Violating the general acidity–activity correlation: Computational evidence in a CpNa-modified HMCM-22 zeolite for ethene protonation

The catalytic activity of zeolite is well known to be associated with proton-donating ability at the Brønsted acid sites, i.e., high acidity versus high activity. In this paper, we firstly show that for modified zeolites, this simple acidity–activity correlation might be biased. We found that the proton hopping barrier (21.42 kcal/mol) within CpNa-modified HMCM-22 (Cp: cyclopentadienyl) is significantly higher than that of HMCM-22 (13.46 kcal/mol), indicating a clear lowering of acidity upon CpNa-modification. Surprisingly, the activation energy of ethene protonation is also significantly reduced from 30.43 kcal/mol in HMCM-22 to 24.74 kcal/mol in CpNa-modified HMCM-22, suggesting an enhanced catalytic activity upon CpNa-modification. The cause of violation from traditional “acidity–activity” correlation can be ascribed to the dual factor of CpNa, namely, Na+ can “activate” the proton due to its influence on the charges of proton and oxygen atoms, whereas Cp− can “deactivate” the proton due to its effective interaction with proton.

For the ethylene protonation in HMCM-22, the CpNa-modification can cause the violation of the general acidity-activity correlation.Figure optionsDownload as PowerPoint slideResearch highlights
► CpNa-modified HMCM-22 possesses high catalytic activity, but low acidity.
► Guest molecule is the main reason to cause violation of traditional acidity–activity.
► The enhanced “activity” is not always correlated with enhanced “acidity”.