Core-shell and egg-shell zeolite catalysts for enhanced hydrocarbon processing

Influence of aluminum zoning toward external surfaces in MFI zeolites on propene oligomerization catalysis

Brønsted acid zeolites catalyze alkene oligomerization reactions, an important route to produce fuels and chemicals from light hydrocarbon feedstocks. Propene dimerization rates (per H+, 503 K) decrease monotonically with increasing crystallite size in MFI zeolites because heavy oligomer products remain occluded within microporous voids and restrict intrazeolite diffusion of reactants and products. Here, we show that the preferential zoning of framework Al centers and their associated H+ sites toward exterior surfaces of MFI crystallites in an "egg-shell" architecture minimizes the extent of diffusion-enhanced secondary reactions within a given crystallite, which increases both propene dimerization rates (per H+) and selectivity to true oligomer products. These results show that tailoring Al distributions to be spatially zoned toward external surfaces of medium-pore zeolite crystallites is efficacious at minimizing diffusion path lengths to increase alkene oligomerization rates and selectivity to true oligomer products.

Hollow Zeolite Nanoreactor with Double Shells for Methanol Aromatization: Explicit Recognition on Catalytic Function of Inverse Elemental Zone and Shell-Cavity

Core@shell catalyst composited of dual aluminosilicate zeolite can effectively regulate the distribution of acid sites to control hydrocarbon conversion process for the stable formation of target product. However, the diffusion restriction reduces the accessibility of inner active sites and affects synergy between core and shell. Herein, hollow ZSM-5 zeolite nanoreactor with inverse aluminum distribution and double shells are prepared and employed for methanol aromatization. It is demonstrated that the intershell cavity alleviated the steric hindrance from zeolites channel and provided more paths and pore entrance for guest molecule. Correspondingly, olefin intermediates generated from methanol over the external shell are easier to adsorb at internal acid sites for further reactions. Importantly, the diffusion of generated aromatic macromolecules to the external surface is also promoted, which slows down the formation of internal coke, and ensures the use of internal acid sites for aromatization. The aromatics selectivity of the nanoreactor remained at 8% after 154 h, while that of solid core@shell catalyst decreased to 2% after 75 h. This finding promises broader insight to improve internal active site utilization of core@shell catalyst at the diffusion level and can be great aid in the flexible design of multifunctional nanoreactors to enhance the relay efficiency.

Design Synthesis of Low-Silica SAPO-34 Nanocrystals by Constructing Isomorphous Core-Shell Structure: An Effective Catalyst for Improving Catalytic Performances in Methanol-to-Olefins Reaction

It is well known that low-silica SAPO-34, with an extra porosity (meso- and/or macropores) system, affords excellent catalytic performance in the methanol-to-olefins (MTO) reaction, while the direct synthesis of low-silica SAPO-34 with a hierarchical structure is difficult to achieve, principally because the crystal impurities are usually formed under a low silica content in a gel precursor. Herein, low-silica SAPO-34 nanocrystals were successfully fabricated for the first time by constructing an isomorphous core-shell structure in an epitaxial growth manner. In which, low-silica, ultrasmall nanosquare-shaped SAPO-34 crystals with the same growth orientation along the (100) crystal plane compactly grow on the monocrystal SAPO-34 cores. Crucially, the external surface acid properties of the core SAPO-34 with the Si-rich outer layer are effectively modified by the low-silica SAPO-34 shell. Furthermore, the growth process and Si-substitution mechanism of the shell zeolite were comprehensively investigated. It was found that with the prolonged crystallization time, more and more coordinated Si(4Al) and Si(3Al) structures via two substitution mechanisms (SM2 and SM3) are generated in the nanocrystalline SAPO-34 shell, which endow moderate acidity of the core-shell SAPO-34. Compared to the uncoated SAPO-34, the core-shell SAPO-34 performs a longer lifespan and a higher average selectivity of light olefins (ethylene plus propylene) when applied to the MTO reaction, which is attributed to the positive effects of the luxuriant interstitial pores offering a fast diffusion channel and the moderate acid density depressing the hydrogen transfer reaction of light olefins. This work provides new insights into the fabrication of low-silica SAPO-34 nanocrystals, which are based on the rational design of the isomorphous core-shell zeolite.

Silicalite-1 Layer Secures the Bifunctional Nature of a CO2 Hydrogenation Catalyst

Close proximity usually shortens the travel distance of reaction intermediates, thus able to promote the catalytic performance of CO₂ hydrogenation by a bifunctional catalyst, such as the widely reported In₂O₃/H-ZSM-5. However, nanoscale proximity (e.g., powder mixing, PM) more likely causes the fast deactivation of the catalyst, probably due to the migration of metals (e.g., In) that not only neutralizes the acid sites of zeolites but also leads to the reconstruction of the In₂O₃ surface, thus resulting in catalyst deactivation. Additionally, zeolite coking is another potential deactivation factor when dealing with this methanol-mediated CO₂ hydrogenation process. Herein, we reported a facile approach to overcome these three challenges by coating a layer of silicalite-1 (S-1) shell outside a zeolite H-ZSM-5 crystal for the In₂O₃/H-ZSM-5-catalyzed CO₂ hydrogenation. More specifically, the S-1 layer (1) restrains the migration of indium that preserved the acidity of H-ZSM-5 and at the same time (2) prevents the over-reduction of the In₂O₃ phase and (3) improves the catalyst lifetime by suppressing the aromatic cycle in a methanol-to-hydrocarbon conversion step. As such, the activity for the synthesis of C₂ ⁺ hydrocarbons under nanoscale proximity (PM) was successfully obtained. Moreover, an enhanced performance was observed for the S-1-coated catalyst under microscale proximity (e.g., granule mixing, GM) in comparison to the S-1-coating-free counterpart. This work highlights an effective shielding strategy to secure the bifunctional nature of a CO₂ hydrogenation catalyst.

Tunable hybrid zeolites prepared by partial interconversion

Zeolite interconversion is a widely used strategy due to its unique advantages in the synthesis of some zeolites. By using a long-chain quaternary amine as both a structure-directing agent and porogen, we have produced superior catalysts, which we named Hybrid Zeolites, as their structures are made of building units of different zeolite types. The properties of these materials can be conveniently tuned, and their catalytic performance can be optimized simply by stopping the interconversion at different times. For cracking the 1,3,5-triisopropylbenzene, Hybrid Zeolites made of FAU and MFI units show a 5-fold increase in selectivity towards the desired product, that is, 1,3-diisopropylbenzene, compared to the commercial FAU, and a 7-fold increase in conversion at constant selectivity compared to MFI zeolite.

Recent Advances of Core-Shell Cu-Based Catalysts for the Reduction of CO2 to C2+ Products

Copper is a key metal for carbon dioxide (CO₂ ) reduction reaction, which can reduce CO₂ to value-added products. The core-shell structure can effectively promote the C-C coupling process due to its strong synergistic effect originated from its unique electronic structure and interface environment. Therefore, the combination of copper and core-shell structure to design an efficient Cu-based core-shell structure catalyst is of great significance for electrocatalytic CO₂ reduction (CO₂ RR). In this review, we first briefly summarize the basic principle of CO₂ RR. In addition, we outline the advantages of core-shell structure for catalysis. Then, we review the recent research progresses of Cu-based core-shell structures for the selective reduction of multi-carbon (C₂₊ ) products. In the end, the challenges of using core-shell catalyst for CO₂ RR are described, and the future development of this field is prospected.

Regulation of the Si/Al ratios and Al distributions of zeolites and their impact on properties

Zeolites are typically a class of crystalline microporous aluminosilicates that are constructed by SiO₄ and AlO₄ tetrahedra. Because of their unique porous structures, strong Brönsted acidity, molecular-level shape selectivity, exchangeable cations, and high thermal/hydrothermal stability, zeolites are widely used as catalysts, adsorbents, and ion-exchangers in industry. The activity, selectivity, and stability/durability of zeolites in applications are closely related to their Si/Al ratios and Al distributions in the framework. In this review, we discussed the basic principles and the state-of-the-art methodologies for regulating the Si/Al ratios and Al distributions of zeolites, including seed-assisted recipe modification, interzeolite transformation, fluoride media, and usage of organic structure-directing agents (OSDAs), etc. The conventional and newly developed characterization methods for determining the Si/Al ratios and Al distributions were summarized, which include X-ray fluorescence spectroscopy (XRF), solid state ²⁹Si/²⁷Al magic-angle-spinning nuclear magnetic resonance spectroscopy (²⁹Si/²⁷Al MAS NMR), Fourier-transform infrared spectroscopy (FT-IR), etc. The impact of Si/Al ratios and Al distributions on the catalysis, adsorption/separation, and ion-exchange performance of zeolites were subsequently demonstrated. Finally, we presented a perspective on the precise control of the Si/Al ratios and Al distributions of zeolites and the corresponding challenges.

Hydrocarbon Products Occluded within Zeolite Micropores Impose Transport Barriers that Regulate Brønsted Acid-Catalyzed Propene Oligomerization

Brønsted acid zeolites catalyze alkene oligomerization to heavier hydrocarbon products of varied size and branching. Propene dimerization rates decrease monotonically with increasing crystallite size for MFI zeolites synthesized with fixed H⁺-site density, revealing the strong influence of intrazeolite transport limitations on measured rates, which has gone unrecognized in previous studies. Transient changes in dimerization rates upon step-changes in reactant pressure (150-470 kPa C₃H₆) or temperature (483-523 K) reveal that intrazeolite diffusion limitations become more severe under reaction conditions that favor the formation of heavier products. Together with effectiveness factor formalisms, these data reveal that product and reactant diffusion, and consequently oligomerization rates and selectivity, are governed by the composition of hydrocarbon products that accumulate within zeolitic micropores during alkene oligomerization. This occluded organic phase strongly influences rates and selectivities of alkene oligomerization on medium-pore zeolites (MFI, MEL, TON). Recognizing the coupled influences of kinetic factors and intrazeolite transport limitations imposed by occluded reaction products provides opportunities to competently tailor rates and selectivity in alkene oligomerization and other molecular chain-growth reactions through judicious selection of zeolite topology and reaction conditions.

Molecular transport in zeolite catalysts: depicting an integrated picture from macroscopic to microscopic scales

Increasing social sustainability triggers the persistent progress of industrial catalysis in energy transformation and chemical production. Zeolites have been demonstrated to be pivotal catalysts in chemical industries due to their moderate acidity and versatile well-defined pore structures. However, in the context of enhancing the performances of zeolite catalysts, the perspectives on the diffusion regulations within the pores and channels in the bulk phases or external surfaces of the zeolites are often overlooked. Establishing the structure-transport-reactivity relationships in heterogeneous catalysis can provide rational guidelines to design high-performance catalysts. Herein, this tutorial review attempts to systematically depict an integrated picture of molecular transport behaviors in zeolite catalysts from macroscopic to microscopic perspectives. The advances in the accurate diffusion measurements employing both macroscopic and microscopic techniques are briefly introduced. The diffusion characteristics in zeolite catalysts under working conditions (e.g., high temperature, multi-components, and reaction coupling) are then addressed. The macroscopic internal diffusion and the microscopic diffusion occurring in the micro-zones of zeolite crystals (e.g., surface diffusion, diffusion anisotropy, and confined diffusion) are reviewed and discussed in more detail. These diffusion behaviors highly impact the underlying reaction mechanism, catalytic performances, and catalyst optimization strategies. Finally, the multi-type pore systems of practical zeolite catalysts in industrial reactors and their transport behaviors are analyzed. The fully-crystalline monolithic zeolites in the absence of binders are highlighted as rising-star catalytic materials for industrial applications. The research challenges in this field and the potential future development directions are summarized.