Synthesis, characterization and reactivity of metal modified heteropolyacid catalysts for organic transformation reactions.

Abstract

In the present investigation, eco-friendly, sustainable and recyclable heteropolyacid catalysts have been developed for a series of organic transformations such as the production of ethyl acetate by solvent free liquid phase esterification and the formation of benzaldehyde by the oxidation of styrene and benzyl alcohol. All these reactions were carried out at room temperature. Various metal exchanged heteropolyacid catalysts with different metal loadings were prepared by the ion-exchange method and characterized by spectroscopic and adsorptions methods such as powder X-ray diffraction, scanning electron and transition electron microscopy, 31P NMR spectroscopy, elemental analysis, Bright Field-STEM analysis, infrared spectroscopy, ex-situ pyridine adsorbed infrared spectroscopy, pore size distribution studies and BET surface area measurements to understand their structural and surface properties, together with their acidity and to relate this to the catalytic performance. A series of K and Cs exchanged phosphotungstic acid (PTA) and phosphomolybdic acid (PMA) catalysts were synthesized for the esterification reaction. XRD patterns and infrared spectra showed that the crystallites and characteristic bands of the Keggin ion, respectively, were maintained after exchange of protons in the heteropolyacids with metal at all loadings of the catalysts. From pyridine adsorbed infrared spectroscopy, Brønsted and Lewis acid sites were present on the surface of the catalyst. Bright field STEM analysis showed a uniform distribution of the chemical composition which correlated well with theoretical atomic values of the metal loadings of all the catalysts, verified by ICP results. The efficiency of various metal exchanged heteropolyacid catalysts were assessed for the esterification reaction using various substrates and the Cs exchanged phosphotungstic acid catalysts showed superior activity compared to the other catalysts. In particular, the Cs exchanged phosphotungstic acid with a 1 wt.% loading showed the highest activity and was most tolerant to the presence of water that was produced in the reaction. The catalytic activity showed some correlation to the Brønsted and Lewis acidity, as well as Keggin ion density of catalysts. A series of alkali metal exchanged phosphomolybdic acid catalysts were synthesized by ion exchange, characterized by various physico-chemical techniques and used in the solvent freeoxidation of styrene to benzaldehyde. XRD and infrared results showed that the primary structure of the Keggin ion usually present in phosphomolybdic acid is retained after metal exchange. HR-TEM analysis show a well-constructed spherical morphology of the materials with a lower degree of crystalinity. Type IV isotherms with mesoporous structure are observed from nitrogen adsorption-desorption isotherm studies and ex-situ pyridine adsorption experiments reveal that Brønsted acidic sites increased after metal exchange. The K exchanged phosphomolybdic acid catalysts were most efficient in the conversion of styrene to benzaldehyde and the order of reactivity of the alkali metal exchanged phosphomolybdic acid catalysts was K > Rb > Cs. Insight into the reaction pathway was obtained by investigating the oxidation styrene oxide. The results showed that phenyl acetaldehyde together with benzaldehyde are produced, providing some evidence that styrene oxidation proceeds via C=C cleavage to selectively produce benzaldehyde. The catalyst was easily recovered and was reused for up to three cycles showing stable activity. Copper and zinc exchanged heteropolyacid catalysts were used in the oxidation of benzyl alcohol to benzaldehyde. The Zn exchanged phosphomolybdic acid catalysts showed higher oxidation activity that correlated well with the acidity of the catalysts. The catalysts were synthesized by the ion exchange method and were subjected to X-ray diffraction, infrared spectroscopy, pyridine adsorbed infrared spectroscopy, BET surface studies, 31P NMR spectroscopy and inductively coupled plasma-optical emission spectroscopy to determine its physico-chemical properties. XRD diffractograms showed that crystallites of the Keggin ion of the heteropolyacid were present and were retained after metal modification, as observed by infrared spectroscopy. From pyridine adsorbed infrared, the Brønsted/Lewis acid site ratio increased in the metal exchanged phosphomolybdic acid catalysts, whereas there was a decrease in the ratio for the modified phosphotungstic acid catalysts. 31P NMR spectra results showed that the metal was successfully exchanged with protons and was not incorporated in the primary structure of heteropolyacid catalyst, thus maintaining the Keggin ion. ICP analysis indicated a correlation of the theoretical metal content with the experimental loadings of the catalysts.