Isomerization of n-hexane on the Pt-promoted Keggin and Dawson tungstophosphoric heteropoly acids supported on zirconia
Introduction
Among various catalysts for C4–C6 alkane isomerization, systems based on heteropoly acids is most promising. Bulk heteropoly acids (HPA) and salts have strong acidic properties and can activate alkanes via the formation of carbocations. The H3PW12O40 (HPW) heteropoly acid is the strongest acid of this family and more oftenly used [1], [2], [3], [4]. A significant drawback of the bulk HPW systems is the low surface area. An increase in the concentration of active sites could be achieved by the preparation of the HPW salts of group IA element or by the support of HPW on suitable carriers with high surface areas. However, although the HPW salts are widely used for the isomerization of n-alkanes [5], [6], [7], [8], [9], the supported systems are studied inadequately [10], [11].
A very important step in the development of active catalysts is a proper choice of the support. Because the Keggin structure is responsible for the unique acidic properties of HPA, it is necessary to save these species on the support surface. It was shown that the support of HPW on carriers with strong basic sites (Al2O3, MgO) leads to the partial or complete destruction of the Keggin ions and results in the deactivation of the catalyst in n-pentane isomerization and isopropanol dehydration [1], [2], [11], [12]. The neutral or acidic supports are more suitable for the preparation of the active systems due to low concentrations and strengths of basic sites. HPW supported on almost neutral SiO2 or MCM-41 was more active than the bulk system in isopropanol dehydration [12], trans-de-t-butylation of 2,6-di-t-butyl-4-methylphenol [13] or conversion of 1,3,5-triisopropylbenzene [14]. It was shown that the support of HPW on fluorinated alumina increases the activity in alkane isomerization compared to bulk or SiO2 supported systems [11]. The specificity of Al2O3-F was found to be the presence of strong Lewis acid sites (LAS), medium Brønsted acid sites (BAS), and weak basic sites. Hence, the other systems having weak basicity (ZrO2, TiO2) could also be promising as supports for isomerization catalysts. This idea is supported by the data on the enhanced activity of HPW supported on ZrO2 and TiO2 in the isopropanol dehydration [12], esterification of acetic acid with isoamyl alcohol [15], and n-butane isomerization [10].
Unfortunately, the lack of information on the interaction of HPW with the surface sites of oxides exists. The interaction of Keggin ions with hydroxyls is known to lead to the formation of distorted grafted HPA compounds even with weak bases such as SiO2 or MCM-41 [13], [16], [17]. The grafted (MOH2+)(H2PW12O40−) species were found to be the main states of HPW supported in low concentration on the oxide surface among of bulk crystalline form. The distorted PW11O397− species could also be formed due to the deeper interaction with OH-groups [14]. Additionally, the formation of dimeric H6P2W18O62 (HP2W18) or H6P2W21O71 (HP2W21) Dawson-type structures was found to proceed for silica-supported HPW [13], [18]. These species were eight times more active in the trans-de-t-butylation of 2,6-di-t-butyl-4-methylphenol than the Keggin ions [13]. The dimeric HP2W21 acid anions and distorted PW11O397− species were also found on the surface of the HPW/ZrO2 system [12]. Because the Dawson systems have high activity in acid–catalyzed reactions, the potential of these systems in n-alkane isomerization was very important to study.
Thus, the main goal of this study was to compare the activities of zirconia supported catalysts prepared on the basis of Keggin HPW, H5PW11ZrO40, and Dawson H6P2W18 or H6P2W21 heteropoly acids in n-hexane isomerization. The nature of active sites is responsible for the activity of each system is also very important.
Section snippets
Experimental
Commercial H3PW12O40·xH2O (across chemical) was dried under vacuum conditions at 50 °C for 4 h to obtain H3PW12O40·6H2O H6P2W18O62 (HP2W18), H6P2W21O71 (HP2W21) and H5PW11ZrO40 (HPW11Zr) heteropoly acids were synthesized by a known procedure [19], [20] and purified by repeated extraction with diethyl ether.
ZrO2 was prepared by the calcination of commercial Zr(OH)4 (Ssurf=150 m2/g, Magnesium Electron Co., grade XZO632/03) at 500 °C for 2 h in an air flow. The BET area of ZrO2 obtained was 75 m2/g.
The thermal behavior of bulk and supported HPAs
The DTA curves of bulk HPAs studied in the temperature region from 20 to 900 °C are shown in Fig. 1. The pattern of HPW shows two endothermic peaks at 101 and 229 °C and one exothermic peak at 612 °C. The endothermic peaks appear because of the loss of physically sorbed and structural water and the formation of HPW·6H2O and HPW crystals, respectively [21]. The exothermic peak is characteristic of the decomposition of the Keggin structure of HPW. The thermal behavior of HPW11Zr differs from that of
Discussion
HPAs belong to a new class of very active catalysts such as sulfated and tungstated zircona or β-zeolite [22]. The isomerization of n-hexane on these catalysts was proposed to proceed via a monomolecular mechanism on acid sites [23], [24], [25]. The Pt additives play the role of primary activators of alkanes on Pt0 or Pt–Hδ+ species, or they can also be the terminators of side processes of alkylation.
The strength and density of the acid sites are two main factors responsible for the activity.
Conclusion
The catalytic system based on HPW supported on ZrO2 exhibits the high activity in n-hexane isomerization. The yield of isohexanes is close to 80% and the selectivity is 96–98%. The grafted and partially distorted Keggin ions can be responsible for the catalytic activity. These species cannot be formed by the dispersion of P2Wn6− and HPW11Zr systems because of insufficient stability of initial acids in contact with a support. The catalytic systems prepared on the basis of HP2Wn or HPW11Zr
Acknowledgements
Authors would like to acknowledge Dr. V. Makhlyarchuk for the help with paper preparation.
References (27)
- et al.
Adv. Catal.
(1996) - et al.
J. Mol. Catal.
(1999) - et al.
Catal. Today
(2001) - et al.
Catal. Today
(2001) - et al.
J. Catal.
(2001) - et al.
Appl. Catal. A: Gen.
(1998) - et al.
J. Mol. Catal. A: Chem.
(1996) - et al.
J. Mol. Catal.
(1990) - et al.
J. Catal.
(1988) - et al.
J. Catal.
(1998)