Abstract:

A theoretical-experimental study about the behaviour of the protons and their interactions with water molecules on the Wells-Dawson acid and their influence on the catalytic activity was done. The Wells-Dawson acid (H6P2W18 O62·nH2O) was characterised by TGA analysis. Besides, DRIFTS and 1H MAS-NMR measurements as a function of acid treatment temperature were done. These experimental results, consistent with prior studies, suggest that the protonic acidity is related to the presence of water molecules in the heteropolyoxoanion (HPA) structure. In particular, the H5O2+ species associated with the last two H2O molecules determined by TGA analysis and the 1H MAS-NMR measurements show that the presence of H5O2+ species plays an important role in the acidity - catalytic activity relationship. The Wells-Dawson acid structure formation was studied by calculating the relative energy of the system. The possible acid molecular structure (P2W18O62 H6·nH2O) was analysed considering different steps by associating to the Wells-Dawson anion (P2W18O62)-6, the protons, water molecules and secondary structures. The theoretical calculus by extended Hückel method (EHMO) was done analysing the most energetically favourable positions for the protons. The theoretical results indicate that three different water species associated to the Wells-Dawson acid structure can exist. Besides, these results show that the H5O2+ species are the most energetically stable ones in the acid structure. These species bonds two secondary structures of Wells-Dawson acid leading to minimum system energy. It can be expected that these species are the last water molecules lost in the acid structure at the end of acid dehydration process. This assumption is in agreement with the results obtained with the three experimental techniques previously mentioned. © 2002 Elsevier Science B.V. All rights reserved.