Deep hydrogenation of coal tar over a Ni/ZSM-5 catalyst

Article abstract of DOI:10.1039/c3ra47701k

We have developed a Ni/ZSM-5 catalyst and utilised it to completely hydrogenate a series of condensed arenes, including naphthalene, anthracene and phenanthrene, under relatively mild conditions. The yields of decalin, perhydroanthracene and perhydrophenanthrene reached 100%, 98.8% and 25.8%, respectively. By analyzing the isomer distribution of the perhydroarenes, we proposed a mechanism of biatomic hydrogen transfer, which was further proven via the hydrogenation of 9,10-diphenylanthracene. Through hydrotreatment over the Ni/ZSM-5 catalyst, both high-temperature coal tar rich in condensed arenes and low-temperature coal tar mixing arenes with alkanes were greatly upgraded. The majority of arenes in the coal tars were deeply and even completely hydrogenated, which may open up a route for further processing and application of coal tar as clean fuels.

Full text of DOI:10.1039/c3ra47701k

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Deep hydrogenation of coal tar over a Ni/ZSM-5
catalyst†
Cite this: RSC Adv., 2014, 4, 17105
a
a
a
b
a
Shi-Chao Qi, Lu Zhang, Xian-Yong Wei,* Jun-ichiro Hayashi, Zhi-Min Zong
a
and Lu-Lu Guo
Received 17th December 2013
Accepted 14th February 2014
DOI: 10.1039/c3ra47701k
www.rsc.org/advances
16–18
We have developed a Ni/ZSM-5 catalyst and utilised it to completely synthesis, oil processing and coal hydroliquefaction.
Coal
hydrogenate a series of condensed arenes, including naphthalene, tar, the viscous liquid from coal carbonization, can be classied
anthracene and phenanthrene, under relatively mild conditions. The into high-temperature coal tar (HTCT) and low-temperature
yields of decalin, perhydroanthracene and perhydrophenanthrene coal tar (LTCT). The annual output of coal tar has been higher
reached 100%, 98.8% and 25.8%, respectively. By analyzing the isomer than 20 million tons around the world. At present, using coal tar
distribution of the perhydroarenes, we proposed a mechanism of as a fuel faces a huge challenge because of the presence of CAs,
19
biatomic hydrogen transfer, which was further proven via the hydro- especially in HTCT. On the other hand, it is so difficult to
genation of 9,10-diphenylanthracene. Through hydrotreatment over upgrade coal tar that there are rarely ideal deep hydrogenation
the Ni/ZSM-5 catalyst, both high-temperature coal tar rich in results, even though the hydrogenation of coal tar has been
20
condensed arenes and low-temperature coal tar mixing arenes with extensively investigated. Kusy et al. used Co–Mo/Al O3 to
2
alkanes were greatly upgraded. The majority of arenes in the coal tars hydrogenate a coal tar, but there were only few saturates in the
were deeply and even completely hydrogenated, which may open up a product. It can be predicted that hydrogenated HTCT (HHTCT)
route for further processing and application of coal tar as clean fuels. and hydrogenated LTCT (HLTCT) can be used as clean fuels
21,22
once they are successfully saturated.
In previous work, we developed highly active Ni/ZSM-5.
23
The risk of petroleum shortage has stimulated the development
Here, the Ni/ZSM-5 is applied to catalyze the hydrogenation of
CAs (i.e. naphthalene, anthracene, phenanthrene and 9,10-
diphenylanthracene (DPA)), HTCT and LTCT. All the CAs are
deeply hydrogenated and both HTCT and LTCT are greatly
upgraded; moreover, biatomic hydrogen transfer (BAHT) is put
forward as the pathway for CA hydrogenation over nickel.
The Ni/ZSM-5 was prepared according to the method repor-
1
–3
of coal to oil technologies, but the high concentration of
condensed arenes (CAs) in the liquids resulting from these
4–7
technologies limits their application as clean fuels. Deep
hydrogenation of CAs has thus been a major technical
4,8,9
subject.
However, only few reports were issued on the cata-
lytic hydrogenation of CAs to cyclanes in high yields under
8–10
relatively mild conditions.
For instance, less than 10% of
23
ted. Cyclohexane (20 mL), Ni/ZSM-5 (0.5 g) and a substrate
phenanthrene was hydrogenated to perhydrophenanthrene
(CA, 5 mmol, or the soluble fraction of coal tar in cyclohexane,
ꢀ
11
12
(
PHP) at 250 C over a Pt–Pd/g-Al O catalyst. Tang et al.
2 3
0.5 g) were fed into an autoclave. Aer replacing air in the
completely converted naphthalene over a Pd/Al-MCM-41 cata-
lyst, with the main product being tetralin rather than decalin. In
addition, quite a few studies on the kinetics of arene hydroge-
autoclave and pressurizing with hydrogen to 5.0 MPa at room
temperature, the autoclave was heated to an indicated temper-
ature within 20 min and maintained at this temperature for a
prescribed period of time with rapid agitation (250 rpm). The
total pressure inside the autoclave was maintained at higher
13–15
nation over metallic catalysts have been reported
and there
are only few articles about the specic pathways for hydrogen
transfer to aromatic rings, which is a crucial subject for organic
than 6.0 MPa via timely H
tion. The reaction mixture was taken out of the autoclave and
ltered aer cooling the autoclave in a water bath. The ltrate
2
supplementation during the reac-

a
Key Laboratory of Coal Processing and Efficient Utilization, Ministry of Education,
was analyzed with a Hewlett-Packard 6890/5973 GC/MS.
Naphthalene hydrogenation over Ni/ZSM-5 at 180
Division of Advanced Device Material, Institute for Materials Chemistry and produced tetralin and decalin (Fig. 1, le). The highest yield of
China University of Mining & Technology, Xuzhou 221116, P. R. China. E-mail:
ꢀ
wei_xianyong@163.com
b
C
Engineering, Kyushu University, 6-1, Kasuga Koen, Kasuga, Fukuoka 816-8580, Japan
tetralin was obtained aer 10 h, then the molar fraction of
tetralin decreases rapidly. Correspondingly, a rapid increase in
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c3ra47701k
1
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the molar fraction of decalin is also observed. Deep hydroge- the cis-isomers share the highest yields. The isomer distribution
nation of naphthalene over Ni/ZSM-5 produced trans- and cis- of PHPs is similar to yields. Moreover, the yields of (4aR,8a-
decalin simultaneously (Fig. 1, right). The concentration of cis- S,9aS,10aR)-PHA and PHPs are higher than that of (4aR,8a-
decalin is higher than that of trans-decalin throughout because R,9aS,10aS)-PHA. These facts imply that the intermediates with
8,24
the BAHT over the metallic catalyst is favorable.
The trans- smaller steric hindrance are more easily adsorbed on the nickel
isomer is generally more thermodynamically stable than the cis- surface for the subsequent hydrogenation step than those with
isomer and the molar ratio of trans-/cis-isomer rises slowly from bigger steric hindrance and that the predominant hydrogena-
0.33 to 0.87 with reaction time, indicating the conguration tion pathway of arenes on the nickel surface is BAHT.
inversion from cis-decalin to trans-decalin. Monatomic hydrogen transfer to CAs favors the generation
Perhydroanthracenes (PHAs) and perhydrophenanthrenes of thermodynamically stable trans-isomers, whereas Ni/ZSM-5,
ꢀ
(
2
PHPs) were obtained in high yields over Ni/ZSM-5 at 160 C for as a metallic catalyst, mainly catalyzes BAHT to aromatic
25,26
4 h. As shown in Table 1, PHAs almost makes up the whole rings.
H adsorbed on the Ni surface is parallelly added to
2
composition of the products. Deep hydrogenation of phenan- C]C and saturates it, which favors the production of cis-
1
–4
threne is difficult due to its steric conguration. However, the isomers. During naphthalene hydrogenation, trans-decalin can
yield of PHP also reached 25.8 mol%, and the total yield of be produced via BAHT to the 1- and 8a-positions from one side
octahydrophenanthrenes (OHPs) reached 62.9 mol%. As shown of the molecular plane and subsequent BAHT to the 4- and 4a-
in Fig. S1 to S7,† all hydroanthracenes and hydrophenanthrenes positions from another side (Scheme 1, Path III). Cis-decalin
are conrmed via GC/MS analysis. According to the isomer would be generated if 4- and 4a-positions were hydrogenated via
distribution of the PHAs (Table 2), four congurational isomers BAHT from the same side as 8a-H (Scheme 1, Path II). Clearly,
manifest the diverse pathways of complete hydrogenation and sym-octalin can be hydrogenated only to cis-decalin via BAHT
(
Scheme 1, Path I). Moreover, because of the reversible
8,9
processes of arene hydrogenation, the cis-isomer can trans-
form into the trans-isomer via the inverse process of Path II
cooperating with Path III, which explains the increase in the
molar ratio of trans-/cis-decalin with prolonging the reaction
(Fig. 1, right).
In order to certify BAHT by an easy and effective method,
ꢀ
DPA was partially hydrogenated over Ni/ZSM-5 at 150 C for 5 h.
As shown in Fig. 2, DPA was completely converted but not fully
saturated due to the controlled reaction conditions. All the
products from DPA hydrogenation contain benzene ring in the
27
center. According to the theory of superdelocalizability, the 9-
and 10-positions on anthracene ring are most readily hydroge-
nated. However, as shown in Fig. 3, the bond length between the
Fig. 1 Time profiles of molar fraction for naphthalene hydrogenation
9- and 10-positions of gaseous anthracene is ca. 300 pm and the
(
left) and the molar fraction changes of total, cis- and trans-decalins
lattice spacing of Ni (111), which is the main catalytic lattice
ꢀ
from catalytic hydrogenation of naphthalene over Ni/ZSM-5 at 180 C
2
8–30
plane of Ni for arene hydrogenation, is only ca. 250 pm.
According to the multiplet theory of catalysis,
(
right).
31,32
it is impos-
sible for the Ni adsorbing and cracking H to be added to the
2
two carbon atoms at the 9- and 10-positions. Such impossibility
results in the difficulty in hydrogenating the central benzene
ring in DPA via BAHT.
Table 1 The product yields (mol%) from anthracene and phenan-
threne hydrogenation over Ni/ZSM-5
a
b
Substrate
USU
UUS
USS
SUS
SSS
As demonstrated in Figs 4 and 5, the dark brown liquids
HTCT and LTCT were hydrogenated to the colorless and
transparent liquids HHTCT and HLTCT, respectively, over fresh
Anthracene
Phenanthrene
0.0
0.7
0.0
9.4
0.0
15.0
1.2
47.9
98.8
25.8
ꢀ
Ni/ZSM-5 three times at 160 C (each for 15 h). This implies that
a
b
U denotes unsaturated ring. S denotes saturated ring.
Table 2 Yields (mol%) of different PHAs and PHPs
PHA
PHP
3
1.5
16.2
6.2
44.9
1.2
16.5
4.2
3.3
0.6
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Scheme 1 The formation of cis- and trans-decalins via BAHT to naphthalene over Ni/ZSM-5.
most of the unsaturated species in both HTCT and LTCT were cyclanes, such as decalin (20.896 wt%), hexadecahydro-
hydrogenated. As shown in Fig. S8 and Table S1,† HTCT is rich uoranthene (8.143 wt%) and dodecahydrouorene (5.288 wt
in a variety of CAs, such as naphthalene (25.484 wt%), phen- %), and a few CAs were deeply hydrogenated to hydroarenes
anthrene (12.850 wt%), uoranthene (7.019 wt%) and uorene (HAs) containing single benzene ring, such as decahydropyrene
(6.266 wt%). Furthermore, there are also a few heteroatomic (3.203 wt%) and dodecahydrochrysene (1.933 wt%). Hydroge-
compounds (HACs) detected in HTCT, such as dibenzofuran nating the remaining benzene ring is extremely difficult due to
(
(
3.088 wt%), benzothiophene (1.057 wt%) and quinoline the severe steric hindrance caused by the cyclane ring on the
8,9,23
0.548 wt%), which would passivate and poison the catalyst. The benzene ring,
which is in agreement with the results of
catalyst was therefore renewed during the hydrogenation phenanthrene hydrogenation (Table 1). Moreover, compared
process. Neglecting the impact on the deactivation of the cata- with HTCT, there are no obvious HACs detected in HHTCT
lyst by the HACs HTCT is still difficult to be hydrogenated because heteroatoms could be deprived from HTCT via reaction
38–40
because the blended CAs in HTCT severely restrain mutual with metallic species.
Indeed, the nickel sites located
reactivity toward hydrogenation owing to the competitive on ZSM-5, which are poisoned by adsorbed sulde, may be
3
3–37
adsorption on the catalyst used.
However, as shown in partially recovered by spillover hydrogen from the acidic
41,42
Fig. S9 and Table S2,† most of the arenes were converted to support.
ꢀ
Fig. 2 Total chromatogram of partially hydrogenated DPA (150 C, 5 h) and mass spectra of the products.
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Table 3 Elemental analysis of the samples
Content (wt%)
Sample
C
H
O
N
S
HTCT
HHTCT
LTCT
93.14
87.83
87.68
85.77
5.81
0.44
0
1.25
0.22
0.15
0.46
12.17
11.07
14.00
0
0
0
0
0
0
HLTCT
Fig. 3 Lengths of different bonds in gaseous anthracene.
alkanes which make up most of LTCT with small amounts of
alkenes, arenes and HACs. As exhibited in Fig. S11 and Table
S4,† all the alkenes and most of the arenes were saturated by Ni/
ZSM-5 catalyzed hydrogenation and only very small amounts of
HAs and HACs appear in HHLTCT. These facts indicate that the
Ni/ZSM-5 also plays an important role in upgrading LTCT. As
displayed in Table 3, carbon contents in both HTCT and LTCT
was obviously reduced by the catalytic hydrogenation and the
ratio of C/H of HTCT dropped more obviously than that of LTCT
because more arenes are saturated in HTCT than in LTCT.
Meanwhile, the heteroatoms in HTCT and N and S in LTCT were
completely removed and oxygen content in LTCT was signi-
cantly reduced by the catalytic hydrogenation. These facts are in
agreement with the results of GC/MS analysis (Fig. S8 to S11†).
In summary, the Ni/ZSM-5 we developed displayed a very
high activity for the hydrogenation of CAs, including naphtha-
lene, anthracene and phenanthrene. The CAs were completely
converted to afford high yields of cyclanes under relatively mild
conditions. On the nickel surface, CAs were hydrogenated via a
BAHT mechanism, which favors the generation of cis-cyclanes,
which was further proved via the hydrogenation of DPA. HTCT
Fig. 4 Images of HTCT and HHTCT.
As shown in Fig. S10 and Table S3,† LTCT contains a series of and LTCT were well upgraded over the catalyst and the majority
CAs like HTCT, but their molecular weights are apparently of the CAs in both HTCT and LTCT were completely saturated to
smaller than those in HTCT. In addition, there are a series of cyclanes, which may open up a route for further processing and
application of coal tar as clean fuels.
Acknowledgements
This work was supported by National Basic Research Program of
China (Grant 2011CB201302), National Natural Science Foun-
dation of China (Grants 20936007, 51221462 and 51074153), the
Key Project of Coal Joint Fund from National Natural Science
Foundation of China and Shenhua Group Corporation Limited
(Grant 51134021), the Strategic Chinese-Japanese Joint
Research Program (Grant 2013DFG60060), the Fundamental
Research Fund for the Doctoral Program of Higher Education
(Grant 20120095110006) and the Priority Academic Program
Development of Jiangsu Higher Education Institutions.
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