Facile and green aerosol-assisted synthesis of zeolites

Abstract

Here we report a facile and green aerosol-assisted method to synthesize zeolites. Compared with the conventional hydrothermal route, the new method is simpler and less polluting, and requires lower template amount, reaction volume, and crystallization temperature.

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Zeolites are widely used as catalysts in industrial chemical processes due to their large porosity, uniform channels, and excellent thermal and hydrothermal stabilities.^1–4 These materials are typically synthesized under solvo/hydrothermal conditions, which usually require a large amount of solvent and relatively high crystallization temperature. The solvent normally results in the discharge of polluted water and lower synthesis efficiency owing to the occupation of the autoclave space by the solvent.^2,5 In 1990, a dry gel method was proposed by Xu et al. to reduce pollution and enhance the yield of the products.^6,7 However, the method is difficult to scale up due to the complicated procedure. Recently, Xiao et al. presented a solvent-free synthesis of various zeolites. The method is very simple and less polluting, but mainly focused on the synthesis of micro-size zeolites.^2,5 The findings from the above methods, which have their own merits and weaknesses, are able to guide further research.

As a green, adaptable and scalable route, aerosol methods have been widely used to synthesize mesoporous and macroporous molecular sieves.^8–15 Heteroatoms can be finely dispersed in the silica or silica–alumina matrix by the method.^8,15 Nevertheless, to the best of our knowledge, an aerosol method has not been employed for the synthesis of microporous silica–alumina or transition metal substituted zeolites.

In this communication we show a generalized aerosol-assisted method for the synthesis of various zeolites (TS-1, silicalite-1, Beta, ZSM-5, Zn-ZSM-5, Fe-ZSM-5). The detailed synthesis methods and characterizations of the above mentioned zeolites are given in the ESI.† The method includes two steps: (1) The precursor solutions were prepared by mixing raw materials. With an aerosol-dry apparatus (Fig. S6†), the precursor solutions were quickly transformed into amorphous powder. (2) The amorphous powder and a given amount of template agent were loaded into a Teflon-lined autoclave and crystallized under autogenous pressure. No extra water was added, which maintained a high concentration of template agent. Here, as a model study, the synthesis of TS-1 by an aerosol-assisted method is presented.

Among zeolites reported to date, TS-1 has received great attention due to its excellent catalytic properties in a series of selective oxidation reactions with hydrogen peroxide under mild conditions.^16–19 It has been successfully used as an industrial catalyst in the processes of propylene epoxidation, cyclohexanone ammoximation, and phenol hydroxylation.^16,20,21 However, there are still some problems that limit the practical applications of the conventional hydrothermal synthesis of TS-1:

(i) A relatively high cost of TS-1 due to the use of the expensive template: TS-1 was firstly synthesized by using tetrapropylammonium hydroxide (TPAOH) as the template.²² However, the industrial application of TS-1 is limited by a relatively high price of TPAOH.^16,17,23 Although other inexpensive organic amines were used to synthesize low-cost TS-1, their performances are not as good as that of the classical TS-1.^16,24

(ii) The synthesis procedure is relatively complicated and time-consuming. The catalytic performance of the TS-1 zeolite greatly depends on the coordination state of the titanium species.^16,25 However, the hydrolysis of Ti alkoxide is so fast that it easily leads to the formation of the TiO₂ precipitates. To avoid this problem, the Ti alkoxide is hydrolyzed following some strict procedures: dripping TPAOH slowly at 0 °C, dissolving TBOT in isopropyl alcohol, and operating under a CO₂ free atmosphere.^26–29

The above problems can be resolved by an aerosol-assisted method. Fig. 1A shows the UV-vis spectrum and SEM (scanning electron microscopy) image of SiO₂–TiO₂ amorphous powder, which was obtained in the first step. The UV-vis spectrum exhibits a strong absorption at 218 nm, which is assigned to the isolated tetrahedral titanium species.^23,25,30 This indicates that titanium ions are already well dispersed in the SiO₂–TiO₂ amorphous powder. The magnification image (Fig. 1B) shows that the SiO₂–TiO₂ amorphous powder has a spherical morphology. The particle size ranges from 50 nm to 10 μm.

Fig. 1 (A) UV-vis spectrum of the SiO₂–TiO₂ amorphous powder, (B) SEM image of the SiO₂–TiO₂ amorphous powder.

Fig. 2 shows X-ray diffraction (XRD) pattern, UV-vis spectrum, N₂ sorption isotherm and SEM image of the final product, TS-1-0.05 sample (TPAOH/Si = 0.05). The XRD pattern of TS-1-0.05 (Fig. 2A) shows the characteristic peaks at 2θ = 7.8, 8.8, 23.0, 23.9 and 24.4°, which are typical for MFI topology.^31,32 The UV-vis spectrum of the TS-1 zeolite shows a strong band at 213 nm, indicating the presence of the framework Ti species. The amorphous Ti species and anatase TiO₂, corresponding to the absorptions at 270 nm and 320 nm, are not observed.³⁰ This suggests that titanium ions are well dispersed in the framework of the TS-1-0.05 sample. Fig. S7† exhibits the Raman spectrum of TS-1-0.05 excited with a 244 nm laser line. The Raman peaks at 490, 530, and 1125 cm⁻¹ are associated with the framework titanium species.³³ Fig. S8† shows the Raman spectrum of TS-1 collected with a 325 nm laser line which is sensitive to anatase TiO₂. Peaks at 144, 390, 515 and 637 cm⁻¹ are not observed, indicating the absence of anatase TiO₂. The X-ray fluorescence spectroscopy indicates that the SiO₂/TiO₂ ratio of the TS-1 is 63.6, which is close to the mole ratio (60) of the starting precursor solution. The N₂ adsorption–desorption isotherm of the TS-1-0.05 sample (Fig. 2C) shows a sharp increase at a relative pressure below 0.02, indicating the presence of micropores. A small hysteresis loop and a steep increase at a relative pressure of 0.9 <P/P₀ < 1 can be observed, which is due to the intercrystal pore formation. The BET surface area and pore volume are 466 m² g⁻¹ and 0.3 cm³ g⁻¹, respectively. The magnification image (Fig. 2D) shows that TS-1-0.05 sample has a crystal morphology with diameters of 350–800 nm. However, if the crystallization process is conducted at 110 °C for 48 h, the crystal size of TS-1 is reduced to 150–400 nm (Fig. S9†), indicating that the crystal size can be adjusted by the synthesis conditions.

Fig. 2 Analytical data for the TS-1-0.05. (A) XRD pattern, (B) UV-vis spectrum, (C) N₂ sorption isotherm, (D) SEM image.

Table 1 shows the catalytic performance of the calcined TS-1 in propylene epoxidation. The TS-1, which was prepared according to an improved conventional method, was used as a reference.²⁹ The X-ray diffraction (XRD) pattern, UV-vis spectrum, and scanning electron microscopy (SEM) image of the TS-1 (convention) are shown in Fig. S10.† The results suggest that both H₂O₂ conversion and selectivity of the propylene oxide are almost the same for the two samples. However, the utilization of H₂O₂ for the TS-1-0.05 (aerosol) is higher than that of the TS-1 (convention).

Table 1 Catalytic performance of calcined TS-1 for the epoxidation of propylene^a

Cat.	XH2O2/%	SPO/%	UH2O2/%	YPO/%
a Reaction conditions: catalyst 0.2 g, methanol 34 mL, H2O2 1 mol L^−1, propylene pressure 0.4 MPa, 333 K, 1 h.
TS-1 (aerosol)	96.0	90.1	97.5	84.3
TS-1 (convention)	95.1	90.6	93.5	80.6

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Typical processes of the conventional hydrothermal method and the aerosol-assisted method are shown in Fig. 3. Compared with conventional hydrothermal synthesis, the aerosol-assisted method has the following obvious features: (1) a simpler synthetic process. The aerosol-dry process makes titanium ions highly distributed without complex operations to avoid the formation of TiO₂ precipitates. As shown in Fig. 2 and S11,† the extra-framework titanium species are absent when the SiO₂/TiO₂ ratio ranges from 40 to 60. Furthermore, due to the presence of HCl, TEOS can hydrolyze completely at room temperature in a short time (<15 minutes). By the aerosol-dry process, HCl and ethyl alcohol were removed in a short time. Therefore, the time-consuming process of evaporating alcohol can be avoided. (2) The template amount is ultralow. TS-1 can be completely crystallized at the ratio of TPAOH/SiO₂ = 0.04 (Fig. S12†), which is much lower than that for the conventional hydrothermal method. (3) Less pollution. The only water comes from the template agent (25% TPAOH solution). During the crystallization processes, the TPAOH aqueous solution is absorbed due to the relatively high porosity of the MFI structure. As a result, almost no waste solution is discharged. (4) Low pressure. In conventional hydrothermal synthesis, a large amount of Ti species will be lost at a low crystallization temperature. Therefore, TS-1 is usually crystallized at 170 °C.^{17,19,23,26,31} In this work, most of Ti species can be successful incorporated into the framework of TS-1 at 130 °C. The saturated vapor pressure of water is 270.02 kPa at 130 °C and 791.47 kPa at 170 °C. Hence, the decreased pressure reduces the equipment requirements. (5) Better utilization of autoclaves. During the hydrothermal crystallization process, H₂O occupies most of the space of the autoclaves in the conventional synthesis. However, in an aerosol-assisted method, only a small amount of space is required for crystallization. For example, in a 3 mL autoclave, 1.4 g TS-1 can be obtained by the aerosol-assisted method, which is about 8 times that for a typical hydrothermal synthesis (0.173 g).^3,5

Fig. 3 Typical processes of TS-1 synthesized by the conventional hydrothermal method (a) and the aerosol-assisted method (b).

In conclusion, various zeolites have been successfully synthesized by an aerosol-assisted method. Compared with the conventional route, the new method is simpler and less polluting, and requires a lower template amount, reaction volume and crystallization temperature. This method is particularly useful for the synthesis of transition metal substituted zeolites, in which heteroatoms are required to be finely dispersed in the silica matrix. As a model case, TS-1 exhibits a good catalytic performance in the epoxidation of propylene. This green aerosol route can be extended to the synthesis of other zeolites and has potential applications in future synthesis of industrial zeolites.

Acknowledgements

This work was financially supported by the National Science Foundation of China (NSFC, Grant 21473016, 21206017, 20603004, and 20773019).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI:10.1039/c5ra12372k

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