Interaction of aromatic compounds with zeolite HZSM-5 studied by UV/VIS spectroscopy

Solid-state nuclear magnetic resonance investigations of the nature, property, and activity of acid sites on solid catalysts

“…19b) show bands at 270 and 400 nm due to neutral aromatics (ethylbenzene) and ethylcyclohexadienyl carbenium ions, respectively (Fig. 19b) [210]. The latter species are caused by a ring protonation of the ethylbenzene molecules (strong UV/ vis band at 400 nm), which is an intermediate step of the exchange of hydrogen atoms bound to ring carbon atoms with the hydroxyl protons of Si(OH)Al groups occurring in the temperature range of 393-423 K [211].…”

“…450 nm (Fig. 19b) hinting to the formation of sec-ethylphenyl carbenium ions (C 6 H 5 (CD) þ CD 3 ) [210,211]. To study the activation energy E a of the above-mentioned regioselective H/D exchange on zeolite deH,Na-Y/81.5, in situ pulsed-flow 1 H MAS NMR experiments were performed at 443-463 K. As an example, Fig.…”

Electrophilic aromatic substitution over zeolites generates Wheland-type reaction intermediates

“…2a). The observed bands at ~295, 334 and 415 nm are attributed to neutral alkylated benzenes, carbenium ions with alkyl side chain carbocation and aromatic ring protonation, respectively, whereas the band at 596 nm presumably develops due to the existence of π-complexes between the zeolite and aromatic species (vide infra for NMR results) [31][32][33][34][35][36][37][38][39] . The on-line MS data reveal the existence of diethyl ether, lower olefins (ethylene and propylene) and ethylbenzene ( Fig.…”

“…mediated formation of cyclohexadienyl (arenium) ions/σ-complex (that is, a Wheland-type intermediate) via protonation/alkylation of the aromatic ring (equation (1)) and (ii) Lewis acid side-promoted formation of alkylated carbenium ions via hydride abstraction from the alkyl chain of aromatics (equation (2)) 33,37 .…”

“…3, Supplementary Figs. 3-6 and Supplementary Methods II) [31][32][33][34][35][36][37][38] . Figure 3a-d reflects a redshift of the absorption maxima of a σ-complex (Wheland species), either upon ethylation of the zeolite or in the presence of an electron-donating group attached to the aromatic ring.…”