Jeffrey Kenvin

Structural analysis of hierarchically organized zeolites.

Hierarchically organized zeolites are materials retaining the crystalline order and associated functionality of bulk zeolites while also integrating a multilevel pore network. Here, the authors review the raft of techniques applied to characterize their crystal, pore and active site structures.

Supported catalysts prepared from mononuclear copper complexes: Catalytic properties

Abstract Model surfaces of copper on silica (Cab-O-Sil) were prepared and characterized by four probe reactions: methyl acetate hydrogenolysis, ethanol and methanol decomposition, and acetaldehyde hydrogenation. The model catalysts were prepared by thermal decomposition of copper acetylacetonate {Cu(acac) 2 } deposited on silica either as a monolayer or as multiple layers. The prior characterizations suggested that thermal decomposition of monolayer films of Cu(acac) 2 produced isolated copper species which could be oxidized and reduced up to 10 times without changing the dispersion of the copper. Thermolysis of the multiple layers of Cu(acac) 2 produced a copper surface of lower dispersions (ca. 70%) which decreased to 30% after only five oxidation/reduction cycles. These two surfaces were appropriate models to study the catalysis of oxygenates over Cu ensembles.

Selective chemisorption and oxidation/reduction kinetics of supported copper oxide catalysts prepared from copper(II) acetylacetonate

Copper(II) oxide catalysts, prepared by non-aqueous adsorption of Cu(acac)2 on Cab-O-Sil followed by thermal decomposition, were titrated by NO and N20 to characterize the dispersion of the copper ions. These catalysts showed molar ratios of NO/Cu close to unity when the Cu loadings were less than 2.5 wt%. For samples having loadings greater than 3.8 wt% Cu, the NO/Cu molar ratios were near 0.7. The NO/Cu molar ratio also depended upon the catalyst preparation technique subsequent to the initial impregnation with Cu(acac)2 when the Cu loadings were ≥ 3.5 wt%. Samples washed with fresh acetonitrile showed NO/Cu ratios close to unity, whereas, those not so washed showed NO/Cu ratios near 0.7. IR spectra of NO sorbed on partially decomposed samples showed only “bent” CuNO, whereas, NO sorbed to the sample which was totally decomposed showed both linear and bent CuNO. Selected samples (3.8 and 8.6 wt% Cu) were reacted with N2O to determine the dispersion of the Cu. The sample having 8.6 wt% Cu reacted with the N2O to give a dispersion of 0.43; whereas the other sample (3.8 wt%) did not react with the N2O. This dispersion determined by N2O agreed with that calculated from NO titration (0.47) if the NO/Cu stoichiometry was assumed equal to unity. Subsequently, these catalysts were reduced in H2 and reoxidized in O2 to determine the oxidation and reduction kinetics as a function of copper loading. The 3.8 wt% Cu sample lost 1 O/Cu upon reduction in H2 and gained l O/Cu for reoxidation in O2 for up to five redox cycles; whereas, the 8.6 wt% Cu sample showed a stoichiometry of O/Cu which decreased from 1.00 to 0.57 after five redox cycles.

Unified Method for the Total Pore Volume and Pore Size Distribution of Hierarchical Zeolites from Argon Adsorption and Mercury Intrusion

A generalized approach to determine the complete distribution of macropores, mesopores, and micropores from argon adsorption and mercury porosimetry is developed and validated for advanced zeolite catalysts with hierarchically structured pore systems in powder and shaped forms. Rather than using a fragmented approach of simple overlays from individual techniques, a unified approach that utilizes a kernel constructed from model isotherms and model intrusion curves is used to calculate the complete pore size distribution and the total pore volume of the material. An added benefit of a single full-range pore size distribution is that the cumulative pore area and the area distribution are also obtained without the need for additional modeling. The resulting complete pore size distribution and the kernel accurately model both the adsorption isotherm and the mercury porosimetry. By bridging the data analysis of two primary characterization tools, this methodology fills an existing gap in the library of familiar methods for porosity assessment in the design of materials with multilevel porosity for novel technological applications.