From Supported Metals To Covalently Complexes - The Way To Increase Catalytic Selectivity
FROM SUPPORTED METALS TO COVALENTLY COMPLEXES - THE WAY TO INCREASE CATALYTIC SELECTIVITY
V. N. Zaitsev, V. V. Skopenko, V. D. Oleynik
Department of Inorganic Chemistry T. Shevchenko University,
60 Vladimirskaya str. Kiev, Ukraine.
Selectivity of heterogeneous catalysis is the main question of their application. One method used to increase their selectivity is to surround the supported metal with ligand environments. However, limited degrees of freedom do not allow substantial improvement in selectivity. A new generation of heterogeneous catalysis that involved immobilized complexes where catalytic centres were removed from the surface but remain bound with it via a spacer was proposed. For this type of catalysts the selectivity increase tremendously but they become less stable or catalytically inactive. We studied this phenomenon and showed that in many such cases complexes with saturated coordination spheres were obtained and it was the reason of their low catalytic activity. Thus from Mossbauer spectroscopy was found that the complexes MQ2 (where Q=Phen-SiO2 or Dipy-SiO2) dominate when chlorides were used instead to MQ3 for tetrafluoroborates and perchlorates. Five‑coordinate complexes of copper [CuQ2Cl]Cl have been isolated and tetrahedral [CoQ2]Cl2. Only octahedral [NiQ3](BF4)2 and square planar [PdQ2]Cl2 bonded complexes have been found on the Phen-SiO2 and Dipy-SiO2 surfaces. We also showed that most available chemically modified surfaces have a cluster type distribution of covalently bonded ligands. This promotes a formation of metal clusters during the catalytic process and, ultimately, degradation of catalysis. The negative effects can be overcome in the next generation of heterogeneous catalysts. These are two dimensional immobilized complexes where a metal ion has the desirable ligand environment, but additionally, catalytic centres are isolated from each other on the surface. The general procedure to prepare such catalysts is proposed. It was found that the distance between covalently bonded complexes can be regulated in order to receive a highly selective uniformal immobilized catalyst.
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