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Energy & Fuels 2009, 23, 1534–1538
Adsorption Behavior of Carbon Dioxide and Methane on AlPO4-14: A Neutral Molecular Sieve Xing-Xiang Zhao,† Xiao-Liang Xu,† Lin-Bing Sun,† Li-Li Zhang,‡ and Xiao-Qin Liu*,† State Key Laboratory of Materials-oriented Chemical Engineering, Nanjing UniVersity of Technology, Nanjing 210009, China, and Jilin Oilfield Corporation of Petro-China: Reconnaissance and Design Institute, Songyuan 138000, China ReceiVed October 10, 2008. ReVised Manuscript ReceiVed December 31, 2008
AlPO4-14 molecular sieve was prepared by hydrothermal crystallization of saturated gels. The synthesized molecular sieve was characterized by X-ray diffraction and scanning electron microscopy. For the first time, the adsorption behavior of CO2 and CH4 was investigated on AlPO4-14, a neutral molecular sieve with no extra-framework cations. Adsorption isotherms of CO2 and CH4 on AlPO4-14 were examined at 273 and 300 K. Henry’s law constants and isosteric heats of adsorption of all adsorbates were determined from adsorption isotherms. It was found that CO2 was strongly adsorbed on the molecular sieve, and the selectivity of CO2/ CH4 can reach as high as 21.77 at 273 K. The adsorbate-adsorbent interaction and the steric effect should be responsible for the preferential adsorption of CO2 on AlPO4-14.
As energy problems become more and more serious, the use of natural gas has been economically attractive from the environmental point of view. However, the presence of nonburning components in natural gas such as CO2 and N2 do not fit the pipeline quality for minimum heating value specifications (the maximum amount of CO2 and N2 cannot exceed 4%). The existence of CO2 in natural gas can also corrode equipments and pipelines. Moreover, CO2 is one of the greenhouse gases that causes global warming. Therefore, the capture and separation of CO2 are significant for the application of natural gas and for the improvement of environment quality. The typical technology for CO2 removal from CH4 is amine absorption; however, the process is energy-consuming and cost-intensive.1 Pressure swing adsorption (PSA) has emerged as a successful technology for the purification and bulk separation of gas mixtures by either equilibrium or kinetic driving factors. To remove CO2 efficiently by using PSA technology, it is necessary to develop new adsorbents with enhanced performance. To date, various molecular sieves have been employed for CO2/CH4 separation.2-9 Cavenati et al.2 investigated the adsorp-
tion of CO2 on zeolite 13X, and their results showed that the natural gas after purification could meet the pipeline quality. Himeno et al.3 determined the isosteric heats of adsorption and Henry’s law constants on an all-silica molecular sieve DD3R using pure components, and the molecular sieve exhibited a high selectivity of CO2 over CH4. Delgado et al.4 reported the removal of CO2 in natural gas by means of mordenite. They found that the selectivity of CO2/CH4 on Na-mordenite is much larger than that on H-mordenite, which can be ascribed to the electrostatic interaction of CO2 with Na+ ions. Additionally, other aluminosilicates such as β-zeolite5 and ZSM-56,7 as well as silicoaluminophosphates (SAPOs) such as SAPO-510 and SAPO-3411 have also been used as adsorbents for the separation of CO2/CH4. In comparison with the research on molecular sieves possessing exchangeable cations, however, little attention has been paid to CO2/CH4 separation using a neutral molecular sieve with no extra framework cations. Aluminophosphates (AlPOs) are molecular sieves with special characteristics. They possess a neutral framework with no exchangeable cations. AlPOs contain both Lewis and protonic acid sites.12,13 These characteristics may make AlPOs useful in the process of separation. As a member of AlPOs, AlPO-5 has
* To whom correspondence should be addressed. Telephone: +86 25 83587176. Fax: +86 25 83587191. E-mail:
[email protected]. † Nanjing University of Technology. ‡ Jilin Oilfield Corporation of Petro-China. (1) Baker, R. Future Directions of Membrane Gas Separation Technology. Ind. Eng. Chem. Res. 2002, 41, 1393–1411. (2) Cavenati, S.; Grande, C. A.; Rodrigues, A. E. Adsorption Equilibrium of Methane, Carbon Dioxide, and Nitrogen on Zeolite 13X at High Pressures. J. Chem. Eng. Data 2004, 49, 1095–1101. (3) Himeno, S.; Tomita, T.; Suzuki, K. Characterization and Selectivity for Methane and Carbon Dioxide Adsorption on the All-silica DD3R Zeolite. Microporous Mesoporous Mater. 2007, 98, 62–69. (4) Delgado, J.; Uguina, M.; Gomez, J. Adsorption Equilibrium of Carbon Dioxide, Methane and Nitrogen onto Na- and H-mordenite at High Pressures. Sep. Purif. Technol. 2006, 48, 223–228. (5) Li, P.; Tezel, F. H. Adsorption Separation of N2, O2, CO2 and CH4 Gases by β-zeolite. Microporous Mesoporous Mater. 2007, 98, 94–101. (6) Harlick, P. J.; Tezel, F. H. Adsorption of Carbon Dioxide, Methane and Nitrogen: Pure and Binary Mixture Adsorption for ZSM-5 with SiO2/ Al2O3 Ratio of 280. Sep. Purif. Technol. 2003, 33, 199–210.
(7) Harlick, P. J.; Tezel, F. H. Adsorption of Carbon Dioxide, Methane, and Nitrogen: Pure and Binary Mixture Adsorption by ZSM-5 with SiO2/ Al2O3 Ratio of 30. Sep. Sci. Technol. 2002, 37, 33–60. (8) Liu, X.; Li, J.; Zhou, L. Adsorption of CO2, CH4 and N2 on Ordered Mesoporous Silica Molecular Sieve. Chem. Phys. Lett. 2005, 415, 198– 201. (9) Liu, X.; Zhou, L.; Fu, X. Adsorption and Regeneration Study of the Mesoporous Adsorbent SBA-15 Adapted to the Capture/separation of CO2 and CH4. Chem. Eng. Sci. 2007, 62, 1101–1110. (10) Choudhary, V. R.; Mayadevi, S. Sorption Isotherms of Methane, Ethane, Ethylene, and Carbon Dioxide on ALPO-5 and SAPO-5. Langmuir 1996, 12, 980–986. (11) Li, S.; Falconer, J. L.; Noble, R. D. SAPO-34 Membranes for CO2/ CH4 Separation. J. Membr. Sci. 2004, 241, 121–135. (12) Choundhary, V. R.; Akolekar, D. B. Site Energy Distribution and Catalytic Properties of Microporous Crystalline AlPO4-5. J. Catal. 1987, 103, 115–125. (13) Bond, G. C.; Gelsthorpe, M. R.; Sing, K. S. Incorporation of Zinc in an Aluminophosphate Microporous Phase. J. Chem. Soc., Chem. Commun. 1985, 1056–1057.
1. Introduction
10.1021/ef8008635 CCC: $40.75 2009 American Chemical Society Published on Web 02/17/2009
CO2 and CH4 Adsorption on AlPO4-14
been first applied for CO2/CH4 separation by Choudhary and Mayadevi.10 However, both CO2 (3.30 Å) and CH4 (3.80 Å) can easily enter the channel of AlPO-5 (with an aperture of 7.3 Å14), which results in a low selectivity of CO2/CH4 (about 2 at 305 K).10 As a member of the AlPO family, AlPO4-14 has a channel aperture of 3.80 Å, and the separation is supposed to be accomplished by the difference in kinetic diameters of CO2 and CH4. In the present study, we used AlPO4-14 as an adsorbent for the separation of CO2 and CH4 for the first time. Adsorption isotherms of CO2 and CH4 on AlPO4-14 were measured and the adsorption behavior was explored in detail. Our results show that this molecular sieve with a neutral framework exhibited an excellent separation effect, and the selectivity of CO2/CH4 can reach 21.77 at 273 K. This indicates that the AlPO4-14 molecular sieve may be a suitable candidate for the separation of CO2/CH4. 2. Experimental Section 2.1. Adsorbent Synthesis. AlPO4-14 was prepared by hydrothermal crystallization of saturated gels containing a structuredirecting agent.15,16 The molar composition ratio for Al2O3 (pseudoboehmite), P2O5 (85% of aqueous solution H3PO4), isopropylamine, and H2O was 1:1:1:35. The mixture was heated to 473 K in a sealed Teflon-lined autoclave under autogenous pressure for 48 h. The resulting solid was then filtered, washed with deionized water, and dried at 373 K for 24 h. The template isopropylamine was removed by calcination at 823 K for 4 h using a flow of air in a computercontrolled muffle furnace. 2.2. Characterization. The X-ray powder diffraction (XRD) pattern was collected using a Bruker D8 Advance diffractometer with a Cu KR monochromatized radiation source, which was operated at 40 kV and 30 mA with a scan speed of 0.05° per 0.2 s. The morphology features were observed by means of a scanning electron microscope (SEM, QUANTA-2000). The sample was coated with Au film to improve the conductivity prior to imaging. The N2 adsorption-desorption isotherms were measured using a Micromeritics ASAP 2020 system at 77 K. The sample was outgassed at 673 K for 4 h prior to analysis. The Brunauer-EmmettTeller (BET) surface area was calculated using adsorption data in a relative pressure ranging from 0.04 to 0.20. Measurement of the bulk density for the adsorbent was referring to ASTM D6683-01.17 2.3. Adsorption Equilibrium Isotherms. Adsorption isotherms were measured by the Micromeritics ASAP 2020 outfitted with a turbo molecular drag pump. CO2 (99.99%) and CH4 (99.99%) were used as adsorbates. The samples were activated under vacuum (5 × 10-1 mmHg) at 673 K for 8 h before measurement. Equilibrium and uptake runs were performed at 273 and 300 K. The temperatures were kept constant by means of either an ice-water bath or water bath.
3. Results and Discussion 3.1. Adsorbent Characterization. Figure 1 shows the XRD pattern of as-synthesized molecular sieve. The intense diffraction lines imply a good crystallization of the molecular sieve. All of the lines can be ascribed to AlPO4-14, matching well with (14) Yang, R. T. Adsorbents: Fundamentals and Application; Wiley: New York, 2003; pp 169-170. (15) Broach, R. W.; Wilson, S. T.; Kirchner, R. M. Proceedings of the 12th International Zeolite Conference, Warrendale, PA, 1999; p 1715. (16) Broach, R. W.; Wilson, S. T.; Kirchner, R. M. Corrected Crystallographic Tables and Figure for As-synthesized AlPO4-14. Microporous Mesoporous Mater. 2003, 57, 211–214. (17) American Society of Testing Materials, Test Method for Measuring Bulk Density Values of Powders and Other Bulk Solids, ASTM D668301, 2005.
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Figure 1. XRD pattern of as-synthesized AlPO4-14.
that reported in the literature.18 Figure 2 presents SEM images of the molecular sieve. The crystal size of synthesized molecular sieve is about 5 µm. And the typical crystal habit is tabular, which is consistent with the report of Girnus et al.19 Additionally, the bulk density and N2 adsorption analysis was also performed; the results show that AlPO4-14 zeolite possesses a bulk density of 0.21 g · cm-3 and a BET specific surface area of 138 m2 · g-1. 3.2. Adsorption Performance. Figure 3 shows adsorption isotherms of pure CO2 and CH4 on AlPO4-14 at pressures up to 100 kPa and different temperatures. The isotherm of CO2 at 273 K shows a steep slope at low pressures, whereas the isotherm at high temperatures (e.g., 300 K) becomes gentle. The adsorption amounts of CO2 increase slightly with the increasing pressure at the temperatures investigated (ranged from 273 to 300 K). It is known that the isotherm curve is mainly affected adsorption. With the increase of temperature, the adsorption amount decreases (adsorption is an exothermic process). As a result, the isotherm curve of CO2 exhibits different shapes of isotherm at different temperatures. As presented in Figure 3, the adsorption amount of CO2 (p ) 100 kPa) at 273 and 300 K is 2.71 and 2.02 mmol · g-1 respectively, whereas the amount of CH4 is only 0.52 and 0.31 mmol · g-1 respectively under the same conditions. In other words, the adsorption amounts of CO2 are 5.2-6.5 times higher than those of CH4. To examine the factors influencing the adsorption difference of CO2 and CH4 on AlPO4-14, the interaction between adsorbate and adsorbent is first taken into consideration. As is known, the Langmuir model is frequently used to describe the adsorption isotherm of a single component. The mathematical form of the Langmuir model is given in eq 1, n ) nm
bp 1 + bp
(1)
where m is the amount adsorbed at pressure p and temperature T, and nm and b are the saturated adsorbed amount and Langmuir constant, respectively. The isotherm data of CO2 and CH4 can be well described by Langmuir models, and corresponding fitting parameters are listed in Table 1. The Langmuir constants (b) of CO2 are much higher than those of CH4, implying that the interaction of CO2 with adsorbent is much stronger. It is known that the quadrupole moment of CO2 is 4.30 × 10-26 esu · cm2, (18) Fyfe, C. A.; Altenschildesche, H. M.; Wong-Moon, K. C. 1D and 2D Solid State NMR Investigations of the Framework Structure of Assynthesized AlPO4-14. Solid State Nucl. Magn. Reson. 1997, 9, 97–106. (19) Girnus, I.; Lo¨ffler, E.; Lohse, U. Small Pore AlPO4/SAPO Structures Formed in Dependence on the Silicon Content of the Synthesis Gel. Collect. Czech. Chem. Commun. 1991, 57, 946–958.
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Zhao et al.
Figure 2. SEM images of synthesized AlPO4-14.
Figure 3. Pure component adsorption isotherms of CO2 (9) and CH4 (b) on AlPO4-14 at (a) 273, (b) 283, (c) 290, and (d) 300 K. Lines are fitting curves of the Langmuir isotherm model. Table 1. Langmuir Parameters of AlPO4-14 Molecular Sieve adsorbate
T (K)
nm (mol · kg-1)
b (kPa-1)
r2
CO2
273 300 273 300
3.254 3.737 1.595 1.166
0.0529 0.0117 0.0048 0.0036
0.999 0.999 0.999 0.999
CH4
whereas that of CH4 is 0.20 The higher quadrupole moment of CO2 thus leads to a stronger interaction between CO2 and adsorbent, which is beneficial to the adsorption. The steric effect is considered to be another factor affecting the adsorption behavior. The kinetic diameters of CO2 and CH4 are 3.30 and 3.80 Å respectively, whereas the channel aperture of AlPO4-14 is 3.80 Å. Apparently, CO2 can enter the channel of adsorbent easily, whereas the entrance of CH4 to the channel should be quite difficult, if possible. Similar application of AlPO4-14 to separate propylene (3.6 Å) and propane (3.80 Å) by steric effect is reported by Yang.14 Therefore, the stronger (20) Golden, T. C.; Sircar, S. Gas Adsorption on Silicalite. J. Colloid Interface Sci. 1994, 162, 182–188.
interaction of adsorbate-adsorbent and the smaller molecular size are responsible for the higher adsorption amount of CO2 than CH4 on AlPO4-14. 3.3. Isosteric Heats of Adsorption and Henry’s Law Constant. The isosteric heats of adsorption and their variations with surface coverage can provide evidence about the interactions of adsorbate with adsorbent. Isosteric heats of adsorption can be calculated from the pure component isotherms at different temperatures using the Clausius-Clapeyron equation as shown in eq 2,21 qst RT 2
)-
[ ∂ln∂Tp ]
n
(2)
where qst is the isosteric heat of adsorption. The isosteric heat is fitted with the virial type as displayed in eq 3.22 qst ) R(k1 + b1n + c1n2 + d1n3)
(3)
(21) Ruthven, D. M. Principles of Adsorption and Adsorption Processes; John Wiley and Sons: New York, 1984; pp 34-44.
CO2 and CH4 Adsorption on AlPO4-14
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Figure 4. Isosteric heats of adsorption for CO2 (9) and CH4 (b) on AlPO4-14. (a) Low coverage, (b) Full coverage. Lines show predicted isotherm by eq 3. Table 2. Henry’s Law Constants for Different Adsorbents KH (mol · kg-1 · kPa-1) adsorbent
T (K)
CO2
CH4
KHCO2/KHCH4
ref
AlPO4-14 DD3R BPLa DD3R SBA-15 AlPO4-14 silicalite
273 273 273 298 298 300 304
0.1676 0.1108 0.0741 0.0421 0.0165 0.0412 0.0377
0.0077 0.0117 0.0204 0.0057 0.0029 0.0066 0.0079
21.77 9.47 3.63 7.39 5.70 6.24 4.79
this work 3 24 3 8 this work 20
a
An activated carbon.
Figure 4 shows the results of the isosteric heats for CO2 and CH4 on AlPO4-14. The isosteric heats estimated for CO2 and CH4 are 35.2 and 12.8 kJ · mol-1 respectively at adsorption amount of zero. Besides, the isosteric heats of CO2 are obviously higher than CH4 at different adsorption amounts, indicating that the adsorbate-adsorbent interaction between CO2 and AlPO414 is much stronger. At low surface coverage, the isosteric heats for CH4 exhibit an increasing trend, which can be attributed to the adsorbate-adsorbate interaction;10 whereas those for CO2 show plateau behavior, suggesting a homogeneous adsorption model. At full coverage, the heats of adsorption decrease slightly with increasing adsorption amount of CO2. Strong adsorption sites are occupied first at low coverage due to the high affinity of CO2. Along with the increase of loading, the weaker adsorption sites will be occupied, and the isosteric heats thus decline. The Henry’s law constant has been used at a low surface coverage, which determines the interaction between adsorbate molecules and adsorbent surface at infinite dilution. It can be evaluated by the extrapolation of isothermal ln(p/n) versus n data to zero adsorbed concentration.23 As Table 2 presents, the Henry’s law constants (KH) of CO2 on AlPO4-14 are 0.1676 and 0.0412 mol · kg-1 · kPa-1 at 273 and 300 K respectively, which is much higher than those of CH4 (0.0077 and 0.0066 mol · kg-1 · kPa-1). Hence, the isosteric heats of adsorption and Henry’s law constant are in good agreement with the results of Langmuir constant, indicative of a stronger adsorbate-adsorbent interaction of CO2 with AlPO4-14. It is worth noting that the separation coefficient of CO2 and CH4 (KHCO2/KHCH4) can reach 21.77 on AlPO4-14 at 273 K (Table 2), which is much higher than DD3R (9.47)3 and (22) He, Y.; Seaton, N. A. Heats of Adsorption and Adsorption Heterogeneity for Methane, Ethane, and Carbon Dioxide in MCM-41. Langmuir 2006, 22, 1150–1155. (23) Ruthven, D. M. Principles of Adsorption and Adsorption Processes; John Wiley and Sons: New York, 1984; pp 62-64.
Figure 5. XRD patterns of AlPO4-14 molecular sieve; (a) calcined sample, (b) thermal treatment at 823 K, (c) treated with nitric acid (pH 4.5).
BPL (3.63)24 at the same adsorption temperature. To the best of our knowledge, this is the first investigation of the adsorption behavior of CO2 and CH4 on AlPO4-14, a neutral molecular sieve with no exchangeable cations. We believe that the appropriate pore apertures and surface characteristics of AlPO4-14 are responsible for such a high selectivity of CO2/CH4. Some literature reported the adsorption of CO2 and CH4 on the general adsorbent (e.g., zeolite 13X and A type zeolite), whereas only the selectivity of CO2/CH4 from saturation adsorption amounts can be obtained. To compare these general adsorbents with AlPO4-14 molecular sieve, the selectivity of CO2/CH4 on AlPO4-14 was calculated from the saturation adsorption amounts rather than the Henry’s law constants. The results show that the selectivity of CO2/CH4 on AlPO4-14 is 6.45 at 300 K, which is higher than that on 13X zeolite at 299 K (5.75)25 and on A type zeolite at 298 K (4.44).26 Our results demonstrate that the neutral molecular sieve AlPO4-14 has potential applications in the separation of CO2 and CH4. The Henry’s law constants of two adsorbates on AlPO4-14 and some other adsorbents at a relatively high temperature are also listed in Table 2. The selectivity of CO2/ CH4 decreases with increasing temperature, implying that the adsorption behavior is dependent on temperature. To evaluate the stability of AlPO4-14, the calcined molecular sieve was thermally treated at 823 K for 4 h. The (24) Himeno, S.; Komatsu, T.; Fujita, S. High-Pressure Adsorption Equilibria of Methane and Carbon Dioxide on Several Activated Carbons. J. Chem. Eng. Data 2005, 50, 369–376. (25) Cavenati, S.; Grande, C. A.; Rodrigues, A. Removal of Carbon Dioxide from Natural Gas by Vacuum Pressure Swing Adsorption. Energy Fuels 2006, 20, 2648–2659. (26) Hernandez-Maldonado, A. J.; Yang, R. T. Partially Calcined Gismondine Type Silicoaluminophosphate SAPO-43: Isopropylamine Elimination and Separation of Carbon Dioxide, Hydrogen Sulfide, and Water. Langmuir 2003, 19, 2193–2200.
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XRD pattern of molecular sieve after thermal treatment is in good agreement with the one before treatment (Figure 5), implying a good thermal stability of AlPO4-14. It is known that the moisture adsorptive property and acid resistance are important for the separation of natural gas. The adsorption amounts of moisture for 3A, 4A, 5A, and 13X, the molecular sieves potential for natural gas separation, are in the range of 0.21-0.36 cm3 · g-1.27 The moisture adsorptive property for AlPO4-14 was reported to be 0.27 cm3 · g-1,28 indicating an appropriate moisture adsorption ability. According to the contents of moisture and CO2 in natural gas, the pH value was calculated to be about 5.6. To assess the acid resistance of AlPO4-14, an aqueous nitric acid with a pH value of 4.5 was prepared. The molecular sieve before and after acid treatment kept similar weight and XRD patterns (Figure 5), indicative of a good acid resistance. On the basis of these (27) Inglezakis, V. J.; Poulopoulos, S. G. Adsorption, Ion Exchange and Catalysis: Design of Operations and EnVironmental Applications; Elsevier Science: New York, 2006; p 251. (28) Zibrowius, B.; Lohse, U. Richter-Mendau, Characterization of AlPO4-14 by Magic-Angle-Spinning Nuclear Magnetic Resonance Spectroscopy and Thermoanalytical and Adsorption Measurements. J. Chem. Soc., Faraday Trans. 1991, 87, 1433–1437.
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results, it is conclusive that AlPO4-14 molecular sieve may be a good candidate for the separation of natural gas. 4. Conclusions Adsorption isotherms of CO2 and CH4 on AlPO4-14 molecular sieve with a neutral framework were measured in wide ranges of pressure and temperature. The results show that the isotherms fit the Langmuir model well. Both isosteric heats of adsorption and Henry’s law constants of CO2 are higher than those of CH4 at different temperatures. The selectivity of CO2/ CH4 can reach 21.77 at 273 K. The adsorbate-adsorbent interaction and the steric effect should be responsible for the preferential adsorption of CO2 on AlPO4-14. Our study indicates that AlPO4-14 may be a good candidate for the purification of natural gas. Acknowledgment. The authors are grateful for financial supports of the Doctoral Program of Higher Education (Project 200402910050), the Program for Changjiang Scholars and Innovative Research Team in University (No. IRT0732), and the Major Basic Research Project of Natural Science Foundation of Jiangsu Province Colleges (No. 08KJA530001). EF8008635
