Preparation of Hierarchical Porous HZSM-5 and Its Application in Light Paraffin Aromatization†

doi:10.7503/cjcu20140987

Abstract

Abstract:

The HZSM-5 molecular sieves were modified by 0.2 mol/L NaOH aqueous solution for 30, 120 and 300 min, respectively and the as-prepared catalysts were characterized by the X-ray diffraction, N₂ adsorption-desorption, transmission electron microscopy, ammonia temperature-programmed desorption(NH₃-TPD) and Fourier transform infrared spectroscopy of adsorbed pyridine. The catalytic activity, stability and BTEX selectivity of alkali-treated HZSM-5 in the light paraffin aromatization were also investigated. The results showed that the degree of crystallization decreased and the mesopores were produced in the HZSM-5 molecular sieves with the increase of alkali-treatment time, and at the same time the acidic properties of HZSM-5 could also be tuned by alkali treatment, the Brönsted acid sites were decreased while the Lewis acid sites were increased with the increase of alkali modification time. Catalytic evaluation of light paraffin aromatization over modified HZSM-5 revealed that HZSM-5 treated by NaOH for 120 min exhibited the best aromatization activity, stability and BTEX selectivity due to its suitable pore structure and acidic distribution.

Key words: Alkali treatment, Hierarchical porous HZSM-5 molecular sieve, Light paraffin aromatization, Stability, Brösted acid and Lewis acid, Selectivity

CLC Number:

O643

TrendMD:

ZHANG Ruizhen, WANG Cui, XING Pu, WEN Shaobo, WANG Jian, ZHAO Liangfu, LI Yuping. Preparation of Hierarchical Porous HZSM-5 and Its Application in Light Paraffin Aromatization^†[J]. Chem. J. Chinese Universities, 2015, 36(4): 725.

Figures/Tables 14

Fig.1 XRD patterns of catalysts a. HZSM-5; b. 30 minAT-HZSM-5; c. 120 minAT-HZSM-5; d. 300 minAT-HZSM-5.

Fig.2 N2 adsorption-desorption isotherms(A) and pore distributions of catalysts(B) a. HZSM-5; b. 30 minAT-HZSM-5; c. 120 minAT-HZSM-5; d. 300 minAT-HZSM-5.

Table 1 Structure parameters of different samples

Sample	D_average/nm	S_BET/(m²·g^-1)	$S micropore a$ /(m²·g^-1)	$S external a$ /(m²·g^-1)	$V total b$ /(cm³·g^-1)	$V micropore a$ /(cm³·g^-1)
HZSM-5	1.89	401.79	310.38	91.41	0.19	0.12
30 minAT-HZSM-5	1.94	402.15	296.83	105.32	0.20	0.12
120 minAT-HZSM-5	3.14	409.18	239.72	169.46	0.32	0.10
300 minAT-HZSM-5	4.20	408.41	259.57	148.84	0.43	0.11

Table 1 Structure parameters of different samples

Sample	D_average/nm	S_BET/(m²·g^-1)	$S micropore a$ /(m²·g^-1)	$S external a$ /(m²·g^-1)	$V total b$ /(cm³·g^-1)	$V micropore a$ /(cm³·g^-1)
HZSM-5	1.89	401.79	310.38	91.41	0.19	0.12
30 minAT-HZSM-5	1.94	402.15	296.83	105.32	0.20	0.12
120 minAT-HZSM-5	3.14	409.18	239.72	169.46	0.32	0.10
300 minAT-HZSM-5	4.20	408.41	259.57	148.84	0.43	0.11

Fig.3 TEM images of 120 minAT-HZSM-5(A—C) and 300 minAT-HZSM-5(D—F) with different magnifications

Fig.4 NH3-TPD spectra of catalysts a. HZSM-5; b. 30 minAT-HZSM-5; c. 120 minAT-HZSM-5; d. 300 minAT-HZSM-5.

Fig.5 Py-IR spectra of catalysts at 200 ℃(A) and 450 ℃(B) a. HZSM-5; b. 30 minAT-HZSM-5; c. 120 minAT-HZSM-5; d. 300 minAT-HZSM-5.

Table 2 Distribution of Brönsted and Lewis acid sites of catalysts*

Sample	Acidic amount(c) at 200 ℃/(μmol·g^-1)			Acidic amount(c) at 450 ℃/(μmol·g^-1)
Sample	B	L	B/L	B	L	B/L
HZSM-5	161.16	12.04	13.39	135.08	16.22	8.33
30 minAT-HZSM-5	158.19	21.92	7.22	124.82	25.54	4.89
120 minAT-HZSM-5	125.97	43.93	2.87	89.03	39.94	2.23
300 minAT-HZSM-5	94.37	48.00	1.97	49.76	36.79	1.35

Table 3 Distribution of weak, mediate and strong acid of catalysts

Sample	Acidic amount/(μmol·g^-1)
Sample	Weak and mediate(200—450 ℃)	Strong(450 ℃)	Total(200 ℃)	Stong/Total(%)
HZSM-5	21.90	151.30	173.20	87.36
30 minAT-HZSM-5	29.75	150.36	180.11	83.48
120 minAT-HZSM-5	40.93	128.97	169.90	75.91
300 minAT-HZSM-5	55.82	86.55	142.37	60.79

Fig.6 Conversion of propane(A) and butane(B) over different catalysts a. HZSM-5; b. 30 minAT-HZSM-5; c. 120 minAT-HZSM-5; d. 300 minAT-HZSM-5.

Fig.7 Liquid yield with reaction time a. HZSM-5; b. 30 minAT-HZSM-5; c. 120 minAT-HZSM-5; d. 300 minAT-HZSM-5.

Fig.8 Accumulated liquid yield for 104 h a. HZSM-5; b. 30 minAT-HZSM-5; c. 120 minAT-HZSM-5; d. 300 minAT-HZSM-5.

Fig.9 Concentration of methane(A) and ethane(B) in the tail gas of aromatization reaction a. HZSM-5; b. 30 minAT-HZSM-5; c. 120 minAT-HZSM-5; d. 300 minAT-HZSM-5.

Fig.10 Selectivities of C5(A) and aromatic hydrocarbon(B) in the liquid product a. HZSM-5; b. 30 minAT-HZSM-5; c. 120 minAT-HZSM-5; d. 300 minAT-HZSM-5.

Fig.11 Selectivity of BTEX(A) and C9+(B) in aromatic hydrocarbon over different catalysts a. HZSM-5; b. 30 minAT-HZSM-5; c. 120 minAT-HZSM-5; d. 300 minAT-HZSM-5.

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