Effect Of Br(?)nsted/Lewis Acid And Modification Of Zn、Cu Species On The Formation Of Para-Xylene During Methylation Of Toluene With Methanol Over HZSM-5 Zeolite

Para-xylene（PX）is an important raw material in coal chemical industry.However,toluene disproportionation,heavy aromatics（C9）transalkylation and xylene isomerization produce a large number of by-products,which is not conducive to the formation of PX.The methylation of methanol with toluene to xylene over HZSM-5 catalyst will produce PX more directly and effectively,also avoiding the formation of abundant by-products.However,there are still some problems,such as high dissociation activation energy of methanol and poor selectivity of PX in three xylenes.Therefore,in order to improve the activity to PX formation in methylation reaction,it is necessary to have a modification to HZSM-5 zeolite.This will have a great theoretical significance for the design of high performance catalysts.Based on the DFT calculation method,the methylation of methanol with toluene to xylene is studied in detail over Br（?）nsted acid（BAS）,Lewis acid（LAS）,Znand Cumodified HZSM-5 catalysts.The effects of Bader charge（e）,bond length（（?））,acid strength and the synergistic effect of BAS and LAS for the dissociation of methanol and activity with selectivity to PX are investigated when different acid sites existed.The microcosmic reasons for methanol dissociation and the activity to PX formation are elucidated from the microcosmic level,which provide the theoretical guidance for the actual reaction process.And the main research contents and conclusions are as follows:1.The synergistic effect of BAS and LAS formed by extra-framework AlOH²⁺on the formation of methoxyl group and PX is elucidated:BAS/AlOH²⁺-HZSM-5 catalyst with BAS and extra-framework AlOH²⁺is constructed and used for the formation of xylenes.Firstly,the synergistic effect of BAS and AlOH²⁺changed the route and activity of methoxy formation.On BAS-HZSM-5,methanol and methoxy group are adsorbed on BAS;On BAS/AlOH²⁺-HZSM-5 catalyst,methanol is adsorbed on BAS,and the methoxy group formed by dissociation of methanol is more easily adsorbed on AlOH²⁺;The synergistic effect of BAS and AlOH²⁺greatly reduces the activation energy of methoxy formation from 227.3 k J·mol^-1 on BAS-HZSM-5 to 65.1k J·mol^-1 on BAS/AlOH²⁺-HZSM-5.Moreover,the selectivity of PX on BAS/AlOH²⁺-HZSM-5 catalyst is higher than that to OX and MX.On BAS-HZSM-5catalyst,the formation of PX and MX is a competitive reaction.It can be seen that the synergistic effect of BAS and AlOH²⁺in BAS/AlOH²⁺-HZSM-5 is not only conducive to the dissociation of methanol,but also beneficial to the formation of PX.2.The effects of different Znspecies modification and the distance between ZnOH⁺and BAS on the methylation reaction of methanol with toluene are clarified:ZnH⁺、ZnO、Zn²⁺and ZnOH⁺are loaded on HZSM-5 catalyst to explore the effects of different Znspecies on the formation of methoxy and PX.On the ZnH⁺modified catalyst,the dissociated methyl group of methanol is combined with H proton,resulting in the formation of methane;On ZnO and Zn²⁺modified HZSM-5 catalysts,the formation of methoxy group requires high activation energy.The introduction of ZnOH⁺into the five membered ring of HZSM-5,which can improve the activity of methanol dissociation through the synergistic effect of BAS and ZnOH⁺compared to the BAS-HZSM-5.The effect of the distance between ZnOH⁺and BAS(D⁺_ZnOH-_BAS)on the methylation of methanol with toluene is further investigated.Considering the activity to methanol dissociation and PX formation on BAS/ZnOH⁺-HZSM-5 with five different distances,it can be concluded that BAS/ZnOH⁺-HZSM-5（a）with D⁺_ZnOH-_BAS of 4.340（?）,which is beneficial to the high selectivity and activity to PX formation.3.The methylation of methanol with toluene to xylenes over HZSM-5 catalysts modified by different Cuspecies is investigated:The dissociation of methanol on BAS/Cu⁺-HZSM-5 and BAS/CuOCu²⁺-HZSM-5catalysts,which loaded with Cu⁺and CuOCu²⁺species in HZSM-5 channel is studied.The results show that Cu⁺would preferentially participate in the adsorption and dissociation of methanol on BAS/Cu⁺-HZSM-5 catalyst,and the dissociated methyl and hydroxyl groups are adsorbed on Cu⁺,while the BAS did not participate in the reaction.And there is no synergistic effect of BAS/LAS.On BAS/CuOCu²⁺-HZSM-5 catalyst,CH₃OH is preferentially adsorbed on BAS,but the dissociated methyl is adsorbed on CuOCu²⁺.By comparing the activation energy of methanol dissociation on the two catalysts and the distance DC-O（Cu）between methanol C atom and Cuactive site in transition state structure of methanol dissociation,it is concluded that the activation energy of methanol dissociation is smaller on BAS/Cu⁺-HZSM-5 with shorter DC-O（Cu）,which is 109.7 k J·mol^-1.On BAS/Cu⁺-HZSM-5,the activation energies of three kinds of xylenes are small,and the difference is not big,so the selectivity to PX is not high.4.The effects of different acid types with strength on catalysts for the dissociation of methanol and production of PX are determined:By analyzing the acid strength of different species modification and pure BAS catalysts,the order of four catalysts is as follows:BAS/Cu⁺-HZSM-5>BAS/AlOH²⁺-HZSM-5>BAS/ZnOH⁺-HZSM-5（a）>BAS-HZSM-5.The high activation energy of 227.3 k J·mol^-1for methanol dissociation on BAS-HZSM-5 with the weakest acid strength is not conducive to the formation of methoxy and PX in the subsequent methylation.The activation energy of methanol dissociation is relatively low in BAS/Cu⁺-HZSM-5 catalyst with the strongest acid strength,but the formation of three xylenes is competitive.The activity of methanol dissociation and the selectivity to PX is good in BAS/AlOH²⁺-HZSM-5 and BAS/ZnOH⁺-HZSM-5（a）with the moderate acid strength,but the activation energy of PX formation in BAS/ZnOH⁺-HZSM-5（a）is less than that of 46.2 k J·mol^-1 in BAS/AlOH²⁺-HZSM-5 catalyst.So the BAS/ZnOH⁺-HZSM-5（a）catalyst with moderate acid strength is most favorable to the formation of PX in methylation of methanol with toluene.