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Papers by Max Bols
Journal of the American Chemical Society
Reference Module in Chemistry, Molecular Sciences and Chemical Engineering, 2021
This Excel spreadsheet contains all Mössbauer and resonance Raman data presented in the main text... more This Excel spreadsheet contains all Mössbauer and resonance Raman data presented in the main text of Snyder et al., Science 2021.
Chemical Reviews, 2022
Transition-metal-exchanged zeolites perform remarkable chemical reactions from low-temperature me... more Transition-metal-exchanged zeolites perform remarkable chemical reactions from low-temperature methane to methanol oxidation to selective reduction of NOx pollutants. As with metalloenzymes, metallozeolites have impressive reactivities that are controlled in part by interactions outside the immediate coordination sphere. These second-sphere effects include activating a metal site through enforcing an "entatic" state, controlling binding and access to the metal site with pockets and channels, and directing radical rebound vs cage escape. This review explores these effects with emphasis placed on but not limited to the selective oxidation of methane to methanol with a focus on copper and iron active sites, although other transitionmetal-ion zeolite reactions are also explored. While the actual active-site geometric and electronic structures are different in the copper and iron metallozeolites compared to the metalloenzymes, their second-sphere interactions with the lattice or the protein environments are found to have strong parallels that contribute to their high activity and selectivity.
Science, 2021
Fencing in radicals Zeolite catalysis could potentially offer a more direct route from methane to... more Fencing in radicals Zeolite catalysis could potentially offer a more direct route from methane to methanol. However, current catalysts tend to deactivate too quickly for practical use. Snyder et al. investigated the deactivation mechanism using Mössbauer and Raman spectroscopy and accompanying simulations (see the Perspective by Scott). Their results suggest that in zeolites with large apertures, after iron active sites strip hydrogen from methane, the resulting methyl radicals can leak away and deactivate other iron centers. Zeolites with tighter apertures can keep the radicals nearby longer, favoring the formation of methanol. Science , abd5803, this issue p. 327 ; see also abj4734, p. 277
Nature Catalysis, 2021
Iron-containing zeolites are heterogeneous catalysts that exhibit remarkable activity in the sele... more Iron-containing zeolites are heterogeneous catalysts that exhibit remarkable activity in the selective oxidation of inert hydrocarbons and catalytic decomposition of nitrous oxide (N2O). The reduction of N2O is critical to both of these functions, however direct experimental data tracking the iron active sites during N2O binding and activation are limited. Here, the N2O-ligated Fe(II) active site in iron-exchanged zeolite beta is isolated and characterised by variable-temperature Mössbauer, diffuse reflectance UV-Vis-NIR, and FTIR spectroscopy. N2O is found to bind through the terminal nitrogen atom, with significant backbonding from the Fe(II) centre at low temperature. At higher temperatures the Fe-N2O interaction is weakened, facilitating isomerization to the O-bound form which is competent in O-atom transfer. DFT calculations elucidate the geometric and electronic structure requirements for N2O binding and activation. A geometric distortion imposed by the zeolite lattice plays an important role in activating N2O. This highlights a mechanism for structural control over function in Fe-zeolite catalysts. Introduction. N2O is a potent and selective oxidant, but also a major cause of stratospheric ozone depletion and the third largest contributor to anthropogenic global warming. 1 It is generated in large quantities as a side product in the production of adipic acid and nitric acid, but it remains underutilised due to its high kinetic stability. Understanding the binding and activation of N2O by transition metal catalysts is therefore of significant interest for the use of N2O as an oxidant, and for its catalytic decomposition to environmentally benign O2 and N2. However, the N2O ligand interacts only weakly both as a donor and as a acceptor. Consequently only a handful of N2O-ligated transition metal centres have been successfully isolated and characterised. 2-6 Binding through the terminal nitrogen is generally preferred due to
Journal of the American Chemical Society, 2021
Using UV-Vis and resonance Raman spectroscopy, we identify a [Cu2O] 2+ active site in O2 and N2O ... more Using UV-Vis and resonance Raman spectroscopy, we identify a [Cu2O] 2+ active site in O2 and N2O activated Cu-CHA that reacts with methane to form methanol at low temperature. The Cu-O-Cu angle (120°) is smaller than for the [Cu2O] 2+ core on Cu-MFI (140°) and its coordination geometry to the zeolite lattice is different. Site-selective kinetics obtained by operando UV-Vis show that the [Cu2O] 2+ core on Cu-CHA is more reactive than the [Cu2O] 2+ site in Cu-MFI. From DFT calculations we find that the increased reactivity of Cu-CHA is a direct reflection of the strong [Cu2OH] 2+ bond formed along the H-atom abstraction reaction coordinate. A systematic evaluation of these [Cu2O] 2+ cores reveals that the higher O-H bond strength in Cu-CHA is due to the relative orientation of the two planes of the coordinating bidentate O-Al-O T-sites that connect the [Cu2O] 2+ core to the zeolite lattice. This work along with our earlier study (J.
Dalton Transactions, 2020
Strategies for further research and developments on active sites in Fe-zeolites for redox catalysis.
Chemistry of Materials, 2019
Proceedings of the National Academy of Sciences, 2018
Significance Fe zeolites are heterogeneous catalysts that show potential in a number of important... more Significance Fe zeolites are heterogeneous catalysts that show potential in a number of important industrial applications, including the selective partial oxidation of methane to methanol at room temperature, and the selective conversion of benzene to phenol. There are practical limitations associated with Fe-zeolite catalysts that may be resolved with mechanistic insight; however, reliable experimental data on Fe zeolites are limited. This study defines the mechanism of selective benzene hydroxylation catalyzed by Fe zeolites, clarifying the relationship between active site structure and catalytic performance (activity, selectivity). Mechanistic insight from this study represents an important step toward synthetic control over function in selective hydrocarbon oxidation catalysis.
Journal of the American Chemical Society, Jan 26, 2018
The formation of single-site α-Fe in the CHA zeolite topology is demonstrated. The site is shown ... more The formation of single-site α-Fe in the CHA zeolite topology is demonstrated. The site is shown to be active in oxygen atom abstraction from NO to form a highly reactive α-O, capable of methane activation at room temperature to form methanol. The methanol product can subsequently be desorbed by online steaming at 200 °C. For the intermediate steps of the reaction cycle, the evolution of the Fe active site is monitored by UV-vis-NIR and Mössbauer spectroscopy. A B3LYP-DFT model of the α-Fe site in CHA is constructed, and the ligand field transitions are calculated by CASPT2. The model is experimentally substantiated by the preferential formation of α-Fe over other Fe species, the requirement of paired framework aluminum and a MeOH/Fe ratio indicating a mononuclear active site. The simple CHA topology is shown to mitigate the heterogeneity of iron speciation found on other Fe-zeolites, with FeO being the only identifiable phase other than α-Fe formed in Fe-CHA. The α-Fe site is forme...
Proceedings of the National Academy of Sciences of the United States of America, May 2, 2018
Iron-containing zeolites exhibit unprecedented reactivity in the low-temperature hydroxylation of... more Iron-containing zeolites exhibit unprecedented reactivity in the low-temperature hydroxylation of methane to form methanol. Reactivity occurs at a mononuclear ferrous active site, α-Fe(II), that is activated by NO to form the reactive intermediate α-O. This has been defined as an Fe(IV)=O species. Using nuclear resonance vibrational spectroscopy coupled to X-ray absorption spectroscopy, we probe the bonding interaction between the iron center, its zeolite lattice-derived ligands, and the reactive oxygen. α-O is found to contain an unusually strong Fe(IV)=O bond resulting from a constrained coordination geometry enforced by the zeolite lattice. Density functional theory calculations clarify how the experimentally determined geometric structure of the active site leads to an electronic structure that is highly activated to perform H-atom abstraction.
Chemical reviews, Jan 14, 2018
Metal-exchanged zeolites are a class of heterogeneous catalysts that perform important functions ... more Metal-exchanged zeolites are a class of heterogeneous catalysts that perform important functions ranging from selective hydrocarbon oxidation to remediation of NOpollutants. 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 id...
Inorganic Chemistry, 2017
α-Fe is the precursor of the reactive Fe(IV)=O core responsible for methane oxidation in Fe-conta... more α-Fe is the precursor of the reactive Fe(IV)=O core responsible for methane oxidation in Fe-containing zeolites. To get more insight into the nature and stability of α-Fe in different zeolites, the binding of Fe(II) at six-membered cation exchange sites (6MR) in ZSM-5, zeolite beta and ferrierite was investigated using DFT and multi reference ab initio methods (CASSCF/CASPT2). CASPT2 ligand field (LF) excitation energies of all sites were compared with the experimental DR-UV-vis spectra reported by Snyder et al. [Nature, 536 (7616):317-321, 2016]. From this comparison it is concluded that the 16 000 cm-1 band of α-Fe, observed in all three zeolites, can uniquely be assigned to a high spin square planar (SP) Fe(II) located at a 6MR with Al-Si-Si-Al sequence, where the Al are positioned opposite in the ring and as close to each other as possible. The stability of such conformations is also confirmed by the binding energies obtained from DFT. The bands at 10 000 cm-1 in the experimental spectra, assigned to spectator Fe(II), are attributed to six coordinated trigonal prismatic Fe(II) species, as calculated for the γ-site in ZSM-5. The entatic effect of the zeolite lattice on the stability of the SP sites was investigated making use of an unconstrained Fe(II) model complex FeL 2 (with L = [Al(OH) 4 ]-). The SP conformer is approximately 2 kcal/mol more stable than the tetrahedral one, indicating that the SP coordination environment of α-Fe is not imposed by the zeolite lattice but rather electronically preferred by Fe(II) in the environment of four O ligands. A significant contribution to the stability of the SP conformer is provided by mixing of the doubly occupied 3d z 2 orbital with the higher lying 4s.
Nature, 2016
An efficient catalytic process for converting methane into methanol could have far-reaching econo... more An efficient catalytic process for converting methane into methanol could have far-reaching economic implications. Iron-containing zeolites (microporous aluminosilicate minerals) are noteworthy in this regard, having an outstanding ability to hydroxylate methane rapidly at room temperature to form methanol 1-3. Reactivity occurs at an extra-lattice active site called α-Fe(ii), which is activated by nitrous oxide to form the reactive intermediate α-O 4,5 ; however, despite nearly three decades of research 5 , the nature of the active site and the factors determining its exceptional reactivity are unclear. The main difficulty is that the reactive species-α-Fe(ii) and α-O-are challenging to probe spectroscopically: data from bulk techniques such as X-ray absorption spectroscopy and magnetic susceptibility are complicated by contributions from inactive 'spectator' iron. Here we show that a site-selective spectroscopic method regularly used in bioinorganic chemistry can overcome this problem. Magnetic circular dichroism reveals α-Fe(ii) to be a mononuclear, high-spin, square planar Fe(ii) site, while the reactive intermediate, α-O, is a mononuclear, high-spin Fe(iv)=O species, whose exceptional reactivity derives from a constrained coordination geometry enforced by the zeolite lattice. These findings illustrate the value of our approach to exploring active sites in heterogeneous systems. The results also suggest that using matrix constraints to activate metal sites for function-producing what is known in the context of metalloenzymes as an 'entatic' state 6-might be a useful way to tune the activity of heterogeneous catalysts. No spectroscopic feature of α-Fe(ii) has thus far been discovered 7. We have now identified Fe(ii) ligand-field bands in the diffuse reflectance ultraviolet-visible (DR-UV-vis) spectra of three iron-containing zeolites (Fe-zeolites) that are known to contain α-Fe(ii) (see Extended Data Fig. 1) 4,8-10. Of these, we chose the Fe(ii)-beta (BEA)
Journal of the American Chemical Society, 2021
α-Fe(II) active sites in iron zeolites catalyze N 2 O decomposition and form highly reactive α-O ... more α-Fe(II) active sites in iron zeolites catalyze N 2 O decomposition and form highly reactive α-O that selectively oxidizes unreactive hydrocarbons, such as methane. How these α-Fe(II) sites are formed remains unclear. Here different methods of iron introduction into zeolites are compared to derive the limiting factors of Fe speciation to α-Fe(II). Postsynthetic iron introduction procedures on small pore zeolites suffer from limited iron diffusion and dispersion leading to iron oxides. In contrast, by introducing Fe(III) in the hydrothermal synthesis mixture of the zeolite (one-pot synthesis) and the right treatment, crystalline CHA can be prepared with >1.6 wt % Fe, of which >70% is α-Fe(II). The effect of iron on the crystallization is investigated, and the intermediate Fe species are tracked using UV−vis-NIR, FT-IR, and Mossbauer spectroscopy. These data are supplemented with online mass spectrometry in each step, with reactivity tests in α-O formation and with methanol yields in stoichiometric methane activation at room temperature and pressure. We recover up to 134 μmol methanol per gram in a single cycle through H 2 O/CH 3 CN extraction and 183 μmol/g through steam desorption, a record yield for iron zeolites. A general scheme is proposed for iron speciation in zeolites through the steps of drying, calcination, and activation. The formation of two cohorts of α-Fe(II) is discovered, one before and one after high temperature activation. We propose the latter cohort depends on the reshuffling of aluminum in the zeolite lattice to accommodate thermodynamically favored α-Fe(II).
Journal of the American Chemical Society
Reference Module in Chemistry, Molecular Sciences and Chemical Engineering, 2021
This Excel spreadsheet contains all Mössbauer and resonance Raman data presented in the main text... more This Excel spreadsheet contains all Mössbauer and resonance Raman data presented in the main text of Snyder et al., Science 2021.
Chemical Reviews, 2022
Transition-metal-exchanged zeolites perform remarkable chemical reactions from low-temperature me... more Transition-metal-exchanged zeolites perform remarkable chemical reactions from low-temperature methane to methanol oxidation to selective reduction of NOx pollutants. As with metalloenzymes, metallozeolites have impressive reactivities that are controlled in part by interactions outside the immediate coordination sphere. These second-sphere effects include activating a metal site through enforcing an "entatic" state, controlling binding and access to the metal site with pockets and channels, and directing radical rebound vs cage escape. This review explores these effects with emphasis placed on but not limited to the selective oxidation of methane to methanol with a focus on copper and iron active sites, although other transitionmetal-ion zeolite reactions are also explored. While the actual active-site geometric and electronic structures are different in the copper and iron metallozeolites compared to the metalloenzymes, their second-sphere interactions with the lattice or the protein environments are found to have strong parallels that contribute to their high activity and selectivity.
Science, 2021
Fencing in radicals Zeolite catalysis could potentially offer a more direct route from methane to... more Fencing in radicals Zeolite catalysis could potentially offer a more direct route from methane to methanol. However, current catalysts tend to deactivate too quickly for practical use. Snyder et al. investigated the deactivation mechanism using Mössbauer and Raman spectroscopy and accompanying simulations (see the Perspective by Scott). Their results suggest that in zeolites with large apertures, after iron active sites strip hydrogen from methane, the resulting methyl radicals can leak away and deactivate other iron centers. Zeolites with tighter apertures can keep the radicals nearby longer, favoring the formation of methanol. Science , abd5803, this issue p. 327 ; see also abj4734, p. 277
Nature Catalysis, 2021
Iron-containing zeolites are heterogeneous catalysts that exhibit remarkable activity in the sele... more Iron-containing zeolites are heterogeneous catalysts that exhibit remarkable activity in the selective oxidation of inert hydrocarbons and catalytic decomposition of nitrous oxide (N2O). The reduction of N2O is critical to both of these functions, however direct experimental data tracking the iron active sites during N2O binding and activation are limited. Here, the N2O-ligated Fe(II) active site in iron-exchanged zeolite beta is isolated and characterised by variable-temperature Mössbauer, diffuse reflectance UV-Vis-NIR, and FTIR spectroscopy. N2O is found to bind through the terminal nitrogen atom, with significant backbonding from the Fe(II) centre at low temperature. At higher temperatures the Fe-N2O interaction is weakened, facilitating isomerization to the O-bound form which is competent in O-atom transfer. DFT calculations elucidate the geometric and electronic structure requirements for N2O binding and activation. A geometric distortion imposed by the zeolite lattice plays an important role in activating N2O. This highlights a mechanism for structural control over function in Fe-zeolite catalysts. Introduction. N2O is a potent and selective oxidant, but also a major cause of stratospheric ozone depletion and the third largest contributor to anthropogenic global warming. 1 It is generated in large quantities as a side product in the production of adipic acid and nitric acid, but it remains underutilised due to its high kinetic stability. Understanding the binding and activation of N2O by transition metal catalysts is therefore of significant interest for the use of N2O as an oxidant, and for its catalytic decomposition to environmentally benign O2 and N2. However, the N2O ligand interacts only weakly both as a donor and as a acceptor. Consequently only a handful of N2O-ligated transition metal centres have been successfully isolated and characterised. 2-6 Binding through the terminal nitrogen is generally preferred due to
Journal of the American Chemical Society, 2021
Using UV-Vis and resonance Raman spectroscopy, we identify a [Cu2O] 2+ active site in O2 and N2O ... more Using UV-Vis and resonance Raman spectroscopy, we identify a [Cu2O] 2+ active site in O2 and N2O activated Cu-CHA that reacts with methane to form methanol at low temperature. The Cu-O-Cu angle (120°) is smaller than for the [Cu2O] 2+ core on Cu-MFI (140°) and its coordination geometry to the zeolite lattice is different. Site-selective kinetics obtained by operando UV-Vis show that the [Cu2O] 2+ core on Cu-CHA is more reactive than the [Cu2O] 2+ site in Cu-MFI. From DFT calculations we find that the increased reactivity of Cu-CHA is a direct reflection of the strong [Cu2OH] 2+ bond formed along the H-atom abstraction reaction coordinate. A systematic evaluation of these [Cu2O] 2+ cores reveals that the higher O-H bond strength in Cu-CHA is due to the relative orientation of the two planes of the coordinating bidentate O-Al-O T-sites that connect the [Cu2O] 2+ core to the zeolite lattice. This work along with our earlier study (J.
Dalton Transactions, 2020
Strategies for further research and developments on active sites in Fe-zeolites for redox catalysis.
Chemistry of Materials, 2019
Proceedings of the National Academy of Sciences, 2018
Significance Fe zeolites are heterogeneous catalysts that show potential in a number of important... more Significance Fe zeolites are heterogeneous catalysts that show potential in a number of important industrial applications, including the selective partial oxidation of methane to methanol at room temperature, and the selective conversion of benzene to phenol. There are practical limitations associated with Fe-zeolite catalysts that may be resolved with mechanistic insight; however, reliable experimental data on Fe zeolites are limited. This study defines the mechanism of selective benzene hydroxylation catalyzed by Fe zeolites, clarifying the relationship between active site structure and catalytic performance (activity, selectivity). Mechanistic insight from this study represents an important step toward synthetic control over function in selective hydrocarbon oxidation catalysis.
Journal of the American Chemical Society, Jan 26, 2018
The formation of single-site α-Fe in the CHA zeolite topology is demonstrated. The site is shown ... more The formation of single-site α-Fe in the CHA zeolite topology is demonstrated. The site is shown to be active in oxygen atom abstraction from NO to form a highly reactive α-O, capable of methane activation at room temperature to form methanol. The methanol product can subsequently be desorbed by online steaming at 200 °C. For the intermediate steps of the reaction cycle, the evolution of the Fe active site is monitored by UV-vis-NIR and Mössbauer spectroscopy. A B3LYP-DFT model of the α-Fe site in CHA is constructed, and the ligand field transitions are calculated by CASPT2. The model is experimentally substantiated by the preferential formation of α-Fe over other Fe species, the requirement of paired framework aluminum and a MeOH/Fe ratio indicating a mononuclear active site. The simple CHA topology is shown to mitigate the heterogeneity of iron speciation found on other Fe-zeolites, with FeO being the only identifiable phase other than α-Fe formed in Fe-CHA. The α-Fe site is forme...
Proceedings of the National Academy of Sciences of the United States of America, May 2, 2018
Iron-containing zeolites exhibit unprecedented reactivity in the low-temperature hydroxylation of... more Iron-containing zeolites exhibit unprecedented reactivity in the low-temperature hydroxylation of methane to form methanol. Reactivity occurs at a mononuclear ferrous active site, α-Fe(II), that is activated by NO to form the reactive intermediate α-O. This has been defined as an Fe(IV)=O species. Using nuclear resonance vibrational spectroscopy coupled to X-ray absorption spectroscopy, we probe the bonding interaction between the iron center, its zeolite lattice-derived ligands, and the reactive oxygen. α-O is found to contain an unusually strong Fe(IV)=O bond resulting from a constrained coordination geometry enforced by the zeolite lattice. Density functional theory calculations clarify how the experimentally determined geometric structure of the active site leads to an electronic structure that is highly activated to perform H-atom abstraction.
Chemical reviews, Jan 14, 2018
Metal-exchanged zeolites are a class of heterogeneous catalysts that perform important functions ... more Metal-exchanged zeolites are a class of heterogeneous catalysts that perform important functions ranging from selective hydrocarbon oxidation to remediation of NOpollutants. 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 id...
Inorganic Chemistry, 2017
α-Fe is the precursor of the reactive Fe(IV)=O core responsible for methane oxidation in Fe-conta... more α-Fe is the precursor of the reactive Fe(IV)=O core responsible for methane oxidation in Fe-containing zeolites. To get more insight into the nature and stability of α-Fe in different zeolites, the binding of Fe(II) at six-membered cation exchange sites (6MR) in ZSM-5, zeolite beta and ferrierite was investigated using DFT and multi reference ab initio methods (CASSCF/CASPT2). CASPT2 ligand field (LF) excitation energies of all sites were compared with the experimental DR-UV-vis spectra reported by Snyder et al. [Nature, 536 (7616):317-321, 2016]. From this comparison it is concluded that the 16 000 cm-1 band of α-Fe, observed in all three zeolites, can uniquely be assigned to a high spin square planar (SP) Fe(II) located at a 6MR with Al-Si-Si-Al sequence, where the Al are positioned opposite in the ring and as close to each other as possible. The stability of such conformations is also confirmed by the binding energies obtained from DFT. The bands at 10 000 cm-1 in the experimental spectra, assigned to spectator Fe(II), are attributed to six coordinated trigonal prismatic Fe(II) species, as calculated for the γ-site in ZSM-5. The entatic effect of the zeolite lattice on the stability of the SP sites was investigated making use of an unconstrained Fe(II) model complex FeL 2 (with L = [Al(OH) 4 ]-). The SP conformer is approximately 2 kcal/mol more stable than the tetrahedral one, indicating that the SP coordination environment of α-Fe is not imposed by the zeolite lattice but rather electronically preferred by Fe(II) in the environment of four O ligands. A significant contribution to the stability of the SP conformer is provided by mixing of the doubly occupied 3d z 2 orbital with the higher lying 4s.
Nature, 2016
An efficient catalytic process for converting methane into methanol could have far-reaching econo... more An efficient catalytic process for converting methane into methanol could have far-reaching economic implications. Iron-containing zeolites (microporous aluminosilicate minerals) are noteworthy in this regard, having an outstanding ability to hydroxylate methane rapidly at room temperature to form methanol 1-3. Reactivity occurs at an extra-lattice active site called α-Fe(ii), which is activated by nitrous oxide to form the reactive intermediate α-O 4,5 ; however, despite nearly three decades of research 5 , the nature of the active site and the factors determining its exceptional reactivity are unclear. The main difficulty is that the reactive species-α-Fe(ii) and α-O-are challenging to probe spectroscopically: data from bulk techniques such as X-ray absorption spectroscopy and magnetic susceptibility are complicated by contributions from inactive 'spectator' iron. Here we show that a site-selective spectroscopic method regularly used in bioinorganic chemistry can overcome this problem. Magnetic circular dichroism reveals α-Fe(ii) to be a mononuclear, high-spin, square planar Fe(ii) site, while the reactive intermediate, α-O, is a mononuclear, high-spin Fe(iv)=O species, whose exceptional reactivity derives from a constrained coordination geometry enforced by the zeolite lattice. These findings illustrate the value of our approach to exploring active sites in heterogeneous systems. The results also suggest that using matrix constraints to activate metal sites for function-producing what is known in the context of metalloenzymes as an 'entatic' state 6-might be a useful way to tune the activity of heterogeneous catalysts. No spectroscopic feature of α-Fe(ii) has thus far been discovered 7. We have now identified Fe(ii) ligand-field bands in the diffuse reflectance ultraviolet-visible (DR-UV-vis) spectra of three iron-containing zeolites (Fe-zeolites) that are known to contain α-Fe(ii) (see Extended Data Fig. 1) 4,8-10. Of these, we chose the Fe(ii)-beta (BEA)
Journal of the American Chemical Society, 2021
α-Fe(II) active sites in iron zeolites catalyze N 2 O decomposition and form highly reactive α-O ... more α-Fe(II) active sites in iron zeolites catalyze N 2 O decomposition and form highly reactive α-O that selectively oxidizes unreactive hydrocarbons, such as methane. How these α-Fe(II) sites are formed remains unclear. Here different methods of iron introduction into zeolites are compared to derive the limiting factors of Fe speciation to α-Fe(II). Postsynthetic iron introduction procedures on small pore zeolites suffer from limited iron diffusion and dispersion leading to iron oxides. In contrast, by introducing Fe(III) in the hydrothermal synthesis mixture of the zeolite (one-pot synthesis) and the right treatment, crystalline CHA can be prepared with >1.6 wt % Fe, of which >70% is α-Fe(II). The effect of iron on the crystallization is investigated, and the intermediate Fe species are tracked using UV−vis-NIR, FT-IR, and Mossbauer spectroscopy. These data are supplemented with online mass spectrometry in each step, with reactivity tests in α-O formation and with methanol yields in stoichiometric methane activation at room temperature and pressure. We recover up to 134 μmol methanol per gram in a single cycle through H 2 O/CH 3 CN extraction and 183 μmol/g through steam desorption, a record yield for iron zeolites. A general scheme is proposed for iron speciation in zeolites through the steps of drying, calcination, and activation. The formation of two cohorts of α-Fe(II) is discovered, one before and one after high temperature activation. We propose the latter cohort depends on the reshuffling of aluminum in the zeolite lattice to accommodate thermodynamically favored α-Fe(II).