Metal supported on carbon based materials for adsorptive desulphurisation of fuels

Abstract

The transport industry is one of the biggest contributors to air pollution, with the major air pollutants being COx, NOx, and SOx. SOx is produced by the combustion of the organic sulphur compound found in fuels, e.g. thiophene, mercaptan and sulfides. Adsorptive desulphurization with novel adsorbents was investigated at ambient conditions in this study, to find an economically viable and effective alternative method of removing sulphur from diesel fuel. First, a review of adsorbent improvement strategies was done, including their effects on thermodynamics, kinetics and equilibrium adsorption isotherms, as well as the screening of the most promising adsorbent. Activated carbon (AC) was the most promising adsorbent, based on the figure of merit (FoM), while metal-organic frameworks (MOFs) were the most active. There is no consensus on the parameters that have the most influence on adsorption activity, and machine learning presents an opportunity to investigate this phenomenon. In this work, three regression techniques were used in research: linear regression; multiple regression; random forest. The findings suggest that adsorbent properties (metal ion, metal properties, surface area and pore volume) need the most attention in order to improve adsorbent activity. The first experimental work done was to screen a number of commercial adsorbents using both model and conventional diesel. AC showed good activity with both model diesel and conventional diesel. For the first time, the sulphur adsorption order for conventional diesel produced in South Africa was reported on, with decreasing order of: 4-MDBT>> 4,6-DMDBT̴̴̴̴̴̴̴̴̴ 4 E,6-MDBT̴̴̴̴̴̴ 2,4,6-TMDBT̴̴̴̴̴̴1,4,6-TMDBT. The study also investigated the effect of different supports and Lewis acid metals, and the analysis indicated that AC and NiO were the most promising. The high activity level of NiO was attributed to it having the lowest acidity level, based on the ionic-covalent parameter. MOFs are an emerging class of porous materials that are constructed from metal-containing nodes and organic linkers. They have the potential to be easily manipulated to synthesize an adsorbent with unique properties. In this study, Ni-doped MOFs (Ni-BDC) and the composites of AC@Ni-BDC were synthesized in the presence of formic acid, which has evidenced three effects, namely: i) accelerating MOF synthesis; ii) modulating crystallite size; iii) controlling crystallinity. The experimental results showed that modulated synthesis of Ni-based MOFs using formic acid improved the overall adsorptive activity of MOF almost twofold. The adsorption activity of the composite towards thiophene (TH) was the average of the two materials (i.e. AC and Ni-BDC), while the activity doubled for dibenzothiophene (DBT) and 4, 6-dimethyldibenzothiophene (4, 6 DMDBT) with respect to the expected average. The improved activity was attributed to enhanced pore structure, crystallinity and synergistic effects that produce stronger acidic sites. Finally, the synthesized composite has the potential to remove the sulphur compounds in a broad spectrum.