Tuning the Porosity and Photocatalytic Performance of Triazine-Based Graphdiyne Polymers through Polymorphism.

Crystalline and amorphous organic materials are an emergent class of heterogeneous photocatalysts for the generation of hydrogen from water, but a direct correlation between their structures and the resulting properties has not been achieved so far. To make a meaningful comparison between structurally different, yet chemically similar porous polymers, two porous polymorphs of a triazine-based graphdiyne (TzG) framework are synthesized by a simple, one-pot homocoupling polymerization reaction using as catalysts CuI for TzGCu and PdII /CuI for TzGPd/Cu . The polymers form through irreversible coupling reactions and give rise to a crystalline (TzGCu ) and an amorphous (TzGPd/Cu ) polymorph. Notably, the crystalline and amorphous polymorphs are narrow-gap semiconductors with permanent surface areas of 660 m2 g-1 and 392 m2 g-1 , respectively. Hence, both polymers are ideal heterogeneous photocatalysts for water splitting with some of the highest hydrogen evolution rates reported to date (up to 972 µmol h-1 g-1 with and 276 µmol h-1 g-1 without Pt cocatalyst). Crystalline order is found to improve delocalization, whereas the amorphous polymorph requires a cocatalyst for efficient charge transfer. This will need to be considered in future rational design of polymer catalysts and organic electronics.

Fluorescent Sulphur- and Nitrogen-Containing Porous Polymers with Tuneable Donor-Acceptor Domains for Light-Driven Hydrogen Evolution.

Light-driven water splitting is a potential source of abundant, clean energy, yet efficient charge-separation and size and position of the bandgap in heterogeneous photocatalysts are challenging to predict and design. Synthetic attempts to tune the bandgap of polymer photocatalysts classically rely on variations of the sizes of their π-conjugated domains. However, only donor-acceptor dyads hold the key to prevent undesired electron-hole recombination within the catalyst via efficient charge separation. Building on our previous success in incorporating electron-donating, sulphur-containing linkers and electron-withdrawing, triazine (C3 N3 ) units into porous polymers, we report the synthesis of six visible-light-active, triazine-based polymers with a high heteroatom-content of S and N that photocatalytically generate H2 from water: up to 915 µmol h-1 g-1 with Pt co-catalyst, and-as one of the highest to-date reported values -200 µmol h-1 g-1 without. The highly modular Sonogashira-Hagihara cross-coupling reaction we employ, enables a systematic study of mixed (S, N, C) and (N, C)-only polymer systems. Our results highlight that photocatalytic water-splitting does not only require an ideal optical bandgap of ≈2.2 eV, but that the choice of donor-acceptor motifs profoundly impacts charge-transfer and catalytic activity.

Microporous Lead-Organic Framework for Selective CO2 Adsorption and Heterogeneous Catalysis.

A novel microporous metal-organic framework, {[Pb4(µ8-MTB)2(H2O)4]·5DMF·H2O}n (1; MTB = methanetetrabenzoate and DMF = N,N'-dimethylformamide), was successfully synthesized by a solvothermal reaction and structurally characterized by single-crystal X-ray diffraction. The framework exhibits a unique tetranuclear [Pb4(µ3-COO)(µ2-COO)6(COO)(H2O)4] secondary building unit (SBU). The combination of the SBU with the tetrahedral symmetry of MTB results in a three-dimensional network structure, with one-dimensional jarlike cavities having sizes of about 14.98 × 7.88 and 14.98 × 13.17 Å2 and propagating along the c axis. Due to the presence of four coordinately unsaturated sites per one metal cluster, an activated form of compound 1 (i.e., desolvated form denoted as 1') was tested in gas adsorption and catalytic experiments. The studies of gas sorption revealed that 1' exhibits a surface area (Brunauer-Emmett-Teller) of 980 m2·g-1. This value is the highest reported for any compound from the MTB group. Interactions of carbon dioxide (CO2) molecules with the framework, confirmed by density functional theory calculations, resulted in high CO2 uptake and significant selectivity of CO2 adsorption with respect to methane (CH4) and dinitrogen (N2) when measured from atmospheric pressure to 21 bar. The high selectivity of CO2 over N2 is mostly important for capturing CO2 from the atmosphere in attempts to decrease the greenhouse effect. Moreover, compound 1' was tested as a heterogeneous catalyst in Knoevenagel condensation of active methylene compounds with aldehydes. Excellent catalytic conversion and selectivity in the condensation of benzaldehyde and cyclohexanecarbaldehyde with malononitrile was observed, which suggests that accessible lead(II) sites play an important role in the heterogeneous catalytic process.

Twinned Growth of Metal-Free, Triazine-Based Photocatalyst Films as Mixed-Dimensional (2D/3D) van der Waals Heterostructures.

Design and synthesis of ordered, metal-free layered materials is intrinsically difficult due to the limitations of vapor deposition processes that are used in their making. Mixed-dimensional (2D/3D) metal-free van der Waals (vdW) heterostructures based on triazine (C3 N3 ) linkers grow as large area, transparent yellow-orange membranes on copper surfaces from solution. The membranes have an indirect band gap (Eg,opt = 1.91 eV, Eg,elec = 1.84 eV) and are moderately porous (124 m2 g-1 ). The material consists of a crystalline 2D phase that is fully sp2 hybridized and provides structural stability, and an amorphous, porous phase with mixed sp2 -sp hybridization. Interestingly, this 2D/3D vdW heterostructure grows in a twinned mechanism from a one-pot reaction mixture: unprecedented for metal-free frameworks and a direct consequence of on-catalyst synthesis. Thanks to the efficient type I heterojunction, electron transfer processes are fundamentally improved and hence, the material is capable of metal-free, light-induced hydrogen evolution from water without the need for a noble metal cocatalyst (34 µmol h-1 g-1 without Pt). The results highlight that twinned growth mechanisms are observed in the realm of "wet" chemistry, and that they can be used to fabricate otherwise challenging 2D/3D vdW heterostructures with composite properties.

Tailored Band Gaps in Sulfur- and Nitrogen-Containing Porous Donor-Acceptor Polymers.

Donor-acceptor dyads hold the key to tuning of electrochemical properties and enhanced mobility of charge carriers, yet their incorporation into a heterogeneous polymer network proves difficulty owing to the fundamentally different chemistry of the donor and acceptor subunits. A family of sulfur- and nitrogen-containing porous polymers (SNPs) are obtained via Sonogashira-Hagihara cross-coupling and combine electron-withdrawing triazine (C3 N3 ) and electron-donating, sulfur-containing linkers. Choice of building blocks and synthetic conditions determines the optical band gap (from 1.67 to 2.58 eV) and nanoscale ordering of these microporous materials with BET surface areas of up to 545 m2 g-1 and CO2 capacities up to 1.56 mmol g-1 . Our results highlight the advantages of the modular design of SNPs, and one of the highest photocatalytic hydrogen evolution rates for a cross-linked polymer without Pt co-catalyst is attained (194 µmol h-1 g-1 ).

Post-Synthesis Stabilization of Germanosilicate Zeolites ITH, IWW, and UTL by Substitution of Ge for Al.

Germanosilicate zeolites often suffer from low hydrothermal stability due to the high content of Ge. Herein, we investigated the post-synthesis introduction of Al accompanied by stabilization of selected germanosilicates by degermanation/alumination treatments. The influence of chemical composition and topology of parent germanosilicate zeolites (ITH, IWW, and UTL) on the post-synthesis incorporation of Al was studied. Alumination of ITH (Si/Ge=2-13) and IWW (Si/Ge=3-7) zeolites resulted in the partial substitution of Ge for Al (up to 80 %), which was enhanced with a decrease of Ge content in the parent zeolite. In contrast, in extra-large pore zeolite UTL (Si/Ge=4-6) the hydrolysis of the interlayer Ge-O bonds dominated over substitution. The stabilization of zeolite UTL was achieved using a novel two-step degermanation/alumination procedure by the partial post-synthesis substitution of Ge for Si followed by alumination. This new method of stabilization and incorporation of strong acid sites may extend the utilization of germanosilicate zeolites, which has been until now been limited.

Fabrication of Hybrid Organic-Inorganic Materials with Tunable Porosity for Catalytic Application.

Novel layered organic-inorganic materials functionalized with amino groups have been synthesized by using a two-dimensional zeolitic precursor, IPC-1P, prepared by a top-down approach from zeolite UTL. The formation of porous materials containing silsesquioxane linkers covalently bonded to zeolite layers in the interlayer space was confirmed by a variety of characterization techniques (N2  sorption, XRD, TEM). The textural properties and catalytic behavior of functionalized hybrid materials synthesized by direct pillaring of IPC-1P or by grafting of (3-aminopropyl)silyl groups to the IPC-1P precursor preliminarily pillared with tetraethoxysilane (TEOS) were compared. The use of a mixture of aminosilsesquioxanes and TEOS for pillaring of IPC-1P led to the formation of functionalized materials, which are characterized by excellent textural properties (SBET =154-435 m2 g-1 , Vtotal =0.336-0.630 cm3 g-1 ) and provide a 100 % yield of target benzylidenemalononitrile in the Knoevenagel condensation of benzaldehyde and malononitrile.