Article pubs.acs.org/IECR
Adsorption of CO2, CH4, and N2 on 8‑, 10‑, and 12-Membered Ring Hydrophobic Microporous High-Silica Zeolites: DDR, Silicalite-1, and Beta Jiangfeng Yang, Junmin Li, Wei Wang, Libo Li, and Jinping Li* Research Institute of Special Chemicals, Taiyuan University of Technology, Taiyuan 030024, Shanxi, P. R. China S Supporting Information *
ABSTRACT: Three hydrophobic microporous high-silica zeolites, DDR (with an 8-membered ring), silicalite-1 (a 10membered ring), and beta (a 12-membered ring) were synthesized. The Si/Al ratios were 230, 1350, and 35, respectively. The samples were characterized by X-ray diffraction, scanning electron microscopy, thermal gravimetric analysis, water vapor adsorption, and volumetric nitrogen adsorption. They were tested for their CO2, CH4, and N2 adsorption properties at pressures of up to 10 bar at 288−313 K after activation, and the results were correlated with the Langmuir model. The heat of adsorption was calculated using the Clausius−Clapeyron equation based on the adsorption isotherms. These data were used to estimate the separation selectivities for CO2/CH4 and CH4/N2 binary mixtures at 298 K, using the ideal adsorbed solution theory (IAST) model. Experimental results showed that DDR and beta have good selectivities for CO2/CH4, because they have narrow pores (DDR) or more balance metal ions (relatively low Si/Al ratios beta) effect separately. The synthesized silicalite-1 has the lowest SCO2/CH4 but has the most suitable orifices for methane adsorption and the highest SCH4/N2. In addition, the breakthrough data for CH4/N2 mixtures further indicates that silicalite-1 is more suitable for the CH4 enrichment than the commercially used sorbents zeolite-5A and 13X. From the reproducibility of CH4 and N2 adsorption isotherms on silicalite-1, we can infer that which has the potential to be a commercial sorbent by the stable adsorption properties.
1. INTRODUCTION Some of the technological problems in using clean energy sources are the separation and recovery/reuse of vapors and gases. Several very important examples that are currently of interest are the purification of natural gas and biogas and the enrichment of low concentrations of coal bed methane.1,2 Many processes, such as the removal of carbon dioxide, water, and nitrogen from gas mixtures, include an adsorption step as part of the pressure swing adsorption (PSA) process, in which microporous adsorbents, such as activated carbon and zeolites, are used.3−5 It is normally necessary to first completely remove water from the mixed gases, because this can affect the adsorption efficiency of the adsorbents, especially when the adsorbents are hydrophilic materials.6 Therefore, if we can make use of a hydrophobic adsorbent, this will avoid destructive absorbance of water molecules, so the use of an adsorbent to remove trace amounts of water molecules is not necessary. Pure-silica or high-silica zeolites such as DDR, silicalite-1, and beta are important hydrophobic molecular sieves because they can be used for the separation of organics from water.7−9 Of these, the silica zeolite DDR is especially attractive. The 8membered windows are 0.36 × 0.44 nm in size, similar to CH4 (0.38 nm) but larger than CO2 (0.33 nm) and N2 (0.36 nm).6,10 Silicalite-1, an all-silica version of the MFI type zeolite ZSM-5, consists of a 2-dimensional network of interconnecting channels formed by two different sizes of 10-membered tetrahedral rings (Figure 1).11,12 The channel dimensions are approximately A, 0.51 × 0.56 nm and B, 0.53 × 0.56 nm, so small molecules (CO2, CH4, and N2) can penetrate these pores, where they become adsorbed on to the internal surfaces. The © 2013 American Chemical Society
zeolite beta has the largest pores of these three molecular sieves and has two sizes of two 12-membered rings, A 0.66 × 0.77 nm and B 0.56 × 0.56 nm.13 Many previous studies have investigated gas adsorption behaviors and selected separations using the three kinds of structure,14−18 but there have been few studies using the pure or high-silica structures because of the traditional idea that these have lower adsorption potential and fewer adsorption sites on the surface, so the gas molecules are not easily adsorbed on the surface or the pores. Pure or high-silica zeolites have a ‘smooth surface’ and few adsorption sites, so adsorption of gas molecules depends almost exclusively on their intrinsic gas diffusion adsorption potential. Some researchers have studied the adsorption and diffusion of light gases (CO2, CH4, O2, N2 or Ar) using a single pure or high-silica zeolite.19−21 Our interest, however, has been to focus on CO2, CH4, and N2 diffusion and selective adsorption using three different pore size hydrophobic high-silica zeolites. In the present study, we have synthesized the 8-, 10-, and 12membered high-silica zeolites DDR, silicalite-1, and beta and studied the high pressure adsorption behavior with CO2, CH4, and N2. The equivalent adsorption selectivities were calculated by single gas adsorption isotherms, and the effect of adsorption heat on the materials and adsorption properties was also studied. We also carried out a novel mixed gas experiment to elucidate the adsorption selectivity of these materials and Received: Revised: Accepted: Published: 17856
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the autoclave; finally, the autoclave was sealed when the reaction stopped, and it was put into an oven at 453 K for 12 days for crystallization. The as-synthesized samples were washed with distilled water and dried in the atmosphere. Similar with the DDR, colorless and transparent crystals with the size of 100−200 μm were produced (see Figure S1 in the Supporting Information). 2.1.3. Synthesis of Beta. First, the reactive gel was prepared by mixing 1.67 mL of silica sol (1 mmol SiO2) and 6 mL of tetraethyl ammonium hydroxide (TEAOH, 1 mmol, Aladdin, 25% aqueous solution) with stirring in a Teflon-lined autoclave for 5−10 min; 0.2 mL of hydrogen fluoride (1 mmol, SCRC, 40% aqueous solution) was trickled into the autoclave, and stirring was maintained for 10 min until the reactive gel was transparent and uniform. The autoclave was sealed and put into an oven at 413 K for 7 days for crystallization. The assynthesized samples were washed with distilled water and dried in the atmosphere. White nanosize powder was produced which was different from the former two samples. 2.2. Characterization. The crystallinity and phase purity of the molecular sieves were measured by powder X-ray diffraction (XRD) with a Rigaku Mini Flex II X-ray diffractometer with Cu Kα radiation operated at 30 kV and 15 mA. The scanning range was from 5 to 40° (2 theta) at 1°/min. Morphological data were acquired through scanning electron microscopy (SEM) using a JEOL JSM-6700F scanning electron microscope operated at 15.0 kV. The samples were coated with gold in order to increase their conductivity before scanning. The Si/Al ratio of the zeolites was determined by elemental analysis with atomic absorption spectrophotometer-S2 (AAS, thermo, USA). Elemental analysis (CHN, the content of carbon element (C), hydrogen element (H), and nitrogen element (N)) was carried out on an Elementar vario EL analyzer. Thermal gravimetric analysis (TGA) was carried out in air at a heating rate of 10 K/min using a Netzsch STA409C balance. N2 adsorption was measured using a micromeritics ASAP 2020 gas-adsorption apparatus at liquid-nitrogen (77 K) temperature (the material was activated and outgassed between each experiment under secondary vacuum at 473 K with a temperature ramp of 5 K/min, and then holding 6 h). Water vapor adsorption was measured using a Belsorp max gasadsorption apparatus at 298 K (the same processing method as the N2 adsorption). 2.3. High Pressure Gas Adsorption Measurements. The purity of the carbon dioxide was 99.99%, methane was 99.95%, and that of the nitrogen was 99.99%. The adsorption isotherms under high pressure were measured on an Intelligent Gravimetric Analyzer (IGA 001, Hiden, UK). Prior to measuring the isotherm, a 50 mg sample was predried under reduced pressure and then outgassed overnight at 673 K under a high vacuum until no further weight losses were observed. Each adsorption/desorption step was allowed to approach equilibrium over a period of 30−40 min, and all the isotherms for each gas were measured on a single sample. 2.4. Novel Mixed Gases Tests. The samples were extruded, ground, sieved into 40−80 mesh particles (diameter of about 0.5−0.8 mm), and then oven-dried for 24 h at 393 K. Samples were then introduced to the adsorption bed (φ9 mm × 150 mm). A carrier gas (He ≥ 99.999%) was used to purge the adsorption bed for more than 1 h to ensure that the adsorption bed was saturated with He. The He flow was then stopped. The raw mixed gases (CH4:N2 = 50%:50%) at a flow rate of 5 cm3/min (STP) was passed through the adsorption
Figure 1. The main channels of zeolite A) DDR, B) silicalite-1, and C) beta.10−13
compared our results with the commercially used sorbents zeolite-5A and 13X.
2. EXPERIMENTAL SECTION 2.1. Materials and Methods. The high-silica zeolites of DDR, silicalite-1, and beta samples were synthesized in our laboratory following synthesis procedures reported previously.22−24 A brief description is given below. 2.1.1. Synthesis of DDR. One mL of silica sol (0.6 mmol SiO2), 0.45 g of 1-adamantanamine (0.3 mmol, Aldrich, 97%), and 0.18 g of potassium fluoride (0.3 mmol, SCRC, 99%) were mixed in a container with 9 mL of distilled water and vigorously stirred for 2 h until homogeneous. Then, the mixture was transferred to a 23 mL Teflon-lined autoclave which was sealed and heated in an oven at 453 K for 9 days in order to crystallize the sample. After crystallization, the synthesized samples were washed with distilled water and dried in the atmosphere. Finally, colorless and transparent crystals with the size of 100− 200 μm were produced (see Figure S1 in the Supporting Information). 2.1.2. Synthesis of Silicalite-1. The reactive gel was prepared by mixing 0.6 g of fumed silica (10 mmol SiO2), 0.83 mL of propylamine (10 mmol, SCRC, 98.5%), 8.66 mL of triethylamine (60 mmol, Tianjin chemical, China, 99%), and 0.66 g of tetramethylammonium (2.5 mmol, SCRC, 99%) bromide; this mixture was stirred in a Teflon-lined autoclave until homogeneous, and then 0.5 mL of hydrogen fluoride (2.5 mmol, SCRC, 40% aqueous solution) was carefully trickled into 17857
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bed. The recovery gas was passed to an analyzer port and analyzed by gas chromatography (Shimadzu, GC-2014C, Japan). The mass of the samples used were as follows: 5A, 5.30 g; 13X, 5.21 g; silicalite-1, 4.27 g. The experimental temperature was about 293 K, and the pressure was 100 kPa. The full experimental setup of these novel gas experiments is shown in Figure S2.
3. RESULTS AND DISCUSSION 3.1. Synthesized and Activated Zeolites. Figure 2 shows the experimental and simulated XRD patterns of the activated zeolites DDR, silicalite-1, and beta. It was found that the peak positions and relative diffraction intensities of DDR and silicalite-1 were similar to the standard data but the beta with little difference from the simulated pattern. By comparing with previously published XRD data of beta,25,26 there was no difference, so this proves that the specified zeolites had been synthesized. The Si/Al ratios for the three zeolites were 230 (DDR), 1350 (silicalite-1), and 35 (beta) with errors ≤1%. The experimental results show that all of the samples were highsilica materials, with silicalite-1 the highest. Removing the organic template of the molecular sieves by high temperature is also known as activation. This is the key step before adsorption testing, because otherwise it is difficult to determine the role of the molecular sieve itself versus that of the organic template in the pores. The all-silica or high-silica zeolites have high thermal stability, generally higher than 1300 K. TGA curves show the weight loss of the zeolites from 298 to 1023 K (Figure 3), and it is assumed that this represents the template removal process. The temperature range for weight loss for silicalite-1 was narrow, being from 673 to 723 K, so it suggests most of its template material is quickly removed. The ranges for DDR and beta were much wider, occurring over 373−1023 K, so the removal of all of their templates needs higher temperatures and more time. However, we were worried that the surface of the crystal would be damaged during template removal at too high a temperature. In summary, we chose 873 K as the activation temperature, based on the TGA curves of beta and silicalite-1, which were more stable than samples activated at lower temperature. It was found that the structure of the three zeolites did not change any further after activation for 10 h. The XRD patterns of the activated zeolites are shown in Figure 2. The CHN analyses of the two versions (synthesized and activated) of DDR, silicalite-1, and beta are shown in Table 1. The percentage weight loss can be calculated from the TGA curves, and for DDR it was 28% when the temperature up to 1000 K, which is consistent with the CHN changes before and after activation (873 K and10 h). This proves that all the weight loss in DDR was due to the template removal process, and the percentage of template removal was 95.4% after activation (based on the loss of elemental N), which means lower temperature activated need longer time. The same situation applied to silicalite-1. The weight loss was 11%, and the percentage of template removal was 90%. Beta had the highest percentage (96.2%) of template removal of the three materials, but the weight loss rate was not consistent with the CHN data changes (weight loss was 25% and CHN changes 17%). First, we considered that parts of the weight loss were due to changes in oxygen content after activation because O exists in TEAOH; the loss was 2.4% based on the molecular weight. Another portion of the weight loss was presumed to be due to H2O; this was about 5% below 373 K, as can be seen from the TGA curve.
Figure 2. XRD patterns of a) DDR, b) silicalite-1, and c) beta with their synthesized and activated.
Therefore, we speculate that the beta sample that was synthesized was no more hydrophobic than DDR and silicalite-1. In addition, a finite amount of template in the zeolite was not removed completely at temperatures below 873 17858
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hydrophobic. This result means that trace amounts of template do not affect the results, confirming previous assumptions. From the liquid N2 adsorption (Figure 5), the BET, Langmuir surfaces, pore width, and volumes of the 8-, 10-,
Figure 3. TG curves of samples DDR, silicalite-1, and beta.
Table 1. Elemental Analysis (CHN) of Synthesized and Activated High-Silica Zeolites name
C (%)
H (%)
N (%)
DDR-synthesized DDR-activated silicalite-1-synthesized silicalite-1-activated beta-synthesized beta-activated
22.52 0.32 8.85 1.25 15.20 2.60
3.53 0.30 1.84 0.35 3.90 1.45
2.79 0.13 2.31 0.23 2.10 0.08
Figure 5. N2 adsorption isotherms of DDR, silicalite-1, and beta (77 K).
and 12-membered ring zeolites were calculated and are shown in Table 2. Beta had the highest surface and pore volume of the Table 2. Surface Area and Pore Width of DDR, Silicalite-1, and Beta
K. We assumed that this would not affect the results of this research, and proof is provided for this assertion. Figure 4 shows the adsorption of water vapor at 298 K by the high-silica zeolites. Over the pressure range from 0 to 3.16 kPa
zeolites DDR silica lite-1 beta
no. of atoms in ring
BET area (m2/g)
Langmuir surface area (m2/g)
pore width (nm)
pore volume (cm3/g)
8 10
340 439
433 628
0.38−0.43 0.46−0.54
0.12 0.19
12
568
749
0.47−0.62
0.31
three samples, followed by silicalite-1 and DDR. This means the surface areas and pore volumes are consistent with the pore ring number sequence. From the SEM, it can be found another reason not the key factor but which does exist, the synthesized beta in nano sizes, DDR and silicalite-1 with micrometer size (Figure 3S), we know nanoparticles have larger surface than bigger size materials because they have richer outer surfaces. However, the proportion of micropores follow the sequence silicalite-1 > DDR > beta. In Figure 6, we also found that the pore widths were much closer to the data found by crystal analysis (from Figure 1). The micropores in DDR are distributed over the range 0.38−0.43 nm (calculated from the CO2 adsorption at 273 K (Figure S4)). The range for silicalite1 is 0.46−0.54 nm and beta 0.47−0.62 nm. The calculation method is described in the Supporting Information. It was also found that the zeolite pore sizes were slightly affected by small amounts of template. 3.2. Adsorption Isotherms at Elevated Pressures. The adsorption and desorption equilibrium isotherms of CO2, CH4, and N2 on all three adsorbents at 288, 298, and 313 K are plotted in Figure 7. It can be seen that the adsorption and desorption curves coincide, indicating that the adsorption is reproducible, which is a very important factor in PSA technology.28 From the point of view of kinetics, easy and quick adsorption and desorption is another key factor. Figures
Figure 4. Water vapor adsorption isotherm of samples DDR, silicalite1, and beta at 298 K (saturated vapor pressure P0 = 3.167 KPa).
(saturated vapor pressure of H2O at 298 K), the respective amounts of water adsorbed were 16.99, 9.02, and 7.24 cm3/g for beta, DDR, and silicalite-1, respectively. We conclude that all of these materials are hydrophobic compared with the highsilica zeolites described in the literature.27 The order of water vapor adsorption was beta > DDR > silicalite-1, so the experimental results verify the inference above TG and CHN analysis. We also found that the order was opposite to the order of the Si/Al ratio, so the higher ratio of Si/Al is more strongly 17859
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S6−S8 show the CO2, CH4, and N2 adsorption and desorption kinetics on the three high-silica zeolites. It was found that the weight increase and loss changed rapidly with change of pressure, and not much time is needed to reach the equilibrium, especially in a lager pore size zeolite. This is further discussed in the Supporting Information. To design and operate a gas adsorption process, the isosteric heat of adsorption (Qst) is always taken into account, in order to estimate the temperature change in the adsorption process. Usually, in a gas adsorption process, Qst is the adsorption isosteric enthalpy (−ΔH), which can be calculated as a function of loading using adsorption data at different temperatures via the Clausius−Clapeyron equation:29 ln
P2 ΔH ⎛ 1 1⎞ = ⎜ − ⎟ P1 R ⎝ T1 T2 ⎠
In addition, the CO2 adsorption capacities were greater than for CH4 and N2 under the same conditions. This order is consistent with the gas polarization data30 and the heats of adsorption. In Figure 8, the heats of adsorption of the zeolites
Figure 6. Microporous distribution in DDR, silicalite-1, and beta. (Horvath−Kawazoe differential pore volume plot, slit pore geometry (original H−K).)
Figure 7. CO2 (blue, ▲), CH4 (red, ■). and N2 (black, ●) adsorption and desorption isotherms of DDR, silicalite-1. and beta under 10 bar at 288− 313 K (solid: adsorption, hollow: desorption, solid line: Langmuir fitting curve). 17860
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high-silica construction with an energetically homogeneous surface. The gas adsorption capacities of the three high-silica zeolites at low and high pressure are shown in Figure 7. It can be seen that all the materials have a higher adsorption at low temperatures, decreasing with increasing temperature. CO2 adsorption for the beta sample showed the greatest adsorption, 85 cm3/g at 10 bar, as a result of its larger surface area and pore size, followed by silicalite-1 and DDR. This shows that the absorption of CO2 is highly correlated with the material surface area and is also consistent with the heat of adsorption of CO2 on the high-silica zeolites. However, the CH4 adsorption capacity showed a different order for the three zeolites. Silicalite-1 had the highest capacity, both at low and high pressure, followed by beta and DDR. This means that the CH4 adsorption is not just related to the surface areas but also the pore size and the proportion of micropores in the materials, silicalite-1 has the most suitable pore size and the highest proportion of micropores.31 From the heat of adsorption data, we can also see that DDR and silicalite-1 show better results than beta, although DDR cannot adsorb more CH4 than silicalite-1, even to high pressure, because of its smaller pore volume. There are differences in the order of N2 adsorption at low and high pressure for the three zeolites. First, materials with a large surface area show high nitrogen adsorption at pressures below 1 bar (the beta sample adsorbs 6.8 cm3/g). However, silicalite-1 shows greater adsorption when the pressure is increased to 10 bar, and the lower the temperature, the more obvious was this effect. We can see that the heat of adsorption of N2 on the beta sample is greater than on silicalite-1 at low pressure but shows a higher adsorption capacity (more than 20 cm3/g). Table 3 shows the CO2, CH4, and N2 adsorption Table 3. Adsorb Capacities of CO2, CH4, and N2 in Silicalite1 at Different Temperatures cm3(STP)/g Si/Al ratio
CO2
CH4
N2
370 4240 1230 thousands 1350 1350 1350
35.8 44.8 34.1 42.9 36.5 27.3
13.4 15.2 15.7 12.5 17.7 14.6 10.9
4.5 4.7 4.5 4.4 5.6 4.7 3.6
pressure 1 1 1 1 1 1 1
bar bar bar bar bar bar bar
temp 298 298 296 313 288 298 313
K K K K K K K
ref 21 21 32 33 this work this work this work
capacity in silicalite-1 compared with other reported data,21,32,33 and we found they were very close; it verifies the accuracy of the adsorption data of the silicalite-1 sample in this research. 3.3. Adsorption Selectivity. Adsorbents must have sufficient adsorption capacity or good adsorption separation performance, but high selectivity plays a decisive role. Generally, the most accurate method of testing the separation of gas mixtures is to use a binary adsorption equilibrium system, giving the separation selectivities for CO2/CH4 and CH4/N2 binary mixtures at different pressures according to the ideal adsorbed solution theory (IAST) model.34 Analytical expressions of the adsorption isotherm are needed to apply the method, and in the present work a virial equation was used. The constants used in the virial equation are given in the Supporting Information. The Gibbs free energy of the desorption phase diagrams for a given pressure, that is, the
Figure 8. Adsorption heats of CO2 (blue, ▲), CH4 (red, ■), and N2 (black, ●) on a) DDR, b) silicalite-1, and c) beta.
show best results for CO2, followed by CH4 and N2. These data are consistent with the polarizabilities (CO2, 26.5 × 10−25 cm3; CH4, 26.0 × 10−25 cm3; N2, 17.6 × 10−25 cm3). We can also see that the heat of adsorption curves of CO2, CH4, and N2 on the higher Si/Al silicalite-1 and DDR were more stable than on beta, so we conclude that a good molecular sieve should have a 17861
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evolution of the composition of the gas phase as a function of the composition of the adsorbed phase is used to determine the standard-state loadings (n0i ) for each of the two pure gases (1 and 2) at a given value of G. The two-equation system is then solved simultaneously for the phase equilibrium to determine the component molar fractions in the gas (xg) and adsorbed phases (xa), as follows: pxig = pi (ni0)xia
The selectivity values were calculated using the following relation: S1,2 =
(x1a /x1g) (x 2a /x 2g)
The IAST theory assumes that the mixing of the adsorbed phases of the two components is ideal.35 The separation selectivities (S1,2) were calculated and are shown in Figure 9. We can see that beta has the highest value of SCO2/CH4 under 1 bar, the lower pressure the higher selectivity. DDR and silicalite-1 give poor results, because some Na+ is present in the beta sample, which improves the selectivity of CO2.36 Due to the small pore sieving effect, DDR produced higher values than silicalite-1.20 Figure 9b shows that silicalite-1 was the most suitable of the three adsorbents for the separation of CH4/N2, because the SCH4,N2 value was the highest, both at low and high pressure, followed by DDR and beta. The experimental adsorption isotherms of the gas mixture were also predicted by IAST (Figure S5); the adsorption capacities of CO2 and CH4 in the mixed gases CO2/CH4 = 50%/50% showed that beta has a higher CO2 adsorption volume 30 cm3/g under 1 bar, the silicalite-1 and DDR are 21 and 18 cm3/g separately, and the CH4 adsorption volume in three zeolites was 4.3, 6.0, and 3.5 cm3/g. So it inferred that beta is suitable for CO2/CH4 mixtures separation. From the CH4 and N2 adsorption capacities of CH4/N2 = 50%/50% mixture, silicalite-1 has more CH4 adsorption than DDR and beta. Based on these calculations, we conclude that suitable pore sizes and high proportion of micropores were important factors in attaining the best CH4 adsorption storage and selectivity relative to N2 in high-silica zeolites. The pore sizes in DDR were too small to be suitable for CH4 adsorption because the kinetic diameter of CH4 is larger than N2. The beta zeolite has lower Si/Al, so we deduce that more balance ions reduce the selectivity for CH4/N2. To further analyze this behavior in high silica zeolites, we compared our data with two kinds of commercial adsorbents, low-silica zeolite-5A (Si/Al = 1, Aladdin, China) and zeolite13X (Si/Al = 1.2, Aladdin, China). The CO2, CH4, and N2 adsorption isotherms were also measured for these zeolites, and the results are shown in Figure S7 (Supporting Information). Figure 9c shows values of SCH4/N2 for 5A and 13X (the data of adsorption in Figure 9S), calculated by the same method. We conclude that a molecular sieve with a high-silica construction has higher separation selectivity for CH4/N2 than a low-silica sieve. 3.4. Novel Mixed Gases Test. From measurements of the adsorption selectivities for CO2/CH4 and CH4/N2, we found that the separation of CO2/CH4 was not one of the advantages of high-silica zeolites37 but the separation of CH4/N2. Low concentration coal bed methane from Chinese coal mines consists of 30−40% N2 and 50−60% CH4, so CH4 enrichment or complete separation of CH4 from mixtures of CH4 and N2 is
Figure 9. Adsorption selectivity of mixtures CO2/CH4 and CH4/N2 on DDR, silicalite-1, beta, zeolite-5A, and 13X.
very important.38 Also, from the results of the novel separation experiments, we suggest using silicalite-1, which has a high SCH4/N2 compared with the commercial low-silica adsorbents zeolite-5A and 13X. Figure 10 shows there is only modest separation of CH4/N2 using 5A and 13X; the gas penetration time is very short, although equilibrium is reached quickly. Silicalite-1, which shows superior separation of CH4/N2, has a penetration time for N2 of 250 s, but the retention time is very long, about 250 s, and then CH4 is released. As a result, highsilica silicalite-1 not only has a higher equilibrium separation 17862
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conclude that regeneration of the adsorbent was very effective; silicalite-1 can be the CH4/N2 separation adsorbent further.
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CONCLUSIONS Three high-silica microporous zeolites, DDR, silicalite-1, and beta, which have 8-, 10-, or 12-membered rings in their main channels, respectively, were synthesized and characterized using XRD, TGA, and CHN analysis. Observation of low water vapor adsorption volumes proved that the zeolites we synthesized are all hydrophobic materials. The samples were compared for their CO2, CH4, and N2 adsorption. The separation selectivity for the mixed gases CO2/CH4 and CH4/N2 showed that their separation performances were comparable, depending on the single gas adsorption characteristics. Experimental results showed that the zeolites DDR and beta have higher separation selectivities for CO2/CH4 than siliclaite-1, because of the presence of narrow pores and larger concentrations of balance metal ions. Silicalite-1 has higher CH4 adsorption capacity and separation selectivity for CH4/N2 than the other two high-silica zeolites. The value of SCH4/N2 is even higher than for commercial low-silica zeolites. In addition, the results of a novel experiment using CH4/N2 further indicate that silicalite-1 is more suitable for the CH4 enrichment in low concentration coal bed methane or natural gas than the commercially available sorbents zeolite-5A and 13X.
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ASSOCIATED CONTENT
S Supporting Information *
Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Phone: 86-351 6010908. Fax: 86-351 6010908. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We gratefully acknowledge financial support from the National Natural Science Foundation of China (No. 21136007, 51302184).
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REFERENCES
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Figure 10. Breakthrough data of zeolite-5A (a), 13X (b), and silicalite1 (c) at 298 K and 1 bar for an equimolar CH4/N2 mixture (C, the content of outlet gas; C0, the content of raw gas).
factor for CH4/N2 compared with low-silica sieves but also performs well. Figure S10 also shows a second test of gas adsorption in silicalite-1. It can be seen that the adsorption volume is nearly the same as in the first test, so we can 17863
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