Multiphase catalytic hydrogenation technology: from fundamental principles to industrial applications
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In various fields such as fine chemicals, pharmaceutical synthesis, and lipid chemistry, hydrogenation reactions are a key class of organic transformation processes aimed at selectively reducing unsaturated functional groups (such as C=C, C=O, -NO₂, etc.) to produce high-value target products. Due to advantages such as easy separation and recovery of the catalyst, reusability, and environmental friendliness, multiphase catalytic hydrogenation technology has become the mainstream choice for industrial production. 1 Parameter The multiphase catalytic hydrogenation reaction follows the Horiuti-Polanyi mechanism, and the process includes: a. Adsorption of the reactants on the catalyst surface (through π-bonds or σ-bonds); b. Addition of chemically adsorbed hydrogen, as well as further hydrogenation or dehydrogenation of the intermediate products (possibly accompanied by double bond migration, cis-trans isomerization, etc.). There are numerous key factors that influence reaction selectivity and efficiency, among which precise control of the core technical parameters is essential for achieving efficient catalysis. The following provides a detailed discussion of these key parameters. 1.1 Catalysts • Types and ratios of active metals: Different metals exhibit significant differences in their selectivity for the hydrogenation of various functional groups. For example, Ir and Os catalysts exhibit higher selectivity for the hydrogenation of the C=O bond in α,β-unsaturated aldehydes, whereas Pd, Rh, and Ni tend to hydrogenate the C=C bond instead ； Ag and Au catalysts exhibit excellent performance in the selective hydrogenation of the C=O bond in simple α,β-unsaturated aldehydes such as acraldehyde. For bimetallic catalysts, the metal ratio is a key parameter for regulation: in Ag-In/SiO₂ catalysts, when the optimal mass ratio of Ag to In is 9:0.75, the selectivity for the hydrogenation of acraldehyde to allyl alcohol can reach 75%, with a yield of 70% ； In the Rh-Sn/SiO₂ catalyst, when the loadings of Rh and Sn are 1.1 wt.% and 1.3 wt.% respectively, the selectivity for the hydrogenation of citral to geraniol/oranial reaches 98%. •Catalyst particle size and dispersion: This parameter has a significant impact on the structural sensitivity of the reaction. Taking the Pt catalyst as an example, in the hydrogenation of citral, when the particle diameter of Pt is in the range of 1–5 nm, the turnover frequency (TOF) increases as the particle size grows; within the range of 5–30 nm, however, this value changes relatively slowly ； In the hydrogenation of crotonaldehyde, an increase in the size of Pt particles leads to improved selectivity for unsaturated alcohols. This is because the proportion of the Pt(111) crystal plane increases in large-particle Pt, which facilitates the selective adsorption and activation of C=O bonds. For Au-based catalysts, when the particle size is less than 2 nm, the quantum size effect alters their electronic properties, resulting in changes in the selectivity for acraldehyde hydrogenation ； As the Au particle size increased from 4.0 nm to 7.7 nm, the allyl alcohol selectivity rose from 15% to 35%. •Carrier: The carrier not only serves to disperse the active metal but also regulates catalytic performance through metal-carrier interactions. Reductible carriers (such as TiO₂, ZrO₂, Nb₂O₅) form defect sites (such as Ti³⁺, oxygen vacancies) under high-temperature reduction (773 K). These Lewis acid sites can lower the energy level of the antibonding orbitals of the C=O bond, thereby promoting its activation. For example, after high-temperature reduction at 773 K, the selectivity of Pt/TiO₂ catalysts for the hydrogenation of cinnamaldehyde to cinnamol increased from 12.6% to 37.2% ； The graphite carrier can transfer electrons to the antibonding orbitals of the metal clusters, weakening the interaction between the metal and the C=C bonds, thereby significantly improving the selectivity for unsaturated alcohols compared to activated carbon carriers.
1.2 Reaction parameters • Hydrogen partial pressure: The hydrogen partial pressure directly determines the hydrogen coverage on the catalyst surface, thereby affecting reaction selectivity. At high hydrogen coverage, the reactants are more prone to hydrogenation reactions ； A low hydrogen coverage may lead to isomerization or hydrogenolysis. In the reaction of vapor-phase hydrogenation of acraldehyde to produce allyl alcohol, increasing the pressure from 7.5 mbar to 20 bar significantly improves the selectivity for allyl alcohol, which can reach up to 42% ； In the hydrogenation of halogenated nitrobenzenes, a high hydrogen partial pressure (such as above 2 MPa) must be maintained to suppress the hydrolysis of C-halogen bonds, which tends to occur at low hydrogen coverage levels, thereby ensuring high selectivity for amine products. •Reaction temperature: The temperature must strike a balance between ensuring the reaction rate and suppressing side reactions as well as catalyst deactivation. Excessively high temperatures can lead to the non-selective hydrogenation of all functional groups, and may also cause catalyst deactivation (for example, in the hydrogenation of α,β-unsaturated aldehydes, high temperatures can cause the unsaturated alcohols to decarboxylate to form CO, thereby blocking the active sites of the catalyst). For example, when Pt/TiO₂-LTR and Pt/SiO₂ were used to catalyze the hydrogenation of citral, the lowest activity was observed at 100°C; as the temperature increased, the amount of CO produced increased. Further elevation of the temperature caused the CO desorption rate to exceed the decarboxylation rate, after which the activity increased again. The commonly used reaction temperature range in industry is 303–593 K; for example, when crotonaldehyde is hydrogenated over a Rh-Sn/SiO₂ catalyst, 413 K is an appropriate temperature that balances activity and selectivity. •Concentration and solvent: In liquid-phase hydrogenation, the concentration of the reactants affects their adsorption configuration on the catalyst surface, thereby altering selectivity. For example, in the hydrogenation of cinnamaldehyde using Pt/SiO₂ catalyst, an increase in the initial concentration of the reactant enhances the selectivity for cinnamyl alcohol; this is because at high concentrations, the aromatic rings tend to self-assemble, allowing cinnamaldehyde to bind to the catalyst surface in a terminal-adsorption mode. The polarity and acidity/basicity of the solvent are also crucial; in isopropanol, the selectivity for the hydrogenation of geraniol to citronellol under Cu/Al₂O₃ catalysis can exceed 98%, whereas it is 56% in n-heptane ； In hydrochloric acid solution, the selectivity for the hydrogenation of isoquinoline to form cyclohexyl derivatives is 97%, while in methanol it is 87% for the formation of phenyl derivatives. •Stirring rate and mass transfer: In gas-liquid-solid (G/L/S) multiphase systems, the stirring rate determines the dissolution rate of hydrogen and the suspension state of the catalyst, thereby affecting the hydrogen coverage. An insufficient stirring rate can lead to inadequate hydrogen supply, reducing the rate of the hydrogenation reaction and even causing side reactions such as isomerization. In industry, the stirring speed is typically maintained between 500 and 1500 revolutions per minute; it needs to be optimized based on the viscosity of the reaction mixture and the particle size of the catalyst, in order to ensure that the solubility of hydrogen in the liquid phase meets the requirements of the reaction.
1.3 Reactor The type of reactor and its operating parameters directly affect the efficiency of mass and heat transfer within the reaction, as well as the residence time distribution, thereby influencing selectivity and production capacity. •Type: Trickle-bed reactors are suitable for liquid-phase hydrogenation reactions (such as the hydrogenation of citral and crotonaldehyde). Using a Pt-Sn/MgO catalyst, at 100°C and a hydrogen partial pressure of 20 bar, a conversion rate of 97% for citral can be achieved, with selectivities of 97% for nerol and geraniol ； In benzaldehyde hydrogenation, due to the narrow residence time distribution and lower pressure in monolithic reactors, the Ni-cordierite monolithic catalyst with a channel density of 600 cpsi exhibits higher selectivity than one with 400 cpsi ； Microstructured reactors exhibit high efficiency in heat and mass transfer. In the gas-phase hydrogenation of acraldehyde, carbon-coated microreactors loaded with Ru catalysts yield a product distribution comparable to that of fixed-bed reactors, but offer greater safety and easier control over selectivity. •Residence time and space velocity: In continuous reactors, the residence time must be precisely matched to the reaction kinetics in order to avoid over-hydration or insufficient conversion. For example, in a packed-bed membrane reactor (PBMR), by using distributed feeding to control local reactant concentrations and optimize the residence time distribution, the yield of allyl alcohol can be significantly increased ； In a fixed-bed reactor, when the space velocity for acraldehyde hydrogenation (W/F⁰) is controlled at 15.3 g·h·mol⁻¹, the selectivity for allyl alcohol can reach 35–37%.
2. Characteristics 2.1 Advantages: The catalyst is easy to separate and recover, and it is environmentally friendly. Since heterogeneous catalysts are in solid form, they can be separated from the products using simple methods such as filtration or centrifugation after the reaction. This avoids product contamination and catalyst waste that occur with homogeneous catalysts, reducing environmental protection costs and aligning with the trends in green chemistry. For example, after catalyzing the esterification of fatty acids, ion exchange resins can be directly retained in the fixed bed for reuse without the need for additional separation steps. It offers a wide range of selective tunability to meet diverse requirements: by adjusting the catalyst composition (single/metallic, metal ratios), particle size, carrier type, as well as reaction parameters (hydrogen pressure, temperature, solvent), precise selective hydrogenation of different functional groups can be achieved. For example, the Ag-In/SiO₂ catalyst can achieve selective hydrogenation of acraldehyde to produce allyl alcohol with a selectivity of 75%, while the Rh-Sn/SiO₂ catalyst can enable selective hydrogenation of citral to yield geraniol/orange aldehyde with a selectivity of 98%, meeting the demand for high-purity intermediates in industries such as fine chemicals and pharmaceuticals. It features strong process stability and is easy to scale up for industrial use: The multiphase catalytic reaction system is stable, with catalysts having a long service life (for example, the Ru-Fe/C catalyst can operate stably for 440 hours in the continuous citral hydrogenation process), and reactor types are well-established (fixed-bed, trickle-bed, monolith-type, etc.), making it easy to scale up for industrial application according to production requirements. For example, the space-time yield (STY) of allyl alcohol using Ag-In/SiO₂ catalysts can reach 2.2–6.8 g·g⁻¹·h⁻¹, which is far higher than the 1 kg·L⁻¹·h⁻¹ value achieved in industrial propylene oxide isomerization processes; thus, these catalysts have potential for industrial application. Adapted to green solvent systems to expand application scenarios: It is compatible with green solvents such as ionic liquids and supercritical CO₂, further reducing the environmental impact of the process. For example, Pd/C catalysts can catalyze the hydrogenation of crotonaldehyde and cinnamaldehyde in supercritical CO₂, achieving a conversion rate of 100%, and the catalyst can be recovered and reused ； The hydrogenation of cinnamaldehyde using Pd/C in ionic liquids allows the selectivity for the hydrogenation of C=C bonds to approach 100% by adjusting the anion type or temperature.
2.2 Limitations: Significant mass transfer limitations affect reaction efficiency: In the G/L/S multiphase system, the dissolution of hydrogen from the gas phase into the liquid phase, its diffusion from the liquid phase to the catalyst surface, as well as the diffusion of reactants/products within the catalyst pores, can all become rate-limiting steps. Especially for macromolecular substrates (such as lipid derivatives), the high diffusion resistance within the catalyst pores leads to a decrease in catalytic activity. For example, in the hydrogenation of fatty acid esters, the immiscibility between fatty acids and glycerol further increases the mass transfer resistance, affecting selectivity and reaction rate. The cost of catalyst preparation is high, and some metal resources are scarce: High-performance heterogeneous catalysts often use precious metals such as Pt, Rh, Ir, and Au; these metals are scarce and expensive, thereby increasing the production costs ； The preparation process of bimetallic catalysts is complex, requiring precise control over the metal ratios and dispersion level, which further increases the difficulty and cost of production. For example, although Ir-based catalysts exhibit excellent selectivity, their high cost limits their widespread use. The selectivity for specific reactions still needs to be improved: For the selective hydrogenation of the C=O bond in simple α,β-unsaturated aldehydes (such as acraldehyde), although efficient catalysts such as Ag-In and Au-In have been developed, it is still necessary to maintain high selectivity at high conversion rates ； For the regioselective hydrogenation of polycyclic aromatic hydrocarbons and diketone compounds, the selectivity of the target products in some reactions is less than 80%, which makes it difficult to meet the requirements of high-end applications. The catalyst is prone to deactivation, and its stability needs to be improved: During the reaction, the catalyst may become deactivated due to coke deposition, sintering of active metals, or poisoning (by substances such as CO and sulfides). For example, in the hydrogenation of α,β-unsaturated aldehydes, the CO generated by the decarboxylation of unsaturated alcohols can reversibly block the active sites of the catalyst ； In supercritical CO₂ systems, the Pd/C catalyst loses its activity due to carbon deposition and metal sintering, requiring regular regeneration.
3 Development Trends
3.1 Innovations in catalyst design and preparation technologies
• Development of low-cost non-precious metal catalysts: Based on non-precious metals such as Ni, Cu, and Co, their catalytic activity and selectivity are enhanced through methods like alloying, support modification, and surface treatment. For example, optimizing the preparation process of the Cu-Al₂O₃ catalyst and introducing small amounts of promoters such as Fe and Zn can improve its hydrogenation selectivity toward C=O bonds ； Develop non-precious metal catalysts such as transition metal nitrides and carbides to replace precious metals in the hydrogenation of nitroaromatics and aldehyde ketone compounds. •Precise control of catalyst microstructure: By utilizing advanced preparation techniques such as atomic layer deposition (ALD), sol-gel method, and template method, it is possible to precisely control the particle size, crystal phase structure, pore structure, and distribution of active sites in catalysts. For example, single-atom or dual-atom dispersed noble metal catalysts can be prepared using ALD technology to improve atomic utilization and selectivity ； Design multi-level porous structure carriers to reduce the diffusion resistance of macromolecular substrates. •Functionalization and intelligent modification of catalysts: Introducing specific functional groups (such as amino or hydroxyl groups) or molecular recognition units onto the catalyst surface to achieve selective adsorption of reactants, thereby enhancing regioselectivity and stereoselectivity ； Develop intelligent catalysts with self-repairing capabilities, which can alleviate deactivation caused by carbon deposition and sintering through dynamic regulation of the structure of active sites. 3.2 Reaction Process and Reactor Optimization • Improving mass and heat transfer efficiency: Developing new types of efficient reactors (such as microchannel reactors and rotating packed bed reactors) to reduce the mass transfer distance and enhance the contact efficiency among gas, liquid, and solid phases ； Auxiliary methods such as ultrasound and microwaves are employed to enhance mass and heat transfer in the reaction system, thereby increasing the reaction rate. For example, the mass transfer efficiency for acraldehyde hydrogenation in microchannel reactors is 10–100 times that of conventional fixed-bed reactors, which can significantly increase production capacity. •Development of green process systems: Promoting solvent-free hydrogenation processes or using green solvents such as water, ionic liquids, and supercritical CO₂ to reduce the use of organic solvents ； Develop continuous and integrated processes that combine steps such as reaction, separation, and catalyst regeneration, thereby improving process efficiency and reducing energy consumption. For example, the Ag-In/SiO₂ catalyst facilitates solvent-free hydrogenation of citral, achieving a conversion rate of 93% and a selectivity of 76% for geraniol/nerol, thereby simplifying the subsequent separation process. •Intelligent control of process parameters: By combining online detection techniques such as in-situ infrared and Raman spectroscopy with artificial intelligence algorithms, it is possible to monitor in real time the concentrations of reactants, the distribution of products, and the condition of the catalyst during the reaction process. This enables intelligent adjustment of process parameters such as hydrogen partial pressure, temperature, and space velocity, thereby achieving precise control and optimization of the reaction process.
3.1 Innovations in catalyst design and preparation technologies
• Development of low-cost non-precious metal catalysts: Based on non-precious metals such as Ni, Cu, and Co, their catalytic activity and selectivity are enhanced through methods like alloying, support modification, and surface treatment. For example, optimizing the preparation process of the Cu-Al₂O₃ catalyst and introducing small amounts of promoters such as Fe and Zn can improve its hydrogenation selectivity toward C=O bonds ； Develop non-precious metal catalysts such as transition metal nitrides and carbides to replace precious metals in the hydrogenation of nitroaromatics and aldehyde ketone compounds. •Precise control of catalyst microstructure: By utilizing advanced preparation techniques such as atomic layer deposition (ALD), sol-gel method, and template method, it is possible to precisely control the particle size, crystal phase structure, pore structure, and distribution of active sites in catalysts. For example, single-atom or dual-atom dispersed noble metal catalysts can be prepared using ALD technology to improve atomic utilization and selectivity ； Design multi-level porous structure carriers to reduce the diffusion resistance of macromolecular substrates. •Functionalization and intelligent modification of catalysts: Introducing specific functional groups (such as amino or hydroxyl groups) or molecular recognition units onto the catalyst surface to achieve selective adsorption of reactants, thereby enhancing regioselectivity and stereoselectivity ； Develop intelligent catalysts with self-repairing capabilities, which can alleviate deactivation caused by carbon deposition and sintering through dynamic regulation of the structure of active sites. 3.2 Reaction Process and Reactor Optimization • Improving mass and heat transfer efficiency: Developing new types of efficient reactors (such as microchannel reactors and rotating packed bed reactors) to reduce the mass transfer distance and enhance the contact efficiency among gas, liquid, and solid phases ； Auxiliary methods such as ultrasound and microwaves are employed to enhance mass and heat transfer in the reaction system, thereby increasing the reaction rate. For example, the mass transfer efficiency for acraldehyde hydrogenation in microchannel reactors is 10–100 times that of conventional fixed-bed reactors, which can significantly increase production capacity. •Development of green process systems: Promoting solvent-free hydrogenation processes or using green solvents such as water, ionic liquids, and supercritical CO₂ to reduce the use of organic solvents ； Develop continuous and integrated processes that combine steps such as reaction, separation, and catalyst regeneration, thereby improving process efficiency and reducing energy consumption. For example, the Ag-In/SiO₂ catalyst facilitates solvent-free hydrogenation of citral, achieving a conversion rate of 93% and a selectivity of 76% for geraniol/nerol, thereby simplifying the subsequent separation process. •Intelligent control of process parameters: By combining online detection techniques such as in-situ infrared and Raman spectroscopy with artificial intelligence algorithms, it is possible to monitor in real time the concentrations of reactants, the distribution of products, and the condition of the catalyst during the reaction process. This enables intelligent adjustment of process parameters such as hydrogen partial pressure, temperature, and space velocity, thereby achieving precise control and optimization of the reaction process.
3.3 Deep Integration of Fundamental Research and Industrial Applications • Deepening understanding of reaction mechanisms: By utilizing in-situ characterization techniques (such as in-situ EXAFS, XPS, HRTEM) and theoretical calculations (DFT, microkinetic simulations), the structure of catalyst active sites, the adsorption patterns of reactants, and the evolution patterns of intermediate products are elucidated, providing theoretical guidance for catalyst design and process optimization. For example, the surface structure changes of Ag-In catalysts during propylene hydrogenation were studied using in-situ EXAFS to clarify the mechanism of action of the active sites. •Focus on personalized solutions for specific fields: Develop customized catalysts and processes to meet the demands for high-purity, highly stereoselective products in high-end industries such as pharmaceuticals and electronic chemicals ； For fields such as lipid chemistry and the utilization of renewable resources like biodiesel, efficient catalytic systems are developed to achieve low-cost conversion. For example, chiral heterogeneous catalysts are developed for the asymmetric hydrogenation reactions to produce pharmaceutical intermediates ； Optimize plant oil hydrogenation catalysts to improve the quality and yield of biodiesel. •Catalyst regeneration and resource recycling: Developing efficient and low-cost catalyst regeneration techniques (such as oxidative carbonization, hydrogen reduction, plasma treatment) to extend the service life of catalysts ； Establish a catalyst recycling system to achieve efficient recovery and reuse of precious metals, thereby reducing resource consumption and costs. IV. Conclusion As one of the core technologies in green chemistry, multiphase catalytic hydrogenation plays an irreplaceable role in fields such as fine chemicals, pharmaceuticals, and the utilization of renewable resources. Although challenges such as mass transfer limitations, high catalyst costs, and insufficient stability still exist, ongoing innovations in catalyst design, reactor technology, and process optimization will continue to expand its scope of application and improve its technical and economic viability. In the future, through the close integration of fundamental research and industrial applications, multiphase catalytic hydrogenation technology will evolve toward being more efficient, greener, and smarter, providing strong support for the transformation and sustainable development of the chemical industry. (Note: Some content has been organized by AI)
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