Research On Electronically-regulated ZnO For The Catalytic Synthesis Of C₂₊ Alcohols From Syngas

The catalytic synthesis of alcohols from syngas using coal or natural gas as raw materials is one of the challenging and attractive topics in the field of C1 chemistry.The C₂₊alcohols obtained from this process can be used as fuels,fuel additives to enhance octane or hexadecane numbers,and intermediates for value-added chemicals such as pharmaceuticals,cosmetics,and polyester.In addition,the synthesis of C₂₊alcohols involves the retention and fracture of C-O bonds,as well as the formation and control growth of C-C chains in science.However,up to now,the catalyst with low cost,high CO conversion,high total alcohol selectivity and high C₂₊alcohols ratio has not been developed.The"seesaw"problem of the above performance indicators of the catalyst has not been effectively solved.Therefore,We need to explore a new way to examine and study this complex process from a new perspective in order to make a breakthrough.The preparation of alcohols from syngas is a complex and typical redox process,which involves the transfer of electrons from one molecule to another.Therefore,the electron transport performance of the catalyst will play an extremely important role in this process.From the perspective of currently used catalysts,most of them involve transition metal oxides,and most of these transition metal oxides are semiconductors.The defects and impurities（doping）in the crystalline phase of these semiconductor materials will affect their Fermi levels,and then affect their electric work function,thus affecting the transmission of electrons.These changes will affect its catalytic performance.This article focuses on Zn O,which has not been given sufficient attention in previous work,by doping other metal elements to regulate its electron transfer transmission of Zn O.Combining the knowledge of the electron theory of catalysis,it explores the impact of catalyst electronic performance on the synthesis of alcohols from syngas The main research contents and conclusions are as follows:（1）On the basis of the ternary Cu Zn Al catalyst with the ability to efficiently synthesize ethanol which was discovered by our research group,Zn O doped with different metals（3 mol%Cu,Al and Ga）is used as the zinc source to investigate its influence on the performance of Cu Zn Al catalyst for synthesising alcohols from syngas,and compare with the catalyst using pure Zn O as zinc source.The results show that Zn O doped with different metals as zinc source can regulate the number of strong alkaline sites on the surface of the catalyst,change electron-donating ability of the catalyst,and thus affect the formation of C₂₊alcohols.But the main product of the catalyst is methanol with weak carbon chain growth ability.The research shows that the enhancement of electron-donating ability is conducive to the the synthesis of C₂₊alcohols on the ternary Cu Zn Al catalyst,and the proportion of C₂₊alcohols increases.The growth of carbon chain is achieved through the aldol condensation reaction promoted by alkaline sites.（2）Zn O doped with different metals（3 mol%）is directly used as a catalyst for the synthesis of alcohols from syngas.The introduction of electron donors or acceptors into Zn O can regulate the Fermi level of Zn O.Compared with pure Zn O,Cu,as a p-type doped element,is an acceptor impurity,which can reduce the Fermi level,and increase the electronic work function.Catalytic performance testing shows that it can promote carbon chain growth and significantly increase the proportion of C₂₊alcohols（46.5%）.As an n-type doped element,Al doping will strengthen the donor characteristics of Zn O,resulting in the increase of Fermi level and the decrease of electronic work function,which is not conducive to the generation of C₂₊alcohols.The proportion of C₂₊alcohols in total alcohols is only 11.1%.Similarly,Ga is also an n-type doping element,which also enhance the donor characteristics of Zn O.The catalytic performance test shows that its ability to produce C₂₊alcohols is reduced.According to the content of the electron theory of catalysis,the increase of electrons is not conducive to the p-type reaction.Therefore,the preparation of higher alcohols from syngas on doped Zn O is a reaction promoted by p-type semiconductors,and p-type doping is conducive to the generation of C₂₊alcohols.（3）By changing the doping amount of Cu,When the doping amount of Cu is less than 4mol%,Cu²⁺replaces Zn²⁺and enters the lattice of Zn O,reducing the Fermi level of Zn O,promoting the adsorption and dissociation of CO and the surface of Zn O is more likely to accept electrons to form CH_x species.In situ infrared testing showed that the CH₂*absorption peak on the surface increased with the progress of the reaction,indicating that the hydrogenation of CH_xO species to methanol was limited.The product analysis shows that the alcohol products were mainly linear alcohols,which indicates that the reaction of producing higher alcohols on Cu doped Zn O conforms to the insertion mechanism.When the doping amount of copper in Zn O reaches 7 mol%or more,the copper species gathered on the surface of Zn O cover up the catalytic performance of Zn O semiconductor,which is not conducive to the dissociation of CH_xO,and the proportion of higher alcohols in the product decreases,from 48.1%to 3.2%.（4）The doping state of Cu significantly affects the performance of the catalyst for the synthesis of higher alcohols,and the preparation methods can change the doping form of copper species,thus regulating the electronic structure of Zn O.The modified Zn O catalysts with different forms of Cu were obtained by four different preparation methods with 4 mol%copper doping.Experimental research shows that the catalysts prepared by coprecipitation method and urea hydrolysis method have isolated Cu²⁺.The Cu²⁺entering the Zn O lattice regulates the electronic structure of semiconductors,making the electronic density around the zinc atom lower and the Fermi level lower,which is conducive to the formation of higher alcohols.Due to the existence of more isolated Cu²⁺,the Fermi level of catalyst prepared by urea hydrolysis is the lowest among the four catalysts,and the proportion of C₂₊alcohols is the highest,up to69.9%.There is a negative correlation between the Fermi level and the formation of C₂₊alcohol.The lower Fermi level promotes the electron transfer process from the reactant molecule to the catalyst surface,which is conducive to the fracture of C-O bond and the formation of C-C bond.In the catalysts prepared by impregnation method and solid phase method,copper species only exist on the surface of Zn O.And the higher Fermi level is not conducive to the occurrence of p-type reaction,and the main alcohol product is methanol.（5）Change the doping amount of n-type Ga from 1 mol%to 13 mol%,the research shows that when the doping amount of Ga is 1 mol%to 3 mol%,Ga³⁺first replaces the position of Zn²⁺in Zn O lattice.The tetrahedron configuration is mainly.As a donor impurity,Ga doping increases the Fermi level and decreases the proportion of higher alcohols.At this time,the catalytic active site is mainly Zn O,and partial Ga is an electronic regulator.As the amount of Ga doping increases,the donor characteristics of Zn O are enhanced.The increase of electrons is not conducive to the p-type reaction,and the proportion of higher alcohols decreases.When the doping amount of Ga is 4 mol%,the carrier concentration becomes saturated,and the amount of Ga entering the octahedral gap increases,resulting in greater lattice distortion.At this point,the octahedral gap Ga and Zn O serve as the main catalytic sites,and the proportion of C₂₊alcohols increases again to 30.5%,indicating that a certain amount of Octahedron Ga can also promote the carbon chain growth.However,when the doping amount of Ga continues to increase to 7 mol%～13 mol%,excessive octahedral gap Ga is not conducive to the formation of C₂₊alcohols.Moreover,the Fermi level of the system increases,which is not conducive to the electron transfer in the p-type reaction.