Sihai Yang

Cation-induced kinetic trapping and enhanced hydrogen adsorption in a modulated anionic metal–organic framework

Metal–organic frameworks (MOFs)—microporous materials constructed by bridging metal centres with organic ligands—show promise for applications in hydrogen storage, which is a key challenge in the development of the ‘hydrogen economy’. Their adsorption capacities, however, have remained insufficient for practical applications, and thus strategies to enhance hydrogen–MOF interactions are required. Here we describe an anionic MOF material built from In(iii) centres and tetracarboxylic acid ligands (H4L) in which kinetic trapping behaviour—where hydrogen is adsorbed at high pressures but not released immediately on lowering the pressure—is modulated by guest cations. With piperazinium dications in its pores, the framework exhibits hysteretic hydrogen adsorption. On exchange of these dications with lithium cations, no hysteresis is seen, but instead there is an enhanced adsorption capacity coupled to an increase in the isosteric heat of adsorption. This is rationalized by the different locations of the cations within the pores, determined with precision by X-ray crystallography. Porous metal–organic frameworks are promising for hydrogen storage applications, but adsorption capacities have remained too low for practical use. Now, the adsorption behaviour of such a framework has been modulated by exchanging cations within its pores resulting in either kinetic trapping or enhanced hydrogen affinity.

A partially interpenetrated metal–organic framework for selective hysteretic sorption of carbon dioxide

The selective capture of carbon dioxide in porous materials has potential for the storage and purification of fuel and flue gases. However, adsorption capacities under dynamic conditions are often insufficient for practical applications, and strategies to enhance CO(2)-host selectivity are required. The unique partially interpenetrated metal-organic framework NOTT-202 represents a new class of dynamic material that undergoes pronounced framework phase transition on desolvation. We report temperature-dependent adsorption/desorption hysteresis in desolvated NOTT-202a that responds selectively to CO(2). The CO(2) isotherm shows three steps in the adsorption profile at 195 K, and stepwise filling of pores generated within the observed partially interpenetrated structure has been modelled by grand canonical Monte Carlo simulations. Adsorption of N(2), CH(4), O(2), Ar and H(2) exhibits reversible isotherms without hysteresis under the same conditions, and this allows capture of gases at high pressure, but selectively leaves CO(2) trapped in the nanopores at low pressure.

Exceptionally high H2 storage by a metal-organic polyhedral framework

The desolvated polyhedral framework material NOTT-112 shows an excess H(2) uptake of 7.07 wt% between 35 and 40 bar at 77 K, and a total H(2) uptake of 10 wt% at 77 bar and 77 K.

Metal−Organic Polyhedral Frameworks: High H2 Adsorption Capacities and Neutron Powder Diffraction Studies

Neutron powder diffraction experiments on D(2)-loaded NOTT-112 reveal that the axial sites of exposed Cu(II) ions in the smallest cuboctahedral cages are the first, strongest binding sites for D(2) leading to an overall discrimination between the two types of exposed Cu(II) sites at the paddlewheel nodes. Thus, the Cu(II) centers within the cuboctahedral cage are the first sites of D(2) binding with a Cu-D(2) distance of 2.23(1) A.

High capacity gas storage by a 4,8-connected metal-organic polyhedral framework.

The polyhedral complex [Cu(4)L(H(2)O)(4)]solv (NOTT-140) shows a 4,8-connected structure of rare scu topology comprising octahedral and cuboctahedral cages; desolvated NOTT-140a shows a total CO(2) uptake of 314.6 cm(3) (STP) cm(-3) at 20 bar, 293 K, and a total H(2) uptake of 6.0 wt% at 20 bar, 77 K.

Supramolecular binding and separation of hydrocarbons within a functionalized porous metal–organic framework

Supramolecular interactions are fundamental to host-guest binding in many chemical and biological processes. Direct visualization of such supramolecular interactions within host-guest systems is extremely challenging, but crucial to understanding their function. We report a comprehensive study that combines neutron scattering, synchrotron X-ray and neutron diffraction, and computational modelling to define the detailed binding at a molecular level of acetylene, ethylene and ethane within the porous host NOTT-300. This study reveals simultaneous and cooperative hydrogen-bonding, π···π stacking interactions and intermolecular dipole interactions in the binding of acetylene and ethylene to give up to 12 individual weak supramolecular interactions aligned within the host to form an optimal geometry for the selective binding of hydrocarbons. We also report the cooperative binding of a mixture of acetylene and ethylene within the porous host, together with the corresponding breakthrough experiments and analysis of adsorption isotherms of gas mixtures.

Enhancement of H2 adsorption in Li+-exchanged co-ordination framework materials

H(2) adsorption in (Me(2)NH(2))[In(L)] is enhanced by exchange of Me(2)NH(2)(+) for Li(+) cations; the Li(+)-exchanged material displays a lower isosteric heat for H(2) adsorption than the parent material, indicating that the increase in H(2) capacity is due to an increase in the accessible pore volume on cation exchange, while the lower adsorption enthalpy is consistent with increased pore size.

A Robust Binary Supramolecular Organic Framework (SOF) with High CO2 Adsorption and Selectivity

A robust binary hydrogen-bonded supramolecular organic framework (SOF-7) has been synthesized by solvothermal reaction of 1,4-bis-(4-(3,5-dicyano-2,6-dipyridyl)dihydropyridyl)benzene (1) and 5,5′-bis-(azanediyl)-oxalyl-diisophthalic acid (2). Single crystal X-ray diffraction analysis shows that SOF-7 comprises 2 and 1,4-bis-(4-(3,5-dicyano-2,6-dipyridyl)pyridyl)benzene (3); the latter formed in situ from the oxidative dehydrogenation of 1. SOF-7 shows a three-dimensional four-fold interpenetrated structure with complementary O–H···N hydrogen bonds to form channels that are decorated with cyano and amide groups. SOF-7 exhibits excellent thermal stability and solvent and moisture durability as well as permanent porosity. The activated desolvated material SOF-7a shows high CO2 adsorption capacity and selectivity compared with other porous organic materials assembled solely through hydrogen bonding.

A mesoporous metal–organic framework constructed from a nanosized C3-symmetric linker and [Cu24(isophthalate)24] cuboctahedra

The mesoporous framework [Cu(3)(L)(H(2)O)(3)]·(DMF)(35)·(H(2)O)(35) (NOTT-119) shows on desolvation a BET surface area of 4118(200) m(2) g(-1), a pore volume of 2.35 cm(3) g(-1), a total H(2) uptake of 101 mg g(-1) at 60 bar, 77 K and a total CH(4) uptake of 327 mg g(-1) at 80 bar, 298 K.

Direct hydrodeoxygenation of raw woody biomass into liquid alkanes

Being the only sustainable source of organic carbon, biomass is playing an ever-increasingly important role in our energy landscape. The conversion of renewable lignocellulosic biomass into liquid fuels is particularly attractive but extremely challenging due to the inertness and complexity of lignocellulose. Here we describe the direct hydrodeoxygenation of raw woods into liquid alkanes with mass yields up to 28.1 wt% over a multifunctional Pt/NbOPO4 catalyst in cyclohexane. The superior performance of this catalyst allows simultaneous conversion of cellulose, hemicellulose and, more significantly, lignin fractions in the wood sawdust into hexane, pentane and alkylcyclohexanes, respectively. Investigation on the molecular mechanism reveals that a synergistic effect between Pt, NbOx species and acidic sites promotes this highly efficient hydrodeoxygenation of bulk lignocellulose. No chemical pretreatment of the raw woody biomass or separation is required for this one-pot process, which opens a general and energy-efficient route for converting raw lignocellulose into valuable alkanes.