Stable Rhodium Pair Sites on MgO: Influence of Ligands and Rhodium Nuclearity on Catalysis of Ethylene Hydrogenation and H–D Exchange in the Reaction of H₂ with D₂

Understanding of supported metal pair-site catalysts is largely lacking, because most are structurally nonuniform or incorporate stabilizing ligands that inhibit catalysis. We synthesized rhodium pair-sites on MgO from Rh₂(μ-OMe)₂(COD)₂ (OMe = methoxy; COD = cyclooctadiene), characterizing the surface species with infrared and X-ray absorption spectroscopies. Catalytic properties for ethylene hydrogenation and H–D exchange in the H₂ + D₂ reaction show that surface pair sites were retained when organic ligands were removed, giving catalysts that facilitate H₂ dissociation much more rapidly than single-site rhodium and catalyze ethylene hydrogenation 2 orders of magnitude faster than single-site rhodium on MgO at 298 K

Zeolite- and MgO-Supported Molecular Iridium Complexes: Support and Ligand Effects in Catalysis of Ethene Hydrogenation and H–D Exchange in the Conversion of H₂ + D₂

Zeolite- and MgO-supported mononuclear iridium diethene complexes were formed by the reaction of Ir(C₂H₄)₂(acac) (acac = acetylacetonate, C₅H₇O₂^–) with each support. Changes in the ligand environment of the supported iridium complexes were characterized by infrared, X-ray absorption near edge structure, and extended X-ray absorption fine structure spectroscopies as various mixtures of H₂, C₂H₄, and CO flowed over the samples. In contrast to the nonuniform metal complexes anchored to metal oxides, our zeolite-supported metal complexes were highly uniform, allowing precise determinations of the chemistry, including the role of the support as a macroligand. Zeolite- and MgO-supported Ir(C₂H₄)₂ complexes are each rapidly converted to Ir(CO)₂ upon contact with a pulse of CO, and the ν_CO frequencies indicate that the iridium is more electron-deficient when the support is the zeolite. The Ir(CO)₂ complex supported on MgO was highly stable in the presence of various combinations of CO, C₂H₄, and helium. In contrast, the zeolite-supported Ir(CO)₂ complex was found to be highly reactive, forming Ir(CO)₃, Ir(CO)(C₂H₄), Ir(CO)₂(C₂H₄), and Ir(CO)(C₂H₄)₂. The π-bonded ethene ligands of the zeolite-supported Ir(C₂H₄)₂ in H₂ were facilely converted to σ-bonded ethyl when treated. However, the stability of the ethene ligands was markedly increased when the support was changed to MgO or when a CO ligand was simultaneously bonded to the iridium. The rates of catalytic ethene hydrogenation and H₂/D₂ exchange in the presence of a catalyst initially consisting of Ir(C₂H₄)₂ on the zeolite were found to be more than an order of magnitude higher than when MgO was the support. The iridium complexes containing one or more CO ligands were found to be inactive for H₂/D₂ exchange reactions when the support was MgO, but they were moderately active when it was the zeolite. The effects of the MgO and zeolite supports on reactivity and catalytic activity are attributed to their differences as ligands donating or withdrawing electrons, respectively

Tuning Catalytic Selectivity: Zeolite- and Magnesium Oxide-Supported Molecular Rhodium Catalysts for Hydrogenation of 1,3-Butadiene

Regulation of the catalytic selectivity of rhodium for the industrially important hydrogenation of 1,3-butadiene to <i>n</i>-butenes has been achieved by controlling the structure of essentially molecular rhodium species bound to supports. The selectivity for <i>n-</i>butene formation increases as the nuclearity of the metal species decreases from several Rh atoms to one, but these catalysts form the undesired product <i>n</i>-butane, even at low diene conversions. The <i>n</i>-butene selectivity increases when the rhodium is selectively poisoned with CO ligands, and it is highest when the support is the electron-donor MgO and the rhodium is in the form of clusters that are well approximated as dimers. The electron-donor support is crucial for stabilization of the rhodium carbonyl dimer sites, as it limits the oxidative fragmentation of the clusterswhich is facilitated when the support is HY zeolite (a poor electron donor)that leads to decreased catalytic activity and selectivity. The selective MgO-supported rhodium carbonyl dimers suppress the catalytic routes that yield butane, limiting the activity for H₂ dissociation to avoid butane formation via primary reactions and also favoring the bonding of 1,3-butadiene over butenes to limit secondary reactions giving butane. With this catalyst, selectivities to <i>n-</i>butene of >99% were achieved at 1,3-butadiene conversions as high as 97%. This selectivity matches that of any reported for this reaction, and the catalyst works under milder conditions (313 K and 1 bar) than others that are selective for this reaction

Homogeneity of Surface Sites in Supported Single-Site Metal Catalysts: Assessment with Band Widths of Metal Carbonyl Infrared Spectra

Determining and controlling the uniformity of isolated metal sites on surfaces of supports are central goals in investigations of single-site catalysts because well-defined species provide opportunities for fundamental understanding of the surface sites. CO is a useful probe of surface metal sites, often reacting with them to form metal carbonyls, the infrared spectra of which provide insights into the nature of the sites and the metal–support interface. Metals bonded to various support surface sites give broad bands in the spectra, and when narrow bands are observed, they indicate a high degree of uniformity of the metal sites. Much recent work on single-site catalysts has been done with supports that are inherently nonuniform, giving supported metal species that are therefore nonuniform. Herein we summarize values of ν_CO data characterizing supported iridium <i>gem</i>-dicarbonyls, showing that the most nearly uniform of them are those supported on zeolites and the least uniform are those supported on metal oxides. Guided by ν_CO data of supported iridium <i>gem</i>-dicarbonyls, we have determined new, general synthesis methods to maximize the degree of uniformity of iridium species on zeolites and on MgO. We report results for a zeolite HY-supported iridium <i>gem</i>-dicarbonyl with full width at half-maximum values of only 4.6 and 5.2 cm^–1 characterizing the symmetric and asymmetric CO stretches and implying that this is the most nearly uniform supported single-site metal catalyst

Isostructural Zeolite-Supported Rhodium and Iridium Complexes: Tuning Catalytic Activity and Selectivity by Ligand Modification

A family of isostructural, essentially molecular complexes of rhodium and of iridium anchored to HY zeolite was synthesized from M­(C₂H₄)₂(acac) and M­(CO)₂(acac) (M = Rh, Ir; acac is acetylacetonate), with the initial supported species being M­(C₂H₄)₂ and M­(CO)₂, each bonded to the zeolite through two M–O bonds. Each was used as a catalyst at 300 and 373 K and atmospheric pressure for the conversion of ethylene in the presence of H₂ (and sometimes D₂), giving ethane and, when the metal was rhodium, butenes, and, when D₂ was present, HD. The high degree of uniformity of the metal complexes allowed a precise spectroscopic elucidation of the predominant species present during catalysis. The CO ligands were inhibitors of the catalytic reactions, with the metal dicarbonyl complexes lacking measurable activity under our conditions. The CO ligands also served as probes helping to characterize the structures and electronic properties of the catalytic metal complexes. The data show that subtle changes in the bonding of the ligands markedly affect the catalytic performance

2‑Propanol Dehydration on the Nodes of the Metal–Organic Framework UiO-66: Distinguishing Catalytic Sites for Formation of Propene and Di-isopropyl Ether

2-Propanol dehydration was used as a test reaction to probe the catalytic properties of metal–organic framework (MOF) UiO-66. Experiments were performed with a flow reactor operated at atmospheric pressure and 510 K, showing (a) how the catalytic activity increased and then decreased, depending on the nature of ligands on the Zr6O8 MOF nodes (such as formate, acetate, hydroxyl, or alkoxy groups); and (b) how the selectivity changed with changing node ligands, which were characterized by IR spectroscopy, 1H NMR spectroscopy of digested MOF samples, and other techniques. The selectivity is sensitive to the node ligand composition, with the dehydration reaction initially facilitated by the removal of adventitious node formate and acetate ligands formed in the MOF synthesis and concomitant formation of node OH ligands from water formed in the catalysis. Node pair sites consisting of a node Zr-μ1-OH site and a neighboring node zirconium vacancy site are inferred to be active for propene formation. The ether formation rate increased with an increasing density of node 2-propoxy ligands, leading to the suggestion that these ligands at a paired zirconium defect site react with adjacent 2-propanol molecules to form di-isopropyl ether in a bimolecular nucleophilic substitution mechanism. These results show how the selectivity of UiO-66 can be modulated simply by changing the node ligands though postsynthetic modifications, without changing the node motif, oxidation state of the node metal atoms, pore structure, MOF topology, or linker chemistry

Oxide- and Zeolite-Supported Isostructural Ir(C₂H₄)₂ Complexes: Molecular-Level Observations of Electronic Effects of Supports as Ligands

Zeolite Hβ- and γ-Al₂O₃-supported mononuclear iridium complexes were synthesized by the reaction of Ir­(C₂H₄)₂(acac) (acac is acetylacetonate) with each of the supports. The characterization of the surface species by extended X-ray absorption fine structure (EXAFS) and infrared (IR) spectroscopies demonstrated the removal of acac ligands during chemisorption, leading to the formation of essentially isostructural Ir­(C₂H₄)₂ complexes anchored to each support by two Ir–O_support bonds. Atomic-resolution aberration-corrected scanning transmission electron microscopy (STEM) images confirm the spectra, showing only isolated Ir atoms on the supports with no evidence of iridium clusters. These samples, together with previously reported Ir­(C₂H₄)₂ complexes on zeolite HY, zeolite HSSZ-53, and MgO supports, constitute a family of isostructural supported iridium complexes. Treatment with CO led to the replacement of the ethylene ligands on iridium with CO ligands, and the ν_CO frequencies of these complexes and white line intensities in the X-ray absorption spectra at the Ir L_III edge show that the electron density on iridium increases in the following order on these supports: zeolite HY < zeolite Hβ < zeolite HSSZ-53 ≪ γ-Al₂O₃ < MgO. The IR spectra of the iridium carbonyl complexes treated in flowing C₂H₄ show that the CO ligands were replaced by C₂H₄, with the average number of C₂H₄ groups per Ir atom increasing as the amount of iridium was increasingly electron-deficient. In contrast to the typical supported catalysts incorporating metal clusters or particles that are highly nonuniform, the samples reported here, incorporating uniform isostructural iridium complexes, provide unprecedented opportunities for a molecular-level understanding of how supports affect the electronic properties, reactivities, and catalytic properties of supported metal species

Tracking Rh Atoms in Zeolite HY: First Steps of Metal Cluster Formation and Influence of Metal Nuclearity on Catalysis of Ethylene Hydrogenation and Ethylene Dimerization

The initial steps of rhodium cluster formation from zeolite-supported mononuclear Rh­(C₂H₄)₂ complexes in H₂ at 373 K and 1 bar were investigated by infrared and extended X-ray absorption fine structure spectroscopies and scanning transmission electron microscopy (STEM). The data show that ethylene ligands on the rhodium react with H₂ to give supported rhodium hydrides and trigger the formation of rhodium clusters. STEM provided the first images of the smallest rhodium clusters (Rh₂) and their further conversion into larger clusters. The samples were investigated in a plug-flow reactor as catalysts for the conversion of ethylene + H₂ in a molar ratio of 4:1 at 1 bar and 298 K, with the results showing how the changes in catalyst structure affect the activity and selectivity; the rhodium clusters are more active for hydrogenation of ethylene than the single-site complexes, which are more selective for dimerization of ethylene to give butenes

Catalytic Conversion of Guaiacol Catalyzed by Platinum Supported on Alumina: Reaction Network Including Hydrodeoxygenation Reactions

The conversion of guaiacol catalyzed by Pt/γ-Al₂O₃ in the presence of H₂ was investigated with a flow reactor at 573 K and 140 kPa. Dozens of reaction products were identified, with the most abundant being phenol, catechol, and 3-methylcatechol. The kinetically significant reaction classes were found to be hydrogenolysis [including hydrodeoxygenation (HDO)], hydrogenation, and transalkylation. Selectivity–conversion data were used to determine an approximate quantitative reaction network accounting for the primary products, and a more detailed qualitative network was also inferred. Catalytic HDO was evidenced by the production of anisole and phenol. The HDO selectivity increased with an increasing H₂ partial pressure and a decreasing temperature. Products formed by transalkylation reactions match those produced in the conversion catalyzed by HY zeolite, in which no deoxygenated products were observed

Hydrogen Activation and Metal Hydride Formation Trigger Cluster Formation from Supported Iridium Complexes

The formation of iridium clusters from supported mononuclear iridium complexes in H₂ at 300 K and 1 bar was investigated by spectroscopy and atomic-resolution scanning transmission electron microscopy. The first steps of cluster formation from zeolite-supported Ir­(C₂H₄)₂ complexes are triggered by the activation of H₂ and the formation of iridium hydride, accompanied by the breaking of iridium–support bonds. This reactivity can be controlled by the choice of ligands on the iridium, which include the support