Brian M. Wiers

Separation of Hexane Isomers in a Metal-Organic Framework with Triangular Channels

Telling Hexanes Apart The efficiency of modern internal combustion engines depends on the relative reactivity of the hydrocarbons that comprise the fuel. In particular, branched hydrocarbons are less likely than their linear counterparts to react prematurely—a property reflected in the fuel mixtures octane number. Herm et al. (p. 960) report a metal organic framework material with triangular pore channels that discriminate among the differently shaped isomers of hexane more finely than the commercial standard. A porous material shows preliminary promise for enhancing a separations process central to gasoline production. Metal-organic frameworks can offer pore geometries that are not available in zeolites or other porous media, facilitating distinct types of shape-based molecular separations. Here, we report Fe2(BDP)3 (BDP2– = 1,4-benzenedipyrazolate), a highly stable framework with triangular channels that effect the separation of hexane isomers according to the degree of branching. Consistent with the varying abilities of the isomers to wedge along the triangular corners of the structure, adsorption isotherms and calculated isosteric heats indicate an adsorption selectivity order of n-hexane > 2-methylpentane > 3-methylpentane > 2,3-dimethylbutane ≈ 2,2-dimethylbutane. A breakthrough experiment performed at 160°C with an equimolar mixture of all five molecules confirms that the dibranched isomers elute first from a bed packed with Fe2(BDP)3, followed by the monobranched isomers and finally linear n-hexane. Configurational-bias Monte Carlo simulations confirm the origins of the molecular separation.

A Solid Lithium Electrolyte via Addition of Lithium Isopropoxide to a Metal–Organic Framework with Open Metal Sites

The uptake of LiO(i)Pr in Mg(2)(dobdc) (dobdc(4-) = 1,4-dioxido-2,5-benzenedicarboxylate) followed by soaking in a typical electrolyte solution leads to the new solid lithium electrolyte Mg(2)(dobdc)·0.35LiO(i)Pr·0.25LiBF(4)·EC·DEC (EC = ethylene carbonate; DEC = diethyl carbonate). Two-point ac impedance data show a pressed pellet of this material to have a conductivity of 3.1 × 10(-4) S/cm at 300 K. In addition, the results from variable-temperature measurements reveal an activation energy of just 0.15 eV, while single-particle data suggest that intraparticle transport dominates conduction.

Ammonia Capture in Porous Organic Polymers Densely Functionalized with Brønsted Acid Groups

The elimination of specific environmental and industrial contaminants, which are hazardous at only part per million to part per billion concentrations, poses a significant technological challenge. Adsorptive materials designed for such processes must be engendered with an exceptionally high enthalpy of adsorption for the analyte of interest. Rather than relying on a single strong interaction, the use of multiple chemical interactions is an emerging strategy for achieving this requisite physical parameter. Herein, we describe an efficient, catalytic synthesis of diamondoid porous organic polymers densely functionalized with carboxylic acids. Physical parameters such as pore size distribution, application of these materials to low-pressure ammonia adsorption, and comparison with analogous materials featuring functional groups of varying acidity are presented. In particular, BPP-5, which features a multiply interpenetrated structure dominated by <6 Å pores, is shown to exhibit an uptake of 17.7 mmol/g at 1 bar, the highest capacity yet demonstrated for a readily recyclable material. A complementary framework, BPP-7, features slightly larger pore sizes, and the resulting improvement in uptake kinetics allows for efficient adsorption at low pressure (3.15 mmol/g at 480 ppm). Overall, the data strongly suggest that the spatial arrangement of acidic sites allows for cooperative behavior, which leads to enhanced NH3 adsorption.

Ionic conductivity in the metal-organic framework UiO-66 by dehydration and insertion of lithium tert-butoxide.

Shields up! Post-synthetic modification of the secondary building units in the metal-organic framework UiO-66 (Zr6O4(OH)4(O2CR)12) by dehydration and subsequent grafting of LiOtBu yields a solid Li(+) electrolyte with a conductivity of 1.8×10(-5) S cm(-1) at 293 K. As the grafting leads to screening of the anionic charge, the activation energy for ionic conduction is significantly lower than when Li(+) is introduced through deprotonation.

Metal–organic frameworks as solid magnesium electrolytes

A series of solid magnesium electrolytes were synthesized via the transmetallation of magnesium phenolates to coordinatively unsaturated metal sites lining the pores of the metal–organic frameworks Mg2(2,5-dioxidobenzene-1,4-dicarboxylate) and Mg2(4,4′-dioxidobiphenyl-3,3′-dicarboxylate). The resulting materials represent a new class of solid magnesium electrolytes that are both crystalline, and exhibit room-temperature ionic conductivities up to 0.25 mS cm−1. The materials reported herein are one-hundred times more conductive at room temperature than any other solid magnesium electrolyte and represent the only class of materials sufficiently conductive for practical consideration in magnesium batteries.

Homodiamine-functionalized metal–organic frameworks with a MOF-74-type extended structure for superior selectivity of CO2 over N2

A porous Mg2(dondc) framework (H4dondc = 1,5-dioxido-2,6-naphthalenedicarboxylic acid) with open metal sites was prepared and functionalized with primary or secondary diamines (en = ethylenediamine, mmen = N,N′-dimethylethylenediamine, or ppz = piperazine). The CO2 adsorption was substantial under post-combustion flue gas conditions as compared to other reported metal–organic frameworks. Interestingly, the IR spectroscopic measurements demonstrated that the CO2 adsorption mechanism is based on the combination of physisorption and chemisorption. The CO2 adsorption capacity of 1-mmen was greater than that of 1-en and 1-ppz, which can likely be attributed to the basicity of the free amine groups tethered to the open coordination sites. Ultrahigh selectivity and superior dynamic separation of CO2 over N2 were evident in 1-ppz. Such exceptional CO2 uptake and CO2/N2 selectivity of diamine-functionalized materials hold potential promise for post-combustion CO2 capture applications.

Electron delocalization and charge mobility as a function of reduction in a metal–organic framework

Conductive metal–organic frameworks are an emerging class of three-dimensional architectures with degrees of modularity, synthetic flexibility and structural predictability that are unprecedented in other porous materials. However, engendering long-range charge delocalization and establishing synthetic strategies that are broadly applicable to the diverse range of structures encountered for this class of materials remain challenging. Here, we report the synthesis of KxFe2(BDP)3 (0 ≤ x ≤ 2; BDP2− = 1,4-benzenedipyrazolate), which exhibits full charge delocalization within the parent framework and charge mobilities comparable to technologically relevant polymers and ceramics. Through a battery of spectroscopic methods, computational techniques and single-microcrystal field-effect transistor measurements, we demonstrate that fractional reduction of Fe2(BDP)3 results in a metal–organic framework that displays a nearly 10,000-fold enhancement in conductivity along a single crystallographic axis. The attainment of such properties in a KxFe2(BDP)3 field-effect transistor represents the realization of a general synthetic strategy for the creation of new porous conductor-based devices.A conducting metal–organic framework with charge delocalization by reductive potassium insertion is demonstrated. Integration into a field-effect transistor shows similar mobilities to semiconductors, with a mobility estimated to be at least 0.84 cm2 V–1 s–1.