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Surface characterization of adsorbents in ultrasound-assisted oxidative desulfurization process of fossil fuels
Journal of Colloid and Interface Science 313 (2007) 18–25 www.elsevier.com/locate/jcis
Surface characterization of adsorbents in ultrasound-assisted oxidative desulfurization process of fossil fuels Omid Etemadi, Teh Fu Yen ∗ Department of Civil and Environmental Engineering, Viterbi School of Engineering, University of Southern California, Los Angeles, CA 90089-2531, USA Received 10 January 2007; accepted 16 April 2007 Available online 31 May 2007
Abstract Surface properties of two different phases of alumina were studied through SEM images. Characterization of amorphous acidic alumina and crystalline boehmite by XRD explains the differences in adsorption capacities of each sample. Data from small angle neutron scattering (SANS) provide further results regarding the ordering in amorphous and crystalline samples of alumina. Quantitative measurements from SANS are used for pore size calculations. Higher disorder provides more topological traps, irregularities, and hidden grooves for higher adsorption capacity. An isotherm model was derived for adsorption of dibenzothiophene sulfone (DBTO) by amorphous acidic alumina to predict and calculate the adsorption of sulfur compounds. The Langmuir–Freundlich model covers a wide range of sulfur concentrations. Experiments prove that amorphous acidic alumina is the adsorbent of choice for selective adsorption in the ultrasound-assisted oxidative desulfurization (UAOD) process to produce ultra-low-sulfur fuel (ULSF). © 2007 Elsevier Inc. All rights reserved. Keywords: Desulfurization; Langmuir–Freundlich isotherm; Alumina; Adsorption; Ultra-low-sulfur fuel
1. Introduction Ultradeep desulfurization methods for fossil fuels have drawn broad attention in recent years. Emission control standards are being imposed on gasoline and diesel fuel products in order to reduce their environmental impacts. Global petroleum reserves contain more sour crudes, and the demand for greater efficiency improvements in desulfurization of the stream are of concern. The maximum level of sulfur compounds in gasoline and diesel fuels has been reduced from 500 ppmw to 30 and 15 ppmw, respectively, by June 2006 in the United States [1,2]. Desulfurization techniques need optimization and fresh approaches to reach very low sulfur content in fossil fuels. Organic sulfur compounds (OSCs) in fossil fuels are converted to SOx upon combustion. These compounds are sources of secondary pollutants that produce acid rain. Due to their higher selectivity, SOx are adsorbed to catalytic converters and occupy the sites that are designed for CO and NOx reduction. There* Corresponding author.
E-mail address: [email protected] (T.F. Yen). 0021-9797/$ – see front matter © 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2007.04.033
fore, ultradeep desulfurization of fossil fuels will avoid the poisoning of catalysts. On the other hand, one of the economic processes of hydrogen production for fuel cell applications is through cracking of hydrocarbons in high-energy-density fuels such as diesel. However, the sulfur concentration in the fuel should be reduced to less than 1 ppmw for proton exchange membrane fuel cells (PEMFC) and less than 10 ppmw for solid oxide fuel cells (SOFC) [3]. A combined technique of selective oxidation and adsorption with no consumption of hydrogen shows acceptable results for producing ULSF. Oxidative desulfurization has been studied extensively in recent years [4–7]. The principle depends on sulfur compounds having more affinity to oxidation than their analogue hydrocarbons in fossil fuels. High conversions of sulfides to sulfones and sulfoxides provide a difference in polarity that can be used for selective removal of OSCs with solid adsorbents. Both oxidation and adsorption in the ultrasound-assisted oxidative desulfurization (UAOD) process at ambient temperature and atmospheric pressure can provide a clean fuel that meets the new emission control standards with minimum damage to membranes and catalytic converters [8]. Ultrasound irradiation can enhance the oxidation process through the biphasic
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transfer of oxidants [9–11]. Sonication provides an emulsion that increases the concentration of reactive species and provides more interfacial surface for reaction [12]. Solid adsorption on alumina is an important component of the UAOD process. Slightly more polar OSCs after oxidation are selectively adsorbed and removed at low temperature and atmospheric pressure by alumina. This developed process is an attempt to upgrade fossil fuels as a low-cost source of reformed fuel that can be used in fuel cells. Often calcining is required for a successful UAOD process, since the adsorbent can be regenerated for reuse. A better understanding of adsorption in the UAOD process is provided by studying the characteristic properties of alumina adsorbents. Characterization of two different phases of alumina (amorphous and boehmite) by SEM, XRD, and SANS clarifies the mechanism of adsorption. Images from SEM show the surface property differences between the two samples. The crystallinity of alumina samples is confirmed by XRD, and SANS analysis shows the ordering of the structure in each sample. Quantitative parameters obtained from SANS can complement the visual information gained from SEM of the two phases of alumina samples. Previous results prove that amorphous acidic alumina has relatively better adsorption capacity toward oxidized sulfur compounds in fossil fuels than the crystalline α-phase nanopowder of boehmite. Moreover, the amorphous phase alumina sample is more stable than boehmite after calcining at 550 ◦ C, where the amorphous sample maintains 98% of its adsorption capacity after 4 h of calcining and 91% after 24 h, whereas the crystalline sample has 84% of its original capacity [13]. Similar inorganic adsorbents such as silica have shown better reactivity in amorphous form rather than in crystalline structure [14], due to availability of higher surface area and active sites. Nanoiron, as a form of reactant or catalyst, performs better in the amorphous phase than in the crystalline phase [15]. The reason for the difference in adsorption capacities between the two phases of alumina is reported in this paper from characterization studies by SEM, XRD, and SANS. The phase transfer of boehmite to γ -alumina at 500 ◦ C [16] is another reason for reduced capacity after calcining. Experiments have shown lower acidity for γ -alumina [17] and boehmite [18]. Our test of DBTO adsorption on an alumina sample with phase composition of 70% δ- and 30% γ -alumina with particle size 30–40 nm resulted in much lower affinity toward DBTO compared to that for the amorphous phase. The isotherm pattern of amorphous acidic alumina for DBTO adsorption is studied in this paper. The adsorption capacity of acidic alumina has been calculated from packed column experiments and is described elsewhere [13]. Here, maximum adsorption capacity and binding affinity are measured from the isotherm model. Selective adsorption of oxidized sulfur compounds relative to hydrocarbons and nonoxidized forms of sulfur molecules [13] is the reason for choosing alumina as the adsorbent for the UAOD process. The point of zero net charge (p.z.n.c.) of alumina is at pH 8.7 [19]. Thus, alumina is positively charged except under highly alkaline conditions. Therefore, oxidation in UAOD is a prior action before alumina
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adsorption to provide more polar adsorbates. Samples with sulfur concentrations from 5 to 700 ppmw were prepared, and an isotherm pattern was found by using the collected data from GC-SCD injections. A hybrid Langmuir–Freundlich isotherm model fits the result of this experiment. This generalized form of isotherm has also been identified in a heterogeneous catalyst by statistical distribution of adsorption energies of the active centers that exist on the surface [20,21]. The heterogeneity coefficient (m) is a key factor in modeling the adsorption pattern of acidic alumina. Molecules adsorbed in mineral micropores are subjected to stronger adsorption energies due to compound interaction with pore walls [22]. 2. Experimental methods 2.1. Materials Amorphous-phase aluminum oxide (activated, acidic, Brockmann I, standard grade, ∼150 mesh, 58 Å), boehmite-phase aluminum oxide (nanopowder, whiskers, 2–4 nm), and DBTO were obtained from Aldrich Chemical Inc., Milwaukee, WI. 2.2. SEM, XRD, and SANS analysis for two phases of alumina The Cambridge 360 scanning electron microscope and Rigaku X-ray diffractometer were at the Center for Electron Microscopy and Microanalysis (CEMMA) at the University of Southern California. The Cambridge 360 has capabilities for image storage and analysis, with a resolution of 30 Å, with an Oxford Instrument energy dispersive spectrometer (Link EDS) system that is attached to the SEM to detect and analyze X-rays emitted from the sample surface when it is impinged upon by the primary electron beam. An interface has been designed for the microscope that allows electron beam writing to be done in the SEM. Image files are captured digitally onto a personal computer. The X-ray source in the XRD is a 12-kW rotating anode X-ray generator. A θ –2θ diffractometer with an intrinsic germanium solid-state detector is used for routine powder diffraction. There are also a thin film attachment working at low incidence angles as required for thin films and a texture goniometer for texture and stress measurements. A searchable database of powder diffraction files (JCPDS) for minerals and inorganic chemical compounds is connected to the instrument. The powdered sample theoretically provides all possible orientations of the crystal lattice, the goniometer provides a variety of angles of incidence, and the detector measures the intensity of the diffracted beam. The exact angle and intensity of a set of peaks are unique to the crystal structure being examined. Amorphous acidic alumina and nanopowder boehmite samples were sent to the intense pulsed neutron source (IPNS) facility at Argonne National Lab in Chicago to gain information about their structural properties through the SANS technique. The small-angle neutron diffractometer (SAND) is an improved version of a small-angle scattering instrument, which has a larger area detector, a wide-angle linear position-sensitive detector bank, and a larger sample-to-detector distance. This
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enables SAND to obtain higher quality data in a much wider Q range (0.0035 to 2 Å−1 ) in a single measurement. Alumina samples were measured on SAND in 1-mm-path-length quartz cells with measurement time 1 h per sample. Physical characterizations of different types of alumina are shown in Table 1. The two phases of alumina have different particle sizes; therefore, they have different surface area, which is an important parameter for adsorption. Alumina samples were prepared after activation at 250 ◦ C and also after calcining at 550 ◦ C for SEM imaging. The samples were stored in the vacuum chamber of the SEM for image processing. Alumina samples were prepared for XRD measurements after activation at 250 ◦ C. 2.3. Isotherm studies of DBTO adsorption onto amorphous acidic alumina In order to predict the adsorption of aromatic sulfur compounds in fossil fuels, a model compound solution of DBTO was prepared to mimic the corresponding compound in fossil fuel. Twelve samples with sulfur concentrations from 5 to 700 ppmw were prepared in 30-ml vials from DBTO in toluene solvent. Acidic alumina was activated overnight at 200 ◦ C, and then 400 mg of alumina was measured and added to each vial of DBTO solution. The samples were shaken for 4 h in order to Table 1 Physical properties of two alumina phases (amorphous and boehmite) Type
Properties
Measurements
Acidic alumina (amorphous phase)
Particle size (µm) Pore size (Å) Surface area (m2 /g)
104 58 155
Nanopowder alumina (boehmite phase)a
Particle size (nm) Interstitial spacingb Surface area (m2 /g)
2–4 2–4 350–720
a Interstitial spaces between packed nanoparticles (instead of pores) are close to particle sizes. b α-Monohydrate, AlO·OH.
reach equilibrium after adsorption. Liquid samples were diluted and subjected to GC-SCD for quantifying sulfur concentration after adsorption onto alumina. 3. Results and discussion 3.1. SEM for two phases of alumina The two phases of alumina have already been tested for adsorption capacity of OSCs in diesel fuel before and after calcining at 550 ◦ C [13]. The purpose of using calcination for regeneration was to eliminate organic solvent wash for alumina recovery, presenting a feasible method for desulfurization of diesel. According to Table 1, the alumina samples differ in their particle size and effective surface area, and they also have different phases. The effect of surface area and phase of alumina on their adsorption capacity has been discussed previously [13]. The results of packed column experiments with 500 ppmw sulfur concentration of DBTO solution after activation at 200 ◦ C and after calcination at 550 ◦ C for 24 h are presented in Fig. 1. This figure illustrates the effect of particle size and surface area on adsorption capacity and also the effect of calcining on adsorbent structure and alumina phase. The initial adsorption capacity of boehmite nanopowder alumina is 56% higher than that of amorphous acidic alumina with particle size 104 µm. This is due to the higher surface area provided by the nanoparticles (350–720 m2 /g), compared to 155 m2 /g for amorphous alumina. Recovering the media from the DBTO is achieved by calcining the alumina at 550 ◦ C through temperature programming for 24 h. The amorphous acidic alumina with particle size 104 µm has 91% recovery, while the boehmite alumina with particle size 2–4 nm recovers 84%. The higher capacity reduction of boehmite compared with the amorphous phase alumina must be studied according to their crystalline structure. The transition of boehmite to γ -alumina above 500 ◦ C is a reason for losses in adsorption sites, which were tested for DBTO for
Fig. 1. Effect of adsorption capacity on particle size for alumina phase. The changes upon calcining (550 ◦ C, 24 h) are noted.
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Fig. 2. SEM images of acidic amorphous alumina before and after calcining. (Insets are large magnifications.)
Fig. 3. SEM images of boehmite nanopowder alumina before and after calcining. (Insets are large magnifications.)
a sample with mixed phase composition of 70% δ- and 30% γ -alumina with particle size 30–40 nm. Images of alumina powders from SEM will qualitatively clarify their structural properties as an adsorbent. Fig. 2 shows the difference between the particles of amorphous acidic alumina before and after heating the sample at 550 ◦ C for removing the sulfur compound adsorbates and recovering the alumina. Higher magnification shows that the heterogeneity of the surface of the particles becomes smaller, with smoother surfaces, after calcining, which can be a reason for a slight loss of adsorption capacity (the amorphous acidic alumina sample has a loss of 9% in adsorption capacity). However, the sample almost maintains its average particle size (104 µm) even after calcining. Fig. 3 shows the SEM images of boehmite nanopowder alumina with their magnification corresponding to that for amorphous acidic alumina. It must be noted that the nanopowder particles are agglomerated to large sizes; therefore, the particles are in micrometer scale. Although the surface areas are between 350 and 720 m2 /g, which is 2.3 to 4.6 times more than the acidic amorphous sample surface area, the adsorption ca-
pacity of nanopowder is only 1.6 times higher. Agglomeration of particles can reduce the effective surface area of nanopowder alumina. Calcining the nanopowder sample can break these particles to smaller sizes with higher surface area, but the phase transfer of boehmite to γ -alumina reduces the original adsorption capacity of the sample. 3.2. XRD for two phases of alumina X-ray scattering is used to study structural characteristics of minerals and to identify mineral powders. Therefore, XRD can differentiate between the amorphous and crystalline materials. Images of the two alumina powders from SEM qualitatively showed their structural properties as an adsorbent. The amorphous acidic alumina almost maintains its average particle size (104 µm) even after calcining. In the case of boehmite nanopowder the particles are agglomerated in large sizes; therefore, the adsorption capacity of nanopowder is only 56% higher than the acidic amorphous alumina. The higher loss of capacity in a nanopowder sample compared to the amorphous acidic alumina is 16% and 9%, respectively, which might be
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(a)
(b) Fig. 4. Uncalibrated XRD scan of (a) acidic alumina (amorphous phase) and (b) nanopowder of alumina (boehmite phase). (Both peaks at B are assigned from AlO·OH.)
related to the crystalline phase of nanopowder. Amorphous phase alumina can offer relatively better adsorption sites in alumina. The resulting analysis of XRD is described graphically with counts per second (cps) intensity on the Y -axis and goniometer angle on the X-axis. Fig. 4a shows the XRD pattern of the amorphous acidic alumina sample, which was plotted from the collected raw data by the Rigaku X-ray diffractometer. Data were acquired over the range 2θ = 13◦ –40◦ , and no peaks occurred in this interval, which indicates the amorphous nature of acidic alumina powder. The XRD pattern for the nanopowder alumina sample is shown in Fig. 4b. Data were acquired over the range 2θ = 13◦ –40◦ range, and the two identified peaks represent the aluminum oxyhydroxide (AlO·OH) or boehmite, which is an α-monohydrate crystalline alumina. The crystallinity of the nanopowder sample is confirmed by the XRD pattern. In order to confirm the effect of crystalline structure on adsorption capacity, another sample of alumina with phase composition of 70% δ- and 30% γ -alumina (particle size 30–40 nm) was tested for adsorption capacity, which resulted in a much lower affinity toward DBTO than that of the amorphous acidic alumina. Therefore, the amorphous phase adsorbent offers relatively higher adsorption capacities and a more stable adsorbent for sulfur compounds than the crystalline phase adsorbents.
3.3. SANS for two phases of alumina The SAND instrument is suitable for structural determination of porous materials and provides information about the order/disorder transition in different samples. The SANS patterns were measured on samples and Fig. 5 depicts the characteristic sizes of two phases of alumina. The amorphous acidic alumina sample has a radius of gyration (Rg ) of 51 Å. If one assumes spherical pores in alumina particles, then the average radius (R) is equal to 3 Rg = (1) R → R = 65.6 Å. 5 The radius presented by Aldrich for the amorphous alumina sample is 58 Å. Our result from SANS, adopting the spherical assumption, is a reasonable measurement. For the boehmite nanopowder sample, Rg for the smallest pore or particle is 15 Å, and subsequently, assuming spherical particles, R is equal to 19 Å. Moreover, larger particles or pores are evident from the SANS data. 3.4. Isotherm studies of DBTO adsorption to amorphous acidic alumina Concentration results from the injected samples to GC-SCD were used to calculate the amount of sulfur adsorbed onto acidic
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Fig. 5. SANS data for alumina samples. (Calculation is based on [24].)
alumina per gram of adsorbent (qe ). Theoretical concentration of adsorbed sulfur per gram of adsorbent was calculated from the Langmuir–Freundlich isotherm. This model describes the relationship between the concentration of sorbed sulfur in DBTO (qe ) and the equilibrium sulfur concentration of DBTO2 in solution (C) such that qe =
qm kC m , 1 + kC m
(2)
where qm , k, and m are three fitting coefficients that represent the adsorption capacity, the binding affinity of the compound, and the heterogeneity of site energies, respectively. As k approaches zero at low binding affinities, the equation reduces to the classical Freundlich equation. As m approaches unity, indicative of a completely homogeneous sorbent surface (i.e., energetic equivalence of all binding sites), the equation reduces to the classical Langmuir equation. Thus, a hybrid Langmuir– Freundlich isotherm is able to model adsorption of solutes at high and low concentrations onto homogeneous and heterogeneous sorbents. The concentration results are shown in Table 2. Thermodynamic binding properties of a sorbate can be extracted directly from fitting parameters of isotherm models, such as the Langmuir model, assuming a homogeneous sorbent. Energetically heterogeneous surfaces, however, must be characterized by an affinity distribution function. Adsorption capacity (qm ) and binding affinity (k) were derived from plotting 1/qe vs 1/C m . The heterogeneity coefficient (m) was found through trial and error in the solver function of Microsoft Excel 2002 to maximize the coefficient of determination (R 2 ). Fig. 6 shows the plot of 1/qe vs 1/C m , from which qm and k are calculated. For m = 0.83 and from Eq. (2),
Table 2 Equilibrium and adsorbed sulfur concentrations of DBTO on acidic alumina Original conc. (ppmw)
C, equilibrium conc. (g/L)
qe (mg/g) Experimental
Theoretical
Error (%)
5 10 20 25 50 100 200 300 400 500 600 700
0.002 0.003 0.009 0.010 0.024 0.050 0.115 0.184 0.260 0.344 0.425 0.507
0.10 0.20 0.32 0.43 0.74 1.38 2.11 2.72 3.06 3.06 3.15 3.26
0.10 0.18 0.37 0.42 0.77 1.27 2.05 2.55 2.92 3.21 3.42 3.59
2 −11 16 −2 5 −8 −3 −6 −5 5 9 10
Fig. 6. Plotting qm vs 1/C m for qm and k calculations.
1 1 1 1 = · + , qe kqm C m qm where qm = 5.2 mg sulfur/g alumina and k = 3.92.
(3)
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Adsorption capacity from the breakthrough curve of the packed column study [23] is qm = 5.7 mg sulfur/g alumina. The Langmuir model assumes a constant energy of adsorption at all sites, while the Freundlich model assumes an infinite supply of unreacted adsorption sites. Therefore, neither can model this saturation behavior. The Langmuir–Freundlich model behaves as a Freundlich equation at low concentrations and as a Langmuir equation at high solute concentration. The adsorption isotherm modeling of DBTO on amorphous acidic alumina can be illustrated by plotting qe vs C, the equilibrium concentration. 4. Conclusions Complementary studies were performed by SEM to explain the adsorption capacity of amorphous and boehmite alumina samples. These images can provide qualitative information about the structural properties of alumina with different particle size and surface areas. Moreover, the effect of calcining on alumina particles can be seen through SEM images. Amorphous acidic alumina with particle size 104 µm is compared with boehmite nanopowder alumina with particle size 2–4 nm, before and after calcining. Adsorption capacity of amorphous acidic alumina is originally 1.6 times lower than the nanopowder sample, though the surface area is 2.3 to 4.6 times smaller. Therefore, the amorphous sample has a relatively better capacity for adsorbing sulfur compounds. Additionally, it has been shown that calcining has less affect on adsorption reduction of amorphous sample compared to boehmite (9% and 15%, respectively), and amorphous phase alumina is more stable than the boehmite sample after calcining. Images of SEM clarify the agglomeration of nanopowders in the boehmite sample to micrometer-scale particles that can reduce the available adsorption sites. These images also differentiate between the surface topology of the two samples. For using nanopowder as an adsorbent of OSCs and to avoid agglomeration, particles must be nullified and balanced through proper functionalizing. Studies were performed by XRD to explain the adsorption capacity phenomena of two alumina samples. The diffraction patterns can provide qualitative information about the crystallinity of alumina samples. Crystallinity of the adsorbent provided by XRD results has a decrease in adsorption capacity. Higher disorder provides more topological traps, irregularities, and hidden grooves for higher adsorption capacity. Therefore, the amorphous phase provides more sites for adsorption. This can be the result of slow diffusion and tortuosity of amorphous acidic alumina. Increased attractive interaction between compounds sorbed to internal mesopore surfaces in monolayer or near-monolayer coatings may occur due to surface curvature that brings unbound moieties closer together. Two samples were subjected to SAND instrument for ordering studies through SANS measurements. Quantitative measurements from SANS patterns were used to calculate the pore size in alumina particles.
Moreover, a theoretical study demonstrates a hybrid Langmuir–Freundlich isotherm model for adsorption of DBTO on amorphous acidic alumina. Results show reasonable fitness between experimental and theoretical values of adsorption in a wide range of sulfur concentrations from 5 to 700 ppmw sulfur. Acknowledgments The authors thank NAVSEA-ONR for their financial contribution through Northrop Grumman and CalNova Tech. We are also very grateful for the funding provided by the Army Research Laboratory (ARL). Special thanks go to Dr. Pappannan Thiyagarajan from the Intense Pulsed Neutron Source (IPNS) at Argonne National Laboratory for the SANS pattern measurements. Appendix A. Nomenclature ARL Army Research Laboratory CEMMA Center for Electron Microscopy and Microanalysis DBTO dibenzothiophene sulfone EDS energy-dispersive spectrometer GC-SCD gas chromatography sulfur chemiluminescence detection IPNS intense pulsed neutron source JCPDS Joint Committee on Powder Diffraction Standards OSC organic sulfur compounds p.z.n.c. point of zero net charge PEMFC proton exchange membrane fuel cell SAND small-angle neutron diffractometer SANS small-angle neutron scattering SEM scanning electron microscope SOFC solid oxide fuel cell UAOD ultrasound-assisted oxidative desulfurization ULSF ultra-low-sulfur fuel XRD X-ray diffraction References [1] US EPA, Marks Historic Milestone in Clean Diesel, 2006. [2] US EPA, Program Update: Introduction of Cleaner-Burning Diesel Fuel Enables Advanced Pollution Control for Cars, Trucks and Buses, 2006. [3] J.H. Kim, X. Ma, A. Zhou, C. Song, Catal. Today 111 (2006) 74–83. [4] A. Attar, W.H. Corcoran, Ind. Eng. Chem. Res. 17 (1978) 102–109. [5] F. Zannikos, V. Vignier, Fuel Process. Technol. 42 (1995) 35–45. [6] S. Otsuki, T. Nonaka, N. Takashima, W.H. Qian, A. Ishihara, T. Imai, T. Kabe, Energy Fuels 14 (2000) 1232–1239. [7] F.M. Collins, A.R. Lucy, C. Sharp, J. Mol. Catal. A Chem. 117 (1997) 397–403. [8] O. Etemadi, T.F. Yen, Prepr. Pap. Am. Chem. Soc. Div. Fuel Chem. 51 (2006) 820. [9] L.H. Thompson, L.K. Doraiswamy, Ind. Eng. Chem. Res. 38 (1999) 1250. [10] J.L. Luche, Synthetic Organic Sonochemistry, Plenum, New York, 1998. [11] Y.T. Shah, A.B. Pandit, V.S. Moholkar, Cavitation Reaction Engineering, Kluwer Academic/Plenum, New York, 1999. [12] T.F. Yen, R.D. Gilbert, J.H. Fendler, Membrane Mimetic Chemistry and Its Applications, Plenum, New York, 1994. [13] O. Etemadi, T.F. Yen, Energy Fuels, in press. [14] P.W. Lednor, R. Ruiter, J. Chem. Soc. Chem. Commun. (1989) 320–321.
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[15] Y. Liu, C. Choi, D. Dionysiou, G.V. Lowry, Chem. Mater. 17 (2005) 5315– 5322. [16] H.C. Lee, H.J. Kim, S.H. Chung, K.H. Lee, H.C. Lee, J.S. Lee, J. Am. Chem. Soc. 125 (2003) 2882–2883. [17] K. Sohlberg, S.T. Pantelides, S.J. Pennycook, J. Am. Chem. Soc. 123 (2000) 26–29. [18] K.M. Reddy, C. Song, Stud. Surf. Sci. Catal. 117 (1998) 291–299.
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[19] G. Sposito, The Surface Chemistry of Soils, Oxford Univ. Press, New York, 1984. [20] R. Sips, J. Chem. Phys. 16 (1948) 490. [21] R. Sips, J. Chem. Phys. 18 (1950) 1024. [22] J. Farrell, M. Reinhard, Environ. Sci. Technol. 28 (1994) 53–72. [23] O. Etemadi, T.F. Yen, Energy Fuels, in press. [24] A. Guinier, G. Fournet, Small Angle Scattering of X-Rays, Wiley, 1955.
Surface characterization of adsorbents in ultrasound-assisted oxidative desulfurization process of fossil fuels Omid Etemadi, Teh Fu Yen ∗ Department of Civil and Environmental Engineering, Viterbi School of Engineering, University of Southern California, Los Angeles, CA 90089-2531, USA Received 10 January 2007; accepted 16 April 2007 Available online 31 May 2007
Abstract Surface properties of two different phases of alumina were studied through SEM images. Characterization of amorphous acidic alumina and crystalline boehmite by XRD explains the differences in adsorption capacities of each sample. Data from small angle neutron scattering (SANS) provide further results regarding the ordering in amorphous and crystalline samples of alumina. Quantitative measurements from SANS are used for pore size calculations. Higher disorder provides more topological traps, irregularities, and hidden grooves for higher adsorption capacity. An isotherm model was derived for adsorption of dibenzothiophene sulfone (DBTO) by amorphous acidic alumina to predict and calculate the adsorption of sulfur compounds. The Langmuir–Freundlich model covers a wide range of sulfur concentrations. Experiments prove that amorphous acidic alumina is the adsorbent of choice for selective adsorption in the ultrasound-assisted oxidative desulfurization (UAOD) process to produce ultra-low-sulfur fuel (ULSF). © 2007 Elsevier Inc. All rights reserved. Keywords: Desulfurization; Langmuir–Freundlich isotherm; Alumina; Adsorption; Ultra-low-sulfur fuel
1. Introduction Ultradeep desulfurization methods for fossil fuels have drawn broad attention in recent years. Emission control standards are being imposed on gasoline and diesel fuel products in order to reduce their environmental impacts. Global petroleum reserves contain more sour crudes, and the demand for greater efficiency improvements in desulfurization of the stream are of concern. The maximum level of sulfur compounds in gasoline and diesel fuels has been reduced from 500 ppmw to 30 and 15 ppmw, respectively, by June 2006 in the United States [1,2]. Desulfurization techniques need optimization and fresh approaches to reach very low sulfur content in fossil fuels. Organic sulfur compounds (OSCs) in fossil fuels are converted to SOx upon combustion. These compounds are sources of secondary pollutants that produce acid rain. Due to their higher selectivity, SOx are adsorbed to catalytic converters and occupy the sites that are designed for CO and NOx reduction. There* Corresponding author.
E-mail address: [email protected] (T.F. Yen). 0021-9797/$ – see front matter © 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2007.04.033
fore, ultradeep desulfurization of fossil fuels will avoid the poisoning of catalysts. On the other hand, one of the economic processes of hydrogen production for fuel cell applications is through cracking of hydrocarbons in high-energy-density fuels such as diesel. However, the sulfur concentration in the fuel should be reduced to less than 1 ppmw for proton exchange membrane fuel cells (PEMFC) and less than 10 ppmw for solid oxide fuel cells (SOFC) [3]. A combined technique of selective oxidation and adsorption with no consumption of hydrogen shows acceptable results for producing ULSF. Oxidative desulfurization has been studied extensively in recent years [4–7]. The principle depends on sulfur compounds having more affinity to oxidation than their analogue hydrocarbons in fossil fuels. High conversions of sulfides to sulfones and sulfoxides provide a difference in polarity that can be used for selective removal of OSCs with solid adsorbents. Both oxidation and adsorption in the ultrasound-assisted oxidative desulfurization (UAOD) process at ambient temperature and atmospheric pressure can provide a clean fuel that meets the new emission control standards with minimum damage to membranes and catalytic converters [8]. Ultrasound irradiation can enhance the oxidation process through the biphasic
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transfer of oxidants [9–11]. Sonication provides an emulsion that increases the concentration of reactive species and provides more interfacial surface for reaction [12]. Solid adsorption on alumina is an important component of the UAOD process. Slightly more polar OSCs after oxidation are selectively adsorbed and removed at low temperature and atmospheric pressure by alumina. This developed process is an attempt to upgrade fossil fuels as a low-cost source of reformed fuel that can be used in fuel cells. Often calcining is required for a successful UAOD process, since the adsorbent can be regenerated for reuse. A better understanding of adsorption in the UAOD process is provided by studying the characteristic properties of alumina adsorbents. Characterization of two different phases of alumina (amorphous and boehmite) by SEM, XRD, and SANS clarifies the mechanism of adsorption. Images from SEM show the surface property differences between the two samples. The crystallinity of alumina samples is confirmed by XRD, and SANS analysis shows the ordering of the structure in each sample. Quantitative parameters obtained from SANS can complement the visual information gained from SEM of the two phases of alumina samples. Previous results prove that amorphous acidic alumina has relatively better adsorption capacity toward oxidized sulfur compounds in fossil fuels than the crystalline α-phase nanopowder of boehmite. Moreover, the amorphous phase alumina sample is more stable than boehmite after calcining at 550 ◦ C, where the amorphous sample maintains 98% of its adsorption capacity after 4 h of calcining and 91% after 24 h, whereas the crystalline sample has 84% of its original capacity [13]. Similar inorganic adsorbents such as silica have shown better reactivity in amorphous form rather than in crystalline structure [14], due to availability of higher surface area and active sites. Nanoiron, as a form of reactant or catalyst, performs better in the amorphous phase than in the crystalline phase [15]. The reason for the difference in adsorption capacities between the two phases of alumina is reported in this paper from characterization studies by SEM, XRD, and SANS. The phase transfer of boehmite to γ -alumina at 500 ◦ C [16] is another reason for reduced capacity after calcining. Experiments have shown lower acidity for γ -alumina [17] and boehmite [18]. Our test of DBTO adsorption on an alumina sample with phase composition of 70% δ- and 30% γ -alumina with particle size 30–40 nm resulted in much lower affinity toward DBTO compared to that for the amorphous phase. The isotherm pattern of amorphous acidic alumina for DBTO adsorption is studied in this paper. The adsorption capacity of acidic alumina has been calculated from packed column experiments and is described elsewhere [13]. Here, maximum adsorption capacity and binding affinity are measured from the isotherm model. Selective adsorption of oxidized sulfur compounds relative to hydrocarbons and nonoxidized forms of sulfur molecules [13] is the reason for choosing alumina as the adsorbent for the UAOD process. The point of zero net charge (p.z.n.c.) of alumina is at pH 8.7 [19]. Thus, alumina is positively charged except under highly alkaline conditions. Therefore, oxidation in UAOD is a prior action before alumina
19
adsorption to provide more polar adsorbates. Samples with sulfur concentrations from 5 to 700 ppmw were prepared, and an isotherm pattern was found by using the collected data from GC-SCD injections. A hybrid Langmuir–Freundlich isotherm model fits the result of this experiment. This generalized form of isotherm has also been identified in a heterogeneous catalyst by statistical distribution of adsorption energies of the active centers that exist on the surface [20,21]. The heterogeneity coefficient (m) is a key factor in modeling the adsorption pattern of acidic alumina. Molecules adsorbed in mineral micropores are subjected to stronger adsorption energies due to compound interaction with pore walls [22]. 2. Experimental methods 2.1. Materials Amorphous-phase aluminum oxide (activated, acidic, Brockmann I, standard grade, ∼150 mesh, 58 Å), boehmite-phase aluminum oxide (nanopowder, whiskers, 2–4 nm), and DBTO were obtained from Aldrich Chemical Inc., Milwaukee, WI. 2.2. SEM, XRD, and SANS analysis for two phases of alumina The Cambridge 360 scanning electron microscope and Rigaku X-ray diffractometer were at the Center for Electron Microscopy and Microanalysis (CEMMA) at the University of Southern California. The Cambridge 360 has capabilities for image storage and analysis, with a resolution of 30 Å, with an Oxford Instrument energy dispersive spectrometer (Link EDS) system that is attached to the SEM to detect and analyze X-rays emitted from the sample surface when it is impinged upon by the primary electron beam. An interface has been designed for the microscope that allows electron beam writing to be done in the SEM. Image files are captured digitally onto a personal computer. The X-ray source in the XRD is a 12-kW rotating anode X-ray generator. A θ –2θ diffractometer with an intrinsic germanium solid-state detector is used for routine powder diffraction. There are also a thin film attachment working at low incidence angles as required for thin films and a texture goniometer for texture and stress measurements. A searchable database of powder diffraction files (JCPDS) for minerals and inorganic chemical compounds is connected to the instrument. The powdered sample theoretically provides all possible orientations of the crystal lattice, the goniometer provides a variety of angles of incidence, and the detector measures the intensity of the diffracted beam. The exact angle and intensity of a set of peaks are unique to the crystal structure being examined. Amorphous acidic alumina and nanopowder boehmite samples were sent to the intense pulsed neutron source (IPNS) facility at Argonne National Lab in Chicago to gain information about their structural properties through the SANS technique. The small-angle neutron diffractometer (SAND) is an improved version of a small-angle scattering instrument, which has a larger area detector, a wide-angle linear position-sensitive detector bank, and a larger sample-to-detector distance. This
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enables SAND to obtain higher quality data in a much wider Q range (0.0035 to 2 Å−1 ) in a single measurement. Alumina samples were measured on SAND in 1-mm-path-length quartz cells with measurement time 1 h per sample. Physical characterizations of different types of alumina are shown in Table 1. The two phases of alumina have different particle sizes; therefore, they have different surface area, which is an important parameter for adsorption. Alumina samples were prepared after activation at 250 ◦ C and also after calcining at 550 ◦ C for SEM imaging. The samples were stored in the vacuum chamber of the SEM for image processing. Alumina samples were prepared for XRD measurements after activation at 250 ◦ C. 2.3. Isotherm studies of DBTO adsorption onto amorphous acidic alumina In order to predict the adsorption of aromatic sulfur compounds in fossil fuels, a model compound solution of DBTO was prepared to mimic the corresponding compound in fossil fuel. Twelve samples with sulfur concentrations from 5 to 700 ppmw were prepared in 30-ml vials from DBTO in toluene solvent. Acidic alumina was activated overnight at 200 ◦ C, and then 400 mg of alumina was measured and added to each vial of DBTO solution. The samples were shaken for 4 h in order to Table 1 Physical properties of two alumina phases (amorphous and boehmite) Type
Properties
Measurements
Acidic alumina (amorphous phase)
Particle size (µm) Pore size (Å) Surface area (m2 /g)
104 58 155
Nanopowder alumina (boehmite phase)a
Particle size (nm) Interstitial spacingb Surface area (m2 /g)
2–4 2–4 350–720
a Interstitial spaces between packed nanoparticles (instead of pores) are close to particle sizes. b α-Monohydrate, AlO·OH.
reach equilibrium after adsorption. Liquid samples were diluted and subjected to GC-SCD for quantifying sulfur concentration after adsorption onto alumina. 3. Results and discussion 3.1. SEM for two phases of alumina The two phases of alumina have already been tested for adsorption capacity of OSCs in diesel fuel before and after calcining at 550 ◦ C [13]. The purpose of using calcination for regeneration was to eliminate organic solvent wash for alumina recovery, presenting a feasible method for desulfurization of diesel. According to Table 1, the alumina samples differ in their particle size and effective surface area, and they also have different phases. The effect of surface area and phase of alumina on their adsorption capacity has been discussed previously [13]. The results of packed column experiments with 500 ppmw sulfur concentration of DBTO solution after activation at 200 ◦ C and after calcination at 550 ◦ C for 24 h are presented in Fig. 1. This figure illustrates the effect of particle size and surface area on adsorption capacity and also the effect of calcining on adsorbent structure and alumina phase. The initial adsorption capacity of boehmite nanopowder alumina is 56% higher than that of amorphous acidic alumina with particle size 104 µm. This is due to the higher surface area provided by the nanoparticles (350–720 m2 /g), compared to 155 m2 /g for amorphous alumina. Recovering the media from the DBTO is achieved by calcining the alumina at 550 ◦ C through temperature programming for 24 h. The amorphous acidic alumina with particle size 104 µm has 91% recovery, while the boehmite alumina with particle size 2–4 nm recovers 84%. The higher capacity reduction of boehmite compared with the amorphous phase alumina must be studied according to their crystalline structure. The transition of boehmite to γ -alumina above 500 ◦ C is a reason for losses in adsorption sites, which were tested for DBTO for
Fig. 1. Effect of adsorption capacity on particle size for alumina phase. The changes upon calcining (550 ◦ C, 24 h) are noted.
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Fig. 2. SEM images of acidic amorphous alumina before and after calcining. (Insets are large magnifications.)
Fig. 3. SEM images of boehmite nanopowder alumina before and after calcining. (Insets are large magnifications.)
a sample with mixed phase composition of 70% δ- and 30% γ -alumina with particle size 30–40 nm. Images of alumina powders from SEM will qualitatively clarify their structural properties as an adsorbent. Fig. 2 shows the difference between the particles of amorphous acidic alumina before and after heating the sample at 550 ◦ C for removing the sulfur compound adsorbates and recovering the alumina. Higher magnification shows that the heterogeneity of the surface of the particles becomes smaller, with smoother surfaces, after calcining, which can be a reason for a slight loss of adsorption capacity (the amorphous acidic alumina sample has a loss of 9% in adsorption capacity). However, the sample almost maintains its average particle size (104 µm) even after calcining. Fig. 3 shows the SEM images of boehmite nanopowder alumina with their magnification corresponding to that for amorphous acidic alumina. It must be noted that the nanopowder particles are agglomerated to large sizes; therefore, the particles are in micrometer scale. Although the surface areas are between 350 and 720 m2 /g, which is 2.3 to 4.6 times more than the acidic amorphous sample surface area, the adsorption ca-
pacity of nanopowder is only 1.6 times higher. Agglomeration of particles can reduce the effective surface area of nanopowder alumina. Calcining the nanopowder sample can break these particles to smaller sizes with higher surface area, but the phase transfer of boehmite to γ -alumina reduces the original adsorption capacity of the sample. 3.2. XRD for two phases of alumina X-ray scattering is used to study structural characteristics of minerals and to identify mineral powders. Therefore, XRD can differentiate between the amorphous and crystalline materials. Images of the two alumina powders from SEM qualitatively showed their structural properties as an adsorbent. The amorphous acidic alumina almost maintains its average particle size (104 µm) even after calcining. In the case of boehmite nanopowder the particles are agglomerated in large sizes; therefore, the adsorption capacity of nanopowder is only 56% higher than the acidic amorphous alumina. The higher loss of capacity in a nanopowder sample compared to the amorphous acidic alumina is 16% and 9%, respectively, which might be
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(a)
(b) Fig. 4. Uncalibrated XRD scan of (a) acidic alumina (amorphous phase) and (b) nanopowder of alumina (boehmite phase). (Both peaks at B are assigned from AlO·OH.)
related to the crystalline phase of nanopowder. Amorphous phase alumina can offer relatively better adsorption sites in alumina. The resulting analysis of XRD is described graphically with counts per second (cps) intensity on the Y -axis and goniometer angle on the X-axis. Fig. 4a shows the XRD pattern of the amorphous acidic alumina sample, which was plotted from the collected raw data by the Rigaku X-ray diffractometer. Data were acquired over the range 2θ = 13◦ –40◦ , and no peaks occurred in this interval, which indicates the amorphous nature of acidic alumina powder. The XRD pattern for the nanopowder alumina sample is shown in Fig. 4b. Data were acquired over the range 2θ = 13◦ –40◦ range, and the two identified peaks represent the aluminum oxyhydroxide (AlO·OH) or boehmite, which is an α-monohydrate crystalline alumina. The crystallinity of the nanopowder sample is confirmed by the XRD pattern. In order to confirm the effect of crystalline structure on adsorption capacity, another sample of alumina with phase composition of 70% δ- and 30% γ -alumina (particle size 30–40 nm) was tested for adsorption capacity, which resulted in a much lower affinity toward DBTO than that of the amorphous acidic alumina. Therefore, the amorphous phase adsorbent offers relatively higher adsorption capacities and a more stable adsorbent for sulfur compounds than the crystalline phase adsorbents.
3.3. SANS for two phases of alumina The SAND instrument is suitable for structural determination of porous materials and provides information about the order/disorder transition in different samples. The SANS patterns were measured on samples and Fig. 5 depicts the characteristic sizes of two phases of alumina. The amorphous acidic alumina sample has a radius of gyration (Rg ) of 51 Å. If one assumes spherical pores in alumina particles, then the average radius (R) is equal to 3 Rg = (1) R → R = 65.6 Å. 5 The radius presented by Aldrich for the amorphous alumina sample is 58 Å. Our result from SANS, adopting the spherical assumption, is a reasonable measurement. For the boehmite nanopowder sample, Rg for the smallest pore or particle is 15 Å, and subsequently, assuming spherical particles, R is equal to 19 Å. Moreover, larger particles or pores are evident from the SANS data. 3.4. Isotherm studies of DBTO adsorption to amorphous acidic alumina Concentration results from the injected samples to GC-SCD were used to calculate the amount of sulfur adsorbed onto acidic
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Fig. 5. SANS data for alumina samples. (Calculation is based on [24].)
alumina per gram of adsorbent (qe ). Theoretical concentration of adsorbed sulfur per gram of adsorbent was calculated from the Langmuir–Freundlich isotherm. This model describes the relationship between the concentration of sorbed sulfur in DBTO (qe ) and the equilibrium sulfur concentration of DBTO2 in solution (C) such that qe =
qm kC m , 1 + kC m
(2)
where qm , k, and m are three fitting coefficients that represent the adsorption capacity, the binding affinity of the compound, and the heterogeneity of site energies, respectively. As k approaches zero at low binding affinities, the equation reduces to the classical Freundlich equation. As m approaches unity, indicative of a completely homogeneous sorbent surface (i.e., energetic equivalence of all binding sites), the equation reduces to the classical Langmuir equation. Thus, a hybrid Langmuir– Freundlich isotherm is able to model adsorption of solutes at high and low concentrations onto homogeneous and heterogeneous sorbents. The concentration results are shown in Table 2. Thermodynamic binding properties of a sorbate can be extracted directly from fitting parameters of isotherm models, such as the Langmuir model, assuming a homogeneous sorbent. Energetically heterogeneous surfaces, however, must be characterized by an affinity distribution function. Adsorption capacity (qm ) and binding affinity (k) were derived from plotting 1/qe vs 1/C m . The heterogeneity coefficient (m) was found through trial and error in the solver function of Microsoft Excel 2002 to maximize the coefficient of determination (R 2 ). Fig. 6 shows the plot of 1/qe vs 1/C m , from which qm and k are calculated. For m = 0.83 and from Eq. (2),
Table 2 Equilibrium and adsorbed sulfur concentrations of DBTO on acidic alumina Original conc. (ppmw)
C, equilibrium conc. (g/L)
qe (mg/g) Experimental
Theoretical
Error (%)
5 10 20 25 50 100 200 300 400 500 600 700
0.002 0.003 0.009 0.010 0.024 0.050 0.115 0.184 0.260 0.344 0.425 0.507
0.10 0.20 0.32 0.43 0.74 1.38 2.11 2.72 3.06 3.06 3.15 3.26
0.10 0.18 0.37 0.42 0.77 1.27 2.05 2.55 2.92 3.21 3.42 3.59
2 −11 16 −2 5 −8 −3 −6 −5 5 9 10
Fig. 6. Plotting qm vs 1/C m for qm and k calculations.
1 1 1 1 = · + , qe kqm C m qm where qm = 5.2 mg sulfur/g alumina and k = 3.92.
(3)
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Adsorption capacity from the breakthrough curve of the packed column study [23] is qm = 5.7 mg sulfur/g alumina. The Langmuir model assumes a constant energy of adsorption at all sites, while the Freundlich model assumes an infinite supply of unreacted adsorption sites. Therefore, neither can model this saturation behavior. The Langmuir–Freundlich model behaves as a Freundlich equation at low concentrations and as a Langmuir equation at high solute concentration. The adsorption isotherm modeling of DBTO on amorphous acidic alumina can be illustrated by plotting qe vs C, the equilibrium concentration. 4. Conclusions Complementary studies were performed by SEM to explain the adsorption capacity of amorphous and boehmite alumina samples. These images can provide qualitative information about the structural properties of alumina with different particle size and surface areas. Moreover, the effect of calcining on alumina particles can be seen through SEM images. Amorphous acidic alumina with particle size 104 µm is compared with boehmite nanopowder alumina with particle size 2–4 nm, before and after calcining. Adsorption capacity of amorphous acidic alumina is originally 1.6 times lower than the nanopowder sample, though the surface area is 2.3 to 4.6 times smaller. Therefore, the amorphous sample has a relatively better capacity for adsorbing sulfur compounds. Additionally, it has been shown that calcining has less affect on adsorption reduction of amorphous sample compared to boehmite (9% and 15%, respectively), and amorphous phase alumina is more stable than the boehmite sample after calcining. Images of SEM clarify the agglomeration of nanopowders in the boehmite sample to micrometer-scale particles that can reduce the available adsorption sites. These images also differentiate between the surface topology of the two samples. For using nanopowder as an adsorbent of OSCs and to avoid agglomeration, particles must be nullified and balanced through proper functionalizing. Studies were performed by XRD to explain the adsorption capacity phenomena of two alumina samples. The diffraction patterns can provide qualitative information about the crystallinity of alumina samples. Crystallinity of the adsorbent provided by XRD results has a decrease in adsorption capacity. Higher disorder provides more topological traps, irregularities, and hidden grooves for higher adsorption capacity. Therefore, the amorphous phase provides more sites for adsorption. This can be the result of slow diffusion and tortuosity of amorphous acidic alumina. Increased attractive interaction between compounds sorbed to internal mesopore surfaces in monolayer or near-monolayer coatings may occur due to surface curvature that brings unbound moieties closer together. Two samples were subjected to SAND instrument for ordering studies through SANS measurements. Quantitative measurements from SANS patterns were used to calculate the pore size in alumina particles.
Moreover, a theoretical study demonstrates a hybrid Langmuir–Freundlich isotherm model for adsorption of DBTO on amorphous acidic alumina. Results show reasonable fitness between experimental and theoretical values of adsorption in a wide range of sulfur concentrations from 5 to 700 ppmw sulfur. Acknowledgments The authors thank NAVSEA-ONR for their financial contribution through Northrop Grumman and CalNova Tech. We are also very grateful for the funding provided by the Army Research Laboratory (ARL). Special thanks go to Dr. Pappannan Thiyagarajan from the Intense Pulsed Neutron Source (IPNS) at Argonne National Laboratory for the SANS pattern measurements. Appendix A. Nomenclature ARL Army Research Laboratory CEMMA Center for Electron Microscopy and Microanalysis DBTO dibenzothiophene sulfone EDS energy-dispersive spectrometer GC-SCD gas chromatography sulfur chemiluminescence detection IPNS intense pulsed neutron source JCPDS Joint Committee on Powder Diffraction Standards OSC organic sulfur compounds p.z.n.c. point of zero net charge PEMFC proton exchange membrane fuel cell SAND small-angle neutron diffractometer SANS small-angle neutron scattering SEM scanning electron microscope SOFC solid oxide fuel cell UAOD ultrasound-assisted oxidative desulfurization ULSF ultra-low-sulfur fuel XRD X-ray diffraction References [1] US EPA, Marks Historic Milestone in Clean Diesel, 2006. [2] US EPA, Program Update: Introduction of Cleaner-Burning Diesel Fuel Enables Advanced Pollution Control for Cars, Trucks and Buses, 2006. [3] J.H. Kim, X. Ma, A. Zhou, C. Song, Catal. Today 111 (2006) 74–83. [4] A. Attar, W.H. Corcoran, Ind. Eng. Chem. Res. 17 (1978) 102–109. [5] F. Zannikos, V. Vignier, Fuel Process. Technol. 42 (1995) 35–45. [6] S. Otsuki, T. Nonaka, N. Takashima, W.H. Qian, A. Ishihara, T. Imai, T. Kabe, Energy Fuels 14 (2000) 1232–1239. [7] F.M. Collins, A.R. Lucy, C. Sharp, J. Mol. Catal. A Chem. 117 (1997) 397–403. [8] O. Etemadi, T.F. Yen, Prepr. Pap. Am. Chem. Soc. Div. Fuel Chem. 51 (2006) 820. [9] L.H. Thompson, L.K. Doraiswamy, Ind. Eng. Chem. Res. 38 (1999) 1250. [10] J.L. Luche, Synthetic Organic Sonochemistry, Plenum, New York, 1998. [11] Y.T. Shah, A.B. Pandit, V.S. Moholkar, Cavitation Reaction Engineering, Kluwer Academic/Plenum, New York, 1999. [12] T.F. Yen, R.D. Gilbert, J.H. Fendler, Membrane Mimetic Chemistry and Its Applications, Plenum, New York, 1994. [13] O. Etemadi, T.F. Yen, Energy Fuels, in press. [14] P.W. Lednor, R. Ruiter, J. Chem. Soc. Chem. Commun. (1989) 320–321.
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[19] G. Sposito, The Surface Chemistry of Soils, Oxford Univ. Press, New York, 1984. [20] R. Sips, J. Chem. Phys. 16 (1948) 490. [21] R. Sips, J. Chem. Phys. 18 (1950) 1024. [22] J. Farrell, M. Reinhard, Environ. Sci. Technol. 28 (1994) 53–72. [23] O. Etemadi, T.F. Yen, Energy Fuels, in press. [24] A. Guinier, G. Fournet, Small Angle Scattering of X-Rays, Wiley, 1955.