Yaopeng Zhao

Polystyrene sulphonic acid resins with enhanced acid strength via macromolecular self-assembly within confined nanospace

Tightening environmental legislation is driving the chemical industries to develop efficient solid acid catalysts to replace conventional mineral acids. Polystyrene sulphonic acid resins, as some of the most important solid acid catalysts, have been widely studied. However, the influence of the morphology on their acid strength--closely related to the catalytic activity--has seldom been reported. Herein, we demonstrate that the acid strength of polystyrene sulphonic acid resins can be adjusted through their reversible morphology transformation from aggregated to swelling state, mainly driven by the formation and breakage of hydrogen bond interactions among adjacent sulphonic acid groups within the confined nanospace of hollow silica nanospheres. The hybrid solid acid catalyst demonstrates high activity and selectivity in a series of important acid-catalysed reactions. This may offer an efficient strategy to fabricate hybrid solid acid catalysts for green chemical processes.

Nature of supramolecular complexes controlled by the structure of the guest molecules: formation of octa acid based capsuleplex and cavitandplex.

Factors that govern inclusion of organic molecules within octa acid (OA), a synthetic deep cavity cavitand, have been delineated by examining the complexation behavior of a number of organic molecules with varying dimensions and functionalities with OA. The formation of two types of complexes has been noted: the one which we call cavitandplex is a partially open complex in which a part of the guest molecule remains exposed to water, and the other termed capsuleplex is formed through assembly of two OA molecules. In capsuleplex, the guest is protected from water. Generally, guest molecules that possess ionic head groups form cavitandplex, and all others form capsuleplex. Capsuleplex may contain one or two guest molecules within the capsule. Small organic molecules (<10 A in length) may form both 2:1 and 2:2 capsuleplex, while longer ones (>12 A) preferentially form 2:1 capsuleplex. Extensive 1H NMR experiments have been carried out to characterize host-guest complexes. In the absence of the guest, OA tends to aggregate in water. The extent of aggregation depends on the concentration of OA and the presence of salts in solution. We expect the information obtained from this study to be of great value in predicting the nature of complexes with a given guest and facilitating appropriate guest chosen by researchers.

Semiconducting Neutral Microstructures Fabricated by Coordinative Self‐Assembly of Intramolecular Charge‐Transfer Tetrathiafulvalene Derivatives

A new class of tetrathiafulvalene (TTF)-based microstructures fabricated by coordinative self-assembly has been successfully prepared by a solution process. The morphology of the TTF coordination polymers is readily tuned by the variation of metal ions. Upon incorporation of Pb(2+) and Zn(2+) ions, one-dimensional (1D) wirelike microstructures and spherical polymer particles were achieved, respectively. These results indicate that the coordinative approach pursued in this work, in which the building blocks of 1 are linked in a coordination polymer chain by association with metal ions, is an efficient and versatile approach to produce more mechanically robust micro- and nanometer-sized coordination polymer materials. More interestingly, the neutral coordination polymers are conductive and magnetic at room temperature without external manipulation. Such conductivity is reminiscent of the behavior of the neutral conductive TTF in single crystals.

Spectroscopic and crystallographic investigations of novel BODIPY-derived metal-organic frameworks.

To explore new 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY)-derived metal-organic frameworks (MOFs), we employed 2,6-dicarboxyl-1,3,5,7-tetramethyl-8-phenyl-4,4-difluoroboradiazaindacene (H2L) as a ligand to successfully synthesize five coordination polymers, namely, {[Zn2(L)2(bpp)]·2H2O·2EtOH}n (1), {[Cd2(L)2(bpp)]·2H2O·EtOH}n (2), {[Cd2(L)(bpe)3(NO3)2]·2H2O·DMF·EtOH}n (3), {[Cd(L)(bpe)0.5(DMF)(H2O)]}n (4), and {[Cd(L)(bpe)0.5]·1.5H2O·DMF}n (5) (bpp = 1,3-bi(4-pyridyl)propane, bpe = 1,2-bi(4-pyridyl)ethane). Except for two 2D-layer coordination polymers 3 and 4, the rest samples exhibit 3D metal-organic frameworks with certain pore sizes, especially MOFs 1 and 5. Spectroscopic and crystallographic investigations demonstrate that the absorption and emission energies of the BODIPY chromophores are sensitive to the coordination modes. Moreover, in case 2, the transition metal centers coordinated with the dicarboxylate ligands L(2-) are capable of forming the two BODIPY units in coplanar arrangements (θ = 37.9°), simultaneously suppressing the uncommon J-dimer absorption band centered at 705 nm with a long tail into the near-infrared region at room temperature. On the other hand, in comparison with the ligand H2L, the emission of monomer-like BODIPY in case 3 is enhanced in the solid state by a considerably long distance between the parallel BODIPY planes (about 14.0 Å).

Systematic morphology and phase control of Mg-ptcda coordination polymers by Ostwald ripening and self-templating

Mg-ptcda (ptcda = perylene-3,4,9,10-tetracarboxylic dianhydride) coordination polymer particles (CPPs) with special hexagonal tubes morphology were manufactured by a self-assembly and hydrothermal method. The growth process from unstable nanoribbons to metastable hexagonal rods to a core–shell structure and finally to the stable hexagonal tubes was achieved and investigated by SEM and XRD characterizations. The morphology of Mg-ptcda is mainly controlled by Ostwald ripening and self-templating mechanism. Mg-ptcda CPPs with various morphologies, such as hexagonal rings and snowflakes, and particle sizes from micro to nanoscale can be finely tuned using organic solvent as additives. The optical properties of Mg-ptcda CPPs show that incorporation of ptcda dyes in coordination polymers can efficiently reduce their aggregation and interaction in the solid state. Furthermore, porous MgO nanomaterials with various morphologies can be fabricated from Mg-ptcda CPP precursors by a simple thermal treatment process.

Designing systems for one-way trans to cisphotoisomerization for solar reactions

We have examined triplet photosensitized isomerization of sterically crowded derivatives of stilbene, diene, triene and styrene. Under selective triplet sensitization, several such compounds proceeded in the synthetically desirable manner of complete one-way trans to cis conversion or in a highly stereoselective manner. In the process, we have identified several systems suitable for solar irradiation as a “green” procedure for preparation of hindered olefinic isomers. A simple procedure for determining the relative efficiency of a solar reaction has also been introduced.

Highly active water oxidation on nanostructured biomimetic calcium manganese oxide catalysts

Water oxidation is a crucial reaction step in solar-to-chemical energy conversion processes such as photocatalytic water splitting and carbon dioxide reduction. In natural photosynthesis, the water oxidation reaction is catalyzed by μ-oxido-Mn4Ca clusters in photosystem II (PSII). Herein, we report the fabrication of nanostructured biomimetic calcium manganese oxides (CaxMnOy) via a simple process under mild conditions utilizing H2O2 as an oxidant and TMAOH (tetramethylammonium hydroxide) as an alkaline source. CaxMnOy materials with x higher than 0.26 are composed of nanoparticles with particle sizes ranging from 15 to 30 nm according to the result of HRTEM. The results of X-ray absorption fine structure (XAFS) indicate that calcium manganese oxides have similar structural motifs to the catalytically active site for water oxidation in PSII. It was also found that the content of Ca and the concentration of H2O2 in the initial mixture could affect the crystallinity and the average Mn valence state of calcium manganese oxides. Water oxidation experiments for both chemical and photocatalytic systems suggest that the disordered structure of calcium manganese oxides and a modest valence state of Mn (+3.7 to +3.8) are necessary for achieving high activity. Our method provides a strategy for synthesis and modulation of nanostructured biomimetic water oxidation catalysts.

Asymmetric hydrogenation in nanoreactors with encapsulated Rh-MonoPhos catalyst

The asymmetric multicomponent catalyst, Rh-MonoPhos, was successfully encapsulated in the nanocages of mesoporous silicas with cage-like structure (FDU-12 and C-FDU-12). The resulting solid catalyst exhibits excellent activity (TOF 2052 h−1) and enantioselectivity (93% ee) in the asymmetric hydrogenation of olefin derivatives. The solid catalyst with Rh(MonoPhos)2 in nanocages affords considerably higher activity than that with Rh(MonoPhos)1 or Rh(MonoPhos)3 in nanocages. Combined with the ESI-MS results, it could be supposed that RhL2(nbd) may be active species or the precursors for active species. Moreover, the solid catalyst with C-FDU-12 (inner surface modified by a thin carbon layer) as host material shows considerably higher activity and enantioselectivity than that with pure silica FDU-12 as host material in the asymmetric hydrogenation of different types of olefin derivatives, indicating that the microenvironment modification is one of the key factors for improving the catalytic performance of the solid catalyst. The solid catalysts possess high stability and could be reused for at least 7 times.

Asymmetric hydrogenation by RuCl2(R-Binap)(dmf)(n) encapsulated in silica-based nanoreactors

The Noyori catalyst RuCl2(R-Binap)(dmf)n has been successfully encapsulated in C-FDU-12 by using the active chlorosilane Ph2Cl2Si as the silylating agent. 31P-NMR results show that there is no strong interaction between the molecular catalyst and the solid support, thus the encapsulated molecular catalyst could move freely in the nanoreactor during the catalytic process. The solid catalyst exhibits high activity and enantioselectivity for the asymmetric hydrogenation of a series of β-keto esters due to the preserved intrinsic properties of RuCl2(R-Binap)(dmf)n encapsulated in the nanoreactor. The solid catalyst could be recycled by simple filtration and be reused at least four times.

Facile Synthesis of Hybrid Core–Shell Nanospheres for the Asymmetric Transfer Hydrogenation of Aromatic Ketones

The polymer–inorganic hybrid core–shell nanospheres with N‐(para‐toluenesulfonyl)‐1,2‐diphenylethylenediamine in the core and the poly(methyl acrylate) (PMA) polymer in the shell were prepared by using a sol–gel process. The surface properties of solid catalysts were modified by controlling PMA and the cetyltrimethylammonium bromide surfactant in the shell. The water contact angle results suggest that the presence of PMA and cetyltrimethylammonium bromide in the shell increases the surface hydrophobicity. In the Rh‐catalyzed transfer hydrogenation of aromatic ketones in aqueous HCOONa, the solid catalyst with higher surface hydrophobicity demonstrates higher activity, which suggests that suitable surface properties increase the reaction rate by increasing the diffusion rates of hydrophobic substrates. Furthermore, this heterogeneous catalyst can be reused conveniently without loss of ee values.