Justin M. Notestein

Adsorption of n -butanol from dilute aqueous solution with grafted calixarenes

Materials were synthesized for the recovery of n-butanol from dilute aqueous solutions, as may be useful for applications in biofuel-water separations. These materials are composed of hydrophobic, cavity-containing calixarenes covalently bound directly to porous, hydrophilic silica supports through a Si linker atom rather than a flexible organic linker, as is common, at surface coverages of up to ∼0.25 calixarenes/nm(2) (∼250 μmol calix/g matl). The calixarene ring size, upper rim groups, bridging group (calixarene vs thiacalixarene), and surface density were varied. The materials were characterized by NMR, UV-vis, and TGA. The absolute butanol uptake reached ∼0.16 mmol butanol per gram of material at equilibrium concentrations below 0.12 M and increased monotonically with the calixarene surface density. The background adsorption onto the silica surface was small at high calixarene loading. At 298 K, the free energy of adsorption in the calixarene cavities became more favorable by 3 kJ/mol as the surface area of the hydrophobic calixarene upper rim groups increased from H to methyl to tert-butyl, consistent with adsorption driven by van der Waals interactions. A thiacalix[4]arene-SiO(2) material, containing polarizable sulfur bridges and a larger, more conformationally mobile calixarene structure, had slightly stronger adsorption still. All materials except this thiacalixarene exhibited fully reversible adsorption into solution. As a representative material, the adsorption of n-butanol from aqueous solution at a tert-butylcalix[4]arene site was accompanied by a negligible enthalpy change but a small, favorable entropy change of +50 ± 20 J/mol/K, indicating that adsorption is driven by desolvation. Butanol desorbed from tert-butylcalix[4]arene materials at ∼150 °C into the gas phase, well within the range of stability of calixarenes (<300 °C), indicating that these materials have promise as regenerable adsorbents.

Manganese Triazacyclononane Oxidation Catalysts Grafted under Reaction Conditions on Solid Cocatalytic Supports

Manganese complexes of 1,4,7-trimethyl-1,4,7-triazacyclononane (tmtacn) are highly active and selective alkene oxidation catalysts with aqueous H(2)O(2). Here, carboxylic acid-functionalized SiO(2) simultaneously immobilizes and activates these complexes under oxidation reaction conditions. H(2)O(2) and the functionalized support are both necessary to transform the inactive [(tmtacn)Mn(IV)(μ-O)(3)Mn(IV)(tmtacn)](2+) into the active, dicarboxylate-bridged [(tmtacn)Mn(III)(μ-O)(μ-RCOO)(2)Mn(III)(tmtacn)](2+). This transformation is assigned on the basis of comparison of diffuse reflectance UV-visible spectra to known soluble models, assignment of oxidation state by Mn K-edge X-ray absorption near-edge spectroscopy, the dependence of rates on the acid/Mn ratios, and comparison of the surface structures derived from density functional theory with extended X-ray absorption fine structure. Productivity in cis-cyclooctene oxidation to epoxide and cis-diol with 2-10 equiv of solid cocatalytic supports is superior to that obtained with analogous soluble valeric acid cocatalysts, which require 1000-fold excess to reach similar levels at comparable times. Cyclooctene oxidation rates are near first order in H(2)O(2) and near zero order in all other species, including H(2)O. These observations are consistent with a mechanism of substrate oxidation following rate-limiting H(2)O(2) activation on the hydrated, supported complex. This general mechanism and the observed alkene oxidation activation energy of 38 ± 6 kJ/mol are comparable to H(2)O(2) activation by related soluble catalysts. Undesired decomposition of H(2)O(2) is not a limiting factor for these solid catalysts, and as such, productivity remains high up to 25 °C and initial H(2)O(2) concentration of 0.5 M, increasing reactor throughput. These results show that immobilized carboxylic acids can be utilized and understood like traditional carboxylic acids to activate non-heme oxidation catalysts while enabling higher throughput and providing the separation and handling benefits of a solid catalyst.

A heterogeneous, selective oxidation catalyst based on Mn triazacyclononane grafted under reaction conditions

A unique method has been developed to synthesize an active heterogeneous oxidation catalyst by the in situ grafting of a 1,4,7-trimethyl-1,4,7-triazacyclononane manganese complex on carboxylic acid-functionalized supports serving dual roles as surface tether and necessary co-catalyst, massively increasing total turnovers as compared to the homogeneous analog.

MOFs and their grafted analogues: regioselective epoxide ring-opening with Zr6 nodes

Zirconium(IV)-containing metal–organic framework (MOF) catalysts NU-1000 and MOF-808 were compared to analogous catalysts synthesized by grafting the nodes onto bare and functionalized silica. As-synthesized, calcined, and activated catalysts all exhibit similar selectivity in the ring-opening of 1,2-epoxyoctane with isopropanol, but MOF-808 gives exceptionally high rates and yields per gram catalyst.

Counting Active Sites on Titanium Oxide-Silica Catalysts for Hydrogen Peroxide Activation through In Situ Poisoning with Phenylphosphonic Acid

Quantifying specific active sites in supported catalysts improves our understanding and assists in rational design. Supported oxides can undergo significant structural changes as surface densities increase from site‐isolated cations to monolayers and crystallites, which changes the number of kinetically relevant sites. Herein, TiOx domains are titrated on TiOx–SiO2 selectively with phenylphosphonic acid (PPA). An ex situ method quantifies all fluid‐accessible TiOx, whereas an in situ titration during cis‐cyclooctene epoxidation provides previously unavailable values for the number of tetrahedral Ti sites on which H2O2 activation occurs. We use this method to determine the active site densities of 22 different catalysts with different synthesis methods, loadings, and characteristic spectra and find a single intrinsic turnover frequency for cis‐cyclooctene epoxidation of (40±7) h−1. This simple method gives molecular‐level insight into catalyst structure that is otherwise hidden when bulk techniques are used.

Recovery of dilute aqueous acetone, butanol, and ethanol with immobilized calixarene cavities

Macrocyclic calixarene molecules were modified with functional groups of different polarities at the upper rim and subsequently grafted to mesoporous silica supports through a single Si atom linker. The resulting materials were characterized by thermogravimetric analysis, UV-visible spectroscopy, nitrogen physisorption, and solid-state NMR spectroscopy. Materials were then used to separate acetone, n-butanol, and ethanol from dilute aqueous solution, as may be useful in the recovery of fermentation-based biofuels. For the purpose of modeling batch adsorption isotherms, the materials were considered to have one strong adsorption site per calixarene molecule and a larger number of weak adsorption sites on the silica surface and external to the calixarene cavity. The magnitude of the net free energy change of adsorption varied from approximately 15 to 20 kJ/mol and was found to decrease as upper-rim calixarene functional groups became more electron-withdrawing. Adsorption appears to be driven by weak van der Waals interactions with the calixarene cavity and, particularly for butanol, minimizing contacts with solvent water. In addition to demonstrating potentially useful new sorbents, these materials provide some of the first experimental estimates of the energy of interaction between aqueous solutes and hydrophobic calixarenes, which have previously been inaccessible because of the insolubility of most nonionic calixarene species in water.

Rate and Selectivity Control in Thioether and Alkene Oxidation with H2O2 over Phosphonate-Modified Niobium(V)–Silica Catalysts

Supported metal oxide catalysts are versatile materials for liquid‐phase oxidations, including alkene epoxidation and thioether sulfoxidation with H2O2. Periodic trends in H2O2 activation was recently demonstrated for alkene epoxidation, highlighting Nb‐SiO2 as a more active and selective catalyst than Ti‐SiO2. Three representative catalysts are studied consisting of NbV, TiIV, and ZrIV on silica, each made through a molecular precursor approach that yields highly dispersed oxide sites, for thioanisole oxidation by H2O2. Initial rates trend Nb>Ti≫Zr, as for epoxidation, and Nb outperforms Ti for a number of other thioethers. In contrast, selectivity to sulfoxide vs. sulfone trends Ti>Nb≫Zr at all conversions. Modifying the Nb‐SiO2 catalyst with phenylphosphonic acid does not completely remove sulfoxidation reactivity, as it did for photooxidation and epoxidation, and results in an unusual material active for sulfoxidation but neither epoxidation nor overoxidation to the sulfone.

Predicting NOx Catalysis by Quantifying Ce3+ from Surface and Lattice Oxygen

Our work introduces a novel technique based on the magnetic response of Ce3+ and molecular oxygen adsorbed on the surface of nanoceria and ceria-based catalysts that quantifies the number and type of defects and demonstrates that this information is the missing link that finally enables predictive design of NOx catalysis in ceria-based systems. The new insights into ceria catalysis are enabled by quantifying the above for different ceria nanoparticle shapes (i.e., surface terminations) and O2 partial pressure. We used ceria nanorods, cubes, and spheres and evaluated them for catalytic reduction of NO by CO. We then demonstrated the quantitative prediction of the reactivity of nanomaterials via their magnetism in different atmospheric environments. We find that the observed enhancement of reactivity for ceria nanocubes and nanorods is not directly due to improved reactivity on those surface terminations but rather due to the increased ease of generating lattice defects in these materials. Finally, we demonstrate that the method is equally applicable to highly topical and industrially relevant ceria mixed oxides, using nanoscale alumina-supported ceria as a representative case-a most ill-defined catalyst. Because the total oxide surface is a mixture of active ceria and inactive support and ceria is not likely present as crystallographically well-defined phases, reactivity does not easily scale with surface area or a surface termination. The key parameter to design efficient NO reduction in ceria-based catalysts is knowing and controlling the surface localized excess Ce3+ ion areal density.

Pushing the Limits on Metal–Organic Frameworks as a Catalyst Support: NU-1000 Supported Tungsten Catalysts for o-Xylene Isomerization and Disproportionation

Acid-catalyzed skeletal C-C bond isomerizations are important benchmark reactions for the petrochemical industries. Among those, o-xylene isomerization/disproportionation is a probe reaction for strong Brønsted acid catalysis, and it is also sensitive to the local acid site density and pore topology. Here, we report on the use of phosphotungstic acid (PTA) encapsulated within NU-1000, a Zr-based metal-organic framework (MOF), as a catalyst for o-xylene isomerization at 523 K. Extended X-ray absorption fine structure (EXAFS), 31P NMR, N2 physisorption, and X-ray diffraction (XRD) show that the catalyst is structurally stable with time-on-stream and that WO x clusters are necessary for detectable rates, consistent with conventional catalysts for the reaction. PTA and framework stability under these aggressive conditions requires maximal loading of PTA within the NU-1000 framework; materials with lower PTA loading lost structural integrity under the reaction conditions. Initial reaction rates over the NU-1000-supported catalyst were comparable to a control WO x-ZrO2, but the NU-1000 composite material was unusually active toward the transmethylation pathway that requires two adjacent active sites in a confined pore, as created when PTA is confined in NU-1000. This work shows the promise of metal-organic framework topologies in giving access to unique reactivity, even for aggressive reactions such as hydrocarbon isomerization.

Photo-Initiated Reduction of CO2 by H2 on Silica Surface

The reduction of CO2 is a promising route to produce valuable chemicals or fuels and create C-neutral resource cycles. Many different approaches to CO2 reduction have been investigated, but the ability of vacuum UV (VUV) irradiation to cleave C-O bonds has remained largely unexplored for use in processes that convert CO2 into useful products. Compared with other photo-driven CO2 conversion processes, VUV-initiated CO2 reduction can achieve much greater conversion under common photochemical reaction conditions when H2 and non-reducible oxides are present. Infrared spectroscopy provides evidence for a chain reaction initiated by VUV-induced CO2 splitting, which is enhanced in the presence of H2 and silica. When the reaction is carried out in the presence of silica or alumina surfaces, CO yields are increased and CH4 is formed as the only other detected product. CH4 production is not promoted by traditional photocatalysts such as TiO2 under these conditions. Assuming improvements in lamp and reactor efficiencies with scale up, or coupling with other available CO/CO2 hydrogenation techniques, these results reveal a potential, simple strategy by which CO2 could be valorized.