On the preferential location of Al and proton siting in zeolites: A computational and infrared study

Force field atomistic simulations have been used to investigate the Al preferential location and proton siting in zeolites, and the role of the template is shown to be crucial in order to elucidate the Al and acid site distributions. Therefore, it is not the energy of the final structure what controls the Al distribution, but rather the energetics during the synthesis process (zeolite+SDA) that has to be considered, and in this sense the structure directing agent (SDA) accommodation within the microporous space acts not only as a structure director but also as a director of the Al distribution. Once the Al distribution is known, the positively charged SDA can be removed from the calculation and substituted by protons and the population of the four protons in each Al center can be calculated from the corresponding energies. Also, this methodology allows to compute the OH stretching frequencies and, therefore, a map of acid site frequencies, and relative population is obtained which gives a simulation of the OH stretching IR bands in terms of frequencies and intensities. The methodology has been validated by applying this general approach to the case of the newly synthesized ITQ-7 zeolite and the corresponding calculated IR spectrum shows a reasonable agreement with the experimental spectrum. Two bands centered at 3577 and 3624 cm(-1) result from the simulation, which compare to the experimental values of 3595 and 3629 cm-1. Preferential proton siting has been also studied and energy differences in each AlO4 tetrahedra of about 30-40 kJ/mol are found, in qualitative agreement with H-1 NMR results.