Heterogeneous Catalysts For Biodiesel Production

Biodiesel, derived from renewable resources like animal fats and vegetable oils, is a clean biofuel, which can be used in compression ignition engines. The first-generation biodiesel usually comprises fatty acid methyl esters that are produced by transesterification of triglycerides with methanol or esterification of fatty acids with methanol. The second-generation biodiesel, also called renewable diesel fuel, refers to the aliphatic hydrocarbons obtained by deoxygenation. Biodiesel is considered as a promissing alternative fuel for diesel because of its reduction of most exhaust emissions like sulfur oxides and aromatic hydrocarbon, non-toxicity, having a higher cetane number and a better combustion performance. The vegetable oils being exploited commercially for biodiesel production constitute the edible fatty oils derived from rapeseed, soybean, palm, sunflower, etc. Owning to the high price and a big gap in demand and supply of such oils, it is not feasible to produce biodiesel. This highlights the need for the development of environment-friendly and more effective catalysts allowing the use of non-edible and low-price vegetable oils. In this work, heterogeneous catalysts were developed for transformation of hempseed oil to fatty acid methyl ester, fatty acid and renewable diesel fuel, and the relationship between the structural characteristics and catalytic performances were discussed.Alkaline earth metal oxide supported copper oxide was prepared using chemisorption-hydrolysis method as a bi-function catalyst for one-pot process combining transesterification and selective hydrogenation. XRD, SEM, N2 adsorption-desorption, CO2-TPD, H2-TPR, XPS and Hammett indicator method were used to characterize the as-preparaed catalysts. The dependence of catalytic performances for transesterification and selective hydrogenation on copper oxide loading and reaction conditions was given. And the correlation of structure characteristics and catalytic performance was discussed. Furthermore, the relationship between iodine value, cetane number and the degree of unsaturation was estabilished. Alkaline earth metal oxide supported copper oxide catalyst formed alkaline earth metal cupretes after high temperature calcination in vacuum. Cu/SrO catalysts yielded a strontium-rich solid compound of Sr2CuO3at a low CuO loading of1-10wt.%and a copper-rich solid compound of SrCuO2at a high CuO loading of10-30wt.%. Cu/SrO catalyst particles consisted of many interconnected nano-sheet with thicknesses of50-100nm, which were formed by solid-state reactions of the uniform spherical particles with the average particle size of35-70nm. Basic strength of alkaline earth metal oxide supported copper oxide catalyst increased in the order of Cu/MgO<Cu/CaO<Cu/SrO<Cu/BaO. Strontium oxide supported copper oxide samples showed more amount of basic sites than other alkaline earth metal oxides supported copper oxides. With the increase of copper oxide content, the strength and amount of basic sites of Cu/SrO catalyst decreased. Cu/SrO exhibited the largest amount of reducible copper-containing species. The copper species in the10%Cu/SrO catalyst before the reaction is mainly in the form of CuSr2O3, which has been reduced to metallic copper after reaction under hydrogen pressure of3.0MPa at180℃. Alkaline earth metal oxides supported with copper oxide were both active for transesterification of hempseed oil with methanol and selective hydrogenation of methyl esters and Cu/SrO catalysts exhibited the superior catalytic activity for transesterification with a biodiesel yield of96%and hydrogenation with a reduced iodine value of113and it also exhibited a promising selectivity for eliminating methyl linolenate and increasing methyl oleate without rising methyl stearate in the selective hydrogenation. The catalytic performance toward transesterification is closely correlated to the basic strengths and basic sites amounts of alkaline earth metal oxide supporting copper oxide. The catalytic activity for selective hydrogenation was closely related to the amount of reduced surface copper species. The iodine value increased and the cetane number decreased with the increase of degree of unsaturation. After the one-pot process combining transesterification and selective hydrogenation catalyzed by10wt.%CuO catalyst, the obtained biodiesel presented a reduced iodine value of113and a increasd cetane number of55, and also exhibited a promising selectivity for eliminating methyl linolenate and increasing methyl oleate without rising methyl stearate in the selective hydrogenation. The fuel properties of the selective hydrogenated methyl esters were within biodiesel specifications.Cordierite honeycomb ceramic was firstly treated with oxalic acid to obtain a high-surface-area porous support, which was deposited by a coating of Y2O3, ZrO2, and CeO2through pore volume impregnation as a second support to activate oxy-groups in the oxygen-containing compounds, and then decorated with a bimetallic nickel and copper by pore volume impregnation to adsorb and activate dihydrogen to obtain monolithic catalysts for hydrodeoxygenation. Metal oxides with variable valances (MO) grew not only on the external surface of cordierite supports but also inside the macroporous location. Ni-Cu/MO/Cor monolithic catalyst showed specific surface area in the range of7-14m2·g-1, average pore diameter ranging from5.9to21.9nm. NiO-CuO crystals formed over zirconium oxide, yttrium oxide and cerium oxide presented average size of250nm,200nm and150nm, respectivity. Ni-Cu/CeO2/Cor catalyst showed lower reduction temperature and more reducible species than the others. Increaseing NiO-CuO loading led to the enhanced interaction of cerium oxide and nickel copper metal oxides and increase of reducible species amount. Bimetallic nickel and copper monolithic catalysts presented conversions of fatty acid methyl esters between54-92%in the continuous hydrodeoxygenation of palm oil methyl esters, which were carried out in a monolithic reactor at350℃and3.0MPa of H2for3h. The liquid products were linear hydrocarbons C14-C18, with high percent of heptadecane (C17H36) and pentadecane (C15H32), and methane is the main product in the gas phase. The high selectivity toward heptadecane formation was veried with the C17H36content of60-80%in the liquid products, which suggested that the hydrodeoxygenation of biodiesel of palm oil involves the decarboxylation (DCO) step. Deoxygenation of fatty acid methyl esters occurs via the hydrdeogenation over Ni-Cu/CeO2/Cor, while in the presence of the Ni-Cu/Y2O3/Cor sample, HDO proceeds via decarboxylation. Metal oxides with variable valances supported bi-metallic nickel and copper monolithic catalyst showed conversion of fatty acid of hempseed oil in the range of34.9%-52.4%, with heptadecane (C17H36) as the main liquid product.Solid super-acid SO42-/SnO2catalyst was applied for hydrolysis of hemp seed oil to produce fatty acids, which were used as starting materials for second-generation biodiesel production. The relationship between structure properties and catalytic performance for hydrolysis reaction was discussed. Solid super-acid SO42-/SnO2catalyst showed the tetragonal SnO2characteristic diffraction peaks, and the samples calcined at400and500℃showed partially amorphous characteristics. The as-synthesised catalysts presented particle size in the range of10-30nm, specific surface area in the40-133m2·g-1range, and average pore diameter in the range of2.32-34.57nm. Sulfuric group combined to the SnO2in two ways:chelation and bridging connection, forming the main super acid active sites. Increaseing concentration of sulfuric acid resulted in increase of sulfate loadings and the acid strength. With the increase of calcining temperature, sulfate loading and the acid strength increases first and then decreases, the sample calcined at500℃exhibited the highest sulfate loading and the acid strength. Catalytic activity toward hydrolysis reaction in term of hempseed oil conversion increased with the increase of acid strength. SO42-/SnO2catalyst prepared with3mol·L-1sulfuric acid solution immersion followed by the500℃calcining for3h showed conversin of95%in the hydrolysis of hempseed oil at100℃, reaction time of10h, weigh rate of catalyst/oil of5%, emulsifier (dodecylbenzenesulfonic acid sodium) dosage of1%.Attapulgite (ATP) and Pd/ATP catalyst presented rod-like particles in diameter of40-90nm, and the PaCl2spherical particles in diameter of2-5nm were uniformly dispersed over the surface of attapulgite, which were reduced to metallic palladium particles and uniformly dispersed on the surface. The samples showed a wide pore distribution, with micropores centered at1-2nm, mesopores at2.5-50nm and marcopores at50-200nm, specific surface area in the range of100.3-144.7m2·g-1, and average pore size in the range of9.6-14.4nm. Pd/ATP catalyst can be reduced at room temperature in hydrogen, and then palladium adsorps hydrogen atoms to generate palladium hydride (PdHx), which will be decomposed with increasing temperature leading a low-temperature hydrogen release peak. With the increase of palladium loading, the interaction between palladium and palladium hydride enhanced, and the amount of palladium hydrides increased. Palladium compound of Pd/ATP catalyst exists in the form of palladium chloride, which was reduced to palladium after reduction. Palladium loading significantly affected catalytic performance of Pd/ATP catalyst for deoxygenation of fatty acid to produce renewable diesel fuel. With Pd loading amount increasing from0.5%to4%, the conversion of fatty acids increased rapidly from64%to92%. A further increase of palladium loading from4%to8%resulted in a slight increase of fatty acids conversion. The liquid product of deoxygenation catalyzed by Pd/ATP catalyst is C14-C18aliphatic hydrocarbons, with heptadecane (C17H36) content of about70%, while the gas phase products were primarily carbon dioxide and water.