Bifunctional Catalysts for One-Step Conversion of Syngas into Aromatics with Excellent Selectivity and Stability

Recent advances in bifunctional synthesis gas conversion to chemicals and fuels with a comparison to monofunctional processes

In order to meet the climate goals of the Paris Agreement and limit the potentially catastrophic consequences of climate change, we must move away from the use of fossil feedstocks for the production of chemicals and fuels. The conversion of synthesis gas (a mixture of hydrogen, carbon monoxide and/or carbon dioxide) can contribute to this. Several reactions allow to convert synthesis gas to oxygenates (such as methanol), olefins or waxes. In a consecutive step, these products can be further converted into chemicals, such as dimethyl ether, short olefins, or aromatics. Alternatively, fuels like gasoline, diesel, or kerosene can be produced. These two different steps can be combined using bifunctional catalysis for direct conversion of synthesis gas to chemicals and fuels. The synergistic effects of combining two different catalysts are discussed in terms of activity and selectivity and compared to processes based on consecutive reaction with single conversion steps. We found that bifunctional catalysis can be a strong tool for the highly selective production of dimethyl ether and gasoline with high octane numbers. In terms of selectivity bifunctional catalysis for short olefins or aromatics struggles to compete with processes consisting of single catalytic conversion steps.

Direct and stable hydrogenation of CO2 to aromatics over a tandem catalyst Zn0.1Ti0.9Ox/HZSM-5

Direct and stable conversion of CO2 to aromatics (CTA) is an attractive route for reducing CO2 emissions. However, due to the chemical inertness of CO2, direct CTA reaction with high aromatics selectivity is still challenging. In this work, a tandem catalyst Zn0.1Ti0.9Ox/HZSM-5 with appropriate density and strength of acid sites exhibits a high aromatics selectivity of 67.2% and long-term stability over 100 h. Furthermore, the total selectivity of benzene, toluene, and xylene achieves 24.1% over Zn0.1Ti0.9Ox/HZSM-5 with a modified hydrophilic surface. In addition, the CTA via the formate route has been determined in this reaction system.

Fischer-Tropsch Synthesis of C4-C5 Isoparaffins via CoxMn1-xO Nanocomposites with Suppressed CO2

The synthesis of a specific product via the Fischer-Tropsch synthesis remains challenging due to the uncontrollable coupling of CHx on active sites. Isoparaffins, essential high-quality petroleum additives for improving octane numbers, are primarily derived from petroleum or natural gas. With petroleum reserves dwindling and the associated low selectivity, the direct conversion of syngas to isoparaffins has emerged as a promising alternative. This study presents a tandem catalyst comprising CoxMn1-xO and zeolites for catalyzing the direct conversion of syngas to C4-C5 isoparaffins. The relay catalyst exhibited an impressive selectivity of 55.6% toward the desired products while maintaining a low CO2 selectivity of approximately 20%. Notably, the selectivity of isobutane reached 43.5%, exceeding predictions based on the Anderson-Schulz-Flory distribution. Syngas undergoes conversion into olefins on CoxMn1-xO nanocomposites, diffuses into microporous zeolites, and interacts with Brønsted acids to produce isoparaffins. The stability of the relay catalyst relied significantly on the pore characteristics and acidic density of the zeolites.

Unraveling the Structure-Activity-Stability Relationship over Gallium-Promoted HZSM-5 Nanocrystalline Aggregates for Propane Aromatization

The aromatization of light alkane is an important process for increasing the aromatic production and utilization efficiency of light alkane resources simultaneously. Herein, Ga-modified HZSM-5 catalysts were prepared and investigated by a series of characterization techniques such as X-ray diffraction, nuclear magnetic resonance spectroscopy, transmission electron microscopy, N2 adsorption-desorption, and NH3 temperature-programmed desorption to study their physicochemical properties. The catalytic performance in propane aromatization was also tested. Importantly, the structure-activity relationship, reaction pathway, and coke formation mechanism in propane aromatization were systematically explored. It was found that different Ga introduction methods would affect the amounts of Brønsted and Lewis acid sites, and Ga-HZSM-5 prepared by the hydrothermal method exhibited higher amounts of Brønsted and Lewis acid sites but a lower B/L ratio. As a result, Ga-HZSM-5 showed higher propane conversion and benzene, toluene, and xylene yield compared with that of Ga2O3/HZSM-5. The propane aromatization reaction pathway indicated that propane dehydrogenation to propene was a crucial step for aromatic formation. The increase of the Lewis acid density in Ga-HZSM-5 can effectively improve the dehydrogenation rate and promote the aromatization reaction. Furthermore, the formation of coke species was studied by thermogravimetry-mass spectrometry and Raman approaches, the results of which indicated that the graphitization degree of coke formed over spent Ga-HZSM-5 is lower, resulting in enhanced anticoking stability.

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.

Ferrocene-functionalized zirconium-oxo clusters for achieving high-performance thermocatalytic redox reactions

The Zr(IV) ions are easily hydrolyzed to form oxides, which severely limits the discovery of new structures and applications of Zr-based compounds. In this work, three ferrocene (Fc)-functionalized Zr-oxo clusters (ZrOCs), Zr9Fc6, Zr10Fc6 and Zr12Fc8 were synthesized through inhibiting the hydrolysis of Zr(IV) ions, which show increased nuclearity and regular structural variation. More importantly, these Fc-functionalized ZrOCs were used as heterogeneous catalysts for the transfer hydrogenation of levulinic acid (LA) and phenol oxidation reactions for the first time, and displayed outstanding catalytic activity. In particular, Zr12Fc8 with the largest number of Zr active sites and Fc groups can achieve > 95% yield for LA-to-γ-valerolactone within 4 h (130 °C) and > 98% yield for 2,3,6-trimethylphenol-to-2,3,5-trimethyl-p-benzoquinone within 30 min (80 °C), showing the best catalytic performance. Catalytic characterization combined with theory calculations reveal that in the Fc-functionalized ZrOCs, the Zr active sites could serve as substrate adsorption sites, while the Fc groups could act as hydrogen transfer reagent or Fenton reagent, and thus achieve effectively intramolecular metal-ligand synergistic catalysis. This work develops functionalized ZrOCs as catalysts for thermal-triggered redox reactions.

Application of metal-organic frameworks and their derivates for thermal-catalytic C1 molecules conversion

One-carbon (C1) catalysis refers to the conversion of compounds with a single carbon atom, especially carbon monoxide (CO), carbon dioxide (CO2), and methane (CH4), into clean fuels and valuable chemicals via catalytic strategy is crucial for sustainable and green development. Among various catalytic strategies, thermal-driven process seems to be one of the most promising pathways for C1 catalysis due to the high efficiency and practical application prospect. Notably, the rational design of thermal-driven C1 catalysts plays a vital role in boosting the targeted products synthesis of C1 catalysis, which relies heavily on the choice of ideal active site support, catalyst fabrication precursor, and catalytic reaction field. As a novel crystalline porous material, metal-organic frameworks (MOFs) has made significant progress in the design and synthesis of various functional nanomaterials. However, the application of MOFs in C1 catalysis faces numerous challenges, such as thermal stability, mechanical strength, yield of MOFs, and so on. To overcome these limitations and harness the advantages of MOFs in thermal-driven C1 catalysis, researchers have developed various catalyst/carrier preparation strategies. In this review, we provide a concise overview of the recent advancements in the conversion of CO, CO2, and CH4 into clean fuels and valuable chemicals via thermal-catalytic strategy using MOFs-based catalysts. Furthermore, we discuss the main challenges and opportunities associated with MOFs-based catalysts for thermal-driven C1 catalysis in the future.

Cu-nanocluster-loaded N-doped porous graphitic carbon for electrochemical CO2 reduction towards syngas generation

In this study, a novel electrocatalyst, namely Cu/N-pg-C derived from Cu-doped ZIF-8, was investigated for making syngas products with various H2/CO ratios. Different ratios of the electrocatalytic syngas products CO and H2 could be selected by adjusting the applied potential and hence tuning the transfer of electrons from N-doped graphitic carbon to the well-dispersed Cu nanoclusters.

Coupled conversion of polyethylene and carbon dioxide catalyzed by a zeolite-metal oxide system

Zeolite-catalyzed polyethylene (PE) aromatization achieves reduction of the aromatic yield via hydrogenation and hydrogenolysis reactions. The hydrogen required for CO2 hydrogenation can be provided by H radicals formed during aromatization. In this study, we efficiently convert PE and CO2 into aromatics and CO using a zeolite-metal oxide catalyst (HZSM-5 + CuZnZrOx) at 380°C and under hydrogen- and solvent-free reaction conditions. Hydrogen, derived from the aromatization of PE over HZSM-5, diffuses through the Brønsted acidic sites of the zeolite to the adjacent CuZnZrOx, where it is captured in situ by CO2 to produce bicarbonate and further hydrogenated to CO. This favors aromatization while inhibiting hydrogenation and secondary hydrogenolysis reactions. An aromatic yield of 62.5 wt % is achieved, of which 60% consisted of benzene, toluene, and xylene (BTX). The conversion of CO2 reaches values as high as 0.55 mmol gPE-1. This aromatization-hydrogen capture pathway provides a feasible scheme for the comprehensive utilization of waste plastics and CO2.

Selectivity Control by Relay Catalysis in CO and CO2 Hydrogenation to Multicarbon Compounds

ConspectusThe hydrogenative conversion of both CO and CO2 into high-value multicarbon (C2+) compounds, such as olefins, aromatic hydrocarbons, ethanol, and liquid fuels, has attracted much recent attention. The hydrogenation of CO is related to the chemical utilization of various carbon resources including shale gas, biomass, coal, and carbon-containing wastes via syngas (a mixture of H2 and CO), while the hydrogenation of CO2 by green H2 to chemicals and liquid fuels would contribute to recycling CO2 for carbon neutrality. The state-of-the-art technologies for the hydrogenation of CO/CO2 to C2+ compounds primarily rely on a direct route via Fischer-Tropsch (FT) synthesis and an indirect route via two methanol-mediated processes, i.e., methanol synthesis from CO/CO2 and methanol to C2+ compounds. The direct route would be more energy- and cost-efficient owing to the reduced operation units, but the product selectivity of the direct route via FT synthesis is limited by the Anderson-Schulz-Flory (ASF) distribution. Selectivity control for the direct hydrogenation of CO/CO2 to a high-value C2+ compound is one of the most challenging goals in the field of C1 chemistry, i.e., chemistry for the transformation of one-carbon (C1) molecules.We have developed a relay-catalysis strategy to solve the selectivity challenge arising from the complicated reaction network in the hydrogenation of CO/CO2 to C2+ compounds involving multiple intermediates and reaction channels, which inevitably lead to side reactions and byproducts over a conventional heterogeneous catalyst. The core of relay catalysis is to design a single tandem-reaction channel, which can direct the reaction to the target product controllably, by choosing appropriate intermediates (or intermediate products) and reaction steps connecting these intermediates, and arranging optimized yet matched catalysts to implement these steps like a relay. This Account showcases representative relay-catalysis systems developed by our group in the past decade for the synthesis of liquid fuels, lower (C2-C4) olefins, aromatics, and C2+ oxygenates from CO/CO2 with selectivity breaking the limitation of conventional catalysts. These relay systems are typically composed of a metal or metal oxide for CO/CO2/H2 activation and a zeolite for C-C coupling or reconstruction, as well as a third or even a fourth catalyst component with other functions if necessary. The mechanisms for the activation of H2 and CO/CO2 on metal oxides, which are distinct from that on the conventional transition or noble metal surfaces, are discussed with emphasis on the role of oxygen vacancies. Zeolites catalyze the conversion of intermediates (including hydrocracking/isomerization of heavier hydrocarbons, methanol-to-hydrocarbon reactions, and carbonylation of methanol/dimethyl ether) in the relay system, and the selectivity is mainly controlled by the Brønsted acidity and the shape-selectivity or the confinement effect of zeolites. We demonstrate that the thermodynamic/kinetic matching of the relay steps, the proximity and spatial arrangement of the catalyst components, and the transportation of intermediates/products in sequence are the key issues guiding the selection of each catalyst component and the construction of an efficient relay-catalysis system. Our methodology would also be useful for the transformation of other C1 molecules via controlled C-C coupling, inspiring more efforts toward precision catalysis.

Tandem catalysts of different crystalline In2O3/sheet HZSM-5 zeolite for CO2 hydrogenation to aromatics

In tandem catalysts, not only good synergy between the two active components is required, but also the precise control of the spatial distribution between the two active components of metal oxides and zeolite is crucial for the migration and conversion of reaction intermediates in the direct conversion of CO2 to hydrocarbons. The correlation between the metal and the acidic site of zeolite has traditionally been simplified as "the closer, the better". However, it should be noted that this principle only holds true for a portion of tandem catalysts. Therefore, this paper studied the effect of different crystalline In2O3 (cubic phase, hexagonal phase, and mixed cubic/hexagonal phase) and sheet HZSM-5 zeolite tandem catalysts on the activity of CO2 hydrogenation reaction under different spatial distribution. The generalized gradient approximation (GGA) of density functional theory (DFT) were used to simulate the adsorption energy of CO2 by oxygen vacancy on c-In2O3(111) and h-In2O3(104) planes, it was found that Ov1 on c-In2O3(111) and Ov4 on h-In2O3(104) had the strongest adsorption energy for CO2. In addition, it has been observed that the proximity of the two active components (e.g., during mortar mixing) results in decreased catalytic performance. This is due to the migration of metal In, which neutralizes the acid sites of zeolites and leads to inefficient conversion of methanol reaction intermediates to aromatics. As a result, CO2 conversion and aromatic selectivity are decreased.

Interplay Between Particle Size and Hierarchy of Zeolite ZSM-5 During the CO2 -to-aromatics Process

The CO2 -to-aromatics process is a chemical reaction that converts carbon dioxide (CO2 ) into valuable petrochemical, i. e., aromatics (especially, benzene, toluene, and xylene) over the metal/zeolite bifunctional catalytic systems. These aromatics are used in producing plastics, fibers, and other industrial products, which are currently exclusively sourced from fossil-derived feedstocks. The significance of this process lies in its potential to mitigate climate change by reducing greenhouse gas emissions and simultaneously producing valuable chemicals. Consequently, these CO2 -derived aromatics can reduce the reliance on fossil fuels as a source of feedstocks, which can help to promote a more sustainable and circular economy. Owing to the existence of a wider straight channel favoring the aromatization process, zeolite ZSM-5 is extensively used to yield aromatics during CO2 hydrogenation over bifunctional (metal/zeolite) catalytic systems. To provide a better understanding of this unique property of zeolite ZSM-5, this work investigates the impact of particle size and hierarchy of the zeolite and how these govern the reaction performance and the overall selectivity. As a result, an improved understanding of the zeolite-catalyzed hydrocarbon conversion process has been obtained.

Insights into the Diffusion Behaviors of Water over Hydrophilic/Hydrophobic Catalysts During the Conversion of Syngas to High-Quality Gasoline

Although considerable efforts towards directly converting syngas to liquid fuels through Fischer-Tropsch synthesis have been made, developing catalysts with low CO2 selectivity for the synthesis of high-quality gasoline remains a big challenge. Herein, we designed a bifunctional catalyst composed of hydrophobic FeNa@Si-c and HZSM-5 zeolite, which exhibited a low CO2 selectivity of 14.3 % at 49.8 % CO conversion, with a high selectivity of 62.5 % for gasoline in total products. Molecular dynamic simulations and model experiments revealed that the diffusion of water molecules through hydrophilic catalyst was bidirectional, while the diffusion through hydrophobic catalyst was unidirectional, which were crucial to tune the water-gas shift reaction and control CO2 formation. This work provides a new fundamental understanding about the function of hydrophobic modification of catalysts in syngas conversion.

Xylene Synthesis Through Tandem CO2 Hydrogenation and Toluene Methylation Over a Composite ZnZrO Zeolite Catalyst

Selective synthesis of specific value-added aromatics from CO2 hydrogenation is of paramount interest for mitigating energy and climate problems caused by CO2 emission. Herein, we report a highly active composite catalyst of ZnZrO and HZSM-5 (ZZO/Z5-SG) for xylene synthesis from CO2 hydrogenation via a coupling reaction in the presence of toluene, achieving a xylene selectivity of 86.5 % with CO2 conversion of 10.5 %. A remarkably high space time yield of xylene could reach 215 mg gcat -1  h-1 , surpassing most reported catalysts for CO2 hydrogenation. The enhanced performance of ZZO/Z5-SG could be due to high dispersion and abundant oxygen vacancies of the ZZO component for CO2 adsorption, more feasible hydrogen activation and transfer due to the close interaction between the two components, and enhanced stability of the formate intermediate. The consumption of methoxy and methanol from the deep hydrogenation of formate by introduced toluene also propels an oriented conversion of CO2 .

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.

Photocatalytic Production of Syngas from Biomass

ConspectusAs a renewable solar energy and carbon carrier, biomass exploration has received global attention. Photocatalytic valorization of biomass into fuels and chemicals is a promising and sustainable method for future chemical production. Photocatalysis has the potential to accomplish reactions under ambient conditions due to the unique reaction mechanisms involving photoinduced charge carriers and has recently been recognized as an efficient and feasible technology for biomass conversion. Biomass is widely used as sacrificial agent to scavenge holes in photocatalytic hydrogen evolution, and the carbon is eventually degraded to CO₂ with a minor amount of CO. The generation of CO instead of CO₂ is more economical and promising but also a challenge under photoreforming conditions.This is a new research direction, while until now there has still been the lack of a comprehensive review article to summarize and provide prospects for this topic. This Account will highlight our contributions in the research direction of the photocatalytic reforming of biomass into syngas (CO + H₂). In 2020, we first reported the photocatalytic conversion of biopolyols and sugars into syngas by employing a defect-rich Cu-TiO₂ nanorod photocatalyst and found that formic acid is a key intermediate to CO. Further study revealed that a facet-dependent electron-trapping state on anatase TiO₂ will affect the photocatalytic dehydration activity for formic acid intermediates by regulating the electron transfer process during the reaction, and the selective generation of FA or CO from photocatalytic biomass reforming was achieved via exposing the (100) or (101) facets, respectively. Visible light-driven syngas generation was further achieved over a CdS-based photocatalyst. Sulfate modification of CdS ([SO₄]/CdS) was constructed as the proton acceptor, thus efficiently facilitating the proton-coupled electron transfer process. Besides, we put forward an oxygen-controlled strategy to increase the CO generation rate without a significant decrease in CO selectivity via controlling the O₂/substrate ratio. Based on this system, a Z-scheme CdS@g-C₃N₄ core-shell structure and CdO-CdS semicoherent interface were created to facilitate charge transfer and enhance the O₂ activation, thus increasing the CO generation rate. Moreover, we also developed a photoelectrochemical approach to separately produce CO and H₂ from biomass. Nitrogen doping of a hexagonal WO₃ nanowire array was used to produce the photoanode. The built-in electric field generated via nitrogen doping promoted charge transfer, hence improving the efficiency of PEC reforming of biopolyols and sugars. This Account will systematically analyze the challenges in this research direction, the reaction route in the photocatalytic biomass reforming, and the factors affecting CO selectivity and give insight into the design of efficient photocatalytic systems.

Nanomaterials as catalysts for CO2 transformation into value-added products: A review

Carbon dioxide (CO₂₎ is the most important greenhouse gas (GHG), accounting for 76% of all GHG emissions. The atmospheric CO₂ concentration has increased from 280 ppm in the pre-industrial era to about 418 ppm, and is projected to reach 570 ppm by the end of the 21^st century. In addition to reducing CO₂ emissions from anthropogenic activities, strategies to adequately address climate change must include CO₂ capture. To promote circular economy, captured CO₂ should be converted to value-added materials such as fuels and other chemical feedstock. Due to their tunable chemistry (which allows them to be selective) and high surface area (which allows them to be efficient), engineered nanomaterials are promising for CO₂ capturing and/or transformation. This work critically reviewed the application of nanomaterials for the transformation of CO₂ into various fuels, like formic acid, carbon monoxide, methanol, and ethanol. We discussed the literature on the use of metal-based nanomaterials, inorganic/organic nanocomposites, as well as other routes suitable for CO₂ conversion such as the electrochemical, non-thermal plasma, and hydrogenation routes. The characteristics, steps, mechanisms, and challenges associated with the different transformation technologies were also discussed. Finally, we presented a section on the outlook of the field, which includes recommendations for how to continue to advance the use of nanotechnology for conversion of CO₂ to fuels.

Comprehensive Study of Oxygen Vacancies on the Catalytic Performance of ZnO for CO/H2 Activation Using Machine Learning-Accelerated First-Principles Simulations

Oxygen vacancies (OVs) play important roles on any oxide catalysts. In this work, using an investigation of the OV effects on ZnO(101̅0) for CO and H₂ activation as an example, we demonstrate, via machine learning potentials (MLPs), genetic algorithm (GA)-based global optimization, and density functional theory (DFT) validations, that the ZnO(101̅0) surface with 0.33 ML OVs is the most likely surface configuration under experimental conditions (673 K and 2.5 MPa syngas (H₂:CO = 1.5)). It is found that a surface reconstruction from the wurtzite structure to a body-centered-tetragonal one would occur in the presence of OVs. We show that the OVs create a Zn₃ cluster site, allowing H₂ homolysis and C-O bond cleavage to occur. Furthermore, the activity of intrinsic sites (Zn_3c and O_3c sites) is almost invariable, while the activity of the generated OV sites is strongly dependent on the concentration of the OVs. It is also found that OV distributions on the surface can considerably affect the reactions; the barrier of C-O bond dissociation is significantly reduced when the OVs are aligned along the [12̅10] direction. These findings may be general in the systems with metal oxides in heterogeneous catalysis and may have significant impacts on the field of catalyst design by regulating the concentration and distribution of the OVs.

Advanced zeolite and ordered mesoporous silica-based catalysts for the conversion of CO2 to chemicals and fuels

For many years, capturing, storing or sequestering CO₂ from concentrated emission sources or from air has been a powerful technique for reducing atmospheric CO₂. Moreover, the use of CO₂ as a C1 building block to mitigate CO₂ emissions and, at the same time, produce sustainable chemicals or fuels is a challenging and promising alternative to meet global demand for chemicals and energy. Hence, the chemical incorporation and conversion of CO₂ into valuable chemicals has received much attention in the last decade, since CO₂ is an abundant, inexpensive, nontoxic, nonflammable, and renewable one-carbon building block. Nevertheless, CO₂ is the most oxidized form of carbon, thermodynamically the most stable form and kinetically inert. Consequently, the chemical conversion of CO₂ requires highly reactive, rich-energy substrates, highly stable products to be formed or harder reaction conditions. The use of catalysts constitutes an important tool in the development of sustainable chemistry, since catalysts increase the rate of the reaction without modifying the overall standard Gibbs energy in the reaction. Therefore, special attention has been paid to catalysis, and in particular to heterogeneous catalysis because of its environmentally friendly and recyclable nature attributed to simple separation and recovery, as well as its applicability to continuous reactor operations. Focusing on heterogeneous catalysts, we decided to center on zeolite and ordered mesoporous materials due to their high thermal and chemical stability and versatility, which make them good candidates for the design and development of catalysts for CO₂ conversion. In the present review, we analyze the state of the art in the last 25 years and the potential opportunities for using zeolite and OMS (ordered mesoporous silica) based materials to convert CO₂ into valuable chemicals essential for our daily lives and fuels, and to pave the way towards reducing carbon footprint. In this review, we have compiled, to the best of our knowledge, the different reactions involving catalysts based on zeolites and OMS to convert CO₂ into cyclic and dialkyl carbonates, acyclic carbamates, 2-oxazolidones, carboxylic acids, methanol, dimethylether, methane, higher alcohols (C₂₊OH), C₂₊ (gasoline, olefins and aromatics), syngas (RWGS, dry reforming of methane and alcohols), olefins (oxidative dehydrogenation of alkanes) and simple fuels by photoreduction. The use of advanced zeolite and OMS-based materials, and the development of new processes and technologies should provide a new impulse to boost the conversion of CO₂ into chemicals and fuels.

Promoted Photothermal Catalytic CO Hydrogenation by Using TiC-Supported Co-Fe5 C2 Catalysts

Photothermal catalytic CO hydrogenation offers the potential to synthesize light hydrocarbons by using solar energy. However, the selectivity and activity of the reaction are still far below those achieved in conventional thermal catalytic processes. Herein, we report that the Co-modified Fe₅ C₂ on TiC catalyst promotes photothermal catalytic CO hydrogenation with a 59 % C₂₊ selectivity in the produced hydrocarbons and a 30 % single-pass CO conversion at a high gas hourly space-time velocity of 12 000 mL g^-1  h^-1 . Using in-situ-irradiated XPS, we show that light-induced hot electron injection from TiC to Fe₅ C₂ modulates the chemical state of Fe, thereby increasing the CO-to-C₂₊ conversion. This work suggests that it is possible for plasmon-mediated surface chemistry to enhance the activity and selectivity of photothermal catalytic reactions.

Influence of the ZnCrAl Oxide Composition on the Formation of Hydrocarbons from Syngas

The conversion of syngas into value-added hydrocarbons gains increasing attention due to its potential to produce sustainable platform chemicals from simple starting materials. Along this line, the "OX-ZEO" process that combines a methanol synthesis catalyst with a zeolite, capable of catalyzing the methanol-to-hydrocarbon reaction, was found to be a suitable alternative to the classical Fischer-Tropsch synthesis. Hitherto, understanding the mechanism of the OX-ZEO process and simultaneously optimizing the CO conversion and the selectivity toward a specific hydrocarbon remains challenging. Herein, we present a comparison of a variety of ZnCrAl oxides with different metal ratios combined with a H-ZSM-5 zeolite for the conversion of syngas to hydrocarbons. The effect of aluminum on the catalytic activity was investigated for ZnCrAl oxides with a Zn/Cr ratio of 4:1, 1:1, and 1:2. The product distribution and CO conversion were found to be strongly influenced by the Zn/Cr/Al ratio. Although a ratio of Zn/Cr of 1:2 was best to produce lower olefins and aromatics, with aromatic selectivities of up to 37%, catalysts with a 4:1 ratio revealed high paraffin selectivity up to 52%. Notably, a distinct effect of aluminum in the oxide lattice on the catalytic activity and product selectivity was observed, as a higher Al content leads to a lower CO conversion and a changed product spectrum. We provide additional understanding of the influence of different compositions of ZnCrAl oxides on their surface properties and the catalytic activity in the OX-ZEO process. Furthermore, the variation of the zeolite component supports the important role of the channel topology of the porous support material for the hydrocarbon production. In addition, variation of the gas hourly space velocity showed a correlation of contact time, CO conversion, and hydrocarbon selectivity. At a gas hourly space velocity of 4200 mL/g_cat h, CO conversion as high as 44% along with a CO₂ selectivity of 42% and a lower paraffin (C₂ ⁰-C₄ ⁰) selectivity of 41% was observed.

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.

A review of catalytic hydrogenation of carbon dioxide: From waste to hydrocarbons

With the rapid development of industrial society and humankind’s prosperity, the growing demands of global energy, mainly based on the combustion of hydrocarbon fossil fuels, has become one of the most severe challenges all over the world. It is estimated that fossil fuel consumption continues to grow with an annual increase rate of 1.3%, which has seriously affected the natural environment through the emission of greenhouse gases, most notably carbon dioxide (CO₂). Given these recognized environmental concerns, it is imperative to develop clean technologies for converting captured CO₂ to high-valued chemicals, one of which is value-added hydrocarbons. In this article, environmental effects due to CO₂ emission are discussed and various routes for CO₂ hydrogenation to hydrocarbons including light olefins, fuel oils (gasoline and jet fuel), and aromatics are comprehensively elaborated. Our emphasis is on catalyst development. In addition, we present an outlook that summarizes the research challenges and opportunities associated with the hydrogenation of CO₂ to hydrocarbon products.

A Dual-Bed Strategy for Direct Conversion of Syngas to Light Paraffins

The authors studied the direct conversion of syngas to light paraffins in a dual-bed fixed-bed reactor. A dual-bed catalyst composed of three catalysts, a physically mixed methanol synthesis catalyst (CZA), and a methanol dehydration to dimethyl ether (DME) catalyst (Al2O3(C)) were put in the upper bed for direct conversion of syngas to DME, while the SAPO-34 (SP34-C) zeolite was put in the lower bed for methanol and DME conversion. The effects of the mass ratio of CZA to Al2O3(C), the H2/CO molar ratio, and the space velocity on catalytic performance of syngas to DME were studied in the upper bed. Moreover, a feed gas with a CO/CO2/DME/N2/H2 molar ratio of 9/6/4/5 balanced with H2 was simulated and studied in the lower bed over SP34-C; after optimizing the reaction conditions, the selectivity of light paraffins reached 90.8%, and the selectivity of propane was as high as 76.7%. For the direct conversion of syngas to light paraffins in a dual bed, 88.9% light paraffins selectivity in hydrocarbons was obtained at a CO conversion of 33.4%. This dual-bed strategy offers a potential route for the direct conversion of syngas to valuable chemicals.

Hydrogenation of Carbon Dioxide to Value-Added Liquid Fuels and Aromatics over Fe-Based Catalysts Based on the Fischer–Tropsch Synthesis Route

Hydrogenation of CO2 to value-added chemicals and fuels not only effectively alleviates climate change but also reduces over-dependence on fossil fuels. Therefore, much attention has been paid to the chemical conversion of CO2 to value-added products, such as liquid fuels and aromatics. Recently, efficient catalysts have been developed to face the challenge of the chemical inertness of CO2 and the difficulty of C–C coupling. Considering the lack of a detailed summary on hydrogenation of CO2 to liquid fuels and aromatics via the Fischer–Tropsch synthesis (FTS) route, we conducted a comprehensive and systematic review of the research progress on the development of efficient catalysts for hydrogenation of CO2 to liquid fuels and aromatics. In this work, we summarized the factors influencing the catalytic activity and stability of various catalysts, the strategies for optimizing catalytic performance and product distribution, the effects of reaction conditions on catalytic performance, and possible reaction mechanisms for CO2 hydrogenation via the FTS route. Furthermore, we also provided an overview of the challenges and opportunities for future research associated with hydrogenation of CO2 to liquid fuels and aromatics.

Direct Conversion of Syngas to Higher Alcohols via Tandem Integration of Fischer-Tropsch Synthesis and Reductive Hydroformylation

The selective conversion of syngas to higher alcohols is an attractive albeit elusive route in the quest for effective production of chemicals from alternative carbon resources. We report the tandem integration of solid cobalt Fischer–Tropsch and molecular hydroformylation catalysts in a one‐pot slurry‐phase process. Unprecedented selectivities (>50 wt %) to C₂₊ alcohols are achieved at CO conversion levels >70 %, alongside negligible CO₂ side‐production. The efficient overall transformation is enabled by catalyst engineering, bridging gaps in operation temperature and intrinsic selectivity which have classically precluded integration of these reactions in a single conversion step. Swift capture of 1‐olefin Fischer–Tropsch primary products by the molecular hydroformylation catalyst, presumably within the pores of the solid catalyst is key for high alcohol selectivity. The results underscore that controlled cooperation between solid aggregate and soluble molecular metal catalysts, which pertain to traditionally dichotomic realms of heterogeneous and homogeneous catalysis, is a promising blueprint toward selective conversion processes.

Metal-Acid Bifunctional Catalysts toward Tandem Reaction: One-Step Hydroalkylation of Benzene to Cyclohexylbenzene

The one-step hydroalkylation of benzene to cyclohexylbenzene (CHB) is a technically challenging and economically interesting reaction with great industrial importance, where bifunctional catalysts play a crucial role in such a tandem reaction. In this work, we report H₃PW₁₂O₄₀ (HPW) modified Ni nanoparticles (NPs) supported on mixed metal oxides (Ni/MMOs), which are featured by HPW species localized on the surface of Ni NPs (denoted as HPW-Ni/MMOs). The optimal catalyst (0.3HPW-Ni/MMOs) exhibits a satisfactory catalytic performance toward benzene hydroalkylation to CHB with a CHB yield of up to 41.2%, which is the highest standard among previously reported catalysts to date. A combination investigation based on HR-TEM, XPS, XANES, and in situ FT-IR verified the electron transfer from the W atom to the adjacent Ni atom, which facilitated the formation and desorption of cyclohexene (CHE) from Ni followed by the alkylation reaction of benzene and CHE at the interfacial Brønsted (B) acid sites of HPW, accounting for the significantly enhanced catalytic behavior. It is proposed that the HPW-Ni interface structure in xHPW-Ni/MMOs samples provides unique adsorption sites for benzene and CHE with a moderate adsorption strength, which serve as the intrinsic active center for this reaction: the Ni site promotes the hydrogenation of benzene to CHE, while the B acid site in HPW facilitates the alkylation of CHE and benzene to produce CHB. This work provides a fundamental understanding of the metal-acid synergistic catalysis toward the hydroalkylation reaction, which can be extended to the design and preparation of high-performance catalysts used in tandem reactions.

Fischer-Tropsch synthesis to olefins boosted by MFI zeolite nanosheets

Catalytic reactions are severely restricted by the strong adsorption of product molecules on the catalyst surface, where promoting desorption of the product and hindering its re-adsorption benefit the formation of free sites on the catalyst surface for continuous substrate conversion^1,2. A solution to this issue is constructing a robust nanochannel for the rapid escape of products. We demonstrate here that MFI zeolite crystals with a short b-axis of 90-110 nm and a finely controllable microporous environment can effectively boost the Fischer-Tropsch synthesis to olefins by shipping the olefin molecules. The ferric carbide catalyst (Na-FeC_x) physically mixed with a zeolite promoter exhibited a CO conversion of 82.5% with an olefin selectivity of 72.0% at the low temperature of 260 °C. By contrast, Na-FeC_x alone without the zeolite promoter is poorly active under equivalent conditions, and shows the significantly improved olefin productivity achieved through the zeolite promoter. These results show that the well-designed zeolite, as a promising promoter, significantly boosts Fischer-Tropsch synthesis to olefins by accelerating escape of the product from the catalyst surface.

Tunable green syngas generation from CO2 and H2O with sunlight as the only energy input

The carbon-neutral synthesis of syngas from CO₂ and H₂O powered by solar energy holds grand promise for solving critical issues such as global warming and the energy crisis. Here we report photochemical reduction of CO₂ with H₂O into syngas using core/shell Au@Cr₂O₃ dual cocatalyst-decorated multistacked InGaN/GaN nanowires (NWs) with sunlight as the only energy input. First-principle density functional theory calculations revealed that Au and Cr₂O₃ are synergetic in deforming the linear CO₂ molecule to a bent state with an O-C-O angle of 116.5°, thus significantly reducing the energy barrier of CO₂RR compared with that over a single component of Au or Cr₂O₃. Hydrogen evolution reaction was promoted by the same cocatalyst simultaneously. By combining the cooperative catalytic properties of Au@Cr₂O₃ with the distinguished optoelectronic virtues of the multistacked InGaN NW semiconductor, the developed photocatalyst demonstrated high syngas activity of 1.08 mol/g_cat/h with widely tunable H₂/CO ratios between 1.6 and 9.2 under concentrated solar light illumination. Nearly stoichiometric oxygen was evolved from water splitting at a rate of 0.57 mol/g_cat/h, and isotopic testing confirmed that syngas originated from CO₂RR. The solar-to-syngas energy efficiency approached 0.89% during overall CO₂ reduction coupled with water splitting. The work paves a way for carbon-neutral synthesis of syngas with the sole inputs of CO₂, H₂O, and solar light.