Surface Modification Of Activated Carbon Using For Adsorption Denitrogenation From Liquid Hydrocarbon Stream

Due to stringent worldwide environmental regulations, refiners are facing continuouschallenges in producing increasingly cleaner fuels. However, it is extremely difficult forrefractory to produce such high purified fuels through the conventional hydrodesulfurization.The main reason for difficult to pursue ultra-low sulfur hydrocarbon steams is the hindereffect caused by the co-existed nitro-compounds. Thus, many researchers focused their workon the denitrogenation using both hydrogenation and adsorption process. Among them,adsorptive denitrogenation （HDN） showed surperiorty for the selective adsorption of nitrogencompounds and mild conditions compare with hydrodenitrogenation process. Activatedcarbon （AC） was considered as the best candidator for such application due to the feacheres,such as cheap, stable, high adsorption capacity and high selectivity. However, few works havebeen done on the fundermental understanding for the adsorption behavior of AC for nitrogencompounds in liquid hydrocarbon steams.The current thesis focused the work to develop a fundamental understanding on theselective adsorption of nitrogen compounds that are common in diesel-range hydrocarbonstreams over activated carbon. To acquire deep insight into the adsorptive denitrogenationmechanism, it is essential to explore the role of oxygen functional groups on the surface ofactivated carbon for adsorption of nitrogen compounds from liquid hydrocarbons. Thissubject has not been well described in the literature. Therefore, systematic studies foradsorption removal of nitrogen compounds from liquid hydrocarbon streams over variousactivated carbons were investigated. The specific works of the investigation were to:1. Identification and quantification of nitrogen compounds in five typical liquidhydrocarbon streams derived from petroleum and coal have been conducted by a combinationof the solid extraction pre-separation, GC-MS and GC-NPD analysis. The study shows thatusing the solid phase extraction with activated alumina as a stationary phase, and benzene andbenzene+methanol as a mobile phase, respectively, is a simple and efficient method forpre-separating and concentrating the nitrogen compounds from the petroleum-derived liquidhydrocarbon streams, while using a solid phase extraction with activated alumina as astationary phase, and benzene, chloroform, and methanol as a mobile phase in turn isnecessary for efficient pre-separating and concentrating the nitrogen compounds from the coal liquids. The identification and quantification of the nitrogen compounds in the five typicalliquid hydrocarbon streams show that the coal-derived streams not only have much highernitrogen content than the petroleum-derived ones, but also contain quite different types of thenitrogen compounds. The major nitrogen compounds in the coal-derived streams are the basicnitrogen-containing compounds; including aniline, quinoline, and their derivatives, while inthe petroleum-derived ones are the neutral nitrogen compounds, such as carbazole and itsalkyl substituted derivatives. The identification and quantification of the nitrogen compoundsin different streams in this study provide essential and valuable information for clarifying thepotential role and behavior of the nitrogen compounds in the subsequent fuel processing andapplications, and for developing the novel and more efficient method to produce ultracleanliquid hydrocarbon fuels.2. The charcoal based activated carbon was oxidized by ammonium persulfate. Thetype and distribution of oxygen-containing surface functional groups （OCFGs） on activatedcarbon before and after oxidation was monitored using elementary analysis, FT-IR,temperature-programmed desorption coupled with mass spectrometry （TPD-MS）, X-rayphotoelectron spectroscopy （XPS）, and X-ray absorption near edge structure （XANES）spectroscopy. The results obtained from these methods were compared with each other. It wasfound that using the TPD-MS method currently reported in the literature to quantify variousOCFGs on the carbon surface on the basis of TPD-CO₂and TPD-CO profiles is significantlysuperior to others due to its good accuracy and reproduction along with simple experimentalconditions. Both of the XPS and XANES are under estimated the concentration of OCFGs.3. APS is an excellent oxidant for introducing oxygen-containing functional groupsonto the surface of carbon materials, including phenol, carboxyl and carboxylic anhydridegroups, especially for formation of the carboxylic acid groups. The oxidation conditions,including temperature, time and concentration of APS in the solution, strongly influence thephysical and chemical properties of the treated carbon sample. The maximum oxygen contentof18.7mmol/g can be achieved by oxidation with the2.0mol/L APS solution at60oC for3h.The amount of the oxygen-containing functional groups and their distribution on carbonsurface can be tailored by changing the oxidation conditions. Using TPD-CO₂and TPD-COprofiles alone to quantify various OCFGs on carbon surface may underestimate the amount ofcarboxyl groups, and overestimate amount of anhydrides and lactones due to some in-situreactions during the TPD-MS analysis. The released H2O and CO in the temperature rangecorresponding to decomposition of carboxyl groups should been taken into account inestimating the amount of carboxyl groups.4. The APS oxidative modification could significantly enhance the concentration of various oxygen compounds, and simultaneously reduced the surface area of the activatedcarbons. The heat treatment of the oxidized sample at350oC under N2removed all thecarboxylic and part of the anhydride groups, but almost had no influence on the concentrationof other groups. Furthermore, almost no changes were seen for the physical properties afterthe heat treatment process. It is possible to selectively tailor the oxygen functional groups foractivated carbon by a combination of oxidative modification and heat treatment. The physicalproperties do not have obvious effect on the adsorption performance of these samples forMDF. But the increase of the concentration of the oxygen functional groups by APS oxidationmodification could significantly enhance the adsorption capacity and selectivity of theactivated carbon for both the basic and neutral nitrogen compounds in MDF. The acidicgroups, especially the carboxylic acid groups, contribute more towards the adsorption of basicnitrogen compounds. Multiple mechanisms work together to determine the adsorptionperformances of the activated carbon for various nitrogen compounds. The acid-base and Hbonding interaction play more roles in the adsorption process.5. The adsorptive denitrogenation （ADN） of liquid hydrocarbon streams usingcarbon-based adsorbents is considered as a promising process. Twelve activated carbon （AC）samples with different physical and chemical properties are selected to investigate theadsorption property using two model fuels containing typical nitrogen compounds, indole andquinoline in decane. The surface oxygen containing groups on the surface of ACs arecharacterized by TPD technique with a mass spectrometer to identify and quantify the typeand concentration of the oxygen-containing functional groups on the basis of the CO₂-andCO-evolution profiles. The adsorption was found to obey the Langmuir adsorption isotherm.The adsorption parameters （the maximum capacity and adsorption constant） were estimated.With these results, the correlation between the adsorption performance of AC samples andtheir chemical property was conducted by a multiple linear regression analysis. It was foundthat that the type and number of the oxygen-containing functional groups play a decisive rolein ADN on AC adsorbents. The acidic, especially the carboxylic acidfunctional groups appearto show high responsibility for adsorption of both the basic and neutral nitrogen compounds.The present study implies that the adsorptive denitrogenation performance of thecarbon-based adsorbents can be improved significantly by enhancement of the special oxygenfunctional groups on the surface.