Effect of Aqueous Solvent on Aldol Condensation of Formaldehyde and that the C-C coupling step is kinetically more feasible than Acetone on Anatase TiO (101) Surface the α-H abstraction step with a lower barrier of 44 kJ/mol. 2 Upon the C-C coupling step, two hydrogenation/dehydration

pathways were examined. On one hand, as shown in Figure 2, Yuntao Zhao1, 2, Xinli Zhu1, Hua Wang1, Jinyu Han1, Qingfeng Ge1, 3,* and Donghai Mei2,* the calculated barrier of 190 kJ/mol for the 1Collaborative Innovation Center of Chemical Science and Engineering, School of Chemical CH COCH CH OH* dehydration makes this direct Engineering and Technology, Tianjin University, Tianjin 300350, China 3 2 2 hydrogenation pathway kinetically unlikely. On the other 2School of Chemistry and Chemical Engineering, Tianjin Polytechnic University, Tianjin hand, the alternative tautomerization pathway is also difficult China to proceed due to the higher barrier of 110 kJ/mol for the 3Department of Chemistry and Biochemistry, Southern Illinois University, Carbondale, Illinois first tautomerization step. As a result, a mixed reaction route 62901, United States with the first direct hydrogenation step, followed by the *[email protected], [email protected] dehydration step in tautomerization pathway is identified in

gas phase. Introduction In aqueous phase, it is found that the α-H abstraction Aldol condensation plays a vital role in the catalytic transformation of biomass-derived could only be achieved via proton transfer mechanism. The oxygenates to useful and value-added chemical products by reducing the oxygen content and calculated kinetic barriers suggest that both α-H abstraction increasing carbon chain length through C-C coupling [1,2]. Most study on aldol condensation and the C-C coupling steps are comparable to those in vapor focused on vapor phase process although biomass-derived oxygenates, including carboxylic phase. However, the following reaction steps after the C-C coupling are largely affected by the acid, aldehydes, ketones, are stable in an aqueous solution. Water itself is one of the products aqueous phase. The dehydration barrier in the direct hydrogenation pathway is lowered by 88 from aldol condensation and its effect on the reaction is expected to increase as the reaction kJ/mol, making it a competitive route to form methyl vinyl ketone. The activation barrier of progresses. Aldol condensation of acetone and formaldehyde co-feeding with water had been tautomerization route via proton transfer is also lowered by 63 kJ/mol. Therefore, in aqueous previously reported [3,4]. However, the role of water is not clearly understood. In this work, phase, these two reaction routes, hydrogenation and tautomerization, are parallel in the aldol condensation of formaldehyde and acetone on an anatase TiO (101) surface in vapor and in 2 condensation of formaldehyde and acetone. liquid water environment are comparatively studied using density functional theory (DFT) calculations. The effects of aqueous phase on the reaction mechanism and the kinetically relevant steps including α-H abstraction, C-C coupling, and hydrogenation steps are elucidated. The results showed that the hydrogen-bonding network in the aqueous phase around the active intermediates is important, not only providing different additional stabilities for reaction and transition states, but also altering the reaction pathways via proton transfer mechanism. Materials and Methods All calculations were performed using the spin-polarized gradient-corrected functional of Perdew, Burke, and Ernzerhof (PBE) implemented in the CP2K package. The wave functions were expanded in an optimized double-ζ Gaussian basis set (DZVP). An energy cutoff of 500 Ry was used for the electrostatic energy calculation. Only the Gamma k-point sampling was adopted. The transition states of the surface reaction were searched using the climbing-image nudged elastic band method. The maximum force was converged to less than 0.05 eV/Å. The semiempirical van der Waals correction proposed by Grimme was included in all calculations. Results and Discussion Figure 2. Free energy profile from formaldehyde and acetone to methyl vinyl ketone on Figure 1 shows the adsorbed formaldehyde and acetone on anatase TiO2 (101) surface in anatase TiO2 (101) in gas phase (left), and aqueous phase (right). both gas phase and aqueous phase. A periodic four atomic layer surface slab consisting of 48 Significance Ti and 96 O atoms was chosen. To simulate the aqueous phase system, 47 explicit H2O This work provides the fundamental insights into how the aqueous phase affects aldol molecules were put above the surface slab (15 Å in the z direction) on the basis of liquid water condensation reaction mechanisms in the catalytic conversion of biomass-derived oxygenates. density under typical aldol condensation reaction condition, followed by 10 ps of AIMD References simulation for equilibrium. The elementary steps of aldol condensation, including α-H 1. C. J. Barrett, et al. Appl. Catal. B: Environmental 66 (2006) 111-118. abstraction of acetone, C-C coupling of enolate with formaldehyde, and dehydration to form 2. G.W. Huber, et al. Chem. Rev. 106 (2006) 4044-4098. methyl vinyl ketone, were studied in the vapor and aqueous phases with or without proton 3. E. Suzuki, et al. Bull. Chem. Soc. Jpn. 61(1988) 1008-1010. transfer. The overall Gibbs free energy profiles are shown in Figure 2. Our DFT results show 4. K. K. Rao, et al. J. Catal. 173 (1998) 115-121.