Hydrodeoxygenation of Stearic Acid Over Supported Metal Catalysts for the Production of Green Diesel

Kumar, Pankaj and Maity, Sunil Kumar (2019) Hydrodeoxygenation of Stearic Acid Over Supported Metal Catalysts for the Production of Green Diesel. PhD thesis, Indian institute of technology Hyderabad.

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Abstract

The renewable energies have been attracted considerable attention throughout the world due to the rapid depletion of fossil fuels and the continuous degradation of the environment by greenhouse gases. The transportation fuels are the leading energyconsuming sector - accounting for about 28% of global energy. Biomass is the promising carbon-neutral renewable energy source in the globe. It also has tremendous potentials to provide renewable transportation fuels commonly known as bio-fuels. The bioethanol and biodiesel are the promising bio-fuels at the moment. The poor fuel properties of these bio-fuels, however, limit their application for blending with petrol and diesel, respectively, up to 15-20% only in the unmodified combustion engine. Therefore, there is a need for the production of hydrocarbon analogous bio-fuels from biomass commonly known as hydrocarbon bio-fuel. Triglycerides are the promising feedstock for the production of hydrocarbon biofuels because of their lower oxygen content and long carbon chain length compared to cellulosic and starchy biomass. Besides, the triglycerides are composed of C8-C24 fatty acids with the majority of C16 and C18 fatty acids. Therefore, the removal of oxygen from triglycerides results in the formation of diesel-range hydrocarbon biofuel generally known as green diesel. The hydrodeoxygenation (HDO) is the promising technology for the removal of oxygen from triglycerides and fatty acids. This technology produces green diesel with high yield and minimal carbon loss. The HDO is a catalytic process and carried out in the presence of hydrogen over supported metal catalysts. The catalysts thus play a vital role in this process. The present work is thus focused on HDO of stearic acid as a model fatty acid over alumina-supported transition metal (Ni, Co, NiMo, and CoMo) catalysts. In the present work, Ni, NiMo, Co, and CoMo catalysts were prepared by incipient wetness impregnation and co-impregnation method. The catalysts were characterized by BET, temperature programmed reduction (TPR), temperature programmed desorption of NH3 (NH3-TPD), pulse chemisorption, powder XRD, and Raman and UV-vis-NIR spectroscopy to identify the catalytically active different surface species. HDO of stearic acid was conducted in a high-pressure batch reactor using n-dodecane as a solvent. The products of the liquid sample were identified by GC-MS and quantified by GC-FID. The gaseous products were quantified by GCTCD. In this work, C15-C18 alkanes, octadecanol (C18-OH), and octadecanal (C17- CHO) were observed as the products during HDO of stearic acid. In this reaction, the stearic acid first reduces to C18-OH in the presence of hydrogen over supported metal catalysts. The C18-OH then undergoes further chemical transformation following two different pathways. In the first pathway, C18- OH undertakes dehydrogenation followed by decarbonylation reaction (elimination of CO) to form C17 alkane (decarbonylation pathway). This reaction pathway was dominant over the alumina-supported Ni catalyst. In the second pathway, C18-OH undergoes dehydration followed by hydrogenation reaction to form C18 alkane (HDO pathway). This reaction pathway was dominant over HZSM-5 supported Ni and alumina-supported NiMo and CoMo catalysts. Both the reaction pathway was, however, significant over the alumina-supported Co catalyst. A suitable kinetic model was further established based on the reaction mechanism for aluminasupported Ni, NiMo, and CoMo catalyst. The calcination temperature plays a vital role in the performance of supported metal catalysts. HDO of stearic acid was investigated over alumina-supported Ni, NiMo, Co, and CoMo catalysts at different calcination temperatures to understand the role of calcination temperature on the catalytic performance of these catalysts. With increasing calcination temperature, the metal dispersion was increased for Ni, NiMo, and CoMo catalyst (up to 973 K) and decreased for Co catalyst. With increasing calcination temperature, the catalytic activity was thus increased for Ni and NiMo catalyst and decreased for Co catalyst. The activity of CoMo catalyst was, however, enhanced with rising calcination temperature up to 973 K and declined slightly after that. The optimum calcination temperature for Ni, NiMo, Co, and CoMo catalyst was found to be 1023 K, 973 K, 773 K, and 973 K. The reaction followed the decarbonylation route over active metallic centers (Ni and Co) and the HDO route over oxophilic M2+.MoO2 (M=Ni/Co) and reducible cobalt oxide species. HDO of stearic acid was also investigated over SiO2, -Al2O3, and HZSM-5 supported Ni catalysts. Characterization studies revealed that only dispersed NiO was present up to 3.4 mmol Ni loading on -Al2O3. The acidity of calcined catalysts was increased with increasing Ni loading up to 3.4 mmol. The catalytic activity and selectivity to products for HDO of stearic acid depend strongly on the acidity of the supports. The conversion of stearic acid was decreased in the order of 2.2NiZSM>2.2NiAl~2.2NiSi. The 2.2NiZSM-5 showed about 85% conversion of stearic acid compared to only ~41% for 2.2NiAl and 2.2NiSi at 360 min of reaction time. The conversion of stearic acid was increased with increasing reaction time, Ni loading on -Al2O3, temperature, and catalyst loading. The complete conversion of stearic acid was accomplished with more than 80% selectivity to C17 alkane at 563 K over 3.4 mmol Ni/-Al2O3 catalyst. HDO of stearic acid was further studied to depicts the role of Ni/Mo mole ratio in the performance of alumina-supported NiMo catalysts. Both Ni and NiMo alloy co-exist in the NiMo catalysts depending on the Ni and Mo content. With increasing Ni/Mo mole ratio, the NiMo alloy content in the catalyst was increased with the simultaneous decrease in the Ni content. The activity of NiMo catalysts was thus enhanced with increasing Ni/Mo mole ratio. C17 and C18 alkanes are thus observed as the dominating hydrocarbon product over Ni and NiMo alloy-rich catalysts, respectively. The activity of the NiMo catalyst was further enhanced with the increasing reaction temperature and metals loading. The selectivity to alkanes was, however, not affected by metal loading. HDO of stearic acid was also extended to ascertain the role of Co/Mo mole ratio on the catalytic performance of alumina-supported CoMo catalysts. The calcined CoMo catalysts showed the presence of Mo, Co, and mixed metal oxides depending on the relative metal content. The reduced CoMo catalysts were gradually enriched with CoMo alloy with increasing Co content up to 2.4 mmol and slightly decreased for 3.1 mmol Co. The catalytic activity of CoMo catalysts was thus enhanced with increasing Co content. The reaction follows the HDO mechanism over CoMo alloy and reducible Co oxide species. The selectivity to C18 alkane was enhanced with increasing Co content up to 2.4 mmol due to the enrichment of CoMo alloy in the catalyst. The catalytic activity of CoMo catalysts was further enhanced with increasing reaction temperature and metals loading without affecting the selectivity to alkanes much.

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