Abstract
Aldol condensation reaction is usually catalysed using homogeneous catalysts. However, the heterogeneous catalysis offers interesting advantages and the possibility of cleaner biofuels production. Nowadays, one of the most used kinds of heterogeneous catalysts are hydrotalcites, which belong to a group of layered double hydroxides. This paper describes the aldol condensation of cyclohexanone (CH) and furfural (F) using Mg/Al mixed oxides and rehydrated mixed oxides in order to compare the catalyst activity after calcination and rehydration, as well as the possibility of its regeneration. The catalysts were synthesized by calcination and subsequent rehydration of the laboratory-prepared and commercial hydrotalcites, with Mg:Al molar ratio of 3:1. Their structural and chemical properties were determined by several analytical methods (inductively coupled plasma analysis (ICP), X-ray diffraction (XRD), diffuse reflectance infrared Fourier transform spectroscopy (DRIFT), specific surface area (BET), thermogravimetric analysis (TGA), temperature programmed desorption (TPD)). F-CH aldol condensation was performed in a continuous fixed-bed reactor at 80 °C, CH:F = 5:1, WHSV 2 h−1. The rehydrated laboratory-prepared catalysts showed a 100% furfural conversion for more than 55 h, in contrast to the calcined ones (only 24 h). The yield of condensation products FCH and F2CH was up to 68% and 10%, respectively. Obtained results suggest that Mg/Al mixed oxides-based heterogeneous catalyst is suitable for use in the aldol condensation reaction of furfural and cyclohexanone in a fixed-bed reactor, which is an interesting alternative way to obtain biofuels from renewable sources.
Highlights
Because of the increasing environmental awareness and dwindling supplies of fossil resources as a traditional feedstock, new alternative methods to obtain chemical compounds and fuels are recently given significant research attention [1]
Our results suggest that investigated Mg–Al hydrotalcite precursor (Mg)/Al mixed oxides-based heterogeneous catalysts are suitable for the aldol condensation reaction, considering their relatively low costs, high activity and stability at high temperatures as well as in aqueous solution
Diffuse reflectance infrared Fourier transform spectroscopy (DRIFT) analysis of the hydrotalcite samples shows the differences between laboratory prepared and commercial hydrotalcites before and after rehydration and calcination (Figure 1a)
Summary
Because of the increasing environmental awareness and dwindling supplies of fossil resources as a traditional feedstock, new alternative methods to obtain chemical compounds and fuels are recently given significant research attention [1]. The substitution of fossil fuels from renewable resources has been proposed in the European Union as a part of the strategy to eliminate greenhouse gas emissions from the transport. The main goal of these efforts is to lower environmental impacts of the traditional methods such as CO2 production, air pollution, etc., and to develop sustainable methods of maximum utilization of the alternative biomass resources. One of the alternative ways to obtain fuels from renewable resources is cyclohexanone condensation reaction. This reaction of organic synthesis is known as a tool for producing complex molecules.
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