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Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Ultramicroporous Metal−Organic Framework with Polar Groups for Efficiently Recovering Propylene from Polypropylene Off-Gas Qiang Tan,† Yaguang Peng,† Wenjuan Xue,‡,§ Hongliang Huang,*,‡,§ Dahuan Liu,*,† and Chongli Zhong†,‡,§ †
State Key Laboratory of Organic−Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, China State Key Laboratory of Separation Membranes and Membrane Processes and §School of Chemistry and Chemical Engineering, Tianjin Polytechnic University, Tianjin 300387, China
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S Supporting Information *
ABSTRACT: Recovery of propylene from polypropylene (PP) purge stream is of great importance due to both more efficient conversion of the monomers and environmental protection. Herein, eight stable metal−organic frameworks (MOFs) with various topological structures were synthesized and their C3H6/N2 separation performances were studied.The results indicate that CAU-1, an Al-MOF with ultramicroporous structure and amine-decorated function site, shows prominent C3H6/N2 liming selectivity among these MOFs. The ideal adsorbed solution theory (IAST) selectivity of CAU-1 is up to 236 under ambient condition, surpassing other reported porous materials. In addition, the high recovery efficiency (>99%) obtained by breakthrough experiments, good regenerability, and high stability under harsh chemical conditions suggest the great potential of CAU-1 in practical industrial application. Our work demonstrates that the ultramicroporous structures and polar groups in CAU-1 play key roles for C3H6/N2 separation, which provides a guideline to design and synthesize high-performance MOFs in propylene recovery from polypropylene off-gas.
1. INTRODUCTION Polypropylene (PP) is one of the widely used polymer products in the world.1−3 However, during the production process of polypropylene, the unreacted C3H6 monomer was purged with nitrogen, forming a rich nitrogen purge stream (70% N2 and 30% C3H6).4 This purge gas is usually burned or discharged into the air, causing the loss of high value monomers and air pollution.5 Therefore, considering the value of the increase of utilization efficiency for the C3H6 monomers and environmental protection, it is imperative to recover propylene from the polypropylene (PP) purge stream.6 To date, several approaches have been proposed for propylene recovery from polypropylene off-gas, such as membrane separation,7,8 adsorption separation,9,10 and cryogenic distillation.11 Compared with other technologies, membrane and adsorption separation processes have received great attention due to their simple operation and low energy consumption.5,6,12 Although much progress has been made on membranes for the separation of C3H6/N2, the low selectivity (10−20) for the C3H6/N2 gas mixture has been revealed.13,14 Taking these into account, the adsorptive separation process has received tremendous attention. However, the adsorption selectivities of conventional adsorbents (zeolites15−18 and activity carbons19,20) are not sufficient for practical industrial application due to their limited surface areas as well as chaotic pore structures. Thus, new adsorbents with high selectivity and stable structure should be developed for propylene recovery from polypropylene off-gas. © XXXX American Chemical Society
Metal−organic frameworks (MOFs), as a kind of novel porous materials, have received extensive attention in gas storage/purification due to large surface areas, ordered and permanent pore structures, tunable surface functionalities, and adjustable pore sizes.21−27 Progress in a series of challenges has been realized by using MOFs for the separation of C2H4/ C2H6,28,29 C2H2/C2H4,30 C2H2/CO2,31 and C3H6/C3H8.2 However, porous materials based on MOFs for separation of subcritical−supercritical gases was rarely published,32,33 especially for propylene recovery from polypropylene off-gas. Herein, several typical MOFs with high stabilities were prepared to systematically evaluate their C3H6/N2 separation performances. As a result, CAU-1, which has ultramicropore structures and amine groups, shows the highest limiting selectivity among these MOF materials. The C3 H6 /N 2 selectivity predicted by the ideal adsorbed solution theory (IAST) in CAU-1 is 236 at 298 K and 1 bar, surpassing other reported porous materials. The control experiments and theoretical calculations suggest the high selectivity of CAU-1 can be attributed to the combination of ultramicroporous structures and amino groups. The experimental breakthrough curves indicate that CAU-1 could recover C3H6 from the gas mixture of C3H6/N2 with an efficiency of over 99%. Besides, Received: Revised: Accepted: Published: A
April 5, 2019 June 16, 2019 July 9, 2019 July 9, 2019 DOI: 10.1021/acs.iecr.9b01877 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research
process was repeated three times. Subsequently, the green product was stirred in methanol at room temperature for 24 h. The activation process was also repeated three times. The final sample was collected and dried at 150 °C for 12 h under vacuum.37 MIL-101-NH2. Cr(NO3)3·9H2O (1 g, 2.5 mmol), H2BDCNH2 (0.415 g, 2.5 mmol), sodium hydroxide (4 mmol), and deionized water (15 mL) were mixed together in a Teflon-line stainless steel autoclave and ultrasonically mixed for 30 min. Then the mixture was heated to 150 °C for 12 h under vacuum. The activation process was the same as MIL-101.38 MOF-808. 1,3,5-Benzenetricarboxylic acid (2.1 g, 10 mmol), ZrOCl·8H2O (9.7 g, 30 mmol), DMF (450 mL), and formic acid (450 mL) were poured into a 1000 mL flask. The reaction was heated at 130 °C for 48 h. The sample was obtained by filtration and washed with DMF and then washed with acetone. The activated sample was obtained by drying at room temperature for 1 day and at 150 °C for 12 h.39 ZIF-8. A mixture of 1.078 g of ZnCl2, 0.972 g of 2methylimidazole, 0.54 g of HCOONa, and 80 mL of methanol was transferred into a Teflon-line stainless steel autoclave and heated at 140 °C for 24 h. Then the obtained white sample was soaked in methanol for 24 h and dried at 150 °C for 12 h under vacuum.40 2.3. Gas Adsorption Measurements. Gas adsorption− desorption measurements were performed on a Micromeritics ASAP 2020 instrument. Prior to measurement, the MOF materials were further activated at a degas station at 150 °C overnight. 2.4. Chemical Stability. In order to investigate the stability of CAU-1 in harsh chemical condition, about 60 mg of activated samples were soaked in 20 mL of water, boiling water, acidic aqueous solution (pH 1), and alkaline aqueous solution (pH 11), respectively. The samples were immersed in these solutions for 1 day. Afterward, the CAU-1 samples were centrifuged and washed with deionized water three times. 2.5. Breakthrough Experiment. In a typical experiment, CAU-1 powder (about 0.78 g) was packed into a column (φ 6 mm × 63 mm) held in place using quartz wool, and then activated at 423 K for 1 h. A helium flow (He ≥ 99.999%) was introduced into the column with a total rate of 20 mL min−1 as carrier gas. Then the raw mixed gases C3H6/N2 (30:70) with a total rate of 13.3 mL min−1 were introduced. The outlet gas concentrations were monitored by a mass spectrometer (BSDMass 100). The experimental temperature was about 298 K, and the pressure was 100 kPa. 2.6. Density Functional Theory (DFT) Calculation. The DFT method was used to calculate the interactions between CAU-1 and N2/C3H6/C3H8, and DMOL3 code in Materials Studio (v7.0) was used for the calculations. Spin-polarized density functional calculations were performed at the Becke three parameter hybrid exchange−correlation functional41,42 (B3LYP) level of theory. DFT semicore pseudopots were utilized for the core treatment. The double numeric polarization (DNP) basis set was used to describe atomic orbitals. Geometry optimization of CAU-1, N2, C3H6, and C3H8 was conducted before calculation. The binding energies (Ebd) between N2/C3H6/C3H8 and CAU-1 were calculated according to the following equation:
the good recyclability and the high stability under harsh chemical conditions demonstrate that CAU-1 is a potential candidate for C3H6/N2 separation in practical industrial application. This work shows that both the ultramicroporous structure and polar functional site are key factors for C3H6/N2 separation in MOF. More importantly, this work could also provide a guideline for the design and synthesis of highperformance MOFs for recovering propylene from polypropylene off-gas.
2. MATERIALS AND METHODS 2.1. Apparatus. The powder X-ray diffraction (PXRD) patterns of MOF samples were measured on a D8 Advanced X diffractometer equipped with a Cu sealed tube (λ = 1.5406 Å) with a step of 0.02° at room temperature. Nitrogen adsorption−desorption at 77 K was collected by a Micromeritics ASAP 2020 instrument. Fourier transform infrared (FTIR) spectra were examined with a Nicolet 6700 FT-IR spectrophotometer. Thermostabilities of the MOF samples were measured on a TGA-50 (SHIMADZU) instrument with a heating rate of 10 °C/min under air atmosphere. The morphologies of the MOFs were characterized by using a Hitachi S-4700 scanning electron microscope. 2.2. Preparation of MOFs. All MOFs studied in this work were prepared as previously referenced. Their detailed synthetic procedures are given below. CAU-1. Briefly, a mixture of AlCl3·6H2O (2.967 g, 12.3 mmol) and 1,4-benzenedicarboxylate acid (0.746 g, 4.1 mmol) was dissolved in 30 mL of methanol in a Teflon-line stainless steel autoclave and then heated at 125 °C for 5 h. Subsequently, the resulted microcrystalline powder was separated by filtration. The yellow product was dispersed in deionized water and stirred for 24 h, and the washing process was repeated three times to completely remove the residual Cl−. After that, the microcrystalline product was soaked and stirred in fresh methanol two times over 2 days. Finally, the activated sample was obtained by drying at 150 °C overnight under vacuum.34 UiO-66. 1,4-benzenedicarboxylate acid (H2BDC, 0.123 g, 0.75 mmol) and ZrCl4 (0.125 g, 0.54 mmol) were added to the mixed solution with N,N-dimethylformamide (DMF, 15 mL) and concentrated hydrochloric acid (1 mL). The mixture was sonicated for 30 min and added to a Teflon-line stainless steel autoclave and then heated at 80 °C for 12 h. The white solid was obtained by filtration and washed first with DMF (2 × 30 mL) and then with EtOH (2 × 30 mL). The activated sample was obtained by heating to 150 °C overnight under vacuum.35 UiO-66-NH2. The sample was synthesized analogously by replacing H2BDC with the equivalent molar amounts of 2aminoterephthalic acid (H2BDC-NH2).35 PCN-222. ZrOCl2·8H2O (2.0 g) and (4-carboxyphenyl) porphyrin (0.4 g) were dissolved into 800 mL of DMF/formic acid (5:3, v/v) in a flask. The flask was stirred and heated to reflux at 135 °C for 72 h. In the activation process, the initial sample was soaked in a mixture solution (400 mL of DMF and 15 mL of 8 M HCl) and heated at 120 °C for 24 h. Then this red product was washed six times by acetone and activated at 150 °C for 12 h under vacuum.36 MIL-101. Cr(NO3)3·9H2O (1 g) and H2BDC (0.415 g) were dispersed in deionized water (10 mL). Then the mixture was added to a Teflon-line stainless steel autoclave and heated to 200 °C for 24 h. In the activation process, the sample was placed in DMF and stirred at 100 °C for 24 h. The activation
E bd = EAB − EA − E B
(1)
where EAB is the total energy for the adsorption of N2, C3H6, or C3H8 to CAU-1; EA and EB are the energies of adsorbent B
DOI: 10.1021/acs.iecr.9b01877 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research
Figure 1. Illustration of crystal structures of MOFs: (a) UiO-66-NH2; (b) UiO-66; (c) PCN-222; (d) MOF-808; (e) MIL-101-NH2; (f) MIL-101; (g) ZIF-8; (h) CAU-1.
Figure 2. Adsorption isotherms of C3H6 and N2 in eight MOFs at 298 K: (a) CAU-1; (b) UiO-66-NH2; (c) UiO-66; (d) PCN-222; (e) MIL-101NH2; (f) MIL-101; (g) MOF-808; (h) ZIF-8. (I) Limiting selectivity of C3H6/N2 in these MOFs.
(CAU-1) and adsorbate (N2, C3H6, or C3H8), respectively. A
3. RESULTS AND DISCUSSION
more negative value of Ebd represents a stronger binding of N2,
To systematically evaluate the prospect of MOFs in propylene recovery, eight stable MOFs (Figure 1), including microporous, mesoporous, and functionalized MOFs, were synthe-
C3H6, or C3H8 with CAU-1. C
DOI: 10.1021/acs.iecr.9b01877 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Industrial & Engineering Chemistry Research
Figure 3. (a) C3H6 and N2 adsorption isotherms for CAU-1 at 273 and 298 K, respectively. (b) IAST-predicted selectivity of CAU-1 for C3H6/N2 (30/70) gas mixture at 273 and 298 K. (c) Qst of C3H6 and N2 in CAU-1.
Figure 4. Preferential adsorption sites and binding energies of C3H6 (a), C3H8 (b), and N2 (c) in CAU-1 obtained from DFT-optimized structural model.
2a−h) and the limiting selectivities of C3H6/N2 were also calculated (Figure 2I and Figure S3). As a result, CAU-1 exhibits the highest limiting selectivity among these MOF materials. The C3H6 adsorption isotherm of CAU-1 has a sharp increase at low pressure compared with other MOFs, suggesting the strong interaction between CAU-1 and C3H6 molecules. Considering that CAU-1 possesses the highest limiting selectivity, we further investigate the adsorption performance, adsorption mechanisms, and potential industrial application of CAU-1. To further explore the property of CAU-1 in gas separation, C3H6 and N2 adsorption isotherms were collected at 273 and 298 K. The C3H6 uptake amount of CAU-1 is 126 cm3/g at 1 bar and 273 K (Figure 3a). By contrast, the N2 uptake amount (8.8 cm3/g) is negligible at the same condition. As the temperature grows up to 298 K, the C3H6 uptake amount of CAU-1 reaches 115 cm3/g, outperforming those of UPC-21 (110.1 cm3/g),43 UPC-32 (98.2 cm3/g),44 UPC-33 (94.3 cm3/g),45 and UTSA-35a (73.9 cm3/g).46 Meanwhile, the uptake amount of N2 is only 4.4 cm3/g. The separation selectivity of the C3H6/N2 mixture (30:70, v:v) was evaluated by the ideal adsorbed solution theory (IAST). As expected, values of the IAST selectivity of CAU-1 for the C3H6/N2 mixture are 236 and 313, respectively, at 298 and 273 K under atmosphere pressure (Figure 3b). To further
Figure 5. Breakthrough curves of CAU-1 for C3H6/N2 gas mixture separation.
sized. As shown in Figure S1, the experimental XRD data of MOFs match well with those of the simulated one, confirming high crystallinity of these MOFs. The porosity structures of these MOFs were investigated by N2 adsorption (Figure S2), and the corresponding results are shown in Table S1. Subsequently, to screen out the optimal material for C3H6/ N2 separation, the pure-component adsorption isotherms of C3H6 and N2 in these MOFs were collected at 298 K (Figure D
DOI: 10.1021/acs.iecr.9b01877 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research
Figure 6. (a) PXRD patterns and (b) C3H6 adsorption of CAU-1 before and after being immersed in various harsh conditions. (c) Five cycles of C3H6 adsorption for CAU-1 at 298 K.
interactions between −NH2 groups in CAU-1 and C3H6, C3H8, and N2 molecules, respectively. As shown in Figure 4, the binding energy of C3H6 (−18.01 kJ/mol) is greater than those of C3H8 (−15.44 kJ/mol) and N2 (−10.64 kJ/mol). The distances between the H atoms in the −NH2 group and the C atoms of the CC bond in C3H6 are 2.542 and 2.734 Å, respectively. However, due to the H atom of C3H8 surrounding the C atom, the C atom cannot directly interact with the −NH2 group. The distance between the H atom in the −NH2 group and the C atom in C3H8 is 3.175 Å, which is larger than that in C3H6. These results confirm that amino groups can improve the electrostatic interactions between CAU-1 and C3H6. Therefore, the high C3H6/N2 selectivity in CAU-1 can be attributed to the ultramicroporous structures and polar functional sites. To explore the feasibility of CAU-1 for recovery of propylene from polypropylene off-gas, dynamic column breakthrough experiments were performed. As demonstrated in Figure 5, the N2 eluted through the fixed bed at the beginning, whereas propylene was retained in the fixed bed at first, until 14 min/g, and then reached equilibrium at 22 min/g. The experimental breakthrough curves confirm that CAU-1 can efficiently recover propylene from the C3H6/N2 mixture even at a dynamic condition. Moreover, CAU-1 can retain high C3H6 recovery efficiency (>99% (vol)) for about 13 min/g. As far as we know, CAU-1 is the first case of MOFs that can capture C3H6 from C3H6/N2 mixture through breakthrough experiments. It is noteworthy that the stability of adsorbent under harsh chemical conditions is vital to practical industrial application. To test the chemical stability of CAU-1, the samples were soaked in water, boiling water, HCl solution (pH 1), and NaOH solution (pH 11) for 1 day, respectively. As depicted in Figure 6a, the PXRD patterns of CAU-1 treated with various harsh conditions show similar characteristic peaks compared with those of pristine CAU-1. The pure component adsorption isotherms of C3H6 of these samples were also measured. As depicted in Figure 6b, C3H6 uptake of these samples shows no noticeable loss. These results demonstrate that CAU-1 exhibits good stability under harsh chemical conditions. Considering an ideal adsorbent should also possess good regenerability for practical applications,55,56 five adsorption−desorption cycles of C3H6 on CAU-1 were carried out. The propylene adsorption capacity on CAU-1 remains unchanged after five cycles (Figure 6c). All of these results indicate that CAU-1 is an ideal
confirm the excellent performance of C3H6/N2 for CAU-1, we have measured C3H6 and N2 adsorption isotherms of other porous materials (zeolite 5A and microporous activated carbon) with similar pore sizes at 298 K (Figure S4) and calculated the IAST selectivity of C3H6/N2 (Figure S5). As a result, CAU-1 exhibits higher selectivity than those of the zeolite 5A and microporous activated carbon. Furthermore, it should be noted that the C3H6 uptake is 115 cm3/g at 1 bar and 298 K in CAU-1, which is much higher than that of zeolite 5A. As far as we know, the C3H6/N2 selectivity in CAU-1 is higher than most porous materials (Table S2), confirming that CAU-1 is an ideal material for effective recovery of propylene from polypropylene off-gas. Furthermore, the isosteric heat of adsorption (Qst) and DFT calculation were used to explore the related adsorption mechanisms. To evaluate the adsorption affinity of C3H6 and N2 in CAU-1, the Qst was estimated by using the Clausius− Clapeyron equation. As depicted in Figure 3c, at zero coverage, the Qst of C3H6 of CAU-1 is 33.3 kJ/mol, higher than some other materials such as MFM-202a,47 UTSA-35a,45 LIFM38,48 and UPC-35.49 It is noticed that the Qst of C3H6 is constantly higher than that of N2, suggesting stronger interactions of CAU-1 with C3H6. Considering the larger molecular size of C3H6, the van der Waals interactions between C3H6 and frameworks are more sensitive than those of N2.50 Specifically, in the case of the ultramicroporous (
