MFI/MEL intergrowth and its effect on n-decane cracking

A series of intergrowths of zeolite MFI and MEL structures were synthesized with various Si/Al ratios from as low as 8 up to 17 and various degree of intergrowth. The solids were synthesized from gels containing mixtures of tetrapropylammonium bromine and tetrabutylammonium bromine as structure directing agents (T1 and T2), at a chemical molar composition of 100SiO2:xAl2O3:10Na2O:30T:4200H2O. The solids were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), adsorption/desorption of N2 at 77 K, electron diffraction diffuse scattering, chemical analysis by energy-dispersive X-rays and acidity assessment by step-wise temperature-programmed desorption (TPD) of NH3. The percentage of intergrowth was determined by comparison of the experimental diffraction pattern with simulated patterns using DIFFaX software. Well-crystallized samples were obtained from gels with nominal Si/Al ratios of 11 and 25; for Si/Al = 10, the samples were less crystalline. The Si/Al ratios obtained were in most cases lower than that of the starting gel and the intergrowth percentages obtained were lower than expected based on the relative amounts of structure directing agents in the gel; the highest degree of intergrowth achieved was 85% MFI/15% MEL. The effects of intergrowth structures on the activity and selectivity of the n-decane cracking reaction was studied at 400 °C and atmospheric pressure. The catalyst activity was a linear function of the concentration of strong acid sites. The selectivity showed that although the pentasil zeolite pore structure limits isomerization and hydrogen transfer reactions, MFI has a more open structure than MEL and intergrowth shows intermediate behavior between these two structures.