Microwave-assisted single-surfactant templating synthesis of mesoporous zeolites

Abstract

A single-surfactant templating method was explored for the synthesis of mesoporous zeolites under microwave irradiation, which allowed programming of temperature and time over a wide range of conditions and resulted in a significant reduction of the synthesis time. Most importantly, this approach eliminated the use of an organic molecular template for creating microporous zeolitic structure. Two preparation methods were examined: one-pot and two-step synthesis routes. The synthesized materials were shown to be micro-mesoporous structures consisting of zeolitic crystallites (ZSM-5/Mordenite-type) embedded into amorphous domains of hexagonally ordered mesoporous silica, MCM-41, with properties comparable with those reported for the samples obtained by dual templating and autoclave heating. Mesopores were created by soft-templating, whereas the crystalline microporous framework was formed in the absence of organic molecular template under microwave irradiation. One of the samples was tested as a catalyst in the cracking of vacuum gas oil, which resulted in obtaining hydrocarbons in the range of liquefied petroleum gas, gasoline and diesel, indicating that the mesoporous zeolitic materials prepared via single-surfactant templating under microwave irradiation are effective catalysts for the pyrolysis of vacuum gas oil. Both the synthesis procedure and the presented application of mesoporous zeolites address a few aspects of green chemistry and sustainability.

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Introduction

Significant variations in crude oil quality around the world drive research to develop catalysts for the efficient conversion of heavy fractions of petroleum to light fractions of hydrocarbons.¹ Nowadays, the catalytic cracking of hydrocarbons in industry is performed using zeolite-based materials because they exhibit strong intrinsic acidity, which is an important factor affecting the catalytic efficiency in light olefin production. Unfortunately, the use of aluminosilicate zeolites such as Y and ZSM-5 has a drawback because of low accessibility of reactants to micropores (pore widths below 2 nm), especially those with diameters smaller than 1 nm. To overcome this problem, mesopores (pore widths between 2 and 50 nm) could be introduced to the structures of zeolitic materials to facilitate mass transfer of reactants to the catalytic sites. Although mesoporous materials such as MCM-41 ^2–4 and SBA-15 ^5–7 could be potentially useful for catalytic cracking because they possess mesopores facilitating mass transfer, their major drawback is weak acidity. Therefore, mesoporous zeolites (MZ) that combine both: high catalytic activity because of their strong acidity and superior accessibility to the reactants because of the presence of mesopores,^8–11 are very interesting catalytic materials from the viewpoint of industrial applications. So far, several synthetic strategies have been developed to create mesoporous materials with crystalline walls (zeolitic phase), namely desilication,¹² dealumination,¹³ nanocasting with help of carbon particles,¹⁴ carbon nanoparticles¹⁵ and colloid-imprinted carbons,^16–18 one-pot synthesis employing organosilanes and micropore structure directing agents (SDA),^19–21 and dual templating,²² however, none of these techniques delivered a desired outcome – highly ordered mesoporous materials with highly crystalline pore walls.

Recently, some special structure-directing templates – surfactant²³ and non-surfactant polymers²⁴ – have been developed, which are able to direct the formation of micro- and mesoporous phases, but these templates are costly to be used in a large scale synthesis of catalysts because the lifetime of these materials in the fluid catalytic cracking unit was shown to be about 5 minutes due to the fast deactivation.²⁵ To reduce the synthesis cost, one needs to limit both the use of surfactants and the synthesis time, which can be achieved by organic-free synthesis^26–28 under microwave irradiation.^7,29 The only methods to produce MZ without templating are desilication and dealumination,^12,13 however dealumination significantly reduces the amount of acidic sites crucial for high catalytic activity,³⁰ while desilication promotes amorphization of pore walls, and leads to the insertion of counter-ions to the structure, which have to be removed in an additional step.^31,32 An improvement in the catalytic activity of mesoporous silica-based materials, such as MCM-41, can be achieved by transformation of the pore walls into zeolitic structure.^22,33,34 Usually, this transformation makes the mesophase unstable and prone to collapse during zeolite crystallization,^35,36 although to some extend of crystallization it is possible to achieve a short-range molecular ordering and preserve an appreciable amount of mesoporosity. Such materials possess zeolitic domains in the mesoporous walls,³⁷ and are commonly known as micro-mesostructured solids, hierarchical zeolites, and micro-mesoporous hybrid materials. Because the zeolitic domains embedded in ordered mesoporous materials form micro-mesoporous hybrids, their properties originate from both components, zeolitic domains and ordered mesopores,³⁸ which results in high catalytic activity and selectivity, for instance, in processing bulky molecules present in heavy and ultra-heavy petroleum.²² Additionally, these materials exhibit high surface areas, large pore volumes, uniform microporous channels, excellent thermal and hydrothermal stability. All these features make MZ interesting catalytic materials.

This work is the first report on the preparation of high-quality MZ by microwave-assisted synthesis using single-surfactant templating. In previous reports, micro-mesoporous hybrid materials were developed by dual templating mechanism.²² Here we present the synthesis route that eliminates the use of organic templates, acids, organic solvents, and is accomplished by using the easily-programmable microwave, which is environmentally friendly, reproducible and inexpensive. The MZ samples synthesized in this work are based on ordered mesoporous material such as MCM-41 with embedded small zeolitic domains. MCM-41 acts as a support for the growth of zeolitic nanocrystals of ZSM-5/Mordenite-type. The crystalline zeolitic aluminosilicate nanodomains are created in the absence of organic templates, where H₂O and Na⁺ cation control their formation.²⁸ Thus, the resulting material can be considered as a mesoporous zeolite consisting of zeolitic domains embedded into ordered mesoporous structure. The main advantages of this synthesis include the elimination of organic molecular template and short synthesis time as compared with conventional autoclave-based solvothermal method. This strategy affords the MZ samples with high surface area and large pore volume that perform well as catalysts in cracking of vacuum gas oil.

Experimental

Two routes were explored for the synthesis of mesoporous zeolites under microwave irradiation by using single-surfactant templating mechanism: one-pot synthesis (MZ-OS) and two-step synthesis (MZ-TS). First, a microwave-assisted synthesis of MFI-type zeolite without organic molecular template was performed according to the previously patented method²⁸ but under microwave irradiation instead of conventional oven heating.

Microwave-assisted synthesis of MFI-type zeolite without organic molecular template

In a typical synthesis, an aqueous mixture of silica gel (TLC high purity grade, Aldrich; particle sizes ≈ 5–25 μ, BET surface area ≈ 500 m² g⁻¹, pore volume ≈ 0.75 cm³ g⁻¹, average pore diameter ≈ 60 Å) was combined with a solution of sodium aluminate (NaAlO₂, Strem Chemicals, 99.9% Al), sodium hydroxide (NaOH, Fisher Scientific, 98.4%) and deionized water to achieve the following molar composition: 12Na₂O : 100SiO₂ : 2Al₂O₃ : 2500H₂O. The reaction mixture was stirred at 70 °C for 5 hours to obtain a homogeneous gel, which was transferred into microwave Teflon vessels. The vessels containing synthesis gels were kept at 180 °C under microwave irradiation for a period of time ranging from 18 to 24 hours. The resulting materials were filtered, washed and dried. The samples were labeled: ZSM-5-MW-X, where “X” represents time of the microwave treatment. A commercial ZSM-5 sample (Sentex Industrial Ltd.) was used for the purpose of comparison.

Microwave-assisted synthesis of mesoporous zeolites by using single-surfactant templating

One-pot synthesis. The MZ samples were obtained by single-surfactant templating by adding cetyltrimethylammonium bromide (CTAB) to the hydrogel prepared according to the procedure described in the previous section. Simply, an aqueous solution of CTAB (Aldrich, 98%) was added to the initial gel (having the same composition as that provided in previous section) under stirring, which was continued for additional 3 hours. The resulting hydrogel, having the following molar composition: 16CTAB : 12Na₂O : 100SiO₂ : 2Al₂O₃ : 2500H₂O, was transferred into microwave Teflon vessels, which were kept at 180 °C under microwave irradiation for a period of time ranging from 24 to 36 hours. The obtained materials were filtered, washed, dried and calcined in air at 540 °C for 4 hours (heating rate of 2 °C min⁻¹). The resulting samples were labeled: MZ-OS-X, where “X” represents the time of microwave treatment. For the purpose of comparison, the acidic form of the aluminum-containing MCM-41 was obtained as in³ with the following molar composition: 4SiO₂ : 1Na₂O : 0.13Al₂O₃ : 1CTAB : 200H₂O and the ZSM-5/AlMCM-41 hybrid micro-mesoporous material with the molar composition of 1SiO₂ : 0.32Na₂O : 0.03Al₂O₃ : 0.20TPAB : 0.16CTAB : 55H₂O²² was prepared by hydrothermal method using two templates of tetrapropylammonium bromide (TPAB) and cetyltrimethylammonium bromide (CTAB).

Two-step synthesis. The MZ samples were also synthesized by single-surfactant templating mechanism using a two-step procedure. Typically, an aqueous solution of silica gel (TLC, Aldrich; the same as that described in the first section of experimental) was combined with a solution of NaAlO₂, NaOH, H₂O and then, the reaction mixture was stirred at 70 °C for 5 hours to obtain a homogeneous gel. This hydrogel was transferred into microwave Teflon vessels, which were kept at 180 °C under microwave irradiation for 22 hours (first step). Then, the mixture was allowed to cool down to 35 °C and aqueous solution of CTAB (Aldrich, 98%; a soft template often used for the preparation of mesoporous molecular sieves) was added to the initial gel. The molar composition of this mixture was: 16CTAB : 12Na₂O : 100SiO₂ : 2Al₂O₃ : 2500H₂O. After transferring the vessels back to microwave, the hydrothermal treatment was conducted with fast stirring (option 3 in MARS5) at 70 °C for 8 hours; then, temperature was ramped to 100 °C and the samples were kept at this temperature for 8, 10 and 14 hours without stirring. The obtained materials were filtered, washed and calcined in air at 540 °C for 4 hours (heating rate of 2 °C min⁻¹). The samples were labeled: MZ-TS-X, where “X” represents the time of microwave treatment (22 h at 180 °C plus 8, 10 and 14 hours at 100 °C). Characterizations and testing of zeolitic materials are available in ESI.†

Results and discussion

MFI-type zeolites obtained without organic molecular template under microwave conditions

Wide-angle X-ray diffraction patterns of the MFI-type zeolite samples synthesized without organic molecular template are shown in Fig. 1. Additionally, the XRD pattern of the standard commercial ZSM-5 sample is shown to identify the zeolitic structure. The lack of intense peaks for the samples prepared below 20 hours of microwave treatment indicates that the long range crystallization starts to take place after 22 hours. The best formation of the MFI-type zeolite is observed for the sample kept under microwave irradiation for 24 hours. There is a significant reduction in the synthesis time to form the MFI structure as compared to the hydrothermal procedure without microwave irradiation presented elsewhere.²⁸ Fig. 2 illustrates N₂ adsorption–desorption isotherms for the ZSM-5-MW-22 and ZSM-5-MW-24 samples, and Table S1 (ESI†) shows the textural properties of ZSM-5 zeolites and commercial ZSM-5 sample. The MFI-type zeolite samples synthesized without organic molecular template under microwave irradiation exhibit a substantial N₂ adsorption at low relative pressures. N₂ adsorption isotherms are type I, typical for microporous materials according to the IUPAC classification¹⁹ and textural properties of these materials are similar to a commercial ZSM-5.

Fig. 1 Wide-angle XRD spectra of the ZSM-5 zeolite samples.

Fig. 2 N₂ adsorption–desorption isotherms of the ZSM-5 zeolite samples.

Both synthesized materials possess high specific surface areas along with large pore volumes, which mainly come from the presence of micropores. Textural properties of the prepared samples are comparable to the commercial ZSM-5 material; however, it is noteworthy that ZSM-5-MW-22 sample exhibit a slightly higher total pore volume as compared to commercial ZSM-5. This result is in line with literature³⁹ and confirms that a decrease in the total pore volume of MFI-type zeolites is usually accompanied by an increase in their crystallinity.³⁹

Single-surfactant templated mesoporous zeolites obtained under microwave conditions

One-pot synthesis. Low-angle and wide-angle X-ray diffraction spectra of the calcined MZ samples, standard AlMCM-41, and commercial ZSM-5 are presented in Fig. 3(A) and (B) respectively.

Fig. 3 (A) Low-angle and (B) wide-angle XRD spectra of the MZ samples obtained by one-pot synthesis.

The low-angle XRD patterns for the MZ-OS-28 and MZ-OS-34 samples show a peak between 1.5° and 2.5°, which is in the range of (100) peak usually present in the XRD patterns of MCM-41, indicating that the samples studied possess uniform mesopores. The typical (110), (200), and (210) peaks characteristic for hexagonally ordered materials are not clearly observed, what indicates the lack of mesostructural ordering in the zeolites studied. The MZ-OS-24 and MZ-OS-36 samples do not show peaks in the 1.5–2.5° ranges, therefore 24 hours and 36 hours syntheses do not produce samples with uniform mesopores. The XRD patterns recorded at wide angles show a transition from an amorphous to crystalline phase after 34 hours of microwave treatment – the phases present in the material are identified as MFI/MOR-type structures, while pure MFI-type structure was obtained when time of the synthesis was extended to 36 hours. Thus, the best preservation of uniform mesostructure along with formation of the zeolite MFI/MOR-type domains was observed for the crystallization time of 34 hours. This kind of behavior was reported for other synthetic strategies,²² where depending on the synthesis conditions the mesostructural ordering disappeared.

Fig. 4(A) and (B) illustrate the N₂ adsorption–desorption isotherms and the PSD curves for the MZ-OS materials. All the samples possess hysteresis loops characteristic for mesoporous materials,⁴⁰ except for MZ-OS-36. The MZ-OS-34 sample exhibits a large amount of micropores, which can be attributed to the presence of zeolitic structure, and the highest amount of mesopores with pore sizes around 4 nm. The textural and structural properties of the MZ-OS materials are summarized in Table 1.

Fig. 4 (A) N₂ adsorption–desorption isotherms and the corresponding pore size distributions (B; inset) for the MZ samples obtained via one-pot synthesis.

Table 1 Textural properties of the MZ samples obtained via one-pot synthesis under microwave irradiation and ZSM-5/AlMCM-41 prepared in conventional oven^22a

Sample	Vt Pore volume (cm^3 g^−1)	SBET (m^2 g^−1)	wKJS pore size at PSD max (nm)
a *By one-pot, dual templating and conventional autoclave heating, Vt = amount of N2 adsorbed at the P/P0 = 0.98; SBET = obtained from adsorption data at 0.05 < P/P0 < 0.2; wKJS = calculated by the KJS method.^41
MZ-OS-24	0.80	828	3.64
MZ-OS-28	0.71	569	4.22
MZ-OS-34	0.57	648	3.86
MZ-OS-36	0.24	496	3.49
ZSM-5/AlMCM-41*	0.70	529	1.00

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Two-step synthesis. Analysis of the low-angle and wide-angle X-ray spectra of the calcined MZ samples obtained via single-surfactant templating under microwave irradiation, the standard AlMCM-41, and commercial ZSM-5 are presented in Fig. 5(A) and (B). Sample MZ-TS-30 exhibits the best mesostructural ordering with zeolitic MFI/MOR-type domains. The identification of zeolitic phase was performed by comparison of the XRD spectra recorded for the synthesized samples and the commercial Mordenite and ZSM-5 zeolites (see Fig. 6). As can be seen from this figure, the samples synthesized during 30 and 34 hours show a mixture of MFI and MOR type structures, which becomes the MFI type structure when synthesis time is extended to 36 hours. Fig. 3 and 5 show that the mesophase becomes instable and disappears upon prolonged crystallization time.

Fig. 5 (A) Low-angle and (B) wide-angle XRD spectra of the MZ samples obtained via two-step synthesis.

Fig. 6 Wide-angle XRD spectra of the MZ-OS-34 and MZ-TS-30 samples, and commercial Mordenite and ZSM-5.

Fig. 7(A) displays N₂ adsorption–desorption isotherms and Fig. 7(B; inset) the PSD curves for the MZ-TS materials. For all MZ-TS samples, there is a substantial amount of N₂ adsorbed at very low relative pressures (P/P₀ < 0.05), which further confirms the presence of microporosity presumably originating from the zeolitic structure. Samples MZ-TS-32 and MZ-TS-36 do not show adsorption hysteresis, while in the case of MZ-TS-30, even though hysteresis is not observed, the isotherm shape is similar to the MCM-41 structure with mesopores below 4 nm. The PSD curve of the MZ-TS-30 material (Fig. 7(B)) shows a sharp peak with a maximum around 4 nm; therefore, the sample studied possesses small mesopores. The textural and structural properties of the MZ-TS materials are summarized in Table 2.

Fig. 7 (A) N₂ adsorption–desorption isotherms and the corresponding pore size distributions (B; inset) for the MZ samples obtained via two-step synthesis.

Table 2 Textural properties of the MZ samples obtained via two-step synthesis under microwave irradiation and ZSM-5/AlMCM-41 prepared in conventional oven^22a

Sample	Vt Pore volume (cm^3 g^−1)	SBET (m^2 g^−1)	wKJS Pore size at PSD max (nm)
a *By one-pot, dual templating and conventional autoclave heating, Vt = amount of N2 adsorbed at the P/P0 = 0.98; SBET = obtained from adsorption data at 0.05 < P/P0 < 0.2; wKJS = calculated by the KJS method.^41
MZ-TS-30	0.51	757	3.53
MZ-TS-32	0.20	390	1.02
MZ-TS-36	0.22	407	1.02
ZSM-5/AlMCM-41*	0.70	529	1.00

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CO₂ adsorption

The CO₂ adsorption capacities for the MZ samples were investigated at 23 °C under ambient pressures.

Fig. 8 shows the CO₂ adsorption isotherms measured for the following samples: MZ-TS-30, MZ-OS-34, ZSM-5-MW-24 h, ZSM-5/AlMCM-41, and aluminum-containing MCM-41. The CO₂ uptakes at 23 °C and 760 mmHg for all samples except AlMCM-41 are in the range 1.5–2.3 mmol g⁻¹ (Table S2, ESI†) and analogous to the data reported for different zeolites.^42,43 The highest uptake of 2.3 mmol g⁻¹ for MZ-TS-30 can be attributed to the small micropores present in ZSM-5 domains and large surface area of this sample. Note that AlMCM-41 features also large surface area but does not have small zeolitic micropores, therefore its CO₂ uptake is smallest among the samples studied.

Fig. 8 CO₂ adsorption isotherms obtained at 23 °C for the samples studied.

Pyrolysis of VGO

The spectrum showing the distribution of the hydrocarbon products obtained by coupled catalytic pyrolysis-GC/MS and the spectrum obtained for thermal pyrolysis of VGO alone (analysis without catalyst) are shown in Fig. 9(a) and (b) respectively. Thermal pyrolysis of the VGO generates hydrocarbons in the carbon range from C₁₇ to C₄₁ with high activation energy,⁴⁴ however; when the MZ-TS-30 was added to VGO, the light fraction could be obtained. The results show that the catalytic pyrolysis of VGO physically mixed with 10 wt% of MZ-TS-30 results in hydrocarbon products in the range of C₃–C₅ (liquefied petroleum gas, LPG), the middle distillates, C₆–C₁₀ (gasoline) and C₁₁–C₁₆ (diesel) with low activation energy. The LPG and gasoline fractions were obtained due to strong acid sites⁴⁵ combined with microporosity of the MFI/MOR-type zeolite, whereas the diesel fraction was attributed to the mesoporosity associated with mild acid sites of AlMCM-41.⁴⁶ Overall, MZ-TS-30 shows a great potential in production of light fractions of hydrocarbons and could be successfully employed as a catalyst in cracking vacuum gas oil.

Fig. 9 Distribution of the hydrocarbon products obtained by coupled pyrolysis-GC/MS: the catalytic pyrolysis of VGO physically mixed with 10 wt% of MZ-TS-30 (a), and pyrolysis of VGO⁴³ without catalyst (b).

SEM and TEM analysis

Fig. S1(A) and S2(B)† show the SEM images of the MZ-TS-30 and MZ-OS-34 samples and Fig. S1(C) and (D)† show the SEM images of the ZSM-5-MW-24 zeolite. Both samples shown in Fig. S1(A) and (B)† consist mostly of plate-like particles with a hexagonal crystal shape with size up to 400 nm; this morphology is commonly reported for MCM-41 aluminosilicates.⁴⁷ The SEM image of MZ confirms the formation of the hybrid structure, while isolated crystals as those visible for the ZSM-5-MW-24 zeolite (Fig. S1(C) and (D)†), are not observed. The presence of such isolated crystals would indicate the growth of ZSM-5 independently of the mesoporous structure formation,⁴⁸ whereas in the case of MZ-TS-30 only homogeneous material is visible.

Fig. 10 and S2 (ESI†) show the TEM images of the obtained mesoporous zeolites under two different crystallization times and synthesis routes: MZ-OS-34 (Fig. 10, top panels and Fig. S2(A) and (B) in ESI†), as well as MZ-TS-30 (Fig. 10, bottom panels, and Fig. S2(C) and (D) in ESI†). As can be seen from these figures, the hybridization of the zeolitic crystalline phase did not significantly alter the mesostructural ordering. The corresponding Fast Fourier Transform (FFT) patterns (insets in Fig. S2, ESI†) were also obtained at different foci for MZ-OS-34 (insets in Fig. S2(A) and (B), ESI†) and MZ-TS-30 (insets in Fig. S2(C) and (D), ESI†). In the case of MZ-OS-34 these patterns show the presence of mesopores with diameters of ca. 3.63 nm (inset in Fig. S2(A), ESI†), and the presence of micropores with diameters of ca. 0.56 nm (inset in Fig. S2(B), ESI†), which is characteristic for the MFI structure. Similarly, the FFT patterns for MZ-TS-30 show mesopores with diameters of ca. 3.15 nm (inset in Fig. S2(C), ESI†) and the d-value between the (011) MFI structure planes of ca. 1.1 nm (inset in Fig. S2(D), ESI†). Additionally, the estimated sizes of mesopores in both samples are analogous to those obtained by N₂ adsorption analysis. Both adsorption and TEM analyses confirmed the co-existence of mesoporosity and microporosity (zeolitic structures), which is essential for the composite materials with improved diffusion properties and capable for efficient cracking of heavy oil. In the both synthesis routes, the intracrystalline mesopores are nearly of the same size, around 4 nm, due to the effect of the structure-directing agent (Fig. 10).

Fig. 10 TEM images of the mesoporous zeolites prepared using different crystallization times and synthesis routes: MZ-OS-34 (top) and MZ-TS-30 (bottom).

Conclusions

In this work, mesoporous zeolites were synthesized by single-surfactant organic templating and hydrothermal treatment under microwave irradiation. The resulting hybrid materials consist of the zeolitic crystallites (MFI/MOR-type crystalline phase) embedded into the mesoporous material (MCM-41 non-crystalline phase), where MCM-41 acts as a support for growth of the zeolitic nanocrystals of Mordenite and ZSM-5. Mesopores were created by using soft templating and micropores were formed in the absence of an organic molecular template. Nitrogen adsorption–desorption showed high surface area of mesoporous zeolites when compared with the literature data for the samples obtained by dual templating mechanism and conventional autoclave heating. The catalytic test of the new materials involving pyrolysis of VGO indicates that the products were in the range of liquefied petroleum gas, gasoline and diesel – light fractions of higher commercial value.

Acknowledgements

This work was supported by the National Council of Technological and Scientific Development (CNPq, Brazil). The TEM data were obtained at the (cryo) TEM facility of the Liquid Crystal Institute, Kent State University, supported by the Ohio Research Scholars Program Research Cluster on Surfaces in Advanced Materials. The authors thank Dr Miroslaw Salamonczyk and Michal Marszewski for taking images and Dr Min Gao for technical support with the TEM experiments.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Details of characterization procedures; 2 figures showing the SEM and TEM images of the samples studied; and 2 tables listing adsorption parameters and CO₂ uptakes. See DOI: 10.1039/c6ra06554f

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