Alkali Metal Cation Control of Oxidation Reactions of Radicals in Zeolites

Abstract

The dynamics of the xanthyl radical in a series of alkali-metal cation exchanged (Li+, Na+, K+, Rb+, and Cs+) Y faujasites in the absence and presence of molecular oxygen are examined by using laser flash photolysis. Upon laser photolysis of xanthene-9-carboxylate incorporated within the supercages of NaY in the absence of oxygen, prompt formation of the xanthyl radical and the xanthylium cation is observed. The xanthyl radical is formed by photoionization of xanthene-9-carboxylate to the corresponding acyloxy radical that then rapidly decarboxylates. The prompt xanthylium cation is produced by photoionization of the xanthyl radical. This mechanism for the prompt xanthylium cation is supported by results from two-color laser and photoinduced electron transfer experiments. When oxygen is introduced into the sample, the radical is almost completely quenched. In addition, a slow growth of the xanthylium cation at 375 nm is observed. This growth is due to heterolytic cleavage of the peroxyl radical formed upon reaction of the xanthyl radical with molecular oxygen. The formation and resulting growth of the xanthylium cation in the presence of oxygen is found to be highly dependent on the zeolite counterion with significant carbocation formation occurring in both LiY and NaY, little carbocation formation in KY, and no carbocation formation in RbY and CsY. These are the first results showing how the oxidation of radicals to carbocations within zeolites can be controlled by simple alkali metal exchange.

Dynamics of the transient species generated upon photolysis of diarylmethanes within zeolites — Deprotonation and oxidation reactions

“…In particular, the oxidizing ability of Y zeolites follows the order HY > NaY > KY > CsY (16), and this order matches the yield of time-resolved carbocation formation as a function of counterion. Furthermore, An 2 CH + was generated under milder conditions than Tol 2 CH + , while Ph 2 CH + was not detected even in the highly oxidizing HY zeolite.…”

“…The high density of negative charge on the zeolite framework is thought to provide an environment well-suited for the generation of positively charged species and in fact, carbocations can be thermally generated within proton-exchanged zeolites (15). The redox properties of the zeolite are highly dependent on the nature of the chargebalancing cations (16), and it has been shown that protonexchanged zeolites are much more powerful oxidizing agents than alkali cation-exchanged zeolites (17). These acidic solids have been the subject of extensive research because of their catalytic ability and wide use as industrial catalysts, particularly for the refinement of petroleum products (4,18,19).…”

Evidence for Encaging Luminescent Guest Molecules in the Inner Cages of Zeolite Host

“…In order to systematize nanohybrid chemistry using host-guest systems, it is required to elucidate the origin of the specific properties of nanohybrid systems, which usually appear only when the guests are encaged into the hosts. NMR, [10][11][12][13] IR, 12,14,15 SEM, 16,17 optical microscopy, 18 and TEM 1,16,17,19 are used as the conventional methods for the distinction. NMR and IR are important tools that give information of local structures or environments of guests and hosts in the range from atomic size to molecular size.…”

“…NMR and IR are important tools that give information of local structures or environments of guests and hosts in the range from atomic size to molecular size. In studies on nanohybrid systems using organic host materials (e.g., cyclodextrin 20 or crown ether 21 ), NMR and IR measurements provide us clear information on the position of guest molecules, because these organic hosts have small and simple structures compared to inorganic host materials (e.g., zeolite [9][10][11][12][13][14][15] or mesoporous silica 2,16 ). Only partial information on the interaction for the nanohybrid systems containing inorganic hosts can be obtained by using these methods.…”

“…Although much discussion has been made on interactions between hosts and guests in studies on nanohybrid systems using inorganic host materials, [12][13][14][15] only a few reports can be found for distinction of guests encaged in hosts or adsorbed on the surface of hosts, 19,22,23 because it is difficult to determine clearly the guests' position by using conventional methods such as NMR, [10][11][12][13] IR, 12,14,15 SEM, 16,17 and optical microscopy. 18 Beutel and Su, for example, have measured NMR of an organic molecule (phenol) adsorbed onto FAU-type zeolite.…”