Fine‐Tuning the Acid–Base Properties of Boron‐Doped Magnesium Oxide Catalyst for the Selective Aldol Condensation

Abstract

Controlled incorporation of dopants into the structure of metal oxide catalysts can be used to fine‐tune their topological, chemical, and electronic properties. Conventional preparation methods (coprecipitation and impregnation) tend to produce materials with a limited extent of chemical interaction between metal oxide and dopant, small pore volume, and disconnected pore structure. Herein, we describe a combustion method to optimize both the porous structure of metal oxides and the chemical interaction with dopants. By exploiting the differences in melting points between metal oxide (MgO) and dopant (B), it is possible to create hierarchical mixed oxides with tailored chemical structure. As a model reaction, we employed the aldol condensation of acetaldehyde to crotyl alcohol, a key intermediate step in the conversion of bioethanol to 1,3‐butadiene, to show the unique characteristics of these materials. The B–Mg mixed oxide prepared by combustion exhibits an order of magnitude higher productivity of C4 products than conventional MgO and B–MgO. We anticipate that our approach could be extended to the development of other dual oxides with open structure for unrestricted diffusion and enhanced chemical interaction of acid and basic metal oxides.

Interfacial Control of Catalytic Activity in the Aldol Condensation: Combining the Effects of Hydrophobic Environments and Water

“…A reaction that has been extensively used for this purpose is the aldol condensation. ,− This reaction has been recently included in the design of many biomass-processing schemes, − and its study has provided clear examples of complex behaviors, such as acid–base cooperativity. Several groups have investigated the effects of different types of acidic sites on the activity of immobilized base catalysts, demonstrating that cooperativity is affected by the strength, relative position, and structure of acid and base functionalities. ,− , Additional studies have shown the role of properties more relevant to interfacial interactions such as the effects of support hydrophobicity ,− and solvents on the reaction. ,,− For example, it has been shown that the catalytic activity of amine-functionalized mesoporous silicas decreases with increasing solvent polarity. , This drop in activity has been attributed to the increasing stabilization of ion pairs formed between acidic groups and amines that blocks a fraction of the active sites by turning them into non-nucleophilic ammonium cations. ,, Interestingly, in spite of its high dielectric constant, water deviates from this trend: the catalytic activity of primary amines is higher in water than in methanol. , This anomalous behavior has been explained by the role of water in limiting competing reactions such as the formation of the non-enolizable Schiff base 1 (pathway a, Scheme ) , and the conjugated dehydration product 2 (pathway b, Scheme ) that deactivate another fraction of the active amine sites. In addition, water is required for the hydrolytic desorption of the product in the last step of the reaction (step c, Scheme ), which drives the equilibrium toward completion …”

“…Several groups have investigated the effects of different types of acidic sites on the activity of immobilized base catalysts, demonstrating that cooperativity is affected by the strength, relative position, and structure of acid and base functionalities. 19,[21][22][23][24][25][26][27][28][29]37 Additional studies have shown the role of properties more relevant to interfacial interactions such as the effects of support hydrophobicity 24,38−40 and solvents on the reaction. 18,19,41−43 For example, it has been shown that the catalytic activity of amine-functionalized mesoporous silicas decreases with increasing solvent polarity.…”

Synthesis of α,β‐ and β‐Unsaturated Acids and Hydroxy Acids by Tandem Oxidation, Epoxidation, and Hydrolysis/Hydrogenation of Bioethanol Derivatives

“…To selectively convert the C 2 aldehydes into the corresponding C 4 coupling products (Scheme b) we have developed a number of catalytic systems based on basic K/Na‐X and K/Na‐Y, and MgO‐K/NaX, zeolites as well as B‐doped MgO oxides . The Mg‐Al Hydrotalcite, Al‐Beta zeolite, which along with the Mg‐B catalyst previously employed have shown the highest C 4 productivities among all the catalysts investigated, with productivities of 8.3, 23.4, and 24.2 mmol of C 4 g cat −1 h −1 , respectively, at 50–60 % conversion level and 180 °C (Table S2).…”

Synthesis of α,β‐ and β‐Unsaturated Acids and Hydroxy Acids by Tandem Oxidation, Epoxidation, and Hydrolysis/Hydrogenation of Bioethanol Derivatives

“…To selectively convert the C 2 aldehydes into the corresponding C 4 coupling products (Scheme 1 b) we have developed a number of catalytic systems based on basic K/Na-X and K/Na-Y, [11] and MgO-K/NaX, [12] zeolites as well as Bdoped MgO oxides. [13] The Mg-Al Hydrotalcite, Al-Beta zeolite, which along with the Mg-B catalyst previously employed have shown the highest C 4 productivities among all the catalysts investigated, with productivities of 8.3, 23.4, and 24.2 mmol of C 4 g cat À1 h À1 , respectively, at 50-60 % conversion level and 180 8C (Table S2). Notably, on the 7.5 wt.…”

“…We employed 13 C-NMR for product identification and HPLC for quantification. The results showed that before reaction only the crotonic acid can be detected by 13 C-NMR ( Figure S2). After 3 h of reaction at pH 2.9 additional chemical shifts appeared in the 13 C-NMR spectra that corresponded to the formation of the epoxide (3methyloxirane-2-carboxylic acid) and a,b-dihydroxy acid (2,3-dihydroxybutanoic acid).…”