Greener Catalysis for Environmental Applications

Catalytic reactions account for approximately 85% of all chemical reactions, and they are particularly significant in environmental science. Anastas and Warner introduced their 12 postulates of green chemistry, the ninth of which is catalysis, more than 20 years ago. The catalysts can be further made and used in such a way that the environmental benefits could be even more.

This Special Issue is devoted to “greener catalysis for environmental applications”, and primarily covers the catalytic synthesis of value-added chemicals, as well as the catalytic removal of pollutants.

One of the examples of catalytic removal of contaminants in water by the activation of an oxidant was presented by Krawczyk et al. [1]. The mechanism of decolorization of Acid Blue 129 was explained experimentally and theoretically, and the toxicity decline (evaluated using Daphnia magna and Lemna minor) of the solution after the oxidation was observed. However, this is not always the case with advanced oxidation processes, as noted by Kudlek [2]. The catalytically activated oxidants, and the radicals created in that process, can generate disinfection byproducts, which are often substantially more toxic compared to the original substrate (e.g., case of ibuprofen in the article of Kudlek [2]). Nonetheless, after a sufficient treatment period, UV-catalyzed processes may decrease the toxic nature of post-processed water solutions. The biodegradation of some compounds, such as nonylphenol ethoxylates (NPEOs), can also lead to toxic intermediates, which further require more invasive treatment. The heterogeneous catalytic activation of hydrogen peroxide was found to be a very efficient system for the decomposition of NPEOs, with an average of nine ethylene oxide units (NP9EO) [3]. A nanocrystalline Cu-based heterogeneous catalyst, in a dose of 0.3 g/L and an H₂O₂ concentration of 0.05 mM, has resulted in NP9EO and total organic carbon (TOC) removal efficiency of 83.1% and 70.6%, respectively. Other catalysts for the heterogeneous activation of hydrogen peroxide are iron materials such as magnetite. Zeolite-supported magnetite was used to activate H₂O₂ for the first time for the removal of ofloxacin [4]. This system achieved 88% of ofloxacin degradation efficiency and 51% of TOC removal efficiency under optimized reaction conditions. Furthermore, after five runs, reusability tests showed only a slight decrease in the catalytic activity.

The catalyst’s reusability was also presented in work focused on the reductive removal of pollutants [5]. Therein the authors have utilized elemental iron as a catalyst for the reduction of chloroacetamides. By varying the amount of catalyst or reducing agent before the reaction, it was possible to obtain conditions for the complete dechlorination of these pollutants to nontoxic substances. The reductive treatment of dibenzothiophene and naphthalene was presented by Puello-Polo et al. [6]. They have determined that the addition of gallium and vanadium as structural promoters in the NiMo/Al₂O₃ catalysts allows for the largest generation of sites for the hydrogenation and desulfurization of contaminants.

Greener removal of contaminants can be turned even more sustainable by pairing it with the simultaneous synthesis of value-added products. The products of complete and incomplete combustion of hydrocarbons, i.e., carbon dioxide (CO₂) and carbon monoxide (CO), are considered pollutants harmful to humankind and the environment. In such a sense, authors [7] have demonstrated the substitution of La with K cations in LaNiO₃ perovskite that exhibited a 100% selectivity towards the methanation of CO₂ at all temperatures investigated. On the other hand, reducing CO to value-added products such as gasoline and jet fuel range hydrocarbons by two different groups of CoMn catalysts derived from hydrotalcite-like precursors was reported by Gholami et al. [8]. The catalysts prepared using a KOH + K₂CO₃ mixture as a precipitant agent exhibited a high selectivity of 51–61% for gasoline (C₅–C₁₀) and 30–50% for jet fuel (C₈–C₁₆) range hydrocarbons compared with catalysts precipitated by KOH.

In a typical batch chemical process, solvents account for fifty to eighty percent of the mass, and they also drive its majority energy consumption. In this regard, for the greener synthesis of benzimidazole derivatives, the authors have reported solvent-free conditions and short synthesis time by a green montmorillonite K10 catalyzed method [9]. They have claimed that this method does not require the use of solvents, and can substantially reduce energy consumption in comparison to recently published procedures. Similarly, a greener solvent-free process of used cooking oil epoxidation has been developed [10]. An additional advantage of this method can be the catalyst’s enhanced activity after reusing (for not more than four times).

In conclusion, this Special Issue gathered articles of substantial quality and broad scientific interest to the Catalysts research community on various ways for making catalytic processes greener. Catalysis sustainability improvement can be obtained, among other things, by careful toxicity assessment during the catalyst preparation and the use of solvent-free conditions, catalyst recyclability, and pairing the removal of contaminants with the synthesis of value-added products (Figure 1).