Article pubs.acs.org/jced
Adsorption of Lower Alkanes on a Zinc Based Metal Organic Framework Prashant Mishra, Hari Prasad Uppara, Bishnupada Mandal, and Sasidhar Gumma* Department of Chemical Engineering, Indian Institute of Technology Guwahati, Guwahati 781039, Assam, India ABSTRACT: In this work, we report the gas adsorption properties of C 2 H 6 , C 3 H 8 , i-C 4 H 10 , and n-C 5 H 12 on Zn 2 (bdc) 2 (dabco)(H 2 O) 0.5 (DMF) 4 commonly known as ZnDABCO metal organic framework (MOF). Gravimetric adsorption measurements were performed at three different temperatures (294, 314, and 350 K) and a wide pressure range. For the linear alkanes, the isotherms were of Type-I; however, iC4H10 exhibits a Type-IV isotherm at 294 K. Adsorption capacities on ZnDABCO are found to be higher than that on convention adsorbents like activated carbons and silicalites. The isotherms for linear alkanes were fit to a modified virial model. Enthalpy of adsorption was calculated using model parameters; it increases with loading indicating the role of lateral interactions. As is to be expected, a linear correlation was observed between the polarizability of the adsorbates and the enthalpy at zero loading (vertical interactions).
1. INTRODUCTION In recent years, a new class of porous materials known as metal organic framework (MOF) has been widely synthesized and investigated for the gas separation, gas storage, and catalysis applications.1−10 MOFs are synthesized by covalent bonding of metal atoms with bulky organic linkers; both degree of metal atom unsaturation and organic linkers have distinct effects on MOFs adsorption characteristics. MOFs have attracted a lot of attention due to some of their interesting characteristics like tunable structure and lower enthalpy of adsorption. However, some MOFs have poor stability and degenerate upon exposure to water vapor.11 Adsorption isotherms of lower alkanes on activated carbons and zeolites have been reported by several authors.12−15 However, literature for their adsorption on MOFs is limited. Adsorption of alkanes is of particular interest in natural gas storage and delivery where the effect of higher alkanes on a cyclic charge/discharge process is required in addition to methane adsorption capacity.16 Another motive to study alkane adsorption is to evaluate adsorption characteristics of saturated metal site containing ZnDABCO MOF as a function of polarizability. In this work, we have chosen ZnDABCO MOF for C2H6, C3H8, i-C4H10, and n-C5H12 adsorption study because of its known characteristics like coordinatively saturated metal sites and lower enthalpy of adsorption at zero coverage. This MOF was first synthesized by Dybtsev et al.17 Two distinct pore sizes (7.5 Å × 7.5 Å along the c-axis and 4.8 Å × 3.2 Å along the aand b-axes)17 were observed. Moderate thermal stability (up to 573 K)18 and water tolerance (30% relative humidity water vapor sorption at 25 °C)19 are reported for this MOF. All the metal atoms are coordinatively saturated. Due to the absence of coordinatively unsaturated metal sites and the nonpolar adsorbates used in this work, low values of enthalpy of © XXXX American Chemical Society
adsorption are expected. Several articles available in the literature primarily focus on CO2 adsorption on ZnDABCO.18,19 The adsorption of CH4 on ZnDABCO has also been widely studied.18,19 However, to the best of our knowledge to date no data are available on adsorption of the hydrocarbons studied in this work. Here, we report equilibrium adsorption isotherms of C2H6, C3H8, i-C4H10, and n-C5H12 on this MOF at three different temperatures. Adsorption enthalpies are also calculated from model fits for the experimental data.
2. EXPERIMENTAL SECTION ZnDABCO was synthesized as per procedure suggested by Dybtsev et al.17 TGA characteristics and BET surface area analysis were already reported in our earlier work.18 A gravimetric method was used to measure adsorption equilibria of C2H6, C3H8, i-C4H10, and n-C5H12 at 294, 314, and 350 K. The adsorbent was activated prior to each isotherm measurement by heating it at 433 K under vacuum (and a purge flow of 20 cm3 min−1 of helium). Equilibrium measurement has been carried out as per the usual procedure.18 Adsorption is measured gravimetrically in this work with proper buoyancy corrections. For each isotherm, measurement is started from complete vacuum, followed by stepwise dosing of gas/vapors to increase the pressure to desired target value. Three absolute pressure transducers (make MKS, model 627) with different full scale ranges (10, 1000, and 20 000 Torr) were used to measure the pressure accurately. All isotherms are reported as excess amount adsorbed (mol kg−1) against fugacity (kPa); Received: June 29, 2012 Accepted: August 27, 2012
A
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fugacity is used instead of pressure to account for bulk-phase nonideality at higher pressures.
3. RESULTS AND DISCUSSION Adsorption isotherms of C2H6, C3H8, i-C4H10, and n-C5H12 adsorbates on ZnDABCO at three different temperatures are shown in Figures 1 to 4. Isotherms for C2H6, C3H8, and n-
Figure 3. i-C4H10 isotherms. Symbols are experimental data at 294 K (▲), 314 K (◆), and 350 K (●).
Figure 1. C2H6 isotherms. Symbols are experimental data at 294 K (▲), 314 K (◆), and 350 K (●); lines are fits obtained using modified virial isotherm parameters from Table 2.
Figure 4. n-C5H12 isotherms. Symbols are experimental data at 294 K (▲), 314 K (◆), and 350 K (●); lines are fits obtained using modified virial isotherm parameters from Table 2.
MgDOBDC.27 For C3H8, loading on ZnDABCO at ambient temperature and about 10 kPa is higher than that on activated carbon F30/470,28 silica gel KC,14 silicalite;22 comparable to that on CuBTC26 and lower than that on MgDOBDC.27 However, saturation loading of C2H6 and C3H8 on both ZnDABCO and MgDOBDC would be similar due to their similar pore volumes.27 The adsorption capacities for i-C4H10 are higher than those in silica gel KC,14 silicalite;22 comparable to those in CuBTC,20 activated carbon ASA-H.29 The loading capacity for n-C5H12 on ZnDABCO is significantly higher than that on BAX activated carbon,15 microcrystalline rutile,30 and silicalite.31 Detailed comparison of adsorption capacities of ZnDABCO with that of other reported materials for all of the studied adsorbates is presented in Table 1. Isotherm modeling gives insight into adsorption characteristics. As in the case of our earlier work,18 we used modified virial equation (eq 1) to model C2H6, C3H8, and n-C5H12 isotherms.
Figure 2. C3H8 isotherms. Symbols are experimental data at 294 K (▲), 314 K (◆), and 350 K (●); lines are fits obtained using modified virial isotherm parameters from Table 2.
C5H12 are of regular Type-I shape. For the branched alkane, iC4H10, obtained isotherm at 294 K is of Type-IV shape. Earlier works also report similar trends for adsorption of i-C4H10 on CuBTC20 and silicalite-1.21,22 The larger kinetic diameter of iC4H10 molecule combined with the pore size of the ZnDABCO MOF may lead to this type of behavior. However, other detailed microscale experiments (see ref 23, for example) may be necessary to identify the exact reason for such a behavior. Adsorption at low pressure increases with an increase in carbon chain length attributed to an increase of polarizability. However, saturation adsorption capacity decreases with the increase of carbon number of the alkane due to increase of molecular size. The loading for C2H6 on ZnDABCO, measured in this work is higher than that on activated carbon,12 zeolite 5A,13 silica gel KC,14 silicalite,22 STH-2 GCB,24 and MOF-5;25 it is comparable to that on CuBTC26 and lower than that on
f=
NM exp(bN + cN 2) β (M − N )
(1)
−1
where N (mol kg ) is the amount adsorbed, f (kPa) is the fugacity, β (mol kg−1 kPa−1) is the Henry constant, M (mol kg−1) is the saturation capacity, and b (mol−1 kg) and c (mol−2 kg2) are the second and third virial coefficients, respectively. B
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Table 1. Comparison of Adsorption of Lower Alkanes on Different Adsorbents temperature
pressure
N
adsorbent
K
kPa
mol kg−1
ref
ZnDABCO silica gel KC activated carbon zeolite 5A STH-2 GCB MOF-5 CuBTC MgDOBDC
294 293 323 293.15 293.2 297 295 298
5.62 0.99 1.53 1.36 0.168 1.9 4.8 6.8
this work 14 12 13 24 25 26 27
ZnDABCO silica gel KC silicalite CuBTC MgDOBDC
294 293 308 283 298
4.33 0.45 1.8 5.8 6.0
this work 14 22 26 27
ZnDABCO silica gel KC silicalite AC ASA-H CuBTC
294 293 308 293 303
4.88 0.54 0.92 4.5 5
this work 14 22 29 20
ZnDABCO rutile silicalite AC BAX-1100
294 303 303 293
4.85 0.03 1.45 4.80
this work 30 31 15
C2H6 120 243.75 92.94 92.96 120 100 100 100 C3H8 10.4 19.5 10 10 10 i-C4H10 6.92 13.19 20 10 3 n-C5H12 2.8 5 1.32 13.4
Δhads = R
N
Figure 5. Variation of adsorption enthalpy with loading for CH4 (■), C2H6 (●), C3H8 (◆), and n-C5H12 (▲).
comparison CH4 adsorption enthalpy is also included here from our earlier work.18 Enthalpy of adsorption on ZnDABCO for these hydrocarbons at zero occupancy is lower than that on other adsorbents like silicalite,22 CuBTC,26 microcrystalline rutile,30 zeolites (MOR, MFI)33 and MgDOBDC;27 this indicates weaker host−guest interaction for ZnDABCO and will result in easier regeneration. Enthalpies of adsorption show a slight increase with loading due to an increase in lateral interactions between the adsorbate molecules. Figure 6, shows enthalpy at zero coverage as a function of polarizability of the alkane; as expected, the relationship was nearly linear.
(2)
4. CONCLUSIONS In this work, adsorption of C2H6, C3H8, i-C4H10, and n-C5H12 was carried out over a wide range of pressures and temperatures on ZnDABCO. For linear adsorbates Type-I isotherm were observed whereas i-C4H10 had shown Type-IV shape at 294 K. Adsorption capacities reported here are higher than most of the
where T is temperature in K. The units used for all other parameters in eq 2 are given in Table 2. The saturation loading Table 2. Model Parameters for Adsorption Isotherms Adsorbates C2H6
C3H8
n-C5H12
8.2 2537 −0.53 81.4 0.154 −39.4 9.8
2.4 3469 −0.57 31.4 0.225 −51.2 7.5
0.86 4454 −1.97 124 0.572 −90.6 5.4
(3)
−1
where R is the gas constant in kJ mol K . Variation of enthalpy of adsorption with loading for C2H6, C3H8, and n-C5H12 is shown in Figure 5. For the sake of
b = b0 + b1/T
c = c0 + c1/T
β0·106, mol kg−1 kPa−1 β1, K b0, mol−1 kg b1, mol−1 kg K c0, mol−2 kg2 c1, mol−2 kg2 K M, mol kg−1
= R( −β1 + b1N + c1N 2) −1
The usual temperature dependency was considered for these parameters (eq 2). β = β0 exp(β1/T )
∂(ln P) ∂(1/T )
M was considered to be independent of temperature. The obtained parameters from the isotherm fits are listed in Table 2. These fits are included in Figures 1, 2, and 4 along with the experimental data. The isotherm for i-C4H10 exhibits Type-IV behavior and cannot be modeled using these equations. For modified form of virial equation used in this work, the enthalpy of adsorption Δhads (kJ mol−1) can be readily derived32
Figure 6. Enthalpy of adsorption at zero occupancy as a function of polarizability of the adsorbate; linear trend line is also shown. C
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convention silicalites and activated carbons. The modified virial model was used to model Type-I isotherms. Fitting parameters were utilized to calculate enthalpies of adsorption. An increase in adsorption enthalpy is observed with loading, attributed to lateral interactions. An increase in enthalpy of adsorption at zero coverage was also observed with an increase in polarizability. The adsorption enthalpies at zero coverage for ZnDABCO are lower than that for MOFs containing open metal centers like MgDOBDC and MIL-101, making easier regeneration of this MOF.
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AUTHOR INFORMATION
Corresponding Author
*Tel/Fax: +91 361 2582261. E-mail address: s.gumma@iitg. ernet.in. Notes
The authors declare no competing financial interest.
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REFERENCES
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