Facile one-pot solvent-free synthesis of hierarchical ZSM-5 for methanol to gasoline conversion

Abstract

A hierarchical ZSM-5 zeolite with high crystallinity, high surface area, adjustable Si/Al ratio and suitable acidic properties was synthesized by a one-pot solvent-free method. The resultant zeolite exhibited complete methanol conversion, high C₅⁺ selectivity (>60%) and a long lifetime (350 h) in the MTG process.

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Methanol to gasoline (MTG) technology, which relies on non-petroleum resources, has been considered as an increasingly important route to balance the dwindling levels of petroleum.¹ It can not only relieve the situation of methanol overcapacity but also increase the supply of gasoline. ZSM-5 zeolite is the most commonly applied catalyst in the MTG process for its moderate acidic properties and uniform microporosity.^2–6 Furthermore the pore structure (0.56 × 0.53 nm) of ZSM-5 offers shape selectivity toward gasoline-range hydrocarbons. However, the conventional ZSM-5 catalyst for the MTG reaction suffers from a short lifetime due to rapid coke formation on BrØnsted acidic sites and diffusion limitation in the microporous structure of ZSM-5.⁷ Recently, ZSM-5 with hierarchical structure was developed to improve the mass diffusion and the tolerance of coking, which subsequently prolonged the lifetime of MTG catalysts.^8–15 For example, Svelle et al. prepared hierarchical ZSM-5 by desilication, obtaining the prolonged lifetime during the conversion of methanol to hydrocarbons.¹¹ Zhang et al. synthesized hierarchical ZSM-5 with the help of tetrapropylammonium hydroxide and obtained 100% methanol conversion and 59% selectivity towards the gasoline-range hydrocarbons (C₅⁺).¹⁴

Up to now, several methods for the preparation of hierarchical ZSM-5 have been reported, including post treatments such as dealumination and desilication,^16,17 hard-template synthesis method^18,19 and soft template synthesis methods e.g. dual templating and amphiphilic organosilanes.^20,21 However, all of them were synthesized with the presence of solvent, such as water and alcohols, which resulted in poor zeolite yield and serious pollution.^22–24 Xiao et al. developed a solvent-free route for the synthesis of microporous zeolites,^25,26 which led to high yield of zeolites, significant reduction of waste production, and avoided environmental pollution. However, the one-pot solvent-free synthesis of hierarchical ZSM-5, especially for MTG process, is still of great challenge due to the requirements of both hierarchical pore structure and suitable acidic property for MTG reaction.

Herein, we report the synthesis of hierarchical ZSM-5 using activated carbon (AC) as hard-template via a one-pot solvent-free route (named as SH-ZSM-5) for the first time. The AC can be employed to produce nanosized or hierarchical zeolite crystals. The nanosized zeolite crystals are formed when zeolite crystallization is limited in the confined space of AC. On the other hand, hierarchical zeolite crystals can be obtained when AC was embedded within the crystals and finally removed by combustion.¹⁹ As depicted in Scheme 1, a series of hierarchical SH-ZSM-5 zeolites with various Si/Al ratios were synthesized by mixing the solid raw materials of Na₂SiO₃·9H₂O, fumed silica, NaAlO₂, activated carbon, NH₄Cl and tetrapropyl-ammonium bromide (TPABr). The mixture was rapid grinded for 20 s in a mechanical grinder and then transferred to autoclave and heated to 180 °C. The resulting samples were noted as SH-ZSM-5-x after calcination, where x represented the ratio of Si/Al in the products. SH-ZSM-5-45 was selected as the typical example in the following section. Experimental details including AC structure, synthesis process and characterizations can be found in ESI.†

Scheme 1 Solvent-free synthesis of hierarchical ZSM-5 zeolite.

Fig. 1 shows the X-ray diffraction (XRD) pattern, N₂ adsorption–desorption isotherms, mercury intrusion porosimetry (MIP) analysis, scanning electron microscopy (SEM), high resolution transmission electron microscopy (TEM) and ²⁷Al NMR of SH-ZSM-5-45. The XRD pattern (Fig. 1a) exhibits well-resolved diffractions in the ranges of 7–10° and 22.5–25°, which is in good agreement with those of ZSM-5 zeolite,²⁷ indicating the successful synthesis of ZSM-5 through the solvent-free route. N₂ adsorption–desorption isotherms and MIP were employed to test the pore structure of SH-ZSM-5-45 (Fig. 1b and c). Fig. 1b reveals the presence of micropores in SH-ZSM-5-45 (sharply increase at P/P_o < 0.05), and the appearance of a hysteresis loop at P/P_o = 0.4–0.9 infers the existence of mesopores in the sample.²⁸ The existence of mesopores is further confirmed by the BJH pore size distribution (PSD) (Fig. 1b inset). An obvious peak for mesopores at ca. 2.6 nm besides the one for micropores at ca. 0.6 nm was observed. The Brunauer–Emmett–Teller (BET) surface area, total pore volume and micropore volume are 377 m² g⁻¹, 0.16 cm³ g⁻¹ and 0.14 cm³ g⁻¹ respectively, corresponding with those of hierarchical ZSM-5 from hydrothermal synthesis.¹⁰ Notably, MIP result (Fig. 1c) reveals the appearance of macropores with an average pore size of 68 nm besides the micro- and mesopores. Meanwhile, a uniform particle size of 1–2 μm for SH-ZSM-5-45 is shown by SEM in Fig. 1d, and the presence of macropores at about 40 nm is confirmed by TEM image in Fig. 1e. The difference of the PSD for macropores between MIP and TEM may be caused by their inherent method, where the MIP method gives the average PSD of the whole sample while TEM just shows a part of the sample. Moreover, regular pore channels (2–3 nm) can be found in the inserted HRTEM image in Fig. 1e, corresponding well with the N₂ adsorption data. Thus, the XRD, BET and PSD analysis together with TEM observation proved that a real hierarchical ZSM-5 with micro-, meso-, and macro-pores was successfully prepared through the solvent-free route. ²⁷Al NMR spectrum (Fig. 1f) shows that only one sharp band at 54 ppm associating with tetrahedral aluminium in the framework can be observed.²⁹ Compared with the hierarchical ZSM-5 from hydrothermal route, the disappearance of the peak at about 0 ppm corresponding to octahedral aluminium indicates that no Al atoms exist in the extra framework, which can improve the hydrothermal stability of zeolite catalyst¹⁰ and favour the subsequent MTG reaction. Fig. S1† further proves that all the SH-ZSM-5 samples exhibit just one peak at 54 ppm in ²⁷Al NMR spectrum no matter of the Si/Al ratio. Fig. S2† shows the ²⁹Si NMR spectrum results and SH-ZSM-5-45 exhibits a strong band at −112 ppm assigned to Q⁴ silica species [Si(SiO)₄], and a weak band at −106 ppm corresponding to Q³ species [Si(SiO)₃(OH)] and [Si(SiO)₃Al].³⁰

Fig. 1 (a) XRD pattern, (b) N₂ adsorption–desorption isotherms (inset: pore size distribution), (c) macropore size distribution by MIP test, (d) SEM image, (e) HRTEM image and (f) ²⁷Al MAS NMR spectrum of SH-ZSM-5-45.

In addition, this facile solvent-free route can be extended to prepare hierarchical SH-ZSM-5 with controlled Si/Al ratio. The samples with Si/Al ratio of 20–100 still exhibit MFI structure with high crystallinity (Fig. S3†) and uniform particle size of 1–2 μm (Fig. S4†). The type-IV isotherms in Fig. S5† infer the presence of developed micropores and mesopores in these samples, which are further confirmed by corresponding PSD curves. Table S1† listed the textural data of prepared materials. The BET surface area increases from 299 m² g⁻¹ for SH-ZSM-5-20 to 377 m² g⁻¹ for SH-ZSM-5-45 and then decreases to about 340 m² g⁻¹. The total pore volume deceases monotonically from 0.20 to 0.15 cm³ g⁻¹, while retaining large micropore volume (above 0.11 cm³ g⁻¹), indicating the micropore structure was reserved during the synthesis process. SH-ZSM-5-45 shows the highest micropore volume of 0.14 cm³ g⁻¹ among the SH-ZSM-5 samples. Besides, the Q⁴/Q³ ratio in Fig. S2† decreases with the order of SH-ZSM-5-100 > SH-ZSM-5-60 > SH-ZSM-5-45 > SH-ZSM-5-30 > SH-ZSM-5-20. Moreover, different tetraalkylammonium bromides instead of TPABr were conducted under solvent-free synthesis, the XRD pattern shows that the prepared materials using (tetraethylammonium bromide, TEABr and tetrabutyl ammonium bromide, TBABr) also have well-resolved diffractions of MFI structure of ZSM-5 (Fig. S6†).

The crystallization process of SH-ZSM-5-45 was further investigated, and corresponding XRD patterns, SEM images as well as N₂ sorption–desorption isotherms are shown in Fig. 2. The samples at various crystallization time were directly analyzed in solid state. Fig. 2A shows that the starting materials exhibit sharp peaks associated with raw materials before crystallization (ca. one peak for NH₄Cl at 32°). After being heated at 180 °C for 0.5–1 h, the peaks of raw materials disappeared, meanwhile, a broad refraction at 20–25° for an amorphous phase appeared, indicating the process of solid transformation. The N₂ isotherm of the sample after being crystallized for 1 h (Fig. 2C-c) exhibits only a small amount of micropores (26 m² g⁻¹) and the morphology is still amorphous from SEM image of Fig. 2B-c. When the crystallization time was increased to 2 h, however, the sample gives well resolved refraction peaks associated with MFI structure, revealing the formation of ZSM-5 crystal, which can be further proved by SEM image (Fig. 2B-d), and N₂ isotherm (Fig. 2C-d) shows that the BET surface area increased sharply to 304 m² g⁻¹. The intensity of XRD peaks gradually increased and dominant zeolite crystals were observed from SEM images (Fig. 2A and B) as the crystallization time increased from 2 to 6 h. Further prolonging the crystallization time to 24 h results in no obvious change in XRD pattern, SEM images and BET data (Fig. 2), indicating the solid phase transformation is finished.

Fig. 2 (A) XRD patterns, (B) SEM images and (C) N₂ adsorption–desorption isotherms of SH-ZSM-5-45 samples crystallized for (a) 0 h, (b) 0.5 h, (c) 1 h, (d) 2 h, (e) 4 h, (f) 6 h and (g) 24 h.

Fig. 3 presents the catalytic performance of methanol to gasoline over hierarchical SH-ZSM-5 at 370 °C, WHSV of 1.0 h⁻¹ and atmospheric pressure. All the SH-ZSM-5 catalysts exhibit complete methanol conversion (100%) within 100 h in Fig. 3a. But the lifetime of catalysts differs with the Si/Al ratio, and 200 h for SH-ZSM-5-20, 180 h for SH-ZSM-5-30, 350 h for SH-ZSM-5-45, 250 h for SH-ZSM-5-60 and 200 h for SH-ZSM-5-100 were found when the conversion of methanol is over 95%. Fig. 3b gives the distribution of MTG products. As can be seen, the selectivity to C₅⁺ is over 60% on SH-ZSM-5-45, which is the highest one among these catalysts with different Si/Al ratio. Fig. 3b also shows that the selectivity of LPG and aromatics on SH-ZSM-5-45 is 37% and 28% respectively, which is the lowest LPG selectivity and highest aromatics selectivity among these samples. LPG can be further converted to C₅⁺ after being recycled back to the reactor to increase the gasoline yield and aromatics can be separated directly as value-added products.

Fig. 3 (a) Catalytic conversion of methanol and (b) product selectivity (LPG, aromatics and C₅⁺) of hierarchical SH-ZSM-5 for MTG reaction at 370 °C with WHSV of 1 h⁻¹ at atmospheric pressure. (Note: green, pink, blue, red, black line in (a) and the samples from left to right in (b) are SH-ZSM-5-20, SH-ZSM-5-30, SH-ZSM-5-45, SH-ZSM-5-60 and SH-ZSM-5-100 respectively).

The methanol conversion and the lifetime of SH-ZSM-5 is related to their pore structure and acidic properties of these samples.¹⁴ Fig. 4a shows NH₃-TPD profiles of fresh SH-ZSM-5. Two peaks of NH₃ adsorption were identified with the temperature range of 150–250 and 400–500 °C (except SH-ZSM-5-100), which can be attributed to weak acidic sites and strong acidic sites, respectively.³¹ The centres of the adsorption peaks move to lower temperature with the increase of the Si/Al ratio, indicating the gradually decrease of the strength of strong acidic sites. And no strong acidic sites were detected obviously for SH-ZSM-5-100, inferring primary weak acidic sites exist in the sample. Increasing the Si/Al ratio also leads to the decrease in the amount of weak acidic sites and strong acidic sites due to the decrease of the aluminium content in the catalysts, as shown by the dwindling areas of corresponding peaks. SH-ZSM-5-20 and SH-ZSM-5-30 exhibit shorter lifetime because they possess much more amount of strong acidic sites, which can enhance the conversion of light intermediates to gasoline-ranged hydrocarbons, but causing severely coke formation in the pores.³² However, there are almost no strong acidic sites in SH-ZSM-5-100, leading to the rapid deactivation. SH-ZSM-45 exhibits comparable strong acidic sites and similar hierarchical pore systems compared to SH-ZSM-5-60, but its higher micropore surface area (302 m² g⁻¹) and abundant micropore volume are beneficial to prolong its lifetime for MTG.

Fig. 4 (a) NH₃-TPD profiles of fresh SH-ZSM-5 and (b) TGA analysis of spent SH-ZSM-5 after MTG reaction (note: green, pink, blue, red, black lines are for SH-ZSM-5-20, SH-ZSM-5-30, SH-ZSM-5-45, SH-ZSM-5-60 and SH-ZSM-5-100 respectively).

Fig. 4b shows the total weight loss over spent SH-ZSM-5 after the MTG reaction. The weight losses are 10.74% for SH-ZSM-5-20, 9.5% for SH-ZSM-5-30, 6.81% for SH-ZSM-5-45, 6.46% for SH-ZSM-5-60 and 2.64% for SH-ZSM-5-100 respectively. It is found that spent catalysts with lower Si/Al ratio contained much more amount of cokes than those with higher Si/Al ratio, which is in consistence with the NH₃-TPD analysis. The amount of coke and the coke formation rate on the spent catalysts were calculated based on TGA analysis.³³ SH-ZSM-5-45 gives a coke formation rate of 0.14 mg g_cat h⁻¹, which is much lower than those of SH-ZSM-5-20 and SH-ZSM-5-30 (0.47 and 0.34 mg g_cat h⁻¹, respectively), further confirming that stronger acidic strength of the catalyst brings coke formation more quickly. SH-ZSM-5-45, the catalyst with hierarchical pore system, suitable acidic sites and highest BET surface area as well as micropore volume, prepared through solvent-free route in this work is a promising catalyst for MTG reaction.

Conclusions

In summary, we demonstrated a novel and facile one-pot solvent-free route to prepare hierarchical ZSM-5 using simply mechanical mixing of solid raw materials followed by heating treatment. The resultant ZSM-5 exhibited high crystallinity, high BET surface area and adjustable Si/Al ratio, which led to suitable acidic property for methanol conversion to gasoline. Catalytic tests showed that hierarchical SH-ZSM-5-45 exhibited excellent MTG performance with a methanol conversion of 100%, a high C₅⁺ selectivity of 60%, the longest lifetime of 350 h under experimental conditions. Moreover, these catalysts from current route can be scaled up easily. Such a facile one-pot solvent-free method would significantly improve the zeolite yield and avoid huge wastewater pollution, which are attractive in industrial applications of hierarchical zeolites.

Acknowledgements

The authors acknowledge the financial supports from NSFC (No. 21507141 & No. 21506243), Science and Technology Commission of Shanghai Municipality (No. 14DZ1203700 & 14DZ1207602), and Lu'an Mining Group (Changzhi, China).

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Footnote

† Electronic supplementary information (ESI) available: Experimental details, SEM, BET, pore size distribution, NRM. See DOI: 10.1039/c6ra00247a

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