Steam reforming of acetic acid as a biomass derived oxygenate: Bifunctional pathway for hydrogen formation over Pt/ZrO2 catalysts
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The selective catalytic conversion of biomass-derived syngas into ethanol is thermodynamically feasible at temperatures below roughly 350 °C at 30 bar. However, if methane is allowed as a reaction product, the conversion to ethanol (or other oxygenates) is extremely limited. Experimental results show that high selectivities to ethanol are only achieved at very low conversions, typically less than 10%. The most promising catalysts for the synthesis of ethanol are based on Rh, though some other formulations (such as modified methanol synthesis catalysts) show promise. (Critical review—173 references.)
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Synthesis gas (syngas) is mostly known by its use on ammonia and hydrocarbons (Fischer-Tropsch process) production. However, a less explored route to produce chemical products, among them alcohols and other oxygenates, from syngas is gaining attention over the last few years. In this route, an initial feedstock as biomass is firstly gasified to synthesis gas, which is reformed, cleaned, compressed and finally catalytically converted in a mixture of alcohols and oxygenated products that after separation steps attain sufficient purity to be sold. In this case of study, the commercial simulator ASPEN Plus v7.3 is used to evaluate the application of a Rh-based (Rh-Mn-Li-Fe/SiO2) catalyst in a small scale plant with processing capacity of 100 kmol/h of pure syngas. This plant, besides methane, water, and CO2 produces 8 oxygenated products: methanol, ethanol, propanol, butanol, acetic acid, acetaldehyde, methyl and ethyl acetates, being necessary 9 further separation or concentration steps in order to obtain the products in their desired purity. The main goals of this work were to design and optimize a process so as to produce alcohols and other oxygenates using syngas as feedstock. After conceiving the process, an optimization was performed, which started by evaluating the reactor conversion/selectivity in order to produce more add-value products. Then, the downstream separation processes were optimized searching for less energy consumption and recovering as much as possible add- value products. Lastly, we aimed at possible solutions and improvements concerning sustainability, feedstock and energy integration, and utilities consumption.
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Co-processing of H2O, CO2, and light (C1–2) oxygenates with CH4 at 950 K over Mo/H-ZSM-5 catalysts results in complete fragmentation of the oxygenate and CO as the sole oxygen-containing product. The C/Heff accounts for removal of O as CO and describes the net C6H6 and total hydrocarbon synthesis rates at varying (0.0–0.10) oxygenate and H2 to CH4 co-feed ratios.
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Steam reforming of acetic acid over Pt/ZrO2 catalysts was studied as a model reaction of steam reforming of biomass derived oxygenates. Pt/ZrO2 catalysts were very active; however, the catalyst deactivated in time by formation of oligomers which block the active sites for steam reforming. Temperature programmed oxidation of the used catalysts revealed that there are three types of deposits, on Pt, on ZrO2 close to Pt particles, and on ZrO2. The removal of the second type of deposit was essential to regenerate the catalysts. Thus, it was suggested that the Pt-ZrO2 boundary sites were active sites for steam reforming, where both Pt and ZrO2 participates in the reforming, to activate acetic acid and water, respectively. This is an abstract of a paper presented at the 231st ACS National Meeting (Atlanta, GA 3/26-30/2006).
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As one of the alternative methods for preparing clean fuels and raw chemicals from non-petroleum sources,the catalytic conversion of syngas derived from coal,natural gas and biomass to C2-oxygenates such as alcohol,acetaldehyde and acetic acid has attracted great attention.Rhodium-based catalyst is one of the most effcctive catalytic system for the synthesis of C2-oxygenates from syngas.The recent studies of rhodium-based catalysts mainly including promoters,supports and preparation methods and conditions for synthesis of C2-oxygenates from syngas were reviewed.The perspective for further study was presented.
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Carbon Chain Growth via Formyl Insertion on Rh and Co Catalysts in Syngas Conversion
Yonghui Zhao and Wei-Xue Li
State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Science, Dalian 116023, China
Introduction
Syngas (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.
Formyl, 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.
Computational Methods
All 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 A. 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/A, 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.
Results and Discussion
We 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.
Further 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.
Figure 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.
Conclusions
In 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.
Acknowledgement. We thank finical supports by NFSC (20873142, 20733008, 20923001), MOST (2007CB815205, 2011CB932704), and fruitful discussions with Prof. Xin-He Bao and Ding Ma
References
(1) Y. 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)
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