Solid Phosphoric Acid Catalyst:  A Multinuclear NMR and Theoretical Study

Abstract

The synthesis, structure, and acid function of solid phosphoric acid (SPA) catalyst were studied in detail. 31P and 29Si MAS NMR and X-ray powder diffraction identified the following crystalline silicon phosphate phases in SPA: Si5O(PO4)6, hexagonal-SiP2O7, Si(HPO4)2·H2O, and SiHP3O10. The acidity of SPA is due to a liquid or glassy solution of phosphoric acid oligomers supported on the silicon phosphate phases. 15N MAS NMR of adsorbed pyridine-15 N and 13C MAS NMR of adsorbed acetone-2-13 C showed Brønsted acid sites and no Lewis acid sites. 1H→15N→31P and 1H→13C→31P double cross polarization MAS NMR of the probe molecules provided a rare opportunity to use NMR to unambiguously localize chemisorption sites; the probe molecules are complexed to phosphoric acid and pyrophosphoric acid but not to the silicon phosphate phases. In situ NMR of the oligomerization of propene on SPA suggests that propene quantitatively reacts with phosphoric acid and its oligomers to form isopropyl phosphate, and formation of this very stable intermediate accounts for the lower olefin oligomerization activity of SPA relative to acidic zeolites. Theoretical calculations including geometries at B3LYP/6-311+G(d,p) and chemical shifts at GIAO-MP2/tzp/dz were used to model complexation of acetone or propene to SPA, and these support our conclusions.

HDO of Methyl Palmitate over Silica-Supported Ni Phosphides: Insight into Ni/P Effect

“…Two main regions of the spectra of all the measured nickel phosphide catalysts can be highlighted. The first region from +500 to +5000 ppm corresponds to Ni-P intermetallic compounds such as Ni 12 P 5 , Ni 3 P, Ni 2 P, NiP, and Ni 5 P 4 [15,29,39] and the second one from +4 to −40 ppm corresponds to the different phosphate species (denoted in this study overall as PO x groups), i.e., phosphoric acid H n PO 4 (3−n)− (~0 ppm), terminal P 2 O 7 4− (~−7 ppm), and internal (PO 3 − ) n (~−22 ppm) phosphate groups in phosphoric acid oligomers [15,29,32,40,41]. For all of the applied catalysts, we detected the lack of the signals with chemical shifts from −30 to −50 ppm that could be attributed to phosphorus-containing species in the form of silicon phosphate [40].…”

Role of Vanadium and Phosphorus in Substituted Keggin‐Type Heteropolyoxo Molybdates Supported on Silica SBA‐15 in Selective Propene Oxidation

“…In the 31 PMAS NMR spectra of activated PV 2 Mo 10 -SBA-15 studied herein, ab road resonance indicated structuralr earrangement and ap artial decomposition of Keggin ions during thermal treatment under catalytic conditions. Moreover,t he 31 PMAS NMR spectra of activated PV 2 Mo 10 -SBA-15 resembled those of aV PO-SBA-15 sample treatedu nder oxidative and reductive conditions. [34] Ab road resonance observed for the VPO-SBA-15 sample between À12 and À38 ppm was attributed to various vanadyl orthophosphates phases (À8.4 to 21.2 ppm) and P bound to the SBA-15 support (%À38 ppm).…”

“…However,t he formation of phosphorous oxide or silicon hydrogen tripolyphosphate exhibiting linked PÀOÀPs tructures could be excluded. In total, the 31 PMAS NMR results indicated various structuralm otifs in the activated samples studied herein. Apparently,Premained connected to the molybdate and/or vanadate species of the [(Mo,V)O x ]u nits under propene oxidation conditions.…”

“…[30] Krawietz et al assigned the peak at À35.9 ppm to silicon hydrogen tripolyphosphate (À35 ppm). [31] For H 3 [33] The authors assigneda na dditional peak at À30 ppm to phosphorous oxidese xhibiting PÀOÀPm otifs. In the 31 PMAS NMR spectra of activated PV 2 Mo 10 -SBA-15 studied herein, ab road resonance indicated structuralr earrangement and ap artial decomposition of Keggin ions during thermal treatment under catalytic conditions.…”

“…[31] For H 3 [33] The authors assigneda na dditional peak at À30 ppm to phosphorous oxidese xhibiting PÀOÀPm otifs. In the 31 PMAS NMR spectra of activated PV 2 Mo 10 -SBA-15 studied herein, ab road resonance indicated structuralr earrangement and ap artial decomposition of Keggin ions during thermal treatment under catalytic conditions. Moreover,t he 31 PMAS NMR spectra of activated PV 2 Mo 10 -SBA-15 resembled those of aV PO-SBA-15 sample treatedu nder oxidative and reductive conditions.…”