Catalytic consequences of hydroxyl group location on the kinetics of n-hexane hydroisomerization over acidic zeolites

Selective regulation of n-dodecane isomerization and cracking performance in Pt/beta catalysts via orientation control of Brønsted acid site distribution

The isomerization and cracking performance of n-dodecane have been successfully regulated by the orientation control of Brønsted acid site distribution in Beta zeolites.

Dehydration of butanol towards butenes over MFI, FAU and MOR: influence of zeolite topology

The effect of zeolite topology on the kinetics, selectivity and catalyst stability for the dehydration of butanol is studied experimentally and through microkinetic modeling.

New trends in tailoring active sites in zeolite-based catalysts

This review discusses approaches for tailoring active sites in extra-large pore, nanocrystalline, and hierarchical zeolites and their performance in emerging catalytic applications.

Fourier self-deconvolution of the IR spectra as a tool for investigation of distinct functional groups in porous materials: Brønsted acid sites in zeolites

For many decades, IR and FT-IR spectroscopy has generated valuable information about different functional groups in zeolites, metal-organic frameworks (MOFs), and other porous materials. However, this technique cannot distinguish between functional groups in different local environments. Our study demonstrates that this limitation could be overcome by using Fourier self-deconvolution of infrared spectra (FSD-IR). We apply this method to study three acidic mordenite zeolites and show (i) that these zeolites contain six distinct Brønsted acid sites (BAS) as opposed to 2-4 different BAS previously considered in literature and (ii) that the relative amounts of these BAS are different in the three zeolites examined. We then analyze possible locations of six BAS in the mordenite structure and explain a number of conflicting results in literature. On this basis, we conclude that the FSD-IR method allows direct visualization and examination of distributions of distinct BAS in zeolites, thus providing a unique research opportunity, which no other method can provide. Given the similarities in the IR analysis of different functional groups in solids, we expect that the FSD-IR method will be also instrumental in the research into other porous materials, such as solid oxides and MOFs. The latter point is illustrated by FSD of the IR spectrum of hydroxyl groups in a sample of α-alumina.

Contaminant adsorption on nanoscale particles: structural and theoretical characterization of Cu2+ bonding on the surface of Keggin-type polyaluminum (Al30) molecular species

The adsorption of contaminants onto metal oxide surfaces with nanoscale Keggin-type structural topologies has been well established, but identification of the reactive sites and the exact binding mechanism are lacking. Polyaluminum species can be utilized as geochemical model compounds to provide molecular level details of the adsorption process. An Al30 Keggin-type species with two surface-bound Cu(2+) cations (Cu2Al30-S) has been crystallized in the presence of disulfonate anions and structurally characterized by single-crystal X-ray diffraction. Density functional theory (DFT) calculations of aqueous molecular analogues for Cu2Al30-S suggest that the reactivity of Al30 toward Cu(2+) and SO4(2-) shows opposite trends in preferred adsorption site as a function of particle topology, with anions preferring the beltway and cations preferring the caps. The bonding competition was modeled using two stepwise reaction schemes that consider Cu2Al30-S formation through initial Cu(2+) or SO4(2-) adsorption. The associated DFT energetics and charge density analyses suggest that strong electrostatic interactions between SO4(2-) and the beltway of Al30 play a vital role in governing where Cu(2+) binds. The calculated electrostatic potential of Al30 provides a theoretical interpretation of the topology-dependent reactivity that is consistent with the present study as well as other results in the literature.

Local surface structure effect on reactivity of molecules confined between metallic surfaces

Interactions between metallic surfaces separated by nanometer distances create an unusual reactivity environment. Here we evaluate the effect of the geometry given by differences in the structures of the interacting surfaces and by the presence of steps. Adsorption of an oxygen molecule and its dissociation is examined in gaps defined by interacting platinum surfaces that have separations between 5.36 and 4.70 Å, and by comparing the effect of the different gap geometries on the adsorption strength and barriers for dissociation. It is found that specific surface-surface configurations influence the electronic structure of the surface where the molecule is adsorbed, modifying the width of its d-orbital and therefore the adsorption strength due to changes in the overlap of the adsorbate molecular orbitals with the metal d-band. In addition, the degree of the molecule-metal interaction with the other surface may restrict the adsorbate mobility and its dissociation. The presence of defects may decrease the adsorbate-surface interaction strength, but the net result depends on the specific reaction and nature of the intermediates since in some cases weaker adsorptions may result in lower dissociation barriers.