6538
Ind. Eng. Chem. Res. 2008, 47, 6538–6546
Hydroisomerization of n-Octane over Bifunctional Ni-Pd/HY Zeolite Catalysts Dhanapalan Karthikeyan,* Nachiyappan Lingappan, and Bommasamudram Sivasankar Department of Chemistry, Anna UniVersity, Chennai - 600025, India
Navamoney John Jabarathinam Department of Chemistry, Jerusalem College of Engineering, Chennai - 601302, India
Bifunctional catalysts containing 0.1-0.5 wt % nickel and 0.1 wt % palladium supported on HY zeolite were prepared through incipient wetness impregnation (IWI) and characterized by X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET) surface area, temperature-programmed desorption of ammonia (NH3-TPD), temperature-programmed reduction (TPR), X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM). Hydroisomerization of n-octane was carried out between 200 and 450 °C under 1 atm of pressure, and it was found that Ni addition up to 0.3 wt % over 0.1 wt % Pd/HY zeolite enhanced the n-octane conversion and isomerization selectivity with a low percentage of cracked products. As the Ni amount exceeds the threshold values, the conversion decreases with an increase in cracked products, and also, the selectivity of mono- and dibranched isomers was improved, suggesting the operation of a protonated cyclopropane (PCP) intermediate mechanism. As a conclusion, the bimetallic catalysts were more selective toward the formation of dibranched isomers containing higher octane number. The role of noble metal over HY zeolite and recyclability of the catalyst were also studied. Introduction
Scheme 1. Mechanism of n-Octane Hydroisomerization
The current environmental requirements are giving rise to a general reduction in both reid vapor pressure and aromatics, alkenes, sulfur and methyl terbutyl ether (MTBE) contents, which have a negative impact on the octane number of the gasoline pool.1 Isomerization of long-chain n-alkanes appears to be an interesting alternative since it provides branched molecules, which possess higher octane numbers than linear molecules. Several studies have been made in C4-C7 isomerization processes;2 however, the multibranched C8 alkanes are the most vital isomers owing to their high octane number. Many bifunctional catalysts that have been commercially utilized to isomerize alkanes contain a zeolitic component in combination with a noble metal, which provides the hydrogenatingdehydrogenating function. In addition, several investigations have been performed to evaluate the most suitable combination of acidic and metallic components to improve catalytic activity, selectivity, and stability of the catalysts.3,4 Most industrial hydroisomerization catalysts are based on zeolites, which include the acid function. As the hydrogenatingdehydrogenating function, several metals have been tested, including Pt, Pd, Rh, Ir, Ru, Re, and Ni, mostly associated with mordenite or CaY zeolite.5 From the above metals, platinum was singled out as exhibiting both high activity and good selectivity.6 However, palladium, although rarely studied, was shown to be the most selective in butane hydroisomerization.7,8 The hydroisomerization mechanism is as follows (Scheme 1): the metallic function converts the linear alkane into the corresponding alkene. The acid function is responsible for the isomerization of the linear alkene into the corresponding isoalkene, via formation of secondary carbenium ion, first formed upon addition of the linear alkene to the Brønsted acid sites. The secondary carbenium ion is driven by thermodynamics to rearrange into the tertiary carbenium ion, which is likely to * To whom correspondence should be addressed. Tel: +91-4422203142. Fax: +91-44-22200660. E-mail: dkarthikeyan05@ yahoo.co.in.
restore the Brønsted acid sites and to release the isomerized alkene or to crack via β-scission, providing a lower alkene and a new carbenium fragment.9 An ideal mechanism is characterized by controlling the acid function; the acid function has to be as strong as possible to convert the intermediate alkene into the corresponding carbenium ion at the lowest possible temperature to favor isomerization rather than cracking. Zeolites like mordenite, β, and ZSM-5 have shown to be very promising as far as the hydroisomerization is concerned.1–3 As the hydrogenating-dehydrogenating function, it has been demonstrated that platinum can remove coke precursors by hydrogenation, increasing the catalyst stability, and promote alkene production, allowing setup of the bifunctional mechanism characteristic of the alkane hydroisomerization.10 Many researchers have reported that hydroisomerization of n-octane was carried out with various catalysts, namely, silicoaluminophosphates SAPO-5, SAPO-11, SAPO-31 and SAPO-41;11 Pt-Pdloaded SAPO-31 and ZSM-48;12 SO42--ZrO2;10 tungstate zirconia (WZ);13 Ni-containing ZSM-5, mordenite, and β zeolites;14 ZSM-12, USY, and β zeolites;6 Pt/HY zeolites;15 and Pt-containing mordenite, β, and ZSM-5 zeolites.16 But only a few reports are available about the addition of Ni as the second metal to Pd, with the concentration of Ni loading kept at a fairly high range (>1.0 wt %).17–19 Hence, in the present investigation, nickel is introduced as the second metal in a low concentration range (
