Search

Recently reported groundbreaking discovery of efficient delamination of zeolite MCM-56, producing colloidal suspensions of MWW monolayers dispersed in the liquid phase, created unprecedented possibilities for the synthesis of a zeolite catalyst. Based on this innovation, the concept of using MWW monolayers to prepare silica-supported zeolite nanosheet catalysts suitable for transformations of large organic molecules was explored in this work. A series of silica-MWW preparations was synthesized from colloidal suspensions of the monolayers, using both solid and colloidal silica sources. The synthesized solids were thoroughly characterized with various physicochemical methods and their catalytic performance was tested in alkylation of mesitylene with benzyl alcohol. The obtained results indicate that solids containing MWW layers dispersed on silica show promising catalytic properties. The mixed MWW:silica catalysts synthesized from dispersions of MWW monolayers and liquid silica were found to exhibit high specific catalytic activity (with TOF values of 3.4 × 10−3 to 4.8 × 10−3 s−1), despite the high content of inactive amorphous silica support (40–60%). Materials synthesized from solid fumed and precipitated silicas showed low or negligible overall activity, which could be attributed to the small incorporation of the zeolitic active phase. For one of such materials, a notable high TOF (4.8 × 10−3 s−1) was found. It was found earlier that ethanol is an effective flocculent for zeolite layers by themselves, but in the presence of solid silica its efficiency was reduced. [ABSTRACT FROM AUTHOR]

Structural defects in metal–organic frameworks can be exploited to tune material properties. In the case of UiO-66 material, they may change its nature from hydrophobic to hydrophilic and therefore affect the mechanism of adsorption of polar and non-polar molecules. In this work, we focused on understanding this mechanism during adsorption of molecules with different dipole moments, using the standard volumetric adsorption measurements, IR spectroscopy, DFT + D calculations, and Monte Carlo calculations. Average occupation profiles showed that polar and nonpolar molecules change their preferences for adsorption sites. Hence, defects in the structure can be used to tune the adsorption properties of the MOF as well as to control the position of the adsorbates within the micropores of UiO-66. Structural defects in metal–organic frameworks can be exploited to tune material properties. Here, a combined theoretical and experimental study demonstrates that the introduction of defects to UiO-66 alters its nature from hydrophobic to hydrophilic, affecting the adsorption mechanism of polar and non-polar molecules. [ABSTRACT FROM AUTHOR]

Quasi equilibrated temperature programmed desorption and adsorption (QE-TPDA) of hexane and cyclohexane was applied for characterization of zeolites 5A, ZSM-5, 13X, Y, NaMOR and ordered mesoporous silicas MCM-41, MCM-41/TMB, SBA-15 and HMS. Similar QE-TPDA profiles of hexane and cyclohexane with a single desorption maximum were observed for the wide pore zeolites. No adsorption of cyclohexane for zeolite 5A and a single desorption maximum for ZSM-5 were found, while two-step desorption profiles of hexane were observed for these zeolites. Similar values of the adsorption enthalpy and entropy of hexane and cyclohexane were obtained by fitting the Langmuir model functions for the zeolites X and Y. For NaMOR and ZSM-5 larger differences in these parameters were found. A single desorption peak found at low temperatures in the QE-TPDA profiles of hexane and cyclohexane for the studied silicas was attributed to the multilayered adsorption on their mesopore surface. The adsorption isobars calculated from the thermodesorption profiles were fitted with the BET function. This way values of the specific surface area and the adsorption heat were calculated. Additionally values of the initial heat of adsorption were found by fitting the Henry's law to the high-temperature sections of the linearized isobars. The largest deviations from the BET and Henry functions and the largest values of the adsorption heats found for SBA-15 indicated the greatest heterogeneity of the adsorption sites on its surface. [ABSTRACT FROM AUTHOR]

Thermoporosimetry, i.e., DSC measurements of melting point depression of water and heptane confined in mesopores, has been used for determination the pore size distribution of several mesoporous silicas synthesized with the use of micelle templates. Porosity of these materials was additionally characterized by low-temperature nitrogen adsorption and quasi-equilibrated thermodesorption of nonane. The pore size distributions obtained using the water thermoporosimetry were similar to those determined using the other methods, but the pore size values found for the narrow pore materials were underestimated by ca 1 nm. Too large pore sizes obtained for the wide pore silica from heptane thermoporosimetry were attributed to nonlinear dependence of the melting point depression on the reciprocal of the pore size. [ABSTRACT FROM AUTHOR]

dsorption of n-hexane and n-nonane on mesoporous micelle-templated silicas (SBA-15 and MLV-multilayer vesicle) and on their carbon replicas (CMK-3 and OCM-onion-like carbonaceous material) was studied by means of quasi-equilibrated thermodesorption technique, utilising a standard TPD setup with a chromatographic detector. Analysis of n-hexane adsorption isobars, determined from the thermodesorption profiles, revealed substantial heterogeneity of the adsorption sites present in the carbonaceous materials. The pore size distributions calculated from the thermodesorption profiles of n-nonane for both siliceous and carbonaceous materials were in agreement with those obtained from the low-temperature nitrogen adsorption isotherms. They confirm an uniform structure of mesopores in SBA-15, CMK-3 and OCM as well as a more complex structure of mesopores in the MLV materials. [ABSTRACT FROM AUTHOR]

Equilibrated thermodesorption (TPED) and quasi-equilibrated temperature programmed desorption and adsorption (QE-TPDA) were employed as methods for studying the influence of different extraframework cations (Na, K, Li, Cu, Zn, or Mg) on adsorption of n-hexane on ZSM-5 zeolite with high Al content (Si/Al = 15). Considerable influence of the cations on both initial adsorption in the micropores and ordering of the adsorbed molecules, occurring at high coverages, has been observed. This influence is reflected by the values of the adsorption enthalpy and entropy, determined by fitting the dual site Langmuir (DSL) adsorption function to the equilibrated thermodesorption profiles. However, no clear correlation between the determined parameters and properties of the extraframework cations could be found. [ABSTRACT FROM AUTHOR]

Thermodesorption of n-hexane and n-nonane from the acidic and non-acidic zeolites ZSM-5 and Y was studied by means of the thermogravimetric temperature programmed equilibrated desorption (TPED) and quasi-equilibrated temperature programmed desorption and adsorption (QE-TPDA). Micropore volumes determined from the adsorption capacity of n-hexane were close to those determined by N2 adsorption. Content and strength of acid sites in the acidic zeolites estimated by fitting the Arrhenius equation to the high temperature parts of QE-TPDA profiles of n-hexane attributed to its cracking were in agreement with their acidity characteristics obtained by IR spectroscopy of chemisorbed pyridine. The mesopore volume was determined from the QE-TPDA profiles of n-nonane. Coking observed only for QE-TPDA of n-nonane on H-USY zeolite resulted in blocking the micropores without affecting the mesopores. [ABSTRACT FROM AUTHOR]

High selectivity of VO/SiO catalysts prepared from fumed silica, observed in oxidation of CH to HCHO at 550-590°C, was attributed to the content of highly dispersed, easily reducible surface vanadia species. [ABSTRACT FROM AUTHOR]