Ivo Vankelecom

Metal-free doped carbon materials as electrocatalysts for the oxygen reduction reaction

Carbon materials such as graphite, graphene, carbon nanotubes and ordered mesoporous carbon have attracted a lot of attention for their use in fuel cells, due to beneficial properties like high conductivity, high mechanical and chemical stability and, for the latter, high surface area. Doping these materials with nitrogen or, less commonly, other elements alters their (electronic) properties, making them particularly suitable for application as electrocatalysts for the oxygen reduction reaction (ORR) in a fuel cell. This paper reviews the synthesis methods employed for the doping of these different types of carbon materials with various elements and the characterization techniques used to investigate their physicochemical properties such as degree of graphitization, dopant content, dopant configuration and surface area. Furthermore, their application as electrocatalysts for the oxygen reduction in a fuel cell is reviewed. Finally, the possible mechanisms for the ORR on N-doped carbon materials are critically discussed and compared to the mechanisms of commercial Pt/C electrocatalysts.

Forward and pressure retarded osmosis: potential solutions for global challenges in energy and water supply

Osmotically driven membrane processes (ODMP) have gained renewed interest in recent years and they might become a potential solution for the worlds most challenging problems of water and energy scarcity. Though the concept of utilizing osmotic pressure difference between high and low salinity streams across semipermeable membranes has been explored for several decades, lack of optimal membranes and draw solutions hindered competition between forward osmosis (FO) and pressure retarded osmosis (PRO) with existing water purification and power generation technologies, respectively. Driven by growing global water scarcity and by energy cost and negative environmental impacts, novel membranes and draw solutions are being developed for ODMPs, mass and heat transfer in osmotic process are becoming better understood, and new applications of ODMPs are emerging. Therefore, OMDPs might become promising green technologies to provide clean water and clean energy from abundantly available renewable resources. This review focuses primarily on new insights into osmotic membrane transport mechanisms and on novel membranes and draw solutions that are currently being developed. Furthermore, the effects of operating conditions on the overall performance of osmotic membranes will be highlighted and future perspectives will be presented.

Layer-by-layer preparation of polyelectrolyte multilayer membranes for separation

Polymer membranes provide a highly promising platform for the development of an efficient and sustainable technique for separation. Ideally such membranes combine a high flux with a high selectivity requiring thin defect-free membranes. The layer by layer (LBL) assembly technique has proven to be a versatile and simple method for the fabrication of very thin polyelectrolyte multilayers making it highly suitable for the preparation of separation membranes. Recent developments in this field related to membrane preparation and their applications in separation processes are presented and discussed in this review. An overview of the different fabrication techniques of such membranes will be first provided. In addition, the formation mechanism and the parameters that can be varied to tune the properties of the membranes will be discussed. Finally, the potential applications of these membranes in different separation areas such as pervaporation, nanofiltration, solvent resistant nanofiltration, reverse osmosis, gas separation and forward osmosis will be addressed.

Harvesting microalgal biomass using submerged microfiltration membranes.

This study was performed to investigate the applicability of submerged microfiltration as a first step of up-concentration for harvesting both a freshwater green algae species Chlorella vulgaris and a marine diatom Phaeodactylum tricornutum using three lab-made membranes with different porosity. The filtration performance was assessed by conducting the improved flux step method (IFM) and batch up-concentration filtrations. The fouling autopsy of the membranes was performed by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX) and Fourier transform infrared spectroscopy (FTIR). The cost analysis was estimated based on the data of a related full-scale submerged membrane bioreactor (MBR). Overall results suggest that submerged microfiltration for algal harvesting is economically feasible. The IFM results indicate a low degree of fouling, comparable to the one obtained for a submerged MBR. By combining the submerged microfiltration with centrifugation to reach a final concentration of 22% w/v, the energy consumption to dewater C. vulgaris and P. tricornutum is 0.84 kW h/m(3) and 0.91 kW h/m(3), respectively.

Membrane technology in microalgae cultivation and harvesting: a review.

Membrane processes have long been applied in different stages of microalgae cultivation and processing. These processes include microfiltration, ultrafiltration, dialysis, forward osmosis, membrane contactors and membrane spargers. They are implemented in many combinations, both as a standalone and as a coupled system (in membrane biomass retention photobioreactors (BR-MPBRs) or membrane carbonation photobioreactors (C-MPBRs). To provide sufficient background on these applications, an overview of membrane materials and membrane processes of interest in microalgae cultivation and processing is provided in this work first. Afterwards, discussion about specific aspects of membrane applications in microbial cultivation and harvesting is provided, including membrane fouling. Many of the membrane processes were shown to be promising options in microalgae cultivation. Yet, significant process optimizations are still required when they are applied to enable microalgae biomass bulk production to become competitive as a raw material for biofuel production. Recent developments of the coupled systems (BR-MPBR and C-MPBR) bring significant promises to improve the volumetric productivity of a cultivation system and the efficiency of inorganic carbon capture, respectively.

Observations on solvent flux and solute rejection across solvent resistant nanofiltration membranes

Abstract Organic solvent nanofiltration is an emerging technology made possible by the recent development of solvent resistant nanofiltration (SRNF) membranes. These membranes have many potential applications from continuous operation over many months in refinery systems [1,2] to short term operation for a few hours in batch chemical processes [3]. In this paper, solvent flux decline and membrane separation properties are investigated (including their dependence on pressure), using methanol with quaternary alkyl ammonium bromide salts with molecular weights (MW) in the range 322 to 547 Daltons as solutes. The membranes are characterised in terms of an equivalent uniform pore size using three simple pore flow models: Ferry model, Steric Hindrance Pore (SHP) model and Verniory model.

Modulated UiO-66-Based Mixed-Matrix Membranes for CO2 Separation

Mixed-matrix membranes (MMMs) composed of polyimide (PI) and metal-organic frameworks (MOFs) were synthesized using Matrimid as the polymer and zirconium terephthalate UiO-66 as the filler. The modulation approach, combined with the use of amine-functionalized linkers, was used for synthesis of the MOF fillers in order to enhance the intrinsic separation performance of the MOF and improve the particle-PI compatibility. The presence of amine groups on the MOF outer surface introduced either through the linker, through the modulator, or through both led to covalent linking between the fillers and Matrimid, which resulted in very stable membranes. In addition, the presence of amine groups inside the pores of the MOFs and the presence of linker vacancies inside the MOFs positively influenced CO2 transport. MMMs with 30 wt % loading showed excellent separation performance for CO2/CH4 mixtures. A significant increase in the mixed-gas selectivity (47.7) and permeability (19.4 barrer) compared to the unfilled Matrimid membrane (i.e., 50% more selective and 540% more permeable) was thus achieved for the MMM containing the MOF prepared from 2-aminoterephthalic acid and 4-aminobenzoic acid, respectively used as the linker and as the modulator.

Performance of PDMS membranes in pervaporation: effect of silicalite fillers and comparison with SBS membranes.

Laboratory-made silicalite filled PDMS membranes were tested by means of concentration and temperature influence on the membrane performance in removal of ethanol from ethanol/water mixtures. This allowed studying the applicability of solution-diffusion model in the transport mechanism description. Experiments were performed by varying the ethanol concentration in the feed and temperature. Two types of fillers were incorporated into the PDMS network: commercial zeolite silicalite (CBV 3002) and laboratory-made colloidal silicalite-1. Obtained results were then compared with data gathered for unfilled PDMS membranes to examine the effect of fillers incorporation. Moreover, the comparison with novel block co-polymer based porous and dense SBS membranes was done. It was found that the solution-diffusion model was a good representation of ethanol transport through both filled and unfilled PDMS membranes, whereas the water flux did not obey this model due to the swelling effects. Incorporation of the fillers increased membrane stability and improved the selectivity. Performance of the SBS membranes characterized by a dense structure was found to be similar to the performance of filled PDMS membranes.

A dimeric form of Jacobsen's catalyst for improved retention in a polydimethylsiloxane membrane

Abstract A dimeric form 1 of Jacobsens catalyst was synthesized for better steric occlusion in a polydimethylsiloxane membrane. In homogeneous conditions, the dimer is about as active and enantioselective as Jacobsens catalyst itself. The relationship between leaching of the complex out of the membrane on one hand and the solubility of the complex and the swelling of the membrane in the solvent used on the other, showed that leaching could be avoided only if low solubility was combined with low swelling or in the case of complete insolubility. As the dimer is less soluble and larger than the monomeric form, this form leaches less. The yields and enantioselectivities of the heterogenised system are comparable to those of the homogeneous monomer.

Membrane occluded catalysts: a higher order mimic with improved performance

Abstract A general method to immobilise homogeneous catalysts and to improve the performance of heterogeneous catalysts is discussed. The method consists in embedding the catalysts in hydrophobic PDMS (polydimethylsiloxane)-membranes. Inspired on a complete structural mimic of cytochrome P-450 up to the level of the membrane, this technique gives superior properties to the membrane resident catalyst. The scope and limitations of this method are discussed by two examples of heterogeneous catalysts, i.e., FePc-Y (iron phthalocyanine zeolite Y) and [Mn(bpy) 2 ] 2+ -Y (manganese bis(bipyridyl) zeolite Y), and three examples of homogeneous complexes, i.e., FePc, Ru-binap ([2,2′-bis(diphenylphosphino-1,1′-binaphtyl]chloro( p -cymene)-ruthenium chloride) and the Jacobsen catalyst ( N,N′ -bis(3,5-di- tert -butylsalicylidene)-1,2-cyclohexane-diamine manganese chloride). Due to changed sorption in the zeolites, catalyst activity is enhanced and deactivation is suppressed. Furthermore, the membrane incorporation makes the use of a solvent redundant. For homogenous complexes, this procedure represents a general method for heterogenisation. Moreover, the technique opens new ways in the field of oxidation chemistry, where solvents are necessary to homogenise reagents which usually differ in polarity.