Evaluation of Zeolite Acidity by ³¹P MAS NMR Spectroscopy of Adsorbed Phosphine Oxides: Quantitative or Not?

Abstract

31 P magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy of adsorbed alkyl-substituted phosphine oxides has witnessed tremendous progress during the last years and has become one of the most informative and sensitive methods of zeolite acidity investigation. However, quantitative evaluation of the number of sites is still a challenge. This study clarifies the main origin of errors occurring during NMR experiments, introduces the appropriate standards (both internal and external), and determines the relaxation parameters and the conditions for the acquisition and integration of spectra. As a result, a methodology for the quantitative measurement of the content of Brønsted and Lewis sites and the amount of internal and external silanol groups is established. The application of probe molecules of different sizes (namely, trimethylphosphine oxide (TMPO), tri- n -butylphosphine oxide (TBPO), and tri- n -octylphosphine oxide (TOPO)) is shown to be a good tool for distinguishing between the active sites inside the zeolite pores, mesopores, and on the outer crystal surface. The methodology proposed is verified on BEA zeolites different in composition, texture, and morphology.

Solid-State NMR Probe Molecules for Catalysts and Adsorbents: Concepts, Quantification, Accessibility, and Spatial Distribution

“…Systematic errors arise usually from the applied measurement or integration procedure. Main reasons for them are too low recycle delays or unsuited standards . Again, as rule of thumb, the applied quantitative standard should chemically be as close as possible to the investigated solid.…”

“…For a quantitative determination of the spatial distribution, probe molecules of increasing diameter are a frequent choice. Examples used in solid-state NMR spectroscopy are the 31 P probe molecules based on the alkylphosphine oxide motif (OPR 3 ) ,, or those on the triphenylphosphine motif (P­(Ph-4-R) 3 ). , However, the size variations utilized for these investigations are naturally limited by the chemical dimensions of these molecules in relation to the solid structure. In other words, the dimension of probe molecules and structural units on the solid must fit.…”

“…If the sites are only partially covered, results are not quantitative, and conclusions from such investigations must be treated carefully. Thus, the choice of the probe molecule size must be based on the geometric structures of the solid under investigation and the later reactant. Long T 1 relaxation times exceeding 30 s might occur for physisorbed species, for example, in 31 P MAS NMR spectroscopy. ,, However, for the sake of short measurement times and high accuracy, probe molecules should be applicable with short delays. Fortunately, the peaks that are quantified are often directly bound to other nuclei that speed up the relaxation, like metals or protons.…”

“…More voluminous alkylphosphine oxide analogues with the binding motif OPR 3 are triethylphosphine oxide (TEPO, 0.60 nm), tri- n -butylphosphine oxide (TBPO, 0.86 nm), ,, and tri- n -octylphosphine oxide (TOPO, 1.16 nm). , It should be considered that the calculated values do not take into account the high flexibility of the linear butyl or octyl groups at elevated temperatures. This flexibility makes access into smaller pores more probable for larger alkylphosphine oxides.…”

“…Thus, large probe molecules can block nearby active sites for other probe molecules, which results in an underestimation of sites. The probe must also be able to 96,97,157 However, for the sake of short measurement times and high accuracy, probe molecules should be applicable with short delays. Fortunately, the peaks that are quantified are often directly bound to other nuclei that speed up the relaxation, like metals or protons.…”