A MoO3âMetalâOrganic Framework Composite as a Simultaneous

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MoO3-Metal-organic Framework Composite as Simultaneous Photocatalyst and Catalyst in PODS Process of Light Oil Minoo Bagheri, Mohammad Yaser Masoomi, and Ali Morsali ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b02581 • Publication Date (Web): 30 Aug 2017 Downloaded from http://pubs.acs.org on August 30, 2017

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ACS Catalysis

MoO3-Metal-organic Framework Composite as Simultaneous Photocatalyst and Catalyst in PODS Process of Light Oil Minoo Bagheri‡, Mohammad Yaser Masoomi‡ and Ali Morsali* Department of Chemistry, Faculty of Sciences, Tarbiat Modares University, P.O. Box 1411713116, Tehran, Iran.

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Abstract: Photo-oxidative desulfurization (PODS) properties of MoO3-metal-organic framework composite photocatalysts were investigated by introducing the proper wt% MoO3 into a Zn(II) based

MOF,

[Zn(oba)(4-bpdh)0.5]n·1.5DMF

(TMU-5),

for

the

mineralization

of

dibenzothiophene from model oil. The addition of 3 wt % MoO3 into TMU-5 host acting as the crystal growth inhibitor was confirmed by PXRD and BET results. For the first time, under mild and green reaction conditions, 5 wt % MoO3-TMU-5 composite (MT-5) exhibited good photocatalytic activity in the model oil PODS reaction, which has no limitations in the current oxidative desulfurization catalytic systems . Only 3% of the total amount of MoO3 content in the MT catalyst is leached during the reaction. Also, the rate of PODS of MT-5 obeys the pseudofirst order equation with an apparent rate constant of 0.0305 min-1 and half-life t1/2 of 22.7 min. The radical scavengers experiments and terephthalic acid fluorescence technique confirmed that OH. and O2-.are the main reactive species in the dibenzothiophene photocatalytic degradation. The synergic effects of the active surface of TMU-5 (organic linkers as antenna) together with the active sites of MoO3, may lead to the further enhancement of the PODS activity of the MT-5 photocatalyst. Moreover, a possible photocatalytic desulfurization mechanism was proposed in the presence of MoO3-TMU-5 composites. Keywords: MOFs; Photocatalyst; Desulfurization; Light oil; MoO3


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ACS Catalysis

INTRODUCTION Petroleum, as a major source of world energy supply, consists of various sulfur compounds in four forms including mercaptans, sulfides, disulfides and thiophenes. The sulfur oxides, emitted from the combustion of these compounds, eventually cause acid rains, and tend to deactivate some catalysts. Therefore, the removal of the sulfur element from crude oil is essential from the point of view of air quality control in the industrial applications.1-2 Different techniques are applied to the desulfurization of crude oil such as hydrodesulfurization (HDS), Oxidative desulfurization (ODS), oxidation–extraction desulfurization (OEDS), adsorptive desulfurization (ADS) and bio-desulfurization (BDS).3-8 HDS is a common catalytic process in industry which needs high temperature and pressure. However, this method is not able to remove heterocyclic sulfur compounds (dibenzothiophene (DBT) and its derivatives).9-11 Other methods (e.g. ODS, OEDS, etc.) are applied for the deep desulfurization.12-13 In such methods, sulfur compounds can be oxidized by an organic or inorganic oxidant such as tert-butyl-hydroperoxide, peroxy salts, nitrogen dioxides, ozone, and hydrogen peroxide at low temperatures and atmospheric pressure. Moreover, a second step for the separation of the products is necessary in the above-mentioned techniques.14-16 Among these desulfurization approaches, photo-oxidation desulfurization (PODS) is a green method for removal of heterocyclic sulfur compounds from the crude oil. Instead of using organic or inorganic oxidants, molecular oxygen in the PODS process leads to a more practical and beneficial potential as for environmental and economical considerations. Moreover, PODS can be applied not only to the conversion of the original sulfur compounds into oxidative derivatives, but also to the degradation and mineralization of these products in an ambient


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temperature and atmospheric pressure. The decomposed products could be transferred into aqueous phase as SO42- ions.17 On the other hand, metal–organic frameworks (MOFs), as one class of the emerging porous inorganic-organic hybrid materials, became more and more popular due to their designability of incorporating the rigidity of inorganic secondary building units (SBUs) with the ﬂexibility and tunability of organic linkers.18-21 The combination of both inorganic and organic chemistry in MOFs have made these materials ideal candidate for gas storage/separation, ion exchange and sensing, drug delivery, catalysis and photocatalysis.22-32 Besides, some reports are available so far regarding to the usage of MOFs in desulfurization process.33-36 Todays, various heterogeneous catalysts have been recognized for ODS reactions,37-40 while there are only a few reports on PODS using TiO2,41 mesoporous graphitic carbon nitride42 and mixed-phase Fe2O3.43 To the best of our knowledge, no study on the usage of MOFs as a photocatalyst in PODS process was reported. Herein, we employed the synergic effects of the active surface of MOF (organic linkers as antenna) together with active center of MoO3 catalyst to enhance the desulfurization efficiency of PODS under UV and visible light irradiation conditions. This study highlights the potential of using MOFs as a simultaneous photocatalyst and catalyst in the oxidation of the sulfur compound from light oil.


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EXPERIMENTAL SECTION Materials and Physical Techniques. All reagents for the synthesis and analysis were commercially available from Aldrich and Merck Company and used as received. DBT, n-octane and methanol were purchased from Merck Company. Apparatus. Melting points were measured on an Electrothermal 9100 apparatus. IR spectra were recorded using Thermo Nicolet IR 100 FT-IR. Thermal behavior was measured with a PL-STA 1500 apparatus with the rate of 10 ºC.min-1 in a static atmosphere of nitrogen. X-ray powder diffraction (XRD) measurements were performed using a Philips X’pert diffractometer with monochromated Cu-Kα radiation. The ultraviolet-visible diffuse reflectance spectra were recorded on Lambda 1050 using BaSO4 as reference. The samples were characterized with a field emission scanning electron microscope (FE-SEM) TESCAN MIRA (Czech). HRTEM (high-resolution transmission electron microscopy) analysis was performed on a JEOL 2100 electron microscope at an operating voltage of 200 kV. A sorption study was performed using the TriStar II 3020 surface area analyzer from Micrometrics Instrument Corporation: N2 at 77K. Synthesis of compounds Synthesis of TMU-5 TMU-5, as a host, was synthesized according to a recipe reported in our previous study44-45 by mixing Zn(OAc)2.2H2O (0.3 mmol), H2oba (0.5 mmol) and 4-bpdh (0.5 mmol) in 30 mL DMF under ultrasonic irradiation for 60 min. The yellow powder was collected by centrifugation, washed three times with 3 mL DMF and dried at 80 °C for 24 h. Yield: 0.220 g (81 % based on oba). Elemental analysis (%) calculated for [Zn(C14O5H8)(C14H14N4)0.5]·(C3NOH7)1.5: C: 55.6, H: 4.7, N: 8.9; Found: C: 54.8, H: 4.2, N: 8.8.


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Synthesis of MoO3 In a typical experiment, 2 g of (NH4)6Mo7O24.4H2O was dissolved in 25 mL double-distilled water under vigorous stirring for 10 minutes at room temperature (RT) and then the pH of the solution was adjusted to (approximately) 8 by addition of HNO3 (67%). While stirring at 60 °C for 6 h, the metal hydroxides precipitated out of the solution. After cooling the solution to RT, the precipitate was collected by centrifugation. The light blue precipitate was washed several times with double-distilled water and ethanol, and subsequently dried in an oven at 60 °C under vacuum for 3 h. The sample was finally calcined at 500 °C for 2 h in air. Synthesis of MoO3-TMU-5 composites Typically, an appropriate amount of MoO3 (m: the mass required to achieve 0, 3, 5, 8 and 10 wt% of MoO3) was dispersed in 30 mL DMF under sonication for 5 minutes at room temperature (RT). Then a mixture of Zn(OAc)2.2H2O (0.6 mmol), H2oba (1 mmol) and 4-bpdh (1 mmol) were added while sonicated for 60 min. The bright/dark yellow precipitate was collected by centrifugation. The precipitate was washed four times with DMF, and subsequently dried in an oven at 80 °C for 24 h. All the samples were finally activated at 120 °C for 72 h under vacuum. The samples were denoted by MT-x, where “x” stands for percentage of MoO3. MT-3: Yield: 0.45 g (81 % based on oba). Elemental analysis (%) calculated for [Zn(C14O5H8)(C14H14N4)0.5]·(C3NOH7)1.5(MoO3)0.12: C, 54.0; H, 4.5; N, 8.6; Found: C, 53.6; H, 4.2; N: 8.5. MT-5: Yield: 0.48 g (83 % based on oba). Elemental analysis (%) calculated for [Zn(C14O5H8)(C14H14N4)0.5]·(C3NOH7)1.5(MoO3)0.2: C, 52.9; H, 4.4; N, 8.5; Found: C, 51.8; H, 4.2; N: 8.4.


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MT-8: Yield: 0.49 g (80 % based on oba). Elemental analysis (%) calculated for [Zn(C14O5H8)(C14H14N4)0.5]·(C3NOH7)1.5(MoO3)0.33: C, 51.2; H, 4.3; N, 8.2; Found: C, 50.4; H, 4.0; N: 7.9. MT-10: Yield: 0.50 g (79 % based on oba). Elemental analysis (%) calculated for [Zn(C14O5H8)(C14H14N4)0.5]·(C3NOH7)1.5(MoO3)0.42: C, 50.1; H, 4.2; N, 8.0; Found: C, 49.8; H, 4.0; N: 7.8. Evaluation of PODS efficiency PODS activity of samples was evaluated on DBT photo-decomposition as sulfur source in model oil. For this purpose, a solution of 521 ppm sulfur obtained from DBT in 50 ml n-octane was prepared in a cylindrical quartz UV reactor. Then, 0.025 g photocatalyst and 0.3 mL of ndodecane as internal standard were mixed with 50 mL of water in the vessel (volumetric ratio of organic to aqueous phases; 1: 1). The mixture was stirred vigorously at a constant speed (500 rpm) by a magnetic agitator at room temperature (RT) and subsequently stirred in the dark for 20-45 min (depending on the type of samples and their darkness time found based on the absorption experiments), to establish the adsorption/desorption equilibrium on the surface of photocatalyst before UV and / or visible light irradiation. The irradiation was done with a UV (30W, UV-C, λ = 253.7 nm, 4.89 eV, Philips (The Netherlands) and visible (300 W Xe lamp (PLS-SXE300, Beijing Changtuo Co., Ltd., China)) light. The reaction temperature was kept constant at 20 °C using the circulation of tap water in the jacket of the UV reactor. Perpendicular UV irradiation was applied when the distance between the UV source and the reaction mixture was almost 15 cm (Figure S1). The mixed solution was photo-irradiated for a certain time with air bubbling flow at about 150 ml min-1. Samples for analyses were taken from the oil phase at specified reaction times and immediately centrifuged at 6000 rpm for 10 min to remove the particles and were further analyzed by gas chromatography coupled with a flame ionization detector (GC-FID (hp)) to determine their sulfur content. The data gathered from the experiments were used to calculate the conversion percentage and the adsorption capacity of DBT according to the batch equilibrium method as follows:


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% desulfurization = (C0-Ct/C0 )×100

(1)

qe=(C0-Ce)V/M

(2)

where C0 and Ce are the initial concentration and concentration of DBT at any given time, qe is the equilibrium amount of DBT converted per unit mass of adsorbent (mg.g-1), V is the volume of the initial solution (ml) and M is the mass of the photocatalyst in gram. The recycled photocatalyst was filtrated, washed with methanol, dried at 110 ° for 5h and reused in the same way for five times. For determination of PODS mechanism, controlled experiments were performed similar to the above PODS reaction except that the radical scavengers (2 mM) were added to the reaction system: KI as scavenger for photo-generated holes, AgNO3¬ as scavenger for electrons, tert-butyl alcohol (t-BuOH) as scavenger for hydroxyl radicals, and benzoquinone (BQ) as scavenger for superoxide radical species.46-47 The chemical oxygen demand (COD) test is extensively employed as an effective technique to measure the organic strength of wastewater. This test allows measurement of waste in terms of the total quantity of oxygen required for oxidation of organic matter to CO2, water and mineral compounds. Here, the open reflux method was used for COD determination.48 Preparation of samples for ICP A powder sample of MoO3-TMU-5 composites (5 mg) was digested with NaOH (2.5 mL, 2 M) and the resulting clear solution was diluted to 100 mL and adjusted to a pH of 7 with HCl. The concentration of Mo was determined by ICP. A simultaneous inductively coupled plasma-optical emission spectrometry (ICP-OES, Varian Vista-PRO, Springvale, Australia) with a radial torch coupled to a concentric nebulizer and Scott spray chamber and equipped with a charge-coupled detector (CCD) was used for ICP measurements.


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RESULTS AND DISCUSSION Characterization TMU-5 can be obtained fast and easily via sonochemical method on a large scale at low cost.45, 49 This MOF structure is constructed based on Zn2(-COO)4 paddlewheel SBU which contains threedimensional azine-functionalized pores with an aperture size of 4.4 × 6.2 Å and 24.1% void space (nitrogen probe) per unit cell (Figure 1). TMU-5 shows structural stability in water and typical organic solvents after 24 h confirmed by the XRD patterns (Figure S2). Composites of MoO3-TMU-5 were synthesized by adding appropriate amounts of MoO3 during the sonochemical synthesis of TMU-5 procedure. FE-SEM images show that all composites have plate-like morphology (Figure S3). Also, the data of element mapping shows that the uploaded MoO3 distributes homogeneously in all composites (Figure S4). The uploaded amount of MoO3, as determined by inductively coupled plasma (ICP) measurements, are 2.8, 4.7, 7.6 and 9.5 wt % for MT-3, MT-5, MT-8 and MT-10, respectively. The percentages of MoO3 were also confirmed by energy-dispersive X-ray spectrum (EDS) and thermogravimetric analyses (TGA) measurements and are in good agreement with the nominal composition of the samples (Figure S5 and S6). The simulated (derived from the single crystal structure of TMU-5) and experimental powder Xray diﬀraction (PXRD) patterns of all TMU-5 composites are coincident (Figure 2), indicating that the structure of TMU-5 remains intact after adding MoO3. Specifically, in the XRD pattern of the MT-3 sample, no peak can be assigned to the known phase of MoO3, probably because of small amounts of MoO3 and/or the small crystallite size, which is not detectable in XRD


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technique. Upon adding greater amounts of MoO3 to the MOF host, the height of the reflexes at 2θ = 12.8° (020), 23.3° (110), 25.8° (040) and 27.5° (021) increase (Figure 2).

Figure 1. Single crystal X-ray structure of TMU-5 (top); and of the azine functionalized pores; the interaction between the framework and DBT highlighted (bottom). Color code: O: red; N: blue; C: black; and Zn: green polyhedra.


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ACS Catalysis

Figure 2. Comparison of PXRD patterns for TMU-5, its composites and MoO3. High-resolution transmission electron microscopy (HRTEM) was used to further investigate the structure of MoO3-TMU-5 composites (Figure 3). The images show that MoO3 particle sizes range from 15 to 20 nm. The HRTEM image of a MoO3 demonstrates a lattice distance of about 0.35 nm, corresponding to the (040) plane of MoO3 (inset in Figure 3b). Considering that MoO3 nanoparticles are larger than the pore size of TMU-5, the MoO3 nanoparticles might not be located inside the MOF pores.


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Figure 3. (a) TEM images of the MT-5, (b) magnified TEM image of MT-5, (Inset) HRTEM image of MoO3. The permanent porosity of TMU-5 and its composites were verified by N2 adsorption at 77 K. For all frameworks, isotherms reveal a type-I behavior which is a characteristic of a microporous material (Figure S7 and Table S1). TMU-5 shows a hysteresis type H4 which is often associated with narrow pores in microporous materials.50 The results show that except for MT-3, the surface areas of the samples are decreased upon adding more MoO3 to TMU-5. In the case of MT-3, it is evident that the addition of 3 wt% MoO3 to the host can act as crystal growth inhibitor causing the increased specific surface area. It is also in good agreement with the results of its PXRD and crystallinity (Table S1).51-53 Pure TMU-5 exhibits a strong absorption at approximately 551 nm with an optical band gap at roughly 2.25 eV. This is due to the light absorption caused by the excitation of electrons from the VB to the CB of TMU-5. The optical band gaps of the MT-x composites slightly increase upon adding MoO3 (Figure S8 and Table S1).


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ACS Catalysis

Evaluation of PODS performance Evaluation of adsorption and PODS efficiency over TMU-5 Type of absorbent is one of the crucial factors in photocatalysis reactions. The role of TMU-5 as the host on the adsorption properties and PODS efficiencies is presented in Table 1. These results show that percentage adsorption for TMU-5 decreases with increasing DBT concentration. However, its maximum adsorption capacity increases up to 2000 ppm of DBT and remains constant afterwards (Figure 4a and Table 1). The maximum adsorption capacity in presence of TMU-5 is 101.5 mgS/g MOF. Investigation of DBT adsorption curves show that DBT desorption takes place for 4000 and 5000 ppm of DBT after 60 min (Figures 4b-d).

Figure 4. (a) The adsorption capacity of TMU-5 in presence of different DBT concentrations. (b) The DBT concentration of solution in presence of host TMU-5 for 90 min. (c and d) Effect of the


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contact time on DBT adsorption over TMU-5 at (c) 500-2000 ppm DBT and (b) 3000-5000 ppm DBT at room temperature for 90 min. (madsorbent 0.025 g, Voil= 25ml).

Moreover, the PODS process was evaluated over host TMU-5 in presence of different concentrations of DBT in model oil. The maximum percentage of conversion is observed at about 38.5 % with 3000 ppm of DBT for 60 min (Table 1). It shows that TMU-5 is an acceptable adsorbent, yet this adsorption content is suitable for photo-oxidation process. Further experiments were carried out on 3000 ppm DBT (521 ppm S) as the best concentration in presence of host TMU-5. The molecule size and interaction between adsorbate and adsorbents influence the adsorption capacity. We reasoned that DBT molecules can easily diffuse into the pores of TMU-5, and the effective interaction between the methyl groups of 4-bpdh and the benzene ring of DBT cause the acceptable adsorption capacity over TMU-5 for accomplishment of PODS reaction.34, 54 Table 1. Effect of TMU-5 host on PODS of model oil.

DBT concentration (ppm)

Ads. (%)

Tdark (min)

Qmax (mg DBT/g MOF)

Qmax (mg S/g MOF)

Degradation under UV light (%) 30 min

60 min

500

30

20

150

26.1

13

23

1000

29

25

290

50.4

12

21

2000

29

30

580

100.7

10

23

3000

19.5

30

585

101.6

29

38.5

4000

14.5

45

584

101.4

20

27

5000

11.6

45

585

101.6

12

17.4


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ACS Catalysis

Effects of molybdenum trioxide content on MoO3-TMU-5 composite PODS efficiencies The effect of MoO3 content on photooxidative efficiencies of the nanocomposite photocatalysts, i.e. MT-3, MT-5, MT-8 and MT-10, toward 3000 ppm DBT (521 ppm S) under UV and/or visible light irradiation at room temperature was also evaluated (Table 2 and Figure S9). Except for MT-3 sample, upon adding different wt% of MoO3 to TMU-5 host, a gradual decrease in the adsorption capacity is observed which is probably due to the decreasing accessibility of active surface after MoO3 loading. This observation is also confirmed by BET surface area results. In the absence of photocatalysts, there is negligible change in the DBT concentration for 90 min under UV light irradiation. The PODS efficiencies in presence of MT-x photocatalysts are improved compare to TMU-5 host. Also, the PODS rate of DBT in model oil increases rapidly and reaches the maximum when the MoO3 content increases from 3 to 5 (Table 1 and Figure S9a) under UV light irradiation at a shorter time, whereas a gradual decrease is observed afterwards. This regularity is observed in a longer timeline under visible light irradiation (Figure S9b). The highest photooxidation of DBT is 95.6 % and 71.3 % for 60 and 90 min in the presence of MT-5 photocatalyst under UV and visible light irradiation, respectively (Table 1). Therefore, the variation of the PODs activity should be related to the content and dispersion of MoO3 on the host surface. A decrease in PODS rate with 8 % and 10 % MoO3 contents is attributed to the decreasing accessibility of active surface, probably due to the higher amounts of MoO3 in two weight percentages.55 Comparison between PODS efficiencies by MoO3-TMU-5 composite and other compounds clearly reveals its superiority and advantages of using MoO3-MOF composite over other reported


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compounds so far (Table 3). MT-5 photocatalyst shows the highest efficiency of 96% in presence of higher sulfur concentration of 521 ppm and shorter PODS reaction time of 60 min. Crystallite size of MoO3 was calculated from XRD pattern at 2θ = 27.5° for bulk MoO3 and its composites. The data show that upon decreasing MoO3 crystallite size PODS efficiencies have been increased (Table 2). Also, conversion of DBT did not proceed in presence of catalytic amount (25 mg) of MoO3 bulk in the PODS reaction condition.

Table 2. The PODS properties of different photocatalysts in presence of 3000 ppm DBT under UV and/or visible light.* TDegradation (min)

Adsorption (%)

Tadsorption (min)

Photocatalyst

UV*

Degradation (%)**

Visible*

UV

Visible

Crystallite size of MoO3 (nm)

Bulk MoO3

-

0

-

-

0

0

105

MT-3

30

27

90

120

44.8

31.7

17.9

MT-5

30

14.3

60

90

95.6

71.3

9.7

MT-8

30

9.5

90

120

60.5

41.6

12.4

MT-10

30

5.6

120

120

44

33.8

13.2

Reaction condition: mphotocatalyst 0.025 g, Voil= 25ml, Temperature = RT *The time after the adsorption time (Tadsorption) as initial time (T0). ** Degradation (%) = Conversion (%) – Adsorption (%)


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ACS Catalysis

Table 3. Comparison of PODS efficiency of DBT in various compounds.

Compounds 5 wt% MoO3-TMU-5 Mixed phase Fe2O3 (63.4 % α/36.6 % β Fe2O3) Graphitic carbon nitride TiO2/hectorite thin film TiO2 TiO2

Sulfur concentration (ppm) 521

PODS efficiency (%) 95.6

PODC Time (min)

Ref.

60

This work

478

92.3

90

43

100 37.1 0.2 92.1

100 86 98 40

90 480 30 600

42 56 41 57

Leaching of MoO3 from MT-5 photocatalyst framework To understand the heterogeneity of MT-5 photocatalyst system, a filtration experiment was carried out under UV light irradiation (Figure S10). After 15 min UV irradiation, the MT-5 photocatalyst was filtered which resulted in the complete shutdown of the reaction. The leaching of molybdenum in water was calculated, ca. 1 ppm (3 wt% of MoO3), by ICP at the end of the reaction. Based on the obtained data, the possible contribution of the leached molybdenum to the PODS efficiency was investigated by the same amount of MoO3 (ca 0.0375 mg) under the same reaction conditions. There is no change in conversion of DBT, indicating molybdenum ions have no contribution to the PODS of DBT. Thus, the systems are totally heterogeneous in nature under the reaction conditions. PODS mechanism Scavengers for determination of active species In a photocatalytic process, the adsorbed photons with appropriate energy produce hole-electron pairs in the valance (VB) and conduction bands (CB). The positive hole could either oxidize the adsorbed organic contaminants directly, or produce very reactive hydroxyl radicals (OH˙). The electron in the CB reduces the adsorbed oxygen to create superoxide anion (O. ) on photocatalyst as the main and active oxidizing species.


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Here, presence of .OH radicals was detected by terephthalic acid fluorescence approach.58 In presence of MT-5 photocatalyst, the fluorescence intensity gradually increased over time reaction indicating attendance of the active species of .OH radicals (Figure 5a). To further verify the effective role of h+, O2-., .OH and electron in PODS process, related scavengers including KI, BQ, t-BuOH and AgNO3 are investigated (Figure 5b). The addition of hole scavenger (KI) shows negligible effect on PODS efficiencies. Thus in this study, h+ species are not responsible for oxidation in PODS. Conversely, the PODS efficiencies suffer a significant reduction in presence of BQ (O2-. scavenger) and t-BuOH (.OH scavenger) from 95.6 % to 23 % and 54.5 %, respectively. Also, adding AgNO3 as electron scavenger reduces PODS efficiency in presence of MT-5. Electron quenching by addition of AgNO3 scavenger will hinder the activation of molecular oxygen for the formation of superoxide radicals by molecular oxygen.46, 58 This possibility was investigated by introducing N2 instead of air to the reactor for 60 min. The PODS efficiency decreased up to 12.5 %, indicating the need for O2 in the desulfurization process. These observations confirmed that .OH and O2-. are the main oxidative species in PODS process.


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ACS Catalysis

Figure. 5. (a) OH radical trapping fluorescence spectra on terephthalic acid solution with MT-5. (Reaction conditions: Vterephthalic acid =100 mL, [terephthalic acid] = 2 mM, MT-5 amount =0.025 g, T= RT). (b) Effects of radical scavengers on the PODS efficiencies over MT-5 photocatalyst. Possible PODS mechanism over MT-5 photocatalyst As mentioned above, in the case of MT-5 photocatalyst, hydroxyl and superoxide radicals are active species in PODS process which may be formed by UV and / or visible light irradiation according to the Eqs. 5- 9 in aqueous phase.59


ACS Catalysis

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MT-5 MT-5 (e- + h+)

(5)

2 H2O + 4h+ → O2 + 4 H+

(6)

H2O + h+ → OH . + H+

(7)

O2 + e- → O.

(8)

H+ + O. → HO.

(9)

then, the Mo-peroxo species are generated in presence of MoO3 from the reaction of Mo(VI) with hydroxyl and superoxide radicals in a reversible reaction to generate the catalyst: . Mo=O + HO. (OH . or O. ) ⇄ OH + Mo-peroxo

(10)

nucleophilic attack of the active Mo-peroxo species leads to the oxidation of the sulfur atom of DBT in organic phase:60 Mo-peroxo + DBT → Mo=O + DBTO

(11)

other active Mo-peroxo species can oxidize dibenzothiophene sulfoxide to form corresponding sulfone and finally they may convert DBT to mineral compounds (Scheme 1): Mo-peroxo + DBTO → Mo=O + DBTO2

(12)

Mo-peroxo + DBT → Mo=O + CO2 + SO + etc

(13)

In other words, the synergic effects of TMU-5 active surface with corresponding light photon absorption along with MoO3 content up to a certain wt% as active center, may lead to further enhancement of the PODS activity of the MT-5 photocatalyst.


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Scheme 1. Schematic illustration of DBT degradation in PODS process photocatalyzed by MT5. Kinetics of total mineralization of the DBT Kinetics of total mineralization of the DBT has been followed using COD technique with disappearance of chemical oxygen demand for MT-5 photocatalyst in presence of 3000 ppm DBT (Figure S11). The COD value of the initial DBT solution declines after 24 h and remains approximately constant after 72h, indicating the great potential of MT-5 photo-degradation process for removal of DBT. Also, the PODS products in presence of 3000 ppm DBT after 72h were characterized using inspection of pH and addition of KMnO4, BaCl2 as indicators of the


ACS Catalysis

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water layer after reaction. The changes of pH from 6.93 to 3.50 may be related to the pH of saturated H2CO3 at room temperature. On the other hand, KMnO4 did not decay after being added into the water, indicating lack of SO2 as PODS product. Thus, CO2 may be one of the final products. By introducing BaCl2 solution into the lower water layer, a white precipitate (BaSO4) was formed. This phenomenon can confirm that sulfate ion is present in the water. So, MT-5 photocatalyst can convert DBT to mineral compounds during PODS process. Kinetic of PODS process over MT-5 photocatalyst The PODS rate in presence of MT-5 as the best photocatalyst has been investigated based on the adsorption equilibrium capacity using two common semi-empirical kinetic models, pseudo-first order and pseudo-second order as follows:7

log( − ) = log − .

!

=

" # # $

+

"

(3)

(4)

$

where qe and qt (mg/g) are the converted amounts of DBT at equilibrium and time t (min), respectively; k1 (min-1) and k2 (g/mg min) are the pseudo-first and/or second- order kinetic constant. The linear behavior of two plots presented in Figure S12a and b shows that the rate of PODS over MT-5 follows the pseudo-first order equation due to the good correlation obtained from its linear fit. Experimental data of PODS at equilibrium conditions within 60 min are summarized in Table S2. The k is 0.0305 min-1 and half-life t 1/2 is only 22.7 min with 0.025 g of MT-5.


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ACS Catalysis

Turnover number is defined as [(moles of decomposed pollutant) / (initial mole of the catalyst)].61-62 In this work, after 60 min of UV irradiation, 95.6 % of the initial concentration of DBT was degraded by MT-5, corresponding to a turnover number of about 8.2 × 10-3. Recyclability of MT-5 MT-5 photocatalyst was recovered by centrifugation and washed several times with DMF. The recovered photocatalyst was subsequently used in successive runs. The recyclability of MT-5 in presence of 3000 ppm DBT under the same reaction conditions was investigated (Figure S13a). The PODS efficiency of MT-5 remains approximately constant after 3 cycles and it reaches efficiency values of 90 % and 85 % after 4 and 5 runs, respectively. As mentioned above, the results of leaching tests suggest that the MT-5 photocatalyst is a stable photocatalyst in the reaction condition. Therefore, this reduced efficiency is probably related to the poisoning and inactivation of blocked MT-5 surfaces. PXRD pattern for the used MT-5 after 3 cycles suggests that structural integrity of the photocatalyst was mostly maintained with a slight reduction in its diffraction intensity (Figure S13b).63-64 CONCLUSIONS The photocatalytic oxidative desulfurization (PODS) of DBT to mineral compounds under UV and/or visible light irradiation is for the first time realized over a photoactive MoO3-TMU-5 composite, which is fabricated by a facile in situ synthesis method. This study highlights the potential of MOFs as simultaneous photocatalyst and catalyst in the oxidation of sulfur from light oil. The PODS efficiency of DBT over the current 5% MoO3-TMU-5 is excellent (95.6 %) at a short time of about 1h but its recyclability as a long-life photocatalyst is still low. Although, we believe that highly-efficient MOF-based photocatalysts for the PODS of DBT can be obtained via a more principled study of the photocatalyst structure. The present study will open new perspectives for the development of photocatalysts for PODS process.


ACS Catalysis

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ASSOCIATED CONTENT Supporting Information FE-SEM images, Elemental map images, EDS spectra, Thermogravimetric profiles, UV-Vis spectra, N2 isotherm at 77 K, DRS UV-Vis spectra, XRPD patterns and PODS efficiencies are provided in the Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]: (+98) 21-82884416 Author Contributions ‡These authors contributed equally to this work. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT Support of this investigation by Tarbiat Modares University is gratefully acknowledged. References 1. Gary, J. H.; Handwerk, G. E.; Kaiser, M. J., Petroleum refining: technology and economics. CRC press: Boca Raton, 2007; p 488. 2. Annual Energy Review, U.S. Energy Information Administration. U.S. Government Printing Office: Washington, DC, 2011. 3. Houalla, M.; Nag, N.; Sapre, A.; Broderick, D.; Gates, B., AlChE J. 1978, 24, 1015-1021. 4. Masoomi, M. Y.; Bagheri, M.; Morsali, A., Inorg. Chem. 2015, 54, 11269-11275. 5. Mei, H.; Mei, B.; Yen, T. F., Fuel 2003, 82, 405-414. 6. Yu, G.; Lu, S.; Chen, H.; Zhu, Z., Carbon 2005, 43, 2285-2294. 7. Bagheri, M.; Masoomi, M. Y.; Morsali, A., J. Hazard. Mater. 2017, 331, 142-149. 8. Lü, H.; Gao, J.; Jiang, Z.; Jing, F.; Yang, Y.; Wang, G.; Li, C., J Catal 2006, 239, 369-375. 9. Satterfield, C. N., Heterogeneous catalysis in industrial practice. 2nd ed.; New York, NY (United States); McGraw Hill Book Co.: 1991. 10. Kwak, C.; Lee, J. J.; Bae, J. S.; Choi, K.; Moon, S. H., Appl. Catal., A 2000, 200, 233-242.


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11. Hermann, N.; Brorson, M.; Topsøe, H., Catal Lett 2000, 65, 169-174. 12. Zhu, W.; Huang, W.; Li, H.; Zhang, M.; Jiang, W.; Chen, G.; Han, C., Fuel Process. Technol. 2011, 92, 1842-1848. 13. Zhu, W.; Wu, P.; Chao, Y.; Li, H.; Zou, F.; Xun, S.; Zhu, F.; Zhao, Z., Ind. Eng. Chem. Res. 2013, 52, 17399-17406. 14. Stanislaus, A.; Marafi, A.; Rana, M. S., Catal. Today 2010, 153, 1-68. 15. Babich, I.; Moulijn, J., Fuel 2003, 82, 607-631. 16. Abotsi, G. M.; Scaroni, A. W., Fuel Process. Technol. 1989, 22, 107-133. 17. Srivastava, V. C., RSC Adv. 2012, 2, 759-783. 18. Falcaro, P.; Ricco, R.; Doherty, C. M.; Liang, K.; Hill, A. J.; Styles, M. J., Chem. Soc. Rev. 2014, 43, 5513-5560. 19. Tranchemontagne, D. J.; Mendoza-Cortes, J. L.; O'Keeffe, M.; Yaghi, O. M., Chem. Soc. Rev. 2009, 38, 1257-1283. 20. Wang, S.; Yao, W.; Lin, J.; Ding, Z.; Wang, X., Angew. Chem. Int. Ed. 2014, 53, 1034-1038. 21. Zhou, T.; Du, Y.; Borgna, A.; Hong, J.; Wang, Y.; Han, J.; Zhang, W.; Xu, R., Energy Environ Sci 2013, 6, 3229-3234. 22. Della Rocca, J.; Liu, D.; Lin, W., Acc. Chem. Res. 2011, 44, 957-968. 23. Wu, X.-Q.; Huang, D.-D.; Zhou, Z.-H.; Dong, W.-W.; Wu, Y.-P.; Zhao, J.; Li, D.-S.; Zhang, Q.; Bu, X., Dalton Trans. 2017, 46, 2430-2438. 24. Wu, Y.-P.; Wu, X.-Q.; Wang, J.-F.; Zhao, J.; Dong, W.-W.; Li, D.-S.; Zhang, Q.-C., Cryst. Growth Des. 2016, 16, 2309-2316. 25. Zhao, J.; Dong, W.-W.; Wu, Y.-P.; Wang, Y.-N.; Wang, C.; Li, D.-S.; Zhang, Q.-C., J. Mater. Chem. A 2015, 3, 6962-6969. 26. Gao, J.; Bai, L.; Zhang, Q.; Li, Y.; Rakesh, G.; Lee, J.-M.; Yang, Y.; Zhang, Q., Dalton Trans. 2014, 43, 2559-2565. 27. Wang, S.; Wang, X., Small 2015, 11, 3097-3112. 28. Lee, Y.; Kim, S.; Kang, J. K.; Cohen, S. M., Chem. Commun. 2015, 51, 5735-5738. 29. Wang, S.; Wang, X., Angew. Chem. Int. Ed. 2016, 55, 2308-2320. 30. Long, J.; Wang, S.; Ding, Z.; Wang, S.; Zhou, Y.; Huang, L.; Wang, X., Chem. Commun. 2012, 48, 11656-11658. 31. Wang, S.; Wang, X., Appl Catal B, Environ 2015, 162, 494-500. 32. Fu, Y.; Sun, D.; Chen, Y.; Huang, R.; Ding, Z.; Fu, X.; Li, Z., Angew. Chem. Int. Ed. 2012, 51, 33643367. 33. Tang, W.; Gu, J.; Huang, H.; Liu, D.; Zhong, C., AlChE J. 2016, 62, 4491-4496. 34. Jia, S.-Y.; Zhang, Y.-F.; Liu, Y.; Qin, F.-X.; Ren, H.-T.; Wu, S.-H., J. Hazard. Mater. 2013, 262, 589597. 35. McNamara, N. D.; Hicks, J. C., ACS Appl. Mater. Interfaces 2015, 7, 5338-5346. 36. McNamara, N. D.; Neumann, G. T.; Masko, E. T.; Urban, J. A.; Hicks, J. C., J Catal 2013, 305, 217226. 37. Samokhvalov, A., ChemPhysChem 2011, 12, 2870-2885. 38. Liu, Y.; Liu, S.; Liu, S.; Liang, D.; Li, S.; Tang, Q.; Wang, X.; Miao, J.; Shi, Z.; Zheng, Z., Chemcatchem 2013, 5, 3086-3091. 39. Shiraishi, Y.; Taki, Y.; Hirai, T.; Komasawa, I., Chem. Commun. 1998, 2601-2602. 40. Zhang, M.; Zhu, W.; Xun, S.; Li, H.; Gu, Q.; Zhao, Z.; Wang, Q., Chem Eng J 2013, 220, 328-336. 41. Vargas, R.; Núñez, O., J. Mol. Catal. A: Chem. 2008, 294, 74-81. 42. Zhu, Y.; Li, X.; Zhu, M., Catal. Commun. 2016, 85, 5-8. 43. Li, F.-t.; Liu, Y.; Sun, Z.-m.; Zhao, Y.; Liu, R.-h.; Chen, L.-j.; Zhao, D.-s., Catal. Sci. Tech. 2012, 2, 1455-1462.


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Table of contents


ACS Catalysis

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A MoO3âMetalâOrganic Framework Composite as a Simultaneous

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