Microwave-assisted catalytic dehydration of glycerol for sustainable production of acrolein over a microwave absorbing catalyst
Graphical abstract
Introduction
Recently, increasing studies have been conducted on renewable energy, as a solution to current energy and environmental issues caused by traditional fossil fuels use. Biodiesel has attracted much interest as an alternative energy to non-renewable energy sources [1,2] since it is environmental friendly, technically feasible, and biodegradable [3]. Glycerol is the major by-product of biodiesel synthesis, which accounts for approximately 10 wt% of the total transesterification process production [4,5]. The tremendous increase in biodiesel production leads to a glut of glycerol in the market. The applications of glycerol to value-added chemicals have gained much attention, not only due to the surplus of glycerol available, but also because glycerol is edible, bio-sustainable, and non-toxic [6,7].
Glycerol has a wide variety of applications through different reaction pathways, due to its multi-functional structure and physico-chemical characteristics [[8], [9], [10]]. As one of the most important value-added chemicals that can be produced from glycerol, acrolein is a significant and flexible intermediate for the production of super-absorber polymers, detergents, textiles, and acrylic acid esters [11]. In addition, acrolein is utilized as the feedstock to produce DL-Methionine [12] and 1,3-propanediol [13] which are widely used in meat and material manufacturing, respectively.
Gas-phase catalytic dehydration of glycerol is widely considered as an economical and environmentally friendly approach to substitute conventional petrochemical process for acrolein production. Various types of catalysts were reported in previous studies on glycerol dehydration to acrolein, including heteropoly acids [14], zeolites [15], mixed metal oxides [16,17], phosphates [18], and pyrophosphates [19]. Particularly, metal oxide catalysts such as aluminum oxide (Al2O3) [20], niobium oxide (Nb2O5) [21], and tungsten oxide (WO3) [22] have been widely used in acrolein production from glycerol and 60%–80% acrolein selectivity at almost complete glycerol conversion could be achieved. However, the main obstacle for industrial application of these catalysts is rapid deactivation due to coke formation on the catalyst surface [6,23]. The uneven temperature distribution within catalyst particles caused by conventional electric heating method is one of the most important reasons for the coke formation.
Microwave irradiation is an alternative heating method and has been successfully applied to many fields such as drying, food processing, and biomass pyrolysis and gasification [[24], [25], [26]]. Different from conventional heating processes where heat is transferred from the surface to the core of the material through conduction driven by temperature gradients, microwaves induce heat at the molecular level by direct conversion of electromagnetic energy into heat [27], and hence they can provide uniform internal heating for material particles. Therefore, microwave heating is expected to reduce the coke formation and improve the catalyst stability. In addition, the new heating method offers many other advantages over traditional processes, including simple set-up, rapid and convenient start-up and shut-down, and low cost [28]. However, no studies on glycerol dehydration to acrolein by microwave heating were reported.
A microwave heating and reaction system was previously developed by our group [29]. Using this system, microwave-assisted dehydration of glycerol for acrolein production over a novel coated microwave absorbing catalyst was conducted in this study. The effects of temperature, support coating to microwave absorbent ratio, and weight hourly space velocity (WHSV) on glycerol conversion and acrolein selectivity were investigated. The catalyst stability in microwave-heating and electric-heating processes was examined and compared. Moreover, microwave-assisted in-situ regeneration of deactivated catalyst was performed.
Section snippets
Preparation of coated microwave absorbing catalyst
In this study, a coated microwave absorbing catalyst was used (some preliminary data on the superiority of coated catalyst over physically mixed catalyst are shown in Table S1 and Fig. S1). SiC with particle size of 500 nm was used as the microwave absorbent, whose temperature was increased very quickly when it absorbed the microwaves. A certain amount of SiC was dispersed in deionized water with liquid/solid ratio of 30 mL/g. Small amount of Na2SiO3 as dispersant was added to the SiC solution
Characterization of the catalysts
The specific surface area of the microwave absorbent SiC was only 13 m2/g, making it unsuitable to be catalyst support. For the coated catalyst support ZrO2@SiC prepared in this study, the ZrO2 coating with certain pore channel and structure obviously increased the specific surface area (55–65 m2/g), which could improve the loading of active component WO3.
The powder XRD patterns of the WO3/ZrO2@SiC catalysts with various ZrO2/SiC weight ratios are shown in Fig. S2. The characteristic peaks of
Conclusions
In this study, microwave-assisted catalytic dehydration of glycerol for sustainable acrolein production over a coated microwave absorbing catalyst WO3/ZrO2@SiC was investigated. Microwave heating provided more even temperature distribution within the catalyst bed than conventional electric heating, resulting in higher acrolein yield at lower temperature and better catalyst stability. The technique of microwave heating coupled with microwave absorbing catalyst provides a novel and effective
Acknowledgements
The authors would like to express their great appreciation to the National Natural Science Foundation of China (21808203) and the Natural Science Foundation of Zhejiang Province (LQ17B060003) for their financial support for this work.
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