Author's personal copy Applied Catalysis A: General 431–432 (2012) 104–112 Contents lists available at SciVerse ScienceDirect Applied Catalysis A: General jo u r n al hom epage: www.elsevier.com/locate/apcata Interaction of Zn 2+ with extraframework aluminum in HBEA zeolite and its role in enhancing n-pentane isomerization Nur Hidayatul Nazirah Kamarudin a , Aishah Abdul Jalil a , Sugeng Triwahyono b,∗ , Rino R. Mukti c , Muhammad Arif Ab Aziz a , Herma Dina Setiabudi a , Mohd Nazlan Mohd Muhid d , Halimaton Hamdan d a Institute of Hydrogen Economy, Faculty of Chemical Engineering, Universiti Teknologi Malaysia, 81310 UTM Johor Bahru, Johor, Malaysia b Ibnu Sina Institute for Fundamental Science Studies, Faculty of Science, Universiti Teknologi Malaysia, 81310 UTM Johor Bahru, Johor, Malaysia c Division of Inorganic and Physical Chemistry, Faculty of Mathematics and Natural Science, Institut Teknologi Bandung, Jl Ganesha No 10, Bandung 40132, Indonesia d Department of Chemistry, Faculty of Science, Universiti Teknologi Malaysia, 81310 UTM Johor Bahru, Johor, Malaysia a r t i c l e i n f o Article history: Received 25 February 2012 Received in revised form 9 April 2012 Accepted 15 April 2012 Available online 1 May 2012 Keywords: Zn-HBEA Extraframework aluminum Bridging hydroxyl groups Protonic acid sites n-Pentane isomerization a b s t r a c t The electrodeposition method was used to produce Zn 2+ cation precursors, followed by the introduc- tion of Zn 2+ cation precursors to HBEA by the ion exchange technique. The introduction of Zn 2+ cations slightly changed the specific surface area and crystallinity of HBEA. IR, XPS and solid state MAS NMR results showed that Zn 2+ cations interacted with (AlO) + extraframework aluminum to form Zn(OAl ) 2 and simultaneously induced the formation of bridging hydroxyl groups, Si(OH)Al. The pyridine adsorbed IR study revealed that the presence of Zn 2+ cations fully eliminated weak and partially eliminated strong Brønsted acid sites. As a result, strong and relatively weak Lewis acid sites were formed in which the pyridine probe molecule desorbed at 623 K and below. The presence of Zn 2+ cations enhanced the cat- alytic activity of HBEA in n-pentane isomerization due to the presence of strong Lewis acid sites; the sites may facilitate the formation and maintenance of active protonic acid sites through a hydrogen spillover mechanism. At 598 K, the yield of isopentane for Zn-HBEA was 25.7% higher than that of HBEA. Within a reaction temperature range of 373–648 K, the apparent activation energy for isomerization of n-pentane over HBEA and Zn-HBEA was 118.76 and 90.79 kJ/mol, respectively. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Hydroisomerization of paraffins to enhance the octane num- ber of gasoline is an environmentally more acceptable method compared to other technologies such as blending with oxygenates like MTBE, which has been identified as a source of ground- water contamination. It is clear that branched paraffins are the preferred gasoline component; therefore, it is highly desirable to increase the contribution of high octane branched paraffins to the gasoline pool [1]. Alkane isomerization is typically car- ried out over bifunctional metal-acid catalysts [2] and it is well established that the isomerization proceeds through consecu- tive branching reactions. Isomerization takes place at the acid sites of the bifunctional catalyst, whereas the metal site provides hydrogenation–dehydrogenation capability. Several types of solid acid catalyst such as MOR [3], SAPO-11 [4], ZSM-22 [5], ZSM-5 [6,7], Beta [8,9], HY [10,11] and oxoanions loaded on ZrO 2 [2,12–14] have been used for the isomerization process. The addition of noble or transition metals to catalysts and the presence of hydrogen in the ∗ Corresponding author. Tel.: +60 7 5536076; fax: +60 7 5536080. E-mail addresses: sugeng@utm.my, sugengtw@gmail.com (S. Triwahyono). gas phase markedly improve the activity and stability of the cata- lyst. Transition metals such as platinum, zinc and iridium cations loaded on support catalysts have also been reported to improve the production of isoproducts and to suppress the cracking reac- tion in n-heptane isomerization over Pt/HY zeolite [7,15,16]. These cationic transition metals might play an important role in speeding up the desorption of olefinic species from the surface of the cat- alyst, thereby preventing the secondary reaction and/or thermal decomposition of reactants and products. Particularly, the roles of Zn metal in solid catalysts have been discussed by several research groups in recent years. Saberi et al. reported on n-heptane isomerization over Pt–Zn–HY trifunctional catalysts in which they concluded that each reaction temperature needs a specific Zn loading in order to obtain the maximum activity for isomerization [9]. Freude and co-workers explored the proper- ties of Zn/H–BEA zeolite by 1 H MAS NMR and IR spectroscopy of adsorbed CO [17,18]. Kazansky et al. reported that Zn 2+ ions intro- duced into ZSM-5 zeolites resulted in the partial localization of Zn 2+ ions at negatively charged tetrahedral AlO 4 - , which led to a partial decrease in the Brønsted acidic sites and obstruction of the remaining bridging OH groups [19]. This factor led to a con- siderable suppression of the cracking activity of zeolites which certainly needs proton donor centers. An extension of the study 0926-860X/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apcata.2012.04.020