Superacid catalysts, cracking

Subsequently, the same authors138 described the preparation of a solid superacid catalyst with acid strength of H0 = —16 with a sulfuric acid-treated zirconium oxide. They exposed Zr(OH)4 to 1A sulfuric acid and calcined it in air at approximately 600°C. The obtained catalyst was able to isomerize (and crack) butane at room temperature. The acidity was examined by the color change method using Hammett indicators added to a powdered sample placed in sulfuryl chloride. The... 

It is well known that zeolites and solid superacid catalysts suffer rapid deactivation within the first few minutes during alkylation reaction [4-7], Therefore, the product distribution during initial few minutes of reaction was monitored on the dealuminated Y zeolites. During preliminary experiments it was found that appreciable alkylation activity is observed only at temperatures above 50 C. Unlike liquid-phase reactions, where alkylates were observed in the product within 1 min time-on-stream [5,7], in our experiment no Cg hydrocarbons was observed during first 5 minutes and the product mainly consists of only Cs fiactions, i.e., cracked products. After 5 minutes, Cg fiactions started showing up in the product, reached a maximum and then again decreased. This indicates that the alkylate formed initially were... 

These polymers have been referred to as organometallic superacid catalysts. Electron microprobe X-ray analysis has shown that bound aluminium was uniformly distributed in the polymer beads. Such acidic catalysts may prove useful as insoluble Friedel-Craft catalysts, although many of the cracking and isomerizations reported can be conveniently done with zeolites (molecular sieves) (Meier and Uytterhoeven, 1973) albeit at higher temperatures. 

It has been shown, however, that such catalysts may contain protons, either by design or because of the difficulty in removing all traces of moisture, and these protons have been shown to be superacidic with Hammett acidities up to —18. These protons will also play some role in the catalytic activity of these ionic liquids in practical situations. Ionic liquids in which superacidic protons have deliberately been generated by addition of small amounts of water, HCl or H2SO4 have been used to catalytically crack polyethene under relatively mild conditions. The main products are mixed C3-C5 alkanes, which would be a useful feedstock from waste polyethene recycling. In contrast to other cracking procedures no aromatics or alkenes are produced, although small amounts of polycyclic compounds are obtained. 

A variety of solid acids besides zeolites have been tested as alkylation catalysts. Sulfated zirconia and related materials have drawn considerable attention because of what was initially thought to be their superacidic nature and their well-demonstrated ability to isomerize short linear alkanes at temperatures below 423 K. Corma et al. (188) compared sulfated zirconia and zeolite BEA at reaction temperatures of 273 and 323 K in isobutane/2-butene alkylation. While BEA catalyzed mainly dimerization at 273 K, the sulfated zirconia exhibited a high selectivity to TMPs. At 323 K, on the other hand, zeolite BEA produced more TMPs than sulfated zirconia, which under these conditions produced mainly cracked products with 65 wt% selectivity. The TMP/DMH ratio was always higher for the sulfated zirconia sample. These distinctive differences in the product distribution were attributed to the much stronger acid sites in sulfated zirconia than in zeolite BEA, but today one would question this suggestion because of evidence that the sulfated zirconia catalyst is not strongly acidic, being active for alkane isomerization because of a combination of acidic character and redox properties that help initiate hydrocarbon conversions (189). The time-on-stream behavior was more favorable for BEA, which deactivated at a lower rate than sulfated zirconia. Whether differences in the adsorption of the feed and product molecules influenced the performance was not discussed. 

Cracking according to the second mechanism occurs by direct protonation of the paraffin with the solid catalyst acting as a superacid ... 

The natural clay minerals are hydrous aluminum silicates with iron or magnesium replacing aluminum wholly or in part, and with alkali or alkaline earth metals present as essential constituents in some others. Their acidic properties and natural abundance have favored their use as catalysts for cracking of heavy petroleum fractions. With the exception of zeolites and some specially treated mixed oxides for which superacid properties have been claimed, the acidity as measured by the color changes of absorbed Hammett bases is generally far below the superacidity range. They are inactive for alkane isomerization and cracking below 100 °C and need co-acids to reach superacidity. 

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,... 

Haag proposed an explanation of hydrogen, methane and ethane formation during the alkane cracking over H-ZSM-5 and H-ZSM-11 catalysts by the reactions typical of superacids ... 

As a result of this importance, great efforts have been made to understand the interplay between structure and chemistry to produce optimised acid catalysts for processes such as cracking, alkylation and isomerisation. It is now well established that zeolites are not superacidic, so that the apparent carbenium ion controlled conversions are thought to pass through carbenium-ion-like transition states stabilised by the zeolite framework. For methanol-to-hydrocarbon reactions, elegant in situ NMR has demonstrated that a reactive hydrocarbon pool that forms within the pores is observed to be responsible for the formation of the first C-C bonds, and it is likely that reactive hydrocarbon intermediates have a greater role in add-catalysed reactions than previously spelt out. 

A superacid polymer catalyst was obtained by binding aluminium chloride to sulfonated, macroporous co(polystyrene-DVB) (Magnotta et u/., 1976 Magnotta and Gates, 1977a,b). The catalyst was active in bringing about cracking and isomerization of -hexane at 357 C at atmospheric pressure. 

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