Youlong Zhu

Desymmetrized Vertex Design for the Synthesis of Covalent Organic Frameworks with Periodically Heterogeneous Pore Structures

Two novel porous 2D covalent organic frameworks (COFs) with periodically heterogeneous pore structures were successfully synthesized through desymmetrized vertex design strategy. Condensation of C(2v) symmetric 5-(4-formylphenyl)isophthalaldehyde or 5-((4-formylphenyl)ethylene)isophthalaldehyde with linear hydrazine linker under the solvothermal or microwave heating conditions yields crystalline 2D COFs, HP-COF-1 and HP-COF-2, with high specific surface areas and dual pore structures. PXRD patterns and computer modeling study, together with pore size distribution analysis confirm that each of the resulting COFs exhibits two distinctively different hexagonal pores. The structures were characterized by FT-IR, solid state (13)C NMR, gas adsorption, SEM, TEM, and theoretical simulations. Such rational design and synthetic strategy provide new possibilities for preparing highly ordered porous polymers with heterogeneous pore structures.

Reversible tuning of pore size and CO2 adsorption in azobenzene functionalized porous organic polymers

A series of responsive porous organic polymers (POPs), UCBZ-1, 2, 3 and 4, comprising azobenzene moieties, were synthesized through dynamic imine chemistry and their structure–property relationship was studied. The resulting porous organic polymers showed permanent porosity with BET surface areas of around 1000 m2 g−1. More importantly, these polymers exhibit fully reversible responsive pore size distribution and CO2 uptake triggered by UV irradiation or heat treatment. We observed the transformation of some micropores with pore size around 14 A to larger ones (17 A) upon UV irradiation. Although the BET surface areas remain almost constant during the isomerization process, trans to cis conversion of the azobenzene groups significantly increases CO2 uptake (up to 29% at 273 K and 1 atm) of the frameworks. Our study on the structure–property relationship shows that the bulkiness of the azobenzene moieties reduces the extent of responsiveness of the materials. Such a photoresponsive behavior is largely attributed to the enhanced dipole–quadrupole interaction between CO2 and the frameworks due to the increased dipole moment of cis –NN– groups compared to trans –NN–. Such responsiveness significantly differs from the previously reported responsive MOFs containing azobenzene groups, which release CO2 upon trans to cis isomerization. These polymers thus represent a new class of porous polymers having intriguing stimuli-responsive properties for advanced applications.

Synthesis of a conjugated porous Co(II) porphyrinylene–ethynylene framework through alkyne metathesis and its catalytic activity study

The development of efficient catalysts for the oxygen reduction reaction (ORR) is crucial for a number of emerging technologies, to counter energy and environment crises. Herein, we report an alkyne metathesis polymerization protocol to synthesize a conjugated microporous metalloporphyrin-based framework containing interconnected ORR catalytic centers. A simple composite of the framework and carbon black shows excellent ORR electrocatalytic activity and specificity through a four-electron reduction mechanism under both acidic and alkaline conditions. The pyrolysis of the catalyst, which is commonly involved in the preparation of ORR catalytic systems, is not necessary. Compared to monomeric metalloporphyrins, the framework shows enhanced ORR catalytic activity, presumably due to the porous and conjugated nature of the framework structure, which allows better exposure of the catalytically active sites, and efficient electron/mass transport. More importantly, the composite electrocatalyst exhibits superior durability and methanol tolerance over commercial Pt/C and metalloporphyrin monomers. Given the highly structural tunability of conjugated microporous polymers, it is conceivable that such a non-pyrolytic approach could enable the systematic exploration of the structure–activity relationship of organic framework-based ORR catalysts and eventually lead to the development of cost-effective replacements for Pt/C.

Aromatic-rich hydrocarbon porous networks through alkyne metathesis

Purely hydrocarbon-based porous polymers have generally been prepared through various irreversible transition metal-catalyzed cross-coupling reactions forming C–C bonds. Herein, we report an alternative synthetic approach, namely reversible alkyne metathesis, for the preparation of ethynylene-linked porous polymers. Planar and tetrahedral-shaped monomers were explored to construct poly(aryleneethynylene) (PAE) networks. We systematically varied the size of the monomers and studied the structure–property relationships. The resulting polymers exhibit high Brunauer–Emmett–Teller (BET) surface areas in the range of 736 m2 g−1 to 2294 m2 g−1. The advantages of such aromatic-rich PAE networks are their lightweight, high thermal/chemical stabilities, and superior hydrophobicity, which are beneficial for their application in adsorption/separation of toxic organic pollutants from water. We found that PAEs can adsorb a significant amount of common aromatic solvents, e.g. up to 723 wt% of nitrobenzene. Our study thus demonstrates an encouraging novel approach to prepare purely hydrocarbon-based porous materials.

Highly CO2 selective pillar[n]arene-based supramolecular organic frameworks

Abstract Finding a cost-effective, environmental benign, economical and technologically viable way to selectively capture, storage and/or purify energy and light gases represents one of the most urgent needs from energetic, biological, and environmental standpoints. Two 3D low-density supramolecular organic frameworks (SOFs) with solution-processibility were constructed using low-cost and readily available synthetic macrocyclic compounds, namely pillar[5]arene (P5) and pillar[6]arene (P6), via strong O-H···O, C-H···O, C-H···π, π···π interactions. These SOFs offer unusual topology, interconnecting pores and extremely high selective CO2 capture and separation properties for multiple gas mixtures (e.g. 30/70 mixture of CO2/H2, up to 3733/1, 298 K), making them great promising candidates for practical gas separation of steam-methane re-former (SMR) off-gas, flue gas, natural gas, biogas, syngas, paraffin/olefin, etc.