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Abstract
S
The catalytic behavior of several H-ZSM-5 zeolites and alumina samples were evaluated. The investigated catalysts namely, H-Z, Pt/H-Z, DH-Z, Pt/DH-Z, Pt/(Al2O3+H-Z), Pt/(Al2O3+DH-Z) and Pt/Al2O3, were characterized and catalytically tested through the hydroconversion of C7 and C8 n-alkanes. The reactions were conducted at temperatures within the range of 250º-450 ºC, using the pulse technique. The conclusions drown from this study may be summarized as follows:
1) Reaction temperature is an important factor influencing the yields and constituents of the products. n-Paraffins mainly undergo hydroisomerization at relatively lower to moderate temperature range. Higher temperature hinders the formation of isomerates, but favors both cracking and aromatization reactions due to their endothermicity.
2) The dual modification of H-Z zeolite, via dealumination and Pt incorporation is found to enhance the hydroconversion activities of the catalysts.
3) Pt-incorporation is greatly effective, for improving the hydroisomerization of n-alkane.
4) H-Z zeolite subjected to dealumination a quires acid sites number with higher strength, compared to that of the parent H-Z. Moreover, the unloaded zeolites (whether modified or not) possess higher acidities than the corresponding Pt-loaded versions
5) There is a strong parallelism between the occurrence of a certain reaction and the presence of acidic sites of a given strength. Sufficiently strong acid sites are responsible for cracking reaction, whereas acidic centers with relatively intermediate strength levels are required to promote isomerization reaction.
6) H-ZSM-5 zeolite is found to be an excellent catalyst for hydrocracking reaction even at relatively low temperatures. Evidently, the high strength of acid sites and the large surface area of this type of zeolite are the main accountable for cracking enhancement. The catalytic Selectivity to hydrocracking the longer chain octanes exceeds that of heptanes with the preference of centric C-C bond rupture rather than peripheral. On the other, alumina-containing catalyst has the lowest efficiency to alkane dissociation, compared to zeolites, as it possesses the least concentration of strong acid centers.
7) The pore structure of a catalyst plays a very important role in the dehydrocyclization reaction. The abundance of cyclic components, in the converted product of n-alkanes over Pt/Al2O3, is referred essentially to the open structure of alumina support. Contrarily, the limited formation of such compounds with H-Z carrier can be ascribed to the narrow opertures of the zeolite. Consequently, the various supports can be arranged ascendingly according to the selectivity of their corresponding catalysts to dehydrocyclization reaction in the order of:
H-Z < DH-Z < H-Z +Al2O3 ¬< DH-Z+ Al2O3 < Al2O3
8) In order to boost the catalytic selectivity to a certain reaction, the following trend must be implemented:-
a. Highly acidic H-ZSM-5 zeolites are recommended for cracking and isomerization reactions, for producing light hydrocarbons in good yields.
b. To obtain a high catalytic selectivity towards the dehydrocyclization reaction, it is necessary to use alumina as a support.
c. To maximize the selectivity to the uncracked iso-paraffins formation, Pt/alumina-containing catalysts are selected.