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(Zeolite Socony Mobil)-5分子筛和12元环β分子筛,并考察了其催化裂解正庚烷的性能。引入介孔后分子筛酸强度更低且扩散能力更强,能够有效减少氢转移的副反应发生,多级孔ZSM-5-HTS(其中HTS为十六烷基三甲氧基硅烷)分子筛与β-HTS分子筛可分别将低碳烯烃收率提高16.78%和21.63%。多级孔β-HTS分子筛在优化反应条件下,双烯收率为50.29%,比多级孔ZSM-5-HTS分子筛高10.37%,且丙烯选择性可达32.68%,丙烯选择性较高归因于其独特的三维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-HTS and β could increase the yield of light olefins by 16.78% and 21.63%, respectively. Hierarchical β-HTS zeolite outperformed hierarchical ZSM-5-HTS under identical operation conditions. Under optimized 680 ℃ and space velocity of 10 h−1, the yield of light 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-HTS.
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Key words:
- n-heptane /
- olefin /
- catalytic cracking /
- solvent evaporation induced self-assembly /
- hierarchical zeolite /
- β zeolite /
- ZSM-5
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表 1 不同分子筛的孔结构参数
Table 1. Pore structure parameters of different zeolites
Sample n(Si)/n(Al)1) SBET/(m2·g−1) Smic/(m2·g−1) Vmic/(cm3·g−1) Vmeso/(cm3·g−1) Vtotal/(cm3·g−1) D/nm β-HTS 33.12 554.23 361.00 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 1) Determined by ICP-AES measurement; SBET—BET Specific surface area; Smic—Micropores specific surface area; Vmic—Microporous volume; Vmeso—Mesopore volume; Vtotal—Total pore volume 表 2 不同分子筛的酸性Py-IR结果
Table 2. Py-IR measurements of acidity for different zeolites
Samples 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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