(720g) Structure, Reactivity and Selective Tuning of Well-Defined Rh Complexes in HY Zeolites

K. Khivantsev,^a A. D. Vityuk,^a H. A. Aleksandrov,^b G. N. Vayssilov,^b O. S. Alexeev,^a,* 

M. D. Amiridis^a

^aDepartment of Chemical Engineering, University of South Carolina, Columbia, 29208, USA

^bFaculty of Chemistry and Pharmacy, University of Sofia, Sofia, BG-1126, Bulgaria

*Corresponding author: alexeev@cec.sc.edu

1. Introduction

Rhodium is one of the most important metals in homogeneous and heterogeneous catalysis, with Rh carbonyl complexes catalyzing a number of reactions of industrial importance primarily in the liquid phase, including hydroformylation, hydrogenation, hydrosilylation, dehydrogenation, and carbonylation.¹ The search for new heterogeneous Rh catalysts that resemble the properties of their homogeneous analogs has constituted an active area of research for years. Such catalytic materials could offer not only easy separation of products from a catalyst but also the opportunity more efficiently perform the same organic reactions in flow rather than in batch reactors. Metal oxides frequently used as catalyst supports have nonuniform surfaces, and therefore, a variety of binding sites with different structural and electronic properties. As a result, metal complexes grafted on such supports yield catalytic sites that are nonuniform in structure and composition and exhibit different electronic properties, impacting substantially their catalytic performance. In this respect, the use of zeolites as supports is more promising, since these crystalline materials offer highly ordered arrays of binding sites for metal complexes and, therefore, allow for the preparation of catalytic materials with well-defined and nearly uniform structures.

Our goals were to prepare catalytic materials incorporating highly uniform and molecular in nature Rh(CO)₂ complexes anchored to the zeolite framework, to precisely examine the structure and chemical properties of such complexes, and to determine how the reactivity of the ligands in these complexes can be used for the surface mediated synthesis of other Rh species. FTIR, XPS, STEM, EXAFS, mass spectrometry, isotope labeling, and DFT calculations were used to monitor ligand exchange reactions and to understand the structure and composition of the species formed at the molecular level. New results presented herein demonstrate selective pathways for the conversion of HY zeolite-supported Rh(CO)₂ into well-defined and structurally uniform Rh(CO)(C₂H₄), Rh(NO)₂, and Rh(CO)(H)_x complexes and disclose the stability, chemical reactivity, and catalytic performance of these complexes for the hydrogenation of C₂H₄.

2. Experimental

1% Rh(CO)₂/HY samples were prepared by slurrying a Rh(CO)₂(C₅H₇O₂) precursor with dealuminated HY zeolites (Si/Al=2.7-30 atomic) in dry n-pentane under N₂ at 25°C for 16 h. Ligand exchange reactions taking place during exposure to NO, C₂H₄, and H₂ were followed by in-situ FTIR at 25°C and atmospheric pressure. Isotopically labeled ¹³CO and D₂ were used to confirm assignments of FTIR bands and to identify the nature of the complexes formed. An Inficon Transpector 2 residual gas analyzer equipped with a MDC variable leak valve was used for the analysis of products formed during ligand exchange reactions. XAS spectra were collected at beamline 4-1 at SSRL. XPS measurements were conducted using a Kratos AXIS Ultra DLD system. STEM images were recorded using a JEOL-2100F instrument equipped with a CEOS aberration corrector on the illuminating optics and operating at 200 keV. Kinetic measurements were performed in a quartz tubular micro reactor with various feed compositions, while products were analyzed with an on-line GC (HP 7890 A) system. Periodic DFT calculations were performed with the PW91 exchange-correlation functional using a Vienna ab initio simulation package (VASP).

3. Results and discussion

We found that grafting of Rh(CO)₂(acac) on dealuminated HY zeolites leads to the formation of two types of Rh(CO)₂ species with characteristic ν_CO bands at 2117/2053 and 2113/2048 cm^-1 that have similar structural properties (evidenced by EXAFS). However, their thermal stabilities are different, and the fraction of each species formed strongly depends on the Si/Al ratio, as zeolites with a higher Al content favor the formation of the latter species in larger amounts. Our FTIR results strongly suggest that Rh(CO)₂ species with the ν_CO bands at 2113/2048 cm^-1 cannot be linked to unreacted and partially reacted Rh(CO)₂(acac) complexes or to the formation of Rh(CO)₂(H₂O)_x species. DFT calculations further indicate that binding sites of different nature in dealuminated faujasites are responsible for their formation.

Carbonyl ligands in both types of zeolite-grafted Rh(CO)₂ complexes are capable of reacting with gas phase C₂H₄ and NO to form Rh(CO)(C₂H₄) and Rh(NO)₂ species, respectively. Nevertheless, the conversion rate is substantially higher for Rh(CO)₂ complexes with the ν_CO bands at 2117/2053 cm^-1, suggesting that electronic properties of Rh sites control the reactivity of carbonyl ligands. Our results further show that the exposure of Rh(CO)(C₂H₄) species to H₂ selectively yields well-defined Rh(CO)(H)_x complexes. The Rh(CO)(H)_x species thus formed are stable, site-isolated, mononuclear, bound to oxygen atoms of the zeolite framework (evidenced by EXAFS), and are characterized by a set of well-defined ν_CO and ν_RhH bands in their FTIR spectra. DFT results further confirm that experimentally observed ν_CO and ν_RhH bands correspond to Rh carbonyl hydride complexes incorporating dissociated and molecular H₂ and having different locations in the zeolite framework. In fact, HY zeolite-supported Rh(CO)(H)_x species described herein are the first known heterogeneous analogs of molecular rhodium carbonyl hydride complexes, which are believed to be key intermediates for the hydroformylation and hydrogenation of alkenes in solution. 

The hydride ligands in grafted Rh(CO)(H)_x can be displaced by CO or N₂ to form Rh(CO)₂ and Rh(CO)(N₂) complexes, respectively, although the displacement of hydrides with N₂ is slow and non-selective. In contrast, C₂H₄ does not displace the hydride ligands in Rh(CO)(H)_x, but reacts with them to form C₂H₆ and unstable Rh(CO) intermediates. The latter species readily react with C₂H₄ from the gas phase to form the more stable Rh(CO)(C₂H₄) complexes. Our results further suggest that Rh(CO)(H)_x species lose hydride ligands at approximately 130°C, with unstable Rh(CO) species formed during this process. Subsequent redistribution of CO between Rh(CO) species leads to the formation of more stable Rh(CO)₂ complexes and Rh sites free of CO ligands. Furthermore, the HY zeolite-grafted Rh(CO)₂, Rh(CO)(C₂H₄), Rh(CO)(H)_x species were found to be catalytically active for the hydrogenation and dimerization of C₂H₄ at 25°C. Since both reactions occur simultaneously, experiments with the selective blockage of Rh coordination sites with bulky ligands and support sites with (CH₃)₃SiCl allowed us to clarify the role and function of the Rh and support sites during catalysis and tune the selectivity of ethylene transformations. Finally, we demonstrated that NO ligands attached to Rh sites exhibit a different type of surface chemistry. Regardless of this fact, however, Rh(NO)₂/HY was also found to be active in the hydrogenation of C₂H₄, which is a rare example of alkene hydrogenation catalysis by grafted rhodium nitrosyl complexes.

4. Conclusions

We have shown that the use of organometallic precursors and highly crystalline supports (zeolites) provides an excellent opportunity for the surface mediated synthesis of a family of grafted Rh(CO)₂, Rh(NO)₂, Rh(CO)(C₂H₄), Rh(CO)(H)_x complexes, all of which are well-defined, site-isolated, molecular in nature, and catalytically active. We have established that coordination environment of Rh sites controls the catalytic activity of these complexes in the conversion of alkenes. This is an example of fine tuning catalytic properties of supported metal sites by altering the ligand environment on a single atom scale. Thus, a new pathway is opened to examine the role of such complexes and the resulting structure-reactivity relationships for a wide spectrum of industrially relevant catalytic applications. 

References

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