The use of novel mesoporous molecular sieve MCM-41 for the sorption and separation of neutral and overbased sulphonate and phenate lubricant detergents
An area of industry that exploits zeolites for separation purposes is that of the petroleum industry where zeolites are used in catalytic conversions as well as chromatographic separations of hydrocarbons. The development of large pore sized materials such as MCM-41 enables larger molecules to be separated by the process of size exclusion than by traditional zeolites. In this work, mesoporous materials were synthesised and characterised using powder x-ray diffraction and nitrogen surface studies and shown to have different pore sizes in the region of 2 nm to 6 nm. They were then compared to traditional size exclusion gels and zeolites for their ability to sorb overbased and neutral sulphonate and phenate detergents, which were Soxhlet extracted from base oil, characterised by infrared spectroscopy and the results used to predict possible separations of binary detergent mixtures. The sorption properties of the mesoporous and silica gel materials were determined as follows: - (i) batch slurry sorption uptake to determine the maximum amount of detergent adsorbed within the pores of the material which showed an uptake of neutral sulphonate in the range of 0.12 to 0.37 g/g of mesoporous material and neutral phenate in the range of 0.11 to 0.30 g/g of mesoporous material with the uptake apparently increasing with pore diameter and aluminium content. Only surface adsorption occurred on the silica gel materials in the range of 0.3 to 0.6 gig of silica gel. (ii) The materials were prepared as liquid chromatography columns which showed that the neutral detergents were retained by the mesoporous materials and not the silica gels whereas the overbased detergents were not retained within the pores of either type of material. (iii) Using the mesoporous materials as the packing in gas chromatography columns to determine the heats of adsorption of molecules which were constituents of the detergent molecules investigated. It was found that the heat of adsorption of alkyl chains were twice as large for the MCM than for the silica gel materials for example, dodecane with a heat of adsorption in the range of 101 to 111 KJ/mol on the mesoporous materials and in the range of 44 to 47 KJ/mol on the silica gel materials. The data also showed that very polar compounds such as phenol were completely retained by the columns suggesting strong interaction with the silicate material. The sorption data was then applied to the column separation of a series of overbased and neutral detergent mixtures where it was found that the neutral detergents entered and adsorbed in the iii bstract mesoporous materials whereas the overbased sulphonate was completely excluded from all the mesoporous materials and the overbased phenate adsorbed on the external surfaces of the 4 nm and 6 nm materials. The mesoporous materials were therefore able to separate the detergent mixtures whereas the silica gel materials failed to separate any of the detergent mixtures. The detergents were characterised primarily by infrared spectroscopy and the equilibrium established between the neutral detergent monomer and the overbased micelle was investigated by radiochemical labelling of the neutral sulphonate and measured using liquid scintillation techniques. The results of the equilibrium studies using radioactive neutral sulphonate suggested that exchange had occurred. The sorption uptake of radioactive neutral sulphonate determined using the liquid scintillation technique was found to be in agreement with the uptake value determined by infrared spectroscopy of 0.21grams per gram of material.