Acrylic acid (AA), CH2=CHCO2His the first representative of α,β-unsaturatedcarboxylic acids. Anhydrous (glacial) AA is a colorless liquid with a boiling point of 141.0 °C and a melting point of 13.5 °C. In terms of acidity, AA is slightly stronger than acetic acid with a dissociation constant of 5.5 × 10-5 (pKa 4.26). It is completely miscible with water and has the lowest freezing point of -12.5 °C with 37% water content. In chemical properties, AA is similar to both carboxylic acids and unsaturated compounds, displaying a significant ability to polymerize. Due to this, the surface liquid where the polymerization inhibitor (phenothiazine, hydroquinone or its monomethyl ester) is concentrated must not be removed from the solidified AA and the solidified acid must not be heated above 30 °C[1].
Despite its high chemical activity, AA causes little harm to the environment and living organisms. Additionally, AA is a biodegradable compound that degrades rapidly in light, soil and water. Apparently, due to this, AA is rare in nature. It is known to be produced by seaweeds and has also been found in the rumen (first stomach) fluid of sheep.
As a member of an important class of carboxylic acids, AA has high relative content of carbon dioxide, up to 61% of the gross weight of the AA molecule. It serves as an excellent example of efficient CO2 utilization. Notably, among various products of organic synthesis obtained from CO2, organic acids such as formic acid, hydroxybenzoic acids, adipic acid, etc. constitute a significant part (Figure 1)[2-16]. Additionally, CO2 has several advantages as a reagent, including its affordability, non-flammability, non-toxicity, widespread availability and inexhaustibility as a main component of industrial gases used in chemical production, thermal and power plants, etc. and as a constituent of air (> 0.04%). The tools for reactions with CO2 is constantly expanding to include a wide range of catalytic, photocatalytic and electrochemical methods[17]. This field of chemistry is progressing rapidly, with new CO2 carboxylation methods that do not even require special equipment and can operate at normal pressure and temperature. In addition to the obvious advantages of this C1-synthon (see Figure 1), there is another powerful incentive for its use in industrial synthesis.
As the primary greenhouse gas in the atmosphere, CO2 is responsible for ongoing global climate change and the resulting negative impacts on human life[18-20]. The implementation of the 2015 Paris Climate Agreement, which aims to reduce CO2 emissions, requires a fundamental restructuring of the technological sector. Energy-efficient, atom-efficient processes based on CO2 recycling will be crucial in this transition. In this technological paradigm, precise control over the amount of CO2 emitted is a key factor that defines the quality of the technology and even the products themselves.
The proposed methodology for estimating emitted CO2 includes not only the production stages, but also the entire ‘life cycle’ of the product, including its processing and decomposition as waste[6]. Creating products with high CO2 content is undoubtedly relevant and economically feasible. However, achieving this goal is challenging because many chemical transformations of CO2 are thermodynamically prohibited. This is due to the high stability of the carbon dioxide molecule, which in the potential energy well is located much lower than other compounds (ΔGR0 = -396 kJ mol-1). Similar to the water molecule, it is the most stable product of combustion and oxidation processes[2]. Consequently, the Gibbs energy of many of its reactions turns out to be positive, primarily due to the energy costs of CO2 activation and changes in the degree of oxidation of carbon atom.
Based on CO2 and other small molecules, one can envision several highly atom-efficient pathways for synthesizing compounds of great value to industry. However, these pathways may be thermodynamically or due to other energy factors or technologically unattainable. In this case such processes are often referred to as ‘dream reactions’ because they are highly desirable yet challenging to achieve. Examples of ‘dream reactions’ include the synthesis of methanol by direct oxidation of methane, where the problem of selectivity is of fundamental importance, because methanol oxidizes much easier than the starting methane (Scheme 1). CO2 endothermic syntheses of formic acid (ΔGR0 = +32.9 kJ mol-1), acetic acid (ΔGR0 = +55.0 kJ mol-1), adipic acid (ΔGR0 = +77.4 kJ mol-1), dimethyl carbonate (ΔGR0 = +26.2 kJ mol-1), phenylisocyanate (ΔGR0 = +306.6 kJ mol-1), and other aryl derivatives are also considered ‘dream reactions’. These reactions require a combination of special approaches, to alter the thermodynamics of the process and significantly increase their rate.
Therefore, the most important role in the implementation of such transformations is assigned to catalytic systems. Although they do not change the thermodynamics of the process, they allow for lowering the energy barrier, thus, opening the possibility of more effectively controlling the chemical equilibrium and shifting it toward the reaction products. In particular, in the catalytic synthesis of formic acid, its conversion to the solvated salt form allowed for making the process not only thermodynamically favorable but also catalytically efficient with TON values reaching several million[21, 22].
Among the ‘dream reactions’ (see Scheme 1), the synthesis of AA holds a special significance and a revealing history, from the initial idea to the testing of the technological scheme of synthesis in a laboratory. Several scientific groups have achieved remarkable success in implementing this challenging reaction[23-27], although one step still separates researchers from turning the dream into reality. Given this, each of us can contribute to taking this important step by combining the achievements of many chemists to improve this breakthrough reaction and create an efficient industrial technology based on it. Therefore, this paper will analyze the most important stages in the development of this process and the existing obstacles that complicate the transition to an efficient industrial synthesis of AA.
AA is an important industrial product, as evidenced by its impressive global production forecasted at 7.8 million tons in 2024. Derivatives of AA such as esters and salts as well as polymers based on it are widely used in various industries and households. These derivatives are utilized in coatings, paints, superabsorbents, adhesives, sealants, polymer fibers (such as Acrilan, Creslan, Dralon, Vonnel, etc.), components for 3D printing, reagents for oil production, water purification and more.1 Thanks to the ease of radical poly- and copolymerization, a wide variety of acrylic polymers can be achieved by using different types of initiating systems. The choice of the type of ether radical controls the degree of polymer adhesion or fiber smoothness. Polyacrylates hardly ever yellow in direct sunlight because they absorb UV radiation below 290 nm. Copolymers of acrylate ester and methacrylate ester (‘pure acrylates’) have excellent UV light resistance, making them ideal binders for transparent coatings with low pigment content, which is among the important advantages of polyacrylates[28]. Acrylic fibers are also a major source for pyrolysis into carbon fiber[29]. Water-soluble polyacrylic acid and its salts with a molecular weight of 2 – 5 thousand are utilized as deposit inhibitors, sludge dispersants in cooling water systems, pigments or paper coating materials[30]. Copolymers with a small number of hydrophobic side groups are also useful as fracturing fluids in oil drilling[31]. Cross-linked sodium polyacrylate is a common absorbent in baby and adult diapers[32-34], feminine hygiene products[35]. It should be noted that the market share of sodium (poly)acrylate involved in the production of superabsorbents[36] reaches 70% and is an important component of the total volume of AA produced. Cross-linked polymers of acrylic or methacrylic acid neutralized to more than 50 mol.% are also used to provide high water retention capacity and viscosity of gypsum[37]. The cross-linked water-absorbing AA polymer is integrated into the formulations of prolonged-acting tablets[38]. A mixture of water swelling particles of anionic AA polymer or methacrylic acid and their soluble salts and cationic aminoacrylate and aminoalkylacrylamide polymers are used in precoated adhesives for wallpapers[39].
