Monofunctional catalysts zeolites

Methylcyclopentane (MCP) is a convenient probe molecule for interrogating the metal and acid sites of a bifunctional catalysts. For instance, metal clusters are formed in the cavities of zeolite Y by ion exchange, followed by calcination and reduction with hydrogen. Protons which act as Bronsted acid sites are formed during reduction of the metal ions. A monofunctional catalyst can be obtained by neutralizing these protons with NH3 or by secondary exchange with Na ions. With this acid-free form of such catalysts the ring-... 

Carbonium ions and isoparaffins are formed by hydride ion abstraction and hydride ion transfer reactions. This mechanism has been described for HF.SbFg (5). Isomerization of n-paraffins over monofunctional acidic catalysts has also been claimed for mordenite (6, 7), for sieve Y (8), and for the base of the catalyst of undisclosed composition applied in the isomerization process using a noble metal on an acidic zeolite base (3). 

Dehydrocyclization of n-hexane to form benzene has been a subject of considerable academic and industrial interest since Bernard first reported that platinum clusters supported inside the channels of zeolite L catalyze the reaction with exceptional activity and selectivity (7). The nonacidic nature of the Pt-zeolite L catalyst and correlation of reaction rate with Pt content are consistent with the accepted view that the catalyst is monofunctional, depending solely on Pt metal for catalytic activity (7). However, comparison of aromatization reactivity over nonacidic Pt-zeolites to conventional non-zeolitic catalysts revealed that additional factors contribute to the unusual performance of Pt-zeolites (2). 

Noble metal zeolite catalysts are used in various processes, most of them occurring through bifunctional hydrogenating/acid catalysis. One exception, however, is the selective aromatization of n-alkanes (e.g. n-hexane into benzene) proceeding through monofunctional metal catalysis. Indeed the PtLTL catalyst used commercially does not present any protonic sites. 

Ethylbenzene disproportionation catalyzed by add zeolites was studied by Karge et al. [887, 888] and recommended as a versatile test reaction for acid (monofunctional or bifunctional) catalysts such as add zeolites or related materials and is frequently used for this purpose. Also, this reaction was studied in situ by IR spectroscopy in combination with gas chromatography for determination of conversion and selectivity [888]. In these experiments, a flow-reactor quartz glass cell as described in Ref. [152] was used, which could be operated imder ultra-high vacuum during the pretreatment of the thin catalyst wafers of pressed zeolite powder at, e.g., 670-870 K and 10 Pa. After pretreatment, the cell was used as a differential fixed-bed micro-flow reactor (cf. also [ 152,158]). Results are illustrated by Fig. 52. 

Zeolites were used in various processes that convert paraffins or olefins into alkyl monoaromatics containing chiefly from six to nine carbon atoms.There are various catalysts, and they involve a base or an acid zeolite, according to the type of process. The first catalyst, identified at the end of the 1970s. is composed of Pt deposited on the L zeolite (Table 2) exchanged with large alkaline ions such as potassium or alkaline earths, such as Ba. which gives it an alkaline nature. This cataiyst is monofunctional and is conceptually different from the conventional bifunctional acid catalysts based on Pt on chlorinated alumina. It selectively dehydrocyclizes the paraffins into aromatics, particularly hexane, which is the least reactive of them. The reaction takes place on the metal, which develops a special selectivity in contact with the alkaline zeolite. This aromatization process has not been successful so far. partly because of the extreme sensitivity of the catalyst to the slightest trace of sulfur compounds. 

Ben Taarit and Bandiera (138), have proposed that in monofunctional zeolite catalysts both cracking and dehydrogenation occur on the same type of sites being the attack of the proton the controlling step. The selectivity to these two reactions could be changed by changing the acidity of the sites, opening a possibility to improve the selectivity of a cyclar catalyst. 

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