Synthesis Gas Conversion to Aliphatic Alcohols: Study of MoS2 catalytic systems

Abstract

The recent energy debate and demand for renewable fuels has intensified research activities for conversion of biomass derived feedstocks to fuels and fuel additives. Synthesis of ethanol and higher aliphatic alcohols from syngas (CO + H2) is therefore receiving renewed interest. An important objective of this thesis was to develop commercially competitive catalysts and understand the fundamental issues affecting their performance. Molybdenum Sulfide (MoS2) class of catalysts were synthesized by sulfidation of ammonium thiomolybate and acetate salts of co-promoters. Several catalyst formulations were prepared by calcination, followed by doping with alkali promoters. Solid state modifications were made in some cases to dilute the active MoS2 material in supported catalysts. By modifying synthesis procedures, homogeneous and narrow size distribution of the sulfide material was obtained. Size control 2 agglomerates by a novel chelation synthesis technique, which was particularly useful in enhancing alcohol yields. The catalyst performance was studied in a fixed bed rector, operating in a range of 280-350oC, 69-92 bar and typical gas space velocities of 3000-10000 L/g.cat/hr. Analytical equipment with the capability to perform online gas phase analysis of oxygenates along with permanent gases was setup. This led to the quantitative determination of reactants/products concentrations in a time-on-stream run and detection of oscillations in concentration profiles of oxygenates. Such an oscillatory behavior has not been reported before and it could be important for safe operation and understanding the scale up parameters of the process. Alcohol active catalysts were thoroughly tested for steady state activity and evaluated for parametric effects on conversion and selectivity. Depending upon the composition, a catalyst showed varying sensitivity to operating conditions. Temperature, space velocity and CO:H2 ratio in the syngas possessed the greatest capability in altering the product portfolio and overall reaction rates. Use of co-promoters like Cu, led to increased alcohol selectivity of >85%, whereas promoters like Rh significantly improved the yield of alcohols (alcohol yields > 500 g/kg.cat/hr). Such high yields and selectivities are indicative of a major advancement over the existing MoS85%, whereas promoters like Rh significantly improved the yield of alcohols (alcohol yields > 500 g/kg.cat/hr). Such high yields and selectivities are indicative of a major advancement over the existing MoS 500 g/kg.cat/hr). Such high yields and selectivities are indicative of a major advancement over the existing MoS2 based catalyst systems. By changing the support type and modifying support basicity, selectivity for alcohols as well as overall alcohol yield could be improved by 3 times at temperatures > 300 300oC. Modification of aluminosilicate supports by interchanging framework cations has not been reported for higher alcohol synthesis and offers a very simple technique for enhancing performance of supported catalysts. Use of zeolites as supports offers increased C2+/C1 alcohol ratios from nominal value of 2 to 4. Active catalysts were characterized by SEM, TEM, EDS and XRD, which revealed that the final catalyst morphology greatly affects the alcohol synthesis performance of the catalyst.