A pinboard by
Yingxin He

Research Assistant, Missouri University of Science and Technology


Engineering Porous Polymer Hollow Fiber Microfluidic Reactors for Sustainable C-H Functionalization

The highly hydrophilic and solvent-stable porous polyamide-imide (PAI) hollow fibers were created by cross-linking of bare PAI hollow fibers with 3-aminopropyl trimethoxy silane (APS). The obtained composite hollow fibers were used as continuous-flow process for heterogeneously catalyzed the aldol and nitroaldol condensations of aromatic aldehydes with acetone and nitomethane at 50 and 40 °C, respectively. Furthermore, the APS-grafted PAI hollow fibers were also functionalized with salicylic aldehyde for binding catalytically active Pd(II) ions through covalent postmodification method. The catalytic activity of the composite hollow fiber microfluidic reactors (Pd(II) immobilized APS-grafted PAI hollow fibers) was tested via heterogeneous Heck coupling reaction of aryl halides under both batch and continuous flow reaction in polar aprotic solvents at high temperature (120 °C) and low operating pressure. XPS and ICP analyses of the starting and recycled composite hollow fibers indicated that the fibers contain very similar loading of Pd(II) implying no degree of catalyst leaching from hollow fibers during reaction. The composite hollow fiber microfluidic reactors showed long-term stability and strong control over the leaching of Pd species.


Surfactant- and Binder-Free Hierarchical Platinum Nanoarrays Directly Grown onto a Carbon Felt Electrode for Efficient Electrocatalysis.

Abstract: The future of fuel cells that convert the chemical energy to electricity relies mostly on the efficiency of oxygen reduction reaction (ORR) due to its sluggish kinetics. By effectively bypassing the use of organic surfactants, post-synthesis steps for the immobilization onto electrodes, the catalytic inks preparation using binders, and the common problem of nanoparticles detachment from supports involved in traditional methodologies, we demonstrate a versatile electrodeposition method for growing anisotropic microstructures directly onto a three-dimensional (3D) carbon felt electrode, using platinum nanoparticles as elementary building blocks. The as-synthesized materials are extensively characterized by integrating methods of physical (TGA, XRD, SEM, ICP, XPS) and electroanalytical (voltammetry, EIS) chemistry to examine the intricate relationship of material-to-performance and select the best-performing electrocatalyst to be applied in the model reaction of ORR for its practical integration into a microbial fuel cell (MFC). A tightly optimized procedure enables decorating an electrochemically activated carbon felt electrode by 40-60 nm ultrathin 3D-interconnected platinum nanoarrays leading to a hierarchical framework of ca. 500 nm. Half-cell reactions reveal that the highly rough metallic surface exhibits improved activity and stability towards ORR (Eonset ~1.1 V vs. RHE, p(HO2‒) < 0.1%) and hydrogen evolution reaction (HER, -10 mA cm-2 for only 75 mV overpotential). Owing to its unique features, the developed material shows distinguished performance as an air-breathing cathode in a garden compost MCF exhibiting better current and faster power generation than its equivalent classical double chamber. The enhanced performance of the material obtained herein is explained by the absence of any organic surfactant on the surface of the nanoarrays, the good metal-support interaction, the particular morphology of the nanoarrays, and the reduced aggregation/detachment of particles. It promises a radical improvement in current surface reactions and paves a new way towards electrodes with regulated surface roughness allowing for their successful application in heterogeneous catalysis.

Pub.: 18 Jun '17, Pinned: 03 Jul '17

Synergistic effects of Gold-Palladium nanoalloys and reducible supports on the catalytic reduction of 4-nitrophenol.

Abstract: Herein we report on the catalytic activity of mesoporous nickel, iron, cerium, cobalt and manganese oxides prepared using KIT-6 as a hard-template via evaporation assisted precipitation. The mesoporous metal oxides (MMOs) were characterized and used as heterogeneous catalysts in the reduction of 4-nitrophenol (4-Nip) by sodium borohydride (〖BH〗_4^-). Furthermore, polyamidoamide (PAMAM) dendrimers were used to synthesize Gold-Palladium nanoalloy particles. The size of AuPd/PAMAM was found to be 3.5 ± 0.8 nm in diameter before being immobilized on the aforementioned mesoporous metal oxides and used as catalysts in the reduction of 4-Nip. Prior to catalytic evaluation, the reduction profiles of the mesoporous metal oxides were investigated by hydrogen-temperature programmed reduction (H2-TPR), and showed that mesoporous metal oxides can be easily reduced at lower temperatures and that the immobilization of Gold-Palladium nanoalloy particles lowers their reduction temperatures. Mesoporous cobalt and manganese oxides showed catalytic activity towards 4-Nip reduction and the activity was enhanced after immobilization of the Gold-Palladium nanoalloys. Isolation of nanoparticles activity was achieved by immobilization of the Gold-Palladium nanoalloys on the inert silica support. From this we postulated an electron relay mechanism for the reduction of 4-nitrophenol. With the use of power rate law we showed that 4-Nip reduction follows pseudo-first order kinetics.

Pub.: 27 Jun '17, Pinned: 03 Jul '17

Block Polymer Membranes Functionalized with Nanoconfined Polyelectrolyte Brushes Achieve Sub-Nanometer Selectivity

Abstract: The well-defined nanostructure of membranes manufactured from self-assembled block polymers enables highly selective separations; however, recent efforts to push the pore size of block polymer-based membranes to the lower end of the size spectrum have only been moderately successful for a variety of reasons. For instance, the conformational changes of the stimuli-responsive functional groups lining the pore walls of some block polymer membranes result in varied pore sizes that limit their operational range. Here, we overcome this challenge through the directed design of the third moiety of an A-B-C triblock polymer. The use of this macromolecular design paradigm allows for the preparation of a 500 nm thick polyisoprene-b-polystyrene-b-poly(2-acrylamido-ethane-1,1-disfulonic acid) (PI-PS-PADSA) coating atop a hollow fiber membrane support. This nanoporous test bed, which exhibits an average pore radius of 1 nm, demonstrates an extremely high solute selectivity by fully gating solutes that have only an 8 Å size difference, a separation that is based solely on a sieving mechanism. Furthermore, the nanoscale structural characteristics of the solvated PADSA pore walls are elucidated by quantifying the rejection of neutral solutes and calculating the hydraulic permeability values in solutions of high ionic strength (1 mM ≤ I ≤ 3 M) and over a broad range of solution pH (1 ≤ pH ≤ 13). These key results provide a solid foundation for defining structure–property–performance relationships in the emerging area of nanoporous triblock polymer thin films. Moreover, the successful demonstration of the test bed separation device offers a tangible means by which to manufacture next-generation nanofiltration membranes that require a robust performance profile over a dynamic range of conditions.

Pub.: 23 Jun '17, Pinned: 03 Jul '17

The Surface Chemistry of Metal-Based Hydrogenation Catalysis

Abstract: The promotion of hydrogenation reactions by transition-metal-based heterogeneous catalysts was established many decades ago but is still quite common in the chemical industry. Because of their importance, these processes have been studied in great detail from both fundamental and practical points of view, and much has been learned about them. However, some key questions remain unanswered, and solutions to specific industrial needs are still pending. In this Perspective, we discuss the state-of-the-art of our understanding of some of the fundamental issues associated with hydrogenation catalysis. From the mechanistic point of view, we use the example of olefin hydrogenation to assess the status of our knowledge on the adsorption of the organic reactants, the role that the strongly adsorbed carbonaceous deposits that form during reaction play in defining the catalytic kinetics, the mechanistic details of the hydrogen dissociative uptake and surface mobility during reaction, and the dynamic changes of the structure of the surface induced by the catalytic conditions. We then introduce the issue of selectivity in connection with the hydrogenation of alkynes, dienes, trienes, and aromatics; unsaturated aldehydes and imines; and cases where hydrogenation competes with other types of reactions such as dehydrogenations, skeletal rearrangements, cyclizations, and hydrogenolysis. Two general approaches to the control of selectivity are discussed: via the tuning of the structure of the catalytic surface, which can now be addressed by using nanoparticles with specific sizes and shapes, and by modifying the electronic properties of the metal, via the addition of a second element. Finally, we make reference to the interest in designing enantioselective hydrogenation processes using heterogeneous metal catalysts, and we briefly summarize the ideas that have developed from surface-science studies toward this goal and the advances made in understanding the most promising approach to date, which involves the addition of molecular chiral modifiers to the reaction mixtures.

Pub.: 30 Jun '17, Pinned: 03 Jul '17

Effects of coating solvent and thermal treatment on transport and morphological characteristics of PDMS/Torlon composite hollow fiber membrane

Abstract: A new approach for formation of the polydimethylsiloxane (PDMS) layer on Torlon polyamide-imide hollow fiber (PAI-HF) support has been developed by directly after fiber spinning without the need to undergo the final conventional solvent exchange and drying step, thereby saving postspinning processing steps. The produced PDMS/PAI-HF composite membranes were found to have high CO2 permeance (i.e., 1100 GPU) and exhibited good CO2/N2 selectivities of 8–10 which is close to 90% of that of a PDMS dense film. The effects of coating solution, rewetting and crosslinking temperature on the PAI-HF morphological features, that is, gas transport, skin thickness, skin integrity, and substructure resistance are investigated. The rewetting and thermal treatment of the PAI-HF caused the densification of the skin layer and reduced the pore sizes on the top layer. In addition, the potential use of the PAI-HF support with polymers that are insoluble in hexane is also considered. Effects of water, methanol, and hexane exposure of PAI-HF to these solvents are considered. This evaluation calls attention to issues that must be addressed in any eventual use of the PAI-HF with water-soluble or methanol-soluble selective layer polymers, rather than simple hexane-soluble polymers such as PDMS. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 45418.

Pub.: 23 Jun '17, Pinned: 03 Jul '17

Modification of Polysulfone (PSF) Hollow Fiber Membrane (HFM) with Zwitterionic or Charged Polymers

Abstract: Membrane fouling is a critical problem limiting membrane performance and application lifetime. Zwitterionic polymers are highly resistant to irreversible membrane foulants because of their hydrated structure and charge neutral characteristics. The objectives of this study were to improve membrane antifouling performance with zwitterionic polymer modifications and to gain a better understanding of the factors affecting the process. We used polysulfone hollow fiber, instead of flat-sheet membranes, for the study because of its common industrial applications for ultrafiltration. Different characterization methods including scanning electron microscopy, nuclear magnetic resonance, attenuated total reflectance Fourier-transform infrared spectroscopy, contact angle goniometry, and cyclic filtration tests of deionized water and bovine serum albumin were applied to characterize the membrane and evaluate the performance of pristine and modified HFMs. Results showed that membrane fouling was significantly decreased by grafting the zwitterion carboxybetaine methacrylate (CBMA) via atom transfer radical polymerization. Positively charged [2-(acryloyloxy)ethyl] trimethylammonium chloride with a quaternary amine end group and negatively charged 2-carboxyethyl acrylate with a carboxyl end group were grafted separately as charged polymer controls. These charged polymers improved the membrane antifouling property but not to the same extent as the membrane with the zwitterionic polymers. Compared with the pristine PSF HFMs, all modified membranes exhibited enhanced hydrophilicity and antifouling property.

Pub.: 13 Jun '17, Pinned: 03 Jul '17