Catalytic activity of silver–ion exchanger nanocomposites in methanal oxidation reaction with molecular oxygen

Abstract

oxides; aluminum; mesoporous mesostructured silicates MCM 41 and SBA 15 and NaY zeolites-are widely used as catalysts of the methanal (formaldehyde) oxi dation with oxygen [1][2][3][4]. The catalytic activity of sil ver containing catalysts in this reaction is caused by the formation, on the silver particle surface, of highly active oxygen incorporated in the surface oxide [5][6][7]. It is known that the presence of hydroxyl ions in the reaction medium facilitates the rupture of the C-H bond and promotes methanal oxidation with oxygen [8][9][10]. Nevertheless, high conversions of methanal are observed only at elevated temperatures. Owing to the surface location of active silver sites and the high concentration of ОН -ions in the matrix, silver-anion exchanger composites are promising catalytic materi als of methanal oxidation with oxygen.The aims of this work are to synthesize silver con taining nanocomposites based on various highly basic anion exchangers and to study their catalytic activity in the oxidation of methanal dissolved in water with oxygen.For synthesis of silver containing composites, we used granular 3D cross linked highly basic anion exchange resins (in the Cl -form) AH 17 8 and AH 17 2P (Azot, Ukraine) and Lewatit M 500 and Lewatit MP 500 (Lanxess, Germany) with a gel and porous structure and, for comparison, sulfonated cation exchange resin (in the Na + form) KU 2 8 (Azot, Ukraine). Matrix grains were 0.8-1.0 mm in size.After acid-base conditioning to constant total exchange capacity, silver was chemically deposited into ion exchangers by adding successively silver nitrate and a reductant (NaBH 4 + NaOH) under dynamic conditions. In catalytic experiments, com posites with the active oxygen surface layer were used. To generate such a layer, oxygenated water was passed through the layer of the prepared composite.The composite morphology and the phase compo sition of the deposited particles were characterized on the basis of X ray powder diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive X ray analysis (EDX) data. The prepared composites had mainly a surface distribution of silver particles (Fig. 1). Basic characteristics of the composites are summarized in Table 1.The catalytic activity of the synthesized composites was studied for the reaction of oxidation of methanal dissolved in water at 23 ± 2°C. The methanal concen tration was 30 ± 3 mmol/L. To elucidate the effect of ОН -ions on the catalytic activity of composites, the methanal oxidation was also carried out in a 0.1 M alkaline solution. The composite volume to methanal solution volume ratio was 1 : 10. The solution was con tinuously stirred by bubbling oxygen or argon through it. The reaction course was monitored by measuring changes in concentration: the oxygen concentration was determined by a gasometric method [11], and the methanal concentration was determined by titrimetric and photocolorimetric methods [12,13]. From the PHYSICAL CHEMISTRY Abstract-Silver containing ion exchangers with the surface distributio...