Effect of Si/Al Ratio on the Nature and Reactivity of HY Zeolite-Supported Rhodium Dicarbonyl Complexes

Abstract

Fourier transform infrared (FTIR), extended Xray absorption fine structure (EXAFS), X-ray photoelectron spectroscopy (XPS) measurements, and DFT calculations were used to characterize the species formed following reaction of a Rh(CO) 2 (acac) precursor with dealuminated HY zeolites with different Si/Al ratios. The results indicate the formation of two types of Rh(CO) 2 species with characteristic ν CO bands at 2117/2053 and 2110/2043 cm −1 . Both species are attached to the zeolite framework and have similar structural properties. However, their thermal stabilities are different, and the fraction of each species formed strongly depends on the Si/Al ratio of the zeolite. The carbonyl ligands in both types of these zeolite-bound Rh(CO) 2 complexes can react with gas-phase C 2 H 4 to form Rh(CO)(C 2 H 4 ) species. However, the reaction rate is substantially higher for the Rh(CO) 2 complex with the ν CO bands at 2117/2053 cm −1 , suggesting that the electronic properties of the Rh site affect the reactivity of the carbonyl ligands. The results further indicate that the two types of Rh(CO) 2 species are not associated with unreacted or partially reacted Rh(CO) 2 (acac) complexes or with the formation of a hydrated Rh(CO) 2 (H 2 O) x -type species. Instead, the results of the DFT calculations suggest that the differences observed can be attributed to differences in the structure of the binding sites in the dealuminated faujasites framework.

Synthesis, Modeling, and Catalytic Properties of HY Zeolite-Supported Rhodium Dinitrosyl Complexes

“…These νNO bands can be assigned to the symmetric and asymmetric vibrations of new NO ligands 61 and are similar to those previously observed for ZSM-5-supported Rh(NO)2 dinitrosyl complexes (i.e., at 1862 and 1785 cm -1 ), the structure of which has been confirmed by 14 NO/ 15 NO exchange and DFT calculations. 33 DFT calculations also predict that NO binds 41 kJ/mol stronger than CO to Rh + cations 18,33 Absorbance (a.u.) in acetone solution with CO yields [Rh(PPH3)2(CO)3]ClO4.…”

“…58 faujasite surfaces has been examined extensively in the past and attributed to the protonation and removal of the acac ligands from these complexes by acidic OH groups of the faujasite, yielding Rh(CO)2 species anchored to the zeolite framework. [17][18][19]34 To build upon these previous reports, a Rh(CO)2(acac) precursor was used to prepare a Rh(CO)2/HY30 material that was extensively characterized by FTIR and EXAFS to establish the uniformity and single-site nature of the surface Rh(CO)2 complexes formed. [17][18][19] When this Rh(CO)2/HY30 sample was further exposed to a pulse of a 1% NO/He mixture at 25°C, significant changes were observed in the FTIR spectra, as depicted in Fig.…”

“…1 for the formation of these species. 18 Since the replacement of carbonyl ligands in supported Rh(CO)2 complexes by NO is a facile substitution process, one could also expect similar formation of at least two types of zeolite-supported Rh(NO)2 complexes. Figs.…”

“…16 In this case, both the high reactivity of CO ligands and the almost instantaneous conversion of Rh(CO)2 into Rh(CO)(C2H4) complexes in the presence of gas-phase C2H4 appear to eliminate any potential effects of different ligands that are initially present on the Rh sites. [17][18][19] As a result, both complexes produce the same type of intermediates under reaction conditions and, therefore, exhibit similar catalytic properties.…”

“…Furthermore, we have also shown that these HY zeolite-supported Rh(CO)2 species can be used to selectively form Rh(CO)(C2H4) or Rh(CO)(H)x complexes upon subsequent exposure to C2H4 and H2 at room temperature. [17][18][19] Since literature examples also indicate that NO can replace the CO ligands of supported Rh(CO)2 complexes, 17,36,37 (acac = C5H7O2) (Strem, 98% purity) was used as supplied. n-Pentane (Aldrich, 99% purity) was refluxed under N 2 in the presence of Na/benzophenone ketyl to remove traces of moisture and deoxygenated by sparging of dry N2 prior to use.…”