Carbon chain growth via formyl insertion on Rh and Co catalysts in syngas conversion

Carbon Chain Growth via Formyl Insertion on Rh and Co Catalysts in Syngas Conversion \n\nYonghui Zhao and Wei-Xue Li\n\nState Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Science, Dalian 116023, China\n\n Introduction\n\nSyngas (carbon monoxide and hydrogen), producing from coal, natural gas or biomass, has attracted much attention as alternative to petroleum-derived fuels and chemicals. Syngas can be selectively converted towards either oxygenates such as alcohols, aldehydes and acids etc, or hydrocarbons via Fischer-Tropsch synthesis (FTS). Industrially, rhodium-based and cobalt-based catalysts are often used for C2−oxygenates and hydrocarbons formation. Despite numerous studies so far, the exact mechanism remains in much debate, which represents a major challenge in catalysis. \nFormyl, formed by CO hydrogenation, has been implicated to be one of the key reactive intermediates in syngas conversion. It has been proposed that the hydrogenation of HCO followed by C=O bond scission leads to the CHx monomer formation. Then chain growth proceeds either by CO insertion in CHx monomer, or by carbene coupling, or by condensation of C1-oxygenates with elimination of water, leading to the formation of Cn(n≥2) −oxygenates or hydrocarbons. However, the short lifetime of the HCO species prevents the characterization typically requiring the elevated pressures and the identification of its role in the syngas conversion. Recently, the direct evidence of HCO as the key intermediate for CO methanation was successfully obtained by in situ spectroscopic experiments on supported Ru catalysts. Herein, we report on the use of density functional theory (DFT) calculations to explore the underlying role of HCO in syngas conversion and its dependence on the catalysts.\n\nComputational Methods\n\nAll calculations were performed using Vienna ab initio simulation package (VASP) and PAW potential. The wave function was expanded by plane wave with kinetic cutoff 400 eV. The exchange-correlation energy and potential were described by generalized gradient approximation in form of the PW91 and spinpolarized calculations were performed throughout the present paper. Rh(111) and Co(0001) surfaces were simulated by a four layers slab with a p(3х3) periodicity separated by a vacuum of 15 Å. Adsorption was only allowed on one side of the slabs. The chemisorbed species and metal atoms of the uppermost two layers were allowed to relax till the residual forces less than 0.03 eV/Å, while the remained atoms were fixed at their bulk truncated positions. Transition states (TSs) were located by constrained minimization method and climbing-image nudged elastic band method (CI-NEB). All TSs were confirmed by the frequency analysis.\n\nResults and Discussion\n\nWe first investigate the competitive CO versus HCO insertion in CHx(x=1−3) on Rh (111) as shown in Figure 1a. The activation energy barriers for CO insertion in CH, CH2 and CH3 are calculated to be 1.34, 1.25 and 1.55 eV respectively, significantly higher than the corresponding barriers for HCO insertion (0.89, 0.75 and 1.02 eV). Compared to the most commonly studied CO insertion pathway, the kinetic preference for the novel HCO insertion pathway can be immediately seen. Moreover, the HCO insertion in CHx is slightly endothermic or exothermic, with the reaction energies of 0.27, −0.10 and −0.04 eV, whereas the CO insertion is endothermic by 1.11, 0.69 and 0.35 eV, respectively. Therefore, the HCO insertion pathway would be preferable on thermochemical grounds. Similar results has also been found on Co(0001) surfaces, as shown in Figure 1b. The calculated barriers for HCO insertion are comparable to the calculated barriers of carbene coupling reported in literatures, and this indicates that HCO insertion are competitive to the carbene coupling. This would open a new reaction channel for chain growth on Rh and Co catalysts considered.\nFurther calculations found that the C=O scission of CHxCHO(x=1-3) formed from HCO insertion present distinct dependence on the catalysts. Compared to Co(0001) catalysts, we found that corresponding barriers for C=O scission on Rh(111) was much lowered due to its lower affinity toward oxygen. These calculations are consistent with the observation experimentally that Co and Rh catalysts exhibit excellent selectivity towards hydrocarbons and oxygenates respectively.\n\n\n\n\n\n\n\n\n\n\n\n\n\n\nFigure 1. Calculated barriers and reaction energies for CO (dashed line) and HCO (solid line) insertion in CHx(x=1−3) on Rh(111) (a) and Co(0001) (b) surfaces.\n \nConclusions\n\nIn summary, we present a density functional theory study of the underlying role of formyl in syngas conversion. The HCO insertion exhibits superior or similar activity to CO insertion and carbene coupling. This result opens a new reaction channel for the chain growth in syngas conversion. Co-catalysts and/or the promoters with lower affinity of oxygen would retard the C=O bond scission (boost formyl insertion), leading to an improved selectivity to oxygenates. \n\nAcknowledgement. We thank finical supports by NFSC (20873142, 20733008, 20923001), MOST (2007CB815205, 2011CB932704), and fruitful discussions with Prof. Xin-He Bao and Ding Ma\n\nReferences\n (1)\tY. H. Zhao, K. J. Sun, X. F. Ma, J. X. Liu, D. P. Sun, H. Y. Su, W. X. Li, Angew. Chem. Int. Ed. 2011 (in press)