纳米钒铬复合氧化物的溶剂热合成、表征及催化2,6-二氯甲苯氨氧化反应（英文）

甲基芳烃气相氨氧化反应制备对应的芳香腈被认为是丙烯氨氧化制备丙烯腈之后化工领域又一重大进展,芳香腈是重要的精细化学品,广泛应用于医药、农药、颜料、染料、橡胶、光电材料等领域.其中2,6-二氯甲苯氨氧化反应制备2,6-二氯苯腈是特别重要的反应,2,6-二氯苯腈工业上可用于制备高效除草剂、杀菌剂及各种特种工程塑料;然而相较于其它的甲基芳烃,2,6-二氯甲苯由于甲基邻位有两个较大位阻且较强吸电子的氯原子影响,甲基活性较低,较难发生氨氧化反应,原料转化率和产品收率均较低.本课题组一直致力于发展高活性和选择性的氨氧化催化剂以及有效的策略实现甲基芳烃高效转化为芳香腈,我们曾以硅胶负载的钒磷氧化物(VPO/SiO_2)和钒铬氧化物(VCrO/SiO_2)为催化剂,成功实现了2,6-二氯甲苯氨氧化反应制备2,6-二氯苯腈.钒铬复合氧化物(VCrO)具有广泛的应用,可用于多相催化、气体传感、能量储存等领域.VCrO通常通过高温固相反应制备,然而一般得到的是混合相,产品形态和颗粒大小也不能很好控制;当用于氧化或氨氧化反应时,需要较高的反应温度,原料也容易发生过度氧化,导致积碳及活性降低.我们以V_2O_5和CrO_3为原料,在醇或者醇水溶液中于180℃进行溶剂热反应制备了无定形的VCrO前驱体,然后将前驱体在不同温度下氮气气氛中煅烧,产品通过粉末X射线衍射、透射电镜和X射线光电子能谱等进行表征.当以甲醇或甲醇水溶液为溶剂热反应介质,并且前驱体700℃进行煅烧后,产品为纯的正交晶系CrVO4纳米晶相;当以甲醇为溶剂时,CrVO_4晶相的尺寸大约为500 nm;而改为甲醇水溶液为溶剂时,产品尺寸急剧减小到50 nm以下,而且通过改变甲醇和水的体积比分别为10:1,5:1,1:1和1:5时,CrVO_4纳米晶相的尺寸从50 nm逐渐减小到30,20和10 nm,能够进行有效调控.据我们所知,这是首次合成纯的CrVO_4纳米晶相.我们以该纳米CrVO4为催化剂催化2,6-二氯甲苯氨氧化反应制备2,6-二氯苯腈,在335℃的相对较低温度下反应,原料转化率为84%,产品收率为75%;进一步升高温度到390℃,原料转化率为99%,产品收率可达81%.在所有已报道的二元复合氧化物催化剂中,纳米CrVO_4显示了最高的催化活性,主要归功于它较小的粒子尺寸、较大的表面积和更多暴露的活性中心

Solvothermal synthesis and characterization of nanocrystalline vanadium-chromium composite oxides and catalytic ammoxidation of 2,6-dichlorotoluene

Vanadium-chromium oxides (VCrO) were usually prepared by high-temperature solid-state reactions; however, mixed phases were frequently produced and the morphology of the products was not well controlled. In this work, we prepared amorphous VCrO precursors by using V₂O₅ and CrO₃ and alcohols or mixtures of alcohol and water via solvothermal reaction at 180 °C. The precursors were then calcined under nitrogen at various temperatures. The products were characterized by powder X-ray diffraction, transmission electron microscopy, and X-ray photoelectron spectroscopy. It was revealed that pure-phase nanocrystalline orthorhombic CrVO₄ was obtained when methanol or methanol/water was used as the solvothermal medium and the precursor was calcined at 700 °C. The size of the CrVO₄ crystals was around 500 nm when methanol was used, whereas it reduced significantly to less than 50 nm when a mixture of methanol and water was used. The sizes could be effectively tuned from 10 to 50 nm by varying the methanol/water volume ratio. To the best of our knowledge, this is the first report on the synthesis of pure-phase CrVO₄ nanocrystals. The nano-CrVO₄ showed almost the highest catalytic activity for the ammoxidation of 2,6-dichlorotoluene to 2,6-dichlorobenzonitrile among the reported bi-component composite oxides, owing to its smaller particle size, larger specific surface area, and more exposed active centers.