Iron doped zeolitic imidazolate framework (Fe-ZIF-8): synthesis and photocatalytic degradation of RDB dye in Fe-ZIF-8 Mai Thi Thanh, Tran Vinh Thien, Pham Dinh Du, Nguyen Phi Hung & Dinh Quang Khieu Journal of Porous Materials ISSN 1380-2224 J Porous Mater DOI 10.1007/s10934-017-0498-7 Your article is protected by copyright and all rights are held exclusively by Springer Science+Business Media, LLC This e-offprint is for personal use only and shall not be selfarchived in electronic repositories If you wish to self-archive your article, please use the accepted manuscript version for posting on your own website You may further deposit the accepted manuscript version in any repository, provided it is only made publicly available 12 months after official publication or later and provided acknowledgement is given to the original source of publication and a link is inserted to the published article on Springer's website The link must be accompanied by the following text: "The final publication is available at link.springer.com” J Porous Mater DOI 10.1007/s10934-017-0498-7 Author's personal copy Iron doped zeolitic imidazolate framework (FeZIF-8): synthesis and photocatalytic degradation of RDB dye in Fe-ZIF-8 1,2 Mai Thi Thanh · Tran Vinh Thien3 · Pham Dinh Du4 · Nguyen Phi Hung5 · Dinh Quang Khieu1 © Springer Science+Business Media, LLC 2017 Abstract This paper presents a study on the synthesis of iron doped ZIF-8 with different molar ratio of Zn/ Fe (Fe-ZIF-8) and sunlight driven photocatalytic activity of obtained materials The materials were characteristic of X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), nitrogen adsorption/desorption isotherms, diffusive reflectance UV–Vis (DR-UV–Vis) and atomic absorption spectroscopy (AAS) The results showed that Fe(II) as iron source could be directly introduced into ZIF-8 to form Fe- ZIF-8 Depending on the amount of iron(II) introduced, the Fe(II) or both Fe(II) and Fe(III) may exist in ZIF-8 Fe- ZIF-8 was selected as photocatalyst to decompose Remazol deep black B (RDB), a model of dye contaminant, under sunlight illumination Undoped ZIF-8 seems not to catalyze for degradation of RDB while Fe-ZIF-8 exhibited sunlight- driven photocatalytic degradation of RDB The kinetics of photocatalytic reaction were also addressed This study sug- gests iron doped zeolite-imidazole framework Fe-ZIF-8 to be promising catalyst for the heterogeneous photo-catalytic dye degradation technique in visible region * Dinh Quang Khieu dqkhieu@hueuni.e du.vn College of Science, Hue University, Hue 530000, Vietnam Faculty of Physics-Chemistry-Biology, Quang Nam University, Tam Kỳ 560000, Vietnam Faculty of Natural Science, Phu Yen University, Tuy Hòa 620000, Vietnam Faculty of Natural Science, Thu Dau Mot University, Thủ Dầu Một 590000, Vietnam Department of Chemistry, Qui Nhon University, Qui Nhon 820000, Vietnam Keywords Remazol deep black B · Fe-ZIF-8 · ZIF-8 · Photocatalytic degradation · Sunlight Introduction Metal–organic frameworks (MOFs), as a new class of porous solids, have received considerable attention because of their potential applications in gas adsorption and separa- tion processes [1, 2], catalysis [3–5], templating [6] Zeolite imidazolate frameworks (ZIFs), a sub-class of MOFs, the compose of transition metal ions [Zn(II), Co(II)] and imidazolate linkers which form 3D tetrahedral frameworks They have recently attracted growing interest due to their unique and highly desirable properties such as high surface areas, crystallinity as well as thermal and chemical stabilities [7, 8] Among of them, ZIF8 [Zn(2-methylimidazole)2·2H2O] is constructed from 2-methylimidazole ligands and Zn(II) center ions, which exhibits higher thermal and chemical sta- bility than other MOFs [9, 10] It exhibits a sodalite topol- ogy with internal cavities which are accessible through small apertures with a diameter of 0.34 nm [10] The heterostruc- tures integration of ZIF-8 can be achieved by deposition of metal (oxide) into pores, and coating ZIF-8 as a shell on metal (oxide) as well Generally, metals or oxide incorpo- rated ZIF-8 have been synthesized by a indirect method Zou et al [11] reported the synthesis of Fe3O4@ZIF-8 for the determination of inorganic arsenic by hydride generationatomic fluorescence spectrometry Jiang et al [12] presented Fe3O4 incorporated ZIF-8 nanocrystals with high adsorption capacity towards hydroquinone in which magnetic Fe3O4 nanoparticles were prepared by adding surfactant-capped Fe3O4 nanoparticles into the ZIF-8 Recent studies suggested that MOFs were semiconductors or photocatalysts The photo-catalysis of MOFs are 13 considered to be achieved by the electron transfer from the photoexcited organic ligands to metallic clusters within MOFs, which is termed as ligand to cluster charge transfer (LCCT) [13–15] Photocatalytic activity of ZIF-8 and its modified ZIF-8 has been recently explored ZIF-8 photocatalyst showed excellent photocatalytic activity for methylene blue (MB) degradation under UV irradia- tion, which was evidenced by the detection of hydroxyl radicals by a fluorescence method [12 ] ZnO@zeolitic imidazole frameworks-8 (ZnO@ZIF-8) showed molecule size selectivity and good photocatalytic activity for MB under UV irradiation [16] Commonly, MOFs exhibit very large band gaps due to the ultraviolet absorptive behavior Therefore, developing the band gap engineering of these catalysts into the visible-light active area is of interest and importance for their further sun lightdriven photo- catalytic applications To the best of our knowledge, so far photocatalytic performance of Fe doped ZIF-8 under sun light has not been reported Remazol black B (RDB) is a common diazo reactive dye, in aqueous solution and used widely in textile indus- tries [17 ] It is stable and hardly biological degradation due to the presence of aromatic ring Thus, much research attention has been paid to investigate to eliminate RDB from aqueous solution [18, 19 ] In this work, we demonstrate a facile strategy to enhance the photocatalytic activity of ZIF-8 by in situ incorporating iron in its structural compositions (Fe- ZIF-8) The catalytic activity of obtained ZIF-8 and Fe- ZIF8 were tested by photocatalytic degradation of RDB under sun light illumination Experimental 2.1 Materials Zinc nitrate hexahydrate (Zn(NO ) ·6H O, Daejung, Korea, ≥99%), iron(II) sulfate heptahydrate (FeSO4·7H2O, Merck, Germany >99%), methanol (CH3OH, Merck, Ger- many) and 2-methylimidazole (C4 H6N 2, Aldrich, USA 99%) were utilized in this paper Remadazol deep black B (denoted as RDB) (C 26 H 21 N Na O 19 S , Molecular Weight = 991.82) was obtained from Thuy Duong textile Company (Vietnam) The structure of RDB is shown in Scheme 2.2 Preparation of ZIF-8 and iron doped ZIF-8 (Fe-ZIF-8) Fe-ZIF-8 was synthesized as procedure of ZIF-8 [7, 20, 21] Briefly, 2.8 mmol of zinc(II) and iron(II) [Fe(II)/Zn(II) = 0/2.8; 0.28/2.52; 0.56/2.24; 0.84/1.96; 1.12/1.68 (mmol)] were dissolved in 1.4 mmol of methanol A solution consisting of 64.4 mmol of 2-methylimidazole and 1400 mmol of metha- nol was added to the Zn–Fe based solution and vigorously stirred for 24 hs at ambient temperature Nitrogen was bubbled through the solution to minimize the oxidation reaction of Fe(II) to Fe(III) species Finally, the solid was collected by centrifugation at 300 rpm, and washed thoroughly with methanol This washing procedure was repeated three times The resultant crystals were dried overnight at 120 °C The obtained Fe-ZIF-8 with the initial molar ratios of Fe/Zn being 0/10, 1/9, 2/8, 3/7 and 4/6, were named ZIF-8(0:10), ZIF8(1:9), ZIF- 8(2:8), ZIF-8(3:7), and ZIF-8(4:6) respectively The obtained sample possessed while color for ZIF-8 [e.g ZIF-8(0:10)] and light brown color for Fe-ZIF-8 2.3 Photocatalytic degradation of RDB The photocatalytic activity of the ZIF-8 and FeZIF-8 was tested in the decomposition of RDB at ambient temperature In order to distinguish the adsorption and photocatalytic degrada- tion of obtained catalysts both experiments of dark adsorption and photocatalytic degradation reaction were conducted Adsorption studies for the removal of RDB dye from aque- ous solutions were carried out under the dark For these experi- ments, 0.0753 g of ZIF-8 or Fe-ZIF-8 was placed in a 500 cm beaker containing−1 300 cm3 of dye solutions (30–40 mg L ), which were stirred magnetically at 30 °C mL samples were withdrawn at different time intervals and determined the RDB concentration using a Perkin–Elmer UV–vis spectrophotom- eter corresponding to λmax of dye = 600 nm For photocatalytic test, prepare 500 mL beaker with 0.075 g of catalyst and 300−1cm3 of RDB solution (10, 20, 30, 40 mg L ) Then, the mixture solution was stirred magnetically and illuminated under natural sunlight (temperature around 30–33 °C, UV index: 12; brightness (L): 167.107 cd m−2; illuminance (E): 100,000 lux [Digital lux meter (19,990 Lux)-Manufacturer)] Four milliliters of sam- ples were withdrawn at different time intervals to determine the RDB concentration All experiments were performed in the same time, place and experimental setup The catalytic activity of Fe-ZIF-8 under UV irradiation and visible light OHNH2 Scheme Molecular structure of RDB NaO3SOCH2 CH2O2S N N N N SO2CH2CH2OS O3Na Na O3 S SO Na J Porous Mater Author's personal copy (500–720 nm) was also performed using lamp TN-BOsRam (28 W, λ = 356 nm), GPH1554T5L/4P (254 nm; 75 W) Halogen lamp Philips ML (λ = 500–720 nm, 160 w) for the sake of comparison 50 cp s 2.4 Characterization Int en sit y (ar b.) The powder X-ray diffraction (XRD) patterns were recorded by a D8 Advance, Bruker— Germany with CuKα radiation (λ = 1.5406 Å) Elemental analysis of Fe and Zn were performed by atomic adsorption spectroscopy (AAS) using AA6800 (Shimadzu) Nitrogen adsorption/desorption isotherm measurements were conducted using a Micromerit- ics ASAP 2020 Samples were pretreated by heating under vacuum at 150 °C for h before the measurements Dif- fusive reflectance UV–Vis spectroscopy (DR-UV–Vis) was conducted in JASCO-V670 at 200–800 nm X-ray photo- electron spectroscopy (XPS) was performed by Shimadzu Kratos AXISULTRA DLD spectrometer Peak fitting was conducted by CASA XPS software (Scheme 2) J Porous Mater Fe-ZIF-8(4:6) (11 1) ( 002) ( 114) ( 112) Fe -ZIF-8(2:8) Fe-ZIF-8(1:9) ZIF-8 10 15 20 25 30 theta (degree) Fig XRD patterns of ZIF-8 and Fe-ZIF-8 exist in amorphous form or replaced partly with Zn(II) in ZIF-8 framework Based on ZIF-8 with space group of I4̄ 3m [23] the cell parameter of ZIF-8 and Fe-ZIF-8 was expressed by 3R e s ul ts a n d di s c u s si o n 13 Fe- ZIF-8(3:7) ( 233) Figure shows XRD patterns of ZIF-8 and Fe-ZIF-8 The XRD patterns of ZIF-8 in this work were agreed well with references [4, 21, 22] There was a welldefined diffraction (111) at two theta = 7.16° in the XRD pattern of ZIF-8, indicating that the crystallinity of ZIF-8 in this work was relatively high The XRD patterns of FeZIF-8 exhibited characteristic peaks of ZIF-8 and no characteristic peaks of iron oxide were observed However, the intensity of these diffractions decreased with an increase in the amount of iron incorporated The 1= d characteristic diffractions of ZIF-8 were not observed as the molar ratio of Fe/Zn reached 4/6 indicating that the large amount of incorporated iron oxide caused the collapse of ZIF-8 framework Iron oxides may 13 J Porous Mater where d is spacing distance, a cell parameter and h, k, l Miller index of diffraction plane The calculated cell parameters changed insignificantly as shown in Table indicating that the iron incorporation effected negligibly on dspacing of ZIF-8 framework The zinc and iron compositions were analyzed by AAS and XPS The results S c h e m e Author's personal copy are presented in Table The molar ratio of Fe/Zn in synthetic gel and molar ratio of Fe/Zn in product are very different and the higher initial molar ratio the higher the final molar ratio For example, in Fe- ZIF8(3:7) sample, initial molar ratio of Fe/Zn was 3/7 or 0.429 then final molar ratio of Fe/Zn increased to 0.804 a l y s t u n d e r s u n l i g h t i l l u m i n a t i o n A p r o p o s e d m o d e l o f p h o t o c a t a l y t i c r e a c t i o n m e c h a n i s m o f R D B o n F e Z I F c a t 13 Table Cell parameters of ZIF-8 and Fe-ZIF-8 Notation ZIF-8 Fe-ZIF-8(1:9) Fe-ZIF-8(2:8) Fe-ZIF-8(3:7) a (Ǻ) 16.800 16.977 16.978 16.820 Although nitrogen was bubbled through the solution a part of amount of Fe(II) was reduced to Fe(III) species during synthetic process The much difference could be attributed to by two reasons: (a) Since the greater the amount of iron was introduced the higher the acidity of synthetic gel, then Zn(II) tended to dissolve in liquid phase Meanwhile iron, mainly Fe(III) will precipitate the solid phases due to −17 low solubility [Ksp at 25 °C of Zn(OH)2 = 5.10 ; −15 −38 Fe(OH)2 = 2.10 ; Fe(OH)3 = × 10 ]; (b) as the high amount of iron was introduced the iron competed with Zn in associa- tion with N due to its higher affinity for free electron pairs of nitrogen in imidazole This caused the collapse of ZIF-8 structure as shown in XRD analysis of Fig in resulting Zn(II) species dissolved into liquid phase (electronegativ- ity of Fe = 1.85 and Zn = 1.65) The XPS spectra indicated a chemical state of element, e.g iron (Fe2p), and zinc [Zn(2p)] The peaks of Zn 2p1/2 (1045 eV) and Zn2p3/2 (1021 eV), observed for all of sam- ples confirmed the existence of Zn(II) (Fig 2) For ZIF-8, the peak of Fe 2p3/2 was inconspicuous, indicating that iron was a very minor component Only peak Fe2p3/2 at 710.5 eV was detected implying the main iron in Fe-ZIF-8(1:9) was Fe(II) The peaks of Fe2p3/2 and Fe2p3/2 were detected at 710 and 709 eV (Fig 2) This means that Fe(II) and Fe(III) coexisted in Fe-ZIF-8(2:8) and Fe-ZIF-8(3:7) The propor- tion of iron oxidation state, calculated from peak area was listed in Table It was worth noting that unlike Fe(II) the initial iron source of Fe(III) was also tested to introduce into ZIF-8 but the solid product were not obtained This means that the present of Fe(III) in initial gel synthesized mixture was not favorable for the formation of ZIF-8 structure The iron in ZIF-8(1:9) existed mainly at oxidation state of Fe(II) while that in ZIF-8(2:8) and ZIF-8(3:7) exist mainly at oxidation state of Fe(III) Since 2+ the charge and radii of Zn 2+ (0.75 Å) and Fe (0.74 Å) are similar and the charge and radii of Zn2+ (0.75 Å) and Fe3+ (0.74 Å) are rather different it is likely that Fe(II) can substitute for Zn(II) in ZIF-8 and the most likely place for Fe(III) is to stay in the grain boundary From the XRD and XPS analysis, it is possible that at low iron composition, Fe(II) could isomophously replace com- pletely Zn(II) but as high iron amount was introduced then only a part of iron (II) was introduced into ZIF-8 framework and other part of iron (III) exist as iron oxide (III) that was highly dispersed on ZIF-8 Figure shows the nitrogen adsorption/desorption iso- therms of ZIF-8 and Fe-ZIF-8 All samples exhibited a type IV with H4 which is characteristic for mesoporous materials The Fe-ZIF-8(1:9) possessed the shape slight different from ZIF-8 at high relative pressure This result suggested that the porous structure was distorted due to the incorporation of iron oxides ZIF-8 exhibited2 a high specific surface area of 1484 m /g (calculated by BET model), which was similar with that in the previous literatures [4, 21, 22] (Table 3) The introduction of iron into ZIF-8 lowers the specific surface area, pore diameter, as well as pore volume The specific sur- 2face areas were 1484, 1469, 1104, and 735 m g−1 for ZIF-8, Fe-ZIF-8(1:9), Fe-ZIF-8(2:8) and FeZIF-(3:7), respectively This also proved the encapsulation of iron oxides within the pores of framework, which bring about lowering of acces- sible void space for N2 gas molecules The band gaps of ZIF-8 and Fe-ZIF-8 were investigated by a UV–Vis diffuse reflection measurement at room tem- perature (Fig 4a) The curve of (αE)2 versus E (where α is an absorption coefficient and E is the photon energy) [24] was plotted as shown in Fig 4b The energy band gap of samples was determined by the extrapolation of the straight line down to zero on the x-axis (where E = Eg) and the results are shown in Table ZnO and ZIF-8 show absorption edge around 350–390 nm corresponding to the band gaps of 3.5–3.2 eV, respectively The very high absorption peak around 230 nm involved π → π* transitions in imidazole ring The Eg value of ZIF-8 assessed by absorption edge was 5.2 eV (previous reported is 4.9 eV [25] and 5.16 eV [26]) The weak absorption of ZIF-8 was also observed in visible light at around 590 and 700 nm corresponding to Eg of 2.1 and 1.8 eV, respectively (the inset in Fig 4b) which is possible due to doped centers of nitrogen Eg of Table Chemical compositio n Notation Analyzed by AAS Analyzed by XPS of ZIF-8 and Fe-ZIF-8 analyzed b y A A S a n d X P S Zn (mol g−1) Fe (mol g−1) Molar ratio Initi al mol ar rati o (Fe/ Zn) Fe(II) (%) Fe(III) (%) (Fe/Zn) ZIF-8 Fe-ZIF8(1:9) Fe-ZIF- 0.043 0.038 – 0.005 0.033 0.012 Fig XPS Fe2p and Zn2p core level spectra of ZIF-8 and Fe-ZIF-8 ZIF-8 Fe - ZIF-8(1:9) Fe - ZIF-8(2:8) Fe - ZIF-8(3:7) 800 750 700 Table The band gap energies of ZnO, ZIF-8 and Fe-ZIF8 Notation Eg1 (eV) Eg2 (eV) Eg3 (eV) Eg4 (eV) 400 ZnO ZIF-8 Fe-ZIF-8(1:9) Fe-ZIF-8(2:8) / 5.2 4.7 – 3.2 3.5 – – – 2.1 2.2 2.2 – 1.8 – – 350 Fe-ZIF-8(3:7) – – 2.1 – 650 Ad sor be d (c m g-1 ST P) 600 550 500 450 300 250 200 150 0.0 0.2 0.4 0.6 0.8 Relative pressure(P/P ) 1.0 expected that Fe-doped ZIF-8 can perform photocatalytic activity in visible light The thermal stability of materials was tested by ther- mal gravimetric analysis The ZIF-8, Fe-ZIF-8(1:9), Fe- ZIF-8(2:8) and Fe-ZIF8(3–7) were found to be highly stable Fig Nitrogen up to 220, 250, 289 adsorption/desorption and 325 °C isotherms of ZIF-8 and FeZIF-8 respectively Beyond this temperature the framework slowly started to decompose ZIF-8(1:9) was 4.9 and and a flat valley was 2.2 eV band at 250 obtained till 700 °C and 560 nm Those of (Fig 5) The incorporaFe-ZIF-8(2:8) and Fetion of iron in ZIF-8 ZIF-8(3:7) were 2.2 seems to make the and material more stable 2.1 eV, respectively This phenomenon was Then the Fe doped ZIFalso observed as TiO2 showed a remarkable is doped ZIF-8 [28] absorption band shift The photocatalytic toward the longer activity of ZIF-8 and wave- length region Fe-ZIF-8 was examined The observed decrease by observing the in the band gap of the degradation of RDB iron doped ZIF-8 may with the illumination be attributed to the of natural sunlight as excitation of 3d the energy source To electrons of Fe(III) or confirm Fe(II) to the co photocatalytic activity nduction band level of of materials, both dark ZnO by a charge adsorption and transfer transition photocatalytic [27] These results degradation of RDB were conducted together Figure shows a comparison of kinetics of dark Table Textural properties of ZIF-8 and Fe-ZIF8 ab so rb( %) Notation SBET (m2 g−1) SLangmuir (m2 g−1) ZIF-8 Fe-ZIF-8(1:9) 1484 1469 1909 1599 Fe-ZIF-8(2:8) Fe-ZIF-8(3:7) 2.09 1104 735 0.38 1251 945 0.5 0.4 ( a ) 0.8 0.7 0.6 0.3 0.2 0.1 0.0 -0.1 -0.2 ZIF-8 ( Fe-ZIF-8(1:9) b Fe-ZIF-8(2:8) ) Fe-ZIF-8(3:7) 0 ZnO 0 w a v el e n gt h( 0 n 3m 0 ) Fig 4a DRUV– Vis spect (left) and b Tauc’ s plots (right ) of ZnO, ZIF-8 and FeZIF-8 Author's personal copy J Porous Mater 11 -0.447mg -13.550% 3.5 -0.641mg -6.282% 10 -2.524mg -24.735% -1.649mg -49.985% 3.0 2.5 -4.444mg -43.552% TG A (m g) TG 2.0 A (m 1.5 g) 1.0 Fe-ZIF-8(1:9) 0.5 ZIF-8 200 400 600 800 0.0 200 Temperature( C) -0.452mg -10.834% 4.5 -0.640mg -21.484% 3.0 400 Temperature( C) 4.0 -1.333mg -44.747% 2.5 2.0 600 800 -0.503mg -12.057% -1.497mg -35.882% 3.5 3.0 TG A( 1.5 mg ) TG 2.5 A( mg 2.0 ) 1.5 1.0 1.0 0.5 Fe-ZIF-8(2:8) 0.0 Fe-ZIF-8(3:7) 0.5 0.0 200 400 600 Temperature( C) 800 200 400 600 800 Temperature( C) Fig TGA of ZIF-8 and Fe-ZIF-8 adsorption and photocatalytic reaction in which the sun light was illuminated naturally during 300 The RDB removal increased quickly around early 25 and reached equilibrium at 210 for ZIF-8 RDB removal ability over ZIF-8 in both cases of dark adsorption and photocatalytic degradation was similar After 300 of sun- light illumination, the removal fraction of Ct/Co reduced to around 45% and almost as dark adsorption It means that the RDB removal was only contributed by the adsorption and ZIF-8 did not exhibit photocatalytic activity in the present study A desorption in dark adsorption might be occurring where the RDB concentration appears to fluctu- ate or even reduce a little as shown in Fig This behav- ior could be attributed to either a reversible adsorption [29, 30] For Fe-ZIF-8, the decolorization rate increased as iron amount doped increased to the molar ratio of 1/9 and slightly decreased for further increase in doped iron The fraction (Ct/Co) of photocatalytic decolorization decreased continu- ously and reached around 90% at 300 while those of dark adsorption reached equilibrium at around 40– 50% These observations occurred for all cases of Fe doped ZIF-8 confirming that the introduction of iron into ZIF-8 improved photocatalytic activity under visible light illumination Leaching experiment was also conducted in which Fe- ZIF-8 catalyst was filtered after 60 reaction minutes; the decolorization of dye was stopped despite of still remaining sunlight illumination (Fig 7) The decolorization of RDB without Fe-ZIF-8 was also not observed during sunlight illumination indicating RDB was stable for sunlight Above results had confirmed that Fe-ZIF-8 was a heterogeneous catalyst in the degradation reaction of RDB In addition, the decolorization reaction under UV irradiation (λ = 365 and 254 nm) and visible light (500–720 nm) were also con- ducted The results showed that ZIF-8 and Fe-ZIF-8 did not exhibit photocatalytic activity under UV-irradiation (λ = 356 nm) (not shown in Fig 7) However, the photo- chemical reaction occurred under UV (λ = 254 nm) irra- diation and visible light but with low intensity as shown in Fig The combine of UV and Vis-light in sunlight could explained that fact that the photochemical reaction for RDB 13 Fe-ZIF-8(1:9) ZIF-8 1.0 1.0 -1 Dark adsorptionï, C = 30 mg.L -1 Dark adsorptionï, C = 40 mg.L -1 Sunlight illumination, C = 30 mg.L -1 Sunlight illumination, C = 40 mg.L 0.9 0.8 0.7 C/ C0 0.6 -1 Dark adsorption, C = 30 mg.L -1 Dark adsorption, C = 40 mg.L -1 Sunlight illumination, C = 30 mg.L 0.4 -1 Sunlight illumination, C = 40 mg.L 0.8 C/ C0 0.6 0.5 0.2 0.4 0.0 0.3 50 100 150 200 250 300 Time(minute) 50 100 Fe-ZIF-8(2:8) 1.1 -1 Dark adsorption, C = 30 mg.L -1 Dark adsorption, C = 40 mg.L -1 Sunlight illumination, C = 30 mg.L -1 Sunlight illumination, C = 40 mg.L 0.9 0.8 0.7 C/C 0.6 0.5 200 250 300 Fe-ZIF-8(3:7) 1.0 1.1 1.0 150 Time(minute) 0.4 -1 Dark adsorption, C = 30 mg.L -1 Dark adsorption, C = 40 mg.L -1 Sunlight illumination, C = 30 mg.L -1 Sunlight illumination, C = 40 mg.L 0.9 0.8 0.7 C/C 0.6 0.5 0.4 0.3 0.3 0.2 0.2 0.1 0.1 0.0 0.0 50 100 150 200 250 300 50 100 Time(minute) Fig The comparison of kinetics of dark adsorption and photo- catalytic reaction (dark adsorption: V: 300 mL, initial concentrations: 30 and 40 mg L −1, mass of adsorbent: 0.075 g, temperature: 30 °C, agitation speed: 500 rpm; photocatalytic reaction: V: 300 mL, initial degradation occurred more effectively under sunlight than n dC = − dt 200 250 300 concentrations: 30 and 40 mg L−1, mass of catalyst: 0.075 g, ambient temperature: 30–33 °C, sun light with UV index 12, agitation speed: 500 rpm) Ct = −rin × t + C individual light The photocatalytic degradation rate depends on the ini- tial concentration of the dye [31, 32] The influence of the initial concentration on the photocatalytic degradation rate of RDB over various catalysts was shown in Fig For Fe- ZIF-8(1:9) catalyst the concentrations −1 of RDB at 300 were 0; 0; 4.6 and 6.7 mg L −1 for Co = 10, 20, 30, 40 mg L , respectively while those for Fe-ZIF-8(2:8) were 0; 1.3; 6.5; and 9.2 mg L−1 and Fe-ZIF-8(3:7) was 1.2; 2.9; 10.1 and 15.7 mg L−1 Therefore, the catalytic activity decreased in the order of Fe-ZIF-8(1:9) > Fe-ZIF8(2:8) > Fe-ZIF-8(3:7) The instantaneous reaction rate was calculated from the equation: ri 150 Time(minute) (2 ) (3) where C0 and Ct are the initial concentration and the concen- tration at time t, respectively The initial rate (r0) of a reaction is the instantaneous rate at the start of the reaction (when t = 0) The initial rate is equal to the negative of the slope of the curve of reactant concentration versus time at t = Figure illustrated the plots of Ct against t at C0 (t = 0) and the values of r0 corresponding to each initial concentration C0 was listed in Table Since sunlight intensity changes during illumination time, initial rate method uses only kinetic data at initial short period at which light intensity is considered constant Therefore, this method is used to investigate decolorization kinetics in the present paper [33, 34] The rate equation for decolourization of dye can be writ- ten as: Integrating Eq (2) for the boundary conditions t → then C → C0 gives: r = − dC dt = k.Cn (4) Author's personal copy J Porous Mater J Porous Mater 1.0 1.0 Fe-ZIF-8(1:9) Fe-ZIF-8(2:8) 0.8 illumination without catalyst + UV(254 nm) irradation Fe-ZIF-8(2:8) + visible light illumination Fe-ZIF-8(2:8) + sunlight illumination sunlight illumination without catalyst removing sunlight Fe-ZIF-8(2:8) after 60 minutes Fe-ZIF-8(2:8) removing Fe-ZIF-8(9:1) after 60 minutes Fe-ZIF-8(1:9) + UV(254 nm) irradation 0.6 Fe-ZIF-8(1:9) + visible light illumination Fe-ZIF-8(1:9) + sunlight illumination 0.8 Ct / C0 0.6 0.4 Ct/ C0 0.4 0.2 0.2 0.0 0.0 100 150 200 250 100 300 Time (minute) 5060 5060 150 200 250 300 Time (minute) 1.1 1.0 Fe-ZIF-8(3:7) sunlight illumination without catalyst removing Fe-ZIF-8(3:7) after 60 minutes Fe-ZIF-8(3:7) + UV(254 nm) irradation Fe-ZIF-8(3:7) + visible light illumination Fe-ZIF-8(3:7) + sunlight illumination 0.9 0.8 0.7 Ct/ C0 0.6 0.5 0.4 0.3 0.2 0.1 0.0 5060 100 150 200 250 300 Time (minute) Fig Leaching tests of Fe-ZIF-8 (mass of adsorbent: 0.075 g, agitation speed: 500 rpm; V: 300 mL, initial concentrations: 20 mg L−1) where C is the dye concentration at time, t, the reaction time, and k, the kinetic rate constant, n, the order of the reaction, r, the reaction rates On the other hand, the initial rate for a reaction can be written: Langmuir–Hinshelwood (LH) model [37] is the most commonly used kinetic expression to explain the kinetics of the heterogeneous catalytic processes and is given by: 1r 1 = k r + krK.C 0 r0= k Co (7) (5 where k reaction rate constant (mg L K RDB ) r where ki is the overall observed rate constant for the reac−1 tion, n the order of the reaction with respect to concentra- tion Linearizing Eq (5) by taking natural logarithms on (6 both size, the equation ) yields lnr0 = lnki + nlnC0 Therefore, the linear regression of the lnro against lnCo gives a straight line with slope equals n and the intercept on the ordinate provides lnki (inset of Fig 8) The values of n and k calculated were listed in Table Linear 13 adsorption constant (L mg above −1 s −1 ), and ) and Co and ro are denoted as The linear regression of 1/ro vesus 1/Co will provide the values of kr and K These parameters present at Table The high coefficient of determination and p value