Direct Oxidation Of Primary Alcohols To Methyl Esters Catalyzed By Nano-Au/SBA-16 Catalyst

Carboxylic acid esters are industrially important fine chemicals, which are widely used in the perfume industry, diluents, lubricants, resin, surfactant, polycarboxylate superplasticizer, plasticizer, paint, perfume, cosmetics and as intermediates of medicines. Carboxylic esters can be prepared by five methods:the reaction of activated acid derivatives (acyl chlorides and anhydrides etc.) with alcohols/phenol, transesterification between carboxylic acids with a variety of esters, cross-coupling reactions involving the carbonylation of aryl halides and CO and direct conversion of aldehydes/alcohols into esters. Among these methods, oxidative esterification of alcohols represents an important advance toward green chemistry because of mild reaction conditions and simplified process. Recently, there has been a considerable focus on this reaction. It was found that gold naonoparticle is an active catalyst in oxidative esterification of alcohols. However, Au naonoparticles are prone to aggregating into large particles during the reaction, leading to the significant decrease in the catalytic activity.In this work, the mesoporous cage-like material SBA-16 was chosen as a support. Gold nanoparticles are loaded on SBA-16 through modification with amino-groups, adsorption of HAuCl4 and reduction with NaBH4. The resultant catalyst Au/SBA-16 was characterized with N2 sorption, XRD, TEM as well as FT-IR.It was found that that the structure of support and Au loading have an impact on the activity in oxidation of benzyl alcohol to methyl benzoate. The catalytic activity decreases in the order:Au/SBA-16>Au/silica> Au/SBA-15. The catalysts with Au loading of 5-6 wt%, corresponding to 2-3 nm of Au, show the highest activity.The Au/SBA-16 catalyst was effective for oxidation of various other alcohols. Cinnamyl alcohol was converted to methyl cinnamate with 99% conversion and 95% selectivity in 7 h of reaction at 1 atm O2 and 25℃. The conversion rate of n- hexanol was 62% at 130℃and 6 atm O2.Au/SBA-16 can easily recovered by filtration and consecutively reused After eight reaction cycles, an 80% yield of Carboxylic esters is still achieved. The recyclability of Au/SBA-16 is much better than that of Au/silica, over which only 50% yield is obtained after six reaction cycles. The TEM images show that the gold particle sizes on Au/SBA-16 have less changes, whereas gold nanoparticles on Au/silica undergoes serious agglomerations. It is can be explained by the fact that the smaller pore entrances of SBA-16 efficiently inhibit the motions of Au nanoparticles and thereby prevent the undesired growth of Au nanoparticles.