Aromatic bromination catalysis on zeolites. I-Raman spectra of activated adsorbed bromine

Laser Raman spectra of Br2 adsorbed on zeolites X, Y and A reveal features characteristic of physisorbed Br2 and of Br2 on cation oxide sites. Specifically, Br2 on zeolites X adsorbs on sites III (ν(BrBr), 318–280 cm−1) and on sites II (ν(BrBr), 280–240 cm−1). The data indicate that the cations in sites II are more effectively shielded by the oxide framework than those in sites III. The greater participation of the oxide framework in the adsorption at sites II is also reflected in the detection of the adsorbate–adsorbent frequency, ν(O Br) at ∼ 160 cm−1. The appearance of this line is indicative of significant donor (oxide)–acceptor (bromine) electron-transfer interaction. The spectra of the adsorption complex NaY + Br2 confirm the assignments for MX + Br2 as NaY has no cations in sites III and fewer cations in sites II than NaX, with the result that only Br2 on sites II (271 cm−1, NaY; 270 cm−1, NaX) and physisorbed Br2 is detected. For NaA, no ν(O Br) is detected, which is consistent with the previously reported absence of charge-transfer interactions between the oxide framework and Br2.