Aldol condensation is a key C-C coupling reaction for upgrading of biomass-derived oxygenates to fuels and chemicals. Here, we investigate the effects of added water on the condensation of cyclopentanone (CPO) on a series of organosilane-functionalized acid- and base- catalyzed catalysts. We have found that the role of water strongly depends on the degree of functionalization of the materials. For example, while on non-functionalized (hydrophilic) MgO the rate decreases with the addition of water, the rate actually increases with added water on MgO hydrophobized via silylation with octadecyltrichlorosilane (OTS). A concomitant change in kinetics is observed from the pristine (hydrophilic) MgO to the hydrophobized sample. Specifically, on the hydrophilic sample, the reaction is first-order, as expected if the rate-limiting step is the formation of an enolate intermediate via α-H abstraction at a basic site, as widely reported in previous literature. By contrast, on the hydrophobized sample the reaction becomes second-order, indicating a shift in rate-limiting step to the bimolecular C-C coupling. We propose that grafted OTS molecules interfere between active sites, making the adsorbate-adsorbate interaction on the surface less likely and reducing the rate of C-C coupling. The promotional role of water seems to be the assistance of the C-C bond-forming step. Similarly, when MCM-41 functionalized with sulfonic groups is used for the acid-catalyzed CPO self-condensation reaction water shows a promoting effect on activity by assisting the C-C coupling step acting as a bridge that makes the electrophile molecule more favorably attacked by the enol. Therefore, the reaction pathway switches from Eley-Rideal to Langmuir-Hinshelwood. This effect is only observed with low acid-site density catalysts, for which the C-C coupling is rate limiting. On catalysts with high density of sulfonic groups, the C-C coupling is highly favored and the rate limiting step is the enol formation (first order).

Promoting effect of water on the acid- and based-catalyzed aldol condensation of Cyclopentanone

2019 Fall National Meeting

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