minh post mot bai viet ve mot ung dung khac cua zeolite: thuc hien cac phan ung hydroisomer hoa va phan ung hydrocracking trong pha long cua hon hop cua ankan tren xuc tac zeollite. Cac ban tham khao nhe...
Hydroisomerization and Hydrocraking in liquid phase of n-alkane mixtures on
zeolites
J. A. Muñoz Arroyo1, J. A. Martens2, G. B. Marin3
1Instituto Mexicano del Petróleo, Eje Central Lázaro Cárdenas No. 152, San Bartolo
Atepehuacan, México, D. F., 07730, Centrum voor Oppervlaktechemie en Katalyse,
Departement Interfasechemie, K.U. Leuven, Kard. Mercierlaan 92, B-3001 Heverlee,
Belgium, 3Laboratorium voor Petrochemische Techniek, Ghent University, Krijgslaan 281,B-9000 Gent, Belgium.
Introduction
The increase of heavy oil processing in the world, has made hydrocracking an important oil refinery process inside of a modern refinery. The conversion of residuals into more valuable products allows to equilibrate the supply and demand of gasoline, jet fuel and diesel is equilibrated. In this process, bifunctional catalysts consisting of a noble metal and acid support such as zeolite catalysts are used to carry on the isomerization and cracking reactions involved [1].
The hydrocracking of n-paraffins on Pt supported on faujasites such as Y zeolites exhibits a considerable amount of cracking products at high conversions which limits the production of branched hydrocarbons for the C8-C12 fractions [2, 3]. However an increase of the yield of branched hydrocarbons together with low hydrocracking product yields from n-paraffin can be obtained using shape selective zeolites such as Pt/H-ZSM-22. On these catalysts the cracking reactions are suppressed, owing to the sterical constraints in the micropores of the zeolite [4-8].
Shape selectivity on Pt/H-ZSM-22 via the pore mouth and key-lock catalysis has been observed for the hydrocracking of paraffin and naphthene model components in vapour phase [6, 7]. A typical feature of Pt/H-ZSM-22 in the hydrocracking of n-paraffin is the suppression of di- and tribranched hydrocarbons together with the decrease in the formation of cracking products and centrally branched hydrocarbons with respect to the isomers with a branch in the C2 position.
The hydrocracking of n-paraffins on Pt/H-ZSM-22 leads to the formation of linear cracking products, which can be explained by the suppression of di- and –tribranched isomers susceptible to (s,t), (t,s) and (t,t) β-scission and the predominance of the (s,s) β-scission. Based on these results in this work a comparative study for the hydroisomerization and hydrocraking of a n-paraffin mixtures on Pt/H-ZSM-22 and Pt/USY zeolite catalysts at industrially relevant conditions and in particular in the liquid phase is performed (490 to 533 K, 6 to 9 MPa and a H2/HC ratio from 4 to 5).
Results and Discussion
A comparison of the product yields obtained from n-alkane mixtures on Pt/USY and Pt/ZSM-22 is performed. Monobranching, dibranching and cracking yields increase with increasing conversions on both catalysts. On Pt/ZSM-22 the net yield of single branched isoparaffins is slightly higher than on Pt/USY, whereas on the Pt/ZSM-22 catalyst the cracking is somewhat more important than on Pt/USY at conversions below 30 %, the situation is reversed at higher conversions, since the formation of tribranched isomers on Pt/USY is favoured.
An analysis of the distribution for the single branched and dibranched isoparaffins present in the hydrocracked effluent is also carried out. Isoparaffins with carbon numbers C9-C18 are obtained by skeletal isomerization of the n-paraffins in the feedstocks. Preferential formation of the 2-MeCn by isomerization of n-paraffins on Pt/ZSM-22 are observed. The selective formation of these isomers is a manifestation of pore mouth catalysis. On Pt/USY, more centrally branched isoparaffins are produced as expected in the absence of shape selectivity. On Pt/ZSM-22 for each carbon fraction of the mixture, only small quantities of dibranched isoparaffins are produced in comparison with the amounts observed for Pt/USY. The selectivity pattern for skeletal branching of n-paraffins for both catalysts does not show
important differences with respect to that already established in the vapor phase for pure nparaffins. In the liquid phase, the same tendencies are observed as in the vapour phase [8-10], although they are less pronounced. Skeletal isomerization of n-C10 leads to the selective formation of 2,7-diMe-C8 and 2,6-diMe-C8. For n-C11, the preferred dibranched isomers are 2,8-diMeC9 and 2,7-diMeC9. The formation of these specific isomers was explained by a mechanism in which the preferred 2-MeCn single branched isoparaffins are stretched across an external [001] face of a ZSM-22 crystal such that the first methyl group is pinned in one pore mouth, while the second methyl branching is generated at the other end of the carbon chain where it reaches a neighbouring pore mouth, this mechanism correspond to the keylock catalysis reported previously in the vapour phase. Several differences for the cracking products on Pt/H-ZSM-22 and Pt/USY were observed, whereas on Pt/USY the predominant mechanism was (t,t) β-scission, on Pt/H-ZSM-22 the (s,s) β-scission mechanism is prevailing. For Pt/USY the central cracking prevails producing mainly C5 and C6. On Pt/H-ZSM-22 the production of C3 is more pronounced together with C4 and C5 products.
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