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

Abstract

By anchoring metal complexes to supports, researchers have attempted to combine the high activity and selectivity of molecular homogeneous catalysis with the ease of separation and lack of corrosion of heterogeneous catalysis. However, the intrinsic nonuniformity of supports has limited attempts to make supported catalysts truly uniform. We report the synthesis and performance of such a catalyst, made from [Rh(C(2)H(4))(2)(CH(3)COCHCOCH(3))] and a crystalline support, dealuminated Y zeolite, giving {Rh(C(2)H(4))(2)} groups anchored by bonds to two zeolite oxygen ions, with the structure determined by extended X-ray absorption fine structure (EXAFS) spectroscopy and the uniformity of the supported complex demonstrated by (13)C NMR spectroscopy. When the ethylene ligands are replaced by acetylene, catalytic cyclotrimerization to benzene ensues. Characterizing the working catalyst, we observed evidence of intermediates in the catalytic cycle by NMR spectroscopy. Calculations at the level of density functional theory confirmed the structure of the as-synthesized supported metal complex determined by EXAFS spectroscopy. With this structure as an anchor, we used the computational results to elucidate the catalytic cycle (including transition states), finding results in agreement with the NMR spectra.

Beyond Relationships Between Homogeneous and Heterogeneous Catalysis

“…In another example, a zeolite-supported rhodium complex catalyzing acetylene cyclotrimerization was represented by DFT calculations mapping the potential energy surface (PES). Calculations confirmed the experimentally determined structure of the initial complex, used as an anchor for the mapping of the reaction PES [16].…”

“…Simple Al(OH) 4 -and more complex 8 T-atom (T = Si, Al) models of a zeolite were used to predict the NMR and vibrational spectra of zeolite-supported complexes. The vibrational spectra are accounted for by the metal atom and the ligand atoms immediately bonded to it; the rest of the zeolite does not strongly influence these spectra [16,17]. The ligand dissociation energies (LDEs) are more strongly influenced by the size of the model representing the zeolite, depending on whether the zeolite framework rearranges when ligands are removed to fill the vacant site and stabilize the product to reduce the LDE [18,19].…”

Preparation and Catalytic Performances of a Molecularly Imprinted Ru‐Complex Catalyst with an NH2 Binding Site on a SiO2 Surface

“…[1][2][3][4][5][6][7] Metal-complex precursors can either be directly attached to oxide surfaces by chemical reactions with surface hydroxyl groups [8][9][10][11][12][13] or else grafted onto such surfaces through organic branches. [14][15][16][17][18][19] Zeolite, [20,21] MCM, [22] and SBA, [23][24][25] which are porous oxide supports, as well as ionic-liquid-modified SiO 2 , [26,27] can be utilized as supports for metal complexes. The potential of immobilized metal-complex catalysts remarkably interplays with the nature of support surfaces, resulting in significant rate enhancements and unique catalytic performances that the analogous homogeneous complexes do not exhibit.…”

Highly Efficient Hydroformylation of 1‐Hexene over an ortho‐Metallated Rhodium(I) Complex Anchored on a 2D‐Hexagonal Mesoporous Material

“…The heterogenised catalysts on these solids are used in several chemical transformation, for example, oxidation, [14] epoxidation, [15] alkane oxidation, [16] hydrogenation, [17] carbonylation, [18] polymerisation, [19] C-C coupling reaction [20,21] and many others. [22,23] A unique advantage of these functionalised mesoporous materials as solid catalyst supports is their high surface areas (Ͼ700 m 2 g -1 ) and tunable pore diameters (2-10 nm).…”