Alexander Okrut

Selective molecular recognition by nanoscale environments in a supported iridium cluster catalyst

The active sites of enzymes are contained within nanoscale environments that exhibit exquisite levels of specificity to particular molecules. The development of such nanoscale environments on synthetic surfaces, which would be capable of discriminating between molecules that would nominally bind in a similar way to the surface, could be of use in nanosensing, selective catalysis and gas separation. However, mimicking such subtle behaviour, even crudely, with a synthetic system remains a significant challenge. Here, we show that the reactive sites on the surface of a tetrairidium cluster can be controlled by using three calixarene-phosphine ligands to create a selective nanoscale environment at the metal surface. Each ligand is 1.4 nm in length and envelopes the cluster core in a manner that discriminates between the reactivities of the basal-plane and apical iridium atoms. CO ligands are initially present on the clusters and can be selectively removed from the basal-plane sites by thermal dissociation and from the apical sites by reactive decarbonylation with the bulky reactant trimethylamine-N-oxide. Both steps lead to the creation of metal sites that can bind CO molecules, but only the reactive decarbonylation step creates vacancies that are also able to bond to ethylene, and catalyse its hydrogenation.

Catalytic consequences of open and closed grafted Al(III)-calix[4]arene complexes for hydride and oxo transfer reactions

An approach for the control and understanding of supported molecular catalysts is demonstrated with the design and synthesis of open and closed variants of a grafted Lewis acid active site, consisting of Al(III)-calix[4]arene complexes on the surface of silica. The calixarene acts as a molecular template that enforces open and closed resting-state coordination geometries surrounding the metal active sites, due to its lower-rim substituents as well as site isolation by virtue of its steric bulk. These sites are characterized and used to elucidate mechanistic details and connectivity requirements for reactions involving hydride and oxo transfer. The consequence of controlling open versus closed configurations of the grafted Lewis acid site is demonstrated by the complete lack of observed activity of the closed site for Meerwein-Ponndorf-Verley (MPV) reduction; whereas, the open variant of this catalyst has an MPV reduction activity that is virtually identical to previously reported soluble molecular Al(III)-calix[4]arene catalysts. In contrast, for olefin epoxidation using tert-butyl-hydroperoxide as oxidant, the open and closed catalysts exhibit similar activity. This observation suggests that for olefin epoxidation catalysis using Lewis acids as catalyst and organic hydroperoxide as oxidant, covalent binding of the hydroperoxide is not required, and instead dative coordination to the Lewis acid center is sufficient for catalytic oxo transfer. This latter result is supported by density functional theory calculations of the transition state for olefin epoxidation catalysis, using molecular analogs of the open and closed catalysts.

Epoxidation of 1-octene under harsh tail-end conditions in a flow reactor I: a comparative study of crystalline vs. amorphous catalysts

Amorphous silica versus crystalline delaminated-zeolite catalysts consisting of grafted Ti(IV) Lewis-acid active sites were investigated from the perspective of 1-octene olefin epoxidation with ethylbenzene hydroperoxide (EBHP) as oxidant. Reactions were performed at conditions of temperature and concentrations of organic hydroperoxide and inhibitors (epoxide product and alcohol co-product) that mimic the harsh conditions found at the tail-end of the flow reactor for industrial propylene-oxide (PO) synthesis, where there is a current need to improve activity and selectivity, because of deactivation. Catalyst synthesis was performed by grafting a Ti-alkoxide precursor onto framework vacancies (“silanol nests”) of the delaminated zeolite UCB-4, as well as onto amorphous SiO2. Both catalysts were characterized by powder X-ray diffraction (PXRD), nitrogen physisorption at 77 K, and UV-visible spectroscopy before and after catalysis. Experiments at different conversions were performed, and show that crystalline Ti-UCB-4 exhibits a ∼9% higher average selectivity (73% versus 64%) and greater conversion, stability, and robustness upon increasing time on stream relative to amorphous Ti–SiO2. UV-vis spectra are discussed for fresh, spent, and spent/calcined materials and demonstrate that Ti sites in Ti-UCB-4 exist as isolated grafted complexes with four-fold coordination to the zeolite framework, whereas Ti–SiO2 consists of grafted Ti-sites on the silica surface, some of which are isolated but a dominant proportion of which are TiO2 oligomers. The observed increased stability of the crystalline catalyst under tail-end reactor conditions is attributed to the surface pockets of the crystalline material, in which Ti is grafted.

Epoxidation of 1-octene under harsh tail-end conditions in a flow reactor II: impact of delaminated-zeolite catalyst surface area and structural integrity on catalytic performance

Building on our previous study of delaminated-zeolite catalysts for harsh tail-end conditions in an epoxidation flow reactor employing organic hydroperoxide as oxidant, this manuscript compares approaches for delamination in catalyst prepration. In one, a mild method of delamination is used for synthesis of Ti-UCB-4, in which fluoride in organic solvent is used as a mineralizing agent to affect delamination, while in another, catalyst Ti-SSZ-70-DEL-HIGHPH is synthesized by delamination under high-pH conditions and results in the highest external surface area, similar to that previously reported for ITQ-2. We also compare both materials to the calcined 3-D zeolite consisting of Ti-SSZ-70, a control which underwent no delamination treatment. Results of long-term flow reactor catalytic testing demonstrate a distinct 2.5-fold increase in reaction-rate constant k′ on a mass basis for Ti-UCB-4 relative to 3-D Ti-SSZ-70, and a lack of long-term deactivation for both catalysts. In contrast, for Ti-SSZ-70-DEL-HIGHPH, due to deactivation, no steady-state behavior is observed for either conversion or selectivity with increasing times on stream. A combination of data from powder X-ray diffraction (PXRD), nitrogen physisorption at 77 K, high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), and UV-visible (UV-vis) spectroscopy demonstrate Ti-SSZ-70-DEL-HIGHPH to be comprised of a combination of crystalline and amorphous phases. Control experiments demonstrate a negative synergy on catalysis when both phases are combined in a single catalyst, which leads to decreased conversion, at levels below values predicted based on the linear combination of the two phases present.

Role of N-Heterocyclic Carbenes as Ligands in Iridium Carbonyl Clusters

The low-energy isomers of Irx(CO)y(NHC)z (x = 1, 2, 4) are investigated with density functional theory (DFT) and correlated molecular orbital theory at the coupled cluster CCSD(T) level. The structures, relative energies, ligand dissociation energies, and natural charges are calculated. The energies of tetrairidium cluster are predicted at the CAM-B3LYP level that best fit the CCSD(T) results compared with the other four functionals in the benchmark calculations. The NHCs behave as stronger σ donors compared with COs and have higher ligand dissociation energies (LDEs). For smaller isomers, the increase in the LDEs of the COs and the decrease in the LDEs of the NHCs as more NHCs are substituted for COs are due to π-back-bonding and electron repulsion, whereas the trend of how the LDEs change for larger isomers is not obvious. We demonstrate a μ3-CO resulting from the high electron density of the metal centers in these complexes, as the bridging COs and the μ3-COs can carry more negative charge and stabilize the isomers. Comparison of calculations for a mixed tetrairidum cluster consisting of two calixarene-phosphine ligands and a single calixarene-NHC ligand in the basal plane demonstrated good agreement in terms of both the ligand substitution symmetry (C3v derived), as well as the infrared spectra. Similar comparisons were also performed between calculations and experiment for novel monosubstituted calixarene-NHC tetrairidium clusters.