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Key role of surface hydroxyl groups in CO activation during Fischer-Tropsch synthesis G. T. Kasun Kalhara Gunasooriya, Alexander P. van Bavel, Herman P. C. E. Kuipers, and Mark Saeys ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b00634 • Publication Date (Web): 03 May 2016 Downloaded from http://pubs.acs.org on May 7, 2016
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Key role of surface hydroxyl groups in C-O activation during Fischer-Tropsch synthesis G.T. Kasun Kalhara Gunasooriya,† Alexander P. van Bavel,§ Herman P.C.E. Kuipers,§ Mark Saeys*,† † §
Laboratory for Chemical Technology, Ghent University, Technologiepark 914, 9052 Gent, Belgium Shell Technology Centre Amsterdam, P.O. Box 38000, 1030 BN Amsterdam, Netherlands
ABSTRACT: C-O activation is a crucial step in Fischer–Tropsch synthesis (FTS). Several pathways have been proposed to activate CO, namely direct C-O dissociation, activation via hydrogenation, and activation by insertion into growing chains. Invariably, very high barriers are calculated for both direct C-O dissociation and for hydrogenation at the O-atom in CO* and RCO*, while hydrogenation at the C-atom leads to oxygenates. We demonstrate that surface hydroxyl groups open a new pathway for CO* and RCO* activation via proton transfer to the O-atom. In combination with the CO insertion mechanism, the calculated rate for this new pathway is consistent with the selectivity in FTS, and agrees with the kinetic effect of water. Hydroxyl group formation from O* is sufficiently fast to be quasi-equilibrated, and much faster than CO2 formation. The role of surface hydroxyl groups as hydrogenating species is likely general, and involved in several oxygenate transformation reactions. KEYWORDS: catalysis, cobalt, Fischer-Tropsch synthesis, mechanism, C-O activation, Hydroxyl species, proton transfer Fischer-Tropsch synthesis (FTS) is an attractive technology to convert natural gas, CO2 and waste biomass to clean transportation fuels and chemicals.1 FTS transforms synthesis gas, a mixture of CO and H2, to long-chain alkanes, alkenes, oxygenates and water.1-3 Supported cobalt catalysts are often preferred due to their high activity, selectivity towards long chain hydrocarbons, low CO2 selectivity and low water-gas shift activity. Although FTS was discovered almost a century ago and has been industrialized for many years, the reaction mechanism4-6 and the kinetic role of water7-9 remain debated. Two general classes of reaction mechanisms have been proposed; the carbide mechanism10 and the CO insertion mechanism.11 In the carbide mechanism, originally proposed by Fischer and Tropsch10, C-O activation occurs via direct CO dissociation. The carbide species undergo hydrogenation to CHx groups, the monomers for the growth of alkyl chains. In the carbide mechanism, C−O scission occurs before C−C coupling and the CHx coverage needs to be sufficiently high to favor chain growth over chain termination by hydrogenation.12 This requires a sufficiently fast CO dissociation rate. Direct CO dissociation is a difficult step on Co terraces and barriers above 220 kJ/mol are consistently reported.4, 13-15 Significantly lower barriers are calculated on stepped Co surfaces.16 However, the FTS turn over frequency is independent of the particle size above 10 nm,17-19 and therefore the kinetically relevant steps occur on terraces. Moreover, due to their low thermodynamic stability and high affinity for reaction intermediates20 and carbon,21 corner and step sites might not be available under FTS conditions. Therefore, alternative C-O activation pathways have been proposed. In the hydrogen-assisted CO activation mechanism 4, 11, 15, 22-23 (Figure 1a), CO* is hydrogenated to formyl (HCO*) and to formaldehyde (H2CO*) or hydroxyl methylene (HCOH*) before C-O scission. Hydrogenation weakens the C–O bond and is calculated using DFTvdW-DF24 to lower the C-O scission barrier from 235 kJ/mol in CO to 58 kJ/mol in HCO* and about 40 kJ/mol in H2CO* and HCOH*. The involvement of hydrogen in the rate-limiting
initial C-O activation is furthermore consistent with the positive hydrogen reaction order.4, 25 However, CO* hydrogenation is highly endothermic by 120 kJ/mol and the effective C-O activation energy for the hydrogen-assisted pathway, 178 kJ/mol, remains too high to be consistent with the observed reaction kinetics. Moreover, the formation of formaldehyde (H2CO) is kinetically preferred over C-O activation and formaldehyde is predicted incorrectly as the primary product in the hydrogen-assisted pathway. In the CO insertion mechanism, RCHx* groups couple with CO* before C−O scission.6, 11, 15 For a realistic CO coverage of 1/3 ML, an effective barrier of 110 kJ/mol was calculated for a pathway via RCH2CO*, leading to a turnover frequency (TOF) on Co terraces consistent with experimental kinetic data.6 In the dominant mechanism, RC* groups undergo hydrogenation and coupling with CO* to form RCH2CO*. C-O scission in RCH2CO* is the rate-limiting step in the catalytic cycle. Experimental evidence for the kinetically fast C−O scission in RCH2CO species on Co terraces at FTS conditions came from temperature programmed X-ray photoelectron spectroscopy studies of ethanol decomposition.26 Chemical transient27 and isotope labeling28 studies provide further support for the CO insertion mechanism. However, also in the CO insertion mechanism, the formation of the chain initiating CH* species requires sufficiently fast CO activation. It is important to recognize that CO activation is an important step in both the carbide and the CO insertion mechanism. For the CO insertion mechanism, the selectivity between oxygenates and hydrocarbons is determined by the reactions of the RCH2CO* intermediate (Figure 1b).6 In the kinetically preferred pathway, hydrogenation occurs at the C-atom, leading to aldehyde (oxygenate) products, as in the hydrogen-assisted CO activation pathway. The barrier for C-O scission and further chain growth is indeed slightly higher than the barrier for hydrogenation at the C-atom (Figure 1b). The selectivity between these two pathways is however affected by the CO coverage and by the reaction conditions, as discussed by Zhuo et al.6
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ACS Catalysis As the primary oxygen-containing product in FTS over Co, water is present in varying quantities and plays a kinetic role. Water, whether indigenous or co-fed, has been shown to increase29-32, decrease33-36 or not affect37-39 TOFs, and to increase the C5+ selectivity.29-32, 40 For Ru catalysts, the kinetic effect of water has been attributed to a water-mediated proton-transfer mechanism via an H3O+ transition state,8 although other pathways can be envisioned. CO* and RCO* are preferentially hydrogenated at the C-atom. The barriers for hydrogenation at the O-atom are about 50 kJ/mol higher, even though COH* and RCOH* are more stable than HCO* and RCH2O* (Figure 1a and 1b). The high barrier for hydrogenation at the O-atom obviously results from the distance that the H-atom needs to travel from the cobalt surface to the O-atom. The energy penalty to partially break the Co-H bond is 263 kJ/mol and the penalty to tilt the CO molecule is 49 kJ/mol (Table S5, Supporting Information). In Table 1, we therefore evaluate several alternative mechanisms to hydrogenate the O-atom in CO*. In the proton-shuttling pathway, a water molecule bridges the gap between the surface H* and the O-atom in CO*. This significantly lowers the hydrogenation activation barrier from 197 kJ/mol to 82 kJ/mol. The involvement of a water molecule from the gas phase however leads to a high entropy penalty and the free energy barrier for this three-body proton shuttling pathway, 153 kJ/mol, is higher than the free energy barrier for hydrogenation at the C-atom, 137 kJ/mol. A second pathway considers hydrogenation by a weakly adsorbed water molecule. This pathway also has a low barrier of 70 kJ/mol, but the free energy barrier, 139 kJ/mol, remains high due to the entropy lost by the gas phase water molecule. In a third pathway, CO* is hydrogenated by a surface hydroxyl group. The activation barrier for this pathway, 96 kJ/mol, is higher than for water-assisted pathways. The free energy barrier for this pathway, using adsorbed CO, gas phase H2O and H2 at FTS conditions as the reference state, is only 94 kJ/mol because of the very small entropy penalty. This pathway is hence kinetically much faster than direct hydrogenation at the C-atom and reverses the selectivity of this CO hydrogenation step. The very small entropy penalty for this pathway results from the favorable reaction Gibbs free energy for the formation of surface hydroxyl groups from water, H2O(g) + * ↔ OH* + ½ H2(g) (Table S6, Figure S1 Supporting Information).
Figure 1. Electronic energy profile calculated with the DFTvdW-DF functional24 for (a) the direct CO dissociation and the hydrogen-assisted CO activation pathway; (b) the CO insertion pathway. Effective activation barriers are indicated and selected transition state structures are shown. The dominant CO insertion pathway starts from CH* and CO* to form CH3CO*, which undergoes a rate-limiting C-O scission step. The energy profile therefore starts from CH3CO*. Adsorption energies, entropies, Gibbs free adsorption energies for the most stable reaction intermediates, activation barriers, Gibbs free energy barriers and geometries for all the reactions are presented in Supporting Information (Table S1, S2, S3, S4).
Table 1. Energy barriers and Gibbs free energy barriers for CO hydrogenation by various species (H*, H2O*, H2O—H**, OH*). CO* + H*
CO* + H* + H2O(g)
CO* + H2O(g) + * ↔
COH* + H2O(g) + *
COH* + OH*
COH* + O*
CO* + H* ↔
↔
COH* + *
CO* + OH* ↔
↔
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HCO* + *
Transition state
Activation energy# 197 82 70 96 143 Ef (kJ/mol) Gibbs free energy# 189 153 139 94 137 ∆Gf (kJ/mol) # Energy barriers correspond to the energy of the TS relative to adsorbed CO*, H*, and OH* or gas phase H2O, as indicated by the reaction. Gibbs free energy barriers are relative to adsorbed CO*, gas phase H2 at 9 bar and gas phase H2O at 7 bar under FTS conditions (500 K, 20 bar, 60% conversion), and include the effects of pressure, composition, and temperature.24
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The new OH-mediated pathway dramatically changes the reaction path. In the first step (Figure 2a), a surface hydroxyl group hydrogenates (protonates) CO*, forming COH*. The barrier for this pathway is 47 kJ/mol lower than the barrier for hydrogenation at the C-atom. COH* is hydrogenated to hydroxyl methylene (HCOH*), which undergoes C-O scission with a low barrier. This pathway avoids the formation of formaldehyde (H2CO); the effective barrier for the second hydrogenation at the C-atom is 20 kJ/mol higher than the barriers along this pathway. In the alternative formyl pathway, CO is first hydrogenated at the C-atom, and subsequently at the Oatom by a hydroxyl species. Also this pathway avoids formaldehyde. To compare the relative importance of both pathways, Gibbs free energies were computed for the transition states (Figure 3a). While the electronic energy profile (Figure 2a) suggests that both pathways are competitive, accounting for entropy losses and pressure effects to reach the respective transition states suggests that the dominant reaction path goes via HCO* and HCOH*. The highest free energy along the reaction path, 159 kJ/mol, corresponds to the HC-OH scission transition state. Free energy barriers for hydrogenation at the C-atom by hydroxyl groups are very high and do not contribute to the reaction mechanism (Table S3). The introduction of hydroxyl groups into the hydrogenation mechanism also impact the selectivity of the CO insertion mechanism (Figure 2b) by facilitating the hydrogenation of the O-atom in RCH2CO* with a low barrier of 44 kJ/mol, followed by RCH2C-OH scission with a low effective barrier of 56 kJ/mol. The selectivity of the CO insertion mechanism is however complex, and detailed microkinetic simulations, accounting for coverage and reaction conditions, would be required41 to compare the rates of the various pathways. It should be indeed appreciated that FTS produces a wide range of products, and several pathways must be kinetically competitive. To evaluate the effect of the reaction conditions and coverage, we computed Gibbs free energies for several transition states along the CO insertion pathway, considering a realistic CO coverage of 1/3 ML (Figure 3b).6, 42 For those conditions, the transition states for RCH2C-OH and RCH2C-O scission have comparable Gibbs free energies of 134 and 131 kJ/mol, while the transition state for hydrogenation at the Catom in RCH2CO* and leading to aldehydes, has a higher Gibbs free energy of 139 kJ/mol. The Gibbs free energy barriers in Figure 3 can be used to compute TOFs for FTS reaction conditions (Table S7, Supporting Information). For CO activation via the hydroxylassisted pathway, the calculated TOF is 2x10-3 s-1. This value is lower than the CO TOF for the CO insertion mechanism assuming a total RC* coverage of 0.1 ML, 4x10-2 s-1, and also lower than the experimental TOF of about 1x10-2 s-1. 17-19 This suggests that the chain initiating CH* species are formed via hydroxyl-assisted CO activation and grow via CO insertion. The ratio between these TOFs for hydroxyl-assisted CO activation and CO insertion leads to a chain growth probability, α, of 0.95, close to typical experimental values on Co.43 The proposed OH-assisted pathway produces one surface O-atom per turn over and regenerates the catalytic OH* group. These surface O-atoms must be quickly converted to hydroxyl groups. The calculations suggested two competing mechanisms (Table 2). Direct hydrogenation of O* has a barrier of 121 kJ/mol and a TOF of 25 s-1 under FTS conditions.
Figure 2. Electronic energy profile calculated with the DFT-vdWDF functional24 for the hydrogen and hydroxyl assisted (a) CO activation pathway and (b) CO insertion pathway. Effective activation barriers are indicated and selected transition state structures are shown.
Figure 3. Gibbs free energies for selected transition states along the (a) CO activation pathway and (b) for the CO insertion pathway. The reference state for the CO activation pathway is adsorbed CO, gas phase H2 and gas phase H2O under FTS conditions (500 K, 20 bar, 60% conversion).24 The reference state for the CO insertion pathway is CH* in the presence of co-adsorbed CO, CO* at a coverage of 1/3 ML, gas phase H2 and gas phase H2O under FTS conditions. Gibbs free energies account for entropy losses and pressure effects to reach the transition state and provide a more accurate picture of the relative kinetics of the pathways. The dominant pathways for CO activation and for CO insertion are indicated in green and connected by arrows. High CO coverage TS geometries are presented in Supporting Information (Table S8).
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Table 2. Energy barriers and Gibbs free energy barriers for various OH* formation and O*/OH* removal pathways. O* + H2(g) + *
O* + H*
OH* + H*
OH* + OH*
O* + CO*
OH* + CO*
↔
↔
↔
↔
↔
↔
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H* + OH*
OH* + *
H2O* + *
H2O* + O*
CO2* + *
COOH* + *
Transition state
69 121 147 75 138 176 Activation energy# (kJ/mol) Gibbs free ener110 115 126 78 136 169 gy## (kJ/mol) # Energy barriers correspond to the energy of the TS relative to the adsorbed species indicated by the reaction. ## Gibbs free energy barriers are relative to adsorbed O*/OH* (bold and underlined), adsorbed CO*, gas phase H2 at 9 bar, gas phase H2O at 7 bar under FTS conditions (500 K, 20 bar, 60% conversion), and include effects of pressure, composition, and temperature.25
Alternatively, heterolytic H2 dissociation, O*_* + H2(g) OH* + H*, has a lower activation barrier of 69 kJ/mol, but the loss of entropy in the TS leads to a free energy barrier of 110 kJ/mol and a TOF of 1600 s-1. Reaction of O* with water, O* + H2O(g) + * 2 OH* is potentially even faster, with a free energy barrier of 66 kJ/mol. These reactions are sufficiently fast to assume that O* and OH* are quasi-equilibrated under FTS conditions. The conversion of O* to OH* is also three orders of magnitude faster than the reaction of O* with CO* to form CO2, in agreement with the low CO2 selectivity in FTS over Co catalysts. Also the coupling between CO* and OH* to form COOH* is slow on Co, with a free energy barrier of 169 kJ/mol (Table 2), and 8 orders of magnitude slower than the OH-assisted hydrogenation of CO* and RCO*. The calculated activation energy barrier for OH* hydrogenation, 147 kJ/mol, can be compared with the experimentally observed barrier of 129 kJ/mol for water formation from O*.44 The total O* and OH* concentration is determined by the kinetics of O*/OH* formation via C-O(H) scission and dissociative water adsorption, and the consumption of O*/OH* via hydrogenation and CO oxidation, and microkinetic simulations would be required to determine the coverage of O*/OH* groups. Experimentally, a considerable O*/OH* coverage has been reported under FTS conditions on cobalt. SSITKA studies by den Breejen et al.18 determined an OHx (i.e., O* and OH*) coverage of 0.05 – 0.1 ML and surface science studies44-47 reported a substantial O* coverage after dosing O2. Moreover, previous experimental48-49 and theoretical50 studies observed surface oxidation of supported Co particles and slow O*/OH* removal rates51-52, suggesting a considerable O*/OH* coverage under FTS conditions. In conclusion, surface hydroxyl groups are found to be potent hydrogenating species during FTS. Direct C-O dissociation and direct hydrogenation at the O-atom in CO* and RCO* have high barriers, while hydrogenation at the C-atom leads to oxygenates. Hydroxyl groups have the correct geometry to hydrogenate the O-atom in both CO* and RCO* with low activation barriers. The resulting TOFs support a hydroxylassisted CO activation step to form chain-initiating CH* species, followed by chain growth via partially hydroxyl-assisted CO insertion. The regeneration of hydroxyl groups is fast enough to be considered quasi-equilibrated. This new role of
surface OH groups as hydrogenating species is likely general and involved in several oxygenate transformation reactions.
ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website at DOI: Details of computational methods, adsorption energies, entropies, and Gibbs free adsorption energies for reaction intermediates under Fischer-Tropsch conditions and their configurations, energy barriers (Ef), Gibbs free energy barriers (∆Gf) under Fischer-Tropsch conditions for the hydrogenation (H*, H2O—H**, OH* as hydrogenating species) and C-O scission reactions and their configurations, Bond analysis for the CO* hydrogenation at O-atom, rate equations for the CO activation and CO insertion mechanisms, Gibbs free energy of formation for O*/OH* as function of temperature and ratio between pH2O and pH2 are presented.
AUTHOR INFORMATION Corresponding Author * E-mail:
[email protected] Homepage: http://www.lct.ugent.be/
Notes The authors declare no competing financial interest.
ACKNOWLEDGMENT This work was supported by Shell Global Solutions International B.V. and by an Odysseus grant from the Research FoundationFlanders. The computational resources (Stevin Supercomputer Infrastructure) and services used in this work were provided by Ghent University.
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