Study On The Catalytic Dehydration Of Glycerol To Acrolein

The advancing development of biodiesel production has provided a surplus supply ofglycerol with reasonably lower price. Efficient utilization of excess glycerol can not onlyimprove economic benefit of biodiesel plants, but also provide a green production route forchemicals. Glycerol is one suitable feedstock for the production of several high valuechemicals through catalytic conversions. One of the most promising ways of glycerolapplication is double-dehydration to acrolein. Although very efficient catalysts for thereaction can be prepared, unfortunately, most of these catalysts suffer from fast deactivationphenomena due to extensive coking, especially supported heteropolyacids. And the effects ofcatalyst acidity and structure properties on the dehydration of glycerol are uncertain up to now.Systematic studies have been conducted over microporous H-zeolites and mesoporoussupported metal sulfate catalysts using activity measurement, XRD, SEM, BET, FT-IR,TG-DTA-MS, NH3-TPD, H2-TPR-MS and XPS in this dissertation.Acidity is one determinant factor of glycerol dehydration, and lower acid strength andmoderate acid density are conducive to efficient conversion of glycerol to acrolein overH-zeolites. The acid density of HZSM-5declined obviously after hydrothermal treatment,while the selectivity to acrolein increased significantly. However, serious destruction ofzeolite crystalline structure was found during hydrothermal treatment. Phosphorusmodification could improve hydrothermal stability of H-zeolites. The combination ofphosphorus modification and hydrothermal treatment could lower strong acid amount andacid density of H-zeolites, reducing secondary reactions, thus improve acrolein selectivity.Diffusion limitation was the other determinant factor for H-zeolites besides acidity.HZSM-5(11), with channel diameter larger than the diameter of glycerol, showed superiorcatalytic performance in comparison with HZSM-34(channel diameter=0.36nm). Thecatalytic performance of different H-zeolite catalysts implied that low channel complexity, large channel diameter and small particle size were essential factors for the production ofacrolein over H-zeolites, indicating that lower diffusion resistance was favorable for acroleinproduction from glycerol dehydration reaction.Excellent catalytic performance was discovered over metal sulfate catalysts supported onmesoporous silica, the diffusion resistance of which was significantly lower than that ofH-zeolites. Large quantities of acid sites with medium and weak strength were found oversupported metal sulfate catalysts. At a GHSV of glycerol of873h-1, glycerol conversion wasalways higher than90%at45h on stream, and selectivity to acrolein remained at ca.70mol%over supported zinc sulfate catalyst.Bronsted and Lewis acid sites played different roles in glycerol dehydration reaction, itcould be concluded that Br nsted acid sites were active sites for acrolein production fromglycerol dehydration, and that Lewis acid sites should be responsible for the formation ofacetol. Supported nickel sulfate was one catalyst containing both Bronsted and Lewis acidsites. Cesium carbonate was able to modify Bronsted acidity of nickel sulfate. The catalyticactivity and acrolein selectivity decreased significantly, while selectivity to acetol increasedafter cesium modification.In a word, nano HZSM-11, with smaller diffusion resistance, was favorable for thediffusion of glycerol into and product molecules out of channels of H-zeolite, leading tosuperior catalytic performance for glycerol dehydration. Moreover, nano HZSM-11wassuitable for continuous dehydration of glycerol through reaction-regeneration cycle, whichmade it a promising catalyst in future. Although excellent catalytic performance was obtainedover supported zinc sulfate catalyst, its regenerability remained to be further improved.