Alkali-modified FAU Zeolite Catalysts For Side-chain Alkylation Of Toluene With Methanol To Produce Styrene And Ethylbenzene

Styrene is an important basic organic chemical feedstock.At present,styrene is produced commercially by alkylation of benzene with ethylene to ethylbenzene followed by dehydrogenation of ethylbenzene.By using the excess amount of toluene and abundant methanol as raw materials,side-chain alkylation of toluene with methanol is a potential low-cost and less-energy intensive process to synthesize styrene.This thesis focus on the side-chain alkylation of toluene with methanol to produce styrene using alkali-modified FAU zeolites catalysts,which is the most effective catalytic material for this reaction.Furthermore,this thesis examines the influences of zeolite topology structure and property on the catalytic performance,the catalytic activity of hierarchical X zeolite obtained by post treatment methods,the synergistic catalytic effects of cesium cations and cesium oxide over CsX zeolite,and the promoting effects of alkali metal oxide in side-chain alkylation of toluene with methanol.The main results are summarized as follows:Zeolites with different topological structures were modified by cesium ion-exchange(CE)and cesium ion-exchange plus cesium impregnation(CE+CI)methods,respectively.The catalytic performance of different catalysts for side-chain alkylation of toluene with methanol was investigated.It was found that X zeolite with FAU topology structure exhibited the best catalytic activity.The catalytic performance of FAU zeolite with different silicon to aluminum ratio and crystal sizes was comparatively examined.The X zeolite with low silicon to aluminum ratio and large crystal sizes generally exhibited better catalytic activity.For CE modified X zeolite catalyst,the conversion of toluene(XT)?selectivity of styrene(SST)and total selectivity of styrene and ethylbenzene(SST+EB)could be purposely adjusted by changing reaction condition within limits.However,for CE+CI modified X zeolite catalyst,changing reaction condition could only influence XT with very little impact on SST and SST+EB.A series of hierarchical X zeolite with different micro/mesoporosity were prepared through the post treatment of ammonium exchange(AE),dealumination(DA)and the combination of ammonium exchange and dealumination(AE+DA).Different X zeolite catalysts were modified by CE and CE+CI,respectively.By comparing the catalytic performance of NaX zeolite with those of hierarchical zeolite X prepared by post treatment method,it was found that most of the post treated samples' catalytic activity decreased compared with that of the parent NaX.Although the mesopore structure of X zeolite was increased by post-treatment process,the inherent micropore structure of X zeolite was destoryed.However,the catalytic performance of basic X zeolite catalysts was closely related to the microporosity of zeolite X.Losing microporosity caused the decreasing of catalytic performance.Post treatment process resulted in the reduction of the number of alkali metal ion-exchange sites.Thus,the amount of basic sites decreased and the strength of basic sites weakened.The catalytic performance of hierarchical zeolite X was not improved.CsX-Hex and CsX-Him catalysts were prepared by ion-exchange and impregnation methods,respectively.When T=698 K,P=1 atm,WHSV=2 h-1,nT/nM?2,nN2/n(T+M)=2,the average catalytic performance of CsX-Hex were XT=1.6%,SST=39.8%,SST+EB?65.1%while the average catalytic performance of CsX-Him were XT=43%,SST=6.9%,SST+EB=97.6%.The results indicated that both cesium cations(Cs+)balanced negative charge of zeolite framework and cesium oxide(Cs2O)dispersed over the zeolite particle surface or inside the pore existed in CsX zeolite modified by ion-exchange or impregnation.They played different roles in the process of catalyzing side-chain alkylation of toluene with methanol:Cs+adsorbed and activated toluene while Cs2O dehydrogenated methanol to formaldehyde as intermediates.Styrene was formed by side-chain alkylation of toluene which adsorbed over Cs+with formaldehyde.Thus,there was synergistic catalytic effect between Cs+and Cs2O.The catalytic performance of CsX was determined by the absolute and relative contents of cesium cations and cesium oxides.A series of basic X zeolite catalysts with different compositions were prepared by adding appropriate amount of alkali metal oxides(K2O or CS2O)into alkali metal cations(K+or Cs+)ion-exchanged zeolite X.Under the reaction condition of T=698 K,P=1 atm,WHSV=2 h-1,nT/nM?6,nN2/n(T+M)?2,K2O/KX catalyst demonstrated relatively high YST?0.5%(YST+EB=1.4%)while Cs2O/CsX catalyst demonstrated relatively high YST+EB?3.5%(YEB?3.3%).By introducing alkali metal oxides,acid sites were neutralized according to the amount of alkali while strong basic sites were formed.Side reactions which were caused by the acid sites and coke deposition of the catalysts were inhibited.The alkali metal oxides could promote the sequential dehydrogenation process from methanol to formaldehyde to carbon monoxide.The synergistic catalytic effect between alkali metal cations and alkali metal oxides promoted the subsequent side-chain alkylation of toluene with formaldehyde.The YST and YST+EB of the catalysts could be adjusted and controlled by choosing different combinations of alkali metal cations and alkali metal oxides modification.