Fabrication of cuprous nanoparticles in MIL-101: an efficient adsorbent for the separation of olefin–paraffin mixtures

Abstract

Various amounts of Cu⁺ nanoparticles were successfully deposited to the pores of metal–organic frameworks MIL-101 with a double-solvent method. An optimized, cuprous-loaded MIL-101 was shown to have an enhanced ethylene adsorption capacity and higher ethylene–ethane selectivity (14.0), compared to pure MIL-101 (1.6). The great improvement in selectivity can be attributed to the newly generated nano-sized cuprous chloride particles that can selectively interact with the carbon–carbon double bond in ethylene through π-complexation.

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Separations of olefin–paraffin mixtures are one of the most energy-intensive processes in the chemical industry due to their very small relative volatilities and similar molecular sizes.¹ Their separations, performed at a large scale, are currently achieved by cryogenic distillations, which are operated at low temperature (−30 °C) and high pressure (20 bar).² An alternative, low-energy consumption olefin–paraffin separation process would significantly reduce operating expenses. Among all the competing techniques toward this end, the adsorptive separation appears to be one of the most promising and energy-efficient processes.^2b Adsorbents with adequate selectivity and capacity play a crucial role in designing a practical adsorption process. To date, a number of solid adsorbents, including zeolites,³ activated carbon,⁴ carbon molecular sieves,⁵ and Ag(I) or Cu(I)-doped adsorbents,⁶ have been evaluated for the adsorptive separation of olefin–paraffin mixtures. Recently, an emerging class of solid materials known as metal–organic frameworks (MOFs), which offer a high surface area, adjustable pore dimensions, and chemical tunability, have also been examined for olefin–paraffin separation.⁷ In particular, MOFs with open-metal sites, like MOF-74 and HKUST-1, to some extent, show a potential in selective interactions with olefin by π-complexation.⁸ However, the MOFs with open-metal sites also have strong affinity toward water and are unstable when exposed to water vapor.⁹ Besides, the ultra-microporous MOFs with pore sizes similar to the kinetic diameter of olefin, such as zeolitic imidazolate frameworks, show a remarkable kinetic effect in the separation of olefin from paraffin.¹⁰

Over the past decades, research efforts have been mostly aimed at preparing new MOF structures and studying their applications in molecular storage and separation.¹¹ Currently, the loading of metal ions inside the porous matrices of MOFs is of interest, and have been used as catalysts.¹² It has been reported that metal ions, like Cu⁺, Ag⁺, Pd²⁺ and Pt²⁺, have a highly selective adsorption capability for compounds containing the C=C bond through a π-complex formation.¹³ Among the numerous MOFs reported so far, MIL-101, synthesized by Ferey et al.,¹⁴ is one of the excellent MOF materials due to its extra high specific surface area, pore volume, and high thermal and chemical stability in water as well as common organic solvents. The combination of these outstanding features makes it an interesting candidate for adsorption and catalysis. Moreover, the mesoporous cages can be used to confine the metal nanoparticles and to restrict their growth, which are promising features for their applications in adsorption. However, to the best of our knowledge, no reports exist on the effect of Cu⁺ nanoparticles in MOFs for ethylene and ethane separation. Herein, we reported the synthesis of a new π-complexation adsorbent using MIL-101 as the support and cuprous chloride as the active component, in which CuCl was introduced to MIL-101 by the “double-solvent” method.^12b Furthermore, the Cu⁺-supported MIL-101 (denoted as Cu⁺@MIL-101) was evaluated for the adsorptive separation properties of ethylene and ethane to take advantage of the nano-sized Cu⁺ sites for adsorption.

MIL-101, a highly crystallized green powder, was synthesized and purified according to the reported methods.^14,15 To avoid metal nanoparticle (NP) aggregation on external surfaces of the MIL-101 framework, we used the double-solvent method for the synthesis of Cu⁺@MIL-101.^12b Scanning electron microscopy (SEM) images (Fig. 1a) confirm that the synthesized MIL-101 is a highly crystallized regular octahedron with perfect cubic symmetry. The transmission electron microscope (TEM) image (Fig. 1d) confirms the cuprous loading and shows that the sizes of most Cu⁺ nanoparticles are mainly in the range of 1.5–4.0 nm, which are small enough to be accommodated in the mesoporous cavities of MIL-101.

Fig. 1 SEM images of (a and b) MIL-101 and TEM images of (c) MIL-101, (d) 40 wt% Cu⁺@MIL-101.

The powder X-ray diffraction (PXRD) patterns shown in Fig. 2 also confirm the structural integrity of the purified MIL-101. The main peaks of the MIL-101 match well with the PXRD pattern of the MIL-101 previously published by some investigators.^15,16 Fig. 2b and c show XRD patterns after Cu⁺ loading. Clearly, when the loading amount is less than 50 wt%, Cu⁺ can obtain good dispersion on MIL-101, and the dispersion is under the monolayer dispersion amount.¹⁷ However, when the loading amount exceeds 50 wt%, three major diffraction peaks at 28.5°, 47.5°, and 56.3° are observed. These peaks can be assigned to CuCl for indicating the aggregation of Cu⁺ on the surface of MIL-101. Besides, the main peaks of the MIL-101 still remain, demonstrating the constant intact structure of MIL-101 after Cu⁺ loading.

Fig. 2 X-ray patterns for Cu⁺-loaded MIL-101 samples with different CuCl loadings. (a) Raw; (b) 10%, 20%, 40 wt% loading; (c) 60%, 80%, 100 wt% loading; (d) simulated.

To further confirm the cuprous loading, X-ray photoelectron spectroscopy (XPS) was also performed. The presence of Cu⁺, which is similar to that of CuCl rather than CuCl₂, is supported by the XPS spectrum (Fig. 3). In addition, the BET surface area and pore volume of the Cu⁺@MIL-101 are analysed and summarized in Table S1.† For Cu⁺@MIL-101 with 40 wt% loading, the BET surface area significantly decreases from 3120 to 1587 m² g⁻¹, and the pore volume decreases from 2.12 to 1.01 cm³ g⁻¹ after loading the CuCl, which confirms that the Cu⁺ nanoparticles are successfully dispersed within the pores of MIL-101. Furthermore, the sacrificed cumulative volume contributed by the mesopores around 2.5 nm can be observed from the pore-size distribution curve shown in Fig. S1.†

Fig. 3 XPS spectra of Cu regions of Cu⁺@MIL-101. (a) CuCl₂; (b) CuCl; (c) 40 wt% Cu⁺@MIL-101.

Single-component adsorption isotherms of ethylene and ethane at 303 K and 1 bar, respectively, on the pure and composite MIL-101 adsorbents loaded with different Cu⁺ amounts are shown in Fig. S4;† all the adsorption isotherms could be well described by the double-site Langmuir model. The Henry constant (H), equilibrium selectivity (α), and adsorbent selection parameter (S) are summarized in Table S1.† The introduction of Cu⁺ leads to a preference for C₂H₄ adsorption at 1 bar (from 2.2 to 2.75 mmol g⁻¹) and an obviously enhanced selectivity of ethylene over ethane (from 1.6 to 16.5), and the selectivity also increases with the loading of Cu⁺. Fig. 4 compares the isotherms on the undoped MIL-101, 40 wt% Cu⁺@MIL-101, and pure CuCl. Interestingly, the pure CuCl shows almost no ethylene adsorption due to its negligible specific surface area. Therefore, for enhancing ethylene adsorption, well dispersed and nano-sized particles of Cu⁺, which offer larger surface areas, are particularly important. The enhanced C₂H₄ uptake and selectivity can be ascribed to the well-dispersed Cu⁺ that generates additional adsorption active sites on MIL-101 and increases the energetic heterogeneity of the surface, while these newly generated Cu⁺ sites can selectively interact with the C=C bond in ethylene through a π-complexation. Meanwhile, the original physical adsorption sites on MIL-101 are covered by Cu⁺, which leads to the decrease in ethane uptake. Furthermore, the more Cu⁺ ions are introduced, the fewer physical adsorption sites remain. As a result, the Cu⁺-loaded MIL-101 samples exhibit selective adsorption of ethylene over ethane. However, when Cu⁺ loading is relatively high (>50 wt%), some pores were blocked by the larger aggregates formed from the growth of the smaller Cu⁺ nanoparticles, thus decreasing the ethylene uptake.

Fig. 4 Ethylene and ethane isotherms for the pure, 40 wt% Cu⁺-loaded MIL-101, and pure CuCl at 30 °C and 1 bar.

Additionally, the Ideal Adsorbed Solution Theory (IAST)¹⁸ was applied to predict the selectivity for separating a mixture of ethylene and ethane. The selectivity of C₂H₄/C₂H₆ with different molar fractions and varied total pressures are shown in Fig. S6.† The C₂H₄/C₂H₆ selectivity of 40 wt% Cu⁺-loaded MIL-101 is much higher than that of the undoped MIL-101, despite different molar fractions of the two adsorbates. For example, the C₂H₄/C₂H₆ selectivity is 1.76 on the undoped MIL-101 at a C₂H₄ molar fraction of 0.2, while a selectivity of 8.7 is obtained on Cu⁺-loaded MIL-101. The selectivity with total pressures ranging from 1 to 4 bar were also calculated, in which the composition was fixed at 50 : 50% and reaffirmed that C₂H₄/C₂H₆ selectivity could be largely enhanced by loading Cu⁺. We summarized the most widely used adsorbents for C₂H₄/C₂H₆; their equilibrium uptake at about 30 °C and 1 bar are listed in Table S2.† Obviously, the traditional adsorbents, like activated carbon and zeolites, have a low uptake capacity, although some have a high selectivity of up to 2.52. In contrast, metal–organic frameworks with open-metal sites usually have a high adsorption amount, but poor selectivity is impractical for an industrial adsorption process.

To further investigate the affinity after the modification of MIL-101, single-component gas adsorption isotherms (Fig. S5†) were measured on Cu⁺@MIL-101 with 40 wt% loadings of CuCl at 1 bar pressure and temperatures varying from 303 K to 323 K. The initial isosteric heats (Fig. S7†) for ethylene and ethane are determined to be about 40 and 25 kJ mol⁻¹, respectively. These moderate values are comparable with those of 13X zeolite,^3b Cu₃(BTC)₂,^8b and Mg-MOF-74.^8c The kinetic adsorption profiles of ethane and ethylene recorded at pressures of 60 mmHg and 298 K, respectively, are shown in Fig. S8.† There is no significant reduction in diffusion kinetics for ethane–ethylene adsorption after the Cu⁺ nanoparticles were confined in the cages. The intracrystalline diffusivity D_c/r_c² for ethane and ethylene in the undoped and 40 wt% Cu⁺@MIL-101 studied in this work were 5.72 × 10⁻², 2.85 × 10⁻², 3.79 × 10⁻², and 2.06 × 10⁻² s⁻¹, respectively. The decreased diffusion time constants for the ethane–ethylene pair in the Cu⁺@MIL-101 also double-check the successful deposition of Cu⁺ NPs. Compared to ethane, slower kinetics were observed in ethylene in both undoped and Cu⁺@MIL-101, which can be explained by the enhanced surface interaction energy between Cu⁺ and ethylene by the π-complexation.¹⁹

To compare the adsorptive separation between raw MIL-101 and Cu⁺-doped MIL-101, we performed transient packed-bed breakthrough calculations using the Aspen™ Adsorption module. Fig. 5 provides a comparison of the breakthrough characteristics for a 50/50 ethane–ethylene mixture at 303.15 K and 1 bar for the MIL-101 and Cu⁺-doped MIL-101. It was obvious that the ethylene adsorption capacity significantly improved in the case of Cu⁺-doped MIL-101. In summary, we demonstrated the successful deposition of Cu⁺ nanoparticles in porous MIL-101 through a double-solvent method. A well-dispersed Cu⁺-loaded MOF maintained its structure, and exhibited higher C₂H₄ uptake and higher ethylene–ethane selectivity (14.0), compared to the undoped MIL-101 (1.6). This can be attributed to the newly generated Cu⁺ sites that can selectively interact with the C=C bond in ethylene through the π-complexation.

Fig. 5 Breakthrough curves of equimolar mixtures of ethane–ethylene in columns of MIL-101 (a) and Cu⁺@MIL-101 (b) at 303.15 K and 1 bar calculated using Aspen™ (black: ethylene; red: ethane).

This work was supported by the National Natural Science Foundation of China (21376205 and 20936005), the Zhejiang Provincial Natural Science Foundation of China (LR13B060002), and the Zhejiang Province Key Science and Technology Innovation Team (2011R50002).

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Footnote

† Electronic supplementary information (ESI) available: Details of synthesis, adsorption experiments and characterization. See DOI: 10.1039/c4ra02125h

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