Roberto Buzzoni

Hydroxyls nests in defective silicalites and strained structures derived upon dehydroxylation: vibrational properties and theoretical modelling

Defective silicalite, an efficient and selective catalyst in the gas phase Beckmann rearrangement reaction, has been characterised by infrared spectroscopy and by molecular modelling techniques. We report a detailed IR study on the effect of outgassing treatments at increasing temperature on silanols bands and on framework modes. The effect of a temperature decrease up to 100 K (during the IR measurement) on the H-bonding interactions has also been investigated. The interaction of silanols with mesitylene, a probe molecule which cannot penetrate the channels, has been studied in order to distinguish between internal and external OH groups. Molecular mechanics and ab initio methods have also been used to model the structure and the vibrational features of a properly designed nest in order to support the assignments of the IR spectrum.

Characterisation of defective silicalites

The structure and the adsorptive properties of internal defects in two chemically very pure silicalite samples, characterised by a high and low concentration of defects (internal SiOH nests generated by Si vacancies), are investigated by means of several physical methods (neutron diffraction, IR spectroscopy of adsorbed NH3, microcalorimetry of NH3 adsorption and quantum chemical computation). The role of cooperative effects in hydrogen bonded chains and rings of silanols covering the internal defective nanocavities in influencing the absorptive properties of the material is elucidated. This study provides an explanation of the less hydrophobic character of defective silicalite relative to perfect silicalite and sheds light on the reason why defective silicalite acts as an efficient and selective catalyst for the gas phase Beckman rearrangement reaction where cyclohexanone oxime is converted into caprolactam.

Atom pair potential for molecular dynamics simulations of structural and dynamical properties of aluminosilicates: test on silicalite and anhydrous Na-A and Ca-A zeolites and comparison with experimental data

An empirical atom pair potential for aluminosilicates including coulombic interactions explicitly, and suitable for molecular dynamics simulations, has been tested on an aluminium-free molecular sieve (silicalite) and on the anhydrous aluminium-rich zeolite A, containing Na or Ca cations. This represents a first step towards a realistic potential that is able to reproduce the details of the structure and dynamics of aluminosilicates. The results are in reasonable agreement with experimental data, both structural (taken from the literature) and spectroscopic (most of which are new). For structures showing disordered extraframework ion distributions, point-charge models are insufficiently accurate and polarization effects should be taken into account. Previously proposed model potentials are discussed.

Shape selective conversion of 1,2,4-trimethylbenzene over different zeolite frameworks

Abstract The upgrading of Pseudocumene (1,2,4-TMB) to more valuable methylbenzenes over acidic zeolites with variable acidic and diffusive properties is studied, using reactivity tests and IR spectroscopy. The concurrent reactions investigated include isomerization and transalkylation. At low conversion levels the isomer distribution in C 8 , C 9 and C 10 products is not thermodynamically controlled, being different over different zeolite frameworks, indicating shape selectivity. Variable temperature experiments allow to identify the location of active sites for the different reactions on different catalysts, and the nature of shape selectivity.