Role of Shape Selectivity in n-Heptane Cracking and Aromatization Reaction on Modified ZSM-5

Abstract

Cracking and aromatization reaction of n-heptane was carried out on ZSM-5 modified by three methods: They are (i) Extra framework alumina obtained by steaming at high temperature, (ii) Altering the pore size by coke deposition as a function of time on stream, and (iii) Presence of amorphous material in the zeolite. The effect of these modifications on the shape selective product p-xylene is investigated. In all the cases the shape selective properties are profoundly influenced. The presence of extra framework alumina, and coke increased the concentration of para xylene in the product. In the case of ZSM-5 containing amorphous material obtained by synthesizing the zeolite without the aid of template, the para xylene in the product was higher than that obtained over zeolite synthesized using the template. In all the cases the increase in shape selective product may be resulted by decrease in channel dimensions due to the presence of the extraneous materials in the channel. Pore size distribution studies on deactivated catalyst provided evidence for the operation of molecular traffic control (MTC) mechanism in the case of n-heptane aromatiztion reaction.Ever since weisz and co-workers from Mobil coined the word shape selectivity and demonstrated its commercial potential using small pore molecular sieves, shape selective properties of zeolites have been studied with great interest [/]. The availability of MFI type medium pore zeolites of ZSM family with pore sizes compatible with molecules in gasoline range has opened up new vistas in hydrocarbon transformations [2,3]. Based on their acidic and shape selective properties a number of processes were developed in the recent past [2]. Xylene isomerization, Methanol to gasoline, Distillate and lube dewaxing, M-forming, Mobil selective toluene disproportionation, manufacture of para diethyl benzene are 130

Review on the Catalytic Conversion of Naphtha to Aromatics: Advances and Outlook

“…The optimized zeolite catalyst exhibited higher olefin selectivities and lower aromatic selectivities. The ZSM-5 zeolites modified 18 using different methods, i.e., steaming, coke deposition, and synthesis of the zeolite without using a template (amorphous material in the zeolite), were used for aromatization of n-C 7 . All three modified catalysts resulted in higher ratios of para-xylene (shape-selective production) due to the decreased channel dimensions.…”