Brönsted equation kinetic modeling for quantitative relationship between activity and acidity strength distribution in oligomerization and aromatization of ethylene over ZSM-5 catalyst

doi:10.11949/0438-1157.20191536

Abstract

Abstract:

Ethylene catalyzed oligomerization and aromatization are the key processes for the high value of olefins and the synthesis of liquid fuels. The desorption activation energy of the probe molecule calculated in NH₃-TPD was used as a parameter of the acid site intensity distribution of the catalyst, which correlate the information of the acid intensity distribution with the acid-catalyzed reaction activity. According to the linear free energy theory, the Br?nsted equation theory was further developed as the reaction kinetic model for establishing quantitative relationships between different acid strength sites and hydrogen transfer, oligomerization, and aromatization elementary steps corresponding to alkane，light olefins and aromatic products in the oligomerization and aromatization of ethylene on the ZSM-5 catalyst with the different Si/Al molar ratios. Both the elementary steps of hydrocarbon conversion and the basic probe molecule adsorption/desorption on acid sites follow the carbocation mechanism. Therefore, the desorption activation energy is related to the reaction activation energy based on the linear free energy by introducing γ as the kinetic parameter of reaction sensitivity to the acid strength of catalyst, which has a definite physical meaning in catalytic reaction kinetic modeling. The analysis of γ of different acid strength sites can conclude that acid sites with different strengths have different coefficients of action on ethylene oligomerization and aromatization. The acid site with ammonia desorption activation energy (DAE) of 90 kJ·mol^-1 is the main active site for hydrogen transfer reaction, while the main active acid site for ethylene oligomerization is DAE 90 kJ·mol^-1 and 124 kJ·mol^-1.The acid site with DAE of 150 kJ·mol^-1 is the main active site for the aromatization reaction.

Key words: acidity, catalysis, zeolite, kinetic modeling, oligomerization, aromatization

摘要：

乙烯催化齐聚和芳构化是烯烃高值化和合成液体燃料的关键过程，采用NH₃-TPD谱图分析计算方法得到的探针分子脱附活化能作为催化剂的酸位点强度分布的参数，结合线性自由能理论在Br?nsted方程中引入酸强度对反应的影响因子γ作为动力学方程参数关联探针分子脱附活化能与反应活化能，明确固体酸酸强度的基准在催化动力学方程中的物理意义。建立同为MFI拓扑结构 Si /Al摩尔比为12.5、19、25、60、70的ZSM-5分子筛催化剂上不同酸强度位和乙烯齐聚及芳构化中氢转移、齐聚、芳构化各单元步骤以及对应的烷烃、低碳烯烃和芳烃产物分布的定量关系。根据拟合得到动力学参数γ可以发现不同强度的酸位点对乙烯齐聚及芳构化具有不同的作用系数。氨脱附活化能(DAE)为90 kJ·mol^-1的酸位点是氢转移反应的主要活性位点，而对于乙烯齐聚反应中主要的活性酸位点为DAE 90 kJ·mol^-1和124 kJ·mol^-1，DAE为150 kJ·mol^-1的酸位点是芳构化反应的主要活性位点。

关键词: 酸性, 催化, 沸石, 动力学模型, 齐聚, 芳构

CLC Number:

TQ 028.8

Fang JIN,Peng ZHANG,Guiying WU,Di WU. Brönsted equation kinetic modeling for quantitative relationship between activity and acidity strength distribution in oligomerization and aromatization of ethylene over ZSM-5 catalyst[J]. CIESC Journal, 2020, 71(5): 2076-2087.

金放,张鹏,吴桂英,吴迪. Brönsted方程动力学模型研究ZSM-5催化乙烯齐聚及芳构化活性和酸强度分布之间的定量关系[J]. 化工学报, 2020, 71(5): 2076-2087.

Figures/Tables 10

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T₀/K	(T₀+ΔT)/K	E/(kJ·mol^-1)	K_E/s^-1
348	398	51.9	1.1×10^-7
373	423	62.9	1.3×10^-8
398	448	71.7	1.8×10^-8
423	473	88.9	5.1×10^-9
448	498	105.6	9.4×10^-11
473	523	128.5	2.2×10^-11
523	573	145.5	6.8×10^-12
573	623	176.2	1.10×10^-13

T₀/K	(T₀+ΔT)/K	E/(kJ·mol^-1)	K_E/s^-1
348	398	51.9	1.1×10^-7
373	423	62.9	1.3×10^-8
398	448	71.7	1.8×10^-8
423	473	88.9	5.1×10^-9
448	498	105.6	9.4×10^-11
473	523	128.5	2.2×10^-11
523	573	145.5	6.8×10^-12
573	623	176.2	1.10×10^-13

样品	TOF/(mmol·h^-1·g^-1)	A_c_i/(mmol·h^-1·g^-1)
样品	TOF/(mmol·h^-1·g^-1)	C2	C3	C3⁼	C4⁼	C6A	C7A	C8A
19ZSM-5	67.76	0.51	0.37	28.20	22.67	1.93	12.66	11.65
25ZSM-5	62.06	0.45	0.54	23.68	15.36	1.75	9.80	12.46
60ZSM-5	48.91	0.25	0.24	18.08	10.68	1.32	6.17	6.23
70ZSM-5	45.41	0.16	0.25	13.78	9.87	0.83	5.33	5.63

样品	TOF/(mmol·h^-1·g^-1)	A_c_i/(mmol·h^-1·g^-1)
样品	TOF/(mmol·h^-1·g^-1)	C2	C3	C3⁼	C4⁼	C6A	C7A	C8A
19ZSM-5	67.76	0.51	0.37	28.20	22.67	1.93	12.66	11.65
25ZSM-5	62.06	0.45	0.54	23.68	15.36	1.75	9.80	12.46
60ZSM-5	48.91	0.25	0.24	18.08	10.68	1.32	6.17	6.23
70ZSM-5	45.41	0.16	0.25	13.78	9.87	0.83	5.33	5.63

产物	δ	γ	R²	RSS	TSS
C2	0.0005	0.0031	0.95	3.36×10^-9	6.35×10^-8
C3	0.0003	0.0072	0.77	1.17×10^-8	5.15×10^-8
C3⁼	0.0350	0.0025	0.86	1.62×10^-5	1.20×10^-4
C4⁼	0.0250	0.0021	0.85	1.15×10^-5	7.49×10^-5
C6A	0.0026	0.0018	0.99	1.24×10^-8	1.03×10^-6
C7A	0.0186	0.0010	0.94	2.07×10^-6	3.40×10^-5
C8A	0.0057	0.0100	0.98	1.29×10^-7	3.91×10^-5