Aldol condensation reactions provide pathways to produce chemical building blocks by forming C-C bonds within reactant pools (e.g., ethanol and acetaldehyde). The rates and selectivities among these pathways depend upon the solvating environment, and therefore, Ti substituted zeolites with distinct void dimensions and polarity can influence the average carbon number of coupling products to favor desired species. Here, we synthesized, characterized, and catalytically interrogated a series of Ti-silicates including Ti-MFI, BEA, FAU and SBA-15, which present different topologies and silanol (i.e., SiOH) densities. Turnover rates on all materials increase superlinearly with [CH3CHO] and decrease with increasing [C2H5OH] and [H2O] at 503 K, which implies a consistent mechanism and that sites are primarily unoccupied across these materials. Rates increase upon substitution of CD3CDO for CH3CHO (kH/kD ~ 0.8-0.9) in all Ti-BEA. Collectively, these findings indicate that C-C bonds form by kinetically relevant nucleophilic attack of enolates to aldehydes in these catalysts, nevertheless, turnover rates and selectivities for aldol condensation increase with SiOH density on Ti-BEA. Furthermore, both activation enthalpies and entropies for aldol condensation become more positive with SiOH density. At 503 K, the entropy contributions dominate and give larger turnover rates in SiOH-rich Ti-BEA. In situ infrared spectroscopy shows C2H5OH adsorbs and irreversibly reacts with SiOH groups of Ti-BEA to form ethoxysilane (i.e., Si-OC2H5), and the interactions between these and reactive species bound to Ti-atoms appear responsible for the influence of SiOH density on kinetics. Pore topology also affects reaction rates and distributions of aldol condensation products. At isoconversion, aldol condensation rates for SiOH-rich Ti-FAU are a factor of 4-7 greater than rates for the other Ti-silicates. Selectivities to aldol condensation decrease with characteristic pore diameter from 90% in Ti-MFI to 46% in Ti-SBA-15. Tighter pores also seem to limit growth of aldol condensation products beyond C4, the selectivities of which increase from Ti-SBA-15 to Ti-MFI. The findings here suggest pore topology and polarity of Ti-silicates influence barriers of the reactions differently, which can be utilized to control the selectivity to value-added aldol condensation products.

Effects of pore topology and polarity on rates and selectivities for aldol condensation on Ti-Silicates

ACS Spring Meeting 2021

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