Stable Mechanistically-Relevant Aromatic-Based Carbenium Ions in Zeolite Catalysts

Abstract

The existence of carbenium ion species is assumed in many zeolite catalysis mechanisms. Using computational techniques that include environmental effects, a benzenium-type carbenium ion is identified in zeolite catalysts for the first time. Localization of nearby transition states indicate that this species may play an important role as an intermediate in the bimolecular m-xylene disproportionation reaction. The barrier to back-donation of the proton from the benzenium ion is at least 50 kJ/mol, meaning that this species may be spectroscopically observable. An additional carbenium ion intermediate, formed by abstraction of a hydride from m-xylene, is also predicted. The stability of this second new carbenium ion suggests that aromatic-based carbenium ions are likely to be intermediates in many zeolite-catalyzed reactions. Two types of fundamentally different fully periodic calculations support the stability predictions.

Adsorption and separation of linear and branched alkanes on carbon nanotube bundles from configurational-bias Monte Carlo simulation

“…However, the interaction governing adsorption behavior here is mainly dispersive van der Waals forces, which are difficult to calculate accurately from quantum chemistry unless with sufficiently high level theory, very large basis set, and a great deal of CPU time is used. Alternatives are to use hybrid techniques, as proposed by Clark et al, 109 in which the dispersive interactions are incorporated by ad hoc empirical potentials; or using the HM-IE method 110 as we have used to calculate the ab initio potentials of O 2 and N 2 with C 168 schwarzite and C 60 , respectively. [111][112][113] While we expect the use of a more accurate potential might lead to some quantitative differences from the results using the empirical potential, we do not expect any qualitative differences.…”

Proton Transfer in Zeolites

“…Hydride abstraction from xylene is assumed to be the initial step in its disproportionation into toluene and trimethylbenzene [9]. The parent compound (7, Fig.…”

“…However, the lifetime of carbenium ions in zeolites is not only limited by proton transfer, but also formation of a C-O bond between the carbenium ion and a framework oxygen atom, yielding an alkoxide, needs to be considered. In ferrierite (FER) the alkoxide of 6 is found to be 50 to 60 kJ mol -1 more stable than the carbenium ion [9]. Table 22.1 lists three examples of cyclic alkenyl carbenium ions that live long enough in zeolites to be detected by NMR [6].…”

Das tert‐Butylkation in H‐Zeolithen: Deprotonierung zu Isobuten und Umwandlung in Oberflächenalkoxide

“…Ursprünglich wurde angenommen, dass diese Reaktionen den bekannten Mechanismen der Chemie in supersauren Medien folgen würden und somit Carbokationen als Intermediate gebildet werden.[1] Die Existenz von Carbeniumionen konnte bisher durch NMR-, [2] UV/Vis- [3] und IR-Spektroskopie [4,5] sowie rechnergestützte Techniken [6] nur für cyclische Alkenyl-oder aromatische Carbeniumionen nachgewiesen werden, nicht aber für Carbeniumionen, die von kleinen Alkenen abgeleitet sind. Diese bilden vielmehr mit an der Oberfläche vorhandenen Sauerstoffatomen Alkoholate.…”

“…Diese bilden vielmehr mit an der Oberfläche vorhandenen Sauerstoffatomen Alkoholate. [7][8][9] Nicholas und Haw folgerten, dass stabile Carbeniumionen in Zeolithen durch NMR-Spektroskopie beobachtet werden können, wenn die Ausgangssubstanz (aus der das Carbeniumion durch Protonierung erhalten wird) eine Protonenaffinität von mindestens 875 kJ mol À1 hat.[10] Quantenchemische Studien zeigten, dass diese Aussage weiter verallgemeinert werden kann: Der Protonentransfer von einem HZeolith auf ein Molekül [6] oder einen molekularen Cluster [11] findet statt, wenn dessen Protonenaffinität größer oder gleich der von Ammoniak (854 kJ mol …”