Paraffin Mixtures with ZIF-4 - Langmuir

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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

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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).

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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

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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

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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.


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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 =


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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


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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)

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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


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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

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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).


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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


0.8

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.


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


0.8

1.0


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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).


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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.


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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

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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

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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.


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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



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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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

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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

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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

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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

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Paraffin Mixtures with ZIF-4 - Langmuir

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