Investigation Of Supported Metal Catalysts On The Modified Zeolite ZSM-22 For The Hydroisomerization Of N-dodecane

With the increasingly strict environment laws and regulations and the rapid development of the automobile industry,these requirements for the quality of oil products are becoming higher and higher.Hydroisomerization technology plays an important role in improving oil quality,such as octane number of gasoline and low temperature fluidity of diesel oil.At present,acidic porous zeolite supported precious metal Pt catalyst is mainly used in industry,its catalytic performance needs to be further improved due to its low catalytic activity and products yield.Therefore,it is very important to preparehigh performance hydroisomerization catalyst.in this work,we modified zeolite ZSM-22 to adjust its acidity and loaded precious metal Pt and non-precious metal Ni based catalysts to study the hydroisomerization performance of n-dodecane.These prepared catalysts were characterized by XRD,N₂-adsorption,NH₃-TPD,SEM,TEM,H₂-TPR,CO chem-adsorption and Py-IR characterization,and the detailedexperimental research content is as follows:In the first part,ZSM-22 zeolite was modified with citric acid solution(CA)with different concentrations and then supported with Pt catalyst(Pt/ZSM-22-CA-x,x=0,1,2,3)to study the hydroisomerization performance of n-dodecane.The results showed that the catalytic effects of Pt/ZSM-22-CA-2 and Pt/ZSM-22-CA-3 catalysts were similar because of the similar properties of the B acid centers.In addition,its catalytic performance is better than the traditional Pt catalyst prepared by NH₄⁺-exchanged ZSM-22 zeolite.In the second part,the traditional non-noble metal Ni-based catalyst was used to replace Pt catalyst.After the zeolite ZSM-22 was modified with diluted nitric acid,the Ni_xP_y catalyst was prepared by impregnation method.The catalytic performance of these Ni_xP_y/ZSM-22-HNO₃-x(x=0,1.0,1.5,2.0,4.0)for the hydroisomerization of n-dodecane was investigated.The results showed that when the Ni loading was 1.0 wt.%with the Ni:P=1:1,the catalytic performance is the best,this is because that Ni_xP_y particles provided sufficient metal sites for dehydrogenation-hydrogenation,and is highly dispersed on the surface of the catalyst.The calculated average particle size on these catalysts were increased as followed:0.5Ni_xP_y/ZSM-22-HNO₃(3.6 nm)<1.0Ni_xP_y/ZSM-22-HNO₃(3.9 nm)<1.5Ni_xP_y/ZSM-22-HNO₃(4.2 nm)?2.0Ni_xP_y/ZSM-22-HNO₃(4.3 nm)<4.0Ni_xP_y/ZSM-22-HNO₃(5.9 nm).It can be seen that when the appropriate Ni loading is 1.5 wt.%and 2.0 wt.%,the average particle size of the two catalysts can be considered to be the same,indicating that the number of metal/acid center sites of the two catalysts is similar,therefore,the catalytic performance of the two catalysts is similar.The third part is to ensure that the Ni loading is 1.0wt.%with the Ni:P=1:1.The ZSM-22 zeolite was modified by metal oxide MgO with different loading(Ni_xP_y/ZSM-22-Mg-x,x=0.5,1.0,2.0,3.0,4.0)was investigated.In other words,the ZSM-22 zeolite was modified by thedifferent concentrations of MgO.The results showed that when the Mg load was 0.5 wt.%and 1.0 wt.%,the acidity and acid content of the two catalysts were similar,so the catalytic performance of the two catalysts was similar.However,with the further increase of the MgO loading,the prepared catalyst showed a worse catalytic performance.In the fourth part,some new ideas were proposed based on the above research workds,and the hydroisomerization performance of new hydroisomerization catalysts Mo_xP_y,W_xP_y and Mo S_x relative to n-dodecane was designed initially.The research results showed that W_xP_y/ZSM-22catalyst had no catalytic activity.However,the catalytic activity of Mo_xP_y/ZSM-22 catalysts was significantly lower than that of Ni_xP_y catalysts.In addition,the selectivity of the catalysts may be related to the existence of Mo species in the impregnation solution.