Ammonia Adsorption on Keggin-Type Heteropolyacid Catalysts Explored by Density Functional Quantum Chemistry Calculations

Abstract

Density functional quantum chemical calculations have been used to compare the acid strengths of phosphotungstic and phosphomolybdic acids by computing the adsorption energy of ammonia on model clusters of each heteropolyacid. The adsorption of ammonia on a phosphotungstic acid cluster was stronger than the adsorption on a phosphomolybdic acid cluster. The predicted adsorption energies were near −150 and −106 kJ mol-1 for phosphotungstic and phosphomolybdic acid, respectively. This compares well with the experimental heats of ammonia sorption determined from microcalorimetry. An analysis of different adsorption modes of ammonia on phosphotungstic acid showed that bidentate adsorption of ammonia on the heteropolyacid clusters aided in proton transfer and yielded stronger adsorption energies than did a monodentate adsorption mode. In addition, we report the computed adsorption energies of pyridine on a heteropolyacid cluster.

The relationship between adsorption and solid acidity of heteropolyacids

“…The adsorption energies (DE ads ), as well as the individual components of the thermodynamic cycle for adsorption on a solid acid (ÀPA ads and DE int ), are reported in Table 1. The agreement between the calculated adsorption energies and values measured by microcalorimetry has been previously discussed for the adsorption of ammonia [37] and water [32]. A detailed demonstration of the agreement between computational and experimental adsorption energies of alkenes is published separately [38].…”

Support enhanced α-pinene isomerization over HPW/SBA-15

“…S2). [32,51,53] and their subsequent thermal transformation to tungstate and phosphate species, accompanied by the liberation of reactively-formed protons; [52,[54][55][56] these are unlikely to play a role in low temperature catalytic processes, wherein the primary Keggin structure and entrained water remain intact. Integrated peak intensities for the 200 C desorption (Fig.…”

“…[30] has attracted much interest due its strong Brönsted acidity and broad application to acid catalysed reactions. [31][32][33][34][35][36][37] However, bulk HPAs have a low surface area (1-5 m 2 .g 1 ), which hinders their performance as a heterogeneous catalyst, and hence there is an exigency to develop supported analogues. [30,34,[38][39][40][41] For example, the encapsulation of phosphotungstic acid clusters within the cages of a MIL-101 MOF enhances selectivity towards the aldol self-condensation of cyclic ketones for high-density biofuels production.…”

Accurate calculation of31P NMR chemical shifts in polyoxometalates

“…or a TM (Co, Fe, etc.). [12][13][14][15][16][17][18][19] Nuclear magnetic resonance (NMR) of the different active nuclei constituting IPAs and HPAs is nowadays considered to be a very powerful method to elucidate their molecular structures both in solution and in the solid state. This is an extraordinarily versatile family of inorganic compounds with unmatched tuneable physicochemical properties that result in many applications.…”