Development of zeolite based acid catalysts for the hydroisomerization of n-paraffins

• Main Author: Saberi, Mohammad Ali
• Format: Others
• Published: 2002
• Online Access: http://spectrum.library.concordia.ca/1840/1/NQ73338.pdf; Saberi, Mohammad Ali <http://spectrum.library.concordia.ca/view/creators/Saberi=3AMohammad_Ali=3A=3A.html> (2002) Development of zeolite based acid catalysts for the hydroisomerization of n-paraffins. PhD thesis, Concordia University.

There are some abundant long chain n -alkanes in refinery stream fuels which have a lower octane number and a higher pour point. It would be thus desirable to convert these normal paraffins to their branched isomers (which have higher octane number). Novel classes of cation incorporated Pt-M-HY (M being cations such as Zn 2+ , Al 3+ , Cd 2+ , Ni 2+ , Cr 3+ and some others) and bifunctional Pt-HY zeolite catalysts have been prepared and tested for hydroisomerization of n -Paraffins ( n -heptane as a model). The modified zeolite catalysts showed superiority to that of the parent bifunctional Pt-HY zeolite catalyst in term of isomerization of n -heptane. This is due to the desorption-transfer promoting behavior of the mentioned cations which cause fast removal of intermediate alkylcarbocation from Brønsted acid sites (decreasing the residence time of these species on acid sites). By decreasing the residence time of the intermediate alkylcarbocations on Brønsted acid sites, these species did not undergo further cracking reactions. As a result the isomerization products increased significantly and cracked products decreased. Such behavior was ascribed to the formation of new desorption-transfer promoting sites. A triangular "acid/dehydro-hydrogenation/desorption-transfer promoting" site configuration (trifunctional catalyst) was proposed. The data obtained from characterization studies such as BET, XRD, FTIR and NH 3 -TPD show that cationic species do not effect: pore size distribution, surface area, crystallinity, dehydrogenation-hydrogenation (Pt) site, and Brønsted acid site density and strength. No significant ion-exchange was observed upon Zn 2+ , Al 3+ , Cd 2+ , Ni 2+ and Cr 3+ loading at the reaction conditions. These species only create some Lewis acid sites of which the acid character appeared to play an important role in the enhancement of the isomerization activity. Our concept of trifunctional catalyst suggests that Zn 2+ and other ions behave as sorption competitors to the zeolite Brønsted acid sites. As a result, the isomerization activity of zeolite increased where the effect of Zn 2+ was higher than the other cations. In addition the simultaneous incorporation Zn 2+ or the other cations with Pt species gave better results than that of the sequential loading. With increasing reaction temperature more acid sites were activated and became involved in the reaction, thus, each reaction temperature needed a specific Zn 2+ loading in order to provide the best isomerization activity, and a higher amount of zinc was required at higher reaction temperature where the cracking activity is stronger. The kinetic studies showed that the behavior of the trifunctional Pt-Zn y -HY was significantly different than that of the bifunctional Pt-HY zeolite catalyst. All kinetic data showed a drastic reduction of cracking (Ý-scission) within the temperature range of 195C̕-240C̕.