Catalytic dehydrogenation of alcohol over solid-state molybdenum sulfide clusters with an octahedral metal framework

Article abstract of DOI:10.1016/j.materresbull.2015.07.027

Abstract Solid-state molybdenum sulfide clusters with an octahedral metal framework, the superconducting Chevrel phases, are applied to catalysis. A copper salt of a nonstoichiometric sulfur-deficient cluster, Cu_xMo₆S_8-δ (x = 2.94 and δ ≈ 0.3), is stored in air for more than 90 days. When the oxygenated cluster is thermally activated in a hydrogen stream above 300 °C, catalytic activity for the dehydrogenation of primary alcohols to aldehydes and secondary alcohols to ketones develops. The addition of pyridine or benzoic acid decreases the dehydrogenation activity, indicating that both a Lewis-acidic coordinatively unsaturated molybdenum atom and a basic sulfur ligand synergistically act as the catalytic active sites.

Full text of DOI:10.1016/j.materresbull.2015.07.027

Accepted Manuscript
Title: Catalytic dehydrogenation of alcohol over solid-state
molybdenum sulfide clusters with an octahedral metal
framework
Author: Satoshi Kamiguchi Kazu Okumura Sayoko
Nagashima Teiji Chihara
PII:
S0025-5408(15)30048-9
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.materresbull.2015.07.027
MRB 8335
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Revised date:
Accepted date:
23-3-2015
29-6-2015
20-7-2015
Please cite this article as: S. Kamiguchi, K. Okumura, S. Nagashima, T.
Chihara, Catalytic dehydrogenation of alcohol over solid-state molybdenum sulfide
clusters with an octahedral metal framework, Materials Research Bulletin (2015),
http://dx.doi.org/10.1016/j.materresbull.2015.07.027
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Catalytic dehydrogenation of alcohol over solid-state molybdenum sulfide clusters with an
octahedral metal framework
Satoshi Kamiguchi a,b,*, Kazu Okumurac, Sayoko Nagashimad, and Teiji Chiharad
____________________
a Advanced Catalysis Research Group, RIKEN Center for Sustainable Resource Science, 2-1
Hirosawa, Wako City, Saitama 351-0198, Japan
b Organometallic Chemistry Laboratory, RIKEN, 2-1 Hirosawa, Wako City, Saitama 351-0198,
Japan
c School of Advanced Engineering, Kogakuin University, Nakano-machi, Hachioji City, Tokyo
192-0015, Japan
d Graduate School of Science and Engineering, Saitama University, Shimo-Okubo, Sakura-ku,
Saitama City, Saitama 338-8570, Japan
* Corresponding author: Satoshi Kamiguchi
* Phone: (+81)48-467-1349
* Fax: (+81)48-467-9391
* E-mail: kamigu@riken.jp
* Address: RIKEN, Wako, Saitama 351-0198, Japan
1
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Abstract
Solid-state molybdenum sulfide clusters with an octahedral metal framework, the
superconducting Chevrel phases, are applied to catalysis. A copper salt of a nonstoichiometric
sulfur-deficient cluster, CuxMo6S8–δ (x = 2.94 and δ ≈ 0.3), is stored in air for more than 90 days.
When the oxygenated cluster is thermally activated in a hydrogen stream above 300 ºC, catalytic
activity for the dehydrogenation of primary alcohols to aldehydes and secondary alcohols to
ketones develops. The addition of pyridine or benzoic acid decreases the dehydrogenation
activity, indicating that both a Lewis-acidic coordinatively unsaturated molybdenum atom and a
basic sulfur ligand synergistically act as the catalytic active sites.
KEYWORDS: A. Chalcogenides, A. Inorganic compounds, A. Superconductors; D. Catalytic
properties, D. defects.
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1. Introduction
Solid-state molybdenum sulfide clusters with an octahedral metal framework, the Chevrel
phases, are a common group of inorganic compounds. Studies on these clusters have focused
mainly on their physical properties such as superconductivity and magnetism [1–3]. There are
only a few reports on their use for catalysis [4–6], although these clusters do have some
characteristic features as catalysts, for example: multimetal, multielectron systems; unique
intermediate oxidation states of the metal atoms around 2+; and high thermal stability expected
from high synthesis temperatures (700–1000 °C). Considering these features, we have been
studying the application of these clusters to various catalytic reactions such as the ring opening of
tetrahydrofuran [7] and decomposition of methyl tert-butyl ether [8]. This paper describes the
oxidant-free dehydrogenation of alcohols over the clusters with a sulfur-deficient molybdenum
atom. The development of a new catalytic active site is also discussed.
2. Experimental
Solid-state molybdenum sulfide clusters CuxMo6S8–δ (x = 2.94, δ ≈ 0.3) (1) [9], Mo6S8–δ (δ
≈ 0.4) (2) [10], Li4Mo6S8 (3) [11], and Ni2Mo6S8 (4) [12] were synthesized according to the
published procedures. The amounts of sulfur deficiency in 1 and 2 were determined by elemental
analyses [7]. Crystals of these clusters were crushed and screened with a 150–200 mesh sieve.
They were stored in air for more than 90 days before use as catalysts. The reactions were
performed using a conventional continuous-flow microreactor operated at atmospheric pressure
[13]. Typically, a weighed crystalline cluster sample (10 mg) of 1 stored in air was packed in a
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borosilicate glass tube (3 mm i.d.) and placed in the center of an electric furnace. The catalyst
sample was initially activated from room temperature to a fixed temperature between 150 and
500 °C over 15 min in a hydrogen stream (1.2 L/h) and then held at this temperature for 45 min.
Without changing the temperature, the reaction was initiated by adding 1-butanol (92 μL/h, 1.0
mmol/h) into the hydrogen stream using a syringe pump. The reaction was monitored every 20
min by sampling the reaction gas (1 mL) via a six-way valve held at 140 °C, followed by analysis
using an online gas–liquid chromatography. The reactor effluent was frozen in a dry-ice trap for
determination of material balance and subsequent analysis of the products. Catalytic reactions
using the other catalysts were performed in the same way.
3. Results and Discussion
3.1 Catalytic reaction
The cluster, stored in air, was thermally activated in a hydrogen stream, followed by the
reaction of 1-butanol at the activation temperature. A typical reaction profile at 300 ºC is plotted
in Fig. 1. The catalytic activity slightly decreased with time, but the selectivity remained constant
throughout the reaction. Dehydrogenation of 1-butanol to butyraldehyde was the main reaction,
with 91% selectivity. In contrast, the selectivity for dehydration to butenes and dibutyl ether was
very low, with 8 and 0.5% selectivity, respectively. The turnover frequency per Mo6 cluster core
over a period of 3–4 h was 26.2 h–1, assuming that all the cluster cores were active. The material
balance was 97% at 3–4 h after the start of the reaction. No appreciable changes in the X-ray
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diffraction pattern were observed after 3 h reaction with 1-butanol (see supplementary data, Fig.
S1).
The catalytic activity for various alcohols over 1 is tabulated in Table 1. Primary alcohols
such as ethanol and propanol were also selectively dehydrogenated to the corresponding
aldehydes with 88–89% selectivity. Secondary alcohols such as 2-propanol and 3-pentanol were
predominantly dehydrogenated to the corresponding ketones.
The catalytic activity of some molybdenum sulfide clusters and related compounds for the
dehydrogenation of 1-butanol is tabulated in Table 2. All the clusters that were tested selectively
catalyzed the dehydrogenation with 72–97% selectivity (entries 1–4). The activity decreased with
a decreasing amount of sulfur deficiency (1 ≈ 2 >> 3 ≈ 4) [7]. When molybdenum metal and
molybdenum disulfide were applied to the reaction, the dehydrogenation selectivities were <67%
(entries 5 and 6). Some clusters with a Mo6S8 unit have been reported to decompose to
molybdenum disulfide, molybdenum metal, or both, by thermal treatment under various
atmospheres [6, 9, 14]. Even if these decomposed species were formed, they were not active in
the reaction.
The effect of the reaction temperature on activity and selectivity over 1 at 3 h after the start
of the reaction is presented in Fig. 2. Substantial catalytic activity appeared above 300 ºC and
increased with increasing temperature with a maximum at 350 ºC: the conversion at this
temperature was 46.3%, although it slightly decreased with time (38.3% at 4.67 h). The
dehydrogenation to butyraldehyde was the main reaction over the temperature range 300–500 ºC,
with >79% selectivity. At 350 ºC, the selectivity was highest (90.5%), and was almost constant
with time (90.2% at 4.67 h).
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3.2. Catalytic active site
We have investigated the activation process of 1, using XRD patterns, thermogravimetric
analyses, Fourier transform infrared spectra, X-ray photoelectron spectra, temperature
programmed desorption of ammonia, and elemental analysis [7, 8]. A proposed mechanism for
the activation can be summarized as follows. Upon storage in air for more than 90 days, 1
incorporates oxygen as an oxo ligand at the sulfur-deficient site on the cluster anion. When this
oxygenated cluster is heated in a hydrogen stream, it releases the oxo ligand up to 300 C,
leading to the re-formation of sulfur-deficient 1 [7, 8]. As Fig. 2 shows, the dehydrogenation
activity developed above 300 ºC, indicating the participation of the sulfur-deficient site or, in
other words, the coordinatively unsaturated molybdenum atom in the reaction. When 1-butanol
was allowed to react in the presence of 4 equivalents (eq) of pyridine at 300 °C, the conversion
was reduced to 4%. This observation indicates that the Lewis acid site is catalytically active for
the reaction.
It has been reported that acidic oxides (e.g., γ-Al2O3) are active for the dehydration of
alcohols and basic oxides (e.g., MgO, CaO) for the oxidant-free dehydrogenation [15], where
alcohol is adsorbed on metal as an alkoxide and on oxygen as a proton, followed by the
elimination of an α-hydrogen of the alkoxide to afford a carbonyl compound and molecular
hydrogen [16]. When 0.18 eq of benzoic acid was added to 1-butanol over 1, the conversion
decreased to 9%. The sulfur ligand in the activated 1 acts as a basic site; density functional theory
(DFT) calculations of a discrete Mo6S8 cluster have shown that methanol is adsorbed on basic
sulfur as a proton and on Lewis acidic molybdenum as a methoxide [17]. There are many reports
on the reactivities of molybdenum sulfide clusters in which a sulfur ligand acts as a base [18].
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Thus, both the Lewis acidic coordinatively unsaturated molybdenum cation and basic sulfide
anion in 1 participate in the oxidant-free dehydrogenation of alcohols (Scheme 1). The catalytic
activity of 2–4 is also attributable to the molybdenum cation and sulfide anion.
The dehydrogenation selectivity of the copper salt 1 was higher than that of the neutral
cluster 2 (Table 2, entries 1 and 2). It has been reported that hydrotalcite-supported copper
nanoparticle selectively catalyzes the dehydrogenation of alcohols [19]. In cluster 1, two or three
copper cations aggregate, with Cu–Cu distances of ca. 3 Å [20]. This copper aggregate was
retained during the dehydrogenation of 1-butanol; the XRD pattern of 1 was the same before and
after the reaction (Figure S1). Thus, the higher dehydrogenation selectivity of 1 is attributable to
this copper aggregate.
4. Conclusions
Solid-state molybdenum sulfide clusters, the Chevrel phases, which are not metal oxide
catalysts, catalyzed the oxidant-free dehydrogenation of alcohols. Both the Lewis-acidic
coordinatively unsaturated molybdenum atom and basic sulfur ligand synergistically acted as the
catalytic active sites. In this way, the Chevrel phases function as bifunctional acid–base catalysts.
Acknowledgments
This work was supported by a Grant-in-Aid (25410079) for Scientific Research (C) from
the Ministry of Education, Science, and Culture.
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Supplementary data
Supplementary data associated with this article can be found, in the online version.
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References
[1] R. Chevrel, M. Sergent, J. Prigent, J. Solid State Chem. 3 (1971) 515–519.
[2] Ø. Fischer, M. B. Maple, Superconductivity in Ternary Compounds I, Topics in Current
physics 32, Springer, Berlin, 1982.
[3] M. B. Maple, Ø. Fischer, Superconductivity in Ternary Compounds II, Topics in Current
physics 34, Springer, Berlin, 1982.
[4] M. Wakihara, H. Hinode, C. Inoue, Solid State Ionics 53–56 (1992) 413–417.
[5] K. F. McCarty, G. L. Schrader, Ind. Eng. Chem. Prod. Res. Dev. 23 (1984) 519–524.
[6] T. J. Paskach, G. L. Schrader, R. E. McCarley, J. Catal. 211 (2002) 285–295.
[7] S. Kamiguchi, K. Takeda, R. Kajio, K. Okumura, S. Nagashima, T. Chihara, J. Clust. Sci. 24
(2013) 559–574.
[8] S. Kamiguchi, Y. Seki, A. Satake, K. Okumura, S. Nagashima, T. Chihara, J. Clust. Sci. 26
(2015) 653–660.
[9] M. Rabiller-Baudry, M. Sergent, R. Chevrel, Mat. Res. Bull. 26 (1991) 519–526.
[10] E. Gocke, R. Schöllhorn, G. Aselmann, W. Müller-Warmuth, Inorg. Chem. 26 (1987) 1805–
1812.
[11] R. J. Behlok, W. R. Robinson, Mat. Res. Bull. 18 (1983) 1069–1072.
[12] V. Harel-Michaud, G. Pesnel-Leroux, L. Burel, R. Chevrel, C. Geantet, M. Cattenot, M.
Vrinat, J. Alloys Compd. 317–318 (2001) 195–200.
[13] S. Kamiguchi, M. Noda, Y. Miyagishi, S. Nishida, M. Kodomari, T. Chihara, J. Mol. Catal.
A 195 (2003) 159–171.
[14] R. Chevrel, M. Sergent, J. Prigent, Mater. Res. Bull. 9 (1974) 1487–1498.
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[15] A. Cimino, F. S. Stone, in: G. Ertl, H. Knözinger, J. Weitkamp (Eds.), Handbook of
Heterogeneous Catalysis, vol. 2, Wiley-VCH, Weinheim, 1997.
[16] Y. Shinohara, T. Nakajima, S. Suzuki, S. Mishima, H. Ishikawa, J. Chem. Software 4 (1998)
89–100.
[17] P. Liu, Y. Choi, Y. Yang, M. G. White, J. Phys. Chem. A 114 (2010) 3888–3895.
[18] M. Hidai, S. Kuwata, Y. Mizobe, Acc. Chem. Res. 33 (2000) 46–52.
[19] T. Mitsudome, Y. Mikami, K. Ebata, T. Mizugaki, K. Jitsukawa, K. Kaneda, Chem.
Commun. (2008) 4804–4806.
[20] K. Yvon, A. Paoli, Acta Crystallogr. B 33 (1977) 3066–3072.
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Figure captions
Fig. 1 A typical reaction profile of 1-butanol dehydrogenation catalyzed by CuxMo6S8–δ (x =
2.94, δ ≈ 0.3) (1) in a hydrogen stream at 300 °C. Conversion = products/(products +
recovered 1-butanol) × 100 (%); selectivity = product/(total amount of products) × 100(%).
Legend: conversion (●); selectivity for butyraldehyde (○); selectivity for butenes (∆), and
selectivity for butyl ether (□).
Fig. 2 Effect of temperature on activity and selectivity in dehydrogenation of 1-butanol over
CuxMo6S8–δ (x = 2.94, δ ≈ 0.3) (1) in a hydrogen stream at 3 h after the start of the reaction.
Legend: conversion (●); selectivity for butyraldehyde (○); selectivity for butenes (∆), and
selectivity for butyl ether (□).
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Graphical Abstract (for review)
Graphical abstract
O
+ H2
OH
H
H
H
O
H
Mo
S
Cu
Page 12 of 19

Highlights (for review)
Highlights:
• Solid-state molybdenum sulfide clusters catalyzed the dehydrogenation of alcohol.
• The dehydrogenation proceeded without the addition of any oxidants.
• The catalytic activity developed when the cluster was activated at 300–500 ºC in H2.
• The Lewis-acidic molybdenum atom and basic sulfur ligand were catalytically active.
• The clusters function as bifunctional acid–base catalysts.
1
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Figure
Fig. 1
100
80
60
40
20
0
0
2
4
6
8
t / h
11
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Fig. 2
100
80
60
40
20
0
60
40
20
0
200
300
400
500
Reaction temperature / ºC
12
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Scheme 1
O
+ H2
H
OH
S
Mo
S
H
H H
H
H
O
O
H
S
Mo
S
S
Mo
S
13
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Table
Table 1. Dehydrogenation of alcohols over CuxMo6S8 (1) at 300 ºCa
Alcohol
Conversion (%)
Selectivity (%)
Aldehyde
Ketone
Olefin
9.0
Ether
1.8
Othersb
1.1
Ethanol
15.7
13.2
25.7
72.8
63.0
88.1
89.3
90.5
-
-
-
1-Propanol
1-Butanol
2-Propanol
3-Pentanol
9.9
0.0
0.8
-
8.3
0.5
0.7
82.0
54.2
17.2
44.8
0.1
0.7
-
0.0
1.0
a At 3 h after the start of the reaction. b Methane, ethylene, etc.
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Table 2. Dehydrogenation of 1-butanol over various molybdenum sulfide cluster catalysts at 300 ºCa
Entry
Catalyst
Conversion (%)
Selectivity (%)
Butyraldehyde
Butenes
8.3
Butyl ether
Othersb
0.7
1
2
3
4
5
6
7
CuxMo6S8–δ (x = 2.94, δ ≈ 0.3) (1)
Mo6S8–δ (δ ≈ 0.4) (2)
Li4Mo6S8 (3)
25.7
24.0
6.3
90.5
74.9
97.4
72.1
52.6
67.2
-
0.5
0.5
0.7
4.7
2.4
0.9
-
24.1
0.7
0.5
1.2
Ni2Mo6S8 (4)
2.6
15.4
12.1
22.3
-
7.8
Mo metalc
3.7
32.9
9.6
d
MoS2
14.0
0.0
None
-
a At 3 h after the start of the reaction. b Methane, ethylene, etc. c Mo metal 80 mg. d MoS2 50 mg.
15
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Supplementary Material
Click here to download Supplementary Material: AlcoholDehydrogenation_SupplementaryData_FigS1_XRD.pdf
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