The conviction that the world is warming because of a human-enhanced greenhouse effect has been backed up by current scientific observations (Demiral et al., 2021; Ozturk et al., 2022). The planet's average temperature increased by 0.6 to 0.9°C annually between 1906 and 2006. This change is linked with the enhanced emission of carbon dioxide (CO2) which is considered a major anthropogenic greenhouse gas (GHG) after which methane, nitrous oxide, and water vapors also belong to the same group. The global monitoring reported that the average concentration of CO2 was 338.8 ppmv in 1980 which increased to 405.02 ppmv in 2018 (Vorokhta et al., 2019). Undoubtedly, CO2 levels are higher than in at least the past 800,000 years (Ramirez et al., 2019). CO2 emission results from human activities such as the combustion of carbon-based fuels (e.g., oil, natural gas, and coal), soil erosion, animal agriculture, deforestation, etc. (Lin & Raza, 2020). Furthermore, the Intergovernmental Panel on Climate Change (IPCC) investigated that CO2 concentration will increase up to 570 ppmv, consequently increasing the global mean temperature by 2°C, which will cause a 3 m increase in sea level (Yaumi et al., 2017) and other major environmental changes (such as melting of glaciers and increasing ocean acidity) by the year 2100. In the year 2010, the energy sector alone contributed about 68% of global GHGs emissions to the atmosphere (Lamb et al., 2021). An unceasing increase in its concentration in the air is of great concern to the scientific community due to its environmental impacts (Modak & Jana, 2019).
This significant environmental concern motivated researchers and governments in the direction of the development of more efficient carbon capture techniques (Morales-Ospino et al., 2020; Yaumi et al., 2017). Carbon capture and storage (CCS) technologies are designed to capture, transport, and store CO2. Post-combustion CO2 capture technology is the most easily adaptable of the currently known technologies to existing emission sources (Lee & Park, 2015). Among these methods, the preparation of sorbent material and adsorption technique is one of the best choices and has proven to be inexpensive and practical, owing to its easy and cheap process (Alkayal et al., 2022; Ibrahim et al., 2022).
Wet/dry sorbents are used to segregate gases and collect CO2 via a sorption/desorption process (Zhang et al., 2010). Each adsorption technique (wet or dry) possesses its benefits and shortcomings. Wet adsorption needs high energy and suffers from erosion and sluggish solid-to-gas reaction while it can treat large volumes of combustion emissions (Ben-Mansour et al., 2016). Dry adsorption possesses a high level of environmental and energy efficiency and requires only a basic device, yet it does not perform well for huge quantities of emissions and has a low separation efficiency (Kim et al., 2016). The most appropriate adsorbents should have the ability to capture CO2 from a gas stream with fast kinetics, selectivity, resilience, and an excellent adsorption capacity (Tan et al., 2015; Wang et al., 2015). It is almost impossible to find adsorbent with all the above stated qualities however, it is optimistic that the properties of adsorbents could be altered with small quantities of other materials, which makes composites adsorbents. These composites are expected to perform well compared to the parent material. A study of such materials’ efficiency under identical experimental conditions will help in selecting and comparing different adsorbents.
Chitosan (CS) is a biological waste product that is a main derivative of chitin, the second most prevalent natural polysaccharide after cellulose on the earth. Chitin is a -(1–4)-linked 2-acetamido-deoxy-d-glucose (N-acetyl glucosamine) that is structurally like cellulose except for the acetamide groups in the C-2 position, which in cellulose are replaced by hydroxyl groups. CS has several advantages, including easy availability, low cost, renewability, lower energy consumption, environmentally friendly nature, non-toxicity, biocompatibility, and biodegradability (Irani et al., 2017). Given the acidic nature of CO2 and the basic nature of the nitrogen-containing polymer, will eventually lead to acid-base interaction between acidic CO2 molecules and the basic site of CS polymer. CS has been utilized to generate functional derivatives by chemical modification, graft reaction, and ionic contact to increase its adsorption efficiency.
The name zeolite refers to a tetrahedrally coordinated network of atoms (Abdullahi et al., 2017). The zeolites are aluminosilicates with a microporous crystalline structure that occur naturally and/or are synthesized. Two essential structural components make up the Zeolite framework: aluminum or silicon atoms linked to four oxygen atoms to produce a tetrahedral shape, and an oxygen atom coupled to two tetrahedral atoms (usually bent 145°) (Busca, 2014). Aluminum (Al) present in zeolites induces a negative charge that is compensated by exchangeable cations (often alkali) in the pore spaces. These alkali cations cause zeolites to adsorb acidic gases such as CO2 (Spigarelli & Kawatra, 2013). Zeolite-based adsorbents are divided into various categories based on their pore size, including zeolite A, zeolite X, zeolite Y, ZSM-5, zeolite P, etc. (Sodha et al., 2022). Hollow zeolites (HZ) are a form of hierarchical zeolites with a low density and large surface area, as well as a changeable shell thickness and macropores in the core of the particles. Because of their great thermal stability, homogeneous structure of porosity, and good performance in mass transfer, these materials have garnered a lot of attention (Zhang & Che, 2019). Compared to hierarchical zeolites or ordinary nanocrystals, they offer numerous advantages, such as a crystalline structure that outperforms amorphous silica analogs in terms of hydrothermal and chemical stability, and a system of micropores functioning like a shape-selective membrane. Furthermore, their characteristics may be continually adjusted by altering the composition, notably the aluminum framework (Pagis et al., 2016). The advantages of the amine-modified adsorbents are all the same other than that they are less expensive and can reduce the amount of energy required for regeneration, improve adsorption capability, CO2 selectivity, corrosion resistance, and cause minimal pollution to the environment (Fashi et al., 2019; Pagis et al., 2016; Wang et al., 2019).
In the present study, two classes of adsorbents i.e. polymer-based chitosan and mineral-based zeolite both modified with monoethanolamine (MEA), have been developed and compared to study CO2 adsorption capacity of these adsorbents under similar reaction conditions. The amines are known to possess an affinity for CO2 and hence are selected to enhance the selectivity of adsorbents. An amine solution has higher capture selectivity to CO2 than other gases and it is not affected strongly by the CO2 partial pressure. This study will be helpful to compare the performance of MEA-modified chitosan beads and hollow zeolites as promising CO2 adsorbents evaluated under similar conditions.