(521ch) Zeolites As Selective Catalysts for Cross Aldol Reactions | AIChE

(521ch) Zeolites As Selective Catalysts for Cross Aldol Reactions

Authors 

Jentoft, F., University of Massachusetts Amherst
Aldol condensations are useful in chemical synthesis and have potential for bio-oil upgrading to transportation fuels or platform chemicals. Depending on the reactant(s), controlling selectivity presents a challenge, since various self- and cross-aldol condensation products may form. Further, a side reaction to an olefin and a carboxylic acid (called fission) is attracting interest, as a route for producing isobutene (an important monomer, e.g. for synthetic rubber) via acetone self-condensation. Soluble acid-base catalysts combined with special reagents permit selectivity control, but present the well-known disadvantages of corrosion, waste generation and expensive catalyst recovery. Also, efforts in heterogeneously catalyzed aldol reactions have focused on catalyst stability and activity. Hence, there is a demand for selective, solid catalysts that will make aldol processes more sustainable. This work explores zeolites with different frameworks (MFI, MWW, MOR, BEA and FAU) as catalysts to investigate the effect of microporosity on selectivity. Benzenesulfonic acid serves as a soluble benchmark catalyst devoid of the spatial constraints imparted by zeolite pores. Results obtained for the zeolite-catalyzed cross-aldol reaction between benzaldehyde and diverse ketones (C3 – C7) indicate that the zeolites promote formation of the fission products. Maximum fission product selectivities were 92% at 88% conversion with the large-pore FAU framework and 67% at 26% conversion with the medium-pore MFI framework. The MOR framework favored the fission pathway; however, it was less active with 30% benzaldehyde conversion and is in this respect comparable to the medium-pore zeolites. The soluble acid catalyst was highly selective to the classical aldol condensation product (Figure 1). Regarding the role of ketone size for selectivity, higher ketones resulted in greater selectivity towards fission products, which suggests an interplay of confinement effects from making the ketone bulkier and electronic effects from the variation of substitution at the alpha carbon.