Azfar Hassan

A comparison between β- and USY-zeolite-based hydrocracking catalysts

Abstract Two series of zeolite-based hydrocracking catalysts were prepared to study the effects of the support type, preparation method and metal loading on catalyst properties and hydrocracking activity for hydrotreated vacuum gas oil (HT-VGO). The support used was γ-Al2O3 and β-zeolite in the first series and γ-Al2O3 and USY-zeolite in the second series. Nickel and tungsten were loaded as active metals on these supports. The prepared catalysts were characterized as to their surface area, pore volume, thermal stability, reducibility and acidity characteristics. The characterization results revealed that catalysts displayed significant differences in properties dependent on the preparation method and the type of support used. Catalysts from both series showed promising results for HT-VGO hydrocracking in the batch reactor. A correlation exists between the reducibility of oxidic form and the hydrogenation activity of the sulfided form of the catalysts. The higher the reducibility, the higher the hydrogenation activity. Catalysts prepared on mixed supports gave higher amounts of saturates.

Hydrocarbon group types analysis of petroleum products: A comparative evaluation of HPLC and TLC analytical performance

ABSTRACT Four petroleum samples representing a range of hydrocarbon composition were analyzed using the HPLC and TLC–FID techniques that provided separation and quantitation of saturates, aromatics, polars (resins) and asphaltenes. The results obtained from the two methods, for residues and crude oil samples, were not comparable because these samples contain high amount of polars and asphaltenes whereas VGO shows a very good comparison between the results obtained from the two techniques because of its asphaltenes-free and low polar composition. TLC–FID separation required a number of solvents while in HPLC, only one type of solvent was used. In TLC method, a number of samples can be analyzed simultaneously while in HPLC, only one sample is analyzed at a time. HPLC system analyzes a large number of sample in a complete automated fashion without operator involvement while the TLC method needs more involvement and attention of the operators. In HPLC, the aromatics can be further classified based on number of rings while in TLC–FID, aromatics were detected as one peak. The reproducibility of TLC–FID analysis for individual fractions was found good.

Nitrogen inversion and N–O bond rotation processes in di-and tri-substituted hydroxylamines. A dynamic NMR study

The barriers to inversion in several acyclic di- and tri-substituted hydroxylamines are determined by 1H NMR band shape analysis. The barriers range from 49.1 to 66.8 kJ mol–1 and are discussed in terms of a conformational process which involves nitrogen inversion and rotation around the N–O bond. The N–benzyl group with an ortho hydroxy substituent increases the nitrogen inversion barrier by 10 kJ mol–1, which indicates the requirement of breaking of the intramolecular hydrogen bond prior to inversion.

NMR study of the anomeric effect and nitrogen inversion in some isoxazolidines

Abstract A series of isoxazolidines with various alkyl substituents at N and an ethoxy group at C(5) have been synthesised and their NMR spectra recorded over a range of temperatures. The NMR spectra at low temperature indicate the presence of two isomers due to slow nitrogen inversion. The nitrogen inversion barriers are determined using complete band shape analysis and are found to be in the range 59.3–65.6 kJ mol −1 . The strong anomeric effect exerted by the C(5) ethoxy group locks the substituent in the pseudoaxial orientation. The cis isomers are found to be more stable than the trans isomers.

Oxidation of N-benzyl-N-methylhydroxylamines to nitrones. A mechanistic study

Oxidation of various N- (o-, m-, p- substituted benzyl)-N-methylhydroxylamines has been carried out using mercury(II) oxide and p-benzoquinone (p-BQ) as oxidants. Hammett plots have been obtained with negative ρ values, showing the development of a positive centre in the transition state. The unstable E nitrones, which readily isomerize to the more stable Z nitrones, are obtained in appreciable quantities and in some cases as the major product. A considerable deuterium isotope effect is observed in the oxidation process. The overall picture of the mechanistic pathway involves electron transfer from nitrogen to the oxidant followed by hydrogen abstraction.

Nitrogen inversion and N–O bond rotation in some hydroxylamine and isoxazolidine derivatives

A series of trisubstituted hydroxylamine derivatives, both cyclic and acyclic, has been prepared. The energy barriers in these hydroxylamines are found to be dominated either by nitrogen inversion or N–O bond rotation depending on the nature of the substituents attached to the nitrogen. In several series of compounds, having XC6H4CH2 substituents attached to nitrogen, Hammett free energy correlations are obtained with positive ρ values, indicating increased electron density at the transition state for the inversion process. Isoxazolidines with C(5) ethoxy substituents demonstrate a strong anomeric effect.