Characterization of the Structure and Catalytic Behavior of AlF3−x(OH)xwith Aluminum Successively Replaced by Chromium and Magnesium
Characterization of the Structure and Catalytic Behavior of AlF3−x(OH)xwith Aluminum Successively Replaced by Chromium and Magnesium
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DOI：
10.1006/jcat.1996.0095
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发表时间：
1996-04
影响因子：
7.3
通讯作者：
E. Kemnitz;A. Hess;G. Rother;S. Troyanov
中科院分区：
文献类型：
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作者：
E. Kemnitz;A. Hess;G. Rother;S. Troyanov
α-AlF3·3H2O的煅烧形成β-AlF3相，其对于C1-烃（例如二氯二氟甲烷）的歧化具有催化活性。铬和镁逐步取代铝会导致煅烧产品的结构和表面性能发生显着变化。这伴随着催化活性的显着变化。催化剂的合成是通过混合金属氟化物三水合物的共沉淀和随后的煅烧过程进行的。铬的逐步取代导致晶格从 β-AlF3 的伪六方钨青铜 (HTB) 结构重建为 CrF3−x(OH)x 的立方烧绿石结构。该路线没有得到六方β-CrF3或混晶β-(Al,Cr)F3。 50% 铬样品获得了最大的 CCl2F2 歧化催化活性，同时具有最大的 BET 表面积和最大数量的路易斯酸位点。 CrF3−x(OH)x 表现出催化活性和 BET 表面积的显着损失。通过比较准六方钨青铜结构与立方烧绿石结构在路易斯酸金属阳离子的可及性方面给出了可能的解释。烧绿石晶格内存在的羟基能够形成氢桥键，同时屏蔽金属阳离子。在用镁替代的情况下，目的是调节路易斯酸度的强度。煅烧样品的催化活性在 10% 镁样品处达到最大值。由于随着Mg含量的进一步增加，路易斯酸位点的强度降低，催化活性越来越减弱。当 Mg 含量为 50% 或更高时，路易斯酸位点的强度不再足以催化歧化。
The calcination of α-AlF3·3H2O forms a β-AlF3phase which is catalytically active for dismutation of C1-hydrocarbons such as dichlorodifluoromethane. The stepwise replacement of aluminum by chromium and magnesium leads to considerable alterations in structure and surface properties of the calcination products. This is accompanied by significant changes in the catalytic activity. The synthesis of the catalysts was carried out by coprecipitation of mixed metal fluoride trihydrates and subsequent calcination procedures. The stepwise replacement with chromium leads to a rebuilding of the lattice from the pseudo-hexagonal tungsten bronze (HTB) structure of β-AlF3into a cubic pyrochlore structure of CrF3−x(OH)x. Hexagonal β-CrF3or mixed crystals β-(Al,Cr)F3were not obtained via this route. The maximum catalytic activity for CCl2F2dismutation was obtained for the 50% chromium sample, which is accompanied by a maximum BET surface area and a maximum number of Lewis acid sites. CrF3−x(OH)xexhibits a dramatic loss of catalytic activity as well as BET surface area. Possible explanations are given by comparing the pseudo-hexagonal tungsten bronze structure with the cubic pyrochlore structure with regard to accessibility of the Lewis acid metal cations. The presence of hydroxyl groups within the pyrochlore lattice enables the formation of hydrogen bridge bonds which is accompanied by a shielding of the metal cations. In the case of the replacement with magnesium the aim was to tune the strength of the Lewis acidity. The catalytic activity of the calcined samples passes through a maximum at the 10% magnesium sample. Due to the decrease of the strength of Lewis acid sites with further increasing Mg content, the catalytic activity is more and more diminished. At 50% Mg and higher the strength of the Lewis acid sites is no longer sufficient to catalyze the dismutation.