Cong-Yan Chen

Studies on mesoporous materials: I. Synthesis and characterization of MCM-41

Abstract The mesoporous molecular sieves MCM-41 are synthesized and characterized by X-ray powder diffraction, nitrogen adsorption/desorption, cyclohexane and water adsorption, transmission electron micrographs (TEM), thermogravimetric analysis (TGA), ammonia temperature-programmed desorption (TPD), FTIR, FT-Raman, 29Si, 27Al and 13C magic angle spinning (MAS) NMR spectroscopy. Raman spectra from these materials exhibit a common band at ∼610 cm−1 assignable to cyclic trisiloxanes (3-membered rings), indicating in combination with TEM, IR and NMR results that the inorganic portion of MCM-41 resembles amorphous silicas or aluminosilicates rather than crystalline molecular sieves in terms of the local structure and bonding. Pure-silica MCM-41 can be heated to at least 850°C in dry air or 800°C in air with 8 Torr water vapor before structural collapse begins. Using sodium aluminate as the aluminum reagent, aluminosilicate MCM-41 can be prepared with a Si/Al ratio as low as 29 without observing the presence of octahedral aluminum. Such is not the case when using Catapal alumina. Adsorption of cyclohexane and water reveals that pure-silica and aluminosilicate (Si/Al = 39) MCM-41 are both hydrophobic. Based on ammonia TPD results, aluminosilicate MCM-41 has acidity similar to that of amorphous aluminosilicates.

Studies on mesoporous materials II. Synthesis mechanism of MCM-41

The solids obtained during the synthesis of MCM-41 are investigated by X-ray powder diffraction, thermogravimetric analysis and 29Si NMR spectroscopy. These data, in combination with those obtained from in situ14N NMR spectroscopy, are used to elucidate the synthesis mechanism of MCM-41. The results shown here reveal that the liquid crystalline phase, H1, is not present in the synthesis medium during the formation of MCM-41 and therefore cannot be the structure-directing agent for MCM-41. Rather, the data suggest that randomly ordered rod-like organic micelles interact with silicate species to yield approximately two or three monolayers of silica encapsulation around the external surfaces of the micelles. Subsequently, these composite species spontaneously assemble into the long-range ordered structure (hexagonal packing) characteristic of MCM-41. With further heating, the silicate species in the interstitial spaces of the ordered organic—inorganic composite phase continue to condense. Complete condensation of the silicate species is not possible because SiO− species are necessary for charge compensation of the occluded alkylammonium ions.

Studies on ordered mesoporous materials III. Comparison of MCM-41 to mesoporous materials derived from kanemite

Abstract MCM-41 and mesoporous materials derived from kanemite are synthesized and characterized by X-ray powder diffraction (XRD), N 2 adsorption-desorption, cyclohexane and water physical adsorption, scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermogravimetric analyses (TGA), Fourier transform infrared (FTIR) and 13 C and 29 Si magic-angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy. Both preparations yield mesoporous materials with narrow-pore-size distributions and somewhat similar physicochemical properties. However, due to a higher degree of condensation in the silicate walls of the materials derived from kanemite, these samples have higher thermal and hydrothermal stability than MCM-41. Additionally, the mechanisms by which these two types of materials are formed are dramatically different.

Base catalysis by intrazeolitic cesium oxides

Abstract Zeolites X and Y containing intrazeolitic cesium oxide particles are synthesized and characterized by 133 Cs nuclear magnetic resonance (NMR) spectroscopy, infrared (IR) spectroscopy of adsorbed CO 2 and CO 2 temperature- programmed desorption. The isomerization of 1-butene to give cis/trans ratios of 2-butene above 10 at 150°C shows that these materials are solid base catalysts. At the same reaction conditions, cesium ion-exchanged zeolites X and Y are inactive. The combined physicochemical properties are used to develop structure-property relationships and to rationalize the origin of the strong basicity achieved by the intrazeolitic cesium oxide particles. These novel catalysts offer new possibilities for shape-selective solid superbase catalysis.

A Single‐Site Platinum CO Oxidation Catalyst in Zeolite KLTL: Microscopic and Spectroscopic Determination of the Locations of the Platinum Atoms

A stable site-isolated mononuclear platinum catalyst with a well-defined structure is presented. Platinum complexes supported in zeolite KLTL were synthesized from [Pt(NH3)4](NO3)2, oxidized at 633 K, and used to catalyze CO oxidation. IR and X-ray absorption spectra and electron micrographs determine the structures and locations of the platinum complexes in the zeolite pores, demonstrate the platinum-support bonding, and show that the platinum remained site isolated after oxidation and catalysis.

Searching for new high silica zeolites through a synergy of organic templates and novel inorganic conditions

Abstract The synergy of template molecule size and hydrophobicity and the sources of inorganic reagents affecting zeolite nucleation are described. In particular, the discovery of high silica zeolites SSZ-26, 41, 42 and UTD-1 is considered in this light. Of particular value is the use of zeolites themselves as reactants; three different examples are given. Some general rules for lattice substitution and zeolite product type are given. An unexpected use of the template used in the discovery of zeolite SSZ-42 is described now that the structure of the zeolite has been resolved, largely from single crystal data. Finally some new synthetic conditions with considerable economic advantage are described for zeolite SSZ-25. Some new questions about the effect of organic cations on solution instability are raised.

Little energetic limitation to microporous and mesoporous materials

A series of pure-silica or high-silica zeolites, several mesoporous silicas, moganite, and the conventional silica polymorphs (tridymite, cristobalite, coesite, glass) are energetically, at most, 15 kJ/mol higher than quartz, with the energy versus volume curve leveling off about 15 kJ per mol of SiO2 above quartz for large volumes. For AlPO4 polymorphs, a similar trend holds, with energy leading off at an even less metastable value, namely about 10 kJ per mol of Al0.5P0.5O2 above berlinite. These small energy differences are consistent with self assembly of these materials being geometrically and kinetically, rather than energetically, controlled. Thus there is very little energetic limitation to the possibility of synthesizing various micro- and mesoporous framework structures.

Reactions of meta-xylene on zeolites with intersecting medium and large pores I. Basic studies

The reactions of m-xylene are conducted over all known zeolites with intersecting medium and large pores (MCM-22, NU-87, CIT-1, SSZ-33, SSZ-26) and the results compared to those from large (SSZ-24, ZSM-12, zeolite beta, zeolite L) and medium (ZSM-5, EU-1) pore zeolites. Each of the materials MCM-22, NU-87 and CIT-1/SSZ-33/SSZ-26 reveals a unique reaction behavior in terms of the p-xyleneo-xylene and isomerization/disproportionation ratios, the distribution of formed trimethylbenzenes and the rate of deactivation. The reaction activities and selectivities of MCM-22 and CIT-1/SSZ-33/SSZ-26 are somewhat like medium and large pore zeolites, respectively, as is expected from their crystal structures. NU-87 displays a lower p-xyleneo-xylene ratio and a higher turnover frequency than predicted from the crystal structure.

Tracking Iridium Atoms with Electron Microscopy: First Steps of Metal Nanocluster Formation in One-Dimensional Zeolite Channels

Using aberration-corrected scanning transmission electron microscopy (STEM), we imaged iridium atoms in isolated iridium complexes in the one-dimensional nonintersecting 14-ring channels of zeolite SSZ-53. STEM allows tracking of the movement of atoms in the channels, demonstrating the interaction of iridium with the zeolite framework (channel confinement) and providing a direct visualization of the initial steps of metal nanocluster formation. The results demonstrate how STEM can be used to help design improved catalysts by identifying the catalytic sites and observing how they change in reactive atmospheres.

Zeolite-supported metal complexes of rhodium and of ruthenium: a general synthesis method influenced by molecular sieving effects

A general method for synthesis of supported metal complexes having a high degree of uniformity is presented, whereby organometallic precursors incorporating acetylacetonate (C(5)H(7)O(2)(-), acac) ligands react with zeolites incorporating OH groups near Al sites. The method is illustrated by the reactions of Rh(acac)(CO)(2) and of cis-Ru(acac)(2)(eta(2)-C(2)H(4))(2) with zeolites slurried in n-pentane at room temperature. The zeolites were H-Beta, H-SSZ-42, H-Mordenite, and HZSM-5. Infrared (IR) and extended X-ray absorption fine structure spectra of the zeolites incorporating rhodium complexes indicate the formation of Rh(CO)(2)(+) bonded near Al sites; similar results have been reported for the formation of zeolite-supported Rh(eta(2)-C(2)H(4))(2)(+) from Rh(acac)(eta(2)-C(2)H(4))(2). IR spectra of the supported rhodium gem-dicarbonyls include sharp, well-resolved nu(CO) bands, demonstrating that the sites surrounding each metal complex are nearly equivalent. The frequencies of the nu(CO) bands show how the composition of the zeolite influences the bonding of the supported species, demonstrating subtle differences in the roles of the zeolite as ligands. When the zeolite has pore openings larger than the critical diameter of the precursor organometallic compound, the latter undergoes facile transport into the interior of the zeolite, so that a uniform distribution of the supported species results, but when the precursors barely fit through the zeolite apertures, the mass transport resistance is significant and the supported metal complexes are concentrated near the pore mouths.