A Highly Water-Stable meta-Carborane-Based Copper Metal-Organic Framework for Efficient High-Temperature Butanol Separation

Arunraj Chidambaram, Kyriakos Stylianou
AMER CHEMICAL SOC, 2020
Journal paper

Abstract

Biofuels are considered sustainable and renewable alternatives to conventional fossil fuels. Biobutanol has recently emerged as an attractive option compared to bioethanol and biodiesel, but a significant challenge in its production lies in the separation stage. The current industrial process for the production of biobutanol includes the ABE (acetone-butanol-ethanol) fermentation process from biomass; the resulting fermentation broth has a butanol concentration of no more than 2 wt% (the rest is essentially water). Therefore, the development of a cost-effective process for separation of butanol from dilute aqueous solutions is highly desirable. The use of porous materials for the adsorptive separation of ABE mixtures is considered a highly promising route, as these materials can potentially have high affinities for alcohols and low affinities for water. To date, zeolites have been tested toward this separation, but their hydrophilic nature makes them highly incompetent for this application. The use of metal-organic frameworks (MOFs) is an apparent solution; however, their low hydrolytic stabilities hinder their implementation in this application. So far, a few nanoporous zeolitic imidazolate frameworks (ZIFs) have shown excellent potential for butanol separation due to their good hydrolytic and thermal stabilities. Herein, we present a novel, porous, and hydrophobic MOF based on copper ions and carborane-carboxylate ligands, mCB-MOF-1, for butanol recovery. mCB-MOF-1 exhibits excellent stability when immersed in organic solvents, water at 90 degrees C for at least two months, and acidic and basic aqueous solutions. We found that, like ZIF-8, mCB-MOF-1 is non-porous to water (type II isotherm), but it has higher affinity for ethanol, butanol, and acetone compared to ZIF-8, as suggested by the shape of the vapor isotherms at the crucial low-pressure region. This is reflected in the separation of a realistic ABE mixture in which mCB-MOF-1 recovers butanol more efficiently compared to ZIF-8 at 333 K.

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Arunraj Chidambaram, Kyriakos Stylianou
AMER CHEMICAL SOC, 2020
Journal paper

Abstract

Official source

https://infoscience.epfl.ch/record/286677?ln=en

About this result

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Robust metal-organic frameworks for dry and wet biogas upgrading

Arunraj Chidambaram, Kyriakos Stylianou

Biofuels are considered as viable, sustainable and renewable alternatives to fossil fuels. One amongst them is biomethane which has emerged as one of the valuable contributors in facilitating the essential transition towards green energy sources. Industrially, anaerobic digestion of biomass results in the production of crude biogas that comprises mainly of methane (CH4) and other components like carbon dioxide (CO2) and water (H2O). Biogas is saturated in water vapour which must be removed to ensure the operational stability and safety of the technology systems that handles wet biogas and employs the end-product biomethane. Through a series of separation processes, the biogas is cleaned (drying) and upgraded (CO2/CH4 separation) to biomethane. The adsorptive separation technology with nanoporous materials for biogas upgrading has become progressively competitive due to their meritorious features. In particular, metal-organic frameworks (MOFs) have shown a tremendous potential in upgrading biogas under dry conditions. However, their affinity for water promotes competitive co-adsorption, and this, coupled with their relatively low hydrolytic stability affects their upgrading performance negatively. This fact requires the complete removal of moisture before the adsorptive based gas separation upgrading process can be carried out. A similar situation applies for highly hydrophilic zeolite adsorptive systems. In this work, we present two permanently porous MOFs made of aluminium metal ions (Al(III)) and tetracarboxylate ligands with strong CO2 binding pockets comprising of two aromatic rings which are 7 angstrom apart. Both MOFs are robust, and adsorption isotherms revealed that they are porous to CO2 and CH4 at 303 K with isosteric heat of adsorption of -25 to -40 and -13 to -16 kJ/mol, respectively. Interestingly, these materials are found to be non-porous to water at low relative humidity as shown by water vapour isotherms. Inspired by their single component sorption behaviours, we demonstrate that their biogas upgrading performance is not affected regardless of the high water vapour content (4 vol% and 85% RH) in the feed gas of the multicomponent fixed bed breakthrough experiments. This is due to the lack of competitive co-adsorption of CO2 and H2O within the hydrophobic CO2 binding pockets. This makes both MOFs presented in this work as industrially relevant candidates for biogas upgrading application which can withstand residual moisture of dried biogas as well as the momentary breakthroughs of wet biogas. (C) 2021 Elsevier Ltd. All rights reserved.

ELSEVIER2021

The world needs to think about the after fossil fuel era and while the sun baths the earth with a tremendous amount of energy, man must learn how to harvest and store it in order to dispose of a continuous supply meeting his needs. Water photo-electrolysis is a route toward the conversion of solar energy and its storage into chemical energy. Iron oxide, in the form of hematite, has been identified as a promising material, showing an excellent bandgap coupled with a high stability and abundance. However, its use is impeded by a very short charge carrier lifetime and poor oxygen evolving reaction kinetics. The former point creates the incentive for a fine nanostructuring, allowing the absorption of light close enough to the semiconductor-liquid interface for the photo-generated hole to be able to diffuse and react with water. The kinetics limitations can be overcome by mean of catalysis. In this thesis, I use a technique called atmospheric pressure chemical vapor deposition (APCVD), developed by Kay et al. in 2006 in order to fabricate hematite photoanodes that are highly nanostructured and show record solar-to-hydrogen conversion efficiencies. My work focuses on the improvement of the nanostructure in order to reduce the energy loss by deep light absorption in the material. First, the deposition parameters of the APCVD are optimized in order to improve the crystallinity of the photoanodes. The key parameter modified is the residence time of the precursor in the gas stream above the substrate. The process results in an improved overall solar energy conversion and a better electric conductivity of the material. The further application of the known best oxygen evolving catalyst – Iridium oxide – results in a record photocurrent of 3.0 mA cm−2 at 1.23 V bias vs. RHE for this class of material. In a second phase, the electrodes prepared using the newly developed conditions are analyzed by X-Ray diffraction and I find that the reduction of the residence time leads to an increased orientation of the iron oxide crystal planes. Since hematite presents a strong conduction anisotropy, and the better conducting basal planes are found perpendicular to the substrate, a correlation is made between the improved conductivity of the photoanodes prepared by reducing the residence and the preferential orientation. In order to better control the morphology of the hematite photoanodes, I present strategies of directed nucleation for the particle-assisted chemical vapor deposition. A flat model substrate is prepared as a platform for the elaboration of directing patterns. Gold nanoparticles are homogeneously deposited on the substrate with a controlled surface density. However, the chosen material reveals to be unable to initiate the iron oxide precursor decomposition and the nanoparticles show no effect on the morphology. Another patterning technique – the nanosphere lithography – is employed to create highly ordered nano-islands of a FePt alloy on the flat substrate. The dimensions of the patterning nanospheres used for the lithography controls the size and spacing of the imprints. The deposition of hematite by APCVD is successfully affected by the pattern and the size of the iron oxide nanostructures is controlled by the imprinted substrate. Finally, I combine the knowledge acquired on the directed nucleation and modified deposition conditions in order to use the APCVD in the so-called host-guest strategy. A transparent, conductive and macroporous host is built using templating TiO2 nanoparticles and a conductive niobium doped tin oxide scaffold. The host is then infiltrated by hematite using different combinations of parameters in the APCVD setup. A maximum photocurrent – equivalent to the state-of-the-art hematite host-guest strategy – is reached with a hybrid nanostructure. A dendritic formation growing atop of the macroporous host while Nb-SnO2 is conformally coated with a thin layer of hematite. By mean of parameter tuning in the APCVD system, a selective improvement of the conformal hematite layer is achieved, showing a better performance when the sample is illuminated from the substrate side as well as a modified surface morphology.

EPFL2013

Electrochemical Reduction of CO2 Catalyzed by Metal Oxides

Yeonji Oh

A rapidly growing population and industrialization have caused the exploitation of natural resources in keeping with fossil fuels shortage. Due to the combustion of fossil fuels, the concentration of CO2 in the earth's atmosphere, which is a potent greenhouse gas, has been increasing dramatically and contributes to the current climate change. Meanwhile, CO2 can be considered as an abundant and inexpensive carbon feedstock, especially if carbon sequestration is required for future fossil fuel-based power stations. Electrochemical reduction of CO2 to form useful chemicals or fuels is a potentially efficient method of CO2 utilization and recycling. The advantage of the electrochemical CO2 reduction is that natural renewable sources, like solar power, wind power, hydropower or nuclear power can supply electric power for CO2 conversion. Various products, such as carbon monoxide, oxalic acid, formic acid, alcohols and hydrocarbons have been reduced from CO2 First in chapter 1, the general remarks about electrochemical reduction of CO2, especially on metal electrodes are introduced. It also includes the introduction of photoelectochemical CO2 reduction and literature review study about electrocatalytic CO2 reduction with organic molecules as mediators and catalysts. In chapter 2, our interest in Mo-based electrocatalysts led us to study the activity of MoO2 for CO2 reduction. MoO2 microparticles indeed act as an active catalyst for the electrochemical reduction of CO2 in organic solvents such as acetonitrile (MeCN) and dimethylformamide (DMF). In the context of this study, it was found that the CO2 reduction activity is much higher at -20 oC than at RT, and it can be promoted by a small amount of water. The selectivity of CO2 reduction depends on the temperature, overpotential and water concentration. Chapter 3 describes the electrochemical reduction of CO2, improved and modified by using imidazolium ionic liquids (ILs) as an electrolyte in organic solvent with MoO2/Pb electrode. The important role of ILs in changing the pathway of the reduction was observed. Particularly, the use of imidazolium ILs instead of tetraalkylammonium salt as an electrolyte altered the product from oxalate to CO. Besides, the overpotential of the CO2 reduction was shifted to the less negative potentials compared to previous work, and the current density as well increased. At a low water concentration, the catalytic activity was promoted and the total Faradaic efficiency (FE) was up to 100% with more than 60% of CO formation. Furthermore, imidazolium ILs together with our MoO2/Pb electrode lowered the overpotential of CO2 reduction by about 40 mV. In addition to the high selectivity for CO formation, the imidazolium ILs can perform a role as co-catalyst for lowering the CO2 reduction potential. Finally, chapter 4 shows the possibility of photoelectrochemical CO2 reduction to extend our research with various catalysts. In this work, we could derive a photoelectrochemical method to deposit Sn/SnOx catalyst on the surface protected cuprous oxide (Cu2O) photocathodes. The deposition of Sn/SnOx catalyst was optimized on the photocathode and it exhibited reliable CO2 reduction activity in a mild aqueous condition. Sn/SnOx-Cu2O photocathode showed the onset potential of the photocurrent at +0.1 V vs. RHE, which is very promising result. At a constant potential electrolysis of -0.1 V vs. RHE, a FE of 50% for formate formation was obtained and the total FE was around 86%.

EPFL2015