Polydopamine-Mediated Formation of MnSn(OH)6 on Cryptomelane

Jul 13, 2017 - Efficient isomerization of glucose into fructose contributes to the utilization of biomass producing 5-hydroxymethylfurfural (HMF). In ...
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Polydopamine-Mediated Formation of MnSn(OH)6 on CryptomelaneType Manganese Oxide for Catalyzing Glucose Isomerization to Fructose Xianlin Meng,† Peng Li,† Mengmeng Du, and Peijun Ji* Department of Chemical Engineering, Beijing University of Chemical Technology, Beijing, China ABSTRACT: Efficient isomerization of glucose into fructose contributes to the utilization of biomass producing 5-hydroxymethylfurfural (HMF). In this work, we have explored a novel catalyst for isomerization of glucose into fructose. By the mediation of polydopmaine (PDA), MnSn(OH)6 was formed on cryptomelanetype manganese oxide (OMS-2). The composite MnSn(OH)6/PDA@ OMS-2 can efficiently catalyze the glucose isomerization to fructose, with a yield of fructose 47 ± 1.6%. When used together with the Brønsted acid catalyst Amberlyst-15, the HMF yield from glucose was 53 ± 2.5%. Consecutive use of the composite demonstrated that after eight cycles the activity can be retained in terms of the yield of fructose. The mechanism of polydopamine-mediated formation of MnSn(OH)6 has been investigated. The desired yield of fructose has been obtained by MnSn(OH)6/PDA@OMS-2, and the preparation of the catalyst has also shown advantages, including low cost and easy preparation, short preparation time (36 h), and low temperature (≤70 °C). exchange resins,15 phosphoric acid treated metal oxides,2,16 and tin based catalysts, including Lewis acidic metal salt SnCl4 and Sn-beta.17−23 Isomerization of glucose into fructose over Mg− Al hydrotalcite achieved a yield of fructose of about 30%, and the glucose conversion was about 50%.11 Carraher et al. demonstrated that organic Brønsted bases, specifically tertiary amines, catalyze the isomerization of glucose to fructose with a yield of 32%.5 Yue et al. reported a hydrothermally synthesized layered zirconosilicate, and the yield of fructose was about 25%, and the glucose conversion was about 50%.13 Many problems, such as leaching of caustic species and difficult and/or expensive catalyst syntheses, are still waiting to be solved.5,24 The Lewis acidic metal salt SnCl4 has been demonstrated to be effective for isomerization of glucose to fructose.17,18 When used together with a Brønsted-acidic ionic liquid, the HMF yields can be higher than 47.5%. Although the use of homogeneous metal halides with ionic liquids results in high selectivity and yield of HMF from glucose, catalyst handling, separation, and reuse remain a concern in scaling up for industrial application.12,16 Catalytic conversion of glucose to fructose and further to 5-HMF over solid acid catalysts is highly recommended considering these catalysts eliminate the corrosiveness of homogeneous acids, are environmentally benign, and allow for easy separation and recovery.12,16,25 Instead, solid Lewis acid zeolites Sn-beta have been investigated

1. INTRODUCTION 5-Hydroxymethylfurfural (HMF) is a biomass-derived important platform chemical that can be converted to biofuel and numerous value-added chemicals, such as maleic anhydride, 2,5bis(ethoxymethyl)furan, levulinic acid, 2,5-furandicarboxylic acid, 2,5-diformylfuran, 2,5-dihydroxymethylfuran, and 5hydroxy-4-keto-2-pentenoic acid, with a wide range of applications in pharmaceuticals, petroleum industry, and furanose-based polymers.1,2 HMF is accessible from the C6 sugars in lignocellulosic biomass.3 Fructose dehydration to HMF is easy to realize over homogeneous or heterogeneous catalysts.4 Compared with fructose, glucose as a feedstock for HMF has the advantages of abundance, easily available, and low cost, because glucose can be obtained from cellulose by hydrolysis with yields of 98−100%, while it is economically limited to 42% for isomerization of glucose to fructose which requires additional and expensive separation steps.5,6 In the catalytic conversion of glucose to HMF, glucose first isomerizes to fructose and then the resulting fructose loses three molecules of H2O to form HMF. The isomerization of glucose into fructose is the key step for the efficient formation of HMF, because the stability of the glucose ring makes processing difficult.7−9 Fructose is typically industrially produced through the enzymatic isomerization process, which has several drawbacks, including high cost, longer reaction time, and irreversible deactivation.10 A broad range of homogeneous and heterogeneous chemical catalysts have been reported for the isomerization of glucose to fructose, including hydrotalcite catalyst,11 organic bases,5,12 alkali cation-exchanged zeolites,13,14 anion © XXXX American Chemical Society

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April 3, 2017 July 8, 2017 July 13, 2017 July 13, 2017 DOI: 10.1021/acs.iecr.7b01353 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research as catalysts for the isomerization of glucose to fructose.19−24 Snbeta zeolites together with Brønsted acid catalysts, such as hydrochloric acid and Amberlyst resins, could efficiently catalyze the conversion of glucose to 5-HMF with 5-HMF yields around 60%. The classic synthesis of Sn-beta zeolites generally requires about 40 days and the necessity to use fluoride as mineralizing agent to effectively incorporate SnIV in framework positions.19,20 To reduce the crystallization time several approaches have been proposed, including seeded-growth procedure21 and postsynthesis modification.22 Chang et al. prepared the Sn-beta zeolites at 550 °C for 12 h, and the overall preparation time can be reduced to 6 days.21 However, the catalyst exhibited a lower catalytic activity in glucose isomerization. In the postsynthesis procedures, H-form Meso-beta were obtained by ion exchanges and the subsequent calcination in air at 550 °C for 5 h.2 The substitution with SnCl4 vapor had to be carried out at elevated temperature above 500 °C for 6 h. The overall catalyst preparation time could be reduced to 6 days. Using the abovementioned methods, noticeable reduction of crystallization time was achieved. The Sn-beta zeolites prepared via different synthetic protocols possess different physicochemical properties and distributions of Sn species, as well as catalytic activity,26,27 compared to the Sn-beta zeolites of 40 days preparation. Tin(IV) is prone to fast and uncontrolled hydrolysis in solution, as noted by the fuming precipitation of tin oxide when SnCl4 is exposed to humid atmospheres.28 Sn species for the preparation of Sn based catalysts should be preferentially not in octahedral coordination like in SnO2, which showed poor catalytic activities.29 Therefore, synthesis of the Sn based catalyst with high Sn loading without the formation of bulk SnO2 remains a challenge. Moreover, decreasing the preparation time is also pursued by researchers. The cryptomelane-type manganese oxideoctahedral molecular sieve (OMS-2) can form a MnO6 octahedral unit and a square tunnel, with the coexistence of Mn4+, Mn3+, and Mn2+.30 The open tunnel structure and the mixed-valence Mn ions make the OMS-2 become an active and selective oxidation catalyst and has been widely used in many chemical processes.31 In this work, we intend to synthesize heterogeneous Sn-based catalysts on OMS-2. OMS-2 was first coated with a thin layer of polydopamine (PDA). Mediated by polydopamine, MnSn(OH)6 was formed on PDA@OMS-2 by impregnating PDA@ OMS-2 in the SnCl4 aqueous solution. MnSn(OH)6/PDA@ OMS-2 has been found to be effective for the isomerization of glucose to fructose.

2.3. Preparation of Polydopamine (PDA) Coated OMS2. A total of 1.0 g of OMS-2 was added into 100 mL of Trisbuffer (pH 8.5) under sonication for 15 min, and then 200 mg of dopamine was added. The mixture was sonicated at room temperature for 5 min. The polydopamine-coated OMS-2 (PDA@OMS-2) was collected by filtration, washed with double-distilled water, and then dried under vacuum at 60 °C overnight. 2.4. Preparation of MnSn(OH)6/PDA@OMS-2 Catalyst. With PDA@OMS-2 as the support, Sn-based catalysts were prepared by impregnation method. A total of 1.0 g of PDA@ OMS-2 and SnCl4·5H2O with different concentrations were added to 30 mL of deionized water and stirred for 10 h. The prepared catalysts were collected by filtering through a 450 nm polycarbonate membrane, and then dried under vacuum at 80 °C overnight. 2.5. Characterization and Measurement. XPS spectra were measured using a X-ray photoelectron spectrometer (Thermo VG ESCALAB250). The measurement was carried out at the pressure of 2 × 10−9 Pa using Mg Ka X-ray as the excitation source. Infrared spectra were recorded on a Bruker Tensor 27 FTIR spectrometer equipped with a liquid-nitrogencooled MCT detector at a nominal resolution of 2 cm−1. The XRD patterns were obtained with a powder diffractometer of X’Pert PRO MPD with a Gu anode at 40 kV, wavelength 0.154 nm. The powder diffractograms were operated at a scan rate of 1°/min from 2θ = 5° to 2θ = 90°. JCPDS powder diffraction data files were employed as references for crystal phase identification of samples. TEM images for the catalysts were obtained by using a JEOL JEM3010F transmission electron microscope operating at 300 kV. The FT-IR spectra after pyridine adsorption were studied to evaluate the Lewis acid characteristics on the surface of the MnSn(OH)6/PDA@OMS-2, which was performed on a Bruker Tensor 27 with a DTGS detector and a resolution of 4 cm−1. Tin cations loaded on MnSn(OH)6/PDA@OMS-2 and SnO2/ PDA@OMS-2 were analyzed with inductively coupled plasmaatomic emission spectroscopy (ICP-AES). 2.6. Glucose Isomerization to Fructose. A total of 1 g of glucose was added to 10 mL of DMSO to prepare the glucose solution. Then 100 mg of MnSn(OH)6/PDA@OMS-2 was added to the solution. The mixture was heated to 150 °C under stirring. The reaction was carried out for 5 h. Consecutive use of MnSn(OH)6/PDA@OMS-2 was performed. The catalyst after each run was recovered by filtration, thoroughly washed with water and ethanol, and then dried at 60 °C under vacuum overnight. The glucose isomerization reaction for each cycle was carried out for 5 h. For catalyzing the conversion of glucose to HMF, 1 g of glucose was dissolved in 10 mL of DMSO to prepare the glucose solution. Then MnSn(OH)6/PDA@OMS-2 and 0.2 g of Amberlyst-15 (dried under vacuum before use) were added to the solution. The reaction mixture was heated to desirable temperature with an oil bath under strong stirring. Samples were removed and centrifuged to remove all insoluble particulates. A portion of the reaction mixture (50 μL) was diluted in a 1:250 ratio with distilled water and taken for analysis. Glucose, fructose, formic acid, and levulinic acid were analyzed by high-performance liquid chromatography (Shimadzu LC-10A) fitted with a refractive index (RI) detector and an Aminex HPX-87H ion exclusion column. The mobile phase was 5 mM H2SO4 flowing at a rate of 0.6 mL/min. The column

2. METHODS 2.1. Materials. All chemicals required for the synthesis of catalyst and catalytic reactions were purchased from SigmaAldrich (Shanghai, China) or Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China) and used as received without further purification. 2.2. Catalyst Synthesis: Preparation OMS-2. OMS-2 was synthesized by the reflux method.30 89 mL of the KMnO4 solution (44.3 mg/mL) was added to a mixture of 40 mL of MnSO 4 ·H 2 O solution (66 mg/mL) and 1.36 mL of concentrated HNO3. The suspension was stirred vigorously and refluxed at 100 °C for 20 h. After filtration, the precipitate was washed with distilled water until neutral pH and dried at 120 °C for 6 h. B

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Industrial & Engineering Chemistry Research oven was set to 65 °C. The column can efficiently resolve glucose and fructose. HMF was detected using Diamonsil C18 column and an UV−vis detector at 284 nm. The column oven temperature was set at 25 °C, and the mobile phase was methanol/water = 70:30 (v/v) at a flow rate of 1.0 mL/min. Each reaction was performed a minimum of three times, and differences between runs are reflected in the error bars. The yield of fructose (for example) is defined as amount of fructose produced/initial amount of glucose. 2.7. UV−vis Spectra Measurement for the Supernatants. The Mn2+ ions were monitored by measuring UV−vis spectra, according to the formaldoxime method.32,33 The formaldoxime solution (0.575 M hydroxylamine HCl and 0.2 M formaldehyde) was prepared and used as the manganese analytical reagent. A total of 50 mg of OMS-2 or 50 mg of PDA@OMS-2 was impregnated in the 90 mM SnCl4 solution for 12 h. Then the mixtures were centrifuged, and the supernatants were removed. A total of 1 mL of formaldoxime solution, 1 mL of 10% hydroxylamine hydrochloride, and 14 mL of H2O were added to 6 mL of the supernatants. After shaking by hand for 5 min, the mixtures stood for 20 min. Then the mixtures were subjected to UV−vis spectra measurements. Triplicate measurements were carried out. In separate experiments, the samples OMS-2 (50 mg) and PDA@OMS-2 (50 mg) were treated by separately immersing in 50 mL of diluted H2SO4 for 12 h. After centrifugation, the supernatants were removed. Then 6 mL of the supernatants was used for the experiments of detection of Mn2+.

standard card (JCPDS no. 72-007). While in the SnCl4·5H2O solution (181 mM), SnO2 was formed on PDA@OMS-2, as indicated by the diffraction peaks around 26.5° and 33.8°, and they are indexed to the SnO2 standard card (JCPDS no. 77449). Figure 2 shows the transmission electron microscope (TEM) images. OMS-2 exhibited continuous lattice fringes with a

3. RESULTS AND DISCUSSION 3.1. Formation of MnSn(OH)6 on PDA@OMS-2. Figure 1 shows the XRD patterns for the samples. The synthesized

Figure 2. TEM images for OMS-2 (a), PDA@OMS-2 (b), MnSn(OH)6/PDA@OMS-2 (c), and SnO2/PDA@OMS-2 (d). The red arrows indicate the thin PDA layers.

spacing of about 0.7 nm (Figure 2a), which corresponds to the lattice of OMS-2.30 Coating OMS-2 with PDA was carried out at room temperature. The thickness of the PDA film was affected by the concentration of dopamine, pH condition, and incubation time.34 Once the concentration of dopamine and pH condition were optimized and determined as described in the experimental section, we found that 5 min incubation is appropriate to coat a thin layer of PDA. Figure 2b shows that the OMS-2 nanorods were uniformly coated with a thin layer of polydopamine (PDA). MnSn(OH)6 on PDA@OMS-2 exhibited a morphology of nanoparticles (Figure 2c). Similarly, SnO2 also exhibited nanoparticles on PDA@OMS-2 (Figure 2d). Figure 3a shows the XPS spectra. Compared to the XPS spectrum of OMS-2, the peaks of C 1s and N 1s appeared in the spectrum of PDA@OMS-2, confirming the coating OMS-2 with PDA. In the spectra of MnSn(OH)6/PDA@OMS-2 and

Figure 1. XRD patterns for OMS-2, PDA@OMS-2, MnSn(OH)6/ PDA@OMS-2, and SNO2/PDA@OMS-2. O, typical patterns for OMS-2; V, MnSn(OH)6; +, SnO2.

OMS-2 exhibited typical peaks that are consistent with the OMS-2 standard card (JCPDS file no. 29-1020).30 It is manganese oxide octahedral molecular sieve. After coating with polydopamine (PDA), the XRD pattern of PDA@OMS-2 resembles that of OMS-2. The CHN analysis for the samples PDA@OMS-2 showed that the loading of PDA was 13.13 ± 0.62%. Different compounds on PDA@OMS-2 had been obtained by impregnating PDA@OMS-2 in the SnCl4·5H2O solutions with different concentrations. In the SnCl4·5H2O solution (90 mM), MnSn(OH)6 was formed on PDA@OMS-2, as indicated by the sharp diffraction peaks at 19.5°, 22.5°, 32.1°, and 51.8°. They are indexed to wickmanite, the MnSn(OH)6 C

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Figure 4. FT-IR spectra after pyridine adsorption for MnSn(OH)6/ PDA@OMS-2 (blue) and SnO2/PDA@OMS-2 (red).

and SnO2/PDA@OMS-2 exhibited two prominent peaks at 1445 and 1588 cm−1. The peaks at 1445 cm−1 could be attributed to surface coordinated pyridine molecules with Lewis acid sites.16,36 The peak at 1588 cm−1 was assigned to hydrogen-bonded pyridine on Lewis acid sites. The absence of an obvious peak around 1540 cm−1 implied the lack of Brønsted acid sites on samples MnSn(OH)6/PDA@OMS-2 and SnO2/PDA@OMS-2. Based on the experiment of temperature-programmed desorption (TPD) of ammonia, the density of Lewis acid sites for MnSn(OH)6/PDA@OMS-2 is 0.313 mmol/g, in comparison to 0.142 mmol/g for SnO2/ PDA@OMS-2. N2 adsorption measurements were performed for the samples. The isotherms are shown in Figure 5. After coating with PDA, the BET surface area of PDA@OMS-2 was decreased in comparison to OMS-2, whereas the samples MnSn(OH)6/PDA@OMS-2 and SnO2/PDA@OMS-2 exhibited higher BET surface area than OMS-2. Among the samples, MnSn(OH)6/PDA@OMS-2 has the highest BET surface area. The existing states of Sn species for SnO2/PDA@OMS-2 and MnSn(OH)6/PDA@OMS-2 samples were investigated by measuring XPS spectra as shown in Figure 6. The SnO2/PDA@ OMS-2 sample displays two peaks of 3d5/2 and 3d3/2 at 494.4 and 486.0 eV, which are characteristics of the octahedral Sn species existing in the form of SnO2.29,37 The octahedral Sn species normally show poor catalytic activity.29 The spectra of the MnSn(OH)6/PDA@OMS-2 sample displays two peaks of 3d5/2 and 3d3/2 at 495.0 and 486.5 eV, respectively. The peak splitting of 8.5 eV is observed, which indicates the Sn being present in the Sn(IV) state. Compared to the peaks of the SnO2/PDA@OMS-2 sample, about 0.5 eV chemical shift of Sn 3d has been observed for MnSn(OH)6/PDA@OMS-2, which is ascribed to the presence of Mn cations in the Sn−O lattice. The results shown in Figures 5 and 6 indicate that the existing states and properties of Sn species of the MnSn(OH)6/PDA@OMS-2 and SnO2/PDA@OMS-2 samples are different. 3.2. Catalysis Using the Composite MnSn(OH)6/PDA@ OMS-2. Figure 7 shows the glucose conversion and fructose yield with reaction time under the catalysis of MnSn(OH)6/ PDA@OMS-2. After 4 h of reaction, 64.05% of glucose was converted, and the fructose yield was 47.35%. The fructose selectivity is 73.9%. The yield of HMF was about 4% after 4 h. Consecutive use of MnSn(OH)6/PDA@OMS-2 was performed. Figure 8 shows the yield of fructose in eight consecutive runs. Consecutive use of the composite demon-

Figure 3. XPS (a) and FTIR (b) spectra for OMS-2, PDA@OMS-2, MnSn(OH)6/PDA@OMS-2, and SnO2/PDA@OMS-2.

SnO2/PDA@OMS-2, the peaks for Sn 3d and Sn 3p3/2 appeared, and the intensity of the oxygen band is relatively increased compared to that of PDA@OMS-2. The XPS spectra confirmed that the Sn species were loaded on PDA@OMS-2. In Figure 3b for FTIR spectra, OMS-2 nanorods exhibited a characteristic peak at 720 nm. In the spectrum for the OMS-2 nanorods coated with PDA, the band at 1588 cm−1 was assigned to νring (CC) stretching modes,34 confirming the coating of polydopamine. In the spectrum for MnSn(OH)6/ PDA@OMS-2, the band at 1620 cm−1 can be assigned to hydroxyl deformation due to the formation of MnSn(OH)6 nanoparticles.35 The absorbance around 3322 cm−1 is due to −OH stretching. This band had a relatively larger absorbance in the spectrum of MnSn(OH)6/PDA@OMS-2 due to the formation of MnSn(OH)6. The sample of SnO2/PDA@ OMS-2 exhibited a similar FTIR spectrum to that of PDA@ OMS-2, as the formation of SnO2 nanoparticles did not contribute to the absorbance at the aforementioned bands. To investigate the existence of Lewis and Brønsted acid sites on the composite catalysts, the FTIR spectra for the samples after pyridine adsorption were measured as illustrated in Figure 4. The FTIR spectra for samples MnSn(OH)6/PDA@OMS-2 D

DOI: 10.1021/acs.iecr.7b01353 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 5. Nitrogen adsorption−desorption isotherms of as-prepared samples. BET surface area for OMS-2, PDA@OMS-2, MnSn(OH)6/PDA@ OMS-2, and SnO2/PDA@OMS-2 are 103.4, 75.0, 130.4, and 107.8 m2 g−1, respectively.

Figure 8. Consecutive use of MnSn(OH)6/PDA@OMS-2 for glucose isomerization. All of the reactions were carried out at the same conditions, and the reaction time was 5 h.

strated that, after 8 cycles, the activity of MnSn(OH)6/PDA@ OMS-2 can be retained in terms of the yield of fructose. Figure 9 shows the results using additional three catalysts OMS-2, PDA@OMS-2, and SnO2/PDA@OMS-2. The com-

Figure 6. XPS of MnSn(OH)6/PDA@OMS-2 (blue) and SnO2/ PDA@OMS-2 (red).

Figure 7. Glucose conversion, fructose yield, HMF yield, and levulinic acid and formic acid yields as a function of reaction time.

Figure 9. Comparison among the four catalysts. MnSn(OH)6/PDA@ OMS-2, OMS-2, PDA@OMS-2, and SnO2/PDA@OMS-2. All of the reactions were carried out at the same conditions, and the reaction time was 5 h. E

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by impregnating OMS-2 in the SnCl4 solution with different concentrations, and for all of the concentrations, only SnO2 was formed on OMS-2 as shown in Figure 11a. Figure 11b shows

mon feature for the three catalysts is that the fructose yields are much lower. These results indicate that formation of MnSn(OH)6 on the support PDA@OMS-2 is vital to glucose isomerization to fructose. A fructose yield of 47.4% by MnSn(OH)6/PDA@OMS-2 is obtained. The detection of Sn leaching was carried out by repeatedly using the catalyst MnSn(OH)6/PDA@OMS-2 five times. The catalyst after each time of reaction was recovered by filtration, then thoroughly washed with water and ethanol, and then dried at 60 °C under vacuum overnight. The ICP-AES elemental analyses showed that the loading amount of Sn in the collected MnSn(OH)6/PDA@OMS-2 is 0.0311 ± 0.0012 mol/100 g. Compared to the initial loading amount of Sn 0.0315 mol/100 g, the change of loading amount of Sn after five times reaction was about 1.3%. MnSn(OH)6/PDA@OMS-2 has been further used together with the Brønsted acid catalyst Amberlyst-15 for the conversion of glucose producing HMF. MnSn(OH)6/PDA@OMS-2 catalyzed the isomerization of glucose to fructose, and Amberlyst-15 catalyzed the dehydration of fructose to HMF. Here Amberlyst-15 is a commercial Brønsted acid catalyst.38−40 Figure 10 shows that, after 4 h of reaction, the glucose

Figure 11. XRD patterns (a) and SEM image (b) for SnO2/OMS-2. O, typical patterns for OMS-2; +, SnO2.

that some SnO2 was scattered in the solution, not on the OMS2 nanorods, indicating a weak interaction of SnO2 with OMS-2, whereas with the mediation of PDA, both MnSn(OH)6 and SnO2 can interact with OMS-2 with a good adsorption as shown in Figure 2b,c. When SnCl4 is placed in water, it dissociates into Sn4+ and − Cl , followed by the complexation of Sn4+ with water molecules forming aquachlorotin(IV) and hexaaquatin(IV).41−43 When the SnCl4 concentration is lower than 100 mM, only hexaaquatin(IV) formed because any chloride-containing complexes are completely dissociated. Hexaaquatin(IV) is prone to undergo fast hydrolysis, as expressed by reaction 1.42,43 Two conditions are required for forming MnSn(OH)6 as shown in reaction 2: the concentration of SnCl4 is lower than 100 mM and the concentration of Mn2+ ions matches the concentration of [Sn(OH)6]−2. Otherwise, reaction 3 will follow reaction 1 and SnO2 is formed.

Figure 10. Glucose conversion, HMF yield, and levulinic acid and formic acid yields under the catalysis of MnSn(OH)6/PDA@OMS-2 and Amberlyst-15.

conversion was 81 ± 2.8%, HMF yield was 53 ± 2.5%, and fructose yield was 15 ± 0.7%. In addition, the yields of levulinic acid and formic acid were 7.7% and 6.8%, respectively. 3.3. Mechanism of the Polydopamine-Mediated Formation of MnSn(OH)6 on OMS-2. The ICP-AES elemental analyses showed that the loading amounts of Sn(IV) in SnO2/PDA@OMS-2 and MnSn(OH)6/PDA@OMS-2 are 0.0594 mol/100 g and 0.0315 mol/100 g, respectively. The ICP-AES analysis results indicate that SnO2/PDA@OMS-2 loads more Sn(IV) than MnSn(OH)6/PDA@OMS-2. However, the difference in the loading amount of Sn(IV) cannot be used to explain the activity difference, because MnSn(OH)6/ PDA@OMS-2 exhibited much larger catalytic activity than SnO2/PDA@OMS-2 as shown in Figure 9. It is postulated that the formation of MnSn(OH)6 on PDA@OMS-2 is vital for achieving a high catalytic activity. Figure 1 demonstrated that, in the solution of SnCl4·5H2O with a concentration 90 mM, MnSn(OH)6 was formed on PDA@OMS-2. While when the concentration was increased to 181 mM, SnO2 instead of MnSn(OH)6 was formed on PDA@OMS-2. It is indicated that the SnCl4·5H2O concentration has an effect on the formation of MnSn(OH)6. Additional experiments were also performed

[Sn(OH)y (H 2O)6 − y ](4 − y) ⇔ [Sn(OH)y + 1(H 2O)5 − y ](3 − y) + H+

(1) −2

[Sn(OH)6 ]

+ Mn

2+

⇔ MnSn(OH)6

(2)

[Sn(OH)y + 1(H 2O)5 − y ](3 − y) ⇔ SnO2 + (y + 1)H 2O + (3 − y)H3O+

(3)

When OMS-2 was impregnated in the SnCl4 solution (90 mM), no MnSn(OH)6 was formed on OMS-2 (Figure 11a), in contrast to MnSn(OH)6 formed on PDA@OMS-2 (Figure 1). It is implied that the PDA layer of PDA@OMS-2 played an F

DOI: 10.1021/acs.iecr.7b01353 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Some of the Mn2+ ions may be on the surface of OMS-2 but with a relative low concentration, and other Mn2+ ions were in the solution. As a result of low Mn2+ ions concentration, MnSn(OH)6 could not be formed on OMS-2. Instead, SnO2 was formed on OMS-2. UV−vis spectra have been measured to investigate the relative abundance of the Mn2+ ions in the supernatants of the solutions. The complex of Mn2+ ions with formaldoxime produced in the colorimetric reaction has a characteristic absorbance peak at 450 nm.32,33 A larger intensity at 450 nm means a higher concentration of Mn2+ ions in the solution. Four systems have been investigated. SnO2@OMS-2 is compared with MnSn(OH)6/PDA@OMS-2. The spectra (Figure 11a) indicate that the supernatant of SnO2@OMS-2 contained a higher concentration of Mn2+ ions than that of MnSn(OH)6/PDA@OMS-2. In the latter supernatant, the concentration of Mn2+ ions was lower due to the formation of MnSn(OH)6 on the surface of PDA@OMS-2. It is consistent with the results of XRD pattern (Figure 1). To investigate whether Mn2+ ions can be enriched on the surface of PDA@ OMS-2, the two samples OMS-2 and PDA@OMS-2 were separately immersed in diluted H2SO4 as described in the experimental section. The supernatant of the OMS-2 system contained a higher concentration of Mn2+ ions than that of the PDA@OMS-2 system (Figure 13b). It is implied that, in the system of PDA@OMS-2, the Mn2+ ions interacted with the PDA layer and enriched on the surface of PDA@OMS-2. The results in Figure 13 together with the XRD patterns in Figure 1 support the mechanism of formation of MnSn(OH)6 on the surface of PDA@OMS-2 as illustrated in Figure 12a.

important role in the formation of MnSn(OH)6. For the formation of MnSn(OH)6 mediated by PDA, the mechanism is proposed as illustrated in Figure 12a. As the PDA layer is

Figure 12. (a) Schematic presentation for the polydopamine (PDA)mediated formation of MnSn(OH)6 on PDA@OMS-2. (b) Schematic presentation for the formation of SnO2 on OMS-2.

4. CONCLUSIONS Cryptomelane-type manganese oxide (OMS-2) is not expensive and can be easily prepared. Coating a thin layer of polydopmaine (PDA) on OMS-2 can be achieved at room temperature in a short time (5 min) due to the oxidative property of OMS-2. It has been found that the PDA thin layer plays a vital role for the formation of MnSn(OH)6 on OMS-2. The composite MnSn(OH)6/PDA@OMS-2 is a novel catalyst for catalyzing glucose isomerization to fructose. Under the catalysis of this composite, the yield of fructose was 47 ± 1.6%. Consecutive use of the composite demonstrated that, after eight cycles, the activity can be retained in terms of the yield of fructose. When used together with the Brønsted acid catalyst Amberlyst-15, the HMF yield from glucose was 53 ± 2.5%. The composite MnSn(OH)6/PDA@OMS-2 has exhibited advantages, including not being expensive, being easily prepared, and

porous,34 the Mn2+ ions diffused throughout the layer of PDA. The catechol side chain (groups) of PDA can form a complex with the Mn2+ ions.44 Thus, the Mn2+ ions can be enriched on the surface of PDA@OMS-2. The concentration of the Mn2+ ions matched the hydrolysis of [Sn(OH)5(H2O)]−1,28 and MnSn(OH)6 was formed. When the concentration of SnCl4 is larger than 100 mM, no MnSn(OH)6 was formed. This is possibly ascribed to the fact that the concentration of the Mn2+ ions on the surface of PDA@OMS-2 did not match the hydrolysis of SnCl4. Further hydrolysis of [Sn(OH)6]−2 resulted in SnO2 as explained by reaction 3. Figure 12b shows the control by using OMS-2. When OMS-2, instead of PDA@OMS-2, was impregnated in the SnCl4 solutions, the Mn2+ ions could not be enriched on the surface of OMS-2.

Figure 13. UV−vis spectra for the detection of Mn2+ in the supernatants. Systems: (a) SnO2/PDA@OMS-2 and MnSn(OH)6/PDA@OMS-2; (b) OMS-2+dilute H2SO4 and PDA@OMS-2+dilute H2SO4. G

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(12) Liu, C.; Carraher, J. M.; Swedberg, J. L.; Herndon, C. R.; Fleitman, C. N.; Tessonnier, J. Selective base-catalyzed isomerization of glucose to fructose. ACS Catal. 2014, 4, 4295. (13) Yue, C.; Magusin, P. C. M. M.; Mezari, B.; Rigutto, M.; Hensen, E. J. M. Hydrothermal synthesis and characterization of a layered zirconium silicate. Microporous Mesoporous Mater. 2013, 180, 48. (14) Saravanamurugan, S.; Paniagua, M.; Melero, J. A.; Riisager, A. Efficient isomerization of glucose to fructose over zeolites in consecutive reactions in alcohol and aqueous media. J. Am. Chem. Soc. 2013, 135, 5246. (15) Chakrabarti, A.; Sharma, M. M. Cationic ion exchange resins as catalyst. React. Polym. 1993, 20, 1. (16) Atanda, L.; Mukundan, S.; Shrotri, A.; Ma, Q.; Beltramini, J. Catalytic conversion of glucose to 5-hydroxymethylfurfural with a phosphated TiO2 catalyst. ChemCatChem 2015, 7, 781−790. (17) Tian, G.; Tong, X. L.; Cheng, Y.; Xue, S. Tin-catalyzed efficient conversion of carbohydrates for the production of 5-hydroxymethylfurfural in the presence of quaternary ammonium salts. Carbohydr. Res. 2013, 370, 33. (18) Hu, S.; Zhang, Z.; Song, J.; Zhou, Y.; Han, B. Efficient conversion of glucose into 5-hydroxymethylfurfural catalyzed by a common Lewis acid SnCl4 in an ionic liquid. Green Chem. 2009, 11, 1746. (19) Moliner, M.; Román-Leshkov, Y.; Davis, M. E. Tin-containing zeolites are highly active catalysts for the isomerization of glucose in water. Proc. Natl. Acad. Sci. U. S. A. 2010, 107, 6164. (20) Deval, R. B.; Assary, R. S.; Nikolla, E.; Moliner, M.; RománLeshkov, Y.; Hwang, S.; Palsdottir, A.; Silverman, D.; Lobo, R. F.; Curtiss, L. A.; Davis, M. E. Metalloenzyme-like catalyzed isomerizations of sugars by Lewis acid zeolites. Proc. Natl. Acad. Sci. U. S. A. 2012, 109, 9727. (21) Chang, C.; Cho, H. J.; Wang, Z.; Wang, X.; Fan, W. Fluoridefree synthesis of a Sn-BEA catalyst by dry gel conversion. Green Chem. 2015, 17, 2943. (22) Jin, J.; Ye, X.; Li, Y.; Wang, Y.; Li, L.; Gu, J.; Zhao, W.; Shi, J. Synthesis of mesoporous beta and Sn-Beta zeolites and their catalytic performances. Dalton Trans. 2014, 43, 8196. (23) Liu, M.; Jia, S.; Li, C.; Zhang, A.; Song, C.; Guo, X. Facile preparation of Sn-β zeolites by post-synthesis (isomorphous substitution) method for isomerization of glucose to fructose. Chin. J. of Catal. 2014, 35, 723. (24) Zhao, S.; Guo, X.; Bal, P.; Lv, L. Chemical isomerization of glucose to fructose production. Asian J. Chem. 2014, 26, 4537. (25) Jiang, Y.; Yang, L.; Bohn, C. M.; Li, G.; Han, D.; Mosier, N. S.; Miller, J. T.; Kenttämaa, H. I.; Abu-Omar, M. M. Speciation and kinetic study of iron promoted sugar conversion to 5-hydroxymethylfurfural (HMF) and levulinic acid (LA). Org. Chem. Front. 2015, 2, 1388. (26) Wolf, P.; Liao, W.; Ong, T.; Valla, M.; Harris, J. W.; Gounder, R.; van der Graaff, W. N. P.; Pidko, E. A.; Hensen, E. J. M.; Ferrini, P.; Dijkmans, J.; Sels, B.; Hermans, I.; Coperet, C. Identifying Sn site heterogeneities prevalent among Sn-Beta zeolites. Helv. Chim. Acta 2016, 99, 916. (27) Yakimov, A. V.; Kolyagin, Y. G.; Tolborg, S.; VennestrØm, P. N. R.; Ivanova, I. I. Accelerated synthesis of Sn-BEA in fluoride media: effect of H2O content in the gel. New J. Chem. 2016, 40, 4367. (28) Neilson, J. R.; Kurzman, J. A.; Seshadri, R.; Morse, D. E. Ordering double perovskite hydroxides by kinetically controlled aqueous hydrolysis. Inorg. Chem. 2011, 50, 3003. (29) Tang, B.; Dai, W.; Wu, G.; Guan, N.; Li, L.; Hunger, M. Improved postsynthesis strategy to Sn-Beta zeolites as Lewis acid catalysts for the ring-opening hydration of epoxides. ACS Catal. 2014, 4, 2801. (30) Deguzman, R. N.; Shen, Y.; Neth, E. J.; Suib, S. L.; O’Young, C.; Levine, S.; Newsam, J. M. Synthesis and characterization of octahedral molecular sieves (OMS-2) having the hollandite structure. Chem. Mater. 1994, 6, 815.

having a high catalysis efficiency. In addition, the preparation of MnSn(OH)6/PDA@OMS-2 has been carried out at low temperatures (≤70 °C), and high temperature calcinations are not needed. The whole processing, including the preparations of OMS-2, coating of PDA, and formation of MnSn(OH)6 on PDA@OMS-2, can be accomplished within 36 h. The catalyst can be further explored and has a potential application for glucose isomerization.

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AUTHOR INFORMATION

Corresponding Author

*Tel. +86 10 64446249. E-mail: [email protected]. cn. ORCID

Peijun Ji: 0000-0001-9449-4213 Author Contributions †

X.J. and P.L. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Science Foundation of China (21476023).

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DOI: 10.1021/acs.iecr.7b01353 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX