Heterolytic H2 Research Articles

Efficient Hydrogenation of Methyl Palmitate to Hexadecanol over Cu/m-ZrO2 Catalysts: Synergistic Effect of Cu Species and Oxygen Vacancies

Renewable fatty esters are a class of clean feedstock to produce high value-added fatty alcohols. Herein, the hydrogenation of methyl palmitate (MP) to hexadecanol over defect-rich Cu/m-ZrO2 catalysts was studied systematically. A series of Cu/m-ZrO2 catalysts with different concentrations of oxygen vacancies (Ovs) were fabricated by adjusting the calcination temperature of m-ZrO2 support. Among the catalysts, Cu/m-ZrO2-350 displays the highest Cu dispersion, the largest number of Cu0 sites and surface Ov concentration, which exhibits the best activity for MP hydrogenation. It gives the MP conversion of 94.6% and hexadecanol yield of 93.3% at 300 °C and 7.5 MPa of H2. Spectroscopic characterizations and theoretical calculations revealed that the highly dispersed Cu species and abundant Ov sites synergistically boost the hydrogenation of MP. Specifically, the Cu0 sites facilitate the heterolysis of H2, and the Ov sites contribute to the dissociation of MP. The synergistic effect of Cu species and Ov sites disclosed in this work provides deep insights into the design of catalysts for efficient conversion of fatty esters to fatty alcohols.

Mononuclear Re sites on In2O3 catalyst for highly efficient CO2 hydrogenation to methanol

Catalytic hydrogenation of CO2 to methanol utilizing H2 derived from renewable energy represents one of the most prospective strategies for the sustainable recycle of carbon. Accordingly, the design and preparation of high-performance catalysts for methanol production has received broad attention. Herein, In2O3 supported mononuclear Re is reported to be an efficient catalyst for methanol production which exhibits methanol selectivity of up to 80% at CO2 conversion of 11.5%, providing a methanol STY of 758 mg·h−1·gcat−1 under 5 MPa and 300 °C, more than twice as that for pristine In2O3 (376 mg·h−1·gcat−1). The apparent reaction orders of CO2 and H2 were determined to be around 0 and 0.6 respectively, suggesting the activation of H2 is more crucial compared with CO2. In-depth characterizations reveal that the remarkably improved performance originates from the mononuclear Ren+ sites strongly interacting with In2O3, which significantly enhance the ability of H2 dissociation. Combined experimental and theoretical studies proved the feasibility of formate pathway over In2O3 supported mononuclear Re catalyst. The results of DFT calculations suggest that mononuclear Re site on In2O3(111) model plays a vital role on heterolytic H2 dissociation and intermediates transformation, boosting the formation of HCOO* and lowering the energy barriers of rate-determining step for methanol formation.

Cyclic Amide-Anchored NHC-Based Cp*Ir Catalysts for Bidirectional Hydrogenation–Dehydrogenation with CO2/HCO2H Couple

H2 storage in carbon dioxide (CO2) or bicarbonate (HCO3–) in the form of formic acid (HCO2H) or formate (HCO2–) and the reverse H2 liberation allows, in principle, to develop a rechargeable hydrogen carrier system along with a CO2-recycling mechanism. The key to such an alluring approach toward the realization of a carbon-neutral H2-based fuel option is the development of efficient bidirectional catalysts for CO2 (or HCO3–) hydrogenation and HCO2H (or HCO2–) dehydrogenation. With an aim toward (i) structurally robust catalysts under variable reaction conditions, (ii) metal–ligand bifunctionality-triggered heterolytic H2 splitting and H+/H– transfer during hydrogenation/dehydrogenation reactions, (iii) electron-rich catalytic metal center for facilitating hydride delivery, and (iv) water solubility of the catalysts via second coordination sphere interactions, herein, we applied a series of “cyclic amide–NHC” hybrid bidentate ligand-bound Cp*Ir(III) complexes (Ir-1–Ir-4) in bidirectional hydrogenation–dehydrogenation of CO2 (HCO3–)/HCO2H (HCO2–) couple in water as a “green” solvent without the use of organic additives/solvents. Notably, with the catalyst Ir-1, hydrogenation of CO2 achieving a turnover number (TON) of 16 680 at 60 °C in 6 h and dehydrogenation of formic acid with a turnover frequency (TOF5min) of 70 674 h–1 at 80 °C can be efficiently carried out. Key control and mechanistic studies emphasized the following aspects of the current system: (i) pH of the solution played a crucial role in controlling the rate of hydrogenation/dehydrogenation reactions, (ii) H2 was cleaved readily by the catalyst to form the iridium hydride intermediate, which could react with CO2 to furnish the formate product, (iii) pH (acid/base)-switchable on-demand formic acid dehydrogenation was devised, and (iv) the liberated H2 and CO2 gas from the Ir-1-catalyzed formic acid dehydrogenation reaction were reutilized in secondary reactions in a tandem fashion, signifying the suitability of the system to demonstrate the utility of formic acid as a typical H2/CO2 storage liquid, as it is advocated for.

Environment Molecules Boost the Chemoselective Hydrogenation of Nitroarenes on Cobalt Single-Atom Catalysts

Single-atom catalysts (SACs) have received intensive interest due to the utmost atom utilization and unique catalytic behaviors. However, the catalytic mechanism of SACs is still unclear, especially in the reactions involving multiple substrates. Here, we report that the environment moieties, protic solvents, and external bases can boost the cobalt SAC-catalyzed chemoselective hydrogenation of nitroarenes. Systematical studies clearly reveal that the heterolysis of H2 between the metal center and the neighboring coordination sphere is significantly facilitated via the H-shuttling or deprotonation route with the participation of protic solvents or bases. Besides, the derived single-atom cobalt hydride species are confirmed to be specifically reactive for the reduction of the nitro group, affording a superior activity and general chemoselectivity in the hydrogenation of various nitroarenes. Finally, a catalytic cycle is proposed to elucidate the activity origin of SACs, which sheds light on the rational design of SACs by engineering the microenvironment of active sites.

Suppression of Methane Formation by Regulating the Hydrogenation Capability of Metal Oxides in Alkylation of Benzene with Syngas

In alkylation of benzene with syngas, the inevitable converting of syngas into methane, as a side reaction, severely hinders the effective utilization of carbon monoxide (CO), which is a major challenge to circumvent. In this work, a series of bifunctional catalysts composed of various zinc/zirconium/cerium metal oxides and HZSM-5 zeolite were prepared in order to explore the key factor of methane formation and achieve high CO efficiency. The results show that methane selectivity mostly depends on the hydrogenation capability, which can be regulated via altering compositions of the metal oxides. Combined with density functional theory calculation and characterizations, it is found that the hydrogenation capability of metal oxide is related to the energy barrier of hydrogen (H2) heterolysis. Compared to CeO2, the addition of zinc favors a lower energy barrier and a stronger ability toward H2 heterolysis, while the addition of zirconium makes this ability weaker. Too strong heterolysis capability consequently leads to excessive hydrogenation, which further causes high methane selectivity; however, too weak heterolysis capability will lead to low catalytic activity. Hence, proper hydrogenation capability is an important element for low methane selectivity and high catalytic activity. Our findings reveal the formation mechanism of methane and provide a new strategy to reduce methane content.

Hydride Generation on the Cu-Doped CeO2(111) Surface and Its Role in CO2 Hydrogenation Reactions

Ceria-based catalysts exhibit great activity in catalyzing selective hydrogenation of CO2 to methanol. However, the underlying mechanism of this reaction, especially the generation of active H species, remains unclear. In this work, we performed extensive density functional theory calculations corrected by on-site Coulomb interaction (DFT + U) to investigate the H2 dissociation and the reaction between the active H species and CO2 on the pristine and Cu-doped CeO2(111) (denoted as Cu/CeO2(111)) surfaces. Our calculations evidenced that the heterolytic H2 dissociation for hydride generation can more readily occur on the Cu/CeO2(111) surface than on the pristine CeO2(111) surface. We also found that the Cu dopant can facilitate the formation of surface oxygen vacancies, further promoting the generation of hydride species. Moreover, the adsorption of CO2 and the hydrogenation of CO2 to HCOO* can be greatly promoted on the Cu/CeO2(111) surface with hydride species, which can lead to the high activity and selectivity toward CO2 hydrogenation to methanol.

Advanced heterolytic H2 adsorption of K-added Ru/MgO catalysts for accelerating hydrogen storage into aromatic benzyltoluenes

Herein, we report a highly active K-added Ru/MgO catalyst for hydrogen storage into aromatic benzyltoluenes at low temperatures to advance liquid organic hydrogen carrier technology. The hydrogenation activity of Ru/K/MgO catalysts exhibits a volcano-shaped dependence on the K content at the maximum with 0.02 wt%. This is in good agreement with the strength and capacity of H2 adsorption derived from basicity, despite a gradual decrease in the textural property and the corresponding increase in the Ru particle size with increasing the K content. Density functional theory calculations show that heterolytic hydrogen adsorption properties (strength and polarization) are facilitated up to a specific density of K on the Ru–MgO interface and excessive K suppresses heterolytic H2 adsorption by direct interaction between K and hydrogen, assuring the hydrogenation activity and H2 adsorption capability of Ru/K/MgO catalysts. Hence, the Ru/K/MgO catalyst, when K is added in an optimal amount, is highly effective to accelerate hydrogen storage kinetics at low temperatures owing to the enhanced heterolytic H2 adsorption.

Heterolytic Dissociation of H2 in Heterogeneous Catalysis

H2 dissociation plays crucial roles in catalytic hydrogenation reactions and hydrogen storage. Metal-nanoparticle-based heterogeneous catalysts often dissociate H2 via a homolytic pathway. The heterolytic H2 dissociation pathway was identified in several heterogeneous catalytic systems, including single-atom catalysts, metal–support interfaces, and bulk metal oxides/sulfides/nitrides/phosphides. The active site structures of these heterogeneous catalysts resemble homogeneous catalysts where the metal centers (Lewis acids) are coordinated with O/S/N/P atoms (Lewis bases). These Lewis acid–base pairs dissociate H2 molecules heterolytically into proton–hydride pairs, which favor the hydrogenation of polar functional groups in unsaturated hydrocarbons. In this review, we summarize the common structural features of heterogeneous, homogeneous, and enzyme catalysts in the heterolytic dissociation of H2. The active sites, Lewis acid–base pairs, are discussed throughout this review. The energy barriers and kinetic contributions of heterolytic and homolytic H2 dissociation pathways in heterogeneous catalytic systems are discussed. The spectroscopic evidence of the heterolytic H2 dissociation pathways is critically reviewed.

Frustrated Lewis Pair in Zeolite Cages for Alkane Activations

AbstractThe frustrated Lewis pair (FLP) concept in homogeneous catalysis was extended to heterogeneous catalysis via the supramolecular system of FLP between deprotonated zeolite framework oxygens and confined carbocations in methanol‐to‐olefin (MTO) reactions. In this FLP, the polymethylbenzenium (PMB+) functioned as the Lewis acid to accept an electron pair, and the deprotonated framework oxygen site acted as the Lewis base to donate an electron pair. This FLP theoretically demonstrated the ability to undergo H2 heterolysis and alkanes dehydrogenation, and this was further confirmed by gas chromatography–mass spectrometer (GC‐MS) catalytic experiments inside FLP‐containing chabazite zeolites. All these findings not only bring new recognition to the carbocation chemistry in zeolite cages but also put forward a new reaction pathway as one part of MTO reactions.

Frustrated Lewis Pair in Zeolite Cages for Alkane Activations.

The frustrated Lewis pair (FLP) concept in homogeneous catalysis was extended to heterogeneous catalysis via the supramolecular system of FLP between deprotonated zeolite framework oxygens and confined carbocations in methanol-to-olefin (MTO) reactions. In this FLP, the polymethylbenzenium (PMB+ ) functioned as the Lewis acid to accept an electron pair, and the deprotonated framework oxygen site acted as the Lewis base to donate an electron pair. This FLP theoretically demonstrated the ability to undergo H2 heterolysis and alkanes dehydrogenation, and this was further confirmed by gas chromatography-mass spectrometer (GC-MS) catalytic experiments inside FLP-containing chabazite zeolites. All these findings not only bring new recognition to the carbocation chemistry in zeolite cages but also put forward a new reaction pathway as one part of MTO reactions.

Rationally designed novel multifunctional poly(ionic liquid)s for ultra-selective valorization of Yiwu lignite to monocyclic aromatic compounds

The selective valorization of lignite into monocyclic aromatic compounds (MCACs) is extremely challenging due to its chemical inertness and geometrical complexity. The catalytic efficiency of lignite valorization into MCACs over solid acids is significantly reduced by the presence of H2O resulting in the leaching and deactivation of acid sites. Moreover, the precise fabrication of poly(ionic liquid)s (PILs) containing Brønsted-Lewis acidic sites, well-defined mesoporous channels, and hydrophobicity remains a considerable challenge. Herein, a novel hydrophobic mesoporous PILs with strong Brønsted-Lewis acid sites was successfully constructed by a synergistic regulation-induced copolymerization strategy for the first time. More exotically, the resultant multifunctional PILs was found to be ultra-efficient catalyst for the selective valorization of Yiwu lignite (YL) into MCACs with 23.5 wt.% yield, which is the highest yield reported for lignite catalytic valorization by precious metal-free catalysts under mild conditions. The co-catalysis mechanisms of YL valorization into MCACs over the multifunctional PILs are confirmed by characterization methods and the synergistic catalysis mechanisms are as follows: (1) hydrophobic micro-environment: the hydrophobic micro-environment created by hydrophobic block is favorable for preventing the leaching and deactivation of Brønsted-Lewis acid sites; (2) mesoporous confinement: the well-defined mesoporous channels strengthen the adsorption amount of H2 and increases the concentration of substrates; (3) synergistic catalytic active hydrogen transfer of Brønsted-Lewis acid sites: Brønsted-Lewis acid sites synergistically promote H2 heterolysis to the mobile H+ and immobile H−, and then the resulting mobile H+ transfers to oxygen atom in lignite to produce MCACs via C–O bond cleavage. This work not only sheds some light on developing the multifunctional solid acid catalysts, but also demonstrates an efficient catalyzing method to produce lignite-based MCACs by co-catalysis of Brønsted-Lewis acidic sites, mesoporous confinement, and hydrophobic micro-environment.

Au3+ Species Boost the Catalytic Performance of Au/ZnO for the Semi-hydrogenation of Acetylene.

Au nanoparticles have garnered remarkable attention in the chemoselective hydrogenation due to their extraordinary selectivity. However, the activity is far from satisfactory. Knowledge of the structure-performance relationship is a key prerequisite for rational designing of highly efficient Au-based hydrogenation catalysts. Herein, diverse Au sites were created through engineering their interactions with supports, specifically via adjusting the support morphology, that is, flower-like ZnO (ZnO-F) and disc-like ZnO (ZnO-D), and the catalyst pretreatment atmosphere, that is, 10 vol % O2/Ar and 10 vol % H2/Ar (denoted as -O and -H, respectively). The four samples of Au/ZnO were characterized by various techniques and evaluated in the semi-hydrogenation of acetylene. The transmission electron microscopy results indicated that the Au particle sizes are almost similar for our Au/ZnO catalysts. The charge states of Au species demonstrated by X-ray photoelectron spectroscopy, diffuse reflectance infrared Fourier transform spectroscopy with CO as the probe molecule, and simulation based on density functional theory, however, are greatly dependent on the ZnO shape and pretreatment atmosphere, that is, the percentage of Au3+ reduces following the order of Au/ZnO-F-O > Au/ZnO-F-H > Au/ZnO-D-O > Au/ZnO-D-H. The testing results showed that the Au/ZnO-F-O catalyst containing maximum of Au3+ possesses the optimal activity with 1.8 × 10-2 s-1 of specific activity at 200 °C, around 16.5-fold of that for Au/ZnO-D-H. More interestingly, the specific rate at 200 °C and the average conversion/selectivity in the entire operating temperature range are well correlated with the redox states of the Au species, indicating that Au3+ sites are more active for acetylene hydrogenation. A plausible explanation is that the Au3+ species not only facilitate acetylene adsorption via electrostatic interactions but also favor the heterolysis of H2 via constructing frustrated Lewis pairs with O.

Terminal Mo Carbide and Carbyne Reactivity: H2 Cleavage, B–C Bond Activation, and C–C Coupling

Transition-metal carbides have been posited as intermediates in the upgrading of C1 feedstocks, including the Fischer–Tropsch synthesis of higher olefins from CO and H2. Still, molecular examples remain rare, and their reactivity is poorly understood. In a molecular platform supported by a flexible terphenyl diphosphine ligand, the important C–C coupling step was previously demonstrated on the reaction of Mo carbide and carbene species with CO. Methylidyne and methylidene complexes were accessible on sequential treatment of the carbide with sources of H+ and H–, surrogates for heterolyzed H2. Herein, we demonstrate that the terminal carbide complex is also capable of directly activating H2, yielding a P–C-bonded ylide complex. The carbide moiety also inserts into the B–C bond of triphenylborane, yielding an unusual example of a borylcarbene with a direct Mo–B contact. C–C coupling from the terminal methylidyne, a potential intermediate formed on heterolytic H2 cleavage and H+ transfer to the carbide, yields a Mo ketenyl complex, thus giving information for the first time on the relative rates of coupling from carbide, carbyne, and carbene species in the same molecular platform.

Promoted H2 splitting on ZnO by pre-adsorbed formic acid

Activation of hydrogen on oxide surface is of both practical and fundamental importance for broad fields of syngas-based chemical industries as well as clean energy utilizations. Previously we found ZnO can catalyze the heterolysis of H2, which only occurs at low temperatures (<50 K) due to the quick kinetics of the reversed reaction. Here in this work, we demonstrate that the heterolytic adsorption of H2 can be significantly enhanced by pre-adsorbed formic acid molecules. The formic acid molecule can readily dissociate on the ZnO(10−10) surface and form strongly-bound formate and hydroxyl species, which induce charge accumulation at the nearest-neighbored Zn ions and initiate the heterolysis of H2 molecules as well as the growth of H-chain structure along the [0001] direction on the ZnO surface. With such stabilization effect of formic acid, the dissociative adsorption of H2 can be significantly raised up to 200 K under UHV conditions. These results strongly emphasize the important impact of co-adsorbates on the surface reaction process. Particularly, our findings may shed new light on the atomistic mechanisms for a variety of H2-involved reactions on oxide catalysts.

Lewis Acidity of Carbon in Activated Carbonyl Group vs. B(C6 F5 )3 for Metal-Free Catalysis of Hydrogenation of Carbonyl Compounds.

In this work, using DFT calculations, we investigated Lewis acidities of carbon (in activated carbonyl group) in comparison to the B(C6 F5 )3 in combination with dioxane as the Lewis base (LB) for metal-free catalysis of heterolytic H2 splitting and hydrogenation of carbonyl compounds. We found that in case of carbon as the Lewis acid (LA) the reaction is controlled by frontier molecular orbital interactions between the H2 and LA-LB fragments at shorter distances. The steric effects can be reduced by electrophilic substitutions on the carbonyl carbon. Synergic combination between stronger orbital interactions and reduced steric effects can lower the barrier of the H2 splitting below 10 kcal/mol. With the B(C6 F5 )3 , the H2 splitting is controlled by electrostatic interactions, which cause to form an early transition state. An advantage of employing Lewis acidity of the activated carbonyl carbon for hydrogenation is that the hydride-type attack and hydrogenation of the C=O bond occur in a single step throughout H2 splitting. Hence, stronger Lewis acidity of the C(C=O) reinforces hydrogenation without prohibition of the hydride delivery.

Pd-phosphite and Pd-Pd tetrazolyl paddlewheel complexes as catalysts for CO2 hydrogenation

Hydrogenation of CO2 to products such as formic acid has gained attention in recent times as one of the means of addressing the environmental impact of CO2 as well as H2 storage. Herein we report the synthesis of a tetrazolyl ligands bearing PS bonds which undergoes a PS heterolysis during complexation with [PdCl2(MeCN)2] resulting in the accidental synthesis of palladium-tetrazolyl paddlewheel complexes and corresponding palladium phosphite complexes with P-OH bonds. This PS heterolysis depends on the nature of substituents present on the phosphorus atom present in the ligand since P^N bidentate coordination is possible. These palladium complexes were then evaluated as precatalysts for CO2 hydrogenation where TONs of around 564 were observed. Presence of proton responsive P-OH groups contribute towards catalytic activity as a result of an assisted H2 heterolysis. A plausible mechanism of action indicates bicarbonate coordinates to Pd followed by elimination of water which subsequently results in the generation of formate.

Role of the Cu-ZrO2 interface in the hydrogenation of levulinic acid to γ-valerolactone

Here we exquisitely fabricated Cu/ZrO2-dp catalysts with plentiful Cu-ZrO2 interfaces by depositing amorphous ZrO2 onto Cu nanoparticles for the hydrogenation of levulinic acid (LA) to γ-valerolactone (GVL). With the created plentiful Cu-ZrO2 interfaces, the optimal catalyst 3Cu/ZrO2-dp exhibited exceptional catalytic performance under mild reaction conditions, and achieved the highest GVL mass productivity of 266.0 mmol GVL·h−1·g−1 Cu, which was 12.5 and 2.3 times of Cu/ZrO2 catalysts with equivalent Cu loadings prepared by traditional impregnation (3Cu/ZrO2-im) or co-precipitation (3Cu/ZrO2-cp). As far as we know, this GVL mass productivity stood at the highest level compared with those obtained using non-noble metal catalysts under similar reaction conditions. By systematic investigation with multiple characterizations, density functional theory (DFT) calculations, and kinetic studies, it was found that interfacial active centers were created at Cu-ZrO2 interfaces, which contained oxygen vacancies (Ov), negatively charged Cuδ− and partially reduced Zr3+. The Ov favored the adsorption and activation of LA via its ketone group, while negatively charged Cuδ− was able to enhance heterolysis of H2, which resulted in the formation of H+-Cuδ− and Zr3+-H− active species via hydrogen spillover. Also, plentiful acid sites, which derived from coordinatively unsaturated and defective Zr species, generated at Cu-ZrO2 interfaces. With the cooperation of interfacial active centers (Cuδ−-Ov-Zr3+) and acid sites, the fabricated 3Cu/ZrO2-dp with plentiful Cu-ZrO2 interfaces achieved excellent catalytic performance for the hydrogenation of LA to GVL. Hence, the synergistic catalysis of Cu-ZrO2 interfaces provided an effective strategy for designing catalysts with a satisfactory performance for the hydrogenation of LA, which also can be expanded to other hydrodeoxygenation reactions.

Remarkably fast low-temperature hydrogen storage into aromatic benzyltoluenes over MgO-supported Ru nanoparticles with homolytic and heterolytic H2 adsorption

Hydrogen storage into aromatic compounds under mild conditions is a stringent issue in liquid organic hydrogen carrier (LOHC) systems. Herein, we report a highly active Ru/MgO catalyst in the hydrogenation of monobenzyltoluene and dibenzyltoluene at low temperatures. When MgO with basic surface oxygen was employed as a support, Ru/MgO showed a faster H2 storage rate and superior kinetic parameters than the other supported Ru catalysts. The better catalytic performance of Ru/MgO was explained by the results of characterization and control experiments. Ru/MgO could adsorb the large amounts of monobenzyltoluene and hydrogen with higher strength. Particularly, homolytic and heterolytic hydrogen adsorption modes were identified in Ru/MgO, unlike Ru/Al2O3 showing homolytic H2 adsorption. Density functional theory calculations confirmed heterolytic H2 dissociation near the Ru-MgO interface, which assured the hydrogenation efficiency of Ru/MgO. Consequently, Ru/MgO is highly recommended for fast hydrogen storage into aromatic LOHC compounds at low temperatures.

Heterolytic cleavage of dihydrogen (HCD) in metal nanoparticle catalysis

Supports, ligands and additives can promote heterolytic H2splitting by a cooperative mechanism with metal nanoparticles.

Energetics for Proton Reduction in FeFe Hydrogenase.

The energetics for proton reduction in FeFe-hydrogenase has been reinvestigated by theoretical modeling, in light of recent experiments. Two different mechanisms have been considered. In the first one, the bridging hydride position was blocked by the enzyme, which is the mechanism that has been supported by a recent spectroscopic study by Cramer et al. A major difficulty in the present study to agree with experimental energetics was to find the right position for the added proton in the first reduction step. It was eventually found that the best position was as a terminal hydride on the distal iron, which has not been suggested in any of the recent, experimentally based mechanisms. The lowest transition state was surprisingly found to be a bond formation between a proton on a cysteine and the terminal hydride. This type of TS is similar to the one for heterolytic H2 cleavage in NiFe hydrogenase. The second mechanism investigated here is not supported by the present calculations or the recent experiments by Cramer et al., but was still studied as an interesting comparison. In that mechanism, the formation of the bridging hydride was allowed. The H–H formation barrier is only 3.6 kcal/mol higher than for the first mechanism, but there are severe problems concerning the motion of the protons.