Abstract

Different solid catalysts were prepared by impregnation of tungsten species on commercial mesoporous silica. The resulting materials were characterized in order to elucidate their composition, properties and acidity. Moreover, the catalytic results were carefully studied, in an attempt to reach insights about the catalytic active species involved in the investigated reactions. The obtained solids were nominated Q15W-X, where X = 1 means an impregnation using ammonium paratungstate (APT), X = 2 sodium tungstate (Na₂WO₄) and X = 3 tungstic acid dihydrate (W-2) as tungsten source. They were evaluated as catalysts towards the esterification of palmitic acid by methanol, the average conversion was found to be between 80 and 90% and the catalysts were reused in four more reaction cycles, keeping similar conversion results as in the first use. The spectroscopic analyses suggested that the formation of polytungstate species over the silica with different tungsten species, mainly W⁵⁺ and W⁶⁺ is probably influence the esterification under mild conditions. Such species are not present in the solid resulting from the impregnation of W-2 on glass beads (GB) and the solid WO₃ obtained by the calcination of W-2 (WO₃-C), which presents poor catalytic activity and are mostly constituted by monoclinic WO₃.

摘要

通过在商业介孔二氧化硅上浸渍钨物种来制备不同的固体催化剂。对所得材料进行表征，以阐明它们的组成、性质和酸度。此外，对催化结果进行了仔细研究，以试图深入了解所研究反应中涉及的催化活性物质。得到的固体命名为Q15W-X，其中X  =  1 表示使用仲钨酸铵 (APT) 进行浸渍，X  =  2 钨酸钠 (Na ₂ WO ₄ ) 和X  =  3 钨酸二水合物 ( W-2) 作为钨源。它们被评估为甲醇对棕榈酸进行酯化的催化剂，发现平均转化率在 80% 到 90% 之间，并且催化剂在另外四个反应循环中重复使用，保持与第一次使用时相似的转化结果。光谱分析表明，在具有不同钨种类（主要是W ⁵⁺和W ⁶⁺ ）的二氧化硅上形成的多钨酸盐可能影响温和条件下的酯化反应。此类物质不存在于由W-2浸渍在玻璃珠 (GB) 上产生的固体和通过W-2 ( WO ₃ -C )煅烧获得的固体 WO _3中)，其催化活性较差，主要由单斜晶系WO ₃构成。