Design And Performance Of Catalyst For Catalytic Cracking Of Alkanes

Naphtha catalytic cracking is one of the representative technologies for the new production processes,and is expected to be the main source of light olefins in future.In this paper,naphtha cracking over the zeolite based catalysts has been systematically investigated,including analysis of the reaction network for n-pentane catalytic cracking,exploration of influences of element incorporation and high-temperature regeneration on the element reactions and cracking performance,and design of SO₄^2-/TiO₂/ZSM-5 catalyst directed by the surface modification mechanism.The purpose is to elucidate the intrinsic relationship between catalysts,element reactions,and cracking performance,providing theoretical guidance for catalyst design in naphtha catalytic cracking process.Reaction network for alkanes cracking over zeolites is complex,and it is hard to define the relationship between products and reaction pathways.In order to solve the problems,the element reactions for n-pentane catalytic cracking have been figured out based on B acid initiation mechanism.It was found that the initial cleavage was mainly at C-H/C₂-C₃ bond,generating carbenium ions,which triggered the hydride transfer,Beta scission,and some side reactions.Modulation of the initial cleavage and the reaction pathways related to carbenium ions made a difference on the product distribution.Compared with HZSM-35 and H-Beta zeolites,HZSM-5 possessed a middle pore size,which exhibited an optimized role in the initial cleavage,hydride transfer,and side reactions,and thus were more suitable for naphtha catalytic cracking to produce light olefins.The selective modulation of reaction pathways and explanation of its effects on the cracking performance are facing challenges in the present study.In this paper,the ZSM-5 based catalysts were modified by the element incorporation and high-temperature regeneration.It was found that the Sr-,Zr-and La-incorporation selectively enhanced the hydride transfer reactions and promoted the cracking activity of the ZSM-5 based catalysts.The promoting effect gradually strengthened in the order of Sr<La<Zr.Ag sites and redox sites generated by Ag-incorporation and high-temperature regeneration respectively initiated the dehydrogenation and redox cracking routes,which selectively enhanced C-H bond breaking and suppressed hydride transfer,promoting the selectivity to light olefins;however,they also accerrated some side reactions,leading to the decrease of propylene/ethylene ratio and the rapid deactivation of catalysts.CLD strategy forced the incorporated atoms enriching on the external surface of ZSM-5 zeolite,retaining its native MFI-structure,micropore system and acid properties.CLD surface modification enhanced the adsorption diffusion process and promoted the cracking activity of the ZSM-5 based catalysts.SO₄^2-/TiO₂/ZSM-5catalysts were prepared by introducing SO₄^2-group into TiO₂-CLD layer,which further enhanced the adsorption diffusion process,and promoted the cracking activity;at the same time,it selectively hindered the routes from light olefins to cyclo-species,aromatics,and thus,effectively inhibited the formation of external coke in the normal-alkanes catalytic cracking.