Role of Water in Aldol Condensation Reactions Catalyzed by MCM-41 Interestingly, when water was added to the reaction mixture with this low-acid-density catalyst, Functionalized with Sulfonic Groups not only the activity greatly increased, but now the best fit switched to a combination of ER and LH dominated by the latter (see Fig. 1b).

Gengnan Li, Duong T. Ngo, Tuong V. Bui, Bin Wang, Daniel E. Resasco* School of Chemical, Biological and Materials Engineering and Center for Interfacial Reaction Engineering (CIRE), University of Oklahoma, Norman, Oklahoma, 73019, USA *[email protected]

Introduction Aldol condensation is an important carbon-carbon bond formation reaction with Figure 1. Comparison of reaction rates under water-free (a) and external water-added (b) potential applications in biomass conversion. Molecular weight of short carbonyl compounds conditions; DFT calculation of the interaction between catalyst and external water (c). derived from biomass (e.g. cyclopentanone, acetone, etc.) can be shifted to a range more appropriate for fuels. For this reaction that can be catalyzed by bases or acids, the presence of We hypothesize that water may act as a bridge that helps polarizing the electrophile molecule water is typically undesirable since it competes for active sites, inhibiting the intrinsic activity. receiving the attack from enol. That is, in the LH mode, the carbonyl of the adsorbed CPO is However, water is often unavoidable, particularly in biomass upgrading processes. Therefore, polarized by the acid site on the surface facilitating the attack and C-C coupling. However, this we have recently investigated functionalized catalysts which not only are tolerant to water but cannot happen when the electrophile is in the bulk (ER mode). As shown by DFT (see Fig. 1c), they may show an enhancement in activity in the presence of water [1,2]. We have investigated water has the ability to bridge a nucleophile with an acid site via H-bonding. The promoting the aldol condensation of cyclopentanone mesoporous MCM-41 catalysts functionalized with effect of water is illustrated in Scheme 1 below. Ab initio molecular dynamics (AIMD) sulfonic acid groups with varying acid density. Contrary to the behavior exhibited by simulations and DFT calculations show that the water molecular can form H-bonds with conventional base (MgO) or acid (H-ZSM-5) catalysts, for which water is detrimental, these adjacent acid sites, and that the proton in the sulfonic moiety may dissolve in water and functionalized catalyst shows an enhancement in catalytic activity by the presence of water. In become delocalized in the proximity of the functional group. These dissolved protons may thus this contribution, the role of water in kinetically-relevant steps have been investigated by polarize the second ketone and facilitates the C-C coupling in the L-H reaction pathway. combining the experimental kinetic data with DFT calculations. Therefore, L-H reaction pathway is more favorable in presence of water, as experimentally observed. Materials and Methods A mesoporous MCM-41-Type A sample was obtained from ACS Materials and functionalized with 3-mercaptopropyltrimethoxysilane (MPTS) to varying densities. After oxidization with H2O2, a series of sulfonic-group-functionalized MCM-41 acid catalysts with controlled acid density was obtained. The self-condensation of cyclopentanone (CPO) was conducted in a Parr reactor (100mL, 200 mg catalyst, 200 ºC, 500 psi N2, 3 h reaction). Using an internal standard, the products were analyzed by GC-MS and GC-FID. The acidities of the catalysts were quantified by pyridine titration and TGA/MS. Scheme 1 Proposed mechanism for self-condensation of cyclopentanone based on Langmuir- Hinshelwood with water-added in the feed. Results and Discussion It is well-known that the acid-catalyzed aldol condensation reaction starts with the Significance formation of an enol via protonation and tautomerization of the first CPO, followed by attack Water molecules can play an important role in determining the energy barrier of kinetically to the electrophilic C of the other carbonyl. The kinetic analysis in our work shows that this relevant steps. Understanding the reaction mechanisms and finding the role of water molecules attack is the rate limiting step. Since the enol intermediate can either react with a is of crucial significance to control the activity and selectivity. In this contribution, we have cyclopentanone molecule adsorbed on an adjacent acid site (LH) and/or in the liquid phase concentrated in the effect of added water on C-C coupling reactions over HSO3-functionalized (ER), we have conducted our kinetic analysis allowing both contributions to occur MCM-41 catalysts through a combination of experimental and theoretical work. simultaneously. On the catalysts with high density of sulfonic groups, the best fitting of the data is obtained with only the LH contribution to the LH+ER fit. Evidently, the close proximity References of sites on the surface makes this mechanism more plausible. By contrast, when a low acid 1. D. Ngo, T. Sooknoi, D. Resasco, Appl. Catal. B: Environ., 2018, 237, 835-843. density catalyst was used, the fitting with only LH was unsatisfactory and it can only be fitted 2. P. Zapata, J. Faria, M. Ruiz, R. Jentoft, D. Resasco, J. Am. Soc. Chem., 2012, 134, 8570- satisfactorily with a combination of ER and LH, dominated by the former (see Fig. 1a). 8578.