Search

Covalent immobilization of molecular catalysts onto metal oxide surfaces through linker groups is a common strategy for heterogenizing homogeneous catalysts with the expectation that the immobilized catalyst will have properties similar to those of its molecular counterpart. However, the catalytic properties of the immobilized species are often quite different compared to their soluble counterparts in ways that are difficult to predict. This phenomenon is poorly understood and could be due to a variety of factors, including steric shielding of the complex by the surface, changes to the coordination sphere upon immobilization, or a lack of conformational flexibility of the immobilized complexes. Here, we tested the effect of surface immobilization on the catalytic activity and selectivity of atom transfer radical additions and cyclizations. In this study, we varied the proximity of the phosphonate anchoring group to the Cu center by attachment at varying positions of chelating nitrogen ligands such as 1,10-phenanthroline (phen), tris(pyridylmethyl)amine, and 2,9-dimethyl-1,10-phenanthroline as ligand scaffolds. Catalytic testing revealed that in cases where the anchoring group is remote from the catalytic center, as is the case for Cu(phen), the immobilized catalyst functions overall slightly better than its homogeneous counterpart (resulting in higher yields). However, for complexes in which the linker group is close to the active center, the catalytic performance of the immobilized complex was generally worse when immobilized than when in solution (decreased yield upon immobilization). Potential explanations of these observations are discussed. This study very clearly demonstrates the highly complex nature of immobilized catalysts and highlights the need for more in-depth comparisons between immobilized and soluble organometallic catalysts.

Organometallic complexes are frequently deposited on solid surfaces, but little is known about how the resulting complex–solid interactions alter their properties. Here, a series of complexes of the type Cu(dppf)(Lx)+(dppf = 1,1′-bis(diphenylphosphino)ferrocene, Lx= mono- and bidentate ligands) were synthesized, physisorbed, ion-exchanged, or covalently immobilized on solid surfaces and investigated by 31P MAS NMR spectroscopy. Complexes adsorbed on silica interacted weakly and were stable, while adsorption on acidic γ-Al2O3resulted in slow complex decomposition. Ion exchange into mesoporous Na-[Al]SBA-15 resulted in magnetic inequivalence of 31P nuclei verified by 31P-31P RFDR and 1H-31P FSLG HETCOR. DFT calculations verified that a MeCN ligand dissociates upon ion exchange. Covalent immobilization via organic linkers as well as ion exchange with bidentate ligands both lead to rigidly bound complexes that cause broad 31P CSA tensors. We thus demonstrate how the interactions between complexes and functional surfaces determine and alter the stability of complexes. The applied Cu(dppf)(Lx)+complex family members are identified as suitable solid-state NMR probes for investigating the influence of support surfaces on deposited inorganic complexes.

We identify favorable parameters for the formation of aromatics from ethanol-based feeds over ZSM-5. An ethanol partial pressure of 0.3 bar, a reaction temperature of ∼700 K, a low weight hourly space velocity (WHSV), and a high Brønsted acid site density increase the content of aromatics, in the latter case, on the expense of lifetime. The aromatic fraction composition changes with the WHSV and nSi/nAlratio. Water cofeed increases the content of aromatics in the C2:H2O stoichiometry of 1:0.5 (equals diethyl ether as feed) but decreases it if this is exceeded. Diethyl ether and ethylene lead to more aromatics than ethanol feed. The lifetime until deactivation increases in the order ethanol < diethyl ether < ethylene. This points at different reaction and/or deactivation mechanisms in converting the three feeds. Thus, a water-poor feed effectively improves the catalyst lifetime and productivity in the ethanol conversion to aromatics.

In molecular heterogeneous catalysis knowledge about the location and accessibility of the immobilized metal complex inside porous solids is important to assess the catalytic efficiency. Here, we developed a method to quantify accessible Rh complexes in SBA-15 mesopores viasolid state NMR spectroscopy. In order to identify suitable probe molecules several phosphines were screened in solution and their diameters were determined by DFT calculations. The ligation of phosphines at immobilized Rh complexes was then monitored by quantitative 31P MAS NMR spectroscopy. For the first time a polymer too large to enter the mesopores, poly(styrene-co-vinyltriphenylphosphine), PS-co-PVPP, was used to probe selectively Rh complexes immobilized on the external surface, in pore mouths or defects. By combining 31P MAS NMR and ICP-OES the P/Rh ratio was determined revealing that only up to 39% of the immobilized Rh complexes were accessible. Therefore, corrected turnover numbers (TONs) of up to 55 could be calculated for the asymmetric 1,2-addition with immobilized Rh catalysts. With the herein presented methodology the accessibility and spatial distribution of immobilized molecular catalysts can for the first time be evaluated quantitatively.

Trimethylphosphine (TMP) is demonstrated as a suitable 31P MAS NMR probe molecule for determining accessibility, environment, and spatial distribution of oxidation-active oxidic metal species on solid catalysts quantitatively. It oxidizes to trimethylphosphine oxide (TMPO) at oxygen donor sites, which is demonstrated for oxides of copper, manganese, cobalt, and molybdenum. At loadings <2 wt % of Mo a direct quantitative correlation between TMPO quantity and accessible metal oxide content is observed. Exceeding 2 wt % results in a gradual agglomeration and thus decreases the amount of available oxidative sites, probed as a decay of the amount of TMPO formed. Additionally, the spatial distribution of oxides neighboring species could be inferred. The solid TMPO deposited near MoOxspecies was very sensitive to extra framework aluminum (EFAL) as well as Brønsted acid sites in close proximity, depending on Mo loading. Thus, the TMP method provides unprecedented insights into the surface chemistry of oxidative metal oxide catalysts.

We herein investigate methanol adsorbates on a variety of heterogeneous catalysts. We quantitatively desorb methanol from saturated MFI zeolites, SBA-15 materials and silicotungstic acid (STA) supported on silica, all in the respective siliceous, Na- and H-forms. Surface species are identified by 1H and 13C MAS NMR and DRIFTS. On saturated surfaces, we find liquid-like methanol in weak surface interaction. For siliceous materials, adsorption on silanol Si(OH) groups is dominant, especially on materials with amorphous pore walls like SBA-15. Weak methanol binding on microporous silicalite is caused by a repulsive effect due to micropore confinement. For Na-form materials, methanol complexes at Na+counter ions dominate the adsorption of methanol in Na-ZSM-5 micropores. The strong confinement leads to stronger methanol adsorption compared to less confined systems. Without confinement, no complex at Na+is observed and Si(OH) groups dominate adsorption. On H-form materials, methanol complexes at acid sites form in a higher quantity under confinement in H-ZSM-5. After treatment at 423 K, the formation of dimethyl ether (DME) was evidenced by IR and 13C MAS NMR spectroscopy and the acid site proton peak found at δ1H= 14.4 ppm. Si(OH) groups bind methanol stronger than counter ions and acid sites (Na+and H+). This explains why defects and the Si(OH) density influence heterogeneous reactions. Our findings show that confinement in micropores is crucial for the stabilization of methanol complexes at counter ions Na+and acid sites H+.

Water and alcohols like methanol are key substrates in catalysis. Thus, adsorption on catalysts plays a key role in chemical reactions. Herein we investigate how surface sites and confinement influence the formation of water surface species, their adsorption strength, and complex stability. We compare adsorption on microporous MFI zeolites, mesoporous SBA-15 materials and silica A200 as supports for silicotungstic acid (STA) in their isostructural silica, Na-, and H-forms. A systematic variation of confinements, surface sites and adsorbates enables a separation of confinement from other effects. In saturation, the quantity of bound water is determined by the available surface area and maximized for materials with high Si(OH) densities on the surface. The strong binding of water at Si(OH) explains why the Si(OH) density influences heterogeneous catalysis. Complexes between water and counter ions (Na+) or acid sites (H+) form under micropore confinement in Na- and H-ZSM-5. Complexes are absent or decompose quickly on aluminated SBA-15 and STA in Na- or H-forms. More methanol than water molecules bind to counter ions and acid sites on ZSM-5. This explains why acid sites in the methanol-to-olefin conversion are just partially blocked by water. We identify confinement as the key parameter for the formation of surface complexes and for adjusting the efficiency of their protonation. Our finding helps to rationalize reactions with water and/or alcohol reactants.

Carbon recycling reusing emitted carbon dioxide (CO2) as raw materials is nowadays recognized as a concept ahead of CO2capture and storage for controlling its emission to the air. Here, recently discovered alkali-metal-based instantaneous ex situ mineral carbonation under ambient conditions is studied for saponite clay nanoparticles. The cationic concentration of Cs+is varied for seeking the possibility of reusing Cs-contaminated soil in view of carbon recycling. The ex situ mineral carbonation dominantly occurring at the interior surfaces of 10 Å open spaces formed by overlapped nanosheets increases with increasing the Cs concentration up to 0.580 mmol g–1due to the enlargement of the surface area. The ability of the ex situ mineral carbonation, however, deteriorates with further increasing to 0.585 mmol g–1since the 10 Å open spaces are effectively occupied by Cs compounds as Cs2O reducing their fraction. The present findings of the ex situ mineral carbonation relevant to the cationic concentration imply that the practical soil associated with a Fukushima nuclear accident can stabilize CO2without energy consumption as well as chemical solution.

Herein, we describe a method for the quantification of Brønsted acid sites located on surfaces and in pores of hierarchical zeolite catalysts. The probe triphenylphosphine (TPP) accesses only pores bigger 0.72 nm. The signal of protonated TPP is baseline separated from other signals and can be directly quantified by 31P MAS NMR spectroscopy. Results are robust and are not affected by the total TPP loading nor by remaining solvent traces. The error of the Brønsted acid site density evaluation is below ±10% for amorphous silica–alumina and below ±5% for probing crystalline materials like MCM-22 or hierarchical zeolites. On amorphous silica–alumina, only 12.5% of all acid sites were accessible by TPP, which binds near tetrahedral and pentahedral aluminum. The 47 ± 2 μmol/g acid sites on the surface and in pore mouths of zeolite MCM-22 represent 12% of the total acidity. On TNU-9, 2% of the total acidity is located on the surface. On commercial zeolite ZSM-5, no surface acidity was found. Desilication of ZSM-5 and TNU-9 zeolites introduced an additional 20 ± 1 and 29 ± 1 μmol/g of Brønsted acid sites, respectively. These additional acid sites are located in introduced mesopores of hierarchical ZSM-5 and TNU-9 zeolites and account for 6–7% of the total sites present. The location in mesopores can cause undesired byproducts in catalysis due to the absence of shape selectivity effects. The techniques described herein will aid the understanding of the acid site density in hierarchical systems and lead to improvements of catalyst synthesis and performance.

We herein investigate the selective dehydration of ethanol at high conversions (>99.5%) and a moderate reaction temperature of 493 K over a silicotungstic acid (STA) catalyst supported on silica. The variation of STA loadings resulted in a linearly decreasing BET surface area with increasing STA content and the formation of negligible quantities of crystalline STA. In the 1H MAS NMR spectra, two distinct NH4+species were identified after NH3adsorption, i.e.Brønsted acid sites on the external surface of the STA nanoparticles corresponding to a chemical shift of δ1H= 6.6 ppm and Brønsted acid sites within the pseudo-liquid phase (PLP) corresponding to a chemical shift at δ1H= 5.6 ppm. At 36 wt% loading, all Brønsted acid sites were easily accessible, while at higher STA-loadings the intensity at 5.6 ppm increased slowly with exposure time to NH3due to diffusion limitations. The lifetime of the STA catalyst has a maximum at a STA loading of 36 wt% exhibiting a TOS of 15.7 h. Impregnation with CsOH led to a slightly increased BET surface area. The preferred inhibition of external Brønsted acid sites is reflected by a decreasing intensity of the 6.6 ppm 1H MAS NMR peak of NH4+ions. The inhibition increases the lifetime to up to 30 h at conversions above 99.5%, as a result of the suppressed formation of higher olefins as by-products. Thus, the impregnation with CsOH is an efficient way to reduce external Brønsted acid sites of supported STA catalysts and leads to a longer lifetime and selectivity in the ethanol dehydration process under industrially relevant conditions.

The synthesis of macrocycles is severely impeded by concomitant oligomer formation. Here, we present a biomimetic approach that utilizes spatial confinement to increase macrocyclization selectivity in the ring-closing metathesis of various dienes at elevated substrate concentration up to 25 mM using an olefin metathesis catalyst selectively immobilized inside ordered mesoporous silicas with defined pore diameters. By this approach, the ratio between macro(mono)cyclization (MMC) product and all undesired oligomerization products (O) resulting from acyclic diene metathesis polymerization was increased from 0.55, corresponding to 35% MMC product obtained with the homogeneous catalyst, up to 1.49, corresponding to 60% MMC product. A correlation between the MMC/O ratio and the substrate-to-pore-size ratio was successfully established. Modification of the inner pore surface with dimethoxydimethylsilane allowed fine-tuning the effective pore size and reversing surface polarity, which resulted in a further increase of the MMC/O ratio up to 2.2, corresponding to >68% MMC product. Molecular-level simulations in model pore geometries help to rationalize the complex interplay between spatial confinement, specific (substrate and product) interaction with the pore surface, and diffusive transport. These effects can be synergistically adjusted for optimum selectivity by suitable surface modification.

ZSM-23 (MTT) is a silicon-rich zeolite with one-dimensional, 10-membered ring channels, which has recently attracted interest as a promising catalyst in aromatic-free methanol-to-hydrocarbons conversion. To obtain a better understanding of the catalytic activity and ultimately to design a better catalyst, it is crucial to locate the active sites in the zeolite framework. This work investigates the tetrahedral aluminum framework sites in two zeolite H-ZSM-23 samples by experimental and computational 27Al NMR spectroscopy. 27Al MQMAS NMR experiments at six different magnetic fields (4.7–22.3 T) were utilized to resolve distinct Al sites. The detected tetrahedral framework Al sites were assigned to the specific tetrahedral sites in the crystal structure by DFT calculations of the 27Al chemical shieldings. A comprehensive investigation of the structural model, basis set, and exchange–correlation potential used in the DFT calculations was performed. Two avenues were pursued for calculating the 27Al isotropic chemical shifts: the isolated-sites approach where clusters are extracted from large supercells with high Si/Al ratios and an approach targeting lower Si/Al ratios with a fully periodic model. It is found that for the ZSM-23 zeolites with Si/Al = 24 and 37 investigated here, the latter approach gives the best agreement with experiment.

The direct conversion of methane to methanol (MTM) is a reaction that has the potential to disrupt a great part of the synthesis gas-derived chemical industry. However, despite many decades of research, active enough catalysts and suitable processes for industrial application are still not available. Recently, several copper-exchanged zeolites have shown considerable activity and selectivity in the direct MTM reaction. Understanding the nature of the active site in these materials is essential for any further development in the field. Herein, we apply multivariate curve resolution analysis of X-ray absorption spectroscopy data to accurately quantify the fraction of active Cu in Cu-MOR (MOR = mordenite), allowing an unambiguous determination of the active site nuclearity as a dicopper site. By rationalizing the compositional parameters and reaction conditions, we achieve the highest methanol yield per Cu yet reported for MTM over Cu-zeolites, of 0.47 mol/mol.

The reactivity of partially alkali metal ion-exchanged X,H-ZSM-5 (X = Li, Na, Cs) catalysts was evaluated in methanol-to-olefin (MTO) conversion. 1H MAS NMR spectroscopy using acetonitrile-d3and ammonia as probes revealed decreased Brønsted acid site density (ASD) and maintained acid site strength of alkali metal ion-exchanged X,H-ZSM-5 catalysts. Optimized ASD and increasing counter ion diameter increase the propene content and decrease the selectivity to alkanes and aromatics. Decreasing the ASD to 0.12 mmol g−1by exchange with Cs resulted in the best propene/ethene ratio of 5 and the lowest hydrogen transfer index (HTI) of 0.08. Smaller counter ions Na and Li yielded less alkene cycle products but more than those of the parent (ASD = 0.78 mmol g−1). The X,H-ZSM-5 lifetime, conversion capacity, and coke amount correlate with the ASD. Their lifetime was outperformed by directly synthesized ZSM-5 catalysts. It is proposed that the ion size limits the space in the pores and introduces additional shape selectivity at the expense of lifetime.

Low-silica AlPO-34 materials with similar crystal sizes but different Brønsted acid site densities were prepared and investigated as catalysts in methanol-to-olefin (MTO) conversion. The effect of Brønsted acid site density on catalyst activity and the dominant reaction mechanism during the MTO conversion was investigated viaTGA, GC-MS, solid-state NMR spectroscopy, and in situUV/vis spectroscopy together with the catalytic performance. For the catalysts with lower Brønsted acid site densities, the olefin-based cycle mechanism is the dominant mechanism during the MTO conversion. Long-chain alkenes, e.g., C5–C6alkenes, act as intermediates that are cracked to lower olefins, or are converted to dienes viahydride transfer reactions, and can also diffuse out of the cages of low-silica AlPO-34 catalysts as the products. With decreasing Brønsted acid site density or reaction temperature, the methylation route of the olefin-based cycle was found to be much more favored than the cracking route. Therefore, a higher selectivity to C5–C6alkenes (∼50%) is achieved. Simultaneously, dienes are the predominant deposits occluded in the used catalysts. For catalysts with slightly higher Brønsted acid site densities, the long-chain alkenes are rapidly transformed to aromatics and, subsequently, an aromatic-based cycle mechanism contributes to the MTO conversion. Interestingly, the catalyst with the most suitable Brønsted acid site density can well balance the above-mentioned two reaction cycles accompanied by a low deactivation rate, leading to a long catalyst lifetime of up to 15 h.

The application of probe molecules is the state-of-the-art method to quantitatively determine accessibility and spatial distribution of catalytically active sites on solid materials. Knowing the spatial distribution of active sites on solid catalysts is crucial for increasing their performance and shape-selectivity. In this review, general aspects of the use of probe molecules in combination with quantitative spectroscopy are discussed. Then, the currently available families of probe molecules applied in solid-state NMR are described in detail. Properties and chemical shifts are provided for exemplary molecules interacting with active sites. Literature examples show which probe molecules can efficiently be used quantitatively on which system. Then, approaches for evaluating the spatial distribution of active sites are presented. It is shown how an intelligent combination of probe molecules and postmodification techniques provides a quantitative picture of sites in most spatial regimes. This shall lead the way for further methodological developments regarding solid-state NMR probe molecules and catalytic investigations.

We herein conduct the first 1H and 13C MAS NMR investigations that explore the formation of surface methoxy groups (SMG) on defect Zr(OH) groups of UiO-66 metal–organic frameworks. Loading acetone-2-13C indicates that UiO-66 contains weakly acidic Zr(OH) groups. These react at room temperature with adsorbed methanol-13C to SMG. Quantitative formation of SMG and methanol removal are achieved at 473 K. 1H–13C heteronuclear correlation proves close proximity between persistent, inaccessible Zr(OH) groups at δ1H= 1.2 ppm and SMG groups. SMG react with water to free methanol; however, the SMG cannot methylate hydrocarbons like toluene. This is explained by the weak acidity of Zr(OH) groups hosting the SMG. The 0.17 mmol/g SMG that was formed corresponds to 13% missing linkers, in good agreement with 11% missing linkers calculated from TGA. Thus, the formation and quantification of SMG by 13C MAS NMR spectroscopy are demonstrated as alternative measures for the amount of Zr(OH) in defects.

The intrinsic hydrogenation activities of homologous series of noble-metal-containing zeolites Y were studied by in situ solid-state NMR spectroscopy under semibatch conditions. For the hydrogenation of acrylonitrile, reaction rates in the sequence Pd/H,Na–Y > Rh/H,Na–Y > Pt/H,Na–Y > Ir/H,Na–Y were determined. The dehydrogenation of propane at these zeolites gave a sequence of the turnover frequencies of Ir/H,Na–Y > Rh/H,Na–Y > Pd/H,Na–Y, while Pt/H,Na–Y zeolites showed significantly higher activities. The temperature-programmed desorption of hydrogen (H2-TPD) was utilized for studying the strength of H2/metal interactions. The positions of the high-temperature peaks were arranged according to 2.8Pd/H,Na–Y (723 K) > 2.3Rh/H,Na–Y (713 K) > 4.7Ir/H,Na–Y (663 K). Comparison of these data indicates that strong H2/metal interactions are accompanied by a preferred formation of surface hydrogen atoms, which are the reason for the high hydrogenation activity of Pd/H,Na–Y zeolites compared with Rh/H,Na–Y and Ir/H,Na–Y zeolites. In the case of the propane dehydrogenation, the strong H2/Pd interactions in Pd/H,Na–Y zeolites hinder the desorption of the reaction product H2, explaining the lower dehydrogenation activity of these zeolites compared with Rh/H,Na–Y and Ir/H,Na–Y zeolites. For the high catalytic activities of the Pt/H,Na–Y zeolites, an effect of strongly chemisorbed hydrogen atoms inside the Pt clusters is discussed.

In the present work, the generation and properties of Brønsted acidic bridging OH groups (Si(OH)Al) in Rh-, Ir-, Pd-, and Pt-containing zeolites Y were quantitatively investigated by multinuclear solid-state NMR spectroscopy. The number of Si(OH)Al groups formed per noble metal atom upon their reduction was found to be 0.3 to 1.3 instead of the theoretically expected values of 2 (Pd2+, Pt2+) or 3 (Rh3+, Ir3+). For zeolites Y with low noble metal loadings, a significantly lower acid strength of Si(OH)Al groups in comparison with those of H,Na–Y zeolites occurred. Interestingly, noble metal-containing zeolites Y with lower acid strength have a significant content of not completely reduced noble metal atoms with partially cationic character. The spatial distribution of Si(OH)Al groups in noble metal-containing zeolites Y was found to differ significantly from that in H,Na–Y zeolites. This effect is explained by the migration of a significant content of noble metal atoms into the sodalite cages of zeolites Y already upon calcination of their noble metal complexes. Therefore, a much higher content of Si(OH)Al groups generated by the reduction of noble metal species in zeolites Y are located in sodalite cages compared to those in H,Na–Y zeolites.

The formation and evolution of initial reaction intermediates as well as the reaction mechanism during the early period of the methanol conversion on the silicoaluminophosphate SAPO-41 with one-dimensional and 10-numbered ring pore system was elucidated. According to in situ UV–vis spectroscopy, the formation and nature of intermediates formed on the catalysts in the methanol conversion process were monitored. The intermediates remaining on the catalysts after quenching the methanol conversion were determined by ex situ UV–vis, 1H MAS NMR, and 13C MAS NMR spectroscopy, and the reactivity of these species was investigated by adsorption of ammonia and subsequent solid-state NMR spectroscopy. The above-mentioned spectroscopic studies gave a detailed mechanistic insight into the induction period of the methanol conversion on SAPO-41. Monoenylic carbenium ions, being the dominating species during the initial period of the methanol conversion, were rapidly formed and gradually transferred to dienylic carbenium ions, benzene-based carbenium ions, and trienylic carbenium ions, in addition to three-ring compounds and dienes with different chain lengths. On the basis of these spectroscopic observations and the catalytic results, the olefin-based reaction cycle is disclosed to be the dominating reaction mechanism in the initial period of the methanol conversion on SAPO-41.

1H and 27Al MAS NMR spectroscopies have been applied for studying the effect of water molecules, nitrogen bases, and o-xylene on the hydroxyl protons of bridging AlOH groups and framework aluminum atoms in the metal−organic framework (MOF) MIL-53. For water molecules adsorbed on the low-temperature form MIL-53lt, two 1H MAS NMR signals were found indicating the formation of different O−H···O hydrogen bonds to neighboring oxygen atoms, such as to carboxylic oxygens. Upon adsorption of the nitrogen bases acetonitrile, ammonia, and pyridine, a linear increase of the quadrupole coupling constant, CQ, of the framework aluminum atoms in dehydrated MIL-53 from CQ= 8.5 MHz (unloaded material) to maximum 10.8 MHz (pyridine-loaded material) as a function of the proton affinity of the adsorbates was observed. Adsorption of o-xylene led to three stepwise changes of the quadrupole coupling constants, CQ, of framework aluminum atoms in dehydrated MIL-53. While the first two stepwise changes of the CQvalues (CQ= 8.0 and 8.7 MHz) occur for o-xylene loadings of lower than 4 molecules per unit cell and for all AlO4(OH)2centers, the third change of the CQvalue to 9.4 MHz was observed for o-xylene loadings higher than 4 o-xylene molecules per unit cell and for maximum 50% of the framework aluminum atoms. This third adsorbate-induced change of the CQvalue of framework aluminum atoms in MIL-53 is accompanied by a significant decrease of the adsorbate mobility.

Homogeneous catalysts immobilized on metal oxides often have different catalytic properties than in homogeneous solution. This can be either activating or deactivating and is often attributed to interactions of catalyst species with the metal oxide surface. However, few studies have ever demonstrated the effect that close associations of active sites with surfaces have on the catalytic activity. In this paper, we immobilize H2Ru(PPh3)2(Ph2P)2N–C3H6–Si(OEt)3(3) on SiO2, Al2O3, and ZnO and interrogate the relationship to the surface using IR, MAS NMR, 1H–31P HETCOR, and XAS spectroscopies. We found that while there are close contacts between the P atoms of the complex and all three metal oxide surfaces, the Ru–H bond only reacts with oxygen bridges on SiO2and Al2O3, forming new Ru–O bonds. In contrast, complex 3stays intact on ZnO. Comparison of the catalytic activities of our immobilized species for CO2hydrogenation to ethyl formate showed that Lewis acidic metal oxides activate, rather than deactivate, complex 3in the order Al2O3> ZnO > SiO2. The Lewis acidic sites on the metal oxide surfaces most likely increase the productivity by increasing the rate of esterification of formate intermediates.

Platinum hydride species catalyze a number of interesting organic reactions. However, their reactions typically involve the use of high loadings of noble metal and are difficult to recycle, making them somewhat unsustainable. We have synthesized surface-immobilized Pt–H species via oxidative addition of surface OH groups to Pt(PtBu3)2(1), a rarely used immobilization technique in surface organometallic chemistry. The hydride species thus made were characterized by infrared, magic-angle spinning nuclear magnetic resonance, and X-ray absorption spectroscopies and catalyzed both olefin isomerization and cycloisomerization of a 1,6 enyne (5) with a high selectivity and low Pt loading.