Article pubs.acs.org/IECR
Synthesis, Characterization, and Evaluation of Activated Carbon Spheres for Removal of Dibenzothiophene from Model Diesel Fuel Changming Zhang,†,‡ Wen Song,§ Guohua Sun,† Lijing Xie,†,‡ Liu Wan,†,‡ Jianlong Wang,*,† and Kaixi Li*,† †
(Key Laboratory of Carbon Materials, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, People’s Republic of China) ‡ (Graduate University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China) § (ShanXi XinHua Chemical Company, Ltd., Taiyuan 030008, People’s Republic of China) ABSTRACT: Activated carbon spheres (ACSs) with high surface area and different porous structure were prepared from the fundamental chemical materials, 3-methylphenol and formaldehyde, by suspension polymerization and steam activation. The effects of two organic additives, ethylene glycol and poly(ethylene glycol), to the textural structure and adsorptive dibenzothiophene (DBT) of ACSs were investigated. The pyrolysis behavior of the resin spheres (RSs) was characterized by thermal gravimetric analysis. The texture properties of the obtained ACSs were characterized by N2 adsorption−desorption and scanning electron microscope (SEM) techniques. The as-prepared ACSs reached a Brunauer−Emmet−Teller (BET) surface area value as high as 1501 m2/g and a total pore volume of 0.72 cm3/g. The BET surface areas and pore volumes increased after adding two organic additives. The adsorptive capacity of DBT for model oil had a good linear correlation with the volume of small micropores (0.6 −1.2 nm).
1. INTRODUCTION Sulfur compounds in fuels are converted to SOx during combustion, which not only result in acid rain but also poison catalysts in catalytic converters for reducing CO and NOx.1−3 Deep desulfurization of transportation fuels is receiving increasing attention in the research community worldwide due to increasingly stringent regulations and fuel specifications in many countries for environmental protection purposes. The conventional approach for a deep desulfurization of fuel oil is called hydrodesulfurization (HDS), which not only requires high temperature and high pressure but also has some difficulty in removing aromatic thiophenes and their derivatives.4−7 Therefore, extensive research is carried out to propose alternative technologies to obtain low-sulfur fuels. Among various alternative methods, adsorptive desulfurization is considered to be more promising due to its ambient operation temperature and pressure, availability of regeneration, and lowenergy consumption.8−13 Activated carbons are the most often used materials for adsorption, which can be attributed to their high surface area, large pore volume, low cost, and easy regeneration.14−18 Among activated carbons, activated carbon spheres (ACSs) show many charming characteristics, such as a smoother surface, suitable ball size, better fluidity, high loading density, and high mechanical strength.19−24 ACSs are considered promising and valuable adsorbents for adsorption of harmful materials in liquid or gas phase, and in catalyst support.19,24,25 Several precursor materials were employed to prepare spherical activated carbon, for example, pitch, phenolic resin, and polystyrene polymer spheres.26−28 Among them, phenolic resins are promising polymeric precursor materials for producing high surface area ACSs because of the low inorganic impurities and negligible ash content. But the pore structure of © 2014 American Chemical Society
the prepared ACSs with phenolic resins as precursors cannot be controlled well. In this study, we report a new method to prepare ACSs from 3-methylphenol and formaldehyde as precursor materials with the addition of ethylene glycol or poly(ethylene glycol) as additives by suspension polymerization and steam activation. The preparation of ACSs from these resin beads has few reports to our knowledge. The prepared ACSs showed a high surface area of 1501 m2/g and a diameter of about 2 mm. The pore size was controlled by decomposing different molecular weight additives at the heat processing. The dibenzothiophene (DBT) adsorption properties of ACSs were also investigated.
2. EXPERIMENTAL SECTION 2.1. Preparation of ACSs. Spherical resin beads were prepared from 3-methylphenol and formaldehyde by suspension polymerization. In a typical run, the polymerization reaction was carried out in a 1000 mL round-bottomed fourneck reaction vessel with a Teflon stirrer, a reflux condenser, and a thermocouple. The 3-methylphenol was polymerized with an aqueous solution (37−41% (w/v)) of formaldehyde in a molar ratio of 1:4 in the presence of triethylamine (TEA, 1.5 wt %), followed by dispersing the resulting mixture into 200 mL of deionized water via stirring at 500 rpm. Then, poly(vinyl alcohol) (PVA, 5.0 wt %) was added to the above mixture at 90 °C by stirring at 500 rpm for 30 min. Afterward, hexamethylene tetraamine (HMTA, 3 wt %) was added to the reaction vessel and polymerization reaction was carried out at the same Received: Revised: Accepted: Published: 4271
November 7, 2013 February 13, 2014 February 21, 2014 February 21, 2014 dx.doi.org/10.1021/ie403773f | Ind. Eng. Chem. Res. 2014, 53, 4271−4276
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Figure 1. Thermal stability tests of the samples (a) TG and (b) DTG curves.
Figure 2. N2 adsorption−desorption isotherms and DFT pore size distributions of the samples.
temperature and fixed agitation rate (500 rpm) for 3 h. After cooling to room temperature, the resin sphere (RS) was obtained via solid−liquid separation. In addition, two types of RSs were prepared by adding two organic additives, ethylene glycol (EG, 30 wt %, named EG30) and poly(ethylene glycol) (PEG, 35 wt %, named PEG35). Then, the three types of spherical resin beads were further carbonized at 850 °C for 30 min in nitrogen atmosphere and activated at 850 °C with steam for 1 h. These ACSs will be denoted as ACS, ACSEG30, and ACSPEG35, respectively. 2.2. Characterization. Thermogravimetric (TG) analysis of the resin spheres was carried out using a Perkin-Elmer TG/ DTG-6300 instrument in a temperature range of 30−850 °C under nitrogen with a heating rate of 10 °C min−1. The textural parameters of all of the studied activated carbons were carried out by N2 adsorption/desorption at −196 °C using a ASAP 2020 instrument. The samples were separately degassed at 250 °C in a vacuum environment for a period of at least 4 h prior to measurements. Experimental adsorption data at the relative pressure (P/P0) less than 0.3 was used to calculate surface area values using the standard Brunauer, Emmett, and Teller (BET) equation. The pore size distribution (PSD) was determined by applying density functional theory (DFT) method based on nitrogen adsorption data. The surface morphology of the samples was observed on a Hitachi S-4800 field emission scanning electron microscope (SEM) operating at 10 kV. 2.3. Adsorption of DBT. Model oil was prepared by dissolving DBT in n-octane. In a typical run, 0.1 g of sample and 20 mL of model oil were added in a flask. The sulfur concentrations of DBT solution were from 50 to 500 μg/mL. The covered flasks were placed in a shaking bath and allowed to shake for 72 h at room temperature (25 °C). After equilibration
the concentration in the solution was determined using a microcoulometric detector. The adsorbed amount was calculated from the formula qe = V(C0 − Ce)/m, where qe is the equilibrium amount adsorbed, V is the volume of the liquid phase, C0 is the concentration of solute in the bulk phase before it comes in contact with the adsorbent, Ce is the concentration of the solute in the bulk phase at equilibrium, and m is the amount of the adsorbent. The DBT adsorption was interpreted according to the Langmuir−Freundlich equation.
3. RESULTS AND DISCUSSION 3.1. Thermogravimetric Analysis. Thermogravimetric (TG) and differential thermogravimetric (DTG) curves of the resin spheres are shown in Figure 1, which are obtained during the thermal stability tests. From the TG curves, it can be seen that RS shows a total mass loss of 57.6 wt %, which is the highest thermal stability among the synthesized resin spheres. EG30 and PEG35 show mass losses of 74 and 87 wt %, respectively. About the heat decomposition of phenolic resin, it is well-known that three stages were taken.29−34,43 According to the DTG curves, three thermal degradation ranges are concluded as follows: (i) 100−350, (ii) 350−500, and (iii) 500−700 °C. The mass loss of the first stage can be attributed to the departure of unreacted groups (mainly methylol) and the loss of moisture present in the resin spheres.29,30,43 It should be noted that EG30 shows a significant mass loss during this stage, which can be assigned to the presence of a little unreacted ethylene glycol (boiling point 196−198 °C) in this resin sphere.43 In the second stage, a cross-linked network of the resin spheres is broken, and the evolution of light gases (CH4, CO, and CO2, etc.) takes place, as well as the formation of 3methylphenol and methyl derivatives.31,32,43 Two organic 4272
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Table 1. Porous Structure Parameters of the ACSs
a
samples
SBET (m2/g)
Smicro (m2/g)
Vtotal (cm3/g)
Vmicro (cm3/g)
Vmeso (cm3/g)
Vspecific‑microa (cm3/g)
DP (nm)
ACS ACSEG30 ACSPEG35
993 1467 1501
699 1026 464
0.42 0.68 0.72
0.32 0.47 0.25
0.05 0.09 0.35
0.10 0.18 0.21
1.69 1.86 1.9
Vspecific‑micro: volume of specific micropores (0.6−1.2 nm).
Figure 3. SEM images of (a, b) ACS, (c, d) ACSEG30, and (e, f) ACSPEG35.
volumes of samples in Table 1 prove that high Vmeso (0.35 cm3/ g) makes an exception of the ACSPEG35 isotherm. According to Table 1, the BET surface areas, total pore volumes, and mesoporous volumes of ACSEG30 and ACSPEG35 increase markedly compared with those of ACS, in the following the order: ACSPEG35 > ACSEG30 > ACS. However, micropore surfaces and micropore volumes of the samples have a different trend, in the following the order: ACSEG30 > ACS > ACSPEG35. In addition, all of the samples have a pore size distribution less than 4 nm, ranging from micropores to mesopores. The results are consistent with the N2 adsorption/desorption isotherms. ACS and ACSEG30 have mainly supermicropores and less mesopores. ACSPEG35 exhibits some supermicropores with a high amount of mesopores, which indicated that the additions of EG and PEG result in different increases of micropore and mesopore, respectively. The two additives, which have different
additives of ethylene glycol and poly(ethylene glycol) may result in the important rate of mass losses of EG30 and PEG35 during this stage. In the last stage, hydrogen atoms are removed from the C−H bond in 3-methylphenol and hydrogen gas is evolved.33,34 3.2. Textural Structure. N2 adsorption−desorption isotherms and PSD curves of the samples are shown in Figure 2. And the corresponding structure parameters are summarized in Table 1. The isotherms of ACS and ACSEG30 are similar and present type I isotherms with an abrupt knee at low relative pressure. It demonstrates that the pore structure of the samples is mainly composed of well-developed micropore volumes.35,36 The isotherm of ACSPEG35 is an exception, as it may be considered as being an intermediate form between type I and type IV.37 The hysteresis loops at P/P0 > 0.4 indicates the presence of a certain mesopore in the sample. The mesopore 4273
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the Freundlich and Langmuir models in its consideration of the nonuniformity of the energy of the solid surface, is suitable for the adsorption of single solute from dilute solutions. The isotherm parameters were calculated by the fitting of experimental data using a nonlinear method and are listed in Table 2. As can be observed from Table 2, the better R2 coefficients are obtained in all cases when the Langmuir− Freundlich isotherm is used. The high correlation coefficients indicate that the Langmuir−Freundlich equation is suitable for the adsorptive investigation. The values of parameters K and n are related to the Langmuir-type constant and the heterogeneity of the site energies, respectively. The K values are similar for all samples. The n values of the samples are close to 1, which indicates that the surface is energetically homogeneous and all of the active sites have the same energy toward DBT adsorption. It also suggests a Langmuir-type, monolayer adsorption mechanism.41 It can be seen from Figure 4 that the adsorptive capacities of ACSEG30 and ACSPEG35 for DBT are higher than that of ACS. It demonstrates that incorporating EG and PEG is effective to enhance the adsorptive capacity for DBT. However, according to the results from Table 2 and Figure 4, the adsorptive capacities of the samples for DBT present that BET surface area, micropore surface area, total pore volume, and micropore volume obtained from N2 adsorption do not provide good linear correlation with sulfur adsorption capacity. As the diameter of DBT is known to be about 0.65 nm, its adsorption capacity is greatly governed by the micropore volume.11 According to the PSD of the samples in Figure 2, it can be seen that an overlapping micropore range is obvious around 0.6−1.2 nm. As illustrated in Figure 5, it was found that the
molecular weights, conduce the different porous structure of ACSs at the process of heat decomposition. 3.3. Surface Morphology. The SEM images of the samples are shown in Figure 3. The ACS has a smooth surface and a good spherical shape. After the addition of EG and PEG, ACSEG30 and ACSPEG35 retain a spherical structure, but their exterior surfaces become rough and have some pores. From images of the inner structure, it can be observed that ACSEG30 and ACSPEG35 have different degrees of developed pore structure compared with that of ACS. In addition, ACSPEG35 shows the highest textural development among the samples, which suggested the differences in the effectiveness of EG and PEG on the pore structure. 3.4. DBT Adsorption from Liquid Hydrocarbon Solution. The DBT adsorption isotherms on different ACSs are illustrated in Figure 4. The shapes of the DBT adsorption
Figure 4. Adsorption isotherms of the samples to DBT (Langmuir− Freundlich model fitting).
isotherms have a concavity toward the abscissa axis. It indicates that it belongs to the L type according to the Giles classification. It demonstrates that the adsorbed DBT on the ACSs surface is parallel. And it becomes increasingly difficult for a fresh solute molecule to find a vacant site when more sites in the substrate are filled. A great number of different isotherm equations were proposed in the literature to describe the singlesolute adsorption from dilute solutions on energetically heterogeneous solids.39,40 The relation between the equilibrium concentrations (Ce) and the equilibrium adsorption capacity (qe) was correlated by the Langmuir, Freundlich, and Langmuir− Freundlich models. The adsorption isotherms of DBT on all tested samples are presented in Figure 4 along with the fit to the Langmuir−Freundlich equation to correlate the relation between Ce and qe. The Freundlich isotherm is an empirical model that can be employed to nonideal adsorption on heterogeneous surfaces. The Langmuir isotherm model refers to the system with monolayer adsorption. The Langmuir−Freundlich model, which has the advantage versus
Figure 5. Correlation between the adsorption capacities and specific micropore volume (0.6−1.2 nm).
adsorptive capacity has a good linear correlation with the volume of specific micropores (0.6−1.2 nm) for all of the test samples. These results suggest that pore size rather than surface
Table 2. t Fitting Parameters to Isotherm Equations and the Goodness of the Fi Freundlich
Langmuir
2
Langmuir−Freundlich 2
sample
Kf(mg/g)
1/n
R
q0((mg S)/g)
KL(L/mg)
R
ACS ACSEG30 ACSPEG35
16.39 19.87 21.83
0.31 0.45 0.48
0.8316 0.8342 0.8554
17.65 20.23 22.52
0.0083 0.0102 0.0113
0.9171 0.9452 0.9675
q0((mg S)/g)
K (L/mg)
n
R2
18.84 20.44 23.01
0.032 0.028 0.029
1.09 0.86 0.95
0.9927 0.9983 0.9989
q0 values are the maximum adsorption per unit weight of the adsorbent. 4274
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