Chen Yongsheng, Fulton John L, Partenheimer Walter
Chemical Sciences Division, Pacific Northwest National Laboratory, Richland, Washington 99352, USA.
J Am Chem Soc. 2005 Oct 12;127(40):14085-93. doi: 10.1021/ja053421v.
Extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) spectroscopies were used to probe the first-shell coordination structure about Mn(II) and Br(-1) ions that exist as contact ion pairs in supercritical water. This work was performed to clarify why solutions of MnBr2 in supercritical water are known to effectively catalyze the aerobic oxidative synthesis of terephthalic acid from p-xylene as well as a number of other methylaromatic compounds. The Mn and Br K-edge spectra were collected at the bending magnet beamline (sector 20) at the Advanced Photon Source, Argonne National Laboratory. The first-shell coordination structure about the Mn(II) ion changes from octahedral at ambient conditions to tetrahedral at supercritical conditions. Under supercritical conditions, the measured bond distances of Mn-OH2 and Mn-Br are 2.14 and 2.46 A, respectively. Direct contact ion pairs form with about 2 Br(-1) ions present in the first coordination shell of the Mn(II) ion. The structure of dissolved MnBr2, below 1.0 m, changes from essentially [Mn(II)(H2O)6]+2 to [Mn(II)(H2O)2(Br(-1))2] in supercritical water (scH2O). When an excess of Br(-1) ion is added, the bromide coordination number increases and the number of water molecules decreases. The results show that the initial MnBr2 catalyst in scH2O is tetrahedral with two Mn-Br contact ion pairs. The presence of the acetate anion deactivates the catalyst by formation of insoluble MnO.
扩展X射线吸收精细结构（EXAFS）和X射线吸收近边结构（XANES）光谱被用于探测超临界水中以接触离子对形式存在的Mn(II)和Br(-1)离子的第一配位层结构。开展这项工作是为了阐明为何已知超临界水中的MnBr2溶液能有效催化对二甲苯以及其他多种甲基芳烃化合物的有氧氧化合成对苯二甲酸。Mn和Br的K边光谱是在阿贡国家实验室先进光子源的弯曲磁铁光束线（20号扇区）采集的。Mn(II)离子的第一配位层结构在环境条件下为八面体，在超临界条件下变为四面体。在超临界条件下，测得的Mn-OH2和Mn-Br键长分别为2.14 Å和2.46 Å。直接接触离子对由Mn(II)离子第一配位层中存在的约2个Br(-1)离子形成。在超临界水（scH2O）中，浓度低于1.0 m的溶解态MnBr2结构从基本上的[Mn(II)(H2O)6]+2变为[Mn(II)(H2O)2(Br(-1))2]。当加入过量的Br(-1)离子时，溴化物配位数增加，水分子数量减少。结果表明，scH2O中的初始MnBr2催化剂为四面体结构，带有两个Mn-Br接触离子对。乙酸根阴离子的存在通过形成不溶性MnO使催化剂失活。
