Study On Glycerol Dehydration To Acrolein And Aldol Condensation Of Acetic Acid(Methyl Acetate)-formaldehyde To Acrylic Acid Over The Vanadium Phosphorus Oxides

Many chemicals originated from petroleum industry have become the essentials in our daily life.Petroleum,as the non-renewable resource,its consumption is continuously increased in recent years.It is a common interest to find out new raw materials for chemicals production in view of sustainable development of economy.As a result,there is a strong trend to manufactory chemicals by using biomass-derived resource.Acrylic acid is currently produced through two step oxidation of propylene,however,the process is complicated and different catalysts need to be used.One step partial oxidation of propane to acrylic acid is highly desirable and has received numerous attentions in recent years,and the researches are focused on the VPO,MMO,and HPA types of catalysts.For the MMO type of catalyst,catalyst nature is very sensitive to preparation parameters and the "window" of synthesizing active catalyst is rather narrow.Thus the reproducibility of catalyst structure/performance is poor.The route for acrylic acid production via bio-mass deriveable glycerol and methyl acetate as byproducts in industry are thus of great importance and have attracted extensive attentions.(1)In the present study,we synthesized different VPO catalysts by changing the preparation variables,particularly the introduction of PEG additive in preparation media,to modify catalyst nature and to figure out the key factors that affect catalyst performance for glycerol dehydration.Based on the results of characterization and activity evaluation,the following observations were made:(a)In fabrication of VPO catalyst,the mixed isobutanol and benzyl alcohol showed better outcome than the single benzyl alcohol.Introduction of PEG additive to preparation media can significantly enhance the as-synthesized VPO catalyst performance.Acrolein yield of 70.1 mol%can be obtained over VPOPEG-a under the following conditions:aqueous glycerol solution feed(36.5 wt%)with LHSV=4 h-1 and reaction temperature of 320?.The VPOPEG-a catalyst can handle heavy loading of reaction feed,for instance,a concentrated glycerol solution(50.0 wt%)or a notably higher LHSV of 12 h-1,and still retain reasonable acrolein yields(48-66 mol%),giving significantly high acrolein formation rates(16.4-35.3 mmol gcat-1h-1).(b)Within a time on stream of 50 h at 300?,glycerol conversion declined slowly.Acrolein yield decreased steadily in the first 10 h,and kept more stable after 20 h reaction,maintaining the value of approximately 40%.(c)The type of alcohol and addition of PEG in preparation media showed notable impact on the crystallinity and morphology of both precursors and activated VPOs,the strength of V-O bonding,and the surface V5/V4+ ratio.Raman spectroscopic study indicates the presence of V5+specimen and the change of its content during the reaction course.XPS study revealed significant decrease in the V5+/V4+ratio and the surface carbonaceous deposits after reaction,which may account for the observed catalyst deactivation.The balanced surface V5+/V4+ratio and the medium strong acid sites of high density in the VPOPEG-a catalyst are found to be critical to accomplish superior activity.(2)In this study,we used PEG-derived VPO type catalysts for aldol-condensation of acetic acid(methyl acetate)and formaldehyde to produce acrylic acid(AA)and methyl acrylate(MA).Based on the results of characterization and activity evaluation,the following conclusions were obtained:(a)A VPO catalyst,prepared using single benzyl alcohol and PEG additive and activated in an atmosphere with low concentration of butane in air(0.3 vol%),mainly comprised ?-VOPO4 and(VO)2P2O7 constitutes and showed superior performance.The highest formation rate(15.0?mol gcat-1min-1)was achieved when acetic acid was used as feedstock and N2 as carrier.When methyl acetate was use as feedstock and N2-air mixture(1:1,v/v)was used as carrier,the highest rate(18.2?mol gcat-1 min-1)was obtained at 360?.When certain amount of methanol was added into the feedstock,the formation rate of(AA+MA)can reach 19.5?mol gcat-1 min-1 at 360?.(b)Within a time on stream of 70 h at 360?,methyl acetate conversion essentially maintained while(AA+MA)formation rate slightly declined in the initial stage and retained afterwards.In case acetic acid and HCHO were used as feedstock,the initial activity dropped more quickly within 24 h.(c)The systematic XRD,Raman,H2-TPR,and XPS characterizations clearly revealed the reactivity order of various VPO species and the desired structural/redox feature of an efficient catalyst surface demand for the target reaction.The catalyst activation was found to critically determine catalyst structure and composition,especially the vanadium oxidation states and their relative contents.The catalyst performance as well as stability was significantly affected by the ratio of VOPO4 to(VO)2P2O7 in catalyst constitution,the composition of feedstock(methyl acetate vs.acetic acid,and methanol addition),and the oxygen concentration in carrier.The highest(AA+MAA)formation rate can be accomplished under the optimized conditions.These findings are informative for better understanding the process chemistry and helpful for further catalyst development.