Micro-Mesoporous Metallosilicates E-Book

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Veröffentlichungsjahr: 2024

Beschreibung

Micro-Mesoporous Metallosilicates

Up-to-date and in-depth text bridging the technology gap between fundamental research and industry-scale applications of porous materials for catalysis

Micro-Mesoporous Metallosilicates: Synthesis, Characterization, and Catalytic Applications comprehensively introduces the chemistry and catalytic technologies of metallosilicates, an important family of microporous crystalline zeolite and heteroatom-containing mesoporous materials, with a primary focus on design synthesis, characterization, theoretical studies, and catalytic applications of titanosilicates, tin-silicates, germanosilicates and Ti-mesosilica, and more.

The text covers recent advances in the synthesis of titanosilicates, including hydrothermal synthesis, dry-gel conversion, fluoride-assisted synthesis, and post-synthesis methods, along with the synthesis of metallosilicates with two-dimensional lamellar structures and their structural modifications as well as applications in selective oxidation reactions.

The text also discusses synthesis of germanosilicates with specially designed organic structure-directing agents, synthesis and catalytic applications of heteroatom-containing mesoporous silica, and dendritic mesoporous silica nanoparticles with unique wrinkled center-radial structures.

Overall, every important porous metallosilicate and its synthesis, characterization, pore engineering, catalytic application, and industrial technique and process are covered.

Specific sample topics discussed in Micro-Mesoporous Metallosilicates include:

• Chemical post-modifications of titanosilicates, in terms of the effects on transfer, adsorption/desorption, and surface reactions
• X-Ray based techniques, ultraviolet-visible-near infrared spectroscopy, Raman spectroscopy, and solid-state NMR spectroscopy
• Theoretical calculation as an effective tool and supplement to understand the catalytic active center, structural character, and Brønsted/Lewis acidity
• Titanosilicates in the liquid-phase epoxidation reaction of propylene and propylene chloride to corresponding epoxides
• Effects of particle sizes, oxidation state, and location sites of Au nanoparticles, and epoxidation performance of Ti-containing materials

Delivering cutting-edge research and bridging the technology gap between fundamental research and industrial applications, Micro-Mesoporous Metallosilicates is a valuable resource for chemists, materials scientists, chemical engineers, and experienced researchers in related fields.

Leseprobe

Micro-Mesoporous Metallosilicates

Synthesis, Characterization, and Catalytic Applications

Edited by Peng Wu and Hao Xu

Editors

Prof. Peng WuEast China Normal UniversityNorth Zhongshan Rd. No. 3663Shanghai, 200062China

Prof. Hao XuEast China Normal UniversityNorth Zhongshan Rd. No. 3663Shanghai, 200062China

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Cover Image: Courtesy of Dr. Hao Xu

Preface

Zeolites have experienced tremendous progress in the last 60 years as key materials for a variety of applications, ranging from adsorbents to heterogeneous catalysts. The metallosilicates or heteroatom-containing microporous and mesoporous materials established a universally acknowledged milestone, with the most representative TS-1 titanosilicate as selective and clean oxidation catalyst using hydrogen peroxide as an oxidant. The discovery of TS-1 shattered our limited understanding that zeolites were solely confined to being heterogeneous solid-acid catalysts used for gas-phase hydrocarbon transformations. Innovative selective liquid-phase oxidation processes have been incessantly commercialized based on the TS-1/hydrogen peroxide catalytic system since its emergence in the 1980s, including phenol hydroxylation, ketone ammoximation, and propylene epoxidation. A great progress has occurred in the domain of these specific zeolites in past decades, resulting in the successful synthesis of other metallosilicates with either different structure topologies or changeable heteroatoms. The metallosilicates diversify in structure, composition, and multifunctional uses, substantially endowing these types of novel porous materials with new interesting properties and functionalities. They are not only useful for selective oxidations but also promising for dehydrogenation of propane and biomass conversion. This book is edited to comprehensively introduce the chemistry and catalytic technologies of metallosilicates, with a focus on design synthesis, active site construction, structural modification, characterization of active sites, and catalytic applications of titanosilicates, stannosilicates, germanosilicates, etc. This book will attract broad interests from both academia and industry, and it is also intended to bridge the technology gap between basic research and industrial requirements.

This book consists of 16 chapters. Chapter 1 concentrates on recent advances in the synthesis of titanosilicates, including hydrothermal synthesis, dry-gel conversion, fluoride-assisted synthesis, and post-synthesis methods. This is followed by a chapter by H. Xu et al. (Chapter 2), which provides the synthesis of metallosilicates with two-dimensional lamellar structures and their structural modifications as well as applications in selective oxidation reactions. The approaches to the synthesis of Sn and Zr zeolites and their applications in catalysis are summarized by Z. Zhu (Chapter 3). The synthesis of germanosilicates with specially designed organic structure-directing agents is presented by J. Jiang (Chapter 4). Germanosilicates are structurally unstable in humid environment, which inversely endows them with structurally modifiable properties. O. Veselý et al. demonstrate the application of germanosilicates as starting compounds for preparation of materials with highly controlled chemical and structural properties via methods of intercalation, pillaring, or the ADOR transformations (Chapter 5). In addition, the synthesis and catalytic applications of heteroatom-containing mesoporous silica are also included in this book. K. Zhang et al. (Chapter 6) specifically focus on the dendritic mesoporous silica nanoparticles with unique wrinkled center-radial structures. Post-modifications are effective in regulating the microenvironment of Ti active sites to enhance the catalytic activity and selectivity of titanosilicates. Y. Liu et al. (Chapter 7) present an overview of current developments in chemical post-modifications of titanosilicates in terms of the effects on transfer, adsorption/desorption, and surface reactions. The characterizations of heteroatom species in metallosilicates are critical to obtain structure–performance relationships and the accurate design synthesis of high-performance metallosilicate catalysts. J. Xu and F. Deng (Chapter 8) focus on the X-ray-based techniques, ultraviolet–visible–near infrared spectroscopy, Raman spectroscopy, and solid-state NMR spectroscopy, which provide information about the coordination, valence states, and locations of heteroatoms in zeolites, as well as their host–guest interactions with zeolite frameworks. W. Chen and A. Zheng (Chapter 9) stress the importance of theoretical calculation as an effective tool and supplement to understand the catalytic active center, structural character, and Brønsted/Lewis acidity of heteroatom-containing zeolites, and further they clarify the reaction mechanism catalyzed by metallosilicates. The remaining chapters are exclusively devoted to the application of metallosilicates in catalysis. P. Wu et al. (Chapter 10) concentrate on the application of titanosilicates in ammoximation reaction of ketones and aldehydes and the successful construction of large-scale plants for clean production of cyclohexanone ammoximation with NH3 and H2O2. After this, C. Xia (Chapter 11) presents the application of titanosilicates in the liquid-phase epoxidation reaction of propylene and propylene chloride to corresponding epoxides as well as the ethylene epoxidation to ethylene oxide/ethyl glycol using H2O2 as the oxidant. The usage of cumene hydroperoxide as the oxidant and titanosilicates as the catalysts can also construct a highly efficient catalytic system for propylene oxide production, and the developments in both academic and industrial fields are outlined by L. Xu in Chapter 12. W. Fan et al. (Chapter 13) describe the applications of metallosilicates in the hydroxylation reactions of aromatics. The in situ formation of H2O2 is more appealing for the alkene epoxidation reactions due to the high cost and explosive risk of H2O2. The gas-phase propylene epoxidation with H2 and O2 over the bifunctional Au–Ti catalysts is presented by X. Duan and X. Zou in Chapter 14. The authors focus on the effects of particle sizes, oxidation state, and location sites of Au nanoparticles, and the Ti-containing materials with different structures and hydrophobicity on the epoxidation performance. The bifunctional metallosilicates have also found applications in the activation and conversion of low-carbon alkanes as demonstrated by L. Wang and F. Xiao in Chapter 15. The final chapter by the authors from the group of H. Yamashita presents possibilities of application of metallosilicates in photocatalysis.

These excellent chapters review and describe the cutting edge of basic research and R&D practices of metallosilicates, enlightening an optimistic view for their bright future. The number and type of new metallosilicates are ever increasing with the advancement of synthetic chemistry. Direct synthesis still dominates the preparation of metallosilicates. Post-synthesis proves to be useful to develop the materials with novel pore and crystalline structures as well as with desirable active sites that are difficult to simultaneously realize by direct synthesis. Diversifying the isomorphously substituted framework heteroatoms greatly broadens the applications of metallosilicates from traditional selective oxidation to Lewis acidic catalysis and photocatalysis. These new materials with extra-large pores or even mesopores, highly required by practical applications, should be useful in developing new green catalytic processes that enable processing bulkier substrates. Combining the intrinsic catalytic properties of metallosilicates with the second active site of noble metal will open new possibility to develop greener processes with intensification features, simplifying complex reaction steps and saving separation energy. In addition, spectroscopical techniques and theoretical calculations provide new information about the chemistry and coordination of framework heteroatom active sites and their catalysis mechanism involved in each reaction. Exciting achievements beyond our imagination are highly expectable by taking the advantage of the experimental and theoretical knowledge achieved so far. Last but not least, novel approaches in the synthesis, manipulation, and modification of metallosilicates clearly evidence a significant potential of these zeolites in new application areas.

It is our great pleasure and honor to work together with the excellent experts and researchers in heteroatom zeolite field to compile this thought-provoking book. We would like to express our sincere thanks to all of them for their great efforts to put together timely reviews on selected topics. This book, concentrated on the specific zeolites of metallosilicates, is expected to be helpful for the experienced academic researchers. We also wish the industrial researchers and the newcomers would find it as a useful reference book.

June 2023Shanghai, China

Peng Wu

Hao Xu

1Synthesis of Titanosilicates

Xinqing Lu

Zhejiang Normal University, Key Laboratory of the Ministry of Education for Advanced Catalysis Materials, Department of Chemistry, Yingbin Avenue, Jinhua, Zhejiang 321004, P.R. China

1.1 Introduction

Zeolites are microporous crystals that are constructed by tetrahedral SiO4 and AlO4 species interlinked by sharing O atoms, and they demonstrate remarkable application prospects in adsorption, separation, ion exchange, and heterogeneous solid-acid catalysis [1]. Typically, a part of the framework of Si and Al atoms can be replaced by heteroatoms, such as Ti, Sn, Ge, Zr, B, P, V, and Ga, via isomorphous substitution, resulting in heteroatomic zeolites or metallosilicates [2–4]. Among these heteroatomic zeolites, titanosilicate is the most representative one, and it can catalyze diverse selective oxidation reactions, such as alkene epoxidation, aldehyde or ketone ammoxidation, benzene or phenol hydroxylation, 1,4-dioxane oxidation, selective oxidation of pyridine derivatives, and oxidation desulfurization [5–9], as well as acid-catalyzed reactions, such as ring-opening reactions of epoxides [10–12], ethylenediamine condensation [13], and Beckmann rearrangement of oxime [14] (as shown in Figure 1.1). Moreover, the discovery of titanosilicates has expanded the application scope of zeolites, as heterogeneous catalysts, from acid catalysis to the redox field. Several reviews and monographs have proposed opportunities and challenges for titanosilicates in synthetic and catalytic applications [3–9, 15–18]. As depicted in Figure 1.2, the number of annual publications related to titanosilicates has rapidly increased from 1983 to 2023, and this number has remained at approximately 200–350 over the last decade.

Notably, titanosilicates can be divided into microporous, mesoporous, and hierarchical types based on their textural properties and pore sizes. Among these, microporous titanosilicates, with isolated tetrahedral Ti species, possess pores that are <2 nm in size, and these include small- and medium-pore titanosilicate zeolites with 8- or 10-membered ring (MR), 12-MR large-pore zeolites, and extra-large-pore zeolites with ≥14 MRs. Among the 255 ordered zeolite framework structures with three-letter codes and the partially disordered zeolite structures recognized by the International Zeolite Association Structure Commission (IZA), 28 structures can be synthesized as microporous titanosilicates. Owing to their unique porosity and hydrophobicity, microporous titanosilicates can activate H2O2 molecules and catalyze selective oxidation reactions. Titanosilicalite-1 (TS-1), with MFI topology, was the first microporous titanosilicate to be employed as a commercial catalyst. For example, the application of TS-1 in the liquid-phase epoxidation of propylene to propylene oxide using H2O2 as the oxidant was first reported by EniChem in 1983 [19] and was implemented on a commercial scale by Evonik and SKC in South Korea in 2008. Mesoporous titanosilicates, such as Ti-MCM-41, Ti-MCM-48, Ti-KIT-5, Ti-SBA-15, and Ti-SBA-16, possess pores that are >2 nm in size and amorphous pore walls [5]. They are more active than microporous materials in the oxidation of bulky substrates with cumene hydroperoxide or tert-butyl hydroperoxide (TBHP) as the oxidant. However, they are much less active in oxidation reactions using hydrogen peroxide as the oxidant owing to their extremely high hydrophilicity derived from abundant surface silanols on their amorphous pore walls [20]. Hierarchical titanosilicates contain both micropores and mesopores and exhibit better catalytic properties than their microporous counterparts, particularly in catalytic reactions involving bulky substrates and/or organic hydroperoxide oxidants [5, 18].

Figure 1.1 Reactions catalyzed by titanosilicates.

Figure 1.2 Change trend of annual publication number for titanosilicates.

Source: SciFinder.

Titanosilicates are primarily synthesized via hydrothermal synthesis (HTS), dry-gel conversion (DGC), fluoride-assisted synthesis, and post-synthesis methods (see Figure 1.3). Among these, HTS has been the most widely adopted approach for zeolite synthesis. This is because the contents and distributions of Ti species, crystal sizes, morphologies, and other physicochemical properties of titanosilicates can be tailored by adjusting the composition of synthetic gels and the crystallization conditions [15]. Particularly, the formation of anatase TiO2 via the oligomerization of Ti monomers in a HTS process is generally easy owing to the faster hydrolysis rate of the Ti precursor compared to that of the Si precursor. However, the formation of anatase TiO2 results in low activity and selectivity in catalytic reactions. Consequently, several strategies have been proposed to inhibit the generation of the anatase phase by using additional additives, such as H2O2, isopropanol, Triton X-100, Tween-20, and (NH4)2CO3, as well as by accurately adjusting the feeding rate [21–27]. These methods generally slow the hydrolysis of Ti precursors to match that of the Si precursors, thereby lowering the anatase content. As an example, Lin et al. [28] proposed a reversed-oligomerization synthesis strategy to address the mismatched hydrolysis rates between Si and Ti precursors, which was implemented by the fast oligomerization of Ti monomers and subsequent de-oligomerization to Ti monomers with the aid of hydroxyl free radicals (•OH) generated in situ by ultraviolet (UV) irradiation.

Figure 1.3 Overview of the synthesis methods of titanosilicates with different structures. HTS indicates hydrothermal synthesis, PS indicates post-synthesis, DGC indicates dry-gel conversion, F− indicates fluoride-assisted method. Small and medium-pore stands for 8-MR and 10-MR titanosilicates, large-pore for 12-MR and extra-large pore for ≥14-MR.

The DGC method can be classified into vapor-phase transport (VPT) and steam-assisted crystallization (SAC) based on the volatility of structure-directing agents (SDAs) [9]. The VPT approach is applicable to volatile SDAs, where SDAs and water are not present in dried synthetic gels but are transferred to them via the vapor phase. The SAC approach can be realized by adding non-volatile SDAs to the dried synthetic gels with water placed below them; subsequently, the synthetic gels can be crystallized using steam. Compared with traditional HTS methods, the DGC method presents several advantages, such as lower SDA consumption, higher product yield, and shorter crystallization time. The crystal sizes obtained by DGC methods can differ from those obtained by direct HTS. For instance, for Ti-Beta synthesized through the DGC method, the crystal size is much smaller than that obtained by the HTS method [29], whereas the opposite is true for MWW-type titanosilicates [30]. In addition, DGC can be used to prepare hierarchical titanosilicate zeolites [31–33].

In fluoride-assisted synthesis, the presence of F− can accelerate the crystallization process; however, titanosilicate crystals are usually larger than those obtained via HTS [34]. In contrast to aluminosilicates, the concentrations of alkali metal ions (such as Na+ and K+) in synthetic gels should be limited to obtain titanosilicates with high activities. Notably, MOR- and MSE-type zeolites barely crystallize in siliceous gels in the absence of Al3+ and alkali metal ions. Thus, the post-synthesis method is another available approach for titanosilicates. The post-synthesis method can proceed in the gas–solid [35], liquid–solid [36], or solid–solid phase [37], depending on the phase of the Ti source. In this chapter, we highlight the most remarkable achievements in the synthesis of titanosilicates with different pore topologies, including medium-pore, large-pore, extra-large-pore, mesopore, and Engelhard Ti silicates (ETS).

1.2 Synthesis of Medium-Pore Titanosilicates

1.2.1 TS-1 Synthesis

TS-1 (MFI topology), which possesses a three-dimensional (3D) medium-pore system (10-MR, ∼0.55 nm), is one of the most studied titanosilicates and has been applied in many industrial processes, such as propylene epoxidation [19], phenol hydroxylation [38], and cyclohexanone ammoximation [39]. The first discovery of a TS-1 zeolite can be dated back to the patent disclosed by Taramasso et al. in 1983 [19], which was based on the matching hydrolysis of tetraethylorthosilicate (TEOS) and tetraethylorthotitanate (TEOT) using tetrapropylammonium hydroxide (TPAOH) as the SDA.

Extra-framework Ti species tend to form in TS-1 in the presence of Na+ and K+ from commercial aqueous TPAOH solutions as impurities [40, 41]. The diffraction peaks ascribed to the extra-framework anatase TiO2 can even be detected in the X-ray diffraction patterns of TS-1 zeolites once the Na+ concentration in the synthetic gel reaches a very high level. Moreover, residual alkali metal cations in the synthesized titanosilicates are detrimental to their reactivity in selective catalytic oxidation reactions [42].

In addition to alkali metal cations, the Ti content in synthetic gels and crystallization temperature were also found to be key factors for TS-1 synthesis [43]. The Ti content of the TS-1 samples is always lower than that of synthetic gels, implying that the Ti atoms in the synthetic gels could not be completely introduced into the MFI framework. Excess Ti in the synthetic gel increases the risk of forming extra-framework anatase TiO2 as an impurity. Hence, several attempts have been made to maximize the amount of tetrahedrally incorporated Ti species in the framework to achieve a higher activity. It has been found that TPAOH should be slowly added into the synthetic gels at low temperatures under vigorous stirring to inhibit the formation of extra-framework Ti species [44]. In other words, it is necessary to match the hydrolysis rates between the Si and Ti precursors to obtain abundant silanol groups for condensation with the monomeric Ti species generated by hydrolysis. Fan et al. proposed that the Si/Ti molar ratio in the framework of the TS-1 zeolite can be decreased to 34 using (NH4)2CO3 as a crystallization-mediating agent to match the hydrolysis rate of the Ti precursor with that of the Si precursor and the crystallization rate [23]. In contrast, the Si/Ti ratio was 58 in the absence of (NH4)2CO3.

Recent research on the synthesis of TS-1 has mainly focused on (a) enhancing the accessibility of the framework Ti species by constructing hierarchical TS-1 zeolites by tuning the particle size and morphology and (b) developing alternative SDAs to replace expensive TPAOH for practical applications. Representative approaches for constructing hierarchical TS-1, such as mesoporogen-directed methods, mesoporogen-free methods (kinetic-regulated or DGC methods), and demetallization methods, have been reviewed previously [2, 5, 18]. On the other hand, the high cost of TPAOH has stimulated many researchers to concentrate on reducing its dosage or finding an alternative SDA. TS-1 can be prepared using a cheaper SDA of tetrapropylammonium bromide (TPABr); however, the large zeolite crystals (15 × 8 × 1.5 μm) obtained in earlier studies are undesirable [45]. Zuo et al. reported the synthesis of TS-1 with a crystal size of 200 nm using TPABr as the SDA, where seeding played a crucial role in reducing the crystal size [46].

TS-1 zeolites with novel morphologies have also been reported. In contrast to the raspberry morphology of traditional TS-1 zeolites, a lamellar TS-1 zeolite with oriented growth and sheet-like morphology (0.5–1.0 μm length and 20–50 nm thickness) was synthesized with a bifunctional surfactant as the SDA [47]. Due to its hierarchical structure, layered TS-1 showed a higher activity than conventional TS-1, Ti-MWW, Ti-Beta, or even mesoporous titanosilicate Ti-MCM-41 in cyclohexene epoxidation with TBHP as the oxidant. Wang et al. [48] reported the one-pot synthesis of TS-1 microspheres that were approximately 10 μm using a triblock copolymer (F127) as the supporting additive through a temperature-programed self-assembly strategy. The TS-1 microspheres were self-assembled from TS-1 nanoparticles (50–100 nm) and possessed more intercrystal mesopores than traditional TS-1 zeolite, resulting in higher activities during the oxidation of bulky substrates, such as 3-picoline and cyclohexene. Moreover, these TS-1 microspheres exhibited high mechanical stability under harsh thermal and hydrothermal conditions. Thus, because of their high reactivity and mechanical strength, TS-1 microspheres have potential applications in slurry reactors [49].

1.2.2 Ti-MWW Synthesis

Ti-MWW is another medium-pore titanosilicate with a two-dimensional (2D) pore system consisting of intralayer 10-MR sinusoidal pore channels and an interlayer 10-MR pore channel linked to intracrystalline 12-MR supercages [50]. Direct HTS of Ti-MWW is challenging [51]. A key breakthrough was achieved by introducing H3BO3 as a structure-supporting agent for the synthesis of Ti-MWW zeolites with high activity, inspired by the HTS of the borosilicate MWW-type zeolite ERB-1. Nevertheless, a higher B content in the synthetic gel is required for Ti-MWW zeolite than for ERB-1 because the introduction of Ti4+ ions considerably hinders the crystallization of the MWW framework [52]. The UV–visible spectra of the as-synthesized Ti-MWW had an adsorption band at ca. 220 nm, attributed to the tetrahedrally coordinated framework Ti species, together with another band at 260 nm, related to the extra-framework octahedral Ti species on the external surface of the Ti-MWW zeolites [52]. Irrespective of the Ti content in the synthetic gel, a band at approximately 330 nm was not observed in the UV–visible spectra of Ti-MWW, implying that the generation of the anatase TiO2 phase was effectively inhibited.

Hexamethyleneimine (HMI) and piperidine (PI) can be employed as SDAs for Ti-MWW synthesis, and the obtained samples are denoted as Ti-MWW(HMI) and Ti-MWW(PI), respectively. The two Ti-MWW zeolites have similar specific surface areas but significantly different Ti4+ distributions owing to their different particle sizes. Irrespective of the Si/Ti molar ratio in the zeolites, the Ti-MWW(PI) zeolite showed the main adsorption band at approximately 260 nm, together with a band at approximately 220 nm, indicating more extra-framework Ti species (Figure 1.4). In contrast, the main absorption band in the UV–visible spectrum of Ti-MWW(HMI) shifted from 260 to 220 nm with increasing Si/Ti molar ratio, indicating that the incorporation of Ti species preferentially occurred in the framework. After calcination of the as-synthesized Ti-MWW zeolites, a new band at approximately 330 nm, attributed to the anatase TiO2 phase, emerged in the UV–visible spectra for both Ti-MWW(HMI) and Ti-MWW(PI), signifying that the neighboring external surface Ti species partially condensed and aggregated upon calcination. The formed anatase TiO2 in the calcined Ti-MWW zeolite was barely removed by acid treatment. Therefore, acid treatment is usually performed on as-synthesized Ti-MWW zeolites rather than on calcined zeolites to remove the extra-framework Ti species on the surface, accompanied by a small amount of framework Ti species.

Figure 1.4 UV-visible spectra of as-synthesized (A) and calcined Ti-MWW(PI) (C), and as-synthesized (B) and calcined Ti-MWW(HMI) (D) with the Si/Ti molar ratios of 100 (a), 50 (b), 30 (c), and 10 (d).

Source: Reprinted with permission from Wu et al. [52]. Copyright American Chemical Society (2001).

The B content in the Ti-MWW zeolite framework was far below that of the synthetic gel, implying that most of the B species were not involved in building the MWW structure. Additionally, owing to the weak acidity and increased electronegativity of the zeolite framework, the presence of residual framework boron is detrimental to selective oxidation reactions, although most of the framework boron atoms are extracted from the MWW framework by acid treatment. Hence, many other methods, such as the fluoride-assisted method [9] and DGC method [30], have been developed to synthesize Ti-MWW zeolites with reduced boron content in synthetic gels. Wu et al. developed a post-synthesis method to prepare boron-free Ti-MWW by incorporating Ti species into the framework of deboronated MWW via PI-assisted reversible 3D–2D–3D structure conversion [53]. However, boron-free Ti-MWW synthesized via the gas-phase atom planting strategy showed relatively low reactivity in oxidation reactions because of the presence of extra-framework octahedral Ti species and the anatase TiO2 phase [54]. Lu et al. [36] recently reported the boron-free synthesis of Ti-MWW by introducing framework Ti species into an acidic medium, and the obtained Ti-MWW zeolites showed outstanding catalytic properties for 1-hexene epoxidation. In addition to post-synthesis methods, boron-free Ti-MWW can be one-pot hydrothermally synthesized using HMI and N,N,N-trimethyl-1-adamantammonium hydroxide (1-TMAdaOH) as dual-SDAs with a small amount of K+[55]. This method was inspired by the synthesis of a highly siliceous MWW zeolite (ITQ-1). After the detrimental K+ was removed by mild acid treatment, the obtained Ti-MWW catalyst showed high activity in epoxidation reactions.

1.2.3 TS-2 Synthesis

TS-2 with its MEL topology of two intersecting 10-MR pore channels shows a catalytic performance similar to that of the TS-1 zeolite because of the similar secondary building units. In contrast to the abundant literature on TS-1 synthesis, studies on TS-2 synthesis are limited. The synthesis of TS-2 was first reported by Reddy et al. via the crystallization of synthetic gels containing TEOS, tetrabutyl orthotitanate (TBOT), and tetrabutylammonium hydroxide (TBAOH) [56, 57]. Moreover, the crystallization process can be accelerated by microwave-assisted heating [58]. Tuel et al. [59] found that tetrabutylphosphonium hydroxide was an alternative SDA for TS-2 synthesis. In addition to HTS, DGC [60] and F−-assisted methods [61] have also been applied to TS-2 synthesis. Moreover, TS-2 can be prepared by the solid–gas reaction of the borosilicate MEL-type zeolite B-ZSM-11 with saturated TiCl4 vapor; however, it shows poor catalytic reactivity in epoxidation reactions owing to the presence of anatase TiO2[62]. Xu et al. recently reported an intergrowth-triggered TS-2 microsphere formed by adjusting the synthetic parameters [63]. The initially formed microspheres were well preserved during the crystallization process because of the special intergrowth stacking style occurred in the synthetic gels with high alkalinity.

1.2.4 Synthesis of Other Medium-Pore Titanosilicates

Ti-FER zeolites with 2D-intersecting 10 × 8-MR pore channels can be synthesized using HF/pyridine as the mineralizing agent with [64] or without seeds [65]. FER zeolites can also be synthesized as 2D-layered zeolites, on which structural modifications are applicable to construct Ti-FER zeolites with larger pore systems. The structural diversity of layered Ti-FER zeolites is available in Chapter 2. Ti-ZSM-48, with non-interpenetrating linear 10-MR pore channels (5.3 × 5.6 Å in diameter) and Si/Ti molar ratios ≥30, can be hydrothermally synthesized using diaminooctane, hexamethonium hydroxide, or trimethylpropylammonium hydroxide as SDAs [66–68]. However, Ti-ZSM-48, which has a partially disordered structure, is inactive for phenol hydroxylation with H2O2 because of diffusional limitations or subtle differences in the Ti microenvironment compared to TS-1 and TS-2 [68, 69]. In addition, small- and medium-pore titanosilicates with other topologies, such as STF [70], CHA [71–74], LTA [75, 76], CDO [77], AEI [78], STT [79], and PCR [80, 81], have also been successfully synthesized, but were inactive or showed less attractive activity in catalytic reactions because of mass transfer limitations.

1.3 Synthesis of Large-Pore Titanosilicates

1.3.1 Ti-Beta Synthesis

Titanoaluminosilicate Ti-Al-Beta, with 3D 12-MR pore channels, was first prepared by a HTS method via the isomorphous substitution of Si4+ by Ti4+ in the aluminosilicate Al-Beta. It showed higher activity than TS-1 in the selective oxidation of cycloalkanes with H2O2 as the oxidant [22], indicating that titanosilicates with relatively large pores are more efficient for oxidation reactions involving bulky substrates. Further study on the synthesis and characterization of Ti-Al-Beta was conducted to illustrate the influence of synthetic parameters on the Ti-coordinated states and catalytic properties in selective oxidation reactions [82, 83]. The Ti species introduced in Ti-Al-Beta tends to adopt extra-framework octahedral coordination rather than framework tetrahedral coordination. Moreover, the acidity derived from the framework tetrahedral Al in Ti-Al-Beta easily accelerates the ring opening of epoxides, decreasing epoxide selectivity in alkene epoxidation [83]. Thus, the amount of Al in Ti-Al-Beta should be decreased to eliminate the adverse effects of Al on catalytic performance. Al-free Ti-Beta titanosilicate has been successfully synthesized with tetraethylammonium hydroxide (TEAOH) and dealuminated Beta as the SDA and seed, respectively, as reported by Corma et al. [84]. However, Al-free Ti-Beta obtained by the above seeding methodology still showed very low-epoxide selectivity in alkene epoxidation. A similar phenomenon was observed for an Al-free Ti-Beta zeolite prepared by a solid–gas phase reaction between borosilicate B-Beta and saturated TiCl4 vapor [85]. This phenomenon can be illustrated by the presence of numerous internal Si—OH groups (framework defects) in the Al-free Ti-Beta (Ti-Beta(OH)) obtained in OH− media. Some Si—OH groups are formed after the elimination of TEA+ and are balanced by SiO− groups upon calcination [84]. Corma et al. found that TEA+ can be neutralized by F− instead of SiO− for synthesis in an F− medium, resulting in the formation of defect-less Ti-Beta zeolites (Ti-Beta(F)) with high crystallinity, hydrophobicity, and hydrothermal stability [34, 86]. Because of its hydrophobicity, Ti-Beta(F) is superior to Ti-Beta(OH) in unsaturated fatty oil epoxidation reactions in terms of reactivity and epoxide selectivity [34, 87]. Additionally, several other approaches, such as the DGC method, interzeolite transformation, and dissolution–recrystallization methods, have also been reported for the synthesis of Ti-Beta zeolites [29, 88–90]. Inspired by the DGC method for preparing high-silica Beta zeolite [91], large-pore Ti-Beta with high hydrophobicity has been prepared by the DGC method, even in the presence of alkali metal cations [29, 88]. The hydrophobic Ti-Beta obtained by the DGC method showed higher catalytic reactivity and selectivity to epoxides than the hydrophilic Ti-Beta(OH) in selective oxidation reactions [29, 92], which is in accordance with previous conclusions related to the effects of zeolite polarity on the catalytic properties in selective oxidation reactions [34, 86]. Zhu et al. [89] reported the interzeolite transformation of Ti-MWW zeolite into Ti-Beta zeolite (as shown in Figure 1.5), which showed excellent catalytic properties for cyclohexene epoxidation with H2O2 as the oxidant. The framework structure similarity between MWW and Beta zeolites and the addition of Beta seeds play crucial roles in the crystallization of Ti-Beta. The interzeolite transformation of Ti-MWW (10 × 10-MR pore channels) to large-pore Ti-Beta (12 × 12 × 12-MR pore channels) breaks the traditional interzeolite transformation concept (spontaneous transformation toward a denser zeolite framework) of zeolite synthesis [93–95]. The dissolution–recrystallization of dealuminated-Beta zeolite in a mixture of a tetraethylammonium aqueous solution and a Ti precursor produced Ti-Beta with nanosized crystals, high Ti content, and intercrystal mesoporosity, which showed outstanding catalytic properties in cyclohexene epoxidation with hydrogen peroxide or TBHP as the oxidant [90].

1.3.2 Ti-MOR Synthesis

Ti-MOR, which contains 12-MR pore channels, has shown unique catalytic properties both in the hydroxylation of toluene and ammoximation of ketones [96, 97]. Until now, MOR structures have not been crystallized in siliceous gels without the assistance of Al3+[98]. Thus, a highly active Ti-MOR can only be prepared by dealumination of the aluminosilicate zeolite Al-MOR and the subsequent gas–solid reaction between TiCl4 vapor and the dealuminated MOR at elevated temperature [99]. The transport of TiCl4 molecules and reactants is strongly related to the length of the 12-MR pore channel along the c-axis. In this sense, Yang et al. investigated Ti-MOR zeolites with different crystal lengths along the c-axis to understand the role of the 12-MR pore channel length on the catalytic performance (Figure 1.6) [100]. The crystal length along the c-axis plays a critical role in the diffusion of framework Al species out of the 12-MR pore channels and the diffusion of Ti species into the 12-MR pore channels, which affects the amount of Ti inserted and the catalytic activity.

Figure 1.5 Flow Diagram for the interzeolite transformation of Ti-MWW zeolites into Ti-Beta zeolites.

Source: Reprinted with permission from Zhu et al. [89]. Copyright Royal Society of Chemistry (2019).

Figure 1.6 The SEM images of MOR zeolites with different crystal lengths of 110 nm (a), 230 nm (b), 630 nm (c), 2450 nm (d), 5160 nm (e) and the pore channels of MOR zeolite (f).

Source: Yang et.al. [100]/Reproduced with permission from Elsevier.

1.3.3 Ti-MSE Synthesis

The Ti-containing MSE zeolite is another large-pore titanosilicate with 3D 12 × 10 × 10-MR pore channels and an 18 × 12-MR supercage linked to the 10-MR pore channel. Similar to the MOR, the MSE structure cannot be obtained via hydrothermal crystallization without the assistance of Al3+. Kubota et al. attempted to synthesize Ti-MSE by introducing a Ti tetrabutoxide and H2O2 aqueous solution into an aluminosilicate gel with N,N,N′,N′-tetraethylbicyclo[2.2.2]oct-7-ene-2,3:5,6-dipyrrolidinium diiodide (TEBOP2+(I−)2) as the SDA [101]. Most framework Al atoms were extracted from the zeolite framework via acid treatment. However, titanoaluminosilicates show low phenol hydroxylation activity after acid treatment, possibly because of their hydrophilic nature. Thus, highly active Ti-MSE zeolites can only be synthesized using the post-atom planting method. Ti-MSEs are known as Ti-MCM-68 [101–105], Ti-UZM-35 [106], and Ti-YNU-2 [107] according to the parent materials used for Ti planting. Ti-MCM-68 was first synthesized by dealumination of Al-MCM-68 (HTS using TEBOP2+(I−)2 as the SDA) and a subsequent gas–solid reaction between TiCl4 vapor and dealuminated MCM-68 [104]. The obtained Ti-MCM-68 showed superior catalytic performance compared to that of TS-1 for phenol hydroxylation with hydrogen peroxide in terms of reactivity and para-product selectivity. However, the HTS of Al-MCM-68 requires an extremely long crystallization period (>14 days) using TEBOP2+(I−)2 as the SDA. Peng et al. [105] found that Al-MCM-68 could be rapidly crystallized in only one day via the inter-zeolite transformation method using siliceous Beta zeolite as the silica source. The corresponding Ti-MCM-68 showed high catalytic activity for anisole hydroxylation. In addition, another MSE titanosilicate, Ti-UZM-35, synthesized by the combination of the dealumination of Al-UZM-35 (synthesized using dimethyldipropylammonium hydroxide (DMDPAOH) as the SDA) and a subsequent gas–solid reaction [106] showed comparable reactivity and product selectivity to the Ti-MCM-68 zeolite in the hydroxylation of phenol. Ti-YNU-2 exhibited remarkably enhanced catalytic properties compared to Ti-MCM-68 although the preparation process was relatively complex [107]. To prepare Ti-YNU-2, an all-silica YNU-2P precursor with abundant internal defects was stabilized by steam treatment and subsequently treated with TiCl4 vapor. Kubota et al. claimed that new five- or six-coordinated Ti species were present in Ti-YNU-2, which accounted for the enhanced catalytic reactivity and para-selectivity in phenol hydroxylation. In addition to the gas-phase Ti source, Ti-MCM-68, which has a high catalytic reactivity and para-selectivity for phenol hydroxylation, has been prepared using a liquid-phase source [102].

1.3.4 Synthesis of Other Large-Pore Titanosilicates

Other large-pore titanosilicates, such as Ti-ZSM-12 (unidimensional 12-MR pore channel) [108], Ti-ITQ-39 (3D 12 × 12 × 10-MR pore channel) [109], Ti-ITQ-7 (3D 12 × 12 × 12-MR pore channel) [110, 111], Ti-SSZ-33 (3D 12 × 12 × 10-MR pore channel) [112], Ti-IPC-2 (2D 12 × 10-MR pore channel) [80, 113], Ti-SSZ-42 (unidimensional 12-MR pore channel) [114], Ti-SAPO-5 (unidimensional 12-MR pore channel) [115], and Ti-ITQ-17 (3D 12 × 12 × 12-MR pore channel) [116], have also been successfully synthesized. Ti-ZSM-12 has been prepared using hexamethylene bis(diethylmethylammonium) hydroxide as the SDA via HTS [108]. However, the catalytic activities of Ti-ZSM-12 in cyclohexene epoxidation with H2O2 and TBHP were both much lower than those of Ti-Beta, possibly due to its smaller unidimensional micropore channels (5.6 × 6.0 Å) compared to Beta. Ti-ITQ-39, with its combination of medium- and large-pore channels, was synthesized using the HTS method [109]. Ti-ITQ-39 was highly active in linear and cyclic alkene epoxidation with H2O2, showing unique epoxide selectivities compared to TS-1 (medium-pore) and Ti-Beta (large-pore). Ti-SSZ-33 and Ti-SSZ-42, with topologies of CON [112] and IFR [114], respectively, were post-synthesized by the insertion of Ti species into the corresponding borosilicates and were active for cycloalkene epoxidation. Similar to the Beta zeolite, ITQ-7 possesses 3D 12-MR pore channels with one sinusoidal 12-MR pore channel along the c-axis and two straight 12-MR pore channels along the a- and b-axes [117]. Ti-ITQ-7 was synthesized using 1,3,3-trimethyl-6-azonium-tricyclo[3.2.1.46,6]dodecane as the SDA in F− media [111] and showed catalytic properties comparable to those of Ti-Beta(F) owing to the similar secondary building units in the two zeolite structures. The crystallization time was reduced from 12 days to 12 hours, and the Ti incorporation efficiency was increased from 17.2% to 51% by introducing Ge atoms into the synthetic gels because of the enhanced structural stability of the Ge atoms located in the double-four ring cages [110, 118]. With a higher Ti content, Ti-Ge-ITQ-7 showed superior activity in the epoxidation of various alkenes compared to Ti-ITQ-7. ITQ-17, with the BEC topology, has a 3D 12-MR pore channel system (6.3 × 7.5 Å and 6.0 × 6.9 Å). However, the titanogermanium silicate ITQ-17 shows very low hydrothermal stability [116], and ITQ-17 with its stable zeolite structure can only be in the form of germanate [119] or silicogermanate [120]. Nevertheless, it was expected that Ge-free Ti-ITQ-17 should exhibit a superior catalytic behavior to Ti-Beta because the former has a larger pore opening compared to Ti-Beta (6.6 × 6.7 Å and 5.6 × 5.6 Å). All-silica ITQ-17 was first synthesized using 4,4-dimethyl-4-azonia-tricyclo[5.2.2.02,6]undec-8-ene Iodide as the SDA in buffered media containing hexafluorosilicate species and K+[121]. After the successful synthesis of all-silica ITQ-17, Ti-ITQ-17 was synthesized under similar conditions. However, the incorporation of Ti species into the zeolite framework was inhibited by K+[116]. Thus, further theoretical molecular modeling using different organic molecules as the SDA was performed to find a suitable SDA for Ti-ITQ-17 synthesis that would allow crystallization in the absence of K+. Among the nine designed SDAs, SDA9 showed an optimum directing ability for the crystallization of Ti-ITQ-17 (see Figure 1.7) [116]. The obtained Ti-ITQ-17 exhibited higher reactivity and epoxide selectivity than Ti-Beta for the epoxidation of bulky cycloalkenes.

Figure 1.7