Determination of mass transfer behavior of typical products of MTO (methanol to olefins) reactions over HZSM-5 zeolite
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摘要: 分子筛催化甲醇制烯烃反应（MTO）是典型的扩散主导反应过程，运用频率响应技术系统研究了几种典型产物分子（乙烯/乙烷、丙烯/丙烷、苯）在HZSM-5分子筛上的扩散行为。结果表明，频率响应法成功辨析了不同产物分子的传质规律，证实C2和C3烃分子在HZSM-5微孔孔道内具有相近的扩散速率，但由于受晶体表面阻碍效应影响不同，乙烷分子可自由进出HZSM-5分子筛孔道，而丙烷分子则受到较显著的微孔孔道扩散限制。另外，苯分子的扩散速率显著小于C2和C3分子，且苯分子受晶体表面阻抗效应的影响较小。本研究结果可用于解释HZSM-5分子筛在MTO反应中产物选择性的特点及表面结焦原因，进而从传质角度为高活性、选择性以及稳定性的高效甲醇转化制烃催化剂的定向开发提供理论指导。Abstract: Methanol conversion to olefins (MTO) catalyzed by zeolite catalysts is a typical diffusion dominated reaction process. In this paper, the diffusion behavior of several typical product molecules (ethylene/ethane, propylene/propane, benzene) on a HZSM-5 zeolite was systematically studied by using Frequency Response method. The results show that the mass transfer regularity of the product molecules have been successfully determined by the Frequency Response method. It is confirmed that the diffusion rates of C2 and C3 hydrocarbon molecules within the HZSM -5 micropores are similar, but the effects of the surface resistance are different. So, the C2 molecules can freely go in and out of the channels of the HZSM-5 zeolite, while the diffusion of C3 molecules is significantly affected by the channel diffusion limitation. In addition, the diffusion rate of benzene molecules is observably lower than that of C2 and C3 molecules, and the resistant effects of benzene molecules caused by the zeolite crystal surface are not serious. The conclusions obtained in this study can be used to explain the product selectivity of MTO reaction over HZSM-5 zeolites and the coking mechanism of the catalyst, and provide the mass transfer theoretical guidance for the preparation of the MTO catalysts with excellent performance.
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Key words:
- HZSM-5 /
- methanol conversion /
- mass transfer behavior /
- frequency response method /
- surface barrier
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图 1 频率响应装置示意图[27]
1: sorbate inlet; 2: valve; 3: electromagnet; 4: armature; 5: bellow; 6: rotary and turbo drag pump; 7: computer workstation; 8: sample cell; 9: vacuum flange connections; 10: pressure transducer; 11: type 270 signal conditioner; 12: pressure reference
Figure 1 Schematics of the frequency response apparatus[27]
图 2 扩散(a)和吸附(b)过程频率响应(FR)谱图分析
Figure 2 Frequency response (FR) spectrum analysis of diffusion (a) and adsorption (b) process
图 3 表面阻抗扩散的理论FR谱图
Figure 3 Theoretical FR spectra for diffusion with surface resistance
图 4 HZSM-5分子筛物性表征数据
(a)-(c): SEM images; (d): XRD pattern; (e): NH3-TPD profile; (f): nitrogen adsorption and desorption isotherm
Figure 4 Physical characterization results of the HZSM-5 zeolite
图 5 乙烯在HZSM-5分子筛上的FR谱图
Figure 5 FR curves of ethene in the HZSM-5 zeolite at 303 K under 66 Pa (a) and 133 Pa (b)
(in-phase (□) and out-of-phase (○) characteristic function)
图 6 乙烷在HZSM-5分子筛上的FR谱图
Figure 6 FR curves of ethane in the HZSM-5 zeolite at 303 K under 66 Pa (a) and 133 Pa (b)
(in-phase (□) and out-of-phase (○) characteristic function)
图 7 丙烯在HZSM-5分子筛上的FR谱图
Figure 7 FR curves of propene in the HZSM-5 zeolite at 303 K under 66 Pa (a) and 133 Pa (b)
(in-phase (□) and out-of-phase (○) characteristic function)
图 8 丙烷在HZSM-5分子筛上的FR谱图
Figure 8 FR curves of propane in the HZSM-5 zeolite at 303 K under 66 Pa (a) and 133 Pa (b)
(in-phase (□) and out-of-phase (○) characteristic function)
图 9 苯在HZSM-5分子筛上的FR谱图
Figure 9 FR curves of benzene in the HZSM-5 zeolite at 303 K under 66 Pa (a) and 133 Pa (b)
(in-phase (□) and out-of-phase (○) characteristic function)
表 1 乙烯和乙烷在HZSM-5分子筛上传质过程的时间常数和响应强度值
Table 1 Time constant and response intensity values of ethene and ethane in the process of mass transfer in the HZSM-5 zeolite
p/Pa Ethylene Ethane f1/ s-1 f2/ s-1 K1 K2 f1/ s-1 f2/ s-1 K1 K2 66 0.06 15.9 0.012 0.19 0.05 12.48 0.009 0.085 133 0.08 27.1 0.018 0.15 0.047 12.28 0.009 0.07 
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表 2 丙烯和丙烷在HZSM-5分子筛上传质过程的时间常数和响应强度值
Table 2 Time constant and response intensity values of propene and propane in the process of mass transfer in the HZSM-5 zeolite
p/Pa Propylene Propane f1/ s-1 f2/ s-1 f3/ s-1 K1 K2 K3 f1/ s-1 f2/ s-1 f3/ s-1 K1 K2 K3 66 0.08 0.80 15.93 0.09 0.07 0.67 0.03 0.51 6.43 1.05 0.57 1.87 133 0.06 0.48 35.04 0.04 0.03 0.44 0.03 0.88 15.92 0.69 0.41 1.15
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表 3 苯在HZSM-5分子筛上传质过程的时间常数和响应强度值
Table 3 Time constant and response intensity values of benzene in the process of mass transfer in the HZSM-5 zeolite
p/Pa Benzene f1/ s-1 f2/ s-1 K1 K2 66 0.03 1.84 0.47 0.09 133 0.036 0.48 0.23 0.08
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