This review article deals with recent progress in the preparation of sulfated zirconia (SZ)-bassed, strong solid-acid catalysts, the characterization of their physicochemical properties and the evaluation of their catalytic performance in various promising applications. Strong emphasis was put on discussion of controversial issues such as the strength of acid sites, the nature of active sites, the reaction mechanism, and the role and state of supported platinum. An important part of this work was devoted to recent catalytic applications.

Sulfated Zirconia-Based Strong Solid-Acid Catalysts: Recent Progress

The review is a summary and analysis of the data characterizing CO adsorption on surface cationic sites of oxides including supported materials and microporous and mesoporous materials. The contributions of various types of CO bonding to the IR frequency shifts of carbon-bonded molecules are analyzed, namely, the increase of the CO stretching frequency in cases of electrostatic and σ bonding and the decrease of the frequency with π bonding. Polycarbonyls, bridging CO, oxygen-bonded CO, and tilted CO are also considered. The main part of the review is a collection of the experimental results characterizing carbonyls of individual metal ions. The spectral behavior of CO bonded to metal atoms is also assessed in the cases when the metal ions are easily reduced to metal (Cu, Ag, Au, Pd, or Pt) or cationic carbonyls are produced after CO adsorption on supported metals (Ru, Rh, Ir, and Os). The interaction of CO with surface OH groups is also considered. It is demonstrated that IR spectroscopy of adsorbed CO is an efficient methodology to characterize cationic surface sites in terms of their nature, oxidation states, coordination environment and coordinative unsaturation, and location at faces, edges or corners of microcrystallites. When applied to materials with surface hydroxyl groups CO undergoes hydrogen bonding and information can be collected on the proton acid strength.

Characterization of oxide surfaces and zeolites by carbon monoxide as an IR probe molecule

State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, People’s Republic of China; Department of Materials Science and Engineering, East China University of Science and Technology, 130 Meilong Road, Shanghai 200233, People’s Republic of China; and National Engineering Research Center for Nanotechnology, 28 East Jiangchuan Road, Shanghai 200241, People’s Republic of China

On the synergetic catalytic effect in heterogeneous nanocomposite catalysts.

The bases of IR spectroscopic methods for the characterization of the surface acidity, of the Lewis type and of the Bronsted type, of solid, simple and mixed oxides are briefly reviewed. The results are described and a systematization is proposed. The surface acidity is essentially associated with the ionicity/covalency of the element–oxygen bond, mainly affected by the size and charge of the cation.

The surface acidity of solid oxides and its characterization by IR spectroscopic methods. An attempt at systematization

IR spectroscopy in catalysis

n-butane isomerization on sulfated zirconia was studied in the temperature range 393–473 K. Rapid deactivation occurs, when the reaction is carried out in He carrier gas. In contrast, stable stationary activity was observed in H2 carrier gas. In situ Raman and UV-VIS diffuse reflectance spectroscopy and ESR showed that the deactivation is caused by the formation of allylic and polyenylic cations and polycyclic aromatic compounds, the formation of which is largely prevented in the presence of H2. Deactivated catalysts can be fully regenerated by treatment at 723–753 K in flowing O2. The regeneration process was also followed by in situ spectroscopies.

n-butane isomerization on sulfated zirconia. Deactivation and regeneration as studied by Raman, UV-VIS diffuse reflectance and ESR spectroscopy

Large 1H chemical shifts are suggestive of protonic superacidity of sulfated zirconia catalysts, this property being related to the structure of surface sulfate species.

Superacid properties of sulfated zirconia as measured by Raman and 1H MAS NMR spectroscopy

Ceria on silica and alumina catalysts: dispersion and surface acid-base properties as probed by X-ray diffractometry, UV-Vis diffuse reflectance and in situ IR absorption studies

The acid properties of pure and sulfated zirconias were studied by FTIR spectroscopy of adsorbed CO and NH3 as probe molecules. Whereas pure monoclinic zirconia shows only Lewis acidity, sulfation of tetragonal and monoclinic zirconia creates new bridging OH groups. Two types of Bronsted-acidic centers and two types of Lewis-acidic centers with enhanced acid strength were identified. A communication between the different types of Lewis-acidic sites and the related adsorbed sulfate molecules could be shown. The coordination of basic molecules such as CO onto Lewis-acid sites induces a decrease of the intrinsic Bronsted-acidity of the bridging OH groups. These effects are discussed on the basis of a model of the acidic centers that was previously proposed.

Gamal A.H. Mekhemer

Papers

n-butane isomerization on sulfated zirconia. Deactivation and regeneration as studied by Raman, UV-VIS diffuse reflectance and ESR spectroscopy

Superacid properties of sulfated zirconia as measured by Raman and 1H MAS NMR spectroscopy

Ceria on silica and alumina catalysts: dispersion and surface acid-base properties as probed by X-ray diffractometry, UV-Vis diffuse reflectance and in situ IR absorption studies

Acidity of sulfated zirconia as studied by FTIR spectroscopy of adsorbed CO and NH3 as probe molecules

Characterization of phosphated zirconia by XRD, Raman and IR spectroscopy