Construction Of Hollow Mesoporous Zn_1-xMg_xO Solid Alkali Nanospheres For The Highly Selective O-Methylation Of Catechol

Selective O-alkylation of catechol is widely used in the synthesis of various chemicals and high value-added intermediates.Alkylation reagents such as sulfuric acid esters and alkyl halides are commonly used in industry,which react to generate a large amount of strong acid,after neutralization,high-salt organic wastewater is produced which is difficult to biochemically treat,causing environmental pollution.Switching to carbonate,solid base-catalyzed selective O-alkylation of catechol is a green process currently recommended,but there are still many difficulties have not been well resolved in the further industrialization of this process:（1）Carbonate has low activity,requiring high temperature and solid base catalyst;（2）There are shortcomings to the solid base such as lack of pore structure,low specific surface,and active site embedding,which seriously affects the mass transfer kinetic properties of the reaction process;（3）It has been reported that the highest selectivity and yield of catechol to guaiacol is 84%,which cannot meet the demand for high selectivity.Therefore,it is necessary to Construct a solid base catalyst with excellent catalytic performance and high O-alkylation selectivity.In this paper,carbon spheres are used as a template to adsorb Mg²⁺and Zn²⁺,the carbon sphere template is removed by high-temperature calcination to prepare hollow mesoporous Zn_1-xMg_xO（x=0,0.012,0.030,0.052,0.089,1）nano-microsphere solid alkali,named Zn_1-xMg_xO HMNSs solid base.At the same time,Zn O-Mg O（1:5）Cs and Zn O-Mg O（0.948:0.052）Cs were prepared by traditional method.Through SEM and TEM characterization,it is determined that the Zn_1-xMg_xO HMNSs solid base has a regular hollow spherical structure with a particle size of 330-420 nm and good dispersibility.The five characteristics of XRD,XPS,FT-IR,HRTEM,and EDS-mapping confirm each other,showing that in this type of solid base,Mg²⁺doping in the hexagonal wurtzite lattice of Zn O forms a series of perfection with different Mg content Eutectic structure,while the comparative sample Zn O-Mg O（0.948:0.052）Cs solid base has only partial eutectic structure and Zn O-Mg O（1:5）Cs solid base has no eutectic structure.BET adsorption results determined that the total surface area of the series of Zn_1-xMg_xO HMNSs solid base was(22.7～46.2 m² g^-1);the pore volume was(0.13～0.31 cc g^-1),and the irregular morphology of Zn O-Mg O（1:5）Cs solid base and Zn O-Mg O（0.948:0.052）Cs solid base has a smaller specific surface area:13.4 m² g^-1and10.8 m² g^-1,and a smaller pore volume:0.07 cc g^-1and 0.09 cc g^-1.CO₂-TPD and NH₃-TPD shows that the prepared series of Zn_1-xMg_xO HMNSs solid base nanospheres all have abundant base/acid sites（including medium strong base/acid sites,strong base/acid sites,Super base/acidic site）,as x increases from 0.012 to 0.089,the content of medium-strong base sites increases in turn,from 0.58 mmol g^-1 to 2.27 mmol g^-1.Similarly,medium-strong acid sites,The content also increased from 0.64 mmol g^-1 to 2.27 mmol g^-1.The comparative sample Zn O-Mg O（0.948:0.052）Cs solid base has only 0.42 mmol g^-1and0.63 mmol g^-1 of strong base/medium strong acid sites.In the selective O-methylation of catechol,Zn_1-xMg_xO HMNSs（x=0.052）solid base catalyst showed the best catalytic performance.Only at 180℃,for 16 h,catechol conversion is as high as 100%,and the guaiacol selectivity and GC yield are both>95%,which is the highest selectivity among all reports so far.Kinetic tracking also proved that the hollow mesoporous microspheres of Zn_1-xMg_xO HMNSs（x=0.052）solid base have excellent catalytic rate.Compared with comparison solid base,the hollow structure significantly accelerates the reaction rate.This article also conducted experiments of the catalyst cycle life:catalytic cycle 3 times,the conversion rate of catechol>98%,the selectivity of guaiacol>93%;after sixth cycle,the conversion of catechol still be maintained at 95%,but the selectivity of guaiacol is significantly reduced to 68%.XRD proved that the destruction of the eutectic structure is the main reason for the decrease in the selectivity of guaiacol.in this paper,the Zn_1-xMg_xO HMNSs solid base catalyst has the following characteristics:（1）Hollow mesoporous nanospheres—beneficial to improve the mass transfer of the catalytic reaction and increase the reaction activity;（2）The acid-base sites are coordinated and adjustable—significantly improved the selectivity of catechol O-alkylation;（3）Regular spherical morphology—used in industrial bed reactors to reduce wear and pulverization and prolong cycle life.It solves the problems of traditional solid base that has few active sites,high reaction temperature and low selectivity catalysts in the selective O-methylation reaction of catechol,shows industrial application prospects.