Modification Of HZSM-5 And Its Catalytic Performance In Co-aromatization Of Methanol And N-hexane

There are mainly two routes for aromatics production:petroleum route and non-petroleum route.The traditional route for preparation of aromatics such as benzene,toluene and xylene(BTX)is mainly the naphtha reforming,petroleum thermal cracking and other processes.Coal,natural gas and biomass in non-petroleum routes can be efficiently converted into aromatics,in which the novel process of coalbased methanol to aromatics(MTA)uses the coal-based methanol with the excess production in China and is clean,energy saving,efficient and low-cost.In addition,more low-value light hydrocarbons are by-produced in coal and petroleum chemical industry of China,while are difficult to effectively utilize due to lower octane value and lower saturated vapor pressure.Co-aromatization might be an efficient route for utilizing them and this route had been thermal-dynamically verified.However,the reported researches provide low aromatics yields.In this work,the co-aromatization of light hydrocarbons(using n-hexane as a model)and methanol into BTX are studied and the effects of modification on HZSM-5 including impregnation by Zn and Mo,alkali treatment and core-shell structure on the properties of catalyst and the aromatics production distribution.1.The series of[Zn,Mo]/HZ-5 catalysts were prepared by impregnation method.The introduction of Zn and Mo regulated the product distribution and improved the aromatic yield.The modification of Zn and Mo turned the acidity of HZSM-5,resulting in the decrease in strong acid amount and the increase of weak acid amount.Br?)nsted acid amount decreased,and the L/B value increased significantly from 0.11 to 1.51 over HZ-5.The Zn-3Mo/HZ-5 catalyst showed the best catalytic performance giving the aromatic yield of 54.5%under the conditions of methanol/n-hexane ratio of 7:3,430℃ and 1.0 h-1.It was found that the increase of reaction temperature would lead to the change of product distribution,and the appropriate increase of reaction temperature was beneficial to the aromatization reaction.The aromatic yield further reached 64.0%at 470℃.2.The hierarchical M/HZ-5-x catalysts were prepared by treating[Zn,Mo]/HZ-5 with alkali and were characterized by XRD,N2 adsorption/desorption,TEM,NH3-TPD,Py-FTIR,FT-IR,ICP-OES,and GCMS for its pore structure and acidity.The results showed that alkali treatment decreased Br?)nsted acid amount and increased Lewis acid amount,leading to a modulation of L/B value from 1.12 for M/HZ-5 to 2.24 for M/HZ-5-Na(alkali treatment).The aromatic yield over M/HZ-5-Na was increased from 40.7%to 44.0%compared with M/HZ-5.The catalyst lifetime was prolonged and the lifetime of M/HZ-5-Na was 2.5 times that of M/HZ-5.The deactivation behavior of M/HZ-5-Na catalyst was also investigated.The enhanced stability of catalysts might be relatve with its larger capacity of carbon deposit for M/HZ5-Na catalyst.3.To increase the selectivity of pX,core-shell HZSM-5@Silicalite-1(x)(x is the number of wrapping times)catalysts with different wrapping times were prepared by hydrothermal method.HZSM-5@S-1(x)still kept the typical MFI-topologhy structure,and its pore structure did not change significantly.The characterization tests of Py-FTIR and DTBPy-FTIR showed the reduction in total acid amount,Lewis acid amount and Br?)nsted acid amount.Eapecially,the acid sites on the outer surface of HZSM-5@S1(2)decreased dramatically compared with HZSM-5.The catalyst HZSM-5@S-1(2)has excellent catalytic performance with 89.9%of n-hexane conversion and 83.8%of to pX selectivity(in xylene)in the co-aromatization reaction of methanol with n-hexane at 430℃ and WHSV of 1.0 h-1.The lower carbon deposit on HZSM-5@S-1(2)compared with HZSM-5 indicates that the coating of silicalite-1 shell inhibits the coking reaction and reduces the carbon deposit on the catalyst.