**3. Metal-organic frameworks (MOFs)**

Metal-organic frameworks (MOFs) are a class of functional materials synthesized by the assembly of the metal ions/clusters and organic linkers like cyano and pyridyl, carboxylates, phosphonates and crown ethers, which are connected to metal ions/clusters through coordination bonds of moderate strength [59, 60]. The MOFs have been studied since 1990s with more than 20,000 structures synthesized and evaluated for applications such as adsorption, catalysis, drug delivery, sensing, separation, gas storage, bioimaging and so on [61–64]. These structures have shown huge potential in these areas due to distinctive features, such as high porosity and surface area, chemically adjustable pore, uniform structures, tunable surface properties (functional groups), good thermal stability, unsaturated metal centers and even the catalytically active organic linkers [65–68]. The most typical metal-organic

frameworks are MIL (Materials of Institute Lavoisier), based on lanthanides or transition metals; UiO (University of Oslo), built up with Zr; MOF-5, composed of Zn; and Cu-BTC, based on Cu [69].

These structures have been applied to adsorption of organic and inorganic contaminants in water and wastewater with better efficiencies when compared with conventional adsorbents, probably due to large surface area (1000–10,000 m2 •g<sup>−</sup><sup>1</sup> ); presence of central metal ions, open metal sites, coordinatively unsaturated sites and functional groups on the organic linkers; and easily tunable structure and flexible framework, since it is possible to modify the pores surface [66, 70–72]. The adsorption mechanisms for removal of organic pollutants by MOFs in water mainly include electrostatic, hydrophobic, acid-base π-π interactions and hydrogen bonding [73–75].

MOFs photocatalytic properties are exploited due to high capacity of the organic linkers in absorbing photons, such as antennas to harvest light, which transfer the energy to the metal sites by transition from ligand to metal cluster charge under UV or visible light radiation [76, 77]. Therefore, these structures can act as semiconductor, since MOFs contain conduction and valence bands, with band gaps in the range of 1.0–5.5 eV, or the band gaps are related to the energy levels of the highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs) of the linker molecules, which leads to the formation of charge carriers (e<sup>−</sup>/h+ ) that can subsequently be transferred to the surface [60, 68]. Furthermore, the easily tailorable physical and chemical functions, together with the large surface area and permanent pores/channels to potentially anchor/encapsulate photosensitizers and catalytic moieties, make MOFs potential candidates for application in photocatalytic processes [78].

The photocatalytic and adsorption features of the metal-organic frameworks are directly related to synthetic routes and parameters such as temperature, reagent concentration, solvent, pH and pressure [79]. Many techniques have been studied to produce distinct structures with specific properties, such as hydro/solvothermal, electrochemical, mechanochemical and solvothermal [80–82].

Electrochemical method to synthesize MOFs is based on the dissolution of a anodic metal that supply metal ions into a reaction medium that contains the organic linkers and electrolytes, avoiding the use of metal salts. This reaction between metal ions and organic linkers produces structures with high purity, due to the absence of nitrate, perchlorate or chloride presents in metal salts, in a short time when compared to other synthetic methods [83–85]. The electrochemical approach was applied by [86] to produce Cu3(BTC)2 (HKUST-1) for CO2 and CH4 adsorption and separation. Two copper electrodes were used into a solution that contained benzene-1,3,5-tricarboxylate (H3BTC) and tetrabutylammonium tetrafluoroborate (TBAFB) as electrolytes to form the MOF. The characterization confirmed the high textural properties, high crystallinity and good thermal stability of the structure, which showed satisfactory CO2 adsorption. A microseparator device containing a metal-organic framework synthesized by electrochemical method was produced by [87]. HKUST-1 was prepared by copper electrodes, benzene-1,3,5-tricarboxylate (BTC) linker and methyl-tributyl-ammonium methyl sulfate (MTMS) electrolyte and was applied to the separation of methanol and n-hexane. The breakthrough curves analysis demonstrated that up to 400 mg•g<sup>−</sup><sup>1</sup> of methanol can be adsorbed to the electrochemically synthesized Cu-BTC coating. Yang et al. [88] synthesized MOF-5 using zinc anodes, terephthalic acid (H2BDC) linker, zinc nitrate hexahydrate [Zn(NO3)2•6H2O] electrolyte and 1-butyl-3-methylimidazolium chloride ionic liquid, which was employed to a template to induce the porous structure. This structure was mixed with BiOBr by ultrasound treatment to create a composite that was evaluated for methyl orange dye degradation by photocatalysis under simulated solar light irradiation. The results showed that this new composite can achieve degradation more than 90% that occurs mainly because of the H+ and •O2 <sup>−</sup> active species.

**69**

*Photocatalytic Adsorbents Nanoparticles DOI: http://dx.doi.org/10.5772/intechopen.79954*

•g<sup>−</sup><sup>1</sup>

area near 1200 m2

Mechanochemical method is a solvent free methodology to produce MOFs that employ a direct mechanical grinding of the metal salts and linker precursors either in a mortar or a ball mill [89]. This technique can occur at room temperature in short reaction times [90]. The compound MOF-14 [Cu3(BTB)2] was synthesized by [91] using ball milling to copper acetate monohydrate (metal salt) and H3BTB (organic linker) mixture, which were placed in a ball mill with three balls and grinded together for 10 min and later activated by a single post-synthetic washing step with ethanol. This structures showed high micropore volume and a specific surface

Zn2(oba)2(4-bpdh)•(DMF)y (TMU-5) were prepared by mechanochemical by [92] from a mixture of 4,4′-oxybisbenzoic acid (H2oba) and 1,4-bis(4-pyridyl)-2,3-diaza-1,3-butadiene (4-bpdb) or 2,5-bis(4-pyridyl)-3,4-diaza-2,4-hexadiene (4-bpdh), respectively, for 15 min. The structures obtained possess different structural topologies, metal-ligand connectivities and therefore different pore sizes. Furthermore, the pore surface in both networks were capable to capture CO2, achieving 60 cm3

Chen et al. [93] synthesized [In2(OH)2(BPTC)]•6H2O (InOF-1) by mechanochemical route from indium acetate hexahydrate In(OAc)3•6H2O and organic linker 3,3′,5,5′-biphenyltetracarboxylic acid (H4bptc), which were filled in a stainless steel milling jar for different times (10–60 min). InOF-1 showed moderate adsorption capacity for CO2 and high for CO2/CH4 and CO2/N2 adsorption selectivities.

In the sonochemical technique, the MOF synthesis occurs due to extremely high temperature (≈ 4000 K) and pressure (≈ 1000 atm) in microenvironment formed by acoustic cavitation generated by ultrasound, which starts the chemical bonds breakage of the elements in the solution and allows interaction between metal salts and organic linkers [94, 95]. This is an environment friendly method to produce homogeneous nucleation centers in a short time and with low energy consumption [79]. Zn(II)-based metal-organic framework [Zn(TDC)(4-BPMH)]n•n(H2O) was produced by [96] from 2,5-thiophene dicarboxylic acid (TDC) linker, N,Nbis-pyridin-4-ylmethylene-hydrazine (4-BPMH) as pillar spacer and zinc nitrate hexahydrate [Zn(NO3)2•6H2O] as metal precursor. The MOFs nanoparticles were sonochemically synthesized under atmospheric pressure using an ultrasonic bath with different irradiation powers, irradiation time, precursor concentrations and temperatures. Adsorption capacities of 2,4-dichlorophenol and amoxicillin were evaluated in aqueous media and their related results demonstrated an efficiency nearly 90% for both cases. Sonochemical method was employed by [97] to synthesize [Cd(oba)(4-bpdh)]n•1DMF (TMU-7) using the ligand 4,4-oxybisbenzoic acid (H2oba), the N-donor ligand 2,5-bis(4-pyridyl)-3,4-diaza-2,4-hexadiene (4-bpdh) and the metal precursor zinc acetate tetrahydrate [Zn(OAc)2•4H2O]. Synthesis of TMU-7 was carried out in an ultrasonic bath at ambient temperature and atmospheric pressure with different reaction times and concentrations of metal and ligands to evaluate the morphology. The results showed that the Congo red dye was efficiently removed when this nanostructures were used, achieving an adsorption approximately equal 97%. Abdollahi et al. [98] produced Zn4(oba)3(DMF)2 from Zn(NO3)2•6H2O and H2oba dispersed in DMF. MOFs syntheses were carried out in an ultrasonic bath at ambient temperature and atmospheric pressure for different reaction times and concentrations of initial precursors. In comparison of samples synthesized by solvothermal and sonochemical methods, the nanostructures synthesized by ultrasound have been more efficient to remove Congo red and Sudan red dyes, with an adsorption efficiency approximately 53 and 87%, respectively. Hydro/solvothermal synthesis is the most classical route to produce metalorganic frameworks nanoparticles. This method involves the heterogeneous reaction between organic linkers and metal salts that are dissolved in water or organic solvents (e.g. alcohols and pyridine) under moderate to high temperatures

. Framework Zn2(oba)2(4-bpdb)•(DMF)x (TMU-4) and

•g1 .

#### *Photocatalytic Adsorbents Nanoparticles DOI: http://dx.doi.org/10.5772/intechopen.79954*

*Advanced Sorption Process Applications*

Zn; and Cu-BTC, based on Cu [69].

frameworks are MIL (Materials of Institute Lavoisier), based on lanthanides or transition metals; UiO (University of Oslo), built up with Zr; MOF-5, composed of

These structures have been applied to adsorption of organic and inorganic contaminants in water and wastewater with better efficiencies when compared with conventional adsorbents, probably due to large surface area (1000–10,000 m2

the linker molecules, which leads to the formation of charge carriers (e<sup>−</sup>/h+

electrochemical, mechanochemical and solvothermal [80–82].

curves analysis demonstrated that up to 400 mg•g<sup>−</sup><sup>1</sup>

tion more than 90% that occurs mainly because of the H+

subsequently be transferred to the surface [60, 68]. Furthermore, the easily tailorable physical and chemical functions, together with the large surface area and permanent pores/channels to potentially anchor/encapsulate photosensitizers and catalytic moieties, make MOFs potential candidates for application in photocatalytic processes [78]. The photocatalytic and adsorption features of the metal-organic frameworks are directly related to synthetic routes and parameters such as temperature, reagent concentration, solvent, pH and pressure [79]. Many techniques have been studied to produce distinct structures with specific properties, such as hydro/solvothermal,

Electrochemical method to synthesize MOFs is based on the dissolution of a anodic metal that supply metal ions into a reaction medium that contains the organic linkers and electrolytes, avoiding the use of metal salts. This reaction between metal ions and organic linkers produces structures with high purity, due to the absence of nitrate, perchlorate or chloride presents in metal salts, in a short time when compared to other synthetic methods [83–85]. The electrochemical approach was applied by [86] to produce Cu3(BTC)2 (HKUST-1) for CO2 and CH4 adsorption and separation. Two copper electrodes were used into a solution that contained benzene-1,3,5-tricarboxylate (H3BTC) and tetrabutylammonium tetrafluoroborate (TBAFB) as electrolytes to form the MOF. The characterization confirmed the high textural properties, high crystallinity and good thermal stability of the structure, which showed satisfactory CO2 adsorption. A microseparator device containing a metal-organic framework synthesized by electrochemical method was produced by [87]. HKUST-1 was prepared by copper electrodes, benzene-1,3,5-tricarboxylate (BTC) linker and methyl-tributyl-ammonium methyl sulfate (MTMS) electrolyte and was applied to the separation of methanol and n-hexane. The breakthrough

to the electrochemically synthesized Cu-BTC coating. Yang et al. [88] synthesized MOF-5 using zinc anodes, terephthalic acid (H2BDC) linker, zinc nitrate hexahydrate [Zn(NO3)2•6H2O] electrolyte and 1-butyl-3-methylimidazolium chloride ionic liquid, which was employed to a template to induce the porous structure. This structure was mixed with BiOBr by ultrasound treatment to create a composite that was evaluated for methyl orange dye degradation by photocatalysis under simulated solar light irradiation. The results showed that this new composite can achieve degrada-

presence of central metal ions, open metal sites, coordinatively unsaturated sites and functional groups on the organic linkers; and easily tunable structure and flexible framework, since it is possible to modify the pores surface [66, 70–72]. The adsorption mechanisms for removal of organic pollutants by MOFs in water mainly include electrostatic, hydrophobic, acid-base π-π interactions and hydrogen bonding [73–75]. MOFs photocatalytic properties are exploited due to high capacity of the organic linkers in absorbing photons, such as antennas to harvest light, which transfer the energy to the metal sites by transition from ligand to metal cluster charge under UV or visible light radiation [76, 77]. Therefore, these structures can act as semiconductor, since MOFs contain conduction and valence bands, with band gaps in the range of 1.0–5.5 eV, or the band gaps are related to the energy levels of the highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs) of

•g<sup>−</sup><sup>1</sup> );

) that can

of methanol can be adsorbed

<sup>−</sup> active species.

and •O2

**68**

Mechanochemical method is a solvent free methodology to produce MOFs that employ a direct mechanical grinding of the metal salts and linker precursors either in a mortar or a ball mill [89]. This technique can occur at room temperature in short reaction times [90]. The compound MOF-14 [Cu3(BTB)2] was synthesized by [91] using ball milling to copper acetate monohydrate (metal salt) and H3BTB (organic linker) mixture, which were placed in a ball mill with three balls and grinded together for 10 min and later activated by a single post-synthetic washing step with ethanol. This structures showed high micropore volume and a specific surface area near 1200 m2 •g<sup>−</sup><sup>1</sup> . Framework Zn2(oba)2(4-bpdb)•(DMF)x (TMU-4) and Zn2(oba)2(4-bpdh)•(DMF)y (TMU-5) were prepared by mechanochemical by [92] from a mixture of 4,4′-oxybisbenzoic acid (H2oba) and 1,4-bis(4-pyridyl)-2,3-diaza-1,3-butadiene (4-bpdb) or 2,5-bis(4-pyridyl)-3,4-diaza-2,4-hexadiene (4-bpdh), respectively, for 15 min. The structures obtained possess different structural topologies, metal-ligand connectivities and therefore different pore sizes. Furthermore, the pore surface in both networks were capable to capture CO2, achieving 60 cm3 •g1 . Chen et al. [93] synthesized [In2(OH)2(BPTC)]•6H2O (InOF-1) by mechanochemical route from indium acetate hexahydrate In(OAc)3•6H2O and organic linker 3,3′,5,5′-biphenyltetracarboxylic acid (H4bptc), which were filled in a stainless steel milling jar for different times (10–60 min). InOF-1 showed moderate adsorption capacity for CO2 and high for CO2/CH4 and CO2/N2 adsorption selectivities.

In the sonochemical technique, the MOF synthesis occurs due to extremely high temperature (≈ 4000 K) and pressure (≈ 1000 atm) in microenvironment formed by acoustic cavitation generated by ultrasound, which starts the chemical bonds breakage of the elements in the solution and allows interaction between metal salts and organic linkers [94, 95]. This is an environment friendly method to produce homogeneous nucleation centers in a short time and with low energy consumption [79]. Zn(II)-based metal-organic framework [Zn(TDC)(4-BPMH)]n•n(H2O) was produced by [96] from 2,5-thiophene dicarboxylic acid (TDC) linker, N,Nbis-pyridin-4-ylmethylene-hydrazine (4-BPMH) as pillar spacer and zinc nitrate hexahydrate [Zn(NO3)2•6H2O] as metal precursor. The MOFs nanoparticles were sonochemically synthesized under atmospheric pressure using an ultrasonic bath with different irradiation powers, irradiation time, precursor concentrations and temperatures. Adsorption capacities of 2,4-dichlorophenol and amoxicillin were evaluated in aqueous media and their related results demonstrated an efficiency nearly 90% for both cases. Sonochemical method was employed by [97] to synthesize [Cd(oba)(4-bpdh)]n•1DMF (TMU-7) using the ligand 4,4-oxybisbenzoic acid (H2oba), the N-donor ligand 2,5-bis(4-pyridyl)-3,4-diaza-2,4-hexadiene (4-bpdh) and the metal precursor zinc acetate tetrahydrate [Zn(OAc)2•4H2O]. Synthesis of TMU-7 was carried out in an ultrasonic bath at ambient temperature and atmospheric pressure with different reaction times and concentrations of metal and ligands to evaluate the morphology. The results showed that the Congo red dye was efficiently removed when this nanostructures were used, achieving an adsorption approximately equal 97%. Abdollahi et al. [98] produced Zn4(oba)3(DMF)2 from Zn(NO3)2•6H2O and H2oba dispersed in DMF. MOFs syntheses were carried out in an ultrasonic bath at ambient temperature and atmospheric pressure for different reaction times and concentrations of initial precursors. In comparison of samples synthesized by solvothermal and sonochemical methods, the nanostructures synthesized by ultrasound have been more efficient to remove Congo red and Sudan red dyes, with an adsorption efficiency approximately 53 and 87%, respectively.

Hydro/solvothermal synthesis is the most classical route to produce metalorganic frameworks nanoparticles. This method involves the heterogeneous reaction between organic linkers and metal salts that are dissolved in water or organic solvents (e.g. alcohols and pyridine) under moderate to high temperatures and pressures [84]. This reaction occurs in sealed reactor vessels (autoclave) under conditions above the boiling point of the solvent and usually takes place over days or hours [99, 100]. The frameworks synthesized by hydro/solvothermal synthesis are nanoparticles with high crystallinity and good size control, which are insoluble in the solvent [70]. Mn (II) ions based metal-organic framework was synthesized by solvothermal method described in [101]. A solution of 1,3,5-tris(4-carboxyphenyl) benzene acid (H3BTB) as linker, manganese (II) acetate [Mn(OAc)2] as metal salt, imidazole and d L-N-tert-butoxycarbonyl-2-(imidazole)-1-pyrrolidine (L-BCIP) as chiral adduct were sealed in a Teflon stainless steel vessel at 110°C for 3 days. The adsorption rate for methylene blue was nearly 90% after 120 min, when the MOF was used. The zinc-based MOF ([Zn5(FODC)2(OCH2CH2O)3(H2O)]•(sol)n) was synthesized by solvothermal method [102] and evaluated for adsorption and selective separation of methylene blue (MB), crystal violet (CV) and rhodamine B (RhB). Fluorenone-2,7-dicarboxylate (H2FODC) ligand and Zn(OAc)2•4H2O were mixed and transferred to stainless steel reactor with a Teflon stainless steel autoclave, sealed and heated up to 110°C for 3 days. The structures produced showed high affinity to cationic dyes, with removal rates of MB, CV and RhB on the Zn-MOF equal to 98.44, 90.77 and 41.99%, respectively. The photocatalytic response of [Ni(azp)(ppa)(H2O)2]n metal-organic framework was studied by [103]. This structure was produced from the salt metal nickel nitrate hexahydrate ([Ni(NO3)2]•6H2O), the O-donor ligand 1,4-phenylenedipropionic acid (PPA) and four N,N′-donors ligands [4,4′-azodipyridine (AZP); 4,4′-trimethylenedipyridine (TMDP); 1,2-bis-(4-pyridyl)ethane (BPETHA) and 4,4′-bipyridine (BPY)]. The solutions were individually placed in a Teflon-lined stainless steel heated at 80°C for 10 h and then continuously heated at 120°C for 24 h. The new structures formed displayed diverse structural architectures due to a variety of the length and flexibility of N,N′-donors ligands and the varied coordination modes of PPA. The band gaps of the MOFs were between 3.3 and 3.7 eV, with a photocatalytic degradation of MB roughly in the range of 65–97%. Gao et al. [104] evaluated the photocatalytic degradation of Acid Orange 7 dye in aqueous solution over MIL-53(Fe) under visible LED light irradiation and in the presence of persulfate oxidant. The framework was synthesized by solvothermal technique from a mixture of FeCl3•6H2O, 1,4-benzenedicarboxylic acid (H2BDC) organic linker and DMF. The reactant mixture was transferred into a Teflon-lined stainless steel autoclave and heated at 150°C for 15 h. MIL-53(Fe) band gap was determined equal to 2.62 eV and the photocatalytic degradation response demonstrated that this structure can degrade Acid Orange 7 via the direct hole oxidation pathway under visible light, achieving a removal efficiency upper 90% after 90 min.

Nevertheless, such as above mentioned, for the removal and degradation processes is interesting the evaluation of adsorption and photocatalytic effects coupled. In this context, Gao et al. [105] evaluated the adsorption and visible light photodegradation of aqueous clofibric acid (CA) and carbamazepine (CBZ) by MIL-53(Fe) metal-organic framework prepared by solvothermal method. MOF synthesis was performed from a mixture of FeCl3•6H2O, terephthalic acid and DMF that was introduced in a Teflon-lined steel autoclave and maintained at 120°C for 3 days. The adsorption and photocatalytic experiments evaluated the pH effect and the related results suggested that the adsorption of CA and CBZ were mainly ascribed to electrostatic interactions and π–π interactions, respectively. In pH equal 3.0, the maximum adsorption capacity of CA and CBZ on MIL-53(Fe) were 0.80 and 0.57 mmol•g<sup>−</sup><sup>1</sup> , respectively, and the photodegradation efficiency for CA and CBZ was greater than 90%, when H2O2 was used as oxidant.

Araya et al. [106] synthesized the FeBTC MOF modified with Amberlite IRA-200 resin to yield a novel heterogeneous photocatalyst, A@FeBTC, to degrade

**71**

*Photocatalytic Adsorbents Nanoparticles DOI: http://dx.doi.org/10.5772/intechopen.79954*

catalyst dosage equal 0.5 g•l

tion responses upper 95% for A@FeBTC and FeBTC.

−1

only by adsorption removal and 96.6% after photocalysis process.

separated electrons and holes in the MIL-100(Fe)@MIL-53(Fe).

**4. α-Fe2O3@TiO2, MOF@TiO2 and others TiO2 nanocomposites**

TiO2 application as a photocatalyst is of great interest because this compound is nontoxic, economically viable, chemically inert, photostable to corrosion, besides having high thermal stability and intense photocatalytic activity and oxidation power [22, 109]. The main crystalline structures of titanium dioxide are anatase, rutile and brookite, but the last one is difficult to be synthesized in laboratory [110]. However, not all crystalline structures have the same efficiency in the absorption of light for catalysis, and rutile, although the polymorph thermodynamically more stable has reduced photocatalytic activity in comparison to anatase [111]. This occurs possibly due to the high temperature required for its preparation, resulting in an increase in the particle size, lower electron mobility in relation to anatase

Rhodamine B dye. The iron-based framework was synthesized using the hydrothermal method from H3BTC, iron powder, hydrofluoric acid, nitric acid and water. The reaction mixture was transferred to a Teflon-lined pressure vessel and maintained at 160°C for 12 h. A@FeBTC catalyst was prepared by grinding and stirring the powdered resin in an aqueous suspension of FeBTC for 24 h, with different resin/ FeBTC mass ratio. All samples (with or without Amberlit resin) were evaluated to dye removal and showed adsorption efficiency greater than 20% and photodegrada-

Simultaneously efficient adsorption and photocatalytic degradation of tetracycline by Fe-MIL-101, Fe-MIL-100 and Fe-MIL-53 was studied by [107]. MOFs were synthesized by a hydrothermal method from FeCl3•6H2O and H2BDC dissolved in DMF. The solution was sonicated, transferred to a Teflon-lined stainless steel autoclave and maintained at 110°C in an oven for 20 h for Fe-MIL-101 and maintained at 150°C for 12 h for Fe-MIL-53. In the synthesis of Fe-MIL-100, a mixture of FeCl3•6H2O, H2BDC and hydrofluoric acid was dissolved in DMF and magnetically stirred. The solution was heated at 160°C for 12 h into a Teflon-lined stainless steel autoclave. The effects of adding dosage and initial concentration of tetracycline on degradation efficiency were examined and the results revealed that a Fe-MIL-101

was efficiently adsorbed and degraded by Fe-MIL-101, reaching approximately 55%

Abdpour et al. [108] evaluated the MIL-100(Fe)@MIL-53(Fe) photocatalytic

performance of methyl orange degradation under visible light. Firstly, MIL-100(Fe) was synthesized by solvothermal method from H2BTC and FeCl3•6H2O dissolved in distilled water in the presence of HNO3 and HF. The resulted solution was stirred, transferred to the autoclave and heated at 150°C for 20 h. Then, different amounts of MIL-100(Fe) nanoparticles were dispersed in DMF by ultrasonication. FeCl3.6H2O and H2BDC were added in this solution, which was stirred and placed in the ultrasonic generator probe for 15 min at 50% of the maximum power. The sonochemically synthesized the metal-organic framework was denominated MIL-100(Fe)@MIL-53(Fe). The band gaps energies calculated for the samples were around 2.5 eV. The results showed that all samples reached adsorption efficiencies near 10–15%, whereas the samples that contained 0.03 and 0.04 g of MIL-100(Fe) achieved the highest photocatalytic degradation (≈70%). The reduction of the photogenerated electron-hole pairs recombination probably occurs due to the decrease in photoluminescence intensity because excited electron transfer from the conduction band of MIL-100(Fe) to MIL-53(Fe) and hole transfer from the valance band of MIL-53(Fe) to MIL-100(Fe), which prolong the lifetime of the

showed the best photocatalytic efficiency. Tetracycline

#### *Photocatalytic Adsorbents Nanoparticles DOI: http://dx.doi.org/10.5772/intechopen.79954*

*Advanced Sorption Process Applications*

and pressures [84]. This reaction occurs in sealed reactor vessels (autoclave) under conditions above the boiling point of the solvent and usually takes place over days or hours [99, 100]. The frameworks synthesized by hydro/solvothermal synthesis are nanoparticles with high crystallinity and good size control, which are insoluble in the solvent [70]. Mn (II) ions based metal-organic framework was synthesized by solvothermal method described in [101]. A solution of 1,3,5-tris(4-carboxyphenyl) benzene acid (H3BTB) as linker, manganese (II) acetate [Mn(OAc)2] as metal salt, imidazole and d L-N-tert-butoxycarbonyl-2-(imidazole)-1-pyrrolidine (L-BCIP) as chiral adduct were sealed in a Teflon stainless steel vessel at 110°C for 3 days. The adsorption rate for methylene blue was nearly 90% after 120 min, when the MOF was used. The zinc-based MOF ([Zn5(FODC)2(OCH2CH2O)3(H2O)]•(sol)n) was synthesized by solvothermal method [102] and evaluated for adsorption and selective separation of methylene blue (MB), crystal violet (CV) and rhodamine B (RhB). Fluorenone-2,7-dicarboxylate (H2FODC) ligand and Zn(OAc)2•4H2O were mixed and transferred to stainless steel reactor with a Teflon stainless steel autoclave, sealed and heated up to 110°C for 3 days. The structures produced showed high affinity to cationic dyes, with removal rates of MB, CV and RhB on the Zn-MOF equal to 98.44, 90.77 and 41.99%, respectively. The photocatalytic response of [Ni(azp)(ppa)(H2O)2]n metal-organic framework was studied by [103]. This structure was produced from the salt metal nickel nitrate hexahydrate ([Ni(NO3)2]•6H2O), the O-donor ligand 1,4-phenylenedipropionic acid (PPA) and four N,N′-donors ligands [4,4′-azodipyridine (AZP); 4,4′-trimethylenedipyridine (TMDP); 1,2-bis-(4-pyridyl)ethane (BPETHA) and 4,4′-bipyridine (BPY)]. The solutions were individually placed in a Teflon-lined stainless steel heated at 80°C for 10 h and then continuously heated at 120°C for 24 h. The new structures formed displayed diverse structural architectures due to a variety of the length and flexibility of N,N′-donors ligands and the varied coordination modes of PPA. The band gaps of the MOFs were between 3.3 and 3.7 eV, with a photocatalytic degradation of MB roughly in the range of 65–97%. Gao et al. [104] evaluated the photocatalytic degradation of Acid Orange 7 dye in aqueous solution over MIL-53(Fe) under visible LED light irradiation and in the presence of persulfate oxidant. The framework was synthesized by solvothermal technique from a mixture of FeCl3•6H2O, 1,4-benzenedicarboxylic acid (H2BDC) organic linker and DMF. The reactant mixture was transferred into a Teflon-lined stainless steel autoclave and heated at 150°C for 15 h. MIL-53(Fe) band gap was determined equal to 2.62 eV and the photocatalytic degradation response demonstrated that this structure can degrade Acid Orange 7 via the direct hole oxidation pathway under visible light, achieving a removal efficiency

Nevertheless, such as above mentioned, for the removal and degradation processes is interesting the evaluation of adsorption and photocatalytic effects coupled. In this context, Gao et al. [105] evaluated the adsorption and visible light photodegradation of aqueous clofibric acid (CA) and carbamazepine (CBZ) by MIL-53(Fe) metal-organic framework prepared by solvothermal method. MOF synthesis was performed from a mixture of FeCl3•6H2O, terephthalic acid and DMF that was introduced in a Teflon-lined steel autoclave and maintained at 120°C for 3 days. The adsorption and photocatalytic experiments evaluated the pH effect and the related results suggested that the adsorption of CA and CBZ were mainly ascribed to electrostatic interactions and π–π interactions, respectively. In pH equal 3.0, the maximum adsorption capacity of CA and CBZ on MIL-53(Fe) were 0.80

CBZ was greater than 90%, when H2O2 was used as oxidant.

, respectively, and the photodegradation efficiency for CA and

Araya et al. [106] synthesized the FeBTC MOF modified with Amberlite IRA-200 resin to yield a novel heterogeneous photocatalyst, A@FeBTC, to degrade

**70**

upper 90% after 90 min.

and 0.57 mmol•g<sup>−</sup><sup>1</sup>

Rhodamine B dye. The iron-based framework was synthesized using the hydrothermal method from H3BTC, iron powder, hydrofluoric acid, nitric acid and water. The reaction mixture was transferred to a Teflon-lined pressure vessel and maintained at 160°C for 12 h. A@FeBTC catalyst was prepared by grinding and stirring the powdered resin in an aqueous suspension of FeBTC for 24 h, with different resin/ FeBTC mass ratio. All samples (with or without Amberlit resin) were evaluated to dye removal and showed adsorption efficiency greater than 20% and photodegradation responses upper 95% for A@FeBTC and FeBTC.

Simultaneously efficient adsorption and photocatalytic degradation of tetracycline by Fe-MIL-101, Fe-MIL-100 and Fe-MIL-53 was studied by [107]. MOFs were synthesized by a hydrothermal method from FeCl3•6H2O and H2BDC dissolved in DMF. The solution was sonicated, transferred to a Teflon-lined stainless steel autoclave and maintained at 110°C in an oven for 20 h for Fe-MIL-101 and maintained at 150°C for 12 h for Fe-MIL-53. In the synthesis of Fe-MIL-100, a mixture of FeCl3•6H2O, H2BDC and hydrofluoric acid was dissolved in DMF and magnetically stirred. The solution was heated at 160°C for 12 h into a Teflon-lined stainless steel autoclave. The effects of adding dosage and initial concentration of tetracycline on degradation efficiency were examined and the results revealed that a Fe-MIL-101 catalyst dosage equal 0.5 g•l −1 showed the best photocatalytic efficiency. Tetracycline was efficiently adsorbed and degraded by Fe-MIL-101, reaching approximately 55% only by adsorption removal and 96.6% after photocalysis process.

Abdpour et al. [108] evaluated the MIL-100(Fe)@MIL-53(Fe) photocatalytic performance of methyl orange degradation under visible light. Firstly, MIL-100(Fe) was synthesized by solvothermal method from H2BTC and FeCl3•6H2O dissolved in distilled water in the presence of HNO3 and HF. The resulted solution was stirred, transferred to the autoclave and heated at 150°C for 20 h. Then, different amounts of MIL-100(Fe) nanoparticles were dispersed in DMF by ultrasonication. FeCl3.6H2O and H2BDC were added in this solution, which was stirred and placed in the ultrasonic generator probe for 15 min at 50% of the maximum power. The sonochemically synthesized the metal-organic framework was denominated MIL-100(Fe)@MIL-53(Fe). The band gaps energies calculated for the samples were around 2.5 eV. The results showed that all samples reached adsorption efficiencies near 10–15%, whereas the samples that contained 0.03 and 0.04 g of MIL-100(Fe) achieved the highest photocatalytic degradation (≈70%). The reduction of the photogenerated electron-hole pairs recombination probably occurs due to the decrease in photoluminescence intensity because excited electron transfer from the conduction band of MIL-100(Fe) to MIL-53(Fe) and hole transfer from the valance band of MIL-53(Fe) to MIL-100(Fe), which prolong the lifetime of the separated electrons and holes in the MIL-100(Fe)@MIL-53(Fe).
