ARTICLE pubs.acs.org/IECR
Modification of Carbon Nanotubes for H2S Sorption Ali Mohamadalizadeh,*,† Jafar Towfighi,‡ Alimorad Rashidi,§ Ali Mohajeri,† and Mohamadmehdi Golkar† †
Gas Research Division and §Nanotechnology Research Center, Research Institute of Petroleum Industry, West Boulevard, Azadi Sport Complex, Tehran, Iran 14665-1998 ‡ Department of Chemical Engineering, Tarbiat Modares University, Aleahmad Highway, Tehran, Iran 14115-143 ABSTRACT: Multiwall carbon nanotubes (MWNTs), prepared with methane as carbon source through the chemical vapor deposition (CVD) method at 900 °C, were functionalized to have amino and amido functional groups and then deposited with tungsten (W) nanoparticles. Pure H2S sorption on MWNTs, MWNTs loaded with W, functionalized MWNTs, and activated carbon (prepared from walnut shell) was investigated at 20 °C and a pressure range of 110 bar. X-ray diffraction, transmission and scanning electron microscopies, Raman spectroscopy, ASAP, inductively coupled plasma, and temperature-programmed reduction analysis were performed to determine the properties of the sorbent. The presence of amino functional groups was found to increase the sorption capacity of MWNTs, while the decoration of MWNTs with W nanoparticles decreased the sorption capacity. Results of the research revealed that functionalized MWNTs have a higher H2S sorption capacity than activated carbon. Langmuir, Freundlich, Unilan, Sips, and Toth isotherm models were used for a mathematical description of hydrogen sulfide equilibrium adsorption, and the experimental data for all four adsorbents showed the best accordance with the Freundlich model, while the Unilan model had the least accordance with the experimental data obtained for adsorbents based on MWNTs.
1. INTRODUCTION The term acid gases is used to refer to gaseous mixtures containing H2S and CO2. H2S is a poisonous gas that damages gas processing facilities as well as transportation equipment. Therefore, different adsorbents have been developed for the separation of such species from gas streams.15 During production of natural gas for marketing, the necessity of reducing hydrogen sulfide content to some parts per million levels is essential for safety reasons.6 Numerous researches are conducted to generate adsorbents which are capable of adsorbing more hydrogen sulfide.710 Several technologies developed to eliminate hydrogen sulfide from gas streams can be categorized as11 chemical adsorption in aqueous solutions, H2S conversion into elemental sulfur, H2S conversion to low solubility metallic sulfides, chemical adsorption of H2S on solid adsorbents like iron oxides which can be activated (with copper, silver, gold, platinum, nickel, cadmium, lead, mercury, tin, and cobalt), adsorption and oxidation on activated carbon (AC) and doped AC, sludgederived adsorbents, chemical scrubbing adsorption plus reaction, biological, aerobic, and microbiological methods, and absorption with liquids. Among all, carboneous materials such as activated carbon have also been widely used because of their large surface area and high pore volume.1216 Activated carbon can be used in the form of catalytic-impregnated, impregnated, and nonimpregnated forms.11 Catalytic AC is usually treated by a chemical containing nitrogen such as NH3. Impregnated AC is manufactured by mixing a carbon substrate with solids or liquid chemicals before or after activation. These chemicals serving as impregnates are NaOH, Na2CO3, NaHCO3, KOH, KI, KMnO4, and H2SO417 which is further impregnated by iron, zinc, copper, and nickel.18 Local pH has a significant role on adsorption capacity of hydrogen sulfide, and total sorption capacity decreases with increasing acidity.11,19 These “modifications” usually promote selective oxidation of H2S into elemental sulfur. r 2011 American Chemical Society
Various sources like oil-palm shell and wood-based materials are used to increase physical and chemical sorption of H2S onto new adsorbents which leads to increasing the capacity and producing regenerable adsorbents.18,20 It has been claimed that activated carbon, with a greater specific micropores volume size range between 0.5 and 1 nm, has higher H2S adsorption capacity.11 Moreover, the nature and density of the chemical groups at the surface of activated carbon play an equally important role. Optimization of the operating conditions like temperature, O2/H2S ratio, and space velocity for minimizing the byproduct such as COS and SO2 has been the target of many research studies aimed at selective oxidation of H2S into elemental sulfur.2025 Other materials like multiwall carbon nanotubes (MWNTs) and SiC tubes impregnated with nickel sulfide nanoparticles are used for selective oxidation of H2S into elemental sulfur in the presence of steam with significant increases in selectivity and conversion of H2S into elemental sulfur and better performance for MWNTs.2628 CNTs also can be functionalized for increasing surface area and promoting adsorption properties.2931 In this paper, the sorption of pure hydrogen sulfide on activated carbon prepared from nut shell, MWNTs, CNTs decorated with tungsten nanoparticles (W-CNTs) and amino-functionalized CNTs (F-CNTs) was investigated. Previous research in our laboratory was focused on producing hybrid nanosorbents to adsorb H2S.32 For the first time, the performance of these nanosorbents was compared with that of activated carbon, and the amount of adsorbed hydrogen sulfide and the procedure of regeneration were studied. Also the Freundlich,33 Langmuir,34 Unilan,35 Sips,36 and Toth37 isotherms were employed to mathematically Received: December 28, 2010 Accepted: May 14, 2011 Revised: May 9, 2011 Published: May 16, 2011 8050
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Figure 1. (a) SEM and (b) TEM images of prepared MWNTs.
describe sorption of hydrogen sulfide. The parameters of models were calculated, and the ability of these correlations in the description of hydrogen sulfide sorption was presented.
2. MATERIALS AND METHODS 2.1. Preparation of MWNTs. MWNTs were obtained with a special chemical vapor deposition (CVD) method developed by our group.38 In this method MWNTs were prepared with a CoMo/MgO catalyst in a horizontal furnace consisting of a quartz reactor with a diameter and length of 45 mm and 100 cm, respectively. The reaction was carried out using a methane flow with the rate of 50 (mL/min) as the carbon source, a hydrogen flow with the flow rate of 250 (mL/min) as the career gas at 900 °C for 30 min. After the reaction, the furnace was cooled at a nitrogen atmosphere. To eliminate the remnants of the catalyst metal nanoparticles, the synthesized material was purified. The prepared material was dissolved in 18% HCl solution for 16 h, and it was rinsed with distilled water several times after filtration. It was then dissolved in a 6 M nitric acid solution for 6 h at 70 °C. After filtration and drying, the purified materials were heated in a furnace at 400 °C for 30 min in order to eliminate amorphous carbon. Transmission electron micriscopy (TEM) and scanning electron microscopy (SEM) images of the MWNTs used for the sorption of H2S are shown in Figure 1. Also Raman spectrum of the produced MWNTs is shown in Figure 2. As it is obvious, this sample IG/ID = 2 (IG, graphite; ID, disorder band) indicates the high quality of produced MWNTs. 2.2. Preparation of W-CNT. First the prepared MWNTs were washed and stirred in a 30% nitric acid solution at room temperature for 16 h. They were then filtered and dried before being washed and stirred again at 100 °C in another 30% nitric acid solution for 18 h and subsequently filtered again. Ammonium meta-tungstate ((NH4)6W12O39.xH2O) was used to load 3 wt % of tungsten nanoparticles on the MWNTs as follows. It was dissolved in distilled water and ethanol mixture with a 1:1 volume ratio and the resulting solution was poured on pure MWNTs. The solution was then dried for 2 h at110 °C and then the mixture was calcinated at room temperature to 500 °C with a slope of 2 °C/min in the presence of O2 and He. To prevent the oxidation of MWNTs, the oxygen inlet was cut at 300 °C. After calcinations, the product was reduced at 560 °C in the presence of H2 prior to ICP analysis for determination of loading metal on
Figure 2. Raman spectrum of produced MWNTs.
Figure 3. TEM image of deposited tungsten nanoparticles on MWNTs.
the MWNTs. In this method, tungsten nanoparticles were deposited on MWNTs. (Figure 3) The catalyst reduction was necessary to attain information on the metalsupport interaction.39 H2-TPR was used to characterize the interaction between the support and the supported species. The H2-TPR profile of a sample is shown in Figure 4. Results showed the optimum temperature for reduction of 3 wt % tungsten decorated on MWNTs was 559.4 °C. 2.3. Preparing Amino-functionalized MWNTs. The following modified procedure was used to graft the desired organic functional group on MWNTs.40 The support material was sonicated in a 36 N sulfuric acid and 15.8 N nitric acid solution with a ratio of 60:40 for approximately 3 h. The solution was then cooled to room temperature with distilled water before being washed and then dried for approximately 6 h at 120 °C. A 10 g 8051
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Figure 4. TPR test for MWNTs decorated with 3 wt % tungsten.
Figure 5. XRD patterns of (a) MWNTs and (b) aminofunctionalized MWNTs.
portion of urea was added to each 0.5 g of MWNTs, and the samples were heated to approximately 150 °C for 15 min. Distilled water was added and then centrifuged. An extraction step was performed using sodium perchlorate and the resulting product was washed with distilled water and then dried. Through this procedure, amino functional groups were grafted onto the MWNTs. Figure 5 shows XRD patterns of the functionalized MWNTs. It is clear that functionalized MWNTs have similar patterns with MWNTs before functionalization. This means functionalization does not change the structure of MWNTs and they both have similar peaks in 2θ = 26.6 and 2θ = 42.4, but the crystalinity has been improved after this procedure. This result has also been shown by Safari et al.41 Back titration was used for determination of amino-functional groups and amido-functional groups on MWNTs and the amount of NH2 per gram of MWNTs was calculated.40,42 A 7 mL portion of H2SO4 titrisol (Merck, Art 9984) 0.1 N was added to 0.1 g of functionalized MWNTs. The mixture was sonicated for 15 min at room temperature to achieve better dispersion of the MWNTs in the solution, and it was kept at room temperature for 24 h. Then the mixture was filtered and washed with distilled water for several times. The filtrate was titrated with NaOH titrisol 0.1 N (Merck, Art. 9959) in the presence of phenolphthalein as pH indicator. The same experiment was done for the blank sample; the differences between two experiments reflected the amount of titratable amino function. According to the calculation (not shown here), 3.1 mL of NaOH was used, and therefore the NH2 was equal to 3.9 mmol/g per gram of MWNTs. The above experiment showed that an amino group was formed with the urea melt method.40 Also, FTIR studies showed a peak at 1634 cm1, which confirmed the existence of an amide group; therefore, amide and an amine group were formed in the urea melt method.
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Raman spectrum of the functionalized MWNTs and MWNTs decorated with tungsten particles (not shown here) were reproducible which indicates no changes in the structure of MWNTs after these modifications. 2.4. Preparing of Activated Carbon from Nut Shell. To prepare activated carbon samples, a 1.5:1 mixture of H3PO4 and nutshell was stirred at 80 °C for 2 h, and then the mixture was heated in an oven at 110 °C overnight. The prepared material was heated in the presence of N2 at 450 °C for 1 h and then washed with HCl and neutralized with distilled water at 60 °C to pH = 7.43 2.5. Characterization Techniques. Scanning electron microscopy (SEM) images were done with a Philips, XL30 device. Gold was used as the conductive material for sample coating. The pore size and surface area measurements were performed with a Micrometrics ASAP 2010 instrument by adsorption of nitrogen at 77 K. Samples were degassed at 300 °C for 5 h prior to N2 physisorption. The metal loading was analyzed through atomic absorption spectroscopy (AAS at the analysis center of the Research Institute of Petroleum Industry). X-ray diffraction measurements were conducted using the standard powder diffraction procedure carried out with a Philips diffractometer (PW1840) (Lump 3ukR, λ = 1.54 Å). TEM images were obtained with a Philips, CM 200 device in the Department of Material Science at Sharif University of Technology. Hydrogen temperature programmed reduction (H2-TPR) experiments were carried out for the determination of the catalysts reduction behavior. The samples were heated at room temperature to 623 K in a N2 flow until no desorption gases were detected. The samples were then cooled to room temperature, and the carrier gas was replaced by the reaction mixture (5% H2/Ar) at a flow rate of 30 mL/min. A linear increase in temperature at room temperature to 1200 K at a rate of 10 K/min was adopted. The amount of consumed H2 was detected using a gas chromatograph (Shimadzu GC-14A) equipped with a thermal conductivity detector connected to a data processor (Shimadzu C-R4A). 2.6. H2S Adsorption Setup. The schematic flow diagram of the gas absorption setup is shown in Figure 6. Two grams of the nano adsorbent were placed in the sample column attached to the setup. The system was connected to a vacuum line and the sample column was heated for about 2.5 h at 200 °C in order to have all the preadsorbed gases desorbed. Then, valves V2 and V4 were closed, and H2S was injected from the gas container to the gas charge tank. To carry out the tests in constant temperature, the gas charge tank and sample column were placed in a water bath of 20 °C temperature. When the thermal equilibrium was reached, the initial gas pressure was read by closing V1 and opening V2 and V3 valves to let H2S into the sample column, and from then on the changes in the gas pressure were plotted versus time until the pressure reached a fixed value. A blank test was conducted with helium. Performing the experiment with He revealed that part of the pressure drop is due to the gas expansion after opening V3, and since there is no relation between this pressure drop and gas adsorption, it has to be subtracted from the total pressure drop. To evaluate the H2S sorption value, the Z factor was calculated using the SRK equation. All the experiments were carried out under atmospheric pressure up to about 10 bar. According to RIPI safety regulations, the experiments were not performed to more than 10 bars. For safety reasons, H2S was passed through a NaOH solution after the tests. 8052
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Figure 6. Schematic diagram of adsorption system.
Figure 7. Isotherms for adsorption of H2S onto MWNTs, CNT decorated with tungsten nanoparticles (W-CNT), amino functionalized CNTs (F-CNT), and activated carbon (AC) (at 20 °C).
Figure 8. Nitrogen adsorption and desorption isotherm measured at 76.15 K for MWNTs, W-CNTs, F-CNTs, and AC.
Table 1. H2S Adsorption Data on MWNTs, W-CNTs, FCNTs, and Activated Carbon BET surface
average pore diameter
pore volume of
adsorbent
area (m2/g)
by BET (nm)
pores (cm3/g)
MWNTs
140
9.9
0.483
W-CNTs
100
11.5
0.395
179
11.6
0.755
1832
2.9
1.129
F- CNTs AC
3. RESULTS AND DISCUSSION 3.1. H2S Sorption onto Adsorbents. The H2S isotherm adsorption curves for MWNTs, W-CNTs, F-CNTs, and activated carbon are shown in Figure 7. Only seven experiments were carried out for each adsorbent at the pressure range of 110 bar, because of hazardous effects and problems of working with pure H2S. Several factors are known to affect H2S sorption on the adsorbents. It is clear that the H2S sorption on functionalized MWNTs is more than those of MWNTs and W-CNTs. Nitrogen adsorption isotherms were measured using ASAP 2010 at 76.15 K. BET results are shown in Table 1, which indicate
Figure 9. H2S adsorption mechanism on amino-functionalized MWNTs.
the surface area of MWNTs is increased from 140 to 179 m2/g with the functionalization process. Functionalization causes debundling of carbon nanotubes which increases surface area.30,31 Analysis showed that pore volume is increased from 0.48 to 0.757 cm3/g and so the hydrogen sulfide sorption ability is increased also. The results in Table 1 show that the surface area and pore volume of W-CNTs is reduced from 140 to 100 m2/g and 0.48 to 0.396 cm3/g, respectively, owing to the deposition of metal particles on MWNTs, which leads to lower hydrogen sulfide adsorption. Reduction in the surface area and pore volume of W-CNTs indicates that adding metal particles might block the accessibility of mesoporousities. The presence of metal particles usually increases the sorption of H2S in the presence of water and air/O2, which accelerates 8053
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Figure 10. Pore size distributions for (a) MWCNTs, W-CNTs, F-CNTs, and (b) activated carbon.
metal reaction with hydrogen sulfide.43 These experiments were done in the absence of water and air/O2; however, by adding metal particles, the surface area of the adsorbent and the physical absorption of H2S decreased, which shows absorption is more probable than adsorption. The efficiency of activated carbon in H2S adsorbing depends on the combination of surface chemical properties and porosity.44 The surface area and pore volume of activated carbon were 1832 (m2/g) and 1.129 (cm3/g) (Table 1), respectively, as compared to MWNTs and W-CNTs. Also the curves in Figure 8 exhibit higher surface areas for activated carbon in comparison to adsorbents based on MWNTs, which indicates that the total amount of H2S sorption is greater for activated carbon. However, the adsorption capacity of MWNTs became greater than that of activated carbon, functionalized with amino and amido groups. Two mechanisms are probable for H2S sorption due to the presence of these groups. One mechanism is chemisorption on amino groups and another one is physisorption on amido groups. The main mechanism is an acidbase reaction between H2S and amino groups which increases H2S adsorption on MWNTs (Figure 9). Also, an amido group accelerates the adsorption of H2S by making hydrogen bonds. Surface functional groups provide a polar environment and increase the chemical adsorption. There is a competition between the physical and chemical adsorption phenomena; therefore the summation of the physical and chemical adsorptions determines the capacity of the adsorbents.45 These reasons indicate that surface chemical properties are important factors in determining the final adsorption ability of MWNTs rather than specific surface areas or pore volume. Increasing the surface area does not necessarily indicate that the potential for H2S adsorption increased. This behavior is also observed for H2S sorption into activated carbon.44 Figure 7 indicates that H2S sorption on activated carbon was greater than functionalized MWNTs at pressures less than 2 bar, but the condition reverses at pressures above this. It is clear from nitrogen adsorption isotherms (Figures 10a, b) that the distribution of pores size of MWNTs, W-CNTs, and F-CNTs is in mesopore range and the pore distributions of activated carbon are not sharp and show a range of micropore sizes. Pore diameter values of activated carbon and functionalized MWNTs were 2.9 and 11.6 nm (Table 1). Some pores were blocked as pressure increased due to the smaller pore diameter of activated carbon which could have caused the reduction of H2S sorption on activated carbon as compared to F-CNTs.18 Also smaller pore diameters can increase the mass transfer resistance and cause less sorption of H2S. However, surface chemical properties have a major
Figure 11. H2S sorption experiments of activated carbon and W-CNT for the second time.
role in increasing the capacity of functionalized MWNTs compared with activated carbon. H2S adsorption experiments on activated carbon and W-CNTs were performed again, and adsorbents were not degassed before the test at 200 °C. Results indicated that at 9 bar the amount of H2S sorption decreases about 0.128 (mmol/g) for W-CNTs and 0.32 (mmol/g) for activated carbon (Figure 11). Adsorption and desorption of H2S on MWNTs occurs readily in correspondence to pore distribution in the mesopore range, and low amounts of the gases are captured. On the contrary, the majorities of the pores in the activated carbon are in micropore range and are active for H2S immobilization. Therefore high amounts of gas are trapped, which remain in pores after desorption.11,14 3.2. Modeling of Adsorption Isotherms. Langmuir, Freundlich, Unilan, Sips, and Toth models were used for mathematical description of hydrogen sulfide equilibrium adsorption. In nonlinear forms of models (Table 2), q (mmol/g) is the amount of adsorbed gas per adsorbent, qm is the maximum capacity of adsorbent, P (bar) is the pressure of the gas and s, b, n, k, and c are equation parameters which are dependent on the nature of the gas. As mentioned, the capacity of adsorbents decreases in the order of F-CNT > AC > CNT > W-CNT, and the numerical values of the maximum capacity of each adsorbent are 1.85, 1.64, 1.39, 1.32 mmol/g adsorbent, respectively. Table 2 illustrates the results of H2S adsorption modeling for several adsorbents. The values of parameters and regression coefficients were calculated with corresponding mathematical methods. The value of regression coefficients were between 0 and 1 (1 being ideal regression) so the adaptability of the isotherm models could be evaluated. Results in Table 2 show that 8054
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Table 2. Determined Isotherm Model Constants for the Adsorption of H2S on MWNTs, W/MWNTs, Amino-functionalized MWNTs and Activated Carbon adsorbents isotherm
parameter
Freundlich
k
Langmuir
expression q qm
¼ kP1=n
AC
0.3713
0.3303
0.4341
2.0081
2.1976
2.0039
2.6632
R2 b
0.9926 0.6819
0.996 0.7021
0.9991 0.6359
0.9995 0.845
0.9577
0.9643
0.9666
0.9801
0.5183
0.5002
0.5121
0.8981
1.26 103
1.25 104
1.89 103
3.88 103
0.8672
0.8333
0.9098
0.9796
0.4766
0.4923
0.5066
0.7151
n
0.5456
0.5627
0.6573
0.7514
R2 b
0.9843 0.2376
0.9796 0.2636
0.9723 0.2736
0.9879
c
2.6434
2.3091
1.9701
R2
0.9901
0.9847
0.9787
q qm
¼
bP 1 þ bP
b
q qm
¼
1 2s
q qm
¼
h ln
1 þ bes P 1 þ be s P
R2
Toth
F-CNTs
0.3414
s Sips
W-CNTs
n
R2 Unilan
MWNTs
b
q qm
¼
ðbPÞ1=n 1 þ ðbPÞ1=n
bP ½1 þ ðbPÞc 1=c
i
Figure 12. The experimental data of H2S adsorption on (a) MWNTs (b) W-CNTs (c) F-CNTs and (d) AC in accordance with adsorption isotherms.
the experimental data for all adsorbents based on MWNTs were in perfect accordance with Freundlich equation while such was not the case with the Unilan equation. Adsorption isotherm curves were plotted for all the equations (Figure 12ad). Comparison of adsorption curves with the experimental data indicated that Toth and Sips equations can better predict the experimental data at the beginning of the H2S adsorption process only for MWNTs and W-CNTs (Figure 12a, b). In addition to Toth and Sips equations, the Unilan equation also showed an acceptable accordance with the experimental data
at the beginning of the process of hydrogen sulfide adsorption only for F-CNTs (Figure 12c). The experimental data for activated carbon at the beginning of the process could be better described by the Sips equation (Figure 12d). Unlike the case of three adsorbents based on MWNTs, the Unilan equation has a more acceptable accordance with the experimental data for activated carbon, and the value of regression coefficient (0.9796) is greater than the calculated values for the other adsorbents. Therefore the Unilan equation describes the experimental data better for the nonhomogeneous materials, 8055
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Industrial & Engineering Chemistry Research with a complex and wide range of pore size distribution, such as activated carbon. Most probably the Unilan equation did not work well for the adsorbents based on MWNTs because of low mass transfer resistance of MWNTs caused by their porosity in mesopore range. According to the results of the mathematical modeling, the Toth equation could not describe the experimental data for activated carbon well. Altogether, the values of regression coefficients indicated that the Freundlich equation generally had a better compatibility with the experimental data (Table 2). The Freundlich model is an empirical equation based on a sorption heterogeneous surface through a multilayer adsorption mechanism.46,47 From the modeling of experimental data, it is concluded that the assumption of monolayer and capillary adsorption does not agree with the experiments in the studied pressure range. Although, regarding the structure of MWNTs, the multilayer diffusion mechanism can be probable, and we will study more about these mechanisms in future researches.
4. CONCLUSION The sorption of H2S depends on the structural and chemical characteristics of the surface of the adsorbents. Modification of MWNTs with amino and amido groups enhanced the H2S sorption capacity as compared to activated carbon, which indicates surface chemical properties affect the ability of MWNTs to adsorb more H2S. Desorption occurs more readily on MWNTs than on activated carbon, due to the pore size distribution (in mesopore ranges) of MWNTs and lower mass transfer resistance. Study of adsorption isotherms show that, in order, Freundlich, Toth, Sips, Langmuir, and Unilan equations have the best accordance with the experimental data of H2S sorption for adsorbents based on MWNTs. But for activated carbon, Freundlich, Sips, Langmuir, and Unilan equations show the best compatibility with the experimental data, and the Toth equation does not overlap with the experimental data at all. Because of the differences in the structure, the compatibility of the Unilan equation was more applicable for activated carbon as compared with MWNTs. From the modeling of experimental data, it is concluded that multilayer adsorption does agree with the experiments in the studied pressure range. ’ AUTHOR INFORMATION Corresponding Author
*Tel: þ982148252411. Fax: þ982144739716. Email: alizadehsa@ ripi.ir.
’ ACKNOWLEDGMENT The authors appreciate the financial supports of the Research Institute of Petroleum Industry (RIPI). ’ REFERENCES (1) Wang, R. Investigation of a New Liquid Redox Method for Removal of H2S and Sulfur Recovery with Heteropoly Compound. Sep. Purif. Technol. 2003, 31, 111. (2) Mikhail, S.; Zaki, T.; Khalil, L. Desulfurization by an Economically Adsorption Technique. Appl. Catal., A 2002, 227, 265. (3) Stepova, K.; Maquarrie, D.; Krip, I. Modified Bentonites as Adsorbents of Hydrogen Sulfide Gases. Appl. Clay Sci. 2009, 42, 625.
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