Hydrodeoxygenation of Triglyceride and Karanja oil for the Production of Green Diesel over Supported Metal and Metals-support Composite Catalysts

Yenumala, S R and Maity, Sunil Kumar (2016) Hydrodeoxygenation of Triglyceride and Karanja oil for the Production of Green Diesel over Supported Metal and Metals-support Composite Catalysts. PhD thesis, Indian institute of technology Hyderabad.

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Abstract

Transportation fuels play an extremely vital role in the daily life of today’s modern civilization. At present, the transportation fuels are predominantly derived from finite fossil fuels. It is one of the leading energy-consuming sectors with the stake of about 28% of the energy consumption in the world. The bio-fuels are the attractive alternatives to fossil fuels derived transportation fuels. The biodiesel has thus been attracted huge attention globally as a potential substitute of petro-diesel. The lower calorific value and unfavorable cold flow properties, however, limit the application of biodiesel as blending with petro-diesel to the extent of 20 wt% only for direct use in an unmodified combustion engine. Therefore, the methods of production of hydrocarbon analogous liquid transportation fuels from biomass are highly essential for shifting dependency away from limited fossil fuels. Triglycerides are the promising feedstock for the production of hydrocarbon transportation fuels due to their simplicity in chemical structure and lower content of oxygen compared to cellulosic biomass. Moreover, the triglycerides are composed of C8-C24 fatty acids with the majority being C16 and C18 fatty acids. Therefore, the removal of oxygen heteroatoms from triglycerides will lead to diesel range hydrocarbons commonly known as green diesel. Hydrodeoxygenation (HDO) in the presence of high hydrogen pressure is a promising approach for the production of green diesel in high yield from triglycerides. India has estimated annual production potential of 20 million tons of inedible oil seeds (e.g. karanja, neem, mahua etc) with only a few percentage of utilization with the share of karanja oil seeds being 0.2 million tons alone. In the present work, the HDO of pure triglyceride and karanja oil was investigated using supported nickel catalyst and ordered mesoporous Ni-alumina and NiMo-alumina composite catalysts. The objectives of the present work are (i) HDO of pure triglyceride over alumina supported nickel catalyst to delineate a comprehensive reaction mechanism and develop a mechanistic kinetic model over a wide range of process conditions, (ii) HDO of neat karanja oil over supported (γ-Al2O3, SiO2, and HZSM-5) nickel catalyst to articulate the roles of acidity of the catalysts, nickel loading on γ-Al2O3, and temperature on conversion of oxygenates and product distribution and to demonstrate the suitability of the green diesel for direct application as transportation fuel, (iii) HDO of karanja oil using ordered mesoporous nickel-alumina composite catalyst to demonstrate its superior catalytic activity compared to γ-Al2O3 and mesoporous alumina supported nickel catalyst in relation to their structural properties, and (iv) HDO of karanja oil using NiMo-alumina composite catalyst to demonstrate its superior catalytic activity compared to mesoporous alumina supported NiMo catalyst in relation to their structural properties. The supported nickel catalysts were prepared by incipient wetness impregnation method. The ordered mesoporous Ni-alumina and NiMo-alumina composite catalysts were prepared by one-pot evaporation-induced self-assembly method. The catalysts were characterized by BET, temperature programmed reduction (TPR), temperature programmed desorption of NH3 (NH3-TPD), pulse chemisorption, UVvis-NIR, Raman spectroscopy, high-resolution transmission electron microscopy (HR-TEM), and 27Al solid state NMR. The spent catalysts were further characterized using Fourier transform infrared (FTIR) spectroscopy and thermo gravimetric analysis (TGA). The physicochemical properties of karanja oil such as acid value, iodine value, FAME composition, density, viscosity, and elemental composition (CHNS-O) were measured using standard methods. Similarly, green diesel was characterized to measure density, viscosity, pour point, flash and fire point, elemental composition, lower calorific value, and chemical composition. HDO studies were performed in a 300 ml stainless steel high-pressure batch reactor in batch and semi-batch mode. The products of the liquid samples were identified with a gas chromatography (GC) equipped with a mass spectrometer detector and quantified by GC equipped with a flame ionization detector. The gas samples were analyzed by online GC equipped with a thermal conductivity detector and flame ionization detector. A wide range of linear alkanes (C12-C22) was observed as hydrocarbon products during HDO of karanja oil. Palmitic acid, stearic acid, hexadecanol, octadecanol, octadecanal, mono-palmitate, mono-stearate, and fatty esters were observed as oxygenated intermediates depending upon the nature of the catalyst. The gas phase analysis further revealed the formation of CO, CO2, and C1- C5 hydrocarbons. During HDO of triglyceride (1:2 molar mixtures of tripalmitin and tristearin), it was observed that triglyceride instantaneously converted to respective fatty acids through di-glyceride and mono-glyceride intermediates with propane as a coproduct. The fatty acids then undergo reduction under hydrogen atmosphere over the metallic site of the catalyst to form the fatty aldehyde. The fatty aldehyde further converted to alkane mainly through two different reaction pathways (RP-I and RP-II). Following RP-I, the fatty aldehydes subsequently converted to the corresponding alkane through decarbonylation reaction with the release of one mole of CO. The HDO of triglycerides was found follow RP – I predominately over supported nickel catalyst. In RP – II, the fatty aldehydes further reduced to corresponding fatty alcohols. The fatty alcohols then undergo dehydration to corresponding olefins followed by its hydrogenation to the alkane. The RP – II was dominating one over NiMo-alumina composite catalyst. The formation of fatty esters (RP – III) was observed over NiMo-alumina composite catalysts. The cracking reaction (RP – IV) was observed only at elevated reaction temperatures. The rate constants of the kinetic model were estimated based on the experimental results at the different temperatures for HDO of triglyceride. The rate constant corresponding to RP-I was significantly higher than that of RP-II. The activation energy for the reduction of fatty acids to fatty aldehydes, reduction of fatty aldehydes to fatty alcohols, decarbonylation of fatty aldehydes to n-alkane were 80 kJ/mol, 85 kJ/mol and 90 kJ/mol respectively. The rate constants were further correlated with catalyst loading by a linear correlation with zero intercept. The product distribution profiles obtained for estimated kinetic parameters were agreed well with the experiments. The developed kinetic model was, further, validated using experimental data at various hydrogen-to-nitrogen mole ratios in the gas phase. HDO of neat karanja oil was investigated in a semi-batch reactor over supported (γ-Al2O3, SiO2, and HZSM-5) nickel catalyst, The catalysts were associated with both dispersed and bulk nickel/nickel oxide depending on the extent of nickel loading and nature of support. Virgin karanja oil was composed of 76 wt% C18 fatty acids with 15 wt% oxygen. Nickel exhibited stronger interaction with γ-Al2O3 than SiO2 and HZSM-5. γ-Al2O3 supported nickel catalyst thus demonstrated superior HDO activity with least tendency towards cracking. On the other hand, catalytic cracking become significant over strongly acidic HZSM-5 and 5.7 mmol Ni/HZSM-5 leading to the formation of the larger extent of lighter alkanes. γ-Al2O3 supported nickel catalyst with low nickel loading (≤3.0 mmol) led to a large extent of cracking with high wt% of lighter alkanes. With ≥4.3 mmol nickel loading on γ-Al2O3, the reaction, however, proceeded largely through HDO pathway. The thermal cracking became prominent above 653 K. The optimal process conditions for maximizing HDO pathway were 653 K and 5.7 mmol nickel loading on γ-Al2O3 at 35 bars H2. ∼80 wt% of KO converted to the liquid product with 65 wt% C15-C18 hydrocarbons and H/C atomic ratio of 1.97. Physicochemical properties of the liquid product were matched reasonably well with the specification of light diesel oil. Mesoporous nickel-alumina composite catalysts are mainly associated with dispersed tetrahedral (or octahedral) coordinated nickel aluminate with strong metal-support interaction and a negligible amount of extra-framework nickel. Mesoporous nickel-alumina composite catalysts thus demonstrated superior catalytic activity over γ-Al2O3 and mesoporous alumina supported nickel catalysts. The NiMo-alumina composite catalysts demonstrated superior catalytic activity for HDO of karanja oil over mesoporous alumina supported nickel catalyst. It was mainly due to (i) stronger metal-support interaction with a lesser extent of bulk nickel oxide, (ii) the presence of larger extent of nickel-molybdenum alloy and lesser extent of Al2(MoO4)3, (iii) slightly higher surface area than mesoporous alumina supported NiMo. The RP-II was the dominating reaction route over NiMo-alumina composite catalyst with octadecane as the dominating product. The selectivity towards octadecane was increased with increasing molybdenum content in NiMoalumina composite catalyst.

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