The Preparation And Application Of Zeolite Catalyst For Toluene Alkylation With Methanol

The alkylation reaction of toluene with methanol combines the production of petroleum chemical aromatics with the comprehensive utilization of coal chemical methanol resources,further reactions such as shape-selective alkylation and side-chain alkylation are derived,which can selectively produce xylene,styrene and other chemicals.In the shape-selective alkylation of toluene with methanol,the shape-selective pore of ZSM-5 zeolite and the oxide modification can effectively shield the external surface acid sites,which can significantly increase the content of para-xylene in xylene products.However,the modification of zeolite catalysts by oxides reduces the one-way conversion of toluene and increases the cost of subsequent cycle processing of toluene.The side-chain alkylation of toluene with methanol is an acid-base catalytic process.Styrene,ethylbenzene and other products can be selectively produced by the adsorption of toluene at acid sites and the selectively activate methyl at base sites of CsX zeolites.The acid-base properties of CsX zeolites in side-chain alkylation also lead to its low toluene conversion.Therefore,this paper focused on the development of catalysts used in toluene alkylation with methanol,which was devoted to high activity and high selectivity,and actively explored the shape-selective alkylation and side-chain alkylation catalysts respectively,including the following aspects.In this paper,twin ZSM-5 zeolite（TZSM-5）was successfully synthesized by hydrothermal method assisted by two templating agents（TPABr and n BA）with b-axis vertical channels effectively covered.Furthermore,the crystal structure of TZSM-5 zeolite and the shape-selective alkylation performance of toluene with methanol were optimized by adjusting the dosage and relative proportion of the two template agents in the synthesis process.The results showed that the optimal feeding ratio for hydrothermal synthesis was 300 Si O₂:1 Al₂O₃:1 Na₂O:40.5 TPABr:318 n BA:10160 H₂O.TZSM-5 zeolite synthesized based on this ratio could maintain a relatively complete twin structure and obtain a relatively excellent shape-selective alkylation catalytic performance.The one-way conversion of toluene was more than 20%,and the para-selectivity was about 97%.Then,the Si O₂/Al₂O₃ ratio and crystal size of TZSM-5 zeolites were adjusted to reveal the structure-activity relationship of the shape-selective alkylation reaction.The results showed that the zeolites with high Si O₂/Al₂O₃ had lower acid amount and weak acidity,which could effectively inhibit xylene isomerization and toluene disproportionation,meantime ensure a high para-selectivity（～97%）.But the toluene conversion decreased obviously,so considering comprehensively,the optimal Si O₂:Al₂O₃=300 was preferred.The crystal size of TZSM-5zeolite could be controlled in the range of 2～23μm by both the seed method and the two-stage crystallization method.Under the similar size and surface coverage degree,the acid amount of the zeolite synthesized by the two-stage crystallization method was lower,so the xylene selectivity and para-selectivity of the zeolite synthesized by the two-stage crystallization method were higher than that by the seed method.By further applying TZSM-5 to the shape-selective alkylation of toluene with methanol for co-production of olefin（molar ratio of toluene to methanol=1）,partial low-olefin selectivity（C₂⁼～C₄⁼=27 wt%）could be produced simultaneously with the high selectivity of para-xylene（about 60 wt%for xylene and 92 wt%for para-selectivity）,which improved the economy of traditional alkylation process.Finally,the effective components of side-chain alkylation of toluene with methanol on CsX zeolite catalysts were studied.The results showed that CuO was a kind of efficient agent for CsX to catalyze side-chain alkylation,and the optimum loading amount was about 1 wt%.CuO additive could improve toluene conversion and effectively improve styrene selectivity（～50%）under the condition of maintaining high total selectivity of styrene and ethylbenzene（>90%）.The mechanism of action of CuO additives can be attributed to two aspects:one is to promote the dehydrogenation of methanol to formaldehyde which is the real alkylation active component,so as to enhance toluene conversion;the other is to effectively adjust the acid-base properties of the molecular sieve and inhibit the generation of ethylbenzene by hydrogen transfer reaction of styrene.In addition,CuO additives also had obvious promoting effect on the common side-chain alkylation catalyst systems including Cs₂O/CsX,B₂O₃/CsX and Zn O/CsX,among which CuO and B₂O₃ co-modified CsX zeolite was the best catalyst system.