Production of biofuels and esters - synthetic chemicals widely used for fragrances and fabrics - could potentially be greener, cheaper and more efficient thanks to researchers in the US who have shown how oxygen-coated gold nanoparticles can selectively oxidise a variety of alcohols and aldehydes.

    Heterogeneous catalysis by gold has recently been the subject of renewed interest. In particular, scientists are keen to replace current technology for stoichiometric oxidation of alcohols with a catalytic process that could reduce energy consumption, increase selectivity and minimise production of environmental pollutants.

    Now, Cynthia Friend's lab at Harvard University, Massachusetts, US, has found that oxidative coupling reactions of aldehydes with alcohols using oxygen-coated gold nanoparticles are highly selective and can occur at low temperatures. Furthermore, the reactions occur entirely on the surface of the gold catalyst, in contrast to previous research on supported catalysts that triggered solution phase reactions.

    'These principles apply to a wide range of alcohols and aldehydes, and thus to the synthesis of a wide range of esters,' says Friend. 'These reactions can be accomplished cleanly with a high selectivity and at close to room temperature, which opens up the possibility of efficient and simple reactions to produce esters,' she adds.

    Esters are chemical compounds derived by reacting an oxoacid with a hydroxyl compound such as an alcohol or phenol. The team demonstrated that adsorbed atomic oxygen bound to the gold surface acts as a Br?nsted base, which causes the O-H bond in methanol to break. The resulting adsorbed methoxy intermediate then acts as a nucleophile to attack electron-deficient centres in a number of aldehydes to selectively form methyl esters.

    Schematic mechanism for coupling of methanol and aldehydes

    ? Nature Chemistry

    'In a reacting system of this type, the two alcohols compete for reaction with the adsorbed active oxygen,' explains Friend. 'The challenge will be to determine how to control which of four possible esters will be formed.'

    Graham Hutchings, a catalyst expert at Cardiff University, UK, is enthusiastic about the work. 'This is an excellent model study on gold surfaces - really elegant experimental work,' he says. 'Gold nanoparticles are well known to be active for alcohol oxidation and these model studies may help us understand some of the by-product formation, which will be really helpful.'

    Friend suggests this work could offer environmentally friendly and more efficient alternatives to the production of low molecular weight esters. She also points out that understanding selective oxidation of alcohols in general is relevant to the processing of alternative fuel sources.

    However, it remains to be seen whether the process can be scaled up. 'The major challenges for practical use are the efficiency of delivery of atomic oxygen to the gold, the cost of the gold in the catalyst, and the stability of gold-based catalysts under operating conditions,' explains Friend. 'We are currently investigating vapour-phase esterification reactions over metallic nanoporous gold using O2 as the oxidant at atmospheric pressure with encouraging results,' she says.

    'This work shows that some reactions are possible under very low pressure and this is a very exciting possibility,' comments Hutchings. 'This will also inspire us to look for these types of reactions under higher pressure conditions,' he adds.

    生物燃料和酯，这些广泛用于香料和纺织品的合成化学品的生产可能会变得更环保、更便宜且更有效率。这要归功于美国的研究者们，他们表示镀氧的金纳米粒子能选择性地氧化多种醇和醛。

    最近，金的多种多样的催化作用成为了科学家研究兴趣的新主题。尤其，科学家们热切现在的技术能得到替换，新技术是一种带有催化过程的、按化学计量的醇的氧化，它能减少能量消耗、增加选择性，使环境污染物的产量达到最小。

    现在，在美国马萨诸塞州的哈佛大学，Cynthia Friend的实验室已经发现，用镀氧的金纳米粒子催化的醛和醇的氧化偶合反应是有高选择性的，且能在低温时发生。此外，与之前关于这种支持反应的催化剂只能在溶液相反应中起催化作用相反，反应完全在金催化剂的表面发生。

    Friend说，这些原理对大范围的醇和醛适用，也适用于大范围的酯的合成。她还说，这些反应具有高选择性，在室温下就能干净得完成。这为有效简单地合成酯提供了可能。

    酯是通过氧合酸和羟基化合物如醇或苯酚反应得到的化合物。研究小组证明，吸附到金表面的原子氧相当于布仑斯惕碱，能使甲醇中的O-H键断开，得到的甲氧基中间物被吸附到金表面，相当于一种亲核试剂，它攻击多种醇的缺电子的中心，选择性地形成甲酯。

    Friend解释说，在这种反应体系中，两种醇会竞争从而与被吸附的活性氧反应，难题是如何在四种可能形成的酯中控制形成某一种。

    英国Cardiff大学的催化剂专家Graham Hutchings对这项研究非常热心。他说，这是一个在金表面进行的非常优秀的模型实验，是非常高雅的实验工作。金纳米粒子是为人所熟知的醇氧化的催化剂，这些实验能有助于我们了解副产物的形成，而了解这些是非常有用的。

    Friend建议说，这项研究能为低分子量酯的生产提供更环保有效地选择。同时她指出，总体上了解醇的选择性氧化对代用燃料的加工也是有帮助的。

    然而，加工能否成比例增加还要拭目以待。实际应用的最大挑战是原子氧吸附到金表面的效率，在催化过程中金的消耗，及以金作为基底的催化剂的稳定性。她说，我们在研究蒸汽相的酯化反应，在一定的大气压下，氧气为氧化剂，通过金属纳米孔道反应，其效果良好。

    这项研究证明在低压下，一些反应是可以进行的。这是一种令人兴奋地可能。Hustings说，这也鼓励我们去寻找高压条件下得这种反应。