Subscriber access provided by United Arab Emirates University | Libraries Deanship
Article
Quantum Chemical Characterization of Structural Single Fe(II) Sites in MIL-Type Metal Organic Frameworks for Oxidation of Methane to Methanol and Ethane to Ethanol Jenny G. Vitillo, Aditya Bhan, Christopher J. Cramer, Connie C Lu, and Laura Gagliardi ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b04813 • Publication Date (Web): 10 Jan 2019 Downloaded from http://pubs.acs.org on January 11, 2019
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 11 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Catalysis
Quantum Chemical Characterization of Structural Single Fe(II) Sites in MIL-Type Metal Organic Frameworks for Oxidation of Methane to Methanol and Ethane to Ethanol Jenny G. Vitillo,a* Aditya Bhan,b Christopher J. Cramer,a Connie C. Lua and Laura Gagliardia* aDepartment
of Chemistry, Chemical Theory Center, and Supercomputing Institute, University of Minnesota, 207 Pleasant Street S.E., Minneapolis, Minnesota 55455, United States. bDepartment of Chemical Engineering and Materials Science, University of Minnesota, 421 Washington Avenue S.E., Minneapolis, Minnesota 55455, United States. KEYWORDS. MOFs, C-H bond activation, catalysis, non-heme iron, density functional theory, multireference methods, MIL-100. ABSTRACT: Single non-heme Fe(II) ions present as structural moieties in several metal-organic frameworks (e.g. MIL-100, MIL101, MIL-808) are identified by Kohn-Sham density functional calculations as promising catalysts for C-H bond activation, with energetic barriers as low as 40 kJ mol-1 for ethane and 60 kJ mol-1 for methane following oxidative activation of iron. The rate determining step is the activation of N2O, with a barrier of 140 kJ mol-1. Through consideration of the full reaction profile leading to the corresponding alcohols ethanol and methanol, we have identified key changes in the chemical composition of the node that would modulate catalytic activity. The thermal and chemical stability of these materials together with the scalability of their syntheses make them attractive catalysts for the selective low temperature conversion of light alkanes to higher-value oxygenates.
and pressures are employed to improve yields. A particular challenge, however, is that the C-H bonds of initially oxidized species, e.g. alcohols are more reactive than those in the starting light alkanes,3 making it difficult to suppress overoxidation.1, 4 In the metalloenzyme arena, methane monoxygenases are able to selectively convert methane to methanol at room temperature and atmospheric pressure exploiting one or more copper or two iron atoms in their active sites,5-6 but progress has been slow in translating the selectivity of the enzymes into simpler synthetic systems.7
Conversion of gaseous light hydrocarbons like methane and ethane to liquid fuels is a pivotal missing piece for the full economic exploitation of biogas and natural gas.1 Such conversion would also have important environmental repercussions, as it would reduce gas flaring in oil fields (estimated to exceed 140 billion cubic meters per year),2 which occurs because transportation of natural gas, in absence of a dedicated pipeline, is not economically viable. Efficient catalysts able to convert methane and ethane (the two major components of natural gas) into liquid, or solid, chemical feedstock would thus constitute a turning point in the worldwide energetic supply. Given the importance of this goal, many efforts to catalyze such reactions have been undertaken, but no catalyst to date has advanced to the level of having clear potential in an industrial-scale process.2
From the synthetic point of view, high surface area supports have been extensively studied as matrices for the introduction of biomimetically inspired active sites.8 Among these species, iron based zeolites are particularly intriguing for being able to catalyze the hydroxylation of methane at room temperature,5, 9 with activity attributable to single Fe(II) sites (-Fe(II) sites) characterized by high-spin ground state, constrained coordination geometries and a minimal number of ligands around Fe.5, 10-11 These three characteristics have been determined to be key for activity. The mononuclearity of the Fe active sites distinct in reference to other metal-exchanged zeolite and MOF catalysts,12 especially those based on copper where the active species have been proven to be di- or trinuclear metallic sites.2, 13-15 However in zeolites, the active ions are hosted as extra-framework species. Thus, although the hosting material is crystalline, the Fe ions are disordered and multiple species are present in the solid.9 The relative amounts of those species and their nature can be strongly dependent on the synthetic procedure.9, 16-17 Besides decreasing the selectivity of the reaction, such disorder makes the isolation and characterization of the reacting species particularly difficult, with inactive spectator ions often the majority of deposited Fe. Indeed, the structure of the -Fe(II) species was only recently elucidated after many years of effort.11
C-H bonds in alkanes are apolar implying both heterolytic and homolytic scission pathways require aggressive reaction conditions which bring forth kinetic challenges in selective C-H functionalization. Selective cleavage of methane and ethane C-H bonds (enthalpies associated to the homolytic scission: 439 and 423 kJ mol-1, respectively at room temperature)3 thus require highly active oxidation catalysts and in many instances high temperatures
Metal organic frameworks (MOFs) comprise a relatively new class of materials that has begun to demonstrate its utility for various applications.18 One of their hallmarks is an intrinsic flexibility in design owing to their construction as sets of inorganic nodes interconnected by organic linkers. Thus, modulation of their chemical and physical properties can be achieved by changing their building units. Moreover, most MOFs are crystalline solids, which
1. INTRODUCTION
ACS Paragon Plus Environment
ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
is a characteristic of primary importance for materials design and MOFs have been exploited as catalysts for many different reactions.19 In analogy to the zeolites discussed above, extraframework iron species in MOFs have been demonstrated to be active for C-H bond cleavage20-22 although this extra-framework character poses the same challenges to characterization and optimization that have already been discussed.21 In other instances, iron species at structural positions of inorganic MOFs building units exhibit catalytic function with high selectivity (>80%) under mild conditions, e.g. for the conversion of ethane to ethanol for Fe0.1Mg1.9(dobdc)2 (also called MOF-74-(Mg,Fe), see Figure S8)2324 and methane to methanol for MIL-53-(Al,Fe), see Figure S9) (MIL, Materials of Institute Lavoisier).25-26 The iron species in these MOFs have different oxidation states, Fe(II) for MOF-74 and Fe(III) in MIL-53, and consequently their mechanisms may be expected to differ but these catalysts nevertheless share common features: in both cases, the active sites are single, non-heme iron species in octahedral environments, obtained through high dilution of the iron atoms into the otherwise inert framework established with a different structural metal (note that “structural” implies that the metal is an intrinsic component of the MOF itself, not deposited post-synthetically). While promising, these materials offer room for improvement. Fe0.1Mg1.9(dobdc)2 is not stable in air for long time.27-28 While MIL-53-(Al,Fe) does not suffer from similar instability,28 it has been synthesized only in low yield using an electrochemical approach.25 Finally, the inorganic components of both MOF-74 and MIL-53 consist of one dimensional chains of coordinated metal sites.23, 25 So, in order to better isolate the iron atoms, in both cases the loading in the active Fe has to be kept very low (
