Facile method to disperse nonporous metal organic frameworks: composite formation with a porous metal organic framework and application in adsorptive desulfurization

Microwave- and ultrasonic-assisted synthesis of 2D La-based MOF nanosheets by coordinative unsaturation degree to boost phosphate adsorption

The metal or metal clusters and organic ligands are relevant to the selectivity and performance of phosphate removal in MOFs, and the electron structure, chemical characteristics, and preparation method also affect efficiency and commercial promotion. However, few reports focus on the above, especially for 2D MOF nanomaterials. In this work, two 2D Ln-TDA (Ln = La, Ce) nanosheets assembled via microwave- and ultrasonic-assisted methods are employed as adsorbents for phosphate (H2PO4 -, HPO4 2-) removal for the first time. Their microstructure and performance were characterized using XRD, TEM, SEM, AFM, FTIR, zeta potential, and DFT calculations. The prepared 2D Ln-TDA (Ln = La, Ce) nanosheets exposed more adsorption sites and effectively reduced the restrictions of mass transfer. Based on this, the Langmuir model was employed to estimate the maximum adsorption capacities of the two kinds of nanosheets, which reached 253.5 mg g-1 and 259.5 mg g-1, which are 553 times and 3054 times larger than those for bulk Ln-TDA (Ln = La, Ce), respectively. Additionally, the kinetic data showed that the adsorption equilibrium time is fast, approximately 15 min by the pseudo-second-order model. In addition, the prepared products not only have a wide application range (pH = 3-9) but also offer eco-safety in terms of residuals (no Ln leak out). Based on the XPS spectra, FTIR spectra and DFT calculations, the main adsorption mechanisms included ligand exchange and electrostatic interactions. This new insight provides a novel strategy to prepare 2D MOF adsorbents, achieving a more eco-friendly method (microwave- and ultrasonic-assisted synthesis) for preparing 2D Ln-based MOF nanosheets by coordinative unsaturation to boost phosphate adsorption.

A magnetic solid phase extraction based on UiO-67@GO@Fe3O4 coupled with UPLC-MS/MS for the determination of nitroimidazoles and benzimidazoles in honey

A novel composite material consisting of zirconiumbiphenyldicarboxylate metal-organic framework, graphite oxide and ferroferric oxide was fabricated by a facile one-step method and served as a magnetic solid phase extraction sorbent for the simultaneous determination of nitroimidazoles and benzimidazoles in honey. The amount of graphite oxide for the synthesis of composite material and analysis parameters were optimized. The optimum parameters were: dipotassium hydrogen phosphate buffer solution (pH 6) as diluent solvent, adsorption time 10 min, desorption time 5 min, methanol/acetonitrile (1:1, V/V) as desorption solvent. The targets were detected by ultra-high performance liquid chromatography tandem mass spectrometry. The recoveries of twelve analytes ranged in 70.5%-103.4% with relative standard deviations lower than 12.9% (n = 3). The quantification limits were 0.2-0.6 μg/kg. Using the composite material as sorbent, a simple, rapid and environmental-friendly method based on magnetic solid phase extraction was successfully developed for determination of seven nitroimidazoles and five benzimidazoles in honey.

How neutral nitrogen-containing compounds are oxidized in oxidative-denitrogenation of liquid fuel with TiO2@carbon

Oxidative-denitrogenation (ODN) of indole (IND) and methyl-substituted INDs (methyl-INDs), representative neutral nitrogen-containing compounds (NCCs), was carried out with TiO₂@C and H₂O₂ as heterogeneous catalyst and oxidant, respectively, under ultrasound irradiation. The oxidation of INDs progressed through radical formation, as evidenced by electron spin resonance and radical scavenger experiments. The oxidized position of INDs in the ODN process was checked via characterization of the obtained products. It was observed that the oxidation finally occurred on the carbon rather than on the nitrogen atom of INDs, unlike the oxidation of basic NCCs (e.g., oxidation on the nitrogen atom, as respective N-oxides were formed) and sulfur-containing compounds. To understand the relative reactivity and oxidation position, electron density (ED) on the nitrogen atom of the studied INDs and relative stability of representative intermediates/products were calculated. It could be confirmed that ED on the nitrogen atom of the INDs is very important in the oxidation of INDs since the ODN reactivity of INDs was enhanced with increasing ED on the nitrogen atom of the investigated INDs. Moreover, theoretical analyses of the relative stability of substrate and intermediates/products (especially for IND) can explain the route for the observed final products in ODN. In other words, oxygen on the nitrogen atom, obtained via the first step of oxidation (electrophilic addition of an active oxygen atom on nitrogen), moves to the nearby carbon atom, because of the relative stability of the intermediates and products.

Cu(II)-Containing Metal-Organic Framework with 1D Hexagonal Channels for Cyanosilylation Reaction and Anticancer Activity on Osteosarcoma Cells

A novel metal-organic framework (MOF) has been produced via Cu(NO₃)₂·6H₂O reaction with 3-(1H-tetrazol-5-yl)pyridine (HL) in water, and its chemical formula is {[(Cu(L)₂(H₂O)₂](H₂O)₈) _n } (1). Due to its high density of coordinately unsaturated sites along with large one-dimensional (1D) hexagonal channels, the activated complex 1 (1a) was explored as the solvent-free heterogeneous catalyst for cyanosilylation under mind conditions. The inhibitory function of compound 1a against the survival rate of OS-732 osteosarcoma cells was evaluated via Cell Counting Kit-8 (CCK-8) detection kit. Furthermore, the Annexin V-FITC/PI detection kit and the active oxygen (ROS) detection kit was carried out to determine the cell apoptosis levels and the ROS accumulation in OS-732 osteosarcoma cells after treatment by compound 1a.

Metal-Organic Frameworks for Liquid Phase Applications

In the last two decades, metal-organic frameworks (MOFs) have attracted overwhelming attention. With readily tunable structures and functionalities, MOFs offer an unprecedentedly vast degree of design flexibility from enormous number of inorganic and organic building blocks or via postsynthetic modification to produce functional nanoporous materials. A large extent of experimental and computational studies of MOFs have been focused on gas phase applications, particularly the storage of low-carbon footprint energy carriers and the separation of CO2-containing gas mixtures. With progressive success in the synthesis of water- and solvent-resistant MOFs over the past several years, the increasingly active exploration of MOFs has been witnessed for widespread liquid phase applications such as liquid fuel purification, aromatics separation, water treatment, solvent recovery, chemical sensing, chiral separation, drug delivery, biomolecule encapsulation and separation. At this juncture, the recent experimental and computational studies are summarized herein for these multifaceted liquid phase applications to demonstrate the rapid advance in this burgeoning field. The challenges and opportunities moving from laboratory scale towards practical applications are discussed.

Hybridization with Ti3C2Tx MXene: An Effective Approach to Boost the Hydrothermal Stability and Catalytic Performance of Metal-Organic Frameworks

Metal-organic frameworks (MOFs) have attracted increasing research enthusiasm owing to their tunable functionality, diverse structure characteristics, and large surface area. However, poor hydrothermal stability restricts the utilization of some MOFs in practical applications. Our work aims at improving the hydrothermal stability of a representative MOF, namely, HKUST-1, by incorporating a two-dimensional material Ti₃C₂T_x MXene for the first time. A new type of hybrid material is synthesized through the hybridization of HKUST-1 and Ti₃C₂T_x, and the obtained hybrids show improved hydrothermal stability as well as catalytic performance. The porosity of hybrids is enhanced when incorporating an appropriate amount of Ti₃C₂T_x, and the surface area can reach 1380 m²·g^-1, while the pristine HKUST-1 is 1210 m²·g^-1. After the hydrothermal treatment (hot water vapor, 70 °C), the structure of hybrid materials maintains well, while the framework of HKUST-1 is severely destroyed. When catalyzing the ring-opening reaction of styrene oxide, the conversion reaches 76.7% only for 20 min, which is much higher than that of pure HKUST-1 (23.1% for 20 min). More importantly, the catalytic activity could recover without loss even after six cycles. Our hybrid materials are promising in practical catalytic applications due to their excellent hydrothermal stability, catalytic activity, and reusability.

Removal of a cationic dye from aqueous solution by a porous adsorbent templated from eco-friendly Pickering MIPEs using chitosan-modified semi-coke particles

Porous materials prepared from high internal phase emulsions have been attracting much attention in recent years, but two major defects related to the high consumption of organic solvent and surfactants are always difficult to solve.

Metal Organic Frameworks as Desulfurization Adsorbents of DBT and 4,6-DMDBT from Fuels

Ultradeep desulfurization of fuels is a method of enormous demand due to the generation of harmful compounds during the burning of sulfur-containing fuels, which are a major source of environmental pollution. Among the various desulfurization methods in application, adsorptive desulfurization (ADS) has low energy demand and is feasible to be employed at ambient conditions without the addition of chemicals. The most crucial factor for ADS application is the selection of the adsorbent, and, currently, a new family of porous materials, metal organic frameworks (MOFs), has proved to be very effective towards this direction. In the current review, applications of MOFs and their functionalized composites for ADS are presented and discussed, as well as the main desulfurization mechanisms reported for the removal of thiophenic compounds by various frameworks. Prospective methods regarding the further improvement of MOF's desulfurization capability are also suggested.

High-Efficiency Separation of Aromatic Sulfide from Liquid Hydrocarbon Fuel in Conjugated Porous Organic Framework with Polycarbazole Unit

We synthesized three conjugated polycarbazole porous organic frameworks named o-Cz-POF, m-Cz-POF, and p-Cz-POF for hydrocarbon fuels' adsorptive desulfurization. The carbazole building blocks possessed ortho, meta, and para steric configuration, which resulted in POFs exhibiting adjustable specific surface area and pore structure. Adsorption kinetics experiments and DFT calculations were carried out to understand the competitive adsorption of 3-methylthiophene and octane in the Cz-POF. The instantaneous adsorption rate and adsorption energy calculation analyses gave a convincing demonstration on preferential selective adsorption of 3-methylthiophene in Cz-POFs. Furthermore, the fixed bed breakthrough experiment demonstrated that the Cz-POFs can selectively adsorb 3-methylthiophene efficiently, and hydrocarbon fuel with sulfide content close to 0 ppm was obtained. The features of high stability and high desulfurization efficiency of Cz-POFs make them hold the promise as a new type of porous adsorbent for ultradeep adsorption desulfurization.

Understanding the desulphurization process in an ionic porous aromatic framework

An ionic porous aromatic framework, iPAF-1, was successfully synthesized from a designed monomer with imidazolium functional groups. The iPAF-1 exhibits the highest dibenzothiophene uptake among all reported adsorptive desulphurization adsorbents. The so-called precursor designed synthetic route provides the stoichiometric and homogeneous introduction of desired functional groups into the framework. Molecular dynamics simulation was performed to understand the structure and the desulphurization process within the amorphous iPAF-1. The insight into the key role of the moderate bonding interaction between the adsorbate and the functional groups of iPAF-1 for improved uptake is highlighted in this work.

TiO2-Containing Carbon Derived from a Metal-Organic Framework Composite: A Highly Active Catalyst for Oxidative Desulfurization

A new metal-organic framework (MOF) composite consisting of Ti- and Zn-based MOFs (ZIF-8(x)@H₂N-MIL-125; in brief, ZIF(x)@MOF) was designed and synthesized. The pristine MOF [H₂N-MIL-125 (MOF)]- and an MOF-composite [ZIF(30)@MOF]-derived mesoporous carbons consisting of TiO₂ nanoparticles were prepared by pyrolysis (named MDC-P and MDC-C, respectively). MDC-C showed a higher surface area, larger pore sizes, and larger mesopore volumes than MDC-P. In addition, the TiO₂ nanoparticles on MDC-C have more uniform shapes and sizes and are smaller than those of MDC-P. The obtained MDC-C and MDC-P [together with MOF, ZIF(30)@MOF, pure/nanocrystalline TiO₂, and activated carbon] were applied in the oxidative desulfurization reaction of dibenzothiophene in a model fuel. The MDC-C, even with a lower TiO₂ content than that of MDC-P, showed an outstanding catalytic performance, especially with a very low catalyst dose (i.e., a very high quantity of dibenzothiophene was converted per unit weight of the catalyst), fast kinetics (∼3 times faster than that for MDC-P), and a low activation energy (lower than that for any reported catalyst) for the oxidation of dibenzothiophene. The large mesopores of MDC-C and the well-dispersed/small TiO₂ might be the dominant factors for the superior catalytic conversions. The oxidative desulfurization of other sulfur-containing organic compounds with various electron densities was also studied with MDC-C to understand the mechanism of catalysis. Moreover, the MDC-C catalyst can be reused many times in the oxidative desulfurization reaction after a simple washing with acetone. Finally, composing MOFs and subsequent pyrolysis is suggested as an effective way to prepare a catalyst with well-dispersed active sites, large pores, and high mesoporosity.

Iron Containing Metal-Organic Frameworks: Structure, Synthesis, and Applications in Environmental Remediation

Metal-organic frameworks (MOFs) with Fe content are gradually developing into an independent branch in environmental remediation, requiring economical, effective, low-toxicity strategies to the complete procedure. In this review, recent advancements in the structure, synthesis, and environmental application focusing on the mechanism are presented. The unique structure of novel design proposed specific characteristics of different iron-containing MOFs with potential innovation. Synthesis of typical MILs, NH₂-MILs and MILs based materials reveal the basis and defect of the current method, indicating the optimal means for the actual requirements. The adsorption of various contamination with multiple interaction as well as the catalytic degradation over radicals or electron-hole pairs are reviewed. This review implied considerable prospects of iron-containing MOFs in the field of environment and a more comprehensive cognition into the challenges and potential improvement.

Adsorptive removal and separation of chemicals with metal-organic frameworks: Contribution of π-complexation

Efficient removal and separation of chemicals from the environment has become a vital issue from a biological and environmental point of view. Currently, adsorptive removal/separation is one of the most promising approaches for cleaning purposes. Selective adsorption/removal of various sulfur- and nitrogen-containing compounds, olefins, and π-electron-rich gases via π-complex formation between an adsorbent and adsorbate molecules is very competitive. Porous metal-organic framework (MOF) materials are very promising in the adsorption/separation of various liquids and gases owing to their distinct characteristics. This review summarizes the literature on the adsorptive removal/separation of various π-electron-rich compounds mainly from fuel and gases using MOF materials containing metal ions that are active for π-complexation. Details of the π-complexation, including mechanism, pros/cons, applications, and efficient ways to form the complex, are discussed systematically. For in-depth understanding, molecular orbital calculations regarding charge transfer between the π-complexing species are also explained in a separate section. From this review, readers will gain an understanding of π-complexation for adsorption and separation, especially with MOFs, to develop new insight for future research.

Fabrication of magnetically responsive HKUST-1/Fe3O4 composites by dry gel conversion for deep desulfurization and denitrogenation

Selective adsorption by use of metal-organic frameworks (MOFs) is an effective method for purification of hydrocarbon fuels. In consideration that the adsorption processes proceed in liquid phases, separation and recycling of adsorbents should be greatly facilitated if MOFs were endowed with magnetism. In the present study, we reported for the first time a dry gel conversion (DGC) strategy to fabricate magnetically responsive MOFs as adsorbents for deep desulfurization and denitrogenation. The solvent is separated from the solid materials in the DGC strategy, and vapor is generated at elevated temperatures to induce the growth of MOFs around magnetic Fe₃O₄ nanoparticles. This strategy can greatly simplify the complicated procedures of the well-known layer-by-layer method and avoid the blockage of pores confronted by introducing magnetic Fe₃O₄ nanoparticles to the pores of MOFs. Our results show that the adsorbents are capable of efficiently removing aromatic sulfur and nitrogen compounds from model fuels, for example removing 0.62mmolg^-1S and 0.89mmolg^-1N of thiophene and indole, respectively. In addition, the adsorbents are facile to separate from liquid phases by use of an external field. After 6 cycles, the adsorbents still show a good adsorption capacity that is comparable to the fresh one.

UiO-66-Type Metal-Organic Framework with Free Carboxylic Acid: Versatile Adsorbents via H-bond for Both Aqueous and Nonaqueous Phases

The metal-organic framework (MOF) UiO-66 was synthesized in one step from zirconium chloride and isophthalic acid (IPA), together with the usual link material, terephthalic acid (TPA). UiO-66 with free -COOH can be obtained in a facile way by replacing up to 30% of the TPA with IPA. However, the chemical and thermal stability of the synthesized MOFs decreased with increasing IPA content used in the syntheses, suggesting an increase in the population of imperfect bonds in the MOFs because of the asymmetrical structure of IPA. The obtained MOFs with free -COOH were applied in liquid-phase adsorptions from both water and model fuel to not only estimate the potential applications but also confirm the presence of -COOH in the MOFs. The adsorbed amounts of several organics (triclosan and oxybenzone from water and indole and pyrrole from fuel) increased monotonously with increasing IPA content applied in MOF synthesis (or -COOH in the MOFs). The favorable contribution of free -COOH to adsorption can be explained by H-bonding, and the direction of H-bonds (adsorbates: H donor; MOFs: H acceptor) was confirmed by the adsorption of oxybenzone in a wide pH range. The versatile applications of the MOFs with -COOH in adsorptions from both polar and nonpolar phases are remarkable considering that hydrophobic and hydrophilic adsorbents are generally required for water and fuel purification, respectively. Finally, the presence of free -COOH in the MOFs was confirmed by liquid-phase adsorptions together with general Fourier transform infrared analyses and decreased chemical and thermal stability.

Remarkable adsorptive removal of nitrogen-containing compounds from a model fuel by a graphene oxide/MIL-101 composite through a combined effect of improved porosity and hydrogen bonding

A composite was prepared by combining a highly porous metal-organic framework (MOF), MIL-101 (Cr-benzenedicarboxylate), and graphene oxide (GnO). The porosity of the composite increased appreciably by the addition of GnO up to a specific amount in the MOF, though further increases in the quantity of GnO was detrimental to porosity. The improved porosity of the GnO/MIL-101 composite was utilized for adsorptive denitrogenation (ADN) of a model fuel where indole (IND) and quinoline (QUI) were used as nitrogen-containing compounds (NCCs). It was found that both IND and QUI showed improved adsorption on the composite compared with pristine MIL-101 or GnO due to the improved porosity of the composite. Interestingly, the improvement in adsorption of IND was much higher than the quantity estimated for the porosity. Importantly, GnO/MIL-101 showed the highest adsorption capacities for NCCs. Irrespective of the studied solvents and co-presence of IND and QUI, the composite adsorbent performed ADN most effectively. This remarkable improvement is explained by the additional mechanism of hydrogen bonding between the surface functional groups of GnO and the hydrogen attached to the nitrogen atom of IND. This hydrogen bonding mechanism is also supported by the results of the adsorption of pyrrole and methylpyrrole. On the other hand, QUI does not show hydrogen-bonding capability, and therefore, its enhanced adsorption originates from only the increased porosity of the adsorbents.

Selective Adsorption of n-Alkanes from n-Octane on Metal-Organic Frameworks: Length Selectivity

The liquid-phase adsorption of n-alkanes (from n-octane (C8) solvent) with different chain lengths was carried out over three metal-organic frameworks (MOFs), viz., metal-azolate framework-6 (MAF-6), copper-benzenetricarboxylate (Cu-BTC), and iron-benzenetricarboxylate (MIL-100(Fe)), and a conventional adsorbent activated carbon (AC). MAF-6 and Cu-BTC were found to have significant selectivity for the adsorption of n-dodecane (C12) and n-heptane (C7), respectively, from C8. Selectivity for C12 on MAF-6 was also observed in competitive adsorption from binary adsorbate systems. To understand the selective adsorption of C12 on MAF-6 more, the adsorption of C12 from C8 over MAF-6 was investigated in detail and compared with that over AC. The obtained selectivities over MAF-6 and Cu-BTC for C12 and C7, respectively, might be explained by the similarity between cavity size of adsorbents and molecular length of n-alkanes. In the case of AC and MIL-100(Fe), no specific adsorption selectivity was observed because the cavity sizes of the two adsorbents are larger than the size of the n-alkanes used in this study. The adsorption capacities (qt) of n-alkanes over AC and MIL-100(Fe) decreased and increased, respectively, as the polarity (or length) of the adsorbates increased, probably because of nonpolar and polar interactions between the adsorbents and n-alkanes. On the basis of the results obtained, it can be concluded that matching the cavity size (of adsorbents) with the molecular length (of n-alknaes) is more important parameter than the MOF's hydrophilicity/hydrophobicity for the selective adsorption/separation of alkanes.

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.

Deep adsorption desulfurization of liquid petroleum gas by copper-modified bentonite

Effects of different copper compounds, the amount of Cu²⁺ loading, the baking temperature and the valence state of copper on the adsorption desulfurization of LPG.

Fabrication of a magnetic nanoparticle embedded NH2-MIL-88B MOF hybrid for highly efficient covalent immobilization of lipase

We demonstrate an approach to synthesize an amine-functionalized magnetic metal-organic framework (MOF) for lipase immobilization. The activity of immobilized lipase is enhanced by different parameters like pH, temperature and thermal stability.

Removal of sulfur compounds from LPG by heteropoly acid-modified Al–MCM-41 mesoporous molecular sieves

Al–MCM-41(30) was synthesized using Al₂(SO₄)₃ as the aluminum source, Na₂SiO₃ as the silicon precursor and cetyltrimethylammonium bromide (CTAB) surfactant as the template.

Adsorption of indole and quinoline from a model fuel on functionalized MIL-101: effects of H-bonding and coordination

Adsorptive denitrogenation was carried out with functionalized metal–organic frameworks in order to understand plausible adsorption mechanisms.