Study On Catalysts For Dehydration Of Glycerol To Acrolein

With the increase of biodiesel production,more and more attention has been paid to the effective utilization of glycerol which is a major by-product in the biodiesel synthesis.Among the various conversion routes of glycerol,dehydration of glycerol to acrolein is one of the most promising routes.However,the effect of acidity and structure of catalysts on the reaction performance and the reaction mechanism are still not clear.Furthermore,the reported solid acid catalysts also had some disadvantages.The present thesis explored relative properties of catalysts for the dehydration of glycerol to acrolein.In addition,the acidity and structure of phosphate catalysts were also adjusted to improve their catalytic performance.Meanwhile,the reaction mechanism was further studied.First of all,the reaction performance of metal oxides on the dehydration of glycerol was investigated,it was demonstrated that acidic site and basic site could catalyze the dehydration of glycerol to acrolein by different reaction mechanisms.The reaction on acidic sites is in accordance with the normal carbon ions mechanism,compared with Lewis acid sites,Br?nsted acid sites are more conducive to the formation of acrolein because of the space limitations of catalysts;The reaction on basic sites are in accordance with the negative carbon ion mechanism,basic sites are more conducive to the formation of acetol.Basic sites can catalyze both the dehydration of glycerol and the dehydrogenation of glycerol.The loading amount of niobium phosphate and the pH of preparation solution could adjust the surface coverage and the dispersion of active phase,leading to the increase of acid site amount and specific surface area,changing the parameters of pores,and lowering the acid strength.The presence of phosphate ion could boost the transformation of four-coordinate NbO4 tetrahedra associated with Lewis acid sites to six-coordinate NbO6 octahedra associated with Br?nsted acid sites,which could lead to the improvement of catalytic performance.The catalyst at the loading amount of 20 wt% and the pH of 3 exhibited a high acrolein selectivity up to 80% at a nearly 100% glycerol conversion.The selectivity and the oxidation temperature of coke decreased at the air atmosphere in the dehydration of glycerol,and meanwhile the selectivity to acrolein was also reduced to some extent.Supported zinc phosphate could still keep high glycerol conversion and acrolein selectivity at the air atmosphere.Supported zinc phosphate at the P/Zn ratio of 2 and the CTAB addition amount of 1 wt % showed the best catalytic performance in present thesis.The weak acid strength and the big pore structure could decrease the secondary reaction and diffusion limitation which prevent coke formation.The key to improve the regeneration of the catalysts is to enhance their acid stability.It was interesting to note that the acrolein selectivity was related to the Br?nsted acid density of catalysts.It was proposed that the terminal and internal OH-groups of glycerol molecular were more likely to be protonated simultaneously when glycerol catalyzed by the catalyst with high Br?nsted acid density.Thus,the one-step dehydration of glycerol to acrolein inhibited the formation of acetol and the side-reaction based on 3-HPA,which in turn resulted in a high selectivity to acrolein.