Co-ZSM-5 thermal treatments effect on its acidic properties and catalytic behavior.

CLARA SAUX AND LILIANA B. PIERELLA

Genova, Italia

Congreso; ABC-6 – 6th World Congress on Catalysis by Acid and Bases; 2009

The partial styrene oxidation with hydrogen peroxide was studied over Co-ZSM-5 catalysts which differ in the thermal treatments applied. The MFI structured, ZSM-5 zeolites, were obtained by hydrothermal crystallization in the Na2O-Al2O3-SiO2 system, by known methods. Cobalt was incorporated to the matrix (NH4+-ZSM-5) by wetness impregnation technique followed by different post-thermal treatments: oxidation (Co oxid.) and reduction (Co red.). The analyses performed by Atomic Absorption revealed that a 2.8 Co wt% was effectively incorporated in the samples. XRD and FTIR on the fingerprint zone shown a 98% and 97% of crystallinity for both Co oxid. and Co red. samples, respectively. Structure and orthorhombic symmetry were confirmed. The characterization results of acidic properties by FTIR of adsorbed pyridine (Py) showed  bands at 1452-1456 cm-1 (υ 19B mode) and 1612 cm-1 (υ 8A mode), corresponding to Py interaction with Lewis acid sites (L-Py) could be observed, as well as bands at 1547 cm-1(υ 19B mode) and a shoulder at 1635 cm-1 (υ 8A mode), indicative of the interaction with Brönsted acid sites (PyH+). The band at 1490-1495 cm-1 corresponds to the vibration of adsorbed Py over both Brönsted and Lewis acid sites. New and strong Lewis sites (1452 cm-1), different from those characteristics of aluminium in the matrix (1456 cm-1) were generated by cobalt introduction. When samples are reduced the number of Lewis sites diminished by the disappearance of Co oxide species responsible of some of these sites.   The materials so far characterized from their structural characteristics and acidic properties were then used as catalysts for styrene selective oxidation, to evaluate their catalytic efficiency. The reaction was carried out at 60 ºC varying reaction time, at styrene to H2O2 molar ratio of 3, 0.2 g of catalyst and using acetonitrile as solvent. As seen on Figure 2, styrene conversion shows an important increment in the conversion of the oxidized sample as compared to the reduced one. This behaviour may be explained by the high Lewis acid sites concentration on Co oxid. material in accordance with the idea that strong Lewis acid sites are the active sites for the oxidation and that the basic acetonitrile molecules interact with the Brönsted acid sites “blocking” them.  Benzaldehyde was the main product, even though others could still be found, such as styrene oxide, phenylacetaldehyde, 1-phenyl, 1,2-ethanediol and benzoic acid.