Effects Of Fe Or Sn Additives On Propane Aromatization Performance Of Pt Promoted Zn/HZSM-5 Nano-zeolite Catalyst

Benzene,toluene and xylene(BTX)are important chemical raw materials.It is of great significance to increase the production of BTX via short-chain alkane aromatization.In this study,in order to improve the catalytic performance of Zn/HZSM-5 catalyst in short-chain alkane aromatization,the addition of small amount of platinum onto Zn/HZSM-5 nano-zeolite catalyst was systematically studied.It is expected to accelerate the recombinative desorption of hydrogen atoms over active Zn species of the Zn/HZSM-5 zeolite catalyst to overcome the low aromatics selectivity and high dry gas(methane and ethane)production problems.The main contents and results are as follows:Firstly,Znx/HZSM-5 nano-zeolite catalysts with different Zn loading(x=0.1?8.0 wt.%)were prepared as base catalysts for further modification by impregnation.Then,the performance of propane aromatization was tested by both pulsed reactor and fixed-bed reactor Then,Pt was directly introduced onto Znx/HZSM-5 to study the effect of promoting aromatics selectivity.In propane aromatization,the modification of HZSM-5 nano-zeolite with Zn ion significantly enhanced the aromatics selectivity.As the Zn loading increased from 0.1 to 8.0 wt.%,the optimal aromatics selectivity is only 35%.While the lowest selectivity of dry gas(methane and ethane)is still higher than 65%.The poor dispersion of Pt onto Zn/HZSM-5 nano-zeoite by chloroplatinic acid impregnation could not increase aromatics selectivity of the Znx/HZSM-5 nano-zeolite base catalysts.Therefore,efforts were made to improve Pt dispersion by modifying the Znx/HZSM-5 catalysts with Fe additive.The Zn1.0Fe0.3Pt0.1/HZSM-5 nano-zeolite catalyst was developed on the basis of Zn loading and Fe loading optimization.The average particle size of the Zn1.0Fe0.3Pt0.1/HZSM-5 nano-zeolite catalyst was estimated to be about 2 nm.During the fixed-bed propane aromatization test(of T=550?,P=0.1 MPa and WHSV=0.55 h-1),the Pt containing catalyst exhibited 50%propane conversion and 51%aromatics selectivity approximately.In addition,the selectivity of methane and ethane was lower than 36%.The Zn1.0Fe0.3Pt0.1/HZSM-5 nano-zeolite catalyst was found having excellent dehydrogenation aromatization performance which is great better than that of Zn/HZSM-5 catalyst.The Zn1.0Fe0.3Pt0.1/HZSM-5 catalyst showed anti-coking deactivation ability in a continuous 900 h propane aromatization test.Further effort was made by modifying the Znx/HZSM-5 nano-zeolite base catalysts with Sn additive prior to the impregnation of chloroplatinic acid to improve the stability.Results showed that the pre-treatment of base catalysts with stannic chloride solution was also helpful to enhance Pt dispersion.The Zn6.0Sn0.2Pt0.1/HZSM-5 nano-zeolite catalyst was obtained by optimizing Zn and Sn Pt amount with an average metal particle size about 2 nm.Under the same fixed-bed propane aromatization conditions,the aromatics selectivity of the Sn mediated Pt-containing catalyst was more than 46%with almost 70%propane conversion.In comparasion with the Zn6.0/HZSM-5 catalyst,the aromatics selectivity of the Zn6.0Sn0.2Pt0.1/HZSM-5 nano-zeolite catalyst was 14 percent higher.In addition,the Zn6.0Sn0.2Pt0.1/HZSM-5 catalyst also obtained higher anti-coking deactivation ability.It was seen keeping its catalytic activity stable in a 3000 h test.The stability of Zn6.0Sn0.2Pt0.1/HZSM-5 catalyst was much superior to the above-mentioned Zn1.0Fe0.3Pt0.1/HZSM-5 catalyst,although its aromatics selectivity is relatively lower.Based on the foregoing results,the self-developed dual beam FT-IR spectrometer was employed to run in situ FT-IR spectroscopic study of propane aromatization.In this work,propane aromatization over Zn1.0Fe0.3Pt0.1/HZSM-5 and Zn6.0Sn0.2Pt0.1/HZSM-5 catalysts were studied.The spectroscopic and on-line product mass spectrometry findings were employed to understand the mechanism of propane dehydroaromatization.Results showed that,the propane aromatization over Zn1.0Fe0.3Pt0.1/HZSM-5 and Zn6.0Sn0.2Pt0.1/HZSM-5 nano-zeolite catalysts followed the Br(?)nsted-Lewis acid synergistic catalytic mechanism.Moreover,the catalytic performance of Zn/HZSM-5 catalysts in the propane dehydroaromatization was remarkably improved by the addition of metal dehydrogenation active sites such as FePt and SnPt,and by the assistance of FePt and SnPt sites in accelerating the recombinative desorption of hydrogen atoms from[Zn-O-Zn]2+ Lewis acid sites.In situ spectroscopic study revealed that the absorption band(2983 cm-1)of Zn methyl surface species in both Zn1.0Fe0.3Pt0.1/HZSM-5 and Zn6.0Sn0.2Pt0.1/HZSM-5 nano-zeolite catalysts was notably weaker than that on the reference catalysts.Obovementioned results support the hypothesis that the FePt and SnPt metal active sites have assisted the recombinative desorption of hydrogen atom from[Zn-O-Zn]2+ Lewis acid sites thus improving aromatics selectivity.