Catalytic Cracking of N-Heptane by Hierarchical ZSM-5 and β Zeolites to Increase Yield of Olefins
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摘要: 采用溶剂挥发自组装法和取向连接生长法制备了多级孔结构的10元环ZSM-5分子筛和12元环β分子筛并考察了其催化裂解正庚烷性能。引入介孔后分子筛酸密度更低且扩散能力更强，能够有效减少氢转移副反应发生，多级孔ZSM-5与β可分别将低碳烯烃收率提高16.78%和21.63%。多级孔β分子筛在优化反应条件下，双烯收率为50.29%，比多级孔ZSM-5高10.37%，且丙烯选择性可达32.68%，较高的丙烯选择性是由于其独特的三维12元环孔道结构和较好的介孔与微孔连通性。Abstract: Hierarchical ZSM-5 and β zeolites were synthesized by solvent evaporation induced self-assembly and oriented attachment growth methods, respectively, and their catalytic properties in n-heptane cracking were assessed to determine the impact of hierarchical structure over catalysis. The catalytic cracking of n-heptane to generate light olefins was evaluated and the results demonstrated that high yields of light olefins could be attained on hierarchical zeolites, despite of lower conversions due to the reduction of acid site number. The hierarchical ZSM-5 and β can increase the yield of light olefins by 16.78% and 21.63%. Hierarchical β zeolite outperformed hierarchical ZSM-5 under identical operation conditions. Under optimized 680 ℃ and space velocity of 10.0 h−1·g-N-Heptane·g-Cat.−1, the yield of ligiht olefins by hierarchical β-HTS reached 50.29%, and simultaneously the highest propylene selectivity of 32.68% was achieved. The superior catalytic performance was attributed to a combined effect of reduced acid site density and enhanced diffusion property. The higher propylene selectivity was attributed to the unique large micropore size, better mesopore connectivity and decreased acid site density of hierarchical β.
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Key words:
- n-heptane /
- olefin /
- catalytic cracking /
- solvent evaporation induced self-assembly /
- hierarchical zeolites /
- zeolite β /
- ZSM-5
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图 1 多级孔β-HTS分子筛和普通β分子筛(a)；多级孔ZSM-5-HTS分子筛和普通ZSM-5分子筛(b)的XRD谱图
Figure 1. XRD patterns of hierarchical zeolite β-HTS and conventional β zeolite (a); Hierarchical ZSM-5-HTS and conventional ZSM-5 zeolite (b)
图 3 多级孔β-HTS分子筛、普通β分子筛(a)和多级孔ZSM-5-HTS分子筛、普通ZSM-5分子筛(b)的N2吸附-脱附等温线
Figure 3. N2 adsorption-desorption isotherms of hierarchical β-HTS and conventional β zeolite (a); hierarchical ZSM-5-HTS and conventional ZSM-5 zeolite (b)
图 4 多级孔β-HTS分子筛和传统β分子筛的孔径分布图(a); 多级孔ZSM-5-HTS分子筛和传统ZSM-5分子筛的孔径分布图(b)
Figure 4. Pore-size distribution of hierarchical β-HTS and conventional β zeolite (a); hierachical ZSM-5-HTS and conventional ZSM-5 zeolite (b)
图 6 不同分子筛在200 ℃ (a, b)和350 ℃ (c, d)的吡啶红外光谱图
Figure 6. Py-IR spectra of pyridine after desorbing at 200 ℃ (a, b) and 350 ℃(c, d) for different zeolites
图 12 不同分子筛在正庚烷裂解中的稳定性
Figure 12. The time-on-stream stability of different zeolites in n-heptane cracking
图 13 正庚烷催化裂解反应进行20 h后的分子筛积碳量的热重曲线
Figure 13. The thermogravimetric analysis of coke on spent hierarchical and conventional zeolite catalysts after 20 h time-on-stream tests in n-heptane catalytic cracking
表 1 合成不同分子筛的孔结构参数
Table 1. Pore structure parameters of different zeolites
Sample Si/Al ratio[a] SBET/(m2·g−1) Smic/(m2·g−1) Vmic/(cm3·g−1) Vmeso/(cm3·g−1) Vtotal/(cm3·g−1) D/(nm) β-HTS 33.12 554.23 361.0 0.19 0.70 1.00 15.72 β 24.21 501.54 380.31 0.23 0.11 0.34 3.41 ZSM-5-HTS 39.04 377.00 251.54 0.13 0.27 0.40 8.24 ZSM-5 24.57 244.51 224.60 0.12 0.02 0.14 2.42 [a]Determined by ICP-AES measurement. 
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表 2 合成分子筛的酸量及酸性分布
Table 2. Py-IR Measurements of Acidity for zeolites
Sample c/(10−4mol·g−1) T TL TB SB SL ZSM-5 3.65 0.22 3.43 3.19 0.18 ZSM-5-HTS 3.40 2.13 1.27 1.09 0.85 β 4.39 3.31 1.08 0.84 1.26 β-HTS 1.59 1.09 0.50 0.43 0.23 T-Total acid; TL-Total L acid; TB-Total B acid; SL-Strong L acid; SB-Strong B acid
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