Heteropoly compounds as catalysts for hydrogenation of propanoic acid

• Published in: Journal of catalysis 2008-01, Vol.253 (2), p.244-252
• Main Authors: Benaissa, H., Davey, P.N., Khimyak, Y.Z., Kozhevnikov, I.V.
• Format: Article
• Language: English
• Subjects: Acids; Catalysis; Catalysts; Chemical compounds; Chemical reactions; Chemistry; Exact sciences and technology; General and physical chemistry; Heteropoly compounds; Hydrogenation; Propanal; Propanoic acid; Theory of reactions, general kinetics. Catalysis. Nomenclature, chemical documentation, computer chemistry
• ISSN: 0021-9517; 1090-2694
• DOI: 10.1016/j.jcat.2007.11.011

Bulk Keggin heteropoly acids (HPAs) H 3+ n [PMo 12− n V n O 40] ( n = 0 – 2 ) and their Cs + salts catalyse the vapour-phase hydrogenation of propanoic acid at 350 °C and 1 bar H 2 pressure, yielding propanal together with 3-pentanone and propane as the main products. Catalyst acidity (controlled by Cs substitution) has crucial effect on the reaction selectivity. As the Cs content increases, the selectivity to propanal passes a maximum (74–76%). At the same time, the selectivity to propane sharply decreases, whereas 3-pentanone selectivity increases monotonously. This indicates that 3-pentanone is likely to form via Cs propanoate intermediate. Partial substitution of Mo(VI) by V(V) in the PMo 12O 3− 40 anion has a small effect on the catalyst performance. Initially crystalline, the catalysts become amorphous after reaction, with their surface area significantly reduced. As evidenced by FTIR, H 4[PMo 11VO 40] and its Cs salts, possessing a higher thermal stability, retain the Keggin structure in their bulk after reaction, whereas less stable H 3[PMo 12O 40] and H 5[PMo 10V 2O 40] derivatives undergo decomposition. This shows that the as-made crystalline heteropoly compounds are catalyst precursors rather than the true catalysts. The reaction over Cs 2.4H 1.6[PMo 11VO 40] is zero order in propanoic acid with an activation energy of 85 kJ/mol. The formation of propanal is suggested to occur via a Mars–Van Krevelen mechanism.