Aldol reactions are known to pose a difficulty with respect to controlling product selectivity, since various self and cross aldol reactions are possible. While a great degree of selectivity control has been realized using soluble catalysts, efforts in heterogeneously catalyzed aldol condensations have focused on catalyst stability and activity, hence, there is need for solid catalysts that would make aldol processes more sustainable and enhance selectivity. In this work, zeolites of different frameworks such as MFI, MWW, MOR, BEA and FAU were used for the aldol reaction of benzaldehyde with various ketones, to investigate the role played by porosity in controlling selectivity in cross aldol reactions. Benzenesulfonic acid was used as a soluble acid catalyst to serve as a benchmark devoid of the spatial constraints imparted by the zeolite pores. Regioselectivity was investigated using benzaldehyde and 2-butanone, which may form linear or branched addition and condensation products. Also, the addition products can be cleaved to form an olefin and a carboxylic acid, which will be referred to as fission products. The soluble acid catalyst selectively formed branched condensation product, medium-pore zeolites preferentially formed the linear condensation product with &gt; 70% selectivity obtained with the MFI framework, whereas the large-pore zeolites favored fission with ~70% selectivity achieved with the FAU framework. A study with benzaldehyde and 3-pentanone allowed for a deeper probing into the role of catalyst porosity in directing selectivity in a dehydration versus fission pathway network. Results obtained show that the soluble acid catalyst selectively favored the dehydration pathway, and the zeolites preferentially formed the fission products. Maximum fission product selectivities were 92% at 88% conversion with the large-pore FAU framework and 61% at 26% conversion with the medium-pore MFI framework. The role of the ketone size for selectivity was probed by reacting benzaldehyde with a smaller ketone (acetone). Generally, the soluble acid steers selectivity completely towards the condensation product. In contrast, H-form zeolites promote fission over dehydration in the cross-aldol reactions of benzaldehyde with several ketones except for cases where dehydration is very facile.

Controlling selectivity in liquid-phase cross aldol reactions by using porous solid catalysts

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