The hydrogenation of aromatic-naphthalene with Ni2P/CNTs

Article abstract of DOI:10.1039/c5ra05364a

Ni₂P/CNTs was synthesized using an impregnation method. XPS revealed that CNTs could affect the electronic properties of bulk Ni₂P. The catalyst shows superior activity for HYD of naphthalene with a conversion of 99%, and demonstrates superior tolerance towards potential catalyst poisons, which is higher than Ni/CNTs with a conversion of 89%.

Full text of DOI:10.1039/c5ra05364a

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The hydrogenation of aromatic-naphthalene with
Ni2P/CNTs
Cite this: RSC Adv., 2015, 5, 57700
a
a
a
c
ab
*
Minzhi Ruan, Jun Guan, Demin He, Tao Meng and Qiumin Zhang
Received 26th March 2015
Accepted 16th June 2015
DOI: 10.1039/c5ra05364a
www.rsc.org/advances
Ni2P/CNTs was synthesized using an impregnation method. XPS carriers which have excellent performances.5,6 However, carbon
revealed that CNTs could affect the electronic properties of bulk Ni2P. nanotubes (CNTs) with unique properties have experienced
The catalyst shows superior activity for HYD of naphthalene with a limited use as a support material for Ni2P, and the potential
conversion of 99%, and demonstrates superior tolerance towards industrial application value of Ni2P/CNTs in aromatic HYD has
potential catalyst poisons, which is higher than Ni/CNTs with a not been sufficiently explored. Herein, we have thus applied
conversion of 89%.
Ni2P catalysts supported on carbon nanotubes to examine the
effect of HYD. CNTs was reported to have huge specic areas,
therefore they have a better hydrogen storage function and
strictly mesoporous nature, differing greatly from the classical
activated carbon support.7–13 The CNTs was functionalized
using an acid mixture which led to the unique CNT properties,
and the Ni2P/CNTs particles were synthesized using an
impregnation method.14
X-ray diffraction patterns of reduced Ni2P/CNTs and Ni2P
samples are presented in Fig. 1a. The Ni2P loadings were varied
from 5 to 40 wt%, and the Ni/P mole ratios were varied from 0.5
to 2.0. With the increase of Ni2P loadings and Ni/P mole ratios,
the sharp and symmetric peaks evidently indicate that the
samples were well crystallized. The patterns all display a broad
feature at 20–30ꢀ and 48.8ꢀ due to the carbon nanotube
supports. For all of the catalysts, the peaks at 2q ¼ 40.6ꢀ, 44.5ꢀ,
47.1ꢀ, 54.1ꢀ, 54.8ꢀ (PDF: 74-1385) can be ascribed to Ni2P, and
no additional phase of Ni and P is observed, indicating that the
active phase formed is mainly Ni2P. The 30 wt% Ni2P/CNTs
sample (Fig. 1b) was substantiated by energy dispersive spec-
troscopy (EDS) in Fig. 1c.
The IR spectrum of the acid-treated CNTs and Ni2P/CNTs
catalysts is shown in Fig. 1d. The bands appearing at 1000
cmꢁ1 can be assigned to the Ni2P, and the bands appearing at
1635 and 3450 cmꢁ1 can be assigned to the C]O and O–H
stretching vibration bands, which conrm the presence of
functional groups in the acid-treated CNTs, and could help
adsorb active metal on the surface.15
Table 1 shows the physical properties of the Ni2P/CNTs
catalysts, including the Ni2P loading content, average Ni2P
size and BET surface area. The results showed that the BET
surface area of the catalysts declined with the increase in the
amount of Ni2P loading. This indicated that the Ni2P particles
Due to the increasingly low quality of petroleum feedstocks and
stringent environmental regulations, it is of paramount
importance to develop new environmentally friendly catalysts
for hydrotreatment. The hydrogenation (HYD) and hydro-
desulfurization (HDS) reactions, which transfer polycyclic
aromatic hydrocarbons with sulfur into high RON oil, underpin
many clean-energy technologies. Although Pt or Pd have been
used in hydrotreatment catalysts, they have poor performances
when in contact with sulfur.1 Furthermore Pt and Pd are luxu-
rious and relatively infrequent in the Earth’s shell limiting the
use of them in energy systems deployed on a global scale. More
recently, nanoparticulate lms of Ni2P composed of inexpensive
materials to show high HDS activity. As the nickel phosphorus
catalyst was rstly introduced by Oyama, it has aroused lots of
interest all around the world as it has been used in energy
applications with high performances of HDS and hydro-
denitrogenation (HDN) activity and even has outstanding
performances in fuel cells. This indicated that the sequence of
HYD activity over transition metal phosphides is as follows:
Fe2P < CoP < MoP < WP < Ni2P, where the Ni2P catalyst was
found to be the most active.2–4 Among such classes of catalysts,
various carbon nanostructures, including carbon, carbon
nanotubes, and graphene and so on, have been used for catalyst
aSchool of Chemical Engineering, Dalian University of Technology (DUT), No. 2
Linggong Road, Dalian 116024, P. R. China. E-mail: zhangqm@dlut.edu.cn; Fax:
+86 0411 84986150; Tel: +86 0411 84986150
bState Key Laboratory of Fine Chemicals, Dalian University of Technology (DUT),
China
cBeijing Guodian Longyuan Environmental Engineering Co., LTD, Building No. 1, Yard
No. 16, Xisihuan Mid Road, Haidian District, Beijing 100039, PR China
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Fig. 1 (a) XRD patterns of catalyst samples with different wt% of Ni2P
(Ni/P ¼ 1.25, atomic) and Ni/P proportion (30 wt% Ni2P); (b) SEM image
of 30 wt% Ni2P/CNTs; (c) EDS of 30 wt% Ni2P/CNTs; (d) FTIR of catalyst
samples.
Fig. 2 TEM images and size distribution histograms: (a) 10 wt% Ni2P/
CNTs, (b) 20 wt% Ni2P/CNTs, (c) 30 wt% Ni2P/CNTs and (d) 40 wt%
Ni2P/CNTs.
occupied the surface holes of the CNTs. TEM observations
indicate that the size and the morphology of the Ni2P/CNTs
composites differ from those of the CNTs. Most of the Ni2P
particles are displayed outside of the CNTs, the loading content
could affect the Ni particle size and its distribution.15 In addi-
tion, as the histograms in Fig. 2 show, the mean particle size
increases from 6.7 nm to 20.0 nm with the increasing loading
content. In the following section, it was found that a smaller
Ni2P particle size with a higher dispersion contributes to the
inhibition of the agglomeration of Ni2P particles, which should
favour the hydrogenation reaction.
The XPS Ni 2p and P 2p spectra of 30 wt% Ni2P/CNTs cata-
lysts are presented in Fig. 3a and b. For samples (1)–(3), the Ni
2p core level spectrum includes two contributions (Fig. 3a and
b), which correspond to Ni 2p3/2 and Ni 2p1/2, respectively. The
former one, is assigned to Nid+ in the Ni 2p phase and is
centered at 853.31 eV, and the later one at 857.9 eV is assigned
to oxidized Ni. With regards to the P 2p3/2 binding energy, the
peaks of the P 2p electron binding energy at 129.87 eV and
133.98 eV can be assigned to oxidized and elemental P,
respectively.
spectrum for the 30 wt% Ni2P/CNTs catalyst commonly mirrors
that of the Ni2P catalyst. The fresh Ni2P/CNTs sample displays
two distinct peaks, whose positions correspond to the Ni–P and
Ni–Ni distances in bulk Ni2P. The P (2p) region shows one
signicant difference: the XPS spectrum of the Ni2P catalyst has
a very intense peak at 132.04 eV but there are two peaks in the
XPS spectrum of Ni2P/CNTs that are consistent with the binding
energy for phosphorus in the CNTs. Some of the phosphorus
impregnated onto the CNTs apparently reacts with the support
to form Ni–P-CNTs. The spectrum of fresh Ni2P/CNTs differs
from the Ni2P sample, indicating that the Ni+ peak shied from
855.4 to 853.31 eV (Fig. 3a), and the binding energy assigned to
the P5+ species shied from 131.96 to 129.87 eV (Fig. 3b). The
shi of binding energies of the surface Ni+ and P5+ species
suggests the existence of an electronic effect caused by the
surface –OH or –COOH, or CNTs species, which may result in
the electron enrichment of the surface Ni and P species. As it is
revealed in Fig. 3c and d, the Ni atom is placed on an atop site
instead of at a hole site, there are two stable binding sites for a
Ni2 dimer on a carbon nanotube wall: an atop–atop site and a
bridge–bridge site. The Ni2 dimer when placed on a nanotube
The peaks in the XPS spectrum for the Ni2P catalyst have
been assigned previously.16,17 For Ni2P, the binding energies at
857.9 and 134.8 eV are assigned to Ni2+ and P5+ species. The XPS
Table 1 BET results and pore structure parameters of Ni2P/CNTs catalysts
Samples
Surface areaa (m2 gꢁ1
)
Pore volumea (cm3 gꢁ1
)
dXRDb (nm)
dTEMc (nm)
10 wt%
20 wt%
30 wt%
40 wt%
171
151
143
94
0.33
0.31
0.26
0.23
13.6
14.6
21.9
28.0
6.7
11.0
13.1
20.0
a
Evaluated from N2 adsorption–desorption isotherms. b Calculated using the Scherrer equation from the XRD. c Measured using TEM.
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Table 2 Catalytic hydrogenation of naphthalene and 1-methyl naphthalenea
Samples
Raw materials
Naphthalene
Products
Selectivity [%]
30 wt% Ni2P/CNTs
cis-Decalin
trans-Decalin
Tetralin
1-Methyl tetralin
5-Methyl tetralin
1-Methyl decalin
cis-Decalin
14
69
17
17
36
47
0
30 wt% Ni2P/CNTs
20 wt% Ni/CNTs
1-Methyl naphthalene
Naphthalene
trans-Decalin
Tetralin
28
61
a
Reaction conditions: reactions were carried out with naphthalene or 1-methyl naphthalene (5 wt% in decane), at 613 K, 4.0 MPa, vapour–liquid
ratio ¼ 600, LHSV ¼ 4.0 hꢁ1. The Ni loading of 20 wt% Ni/CNTs is equal to the Ni loading of 30 wt% Ni2P/CNTs. Reactions were carried out in a
continuous-ow microreactor.
Table 3 Hydrotreatment performance of the Ni2P/CNTs catalystsa
Ni2P
(wt%)
Ni/P
(atomic)
LHSV
Conversion
[%]
Entry
(hꢁ1
)
1
2
3
4
40
30
20
10
1.25
1.25
1.25
1.25
4
4
4
4
91
>99
65
30
a
Reaction conditions: reactions were carried out with naphthalene (5
wt%), at 613 K, 4.0 MPa.
Fig. 4 Effect of adding DBT or quinolone to HYD. The content of HDS
and HDN intermediates of DBT and quinoline was 1.0 wt% in 5.0 wt%
naphthalene reactant.
shi in binding energy peaks assigned to the spent phosphorus
species in Ni2P/CNTs was detected, illustrating that the addition
of functionalized CNTs could affect the electronic properties of
bulk Ni2P.19
The hydrotreatment reaction results for Ni2P/CNTs in the
HYD of naphthalene and 1-methyl naphthalene are summa-
rized in Tables 2 and 3. Mass balance is 100 ꢂ 5% for each
reaction type. The reaction of naphthalene on 30 wt% Ni2P/
CNTs (Ni/P ¼ 1.25, mol) occurs with a very high and stable
conversion of 99%, which is much higher than for the other
samples or that of the Ni2P/SiO2 and NiP/C catalysts.8,20 This
may be attributed to 30 wt% Ni2P/CNTs having a higher BET
surface area which could provide more Ni2P active sites on the
surface of the CNTs (Table 1), furthermore, the Ni2P particles of
the 30 wt% samples with a small size were well dispersed on the
CNTs (Fig. 2). There are three major products formed from
naphthalene on these catalysts: cis-decalin, trans-decalin and
tetralin, compared with the Ni2P/SiO2 catalysts, Ni2P/CNTs gave
a higher trans-decalin selectivity of 69% and the proportion of
cis-decalin and trans-decalin is nearly 1 : 4, indicating that it
favours the hydrogenation pathway. In comparison with
Fig. 3 XPS spectra for Ni2P, spent Ni2P/CNTs and fresh Ni2P/CNTs: (a)
Ni 2p core level spectra and (b) P 2p core level spectra. The two stable
binding sites for Ni2P on CNTs: (c) atop–atop site and (d) bridge–
bridge site.
wall has a tendency to separate in a reaction mediated by the
underlying carbon atoms. Thus, the number of Ni2 congura-
tions on a tube varies with the curvature and Ni2 exhibits a
larger range of Ni–Ni bond lengths (2.34–2.78 A).18 A signicant
˚
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naphthalene, tetralin, the nal product of HYD of 1-methyl
naphthalene is less than 50%, this is due to the steric hindrance
effect caused by the methyl group. The Ni/CNTs catalyst exhibits
less activity with a conversion of 84%. Obviously, the existence
of P contributes to the HYD; the joint action of the Ni–P-CNTs
effect could be explained by the XPS results. Liu et al. also
conrmed that the P sites of the phosphide of Ni2P play a
complex and signicant role. First, the Ni–P bonds produce a
weak ligand effect that allows a reasonably high activity of HYD.
Second, the amount of Ni active sites on the surface decreases
owing to an ensemble effect of P, which prevents the reaction
system from deactivating. Third, the P sites are spectators and
provide moderate bonding and the H adatoms are essential for
HYD.21
We next investigated the durability and the resistance to
sulfur and nitrogen solutions of the catalysts. As revealed in
Fig. 4, aer contact with the sulfur and nitrogen solutions, the
initial HYD conversion dropped from 99% to 85%, however
when the solution was switched to naphthalene, the catalyst can
return to its former level, demonstrating superior tolerance
towards potential catalyst poisons, such as DBT and quinoline.
In summary, a novel Ni2P/CNTs catalyst was developed for
HYD. The catalyst exhibited excellent activity and stability, and
was successfully used for HDS or HDN, showing superior
performance to a Pt–Pd catalyst. As aromatic compounds are
important components of fuels, this work is a signicant step
towards the development of more active and practical catalysts
for the upgrading of coal-tar and bio-oil.
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Experimental section
Multi-carbon nanotubes were purchased from CAS (Chengdu,
China). Concentrated nitric acid (70%, Aladdin) was used to
functionalize the CNTs at 60 ꢀC for 6 h. Nickel phosphate
compounds loaded with CNTs were prepared by the following
route: nickel nitrate (60 mg, 99%, Aladdin) and ammonium
hydrogen phosphate (20 mg, 99%, Aladdin) were dissolved in
deionized water. Functionalized CNTs (100 mg) were then
mixed into the solution. Ultrasonication was carried out for 6 h
and followed by impregnation at room temperature. The
sample was then collected and calcined in N2 at 793 K. Reduc-
tion was conducted in 95% H2 mixed with 5% He with a total
ow of 100 mL minꢁ1 at 793 K. Subsequently, the calcined
sample was pelletized and sieved for the HYD reaction.
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Acknowledgements
The authors thank the nancial support of the Major Project of
the National Energy Administration (NY20130302513-1).
This journal is © The Royal Society of Chemistry 2015
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