Discernment and Quantification of Internal and External Acid Sites on Zeolites

Abstract

A new methodology is reported for concurrent qualitative and quantitative characterization of internal and external acid sites in zeolitic catalysts. H−ZSM-5 zeolites with varied Si/Al ratios have been examined by solid-state 31 P MAS NMR using different adsorbed probe molecules, namely trimethylphosphine oxides (TMPO) and tributylphosphine oxide (TBPO), in conjunction with elemental analysis. Up to seven distinct 31 P resonance peaks at 86, 75, 67, 63, 53, 43, and 30 ppm were identified from the 31 P NMR spectra of adsorbed TMPO. The resonance peak at 30 ppm has never been observed previously and may be ascribed to mobile TMPO. The peak at 43 ppm is assigned to physisorbed TMPO. The rest of the peaks result from TMPOH + complexes at Brønsted sites with peaks at higher chemical shifts reflecting acid sites of higher strengths. 31 P NMR experiments, performed with TMPO and TBPO adsorbed on a mesoporous MCM-41 sample, respectively, provide further correlation of the internal and external acid sites. It is concluded that the peaks at 75 and 53 ppm arise exclusively from the internal Brønsted sites, whereas the peaks at 86, 67, and 63 ppm are associated with both internal and external acid sites. While the concentration and distribution of internal sites were found to increase with acid strengths, changing the Si/Al ratio of HZSM-5 has nearly no effect on the strength of the external acid sites.