Experimental And Theoretical Study On The Preparation Of Acrylate By Aldol Condensation

Methyl acrylate(MA)and methyl methacrylate(MMA)are widely used to produce synthetic fibers,paintings,polymer materials and plastic processing additives.The present main production methods for MA and MMA have some disadvantages,such as complicated process,serious pollution and heavy dependence on petroleum.Methyl acetate(Ma)is the main byproduct of terephthalic acid and polyvinyl alcohol production,and formaldehyde(FA)is a cheap industrial raw material.Ma and FA can be used as the feedstock to prepare MA and MMA via aldol condensation reaction.This route not only increases the utilization value of Ma and FA but also decreases the dependence on petroleum.Aldol condensation of Ma and FA to MA is catalyzed by V-P complex oxides or supported alkali metal oxides,for instance Cs oxide supported on silica or SBA-15.The latter attracts more attention since it has the advantages as compared to the former,including simple preparation method,easily adjustable structure and properties and better catalytic performance.Concerning the supported alkali metal oxides,there remain some problems that needed to be further addressed,such as,the interaction of active component with the support that leading to the formation of the active sites on the catalysts,the relationship between the catalytic performance and the acid-base property including the strength and amount of the active sites,the influence of the catalyst's pore texture on its performance,the synergetic effect of the acid-base sites and the reaction mechanism of the aldol condensation of Ma and FA on the catalysts to from MA.Aiming at the above problems,the following work has been performed in this thesis:(1)SBA-15 molecular sieves supported alkali metal(Li,Na,K,Rb and Cs)oxide catalysts(M/SBA-15)were prepared by impregnation method.Catalytic tests in a fixed-bed continuous flow reactor indicated that the support of SBA-15 is inert for this reaction.The yield of MA over M/SBA-15 increases with the increase of the basicity of alkali metal.Under the reaction conditions:T=663 K,LHSV=0.9 h-1,Ma:FA:MeOH=1:2:2,the highest yield of MA obtained on Cs/SBA-15 catalyst is 30.9%.During the reaction,Cs/SBA-15 continuously deactivated with increasing time on stream.The deactivated Cs/SBA-15 catalyst could recover its activity by burning off the deposited carbon,indicating that carbon deposition is the main reason for the deactivation of the catalyst.(2)When the alkali metal loading was 0.37 mmol/g,alkali metal was loaded on SBA-15 by replacing the H atom of surface silanols on SBA-15.There existed weak base and medium base on M/SBA-15,and the density of medium base increased with the increase of the basicity of alkali metal.The initial conversion of Ma increases with the increase of the density of medium base,indicating that medium base plays a decisive role in the aldol condensation reaction.(3)Three kinds of silica gel A-type silica gel(A-silica),B-type silica gel(B-silica),and C-type silica gel(C-silica)with different pore structure were used as the support of Cs oxide to prepare Cs/silica catalysts and their catalytic performance was tested.The catalytic performance of Cs/B-silica was the highest,which was attributed to its larger specific surface area and pore volume.The larger specific surface area favors the contact of the reactants with more active sites and the larger pore volume increases the carbon capacity of the catalyst.Under the same Cs loading and evaluation conditions,the catalytic activity and the stability of Cs/B-silica for vapor phase aldol condensation of Ma with FA is better than Cs/SBA-15.(4)About 50%of the pore volume of A-silica comes from its micropores.There is no micropore found on B-silica and C-silica.Upon the loading of Cs and subsequent calcination,the micropores of Cs/A-silica disappear.The Cs depositing in the micropores is enveloped in the support matrix during the calcination.With the same loading of the Cs,density of base site on Cs/A-silica was significantly lower than that on Cs/B-silica and Cs/C-silica while the TOF value of Cs/A-silica was close to that of Cs/B-silica and Cs/C-silica,indicating that the low catalytic activity observed for Cs/A-silica was mainly resulted from the low density of base site on Cs/A-silica.(5)The study of Cs/B-silica with different Cs loadings indicated that the catalytic performance of Cs/B-silica was related to the Cs loading.The best catalytic performance was obtained at the loading of 0.37 mmol/g.The study of CsNO3 decomposition at the calcining process indicated that CsN03 could uniformly dispersed on the support surface when the Cs loading was low and reacted with the surface silanols,forming Si-O-Cs active sites.When the Cs loading was high,part of the CsNO3 would remain on the surface since they could not contact with the surface silanols and possessed a self-decomposition temperature of near 983 K which was remarkably higher than that of the calcination temperature used in preparation of the catalysts.The remained CsN03 resulted in the deterioration of the catalytic performance.(6)Evaluation results of the catalytic performance of Cs/B-silica on the MP-FA aldol condensation reaction indicated that under the conditions:T=648 K,Ma(MP):FA:MeOH=1:2:2 and the same molar flow,the initial yield of MMA(46.1%)is obviously higher than that of MA(34.4%).(7)By means of quantum chemistry calculations,the active sites on Cs/B-silica catalyst were determined and the reaction mechanism was studied.The surface structure was represented by a Si12 cluster,which was made up of Cs/A-silica was significantly lower than that on Cs/B-silica and Cs/C-silica while the TOF value of Cs/A-silica was close to that of Cs/B-silica and Cs/C-silica,indicating that the low catalytic activity observed for Cs/A-silica was mainly resulted from the low density of base site on Cs/A-silica.(5)The study of Cs/B-silica with different Cs loadings indicated that the catalytic performance of Cs/B-silica was related to the Cs loading.The best catalytic performance was obtained at the loading of 0.37 mmol/g.The study of CsN03 decomposition at the calcining process indicated that CsN03 could uniformly dispersed on the support surface when the Cs loading was low and reacted with the surface silanols,forming Si-O-Cs active sites.When the Cs loading was high,part of the CsNO3 would remain on the surface since they could not contact with the surface silanols and possessed a self-decomposition temperature of near 983 K which was remarkably higher than that of the calcination temperature used in preparation of the catalysts.The remained CsN03 resulted in the deterioration of the catalytic performance.(6)Evaluation results of the catalytic performance of Cs/B-silica on the MP-FA aldol condensation reaction indicated that under the conditions:T=648 K,Ma(MP):FA:MeOH=1:2:2 and the same molar flow,the initial yield of MMA(46.1%)is obviously higher than that of MA(34.4%).(7)By means of quantum chemistry calculations,the active sites on Cs/B-silica catalyst were determined and the reaction mechanism was studied.The surface structure was represented by a Si12 cluster,which was made up of a 5-and a 6-member ring.In this cluster,there were three silanols,which were signed as H-O(1)/B-silica,H-O(2)/B-silica and H-0(3)/B-silica,respectively.Density functional theory(DFT)results indicated that alkali metal atom reacted easily with H-0(2)/B-silica silanol,forming M-0(2)/B-silica.ESP and ALIE analyses for the M/B-silica predicted that the O(2)atom connected with alkali metal atom were the active site for electrophilic reaction.The activity increases with the increase of base strength of the alkali metal,which is in line with the experimental observations.(8)Based on the characterization of B-silica and Cs/B-silica,the most possible formation pathways of MA were proposed and the energy barriers of each elementary reaction were calculated.By comparing the energy barrier of rate determining step,the main MA formation pathway over B-silica and Cs/B-silica was determined.The MA formation pathway over B-silica contained two steps:firstly,a-H of Ma was captured by carbonyl oxygen of FA and a-C of Ma combined with carbonyl carbon of FA,forming the intermediate of methyl 3-hy droxypropanoate.Subsequently,methyl 3-hydroxypropanoate dehydrated to form MA and H20.In this pathway,the first step was the rate determining step,whose energy barrier was 80.3 kcal/mol.The main MA formation pathway over Cs/B-silica contained three steps:firstly,a-H of Ma was captured by carbonyl oxygen of Ma under the attraction of O base site,forming an enol molecule.Subsequently,a-C of enol attacked carbonyl oxygen of FA,at the same time,the captured H in the first step transferred to the carbonyl oxygen of FA,forming methyl 3-hydroxypropanoate intermediate.In the end,methyl 3-hydroxypropanoate dehydrated to form MA and H20.In this pathway,the dehydration step is the rate determining step with an energy barrier of 44.1 kcal/mol.The base site on Cs/B-silica changed the reaction pathway.As a result,the energy barrier of the rate determining step over Cs/B-silica is far lower than that over B-silica.(9)To further improve the catalytic performance of Cs/B-silica,the second element was introduced for the modification of Cs/B-silica.Cs/B-silica was modified with K,Al,P,respectively.By modification with Al,the initial MA yield increased from 36.4%to 40.2%.Characterization results indicated that Al atom doped in the Si02 framework,forming tetra-coordinate aluminum,and promoted the acidity of the catalyst.Quantum chemistry calculation indicated that the formation pathway of MA over Cs-Al/B-silica contained five steps.Among those,the fourth step(dehydration step)was the rate determining step,which needed the energy barrier of 39.9 kcal/mol.In the dehydration step,the synergic catalytic effect of Al-OH acid site and base site further reduced the needed energy barrier of dehydration step than that only in the presence of base site.