Paraffin Mixtures with ZIF-4 - Langmuir

Oct 21, 2015 - Currently, olefin/paraffin mixtures are separated by low-temperature rectification, which is very energy- and cost-intensive. ..... The...
6 downloads 6 Views 1MB Size
Subscriber access provided by Mount Allison University | Libraries and Archives

Article

Adsorptive Separation of Olefin/Paraffin Mixtures with ZIF-4 Martin Hartmann, Ulrike Böhme, Max Hovestadt, and Carolin Paula Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.5b02907 • Publication Date (Web): 21 Oct 2015 Downloaded from http://pubs.acs.org on October 27, 2015

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Langmuir is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 22

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

Langmuir

submitted to Langmuir “Adsorptive Separation of Olefin/Paraffin Mixtures with ZIF-4” Martin Hartmann*, Ulrike Böhme, Max Hovestadt, Carolin Paula Erlangen Catalysis Resource Center (ECRC), Friedrich-Alexander-Universität ErlangenNürnberg, Egerlandstr. 3, 91058 Erlangen, Germany

*Corresponding Author: Prof. Dr. Martin Hartmann E-Mail: [email protected] Tel.: +49-9131-8528792

Abstract The microporous zeolitic imidazolate framework ZIF-4 has been synthesized and its ethylene/ethane as well as propylene/propane separation potential has been evaluated by single-component adsorption isotherms and break-through experiments of the respective binary mixtures. In all experiments, a higher selectivity for the paraffin is observed which is manifested by a steeper equilibrium isotherm as well as a later breakthrough in the fixed-bed adsorber experiments. Microporous adsorbents with paraffin selectivity are rare, but highly interesting for cyclic adsorption processes such pressureswing adsorption (PSA).

1 ACS Paragon Plus Environment

Langmuir

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

Page 2 of 22

Introduction Short-chain olefins are produced from naphtha in a process called steam-cracking. A large part of the produced olefins are used for the production of polypropylene and polyethylene. Thus, the purity of the corresponding monomers ethylene (ethene) and propylene (propene) has to exceed 99.5 wt.-%. Typical steam-cracker product mixtures contain 29.1 wt.-% of ethene, 16.1 wt.-% of propene, 3.9 wt.-% of ethane and 1.2 wt.-% of propane (T = 850 °C, tresidence= 0.2 sec.). Currently, olefin/paraffin mixtures are separated by low-temperature rectification, which is very energy and cost intensive. As an alternative, adsorptive separation employing membranes or fixed-bed adsorbers is considered. The potential of porous coordination polymers (Metal Organic Frameworks, short MOF) as novel adsorbents has been intensively studied in recent years 1-25. A large number of studies is devoted to zeolitic imidazolate frameworks (ZIFs)

7-17

and other

metal carboxylates including CPO-27 17-20 and MIL-53 21-25. ZIFs are porous, crystalline frameworks in which the tetrahedral metal ions are linked by imidazolate (Im) units. ZIF structures are comparable with those of zeolites, as the bridging angle formed by this linkage is analogous to silicon oxygen aluminum units in zeolites. In particular the high thermal and chemical stability of these materials compared to other MOFs is an indispensable prerequisite for their use as economic adsorbents 26. For an efficient purification of the desired olefin, an adsorbent is thought for that has a higher affinity to the corresponding paraffin. Recently, several candidates have been identified in the subfamily of ZIFs, who indeed exhibit a preferential paraffin adsorption over the corresponding olefin. In particular ZIF-8 has been extensively tested for olefin/paraffin

separation

employing

ethane/ethene,

propane/propene,

n-butane/1-butene and n-Octane/1-Octene mixtures. In all studies conducted so far, a higher paraffin selectivity has been observed.

6, 9-12, 17, 27-28

A recent grand canonical

Monte Carlo calculation on a number of ZIFs confirmed the experimentally-observed paraffin-selectivity for ZIF-8. 29 ZIF-4 is constructed from unsubstituted imidazole linkers and Zn2+ cations and crystallizes in the orthorhombic space group Pbca. The cag topology (Figure 1) of ZIF-4, 2 ACS Paragon Plus Environment

Page 3 of 22

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

Langmuir

however, is unknown among zeolites. The diameter of the largest pore and the largest cage correspond to dag = 0.20 nm and dPh = 0.49 nm, respectively 30. Studies of ZIF-4 up till now deal with the synthesis, stability and transformation into other nets (ZIF-zni) 3137.

ZIF-4 is believed to possess some framework flexibility as evident from NMR and XRD

measurements 38 as well as Monte Carlo Simulations 39. The adsorption of hydrogen H2 40

and the separation of ternary mixtures (H2, N2, CO2 and CH4) 41 has been evaluated. In

combination with ionic liquids, ZIF-4 has been investigated for the extraction of pharmaceuticals

42.

The maximal pore opening is smaller than the critical kinetic

diameter of ethane, ethene, propane and propene, which ranges from 0.43 to 0.51 nm 43. However, due to the known structural flexibility of ZIF-4, it is expected that afore mentioned gas are nevertheless able to access the pore system.

Figure 1: Cage structure of ZIF-4 44.

In order to evaluate the potential of ZIF-4 for olefin/paraffin separation, single component isotherms of the C2 and C3 olefins and paraffins as well as breakthrough experiments with ethane/ethene and propane/propene mixtures are performed. ZIF-4 exhibits a clear paraffin selectivity with separation factors around two, which renders this material an interesting candidate for pressure swing adsorption (PSA) experiments.

1

Materials and Methods 1.1 Solvothermal Synthesis of ZIF-4

The synthesis of ZIF-4 was performed according to the procedure published by Park et al. 26 with some modifications developed in our laboratory. In a typical synthesis, 3.641 g (12.24 mmol) of zinc nitrate hexahydrate and 2.400 g (35.25 mmol) of imidazole were 3 ACS Paragon Plus Environment

Langmuir

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

dissolved in 240 ml of dimethylformamide in a 500 ml round bottom flask. The round bottom flask was connected to a reflux condenser, heated to 130 °C using an oil bath and kept for 48 h at this temperature. Thereafter, the synthesis mixture was cooled to room temperature and the precipitated solid was recovered by filtration. The obtained solid was washed three times with 35 ml of ethanol. The resulting white powder (particle size up to 100 µm) was dried in a vacuum oven at 80 °C for 24 h and subsequently activated for 5 h at 150 °C under reduced pressure (10-4 hPa). Powder X-ray diffraction pattern were recorded on a PANalytical X’Pert diffractometer (Cu K α (λ = 1.54 Å)) in the 2θ range of 5 to 50 ° with a step size of 0.02 ° and step length of 1 sec/0.02 °. The obtained powder patterns are compared to the reference ZIF-4 from the Cambridge Structural Database. In order to determine the morphology and the particle size, scanning electron micrographs were collected with a GEMINI® electron microscope (Fa. Carl Zeiss) employing a 3 mm slit and a SE2 detector using an acceleration voltage of 3 and 5 kV. Nitrogen and argon adsorption isotherms were recorded at 77 K employing an ASAP 2010 instrument (Micromeritics).

1.2 Adsorption isotherms and Breakthrough Experiments Single component adsorption isotherms of ethane, ethene, propane and propene were measured volumetrically at 20 °C in the pressure range up to 1 bar employing a modified Micromeritics ASAP 2000 instrument. The equilibrium was reached within 10 minutes; no kinetic limitation was observed. Breakthrough curves of ethane/ethene and propane/propene mixtures were collected in a home-build flow-type apparatus with fixed-bed adsorber. About 450 mg of adsorbent is filled into a stainless steel HPLC column with a length of 125 mm and an inner diameter of 3 mm. The gas composition at the adsorber outlet is determined with a Bruker GC 450 gaschromatograph equipped with an FID analyzer. The experiments were conducted at 20 °C employing a feed volume flow of 0.6 mlN/min. Different feed compositions ranging from (Vethane/Vethene = 0.5/0.5 to 0.1/0.9 and Vpropane/Vpropene = 0.4/0.6, to 0.1/0.9) have been used. For desorption a helium flow of 1 mlN/min was passed over the adsorber bed. After each measurement, the adsorbent was regenerated for 24 h at 100 °C under helium flow.

4 ACS Paragon Plus Environment

Page 4 of 22

Page 5 of 22

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

Langmuir

2

Theory 2.1 Models for describing pure gas adsorption

The Toth model was developed in order to better describe the influence of a heterogeneous surface on the initial slope and the shape of the adsorption isotherm:

 =

  1 +   ⁄

The equation contains the affinity parameter bi for the component i and the parameter of heterogeneity t, which is specific for the adsorbent. In the low pressure range the linear dependence of the surface coating on the partial pressure is described by Henry's law.

 =   The steeper the gradient, the stronger the affinity of the adsorbent. For two components the separation factor αi,j can be calculated from the initial slopes of the isotherms.

, =  ⁄  From the saturation loading qs,i the pore volume qpore can be calculated using the conversion factor Di according to the Gurvich rule.

 =  , Di represents the ratio of liquid density to gas density of the adsorbate.

2.2 Models for the prediction of binary gas adsorption For the determination of binary gas adsorption isotherms the theory of ideal adsorbed solution theory (IAST) by Myers and Prausnitz 45-46 was used. The theory is based on the equilibrium of the chemical potentials of the gas and adsorbed phase

 π  =  

5 ACS Paragon Plus Environment

Langmuir

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

The equilibrium equation contains a fictive partial pressure in the adsorbate phase  , the spreading pressure π , the adsorbate and gas phase composition ( ,  ) and the total

pressure p. For the further calculations, a model must be adapted to the single adsorption data points. The best fit of the experimental single component isotherms was obtained by employing the Toth model. The method and the calculation steps for the mixture isotherms are listed in Table S1 of the supporting information. Moreover the calculated spreading pressures are shown in Figure S2 (cf. Supporting information).

2.3 Selectivity of adsorption The selectivity can be calculated, to assess the separation potential for the paraffin/olefin mixtures. For zero pressure the limit of the selectivity is the separation factor. 47-48

, =

 ⁄ ℎ lim , =  ⁄  →  ⁄

2.4 Breakthrough curves Assuming constant flow velocity and quasi-isothermal behavior of the adsorber bed, binary breakthrough curves follow the mass balance equation including the equilibrium isotherm  = $% , % .

&% 1 − ) & &% +' * ++ =0 & & &, ) Thereby, the typical form, with overshoot, feed plateau and intermediate plateau in the desorption line, can be described. Based on the mass balance multi component isotherms can be calculated for simple linear adsorption models, viz. the Langmuir model. Therefore the so called hodographic transformation can be adduced 49-50. In this approach, the concentration value pairs of the two components are plotted against each other and the occurring linear dependence is used for further calculations. Nonlinear systems differ from this linear behavior. In this work the deviation of the concept of an ideal surface was already considered by using the Toth model, with its heterogeneity factor. By changing the feed concentration of the experiment, the non-ideality with

6 ACS Paragon Plus Environment

Page 6 of 22

Page 7 of 22

respect to diffusive effects, can be confirmed furthermore using the hodographic transformation.

3

Results and Discussion 3.1 Characterization

The powder X-ray diffraction pattern of ZIF-4 Figure 2 (left) is in excellent agreement with the simulated reference pattern from the Cambridge Structural Database

51.

The

absence of additional reflections confirms that phase pure ZIF-4 material is obtained. Scanning electron microscopy (Figure 2 right) shows orthorhombic crystals up to a size of 100 µm.

Intensity/ (arb. units)

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

Langmuir

ZIF-4 as synthesized

ZIF-4 calculated 5

10

15

20

25

30

35

40

45

50

2 Theta / °

Figure 2: Powder X-ray diffraction pattern (left) and electron micrographs (right) of ZIF-4.

While the textural properties of ZIF-4 are not addressed in the original paper by Park et al.26, nitrogen adsorption studies by Bennet and co-workers 31-32 revealed a BET surface area of 300 m²/g. However, the shape of the isotherm is very peculiar (Figure S1), exhibiting almost no uptake of nitrogen below p/p0 = 0. 4 followed by a sharp rise at higher p/p0. Recent results by Wharmby et al.52 showed that ZIF-4 transforms into a dense phase below 140 K accompanied by a contraction of the unit cell volume of ca. 23 %. Because nitrogen adsorption at 77 K onto ZIF-4 is seemingly hindered, studies concerning hydrocarbon adsorption in ZIF-4 are rare up till now.

3.2 Single component adsorption isotherms

7 ACS Paragon Plus Environment

Langmuir

The single component adsorption isotherms of ethane, ethene, propane and propene are displayed in Figure 3. All isotherms show a Langmuir-type behavior with different affinity. The isotherms are fully reversible and thus for sake of clarity only the adsorption branch is displayed. The initial slope of the ethane isotherm is larger than that of the ethene isotherm and lies over the whole investigated pressure range above the ethene isotherm. The propane and propene isotherm are very close to each other and, thus, the separation factor calculated from the initial slope of the respective isotherms

is

small

(αpropane/propene = 1.06).

In

contrast,

a

separation

factor

αethane/ethene= 1.71 is calculated for the C2 fraction, which confirms a significantly higher paraffin affinity.

2.5 Gas Uptake / mmol g-1

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

Page 8 of 22

2.0 1.5 1.0

Ethane Ethene Propane Propene

0.5 0.0 0.0

0.2

0.4 0.6 Pressure / bar

0.8

1.0

Figure 3: Measured single component adsorption isotherms at 20 °C of ethane, ethene, propane and propene.

The maximum amount adsorbed is close for all gases studied and amounts to 2.30 and Assuming 2.20 mmol/g for ethane and ethene respectively and 2.4 mmol/g for propane and propene. The single component isotherms of the C3 hydrocarbons have a steeper initial slope compared to the corresponding isotherms of ethane and ethene, which is ascribed to the increased binding strength with rising chain length of the hydrocarbons. Similar observations are reported for ZIF-8 27-28. The Toth model was used to fit the measured single component adsorption isotherms of ethane and ethene (Figure 4 left) and propane and propene (Figure 5 left). Employing the model the isotherms were expanded up to 12 bar for the C2 fraction (Figure 4 right) and up to 8 bar for the C3 fraction (Figure 5 right).

8 ACS Paragon Plus Environment

Page 9 of 22

2.5 Gas Uptake / mmol g-1

Gas Uptake / mmol g-1

2.5 2.0 1.5 qsingle, ethane (Toth model)

1.0

qsingle, ethene (Toth model) qsingle, ethane

0.5

2.0 1.5 1.0 qsingle, ethane (Toth model)

0.5

qsingle, ethene (Toth model)

qsingle, ethene 0.0 0.0

0.2

0.4 0.6 Pressure / bar

0.8

0.0 0

1.0

2

4

6 8 Pressure / bar

10

12

Figure 4: Adsorption isotherms at 20 °C of ethane and ethene (< 1 bar) fitted with Toth model (left) and expanded to 12 bar (right).

For ethane, the Toth model reaches the saturation at lower pressures compared to ethene.

Gas Uptake / mmol g-1

2.5 2.0 1.5 qsingle, propane (Toth model)

1.0

qsingle, propene (Toth model) qsingle, propane

0.5

qsingle, propene 0.0 0.0

0.2

0.4 0.6 Pressure / bar

0.8

1.0

2.5 Gas Uptake / mmol g-1

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

Langmuir

2.0 1.5 1.0 qsingle, propane (Toth model)

0.5

qsingle, propene (Toth model) 0.0 0

2

4 Pressure / bar

6

8

Figure 5: Adsorption isotherms at 20 °C of propane and propene (< 1 bar) fitted with Toth model (top) and expanded up to 8 bar (bottom).

9 ACS Paragon Plus Environment

Langmuir

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

Page 10 of 22

The model isotherms show a slightly lower capacity for propane then for propene. From a pressure of 0.4 bar onwards, the isotherm of propene lies above the isotherm of propane. Employing the Gurvich rule, the accessible pore volumes can be estimated. In Table 1 the results from nitrogen and argon measurements at 77 K are compared with the results from the ethane, ethene, propane and propene measurements at 20 °C up to 1 bar and the calculated Toth isotherms up to 12 bar (C2 fraction) and 8 bar (C3 fraction). The obtained pore volumes are in nice agreement with the pore volumes determined from the crystal structure (0.197 cm3/g) and the paper by Bennet et al. (ca. 0.4 cm3/g).31 Table 1: Pore volume calculated from nitrogen and argon measurements at 77 K and from ethane, ethene, propane and propene measurements at 20 °C

gas V p/ cm³g-1

N2

Ar

0.38

0.32

C2H6

C2H 4

C3H8

C3H6

1 bar/ 12 bar

1 bar/ 12 bar

1 bar/ 8 bar

1 bar/ 8 bar

0.20/ 0.21

0.18/ 0.19

0.212/ 0.214

0.199/ 0.200

3.3 Binary adsorption Employing the ideal adsorbed solution theory (IAST)

45-46,

mixture isotherms at 20 °C

were calculated for an equimolar ethane/ethene mixture up to 12 bar and for an equimolar propane/propene mixture up to 8 bar. The partial mixture isotherms are displayed in Figure 6. In total, the mixture contains about twice as much ethane as ethene. This follows from the stronger affinity and higher capacity of ZIF-4 for the paraffin and is in good agreement with the results of the single component adsorption measurement. For the C3 mixture, initially (up to 1 bar) a higher uptake of propane is observed. At higher pressure, the propene uptake from the mixture is slightly higher than the propane uptake. The reason for this behavior is at present unclear and has to be confirmed by experiment.

10 ACS Paragon Plus Environment

Page 11 of 22

2.5 Gas Uptake / mmol g-1

Gas Uptake / mmol g-1

2.5 2.0 1.5 1.0 qmixed, ethane (Toth model)

0.5

2.0 1.5 1.0 qmixed, propane (Toth model)

0.5

qmixed, ethene (Toth model) 0.0 0

2

4

6 8 Pressure / bar

10

qmixed, propene (Toth model) 0.0 0

12

2

4 Pressure / bar

6

8

Figure 6: Mixture isotherms at 20 °C of an equimolar ethane/ethene (left) and propane/propene mixture (right) calculated employing the IAST method.

Employing the IAST method the adsorbate and gas phase composition were determined, which allows calculation of the selectivities as function of the total pressure. For the feed composition of y = 0.5 the selectivities are shown in dependency of the total pressure in Figure 7. For the C2 fraction the selectivity decreases with increasing pressure. The C3 fraction decreases to a constant value of ca. 0.9 after an initial rise, which is good agreement with theoretical calculations.29

2.5

2.5 Selectivity αpropane,propene / -

Selectivity αethane,ethene / -

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

Langmuir

2.0 1.5 1.0 0.5 Selectivity (y=0.5) 0.0 0

Figure

2

4

6 8 Pressure / bar

7: Pressure dependent

10

1.5 1.0 0.5 Selectivity (y=0.5) 0.0 0

12

selectivity

2.0

of

2

an equimolar

4 6 Pressure / bar

8

10

ethane/ethene

(left)

and

propane/propene mixture (right), calculated via IAST for 20 °C.

3.4 Breakthrough Curves in a Fixed-Bed Adsorber The breakthrough curves of the ethane/ethene and propane/propene mixtures at 20 °C and for Vparaffin/Volefin = 0.1/0.9 are shown in Figure 8. The fixed-bed was loaded in the adsorption step until saturation was reached. The bed was regenerated in subsequent desorption with helium. The breakthrough curves are plotted in the form of the relative 11 ACS Paragon Plus Environment

Langmuir

concentration c/c0 as a function of time, with the feed concentration c0. At saturation, the concentration equals the feed composition and thus c/c0 = 1.

Rrelative concentration c/c0 / -

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

Page 12 of 22

1.2 Ethane (0.1) Ethene (0.9) Propane (0.1) Propene (0.9)

0.8

0.4

ZIF-4 0.0 10 20 30 40 50 60 70 80

200

400

600

800

Time / min Figure 8: Adsorption and desorption profile of the breakthrough curves from an ethane/ethenemixture and a propane/propene-mixture at 20 °C with a total volume flow of ./ = 1.0 mlN/min on mAds = 0.450 g with Vparaffin/Volefin = 0.1/0.9.

The breakthrough experiments show a favorable higher affinity for the paraffin for the C2 and C3 fraction in agreement with the single component adsorption isotherms. The weaker bound component gets displaced resulting in the typical overshoot. Ethene has a slightly shorter breakthrough time and shows a higher overshoot than propene. Both curves have a similar steep initial slope. Compared to ethane, the curve of propane breaks through earlier, is flatter and reaches saturation later. The breakthrough times of ethene and ethane are t’ethene = 25 min and t’ethane = 33 min. This results in a timeframe of ca. 8 minutes under the prevailing conditions in which a separation is technically possible. Propane and propene both have a breakthrough time of approximately t’ = 30 min. Thus a separation is impossible under these circumstances. The result of the separation potential for ethane/ethene and propane/propene mixtures matches the separation factor obtained from the pure component isotherms. A separation of propane and propene was not expected from the low separation factor of αpropane/propene = 1.06. Attributable to the kinetics and steric hindrance the propane gets more retained and causes the overshoot of the propene. The higher affinity for the ethane leads to a longer regeneration time. Hence the ethane desorption curve lies above the ethene curve, nevertheless the curves of propane and propene are superimposed.

12 ACS Paragon Plus Environment

Page 13 of 22

In Figure 9 (left) the adsorption profile of the breakthrough curves from the ethane/ethene mixtures with different composition is given. The results for different feed compositions are compared at 20 °C. The dependence of the course of the step of desorption is shown in the detail view (Figure 9 (right)). 2.0

0.10

Ethane (0.1) Ethane (0.2) Ethane (0.3) Ethane (0.4) Ethane (0.5)

1.8 1.6 1.4

Rrelative concentration c/c0 / -

Rrelative concentration c/c0 / -

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

Langmuir

Ethene (0.9) Ethene (0.8) Ethene (0.7) Ethene (0.6) Ethene (0.5)

0.08

1.2

Ethene (0.9) Ethene (0.8) Ethene (0.7) Ethene (0.6) Ethene (0.5)

0.06

1.0 0.8

0.04

0.6 0.4

ZIF-4

0.02

0.2 0.0

Ethane (0.1) Ethane (0.2) Ethane (0.3) Ethane (0.4) Ethane (0.5)

ZIF-4 0

30

40

50

60

0.00 100

200

300

Time / min

400

500

600

700

800

900

Time / min

Figure 9: Adsorption (left) and desorption profile (right) of the breakthrough curves from ethane/ethene mixtures at 20 °C with a total volume flow of ./ = 1.0 mlN/min and mAds = 0.450 g.

The ethane is more restrained and, therefore, the ethylene arrives first at the exit of the column, which is preferred for the separation process. With decreasing concentration of the paraffin its overshoot is lower and broader and shows a slower decay to the feed concentration. The breakthrough time and the initial gradient of the curve front of ethene are independent of the composition. For ethane the initial gradient of the breakthrough curve flattens significantly with decreasing paraffin concentration. The composition dependent behavior leads to increasing influence of diffusion with lower paraffin concentration. The desorption properties are also composition dependent. The paraffin curves all stay above the olefin curves. All curves show an exponential decay and become flatter with decreasing paraffin concentration. For a composition of Vethane/Vethene = 0.2/0.8 and 0.1/0.9 an intermediate plateau starts to appear. For the reason of higher weighted diffusion, the regeneration of the column is finished later for decreasing paraffin concentration. The application of hodograph transformation is well established in liquid chromatography and allows a fast assessment of mass transfer limitations which would result in deviation from linear behavior if present.52 The breakthrough curves of the ethane/ethene mixture were subjected to a hodograph transformation shown in Figure 13 ACS Paragon Plus Environment

Langmuir

10 (left). This results in the three characteristic points for the compositions of the feed with a relative concentration equal to one (Cethane/Cethene(1/1)), the overshoot (y-intercept) and the intermediate plateau (x-intercept). 2.0

0.006

Ethane(0.1)Ethene(0.9) Ethane(0.2)Ethene(0.8) Ethane(0.3)Ethene(0.7) Ethane(0.4)Ethene(0.6) Ethane(0.5)Ethene(0.5) auxiliary line

1.6 1.4 1.2

0.005 0.004

CETHENE

1.8

CETHENE

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

Page 14 of 22

1.0

Ethane(0.1)Ethene(0.9) Ethane(0.2)Ethene(0.8) Ethane(0.3)Ethene(0.7) Ethane(0.4)Ethene(0.6) Ethane(0.5)Ethene(0.5) auxiliary line

0.003

0.8

0.002

0.6

0.001

ZIF-4

0.4 0.2 0.0 0.0

0.000 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09

ZIF-4 0.2

0.4

0.6

0.8

1.0

CETHANE

1.2

CETHANE

Figure 10: Hodograph transformation (right) of the breakthrough curves from ethane/ethene mixtures at 20 °C with a total volume flow of ./ = 1.0 mlN/min on mAds = 0.450 g with Vparaffin/Volefin = 0.5/0.5, 0.4/0.6, 0.3/0.7, 0.2/0.8 and 0.1/0.9 and detailed view (right).

During loading of the adsorber bed, the concentration of the overshoot is reached first. The y-intercept possesses larger values with increasing concentration of ethane. With the breakthrough of the stronger interacting ethane, the concentration of ethene decreases. After completion of the loading, the gas mixture on the outlet of the column shows feed composition (Cethane/Cethene(1/1)). Between the y-intercept and the feed point the pairs of concentration values have a linear correlation. The resulting slope decreases with a lesser amount of ethane. The pairs of values between the feed point and the x-intercept show the regeneration of the column. At the beginning they follow a linear gradient of the same pitch and deviate from this line at low concentrations (seen in detail in Figure 6 (right)). With increasing concentration of ethane, the value pairs differ progressively from the linear gradient. This indicates a reduction in column efficiency

49.

The reason for this is an increasing influence of diffusive effects and

matches the results of the concentration-dependent change in the shape of the breakthrough curves. The same approach was used to study the influence on the separation behavior of ZIF-4 by varying the feed compositions of propane and propene (see SI, Figure S3) confirming that ZIF-4 is less suitable for C3 separation.

4

Conclusions 14 ACS Paragon Plus Environment

Page 15 of 22

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

Langmuir

In the present work, the porous coordination polymer ZIF-4 was synthesized and examined for its potential in the olefin/paraffin separation. Therefore, single component adsorption isotherms of ethane, ethene, propane and propene were measured. Mixture isotherms for an equimolar ethane/ethene mixture were calculated using IAST and the Toth model. In addition, the separation behavior was investigated in breakthrough curves experiments for ethane/ethene and propane/propene mixtures with different feed composition. The results were analyzed for their concentration dependence. The breakthrough curves of the ethane/ethene mixture were subjected to a hodograph transformation. The separation behavior in the breakthrough experiments matches the results of the isotherms. The concentration-dependent influence of the diffusion on the separation is evident from the breakthrough curves and was confirmed by the results of the hodograph transformation. Especially the breakthrough experiments with the C2 fraction showed a sufficiently large time frame in which separation is technically possible. This result and the overall paraffin selectivity makes ZIF-4 an interesting new adsorbent for industrially relevant separation tasks.

Supporting Information Available: Nitrogen Sorption isotherm at 77 K, pressured reduced spreading pressure, overview of the IAST calculation procedure; breakthrough curves of propane/propene mixtures at 20 °C. This material is available free of charge via the Internet at http://pubs.acs.org

Author Information Corresponding author Fax: +49 9131 85-67401. E-mail: [email protected] Notes The authors declare no competing financial interest.

Acknowledgments Financial support of this work by Deutsche Forschungsgemeinschaft (DFG) in the frame of the priority program 1570 (Ha2527/12-2) is gratefully acknowledged. 15 ACS Paragon Plus Environment

Langmuir

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

References (1)

Li, J. R.; Kuppler, R. J.; Zhou, H., Selective gas adsorption and separation in metal-

organic frameworks. Chem. Soc. Rev. 2009, 38, 1477-1504. (2)

Lee, C. Y.; Bae, Y.-S.; Jeong, N. C.; Farha, O. K.; Sarjeant, A. A.; Stern, C. L.; Nickias, P.;

Snurr, R. Q.; Hupp, J. T.; Nguyen, S. T., Kinetic Separation of Propene and Propane in Metal-Organic Frameworks: Controlling Diffusion Rates in Plate-Shaped Crystals via Tuning of Pore Apertures and Crystallite Aspect Ratios. J. Am. Chem. Soc. 2011, 133, 5228-5231. (3)

Lamia, N.; Jorge, M.; Granato, M.; Almeida Paz, F.; Chevreau, H.; Rodrigues, A.,

Adsorption of propane, propylene and isobutane on a metal-organic framework: Molecular simulation and experiment. Chem. Eng. Sci. 2009, 64, 3246-3259. (4)

Ploegmakers, J.; Japip, S.; Nijmeijer, K., Mixed matrix membranes containing MOFs

for ethylene/ethane separation-Part B: Effect of Cu3BTC2 on membrane transport properties. J. Membr. Sci. 2013, 428, 331-340. (5)

Hartmann, M.; Kunz, S.; Himsl, D.; Tangermann, O.; Ernst, S.; Wagener, A.,

Adsorptive Separation of Isobutene and Isobutane on Cu3(BTC)2. Langmuir 2008, 24, 8634-8642. (6)

Chmelik, C.; Kärger, J.; Wiebcke, M.; Caro, J.; van Baten, J. M.; Krishna, R.,

Adsorption and diffusion of alkanes in CuBTC crystals investigated using infra-red microscopy and molecular simulations. Microporous Mesoporous Mater. 2009, 117, 2232. (7)

Gücüyener, C.; van den Bergh, J.; Gascon, J.; Kapteijn, F., Ethane/Ethene

Separation Turned on Its Head: Selective Ethane Adsorption on the Metal-Organic Framework ZIF-7 through a Gate-Opening Mechanism. J. Am. Chem. Soc. 2010, 132, 17704-17706. (8)

Van Den Bergh, J.; Gücüyener, C.; Pidko, E.; Hensen, E. J. M.; Gascon, J.; Kapteijn, F.,

Understanding the anomalous alkane selectivity of ZIF-7 in the separation of light alkane/alkene mixtures. Chem. - Eur. J. 2011, 17, 8832-8840. (9)

Bux, H.; Chmelik, C.; Krishna, R.; Caro, J., Ethene/ethane separation by the MOF

membrane ZIF-8: Molecular correlation of permeation, adsorption, diffusion. J. Membr.

Sci. 2011, 369, 284-289. 16 ACS Paragon Plus Environment

Page 16 of 22

Page 17 of 22

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

Langmuir

(10)

Kwon, H. T.; Jeong, H.-K., Highly propylene-selective supported zeolite-

imidazolate framework (ZIF-8) membranes synthesized by rapid microwave-assisted seeding and secondary growth. Chem. Commun. 2013, 49, 3854-3856. (11)

Askari, M.; Chung, T. S., Natural gas purification and olefin/paraffin separation

using thermal cross-linkable co-polyimide/ZIF-8 mixed matrix membranes. J. Membr.

Sci. 2013, 444, 173-183. (12)

Pan, Y.; Li, T.; Lestari, G.; Lai, Z., Effective separation of propylene/propane binary

mixtures by ZIF-8 membranes. J. Membr. Sci. 2012, 390-391, 93-98. (13)

Pan, Y.; Wang, B.; Lai, Z., Synthesis of ceramic hollow fiber supported zeolitic

imidazolate framework-8 (ZIF-8) membranes with high hydrogen permeability. J.

Membr. Sci. 2012, 421-422, 292-298. (14)

Luebbers, M. T.; Wu, T.; Shen, L.; Masel, R. I., Effects of Molecular Sieving and

Electrostatic Enhancement in the Adsorption of Organic Compounds on the Zeolitic Imidazolate Framework ZIF-8. Langmuir 2010, 26, 15625-15633. (15)

Chmelik, C.; Freude, D.; Bux, H.; Haase, J., Ethene/ethane mixture diffusion in the

MOF sieve ZIF-8 studied by MAS PFG NMR diffusometry. Microporous Mesoporous Mater.

2012, 147, 135-141. (16)

Li, K.; Olson, D. H.; Seidel, J.; Emge, T. J.; Gong, H.; Zeng, H.; Li, J., Zeolitic

Imidazolate Frameworks for Kinetic Separation of Propane and Propene. J. Am. Chem.

Soc. 2009, 131, 10368-10369. (17)

Peralta, D.; Chaplais, G.; Simon-Masseron, A.; Barthelet, K.; Chizallet, C.;

Quoineaud, A. A.; Pirngruber, G. D., Comparison of the behavior of metal-organic frameworks and zeolites for hydrocarbon separations. J. Am. Chem. Soc. 2012, 134, 8115-8126. (18)

Bao, Z.; Alnemrat, S.; Yu, L.; Vasiliev, I.; Ren, Q.; Lu, X.; Deng, S., Adsorption of

ethane, ethylene, propane, and propylene on a magnesium-based metal-organic framework. Langmuir 2011, 27, 13554-13562. (19)

Bloch, E.; Queen, W. L.; Krishna, R.; Zadrozny, J. M.; Brown, C. M.; Long, J. R.,

Hydrocarbon separations in a metal-organic framework with open iron(II) coordination sites. Science 2012, 335, 1606-1610.

17 ACS Paragon Plus Environment

Langmuir

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

(20)

Bae, Y. S.; Lee, C. Y.; Kim, K.; Farha, O. K.; Nickias, P.; Hupp, J. T.; Nguyen, S. T.;

Snurr, R. Q., High propene/propane selectivity in isostructural metal-organic frameworks with high densities of open metal sites. Angew. Chem. Int. Ed. 2012, 51, 1857-1860. (21)

Hsieh, J. O.; Balkus Jr, K. J.; Ferraris, J. P.; Musselman, I. H., MIL-53 frameworks in

mixed-matrix membranes. Microporous Mesoporous Mater. 2014, 196, 165-174. (22)

Rosenbach Jr, N.; Ghoufi, A.; Deroche, I.; Llewellyn, P. L.; Devic, T.; Bourrelly, S.;

Serre, C.; Ferey, G.; Maurin, G., Adsorption of light hydrocarbons in the flexible MIL53(Cr) and rigid MIL-47(V) metal-organic frameworks: a combination of molecular simulations and microcalorimetry/gravimetry measurements. PCCP 2010, 12, 64286437. (23)

Rosenbach, N.; Jobic, H.; Ghoufi, A.; Devic, T.; Koza, M. M.; Ramsahye, N.; Mota, C. J.;

Serre, C.; Maurin, G., Diffusion of light hydrocarbons in the flexible MIL-53(Cr) metalorganic framework: A combination of quasi-elastic neutron scattering experiments and molecular dynamics simulations. J. Phys. Chem. C 2014, 118, 14471-14477. (24)

Trung, T. K.; Trens, P.; Tanchoux, N.; Bourrelly, S.; Llewellyn, P. L.; Loera-Serna, S.;

Serre, C.; Loiseau, T.; Fajula, F.; Férey, G., Hydrocarbon Adsorption in the Flexible Metal Organic Frameworks MIL-53(Al, Cr). J. Am. Chem. Soc. 2008, 130, 16926-16932. (25)

Wagener, A. Adsorptive Trennung von Gemischen kurzkettiger Alkane und

Alkene an nanostrukturierten porösen Adsorbentien, Ph.D. thesis, TU Kaiserslautern, 2011. (26)

Park, K. S.; Ni, Z.; Côté, A. P.; Choi, J. Y.; Huang, R.; Uribe-Romo, F. J.; Chae, H. K.;

O'Keeffe, M.; Yaghi, O. M., Exceptional chemical and thermal stability of zeolitic imidazolate frameworks. Proceedings of the National Academy of Sciences of the United

States of America 2006, 103, 10186-10191. (27)

Böhme, U.; Barth, B.; Paula, C.; Kuhnt, A.; Schwieger, W.; Mundstock, A.; Caro, J.;

Hartmann, M., Ethene/ethane and propene/propane separation via the olefin and paraffin selective metal-organic framework adsorbents CPO-27 and ZIF-8. Langmuir

2013, 29, 8592-8600.

18 ACS Paragon Plus Environment

Page 18 of 22

Page 19 of 22

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

Langmuir

(28)

Böhme, U.; Paula, C.; Reddy Marthala, V. R.; Caro, J.; Hartmann, M., Ungewöhnliche

Adsorptions- und Trenneigenschaften des Molekularsiebs ZIF-8. Chem. Ing. Tech. 2013,

85, 1707-1713. (29)

Wu, Y.; Chen, H.; Liu, D.; Qian, Y.; Xi, H., Adsorption and separation of

ethane/ethylene on ZIFs with various topologies: Combining GCMC simulation with the ideal adsorbed solution theory (IAST). Chem. Eng. Sci. 2015, 124, 144-153. (30)

Phan, A.; Doonan, C. J.; Uribe-Romo, F. J.; Knobler, C.; Okeeffe, M.; Yaghi, O. M.,

Synthesis, structure, and carbon dioxide capture properties of zeolitic imidazolate frameworks. Acc. Chem. Res. 2010, 43, 58-67. (31)

Bennett, T. D.; Cao, S.; Tan, J. C.; Keen, D. A.; Bithell, E. G.; Beldon, P. J.; Friscic, T.;

Cheetham, A. K., Facile Mechanosynthesis of Amorphous Zeolitic Imidazolate Frameworks. J. Am. Chem. Soc. 2011, 133, 14546-14549. (32)

Hughes, J. T.; Bennett, T. D.; Cheetham, A. K.; Navrotsky, A., Thermochemistry of

Zeolitic Imidazolate Frameworks of Varying Porosity. J. Am. Chem. Soc. 2012, 135, 598601. (33)

Bennett, T. D.; Keen, D. A.; Tan, J.-C.; Barney, E. R.; Goodwin, A. L.; Cheetham, A. K.,

Thermal Amorphization of Zeolitic Imidazolate Frameworks. Angew. Chem. Int. Ed.

2011, 50, 3067-3071. (34)

Bennett, T. D.; Saines, P. J.; Keen, D. A.; Tan, J.-C.; Cheetham, A. K., Ball-Milling-

Induced Amorphization of Zeolitic Imidazolate Frameworks (ZIFs) for the Irreversible Trapping of Iodine. Chem. - Eur. J. 2013, 19, 7049-7055. (35)

Bennett, T.; Simoncic, P.; Moggach, S.; Gozzo, F.; MacChi, P.; Keen, D.; Tan, J.;

Cheetham, A. K., Reversible pressure-induced amorphization of a zeolitic imidazolate framework (ZIF-4). Chem. Commun. 2011, 47, 7983-7985. (36)

Bennett, T.; Goodwin, A.; Dove, M.; Keen, D.; Tucker, M.; Barney, E.; Soper, A.;

Bithell, E.; Tan, J.-C.; Cheetham, A., Structure and Properties of an Amorphous MetalOrganic Framework. Phys. Rev. Lett. 2010, 104, 115503. (37)

Tian, Y. Q.; Zhao, Y. M.; Chen, Z. X.; Zhang, G. N.; Weng, L. H.; Zhao, D. Y., Design and

generation of extended zeolitic metal-organic frameworks (ZMOFs): Synthesis and crystal structures of zinc(II) imidazolate polymers with zeolitic topologies. Chem. - Eur. J.

2007, 13, 4146-4154. 19 ACS Paragon Plus Environment

Langmuir

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

(38)

Page 20 of 22

Morris, W.; Stevens, C. J.; Taylor, R.; Dybowski, C.; Yaghi, O. M.; Garcia-Garibay, M.,

NMR and X-ray study revealing the rigidity of zeolitic imidazolate frameworks. Journal of

Physical Chemistry C 2012, 116, 13307-13312. (39)

Beake, E. O. R.; Dove, M. T.; Phillips, A. E.; Keen, D. A.; Tucker, M. G.; Goodwin, A. L.;

Bennett, T. D.; Cheetham, A. K., Flexibility of zeolitic imidazolate framework structures studied by neutron total scattering and the reverse Monte Carlo method. J. Phys.:

Condens. Matter 2013, 25, 395403. (40)

Chen, E. Y.; Liu, Y. C.; Zhou, M.; Zhang, L.; Wang, Q., Effects of structure on

hydrogen adsorption in zeolitic imidazolate frameworks. Chem. Eng. Sci. 2012, 71, 178184. (41)

Battisti, A.; Taioli, S.; Garberoglio, G., Zeolitic imidazolate frameworks for

separation of binary mixtures of CO2, CH4, N2 and H2: A computer simulation investigation. Microporous Mesoporous Mater. 2011, 143, 46-53. (42)

Ge, D.; Lee, H. K., Ionic liquid based dispersive liquid-liquid microextraction

coupled with micro-solid phase extraction of antidepressant drugs from environmental water samples. J. Chromatogr. A 2013, 1317, 217-222. (43)

Schönbucher,

A.,

Thermische

Verfahrenstechnik:

Grundlagen

und

Berechnungsmethoden für Ausrüstungen und Prozesse. Springer Berlin Heidelberg: 2012. (44)

Yaghi, O. M.; Furukawa, H.; Wang, B. Preparation of functionalized zeolitic

frameworks, US Patent US8480792 B2 (2010) assigned to The regents of the University of California. (45)

Myers, A. L.; Prausnitz, J. M., Thermodynamics of mixed-gas adsorption. AlChE J.

1965, 11, 121-127. (46)

Schmidt-Traub, H., Preparative Chromatography of Fine Chemicals and

Pharmaceutical Agents. John Wiley & Sons: 2005. (47)

Talu, O., Measurement and Analysis of Mixture Adsorption Equilibrium in Porous

Solids. Chem. Ing. Tech. 2011, 83, 67-82. (48)

Moellmer, J.; Lange, M.; Moller, A.; Patzschke, C.; Stein, K.; Lassig, D.; Lincke, J.;

Glaser, R.; Krautscheid, H.; Staudt, R., Pure and mixed gas adsorption of CH4 and N2 on the metal-organic framework Basolite[registered sign] A100 and a novel copper-based 1,2,4-triazolyl isophthalate MOF. J. Mater. Chem. 2012, 22, 10274-10286. 20 ACS Paragon Plus Environment

Page 21 of 22

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

Langmuir

(49)

Ma, Z., Simple wave effects in two-component nonlinear liquid chromatography.

Application to the measurement of competitive adsorption isotherms. J. Phys. Chem.

1990, 94, 6911-6922. (50)

Ma, Z.; Guiochon, G., Comparison between the hodograph transform method and

frontal chromatography for the measurement of binary competitive adsorption isotherms. J. Chromatogr. A 1992, 603, 13-25. (51)

Cambridge

Structural

Database.

http://www.ccdc.cam.ac.uk/Solutions/CSDSystem/Pages/CSD.aspx. (51) Wharmby, M.T.; Henke, S.; Bennett, T.D.; Bajpe, S. R.; Schwedler, I.; Thompson, S.P., Gozzo, F.; Simoncic, P.; Mellot-Draznieks, C.; Tao, H.; Yue, Y.; Cheetham, A.K., Extreme Flexibility in a Zeolitic Imidazolate Framework: Porous to Dense Phase Transition in Desolvated ZIF-4, Angew. Chem. Int. Ed. 2015, 54, 6447-6451. [52] Basmadjian, D; Ha, K.D.; Pan, C.Y., Nonisothermal Desorption by Gas Purge of Single Solutes in Fixed-Bed Adsorbers. I. Equilibrium Theory, Ind. Eng. Chem. Process Des. Dev.

1975, 14, 328-340.

21 ACS Paragon Plus Environment

Langmuir

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

TOC graphic

22 ACS Paragon Plus Environment

Page 22 of 22