Preparation of Metal-Supported Nanostructured Zeolite Catalysts and their Applications in the Upgrading of Biomass-Derived Furans: Advances and Prospects

The development of platform chemicals derived from biomass, in particular, 5-hydroxymethylfurfural (5-HMF) and furfural (FUR), is of crucial importance in biorefinery. Over the past decades, metal-supported nanostructured zeolites, in particular, metal-supported hierarchically porous zeolites or metal-encapsulated zeolites, have been extensively elaborated because of their multiple functionalities and superior properties, for example, shape-selectivity, (hydro)thermal stability, tunable acidity and basicity, redox properties, improved diffusion, and intimacy of multiple active sites. In this review, the effects of such properties of metal-supported nanostructured zeolites on the enhanced catalytic performances in furanic compound upgrading are discussed. In addition, the recent rational design of metal-supported nanostructured zeolites is exemplified. Consequently, the ongoing challenges for further developing metal-supported nanostructured zeolites-based catalysts and their applications in HMF and FUR upgrading are identified.

Ionic Porous Aromatic Frameworks Embedding Polyoxometalates for Heterogeneous Catalysis

Porous aromatic frameworks (PAFs) are highly promising functional porous solids known for their feasible amenability and extraordinary stability. When the framework was modified by ionic functional groups, these ionic PAFs (iPAFs) exhibited charged channels for adsorption, separation, and catalysis. However, the surface areas of ionic porous frameworks are usually lower than that of neutral frameworks, and their synthesis is limited by specific strategies and complex modification processes. To address these challenges, an intuitive route to construct ionic porous framework with high specific surface area was proposed. Herein, a multivariate ionic porous aromatic framework (MTV-iPAFs, named PAF-270) was synthesized using readily available building units with ionic functional groups through a multivariable synthesis strategy. PAF-270 exhibited hierarchical structure with the highest specific surface area among reported imidazolium-functionalized PAFs. Utilizing its physical and chemical properties, the availability for polyoxometalate loading and heterogeneous catalysis of PAF-270 were explored. PAF-270 exhibited a high adsorption capacity up to 50 % for both H3O40PW12 (HPW) and (NH4)5H6PV8Mo4O40 (V8). HPW@PAF-270 and V8@PAF-270 exhibited excellent catalytic abilities for oleic acid esterification and extractive oxidative desulfurization, respectively. Due to the stability of PAFs, these materials also showed remarkable resistance to temperature and pH changes. Overall, these results underscore the potential application of MTV-iPAFs as versatile functional porous materials.

Acceleration of Zeolite Crystallization: Current Status, Mechanisms, and Perspectives

Zeolites are important classes of crystalline materials and possess well-defined channels and cages with molecular dimensions. They have been extensively employed as heterogeneous catalysts and gas adsorbents due to their relatively large specific surface areas, high pore volumes, compositional flexibility, definite acidity, and hydrothermal stability. The zeolite synthesis normally undergoes high-temperature hydrothermal treatments with a relatively long crystallization time, which exhibits low synthesis efficiency and high energy consumption. Various strategies, e.g., modulation of the synthesis gel compositions, employment of special silica/aluminum sources, addition of seeds, fluoride, hydroxyl (·OH) free radical initiators, and organic additives, regulation of the crystallization conditions, development of new approaches, etc., have been developed to overcome these obstacles. And, these achievements make prominent contributions to the topic of acceleration of the zeolite crystallization and promote the fundamental understanding of the zeolite formation mechanism. However, there is a lack of the comprehensive summary and analysis on them. Herein, we provide an overview of the recent achievements, highlight the significant progress in the past decades on the developments of novel and remarkable strategies to accelerate the crystallization of zeolites, and basically divide them into three main types, i.e., chemical methods, physical methods, and the derived new approaches. The principles/acceleration mechanisms, effectiveness, versatility, and degree of reality for the corresponding approaches are thoroughly discussed and summarized. Finally, the rational design of the prospective strategies for the fast synthesis of zeolites is commented on and envisioned. The information gathered here is expected to provide solid guidance for developing a more effective route to improve the zeolite crystallization and obtain the functional zeolite-based materials with more shortened durations and lowered cost and further promote their applications.

A novel MOFs-induced strategy for preparing anatase-free hierarchical TS-1 zeolite:synthesis routes, growth mechanisms and enhanced catalytic performance

Titanosilicate-1 zeolites (TS-1) as one of the most commonly used catalysts for alkene epoxidation, construction of hierarchical pores as well as elimination of anatase to promote mass transportation and avoid invalid decomposition of hydrogen peroxide are always desirable yet challenging goals. Here, a novel and unique Ti-based metal organic frameworks (MOFs)-induced synthetic strategy for fabricating anatase-free hierarchical TS-1 was first proposed. All the components of MOFs perform different functions: the uniformly distributed Ti nodes replace conventional tetrabutyl titanate (TBOT) to serve as sole Ti source for constructing zeolite crystal; the separated ligands can be embedded in the zeolite framework and act as template to in situ build hierarchical pore structure; the coordination interaction between Ti nodes and ligands can efficiently avoid the anatase generation by balancing the forming rates of Ti-OH and Si-OH. This synthetic strategy is of general applicability, and two different synthetic routes including traditional hydrothermal process and steam assisted crystallization (SAC) procedure are successfully adopted. The obtained hydrothermal TS-1 and SAC anatase-free samples all possess abundant intercrystalline mesopores of 20-50 nm and even macropores between 50 and 150 nm, improving the conversion over 25 % for 1‑hexene epoxidation than TS-1 sample prepared by conventional route. The influences of the amount of Ti MOFs precursor and the crystallization process are studied in detail, and possible synthesis mechanisms are proposed. This MOFs-induced strategy might open up an avenue for the rational design of ideal and hierarchical zeolite to boost the catalytic efficiency.

Tetrapropylammonium Hydroxide Treatment of Aged Dry Gel to Make Hierarchical TS-1 Zeolites for Catalysis

This work presents the development and systematic study of a method to prepare hierarchical titanium silicalite-1 (TS-1) zeolites with high tetra-coordinated framework Ti species content. The new method involves (i) the synthesis of the aged dry gel by treating the zeolite precursor at 90 °C for 24 h; and (ii) the synthesis of hierarchical TS-1 by treating the aged dry gel using tetrapropylammonium hydroxide (TPAOH) solution under hydrothermal conditions. Systematic studies were conducted to understand the effect of the synthesis conditions (including the TPAOH concentration, liquid-to-solid ratio, and treatment time) on the physiochemical properties of the resulting TS-1 zeolites, and the results showed that the condition of a TPAOH concentration of 0.1 M, liquid-to-solid ratio of 1.0, and treatment time of 9 h was ideal to enable the synthesis of hierarchical TS-1 with a Si/Ti ratio of 44. Importantly, the aged dry gel was beneficial to the quick crystallization of zeolite and assembly of nanosized TS-1 crystals with a hierarchical structure (S _ext = 315 m² g^-1 and V _meso = 0.70 cm³ g^-1, respectively) and high framework Ti Species content, making the accessible active sites ready for promoting oxidation catalysis.

Impact of a polymer modifier on directing the non-classical crystallization pathway of TS-1 zeolite: accelerating nucleation and enriching active sites

The crystallization process directly affects the physicochemical properties and active centers of zeolites; however, controllable tuning of the zeolite crystallization process remains a challenge. Herein, we utilized a polymer (polyacrylamide, PAM) to control the precursor structure evolution of TS-1 zeolite through a two-step crystallization process, so that the crystallization path was switched from a classical to a non-classical mechanism, which greatly accelerated nucleation and enriched active Ti sites. The TS-1 crystallization process was investigated by means of various advanced characterization techniques. It was found that specific interactions between PAM and Si/Ti species promoted the assembly of colloidal precursors containing ordered structural fragments and stabilized Ti species in the precursors, leading to a 1.5-fold shortened crystallization time and enriched Ti content in TS-1 (Si/Ti = 29). The PAM-regulated TS-1 zeolite exhibited enhanced catalytic performance in oxidative reactions compared to conventional samples.

Titanium-Doped Mesoporous Silica with High Hydrothermal Stability for Catalytic Cracking Performance of Heavy Oil

With the increasing attention to light oil, the catalytic cracking process of heavy oil is being vigorously developed. The silicon hydroxyl groups on the surface of mesoporous silica materials can be used as weak acid centers to preliminarily crack heavy oil macromolecules. Herein, a strategy of introducing titanium into a silica skeleton for modification is proposed to increase active sites, as well as improve the hydrothermal stability. After titanium modification, the mesoporous silica material has more weak acid sites, and shows better ability in deep cracking heavy oil. Notably, when the content of titanium doping is 2%, the CT(2) catalyst exhibited the best high-temperature hydrothermal stability, which can be used as a suitable heavy oil catalytic cracking catalyst. This kind of titanium-modified mesoporous silica material shows great application prospects in heavy oil catalytic cracking, which may provide a novel idea for subsequent development.

Metal Sites in Zeolites: Synthesis, Characterization, and Catalysis

Zeolites with ordered microporous systems, distinct framework topologies, good spatial nanoconfinement effects, and superior (hydro)thermal stability are an ideal scaffold for planting diverse active metal species, including single sites, clusters, and nanoparticles in the framework and framework-associated sites and extra-framework positions, thus affording the metal-in-zeolite catalysts outstanding activity, unique shape selectivity, and enhanced stability and recyclability in the processes of Brønsted acid-, Lewis acid-, and extra-framework metal-catalyzed reactions. Especially, thanks to the advances in zeolite synthesis and characterization techniques in recent years, zeolite-confined extra-framework metal catalysts (denoted as metal@zeolite composites) have experienced rapid development in heterogeneous catalysis, owing to the combination of the merits of both active metal sites and zeolite intrinsic properties. In this review, we will present the recent developments of synthesis strategies for incorporating and tailoring of active metal sites in zeolites and advanced characterization techniques for identification of the location, distribution, and coordination environment of metal species in zeolites. Furthermore, the catalytic applications of metal-in-zeolite catalysts are demonstrated, with an emphasis on the metal@zeolite composites in hydrogenation, dehydrogenation, and oxidation reactions. Finally, we point out the current challenges and future perspectives on precise synthesis, atomic level identification, and practical application of the metal-in-zeolite catalyst system.

Advances in the synthesis and application of SSZ-39 zeolite

Zeolites, especially aluminosilicate zeolites, have been widely utilized in the process of petroleum refining, environmental protection, and fine chemicals. In the past decades, great attentions have been paid on the...

Mesoporogen-Free Strategy to Construct Hierarchical TS-1 in a Highly Concentrated System for Gas-Phase Propene Epoxidation with H2 and O2

Hierarchical TS-1 has attracted enormous attention from both academia and industry due to its remarkable catalytic performance in epoxidation reactions. However, sustainable synthesis of hierarchical-nanosized TS-1 without mesoporogens is still challenging. In this work, we report a facile and mesoporogen-free strategy to simultaneously manipulate pore structure and particle size of TS-1 employing the concentrated system. Taking advantage of the suspended nuclei in the concentrated system as confirmed by the DLS-PSD and atomic force microscopy, the novel TS-1 is demonstrated to have higher Ti concentration on surface, higher surface area (539 m²/g), abundant mesopores, and reduced crystal size (ca. 150 nm). Moreover, this Au-Ti bifunctional catalyst shows a good PO formation rate with enhanced catalytic stability due to the hierarchical structure. This strategy opens a novel way for the green synthesis of hierarchical-nanosized TS-1 and facilitates industrial development of the Au/TS-1 catalyst for propene epoxidation.

Hierarchical Ti-beta with a three-dimensional ordered mesoporosity for catalytic epoxidation of bulky cyclic olefins

Hierarchical beta zeolites with a three-dimensionally ordered mesoporous-imprinted (3DOm-i) structure and post-synthetic Ti grafting for catalytic epoxidation of cyclic olefins.

The synthesis of pure and uniform nanosized TS-1 crystals with a high titanium content and a high space–time yield

Concentrated basic MFI building unit species played a key role in the crystal synthesis of uniform nanosized anatase-free titanium-rich TS-1.

High-efficiency synthesis of enhanced-titanium and anatase-free TS-1 zeolite by using a crystallization modifier

A high-efficiency strategy to rapidly synthesize enhanced-titanium TS-1 zeolite without extra-framework anatase TiO₂ by using 1,3,5-benzenetricarboxylic acid (H₃BTC) as a crystallization modifier.