Role of Lattice Oxygen and Lewis Acid on Ethanol Oxidation over

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Role of Lattice Oxygen and Lewis Acid on Ethanol Oxidation over OMS-2 Catalyst Junhua Li,* Renhu Wang, and Jiming Hao Department of EnVironmental Science and Engineering, Tsinghua UniVersity, Beijing 100084, China ReceiVed: March 28, 2010; ReVised Manuscript ReceiVed: May 9, 2010

Ethanol-gasoline fuel vehicles result in a statistically significant increase in emissions of ethanol, although they are environmentally friendly due to their lower levels of carbon oxide and hydrocarbon emissions. This work focused on catalytic oxidation of ethanol on octahedral molecular sieve (OMS-2) catalyst, which was synthesized using refluxing method. The catalyst demonstrated great reactivity with acetaldehyde as the major intermediate. Small amounts of formaldehyde were detected while little acetic acid was observed in the presence of oxygen. Results suggested that ethanol adsorption occurred through the breakage of O-H bond forming ethoxide species on Lewis acid sites of the catalyst and OMS-2 could partially oxidize adsorbed ethanol to acetaldehyde and formaldehyde from the consumption of catalyst lattice oxygen in the absence of oxygen flow. Acetaldehyde and formaldehyde formed were directly oxidized to carbon dioxide in the presence of an oxygen flow. The major path of ethanol complete oxidation on OMS-2 catalyst appeared to be the direct oxidation of acetaldehyde to carbon dioxide. 1. Introduction Ethanol-gasoline fuel vehicles are more environmentally friendly than other conventional gasoline fuel vehicles due to their lower levels of carbon oxide and hydrocarbon emissions.1-4 However, the use of highly blended ethanol gasoline results in a statistically significant increase in emissions of ethanol,1 acetaldehyde, and formaldehyde.2 In addition, ethanol and acetaldehyde are quite resistant to decomposition by Pt/Rh-based three-way catalysts (the conversion efficiency of ethanol is lower than acetaldehyde1,4). The majority of hydrocarbons are emitted within the first 200 s of the cold start and warm-up periods until the automotive catalyst reaches its light-off temperature.5 Therefore, more efficient catalysts for the reduction of ethanol emissions are needed during these periods. Great efforts have been made to develop efficient catalysts for complete oxidation of ethanol. Manganese-oxide-based catalysts, which are cheaper alternatives for noble metal catalysts with high sintering resistance,6 are among the most active catalysts in ethanol decomposition and their catalytic activities are known to be as high as or slightly higher than those of the supported noble metals.7-13 Among them, OMS-2 catalysts have been widely examined and were recently found to effectively oxidize benzyl alcohol,14 ethyl acetate,15 benzene,16 2-thiophenemethanol,17 and ethanol.13 The OMS-2 structure are made up of peculiar sharing of 2 × 2 [MnO6] octahedral chains that form one-dimensional tunnel structures with pore size of 0.46 nm × 0.46 nm (see Figure 1) and the excellent catalytic activity is partly due to the mild surface acidity-basicity, porous structure, redox property and ion-exchange ability. Many correlations between the high activities for volatile organic compounds (VOCs) oxidation on manganese oxides and the reaction mechanism and structural explanations have been proposed. Lamaita et al.9 reported that the high activity could be explained by the existence of Mn4+/Mn3+ couple, poor catalyst crystallinity and Mn4+ vacancies. Gandhe et al.15 suggested that the presence of Mn4+-O2- Lewis acid-base * To whom correspondence should be addressed. Tel.: +86 10 62771093. Fax: +86 10 62788013. E-mail address: [email protected].

pairs, the ability of Mn to exist as a redox couple viz. Mn4+/ Mn3+, and the availability of facile lattice oxygen facilitated the oxidation process. The activities of OMS-2 reported in the literature show large differences, and there is no clear understanding of the reaction pathway of ethanol oxidation. In this work, the OMS-2 catalyst was prepared by refluxing method, and it showed great activity for complete oxidation of ethanol so far. The reaction intermediates were analyzed on line, and a mechanism involving the possible reactions of ethanol on OMS-2 was proposed. 2. Experimental Methods 2.1. Catalyst Synthesis. OMS-2 sample was synthesized via the reaction between Mn2+ and MnO4- through a refluxing method. The KMnO4 solution was added into a Mn(CH3COO)2 solution to achieve a MnO4-/Mn2+ molar ratio of 0.72. The mixture was then refluxed for 24 h and the temperature was maintained at about 100 °C. The sample was then filtered, washed with distilled deionized water, dried at 110 °C overnight, and calcined at 400 °C for 4 h. 2.2. Catalytic Performance. The activity tests for ethanol catalytic oxidation were performed in a fixed-bed quartz flow reactor (8 mm in diameter) with a gaseous mixture containing 300 ppm ethanol, 10 vol % O2 and N2 balanced at a total flow rate of 150 cm3 min-1, corresponding to a gas hourly space velocity (GHSV) of 36 000 h-1. With the change of catalyst volume, GHSV were equivalent to 36 000, 72 000, and 144 000 h-1. The concentrations of carbon dioxide and acetaldehyde were analyzed online using an Agilent 7890A gas chromatograph (GC). The outlet gas species and corresponding FT-IR spectra were recorded real-time using a Gasmet DX4000 in transient experiments. 2.3. Catalyst Characterization. Brunauer-Emmett-Teller (BET) surface area and pore parameter of the sample was measured by nitrogen physisorption at liquid nitrogen temperature (77 K) on a Micromeritics ASAP 2010 micropore size analyzer. The pore size distribution was calculated from the desorption branch of the N2 adsorption isotherm using the Barrett-Joyner-Halenda (BJH) method.

10.1021/jp102779u  2010 American Chemical Society Published on Web 05/18/2010

Ethanol Oxidation over OMS-2 Catalyst

J. Phys. Chem. C, Vol. 114, No. 23, 2010 10545

Figure 1. Coordination sphere of manganese, crystal structure model, and octahedron arrangement in crytomelane type oxide.9,15

3. Results and Discussion The results of fresh catalyst activity for ethanol oxidation are shown in Figure 2. The lowest temperature at which ethanol partial oxidation to acetaldehyde was detected on the sample was 60 °C and complete oxidation was achieved at 160 °C with carbon dioxide selectivity to 100%. The lowest temperature at which ethanol oxidation to carbon dioxide was detected was 100 °C. Acetaldehyde was the major intermediate species during ethanol complete oxidation process on OMS-2 catalyst with the highest acetaldehyde selectivity at 120 °C. The oxidation activity can also be represented quantitatively by first-order rate constant (k) (since the reaction is first order with respect to ethanol at excessive oxygen condition18). By assuming plug flow reactor (in a fixed bed of catalyst) and free of diffusion limitation, the rate constant can be calculated from the ethanol conversion (X) by Figure 2. Ethanol conversion, CO2 yield, and acetaldehyde selectivity on fresh OMS-2 catalyst. Reaction conditions: 300 ppm ethanol, 10 vol % O2, N2 as balance gas, GHSV ) 36 000 h-1.

X-ray powder diffraction (XRD) pattern was recorded on a TTR3 diffractometer operated at 40 kV and 40 mA, using Ni filtered Cu Ka radiation by step scanning with a scan rate of 10° 2θ/min. Transmission electron microscopy (TEM) images were obtained with a JEOL JEM-2011LaB6 at an accelerating voltage of 200 kV. The sample was ultrasonically suspended in ethanol and deposited on a copper grid covered with a thin layer of holey carbon. An infrared (IR) spectrum of pyridine adsorbed on the sample was used to determine the type of acidic sites. Around 10 mg of the sample was dehydrated at 400 °C under a vacuum of 10-3 Pa for 1 h. A blank spectrum was recorded at room temperature on a Nicolet Avatar360 FT-IR. The sample was exposed to pyridine vapors at room temperature for 30 min, followed by desorption for 30 min at each temperature, before the IR spectra were recorded. Thermal gravimetric analysis (TGA) was carried out on a TGA Q5000 V3.5 instrument in nitrogen at a heat rate of 10 °C/min from ambient temperature to 900 °C. In situ diffuse reflectance infrared Fourier-transform spectroscopy (in-situ DRIFTS) spectra was recorded on a NEXUS 870-FTIR equipped with a smart collector and a MCT/A detector cooled by liquid nitrogen. The sample was mixed with KBr to achieve a KBr/OMS-2 weight ratio of 9 and then the mixture was finely ground and placed in a ceramic crucible. Prior to the experiment, the mixture was preheated in a flow of nitrogen at 300 °C for 1 h, then cooled to the desired temperature and a spectrum of the sample was recorded as the background value. The spectra were recorded with a resolution of 4 cm-1 and with an accumulation of 100 scans.

k)-

F0 ln(1 - X) [Ethanol]0W

where F0 is the molar ethanol feed rate, [Ethanol]0 is the molar ethanol concentration at the inlet (at the reaction temperature), and W is the amount of catalyst (g). From the ethanol conversions and reaction conditions, first-order rate constants could be calculated and the results are summarized in Table 1. The activities of the OMS-2 catalyst over a wide range of GHSV from 36 000 to 144 000 h-1 were measured, and the results are shown in Figure 3. At 72 000 h-1, the conversions of ethanol decreased minimally and reached 100% at around 180 °C. The ethanol conversions of ethanol were maintained above 95% with temperatures higher than 180 °C even though the GHSV reached 144 000 h-1. This indicates that the OMS-2 catalyst was effective for ethanol oxidation within a wide range TABLE 1: Catalytic Activity of Ethanol Oxidation on OMS-2 conditions without H2O

10 vol % H2O

temperature/ ethanol CO2 k/cm3/ TOFa/ °C conversion/% selectivity/% g/s 10-3 s-1 60 80 100 120 140 160 120 140 160 180 200

12 26 41 73 95 >99 52 70 85 94 >99

0 0 0 27 84 >99 94 86 82 92 >99

2.8 4.5 8.3 22 52 84 12 21 34 53 91

1.7 3.7 5.8 10 13 14 7.4 9.9 12 13 14

a TOF (turnover frequency) is defined as the number of ethanol molecules converted per Mn per second. Reaction conditions: 300 ppm ethanol, 10 vol % O2, N2 as balance gas, GHSV ) 36 000 h-1.

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Figure 3. (A) Ethanol conversion and (B) Acetaldehyde selectivity on fresh OMS-2 catalyst as a function of GHSV. Reaction conditions: 300 ppm ethanol, 10 vol % O2, N2 as balance gas.

of GHSV. Acetaldehyde was the major intermediate and the highest acetaldehyde selectivity appeared at higher temperature under conditions with higher GHSV. The catalytic activity of OMS-2 under the reaction mixture with the addition of water vapor was evaluated and the results are shown in Figure 4 and Table 1. The catalytic activity decreased slightly in the presence of water vapor compared with that without water vapor. The lowest temperature of ethanol complete oxidation on OMS-2 rose from 160 to 200 °C with the addition of water vapor. However, according to recent engine study,5 catalyst in automobiles warmed up very quickly at temperatures lower than 200 °C, which reached 160 °C from ambient temperature within 22 s and 200 °C within 26 s during the cold start period. The catalytic performance is consistent with previous literature results in which OMS-2 was found to possess excellent hydrophobicity and strong affinity for VOCs, exhibiting relatively large uptake of ethanol and small uptake of water.16 The hydrophobicity may make it especially suitable for water-forming catalytic oxidation reactions by readily removing produced water molecules from active sites. As shown in Table 1 and Figure 4B, high carbon dioxide selectivity and only small amounts of acetaldehyde were registered in the presence of water vapor. The suppression effect of water vapor on ethanol conversion and acetaldehyde formation may be due to the competitive adsorption of water vapor versus ethanol on

Li et al.

Figure 4. (A) Ethanol conversion and (B) acetaldehyde selectivity with and without water vapor on OMS-2. Reaction conditions: 300 ppm ethanol, 10 vol % O2, 10 vol % H2O when used, N2 as balance gas, GHSV ) 36 000 h-1.

the active sites of OMS-2 catalyst surface. Similar phenomenon was observed by Einaga and Futamura19 who found that water vapor inhibited the build-up of organic byproducts on MnOx/ γ-Al2O3 catalyst surface since they adsorbed on the same sites of the catalyst surface. The higher carbon dioxide selectivity and smaller amounts of acetaldehyde counterbalance the lower activity since acetaldehyde is more harmful to human health and the environment than ethanol. Taking these results into consideration, it seems that the existence of water vapor benefits the application of OMS-2 for ethanol complete oxidation with limited loss in activity but much lower acetaldehyde emission. OMS-2 had BET surface area of 75.0 m2/g and BJH pore volume of 0.50 cm3/g. XRD result (not shown here) demonstrated that it was essentially the same as the pattern of synthetic cryptomelane (KMn8O16, JCPDS 34-168). Figure 5 shows the TEM micrographs of the prepared sample at low magnification and high magnification. The sample displayed fibrous morphologies with diameters between 10 and 30 nm. Pyridine IR results of OMS-2 are shown in Figure 6. The prominent peak at 1450 cm-1 could be attributed to pyridine adsorbed on Lewis acid sites. The absence of an obvious peak around 1540 cm-1 implied the lack of strong Brønsted acid sites. However, the peak at 1605 cm-1 denoted hydrogen-bonded



Figure 5. TEM images of synthesized OMS-2 catalyst at low magnification (left) and high magnification (right).

Figure 6. Pyridine IR results of OMS-2. (a) Pyridine adsorption at 40 °C and pyridine desorption at (b) room temperature, (c) 50, (d) 100, (e) 150, (f) 200, (g) 300, and (h) 400 °C.

pyridine, which implied very weak Brønsted acidity and the peak at 1485 cm-1 arouse due to contribution of both the Lewis and Brønsted acid sites to pyridine adsorption. Thus, the sample could show the presence of strong Lewis acidity and weak Brønsted acidity, probably due to the predominant existence of Mn4+. As mentioned above, OMS-2 materials are made up of peculiar sharing of [MnO6] octahedral chains. This suggests the existence of Mn4+-O2- type Lewis acid-base pairs in these materials. These results are in agreement with the findings made by Makwana et al.14 and Gandhe et al.15 As seen in Figure 6, the peak of pyridine adsorption on OMS-2 decreased as the desorption temperature increased from room temperature to 400 °C. The profile of TGA of the sample is shown in Figure 7. The profile shows the corresponding weight losses in the following range: 30-280, 280-450, 450-570, and 570-770 °C. The sample gave 1.5% wt losses in the 30-280 °C range, which were generally attributed to adsorbed water, carbon dioxide, as well as some physically absorbed oxygen. Further, there were very slight weight losses of the sample in the 280-450 °C regions, which were believed to have resulted from the chemisorbed oxygen.20 The weight losses of about 1.8% for the sample in the 450-570 °C regions were attributed solely to lattice oxygen evolution from OMS-2 material.20,21 The weight losses in the 570-770 °C regions were generally due to the collapse of OMS-2 and the decomposition of manganese oxide to lower oxidation state with the second lattice oxygen release.16,17,22 TGA profile indicated the facility of lattice oxygen

Figure 7. Thermal profile of OMS-2 catalyst.

dislodged from the OMS-2 framework. It is necessary to mention that the TGA was carried out in a nitrogen atmosphere, while the actual ethanol oxidation was carried out in an oxygen flow condition. It was confirmed that OMS-2 was more thermally stable in oxygen than in a nitrogen atmosphere.20 To investigate the role of lattice oxygen in ethanol oxidation on OMS-2, a transient experiment was performed with realtime monitoring of the outlet gas species. Figure 8 shows the concentration changes of the outlet gas species as a function of temperature. The reaction was carried out in a flow of ethanol and nitrogen from 60 to 180 °C. Then, the flow of ethanol in nitrogen was switched to oxygen in nitrogen. Ethanol was oxidized mostly to acetaldehyde and formaldehyde in the absence of oxygen, as well as trace volumes of acetic acid when the reaction temperature was higher than 140 °C. It demonstrated that OMS-2 could partially oxidize ethanol to acetaldehyde and formaldehyde from the consumption of catalyst lattice oxygen in the absence of oxygen. When the ethanol flow was changed to oxygen, all species including ethanol, acetaldehyde, formaldehyde and acetic acid were quickly and completely oxidized to carbon dioxide. Figure 9 shows the in situ DRIFTS spectra of the OMS-2 sample as a function of temperature in a nitrogen, oxygen, and ethanol flow. Ethanol oxidation at 40 °C gave rise to the formation of strong band at 1050 cm-1, which could be attributed to ethoxide species.23 The band of ethoxide decreased as the temperature increased from 40 to 160 °C due to ethanol desorption and oxidation. This indicates that ethanol adsorption on OMS-2 occurred through the breakage of O-H bond forming

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Figure 8. Concentration changes of the outlet gas species of ethanol oxidation on OMS-2 in a flow of ethanol and nitrogen. (A) (a) Ethanol, (b) acetaldehyde; (B) (a) formaldehyde, (b) acetic acid. Reaction conditions: 300 ppm ethanol when used, 10 vol % O2 when used, N2 as balance gas, GHSV ) 36 000 h-1.

Figure 9. In-situ DRIFTS spectra of adsorbed species in a steady state on OMS-2 in a flow of ethanol, oxygen, and nitrogen at various temperatures: (a) 40, (b) 80, (c) 120, and (d) 160 °C.

ethoxide species rather than the breakage of C-H bond. Similar processes have been reported through methanol oxidation on molybdenum catalysts and iron catalysts.24-26 Mavrikakis and

Li et al. Barteau27 also reviewed that cleavage of alcohol O-H bonds upon their adsorption on transition metals appeared to be a general phenomenon, leading to the formation of stable alkoxide intermediates. A reaction scheme in terms of chemical structures without an oxygen flow is proposed in Scheme 1. On the surface of OMS-2, ethanol can be envisaged to undergo adsorption on the Lewis acid sites through donation of a lone pair of electrons from the oxygen atom of hydroxyl to the surface forming ethoxide intermediate. The adsorbed ethoxide species undergo sequential R-C-H and β-C-H bonds cleavage to produce adsorbed acetaldehyde and formaldehyde, respectively, as shown in Scheme 1. If a primary hydrogen atom of the ethoxide intermediate is attacked by the lattice oxygen and leaves, the β-carbon becomes comparatively electronegative and anchors onto adjacent Lewis acid site with the transfer of electron cloud to the Mn4+. This results in the weakness and breakage of the C-C bond of adsorbed ethoxide and the formation of formaldehyde. If a secondary hydrogen atom is attacked by the lattice oxygen and leaves, the R-carbon becomes partially electronegative and the electron cloud tends toward the oxygen atom, resulting in the formation of adsorbed acetaldehyde. The porous structure of OMS-2 catalyst with pore size of 0.46 nm × 0.46 nm favors ethanol decomposition because the molecular size of ethanol is close to half the pore size. It is in line with Son et al.28 who reported the lower reactivity toward large substrates on OMS-2 catalyst. The formation of acetaldehyde can be completed on one Lewis acid site while the formation of formaldehyde requires the cooperation of two Lewis acid sites, which is very unfavorable in kinetics. Therefore, far more acetaldehydes were detected than formaldehydes and thus the major reaction pathway is believed to be the lattice oxygen attacking on the adsorbed ethoxide at a secondary hydrogen atom, forming acetaldehyde. It is in good agreement with Wang et al.29,30 who identified that adsorbed ethoxide species were kinetically unstable transition intermediates and the one-step concerted dehydrogenation of ethanol to acetaldehyde on Pt was the pathway with the lowest barriers. It is noteworthy that in the absence of an oxygen flow, ethanol decomposition arouses crystal defects and phase transition to lower oxidation state, which is confirmed by Chen et al.31 and Krishnan et al.32

Figure 10. FT-IR spectra of the outlet gas species of ethanol oxidation on OMS-2 in a flow of ethanol and nitrogen stabled at different temperatures: (a) 120, (b) 140, (c) 160, and (d) 180 °C. Reaction conditions: 300 ppm ethanol, N2 as balance gas, GHSV ) 36 000 h-1.



SCHEME 1: Proposed Reaction Pathway in the Absence of Oxygen in Terms of Chemical Structures

However, in the presence of an oxygen flow, no phase change was observed by XRD after the reaction. The reversible transformation of crystalline phase in the presence of an oxygen flow is critical for oxidation process to complete the catalytic cycle. The variation of manganese oxidation and oxygen vacancies allow electron exchange and hence oxygen mobility from the surrounding environment to the surface and bulk. Aguero et al.33 suggested that oxygen vacancies were recognized as adsorption-desorption centers for oxygen flow and consequently acted as active centers in oxidation reactions. It was further suggested that reoxidation proceeded via adsorption of oxygen at a vacancy site to form peroxide14,34 or superoxide28 species. Figure 10 shows the FT-IR spectra of the outlet gas species at different temperatures in a flow of ethanol and nitrogen. Compared with the standard spectra of target species, the bands at 2972 and 1073 cm-1 were ascribed to ethanol, 2740 and 1750 cm-1 to acetaldehyde, and 2362 and 2323 cm-1 to carbon dioxide. As shown in the figure, the band intensities of ethanol decreased sharply as the temperature increased and reached a steady level after 160 °C while the band intensities of acetal-

dehyde increased continuously. The band intensities of carbon dioxide increased as the temperature increased from 120 to 140 °C. However, no acetic acid was detected until the reaction temperature reached 160 °C (see Figure 8B). Therefore, it can be deduced that carbon dioxide was not from the oxidation of acetic acid, but probably from the oxidation of acetaldehyde or formaldehyde. The results well prove the mechanism proposed in Scheme 1 in which acetaldehyde and formaldehyde are intermediates and acetaldehyde is predominant in the process of ethanol oxidation. Figure 11 shows the concentration changes of the outlet gas species as a function of temperature in a flow of ethanol, oxygen, and nitrogen. It can be seen that, almost no acetic acid was detected (less than 0.2 ppm) in the presence of molecular oxygen. However, acetaldehyde and formaldehyde were still detected during the reaction. These results demonstrate that acetaldehyde and formaldehyde, the intermediates of ethanol oxidation, were directly oxidized to carbon dioxide, not via acetic acid. It is in good agreement with Delimaris and Ioannides11 who suggested that the major path of ethanol complete oxidation on Mn-Ce catalysts appeared to be the direct oxidation of acetaldehyde to carbon dioxide.

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Li et al. °C. Acetaldehyde was the major intermediate in the process of ethanol oxidation. Small amount of formaldehyde was detected while little acetic acid was detected in the presence of oxygen. The pathway for complete oxidation of ethanol seems to follow a Mars-van Krevelen mechanism. Acknowledgment. This research was financially supported by the National High-Tech Research and Development (863) Program of China (Grants 2006AA060301 and 2009AA064806). References and Notes

Figure 11. Concentration changes of the outlet gas species of ethanol oxidation on OMS-2 in a flow of ethanol, oxygen, and nitrogen. (A) (a) Ethanol, (b) acetaldehyde; (B) (a) formaldehyde, (b) acetic acid. Reaction conditions: 300 ppm ethanol, 10 vol % O2, N2 as balance gas, GHSV ) 36 000 h-1.

The reaction process seems to involve (i) adsorption of ethanol on manganese cation forming ethoxide species; (ii) hydrogen abstraction by lattice oxygen ions and reduction of manganese cations; and (iii) a reoxidation process at manganese sites by oxygen donors and the replenishment of lattice oxygen vacancies to complete the catalytic cycle. This pathway is known as the Mars-van Krevelen mechanism.35 The partial oxidation of ethanol to acetaldehyde constitutes the first step of the Mars-van Krevelen mechanism where manganese cations act as Lewis acid sites for ethanol adsorption, and lattice oxygen ions that drive the decomposition process serve as basic sites. In the second step, manganese cations, acting as redox sites, undergo reoxidation via the replenishment of lattice oxygen vacancies by oxygen from the feed stream. 4. Conclusions The OMS-2 catalyst, synthesized through refluxing method for complete oxidation of ethanol, is cryptomelane type manganese oxide. The lowest temperature at which ethanol partial oxidation to acetaldehyde was detected on OMS-2 was 60 °C and complete oxidation of ethanol was achieved at 160

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Role of Lattice Oxygen and Lewis Acid on Ethanol Oxidation over

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