The Production Of Vanillin By Selective Oxidizing2-Mthoxy-4-Methyl Phenol Over Layered Double Hydroxides

Vanillin can be used as food additives, pharmaceutical intermediates, and so on. Up till now, the synthesis of vanillin from4-methyl guaiacol is mainly in the from of homogeneous system. The substrate4-methyl guaiacol can be oxidized in the presence of metal salts as catalysts. The disadvantages that they often pose a serious threat on the practical utility because of the large consumption of the catalysts, the low catalytic activity, and difficulties in catalyst-product separation and reuse. By the view of this, we report novel heterogeneous catalysts for the selective oxidation of4-methyl guaiacol to vanillin, which makes it come true that the heterogeneous catalysts can efficiently selectively oxidize4-methyl guaiacol to vanillin and be reused. The main research results are shown as follows:1. The layered double hydroxides are used as a heterogeneous catalyst. The electronic structure of Co element is changed by the modulation of the types of the elements (Co, Ni, Fe, Ce, Sm, Mn, and Al) and the ratios of the elements in the octahedral sheets of the layered double hydroxides. All the samples were used as heterogeneous catalysts for the selective oxidation of 4-methyl guaiacol to vanillin. It is found that the electron density of the Co element is modulated by the method of adding different elements to the octahedral sheets of the layered double. What results in that the ratios of Co2+/Co3+is modulated. The ratios of Co2+/Co3+increased when Ni was introduced into the octahedral sheets of the layered double hydroxides. The ratios of Co2+/Co3+decreased when Fe was introduced into the octahedral sheets of the layered double hydroxides. The ratios of Co2+/Co3+decreased when Ce was introduced into the octahedral sheets of the layered double hydroxides. The ratios of Co2+/Co3+increased when Sm was introduced into the octahedral sheets of the layered double hydroxides. It is interesting to note that the conversion of4-methyl guaiacol increased at first and then decreased as the Co2+/Co3+ratios was increased. A virtual high conversion (nearly100%) was attained when the reaction was performed with Co-Ni-Al-CO32--LDHs and Co-Ni-Fe-CO32-LDHs within2hours.2. The layered double hydroxides intercalated with dodecyl sulfonates were used as a heterogeneous catalyst. The electronic structure of Co elements was changed by the modulation of the kinds of the elements introduced into the octahedral sheets of the layered double hydroxides. The elements are Co, Ni, Fe, Ce, and Sm. The dodecyl sulfonates provide the hydrophobic groups that are helpful to the escpae of the product from the active centers. It is possible to play a promoting role in the contributing to improve the selectivity in the oxidation of4-methyl guaiacol to vanillin. It is found that the electron density of the Co element is modulated by introducing different elements into the octahedral sheets of the layered doubles. What results in that the ratios of Co2+/Co3+are modulated. The ratios of Co2+/Co3+increased when Ni was introduced into the octahedral sheets of the layered double hydroxides. The ratios of Co2+/Co3+decreased when Fe was introduced into the octahedral sheets of the layered double hydroxides. The ratios of Co2+/Co3+decreased when Ce was introduced into the octahedral sheets of the layered double hydroxides. The ratios of Co2+/Co3+increased when Sm was introduced into the octahedral sheets of the layered double hydroxides. It is interesting to note that the conversion of4-methyl guaiacol increased at first and then decreased as the Co2+/Co3+ratios was increased. It is significant to note that the selectivity to vanillin of the layered double hydroxides intercalated with dodecyl sulfonates is better than that of the layered double hydroxides. The cobatl, nickel, and iron layered double hydroxides intercalated with dodecyl sulfonates also display good catalytic performance in the recycle studies.