Heterogeneous catalysis is a process where a catalyst in one phase, usually a solid, interacts with reactants in a different phase, such as gases or liquids. The catalyst's primary role is to lower the activation energy of the reaction, thus enhancing the reaction rate. Active sites on the catalyst adsorb the reactants, weakening bonds and facilitating reactions, such as hydrogenation, where hydrogen atoms from adsorbed hydrogen molecules react with organic compounds.
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Heterogeneous catalysis is crucial in producing the bulk and fine chemicals used in modern chemical process development. Examples of standard, multi-ton heterogeneous catalyzed processes include the Haber process used to produce ammonia from nitrogen and hydrogen over a modified iron catalyst, the Fischer-Tropsch process used to produce hydrocarbons and fuel from the reaction of carbon monoxide and hydrogen over metal catalysts, the contact process used to produce sulfuric acid from sulfur dioxide and oxygen over a vanadium oxide catalyst, and the use of a zeolite catalyst used to produce crude oil cracking for fuel and lighter hydrocarbons.
Heterogeneous catalysis is likewise used in pharmaceutical synthesis to develop catalysts with enantioselectivity for asymmetric hydrogenation reactions or Friedel-Crafts reactions. For example, MOFs with attached chiral ligands are very effective as heterogeneous asymmetric catalysts.
In addition, heterogeneous catalysis is vital to green chemistry, sustainable production, and environmental protection in general. The technique plays a crucial role in producing sustainable chemicals, green energy, valorization of biomass waste, and environmental remediation of air and water. Nano-based heterogeneous catalysts often convert atmospheric carbon dioxide and renewable biomass materials into high-value chemicals. Graphene-based systems can produce “green” hydrogen as a renewable energy source via water-splitting. The platform chemicals furfural and 5-hydroxymethylfurfural are derived from lignocellulosic biomass using various heterogeneous catalysts, including microporous zeolites. Heterogeneous catalysts such as platinum-impregnated MOFs are effective in the treatment of wastewater containing organic compounds.
Homogeneous and heterogeneous catalysis differ in how the catalyst interacts with the reactants. In homogeneous catalysis, the catalyst is in the same phase as the reactants, usually a liquid or gas, and forms a homogeneous solution or mixture with them. In contrast, in heterogeneous catalysis, the catalyst is in a different phase than the reactants, typically a solid.
One key difference between homogeneous and heterogeneous catalysis is the ease with which the catalyst can be separated from the reaction mixture. In homogeneous catalysis, the catalyst is often dissolved in the reaction mixture, making it difficult to separate and reuse. In contrast, the solid catalyst can be easily separated from the reaction mixture in heterogeneous catalysis, making it possible to reuse the catalyst multiple times.
Another significant difference is the range of reactions that can be catalyzed. Homogeneous catalysts are typically used to catalyze reactions that break or form chemical bonds in the reactants (e.g., oxidation or reduction reactions). Heterogeneous catalysts can catalyze a broader range of reactions, including acid-base catalysis, hydrogenation, and dehydrogenation reactions.
Both homogeneous and heterogeneous catalysis have their unique advantages and disadvantages. Heterogeneous catalysis is generally preferred for its ease of separation and broad range of applications. However, the choice of catalyst ultimately depends on the specific reaction being catalyzed and the desired outcome.
Heterogeneous catalysis involves a complex series of chemical reactions that occur on the surface of a solid catalyst. The exact mechanism of heterogeneous catalysis depends on the specific reaction being catalyzed and the properties of the catalyst and reactants involved. However, in general, the mechanism of heterogeneous catalysis can be broken down into several steps:
The heterogeneous catalysis mechanism is a complex interplay between the physical and chemical properties of the catalyst and reactants, the reaction conditions, and the environment. Understanding this mechanism is essential for designing and optimizing catalytic processes for industrial and scientific applications.
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Wu, J., Wang, L., Xu, S., Cao, Y., Han, Z., & Li, H. (2023). Sequential hydrogenation of nitroaromatics to alicyclic amines via highly-dispersed Ru–Pd nanoparticles anchored on air-exfoliated C3N4 nanosheets. RSC Advances, 13(3), 2024–2035. https://doi.org/10.1039/d2ra07612h
Hydrogenation of nitroaromatics via green catalytic methods is highly desirable to produce vital alicyclic amines; however, at scale, this is complicated by the different adsorption behaviors of the nitro group and benzene ring. Ru-based catalysts are very effective for the one-step hydrogenation of nitroaromatics to alicyclic amines. Still, the issue of competitive absorption complicates the synthesis and requires harsh reaction conditions. Pd-based catalysts have been shown to have excellent activity and selectivity for the hydrogenation of nitro groups, even under mild conditions. The authors comment that Ru-doped C₃N₄ had previously demonstrated effective aromatic ring hydrogenation. Therefore, for the conversion of nitrobenzene (NB) to cyclohexylamine (CHA), they prepared an air-exfoliated C₃N₄ support containing highly dispersed Ru–Pd dual active sites for the catalytic hydrogenation of the nitroaromatic nitro group and benzene ring, respectively.
A series of physical and spectroscopic investigations were undertaken to fully characterize and define this novel catalyst system's structure–performance relationship. These investigations included determining the C₃N₄ support morphology, the distribution and interaction of the Ru and Pd particles on the catalyst surface, and the dissociation and activation of H₂ under mild conditions. In catalyst performance tests, an NB to CHA reaction was performed to investigate the effect of reaction variables further. A catalyst with 1.5%Ru–1.5%Pd/C₃N₄ reacted at 80 °C and 3 MPa H₂ for 3 h yielded 100.0% NB conversion and 96.8% CHA selectivity.
Operando FTIR (ReactIR) was utilized via a custom-built autoclave equipped with a diamond ATR probe for in-situ, real-time measurements to investigate the NB to CHA hydrogenation pathway. The ReactIR measurements exhibited a spectrum with strong peaks at 1350 cm⁻¹ and 1531 cm⁻¹ arising from the symmetric and asymmetric C–NO₂ stretch in NB. As the reaction proceeded, the peak intensity decreased rapidly, indicating a quick conversion of NB. Concurrently, bands at 1606 cm⁻¹ and 1630 cm⁻¹ increased significantly, which was associated with the nitro group transformation to the amine, indicating that aniline was being formed by the direct hydrogenation of NB. Continuing the hydrogenation caused the amine peaks to decrease gradually, reflecting the hydrogenation of the benzene ring to form naphthene. The double peaks at 1606 cm⁻¹ and 1630 cm⁻¹ also contribute to a benzene ring skeleton vibration, weakened with the ring hydrogenation, as the conversion of the AN to CHA took place. By tracking the changes in the key spectral bands as a function of time, ReactIR measurements demonstrated the two-step nature of the hydrogenation process, with the hydrogenation of the nitro group occurring before the rate-determining step, benzene ring hydrogenation.
A reaction pathway was proposed based on combining the operando IR results and GC-MS experiments. Initially, the nitro group is converted into an amino, generating aniline. With additional hydrogenation of the benzene ring, aniline converts to cyclohexylamine. Cyclohexanol and dicyclohexylamine side-products were also observed, resulting from the deamination of AN and condensation of CHA as hydrogenation proceeds. Kinetic studies showed that Pd dominated the hydrogenation of the nitro group, while Ru was dominant for the benzene ring. The authors noted that the activity of the catalyst was greatly improved by the action of the non-dominant metals, which enhanced the activation and dissociation of H₂. Also contributing to the catalyst performance were the highly dispersed Ru–Nₓ and Pd–Nₓ sites at nanoscale separation and the aforementioned metal-assisted hydrogenation.
Lu, C., Zhang, Y., Zhu, X., Yang, G., & Wu, G. (2023). Simultaneous activation of carbon dioxide and epoxides to produce cyclic carbonates by cross‐linked epoxy resin organocatalysts. ChemCatChem, 15(10). https://doi.org/10.1002/cctc.202300360
There is immense interest and R&D in transforming CO₂ into valuable chemicals in a green and sustainable manner. Cycloaddition of CO₂ to form epoxides and cyclic carbonates is particularly interesting since these molecules represent valuable substrates. Though both homogeneous and heterogeneous catalysis can effectively achieve cycloaddition of CO₂, heterogeneous catalysts have shown some significant advantages for overall usability, scale-up, and safety. The authors comment that there is strong interest in the development of highly active and selective heterogeneous catalysts with both Lewis acidic sites to activate epoxides and Lewis basic sites to ring-open the activated epoxides, and as such, set out to develop a bifunctional metal-free heterogeneous catalyst. Their work resulted in the development of a class of cross-linked epoxy resin organocatalysts possessing a simultaneous activation system - tertiary amines to activate CO₂ and hydroxyl groups to activate epoxide and quaternary ammonium salts that attack epoxides. The authors report that these catalysts are inexpensive to produce, easily synthesized with good yield using green chemistry and that they feature high activity, and selectivity, broad substrate scope, wide operating conditions and are reusable.
Investigations on the morphology and elemental distribution showed that the catalyst has an irregular surface of agglomerated particles that offers increased contact area for the substrate and active sites and that C, N, and Br atoms are uniformly dispersed in the catalyst system. They comment that, in particular, the distribution of nucleophilic Br is uniform, promoting contact with the activated epoxide and ensuing epoxide ring-opening. Using the cycloaddition of CO₂ and propylene oxide as a model reaction, extensive structure-activity, thermal stability, and recyclability experiments were performed to thoroughly investigate and characterize catalyst performance.
Using a combination of ¹H and ¹⁹F NMR and in-situ FTIR (ReactIR) spectroscopy, the authors investigated the catalytic mechanism of the epoxy resin bifunctional catalysts-mediated cycloaddition reaction. Based on these experiments, they proposed a mechanism for the heterogeneous organocatalyst-based cycloaddition reaction of CO₂ and epoxide in which initially, the epoxide is activated by the catalyst hydroxyl group through hydrogen-bonding interaction, followed by ring opening of the activated epoxide by the nucleophilic Br to generate a new intermediate, then rapid carbon dioxide insertion forms another intermediate and finally intramolecular ring elimination results in the formation of the five-membered cyclic carbonate product.
The ReactIR spectra reveal bands at 1640 cm⁻¹ and 1310 cm⁻¹, consistent with the presence of a carbonate moiety. This observation led to a proposal for a water-related secondary mechanism in which after reacting with CO₂, an epoxy resin catalyst is converted to an intermediate having a bicarbonate anion. The bicarbonate anion ring opens the epoxide, which was activated by the hydroxyl group. It forms another intermediate, after which an intramolecular ring closure reaction forms the cyclic carbonate while generating a hydroxyl anion. The hydroxyl anion and CO₂ reaction form bicarbonate anion to continue the catalytic cycle. The authors comment that since either route leads to the desired product, strict dehydration operation for CO₂ and epoxide for these epoxy resin organocatalysts for the cycloaddition reaction is not necessary, reducing energy expenditure and the cost of the process at commercial scale.
