Key Takeaways & Executive Findings
- •• • GaZrOx/H-SSZ-13 composite achieves highly selective CO2 hydrogenation to propane, with selectivity exceeding 90% at 300°C and 3 MPa, demonstrating the critical role of zeolite pore structure in shape-selective LPG production. • • InZrOx-Beta composite enables selective conversion of CO2 to isobutane-enriched C4 alkanes, with isobutane selectivity above 80% among C4 products, highlighting the influence of Brønsted acid sites on branched hydrocarbon formation. • • The dual-functional mechanism requires precise matching of metal oxide (e.g., ZnZrOx, In2O3) for methanol synthesis and zeolite (e.g., H-ZSM-5, H-SSZ-13) for methanol-to-hydrocarbons conversion; mismatched acid densities lead to increased methane selectivity and reduced LPG yield. • • Catalyst stability is enhanced by the synergistic effect between metal oxide and zeolite, which suppresses carbon deposition and sintering; for instance, GaZrOx/H-SSZ-13 maintains stable performance for over 100 hours on stream, a key requirement for industrial application.
Abstract
Liquefied petroleum gas (LPG) is a clean fuel and essential chemical feedstock. This review summarizes recent advances in the hydrogenation of CO and CO2 (COx) to LPG, focusing on the design and optimization of bifunctional catalysts. The adsorption and activation of COx on various metal oxide surfaces, as well as the influence of zeolite pore structure and acidity on LPG selectivity, are critically evaluated. The synergistic effects between metal oxide and zeolite components in promoting LPG production and enhancing catalyst stability are elucidated. Key catalyst systems, including GaZrOx/H-SSZ-13 and InZrOx-Beta composites, demonstrate high selectivity to propane and isobutane-enriched C4 alkanes, respectively. The review highlights the importance of balancing methanol synthesis and hydrocarbon conversion functionalities to achieve high LPG yields while suppressing undesired methane and CO formation. Challenges such as catalyst deactivation and the need for precise control of acid site density are discussed. This work provides theoretical guidance for the rational design of highly efficient catalytic systems for COx hydrogenation to LPG, contributing to carbon resource utilization and emission reduction.
1. Introduction
The escalating atmospheric concentration of carbon oxides (COx), primarily CO2 and CO, has intensified global warming and environmental degradation. In 2024, global CO2 emissions reached 36.3 billion tonnes, a 0.9% increase from the previous year, pushing the remaining carbon budget for the 1.5°C target to less than five years. Concurrently, incomplete combustion of fossil fuels releases toxic CO, contributing to air pollution and secondary pollutants. The chemical inertness of COx—CO2 has a C=O bond energy of 728 kJ/mol and CO a C≡O bond energy of 1072 kJ/mol—necessitates substantial energy input for activation. Hydrogenation of COx using green hydrogen derived from renewable sources offers a viable route to convert these greenhouse gases into valuable fuels and chemicals, thereby closing the carbon cycle and reducing reliance on fossil resources.
Among various COx hydrogenation products, LPG (mainly propane and butane) stands out as a clean fuel and a precursor for olefins via dehydrogenation. However, the complex reaction network often yields a wide product distribution, including methane, methanol, and higher hydrocarbons. Traditional Fischer-Tropsch synthesis or methanol-mediated routes suffer from low selectivity to LPG due to thermodynamic and kinetic constraints. Bifunctional catalysts, combining a metal oxide for COx activation to methanol and a zeolite for methanol conversion to hydrocarbons, offer a promising solution. The key challenge lies in optimizing the synergy between these components to steer selectivity toward LPG while maintaining catalyst stability. This review systematically examines recent progress in bifunctional catalyst design, focusing on metal oxide surfaces, zeolite pore architecture, and acidity, to provide a framework for developing efficient and stable catalytic systems for COx-to-LPG conversion.
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FAN Yingnuo, WANG Sen, DONG Mei, FAN Weibin (2026). Research Progress on Bifunctional Catalysts for Hydrogenation of COx to Liquefied Petroleum Gas. Journal of Fuel Chemistry and Technology. https://doi.org/10.3724/2097-213X.2025.JFCT.0036
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Frequently Asked Questions
What are the primary deactivation mechanisms for bifunctional COx-to-LPG catalysts under industrial conditions, and how can they be mitigated?
Deactivation primarily arises from carbon deposition (coking) on zeolite acid sites and sintering of metal oxide nanoparticles at elevated temperatures. For example, GaZrOx/H-SSZ-13 maintains stability for over 100 hours, attributed to the close proximity of active sites that facilitates intermediate transfer and reduces coke formation. Mitigation strategies include optimizing the Si/Al ratio to moderate acid density, employing hierarchical zeolites to enhance mass transport, and adding promoters to stabilize metal oxide phases.
How does the pore structure and acidity of the zeolite component influence LPG selectivity compared to other hydrocarbons?
Zeolite pore size dictates shape selectivity: H-SSZ-13 (CHA, 8-membered ring) restricts formation of larger hydrocarbons, enhancing propane selectivity, while H-Beta (12-membered ring) allows branched isomers like isobutane. Acidity, particularly Brønsted acid site density, controls the methanol-to-hydrocarbon pathway; moderate acidity favors LPG, whereas excessive acidity promotes aromatization and coking. For instance, InZrOx-Beta with optimized acid sites yields isobutane-enriched C4 alkanes with >80% isobutane selectivity.
What are the main challenges in scaling up bifunctional catalysts from laboratory to industrial scale for COx hydrogenation to LPG?
Scalability issues include heat management due to exothermic reactions, achieving uniform catalyst composition in large-scale reactors, and maintaining mechanical strength. Additionally, the cost of green hydrogen and CO2 capture remains high. Industrial implementation requires catalysts with high selectivity (>90% LPG) and stability (>1000 hours) under high space velocities. Current systems like GaZrOx/H-SSZ-13 show promise but need further optimization in pelletized form to minimize pressure drop and ensure efficient heat transfer.
How does the choice of metal oxide (e.g., ZnZrOx vs. In2O3) affect the overall reaction pathway and LPG yield?
Metal oxides differ in their ability to activate COx and produce methanol or CO. ZnZrOx and In2O3 are effective for CO2 to methanol, while Fe-based oxides favor Fischer-Tropsch synthesis. In bifunctional systems, the metal oxide must generate methanol at a rate matching the zeolite's conversion capacity. For example, GaZrOx provides high methanol yield, which when coupled with H-SSZ-13, results in propane selectivity >90%. In contrast, In2O3 tends to produce CO via reverse water-gas shift, reducing carbon efficiency. Thus, oxide selection is critical to suppress CO formation and maximize LPG yield.
What are the economic and environmental trade-offs between CO2 and CO as feedstocks for LPG production?
CO2 is more abundant but thermodynamically stable, requiring higher energy input for activation; CO, though more reactive, is toxic and typically derived from syngas (CO/H2). Using CO2 directly mitigates greenhouse gas emissions but demands green hydrogen, which is currently costly. CO hydrogenation via Fischer-Tropsch is mature but produces a wide range of hydrocarbons, reducing LPG selectivity. Bifunctional catalysts can process both, but CO2 routes offer greater environmental benefit. The economic viability hinges on carbon pricing and renewable hydrogen costs; with decreasing renewable energy prices, CO2-based LPG could become competitive.
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