P-Site Structural Diversity and Evolution in a Zeosil Catalyst

Abstract

Phosphorus-modified siliceous zeolites, or P-zeosils, catalyze the selective dehydration of biomass derivatives to platform chemicals such as p-xylene and 1,3-butadiene. Water generated during these reactions is a critical factor in catalytic activity, but the effects of hydrolysis on the structure, acidity, and distribution of the active sites are largely unknown. In this study, the Psites in an all-silica self-pillared pentasil (P-SPP) with a low P-loading (Si/P = 27) were identified by solid-state 31 P NMR using frequency-selective detection. This technique resolves overlapping signals for P-sites that are covalently bound to the solid phase, as well as oligomers confined in the zeolite but not attached to the zeolite. Dynamic Nuclear Polarization provides the sensitivity necessary to conduct 29 Si-filtered 31 P detection and 31 P− 31 P correlation experiments. The aforementioned techniques allow us to distinguish sites with P−O−Si linkages from those with P−O− P linkages. The spectra reveal a previously unappreciated diversity of P-sites, including evidence for surface-bound oligomers. In the dry P-zeosil, essentially all P-sites are anchored to the solid phase, including mononuclear sites and dinuclear sites containing the [Si−O−P−O−P−O−Si] motif. The fully-condensed sites evolve rapidly when exposed to humidity, even at room temperature. Partially hydrolyzed species have a wide range of acidities, inferred from their calculated LUMO energies. Initial cleavage of some P− O−Si linkages results in an evolving mixture of surface-bound mono-and oligonuclear P-sites with increased acidity. Subsequent P− O−P cleavage leads to a decrease in acidity as the P-sites are eventually converted to H 3 PO 4 . The ability to identify acidic sites in Pzeosils and to describe their structure and stability will play an important role in controlling the activity of microporous catalysts by regulating their water content.

Diels–Alder Cycloaddition of Biomass-Derived 2,5-Dimethylfuran and Ethylene over Sulfated and Phosphated Metal Oxides for Renewable p-Xylene

“…SiP 300 and SiP 500 exhibited distinct peaks at −0.44 and −0.2 ppm, along with very weak peaks at −11.19 and −11.13 ppm, respectively (Figure 3a). The peaks at −0.44 and −0.2 ppm correspond to neat H3PO4, which is not chemically bound to the surface [20,21], whereas that at −11 ppm can be assigned to pyrophosphate (H3P2O7), which is chemically bound to the silica matrix [21]. This result suggests that phosphoric acid is not chemically bound to the silica surface effectively and exists mainly as a separated active site in the catalyst.…”

“…TiP 300 exhibited four peaks at 0.6, −4.6, −10.3, and −17.6 ppm. The peak at 0.6 ppm corresponds to unbound H 3 PO 4 [21]. According to a previous study on titanium phosphate, the peak at −4.6 ppm can be assigned to the H 2 PO 4 group bound to TiO 2 as the layered structure, whereas the peaks at −10.3 and −17.6 ppm can be assigned to the H 2 PO 4 groups bound to TiO 2 as the framework structure, e.g., α-TiP and γ-TiP [22,23].…”

On the Acid Sites of Phosphorous Modified Zeosils

“…A detailed 31 P NMR study on the different P-environments on P-SPP has recently revealed a host of mono-, and oligomeric phosphorus environments which evolve with the extent of hydration. 17 We have assumed this to be more broadly applicable across different supports and not independently carried out 31 P solid state NMR characterization of P-zeosils reported here. Since up to now only ex-situ NMR has been reported, extrapolating the obtained insights on Penvironments to reaction conditions is challenging as accurately quantifying adventitious water (from liquid feeds, gas lines, and as reaction product) is non-trivial.…”

“…The non-zero rate of dehydration at pyridine saturation, combined with the effect of increasing pyridine partial pressure, present a possibility that P-sites capable of catalyzing isopropanol (IPA) conversions do not bind pyridine irreversibly. Given the catalytic site heterogeneity, 8,17 it is also plausible that higher concentrations of pyridine titrated 'less-reactive' sites, leading to higher acid site counts. However, we find this possibility unlikely given that propene and di-isopropyl ether (DIPE) selectivities remain nearly invariant with pyridine partial pressure after saturation (Figure S12 in Section S3.2 in the SI).…”

“…For example, while Si-O-P linkages have not been detected on SPA catalyts, 16 their presence has been recently established in P-SPP. 17 Moreover, the support for SPA is either kieselguhr, 18 or pyrogenic/mesoporous silica, 19,20 while P-zeosils have been synthesized using both meso-and microporous supports. [7][8][9] Lastly, unlike SPA catalysts where only pyrophosphate phases are present, P-zeosils with low phosphorus loadings (Si/P >10) retain the crystallinity and specific surface area of the zeolitic phase.…”