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.

Structure and acidity of Mo/H-MCM-22 catalysts studied by NMR spectroscopy

“…In other words, samples prepared by physical mixing and/or samples without thermal treatment tend to result in bulkier Mo species that disperse inhomogeneously in Mo/MCM-22. [16,21]. Unlike the other peaks, which arise from TMPO adsorbed on Brønsted or Lewis acid sites, the contributions from the two physisorbed TMPO are irrelevant in terms of acidic features of the catalyst and thus are discarded in Table 2.…”

“…Furthermore, by incorporating the results obtained from elemental analyses by ICP, concentrations of acid sites in each sample can be determined, as also depicted in Table 2. The detailed analysis procedure invoked can be found elsewhere [21].…”

“…Consequently, the density of the electron cloud surrounding the 31 P nucleus neighboring the oxygen atom on TMPO should decrease with increasing acid strength of the Brønsted acid sites, which in turn causes the 31 P resonance to shift towards lower field. Thus, the resonance peak with higher chemical shift would reflect the Brønsted acid site having a stronger acidic strength [21]. Osegovic and Drago derived an empirical relation between the acid strength and the 31 P chemical shift (d) of TMPO [29]:…”

“…From the viewpoint of NMR applications, since the 31 P nucleus possesses a higher sensitivity and a wider chemical shift range (>300 ppm) compared to 13 C and 15 N, thus these phosphorus molecules are advantageous over other conventional NMR probes, such as pyridine or methyl amine [19,20]. In a recent study, we went further to demonstrate that such a 31 P NMR technique, when applied with a proper choice of trialkylphosphine oxides as probe molecules and in conjunction with elemental analysis by ICP-MS, is capable of providing quantitative information on internal and external acid sites [21]. The discernment of internal versus external acid sites is made possible by using phosphorus probe molecules with varied kinetic diameters, for example, for 10-MR zeolites with an average pore aperture of ca.…”

“…Accordingly, detailed acid features, viz. the types, strengths, concentration and location of the acid sites, can be simultaneously determined [21].…”