NEW ADSORBENTS AND MATERIAL MODIFICATIONS- AN APPROACH FOR ULTRACLEAN FUELS Laxmi Gayatri Sorokhaibam, Vinay M. Bhandari and Vivek V. Ranade CSIR-National Chemical Laboratory, Pune, India e-mail: ls.gayatri@ncl.res.in; vm.bhandari@ncl.res.in; vv.ranade@ncl.res.in Abstract Experimental studies on sulfur removal by adsorption method on new forms of adsorbents along with their transition metal incorporated forms are presented. These adsorbents have been specially/selectively modified for removal of sulfur from liquid fuels like diesel and gasoline. The characterisation results revealed the presence of elements like Al, Si, O and successful incorporation of metals like Ni and Cu. The performance of these modified materials on adsorption desulfurization was investigated mainly using benzothiophene and mixtures comprising of thiophene, benzothiophene and dibenzothiophene as an approach to mimic commercial liquid fuel. The adsorption data are modelled by different isotherm equations and mechanism of interaction of the adsorbent surface with the sulfur moieties to provide fundamental understanding on selectivity and role of modification are also analysed. The present work shows that research on tailoring, fabrication, modification of adsorbent materials is a promising approach in an effort to reduce the cost of desulfurization by conventional hydrodesulfurization method when used in combination. Keywords: Desulfurization, Adsorption, Fuel, Separation, Pollution Introduction Ultraclean fuels have created a worldwide heightened interest due to a) growing environmental concern of SOx emission from liquid fuels like gasoline and diesel, b) demand for lower and lower sulfur contents as required by different government regulations and, c) the need for zero sulfur fuels in fuel cell applications to avoid poisoning of the catalyst. The nature and quantities of different sulfur compounds in different types of fuel such as diesel, gasoline or jet fuel are substantially different and therefore warrant specific solutions to each type of fuel as far as sulfur removal is concerned to make it ultra clean. As a result, generalized solutions are not available as yet and there is an urgent need for devising more suitable, techno-economically feasible solutions on deep desulfurization, especially in terms of increased capacities for sulfur removal and for the removal of refractory compounds. Common sulfur compounds in transportation fuels include mercaptans, sulphides, disulfides; and refractory groups like thiophene (T), benzothiophene (BT), dibenzothiophene (DBT) and 4, 6-dimethyldibenzothiophene which are quite difficult to be removed. The conventional method of sulfur removal from liquid fuels is Hydrodesulfurization (HDS), which is considered to be highly cost intensive operation, requiring high temperature (320-3800C) and high pressure of (3-7 Mpa) over Ni/Co catalyst1. Currently, the emission standards as fixed by USEPA are 30 and 15 ppm for gasoline and diesel2 respectively. In European countries, quality of diesel fuels as specified by the EN 590 standard3, the permissible limits for sulfur content in fuels is 10 ppm. As already stated, though HDS process can reduce the sulfur content below 10 ppmw in diesel4, it is very capital intensive and difficult for higher alkyl/cyclic refractory sulfur compounds. Thus in order to meet the demand for ultra-clean fuels for environmental protection, newer approaches and modification of the existing technologies is required for removal of these refractory sulfur compounds. In this context among the various emerging technologies, research in adsorptive desulfurization using new class of adsorbents and modification of the existing materials to increase the efficiency in sulfur removal capacity is taking into new heights. The development of effective adsorbent which can selectively remove sulfur compounds from real liquid fuels is a major challenge as commercial diesel and gasoline comprises of a number of aromatic, cyclic organic compounds apart from these refractory sulfur compounds. The main objective of the present study is to evaluate/ develop/ design suitable adsorbents with high capacity and selectivity to reduce the sulfur levels to ultra-low levels and obtain more insight into sulfur removal behaviour on adsorbent surfaces. Adsorptive Deep Desulfurization For Ultra-clean Fuels The reported adsorbents for desulfurization include activated carbon5,6, metal doped zeolites7,8, silica, alumina and so on. The nature of the adsorbent, presence of specific functional groups, active sites for sulfur moieties to bind, porosity-meso/microporous, surface area etc play different roles in the overall adsorbent development. These elements provide ample scope for developing new adsorbents, tailoring/ modification of the existing materials with greater removal capacity. The incorporation of transition metal ions into zeolite/carbon framework9,10 has attracted considerable interest in modification of the existing materials. In this work both modified carbon based adsorbents, Shirasagi GH2x 4/6, SRCx 4/6 have been evaluated for their sulfur removal- specially for refractory sulfur compounds and further modifications using Ni/Cu loaded forms for understanding surface interactions using synthetic model fuel. These materials are hereafter referred as GH2x, SRCx, Ni-GH2x, Ni-SRCx, Cu-GH2x and Cu-SRCx for convenience. The literature reports have underlined need for research to provide detailed insights into the mechanism of selective adsorption over different adsorbents. Some studies have attempted to provide sulfur removal mechanism in the case of modified zeolites and other adsorbents by proposing π-complexation11, hydrophobic interaction, van der Waals interaction, H-bonding and/ or acid base interaction mechanism. The weak nature of bonding in these types of mechanisms, however, is not reflected in the ease of regeneration and is not thoroughly substantiated. It is expected that since adsorption method is relatively less cost intensive and feasible option for industrial application, it can be intensified with current industrial method of HDS to provide viable solution in reducing sulfur emission and meeting the present norms and regulations. In the present work we have carried out elaborate study on single component adsorption of thiophenic model fuels namely, benzothiophene in octane and multicomponent adsorption of mixture of three common organosulfur compounds, viz., thiophene, benzothiophene and dibenzothiophene in order to provide fundamental understanding, the role of adsorbent modification and ways of improving adsorption efficiency. The sulfur specific adsorbents were then evaluated using synthetic model diesel comprising thiophene, benzothiophene and dibenzothiophene. Experimental Materials And Analysis Modified carbon based adsorbents, Shirasagi GH2x 4/6, SRCx 4/6 and metal impregnated forms (Ni/Cu) were studied. The precursors, GH2x 4/6 and SRCx 4/6 were obtained from Japan Envirochemicals Ltd. GH2x is pelletized form of coconut shell based formulated for adsorption of sulfur compounds while SRCx is impregnated form of modified carbon effective for adsorption of sulfur compound. All chemicals, octane, thiophene, benzothiophene, di-benzothiophene, NiCl2.6H2O and Cu (NO3)2.3H2O used were of high purity obtained from Sigma-Aldrich. Model fuels were prepared in octane solvent having known predetermined initial concentrations and batch equilibrium experiments using predetermined amount of adsorbents were carried out for equilibration time of 6 h. Adsorption experiments were carried out normally at 300C and also using different temperatures for investigating effect of temperature. The adsorbents were activated at a temperature of 2000C for 12 h prior to use. Total sulfur analyser, TN-TS 3000, Thermoelectron Corporation, Netherlands was used for investigating the residual sulfur concentration after adsorption. Synthetic fuel is prepared by mixing equal proportions (100 mg.L-1) of thiophene, benzothiophene and dibenzothiophene and 10 mL of the aliquot was used for batch run using GH2x and SRCx. For analysis of synthetic fuel, Gas Chromatograph (Agilent GC 7980) with FPD was used. Preparation Of Metal Incorporated Adsorbents A series of nickel chloride and copper nitrate salt solutions of different concentrations range 0.1-0.5 M in 0.01 M HCl were prepared. After the initial screening, we have selected 0.5 M, as considerable metal impregnation resulted at this particular concentration as confirmed by EDX data. Further higher concentrations were not attempted as it may also result in blocking of active sites. 3 g each of GH2x and SRCx were treated with 25 mL acidic salt solutions and stirred continuously at 120 rpm maintained at a temperature of 60° C in an orbital shaker for 6 h and thereafter filter and washed to remove excess acid until the pH of the mother liquor is almost neutral. They are referred as Ni-GH2x, Ni-SRCx, Cu-GH2x and Cu-SRCx depending on the nature of the precursor and salt solution we have selected for the purpose of modification. Finally, they are dried at room temperature and activated at 200 °C for 16 h prior to batch run. The main objective of this modification is to test whether there is enhancement in adsorption performance over these already modified forms of activated carbon. Characterization Methods FTIR results were investigated for GH2x and SRCx. In the present work, spectrum for Ni-GH2x using KBr pellet method on Cary 600 FT-IR (Agilent) spectrophotometer is reported. XRD measurements were carried out by using Pan Analytical Diffractometer in wide angle range. Scanning electron microscopic (SEM) studies were done by Leo- Leica, Stereoscan 440, Cambridge, U.K. The BET surface areas of the samples were determined by N2 adsorption isotherm at 77 K using Quantachrome Autosorb Automated gas adsorption system. Energy Dispersive analysis of X-ray (EDX), Bruker, Quanrax-200, Berlin Germany was performed using EDX associated with SEM. Results and Discussion Adsorbent Characterization Characterizations by various technologies were conducted for selectivity analysis and mechanistic understanding over these modified adsorbents. XRD patterns for GH2x and SRCx exhibited well resolved peaks at 2θ value of 24 degree and 43 degree, characteristic of carbonaceous adsorbent matrix. Figure 1 shows the FTIR spectrum of Ni-GH2x. Weak broad band at 1184.06 cm-1 is the stretching vibration of C-N bond while the medium band at 1498.40 cm-1 indicates aromatic C=C stretching. Depending on the nature of modification and the ratio of Al/Si, the peak position for Si-O-Si (Al) which generally present around 1000 cm-1 is shifted below and observed as a broad peak around 971 cm -1. The presence of certain functional groups belonging to C-O stretching, N-H bending and OH group have been indicated in our earlier studies. SEM images of Ni and Cu modified GH2x are presented in Figure 2. It shows disordered structure with pores of non-uniform sizes. Aggregates of metal deposition can be seen from the gray and white patches appearing in contrast. Similar observations can be made from the SEM images of Ni-SRCx and Cu-SRCx, though Cu depositions were found to be higher compared to Ni forms. The EDX report indicated the presence of elements like C, O, Si and Al common to all the four adsorbents while specifically incorporated Ni and Cu are present in Ni modified and Cu modified forms. Among these metal impregnated forms, Ni forms have nearly 78 % carbon while the carbon content is relatively lower for Cu forms. Other important information that could be noted from the data include Al/Si ratio which is comparatively higher in SRCx forms. Al/Si in Ni-SRCx is 4.55, 0.4 for Ni-GH2x while the ratio remains almost same in Cu modified forms. Apart from EDX, proximate analysis method of ash content was measured for Ni modified forms where the % ash content for Ni-SRCx is 1.77 and 1.23 for Ni-GH2x complementing the EDX data for higher inorganic content in Ni-SRCx. All adsorbents, GH2x and SRCx have high surface area and metal impregnated forms namely, Ni-GH2x has slightly lower surface area of 1077 m2. g-1 and relatively lower pore volume of 0.711 cm3.g-1 with average pore diameter of 0.264 nm. These values for the starting material, GH2x and SRCx are 1329 m2.g-1 and 1098m2.g-1 respectively. The lower surface area in the Ni modified forms may be due to blocking of the pores. Figure1. FTIR spectra of Ni-GH2x A B Figure 2. SEM images of A) Ni-GH2x and B) Cu- GH2x Adsorption studies Adsorptive desulfurization performances of the different modified adsorbents, including the adsorptive capacity and selectivity, were examined by using model fuels containing known sulfur compounds in batch adsorption system. Figure 3. Adsorption isotherms for benzothiophene on modified adsorbents As seen from the adsorption isotherms (Figure 3), the order of desulfurization using benzothiophene as model fuel follows SRCx> GH2x> Ni-SRCx> Ni-GH2x. Adsorption modelling with Langmuir and Freundlich isotherm showed Freundlich isotherm giving better fit with very high R2value for benzothiophene model fuel using GH2x, Ni-GH2x and Ni-SRCx while the Langmuir isotherm fitted well for SRCx. The surface oxygen functional groups might play an important role in adsorptive desulfurization over carbon based adsorbents. FTIR studies of GH2x and SRCx also show the presence of surface oxygen groups. The physical characteristics of the adsorbents like BET surface area, pore volume, pore size distribution also has crucial contribution in adsorption process. Ni modified forms of GH2x and SRCx did not show any effective improvement in the removal capacity. On the other hand, the adsorption behaviour here showed lower adsorption at low concentrations while capacity at higher concentrations was similar to that without Ni-modification. Similar trends were indicated in case of Cu-GH2x and Cu-SRCx. It is possible that Ni- or Cu modification does contribute in the sulfur removal capacity however at the cost of existing active sites thereby giving no significant improvement in the overall capacity. This probably also explains the fact that overall capacity remains largely unaffected in spite of possible blocking of the pores as is evident in decreased surface area of Ni modified adsorbents. The lower capacity at low sulfur concentrations for Ni- and Cu-modified adsorbents may be partly attributed the increased reversibility that represents much weaker adsorption at low concentrations. Since GH2x and SRCx have been specially tailored for removal of sulfur compounds, further modification either in Ni or Cu form therefore appears to be ineffective in enhancing its adsorption property or selectivity. In this regard, further research is necessary. Figure 4. Temperature effect of benzothiophene adsorption over A) GH2x and B) SRCx The study on effect of temperature indicated slight changes on adsorption behaviour/capacity (Figure 4) in the range 30 to 50 C, clearly indicating role of impregnation or modification of surface. Although for physical adsorption mechanism, the temperature is expected to have significant impact, the chemical nature of interactions in these modified adsorbents have probably dampened the temperature effect and hence not much difference or adverse effect is seen the case of both GH2x and SRCx, though reduced capacities at higher temperatures in adsorption is logical. as: The selectivity effect on the mixture was investigated in detail using the definition %S x qe ( x ) qe ( x ) qe ( y ) qe ( z ) 100 (1) where, %S x is the % selectivity of the x component in the mixture consisting of x, y, z components. Figure 5. Selectivity for sulfur compounds in synthetic fuel It is evident from Figure 5 that both the modified adsorbents have good capacity for sulfur removal for refractory compounds such as benzothiophene and dibenzothiophene. Further, the difference in the adsorption of different sulfur compounds is not significant, especially at low sulfur concentrations. At higher total sulfur concentration and lower adsorbent dose (0.1 g), the effect of selectivity over GH2x and SRCx adsorbent is higher for dibenzothiophene, followed by benzothiophene and thiophene while the effect is less pronounced with the increased adsorbent dose or lower total sulfur concentration at equilibrium. The nature of modification and selectivity towards the refractory sulfur compounds clearly underlines the utility of such adsorbents in obtaining ultra-clean fuels. It also underlines the importance of understanding surface interactions that dictate the high capacity for such adsorbents over other conventional adsorbents such as activated carbons. The results of deep desulfurization, both in terms of higher capacity and surface modifications indicate promising advantage for commercial application. Conclusion The adsorption performance of modified carbons and metal loaded adsorbents was studied to obtain fundamental understanding of adsorption mechanism and selectivity. 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