TiO2-Containing Carbon Derived from a MetalâOrganic Framework

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TiO2‑Containing Carbon Derived from a Metal−Organic Framework Composite: A Highly Active Catalyst for Oxidative Desulfurization Biswa Nath Bhadra, Ji Yoon Song, Nazmul Abedin Khan, and Sung Hwa Jhung* Department of Chemistry and Green-Nano Materials Research Center, Kyungpook National University, Daegu 41566, Republic of Korea S Supporting Information *

ABSTRACT: A new metal−organic framework (MOF) composite consisting of Ti- and Zn-based MOFs (ZIF-8(x) @H2N-MIL-125; in brief, ZIF(x)@MOF) was designed and synthesized. The pristine MOF [H2N-MIL-125 (MOF)]- and an MOF-composite [ZIF(30)@MOF]-derived mesoporous carbons consisting of TiO2 nanoparticles were prepared by pyrolysis (named MDC-P and MDC-C, respectively). MDC-C showed a higher surface area, larger pore sizes, and larger mesopore volumes than MDC-P. In addition, the TiO2 nanoparticles on MDC-C have more uniform shapes and sizes and are smaller than those of MDC-P. The obtained MDC-C and MDC-P [together with MOF, ZIF(30)@MOF, pure/nanocrystalline TiO2, and activated carbon] were applied in the oxidative desulfurization reaction of dibenzothiophene in a model fuel. The MDC-C, even with a lower TiO2 content than that of MDC-P, showed an outstanding catalytic performance, especially with a very low catalyst dose (i.e., a very high quantity of dibenzothiophene was converted per unit weight of the catalyst), fast kinetics (∼3 times faster than that for MDC-P), and a low activation energy (lower than that for any reported catalyst) for the oxidation of dibenzothiophene. The large mesopores of MDC-C and the well-dispersed/small TiO2 might be the dominant factors for the superior catalytic conversions. The oxidative desulfurization of other sulfur-containing organic compounds with various electron densities was also studied with MDC-C to understand the mechanism of catalysis. Moreover, the MDC-C catalyst can be reused many times in the oxidative desulfurization reaction after a simple washing with acetone. Finally, composing MOFs and subsequent pyrolysis is suggested as an effective way to prepare a catalyst with well-dispersed active sites, large pores, and high mesoporosity. KEYWORDS: mesoporous carbon, MIL-125, MOF composite, pyrolysis, oxidative desulfurization and limited reduction of the S-content,3,7 alternative methods have been sought. Recently, ODS has drawn significant interest from researchers because of its easy processing and high efficacy.11−19 Usually, it consists of two steps: oxidation of the sulfur-containing heteroaromatics into respective sulfones and sulfoxides with suitable oxidants, followed by the removal of oxidized products using an appropriate extractive solvent. tert-Butyl hydroperoxide,15 cyclohexanone peroxide,16 ozone,17 and H2O2,18,19 etc., are used as the oxidant for the oxidation of sulfur-containing heteroaromatics. Among them, H2O2 is quite cheap, readily available, and ecofriendly; therefore, it has become popular as an oxidant for ODS.14,18,19 Various types of catalysts, such as polyoxometalates and their nanocomposites,20−22 ionic liquid-modified silica,23 WOx-supported ZrO2,24 TiO2/porous glass,19 and titanate nanotubes,25 etc., were developed, and they showed their efficiency in ODS via H2O2 activation. Facile synthesis with a low cost, good performance (for low S-content, fast kinetics, and low

1. INTRODUCTION Noticeable amounts of sulfur-containing organic compounds, such as thiophene (Th), benzothiophene (BT), dibenzothiophenes (DBTs), and dimethyldibenzothiophenes (DMDBTs), are present in fuels. The European Union (E.U.) legislation and U.S. Environmental Protection Agency (EPA) set the maximum level of sulfur in fuels as 10 ppmw (in 2009) and 15 ppmw, respectively.1−3 However, at the beginning of 2017, the Tier 3 program of the U.S. EPA also set 10 ppm as the maximum sulfur content in diesel fuels.4 The above rules are already imposed in some areas of the world including Europe, Japan, and some states of the USA.4 Therefore, reducing the Scontent in commercial fuels is in high demand to prevent air pollution (from SOx and resulting acid rain) and catalyst deactivation.1−3 An array of methods, including hydrodesulfurization (HDS),1−3,5−7 adsorption,7−10 biodesulfurization,1,2,11 and oxidative desulfurization (ODS),11−19 have been developed to minimize the S-content in fuels. HDS has been the most widely applied method for desulfurization in industrial processes during the past several decades.1−3,5,6 However, because of some drawbacks of HDS, such as the high energy/high cost, side reactions, low efficiency in removing aromatic thiophenes, © 2017 American Chemical Society

Received: July 15, 2017 Accepted: August 18, 2017 Published: August 18, 2017 31192

DOI: 10.1021/acsami.7b10336 ACS Appl. Mater. Interfaces 2017, 9, 31192−31202

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ACS Applied Materials & Interfaces activation energy (Ea)), and recyclability are important parameters for an efficient catalyst. Therefore, it is still very fascinating to develop advanced catalysts for the ODS reactions, considering that no commercial ODS process has been reported.14 During the past 2 decades, numerous porous materials with fascinating performances have been developed,26−30 and metal−organic frameworks (MOFs) have become known as promising and unique porous materials because of their tunable textural and chemical functionalities. MOFs are mainly composed of metal clusters and organic moieties; therefore, they are fascinating for various potential applications, including adsorption, separation, and heterogeneous catalysis.31−39 Pristine MOFs or functionalized MOFs (via postsynthetic modification)40 have, for example, acidic and basic moieties. Therefore, the metal centers and basic moieties incorporated into MOFs act as Lewis acidic and basic sites for various acid and base catalyses, respectively.37,38 In addition to pristine MOFs,37,38 MOF-derived nanostructures39,40 are applied in various catalytic reactions. Ti-containing carbon (obtained via simple pyrolysis of Ti-MOF41) and pristine MOFs (based on transition metals, such as V, Co, Ti, or Cr42−44) have shown good catalytic activities, especially in ODS. Very recently, heterogeneities in MOFs have been created by combining different MOFs in a single body. These combinations of MOFs, or composite MOFs,45−48 are expected to generate special/new functions or properties by the formation of new interfaces of two MOFs that may vary from those observed in a single MOF. A few composite MOFs have been developed using different synthetic methods, such as one-pot processes,49 MOF-to-MOF conversion,50 core−shell composition,51,52 and embedding MOFs in another MOF.53 MOFs, including composite MOFs, are also used as precursors to yield various functional carbonaceous materials.39,40,43,51−56 The resulting size-/morphology-controlled hybrid materials include various functionalities, including N-, O-, metal-, metal-oxide-, and metal-carbide-doped moieties. Using these important characteristics, the hybrid materials have been applied in the fields of adsorption,54 electrode materials,53,55 molecular encapsulation,53 gas/energy storage,52,53 and catalysis.51,55,56 Therefore, the design and synthesis of novel composite MOFs with well-defined structures may notably broaden the execution toolbox for the construction of enviable metal-/carbon-based advanced functional materials. In this report, we first designed and synthesized a novel MOF-composite, ZIF-8(x)@H2N-MIL-125 (abbreviated as ZIF(x)@MOF), in which the composition of ZIF-8 (ZIF) was tailored from 10 to 40 wt % (x) of the precursors for pristine MOF or H2N-MIL-125. Both the pristine MOF and MOF-composite (ZIF(30)@MOF; 30% ZIF) were then pyrolyzed to obtain MOF-derived carbons (MDC-P and MDC-C, respectively). The obtained materials were thoroughly characterized and applied for ODS of model fuels to show the remarkable applicability of the newly developed advanced materials. The newly developed MDC-C showed a fascinating performance (e.g., the lowest activation energy, fast kinetics, required a very small amount of the catalyst, and good recyclability) in the ODS of sulfur-containing heteroaromatics via H2O2 activation.

Scheme 1. Schematic Diagrams for the (a) Synthesis of ZIF@MOF Compositesa and (b) Pyrolysis of MOF and ZIF@MOFb

a ZIF, zeolitic imidazolate framework-8; MOF, H2N-MIL-125. bKey: dark yellow, MOF; bright yellow, ZIF@MOF; black, carbon; gray, TiO2; white stars, mesopores; white lines, micropores.

First, ZIF was prepared following a literature method.57 Then, the prepared ZIF crystals (the specific weight percent (10−40) of the total amount of major precursors (aminoterephthalic acid and Tiisopropoxide) for the MOF (H2N-MIL-125)) were added to a solvent mixture of methanol and dimethylformamide (DMF; 1/1, v/v) and dispersed by ultrasonic irradiation for 1 h. To the prepared dispersion, 2-aminoterephthalic acid (12.0 mmol) and titanium isopropoxide (6.0 mmol) were immediately added and stirred vigorously for 30 min in a Teflon liner. The liner containing the slurry was then equipped in a stainless steel autoclave and placed in an electric oven at 150 °C for 16 h. The crystalline products (bright to light yellow) were produced and collected by filtration by washing with sufficient DMF and methanol to remove unreacted precursors and to exchange DMF, respectively. The pristine MOF (H2N-MIL-125)58 and TiO259 nanocrystals were prepared following reported methods. The obtained solids were then dried overnight in an oven at 100 °C and stored. Preparation of MDCs. Pyrolysis of prepared MOF materials (pristine and composite) was conducted52 using a tubular furnace (GSL-1500X-50-UL) equipped with a gas in−out system to yield MDC-P and MDC-C. The temperature of the alumina boat containing the MOF or MOF-composite (1.0 g) was increased to a fixed temperature (1000 °C) with a ramping rate of 5 °C·min−1 with continued heating at that temperature for 6 h under a constant N2 (50 mL·min−1) flow. After the furnace was cooled to room temperature (with a cooling rate of 5 °C·min−1), the products were collected. 2.2. Characterization Methods. The physicochemical properties of the synthesized materials were characterized using various analytical methods. The crystal phases were examined using X-ray diffraction (XRD; D2 Phaser, Bruker) assembled with Cu Kα radiation. The N2adsorption isotherms were measured at −196 °C with a surface area and porosity analyzer (Micromeritics, Tristar II 3020) after evacuation of the samples at 150 °C for 6 h. The surface areas and pore-size distributions were estimated using the Brunauer−Emmett−Teller (BET) (at a relative pressure of 0.05−0.20) and Barrett−Joyner− Halenda (BJH) (from the desorption bands) methods, respectively. The morphologies of the selected materials were observed using field emission−scanning electron microscopy (FE-SEM; SU-8220, Hitachi) and field emission−transmission electron microscopy (FE-TEM; Titan G2ChemiSTEM Cs prove). Raman spectra in the range of 200−3000 cm−1 were recorded at room temperature with a UV micro-Raman spectrometer (Renishaw inVia Reflex; excitation at 514 nm, and laser

2. EXPERIMENTAL SECTION 2.1. Synthesis Methods. Synthesis of ZIF(x)@MOF. Scheme 1a shows the schematic diagram for the synthesis of the MOF-composite. 31193


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Figure 1. (a) XRD patterns, (b) N2-adsorption isotherms, and (c) pore-size distribution curves of MDC-C and MDC-C. The purple line of panel a shows the XRD pattern of the synthesized TiO2 nanoparticle. spot size = 100 μm). The spectrometer was equipped with a Leica DMLM microscope allowing for confocal measurements. X-ray photoelectron spectroscopy (XPS) was conducted using a Quantera SXM X-ray photoelectron spectrometer (ULVAC-PHI) with a dual beam charge neutralizer. Additionally, the chemical compositions (C, O, and N) were evaluated using elemental analysis (Thermo Fisher, Flash-2000 with a thermal conductivity detector), and the Ti- and Zncontents were obtained using inductively coupled plasma (ICP) optical emission spectrometry (Thermo Scientific Co., iCAP 6300 Duo). 2.3. Catalytic ODS Experiments. Model fuel solutions (1,000 ppm) were prepared by dissolving Th, BT, DBT, and DMDBT in noctane in separate volumetric flasks. For the oxidation of sulfurcontaining heteroaromatics, the model fuel (10 mL, 1,000 ppm solution), extractive solvent (acetonitrile, 5 mL), oxidant (30% aqueous H2O2, 0.114 mL; oxidant to sulfur molar ratio (O/S) = 15, unless otherwise stated), and dried catalyst (20 mg) were placed in a 30 mL glass reactor equipped with a condenser. The reaction mixtures were then vigorously stirred at 80 °C (unless otherwise stated) for preset times. After finishing the reaction, the content was cooled to room temperature, and the liquid portion was collected by filtration using a syringe filter (polytetrafluoroethylene), hydrophobic, 0.5 μm). The nonpolar (oil or n-octane) and polar (acetonitrile) phases were separated, and the residual concentration of sulfur-containing heteroaromatics in the oil phase was analyzed using UV absorbance (UV-1800, Shimadzu) at 231.2, 297.2, 325.6, and 325.7 nm for Th, BT, DBT, and DMDBT, respectively. For confirmation of the analysis, gas chromatography (GC) with a flame ionization detector (DS Science, IGC 7200) was also used, and the results agreed with those observed using UV. The product that formed upon oxidation of DBT was extracted into the polar acetonitrile solution and was further confirmed using UV and GC-MS (Agilent, 7890A-5975C GC/MSD). The oxidation of DBT was also performed at two other temperatures (40 and 60 °C) using selected catalysts to investigate the reaction kinetics and other parameters to better understand the performances of the catalysts. Again, the ODS of DBT was carried out with different H2O2 contents or O/S ratios (5−20) to examine the effect of the oxidant amounts. Batch adsorption of DBT was also performed using 20 mg of MDC-P or MDC-C in 10 mL of solution (1000 ppm) at 80 °C for 120 min (here, both H2O2 and acetonitrile were absent) with magnetic stirring. After the set time, the collected solution, obtained by filtration, was analyzed using UV absorbance (at 325.6 nm). The recyclability of the MDC-C catalyst was evaluated after regeneration of the spent catalyst via ultrasonic irradiation (1 h) in acetone (1000 mL·(g of used catalyst)−1) and subsequent evacuation at 100 °C overnight. The regenerated MDC-C was applied in the next cycles after successive reactivations following the same method as that applied to the fresh samples.

3. RESULTS AND DISCUSSION 3.1. Synthesis and Characterization. A composite MOF, ZIF(x)@MOF, was prepared using the “MOF embedded in another MOF” strategy,53 whereby ZIF was used as an implant because of its high thermal/chemical stability, well-defined geometric structures, and porosity.60 During the synthesis of composites, the reactant compositions were only varied with respect to the weight percents of ZIF (0, 10, 20, 30, and 40% of the total amount of major precursors for the MOF; 0% represents pure MOF without ZIF). The successful incorporation of ZIF in MOF can be crudely verified from the physical appearances of the pure and composite samples and their increased weights (depending on the weight percent of ZIF) (Supporting Information Table S1). The color of the light yellow MOF powder faded steadily with increasing contents of white ZIF (Figure S1). Then, the obtained products were analyzed based on their XRD patterns and N2-adsorption isotherms, as shown in Figure S2. The results in Figure S2a reveal the successful synthesis of the MOF and ZIF(x)@MOF, where the XRD patterns of the obtained materials are nicely harmonized with the simulated pattern of pristine MIL-125.58 However, the intensity of the XRD peaks decrease as the weight percent of ZIF increases, which was significantly low for ZIF(40)@MOF. Interestingly, ZIF(x)@MOF did not show any XRD peaks of added ZIF even though the applied ZIF was well-crystallized (data not shown). This may suggest the good dispersion of ZIF (via a dynamic equilibrium61 under the synthesis condition) into the MOF or decrystallization of ZIF upon formation of the composites. The porosity of the obtained materials was analyzed from the corresponding N2-adsorption isotherms (Figure S2b), and the estimated textural properties of MOF and ZIF(x)@MOFs are shown in Table S1. The BET surface areas and total pore volumes of the composite MOFs decrease with increasing amounts of ZIF. Even though the XRD patterns of the composites do not show peaks for ZIF, the nearly steady changes in color (Figure S1), relative weight (increase), and porosity (decrease) (Table S1) might indicate successful formation of the ZIF(x)@MOF composite. Among the prepared composite MOFs, ZIF(40)@MOF showed a relatively poor crystallinity and significantly low surface area (392 m2·g−1); however, ZIF(30)@MOF had a good crystallinity and reasonable surface area (654 m2·g−1) (Figure S2b and Table S1). Therefore, ZIF(30)@MOF and pristine MOF (for comparison) were then further studied and used to yield some functional materials to examine the applicability of the new composite MOFs. So far, the MOFs (H2N-MIL-125 or MIL-125)42,62 and MOF (MIL-125)-derived 31194


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Figure 2. FE-TEM images of (a) MDC-P and (b) MDC-C. The images on the right side are the left images zoomed in twice, and white scale bars (left bars, 20 nm; right bars, 10 nm) are shown on each image.

carbon41 (MDC; microporous or mesoporous) have shown potential functionalities and applicabilities in adsorption and catalysis. High-temperature pyrolysis (at 1000 °C for 6 h) of the MOF and ZIF(30)@MOF yielded Ti-containing carbonaceous materials. The powder XRD patterns of the prepared MDC materials are compared in Figure 1a; none of the materials demonstrate peaks attributed to the parent MOF or ZIF(30)@ MOF. The patterns observed for MDC-C and MDC-P show the rutile-TiO2 phase (JCPDF No. 021-1276) as the major component (at ∼27°) with a small amount of anatase-TiO2 (JCPDF No. 021-1272); the TiO2 nanoparticles prepared following a reported method59 are attributed to the pure anatase−TiO2 phase. Interestingly, the XRD diffractions of MDC-C are broader than those of MDC-P, indicating the presence of smaller TiO2 on MDC-C than that on MDC-P. The average sizes of TiO2 nanoparticles in MDC-P and MDCC, estimated using the Scherrer equation, are ∼20 and ∼8 nm, respectively. Figure 1b shows the N2-adsorption isotherms of MDC-P and MDC-C, and the isotherms were further analyzed using the BET and BJH methods to examine the textural properties. The results are summarized in Table S1 together with those for the MOF materials and activated carbon (AC). Very curiously, MDC-C shows a higher surface area (374 m2·g−1) than that of MDC-P (220 m2·g−1), even from the precursors (ZIF(30)@ MOF) with a lower porosity than that of the pristine-MOF. In addition, the total and mesopore volumes of MDC-C (0.44 and 0.30 cm3·g−1, respectively) are slightly higher than those of MDC-P (0.38 and 0.22 cm3·g−1); however, the micropore volumes are in the opposite order (0.18 and 0.14 cm3·g−1 for MDC-P and MDC-C, respectively) (Table S1). In addition, MDC-P and MDC-C exhibit different pore sizes, as observed

from the BJH pore-size analyses (as shown in Figure 1c). MDC-P contains 2.7 and 3.6 nm pores, whereas MDC-C has 3.1 and 3.9 nm pores. Interestingly, very large pores of ∼5−9 nm are only observed in MDC. Therefore, MDC-C shows a larger surface area, pore volume (more importantly mesopore volume), and wider range of pores than those of MDC-P. Fundamentally, porous materials with larger pores are usually favored in catalyses because the active sites on those materials can be rapidly accessed by any substrate for high reactivity.63 The morphologies of the MOF and ZIF(30)@MOF together with the corresponding MDCs were first analyzed using FESEM. The SEM images shown in Figure S3 reveal that the morphologies of the thin and disk-like crystals of the MOF (Figure S3a) are distorted with reduced particle sizes for ZIF(30)@MOF (Figure S3b). Moreover, the morphologies of the obtained MDC-P and MDC-C (Figures S3c,d, respectively) are very similar to those of the parent MOFs, even after hightemperature pyrolysis. To gain a deeper understanding, MDC-P and MDC-C were further analyzed using high-resolution FE-TEM. The FE-TEM images, shown in Figure 2, reveal that TiO2 nanoparticles (see below) in MDC-P have a slightly different shape, size, and arrangement than those in MDC-C. The sizes of the TiO2 nanoparticles on MDC-C are smaller than those of MDC-P, which agrees with the XRD results (8 and 20 nm for MDC-C and MDC-P, respectively). In addition, the nanoparticles in MDC-P are irregularly sized (mixture of small and large particles) and arranged (Figure 2a); however, the TiO2particles in MDC-C are highly homogeneous in shape, size (∼8−15 nm), and arrangement (Figure 2b). Those observations also agree with the SEM-/TEM−energy dispersive X-ray spectroscopy (EDS) mapping of MDC-P and MDC-C (Figures S4 and S5). Particularly, the SEM-EDS and TEM-EDS 31195


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Figure 3. High-resolution (a) C 1s (b) Ti2P, (c) O 1s, and (d) N 1s XPS spectra of MDC-C.

at 1341 and 1597 cm−1, which are called the D- and G-bands, respectively, are caused by disordered carbon materials.64 The D-band originates from the loss of translational symmetry in the disordered carbon, and the G-band originates from the formation of a graphite layer on the MDCs. The intensity ratio of the D-band and G-band (ID/IG) is related to the amount of symmetry loss or defects in the carbon materials.66 The slightly higher ID/IG for MDC-C (0.98) than for MDC-P (0.93) indicates the presence of more defects in MDC-C (which might agree with the higher mesoporosity), which may increase the accessible surface area and the charge transfer (in the adsorption process).67 Figure S7 shows the XPS survey spectrum of MDC-C, which confirms the presence of C, Ti, N, and O elements. The regional C1s spectrum (Figure 3a) shows an unsymmetrical broad peak that can be deconvoluted with four Gaussian curves with binding energies at 284.4, 285.4, 286.5, and 288.8 eV. The main C1s peaks located at 284.4 eV and 285.4 are assigned to sp2 and defect-containing sp2 carbons of MDC-C.66 The weak peaks located at 286.5 and 288.8 eV are attributed to C−O and CO bonds, respectively.66 The oxidation state of the Ti species was also investigated from the Ti2p XPS spectrum. The peaks (Figure 3b) at binding energies of 464.7 and 459.0 eV can be attributed to the Ti 2p1/2 and 2p3/2 energy states of the TiO2 phase (TiIV), respectively, in MDC-C.68 The three peaks of the O1s spectrum at binding energies of 530.1, 530.6, and 531.8, which are observed upon deconvolution of the broad peak in Figure 3c, are caused by oxygen of the Ti−O−Ti (TiO2) lattice,69 CO of carbonyl (or carboxylic),69 and C−O (or Ti−OH), respectively. In addition, the N1s spectrum (Figure 3d) is also deconvoluted at binding energies of 399.1, 401.1, and 402.5 eV which corroborated with the formation of Ti−N, pyridinic (N-6) (or pyrrolic (N-5)), quaternarygraphitic (Q-N), and C−N−O of N-atoms onto the MDC-C, respectively.51,53,70 Therefore, mesoporous-carbon-containing TiO2 (mainly the rutile phase) with various N and O functionalities and a considerably high surface area and larger pores can be obtained

mappings (Figures S4 and S5) of MDC-C (compared with those of MDC-P) illustrate highly dispersed elements, including C-, O-, Ti-, and N-atoms. The incorporation of ZIF (one MOF with a lower oxygen content than that of H2N-MIL-125) into the precursor (for the MOF composite) might be the reason for the regular physical properties of TiO2 (Scheme 1b) and of higher C-content in the MDC-C than that in MDC-P (see below).64 The chemical compositions of the MOF and ZIF(30)@ MOF, including the resulting carbons (MDC-P and MDC-C, respectively), were then estimated by elemental and ICP analyses, and the results are summarized in Table S2. The results show that ZIF(30)@MOF contains higher atomic percentages of C (46%) and N (10.2%) and has a lower Ticontent (11.8%) than the MOF (C, 44%; N, 5.7%; Ti, 16.0%). The presence of a quantitative amount of Zn (5.2%) also points toward the presence of ZIF in the composite or ZIF(30)@ MOF. On the other hand, with the increases in Ti-contents, the C-, O-, and N-contents decrease in both MDC-P and MDC-C compared to the corresponding MOFs. Moreover, MDC-C is more C- and N-rich than MDC-P, while MDC-P has higher Tiand O-contents than MDC-C (Table S2). The results obtained for MDC-P agree with the reported results.41 Even though MDC-C is obtained from Zn-containing ZIF(30)@MOF, MDC-C has a very negligible amount of Zn species. This is because the selected pyrolysis temperature (1000 °C) is high enough to eliminate metallic Zn (the boiling point of Zn is 907 °C) that could have formed upon reduction of produced ZnO by carbon.64 MDC-P and MDC-C were further analyzed using Raman spectroscopy, as shown in Figure S6. The three Raman bands observed from 200−700 cm−1 are attributed to the rutile TiO2 phase (marked with an R in the spectrum, Figure S6), which agrees with the XRD results in Figure 1a. The slight red shifts of the Raman bands from 251, 414, and 599 cm−1 (for MDCP) to 258, 426, and 611 cm−1 (for MDC-C), respectively, ensure the presence of smaller particles in MDC-C than MDCP.65 On the other hand, the occurrence of two stretching bands 31196


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content (Table S2). On the other hand, the conversions of DBT with nanocrystalline TiO2 and AC are 58% and 29%, respectively. More importantly, nearly complete conversion of 1000 ppm DBT (volume, 10 mL) is attained using only 20 mg of the MDC-C catalyst within 30 min of the reaction at 80 °C. To confirm the oxidation of DBT using MDCs, the physical adsorption of DBT over MDC-P and MDC-C was performed for 120 min (at 80 °C), and the results are compared to that for ODS (Figure S8) in terms of the percent removal as a function of time. The removal efficiencies of DBT over MDC-C and MDC-P are similar to each other, however very low compared with those of ODS. The small difference might be due to the higher surface area and/or N-content in MDC-C than MDCP,54 which is also in accordance with the removal of DBT by porous AC. The fragmentation patterns (Figure S9) obtained from the MS spectra of the oxidized product extracted after the ODS reaction were compared with those of the commercial dibenzothiophene oxide (DBTO2). The similar fragmentation patterns (m/z = 79, 115, 136, 168, 187, and 216) and completely matched GC chromatogram and UV spectrum (Figure S10) of commercial and produced DBTO2 confirm the successful oxidation of DBT. To understand the kinetics for the oxidation of DBT over the studied catalysts, the pseudo-first-order kinetic model was applied, and the kinetic constants (k) and correlation parameters (R2) are shown in Table 1. The high correlation

via simple pyrolysis of the ZIF@MOF composite as a precursor. More importantly, MDC consisting of more uniformly dispersed and smaller TiO2 nanoparticles can be obtained from the prepared MOF-composite (compared with MDC from pristine MOF). Larger pores and smaller particle sizes usually increase the accessibility of reactants to the active sites of the catalyst, including hybrid mesoporous-carbon materials; therefore, the obtained materials may be successfully applied in catalysis including ODS. 3.2. Catalytic Oxidation of sulfur-Containing Heteroaromatics. After the development and characterization of the new composite ZIF(x)@MOF and MDC-C (x = 30 is considered as the representative standard composite), practical applications of the MOF-composite and MDC-C were considered. Because Ti-based advanced materials, including WO3/TiO2,18 HPW-TiO2−SiO2,22 TiO2/porous glass,19 Tibased MOFs,62 and MDCs,15,41 have been reported as prospective catalysts for ODS of fuels; ZIF(30)@MOF and MDC-C were also assumed to be suitable catalysts for the ODS reaction. Therefore, ZIF(30)@MOF and MDC-C, along with MOF and MDC-P, were used as heterogeneous catalysts for the ODS reaction of DBT. Because MDC-P and MDC-C are composed of TiO2 on a carbon support, pure TiO2 and AC were also used as catalysts to understand the catalytic reaction with MDC-C. Figure 4 compares the performances (or conversions of DBT)

Table 1. Pseudo-first-order Rate Constants and Correlation Factors of the Oxidation of DBT over Different Catalysts at 80 °C catalyst MOF MDC-P ZIF(30)@MOF MDC-C AC TiO2

k (min−1)a 1.6 4.1 1.3 1.2 6.4 1.3

× × × × × ×

−2

10 10−2 10−2 10−1 10−3 10−2

relative ratio of kb

R2 c

1.0 2.6 0.8 7.5 0.4 0.8

0.995 0.996 0.997 0.996 0.995 0.994

a k = rate constant. bRelative ratios of k values were estimated considering the rate constant over H2N-MIL-125 as a unit. c Correlation parameter.

factors of ∼0.99 confirm that the pseudo-first-order kinetic model can be applied effectively to interpret the experimental results. MDC-C shows the highest kinetic constant and the steepest kinetic plot for DBT oxidation (Figure 4b and Table 1), confirming that MDC-C is the most effective catalyst among the tested ones. By considering the kinetic constant of MOF as unity, MDC-C shows a dramatic improvement in the catalytic activity in the oxidation of DBT (7.5 times that of the MOF). MDC-P shows a higher activity (2.6 times) than pristine MOF; however, it was ∼3 times less than that of MDC-C. In contrast, other catalysts, including ZIF(30)@MOF, TiO2, and AC, show lower kinetics than the MOF (Table 1). Therefore, it is quite interesting to study the ODS over MDC-C together with MDC-P. The amount of oxidant has an important role in the oxidation of sulfur-containing heteroaromatics in fuel.14 The effects of the O/S ratio on the ODS of DBT were estimated for the MDC-C and MDC-P catalysts, and the results are compared in Figure 5. The results at low O/S ratios (4.5− 9.0) clearly confirm MDC-C as a more active catalyst than MDC-P, while the performances nearly merge at a high O/S

Figure 4. (a) Effect of time on conversion and (b) pseudo-first-order kinetic plots for the oxidation of DBT over the MOF, MDC-P, ZIF(30)@MOF, and MDC-C including TiO2 and AC at 80 °C.

of the studied catalysts as a function of reaction time. At the end of the 120 min reactions, the MOF (69%) shows a slightly better performance than ZIF(30)@MOF (60%) (Figure 4a). However, remarkable activity is observed in MDC-C (99.5%; almost complete conversion of DBT; Figure 4a), whereas the activity is a bit lower in MDC-P (95%), even with a higher Ti31197


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activation. Moreover, MDC-C is probably the best catalyst among those reported (including the tested ones) based on the capability to oxidize a maximum amount of DBT using the same weight of the catalysts, even though two other reported catalysts showed higher kinetic constants (Table 3). In those cases, either the applied weight or amount of active site in the catalysts (e.g., polyoxometalate and IL-modified SBA-15) was higher than that of MDC-C. To covert a substrate with any heterogeneous catalyst, facile/ rapid adsorption of the reactants on the surface or active site of the applied catalyst is very important for effective catalysis. The decreased particle size (or highly dispersed particles) of the active component of a solid catalyst supported on carbon having especially hierarchal structures can also play a positive role in catalytic reactions.14 In addition, mesoporosity41 and large pores (with a large pore size distribution) of the catalysts are generally also helpful for efficient mass transfer. Therefore, the higher kinetics of MDC-C than that of MDC-P can be explained by the smaller and uniformly dispersed TiO2 nanoparticles and larger pore size (especially in the 5−9 nm range) and larger mesopore volume. As a result, a newly synthesized MDC (obtained by pyrolysis of the composite) is suggested as an advanced catalytic material for oxidation reactions, including ODS. The oxidation of other sulfur-containing heteroaromatics, such as Th, BT, and DMDBT, was conducted under very similar conditions to understand the oxidation mechanisms over MDC-C, and the results are compared in Figure 8. The conversions of sulfur-containing heteroaromatics and the kinetic constants follow the order: DBT > BT > Th > DMDBT (Table S3). The ODS of various sulfur-containing heteroaromatics has been explained with electron densities of the S-atom on the substrates. The results shown in Figure 8 and Table S3 (together with the electron densities) indicate that the performance of MDC-C in ODS generally increases with increasing electron density on the S-atom (excluding the rate constant for DMDBT oxidation). For example, the rate constants for oxidation of Th (1.4 × 10−2 min−1), BT (2.0 × 10−2 min−1), and DBT (1.2 × 10−1 min−1) over MDC-C are correlated well with corresponding electron densities on the sulfur atom which are 5.696, 5.739, and 5.758, respectively. Therefore, the ODS of sulfur-containing heteroaromatics might be explained by a general mechanism13 of nucleophilic attack of the S-atom to oxygen species, which are produced upon activation of oxidizing agents (e.g., H2O2) by the catalyst.14 The lower activity for DMDBT (than that expected based on the electron density) may be explained by the steric hindrance that originates from the methyl groups on the benzene rings (Scheme S1), which is similar to a previous report.18

Figure 5. Effects of oxidant (H2O2) to sulfur molar ratio (O/S) on the conversions of DBT (catalyst weight, 20 mg of the MDC-P and MDCC at 80 °C in a reaction time of 2 h).

ratio (18.0) (the condition, which is very costly, is not an important condition for oxidation). To further investigate the activities of MDC-C and MDC-P as solid catalysts, the ODS of DBT was conducted at several temperatures (40, 60, and 80 °C), and the results are compared in Figure 6. The results at the lowest temperature (40 °C) demonstrate more evidently that the activity of the MDC-C is significantly higher than that of MDC-P (Figure 6a), even though the difference in the performances is reduced with increasing reaction temperatures (Figure 6b,c). The kinetic constants (k) for both MDC-P and MDC-C (summarized in Table 2) were estimated from the kinetic plots (Figure S11) and increased with increasing temperature, as expected. Moreover, the relative kinetic constants for MDC-C are 3.5, 3.4, and 2.9 times higher, respectively, than those of MDC-P in the reactions at 40, 60, and 80 °C, further confirming the superior catalytic activity of MDC-C (especially at lower temperatures). In addition, the effectiveness of catalysts increases with decreasing apparent activation energy (Ea) in a reaction.14 The Ea values were estimated from the Arrhenius plots (Figure 7) for MDC-P and MDC-C and are 25 and 19 KJ·mol−1, respectively (shown in Table 2). Therefore, the high kinetic constants (especially at low temperatures) observed for MDCC are also in accordance with the low-Ea value. The performance of MDC-C for the catalytic oxidation of DBT is remarkable among the tested and the previously reported results (Table 3). More specifically, 1 g of MDC-C can oxidize 2.71 mmol of DBT (∼100% of the 1000 ppm solution) to DBTO2 in 60 min (as shown in Figure S9b) with a very low Ea (19 KJ·mol−1) and very high kinetic constant (1.2 × 10−1 min−1). MDC-C might be the catalyst with the lowest Ea among the reported catalysts for the ODS of DBT via H2O2

Figure 6. Effect of time on conversion of DBT at (a) 40, (b) 60, and (c) 80 °C with MDC-P and MDC-C. 31198


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Table 2. Pseudo-first-order Rate Constants and Correlation Factors of the Oxidation of DBT at Different Temperaturesa over MDC-P

over MDC-C

temp (°C)

k (min−1)

Ea (KJ·mol−1)

R2

k (min−1)

Ea (KJ·mol−1)

R2

relative ratio of kb

40 60 80

1.4 × 10−2 2.0 × 10−2 4.1 × 10−2

25

0.997 0.998 0.996

4.9 × 10−2 6.9 × 10−2 1.2 × 10−1

19

0.995 0.997 0.996

3.5 3.4 2.9

k, first-order rate constant; Ea, activation energy; R2, correlation parameter. bRelative ratios of k values were estimated by considering the rate constant over MDC-P as a unit. a

Figure 7. Arrhenius plots for the oxidation of DBT with the MDC-P and MDC-C catalysts.

3.3. Reusability of MDC-C. The reusability of a catalyst is very important for commercial applications. The reusability of the MDC-C catalyst for the ODS of DBT has been inspected in several runs after a simple washing with acetone under ultrasound treatment, and the results are shown in Figure 9. The activity decreases slightly during the second run; however, it is very steady in the subsequent runs. The XRD pattern and N2-adsorption isotherm of recycled (after second cycle) MDCC, as shown in Figure S12, are nearly similar to those of the fresh MDC-C. Moreover, Raman spectra and morphology of MDC-C also do not noticeably change after recycling (all the data are not shown). Based on the high and ready reusability, MDC-C is suggested as a potential catalyst that can be applied in ODS reactions.

Figure 8. Effect of time on (a) conversion and (b) plots of the pseudofirst-order kinetics of Th, BT, DBT, and DMDBT over MDC-C at 80 °C.

nanoparticles were obtained by high-temperature pyrolysis of the synthesized ZIF(30)@MOF and the pristine MOF. The prepared MDCs were characterized using several techniques, which showed that MDC-C was composed of crystalline TiO2 supported on a mesoporous carbonaceous material. In addition, the MDC-C was composed of larger pores with TiO 2

4. CONCLUSIONS A MOF composite (ZIF(x)@MOF) was synthesized with a tailored composition following the “MOF-embedding onto another MOF” method. Mesoporous carbons containing TiO2

Table 3. Oxidative Desulfurization of DBT over the Various Catalysts Using H2O2 as Oxidant with Different Reaction Parameters DBT oxidation conditions catalyst WO3/TiO2 TiO2/porous glass NaPW SiW PMo HPW−TiO2−SiO2 HPW−PDMAEMA−SiO2 HPW-IL/SBA-15 H−TiNT MOF MDC-P MDC-C

DBT (mmol)/catalyst (g) 0.34 0.03 2.58 0.81 2.38 0.35 0.03 1.35 0.45 1.85 2.59 2.71

T (°C) 50 100 70 70 70 60 60 60 25 80 80 80

time (min) 60 8 30 30 30 120 180 60 180 120 120 120 31199

conversion (%) 100 100 97.4 30 90 100 100 100 100 68.5 95.6 99.5

k (min−1) 1.1 1.0 4.0 1.1 5.4 − − 1.4 4.6 1.6 4.1 1.2

× × × × ×

× × × × ×

−1

10 10−1 10−1 10−2 10−2

10−1 10−2 10−2 10−2 10−1

Ea (kJ·mol−1) 54 − 30.9 28.3 29.0 − 23 56.6 46.2 − 25 19

ref 17 18 19

20 21 22 24 this work this work this work


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Figure 9. Conversion of DBT with the MDC-C catalyst up to five consecutive cycles after regeneration by ultrasound treatment in acetone (catalyst weight, 20 mg; reaction time, 60 min; reaction temperature, 80 ◦C).

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b10336. Materials, characterization results, and catalyses data (Figures S1−S11, Scheme S1, and Tables S1−S3) (PDF)

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REFERENCES

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nanoparticle of smaller and more uniform sizes (or more welldispersed) than those of MDC-P. The newly developed MDCC and MDC-P, along with their parent MOFs, AC, and TiO2 nanoparticles, were applied to oxidative desulfurization to remove sulfur-containing heteroaromatics, such as dibenzothiophene, from the model fuel. The MDC-C was a remarkable catalyst for the oxidation of dibenzothiophene among the tested and reported catalysts in terms of kinetics. More importantly, the MDC-C is a better catalyst than any reported catalysts, considering its lowest activation energy and highest throughput of catalysis (the highest amount of dibenzothiophene can be oxidized with the same weight of catalyst). Furthermore, the recyclability of MDC-C was also confirmed after a simple washing of the used catalyst. Therefore, MDC-C is a potential catalyst for the oxidative desulfurization reaction because of its low activation energy, ready recyclability, high kinetics, and high throughput. Finally, composing the MOFs, and subsequent pyrolysis, is a competitive way to prepare a functional catalyst, especially with well-dispersed active sites with large pore sizes and a high mesoporosity.

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BT, benzothiophene DBTs, dibenzothiophenes DMDBTs, dimethyldibenzothiophenes EDS, energy dispersive X-ray spectroscopy EPA, Environmental Protection Agency HDS, hydrodesulfurization MDC-C, MOF-composite-derived mesoporous carbons MDC-P, pristine MOF-derived mesoporous carbons MOF, metal−organic framework ODS, oxidative desulfurization Th, thiophene ZIF, zeolitic imidazolate framework

AUTHOR INFORMATION

Corresponding Author

*Tel.: 82-53- 950-5341. Fax: 82-53-950-6330. E-mail: sung@ knu.ac.kr. ORCID

Sung Hwa Jhung: 0000-0002-6941-1583 Funding

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (Grant No. 2017R1A2B2008774). Notes

The authors declare no competing financial interest.

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ABBREVIATIONS AC, activated carbon BET, Brunauer−Emmett−Teller BJH, Barrett−Joyner−Halenda 31200


Research Article

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TiO2-Containing Carbon Derived from a MetalâOrganic Framework

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