Metrics for Evaluation and Screening of Metal-Organic Frameworks for Applications in Mixture Separations

Abstract

For mixture separations, metal-organic frameworks (MOFs) are of practical interest. Such separations are carried out in fixed bed adsorption devices that are commonly operated in a transient mode, utilizing the pressure swing adsorption (PSA) technology, consisting of adsorption and desorption cycles. The primary objective of this article is to provide an assessment of the variety of metrics that are appropriate for screening and ranking MOFs for use in fixed bed adsorbers. By detailed analysis of several mixture separations of industrial significance, it is demonstrated that besides the adsorption selectivity, the performance of a specific MOF in PSA separation technologies is also dictated by a number of factors that include uptake capacities, intracrystalline diffusion influences, and regenerability. Low uptake capacities often reduce the efficacy of separations of MOFs with high selectivities. A combined selectivity-capacity metric, Δq, termed as the separation potential and calculable from ideal adsorbed solution theory, quantifies the maximum productivity of a component that can be recovered in either the adsorption or desorption cycle of transient fixed bed operations. As a result of intracrystalline diffusion limitations, the transient breakthroughs have distended characteristics, leading to diminished productivities in a number of cases. This article also highlights the possibility of harnessing intracrystalline diffusion limitations to reverse the adsorption selectivity; this strategy is useful for selective capture of nitrogen from natural gas.

Figure 1

Sequential steps in the operation of a fixed bed adsorber in the Skarstrom cycle for H₂ purification.^,−

Figure 2

Transient breakthrough of 73/4/3/4/16 H₂/N₂/CO/CH₄/CO₂ mixtures in a fixed bed adsorber packed with (a) AC and (b) LTA-5A zeolite operating at a total pressure of 2 MPa and T = 313 K. For presenting the breakthrough simulation results, we use as x-axis the dimensionless time, τ = tv/L, where L is the length of the adsorber and v is the interstitial gas velocity.^, Further information on input data and simulation details are provided in the Supporting Information.

Figure 3

Boiling points and polarizabilities of noble gases culled from web sources.

Figure 4

Transient breakthrough simulations (indicated by the solid blue line) for separation of 20/80 Xe/Kr mixtures at 298 K and 100 kPa in a fixed bed packed with CoFormate; these simulations include intracrystalline diffusion limitations. The dotted lines represent the shock wave model approximation. The input data and calculation details are available in earlier works.^,,

Figure 5

IAST calculations of (a) component loadings q₂ vs q₁ and (b) separation potential vs adsorption selectivity S_ads for 20/80 Xe(1)/Kr(2) mixture adsorption at 298 K and 100 kPa in six different MOFs: NiMOF-74^, Ag@NiMOF-74, CuBTC,^, SBMOF-2, CoFormate (=Co₃(HCOO)₆), and SAPO-34. (c) Comparison of the transient breakthrough simulations for separation of 20/80 Xe/Kr mixtures at 298 K and 100 kPa in fixed beds packed with CoFormate and Ag@NiMOF-74. The dimensionless concentrations at the exit of the fixed bed are plotted as a function of Q₀t/m_ads, where Q₀ is the volumetric flow rate of the gas mixture at the inlet to the fixed bed, expressed in L s^–1, at STP conditions. (d) Plot of the productivity of pure Kr, determined from breakthrough simulations, vs the IAST calculations of for six different MOFs with y₁₀ = 0.2; y₂₀ = 0.8. Further information on input data and simulation details are provided in earlier works.^,,

Figure 6

Plot of the productivity of pure Xe determined from transient desorption simulations for 20/80 Xe(1)/Kr(2) mixtures vs the IAST calculations of separation potential for six different MOFs with y₁₀ = 0.2; y₂₀ = 0.8. Further information on input data and simulation details are provided in earlier works.^,,

Figure 7

Sequential steps in the operation of a fixed bed adsorber in the Skarstrom cycle for C₂H₂(1)/CO₂(2) separation.

Figure 8

IAST calculations of (a) component loadings q₂ vs q₁ and (b) separation potential vs adsorption selectivity S_ads for the adsorption of C₂H₂(1)/CO₂(2) mixtures in nine different MOFs operating at 298 K and 100 kPa. (c) Simulations of transient desorption (blowdown) under deep vacuum (0.2 Pa total pressure and 298 K). During the time interval indicated by the arrow, the C₂H₂ product containing <1% CO₂ can be recovered. (d) Productivity of 99%+ pure C₂H₂ product determined by transient desorption simulations for PCP-33, HOF-3, TIFSIX-2-Cu-i, JCM-1, DICRO-4-Cu-i, MUF-17, UTSA-74, FJU-90, and FeNi-M′MOF at 298 K and 100 kPa, plotted as a function of the separation potential with y₁₀ = y₂₀ = 0.5. Further information on input data and simulation details are provided in earlier works.^,,,

Figure 9

(a) Transient breakthrough simulations for 1/99 C₂H₂/C₂H₄ mixture adsorption at 298 K and 100 kPa in a fixed bed packed with five different MOFs. The ppm C₂H₂ in the gas mixture at the outlet of the fixed bed is plotted as a function of Q₀t/m_ads, where Q₀ is the volumetric flow rate of the gas mixture at the inlet to the fixed bed, expressed in m³ s^–1, at STP conditions. (b) Productivity of pure C₂H₄, containing less than 40 ppm C₂H₂, plotted as a function of the separation potential determined from IAST with y₁₀ = 0.01; y₂₀ = 0.99. (c) Separation potential, Δq, of SIFSIX-2-Cu-i and SIFSIX-1-Cu, plotted as a function of the % C₂H₂ in the feed mixture. Further information on input data and simulation details are provided in the Supporting Information.

Figure 10

(a) Experimental breakthroughs for CO₂/CH₄ mixtures in a packed bed with Mg₂(dobdc), Co₂(dobdc), MIL-100(Cr), and AC at 298 K. The partial pressures at the inlet are p₁ = 40 kPa, p₂ = 60 kPa, and p_t = 100 kPa. The experimental data, indicated by the symbols, are from Li et al. The % CO₂ and % CH₄ in the exit gas phase are plotted as a function of Q₀t/m_ads. (b) Productivity of 95% pure CH₄ plotted as a function of separation potential.

Figure 11

Different steps in the production of purified CH₄ using an adsorbent such as LTA-4A zeolite, Ba-ETS-4, and clinoptilolites, which rely on kinetic selectivity. The scheme shows the sequence of processing of a single bed in a multibed PSA scheme. Adapted from Bhadra and Farooq and Jayaraman et al.

Figure 12

Comparison of the transient breakthroughs of 20/80 N₂(1)/CH₄(2) mixtures in a fixed bed adsorber packed with MIL-100(Cr) and Ba-ETS-4 operating at 283 K and total pressure p_t = 1 MPa. Further information on input data and simulation details are provided in the Supporting Information.

Figure 13

Five-step PSA process for separating C₃H₆/C₃H₈ mixtures.^,,

Figure 14

Transient breakthrough simulations for (a) adsorption and (b,c) desorption cycles for the separation of C₃H₆/C₃H₈ mixtures in a fixed bed adsorber packed with KAUST-7 operating at 298 K and 100 kPa; the feed compositions are y₁₀ = y₂₀ = 0.5. (c) Three different scenarios for the ratios of diffusivities Đ₁/Đ₂ = 1, 10, and 100 are compared, while maintaining Đ₁/r_c² = 1 × 10^–3 s^–1. Further information on input data and simulation details are provided in the Supporting Information.

Figure 15

(a) IAST calculations of the C₂H₆ uptake q₂ vs the separation selectivity S_ads of 90/10 C₂H₄/C₂H₆ mixture adsorption at 298 K and 100 kPa in four different MOFs. (b) Transient breakthrough simulations for the separation of 90/10 C₂H₄/C₂H₆ mixture adsorption at 298 K and 100 kPa in fixed beds packed with Cu(Qc)₂, MUF-15, CPM-233, and CPM-733. (c) Productivity of 99.95%+ pure C₂H₄ product recovered during the displacement intervals, plotted as function of the separation potential Δq. Further information on input data and simulation details are provided in the Supporting Information.

Figure 16

(a) Currently employed processing scheme for nC6 isomerization and a subsequent separation step using LTA-5A zeolite. (b) Improved processing scheme for the nC6 isomerization process. Further process background details are provided in the Supporting Information.

Figure 17

(a) Simulations of transient breakthrough characteristics for a five-component nC6/2MP/3MP/22DMB/23DMB mixture in a fixed bed adsorber packed with Fe₂(BDP)₃ operating at a total pressure of 100 kPa and 433 K. The partial pressures of the components in the bulk gas phase at the inlet are p₁ = p₂ = p₃ = p₄ = p₅ = 20 kPa. (b) Plot of RON of product gas mixture exiting the fixed bed adsorber packed with ZIF-77 and Fe₂(BDP)₃, plotted as a function of Q₀t/m_ads. Further information on input data and simulation details are provided in the Supporting Information.

Figure 18

Schematic showing the separations of the products from a catalytic reforming unit. Further process background details are provided in the Supporting Information.

Figure 19

Boiling points and freezing points of C8 hydrocarbons, along with molecular dimensions, culled from Torres-Knoop et al.

Figure 20

SMB adsorption technology for the separation of a feed mixture containing o-xylene/m-xylene/p-xylene/ethylbenzene. The SMB technology is depicted here with countercurrent contacting between the down-flowing adsorbent material and up-flowing desorbent (eluent) liquid. Also indicated are the liquid-phase concentrations of a mixture of o-xylene/m-xylene/p-xylene/ethylbenzene using the information presented by Minceva and Rodrigues.

Figure 21

Plot of the separation potential, Δq, vs the gravimetric uptake of p-xylene. Further information on input data and simulation details are provided in the Supporting Information.