铝源对HZSM-5分子筛催化甲醇与正己烷耦合裂解性能的影响

(1.厦门大学化学化工学院,醇醚酯化工清洁生产国家工程实验室,福建 厦门 361102; 2.中国石油天然气股份有限公司大庆化工研究中心,黑龙江 大庆 163000)

铝源; HZSM-5分子筛; 酸量; 多级孔; 低碳烯烃

Influence of aluminum sources in HZSM-5 zeolite on catalytic performance in coupled cracking of n-hexane with methanol
DONG Huanneng1,XU Xianming2,DAI Yueli2,JI Yonggang2,ZHANG Yongjun2,WANG Yefei1,LIAN Yixin1*

(1.National Engineering Laboratory for Green Chemical Productions of Alcohols-Ethers-Esters,College of Chemistry and Chemical Engineering,Xiamen University,Xiamen 361102,China; 2.Daqing Petrochemical Research Center,PetroChina Company Limited,Daqing 16300

aluminum source; HZSM-5 zeolite; acid amount; hierarchical pore; light olefin

DOI: 10.6043/j.issn.0438-0479.202103011

备注

分别以铝酸钠、异丙醇铝、硫酸铝、硝酸铝、氯化铝为铝源,采用水热法合成一系列HZSM-5分子筛,并将其应用于甲醇和正己烷耦合裂解反应中.运用X射线衍射(XRD)、扫描电镜(SEM)、氨程序升温脱附(NH3-TPD)、N2吸附脱附和热重(TG)分析等方法对分子筛进行表征,研究不同铝源对HZSM-5分子筛物理化学性质的影响.结果表明:不同铝源合成的HZSM-5分子筛在酸性、晶粒大小及孔道结构等方面存在较大差异; 丰富的介孔、适中的酸量有利于提高正己烷的转化率、低碳烯烃的选择性以及催化剂的稳定性.其中,以铝酸钠为铝源合成的HZSM-5分子筛在甲醇和正己烷耦合裂解反应中呈现较好的催化性能,反应8 h内正己烷平均转化率达92.93%,低碳烯烃平均选择性为60.28%,具有较强的抗积碳能力.

Objective: At present, naphtha steam cracking is the main route for producing light olefins despite its multiple disadvantages such as high cracking temperature (above 800 °C), extensive energy consumption and low olefin yield. Coupling the highly exothermic methanol to olefins and the strongly endothermic naphtha catalytic cracking to obtain light olefins can achieve energy coupling, which is a promising new technology. In this study, HZSM-5 zeolite catalysts were synthesized and applied to the coupled cracking reaction of methanol and n-hexane (naphtha model reactant). The effects of different aluminum sources on the physical and chemical properties and catalytic performance of HZSM-5 zeolite were studied.
Methods: A series of HZSM-5 zeolite with silicon aluminum ratio (n(SiO2):n(Al2O3))of 55 were synthesized by hydrothermal method with silica sol as silicon source and tetrapropylammonium bromide (TPABr) as template. The selected aluminum sources included sodium aluminate, aluminum isopropoxide (organic aluminum), aluminum sulfate, aluminum nitrate and aluminum chloride (acid anion aluminum). The prepared HZSM-5 zeolites were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), ammonia temperature programmed desorption (NH3-TPD), N2 adsorption desorption and thermogravimetric (TG) analysis. The catalytic activity of the prepared HZSM-5 zeolite was performed within the fixed-bed reactor under the following conditions: 600 °C, 0.1 MPa pressure, 2 h–1 liquid hourly space velocity, V(methanol): V(n-hexane) = 1:1. Before the experiment, the zeolite catalysts were pressed, crushed, and filtered with the 20-40 mesh. The reaction products were analyzed online by gas chromatography.
Results: The effects of different aluminum sources on the physicochemical properties of HZSM-5 zeolites were studied and correlated with their catalytic coupled cracking performance. The results show that the HZSM-5 zeolites synthesized from five different aluminum sources all showed high relative crystallinity. Among them, the HZSM-5 zeolite synthesized with odium aluminate and aluminum isopropoxide as aluminum sources were spherical particles formed by nanocrystals. The irregular accumulation of nanocrystals produced abundant intergranular mesopores. The number of strong acid centers was moderate. On the contrary, the HZSM-5 zeolites synthesized with aluminum sulfate, aluminum nitrate and aluminum chloride as aluminum source had larger grain size and less mesoporous volume because of the order and close packing of nanocrystals. In addition, the acid strength was stronger and the acid amount was greater. In the coupled cracking reaction of methanol and n-hexane, the HZSM-5 zeolite catalysts synthesized with odium aluminate and aluminum isopropoxide as aluminum source presented superior catalytic performance. The higher n-hexane conversion and light olefin selectivity, and more stable catalyst activity were obtained, which could be due to the abundant mesoporous and moderate acid amount that reduced the probability of secondary reaction of light olefins and promoted the migration of carbon deposition precursors. In addition, the results of TG analysis showed that the carbon deposition of HZSM-5 synthesized with odium aluminate and aluminum isopropoxide was significantly lower. In terms of product distribution, the selectivity of ethylene was significantly higher than that of propylene in initial reaction through all HZSM-5 zeolite catalysts. However, as the reaction proceeded, the selectivity of propylene increased, the selectivity of ethylene decreased, and the selectivity of total light olefins tended to be stable.
Conclusion: In the coupled cracking reaction of methanol and n-hexane, the higher acid density promotes the conversion of reactants. However, the strong acid site is easy to produce carbon deposition, which covers the active acid site and deactivates the catalyst. Moreover, more strong acid centers are conducive to the formation of ethylene, and appropriate low acid centers are conducive to the production of propylene because of the reduction in the probability of secondary reaction. For the coupled cracking reaction, carbon deposition is the main reason for the deactivation of HZSM-5 zeolite catalysts. Adjusting the acid properties of HZSM-5 and introducing mesopores will have a positive effect on the performance of the catalysts.

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