Phenol methylation over nanoparticulate CoFe2O4 inverse spinel catalysts: The effect of morphology on catalytic performance

A series of CoFe2O4 nanoparticles have been prepared via co-precipitation and controlled thermal sintering, with tunable diameters spanning 7-50 nm. XRD confirms that the inverse spinel structure is adopted by all samples, while XPS shows their surface compositions depend on calcination temperature and associated particle size. Small (<20 nm) particles expose Fe3+ enriched surfaces, whereas larger (similar to 50 nm) particles formed at higher temperatures possess Co:Fe surface compositions close to the expected 1:2 bulk ratio. A model is proposed in which smaller crystallites expose predominately (1 1 1) facets. preferentially terminated in tetrahedral Fe3+ surface sites, while sintering favours (1 1 0) and (1 0 0) facets and Co:Fe surface compositions closer to the bulk inverse spinel phase. All materials were active towards the gas-phase methylation of phenol to o-cresol at temperatures as low as 300 degrees C. Under these conditions, materials calcined at 450 and 750 degrees C exhibit o-cresol selectivities of similar to 90% and 80%, respectively. Increasing either particle size or reaction temperature promotes methanol decomposition and the evolution of gaseous reductants (principally CO and H-2), which may play a role in CoFe2O4 reduction and the concomitant respective clehydroxylation of phenol to benzene. The degree of methanol decomposition. and consequent H-2 or CO evolution, appears to correlate with surface Co2+ content: larger CoFe2O4 nanoparticles have more Co rich surfaces and are more active towards methanol decomposition than their smaller counterparts. Reduction of the inverse spinel surface thus switches catalysis from the regio- and chemo-selective methylation of phenol to o-cresol, towards methanol decomposition and phenol clehydroxylation to benzene. At 300 degrees C sub-20 nm CoFe2O4 nanoparticles are less active for methanol decomposition and become less susceptible to reduction than their 50 nm counterparts, favouring a high selectivity towards methylation. (C) 2009 Elsevier B.V. All rights reserved.