Polyphosphoric acid (PPA)-modified bitumen:: Disruption of the asphaltenes network based on the reaction of nonbasic nitrogen with PPA

Relatively little is known about the effect of polyphosphoric acid (PPA) on bitumen chemistry and structure. In an effort to increase this understanding, the reaction of indole with PPA was studied by Fourier transform infrared spectroscopy (FTIR). Indole contains a pyrrole functional group very common in bitumen. The results revealed that both the amine and the double bond of indole can be reactive and that the indole concentration affected the reaction pathway. All reactions began with the protonation of the indole double bond, which provided two reactive carbocation intermediates. When the concentration of indole was high, each carbocation coupled with unprotonated indole and two products were obtained. In one case, two N-H groups produced a N-N bond, and in the other case, two indole double bonds produced a cyclobutane ring. When the concentration of indole was reduced by dilution, N-H coupling proceeded without double bonds coupling and a reaction with the PPA anion was found to occur instead. These findings indicate that the effect of PPA in bitumen may be to raise molecular stiffness through N-N bridging, while at the same time, disrupt the hydrogen-bond network where the N-H function of pyrrole groups is involved and reduce the effective molecular weight of asphaltenes.