Research Article Cite This: ACS Catal. 2018, 8, 10051−10057
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Ti3C2Tx MXene Catalyzed Ethylbenzene Dehydrogenation: Active Sites and Mechanism Exploration from both Experimental and Theoretical Aspects Jiangyong Diao,† Minmin Hu,†,‡ Zan Lian,†,‡ Zhaojin Li,† Hui Zhang,† Fei Huang,†,‡ Bo Li,*,† Xiaohui Wang,*,† Dang Sheng Su,† and Hongyang Liu*,† †
ACS Catal. 2018.8:10051-10057. Downloaded from pubs.acs.org by LUND UNIV on 01/07/19. For personal use only.
Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, People’s Republic of China ‡ School of Materials Science and Engineering, University of Science and Technology of China, 96 Jinzhai Road, Hefei 230026, People’s Republic of China S Supporting Information *
ABSTRACT: Two-dimensional materials (such as graphene, MXenes) usually perform remarkably well as catalysts in comparison with their three-dimensional counterparts. In this work, Ti3AlC2-derived Ti3C2Tx MXene (T = O, OH, F) was prepared and employed in the direct dehydrogenation of ethylbenzene. A 92 μmol m−2 h−1 reactivity and a 40 h stability with almost no deactivation were observed, which is much better than those of the previously reported nanocarbon catalysts. The graphene-like layered structure and the C−Ti−O termination groups induced by the preparation process were thought to account for the good catalytic performance of Ti3C2Tx MXene from both experimental and computational perspectives. The discovery expands the application of two-dimensional MXenes and provides more choices in exploring highactivity catalysts for hydrocarbon dehydrogenation reactions. KEYWORDS: MXene, direct dehydrogenation, ethylbenzene, styrene, heterogeneous catalysis
1. INTRODUCTION The limited reserves of fossil resources and the increasingly serious environmental issues have impelled more and more efforts toward highly active catalysts to produce high-valueadded alkenes from hydrocarbons in a green manner.1,2 Styrene, an important and greatly demanded industrial monomer involved in polymer synthesis, is mainly produced by the direct dehydrogenation (DDH) of ethylbenzene over multipromoted iron oxide catalysts presently.3,4 High styrene selectivity and yield can be obtained in such a catalytic system at a very high temperature (∼620 °C), while excess steam must be provided simultaneously as a cofeed to alleviate catalyst coking. Therefore, the energy and water input of the entire process are huge.5−7 Recently, nanodiamond synthesized by an explosive detonation method showed high reactivity and distinguished anticoking ability in the DDH of ethylbenzene under steam-free conditions, which stimulated a research boom in carbon-based catalysts.8−10 Carbon nanotubes,11 graphene,12 mesoporous carbon,13 and onion-like carbons14 have all shown promising catalytic performance in ethylbenzene dehydrogenation reactions at relatively low reaction temperatures (400−550 °C), which obviously reduce the energy consumption of the reaction process. Specifically, the two-dimensional graphene materials possess flat surfaces and abundant functional groups that perform well in both direct and indirect dehydrogenation of ethylbenzene reactions.15,16 © 2018 American Chemical Society
The surface oxygen functional groups, such as CO, on the carbon catalysts are believed to be the active sites for the dehydrogenation reactions and can be tuned by posttreatment.17,18 However, the disadvantages such as handling problems, high pressure drop, hampered heat and mass transfer, and unclear health risks still hinder the practical application of the powdery nanocarbon catalysts technically.19,20 Thus, it is necessary to identify new catalytic materials with enhanced catalytic performance, easy-to-handle ability, and good safety to meet the requirements for large-scale use. Two-dimensional (2D) materials, such as graphene, hexagonal BN, and graphitic C3N4, possess unusual structural and physicochemical properties, potentially allowing for a wide range of applications.21,22 MXenes, a newly discovered category of 2D materials named according to their graphenelike morphology, possess versatile chemical composition, tunable layer thickness, and facile functionalization and have been attracting extensive attention in the field of supercapacitors, lithium/sodium ion batteries, electromagnetic shielding materials, and photocatalysis since they were first synthesized in 2011.23,24 Typically, the MXenes have been Received: May 23, 2018 Revised: August 10, 2018 Published: September 21, 2018 10051
DOI: 10.1021/acscatal.8b02002 ACS Catal. 2018, 8, 10051−10057
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ACS Catalysis obtained by selectively etching off the “A” elements from their counterparts, the MAX phases (Mn+1AXn, n = 1−3, where M represents an early transition metal, A is a group IIIA or IVA element, and X is C and/or N), using HF or LiF/HCl solution (see Scheme 1). When A was extracted, the resulting materials
preparation process can be found in the Experimental Section. The morphology and structure of the samples before and after HF treatment were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The Ti3AlC2 precursor showed a tightly stacked morphology which is similar to that of the unexfoliated graphite, as displayed by the SEM image in Figure 1A. After reaction with
Scheme 1. Schematic Representation of the Etching Process of Ti3AlC2 (MAX) to Ti3C2Tx (MXene)
were spontaneously terminated with O, OH, or F groups, generating X−M−O, X−M−OH or X−M−F surface groups and giving MXenes a Mn+1XnTx formula (T refers to the terminated groups). These terminations can be tuned by changing the preparation or postprocessing method, and they are reported to be the regulators of the electronic structure of MXenes and can bring about distinguished properties to expand the application fields of MXenes.25,26 Several research groups have reported the superior catalytic effects of Ti3C2Tx MXene derivatives toward the hydrogen storage reactions of MgH2 or NaAlH4, revealing the good dehydrogenation/ hydrogenation properties of MXene catalysts.27,28 Moreover, the MAX phase was demonstrated to be an efficient catalyst in the oxidative dehydrogenation of n-butane to butenes and butadiene recently; the O-containing groups resulting from the surface layer oxygen vacancies are considered to be the active sites.29 In addition, TiC-derived carbon (TiC-CDC) was also shown to be an efficient and stable catalyst for the DDH of ethylbenzene in our previous work.30 Hence, MXenes with abundant terminated groups have a great potential to be good dehydrogenation catalysts for hydrocarbons. However, to the best of our knowledge, no such results have been published so far. Herein, the Ti3C2Tx MXene, which is the first reported and most intensively studied MXene, was synthesized by selectively etching off the Al element of Ti3AlC2 and employed in the direct dehydrogenation of ethylbenzene in this work. The reactivity of Ti3C2 MXene was much higher (92 μmol m−2 h−1) than that of the analogous graphene (12 μmol m−2 h−1) and nanodiamond (7 μmol m−2 h−1), while that of the Ti3AlC2 precursor was almost negligible. Characterizations combined with first-principles calculations revealed that the C−Ti−O groups dominating on the MXene surface and the layered structure accounted for the good performance of the Ti3C2Tx MXene catalyzed ethylbenzene dehydrogenation reaction.
Figure 1. SEM images of Ti3AlC2 (A) and Ti3C2Tx (B) particulates and HRTEM (C) and HAADF-STEM (D) images of the multilayered Ti3C2Tx MXene (C, outer surface; D, cross section; the bright dots are Ti atoms, and the weaker dots are termination atoms).
HF, the tightly stacked layers clearly separated from each other, transforming to a loose accordion-like structure with obvious layer spaces due to the loss of the Al atoms (Figure 1B), distinctly different from the untreated Ti3AlC2. This structural transformation was also observed by the XRD patterns shown in Figure S1. It is clearly seen that the (002) peak initially at 2θ = 9.5° broadened and shifted to a lower angle (2θ = 8.8°) after HF treatment, which indicates a larger d spacing.23,33 Furthermore, the generated MXene sheets were investigated by high-resolution transmission electron microscopy (HRTEM). Figure 1C represents the HRTEM image of the as-prepared multilayer sheets of Ti3C2Tx MXene. Layers and layers can be clearly distinguished as shown by the distinct contrast in Figure 1C. In addition, the outer surface of MXene was covered by an amorphous layer (indicated by the white arrows), which indicates the presence of more functional groups and defects in comparison with the inner part.29 Figure 1D shows the high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image of a cross section of Ti3C2Tx MXene sliced by a focused ion beam. The Ti and termination (T) atoms can be clearly distinguished, while the C atoms, which may be covered by the Ti signals, were invisible.23 Figure 1C,D both show that the layers were evenly arranged together, suggesting the good stability of the MXene material. The structural change from Ti3AlC2 to Ti3C2Tx can be further characterized by N2 physisorption. The N2 adsorption−
2. RESULTS AND DISCUSSION Ti3C2Tx MXene was synthesized by selectively etching off the Al element from the ternary Ti3AlC2 precursor using 40% HF solution according to our previous work;31,32 the detailed 10052
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ACS Catalysis
Figure 2. XPS peak fitting of Ti 2p and O 1s spectra of the as-prepared Ti3C2Tx MXene.
annealing;37−39 therefore, the high proportion of C−Ti−O groups as shown in Figure 2B indicated a potentially good dehydrogenation ability. Figure 3 shows the catalytic performance of Ti3C2Tx MXene in the DDH of ethylbenzene (EB), including those of the other
desorption isotherms of Ti3AlC2 and Ti3C2Tx both showed H3 type curves which are commonly seen in materials with a layered structure and macropores (Figure S2).34 Hysteresis loops due to capillary condensation can be observed in the range of P/P0 = 0.5−1.0. However, the hysteresis loop of Ti3C2Tx can be obviously observed while that of Ti3AlC2 was negligible, which can be ascribed to loosely and tightly layered structures, respectively. Meanwhile, the specific surface area increased from 1.6 to 11.6 m2 g−1 (Table S1), consistent with the SEM observations. Functional groups are usually considered to be the real active sites of carbon-related catalysts in catalytic reactions. After Al in the MAX phase was extracted by HF solution, the surface of the resulted MXene phase was spontaneously terminated by O, OH, and F to generate C−Ti−O, C−Ti− OH, and C−Ti−F groups, among which C−Ti−O was the most stable and probably active group for dehydrogenation reactions.35 To reveal the potential catalytic reactivity of the asprepared MXene, X-ray photoelectron spectroscopy (XPS) was employed to characterize the surface composition of Ti3C2Tx. The survey spectrum and the surface element atomic concentration estimated from XPS characterization are shown in Figure S3 and Table S2. The oxygen content was as high as 23.8 atom % (Table S2), which means that the MXene surface was mainly terminated by oxygen functional groups; this result is consistent with the calculation work we reported previously.35 The Ti 2p and O 1s spectra were deconvoluted to identify the specific groups existing on the MXene surface; the results are shown in Figure 2 and Table S3. Typically, the 2D Ti3C2Tx MXene is constituted by Ticentered octahedra which can be categorized as a central homoleptic octahedron (Ti1−C6) and surface heteroleptic octahedra (Ti2−C3T3), and there are two kinds of hybridization of the Ti 2p orbital (2p1/2 and 2p3/2). Considering the complexity of the Ti species, the Ti 2p spectrum was deconvoluted to Ti−C (Ti1), Ti2+/Ti3+ (Ti2, C−Ti−O/OH groups), and TiO2−xFx (Figure 2A),36 among which 49.5% was Ti2+/Ti3+ groups. The deconvolution of the O 1s spectrum reveals five different chemical environments of O, which could be assigned to Ti−O (TiO2, ∼529.9 eV), C−Ti−O (∼531.2 eV), C−Ti−OH (∼532.0 eV), Al−O (Al2O3, 532.8 eV) and OH (adsorbed water, 533.8 eV), respectively. The C−Ti−O groups accounted for 28.4% of the total oxygen species, which is much higher than the amounts of the other four. In previous reports, C−Ti−O is the most stable group on the surface of Ti3C2Tx MXene, and the C−Ti−OH and C−Ti−F groups can be further converted to C−Ti−O under high-temperature
Figure 3. (A) Ethylbenzene dehydrogenation reactivity of Ti3C2Tx MXene, nanodiamond, graphene, and TiC-derived carbon (TiCCDC): data from 20 h steady state reactivity. (B) Long-term stability of Ti3C2Tx MXene on dehydrogenation of ethylbenzene. Reaction conditions: 50 mg of catalyst for (A) and 300 mg of catalyst for (B), T = 550 °C, flow rate 10 mL min−1, 2.8% ethylbenzene with He balance.
three pure carbon catalysts (graphene, nanodiamond, TiCCDC) as a comparison. The EB conversion rate was normalized per specific surface area considering the low value of Ti3C2Tx MXene (11.6 m2 g−1). From the results shown in Figure 3A, it is observed that the Ti3C2Tx MXene had the highest EB conversion rate (92 μmol m−2 h−1), while graphene, nanodiamond, and TiC-CDC only had rates of 12, 7, and 0.8 μmol m−2 h−1, respectively The styrene (ST) 10053
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Figure 4. Reaction pathway and important structures of ethylbenzene dehydrogenation on Ti3C2O2. Ti is shown in light gray, C in dark gray, O in red, and H in white.
of Ti3C2Tx MXene in the catalytic dehydrogenation of hydrocarbons. A computational study was further conducted to verify the favorable performances; the detailed calculation method which is referred to our previous work can be found in the Experimental Section.40 The reaction pathway of DDH at the C−Ti−O site was explored by first-principles calculations, as shown in Figure 4. Two hydrogens need to be abstracted from ethylbenzene to form styrene. The initial state on the reaction pathway is the adsorption of ethylbenzene (C8H10* in Figure 4). The distance of ethylbenzene from the catalyst in the adsorption state is 2.80 Å. The benzene ring in ethylbenzene is nearly parallel to the catalyst surface. The C−H bond in ethylbenzene is activated with the molecule approaching the C−Ti−O site. The barrier of first C−H bond activation is calculated to be 0.66 eV. In the TS1 state, it is indicated that both hydrogen and carbon in the breaking C−H bond interact with oxygen on the catalyst surface with distances of 0.98 and 1.52 Å, respectively. To make room for the second hydrogen abstraction, the hydrogen resulting fromthe first C−H bond activation diffuses to the neighboring
selectivity of Ti3C2Tx MXene was as high as 97.5%, higher than for the other three controlled catalysts, indicating that there are only minor side reactions. On the other hand, the Ti3AlC2 MAX phase showed negligible reactivity in dehydrogenation of ethylbenzene (Figure S4), showing the important role of the structural transformation revealed by the above characterizations in catalytic reactions. A long-term stability test of Ti3C2Tx MXene was also conducted (Figure 3B). Notably, after a short induction period, the EB conversion remained at 21% for almost 40 h without any deactivation. In addition, no obvious coke could be observed from a TEM image of the used MXene (Figure S5), showing the good stability of the Ti3C2Tx MXene catalyst in comparison with the heavy coke on the used commercial iron oxide catalyst under the same reaction conditions.8 The apparent Arrhenius activation energy of the DDH reaction on Ti3C2Tx MXene was also obtained (Figure S6); the value (69.2 kJ mol−1) is close to that of nanodiamond (70.0 kJ mol−1), revealing the similar energy barriers of Ti3C2Tx MXene and nanodiamond in catalyzing the DDH of ethylbenzene, which further suggest the potential application 10054
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quartz reactor at atmospheric pressure. The catalyst was fixed between two quartz wool plugs. The system was heated to reaction temperature and kept for 10 min under helium. The reactant (2.8% EB, total flow rate 10 mL min−1, helium as balance) was then fed to the reactor from a saturator kept at 39.8 °C. The reactants were analyzed by an online gas chromatograph equipped with two columns: a HP-5 capillary column for the hydrocarbons and a CarboPlot capillary column for the permanent gases, coupled to FID and TCD detectors, respectively. The ethylbenzene conversion (XEB) and styrene selectivity (SST) were calculated according to the equations
oxygen site with a barrier of 0.30 eV (TS2 in Figure 4). The intermediate state (C8H9* + H* in Figure 4) after hydrogen diffusion is stable and is downhill by 1.09 eV in comparison with the IM state (C8H9* and H* bonded on the same oxygen). The barrier for the second hydrogen abstraction is calculated to be 0.46 eV (TS3). Therefore, two hydrogens are abstracted stepwise from ethylbenzene and styrene is formed.
3. CONCLUSIONS In conclusion, Ti3C2Tx MXene was first employed in the direct dehydrogenation of ethylbenzene. It showed the highest reactivity (92 μmol m−2 h−1) and selectivity (97.5%) in comparison with the other three carbon catalysts. The active sites were found to be the surface C−Ti−O groups generated during the selective etching process, and the two hydrogens on the ethyl group are abstracted stepwise from ethylbenzene, as revealed by XPS analysis and first-principles calculations. The expanded layered structure facilitates the mass transfer and adsorption−desorption processes. The stability revealed in a long-term test indicates its potential application in the dehydrogenation of hydrocarbons as an industrial catalyst. Considering its low specific surface area, the production of Ti3C2Tx MXene with large surface area will be needed in future work. Since there are over 60 MAX phases available up to now, the preparation of more MXenes with different compositions can be expected. Given the interesting catalytic properties of MXenes as demonstrated herein, MXene-based catalysts or catalyst supports may become a new research hot spot.
XEB = 1 − FC EB,outlet /F0C EB,inlet
SST = CST,outlet /(CST,outlet + C BZ,outlet + C TOL,outlet) YST = XEBSST
where F and F0 are the flow rates of the outlet and inlet and CEB, CST, CBZ, and CTOL denote the concentrations of ethylbenzene, styrene, benzene, and toluene, respectively. 4.4. Theoretical Calculations. The theoretical study of the ethylbenzene dehydrogenation reaction over Ti3C2Tx MXene is simplified to the Ti3C2O2 model, which is a layered structure with a p(4 × 3) supercell. A 2 × 2 × 1 Monkhorst− Pack k-point grid was used to sample the Brillouin zone. The vacuum layer is set to 20 Å to avoid the interaction along the z axis. The calculations reported here were performed by using periodic density functional theory as implemented in the form of the Vienna ab initio simulation package (VASP). For valence electrons, a plane-wave basis set was adopted with an energy cutoff of 500 eV and the ionic cores were described with the projector augmented-wave (PAW) method. RPBE was used as the exchange-correlation functional approximation, and the van der Waals correction was considered by the zero damping DFT-D3 method of Grimme. The climbing nudged elastic band (CI-NEB) method was employed to obtain the reaction pathways and energy barriers. The total energy and band structure energy were converged to an accuracy of 1 × 10−6 eV/atom to obtain the accurate forces; a force tolerance of 0.03 eV/Å was used in all structure optimizations. The Gibbs free energy of a species is calculated by
4. EXPERIMENTAL SECTION 4.1. Catalyst Preparation. The Ti3AlC2 precursor was prepared by a solid−liquid reaction synthesis reported previously. Individual Ti3C2Tx particulates with an accordion-like morphology were synthesized by exfoliating porous Ti3AlC2 in HF (40 wt %) solution for 24 h. After that, the resulting sediment was washed several times with deionized water and immersed therein for 1 day, followed by vacuum filtration. The filtered sample was subsequently dried by supercritical carbon dioxide drying under a condition above the critical point of CO2. The filtered sample was transferred into an autoclave in which ethanol was filled to minimize the evaporation of water from the wet sample during the following processes of heating and pressurizing. As soon as the autoclave was heated to 40 °C, CO2 preheated to that temperature was pumped into the autoclave at a flow rate of 0.54 kg h−1 to a pressure of 100 bar. After the solvent was completely replaced at that pressure by CO2 for 5 h, the CO2 was vented by slowly reducing the pressure to ambient while the temperature was maintained. 4.2. Characterizations. The morphologies of the fresh and used MXene catalysts were characterized by scanning electron microscopy (SEM, FEI Nova NanoSEM 450) and transmission electron microscopy (TEM, FEI Tecnai F20, 200 kV). The crystal structures of the samples were characterized by Xray diffraction (XRD) on a D/MAX-2500 PC X-ray diffractometer with monochromated Cu Kα radiation (λ = 1.5418 Å). N2 physisorption was conducted on a Micrometrics ASAP 2020 system. X-ray photoelectron spectroscopy (XPS) experiments were carried out on an ESCALAB 250 XPS system with a monochromated Mg Kα X-ray source. 4.3. Catalytic Measurements. The direct dehydrogenation of ethylbenzene was carried out at 550 °C in a fixed-bed
G(T , P) = E DFT + ZPE + Eent(T , P)
where EDFT is the total energy given by DFT calculation, ZPE is zero-point energy, and Eent is the correction of entropy and enthalpy. ZPE is calculated by i=1
ZPE =
∑ k
NAhνi 2
where N is the number of atoms, NA is Avogadro’s number, h is Planck’s constant, νi is the vibration frequency calculated by DFT, and i ranges from 1 to 3N − 6(5) for gas species and from 1 to 3N for surface species. Eent for gas is given by Eent(T , P) = ΔH °(0 → T ) − TS(T , P) = ΔH °(0 → T ) − TS°(T ) + RT ln(P /P°)
Eent for surface species is given by Eent(T , P) = ΔU °(0 → T ) − TS°(T ) 10055
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ACS Catalysis The details for calculation of ΔU° and S°(T) can be found in the work of Zhu et al.41
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.8b02002. Characterization and activation calculation results as described in the text (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail for B.L.:
[email protected]. *E-mail for X.W.:
[email protected]. *E-mail for H.L.:
[email protected]. ORCID
Jiangyong Diao: 0000-0001-6266-9322 Bo Li: 0000-0001-8895-2054 Xiaohui Wang: 0000-0001-7271-2662 Hongyang Liu: 0000-0003-2977-2867 Author Contributions
J.D. conducted most of the characterizations and catalytic tests and finished the writing of the paper. M.H. and Z.Li prepared the materials. H.Z. undertook the HRTEM characterization and helped with the analysis of the results. Z.Lian and B.L. designed and conducted the theoretical calculations. X.W. and H.L. initiated and guided this work. F.H. and D.S.S. revised and polished the paper. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Ministry of Science and Technology (2016YFA0204100), the National Natural Science Foundation of China (No. 21703261, 21573254, and 91545110), the Institute of Metal Research, Youth Innovation Promotion Association (CAS) and the Sinopec China. Joint fund between Shenyang National Laboratory for Materials Science and State Key Laboratory of Advanced Processing and Recycling of Nonferrous Metals (No. 18LHPY010).
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REFERENCES
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DOI: 10.1021/acscatal.8b02002 ACS Catal. 2018, 8, 10051−10057
Research Article
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DOI: 10.1021/acscatal.8b02002 ACS Catal. 2018, 8, 10051−10057
