Iron and Copper Active Sites in Zeolites and Their Correlation to Metalloenzymes

Abstract

Metal-exchanged zeolites are a class of heterogeneous catalysts that perform important functions ranging from selective hydrocarbon oxidation to remediation of NO pollutants. Among these, copper and iron zeolites are remarkably reactive, hydroxylating methane and benzene selectively at low temperature to form methanol and phenol, respectively. In these systems, reactivity occurs at well-defined molecular transition metal active sites, and in this review we discuss recent advances in the spectroscopic characterization of these active sites and their reactive intermediates. Site-selective spectroscopy continues to play a key role, making it possible to focus on active sites that exist within a distribution of inactive spectator metal centers. The definition of the geometric and electronic structures of metallozeolites has advanced to the level of bioinorganic chemistry, enabling direct comparison of metallozeolite active sites to functionally analogous Fe and Cu sites in biology. We identify significant parallels and differences in the strategies used by each to achieve high reactivity, highlighting potentially interesting mechanisms to tune the performance of synthetic catalysts.

Monomeric Copper(II) Sites Supported on Alumina Selectively Convert Methane to Methanol

“…Thel ow amount of side products prevented their quantification in the gas phase. However,the analysis of the reacted material and the product by IR and nuclear magnetic resonance (NMR) spectroscopy showed that CH 3 O-species,a long with trace amounts of HCOO-and CO,w ere formed upon reaction with CH 4 ,a s confirmed by 13 CH 4 isotope studies (Supporting Information, Figures S19-S21). Them aterial 2-a 700 had as imilar performance (9 %CH 3 OH yield and 23 %Cureduction with 75% of electrons involved in CH 3 OH formation (Supporting Information, Table S2, Figure S22).…”

“…[6] These enzymes,containing Cu or Fe metal centers,have inspired the design of materials for the conversion of CH 4 to CH 3 OH under mild reaction conditions. [9][10][11][12][13] Thes tructure of the active site is highly debated, with evidence for both (m-oxo) dinuclear copper [9,14] and tris(moxo) trinuclear copper centers (Figure 1a). This hurdle can, in principle,b eo vercome using the concept of chemical looping,w hich entails decoupling the steps of oxidation of Cu sites and their reaction with CH 4 .…”

“…[7] Attempts to perform this reaction catalytically with tailored materials have mostly been unsuccessful because the selectivity toward CH 3 OH is low at high CH 4 conversion. [9][10][11][12][13] Thes tructure of the active site is highly debated, with evidence for both (m-oxo) dinuclear copper [9,14] and tris(moxo) trinuclear copper centers ( Figure 1a). [8] Thel ast step of the cycle utilizes aprotic solvent to desorb the CH 3 Ospecies that are strongly bound to the copper sites.O ft he various systems that have been investigated, Cu-exchanged zeolites have shown potential for the selective oxidation of CH 4 to CH 3 OH using molecular oxygen as an oxidant.…”

“…[8] Thel ast step of the cycle utilizes aprotic solvent to desorb the CH 3 Ospecies that are strongly bound to the copper sites.O ft he various systems that have been investigated, Cu-exchanged zeolites have shown potential for the selective oxidation of CH 4 to CH 3 OH using molecular oxygen as an oxidant. [9][10][11][12][13] Thes tructure of the active site is highly debated, with evidence for both (m-oxo) dinuclear copper [9,14] and tris(moxo) trinuclear copper centers ( Figure 1a). [15] Recent theoretical studies also suggest that monomeric Cu sites should not be excluded.…”

Impact of Intramolecular Hydrogen Bonding on the Reactivity of Cupric Superoxide Complexes with O−H and C−H Substrates

“…Ad rastic enhancement in the reactivity of the cupric superoxide towards phenolic substrates as well as oxidation of substrates possessing moderate C À Hb ond-dissociation energies is observed, correlating with the number and strength of the H-bonding groups.Oxidation and oxygenation reactions are vital for biological and synthetic processes. [1][2][3][4][5][6][7][8][9] In biology,s ome copper-containing metalloenzymes are capable of performing these reactions. [2,10] Forexample,galactose oxidase (GO) is responsible for the oxidation of primary alcohols to aldehydes (Figure 1A); monooxygenases such as peptidylglycine a-hydroxylating monooxygenase (PHM), dopamine b-monooxygenase (DbM), and lytic polysaccharide monooxygenases (LMPOs) catalytically hydroxylate organic substrates containing strong (85-100 kcal mol À1 )C ÀHb ond dissociation energies (BDEs) using dioxygen ( Figure 1B).…”