Thermal Activation of Solid-State Molybdenum Halide Clusters with an Octahedral Cluster Framework and Their Application to Catalytic Synthesis of 3-Methylpyridine from Piperidine and Methanol
作者:Satoshi Kamiguchi、Ryu Kajio、Hitomi Yamada、Hidetaka Yuge、Kazu Okumura、Hajime Iida、Sayoko Nagashima、Teiji Chihara
DOI:10.1246/bcsj.20150055
日期:2015.8.15
Solid-state molybdenum halide clusters with an octahedral metal framework MoX2 (or [Mo6X8]X2X4/2) (X = Cl, Br, I) are applied to catalysis. When these clusters are thermally activated in a hydrogen stream above 300 °C, they exhibit catalytic activity for the dehydrogenative C-methylation of piperidine with methanol, to yield 3-methylpyridine. At 400 °C, the selectivity is as high as 74%. This catalytic behavior is different from that of the molecular clusters [(M6Cl12)Cl2(H2O)4]·4H2O (M = Nb, Ta) and (H3O)2[(M6Cl8)Cl6]·6H2O (M = Mo, W), which exhibit Brønsted acidity after thermal activation; piperidine is N-methylated to yield N-methylpiperidine selectively. Elemental analysis and thermogravimetric analysis demonstrate that the solid-state clusters partially eliminate halogen ligands during the activation. Infrared analysis of adsorbed pyridine on the activated clusters shows the presence of a Lewis acid site. This coordinatively unsaturated site of the molybdenum is catalytically active for dehydrogenative C-methylation. The formation of an η3-1-azaallyl species on the molybdenum facilitates the methylation at the 3-position of piperidine, followed by dehydrogenation to yield 3-methylpyridine.
固态钼卤代簇合物,具有八面体金属骨架MoX2(或[Mo6X8]X2X4/2)(X = Cl, Br, I),应用于催化领域。当这些簇合物在300°C以上的氢气流中热活化时,它们显示出对哌啶与甲醇进行脱氢化C甲基化反应的催化活性,生成3-甲基吡啶。在400°C时,选择性高达74%。这种催化行为与分子簇合物[(M6Cl12)Cl2(H2O)4]·4H2O(M = Nb, Ta)和(H3O)2[(M6Cl8)Cl6]·6H2O(M = Mo, W)不同,后者在热活化后显示出Brønsted酸性;哌啶被N-甲基化,选择性地生成N-甲基哌啶。元素分析和热重分析表明,固态簇合物在活化过程中部分消除了卤素配体。吸附在活化簇合物上的吡啶的红外分析显示存在Lewis酸位点。这种配位不饱和的钼位点对脱氢化C甲基化具有催化活性。在钼上形成η3-1-氮杂烯丙基物种有助于在哌啶的3-位进行甲基化,随后脱氢生成3-甲基吡啶。