface, whereas there was no evidence for Lewis acid sites regardless of the loading amount. The number of Strong-B sites correlated with the catalytic activity for the skeletal isomerization of n-butane and was a maximum when the loading amount was 50 wt %. On the other hand, the number of Mid-B sites was a maximum for 30 wt % H4SiW12O40/SiO2, which had the highest activity for the double-bond shift reaction of 1-butene. Mid-B sites of H4SiW12O40/SiO2 were present in the starting H4SiW12O40, whereas the Strong-B sites formed when H4SiW12O40 was supported on SiO2. There were no Strong-B sites on H4SiW12O40/SiO2 with low loadings, including 5 and 10 wt %. Thus, it was concluded that the direct interaction of H4SiW12O40 with the surface of SiO2 caused the Strong-B sites to form in the second-layer of H4SiW12O40 exposed on the surface.

Comparison of the Acidity of Heteropolyacids Encapsulated in or Impregnated on SBA-15

“…700°C for HSiW@SBA-15. All these results are in agreement with the data reported in the literature [43,44,47]. A more quantitative comparison of the data gives 1.5 mmol NH 3 desorbed per g of H 3 PW 12 O 40 for the bulk HPA, 0.3 mmol NH 3 desorbed per g of SBA-15 and around 2 mmol NH 3 desorbed per g for the SBA-15 supported and encapsulated HPA (Tab.…”

“…Ammonia is often used as a probe molecule for the determination of the acidity of the solids, for example by studying its adsorption by microcalorimetry or its desorption from a catalyst previously saturated with it (TPD) [43][44][45]. We used these two methods in order to compare the acidity of the impregnated and encapsulated catalysts as the catalytic reactivity had shown a difference in the behavior of the HSiW@SBA-15 system compared to the three other ones.…”

Characteristics of post-impregnated SBA-15 with 12- Tungstophosphoric acid and its correlation with catalytic activity in selective esterification of glycerol to monolaurate

“…Acid strengths classification of these developed catalysts were tabulated in Table 2 [14][15][16]. As c , " " increasing acid loadings [17,18]. Thought similar trend was reported [19], we concluded that partial agglomeration of HPW crystal on external surfaces at high HPW loading might have occurred.…”

Diethyl Ether Conversion to Ethene and Ethanol Catalyzed by Heteropoly Acids

“…With silica-supported HPAs at low and medium HPA loadings, both polar and nonpolar substrates typically react via the surface-type catalysis, showing similar dependencies between catalyst activity and HPA loading—the activity, expressed as substrate conversion or reaction rate per total weight of HPA/SiO 2 catalyst, increases with HPA loading at a constant HPA/SiO 2 catalyst weight, reaching a plateau at 25–40% loading, or passes a maximum. 26 , 34 , 38 This is the result of a trade-off between the density of surface proton sites and their strength at varying HPA loading. The proton site density (per total catalyst weight) passes a maximum at a medium HPA loading due to the sharp decrease in catalyst surface area to less than 10 m 2 g –1 for bulk HPA.…”

“…Using the TPD of benzonitrile, which interacts only with the surface acid sites in HPA, the amount of strong surface Brønsted sites in HSiW/SiO 2 has been found to pass a maximum at a HSiW loading of ∼50%. 34 …”