Catalytic cracking of n-dodecane over hierarchical HZSM-5 zeolites synthesized by double-template method

摘要

Hierarchical HZSM-5 zeolites have been prepared by a double-template method with tetrapropylammonium hydroxide and polydiallydimethylammonium chloride (PDDA) as micro- and meso-scale template, respectively, and Al(NO3)(3) as aluminum source. The as-prepared zeolites were characterized by XRD, SEM, TEM, N-2-adsorption-desorption, TGA, ICP, in situ FT-IR and NH3-TPD. The hierarchical HZSM-5 zeolite prepared at the PDDA/TEOS weight ratio of 0.2 in the synthesis gel (HZ-0.2P) had uniform mesopores (11-13 nm) and high acid site accessibility. An increase of the PDDA/TEOS resulted in enlargement of mesopore size and broadening of size distribution, together with appearance of some non-uniform accumulation pores. Moreover, the accessibility of acid sites in hierarchical HZSM-5 decreased. Catalytic cracking of n-dodecane carried out at 500 degrees C and 4 MPa in a wall-coated tubular reactor with HZSM-5 zeolites showed that HZ-0.2P exhibited the superior catalytic activity and stability, with the increase higher than 20% in the conversion of n-dodecane and reduction to ca. 1/2 in the deactivation rate compared with micro-porous HZSM-5 zeolite. This could be ascribed to its highest acid site accessibility, which improves the diffusion of n-dodecane and its cracking product molecules inside pore channels of catalyst.