Microporous materials such as zeolites, metal organic frameworks, activated carbons and aluminum phosphates are suitable for catalysis and separation applications. These high surface area materials ...are invariably produced in particulate forms and need to be transformed into hierarchically porous structures for high performance adsorbents or catalysts. Structuring of porous powders enables an optimized structure with high mass transfer, low pressure drop, good heat management, and high mechanical and chemical stability. The requirements and important properties of hierarchically porous structures are reviewed with a focus on applications in gas separation and catalysis. Versatile powder processing routes to process porous powders into hierarchically porous structures like extrusion, coatings of scaffolds and honeycombs, colloidal processing and direct casting, and sacrificial approaches are presented and discussed. The use and limitations of the use of inorganic binders for increasing the mechanical strength is reviewed, and the most important binder systems, e.g. clays and silica, are described in detail. Recent advances to produce binder-free and complex shaped hierarchically porous monoliths are described and their performance is compared with traditional binder-containing structured adsorbents. Needs related to better thermal management and improved kinetics and volume efficiency are discussed and an outlook on future research is also given.
Thin, binder-less zeolite NaX laminates, with thicknesses ranging between 310 to 750 μm and widths exceeding 50 mm and biaxial tensile strength in excess of 3 MPa, were produced by pulsed current ...processing. The NaX laminates displayed a high CO
adsorption capacity and high binary CO
-over-N
and CO
-over-CH
selectivity, suitable for CO
capture from flue gas and upgrading of raw biogas. The thin laminates displayed a rapid CO
uptake; NaX laminates with a thickness of 310 μm were saturated to 40% of their CO
capacity within 24 seconds. The structured laminates of 310 μm thickness and 50 mm thickness would offer low pressure drop and efficient carbon capture performance in a laminate-based swing adsorption technology.
Stabilization techniques for low-valent organometallic species in acidic forms of large-pore zeolite Y are reported. The intrazeolite surface chemistry and thermal stability of Mesub 3SnCo(CO)sub 4 ...in HY zeolites (Hsub 45Nasub 10Alsub 55Sisub 137Osub 384 and Hsub 16Nasub 39Alsub 55Sisub 137Osub 384) were studied with X-ray absorption spectroscopy (Sn, Co edge EXAFS) and in-situ FTIR/TPD-MS techniques. The acidic zeolite Y host offers a chemically reactive surface that can interact with the Mesub 3Sn moiety of the bimetallic complex. FTIR and EXAFS data indicate attachment of the complex to the zeolite framework. The intrazeolite SnCo complex is accessible toward carbonyl substitution with PMesub 3. One observes growing attachment of the precursor (progressive substitution of methyl ligands at tin) to the zeolite with increasing temperature, while the Sn-Co bond and the Co-carbonyl moiety remain stable up to about 90degreeC. At higher temperatures the Sn-Co bond is cleaved, the carbonyl ligands are dissociated, and cobalt is oxidized in a reaction with the zeolite protons. 25 refs., 13 figs., 1 tab.
We report here our theoretical investigations of the interaction of ammonia (NH
3) with pure and tungsten–doped ceria surfaces. The objective of the work was to understand the modification of surface ...acidity via the use of dopants, at the fundamental level. Surface acidity is an important property that is frequently used to characterize the reactivity of surfaces. Interest in ammonia stems from its use as a probe molecule in experimentally determining the acidity of surfaces, while ceria and tungsten oxide are examples of metal oxides widely used in catalytic systems.
Density functional theory calculations were performed using the Vienna ab-initio simulation package (VASP 4.6.2). The surfaces were modeled using finite number of layers, cleaved from the relaxed bulk structures. The geometries were optimized and resulting binding energies of ammonia were used to correlate to acid site strength.
Our calculations show that ammonia binds to ceria at the Lewis and Brønsted acid sites, however this binding is very weak (<5
kcal/mol). In the presence of tungsten, the binding to the Lewis acid sites continues to be weak, while the binding to the Brønsted acid sites is increased significantly (to ∼10
kcal/mol). We have used the H-terminated surface to study the Brønsted acidity and as a corollary, we also report the significantly different binding of hydrogen to the pure and tungsten–doped ceria surfaces.
In ongoing efforts to develop new catalytic applications involving zeolites, heteronuclear bimetallic complexes were synthesized and anchored in zeolites. The intrazeaolite reactivity of these ...complexes were studied using various techniques such as Extended X-ray Absorption Fine Structure Spectroscopy (EXAFS), Fourier Transform Infrared-Temperature Programmed Desorption-Mass spectroscopy (FTIR-TPD-MS), and UV-Vis spectroscopy. Results obtained from investigating the reactivity of these systems are reported along with their catalytic activity. In another effort, we report successful incorporation of a chiral Manganese(III) salen complex in hexagonal faujasite zeolite (EMT). Due to the size and rigidity of the chiral complex after synthesis inside the zeolite, it is physically entrapped inside the zeolite pore structure "(Ship in a Bottle)." The resulting heterogenous system was simulated using molecular modeling and then physically characterized using a variety of analytical techniques. These techniques include XRF (X-ray Fluorescence), diffuse reflectance UV/VIS, ESR (Electron Spin Resonance), FTIR Spectroscopy and Nitrogen Adsorption Analysis. Various prochiral olefins were used to probe the heterogenous catalytic activity of this system in asymmetric epoxidations. The highest enantiomeric excess (ee) obtained was 80% for cis-$\beta$-methylstyrene oxide. Experimental evidence indicates that the encapsulated complex does not leach.
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