To quantify the spectral contributions of H 2 O within the pores of the zeolites, 2,4,6-tri-tert-butylpyridine (2,4,6-ttbupy; Sigma-Aldrich, 99%) was used as a probe molecule because it cannot fit into the 12membered ring pores of FAU or BEA. After the spectra of H 2 O were obtained on a given catalyst-coated crystal, a stream of He (10 cm 3 min -1 ) was flowed over the catalyst surface at 423 K (5 K min -1 ) for 2 h to remove all H 2 O. The IRE was subsequently cooled to 303 K and a spectrum of 2,4,6-ttbupy (~1 M) in nhexane (Sigma-Aldrich, 99%) was acquired. Comparisons of the recorded spectra over a zeolite-coated IRE to a clean IRE (Section S2.2) were made to distinguish the spectral contributions from within the zeolite pores from the bulk solution.