Iridium Clusters in NaX Zeolite Cages: Synthesis, Characterization, and Selective Catalysis of CO Hydrogenation

Kawi, S.; Chang, J.-R.; Gates, Bruce C.

doi:10.1006/jcat.1993.1233

Cited by 33 publications

(6 citation statements)

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“…Evidently, both MgO 13,15 and γ-Al 2 O 3 have sufficiently basic surfaces to provide efficient media for the synthesis of anionic iridium carbonyl clusters (and the degree of hydroxylation is important). Chemistry similar to that reported here also occurs in the cages of NaX zeolite . In contrast, neutral iridium carbonyl clusters, [Ir 4 (CO) 12 ] and [Ir 6 (CO) 16 ], rather than anions, form as a result of similar reactions in the cages of NaY zeolite, , which is less basic than NaX zeolite, and on the nearly neutral SiO 2 …”

Section: Discussionsupporting

confidence: 78%

Hexairidium Clusters Supported on γ-Al₂O₃: Synthesis, Structure, and Catalytic Activity for Toluene Hydrogenation

Zhao

Gates

1996

J. Am. Chem. Soc.

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[Ir6(CO)15]2- was formed by surface-mediated synthesis on γ-Al2O3 powder by treatment of adsorbed [Ir(CO)2(acac)] in CO at 100 °C and 1 atm. The supported clusters were characterized by infrared and extended X-ray absorption fine structure (EXAFS) spectroscopies and by extraction into solution by cation metathesis. Treatment of γ-Al2O3-supported [Ir6(CO)15]2- in He at 300 °C led to decarbonylation without disruption of the cluster frame, giving Ir6/γ-Al2O3, as indicated by EXAFS data determining a first-shell Ir−Ir coordination number of 4.05 ± 0.07 (where the error bounds represent precision, not accuracy), matching that of the supported [Ir6(CO)15]2-, with a first-shell Ir−Ir coordination number of 4.07 ± 0.03. The supported Ir6 clusters were found to be catalytically active for toluene hydrogenation at temperatures in the range of 60−100 °C; the turnover frequency at 60 °C with a toluene partial pressure of 50 Torr and a H2 partial pressure of 710 Torr was 1.7 × 10-3 s-1. The catalyst was stable in operation in a flow reactor, and consistent with this observation, EXAFS results for the used catalyst indicated that the nuclearity of the supported Ir6 clusters was essentially unchanged during catalysis. The γ-Al2O3-supported Ir6 catalyst is an order of magnitude less active (per total Ir atom) for toluene hydrogenation than a catalyst consisting of small aggregates of iridium (with an average of about 50 atoms each) supported on γ-Al2O3. Because toluene hydrogenation is known to be a structure-insensitive catalytic reaction, the data suggest that the concept of structure insensitivity in catalysis does not extend to clusters as small as Ir6. However, the relatively low activity of the Ir6 clusters may be more an indication of an effect of the support as a ligand changing the electronic properties of the iridium in the cluster than an effect of the cluster size.

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Section: Discussionsupporting

confidence: 78%

Hexairidium Clusters Supported on γ-Al₂O₃: Synthesis, Structure, and Catalytic Activity for Toluene Hydrogenation

Zhao

Gates

1996

J. Am. Chem. Soc.

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“…13 Parallels have been found between the chemistry of iridium carbonyls in solution and that on MgO surfaces; [HIr 4 (CO) 11 ] -, [Ir 8 (CO) 22 ] 2-, and [Ir 6 (CO) 15 ] 2have been formed on MgO from [Ir(CO) 2 (acac)] 4 or from [Ir 4 (CO) 12 ]. 3 Similar chemistry also occurs in the supercages of NaX zeolite, 6 except that the formation of [Ir 8 (CO) 22 ] 2evidently does not occur, likely because the size constraint of the cage prevents its formation. The surfaces of MgO and NaX zeolite evidently provide solvent-like environments with basicities comparable to those of the solvents in which these anions form.…”

Section: Evidence Of Formation Of [Ir 8 (Co) 22 ] 2on γ-Al 2 Omentioning

confidence: 84%

“…Synthesis on a metal oxide surface (in the absence of solvents) typically mimics that occurring in solvents with properties (such as basicity) analogous to those of the surface. For example, MgO and NaX zeolite are sufficiently basic to allow synthesis (as in basic solutions) of a family of anionic iridium carbonyl clusters from [Ir(CO) 2 (acac)] − and rhodium carbonyl clusters from [Rh(CO) 2 (acac)] − and rhodium carbonyl clusters , form instead.…”

Section: Introductionmentioning

confidence: 99%

Probing Metal Oxide Surface Reactivity with Adsorbate Organometallic Chemistry: Formation of Iridium Carbonyl Clusters on γ-Al₂O₃

and

Gates

1997

Langmuir

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Ir8(CO)22] 2-was formed by surface-mediated synthesis by treatment of [Ir(CO)2(acac)] supported on uncalcined γ-Al2O3 under 1 atm of CO at room temperature. The formation of [Ir8(CO)22] 2-completes the synthesis of a family of iridium carbonyl clusters, including [Ir4(CO)12], [HIr4(CO)11] -, [Ir6(CO)15] 2-, and [Ir8(CO)22] 2-, on the surface of γ-Al2O3. The surface chemistry parallels known solution chemistry of the synthesis of the family of iridium carbonyls. The products formed are controlled largely by the basicity of the solution or surface, and the analogy between the solution and surface chemistry provides a preliminary, qualitative basis for characterizing the basicity of γ-Al2O3 and other metal oxide samples. Decarbonylation of [Ir8(CO)22] 2-on γ-Al2O3 led to nearly uniform tetrahedral Ir4 clusters; it is likely that the Ir-Ir single bond linking two iridium tetrahedral moieties in the precursor [Ir8(CO)22] 2-on γ-Al2O3 was broken during the decarbonylation.

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“…We are also not in a position now to comment on the largest possible cluster of CO that may form in the zeolitic pores. It is, however, pertinent to add here that the formation and the entrapment of large noble metal based carbonyl clusters (such as Rh 6 (CO) 16 and Ir 6 (CO) 15 ) in the zeolitic cages have been demonstrated widely. − …”

Section: Discussionmentioning

confidence: 99%

Fourier Transform Infrared Study on the Encapsulation of CO in Zeolite Y under the Moderate Temperature and Pressure Conditions

et al. 1998

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were observed during the room-temperature adsorption of CO over NaY zeolite. This was accompanied by the appearance of a prominent band at 2356 cm -1 along with the sidebands at 2336, 2342-2348, and 2364-2370 cm -1 in the ν 3 region of CO 2 vibrations. The intensity and the frequency of these bands showed monotonic changes on evacuation, temperature rise, and exchange of Na + ions with a proton or a group IIA cation and also on using the isotopically labeled CO. For instance, the adsorption of isotopic 13 C 16 O and 12 C 18 O gases gave rise to a uniform red shift in all the major ν(CO) bands, corresponding to a frequency ratio of ν( 13 CO)/ν( 12 CO) ≈ 0.978 and ν(C 18 O)/ν(C 16 O) ≈ 0.976. Similarly, all the asymmetric stretching bands of CO 2 showed a uniform isotopic shift corresponding to ν( 13 C)/ν( 12 C) ≈ 0.971. Furthermore, the intensity of most of the above-mentioned bands in ν(CO) and ν 3 (CO 2 ) regions exhibited an exponential type growth behavior with increasing adsorbate pressure, though the extent of this growth was different for individual vibrations. No significant change was, however, observed in the frequency and the intensity of the hydroxyl region bands on adsorption of CO under experimental conditions of this study. Arguments are presented to show that the observations of this study are not in harmony with the reported modes of direct CO adsorption at the specific zeolitic sites such as the charge-balancing cations, Al 3+ sites, or the Bronsted acid sites. Instead, the formation of the weakly held (CO) n and (CO 2 ) n molecular clusters occluded in the zeolitic cavities and stabilized under the cation field is in conformity with the data of this study.

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Iridium Clusters in NaX Zeolite Cages: Synthesis, Characterization, and Selective Catalysis of CO Hydrogenation

Cited by 33 publications

References 0 publications

Hexairidium Clusters Supported on γ-Al₂O₃: Synthesis, Structure, and Catalytic Activity for Toluene Hydrogenation

Hexairidium Clusters Supported on γ-Al₂O₃: Synthesis, Structure, and Catalytic Activity for Toluene Hydrogenation

Probing Metal Oxide Surface Reactivity with Adsorbate Organometallic Chemistry: Formation of Iridium Carbonyl Clusters on γ-Al₂O₃

Fourier Transform Infrared Study on the Encapsulation of CO in Zeolite Y under the Moderate Temperature and Pressure Conditions

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Iridium Clusters in NaX Zeolite Cages: Synthesis, Characterization, and Selective Catalysis of CO Hydrogenation

Cited by 33 publications

References 0 publications

Hexairidium Clusters Supported on γ-Al2O3: Synthesis, Structure, and Catalytic Activity for Toluene Hydrogenation

Hexairidium Clusters Supported on γ-Al2O3: Synthesis, Structure, and Catalytic Activity for Toluene Hydrogenation

Probing Metal Oxide Surface Reactivity with Adsorbate Organometallic Chemistry: Formation of Iridium Carbonyl Clusters on γ-Al2O3

Fourier Transform Infrared Study on the Encapsulation of CO in Zeolite Y under the Moderate Temperature and Pressure Conditions

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Hexairidium Clusters Supported on γ-Al₂O₃: Synthesis, Structure, and Catalytic Activity for Toluene Hydrogenation

Hexairidium Clusters Supported on γ-Al₂O₃: Synthesis, Structure, and Catalytic Activity for Toluene Hydrogenation

Probing Metal Oxide Surface Reactivity with Adsorbate Organometallic Chemistry: Formation of Iridium Carbonyl Clusters on γ-Al₂O₃