Structures sulfated zirconia

M. -Trung Tran, N. S. Gnep, G. Szabo, and M. Guisnet, Influence of the calcination temperature on the acidic and catalytic properties of sulphated zirconia, Appl. Catal. A 171, 207-217 (1998). P. Canton, R. Olindo, F. Pinna, G. Strukul, P. Rieflo, M. Meneghetti, G. Cerrato, C. Morterra, and A. Benedetti, Alumina-promoted sulfated zirconia system Structure and microstructure characterization, Chem. Mater. 13, 1634-1641 (2001). 

Considering the impressive amount of literature on sulfated zirconia and solid superacids,125 134-139 it will be difficult to impose a definition a posteriori. On the other hand, due to the large difference in acidity and in structure between various liquid superacids, there is no unique chemistry of hydrocarbons in liquid superacids. For this reason it is not possible to suggest a unequivocal definition of solid superacidity at the present stage. Nevertheless, it seems clear from all the data presently available that at high temperatures the chemical reactivity of the proton bound to the surface shows a close resemblance to the one observed at low temperature in liquid superacidic media as will be seen in Chapter 5. 

Recent Laser Raman spectroscopic studies of sulfated zirconia indicate a monodentate structure (162). 

Sulfated zirconia is a good example of a structural Lewis acid which has been chemically treated to enhance acidity. It has been extensively studied as a solid acid catalyst for vapour phase reactions and we1112 and others14 have found that a mesoporous version of this material is a particularly effective catalyst for liquid phase Friedel-Crafts alkylation reactions and to a lesser extent Friedel-Crafts benzoylations. The commercial (MEL Chemicals Ltd) material SZ999/1 shows a nitrogen isotherm characteristic of a mesoporous solid (surface area 162 m2g, pore volume 0.22 cm3g )- Whereas microporous and mesoporous materials are capable of rapidly catalysing the alkylation of benzene with various alkenes (Table 1), on reuse only the mesoporous... 

We have recently suggested a new approach to the preparation of active sites in sulfated zirconia catalysts [5, 6]. In this case, the catalysts are prepared by deposition of sulfate ions on crystalline zirconium dioxide samples with highly defective structure. According to numerous reports, the monoclinic phase typical for ZrOa is not suitable for this purpose. We have shown that active materials could be obtained by impregnation of zirconia-based oxides with cubic crystalline structure. It should be noted that the cubic structure is not thermodynamically stable for pure zirconia at low temperatures. It can be stabilized by introducing different additives, in particular, alkaline-earth metal cations [7]. Recently, similar results have been obtained for ZrOa stabilized by Y2O3 [8]. 

Thus, we have suggested a new approach not only to the generation of active sites in sulfated zirconia-based catalysts for skeletal isomerization of alkanes, but also to investigation of their formation mechanism. The possibility of synthesis of active surface sites by deposition of sulfate ions on crystalline doped zirconia materials with defective cubic structure without changing the bulk properties of the samples opens many new opportunities for investigation of their nature. 

Solid acid catalysts play an important role in hydrocarbon conversion reactions in the chemical and petroleum industries [1,2]. Many kinds of solid acids have been found their acidic properties on catalyst surfaces, their catalytic action, and the structure of acid sites have been elucidated for a long time, and those results have been reviewed by Arata [3]. The strong acidity of zirconia-supported sulfate has attracted much attention because of its ability to catalyze many reactions such as cracking, alkylation, and isomerization. Sulfated zirconia incorporating Fe and Mn has been shown to be highly active for butane isomerization, catalyzing the reaction even at room temperature [4]. 

The anomalous behavior of the sulfated zirconia catalysts is due to the loss of sulfuric acid at elevated temperatures. The catalyst prepared fi-om calcined zirconia looses its sulfuric acid at 673 K and, consequently, is not active at this temperature. As a result, decreasing the temperature of the catalyst to 473 K does not restore the activity. Also the more highly porous catalyst prepared from zirconium hydroxide releases sulfuric acid, but in narrow pores some sulfuric acid is left. The loss of sulfuric acid at 673 K is obviously irreversible. When the catalyst prepared from zirconium hydroxide is, however, kept at 473 K, the transport of water out of the porous structure is thus low that a stable activity is exhibited. Pre-hydration of the bulk anhydrous zirconium sulfate does not provide an active catalyst. That no catalytic activity is induced in this case is due to the fact that bulk anhydrous zirconium sulfete readily reacts to a stable tetrahydrate, viz., Zr(S04)2.4H20 [1]. As a result the hydrolysis of the sulfate by water vapor is suppressed. 

Al-promoted sulfated zirconias, more active and stable than non-promoted sulfated zirconia catalysts, have been reported in the last years. Addition of AP appears to stabilize the sulfate species at the oxide surface, thereby increasing the number of acid sites with intermediate strength [6], and favoring the tetragonal structure [7]. 

Different solid acid catalysts like zeolite Y [2-6], beta [7-9], MCM-22 [10], solid superacids [11-13], sulphonic acid resins [14], etc. have been proposed as potential alkylation catalysts and some of them are being tested at a pilot plant scale. Zeolites and solid superacids of sulfated zirconia type were found to be the most active but they suffer rapid deactivation after an initial period. Among different zeolites studied large-pore zeolites are prefered over medium-pore type because the former favors the formation and diffusion of bulkier tri-methylpentane isomers. Beside pore size and zeolite structure, the fiamework composition (Si/Al ratio) and acid strength distribution also play an important role on the activity, selectivity and deactivation of the catalysts. It is known that the adsorption behavior of the zeolite and the extent of hydrogen transfer capacity (a crucial factor of alkylation activity) both depend on the aluminium concentration in the framework [15-16]. 

There are three generic routes to sulfated zirconia (sulfation of the oxide/ hydroxide, sol-gel synthesis with Zr alkoxides and sulfuric acid and, much less studied, the thermal decomposition of zirconium sulfate). Furthermore, many potential structures have been suggested on the basis of various studies (Figure Whether one is correct or there are several... 

Figure 4.1 Some of the structures proposed by various authors to describe the properties of sulfated zirconia. A is a singly bound sulfate with a proton attached B represents the polarisation of a ZrOH by a proximal chelating sulfate group C is an anhydride from adjacent singly bound sulfates with the possibility of hydrolysis to give a pair of acidic groups reminiscent of A D and E are species formed by a bidentate sulfate coordinating to a Lewis acidic Zr, as a stronger Lewis base, water can displace the sulfate O and form a Bronsted site. Coordinatively unsaturated Zr sites will also function as Lewis acid sites.

It is important to emphasize that spectroscopic evidence shows that water transforms the Lewis acid sites of sulfated zirconia into Bronsted acid sites [80]. At the same time, water promotes isomerization reactions over sulfated zirconia for a moderate extent of catalyst dehydration. Similarities were reported between the effect of rehydration on the isomerization activity of sulfated zirconia [81] and on that of other oxide catalysts [49] that are consistent with the role of surface donor sites in hydrocarbon isomerization reactions. However, when spectroscopic methods using basic probes were used to compare sulfated zirconia and zeolites in terms of the strength of their acid sites, the results were inconsistent with all catalytic data. These findings illustrate the danger of comparing the acidity of catalyst systems that differ in structure and composition, such as zeolites and sulfated zirconia in these systems the "catalytic" and the "physicochemical" scales for the strength of acid-base interaction may contain significantly different parameters. 

Ahn, H., Nicholas, C. R, and Marks, T. J. 2002. Surface organozirconium electrophiles activated by chemisorption on "super acidic" sulfated zirconia as hydrogenation and polymerization catalysts. A synthetic, structural and mechanistic catalytic study. Organometallics 21 1788-1806. 

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