Hierarchical Beta zeolites as catalysts in a one-pot three-component cascade Prins–Friedel–Crafts reaction†
Hierarchical Beta zeolites obtained from concentrated reaction mixtures (H2O/Si = 2.5–7.0) in the presence of CTAB and their conventional and nanosponge analogues were investigated in a one-pot cascade environmentally friendly Prins–Friedel–Crafts reaction of butyraldehyde with 3-buten-1-ol and anisole under mild conditions (60 °C). The highest yields of the desired products with 4-aryltetrahydropyran structure were achieved when using hierarchical zeolites characterised by well-developed mesoporosity (facilitating the formation of bulky intermediates and products) and by an increased fraction of highly accessible (evaluated by TTBPy method) medium-strength Brønsted acid sites. Acid sites with higher strength promote strong adsorption of bulk O-containing intermediates or products and the formation of byproducts (tetrahydropyranyl ether and 2-propyloxan-4-ol). Therefore, this is an inexpensive and simple synthesis method for preparing hierarchical zeolites with catalytic activity comparable to that of nanosponge Beta, which is however prepared using complex and expensive multi-quaternary ammonium surfactants. Moreover, this synthetic protocol for 4-aryltetrahydropyrans replaces the carcinogenic and toxic chemicals, which have been previously used for Prins–Friedel–Crafts reactions, with green and non-toxic substances.