Robust Carborane-Based Metal–Organic Frameworks for Hexane Separation

Abstract

Hexane isomers play a vital role as feedstocks and fuel additives in the petrochemical industry. However, their similar physical and chemical properties lead to significant challenges in the separation process. Traditional thermal separation techniques are energy-intensive and lead to significant carbon footprint penalties. As such, there is a growing demand for the development of less energy-intensive nonthermal separation methods. Adsorption-based separation methods, such as using solid sorbents or membranes, have emerged as promising alternatives to distillation. Here, we report the successful synthesis of two novel metal–organic frameworks (MOFs), NU-2004 and NU-2005, by incorporating a carborane-based three-dimensional (3D) linker and using aluminum and vanadium nodes, respectively. These MOFs exhibit exceptional thermal stability and structural rigidity compared to other MIL-53 analogues, which is further corroborated using synchrotron studies. Furthermore, the inclusion of the quasi-spherical 3D linker in NU-2004 demonstrates significant advancements in the separation of hexane isomers compared to other MIL MOFs containing two-dimensional (2D) and aliphatic 3D linkers.

A Microporous Multi‐Cage Metal–Organic Framework for an Effective One‐Step Separation of Branched Alkanes Feeds

“…Similar outcomes were also shown by MOF adsorbents possessing a robust framework with optimal pore channels, such as Zn‐tcpt, Al‐bttotb and NU‐2004 [12,28,29] . However, most of these studies with MOFs are currently being performed with only C6 alkane isomers and not all show multicomponent data [29] …”

“…Furthermore, for an equimolar mixture of all seven C5/C6 isomers, the breakthrough times of the C6 isomers follow the same sequence as in the separation of the quinary mixture (Figures 3b, S17, S18). Remarkably, with an adsorption hierarchy of nC6≫nC5≈2MP>3MP>23DMB≈iC5≈22DMB, the high‐octane di‐branched 22DMB and 23DMB and mono‐branched iC5 elute nearly together, much earlier than the elution of the other four LRON components, leading to an ideal 92 RON productivity of 0.67 mol dm −3 (comparable with the one calculated from the experiment with only C6 isomers: 0.74 mol dm −3 ) along with a selectivity of 11.3 (twice higher than that from the mixed‐bed sorption of zeolite 5A and MIL‐160(Al) [29] ). This remarkable desired separation of HRON and LRON molecules is unique for all families of adsorbents, including zeolites and MOFs.…”

“…Remarkably, HIAM‐203 and Ca(H 2 tcpb) (two calcium‐based materials) have the ability to molecular sieve 22DMB from 3MP and nC6, based on a temperature programming separation process due to their structure flexibility [11,27] . Similar outcomes were also shown by MOF adsorbents possessing a robust framework with optimal pore channels, such as Zn‐tcpt, Al‐bttotb and NU‐2004 [12,28,29] . However, most of these studies with MOFs are currently being performed with only C6 alkane isomers and not all show multicomponent data [29] …”

Constraining Flexibility in the MIL-88 Topology through Integration of 3-Dimensional Linkers

“…Previously, our group studied the effects of installing 3-dimensional linkers of increasing bulkiness in MIL-53, which is a class of flexible MOFs composed of 1-dimensional channels featuring chains of single metal ions coordinated to the 2DL 1,4-benzenedicarboxylate (BDC). − While MIL-53 undergoes significant breathing behavior that results in a 40% difference in volume between its narrow pore phase and large pore phase, , we demonstrated that by installing the 3DLs bicyclo[1.1.1]­pentane-1,3-dicarboxylate (BPDCA), bicyclo[2.2.2]­octane-1,4-dicarboxylate (BODCA), and p -carborane-1,12-dicarboxylate ( p CDC), we could limit this change in volume to about 2, 7, and 1%, respectively. − As a result, these 3DL MOFs were able to engage in the size-selective separation of either xylene or hexane isomers, while the 2DL analogue MIL-53 did not show any separation ability under analogous conditions. MIL-88, first reported by Serre and co-workers in 2004, is another well-studied MOF system that shows flexible behavior.…”

“…Recently, our group and others have demonstrated that increasing linker dimensionality from one-dimensional (1D) or two-dimensional (2D) to three-dimensional (3D) is an effective strategy for limiting flexibility in MOFs. − While most MOFs comprise two-dimensional linkers (2DLs), which we define as ditopic ligands with planar, aromatic cores, 2DLs can pose challenges when they are subject to rotational motion in the pore. Upon rotation, 2DLs extend nonuniformly into the pore space, often resulting in significant deviations in pore size (Figure ).…”