The synthesis, characterisation and sorptive properties of functionalised siliceous-MCM 41 materials

Singh, Udayshanker (2003). The synthesis, characterisation and sorptive properties of functionalised siliceous-MCM 41 materials. PhD thesis The Open University.

DOI: https://doi.org/10.21954/ou.ro.0000f6f3

Abstract

A range of chemically-tailored mesoporous MCM 41 materials has been synthesised using two methods namely sol-gel synthesis, and post-synthetic grafting. Purely siliceous-MCM 41, a series of mesitylene-swollen MCM 41, metal-containing MCM 41 (Fe and Ti) and. organically-modified (3-aminopropyl and Schiff base) MCM 41 were synthesised by· the sol-gel synthesis method. Metal-grafted (Fe and Ti) MCM 41, organically-modified (3-aminopropyl, Schiff base and azodye) MCM 41 and the metal(Cu, Fe and Ni)-Schiff base-modified MCM 41 were prepared by the post synthetic grafting method.
The materials were characterised using a number of techniques including powder-X-ray diffraction (p-XRD), infrared spectroscopy, CHN microanalysis, atomic absorption spectroscopy, nuclear magnetic resonance spectroscopy, X-ray photoelectron spectroscopy, transmission electron microscopy, and by the sorption of gases (N₂ and C0₂) and vapours (water, n-hexane,·methanol, n-butanol, t-butanol).
p-XRD showed that all the materials synthesised possessed hexagonal periodicity apart from the sol-gel synthesised 3-aminopropyl-modified MCM 41 and the Schiff base-modified MCM 41 materials, in which the hexagonal structure collapsed after template removal.
The porosity of all the materials was confirmed by the Type IV (mesoporous) and Type I (microporous) isotherms yielded from nitrogen (77 K), C0₂ (195 K) and alcohols (303 K) sorption. A decrease in the surface area (1150 m² g^-1), total pore volume (0.74 cm³ g^-1) and average pore diameter (26 A) of the parent Si-MCM 41 was observed following functionalisation.
The· sorption of water on a number of synthesised materials-yielded Type V isotherms and confirmed that the adsorption of the water molecules on the surface occurred via polar interaction with the surface hydroxyls. The adsorption of C0₂ was thought to occur by a different mechanism to N₂ sorption i.e. via interaction with polar groups on the surface rather than via monolayer formation. The nature of the n-hexane sorption showed the high organophilic character and water sorption demonstrated the hydrophobic character of the surface. The adsorptions of alcohols (MeOH, n-BuOH and t-BuOH) were found to be intermediate in character between those of water and n-hexane suggesting adsorption both by polar interaction with the hydroxyl groups, and by interaction of the alkyl groups of the alcohols with the surface.
Chemisorption of adsorptives, water and alcohols, of the Si-MCM 41 surface was confirmed by infrared spectroscopy. The numbers of surface hydroxyls were found to decrease following modification.
Sorption of two hexene isomers 1-hexene and 3,3-dimethyl-1-butene were carried out on a supplied sample of Ru-MCM 41 in order to investigate the observed enhanced catalytic activity following heat-treatment (at 623 K for 16 h under reduced pressure of <10^-5 torr). The Ru-MCM 41 material showed higher uptake of 1-hexene following heat-treatment. The adsorption of the isomer 3,3-dimethyl-1-butene was found be sterically hindered on the surface.
X-ray photoelectron spectroscopy was evaluated as an analytical technique to investigate the distribution of metal complexes within the pores and on the external surface of the Cu-Schiff base-modified MCM 41 materials. The argon etching technique showed that between 113 and 114 of the copper-Schiff base complex was located in the pores, whereas the remainder was present on the external surface, blocking some of the pores. The quantitative information obtained from XPS was found not to be reliable for this class of materials.