Recent Advances in Direct Synthesis of Value-Added Aromatic Chemicals from Syngas by Cascade Reactions over Bifunctional Catalysts

Dual-engine-driven realizing high-yield synthesis of Para-Xylene directly from CO2-containing syngas

The direct synthesis of light aromatics, especially para-xylene (p-X), from syngas/CO2 is drawing strong interest, but improving the space-time yield (STY) of p-X is a significant challenge. Here, a dynamic "dual-engine-driven" (DED) catalytic system is designed by combining two partners of ZnCr and FeMn (named "dual-engine") with Z5@SiO2 capsule zeolite. The DED catalyst of 1.0%FeMn&[ZnCr&Z5@SiO2] shows an extremely higher p-X STY of 36.1 gp-x·kgcat-1·h-1, about eight times higher than that of [ZnCr&Z5]. DED manipulates ZnCr engine for methanol formation and drives FeMn engine for light olefins generation together, and then the formed methanol and light olefins are coordinately converted in situ into p-X-rich aromatics over Z5@SiO2. The DED model boosts the driving force for syngas/CO2 conversion, simultaneously concerting the cooperation of "dual-engine" for p-X generation, resulting in extremely high STY of p-X. This study achieves non-petroleum p-X production at industrial-relevant level and advances knowledge in designing innovative heterogeneous catalysts.

Development of Multifunctional Catalysts for the Direct Hydrogenation of Carbon Dioxide to Higher Alcohols

The direct hydrogenation of greenhouse gas CO2 to higher alcohols (C2+OH) provides a new route for the production of high-value chemicals. Due to the difficulty of C-C coupling, the formation of higher alcohols is more difficult compared to that of other compounds. In this review, we summarize recent advances in the development of multifunctional catalysts, including noble metal catalysts, Co-based catalysts, Cu-based catalysts, Fe-based catalysts, and tandem catalysts for the direct hydrogenation of CO2 to higher alcohols. Possible reaction mechanisms are discussed based on the structure-activity relationship of the catalysts. The reaction-coupling strategy holds great potential to regulate the reaction network. The effects of the reaction conditions on CO2 hydrogenation are also analyzed. Finally, we discuss the challenges and potential opportunities for the further development of direct CO2 hydrogenation to higher alcohols.

Theoretically Designed Cu10Sn3-Cu-SnOx as Three-Component Electrocatalyst for Efficient and Tunable CO2 Reduction to Syngas

Electrocatalytic transformation of CO2 to various syngas compositions is an exceedingly attractive approach to carbon-neutral recycling. Meanwhile, the achievement of selectivity, stability, and tunability of product ratios using single-component electrocatalysts is challenging. Herein, the theoretically-assisted design of the triple-component nanocomposite electrocatalyst Cu10Sn3-Cu-SnOx that addresses this challenge is presented. It is shown that Cu10Sn3 is a valuable electrocatalyst for suitable CO2 reduction to CO, SnO2 for CO2 reduction to formate at large overpotentials, and that the Cu-SnO2 interface facilitates H2 evolution. Accordingly, the interaction between the three functional components affords tunable CO/H2 ratios, from 1:2 to 2:1, of the produced syngas by controlling the applied potentials and relative contents of functional components. The syngas generation is selective (Faradaic efficiency, FE = 100%) at relatively lower cathodic potentials, whereas formate is the only liquid product detected at relatively higher cathodic potentials. The theoretically guided design approach therefore provides a new opportunity to boost the selectivity and stability of CO2 reduction to tunable syngas.

The Oxygenate-Mediated Conversion of COx to Hydrocarbons─On the Role of Zeolites in Tandem Catalysis

Decentralized chemical plants close to circular carbon sources will play an important role in shaping the postfossil society. This scenario calls for carbon technologies which valorize CO2 and CO with renewable H2 and utilize process intensification approaches. The single-reactor tandem reaction approach to convert COx to hydrocarbons via oxygenate intermediates offers clear benefits in terms of improved thermodynamics and energy efficiency. Simultaneously, challenges and complexity in terms of catalyst material and mechanism, reactor, and process gaps have to be addressed. While the separate processes, namely methanol synthesis and methanol to hydrocarbons, are commercialized and extensively discussed, this review focuses on the zeolite/zeotype function in the oxygenate-mediated conversion of COx to hydrocarbons. Use of shape-selective zeolite/zeotype catalysts enables the selective production of fuel components as well as key intermediates for the chemical industry, such as BTX, gasoline, light olefins, and C3+ alkanes. In contrast to the separate processes which use methanol as a platform, this review examines the potential of methanol, dimethyl ether, and ketene as possible oxygenate intermediates in separate chapters. We explore the connection between literature on the individual reactions for converting oxygenates and the tandem reaction, so as to identify transferable knowledge from the individual processes which could drive progress in the intensification of the tandem process. This encompasses a multiscale approach, from molecule (mechanism, oxygenate molecule), to catalyst, to reactor configuration, and finally to process level. Finally, we present our perspectives on related emerging technologies, outstanding challenges, and potential directions for future research.

Engineering nanoscale H supply chain to accelerate methanol synthesis on ZnZrOx

Metal promotion is the most widely adopted strategy for enhancing the hydrogenation functionality of an oxide catalyst. Typically, metal nanoparticles or dopants are located directly on the catalyst surface to create interfacial synergy with active sites on the oxide, but the enhancement effect may be compromised by insufficient hydrogen delivery to these sites. Here, we introduce a strategy to promote a ZnZrO_x methanol synthesis catalyst by incorporating hydrogen activation and delivery functions through optimized integration of ZnZrO_x and Pd supported on carbon nanotube (Pd/CNT). The CNT in the Pd/CNT + ZnZrO_x system delivers hydrogen activated on Pd to a broad area on the ZnZrO_x surface, with an enhancement factor of 10 compared to the conventional Pd-promoted ZnZrO_x catalyst, which only transfers hydrogen to Pd-adjacent sites. In CO₂ hydrogenation to methanol, Pd/CNT + ZnZrO_x exhibits drastically boosted activity-the highest among reported ZnZrO_x-based catalysts-and excellent stability over 600 h on stream test, showing potential for practical implementation.

COx conversion to aromatics: a mini-review of nanoscale performance

The conversion of CO_x into value-added green aromatics is considered as a promising route to achieve the world's decarbonization due to its considerable thermodynamic driving force and atomic economy where low H/C ratio aromatics are chosen as a product. It is enabled by bifunctional nano-catalysts composed of metal oxides with abundant oxygen vacancies and acid zeolites, thus realizing superior selectivity in hydrocarbons at the single pass of CO_x conversion. In this mini-review, we mainly provide some thought-provoking insights at the nanoscale of this complicated process including the proximity of active sites, reaction mechanism, asymmetric desorption behavior of intermediates and final products and overall thermodynamic analysis. The facile surface diffusion of intermediates owing to the proximity of active sites stimulates the reaction, which follows an autocatalytic process. This positive feedback attributed to the autocatalytic cycle accelerates the transformation of energy and materials in the thermodynamically optimal direction, making the reaction highly selective towards the final products. This complicated coupling process, like a nano-maze constituted by these micro-environment factors, is complicated in terms of the reaction pathway but highly selective to a fixed direction guided by overall thermodynamics. Deep understanding of such an autocatalytic cycle at the nanoscale paves the way for the rational design of next-generation high-performance catalysts.

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.

Highly Enhanced Aromatics Selectivity by Coupling of Chloromethane and Carbon Monoxide over H-ZSM-5

The transformation of methane into high value-added chemicals such as aromatics provides a more desired approach towards sustainable chemistry but remains a critical challenge due to the low selectivity of aromatics and poor stability. Herein, we first report a coupling reaction of CH₃ Cl and CO (CCTA) based on methane conversion, which achieves extremely high aromatics selectivity (82.2 %) with the selectivity of BTX up to ca. 60 % over HZSM-5. The promoting effects have been demonstrated on other zeolites especially 10-membered rings (10 MR) zeolites. Multiple characterizations show that 2,3-dimethyl-2-cyclopentene-1-one (DMCPO) is generated from acetyl groups and olefins. Furthermore, isotopic labeling analysis confirms that CO is inserted into the DMCPO and aromatics rings. A new aromatization mechanism is proposed, including the formation of the above intermediates, which conspicuously weakens the hydrogen transfer reaction, leading to a considerable increase of aromatics selectivity and a dramatic drop in alkanes.

Boosting Mass Exchange between Pd/NC and MoC/NC Dual Junctions via Electron Exchange for Cascade CO2 Fixation

Merging existing catalysts together as a cascade catalyst may achieve "one-pot" synthesis of complex but functional molecules by simplifying multistep reactions, which is the blueprint of sustainable chemistry with low pollutant emission and consumption of energy and materials only when the smooth mass exchange between different catalysts is ensured. Effective strategies to facilitate the mass exchange between different active centers, which may dominate the final activity of various cascade catalysts, have not been reached until now, even though charged interfaces due to work function driven electron exchange have been widely observed. Here, we successfully constructed mass (reactants and intermediates) exchange paths between Pd/N-doped carbon and MoC/N-doped carbon induced by interfacial electron exchange to trigger the mild and cascade methylation of amines using CO₂ and H₂. Theoretical and experimental results have demonstrated that the mass exchange between electron-rich MoC and electron-deficient Pd could prominently improve the production of N,N-dimethyl tertiary amine, which results in a remarkably high turnover frequency value under mild conditions, outperforming the state-of-the-art catalysts in the literature by a factor of 5.9.

Hydrogenation of CO2 into aromatics over ZnZrO–Zn/HZSM-5 composite catalysts derived from ZIF-8

CO2 hydrogenation to aromatics over a ZnZr8O(350)–Zn/Z5 composite catalyst.

Carbon dioxide capture with zeotype materials

This review describes the application of zeotype materials for the capture of CO2 in different scenarios, the critical parameters defining the adsorption performances, and the challenges of zeolitic adsorbents for CO2 capture.

High value utilization of biomass: selective catalytic transformation of lignocellulose into bio-based 2,5-dimethylphenol

A new strategy for the synthesis of high-value biochemical 2,5-dimethylphenol was constructed by lignocellulose catalytic pyrolysis and selective hydroxylation.

Tuning product selectivity in CO2 hydrogenation over metal-based catalysts

Conversion of CO2 into chemicals is a promising strategy for CO2 utilization, but its intricate transformation pathways and insufficient product selectivity still pose challenges. Exploiting new catalysts for tuning product selectivity in CO2 hydrogenation is important to improve the viability of this technology, where reverse water-gas shift (RWGS) and methanation as competitive reactions play key roles in controlling product selectivity in CO2 hydrogenation. So far, a series of metal-based catalysts with adjustable strong metal-support interactions, metal surface structure, and local environment of active sites have been developed, significantly tuning the product selectivity in CO2 hydrogenation. Herein, we describe the recent advances in the fundamental understanding of the two reactions in CO2 hydrogenation, in terms of emerging new catalysts which regulate the catalytic structure and switch reaction pathways, where the strong metal-support interactions, metal surface structure, and local environment of the active sites are particularly discussed. They are expected to enable efficient catalyst design for minimizing the deep hydrogenation and controlling the reaction towards the RWGS reaction. Finally, the potential utilization of these strategies for improving the performance of industrial catalysts is examined.

Towards the development of the emerging process of CO2 heterogenous hydrogenation into high-value unsaturated heavy hydrocarbons

The emerging process of CO₂ hydrogenation through heterogenous catalysis into important bulk chemicals provides an alternative strategy for sustainable and low-cost production of valuable chemicals, and brings an important chance for mitigating CO₂ emissions. Direct synthesis of the family of unsaturated heavy hydrocarbons such as α-olefins and aromatics via CO₂ hydrogenation is more attractive and challenging than the production of short-chain products to modern society, suffering from the difficult control between C-O activation and C-C coupling towards long-chain hydrocarbons. In the past several years, rapid progress has been achieved in the development of efficient catalysts for the process and understanding of their catalytic mechanisms. In this review, we provide a comprehensive, authoritative and critical overview of the substantial progress in the synthesis of α-olefins and aromatics from CO₂ hydrogenation via direct and indirect routes. The rational fabrication and design of catalysts, proximity effects of multi-active sites, stability and deactivation of catalysts, reaction mechanisms and reactor design are systematically discussed. Finally, current challenges and potential applications in the development of advanced catalysts, as well as opportunities of next-generation CO₂ hydrogenation techniques for carbon neutrality in future are proposed.

Bulk and surface theoretical investigation of Nb-doped δ-FeOOH as a promising bifunctional catalyst

The development of bifunctional catalysts is of great interest in fine chemistry, since they are capable of promoting multicatalytic reactions involved in several important industrial processes. Iron oxyhydroxides have been identified as low-cost bifunctional catalysts. However, their applications are limited due to their weak acid character. Thus, elaborated modifications of these systems can significantly contribute to increasing their activities and selectivity. This work consists in the study, through DFT calculations, of the properties of the bulk and the surface of feroxyhyte (δ-FeOOH) doped with niobium, as a potential bifunctional catalyst. We identified the formation of stronger van der Waals interactions among the doped δ-FeOOH layers, which can increase the thermal stability of the catalyst. In addition, evidence has been found that the insertion of Nb increases Brönsted acidity and gives rise to new Lewis acid sites on the catalyst surface.

Catalysts for Production and Conversion of Syngas

Synthesis gas, or syngas for short, is a mixture of CO, CO2, and H2 [...]

Incorporation of Active Metal Species in Crystalline Porous Materials for Highly Efficient Synergetic Catalysis

The design and development of efficient catalytic materials with synergistic catalytic sites always has long been known to be a thrilling and very dynamic research field. Crystalline porous materials (CPMs) mainly including metal-organic frameworks and zeolites with high scientific and industrial impact have recently been the subject of extensive research due to their essential role in modern chemical industrial processes. The rational incorporation of guest species in CPMs can synergize the respective strengths of these components and allow them to collaborate with each other for synergistic catalysis, leading to enhanced catalytic activity, selectivity, and stability in a broad range of catalytic processes. In this review, the recent advances in the development of CPMs-confined active metal species, including metal nanoparticles, metal/metal oxides heteroparticles, metal oxide, subnanometric metal clusters, and polyoxometalates, for heterogeneous catalysis, with a particular focus on synergistic effects between active components that result in an enhanced performance are highlighted. Insights into catalysts design strategies, host-guest interactions, and structure-property relationships have been illustrated in detail. Finally, the existing challenges and possible development directions in CPMs-based encapsulation-structured synergistic catalysts are discussed.

Oxide-Zeolite-Based Composite Catalyst Concept That Enables Syngas Chemistry beyond Fischer-Tropsch Synthesis

Syngas chemistry has been under study since Fischer-Tropsch synthesis (FTS) was invented in the 1920s. Despite the successful applications of FTS as the core technology of coal-to-liquid and gas-to-liquid processes in industry, the product selectivity control of syngas conversion still remains a great challenge, particularly for value-added chemicals such as light olefins. Recent studies show that the catalyst design concept of OXZEO (oxide-zeolite-based composite) enables direct syngas conversion to mixed light olefins with a selectivity reaching 80% and to ethylene with a selectivity of 83% among hydrocarbons. They both well-surpass the limits predicated by the Anderson-Schultz-Flory model via the conventional FTS route (58% and 30%, respectively). Furthermore, this catalyst concept allows one-step synthesis of gasoline-range isoparaffins and aromatic compounds, which is otherwise not possible in conventional FTS. A rapidly growing number of studies demonstrate the versatility of this concept and may form a technology platform for utilization of carbon resources including coal, natural gas, and biomass via syngas to a variety of basic chemicals and fuels. However, the selectivity control mechanism is far from being understood. Therefore, we focus mainly on the catalytic roles of the bifunctionalities of OXZEO while reviewing the development of bifunctional catalysts for selective syngas conversion by taking syngas-to-light olefins as an example. With this, we intend to provide insights into the selectivity control mechanism of the OXZEO concept in order to understand the challenges and prospects for future development of much more active and more selective catalysts.

Boosting the synthesis of value-added aromatics directly from syngas via a Cr2O3 and Ga doped zeolite capsule catalyst

Even though the transformation of syngas into aromatics has been realized via a methanol-mediated tandem process, the low product yield is still the bottleneck, limiting the industrial application of this technology. Herein, a tailor-made zeolite capsule catalyst with Ga doping and SiO2 coating was combined with the methanol synthesis catalyst Cr2O3 to boost the synthesis of value-added aromatics, especially para-xylene, from syngas. Multiple characterization studies, control experiments, and density functional theory (DFT) calculation results clarified that Ga doped zeolites with strong CO adsorption capability facilitated the transformation of the reaction intermediate methanol by optimizing the first C-C coupling step under a high-pressure CO atmosphere, thereby driving the reaction forward for aromatics synthesis. This work not only reveals the synergistic catalytic network in the tandem process but also sheds new light on principles for the rational design of a catalyst in terms of oriented conversion of syngas.

Dehydrogenative Coupling of Alkanes and Benzene Enhanced by Slurry-Phase Interparticle Hydrogen Transfer

The dehydrogenative coupling reaction of alkanes and benzene has attracted attention as a method of direct conversion of alkanes to raw materials for useful chemical products. Here, we report the first combined catalyst system composed of hydrotalcite-supported palladium and solid acid aluminum-exchanged montmorillonite for the direct alkylation of benzene promoted by slurry-phase interparticle hydrogen transfer at 150 °C. The combination of the two catalytic particles showed excellent activity and achieved the maximum benzene conversion of 21% and target product selectivity of 84% in the reaction of n-heptane and benzene. Our results, thus, provide a feasible strategy to design efficient liquid-phase reaction systems employing simple physical mixing of two catalytic particles.

Tailoring the synergistic dual-decoration of (Cu–Co) transition metal auxiliaries in Fe-oxide/zeolite composite catalyst for the direct conversion of syngas to aromatics

Tailoring the crystal lattice and multiple phase interfaces via the feasible accommodation of Cu–Co into the host (Fe) structure, expedited the surface oxygen vacancies that modulated the reduction/chemisorption behavior of active Fe species.

Applications of Zeolites to C1 Chemistry: Recent Advances, Challenges, and Opportunities

C1 chemistry, which is the catalytic transformation of C1 molecules including CO, CO₂ , CH₄ , CH₃ OH, and HCOOH, plays an important role in providing energy and chemical supplies while meeting environmental requirements. Zeolites are highly efficient solid catalysts used in the chemical industry. The design and development of zeolite-based mono-, bi-, and multifunctional catalysts has led to a booming application of zeolite-based catalysts to C1 chemistry. Combining the advantages of zeolites and metallic catalytic species has promoted the catalytic production of various hydrocarbons (e.g., methane, light olefins, aromatics, and liquid fuels) and oxygenates (e.g., methanol, dimethyl ether, formic acid, and higher alcohols) from C1 molecules. The key zeolite descriptors that influence catalytic performance, such as framework topologies, nanoconfinement effects, Brønsted acidities, secondary-pore systems, particle sizes, extraframework cations and atoms, hydrophobicity and hydrophilicity, and proximity between acid and metallic sites are discussed to provide a deep understanding of the significance of zeolites to C1 chemistry. An outlook regarding challenges and opportunities for the conversion of C1 resources using zeolite-based catalysts to meet emerging energy and environmental demands is also presented.

Direct conversion of syngas into aromatics over a bifunctional catalyst: inhibiting net CO2 release

Tandem catalysis via methanol intermediate is a promising route for the direct conversion of syngas into aromatics. However, the simultaneous formation of CO2 is a serious problem. Here, we demonstrate that CO2 was formed by the water-gas shift (WGS) reaction (CO + H2O → CO2 + H2) over a ZnO-ZrO2/H-ZSM-5 catalyst, and the net CO2 formation could be inhibited without affecting the formation of aromatics by co-feeding CO2.

Cu/ZnO Catalysts Derived from Bimetallic Metal-Organic Framework for Dimethyl Ether Synthesis from Syngas with Enhanced Selectivity and Stability

Direct conversion of syngas to dimethyl ether (DME) through the intermediate of methanol allows more efficient DME production in a simpler reactor design relative to the conventional indirect route. Although Cu/ZnO-based multicomponent catalysts are highly active for methanol synthesis in this process, the sintering issue of Cu during the prolonged reaction generally deteriorates their performance. In this work, Cu/ZnO catalysts in a novel octahedron structure are prepared by a two-step pyrolysis of Zn-doped Cu-BTC metal-organic framework (MOF) in N₂ and air. The catalyst CZ-350/A, hybrid of MOF-derived Cu/ZnO sample CZ-350 and γ-Al₂ O₃ for methanol dehydration, displays the best activity for DME formation (7.74% CO conversion and 70.05% DME selectivity) with the lowest deterioration rate over 40 h continuous reaction. Such performance is superior to its counterpart CZ-CP/A made via the conventional coprecipitation method. This is mainly due to the confinement of Cu nanoparticles within the octahedron matrix hindering their migration and aggregation. Besides, partial reduction of ZnO in the activated CZ-350 prompts the formation of Cu⁺ -O-Zn, further facilitating the DME production with the highest selectivity compared to literature results. The results clearly indicate that Cu and ZnO distribution in the catalyst architecture plays an important role in DME formation.

The Effects of the Crystalline Phase of Zirconia on C–O Activation and C–C Coupling in Converting Syngas into Aromatics

Zirconia has recently been used as an efficient catalyst in the conversion of syngas. The crystalline phases of ZrO2 in ZrO2/HZSM-5 bi-functional catalysts have important effects on C–O activation and C–C coupling in converting syngas into aromatics and been investigated in this work. Monoclinic ZrO2 (m-ZrO2) and tetragonal ZrO2 (t-ZrO2) were synthesized by hydrothermal and chemical precipitation methods, respectively. The results of in situ diffuse reflection infrared Fourier transform spectroscopy (DRIFTs) revealed that there were more active hydroxyl groups existing on the surface of m-ZrO2, and CO temperature programmed desorption (CO-TPD) results indicated that the CO adsorption capacity of m-ZrO2 was higher than that of t-ZrO2, which can facilitate the C–O activation of m-ZrO2 for syngas conversion compared to that of t-ZrO2. And the CO conversion on the m-ZrO2 catalyst was about 50% more than that on the t-ZrO2 catalyst. 31P and 13C magic angle spinning nuclear magnetic resonance (MAS NMR) analysis revealed a higher acid and base density of m-ZrO2 than that of t-ZrO2, which enhanced the C–C coupling. The selectivity to CH4 on the m-ZrO2 catalyst was about 1/5 of that on the t-ZrO2 catalyst in syngas conversion. The selectivity to C2+ hydrocarbons over m-ZrO2 or t-ZrO2 as well as the proximity of the ZrO2 sample and HZSM-5 greatly affected the further aromatization in converting syngas into aromatics.

Construction of a tandem HZSM-5 with CuZnAl catalyst for alkylation of benzene with syngas

A bifunctional catalyst composed of ZSM-5 and Cu–Zn–Al shows an outstanding catalytic performance in the alkylation reaction of benzene with syngas.