Dehydration Of Methyl Lactate And Lactic Acid To Methyl Acrylate And/or Acrylic Acid: A Joint Experimental And Theoretical Study

Petroleum,a finite,non-renewable resource,is facing the crisis of depletion.Under the pressure of continuously rising oil price,the chemical industry is trying to find new resources and to develop new processes.Therefore,in recent years close attentions have been paid to making chemicals from biomasses.The production of lactic acid is a mature large-scale industrial fermentation process with high productivity and low cost.The capacity of lactic acid has reached 120 000 tons per yeas in the world.Developing the downstream products of lactic acid is important for the production of chemicals using biomass.Dehydration of lactic acid or esters of lactic acid to acrylic acid or esters of acrylic acid is one potential route for the utilization of lactic acid.Due to the fact that acrylic acid is an important chemical,researches concerning the production of acrylic acid from lactic acid are now very active.At present,the selectivity and yield of lactic acid and lactic acid ester dehydration to acrylic acid and/or acrylates are not high.The research concerning the dehydration of lactic acid and lactic acid esters to acrylates is still in the stage of the laboratory catalyst screening and process optimization.Deep studies regarding the relationship between catalyst performance and catalyst structures are scare.Knowledge about the reaction mechanism of lactic acid dehydration and the origins of the catalytic activity are still lack.The aims of this thesis are to develop high-performance dehydration catalysts and to investigate the relationship between the catalyst structures and their performance as well as the dehydration mechanism of lactic acid and its esters on these catalysts.The extensive screening of dehydration catalyst has been carried out using methyl lactate as a feedstock in a fixed-bed continuous flow reactor. The NaH₂PO₄ supported on silica prepared by the wet impregnation method was found to have a significantly higher catalytic dehydration activity than the other catalysts.In this thesis,studies focus on the phosphoric acid and sodium phosphate catalysts supported on silica.It was found that upon calcination H₃PO₄ and alkali phosphates underwent condensation reactions,forming a variety of condensed products.The differences of catalytic activity and structure among sodium phosphates can be traced back to the differences in Na₂O/P₂O₅ ratios in these compounds.To investigate such influences,catalysts with different ratios of Na₂O/P₂O₅ from 0.77 to 2.0 were prepared.When Na₂O/P₂O₅ ratio is equal to 1.0, sodium polyphosphate chains bearing terminal POH groups are formed from sodium phosphate after calcination.The catalyst gives the highest dehydration selectivity.When Na₂O/P₂O₅ ratio is less than 1.0,POH groups with relatively stronger acidity develop on the catalyst,which promote the decarbonylation and result in a decrease in the selectivity to the acrylates.When Na₂O/P₂O₅ ratio is higher than 1.0,the average length of polyphosphate chains and the density of terminal POH groups as well as the acidity of these POH groups are decreased,which is unfavorable to the dehydration of methyl lactate.For NaH₂PO₄/SiO₂ catalyst,which shows the best selectivity for the dehydration,the influence of catalyst preparation conditions including calcinations temperature,the loading amount of active component, support and reaction conditions including temperature,contact time and feed composition on the methyl lactate dehydration reaction was studied. In addition,the lifetime and regeneration of the catalyst was also investigated.In view of the excellent performance of the NaH₂PO₄/SiO₂ catalyst, the catalytic conversion of methyl lactate on three silica supported mono-alkali phosphates with different alkali cations,RH₂PO₄/SiO₂,(R= Li,Na,K),was investigated.On these mono-alkali phosphates,the greater the electronegativity of alkali element is,the stronger the acidity of catalyst is.On LiH₂PO₄/SiO₂,which POH groups have the strongest acidity,decarbonylation reaction to acetaldehyde is significantly higher; On NaH₂PO₄/SiO₂,which POH groups have the moderate acidity,the selectivity to acrylic acid is the highest;On KH₂PO₄/SiO₂,which POH groups have the weakest acidity,the selectivity to methyl acrylate is the highest.In addition,dehydration of lactic acid to acrylic acid using sodium phosphates with various molar ratio of Na₂O/P₂O₅ was also investigated. It was found that for the dehydration of lactic acid to acrylic acid it is also necessary for the catalyst to have an appropriate acid strength and amount of acid sites.Quantum chemical calculations indicate that the dehydration of lactic acid on the supported sodium polyphosphate is via a concerted reaction mechanism.For the decarbonylation of lactic acid on this catalyst,both a stepwise and a concerted reaction mechanism are equally important.For methyl lactate,dehydration to acrylic acid is via a stepwise reaction mechanism.While conversion of methyl lactate to acetaldehyde is via a stepwise and a concerted reaction mechanism.The formation of methyl acrylate from methyl lactate on the supported sodium polyphosphate is via the esterification of acrylic acid with methanol. Combination of experiment characterization of the catalysts with density function theory simulations was used to elucidate the products and mechanism of the reactions among silica,H₃PO₄ and NaH₂PO₄ during the preparation of silica supported H₃PO₄ and NaH₂PO₄ catalysts. The spectral test results indicate that besides polyphosphoric acid,silicon phosphates on silica supported H₃PO₄ are also formed.On silica supported NaH₂PO₄ only sodium polyphosphates are present.Density functional theory simulations indicate that in the initial stage,reaction of H₃PO₄ with silanol groups on the silica support is more favorable than that between H₃PO₄ itself.In contrast,dimerization and trimerization of NaH₂PO₄ are predicated to be the predominant initial reactions for the silica supported NaH₂PO₄ catalyst.