Hybridization of Synthetic Humins with a Metal-Organic Framework for Precious Metal Recovery and Reuse

The number of synthetic strategies used to functionalize MOFs with polymers is rapidly growing; this stems from the knowledge that non-native polymeric guests can significantly boost MOF performance in a number of desirable applications. The current work presents a scalable and solid-state method for MOF/polymer composite production. This simple method constitutes mixing a MOF powder, namely, Fe-BTC (BTC = 1,3,5-benzenetricarboxylate), with a biomass-derived solid monomer, 5-hydroxymethylfurfural (HMF), and subsequently heating the solids; the latter promotes both solid-state diffusion of HMF into the MOF and the formation of polymeric humin species with a high density of accessible hydroxyl functionality within the MOF pore. The resulting composite, Fe-BTC/humin, was found to selectively extract Ag+ ions from laundry wastewater. Subsequent reduction of the Ag+ species yields a novel catalyst, Fe-BTC/humin/Ag, that is able to drive the organic transformation of cinnamaldehyde in a highly selective manner. Moreover, the catalyst exhibited recyclability up to five cycles, which is in contrast to the Fe-BTC/Ag catalyst without the humin-based polymer. It is envisioned that MOF/polymer composites that are able to selectively extract precious metals from liquid waste streams can be used for the future production of sustainable catalysts; this work was aimed at demonstrating a proof of concept in this regard. Moreover, this study brings more understanding of the impact that MOFs can have on polymer functionalities. Understanding the polymer structure and how it can be manipulated will help us realize the high degree of future potential of this distinct class of composite materials.

Nanofibres de carbone sur filtre métallique comme support catalytique structuré

Pascal Tribolet

The main objectives of this work are 2-fold : The development of a novel type of composite material based on carbon nanofibers supported on sintered metal fibers filters (CNF/SMF) and to explore the use of this material as a catalytic support for a model reaction: selective hydrogenation of acetylene. The advantages of this novel support are: Due to carbon nanofibers, it combines a graphitic structure with a high specific surface area without microporosity. Supported carbon nanofibers can be used in a fixed bed reactor without very high pressure drop. Combination of two materials with high thermal conductivity gives a composite which keeps this property. It is suitable for highly exo/endothermic reactions, diminishing temperature gradients in the catalytic bed. The synthesis of carbon nanofibers on SMF filters was carried out by ethane catalytic decomposition at 655°C directly on metallic filters. An oxidation of SMF followed by a reduction in H2 create surface roughness, leading to a larger area for CNF nucleation. The CNF formation mechanism includes several steps: it starts with carbon deposition on metallic surface followed by its diffusion into the metal leading to carbides formation. When the limit of carbon solubility is attained, a layer of carbon is formed on the surface of metallic fibers. High tension within the bulk metal leads to carbide dissociation and by phase separation to graphite formation, which pushes the metallic particles through the carbon layer. Nickel, stainless steel an Inconel (an alloy of mainly nickel, iron and chromium) filters were used. The latter gave the highest uniformity of the carbon nanofibers layer showing an excellent mechanical stability. In order to control carbon deposition on the filters, a formal kinetic analysis of the decomposition reaction was undertaken. Partial orders of 1 for ethane, -2 for hydrogen and an apparent activation energy of 235 kJ/mol were obtained. The observed kinetics suggests strongly that the C – C bond scission is the rate determining step and not carbon diffusion into the metal which is often reported in the literature. The synthesis procedures like time on stream and reaction temperature were optimized: 1 hour of a mixture C2H6:H2:Ar (3:17:80) decomposition over SMFInconel at 655°C lead to ∼6% of carbon, leading to a CNF layer of 1 µm thickness covering the metallic fibers entirely. This material presents a high specific surface area (22.2 m2/g) and a low pressure drop for the gas flowing through. The carbon nanofibers have a diameter between 20 and 50 nm and a crystalline structure with platelets morphology where the graphene layers are stacked one above the other. Their surface is composed almost exclusively of carbon and hydrogen. Amongst the multiple possible applications of this novel composite material, its use as a catalyst support was assessed. Selective hydrogenation of acetylene was carried out over palladium supported on CNF/SMF and compared with commercial supports (ACF, EGF and AGF). The activated carbon fibers cloth (ACF) led to a Pd activity at least twice as E-type glass fibers tissue (EGF) or EGF covered by an alumina layer (AGF). Reaction kinetics was studied for Pd/ACF, leading to acetylene and hydrogen partial orders of -0.5 and 2.4, respectively and apparent activation energy of 50 kJ/mol. The negative partial order for C2H2 suggests high adsorption strength of this compound compared to other reagents on palladium. This particularity allows Pd to be selective. Hydrogenations are known to be structure sensitive. Therefore the catalyst activity for the model reaction depends on Pd particle size. It increases for large particle sizes. The control of this parameter was a priority. Variations of the palladium precursor, of the support and catalyst treatment were carried out. Micro-wave plasma was used since it is fast and has low energy consumption allowing chemically modifying the carbon support surface and activating catalyst after impregnation. This controls not only palladium sintering, but also decomposes tetraaminopalladate more efficiently, resulting in a catalyst activity of one order of magnitude higher. In order to control the palladium deposition on CNF/SMFInconel composite, it is important to have surface functional groups. The activation of carbon nanofibers was carried out with a 4 hours treatment in a boiling aqueous solution of hydrogen peroxide. This led to an amount on surface oxygen containing groups, per mass of carbon, higher than in ACF. Palladium particle size on the composite was controlled by support activation, the use of several metal precursors or different loadings or by a thermal treatment in argon at 500°C. Optimisation of the procedure allowed to obtain in a controlled way an average palladium particle sizes between 3.6 and 25 nm. The activity and selectivity towards ethylene of Pd/CNF/SMFInconel were observed to be higher compared to palladium supported on activated carbon, alumina or barite. Graphitic nature of carbon nanofibers, leading to strong metal-support interaction, was accounted as responsible for these improvements. The high thermal conductivity of the composite CNF/SMF ensured good heat transfer released due to the exothermicity of the reaction and allowed to work close to isothermal conditions in the catalytic bed. Finally it can be concluded, that for the first time carbon nanofibers synthesis was carried out in one step on sintered metal fibers filters, resulting in a novel structured composite material combining different specific properties and having promising potential applications.

EPFL2006

3D Printing and Supercritical Foaming of Hierarchical Cellular Materials

Matteo Gregorio Modesto Marascio

Hierarchical porous structures have been gathering interest in different fields owing to their unique properties associated with their multi-scale features. The observation of natural materials brought new insights into the functionality of cellular materials and inspired new processes to produce synthetic hierarchical structures. Such hierarchical cellular materials have shown significant potential in many applications, as filtering, tissue engineering and drug delivery. Polymers in particular gathered a burgeoning interest thanks to their ease of processing, which allows to produce structures at high strength/density ratio and high surface area with defined porosity. In the present study, it is proposed to develop novel technologies to manufacture polymer cellular structures. From the application of Supercritical Carbon Dioxide Foaming (ScCO2) to Fused Deposition Modelling / Fused Filament Fabrication (FDM / FFF), an extrusion based additive manufacturing technology, hierarchical porous structures were created. The material transformation phenomena and the processing window of this homothetic foaming processed were studied. The fine tuning of the foaming parameters allowed creating a micro cellular porosity with-in a 3D printed structure, without modifying the 3D topology. This allowed producing a wide range of cellular struc-tures with controlled multi scale porosity and stiffness reduced up to hundred times the stiffness of the 3D printed cellular structures. Homothetic foaming was then applied to biopolymers in order to create gradient hierarchical porous structures. In particular, articular cartilage and bone (osteochondral) defects were targeted. Cartilage repair is a challenging clinical problem because large defects do not regenerate. Tissue engineering offers a solution by implanting a material, defined as scaffold, loaded with cells to induce a regeneration in the tissue that otherwise would not occur. Multi material Poly(lactide-co-caprolactone) â Poly(lactide-BTCP) cellular structures were successfully processed into a scaffold able to replicate the complex gradient mechanical properties of the osteochondral tissue. In particular, the processing windows to process multi material 3D printed and foamed structures was established. The mechanical properties under compression of these scaffolds were compared to the values measured by nanoindentation on human articular cartilage, showing a good correlation between scaffold and target application. Furthermore, from the developed knowledge, a novel additive manufacturing method is proposed from the inte-gration of FDM/FFF and ScCO2 into a single process, named 3D Foam Printing. 3D Foam Printing was applied to process structures with hollow filaments or filaments with radial porosity with live porosity control during the process. The influence of processing parameters on foam morphology was investigated. Different cellular structures achievable by tuning the printing temperature and speed were described for different biomaterials. The processes described in this work allowed to mimic the complex mechanical properties of the osteochondral tissue, a natural hierarchical material. However, they demonstrated to be relevant to a wide range of materials, as polymers, blends and composites. This is particularly true for 3D Foam Printing, which could positively influence many engineering applications, as aerospace, medicine and energy, where tuneable cellular polymers are highly demanded.

EPFL2018

Shape Engineering of Metal-Organic Frameworks via MOF@Polymer Composite Fabrication

Bardiya Valizadeh

Metal-organic frameworks (MOFs) are a fascinating class of porous materials that have become the subject of research owing to their unique characteristics such as high surface areas, low densities, and structural tunability. Recently, the research has been directed towards large-scale synthesis of MOFs and their industrial applications. In terms of synthesis, MOFs are conventionally isolated as crystalline powders that are not industrially favorable due to their limitations in processing. Shaping MOFs into different forms while preserving or improving their properties offers several advantages as they can be easily processed, handled, and stored. Controlling the shape of MOFs in a systematic way can represent a major step forward to bringing them to the market, testing them at industrial scales and realizing their true potential. In this thesis, a new shaping method in the form of MOF@polymer composite beads (M@PB) is presented in which the MOF crystals are dispersed in a porous matrix of polymeric beads. This shaping method, based on phase inversion technique, not only preserves the intrinsic properties of the MOF crystals such as crystallinity and porosity, but also improves their performance for targeted applications. Due to the versatility and flexibility of this method virtually any MOF can be incorporated into any polymer structure. In addition, the method allows for pre- and post-synthetic modification of both the MOF and polymer phases, hence, making the composite material of superior tunability toward targeted applications. Such tunable properties have been illustrated in the context of this thesis for liquid-phase separation and photocatalysis applications. HKUST-1 was used with three different polymers to form M@PB that improve I2 capture and recovery in liquid phase, a process used for recovery of precious metals from waste of electrical and electronic equipment. M@PBs have shown superior mechanical and chemical robustness for advanced adsorption systems that paves the way for their industrial applications. UiO-66, was functionalized with double amino groups and was incorporated into M@PB using polyethersulfone (PES) that was chemically modified with carboxylic acid groups to capture toxic hexavalent chromium (Cr(VI)) from wastewater and reduce it to its non-toxic trivalent form (Cr(III)). The Cr(VI) capture and subsequent photoreduction was carried out, for the first time, in a single integrated adsorption-photoreduction system equipped with a visible light source, demonstrating that the Cr(VI) concentration is brought below drinkable levels and the Cr(VI) subsequently photoreduced to Cr(III) species using light irradiation during adsorbent regeneration. This illustrates the tunability of M@PBs through careful design of the MOF and polymer toward a targeted application.

EPFL2020