Guerbet Reaction Research Articles

The Mechanism of Guerbet Reaction by Metal Ligand Cooperation Catalyst Mn-PCP.

The Guerbet reaction is important for the synthesis of longer-chain monoalcohols like isobutanol through catalytic transfer hydrogenation from short-chain methanol and ethanol. However, the mechanism becomes complicated, especially considering the variations in the different metal-ligand cooperation (MLC) catalysts used. In order to further understand the Guerbet reaction, DFT studies were performed to figure out the detailed mechanism initiated by the unique Mn-PCP MLC Catalyst. Our results suggest that even with the assistance of the carbanion site of the PCP ligand, the direct substitution mechanism is less favored than the condensation-reduction mechanism. The key step of the reaction is the final reduction of the carbonyl, in which the 1,4-reduction of the unsaturated aldehyde is prior to the 3,4-reduction or 1,2-reduction due to the stronger interaction between the catalyst and the substrate. It is found that the production of isobutanol is preferred over n-butanol because of the lower total free energy barrier and lower relative free energy of the product. Finally, by changing the electronic effect of the carbanion site of the catalyst, we found that the relation between the electronic effect and the highest free energy span was not monotonous and a point with optimal electronic effect exists numerically.

Copper-supported hydroxyapatite catalysts for bio-isobutanol synthesis from sustainable short-chain alcohols

Developing high-performance catalysts for the direct production of isobutanol through the Guerbet reaction is a challenge today, due to the potential of isobutanol as fuel. The present study describes the synthesis and characterization of Cu-supported hydroxyapatite catalysts. The synthesis of isobutanol has been investigated through batch experiments and in a continuous-flow reactor operating in a gas-solid phase. This study used ethanol and methanol as the primary feedstock for the synthesis process. It was found that Cu supported hydroxyapatite catalysts are capable of catalyzing the formation of isobutanol from CH3OH and C2H5OH at temperatures (300–500 ºC) and 1 atm in a continuous flow reactor. Notably, the highest selectivity to isobutanol was observed at 400 ºC. The use of Cu10/HAP 1.65 catalyst in a batch reactor greatly enhances the selectivity of iBuOH in the liquid phase, increasing it from 18 % (vapor-phase) to 65 %. The data showed that the reaction of ethanol/methanol was limited by the 1-propanol-to-isobutanol reaction, which is a secondary reaction of the key intermediate. A reaction scheme has been proposed.

Coal-Based Branched Vicinal Diol Ethoxylates Versus Guerbet Alcohol Ethoxylates: Role of Tertiary Hydroxyl Groups.

Branched surfactants exhibit a lower surface tension, excellent low defoaming performance, and better wetting ability compared with linear surfactants, making them promising for applications in industrial cleaning. In this study, 2-hexyl-1-decene (C8 olefin dimer), obtained from the dimerization of 1-octene, was used as the hydrophobe to synthesize branched nonionic surfactants via hydroxylation and ethoxylation. The hydroxylation of the C8 olefin dimer to synthesize 2-hexyldecane-1,2-diol (C8 BD) using H2O2 and HCOOH was investigated systematically. Under the optimal reaction conditions (H2O2/C8 olefin dimer molar ratio: 1.5, HCOOH/C8 olefin dimer molar ratio: 4.0, reaction time: 10 h, reaction temperature: 50 °C), the conversion of the C8 olefin dimer and selectivity toward C8 BD were found to reach 99.96 and 79.89%, respectively. Further, branched nonionic surfactants (C8 BDEn) were synthesized via ethoxylation of C8 BD with ethylene oxide and characterized using FTIR, LCMS, 1H NMR, and 13C NMR techniques. The presence of a tertiary hydroxyl group in C8 BD increases the reactivity of the primary hydroxyl group, leading to a narrower range of products and lower residual substrate content. The physicochemical properties and surface properties of C8 BDEn with different degrees of ethoxylation at various concentrations were investigated and compared with those of commercially available Guerbet alcohol polyoxyethylene ethers (C8 GAEO9 and C6 GAEO9). The results show that, compared with C8 GAEO9 and C6 GAEO9, C8 BDE6 displayed a higher surface activity with a lower equilibrium surface tension (27.14 mN·m-1), superior wettability with a smaller contact angle (39.2°), better emulsification performance with a longer emulsification time of 548 s, and excellent foaming properties (initial foam volume of 11.6 mL). This strategy of utilizing coal-based α-olefins for the synthesis of branched nonionic surfactants presents a route to prepare value-added fine chemicals from coal-based resources.

Synthesis of narrow-distributed Guerbet dodecyl alcohol ethoxylates on Ca(OAc)2 catalyst and theirs surface properties

Despite significant breakthroughs that have been made in the application performance of traditionally distributed Guerbet alcohol ethoxylates (GAEO), research on the characteristics of their narrow distribution products remains relatively limited. Herein, a series of hydrophilic and hydrophobic nonionic surfactants have been synthesized via the ethoxylation of 2-butyl-octanol with ethylene oxide. Ca(OAc)2 was employed as a narrow distribution (NRD) catalyst, while KOH served as a broad distribution (BRD) catalyst. The synthesized products were characterized using Fourier-transform infrared (FT-IR), 1H and 13C nuclear magnetic resonance (1H, 13C NMR) spectroscopy. In addition, a comprehensive assessment of the physicochemical and surface properties of the NRD GAEO surfactants was conducted through systematic investigations. The results were then compared to conventionally distributed GAEO surfactants. The study has demonstrated that NRD GAEO surfactants exhibited lower critical micelle concentration (CMC) values, which have the higher efficiency of the surfactant application. In addition, the NRD products have superior wettability performance, reduced foaming capacity in foam properties, and improved rapid defoaming capacities. In regarding their other properties, they closely mirrored the performance of BRD products. These findings will contribute to a comprehensive understanding of the properties of NRD ethoxylates and have substantial practical implications in optimizing personal care and household cleaning products.

Application of experimental design in the synthesis of 1-butanol from ethanol on mixed oxide catalysts at different mg/al molar ratios

1-Butanol, primarily sourced from petroleum, is currently being considered as a viable alternative to gasoline. Various pathways for synthesizing 1-butanol from renewable raw materials have been explored, with a particular focus on the Guerbet reactions. This study aimed to investigate the synthesis of 1-butanol from ethanol using Mg/Al mixed oxides derived from layered double hydroxides with Mg/Al molar ratios of 3:1, 5:1, 8:1, and 10:1. The catalysts were analyzed through X-ray diffraction, X-ray fluorescence, N2 adsorption, scanning electron microscope and energy-dispersive X-ray spectroscopy. Catalytic reaction tests were performed according to a Central Composite Rotatable Design (CCRD) with three replicates at the central point (CP). Among the catalysts tested at CP, the mixed oxides with Mg/Al molar ratios of 3:1 and 5:1 exhibited the most promising results, displaying similar ethanol conversion, and 1-butanol selectivity. This similarity underscores their well-balanced combination of both acidic and basic sites. However, further assessment through a deactivation test revealed that the mixed oxides with Mg/Al molar ratio of 5:1 held the most potential for long-term 1-butanol synthesis from ethanol, thanks to its superior stability and sustained activity. Using this catalyst, other reaction tests were carried out following the CCRD, varying N2 feed flowrate, ethanol molar content in feed flowrate, and temperature. The results indicated that an increase in ethanol conversion directly correlated with higher temperatures, while the 1-butanol selectivity depended on both the N2 feed flowrate, ethanol molar content in feed flowrate, highlighting the importance of a well-balanced combination of both acidic and basic sites.

Effects of block and random copolymerization on the properties of fatty alcohol polyoxypropylene‐polyoxyethylene ether sulfates for enhanced oil recovery

AbstractEnhanced oil recovery (EOR) by surfactant flooding has been attractive. Among the surfactants developed alcohol polyoxypropylene (PO)‐polyoxyethylene (EO) ether sulfates (APES), also named extended surfactants, have been proved efficient. However, the arrangement of the PO and EO chains as well as the hydrocarbon structure in the molecules may significantly affect their performances. In this paper three APES, C18−16PO25EO10SO4Na (I), C18−16(PO/EO)25+10SO4Na (II), and C16GA(PO/EO)25+10SO4Na (III) were prepared and investigated, in which APES (I) and APES (II) were designed to have same PO and EO numbers but block and randomly copolymerized respectively with linear C18−16 fatty alcohols as starting agent, whereas the APES (III) was derived from double chain C16 Guerbet alcohol (C16GA) with PO and EO randomly copolymerized. The results show that the block copolymerized APES (I) gives much better brine solubility and counterion tolerance than the randomly copolymerized APES (II) and (III). Although all APES synthesized are highly surface‐active and can reduce Daqing crude oil/simulated brine interfacial tension (IFT) to ultralow by mixing with more hydrophobic surfactants in presence or absence of alkali, the APES (III) gives the lowest IFT due to with double hydrocarbon chains. In addition, it is found that for APES (I) gelation occurs in neutralization process and the corresponding nonionic intermediate is highly viscous, whereas the randomly copolymerized two intermediates are liquid‐like with low viscosity, which may be feasible to apply SO3/air falling film sulfation. This study provided useful information for arrangement of embedded nonionic moiety and hydrocarbon structure in designing extended surfactants for EOR.

A DFT study on the formation mechanism of side product 1,2-propanediol in the hydrogenation of dimethyl oxalate over copper catalyst

The costly separation of 1,2-propanediol (1,2-PDO), an unavoidable byproduct in the hydrogenation of dimethyl oxalate (DMO), significantly hampers the economic viability of coal-to-ethylene glycol (EG) technology. To address this challenge, the formation mechanism of the side product 1,2-PDO on the Cu(111) and Cu2O(111) surfaces during DMO hydrogenation was investigated, which focused on the active sites of copper catalyst and the dominant pathway through density functional theory calculation. The thermodynamics of each elementary step and the adsorption behavior of various species involved in the reaction network along with the local density of states and charge density difference were systematically analyzed. The results indicate that 1,2-PDO is generated more favorably on the Cu2O(111) surface than that on the Cu(111) surface, owing to the Lewis acid-base pairs, i.e. Cu+ us and O– suf sites, present on the Cu2O(111) surface, which strengthens the binding of reactants, products, and reaction intermediates to the substrate. EG reacts primarily with methanol (MeOH) to form 1,2-PDO through Guerbet alcohol condensation reaction through three consecutive steps: alcohol dehydrogenation, aldol condensation, and unsaturated aldehyde hydrogenation. The O– suf sites promote the dehydrogenation of alcohols into aldehydes, the generation of enolates during aldol condensation and the hydrogenation of unsaturated aldehydes, while the Cu+ us sites are responsible for the C–C coupling reaction. These findings may shed light on the mechanism of 1,2-PDO formation over Cu catalyst and provide fundamental knowledge for the development of more efficient catalysts and process optimization.

Dilute Alloy Catalysts for the Synthesis of Isobutanol via the Guerbet Route: A Comprehensive Study

This study investigates the potential of bimetallic alloy catalysts, specifically Ni99Pt1, Cu99Ni1, Cu99Fe1, Fe99Pt1, and Fe99Pd1, for the synthesis of isobutanol via the Guerbet route. The catalysts were synthesized with a doping of 1 at% Ni, Fe, Pt, and Pd in the base metals Fe, Cu, and Ni. The catalytic properties of these bimetallic alloy catalysts were explored for their potential for promoting the Guerbet reaction. The study aims to test the usability of the theoretically predicted d-band values in the synthesized bimetallic catalysts, which were prepared by means of incipient wetness impregnation, and shows that doping amounts smaller than 1 at% already significantly improved the catalytic activity of the base metals. In particular, the doping of nickel with platinum yielded an effective catalyst for the synthesis of isobutanol via the Guerbet pathway. The Ni99Pt1/C catalyst from the presented experiments had the highest Space Time Yield (STY) and is, therefore, also a promising catalyst for the hydrogen-borrowing reactions class.

H2 Pressure Dependence in the Homogeneously Catalyzed Guerbet Reaction of Ethanol to Butanol using Mn(I) and Ru(II)

AbstractThe homogeneously catalyzed Guerbet upgrading of ethanol to butanol can be modulated by the application of partial H2 pressure before the reaction. Ruthenium(II) PNP pincer complexes with methylene‐extended acridine‐based ligands and singly‐chelated ruthenium(II) complexes with PN ligands perform best at very low catalyst loadings and upon the application of catalyst‐specific pre‐reaction partial H2 pressure. The combination of ruthenium(II) PNP pincer complexes and manganese(I) pincer complexes with MACHO‐type PNP ligands allows for even lower ruthenium loadings while retaining low base load and consumption, thus enabling high space‐time yields, catalyst productivity and recyclability. Mechanistically, Mn is proposed to stabilize coordinatively unsaturated Ru species and allow coordination of ethoxide to Ru instead of attacking the electrophilic ligand backbone based on NMR spectroscopy and X‐ray crystallography.

Surface Redox Reaction for the Synthesis of NiPt Catalysts for the Upgrading of Renewable Ethanol/Methanol Mixtures

Mixtures of ethanol and methanol being synthesized from CO2 and green H2 can serve as sustainable base chemicals for a number of chemical processes. Amongst these processes, the catalytically supported synthesis of CO2-neutral C4 to C10 alcohols is of increasing importance as, e.g., iso-butanol can be used as a drop-in fuel or after dehydration to produce iso-butene as a feedstock for the synthesis of plastics. 2-ethyl-hexanol can be further refined into solvents, tensides, or monomers. In this respect, NiPt alloys on an activated carbon support were found to be active and stable catalysts for the synthesis of iso-butanol following the Guerbet reaction scheme. In this study, two different routes are applied to the synthesis of these NiPt catalysts: a more conventional one based on the impregnation of Ni and Pt salts and an advanced path with a surface redox reaction between elemental Ni on the support and Pt ions in a polar solution. The experimental evaluation shows that the Pt particles from the surface redox reaction being exposed on the Ni particles are more active than those on the impregnated catalysts due to their high surface energy. Their specific space-time yields are 10–20 times higher.

On the selective transformation of ethanol over Mg- and/or La-containing mixed oxides catalysts

MgO-La2O3 catalysts with different Mg/La molar ratio were synthesized by a precipitation method and a subsequent calcination and then characterized by x-ray diffraction, N2 adsorption-desorption isotherms (at −196 °C), CO2- and NH3-thermoprogrammed desorption and x-ray photoelectronic spectroscopy. The coexistence of acid and basic sites promoted the selective transformation of ethanol into valuable products. Thus, MgO catalyst promoted the Guerbet reaction obtaining n-butanol as product while the incorporation of La2O3 in the catalytic system improved the ethanol conversion notably, obtaining ethylene as the main product due to a dehydration reaction. The highest ethylene yield was obtained for the catalyst with a Mg/La molar ratio of 1.

Metal-Catalyzed Enantioconvergent Transformations.

Enantioconvergent catalysis has expanded asymmetric synthesis to new methodologies able to convert racemic compounds into a single enantiomer. This review covers recent advances in transition-metal-catalyzed transformations, such as radical-based cross-coupling of racemic alkyl electrophiles with nucleophiles or racemic alkylmetals with electrophiles and reductive cross-coupling of two electrophiles mainly under Ni/bis(oxazoline) catalysis. C-H functionalization of racemic electrophiles or nucleophiles can be performed in an enantioconvergent manner. Hydroalkylation of alkenes, allenes, and acetylenes is an alternative to cross-coupling reactions. Hydrogen autotransfer has been applied to amination of racemic alcohols and C-C bond forming reactions (Guerbet reaction). Other metal-catalyzed reactions involve addition of racemic allylic systems to carbonyl compounds, propargylation of alcohols and phenols, amination of racemic 3-bromooxindoles, allenylation of carbonyl compounds with racemic allenolates or propargyl bromides, and hydroxylation of racemic 1,3-dicarbonyl compounds.

Pt/CeCrO2−x: A multifunctional catalyst for tandem catalysis of lignocellulose hydro-liquefaction and Guerbet reaction

Catalytic transfer hydrogenation (CTH) of lignocellulose liquefaction using renewable ethanol presents a promising alternative to conventional hydrogenation techniques. However, the development of efficient and stable multifunctional catalysts remains a significant challenge. Herein, a Pt/Ce9Cr1O2−x catalyst was developed to facilitate the CTH reactions and achieve nearly complete utilization of lignocellulose. Isotope labeling experiments were conducted to elucidate the mechanism of CTH. The introduction of Cr3+ increased the density of surface oxygen vacancies, consequently enhancing the hydrodeoxygenation ability and metal anchoring. The solid solution structure promotes the electrons transfer from Cr atoms to Pt atoms, forming electron-rich Ptδ+ species that promote ethanol dehydrogenation and lignocellulose hydrodeoxygenation. Furthermore, Pt/Ce9Cr1O2−x catalyst provided optimal acid-base sites and hydroxyl groups, thereby improving ethanol upgrading efficiency and wettability.

Molecular catalysed Guerbet reaction: Moving to the larger and the Greener through LCA and scale up simulation approaches

Efficient homologation of bio-ethanol can be performed under mild conditions exploiting a catalytic system based of a ruthenium molecular catalyst, benzoquinone and a base as a co-catalysts. Conversions and selectivity can be tuned by changing the head space of the reactor. On this basis, preliminary design of a full-scale process has been developed based on the experimental results. The process has been characterized with specific energy and performance indicators per unit of produced butanol, which have been used as input for the life cycle assessment analysis. The LCA was applied as scientific methodology to address potential burdens of the baseline configuration, 3X catalytic cycles, electricity from Italian grid and heat from natural gas. Hotspots were identified according to a multi-impact approach method (ReCiPe 2016). The usage of dedicated lignocellulosic biomass as a source of EtOH, the synthesis of the catalytic system and the energy requirements were addressed as the major contributors. Thus, further sensitivity scenarios were created. The best configuration was identified in the use of waste biomasses and in an integrated cogeneration unit. In addition, enabling the recovery of the catalytic system up to five cycles to scenario shows a reduction in the impacts higher than 50% for the categories of global warming potential, −41% for the mineral resource scarcity and around −16% for the fine particulate matter formation. The results were also confirmed by an uncertainty analysis with the Monte Carlo method, which demonstrated the major environmental sustainability of the Guerbet route compared to the oxo synthesis from propylene.

The utilization of bio-ethanol for production of 1-butanol catalysed by Mg–Al mixed metal oxides enhanced by Cu or Co

The Guerbet reaction is a possible way for transformation of ethanol to 1-butanol (important for many kinds of industries), which consists of four steps: dehydrogenation, aldol condensation, dehydration, and hydrogenation. Due to the elimination of possible side-reactions, the selective catalysis is required to favour production of 1-butanol at temperature below 350 °C. The main aim of this work was the ethanol transformation via heterogeneous catalysis using active Mg–Al mixed oxides with copper or cobalt carried out in the microflow reactor in the reaction temperature interval 280–350 °C. The novelty lies in the statistical analysis of results from characterization of catalyst structure and surface with catalysis results providing more sophisticated perspective on the ethanol valorization. The series of Mg–Al catalysts containing copper showed an overall higher conversion of ethanol and selectivity to butanol compared to the series containing cobalt. Major difference of catalytic activity was at low reaction temperatures and at a lower copper content in the Mg–Al matrix, which is significant from the point of view of environmentally clean processes. A multi-step mechanism of the Guerbet reaction involving an aldol condensation was verified for both tested catalysts series and reaction conditions.Graphic abstractValorization of bio-ethanol to 1-butanol by Mg-Al mixed metal oxides enhanced by Cu or Co

Ethanol Upgrading to n-Butanol Using Transition-Metal-Incorporated Poly(triazine)imide Frameworks.

The upgrading of ethanol to n-butanol was performed using a molecular catalyst integrated into a carbon nitride support, one of the first examples of a supported molecular catalyst performing the Guerbet process. Initial studies using crystalline poly(triazine)imide (PTI) with lithium or transition-metal cations imbedded in the support together with a base as the catalyst system did not produce any significant amounts of n-butanol. However, when using the catalyst material formed by treatment of PTI-LiCl with [(Cp*)IrCl2]2 (Cp* = pentamethylcyclopentadienyl) along with sodium hydroxide, a 59% selectivity for butanol (13% yield) was obtained at 145 °C. This PTI-(Cp*)Ir material exhibited distinct UV-vis absorption features and powder X-ray diffractions which differ from those of the parent PTI-LiCl and [(Cp*)IrCl2]2. The PTI-(Cp*)Ir material was found to have a metal loading of 27% iridium per empirical unit of the framework. Along with the formation of n-butanol from the Guerbet reaction, the presence of higher chain alcohols was also observed.

RuPHOX-Ru catalyzed asymmetric Guerbet reaction of secondary alcohols with primary alcohols

An efficient RuPHOX-Ru catalyzed asymmetric Guerbet reaction of secondary alcohols with a broad scope of primary alcohols has been developed, affording long-chain chiral alcohols in high yields with excellent enantioselectivities (up to 99% yield and 96% ee). In particular, alkyl primary alcohols could also react to give the desired products with good results. The protocol could be performed on a gram-scale, providing an alternative pathway with minimal environmental impact and high atom-economy for the preparation of chiral alcohol products.

Boosting the C–C Coupling of Bioethanol to Higher Alcohols by Inhibiting Aqueous Phase Reforming Reaction

A suitable catalyst with an exact match of the acidity/basicity, ingenious geometric structure, and proper electronic environment is vital to the Guerbet reaction for the synthesis of C6+ higher alcohols. Herein, we synthesized a series of Ni/MgAlO and NiSn/MgAlO catalysts based on hydrotalcite laminate for one-pot upgrading of aqueous bioethanol to C6+ higher alcohols. Through the introduction of Sn to form NiSn alloys, the Ni–Ni interactions were attenuated and the cleavage of C–C bonds was suppressed with a yield of 21.9% of C6+ higher alcohols at 250 °C. Meanwhile, by precisely adjusting the ratio of the metal cation Mg2+ to Al3+, the acidity and basicity of the catalysts were optimized and Ni/(4-Mg)AlO with an optimal ratio of 4 presented a considerable C6+ alcohol yield (25.2%) at even 220 °C, as well as an impressive ethanol conversion of 67.7%. Such an excellent activity achieved at lower temperature was attributed to highly dispersed Ni metal with a Ni(Mg)O solid solution structure, leading to the enormously inhibited aqueous ethanol reforming process and significant increased yield of higher alcohols. The results reveal that the electronic structure and coordination environment of metal sites as well as higher moderately basic and strong acid sites were demonstrated to be beneficial for carbon-chain extension of higher alcohols and ethanol selectivity for dehydrogenation/hydrogenation. Moreover, the number of metal, acid, and base sites should be balanced to inhibit side reactions.

The selective ethanol Guerbet condensation over alkali metal-doped sepiolite

In this study, alkali metal-doped sepiolite based catalysts were developed for the up-grading of ethanol through the Guerbet reaction mechanism. The catalysts were prepared by wetness impregnation with aqueous solution containing alkali metal precursors salts over calcined sepiolite, and finally activated at 500ºC in air. The catalysts have been characterized by XRD, SEM-EDS, TGA-DSC, N2-adsorption. In addition, TPD-CO2 and TPD-NH3 were used to determine the acid and basic characteristics of catalysts. The catalysts were tested into a quartz continuous gas-flow reaction system working at atmospheric pressure to investigate the effects of metal loading (wt%), the nature of the alkali metals and of the main reaction parameters (e.g., reaction temperature, weight hourly space velocity), on the production of n-butanol starting from ethanol. The highest yield (18%) was obtained at 400 ºC with the catalyst consisting in sepiolite, calcined at 500ºC, impregnated with 7 wt% of Cs. This study proves that ad hoc modified natural sepiolites with alkali metals are effective catalysts for the Guerbet upgrading of ethanol.

Advances in the homogeneous catalyzed alcohols homologation: The mild side of the Guerbet reaction. A mini-review

The Guerbet reaction would be an optimal solution for bio-ethanol refinery by homologation and it substantially requires the development of active and highly selective catalysts. Although heterogeneous catalysis remains more desirable, homogeneous Guerbet catalytic systems are nowadays acknowledged as challenging proposals for high selectivity and conversion under mild reaction conditions. Several molecular bi-functional catalysts, based on ruthenium, iridium and manganese complexes bearing different ligands such as phosphine, bidentate bisphosphine (P-P) and phosphamine (P-N), pincer (N-N-N), (P-N-P) and cyclopentadienone, are active in the co-presence of a base, which is essential in all the cases for the condensation reaction. This mini-review is aimed to describe the state of the art, with incremental steps forward to understand the role of the catalytic systems and the reaction conditions on conversion and selectivity. While showing different efficiency, mutable behavior by minimal change in the ligand scaffold, variation in water tolerance and only one example of recyclability, metal catalysts are found to be similar in terms of proposed mechanisms. The influence of additives and capability to maintain activity in fermentation broth and bio-ethanol from wine value chain to second generation bio-fuel are also discussed. Although not yet completely rationalized, experimental and theoretical mechanistic investigations pave the way for the design of more stable and efficient catalysts toward improved applicability.