A Site-Isolated Rhodium−Diethylene Complex Supported on Highly Dealuminated Y Zeolite:  Synthesis and Characterization

Abstract

The reaction of Rh(C2H4)2(acac) with the partially dehydroxylated surface of dealuminated zeolite Y (calcined at 773 K) and treatments of the resultant surface species in various atmospheres (He, CO, H2, and D2) were investigated with infrared (IR), extended X-ray absorption fine structure (EXAFS), and 13C NMR spectroscopies. The IR spectra show that Rh(C2H4)2(acac) reacted readily with surface OH groups of the zeolite, leading to loss of acac ligands from the Rh(C2H4)2(acac) and formation of supported mononuclear rhodium complexes, confirmed by the lack of Rh-Rh contributions in the EXAFS spectra; each Rh atom was bonded on average to two oxygen atoms of the zeolite surface with a Rh-O distance of 2.19 A. IR, EXAFS, and 13C NMR spectra show that the ethylene ligands remained bonded to the Rh center in the supported complex. Treatment of the sample in CO led to the formation of site-isolated Rh(CO)2 complexes bonded to the zeolite. The sharpness of the nu(CO) bands in the IR spectrum gives evidence of a nearly uniform supported Rh(CO)2 complex and, by inference, the near uniformity of the mononuclear rhodium complex with ethylene ligands from which it was formed. The supported complex with ethylene ligands reacted with H2 to give ethane, and it also catalyzed ethylene hydrogenation at 294 K.

Molecular Heterogeneous Catalysis: A Single‐Site Zeolite‐Supported Rhodium Complex for Acetylene Cyclotrimerization

“…When hydrogen flowed over this supported complex, ethane was formed; when ethylene and hydrogen flowed over the complex together at 1 bar and 294 K, catalytic hydrogenation to form ethane occurred. [4] Our results also show that propylene was catalytically hydrogenated in the presence of H 2 to give propane, with no evidence of deactivation after 20 turnovers. GC-FID and GC-MS analysis of the product stream confirmed the displacement of ethylene from the catalyst by the first two propylene pulses; conversion of propylene to propane was quantitative.…”

“…Data were communicated previously, [3,4] and we now report a complete analysis of results on the basis of newly measured data of higher quality; the results essentially match those previously communicated. [3,4] EXAFS data characterizing PRE are shown in Figure 3. The initial data fitting with the plausible absorber-backscatterer contributions (RhÀO, RhÀC, RhÀRh, and RhÀAl) led to a narrowed list of candidate fits (models), based on the goodness of fit.…”

“…Models of rhodium complex and bonding site in zeolite: The supported rhodium complexes, including the as-synthesized sample PRE and intermediates in the acetylene trimerization cycle, were investigated with the reactants bonded to 1) a naked Rh + ion, 2) a Rh + ion bonded to the site in the zeolite represented as an Al(OH) 4 À species (thus forming the neutral Al(OH) 4 Rh), and 3) in calculations with the most stable supported rhodium complexes, a cluster of the zeolite incorporating 8T (Si + Al) atoms (as depicted in Figure 2). …”

“…Because the ground state of the atom is a triplet, both the lowest lying singlet (Figure 4) and triplet energy surfaces ( Figure 5) characterizing Rh + and Al(OH) 4 Rh were investigated. The singlet-triplet splittings are shown in Table 3.…”

“…The triplet state is predicted to be the ground state of the Rh + ion, as found experimentally. [16] The calculations predict that triplet Rh + is more stable than singlet Rh + when bound to the model of the zeolite site (Al(OH) 4 À ; Table 3), although bonding of the Rh + ion to this site substantially reduces the singlet-triplet splitting. Addition of acetylene to either the naked ion or this site results in a substantial stabilization of the singlet, so that when two acetylene molecules are p bonded to the Rh + , the singlet is stabilized (and stabilization of the singlet occurs with the addition of just one acetylene molecule).…”