Framework Effects on Activation and Functionalisation of Methane in Zinc‐Exchanged Zeolites

Abstract

The first selective oxidation of methane to methanol is reported herein for zinc‐exchanged MOR (Zn/MOR). Under identical conditions, Zn/FER and Zn/ZSM‐5 both form zinc formate and methanol. Selective methane activation to form [Zn‐CH3]+ species was confirmed by 13C MAS NMR spectroscopy for all three frameworks. The percentage of active zinc sites, measured through quantitative NMR spectroscopy studies, varied with the zeolite framework and was found to be ZSM‐5 (5.7 %), MOR (1.2 %) and FER (0.5 %). For Zn/MOR, two signals were observed in the 13C MAS NMR spectrum, resulting from two distinct [Zn‐CH3]+ species present in the 12 MR and 8 MR side pockets, as supported by additional NMR experiments. The observed products of oxidation of the [Zn‐CH3]+ species are shown to depend on the zeolite framework type and the oxidative conditions used. These results lay the foundation for developing structure–function correlations for methane conversion over zinc‐exchanged zeolites.

The DFT Approach to predict 13C NMR chemical shifts of hydrocarbon species adsorbed on Zn-modified zeolites

“…The experimental chemical shift of methylzinc is characterized by the values of –19 to –21 ppm. 7,55,56,58 Ethylzinc exhibits δ ( 13 C) values of –0.5 ppm and 9.2, 12.5 ppm for Zn–CH 2 – and –CH 3 groups, respectively. 6,67 For n -propylzinc, the chemical shifts at 12.2, 12.4 ppm (Zn–CH 2 –), 19.5 ppm (–CH 2 –), and 20.3, 20.7 (–CH 3 ) were reported.…”

“…aExperimental values of chemical shifts δ( 13 C) are taken from ref. 7,55,56, and 58 for methyl zinc species, refs. 6 and 67 for ethylzinc species and refs.…”

“…Methylzinc species formed on BEA, ZSM-5, FER, and MOR materials are characterized by the chemical shifts in the range of À19 to À21 ppm. 7,[55][56][57][58] Propene p-complexes with the Zn 2+ cations of BEA and ZSM-5 exhibit the d( 13 C) values of 109-110 (QCH 2 ), 165-170 (QCH-), and 19 and 20 (-CH 3 ) ppm. 10,[59][60][61] Representative geometries of these species are presented in Fig.…”

“…13 C Chemical shift values obtained with the TPSS/cc-pVTZ method for 3-methyl-s-allylzinc species 19 to -21 ppm 7,55,56,58. Ethylzinc exhibits d( 13 C) values of -0.5 ppm and 9.2, 12.5 ppm for Zn-CH 2 -and -CH 3 groups, respectively 6,67.…”