Mechanism Of B-doping Ni/SiO₂ Catalysts For The Hydrodeoxygenation Of Dibenzofuran

Processing and upgrading low-rank coal to produce the high-quality fuels and chemicals is a key route to realize the high-value utilization of low-rank coal.The product of coal-based liquefaction contains an appreciable amount of“oxygen”,mainly in the form of phenols and furans.Industrially,phenolic compounds are extracted as valuable chemicals by the alkaline elution.To treat the remaining furans,hydrodeoxygenation（HDO）reaction is regarded as the main route,i.e.,a type of hydro-refining processes.In de-phenolic oil,dibenzofuran（DBF）is the main oxygen-containing compound and has a low reactivity.Therefore,DBF is selected as a model compound to carry out HDO reactions over Ni-based catalysts.Ni-based catalyst has a strong hydrogenation ability but a weak C-O bond scission ability which highlights C-O bond scission as the rate-determining step in HDO reaction of DBF.In order to enhance the ability of C-O bond cleavage,boron was introduced on the Ni/SiO₂ catalyst.the effect of boron doping on the HDO of dibenzofuran and the mechanism of boron doping were explored.the following key three points were observed:1)the introduction of a suitable amount of boron reduces the size of active metal nickel by improving the dispersion;2)the addition of boron increases the acid sites in Ni/SiO₂;3)the occurrence of electron transfer between boron and nickel.The above three conclusions were further explored using DBF and2-Cyclohexylcyclohexanol（CHCHOH）as substrates.The main results are as follows:1.The effect of B doping amount on the hydrodeoxygenation reaction of DBF.In this milieu,we have prepared a series of Ni-xB/SiO₂ catalysts varying in the amount of doped boron.To study the effect of boron doping on the HDO efficiency,HDO reactions of DBF using the prepared Ni/SiO₂ and Ni-xB/SiO₂（x indicates the wt%of boron doping）catalysts were performed in a batch reactor（280 ^oC,6 MPa H₂）.The results indicate that the boron doping enhances the conversion rate of DBF and the production rate of desired products（bicyclohexane,BCHs）.The best result is obtained when Ni/SiO₂ was doped with 5 wt%boron.In comparison,Ni/SiO₂ and Ni-5B/SiO₂ accomplish the conversion rate of DBF as0.79 and 3.12 mmol·min^-1·g^-1,respectively.Regarding the production rate of BCHs,it is observed as 0.05 and 0.87 mmol·min^-1·g^-1 while using Ni/SiO₂ and Ni-5B/SiO₂ as the catalyst,respectively.However,the better catalytic performance of Ni-xB/SiO₂ in HDO of DBF than Ni/SiO₂ may be affected by many factors,including changes in the size of active metal particles,the content of acid sites and the electronic properties of Ni.Therefore,it is necessary to further design experiments to prove the mechanism of a single factor on the HDO of DBF.Further experiments were designed to investigate the changes of Ni metal particle size,acidic sites and electronic properties of the active metal.2.The influence of particle size of nickel.Three different Ni/SiO₂ catalysts varying in the size of nickel particles in the range of 0-5 nm,5-10 nm and 10-15 nm were prepared,for exploring the effect of active metal particle size on HDO reaction of DBF.The activity evaluation was individually carried out using DBF and its hydrodeoxygenation intermediate2-cyclohexylcyclohexanol（CHCHOH）It was found that the conversion rates of DBF and CHCHOH increased by 3.88 and 1.03 mmol·min^-1·g^-1,respectively,when the size of Ni particles decreased to 3.8 nm.The results showed that the reduction of nickel particle size significantly promotes the cleavage of C-O bond of oxygen-containing intermediate and accelerated the reaction rate.In addition,while performing the HDO of CHCHOH under two different atmospheres（H₂ and N₂）,it is found that the reduction in the particle size of nickel in Ni/SiO₂ invariably promotes the conversion of CHCHOH.Under N₂ atmosphere,the major product was cyclohexyl benzene and 2-cyclohexylcyclohexanone.3.The influence of catalyst acidity.A different kind of catalyst（Mix-Ni/SiO₂+B₂O₃/SiO₂）was prepared by the mechanical mixing of B₂O₃/SiO₂（with 1 wt%B₂O₃）and Ni/SiO₂,which has the equivalent amount of acid sites present in Ni-1B/SiO₂.Compared with Ni/SiO₂catalyst,Mix-Ni/SiO₂+B₂O₃/SiO₂achieves the increased DBF conversion and the improved yield of BCHs due to its higher acid content.However,when using CHCHOH as a substrate,the conversion rate of CHCHOH on Ni/SiO₂ and Mix-Ni/SiO₂+B₂O₃/SiO₂ seems similar with the almost identical yield of BCHs,indicating that the increase in the content of weak acid sites has a relatively weak effect on the promotion HDO of DBF decisive pace.4.The verification of the transfer of Ni to B.By XPS,the electron transfer from nickel to boron is confirmed in the prepared catalysts.This electron transfer offers the additional Lewis acid like property to nickel which seems beneficial for the adsorption of reactants and oxygen-containing intermediates over nickel.which is proved by combining CHCHOH pulse experiments and DFT theoretical calculations.In summary,the introduction of boron reduces the size of active metal nickel by improving the dispersion and more active sites are exposed.It enhances the adsorption and activation of oxygen-containing intermediate products,and promotes the cleavage of the C-O bond and accelerates the conversion of CHCHOH in HDO reaction of DBF.