Regulation of the Lewis acidity on matrix and their performance in the catalytic cracking of light hydrocarbons
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摘要: 催化剂分子筛和基质的合理匹配是提高石脑油催化裂化制低碳烯烃产量的最有效路径之一，但是基质表面Lewis酸性对裂化反应的影响尚未明确。本研究通过硼和锌改性γ-Al2O3和锡改性KIT-6介孔氧化硅材料调变表面的Lewis酸，研究基质及其与ZSM-5分子筛配合使用时催化正庚烷和1-己烯裂化制低碳烯烃的性能。采用XRD、TEM、N2物理吸附-脱附以及NH3-TPD等方法探讨了改性γ-Al2O3和KIT-6的结构性质和表面酸性质。结果表明，B可以降低γ-Al2O3的表面Lewis酸性（酸量和酸强度），而Zn可以增强其表面酸性；此外，Sn可以提高有序介孔KIT-6表面Lewis酸性。催化裂化反应结果表明，当基质单独使用时，随基质表面Lewis酸性增强，轻烃反应活化能降低且转化率升高；当基质与ZSM-5配合使用时，基质在上分子筛在下的双床层排布方式对应的转化率最高，且随基质Lewis酸性增强，轻烃转化率升高，但Lewis酸性过强会加速氢转移反应，降低低碳烯烃的选择性。Abstract: The reasonable matching of zeolite and matrix is one of the most effective strategies to increase the yield of light olefins in naphtha catalytic cracking. However, the influence of the surface Lewis acidity within the matrix on the cracking reactions has remained ambiguous. Therefore, in present study, boron and zinc co-modified γ-Al2O3 and tin modified mesoporous silica KIT-6 with tuned surface Lewis acidity were applied to evaluate the cracking reactivity of n-heptane and 1-hexene to light olefins, in which the matrix was used alone and coupled with ZSM-5 zeolite in different packed modes. The effects of the modifiers on the textural properties and surface acidity of γ-Al2O3 and KIT-6 were investigated by XRD, TEM, N2 physical absorption-desorption, and NH3-TPD. The results showed that B doping reduced the Lewis acidity (both in the amount and acid strength) of γ-Al2O3, while the incorporation of Zn doping led to increased Lewis acidity. In addition, the Lewis acidity of ordered mesoporous KIT-6 increased as Sn doping rose. While for pure matrix, the ascend in conversions of n-heptane and 1-hexene was consistent with the increased Lewis acidity of the B and Zn co-modified γ-Al2O3 and xSn/KIT-6 rose, along with decreased activation energy. In contrast, when coupled with ZSM-5 zeolite, the highest conversion was achieved in the dual-bed manner of matrix and zeolite, and the conversion increased concomitantly with the increase in the Lewis acidity of the matrix. However, excessive Lewis acidity can accelerate the hydrogen transfer rate while diminishing the selectivity of light olefins.
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Key words:
- Lewis acid /
- matrix /
- catalytic cracking /
- light olefins
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Figure 1 Three kinds of catalyst bed configurations with ZSM-5 as active component and modified γ-Al2O3 or xSn/KIT-6 as matrix
Figure 3 TEM images and EDS mappings of (a) 0.10B/Al, (b) 0.10Zn/Al and (c) 10Sn/KIT-6
Figure 4 N2 sorption curves and pore size distributions of (a) B and Zn modified γ-Al2O3 and (b) Sn modified KIT-6 catalysts
Figure 5 Py-FTIR spectra of (a) B and Zn modified γ-Al2O3 and (b) Sn modified KIT-6 catalysts
Figure 6 NH3-TPD curves of (a) B and Zn modified γ-Al2O3 and (b) Sn modified KIT-6 catalysts
Figure 7 Conversion versus temperature (a) & (b) and Arrhenius plots (c) & (d) of n-heptane cracking over B and Zn modified γ-Al2O3 and xSn/KIT-6 catalysts
Figure 8 Conversion versus temperature (a) & (b) and Arrhenius plots (c) & (d) of 1-hexene cracking over B and Zn modified γ-Al2O3 and xSn/KIT-6 catalysts
Figure 9 Relationships between the amount of Lewis acid sites and the activation energy for n-heptane and 1-hexene cracking reactions over B and Zn modified γ-Al2O3 (a) and xSn/KIT-6 catalysts (b)
Figure 10 Conversion of n-heptane (a) &( c) and the selectivity of light olefins (b) & (d) over different kinds of catalyst beds
Reaction conditions: 0.1 g catalyst, t=550 °C, WHSV of feed was 0.34 h−1, 50 mL/min Argon as carrier gas, the results were the average value of 5 times detection within time-on-stream of 3 h.
Figure 11 Hydrogen transfer coefficient (HTC) of n-heptane cracking reaction over different kinds of catalyst beds
Figure 12 Conversion of 1-hexene and selectivity of light olefins over catalysts in Cat-1Matrix mode
Reaction conditions: 0.1 g catalyst, t=550 °C, WHSV of feed was 0.34 h−1, 50 mL/min Argon as carrier gas, the results were the average value of 5 times detection within time-on-stream of 3 h.
Figure 13 Hydrogen transfer coefficient (HTC) of 1-hexene cracking reaction over catalysts in Cat-1Matrix mode
Figure 14 Average product selectivity for the catalytic cracking of (a) n-heptane and (b) 1-hexene over catalysts in Cat-1Matrix mode
Table 1 Textural properties and surface acidity of B and Zn modified γ-Al2O3 and xSn/KIT-6 catalysts
Sample BET surface area S/(m2·g−1) Total pore volume v/(cm3·g−1) Average pore size d/nm Weak acid sites
/(μmol·g−1)Strong acid sites
/(μmol·g−1)Total acid sites
/(μmol·g−1)γ-Al2O3 228.7 0.556 9.7 56.9 22.7 79.6 0.05B/Al 254.4 0.573 9.0 47.9 16.4 64.3 0.10B/Al 256.0 0.551 8.6 44.2 10.3 54.5 0.15B/Al 278.8 0.492 7.1 41.6 7.9 49.5 0.05Zn/Al 191.0 0.431 9.0 64.7 29.9 94.6 0.10Zn/Al 184.6 0.418 9.1 78.6 34.3 112.9 KIT-6 573.5 0.756 5.7 ‒ ‒ ‒ 2.5Sn/KIT-6 566.7 0.740 5.6 28.8 11.9 40.7 5.0Sn/KIT-6 560.0 0.721 5.6 44.8 16.3 61.1 7.5Sn/KIT-6 540.1 0.695 5.5 61.6 20.4 82.0 10Sn/KIT-6 523.9 0.651 5.4 92.5 27.8 120.3 15Sn/KIT-6 502.4 0.622 5.1 83.1 21.3 106.4 
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