Supported molybdenum oxides for the aldol condensation reaction of acetaldehyde

•Reduced MoOx sites on supported catalysts are active for aldol condensation.•Pretreatment conditions do not influence steady state aldol condensation activity.•Lewis acid site strength and density are relevant properties.•Moderate/strong acid sites are needed to activate carbonyls for aldol condensation.•MoOx interacts strongly with Al2O3 support and weakly with SiO2 support.