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Oxidative Dehydrogenation on Nanocarbon: Revealing the Catalytic Mechanism Using Model Catalysts Xiaoling Guo, Wei Qi, Wei Liu, Pengqiang Yan, Fan Li, Changhai Liang, and Dangsheng Su ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b02936 • Publication Date (Web): 18 Jan 2017 Downloaded from http://pubs.acs.org on January 18, 2017
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Oxidative Dehydrogenation on Nanocarbon: Revealing the Catalytic Mechanism using Model Catalysts Xiaoling Guo†,‡, Wei Qi*,‡, Wei Liu‡, Pengqiang Yan‡, Fan Li‡, Changhai Liang†, and Dangsheng Su*,‡ † Laboratory of Advanced Materials and Catalytic Engineering, School of Chemical Engineering, Dalian University of Technology, Dalian, 116023 (China) ‡ Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang, 110016 (China) ABSTRACT: Model catalysts offer a direct method to determine the active site structures and mechanisms on nanocarbon catalysts. In this report, we show that the ketonic carbonyl groups are the catalytic center of the nanocarbon-catalyzed alkane oxidative dehydrogenation (ODH) reaction, and we reveal the catalytic mechanism using conjugated polymeric model catalysts containing only ketonic carbonyl groups. An in situ infrared (IR) analysis provides spectroscopic evidence of the “working” configurations of the active sites in association with the redox cycle of the ketonic carbonyl-hydroxyl pairs. Carbonyl reduction (H abstraction from hydrocarbon) is shown to be a kinetically relevant step. An 18O isotope tracer experiment further shows that the activation and exchange of molecular oxygen at the surface active sites are fast processes under common reaction conditions.
KEYWORDS: heterogeneous catalysis, carbon catalysis, alkane oxidative dehydrogenation, model catalysts, kinetics Nanocarbon materials have received attention in the fields of energy storage1, energy conversion2 and catalysis3 because of their advantageous physical and chemical properties. However, the surface chemistry of these non-metallic materials is complex and difficult to control.4 Hence, new nanocarbon materials for applications in catalysis and other energy-related fields have been developed via empirical “trial-and-error” experiments. One problem with the uncertainty of nanocarbon material surface chemistry is the ambiguity of the identity, quantity and working mechanism of the surface active sites in reactions.5 For example, phosphate-modified nanocarbon materials have shown activity comparable to commercial MgVxOy catalysts in alkane oxidative dehydrogenation (ODH) reactions.6 Although this outcome is interesting, the explicit nature of the active centers and catalytic mechanism is largely unknown. Ketonic carbonyl groups (particularly diketone or quinone groups) on nanocarbon materials have been proposed as active sites for alkane ODH reactions based on ex situ temperatureprogrammed desorption (TPD),7 in situ ambient pressure X-ray photoelectron spectroscopy (XPS),6 and chemical titration methods.8 The catalytic reaction mechanism has been ascribed to the redox cycle of the ketonic carbonyl-phenol groups. However, fundamental questions about the nature of carbon catalysis are difficult to answer because there are no direct observations of the reaction intermediate and spectroscopic evidence to verify the proposed active structure and catalytic reaction mechanism has not been obtained. Here, we report on the use of conjugated polymers that contain only ketonic carbonyl groups and have a well-defined chemical composition and structure as model catalysts to reveal the mechanism of the nanocarbon-catalyzed ethylbenzene (EB) ODH reaction. Active site titration, temperature-programmed surface reaction, deuterated EB and 18O isotope tracers and in situ IR measurements were performed using the model catalyst to elucidate the mechanism of the carbon-catalyzed alkane ODH reactions. Compared to nanocarbon materials, the key advantages of synthetic polymer model catalysts are as follows: 1) they are metal-free, which avoids the potential for trace metal
impurities found in nanocarbon materials; 2) the ketonic carbonyl group is the only oxygen group, which eliminates interference from other functionalities and provides an ideal platform for mechanistic studies; 3) their tunable surface area and oxygen content enable active site quantification for subsequent structure-function studies; 4) their spectro-transparency (weak background absorption) provides an unambiguous spectroscopic signal for structure determination during the reaction. As shown in Scheme 1, the polymers were synthesized via a Yamamoto coupling reaction9 using 1,3,5-tribromobenzene (TB) and 3,6-dibromo-phenanthrenequinone (DBPQ) as the monomers. The resulting polymer was named YPB-x, where x is the mole ratio between TB and DBPQ. The polymerization of the two monomers was confirmed using IR and nuclear magnetic resonance (NMR) measurements based on the absence of =C–Br signals, as shown in Figures S1-3. The ketonic carbonyl group was verified as the only oxygen functionality on the polymer. XPS, elemental analysis (weight content of all elements) and energy dispersive X-ray (EDX) spectroscopy results ensured the absence of Ni residue in the purified polymers.10 The final chemical structure of the synthesized polymer is illustrated in Scheme 1.
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Scheme 1. Schematic drawing of the chemical structure of the model catalyst, YPB-x.
First, the physical properties of the YPB-x polymers were characterized. The nitrogen adsorption isotherm shows that the YPB-x polymer series consisted of microporous materials with a typical type-I adsorption isotherm (Figure S4). The surface area and ketonic oxygen group content could be continuously adjusted via tuning the ratio of TB (the linker) and DBPQ (the active site contributor), as shown in Table S1. The oxygen content measured via elemental analysis was consistent with the theoretical value (Table S1). Electron microscopy showed that the synthesized polymer existed in a spherical morphology with an average diameter of 400 - 500 nm (Figure S5). The spherical morphology of the polymer is the result of polymerization in three dimensions and of surface energy minimization. Thermogravimetric analysis coupled with online mass spectrometry (TG-MS) (Figure S6) suggested that the polymer is stable below 400 °C in air, which provides a temperature limit for gasphase reaction studies.
Figure 1. a) EB conversion (triangles) as a function of time on stream during the steady-state activity measurement and in situ titration process. b) EB conversion rate as a function of the cumulative consumption of the PH titrant. 538 K, 0.25 kPa EB, 1.0 kPa O2, 0.2 kPa PH, balance He, and 50 mg catalyst.
The EB ODH reactions on YPB-x were performed under mild reaction conditions in the kinetic regime (265 °C; EB conversion below 10%; without external diffusion) to ensure no significant structural damage to the catalysts via combustion or carbon deposition. As shown in Figure 1a, the model catalyst (YPB-6) exhibited stable activity for the EB ODH reaction after an induction period, and this result proved that the ketonic carbonyl groups are the active sites for the alkane ODH reactions. The quantity of the active sites was accurately measured using an in situ titration process.7 Phenyl hydrazine (PH) was selected as the titrant because of its selective and quantitative reaction with ketonic carbonyl groups (forming hydrazones with a yield over 99%)11 and because its molecular size and polarity are similar to those of EB, which enables PH to quantify the active sites that can contact and react with EB.7 As shown in Figure 1a, the catalytic activity of YPB-6 dramatically decreased after the introduction of PH, and the activity (EB conversion) decreased to
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zero when the uptake of PH reached saturation. This phenomenon is slightly different from the in situ titration results for a nanocarbon catalyst whose activity ends at a non-zero value because the defects on nanocarbon can be oxidized to form new active sites.7b For the proposed model catalysts, only a limited number of oxygen groups can be on the surface, and those groups can be completely consumed by the titrant. Thus, the catalytic activity decreases to zero. In addition, the EB conversion rates monotonically decreased with increasing PH uptake, and the linear dependence (Figure 1b) suggests that this is a site-by-site titration process. As shown in Figure S7, the sharp absorption peak of the ketonic carbonyl groups (C=O at 1683 cm–1) disappeared in the IR spectra of the polymer catalysts after the titration. The appearances of the characteristic C=N (broad band at 1692 cm–1) and N–H (at 1494 cm–1) vibrations suggested a poisoning reaction of the ketonic carbonyl groups with PH molecules. The x-intercept of the line (Figure 1b) represents the quantity of active sites that were “working” under the selected reaction conditions. In some cases, the number of active sites from the titration was consistent with the surface content of the ketonic carbonyl groups (e.g., YPB-6 at 265 °C, 0.25 kPa EB, and 1 kPa O2, 1.62 vs. 1.64 10-3 mol-C=O g-1), but the number could also be slightly lower than the total amount of oxygen species on the polymer surface (e.g., YPB-40 at 265 °C, 0.25 kPa EB, and 1 kPa O2, 6.88 vs. 9.32 10-4 mol-C=O g-1). This result is due to steric hindrance and the limitations of the particular reaction conditions. These results illustrate the importance of in situ structural analysis for carbon catalysts. The analysis can provide detailed kinetic information for the given reaction conditions. The slope of the line in Figure 1b reflects the activity of a single ketonic carbonyl group, i.e., the turnover frequency (TOF) of the synthetic model carbon catalysts. For YPB-6, the TOF was 5.4 x 10-5 molecules-EB C=O-1 s-1, which is slightly lower than that for nanocarbon catalysts (CNTs, OLCs, graphenes, etc., at 8.8 x 10-5 molecules-EB C=O-1 s-1) under identical reaction conditions. This result indicates that the conjugated size (degree of graphitization) may affect the intrinsic activity of carbon materials because the size directly affects the electronic structure and the subsequent redox capability of the carbon catalysts.12
Figure 2. In situ IR spectra of YPB-6 upon the introduction of EB into the reaction system as a function of the reaction time (fresh polymers were used as the background). 583 K; 1.6 kPa EB; balance He; 5 mg catalyst.
One of the key advantages of the proposed model catalyst is its spectro-transparency, which allows the use of in situ MScoupled diffuse reflectance infrared Fourier-transform (DRIFT) spectroscopy to study the interactions and reactions among the substrates and catalysts for mechanistic insights. Figure 2
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shows the IR absorption spectra upon the introduction of EB into the reaction system. The C=O stretching vibration band at 1690 cm-1 gradually decreased, and a new O-H band at 3532 cm-1 correspondingly emerged. The MS signal of the product ST was simultaneously detected, as shown in Figure S8. The IR and MS evidence suggest that EB can be dehydrogenated (forming ST) by reducing ketonic carbonyl into hydroxyl groups, even in the absence of oxygen; this phenomenon was further confirmed using solid-state 1H-NMR (as shown in Figure S9). The reduction and re-oxidation (for the carbon catalyst) parts of the catalytic cycle are separated, similar to the typical Mars-van Krevelen mechanism.13
Figure 3. a) In situ IR spectra and b) MS signals of YPB-6 during the EB TPSR process as a function of the reaction temperature (fresh polymers were used as the background). 1.6 kPa EB; balance He; 5 mg catalyst.
The temperature-programmed surface reaction (TPSR) experiment of EB pre-adsorbed on the model catalyst surface yielded similar results as when EB was introduced, as shown in Figure 3. The partial desorption of EB above 200 °C (Figure 3b) suggests that the chemical adsorption of EB may be a reversible process. The decrease in the absorption intensity at 2930 cm-1 (asymmetric stretching vibration of α-C-H, as shown in Figure 3a) indicates an interaction or reaction of C-H (from the CH2 of the ethyl group on the reactant EB) with the model carbon catalysts. In addition, the new vibration band at 3080 cm-1 that occurred above 200 °C could be assigned to the stretching vibration of the C-H in the alkene molecules (=C-H), which indicates the formation of ST on the model carbon catalyst surface.
Figure 4. a) In situ IR spectra and b) MS signals of reduced YPB6 upon the introduction of O2 (or 18O2) as a function of the reaction time (reduced polymers were used as the background). 538 K; 1.0 kPa O2 (or 18O2); balance He; 5 mg catalyst.
The model catalyst (YPB-6) in the reduced state could be easily re-oxidized in the presence of O2. The 18O isotope (18O2) was used in this experiment to distinguish the surface (16O) and gasphase (18O) oxygen species. In Figure 4a, the hydroxyl groups
were re-oxidized to ketonic carbonyl groups upon the introduction of oxygen, and the re-oxidation was signified by a decrease in ν-O-H at ~3530 cm-1 and an increase in ν-C=O at ~1700 cm-1. This process returned the carbon catalyst to its initial state and completed the catalytic cycle. The in situ IR results indicate that the catalytic ODH reaction relies on the redox cycle of the ketonic carbonyl (-C=O) and hydroxyl (-C-OH) groups on the carbon catalysts, and these two groups are considered the oxidation and reduction states of the active sites, respectively. In Figure 4b, the product analysis from the MS signal shows that the re-oxidation product (H2O) formed immediately upon the introduction of O2. Simultaneously, (the first 10 mins of O2 introduction), the gas-phase 18O2 was totally consumed, which indicates a relatively strong interaction or activation of the carbon catalysts with the oxygen molecules. The formation rate and quantity of H216O and H218O were nearly identical, which may be due to the rapid exchange of the gas-phase and surface oxygen.14 A small amount of 16O2 and 16O18O formed, which suggested that the oxygen molecules may first dissociate (activate) on the carbon surface and that this dissociative adsorption may be a reversible process. The intensities of 16O2 and 16O18O eventually decreased because the surface oxygen (16O) was exhausted, which is consistent with the above assumptions. Unfortunately, the in situ IR measurement did not capture the exact structural information of the intermediates for the re-oxidation process, i.e., the structure of the activated oxygen species on the catalyst surface, and this is likely because the re-oxidation is a relatively fast process.
Figure 5. a) EB ODH rate as a function of the EB (circles) and O2 (triangles) partial pressures; 538 K, 2.0 kPa EB, 1.0 kPa O2, balance He, 50 mg catalyst. b) Comparison of the ODH rates with C8H10/O2 (hollow triangles) and C8D10/O2 (hollow circles) on YPB-6, 538 K, 0.25 kPa EB (or D-EB), 1.0 kPa O2, balance He, 50 mg catalyst.
Figure 5a shows the effect of the O2 and EB partial pressures on the ODH reaction over YPB-6. The EB ODH rates reached nearly constant values at oxygen pressures higher than 1 kPa (Figure 5a), which indicates that the ODH rate does not depend on the O2 partial pressure in this regime. In contrast, the EB ODH rates monotonically increased with the increase in the EB pressure (Figure 5a). In Figure 5b, the kinetic isotopic effect (KIE) values for deuterated EB (D-EB) and EB (REB/RD-EB) are ~3.5, which indicates that the reaction is kinetically controlled by the activation of EB over a relatively wide range because the re-oxidation process (OH + O forming H2O) is a relatively fast step, as shown by the IR-MS results with labeled O2 (18O2). Combining the site titration, kinetic analysis and in situ IR measurements of the model carbon catalysts, a plausible reaction mechanism for the carbon-catalyzed alkane ODH reaction was proposed.5-7 The α-C-H of EB dissociatively adsorbs on the ketonic carbonyl groups on the carbon catalyst in quasi-equilibrated steps.3d ST forms after a sequential H abstraction from
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the ethyl groups upon the reduction of the active sites to hydroxyl groups. Oxygen molecules may need to first be activated (dissociated) on the carbon catalyst surface, and there is a rapid exchange between the gas phase and the surface oxygen species.6 The activated surface oxygen species may diffuse along the conjugated system and eventually re-oxidize the hydroxyl groups into ketonic carbonyl groups to complete the catalytic cycle.3d The abstraction of H is the kinetically relevant step in this reaction process. In conclusion, the present study provides insight into the catalytic mechanism of carbon-catalyzed alkane ODH reactions using polymeric model catalysts. The key advantages of the synthesized model catalysts are their relatively high molecular weights, large conjugated sizes, reasonable surface areas, simple oxygen species and spectro-transparency, which make them ideal analogs for nanocarbon materials in mechanistic studies. The present results indicate that the carbon-catalyzed alkane ODH reactions rely on the redox cycle of ketonic carbonyl/hydroxyl pairs. The entire catalytic process can be separated into a hydrogen abstraction step and a re-oxidation step. However, this process should be distinguished from the typical Mars-vanKrevelen or Langmuir-Hinshelwood mechanisms for transition metal oxide catalyzed ODH reactions because of the unique structure and chemical reactivity of the carbon materials, e.g., the absence of lattice oxygen species and relatively high oxygen activation activity. The present study also sheds light on the applications of model polymers for in-depth investigations of the function of carbon materials in catalysis (as supports), energy storage and conversion.
AUTHOR INFORMATION Corresponding Authors * Email:
[email protected] (W. Qi) * Email:
[email protected] (D. S. Su)
Author Contributions The manuscript was written with contributions from all authors. All authors have approved the final version of the manuscript.
Notes The authors declare no competing financial interests.
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search Program of Natural Science Foundation of Liaoning Province-Shenyang National Laboratory for Materials Science” (2015021010) and the Youth Innovation Promotion Association, CAS.
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ASSOCIATED CONTENT Detailed descriptions of the experimental and characterization methods or additional data, such as the synthesis of the YPB-6 model carbon catalysts, details on the EB ODH process, chemical titration process, and characterization and structural parameters of the YPB-x compounds can be found in the Supporting Information. The Supporting Information is available free of charge via the Internet at http://pubs.acs.org.
ACKNOWLEDGMENTS The authors acknowledge financial support from NSFC (21573256, 91645114, 2161101164, 21303226 and 51521091), the “Joint Re-
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Oxidative Dehydrogenation on Nanocarbon: Revealing the Catalytic Mechanism with Model Catalysts Xiaoling Guo, Wei Qi, Wei Liu, Pengqiang Yan, Fan Li, Changhai Liang, and Dangsheng Su ABSTRACT: Model catalysts offer a direct method to determine the active site structures and mechanisms on nanocarbon catalysts. In this report, we show that the ketonic carbonyl groups are the catalytic center of the nanocarbon-catalyzed alkane oxidative dehydrogenation (ODH) reaction, and we reveal the catalytic mechanism using conjugated polymeric model catalysts containing only ketonic carbonyl groups. An in situ infrared (IR) analysis provides spectroscopic evidence of the “working” configurations of the active sites in association with the redox cycle of the ketonic carbonyl-hydroxyl pairs. Carbonyl reduction (H abstraction from hydrocarbon) is shown to be a kinetically relevant step. An 18O isotope tracer experiment further shows that the activation and exchange of molecular oxygen at the surface active sites are fast processes under common reaction conditions.
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