Solid acids hydrocarbon conversion

Catalytic activity of solid acids in hydrocarbon conversions is often correlated with their acidity. Problems arise from the difficulty to bridge the gap between the equilibrium thermodynamic concept of acidity and the composite kinetic concept of catalytic activity [1], The correlation is meaningful if connected parameters are related to each other, namely, intrinsic activities are correlated with intrinsic acidities or relationship is established between corresponding apparent parameters. 

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. 

Song, W., Marcus, D.M., Fu, H., Ehresmann, J.O., and Haw, ).F. (2002) An ofr-smdied reaction that may have never been direct catalytic conversion of methanol or dimethyl ether to hydrocarbons on the solid acids HZSM-5 or HSAPO-34./. Am. Chem. Soc., 124, 3844-3845. 

Solid and liquid acid-catalyzed hydrocarbon conversions involve by far the largest amount of catalysts and largest economic efforts in oil refining and chemical... 

The preparation of new solid acids, their characterization, mechanistic studies, and theoretical approaches to understand the fundamental aspects of acid-catalyzed hydrocarbon conversion constitute a very large fraction of the topics discussed in the last decade in all journals related to catalysis and physical chemistry. However, in contrast with liquid-acid-catalyzed activation processes, many fundamental questions concerning the initial step, the true nature of the reaction intermediates, and the number of active sites remain open for discussion. For this reason, the results obtained in liquid-superacid-catalyzed chemistry, which can be rationalized by classical reaction mechanisms, supported by the usual analytical tools of organic chemists, represent the fundamental basis to which scientist in the field refer. 

Due to the unclear picture concerning the initial step in hydrocarbon conversion on solid acids, generally one of four pathways can be found in the literature protolysis (1), hydride abstraction by an already existing carbenium ion (2), hydride abstraction by a Lewis acid (M) (3), and oxidation (4) (Scheme 5.2). 

Alkanes and Strong Solid Acids. Since the early reports by Nenizetscu and Dragan67 on alkane isomerization on wet aluminum chloride in 1933, all mechanistic studies have led to a general agreement on the carbenium-ion-type nature of the reaction intermediates involved in acid-catalyzed hydrocarbon conversions. In contrast with this statement, the nature of the initial step is still under discussion and a variety of suggestions can be found in the literature among which direct protolysis of C—H and C—C bonds, protonation of alkenes present as traces, and oxidative activation are the most often quoted.54,55... 

Recently Milczak et al.[57] have reported the nitration of o-xylene using 100% nitric acid over silica supported metal oxide solid acid catalysts with high yields (up to 90 %) but low selectivity to 4-o-NX (40-57 %). Choudary et a/. 5X 591 performed the nitration of o-xylene and other aromatic hydrocarbons by azeotropic removal of water over modified clay catalysts achieving low yields of 4-o-NX and a selectivity of 52%. Better results were obtained when HBeta zeolite was used as catalyst, performing the reaction in dichloromethane at reflux temperature.[60] Conversions of 40 % and maximum selectivity 68 % of 4-o-NX were obtained. Similar conversions and higher selectivities for 4-o-NX (65-75 %) were reported by Rao et al M 1 using a nanocrystaUine HBeta sample and working at 90 °C in the absence of solvent. 

Zeolites and other oxide-based solid acid catalysts are used in hydrocarbon conversion reactions with enormous economic impact. Their lifetime is, however, very... 

Solid acid catalysts play an important role in hydrocarbon conversion reactions in the chemical and petroleum industries [1,2]. Many kinds of solid acids have been found their acidic properties on catalyst surfaces, their catalytic action, and the structure of acid sites have been elucidated for a long time, and those results have been reviewed by Arata [3]. The strong acidity of zirconia-supported sulfate has attracted much attention because of its ability to catalyze many reactions such as cracking, alkylation, and isomerization. Sulfated zirconia incorporating Fe and Mn has been shown to be highly active for butane isomerization, catalyzing the reaction even at room temperature [4]. 

The conversion of propane and cycloalkanes into dialkyl (dicycloalkyl) sulfides under the action of elemental sulfur has been recently reported [59], Sulfur is converted completely into dicyclopentylsulfide in the reaction with cyclopentane in the presence of CX nAlBrs (X = Cl, Br n = 2 or 3) at -20° C for 20 min. Propane forms -Pr2S in 60% at 20 °C after 2 h. Transformations of alkanes and other hydrocarbons under the action of solid acids (metal oxides) occurring at relatively high temperature will be considered in the next chapter. 

His first major discovery in the U.S. was the development of silicophosphoric acid (solid phosphoric acid) as a catalyst for hydrocarbon conversion. 

In 2002 a process for the direct carbonylation of saturated hydrocarbons has been patented (83). The process involves contacting the saturated hydrocarbons, which contain at least one primary, secondary or tertiary carbon atom, with carbon monoxide in the presence of a strong solid acid catalyst to produce an oxygenated saturated hydrocarbon. However, the observed conversions were small. For example, 119 g of isobutane, reacted at 100° C for 12 h with carbon monoxide (68 atm) using sulfated zirconia as the catalyst, produced only 0.14 g of pivalic acid and 0.007 g of methylisopropyl ketone. 

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