A new solid acid for specifically cleaving the CarC alk bond in di(1-naphthyl)methane

Article abstract of DOI:10.1016/j.apcata.2012.03.002

Three catalysts were prepared by impregnating the same volume of pentachloroantimony (PCA), trimethylsilyl trifluoromethanesulfonate (TMSTFMS), or isometric PCA and TMSTFMA into an activated carbon (AC). Di(1-naphthyl) methane (DNM) was used as a coal-related model compound to evaluate their catalytic activity. The results show that C_arC_alk bond in DNM can be specifically cleaved over each catalyst to afford naphthalene and 1-methylnaphthalene under pressurized hydrogen at temperatures up to 300 °C, but as a new solid acid (NSA), PCA-TMSTFMS/AC is significantly more active for DNM hydrocracking than the other two catalysts. FTIR and SEM analyses reveal the strong interactions among PCA, TMSTFMS, and the AC in the NSA. NH ₃-TPD analysis suggests that the NSA should exhibit appreciably stronger acidity than the other two catalysts. The strong interactions may result in the appreciably stronger acidity of the NSA than that of the other two catalysts and thereby facilitate DNM hydrocracking. It is presumed that H ₂ was heterolytically cleaved to immobile H^- and mobile H⁺. The addition of mobile H⁺ to ipso-position of DNM should be crucial step for DNM hydrocracking.

Full text of DOI:10.1016/j.apcata.2012.03.002

Applied Catalysis A: General 425–426 (2012) 79–84
Contents lists available at SciVerse ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
A new solid acid for specifically cleaving the Car Calk bond in
di(1-naphthyl)methane
Xiao-Ming Yuea, Xian-Yong Weia,∗, Bing Suna, Ying-Hua Wanga, Zhi-Min Zonga, Xing Fana, Zi-Wu Liua,b
a Key Laboratory of Coal Processing and Efficient Utilization (Ministry of Education) and Low Carbon Energy Institute, China University of Mining & Technology,
Xuzhou 221008, Jiangsu, China
b School of Chemistry and Chemical Engineering, South China University of Technology, Wushan 381, Guangzhou 510640, Guangdong, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Three catalysts were prepared by impregnating the same volume of pentachloroantimony (PCA),
trimethylsilyl trifluoromethanesulfonate (TMSTFMS), or isometric PCA and TMSTFMA into an activated
carbon (AC). Di(1-naphthyl)methane (DNM) was used as a coal-related model compound to evalu-
ate their catalytic activity. The results show that Car Calk bond in DNM can be specifically cleaved
over each catalyst to afford naphthalene and 1-methylnaphthalene under pressurized hydrogen at
temperatures up to 300 ◦C, but as a new solid acid (NSA), PCA–TMSTFMS/AC is significantly more
active for DNM hydrocracking than the other two catalysts. FTIR and SEM analyses reveal the strong
interactions among PCA, TMSTFMS, and the AC in the NSA. NH3-TPD analysis suggests that the NSA
should exhibit appreciably stronger acidity than the other two catalysts. The strong interactions may
result in the appreciably stronger acidity of the NSA than that of the other two catalysts and thereby
facilitate DNM hydrocracking. It is presumed that H2 was heterolytically cleaved to immobile H−
and mobile H+. The addition of mobile H+ to ipso-position of DNM should be crucial step for DNM
hydrocracking.
Received 30 August 2011
Received in revised form 27 February 2012
Accepted 1 March 2012
Available online 9 March 2012
Keywords:
Coal liquefaction
Di(1-naphthyl)methane
Hydrocracking
Specific cleavage
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
Since coal structures are still not clear, studies on the reac-
tions of coal-related model compounds have become powerful
The cleavage of bridged bonds (BBs), especially C C bridged
bonds, in coals is one of the most important reactions for direct coal
liquefaction (DCL) [1–3]. Conventional DCL is usually conducted
at temperatures higher than 450 ◦C, leading to severe increase
in the yields of both gases and heavy products [4] with huge
energy consumption in addition to significant increase in invest-
ment cost [3,5] caused by the requirement for the equipments
capable of withstanding high temperature and pressure. Prepar-
ing active catalysts for effectively cleaving BBs in coals is needed
for developing low-temperature DCL process to solve the above
problems.
Rapid improvements in the area of carbocation chemistry have
been achieved since the pioneering work of Olah, who used
pentafluoroantimony as a strong Lewis acid [6]. However, such a
strongly corrosive liquid acid erodes equipments and cannot be
effectively recycled. Solid acids have the advantages of low cor-
rosion, larger specific surface area (SSA) and good retrievability
[7].
tools to reveal the mechanisms of coal liquefaction on the molec-
ular level [8–13]. In general, thermal cleavage of Car Calk bond
is much more difficult than that of Calk Calk bond. Consequently,
selective cleavage of Car Calk bond at a lower temperature in the
presence of a proper catalyst is the key of hydrocracking in DCL and
model reactions. Wei et al. found that the metal catalysts such as
Fe, Ni, and Pd/C mainly catalyzed di(1-naphthyl)methane (DNM)
hydrogenation via biatomic hydrogen transfer, whereas their sul-
fides predominantly promoted DNM hydrocracking via monatomic
hydrogen transfer, i.e., radical hydrogen transfer (RHT) [14,15].
RHT to DNM also occurred over an activated carbon (AC) and pro-
moted by adding sulfur [16]. The hydrogenation of aromatic rings
is inevitable in addition to the release of hydrogen sulfide either
over metal sulfides or over AC/sulfur, although the total selectivity
of hydrogenated products was very low from the reaction of DNM.
Specifically cleaving Car Calk BBs in coals under mild conditions is
of great importance both for understanding coal structures (espe-
cially the types of aromatic units and BBs connecting the units) and
for facilitating the separation of reaction products.
Taking the above importance into account, we tried to prepare a
highly active catalyst for specifically cleaving Car Calk BBs in coals.
Here, we report our preliminary results using DNM as a coal-related
model compounds.
∗
Corresponding author. Tel.: +86 0516 83885951; fax: +86 0516 83885951.
E-mail address: wei xianyong@163.com (X.-Y. Wei).
0926-860X/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2012.03.002

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X.-M. Yue et al. / Applied Catalysis A: General 425–426 (2012) 79–84
2. Experimental
2.1. Materials
AC
PCA/AC
DNM was synthesized by heating naphthalene with 1-
chloromethylnaphthalene over zinc powder. Solvent cyclohexane
was commercially purchased and distillated before use. An AC,
trimethylsilyl trifluoromethanesulfonate (TMSTFMS) and pen-
tachloroantimony (PCA) were also commercially purchased. The
AC was ground to <75 ␮m and dried in a vacuum at 80 ◦C for 24 h,
then stored under a nitrogen atmosphere before use.
TMSTFMS/AC
NSA
2.2. Catalyst preparation and characterization
The AC (8 g) and the same amount (30 mL in total) of one or
two acidic species (ASs), i.e., PCA, TMSTFMS or isometric PCA and
TMSTFMS, were placed into an 80 mL CEM Discover microwave
reactor with a condenser and magnetic stirrer. After replacing air
in the reactor with nitrogen three times, the reactor was heated to
80 ◦C and kept at the temperature for 20 min. Then it was cooled to
room temperature and continuously stirred for 12 h. The catalysts,
i.e., PCA/AC, TMSTFMS/AC and PCA–TMSTFMS/AC, were obtained
by filtering the reaction mixtures through a membrane filter of
polytetrafluoroethylene with 0.8 ␮m of pore size followed by des-
iccation at 120 ◦C for 24 h under the protection of nitrogen. Among
the catalysts, PCA–TMSTFMS/AC was investigated with emphasis
as a new solid acid (NSA).
Infrared spectra of the AC and catalysts were recorded on a Nico-
let Magna IR-560 Fourier Transform infrared (FTIR) spectrometer
using KBr pellets. Specific surface area (SSA), total pore volume
(TPV) and average pore diameter (APD) of the samples were deter-
mined by nitrogen adsorption at −196 ◦C on an Autosorb-1-MP
instrument. Scanning electron microscopic (SEM) observations of
the samples and corresponding elemental analysis were performed
with a Hitachi S-3700N scanning electron microscope operating at
200 kV combined with an energy dispersive spectrometer (EDS).
The catalyst acidities were measured by temperature-programmed
desorption of ammonia (NH3-TPD) with a TP-5000 II adsorption
instrument.
4000
3500
3000
2500
2000
1500
1000
500
Wavenumbers (cm-1)
Fig. 1. FTIR spectra of the AC and catalysts.
PCA/AC suggests the reaction of PCA with the AC during the catalyst
preparation. The formation of C Cl bonds also occurred from the
reaction of polyacetylene with PCA when PCA was used as a dopant
[17]. Blue shift of the absorbance for some of C Cl bonds from 769
[17] to 799 cm−1 [18] can be found by comparing FTIR spectrum
of PCA/AC with that of the NSA. Many characteristic absorbances
of CF2 SO2 O (asymmetric) [19] and Si O [20] at 1254 cm−1
(CH3)3Si O [21] at 1173 cm−1, O SO2 O− [22] and F3C SO2
[23] at 1042 cm−1, C F [24] and SO3 (symmetric) [25] at 644 cm−1
,
O
,
CF3 (asymmetric) [25] at 580 cm−1 and SO3 (asymmetric) [25]
at 519 cm−1 from TMSTFMS were confirmed in the NSA in addition
to the absorbance of C Cl bonds at 769 and 799 cm−1, as shown
in Fig. 1 and Table 1. However, based on the absorbance inten-
sity (AI) at 1254 cm−1, the AI at 1173 cm−1 in the NSA is much
weaker than that in TMSTFMS/AC, whereas the AIs at 1042, 644,
580 and 519 cm−1 in the NSA are significantly stronger than those in
TMSTFMS/AC. Neither of the characteristic absorbances except for
the absorbance at 799 cm−1 appeared in PCA/AC. The above results
evidently indicate that there exist strong interactions among PCA,
TMSTFMS and the AC in the NSA, although the detailed interaction
mechanisms need to be further investigated.
2.3. DNM hydrocracking and reaction mixture analysis
DNM (1 mmol), a catalyst (0–0.4 g) and cyclohexane (30 mL)
were put into a 60 mL stainless, magnetically stirred autoclave.
After replacing air in the autoclave with hydrogen, the autoclave
was pressurized with hydrogen to a desired pressure, i.e., initial
hydrogen pressure (IHP between 1 and 5 MPa), at room tempera-
ture (20 ◦C) and heated to an indicated temperature (150–300 ◦C)
in 15 min. After reaction at the temperature for a prescribed period
of time (1–10 h), the autoclave was immediately cooled to room
temperature in an ice-water bath. The products and unreacted
DNM in the reaction mixture taken out from the autoclave were
quantified with a Hewlett-Packard 6890 gas chromatography and
identified with a Hewlett-Packard 6890/5973 gas chromatogra-
phy/mass spectrometry.
As Fig. 2 displays, the surface of AC itself is smooth compared
to the rough surface of three other samples. A number of spher-
ical particles (SPs) with diameter less than 1 ␮m adhered to the
AC surface can be clearly observed only in PCA/AC, while there are
irregular grains (IRGs) with diameter less than 5 ␮m adhered to the
AC surface in TMSTFMS/AC and the NSA. According to EDS analysis
demonstrated in Fig. 3, the following elements were confirmed: (1)
C and O from the AC, C, O, Sb and Cl in the SPs from PCA/AC; (2) C, O,
F, Si and S in the IRGs from TMSTFMS/AC; (3) all the above elements
in IRGs from the NSA. However, O/C ratio in the SPs from PCA/AC
is far higher than that in the AC, although no O contains in PCA.
The higher O/C ratio in the SPs from PCA/AC could be related to the
reaction of PCA with oxygen-containing species in the AC and/or
O2 in air during the catalyst preparation. These observations fur-
ther confirmed the strong interactions among PCA, TMSTFMS and
the AC in the NSA.
3. Results and discussion
As Fig. 1 shows, the absorbances of OH at 3416 cm−1 and >C=C<
moiety at 1620 cm−1 in the AC, PCA/AC and TMSTFMS/AC can be
clearly observed but almost disappeared in the NSA, while the
absorbances of CH3 and >C C< moiety at 2925 and 2857 cm−1
observed in the AC almost disappeared in PCA/AC and completely
vanished in TMSTFMS/AC and the NSA, although both TMSTFMS/AC
and the NSA were prepared by impregnating TMSTFMS, which con-
tains CH3 groups. The appearance of C Cl bonds at 769 cm−1 in
Data listed in Table 2 exhibit that the impregnation of either
PCA or TMSTFMS or both the ASs significantly decreased SSA and
TPV of the resulting catalysts and remarkably decreased APD of the
resulting PCA/AC but appreciably increased APD of the resulting
TMSTFMS and NSA. Especially, the decrease in SSA for the NSA is

X.-M. Yue et al. / Applied Catalysis A: General 425–426 (2012) 79–84
81
Table 1
Comparison of intensity relative to the absorbance at 1254 cm−1
.
WN (cm−1
)
Intensity relative to WN1254
Assignment
NSA
TMSTFMS/AC
1254
1173
1042
799
644
580
1.0000
0.1684
0.3364
0.0598
0.2624
0.0193
0.0602
1.0000
0.2408
0.2272
0
0.2243
0.0147
0.0436
CF2 SO2 O (asymmetric) [19], Si O [20]
(CH3)3Si O [21]
O
C
C
SO2 O− [22], F3C SO2 O [23]
Cl [18]
F [24], SO3 (symmetric) [25]
CF3 (asymmetric) [25]
SO3 (asymmetric) [25]
519
WN – wavenumber; intensity relative to WN1254 – peak area at an indicated wavenumber/peak area at 1254 cm−1
.
Fig. 2. SEM micrographs of the AC and catalysts.
the most significant. The results imply that significant amount of
an AS or two ASs was impregnated into micropores in the AC and
TMSTFMS expanded the micropores to some extent, especially by
synergic interaction with PCA.
NH3-TPD is well used for measuring the surface acidity of porous
samples and more NH3 desorption at higher temperatures indi-
cate stronger acidity of the samples [26]. As Fig. 4 illustrates, there
are sharp desorption peak (SDP) in lower temperature region (LTR,
<300 ◦C) and broad desorption peak in higher temperature region
(HTR, 550–685 ◦C) for PCA/AC and the NSA, but only SDP can be
observed for TMSTFMS/AC. The temperatures corresponding to
maximum intensity in LTR for PCA/AC, TMSTFMS/AC and the NSA
are 87.3, 185.2 and 216.1 ◦C, respectively, and those in HTR for
PCA/AC and the NSA are 601.1 and 606.3 ◦C, respectively. In addi-
tion, the intensities of desorption peaks both in LTR and in HTR from
the NSA are significantly stronger than those from PCA/AC. These
results indicate that the acidity of NSA is substantially stronger than
that of either PCA/AC or TMSTFMS/AC. The enhanced acidity should
be closely related to the strong interactions among PCA, TMSTFMS
and the AC in the NSA, which were characterized by FTIR, SEM
and EDS analyses along with the surface property measurements
mentioned above.
The results illustrated in Fig. 5 show that the activity of NSA
for DNM hydrocracking is appreciably higher than that of PCA/AC
and much higher than that of TMSTFMS/AC under the same
reaction conditions. Dramatic increases in DNM conversion and
the product yield can be observed over any catalyst. Noteworthily,
all the catalysts specifically cleaved the BB in DNM, only affording
naphthalene and 1-metyhylnaphthalene (1-MN). The yield of
naphthalene is appreciably higher than that of 1-MN over any
catalysts and the difference in yield between the two products
increased with raising the reaction temperature, indicating that
the 1-MN demethylation proceeded and became significant at
Table 2
Surface properties of the AC and catalysts.
Sample
SSA (m2 g−1
)
TPV (cm3 g−1
)
APD (nm)
AC
743.0
637.7
526.3
487.6
0.53
0.46
0.26
0.38
2.85
2.91
2.66
3.12
TMSTFMS/AC
PCA/AC
NSA

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X.-M. Yue et al. / Applied Catalysis A: General 425–426 (2012) 79–84
C
C
C
30
30
25
20
15
10
5
TMSTFMS/AC
25
20
O
O
AC
Cl
15
Sb
10
5
Sb
Cl
Sb
PTA/AC
0
0
50
50
PCA/AC
S
40
40
30
20
10
Si
F
O
TMSTFMS/AC
30
Cl
20
Sb
C
S
Sb
Sb
O
Si
10
F
Cl
NSA
0
2
4
6
8
0
0
Energy (keV)
60
60
NSA
Fig. 3. EDS spectra of the AC and catalysts.
50
40
30
20
50
40
30
20
higher temperatures. Dramatic increase both in DNM conversion
and in the product yields above 250 ◦C also deserves attention.
DNM conversion, the product yields and the difference in
yield between naphthalene and 1-MN basically increased with the
increases in the NSA feed (Fig. 6), reaction time (Fig. 7) and IHP
(Fig. 8), but the increases with reaction time at 250 ◦C are insignif-
icant (Fig. 7). There was almost no increase in 1-MN yield with the
increase in IHP at 300 ◦C (Fig. 8). These facts suggest that the NSA
10
0
10
0
87.3
140
180
220
260
300
Temperature (oC)
185.2
601.1
PCA/AC
Fig. 5. DNM conversions and product yields over different catalysts at different
temperatures (catalyst 0.4 g, IHP 5 MPa, 3 h).
activation at temperatures higher than 250 ◦C is needed and the
increase in IHP facilitates 1-MN demethylation to naphthalene.
Thermally cleaving the Car Calk bonds is very difficult because
of the extreme lability of the resulting aryl radical. Hydrogen trans-
fer either from H• or from H+ is crucial for the Car Calk bond
cleavage [27–31], because the addition of H• or H+ to the ipso-
position (IP) of an aryl group with a methylene linkage affords a
stable aromatic molecule rather than a labile aryl radical. As men-
tioned above, synergic interaction of the electrophilic reagents PCA
and TMSTFMS, which were impregnated into the AC in the NSA,
substantially increased the NSA acidity. Strong acidity of the NSA
facilitates heterolytical splitting of H2 to an immobile H− adhered
to the NSA surface and a mobile H+, as displayed in Scheme 1. The
addition of mobile H+ to the IP of a naphthalene ring in DNM leads
to specific cleavage of the BB in DNM, resulting in naphthalene and
naphthalen-1-ylmethylium (NM). The resulting NM can abstract an
216.1
TMSTFMS/AC
606.3
NSA
0
200
400
600
800
Temperature (oC)
Fig. 4. NH3-TPD profiles of the catalysts.

X.-M. Yue et al. / Applied Catalysis A: General 425–426 (2012) 79–84
83
Scheme 1. Possible mechanism for the NSA-catalyzed hydrogen transfer to DNM.
60
60
55
50
45
40
35
100
100
80
60
40
20
0
55
80
50
60
45
40
20
0
40
35
0
0.1
0.2
0.3
0.4
0.5
0.6
0
1
2
3
4
5
6
The NSA used (g)
IHP (MPa)
Fig. 8. Effect of IHP on DNM conversion and product yield (NSA 0.4 g, 300 ◦C, 3 h).
Fig. 6. Effect of the NSA feed on DNM conversion and product yield (IHP 5 MPa,
300 ◦C, 3 h).
4. Conclusions
immobile H− from the NSA surface to afford 1-MN. The demethy-
lation of 1-MN also proceeds via the addition of mobile H+ to the
IP of the naphthalene ring in 1-MN and subsequent abstraction of
an immobile H− by the resulting +CH3 from the NSA surface, but
the 1-MN demethylation is not easy compared to the BB cleavage
in DNM due to less stability of +CH3 than NM.
The strong interactions among PCA, TMSTFMS and the AC in the
NSA were confirmed by FTIR, SEM and EDS analyses along with the
surface property measurements. With stronger acidity than PCA/AC
and TMSTFMS/AC, the NSA exhibited substantially higher activity
than PCA/AC and even much higher activity than TMSTFMS/AC for
specifically cleaving BB in DNM by the strong interactions. The high
activity of NSA for the specific cleavage of BB in DNM could be
ascribed to the catalysis of NSA in effective producing mobile H+
and immobile H−.
80
70
60
50
40
30
20
10
0
80
70
60
50
40
30
20
10
0
300 oC
Acknowledgments
This work was subsidized by the Special Fund for Major
State Basic Research Project (Grant 2011CB201302), the Fund
from National Natural Science Foundation of China for Innovative
Research Group (Grant 50921002), the Key Project of Natural Sci-
ence Foundation of China (Grant 20936007), the Key Project of Coal
Joint Fund from National Natural Science Foundation of China and
Shenhua Group Corporation Limited (Grant 51134021), National
Natural Science Foundation of China (Grant 51074153), the Pro-
gram of the Universities in Jiangsu Province for Development of
High-Tech Industries (Grant JHB05-33), and the Priority Academic
Program Development of Jiangsu Higher Education Institutions.
250 oC
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