Halimaton Hamdan

Solid-state NMR studies of the geometry of brønsted acid sites in zeolitic catalysts

Abstract 1 H, 27 Al and 29 Si solid-state NMR was used to determine the geometry of the Bronsted acid site in zeolites. The mean distance between acidic protons in bridging hydroxyl groups and the nearest framework Al atom is 2.38±0.04 A in zeolite H–Y and 2.48±0.04 A in H-ZSM-5. The quadrupole frequency of framework Al is 1.02±0.05 and 3.1±0.8 MHz in dehydrated Na-Y and H-Y, respectively.

29Si MAS NMR, XRD and FESEM studies of rice husk silica for the synthesis of zeolites

We have used rice husk ash as a silica source for the synthesis of zeolites. Amorphous silica is a highly reactive silicon source, but not all silica which is amorphous to XRD is equally suitable. The local structure of amorphous silica was investigated by 29Si magic-angle spinning nuclear magnetic resonance (MAS NMR) and field emission scanning electron microscopy (FESEM) in order to optimize the process. The results show that amorphous silica extracted by physical combustion with controlled temperature contains only ∗Si(OSi)4 tetrahedral units and is the most reactive silica source in the synthesis of zeolite Y, compared to other silica samples prepared by chemical extraction and uncontrolled burning. The presence of crystalline cristobalite and trydimite phases and SiOH groups render the silica less active.

Alumination of the purely siliceous mesoporous molecular sieve MCM-41 and its hydrothermal conversion into zeolite Na-A

Purely siliceous mesoporous [Si]-MCM-41 has been aluminated using an aqueous solution of sodium aluminate, NaAlO2, to form [Si,Al]-MCM-41 with the framework Si/Al ratio as low as 1.9. 27Al and 29Si magic-angel-spinning (MAS) NMR show that all aluminium is incorporated in the framework. The well resolved X-ray diffraction (XRD) pattern of calcined [Si,Al]-MCM-41 indicates that the mesopore system is stable and that the channel diameter decreases with the increased degree of incorporation of Al. The intensity of lines corresponding to Si(nAl) building blocks in the 29Si MAS NMR spectra reflect changes in the local atomic arrangement upon alumination. Increased reaction temperature at a constant concentration of NaAlO2 enhances the efficiency of the alumination, but increased concentration at constant temperature disrupts the framework structure. Upon treatment with a concentrated (>1 mol 1–1) solution of NaAlO2 at moderate temperatures (100 ± 20 °C), [Si]-MCM-41 is transformed into crystalline zeolite Na-A.

The ionic size of metal atoms in correlation with acidity by the conversion of cyclohexanol over MeAPO-5

The conversion of cyclohexanol was investigated over MeAPO-5 [Me 5 Mg, Co, Mn and Zn]. The acidity of the catalysts was correlated with the ionic radii of Me atoms incorporated into the structure of the catalysts. Based on this correlation, it is suggested that the distortion of structure by the larger Me atoms was associated with the T‐O‐P angle and hence the acid strength.

Isomorphous substitution of framework silicon by aluminium in silicalite: factors determining resolution of 29Si magic-angle-spinning NMR spectra of pentasil zeolites

Abstract The hypothesis that the resolution of 29 Si magic-angle-spinning NMR spectra of zeolite ZSM-5/silicalite is governed by tetrahedral Al content is questioned by showing that the introduction of substantial amounts of framework Al into silicalite does not impair spectral resolution. Possible alternative mechanisms are discussed.

Novel Nanostructured Photocatalyst of Vanadium‐Doped Silica‐Titania Aerogel

An excellent UV and visible irradiation response of vanadium‐doped silica‐titania photocatalysts has been successfully prepared. The designed photocatalyst possesses high surface areas of 610 m2/g. Its outstanding photocatalytic activity is attributed to the presence of tetrahedrally coordinated vanadia species in forms of Ti–O–V bonding. Besides, the incorporation of V extends the light adsorption edge of V‐doped silica titania aerogels and resulting in reduced band gap energy.

1H mas NMR and IR studies of the acidic properties of realuminated zeolite Y

IR and1H MAS NMR confirm that extra-framework aluminium present in dealuminated (ultrastable) zeolite Y is reintroduced into the framework by treatment with strongly basic solutions at elevated temperatures. The realuminated sample contains twice as many Brønsted acid sites than the ultrastable precursor and, with an accuracy of 20%, the same number of acid sites as the parent as-prepared zeolite. However, not as many hydroxyl groups associated with framework Al in the product are acccesible to pyridine as in the parent sample.

Synthesis and characterization of fibrous silica ZSM-5 for cumene hydrocracking

Inspired by the high accessibility and low diffusion limitations of fibrous silica nanoparticles (KCC-1), fibrous silica ZSM-5 (FZSM-5) was engineered using a microemulsion system. The spherical FZSM-5 with a dendrimeric silica fiber possessed a high surface area (554 m2), a wide pore diameter (2–20 nm), abundant strong acid sites and high catalytic activity towards cumene hydrocracking.

Synthesis and Characterization of Rice Husk Ash Derived - Silica Aerogel Beads Prepared by Ambient Pressure Drying

Low density silica aerogel beads were synthesized from rice husk ash via sodium silicate route. The gel beads were prepared by the ball dropping method and dried at ambient pressure after surface modification (silylation) with trimethylchlorosilane (TMCS) solution. The characteristics of porous structure of silica aerogel beads were measured by Brunauer-Emmett-Teller (BET) nitrogen adsorption and desorption method. The silica aerogel beads showed a specific surface area as high as 773 m2/g. They also displayed water-repellant effect (hydrophobic) due to silylation as revealed by Fourier transform infrared spectroscopy (FTIR).

Insulator-to-Metal Transition in Potassium-Loaded Zeolite P

Potassium metal was loaded into porous crystals of potassium-form maximum aluminum zeolite P (K-MAP), and optical absorption, electrical resistivity, and magnetic susceptibility measurements were performed. The average loading density of potassium atoms per GIS cage of zeolite P, n, was systematically changed up to 1.09. Optical absorption bands are observed around 1.3–2 eV and grow to dominate the spectrum with increasing n. These absorption bands are assigned to the optical excitations of s-electrons confined in the zigzag channels of GIS cages. The temperature dependence of electrical resistivity ρ indicates insulating properties for \(n \lesssim 1.05\). At n = 1.09, ρ suddenly decreases by several orders of magnitude, and shows metallic properties indicating that an insulator-to-metal transition occurs between n = 1.05 and 1.09. The samples are basically nonmagnetic at any n. The nonmagnetic and insulating phase at \(n \lesssim 1.05\) is explained by the formation of small bipolarons in the spin-singl...