Computational screening of functional groups for ammonia capture in metal-organic frameworks

Tailoring ammonia capture in MOFs and COFs: A multi-scale and machine learning comprehensive investigation of functional group modification

Our study aims to examine the impact of ligand functionalization on the ammonia adsorption properties of MOFs and COFs, by combining multi-scale calculations with machine learning techniques. Density Functional Theory calculations were performed to investigate the interactions between ammonia (NH3) and a comprehensive set of 48 strategically chosen functional groups. In all of the cases, it is observed that functionalized rings exhibit a stronger interaction with ammonia molecule compared to unfunctionalized benzene, while -O2Mg demonstrates the highest interaction energy with ammonia (15 times stronger than the bare benzene). The trend obtained from the thorough DFT screening is verified via Grand Canonical Monte-Carlo calculations by employing interatomic potentials derived from quantum chemical calculations. Isosteric heat of adsorption plots provide a comprehensive elucidation of the adsorption process, and important insights can be taken for studies in fine-tuning materials for ammonia adsorption. Furthermore, a proof of concept machine learning (ML) analysis is conducted, which demonstrates that ML can accurately predict NH3 binding energies despite the limited amount of data. The findings derived from our multi-scale methodology indicate that the functionalization strategy can be utilized to guide synthesis towards MOFs, COFs, or other porous materials for enhanced NH3 adsorption capacity.

An Integrated Food Freshness Sensor Array System Augmented by a Metal-Organic Framework Mixed-Matrix Membrane and Deep Learning

The static labels presently prevalent on the food market are confronted with challenges due to the assumption that a food product only undergoes a limited range of predefined conditions, which cause elevated safety risks or waste of perishable food products. Hence, integrated systems for measuring food freshness in real time have been developed for improving the reliability, safety, and sustainability of the food supply. However, these systems are limited by poor sensitivity and accuracy. Here, a metal-organic framework mixed-matrix membrane and deep learning technology were combined to tackle these challenges. UiO-66-OH and polyvinyl alcohol were impregnated with six chromogenic indicators to prepare sensor array composites. The sensors underwent color changes after being exposed to ammonia at different pH values. The limit of detection of 80 ppm for trimethylamine was obtained, which was practically acceptable in the food industry. Four state-of-the-art deep convolutional neural networks were applied to recognize the color change, endowing it with high-accuracy freshness estimation. The simulation test for chicken freshness estimation achieved accuracy up to 98.95% by the WISeR-50 algorithm. Moreover, 3D printing was applied to create a mold for possible scale-up production, and a portable food freshness detector platform was conceptually built. This approach has the potential to advance integrated and real-time food freshness estimation.

Computational Investigation of Adsorptive Removal of Pb

Adsorption using metal–organic frameworks (MOFs) such as UiO-66 has shown great promise in remediating water sources contaminated with toxic heavy metals such as Pb2+, but detailed information about the adsorption process remains limited. In this article, we gained mechanistic insights into Pb2+ adsorption using both functionalised and defective UiO-66 by performing density functional theory calculations using cluster models. Our benchmarked approach led to a computational model of solvated Pb2+ (a hemidirected Pb(H2O)62+ complex) fully consistent with experimental reports. The analysis of Pb2+ adsorption using functionalised UiO-66 determined that factors such as electrostatic attraction, chelation, and limited constraints on the Pb2+ coordination geometry lead to enhanced binding affinity. For these reasons, UiO-66-COO– was identified as the most promising functionalised MOF, consistent with experimental literature. We additionally explored a novel aspect of Pb2+ adsorption by UiO-66: the role of missing linker defects that often characterise this MOF. We found that the defects expected to form in an aqueous environment can act as excellent adsorption sites for Pb2+ and the preferred adsorption geometry is again determined by electrostatic attraction, chelation, and constraints on the Pb2+ coordination geometry. Overall, we conclude that functional groups and defect sites can both contribute to Pb2+ adsorption and our study provides crucial design principles for improving the UiO-66 MOF performance in toxic Pb2+ removal from water.

Enhancing toxic gas uptake performance of Zr-based MOF through uncoordinated carboxylate and copper insertion; ammonia adsorption

This study reports the development of a new type of Zr-based MOF by inserting copper and carboxylate into HCl modulated UiO-67 (UiO-67-vac) which gained higher surface area/vacant than UiO-67. Copper was inserted into MOF containing uncoordinated carboxylate group, to create open metal site in the form of -COOCu which called UiO-67-ox-Cu. PXRD, FTIR, BET, SEM, EDS, UV-Vis and XPS were used to characterize the obtained MOFs. As expected, UiO-67-ox-Cu exhibits the highest ammonia capacity (178.3 mg/g) among UiO-67 (104 mg/g) and UiO-67-vac (121 mg/g) at 298 K and 1 bar pressure. In fact, the significant increase in ammonia uptake of UiO-67-ox-Cu is related to the modified binding affinity of -COOCu groups with ammonia. Moreover, UiO-67-vac with the highest surface area showed the hydrogen adsorption capacity of 18.75 mg/g at 77 K, which is comparable or even superior to the previously reported value. Interestingly, adsorption capacities were retained with slight changes around five cycles and three regeneration temperatures, 25, 60 and 120 °C under vacuum pressure which were proved by PXRD after ammonia adsorption/desorption. The good results obtained in the current work clearly show the role of postsynthesis functionalization approach for creation of new metal/active sites into MOFs.

Boosting Ammonia Uptake within Metal-Organic Frameworks by Anion Modulating Strategy

Ammonia with toxic and corrosive features needs advanced protective materials and removal tools, although it is a vital component in human food supply processes. So, to satisfy these requirements, materials with high adsorption capacity and affinity for ammonia should be developed. The present research has been focused on a series zinc-based metal-organic frameworks (MOF) containing mixed ligands, biphenyl dicarboxylic acid (BPDA) and tris(4-(4H-1,2,4-triazol-4-yl)phenyl)amine (TTPA), which are modulated by different anions including CH₃COO^-, CF₃COO^-, and CF₃SO₃^-. Ammonia uptake capacity was measured via static and dynamic conditions under 50% relative humidity. Among all compounds, CF₃SO₃^- anion could enhance the ammonia uptake capacity of MOFs up to 177.85 and 349 mg/g during static and breakthrough measurements, respectively, so that 83.30% of the total uptake capacity (at P/P_o = 1.0 and 298 K) was achieved at low relative pressure range (up to 0.1). The isosteric heats of ammonia adsorption on PFC-27 and derivatives were calculated in the range of 7.03-10.16 kJ mol^-1 so that they increased upon CF₃SO₃^-, CF₃COO^-, and CH₃COO^- ion incorporation. This is potentially beneficial for enhanced ammonia adsorption. Interestingly, adsorption capacities were retained with only slight changes after five cycles and three regeneration temperatures, 25 °C, 60 °C, and 120 °C, under vacuum. The special affinity for NH₃ adsorption and MOF phase stability after desorption is clearly proved by FTIR spectra and PXRD analysis, respectively. Generally, the results suggest that ion insertion modification is an efficient strategy for enhancement of MOF adsorption performance.

In-silico identification of adsorbent for separation of ethane/ethylene mixture

We present here a high-throughput computational screening of 4,821 real metal-organic framework (MOF) structures that do not contain any open metal sites to isolate the best performing candidate for separation of ethane/ethylene mixture at ambient conditions. The MOF structures were assessed on the basis of several adsorption-based separation performance metrics. Some of these metrics were found to correlate strongly among themselves. We have presented various structures-property correlations which unfold useful insights. MOF ATAGEJ is found to be the top performing MOF with highest adsorbent performance score 12.38 mol/kg and regenerability 93.88%. Several other MOFs OTOLIU (MIL-167), UMUMOG (UBMOF-8), and TOVGES (PCN-230) containing tetravalent metal cations such as Zr⁴⁺ and Ti⁴⁺ are found to be potential structures that are thermally, mechanically, and chemically stable and performs better than zeolites. Adsorption selectivity shows exponential correlation with difference of heat of adsorption of ethane and ethene at 0.1 bar and 298 K. We have also presented how various performance metrics correlate among themselves. These correlations unfold useful insights. Graphical abstract.

Green applications of metal–organic frameworks

MOFs as green materials – a highlight of the environmentally conscious or “green” applications of MOFs.

Tailoring the Adsorption and Reaction Chemistry of the Metal-Organic Frameworks UiO-66, UiO-66-NH2, and HKUST-1 via the Incorporation of Molecular Guests

Metal-organic frameworks (MOFs) are versatile materials highly regarded for their porous nature. Depending on the synthetic method, various guest molecules may remain in the pores or can be systematically loaded for various reasons. Herein, we present a study that explores the effect of guest molecules on the adsorption and reactivity of the MOF in both the gas phase and solution. The differences between guest molecule interactions and the subsequent effects on their activity are described for each system. Interestingly, different effects are observed and described in detail for each class of guest molecules studied. We determine that there is a strong effect of alcohols with the secondary building unit of UiO MOFs, while Lewis bases have an effect on the reactivity of the -NH₂ group in UiO-66-NH₂ and adsorption by the coordinatively unsaturated copper sites in HKUST-1. These effects must be considered when determining synthesis and activation methods of MOFs toward various applications.

Highly effective ammonia removal in a series of Brønsted acidic porous polymers: investigation of chemical and structural variations

Efficient removal of ammonia from air is demonstrated in a series of Brønsted acidic porous polymers under dry and humid conditions. The impact of acidic group strength and their spatial distribution on the ammonia uptake is investigated systematically.

Although a widely used and important industrial gas, ammonia (NH₃) is also highly toxic and presents a substantial health and environmental hazard. The development of new materials for the effective capture and removal of ammonia is thus of significant interest. The capture of ammonia at ppm-level concentrations relies on strong interactions between the adsorbent and the gas, as demonstrated in a number of zeolites and metal–organic frameworks with Lewis acidic open metal sites. However, these adsorbents typically exhibit diminished capacity for ammonia in the presence of moisture due to competitive adsorption of water and/or reduced structural stability. In an effort to overcome these challenges, we are investigating the performance of porous polymers functionalized with Brønsted acidic groups, which should possess inherent structural stability and enhanced reactivity towards ammonia in the presence of moisture. Herein, we report the syntheses of six different Brønsted acidic porous polymers exhibiting –NH₃Cl, –CO₂H, –SO₃H, and –PO₃H₂ groups and featuring two different network structures with respect to interpenetration. We further report the low- and high-pressure NH₃ uptake in these materials, as determined under dry and humid conditions using gas adsorption and breakthrough measurements. Under dry conditions, it is possible to achieve NH₃ capacities as high as 2 mmol g^–1 at 0.05 mbar (50 ppm) equilibrium pressure, while breakthrough saturation capacities of greater than 7 mmol g^–1 are attainable under humid conditions. Chemical and structural variations deduced from these measurements also revealed an important interplay between acidic group spatial arrangement and NH₃ uptake, in particular that interpenetration can promote strong adsorption even for weaker Brønsted acidic functionalities. In situ infrared spectroscopy provided further insights into the mechanism of NH₃ adsorption, revealing a proton transfer between ammonia and acidic sites as well as strong hydrogen bonding interactions in the case of the weaker carboxylic acid-functionalized polymer. These findings highlight that an increase of acidity or porosity does not necessarily correspond directly to increased NH₃ capacity and advocate for the development of more fine-tuned design principles for efficient NH₃ capture under a range of concentrations and conditions.

Metal–organic frameworks for the removal of toxic industrial chemicals and chemical warfare agents

Toxic gases can be captured or degraded by metal–organic frameworks.

Computational screening of functional groups for capture of toxic industrial chemicals in porous materials

Functional groups are screened computationally to understand how they bind and capture toxic industrial chemicals.

Adsorptive desulfurization and denitrogenation using metal-organic frameworks

With the increasing worldwide demand for energy, utilization of fossil fuels is increasing proportionally. Additionally, new and unconventional energy sources are also being utilized at an increasing rate day-by-day. These sources, along with some industrial processes, result in the exposal of several sulfur- and nitrogen-containing compounds (SCCs and NCCs, respectively) to the environment, and the exposure is one of the greatest environmental threats in the recent years. Although, several methods were established for the removal of these pollutants during the last few decades, recent advancements in adsorptive desulfurization and denitrogenation (ADS and ADN, respectively) with metal-organic frameworks (MOFs) make this the most promising and remarkable method. Therefore, many research groups are currently involved with ADS and ADN with MOFs, and the results are improving gradually by modifying the MOF adsorbents according to several specific adsorption mechanisms. In this review, ADS and ADN studies are thoroughly discussed for both liquid-phase and gas-phase adsorption. The MOF modification procedures, which are important for improved adsorption, are also described. To improve the knowledge among the scientific community, it is very important to understand the detailed chemistry and mechanism involved in a chemical process, which also creates the possibility and pathway for further developments in research and applications. Therefore, the mechanisms related to the adsorption procedures are also discussed in detail. From this review, it can be expected that the scientific community will obtain an understanding of the current state of ADS and ADN, their importance, and some encouragement and insight to take the research knowledge base to a higher level.

Effective ligand functionalization of zirconium-based metal-organic frameworks for the adsorption and separation of benzene and toluene: a multiscale computational study

The adsorption and separation properties of benzene and toluene on the zirconium-based frameworks UiO-66, -67, -68, and their functional analogues UiO-Phe and UiO-Me2 were studied using grand canonical Monte Carlo simulations, density functional theory, and ideal adsorbed solution theory. Remarkable higher adsorption uptakes of benzene and toluene at low pressures on UiO-Phe and -Me2 were found compared to their parent framework UiO-67. It can be ascribed to the presence of functional groups (aromatic rings and methyl groups) that significantly intensified the adsorption, majorly by reducing the effective pore size and increasing the interaction strength with the adsorbates. At high pressures, the pore volumes and accessible surfaces of the frameworks turned out to be the dominant factors governing the adsorption. In the case of toluene/benzene separation, toluene selectivities of UiOs showed a two-stage separation behavior at the measured pressure range, resulting from the greater interaction affinities of toluene at low pressures and steric hindrance effects at high pressures. Additionally, the counterbalancing factors of enhanced π delocalization and suitable pore size of UiO-Phe gave rise to the highest toluene selectivity, suggesting the ligand functionalization strategy could reach both high adsorption capacity and separation selectivity from aromatic mixtures at low concentrations.

A UiO-66 analogue with uncoordinated carboxylic acids for the broad-spectrum removal of toxic chemicals

A new metal–organic framework analogue is stable under ambient conditions with the ability to remove toxic gases.