Study of pre-coking modified ZSM-5 molecular sieve and its benzene and syngas alkylation properties
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摘要: 苯与合成气的烷基化反应旨在利用非石油资源生产甲苯、二甲苯等烷基苯，本工作采用甲醇蒸汽预积炭制备一系列的ZSM-5分子筛，并与ZnCrOx复合形成双功能催化剂，探索对苯和合成气烷基化制烷基苯的影响。通过对预积炭时间的考察，发现适当的预积炭能显著提高ZSM-5分子筛的催化性能。在450 ℃、4.0 MPa的反应条件下，预积炭改性24 h的催化剂苯转化率达到20.18%。对预积炭改性的ZSM-5分子筛和反应后的复合催化剂进行分析，表明预积炭改变了分子筛的酸性。预积炭覆盖了ZSM-5分子筛上的一些B酸位点，降低了B酸/L酸比，从而提高苯的转化率。最后，通过烃池机理对苯与合成气烷基化反应的失活机理进行合理推测。Abstract: Toluene, xylene and other alkylbenzenes are important basic chemical raw materials, which mainly come from the petrochemical industry. With the increasing foreign dependence of petroleum resources in China, the development of coal-to-aromatics technology can not only alleviate the problem of shortage of petroleum resources in China, but also promote the transformation and upgrading of consumption in the traditional coal chemical industry and promote the clean and efficient use of coal. In this paper, methanol-modified ZSM-5 molecular sieves were used and mechanically milled with ZnCrOx in a mass ratio of 3∶1 as a composite catalyst, so as to investigate the catalytic effect of pre-preg carbon modified ZSM-5 molecular sieves. Under the reaction conditions of 450 ℃ and 4.0 MPa, the catalytic performance of the catalyst modified with pre-built carbon for 24 h was the best, in which the conversion of benzene reached 20.18% and that of CO reached 46.55%. It was found that the catalytic performance of the pre-built carbon modified ZSM-5 sieve was mainly affected by the B acid/L acid ratio, and the pre-built carbon covered some B acid sites on the ZSM-5 sieve, which reduced the B acid/L acid ratio and led to an increase in the benzene conversion, while the crystalline and pore structures of the pre-built carbon modified ZSM-5 sieve did not undergo significant changes. changed significantly. In order to better understand the deactivation mechanism of the carbon-modified ZSM-5 catalyst, TG-MS, Raman and GC-MS were used to further analyze the carbon species, and it was found that the carbon species of the carbon-modified ZSM-5 molecular sieves were mainly low-temperature carbon, which was mainly due to the fact that the strong acid was partially covered by the carbon-modified composite catalyst with the reduced amount of acid, and the carbon species were alkanes, olefins, aromatics and other oligomers mainly. Soluble carbon, high-temperature carbon and graphitic carbon are less, so as to avoid the reduction of catalyst activity caused by excessive carbonisation, while the carbon species on the composite catalyst after the reaction are mainly high-temperature carbon, which is mainly because the carbon species after the reaction of the composite catalyst are mainly large-molecule aromatic hydrocarbons and olefins, and even insoluble carbon such as graphitic carbon. In addition, the deactivation mechanism of ZSM-5 molecular sieve in the alkylation reaction of benzene and syngas was speculated based on the hydrocarbon pool mechanism: in the alkylation reaction of benzene and syngas syngas syngas firstly forms methanol in the active site of metal oxides, and then the methanol migrates in the acidic site of the molecular sieve to generate alkylbenzene with benzene, but as the reaction proceeds, the alkylbenzene will be alkylated deeply in the molecular sieve to form heavy polycyclic aromatic hydrocarbons or graphitic However, as the reaction proceeds, alkylbenzene will be deeply alkylated on the molecular sieve, forming heavy polycyclic aromatic hydrocarbons or graphite carbon, etc. Such substances are not easy to diffuse in the pores and will continuously cover the acid centre and pores of the molecular sieve, which will ultimately lead to catalyst deactivation.
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Key words:
- ZSM-5 /
- pre-accumulation of carbon /
- B acid/L acid ratio /
- phenylalkylation
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图 1 不同预积炭时间改性ZSM-5分子筛的热重曲线
Figure 1 Thermogravimetric curves of modified ZSM-5 molecular sieves with different pre-accumulation times
图 2 不同预积炭时间改性ZSM-5分子筛的NH3-TPD（a）和Py-FTIR谱图（b）
Figure 2 NH3-TPD plots (a) and Py-FTIR plots (b) of modified ZSM-5 molecular sieves with different pre-accumulated carbon times
图 3 不同预积炭时间改性ZSM-5分子筛的XRD谱图
Figure 3 XRD patterns of modified ZSM-5 molecular sieves with different pre-accumulation times
图 4 不同预积炭时间改性ZSM-5 分子筛的N2吸附-脱附等温线
Figure 4 N2 adsorption-desorption isotherms of modified ZSM-5 molecular sieves with different pre-accumulation times
图 5 不同预积炭改性ZSM-5分子筛和ZnCrOx复合催化剂上苯与合成气烷基化反应性能
Figure 5 Reaction performance of benzene with syngas alkylation over different preaccumulated carbon modified ZSM-5 molecular sieves and ZnCrOx composite catalysts. (a) CO conversion and selectivity of gas phase products, (b) benzene conversion and selectivity of liquid phase products
图 6 在100 h内ZnCrOx/ZSM-5-0H的CO转化率及气相产物选择性（a）和苯转化率及液相选择性（c）；ZnCrOx/ZSM-5-24H的CO转化率及气相产物选择（b）和苯转化率及液相选择性（d）
Figure 6 CO conversion and gas-phase product selectivity (a) and benzene conversion and liquid-phase selectivity (c) of ZnCrOx/ZSM-5-0H over 100 h; CO conversion and gas phase product selectivity (b) and benzene conversion and liquid phase selectivity (d) of ZnCrOx/ZSM-5-24H over 100 h
图 7 ZSM-5-24H（a）和ZnCrOx/ZSM-5-24H反应后 （b）的TG-MS谱图（m/z=44为CO2）
Figure 7 TG-MS plots of ZSM-5-24H (a) and ZnCrOx/ZSM-5-24H after reaction (b) (m/z=44 for CO2)
图 8 ZSM-5-0H（a）、ZSM-5-24H（b）和ZnCrOx/ZSM-5-24H反应后（c）的拉曼共聚焦成图像
Figure 8 Raman confocal imaging of ZSM-5-0H (a), ZSM-5-24H (b) and ZnCrOx/ZSM-5-24H after reaction (c)
图 9 ZSM-5-0H、ZSM-5-24H和ZnCrOx/ZSM-5-24H反应后的拉曼光谱谱图
Figure 9 Raman spectra of ZSM-5-0H, ZSM-5-24H and ZnCrOx/ZSM-5-24H after reaction
图 10 不同预积炭时间改性ZSM-5分子筛提取物的GC-MS谱图
Figure 10 GC-MS plots of modified ZSM-5 molecular sieve extracts with different pre-carbon build-up times
图 11 不同预积炭时间改性ZSM-5分子筛和ZnCrOx复合催化剂反应后提取物的GC-MS谱图
Figure 11 GC-MS plots of extracts after the reaction of modified ZSM-5 molecular sieves and ZnCrOx composite catalysts with different pre-accumulation time
图 12 ZSM-5-0H、ZSM-5-24H及其反应3和100 h后的复合催化剂提取物的GC-MS谱图
Figure 12 GC-MS plots of ZSM-5-0H, ZSM-5-24H and their bifunctional catalysts after 3 and 100 h of reaction
图 13 苯与合成气烷基化反应中复合催化剂的失活机理示意图
Figure 13 Schematic mechanism of deactivation of composite catalysts in the reaction of benzene with syngas alkylation
表 1 不同预积炭时间改性ZSM-5分子筛的酸性
Table 1 Acidity of modified ZSM-5 molecular sieves with different pre-carbon build-up times
Sample NH3-TPD/(μmol·g−1) Py-FTIR/(μmol·g−1) weak medium and strong total brønsted lewis B/L ZSM-5-0H 126 154 280 113 75 1.54 ZSM-5-12H 123 146 269 64 94 0.68 ZSM-5-24H 125 118 243 33 64 0.51 ZSM-5-36H 126 103 229 20 50 0.40 
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表 2 不同预积炭时间改性ZSM-5 分子筛的孔道参数
Table 2 Pore parameters of modified ZSM-5 molecular sieves with different pre-accumulation times
Sample $S_{{\mathrm{BET}}}^{\mathrm{a}} $/(m2·g−1) $S_{{\mathrm{meso}}}^{\mathrm{b}} $/(m2·g−1) $S_{\mathrm{micro}}^{\mathrm{c}} $/(m2·g−1) $v_{\mathrm{total}}^{\mathrm{d}} $/(cm3·g−1) $v_{\mathrm{micro}}^{\mathrm{c}} $/(cm3·g−1) ZSM-5 313.9 187.5 126. 4 0.354 0.162 ZSM-5-12H 296.8 171.3 125.5 0.352 0.162 ZSM-5-24H 300.6 184.5 116.1 0.345 0.157 ZSM-5-36H 291.6 177.2 114.4 0.334 0.151 a: BET method; b: Smeso=SBET-Smicro; c: t-plot method; d: Volume adsorbed at p/p0=0.99.
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