Surface structural-chemical characterization of a single-site d ⁰ heterogeneous arene hydrogenation catalyst having 100% active sites

Abstract

Structural characterization of the catalytically significant sites on solid catalyst surfaces is frequently tenuous because their fraction, among all sites, typically is quite low. Here we report the combined application of solid-state 13 C-cross-polarization magic angle spinning nuclear magnetic resonance ( 13 C-CPMAS-NMR) spectroscopy, density functional theory (DFT), and Zr X-ray absorption spectroscopy (XAS) to characterize the adsorption products and surface chemistry of the precatalysts (η 5 O rganometallic molecule-derived heterogeneous catalysts are of increasing interest owing to their enhanced thermal stability and activity vs. their homogeneous analogs, and their atomically precise tailorable metal-ligand structures vs. other heterogeneous catalysts (1, 2). Furthermore, it is becoming increasingly evident that the inorganic support in many systems is noninnocent and can function as both a ligand and an activator, with the chemically important but poorly understood nature of the catalyst-support interaction strongly modulating catalytic activity and selectivity (3, 4). When adsorbed on Lewis acidic, dehydroxylated alumina surfaces, group 4 complexes such as Cp 2 ZrR 2 (Cp = η 5 -C 5 H 5 ; A, R = H; B, R = CH 3 ) and Cp*Zr (CH 3 ) 3 [C, Cp* = η 5 -C 5 (CH 3 ) 5 ] were argued on the basis of highresolution solid-state NMR spectroscopy to transfer an alkyl anion to unsaturated, Lewis acidic surface sites as in Fig. 1 (complexes B, C → qualitative model D) (5, 6). The resulting catalysts are extremely active for olefin hydrogenation and polymerization, and analogous ion-paired species form the basis for large-scale industrial polymerization processes (7,8). However, kinetic poisoning experiments in which the catalytic sites are titrated in situ with H 2 O or t BuCH 2 OH indicate that ≤5% of D-type sites are catalytically significant, likely reflecting, among other factors, the established heterogeneity of alumina surfaces (5, 6, 9), hence rendering active site structural and chemical descriptions necessarily imprecise. In contrast to these results, chemisorption of such organozirconium precursors on SiO 2 , Al 2 O 3 , and SiO 2 -Al 2 O 3 surfaces having appreciable coverage by weakly acidic OH groups predominantly yields covalently bound, poorly electrophilic Etype species via Zr-CH 3 protonolysis with CH 4 evolution (5, 6, 10, 11). Although the E-type sites may be characterized in some detail by high-resolution solid-state NMR and extended X-ray adsorption fine structure spectroscopy (EXAFS), they display minimal catalytic turnover in the absence of added, complicating activators [e.g., methylalumoxane or B(C 6 F 5 ) 3 ], and the fraction of catalytically significant sites is unknown (12, 13). In such situations, it is experimentally impossible to unambiguously distinguish catalytically significant sites from inactive "spectator" sites, hence to fully understand the catalytic chemistry.-In marked contrast to the above results, chemisorption of these same organozirconium molecules on highly Brønsted "super...

Single‐Face/All‐cis Arene Hydrogenation by a Supported Single‐Site d0 Organozirconium Catalyst

“…For o-xylene hydrogenation ( Table 1, entry 1), as ingle product is observed, which, by comparison with commercially available dimethylcyclohexanes,i sd etermined to be cis-1,2-dimethyl-cyclohexane (Figure 2A and D). [12][13][14]17] Likewise, m-a nd p-xylene hydrogenated under identical conditions yield exclusively the respective cis-fully hydrogenated products (Table 1; Figure 2B and E, C and F). [12][13][14]17] Likewise, m-a nd p-xylene hydrogenated under identical conditions yield exclusively the respective cis-fully hydrogenated products (Table 1; Figure 2B and E, C and F).…”

“…No partial hydrogenation products such as o-dimethylcyclohexadienes or o-dimethylcyclohexenes are detected, in agreement with the aforementioned benzene hydrogenation results. [12] B) 13 C{ 1 H} spectra of the cyclohexane derived from Cp*ZrMe 2 /ZrS (top) and Cp*ZrBz 2 /ZrS (bottom) catalyzed hydrogenation of C 6 D 6 .S ymbols x and od enote the two isotopomers. Initial turnover frequencies (TOFs) fall in the order oxylene > m-xylene > p-xylene.…”

“…Owing to the versatility of alcohols and ketones as synthetic intermediates,t hese products can then be modified in many ways to afford avariety of valuable substances. [11] Our group recently reported the kinetics and reaction pathways by which benzene and toluene hydrogenation are catalyzed by Cp*ZrR 3 complexes (Cp* = h 5 -C 5 (CH 3 ) 5 ;R= Me, [12] CH 2 Ph; [13] Figure 1A)chemisorbed on Brønsted acidic sulfated oxides (Cp*ZrR 2 /MS, M = Zr,Al). [10] In particular, electrophilic d 0 catalysts supported on "super" Brønsted acids with large percentages of catalytically significant sites (70 % À98 %) exhibit high activity in technologically relevant olefin polymerization and hydrogenation processes.…”

On the dynamical nature of the active center in a single-site photocatalyst visualized by 4D ultrafast electron microscopy

“…ultrafast electron microscopy | single-site photocatalysis | titanium based photocatalyst | ulatrafast phenomena | time-resolved microscopy S ingle-site catalysts of both the thermally and photoactivated kind now occupy a prominent place in industrial-and laboratory-scale heterogeneous catalysis (1)(2)(3)(4)(5)(6)(7)(8). Among the most versatile of these are the ones consisting of coordinatively unsaturated transition metal ions (Ti, Cr, Fe, Mn.…”

Single atom catalysts on amorphous supports: A quenched disorder perspective

“…Nevertheless, some closely related materials involving amorphous supports have recently been described as single-site catalysts. [10][11][12][13][14] While this is appropriate in the sense that each site is independent of the others, amorphous supports present many slightly different grafting environments for metal ions, so the sites are unlikely to have identical reactivities. Taylor and others recognized very early that even small structural differences between sites can lead to important activity differences.…”