Tailoring the product distribution of n-heptane and shale oil cracking over Fe-La bimetals confined in ZSM-35 zeolite

Abstract

Great progress has been made in the catalytic cracking of shale oil by introducing metals into zeolites for regulating acidity to tailor the distribution of olefin products. However, the interaction between the loaded bimetals and their synergistic effect with the acid sites of the zeolite remains controversial. In this work, catalysts of highly stable Fe-La bimetals confined in ZSM-35 zeolite (Fe-La-ZSM-35) were constructed with appropriate acidic properties. Real shale oil and n-heptane as a representative reactant were chosen to explore the catalytic cracking performance of these materials. The Fe-La species formed metal sites bonded to the zeolitic framework in the confined microenvironment of ZSM-35, which established strong interactions. Comparing Fe-La-ZSM-35 with Fe/La-ZSM-35 synthesized by the impregnation method revealed that the direct doping method could better incorporate La and Fe species within the zeolite, resulting in stronger interactions between the bimetals and framework aluminum to form a large number of medium-strong Lewis acid sites (Fe-OH). The ratio of Brønsted acid sites to Lewis acid sites decreased from 1.56 in Fe/La-ZSM-35 to 0.45 in Fe-La-ZSM-35. Furthermore, the electron cloud density at the active Fe-OH site increased due to electron transfer from La to Fe, which promoted the adsorption of carbocations, improved stabilisation of carbocations and bimolecular reactions were inhibited. And the C3-C4 bonds of n-heptane molecules were more easily attacked due to the weak acidity of these acid sites, resulting in propylene being the main product as the reaction C7H16 → C4H10 + C3H6 was dominant during the catalytic cracking of n-heptane. The propylene yield of Fe-La-ZSM-35 increased to 35.35% compared with La-ZSM-35 (22.62%) and Fe/La-ZSM-35 (26.16%) at 600°C. During three reaction-regeneration cycles, n-heptane conversion and propylene selectivity were as high as 89.33% and 30.85%. For real light shale oil, the catalyst achieved a propylene yield and selectivity of 19.24% and 31.0%, demonstrating excellent feedstock generalizability. In addition, Fe-La-ZSM-35 exhibited excellent stability, indicating considerable potential for use in the catalytic cracking of real oil.