Amine-functionalized zirconium-based metal-organic frameworks and their potential applications in adsorption and separation: A review

Abstract

Metal-organic frameworks (MOFs) refer to porous materials which have received excessive interest due to their unique structural diversity thus helping them maintain high surface area and porosity. Zirconium-based MOFs (UiO-66) can be easily modified with various functional groups, allowing control over how strongly an analyte binds to the MOF. Amine-functionalized UiO-66 MOFs (UiO-66-NH₂) offers higher surface areas and exceptional porosity with potentially enhanced activity compared to traditionally used materials such as base metal oxides and offer shape/size selectivity, which attracts many applications such as gas storage, gas purification and separation, wastewater treatment. In recent studies, UiO-66-NH₂ has been incorporated with nanomaterials such as graphene oxide (GO) (UiO-66-NH₂/GO) to improve its properties for adsorption applications. In this review, we present the synthetic methods, characteristic properties, and adsorption/separation applications of amine-functionalized zirconium-based metal-organic frameworks as well as incorporation of these MOFs in polymeric membranes. Lastly, challenges and future perspectives of UiO-66-NH₂/GO MOFs are discussed.

Keywords

Metal-organic frameworks membranes adsorption selectivity

Introduction

Crystalline porous materials composed of inorganic metal ions or clusters connected to organic linkers or bridging ligands are referred to as metal-organic frameworks (MOFs).¹ The metal ions or clusters act as connecting dots that bond with the organic linkers or ligands via coordination bonds to form cage-like structures.² So far, MOF structures with very high surface areas ranging from 1000– 10,000 m² g⁻¹³ have been designed and explored in diverse applications including gas storage and separation, drug delivery, sensing, and catalysis.^4–9 However, scientists are still struggling to synthesize porous or hollow-structured MOFs with complex structures and certain compositions in a controllable manner.¹⁰

MOFs have advantageous features due to their unique structural diversity in comparison to conventional adsorbents like activated carbon, zeolite, and other porous materials. These features include uniform pore structures, atomic-level structural uniformity, tuneable porosity, extensive varieties, and flexibility in network topology, geometry, dimension, and chemical functionality.^11–15 This allows researchers to successfully control their framework topology, porosity, and functionality. MOFs can be modified to obtain new structures composed of the same secondary building units (SBUs) and different organic ligands (linkers). However, besides all these fascinating features, some MOFs have low hydrothermal and chemical stabilities in aqueous media, and this is one major criterion for good adsorption in wastewater treatment. Consequently, extensive research has been carried out on the development of water-stable MOFs.

The MOF of interest, zirconium dicarboxylate-based metal-organic framework, was discovered by Cavka et al.¹⁶ and is composed of a Zr₆O₄(OH)₄(CO)₁₂ cluster. This structure exhibits high thermal, chemical, and mechanical stability, along with sufficient exposed active sites. Zirconium-based MOFs are porous materials characterized by cubic framework structure formed by cationic Zr₆O₆(OH)₄ nodes and organic linkers like 1,4-benzenedicarboxylate (H₂BDC). Its cubic architecture includes octahedral cages approximately 11 Å in size and tetrahedral cages around 8 Å in size, which are interconnected via narrow triangular openings measuring about 6 Å.¹⁷ Modifications of MOFs have enabled the creation of zirconium-based MOFs such as UiO-MOFs (UiO-66/UiO-67/UiO-68/UiO-69¹⁸ and UiO-69¹⁹) as well as MOF-808 and NU-1000. While UiO-MOFs are highly suitable for applications in acidic conditions, NU-1000 is known for its high alkaline resistance, and MOF-808 is acidic and alkaline resistant to extreme pH conditions, which constrain their application in wastewater treatment and phosphate sequestration.²⁰ UiO-MOFs are three-dimensional porous nanocomposites created by Zr⁴⁺ and carboxylic acids, and their structures are made by increasing the length of the linkers.²¹ The change of the linker in UiO-MOFs does not affect their thermal stability, but changes the pore volume (0.45, 1.05, and 1.82 cm³ g⁻¹) and specific surface area (1087, 3000, and 4170 m² g⁻¹) for UiO-66, UiO-67, and UiO-68, respectively.¹⁶ UiO-66 is the prototype of all UiO MOFs and has been extensively investigated for adsorption applications due to its larger pore size, specific surface area and higher thermal stability as well as active Zr–O clusters compared to other MOFs.^16,22 While many MOFs have a thermal stability range of 300–400 °C, UiO-66 has been reported to decompose at 540 °C.¹⁶

UiO-66 MOFs are synthesized using the standard solvothermal method, in which a zirconium tetrachloride is reacted with carboxylate-type linker in a solvent like N,N-dimethylformamide (DMF) under mild temperatures and pressure.²³ However, due to environmental burdens and costs of organic solvents, it is important to study the synthesis methods and the corresponding properties of UiO-66 for large-scale applications. A good method should reduce the use of solvents while maintaining the high yields of UiO-66 MOF crystals and growth. As such various synthetic methods including but not limited microwave-assisted solvothermal and electrochemical synthesis have been reported.^24,25

Since 2010, a series of MOFs with different functional groups which include but not limited to −NH₂, −NO₂, −Br, −OH and −SO₃H have been synthesized and characterized to improve their chemical affinity and robustness.^26–29 Amine-functionalized MOFs (UiO-66-NH₂) (Figure 1) have captured a major share of the attention that functionalized MOFs are receiving because the −NH₂ group can improve the strength of hydrogen bonds in the structure of MOFs, with most studies reporting increased adsorption capacity due to −NH₂ functionalization of MOFs.^30,31 The basic amine groups in UiO-66-NH₂ show strong attraction to acidic gas molecules and provide active sites for catalysis. UiO-66-NH₂ is also known for its exceptional chemical stability in aqueous media over a wide range of pH³² and tends to be a good platform for post-synthetic modification. However, UiO-66-NH₂ poses challenges such as recovery from aqueous solutions due to their powder form,³³ lower stability compared to UiO-66 alone³⁴ and its larger pore space does not favour gas molecule storage.³⁵ To overcome these limitations, simple and cost-effective nanomaterials such as graphene oxide, zeolites, silica, celites, graphite, etc. can be coupled with UiO-66-NH₂ thereby enhancing its properties.^33,36,37 In polymeric membranes, incorporating these materials individually result in low performance.

Figure 1.

Structure of UiO-66-NH₂ MOF. Reproduced with permission from ref.³⁸ Copyright Royal Society of Chemistry 2016.

Graphene oxide (GO) is a type of porous nanomaterial which is amongst many substrates that can be used to adjust the porous structure and improve the physiochemical properties of UiO-66-NH₂. Compared to other supports such as zeolites, silica, celites, etc., GO has a larger specific surface area, higher stability, plenty of oxygen-containing functional groups (–OH, C–O–C, –C=O)³⁹ and has been proven to be a good carrier due to its modifiable surface properties through chemical functionalization.⁴⁰ GO consists of a layered structure which allows the MOF to attach itself, adjusting the large pore spaces and increasing the stability of UiO-66-NH₂, which could improve the adsorption capacity.⁴¹ The amine-functionalized UiO-66 MOF coupled with GO (UiO-66-NH₂/GO) has also recently gained attention as nanofillers in membranes to enhance surface hydrophilicity for excellent antifouling resistance.^33,36,42

The purpose of this review was to present an overview of the latest experimental studies on the potential of amine-functionalized MOFs (UiO-66-NH₂) and UiO-66-NH₂/GO nanocomposites as emerging adsorbents and nanofillers for separation applications. In comparison with other functionalized UiO-66 MOF such as UiO-66-SO₃H, UiO-66-(COOH)₂, etc., UiO-66-NH₂ has been broadly used as an effective and selective adsorbent due to its diverse structure, good water stability, good biocompatibility, and other desirable properties.^29,35,43 There are few studies reported recently that are focusing on UiO-66-NH₂/GO nanocomposite, which are either briefly highlighting this composite as one of the potential adsorbents, used for photocatalytic oxidation of organic compounds, or are incorporated in membranes for wastewater treatment. There's limited to no studies that are focusing on UiO-66-NH₂ as is, as a composite with GO, and incorporated in membranes for various applications. In this review, a comprehensive presentation of UiO-66-NH₂ and UiO-66-NH₂/GO nanocomposites is undertaken including synthetic methods, characteristic properties, and adsorption applications in the removal of gases, nitrogen compounds and wastewater treatment. Finally, the last section investigates the incorporation of UiO-66-NH₂ and UiO-66-NH₂/GO as nanofillers in polymeric membranes, applications in wastewater and gas separation, and some of the methods being used for fabrication of these membranes. Additionally, relevant concluding remarks and research future potentials for adsorption applications using modified UiO-66-NH₂ were provided in this review paper. Figure 2 shows the summary describing the content of this review.

Figure 2.

Graphical presentation summarizing the content of this review.

Amine-functionalized zirconium-based metal-organic frameworks (UiO-66-NH₂)

Synthesis and characteristic properties of UiO-66-NH₂

Many synthetic methods have been reported for UiO-66-NH₂ including but not limited to solvothermal/hydrothermal,^44,45 microwave-assisted,⁴⁶ and electrochemical⁴⁷ method.

In a solvothermal/hydrothermal process, UiO-66-NH₂ is synthesized either from UiO-66 via solvothermal process in which an organic ligand is combined with metal ions at temperatures below 300 °C. UiO-66-NH₂ is synthesized by simply reacting 2-amino-benzenedicarboxylic acid (NH₂BDC) with Zr ions under solvothermal conditions to obtain a 3D MOF with a crystalline structure as depicted in Figure 3.^{16,26,41,48,49} Modulators such as formic acid, acetic acid and HCl can be used to increase the crystallinity and reproducibility.^50–52 As a result, MOFs produced through this method exhibit consistent size, excellent crystallinity, and well-defined topological morphology.

Figure 3.

Direct synthesis of UiO-66-NH₂ via solvothermal reaction.

The microwave-assisted solvothermal process uses a similar method as the solvothermal synthesis but with a different heating system where heat circulates faster due to electromagnetic waves with moving electric charges, therefore resulting in a shorter reaction time which can take minutes to a few hours.^53–56 The microwave-assisted synthesis has been explored by many researchers.^46,57,58 They all achieved UiO-66-NH₂ with uniform, well-shaped, clear crystals (∼600 nm) in short reaction time (30 min to 4 h) which is achieved in 24–48 h with conventional solvothermal synthesis. Therefore, this method can greatly improve the production efficiency of MOFs. A schematic illustration of the microwave-assisted solvothermal synthesis of UiO-66-NH₂ is depicted in Figure 4.

Figure 4.

Synthesis of UiO-66- NH₂ using microwave-assisted solvothermal method. Reproduced with permission from ref. 46 Copyright ACS 2021.

Electrochemical synthesis has a shorter synthesis time, can take minutes compared to hours or days required in the solvothermal techniques.⁴⁷ Additionally, electrochemical synthesis allows control of reactants concentration, and thus increasing its production.⁵⁹ This technique is mostly used for the synthesis of thin film MOFs. Electrochemical synthesis can be performed in three ways. (1) the anodic method which uses anodic dissolution to generate metal ions under applied electric field.^60,61 The metal ions react with the ligands available in the solution,⁶² leading to fast and eco-friendly synthesis of MOF near the surface of the electrodes. (2) The cathodic method which uses salts as metal source. For instance, Wei et al.⁴⁷ successfully synthesized UiO-66-NH₂ through the electrochemical technique using metallic zirconium as a metal source with a solution containing DMF, NH₂-BDC, acetic acid and tetrabutylammonium bromide (Figure 5). In their study, they synthesized UiO-66-NH₂ with smaller particle size (65 nm) which had a high detection limit (10⁻⁸ mol L⁻¹) of Fe³⁺ compared to solvothermally synthesized MOF (5 × 10⁻⁶ mol L⁻¹). (3) In electrophoretic deposition, charged MOFs particles are suspended in solution containing two identical electrodes under applied electric field.⁵⁶ The applied electric field pushes the particles towards the oppositely charged electrodes which then deposit them.⁶³ Thus, this method allows control of the synthesis rate, enables direct real-time monitoring of the reaction, and offers a novel approach to synthetizing of MOFs.

Figure 5.

Electrochemical synthesis of UiO-66-NH₂. f. 47 Copyright American Chemical Society 2022.

The mixed-ligand (solid-solution) is an important approach to introducing amine functional groups into UiO-66 for chemical and thermal stability. The amine functional groups are generally integrated into the UiO-66 MOF structure via organic linkers to form a solid solution with well-distributed terephthalic acid (BDC) and 2-aminoterephthalic acid (NH₂-BDC) ligands within the framework. This approach has been reported to facilitate tuneable functionality with scalable and uniform characteristics, making the MOF suitable for various applications. However, some of its challenges include attaining accurate control over the density of amine functionalization and potential structural defects, which can precisely change the characteristics of the MOF.^64,65

Generally, the properties of functionalized UiO-66 MOFs are influenced by the extent of functionalization and synthesis approach. The microwave-assisted and electrochemical synthetic methods allow high synthesis rate of UiO-66-NH₂ MOFs with high crystallinity, high porosity, and uniform morphology, and are economically viable compared to the solvothermal synthesis.^55,56 Guan et al.⁴⁴ solvothermally synthesized a series of functionalized UiO-66 (UiO-66-NH₂, UiO-66-NO₂ and UiO-66-Br) by using different functional ligands; H₂N–H₂BDC, O₂N–H₂BDC, and Br–H₂BDC. UiO-66-NH₂ had the highest specific surface area (1072 m² g⁻¹) in the series, and with a comparable thermal stability to other functionalized UiO-66 MOFs (Table 1). Furthermore, a literature comparison on properties of UiO-66-based MOFs is presented in Table 1, which shows the solvothermal synthesis of UiO-66-NH₂ to have the highest specific surface area (1258 m² g⁻¹) and thermal stability (350 °C) compared with other methods.⁴⁶

Table 1.

Characteristic properties of UiO-66-NH₂ vs other functionalized UiO-66-based MOFs.

Functionalized MOFs	Synthetic process	S_pore(nm)	V_pore (cm³g⁻¹)	S_BET (m² g⁻¹)	Thermal stability (°C)	Ref.
UiO-66-NH₂	Solvothermal	3.6, 5.6,7.0	0.43	1072	∼300	⁴⁰
UiO-66-NH₂	Solvothermal	1.21	0.51	1258	350	⁶⁰
UiO-66-NH₂	Solvothermal (ethanol & formic acid)	1.45	0.6	830	-	⁴⁷
UiO-66-NH₂	Hydrothermal (water/acetic acid)	-	0.76	833	∼400	⁴¹
UiO-66-NH₂	Microwave (acetic acid)	1.97	0.38	1169	300	⁴²
UiO-66-NH₂	Microwave (HCl)	7.1	0.35	935	-	⁵⁴
UiO-66-NH₂	Microwave (acetic acid)	-	-	1118	400	⁶¹
UiO-66-NH₂	Electrochemical	-	0.028	844	-	⁴³
UiO-66-Br	Solvothermal	6.7	0.36	927	∼500	⁴⁰
UiO-66-Br₂	Solvothermal (formic acid)	-	0.12	339	390	⁶²
UiO-66-NO₂	Solvothermal	7.9	0.33	814	∼300	⁴⁰
UiO-66-NO₂	Solvothermal (acetic acid & ethanol)	-	0.21	771	397	⁶³
UiO-66-(F)₄	Hydrothermal (water/acetic acid)	-	0.62	833	∼400	⁴¹
UiO-66-F₂	Solvothermal (formic acid)	-	0.28	836	350	⁶²
UiO-66-(OH)₂	Hydrothermal (water/acetic acid)	-	1.08	706	∼400	⁴¹
UiO-66-(OH)₂	Solvothermal (acetic acid & ethanol)	-	0.17	397	207	⁶³
UiO-66-(OH)₂	Hydrothermal (water & acetic acid)	2.5	-	402	∼400	⁶⁴
UiO-66-SO₃H	Solvothermal (formic acid)	-	0.26	769	260	⁶⁵
UiO-66-I	Solvothermal (formic acid)	-	0.27	799	360	⁶⁵
UiO-66-CO₂H	Solvothermal (benzoic acid)	-	0.29	842	340	⁶⁵
UiO-66-(COOH)₂	Hydrothermal (water/acetic acid)	-	0.29	494	∼400	⁴¹
UiO-66-(COOH)₂	Solvothermal (formic acid)	-	0.08	217	290	⁶²
UiO-66-(COOH)₄	Hydrothermal (water/acetic acid)	-	0.42	212	∼400	⁴¹
UiO-66-(OCH₂CH₃)₂	Hydrothermal (water/acetic acid)	-	0.41	405	∼400	⁴¹

UiO-66-NH₂ presents regular octahedral shaped material and has good dispersibility as confirmed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) (Figure 6). The crystals have an average particle size of ∼100 nm.⁶⁶ X-ray diffraction (XRD) is used to confirm the synthesis of UiO-66-NH₂ which may be compared with its simulated XRD spectrum. UiO-66-NH₂ maintains its high and stable crystallinity in a pH range of 1–12.⁵¹ Therefore, its crystalline remains unchanged after adsorption within 2θ = 5–50 °C as confirmed by XRD patterns. Fourier-transform infrared spectroscopy (FTIR) demonstrates the characteristic peaks of UiO-66-NH₂ at 600–900 cm⁻¹ which are attributed to Zr-O stretching and vibration, and a broad −NH₂ peak around 3420 cm⁻¹ which is due to symmetric stretching and asymmetric vibrations of the N-H bond at 3448 and 3368 cm⁻¹, respectively.⁶⁶

Figure 6.

(a) SEM and (b) TEM images of UiO-66-NH₂. Reproduced with permission from ref.⁶⁷ Copyright MDPI 2021.

Fang et al.⁶⁶ identified C, N, O and Zr in UiO-66-NH₂ through x-ray photoelectron spectroscopy (XPS) as shown in the spectrum in Figure 7(a). The C 1s region of UiO-66-NH₂ can be divided into four peaks at 284.5, 285.2, 285.9, and 288.8 eV corresponding to C=C, C–N, C–C, and C=O, respectively (Figure 7(b)). The N 1s region can be divided into three peaks at 398.9, 399.3, and 400.3 eV corresponding to N–C bond, −NH₂, and −NH₃⁺, respectively (Figure 7(c)). The Zr 3d region can be divided into four peaks at 182.7 and 185.1 eV corresponding to Zr–O bonds, and at 183.3 and 185.6 eV corresponding to Zr–Zr bond (Figure 7(d)). Lastly, the O 1s region can be divided into three peaks at 530.63, 531.97 and 533.38 eV corresponding to Zr–O, Zr–OH, and O–C=O, respectively.⁵¹ Therefore, the adsorption process of UiO-66-NH₂ can occur via H-bond, electrostatic, and complexation interactions.⁵¹

Figure 7.

XPS spectra of UiO-66-NH₂ (a) full spectrum, (b) C 1s, (c) N 1s, (d) Zr 3d. Reproduced with permission from ref. 66 Copyright Elsevier 2020.

Adsorption applications of UiO-66-NH₂

The −NH₂ group on MOFs allows chemisorption or physisorption on various sites through hydrogen bonding or van der Waals interactions which substantially increase the adsorptive capacity of UiO-66-NH₂.⁴⁹ The increased adsorptive capacity has been particularly reported for CO₂ and CH₄ (Table 2), with the −NH₂ functional group only increasing the adsorption capacity of CO₂ and CH₄ from the parent UiO-66 at low pressure (1–15 bar) and temperature range of 293–303 K.^68–70 Thus, adsorption capacity is mostly controlled by the −NH₂ group than it is by specific surface area and pore volume. While, at elevated pressures (∼30 bar) of CO₂ adsorption, specific surface area is the only deciding factor, and the −NH₂ group remains the deciding factor for CH₄ under high pressure.⁶⁸ MOFs synthesis approach influences the adsorption properties. Huang et al.⁶⁹ reported the highest CO₂ adsorption capacity (5.8 mmol g⁻¹) at 273 K and 1 bar for UiO-66-NH₂ produced by microwave-assisted solvothermal method, compared with other functionalized UiO-66 MOFs. Similarly, at 298 K and 1 bar, UiO-66-NH₂ showed higher CO₂, CH₄, N₂ and water (at relative humidity less than 20%) adsorption capacity and selectivity compared to other functionalised UiO-66-X MOFs (X = −NO₂, −1,4-Naphthyl and −2,5-(OMe)₂), making it the best option for natural gas sweetening and CO₂ capture from flue gas.⁷¹ Hu et al.⁴⁵ also reported UiO-66-NH₂ as the best MOF for adsorption and selectivity of CO₂, N₂ and water at 273 K and 1 bar compared to UiO-66-X MOFs (X = −(OH)₂, −(COOH)₂, −(COOH)₄, −(OCH₂CH₃)₂, and −(F)₄).

Table 2.

Adsorption studies of UiO-66-NH₂ vs other functionalized UiO-66-based MOFs.

Functionalized MOFs	Conditions	Adsorbate/ analyte	Adsorption capacity (mmol.g⁻¹)	Applications	Ref.
UiO-66-NH₂	273 K, 1 bar	CO₂	5.8	CO₂ capture	⁶¹
UiO-66-NH₂	298 K, 1 bar	CO₂	3.15	CO₂ capture	⁶⁰
UiO-66-NH₂	298 K, 30 bar	CO₂	2.8	CO₂ capture	⁴²
UiO-66-NH₂	303 K, 1 bar	CO₂	52 cm³.g⁻¹	Biogas purification	⁶⁷
UiO-66-NH₂	303 K, 1 bar	CH₄	11 cm³.g⁻¹	Biogas purification	⁶⁷
UiO-66-NO₂	273 K, 25 bar	CO₂	5.6	Biogas and landfill gas purification	⁷²
UiO-66-NO₂	273 K, 25 bar	CH₄	2.2	Biogas and landfill gas purification	⁷²
UiO-66-(OH)₂	273 K, 25 bar	CO₂	4.2	Biogas and landfill gas purification	⁷²
UiO-66-(OH)₂	273 K, 25 bar	CH₄	1.6	Biogas and landfill gas purification	⁷²
UiO-66-(CH₃)₂	273 K, 1 bar	CO₂	130	Post-combustion CO₂ capture	⁷³
UiO-66-(CH₃)₂	273 K, 1 bar	N₂	2 (cm³.g⁻¹)	Post-combustion CO₂ capture	⁷³
UiO-66-NH₂	273 K, 1 bar	CO₂	2.80	Post-combustion CO₂ capture	⁴¹
		N₂	0.26
		Water	645 (cm³.g⁻¹ at 0.9 P/P₀)
UiO-66-(OH)₂	273 K, 1 bar	CO₂	2.78	Post-combustion CO₂ capture	⁴¹
		N₂	0.28
		Water	283 (cm³.g⁻¹ at 0.9 P/P₀)
UiO-66-(COOH)₂	273 K, 1 bar	CO₂	2.46	Post-combustion CO₂ capture	⁴¹
		N₂	0.26
		Water	286 (cm₃.g⁻¹ at 0.9 P/P₀)
UiO-66-(OCH₂CH₃)₂	273 K, 1 bar	CO₂	1.03	Post-combustion CO₂ capture	⁴¹
		N₂	0.11
		Water	243 (cm³.g⁻¹ at 0.9 P/P₀)
UiO-66-(F)₄	273 K, 1 bar	CO₂	2.11	Post-combustion CO₂ capture	⁴¹
		N₂	0.32
		Water	407 (cm³.g⁻¹ at 0.9 P/P₀)
UiO-66-(COOH)₄	273 K, 1 bar	CO₂	0.81	Post-combustion CO₂ capture	⁴¹
		N₂	0.15
		Water	339 (cm³.g⁻¹ at 0.9 P/P₀)
UiO-66-NH₂	-	NO₂	1400 mg.g⁻¹	Pollutant removal	⁷⁰
UiO-66-NH₂	273 K	NH₃	3.01	Air purification	⁷⁴
UiO-66-OH	273 K	NH₃	2.77	Air purification	⁷⁴
UiO-66-NH₂	77 K, 20 bar	H₂	25 mg.g⁻¹	Hydrogen storage	⁶⁹
UiO-66-(OH)₂	77 K, 20 bar	H₂	16.9	Hydrogen storage	⁶⁴
UiO-66-NH₂	298 K, activated, guest molecule	Cl₂	6.6 mol.kg⁻¹	Contaminated air treatment	⁴⁵
UiO-66-NH₂	298 K, activated, guest molecule	Cl₂	5.3 mol.kg⁻¹	Contaminated air treatment	⁴⁵
UiO-66-NH₂	298 K, pH 4, 24 h	Phosphate	154 mg.g⁻¹	Wastewater treatment	⁴⁰
UiO-66-Br	298 K, pH 4, 24 h	Phosphate	133 mg.g⁻¹	Wastewater treatment	⁴⁰
UiO-66-NO₂	298 K, pH 4, 24 h	Phosphate	118 mg.g⁻¹	Wastewater treatment	⁴⁰
UiO-66-NH₂	303 K, natural pH, 3 h	Phosphate	43.9 mg.g⁻¹	Wastewater treatment	⁴⁷
UiO-66-NH₂	321 K, pH 5, h	Cr³⁺	14.8 mg.g⁻¹	Seawater desalination	⁴⁸
UiO-66-NH₂	321 K, pH 5, h	Cr⁴⁺	8.8 mg.g⁻¹	Seawater desalination	⁴⁸
UiO-66-(SH)₂	295 K, pH 3–5, 24 h	Hg²⁺	236 mg.g⁻¹	Wastewater treatment	⁷⁵
UiO-66-NH₂	303 K, pH 5, 3 h	Hg²⁺	113 mg.g⁻¹	Wastewater treatment	⁷⁶
UiO-66-NH₂	298 K, pH 7, 12 h	Sb³⁺	39.2 mg.g⁻¹	Wastewater treatment	²⁹
UiO-66-NH₂	328 K, pH 6, 80 min	Pb²⁺	320.7 mg.g⁻¹	Wastewater treatment	⁷⁷
UiO-66-NH₂	298 K, pH 3, 24 h	Tetracycline	40.2 mg.g⁻¹	Water purification	¹³
UiO-66-NH₂	298 K, pH 10, 24 h	MB dye	55 mg.g⁻¹	Wastewater treatment	⁷⁸
UiO-66-SO₃H	298 K, pH 6, 2 h	MB dye	230 mg.g⁻¹	Wastewater treatment	⁷⁹
UiO-66-NH₂	298 K, 5 h	Quinoline	240 mg.g⁻¹	Denitrogenation of fuels	³⁷
UiO-66-NH₂	298 K, 12 h	Quinoline	122 mg.g⁻¹	Denitrogenation of fuels	⁸⁰
UiO-66-NH₂	298 K, 12 h	Indole	182 mg.g⁻¹	Denitrogenation of fuels	⁸⁰
UiO-66-NH₂	298 K, 2 h	Quinoline	200 mg.g⁻¹	Denitrogenation of fuels	⁸¹
UiO-66-NH₂	298 K, 2 h	Indole	312 mg.g⁻¹	Denitrogenation of fuels	⁸¹
UiO-66-OH	298 K, 12 h	Quinoline	156 mg.g⁻¹	Denitrogenation of fuels	⁸⁰
UiO-66-OH	298 K, 12 h	Indole	174 mg.g⁻¹	Denitrogenation of fuels	⁸⁰
UiO-66-(OH)₂	298 K, 12 h	Quinoline	198 mg.g⁻¹	Denitrogenation of fuels	⁸⁰
UiO-66-(OH)₂	298 K, 12 h	Indole	224 mg.g⁻¹	Denitrogenation of fuels	⁸⁰
UiO-66-SO₃H	298 K, 12 h	Quinoline	128 mg.g⁻¹	Denitrogenation of fuels	⁸⁰
UiO-66-SO₃H	298 K, 12 h	Indole	170 mg.g⁻¹	Denitrogenation of fuels	⁸⁰
UiO-66-NH₃⁺	298 K, 12 h	Quinoline,	218 mg.g⁻¹	Denitrogenation of fuels	⁸⁰
UiO-66-NH₃⁺	298 K, 12 h	Indole	230 mg.g⁻¹	Denitrogenation of fuels	⁸⁰

UiO-66-NH₂ has been reported to have great Cl₂ removal due to reactive removal induced by the −NH₂ group through the electrophilic substitution reaction.⁴⁹ UiO-66-NH₂ is also capable of adsorbing a higher capacity of H₂,⁸² which is an important gas that represent a clean alternative energy source compared to UiO-66-(OH)₂ reported by Chen et al.⁷⁴ This is due to the higher specific surface area of UiO-66-NH₂ (977 m² g⁻¹) compared to that of UiO-66-(OH)₂ (402 m² g⁻¹). The adsorption of NO₂ has been studied and higher capacity was reported, compared to the parent UiO-66, was 1400 mg g⁻¹.⁸⁰ The presence of water has a positive effect on the adsorption of NO₂ as Peterson et al.⁸⁰ reported higher adsorption capacity in humid conditions than in dry conditions. Furthermore, adsorption mechanism of NO₂ by UiO-66-NH₂ has been confirmed via proton nuclear magnetic resonance (¹H-NMR) to occur through many complicated reaction pathways, including reaction with −NH₂ to form diazonium ions and nitrozation of phenyl C-H bonds. The adsorption of NO₂ with UiO-66-NH₂ is irreversible because its reaction occurs at the Zr-O-Zr bridges, leading to framework collapse.⁸¹ UiO-66-NH₂ has been reported to have higher NH₃ adsorption capacities of 3.3 mol kg⁻¹⁸³ and 3.01 mmol g⁻¹⁷⁸ compared to other functionalized UiO-66 MOFs in humid conditions at 273 K due to its high water stability. However, in dry conditions, UiO-66-OH showed the highest adsorption capacity 5.7 mmol g⁻¹ due to −OH being less bulky compared to −NH₂ and other functional groups such as −NO₂, −(OH)₂, −(COOH)₂, and −SO₃H.⁷⁸

Denitrogenation performance of functionalized UiO-66 MOFs for quinoline and indole has also been recently studied. UiO-66-NH₂ has the highest quinoline and indole adsorptive removal compared with other functionalized UiO-66 MOFs (Table 2). Mokgohloa and Ogunlaja⁴¹ reported a quinoline adsorption capacity of 240 mg g⁻¹ with UiO-66-NH₂ at 298 K and 5 h contact time. Meanwhile, Sarker et al.⁸⁴ and Ahmed et al.⁷⁹ achieved lower adsorption capacity (122 and 200 mg g⁻¹) of quinoline with UiO-66-NH₂ at 298 K. It should be noted that Mokgohloa and Ogunlaja⁴¹ synthesized the UiO-66-NH₂ with the traditional solvothermal method while Sarker et al.⁸⁴ and Ahmed et al.⁷⁹ added a modulator (HCl) in their solvothermal synthesis method. The difference between quinoline adsorption capacities could be attributed to different surface areas (445, 806 and 750 m² g⁻¹) and pore volume (0.15, 0.40 and 0.31 cm³ g⁻¹) for Makgohloa and Ogunlaja,⁴¹ Sarker et al.⁸⁴ and Ahmed et al.,⁷⁹ respectively. The highest adsorption capacity of 312 mg g⁻¹ for indole with UiO-66-NH₂ was reported by Ahmed et al.⁷⁹ compared with 182 mg g⁻¹ reported by Sarker et al.⁸⁴ This difference could be attributed to different compositions of aminoterephthalate used during synthesis of UiO-66-NH₂. UiO-66-NH₃⁺, prepared by protonation of UiO-66-NH₂ with HCl, showed higher quinoline (218 mg g⁻¹) and indole (230 mg g⁻¹) adsorption capacity compared to other functionalized UiO-66 MOF, including UiO-66-NH₂.⁸⁴ The high indole adsorption with UiO-66-NH₃⁺ was explained by strong cation-π interactions and weak H-bonding.

The UiO-66-NH₂ MOFs have also received attention for liquid-phase separation by adsorption due to their good thermal, chemical and water stabilities making them great pollutant adsorbents widely used in wastewater treatment.⁸⁵ They are well known for their exceptional stability in aqueous media over a wide range of pH. The main factors that affect pollutant adsorption in wastewater include temperature, pH, adsorbate dosage, and contact time. Guan et al.⁴⁴ conducted the adsorption of phosphate in synthetic urine to reduce the amount of phosphorus in water using functionalized UiO-66-X MOFs (X = −NH₂, −NO₂ and −Br) at a pH range of 2–11, adsorbate dosage of 1.5–18 g L⁻¹ and at 298 K for 24 h. They achieved the highest adsorption capacity (153.9 mg g⁻¹) with UiO-66-NH₂ compared to the other two reaching >135 mg g⁻¹ at pH 4 and adsorbate dosage of 9 g L⁻¹. The difference in adsorption capacities of these MOFs is only due to the different Zr content used in their syntheses, with UiO-66-NH₂ having the highest and subsequent high specific surface area and pore volume. A reason for this is that the Zr-based ligand (benzenedicarboxylic acid) is what attracts the use of these functionalized UiO-66 MOFs for phosphorus uptake due to its accessibility to phosphate.

Studies on the adsorption of organic pollutants have also been conducted. Zhang et al.⁸⁶ studied the adsorption selectivity of dye solution consisting of Methylene Blue (MB), Methyl Orange (MO) and Rhodamine B (RB) with UiO-66-NH₂ at an adsorbate dosage range of 0–50 mg L⁻¹, at 298 K for 24 h. Their results showed that UiO-66-NH₂ has the highest selectivity adsorption for MO followed by MB > RB and this preferential was due to HCl which was used in the synthesis. Therefore, the exposure of hydrogen ions onto the surface of UiO-66-NH₂ resulted in the preferential selectivity of MO > MB > RB. They reported an MB adsorption capacity of 55 mg g⁻¹ in their studies, which is comparable to 43.9 mg g⁻¹ reported by Zhang et al.⁵¹ However, Chen et al.⁸⁷ reported the highest adsorption capacity of MB (96.5 mg g⁻¹) in their similar adsorption selectivity studies of UiO-66-NH₂. This suggests that the acidic UiO-66-NH₂ surface (pH < 6) hinders the adsorption of cationic MB dye and rather favours the anionic dyes (MO). A comparative study using UiO-66-SO₃H and UiO-66-NH₂ has been reported in which the adsorption capacity of MB was ∼230 mg g⁻¹ and MO was ∼220 mg g⁻¹ respectively.⁷⁶ Therefore, UiO-66-NH₂ favours the anionic MO dye due to strong H-bond interactions of the sulfonic acid with the µ₃-OH and −NH₂ group than cationic MB.⁷⁵

The global challenge of heavy metal ions pollution calls for effective removal of these toxic pollutants from domestic effluent and industrial wastewater. Many removal technologies exist, however, most of them are expensive and involve complex operation processes, which limit their industrial applications.⁷⁷ Hence the adsorption strategies with porous materials such as MOFs have gained interest as cost-effective and high-efficient adsorbents for treatment of wastewater. The UiO-66 MOFs have therefore gained most of this attention due to their various active binding sites compared to other MOFs. Modifying the UiO-66 MOFs with functional groups such as amines (−NH₂) can increase the active sites and improve the chelating adsorption of heavy metal ions.⁷⁷

Studies on selective removal of heavy metal ions have been reported which show that UiO-66-NH₂ has a strong affinity for most heavy metal ions such as Cr³⁺, Cr⁴⁺,⁵² Sb³⁺,³³ and Pb²⁺⁷² compared with pristine UiO-66. However, Zhao et al.,⁷³ reported that UiO-66-NH₂ showed lower selective adsorption of Hg²⁺ compared with UiO-66-(SH)₂ reported by Leus et al.⁸⁸ This shows that the thiolate has a strong affinity for Hg²⁺ than the amine group. Some of the factors that influence the adsorption performance of heavy metal ions include but are not limited to inner linker defects, large amount of functional groups, porous structure with large specific area, thermal and chemical stabilities, and charge interactions between linker or functional group and adsorbent.^33,52,77,89 Table 2 reports some data on adsorption capacities of functionalized MOFs under varying temperature and pressure conditions.

UiO-66-NH₂-based composites

Some limitations presented by the pristine UiO-66-NH₂ MOF are overcome by combining it with other materials. This improves properties such as selectivity, thermal tolerance and stability, water and humidity resistance, mechanical strain, and chemical resistance.^{37,41,90–94} Studies have shown that composites of UiO-66-NH₂ have either larger surface areas, larger mesopores, or all of the above.^93,95 Of all UiO-66-NH₂-based composites, UiO-66-NH₂/GO and its derivatives are the most studied.^{41,92,93,96–98} This is due to its ease of synthesis, availability, and astonishing properties of GO.^99–102 The UiO-66-NH₂/GO composite and its derivatives have recently been the subject of many studies.^41,103–105 They are prepared primarily with UiO-66-NH₂ and GO or GO derivatives, these composites are favoured due to their increased thermal stability, electrical conductivity, and physical stability.^106,107 Furthermore, UiO-66-NH₂/GO composites have shown great adsorption capabilities under a variety of different conditions.^93,98,104 Consequently, these composites have applications in water purification, gas removal, and adsorption of chemicals to name a few.^{37,41,91,106,108–110} Work on existing and novel UiO-66-NH₂-based composites is vastly gaining momentum.¹¹¹

Synthesis and characteristic properties of UiO-66-NH₂/GO composites

Typically, UiO-66-NH₂/GO composites are synthesized solvothermally, in a method like that described in the pristine MOF UiO-66-NH₂. The methods described typically use 2-amino terephthalic acid and ZrCl₄ dissolved in DMF as the main reactants (Figure 8).^37,41,90 The additional compound is either added in a reactor with the main reactants or mixed separately after preparation of the MOF.^108,112 The latter is often done when the additional compound is temperature sensitive or reactive to the main reactants.¹¹² Acid modulation with formic acid, acetic acid or hydrochloric acid has been used, and can increase the surface area of the composite.^{46,96,106,113} The reaction mixture is then heated between 120–140 °C for 24 h in a hydrothermal Teflon-lined autoclave. Methanol or ethanol and DMF are used to wash the composite and dried under vacuum at 120 °C from 6 to 24 h. Ning et al.⁴⁶ reported the use of the microwave synthesizer to cut down on the synthesis time of the composite.

Figure 8.

Typical synthesis of UiO-66-NH₂/GO. Reproduced with permission from ref.¹¹⁴ Copyright Elsevier 2019.

Characterization of UiO-66-NH₂/GO composites has mostly relied on XRD, SEM, FTIR, and energy dispersive spectroscopy (EDS).^115,116 XRD reflections indicate that the addition of GO to UiO-66-NH₂ does not disrupt the crystal structure, and confirms that the MOF retains its structure.^106,117 Thus, the octahedral structure of UiO-66-NH₂/GO shown in field emission scanning electron microscopy (FESEM) images remains unchanged as reported in a recent study by Kumar et al.¹¹⁸ (See Figure 9). The XRD reflections of GO disappear on the spectra.^106,117 Although the crystallinity is unchanged, SEM studies show that the composite increases in size.⁴¹ The size of the UiO-66-NH₂/GO crystals varies, some studies reported between 80–90 nm,³⁷ whilst others have a larger size of 249–259 nm, which is greater than the UiO-66-NH₂ (150–200 nm).^37,41,106 This is because UiO-66-NH₂ crystals grow on the outer edges of GO sheets attached via oxygen functional groups, and they appear on SEM imaging having fully coated the surface of the GO sheets.¹¹⁷ Cao et al.³⁷ attributed the smaller crystal sizes to stunting caused by interactions of oxygen-based functional groups of GO and the Zr metal at the MOF nodes. FTIR spectroscopy is used to observe the activity of the functional groups on UiO-66-NH₂, GO, and UiO-66-NH₂/GO. The characteristic GO FTIR peaks are not visible on the spectrum of UiO-66-NH₂/GO, thus there are often little or no changes to the spectrum. Additionally, UiO-66-NH₂/GO retains the ultraviolet-visible (UV-Vis) spectroscopic properties of UiO-66-NH₂ as presented in many studies.¹¹⁹ EDS is used to confirm the presence of oxygen, carbon atoms, and zirconium atoms, including their ratios.^41,119 The presence of micropores and macropores is confirmed using nitrogen adsorption/desorption isotherms.^120,121 These often show type I isotherms, which provide evidence of mainly micropores.^122,123 The specific area of the composites and their pore size are larger than that of the UiO-66-NH₂ (Table 3).^106,107 Thermogravimetric analysis (TGA) for thermal studies is conducted to understand the influence of GO on the thermal stability of UiO-66-NH₂.³⁷

Figure 9.

FESEM micrographs of (a) Zr-MOF and (b,c) Zr-MOF/rGO- nanocatalyst and HRTEM micrographs of (d) Zr-MOF and (e,f) Zr-MOF/rGO-nanocatalyst. Reproduced with permission from ref.¹¹⁸ Copyright ACS 2021.

Table 3.

Characteristic properties of UiO-66-NH₂/GO.

Composite	Pore size (nm)	V_pore (cm⁻³g⁻¹)	Surface area (m²g⁻¹)	Thermal stability (°C)	Ref.
UiO-66 NH₂/GO	12	0.345	1052	500–600	³³
UiO-66-NH₂/GO	-	0.68	1475	-	¹⁰⁴
UiO-66 NH₂/GO	0.273	0.337	1048	200–500	¹¹⁵
UiO-66-NH₂/GO-NH₂	5.80	0.05	147	-	³⁷
UiO-66-NH₂/GA	5.50	0.12	381	-	³⁷

GA, Graphene Aerogel.

In Figure 9, pristine Zr-MOF shows a multi-layered structure (Figure 9(a)). Zr-MOF/rGO-nanocatalyst gave irregular nanoparticles that are dispersed over the rGO to form a microporous structure (Figure 9(b) and (c)). High-resolution transmission electron microscopy (HRTEM) images of the Zr-MOF (Figure 9(d)) and Zr-MOF/rGO-nanocatalyst (Figure 9(e)) show large amount of MOF cuboctahedral nanoparticles and wrap up arrays of the crystals are developed on the GO sheet.¹¹⁸

Graphene and graphene oxide as pristine materials have properties that make them suitable for many uses.¹²⁴ Their specific surface area is large (about 2630 m² g⁻¹), they are excellent conductors of electricity and heat and have a high tensile strength.¹²⁴ Ning et al.⁴⁶ reported that the introduction of GO or graphene into MOFs enhances dispersive interactions and accumulation of MOF nanoparticles. Various graphitic materials have been introduced into MOFs to improve dispersive interactions allowing the MOF particles to expand in the pore space.^113,125 Some papers have reported that graphene oxide is an excellent adsorbent of metal ions (such as platinum, lead, cobalt),¹²¹ gases (such as CO₂, NO₂, H₂, NH₃),^46,126 and dyes.¹²⁷ In composites with UiO-66-NH₂, GO is highly electronegative and bonds with UiO-66-NH₂ through the MOFs Zr metal clusters at the node.¹⁰³ Electronegative oxygen-containing groups (COO, CO) with aromatic sp² domains enable the interaction of GO sheets and UiO-66-NH₂.¹²⁸ The surface of the GO sheets is coated by the UiO-66-NH₂ particles.^46,129 Introducing GO into the UiO-66-NH₂ produces composites that have highly valued qualities like high resistance to thermal degradation,⁴⁶ excellent electrical conductivity,^104,119,130 photocatalytic activity,^103,119 and high resistance to water and humidity.^37,93,108

Adsorption applications of UiO-66-NH₂/GO composites

The driving force of adsorption in UiO-66-NH₂-based composites is multidimensional; it adds to the above-mentioned factors that influence adsorption in the pristine UiO-66-NH₂. In addition to factors like hydrogen-bonding, van der Waals forces, and the −NH₂ functional group, UiO-66-NH₂/GO is dependent on a wide range of dynamics including the presence of functional, charged or polar groups, pH, temperature, the percentage composition of the composite, the surface area, pore size and pore abundance of the composite, the initial concentration and adsorption time.^131–133 In many applications, adsorption occurs in conditions that are harsh and counteract the adsorption process, thus making it necessary to develop composites that depend on many of these factors for enhanced adsorption.^{120,132,134,135}

In contrast to the pristine UiO-66-NH₂, the relative ratios of UiO-66-NH₂ to GO in UiO-66-NH₂/GO composites are of great importance and are often taken into consideration during synthesis.^37,92,106 This is because interactions of UiO-66-NH₂/GO and the adsorbate are dependent on the GO content in the composite.^92,106 The addition of GO at different weight percentages influences the surface properties and hence adsorption properties of the composite.^106,110 Interactions of GO with the absorbates are dispersive in nature.¹¹⁰ It is the composites with 5% weight GO that exhibit the most desirable adsorption properties and hence are commonly used.^37,90,106 These properties include increased mesopores and micropores (see Table 3). Moreover, UiO-66-NH₂/GO composites exhibit an increased surface area in comparison to their counterparts.

Surface area and porosity significantly impact the adsorption of gases in both UiO-66-NH₂ and the UiO-66-NH₂/GO.¹¹⁹ However, the adsorption of CO₂ by UiO-66-NH₂/GO (3.37–6.41 mmol g⁻¹) is greater than that of UiO-66-NH₂ (3.15–5.80 mmol g⁻¹) as shown in Table 4.^{37,46,69,70,90} Facilitation of the CO₂ to the high-energy adsorption site is directed by the mesopores, whilst the role of the micropores is to hold the CO₂ particles in place. UiO-66-NH₂/GO has additional adsorption sites including the interaction of −NH₂ groups with CO₂, which adds to the physical adsorption sites.^37,110 The presence of these adsorption sites encourages robust interaction and selectivity during the adsorption process in gases.^37,105 UiO-66-NH₂/GO also showed great adsorption for N₂ (0.32 mmol g⁻¹) but has a higher selectivity to CO₂, uptake decreases with an increase in temperature.³⁷ Cao et al.³⁷ showed that the pristine UiO-66-NH₂ and the UiO-66-NH₂/GO had CO₂/N₂ selectivity of 22.83 and 28.45, respectively. Similarly, the adsorption of liquids is driven by surface area, pore size and pore abundance.¹³⁶ Studies by Mokgohloa and Ogunlaja⁴¹ on the adsorption of the organic liquid quinoline, showed that UiO-66-NH₂/GO (284 mg g⁻¹) exhibited better adsorption capacity compared to the pure UiO-66-NH₂ (240 mg g⁻¹) (Table 2). The reason for this increased adsorption was that GO enhanced adsorption of quinoline via π-π interactions and hydrogen bonding. This enhanced ability to adsorb organic liquids by UiO-66-NH₂/GA and UiO-66-NH₂/GO was also investigated by Li et al.¹¹⁴ with observed adsorption capacity of 147 mg g⁻¹ for chloroform while Ding and Zeng¹³⁷ observed 1653 mg g⁻¹ for perfluorooctanoic acid.

Table 4.

The adsorption applications of various UiO-66-NH₂ composites.

Composite	Conditions	Adsorbate/analyte	Adsorption capacity	Application	Ref.
UiO-66-NH₂/GO	273 K, 1.1 bar	CO₂	6.41 mmol.g⁻¹	Atmosphere	³³
UiO-66-NH₂/GO	298 K, 1 bar	CO₂	3.37 mmol.g⁻¹	Atmosphere	⁸⁸
UiO-66-NH₂/GO	395 K, 30 bar	CO₂	3.95 mmol.g⁻¹	Atmosphere	⁴²
UiO-66-NH₂/GO	273 K, 1.1 bar	N₂	0.32 mmol.g⁻¹	Atmosphere	³³
UiO-66-NH₂/GO-NH₂	273 K, 1 bar	Quinoline	284 mg.g⁻¹	Fuel	³⁷
UiO-66-NH₂/GA	273 K, 1 bar	Quinoline	270 mg.g⁻¹	Fuel	³⁷
UiO-66-NH₂/GA	-	Chloroform	147 mg.g⁻¹	Environment	¹³⁵
UiO-66-NH₂/GO	298 K, pH 3	Pentadecafluorooctanoic acid	1653 mg.g⁻¹	Environment	¹³⁴
UiO-66-NH₂/GO	298 K, pH 7	Sb³⁺	19.9 mg.g⁻¹	Wastewater	²⁹
UiO-66-NH₂/GO	298 K, pH 1	Eriochrome Black T	263.16 mg.g⁻¹	Wastewater	⁹⁰
UiO-66-NH₂/GO	321 K, pH 5	Boron	75 mg.g⁻¹	Seawater	¹¹⁵

The functional groups found in GO, play a significant role in the adsorption of charged particles and compounds like metal ions and dyes.^33,114 In adsorption studies of UiO-66-NH₂/GO with metal ions (Sb³⁺), adsorption interactions are dominated by interactions of oxygen functional groups on GO with the positive metal ions.³³ These interactions increase the adsorption of UiO-66-NH₂/GO (19.89 mg g⁻¹) compared to UiO-66-NH₂ MOFs (19.43 mg g⁻¹). Additionally, electrostatic forces play a pivotal role. The role of pH in adsorption is that it allows Sb³⁺ metal ions to compete with charged ions (OH⁻, H⁺) for binding sites.³³ UiO-66-NH₂/GO has a higher adsorption of Sb³⁺ at low pH compared with UiO-66-NH₂, but this wide margin decreases at high pH.³³ The composite (UiO-66-NH₂/GO) has also shown a great adsorption capacity (263.16 mg g⁻¹) of Eriochrome Black T at extremely acidic conditions (pH 1).¹¹⁷ The adsorption of the metalloid boron on UiO-66-NH₂/GO is also dependent on pH, where low pH provides better adsorption.¹¹⁷ The composite UiO-66-NH₂/GO exhibited a higher boron adsorption (75.03 mg g⁻¹) than pristine UiO-66-NH₂ (24.54 mg g⁻¹).¹¹⁷ In general, the driving force in the adsorption of metals and metalloids, pH is important as it affects the charges of the adsorbates through the presence of H⁺ and OH⁻ ions.

The focus of many researchers has been to exploit the adsorption superiority of UiO-66-NH₂ composites (see Table 4). Most gas adsorption applications are in the fossil fuel industry where the adsorption must be conducted in harsh conditions i.e. high temperatures and pressure. The superior thermal stability of graphene is exploited in the UiO-66-NH₂/GO composite for the improved thermal resistance of the composite.⁹⁰ The composite showed improved adsorption capabilities in the adsorption of CO₂ and organonitrogen compounds compared to the UiO-66-NH₂ MOF.^37,41,110 Some adsorption applications of UiO-66-NH₂/GO composites have been for the adsorption of chemical agents like chlorpyrifos (a pesticide), pinacolyl methylphosphonic acid (PMPA, a nerve agent also called soman).^106,109 Deng et al.³³ studied the application of UiO-66-NH₂/GO composites in the purification of water, where the composite is used to remove heavy metal ions such as Cu²⁺, Pb²⁺, Zn²⁺, Cd²⁺, As³⁺, Sb³⁺, and Ni²⁺. The use of the composite to remove heavy metals exploits the composites’ stability in highly acidic environments.³³

UiO-66-NH₂ MOF as a nanofiller in membranes

UiO-66-NH₂ MOF has also found applications in membranes as a nanofiller mostly in water treatment¹³⁸ and gas separation.¹³⁹ Membrane technology refers to a method that selectively separates pollutants using its pores, which functions as a physical barrier (Figure 10).¹⁴⁰ The interest in membrane technology, in general, arises from its robustness, less footprint, low-energy use, easy integration with other methods, selectivity, flexibility, effectiveness, and reliability.^141,142 The emphasis is primarily on polymeric membranes due to their advantageous features, including an extensive range of pore sizes, scalability, cost-effectiveness, ease of use, and flexible configurations, making them highly suitable for applications like gas separation¹⁴³ Polymeric membranes encompass a broad variety, including non-pressure-driven membranes, such as pressure retarded osmosis (PRO) and forward osmosis (FO), as well as pressure-driven membranes, such as microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO). They are used based on their properties as shown in Table 5, for instance, reverse osmosis is more suitable to produce potable water. Therefore, the aforementioned applications utilize different membrane processes depending on the filtration process required and Table 5 is presenting the properties of the polymeric membranes for water treatment. Some of the polymers being used for the fabrication of these membranes include polyethersulfone (PES),¹⁴⁴ polysulfone (PSf),¹⁴⁵ polyimide (PI),¹⁴⁶ polyvinylidene fluoride (PVDF),¹⁴⁷ etc.

Figure 10.

Theoretical scheme of the flat sheet membrane filtration process.

Table 5.

Properties of pressure-driven membranes for water and wastewater applications.

Membrane properties	Microfiltration	Ultrafiltration	Nanofiltration	Reserve osmosis
Pressure range (bar)	0.1–2	1–10	5–35	10–150
Pore size (nm)	100 nm–10 m	10–100 nm	1–10 nm	0.1–1 nm
Water permeability (Lm⁻²h⁻¹bar⁻¹)	>500	20–500	5–50	0.5–10
Separation mechanism	Sieving	Sieving	Sieving & Donnan exclusion	Solution diffusion
Transport regime	Macroporous	Mesoporous	Microporous	Molecular
Membrane	Porous	Porous	Partly porous & partly dense	Dense
Separation	High molecular weight substances	Cell proteins and fats	Desalination & wastewater treatment	Drinking water

Membrane fabrication methods

Membranes have been fabricated using different methods such as phase inversion, electrospinning, vacuum-assisted self-assembly (VASA), surface grafting and solution coating method, their schemes are shown in Figure 11 –15, respectively. The phase inversion method is defined as a process of controlled polymer transformation from a liquid phase to a solid phase. It is categorized into four types: thermally induced phase separation, precipitation by controlled evaporation, precipitation from vapour phase, as well as immersion precipitation which is the most commonly employed method.^148,149 The electrospinning method is used for the fabrication of consistent and uniform nanofiber membranes, hollow tubes, wires, spheres, and rods.¹⁵⁰ Controlling the electrospinning conditions such as polymer solution (conductivity, viscosity, molecular weight, volatility, as well as surface tension) and parameters (voltage, feeding rate, and tip to collector distance) alters the internal microstructure of the fibers thereby resulting in different morphologies.¹⁵¹ VASA method is utilized in synthesizing large-scale independent nanocomposite membranes from nanoparticles with large aspect ratio. The concentration and volume are the main factors which control the thickness of these nanocomposite membranes. The initial stage of the fabrication consists of highly disordered dispersion; however, the assembly process results in highly ordered films with unique characteristics.^152,153 The method of surface grafting is an attractive, effective, and widely researched technique to incorporate materials onto the desired surface to enhance the membrane properties and ultimately the membrane performance.¹⁵⁴ This casting method also known as the wet processing method is one of the simplest techniques because it doesn’t require special equipment. The method results in a broad range of consistent thicknesses and dimensional stability.¹⁵⁵

Figure 11.

Fabrication of flat-sheet membranes NIPS method. Reproduced with permission from ref.¹⁵⁶ Copyright Elsevier 2019.

Figure 12.

Fabrication of membrane via electrospinning method. Reproduced with permission from ref.¹⁵⁶ Copyright Elsevier 2019.

Figure 13.

Fabrication of membranes by vacuum-assisted self-assembly method.

Figure 14.

Fabrication of membranes via surface grafting method.

Figure 15.

Fabrication of membranes via solution casting method.

Application of UiO-66-NH₂ MOF-based membranes in wastewater treatment

The polymers utilized in wastewater treatment exhibit outstanding thermal and chemical resistance, meeting the stringent standards required for applications in medicine, food, and hygiene.¹⁵⁷ It is widely recognized that membrane technology, despite its advantages, faces certain limitations in wastewater treatment, which restrict its large-scale industrial adoption. For membranes used in wastewater treatment, the fouling phenomenon has been a major setback.^158,159 Fouling occurs when the surface roughness of the membrane is too high; the foulants tend to be trapped on the surface or inside the pores during the separation process resulting in irreversible fouling. It results in reduced permeate flux, quality, high operating costs, and shortened membrane lifespan. Polymeric membranes without the nanofiller have been shown to exhibit high fouling tendencies.¹⁴⁵ Consequently, the polymeric membranes are modified with nanomaterials such as MOFs,¹⁶⁰ zeolites,¹⁶¹ graphene oxide (GO),¹⁶² carbon nanotubes (CNTs),¹⁶³ zwitterions,¹⁶⁴ and so forth to enhance their hydrophilicity. The interest in using UiO-66-NH₂ MOFs as a nanofiller in membranes can be attributed to their outstanding properties such as highly porous structure, water-stability, large surface area, and clearly defined structure with confined pore sizes which are desirable in enhancing the polymeric membranes.^160,165 The strong interactions between the UiO-66-NH₂ and the polymer also help boost the hydrophilicity,¹⁶⁶ The UiO-66-NH₂ functionalities are also boosted by incorporating the MOF with nanomaterials such as GO, CNTs, zwitterion,^36,154,167 etc. to make composites. The interaction of these hydrophilic nanomaterials is achieved through available exposed active sites on the MOF. Besides incorporating hydrophilic nanomaterials into polymeric membranes, another way of mitigating membrane fouling is the pretreatment of wastewater. Several pretreatment systems that are being used include activated carbon, ozone, media filtration (e.g. sand or anthracite), ultraviolet (UV) light, coagulates/flocculates, cartridge filters, etc. Thus, the membrane system can be used as a polishing step to attain potable or non-potable end-use. Consequently, the membrane lifespan can be extended and reduce maintenance and operation costs.

Table 6 depicts UiO-66-NH₂-based membrane treatment systems for wastewater such as dyes, metal ions, oily water, etc., their fabrication methods and removal efficiency. Recently, Cao et al.¹⁶⁸ investigate a membrane system based on UiO-66-NH₂ for the separation of oil from water. The as-fabricated membranes prepared via electrospinning method exhibited enhanced functionalities as the surface roughness and water contact angle (WCA) were 0.56 m and 21.1^o, respectively, and the separation efficiency of n-hexane reached 99.2% with FRR of 94.4%. Li et al.¹⁶⁹ also demonstrated polyacrylonitrile nanofibrous membrane tailored with UiO-66-NH₂ for oil-water emulsion. The superhydrophilicity and underwater superoleophobicity of the membranes was attributed to hydrophilic amino and carboxyl groups of UiO-66-NH₂ which enhanced the antifouling properties. In addition, the small size gaps between the filler contributed as oil drops were large and hardly entered. It was noted that there are no studies using pristine UiO-66-NH₂ or UiO-66-NH₂/GO composite for the treatment of oily water. The interest in UiO-66-NH₂-based membranes on dye removal has also been increasing. Li et al.¹⁷⁰ tested a membrane system based on UiO-66-NH₂ to remove small molecules such as CR, xylene brilliant cyanin G and Rose Bengal sodium salt and the rejection efficiency was more than 99.5%. The fabricated membranes exhibited strong stability, with no notable decline in separation efficiency even after being immersed in water for five days or subjected to intense ultrasonication. Li et al.¹¹⁴ integrated their UiO-66-NH₂ with graphene oxide (GO) nanosheets to remove Congo red (CR), methylene blue (MB), and rhodamine B (RB) dyes. Generally, MB is positively charged, CR is negatively charged, and RB is in zwitterionic form (neutral). Since GO is negatively charged on its edges, it significantly enhanced the removal efficiency of MB, primarily attributed to the electrostatic interactions between the two materials.

Table 6.

UiO-66-NH₂-based membrane systems for wastewater treatment.

Fabrication method	Membrane type	Wastewater	Removal efficiency (%)	Ref.
Electrospinning	UiO-66-NH₂/PAN/PVP	n-hexane-water	99.2	¹⁶⁶
Electrospinning	UiO-66-NH₂	Oily water	>99	¹⁶⁷
Electrospinning	PVP-UiO-66-NH₂/PAN NFM	Oily water	99	¹⁶⁶
In-situ growth	UiO-66-NH₂/PES	Congo Red	99.5	¹⁶⁸
Hot-pressing	UiO-66-NH₂/GO-PUF/PDA	Methylene Blue	97.26	¹¹²
		Rhodamine B	72.65
		Congo Red	43.20
Reactive seeding (RS) method	UiO-66-NH₂/α-Al₂O₃ support	Cr(VI)	1^st cycle: 98	¹⁶⁹
Reactive seeding (RS) method	UiO-66-NH₂/α-Al₂O₃ support	Cr(VI)	20^th cycle: 94	¹⁶⁹
Electrospinning	Nylon-6/UiO-66-NH₂	Cr(VI)	5^th cycle: >80	¹⁷⁰
Cross-linking	PEI/UiO-66-NH₂/PVDF	Cr(VI)	3^rd cycle: 91.83	¹⁷¹
IP	UiO-66-NH₂/PSf/PA	Pb(II)	99	¹⁷²
Vacuum assisted filtration	UiO-66-NH₂/PVA/PTFE	Desalination	99.99	¹⁷³
IP	UiO-66-NH₂/PA	Desalination	97.1	¹⁷⁴
IP	UiO-66-NH₂/PA	Desalination	98.3	¹⁷⁵
IP	UiO-66-NH₂/PA	Na₂SO₄	97.9 ± 0.82	¹⁷⁶
IP	UiO-66-NH₂/PA/PDA	Na₂SO₄	98.1	¹⁷⁷
Phase inversion	UiO-66-NH₂/PEI	MgCl₂	97.4	¹⁷⁸
Phase inversion	UiO-66-NH₂/PEI	MgSO₄	88.1	¹⁷⁸
IP	UiO-66-NH₂/PA/PAN	Methylparaben	50	¹⁷⁹
		Propylparaben	67
		Bisphenol A	75
		Benzylparaben	85
Phase inversion	UiO-66-NH₂/GO/PES	Greywater	99.9	³⁸

PVDF, Polyvinylidene fluoride; PAN, Polyacrylonitrile; PA, Polyamide; PVP, Polyvinylpyrrolidone; PEI, Polyethyleneimine; TPU, Thermoplastic polyurethane; PUF, Polyurethane foams; PDA, Polydopamine; PAA, Polyacrylic acid; PVA, Poly(vinyl alcohol); PTFE, Polytetrafluoroethylene; APTES, 3-aminopropyltriethoxysilane; PVC, Polyvinyl chloride.

UiO-66-NH₂-based membranes were also used to remove metal ions which include Cr(II), Pb(II), Cr(II), Cd(II), and Ga(II) from wastewater. Du et al.,¹⁷¹ Ji et al.,¹⁷² and Zhang et al.¹⁷³ produced UiO-66-NH₂/α-Al₂O₃, Nylon-6/UiO-66-NH₂, and PEI/UiO-66-NH₂/PVDF membranes, respectively, for the separation of Cr(VI) ions in water. UiO-66-NH₂/α-Al₂O₃ membranes exhibited excellent performance as the membranes went through 20 cycles for recyclability studies and maintained efficiency between 98–94%.¹⁷¹ Nylon-6/UiO-66-NH₂ system was tested through 5 cycles, and it showed removal efficiency >80%. Lastly, PEI/UiO-66-NH₂/PVDF system showed removal efficiency of 91.83% after the 3^rd cycle. The excellent recyclability and overall performance of UiO-66-NH₂/α-Al₂O₃ were ascribed to exceptional water and chemical stability of the system.

Abdullah et al.¹⁷⁴ also fabricated UiO-66-NH₂ membranes for the removal of Pb(II), which reached an efficiency of 99%. Several research groups reported UiO-66-NH₂-based membranes for salt rejection^175–180 and their system showed similar performance. The outstanding performance of these systems was associated to high chemical stability, the narrow pore size of the membranes, and antifouling properties. The UiO-66-NH₂ membranes have also found applications in greywater reclamation⁴² and removal of emerging micropollutants such as endocrine disrupting compounds (EDCs).¹⁸¹ The system for greywater reclamation maintained high efficiency throughout seven fouling-washing cycles and this was attributed to high fouling resistance characteristics of the membranes. It was noted that a lot of studies do not test the membrane reusability or lifespan using real wastewater. This is important more especially if the system has potential for pilot-scale studies.

Application of UiO-66-NH₂ MOF-based membranes in gas separation

Figure 16 shows some of the different mechanisms of gas separation which include convective flow, Knudsen diffusion, molecular sieving, and solution-diffusion. The porous membranes separate these gases through small pores of the membrane based on molecular size. The separation of asymmetric (dense) membranes is based on solubility and diffusivity and the solution-diffusion mechanism is mostly suitable for separating CO₂ in polymeric membranes.¹⁸² Table 7 depicts the properties of gas separation membranes. The use of polymeric membranes for gas separation falls short of industry requirements because of the inherent trade-off between selectivity and permeability, as defined by Robeson's upper bounds.^183,184 Robeson's upper bound is an empirical line that is used to determine the maximum gas pair selectivity achievable by a polymeric membrane at a certain permeability value. Dal-Cin et al.¹⁸⁵ and Comesaña-Gándara et al.¹⁸⁶ gave a detailed explanation of Robeson's upper bounds. The trade-off relationship results in low efficacy as some of the fabricated polymeric membranes cannot overcome this issue. Pure polymeric membranes exhibiting high selectivity usually have low permeability and vice versa. Despite this challenge, the membrane technology is still more attractive than conventional methods such as amine absorption, low-temperature distillation, and pressure swing adsorption owing to its ease of preparation method, environmental friendliness, and diversity.¹⁸⁷ One of the proposed solutions to this challenge is the integration of porous inorganic nanofillers into a polymer matrix to produce mixed matrix membranes (MMMs) as it has exhibited promising outcomes. Some of the inorganic nanofillers being used to attain good permselective properties include CNTs,¹⁸⁸ MOFs,¹⁸⁹ covalent-organic frameworks (COFs),¹⁸⁴ metal oxides,¹⁹⁰ carbon nanofibers (CNFs),¹⁹¹ zeolite and zeolite-like materials,¹⁹² etc. MOFs are of interest in MMMs because of their characteristics such as high and tunable porosity, straightforward synthesis, adsorbate affinity, good thermal and mechanical stabilities. Some of the polymers used are shown in Table 6, and these include PES, polyethylenimine (PEI), polyether block amide (PEBA), PI, Pebax, polymer of intrinsic microporosity (PIM-1), polyacrylonitrile (PAN), PVDF, cellulose acetate (CA), etc.¹⁹³

Figure 16.

Different mechanisms of gas separation via porous membranes (i) convection flow, (ii) Knudsen diffusion, (iii) molecular sieving and dense membrane (iv) solution-diffusion.

Table 7.

Properties of gas separation membranes.

Pore size	Pore classification	Separation mechanism	Separation
>1000 Å	Convective flow	Porous	No separation occurs
100–1000 Å	Knudsen diffusion	Porous	Lighter molecules to preferentially diffuse through pores
5–100 Å	Molecular sieving	Porous	Large molecules excluded from the pores
No pores	Solution-diffusion	Dense	Gas dissolves in the membrane and diffuses across it

Studies based on UiO-66-NH₂ and its composite membranes for gas separation are reported in Table 8. Many researchers focus on overcoming undesirable outcomes such as gas leakage, interfacial defects, poor polymer-MOF affinity, and filler uniform dispersion caused by simple dispersing or mechanically mixing of MOF with polymer matrix.

Table 8.

UiO-66-NH₂-based membranes for gas separation.

Fabrication method	Membrane type	P_CO2 (Barrer)	Selectivity		Ref.
Fabrication method	Membrane type	P_CO2 (Barrer)	CO₂/CH₄	CO₂/N₂	Ref.
Vacuum filtration	UiO-66-NH₂/GO/MCE	P_H2 – 3.9 × 10⁻⁸ molm⁻²s⁻¹Pa⁻¹	-	S_H2/CO2 – 6.35	¹⁹²
Vacuum filtration	UiO-66-NH₂/GO/MCE	P_H2 – 3.9 × 10⁻⁸ molm⁻²s⁻¹Pa⁻¹	-	S_H2/N2 – 9.75	¹⁹²
Solution casting	UiO-66-NH₂/GO/PI	7.28	-	52	¹⁹³
Solution casting	UiO-66-NH₂/6FDA-DAM	1223 ± 23	30 ± 1	-	¹⁹⁴
Solution casting	UiO-66-NH₂/6FDA-DAM	521	23.8	-	¹⁹⁵
Solution casting	UiO-66-NH₂/6FDA-Durene	1890	18	-	¹⁹⁶
Solution casting	UiO-66-NH₂-PAN/PU/PIM-1	333	-	138	¹⁸⁵
Solution casting	UiO-66-NH₂/PIM-1	4810	-	22.3	¹⁹⁷
Solution casting	UiO-66-NH₂/PIM-1	4835	-	28.2	¹⁹⁷
Spin-coating	UiO-66-NH₂/PIM-1	1900	-	24.1	¹⁹⁸
Spin-coating	UiO-66-NH₂/PIM-1	2869	-	27.5	¹⁹⁸
Solution casting	PIM-g-UiO-66-NH₂/Pebax	247	-	56.1	¹⁹⁹
Solution casting	UiO-66-NH₂/Pebax^®1657	118.3	30.5	56.6	²⁰⁰
Dip-coating	UiO-66-NH₂/Pebax^®1657/PVDF/PTMSP	338	-	57	²⁰¹
Dip-coating	UiO-66-NH₂/Pebax^®2533/PP/PVDF	25.8 GPU	-	37	²⁰²
Casting method	UiO-66-NH₂/BPPO/PES	125.6	-	50.2 (S_H2/N2 – 302.4)	²⁰³
Solution casting	UiO-66-NH₂-PEBA/PVDF	130.2	-	72.2	²⁰⁴
Solution casting	UiO-66-NH₂/ODPA-TMPDA	142	-	37.1	²⁰⁵
Casting method	UiO-66-NH₂-PVP-PEI	394	-	S_CO2/H2 – 12.7	²⁰⁶
Flat-scraping	UiO-66-NH₂/ICA/Matrimid^®	40.1	64.7	-	²⁰⁷

CNFs, Cellulose nanofibers; BPPO, bromomethylated poly(phenylene oxide); ICA, Imidazole-2-carbaldehyde; 6FDA, 4,4′-(hexafluoroisopropylidene)diphthaliic anhydride; DAM, Diaminomesitylene; PU, Polyurethane; PTMSP, Poly[1-(trimethylsilyl)prop-1-yn].

Jia et al.^194,195 reported UiO-66-NH₂/GO membranes applied in CO₂ and H₂ separation. In their report,¹⁹⁴ two fabrication methods were examined, i.e. layer-by-layer method where GO nanolayers covered UiO-66-NH₂ as well as totally mixed UiO-66-NH₂ and GO. Totally mixed composite membrane exhibited high H₂ separation because of UiO-66-NH₂ and GO close interfacial contact accomplished through electrostatic forces and hydrogen bonding as well as well-dispersion of UiO-66-NH₂ among GO nanolayers via vacuum filtered onto MCE support. In a separate study,¹⁹⁵ UiO-66-NH₂/GO was integrated into a polyimide polymer to enhance CO₂ permeation and CO₂/N₂ selectivity. GO nanosheets enhanced the dispersion of UiO-66-NH₂ nanocrystals in the PI matrix and the high porosity of the nanofiller and inherent adsorption property to CO₂ improved remarkably compared to pristine polyimide membrane. Their optimal membrane demonstrated consistent CO₂/N₂ separation performance, achieving a CO₂ permeability (P_CO2) of 7.28 Barrer and a CO₂/N₂ selectivity of 52.

UiO-66-NH₂/6-FDA-DAM (4,4′-(hexafluoroisopropylidene)diphthaliic anhydride – Diaminomesitylene) membrane system reported by Ahmad's group^208,209 showed good results and this was ascribed to the 6FDA-DAM polymer used. 6FDA-DAM has been reported to be one of the most permeable polyimides for gas separation due to its rigid backbone. The bulky fluorine in the 6FDA structure inhibits polymer chain packing thereby resulting in improved heat resistance, high free volume, as well as good mechanical properties.^196,210 In their work,²⁰⁹ UiO-66-NH₂ was shown to prevent CO₂-induced plasticization and swelling effect which stands as one of the significant hurdles hindering its industrial applications. In their other report,²⁰⁸ no evidence of CO₂-induced plasticization was detected at pressures up to 40 bar. Qian et al.²¹¹ reported imide-functionalized UiO-66-NH₂ nanoparticles in a 6FDA-Durene polymer for improved compatibility of the membranes. The fabricated membranes demonstrated improved permeability and selectivity, which enhanced as the UiO-66-NH₂ loading increased, with the optimal separation performance observed at 40 wt.% loading. The separation performance approached the Robeson upper bound for CO₂/CH₄, with permeability reaching 1890 Barrer and CO₂/CH₄ selectivity achieving a value of 19. In addition, the membranes enhanced CO₂-induced swelling resistance, and this was attributed to a localized decrease in the flexibility of polymer chains at the interface between the polyimide and the imide-functionalized UiO-66-NH₂ MOF.

Another mostly used polymer, PIM-1, is also highly permeable with ultrahigh rigid molecular and highly contorted ladder structure.¹⁹⁶ These characteristics do not allow efficient chain packing resulting in good solubility and processability, glassy structure with consistent microporosity, high surface areas (400 m² g⁻¹, 2 nm pore size), good gas permeability, etc.^197,198,212 Khdhayyer et al.¹⁹⁹ and Ghalei et al.²⁰⁰ incorporated UiO-66-NH₂ into PIM-1 polymer in their systems for CO₂ permeability and CO₂/N₂ selectivity. Khdhayyer et al.¹⁹⁹ probed the effect of UiO-66-NH₂ and UiO-66-(COOH)₂ on the gas (H₂, CO₂, He, CH₄, O₂, and N₂) transport. The order of gas permeation (CO₂ > H₂ > He > O₂ > CH₄ > N₂) remained unchanged in the presence of the MOFs, and the high CO₂ permeability indicated a solubility-controlled transport mechanism, which is characteristic of PIMs for CO₂. UiO-66-(COOH)₂-based systems increased the permeation but decreased the selectivity. A UiO-66-NH₂-based system, on the other hand, maintained the same selectivity when the permeability increased and this was associated with the affinity of UiO-66-NH₂ for PIM-1 improved by NH₂ functionality. Ghalei et al.²⁰⁰ reported CO₂ permeability of 4810 and 4835 Barrer and CO₂/N₂ selectivity of 22.3 and 28.2; the improved selectivity and minimal permeability loss were noted. This different trend was ascribed to the minimization of the UiO-66-NH₂ size during synthesis. In another study, Fan et al.¹⁸⁷ reported rubbery PAN-UiO-66-NH₂-PU/PIM polymer membrane.s The as-fabricated membranes were reported to be strong and flexible. Both UiO-66-NH₂ and PU/PIM layers enhanced CO₂ permeability and CO₂/N₂ selectivity; abundant active sites of UiO-66-NH₂ enhanced selectivity while the addition of a small amount of PIM-1 improved permeability. P_CO2 of 333 Barrer and selectivity of 138 were recorded, and the performance of the membranes could easily exceed the 2008 CO₂/N₂ upper bond. Furthermore, the selectivity performance remained consistent throughout a 60-day aging study, demonstrating that the entanglement of PIM-1 with the molecular chain effectively slows down the aging process.

Lastly, Husana et al.²⁰¹ reported PIM-grafted-UiO-66-NH₂ incorporated into Pebax support. The PIM structure provided additional molecular transport channels and the increase in compatibility between PIM-g-UiO-66-NH₂ filler and polymer enhanced the CO₂ transport for sufficient gas separation. PIM-g-UiO-66-NH₂/Pebax MMM exhibited P_CO2 of 247 Barrer and CO₂/N₂ selectivity of 56.1, which aligns with the 2008 Robeson upper bound. Furthermore, the as-fabricated MMM showed outstanding anti-aging properties for up to 240 days and enhanced mechanical properties. PIM-baased memebrane systems showed better performance than any other system and this may be attributed to its excellent characteristics aforementioned.

Sarmadi et al.²⁰² studied the effect of polymer solvents (DMF and H₂O/EtOH (70/30 wt.%)) for improved efficiency and trade-off between gas permeability and pair gas selectivity of Pebax^®1657 by incorporating UiO-66-NH₂ nanoparticles. The membranes prepared by DMF solvent showed better performance than H₂O/EtOH blend solvent as the CO₂ permeability and CO₂/CH₄ selectivity were 20.4 and 57.4% higher. Pebax^®1657-UiO-66-NH₂ MMM showed an improvement of 49.4% for CO₂ permeability as well as 71.7 and 34.5% CO₂/N₂ and CO₂/CH₄ selectivity, respectively, compared to the pristine membrane and this was attributed to improved CO₂-OH interactions as a result NH₂ functional group. The research on the enhancement of membranes for gas separation efficiency was also done using composite membrane systems such as UiO-66-NH₂/Pebax^®1657/PVDF/PTMSP,²⁰³ UiO-66-NH₂/Pebax^®2533/PP/PVDF,²⁰⁴ UiO-66-NH₂/BPPO/PES,²⁰⁵ UiO-66-NH₂-PEBA/PVDF,²⁰⁶ UiO-66-NH₂/ODPA-TMPDA,²⁰⁷ UiO-66-NH₂-PVP-PEI,²¹³ and UiO-66-NH₂/ICA/Matrimid^®.²¹⁴

Conclusions and perspectives

The UiO-66-NH₂ is a high-quality MOF due to its highly crystalline structure with high porosity, large crystals, and desired morphology. However, the use of organic solvents during the synthesis of the UiO-66-NH₂ represents a major challenge. Although solvent-free and scalable techniques to synthesize UiO-66-NH₂ MOFs have been reported, solvothermal synthesis remains the most straightforward technique to obtain highly crystalline UiO-66-NH₂ MOFs with high surface area (1258 m² g⁻¹). This calls for researchers to modify the existing solvothermal synthesis (Hummer's method) such that it becomes energy efficient and environmentally friendly. UiO-66-NH₂ showed to be the best adsorbent amongst other functionalized UiO-66 MOFs, particularly for CO₂ selectivity due to strong interactions between CO₂ and −NH₂. However, its low adsorption and poor selectivity, especially for most heavy metal ions, can be improved by post-modification of UiO-66-NH₂. Inorganic materials such as graphene oxide (GO), carbon nanotubes, silica and ligand functionalization methods have been introduced for the modification of the structure of UiO-66-NH₂ MOFs. Thus, leading to enhanced adsorption selectivity by improving the dispersion interaction, agglomeration of particles and tuning the pore structure. UiO-66-NH₂-based composites have provided exceptional performance in various adsorption applications. They have been shown to at least rival or even outperform the pristine UiO-66-NH₂ in many cases. They are more tuneable through the addition of various materials which allows the ease of modification to suit a particular application or able adsorption in harsh conditions. All these advantages make composites the preferred choice compared to the pristine MOF. Consequently, a lot of work has been done and there continue to be novel composites produced with only a handful detailing in depth crystalline imperfections.

This review points out potential UiO-66-NH₂-based membranes and how the various UiO-66-NH₂ synthetic routes could influence the MOFs physico-chemical properties such as degree of functionalization, surface area, porosity, etc., as well as being able to compare variations among different nanocomposite materials of UiO-66-NH₂. The incorporation of UiO-66-NH₂ and UiO-66-NH₂/GO in polymeric membranes for water treatment and gas separation shows potential for upscaling. A few studies studied the reusability potentials of the membranes, and the results are promising. There are fewer studies on wastewater treatment using UiO-66-NH₂/GO composite membranes. Incorporating high fouling resistant nanofillers is one of the ways of reducing membrane fouling. UiO-66-NH₂/GO nanofiller with its hydrophilic properties also provide adequate water channels for high water flux. Incorporating a superhydrophilic material such as zwitterions in the backbone of the polymer can also result in high antifouling membranes. The pretreatment of the feed before using the membrane is paramount for prolonged lifespan of the system. Also, using membrane treatment systems is more desirable for industrial application since these industries deal with high water volumes. In gas separation, some of the UiO-66-NH₂-based membrane systems have been shown to overcome the selectivity and permeability trade-off. This was accomplished through the minimization of the UiO-66-NH₂ size during synthesis and incorporating UiO-66-NH₂ on PIM-1 polymer which result in great affinity. PIM-1 and 6FDA polymers with their outstanding properties such as highly permeability with ultrahigh rigid molecular and highly contorted ladder structure, heat resistance, high free volume, as well as good mechanical properties have shown excellent results. Thus, UiO-66-NH₂-based membrane systems have great potential in water treatment and gas separation.

Improving the functionalization methods and characterisation techniques is very critical in enhancing the properties and subsequent adsorption selectivity of UiO-66-NH₂ MOFs and their composites with GO. Research development in this study area is therefore necessary not only to improve but broaden the application of these MOFs in energy, wastewater, environment and catalysis. We hope that this review reaches a broader audience and inspire future developments in this field.

Footnotes

ORCID iDs

Nokuthula E Magida

Mathule C Mokgohloa

Funeka Matebese

Zenixole R Tshentu

Adeniyi S Ogunlaja

Consent to participate

Not applicable.

Author contributions

Conceptualization, Visualization, Abstract, Introduction and Reviewing the UiO-66-NH₂ MOFs, N.E.M.; Conceptualization, Visualization, Reviewing UiO-66-NH₂/GO composites, M.C.M.; Conceptualization, Visualization, Editing, Reviewing the membranes and conclusion and future work, F.M.; Validation, Editing, Supervision, A.S.O.; Validation, Editing, Supervision, Z. T. All authors have read and agreed to the published version of the review.

Funding

Magida, Makgohloa and Matebese would like to disclose receipt of financial support from Research Capacity Development (RCD), Postgraduate Research Scholarship (PGRS) and National Research Foundation (NRF-CPRR) and the NRF-JINR (Grant numbers 129274 and 120478, respectively).

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Nokuthula E Magida is a postdoctoral researcher in the Pyrometallurgy Division at Mintek. Her research interests include clean coal technologies, bioenergy, advanced materials, and carbon capture technologies.

Mathule C Mokgohloa is a PhD student in Chemistry at Nelson Mandela University. His research focuses of development of metal-organic frameworks for denitrogenation of fuels.

Funeka Matebese is a Postdoctoral Researcher at University of South Africa. Her research interests include wastewater treatment, membranes, and metal organic frameworks.

Zenixole R Tshentu is professor and senior lecturer in Chemistry department at Nelson Mandela University. He specializes in Inorganic Chemistry and specifically involved in mineral beneficiation.

Adeniyi S Ogunlaja is a senior lecturer and currently a head of Chemistry department at Nelson Mandela. His research interests include advanced functional materials, chemistry-biology interface, and innovative analytical methodologies.

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Amine-functionalized zirconium-based metal-organic frameworks and their potential applications in adsorption and separation: A review

Abstract

Keywords

Introduction

Amine-functionalized zirconium-based metal-organic frameworks (UiO-66-NH2)

Synthesis and characteristic properties of UiO-66-NH2

Adsorption applications of UiO-66-NH2

UiO-66-NH2-based composites

Synthesis and characteristic properties of UiO-66-NH2/GO composites

Adsorption applications of UiO-66-NH2/GO composites

UiO-66-NH2 MOF as a nanofiller in membranes

Membrane fabrication methods

Application of UiO-66-NH2 MOF-based membranes in wastewater treatment

Application of UiO-66-NH2 MOF-based membranes in gas separation

Conclusions and perspectives

Footnotes

ORCID iDs

Consent to participate

Author contributions

Funding

Declaration of conflicting interests

References

Amine-functionalized zirconium-based metal-organic frameworks (UiO-66-NH₂)

Synthesis and characteristic properties of UiO-66-NH₂

Adsorption applications of UiO-66-NH₂

UiO-66-NH₂-based composites

Synthesis and characteristic properties of UiO-66-NH₂/GO composites

Adsorption applications of UiO-66-NH₂/GO composites

UiO-66-NH₂ MOF as a nanofiller in membranes

Application of UiO-66-NH₂ MOF-based membranes in wastewater treatment

Application of UiO-66-NH₂ MOF-based membranes in gas separation