Metal oxide alkane activation catalysts
From Inorganic Nano-Magnetite Clusters for Methanol synthesis to Organic Methane Mono-Oxygenase Mimics,
Peter-Paul Knops-Gerrits, William A. Goddard III
Material & Process Simulation Center, Beckman Institute (139-74),
California Inst. of Technology, Pasadena CA 91125, USA
Tel 626-395-2440, Fax 626-585-0918, email firstname.lastname@example.org
Inorganic Nano-Hematite-Magnetite Iron Oxide Clusters
a-Oxygen can be formed on iron sites by Nitrous oxide N2O decomposition at elevated temperature. The main parameters that affect the catalytic results are the size of the iron clusters, which are function of the crystallite size, the various Si/Al ratios, the amount of iron in the sample. Furthermore the activity is function of the pretreatment conditions. a-oxygen can be formed on Fe in ZSM-5 and related zeolitic materials (1). This a-oxygen can convert the methane in a stoichiometric reaction to methanol on Fe-ZSM-5 and to a much lesser extent on a wider set of materials.
The geometries of the lattice coordinated transition metal ions can be determined with a Zeolite FF (ZFF), mainly the Universal FF (UFF) (2) supplemented with a FF based on the quantum mechanics for Fe, Ti, Ga, Al, Si,…As such EPR, DRS, Mossbauer, signatures can be related to geometrical signatures, for given valences of transition metal ions..
Organic Methane Mono-Oxygenase Mimics
Methane Mono-Oxygenase (MMO) and Deoxyhemerythrin (DHr) are examples of di-iron enzymes that catalyze the dissociative and non-dissociative binding of molecular oxygen. To mimic the MMO active site with a finite cluster we chose to study the binuclear heptapodate coordinated iron (III)-complexes of
These have active sites of the form [Fe2(HPTP)(m -OH)]4+ (1) and [Fe2(HPTB)(m -OH)]4+ (2). We have characterized these complexes (3) using quantum mechanics and molecular dynamics modeling and with EXAFS, Mössbauer and Catalysis.
The QM was at the ab initio and DFT (B3LYP) levels. All chelating N-atoms and the ligand backbones were included in the quantum mechanical determination of the geometrical structure and electronic properties of ground states of the active site. The geometries of the supported complexes were determined using UFF (2) supplemented with a FF based on the quantum mechanics. The reactivity of these complexes for the alkane functionalization were modeled.
The oxygen bond length of 1.21 A increases to 1.31 A on the model compound and is smaller than the 1.49 A distance in H2O2, consequently this bond is broken in the model compound. The Fe..Fe distances in the model compound of 3.14 A increases to 3.43 A in intermediate P and 3.63 A in intermediate Q. Upon transformation into the bis (m -O2-) oxo bridged dimer, this distance decreases to 2.65 A. The energetics of these different intermediates seem fairly comparable with the peroxo form or intermediate P, being slightly more stable than the ferryl form or intermediate Q by about 18.9 kcal/mol. The methane to methanol oxidation is exothermic by 50.60 kcal/mol
The EXAFS leads to longer Fe-Fe distances for the benzimidazole compared to pyridine containing complexes. When the complexes are supported on mesoporous oxides, the EXAFS finds a lengthening of the Fe-Fe distances. The EXAFS data on the model compounds leads to larger Fe-Fe distances than the QM. This may be due to neglecting in the QM the large counterbalancing anions associated with the structure of [Fe2(HPTP)(m -OH)(NO3)2](ClO4)2 and [Fe2(HPTB)(m -OH)(NO3)2](ClO4)2. Some examples of the reactivity of these complexes for alkane functionalization [3-4] will be discussed.
Mössbauer spectroscopy allows the oxidation and spin states of the iron atoms to be assessed. Thus the isomer shifts [mm/s] relative to metallic iron at 300 K in the range 0.35-0.60 mm/s are characteristic of 5- or 6-coordinate high-spin di-ferricm -OH complexes, the smaller quadrupolar splitting [mm/s], < 1 mm/s, in the hydroxo-bridged complexes is due to the lengthening of the Fe-O bond upon protonation of the oxo bridge. The Mössbauer pattern of supported complexes are more complex due to complex lattice interactions.
The UFF optimized [Fe2(HPTP)(m -OH)(NO3)2](ClO4)2 model compound :
We thank BP Chemical for their financial support of the program of alkane activation.
The files with the power-point slides are available here.
The first model compound :
The second model compound :