(215aa) New Adsorbents and Material Modifications- An Approach for Ultraclean Fuels

The present research analyses the performance and characterization studies of new class of modified solid adsorbents as an approach for production of ultraclean fuels. The conventional hydrodesulfurization (HDS) process has limitations in producing ultraclean fuel due to high operational cost involving the use of high pressure, temperature and H₂ in catalytic conversion of S compounds in liquid fuels to H₂S and corresponding hydrocarbon. The research is a continuation of our efforts in a) ultralow sulphur emission of transportation fuels as organosulphur compounds are responsible for SOx emission, a highly undesirable air pollutant and b) meeting increasingly stringent Environmental and Government regulations for sulphur content in transportation fuels. The study is also significant for future fuel cell applications which require practically zero sulphur content to avoid poisoning of catalysts.

There have been numerous studies on variety of adsorbents, single as well as modified, for the adsorptive desulfurization of fuels. The adsorbents studied in the literature include simple types such as activated carbons, zeolites, and also more complex types such as modified zeolites, metal oxides, adsorbents derived from biomass and π-complexation adsorbents. It was observed that the capacity for sulphur removal was far low for its commercial use in most cases and/ or performance is severely limited in real fuel desulfurization, mainly due to presence of refractory sulphur compounds and aromatics.

It is necessary to understand interactions of adsorbing components for effective removal of sulphur compounds with increased selectivity to refractory compounds. At present there are no theoretical guidelines in this regard and approach to selection and modification of adsorbent is purely empirical. The present study is directed to evaluate interactions of adsorbing sulphur compounds with the materials and material modifications in terms of nature of modifying metal species, percentage substitution of metal species, combined effect of different species on the adsorbent surfaces and overall improvement in deep desulfurization of fuels for the production of ultraclean fuels. Experimental studies have been carried out initially on single component adsorption of thiophene in iso-octane and benzothiophene in iso-octane using modified SHIRASAGI GH2_X 4/6 and SRC_X 4/6 of Japan EnvironChemicals Ltd. Sulphur content in the feed model fuel was analysed using Total sulphur analyser. The efficiency in terms of sulphur removal/adsorption capacity was found to be substantially higher than most studies reported in literature for activated carbon, zeolites, silica based adsorbents, metal doped porous materials etc. Another main aspect of the experimental investigation is further modification of the adsorbent materials using transition metal salts especially of Ni, Cu etc and evaluation of the modified new material in adsorptive desulfurization. Effect of different parameters such as temperature, adsorbent dose and initial concentration on adsorptive desulfurization was also studied. Two widely used adsorption isotherms, Langmuir and Freundlich model were used to explain the nature of adsorptive desulfurization.

The main characterization studies include BET Surface analysis, FTIR, SEM-EDX and XRD. The adsorbents used in the present study have high specific area above 1000 m².g^-1 and total pore volume of 0.710 and 0.807 cm³.g^-1 for SRCx and GH2x respectively. BJH pore size distribution analysis showed majority of the pores in microporous range with contributions from meso-microporous section in explaining the sorption behaviour of the modified adsorbents. Specific functional groups belonging to –OH, C-O and N-H group were identified by FTIR spectra which are likely to play crucial role in the desulfurization with specific interactions with sulphur moiety. The surface morphology as determined by SEM showed heterogeneous nature and irregular structure at the microscopic level. The surface analysis and composition was further elaborated by Energy Dispersive X-ray spectroscopy confirming the main compositions of the adsorbents as C and O with admixtures of Al and Si in case of GH2x and SRCx whilst Ni modified adsorbents showed the highest impregnation of ~4% in range of concentrations prepared with maximum strength 0.5 M Ni salt deposition. XRD patterns of modified adsorbents depicted two broad bands around 2θ = 24 and 43⁰ indicating amorphous structure, characteristics of the carbonaceous adsorbent matrix. In XRD of GH2x and SRCx , no other peaks that can be assigned to oxides of Al and Si are located which further indicated that these components are well dispersed in the carbon matrix.

The desulfurization behaviour changes with the nature of sulphur compound which is attributed with the specific interaction of modified surfaces with the sulphur compounds. The interactions and their characterization is most critical for the production of ultraclean fuels as all refractory sulphur compounds need to be removed from the fuels for commercial applications. Role of aromatics is also very important in this regard as these are likely to compete in adsorption process thereby reducing removal capacity for sulphur compounds. Thus, there is always a further scope in developing newer material and modified materials that could selectively remove refractory sulphur compounds from diesel and other transportation fuels. The present research therefore addresses key environmental issue of sulphur removal with new and modified adsorbents materials offering promising approach towards environmental sustainability and pollution control. The methodology can also provide insights into production of ultra clean fuels when process intensified with HDS process.