n-Pentane Hydroisomerization On Pt-Promoted Acid Zeolites

Applied Catalysis A: General 372 (2010) 108–113

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Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata

n-Pentane hydroisomerization on Pt-promoted acid zeolites
´ ´ ´ Carmen M. Lopez a,*, Virginia Sazo a, Pedro Perez a, Luis V. Garcıa b
a b

´lisis, Petro ´leo y Petroquı´mica, Apartado 47102, Caracas 1020-A, VenezuelaUniversidad Central de Venezuela, Facultad de Ciencias, Escuela de Quı´mica, Centro de Cata Universidad Central de Venezuela, Facultad de Ingenierı´a, Escuela de Ingenierı´a Quı´mica, Apartado 48057, Caracas 1020-A, Venezuela

A R T I C L E I N F O

A B S T R A C T

Article history: Received 4 May 2009 Received in revised form 13 October 2009 Accepted 15 October 2009 Available online 23October 2009 Keywords: n-Pentane Hydroisomerization Supported-metal catalysts Zeolites

Hydroisomerization of n-pentane over platinum promoted acid zeolites was studied. The effect of structure and strength of acid sites of the support was investigated on the reaction carried out under atmospheric pressure at 300 8C. In order to accomplish the above purpose, some zeolitic catalytic systems werestudied, which have the same structure but different acidity and with similar acidity but different structure. ß 2009 Elsevier B.V. All rights reserved.

1. Introduction Light straight run (LSR) gasoline consists of the C5–88 8C fraction of the naphtha cuts from the atmospheric crude still. The octane number of the LSR naphtha can be improved using an isomerization process to convert the normalparaffins to their branched isomers. It results in a significant octane number increase because n-pentane has an unleaded research octane number (clear RON) of 61.7 and iso-pentane has an average of 92.3. The isomerization of LSR is successfully carried out in the presence of hydrogen and it is referred as hydroisomerization. An atmosphere of hydrogen is used to minimize carbon deposits on the catalystbut hydrogen consumption is negligible [1,2]. In the development of the alkane isomerization process, four catalyst generations were developed and used in succession. The first two (AlCl3 and metal on aluminosilicates) are no longer used. For the catalyst currently in use, Pt/chlorinated Al2O3 (the third generation) and Pt/zeolites (the fourth generation), ratio between the acid and the metallicsite has a determining role in the performance of the process [2]. The zeolites have the advantage of a well-defined porous structure, which allows one to select the suited structure for the process. Several reports have investigated the skeletal isomerization of hydrocarbons C4–C7 in the presence of hydrogen over Pt o Pd/BEA [3–6], MOR zeolite [7,8], Y-zeolite [6,9,10], ZSM-5 zeolite [11,12], andheteropolyacid catalysts [12,13].

* Corresponding author. ´ E-mail address: carmen.lopez@ciens.ucv.ve (C.M. Lopez). 0926-860X/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2009.10.026

The bifunctional mechanism has been accepted for catalysts of platinum supported over zeolites [14–16]. This mechanism requires the dehydrogenation of alkanes on the metallicsites, and the produced olefins protonate on the Brønsted acid sites to the corresponding alkylcarbenium ions. These carbenium ions can undergo to skeletal rearrangement and b-scission followed by de-protonation and hydrogenation over metal sites to produce the isomerization and cracking products. In this mechanism, it is very important to minimize the secondary reactions and to ensure asatisfactory reaction rate. For this requirement, it is necessary to have a defined ratio between the metal and acid sites. It is established that the selectivity to isoparaffins depends on the balance between metal and acid functions [17,18]. Some authors [19,9] have reported optimal values of the ratio metallic site/acid site for the isomerization of lower n-alkanes (C5–C7). Other studies confirm the...