Catalytic performances of b-oriented bi-layered HZSM-5 coatings for cracking of hydrocarbon fuels

Abstract

b-Oriented HZSM-5 bi-layered coatings were prepared on inner wall of stainless steel tubes (ϕ 3×0.5mm) by layer by layer method with the assistance of TiO2 interlayer. The zeolite coating catalysts were characterized by XRD, SEM, EDS, NH3-TPD and in situ FT-IR. The fraction of b-oriented crystals (Fb), quantitatively classifying the orientation of coatings was defined and calculated. The thickness of TiO2 interlayer was shown to be a highly sensitive parameter, affecting Fb. With the thickness of TiO2 interlayer from 0 (without TiO2 interlayer) to 580nm, Fb increased continuously from 38.5% to a maximum value of 89.7%. The permeation of N2 gas through the coating could be affected by Fb, the thickness of zeolite layer and the TiO2 interlayer. The catalytic activity of HZSM-5 zeolite bi-layered coating was tested in the form of structured catalyst using catalytic cracking of n-dodecane (550°C, 4.0MPa) as a model reaction and showed good agreement with their N2 permeation. The HZSM-5 bi-layered coating with relatively high Fb (78.4%) and thin TiO2 interlayer (200nm) exhibited superior N2 permeation of 6.25×10−5mol/m2SPa and highest conversion of n-dodecane with 93% improvement based on a blank sample without active zeolite layer. While the coating with a a-oriented top layer and a b-oriented underlay (Fb=38.5%) showed only a N2 permeation of 6.75×10−6mol/m2SPa and a 25% improvement of conversion. This suggested that the enhanced diffusivity originating from the straight and short channels along b-oriented direction benefits the catalytic performance of HZSM-5 zeolite coating.