Activated carbon supported molybdenum carbides as

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Activated carbon supported molybdenum carbides as cheap and highly efficient catalyst in the selective hydrogenation of naphthalene to tetralin†

Downloaded by Dalian University of Technology on 23 May 2012 Published on 27 February 2012 on http://pubs.rsc.org | doi:10.1039/C2GC35177C

Min Pang,a Chunyan Liu,a Wei Xia,b Martin Muhlerb and Changhai Liang*a Received 7th February 2012, Accepted 24th February 2012 DOI: 10.1039/c2gc35177c

The selective hydrogenation of naphthalene to tetralin has been conducted on Mo2C/AC prepared by microwave irradiation, and achieved a lasting high conversion with 100% selectivity up to 60 hours. The choice of activated carbon as a support is critical in gaining an ideal balance between high activity and good stability of the catalyst. Tetralin is a very useful high boiling point solvent, which is normally obtained by the hydrogenation of naphthalene. Besides being widely used in paint and pharmaceuticals,1 tetralin has shown the possibility of being a hydrogen donor in polymer electrolyte fuel cells owing to its ability of providing hydrogen easily as compared to decalin.2 Although the hydrogenation of naphthalene catalyzed by noble metals shows high selectivity to tetralin, the widespread application of a precious metal in a commercial system is severely retarded due to the high cost and the low abundance. Transition metals, their sulphides or alloys seem to be an alternative, because they are cheap and stable. However, their hydrogenation performances are yet far away from satisfactory;3 thus it is of great interest to look for low-cost catalysts with high activities and good stabilities. Methods to prepare molybdenum carbide, which is best for catalysis, have been intensively explored in order to make full use of its unique catalytic properties which resemble those of noble metals. Meanwhile, due to its much lower price, molybdenum carbide has shown several characteristics of a promising alternative to noble metals. The conventional preparation routes, temperature programmed reduction as a typical case, always fall into the “outside-in” category, where carbides form through the insertion of external carbon atoms into the corresponding metal or metal oxides. In this way, a gradient is generated during the carbonization with a high carbon concentration on the particle surface and low concentration in the bulk. As a result most of a

Laboratory of Advanced Materials & Catalytic Engineering, Dalian University of Technology, Dalian 116024, China. E-mail: [email protected]; Fax: +86 411 84986353; Tel: +86 411 84986353 b Laboratory of Industrial Chemistry, Ruhr University Bochum, Bochum 44780, Germany † Electronic supplementary information (ESI) available: Experimental section, chemical reaction formula of HMT reacting with AHM (eqn (S1)), results of the adsorption test (Table S1), results of the hydrogenation of tetralin with varied contents of naphthalene (Table S2), representative XRD pattern of Mo2C/AC (Fig. S1), division of signals with m/z = 17, 18 (Fig. S2), XRD pattern of the spent Mo2C/AC (Fig. S3), TEM image of the spent Mo2C/AC (Fig. S4). See DOI: 10.1039/ c2gc35177c 1272 | Green Chem., 2012, 14, 1272–1276

the carbon atoms polymerize on the surface and form several carbon layers as contaminants. To tackle this problem, the “inside-out” route has been proposed to reverse the carbon diffusion by initially introducing sufficient carbon into the precursor. The subsequent treatment with additional carbon-containing gases can therefore be avoided. Among the “inside-out” routes, the single-source route to nitrides developed by Afanasiev has gained great popularity and has already extended to the preparation of carbides.4 In parallel with the exploration of novel precursors, the substitution of the conventional heating methods associated with the undesired long-time process under high temperature is carried out to realize the controlled synthesis of carbides. The employment of microwave irradiation as an environmentally benign heating method has achieved appreciable progress.5–7 In spite of some open questions, like the determination of the time profile of temperature, the limitation to microwave-adsorbing materials, microwave has several credentials of green chemistry for its rapid heating and high energy efficiency. Since it is able to complete the preparation within tens of minutes, the synthesized carbides are nearly free of sintering and present high dispersion on the support.8 Moreover, the influence of its unique rapid heating on the final composition of products has been magnified especially in the case of the single-source route. Some phase transformation that seems refractory upon conventional heating can be realized under the microwave irradiation. Herein, we conduct a microwave-assisted single-source route to highly-dispersed Mo2C supported on activated carbon (AC) and use it for the vapor-phase selective hydrogenation of naphthalene. It’s demonstrated that, the absence of decalin in the product results from the moderate hydrogenating capability of Mo2C, rather than the prior adsorption of naphthalene to catalyst in the presence of tetralin, which usually is the root of the high selectivity achieved on noble metals.9 What should be highlighted is that, unlike the quick deactivation of bulk Mo2C reported,10 our catalyst shows a good stability with a lasting 60hour ultrahigh activity and 100% selectivity to tetralin, which we suppose is not only due to the high dispersion of Mo2C realized by microwave irradiation, but also the use of activated carbon as the support. The preparation procedure is a modification of the single-step carburization proposed by Wang but with the assist of microwave irradiation instead.11 The precursor is obtained from hexamethylene tetramine (HMT) reacting with ammonium heptamolybdate (AHM) at a fixed stoichiometry of two (shown in eqn (S1)†). This journal is © The Royal Society of Chemistry 2012


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However, with the identical precursor, we get β-Mo2C here rather than γ-Mo2N as in Wang’s case. Three main diffraction peaks resolved at 39.2, 61.2 and 74.5° match well with the standard pattern of β-Mo2C in the representative XRD pattern (see Fig. S1†) of the prepared catalyst, along with some amorphous or poor-crystallized [Mo,C] species which generally exist in molybedenum carbides as impurities. No diffraction peaks belonging to molybdenum oxides show up in XRD but we do detect a large amount of Mo(VI) species by XPS. This may indicate that the oxides are amorphous on the support and are probably due to the surface oxidation when exposing Mo2C/AC in air. TEM images give a direct insight into the structure properties and distribution of the catalyst. It can be seen in Fig. 1 that the Mo2C nanoparticles are highly dispersed on the activated carbon with 3 nm in diameter. Although the TG-MS of precursor has been done preliminarily by Afanasiev,4 a lack of deeper insight into the thermal decomposition pathway makes the controlled synthesis of carbides–nitrides somewhat beyond guidance. Herein, we re-carry out the TG-MS and try to understand the overall thermal decomposition pathway. The autoreduction of AHM in inert gas has been investigated thoroughly and shows a release of ammonia (eqn (1)).12 4ðNH4 Þ6 Mo7 O24 4H2 O ! 7Mo4 O11 þ 35H2 O þ 7=3N2 þ 58=3NH3

ð1Þ

Here we’ll specify the major differences in our TG-MS analysis by taking the autoreduction of AHM as reference. What we have to emphasize here is that there is no release of ammonia all through the decomposition ever since HMT combines with AHM. In the previous observation, the release of water and

Fig. 1 TEM (a, b) and HRTEM (c, d) images of Mo2C/AC-20 (the number refers to the Mo loading); inset of image (c) shows the (002) facet of β-Mo2C with hexagonal structure; circles in image (d) point to Mo2C particles.

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ammonia in the thermal decomposition of the precursor in inert gas had coinciding maximum at 200 °C and 280 °C according to the signals with the m/z value of 18 and 17.4 But m/z = 17 does not necessarily point to ammonia since it may be due to the OH fragments. So, we do a division on the intensities of the two signals (m/z = 17, m/z = 18). If the result is constant, m/z = 17 is the secondary signal of water; if not, NH3 is released during the decomposition. It’s quite clear that, apart from some fluctuation attributed to noise near the end (shown in Fig. S2†), ammonia is not detected during the whole progress. Another point worthy of notice is that we do not detect any signals with the m/z value of 16, indicating neither the methane nor the ammonia has ever existed. Very much like the autoreduction of AHM in inert gas, the decomposition of precursor also has three water peaks at 130, 200, 300 °C (see Fig. 2). The first peak at 130 °C perfectly matches the dehydration of AHM, whereas the latter two peaks shift to the lower temperature with comparison to those in the autoreduction of AHM. This means, after the introduction of HMT, the reduction of AHM goes more easily. During the autoreduction of AHM, part of (NH4)+ groups release as ammonia and the rest reduce (Mo7O24)6− with the evolution of N2 and H2O; while as to the precursor, (NH4)+ groups decompose to N2 and H2, and most of the H2 is not used for reduction and goes off molecularly with a H2 peak at 230 °C. Here we suppose an intramolecular oxidation–reduction is taking charge of the degradation of (Mo7O24)6−. Specifically, carbon in HMT reacts with (Mo7O24)6− to form CO2 and CO as indicated by the peaks at 200 and 235 °C respectively. In principle, HMT thermally decomposes into H2 and CH4. But due to the chemical bond between HMT and AHM, carbon in HMT no longer combines with H to form CH4 as indicated by the absence of the signal with m/z = 16, but with the neighbouring oxygen contained in (Mo7O24)6− to form COx. Since the internal oxidation–reduction builds on the locally intimate contact between C and O, the reduction of (Mo7O24)6− groups goes more easily and is with a lower on-set temperature. The intramolecular reduction intensifies at the higher temperature region with the evolution of a large amount of CO and CO2. In return, with the elimination of C, N takes dominance in the solid, thus molybdenum nitride is

Fig. 2 Mass spectra of gases evolved upon heating of precursor under He; the part below baseline is attributed to noise and can be ignored; the four curves are divided into two groups for the convenience of presentation.

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favoured at a relatively low temperature, i.e. 650 °C in the single-source route.4 In nitride system, N atoms diffuse from surface to bulk for the metal nitride while the reversed trend is expected simultaneously. The latter would take priority when above 400 °C and becomes stronger with temperature.13 The long tail of m/z = 28 stretching from 700 °C results from the diffusion of N atoms from bulk to surface and the recombination over there into N2. The whole thermal decomposition can be proposed as eqn (2).


ðHMTÞ2 ðNH4 Þ4 Mo7 O24 2H2 O ! 17=4N2 þ ð2 þ X ÞH2 O þ X CO þ ð12 X ÞCO2 þ ð20 X ÞH2 þ 7=2Mo2 N

ð2Þ

It seems from above that the direct pyrolysis of the precursor would under no circumstances lead to molybdenum carbides but nitrides, and it’s true if the pyrolysis was finished by the conventional heating method. While under the microwave irradiation, CO resulting from the intramolecular oxidation–reduction could again be used as the carburization gas at higher temperature to replace N atoms in nitrides by C atoms to form carbides and this has already been testified in our previous work.14 Microwave irradiation is known for its ability to elevate temperature to ultrahigh in a short period of time. From the thermodynamic view, this heating manner is favorable for the release of N2 and the carburization of Mo2N into Mo2C.15 In this regard, we use microwave irradiation in this work and successfully realize the onestep pyrolysis to Mo2C. The hydrogenation of naphthalene is a typical consecutive reaction, with tetralin as the half-hydrogenated product and decalin as the full-hydrogenated one (see Scheme 1). Noble metal catalysts show a high selectivity to tetralin, but its sensitivity to poisons is still to be addressed. In this work, we use Mo2C/AC in the selective hydrogenation of naphthalene in order to find a cheap and effective alternative to noble metals. The catalytic performance is shown in Table 1. It is general that a high space time corresponds to a high conversion providing that less raw material has to be handled with

Scheme 1 The reaction pathway of the hydrogenation of naphthalene; K1, K2 stands for the reaction rate constant of each hydrogenation.

Table 1

per unit. As expected, the conversion of naphthalene tends to larger while increasing the space time, with the maximum conversion of 80, 95, 98% achieved on the catalysts with 10, 20, 30 wt% Mo loading correspondingly when the LHSV is set at 20, 14.3 and 14.3 h−1 respectively. The selectivity to tetralin is most likely a function of adsorption nature and hydrogenating capability of the catalyst. Noble metals and transition metals could easily conduct the hydrogenation to a deeper step, even to ring opening, owing to its high hydrogenating capability. As a result, lots of coke generate and exist as undesired byproduct and detriment to the catalyst.16,17 Introducing heteroatom into metal provides a route to modify the structural and electronic properties of the parent metal, thus the hydrogenation can be pulled back to a shallow level. Unfortunately, as three hot spots, transition metal sulfides, phosphides, silicides still have a lot to improve until their high stability no longer being negated by the low activity or poor selectivity. In this work, tetralin as the exclusive product can be achieved with all the three catalysts under certain condition: an overall 100% selectivity with a maximum 80% conversion on 10 wt%–Mo2C/AC; 100% selectivity with 95% conversion on 20 wt%–Mo2C/AC; 100% selectivity with 90% conversion on 30 wt%–Mo2C/AC. Usually, as to hydrogenation catalyzed by noble metals, the adsorption nature is the key factor for the selectivity. One school of thought drops into the strong interaction between noble metal and naphthalene. That is to say, the adsorption of tetralin on active sites is largely suppressed in the presence of naphthalene, so is the further hydrogenation to decalin.9 But in this work, we do not find any overwhelming adsorption (see Table S1†). On the contrary, even tetralin takes priority in the adsorption to active sites in our catalytic system, the hydrogenation of tetralin is severely hindered with tiny addition of naphthalene (see Table S2†). The hydrogenation of tetralin to decalin is already testified to be an order of magnitude slower than that of naphthalene to tetralin,9 i.e. K1 > K2 in Scheme 1. So in this reaction, Mo2C tends to hydrogenate naphthalene first in the presence of tetralin. The hydrogenation of tetralin shall not be taken into consideration until naphthalene has been converted thoroughly, because it’s much harder for Mo2C to activate tetralin due to its moderate hydrogenating capability. We find in this work that the naphthalene conversion is maintained above 80% during the first 60 hours, but soon after that the Mo2C/AC falls into lasting deactivation (see Fig. 3). First, the XRD pattern of the spent catalyst presents a new, remarkable diffraction peak for lamellar structure (see Fig. S3†). This change may indicate a graphitizing process during the

Hydrogenation of naphthalene over Mo2C/AC samples with different Mo loading LHSVa 20 h−1

16.7 h−1

14.3 h−1

7.2 h−1

3.6 h−1

Entry

Mo loading

cb

sc

c

s

c

s

c

s

c

s

1 2 3

10 wt% 20 wt% 30 wt%

80 95 95

100 93 87

60 97 97

100 95 88

42 95 98

100 100 90

21 94 97

100 100 93

10 52 90

100 100 100

a

LHSV = liquid hourly space velocity. b c = conversion of naphthalene. c s = selectivity of tetralin.

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Fig. 3 Hydrogenation performance of Mo2C/AC-20 as a function of time on stream in the hydrogenation of naphthalene.

Fig. 4 XPS Mo 3d spectra of Mo2C/AC-20 before and after use.

hydrogenation. As can be seen in the TEM images (see Fig. S4†) of the used catalyst, Mo2C is wrapped by a couple of carbon layers, and most of the active sites are covered up as a result. Further support for the surface graphitization comes from the comparison of XPS data between the fresh and the used Mo2C/ AC by looking at the Mo 3d region (shown in Fig. 4). The peaks at binding energy of 232.6, 235.7 eV belong to Mo(VI) species in MoO3 caused by surface oxidation when exposing Mo2C/AC in air. The peak at BE = 228.2 eV is in good agreement with literature value for the carbide-alloyed Mo.18 The high ratio of Mo (VI) to the carbide Mo gives an extra demonstration for the high dispersion and small size of Mo2C on the support. It makes sense that the content of MoO3 drops after hydrogenation since the graphite carbon generated during the reaction shields the surface of Mo2C from oxidation to some extent. An interesting question arising from the increased content of the carbide Mo is whether there is a formation of Mo2C during the hydrogenation. We try to picture one scenario to give a plausible explanation for the enhancements both XPS and XRD of the used Mo2C/AC. The surface MoO3 and [Mo,O,C] species from the exposure of This journal is © The Royal Society of Chemistry 2012

fresh Mo2C/AC in air is reduced by H2 into metallic Mo and [Mo,C] species in the pre-reduction step. In this progress, not only the H2 performs the reducing reagent, but also does the carbide-alloyed C participate to share the elimination of neighbouring oxygen and release in the form of COx. The elimination of carbon leads to carbon vacancies or deficiencies, which appear to be highly effective in adsorbing and activating the reactants. This kind of activity increases dramatically with the amount of deficiencies, but unfortunately it is along with strong bonding between the reactants and the catalyst surface.19 In this regard, the reactants which strongly adsorb on the surface dissociate to give active carbon- containing species. These species may polymerize to form carbon layers on the surface and cover the active sites, and may also gradually diffuse into sublayers to fill the “vacancies”, which leads to newly formed Mo2C. As more “vacancies” are filled with carbon, the catalyst surface is more closed to a perfect Mo2C surface. As a consequence, the bonding of reactants to the Mo2C surface with fewer defects becomes weak, thus the activity drops down due to the low efficiency in activating the reactants. From the view above, the high initial activity contradicts the slow deactivation rate on Mo2C and this is still a problem to be addressed in carbide system. While in this work, we choose the activated carbon as the support. During the pre-reduction of catalyst, the active carbon atoms in the form of CHx generated through the reaction between AC and H2 would be used to carbonize Mo, as indicated by the carbothermal hydrogen reduction,20 to fill the “carbon vacancies” and perfect the surface of Mo2C to a certain degree. That may explain why the Mo2C/AC shows a slow deactivation rate in the first 60 hours without sacrificing much of the high initial activity in comparing to the bulk Mo2C in whose pre-reduction no carbon supplement exists, thus leading to a quick deactivation due to a surface with more carbon deficiencies. In conclusion, Mo2/AC has been prepared through the onestep pyrolysis of a precursor under microwave irradiation and proves a high efficiency in the selective hydrogenation of naphthalene to tetralin. It’s noted in this case that the high initial activity and the slow deactivation of the catalyst can be obtained by using activated carbon as a support, and because the hydrogenation is conducted in a fixed-bed reactor at successive state, we believe that the use of Mo2C/AC as a low cost catalyst for the production of tetralin is of great industrial importance, although further improvement of the catalytic stability is still needed.

Acknowledgements We gratefully acknowledge the financial support provided by the National Natural Science Foundation of China (no. 21073023 and 20973029) and Project Based Personnel Exchange Program with CSC and DAAD.

Notes and references 1 Y. Cheng, H. L. Fan, S. X. Wu, Q. Wang, L. Guo, L. Gao, B. N. Zong and B. X. Han, Green Chem., 2009, 11, 1061–1065. 2 S. Hodoshima, H. Nagata and Y. Saito, Appl. Catal., A, 2005, 292, 90–96.

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3 A. C. A. Monteiro-Gezork, A. Effendi and J. M. Winterbottom, Catal. Today, 2007, 128, 63–73. 4 P. Afanasiev, Inorg. Chem., 2002, 41, 5317–5319. 5 L. Carassiti, A. Jones, P. Harrison, P. S. Dobson, S. Kingman, I. MacLaren and D. H. Gregory, Energy Environ. Sci., 2011, 4, 1503– 1510. 6 S. R. Vallance, S. Kingman and D. H. Gregory, Chem. Commun., 2007, 742–744. 7 D. Obermayer, B. Gutmann and C. O. Kappe, Angew. Chem., Int. Ed., 2009, 48, 8321–8324. 8 C. H. Liang, L. Ding, C. Li, M. Pang, D. S. Su, W. Z. Li and Y. M. Wang, Energy Environ. Sci., 2010, 3, 1121–1127. 9 K. Ito, Y. Kogasaka, H. Kurokawa, M. Ohshima, K. Sugiyama and H. Miura, Fuel Process. Technol., 2002, 79, 77–80. 10 S. J. Ardakani, X. B. Liu and K. J. Smith, Appl. Catal., A, 2007, 324, 9–19.

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11 H. M. Wang, X. H. Wang, M. H. Zhang, X. Y. Du and K. Y. Tao, Chem. Mater., 2007, 19, 1801–1807. 12 C. Thomazeau, V. Martin and P. Afanasiev, Appl. Catal., A, 2000, 199, 61–72. 13 J. G. Chen, Chem. Rev., 1996, 96, 1477–1498. 14 M. Pang, C. Li, L. Ding, J. Zhang, D. S. Su, W. Z. Li and C. H. Liang, Ind. Eng. Chem. Res., 2010, 49, 4169–4174. 15 E. Furimsky, Appl. Catal., A, 2003, 240, 1–28. 16 S. D. Lin and C. S. Song, Catal. Today, 1996, 31, 93–104. 17 C. S. Song and A. D. Schmitz, Energy Fuels, 1997, 11, 656–661. 18 C. A. Wolden, A. Pickerell and T. Gawai, ACS Appl. Mater. Interfaces, 2011, 3, 517–521. 19 K. J. Leary, J. N. Michaels and A. M. Stacy, J. Catal., 1986, 101, 301– 313. 20 C. H. Liang, P. L. Ying and C. Li, Chem. Mater., 2002, 14, 3148–3151.
