Selective dealumination of ZSM-5 by steaming and its effect on ethanol to propene
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摘要: 采用水蒸气对ZSM-5分子筛进行后处理,系统考察了水蒸气处理时间和处理温度对分子筛孔结构、骨架铝分布、酸性及乙醇转化制丙烯的影响。研究结果表明,随着水蒸气处理时间的延长及处理温度的升高,分子筛结晶度和比表面积明显下降。27Al NMR及Co(Ⅱ)交换-ICP结果显示,在水蒸气作用下,分子筛骨架上孤立铝(Alsingle)物种被优先脱除,但临近铝(Alpairs)物种较为稳定。同时,吡啶吸附红外光谱结果显示,在水蒸气作用下,分子筛酸量和酸强度均显著降低。乙醇催化转化结果显示,随着水蒸气处理时间的延长和处理温度的升高,乙烯转化率和烷烃选择性迅速下降,但丙烯和丁烯等高碳烯烃的选择性明显提高。同时发现,乙烯转化率和孤立铝含量之间存在较好的正相关线性关系,而丙烯的生成则与孤立铝和邻近铝的共同作用有关。Abstract: The effects of steaming at varying times and temperatures on ZSM-5 pore structures, framework Al distribution, acid properties and ethanol to propene (ETP) catalytic performance were systematically studied in this study. The results show that the crystallinity and specific surface areas of steam treated ZSM-5 samples decrease with the increase treatment time and temperature. 27Al MAS NMR and Co(Ⅱ) exchange-ICP results show that the isolated framework Al species (Alsingle) can be preferentially removed from the zeolite framework, while the paired Al sites (Alpairs) remain relatively stable after steam treatment. The characterization of Py-IR reveals that the concentration of B acid sites and the acid strength are all declined with the steaming time or temperature increase. The catalytic results of ETP at 450 ℃ show that the sample after steaming gives improved selectivities to propene and butene at the expense of ethene conversion and alkanes selectivity relative to the unmodified zeolite. Besides, a good positive correlation between ethylene conversion and Alsingle concentration is found, whereas the propene formation is influenced by the combination effect of Alsingle and Alpairs sites.
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Key words:
- ZSM-5 zeolite /
- steaming /
- dealumination /
- ethanol to propene /
- Al distribution
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表 1 水蒸气处理时间及温度对ZSM-5分子筛织构性质的影响
Table 1 The effects of steaming time and temperature on textural properties of ZSM-5 samples
Sample Si/Ala Crystallinity/%b Surface area/(m2·g−1) Pore volume/(cm3·g−1) total micropore external total micropore ZSM-5 25 100 387 269 118 0.32 0.13 S-600-4 25 94 358 254 104 0.30 0.12 S-600-8 25 94 348 257 91 0.29 0.11 S-600-12 25 91 333 259 74 0.28 0.11 S-600-24 25 88 337 275 62 0.29 0.12 S-600-48 25 83 329 276 53 0.29 0.12 S-450-8 25 95 365 240 125 0.33 0.11 S-500-8 25 92 369 246 123 0.31 0.11 S-550-8 25 86 381 259 122 0.32 0.12 S-650-8 25 77 354 274 80 0.29 0.12 a: Determined by ICP-AES; b: The relative crystallinity of a sample determined by XRD was calculated by comparing its peak area in the 2θ range of 22°−25° to that of ZSM-5 (ZSM-5, assuming that it had a crystallinity of 100%) 表 2 27Al MAS NMR谱图拟合
Table 2 Results of deconvolution of 27Al MAS NMR spectra
Sample Relative peak areas/% 55.6
(chemical shift)52.6
(chemical shift)0
(chemical shift)ZSM-5 51 46 3 S-600-4 31 63 6 S-600-8 24 63 14 S-600-12 22 61 17 S-600-24 23 53 24 S-600-48 21 48 31 表 3 水蒸气处理后分子筛的酸性质
Table 3 Acidic properties of ZSM-5 samples after steaming
Sample Acidity by type/(μmol·g−1) Brønsted acid distributiona External Acidity/(μmol·g−1) B L B/L strong medium weak B proportion/% ZSM-5 327 88 3.7 82 10 8 28 8 S-600-4 230 118 2.0 83 16 1 31 14 S-600-8 211 136 1.6 89 0 11 38 18 S-600-12 176 149 1.2 66 15 19 39 22 S-600-24 115 139 0.8 49 45 5 29 25 S-600-48 78 78 1.0 49 26 25 30 39 S-450-8 321 121 2.7 87 12 2 22 7 S-500-8 318 107 3.0 78 15 7 24 8 S-550-8 244 92 2.7 76 18 6 38 16 S-650-8 117 128 0.9 50 28 22 9 8 a: Brønsted acidic sites (BAS) distribution were calculated from the amounts of BAS determined by Py-FTIR at different temperatures (weak, pyridine desorbed at 150−250 ℃; medium, pyridine desorbed at 250−350 ℃; strong, pyridine remained at 350 ℃) -
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