The Significance of Solid Chemical Catalysts in Global Decarbonization Efforts
Alexandra Velty
Instituto de Tecnología Química, Universitat Politècnica de València, Consejo Superior de Investigaciones Científicas, Spain
“Understanding solid catalyst molecular design and reaction aspects are crucial for chemical processes and sustainable planetary practices”
Climate change is one of the main challenges facing humanity today. The 2015 Paris agreements set "global average temperature increase well below 2°C above pre-industrial levels" by mid-century and continued efforts "to limit the temperature increase to 1.5°C above pre-industrial levels." Greenhouse gases are responsible for global warming and reaching net zero emissions and stabilizing climate change requires long-term planning. Carbon dioxide (CO₂) is the most important greenhouse gas and human activities alter the carbon cycle by adding more CO₂ to the atmosphere, and influence the ability of natural sinks, such as forests and soils, to remove and store CO₂ from the atmosphere. Thus, decarbonization has become a global imperative and a priority for governments, businesses and society to avoid the catastrophic effects of climate change and to achieve net zero emissions targets. The transition to a world with zero net emissions requires a complete transformation of the way we produce, consume and move, and a crucial step is to replace fossil energy with renewable sources of energy (such as wind, solar, hydropower, geothermal and biomass). The U.S. energy information administration projects that world energy consumption should grow by nearly 50% between 2020 and 2050 as a result of population and economic growth (EIA projects increases in global energy consumption and emissions through 2050). Global chemical and petrochemical production amounted to USD 5.7 trillion in 2017 and is projected to quadruple by 2060 (Saving Costs in Chemicals Management). Fossil fuel and feedstock-based production leads to CO₂ emissions during the production, use and end-of-life stages, making the chemical and petrochemical sectors the third largest contributor to industrial CO₂ emissions, behind steel and cement production (Saygin, Deger, and Dolf Gielen, 2021).
In recent decades, the path to decarbonization has involved different strategies based directly on the carbon capture and storage in permanent geological CO₂ reservoirs (deep geological formations such as depleted oil and gas reservoirs or saline aquifers) and on the carbon capture and conversion into valuable chemicals and fuels. Renewable energies such as wind, hydropower, sunlight and geothermal, will play a key role in the decarbonization. Other approach is the use of biomass derived from trees, plants, and agricultural and urban waste as renewable feedstock for the production of fuels, chemicals and functional materials that has received much attention by the scientific community to ensure the sustainable development of our society.
Unlike petroleum, biomass is renewable and "carbon neutral" because CO₂ released is part of the natural carbon cycle and equal to the amount of CO₂ absorbed during its growth. Trees and plants are grown, harvested and regrown in a relatively short period, turning biomass into a renewable source of energy and chemicals. In this way, biomass contributes to decarbonization at different stages, during its growth, biomass removes carbon dioxide from the atmosphere. Once sustainably managed and harvested, biomass can be converted, through sustainable processes, into chemicals and energy. Besides being a source of energy, biomass remains a valuable carbon source for producing a wide variety of chemicals.
Figure 1. The path to decarbonization involves different strategies based on carbon capture, storage and use, biomass conversion for the production of fuels, chemicals and functional materials, and the use of renewable energies.
Among various biomass feedstocks, including vegetable oils, chitin (a component of arthropod exoskeletons), and starch, lignocellulose, which is derived from plant dry matter, stands out as the most abundant form of biomass. With an annual production of approximately 170 billion metric tons, lignocellulose emerges as the most promising renewable candidate for the production of chemicals and fuels. Lignocellulose is composed of two primary carbohydrate (sugar) polymers, cellulose and hemicellulose, along with an aromatic-rich polymer known as lignin. The composition of lignocellulose varies depending on the wood/plant species, typically consisting of 30-50% cellulose, 20-35% hemicellulose, and 15-30% lignin. The decomposition of these natural polymers gives rise to a wide range of valuable substances, which provide a base for the production of chemicals4 and high-quality fuels by chemical transformation to meet global demand (Corma, Avelino, Sara Iborra, and Alexandra Velty, 2007).
Over the last twenty years, much effort has been devoted to developing efficient and eco-friendly methods to convert biomass into targeted chemicals and energy. The efficient and selective conversion of each lignocellulose component into the desired product or fuel is usually based on chemical catalysis, since catalysis allows the rate of a chemical reaction to be increased by the addition of a substance that is not consumed during the reaction. Biomass is usually first treated in the presence of a soluble or homogeneous catalyst to obtain simpler components easier to convert in subsequent steps in the presence of solid or heterogeneous catalysts into upgraded chemicals and fuels. Due to its diversity, in terms of composition and functional groups, biomass conversion requires catalysts that are selective in promoting specific chemical reactions and that actively facilitate the conversion process into the desired products. Consequently, heterogeneous catalysis, due to its recyclable and environmentally friendly nature as well as its applicability to continuous reactor operations, has received special attention for biomass conversion.
Over the last decades, heterogeneous catalysis has made great progress not only through the accumulation and rigorous interpretation of results based on knowledge and trial/error, but also through the development of fundamental knowledge. The systematic study and understanding of the reaction mechanism, kinetics, thermodynamics, determination of reaction intermediates and active species due to the benefits of computational chemistry, and the spectroscopic characterization of the catalytic material during reaction with the simultaneous measurement of catalytic activity/selectivity have allowed great insights in the preparation of materials and the development of new catalysts (Liu, Lichen, and Avelino Corma, 2021; Corma Canós, Avelino, 2016). More recently, a new design method of solid catalysts called the “ab initio design” allowing the positioning of active sites in the framework that selectively drive the reaction to the desired product has been a new breakthrough in the preparation of highly active and selective catalysts (Gallego, Eva María, et al., 2017). These advances demonstrate the importance of deep and fundamental understanding in driving the catalytic performance of chemical processes. Owing to high surface area, tunable composition, adjustable particle morphology and porous architecture, heterogeneous catalysts offer versatile physicochemical and adsorption/desorption properties for the design and development of efficient and green catalytic processes (Gallego, Eva María, et al., 2017).
Facing the era of sustainable development and to contribute to a healthy planetary development, over the last two decades we have worked on improving the efficiency and sustainability of chemical processes based on the design and preparation of new efficient and robust solid catalysts and on the use of sustainable raw materials such as lignocellulose, terpenes and vegetable oils (Liu, Lichen, and Avelino Corma, 2018). These studies concentrate on the redesign/re-engineering of current industrial processes or explore new avenues for the production of commodities and fine chemicals from renewable resources. We also found a route to convert CO₂ into methane at lower temperatures than conventional processes, with excellent long-term catalyst stability. In this way, methane is produced from waste CO₂ and green hydrogen, offering an alternative to fossil natural gas as a renewable fuel with a lower carbon footprint or as a H₂ energy carrier. These endeavors play a vital role in enhancing the sustainability of chemical processes and therefore contributing to the much-needed global decarbonization.
Photo 1. Alexandra Velty and Avelino Corma, authors of the nominated work “Advanced zeolite and ordered mesoporous silica‐based catalysts for the conversion of CO₂ to chemicals and fuels”.
References
Saygin, Deger, and Dolf Gielen. Zero-emission pathway for the global chemical and petrochemical sector. Energies 14, no. 13, 2021. p.3772.
Corma, Avelino, Sara Iborra, and Alexandra Velty. Chemical routes for the transformation of biomass into chemicals. Chemical reviews 107, no. 6, 2007. pp.2411-2502.
Liu, Lichen, and Avelino Corma. Identification of the active sites in supported subnanometric metal catalysts. Nature Catalysis 4, no. 6, 2021. pp.453-456.
Corma Canós, Avelino. Heterogeneous catalysis: understanding for designing, and designing for applications. Angewandte Chemie International Edition 55, no. 21. 2016. pp.6112-6113.
Gallego, Eva María, M. Teresa Portilla, Cecilia Paris, Alejandro León-Escamilla, Mercedes Boronat, Manuel Moliner, and Avelino Corma. “Ab initio” synthesis of zeolites for preestablished catalytic reactions. Science 355, no. 6329. 2017. pp.1051-1054.
Liu, Lichen, and Avelino Corma. Metal catalysts for heterogeneous catalysis: from single atoms to nanoclusters and nanoparticles. Chemical reviews 118, no. 10. 2018. pp.4981-5079.
