Selective conversion of methanol to propylene over highly dealuminated mordenite: Al location and crystal morphology effects

doi:10.1016/S1872-2067(20)63726-3

Abstract

Abstract:

The growing consumption of light olefins has stimulated intensive researches on methanol to olefin (MTO) process which possesses great advantages for coal conversion to value-added chemicals in an environmentally benign way. The catalysts commonly used for MTO process faces several challenges such as poor selectivity control, low hydrothermal stability and short lifetime. In the present study, we prepared a series of mordenite zeolites with variable Al contents (Si/Al molar ratios of 51-436) by a sequential dealumination treatment of air-calcination and acid leaching. The textural properties, acidity and Al location before and after the dealumination treatment have been systematically studied and their effect on MTO especially the methanol to propylene (MTP) performance was thoroughly investigated. The mordenite zeolites with the Si/Al ratios over 150 selectively catalyzed methanol conversion in the MTP pathway, providing a high propylene selectivity of 63% and propylene/ethylene ratio of > 10. Compared to the low-silica MOR catalysts, highly dealuminated MOR showed much higher stability and longer lifetime, which can be further enhanced via harsh hydrothermal pretreatment. Furthermore, the lifetime was highly related to the crystal size along c-axis. The excellent performance of highly dealuminated MOR is likely ascribed to the mesopores formed upon dealumination and the scarce Al sites located in the T sites shared by the 8-member ring (MR) side pockets and 12-MR pore channels.

Key words: Mordenite, Methanol to propylene, Dealumination, Al location, Crystal size

Li Ren, Bowen Wang, Kun Lu, Rusi Peng, Yejun Guan, Jin-gang Jiang, Hao Xu, Peng Wu. Selective conversion of methanol to propylene over highly dealuminated mordenite: Al location and crystal morphology effects[J]. Chinese Journal of Catalysis, 2021, 42(7): 1147-1159.

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URL: https://www.cjcatal.com/EN/10.1016/S1872-2067(20)63726-3

https://www.cjcatal.com/EN/Y2021/V42/I7/1147

Figures/Tables 14

Fig. 1. XRD patterns of the M-200 samples with different Si/Al ratios. (1) M-200(6); (2) M-200(51); (3) M-200(98); (4) M-200(171); (5) M-200(274); (6) M-200(436). The numbers in parentheses indicate the Si/Al atomic ratios, the same below.

Table 1 Textural properties of various M-200 samples.

Sample	Si/Al ratio	Surface area (m² g^-1)			Pore volume (cm³ g^-1)
Sample	Si/Al ratio	S_total^a	S_micro^b	S_ext^c	V_total^d	V_micro^b	V_meso^e
M-200(6)	6	494	481	13	0.27	0.19	0.08
M-200(51)	51	496	446	50	0.30	0.18	0.12
M-200(98)	98	493	445	48	0.32	0.18	0.14
M-200(171)	171	526	463	63	0.36	0.18	0.18
M-200(274)	274	525	477	48	0.35	0.19	0.16
M-200(436)	436	525	477	48	0.34	0.19	0.15

Scheme 1. (a) Mordenite (MOR topology) structure showing four tetrahedral atoms (T1-T4) and ten oxygen atoms (O1-O10) with different locations. The main 12-MR channels and small 8-MR channels, parallelly running along the c-axis, are interconnected by 8-MR side pockets (marked in green and orange) along the b-axis. T1 is located in the junction of 8-MR and 12-MR pores, T2 and T4 are located in the intersection of 8-MR side pockets and 12-MR channels, while T3 is situate in the intersection of 8-MR side pockets and 8-MR channels, (b) 8-MR channels (marked in blue), 12-MR channels (marked in green) and the 8-MR side pockets interconnecting 12- and 8-MR channels (marked in orange) in the MOR structure.

Fig. 2. Deconvoluted FTIR spectra of M-200(6) (A) and M-200(98) (B) in the OH vibration region.

Fig. 3. CD3CN adsorbed FT-IR spectra of various M-200 samples at 423 K. (a) M-200(6); (b) M-200(51); (c) M-200(98); (d) M-200(171); (e) M-200(274); (f) M-200(436).

Fig. 4. The changes of relative Al distributions with the Al content. ASP: the sum area of the 2315 and 2277 cm-1 bands related to the Al ions in 8-MR side pockets; A12MR: the area of 2297 cm-1 band associated to the Al ions in 12-MR; AF: the total area of 2277, 2297 and 2315 cm-1 bands assigned to the whole Al ions in framework.

Fig. 5. Methanol conversion and the product selectivities as a function of time on stream over M-200(6) (a) and M-200(274) (b). Reaction conditions: catalyst, 0.1 g; WHSV = 1 h-1; temperature, 723 K; N2 gas flow rate, 20 mL min-1.

Table 2 The product distribution in the MTP reaction over the M-200(x) samples.

Catalyst	MeOH conv. (%)	P/E ratio	HTI	ethylene/2MBu	Product selectivity (%)
Catalyst	MeOH conv. (%)	P/E ratio	HTI	ethylene/2MBu	CH₄	C_2-4⁰	C₂⁼	C₃⁼	C₄⁼	C₅₊^N	Ar.	C_2-4⁼
M-200(6)	100	0.3	0.40	129.7	32.8	12.8	38.9	12.0	2.3	0.3	0.9	53.2
M-200(51)	100	3.7	0.38	8.6	12.2	6.0	12.2	44.9	10.0	4.8	9.9	67.1
M-200(98)	100	6.0	0.14	6.0	2.1	4.0	9.8	58.2	16.1	5.1	4.7	84.1
M-200(171)	99.1	9.5	0.13	2.9	0.7	3.9	6.7	62.6	17.6	7.9	0.6	86.9
M-200(274)	98.9	10.9	0.08	2.3	0.5	2.9	6.2	63.2	18.6	8.0	0.6	88.0
M-200(436)	98.9	11.6	0.08	2.1	0.5	2.7	5.5	63.9	18.5	8.6	0.3	87.9

Fig. 6. (a) Dependence of propylene selectivity on the relative percentage of Al ions in 12-MR vs. the total Al ions; (b) Dependence of propylene selectivity, P/E ratio and MTP lifetime on the Al content. The propylene selectivity and P/E ratio were obtained at TOS = 2 h, while the lifetime is defined as the time period with methanol conversion over 80%. Reaction conditions: catalyst, 0.1 g; WHSV = 1 h-1; temperature, 723 K; N2 gas flow rate, 20 mL min-1.

Fig. 7. Methanol conversion and product selectivity as a function of reaction temperatures (a) and weight space velocities (b) over M-200(274) catalyst. Reaction conditions: catalyst, 0.1 g; N2 gas flow rate, 20 mL min-1; (a) WHSV = 1 h-1; (b) temperature, 723 K.

Table 3 The product distribution in the MTP reaction over the MOR zeolites with different morphologies at comparable Si/Al ratios.

Catalyst	MeOH conv. (%)	P/E ratio	Product selectivity (%)
Catalyst	MeOH conv. (%)	P/E ratio	C_1-4⁰	C₂⁼	C₃⁼	C₄⁼	C₅₊^N	Ar.
M-50(154)	98.5	9.9	4.7	5.8	57.2	20.4	11.4	0.5
M-200(171)	99.1	9.5	4.7	6.7	62.6	17.5	7.9	0.6
M-400(190)	98.8	11.1	2.2	5.7	63.4	19.1	8.9	0.7
M-R(196)	99.3	10.0	3.8	6.2	62.6	18.1	8.8	0.5

Fig. 8. (a) Methanol conversion and propylene selectivity as a function of TOS over M-50(154) (1), M-200(171) (2), M-400(190) (3) and M-R(196) (4); (b) The propylene selectivity (TOS = 2 h) and the lifetime as a function of the crystal length along c-axis. Reaction conditions: catalyst, 0.1 g; WHSV = 1 h-1; temperature, 723 K; N2 gas flow rate, 20 mL min-1.

Fig. 9. GC-MS chromatograms of the hydrocarbon species retained on various used M-200 catalysts after MTP reaction at different temperature at TOS of 2 min. (1) M-200(6), 623 K; (2) M-200(274), 623 K; (3) M-200(274), 723 K. The MTP reaction conditions: catalyst, 0.1 g; WHSV = 1 h-1; N2 gas flow rate, 20 mL min-1.

Scheme 2. The reaction mechanism of the methanol to propylene over the dealumianted mordenite catalyst.

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