Alexandre F. P.
Ferreira
ab,
Marjo C.
Mittelmeijer-Hazeleger
b,
Miguel Angelo
Granato
a,
Vanessa F. Duarte
Martins
a,
Alírio E.
Rodrigues
a and
Gadi
Rothenberg
*b
aLaboratory of Separation and Reaction Engineering, Associate Laboratory LSRE/LCM, Department of Chemical Engineering, Faculty of Engineering, University of Porto, Rua Dr Roberto Frias, Porto, Portugal
bVan't Hoff Institute for Molecular Sciences, University of Amsterdam, Science Park 904, Amsterdam, The Netherlands. E-mail: g.rothenberg@uva.nl; Web: http://hims.uva.nl/hcsc
First published on 2nd May 2013
We study the adsorption equilibrium isotherms and differential heats of adsorption of hexane isomers on the zeolitic imidazolate framework ZIF-8. The studies are carried out at 373 K using a manometric set-up combined with a micro-calorimeter. We see that the Langmuir model describes well the isotherms for all four isomers (n-hexane, 2-methylpentane, 2,2-dimethylbutane and 2,3-dimethylbutane). The linear and mono-branched isomers adsorb well, but 2,2-dimethylbutane is totally excluded. Plotting the differential heat of adsorption against the loading shows an initial plateau for n-hexane and 2-methylpentane. This is followed by a slow rise, indicating adsorbate–adsorbate interactions. For the di-branched isomers the differential heat of adsorption decreases with loading. To gain further insight, we ran molecular simulations using the grand-canonical Monte Carlo approach. Comparing the simulation and the experimental results shows that the ZIF framework model requires blocking of the cages, since 2,2-dimethylbutane cannot fit through the sodalite-type windows. Practically speaking, this means that ZIF-8 is a highly promising candidate for enhancing gasoline octane numbers at 373 K, as it can separate 2,2-dimethylbutane and 2,3-dimethylbutane from 2-methylpentane. Our results prove the potential of ZIF-8 as a new adsorbent that can be employed in the upgrade of the Total Isomerization Process for the production of high octane number gasoline, by blending di-branched alkanes in the gasoline.
Thus, the molecular separation of hexane isomers is a key step in gasoline enrichment and octane number enhancement. Traditionally, this is an area where both zeolites and metal–organic frameworks (MOFs) are well known.5–7 Especially zeolites are extensively studied in this context, due to their well-defined pores, the diameters of which are close to those of alkane molecules.7 The most studied structures for separating hexane isomers are ZSM-5,8–11 silicalite-1,11–22 zeolite beta,23–29 and mordenite.30–32 Compared to this, there are relatively few studies on applying MOFs for separating hexane isomers, mainly using UiO-66(zr),33 Zn(BDC)(DABCO)0.5,34,35 MIL-47,36 Cu(hfipbb)(H2hfipbb)0.5,37 and Zn(BDC)(4,4′-Bipy)0.5 (MOF-508).38
The problem is that cleanly separating fuel-grade hexanes remains a challenge, particularly where the cut between monobranched and dibranched isomers is concerned. Here we report the discovery that zeolitic imidazolate framework 8 (ZIF-8) can separate even 2,2-dimethylbutane from 2,3-dimethylbutane. Using a manometric set-up combined with a micro-calorimeter, we show that the sieving radius of ZIF-8 is between 5.8 and 6.3 Å. This value is much higher than the usually reported 5.3 Å.39 To the best of our knowledge, this is the first report that compares the adsorption isotherms of all four isomers. Moreover, we present the differential heats of adsorption of the four components, emphasizing the higher affinity of ZIF-8 for the linear and monobranched isomers, which was also translated by higher adsorption enthalpies. The gate opening effect, due to the flipping of the imidazole linkers, has been mimicked in molecular simulations using the well-established blocking strategy, which has the major advantage of being rather simple to implement. As our work deals with the fundamentals of adsorption on ZIFs, we give a short overview to enable all readers to place the results in the right context.
Zeolitic imidazolate frameworks (ZIFs) combine the properties of both zeolites and MOFs, such as high porosity, high surface area, thermal and chemical stability, and tuneable metal clusters and organic linkers.7,40,41 This gives a wide range of highly interesting structures. ZIF-8 has the formula Zn(mim)2 (where mim = 2-methylimidazole) with a sodalite-related type structure41 (see Fig. 1). At 3.4 Å diameter, the six-membered-ring pore windows of ZIF-8 are narrow, but the cages inside (11.4 Å diameter) are much larger.42 Thermogravimetric analyses of ZIF-8 showed a gradual weight-loss of 28.3% when the temperature was increased from 298 to 723 K, corresponding to the loss of guest species, then the structure was stable up to a temperature of 823 K,41 identical to the temperatures at which the structural collapse of zeolites takes place.43
Fig. 1 Optimised three-dimensional views of the ZIF-8 structure. Nitrogen, carbon and hydrogen atoms are shown in blue, light grey and white, respectively: (a) viewed along axis [001], (b) viewed along axis [111], (c) ZIF-8 sodalite-type cage, (d) SOD topology, and (e) 6-membered-ring window. |
Several studies addressed the chromatographic separation of hexane isomers using ZIF-8 as a stationary phase. Chang et al.7 reported that branched alkanes eluted much earlier than their linear analogues on a ZIF-8 coated capillary. Moreover, 2,2-dimethylhexane eluted even earlier than hexane and heptane, despite its higher boiling point. In traditional stationary phases such as 5%-phenylmethylpolysiloxane, however, the elution followed the order of boiling points.7 Large-pore MOFs such as MOF-57 and MOF-50838 also showed the traditional elution sequence. Even though the diameter of the ZIF-8 pore window is only 3.4 Å, the framework is flexible, with no sharp sieving at 3.4 Å (methane can enter the pores).7,42,44 But bulkier branched alkanes (see Fig. 2) cannot pass through the narrow pore windows, giving a shorter retention time.7 Elsewhere, Lu and Hupp45 observed that a ZIF-8 sensor displayed some chemical selectivity for linear n-hexane over the bulkier cyclohexane,45 and Luebbers et al.46 reported that branched alkanes (except i-butane) were excluded from the pores. Thus, 2-methylbutane eluted very quickly, with practically no retention compared to n-pentane. Similar results were seen for 2-methylheptane.46 More recently, Peralta et al.39 reported the separation of linear and branched hexane isomers by breakthrough experiments of binary mixtures in three different ZIF materials. They concluded that ZIF-8 acts as a molecular sieve: linear alkanes diffuse freely into the pores, while mono-branched alkanes are adsorbed under strong diffusional limitation, and di-branched alkanes are excluded from the pores.39 They also concluded that the effective pore size of ZIF-8 is comparable to the kinetic diameter of mono-branched alkanes, 5.3 Å. Since this is higher than the formal pore size of 3.4 Å, it means that the flexibility of the pore aperture in ZIF-8 is much higher than anticipated.39
Fig. 2 Three-dimensional views and kinetic diameters of hexane isomers: (a) n-hexane, (b) 2-methylpentane, (c) 2,3-dimethylbutane and (d) 2,2-dimethylbutane. Carbon and hydrogen atoms are in light grey and dark grey, respectively. The optimized structures were generated using ChemBio3D Ultra (CambridgeSoft). The kinetic diameters were reported by Gobin et al.53 |
Several molecular simulation studies address the adsorption properties of ZIFs for separating CH4, CO2, H2, and N2 binary mixtures,47–49 as well as ethane–ethene mixtures.50,51 The method used for methane adsorption on ZIF-8 reproduces well the experimental results, with slight overestimations.48,52
The above studies highlight the theoretical potential of ZIF-8 for separating hexane isomers. However, to prove this potential, we must obtain complete information on the adsorption equilibrium properties. To do this, we measured here the adsorption isotherms and differential heats of adsorption of n-hexane (n-C6), 2-methylpentane (2MP), 2,3-dimethylbutane (23DMB) and 2,2-dimethylbutane (22DMB) at 373 K. The experimental isotherms were modelled using the Langmuir model. Additionally, Henry's constants were calculated for the four components. The results prove the potential of ZIF-8 as a new adsorbent in the Total Isomerization Process for the production of high octane number gasoline, by blending di-branched alkanes in the gasoline.
Structure optimisations for Fig. 1 and 2 were generated using the ChemBio3D Ultra software (CambridgeSoft). Monte-Carlo simulations were carried out using the open source package RASPA 1.0 developed by Dubbeldam et al.55 This code was successfully applied in a large number of simulation studies.56–59 The simulations were performed using a 2 × 2 × 2 supercell of dimensions 33.986 × 33.986 × 33.986 Å3, typically using two million Monte Carlo steps.
Fig. 3 Schematic and photograph of the experimental set-up, showing the micro-calorimeter in a thermostatic oven, the manometric introduction system, and the vacuum turbo-molecular pump. Photographs of detailed parts of equipment are presented in the ESI† – Fig. S4. |
(1) |
In the Henry's law region (low pressures) adsorption loading is directly proportional to the pressure (eqn (2), where KH denotes the Henry constant).
q = KHp | (2) |
Since the Langmuir model is thermodynamically consistent, we obtain eqn (3):
KH = nasatk | (3) |
All non-bonded interactions are then described with the Lennard-Jones (LJ) 12–6 potential (eqn (4)),
(4) |
(5) |
Tables 1 and 2 give the forcefield parameters. The LJ parameters of the ZIF-8 framework were taken from the UFF forcefield. Potential parameters for the adsorbates were taken from the TraPPE UA forcefield. Details of the force fields are found in Rappe et al.,60 Martin and Siepmann,61 and Calero et al.62 We use a truncated and shifted potential (cut-off radius = 12 Å) without tail corrections. Since the adsorbate molecules are non-polar, electrostatic interactions were ignored. Guo et al. simulated the adsorption of methane/hydrogen on ZIFs using the same set of parameters.47
σ(Å) | ε/kB (K) | |
---|---|---|
United atom | ||
CH3-sp3 | 3.76 | 108.00 |
CH2-sp3 | 3.96 | 56.00 |
CH-sp3 | 4.68 | 17.00 |
C-sp3 | 0.80 | 6.38 |
Framework | ||
C | 3.43 | 52.84 |
H | 2.57 | 22.14 |
N | 3.26 | 34.72 |
O | 3.12 | 30.19 |
Zn | 2.46 | 62.40 |
Bond | |||||
Bend | |||||
Torsion | |||||
η 0 | η 1 | η 2 | η 3 | η 4 | η 5 |
1204.654 | 1947.740 | −357.854 | −1944.666 | 715.690 | −1565.572 |
Fig. 4 N2 adsorption isotherm on ZIF-8 at 77 K. |
Applying the BET equation to the data gives a surface area value of 1800 m2 g−1. This value affirms the manufacturer's specification of 1300–1800 m2 g−1.54 It also confirms that the activation procedure has regenerated the sample to its original state, removing any component (e.g. water) from the pores.
Fig. 5 Hexane isomer adsorption isotherms, on ZIF-8, at 373 K: △: n-hexane, ○: 2-methylpentane, □: 2,3-dimethylbutane, and ⋄: 2,2-dimethylbutane. The curves show the Langmuir model fit for each isotherm, calculated using eqn (1). |
Fig. 5 shows that ZIF-8 adsorbs n-hexane selectively. However, 2-methylpentane and 2,3-dimethylbutane present similar adsorption saturation loading, but with a lower Langmuir equilibrium constant (about half for the mono-branched isomer and twenty times lower for the di-branched isomer). Thus, we consider that the maximum capacity of the ZIF-8 cages is around ∼2.7 mmol g−1 (7.37 molecules (u.c.)−1). The 2,2-dimethylbutane is almost excluded. Langmuir's model describes well the isotherms for all four isomers (see parameters in Table 3). Plotting the differential heat of adsorption against the loading (see Fig. 6) gives an initial plateau for n-hexane at around 42–44 kJ mol−1. This plateau is followed by a slow rise until about 60 kJ mol−1. We attribute this to increasing adsorbate–adsorbate interactions.
Parameters | ||
---|---|---|
n asat* [mmol g−1] | K [kPa−1] | |
n-Hexane | 2.70 | 0.40 |
2-Methylpentane | 2.57 | 0.20 |
2,3-Dimethylbutane | 2.72 | 0.02 |
2,2-Dimethylbutane | 0.13 | 0.09 |
Fig. 6 Differential heats of adsorption for hexane isomers on ZIF-8 at 373 K: △: n-hexane, ○: 2-methylpentane, □: 2,3-dimethylbutane, and ⋄: 2,2-dimethylbutane. |
The differential heats of adsorption of 2-methylpentane present similar trends to n-hexane. For the di-branched isomers the differential heat of adsorption decreases with loading. This decrease is sharper for the di-branched isomer. Overall, ZIF-8 shows a promising behaviour at 373 K for gasoline octane number enhancement, since it presents selectivity between 2-methylpentane and 2,3-dimethylbutane.
K H [mmol g−1 kPa−1] | n asat k [mmol g−1 kPa−1] | |
---|---|---|
n-Hexane | 0.56 | 1.07 |
2-Methylpentane | 0.26 | 0.51 |
2,3-Dimethylbutane | 0.06 | 0.06 |
2,2-Dimethylbutane | 0.005 | 0.011 |
As Fig. 7a shows, if no blocking is used, all isomers quickly saturate the adsorbent. Almost no selectivity is observed. A zoom-in of the region with pressures up to 20 kPa is included in the ESI† (Fig. S2). The maximum saturation is about 2.9 mmol g−1, corresponding to eight molecules per unit cell. When appropriate blocking is applied, the simulations correctly reproduce the experimental isotherms (see Fig. S3 ESI†). The blocking strategy comprises rejection of all Monte Carlo moves that attempt to insert a molecule inside a certain radius. Fig. 8 shows a snapshot of the maximum adsorption loading of n-hexane in ZIF-8. The most favourable adsorption site for the n-hexane molecules is inside the cages. No adsorption occurs at the windows.
Fig. 7 Molecular simulation of the hexane isomer adsorption isotherms on ZIF-8 at 373 K: (a) without blocking and (b) with blocking. (△: n-hexane, ○: 2-methylpentane, □: 2,3-dimethylbutane, and ⋄: 2,2-dimethylbutane). |
Fig. 8 Snapshot of adsorbed n-hexane in one ZIF-8 unit cell at 102 kPa. |
Peralta et al.39 concluded, by means of binary vapour phase breakthrough experiments of n-hexane and 3-methylpentane, that the mono-branched isomer has a slow diffusion (three orders of magnitude lower than the n-hexane) and is also thermodynamically less favoured, with adsorption constant half of the linear isomer. The parameters obtained by fitting our data to a Langmuir isotherm model give an adsorption constant for n-hexane of 0.40 kPa−1 and for 2-methylpentane of 0.20 kPa−1. These values are in good agreement with the conclusions of Peralta et al.39 Although we haven't performed up-take experiments to assess the diffusion constants, when we compare the equilibration times required for both isomers (see Tables in the ESI†) we see that on average the 2-methylpentane takes about four times longer per point than the n-hexane. We attribute this to the slower diffusion of the mono-branched isomer.
The effective adsorption of 2,3-dimethylbutane and exclusion of 2,2-dimethylbutane prove that the effective pore size of ZIF-8 is about 5.8 Å, up to a maximum of 6.3 Å. This is at least half an Ångström higher than the value presented by Peralta et al.,39 and almost double of the reported window size (3.4 Å).
Table 4 shows that the Langmuir model over-predicts the adsorption capacity at very low pressures for the n-hexane and 2-methylpentane. This is due to an inflection on the isotherm of those two isomers that happens at about 1 kPa (see Fig. S1 in ESI†). This might be due to a first step of adsorption in the cage windows at the surface of the crystals, since the inflection occurs at about 0.25–0.5 mmol g−1, which is similar to the total amount of 2,2-dimethylbutane adsorbed that can be attributed to surface adsorption. Practically speaking, the Langmuir model represents well the adsorption equilibrium of the four isomers, since for pressures above 2 kPa the calculated adsorbed amount is close to the experimental value (note that in the regeneration step of the Total Isomerization Process, such low partial pressures are probably irrelevant).
The very low (yet still measurable) adsorption capacity of 2,2-dimethylbutane is attributed to “outer surface” adsorption. These molecules are too bulky to enter the cages, but it is possible that the ethyl group “anchors” in the window, leading to a certain amount of “external surface” adsorption that will depend on the crystallite size.
As we explained above, when no blocking is applied, the simulations show a slightly lower capacity for the linear isomer. The more compact 2,2-dimethylbutane presents the highest adsorption capacity. This is easily understood as the ZIF-8 cages are big when compared with the adsorbate molecules, and therefore, the packing effects (entropic) favour the di-branched isomers. This kind of effect was already observed in large-pore zeolites, such as mordenite.64 Nevertheless, we reasoned from experimental vs. simulation results that the adsorption behaviour of hexane isomers in ZIF-8 is not controlled by the cage accessible volume, but by the windows size and its “gate-opening” effect. This “gate-opening” effect is characteristic of some ZIF materials, as reported by van den Bergh et al.65,66 To reproduce the experimental results by CBMC simulations, we designed an appropriate blocking strategy. Blocking strategies are well known and simple to be employed. We obtained a good agreement between experimental and simulation results (see Fig. S3, ESI†). Note that for simulation of the n-hexane isotherm no blocking was necessary. This might be because of the smaller kinetic diameter of n-hexane, therefore the gate-opening effect requires very low pressures for the n-hexane or even its adsorption is not affected by the window size at all.
ΔHads | Heat of adsorption |
k | Adsorption equilibrium constant |
K 1, Kθ | Constants related to the bonded interactions: bond stretching and bond bending, respectively |
K B | Boltzmann's constant |
K H | Henry constant |
n a | Loading (adsorbate concentration in the particles) |
p | Pressure (adsorbate concentration in the gas phase) |
r | Bond length |
T | Temperature |
U | Potential energy related to the bond, bend and torsion potentials, respectively |
V | Volume |
ε | Characteristic energy in LJ potential |
ϕ | Torsion angle |
η | Constants related to torsional configurations |
μ | Chemical potential |
θ | Bending angle |
σ | Characteristic distance in LJ potential |
ads | adsorption |
sat | saturation |
bend | Related to bending potential |
bond | Related to bonding potential |
torsion | Related to torsion potential |
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c3cp44381g |
This journal is © the Owner Societies 2013 |