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Review

Hydrogenation of CO2 or CO2 Derivatives to Methanol under Molecular Catalysis: A Review

by
Wenxuan Xue
1 and
Conghui Tang
1,2,*
1
Key Laboratory of Material Chemistry for Energy Conversion and Storage, Ministry of Education, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, 1037 Luoyu Road, Wuhan 430074, China
2
State Key Laboratory of Natural and Biomimetic Drugs, School of Pharmaceutical Sciences, Peking University, Beijing 100191, China
*
Author to whom correspondence should be addressed.
Energies 2022, 15(6), 2011; https://doi.org/10.3390/en15062011
Submission received: 13 February 2022 / Revised: 3 March 2022 / Accepted: 7 March 2022 / Published: 9 March 2022
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Abstract

:
The atmospheric CO2 concentration has been continuously increasing due to fossil fuel combustion. The transformations of CO2 and CO2 derivatives into high value-added chemicals such as alcohols are ideal routes to mitigate greenhouse gas emissions. Among alcohol products, methanol is very promising as it fulfills the carbon neutral cycle and can be used for direct methanol fuel cells. Herein, we summarize the recent progress in the hydrogenation of CO2 or CO2 derivatives to methanol, and focus on those systems with homogeneous catalysts and molecular hydrogen as the reductant. Discussions on the catalytic systems, efficiencies, and future outlooks will be given.

1. Introduction

Global warming has been a growing problem for our ecological environment during the last several decades, due to the continuous combustion of fossil fuels and CO2 emissions [1]. The atmospheric CO2 concentration has increased to 415 ppm; therefore, CO2 capture and utilization is not only important to academic research, but to the whole environment and ecosystem on Earth [2,3,4].
Transforming CO2 into high value-added chemicals or alcohols, which can then be used as biofuel, is an efficient and profitable way to fulfil a carbon-neutral cycle. Nobel Laureate Prof. George A. Olah proposed the concept of the “methanol economy” [5,6,7], which advocated for the catalytic conversion of CO2 to methanol by reducing the greenhouse gas in order to produce methanol for energy applications such as direct methanol fuel cells (DMFC), as well as raw materials for the synthesis of olefins and other hydrocarbons. In addition, methanol can act as a hydrogen energy carrier since it contains 12.6 wt% of hydrogen and can be easily stored, transported, and distributed using existing pipelines because of its liquid nature [8,9]. Under this concept, methanol is used as a liquid organic hydrogen carrier (LOHC) which can reversibly store and release molecular hydrogen whenever and wherever it is in demand [10,11,12,13,14,15].
Practical methods of CO2 hydrogenation to methanol were usually based on heterogeneous catalysis, and these reactions were traditionally carried out at high temperatures (200–300 °C) [16,17,18,19,20]. Studies showing homogeneous catalytic CO2 hydrogenation under relatively mild conditions (<150 °C) are of great significance, and considerable efforts have been made in the last decade to achieve this [21,22,23,24,25,26,27,28].
Among the homogeneous catalytic reactions of CO2 hydrogenation to methanol, the major catalytic systems are transition-metal catalysts [21,22,23,24,25,26,27,28,29], frustrated Lewis-pairs (FLPs) [30,31,32,33] or N-heterocyclic carbenes (NHC) [34,35,36], and reductants such as H2 [22,29], silanes [34,35], or boranes [37,38] are often employed. As a result of its atom efficiency and low cost, molecular hydrogen is used in CO2 hydrogenation more often. Consequently, this review will focus on the hydrogenation of CO2 and CO2 derivatives with H2 as the reductant for molecular catalysis, and especially on those systems that have employed transition-metal pincer complexes and metal–ligand cooperation over the past ten years.

2. Hydrogenation of CO2 or CO2 Derivatives

2.1. Ru-Based Catalysts

The first homogeneous CO2 hydrogenation to methanol was reported by Sasaki and coworkers in 1993 (Figure 1) [39]. In this report, methanol was observed to be an intermediate during the hydrogenation of CO2 to methane. The reaction was carried out using a 1:3 CO2/H2 mixture with an initial pressure of 80 atm, which was gradually heated to 240 °C. The catalytic system consisted of Ru3(CO)12 and KI, which produced methanol with a TON of 32 based on a ruthenium atom. However, the selectivity was poor, different components such as carbon monoxide, methane, and ethane were generated, and the ratio was hard to control. In 1995, the same group studied the detailed mechanisms of this reaction [40]. Under 240 °C, the trinuclear Ru precursor Ru3(CO)12 was transformed into the tetranuclear species [H2Ru4(CO)12]2− and [H3Ru4(CO)12]−, which were the catalytic active species.
This Ru3(CO)12–KI catalytic system demonstrated the feasibility of gaseous CO2 hydrogenation to methanol; however, some limitations such as high temperature and pressure, low turnover efficiency, and poor selectivity have to be considered. Therefore, developing CO2 hydrogenation under relatively mild conditions is urgent and of great significance, though great improvements have been made in the last ten years.
A cascade synthesis of methanol from CO2 was reported by the Sanford group in 2011 (Figure 2) [41]. Three different homogeneous catalysts were employed, including (PMe3)4Ru(Cl)(OAc) ([Ru]-1), Sc(OTf)3, and (PNN)Ru(CO)(H) ([Ru]-2). The three-step cascade reaction was realized in one reaction apparatus with an inner and outer vessel, without isolating the corresponding intermediates. The first and second steps were carried out in the inner vessel, and the hydrogenation of CO2 to formic acid with [Ru]-1 and Sc(OTf)3- catalyzed esterification to generate methyl formate. The reaction temperature was then increased to 135 °C, and the formate was transferred to the outer vessel automatically. The last step was a ruthenium–pincer complex; [Ru]-2 catalyzed hydrogenation of the formate ester to methanol. The inner–outer vessel transfer approach also enhanced the overall turnover number of the reaction significantly, with a turnover number of 21, which is eight times greater than the reaction carried out in one pot without an inner–outer vessel. The difference in catalytic efficiency was ascribed to the deactivation of the [Ru]-2 catalyst in the first and second steps. A similar protocol was also developed by Goldberg and coworkers in 2019, with a combination of Ru(H)2[P(CH2CH2PPh2)3]/Sc(OTf)3/Ir-(tBuPCP)(CO), and an overall TON of 428 was obtained [42].
The above approach, developed by the Sanford group, opened a new way for CO2 hydrogenation to methanol, and used structurally well-defined metal–ligand complexes as a catalyst. However, the combination of three different catalysts was complicated. A one-pot CO2 hydrogenation to methanol with a single catalyst was still desirable.
In 2012, Klankermayer, Leitner, and others presented a single ruthenium–phosphine complex that catalyzed the CO2 hydrogenation to methanol with high efficiency (Figure 3) [43]. Two catalytic systems, [Ru(acac)3]/Triphos and [(Triphos)Ru-(TMM)]2 ([Ru]-3), were able to promote formate hydrogenation to methanol independently. Acid additives were reported to have facilitated the generation of active ruthenium species from Ru precursors, thus, [Ru(acac)3]/Triphos/MSA/EtOH or [Ru]-3/HNTf2/EtOH were tested for direct CO2 hydrogenation in one pot, respectively. These two multifunctional catalysts worked on CO2 hydrogenation to formic acid, and hydrogenation of formate ester to methanol. The highest TONs were 135 with [Ru(acac)3] and 221 with [Ru]-3 under 140 °C, with 20 bar CO2 and 60 bar H2. These results clearly demonstrate that CO2 hydrogenation to methanol can be realized with a single catalyst in one pot, and that they exhibit comparable turnover numbers. The metal triphos system was further expanded by Klankermayer and others in 2020; a tridentate tdppcy (cis,cis-1,3,5-tris-(diphenylphosphino)cyclohexane) ligand was introduced as a structurally tailored ligand, in combination with a ruthenium precursor (cod)Ru(methallyl)2. The TON of CO2 hydrogenation was around 1100, and switching the solvent from THF to EtOH resulted in an unprecedented TON up to 2100 [44].
The Sanford group also disclosed another elegant approach of tandem CO2 hydrogenation to methanol in 2015, by using a single ruthenium catalyst [Ru]-4 (Figure 4) [45]. The key to success was introducing a CO2 capture process to generate carbamate intermediates, which underwent an additional hydrogenation step to produce methanol. In this report, dimethylamine was used as a CO2 capture reagent for dimethylammonium dimethylcarbamate (DMC), which was tested as a model substrate for methanol synthesis. The optimal conditions for DMC hydrogenation were found to be Ru-MACHO-BH ([Ru]-4), with K3PO4 as base under 50 bar H2 at 155 °C, and a TON of 19 was observed. Two possible mechanistic routes were suggested; the first was DMC hydrogenation to DMF, the other was a CO2 release from DMC and subsequent CO2 hydrogenation to formic acid, followed by an amidation reaction to generate DMF. Notably, this method was able to occur under 2.5 bar CO2, which was a greatly reduced pressure compared with others. Under the optimal conditions, CO2 (2.5 bar)/H2 (50 bar) was heated at 95 °C to form DMF, after which, the temperature was increased to 155 °C in order to enable the hydrogenation of DMF to methanol. The highest TON of 550 was achieved, and the conversion of CO2 was as high as 96%.
Meanwhile, Milstein and coworkers developed a combined CO2 capture/utilization protocol for methanol synthesis (Figure 5) [46]. They focused their efforts on the tandem reaction of CO2 to oxazolidinone and oxazolidinone hydrogenation, and a variety of amino alcohols were used as CO2 capture reagents. Valinol was found to be an ideal reagent to give higher yields and to shorten reaction time. The second step took place under Ru complex ([Ru]-5) and tBuOK, and methanol was obtained in 92% yield at 135 °C. In this work, they also showcased a one-pot reaction of CO2 capture/hydrogenation, where 1–3 bar of CO2 was transformed into oxazolidinone, and underwent the hydrogenation step without isolation, using a [Ru]-5 catalyst to obtain methanol in 53% yield.
In 2015, Ding and coworkers developed elegant examples of ruthenium catalyzed by N-formylation reactions of morpholine with H2/CO2 to generate formamide, which was able to undergo a second step of hydrogenation, which produced methanol with high efficiency (Figure 6) [47]. The highest turnover number achieved for the N-formylation reaction was 1,940,000, with a 0.00005 mol% [Ru]-6 catalyst. A one-pot, two-step CO2 hydrogenation to methanol was realized under 35 atm CO2 and 35 atm H2 at 120 °C in THF with a [Ru]-7 catalyst, and then 50 atm H2 at 160 °C to produce methanol in 36% yield. Notably, another Ru-catalyzed amine-assisted CO2 hydrogenation to methanol via formamide was later reported by Prakash [48].
These three methods developed by Sanford [45], Milstein [46], and Ding [47] provided a new way for CO2 capture and utilization (CCU) using a homogeneous ruthenium catalyst and an amine as a capture reagent. In early 2016, the Olah and Prakash group reported a similar strategy (Figure 7) [49]. CO2 was captured by pentaethylenehexamine (PEHA), which has low volatility, is a highly basic reagent, and showed good solubility in THF. A variety of ruthenium pincer catalysts were examined for this CO2 hydrogenation to methanol with PEHA as an amine and K3PO4 as an additive at 75 bar CO2/H2 (1:3). It was found that a significantly higher TON (1060) was obtained when employing Ru-MACHO-BH ([Ru]-4) as a catalyst, and keeping the reaction temperature at 155 °C consistently resulted in a 23% CO2 conversion to CH3OH. The catalyst was recycled and reused by a simple distillation of the mixture after the reaction; five more cycles were performed and a total TON of 1850 was reached. By increasing the CO2/H2 ratio from 1:3 to 1:9, methanol was formed in 65% NMR yield. Remarkably, direct CO2 capture from an ambient atmosphere, with PEHA and in situ hydrogenation, was achieved with up to 79% yield of methanol. This work was a significant step forward in terms of capturing CO2 from an ambient atmosphere in order to transform it into methanol.
Based on this work, the Prakash group further demonstrated a recyclable system for a CO2 capture/in situ hydrogenation system [50]. Methanol was generated in a biphasic 2-methyl THF/water system, which allows for regeneration and an easy separation of catalyst ([Ru]-4) and amine (PEHA). The catalytic system was recycled at least three times without significant loss of activity. CO2 from the air can also be transformed into methanol under this protocol.
Very recently, Prakash and coworkers introduced an alkaline, hydroxide-based system for CO2 capture and hydrogenation to methanol (Figure 8) [51]. Inexpensive inorganic bases such as NaOH, KOH, Ca(OH)2 were combined with ethylene glycol for the CO2 capture process. The in situ formed bicarbonate and formate were hydrogenated to methanol with a high yield; the inorganic base and ethylene glycol were regenerated, along with a methanol formation. The optimal catalytic system was found to be 0.5 mol% Ru-MaCHO-BH ([Ru]-4) under 70 bar H2 at 140 °C, the NMR yield of methanol reached up to 100%, and TON reached up to 480 with 0.1 mol%. In an integrated one-pot system, CO2 capture from ambient air and subsequent hydrogenation was realized to give a 100% NMR yield of methanol based on captured CO2 [52].
Another amine-assisted CO2 hydrogenation to methanol without a tridentate pincer-type ligand was reported by the Wass group in 2017 (Figure 9) [53]. Several new catalyst systems containing ruthenium and amine auxiliaries were investigated, and an exceptional TON of 8900 and TOF of 4500 h−1 were obtained by employing [RuCl2(Ph2PCH2CH2NHMe)2] ([Ru]-8) and diisopropylamine under 100 °C in 2 h.
In addition to small molecular organic amines and inorganic bases promoting CO2 hydrogenation, polymeric amines with a molecular weight ranging from 600 to 250,000 were investigated by Kayaki and their coworkers in 2019 [54]. Branched and linear poly(ethyleneimine)s (PEIs) were used as an amine source to be formylated, with the [Ru]-4 catalyst at 100 °C in THF. The formamide group that was attached to the polymer backbones were hydrogenated to recover the PEI and obtain methanol. A one-pot protocol was also used so that methanol could be easily separated from the mixture. The highest TON was 689 under 20 bar CO2 and 60 bar H2 at 150 °C.
From the examples above, it is evident that the direct CO2 hydrogenation to methanol is still demanding, and requires additional components, such as an organic amine or an inorganic base to promote the CO2 fixation and transformation, which is due to the high activation barrier of CO2. Cascade CO2 hydrogenation, or one-pot two-step hydrogenation, were subsequently investigated. Alternatively, employing CO2 derivatives as a substrate will significantly lower the activation barrier. One can use a different capture reagent to react with CO2 in order to make a number of CO2 derivatives, such as ureas, formates, carbamates, and carbonates; these derivatives then undergo hydrogenation to obtain methanol under transition metal catalysts.
Based on this conception, in 2011, the Milstein group hydrogenated various CO2 derivatives, including formates, carbamates, and carbonates (Figure 10) [55]. When applying dimethyl carbonate as a substrate, the hydrogenation was achieved with [Ru]-2 at 145 °C, 60 bar H2, resulting in a TOF of 2500 h−1, and with [Ru]-9 at 110 °C, 50 bar H2, a TOF of 314 h−1 was given. For methyl formate, [Ru]-2 showed the highest efficiency and lead to a TON of 4700 at 50 bar H2. A variety of substituted methyl carbamates as substrates were also able to be hydrogenated with [Ru]-2 under 10 bar H2 at 110 °C in THF, to deliver methanol in 94–98% yield.
Shortly afterwards, the hydrogenation of another important CO2 derivative, urea to methanol, was developed by Milstein and coworkers in the same year (Figure 11) [56]. [Ru]-2 was found to be the most effective for urea hydrogenation, with 2 mol% [Ru]-2 under 13.6 atm of H2, and urea derivatives were successfully transformed into methanol with a 46–94% yield. Preliminary mechanism studies showed that the reaction first proceeded with the hydrogenolysis of urea to obtain formamide and amine, then the formamide was hydrogenated to deliver methanol.
Another key intermediate in the chemical industry, ethylene carbonate, has been used for ethylene glycol synthesis as a key step in Shell’s omega process. In 2012, the Ding group reported that a PNP pincer ruthenium complex catalyzed the hydrogenation of ethylene carbonate and other cyclic carbonates (Figure 12) [57]. Specifically, Ru-MACHO ([Ru]-7) was found to be the most efficient, and a remarkable TON of 87,000 was achieved by employing only 0.001 mol% [Ru]-7 under 60 atm H2 at 140 °C, which produced methanol in 84% yield. A number of substituted cyclic carbonates were hydrogenated to methanol with a high yield.

2.2. Co-Based Catalysts

The aforementioned CO2 hydrogenation reactions were realized by ruthenium catalysts; in sharp contrast, the first-row transition metals are less developed for this transformation, and their high abundance, low cost, and low toxicity have also drawn much attention in the past five years [58,59,60].
Homogeneous CO2 hydrogenation to methanol by non-pincer-type base metal catalysis was first discovered by Beller and coworkers in 2017 (Figure 13) [61]. The active catalytic species contains Co(acac)3, triphos, and HNTf2, with 20 bar of CO2 and 60 bar of H2 at 140 °C, which achieved a TON of 31. The continued increase in H2 pressure to 70 bar, in addition to more Brønsted acid, improved the TON to 50, even with the lower temperature of 100 °C, which is one of the lowest temperatures to be recorded during a successful CO2 hydrogenation to methanol reaction.
Two years later, the Beller group modified the aforementioned molecularly defined cobalt catalyst system, and a TON of up to 125 was reached, which was more than two-fold greater than the TON reached in the previous reaction [62]. The key to the increase was the modification of triphos by adding substituents on the phenyl ring of the ligand. Triphos(p-tol) (Figure 13) was found to be the most efficient, which was attributed to the increased electron density on phosphorus, which made the cobalt center more electron-rich to activate H2. The additive, HNTf2, could also be removed by using Co(NTf2)2 as the catalyst instead of Co(acac)2.

2.3. Mn-Based Catalysts

Prakash and their colleagues discovered a Mn(I)–PNP pincer complex ([Mn]-1) that catalyzed a one-pot CO2 hydrogenation in 2017 (Figure 14) [63]. Similar to previous works catalyzed by ruthenium catalysts, this procedure also contained two steps: (1) N-formylation of amines by CO2 and H2; and (2) formamide hydrogenation to methanol under H2. In this work, a well-defined Mn–PNP catalyst ([Mn]-1) exhibited good activity in both steps. Morpholine and benzylamines were better amine sources than amines with a long alkyl chain, such as amylamine. The optimal conditions were found to be benzylamine under CO2 (30 bar) and H2 (30 bar) with 2 mol% [Mn]-1 catalyst at 110 °C in THF, which gave the N-formylation product, and then an increase in the total pressure to 80 bar with another 36 h to obtain methanol in 84% yield. However, the turnover number and turnover efficiency were not able to be compared with noble metal catalysts. This study demonstrates the feasibility of using base-metal pincer catalysts in homogeneous CO2 hydrogenation to methanol reactions.
In 2018, the groups of Milstein, Rueping, and Leitner independently reported the manganese-catalyzed hydrogenation of carbonates (Figure 15) [64,65,66]. In Milstein’s report, a PNN pincer manganese complex ([Mn]-2) was chosen as the optimal catalyst at under 30–50 bar of H2 at 110 °C, and a wide range of cyclic and acyclic carbonates were hydrogenated to methanol smoothly. Unsymmetrical acyclic carbonates were also suitable substrates for this transformation, although they achieved lower yields. Moreover, poly(propylene carbonate) from waste plastic was able to undergo depolymerization and hydrogenation to afford methanol in 59% yield [64]. Rueping and others mainly focused on cyclic carbonates as substrates, with a 0.25 mol% PNN pincer manganese complex([Mn]-3) and KOtBu as a base under 50 bar of H2 at 140 °C. Ethylene carbonate was hydrogenated to >99% ethylene glycol and 92% methanol, and poly(propylene carbonate) was hydrogenated with a catalyst [Mn]-3 to produce methanol in 84% yield, which also provides new avenues for plastic degradation. Density functional theory also confirmed the heterolytic cleavage of the dihydrogen molecule. [65] The Leitner group made use of the PNP pincer manganese complex [Mn]-1 and NaOtBu as a base under 30 bar H2, to obtain methanol in 41–94% yields from different cyclic carbonates. These reports demonstrate the feasibility of using non-noble metal pincer-type catalysts in the hydrogenation of carbonates, which, combined with CO2 fixation, could complete the CO2 hydrogenation to methanol [66].
The hydrogenation tendency of polar unsaturated chemical bonds depends on their polarity. Compared to carbonate, carbamates, and ureas, they are more challenging carbonyls in terms of hydrogenation. As common products that result from the reaction of CO2 with amines, or with amines and alcohols, the hydrogenation of carbamates and ureas is highly attractive, and will provide milder conditions for an indirect CO2 hydrogenation to methanol (Figure 16) [67]. The conditions for the hydrogenation of carbamates developed by Milstein were 2 mol% [Mn]-4, 3 mol%, tBuOK under 20 bar, H2 at 130 °C, which led to a series of aryl amine- and alkyl amine-derived carbamates to be hydrogenated in excellent yields. Under similar conditions, the most challenging urea derivatives were hydrogenated, obtaining both amines and methanol successfully.

2.4. Fe-Based Catalysts

Recently, Bernskoetter and coworkers represented a homogeneous Fe–PNP complex ([Fe]-1), which catalyzed CO2 hydrogenation to methanol in a two-step fashion (Figure 17) [68]. The reaction was attempted in a single batch and was found to be inefficient due to catalyst poisoning by CO2 during the formamide hydrogenation step. More specifically, the catalyst formed a stable iron(II) formate complex under the CO2 atmosphere and was not active anymore. By switching the conditions to a two-step procedure, a net TON of 590 was achieved, which was superior to the base-metal catalysts reported for this reaction.

3. Conclusions

As previously discussed, using CO2 and CO2 derivatives for hydrogenation is indicative of state-of-the-art research in this field (see Table 1 and Table 2). The homogeneous catalytic systems show benefits such as relatively mild conditions, low catalyst loadings and clear reaction mechanistic pathways. However, there is still a long way to go in order for these systems to be practical; the highest TONs achieved so far are around 9000 with ruthenium catalysis, and therefore the identification of new catalytic conditions, especially the development of non-noble catalysts with high catalytic efficiencies, is of great significance to this realm. Furthermore, the advantages shown by the homogeneous systems can be explored in more detail; given that the reaction is expected to occur under 100 °C or even lower temperatures, the catalyst loadings still have room to be decreased, which in turn may promote an increase in TONs. Finally, the direct capture of CO2 from the flue gas of coal combustion or from an ambient atmosphere is important to the carbon neutral cycle and has been seen in a few examples.
As is evident, regardless of whether a noble or non-noble metal is employed, the majority of the catalysts are tridentate pincer-type complexes, which demonstrates the superiority of pincer complexes for this transformation. In this regard, the modification of the pincer ligand to achieve higher TONs and TOFs is desired, so that the heteroatoms and substituents on the pincer ligand can be tuned based on their electronic and steric effects, or so that new tridentate coordinating skeletons can be developed for hydrogen activation in milder conditions. In terms of practical applications for CO2 capture and utilization, the recyclability and stability of the catalysts should be considered. Prakash’s biphasic system offers new routes for homogeneous catalysts to recycle in CO2 hydrogenation reactions [50]. We believe that as the community has focused more on reducing CO2 emissions and developing routes for CO2 utilization, the homogeneous CO2 hydrogenation will still be popular in the next decade or so, which will promote the realization of the carbon neutral cycle.

Author Contributions

W.X. and C.T. co-wrote the manuscript. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the National Natural Science Foundation of China (22001086), the Fundamental Research Funds for the Central Universities (HUST 2020kfyXJJS094), and the State Key Laboratory of Natural and Biomimetic Drugs, Peking University (K202011).

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Hydrogenation of CO2 using Ru3(CO)12.
Figure 1. Hydrogenation of CO2 using Ru3(CO)12.
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Figure 2. A tandem CO2 hydrogenation to methanol.
Figure 2. A tandem CO2 hydrogenation to methanol.
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Figure 3. CO2 hydrogenation to methanol by a single ruthenium phosphine complex.
Figure 3. CO2 hydrogenation to methanol by a single ruthenium phosphine complex.
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Figure 4. Amine assisted Ru-catalyzed hydrogenation of CO2.
Figure 4. Amine assisted Ru-catalyzed hydrogenation of CO2.
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Figure 5. Amino alcohol involved Ru-catalyzed hydrogenation of CO2.
Figure 5. Amino alcohol involved Ru-catalyzed hydrogenation of CO2.
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Figure 6. Ru-catalyzed hydrogenation of CO2 with morpholine as the amine source.
Figure 6. Ru-catalyzed hydrogenation of CO2 with morpholine as the amine source.
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Figure 7. PEHA as capture reagent for CO2 hydrogenation from air.
Figure 7. PEHA as capture reagent for CO2 hydrogenation from air.
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Figure 8. Inorganic base aided Ru-catalyzed hydrogenation of CO2.
Figure 8. Inorganic base aided Ru-catalyzed hydrogenation of CO2.
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Figure 9. Ruthenium and amine auxiliaries catalyzed hydrogenation of CO2.
Figure 9. Ruthenium and amine auxiliaries catalyzed hydrogenation of CO2.
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Figure 10. Hydrogenation of carbonates, carbamates and formates under Ru catalysis.
Figure 10. Hydrogenation of carbonates, carbamates and formates under Ru catalysis.
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Figure 11. Hydrogenation of urea using a ruthenium pincer catalyst.
Figure 11. Hydrogenation of urea using a ruthenium pincer catalyst.
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Figure 12. Ruthenium catalyzed hydrogenation of cyclic carbonates.
Figure 12. Ruthenium catalyzed hydrogenation of cyclic carbonates.
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Figure 13. The non-noble cobalt catalysts for direct CO2 hydrogenation.
Figure 13. The non-noble cobalt catalysts for direct CO2 hydrogenation.
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Figure 14. Manganese complexes catalyzed hydrogenation of CO2.
Figure 14. Manganese complexes catalyzed hydrogenation of CO2.
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Figure 15. Manganese catalysts developed for hydrogenation of carbonates.
Figure 15. Manganese catalysts developed for hydrogenation of carbonates.
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Figure 16. Mn-catalyzed hydrogenation of carbamates and ureas.
Figure 16. Mn-catalyzed hydrogenation of carbamates and ureas.
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Figure 17. Iron complexes catalyzed hydrogenation of CO2.
Figure 17. Iron complexes catalyzed hydrogenation of CO2.
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Table
Table 1. Molecular catalysts for CO2 hydrogenation to methanol.
Table 1. Molecular catalysts for CO2 hydrogenation to methanol.
CatalystAuxiliaryT
(°C)		P
(bar)		SolventTime
(h)		TONRef.
Ru3(CO)12KI24080NMP332[39]
[Ru]-1/[Ru]-3Sc(OTf)313540Dioxane1621[41]
Ru(H)2[P(CH2CH2PPh2)3]Sc(OTf)39062Dioxane
EtOH		16428[42]
Ir-(tBuPCP)(CO)
[Ru(acac)3]Triphos MSA14080EtOH24135[43]
[Ru]-3HNTf214080EtOH24221[43]
(cod)Ru(methallyl)2Tdppcy 120120THF201100[44]
Al(OTf)3
(cod)Ru(methallyl)2Tdppcy
Al(OTf)3		120120EtOH202100[44]
Ru-MACHO-BHDimethylamine95~15550THF54550[45]
[Ru]-5Valinol13560DMSO/THF19322[46]
[Ru]-6Morpholine12070THF961,940,000[47]
[Ru]-4PEHA15570THF401060[49]
[Ru]-4PEHA145702-MeTHF/H2O72520[50]
[Ru]-4Ethylene glycol14070THF72480[51]
[Ru]-8Amine10040Toluene208900[53]
[Ru]-4PEIs15080THF20689[54]
Co(acac)3Triphos HNTf214080THF/EtOH2431[61]
Co(acac)3Triphos11090THF/EtOH2450[61]
Co(NTf2)2Triphos(p-tol)10090THF/EtOH24125[62]
[Mn]-1Morpholine11080THF36840[63]
[Fe]-1Morpholine12069Dioxane16590[68]
Table
Table 2. Molecular catalysts for CO2 derivatives hydrogenation to methanol.
Table 2. Molecular catalysts for CO2 derivatives hydrogenation to methanol.
CatalystCO2 derivativesT
(°C)		P
(bar)		SolventTime
(h)		TONRef.
[Ru]-2Dimethyl carbonate14560Dioxane12500[55]
[Ru]-9Dimethyl carbonate11050THF144400[55]
[Ru]-2Methyl formate11050THF144700[55]
[Ru]-2Urea10013.6THF72202[56]
[Ru]-7Ethylene carbonate14060THF4887,000[57]
[Mn]-2Carbonates11050PhMe30390[64]
[Mn]-3Cyclic carbonates14050Dioxane16920[65]
[Mn]-1Cyclic carbonates12030THF40175[66]
[Mn]-4Carbamates, urea13020Toluene4850[67]
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Xue, W.; Tang, C. Hydrogenation of CO2 or CO2 Derivatives to Methanol under Molecular Catalysis: A Review. Energies 2022, 15, 2011. https://doi.org/10.3390/en15062011

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Xue W, Tang C. Hydrogenation of CO2 or CO2 Derivatives to Methanol under Molecular Catalysis: A Review. Energies. 2022; 15(6):2011. https://doi.org/10.3390/en15062011

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Xue, Wenxuan, and Conghui Tang. 2022. "Hydrogenation of CO2 or CO2 Derivatives to Methanol under Molecular Catalysis: A Review" Energies 15, no. 6: 2011. https://doi.org/10.3390/en15062011

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Xue, W., & Tang, C. (2022). Hydrogenation of CO2 or CO2 Derivatives to Methanol under Molecular Catalysis: A Review. Energies, 15(6), 2011. https://doi.org/10.3390/en15062011

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