Viewpoint on the Partial Oxidation of Methane to Methanol Using Cu- and Fe-Exchanged Zeolites-猫眼课题宝

DOI：

10.1021/acscatal.8b01180

发表时间：

2018-09-01

期刊：

ACS CATALYSIS

影响因子：

12.9

通讯作者：

中科院分区：

化学1区

文献类型：

作者：

Dinh, Kimberly T.;Sullivan, Mark M.;Roman-Leshkov, Yuriy

从天然气产量增加中获得的丰富且廉价的甲烷储备可以通过转化为甲醇而毫不费力地纳入当前的石化基础设施，甲醇是石化工业的主要产品。虽然CH 4的氧化C-H键活化在低温下是化学和动力学上可获得的，但很少有催化剂能够防止甲烷过度氧化为二氧化碳。这种选择性的缺乏是由于甲烷分子的高C-H键能（415 kJ mol− 1）与部分氧化产物的较低C-H键能相比，这导致通过连续反应的进一步氧化。目前，通过高温（~ 1170 K）蒸汽重整为合成气和随后在Cu基甲醇合成催化剂上转化的两步法完成的由甲烷催化生产甲醇仅在大规模下有效。迄今为止，不存在可以在单一步骤中使用氧气作为唯一氧化剂以高产率将甲烷转化为甲醇的合成催化剂。相反，生物衍生的甲烷单加氧酶蛋白（MMO）在环境条件下通过使用NADH作为还原剂在甲烷在铁（可溶性MMO）或铜活性中心（颗粒状MMO）上部分氧化之前活化氧气，以显著的选择性1，2将甲烷和氧气转化为甲醇。1因此，使用合成催化剂的最直接的甲烷部分氧化方案集中在Fe-和Cu-氧物种的形成和反应性上，类似于在生物系统中发现的那些。由Fe和Cu离子提供的适当的配位场环境产生亲电金属-氧物种，能够攻击CH 4的强C-H键。迄今为止，最有前途的C-H活化催化剂候选物是Fe基和Cu基沸石，其中FeIV − O和CuIII− O− CuIII被认为是CH 4选择性氧化为CH 3OH的关键活性位点基序。3，4直到最近，人们还认为这些位点只能按化学计量操作，5− 8但Romôn-Leshkov及其同事证明了在各种Cu交换沸石中存在以O2作为唯一氧化剂在稳定状态下催化部分甲烷氧化的位点。尽管这些材料具有巨大的潜力，但高甲醇选择性只能在低转化率（< 0.1%）下实现。实际上，理论研究预测，在CH 4转化率高于0.2%时，在孤立的Cu 2+位点处CH 3OH过度氧化成CO2和H2O将广泛进行。10为了规避这一预测的转化率限制，Ahlquist等人11和最近Ravi等人讨论了通过形成更稳定的产物来保护甲醇，这些产物对随后的氧化更有抗性。12，这些概念将在此强调。值得注意的实施方案是Shilov系统，13其从甲烷生产氯甲烷，以及Periana系统，14其将甲烷活化为硫酸氢甲酯。虽然沸石技术规避了多反应器、高压和高温系统的实验陷阱，但在获得高甲醇产率和抑制过度氧化方面仍然存在许多挑战。我们仍然远远没有开发出工业上可行的催化剂，至少部分是因为自然界使用了难以在人工系统中复制的效应组合。具体来说，MMO具有两个关键特征，导致选择性甲醇生产：（1）配体场，诱导高自旋电子配置在…

Abundant and inexpensive reserves of CH4, obtained from increased production of natural gas, can effortlessly be incorporated into the current petrochemical infrastructure via the conversion to methanol, which is a staple of the petrochemical industry. Although oxidative C− H bond activation of CH4 is thermodynamically and kinetically accessible at low temperatures, few catalysts are capable of preventing overoxidation of methane to carbon dioxide. This lack in selectivity arises from the high C− H bond energy (415 kJ mol− 1) of the methane molecule in comparison to the lower C− H bond energy of the partially oxidized products, which results in further oxidation through consecutive reactions. Currently, the catalytic production of methanol from methane, accomplished through the two-step process of high temperature (∼ 1170 K) steam reforming to syngas and its subsequent conversion over Cu-based methanol synthesis catalysts, is effective only at large scale. To date, no synthetic catalyst exists that can convert methane to methanol in high yields using oxygen as the sole oxidant in a single step. Conversely, biologically derived methane monooxygenase proteins (MMOs) transform methane and oxygen into methanol with remarkable selectivity 1, 2 under ambient conditions through the use of NADH as a reducing agent for the activation of oxygen before the partial oxidation of methane on iron (soluble MMO) or copper active centers (particulate MMO). 1 Accordingly, most direct partial methane oxidation schemes using synthetic catalysts have focused on the formation and reactivity of Fe-and Cu-oxygen species akin to those found in biological systems. The proper ligand field environment afforded by Fe and Cu ions generates electrophilic metal− oxygen species capable of attacking the strong C− H bonds of CH4. The most promising C− H activation catalyst candidates thus far are Fe-and Cu-based zeolites, where FeIV O and CuIII− O− CuIII are believed to be key active site motifs for the selective oxidation of CH4 to CH3OH. 3, 4 Until recently, it was believed that these sites could only operate stoichiometrically, 5− 8 but Román-Leshkov and co-workers demonstrated the existence of sites operating catalytically at steady state for partial methane oxidation using O2 as the sole oxidant in a variety of Cu-exchanged zeolites. 9 Despite the tremendous potential of these materials, high methanol selectivities could only be achieved at low conversions (< 0.1%). Indeed, theoretical studies predict that overoxidation of CH3OH into CO2 and H2O at isolated Cu2+ sites will proceed extensively at CH4 conversions above 0.2%. 10 To circumvent this predicted limit on conversion, the protection of methanol by the formation of more stable products that are more resistant to subsequent oxidation has been discussed by Ahlquist et al. 11 and more recently by Ravi et al., 12 and these concepts will be emphasized herein. Notable implementations are the Shilov system, 13 which produces chloromethane from methane, and the Periana system, 14 which activates methane to methyl bisulfate. While zeolite-based technologies circumvent the experimental pitfalls of multireactor, high-pressure, and high-temperature systems, numerous challenges remain in obtaining high methanol yields and inhibiting excess oxidation.We are still far from developing an industrially viable catalyst for this transformation, at least in part because nature uses a combination of effects that are difficult to replicate in artificial systems. Specifically, MMOs possess two key features that result in selective methanol production:(1) ligand fields that induce high-spin electronic configurations at the …

Viewpoint on the Partial Oxidation of Methane to Methanol Using Cu- and Fe-Exchanged Zeolites

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