Abstract
Hydrotalcites (HTCs) are promising solid base catalysts to produce advanced biofuels by aldol condensation. Their main potential lies in the tunability of their acid-base properties by varying their composition. However, the relationship between the composition of hydrotalcites, their basicity, and their catalytic performance has not yet been fully revealed. Here, we investigate systematically the preparation of HTCs with the general formula of Mg6M3+2(OH)16CO3·4H2O, where M3+ stands for Al, Ga, Fe, and In, while keeping the Mg/M3+ equal to 3. We use an array of analytical methods including XRD, N2 physisorption, CO2-TPD, TGA-MS, FTIR-ATR, and SEM to assess changes in the properties and concluded that the nature of M3+ affected the HTC crystallinity. We show that the basicity of the HTC-derived mixed oxides decreased with the increase in atomic weight of M3+, which was reflected by decreased furfural conversion in its aldol condensation with acetone. We demonstrate that all MgM3+ mixed oxides can be fully rehydrated, which boosted their activity in aldol condensation. Taking all characterization results together, we conclude that the catalytic performance of the rehydrated HTCs is determined by the “host” MgO component, rather than the nature of M3+.
Highlights
The Green Chemistry principles aroused a strong interest in the development of solid base catalysts for important base-catalyzed reaction, such as transesterification, aldol condensation, or alkylation [1,2,3,4,5,6,7,8,9]
The ICP and XRD results confirmed that the coprecipitation of magnesium nitrate and the corresponding M3+ nitrate (M3+ being Al, Ga, In, and Fe, respectively) by the alkaline solution of Na2CO3 and NaOH resulted in the formation of the respective hydrotalcite with the targeted Mg/M3+ ratio of 3
The CO2-Temperature programmed desorption (TPD) study showed that the concentration of the basic sites in MgM3+ mixed oxides differed, and it gradually decreased with the growth in the atomic weight of the M3+ element
Summary
The Green Chemistry principles aroused a strong interest in the development of solid base catalysts for important base-catalyzed reaction, such as transesterification, aldol condensation, or alkylation [1,2,3,4,5,6,7,8,9]. Aldol condensation is currently used to synthesize fine chemicals [5,23,24], and it holds a promising potential for development of sustainable aviation fuels [25,26,27,28] and bio-based monomers [29,30] It is a suitable reaction for probing the acid-base character of catalysts as only the accessible, i.e., catalytically active basic sites can be reached by the reactants in both mixed oxides and reconstructed hydrotalcites. We have synthesized a series of MgM3+ hydrotalcites with M3+ being Al, Ga, Fe, and In using the same synthesis procedure and evaluated the derived mixed oxides and reconstructed hydrotalcites with respect to their physico-chemical properties and catalytic performance in aldol condensation of furfural and acetone This allowed us to elucidate the role of the trivalent cation on the structure, properties, and catalytic performance (including its stability) of the synthesized materials
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