Surface Organometallic Chemistry on Zeolite - Grafting Reaction of Ge(n-C₄H₉)_x Fragments over ZSM-5 outer Surface and Adsorption Properties of the Resulting Materials

The reaction of tetrabutylgermanium with ZSM-5 surface at 230 and 280°C was studied by means of gas volumetry, infrared spectroscopy, solid-state NMR and elemental analysis. The thermal stability and the adsorption properties of the resulting solids were also determined. A condensation occurred between tetrabutylgermanium and the surface hydroxyl groups of ZSM-5, being closly analogous to what had been observed on silica and mordenite. The products in the gas phase are C1-C4 hydrocarbons formed by cracking of the evolved butane on the highly acidic ZSM-5, and in contrast to the report on silica or mordenite, a mixture of [(≡Si-O)_xGe(n-C₄H₉)_x-4)](x=1, 2, 3) species was formed. Increasing the reaction temperature resulted in much more dealkylated species. After heating at 450°C, no butyl ligand remained on germanium. No modification of the solid was detected by nitrogen adsorption, showing that the zeolitic structure was retained upon grafting. The modified materials show that the adsorption properties are quite different from those of the starting ZSM-5. For example, 2,3-dimethyl butane is totally excluded from the modified zeolite while the unmodified one easily adsorbs it. n-Hexane and 2-methyl pentane are also discriminated by this zeolite but, in this case, it is due to a decrease of the sorption rate at the pore entrance due to steric hindrances caused by the grafted fragments.