Hierarchical Porous WO₃@SiO₂ Self-Standing Monolithic Catalysts and Their Efficient Cycling “Cosolvent-Free” Friedel–Crafts Alkylation/Acylation Catalytic Properties

Taking advantage of tungsten oxide inherent Brønsted–Lewis acidity, the first series of WO3–SiO2(HIPE) monolithic (MUB-104(x) series) catalysts bearing multiscale porosity have been designed through integrative chemistry. At the microscopic length scale, beyond X-ray diffraction (XRD) revealing the WO3 monoclinic structure, local electron spin resonance (ESR) spectroscopy addresses g values clearly located at the frontier between extended WO3 phases and polyanion clusters, in agreement with scanning transmission electron microscopy high-angle annular dark field (STEM-HAADF) investigations. NH3-TPD shows that when the tungsten loading is increased from 10 wt % for MUB-104(1) to 24 wt % for MUB-104(3), the amount of acidic sites significantly decreases from 0.35 to 0.08 mmol of NH3 g–1, respectively, in agreement with the particle sizes. The MUB-104(x) series offers efficient cosolvent-less heterogeneous cycling catalysis toward both Friedel–Crafts alkylation and acylation syntheses. MUB-104(1) carries the highest turnover number (TON) of 121 and turnover frequency (TOF) of 20 h–1, while for acylation, MUB-104(1) carries the highest TON of 117 and TOF of 26 h–1. Besides, we find out that the catalysts offer good selectivity, where 30/70% of ortho- and para-substituted alkylated products are obtained. Considering the acylation reaction, the selectivity is even higher, reaching 100% toward the para-substituted acylated product. The catalyst, due to its intrinsic monolithic character, can be easily extracted from the reactive media, recycled, and re-employed further.