Catalytic roles of the acid sites in different pore channels of H-ZSM-5 zeolite for methanol-to-olefins conversion

doi:10.1016/S1872-2067(20)63732-9

Abstract

Abstract:

H-ZSM-5 zeolite is a typical catalyst for methanol-to-olefins (MTO) conversion. Although the performance of zeolite catalysts for MTO conversion is related to the actual location of acid sites in the zeolite framework, the catalytic roles of the acid sites in different pore channels of the H-ZSM-5 zeolite are not well understood. In this study, the MTO reaction network, involving the aromatic cycle, alkene cycle, and aromatization process, and also the diffusion behavior of methanol feedstock and olefin and aromatic products at different acid sites in the straight channel, sinusoidal channel, and intersection cavity of H-ZSM-5 zeolite was comparatively investigated using density functional theory calculations and molecular dynamic simulations. The results indicated that the aromatic cycle and aromatization process occurred preferentially at the acid sites in the intersection cavities with a much lower energy barrier than that at the acid sites in the straight and sinusoidal channels. In contrast, the formation of polymethylbenzenes was significantly suppressed at the acid sites in the sinusoidal and straight channels, whereas the alkene cycle can occur at all three types of acid sites with similar energy barriers and probabilities. Consequently, the catalytic performance of H-ZSM-5 zeolite for MTO conversion, including activity and product selectivity, can be regulated properly through the purposive alteration of the acid site distribution, viz., the location of Al in the zeolite framework. This study helps to elucidate the relation between the catalytic performance of different acid sites in the H-ZSM-5 zeolite framework for MTO conversion, which should greatly benefit the design of efficient catalyst for methanol conversion.

Key words: Methanol-to-olefins, H-ZSM-5 zeolite, Acid site distribution, Density functional theory calculation, Molecular dynamic simulation

Sen Wang, Zhikai Li, Zhangfeng Qin, Mei Dong, Junfen Li, Weibin Fan, Jianguo Wang. Catalytic roles of the acid sites in different pore channels of H-ZSM-5 zeolite for methanol-to-olefins conversion[J]. Chinese Journal of Catalysis, 2021, 42(7): 1126-1136.

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URL: https://www.cjcatal.com/EN/10.1016/S1872-2067(20)63732-9

https://www.cjcatal.com/EN/Y2021/V42/I7/1126

Figures/Tables 9

Fig. 1. 120T cluster models illustrating the intersection cavities, sinusoidal channels, and straight channels (T12, T10, and T8 sites, respectively) of H-ZSM-5 zeolite (Si, yellow; O, red; Al, pink; and H, white). Schematic topological structure of H-ZSM-5 zeolite. Methanol-to-olefins conversion reaction network including aromatic and alkene cycles following the dual-cycle hydrocarbons pool mechanism.

Fig. 2. Reaction networks for the aromatic cycle, alkene cycle, and aromatization process during methanol-to-olefins conversion over H-ZSM-5 zeolite.

Fig. 3. Free energy profiles of (a) ethene (e) and (b) propene (p) formation via the aromatic cycle at 723 K during methanol-to-olefins conversion at the acid sites in the intersection cavities, sinusoidal channels, and straight channels of H-ZSM-5 zeolite. A1 and A2 denote the adsorption of methanol and co-adsorption of p-xylene, respectively, M1-M3 denote methylation reactions, D1 and D2 denote deprotonation reactions, S1-S4 denote alkyl side-chain shifting reactions, and E1 denotes olefin elimination reaction.

Table 1 Calculated free energy barrier (ΔGint≠), rate constant (k), and reaction free energy (ΔGR) of each reaction step for the formation of ethene and propene via the aromatic cycle of methanol-to-olefins conversion at the acid sites in the intersection cavities (T12), sinusoidal channels (T10), and straight channels (T8) of H-ZSM-5 zeolite at 723 K.

Step	ΔG_int^≠ (kJ mol^-1)			k (s^-1)			ΔG_R (kJ mol^-1)
Step	T12	T10	T8	T12	T10	T8	T12	T10	T8
Ethene (e) formation
M1	140	141	171	1.17 × 10³	1.04 × 10³	6.11 × 10⁰	57	60	107
M2	126	136	135	1.23 × 10⁴	3.01 × 10³	3.01 × 10³	-29	-8	-32
D1	91	58	58	3.87 × 10⁶	9.65 × 10⁸	9.75 × 10⁸	39	41	32
S1(e)	50	37	76	3.74 × 10⁹	2.96 × 10¹⁰	4.73 × 10⁷	-30	-39	-13
S2(e)	80	60	101	2.68 × 10⁷	6.70 × 10⁸	7.97 × 10⁵	-16	-2	-1
S3(e)	114	115	117	8.64 × 10⁴	6.75 × 10⁴	5.32 × 10⁴	52	-6	36
S4(e)	70	53	58	1.37 × 10⁸	2.12 × 10⁹	9.92 × 10⁸	-45	-28	-34
E1(e)	103	106	122	5.47 × 10⁵	3.38 × 10⁵	2.50 × 10⁴	-4	-7	-16
Propene (p) formation
M3	114	124	138	8.91 × 10⁴	1.64 × 10⁴	1.50 × 10³	-30	-69	-47
D2	81	77	53	2.03 × 10⁷	4.03 × 10⁷	2.25 × 10⁹	57	49	22
S1(p)	57	36	74	1.05 × 10⁹	4.02 × 10¹⁰	7.05 × 10⁷	-17	-44	-27
S2(p)	82	78	82	1.92 × 10⁷	3.34 × 10⁷	1.85 × 10⁷	7	22	5
S3(p)	75	70	135	5.51 × 10⁷	1.32 × 10⁸	2.85 × 10³	28	36	63
S4(p)	27	42	58	1.67 × 10¹¹	1.32 × 10¹⁰	9.68 × 10⁸	-8	6	-54
E1(p)	30	25	46	9.52 × 10¹⁰	2.34 × 10¹¹	7.10 × 10⁹	-43	-58	-60
AFE ^*	280	293	294

Fig. 4. Free energy profiles for propene formation via the alkene cycle of methanol-to-olefins conversion at the acid sites in the intersection cavities, sinusoidal channels, and straight channels of H-ZSM-5 zeolite at 723 K. A1 and A2 denote the adsorption of methanol and co-adsorption of propene, respectively, M1-M3 denote methylation reactions, D1 and D2 denote deprotonation reactions, and E1 denotes propene elimination reaction.

Table 2 Calculated free energy barrier (ΔGint≠), rate constant (k) and reaction free energy (ΔGR) for each reaction step of the formation of ethene and propene via the alkene cycle of methanol-to-olefins conversion at the acid sites in the intersection cavities (T12), sinusoidal channels (T10), and straight channels (T8) of H-ZSM-5 zeolite at 723 K.

Step	ΔG_int^≠ (kJ mol^-1)			k (s^-1)			ΔG_R (kJ mol^-1)
Step	T12	T10	T8	T12	T10	T8	T12	T10	T8
M1	123	122	134	1.94 × 10⁴	2.30 × 10⁴	3.08 × 10³	38	25	-17
M2	129	120	131	7.17 × 10³	2.97 × 10⁴	5.11 × 10³	27	32	-15
M3	138	137	130	1.73 × 10³	2.02 × 10³	6.07 × 10³	32	38	-29
D1	56	30	39	1.45 × 10⁹	1.06 × 10¹¹	2.26 × 10¹⁰	-24	-37	-54
D2	66	13	17	2.71 × 10⁸	1.73 × 10¹²	9.06 × 10¹¹	-14	-38	-49
E1	109	100	118	1.95 × 10⁵	9.66 × 10⁵	4.74 × 10⁴	39	43	66
E2	156	166	161	8.97 × 10¹	1.41 × 10¹	3.51 × 10¹	104	74	60
AFE ^*	200	208	209

Table 3 Calculated free energy barrier (ΔGint≠), rate constant (k), and reaction free energy (ΔGR) for each reaction step of the formation of benzene via aromatization and the formation of polymethylbenzenes (polyMBs) via successive methylations of benzene at the acid sites in the intersection cavities (T12), sinusoidal channels (T10), and straight channels (T8) of H-ZSM-5 zeolite at 723 K.

Step	ΔG_int^≠ (kJ mol^-1)			k (s^-1)				ΔG_R (kJ mol^-1)
Step	T12	T10	T8	T12	T10	T8		T12	T10			T8
Benzene formation
D1	58	52	32	9.04 × 10⁸	2.68 × 10⁹		7.84 × 10¹⁰	-9		5		-49
HT1	136	159	166	2.43 × 10³	5.15 × 10¹		1.58 × 10¹	90		107		138
C1	75	73	96	5.57 × 10⁷	7.88 × 10⁷		1.79 × 10⁶	-44		-11		24
D2	16	23	30	1.09 × 10¹²	3.17 × 10¹¹		1.07 × 10¹¹	-46		-39		-71
HT2	137	138	148	1.80 × 10³	1.63 × 10³		3.09 × 10²	-41		-1		-11
D3	33	38	25	6.25 × 10¹⁰	2.64 × 10¹⁰		2.39 × 10¹¹	-3		-9		-30
HT3	122	121	132	2.43 × 10⁴	2.78 × 10⁴		4.54 × 10³	-109		-86		-78
D4	2	17	16	1.14 × 10¹³	9.31 × 10¹¹		1.13 × 10¹²	-93		-100		-115
AFE^*	293	311	337
PolyMBs formation
M1	175	161	166	3.71 × 10⁰	3.59 × 10¹	1.52 × 10¹		75		-16		86
M2	157	162	126	7.39 × 10¹	2.86 × 10¹	1.24 × 10⁴		59		61		46
M3	161	135	140	3.80 × 10¹	2.81 × 10¹	1.20 × 10³		79		59		92
M4	159	149	168	5.13 × 10¹	2.73 × 10²	1.06 × 10¹		20		26		30
M5	149	151	170	2.73 × 10²	1.70 × 10²	7.77 × 10⁰		15		20		19
M6	153	186	227	1.37 × 10²	5.21 × 10^-1	5.70 × 10^-4		16			43	21

Fig. 5. Free energy profiles for the formation of (a) benzene and (b) polymethylbenzenes via the successive methylations of the benzene ring during methanol-to-olefins conversion at the acid sites in the intersection cavities, sinusoidal channels, and straight channels of H-ZSM-5 zeolite at 723 K. Here, D1-D4 denote deprotonation reactions, HT1-HT3 denote hydride transfer reactions, C1 denotes cyclization of olefin carbenium ion, and M1-M6 denote benzene ring methylation reactions.

Fig. 6. Contour lines for the diffusion of (a) methanol, (b) propene, (c) p-xylene, and (d) hexamethylbenzene molecules in the straight and sinusoidal channels of H-ZSM-5 zeolite at 723 K. The density maps were constructed according to the two-dimensional projections of the mass centers of guest molecules in a 2 × 2 × 2 supercell. The color bars represent the density of guest molecules distributed in different channels with the unit of 1; the red areas with higher density values indicate higher diffusion probability.

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