Polycrystalline ZSM-5 Aggregates Induced by Seed and Catalytic Performance in Methanol to Hydrocarbon

doi:10.15541/jim20240016

Abstract

Abstract:

Synthesis of ZSM-5 zeolite typically utilizes small molecule polyamines or quaternary ammonium salts as organic structure guiding agent (OSDA). By contrast, the OSDA-free hydrothermal synthesis system eliminates the use of organic templates and the subsequent calcination procedure. This not only reduces the cost of synthesis, but also prevents environmental pollution from the combustion of organic templates, representing an eco-friendly approach. Despite this, literature suggests that even so-called template-free synthesis systems often involve trace amount of organic substances like alcohol. In the present work, a calcined commercial ZSM-5 zeolite was served as seed, with sodium aluminate as aluminum source and silica sol as silicon source, ensuring an entirely template-free synthesis system. Polycrystalline ZSM-5 aggregates consisted of rod-like nanocrystals were successfully prepared in the completely OSDA-free system. Effects of the Si/Al ratio in ZSM-5 seed, dosage and crystallization conditions such as crystallization temperature and crystallization time on ZSM-5 synthesis were investigated. The results show that a highly crystallinity ZSM-5 aggregate consisting of primary nano-sized crystals less than 100 nm is produced from a gel precursor with 5.6% (in mass) seed after hydrothermal treatment for 48 h. Furthermore, the Si/Al ratio in ZSM-5 seed has little effect on the topological structure and pore structure of the synthesized samples. However, the seeds with a low Si/Al ratio facilitate faster crystallization of zeolite and enhance the acidity, especially the strong acid centers, of the catalyst. The catalytic performance of the synthesized polycrystalline ZSM-5 was evaluated during dehydration of methanol and compared with a commercial reference ZSM-5r. The results exhibit that as compared with the reference catalyst, the fabricated sample has a longer catalytic lifetime (16 h vs 8 h) attributed to its hierarchical pores derived from the loosely packed primary nanoparticles. Additionally, the prepared polycrystalline catalyst also exhibits a higher aromatics selectivity (28.1%-29.8% vs 26.5%).

Key words: ZSM-5, polycrystalline aggregate, crystal seed, structure guiding agent

CLC Number:

TQ174

WANG Xuchang, JIAO Chuyu, JI Zhuo, JIAO Qirui, QIN Bo, DU Yanze, ZHENG Jiajun, LI Ruifeng. Polycrystalline ZSM-5 Aggregates Induced by Seed and Catalytic Performance in Methanol to Hydrocarbon[J]. Journal of Inorganic Materials, 2024, 39(8): 945-954.

Figures/Tables 22

Fig. 1 XRD patterns of the as-synthesized samples added with different amounts of the seeds (a) Z518-0-48; (b) Z518-2.8-48; (c) Z518-5.6-48; (d) Z518-11.2-48.

Fig. 2 SEM images of the samples prepared with different amounts of seeds (a) Z518-0-48; (b) Z518-2.8-48; (c) Z518-5.6-48; (d) Z518-11.2-48 The part rendered in red may be MOR zeolite; Colorful figures are available on website

Fig. 3 XRD patterns of the samples with different crystallization time (a) and corresponding crystallization kinetic curve (b) Relative crystallinity (RC) was calculated by comparing the peak areas of XRD patterns in the range of 2θ=22.5°-25° of the samples with those of Z518-5.6-48

Fig. 4 FT-IR spectra of the samples prepared from the same gel precursor with different crystallization time

Fig. 5 SEM and TEM images of the as-synthesized samples with different crystallization time SEM images of (a) Z518-5.6-0, (b) Z518-5.6-4, (c) Z518-5.6-8, (d) Z518-5.6-12, (e) Z518-5.6-14, (f) Z518-5.6-24, (g) Z518-5.6-48 and (h) NK-18; (i, j) TEM images of Z518-5.6-48

Fig. 6 N2 adsorption-desorption isotherms (a) and corresponding BJH pore size distribution curves (b) of the samples prepared from the same gel precursor treated with different crystallization time

Table 1 Textural properties of the as-synthesized samples with different crystallization time

Sample	S_BET/(cm²·g^-1)	S_mic/(cm²·g^-1)	S_ext/(cm²·g^-1)	V_total/(cm³·g^-1)	V_mic/(cm³·g^-1)	V_ext/(cm³·g^-1)	^*RC/%
Z518-5.6-4	66	13	51	0.375	0.002	0.373	16.0
Z518-5.6-8	48	26	22	0.089	0.009	0.080	18.2
Z518-5.6-12	105	84	21	0.093	0.031	0.062	20.5
Z518-5.6-14	396	355	41	0.198	0.134	0.064	90.4
Z518-5.6-24	390	351	40	0.200	0.131	0.069	96.7
Z518-5.6-48	392	351	41	0.213	0.132	0.081	100

Fig. 7 SEM images and corresponding XRD patterns of the samples synthesized at different crystallization temperatures SEM images of (a) Z518-5.6(100), (b) Z518-5.6(120), (c) Z518-5.6(140), (d) Z518-5.6(160) and (e) Z518-5.6(180); (f) Corresponding XRD patterns of the samples synthesized at different crystallization temperatures All samples were obtained from the same gel precursor as the one yielded the sample Z518-5.6

Scheme 1 Formation process of the polycrystalline ZSM-5 aggregates induced by seeds

Table 2 Physical and chemical properties of the samples yielded from the similar gel precursor induced by a crystal seed with different Si/Al ratios

Sample	S_BET/ (cm²·g^-1)	S_mic/ (cm²·g^-1)	V_total/ (cm³·g^-1)	V_mic/ (cm³·g^-1)	V_meso/ (cm³·g^-1)	^*RC/%	^**Si/Al	Acid density/(μmol·g^-1)
Sample	S_BET/ (cm²·g^-1)	S_mic/ (cm²·g^-1)	V_total/ (cm³·g^-1)	V_mic/ (cm³·g^-1)	V_meso/ (cm³·g^-1)	^*RC/%	^**Si/Al	Weak	Medium	Strong	Total
Z518-5.6-48	392	352	0.19	0.13	0.06	100	11.3	218	65	293	576
Z527-5.6-48	376	333	0.19	0.12	0.07	99	11.3	212	83	224	519
Z5200-5.6-48	356	314	0.19	0.12	0.07	99	11.6	205	79	221	505
ZSM-5r	377	343	0.14	0.14	0.03	100	17.4	168	53	236	457

Fig. 8 MTH reaction performance of the samples using crystal seeds with different Si/Al ratios Reaction conditions: T=430 ℃, ptotal=1 atm, WHSV=2.4 h-1, catalysts weight=200 mg

Fig. 9 Selectivity of light olefins over the catalysts with time on stream (a) Z518-5.6-48; (b) Z527-5.6-48; (c) Z5200-5.6-48; (d) ZSM-5r Reaction conditions: T=430 ℃, ptotal=1 atm, WHSV=2.4 h-1, catalysts weight=200 mg

Table S1 Pore structure parameter and relative crystallinity of the as-synthesized samples

Sample	S_BET/(m²·g^-1)	S_mic/(m²·g^-1)	S_ext/(m²·g^-1)	V_total/(cm³·g^-1)	V_mic/(cm³·g^-1)	RC/%
Z518-5.6-48	392	351	41	0.213	0.132	100
Z518-11.2-48	388	346	42	0.199	0.128	99

Fig. S1 SEM images of the crystal seeds (a1, a2) NK-18; (b1, b2) NK-27; (c1, c2) NK-200

Fig. S2 SEM images of (a, b) Z518-2.8-48, (c) Z518-5.6-48, and (d) Z518-11.2-48

Fig. S3 N2 adsorption-desorption isotherms (a) and corresponding BJH pore size distribution curves (b) of the as-synthesized samples

Fig. S4 XRD patterns (a) and N2 adsorption-desorption isotherms (b) of the samples prepared in an OSDA-free system induced by seeds with different Si/Al ratios (a) Z518-5.6-48; (b) Z527-5.6-48; (c) Z5200-5.6-48

Fig. S5 SEM images of the samples yielded from a similar gel precursor induced by seeds with different Si/Al ratios (a, d) Z518-5.6-48; (b, e) Z527-5.6-48; (c, f) Z5200-5.6-48

Fig. S6 EDS analyses of the samples induced by the seeds with different Si/Al ratios

Fig. S7 NH3-TPD profiles of samples Z518-5.6-48, Z527-5.6-48, and Z5200-5.6-48 induced by seeds with different Si/Al ratios and the reference ZSM-5r

Fig. S8 Selectivity of final products over ZSM-5-x catalysts with time on stream (a) Z518-5.6-48; (b) Z527-5.6-48; (c) Z5200-5.6-48; (d) ZSM-5r. Reaction conditions: T=430 ℃, ptotal=1 atm, WHSV=2.4 h-1, catalysts weight=200 mg

Fig. S9 SEM images of the commercial reference ZSM-5r

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