Catalysis and Surface Reactions
Micro-, meso- and unporous material (zeolite ZSM-5, SBA-15, silica-supported silicotungstic acid) in the respective siliceous, Na-, and H-forms was loaded with methanol. The alcohol was subsequently slowly desorbed. It thereby formed intermediates identified by DRIFTS and MAS NMR spectroscopy. Quantification of methanol loadings revealed that the confinement within micropores adds substantial strength to the stability of methanol clusters at H+ and Na+ counter ions. On siliceous materials, a strong binding between methanol and Si(OH) groups was observed.
Silica-supported silicotungstic acid was investigated in the selective dehydration of ethanol at high conversions. An optimum loading was determined and subsequently the impregnation with CsOH solution identified as suitable method to further improve the selectivity of the catalyst. A loading with ammonia combined with quantitative 1H MAS NMR spectroscopy was used to distinguish external acid sites from sites within the pseudo-liquid phase.
This study combines DRIFTS with 13C MAS NMR spectroscopy on labled intermediates present during the methane oxidation over copper mordenite catalysts. Surface methoxy species located next to the copper sites act as overoxidation-preventing traps for the reaction. This explains for the first time why catalyst H-forms outperform their respective counterparts with sodium counter ions.
Partially alkali metal (X=Li, Na, Cs) ion exchanged X,H-ZSM-5 zeolites are investigated. Despite comparable acid site densities were adjusted, the counter ion nature significantly impacted on selectivity and lifetime of MTO catalysts. The impact of acid site density is shown by application of descriptive parameters for pure H-form and partially ion-exchanged ZSM-5 zeolites.
This study combines acid site characterization with the screening of catalytic conditions. A library of 10-MR zeolite catalysts ZSM-5, ZSM-11, and ZSM-22 with different topology (MFI, MEL, TON) and in different acid site densities was synthesized. The optimization of the acid site density resulted in a higher selectivity to small olefins and was able to overcome the shape selectivity expected from the pore system alone. The acid site density optimization is presented as promising tune knob for improving methanol-to-olefin catalysts.