Acidity And Catalytic Performance Of ZnO Modified Nano-Silicalite-1 Zeolite

Silicalite-1(S-1)zeolite is an aluminum-free crystalline silicate which has the same MFI topological structure as the Al-containing ZSM-5 zeolite.Generally,S-1 zeolite has high specific surface area,good thermal stability and hydrothermal stability.However,with the ultra low acid amount and rather weak acid strength,its application in the field of catalysis has been limited.On the other hand,new catalysts are needed for both the non-oxidative dehydrogenation of C3?C4 short-chain alkanes and the aromatization of paraffins such as C6 or C7 alkanes.Therefore,in this dissertation,our works are focused on enhancing the acidity of S-1 zeolite,and on developing novel catalysts with the modified S-1 zeolite for short-chain alkanes dehydrogenation and paraffins aromatization.The main contents and conclusions are as follows:First,a series of ZnO modified nano-size S-1 zeolite catalysts(ZnOx/S-1)were prepared by using the wetness-impregnation method.The acidity of ZnOx/S-1(1.2?x?14.9)catalysts was studied by NH3-TPD and alkaline molecular probes(pyridine or ammonia)adsorption FT-IR spectroscopy.The results indicated that the acidity of S-1 zeolite was dramatically increased by the modification of ZnO.The total acid amount of ZnO10.0/S-1 catalyst reached about 40 times as much as that of parent S-1 zeolite.The acid strength of ZnOx/S-1 catalysts was also obviously enhanced by the modification.Nevertheless,compared to the ZnO modified HZSM-5 zeolite,it should still belong to the category of weak acid.Ammonia(or pyridine)-adsorbed FT-IR spectra showed that the ZnOx/S-1 catalysts have both Lewis and Bronsted acid sites.According to the FT-IR characterization and quantum chemistry calculations,the Bronsted acid sites of ZnOx/S-1 were related to the zinc hydroxyls([Zn-OH]+)of subnanomeric ZnO clusters mainly located in the zeolite pores.The transfer of H protons from the silanols of S-1 zeolite to the subnanomeric ZnO clusters should account for the formation of the[Zn-OH]+groups.In addition to the[Zn-OH]+ groups produced by the ZnO modification,it was found that the dissociative adsorption of isobutane over the subnanomeric ZnO clusters also generated[Zn-OH]+groups which showed similar Brinsted acidity.Therefore,it was found that the ZnVx/S-1 catalysts have both inherent Bronsted acid sites([Zn-OH]+ groups produced by the ZnO modification)and transient Br(?)nsted acid sites([Zn-OH]+groups generated by the dissociative adsorption of alkanes).Second,the catalytic performance of ZnOx/S-1 catalysts for the non-oxidative dehydrogenation of short-chain alkane was investigated with a fixed bed micro-reactor by using isobutane as probe molecule.The results showed that,under the conditions of 550 ?,i-C4H10/N2= 1:1(V/V)and GHSV= 1200 h-1,the parent S-1 zeolite was essentially inert for isobutane conversion.However,up to 60.2%?66.7%isobutane conversion and 84.6%?90.9%butene selectivity were achieved with the ZnOx/S-1(3.7?x?14.9)catalysts.The isobutane conversion was linearly correlated to the total acid amounts of ZnOx/S-1 catalysts.Compared with the ZnO/ZSM-5 reference catalyst,the selectivity of cracking by-products on ZnOx/S-1 catalyst was significantly suppressed.In addition to isobutene,a large portion of butene isomers and some aromatics products were also detected in the products of isobutane non-oxidative dehydrogenation.These results indicated that the Bronsted acid sites([Zn-OH]+)of ZnOx/S-1 catalyst can catalyze the isomerization and aromatization of olefins.Third,ZnO/S-1 supported Pt catalyst(Pt0.1-ZnO7.5/S-1)was prepared by using the wetness-impregnation method.H2PtCl6 was used as precursor.Our experimental characterizations showed that,the Pt0.1-ZnO7.5/S-1 catalyst contained not only Lewis acid-type subnanomeric ZnO cluster active sites([Zn-O-Zn]2+)and Bronsted acid active centers([Zn-OH]+),but also metal active centers(Pt0 and Pt?).Clearly,the Pt0.1-ZnO7.5/S-1 catalyst was a metal-acid bifunctional catalyst.D2 adsorption DRIFT indicated that,the Pt centers of the Pt0.1-ZnO7.5/S-1 catalyst could accelerate the recombination of[Zn-OD]+and[Zn-D]+from the[Zn-O-Zn]2+active sites,which would help to recover the[Zn-O-Zn]2+active sites.The Pt0.1-ZnO7.5/S-1 catalyst showed significantly enhanced n-hexane conversion and aromatization selectivity.Pulse experimental study showed that 61.4%of n-hexane conversion and 71.5%of benzene selectivity were obtained with Pt0.1-ZnO7.5/S-1 catalyst under 500?and atmospheric pressure.Under the same conditions,the corresponding reaction results with Zn07.5/S-1 catalyst were 17.1%and 40.9%,and those with a conventional reforming catalyst(Pt0.3-Re0.3/Al2O3-C11.0)were 33.3%and 51.2%.Moreover,the Pt0.1-ZnO7.5/S-1 catalyst showed good resistance to sulfur poisoning.