Metal confined in metal‐organic framework‐based mixed matrix membranes for efficient butadiene recognition separation

The efficient separation of butadiene (1,3‐C4H6) from C4 hydrocarbons is a critical step in petrochemical processes. However, the traditional cryogenic distillation suffers from energy‐intensity and serious environmental stress, necessitating the development of alternative technologies for efficient 1,3‐C4H6 separation. Herein, a 1,3‐C4H6 recognition mixed matrix membrane is reported via incorporating metal copper encapsulated a metal‐organic framework (CuBTC@Cu) into elastic poly(dimethylsiloxane) (PDMS). The resulting CuBTC@Cu/PDMS membrane can efficient separate 1,3‐C4H6 from various C4 hydrocarbons including 1,3‐C4H6/n‐C4H8, 1,3‐C4H6/iso‐C4H8, 1,3‐C4H6/n‐C4H10 and 1,3‐C4H6/iso‐C4H10, yielding superior selectivity of 5.11, 6.35, 4.78, and 10.30, respectively, with 1,3‐C4H6 permeability of 53240 Barrer. Notably, the appropriate π‐complexation interaction between butadiene molecules and CuBTC@Cu as well as suitable transmission channel size enable the membrane only permeable to 1,3‐C4H6 and block the permeation of other C4 hydrocarbons, showing a unique 1,3‐C4H6 recognition behavior in membrane separation. The concept of affinity‐relying separation combining molecular sieving would open a new direction for designing gas membranes for efficient light hydrocarbon separations.

Preparation of mixed matrix membranes (MMMs) by combining porous fillers such as zeolites and metalorganic frameworks (MOFs) with polymers is considered a promising approach to simultaneously improve selectivity and permeability. [13,14][22][23][24][25] At present, MOFsbased MMMs have been widely used in olefin/paraffin separation, including C 2 H 2 /C 2 H 4 , C 3 H 6 /C 3 H 8 , n-C 4 H 10 /iso-C 4 H 10 and so on. [3]For example, Liu et al. [3,26] prepared a Y-fum-fcu-MOF(30 wt%)/6FDA-DAM MMM to separate C 4 H 10 isomers through the molecular sieving mechanism, the selectivity is 35, and the permeability is 4.1 Barrer at 1.72 bar pressure and 75 • C. Bachman et al. [27] researched a Ni 2 (dobdc)(25 wt%)/6FDA-DAM MMM for C 2 H 4 /C 2 H 6 separation, Ni 2 (dobdc) with a functional surface can induce MOFs-polymer interaction to enhance the separation performance, the ideal separation selectivity and permeability respectively are 4.6 and 128 Barrer at 2.0 bar feed pressure and 35 • C. Japip et al. [28] fabricated a 6FDA-Durene-ZIF-71(20 wt%) MMM for C 3 H 6 /C 3 H 8 separation through molecular sieving effect due to the channel size of ZIF-71 is between the kinetic diameters of C 3 H 6 and C 3 H 8 , the ideal separation selectivity is 8.4, and the permeability is 371 Barrer under 2.0 bar feed pressure and 35 • C. Despite these successes, the separation of 1,3-C 4 H 6 from multiple C 4 hydrocarbons by MOFs-based MMMs has not been reported due to their similar molecular size, boiling point, and polarizability (Table S1). [3,5,6,29]Therefore, the development of MOFs-based MMMs to separate 1,3-C 4 H 6 from other major C 4 hydrocarbons has important application value for the recovery of 1,3-C 4 H 6 .
PDMS is a silicone rubbery material with excellent mechanical properties, low cost, and low chemical reactivity, [30,31] and the PDMS-based membranes have good gas permeability, good stretchability and aging tolerance. [32,33]Moreover, the solubility coefficient and diffusion coefficient are important parameters to assess the separation performance of membrane.Compared with other C 4 hydrocarbons, the 1,3-C 4 H 6 molecule with two π-bonds has the strongest affinity with metal copper (Cu) and can form π-complexation, which helps to enhance the 1,3-C 4 H 6 solubility in the MMMs. [10,34,35]Besides, 1,3-C 4 H 6 has the smallest kinetic diameter among C 4 hydrocarbons (Table S1), and based on the molecular sieving mechanism, [28] it is expected to improve 1,3-C 4 H 6 /other C 4 hydrocarbons separation selectivity of MMMs by designing suitable transmission channel size of MOFs.
Herein, we employ an in situ encapsulation strategy to confine metal Cu in the copper-1,3,5-benzene tricarboxylate (CuBTC) framework, and then combined it with poly(dimethylsiloxane) (PDMS) polymer to fabricate highstrength and thermally stable MMM (CuBTC@Cu/PDMS).The metal Cu nanocrystals encapsulated in the framework strengthen the affinity and π-complexation between the MMMs and 1,3-C 4 H 6 , and diminish the transmission channel size of the framework, which realize the effective recognition of 1,3-C 4 H 6 from other C 4 hydrocarbons and possess

Characterization of CuBTC@Cu
The morphology and size of CuBTC@Cu nanocrystals are consistent with that of CuBTC under Scanning electron microscopy (SEM) observation (Figure S4), indicating that the introduction of Cu does not affect the growth of CuBTC under pre-synthesis strategy. [36]From the Powder X-ray diffraction (PXRD) pattern of Cu-based precursor (Figure 1A), it can be seen that the pre-synthesized product clearly shows the diffraction peaks of metallic Cu (PDF#04-0836) and Cu 2 O (PDF#05-0667). [37]Briefly, the diffraction peaks of Cu are located at 43.3 O is due to the fact that the reduced metal Cu is easily oxidized when exposed to air. [38]By contrast, the peaks intensity of Cu 2 O is stronger than metallic Cu.However, after coordination with 1,3,5-benzenetricarboxylic acid (H 3 BTC), the PXRD pattern (Figure 1B) of CuBTC@Cu matches well with that of CuBTC, and the main diffraction peaks at 6.7 • , 9.5 • , and 11.6 • , respectively, corresponding to the (200), (220), and (222) crystal planes, indicating that the crystalline structures are not changed significantly. [39]However, new peaks appear at 2-theta angles around 43.3 • and 50.5 • , which correspond to the diffraction from the (111) and (200) planes of cubic metallic Cu with Fm-3m space group, respectively. [40]In the High-resolution transmission electron microscopy (HRTEM) measurements (Figure 1C), the lattice fringes with a spacing of 0.21 nm belonging to Cu {111} plane can be observed, which confirms that the metal Cu nanoparticles are confined in the CuBTC framework, [37,41] and the Cu particles with a diameter of 2.45 nm are uniformly distributed in the framework. [42]he chemical valence states of Cu-based precursor, CuBTC and CuBTC@Cu samples were analyzed by X-ray photoelectron spectroscopy (XPS) measurements (Figure S5A and Figure 2A,B).For the Cu-based precursor, the presence of Cu and Cu + was observed, and Cu + was generated from the oxidation of the tested Cu surface by air. [38]For CuBTC particles, the peaks in 2P 3/2 and 2P 1/2 spectra are located at 934.7 eV and 954.4 eV, which are both attributed to divalent Cu. [40] However, the CuBTC@Cu particles socketed with metal Cu exhibit significantly different copper chemical valence states compared with CuBTC.In the Cu 2P 3/2 and Cu 2P 1/2 spectra of CuBTC@Cu, in addition to Cu 2+ at 934.8 and 954.6 eV, the peaks at 932.6 and 952.3 eV prove the presence of Cu 0 , and the atomic contents of Cu 2+ and Cu 0 respectively are 74.39% and 25.61%. [43,44]The XPS analysis results are agree with PXRD and HRTEM, that is, the metallic Cu nanoparticles are successfully socketed in the CuBTC framework. [41,45,46]The Brunauer-Emmett-Teller (BET) surface area and pore size distribution of the Cu-based precursor, CuBTC and CuBTC@Cu particles (Figure S6 and Figure 2C,D) were determined through N 2 adsorption at 77 K. Figure S6 shows that the Cu-based precursor does  not have micropores.However, after coordinate with ligand, the N 2 adsorption-desorption isotherms of CuBTC and CuBTC@Cu can be identified as Type I, confirming the existence of micropores. [17,47]The BET surface area and micropore volume of CuBTC are 934 m 2 /g and 0.40 cm 3 /g, and CuBTC@Cu are 749 m 2 /g and 0.26 cm 3 /g, respectively.Additionally, the minimum pore size of CuBTC@Cu is reduced from 6.79 to 5.89 Å, which further indicates that the Cu nanoparticles are successfully confined in the CuBTC framework and shrink the transmission channel size.As shown in Figure S5B, the FT-IR characteristic bands of prepared CuBTC@Cu crystals are at 729, 1110, 1449, and 1649 cm −1 , which correspond closely with those of CuBTC particles and confirmed that the CuBTC@Cu have the same chemical structures as CuBTC. [47]The adsorption bands at 729, 1110, 1449, and 1649 cm −1 can be respectively F I G U R E 3 (A) PXRD and (B) FTIR spectrum of CuBTC@Cu, pure poly(dimethylsiloxane) (PDMS) membrane, CuBTC/PDMS mixed matrix membrane (MMM) and CuBTC@Cu/PDMS MMM.47][48]

Characterization of MMMs
Subsequently, a certain amount of MOF fillers was incorporated into PDMS, followed by scraping membrane, high-temperature curing and exfoliation processes to fabricate free-standing MMMs with fixed filler content of 15 wt%.As shown in Figure 3A, the broad diffraction peaks in the ranges of 9−14 • of membranes belong to PDMS, [12] indicating that the pristine PDMS membrane was amorphous. [36]Compared with pure PDMS membrane, the CuBTC@Cu/PDMS membrane exhibits new diffraction peaks are ascribe to the CuBTC@Cu, [39,49,50] which indicate that the crystalline structure and internal pore channel of CuBTC@Cu nanoparticles were retained well after the combine with PDMS matrix. [12]Additionally, the Fourier transform infrared spectroscopy (FTIR) spectra of PDMS membrane, CuBTC/PDMS MMM and CuBTC@Cu/PDMS MMM are presented in Figure 3B.The characteristic peaks at 1068 and 1258 cm −1 in all membranes are attributed to the CH 3 symmetric and Si-O stretching vibration of PDMS. [36]After combined with CuBTC@Cu, the CuBTC@Cu MMM exhibits new peaks at 492, 729, 1449 and 1649 cm −1 , which are respectively ascribed to the Cu-O stretching vibrations, C-H out-of-plane bending vibration, C-O asymmetric stretching and C=O symmetric stretching, respectively. [39,45,51]These results can prove that CuBTC@Cu particles are successfully incorporated into PDMS matrix.In addition, no new absorption peaks were observed manifest that only a physical interaction occurs between CuBTC@Cu and PDMS. [30]he surface and cross-section morphologies of pure PDMS and CuBTC@Cu/PDMS membranes were recorded by SEM (Figure 4).The surfaces of PDMS and CuBTC@Cu/PDMS membranes are smooth and defectsfree, and the CuBTC@Cu nanocrystals are homogeneous dispersion in PDMS matrix with no interfacial defects, which indicate the interaction and compatibility between PDMS and CuBTC@Cu are well. [36]Moreover, the thickness of the CuBTC@Cu/PDMS membrane is about 60 µm, and Cu elements are well dispersed in the cross-section from mapping images (Figure S7), which further indicates that CuBTC@Cu particles are evenly dispersed in the PDMS.It is well known that the uniform dispersion of MOFs is beneficial to the diffusion of gas molecules in the MOFs pore channel. [12]he thermal stability information of crystals and membranes can be obtained by Thermogravimetric (TG) curves (Figure 5A).CuBTC and CuBTC@Cu crystals have similar weight drop stages.[47] The first weight drop stage starts at around 70 • C is resulted from the removal of residual solvent or guest molecules in crystals.[52] The second weight drop stage of CuBTC@Cu starts at 310.5 • C was higher than CuBTC (244.5 • C) corresponds to the decomposition of H 3 BTC in the crystals structure and production of copper oxides, [33,52] which indicated that the thermal stability of CuBTC@Cu is better than CuBTC.[53] After 800 • C, about 43.73% mass remained of CuBTC@Cu due to the metal components are not easy to decompose at the test temperature. All embranes exhibited obvious weight loss between 480 and 800 • C owing to the decomposition of PDMS polymer, [54] and the CuBTC@Cu/PDMS membrane still possesses higher thermal stability compared with CuBTC/PDMS membrane.PDMS is a hydrophobic polymer, and wrapping it on the surface of Cu-BTC@Cu can significantly improve the hydrophobicity of Cu-BTC@Cu (Figure S8).[46] The PXRD patterns of the Cu-BTC@Cu membrane before and after immersing in water for 5 days are highly consistent (Figure S9) and the hydrophobicity of the membrane is maintained (Figure S8d), indicating that the framework integrity of CuBTC@Cu is maintained and the CuBTC@Cu/PDMS membrane has excellent water stability.
The mechanical properties of pure PDMS, CuBTC/PDMS, and CuBTC@Cu/PDMS membranes were evaluated by stress−strain curves, and the results are illustrated in Figure 5B and Table S2.The prepared CuBTC@Cu/PDMS membrane has excellent tensile stress and elongation due to the longer flexible segment of PDMS. [36]The ultimate tensile stress for the CuBTC@Cu/PDMS membrane is 3.87 MPa and higher than CuBTC/PDMS membrane (1.25 MPa), which indicated that the CuBTC@Cu/PDMS membrane have well interface compatibility between fillers and polymer matrix than CuBTC/PDMS membrane. [36,45]In addition, the CuBTC@Cu/PDMS membrane can withstand a load of 100 g, which shows its promising industrial applications. [36] I G U R E 4 Surface and cross-sectional SEM images of pure poly(dimethylsiloxane) (PDMS) membrane (A and B) and CuBTC@Cu/PDMS mixed matrix membrane (MMM) (C and D).

Gas separation performance
The single gas permeability and ideal selectivity of pure PDMS, CuBTC/PDMS, and CuBTC@Cu/PDMS membranes were tested by constant-volume method and are shown in Figure 6, Table 1, and Table S3.It can be observed that the incorporation of CuBTC or CuBTC@Cu improves the permeability of 1,3-C 4 H 6 compared with the pure PDMS membrane.The 1,3-C 4 H 6 permeabilities of CuBTC/PDMS and CuBTC@Cu/PDMS membrane respectively are 19060 and 53240 Barrer at 1.0 bar feed pressure, indicating that CuBTC@Cu crystals have stronger affinity for 1,3-C 4 H 6 than CuBTC, [55] which is consistent with the adsorption isotherm data of CuBTC and CuBTC@Cu (Figure S10).Excitingly, the n-C 4 H 8 , iso-C 4 H 8 , n-C 4 H 10 , and iso-C 4 H 10 permeabilities of the CuBTC@Cu/PDMS membrane showed a small increase compared with the pure PDMS membrane.In view of this, the separation of  the membrane remained almost constant during 48 h test, indicating that the CuBTC@Cu/PDMS membrane has good long-term permeation stability (Figure S11). [46]he gas separation mechanism of MMMs can be analyzed by a solution-diffusion model, which assumes that the polymer membrane has no perpetual pore-selective layer, and that the different chemical species separation is based on their solubility and diffusion coefficient differences. [26,55]fter incorporation of CuBTC@Cu into PDMS membrane, the transport properties were changed.We found that the solubility coefficient of 1,3-C 4 H 6 in CuBTC@Cu MMMs was significantly higher than that of the other four C 4 hydrocarbons, thus remarkably improving the separation selectivity of 1,3-C 4 H 6 /other C 4 hydrocarbons.The increased solubility of 1,3-C 4 H 6 in CuBTC@Cu MMMs originates from the strong π-complexation interaction between the two C=C double bonds of unsaturated 1,3-C 4 H 6 and Cu encapsulated in the CuBTC framework. [34,35]Besides, 1,3-C 4 H 6 with rigid and straight molecular shape has the smallest kinetic diameter among C 4 hydrocarbons, [10,11] and metallic Cu encapsulated in the framework pore structure can shrink the transmission channel size of CuBTC to further enhance molecular sieving effect. [28]The synergistic effect of strong affinity and molecular sieving mechanism realizes the CuBTC@Cu MMM to recognize 1,3-C 4 H 6 from C 4 hydrocarbons (Figure 7).Compared with other reported C 4 hydrocarbon membranes, the CuBTC@Cu MMM possess the highest 1,3-C 4 H 6 permeability (Table S4), [3,[9][10][11]26] and there are no reports on the simultaneous separation of 1,3-C 4 H 6 from other four C 4 hydrocarbons.

high 1 , 3 -
C 4 H 6 permeability.This strategy may open a new avenue for designing MOF-based MMMs for efficient C 4 hydrocarbon separation.

F
I G U R E 1 (A) PXRD pattern of the Cu-based precursor, (B) PXRD patterns of CuBTC and CuBTC@Cu particles, and (C) the HRTEM images of CuBTC@Cu.The Cu nanoparticle is marked with red circle, and its corresponding interplanar spacing is shown in the right figure.

F I G U R E 2
XPS Cu 2p spectra of (A) CuBTC and (B) CuBTC@Cu, and (C) N 2 adsorption isotherms at 77 K and (D) pore size distribution of CuBTC and CuBTC@Cu samples.