Methanol to gasoline over zeolite ZSM-5: improved catalyst performance by treatment with HF

Abstract

Zeolite ZSM-5 (SiO₂/Al₂O₃ = 50) has been treated with hydrofluoric acid (HF) solutions, characterized with many techniques and studied in the conversion of methanol to gasoline (MTG). After the HF treatment, the crystallinity of the zeolite is retained; mesopores are generated and the mesopore volumes increase with the HF concentration; the SiO₂/Al₂O₃ ratios increase; the acidities of the samples and the Brönsted acid sites decrease, while the Lewis acid sites increase with the HF concentration. Aluminum from the framework was removed to an extent by the treatment. With increasing the concentrations of HF solution in the treatment, the stabilities of the ZSM-5 samples improve significantly in MTG. The results demonstrate that the acidity and the pore structure are two crucial factors in determining the catalytic properties.

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1. Introduction

Present estimates of the proven oil reserves reveal that they would last only for some 40 years at the current rate of consumption. Natural gas reserves are comparable but somewhat larger. The reserves of coal are even larger, representing about 170 years of supply at the current rate of consumption.^1,2 Based on these perspectives, technologies for the upgrading of coal or gas into liquid fuels or petrochemical building blocks become highly attractive.² Most such routes rely on the transformation of the carbon source into synthesis gas. Fischer–Tropsch synthesis (FTS), a surface catalyzed polymerization process, uses synthesis gas to produce hydrocarbons with a broad range of chain lengths and functionality.³ Alternatively, methanol may be produced from the synthesis gas, which, in turn, may then be reacted into light alkenes for petrochemistry or high octane gasoline upon some catalysts. Therefore, from this aspect, the conversion of methanol to gasoline (MTG), is a complementary rather than a competitor to Fischer–Tropsch synthesis for the upgrading of coal or natural gas into liquid fuels.²

In 1977, Exxon Mobil developed MTG based on ZSM-5 zeolite catalyst. When contacting ZSM-5 zeolite, methanol is converted into hydrocarbons that are limited in fractions similar to traditional gasoline. The first commercialized MTG plant was started in 1980s in New Zealand.⁴ ZSM-5 is an aluminosilicate zeolite having 5.5 Å × 5.1 Å elliptical channels running along the a axis in a sinusoidal manner and 5.6 Å × 5.3 Å elliptical channels running straight along the b axis.⁵ The presence of micropores gives rise to the use of zeolites as shape-selective catalysts.^6–8 In ZSM-5, diffusion of molecules that have a greater critical size than that of durene is difficult, leading to high selectivity in MTG. However, it is the same pore systems that impose limits on the applicability of zeolites as catalyst, due to diffusion limitations and a corresponding inefficient use of the entire zeolite crystals.^2,8 Therefore, improvements and modification of the diffusion properties might lead to improved catalyst performance.^9–11

To prolong the lifetime of zeolite catalysts, many studies used the methods of desilication/dealumination, which offers a simple, versatile and scalable means to prepare hierarchical zeolites with well-defined properties.^12–17 Bjørgen et al.² reported on the formation of accessible intracrystalline mesopores in H-ZSM-5 as a result of selective removal of framework Si by controlled desilication with NaOH at 75 °C for 4 h twice, effectively creating mesopores in the catalysts and improving the product selectivities in MTG. Fathi et al.¹⁸ improved the catalytic performance to produce higher amounts of gasoline range hydrocarbons by post synthesis of HZSM-5 catalyst in CaCO₃, Na₂CO₃ and NaOH solutions at 75 °C for 3 h, and the desilication of the catalyst exhibited improvements in catalytic lifetime, selectivity to C₅₊ hydrocarbons and yield of gasoline rang products. Xiao et al.¹⁷ prepared hierarchical HZSM-5 zeolites by post-synthesis modification of conventional bulk crystals of HZSM-5 with sodium hydroxide solution at 75 °C for 2 h. When the samples were applied in catalyzing the aromatization of glycerol, excellent performance was obtained. Apart from the desilication, there are also some dealumination for ZSM-5. Ashim and Ronald¹⁹ investigated the effect of HF treatment on the acidity of ZSM-5. However, none of other physicochemical properties were studied and the catalysts were not tested for any reactions. Almutairi et al.²⁰ studied the influence of steaming on the acidity and the methanol conversion reaction of HZSM-5 zeolite. They concluded that severe steaming resulted in a strong decrease in the framework Al content and agglomeration of extra-framework Al atoms, and the decrease of the acidity by severe steaming resulted in increased amounts of methanol converted because of a lower rate of coke formation. Campbell et al.²¹ studied the effect of calcination and hydrothermal treatments on the structure and properties of HZSM-5 zeolites. Both calcination and hydrothermal treatment were found to cause dealumination of the zeolite. Kumar et al.²² reported that mild dealumination (with HCl, acetylacetone and ammonium hexafluorosilicate) increases the activity of H-ZSM-5 in the isomerization of m-xylene. The activity was very high with HCl- and acetylacetone-treated samples, which is attributed to the increase of both Lewis and Brönsted acid sites as well as to the creation of mesoporosity. Wloch²³ investigated the purification of ZSM-5 by etching the surface of crystals with an aqueous-acetone solution of hydrogen fluoride. The solution was useful for removing by-products from the samples without affecting morphology or sizes of the crystals. When the samples were used for n-hexane sorption, the rates of the n-hexane sorption with the etched samples were up to two orders of magnitude higher than that for the non-etched one. Qin et al.²⁴ comprehensively studied the effect of fluoride treatments on the secondary pore formation in ZSM-5 crystals, and the catalytic impact was illustrated in the m-xylene conversion. Shao et al.²⁵ reported the catalytic performance of acid treated HZSM-5 catalyst for the conversion of biomass derivates to olefins and aromatics. At the optimized dealumination conditions, the coke yields decreased from 44.1% to 27.4%, the selectivities of ethylene, propylene, and toluene increased, while those of butylene, C₅ and benzene decreased compared with the original ZSM-5 catalyst. However, there is no report of the application of ZSM-5 treated by acids for the reaction of MTG.

Campbell et al.²⁶ have studied the dealumination of ZSM-5 for the reaction of MTG. The dealumination was caused by water produced during methanol conversion. The catalyst lifetime was increased significantly, which was attributed to a decreased rate of coke formation in catalysts containing lower densities of acid sites. In their conclusions, it was significant that, despite the loss of up to ca. 90% of the framework aluminum atoms which were considered to be the active catalytic sites,²⁷ the HZSM-5 samples continued to perform well as MTG catalysts. Therefore, it is significantly needed to study the effect of dealumination or acid treatment of ZSM-5 on the performance of MTG.

In this study we have investigated the effects of hydrogen fluoride (HF) treatment of an HZSM-5 sample on catalyst performance in MTG. In order to maximize the potential for the improvement, a commercial HZSM-5 sample with relatively large crystals and many crystal defects, and thus a fairly short catalyst lifetime, was specifically chosen. Compared with the alkaline-treatment of ZSM-5, HF-treatment is carried at much lower temperature, and the process is more time-saving and simple. The treatment leads to dealumination and desilication, and results in significant improvement in the catalyst lifetime, and the overall performance is better than that of alkaline-treated ZSM-5. Extensive characterization reveals that, for these samples, the effects may be attributed to the decreased rate of coke formation in catalysts which results from the decrease of acid sites, the development of external surface areas and mesopore formation. Although the HF-treatment has some shortages such as corrosion of equipments and the environmental problem of HF, the method is worthy to be investigated from both the academic and industrial points of views.

2. Experimental

2.1 Catalyst preparation

Commercial HZSM-5 zeolite (SiO₂/Al₂O₃ = 50) was purchased from Nankai Catalysts Co., China. 5 g of the zeolite was added into 50 g HF solution with stirring for 45 min. Then, the suspension was washed with deionized water until neutral, dried at 120 °C over night and calcined at 550 °C for 6 h. The molar concentration of HF solution ranged from 0.5–2 M. The resultant zeolites are named as samples ZSM-5-C where C represents the molar concentration of HF.

2.2 Catalyst characterization

The X-ray powder diffraction (XRD) patterns of ZSM-5 were obtained on a Bruker D8-Focus diffractometer with Cu Kα radiation at a scanning rate of 8° min⁻¹ in the 2θ ranges of 5–55°. The parent ZSM-5 was taken as the reference sample and taken as 100% and the relative crystallinity (RC) of all other samples are determined accordingly. Scanning electron microscopy (SEM) images were recorded using a Hitachi S-4800. The transmission electron microscope (TEM) micrographs were recorded using a FEI-Tecnai G2-F20. N₂ adsorption and desorption isotherms were measured at −196 °C on a Micromeritics TriStar 3000 instrument. SiO₂/Al₂O₃ ratios were measured on a Bruker-axs S4 Explorer X-ray fluorescence spectroscopy (XRF). NH₃-TPD measurements were carried out by using a TP-5076 instrument of Tianjin Xianquan Co. to measure the amounts of acid sites and acid strength of the catalysts. Fourier transform infrared spectra (FT-IR) of the catalysts were obtained by a Bruker Vertex 7.0 Fourier transform infrared spectrometer. For IR analysis, the self-supported wafers (10 mg cm⁻²) were degassed for 1 h under vacuum (10⁻³ Pa) at 400 °C, then adsorption of pyridine was carried out at 60 °C for 30 min followed by desorption at 150 °C for 20 min. The transmission spectra were recorded at 150 °C. The amounts of the deposited materials after reaction were determined by a Shimadzu TGA-50 thermogravimetric analyzer. The samples were combusted in the flow of air from room temperature to 750 °C with a ramp of 10 °C min⁻¹ and the weight loss between 300 and 750 °C was attributed to the burning of the deposited coke.

2.3 Catalytic testing

The MTG reaction was carried out at atmospheric pressure in a fixed bed reactor equipped a quartz tube with an internal diameter of 10 mm and a total length of 370 mm. In a typical run, the reactor was packed with 0.5 g catalyst which was sieved to 425–850 μm and diluted with the same sizes of inert quartz particles in a 1 : 2 volume ratio. The catalyst was pretreated in situ at 400 °C for 2 h in a flow of 50 cm³ min⁻¹ N₂ at a heating rate of 10 °C min⁻¹. Then liquid methanol was fed into the reactor at weight hourly space velocity (WHSV) of 10 h⁻¹ by a micropump. The reaction temperature was constantly maintained at 400 °C. The products were separated with an ice-cooled condenser. The gas products were analyzed on a gas chromatograph (GCSP-3420A) equipped with a flame ionization detector (FID) and a KB-PLOT Q (50 m × 0.32 mm × 10.00 μm) capillary column. The liquid products were analyzed on a gas chromatograph (GCSP-3420A) equipped with an FID and a SE-54 (30 m × 0.25 mm × 0.33 μm) capillary column. The methanol conversion was calculated by the following equation:

Methanol conversion (%) = (M_in − M_out)/M_in × 100 (a)

where M_in and M_out denote the amounts of pumped in and unconverted methanol during the reaction, respectively.

The naming conversions were applied when describing the reaction products: C_1–4 represents the gaseous hydrocarbons with carbon numbers ranging between 1 and 4; TMB refers to trimethylbenzene; C₅₊ is the sum of hydrocarbons with carbon numbers of ≥5 (benzene, toluene, xylene and TMB are excluded). The product selectivities are calculated using the following equation:

Product selectivity (%) = M_i/M_HC × 100 (b)

where M_i is the amount of product interested and M_HC is the total amount of hydrocarbon products.

3. Results and discussion

3.1 Catalyst characterization

All the HF-treated samples show typical XRD peaks of the ZSM-5 zeolite, as shown in Fig. 1. However, the relative crystallinity of the parent and HF-treated samples (Table 1) was not changed appreciably with the variation of the treatment conditions, suggesting that the structures of ZSM-5 were not destroyed by the treatment of HF. The result is in accordance with that in the work of Qin et al.²⁴

Fig. 1 XRD patterns of the parent and HF treated ZSM-5.

Table 1 Textural properties of parent ZSM-5 and HF-treated ZSM-5

Samples	RC	Surface area (m^2 g^−1)			Pore volumes (cm^3 g^−1)
		SBET^a	Smicro^b	Sext^b	Vtotal^c	Vmicro^b	Vmeso^d
a Calculated by the BET method in the p/p0 range of 0.01–0.1.b Calculated using the t-plot method.c Calculated by measuring the amount of adsorbed nitrogen at p/p0 = 0.99.d Calculated using Vtotal − Vmicro.
ZSM-5	100	310	237	72	0.167	0.111	0.056
ZSM-5-0.5	100	302	220	82	0.168	0.107	0.061
ZSM-5-1	100	298	211	87	0.168	0.105	0.063
ZSM-5-1.5	98	294	204	90	0.172	0.100	0.072
ZSM-5-2	99	293	201	92	0.180	0.100	0.080

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The SEM images of parent ZSM-5 and the HF-treated samples are shown in Fig. 2. With increasing the concentrations of HF solution, the crystal sizes decrease, which should result from the dissolution by HF solution. The parent ZSM-5 is composed of large aggregates with some impurities and smaller particles. After HF treatment, the amounts of the large aggregates and smaller particles decrease due to the dissolution by HF. According to the literature,²⁴ the defects or domains with different chemical compositions are more vulnerable to acid attack, and these parts of the crystal are first dissolved.

Fig. 2 SEM images of (A) parent ZSM-5, (B) ZSM-5-0.5, (C) ZSM-5-1, (D) ZSM-5-1.5, (E) ZSM-5-2.

The TEM micrographs of the parent and the HF-treated samples are shown in Fig. 3. No mesopores can be observed in the parent ZSM-5. But after the treatment of HF, some mesopores appear within the crystals, and with increasing the concentrations of HF used for treating ZSM-5, the amounts of mesopores increase gradually. All the mesopores within the crystals should result from the dissolution of the crystals by HF.

Fig. 3 TEM images of (A) parent ZSM-5, (B) ZSM-5-0.5, (C) ZSM-5-1, (D) ZSM-5-1.5, (E) ZSM-5-2.

The N₂ adsorption/desorption isotherms are shown in Fig. 4. For the parent sample, a typical type I isotherm is seen, corresponding to a microporous material.²⁸ The isotherms for the HF-treated samples exhibit hysteresis loops, which are associated with the filling and emptying of mesopores by capillary condensation. This feature appears to be more pronounced with the increase of HF concentrations. As shown in Table 1, the BET surface areas decrease slightly, the external surface areas and mesopore volumes increase considerably as a consequence of the exposure to HF solution with increasing concentrations, which are in accordance with the results of TEM. This phenomenon may be attributed to the dissolution of the framework. The mesopore formation is in accordance with the reports from Qin et al.²⁴

Fig. 4 N₂ adsorption/desorption isotherms of the parent and HF treated ZSM-5.

The contents of Al₂O₃ in the HF-treated ZSM-5 samples were measured by XRF technique. The results in Table 1 show that the content of Al₂O₃ decreases with increasing the concentrations of HF used for treating ZSM-5. It is well-known that HF reacts with both silicon and aluminum. However, the results demonstrate that the dealumination is more pronounced than desilication, i.e. the ratios of SiO₂/Al₂O₃ increase with increasing the concentrations of HF used for treating ZSM-5.

NH₃-TPD was used to detect the acid sites, as shown in Fig. 5. The pertaining calculated data are listed in Table 2. All the samples exhibit typical double-peaks in NH₃-TPD profiles. The TPD desorption peaks with maxima in the temperature ranges of 195–225 and 410–450 °C are attributed to the NH₃ molecules chemisorbed on weak and strong acid sites, respectively.^29,30 With increasing the concentrations of HF, the areas of the peaks corresponding to both weak and strong acid sites decrease; the centers of the adsorption peaks corresponding to both weak and strong acid sites move to lower temperature. All the phenomena indicate the gradual decrease of the strength of acid sites.³¹ However, compared with the weak acid sites, the variation of the strong acid sites is less apparent. The decrease of acid sites can be attributed to the decrease of aluminum in ZSM-5 as shown in Table 2.

Fig. 5 NH₃-TPD profiles for the parent and HF-treated ZSM-5.

Table 2 Elemental component and acid amounts of the samples

Samples	SiO2/Al2O3^a	Acid amounts^b (mmol g^−1)			B/L^c
		Total acidity	Weak acidity	Strong acidity
a Measured by XRF.b Measured by NH3-TPD.c The ratio of Brönsted/Lewis acid sites.
ZSM-5	26.1	0.597	0.416	0.181	3.2
ZSM-5-0.5	32.8	0.431	0.278	0.153	2.5
ZSM-5-1	46.2	0.392	0.245	0.147	1.8
ZSM-5-1.5	55.0	0.354	0.202	0.152	1.2
ZSM-5-2	60.4	0.319	0.185	0.134	0.7

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Pyridine-absorbed FTIR was applied to gain further information about the acid sites for the parent and HF-treated ZSM-5 as shown in Fig. 6. Three IR bands at 1540, 1490, and 1450 cm⁻¹ are observed for ZSM-5. The bands at 1540 and 1450 cm⁻¹ are attributable to Brönsted acid and Lewis acid sites, respectively, while the band at 1490 cm⁻¹ could stem from both Brönsted acid and Lewis acid sites.³² With increasing the HF concentrations, the intensities of the IR bands belonging to Lewis acid sites increase slightly, however, those belonging to the Brönsted acid sites decrease gradually. Pérez-Ramírez et al.³³ concluded that the mesopore walls of the hierarchical zeolites contain Lewis acid sites. Therefore, the development of the mesopores by HF treatment contributes to the increase of Lewis acid sites. They also concluded that the micropores are the place where active sites (Brönsted acid sites) locate.³³ Therefore, the decrease of micropores contributes to the decrease of the Brönsted acid sites.

Fig. 6 IR spectra of adsorbed pyridine for the parent and HF treated ZSM-5.

²⁷Al MAS NMR is an effective technique to provide information on the coordination state of Al species. ZSM-5 without HF treatment exhibits a symmetrical and narrow signal at 55.5 ppm (Fig. 7), which is attributable to the tetrahedral Al in the framework of ZSM-5.^34–36 A very small signal at 0 ppm is observed, indicating the existence of a limited number of octahedrally coordinated Al species in ZSM-5.³² After the treatment with HF at the concentration below 1.5 M, the signal at ∼0 ppm does not change appreciably, however, the sever HF-treatment at the concentration of 2 M leads to an increase of the signal, suggesting the increase of the extra-framework Al in ZSM-5-2. With increasing the concentrations of HF, the intensities of the component at 55 ppm assigned to the tetrahedral Al species in the framework decrease. The results agree with the Py-IR characterization.

Fig. 7 ²⁷Al MAS NMR spectra of the parent and HF-treated ZSM-5.

3.2 Catalyst performance in MTG

The methanol conversions as a function of time on stream over the catalysts treated with different HF concentrations are presented in Fig. 8. It should be reiterated that in order to compare the catalysts quickly, the reactions were conducted at high WHSV (10 h⁻¹).¹⁸ The initial methanol conversions of the parent and HF-treated samples are similar, although according to the results shown in Fig. 5 and 6, the amounts of weak and strong acid sites, as well as the Brönsted acid sites decrease. This phenomenon may result from the reason that the acid sites of the samples treated by HF solution are still enough for the conversion of methanol during the initial periods. As being concluded in the work of Campbell et al.,²⁶ only a small amount of active sites is necessary for HZSM-5 to be an efficient and effective MTG catalyst. For the parent ZSM-5, the methanol conversion decreases sharply after 10 h on stream and decrease to 15% after 14 h on stream. While after the treatment of HF, the stabilities of the samples improve significantly. The methanol conversions are almost 100%, and begin to decrease at time on stream (TOS) of 20, 24, 28 and 32 h for samples treated with HF solution of 0.5, 1, 1.5 and 2 M respectively. With increasing the concentrations of HF used for treating ZSM-5, the external surface areas and mesopore volumes increase gradually which facilitate overcoming the inherent diffusion limitations avoiding severe coking.^37–39 As demonstrated in Fig. 10, the amounts of coke formed in the samples significantly decrease with increasing the concentrations of HF used for treating ZSM-5. Therefore, the stabilities of the HF-treated samples increase gradually.⁴⁰ In addition, with increasing the concentrations of HF, both the strong and weak acid sites, as well as the ratios of Brönsted/Lewis acid sites decrease gradually, which also contributes to the stabilities or prolonging of the lifetime according to the literature.^26,41

Fig. 8 Methanol conversions over the parent and HF-treated ZSM-5.

Fig. 9 The yields of LH over the parent and HF-treated ZSM-5.

Fig. 10 TGA profiles of the parent and HF-treated ZSM-5 after undergoing 16 h under the same reaction conditions: 400 °C, 1 atm, WHSV = 10 h⁻¹.

The yields of liquid hydrocarbons (LH) as a function of TOS over the samples are shown in Fig. 9. The initial yield over the parent ZSM-5 is ca. 29%, and that over ZSM-5-0.5 is similar. However, with further increasing the concentrations of HF used for treating ZSM-5, the initial LH yields of the samples decrease gradually. After mild HF treatment, ZSM-5-0.5 may still have acid sites which are enough to supply the reaction with a high yield. With increasing the degree of HF treatment, the strong acid sites or Brönsted acid sites decrease (as shown in Fig. 5 and 6) which leads to the decrease of the conversion of light intermediates to high boiling point hydrocarbons,⁴² therefore the initial LH yield of the samples decrease gradually. Although the parent ZSM-5 and ZSM-5-0.5 have similar initial LH yields, the stability with the same LH yields of ZSM-5-0.5 is doubled. With further increase of the concentrations of HF used for treating ZSM-5, the stabilities of the samples ZSM-5-1, ZSM-5-1.5 and ZSM-5-2 are 2.7, 3.7, 4.3 times of that of the parent ZSM-5, respectively. The increase of the stabilities should be ascribed to the decrease of the strong acid sites or the Brönsted acid sites, and the development of the external surface areas and mesopore volumes.^20,26,37,43

Table 3 lists the methanol conversions and hydrocarbon selectivities for the MTG reactions over the parent (at TOS of 6 h) and HF-treated ZSM-5 (at TOS of 8 h). The parent sample ZSM-5 has the lowest selectivity of C_1–4 and the highest selectivity of C₅₊, LH and BTX (benzene, toluene, and xylenes). Compared with ZSM-5, ZSM-5-0.5 presents similar selectivities. With increasing the concentrations of HF solution used for treating ZSM-5, the selectivity of C_1–4 increases and the selectivities of the others decrease gradually. After the treatment of HF, the strong acid sites and the Brönsted acid sites decrease gradually, which contribute to the increase of C_1–4 and the decrease of the other hydrocarbons.⁴²

Table 3 Methanol conversion and hydrocarbon distribution (TOS = 8 h, 400 °C, 1 atm, WHSV = 10 h⁻¹)

Sample	Methanol conversion (%)	Selectivity (%)						Yield of LH (%)
		C1–4	C5+	Cbene	Ctoluene	Cxylene	CTMB
a The hydrocarbons produced over the parent ZSM-5 are analyzed at TOS of 6 h.
ZSM-5^a	97.0	51.1	30.2	0.6	3.8	9.5	4.7	29.6
ZSM-5-0.5	99.7	53.3	29.4	0.6	3.6	9.4	3.7	29.1
ZSM-5-1	99.9	58.2	26.0	0.5	3.3	8.7	3.3	26.7
ZSM-5-1.5	99.9	58.8	26.4	0.6	3.2	7.6	3.4	25.4
ZSM-5-2	99.9	60.9	25.3	0.5	3.0	7.7	2.6	23.6

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To compare with the alkaline-treated ZSM-5, the catalytic performance is also performed over NaOH-treated ZSM-5. The results (see ESI†) show that, the LH yield over alkaline-treated ZSM-5 is a bit higher than that of HF-treated ZSM-5, however, all the HF-treated ZSM-5 have higher stabilities for the formation of LH, and much longer lifetime regarding the methanol conversions than the alkaline-treated ZSM-5. It is well known that alkaline-treatment leads to the desilication which increases the acidity. However, high acidity results in easy coking in the reaction. HF-treatment increases the mesopores and decreases the acidity of the catalysts, in favor of prolonging the catalytic lifetime significantly. Therefore, HF-treated ZSM-5 is better than the alkaline-treated ZSM-5 for the reaction of MTG.

4. Conclusions

Zeolite ZSM-5 has been treated with HF and studied for the conversion of methanol to gasoline. The physical characterization results indicate that with increasing the concentrations of HF used for treating ZSM-5, the crystallinity and the crystal surface of the HF-treated samples do not show much difference compared with the parent ZSM-5; the external surface areas and mesopore volumes increase; the aluminum in ZSM-5 decreases which leads to the decrease of both the strong and weak acid sites; the removal of aluminum and the decrease of micropores contribute to the decrease of Brönsted acid sites; the development of mesopores results in the slight increase of Lewis acid sites. When the ZSM-5 samples are applied in the methanol to gasoline reaction, with increasing the concentrations of HF from 0.5 to 2 M used for treating ZSM-5, the methanol conversion improves significantly; the stability with the same yields of liquid hydrocarbons (LH) increase by a factor of 2 to 4.3. After the treatment by 0.5 M HF, ZSM-5-0.5 still has the high LH yield as that of the parent ZSM-5, however, when the concentrations of HF used for treating ZSM-5 increase further, the LH yields decrease.

Acknowledgements

This work was supported by National Natural Science Foundation of China (Grant No. 21276183).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra14513b

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