Heterogeneous catalysis - Oxidant-free alcohol dehydrogenation using a reusable hydrotalcite-supported silver nanoparticle catalyst
Heterogeneous catalysis - Oxidant-free alcohol dehydrogenation using a reusable hydrotalcite-supported silver nanoparticle catalyst
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DOI：
10.1002/anie.200703161
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发表时间：
2008-01-01
影响因子：
16.6
通讯作者：
Kaneda, Kiyotomi
中科院分区：
文献类型：
--
作者：
Mitsudome, Takato;Mikami, Yusuke;Kaneda, Kiyotomi
醇脱氢生成羰基化合物是合成化学中最重要的反应之一。[1]最近，由于需要用环境友好的催化氧化体系代替涉及有毒氧化剂如铬盐和锰盐的化学计量反应，已经对使用分子氧（O2）作为氧化剂的醇的高效氧化的均相[2]和非均相[3]催化剂的开发给予了很多关注[Eq.①]。[4]可以设想用于先进的、更原子效率的催化剂系统的替代设计，其不使用分子氧或空气作为用于上述氧化的氧化剂[Eq.②]。我们开发的这种有前途的无O2方法从实用和环境的角度来看都特别令人感兴趣，因为1）它消除了H2O的形成，H2O是一种副产物，其通常使催化剂失活并需要从含水反应混合物中繁琐地纯化产物; 2）它对具有O2敏感官能团的醇具有耐受性; 3）它产生H2，其是用于能量产生的有吸引力的原料;和4）它抑制底物过度氧化成羧酸。尽管在醇的气相脱氢方面取得了工业上的成功，[5]迄今为止，用于无受体醇脱氢的较温和的液相催化剂系统的例子很少，而且这些系统通常存在活性低、底物范围有限和可重复使用性的问题。[6]水滑石（HT）是无机层状材料，由于其不仅具有吸附[7]和药物递送[8]的潜力，而且还具有催化剂的潜力，因此引起了人们的关注。[9]最近，我们报道了通过利用改性的HT用于各种有机反应的固体催化剂设计的新策略，例如使用过氧化氢的烯烃环氧化，[9 e]醇的氧化，[9a]和碳-碳键的形成。[9c]我们现在描述了使用异质水滑石负载的银纳米颗粒[10]（Ag/HT）的醇的高效无受体脱氢。该Ag/HT催化剂体系对多种醇类均表现出很高的催化活性。按文献方法合成了水滑石（HT，Mg_6Al_2（OH）_16CO_3）。[11]为了制备催化剂，将1.0 g HT加入到10 mL 5.0 mm AgNO 3水溶液中，之后将非均相混合物在空气中在38 ℃下搅拌1 h。过滤所得浆液，用去离子水彻底洗涤，并在室温下真空干燥，得到白色粉末。将HT负载的AgI离子在1108 ℃下用H_2还原30分钟得到Ag/HT，然后将其用于脱氢。经上述预处理后的Ag/HT的X射线衍射（XRD）谱中的峰的位置与母体HT的那些相似。Ag/HT复合材料中Ag原子的分数通过元素分析测定为0.005wt%。Ag/HT的透射电子显微镜图像显示平均直径为3.3 nm的Ag纳米颗粒。[12]当1-苯基乙醇和Ag/HT在对二甲苯中的混合物在Ar气氛中在1308 ℃下加热时，1-苯基乙醇在16小时内发生定量脱氢，得到苯乙酮。周转频率（TOF）和周转数（TON）分别达到1375 hk_1和22000（表1，条目1）。反应速率可以在Ar流下加速，并且TOF显著增加到2000 h-1（条目2）。[13]H2与苯乙酮的摩尔比...
The dehydrogenation of alcohols to carbonyl compounds is one of the most important transformations in synthetic chemistry.[1] Recently, much attention has been paid to the development of homogeneous [2] and heterogeneous [3] catalysts for the highly efficient oxidation of alcohols using molecular oxygen (O2) as the oxidant because of the necessity of replacing stoichiometric reactions involving toxic oxidants such as chromium and manganese salts with environmentally benign catalytic oxidation systems [Eq.(1)].[4] Alternative designs can be conceived for advanced, more atom-efficient catalyst systems, which do not use molecular oxygen or air as the oxidant for the above oxidation [Eq.(2)]. Our development of such a promising O2-free methodology is particularly interesting both from a practical and an environmental point of view because 1) it eliminates the formation of H2O, a byproduct which often deactivates catalysts and necessitates tedious purification of products from the aqueous reaction mixture; 2) it is tolerant toward alcohols having O2-sensitive functional groups; 3) it produces H2, which is an attractive feedstock for energy generation; and 4) it suppresses overoxidation of the substrate to carboxylic acids. Despite industrial success in the gas-phase dehydrogenation of alcohols,[5] few examples of milder liquid-phase catalyst systems for the acceptor-free dehydrogenation of alcohols have appeared to date, and these systems have often suffered from low activity, limited substrate scope, and reusability.[6] Hydrotalcites (HTs) are inorganic, layered materials that have attracted attention because of their potential not only for adsorption [7] and drug delivery [8] but also as catalysts.[9] Recently, we reported new strategies for the design of solid catalysts by utilizing modified HTs for various organic reactions such as the epoxidation of olefins using hydrogen peroxide,[9e] the oxidation of alcohols,[9a] and carbon–carbon bond formation.[9c] We now describe the highly effective acceptor-free dehydrogenation of alcohols using heterogeneous hydrotalcite-supported silver nanoparticles [10](Ag/HT). This Ag/HT catalyst system showed remarkably high catalytic activity for a wide range of alcohols. Moreover, the catalyst could be readily reused without any loss of activity and selectivity.Hydrotalcite (HT, Mg6Al2 (OH) 16CO3) was synthesized according to literature procedures.[11] To prepare the catalyst 1.0 g of the HT was added to 10 mL of a 5.0 mm aqueous solution of AgNO3, after which the heterogeneous mixture was stirred for 1 h in air at 38C. The resulting slurry was filtered, washed thoroughly with deionized water, and dried in vacuo at room temperature to yield a white powder. The HT-supported AgI ions were reduced with H2 at 1108C for 30minutes to give Ag/HT before it was used in the dehydrogenation. The positions of the peaks in the X-ray diffraction (XRD) spectrum of Ag/HT after the above pretreatment were similar to those of the parent HT. The fraction of Ag atoms in the Ag/HT composite was determined by elemental analysis to be 0.005 wt%. The transmission electron microscopy image of the Ag/HT showed Ag nanoparticles with a mean diameter of 3.3 nm.[12] When a mixture of 1-phenylethanol and Ag/HT in pxylene was heated at 1308C in an Ar atmosphere, quantitative dehydrogenation of 1-phenylethanol occurred within 16 h to afford acetophenone. The turnover frequency (TOF) and turnover number (TON) reached up to 1375 hÀ1 and 22000, respectively (Table 1, entry 1). The reaction rate could be accelerated under a stream of Ar, and the TOF significantly increased to 2000 hÀ1 (entry 2).[13] The mole ratio of H2 to acetophenone …