Comprehensive Study of Isobutane Selective Oxidation Over Group I and II Phosphomolybdates: Structural and Kinetic Factors

Abstract

Various phosphomolybdates were synthesized using cations from Groups 1 and 2 of the periodic table. These compounds were of the form M x H 3-xn [PMo 12 O 40 ], with n being the cationic charge (?1 or ?2). XRD analysis shows pure phosphomolybdic acid has a triclinic structure. A body centered cubic (BCC) structure gradually develops with addition of Group 1 cations, and the triclinic phase is completely replaced by the BCC phase once metal cations occupy a volume greater than 9-11 Å 3 per phosphomolybdate anion. The Group 2 compounds do not form a cubic phase, however the triclinic phase distorts once cationic volume is greater about 5 or 6 Å 3 and appears to become somewhat amorphous. Isobutane selective oxidation over the compounds yielded methacrolein (primary product), 3-methyl-2-oxetanone (lactone), acetic acid, propene, methacrylic acid, carbon dioxide and water as products. Propene was formed over the Group 1 compounds exclusively and methacrylic acid formation was observed with BaH[PMo 12 O 40 ] only. Products form via two distinct processes: Category 1 product has an exponential profile and coverage is consistent with a Langmuir model, Category 2 formations are consistent with desorptions from within the bulk of the substrates. Methacrolein forms via both Category 1 and 2 processes, whilst all other products are formed by Category 2 exclusively. A rigorous kinetic analysis yielded accurate activation parameters. Category 1 methacrolein formation apparent activation energies ranged from 34.7 ± 1.3 to 119 ± 4 kJ mol -1 . Category 2 formations ranged from 34.3 ± 0.4 to 726 ± 172 kJ mol -1 . No relationship between activity and composition or structure could be ascertained, despite investigation into correlations using several different models.

Gas phase oxidation of 2-methyl-1,3-propanediol to methacrylic acid over heteropolyacid catalysts

“…In this case, XRD is incapable of detecting the dispersed (active) phase, hence it only shows the bodycentered cubic phase which was ascribed to the support. 41 XRD detects two phases related to the dispersed phase and support for x < 2. In our experiments, all catalysts displayed the cubic phase associated with the pure alkaline heteropolysalts.…”

Time- and Temperature-Varying Activation Energies: Isobutane Selective Oxidation to Methacrolein over Phosphomolybdic Acid and Copper(II) Phosphomolybdates

“…Three temperature regions, low, medium, and high, with different apparent activity for methacrolein formation are apparent during temperature-programmed oxidation [2,10,28]. The low-temperature region is assigned to methacrolein desorption.…”

Selective Oxidation of Isobutane to Methacrylic Acid and Methacrolein: A Critical Review

“…Selective oxidation of isobutane to MAA, which would provide a one-step route, has attracted extensive attention because of the simplicity of the process and decreased byproduct generation [26,27]. Moreover, this route is inexpensive and would have a lower environmental impact than other fossil feedstocks-based routes, as it optimally converts isobutane to MAA in a single step [28][29][30][31]. Rohm & Haas Company was the first, in 1981, to claim one-step oxidation of isobutane to MAC and MAA based on Mo/P/Sb oxides, of which the compositions are close to that of Keggin-type heteropoly acids [32].…”