Propylene oxidation over poly(azomethines) doped with heteropolyacids

Kinetic studies of propylene oxidation and isopropanol conversion were unde rtaken using new catalytic systems, namely, conjugated polymer-supported he teropolyacid catalysts. Unsubstituted and dimethoxy-substituted aromatic po ly(azomethines) (PPI and PMOPI) were used as matrices to which selected het eropolyanions (HPAs) were introduced via acid-base-type doping which consis ts of protonation of imine nitrogen. PPI was synthesized via polycondensati on of 1,4-phenylenediamine with terephthaldehyde, whereas PMOPI was prepare d via the reaction of 1,4-phenylenediamine with 2,5-dimethoxyterephthaldehy de. Protonation of PPI and PMOPI with heteropolyacids-H3PW12O40 and H3PMo12 O40-was carried out in the acetonitrile solutions, The protonation level wa s controlled by changing the starting molar ratio of HPA:polymer in the rea ction medium. Both poly(azomethines) in their base forms and protonated pol y(azomethines) were characterized using X-ray diffraction and various spect roscopic methods (FTIR, Raman, XPS). Thermal stability and BET surface area were also studied, X-Ray diffraction patterns show that HPA introduced int o the polymer matrices is molecularly dispersed and does not form a separat e phase. FTIR studies prove that the structural identity of individual hete ropolyanions is preserved on introduction into the polymer matrices. Howeve r, Raman studies prove that protonation Leads to changes in polymer chain c onformation. In the oxidation of propylene the PPI(HPA)(y) and PMOPI(HPA)(y ) catalysts give hexadiene as the main reaction product. Introduction of HP A into the polymer matrices changes the selectivity of the acid since the r eaction carried out in the presence of crystalline H3PMo12O40 results in ac rolein as the main product. For all catalysts studied, only products of non destructive oxidation can be detected (hexadiene, acrolein, propionaldehyde , acetone, benzene). (C) 2000 Academic Press.