Effective Adsorption Separation of n

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Effective Adsorption Separation of n‑Hexane/2-Methylpentane in Facilely Synthesized Zeolitic Imidazolate Frameworks ZIF‑8 and ZIF69 Le Chen, Sheng Yuan, Jun-Feng Qian, Wei Fan, Ming-Yang He, Qun Chen,* and Zhi-Hui Zhang* Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology, Changzhou University, Changzhou 213164, People’s Republic of China S Supporting Information *

ABSTRACT: A facile synthesis procedure was built to synthesize zeolitic imidazolate frameworks (ZIFs) ZIF-8 and ZIF-69 under ambient conditions with the prevention of organic solvents. ZIF-69 adsorbent was first introduced in the liquid-phase separation of hexane isomers. A sieving test was carried out using samples of n-hexane (nHEX) or 2-methylpentane (2MP) over two ZIFs and commercial 5A zeolite materials. According to batch adsorption performances and modeling, ZIF-8 exhibits the highest adsorption selectivity and capacity for nHEX, followed by ZIF-69 and 5A zeolite. The equilibrium data at 25 °C show that a significant amount (0.51 g/g) of nHEX and only 0.09 g/g 2MP can be adsorbed in ZIF-8. And the maximum adsorption capacity of nHEX and 2MP on ZIF-69 are 0.34 g/g and 0.10 g/g, respectively. Breakthrough experiments performed at 150 °C with a binary mixture of nHEX/2MP confirm the shape selectivity and adsorption capacities of ZIF-8 toward nHEX at various total pressures from 0.5 to 1.3 MPa and with different feed concentrations, which is attributed to the flexible architecture of ZIF-8 and its excellent shape match with nHEX. several fractions on a column of Fe2(BDP)3 (BDP2− = 1,4benzenedipyrazolate), a MOF with unique triangular channels increasing entropic costs for the more branched hexanes.9 Zeolitic imidazolate frameworks (ZIFs) represent a new class of MOFs and have tunable pore size and high inner surface area, which frequently possess extended zeolite topologies with larger cages than those of traditional zeolites.11 ZIFs characterize both high stability of inorganic zeolites and functional adjustable MOFs, which make them promising for gas/vapor separations and heterogeneous catalysis,11−15 and the synthesis procedures of ZIFs also attract more and more attention.16−21 ZIF-8 (Zn(MeIM)2, MeIM = 2-methylimidazole) is one of the most extensively studied ZIF materials. Bearing sodalite (SOD) topology, ZIF-8 contains Zn2+ cations linked by 2-methylimidazolate with highly flexible and gate-opening effects to allow sodalite cages to connect to each other by 6-memberedring (MR) windows. These cages have a pore diameter of 1.16 nm and an aperture of 0.34 nm,22,23 whereas for the chlorinated ZIF-69, mixed 5-chlorobenzimidazole and nitroimidazole ligands result in a GME topology that has 12-MR straight channels (the pore size of about 0.78 nm), and is the only

1. INTRODUCTION Separation of n-paraffins and branched paraffins from straight kerosene or naphtha is an important industrial petrochemical production process, including the separation of the straight chain alkanes n-C6 to n-C20, the n-alkanes of n-C6 to n-C14 usually known as light liquid paraffins and n-C14 to n-C20 usually known as heavy liquid wax. n-Alkanes are very important chemical raw materials for the production of alkyl benzene, benzenesulfonate synthetic detergent, lubricant additives, paraffin oil, and a series of chemicals. With the rapid development of industry, there is a huge potential market for liquid paraffins.1 Among the light liquid paraffins, n-hexane (nHEX) is widely used to produce high quality solvent oils and acquire a high yield of ethylene in the steam cracking process.2 High purity nHEX has been obtained by the use of adsorption separation technology with fixed-bed absorbers such as zeolites and MOFs (metal−organic frameworks).3−9 As a type of calcium aluminosilicate, commercially molecular sieve 5A has Linde Type A (LTA) zeolite topology and possesses a 0.51 nm effective aperture diameter between the kinetic diameters of linear and branched paraffins.10 It is a major success even today in the petrochemical industry for zeolite 5A to separate linear alkanes from branched ones due to its suitable aperture diameter.8 Dibranched isomers of hexane are preferred products in an isomerization process. It has been shown experimentally that five hexane isomers can be separated into © XXXX American Chemical Society

Received: June 6, 2016 Revised: August 19, 2016 Accepted: September 19, 2016

A

DOI: 10.1021/acs.iecr.6b02175 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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pressure range from 0.05 to 0.30. The pore size distributions and pore volume were derived from the adsorption branches of the isotherms. 2.5. Batch Adsorption Experimental Procedures. Prior to adsorption, the adsorbents were dried at 150 °C for 4 h. Exact amounts of the adsorbent (around 1.0 g) were added to the isooctane solution (20 mL) with initial concentrations of nHEX (or 2MP) from 2% to 20%. All solutions containing the adsorbents were sealed in 100 mL round-bottomed flasks, then shaken on a shaker water bath operated at a constant temperature of 25 °C for a specified time to determine the adsorbed amount at various times until adsorption equilibrium. nHEX (or 2MP) concentration of the supernatant solutions was analyzed every 2 min by Shimadzu gas chromatography GC-2010. The process under each condition was performed three times to obtain the average value of the adsorption capacities. 2.6. Dynamic Adsorption Breakthrough Experiments. Before adsorption measurements, samples were activated at 150 °C for 4 h to remove the physically adsorbed water molecules. The dynamic adsorption separation of linear and branched paraffins was investigated using breakthrough experiments of binary mixtures of nHEX/2MP. To testify the reproducibility, parallel dynamic experiments were carried out three times. The breakthrough experimental flow diagram is shown in Figure 1.

topology that has both large pores and large windows for ZIFs.24 Previous studies of Yan25 and Luebbers26 showed that ZIF-8 has the robust ability to sieve branched alkanes from linear alkane isomers with a molecular effect. ZIF-8 is also a competitive alternate adsorbent for the separation of linear nHEX from its branched isomers.8 Recently, ZIF-69 membranes have been verified for potential application in gasoline vapor recovery by separating C5−C7 hydrocarbon mixtures.27 However, a systematic study on the adsorption separation of paraffin isomers still needs to be developed among ZIF adsorbents. In this work, we report a facile method for rapid synthesis of ZIF-8 and ZIF-69 in an aqueous system at room temperature. High selectivity for the separation of C6-paraffins was found by comparing two ZIF materials and commercial zeolite 5A in the separation of C6-paraffin isomers HEX and 2MP. Adsorption equilibrium and fixed bed adsorption experiments were carried out to measure the adsorbed amount of a single hexane isomer and binary mixtures of nHEX and 2MP.

2. EXPERIMENTAL SECTION 2.1. Materials. Zinc nitrate hexahydrate (≥99%), 2methylimidazole (≥99%), 5-chlorobenzimidazole (≥98%), 2nitroimidazole (≥98%), isooctane (≥98%), n-hexane (≥98%), and 2-methylpentane (≥98%) were commercially available and used without any pretreatment. Zeolite 5A was kindly provided by Chinese Sinopec Catalyst Co., Ltd. 2.2. Synthesis of ZIF-8. In a typical synthesis, 0.46 g of zinc nitrate hexahydrate was dissolved in 3 mL of deionized (DI) water; then 5.50 g of H-MeIM was dissolved in 20 mL of DI water. Those two solutions were mixed (Zn2+: H-MeIM: H2O = 1:43:840) and stirred for 6 h at room temperature, then the resulting white precipitate were collected by centrifuging, washed with water and methanol subsequently for 3 times, and finally vacuum-dried at 80 °C for 24 h. This sample and the sample with a starting mole ratio of Zn2+/H-MeIM/H2O = 1:43:420 were entitled as ZIF-8 (1) and ZIF-8 (2), respectively. 2.3. Synthesis of ZIF-69. A 0.46 g sample of zinc nitrate hexahydrate was dissolved in 3 mL of deionized water; then 2nitroimidazole (H-NIM) (2.26 g) and 5-chlorobenzimidazole (H-ClBIM) (3.05 g) were suspended in 20 mL of DI water. Those two solutions were mixed (Zn2+/H-NIM/H-ClBIM/ H2O = 1:20:20:840) and stirred for 6 h at room temperature, then the resulting white precipitates were collected by centrifuging, washed with water and methanol subsequently for 3 times, and finally vacuum-dried at 80 °C for 24 h. This sample and the sample with a starting mole ratio of 1:20:20:420 (Zn2+/H-NIM/H-ClBIM/H2O) were entitled as ZIF-69 (1) and ZIF-69 (2), respectively. 2.4. Characterization. X-ray powder diffraction (XRD) patterns were collected at room temperature in a Rigaku D/ MAX-2500PC diffractometer (Rigaku Co., Japan) using Cu Kα1 radiation (λ = 0.15406 nm) operated at 40 kV and 100 mA. Scanning electron microscopy (SEM) images of ZIFs samples were taken at 30 kV with a JSM-6360LA microscope. Thermogravimetric analysis (TGA) (SDTQ600, TA) of the activated ZIFs powder were performed from room temperature to 800 °C with a scan rate of 5 °C·min−1 under air atmosphere. N2 adsorption isotherms were measured at 77 K using a Micromeritics ASAP2020 sorption analyzer. The Brunauer− Emmett−Teller (BET) method was utilized to calculate the specific surface areas (SBET) using adsorption data in a relative

Figure 1. Schematic of breakthrough experimental procedure.

It comprises a feed pump, a stainless steel adsorption column, and a regulating valve at the outlet of the packed bed in order to collect samples. The pump used to inject the mixture is the high-pressure constant-current pump. The stainless steel column (10 cm length, 1.0 cm inner diameter) entirely filled with adsorbent (1−2 g) and quartz sand at the bottom was placed in an oven which regulates the temperature of the adsorption experiment at 150 °C, and is operated by continuously introducing hexane isomer mixtures with known composition [the mass fraction nHEX/2MP = 1:1, 1:2, 1:3, and in the inlet mixture isooctane was used as the solvent (60%)] at a fixed total pressure (0.5, 1.0, and 1.3 MPa) and adjusted to keep a total flow rate of 0.5 mL·min−1. When the saturation is reached, the composition of each sample is analyzed by a chromatographic column and a flame ionization detector (FID), and Shimadzu GC2010 chromatography was used to analyze the composition of samples. The equilibrium dynamic adsorption capacity (qc) of the adsorbents is calculated from the breakthrough curves according to the equation as follows: qc = B

1 (Couρt − CoV ′ρ − ρu m

∫0

t

C i dt )

(1)

DOI: 10.1021/acs.iecr.6b02175 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 2. XRD patterns of simulated ZIF-8 (a), ZIF-69 (b), and as-synthesized ZIFs nanocrystals.

where Ci and Co are the outlet and inlet mass concentrations (%) of the stream through the fixed bed column, respectively; u (mL·min−1) is the volume flow rate of nHEX or 2MP; m (g) is the dosage of the adsorbent; V′ (mL) is the dead space volume of adsorbent bed void and around the pipe; ρ (g·mL−1) is the density of the binary mixture. The feed flow rate u was constant in the liquid phase by highpressure constant-current pump, and the exit flow rate was inconstant since one component was being adsorbed. The mass of component i retained in the adsorbent bed is equal to the total inlet mass of component i minus the total outlet mass of i, also minus the dead space volume of bed. ρu∫ t0 Ci dt is the total outlet mass of component i by integration of the breakthrough curves measured in terms of outlet concentration as a function of time. The selectivity S is given by the quotient between the amounts adsorbed of the two species as follows: q(nHEX) × c0(2MP) S= q2MP × c0(nHEX) (2)

draws little attention in environmental friendly synthesis and related applications, with the exception of its membrane that separates CO2/CO mixtures30 and recovers n-alkane components.27 The synthesis technique of ZIF-69 remains the solvothermal reaction in DMF and needs to be improved. In this work, we present a facile method for rapid synthesis of ZIF69 following a modified process of ZIF-8 by adjusting the concentration of reactants in aqueous solution at room temperature to control the size distribution. XRD patterns of the as-synthesized ZIFs adsorbents were depicted in Figure 2. By a comparison of the XRD patterns to the standard simulation patterns from the literature,30,31 purephase ZIF-8 and ZIF-69 materials were identified. SEM pictures (Figure 3) reveal that the particles are nanocrystals with

where qnHEX and q2MP are the dynamic adsorption capacity of nHEX and 2MP, respectively, c0(nHEX) and c0(2MP) are the mass concentrations of nHEX and 2MP in the feed.

3. RESULTS AND DISCUSSION 3.1. Synthesis and Characterization. To date, the synthesis techniques of ZIFs have been developed extensively16−21 owing to their outstanding stability and zeolite-like topologies, as well as their porosity endowing properties.28 Most of them including ZIF-8 and ZIF-69 have been initially solvothemally synthesized in nonaqueous conditions over a long reaction time.11 ZIF-8 has been reported to be quickly synthesized for the first time in methanol at room temperature.16 Pan et al. extended the rapid and mild synthesis of ZIF8 using water as the only solvent and excess 2-methylimidazole, instead of organic solvents. We developed a similar synthesis procedure to control particle sizes in ZIF-67 [(Co(MeIM)2] nanocrystals.18 Meanwhile the preparation methods of ZIF-8 and ZIF-67 in aqueous solutions also have been documented and they comprise dry-gel, microfluidic methods, and microemulsion,19−21 and even include large-scale production.29 But most of them introduced toxic organic chemicals including different surfactants and amines, indicating not total environmental compatibility. Besides extensive investigation of ZIF-8 and ZIF-67, chlorinated ZIF-69 bearing unusual porosity in GME topology

Figure 3. SEM pictures of ZIF-8 (1) and (2) (a, b) and ZIF-69 (1) and (2) (c, d).

polyhedral shape. Increasing the starting concentration has been successfully applied to control the particle size and morphology, which is in agreement with the previous conclusions for the synthesis of ZIF-67.18 With the doubling of the reagent concentration, particle sizes decreased severely for ZIF-8 and slightly for ZIF-69, respectively. Because the samples somewhat agglomerate under the condition of doubling concentration, ZIF-8 (1) and ZIF-69 (1) nanoparticles were used in further investigations. The obtained nanoparticles can be well dispersed in methanol to form a stable suspension, and can be kept for several weeks without settlement. C

DOI: 10.1021/acs.iecr.6b02175 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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approximately 2 times higher accessible pore volume to accommodate linear paraffins than zeolite 5A in a mass-related capacity. The BET results confirm the effective activation by removing any solvent component (e.g., water) from the pores. 3.2. Batch Adsorption Performances. To investigate the adsorptive performances and kinetics, the single component quantities of nHEX and 2MP adsorbed on adsorbents at various adsorption times were carried out with the initial hexane concentration of 15% in isooctane. As displayed in Figure 5, the quantities of nHEX adsorbed on ZIF-8, ZIF-69, and 5A are much higher than those of 2MP. For three adsorbents, 5A displays the shortest adsorption saturation time. Adsorption saturation of nHEX and 2MP on 5A can be achieved in 8 and 4 min, respectively, due to the smallest pore volume and pore diameter of 5A. ZIF-8 exhibits the highest adsorption capacities for nHEX than those of ZIF-69 and 5A. Adsorption saturation of nHEX on ZIF-8 needed 16 min, while the adsorption of nHEX on ZIF-69 was nearly completed in 22 min (see Figure 5a), suggesting relatively rapid adsorption of nHEX on ZIF-8. The similar adsorption performances of 2MP on ZIF-8 and ZIF-69 are shown in Figure 5b, with an adsorption saturation time of 8 and 9 min, respectively. By comparison with the aspect of nHEX, the lower adsorption capacity of 2MP on ZIF8 and ZIF-69 can be attributed to the larger kinetic diameter of 2MP. The kinetic diameter of the 2MP molecular is 0.55 nm, while that of nHEX is 0.43 nm.32 The sieving radius of ZIF-8 was reported up to 0.58 nm, far larger than its aperture of 0.34 nm, which could take responsibility of its efficiency to sieve branched hexane isomers with nHEX being adsorbed.33 The static kinetic measurements demonstrate that kinetic separation of paraffins is based on molecular dimensions of the hexane isomers, and a small amount of 2MP isomer is adsorbed under slight diffusional limitation. The single component isotherms of nHEX and 2MP adsorbed on ZIFs and 5A are shown in Figure 6. Every equilibrium quantity was obtained after adsorption for 1 h, which was considered to be sufficient time to reach adsorption equilibrium. With the increase of concentration, the adsorption amount of both nHEX and 2MP increases, eventually tending to form a plateau. The adsorption amounts of nHEX on both ZIF-8 and ZIF-69 are superior to those of 2MP. On the other hand, the adsorption amount of 2MP on ZIF-8 is lower than that on ZIF-69, which may be ascribed to the different pore shapes and cage apertures that result from the distinct zeolite topology. Consequently, ZIF-8 displayed better adsorption equilibrium selectivity than ZIF-69.

The TGA curve (Figure S1) of the activated ZIF-8 sample shows a short plateau before 150 °C followed by a slow weight loss of 11.6% up to 500 °C, indicating the removal of guest molecules (mainly water) within cavities or unreacted 2methylimidazole from the surface.17 Then a series of drastic weight losses occurred from 500 °C and did not end at 500 °C, representing the decomposition of the framework. ZIF-8 exhibits a better thermal stability up to 500 °C than common MOFs. The TGA curve (Figure S1) of the activated ZIF-69 sample displays a similar short plateau before 120 °C followed by a slow weight loss of 7.5% in the range of 120 to 222 °C due to a release of residual guest water. Then a slight weight loss of 3.2% indicates the exclusion of unreacted species until 340 °C. After that, the remaining framework decomposes. Thus, ZIF-69 material is thermally stable up to 340 °C, lower than that of ZIF-8 but better than most of the other MOFs. Nitrogen sorption study for the ZIF-8 and ZIF-69 nanocrystals as well as 5A zeolite (Figure 4) reveals reversible type I

Figure 4. N2 adsorpton/desorption isotherms of ZIF-8 (1) and ZIF-69 (1) nanocrystals as well as 5A zeolite.

isotherms, characteristic of microporous materials. The speedy uptake at high relative pressure can be related to physisorbed liquid nitrogen on the crystal surfaces of the nanoparticles. The total pore volume of micropores of ZIF-8 nanocrystals is about 0.436 mL/g, and the BET surface area is 1285.2 m2/g. And the total pore volume of micropores of ZIF-69 nanocrystals is about 0.311 mL/g, and the BET surface area is 845.1 m2/g. For comparison, 5A zeolite has the lowest surface area and pore volume of three adsorbents (see Table S1). ZIF-8 exhibits

Figure 5. Effect of adsorption time on the static adsorbed amount of nHEX (a) and 2MP (b) (initial concentration: 15%) in ZIF-8, ZIF-69, and 5A. D

DOI: 10.1021/acs.iecr.6b02175 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 6. Static adsorption isotherms of nHEX (a) and 2MP (b) in ZIF-8, ZIF-69, and 5A.

The maximum adsorption capacity of nHEX and 2MP on ZIF-8 are 0.51 g/g and 0.09 g/g, respectively (see Figure 6), and the maximum adsorption capacity of nHEX and 2MP on ZIF-69 are 0.34 g/g and 0.10 g/g, respectively (see Figure 6). Volume base is important for an adsorption separation unit. Since total pore volume and BET surface are almost 30% different between ZIF-8 and ZIF-69, ZIF-8 adsorbs nHEX more favorably than ZIF-69 does. Considering a unit cell contains eight sodalite cages, the framework of ZIF-8 allows molecules to efficiently access its pores. Moreover, the adsorption amount of nHEX in ZIF-8 is larger than the theoretical adsorption capacity that is the product of the pore volume and the density of liquid, indicating more flexibility of ZIF-8 than anticipated.8 This result could also be due to the underestimation of the real pore volume of ZIF-8 calculated by the HK model and the existing adsorption of the outer surface during liquid phase experiments. Alternately, ZIF-8 exhibited a much smaller adsorption of 2MP compared to nHEX. Although recent investigations indicated similar adsorbed quantities of monobranched hexane isomers (2MP and 3MP) and the linear nHEX,9,33,34 their vapor adsorption systems of a pure component at lower pressures are totally different than the liquid adsorption systems of an isooctane solution containing respective hexane isomers in this work. On the basis of both kinetics adsorption and isotherms, ZIF-8 is a promising candidate for efficient adsorption separation of nHEX and 2MP in liquid phases. In addition, ZIF-67 has a same structure but different metal centers as that of ZIF-8. It is anticipated to have considerable adsorption ability for nHEX. Bearing a distinct topology with large pores and windows, ZIF-69, in comparison with 5A zeolite, also exhibits a remarkable adsorption ability of nHEX over 2MP. The difference of the shape selectivity for ZIF-8 and ZIF-69 may be attributed to the discrepancy of organic ligands and the related topologies within the ZIFs frameworks. The Langmuir−Freundlich model fits the experimental adsorption data of nHEX for all three adsorbents. Because of the trivial adsorption amount of 2MP in 5A zeolite, all three adsorption models fail in the correlation (see the Supporting Information for details). 3.3. Dynamic Adsorption Breakthrough Experiments of Binary Mixture Composition. Because of the distinguished batch adsorption performances of ZIF-8 adsorbent, a fixed bed packed with ZIF-8 adsorbent is used to investigate the dynamic adsorption behavior of C6-paraffin isomers separation with ZIF-8. The breakthrough time, equilibrium adsorption capacity, and selectivity are all important dynamic adsorption

properties of adsorbents to be evaluated. Generally, the longer breakthrough time results in a higher dynamic adsorption capacity. Two sets of experiments were carried out: (i) at a fixed total pressure and temperature of the isomers in order to study the influence of the feed initial concentration; and (ii) at a fixed feed concentration and temperature of the isomers in order to study the influence of the total pressure. According to the industrial separation conditions,9,35 the dynamic experiments were performed at 150 °C and up to 1.3 MPa in the fixed bed adsorption experiment column. Figure 7 illustrates the breakthrough curves of nHEX/2MP mixtures on ZIF-8 with different feed ratios at 150 °C and 1.3

Figure 7. Breakthrough curves of nHEX and 2MP with different initial concentration ratios in ZIF-8.

MPa. After the injection of a mixture of nHEX and 2MP at the inlet of a column packed with ZIF-8, the linear chain eluted later than its branched isomer, indicating preferentially adsorbing nHEX. The outlet concentration of 2MP increases rapidly to the feed concentration within 10 min. As depicted in Figure 7, the lower is the feed concentration of nHEX injected, the longer is the breakthrough time, from 5 to 12 min; the dynamic saturated adsorption capacities of nHEX increased with the extending of breakthrough time, while 2MP displayed a slight change. In addition, at feed concentration c0(nHEX)/ c0(2MP) = 1:1, the breakthrough curve of nHEX is sharp until saturation; however, a slow approach to saturation of other curves is noted. The shape of the breakthrough curves depends mostly on mass transfer kinetics, indicating the impact of feed concentration. E

DOI: 10.1021/acs.iecr.6b02175 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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dibranched ones as indicated by their breakthrough data.9 In this aspect, ZIF-8 exhibits significantly higher shape selectivity and adsorption capacity toward nHEX than 2MP under our experimental conditions, demonstrating a potential substitute adsorbent for paraffins separation superior to zeolite 5A.

Figure 8 presents the breakthrough curves of binary mixtures (nHEX/2MP = 1:2) on ZIF-8 at different total pressures. The

4. CONCLUSIONS Two zeolitic imidazolate frameworks (ZIFs), ZIF-8 and ZIF-69, were synthesized in an environment-friendly aqueous solution using a facile synthesis procedure. The static adsorption capacities of nHEX on ZIF-8, ZIF-69, and 5A is ZIF-8 > ZIF-69 > 5A. A slight diffusional limitation was found for 2MP on all adsorbents. By comparison with zeolite 5A and ZIF-69, ZIF-8 has the best separation effect of nHEX and 2MP. The separation selectivity of ZIF-8 is larger than that of ZIF-69, probably because of its flexible architecture and excellent shape match with nHEX. A total pressure of 1.3 MPa and feed concentration ratio of 1:3 are the optimal experimental conditions of dynamic adsorption separation. This work not only indicates that ZIF-type materials are promising candidates for separating n-alkanes from their isomers, but also provides an avenue to gasoline vapor recovery.

Figure 8. Breakthrough curves of nHEX and 2MP at different total pressures in ZIF-8.

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Table 1. Experimental Conditions and Dynamic Adsorption Parameters for nHEX/2MP Binary Breakthrough Curves in ZIF-8

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.6b02175. Details on pore structure parameters and TG curves of the adsorbents, batch adsorption modeling, and related fitting data; XRD patterns and SEM images of ZIF-8 and ZIF-69 after breakthrough experiments (PDF)

dynamic adsorption capacity (g/g) feed concentration c0(nHEX)/c0(2MP)

total pressure (MPa)

nHEX

2MP

selectivity

1:1 1:2 1:3 1:2 1:2

1.3 1.3 1.3 1.0 0.5

0.41 0.45 0.48 0.42 0.39

0.07 0.07 0.08 0.08 0.05

5.8 12.8 18.0 11.0 13.6

ASSOCIATED CONTENT

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Fax: 86-519-86330251. Tel: 86-519-86330251.

experimental conditions and results are given in Table 1. Breakthrough curves of nHEX in ZIF-8 indicate that its adsorption is highly favorable at low feed concentration and higher total pressure. As represented in Figure 8, the dynamic adsorption capacity of nHEX increased with the increase of total pressure; the outlet concentration of nHEX adsorbed increased slowly after 10 min, until dynamic equilibrium was reached at 30 min. Considering the adsorption amount and selectivity, a total pressure of 1.3 MPa and a feed concentration ratio of 1:3 were chosen as the optimal experimental conditions of dynamic adsorption separation. With feed concentration, and total pressure taken into account, ZIF-8 displays the best adsorption selectivity for nHEX at the condition of c0(nHEX)/c0(2MP) = 1:3 and 1.3 MPa, as indicated in Table 1. Smaller 2MP molecules loading rather than 3MP (3-methylpentane) serves as a complement of Peralta’s conclusions,8 which is an adsorption hierarchy of nHEX > 3MP (3-methylpentane) > 2,2-DMB (2,2-dimethylbutane). With respect to another important study,9 the separation ability of all hexane isomers in Fe2(BDP)3 was evaluated to achieve a record 92 RON (research octane number) productivity for isomerization. The acute angles (ca. 60°) in the pores of Fe2(BDP)3 leads to weaker interactions and superior entropic costs for the more branched isomers of nHEX.36 As a result, the specific selectivity of nHEX to its monobranched isomers were not as much as those of nHEX to

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the financial support from the National Natural Science Foundation of China (Grant 21201026) and the Natural Science Foundation of Jiangsu Province (Grant BK20130251), a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), and Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology (BM2012110). Jiangsu Advanced Catalysis and Green Manufacturing Collaborative Innovation Center is acknowledged. We also acknowledge the International Cooperation Research Project on Changzhou University of Xuyi Center of Attapulgite Applied Technology, Science and Technology Innovation Work of Huai’an (HAC2015014). J.F.Q. acknowledges the Prospective Joint Research Project on the Industry, Education and Research of Jiangsu Province (BY2015027-16).

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REFERENCES

(1) Kulprathipanja, S. Adsorptive separation process for the purification of heavy normal paraffins with non-normal hydrocarbon pre-pulse stream. U.S. Patent US4992618A, 1991. F

DOI: 10.1021/acs.iecr.6b02175 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.iecr.6b02175 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX