In situ EPR investigation of the addition of persistent benzyl radicals to acrylates on ZSM-5 zeolites. Direct spectroscopic detection of the initial steps in a supramolecular photopolymerization

Abstract

Photolysis of dibenzyl ketone derivatives adsorbed on ZSM-5 zeolites produces persistent benzyl radicals (initiator radicals), which add to methyl acrylates (monomers) to generate persistent adduct radicals. Both initiator and adduct radicals are readily observable by conventional steady-state EPR spectroscopy at room temperature and are persistent for time periods ranging from seconds to many days. The rate of the formation and the amount of the adduct persistent radical formed depends on the structure of the initiator radical (benzyl radical derivative) and the structure of the monomer (acrylate derivative). The lifetimes of the initiator and adduct radicals depend on the supramolecular structure of the radical@zeolite complex and the diffusion and reaction dynamics of the radicals in the complex. The most intense signal and highest addition rate to methyl acrylate were observed for the smallest initiator radical, the benzyl radical, because of its high mobility and relatively rapid diffusion within the internal zeolite surface. With increasing length of an alkyl chain (methyl, ethyl, and pentyl) on either the initiator (alpha position of the radical) or monomer (alkyl group of acrylate ester), the rate of radical addition to the monomer decreased, a result that is consistent with the decreased mobility and diffusion of the initiator radical or monomer. Deuterium isotope experiments and variation of the methyl acrylate concentration demonstrated that the initial adduct radical from methyl acrylate adds to another methyl acrylate to generate a secondary adduct radical, which, in turn, can continue to propagate to form a polymer that is cross-linked to the zeolite crystals. The results demonstrate that EPR can be a powerful tool for the direct in situ analysis of supramolecular photochemistry involving radicals rendered persistent by supramolecular steric effects. The latter eliminate the need for sophisticated flash photolysis equipment to investigate the structure and dynamics of reactive radicals and require only the use of simpler steady-state lamps.

Small pore zeolite catalysts for furfural synthesis from xylose and switchgrass in a γ-valerolactone/water solvent

“…This supports the hypothesis that the catalysts, rather than water dissociation, provide the acidity necessary to catalyze xylose dehydration within shorter reaction times, which was also confirmed by Zhang et al using a FeCl 3 6H 2 O catalyst [15]. [25,38,42,43] Although the zeolites contain a significant amount of acid sites (Table 1), in principle, the pore size of the small pore zeolites (3.6-3.8 Å) should hinder xylose diffusion because the size of xylose (6.8 Å [23,24]) is significantly larger than the pores. Although there may be surface acid sites contributing to the reaction, the sugar molecules may also be able to access the interior acid sites.…”

New Insights into the Mechanism of Triplet Radical-Pair Combinations. The Persistent Radical Effect Masks the Distinction between In-Cage and Out-of-Cage Processes

“…The limitations of eq 1 have been recognized qualitatively by Bohne and co-workers who examined the partitioning of radicals from asymmetrically substituted dibenzyl ketones between aqueous and micellar environments, and quantitatively by Turro and co-workers who altered the probabilities of combination of benzylic radicals by placing one inside and another outside zeolite cages during polymerization reactions (case 1 of Scheme ) . Both of these are examples of microphase separation.…”

Long-lived (minutes) photoinduced charge separation in a structured periodic mesoporous titania containing 2,4,6-triphenylpyrylium as guest

“…4 shows the EPR spectra recorded for the TP@mpTiO 2 samples upon illumination. The poorly resolved structure is characteristic of encapsulated radicals with very restricted mobility, 47 and in this case is much less mobile that what has been reported for benzylic radicals in zeolites. light scattering and reflection from electron excitation.…”