Sulfated metal oxide catalysts

Bullen P.J. et al, Light Paraffin Isomerization Using Sulfated Metal Oxide Catalysts (AIChE Spring National Meeting, Houston, 1999). 

Sulfated metal oxide catalysts represent a class of extremely attractive strong solid acids showing widespread application in different areas of chemical fransformafions. It was reported that sulfated zirconia (SZ) prepared by treatment of zirconia with sulfuric acid or ammonium sulfafe exhibifs exfremely sfrong acidity, and it is able to catalyze the isomerization of bufane to isobutane at room temperature. This behavior... 

An interesting variation on sulfated metal oxide type catalysts was presented by Sun et al. (198), who impregnated a dealuminated zeolite BEA with titanium and iron salts and subsequently sulfated the material. The samples exhibited a better time-on-stream behavior in the isobutane/1-butene alkylation (the reaction temperature was not given) than H-BEA and a mixture of sulfated zirconia and H-BEA. The product distribution was also better for the sulfated metal oxide-impregnated BEA samples. These results were explained by the higher concentration of strong Brpnsted acid sites of the composite materials than in H-BEA. 

Koyande, S.N. Jaiswal, R.G. Jayaram, R.V. Reaction kinetics of benzylation of benzene with benzyl chloride on sulfate-treated metal oxide catalysts. Ind. Eng. Chem. Res. 1998, 37, 908-913. 

The tendency in the past decades has been to replace them with solid acids (Figure 13.1). These solid acids could present important advantages, decreasing reactor and plant corrosion problems (with simpler and safer maintenance), and favoring catalyst regeneration and environmentally safe disposal. This is the case of the use of zeolites, amorphous sihco-aluminas, or more recently, the so-called superacid solids, that is, sulfated metal oxides, heteropolyoxometalates, or nation (Figure 13.1). It is clear that the well-known carbocation chemistry that occurs in liquid-acid processes also occurs on the sohd-acid catalysts (similar mechanisms have been proposed in both catalyst types) and the same process variables that control liquid-acid reactions also affect the solid catalyst processes. 

Several metal oxides could be used as acid catalysts, although zeolites and zeo-types are mainly preferred as an alternative to liquid acids (Figure 13.1). This is a consequence of the possibility of tuning the acidity of microporous materials as well as the shape selectivity observed with zeolites that have favored their use in new catalytic processes. However, a solid with similar or higher acid strength than 100% sulfuric acid (the so-called superacid materials) could be preferred in some processes. From these solid catalysts, nation, heteropolyoxometalates, or sulfated metal oxides have been extensively studied in the last ten years (Figure 13.2). Their so-called superacid character has favored their use in a large number of acid reactions alkane isomerization, alkylation of isobutene, or aromatic hydrocarbons with olefins, acylation, nitrations, and so forth. 

Isomerization of olefins or paraffins is an acid-catalyzed reaction that can be carried out with any number of strong acids, including mineral acids, sulfated metal oxides, zeolites and precious metal-modified catalysts [10]. Often the catalyst contains both an acid function and a metal function. The two most prevalent catalysts are Pt/chlorided AI2O3 and Pt-loaded zeolites. The power of zeoHtes in this reaction type is due to their shape selectivity [11] and decreased sensitivity to water or other oxygenates versus AICI3. It is possible to control the selectivity of the reaction to the desired product by using a zeoHte with the proper characteristics [12]. These reactions are covered in more detail in Chapter 14. 

The benefits of using biodiesel as renewable fuel and the difficulties associated with its manufacturing are outlined. The synthesis via fatty acid esterification using solid acid catalysts is investigated. The major challenge is finding a suitable catalyst that is active, selective, water-tolerant and stable under the process conditions. The most promising candidates are sulfated metal oxides that can be used to develop a sustainable esterification process based on continuous catalytic reactive distillation. 

For a convenient comparison. Table 1 summarizes the pros and cons for each catalyst tested. Clearly, the sulfated metal oxides are the best choice. 

Sulfated tin oxide (STO) is classified as one of the strongest solid acids (STO calcined at 550°C ranks first among sulfated metal oxides according to the Hammett function scale, Hq value = 18). However, the use of STO has been more limited than that of SZ (calcined at 650°C, Hq value = —16.1) due to preparation difficulties and poor yields. However, new preparation routes are making this catalyst more accessible, and recently its use has become more widespread. In a recent study by Furuta et al., STO was compared to SZ in the esterification of n-octanoic acid with methanol. The STO catalyst showed superior activity compared to SZ at temperatures below 150°C. For instance, STO approached a 100% ester yield at 100°C, while SZ required temperatures as high as 150°C to reached similar yields. 

About 10 years have passed since this study began to be seriously undertaken, but the usage of solid superacids as catalysts is still limited. Table IX summarizes the acid-catalyzed reactions on sulfated metal oxides, i.e., cracking, isomerization, alkylation, acylation, esterification,... 

Similarly, by adding metal oxide catalysts to the Ispra Mark 13 sulfnr-bromine cycle. General Atomics snlfnr-iodine cycle and sulfur-iron cycle (Reactions (56) to (64)), a number of new, modified metal sulfate based... 

As discussed above, FSEC s S-NH3 cycle also utilizes decomposition of sulfuric acid as the endothermic step for the absorption of solar thermal heat and production of oxygen. However, high temperature concentration and decomposition of sulfur acid presents daunting materials of construction issues. Like the metal sulfate based TCWSCs, it is possible to modify the S-NH3 cycle and do without the decomposition of H2SO4. There are two ways to accomplish this. The first approach is to decompose ammonium sulfate produced in the hydrogen production step of the S-NH3 cycle (Reaction (111)) to a metal sulfate in the presence of a metal oxide catalyst. The second approach is to convert ammonium sulfate to metal pyrosulfate e.g. Zto 20i)-... 

Initial experiments performed at the INL compared different catalysts, fluids, and operating conditions to determine the effect of SCF on solid acid catalyst alkylation (5). Three sets of studies were performed a catalyst comparison using six different catalysts (i.e., two zeolites, two sulfated metal oxides, and two Nafion catalysts) with methane as a cosolvent an exploration of the effect of varying methane addition on alkylation using a USY zeolite catalyst and a study of the effect of seven cosolvents (i.e., three hydrocarbons, two fluorocarbons, carbon dioxide, and sulfur hexafluoride) at L, ML, NC-L, and SCF conditions on the USY catalyst performance. 

The isomerization of light paraffin using superacid solid catalysts is a clean way to increase the octane number of hydrocarbons. On this basis, sulfated metal oxides have attracted the attention of many research groups owing to their high activity in acid catalyzed reactions [1]. Sulfated zirconia was found to be a promising catalyst in this field and at the industrial level [2],... 

Among the different sulfated metal oxides available, SZ is, by far, the most studied and utilized solid acid catalyst its properties strongly depend on the preparation method (from the nature of the starting materials to calcination conditions). 

Small amounts of sulfate ion on metal oxide catalysts can either enhance or decrease the activity, depending on the nature of the compound formed by the reaction of the sulfate with the metal oxide. The effect of SO2 or SO3 on metal oxide catalysts can also be to enhance the activity by increasing the Brpnsted acidity believed responsible for the active sites on these SCR catalysts 52. Large amounts of condensible sulfate can result in pore blockage. 

On these bases it was accepted that, indeed, a new type of solid superacids was obtained and this opened new perspectives in the use of friendly solid catalysts for carrying out, reactions involving very strong acid sites under mild conditions. However, when research on this subject had progressed it became necessary to question the superacidity of sulfated metal oxides on the basis of the following observations. The use of Ho values for... 

Other preparation methods have recently been developed. Sulfated metal oxides have been prepared by a sol-gel method [42,57,58], which involves the formation of a zirconium-sulfate co-gel by adding sulfuric acid to zirconium n-propoxide in isopropyl alcohol. This one step method appears to be simpler than the two step preparation procedures and allows a better control of the variables. It also allows the direct formation of biiunctional catalysts by the addition of chloroplatinic acid to the gel mixture. A new preparation method, named rapid thermal decomposition of precursors in solution (RTDS), which involves the use of hot pressurized water at hydrotheimal conditions to force metal ion precursors to go into phases of oxyhydroxides and oxyhydrosulfates, has been used to produce sulfated zirconia with crystallite sizes below 100 A [59]. 

The predominant application of metal oxide catalysts is due to their oxidation and acid-base behavior. In the following, these areas are discussed separately, although it is clear that in many materials, for example, heteropolyacids, which combine both strong acidity and oxidation efficacy (37,38), and the sulfated metal oxides, where controversy exists as to whether the observed low temperature isomerization pathways are catalyzed by superacid or redox mechanisms (39-41), the distinction between acid-base and oxidation properties is somewhat arbitrary. To illustrate their principles, a number of different reaction types are discussed. Dehydrogenation reactions, ammoxidation, and the WGS reaction have been included imder oxidation catalysts since they constitute major industrial applications of metal oxide-based catalysts. In the case of acid-base catalysis, some of the recent activity in the area of biodiesel is described as an illustration of the complementarity of both acid catalysis and base catalysis. There are a number of additional applications of oxides as catalysts, such as in photocatalysis (42), which have not been reviewed here because of limitations of space. Oxidation Activity. 

Trogadas and Ramani summarized the modification of PEM membranes, including Nafion modified by zirconium phosphates, heteropolyacids, hydrogen sulfates, metal oxides, and silica. Membranes with sulfonated non-fluorinated backbones were also described. The base polymers polysulfone, poly(ether sulfone), poly(ether ether ketone), polybenzimidazole, and polyimide. Another interesting category is acid-base polymer blend membranes. This review also paid special attention to electrode designs based on catalyst particles bound by a hydrophobic poly-tetrafluoroethylene (PTFE) structure or hydrophilic Nafion, vacuum deposition, and electrodeposition method. Issues related to the MEA were presented. In then-study on composite membranes, the effects of particle sizes, cation sizes, number of protons, etc., of HPA were correlated with the fuel cell performance. To promote stability of the PTA within the membrane matrix, the investigators have employed PTA supported on metal oxides such as silicon dioxide as additives to Nafion. 

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