Selective Adsorption Performances of UiO-67 for Separation of Light

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Selective Adsorption Performances of UIO-67 for Separation of Light Hydrocarbons C1/C2/C3 Yufan Zhang, Huiyu Xiao, Xin Zhou, Xun Wang, and Zhong Li Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b01420 • Publication Date (Web): 07 Jul 2017 Downloaded from http://pubs.acs.org on July 11, 2017

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Industrial & Engineering Chemistry Research

Selective Adsorption Performances of UiO-67 for Separation of Light Hydrocarbons C1/C2/C3 Yufan Zhang1, Huiyu Xiao1, Xin Zhou 2*, Xun Wang1, Zhong Li1*

1 School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, PR China 2 State Key Lab of Subtropical Building Science of China, South China University of Technology, Guangzhou 510640, PR China KEYWORDS：： Adsorption isotherm, Isosteric heats, Selectivity, UiO-67, Light Hydrocarbons.

ABSTRACT: In this work, adsorption performances of UiO-67 for light hydrocarbons separation were investigated. UiO-67 with a BET surface area of 2590 m3·g-1 was prepared by hydrothermal method, and then characterized. Isotherms of CH4, C2H6 and C3H8 （C1, C2 and C3）on UiO-67 were measured by volumetric method, and fixed bed experiments were carried out to evaluate dynamic separation performance of UiO-67. The C2H6/CH4, C3H8/CH4 and C3H8/C2H6 adsorption selectivities of the sample were estimated separately on the basis of ideal adsorbed solution theory (IAST), DIH-based equation and breakthrough curves. The results showed that (a) the adsorption capacities of UiO-67 reached separately 3.00 and 8.18 mmol·g-1 for C2H6 and C3H8 at 1 atm and 298 K, while its CH4 adsorption capacity was only 0.45 mmol·g1

; (b) The isotherms of the light hydrocarbons can be well described by dual-site Langmuir-

Freundlich models; (c) Isosteric heats of these light hydrocarbons adsorption followed the order:

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C3H8 > C2H6 > CH4; and (d) Fixed bed experiments showed that the dynamic adsorption selectivity of UiO-67 toward C2H6/CH4 and C3H8/CH4 binary mixtures reached 7.3 and 69.3, respectively. Hence, UiO-67 is a promising adsorbent for the separation of light hydrocarbons.

1. Introduction Separation of light hydrocarbons such as methane, ethane and propane is a very important industrial process because these light hydrocarbons have been widely utilized as energy sources and raw materials. Natural gas（NG）consumption worldwide is over 3.1 trillion cubic meters (110 trillion standard cubic feet) per year, and is by far the largest industrial gas application. NG is a cleaner alternative to other automobile fuels such as gasoline (petrol) and diesel1-6. It is noteworthy that NG mainly consists of light hydrocarbons, such as methane (70-90%), ethane and propane (approximately 0-22%) as well as butane7. Methane is mainly used as clean fuel, while both ethane and propane are very basic raw materials for various industrial and consumer products such as acetic acid, rubber and plastics. Therefore, the recovery of ethane and propane from NG is very important significance and profitable. The traditional cryogenic distillation separation technology, which is based on their different vapor pressures and thus boiling points, can be applied for separation of light hydrocarbons, but it is very energy-consuming. One of the most promising alternative energy- and cost-efficient separation methods is adsorption separation. An adsorbent is the core of the adsorption technology, and its adsorption performance will pay a vital role in separation efficiency8, 9. Therefore, it is desirable to explore new microporous adsorbents which can selectively separate C2-C3 hydrocarbons from methane at room temperature.


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Among a variety of porous materials, Metal-Organic Frameworks (MOFs), constructed from metal node and organic ligands, are attracting vast attention because of their ultrahigh specific area, flexibility and tunable properties, and great superiorities in diverse potential applications. It has been attracting people to explore the potential applications of MOFs for separation of light hydrocarbons. 10-21 Bao et al. synthesized Mg-MOF-7422, a magnesium-based metal organic framework, and examined its adsorptive performance for adsorption of C2/C3. At 1bar and 298K its adsorption capacities towards C2H6 and C3H8 were 6.6 and 7.2 mmol·g-1, respectively. Böhme et al. tested that adsorption performance of Co−CPO-27, and reported that its adsorption capacities at 1bar and 295K for C2H6 and C3H8 were 3.6 and 3.8 mmol·g-1, respectively. Besides, Böhme et al. examined adsorption performances of zeolitic imidazolate framework ZIF-8 for C2H6 and C3H8, and reported that its adsorption capacities at 1bar and 293K for C2H6 and C3H8 reached 2.5 and 4.4 mmol·g-1 respectively23. Pe´rez et al. calculated adsorption of Cu-BTC for CH4 and C2H6 using Monte Carlo simulations24 and then compared them with experimental adsorption capacities of Cu-BTC. The adsorption capacities of Cu-BTC for CH4 and C2H6 were 0.6 mmol·g1

(experiment)/1.0 mmol·g-1 (simulation) and 4.1 mmol·g-1(experiment)/5.1 mmol·g-

1

(simulation) at 1 bar and 295K, respectively. He et al. synthesized a novel MOF25, UTSA-33,

with pore size of 4.8 to 6.5 Å and BET area of 660.0 m3·g-1, and reported that its adsorption capacities at 296 K and 1 bar for CH4 and C2H6 were 0.53 and 2.53 mmol·g-1 separately.(See Supporting Information S1) These MOFs above showed significantly preferential adsorption of C2H6 and C3H8 over CH4, which could potentially be applied to recover ethane and propane from natural gas. However, the selective separation performance of MOFs were seldom evaluated based on dynamic adsorption experiment for these light hydrocarbons.


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The purpose of this work is to investigate systematically adsorption performance of UiO-67 for separation of ethane and propane from methane. UiO-67 is a zirconium based MOFs with with high robustness and chemical stability, and is comprised of Zr6O4(OH)4 octahedra that are 12fold connected to adjacent octahedra through biphenyl-4,4'-dicarboxylic acid linkers (BPDC) 26. UiO-67 with ethane and propane-selective adsorption was synthesized by hydrothermal method and then characterized by N2 adsorption and X-ray powder diffraction (XRD). CH4, C2H6 and C3H8 （C1, C2 and C3）isotherms of UiO-67 were measured at different temperatures. The isosteric heats of C1, C2 and C3 adsorption on UiO-67 were estimated. The adsorption selectivities of UiO-67 for C2/C1 and C3/C1 were estimated separately by using Ideal adsorbed solution theory (IAST), DIH (difference of the isosteric heats) equation and breakthrough curves of mixtures. The preferential adsorption mechanism of C2 and C3 over C1 of UiO-67 were discussed. The comparisons of UiO-67 with some MOFs in aspect of adsorption capacity of C2 and C3 were made and reported.

2. Experimental

2.1. Materials Zirconium(IV) chloride, (ZrCl4 Reactor Grade, 99.5+%) was purchased from Alfa Aesar Chemicals Biphenyl-4,4'-dicarboxylic acid (BPDC , 98%) was provided by J&K Scientific Co. Ltd. N,N-dimethylmethanamide (DMF), glacial acetic acid and methanol was purchased from Guangzhou Guanghua Scientific Co. Ltd. All reagents were used without further purification. All adsorbates (N2 99.99%, CH4 99.99%, C2H6 99.99%, C3H8 99.99%, He 99.999%) were provided by Guangzhou KODI, and all adsorption test used these gases.


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2.2. Preparation of UiO-67 UiO-67 was synthesized by following reported procedures26. First, ZrCl4 (245 mg, 1.05 mmol), Biphenyl-4,4'-dicarboxylic acid (260 mg, 1.05 mmol), glacial acetic acid (2ml) and N,Ndimethylmethanamide(40 mL) were put into a reaction vessel, and mixed under ultrasonic for 30 mins. Second, the vessel was heated to 120 °C for 24 h, and then cooled to room temperature, followed by filtration. The solid mixture was washed by DMF twice, and then soaked with methanol for 2 days (exchanged with fresh methanol every 12 h), followed by filtration. Finally, the prepared solids were heated to 80°C for 6h, and then dried under vacuum to obtain product of UiO-67 ( yield: 350 mg).

2.3 Adsorbent characterizations N2 adsorption/desorption isotherms were measured by Micrometrics ASAP2460 Surface Characterization Analyzer at 77K. Standard Brunauer-Emmett-Teller equation was applied to calculate BET surface area at the pressure from 0.05 to 0.20 (p/p0)

27

, according to N2 was

padding into microporous at low relative pressure. The single point adsorption total pore volume of pores was calculated based on the saturated N2 adsorption amount at 100kPa, and the median pore width was obtained using the Horvath–Kawazoe method28. Before each measurements, the sample UiO-67 was heated to 150℃ and evacuated for 6h.

Powder X-ray diffraction (PXRD) was characterized by a Bruker D8 Advance X-ray diffractometer with Cu Kα1 emission radiation (λ=1.54056) at 2–50° (2θ) with a scan speed of 2°/min and a step size of 0.02°. Water vapor isotherm was measured by using DVS 2 Surface


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Measurements Systems, a dynamic vapor adsorption analyzer. Before each measurement, the samples were degassed at 150 ℃ in vacuum for 6 h. (See Supporting Information S2)

2.4 Measurement of C1, C2 and C3 adsorption isotherms Adsorption isotherms of C1, C2 and C3 were determined by using Micromeritics 3Flex Surface Characterization Analyzer at different temperatures of 288 K, 298K and 313K. A Dewar with a circulating jacket was connected to a thermostatic bath to control and adjust the temperature of the adsorption system in which the temperatures were strictly controlled with a precision of ± 0.01 K. The free space of the system was measured by dozing the helium gas. Before each measurement, about 100 mg of the sample was degassed at 150 ℃ in vacuum for 6 h.

2.5. Breakthrough experiment To evaluate the dynamic separation performances of UiO-67 for binary gas mixtures such as C2H6/CH4 and C3H8/CH4 mixtures, the breakthrough experiments were carried out by means of a self-assembly experimental setup as shown in Figure S2. Compositions of feed stream were designed as follows: methane: ethane: propane=85:10:5, which are similar to composition of NG. The detail of experiment procedures were shown in Supporting information S3.

3. Results and discussion

3.1 Characterization of samples


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Figure 1. N2 isotherms of UiO-67 at 77 K. Inset shows PXRD patterns of UiO-67 Figure 1 presents N2 adsorption and desorption isotherms at 77K of UiO-67. It exhibited typical type-I profile, suggesting its predominating microporous structures. Its BET surface area and total pore volume separately reached 2590.63 m2·g-1 and 0.94 cm3·g-1, which were calculated with the built-in software of ASAP 2460. Figure S3 shows pore size distribution (PSD) of the sample which were determined using Density Functional Theory (DFT). Two peaks appeared at 10.90 Å and 13.58 Å in the micropore region were clearly observed in the PSD curve of the sample, representing two types of cages in UiO-6729. Figure 1 inset shows the simulated and experimental PXRD pattern of UiO-67, which were concordant with those reported in the literature as well.30

3.2 Adsorption isotherms of C1/C2/C3 on UiO-67


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Figure 2 CH4, C2H6 and C3H8 isotherms of UiO-67 at (a) 288 K; (b) 298 K; (c) 308 K. Figure 2 shows isotherms of C1, C2 and C3 on UiO-67 at different temperatures. With an increase in temperature, these hydrocarbon uptakes decreased, implying that the adsorption behavior of C1, C2 and C3 on UiO-67 are predominantly physical adsorption. It was observed that the isotherms of C3 and C2 were significantly higher than that of C1, suggesting that the amounts adsorbed of C3H8 and C2H6 were than CH4 considerably. For example, at 288K and 100kPa, the amounts adsorbed of UiO-67 for C1, C2 and C3 were 0.56, 4.26 and 9.50 mmol·g-1, respectively, following the order: C1 < C2 < C3, which was related to the molecular size of these hydrocarbons. Molecular dynamics diameter of C1, C2 and C3 are 3.76 Å, 4.44 Å and 5.12 Å, respectively. Generally speaking, the larger the molecule size, the stronger the attraction forces acting on the molecule from the surface force field on the surrounding walls was, for a


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porous material with fixed pore size and volume. As a result, the adsorption capacity of C3H8 on UiO-67 was the highest, that of C2H6 was in the second, and that of CH4 was the lowest among the three hydrocarbons.

3.3. Isosteric heats of CH4, C2H6 and C3H8 adsorption Isosteric heat of adsorption is one of the critical parameters to evaluate the strength of the interaction between an adsorbate and an adsorbent, and can provide credible information about the energetic heterogeneity of an adsorbent surface. The isosteric heats of adsorption can be estimated by using the Clausius-Clapeyron equation.31 Qst ∂lnp = −( ) 2 RT ∂T q

(1)

where the Q s t (kJ/mol) is the isosteric heat of adsorption at a specific surface loading of adsorbate, R (kJ·mol-1·K-1) is the universal gas constant, T (K) is the temperature, p (kPa) is the pressure, and q (mmol/g) is the adsorbate amount adsorbed on the surface. Integrating equation (1) can give

ln p = −

Q st +C RT

(2)

where C is an integral constant. After C1, C2 and C3 isotherms were measured at different temperatures, these isotherms were converted to adsorption isosteres, then the ln p was plotted to

1 at a given amount of T

adsorbed C1, C2 or C3 on the basis of equation (2), and thus a fitted straight line with a slope of


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Q st was yielded. Finally, Q R

st

can be directly calculated from the slope

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Q st . R

Figure 3 Isosteric heats of C1, C2 and C3 adsorption on UiO-67 Figure 3 shows the isosteric heats of C1, C2 and C3 adsorption on UiO-67. The isosteric heats of C2 and C3 are higher than isosteric heat of C1, indicating that the interaction of C2 or C3 with UiO-67 were significantly stronger than CH4 at the same loadings, leading to the preferential adsorption of C2 and C3 over C1. It was noticed that the isosteric heats of C1, C2 and C3 adsorption on UiO-67 decreased with the amounts adsorbed of these hydrocarbons. The isosteric heats of C3 adsorption were in the range of 47.5-33.2 KJ·mol-1, and the isosteric heat of C2 adsorption were in the range of 32.3-25.2 KJ·mol-1, which were greatly higher than that of C1. .

3.4 Simulation of C1/C2/C3 distribution in UiO-67 Materials Studio (MS) is a complete modeling and simulation environment designed to allow researchers in materials science and chemistry to predict and understand the relationships of a


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material’s atomic and molecular structure with its properties and behavior. In this study, we used Sorption modules of MS 5.0 to simulate structure of UiO-67 and relationship between adsorbent and adsorbate. Only using the all-atom model of the COMPASS force field can the adsorption isotherms agree fairly well with experimental data as presented in Figure S4, although there are some slight differences. The slightly higher simulation results are plausibly due to the existence of impurities.

Figure 4. (a) tetrahedron and (b) octahedron cages in UiO-67; simulated absorbate density distributions inside UiO-67 cages for (c) CH4 at 1 kPa, (d) CH4 at 100 kPa, (e) C2H6 at 1 kPa, (f) C2H6 at 100 kPa (g) C3H8 at 1 kPa, and (h) C3H8 at 100 kPa. Figure 4a and 4b shows two types of cages in UiO-67, namely (a) tetrahedron and (b) octahedron cages (Besides, animated graphs of two cages are shown in Supporting Information of Video). The smaller tetrahedron cage imposes stronger non-specific adsorption force on the


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adsorbate than the octahedron cage does, resulting in distinct distributions of adsorbate molecules at different pressures. Figures 4c, 4e and 4g show distributions of C1-C3 molecules on UiO-67 at 1 kPa. C1-C3 molecules are preferentially adsorbed to the strong tetrahedron sites in the low-pressure region, while C1-C3 molecules are rarely adsorbed to the octahedron sites, since the smaller tetrahedron cage imposes stronger non-specific adsorption force on the adsorbate than octahedron cage. Figures 4d, 4f and 4h show distributions of C1-C3 molecules on UiO-67 at 100 kPa. It was observed that as pressure increased, C1-C3 molecules began to be adsorbed on the octahedron sites. It was noticed that the densities of C1-C3 molecules on both tetrahedron and octahedron sites followed the order: C3 > C2 > C1, which was coincident with adsorption capacities of these hydrocarbons on UiO-67. In addition， the simulated isosteric heats of these alkanes adsorption on UiO-67 followed the order: C3 > C2 > C1, which were listed in Table S3. The data in Table S3 indicated that the simulated isosteric heats of the alkanes adsorption were basically consistent with the experimental data.

3.5. C1/C2/C3 adsorption selectivity of UiO-67 The adsorption selectivity is a significant parameter to evaluate the adsorption separation efficiency of an adsorbent in the industrial processes. 3.5.1 Ideal adsorbed solution theory model


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Ideal adsorbed solution theory (IAST) is widely applied to predict the adsorption selectivity of binary gas mixtures from pure component isotherms, which is based on the principle that the adsorbate-adsorbent and adsorbate-adsorbate interactions are sufficiently ideal32. (See Supporting Information S6) Figure 5 shows the IAST-predicted selectivities of UiO-67 for binary gas mixtures at 25 ºC. It was clearly visible that the C2/C1 or C3/C1 selectivity of UiO-67 increased with pressure, being similar with the CO2/CH4 separation behavior of GrO@Cu-BTC33. This phenomenon may be due to the stronger interaction of C2 or C3 with UiO-67 compared to that with C1. As a result, when the increment in C2 or C3 uptake was higher than that in CH4 uptake with the pressure, the C2/C1 or C3/C1 adsorption selectivity of the samples increased with pressure. Figure S5 shows IAST-predicted selectivities of UiO-67 for C3/C2 binary mixtures. It was worth noting that its C3/C2 adsorption selectivity was nearly a constant about 9 except low pressure region.

Figure 5. IAST-predicted selectivities at 298 K of UiO-67 for (a) C2H6/CH4, (b) C3H8/CH4.


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3.5.2 DIH-based equation DIH (difference of the isosteric heats) equation also is a model for predicting adsorption selectivity of gas mixture on an adsorbent, which was proposed by Wang and Cao

34

.

They

thought that the adsorption selectivity of an adsorbent toward binary gas mixture is closely related to the difference in isosteric heats of the two gases and is independent of the molar fraction of the components in the bulk phase at low pressure. DIH-based equation can be expressed as follows34:

Sads (i / j ) = Sideal S0

Sideal =

qi ( p) q j ( p)

(5)

(6)

∆Qst0 ln S0 = 0.716 (7) RT

For the component i and j,

qi ( p) and qj ( p) are the equilibrium adsorption uptake at a pressure

0

p. ∆Qst is the difference between isosteric heats of adsorption for component i and j (kJ·mol-1) when pressure approaching zero. R (=8.314·10-3 KJ·mol-1·K-1) is molar gas constant and T (K) presents temperature. Cao and his coworkers used this method to estimate CO2/CH4 selectivity in MOFs, ZIFs, COFs and PAFs, and reported that the predicted selectivities were close to the IAST-predicted selectivities. In this work, we tried to use DIH-based equation to predict the adsorption selectivities of UiO-67 for C1/C2/C3. Figure 6 shows the C2/C1 and C3/C1 adsorption selectivities of UiO-67. It indicated that the C2/C1 adsorption selectivity of UiO-67


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was nearly constant about 7.5, and the C3/C1 adsorption selectivity was in the range of 72.1 84.4.

Figure 6. The DIH-predicted selectivities of UiO-67 for (a) C2H6/CH4 (10:85) and (b) C3H8/CH4 (5:85) mixtures at 298 K.

3.5.3 Breakthrough curves of C1/C2/C3 gas mixtures on UiO-67 The dynamic separation performances of UiO-67 toward C1/C2/C3 gas mixtures were tested using breakthrough experiments at 298 K. The breakthrough time, for a specified purity of the outlet gas mixtures, represents an appropriate combination of adsorption selectivity (S) and working adsorption capacity that is relevant in practice. Figure 7 shows the breakthrough curves of C1/C2 and C1/C3 binary gas mixtures through the fixed bed of UiO-67. It can be seen that the breakthrough time of CH4 was earlier than those of C2H6 and C3H8, and at the same time the


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breakthrough curve exhibited a small roll-up due to C2H6 or C3H8 desorption because initially adsorbed CH4 was replaced by incoming C2H6 or C3H8 until breakthrough of the latter occurred. The roll-up of CH4 was attributed to its lower adsorption affinity compared to C2H6 or C3H8. Here, the working adsorption capacity and adsorption selectivity (S) were calculated using the Eqs. (S1), (S2) and (S3) on the basis of experimental breakthrough curves, which were listed in Table 1.

Figure 7 Breakthrough curves of (a) CH4/C3H8 binary mixture (methane: propane =85:5) and (b) CH4/C2H6 binary mixture (methane: ethane =85:10) through the fixed bed packed with UiO67 (298 K, 1 atm).


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3.6 Adsorption selectivities of UiO-67 estimated by different methods Table 1 lists the adsorption selectivity of UiO-67 separately for C3/C1 and C2/C1 binary mixtures estimated by using the three methods. The data in Table 1 indicated that adsorption selectivity predicted by DIH equation and IAST model were higher than that dynamic adsorption selectivity estimated on the basis of experimental breakthrough curves. The origination of the deviation could be attributed to the different type of adsorption data used in these models. The selectivities predicted using DIH equation and IAST model were based on thermodynamic equilibrium data, while the dynamic adsorption selectivity was based on dynamic adsorption data. In addition, it was noticed that that the C3/C1 adsorption selectivity of UiO-67 was higher than its C2/C1 adsorption selectivity, which was mainly attributed to the stronger interaction of C3H8 with UiO-67 than C2H6. Table 1 Working capacities and selectivity of UiO-67 for C3/C1 and C2/C1 binary mixtures.

Binary mixtures

IAST selectivity

DIH selectivity

Dynamic adsorption Selectivity

Working capacity (mmol/g)

Composition in feed stream

C3H8/CH4

73.7

72.1

69.3

1.55/0.38

C3H8: CH4=5:85

C2H6/CH4

8.1

7.4

7.3

0.31/0.36

C2H6: CH4=10:85


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4. Conclusions In this work, separation properties of UiO-67 for ethane/methane and propane/methane binary mixtures were evaluated. The adsorption capacities of UiO-67 for C2H6 and C3H8 reached 4.26 and 9.50 mmol·g-1 at 288K and 100kPa, respectively, while its adsorption capacity for CH4 was only 0.56 mmol·g-1. The isosteric heats of CH4, C2H6 and C3H8 adsorption on UiO-67 followed the order: C3H8 > C2H6 > CH4, suggesting the stronger interaction of C3H8 or C2H4 with UiO-67 compared to CH4, resulting in preferential adsorption of C3H8 and C2H4 over CH4. The three methods including IAST model, DIH-based equation and breakthrough curves were applied to estimate the adsorption selectivity of UiO-67 for binary mixtures. The dynamic adsorption selectivity of UiO-67 toward C2H6/CH4 and C3H8/CH4 binary mixtures reached 7.3 and 69.3, respectively. IAST-predicted selectivity and DIH equation-predicted selectivity were slightly higher than the dynamic adsorption selectivity. Fixed bed experiments conformed that C1-C3 gas mixtures can be well separated in the fixed bed of UiO-67. UiO-67 possessed not only high adsorption capacities of C2 and C3, but also high C2/C1 and C3/C1 adsorption selectivity. However, the effects of water vapor as well as some other gases present in natural gas on C1-C3 separation need to be investigated further, and the economy of C1-C3 separation by pressure swing adsorption using UiO-67 as adsorbents is worthy of evaluating before UiO-67 is put into practical application.

ASSOCIATED CONTENT Supporting Information


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The Supporting Information is available free of charge on the ACS Publications website at DOI: ie-2017-01420w. Adsorption capacity of some adsorbents and UiO-67 toward CH4, C2H6 and C3H8, Water vapor isotherm on UiO-67 at 298 K, Breakthrough experiment, Pore-size distribution of UiO-67, isosteric heats of C1/C2/C3 adsorption on UiO-67, Fitting Parameters of the DSLF model for CH4, C2H6 and C3H8 isotherms on UiO-67and Corresponding Correlation coefficients (R2)

Acknowledgements This work was supported by National Natural Science Foundation of China (No. U1662136), the Research Foundation of State Key Lab of Subtropical Building Science of China (C7160190), the Guangdong Natural Science and technology plan project (No. 509164977055), PostDoctoral Innovative Talents Project (BX201600053) from China Postdoctoral Science Foundation (176394), and the Fundamental Research Funds for the Central Universities.

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Selective Adsorption Performances of UiO-67 for Separation of Light

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