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，這些廣泛用于香料和紡織品的合成化學品的生產可能會變得更環保、更便宜且更有效率。這要歸功于美國的研究者們，他們表示鍍氧的金納米粒子能選擇性地氧化多種醇和醛。

    最近，金的多種多樣的催化作用成為了科學家研究興趣的新主題。尤其，科學家們熱切現在的技術能得到替換，新技術是一種帶有催化過程的、按化學計量的醇的氧化，它能減少能量消耗、增加選擇性，使環境污染物的產量達到最小。

    現在，在美國馬薩諸塞州的哈佛大學，Cynthia Friend的實驗室已經發現，用鍍氧的金納米粒子催化的醛和醇的氧化偶合反應是有高選擇性的，且能在低溫時發生。此外，與之前關于這種支持反應的催化劑只能在溶液相反應中起催化作用相反，反應完全在金催化劑的表面發生。

    Friend說，這些原理對大范圍的醇和醛適用，也適用于大范圍的酯的合成。她還說，這些反應具有高選擇性，在室溫下就能干凈得完成。這為有效簡單地合成酯提供了可能。

    酯是通過氧合酸和羥基化合物如醇或苯酚反應得到的化合物。研究小組證明，吸附到金表面的原子氧相當于布侖斯惕堿，能使甲醇中的O-H鍵斷開，得到的甲氧基中間物被吸附到金表面，相當于一種親核試劑，它攻擊多種醇的缺電子的中心，選擇性地形成甲酯。

    Friend解釋說，在這種反應體系中，兩種醇會競爭從而與被吸附的活性氧反應，難題是如何在四種可能形成的酯中控制形成某一種。

    英國Cardiff大學的催化劑專家Graham Hutchings對這項研究非常熱心。他說，這是一個在金表面進行的非常優秀的模型實驗，是非常高雅的實驗工作。金納米粒子是為人所熟知的醇氧化的催化劑，這些實驗能有助于我們了解副產物的形成，而了解這些是非常有用的。

    Friend建議說，這項研究能為低分子量酯的生產提供更環保有效地選擇。同時她指出，總體上了解醇的選擇性氧化對代用燃料的加工也是有幫助的。

    然而，加工能否成比例增加還要拭目以待。實際應用的最大挑戰是原子氧吸附到金表面的效率，在催化過程中金的消耗，及以金作為基底的催化劑的穩定性。她說，我們在研究蒸汽相的酯化反應，在一定的大氣壓下，氧氣為氧化劑，通過金屬納米孔道反應，其效果良好。

    這項研究證明在低壓下，一些反應是可以進行的。這是一種令人興奮地可能。Hustings說，這也鼓勵我們去尋找高壓條件下得這種反應。