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Synthesis 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...