Can Hammett indicators Accurately Measure the Acidity of Zeolite Catalysts with Confined Space? Insights into the Mechanism of Coloration
The acidic properties of zeolite catalysts play a crucial role in governing the catalytic performances, which makes the acidity characterization an important subject in the field of zeolite catalysis. As far, Hammett titration has been a conventional method to determine the acidic strength of liquid and solid acids, however, the reliability of such titration method to measure the acidity of acidic sites inside zeolites is in debate. It is necessary to unveil the origin for the coloration of Hammett indicators adsorbed inside zeolite channels and cages. Herein, by employing the advanced computational method, the adsorbed structures and electronic properties for series of Hammett indicators with various pKa (from -0.29 to -14.52) values has been investigated, concerning the interaction with the Brönsted acidic sites inside ZSM-5, MOR and HY zeolites. On the basis of the theoretical results, the intimate relationship between coloration (traditionally corresponding to the protonation of Hammett indicators) and HOMO-LUMO gap has been established by DFT and time-dependent (TD) DFT calculations for the first time. Furthermore, not only the Brönsted acidity, but the pore confinement derived from zeolite framework could also render the coloration by effectively reducing the HOMO-LUMO gaps, which can be differentiated by the diverse shapes and sizes of zeolite cages. Therefore, the abusement of Hammett method should be prohibition in experiment and the utilization of Hammett indicators should be cautious in the acidity determination of porous zeolite catalysts, and other more accurate approach is needed in the future.