Abstract:
Azobenzenes are a crucial class of molecules used as dyes, food additives, indicators, and therapeutic agents. Conventional methods used for preparation of azobenzene involve use of high stoichiometric/ toxic oxidant amounts which pose several drawbacks such as undesirable overoxidation products, harsh reaction conditions, limited functional group compatibility and environmentally unfriendliness. K-OMS-2 has superbly tailorable functionality which can be adjusted to favor formation of azobenzene. OMS-2 surface chemical property was successfully adjusted via etching with oxalic acid at room temperature and applied for oxidative coupling of anilines to azobenzene. Material characterization was carried out by XRD, SEM, TEM and BET to explore the relationship between material property and catalytic activity after acid treatment. The results show increase in surface area at lower acid concentration as regular K-OMS-2 has 72.6 m2
/g, 0.025M OxH-OMS-2 has 81.9 m2/g and decrease to 53.6 m2/g as acid concentration
was increased to 0.100M. The XRD shows retention of cryptomelane phase structure after
etching and the nanorod morphology of the OMS-2 material is also maintained as is observed from SEM and TEM images. OMS-2 shows higher catalytic conversion of 72% compared to etched OMS-2 which shows 47% in 0.025M OxH-OMS-2, and 41% in 0.100M OxH-OMS-2 at 110o C which is attributed to less Mn4+ content on the catalytic surface due to reduction by oxalic acid from Mn4+ to Mn3+/Mn2+. Selectivity of azobenzene was observed to increase with the increase in concentration of the etchant in the following series, OMS-2 18%, 0.025M OxH-OMS-2 48% and 0.100M OxH-OMS-2 80% which can be ascribed to change on surface chemical state of the material. Calcined 0.100M OxH-OMS-2 shows higher performance of 57% than uncalcined catalyst which is 41%. Moreover, calcination decreased selectivity of azobenzene from 80 to 70% indicating oxidation of Mn2+/Mn3+ back to Mn4+ in air.
