Yongwu Peng

A Modulated Hydrothermal (MHT) Approach for the Facile Synthesis of UiO-66-Type MOFs

Developing a general and economically viable approach for the large-scale synthesis of water-stable metal-organic frameworks (MOFs) with repeatable quality remains the key step for their massive production and commercialization. We herein report a green (aqueous solutions), mild (100 °C, 1 atm), and scalable (can be up to kilograms) modulated hydrothermal (MHT) synthesis of UiO-66, an iconic MOF that has been widely studied recently for its high water stability. More importantly, the MHT synthetic approach can be applied to synthesize other water-stable MOFs with structures identical to UiO-66, such as UiO-66-(F)4, UiO-66-(OCH2CH3)2, and UiO-66-(COOH)4, which cannot be obtained via the traditional solvothermal method. Their performance in postcombustion CO2 capture has also been evaluated. Our MHT approach has clearly depicted a roadmap for the facile synthesis of zirconium-based water stable MOFs to facilitate their massive production and commercialization.

Mixed matrix membranes composed of two-dimensional metal–organic framework nanosheets for pre-combustion CO2 capture: a relationship study of filler morphology versus membrane performance

A facile and scalable bottom-up method is used to synthesize a microporous jungle-gym-like metal–organic framework [Cu2(ndc)2(dabco)]n in the morphologies of nanocubes and nanosheets. The obtained MOFs are blended with polybenzimidazole yielding a series of mixed matrix membranes (MMMs), which are evaluated for their performance in pre-combustion CO2 capture (H2/CO2 separation). Pure gas permeation tests indicate that MMMs with partially oriented nanosheet MOFs possess the largest improvement compared with neat polymers, with the overall H2/CO2 separation performance exceeding the 2008 polymer upper bound.

Modulated Hydrothermal Synthesis of UiO-66(Hf)-Type Metal–Organic Frameworks for Optimal Carbon Dioxide Separation

Recently, there has been growing interest in hafnium (Hf) metal-organic frameworks (MOFs). These MOFs may perform better as gas adsorbents than zirconium (Zr) MOFs due to the presence of Brønsted acid sites with high affinity toward adsorbates, together with the outstanding chemical and hydrothermal stabilities similar to their Zr analogues. However, Hf-MOFs have been rarely reported due to the lack of effective synthetic methods. We herein report a modulated hydrothermal synthesis of UiO-66(Hf)-type MOFs. Among these MOFs, UiO-66(Hf)-(OH)2 possesses a very high CO2 gravimetric uptake of 1.81 mmol g(-1) at 0.15 bar and 298 K, which is 400% higher than that of UiO-66(Hf) (0.36 mmol g(-1)). It also exhibits a record-high volumetric CO2 uptake of 167 v/v at 1 bar and 298 K. Ideal adsorbed solution theory calculations showed a CO2/N2 (molar ratio 15:85) selectivity of 93 and CO2/H2 (molar ratio 30:70) selectivity above 1700. Breakthrough simulations also confirmed its optimal CO2 separation attribute. Our results have demonstrated for the first time the strong potential of Hf-MOFs for advanced adsorbents for high-performance CO2-related separations.

Synthesis of a Sulfonated Two-Dimensional Covalent Organic Framework as an Efficient Solid Acid Catalyst for Biobased Chemical Conversion

Because of limited framework stability tolerance, de novo synthesis of sulfonated covalent organic frameworks (COFs) remains challenging and unexplored. Herein, a sulfonated two-dimensional crystalline COF, termed TFP-DABA, was synthesized directly from 1,3,5-triformylphloroglucinol and 2,5-diaminobenzenesulfonic acid through a previously reported Schiff base condensation reaction, followed by irreversible enol-to-keto tautomerization, which strengthened its structural stability. TFP-DABA is a highly efficient solid acid catalyst for fructose conversion with remarkable yields (97 % for 5-hydroxymethylfurfural and 65 % for 2,5-diformylfuran), good chemoselectivity, and good recyclability. The present study sheds light on the de novo synthesis of sulfonated COFs as novel solid acid catalysts for biobased chemical conversion.

Mechanoassisted Synthesis of Sulfonated Covalent Organic Frameworks with High Intrinsic Proton Conductivity.

It is challenging to introduce pendent sulfonic acid groups into modularly built crystalline porous frameworks for intrinsic proton conduction. Herein, we report the mechanoassisted synthesis of two sulfonated covalent organic frameworks (COFs) possessing one-dimensional nanoporous channels decorated with pendent sulfonic acid groups. These COFs exhibit high intrinsic proton conductivity as high as 3.96 × 10(-2) S cm(-1) with long-term stability at ambient temperature and 97% relative humidity (RH). In addition, they were blended with nonconductive polyvinylidene fluoride (PVDF) affording a series of mixed-matrix membranes (MMMs) with proton conductivity up to 1.58 × 10(-2) S cm(-1) and low activation energy of 0.21 eV suggesting the Grotthuss mechanism for proton conduction. Our study has demonstrated the high intrinsic proton conductivity of COFs shedding lights on their wide applications in proton exchange membranes.

Kinetically controlled synthesis of two-dimensional Zr/Hf metal–organic framework nanosheets via a modulated hydrothermal approach

The kinetically controlled synthesis of two-dimensional (2D) metal–organic framework (MOF) nanosheets in the absence of surfactants is rewarding but challenging. We herein describe such a surfactant-free bottom-up synthesis of 2D stable Zr/Hf MOF nanosheets named NUS-8 composed of Zr6O4(OH)4 or Hf6O4(OH)4 clusters and 1,3,5-benzenetribenzoate (BTB3−) via a modulated hydrothermal approach, which allows fast precipitation and stabilization of intermediate 2D metal–organic nanosheets due to the heterogeneous synthetic conditions. Structural analyses based on synchrotron powder X-ray diffraction data confirm the 2D layered structure of NUS-8 with uniform porosity and highly accessible Lewis acid sites suitable for heterogeneous catalysis. 2D NUS-8 nanosheets exhibit excellent stabilities superior to those of their interlocked 3D MOF analogues synthesized from solvothermal synthesis, which are evidenced by comprehensive stability tests. In particular, dynamic mechanical analysis (DMA) experiments suggest that the stability of 2D NUS-8 nanosheets may come from a combination of interlayer shear sliding deformation and out-of-plane tension/compression modes whereas their interlocked 3D architecture is strictly constrained. Because of the alleviated framework strain and accessible active sites, NUS-8 nanosheets exhibit excellent stability and catalytic activity superior to those of their interlocked 3D MOF counterparts. Our work has demonstrated the potential of a modulated hydrothermal approach in the kinetically controlled synthesis of 2D MOF nanosheets, shedding light on future synthesis of 2D hybrid inorganic–organic materials.

Enhanced catalytic activity of a hierarchical porous metal–organic framework CuBTC

A previously reported mixed ligand strategy was used to synthesize a classic metal–organic framework CuBTC, which exhibited a hierarchical porous structure including micropores and around 3.9 nm. Thanks to the facile mass transfer and denser open metal sites induced by the mixed ligand strategy, the hierarchical porous CuBTC demonstrates enhanced catalytic activity towards both the ring-opening reaction of styrene oxide to 2-methoxy-2-phenylethanol and the cyanosilylation reaction of benzaldehyde to cyanohydrins.

A pH-responsive phase transformation of a sulfonated metal–organic framework from amorphous to crystalline for efficient CO2 capture

We report a pH-responsive phase transformation of a sulfonated MOF from amorphous UiO-66-SO3H to crystalline UiO-66-SO3M (M = Li, Na, K). Such transformation is obtained via neutralizing UiO-66-SO3H with the according alkali hydroxide solutions. EXAFS and FTIR results suggest that the recovered crystallinity can be attributed to the breakage of strong hydrogen bonds among the sulfonic acids in UiO-66-SO3H as well as the subsequent charge repulsion in UiO-66-SO3M to expand the collapsed framework.

A 2D metal–organic framework composed of a bi-functional ligand with ultra-micropores for post-combustion CO2 capture

A two-dimensional (2D) metal–organic framework (MOF) named NUS-5 was synthesized using a bi-functional ligand 4-pyrazolecarboxylic acid (PyC) containing both carboxylate and pyrazole moieties. NUS-5 is composed of porous grids stacked together into a 2D layered structure exhibiting ultra-micropores. It displays remarkable thermostability and excellent selectivity towards sorption of CO2 over N2, making it a good material candidate for post-combustion CO2 capture.

Back Cover: Synthesis of a Sulfonated Two‐Dimensional Covalent Organic Framework as an Efficient Solid Acid Catalyst for Biobased Chemical Conversion (ChemSusChem 19/2015)

The Back Cover picture shows the crystal structure of a sulfonic acid-functionalized 2D covalent organic framework (COF) and its application as a highly effective solid acid catalyst with excellent catalytic activity and chemoselectivity for the conversion of fructose into 5-hydroxymethylfurfural (HMF) and 2,5-diformylfuran (DFF). A sulfonated 2D crystalline COF, termed TFP-DABA, is synthesized directly using 1,3,5-triformylphloroglucinol (TFP) and 2,5-diaminobenzenesulfonic acid (DABA) via a Schiff base condensation reaction followed by irreversible enol-to-keto tautomerization. This COF is highly efficient for fructose conversion with remarkable yields (97% for HMF and 65% for DFF). This study provides encouragement for further exploration of COFs as heterogeneous catalysts for bio-based chemical conversion and related applications. More details can be found in the Communication by D. Zhao et al. on page 3208 in Issue 19, 2015 (DOI: 10.1002/cssc.201500755).