Design And Preparation Of Novel Acid-base Bifunctional Catalysts And Their Application In Side-chain Alkylation Reaction

Styrene has an important application in the petrochemical field.The most conventional route for the large-scale production of styrene is based on a homogeneous Friedel-Crafts alkylation reaction,typically starting from benzene alkylation with ethylene,followed by the dehydrogenation of ethylbenzene.Recently,a potential route for styrene production by toluene side-chain with methanol is proposed.This process of producing styrene offers advantages,such as simple process,lower energy consumption and less environmental pollution compared with the traditional styrene production route.This thesis firstly focused on the review on catalysts and reaction mechanism in side-chain alkylation of toluene with methanol.Furthermore,this thesis explore the side-chain alkylation of toluene with methanol to styrene over the Cs-modified X zeolite by the assistance of basic compounds（2-picoline）as a co-catalyst,the promoting effect of highly dispersed Pt nanoparticles in side-chain alkylation of toluene with methanol,the catalytic activity of X zeolite obtained by post treatment methods,and the cesium-modified hollow ZSM-5 catalyst for side-chain alkylation of 2-picoline with formaldehyde.Firstly,a series of cesium modified zeolite catalysts were synthesized,characterized and the catalytic activities for side-chain alkylation of toluene with methanol were studied.A new way is introduced to improve the catalytic activity by means of assisting basic compounds as co-catalysts during the reaction.Moreover,the effect of 2-picoline concentration on the activity was investigated.With the increase of the 2-picoline concentration from 0 to 2 mol%,both the toluene conversion and styrene selectivity were increased.However,further increase of 2-picoline rapidly caused a drop of catalytic activity.The results indicated that the addition of 2-picoline with low and high concentration have different effects on the toluene side-chain alkylation reaction.Two possible reaction routes on Cs-modified zeolite under the different 2-picoline concentration were proposed:when the concentration of2-picoline is low,the methanol firstly dehydrogenated to formaldehyde,and then attacked the methyl group of toluene to form styrene;while in the high concentration range of 2-picoline,the formaldehyde prefer to further decompose into CO and H₂,thereby dropping the reaction activity.The styrene would be undesirably hydrogenated by H₂ to form excessive ethylbenzene.In addition,the addition of low concentration of 2-picoline is beneficial to the formation of monodenate formate,while bidentate formate is more easily to form under the high concentration of2-picoline.Secondly,a series of Pt-modified CsX catalysts were prepared by vacuum impregnation method and investigated for the physicochemical properties and activities in the reaction of toluene side-chain alkylation with methanol.Among which,the 0.5Pt/Cs X-540-250R catalyst demonstrated superior catalytic activity,leading to a toluene conversion of 9.7%and styrene selectivity of 72.4%.Meanwhile,this Pt modified catalyst exhibited better reaction durability than Cs X.The high performance mainly lies in two aspects:（1）the highly dispersed Pt nanoparticles could assist C-H bond activation in the methyl group of toluene more efficiently to promote the toluene conversion;（2）the inhibition of coke formation by Pt modification to retard the catalyst deactivation.Thirdly,a series of X zeolites with different micro/mesoporosity were synthesized through the post treatment of NH₄F,and then modified by cesium ion exchange and impregnation methods.By comparing the catalytic performance of these Cs modified samples,it was found that the sample with 30 min of NH₄F post treatment exhibited higher catalytic activity than that of the parent sample.However,further increase of etching time to 60 min and 120 min rapidly caused a drop of catalytic activity.We considered that the microporous structure of X zeolites play the critical role in the process of side-chain alkylation of toluene.The sample with 30 min of NH₄F post treatment（X-FT-30）showed larger microporous surface area than parent X,resulting in the increase of available active sites.When etching time is long during the post treatment process,the inherent micropore structure would be destroyed.And the losing microporosity caused the decreasing of catalytic activity.Finally,by studying the side chain alkylation reaction of 2-picoline with formaldehyde on different catalysts,the understanding of the side chain alkylation process is deepened.Hollow ZSM-5 was synthesized by tetrapropylammonium hydroxide（TPAOH）alkali treatment for parent ZSM-5.The side-chain alkylation activity was investigated on the Cs-modified ZSM-5 catalysts.Among these catalysts,Cs₂O/Cs-hollow ZSM-5（Cs₂O/Cs-h ZSM-5）was found to display the highest catalysis performance,which showed the highest 2-picoline conversion of 50.1%and2-vinylpyridine selectivity of 62.5%.Meanwhile,the Cs₂O/Cs-hollow ZSM-5displayed excellent reaction durability.Combining catalytic performance with characterization results（including XRD,N₂ adsorption/desorption,TEM,CO₂-TPD and NH₃-TPD）,the larger BET surface area,increased mesoporous channels and harmonious acid-base property of the Cs modified hollow ZSM-5 catalysts facilitates the transport,adsorption and activation of reactant,thus endowed them with excellent catalytic performance.