(718f) Exploring the Reactivity of Zeolite-Supported Rh Complexes

Artem D. Vityuk, Oleg S. Alexeev, and Michael D. Amiridis

Department of Chemical Engineering

University of South Carlina, Columbia, SC 29208 (USA)

*amiridis@mailbox.sc.edu

Introduction

           The use of model catalytic materials has been proven to be beneficial for understanding structure-catalytic properties relationships which can open new opportunities for the rational design of catalysts for specific applications. Although such materials have been prepared and examined in the past, the synthesis of model catalysts which are structurally uniform and incorporate well-defined active sites remains a major experimental challenge.  In one approach to the synthesis of such materials the support is regarded as a rigid ligand capable of stabilizing organometallic complexes.¹ Single-site catalysts prepared via this organometallic route represent the current state-of-the-art in this area. For example, it has been shown that Rh(CO)₂(acac) complexes are capable of reacting with surfaces of dealuminated Y zeolites and this reaction leads to  the displacement of the “acac” ligand and to the formation of site-isolated and well-defined Rh(CO)₂ species anchored inside the zeolite supercage. It has been also suggested that such Rh(CO)₂ species retain their square-planar geometry upon anchoring and, in fact, two oxygen atoms located in the T4 ring of zeolite Y and coordinated to Al³⁺ cations represent binding sites for these species.² Moreover, it was established that carbonyl ligands in these complexes are quite reactive and exposure to C₂H₄ results in the formation of Rh(C₂H₄)(CO) complexes. In our current work we further explore the reactivity of Y-zeolite supported Rh(CO)₂ species towards C₂H₄, H₂, CO/O₂, and CO/NO gases.

Results and Discussion

          Since zeolite-supported Rh(CO)₂ species have 16 electrons and, therefore, are coordinatively unsaturated, they exhibit a reach surface chemistry, as carbonyl ligands in such complexes can be partially or completely replaced by ligands of a different nature. While transformations between Rh(CO)₂ - Rh(C₂H₄)(CO) – Rh(CO)₂ complexes were documented previously, ³ we show herein that the exposure of Rh(C₂H₄)(CO) to hydrogen leads to hydrogenation of the C₂H₄ ligand and yields well-defined rhodium monocarbonyl hydride complexes Rh(CO)(H)_x. These species were found to be stable in hydrogen atmosphere but undergo decomposition in helium and/or in the presence of water in the gas phase. Zeolite-supported Rh(CO)(H)_x complexes can be readily transformed into well-defined Rh(CO)₂ or Rh(CO)(N₂) species upon CO or N₂ exposure, respectively. Finally, it was demonstrated that supported rhodium monocarbonyl hydride complexes are active in ethylene hydrogenation into ethane. 

          The remarkable ability of Rh sites to modify the coordination environment while retaining the structural identity encouraged us to look at the activity of such species for reactions conventionally performed over supported metal particles (i.e., the oxidation of CO and the reduction of NO by CO).  In the first reaction, we examined properties of zeolite- and alumina-supported Rh(CO)₂ complexes. Both catalysts were found to be inactive for this reaction at temperatures below 200 ºC. However, when the temperature was increased to 220 ºC, the CO conversion over the alumina-based sample was found to be 3 %. EXAFS data collected at the Rh K edge for this sample revealed that the appearance of the activity is associated with a transition of mononuclear rhodium dicarbonyl species to small rhodium clusters. This is in line with XPS results indicating the presence of only reduced Rh at this temperature. Further increase of CO conversion with temperature on this sample was accompanied with gradual decline in the intensity of IR bands designating dicarbonyl ligands with their almost complete disappearance at 280 ºC where CO conversion was close to 100 %.  The zeolite-supported Rh(CO)₂species were evidently less active in this reaction as the conversion of CO remained below 1 % at 220 ºC but increased to 50 % at 280 ºC. In this case, however, Rh sites retained their unique structural properties even at high CO conversions, as evidenced with EXAFS and FTIR data suggesting that mononuclear rhodium species are involved in the CO oxidation.

          In contrast, zeoltie-supported single-site Rh(CO)₂ complexes do not favor the reduction of NO by CO into N₂. We compared activity of Rh(CO)₂ species supported on zeolties with different Si/Al ratios at T = 270 ºC. It was found that zeolite with Si/Al=30 which stabilizes mononuclear Rh(CO)₂ complexes at this temperature is inactive for the reaction. However, NO conversion was increasing in time on Rh(CO)₂/HY15 (Si/Al=15) and Rh(CO)₂/HY3.5 (Si/Al=3.5) samples and was found to be due to the conversion of Rh(CO)₂/Rh(NO)₂ species into the Rh aggregates which were proposed to be the active sites for this reaction.   

 Conclusions 

          Here for the first time we reported selective surface-mediated synthesis and characterization of well defined and highly uniform supported rhodium monocarbonyl hydride species at ambient conditions. We examined stability of supported rhodium carbonyl hydride complexes in different environments and explored their chemical properties. Furthermore, we showed that zeolite supported rhodium complexes are capable of catalyzing reactions of environmental importance such as CO oxidation. Alternatively, such species could be used as model catalysts to probe the structure sensitive character of the NO_x+ CO reaction.

References

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(3)     Liang, A.J.; Craciun, R.; Chen, M.; Kelly, T.G.; Kletnieks, P.W.; Haw, J.F.; Dixon, D.A.; Gates, B.C. J. Am. Chem. Soc. 2009, 131, 8460.