Robust metal-organic frameworks for dry and wet biogas upgrading

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.

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

Arunraj Chidambaram, Kyriakos Stylianou

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.

AMER CHEMICAL SOC2020

Systematic Tuning and Multifunctionalization of Covalent Organic Polymers for Enhanced Carbon Capture

Johanna Maria Huck, Rocio Mercado, Berend Smit, Hui Wang

Porous covalent polymers are attracting increasing interest in the fields of gas adsorption, gas separation, and catalysis due to their fertile synthetic polymer chemistry, large internal surface areas, and ultrahigh hydrothermal stabilities. While precisely manipulating the porosities of porous organic materials for targeted applications remains challenging, we show how a large degree of diversity can be achieved in covalent organic polymers by incorporating multiple functionalities into a single framework, as is done for crystalline porous materials. Here, we synthesized 17 novel porous covalent organic polymers (COPs) with finely tuned porosities, a wide range of Brunauer Emmett Teller (BET) specific surface areas of 430-3624 m(2) g(-1), and a broad range of pore volumes of 0.24-3.50 cm(3) g(-1), all achieved by tailoring the length and geometry of building blocks. Furthermore, we are the first to successfully incorporate more than three distinct functional groups into one phase for porous organic materials, which has been previously demonstrated in crystalline metal organic frameworks (MOFs). COPs decorated with multiple functional groups in one phase can lead to enhanced properties that are not simply linear combinations of the pure component properties. For instance, in the dibromobenzene-lined frameworks, the bi- and multifunctionalized COPs exhibit selectivities for carbon dioxide over nitrogen twice as large as any of the singly functionalized COPs. These multifunctionalized frameworks also exhibit a lower parasitic energy cost for carbon capture at typical flue gas conditions than any of the singly functionalized frameworks. Despite the significant improvement, these frameworks do not yet outperform the current state-of-art technology for carbon capture. Nonetheless, the tuning strategy presented here opens up avenues for the design of novel catalysts, the synthesis of functional sensors from these materials, and the improvement in the performance of existing covalent organic polymers by multifunctionalization.

American Chemical Society2015

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

Arunraj Chidambaram, Kyriakos Stylianou

AMER CHEMICAL SOC2020

Systematic Tuning and Multifunctionalization of Covalent Organic Polymers for Enhanced Carbon Capture

Johanna Maria Huck, Rocio Mercado, Berend Smit, Hui Wang

American Chemical Society2015