| Recently,with gradual depletion of fossil resources,the environmental pollution,CO2 emissions,and the demand for renewable resources is more and more urgent.The study aims to synthesis acrylic acid via a dehydration of bio-based feedstock.Acrylic acid is a key platform molecular in polyacrylic acid and various acrylic esters production through polymerization and co-polymerization.At present,AA production technology is based on a two-step catalytic oxidation of fossil-based propylene via acrolein as intermediate.Acrolein,however,is highly toxic and explosive which drawn environmental concern of the typical AA production technology.Among these resources,the preparation of acrylic acid by the dehydration of bio-based renewable resources has attracted growing attention.Acrylic acid could be synthesized by the lactic acid,methyl lactate,and 3-hydroxypropionic acid.Different catalyst systems with different properties has been developed and investigated in details for the dehydration of the lactic acid,methyl lactate,and 3-hydroxypropionic acid to acrylic acid by different authors.However,the common conclusion has not been obtained for the efficient catalysts used in the three reactions.Therefore,the study aims to investigate the common properties of the catalysts used in the three reactions and develop the high efficient catalysts,which provide the guidance for the industrial production of acrylic acid.The study focuses on the dehydration of lactic acid and others to synthesis acrylic acid.Firstly,the dehydration of methyl lactate to acrylic acid was investigated firstly.The MCM-41 catalysts modified by K2HPO4and A12(SO4)3 were developed.To further improve the raw material utilization,the lactic acid as feedstock was used and carried out.Effect of ZSM-5,Beta,and HAP on catalytic activity was investigated.Finally,the dehydration of 3-hydroxypropionic acid to acrylic acid was investigated,which has a high acrylic acid selectivity and simplify separation process.All of the mentional catalysts were characterized in detail.Effect of catalyst property,especially acidity,on catalytic activity was studied,thus reveling the common property of the cossponding catalysts for the dehydration of bio-based feedstocks to acrylic acid.The dehydration of methyl lactate to acrylic acid was investigated in this study.MCM-41 catalysts modified by K2HPO4and A12(SO4)3 were developed firstly.By changing the loading amount of the two salts and their ratio,the catalytic performance was optimized and the highest AA yield of 72%was achieved over the 45 wt%K2HPO4 and A12(SO4)3(6:4)/MCM-41 catalyst.Combing with the characterizations,it was found that the use of K2HPO4 and A12(SO4)3will improve the adjustment ability on acid property and cooperation effect between K2HPO4 and A12(SO4)3 could be observed.For the multi-composition catalysts,carbon deposition was the main reason for catalyst deactivation.The catalyst could be fully regenerated by air treatment.The best catalyst,showing the highest acrylic acid,has weak acid intensity(peak at 170℃ in NH3-TPD)and small amount of weak acid sites(90 μmol g-1)which are mainly L acid sites.Using the methyl lactate as feedstock to prepare the acrylic acid,methanol was producted which results in the loss of carbon resource.For the aim to improve the raw material utilization,using lactic acid as feedstock to synthesis aclylic acid was investigated.As model catalysts,ZSM-5 and Beta were treated via desilication and dealuminum and impregnation with Na+ and K+to adjust their acid property.Effect of acid property on the catalytic activity was studied.The result indicated that the catalyst with the highest acrylic acid selectivity has weak acid intensity(peak at 180℃ in NH3-TPD)and small amount of weak acid sites(less than 25 μmol g-1)which are L acid sites.Based on the conclusion,the HAP-Ca with weak acid intensity and small amount of acid sites was investigated.The results show that the HAP-Ca catalyst could selectively convert lactic acid to lactic acid.What’s more,the catalysts with smaller size were synthesized and provided a better catalytic performance.The catalysts with smaller size have decreased corbon deposition and has a high acrylic acid of 80%for 100 h time on stream.Meanwhile,the catalyst properties at different reaction time were characterized.The result shows that the deactivation of the catalysts is related to the deposition of aliphatic and aromatic carbon on the surface of the catalysts.With time increasing,the aromatic carbon increases.At the same time,the catalysts have peaks at 175℃ and acid sites of<3.2 μmol g-1,which contribute to the selective conversion of lactic acid to acrylic acid.To further improve acrylic acid yield and simplify the separation process,the 3-hydroxypropionic acid as feedstock was investigated.3-hydroxypropionic acid is isomer of lactic acid and could be dehydrated to form acrylic acid,which has obtained increased attention.ZSM-5,HY,Beta,MCM-41 and silica gel were investigated in the dehydration of 3-hydroxypropionic acid to acrylic acid.It was found that the silica gel gives the best catalytic activity with a 3-hydroxypropionic acid conversion of 100%and an acrylic acid selectivity of 99.0%.At the same time,no deactivation was observed for 200 h time on stream.No loss of the catalytic activity was observed after catalyst generation.Combined with characterizations,effect of acid property of these catalysts on catalytic activity was studied over the ZSM-5,HY,Beta,MCM-41 and silica gel catalysts.It was found that the catalyst with high acrylic acid selectivity has low acid intensity with NH3 peak at 180℃ in NH3-TPD patterns and low amount of L acid sites of 47.7μmol g-1.Besides,it was observed that the L acid sites is beneficial to the high selectivity of lactic acid,while the B acid sites would lead to the formation of by-products.The cause of catalyst deactivation is related to the carbon and the amount of carbon is correlated to the amount of acid sites.All of the lactic acid,methyl lactate,and 3-hydroxypropionic acid have a common hydroxyl functional groups.The acid property plays a crucial role in the dehydration of alcohols.Therefore,adjustment of acid property is important in this study.In this study,different catalysts were developed and investigated for the dehydration of these feedstocks to acrylic acid,revealing the important effect on catalytic activity and stability.The results indicate that the catalysts with high acrylic acid selectivity have common acid property:low intensity with NH3 peak at 170-180℃,L acid sites,small amount of acid sites with lower than 90μmol g-1.The catalysts with such acid property have a high acrylic acid selectivity and/or a long-time life. |
