Solvent-Free Synthesis of Zeolites: Mechanism and Utility - Accounts

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Cite This: Acc. Chem. Res. XXXX, XXX, XXX−XXX

Solvent-Free Synthesis of Zeolites: Mechanism and Utility Qinming Wu,† Xiangju Meng,*,† Xionghou Gao,‡ and Feng-Shou Xiao*,† †

Department of Chemistry, Zhejiang University, Hangzhou 310028, China Petrochemical Research Institute, PetroChina Company Limited, Beijing 100195, China

‡

CONSPECTUS: Zeolites have been extensively studied for years in different areas of chemical industry, such as shape selective catalysis, ion-exchange, and gas adsorption and separation. Generally, zeolites are prepared from solvothermal synthesis in the presence of a large amounts of solvents such as water and alcohols in sealed autoclaves under autogenous pressure. Water has been regarded as essential to synthesize zeolites for fast mass transfer of reactants, but it occupies a large space in autoclaves, which greatly reduces the yield of zeolite products. Furthermore, polluted wastes and relatively high pressure due to the presence of water solvent in the synthesis also leads to environmental and safety issues. Recently, inspired by great benefits of solvent-free synthesis, including the environmental concerns, energy consumption, safety, and economic cost, researchers continually challenge the rationale of the solvent and reconsider the age-old question “Do we actually need solvents at all in zeolite synthesis?” In this Account, we briefly summarize our efforts to rationally synthesize zeolites via a solvent-free route. Our research demonstrates that a series of silica, aluminosilicate, and aluminophosphate-based zeolites can be successfully prepared by mixing, grinding, and heating starting solid materials under solvent-free conditions. Combining an organotemplate-free synthesis with a solvent-free approach maximizes the advantages resulting in a more sustainable synthetic route, which avoids using toxic and costly organic templates and the formation of harmful gases by calcination of organic templates at high temperature. Furthermore, new insights into the solvent-free crystallization process of zeolites have been provided by modern techniques such as NMR and UV-Raman spectroscopy, which should be helpful in designing new zeolite structures and developing novel routes for synthesis of zeolites. The role of water and the vital intermediates during the crystallization of zeolites have been proposed and verified. In addition to a significant reduction in liquid wastes and a remarkable increase in zeolite yields, the solvent-free synthesis of zeolites exhibits more unprecedented benefits, including (i) the formation of hierarchical micro-, meso-, and macrostructures, which benefit the mass transfer in the reactions, (ii) rapid synthesis at higher temperatures, which greatly improve the space−time yields of zeolites, and (iii) construction of a novel catalytic system for encapsulation of metal nanoparticles and metal oxide particles within zeolite crystals synergistically combining the advantages of catalytic metal nanoparticles and metal oxide particles (high activity) and zeolites (shape selectivity). We believe that the concept of “solventfree synthesis of zeolites” would open a door for deep understanding of zeolite crystallization and the design of efficient zeolitic catalysts.

1. INTRODUCTION Zeolites, as one of the most important catalysts types, have been widely applied both in the energy industry and for environmental applications.1−3 Normally, the synthesis of zeolites is carried out under hydrothermal conditions, which involves the use of a large amount of water as a solvent in sealed autoclaves. This typically leads to safety and environmental issues such as the formation of polluted water, dissolution of silica-based species in alkaline media, and consequent reduction in zeolite yields, together with high pressure.4,5 It has long been regarded that a large amount of water solvent is essential in zeolite synthesis, because it is a requirement for efficient transport of reactants in hydrothermal synthesis.5 Besides water, other solvents, such as alcohols and ionic liquids, have also been used in zeolite synthesis.5−9 In particular, the unique features of ionothermal synthesis effectively avoid the safety issues related to high autogenous © XXXX American Chemical Society

pressures and the competition between template−framework and solvent−framework interactions.7−9 Pioneering research to challenge the essentiality of water in zeolite synthesis has been performed using dry-gel conversion (DGC) and vapor-phase transport (VPT) routes, where a sodium aluminosilicate gel suspended above a liquid was treated with a mixed vapor of water and amine at elevated temperature and autogenous pressure in a sealed autoclave.10 It is worth noting that in this case a large amount of water is necessary for the preparation of the gel. Later, Schüth et al. reported the synthesis of zeolites via a gas phase transport in the presence of NH4F.11,12 The mobile species are likely formed by the transformation of the amorphous SiO2 precursor according to reaction 1:12 Received: February 2, 2018

A

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Accounts of Chemical Research SiO2 + 4NH4F = SiF4 + 2H 2O + 4NH3

(1)

The above results offer a belief that it is not necessary to use a large amount of water for the successful synthesis of zeolites. In 2012, a novel and generalized solvent-free route was reported for synthesizing the silica and aluminosilicate zeolites by physical mixture of solid raw materials, followed by grinding and heating (Figure 1), where it is not necessary to add the

Figure 1. Solvent-free synthesis of zeolites. Reprinted with permission from ref 13. Copyright 2012 American Chemical Society. Figure 2. 19F MAS NMR spectra of S-silicalite-1 zeolites with crystallization times of (a) 0, (b) 2, (c) 2.5, (d) 3, (e) 6, (f) 9, and (g) 15 h, respectively. Reprinted with permission from ref 16. Copyright 2015 American Chemical Society.

water solvent.13 Later, this solvent-free route was applied to prepare aluminophosphate-based molecular sieves.14 Combined with an organotemplate-free route, the solvent-free synthesis maximizes the sustainable advantages.15 Furthermore, characterization of solvent-free synthesis shows novel insights into the crystallization process of zeolites.16−18 This Account briefly summarizes these works to date.

the synthetic process, a signal with a chemical shift at −126.6 ppm is observed, which is attributed to the presence of SiF62− species from the interaction between silica and NH4F, as suggested by the reaction as follows:16,20,21

2. THE ROLE OF WATER The results reported by Ren et al.13 have been regarded as a striking success because there have been many attempts to synthesize zeolites using a solvent-free route in the past years.19 They point out that zeolites cannot be formed from a solventfree route if a trace of water (hydrated form of silica) is absent in the solid synthesis process, suggesting the importance of a trace of water for the solvent-free crystallization of zeolites. Normally, the water is used in (i) the stage of depolymerization by hydration of silica species and (ii) the stage of condensation from hydrated silica species, as proposed in the following reactions: Si(OSi)4 + nH 2O → Si(OSi)4 − m (OH)m

SiO2 + 6NH4F = SiF6 2 − + 2NH4 + + 2H 2O + 4NH3 (4)

After crystallization for 2.5 h, an additional peak with a chemical shift at −62.7 ppm is shown, which is associated with F− species in a [415262] cage in the silicalite-1 zeolite (Figure 2).16,20,21 Interestingly, the intensity of this peak gradually increases with decreasing SiF62− species, indicating that the condensation from SiF62− species to the Si(OSi)4 in the [415262] cage and the release of F− species occurs in this stage of the MFI crystallization, as indicated by the following reaction:16,20,21

(1 ≤ m ≤ 4) (2)

Si(OSi)4 − m (OH)m → Si(OSi)4 + nH 2O

SiF6 2 − + 2H 2O + 4NH3 = SiO2 + 4NH4 + + 6F−

(1 ≤ m ≤ 4)

(5)

Similarly, reaction 4 plus reaction 5 suggest that the F− species could serve as “catalysts” for depolymerization of anhydrous solid silica and condensation of the depolymerized silica, where water is also formed in the silica depolymerization along with the consumption of F− species, in good agreement with previous reports.11,12 Distinguishable from the crystallization of silica-based zeolites, it is not necessary to add a trace of water solvent for solvent-free crystallization of silicoaluminophosphate-based zeolites, because the interaction between NH4H2PO4 and boehmite in the starting raw solids can form water as a byproduct that is enough for the formation of the hydrated silica species in the solvent-free crystallization. Recently, various aluminophosphate (APO-11 and APO-5), silicoaluminophosphate (SAPO-43, -34, -20, and -11), and heteroatom-containing (M = Fe, Co, and Mn) aluminophosphate (M-SAPO-5 and MAPO-11) zeolites have been synthesized in the absence of water as the solvent.14,22

(3)

If the above two reactions are combined, water appears on both sides of the equation. In this case, water can be regarded as a “catalyst” in the zeolite crystallization: water is consumed in the hydration of silica species stage and is formed again in the next condensation of the hydrated silica species. If the water could be used as a “catalyst” for zeolite crystallization, it is reasonable that a large amount of water in the zeolite crystallization is not necessary. After successful solvent-free synthesis of zeolites in the presence of a trace of water, we considered that the solvent-free synthesis of zeolites from anhydrous starting materials is possible. To answer this question, a solvent-free synthesis of zeolites is performed from anhydrous solid raw materials in the presence of NH4F, giving the zeolite products with high crystallinity.16 As a typical model, the contribution of F− species in the synthesis of MFI zeolite is investigated with 19F NMR technique, as shown in Figure 2. Notably, in the initial stage of B

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3. THE ADVANTAGES OF SOLVENT-FREE SYNTHESIS OF ZEOLITES

Table 1. STYs of Zeolites Synthesized at High Temperatures

3.1. Sustainable Routes

Compared with the traditional hydrothermal synthesis, the solvent-free synthesis of zeolites is typically sustainable, exhibiting obvious advantages as follows: (i) High yields. Conventional hydrothermal synthesis has nutrients (silicates and aluminates) dissolved in the mother solution, while the solvent-free synthesis greatly reduces these losses. As a result, the yield of MFI zeolite using a solvent-free method is as high as 93−95% (based on SiO2),13 which is much higher than that (80−86%) of the hydrothermal route. (ii) Better utilization of autoclaves. Most of the space in autoclaves has been generally occupied by the large amount of water in the hydrothermal synthesis, which has been excluded in the solvent-free synthesis. For example, the weight of the beta product under solvent-free conditions is almost 14 times that of a conventional hydrothermal synthesis in the same autoclave.15 (iii) A remarkable reduction of pollutants. Avoidance of water addition in the synthesis maximally reduces the formation of liquid wastes. (iv) Low crystallization pressure. The lack of solvent in the zeolite crystallization effectively decreases the autogenous pressure, eliminating many safety concerns. (v) A simple crystallization process. The solvent-free route mainly involves a basic procedure of mixing and heating the raw solids. In addition to these features, the solvent-free method could also be combined with other sustainable strategies for synthesizing zeolites.4,23,24 For example, a cooperation of solvent-free and organotemplate-free routes was successfully applied to synthesize *BEA and MFI zeolites without addition of both solvent and organic templates.15 In this case, avoidance of costly and toxic organic templates as well as solvents reduces the costs of zeolite synthesis and the environmental concerns by mitigating the release of harmful gases formed by the calcination of organic templates and the liquid wastes containing organic templates and silica-based inorganic species, which would be an ultimate goal for the synthesis of zeolites in a sustainable and economical manner.

zeolite

temp (°C)

crystallization time (h)

STY in this work (kg/m3·day)

MFI MOR beta RUB-36

240 240 200 200

0.5 1.5 2 36

11000 4200 2130 178

Reprinted with permission from ref 25. Copyright 2017 Royal Society of Chemistry.

MOR, beta, and MFI are as high as 178, 4200, 2130, and 11000 kg/m3·day, respectively, which are almost 1−2 orders of magnitude higher than those for the corresponding hydrothermal route. To determine the stabilization of organic templates in solvent-free synthesis, 13C MAS NMR spectra of as-made RUB-36 were recorded, and the results showed that the organic template (diethyldimethylammonium, DMDEA+) retains its molecular integrity at 160−200 °C under solvent-free conditions. In contrast, this organic template is partially decomposed at 140 °C under hydrothermal conditions. The absence of water and the confinement of the organic template in the framework of RUB-36 strongly decrease the decomposition of the organic template via a Hofmann elimination, in good agreement with what has been reported previously.26,27 3.3. Formation of Mesoporous Zeolites

Mesoporous zeolites have received much attention in the past decade because of the combination of advantages from zeolites (shape selectivity) and mesoporous materials (fast mass transfer),28,29 and their synthetic strategies can be mainly classified into “top-down” and “bottom-up” strategies.30 The top-down strategy begins with microporous zeolites that are then post-treated to generate the structures of hierarchical zeolite usually by desilication or dealumination or both. Alternatively, the bottom-up strategy constructs mesoporous zeolites by the engineering of microporous and mesoporous domains, which often involves complicated and costly mesoscale templates.31 Based on an economic view, it is highly desirable to develop the bottom-up approach in the absence of the mesoscale organic templates.30 Interestingly, mesoporosity has been found in the solventfree synthesis of SAPO-34 based on N2 sorption isotherms and TEM images, even though the mesoscale template is absent in the synthesis, as given in Figure 3.14 More recently, a solventfree and mesoporogen-free route for synthesizing mesoporous ZSM-5 zeolites was also reported.32 High-resolution TEM images of the zeolite products at different crystallization times, from 6 to 72 h, provide direct evidence for the existence of the adjustable mesoporosity in these samples (Figure 4). When the

3.2. Rapid Synthesis at High Temperatures

Space−time yield (STY) is an important factor for industrial production. Classical hydrothermal synthesis can be regarded as a typically low STY process due to a relatively long crystallization time (low time yield) and the use of water in the autoclaves (low space yield). To improve the space−time yields of zeolites, it is suggested to reduce the amount of solvent and increase the crystallization rate in the synthesis. Increasing the crystallization temperature could effectively accelerate the crystallization rate of zeolites, but it is difficult for conventional hydrothermal synthesis to be performed at very high temperatures due to the decomposition of organic templates in alkaline media and the high autogenous pressure from water at such high temperatures. Recently, solvent-free synthesis has proven to stabilize organic templates along with eliminating the high autogenous pressure at very high temperature.25 As a result, high STYs have been achieved at temperatures higher than 200 °C under solvent-free conditions. For example, RUB-36, MOR, beta, and MFI could be completely crystallized within 36, 1.5, 2.0, and 0.5 h, while conventional hydrothermal synthesis of these zeolites takes 300−350, 24−48, 48−72, and 12−24 h, respectively, as presented in Table 1. Correspondingly, the STYs of RUB-36,

Figure 3. (a) SEM image and (b) TEM image of solvent-free synthesized S-SAPO-34. Reprinted with permission from ref 14. Copyright 2013 Wiley-VCH. C

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in the presence of surfactants (such as CTAB), wherein the amount of surfactant added plays an important role in controlling the thickness of the zeolite crystals.34 Generally, the surfactants are aggregated to form surfactant micelles for templating the mesostructure in the hydrothermal synthesis. In contrast, the surfactant molecules tend to selectively absorb on the external surface of zeolite crystals in the solvent-free route, hindering the growth of specific crystalline facets to produce SAPO-5 crystals with plate-like morphology. Similar phenomenon could also be found in the solvent-free synthesis of SAPO11 crystals in the presence of F127 surfactant.34 In earlier work for solvent-free synthesis of ZSM-5 zeolite, large crystals (5−30 μm) with aggregation are normally produced.13 Recently, Luo et al. were successful in the solvent-free preparation of ZSM-5 zeolite particles composed of nanocrystals (10−40 nm) in the presence of hydrated sodium carbonate, where the formation of nanocrystals is strongly related to adjusting the hydrated sodium carbonate in the synthesis.35 More recently, Geng et al. successfully synthesized the faceted and well-dispersed silicalite-1 zeolite from a solvent-free route assisted by graphene oxide (GO). The selective interactions between the different facets and GO sheets led to oriented growth of silicalite-1 zeolite along the caxis.36

Figure 4. HRTEM images of the S-ZSM-5 zeolites crystallized for (a) 6, (b) 18, (c) 30, and (d) 72 h. Reprinted with permission from ref 32. Copyright 2017 American Chemical Society.

crystallization time is from 6 to 18 h, the mesoporous sizes are in the range 5−8 nm; when the crystallization reaches 30 h, the mesoporous sizes are distributed in 10−20 nm; when the crystallization time is 72 h, the mesoporous sizes become 20− 30 nm. In the aforementioned examples, the mechanism for the formation of mesostructures in the zeolite crystals is still unclear. One possibility is an imprint of gaseous expansion in the solvent-free crystallization of zeolites, and solid evidence is under investigation.

4.2. Mechanism on Zeolite Synthesis

It is strongly desirable to fully understand zeolite crystallization due to the importance in both fundamental research and industrial applications, but it is challenging for conventional hydrothermal synthesis because there are so many different silicate species in the starting gels containing both liquid and solid phases leading to difficult recognition and identification of the key intermediates for zeolite crystallization. A solvent-free route could minimize the amount of water and effectively avoid the intermediates formed in the liquid phase, which offers a promising opportunity to deeply understand the intermediates formed in zeolite crystallization. As an example, the solvent-free crystallization of APO-5 crystals has been well characterized.17 Besides the routine MAS NMR techniques, the J-mediated heteronuclear multiple-

4. UTILIZATION OF SOLVENT-FREE SYNTHESIS 4.1. Morphological Control of Zeolite Crystals

Morphological control of zeolite crystals is one of the key factors for enhancing mass transfer and catalytic performances. Typically morphological control of zeolite crystals includes nanosheets and nanocrystals prepared in the presence of organic templates under a hydrothermal route.33 Recently, we reported that silicoaluminophosphate (SAPO-5) crystals with plate-like morphology could be prepared by a solvent-free route

Figure 5. 31P-{27Al} J-HMQC spectra of washed molecular sieve products crystallized for (A) 0, (B) 1, (C) 1.5, (D) 3, and (E) 24 h and (F) 24 h calcined. Reprinted with permission from ref 17. Copyright 2016 American Chemical Society. D

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Accounts of Chemical Research quantum correlation (J-HMQC) sequence has been applied to directly evaluate the structural connectivity of the P−O−Al species, providing precise information on the intermediates in the crystallization process.37 As shown in Figure 5, there is no relevant signal in the spectrum (Figure 5A) because of the absence of Al−O−P units in the raw mixtures. When the crystallization time is 1 h (Figure 5B), the top cross contours are mainly associated with the signals of P atoms at −18.5 ppm to the signals from an Al atom at −4.8 to −1.0 ppm, which are related to the octahedral Al and P units that are formed to 4- and 6-membered rings (MRs) and branched ring units in the intermediates.38 These 4/6-MRs could be organized to constitute the one-dimensional Al2P2 parent chains and then three-dimensional frameworks around TEA+ templates. Theoretical simulation strongly supports that the 4/6-MR chains as the intermediates assemble to onedimensional chains and then a three-dimensional framework.39 When the crystallization time is enhanced to 1.5 h (Figure 5C), the cross contour shows that all P and Al atoms are in the framework of APO-5. Longer crystallization time results in more perfect crystals. Thus, it can be concluded that the 4/6MRs are key intermediates for the solvent-free crystallization of APO-5 crystals. Encouraged by the investigation on the solvent-free route for synthesizing aluminophosphate AlPO-5, the 2D NMR technique has also been employed to monitor the solventfree crystallization of aluminosilicate zeolite A.18 Figure 6 shows the sheared 27Al 2D triple quantum magic angle spinning (3QMAS) spectra of the S-NaA zeolites at different crystallization times in the absence of water solvent. The experimental second-order quadrupole effect (SOQE) value of the Al atom (2.53 MHz) obtained from crystallization times of 0−2 h is very close to that of the Al atom in double four ring (D4R) units rather than those in S4R units, suggesting that the D4R units are major structural Si−O−Al species in the aged aluminosilicate gels and 2 h heated samples (Figure 6b). Increasing heating time leads to the appearance of new Al signals with smaller SOQE values (2.07 MHz, II), which are assigned to the Al atoms in the crystalline S-NaA zeolites. At the same time, the signals of the Al atoms in D4R units with higher SOQE values (I) are decreasing due to consumption of the D4R species to form LTA structures (Figure 6c−e). After 4 h of crystallization (Figure 6g), all D4R units have been transformed into the framework of LTA structure, and the SOQE value of the Al atom is 1.52 MHz in the framework of crystalline LTA, close to the value of the Al atom in S4R units, which might be related to the α-cage in NaA zeolite by the connections of D4Rs. These results demonstrate that the zeolite A with LTA structure is mainly constructed from the D4R units. Furthermore, the crystallization of zeolites beta and ZSM-5 in the absence of both organotemplates and solvents has also been extensively investigated.15 As shown from in situ UVRaman spectra of the ZSM-5 crystallization, the bands assigned to the building units of 5-membered rings (5MRs) are hardly observed in the starting solid mixtures, and the 5MRs are only formed after the formation of the framework of ZSM-5 zeolite. These results mean that the 5MRs cannot serve as basic building units for the crystallization of MFI structures. The in situ UV-Raman spectra of beta crystallization show that the mixture of solid raw materials is predominantly formed by 4MRs, suggesting that the 4MRs might be the basic building units for the formation of beta zeolite.15 These insights into

Figure 6. 27Al 3Q (MQ) MAS NMR spectra of the S-NaA zeolites crystallized for (a) 0, (b) 2.0, (c) 3.0, (d) 3.25, (e) 3.5, (f) 3.75, (g) 4.0, and (h) 5.0 h in the absence of water solvent. Reprinted with permission from ref 18. Copyright 2017 Elsevier.

zeolite crystallization might be helpful in designing new zeolite structures and developing novel routes for synthesis of zeolites. 4.3. Design of Novel Zeolitic Catalysts

Zeolite catalysts have excellent shape selectivities but shortage of hydrogenated activities due to difficult introduction of active metal nanoparticles in the zeolite frameworks. One solution for this challenge is to support these metal nanoparticles on the zeolite crystals, but the relatively easy aggregation of these metal nanoparticles on the external surface of zeolite crystals strongly hinders catalytic applications.40 Alternatively, active metal nanoparticles encapsulated inside of zeolite crystals have been designed and prepared, but hydrothermal synthesis leads to a significant loss of these metal nanoparticles from the liquid phase (34%). Solvent-free synthesis offers an opportunity for encapsulation of metal nanoparticles into zeolite crystals with highly utilization (>96%) owing to the solid transformation from amorphous phase to zeolite crystals. These metal nanoparticles encapsulated in zeolite crystals synergistically combine the advantages of metal nanoparticles (high activity) E

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Accounts of Chemical Research and zeolites (shape selectivity), developing novel efficient heterogeneous catalysts.41−44 Figure 7 shows the solvent-free synthesis of Pd nanoparticles encapsulated inside of silicalite-1 (designated as S-1) zeolite.41

Figure 7. Solvent-free route for synthesizing Pd@S-1 catalyst. Reprinted with permission from ref 41. Copyright 2016 American Chemical Society.

For the preparation, Pd nanoparticles were encapsulated with amorphous silica; then tetrapropyl ammonium hydroxide (TPAOH) as an organic template was added. After grinding at room temperature, the mixture was transferred into an autoclave, crystallizing at 180 °C for 3 days. After washing at room temperature, drying at 100 °C, and calcining at 550 °C for 4 h, Pd@S-1 was finally achieved. In the hydrogenation of biomass-derived furfural, the main products are tetrahydrofuran, dihydrofuran, furfuryl alcohols, and furan. Because furan is an attractive platform chemical, selective hydrogenation of furfural into furan is strongly desirable. Nevertheless, the typical supported Pd catalysts usually exhibit relatively low selectivity for furan. Interestingly, it is shown that the furan selectivity over the core−shell structure (metal@zeolite, Pd@S-1) is as high as 98.7% in the furfural hydrogenation. In comparison, the S-1 zeolite supported Pd nanoparticles (Pd/S-1) give very low furan selectivity (5.6%). The excellent furan selectivity over the Pd@ S-1 is reasonably related to the distinguishable mass transfer of the hydrogenated products in the micropores of the zeolite (shell of the catalyst), as evidenced by the infrared spectra of adsorbed molecules of the products and their calculated adsorption energies. Besides the shape-selectivity, the stability and wettability of zeolites could also be utilized to improve the catalytic properties. For example, in the oxidation of bioethanol, the catalyst stability and water tolerance are challenging. Notably, after solvent-free encapsulation of gold−palladium bimetallic nanoparticles with an S-1 zeolite shell (AuPd@S-1), AuPd@S-1 shows excellent activity, selectivity, and stability for the oxidation from bioethanol to acetic acid even in the presence of water, as given in Figure 8.42 In comparison, conventional S1 zeolite supported AuPd nanoparticles easily lose activity and selectivity in the oxidation. The extraordinary stability of AuPd@S-1 could be assigned to the confinement of AuPd nanoparticles within the S-1 framework, hindering the aggregation of the AuPd nanoparticles, while the excellent water tolerance of AuPd@S-1 is reasonably assigned to the hydrophobicity of the S-1 zeolite shell, hindering the transfer of water to access the AuPd nanoparticles in the zeolite micropores.

Figure 8. Dependency of (a) ethanol conversion and selectivities for (b) acetic acid, (c) ethyl acetate, and (d) acetaldehyde on the time in the bioethanol oxidation over AuPd@S-1 and AuPd/S-1. Reprinted with permission from ref 42. Copyright 2015 Wiley-VCH.

Besides metal nanoparticles, metal oxides could also be perfectly encapsulated into zeolite crystals by the solvent-free route. For example, manganese oxide encapsulated into S-1 zeolite (MnOx@S-1) exhibits high conversion and selectivity for the catalytic oxidative cyanation of various alkanes to corresponding nitriles by efficiently hindering the side hydration owing to the hydrophobic S-1 zeolite shell;43 when TiO2 or TiO2 loaded with Pt nanoparticles (Pt/TiO2) are fixed into zeolite structures, they show shape-selective degradation of small molecular pollutants but are eco-friendly for bulky organisms under photocatalytic conditions.44

5. CONCLUSIONS AND PERSPECTIVE The solvent-free route is a simple and generalized methodology for synthesis of a great many silica, aluminosilicate, and aluminophosphate-based zeolites (Table 2) with sustainable features such as high yields, low wastes, low pressure, fast crystallization, and organotemplate-free introduction of mesoporosity. A small amount of water or ammonium fluoride plays an important role in the crystallization of zeolite by the solventfree synthesis. In addition, the solvent-free route offers a good opportunity to deeply investigate zeolite crystallization and efficiently encapsulate metal and metal oxide particles, which are important for zeolite synthesis and preparation of zeolitebased catalysts in the future. Despite these advantages, insights into the formation of mesoporosity in the solvent-free synthesis of zeolites are still necessary. Although important intermediates for zeolite crystallization have been directly observed, there is still a shortage of solid evidence for the interaction between the F

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Accounts of Chemical Research Table 2. List of Zeolites Prepared by the Solvent-Free Route and Their Characteristics run

zeolite

IZA code

dimension

pore size

refs

1

ZSM-5

MFI

3

5.1 × 5.5, 5.3 × 5.6

2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

beta mordenite Y sodalite ZSM-39 SAPO-34 SAPO-11 SAPO-5 SAPO-43 SAPO-20 EU-1 ZSM-22 SSZ-13 ITQ-12 ITQ-13

*BEA MOR FAU SOD MTN CHA AEL AFI SOD GIS EUO TON CHA ITW ITH

3 2 3 0 0 3 1 1 0 0 1 1 3 2 3

6.6 × 6.7, 5.6 × 5.6 6.5 × 7.0, 2.6 × 5.7 7.4 × 7.4

17 18 19 20

ITQ-17 RUB-36 ZSM-58 A

BEC

3

4.1 × 5.4 4.6 × 5.7 3.8 × 3.8 2.4 × 5.4, 3.9 × 4.2 4.8 × 5.3, 4.8 × 5.1, 4.0 × 4.8 6.3 × 7.5, 6.0 × 6.9

13, 15, 16, 25 15, 16, 25 13, 25 13 13 13 14 14, 22 22 14 14 16 16 45 46 46

DDR LTA

2 3

3.6 × 4.4 4.1 × 4.1

3.8 × 3.8 4.0 × 6.5 7.3 × 7.3

Xionghou Gao received his Ph.D. degree at Lanzhou Institute of Chemical Physics, Chinese Academic of Science (1997), and has worked in the Lanzhou Petrochemical Research Center, Petrochemical Research Institute, Petrochina Company, since 1997. Currently, he is vice director of Petrochemical Research Institute, Petrochina Company. His research fields include zeolites and catalytic technologies of petrochemical processes. Feng-Shou Xiao obtained his B.S. and M.S. degrees at Jilin University, China. Under a collaborative Ph.D. program, as a Ph.D. student, he studied at Jilin University, Dalian Institute of Chemical Research, and Hokkaido University. After postdoctoral work at the University of California, Davis (UCD), as a faculty member, he joined in Jilin University. He became a full professor in 1998 and distinguished professor in 2001 in the Chemistry Department of Jilin University. In 2009, as a distinguished professor, he moved to Chemistry Department of Zhejiang University. His research is mainly devoted to synthesis and catalysis of porous materials.

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inorganic species and the organic templates. In addition, it is still unclear that such processes could be scaled up. More efforts for the solvent-free synthesis of zeolites should be given much attention in the future.

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Feng-Shou Xiao: 0000-0001-9744-3067 Funding

The authors acknowledge financial support from National Key Research and Development Program of China (2017YFC0211101), National Natural Science Foundation of China (91634201, 91545111, 21703203, and 21720102001), and Zhejiang Provincial Natural Science Foundation under Grant No. LR15B030001. Notes

The authors declare no competing financial interest. Biographies Qinming Wu is a postdoctoral researcher in the Meng and Xiao groups and works on the design and synthesis of zeolites. He got his B.S. degree in Chemistry (2011) at Zhejiang Sci-Tech University and Ph.D. degree (2016) at Zhejiang University under the supervision of Prof. Xiao. Xiangju Meng obtained his B.S. degree (1999) and Ph.D. degree (2004) at Jilin University, China. After postdoctoral research in Tokyo Institute of Technology and National Institute of Advanced Industrial Science and Technology (AIST), he joined Prof. Xiao’s group. He became a full professor at Zhejiang University in 2015. His research interests include zeolites and heterogeneous catalysis. G

DOI: 10.1021/acs.accounts.8b00057 Acc. Chem. Res. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.accounts.8b00057 Acc. Chem. Res. XXXX, XXX, XXX−XXX