Experimental and Theoretical Study of n-Butanal Self-Condensation over Ti Species Supported on Silica

Abstract

Alkali metal oxides supported on SBA-15, M/ SBA-15 (M = Li, Na, K, Rb, and Cs), prepared by the impregnation method were characterized and tested for the aldol condensation of methyl acetate and formaldehyde to methyl acrylate (MA). M/SBA-15 catalysts are active for the formation of MA while the support is inert. The supported alkali metals react with hydroxyl groups on SBA-15 by replacing the H atoms and generate weak and medium basic sites. The latter plays an important role in promoting the reaction. Coke formation results in deactivation of Cs/SBA-15, and the deactivated catalyst can be regenerated by burning off the coke. Two possible reaction pathways for the formation of MA were explored using quantum calculations. The predicated activity for M/SBA-15 is consistent with the experimental observations. A pathway with adsorbed enol molecule as an intermediate was suggested to be predominant for the formation of MA.

The Role of Hydroxyl Group Acidity on the Activity of Silica‐Supported Secondary Amines for the Self‐Condensation of n‐Butanal

“…% Ti from ICP-OES analysis) are shown in Figure 1. These results suggest that, at low Ti loading ( 0.3 Ti atoms nm À2 ), Ti species mainly exist as isolated tetrahedral sites whereas, at high loadings of 0.7 Ti atoms nm À2 , bulk anatase-like structures are formed in the pores of the SBA-15 support [15] (Supporting Information). This observation is consistent with X-ray absorption studies (XANES), for which the spectrum of Ti(0.07)-SBA-15 exhibits a single pre-edge feature at 4971 eV similar to that observed for TS-1 indicating that Ti exist as tetrahedrally coordinated framework sites.…”

Reaction-dependent heteroatom modification of acid–base catalytic cooperativity in aminosilica materials

“…Furthermore, they observed differences in acid strength for the materials depending on the nature of the synthetic conditions, the extent of surface hydroxylation, and Ti dispersion in the silica framework [33]. Titanols on the surface of mesoporous silicas provide stronger Brønsted acidity than silanols, which explains the decreased activity of Ti-A3-SBA-15 compared to the A3-SBA-15 for the aldol condensation, though the pyridine IR results for both the A3-SBA-15 and Ti-A3-SBA-15 indicated very weak Brønsted acidity [48,89].…”

“…M 3+ elements will typically introduce a Brønsted acid site that is a stronger acid than the silanols, while M 4+ elements will introduce new types of hydroxyl species such as titanols [33] or even Lewis acidic sites in the case of coordinatively unsaturated Zr 4+ species [34,35] and tetrahedrally coordinated Ti sites [36]. At present, studies of the effects of heteroatom substitution on cooperative acid-base catalysts have been limited mainly to Al substitution for C-C coupling reactions and one-pot cascade reactions [13,[37][38][39][40][41][42][43][44][45][46][47][48]. In this study, we examine the effects of heteroatom substitution with materials with similar pore diameters, similar organic (amine) loadings, and approximately 5 mol% heteroatom substitution.…”

Condensation and esterification reactions of alkanals, alkanones, and alkanols on TiO2: Elementary steps, site requirements, and synergistic effects of bifunctional strategies

“…Metal oxides (e.g., MgO [12,13], MgOAAl 2 O 3 [14][15][16][17], ZrO 2 [6,18], CeO 2 AZrO 2 [19,20], and TiO 2 [21][22][23]) catalyze aldol condensation reactions. On these oxide catalysts, reactions are limited by thermodynamics [15], leading to low equilibrium concentrations of a,b-unsaturated carbonyl compounds; these compounds tend to undergo subsequent condensation and cyclization reactions that form unreactive residues and block active sites [22,23].…”