Metal sulfates catalytic property

Catalytic Activity and Acidic Property of Solid Metal Sulfates Kozo Tanabe and Tsuneichi Takeshita... 

Hervey H. Voge and Charles R. Adams The Physical-Chemical Properties of Chro-mia-Alumina Catalysts Charles P. Poole, Jr. and D. S. MacIver Catalytic Activity and Acidic Property of Solid Metal Sulfates... 

The sulfate promoted transition metal oxides focussed considerable attention in recent years due to attractive catalytic properties. Most of the research carried out to date centered on sulfated zirconias,1 5 not surprisingly perhaps, as they exhibit the highest surface acidity (Ho <-16.04) among the members of this family of materials and appear to be able to initiate isomerization reactions in temperatures as low as 298 K. Far less interest attracted sulfated porous titanias, mainly owing to a lower surface acidity,6 although it may be a useful property in many catalytic situations. Thus closer inspection of the preparation procedures for sulfated titanias may be of interest, in particular as the reports on preparation and properties of these materials are scarce and we are not familiar with any work dealing with titania-sulfate aerogels. 

Tanabe el al. studied in detail the catalytic action and properties of metal sulfates most of the sulfates showed the maximum acidity and activity by calcination at temperatures below 500°C, with respect to the surface acidity and the acid-catalyzed reaction (118, 119). Other acid-catalyzed reactions were studied with the FeS04 catalyst together with measurement of the surface acidity of the catalyst the substance calcined at 700°C showed the maximum acidity at Ho s 1.5 and proved to be the most active for the polymerization of isobutyl vinyl ether, the isomerization of d-limonene oxide, and the dehydration of 2-propanol (120-122). It is of interest that the catalyst calcined at a slightly higher temperature, 750°C, was completely inactive and zero in acidity in spite of the remarkable activity and acidity when heat treated at 700°C. 

Figure 1 presents the dependence of the catalytic activity and isobutane selectivity of samples precipitated at pH = 9 on the CaO concentration. This study made it possible to evaluate the role of the cubic solid solution in their catalytic properties. One can see an obvious maximum on the dependence of the activity on the alkaline earth metal concentration in the structure of Zrj.xCaxOz-x solid solution. Without calcium the catalytic activity is low. This agrees with the literature data that sulfation of a calcined monoclinic phase without any special treatment does not yield an active catalyst [9]. The maximum activity is observed for the samples with the CaO concentration of 5-10 mol.%. The material with the calcium concentration of 50 mol.% does not show any catalytic activity at all. This sample is mostly composed of perovskite crystalline phase CaZrOs. A comparison of the catalytic activity of the samples with their surface areas clearly indicates that the activity growth is mostly caused by the formation of the cubic phase rather than just by an increased surface area. 

Consider that atoms have a size range of about 1-2 A. Most inorganic solids, with the exception of halides, sulfides (and other pnictides), are based upon the oxygen atom, i.e.- oxide = O", whose atomic radius does not change even when sulfates, phosphates and silicates are formed. Oxide has an atomic diameter of 1.5 A or 0.15 nm. = 0.00015 (om. Nanoparticles are clumps of 1000 to 10,000 atoms. The latter would be a particle of 0.15 (om. in diameter. They can be metal oxides, semiconductors, or metals with novel properties useful for electronic, optical, magnetic and/or catalytic uses. 

In many cases, good correlations have been found between the acidic property of metal sulfates and their catalytic activity, and reaction mechanisms were proposed. There remained an interesting question, namely, how these acid sites are formed from a seemingly neutral salt. A recent paper attempts to answer this question, on the basis of studies using X-rays, infrared, NMR, and ESR spectra (4). 

In the present article, the discussion concentrates on the catalytic activity and selectivity of solid metal sulfates in terms of the acidic property (acidity, acid strength, and Bronsted and Lewis acids) by integrating the kinetic and structural studies. 

The measurement of the acidic property of metal sulfates is a prerequisite for the investigation of their catalytic activity, since many reactions are known to exhibit strong dependence on the acid strength and the acidity of the acid catalyst. These properties have an important bearing on what type of reactions will be catalyzed by metal sulfates. 

Since the metal sulfate catalyst has both Bronsted and Lewis acid sites, it is expected that many n bases with nonbonding electrons such as —O— and —Cl and tt bases hke olefin will undergo acid-base equilibria, thus initiating carbonium ion or carbonium ion-like reactions. Table II summarizes the acid- catalyzed reactions on metal sulfate catalysts and shows the versatility of these systems. Although this table includes some examples of industrial work, our results and others clearly show the general trend in the strength of acid sites required for each specific reaction. But detailed discussion of correlation between the catalytic activity and the acidic property is reserved for the next section. 

The polymerization of aldehydes to high polymer is usually catalyzed not only by bases, but also by both Bronsted and Lewis acids. Takida and Noro (14) found that some metal sulfates do catalyze the polymerization. Measurements of the polymerization activity and the acidic property of solid sulfates of Fe(III), Cr, Zn, Ni, Mn, Mg, Cu, Fe(II), and Ca which were heat-treated at various temperatures revealed that the catalysts having acid sites of pK -f3.3 are effective for the polymerization. Actually, it is seen in Fig. 8 that there exists a fairly good parallelism between the catalytic activity and the acidity at acid strengths pK +3.3. On the other hand, the relation of the number of activated molecules (polymerization rate W divided by averaged... 

The selectivity of the metal sulfate catalyst is influenced by many factors besides its acidic property, such as geometric structure involving a pore structure, arrangement of basic sites, polarity of the surface, etc. For example, the relative values of the first-order rate constants (per imit acidity at pK — 3) of the depolymerization catalyzed by nickel sulfate, cupric sulfate, and silica-alumina were found to be 1100 300 1. The difference may be attributed to the differences in acid-base bi-functional catalysis of these catalysts. This view may be said to have originated in 1948 when Turkevich and Smith (45) showed that the isomerization of 1-butene to 2-butene is catalyzed by metal sulfates, sulfuric acid, phosphoric acid, etc., but little by acetic acid, hydrogen chloride, etc. The high catalytic activity of the catalysts of the former group is considered as due to acid-base bifunctional catalysis as illustrated by Fig. 14. Independently, Horiuti (45a) advanced the same idea... 

We have attempted to present an integrated picture on the acid property of solid metal sulfates as used in various heterogeneously catalyzed reactions, with some view presented on the surface structure. We saw some unusual catalytic activity and selectivity exhibited by these solid sulfates in diversified reactions which may surpass in some aspects those of a more universal catalyst represented by silica-alumina and its related oxides. 

Coelho MA, Rasasco OE, Sikabwe EC, White RL Modificabon of the catalytic properties of sulfated zirconia by addition of metal promoters. C tal Lett 1985, 32 253-262. 

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