Aerobic oxidative esterification of primary alcohols over Pd-Au bimetallic catalysts supported on mesoporous silica nanoparticles

Abstract

We have prepared a series of mesoporous silica nanoparticle (MSN) supported Pd-Au bimetallic catalysts using a newly developed sequential impregnation method. These catalysts were fully characterized by various techniques including nitrogen sorption, powder X-ray diffraction, inductively coupled plasma mass spectrometry (ICP-MS), transmission electron microscopy (TEM) and high angle annular dark-field scanning transmission electron microscopy (HADDF-STEM). By using this synthetic approach, we observed metal nanoparticles (NP) with diameters of 1–2nm homogeneously supported on the MSN. The catalytic performance of these MSN supported metal NPs was tested by aerobic oxidative esterification in a tandem reaction where primary alcohols are oxidized to their corresponding aldehydes and to esters in a subsequent reaction. We determined that Pd NPs are very efficient in the first step of oxidation; however, stagnant in the subsequent oxidation. On the contrary, Au NPs show slow reactivity in converting alcohols to aldehydes, but extraordinarily efficient in the oxidation of aldehydes to esters. By fine tuning the metal ratio, the bimetallic catalyst exhibits better reactivity and selectivity toward a variety of primary alcohols than the corresponding monometallic catalysts. In addition, we also found that the bimetallic Pd-Au@MSN catalysts can be recycled three times without a significant loss in activity.