Synthesis and Catalytic Applications of Advanced Sn- and Zr-Zeolites Materials

The incorporation of isolated Sn (IV) and Zr (IV) ions into silica frameworks is attracting widespread attention, which exhibits remarkable catalytic performance (conversion, selectivity, and stability) in a broad range of reactions, especially in the field of biomass catalytic conversion. As a representative example, the conversion route of carbohydrates into valuable platform and commodity chemicals such as lactic acid and alkyl lactates, has already been established. The zeotype materials also possess water-tolerant ability and are capable to be served as promising heterogeneous catalysts for aqueous reactions. Therefore, dozens of Sn- and Zr-containing silica materials with various channel systems have been prepared successfully in the past decades, containing 8 membered rings (MR) small pore CHA zeolite, 10-MR medium pore zeolites (FER, MCM-56, MEL, MFI, MWW), 12-MR large pore zeolites (Beta, BEC, FAU, MOR, MSE, MTW), and 14-MR extra-large pore UTL zeolite. This review about Sn- and Zr-containing metallosilicate materials focuses on their synthesis strategy, catalytic applications for diverse reactions, and the effect of zeolite characteristics on their catalytic performances.

Defect-Guided Synthesis of Hierarchical Sn-B-Beta Zeolite with Highly Exposed Sn Sites

Selectively anchoring active centers on the external surface for forming highly exposed acid sites is a highly desirable but challenging task in zeolite catalyst synthesis. Herein, a defect-guided etching-regrowth strategy is rationally designed for facilely positioning Sn Lewis acid sites on the outer surface of the Sn-B-Beta while fabricating a bifunctional hierarchical structure. The synthesis was conducted by hydrothermal treatment of the as-made B-Beta (uncalcined), which has intrinsic defects of the BEA structure, with Sn source and basic organic structure directing agent (SDA). Under a moderate SDA concentration, with blocked micropore channels, such SDA-triggered etching-regrowth will proceed along the defect defined pathway, which ensures Sn selectively anchored on the external surface. Moreover, this methodology has exclusively introduced tetrahedrally coordinated framework Sn with open Sn sites as the predominated species. Mono- and disaccharide isomerizations in ethanol over different Sn-Beta catalysts proved the prominent advantages of the hierarchical structure with highly exposed and synergetic acid sites.

"Open" Nonporous Nonasil Zeolite Structure for Selective Catalysis

Nonasil (NON-type framework) zeolite, having inner large cages but only 6-ring (R) apertures, is recognized as a nonporous material without practical application values as catalysts or adsorbents. A novel bottom-up structural construction strategy assisted with well-designed Gemini-type surfactant is proposed to "open" the non cages, constructing two novel NON-related structures with accessible and stable acid sites. The obtained derivant (named as ECNU-27) possessed hierarchical porosity with short-range ordered 8-R micropores and abundant intercrystal mesopores, serving as a promising catalyst for the 1-butene cracking to lower alkenes.

Oxidative desulfurization of model oil over Ta-Beta zeolite synthesized via structural reconstruction

As to metallosilicate zeolites, ions with larger size such as Ta⁵⁺ in the gels greatly retarded their crystallization during the hydrothermal synthesis, affording long-winded synthesis periods, up-limited framework-substituted metal contents, or even frustrated outcome. An efficient hydrothermal synthesis strategy for metallosilicate, in this case of Ta framework-substituted *BEA zeolite, via structural reconstruction was proposed to stride the gap. The Ta content in our developed Ta-Beta-Re-50 zeolite achieved up to 5.48 % (Si/Ta = 52), breaking through the limitation of Ta contents for conventional method (Si/Ta > 100). Additionally, this Ta-Beta-Re zeolite possessed nanosized crystals (20-40 nm) and short crystallization time (8 h), significantly improving space-time yields of practical zeolite production. Through spectroscopic study, it was confirmed that the existence of zeolite structural units intensively facilitated the formation of nucleation and crystal growth. This innovative Ta-Beta zeolite demonstrated high catalytic performances for oxidation desulfurization, far outperforming traditional fluoride-mediated Ta-Beta-F, which was ascribed to its excellent diffusion properties and incredible high isolated Ta contents. Additionally, the catalytic performance of Ta-Beta-Re could be regenerated after simple calcination and the deactivation may be caused by pore blocking of organics. This work provides a new method for rationally design and construction of metallosilicate materials with high activity for catalytic oxidation applications, which can bridge the conceptual and technical gap between periodic trends and zeolite material synthesis.

Hydrothermal synthesis of boron-free Zr-MWW and Sn-MWW zeolites as robust Lewis acid catalysts

An innovative strategy based on dual structure-directing agent-facilitated crystallization was proposed to hydrothermally synthesize boron-free Zr-MWW and Sn-MWW metallosilicates that bear great structural diversity for potential pore engineering. The metallosilicates show distinctive features in Lewis acid-catalyzed reactions as efficient heterogeneous solid catalysts.

Ultrafast synthesis of nanosized Ti-Beta as an efficient oxidation catalyst via a structural reconstruction method

Nanosized Ti-Beta with high hydrophobicity was efficiently prepared via a structural reconstruction method using DA-Beta as the silica source with the assistance of TEAOH and F⁻. The obtained Ti-Beta exhibits high catalytic activity in the epoxidation of cyclohexene.

Advances in porous and nanoscale catalysts for viable biomass conversion

Solid catalysts with unique porosity and nanoscale properties play a promising role for efficient valorization of biomass into sustainable advanced fuels and chemicals.

The synthesis of hierarchical high-silica beta zeolites in NaF media

Hierarchical high-silica beta zeolites were synthesized using an aerosol assisted hydrothermal method in NaF media.

Hydrothermal synthesis of Sn-Beta zeolites in F−-free medium

Sn-Beta zeolites, with high Sn content and smaller crystal size, hydrothermally synthesized in F⁻-free medium using N-cyclohexyl-N,N-dimethylcyclohexanaminium hydroxide as the structure-directing agent with the assistance of Na⁺ and seed, are highly active as Lewis acid catalysts.