Adsorption of CO2, CH4, C3H8, and H2O in SSZ-13, SAPO-34, and T

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Adsorption of CO2, CH4, C3H8, and H2O in SSZ-13, SAPO-34, and T-Type Zeolites Yiwei Luo, Hans Heinrich Funke, John L. Falconer, and Richard D. Noble Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b02034 • Publication Date (Web): 23 Aug 2016 Downloaded from http://pubs.acs.org on September 1, 2016

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Industrial & Engineering Chemistry Research

Adsorption of CO2, CH4, C3H8 and H2O in SSZ-13, SAPO-34, and T-Type Zeolites Yiwei Luo1,2, Hans H. Funke*1, John L. Falconer1, Richard D. Noble1 1

Department of Chemical and Biological Engineering, University of Colorado Boulder, Boulder, CO, USA 80309-0596 2

Department of Chemical Technology, Dalian University of Technology, Dalian, Liaoning Province, P. R. China 116023

Keywords: zeolites, SAPO-34, SSZ-13, T-type, adsorption, zeolites *To whom correspondence should be addressed. Abstract Single-component adsorption isotherms for CO2, CH4, C3H8, and H2O on SAPO-34, SSZ-13, and zeolite T were measured, and isosteric heats of adsorption were estimated. Long equilibration times were required for C3H8 adsorption in SAPO-34 and SSZ-13 because C3H8 diffuses slowly in these zeolites, but C3H8 isotherms were obtained quickly on zeolite T. Thus, when CO2 and C3H8 co-adsorption loadings were measured by temperature-programmed desorption on SSZ-13, CO2 loading were higher than C3H8 loadings at short times, whereas at long times, C3H8 loadings were higher as C3H8 displaced adsorbed CO2. In contrast, adsorption of CO2/C3H8 mixtures equilibrated quickly in zeolite T because of the fast C3H8 diffusion. These adsorption measurements may explain previously-reported zeolite membrane behavior where C3H8 decreased CO2 permeances for SAPO34, SSZ-13, and zeolite T membranes under most conditions, and steady state was only slowly obtained for SAPO-34 and SSZ-13 membranes in the presence of C3H8. Introduction Small-pore zeolites (SAPO-34, SSZ-13, DDR, T-type) can be grown as continuous membranes that are selective for CO2/CH4 separations even at high pressures, and thus these membranes have potential for removing CO2 from natural gas [1–3]. For example, SAPO-34 membranes have been reported with CO2/CH4 separation selectivities greater than 100 at 4.6-MPa feed pressure, and their CO2 permeances were greater than 10-6 mol/(m2 s Pa) at this pressure [4]. The diameters of these zeolites (SAPO-34, 0.38 nm; SSZ-13, 0.38 nm; DDR, 0.44 x 0.36 nm; and T-type, which is an intergrowth of erionite, 0.36 nm x 0.51 nm and offretite, 0.67 x 0.68 nm) are small enough so that CH4, which has a kinetic diameter of 0.38 nm, diffuses much slower than CO2 (0.33-nm kinetic diameter). In order to use small-pore zeolite membranes industrially, they must 1 ACS Paragon Plus Environment


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also be effective in the presence of other impurities present in natural gas such as water and higher hydrocarbons. Most of the moisture is removed from natural gas during pretreatment, because water condensation can be detrimental to process equipment, and water and CO2 can form a corrosive acid. However, water, which has a kinetic diameter of 0.265 nm, readily diffuses into these zeolites, and it decreases gas permeation even at 150 ppm concentrations because it adsorbs strongly [5,6]. Alkanes (mostly C2 –C4) are also present in natural gas, but because their kinetic diameters are relatively large (C2H6, 0.39 nm; C3H8 and larger linear alkanes, 0.43 nm), they might not be expected to adsorb in these zeolites. However, kinetic diameters do not accurately predict adsorption, and ethane, propane, and larger linear alkanes adsorb in CHA and zeolite T pores [7,8]. Indeed, propane decreased CO2 and CH4 permeances and CO2/CH4 separation selectivity in SAPO34 membranes over several days at feed pressures from 0.2 to 4.6 MPa because it slowly adsorbed in SAPO-34 pores [6,9]. The propane permeance equilibrated within few minutes during these measurements and was several orders smaller than the CO2 permeance. Since propane diffuses so slowly into SAPO-34 pores, the small propane permeance was due to permeation through defects. Propane also decreased the low-pressure CO2 and CH4 permeances and CO2/CH4 separation selectivities of SSZ-13 membranes, which have the same CHA pore structure as SAPO-34 membranes but also contain phosphorus[10,11]. However, propane increased CO2 permeances and increased CO2/CH4 selectivities by as much as 40% at high pressures [9]. The SSZ-13 membranes had higher selectivities than SAPO-34 membranes in the absence of propane [12][13]. However, they had lower permeances because they were prepared on supports with larger pores than those used for the SAPO-34 membranes. The SSZ-13 crystals grew into the support so that the SSZ-13 layers were thicker than the SAPO-34 layers, resulting in lower fluxes. Thin SSZ-13 membranes with higher permeances are difficult to grow on smooth supports. The SSZ-13 layer does not penetrate into the smooth support structure, and thus it can crack when the SSZ-13 crystals shrink during calcination [13,14]. Even though zeolite T pores are similar in size to SSZ-13 and SAPO-34 pores, propane apparently diffuses faster into the zeolite T pores because it decreased the CO2 permeance more than 95% after about 5 h. Ethane (0.38-nm kinetic diameter) decreased the CO2 permeance by about 65% within 1 h [1]. Since C3H8 and H2O adsorption affects membrane separations, CH4, CO2, C3H8 and H2O single-component adsorption isotherms were measured on SAPO-34, SSZ-13, and zeolite T 2 ACS Paragon Plus Environment

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crystals, and isosteric heats of adsorption were estimated as a function of loading. Because propane adsorbs slowly in SSZ-13 and SAPO-34 crystals, unusual behavior was observed when adsorption times were not long enough to obtain equilibrium in these zeolites. In contrast, propane adsorption equilibrated quickly in zeolite T. The amounts adsorbed for CO2/C3H8 mixtures in SSZ-13 and zeolite T crystals were also measured using temperature-programmed desorption (TPD). We recently reported adsorption of CO2/C3H8 mixtures in SAPO-34 and 13X zeolites using this method [15]. In 13X zeolite, CO2 adsorbed more strongly than C3H8 and adsorption was fast because C3H8 and CO2 are both much smaller than 13X pores (0.8 nm). In SAPO-34, however, CO2 adsorbed quickly and then C3H8 slowly displaced adsorbed CO2 because C3H8 adsorbed more strongly but diffused much slower into the SAPO-34 pores. Even though overall loadings were larger in SSZ-13 than in SAPO-34, the equilibration times were similar. Thus adsorption measurements alone do not explain the differences in separation properties observed for SAPO-34 and SSZ-13 membranes in the presence of propane. Adsorption of CO2/C3H8 mixtures in zeolite T reached equilibrium within a few minutes, and C3H8 decreased CO2 loadings by 80% compared to single-component loading, whereas the C3H8 loadings in zeolite T for mixtures were similar to single-component loadings. Water adsorption was investigated to determine if SSZ-13 or zeolite T are potentially less susceptible to water than SAPO-34. Both zeolites also adsorb water strongly, however desorption is faster than in SAPO-34, and thus water may affect separations in zeolite T and SSZ-13 membranes less. Experimental Methods Preparation and characterization of zeolite crystals: SSZ-13 crystals with an average size of 10 µm were grown using a procedure similar to that described previously [9]: 0.12 g Al(OH)3 powder (54.72% Al2O3, Sigma-Aldrich), 0.22 g NaOH (98%, Sigma-Aldrich), and 11.78 g deionized water were mixed together and stirred for 1 h. Then 8.92 g TMAdaOH (N,N,N, trimethtyl-1-adamantammonium hydroxide, 25 wt.% in water, Sachem Zeogen 2825) was slowly added with vigorous stirring. After 1 h, 3.96 g colloidal silica (Ludox TM-40, Sigma-Aldrich) was added. The mixture was stirred at 293 K overnight to obtain a homogeneous gel with a molar ratio of 1 SiO2: 0.4 TMAdaOH: 0.025 Al2O3: 0.1 Na2O: 44 H2O. The gel was poured into a Teflon-lined autoclave and placed in an oven at 433 K for 4 days. The resulting crystals were recovered by centrifugation, washed 3 to 4 times with deionized water, dried, 3 ACS Paragon Plus Environment


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and then calcined in air at 823 K for 24 h with heating and cooling rates of 10 K/min. Smaller SSZ13 crystals (0.6 – 1.2 µm, BET surface area 696 +/-16 m2/g) were prepared using the same procedure, except 25 mg of the 10-µm SSZ-13 crystals were added to 25 g of gel, and synthesis was carried out at 433 K for 3 days. These smaller crystals were used for the adsorption studies to decrease the equilibration time for propane adsorption. The SAPO-34 crystals were collected from the bottom of an autoclave after synthesis of a SAPO-34 membrane at 493 K for 6 h with a gel composition of 0.85 Al2O3: 1 P2O5: 0.3 SiO2: 2 TEAOH: 155 H2O, as described previously in detail [4]. The template was removed by calcination in air at 773 K. The average crystal size was ~1 µm, and the BET surface area was 604 +/-15 m2/g. Zeolite T crystals (~1.5 µm) were prepared using procedures described by Chen et al. [16]. First, 1.04 g KOH (> 85 wt.%, Sinopharm Chemical Reagent Co. Ltd), 1.39 g NaOH (> 96 wt.%, Tianjin Kermel Chemical Reagent CO., Ltd), 1.30 g Na2AlO3 (Al2O3: 41 wt.%, Na2O: 24.92 wt.%, Sinopharm Chemical Reagent Co. Ltd) and 9.31 g deionized water were mixed and stirred for 12 h. Next, 2.86 g TMAOH (tetramethylammonium hydroxide, 25 wt.% in water, Zhejiang Ken Te Chemical Co. Ltd) was added to the mixture. Then, 14.29 g colloidal SiO2 (Ludox AS-40, 40 wt.% in water, Aldrich) was added dropwise while stirring vigorously. The gel was aged overnight at room temperature, and synthesis was carried out in a Teflon-lined, stainless steel autoclave at 373 K for 48 h. The crystals were recovered by centrifugation, washed 3 to 4 times with deionized water, dried, and then calcined in air at 823 K for 6 h with heating and cooling rates of 10 K/min. The BET surface area of 466 +/- 10 m2/g was consistent with values reported by others for zeolite T [17,18]. The crystallinities and purities of all crystals were confirmed by XRD (Scintag PAD-V), and the crystal sizes were measured by SEM (Figure S1 in supplemental material). Static adsorption measurements Single-component adsorption isotherms for CO2, CH4, C3H8, and H2O were measured in a volumetric system (Quantachrome Autosorb AS-1C). Samples were outgassed at 473 K for 12 h between measurements, and isotherms for the gases were measured from 293 to 353 K. A 90-min equilibration time was used at each pressure for C3H8 adsorption in SAPO-34 and SSZ-13 because C3H8 diffuses slowly into the CHA pores. Equilibrium was not reached at shorter times, as indicated by the large hysteresis observed during desorption and the unusual temperature dependence of isotherms, as described below. 4 ACS Paragon Plus Environment

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Temperature-programmed desorption (TPD) The amounts adsorbed from mixtures were measured by TPD in the presence of the adsorbing gases to prevent desorption of these weakly-adsorbed molecules before TPD was carried out [15]. A sample (approximately 0.3 g) was heated with a temperature-controlled, wire-wound tubular furnace to 775 K for 3 h in O2 to remove adsorbed species and then cooled to room temperature before the feed was switched to an adsorbate flow. For CO2/C3H8 mixtures, a feed of 37% CO2, 3.7% C3H8, and 59.3% He flowed over the zeolite from 1 to 26 h at room temperature before TPD. The sample was then heated to 775 K at a linear rate of 35 K/min in the presence of the same feed gases, and a mass spectrometer (SRS QMA 200) measured the gas-phase concentrations as a function of time. A He carrier was used so that differences in concentrations of the desorbing species could be measured. The mass spectrometer was calibrated after each TPD by varying the adsorbate feed concentrations in steps for both single-components and mixtures while the sample was held at 673 K so that it did not adsorb the gases. The amounts adsorbed were then calculated from the areas under the TPD curves after they were corrected for changes in total flow rate, as described previously [15]. As previously shown for SAPO-34 and 13X, the amounts desorbed during TPD when only CO2 or C3H8 was adsorbed were typically within 5% of loadings measured by static adsorption. Results and Discussion Single-component adsorption in SSZ-13 Figure 1 - 4 show static adsorption isotherms measured on SSZ-13 zeolite for CH4, CO2, C3H8, and H2O at several temperatures (data points are listed in the supplemental material). Adsorption loadings decreased with increasing temperature for all gases, and at all temperatures, the slopes of the isotherms at low pressures increased in the order: CH4 < CO2 < C3H8 < H2O. Thus, H2O had high loadings at much lower pressures than the other three adsorbates. The water desorption isotherms had a slight hysteresis (Figure S2 in supplemental material). Propane loadings were closer to saturation at a lower pressure than CO2, and CH4 had the lowest loadings. More propane adsorbed at low pressure because it is larger and thus has more interactions with the zeolite surface, but because it is larger, its saturation molecular loading is lower.

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CH4 absorbed [mmol/g]

0.8

0.6 294 K 323 K

0.4

348 K

0.2

0 0

20

40

60

80

100

Pressure [kPa]

Figure 1: Adsorption isotherms of CH4 in SSZ-13 zeolite at three temperatures. The lines are best fits to the dual-site Langmuir equation. 4 CO2 adsorbed [mmol/g]

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294 K 3

323 K

2

348 K 1

373 K

0 0

20

40

60 Pressure [kPa]

80

100

Figure 2: Adsorption isotherms of CO2 in SSZ-13 zeolite at four temperatures. The lines are best fits to the dual-site Langmuir equation.


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3

C3H8 absorbed [mmol/g]

293 K 323 K 348 K 2 373 K

1

0 0

20

40

60

80

100

Pressure [kPa]

Figure 3: Adsorption isotherms of C3H8 in SSZ-13 zeolite at four temperatures. The lines are best fits to the dual-site Langmuir equation. 16 14 12 H2O adsorbed [mmol/g]

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293 K 10 8 323 K 6 4

372 K

2 0 0

0.5

1

1.5

2

2.5

Pressure [kPa]

Figure 4: Adsorption isotherms of H2O on SSZ-13 at three temperatures. The lines are best fits to the dual-site Langmuir equation.



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Single-component adsorption in SAPO-34 Static adsorption isotherms on SAPO-34 zeolite for CO2, CH4, C3H8, and H2O at several temperatures are shown in Figures 5 – 8 (data points are listed in the supplemental material); the SAPO-34 crystals were synthesized at the same conditions used for high-flux SAPO-34 membranes [4]. The adsorption behavior was similar to that observed on SSZ-13. Adsorption loadings decreased with increasing temperature for all gases, and for all temperatures, the slopes of the isotherms at low pressures increased in the order: CH4 < CO2 < C3H8 < H2O. Thus, H2O had high loadings at much lower pressures than the other three adsorbates. The lower-temperature H2O isotherms have an S-shape (Figure 8), which has been reported by others for H2O adsorption on SAPO-34 [19–21]. Chen et al. [21] suggested that the large increase in water loading over a narrow pressure range at 294 and 308 K is the result of capillary condensation in the zeolite pores, consistent with type V isotherms. Condensation apparently also takes place at 323 K in the 0.7-nm cages (Figure 8); the highest P/Psat was 0.17 at 323 K. The CO2 loadings were similar to those reported previously [22], but the CH4 loadings changed more with temperature [23]. Even though the gel compositions used in the previous study were only slightly different from those used here, different aluminum precursors, a different template, and different synthesis procedures apparently increased the CH4 heat of adsorption significantly. Note that for CH4, CO2, and C3H8, the loadings at 100 kPa (the highest pressure used) were lower on SAPO-34 than on SSZ-13 at all temperatures used. Even though SAPO-34 and SSZ-13 have the same CHA structure and similar crystal densities, the BET surface area is 15% higher for SSZ-13, and the single-component CO2 loading at 294 K and 100 kPa for SSZ-13 (~4.2 mmol/g) is about 27% higher than that of SAPO-34 (~3.3 mmol/g) at the same conditions. Similarly, the single-component C3H8 loading on SSZ-13 at 294 K and 100 kPa (~3.1 mmol/g) is 29% higher than the loading on SAPO-34 (~2.4 mmol/g).


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0.8


0.6

294 K

0.4

313 K 0.2 333 K 353 K 0.0 0

20

40 60 Pressure [kPa]

80

100

Figure 5: Adsorption isotherms of CH4 in SAPO-34 at four temperatures. The lines are best fits to the dual-site Langmuir equation. 3.5

3.0

CO2 absorbed [mmol/g]

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293 K

2.5

2.0 313 K 1.5 333 K 1.0 353 K 0.5

0.0 0

20

40

60 Pressure [kPa]

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Figure 6: Adsorption isotherms of CO2 in SAPO-34 at four temperatures. The lines are best fits to the dual-site Langmuir equation.



2.5 293 K 323 K

C3H8 absorbed [mmol/g]

2.0

348 K

1.5 373 K 398 K

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0.0 0

20


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Figure 7: Adsorption isotherms of C3H8 in SAPO-34 at five temperatures. The lines are best fits to the dual-site Langmuir equation. 25

294 K

20 H2O adsorbed [mmol/g]

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308 K 323 K

15

10

338 K 5

353 K 368 K

0 0

0.5

1

1.5 Pressure [kPa]

2

2.5

Figure 8: Adsorption isotherms of H2O on SAPO-34 at six temperatures. The lines are cubic spline fits.


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Single-component adsorption in zeolite T Figure 9 - 12 show static adsorption isotherms on zeolite T for CH4, CO2, C3H8, and H2O at several temperatures (data points are listed in the supplemental material). As seen for SSZ-13 and SAPO-34, adsorption loadings decreased with increasing temperature for all gases, and for all temperatures, the slopes of the isotherms at low pressures increased in the order: CH4 < CO2 < C3H8 < H2O. Water has the highest loading, and CO2 has the next-highest loadings at 100 kPa (and at most pressures), whereas CH4 has the lowest loading. Although C3H8 appears closer to saturation than CO2, it has lower molar loadings because it is a larger molecule. The C3H8 desorption isotherms were similar to the adsorption isotherms, indicating equilibrium was obtained during the adsorption measurements, and adsorption was not limited by C3H8 diffusion. 1.4 294 K

1.2


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1.0 313 K 0.8 0.6

333 K

0.4 353 K 0.2 0.0 0

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40

60 Pressure [kPa]

80

100

Figure 9: Adsorption isotherms of CH4 in zeolite T at four temperatures. The lines are best fits to the dual-site Langmuir equation.



4 293 K

CO2 absorbed [mmol/g]

313 K 3 333 K

353 K 2

1

0 0

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60 Pressure [kPa]

80

100

Figure 10: Adsorption isotherms of CO2 in zeolite T at four temperatures. The lines are best fits to the dual-site Langmuir equation.. 2.5 293 K 313 K 333 K

2.0

353 K C3H8 absorbed [mmol/g]

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1.5

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0.0 0

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Figure 11: Adsorption isotherms of C3H8 in zeolite T at four temperatures. The lines are a combination of cubic spline fits and best fits to the dual-site Langmuir equation.


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14 12 293 K H2O adsorbed [mmol/g]

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10 323 K 8 372 K 6 4 2 0 0

0.5

1

1.5

2

2.5

Pressure [kPa]

Figure 12: Adsorption isotherms for H2O on zeolite T at three temperatures. The lines are best fits to the dual-site Langmuir equation.. Isosteric heats of adsorption The isosteric heats of adsorption were calculated from best-fits of adsorption data since the loadings at different temperatures were not measured at the same pressures, and thus interpolation between the measurements was required. Single-site Langmuir isotherms did not describe any of the adsorption data well. Likewise, two models that have been used for zeolites, the Toth equation [24,25] and a model based on statistical thermodynamics [26,27], did not provide good fits. In contrast, the dual-site Langmuir equation (indicated by the solid lines) fit most of the data in Figures 1 - Figure 12. However, the resulting model parameters were not physically meaningful because different sets of parameters yielded fits of similar quality in some cases, likely because measurements were carried out over a narrow pressure range and loadings were not near saturation for some gases. In addition, the saturation loadings exceeded the unit cell capacity that was previously calculated using molecular dynamics [27], and the saturation loadings strongly depended on temperature. Thus the dual-site Langmuir equation was only used to interpolate between experimental data points in order to calculate isosteric heats of adsorption; no physical significance was assigned to the fit parameters. The dual-site Langmuir equation did not describe some of the H2O adsorption data for SAPO-34 (Figure 8), and thus a cubic spline function was used. Similarly, the C3H8 isotherms for 13 ACS Paragon Plus Environment


zeolite T (Figure 11) were fit with a combination of the dual-site Langmuir equation at low loading and cubic spline functions at higher loadings. As shown in Figure 13, for all three zeolites, the isosteric heats of adsorption increased in the order: CH4 < CO2 < C3H8. For CH4, CO2, and C3H8, the heat of adsorption was higher on zeolite T than on the two CHA zeolites. Indeed, the isosteric heat of adsorption for CH4 of ~30 kJ/mol on zeolite T is more than 50% higher than that on SSZ-13. 50 T C3H8

Isosteric heat of adsorption [kJ/mol]

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T CO2

40

SSZ-13 C3H8

SAPO-34 C3H8

SSZ-13 CO2

30 T CH4

SAPO-34 CO2

SAPO-34 CH4

20 SSZ-13 CH4

10

0 0

0.5

1

1.5

2

Loading [mmol/g]

Figure 13: Isosteric heats of adsorption of CH4, CO2, and C3H8 on SSZ-13, SAPO-34, and zeolite T. The symbols indicate the loadings used to calculate the isosteric heats of adsorption, and the lines are best fits to the data points. The isosteric heats of adsorption for CH4 (~17 kJ/mol), CO2 (~32 kJ/mol), and C3H8 (~39 kJ/mol) on SSZ-13 did not change much with loading, as shown in Figure 13. Hudson et al. [28] reported similar isosteric heats of adsorption for CO2 on Cu-exchanged SSZ-13 (32.5 kJ/mol) and H-exchanged SSZ-13 (33.6 kJ/mol). Pham et al. [29] reported a range of isosteric heats of adsorption for CO2: 26 kJ/mol at low loadings for H-exchanged SSZ-13 with a Si/Al ratio of 12 to 45 kJ/mol on a K-exchanged SSZ-13 with a Si/Al ratio of 6. The heats of adsorption decreased as the loading increased. Pham et al. used the Toth model and Hudsen et al. fit their data with dual-site Langmuir isotherms. On SAPO-34, CO2 had a higher isosteric heat of adsorption (30 kJ/mol) than CH4 (25 kJ/mol), as shown in Figure 13. The CO2 heat of adsorption did not change much with loading. Kim et al. [22] reported isosteric heats of adsorption of 33-41 kJ/mol on SAPO-34, 14 ACS Paragon Plus Environment

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whereas Li et al. reported that CO2 had a heat of adsorption of ~24 kJ/mol and that CO2 adsorbed more strongly than CH4 (∆Hads~16 kJ/mol) [23]. However, the heats of adsorption reported by Li et al. were calculated from the temperature dependence of the adsorption equilibrium constant for a single-site Langmuir model and thus did not depend on loading. In addition, their saturation loadings depended on temperature, even though saturation is not expected to change significantly with temperature. Competitive adsorption appeared to increase CO2/CH4 selectivity in SAPO-34 membranes reported previously because the heat of adsorption for CO2 (~24 kJ/mol) was ~50% larger than that of CH4 (~16 kJ/mol), resulting in mixture selectivities that were higher than ideal selectivities calculated from single-component permeances [23]. More recently, membranes that were prepared with the same synthesis procedure used for the crystals in this study, however, had mixture separation selectivities that were similar to ideal selectivities. The similar heats of adsorption for CO2 (~30 kJ/mol) and CH4 (~26 kJ/mol) measured here (Figure 13) indicate that competition for adsorption sites is less significant for these membranes. Water adsorbs strongly on all three zeolites; as shown in Figure 14, its isosteric heats of adsorption ranged from 30 to 50 kJ/mol. The water heat of adsorption increased with loading for SAPO-34, but decreased with loading for SSZ-13. The shapes of the H2O isotherms on SAPO-34 changed dramatically at temperatures below 338 K. Thus, heats of adsorption could only be calculated accurately for loadings less than ~ 4 mmol/g, even though the maximum loading measured was almost 20 mmol/g. The water isotherms did not change shape at lower temperatures on the other two zeolites. Strongly-adsorbing water significantly affects permeation through zeolite membranes, as previously shown for SAPO-34 membranes, where water concentrations of only 130-170 ppm decreased CO2 permeation by about 11%, and 0.6% water decreased CO2 permeances as much as 96% in 30 min [5,6]. The water loadings on SAPO-34 at 323 K and 2 kPa are about twice those of SSZ-13 and zeolite T, as shown in Figure 15, and thus water at this partial pressure would be expected to affect permeances for SAPO-34 membranes more than for SSZ-13 or zeolite T membranes.



Isosteric heat of adsortpin [kJ/mol]

60

SSZ-13

40 SAPO-34 Zeolite T

20

0 0

2

4

6

8

H2O loading [mmol/g]

Figure 14: Isosteric heats of adsorption of H2O on SSZ-13, SAPO-34, and zeolite T 16 SAPO-34 14 12

H2O adsorbed [mmol/g]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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10 Zeolite T 8

SSZ-13

6 4 2 0 0

0.05

0.1 P/P0

0.15

0.2

Figure 15: Adsorption (solid lines) and desorption (dashed lines) of H2O on SAPO-34, zeolite T, and SSZ-13 at 323 K Adsorbed water appears to be significantly less mobile in SAPO-34, as indicted by the large desorption hysteresis at 323 K in Figure 15. Some of the water remained adsorbed after more than 10 h of evacuation, and heating above 423 K was required to remove the residual water. In contrast, both zeolite T and SSZ-13 exhibited less hysteresis; water desorbed more readily from SSZ-13 and zeolite T. Since SSZ-13 has the same pore structure as SAPO-34, it might be expected to exhibit 16 ACS Paragon Plus Environment

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similar hysteresis, and the reason for the dramatic difference is unclear. Similarly, even though CO2 and H2O have similar heats of adsorption on SAPO-34, CO2 readily desorbed during desorption isotherm measurements. Mixture adsorption of CO2 and C3H8 The CO2 heats of adsorptions (30-33 kJ/mol) are similar on SSZ-13 and SAPO-34 zeolites. Likewise, the C3H8 heats of adsorption are similar on the two zeolites (37-40 kJ/mol). Thus, as expected, TPD of CO2/C3H8 mixtures showed that C3H8 displaced CO2 from both zeolites. Figure 16 compares the CO2 and C3H8 loadings on SSZ-13 and SAPO-34 measured by TPD after different adsorption times for a 37% CO2/3.7% C3H8/59.3% He mixture at 295 K and 85 kPa. The SAPO-34 results in this figure are from a previous study [15]. The time-dependence of CO2 and C3H8 adsorption in the mixture was similar on SSZ-13 and SAPO-34. At short times the CO2 loading was higher than the C3H8 loading, and over approximately 6 h, the CO2 loading decreased and the C3H8 loading increased until it was higher than the CO2 loading. The time constants for C3H8 adsorption in the mixture were similar to those observed for single-component C3H8 adsorption in SAPO-34 and SSZ-13 crystals using the volumetric system (see below). This suggests that adsorbed CO2 does not significantly affect C3H8 adsorption kinetics, but it does affect C3H8 loadings. As shown in Figure 16, the C3H8 loading in the mixture was 80% of the single-component loading for SAPO-34 and less than 70% for SSZ-13. Likewise, the CO2 loading was ~55% of the single-component loading for SAPO-34 and ~40% for SSZ-13. 2.7

SSZ-13 single components

CO2 C3H8

Amount adsorbed [mmol/g]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


SAPO-34 single components [15]

1.8

CO2

C3H8 C3H8

SSZ-13 (mix) CO2

0.9 SAPO-34 (mix) [15]

0.0 0

6

12

18

24

Time [h]


30


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Figure 16: Carbon dioxide and C3H8 loadings on SSZ-13 and SAPO-34 (dashed lines) [15] measured by TPD in a 37% CO2/3.7% C3H8/59.3% He mixture as a function of equilibrium time at 295 K and 85 kPa (o-CO2, ∆-C3H8; solid lines are best fits). The single-component loadings, obtained from static adsorption measurements, are at the same partial pressures used for the mixture adsorption (31 kPa CO2, 3.2 kPa C3H8). The slow replacement of CO2 by C3H8 in SAPO-34 explains why the CO2 permeance in SAPO-34 membranes decreased significantly over days when C3H8 was added to a CO2/CH4 mixture [9] since the SAPO-34 crystals were larger in the membranes. Because C3H8 adsorbed more strongly than CO2 in SAPO-34, the CO2 permeance (and the CO2/CH4 selectivity) was lower at all pressures studied. Similarly, for SSZ-13 membranes at low feed pressures, C3H8 displaced some CO2 from the zeolite when it was added to the feed, but CO2 permeances decreased only slightly [9]. At high pressures, however, adding C3H8 to the feed of an SSZ-13 membrane increased both the CO2 permeance and the CO2/CH4 selectivity. Propane might be expected to decrease the CO2 permeance less at high pressures because CO2 loadings increase more than propane loadings with pressure, as indicated by the differences in slopes of the isotherms in Figure 2 andFigure 3. However, the adsorption measurements at the low pressures used in this study do not explain why C3H8 increased the CO2 permeance at the higher pressures where the permeation measurements were carried out. Propane decreased the CO2 loading in zeolite T more than for the other two zeolites (Figure 17). In contrast to the slow adsorption on SAPO-34 and SSZ-13, the CO2 and C3H8 loadings equilibrated within 15 min because C3H8 diffused faster in zeolite T, and the loadings did not change over 18 h. The CO2 loading in the CO2/C3H8 mixture was only 20% of the single-component loading at the same CO2 pressure, as shown in Figure 17. The differences in heats of adsorption do not explain why C3H8 decreased the CO2 loading in zeolite T more than in SSZ-13 or SAPO-34, which have larger differences in the heats of adsorption of CO2 and C3H8. Moreover, the C3H8 loading was 90% of its single-component loading in zeolite T. These results indicate that the large, rapid decrease in CO2 permeance for zeolite T membranes when 5% C3H8 was added to a CO2 feed [1] is due to the rapid displacement of CO2 by C3H8 in the zeolite pores. Even though all three zeolites preferentially adsorb C3H8 over CO2, equilibration times for SSZ-13 and SAPO-34 are long, and equilibrium adsorption selectivites ((yCO2/yC3H8)/(xCO2 /xC3H8)) are only ~20. 18 ACS Paragon Plus Environment

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4 single component CO2 Amount adsorbed [mmol/g]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3

2

single component C3H8 C3H8 mixture

1 CO2 mixture 0 0

5

10

15

20

Time [h]

Figure 17: Carbon dioxide and C3H8 loadings on zeolite T measured by TPD in a flowing 37% CO2/3.7% C3H8/59.3% He mixture as a function of equilibrium time at 295 K and 85 kPa (o-CO2, ∆-C3H8; solid lines are best fits).The single-component adsorbed amounts were measured in the static adsorption system at the same partial pressures as in the mixture (31 kPa CO2, 3.2 kPa C3H8). Adsorption equilibration times Because C3H8 diffuses slowly in SAPO-34 pores, long times were required to reach steady state when C3H8 was added to CO2/CH4 feeds for SAPO-34 membranes [30], and long times were required to reach equilibrium when measuring C3H8 adsorption on SAPO-34 crystals. As shown in Figure 18, C3H8 uptake did not reach equilibrium for either SAPO-34 or SSZ-13 crystals, even after 10 h. As a result, C3H8 isotherms exhibited large hysteresis behavior and unusual temperature dependencies when insufficient equilibration times were used. The tolerance settings for the static adsorption system that are typical for gas adsorption (less than 0.01% change in loading over a preset time period) were satisfied during these measurements. For example, when 3-min equilibration times were used, C3H8 isotherms in SSZ-13 exhibited large hysteresis during desorption, as shown in Figure 19 for 313 K. Moreover, isotherms that were measured with 3-min equilibration at different temperatures cross each other, and the lowest loading at higher pressure was measured at 273 K because diffusion is slower at the lower temperature. At higher temperatures, where C3H8 diffuses faster, isotherms were closer to equilibrium.



3

Propane adsorbed [mmol/g]

SSZ-13

SAPO-34

2

1

0 0

2

4

6

8

10

Time [h]

:

Figure 18: Amount of C3H8 adsorbed as a function of time for SSZ-13 and SAPO-34 crystals exposed to 58 kPa of propane at 295 K. Measurements were made in a static adsorption system. The pressure after 10 h was about 52 kPa. 3

313 K

3K desorption at 31

Amount adsorbed [mmol/g]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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333 K 293 K 353 K 273 K

2

1

0 0

20

40

60

80

100

Pressure [kPa]

Figure 19: Non-equilibrated adsorption isotherms for C3H8 in SSZ-13 at five temperatures. Also shown is the desorption isotherm at 313 K. Equilibration time for each data point was 3 min. The solid lines are best fits to the dual-site Langmuir equation.


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In contrast, when 99-min equilibration times were used instead of 3 min, loadings were more than 30% higher at lower pressures, loadings decreased with increasing temperature (Figure 3), and the adsorption/desorption hysteresis was small. However, several days were required to measure just the adsorption branch of one isotherm with 99-min equilibrium times. Propane adsorption in zeolite T was much faster, so 10-min equilibration times were used for each data point for zeolite T, and the amounts adsorbed did not change for longer times. Conclusions Isosteric heats of adsorption were measured for CH4, CO2, C3H8, and H2O on SAPO-34, SSZ-13, and zeolite T crystals. Because C3H8 has a higher heat of adsorption than CO2 in zeolite T, SSZ-13 and SAPO-34, C3H8 preferentially adsorbs from mixtures. This preferential adsorption explains why C3H8 decreased CO2 permeation through SAPO-34, zeolite T, and SSZ-13 (at low pressures) membranes. However, it does not explain why C3H8 increased fluxes and selectivities for CO2/CH4 mixtures through SSZ-13 membranes at higher pressures but decreased permeances and selectivities of SAPO-34 membranes at higher pressures. Adsorption of C3H8 and of CO2/C3H8 mixtures on SSZ-13 and SAPO-34 equilibrated slowly because C3H8 slowly diffuses into the small zeolite pores. Several days were required to obtain completely-equilibrated C3H8 adsorption isotherms for SAPO-34 and SSZ-13. Thus, CO2 permeances slowly decrease in SAPO-34 and SSZ13 membranes over several days when C3H8 was added to the feed. Shorter equilibration times during adsorption measurements resulted in strong hysteresis and unrealistic temperature decencies of loadings. Adsorption equilibration was fast in zeolite T because C3H8 diffuses rapidly into the larger T pores, and thus CO2 permeation through zeolite T membranes decreased rapidly when C3H8 was added to the feed. Water strongly adsorbs in all three zeolites, with heats of adsorption of 30-50 kJ/mol. This may explain why low H2O concentrations decrease CO2 permeances through SAPO-34 membranes. The H2O isotherms on SAPO-34 at lower temperatures show strong hysteresis, whereas H2O readily desorbs form zeolite T and SSZ-13, suggesting that zeolite T and SSZ-13 membranes may be less susceptible to H2O contamination. Acknowledgments YL gratefully acknowledge support by the China Scholarship Council. We like to thank Sachem for providing the SSZ-13 template. We also thank Dr. Jianhua Yang from Dalian University of Technology for providing zeolite T crystals. 21 ACS Paragon Plus Environment


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Supporting Information Available: SEM images of zeolite crystals used in this study, adsorption and desorption branches of H2O adsorption on SSZ-13, and data used for isotherm figures. This material is available free of charge via the Internet at http://pubs.acs.org.

TOC Abstract graphic: 0 4

0.5

PH2O [kPa] 1 1.5

SSZ-13 at 323 K

2 H2O C3H8

3

6

CO2

2 1

4 2

CH4

0 0

20

40 60 80 PCO2, CH4, C3H8 [kPa]

2.5 8

100

Adsorbed H2O [mmol/g]

Adsorbed CO2, CH4, C3H8 [mmol/g]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0 120


Adsorption of CO2, CH4, C3H8, and H2O in SSZ-13, SAPO-34, and T

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