Article pubs.acs.org/cm
Effect of Defect Site Preorganization on Fe(III) Grafting and Stability: A Comparative Study of Delaminated Zeolite vs Amorphous Silica Supports Nicolás A. Grosso-Giordano,§ Alexander J. Yeh,§,† Alexander Okrut,§ Dianne J. Xiao,∥,# Fernande Grandjean,‡ Gary J. Long,*,‡ Stacey I. Zones,*,⊥ and Alexander Katz*,§ §
Department of Chemical and Biomolecular Engineering and ∥Department of Chemistry, University of California at Berkeley, Berkeley, California 94720, United States ‡ Department of Chemistry, Missouri University of Science and Technology, University of Missouri, Rolla, Missouri 65409-0010, United States ⊥ Chevron Energy Technology Company, Richmond, California 94801, United States S Supporting Information *
ABSTRACT: The stabilization of isolated grafted Fe3+ sites on siliceous supports is investigated by a comparative study of crystalline versus amorphous materials. Our synthetic approach treats crystalline delaminated zeolite DZ-1 and amorphous silica (SiO2) with an aqueous NaFeEDTA cation precursor complex, to result in grafting of isolated Fe3+ sites via covalent attachment to support hydroxyl groups. Thermogravimetric analysis and UV−visible spectroscopy demonstrate the complete detachment of chelating EDTA ligand upon Fe3+ grafting on both supports. Before calcination treatment, both Fe/DZ-1 and Fe/SiO2 have similar UV−visible spectral features, with absorption bands at 208−225 and 257 nm, characteristic of framework Fe3+ sites in zeolites. Calcination does not affect the UV−visible spectroscopic characteristics of Fe/DZ-1 but changes the spectrum of Fe/SiO2 to a single absorption band at 260 nm, indicating better thermal stability of Fe3+ sites in Fe/DZ-1 as compared to Fe/SiO2. This stability persists for Fe/DZ-1 even during alkane oxidation catalysis in the presence of hydrogen peroxide, which causes aggregation of Fe3+ into oxide oligomers for Fe/SiO2. 57Fe Mössbauer spectroscopy of calcined materials indicates a more uniform distribution of sites in Fe/DZ-1 relative to Fe/SiO2. We thus attribute the greater robustness and site uniformity of Fe/DZ-1 to the chelation of Fe3+ by the rigid crystalline silicate DZ-1 framework, engendered by the spatial preorganization of grafting hydroxyls groups within its uniform defect sites, which are templated by framework B3+ removal during delamination. Such preorganization enables cooperativity between neighboring hydroxyl groups. This contrasts with more randomly distributed hydroxyl groups on SiO2, which lack such preorganization, leading to decreased hydrothermal stability and an Fe3+ grafting density that is ∼7-fold lower for Fe/SiO2 relative to Fe/DZ-1. These observations reveal how the silicate surface onto which a cation is grafted can act as a relevant ligand, capable of controlling material synthesis and functionality akin to ligands in homogeneous metal complexes, and demonstrate the advantages of support crystallinity in having this ligand be hydrothermally stable and tunable via templating.
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INTRODUCTION The grafting of cations in precise oxo-ligand environments on metal-oxide supports is central to preparing functional materials for molecular recognition, sensing, and heterogeneous catalysis. In these materials, isolated cations uniformly distributed on the support are typically desired, as their coordinative unsaturation facilitates substrate bonding and activation. However, achieving such isolation is synthetically challenging, as cations tend to agglomerate into bulk-oxide phases during deposition, activation, and application, leading to the degradation of functional properties.1−7 Many synthetic approaches have used amorphous silica as a convenient support with tunable textural properties,8 while focusing on controlling cation incorporation through careful choice of the ligand sphere of soluble cationic metal precursors that are grafted onto supports.8−14 No efforts, however, have fully addressed the implications that the grafting © 2017 American Chemical Society
ligand environment, defined by the silicate network anchoring the cation to the support, may have on materials synthesis and application.15 In this manuscript, we address these implications from the perspective of incorporating Fe3+ sites on two different siliceous supports, consisting of amorphous silica (SiO2) vs crystalline delaminated zeolite DZ-1. This general class of supported iron oxide materials has found widespread application in alkane oxidation catalysis, with active sites generally attributed to isolated Fe3+ species.4,5,12,16−18 Our synthetic approach is outlined in Scheme 1 and involves treating each support with aqueous NaFeEDTA. Through a comparative study of the Received: May 18, 2017 Revised: June 28, 2017 Published: July 10, 2017 6480
DOI: 10.1021/acs.chemmater.7b02062 Chem. Mater. 2017, 29, 6480−6492
Article
Chemistry of Materials Scheme 1. Synthetic Protocols for the Preparation of Supports and Fe3+ Containing Materialsa
12-MR “cups” represent the upper rim of 12-member ring openings/pockets on the DZ-1 surface; their location within the full MWW zeolitic topology of DZ-1 is shown at the bottom right (MWW layer). Vertices represent individual Si4+ ions at the center of SiO4/2 tetrahedra that constitute the zeolite framework, while lines represent Si−O−Si bonds. Possibly coordinated water is not shown. Zeolite layers are schematically shown as rectangles; delamination causes extensive disordering of these layers and increases the accessible external surface of the zeolite support in DZ-1. Reaction of the NaFeEDTA precursor with silanol ligands on the surface of the support releases EDTA and grafts isolated Fe3+ sites onto the support, as a surface grafted cation in Fe/SiO2 and as a framework incorporated cation in Fe/DZ-1. a
organization of silanol grafting sites and mechanical stabilities of the silica network when comparing crystalline vs amorphous silicates. Typically, compared with the accessible mesoporosity of amorphous silica, zeolitic materials are microporous and consist
resulting Fe/SiO2 and Fe/DZ-1 materials, our goal is to elucidate how the grafting environment influences the Fe3+ incorporation process, the grafted Fe3+ species, and its activity and stability under catalytic application. We anticipate this grafting environment to be defined by the contrasting 6481
DOI: 10.1021/acs.chemmater.7b02062 Chem. Mater. 2017, 29, 6480−6492
Article
Chemistry of Materials
pioneering work of Gates comparing structurally uniform grafted metal complexes in crystalline zeolites to nonuniform ones in amorphous inorganic-oxide supports.34−36 Such uniformity enables more precise control over the design and synthesis of active sites with optimal properties and facilitates fundamental structure−function studies.37 In general, uniformly isolated and stable Fe3+ sites cannot be incorporated onto amorphous SiO2 when simple inorganic Fe3+ salts are used as precursors because unconstrained reaction of Fe3+ from solution with supported sites leads to extensive formation of FeOx phases.12 Instead, successful synthesis has relied upon molecular precursors that are either grafted onto the support by condensation with surface hydroxyl groups10−13 or by support impregnation with protected/multidentate Fe3+ complexes.4,5 In these approaches, the use of bulky organic ligands, such as chelating ethylenediaminetetraacetate (EDTA), has been found to prevent the formation of undesired oxide agglomerates, by a mechanism involving both sterically limiting the approach and hindering the mutual reaction of Fe3+ precursors.4,5 While these elegant strategies demonstrate uniform site isolation and high activity in the catalytic oxidation of hydrocarbons with H2O2, the stability of these sites during catalytic oxidation was not investigated, even though corrosive catalytic conditions are known to cause leaching and agglomeration of grafted Fe3+ sites, leading to catalyst deactivation.7 An additional challenge in these approaches is that full tripodal connectivity of Fe3+ to the support has necessitated oxidative heat treatment for affecting organic-ligand removal via combustion,4,5,12,13 a treatment that may also contribute toward generation of FeOx agglomerates.5 In the absence of this treatment, chelation of Fe3+ by EDTA would prevent Fe3+ connectivity to siloxy ligands anchoring it to the support (no exchange in Scheme 2, bottom).5 In our synthetic approach,
of SiO2 frameworks with well-defined crystallographic topologies. Micropores (
