Value-adding to stranded carbon resources via catalytic conversion of syngas into oxygenated fuels

Abstract

With the increasing demand for fossil fuels in transportation and energy sectors, the pursuit of sustainable technologies for fuel production has increased manifold. In this perspective, the catalytic conversion of syngas to liquid fuels has emerged as a promising strategy to prevail volatile fluctuations in petroleum prices; reduce CO2 emissions and lower sulphur contents in the automotive fuels. As opposed to well-established Fisher-Tropsch and methanol synthesis reactions, heterogeneous catalytic conversion of syngas to higher carbon number alcohols (ethanol, propanol, and butanol) remain challenging till date because of poor selectivity and stringent operating conditions.The present PhD dissertation is aimed to develop a cheap and sustainable catalytic process for higher alcohols synthesis (HAS). Firstly, we undertook a comprehensive literature survey about the available catalytic systems for HAS. Molybdenum based catalysts were opted for carbon monoxide hydrogenation into higher alcohols owing to their noble-metal like properties and excellent resistance to sulphur-poisoning.Secondly, we explored the role of catalyst supports (ɣ-alumina, MgO, activated carbon) for alkali promoted molybdenum carbides (K-Mo2C) in relation to the carbon monoxide conversion and selectivity towards higher alcohols. The mild acidic sites of ɣ-alumina mediated the carbon monoxide dissociation to form C2+ alcohols. The effect of cobalt incorporation into the textural and catalytic properties of alumina supported K-Mo2C was also investigated. The elemental mapping of Co into K-Mo2C structure revealed the presence of segregated Co and Mo2C islands but an interaction was observed at the molecular level, resulting in a different H2 temperature-programmed desorption pattern. With the decreased availability of surface adsorbed hydrogen over Co promoted K-Mo2C/Al2O3 the concentration of methanol was significantly reduced and product selectivity shifted more towards higher alcohols (36 mol. %).Thirdly, carbon-based molybdenum-cobalt nanoparticles were synthesized using environmentally benign raw materials (monosaccharides). The field emission-SEM and high resolution TEM images revealed a spherical morphology with graphitic flakes embedding nanocrystalline MoCo oxides. The resulting bimetallic carbon spheres enabled carbon monoxide hydrogenation to C2+ alcohols under mild operating conditions (270 ⁰C, 30 bar, and 3000 h-1). The results from SEM elemental mapping, x-rays diffraction analysis, XPS and temperature-programmed reduction corroborated a synergism between Co and MoO2 active sites responsible for alkyl radical chain growth and carbon monoxide coupling to form higher alcohols. The addition of potassium promoter (K/Mo = 0.06) during hydrothermal synthesis increased the concentration of high-valence molybdenum (Mo+4) and the selectivity towards C2+OH.Fourthly, the confinement effect of as-prepared carbon spheres for metal nanoparticles was compared with the in situ grown KMoCo crystallites during hydrothermal synthesis. The crystallite size and metal particles distribution were scrutinized via x-rays diffraction analysis, scanning transmission electron microscopy (STEM) and Field emission-SEM. The evaluation of catalyst samples under syngas reaction conditions (30 bar, 270 ⁰C, 3000 h-1) unravelled a superior catalytic performance for in situ KMoCo grown carbon spheres, in terms of carbon monoxide conversion (43 mol. %) and higher alcohols selectivity (30 mol %).Finally, a 3D printed catalyst monolith was designed with in situ grown metal particles. Molybdenum and nickel precursors were incorporated with water-soluble PVA and starch to develop a printable composition. The catalyst monolith was printed via robocasting using a Hyrel 3D (Engine SR) printer. Slow drying under ambient conditions and subsequent pyrolysis of organic compounds resulted in a three-dimensional carbon scaffold with micro/macro interconnected pores. 2D (TEM, SEM) and 3D (X-ray CT) microstructural analyses and catalytic tests (conversion of syngas to alcohols) were carried out for 3D printed catalysts and compared with conventional pelleted catalysts. The structured monolith retained its catalytic performance (CO conversion: 35 mol. %; C2+OH selectivity: 41 mol. %) at much higher space velocity (6000 h-1); whereas, pelleted catalysts with a similar composition exhibited abated CO conversion (16 mol. %) at high feed flow rate. Following the benefits of low pressure drop and minimal diffusional resistances within structured monolith, a highly compact channelled reactor was engineered with very efficient heat and mass transfer characteristics. The successful implementation of the technology would help in overcoming the high capital cost and technology risks associated with commercial gas to liquid (GTL) processing.