Acid Sites of Phosphorus-Modified Zeosils

Abstract

The acid sites of phosphorus-containing zeosils were probed through a combination of solid acid characterization, density functional theory calculations, and kinetic interrogations, establishing their weakly Brønsted-acidic character. Because of the disparity in the acid-site strength, P-zeosils catalyzed the probe chemistry of isopropanol dehydration slower than aluminosilicate zeolites by an order of magnitude on an active-site basis. Propene selectivity during isopropanol dehydration remained 20−30% higher than that of aluminosilicates, illustrating the distinct nature of the weakly acidic phosphorus active sites that favored unimolecular dehydration routes. Regardless of the confining siliceous environment, the nature of phosphorus active sites was unchanged, as indicated by the identical apparent uni-and bimolecular dehydration energy barriers. Kinetic isotope experiments with deuterated isopropanol feeds implicated an E2-type elimination in propene formation on phosphorus-containing materials. The comparison of kinetic isotope effects between phosphorus-containing zeosils and aluminosilicates pointed to an unchanged isopropanol dehydration mechanism, with changes in the apparent energetic barriers attributed to weaker binding on phosphorus-active sites that lead to a relatively destabilized alcohol dimer adsorbate. Both ex situ alkylamine Hofmann elimination and in situ pyridine titration characterization methods exhibited a phosphorous acid site count that was dependent on the probe molecules' identity or concentration, underpinning the limitations of extending common characterization techniques for Brønsted acid catalysis to weakly acidic materials.

Sulfurous zeosils for dehydra-decyclization of tetrahydrofuran to renewable butadiene

“…46 These chemical peaks could explain the differences in BAS densities measured via the Hoffman Elimination of tert -amylamine. The BAS measured for P-MFI are consistent with literature values reported for wet impregnation of P-sites, 32 and the measured BAS densities of B-MFI and S-MFI were significantly lower than P-MFI. As mentioned earlier, Kumar et al measured the BAS density of B-MFI via in situ pyridine titration and obtained a low density ( i.e.…”

“…46 These chemical peaks could explain the differences in BAS densities measured via the Hoffman Elimination of tert-amylamine. The BAS measured for P-MFI are consistent with literature values reported for wet impregnation of P-sites, 32 . 27 To evaluate the idea that inner BAS promote THF dehydration, we measured the performance of 22% sulfuric acid coated silica gel (SSA) for this chemistry.…”

“…Reactive gas chromatography methodology was used in measuring the BAS density using tert-amilamine (tAA) as a titrant, in accordance with existing procedures. 32,33 The XPS measurements were conducted using a PHI Versa Probe III XPS system (ULVAC-PHI) with mono-chromated Al K α X-ray source (1486.6 eV), employing a base pressure of 2.8 × 10 −8 Pa. To position the sample on the sample holder, a piece of double-sided sticking tape was employed. At all times, charge neutralization was conducted before survey and high-resolution measurements.…”

“…Kumar et al effectively estimated the BAS density of self-pillared pentasil (SPP) impregnated with phosphorus at an atomic concentration of Si/P = 27 (169 μmol g −1 ) via the Hoffman Elimination of tert-amylamine. 32 Additionally, Jain et al determined using 31 P-NMR that the active sites in P-SPP consist of a free hydroxyl group (not hydrogen bonded), bonded to a P-site, and also anchored to the siliceous framework. 30 Similar structures have also been observed with the impregnation of boric acid within siliceous MFI.…”

Unravelling Irreversible Adsorbate Thermodynamics through Adsorption-Assisted Desorption

“…While elevated temperatures accelerate the rate of desorption, adsorbates like alkylamines on a zeolite surface become chemically reactive at temperatures necessary for feasible desorption rates. 26 As a result, entropy measurements of chemisorbed moieties on a surface through existing methods are exceedingly challenging and rare. Accelerating the rate of desorption, such that an equilibrium adsorption state can be achieved, is therefore necessary to measure the entropy of molecules that undergo chemisorption.…”