Structure and vibrational features of complexes between unsaturated hydrocarbons and acidic sites in silica and zeolites: An ab initio study

Experimental vibrational data (FTIR) concerning the interaction of ethylene, acetylene, and methylacetylene with the acidic hydroxyls of amorphous silica (SiOH) and H-ZSM5 zeolite (SI(OH)Al) are compared with SCF/DZP and MP2/DZP results concerning the same molecules interacting with H3SiOH (SIL) and H3Si(OH)AlH3 (BRO), the minimal-size clusters mimicking the surface species. The interaction of dimethylacetylene with SIL and BRO has also been studied. Computed harmonic shifts in the O-H stretching frequency Delta nu(h)(OH) are in semiquantitative agreement with experiment, in that the experimental data are systematically underestimated by a factor of approximate to 1/3 at best: the main reason is the less acidic character of SIL and BRO with respect to SiOH and Si(OH)AI, which also causes a systematic underevaluation of the binding energies (BE). All this notwithstanding, calculated BE and Delta nu(h)(OH) values are good descriptors of the interactions, as a number of correlations may be drawn: (i) between the Delta nu(h)(OH) values on SIL and BRO, respectively (showing a constant ratio, identical with the ratio between experimental data); (ii) between BEs on SIL and BRO (showing the same ratio as strong bases on SiOH and Si(OH)AI); (iii) between BE and (Delta nu(h)(OH))(1/2) in agreement with experimental data on Delta H-o of interaction. The possible causes for the less acidic nature of SIL and BRO with respect to SiOH and Si(OH)AI are discussed. The nature of the interaction is predominantly electrostatic, in that SCF interaction energies are in semiquantitative agreement with those computed via a classical Coulombic expression using electrostatic-potential derived charges. However, a small electron transfer takes place from the base molecule to the acidic species, which parallels the strength of interaction and, within the same family of molecules, the ionization energy of the base molecules. The comparison between calculated and experimental values of C-H modes suggests that a secondary interaction takes place between the acidic hydrogen in acetylene and methylacetylene and a basic oxygen atom close to the active site.