A multi-scale porous composite adsorbent with copper benzene-1,3,5-tricarboxylate coating on copper foam

Abstract

A multi-scale porous composite adsorbent with a micropore copper benzene-1,3,5-tricarboxylate coating on macropore copper foam (Cu-BTC/CF) is synthesized in this study through electrochemical deposition. Cu-BTC/CF compensates for the limitations of common metal organic frameworks in terms of low thermal conductivity, high pressure drop, and poor selectivity. The thermal conductivity and adsorption isotherms (N₂, CH₄, and CO₂) in Cu-BTC/CF are experimentally investigated at 20, 40, 60, and 80 pores per linear inch (PPI). The pressure drop, temperature response, and selectivity of CO₂/N₂ and CH₄/N₂ in the Cu-BTC/CF adsorption bed are simulated and predicted accordingly. The pressure drop of Cu-BTC/CF is 3.0–33.8% of pure Cu-BTC powder with a velocity range of 0.1–0.8 m s⁻¹. The effective thermal conductivity of Cu-BTC/CF is 1.59–27.52 times higher than that of pure Cu-BTC powder. The time for Cu-BTC/CF to satisfy the desorption condition (above 373.15 K) is much shorter than that of pure Cu-BTC. The temperature uniformity for the Cu-BTC/CF adsorption bed is correspondingly improved compared with that for a pure Cu-BTC adsorption bed. Moreover, the selectivity of CO₂/N₂ in 60 PPI Cu-BTC/CF is 1.36–7.44 times higher than that in pure Cu-BTC, and the selectivity of CH₄/N₂ in 80 PPI Cu-BTC/CF is 1.41–2.95 times higher than that in pure Cu-BTC at 0–100 kPa and 273.15 K.

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1. Introduction

Metal organic frameworks (MOFs) are crystalline nanoporous materials that consist of metal ions and polyatomic organic bridging ligands. Metal ions function as coordination centers and are connected to ligands by strong coordination bonds. MOFs exhibit potential in gas storage¹ and separation² because of the distinct structures and high thermal and mechanical stability of their frameworks. Copper benzene-1,3,5-tricarboxylate coating (Cu-BTC) is a promising adsorption and separation porous medium because it has good adsorption ability and structural stability except with water. Zhang et al.³ used Cu-BTC in a pressure swing adsorption system to evaluate CO₂ capacity and CO₂/N₂ selectivity. In their study, Cu-BTC adsorbs 12.7 mol kg⁻¹ CO₂ at 298.15 K and 1500 kPa and exhibits a CO₂/N₂ selectivity of approximately 20. Aprea et al.⁴ reported that Cu-BTC possesses higher CO₂ adsorption capacity and lower computational heat release than 13X zeolite under different pressures at ambient temperature.

High adsorption capacity and selectivity, long-term stability, and easy regenerability are the key factors for efficient adsorbent bed design. These factors are influenced by adsorbent crystal growth, thermal conductivity, and temperature uniformity of the adsorbent bed during desorption, respectively.

Gas adsorption and selectivity are important performance indicators of adsorbents. MOFs have been subjected to various modification procedures, particularly through the formation of MOF–inorganic composites, such as MOF/silica,⁵ MOF/carbon nanotubes,⁶ and Cu-BTC/graphite oxide (GO),⁷ to enhance adsorption capacity and selectivity. GO can prevent aggregation and induce the dispersion of nanosized MOF crystallites to improve the porosity and accessibility of the MOF network. Policicchio et al.⁸ added 9% GO to Cu-BTC and achieved about a 30% increase in CO₂ storage capacity. Guo et al.⁹ used a twin copper source to synthesize a MOF membrane with increased selectivity; the fabricated membrane exhibits high permeability and selectivity for recycling H₂. To date, MOF materials are available in powder forms, which cannot be easily retrieved from a sample matrix. Hu et al.¹⁰ applied in situ solvothermal growth method to immobilize MOF-5 on porous foam. The composite demonstrates high extraction sensitivity and good selectivity to plant volatile sulfides owing to the extraordinary porosity of the MOFs and the interaction between S-donor sites and surface cations at the crystal edges. Mao et al.¹¹ fabricated a continuous and well-intergrown Cu-BTC film on a polymer hollow fiber by using solid copper hydroxide nanostrands as the copper source. The hollow fiber composite membrane exhibits good separation performance for binary gases with selectivity 116% higher than the Knudsen values.

Thermal conductivity is also an important property of adsorbents. The exothermic heat of adsorption should be expelled efficiently to maximize adsorbent performance, because gas storage applications require rapid heat uptake. Moreover, high temperatures can reduce storage capacity,¹² and MOFs usually exhibit poor thermal conductivity but high porosity (>80%). For example, the thermal conductivity of MOF-5 crystal is 0.32 W (m⁻¹ K) at 300 K,¹³ whereas the effective thermal conductivity of MOF-5 powder is only 0.091 W (m⁻¹ K).¹⁴ The effective thermal conductivity can be improved using highly conductive adsorbent materials, fins, or other thermal enhancements. For example, adding natural graphite to MOF-5 can improve the effective thermal conductivity of MOF-5 powder.¹⁵

Sorbents must possess long-term stability and regenerability and minimum temperature differences between locations near and far from the heat source in the desorption process for potential practical applications. Thus, inorganic matrix is used as matrix coated by MOFs to enhance the mechanical stability of MOFs. Küsgens et al.¹⁶ adopted in situ synthesis method to coat Cu-BTC on cordierite monoliths, they found that the monolithic structures have a high mechanical stability of 320 N with specific surface areas of 484 m² g⁻¹. Besides, sorbents require short cycle time and heating at high temperatures (above 373.15 K) and must be under vacuum or inert gas purge during desorption. Liu et al.¹⁷ employed helium (He) as the inert gas purge to recover the nickel 2,5-dioxido-1,4-benzenedicarboxylate (Ni/DOBDC) of the adsorbed CO₂; six percent residual CO₂ in Ni/DOBDC is completely removed by heating at 523.15 K with the He purge. Wurzbacher et al.¹⁸ studied the amine-functionalized nanofibrillated cellulose sorbent material within 293.15–368.15 K at a desorption pressure of 6.2 kPa in a packed bed; more than 90% of the captured CO₂ is recovered at >99% purity.

Metal materials can also be used as matrix besides the inorganic matrix. Copper foam (CF) is an excellent macropore material used to exchange large amounts of heat within a small volume. CF exhibits several advantages, such as high specific surface area and high solid thermal conductivity. Cu-BTC grown on skeletons with high thermal conductivity can increase the adsorption ability and easily void the heat accumulation caused by adsorption. Furthermore, suitable fabrication formats can facilitate enrichment procedures and simplify sampling devices. Forming a multi-scale porous composite adsorbent with high thermal conductivity is more practical than using only pure MOFs. Directly coating the MOF material on smooth and nonporous fiber is difficult. The weak adhesion of the MOF coating on the substrate leads to poor coating and low stability. In this study, a multi-scale porous composite adsorbent with micropore Cu-BTC coating on the macropore copper foam (Cu-BTC/CF) is synthesized through electrochemical deposition to compensate for the limitations of pure Cu-BTC. Thermal conductivity and adsorption are investigated at 20, 40, 60, and 80 pores per linear inch (PPI). Pressure drop, desorption temperature, and selectivity in the Cu-BTC/CF adsorption bed are also simulated.

2. Material preparation and characterization

The experiment consists of two main parts, namely, pretreatment and synthesis. In the pretreatment, CFs with different pore densities are cut into pieces (25 mm × 25 mm × 4 mm), as shown in Fig. 1(a). The CF is through-hole without difference between the inside and outside of CF pore. The samples are successively washed with dilute hydrochloric acid (3 mol L⁻¹), acetone, ethanol, and deionized water under ultrasound for 15 min each and then dried at 323.15 K prior to use.

Fig. 1 Schematic of the electrochemical growth of Cu-BTC on porous support. (a) Process of Cu-BTC coating on copper foam. (b) Percent of Cu-BTC loading and the surface area of Cu-BTC/CF and pure Cu-BTC.

In the synthesis, Cu-BTC/CF is prepared using a typical two-electrode system. Fig. 1(a) shows the electrochemical growth process of Cu-BTC crystals on porous copper supports. Tributylmethylammonium methyl sulfate (127 mmol, 4.0 g) and 1,3,5-benzenetricarboxylic acid (9.5 mmol, 2.0 g) are dissolved in 150 mL of ethanol. The solution is heated to 328.15 K and maintained at this temperature during the entire synthesis with constant magnetic stirring under 300 rpm. The pretreated CF substrates are used as electrodes spaced 20 mm apart, and a 50 mA current is passed through the electrochemical cell for 90 min. Bubbles rapidly rise near the CF cathode, and the solution gradually turns light blue, which indicates the formation of copper ions because of CF anode oxidizing. The initial formation of nuclei is considered effectively instantaneous. Copper oxidation and linker concentration are main limitations for the formation of Cu-BTC powder. A saturated solution of the linker and an electrolyte are put into the cell to limit the influence of linker concentration during electrochemical synthesis. Therefore, the formation of Cu-BTC is controlled by adjusting current density, which affects the Cu²⁺ production. Increasing the temperature can also increase diffusion and synthesis yield. Cu-BTC is formed in the double layer around the electrode, leading to the direct formation of Cu-BTC crystals on the electrode surface. Continuing synthesis results in more crystal growth on the uncovered metal surface. When the electrode surface is completely covered, the Cu-BTC film thickens, and the charge transfer and transport of Cu²⁺ and linker become difficult under continuous applied current. Further dissolution of copper can cause cracking of the Cu-BTC membrane. Several large crystals are detached, and a new electrode surface is available for further reaction. The CF supports the copper source for the entire process of Cu-BTC crystal growth. The total amount of copper species is 0.0596 g taken from the CF material according to the charge conservation.¹⁹ Fig. 1(b) shows the amount of Cu-BTC growth on CF and the surface area of Cu-BTC/CF at different pore densities. The amount of Cu-BTC coating on CF at 60 PPI is higher compared with the other CFs. The higher amount of Cu-BTC at 60 PPI leads to a higher surface area compared with the other CFs.

ESI† presents the detailed characteristics of Cu-BTC and Cu-BTC/CF. X-ray diffraction patterns, the method of the effective thermal conductivity test, and N₂ adsorption and desorption isotherms verify the successful Cu-BTC coating on the CF surface. N₂, CO₂, and CH₄ adsorption isotherms at 273.15 K and 0–100 kPa are also provided in ESI.†

Fig. 2(a) shows the scanning electron microscopy images of Cu-BTC on the CF skeleton at different pore densities (20, 40, 60, and 80 PPI). The growth of Cu-BTC crystal is affected by the pore density in CF. Cu-BTC crystals grow with little restriction to cover the copper skeleton with large pore size at 20 and 40 PPI. Same cracks appear at 20 PPI. The reason of cracks appearing in Cu-BTC/CF is because of electrochemical dissolution of copper and the thermal stress of Cu-BTC and CF arisen from the uneven temperature filed. At the beginning of the experiment, the formation of the MOF layer takes place on the CF electrode and in the bulk solution at the same time. As the synthesis continues, the film of Cu-BTC becomes thick and the charge transfer and transport of both Cu²⁺ and the linker become more difficult. The reaction also releases heat, which leads to the temperature gradient from CF to bulk solution, different thermal expansion of CF and MOF causes the crack on Cu-BTC. The octahedral-shaped crystals are observed at 20 and 40 PPI; this crystal shape is typical for Cu-BTC. X-ray diffraction patterns (Fig. S1 in ESI†) also verify the successful growth of Cu-BTC on CF. The growth of Cu-BTC crystals is restrained from forming films by the metallic framework of CF at 60 PPI. When the pore density is increased to 80 PPI, the film is not formed and the crystal growth is restrained seriously by the metallic framework, leading to poorly developed octahedral-shaped crystals. Large aggregates are formed by continuous growth of Cu-BTC crystal. Therefore, by controlling the current density and time of reaction, the microstructure of the Cu-BTC/CF can be reproduced accordingly.¹⁹ The thickness of the formed Cu-BTC layer coating on CF at different pore densities is shown in Fig. 2(b). The thickness of the formed Cu-BTC layer coating on CF is 2.6 μm, 4.0 μm, 2.5 μm and 2.8 μm at 20 PPI, 40 PPI, 60 PPI and 80 PPI, respectively. The different growth morphologies of Cu-BTC crystal can influence the amount of adsorption and selectivity in Cu-BTC/CF. The properties of pure Cu-BTC are indexed from ref. 20, whereas those of Cu-BTC/CF mainly originate from the experiment. Table 1 shows the detailed properties of porosity, solid heat capacity, and solid density for pure Cu-BTC and Cu-BTC/CF. The porosity in Cu-BTC/CF can reach 0.86, which is higher than that of pure Cu-BTC, because the CF with a high porosity is used as the matrix in Cu-BTC/CF.

Fig. 2 SEM images of four samples. (a) SEM images of Cu-BTC coating on CF at different pore densities. (b) Thickness of the formed Cu-BTC layer coating on CF at different pore densities.

Table 1 General physical properties of Cu-BTC and Cu-BTC/copper foam

Property	Cu-BTC^20	Cu-BTC/copper foam
Porosity	0.41	0.86
Solid heat capacity [J (kg^−1 K)]	1456	574
Solid density (kg m^−3)	1379	7518
Particle diameter (m)	2.5 × 10^−3	—

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3. Models of pressure drop and regenerability

Pressure drop in a typical adsorption bed is a key parameter used to characterize viscous energy loss and drop in the kinetic energy. Fig. 3(a) shows a typical adsorption bed filled with pure Cu-BTC powder or Cu-BTC/CF to predict the pressure drop. The adsorbate gases (CO₂, N₂, or CH₄) pass through the adsorption bed from the bottom to the top. The specific pressure drop using Darcy flow considering a quadratic term can be expressed as follows:^21,22where p is the total pressure (kPa), l is the length of the adsorption bed (m), and K_D and K_V are the viscous and kinetic pressure loss, respectively. The relationship between K_D and K_V can be obtained by Ergun equation as follows:where ε is the total porosity (0.41 and 0.86 for pure Cu-BTC and Cu-BTC/CF, respectively), μ is the gas mixture viscosity [1.86 × 10⁻⁵ kg (m⁻¹ s)], d_p is the particle mean diameter, and ρ_g is the gaseous density (1.28 kg m⁻³). d_p for CF can be expressed as follows:where ω is the pore per density (PPI).

Fig. 3 Model of pressure drop and the temperature response model of the adsorption bed. (a) Model of pressure drop in adsorption bed. (b) Adsorption column. (c) Computation model.

Regenerability is also an important property of adsorbents. Desorption is needed to regenerate adsorbed gas materials in the adsorption bed. The conditions to regenerate adsorbed gas through desorption include heating the adsorption bed by a constant heat flux above 373.15 K under a vacuum (low 1.0 × 10⁻⁵ kPa) released by a vacuum pump. Temperature uniformity and heat-up time are important properties in the desorption process. Fig. 3(b) shows a typical bed adsorption column. The bed adsorption column contains the adsorbent (Cu-BTC or Cu-BTC/CF) and a cylindrical heating rod, as described in the 2D model shown in Fig. 3(c); the length of the model is 0.1 m. Without considering radiation, the equation can be expressed as follows:

where ρ_s is the solid density; C_p is the heat capacity; T_s is the solid temperature; x and r are the axial and radial distances, respectively; q is the amount of adsorption, which can be ignored under vacuum (lower than 1.0 × 10⁻⁵ kPa) because the amount of adsorbed gas is less than 0.1% of the captured gas at ambient pressure; ΔH is the isosteric heat of adsorption (kJ mol⁻¹); and k_eff is the effective thermal conductivity of the adsorption bed without considering the gaseous phase under a vacuum. k_eff can be expressed as:

k_eff = (1 − ε)k_s (6)

where k_s is the solid thermal conductivity. The constant heat flux (q̄ = 100 W m⁻²) is placed at the left of the model. The top and bottom of the model are symmetric. The right part is adiabatic, as shown in Fig. 3(c); T_s = 298.15 K at t = 0.

4. Results and discussion

4.1. Pressure drop

Fig. 4 shows the pressure drop values per unit length at four pore densities of 20, 40, 60, and 80 PPI for Cu-BTC/CF. The porosity of Cu-BTC/CF is 0.86 (Table 1) with a velocity in the range of 0–0.8 m s⁻¹. The pressure drop of the pure Cu-BTC at a porosity of 0.41 is also provided for comparison. The pressure drops of the pure Cu-BTC powder and Cu-BTC/CF adsorption beds are predicted from eqn (1)–(4). The pressure drop of the pure Cu-BTC powder adsorption bed increases exponentially with increasing velocity, whereas that of Cu-BTC/CF at all pore densities is not sensitive to the velocity and increases gradually. The pressure drop of Cu-BTC/CF is 3.0–33.8% of pure Cu-BTC at velocities that range from 0 m s⁻¹ to 0.8 m s⁻¹. For example, the pressure drop of Cu-BTC/CF at 20 PPI is only 4.3% of pure Cu-BTC at 0.8 m s⁻¹. Thus, the high porosity of Cu-BTC/CF mainly contributes to the lower pressure drop compared with pure Cu-BTC powder (ε = 0.41, Table 1). For the Cu-BTC/CF with different pore densities, the pressure drop increases with increased pore density. In particular, the pressure drop for 80 PPI is 5.96–11.25 times higher than that for 20 PPI. Thus, high pore density indicates small pore size and results in additional pressure drop.

Fig. 4 Bed pressure drop of different materials.

4.2. Effective thermal conductivity and temperature response

Fig. 5 shows the comparative results for the effective thermal conductivity of Cu-BTC/CF and pure CF at different pore densities. The effective thermal conductivity of Cu-BTC is also provided for reference. The porosities of Cu-BTC/CF, CF, and pure Cu-BTC are 0.86, 0.9, and 0.41, respectively. The effective thermal conductivity of pure Cu-BTC powder is only 0.0678 W (m⁻¹ K⁻¹), whereas that of CF can reach up to 11.0–11.4 W (m⁻¹ K⁻¹). This difference is attributed to the fact that the composition of Cu-BTC is mainly inorganic, whereas CF is metal. Thus, CF can improve the effective thermal conductivity of Cu-BTC. The effective thermal conductivity of Cu-BTC/CF is 1.59–27.52 times higher than that of pure Cu-BTC powder. The effective thermal conductivity of Cu-BTC/CF increases linearly with increasing pore density; increased pore density is associated with an increased percentage of copper skeleton on the surface of Cu-BTC/CF. The variation of the thermal conductivities of Cu-BTC/CF is larger compared to that of CFs at different porosities. This illustrates that the Cu-BTC coating on copper lowers the thermal conductivity of pure copper, but the Cu-BTC occupies the site of the air in CF and the effective thermal conductivity of Cu-BTC is higher compared to that of the air in CF.

Fig. 5 Effective thermal conductivity at different pore densities.

Effective thermal conductivity is determined in the heat diffusion during heating in the process of desorption. Fig. 6(a) shows the temperature distribution in the Cu-BTC powder adsorption bed at different time points (150 and 200 s) during desorption under a vacuum. Fig. 6(b) presents the temperature distribution in the Cu-BTC/CF adsorption bed at 150 s. The temperature distribution in the adsorption bed is obtained using eqn (5) and (6). The minimum temperature in the Cu-BTC powder adsorption bed is only 355 K at 150 s, which cannot satisfy the desorption condition (>373.15 K). The minimum temperature in the pure Cu-BTC adsorption bed is 388 K at 200 s, which satisfies the required desorption condition (above 373.15 K) [Fig. 6(a)]. The minimum temperature in the Cu-BTC/CF adsorption bed can reach 379.5 K at 150 s [Fig. 6(b)] to satisfy the required desorption condition (>373.15 K). The temperature uniformity is significantly improved with increased thermal conductivity. The difference in average temperature between the heating surface and the outside boundary surface can be less than 1 K at 200 s, whereas the temperature is 40 K in pure Cu-BTC adsorption bed at the same time. The high temperature uniformity and effective thermal conductivity in Cu-BTC/CF adsorption bed can improve desorption efficiency.

Fig. 6 Desorption temperature distribution of the adsorption bed. (a) Physical models of pure Cu-BTC. (b) Physical models of Cu-BTC/CF (t = 150 s).

4.3. Gas separation analysis

The selectivity of CO₂/N₂ and CH₄/N₂ in pure Cu-BTC and Cu-BTC/CF adsorption beds at different pore densities is investigated based on Ideal Adsorption Solution Theory (IAST).²³ The mixture adsorption equilibrium in IAST model is predicted based on the single-component isotherms. These isotherms are described by the dual-site Langmuir–Freundlich (DSLF) equation.²⁴ Table 2 lists the fitting parameters of the DSLF isotherm model for the pure isotherms of N₂, CH₄, and CO₂. The relationship between the amount of adsorption and pressure in the DSLF equation agrees with the experimental data for CO₂, N₂, and CH₄ at 273.15 K within 0–100 kPa (Fig. S3 in ESI†).

Table 2 Parameters of fitness

Adsorbate	Conditions	qm,1	qm,2	b1	b2	n1	n2	R^2
N2	20 PPI	2.00 × 10^−2	3.54 × 10^−3	7.25 × 10^−3	1.49 × 10^−2	1.00	1.00	1.00
	40 PPI	6.92 × 10^−2	3.34 × 10^−3	1.79 × 10^−3	1.20 × 10^−2	0.80	0.80	1.00
	60 PPI	6.94 × 10^−2	5.33 × 10^−3	2.80 × 10^−3	9.88 × 10^−3	0.80	0.80	1.00
	80 PPI	3.44 × 10^−2	6.64 × 10^−3	2.70 × 10^−3	1.90 × 10^−2	0.80	0.80	1.00
	Cu-BTC	4.02	3.54 × 10^−4	1.07 × 10^−3	3.06 × 10^−2	0.95	1.00	1.00
CH4	20 PPI	7.13 × 10^−2	1.01 × 10^−3	8.62 × 10^−3	4.88 × 10^−2	0.97	1.00	1.00
	40 PPI	2.63 × 10^−1	4.99 × 10^−4	5.14 × 10^−3	5.24 × 10^−2	0.97	0.99	1.00
	60 PPI	2.58 × 10^−1	1.08 × 10^−2	6.34 × 10^−3	7.97 × 10^−2	0.99	0.96	1.00
	80 PPI	1.47 × 10^−1	9.37 × 10^−3	8.56 × 10^−3	6.97 × 10^−2	0.98	0.99	1.00
	Cu-BTC	4.60	1.15	2.75 × 10^−3	8.01 × 10^−3	1.00	1.53	1.00
CO2	20 PPI	3.04 × 10^−3	2.46 × 10^−2	3.21 × 10^−3	1.03 × 10^−1	0.99	0.97	1.00
	40 PPI	5.67 × 10^−3	4.73 × 10^−2	4.16 × 10^−3	1.39 × 10^−1	0.96	0.88	1.00
	60 PPI	9.08 × 10^−1	4.51 × 10^−2	1.26 × 10^−2	9.03 × 10^−2	1.24	0.86	1.00
	80 PPI	2.98 × 10^−1	7.04 × 10^−2	8.58 × 10^−3	1.12 × 10^−1	1.00	0.99	1.00
	Cu-BTC	12.13	2.64	1.06 × 10^−2	3.70 × 10^−3	1.06	0.70	0.99

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Fig. 7(a) shows the selectivity of equimolar CO₂/N₂ under different pressures (0–100 kPa) at 273.15 K. The selectivity of equimolar CO₂/N₂ in pure Cu-BTC powder and Cu-BTC/CF at 80 PPI is not sensitive to pressure variation. The selectivity of equimolar CO₂/N₂ gradually increases with pressure at 20, 40, and 60 PPI. Under the same pressure, the selectivity of Cu-BTC/CF generally decreases in the following order: 60, 20, 40, and 80 PPI. The selectivity of equimolar CO₂/N₂ at 40 PPI in Cu-BTC/CF is slightly higher than that of Cu-BTC powder, and the selectivity of equimolar CO₂/N₂ at 80 PPI is even lower than that of pure Cu-BTC. The selectivity of CO₂/N₂ is superior at 20 and 60 PPI. For example, the selectivity of CO₂/N₂ at 60 PPI for Cu-BTC/CF is 1.36–7.44 times that for pure Cu-BTC within 0–100 kPa at 273.15 K; this phenomenon can be illustrated by the following description. The aberrant crystals in Cu-BTC (Fig. 2) at 20 and 60 PPI have a larger number of smaller incomplete micropores compared with those of the complete micropores in pure Cu-BTC and Cu-BTC/CF at 40 and 80 PPI. These incomplete micropores can create unsaturated Cu sites and increase the amount of adsorbed CO₂, as shown in Fig. S3(c) in ESI.† The smaller incomplete micropores and increased unsaturated Cu sites at 20 and 60 PPI slightly influence N₂ adsorption. Subsequently, the selectivity of equimolar CO₂/N₂ at 20 PPI in Cu-BTC/CF with pressure greater than 40 kPa is higher than those in pure Cu-BTC and Cu-BTC/CF at 40 and 80 PPI. In addition to the micropores for Cu-BTC/CF at 60 PPI, the existing dense film and the higher amount of Cu-BTC loading on CF (Fig. 2) further enhance the selectivity of CO₂/N₂. Thus, the incomplete micropores, the number of unsaturated Cu sites, and the amount of Cu-BTC loading greatly influence the CO₂ adsorption.

Fig. 7 Selectivity of the equimolar mixture of two different gases. (a) Selectivity of the equimolar mixture of CO₂/N₂. (b) Selectivity of the equimolar mixture of CH₄/N₂.

Fig. 7(b) shows the selectivity of equimolar CH₄/N₂ within 0–100 kPa at 273.15 K. The selectivity of equimolar CH₄/N₂ in pure Cu-BTC decreases slowly with the increase in pressure. The selectivity at 40 PPI and 60 PPI almost keeps constant under pressure variation. The selectivity increases linearly at 20 and 80 PPI. The selectivity of equimolar CH₄/N₂ at pore densities of 40, 60, and 80 PPI are all higher than that of pure Cu-BTC. The selectivity of equimolar CH₄/N₂ at 20 PPI is higher than that of pure Cu-BTC when the operating pressure is higher than 20 kPa. The selectivity of equimolar CH₄/N₂ is the highest at 80 PPI for the studied pore densities. For example, the selectivity of equimolar CH₄/N₂ at 80 PPI is 1.41–2.95 times that of Cu-BTC within 0–100 kPa and 273.15 K. Similarly, the pore structure and size slightly affect N₂ adsorption, and the selectivity of equimolar CH₄/N₂ is determined by the amount of CH₄ adsorption. The CH₄ adsorption is not sensitive to unsaturated Cu sites resulting from the micropores induced²⁵ and is mainly dependent on pore size.²⁶ Based on the highest selectivity of equimolar CH₄/N₂ at 80 PPI, the incomplete pore sizes in Cu-BTC (Fig. 2) at 80 PPI are appropriate for the CH₄ adsorption.

5. Conclusion

Cu-BTC supported on CF is used to synthesize novel multi-scale porous architecture. The pressure drop of Cu-BTC/CF is 3.0–33.8% that of pure Cu-BTC within 0.1–0.8 m s⁻¹ because of the higher porosity in Cu-BTC/CF compared with pure Cu-BTC. The effective thermal conductivity of Cu-BTC/CF is 1.59–27.52 times that of pure Cu-BTC powder. The time needed to reach the desorption condition (above 373.15 K at a vacuum) is 150 s, which is shorter than that of pure Cu-BTC (200 s). The temperature uniformity is also more highly improved for the Cu-BTC/CF adsorption bed compared with that of the pure Cu-BTC adsorption bed. The selectivity of CO₂/N₂ at 60 PPI is 1.36–7.44 times higher than that of pure Cu-BTC because of the large number of unsaturated Cu sites and the high amount of Cu-BTC loading. The suitable pore size results in that the selectivity of CH₄/N₂ at 80 PPI is 1.41–2.95 times higher than that of pure Cu-BTC.

Acknowledgements

This work was financially sponsored by the National Natural Science Foundation of China (No. 51536003), National Program for Support of Top-notch Young Professionals and the Fundamental Research Funds for the Central Universities (No. xjj2012102).

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Footnote

† Electronic supplementary information (ESI) available: Material characterization (i.e., X-ray diffraction patterns, N₂ adsorption and desorption isotherms, the method of the effective thermal conductivity test, N₂, CO₂, and CH₄ adsorption isotherms at 273.15 K and 0–100 kPa and the results of the electric conductivity of CuBTC/CF at different pore densities) information. See DOI: 10.1039/c6ra08622e

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