**1. Introduction**

Carbon dioxide capture and utilization (CCU technologies) has been recognized as a possible and cost-effective way to reduce worldwide greenhouse gas emissions [1–10]. The use of CO2 as a raw material in chemical synthesis is a research area of great scientific, economic and ecological interest [1–14]. The abundance and benignity of carbon dioxide, which is cheap, nontoxic and nonflammable, makes it a very attractive low-cost C1-synthon in organic chemistry. Moreover, the mitigation of CO2 emission from industrial processes in order to reduce CO2 causing the greenhouse effect encourages chemists to carry out research in this area.

CO2 is thermodynamically stable (ΔG<sup>0</sup> <sup>=</sup> −394.228 kJ/mol) [10], however, and needs catalytic activation and a corresponding reactive agent for its possible fixation into the organic molecules. Thermodynamically non-stable three-membered heterocyclic rings such as epoxides serve as ideal reactants for CO2 fixation. The reaction of epoxides and carbon dioxide to produce cyclic carbonates is attractive because CO2 can be incorporated in the epoxide molecule without the formation of any side products (with 100% atom-economy) (Scheme 1) [1,2].

**Citation:** Weidlich, T.; Kamenická, B. Utilization of CO2-Available Organocatalysts for Reactions with Industrially Important Epoxides. *Catalysts* **2022**, *12*, 298. https:// doi.org/10.3390/catal12030298

Academic Editors: Javier Ereña and Ainara Ateka

Received: 16 February 2022 Accepted: 4 March 2022 Published: 6 March 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

**Scheme 1.** CO2 consumption for ethylene carbonate (EC) production applying the carbonation of oxirane produced from ethylene [2].

The epoxides available for carbonation can be classified according to their bound substituents as terminal epoxides (containing at least one unsubstituted CH2 group in the oxirane ring) and internal epoxides (containing substituents on both carbon atoms of the oxirane ring, Figure 1).

**Figure 1.** Epoxides available for cycloaddition reactions.

The reaction conditions for the efficient carbonation of epoxides differ significantly, utilizing terminal (for example, glycidyl derivatives such as epichlorohydrin or glycidol and corresponding glycidyl ethers) [1–3,5–14]) or sterically, much more hindered internal epoxides (for example, epoxidized fatty acids and their derivatives [4,8–14]). From the sustainability point of view, bio-based epoxides ideally serve as suitable reactants for the production of cyclic carbonates utilizing waste CO2.

In particular, the connection of direct air oxidation and carbonation of ethylene oxide serves as a simple and effective technology that is useful for the subsequent efficacious utilization of anthropogenic CO2 (Scheme 1). In case of ethylene carbonate (EC) produced from ethylene, 44 g of CO2 is utilized per mol of produced EC (approximately 2/3 of EC molecular weight creates CO2) according to the stoichiometry of this reaction. On the other hand, CO2 is emitted during the production of ethylene using conventional technologies such as steam cracking (discussed in Section 2). According to the life cycle assessment (LCA) methodology, technology based on the carboxylation of petrochemical ethylene via catalytic air oxidation emits only 0.92 t CO2/t EC [2].

The catalytic carboxylation of epoxides may afford either cyclic carbonates (Scheme 2, Path a) or eventually polycarbonates (Scheme 2, Path b) [4,7,9], depending on the used catalyst and the reaction conditions.

The nucleophile-based ring opening of oxirane activated by the catalyst with the subsequent addition of CO2 and five-membered ring closure accompanied by the release of nucleophile is described in Scheme 1, Path a. The catalyst that activates oxirane (H-bond donor or Lewis acid) decreases the highest reaction barrier of the ring opening (usually the rate-determining step) or/and stabilizes the alkoxide produced by the ring-opening, thus promoting the cycloaddition reaction. Different Lewis bases act as nucleophiles, preferentially bromide or iodide.

Path b. describes the insertion of another oxirane molecule followed by alternate copolymerization with carbon oxide and epoxide. The polymerization occurs in the case of insertion of another epoxide molecule competes with the ring-closure process. Generally,

the utilization of metal-based catalysts is necessary for the direct formation of polycarbonates starting from CO2 and epoxides via ring-open6ing polymerization.

**Scheme 2.** Reaction of CO2 with epoxides producing cyclic carbonates and/or polycarbonates [11].

Utilizing CO2 via cycloaddition to epoxides is the exothermic process that can be carried out under mild reaction conditions using a broad spectrum of catalysts [11,12].

Cyclic carbonates are generally stable liquids, which enables the long-term sequestration of CO2, especially when cyclic carbonates are subsequently used for polymerization. Although the production of cyclic carbonates from CO2 and epoxides has been industrialized since 1958 and uses inexpensive catalysts such as KI, the current production processes still suffer from major disadvantages, such as high reaction temperatures (180–200 ◦C), high pressure (5–8 MPa) and stoichiometric amounts of activating reagents [13,14].

It is known that the increasing of the reaction rate with the increase in CO2 pressure is not only counterproductive due to the high energy consumption, but is even limited. The high increase in CO2 pressure (and the overrunning at a concentration of 0.47 g CO2/mL) is accompanied by sudden decrease in reaction rate due to the dilution effect causing the reduction in epoxide and catalyst concentrations in the reaction mixture [11].

Cyclic carbonates are used for polymer production, as electrolytes in lithium-ion batteries, polar aprotic (ethylene or propylene carbonate) or protic solvents (glycerol carbonate, etc.) and as chemical intermediates in organic synthesis [1–14] (Scheme 3).

**Scheme 3.** Possibilities for the chemical transformation of cyclic carbonates to linear (poly)carbonates, esters, ureas, ethyleneglycols and urethanes (carbamates) [1,2,5,13].

Alluding to cyclic carbonates used as solvents, a grade deal of attention has been given to the application of glycerol carbonate as it possesses low toxicity and good biodegradability, and has a high boiling point and simple availability from lipids and CO2, giving it many applications [10]. Generally, the direct fixation of CO2 in cyclic carbonates and their products is regarded as a greener approach than the existing practices. As North et al. have mentioned, only two reactions of CO2, the dry reforming of methane (for fuel production) and cyclic carbonate chemical production, could consume up to 25% of the anthropogenic CO2 produced annually [12].

The cyclic carbonate formation based on the cycloaddition of CO2 requires oxirane ring opening in the first reaction step, which is followed by insertion of CO2 in the second

reaction step and ring closure to form cyclic carbonate. The general mechanisms of this reaction are illustrated in Scheme 4 [3,12].

**Scheme 4.** Generally accepted mechanisms of cyclic carbonate formation via the cycloaddition of epoxide on CO2 [3,12].

According to the computations, the epoxide ring-opening has the highest energetic barrier. Due to this reason, the applicable catalysts decrease the reaction barrier, acting as Lewis bases (nucleophile, Nu) that are capable of nucleophilic attack and opening the oxirane ring and/or stabilizing intermediates. In addition, the oxirane can be activated by hydrogen bond formation interacting with a Lewis acid or a hydrogen bond donor (HBD), thereby decreasing the energetic barrier of the epoxide ring opening. In order to catalyze cyclic carbonate formation most effectively, the best catalysts often contain a combination of both Lewis base and Lewis acid components or Lewis base/HBD parts. If the catalysis influencing the ring opening is very effective, then the ring closure step can be the rate-determining step with a higher energetic barrier [15].

The action of some described effective catalysts is based even on the activation of CO2, as is assumed. The activation of CO2 comprises formation of carbamate or carbonate, which may also take part in the epoxide ring-opening step [3,5,10–12].

Catalytic cycle 1 in Scheme 4 (CO2 activation mechanism) describes the nucleophilic activation of CO2 using nucleophiles such as tertiary amines, amidines, guanidines or carbenes (Nu1) to form an intermediate A-1 (carbamate or carboxylate), which subsequently promotes oxirane ring opening, leading to the second intermediate (I-1). A-1 attacks oxirane or oxirane activated by Lewis acid (A-2). Cyclization then occurs to produce a cyclic carbonate while the nucleophile (Nu1) is recycled for a new catalytic cycle. The CO2 activation mechanism requires a catalyst nucleophilic towards CO2, but not for epoxide [4,12].

In addition, catalytic cycle 2 demonstrates a more common carbonation pathway through the activation of epoxide. The hydrogen bond donor(s) (HBDs) coordinate(s) to the epoxide ring (formation of intermediate A-2). Subsequently, the Lewis base, for instance, onium halide salts, acting as nucleophile (Nu2) attacks the oxirane to enable the ring opening, producing an intermediate (I-2). Subsequently, upon ring opening, the CO2 insertion occurs in the O-H bond of the intermediate (I-2), producing a new intermediate (I-3). The cyclization of the intermediate (I-3) accompanied by cleavage of the leaving group (Nu2), produces cyclic carbonate as the final product, and nucleophile is recycled for a further reaction. Alternatively, cleavage of nucleophile Nu2 from I-2 before CO2 addition causes the formation of alternative products such as corresponding ketones via a Meinwald rearrangement.

## **2. Sources of Epoxides**

Epoxides are generally accessible either via the oxidation of the C=C double bond in alkenes (using peroxides or O2/Ag-based reaction) or via the dehydrohalogenation of geminal halogenoethanols [5].

Terminal epoxides such as epichlorohydrin (EPIC), glycidol (GL), methyloxirane (propylene oxide, PO) and oxirane (ethylene oxide, EO), and their functional derivatives, are the most studied epoxides for the CO2/epoxide coupling reaction.

EPIC and GL are chemicals that were recently produced from bio-based glycerol obtained as a by-product from the chemical utilization of lipids (Scheme 5). Transesterification or hydrolysis of lipids is the main pathway for the production of bio-based glycerol and fatty acid esters (for biodiesel) and soap (sodium or potassium salts of fatty acids are used as surfactants) [5] (Scheme 6).

**Scheme 5.** Production of glycidol and epichlorohydrin starting from bioglycerol [16–20].

The nucleophilic substitution of OH groups in the glycerol structure with Cl via the action of gaseous HCl enables the formation of monochloropropanediols or dichloropropanols, in the presence of a catalyst, usually a carboxylic acid [2,5,16–18], as the crucial feedstock for production.

The subsequent ring closure induced by an inorganic base such as calcium or sodium hydroxide enables epoxide formation via a dehydrochlorination reaction with inorganic chloride (NaCl or CaCl2) being obtained as a co-product [19,20] (Scheme 5).

Similarly, starting from chloropropanediols, glycidol is produced by basic hydrodechlorination [20].

**Scheme 6.** Synthesis of glycerol and epoxidized methyl oleate from triglyceride of oleic acid (vegetable oil).

EO and PO are still mainly derived from ethylene and propylene produced by the cracking of petrochemical feedstock with subsequent silver-catalyzed direct air oxidation [2].

The development of appropriate sustainable alkenes production is based on the utilization of bio-based syngas (CO + H2) obtained by the gasification of waste biomass [21]. The most promising pathway for alkene production exploits methanol as a crucial intermediate simply available from syngas with its subsequent dehydrogenation/coupling catalyzed by zeolites (methanol-to-olefins technology, MTO) [21]. Apart from the above-mentioned, the direct catalytic cracking of vegetable oils (lipids) may produce propylene [21].

The other sustainable pathway for ethylene or butylene production is based on the dehydrogenation of bio-based alcohols (bioethanol and biobutanol) [22] accessible by fermentation of oligosaccharides. Brasco Co. produces bio-ethylene, for example, through the dehydration of bioethanol for polyethylene production in Brazil [23].

In addition, other types of epoxide can be produced from waste biomass, such as limonene oxide and limonene dioxide, which can be synthesized via the promising epoxidation of bio-based limonene obtained from citrus peels, oak and pine tree, under solvent-free conditions with hydrogen peroxide and a tungsten-based catalyst [24].

In addition, vegetable oil-derived triglycerides and fatty acids contain double bond(s) (Scheme 6), which can undergo epoxidation with peroxides [2]. The epoxidized fatty acid derivatives can be subsequently exploited as the starting materials for cycloaddition or even a subsequent one-pot polymerization reaction with CO2 [25,26]. Scheme 6 shows the transesterification of natural triglycidyl oleate producing methyl oleate and bioglycerol, and the subsequent epoxidation of methyl oleate using hydrogen peroxide.

It is worth noting, however, that epoxidation is a highly exothermic reaction (as ΔH = −55 kcal/mol for each double bond); thus, H2O2 is slowly added or added in a stepwise manner in semi-batch operations, and requires a long reaction time. In order to avoid problems with heat and mass transfer, process intensification using a microreactor was proposed [27]. The microreactor could significantly decrease the reaction time with higher epoxide selectivity.

#### **3. Homogeneous Metal-Free Catalysts**

This review focuses on the recent development of homogeneous organocatalysts for the cycloaddition reaction of CO2 and epoxides (insertion of CO2 into the oxirane ring) with the aim of producing cyclic carbonates. Homogeneous catalytic systems (catalyst and reagent dissolved in the same phase) usually display the highest conversion and selectivity and are usually significantly cheaper compared with heterogeneous ones. Metal-free organocatalysts are usually readily available even from renewable sources, non-recalcitrant, biodegradable, affordable, and inert towards air and moisture. They may, however, be more difficult to separate and recycle from the produced cyclic carbonates. On the other hand, after the discovery of a new effective homogeneous catalysts, subsequent attempts to immobilize it on the insoluble surface have followed in order to solve problems with catalyst separation and recycling [12].

#### *3.1. Catalytically Active Amines and Their Salts*

Generally, the effective absorption of CO2 into the liquid phase and its subsequent activation of chemisorbed CO2 are important steps for the subsequent cycloaddition reaction of CO2 with epoxides.

The alkaline absorbents described in the literature as being capable of effectively absorbing CO2 are different amines [28], including amidines such as 1,8-diazabicyclo [5.4.0]undec-7-ene (DBU) [28–30], guanidines such as 1,5,7-triazabicyclo[4.4.0]dec-5-enium (TBD) [28,29], and azaheterocycles such as pyridines and imidazoles [31,32], which are efficient for the chemisorption and activation of CO2.

The correlation between the structures of different organic amines and their catalytic activity in the cycloaddition of CO2 was studied by Yu et al. [28]. Their article compares the catalytic activity of a variety of nucleophilic aliphatic amines (ethanolamine, bis-(3-aminopropyl)-amine, oleylamine), basic aminoacid arginine, nucleophilic aromatic amines (1,2-phenylenediamine, 2-aminobenzylamine) and non-nucleophilic cyclic amines such as TBD and DBU for the cycloaddition of CO2 to methyloxirane (propylene oxide, PO) [28].

It is well known that the chemisorption ability of CO2 (formation of carbamate) increases with the increasing basicity of amine [33,34]. Yu et al. observed no apparent relationship of the pKa value of conjugated acids of the tested amines with respect to their catalytic activity in the case of propylene carbonate (PC) formation (Figure 2, Table 1). The authors observed the lowest propylene carbonate conversion when applying aliphatic amines.

Low conversion was even observed in the case of amine forming intramolecular hydrogen bonds (1,2-phenylenediamine and 2-aminobenzylamine).

In contrast, aromatic amines (such as 1,4-phenylenediamine) and amidines or guanidines involving conjugated "N=C–N" structures (arginine, TBD) were found to be the most active ones [28].

According to the published information [35,36], tertiary amines are often higher in activity than primary or secondary ones (Table 2, Entries 5, 10–12). It has been proven that amidines containing the "N=C–N" bond in their structures are particularly favorable for the cycloaddition of CO2 with epoxides (Table 1, Entries 6–8, 17 and Table 2, Entries 3 and 12) [2–4,6,32]. The above-mentioned observations are in agreement with the statement of North et al. that for the efficient activation of CO2, compounds that nucleophilically attack CO2 but not the epoxide ring are sought out [12].

**Figure 2.** Structures and abbreviations of the tested amines.

Azzouz et al. demonstrated the effective utilization of 2-aminopyridine (2-NH2-PY) as a catalyst (using 10 molar % of 2-NH2-PY) for the carbonation of different terminal epoxides at 60–85 ◦C and 1 MPa of CO2 [35]. This carbonation was performed even at pilot scale.

Interestingly, corresponding salts with protic (Bronsted) acids (base.HA) of the abovementioned non-nucleophilic amines (DBU.HA, *N*-methylimidazole (MIM.HA), *N*,*N*-dimethylaminopyridine (DMAP.HA)) and, alongside these, even triethanolamine, pyridine and caffeine (Figure 3), are quite catalytically active in the cycloaddition of CO2 with epoxides (Table 3, Entries 3–12, 15–24). Hydrogen halides in particular are the most active in comparison with corresponding free bases [29,30,32,37–39] (Table 3, Entries 3–6, 8–12). The most active seems to be hydroiodides of the corresponding amines (Table 3, Entries 10, 18–24, 26–27). For the above-mentioned catalytic action of amine salts, the reaction mechanism based on the activation of epoxide via the protonation with amine

salt in the role of Bronsted acid (HA) is proposed with subsequent anion-based epoxide ring opening.

**Table 1.** Effect of amine structure on the cycloaddition of CO2 to propylene oxide (PO) producing propylene carbonate (PC).


Abbreviations: PPD—p-Phenylenediamine.




**Table 3.** Comparison of the catalytic effects of amines and their salts on the carbonation of PO.

Abbreviations: [HMIM]Cl—1-methylimidazolium chloride; CAFH.X—caffeinium halide; [Hbet]I— Me3NCH2COO.HI; PANI-HI—Polyaniline hydrogen iodide; [HHMTA]Cl—urotropin hydrogen chloride.

**Figure 3.** Structure of catalytically active caffeine hydrohalides [37].

Sun et al. reported very effective carbonation using triethanolamine hydroiodide, including the simple recyclability of this catalyst without loss of activity even after four recycling steps [38].

The published results indicate that the synergetic effect of hydroxyl groups from protonated aminoalcohol in the role of HBD, together with naked bromide or iodide, significantly influences the cycloaddition of CO2 to the studied epoxides and makes possible the application of this reaction even at ambient conditions (Table 3, Entry 17–18).

Catalytically active ammonium halide activates not only the epoxide ring for opening through the H-bond with hydroxyl groups of triethanolammonium cation and the subsequent addition of intermediate 2-halogenoalkoxide to CO2, but even the next ring closure caused by the facile withdrawal of halide from the produced 2-halogenocarbonate [38] (Schemes 3 and 4).

Apart from the above-mentioned, in the case of caffeine hydrobromide, potassium halides added to the reaction mixture as an additional source of nucleophiles were successfully tested. With the same reaction conditions, the enhanced efficiency of cyclic carbonate formation was observed utilizing equimolar quantities of KX and caffeine.HBr or even

2: 1 KX: caffeine.HBr using DMSO as the reaction solvent at 70 ◦C and 0.7 MPa CO2 pressure [29,37]. The yield of cyclic carbonate increased following the trend KF < KCl < KBr < KI, which was in good agreement with nucleophilicity and nucleofugacity of the corresponding halide anions (Table 4, Entries 17–25).

**Table 4.** Catalyst screening for the cycloaddition of EPIC.


In Table 5, the increase in carbonate yields using CAFH.Br/KI in the case of carbonation of terminal epoxides is documented. In the case of internal epoxide (limonene oxide), however, no carbonation was observed using caffeine hydrobromide or even its mixture with KI (Table 5, Entry 5).

Roshan et al. came to the conclusion that even the addition of a low quantity (a catalytic amount) of H2O significantly enhances CO2-based formation of PC over tertiary heterocyclic amines such as IM, PY and DMAP, giving over 98% selectivity of PC formation (at 120 ◦C, 1.2 MPa, 3 h). The observed results were evaluated by a DFT study comparing energy profiles for free amine in comparison with corresponding amine hydrogencarbonatemediated cycloadditions of CO2 to propylene oxide. Ammonium hydrogencarbonates produced in situ from heterocyclic amine, H2O and CO2 works similarly to the abovementioned amine salts as activators of the epoxide ring. As was argued, the HCO3 − anion generated in the water-CO2-base reaction was the key active species that gave the higher

activity of the base-water systems rather than the carbamate salt (produced by a reaction of R3N with CO2) [32].



Reaction conditions: 7 mol% catalyst at 70 ◦C and 0.7 MPa; reaction time, CAFH.Br = 16 h and CAFHBr/KI = 6 h.

It should be mentioned that low-melting salts (melting point below 100 ◦C) obtained by the neutralization of organic bases with organic or inorganic acids embodies are called protic ionic liquids (PILs). The melting of PILs could enhance the miscibility of catalytically active PILs with reacting epoxide and CO2 compared with solid catalysts, as was published by Zhang et al. in the case of DMAP hydroiodide [42] or by Kumatabara et al. using triethylamine hydroiodide [41] at ambient pressure (Table 3, Entry 10 and Table 4 Entry 26).

Zhang et al. published results obtained even by means of the capture and utilization of CO2 for the cycloaddition into SO using PIL (DMAP hydrobromide) at ambient pressure and 120 ◦C [42]. This PIL has superior catalytic effect compared with other hydrogenhalides of tertiary bases such as DBU, MIM, DABCO or tetramethylguanidine (Table 6). DMAP.HBr is well reusable with no drop in activity after five recycling steps. DMAP.HBr is able to carbonate even internal epoxides such as ChO, although this cycloaddition is quite sluggish.


**Table 6.** Synthesis of SC catalyzed by different PILs [42].

Abbreviations: [HMIm]Br—3-methylimidazolium bromide; [4-MeNH-PyH]—4-methylaminopyridine; [HTMG]Br—*N*,*N*,*N* ,*N* -tetramethylguanidinium hydrogen bromide.

#### *3.2. Two Components Catalysts Based on a Combination of Organic Base and Hydrogen Bond Donor*

As was mentioned in Section 3.1, the combined action of protonated amine with nucleophilic anion positively influences the efficiency of cycloaddition. This synergic action between the Lewis base, such as the amine, and hydrogen bond donors (HBDs) was reported in the literature [2,4,6,32,36,43].

The possible synergic effects of alcohols (glycerol, glycidol, 1,2-propylene glycol (PG), poly(ethylene glycol)-600 (PEG600), poly(ethylene glycol)-400 (PEG400), cellulose, chitosan and β-cyclodextrin (β-CD)) known as HBDs was explored in CO2-based cyclic carbonates synthesis catalyzed by amines, as mentioned in Section 3.1 [36]. For this purpose, the most catalytically active DBU and DMAP were tested in relation to the co-action of the chosen HBDs (Table 7).

Out of the set of experiments, cellulose was recognized as the most effective HBD in the addition of CO2 to propylene oxide [36].

The effective quantity of cellulose used as HBD in the case of the DBU catalysis of the chemical fixation of CO2 into propylene carbonate is very low with respect to the optimal quantity of DBU (15 mg of cellulose + 300 mg of DBU per mL of PO). The effect of the DBU excess on the yield of PC in the DBU-cellulose reaction system was studied. Generally, the yield rises with the increasing of the DBU: cellulose ratio with the maximum conversion and selectivity reached at a mass ratio of 25–30:1 [36]. The high catalytical activity of cellulose was, in all probability, explained by Khiari et al. and Gunnarson et al. [44,45]. Cellulose reacts in co-action with a significant excess of non-nucleophilic DBU, with CO2 producing carbonate by means of a reaction similar to that of cellulose xanthate formation during the production of viscose utilizing a sulfur analogue of CO2, carbon disulfide. The produced carbonate should be a nucleophilic agent that attacks and opens the epoxide ring rather than the known non-nucleophilic DBU.


**Table 7.** Catalyst screening for the base catalyzed cycloaddition reaction of PO; effects of different HBDs [36].

<sup>a</sup> 2nd reuse; <sup>b</sup> 3rd reuse; <sup>c</sup> 4th reuse.

Aoyragi et al. described the markedly increased formation of cyclic carbonates in isopropylalcohol using triphenylphosphine hydroiodide as a catalyst. 1H NMR spectra documented the formation of H-bonds between the used isopropylalcohol and the starting epoxide [46]. The high activity of the above-mentioned hydroiodide (compared with other HXs) was explained by both the high nucleophilicity and even the high leaving ability (nucleofugality) of iodide ion.

Section 3.1 mentioned the significant catalytic activity of triethanolamine [36], which could be explained by the synergy of HBDs (bound alcoholic OH groups) and the Lewis basicity of the tertiary amine.

More advanced catalysts such as 2-hydroxymethylpyridine (2-PY-CH2OH) and 2,6-bis(hydroxymethyl)pyridine (2,6-PY-CH2OH)2 were developed for the high-efficiency cycloaddition of CO2 with epichlorohydrin (EPIC) under a slightly elevated temperature and ambient pressure (T = 25–60 ◦C, 0.1 MPa of CO2; see Table 4, Entry 4) [40]. The high catalytic effect was demonstrated by 1H NMR spectroscope observing the formation of a stable H-bond between the PY-CH2OH and oxygen of epichlorohydrin. The authors demonstrated that the tested compounds with either heterocyclic nitrogen (benzylalcohol PhCH2OH) or hydroxymethyl (CH2OH) groups (PY) catalyzed EC formation only sparingly (PY) or not at all (PhCH2OH) (Table 4, Entries 1–5).

The catalytic activity of nitrogen-doped charcoal for CO2 cycloaddition reactions could be explained by the co-operation of OH groups working as HBDs and tertiary amines bound in the graphitic structure of specially prepared *N*-doped carbons together with the ability of active carbon to adsorb CO2 [47].

#### *3.3. Aminoacids (AAs) as Catalysts*

AAs contain amino and carboxylic groups in their structures. Amino groups can react with CO2 to form *N*-COO− (carbamate) products with low binding energy, which can catalyze the transfer of CO2 to the 2-halogenoalcoholate produced by the halide-based opening of the epoxide ring. The carboxylate group (–COOH) can catalyze the oxirane ring opening as effective HBD analogously to the amino group (–NHR) and hydroxyl (–OH). Some AA salts have been successfully tested in the capture of CO2 from flue gas. In addition, the amino groups can also be utilized in quaternization with the aim of introducing halide as a nucleophile into the engineered catalytically active molecules (Table 8).


**Table 8.** PO catalyzed by AAs.

Abbreviations: L-His—L-Histidine; QGly—glycine quaternized using MeI under 10 min of microwave irradiation.

The yield of PO is dependent on the AA structure; basic AAs such as L-histidine (L-His) and proline (Pro) containing basic additional groups (imidazolium and amino, respectively) provided higher yields than acidic aspartic or glutamic acids (Figure 4).

**Figure 4.** Structures of highly catalytically active AAs.

In addition, the combination of an amino acid with an HBD results in a higher catalytic activity under milder reaction conditions than in the absence of an HBD. The binary catalytic systems formed with the amino acid and H2O produced, for example, more active systems for the synthesis of PO than in the absence of H2O [53]. In the case of L-His, the time taken for the nearly total conversion was reduced from 48 h to 3 h by adding H2O as an HBD (L-His:H2O ratio = 1:29) under similar conditions (Table 8, Entries 1 and 2). The low H2O concentration was used to avoid the hydrolysis of the produced PC [49,53].

Roshan et al. showed that the combination of halide ions as nucleophiles (added in the form of KI, for example) with L-histidine produced highly active catalytic systems for the cycloaddition of CO2 to epoxides [51] (Table 8, Entry 4).

The most effective binary catalytic systems contained KI with basic AAs such as His/KI (Table 8, Entry 4). In a related work, Yang et al. proved the high stability of AA/KI catalytic systems, namely L-Trp/KI, for the cycloaddition of CO2 to PO to form propylene carbonate. After carbonate separation conducted by means of distillation, the catalytic system was reused five times without loss of activity [54].

The catalytic activity of the different KX salts followed the order Cl < Br < I corresponding to the increasing nucleophilicity and leaving group ability [54]. This confirmed the role of these anions in the opening of the epoxide ring [54].
