Improving Alkene Selectivity of Nanocarbon Catalyzed Oxidative

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Improving Alkene Selectivity of Nanocarbon Catalyzed Oxidative Dehydrogenation of n-Butane by Refinement of Oxygen Species Jiaquan Li, Peng Yu, Jingxin Xie, Jie Liu, Zehua Wang, Chongchong Wu, Junfeng Rong, Hongyang Liu, and Dangsheng Su ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b02282 • Publication Date (Web): 14 Sep 2017 Downloaded from http://pubs.acs.org on September 14, 2017

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Improving Alkene Selectivity of Nanocarbon Catalyzed Oxidative Dehydrogenation of n-Butane by Refinement of Oxygen Species Jiaquan Li,† Peng Yu,† Jingxin Xie,† Jie Liu,† Zehua Wang,† Chongchong Wu,† Junfeng Rong*,†, Hongyang Liu,‡ and Dangsheng Su*,§ †

Research Institute of Petroleum Processing, Sinopec, No.18, Xueyuan Road, Haidian District,

Beijing, 100083, China. ‡

Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese

Academy of Sciences, Wenhua Road 72, Shenyang, 110016, China. §

Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Chinese

Academy of Sciences, 457 Zhongshan Road, Dalian, 116023, China.

ABSTRACT. Nanocarbon materials are promising catalysts of oxidative dehydrogenation (ODH) of alkanes, but the improvement of alkene selectivity remains a challenge. Deep understanding and identification of oxygen species on nanocarbons is highly required for approaches of nanocarbons’ modification. Successful application of iodometric titration in quantitative determination of the amount of electrophilic oxygen on the surface of carbon

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nanotubes has been executed in this work. Electrophilic oxygen species have been identified as the main culprits for deep oxidation of ODH of n-butane via a clear correlation between the amount of electrophilic oxygen and combustion reaction rate. By chemical reduction and annealing in nitrogen, the alkene selectivity is significantly improved. Phenol groups are found to play an essential role in improving alkene selectivity. The study reveals that higher alkene selectivity can be achieved by both eliminating deep oxidation active sites and facilitating the formation of phenol and carbonyl groups.

KEYWORDS: carbon nanotubes, electrophilic oxygen, phenol groups, reduction, oxidative dehydrogenation

INTRODUCTION Nanostructured carbon-based materials have shown high potential in several industrial-relevant reactions.1 A typical application of nanocarbon catalysts is the C-H bond activation, such as oxidative dehydrogenation (ODH) of alkanes including ethane,2 propane,3,4 n-butane,5 and ethylbenzene.4,6 As the main component of liquefied petroleum gas (LPG), n-butane is low-cost raw material and abundant in amount. ODH of butane is recognized as an important approach to produce high value-added butenes and butadienes, which are important intermediates in synthetic chemistry.5 Nevertheless, over 40% of the hydrocarbons are burned owing to the rampant deep oxidation side reactions,5 which lead to low atom economy and restrain the alkene selectivity. The inhibition of side reaction and enhancement of alkene yield remain the critical issues for real industrial application of nanocarbon catalysts.


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Efforts have been made to elucidate the reaction mechanism when nanocarbons are used as catalysts.7,8 It is accepted that the nuclephilic ketonic C=O groups are active sites for ODH reactions,5,9,10 while the electrophilic oxygen species (peroxide O22— and superoxide O2—) tend to form at the defect sites or edges of graphic structure and cause deep oxidation.5,11 The electron-rich C=O groups can abstract H atoms from n-butane to form C-OH intermediates, afterwards the remaining H atoms of C-OH groups react with oxygen to form water, closing the ODH cycle.5 Therefore, carbonyl/quinone groups can be formed from phenol groups in oxygen atmosphere.5,12 In addition, considering that phenol groups on hindered phenolic antioxidants can give H atoms to break the degradation cycle of organic polymer materials thus preventing the peroxide radicals from propagating the degradation chain reaction,13,14 we suspect phenol groups on CNTs have similar functionality of preventing deep oxidation caused by electrophilic peroxides. In this sense, phenol groups should be recognized as a kind of latent active sites and play an essential role in ODH reactions. Electrophilic oxygen species are electron-deficient and attack the C=C of desired alkene products, resulting in low C4 alkene selectivity. Peroxides on nanocarbon are formed through oxidation of carbon skeleton of CNTs or by second oxidation of oxygen groups such as carboxyl acids.15 Although electrophilic oxygen species are main culprits of deep oxidation and are detrimental to our target product in ODH reactions, no experimental investigations of quantification and identification of electrophilic oxygen on nanocarbon have been executed other than theoretical analysis.


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Scheme 1. Classification of surface oxygen species on CNTs. In this sense, we classify the oxygen species on CNTs into three types according to their catalytic behavior in ODH reactions (Scheme 1). The oxygen species that play positive role in catalyzing ODH reactions are defined as positive oxygen which include ketone/quinone as active sites and those that can be converted to active sites, for instance, phenol groups. The negative oxygen species that act as deep oxidation sites and have negative effect on ODH refer to electrophilic peroxides and superoxides, as well as carboxyl groups that tend to form peroxides at the presence of oxygen.15 The other species are those that show neutral or unclear effect on catalytic activity, such as ether and lactone. On the basis of this classification, unlike the traditional modification strategies developed by trail-and-error experiments, this study aims to provide new approach to promote alkene yield via oriented controlling of different oxygen species on oxygenated nanocarbons. The elimination of negative oxygen species to inhibit deep


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oxidation of alkenes and converting negative oxygen into positive oxygen species to provide more active sites are considered as two main pathways to achieve higher selectivity in this catalytic reaction. Reduction of nanocarbon materials have been reported using hydrazine, NaBH4 or annealing in vacuum/H2/inert gas.16-20 There were reports on sophisticated investigation of the properties of oxygen groups on reduced graphene oxide, suggesting a complex composition of oxygen species on the surface of nanocarbon materials.21,22 However, the catalytic properties were seldom discussed. Here, the mix-acid oxidized carbon nanotubes (o-CNTs) underwent thermal annealing in nitrogen and wet chemical reduction by LiAlH4 to remove electrophilic oxygen species and turn electrophilic oxygen into phenol groups. Moreover, iodometric titration was performed to provide quantitative results of the amount of electrophilic oxygen on the surface of nanocarbon samples. The C4 alkene selectivity significantly improved after annealing and chemical reduction treatment owing to the elimination of electrophilic oxygen, which provides evidence that electrophilic oxygen species are deep oxidation sites. In addition, phenol groups on CNTs were validated by theoretical analysis and experimental results to play an essential role in gas phase ODH reaction.

EXPERIMENTAL SECTION Materials. Commercially available CNTs are purchased from Shandong Dazhan Nano Materials Co.,LTD (China), and are used directly without any purification. The length, inner diameter and purity of CNTs are: 3-12 µm, 12-15 nm and 96%, respectively. Catalysts preparation. The oxidation of pristine CNTs is performed as following procedure: 10 g CNTs were oxidized in 125 mL of concentrated HNO3 (65-68%) and 375 mL concentrated


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H2SO4 (95-98%) under sonication at 50 ºC for 6 hours, and the precipitate was filtered out and washed with deionized water for several times until the pH of the filtrate reaches 7. The precipitate was dried in atmosphere at 120 ºC for 12 hours to obtain oxidized carbon nanotubes (o-CNT). Wet chemical reduction of o-CNT was conducted using LiAlH4 as reductant. 1 g LiAlH4 (95%, Acros Organics) was dissolved in 20 mL tetrahydrofuran (THF) under N2 protection, then 2 g oCNT was added in the solution. After stirring under N2 protection for 2 hours at 25 ºC, the temperature was raised up to 70 ºC and held for 12 hours. Excess LiAlH4 was eliminated by dripping ethanol into the solution, and then the alkali was neutralized by excess HCl (35-38%). The precipitate was filtered out and washed with deionized water for several times until the pH of the filtrate reaches 7. The precipitate was dried in atmosphere at 120 ºC for 12 hours to obtain oCNT-LiAlH4. Thermal annealing of o-CNT was performed as following procedure: 2 g o-CNT underwent calcination in N2 protection at corresponding temperature (500 ºC, 700 ºC, 900 ºC, 1100 ºC, respectively) for 5 hours, with a heating rate of 5 ºC min-1 to obtain oCNT-500, oCNT-700, oCNT-900 and oCNT-1100. Iodometric titration method. Herein, a quantification method of electrophilic oxygen on the surface of CNTs samples was developed basing on iodometric titration which was used for determination of liquid phase peroxide concentration.23 Electrophilic peroxide and superoxide species are able to oxidize aqueous I— into I2 for subsequent titration with Na2S2O3. In this study, we suppose the electrophilic oxygen species are mostly composed of peroxides and designate peroxide as electrophilic oxygen for convenient calculation. The titration procedure is as follows: 0.3 g CNTs sample was added in KI solution which consists of 10 mL KI (100 g/L), 5 mL H2SO4


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(0.1 mol/L), 30 mL deionized water and 3 drops of (NH4)6Mo7O24 (30 g/L). The reaction between peroxides on CNTs surface and KI is shown in (1). After 30 min under sonication at 25 ºC in the dark, I— was oxidized into I2. Then the precipitate was filtered out and washed 6 times. I2 in filtrate is titrated by Na2S2O3 (0.002 mol/L) as presented in (2) and the concentration of electrophilic oxygen (mol/g) on CNTs is calculated by (3) wherein c (mol/g), V (mL) and m (g) represent for electrophilic oxygen concentration, volume consumption of Na2S2O3 solution and mass of CNTs samples for titration. The obtained iodometric titration data were validated by similar results of repeated experiments. O22-+2KI+2H2SO4 → O2-+2KHSO4+I2+H2O

(1)

I2 + 2Na2S2O3 → 2NaI + Na2S4O6

(2)

c (electrophilic oxygen) = 2×10-6 V/m

(3)

Chemical titration of phenol groups. The titration of phenol groups on oCNT-LiAlH4 was carried out in benzoic anhydride (BA) solution under nidrogen atmosphere, following a procedure similar to that described in the literature.9 The reaction between BA and oCNTLiAlH4 is shown in Scheme S1. The titration is performed as following procedure: 5 g benzoic anhydride (98%, Sinopharm) and 1 g oCNT-LiAlH4 was dissolved in 50 ml of CHCl3. After stirring under N2 protection at 60 ºC for 24 hours, the precipitate was filtered out and washed with 2 L of CHCl3 to remove the physical adsorbed BA molecules. The precipitate was dried at 60 ºC for 24 hours to give oCNT-LiAlH4-BA. Catalytic activity measurement. The catalytic activity of CNTs samples in the reaction of oxidative dehydrogenation of butane was measured at 723 K using a 10 mm diameter fix-bed quartz tube reactor at atmospheric pressure over 300 min. Reactant and product concentrations (weight percent) were measured by online gas chromatography (Agilent 7890B) equipped with a


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HayeSep Q column, a HayeSep N column and a molecular-sieve column connected to a thermal conductivity detector, and a HP-PLOT A12O3 column (50 m×0.53 mm×15 µm) connected to a flame ionization detector. Reactant mixtures (Beijing AP BAIF Gases) contained 0.7 wt.% of butane, 1.4 wt.% of O2 and 97.9 wt.% of N2 with a typical gas flow rate of 4500 mL-gas (h gcat)-1. The selectivity of alkene products is calculated as follows: selbutene =

cbutene ×1.034 ×100 % [cbutane ]in - [cbutane ]out

selbutadiene =

cbutadiene ×1.074 ×100 % [cbutane ]in - [cbutane ]out

where seli (%) and ci (wt. %) are selectivity and concentration of each C4 alkene product in outlet gas, and [cbutane]in and [cbutane]out are n-butane concentration in inlet and outlet gas mixture, respectively. The side-reaction refers to total oxidation of hydrocarbon to form COx. The calculation of sidereaction rates (mol g-1 s-1) are based on the reaction conditions (gas flow rate and butane concentration) and the catalytic performance. The calculating formula for side-reaction rate is presented as: νs = 0.189 ×10-6 conv (1 - sel) The C4 alkenes formation rate is calculated as follows: vc = 0.189 ×10-6 conv * sel The sum of side-reaction rate and C4 alkenes formation rate is the n-butane consumption rate. The yield of each alkene component is calculated as follows: Alkene yield (%) = conv * sel


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where νs (mol g-1 s-1), νc (mol g-1 s-1), conv (%) and sel (%) represent for side-reaction rate, C4 alkenes formation rate, conversion of butane and C4 alkenes selectivity, respectively. Control experiment was conducted under the same reaction condition except there was no catalyst samples added in the reactor. The conversion rate of n-butane was 0 % and neither alkene nor side reaction product was detected by gas chromatography. Characterizations. Transmission electron microscopy (TEM) measurements were conducted on a FEI Tecnai F20 microscope with an accelerating voltage at 200 kV. Raman spectra were obtained under ambient conditions on a JY LabRAM HR Raman spectrometer with a 325 nm laser beam. The weight concentration for element C, H and O was determined on an Elementar Micro Cube elemental analyzer. X-ray photoelectron spectroscopy (XPS) data were obtained with an ESCALab220i-XL electron spectrometer from VG Scientific using 300 W AlKα radiation. The base pressure was about 3×10-9 mbar. The binding energies were referenced to the C1s line at 284.6 eV from defect free graphite. Deconvolution of O1s spectra were performed using mixed Gaussian-Lorentzian component profiles after subtraction of a Shirley background using XPSPEAK41 software. The CO and CO2 TPD profiles were deconvoluted by mutiple Gaussian functions using Origin 8 software.

RESULTS AND DISCUSSION


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Figure 1. ODH activities of pristine CNTs, o-CNT, oCNT-LiAlH4, oCNT-500, oCNT-700, oCNT-900 and oCNT-1100. The ODH reaction conditions: 723 K, 1 atm, O2/butane=2. The oxidation of pristine CNTs by HNO3 and H2SO4 mixture introduces defects and shortens the tubes having more edge sites for anchoring oxygen species.24,25 Reduction by strong reductant LiAlH4 can reduce oxygen groups of high oxidation states such as peroxide, carboxylic acid and carboxylic anhydride to hydroxyl/phenol.20 Oxygenated groups have different thermal stability and decompose from the surface of o-CNT in specific temperature range.4,26,27 The catalytic performance of the LiAlH4 reduced (oCNT-LiAlH4) and annealed o-CNT (oCNT-500, oCNT-700, oCNT-900 and oCNT-1100) catalysts in ODH of n-butane at 723 K is displayed in Figure 1. Oxidation of pristine CNTs leads to an increase of butane conversion from 6.8% to


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28.3% with an increasing of C4 alkenes selectivity from 7.9% to 19.6%. Generally, the gain of alkene selectivity is in conflict with the promotion of butane conversion according to previous work in literature.5 However, the reduction of o-CNT by LiAlH4 increases the alkene selectivity to 41.7%, while the conversion remains as high as 29.7%, even slightly higher than that of oCNT. Also annealed o-CNT can lead to an increase in C4 alkenes selectivity, but this decreases when annealing temperature is higher than 900 ºC. The highest selectivity of 50.8% is obtained over oCNT-900, which is comparable to the best phosphorus modified o-CNTs catalyst.5 As expected, the conversion of butane decreases over annealed catalysts when the selectivity increases. From the point of alkene yield and side-reaction rate (Supporting Information, Figure S1), oCNT-LiAlH4 obtains the highest C4 alkene yield of 12.4% since it achieves high conversion and high selectivity simultaneously. The side-reaction rate of oCNT-LiAlH4 (3.2×108

mol g-1 s-1) is lower than that of o-CNTs (4.2×10-8 mol g-1 s-1). The sample of oCNT-900

possesses an alkene yield of 10.2% while the side-reaction rate is 1.8×10-8 mol g-1 s-1, the lowest among the modified catalysts. The stability of oCNT-LiAlH4 and oCNT-900 is tested in ODH of n-butane for 40 hours (Supporting Information, Figure S2a,b). The catalytic performance of oCNT-900 is stable over 40 hours without apparent deactivation while a drop of conversion is observed on oCNT-LiAlH4. It is possibly because the HNO3/H2SO4 etching and strong reduction by LiAlH4 can generate reactive carbon or oxygen species which induce the decomposition of the catalyst to form CO2 while 900 ºC annealing removes the reactive species on o-CNT and maintain high stability during ODH reaction. TEM images (Figure S3) of o-CNT, oCNT-LiAlH4 and oCNT-900 reveal that wet chemical reduction and annealing in N2 have no obvious impact on the surface structure of o-CNT. The Raman spectra (Figure S4) reveal that o-CNT (ID/IG28,29=0.20) has more structural defects than


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pristine CNTs (ID/IG=0.13), while ID/IG of oCNT-LiAlH4 (ID/IG=0.19) and oCNT-900 (ID/IG=0.20) is close to that of o-CNT (ID/IG=0.20). This indicates that the detailed surface structure of CNTs was maintained well after the reduction and annealing treatments and thus the significant promotion of selectivity to C4 alkenes of oCNT-LiAlH4 and oCNT-900 is not directly related to defects. Table S1 exhibits the oxygen contents of modified samples before and after ODH of butane by using O/C weight ratio obtained from element analysis. After reduction of o-CNT, O/C ratio drops from 0.169 to 0.06 due to the reduction and desorption of high oxidation state peroxides, carboxyls and carboxylic anhydrides. The O/C ratio of oCNT-LiAlH4 after ODH (0.054) exceeds that of o-CNT (0.036) indicating that oCNT-LiAlH4 contains more oxygen species that are stable under the reaction temperature such as phenol and ketone groups which may contribute to the high butane conversion of oCNT-LiAlH4. The O/C ratio of annealed CNTs reduces with the rising annealing temperature. After ODH the O/C increases due to the formation of oxygen groups during ODH reaction.9,30,31 For oCNT-900, the O/C ratio before ODH is low as 0.004 while after ODH it is 0.017, lower than the other annealed samples. We suspect that oCNT-900 avoids to form negative oxygen during ODH and maintains high selectivity. It can be concluded that there is no apparent correlation between catalytic activity and the total oxygen content21 when comparing the catalytic activity with the O/C ratio of the relevant samples (Figure S5), because of the wide variations in composition of oxygen groups via different samples, indicating the significance of the controlling of positive and negative types of oxygen functional groups on o-CNT.


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Figure 2. Amount of electrophilic oxygen determined by iodometric titration on a) o-CNT, oCNT-LiAlH4, annealed o-CNT and pristine CNTs before ODH. The blank titration experiment is operated following the same procedure except that no catalyst was added in. The inset is oCNT-1100 exposed in air for 24 hours. b) o-CNT, oCNT-LiAlH4 and annealed o-CNT after ODH. Electrophilic peroxides and superoxides on surface of CNTs are low in amount. However, considering the decomposition temperature of peroxides on carbon surface is relatively high (550–600 ºC),22,27,32 a part of the electrophilic oxygen species remain stable at the temperature of 450 ºC and cause deep oxidation in ODH reaction. Here we used iodometric determination method for the quantification of electrophilic oxygen on CNT’s surface providing chemical


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evidence for mechanism of o-CNT’s modification.23 Samples of o-CNT, oCNT-LiAlH4 and oCNT-900 before and after ODH of butane were titrated for surface electrophilic oxygen amount. The results in Figure 2a show clearly that the amount of electrophilic oxygen of oCNT-LiAlH4 and annealed o-CNT are lower than that of o-CNT, indicating that both wet chemical reduction and annealing in N2 can eliminate electrophilic oxygen from o-CNT. After ODH reaction, the amount of electrophilic oxygen further decreased for most of the samples (Figure 2b) suggesting the desorption of electrophilic oxygen during the reaction. Also o-CNT was firstly annealed in N2 at 450 ºC and then O2 was added in the stream. The amount of electrophilic oxygen slightly increased after adding O2 (from 1.7×10-6 to 2.0×10-6 mol/g), which indicates that electrophilic oxygen can be formed in O2, as mentioned above, and both decomposition and regeneration of electrophilic oxygen occur during ODH. Therefore, it is the amount of electrophilic oxygen during the reaction that controls alkene selectivity and efforts should be made not only to eliminate electrophilic oxygen from o-CNT but also to prevent it from regenerating during ODH reaction. The titration data for o-CNT annealed at different temperatures reveal a trend that the amount of electrophilic oxygen decreases as the annealing temperature increases with both oCNT-700 and oCNT-900 show extremely low amount, except for oCNT-1100 that contains more electrophilic oxygen groups than oCNT-900. We suppose that there are highly active vacancies and edge sites left on o-CNT after annealing at 1100 ºC. These defects can easily chemisorb O2 to form oxygen groups including electrophilic oxygen species. We then titrated oCNT-1100 samples exposed in the air for different time length (insert in Figure 2a) and observed a sharp increase of the amount of electrophilic oxygen during the first hour and slow formation of electrophilic oxygen afterwards, confirming our conjecture. The amount of electrophilic oxygen for oCNT-900 after ODH appears the lowest among the annealed samples


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and we can infer that the impact of 900 ºC annealing includes decomposition of electrophilic oxygen and keeping the amount of it at low level without regeneration afterwards.

Figure 3. Side-reaction conversion rates of butane on CNTs samples with different amount of electrophilic oxygen after ODH of n-butane (the samples include o-CNT, oCNT-500, oCNT-700, oCNT-900 and oCNT-1100). The ODH reaction conditions: 723 K, 1 atm, O2/butane=2. Combining the catalytic performances (Figure 1) and titration results of CNTs samples after ODH reaction (Figure 2b), it is rational to infer that the amount of electrophilic oxygen existing on CNTs in ODH reaction controls the combustion side reactions. Figure 3 shows an unambiguous trend that side-reaction rates rise with the increasing amount of electrophilic oxygen on CNTs samples after ODH, confirming that electrophilic oxygen species initiate deep


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oxidation side-reaction. It can be seen from Figure 3 that when the amount of electrophilic oxygen is extrapolated to zero, the side-reaction rate intersects the y axis at a value above zero, which may be due to the nature that C=C bonds in C4 alkene products are active to be oxidized by O2 or other aspects of the reaction system that may induce the deep oxidation reaction. It’s also observed in Figure S6 that for the annealed o-CNT samples, the C4 alkenes formation rates increase with the decrease of electrophilic oxygen. From above results, it is clear that chemical reduction and annealing in N2 can remove electrophilic oxygen on o-CNT, enhancing the alkene selectivity by inhibiting side-reactions additional to P, B, or N modification.2,3,5,33 Temperature-programmed desorption (TPD) results of o-CNT, oCNT-LiAlH4 and samples after ODH reactions for oCNT-LiAlH4 and oCNT-900 are presented in Figure S7. We can observe an elimination of carboxyl groups and a significant decline of carboxylic anhydride and lactone groups after reduction of o-CNT by LiAlH4. After ODH of butane, the predominant oxygen species left on the surface of oCNT-LiAlH4 are phenol and carbonyl groups. In the case of oCNT-900 after ODH, the desorption of CO2 and CO before 900 ºC is attributed to the oxygen groups regenerated during ODH reaction. The results indicate the formation of positive oxygen (phenol and carbonyl) on oCNT-900 during ODH reaction that contributes to its high catalytic selectivity.


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Figure 4. Deconvolution of O1s XPS spectra of o-CNT after ODH and oCNT-LiAlH4 after ODH. The ODH reaction conditions: 723 K, 1 atm, O2/butane=2. Additional characterization of surface oxygen is obtained by deconvolution of the O1s XPS.9,34 The surface amount of O=C-O (sum of carboxylic acid, anhydride, lactone and ester groups) on o-CNT drops by around 35% after reduction by LiAlH4 (Figure S8a). For oCNT-LiAlH4, the amount of C-OH drops by 35% and C=O increases by 82% after ODH of butane, indicating a phenol to carbonyl conversion during ODH reaction. Both XPS spectra and element analysis reveal that the oxygen content of oCNT-LiAlH4 after ODH reaction is higher than that of o-CNT after ODH. The sample of oCNT-LiAlH4 after ODH shows a slight increase (8%) of C-OH groups and a remarkable increase of C=O groups compared with o-CNT after ODH (Figure 4).


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We can infer that the increase of phenol groups and carbonyl groups and the elimination of electrophilic oxygen groups result in the observed enhancement of both butane conversion and C4 alkenes selectivity over oCNT-LiAlH4. The reduction treatment of o-CNT optimizes the composition of surface oxygen groups by converting negative and neutral oxygen groups into positive ones, thus improving the catalytic activity. The oxygen content of oCNT-900 is rather low (Figure S8b). After ODH reaction the amount of C-OH and O=C-O increases but the total oxygen content is still lower than that of o-CNT and oCNT-LiAlH4 after ODH. This indicates the formation of new oxygen groups on oCNT-900 explaining the moderate conversion of butane for oCNT-900 in ODH reaction. To study the role of phenol groups, we use benzoic anhydride (BA) titrant to selectively deactivate -C-OH groups and form O=C-O on the surface of oCNT-LiAlH4 (Scheme S1).9,35 The obtained titration derivative is noted as oCNT-LiAlH4-BA. After titration, the amount of -C-O on oCNT-LiAlH4 drops by around 15% according to the peak area of XPS O1s deconvolution, along with a remarkable increase of O=C-O (Figure S9). The ODH of butane was then evaluated over oCNT-LiAlH4-BA. After ODH of butane, there is a significant decrease (59%) of O=C-O and a slight decrease (2%) of C-OH of oCNT-LiAlH4-BA, indicating that the obtained O=C-O groups converted from -C-OH groups can be eliminated in ODH reaction at 450 ºC, but the -COH groups can not be regenerated to interfere with the results. Compared with oCNT-LiAlH4, oCNT-LiAlH4-BA exhibits a significant decrease (from 41.7% to 23.3%) of C4 alkenes selectivity (Figure S10). The decreased alkene selectivity is possibly due to less -C-OH groups converting into C=O active sites and the increased amount of negative oxygen groups. Moreover, as mentioned above, we theoretically suppose that phenol groups on CNTs are similar to the hindered phenolics in structure and have the functionality of inhibiting the formation of peroxide


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radicals.13,14 With the deactivation of phenol, the antioxidation was weakened. It can be inferred that converting -C-OH into O=C-O groups is detrimental to the catalytic performance in ODH. CONCLUSIONS In summary, our work presented detailed analysis and evidence for the impact of electrophilic oxygen and phenol groups on ODH of n-butane. Electrophilic oxygen species have been identified as the main culprits for combustion side reaction, limiting the selective conversion to alkene products. Iodometric titration method was successfully applied to quantify the amount of electrophilic oxygen on the surface of CNTs. Reduction by LiAlH4 and annealing treatment can both eliminate electrophilic oxygen and carboxyl groups which can be oxidized into peroxides, thus improve the selectivity to C4 alkenes. LiAlH4 reduction increased the total amount of positive oxygen to remain high conversion rate. 900 ºC annealing treatment eliminates the electrophilic oxygen species on o-CNT and prevents them from regenerating during ODH reaction. We showed evidence that in ODH of n-butane, phenol groups on CNTs play an essential role for the catalytic performance since it can be converted to carbonyl during ODH reaction. On the basis of identification of the role of electrophilic oxygen and phenol groups, this study proposed new strategies with explicit intention to enhance the catalytic activity of CNTs by both inhibiting side-reactions and facilitating new active site formation.

ASSOCIATED CONTENT Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.


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Supplementary experimental material, characterizations and catalytic results (Table S1, Scheme S1 and Figures S1-S9) as described in the text (PDF). AUTHOR INFORMATION Corresponding Author *J.F. Rong. [email protected] *D.S. Su. [email protected] Notes The authors declare no competing financial interests. Author Contributions J.Q. Li. conducted all of the preparations, most of the characterizations and catalytic tests and finished the writing of this paper. The other authors assisted the characterizations, catalytic tests, results analysis and paper revision. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT This work is financially supported by China Petrochemical Cooperation. (No. S213043). ABBREVIATIONS ODH, oxidative dehydrogenation; CNTs, carbon nanotubes; BA, benzoic anhydride. REFERENCES (1) Su, D. S.; Perathoner, S.; Centi, G. Chemical Reviews 2013, 113, 5782-5816.


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(2) Frank, B.; Morassutto, M.; Schomaecker, R.; Schloegl, R.; Su, D. S. Chemcatchem 2010, 2, 644-648. (3) Frank, B.; Zhang, J.; Blume, R.; Schloegl, R.; Su, D. S. Angewandte Chemie-International Edition 2009, 48, 6913-6917. (4) Rinaldi, A.; Zhang, J.; Frank, B.; Su, D. S.; Hamid, S. B. A.; Schloegl, R. Chemsuschem 2010, 3, 254-260. (5) Zhang, J.; Liu, X.; Blume, R.; Zhang, A.; Schloegl, R.; Su, D. S. Science 2008, 322, 73-77. (6) Zhang, J.; Su, D.; Zhang, A.; Wang, D.; Schloegl, R.; Hebert, C. Angewandte ChemieInternational Edition 2007, 46, 7319-7323. (7) Mestl, G.; Maksimova, N. I.; Keller, N.; Roddatis, V. V.; Schlögl, R. Angewandte Chemie International Edition 2001, 40, 2066-2068. (8) Keller, N.; Maksimova, N. I.; Roddatis, V. V.; Schur, M.; Mestl, G.; Butenko, Y. V.; Kuznetsov, V. L.; Schlögl, R. Angewandte Chemie International Edition 2002, 41, 1885-1888. (9) Qi, W.; Liu, W.; Zhang, B.; Gu, X.; Guo, X.; Su, D. Angewandte Chemie-International Edition 2013, 52, 14224-14228. (10) Qi, W.; Su, D. Acs Catalysis 2014, 4, 3212-3218. (11) Madeira, L. M.; Portela, M. F. Catalysis Reviews-Science and Engineering 2002, 44, 247286.


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(12) Gerber, I.; Oubenali, M.; Bacsa, R.; Durand, J.; Goncalves, A.; Pereira, M. F. R.; Jolibois, F.; Perrin, L.; Poteau, R.; Serp, P. Chem.-Eur. J. 2011, 17, 11467-11477. (13) Vulic, I.; Vitarelli, G.; Zenner, J. M. Polymer Degradation and Stability 2002, 78, 27-34. (14) Tochacek, J. Polymer Degradation and Stability 2004, 86, 385-389. (15) Jefford, C. W.; Boschung, A. F.; Bolsman, T.; Moriarty, R. M.; Melnick, B. Journal of the American Chemical Society 1976, 98, 1017-1018. (16) Pei, S.; Cheng, H.-M. Carbon 2012, 50, 3210-3228. (17) Gao, W.; Alemany, L. B.; Ci, L.; Ajayan, P. M. Nat Chem 2009, 1, 403-408. (18) Nguyen Dien Kha, T.; Choi, J.; Park, C. R.; Kim, H. Chemistry of Materials 2015, 27, 73627369. (19) Gao, X.; Jang, J.; Nagase, S. Journal of Physical Chemistry C 2010, 114, 832-842. (20) Mochalin, V. N.; Shenderova, O.; Ho, D.; Gogotsi, Y. Nat Nano 2012, 7, 11-23. (21) Schwartz, V.; Fu, W.; Tsai, Y.-T.; Meyer, H. M., III; Rondinone, A. J.; Chen, J.; Wu, Z.; Overbury, S. H.; Liang, C. Chemsuschem 2013, 6, 840-846. (22) Dathar, G. K. P.; Tsai, Y.-T.; Gierszal, K.; Xu, Y.; Liang, C.; Rondinone, A. J.; Overbury, S. H.; Schwartz, V. Chemsuschem 2014, 7, 483-491. (23) Greenspan, F. P.; Mackellar, D. G. Analytical Chemistry 1948, 20, 1061-1063. (24) Marshall, M. W.; Popa-Nita, S.; Shapter, J. G. Carbon 2006, 44, 1137-1141.


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(25) Shirazi, Y.; Tofighy, M. A.; Mohammadi, T.; Pak, A. Applied Surface Science 2011, 257, 7359-7367. (26) Figueiredo, J. L.; Pereira, M. F. R.; Freitas, M. M. A.; Orfao, J. J. M. Carbon 1999, 37, 1379-1389. (27) Szymanski, G. S.; Karpinski, Z.; Biniak, S.; Swiatkowski, A. Carbon 2002, 40, 2627-2639. (28) Tuinstra, F.; Koenig, J. L. Journal of Chemical Physics 1970, 53, 1126-1130. (29) Maciel, I. O.; Anderson, N.; Pimenta, M. A.; Hartschuh, A.; Qian, H.; Terrones, M.; Terrones, H.; Campos-Delgado, J.; Rao, A. M.; Novotny, L.; Jorio, A. Nat Mater 2008, 7, 878883. (30) Datsyuk, V.; Kalyva, M.; Papagelis, K.; Parthenios, J.; Tasis, D.; Siokou, A.; Kallitsis, I.; Galiotis, C. Carbon 2008, 46, 833-840. (31) Salzmann, C. G.; Llewellyn, S. A.; Tobias, G.; Ward, M. A. H.; Huh, Y.; Green, M. L. H. Advanced Materials 2007, 19, 883-887. (32) Zielke, U.; Huttinger, K. J.; Hoffman, W. P. Carbon 1996, 34, 983-998. (33) Chen, C.; Zhang, J.; Zhang, B.; Yu, C.; Peng, F.; Su, D. Chemical Communications 2013, 49, 8151-8153. (34) Diao, J.; Liu, H.; Wang, J.; Feng, Z.; Chen, T.; Miao, C.; Yang, W.; Su, D. S. Chemical Communications 2015, 51, 3423-3425.


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Table of Contents In this work, electrophilic oxygen species have been identified as the main culprits for deep oxidation of oxidative dehydrogenation of n-butane. Iodometric titration method was successfully applied in quantitative determination of the amount of electrophilic oxygen on the surface of carbon nanotubes. Phenol groups on carbon nanotubes were found to play an essential role in ODH reaction.


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Figure 1. ODH activities of pristine CNTs, o-CNT, oCNT-LiAlH4, oCNT-500, oCNT-700, oCNT-900 and oCNT1100. The ODH reaction conditions: 723 K, 1 atm, O2/butane=2. 201x141mm (300 x 300 DPI)


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Figure 2. Amount of electrophilic oxygen determined by iodometric titration on a) o-CNT, oCNT-LiAlH4, annealed o-CNT and pristine CNTs before ODH. The blank titration experiment is operated following the same procedure except that no catalyst was added in. The inset is oCNT-1100 exposed in air for 24 hours. b) o-CNT, oCNT-LiAlH4 and annealed o-CNT after ODH. 201x141mm (300 x 300 DPI)


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Figure 2. Amount of electrophilic oxygen determined by iodometric titration on a) o-CNT, oCNT-LiAlH4, annealed o-CNT and pristine CNTs before ODH. The blank titration experiment is operated following the same procedure except that no catalyst was added in. The inset is oCNT-1100 exposed in air for 24 hours. b) o-CNT, oCNT-LiAlH4 and annealed o-CNT after ODH. 201x141mm (300 x 300 DPI)


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Figure 3. Side-reaction conversion rates of butane on CNTs samples with different amount of electrophilic oxygen after ODH of n-butane (the samples include o-CNT, oCNT-500, oCNT-700, oCNT-900 and oCNT1100). The ODH reaction conditions: 723 K, 1 atm, O2/butane=2. 201x141mm (300 x 300 DPI)


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Figure 4. Deconvolution of O1s XPS spectra of o-CNT after ODH and oCNT-LiAlH4 after ODH. The ODH reaction conditions: 723 K, 1 atm, O2/butane=2. 201x141mm (300 x 300 DPI)


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Scheme 1. Classification of surface oxygen species on CNTs. 114x98mm (600 x 600 DPI)


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45x31mm (300 x 300 DPI)


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Improving Alkene Selectivity of Nanocarbon Catalyzed Oxidative

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