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Proposal CO Hydrogenation

M. Sc Thesis proposal (chemical engineering)
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  Department of Chemical Engineering M.S. THESIS PROPOSAL INVESTIGATION OF CO HYDROGENATION CATALYSTS FOR PRODUCING ETHANOL and HYDROCARBON OXYGENATES  by Didem Bü ! ra Kabakçı Supervisors Prof. ZEYNEP LSEN ÖNSAN   Assoc. Prof. AHMET KER  M AVCI Date of Submission: 12/02/2013   Bo # aziçi University Bebek, Istanbul  1. INTRODUCTION A promising alternative to petroleum-derived fuels and chemicals is the utilization of coal, natural gas, and biomass to make syngas for the production of alcohols and hydrocarbons. Indeed, large-scale facilities for the production of methanol and Fischer–Tropsch hydrocarbons continue to be built around the globe [1]. Fischer-Tropsch technology can be briefly defined as the means used to convert synthesis gas containing hydrogen and carbon monoxide to hydrocarbon products. For the above definition the term 'hydrocarbons' includes oxygenated hydrocarbons such as alcohols. The chemistry taking place in a Fischer-Tropsch reactor is complex but can be simplified into the following chemical reactions [2]: The synthesis of higher alcohols from syngas by direct catalysis was recognized in 1923 by Frans Fischer and Hans Tropsch. They reported that a mixture of alcohols, aldehydes, ketones, fatty acids, and esters were formed when the reaction between CO and H 2  was performed at pressures ranging from 10 to 14 MPa and at temperatures of 400–500 $ C in the presence of an alkalized iron oxide catalyst. Variations on this synthesis pathway were soon to follow for the selective production of methanol, mixed alcohols, and isosynthesis products [3,4]. The hydrogenation of synthesis gas to oxygenates is a field of renewed interest due to their properties as gasoline blends and the necessity of finding alternative automotive fuels to meet legal requirements. Among the potential end products ethanol is  particularly attractive since it serves as a clean alternative fuel, a gasoline blend, and a hydrogen carrier. In addition, ethanol has been considered as a feedstock for the emca conceps use or engneerng purposes M. E. Dry Catalysis Research Unit, Department of Chemical Engineering, University of Cape Town, Rondebosch, 7701, South Africa 1. STOICHIOMETRY The term stoichiometry is commonly used to describe the way in which the components in a chemical reaction combine to form products. Thus in the case of the Fischer-Tropsch process the stoichiometry is primarily concerned with the ratio of consumption of hydrogen and carbon monoxide and in some cases also carbon dioxide. In this context, the H2 to CO consumption ratio (or simply the consumption ratio); the H2 to CO usage ratio (or simply the usage ratio) and the stoichiometric ratio (in the absence of CO2 reaction) are synonyms. The simple terminology of the usage ratio is preferred in this text. When CO2 is a reactant, the stoichiometric ratio i.e. the ratio in which reactants are consumed, will involve CO2. The H2 to CO usage ratio may still be of interest but it is then no longer synonymous with the stoichiometric ratio. The chemistry taking place in a Fischer-Tropsch reactor is complex but can be simplified into the following chemical reactions methane CO + 3H2 ---* CH4 + H20 (1) heavier hydrocarbons nCO + 2nil2 ~ (-CH2-)n + nH20 (2) alcohols nCO + 2nil2 ~ CnH2n+ 20 + (n - 1) H20 (3) water gas shift WGS) CO + H20 ~ CO2-k- H2 (4)  synthesis of variety of chemicals, fuels and polymers. Different catalytic systems can  be used for synthesizing higher alcohols from synthesis gas. Product distributions are influenced by temperature, feed gas composition (H 2 /CO), pressure, catalyst type, and catalyst composition [5]. Currently, ethanol is mostly produced via fermentation of biomass-derived sugars . At least three methods are known in the literature for the catalytic conversion of syngas to ethanol and higher alcohols: (i) Direct conversion of syngas to ethanol, wherein selective hydrogenation of CO occurs on a catalyst surface to produce ethanol directly. (ii) Methanol homologation, which involves reductive carbonylation of methanol over a redox catalyst surface to produce ethanol. (iii) A multistep ENSOL  process, wherein syngas is first converted to methanol over a commercial methanol synthesis catalyst followed by methanol carbonylation to acetic acid in the second step and, then, subsequent hydrogenation of acetic acid to ethanol [6]. There are currently no commercial plants producing ethanol or higher alcohols from syngas as an end product. It is well known that syngas conversion to C 2+  oxygenates is often limited by the formation of methane and methanol. Low yield and poor selectivity for ethanol production from syngas stand as the major obstacles associated with the use of known catalysts. In order to make this catalytic conversion route commercially attractive, it is necessary to develop more effective catalysts [5,6]. The catalysts for the production of ethanol and other light alcohols from syngas can  be grouped broadly into 4 categories: (i) Rh-based catalysts, (ii) modified high-temperature and low temperature methanol synthesis catalysts based on ZnO/Cr  2 O 3  and Cu/ZnO/Al 2 O 3 , respectively, (iii) modified Fischer–Tropsch catalysts based on Co, Fe, and Ru, and (iv) modified unsulfided and sulfide Mo-based catalysts. A growing consensus regarding ethanol synthesis from syngas is that supported Rh has a great potential for the reaction, but suitable supports and promoters are needed to enhance the reactivity of Rh and the current high cost of Rh may hinder its commercial utilization [7].  2. LITERATURE SURVEY Recently, there is a growing worldwide interest in the conversion of syngas to higher alcohols, with an emphasis on ethanol. Significant improvements in catalyst design need to be achieved to make this conversion commercially attractive [6]. Substantial research work has been carried out for ethanol and higher alcohols production via CO hydrogenation. Mei et. al. [8] investigated the reaction kinetics of ethanol synthesis from CO hydrogenation over SiO 2 -supported Rh/Mn alloy catalysts. They found that Mn  promoter can exist in a binary alloy with Rh and has a critical role in improving the selectivity toward ethanol and other C 2+  oxygenates, although the barrier toward methane formation is not changed. Then, the effects of various promoters (M = Ir, Ga, V, Ti, Sc, Ca, and Li) on the CO insertion reaction over Rh/M alloy nanoparticles were investigated. They also found alloying the promoters with the electro negativity difference, between the promoter (M) and Rh leads to higher selectivity to ethanol. In another study, Haider et. al. [1] explored the influences of support (silica or titania) and loading of Fe promoter on the activity and selectivity of Rh-based catalysts for the synthesis of ethanol from syngas. Promotion of 2 wt% Rh/SiO 2  by 1 wt% Fe  produced a catalyst that exhibited 22% selectivity to ethanol, with methane being the  primary side-product. Addition of Fe to 2 wt% Rh/titania also improved the selectivity to ethanol with the highest selectivity being 37% for a sample 5 wt% Fe. ! $%& '$()* +, -(./010 20  et. al. [9] CO hydrogenation to ethanol over Rh(1.5)/SiO 2  promoted with La and/or V oxides. The effects of reaction conditions – temperature, H2/CO ratio, space velocity, and pressure are examined – on the activity and selectivity of these catalysts. An ethanol selectivity of 51.8% (close to the highest literature value) at a CO conversion of 7.9% was achieved with a corresponding methane selectivity of 15.4% at 270 °C, 14 bar and H 2 /CO = 2 over the Rh– La/V/SiO 2  catalyst. Combined La/V promotion reduces methane selectivity and increases C 2+  oxygenates selectivity compared to the singly promoted catalysts by increasing the rate of CO insertion.
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