The effect of Ge incorporation on the Brønsted acidity of ZSM-5.

The effect of the isomorphous substitution of some of the Si atoms in ZSM-5 by Ge atoms on the Brønsted acid strength has been investigated by i) DFT calculations on cluster models of the formula ((HO)3SiO)3-Al-O(H)-T-(OSi(OH)3)3, with T=Si or Ge, and ((HO)3SiO)3-Al-O(H)-Si-(OGe(OH)3)(OSi(OH)3)2, ii) a 31P NMR study of zeolite samples contacted with trimethyl phosphine oxide probe molecules and iii) a X-ray photoelectron spectroscopy (XPS) study of ZSM-5 and Ge-ZSM-5 samples. The calculations reveal that the effect of Ge incorporation on the framework acidity strongly depends on the degree of substitution and on the exact T-atom positions that are occupied by Ge. High Ge concentrations allow for enhanced stabilisation of the deprotonated Ge-ZSM-5 through structural relaxation, resulting in a slightly higher acidity as compared to ZSM-5. This structural relaxation is not achievable in Ge-ZSM-5 with a low Ge content, which therefore has a slightly lower acidity than ZSM-5. The NMR study indicates no difference between the Brønsted acidity of ZSM-5(47) and Ge(0.09)ZSM-5(36). Instead, evidence for the presence of a substantial amount of Ge-OH groups in the Ge-containing samples was obtained from the NMR results, which is consistent with earlier FTIR studies. The XPS results do not point to an effect of Ge on the framework acidity of ZSM-5(47), instead, the results can be best interpreted by assuming the presence of additional Ge-OH and Si-OH groups near the surface of the Ge(0.08)ZSM-5(47) sample.