Abstrakt
The one-pot Prins-Friedel-Crafts reaction of aldehydes, a homoallylic alcohol and aromatics catalyzed by large-pore zeolites is an attractive environmentally friendly route towards valuable heterocyclic compounds containing a 4-aryltetrahydropyran moiety. Herein, the catalytic behavior of a set of MWW zeolite catalysts with tunable textural properties (e.g., three-dimensional MCM-22 and MCM-49, layered MCM-56 and MCM-36 materials) and variable chemical compositions was investigated in the Prins-Friedel-Crafts reaction involving either butyraldehyde or benzaldehyde and compared to that of a large pore beta zeolite.
MWW zeolites differing in the concentration of acid sites 0.16-0.55 mmol g(-1) and Bronsted-to-Lewis acid site ratios BAS/LAS = 1.0-1.7 showed similar selectivities towards targeted 4-alkyltetrahydropyran-containing products (71-75% at 67% conversion of butyraldehyde), which exceed the value attained over a hierarchical beta zeolite (55-58% at the same conversion) with similar textural characteristics. While the conversion of relatively small butyraldehyde increased with the total concentration of acid sites in MWW catalysts, it was not affected by textural characteristics of MWW zeolites (98% and 100% over microporous MCM-22 and micro-mesoporous MCM-36, respectively).
In contrast, not only a high concentration of acid sites but also their enhanced accessibility were the key factors, which provide the highest conversion (82%) and selectivity (50%) over micro-mesoporous MCM-36 using benzaldehyde as a substrate. Hierarchical MWW zeolite catalysts offered higher yields of targeted 4-aryltetrahydropyran in comparison with the conventional beta zeolites and previously reported hierarchical beta zeolites due to (1) the improved selectivity conditioned by lower strength of LAS and (2) maintaining high conversion due to the abundance of accessible acid sites on the well-exposed surface.