Energy Fuels 2010, 24, 4793–4796 Published on Web 08/31/2010
: DOI:10.1021/ef100876k
Modified Niobia As a New Catalyst for Selective Production of Dimethoxymethane from Methanol Nayara T. Prado,† Francisco G. E. Nogueria,† Andre E. Nogueira,† Cleiton A. Nunes,† Renata Diniz,‡ and Luiz C. A. Oliveira*,† †
Department of Chemistry, Federal University of Lavras, Caixa Postal 3037, CEP 37200-000, Lavras, MG, Brazil, and ‡ Department of Chemistry, Federal University of Juiz de Fora, Juiz de Fora, Minas Gerais, Brazil Received June 24, 2010. Revised Manuscript Received August 25, 2010
Synthetic niobia, after an innovative surface modification, was utilized for synthesis of dimethoxymethane (DMM) from methanol. A bifunctional niobium oxide, obtained by a previous treament with H2O2, presented a highly selective production of DMM in the liquid phase. The analysis of the product with mass spectrometry showed that dehydration/oxidation takes place to selectively produce DMM. The results strongly suggest that the simultaneous methanol dehydration and oxidation involves bifunctional properties of the niobias with acid sites and oxidizing species generated after the modification with H2O2, which were identified by Raman spectroscopy with a strong signal at 888 cm-1 assigned to ν(O-O) due to the peroxo species generated in modified niobia.
1. Introduction Recently, research has been focused mostly on the investigation of surface properties and catalytic performance of supported niobium oxide catalysts prepared by various methods, rather than on metal oxides deposited on niobia supports.1-3 Versatile catalysts can became more efficient in important industrial scale processes, such as the production of dimethoxymethane (DMM) by direct methanol oxidation.4 DMM has low toxicity and can be used in several industrial processes. Two important uses recently reported were to produce H2 and also a diesel/DMM blend.5 In fact, some preliminary studies revealed that the reduction of particulate emissions and toxic gas pollutants could be achieved when fueling with the diesel/DMM blends.5-7 Results of diesel engine studies suggest that the addition of ethers is more effective in reducing smoke emissions than the addition of alcohols.8 Among the catalysts reported to date, Re/TiO29 and also V2O5/TiO210 were the most effective in the conversion of methanol to DMM. However, the modified niobia catalyst based process is advantageous because the reaction can be *To whom correspondence should be addressed. Telephone: þ55 35 38291626. Fax: þ55 35 38291271. E-mail:
[email protected]. (1) Oliveira, L. C. A.; Zaera, F.; Lee, I.; Lima, D. Q.; Ramalho, T. C.; Silva, A. C.; Fonseca, E. M. B. Appl. Catal., A 2009, 368, 17–21. (2) Oliveira, L. C. A.; Ramalho, T. C.; Gonc-alves, M.; Cereda, F.; Carvalho, K. T. G.; Nazarro, M. S.; Sapag, K. Chem. Phys. Lett. 2007, 446, 133–137. (3) Esteves, A.; Oliveira, L. C. A.; Ramalho, T. C.; Gonc-alves, M.; Anast acio, A. S.; Carvalho, H. W. P. Catal. Commun. 2008, 10, 330–332. (4) Fu, Y.; Shen, J. Chem. Commun. 2007, 21, 2172–2174. (5) Ren, Y.; Huang, Z.; Jiang, D.; Liu, L.; Zeng, K.; Liu, B.; Wang, X. Appl. Ther. Eng. 2006, 26, 327–337. (6) Yuan, Y.; Shido, T.; Iwasawa, Y. Chem. Commun. 2000, 23, 1421– 1422. (7) Yuan, Y.; Iwasawa, Y. J. Phys. Chem. B. 2002, 106, 4441–4449. (8) Sinha, M.; Thomson, J. Combust. Flame 2004, 136, 548–556. (9) Secordel, X.; Berrier, E.; Capron, M.; Cristol, S.; Paul, J.; Fournier M.; Payen E. Catal. Today In press. (10) Zhao, H.; Bennici, S.; Shen, J.; Auroux, A. J. Catal. 2010, 272, 176–189. r 2010 American Chemical Society
Figure 1. XPS profile of the O1s region of pure niobia and after treatment with hydrogen peroxide.
carried out at comparatively lower temperatures (120 °C) and at a low preparation cost, i.e., without impregnation with the active phase. Thus, the use of Nb2O5 as a catalyst can be a new alternative for the dehydration/oxidation of methanol to produce DMM. In this direction, this present paper reports the application of modified Nb2O5 in the decomposition of methanol after treatment with hydrogen peroxide to generate a strong oxidizer group called a peroxo group on the niobia surface.11 The reaction intermediates were monitored by mass spectrometry, (11) Ebbesen, S. D.; Mojet, B. L.; Lefferts, L. J. Catal. 2007, 246, 66–73.
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Energy Fuels 2010, 24, 4793–4796
: DOI:10.1021/ef100876k
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and the peroxo species formation was investigated by Raman spectroscopy.
the charging effect by assuming a constant binding energy for the adventitious C1s peak of 284.6 eV. Fourier-transform Raman spectroscopy was carried out using a Bruker RFS 100 instrument, Nd3þ/YAG laser operating at 1064 nm in the near-infrared, and the CCD detector was cooled with liquid N2. Good signal-to-noise ratios were obtained from 256 scans accumulated over a period of about 30 min and 100 mW of laser power, using 4 cm-1 as the spectral resolution. The powder X-ray diffraction (XRD) data were obtained in a Rigaku model Geigerflex using Cu KR radiation scanning from 2 to 75° at a scan rate of 4° min-1. 2.2. Catalytic Tests. In a typical run, the reaction was carried out in a sealed glass reactor with 10 mg of catalyst at 120 °C with vigorous stirring. A molar ratio of 100:1 of methanol to 50% aqueous hydrogen peroxide (industrial grade) was used. The products were analyzed in a chromatographic system (gas chromatography-mass spectrometry, GC-MS) with an Alltech Econo-Cap SE (30 m 0.32 mm 0.25 μm) capillary column.
2. Experimental Section 2.1. Synthesis and Characterization. Niobia was prepared by slow dropping of a 1 mol L-1 NaOH solution in a 500 mL Teflon beaker containing 100 mL of 0.26 mol L-1 solution of NH4NbO(C2O4)(H2O)](H2O)n, kindly donated by Companhia Brasileira de Metallurgia e Minerac-~ao (CBMM; Araxa, state of Minas Gerais, Brazil) at 70 °C under vigorous stirring. The solids obtained were washed with distilled water until the pH was neutral. The precipitates were washed with water and dried at 150 °C for 12 h. The treatment with hydrogen peroxide was performed treating the previously synthesized niobium oxide (300 mg) with 8 mL of aqueous hydrogen peroxide 30% (v/v) and 80 mL of water for 60 min at room temperature. After these contact times, the solids were washed with distilled water and oven-dried for 12 h at 150 °C. All the chemical agents used in the experimentation were laboratory grade chemicals and supplied by Merck KGaA Darmstadt Germany. The liquid phase reactions were carried out in a 25 cm3 glass reactor purchased from VETEC Company. XPS was performed by using Mg KR radiation (hν = 1253.6 eV) and VG hemispherical electron-energy analyzer using a pass energy of 20 eV. The chamber pressure during the measurement was around 10-9 Torr. The binding energies were corrected for
3. Results and Discussion 3.1. Niobia Characterization. Strong evidence of the peroxo group formation was observed by XPS analysis (Figure 1). The XPS profile presented the formation of additional oxygenated groups on the niobia surface formed after the previous treatment with hydrogen peroxide and identified by
Figure 2. Raman spectra of (a) niobia and (b) modified niobia.
Figure 4. XRD for the synthetic niobia and modified niobia.
Figure 3. Deconvolution of band around 888 cm-1 in modified niobia.
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Figure 5. SEM micrographs (a) and EDS analyses of niobias (b).
agreement with experimental vibrational spectra of similar structures.15 XRD patterns shown in Figure 4 for the samples indicate that the materials are amorphous. These results are in very good agreement with other experimental studies.3 Figure 5 shows the morphologies of the pure niobia (top) and that treated for 60 min (bottom) with H2O2 (modified niobia). These micrographs present small aggregates for all the samples, but it is interesting to observe that the previous treatment with H2O2 significantly modifies the morphology causing a strong agglomeration of the particles. Elemental analysis by EDS confirmed the absence of high amounts of impurities such as Naþ cations that could interfere in the catalytic reaction. The materials showed specific surface areas of 19 and 7 m2 g-1 for pure and modified niobia, respectively. The decreasing in the surface area is in agreement with SEM data that showed particle agglomeration after H2O2 treatment. 3.2. Conversion of Methanol. Methanol conversion was about 58%, and the selectivity of dimethoxymethane was almost 100% using the material treated with hydrogen peroxide (modified niobia). Notably, the activity (TOF) is about 4.8 mmol/(m2 h), which is in agreement with the values reported in the literature on a similar reaction system.16 The untreated niobia or the system methanol//H2O2 (without catalyst) did not present significant methanol conversion. Despite the lower conversion, only the formation of dimethoxymethane was observed without the presence of other products, such as formaldehyde or dimethyl ether. This results are in accordance with other authors where byproducts were not observed in gas phase reactions.17,18 It is
the O1s spectra. The main peak at 529.9 eV related to oxygen anions is accompanied by a peak at 532.1 eV attributed to the formation of additional surface oxygenated groups.2 Similar to XPS, the Raman spectra analysis also suggested that the peroxo species are formed after treatment of niobia with hydrogen peroxide. Figure 2 displays the Raman spectra of niobia before (Figure 2a) and after (Figure 2b) this treatment. In the modified niobia spectra, bands are observed at 888, 670, and 549 cm-1 which can be assigned to the stretching modes of metal coordinated to peroxo ligands.12 In niobia without treatment, these spectra bands are not observed. The band observed at 888 cm-1 is attributed to peroxo stretching [ν(O-O)], and theoretical calculations13 indicated that different stoichiometries in peroxo complex present different modes for ν(O-O). In tetra and triperoxo species, three modes for ν(O-O) are expected, although in di (cis and trans) and monoperoxo complexes two modes and one mode are predicted, respectively. In Figure 2b, a asymmetric band at 888 cm-1 is observed, suggesting the presence of more than one band. The deconvolution analysis (Figure 3) of this band indicates the presence of two bands at 875 and 891 cm-1, which are in agreement with theoretical12 and experimental14,15 data for diperoxo species. The wavenumber difference of these bands (16 cm-1) is similar to experimental vibrational data for the cis-diperoxo species suggesting that in modified niobia the cis-diperoxo species was obtained. The other two bands observed at 670 and 549 cm-1 are assigned to symmetric and antisymmetric stretching mode, respectively, of metal-peroxo {ν[M(O2)]}, in (12) Bayot, D.; Tinant, B.; Devillers, M. Inorg. Chem. 2005, 44, 1554– 1562. (13) Bayot, D.; Devillers, M.; Peeters, D. Eur. J. Inorg. Chem. 2005, 20, 4118–4213. (14) Bayot, D.; Tinant, B.; Devillers, M. Catal. Today. 2003, 78, 439– 447. (15) Bayot, B.; Tinant, B.; Mathieu, B.; Declercq, J.-P.; Devillers, M. Eur. J. Inorg. Chem. 2003, 4, 737–743.
(16) Khandan, N.; Kazemeini, M.; Aghaziarati, M. Appl. Catal., A 2008, 349, 6–12. (17) Hu, H.; Wachs, I. E. J. Phys. Chem. 1995, 99, 10911–10922. (18) Liu, J.; Sun, Q.; Fu, Y.; Shen, J. J. Colloid Interface Sci. 2009, 335, 216–221.
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Figure 7. Schematic model of the simultaneous presence of acid sites and also the peroxo group.
catalyst. Niobia, after five reaction cycles, did not present catalytic activity loss, suggesting that the oxidizing sites are regenerated by the addition of H2O2. The importance of H2O2 addition as oxidizing agent was confirmed by reaction without its addition, where no activity was observed (see cycle number four in Figure 5). Moreover, the H2O2// methanol system (without niobia) also did not present catalytic activity. These results suggest that the role of added H2O2 is for peroxo group regeneration on niobia. The simultaneous presence of acid and oxidant sites characterizing the bifunctional properties of the modified niobia is illustrated in Figure 7.
Figure 6. Reuse of modified niobia in the oxidation of methanol to dimethoxymethane.
interesting to observe that the high selectivity for DMM is probably due to bifunctional (oxidizing and acidic properties) surface characteristics of the modified niobia. Similar results were obtained by Zhao et al.10 utilizing mesoporous V2O5-TiO2-SO42- also in the gas phase. The oxidizer properties are generated after surface modification on niobia by the previous treatment with H2O2 of the synthetic niobia.3 Recently, a new class of niobia was developed by pretreatment of a synthetic niobia with H2O2 to generate a modified niobia. This surface modification generates a strong oxidizer group called a peroxo group on the niobia surface.2,9 These results suggest a new route to obtain dimethoxymethane selectively from a bifunctional catalyst based on niobia. Generally, surface acid sites produce the dehydration product dimethyl ether and acid/oxidant sites produce formaldehyde, methyl formate, and dimethoxymethane.19 In fact, almost all papers presented in the literature present vanadium, molybdenum, or iron oxides supported, but work using pure niobia as the active phase are scarce.18-20 To check for the absence of heat and mass transfer limitations, the Koros-Nowak test21 was employed. This test was carried out with different amounts of catalyst (10, 40, and 100 mg). It was observed that the systems have the same TOF, which indicates that there are no heat or mass transfer limitations in our measurements. Figure 6 shows the conversion of methanol in reaction cycles after 2 h of reaction in order to study the stability of the
4. Conclusions Several authors have studied the kinetics of the methanol partial oxidation. However, most of the previous work was limited to the formation of formaldehyde and not much attention was paid to the formation of DMM. In this work, we have demonstrated that the niobia previously treated wih H2O2 presented a high catalytic activity in the direct oxidation of methanol to DMM. The active phase obtained after the previous treatment with H2O2 was identified by XPS and Raman measurements. These results are novel because they describe, for the first time, the reaction in liquid phase and at a low temperature such as 120 °C using niobium oxide as catalyts in the presence of hydrogen peroxide without metal impregnation. These results are interesting because the dimethoxymethane, with a high oxygen content and cetane number, is considered a promising additive to diesel oil and also can reduce the smoke emissions when added to diesel fuels.
(19) Deo, G.; Wachs, I. E. J. Catal. 1994, 146, 323. (20) Deshmukh, S. A. R. K.; Analand, M.; Kuipers, J. A. M. Appl. Catal., A 2005, 289, 240–255. (21) Koros, R. M.; Nowak, E. J. Chem. Eng. Sci. 1967, 22, 470.
Acknowledgment. This work was supported by the CNPq, CBMM (Brazil), FAPEMIG, FINEP (Grant No. PROINFRA 1124/06), and CAPQ (DQI-UFLA).
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