Study On Ni-based Catalysts For Hydrodeoxygenation Of Fatty Acids To Fatty Alcohols

As one of the important platform compounds,fatty alcohols are the basic chemical raw materials for synthesizing surfactants,detergents,plasticizers and other fine chemicals.It is widely used in textile,daily chemical,paper,food,medicine,leather,etc.China is a big consumer of fatty alcohols.With the improvement of people’s living standards,the demand for fatty alcohols and its derivatives is gradually increasing.However,our productivity for fatty alcohols is low,the variety of fatty alcohols is limited,and the production process is backward,but the market demand is large and production is in short supply.In order to solve such problems,it is important to develop new process methods and catalytic systems.One of the solutions for these problems is to use biotechnology to prepare fatty alcohols.However,it requires a large amount of capital,technology and key equipment for the development and production of bio-based fatty alcohols,and the production threshold is also high.The method of hydrogenating oil to produce fatty acids,followed by hydrodeoxygenating to fatty alcohols and alkanes,not only prevents environmental pollution,ecological damage,but also reduces production costs and achieves energy saving and environmental protection.At present,the key to the preparation of fatty alcohols by hydrodeoxygenation of fatty acids is the selection of inexpensive non-precious metal-based catalysts and the construction of catalytic systems.In this paper,stearic acid was used as a model reactant.Firstly,the electronic effect of Ni/ZrO₂ catalysts in the hydrodeoxygenation of stearic acid to octadecyl alcohol was investigated.By means of catalyst characterization techniques,such as BET,XRD,H₂-TPR,H₂-TPD,NH₃-TPD,CO₂-TPD,HRTEM,FT-IR,etc.,it was proposed that the electron density of metal Ni could be affected by the oxygen vacancies on ZrO₂ support.Consequently,the dissociative adsorption mode of H₂ on the surface of metals was affected:the H atom generated by H₂ homolysis selectively cleaved the C-C bond of stearic acid,thereby generating a hydrocarbon having one carbon less than the carbon number of the fatty acid;while H^δ+and H^δ-produced by H₂ heterolysis selectively cleaved the C-O bond of stearic acid,producing fatty alcohols.We also prepared a high surface area ZrO₂ with monoclinic phase by a hydrothermal method.When it was used as support for Ni catalysts,the metal Ni had much higher electron density than the one supported by ZrO₂ with tetragonal phase prepared by coprecipitation due to the presence of high amount of defects on the monoclinic ZrO₂.The Ni catalysts supported on monoclinic ZrO₂ also had the highest selectivity to stearyl alcohol（up to 94%）,which is the highest value reported for additive-free monometallic Ni catalysts.From an environmental and economic point of view,this contribution not only alleviates the environmental pollution problems caused by the use of commercial Cu-Cr catalysts,but also lowers reaction temperatures and hydrogen pressures from 250-350oC and 10-20 MPa to 240oC and4 MPa,and thus reduces the production cost of fatty alcohols.Secondly,in this paper,the electronic effect of Ni/ZrO₂ catalysts on the selectivity of stearyl alcohol was further investigated in the hydrodeoxygenation of stearic acid.A series of different forms of ZrO₂ and TiO₂ were prepared by a hydrothermal method and used as support for Ni catalysts.After the catalyst evaluation,the best catalyst was identified and the reaction conditions were optimized.Thus,we were able to have 100%conversion of stearic acid and up to 92%selevtivity of stearyl alcohol on 3%Ni/m-ZrO₂ catalyst under the reaction conditions:the mass/volume ratio of stearic acid and n-heptane was 0.4/100,the mass of catalyst was0.036 g,240oC,5.5 MPa H₂,and 9 h reaction.Finally,we tried to control the degree of defects in monoclinic ZrO₂ by varing the hydrothermal temperatures,and then adjust the electron density of the suppoted metal Ni.The catalysts then were characterized by TEM,H₂-TPR,H₂-TPD,NH₃-TPD,CO₂-TPD,etc.And we found that there was a positive correlation between the amount of oxygen vacancy of ZrO₂ support with the hydrothermal temperature,also the increase in the hydrothermal temperature would affect the activity and selectivity of the catalysts in the hydrodeoxygenation of stearic acid.