Methodologies for evaluation of metal–organic frameworks in separation applications

Abstract

Metal-organic frameworks (MOFs) offer considerable potential for separating a wide variety of mixtures. For any given separation, there are several MOFs that could be employed. Therefore, there is a need for reliable procedures for screening and ranking MOFs with regard to their anticipated performance in fixed-bed adsorbers, commonly used in industry. Such fixed-bed adsorbers are invariably operated in a transient mode. The separation performance of fixed-bed adsorbers is governed by a number of factors that include adsorption selectivity, uptake capacity, and intra-crystalline diffusion limitations. We undertake a detailed analysis of the separations of several mixtures that include: C 2 H 2 /CO 2 , CO 2 /N 2 , CO 2 /CH 4 , H 2 S/ CO 2 /CH 4 , H 2 /CO 2 /CO/CH 4 /N 2 , Xe/Kr, C 2 H 2 /C 2 H 4 , C 2 H 4 /C 2 H 6 , C 3 H 6 /C 3 H 8 , O 2 /N 2 , N 2 /CH 4 , hexane isomers, xylene isomers, and styrene/ethylbenzene. For each separation, we compare the performance of a few carefully selected MOFs by using transient breakthrough simulations that are representative of practical operations. These case studies demonstrate that screening MOFs on the basis of adsorption selectivity alone, as is common practice, often leads to wrong conclusions as regards their separation capability in fixed-bed adsorbers. High uptake capacities often compensate for low selectivities.Conversely, low uptake capacities diminish the separation performance of MOFs with high selectivities.Intra-crystalline diffusion limitations lead to distended breakthroughs, and diminished productivities in a number of cases. We also highlight the possibility of harnessing intra-crystalline diffusion limitations to reverse the adsorption selectivity; this strategy is useful for selective capture of nitrogen from natural gas, and in air separations.

A π-electron deficient diaminotriazine functionalized MOF for selective sorption of benzene over cyclohexane

“…Fig. 4c presents IAST calculations for adsorption selectivity, S ads , for equimolar Bz/Cy mixtures with value in excess of about 200, suggesting viability of the present MOF material for vapor phase selective sorption based Bz/Cy separation at 298 K. Transient breakthrough simulations, using the established methodology described in earlier work, 14 confirm that sharp separations are obtained in a fixed bed adsorber; see Fig. 4d.…”

Tracing the origins of transient overshoots for binary mixture diffusion in microporous crystalline materials

“…[1][2][3] Such separations are commonly conducted in fixed bed adsorbers. [1][2][3] Such separations are commonly conducted in fixed bed adsorbers.…”

“…4 The fixed bed devices are operated in a cyclical manner with adsorption and desorption cycles; the operations are intrinsically transient in nature, i.e. 2,7 However, there are some instances where diffusional effects over-ride the influence of adsorption equilibrium of mixtures and are the prime drivers of separations. Intra-particle diffusion limitations cause distended breakthrough characteristics and usually lead to diminished separation effectiveness.…”

“…11,12 For production of purified N 2 , it is desirable to selectively adsorb O 2 . 2,4,9 Another example is the separation of N 2 /CH 4 mixtures, which is important in the context of natural gas upgrading because the presence of N 2 reduces the heating value. [9][10][11][12][13] Due to its smaller cross-sectional dimension, 3.1 Å for O 2 , compared to 3.3 Å for N 2 , the intra-particle diffusivity of O 2 is significantly higher than that of N 2 in both LTA-4A and CMS.…”

Bimodal Functionality in a Porous Covalent Triazine Framework by Rational Integration of an Electron‐Rich and ‐Deficient Pore Surface

“…In view of the significantly higher uptake capacityo fb enzene in CTF-IP10, we should expect that the separation performance of benzene/cyclohexane mixtures with CTF-IP10 would be significantly higher than that of DAT-MOF-1, the separations of which were reported in our earlier work. [11] Breakthrough simulations, carriedo ut with the method describedi no ur earlier work, [12] show that clear separations of equimolar benzene/cyclohexanem ixtures are achievable with CTF-IP10;s ee Figure 5d.Avideo animation( see the Supporting Information) demonstrates the superior selective adsorption ability of CTF-IP10 for benzene over cyclohexane.…”