Effect of seed number of MOR zeolites on the transalkylation reaction and the investigation of the reaction mechanism
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摘要: 以丝光沸石（MOR）分子筛为催化剂，甲苯与三甲苯烷基转移反应为探针反应，系统考察了晶种数量变化对分子筛物化性能和催化性能的影响。研究表明，合成过程中晶种添加量的不同会显著影响催化剂的酸性、比表面积和孔体积，进而影响其催化剂活性和稳定性。当晶种添加量为8%时，MOR分子筛具有最多的B酸含量、最大的比表面积和孔体积，同时对应的催化剂活性和稳定性也最好。本研究还对甲苯和三甲苯烷基转移反应机理进行了深入研究，结果表明，发生在MOR分子筛上的烷基转移反应通过双分子中间体机理进行，并应用实验手段捕捉到了中间体的存在，证实了双分子中间体机理的合理性，同时推导出可能的反应机理路线图。Abstract: The effect of the number of crystal seeds on the physical and chemical properties and catalytic performance of the mordenite (MOR) zeolite was investigated using the transalkylation of toluene and trimethylbenzene as the probe reaction. The results show that the addition of seed crystals in the synthesis process will significantly affect the acidity, specific surface area and pore volume of the catalyst, thus affect the activity and stability of the catalyst. When the addition amount of crystal seeds is 8%, the MOR has the most B acid content, the maximum specific surface area and the pore volume, and the activity and stability of the catalyst are also the best. In addition, the mechanism of the transalkylation reaction of toluene and trimethylbenzene has been studied in detail. The results show that the transalkylation reaction in the MOR molecular sieve channels is carried out by the mechanism of bimolecular intermediates, and the intermediates, confirming the rationality of the mechanism of the bimolecular intermediates and deducing the possible reaction route.
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图 1 不同晶种含量的MOR分子筛的XRD谱图
Figure 1 X-ray diffraction (XRD) patterns of MOR zeolites with different number of seeds
图 2 不同晶种含量合成的MOR分子筛的SEM照片
Figure 2 SEM images of MOR zeolites with different number of seeds
图 3 不同晶种含量合成的MOR分子筛的N2等温吸附-脱附曲线
Figure 3 N2 adsorption and desorption isotherms of MOR zeolites with different number of seeds
图 4 不同晶种含量合成的MOR分子筛的Py-FTIR谱图
Figure 4 Py-FTIR spectra of MOR zeolites with different number of seeds
图 5 不同晶种含量合成的MOR分子筛的NH3-TPD谱图
Figure 5 NH3-TPD spectra of MOR zeolites with different number of seeds
图 6 不同晶种含量合成的MOR分子筛的27Al MAS NMR谱图
Figure 6 27Al MAS NMR spectra of MOR zeolites with different number of seeds
图 7 不同晶种数量MOR分子筛上反应物转化率、二甲苯选择性随时间的变化
Figure 7 Dependence of toluene (a) and 1, 2, 4-TMB (b) conversions and xylene (c) yield on the reaction time catalyzed by MOR zeolite with different number of seeds
reaction condition: toluene/1, 2, 4-TMB＝1 (mol ratio); ρ＝3 MPa; t＝450 ℃; WHSV＝20 h-1; H2/hydrocarbon＝4 (mol ratio)
图 9 甲苯和三甲苯烷基转移双分子机理循环示意图
Figure 9 Bimolecular scheme for the transalkylation of toluene with trimethybenzene
图 10 MOR分子筛上中间体物种捕捉示意图
Figure 10 GC-MS total ion chromatogram of the organic species extracted from MOR with CH2Cl2 after the transalkylation conducted at 150 ℃ for 10 h
表 1 不同晶种含量合成的MOR分子筛的物化性质与酸性质
Table 1 Physicochemical properties and acid properties of MOR zeolites with different number of seeds
Seed crystal Physicochemcal properties Acids properties Si/Al ABET/(m2·g-1) v/(cm3·g-1) average pore diameter d/nm AlF/AlEF peak area* B acids/(mmol·g-1) L acids/(mmol·g-1) weak acids/(mmol·g-1) strong acids/(mmol·g-1) 2% 11.57 288 0.163 4.13 87/13 6 748.68 0.263 0.017 0.348 0.320 4% 8.72 321 0.173 3.53 88/12 6 954.28 0.286 0.017 0.350 0.317 8% 9.06 422 0.228 3.97 89/11 7 517.33 0.296 0.016 0.345 0.323 12% 10.52 355 0.196 3.75 88/12 7 015.42 0.245 0.011 0.341 0.327 16% 10.00 296 0.169 4.09 86/14 6 767.35 0.220 0.017 0.342 0.322 *: peak area of δ=55 in the 27Al MAS NMR test 
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表 2 不同数量晶种合成的MOR分子筛反应后的产物分布
Table 2 Reactants conversion and product distribution catalyzed by MOR zeolite with different number of seeds
Condition Product distribution w/% Catalyst life t/h xylene benzene C1-6 hydrocarbons C2-5 olefins C10+ aromatics 2% 82.44 6.72 0.80 0.07 8.41 168 4% 81.17 8.39 1.59 0.09 6.77 216 8% 81.56 8.29 1.19 0.06 6.78 444 12% 77.99 10.59 1.93 0.08 6.84 240 16% 82.18 7.09 1.53 0.08 7.10 168
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表 3 不同晶种数量MOR新鲜、失活后、再生状态下N2吸附数据以及积炭量的对比
Table 3 Comparison of N2 adsorption data and carbon deposition of the number of different seed crystals MOR fresh, inactivated, after the regeneration
Catalyst ABET/(m2·g-1) Pore volume v/(m3·g-1) Carbon deposition w/% fresh used regeneration fresh used regeneration 2% 288 53.5 277 0.163 0.086 0.156 2.00 4% 321 58.8 308 0.173 0.091 0.162 3.36 8% 422 78.5 409 0.228 0.120 0.204 4.54 12% 355 66.0 336 0.196 0.103 0.185 4.25 16% 296 55.2 283 0.169 0.089 0.157 3.50
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表 4 不同晶种数量MOR内外比表面积的对比
Table 4 Comparison of specific surface area and external specific surface area of MOR
2% 4% 8% 12% 16% BET surface area A/(m2·g-1) 288 321 422 355 296 Micropore area A/(m2·g-1) 237 278 359 304 238 External surface area A/(m2·g-1) 51 43 63 51 58 E/M 0.22 0.15 0.18 0.17 0.24
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