Highly active mesoporous Nb-W oxide solid-acid catalyst.

The synthesis of mesoporous transition-metal oxides has been extensively studied because of their wide range of potential applications. Examples of such compounds include mesoporous TiO2, [2, 3] ZrO2, [2, 4] Nb2O5, [2,3b, 5] Ta2O5, [2, 6] (Nb,Ta)2O5, [2, 7] SnO2, [2, 8] and WO3, [2] which are used as a variety of heterogeneous catalysts, such as solid-acid catalysts, 5d, 6f,g] photocatalysts, 6b,h] oxidation catalysts, and catalyst supports. Solid-acid catalysts, which are reusable and readily separable from reaction products, have been widely investigated as direct replacements for liquid acids to reduce the impact on the environment and to decrease costs. The use of mesoporous transition-metal oxides is an interesting approach to developing a solid-acid catalyst with enhanced activity. The mesopores in the oxide allows the reactants access additional active acid sites in the pores, resulting in improved rates of acid catalysis. Sulfated mesoporous niobium and tantalum oxides have been reported to exhibit remarkable activity in acid-catalyzed Friedel–Crafts alkylation and isomerization. 6f,g] However, the use of the recycled catalyst remains difficult, a result of the leaching of sulfate species, as reported for mesoporous silica and organosilicas bearing sulfonic acid groups. Herein, mesoporous Nb– W mixed oxides are examined as solid-acid catalysts, these give very high catalytic performance in Friedel–Crafts alkylation, hydrolysis, and esterification, which originates from the mesoporous structure and different acid properties formed by specific Nb and W concentrations. Mesoporous Nb–W mixed oxides were prepared from NbCl5 and WCl6 in the presence of a poly block copolymer surfactant Pluronic P-123 as a structure-directing agent. (Additional details are provided in the Supporting Information) Peaks attributable to mesopores were observed from NbxW(10 x) oxides with x values from 2 to 10 in the small-angle powder X-ray diffraction (XRD) pattern (see Figure S1 in the Supporting Information). Peaks attributed to (110) and (200) of the two-dimensional hexagonal structure were observed from an x = 10 sample (mesoporous Nb oxide), which was consistent with previous studies. Wide-angle powder XRD patterns revealed the presence of crystallized tungsten oxide (WO3) in W-rich samples (x = 0 to 2). The presence of mesopores was also indicated by the N2 sorption isotherms (Figure 1) for the same samples (x = 2 to 10). The surface areas were estimated using the Brunauer–Emmett–Teller (BET) method, and pore volumes were obtained by the Barrett–Joyner–Halenda (BJH) method. Although the surface area decreased gradually from 200 (mesoporous Nb oxide) to 52 m g 1 (non-mesoporous W oxide) with increasing addition of W, up to x = 0, the pore volume decreased up to x = 3. Then, the pore volumes increased in the non-mesoporous W-rich oxides (x = 0 to 2) due to the formation of void spaces between particles (Supporting Information, Figure S2). The pore diameter obtained by the BJH method decreased from 7 (mesoporous Nb oxide) to 4.2 nm (mesoporous Nb3W7 oxide) with increasing W content, and mesopores were not observed in the Nb1W9 oxide (Supporting Information, Figure S3). SEM and TEM images of the porous