Atomic Level Control over Surface Species via a Molecular Precursor Approach:  Isolated Cu(I) Sites and Cu Nanoparticles Supported on Mesoporous Silica

Abstract

There is a growing interest in the preparation of catalysts with well-defined and isolated sites, as such systems are ideally suited for structure-function investigations. 1 Recent studies indicate that atomically dispersed metals on high surface area silica materials are effective catalysts. [2][3][4] Thus, an important goal in catalysis research is the development of synthetic methods that provide atomic-level control over the nature of catalytic sites, with formation of isolated single sites or homogeneously distributed and more complicated structures (bimetallic sites, clusters, nanoparticles, etc.) providing new and improved catalysts. [1][2][3][4] It is believed that low-temperature approaches will be most effective for generating tailored structures because of the metastable nature of many of the desired structures. 1 Supported Cu-containing catalysts have been studied for a number of industrially relevant reactions, 5-8 but little effort has been devoted to the generation of tailored sites. The introduction of isolated species onto mesoporous silica using molecular precursors of the form M[OSi(O t Bu) 3 ] n (M ) Ti 3a and Fe 3b ) under nonaqueous conditions has recently been reported. The work presented herein provides direct evidence for atomic-level control over the nature of sites that result from grafting 9 Cu-containing molecular precursors onto the mesoporous silica SBA-15. 10 Reaction of the molecular precursors [CuOSi(O t Bu) 3 ] 4 11 (1) or [CuO t Bu] 4 12 (2) (as solutions in C 6 H 6 ) with the surface hydroxyl groups of SBA-15 under inert conditions provided grafted materials, isolated as light yellow powders after extensive washing with C 6 H 6 and drying in vacuo at 323 K. Quantities of the molecular precursors were used such that the final grafted materials contained ca. 3.5% (CuOSi/SBA(3.5) and CuO t Bu/SBA(3.5) from 1 and 2, respectively) and ca. 5.0% (CuOSi/SBA(5.0) and CuO t Bu/SBA(5.0) from 1 and 2, respectively) Cu by weight. 13 It is known that M[OSi-(O t Bu) 3 ] n species cleanly evolve 3nCH 2 C(CH 3 ) 2 and 3 / 2 nH 2 O upon mild thermolysis (373 to 473 K) to give MSi n O y materials. 1a,14 Previous studies of 1 indicate that it also exhibits loss of CH 2 C-(CH 3 ) 2 and H 2 O upon heating, although some reduction of Cu (to Cu metal) is evident under an inert atmosphere. 11 It was anticipated that grafting 1 onto SBA-15 would provide species that could eliminate CH 2 C(CH 3 ) 2 and H 2 O upon thermolysis to form Si-O surface linkages while maintaining some of the original Cu-OSi linkages to provide stabilized Cu(I) species (in the form of isolated tetramers or as single sites). Use of 2 in similar grafting reactions may also provide isolated sites; however, no additional Si-O surface linkages (and hence extra site stabilization) are provided by this precursor. Scheme 1 illustrates a possible grafting pathway and potential structure types for the resulting surface species.The molecular precursors are presumably well-separated on the surface, as the "OH" site-to-precursor ratios ...

Low-temperature fabrication of Cu(i) sites in zeolites by using a vapor-induced reduction strategy

“…[5][6][7] Cu(I) ionexchanged zeolites show high potential in a variety of applications such as deep desulfurization of fuels, adsorptive separation of olens/paraffins, NO decomposition, and catalytic synthesis of dimethyl carbonate. [13][14][15] This interesting method can achieve high Cu(I) loading, while high preparation temperatures (ca. For the rst method of direct introduction of Cu(I), both vapor phase ion exchange (VPIE) and solid state ion exchange (SSIE) have been employed.…”

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

“…[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). [20,21] Surface organometallic chemistry (SOMC) [22,23] combined with thermolytic molecular precursors (TMP) [24] has emerged as apowerful approach to generate supported isolated metal sites,for abroad range of metals,with tailored nuclearity and oxidation state. [16,17] Amajor challenge in these systems is associated with the presence of as mall fraction of active sites (often below 30-60 %; note that as ystem with 90 %a ctive-site-0.47 mol CH 3 OH/mol Cu-was recently reported.…”

“…[15] Recent theoretical studies also suggest that monomeric Cu sites should not be excluded. [20,21] Surface organometallic chemistry (SOMC) [22,23] combined with thermolytic molecular precursors (TMP) [24] has emerged as apowerful approach to generate supported isolated metal sites,for abroad range of metals,with tailored nuclearity and oxidation state. [18,19] Reaching an unequivocal conclusion regarding the active sites in these systems is therefore challenging.…”

“…Cu supported on non-zeolitic materials (namely amorphous silica) has also demonstrated reactivity for this transformation, indicating that confinement is not required;h owever,t he CH 3 OH yield does not exceed 3.6 %CH 3 OH per Cu. [20,21] Surface organometallic chemistry (SOMC) [22,23] combined with thermolytic molecular precursors (TMP) [24] has emerged as apowerful approach to generate supported isolated metal sites,for abroad range of metals,with tailored nuclearity and oxidation state. [25] This approach consists of grafting tailored molecular precursors on supports with ac ontrolled -OH density,f ollowed by ap ost-treatment that removes organic ligands and generates the desired isolated metal sites.S ince nuclearity and local environment are both key features for the selective CH 4 to CH 3 OH conversion on Cu sites,wereasoned that the SOMC/TMP approach could be ideal for generating monomeric Cu II sites and evaluating their reactivity for this reaction.…”

Syntheses of PAM microgel surfacely covered with alkyl quaternary ammonium surfactant/Keggin‐type polyoxometalate complexes

“…Although many efficient procedures based on the tungsten catalysts for the oxidation with H 2 O 2 have been developed, most of them are homogeneous systems and share common drawbacks, that is, catalyst/product separation and catalyst reuse are difficult. The immobilization of catalytically active species onto solid supports can solve the catalyst recovery and recycle 4–8. For the development, some heterogeneous systems such as insoluble polyoxotungstates,5 immobilized peroxotungstates,9 triphasic phosphotungstate,10 or pseudo‐heterogeneous systems,11 have been reported.…”