Here, we explore how nanopores are used in catalysis, defining their functionality and recent advancements.
Image Credit: ArtemisDiana/Shutterstock.com
Catalysis is a fundamental concept in the areas of chemical, physical, and biological sciences, evolving over the years. The primary function of a catalyst is to decrease the activation energy and increase the selectivity for a chemical reaction to occur.
The use of nanopores in catalysis represents a growing area of research with potential applications in diverse fields, including energy, environmental science, and pharmaceuticals.
What are Nanopores?
The IUPAC categorizes porous materials into three groups: microporous materials, having pores with diameters less than 2 nm; mesoporous materials, characterized by pores ranging between 2 to 50 nm; and macroporous materials, with pores larger than 50 nm. The designation “nanoporous materials” is applied to porous materials with pore diameters measuring less than 100 nm. [1]
The larger specific surface area, confinement-induced selectivity, and regulated mass transfer within nanopores make the nanoporous materials suitable for extensive applications as heterogeneous catalysts, membranes, and adsorbents.
Fundamental Principles of Nanopores in Catalysis
Nanopores, central to catalytic processes, operate based on fundamental principles influencing reaction dynamics.
Acid and redox properties, along with shape-selective behavior, govern all reactions and conversions in nanoporous materials. Acid-catalyzed reactions, particularly involving microporous solid acids, are the most prevalent.
These reactions include fluid catalytic cracking, hydrocracking, dewaxing, aliphate alkylation, isomerization, oligomerization, transformation of aromatics, and the conversion of methanol to hydrocarbons.
The most widely explored nanoporous materials include Zeolites and metal-organic frameworks (MOFs). Zeolites, casually termed “molecular sieves, ” are the most common nanoporous materials used for catalysis-based applications. These are the crystalline inorganic polymers that fall in the family of aluminosilicate substances that have micropores precisely arranged in molecular dimensions.
The other class of nanopores for catalysis include MOFs, also termed as “porous coordination polymers”. These molecular-engineered materials possess high porosity, a mix of organic and inorganic nature, and the presence of coordinated metals and heteroatoms. These materials showcase a blend of organic flexibility and functionality alongside the hydrothermal stability of their inorganic components.
Advancements and Innovations in Nanopore Technology
The era of nanoporous materials is witnessing remarkable advancements and innovations across various scientific domains. One of them is the encapsulation of subnanometer-sized metal clusters in zeolites.
The metal clusters offer unique properties of excellent catalytic performance due to coordinatively unsaturated surface atoms. However, due to high surface energy, these clusters are prone to serious agglomeration. Encapsulating these metal clusters in zeolites is one way to prevent agglomeration from retaining the catalytic performance.
Researchers from Henan University, China, reported a new template-based method for encapsulating Platinum clusters in zeolite. The uniform distribution can be observed due to the electrostatic interaction between negatively charged metal ions and positively charged templates. Enhanced hydrodeoxygenation performance of phenolics and sustained catalytic stability result from a facilitated transfer of hydrogen from platinum to zeolite. [2]
The synthesis strategy for the zeolites and MOFs aims to enhance the catalytic performance of conventional materials. Prof Hongbing Ji and his team from Sun Yat- Sen University, proposed a post-modification method for an easy fabrication of Zeolitic imidazolate framework (ZIF) based material. The multifunctional heterogeneous catalyst ZIF-9-ImBr, owing to the synergistic effect of Co centers and Br anion, demonstrated improved catalytic performance. [3]
Recently, Prof Sansano from Instituto de Síntesis Orgánica, Spain synthesized CuFe2O4 immobilized on the surface of ZIF/67 by an easy hydrothermal method. Its observed catalytic activity was far more than the individual CuFe2O4 and ZIF-67 due to the synergistic behavior of both components. It demonstrated high stability and reusability with minimum loss of catalytic activity. [4]
Developing zeolite-based nanoporous materials has faced challenges, particularly regarding long-term stability, thermal cycling, regeneration, pore size tailoring and complex housing, hindering widespread industrial adoption. However, there is optimism that MOFs could address these issues. MOFs exhibit a unique potential, offering a range of pore dimensions suitable for diverse catalytic reactions, higher surface area (allowing a greater number of active sites) and stability. [5]
Incorporating transition metals into the three-dimensional porous framework of MOFs enhances electronic properties. It provides a large surface area, making catalytically active sites more accessible and allowing for tunable chemical structures. Nanoporous vanadium oxide embedded carbon network (NVC-900) addressed the need for high-efficiency and low-cost metal-based catalysts. [6]
Industrial Applications
Nanoporous materials have widespread applications, including gas adsorption, membranes, catalysts (including photocatalysts and electrocatalysts), biomedical applications, energy harvesting, and energy storage systems such as rechargeable batteries and supercapacitors. The flexibility of polymers and MOFs presents an opportunity for designing materials with enhanced hydrogen adsorption and storage properties.
Parallely, Zeolitic imidazole frameworks (ZIFs), a well-known MOF structure, stand out for energy storage systems due to their high surface area, cost-effectiveness, easy synthesis at room temperature, and intrinsic nitrogen doping on carbon. ZIF-derived carbon and metal oxide materials, especially in electric double-layer supercapacitors (EDLCs) and pseudocapacitors, demonstrate high performance. [7,8]
Also, membranes that have nanoporous materials hold great promise for advancing the efficiency of gas mixture separation, particularly in the context of emerging energy technologies. One compelling application is the separation of gas mixtures containing carbon dioxide (CO2) and hydrogen (H2). [9]
Conclusion
Nanoporous materials emerge as ideal catalysts, offering increased active sites and selectivity based on pore size and shape. It requires further optimization of nanoporous materials for catalytic applications, focusing on improving selectivity, efficiency, and reaction rates. This may involve the design of tailored catalysts for specific reactions and the exploration of new catalytic processes.
While progress in developing new synthesis strategies, there is still considerable research potential in exploring the catalytic applications of these materials. Understanding the origin of product selectivity control and unraveling reaction mechanisms on the surface of the nanopores are crucial for addressing this challenge. [10]
See More: The Challenges with Engineering Functional Nanopores
References and Further Reading
Logar, Nataša Zabukovec, and Venceslav Kaucic. “Nanoporous materials: from catalysis and hydrogen storage to wastewater treatment.” Acta chimica slovenica 53.2 (2006): 117.https://acta-arhiv.chem-soc.si/53/53-2-117.pdf
Tian, Yajie, et al. “Template Guiding for the Encapsulation of Uniformly Subnanometric Platinum Clusters in Beta‐Zeolites Enabling High Catalytic Activity and Stability.” Angewandte Chemie International Edition 60.40 (2021): 21713-21717. https://doi.org/10.1002/anie.202108059
Deng, Yiqiang, et al. “Carbon neutral via catalytic transformation of CO2 into cyclic carbonates by an imidazolium-based ionic zeolitic imidazolate frameworks.” Applied Surface Science 614 (2023): 156250. https://doi.org/10.1016/j.apsusc.2022.156250
Nagshbandi, Zhwan, Mohammad Gholinejad, and José M. Sansano. “Novel magnetic zeolitic imidazolate framework for room temperature enhanced catalysis.” Inorganic Chemistry Communications 150 (2023): 110463. https://doi.org/10.1016/j.inoche.2023.110463
Caro, Juergen. “Are MOF membranes better in gas separation than those made of zeolites?.” Current Opinion in Chemical Engineering 1.1 (2011): 77-83. https://doi.org/10.1016/j.coche.2011.08.007
Mehek, Rimsha, et al. “Metal–organic framework derived vanadium oxide supported nanoporous carbon structure as a bifunctional electrocatalyst for potential application in metal air batteries.” RSC advances 13.1 (2023): 652-664. 10.1039/D2RA06688B
Morris, Russell E., and Paul S. Wheatley. “Gas storage in nanoporous materials.” Angewandte Chemie International Edition 47.27 (2008): 4966-4981. https://doi.org/10.1002/anie.200703934
Kim, Jeonghun, et al. “Nanoarchitecture of MOF-derived nanoporous functional composites for hybrid supercapacitors.” Journal of Materials Chemistry A 5.29 (2017): 15065-15072. https://doi.org/10.1039/C7TA03356G
Castro-Munoz, Roberto, et al. “Towards large-scale application of nanoporous materials in membranes for separation of energy-relevant gas mixtures.” Separation and Purification Technology 308 (2023): 122919. https://doi.org/10.1016/j.seppur.2022.122919
Yang, Hyun Ju, et al. “Selectivity of Electrochemical Reactions Based on Adsorption at Nanoporous Electrodes.” Analytical Chemistry 95.44 (2023): 16216-16224. https://doi.org/10.1021/acs.analchem.3c02991
