Hierarchical hollow zeolite fiber in catalytic applications: A critical review

Zeolites have widely been studied because of the better performance as catalysts and supports. However, the zeolites with only micropores have drawbacks in reactivity and selectivity due to limitation of diffusivity. The hollow zeolite fibers (HZF) with hierarchical porosity however can overcome the problem. The HZF can be synthesized by such methods as incorporated substrate removal method, solid-solid transformation method, co-axial electrospinning technology, dry-wet spinning technology, and hollow fiber incorporation method. The unique hierarchical porous structure leads to the great improvement in the diffusion efficiency of reactants. The catalytic zeolite membrane fibers are the most commonly used as they have stronger catalyst stability and higher catalytic selectivity. The HZFs are suitable in catalytic applications such as selective catalysis, CO preferential oxidation, air purification and wastewater treatment. In order that the HZFs can be applied to industrial operations, more research work should be carried out, such as developments of self-assembly pure HZFs, catalytic substrate incorporated HZFs, HZFs with gradient multicomponent zeolites and HZFs with nanoscale diameters.

Two-Step Dry Gel Method Produces MgAPO-11 with Low Aspect Ratio and Improved Catalytic Performance in the Conversion of Methanol to Hydrocarbons

In this article, the synthesis, characterization and catalytic performance of three MgAPO-11 catalysts with distinct crystal morphologies (sunflower, ball and candy) are presented. Among the three samples, the candy-like MgAPO-11-C, with high crystallinity and uniform particle size (of about 1 µm), was synthesized for the first time by using a unique two-step dry gel method. Despite the similar acid strength of the three samples, the different and distinct morphologies of the catalysts resulted in very different methanol-to-hydrocarbons (MTH) performances. In particular, the candy-like MgAPO-11-C presented the best MTH performance with the highest total conversion capacity (4.4 gMeOH·gcatalyst−1 h−1) and the best selectivity to C5+ aliphatics (64%).

Critical review on microfibrous composites for applications in chemical engineering

Microfibrous composites (MCs) are novel materials with unique structures and excellent functional properties, showing great potential in industrial applications. The investigation of the physicochemical properties of MCs is significant for accommodating the rapid development of high-efficiency chemical engineering industries. In this review, the characteristics, synthesis and applications of different types of previously reported MCs are discussed according to the constituent fibres, including polymers, metals and nonmetals. Among the different types of MCs, polymer MCs have a facile synthesis process and adjustable fibre composition, making them suitable for many complex situations. The high thermal and electrical conductivity of metal MCs enables their application in strong exothermic, endothermic and electrochemical reactions. Nonmetallic MCs are usually stable and corrosion resistant when reducing and oxidizing environments. The disadvantages of MCs, such as complicated synthesis processes compared to those of particles or powders, high cost, insufficient thorough study, and unsatisfactory regeneration effects, are also summarized. As a result, a more systematic investigation of MCs remains necessary. Despite the advantages and great application potential of microfibrous composites, much effort remains necessary to advance them to the industrial level in the chemical engineering industry.

In situ fabrication of core–shell-structured Beta@Silicalite-1 catalysts by a novel steam-assisted crystallization strategy

A green and efficient steam-assisted crystallization (SAC) strategy was employed to synthesize a core–shell zeolite with different frameworks between the core and shell, Beta@Silicalite-1.

Fe2O3-CeO2@Al2O3 Nanoarrays on Al-Mesh as SO2-Tolerant Monolith Catalysts for NO x Reduction by NH3

Currently, selective catalytic reduction of NO _x with NH₃ in the presence of SO₂ is still challenging at low temperatures (<300 °C). In this study, enhanced NO _x reduction was achieved over a SO₂-tolerant Fe-based monolith catalyst, which was originally developed through in situ construction of Al₂O₃ nanoarrays (na-Al₂O₃) on the monolithic Al-mesh by a steam oxidation method followed by anchoring Fe₂O₃ and CeO₂ onto the na-Al₂O₃@Al-mesh composite by an impregnation method. The optimum catalyst delivered more than 90% NO conversion and N₂ selectivity above 98% within 250-430 °C as well as excellent SO₂ tolerance at 270 °C. The strong interaction between Fe₂O₃ and CeO₂ enabled favorable electron transfers from Fe₂O₃ to CeO₂ while generating more oxygen vacancies and active oxygen species, consequently accelerating the redox cycle. The improved reactivity of NH₄⁺ with nitrates following the Langmuir-Hinshelwood mechanism and active NH₂ species that directly reacted with gaseous NO following the Eley-Rideal mechanism enhanced the NO _x reduction efficiency at low temperatures. The preferential sulfation of CeO₂ alleviated the sulfation of Fe₂O₃ while maintaining the high reactivities of NH₄⁺ and NH₂ species. Especially, the SCR reaction following the Eley-Rideal mechanism largely improved the SO₂ tolerance because NO does not need to compete with sulfates to adsorb on the catalyst surface as nitrates or nitrites. This work paves a way for the development of high-performance SO₂-tolerant SCR monolith catalysts.

Boosting the turnover number of core–shell Al-ZSM-5@B-ZSM-5 zeolite for methanol to propylene reaction by modulating its gradient acid site distribution and low consumption diffusion

Hierarchical Al-ZSM-5@B-ZSM-5 core–shell zeolite was prepared to boost turnover number of MTP reaction.

Recent advances in preparation methods for catalytic thin films and coatings

Advancements in the preparation methods and applications of catalytic thin films and coatings are briefly summarized.

Kinetic Modeling of Catalytic Olefin Cracking and Methanol-to-Olefins (MTO) over Zeolites: A Review

The increasing demand for lower olefins requires new production routes besides steam cracking and fluid catalytic cracking (FCC). Furthermore, less energy consumption, more flexibility in feed and a higher influence on the product distribution are necessary. In this context, catalytic olefin cracking and methanol-to-olefins (MTO) gain in importance. Here, the undesired higher olefins can be catalytically converted and, for methanol, the possibility of a green synthesis route exists. Kinetic modeling of these processes is a helpful tool in understanding the reactivity and finding optimum operating points; however, it is also challenging because reaction networks for hydrocarbon interconversion are rather complex. This review analyzes different deterministic kinetic models published in the literature since 2000. After a presentation of the underlying chemistry and thermodynamics, the models are compared in terms of catalysts, reaction setups and operating conditions. Furthermore, the modeling methodology is shown; both lumped and microkinetic approaches can be found. Despite ZSM-5 being the most widely used catalyst for these processes, other catalysts such as SAPO-34, SAPO-18 and ZSM-23 are also discussed here. Finally, some general as well as reaction-specific recommendations for future work on modeling of complex reaction networks are given.

Synthesis of Engineered Zeolitic Materials: From Classical Zeolites to Hierarchical Core-Shell Materials

The term "engineered zeolitic materials" refers to a class of materials with a rationally designed pore system and active-sites distribution. They are primarily made of crystalline microporous zeolites as the main building blocks, which can be accompanied by other secondary components to form composite materials. These materials are of potential importance in many industrial fields like catalysis or selective adsorption. Herein, critical aspects related to the synthesis and modification of such materials are discussed. The first section provides a short introduction on classical zeolite structures and properties, and their conventional synthesis methods. Then, the motivating rationale behind the growing demand for structural alteration of these zeolitic materials is discussed, with an emphasis on the ongoing struggles regarding mass-transfer issues. The state-of-the-art techniques that are currently available for overcoming these hurdles are reviewed. Following this, the focus is set on core-shell composites as one of the promising pathways toward the creation of a new generation of highly versatile and efficient engineered zeolitic substances. The synthesis approaches developed thus far to make zeolitic core-shell materials and their analogues, yolk-shell, and hollow materials, are also examined and summarized. Finally, the last section concisely reviews the performance of novel core-shell, yolk-shell, and hollow zeolitic materials for some important industrial applications.

Microfibrous-Structured Pd/AlOOH/Al-Fiber for CO Coupling to Dimethyl Oxalate: Effect of Morphology of AlOOH Nanosheet Endogenously Grown on Al-Fiber

We report a green, template-free, and general one-pot method of endogenous growth of free-standing boehmite (AlOOH) nanosheets on a 3D-network 60 μm-Al-fiber felt through water-only hydrothermal oxidation reaction between Al metal and H₂O (2Al + 4H₂O → 2AlOOH + 3H₂). Content and morphology of AlOOH nanosheets can be finely tuned by adjusting the hydrothermal oxidation time length and temperature. Palladium is highly dispersed on such AlOOH endogenously formed on Al-fiber felt via incipient wetness impregnation method and as-obtained Pd/AlOOH/Al-fiber catalysts are checked in the CO coupling to dimethyl oxalate (DMO) reaction. Interestingly, Pd dispersion is very sensitive to the thickness (26-68 nm) of AlOOH nanosheet, and therefore the conversion shows strong AlOOH-nanosheet-thickness dependence whereas the intrinsic activity (TOF) is AlOOH-nanosheet-thickness independence. The most promising structured catalyst is the one using a microfibrous-structured composite with the thinnest AlOOH nanosheet (26 nm) to support a small amount of Pd of only 0.26 wt %. This catalyst, with high thermal-conductivity and satisfying structural robustness, delivers 67% CO conversion and 96% DMO selectivity at 150 °C using a feed of CH₃ONO/CO/N₂ (1/1.4/7.6, vol) and a gas hourly space velocity of 3000 L kg^-1 h^-1, and particularly, is very stable for at least 150 h without deactivation sign.

Vapor-phase transport synthesis of microfibrous-structured SS-fiber@ZSM-5 catalyst with improved selectivity and stability for methanol-to-propylene

Microfibrous-structured SS-fiber@HZSM-5 catalyst prepared by cost-effective and high-efficiency VPT method delivers remarkable improvement in selectivity and stability for the MTP reaction due to the improved diffusion in zeolite shell.

A self-supported SS-fiber@meso-HZSM-5 core–shell catalyst via caramel-assistant synthesis toward prolonged lifetime for the methanol-to-propylene reaction

Self-supported SS-fiber@meso-HZSM-5 core–shell catalyst prepared by caramel-assistant hydrothermal synthesis, delivered dramatically prolonged lifetime for MTP reaction due to its hierarchical design and favourably-tuned acidic properties.

Conversion of methanol to propylene over nano-sized ZSM-5 zeolite aggregates synthesized by a modified seed-induced method with CTAB

Nano-sized ZSM-5 zeolite aggregates with mesopores and high catalytic activity were prepared by a modified seed-induced method using silicate-1 as seeds without additional templates in the presence of a trace amount of CTAB.

Microfibrous-structured Al-fiber@ns-Al2O3 core–shell composite functionalized by Fe–Mn–K via surface impregnation combustion: as-burnt catalysts for synthesis of light olefins from syngas

Microfibrous-structured Al-fiber@ns-Al₂O₃ core–shell composite is functionalized by Fe–Mn–K via surface impregnation combustion for synthesis of light olefins from syngas.

Programmable synthesis of mesoporous ZSM-5 nanocrystals as selective and stable catalysts for the methanol-to-propylene process

Programmable synthesis of mesoporous ZSM-5 nanocrystals for a highly efficient methanol to propylene reaction.

Synthesis of microporous/mesoporous core–shell materials with crystalline zeolitic shell and supported metal oxide silica core

Engineered silica@zeolite core–shell composites, possessing a hierarchical porosity in a shape selective manner, were synthesised by deposition of silicalite-1 nanocrystals over various mesoporous silica spheres, in either pure form or loaded with metal guest species.

Catalytic distillation for ethyl acetate synthesis using microfibrous-structured Nafion–SiO2/SS-fiber solid acid packings

We present a microfibrous-structured Nafion–SiO₂/SS-fiber solid acid catalyst and demonstrate its separation and reaction efficiency as catalytic distillation (CD) packings for esterification to produce ethyl acetate from acetic acid and ethanol.

From nano- to macro-engineering of oxide-encapsulated-nanoparticles for harsh reactions: one-step organization via cross-linking molecules

One-step “macro–micro–nano” organization is reported to embed oxide-encapsulated-nanoparticles onto monolithic-substrates via molecularly-defined cross-linking agents, for applications in exothermic and/or high-throughput reaction processes.

Tandem catalytic conversion of 1-butene and ethene to propene over combined mesoporous W-FDU-12 and MgO catalysts

W substituted mesoporous FDU-12 combined with MgO showed enhanced catalytic performance for tandem conversion of 1-butene and ethene to propene.

High-performance PdNi alloy structured in situ on monolithic metal foam for coalbed methane deoxygenation via catalytic combustion

A promising monolithic foam-structured PdNi alloy catalyst has been developed by galvanic deposition and in situ reaction activation, providing exceptionally high activity/selectivity, remarkable stability with oscillation elimination, and enhanced heat transfer.