Superacid sites

The skeletal isomerization of straight-chain paraffins is important for the enhancement of the octane numbers of light petroleum fractions. The isomerization of H-butane to isobutane has attracted much attention because isobutane is a feedstock for alkylation with olefins and MTBE synthesis. It is widely believed that the low-temperature transformation of n-alkanes can be catalyzed only by superacidic sites, and this reaction has often been used to test for the presence of these sites. 

Results of temperature-programmed desorption (TPD) of ammonia125 or argon126 were attributed to superacid sites. TPD of very weak bases such as substituted benzenes has been used successfully to compare the superacid character of a series... 

Zeolites such as HZSM-5 were considered as superacids on the basis of the initial product distribution in accord with C-H and C-C bond protolysis when isoalkanes were reacted at 500°C (the Haag and Dessau mechanism).135 The reactivity was assigned to superacidic sites in the zeolite framework.136 The superacid character of other solid acids was claimed on the basis of Hammett indicator color change137,138 or on the basis of UV spectrophotometric measurements.139,140 In 2000, a special issue of Microporous and Mesoporous Materials141 was devoted to the superacid-type hydrocarbon chemistry taking place on solid acids as suggested by the late Werner Haag. 

Influence of Superacid Sites in Ultrastable Y Zeolites on Gas Oil Cracking... 

In a previous paper (14) we have shown that by an adequate treatment of steam dealuminated Y zeolites with (NH4)2SiF6 it is possible to remove selectively the EFAL. The physicochemical characterization of these samples shows that the presence of EFAL is the responsible for the formation of superacid sites, as well as for the neutralization of a part of framework hydroxyls. 

Superacid sites were not created by impregnation of the molybdate on the crystallized oxide, but rather on the hydroxide, as was observed in the case of the W03 catalyst. The XRD pattern of the inactive material prepared from the crystallized oxide was also completely different from that prepared from the hydroxide, as shown in Fig. 15, i.e., monoclinic for the former and tetragonal for the latter (191). 

Preparation of these kinds of solid supcracids is described first and the effects of the preparation method on the morphology, surface properties, structure of superacid sites, and catalytic performance are discussed. 

The type of superacid sites on SO /metal oxides evacuated at 773 K is only a Lewis type according to the IR absorption bands of adsorbed pyridine [31]. Mortcrra ct al. [32] have shown that pyridine adsorbed on Lewis acid sites dominated the spectra of samples evacuated at 673 K and that the addition of water at 300 K significantly increased the amount of Bronsted acidity. Nascimcnto ct al. [8] report that both Bronsted and Lewis acid sites exist on SO4 /Zr02 treated at 723 K and the ratio of Bronsted to Lewis sites changes with the change of sulfur content. Recently. Lunsford ct al. revealed by use of 31P MAS NMR spectra of adsorbed trimcthylphosphinc that three types of Lewis... 

In the case of the sulfate-treated superacids of Zr, Sn, Ti, Fe, Hf, and Si, superacid sites are not created by the treatment of sulfate ion on the crystallized oxides but rather on the amorphous forms, followed by calcination to crystallization. The superacid of AI2O3 is prepared from the crystallized oxide, Y-AI2O3 [45, 46]. 

Superacid sites are not created by impregnation on the crystallized oxides, but on the amorphous forms whose calcination then converts them to the crystalline forms. Recent work shows that a W03/Zr02 catalyst prepared by impregnation of the crystalUne zirconia (65% tetragonal, 35% monocUnic) exhibits comparable behavior [77, 78]. 

In contrast with conventional acid resins, sulfonated perfluorocarbon-based Nafion polymers can withstand temperatures up to 200 °C and have superacid sites. Nafion beads, however, have an extremely low surface area (< 0.02 m ... 

Scheme 3 Proposed Superacid Sites [40] (Arrows show electron withdrawal).

The uncertainty associated with a clear chemical description of proposed superacid sites makes generalisation difficult This topic is discussed in more detaU in sections dealing with catalyst modffication. 

This is in line with electron withdrawal from the silanol O by EFai, leading to a decreased O-H bond strength and enhanced Br0nsted acidity [36]. An overview of potential EFai species as probed by different techniques is available [37]. However, superacidic sites in zeolites, viz. USY, compared to real superacids at low temperature are unable to activate a-bonds in alkanes. They have strengths only comparable to that of sulfuric acid [38, 39]. 

Superacid sites have also been identified by electron paramagnetic spectroscopy suggesting that certain sites possess enhanced acidity due to inductive effects associated with neighboring acid sites (16),... 

Both free superacid sites and Rh(III) are required. Rh(III) seems to act as an electrophilic reagent attacking the toluene nucleus para to the methyl group, and the free acid sites play a role either in accelerating CO insertion, or in the cleavage of the /7-tolyl carboxylate from the Rh(III) centre. 

It is worth mentioning here that, in contrast to what has been previously fovmd for the skeletal isomerization of n-butane to isobutane, the addition of small amounts of Fe, Mn, and Pt promoters to 804 /Zr02 does not have a marked effect on its alkylation performance (188). From their own results, the authors proposed that the alkylation reaction on this kind of solid acids occurs on very strong (ie, superacidic) sites, probably via -butyl cation formed by H abstraction from isobutane on superacidic Lewds acid sites. 

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