The conversion of α-pinene to: Cis-pinane using a nickel catalyst supported on a discarded fluid catalytic cracking catalyst with an ionic liquid layer

Article abstract of DOI:10.1039/c9ra00675c

The concept of a solid catalyst coated with a thin ionic liquid layer (SCILL) was applied to the stereoselective hydrogenation of α-pinene. Nickel, a non-noble metal, was supported on a discarded fluid catalytic cracking catalyst (DF3C) and then modified with different loadings of the ionic liquid 1-ethanol-3-methylimidazolium tetrafluoroborate ([C₂OHmim][BF₄]). The resulting catalysts showed a range of conversions and selectivities for the hydrogenation of α-pinene. The SCILL catalysts afforded cis-pinane with high selectivity and their activity depended on the ionic liquid loading. For an ionic liquid loading of 10 wt%, although the catalytic activity was suppressed, the selectivity and conversion could reach above 98% and 99%, respectively. In addition, the catalyst remained stable after 13 runs and the activity was almost unchanged with the conversion maintained at approximately 99%. Thus, the ionic liquid layer not only improved the selectivity for cis-pinane but also protected the active site of the catalyst and prolonged the service lifetime of the catalyst. The SCILL catalytic system provides an example of an ionic liquid catalytic system which eliminates organic solvents from the catalytic process.

Full text of DOI:10.1039/c9ra00675c

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The conversion of a-pinene to cis-pinane using
a nickel catalyst supported on a discarded fluid
catalytic cracking catalyst with an ionic liquid
layer†
Cite this: RSC Adv., 2019, 9, 5978
a
ab
Xiaopeng Chen,ab Xiaojie Wei,ab
*
Shunyou Hu,
Zhangfa Tong
Linlin Wang,
and Lijiang Yina
ab
The concept of a solid catalyst coated with a thin ionic liquid layer (SCILL) was applied to the stereoselective
hydrogenation of a-pinene. Nickel, a non-noble metal, was supported on a discarded fluid catalytic
cracking catalyst (DF3C) and then modified with different loadings of the ionic liquid 1-ethanol-3-
methylimidazolium tetrafluoroborate ([C2OHmim][BF4]). The resulting catalysts showed a range of
conversions and selectivities for the hydrogenation of a-pinene. The SCILL catalysts afforded cis-pinane
with high selectivity and their activity depended on the ionic liquid loading. For an ionic liquid loading of
10 wt%, although the catalytic activity was suppressed, the selectivity and conversion could reach above
98% and 99%, respectively. In addition, the catalyst remained stable after 13 runs and the activity was
almost unchanged with the conversion maintained at approximately 99%. Thus, the ionic liquid layer not
only improved the selectivity for cis-pinane but also protected the active site of the catalyst and
prolonged the service lifetime of the catalyst. The SCILL catalytic system provides an example of an ionic
liquid catalytic system which eliminates organic solvents from the catalytic process.
Received 25th January 2019
Accepted 11th February 2019
DOI: 10.1039/c9ra00675c
rsc.li/rsc-advances
and Ir/C catalysts.4 Selka et al. achieved excellent catalytic
activity and selectivity based on their studies into hydrogena-
Introduction
tion of a-pinene over Pd-based catalysts on different supports;
however, the reusability of these catalysts was poor.1 Simakova
et al. investigated the hydrogenation of a-pinene over 4 wt%
palladium on carbon (Pd/C) as a catalyst with n-octane as
a solvent.5 Tanielyan et al. used ethanol as solvent in their
studies of pinene hydrogenation over anchored Wilkinson
catalyst.6 However, noble metal catalysts and organic solvents
are expensive, environmentally unfriendly, and the lack of
reusability of these catalysts poses major challenges.
Discarded uid catalytic cracking catalyst (DF3C) is a kind of
industrial waste product from petroleum rening processes.
DF3C contains heavy metals such as iron, nickel, and vana-
dium, which were contained in the heavy oil subjected to the
reactions. A large amount of DF3C is produced every year, the
majority of which is sent to landll, risking environmental
pollution, from heavy metals seeping into groundwater.7
Recently, attention has been paid to recovering DF3C, and it has
been applied as a catalyst for cracking of waste plastics,8
a source of heavy metals,7 and rare earth elements.11,12 DF3C is
mainly composed of Al2O3, SiO2, and residual nickel, with a rich
pore structure and large specic surface area; hence, DF3C
might act as an effective active metal carrier.
As a natural, green, and renewable product, a-pinene has drawn
great interest for applications in the pharmaceutical, bioenergy,
ne chemistry, and avouring industries.1 There are two kinds
of hydrogenation products of a-pinene, namely, cis-pinane and
trans-pinane, and among these two products, cis-pinane is more
desirable since the content of cis-pinane in raw materials
should be greater than 90% to reduce by-products and simplify
post-treatments. There is a need to design and identify more
effective catalysts for the selective hydrogenation of a-pinene to
improve the yield of cis-pinane. To date, various studies have
focused on hydrogenation of a-pinene, including a report by
Hou et al. on Ru nanoparticles in aqueous micellar micro-
reactors as catalysts with a high selectivity for cis-pinane under
mild conditions.2 Milewska et al. studied biphasic hydrogena-
tion of a-pinene over Pd/C under a high pressure of carbon
dioxide.3 Deliy et al. reported that hydrogenation and isomeri-
zation of pinenes occur simultaneously on Ru/C, Rh/C, Pt/C,
aSchool of Chemistry and Chemical Engineering, Guangxi University, Nanning 530004,
P. R. China. E-mail: wanglinlin1971@sina.com; Fax: +86-771-323-3718; Tel: +86-771-
3272702
bGuangxi Key Laboratory of Petrochemical Resource Processing and Process
Intensication Technology, Nanning 530004, P. R. China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c9ra00675c
Ionic liquid is a new molten salt system that consists of
cation and anion that exist in liquid form at room temperature,
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which can be designed according to specic requirements.
Ionic liquids have many properties different from conventional
organic solvent, such as thermal stability,9 electrochemical
stability,10,11 and adjustable electric elds, which were the
powerful supports for using ionic liquids as catalysts modiers
to achieve excellent catalytic performance and longer service
lives. These features enable ionic liquid to regulate the activity
and selectivity of catalysts. Owing to its unique properties, the
ionic liquid is used in chemical research as a solvent for various
types of reactions,12–16 extractant in separation/purication,17–20
and has applications in the eld of electrochemistry.21–23 Zhao
et al. have reported the synthesis of CoSx nanosheets from ionic
liquid and applied it for the oxygen evolution reaction.24 Zhao
et al. have developed liquid-exfoliation method to produce
stable and high-concentration dispersions of mono- to few-layer
black phosphorus nanosheets using ionic liquid.25 Li et al. has
used the ionic liquid to assist synthesis of Au–Pd bimetallic
particles with enhanced electrocatalytic activity.26 And Brown
et al. have studied the asymmetric hydrogenation and catalyst
recycling using ionic liquid and supercritical carbon dioxide.27
When an ionic liquid is used as a reaction medium, it can alter
the electronic environment of catalyst active sites; thus, ionic
liquids can tune the catalytic effectiveness of non-precious
metal catalysts to be as good as, or even better than those of
noble metal catalysts, particularly in terms of catalyst selectivity.
Ionic liquids can act as selective controllers for organic trans-
formations, such as the selective hydrogenation of nitriles or
alkynes.28 The inherent high viscosity of ionic liquids causes
mass transfer resistance, which severely inhibits the reaction
rate. Additionally, ionic liquids are not generally mass-
produced but rather used on a laboratory scale, which limits
the use of ionic liquids as reaction media. To address the above
problems, the concept of a solid catalyst coated with a thin ionic
liquid layer (SCILL) has been proposed and applied in various
catalytic reactions.29 Notably, selective hydrogenation reactions
show markedly enhanced selectivity compared with those of
conventional solid catalysts. Schwab et al. reported a highly cis-
selective and lead-free hydrogenation of 2-hexyne by a sup-
Scheme 1 The preparation procedure and application of SCILL cata-
lyst. Ni/DF3C (A), NiO/DF3C (B), DF3C (C).
microscope (SEM) imaging, Fourier transform infrared (FT-IR),
N2 adsorption–desorption, and X-ray photoelectron spectros-
copy (XPS).
Experimental
Materials
Nickel(II) nitrate hexahydrate (Ni(NO3)2$6H2O, Shanghai Xingao
Chemical Reagent Co., Ltd., 99.0%), acetone (CH3COCH3,
KESHI, 99.9%), a-pinene (C10H16, Aladdin, 98%), N-methyl-
imidazole (C4H6N2, Aladdin, 99.0%), 2-chloroethanol (C2H5ClO,
Xiya Chemical Co., Ltd., 99.0%), sodium uoroborate (NaBF4,
Macklin, 99.0%), and dichloromethane (CH2Cl2, Beijing
Chemical Works Co., Ltd., 99.0%). DF3C was provided by
Guangxi Tiandong Petrochemical Co., Ltd. All materials were
commercially available and used without further purication
except for the DF3C.
Synthesis of ionic liquid
ported Pd catalyst with an ionic liquid layer.30 Hou et al. re- The ionic liquid 1-ethanol-3-methylimidazolium tetra-
ported on Pd/SiO2 coated with [DMIM][MeHPO3] for the uoroborate [C2OHmim][BF4] was synthesized according to
selective hydrogenation of acetylene.31 The excellent selectivity a literature procedure.33 First, 1-methylimidazole (50.00 g, 0.60
of SCILL-based catalysts can be mainly attributed to the lter mol) was placed in a round-bottom ask (250 mL) with
effect, which regulates the effective concentration of substances a magnetic stirrer and spherical condenser. Then 2-chlor-
at the active sites of catalysts together with the ligand effects of oethanol (60.38 g, 0.75 mol) was added slowly with a constant
the ionic liquid, which change the electron density distribution pressure funnel at a steady rate (two drops a second) for 72 h
ꢀ
of active sites and affect interactions of reactants and interme- under argon atmosphere at 85 C with oil bath heating. Aer
diates with active metals.32
two stable phases formed, the upper liquid phase (containing
In this work, a SCILL catalyst for a-pinene hydrogenation the reactants for the reaction) was poured off and the bottom
with nickel, a non-noble metal, was developed in combination phase was washed with ethyl acetate (6 ꢁ 30 mL). The combined
with the industrial waste carrier DF3C and the ionic liquid 1- washings were dried by rotary evaporation over 48 h at 90 ꢀC in
ethanol-3-methylimidazolium tetrauoroborate ([C2OHmim] a high vacuum environment. The dried product (50 g, 0.3 mol),
[BF4]), as shown in Scheme 1. The effects of temperature, acetone (150 mL), and NaBF4 (38 g, 0.34 mol) were added to
pressure, ionic liquid loading on the hydrogenation of a-pinene a single-neck round bottom ask (250 mL) and stirred at room
were explored. The lifetime of the catalyst with or without ionic temperature for 72 h. The liquid phase product was separated
liquid, under the same reaction conditions was also examined. by vacuum ltration and washed with acetone (4 ꢁ 30 mL) and
The textural and morphological properties of the catalysts were then transferred to the vacuum rotary evaporator to remove the
investigated through X-ray diffraction (XRD), scanning electron solvent and obtain the pure ionic liquid, [C2OHmim][BF4].
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times to ensure that the reactor contained no residual air. The
reactor was heated to a target temperature of 100 C at a rela-
Preparation of IL-Ni/DF3C
ꢀ
First, DF3C was placed in a muffle furnace and calcined at
a high temperature of 500 ꢀC for 5 h to burn off carbon deposits
on its surface. Then the DF3C was impregnated by incipient
wetness with a nickel nitrate solution (10 wt% Ni loading) and
polyvinylpyrrolidone (1.5 wt%). The mixture was le overnight
at room temperature and then dried at 110 ꢀC for 4 h under
vacuum. The dried product was again placed in a muffle furnace
tively low hydrogen pressure (0.05 MPa) and rotation speed
(50 rpm stirring rate) to avoid excessive reaction during the
heating process. The stirring speed and hydrogen pressure were
adjusted to the desired value (5 MPa). During the reaction, the
hydrogen pressure in the reactor was maintained at a constant
value by controlling the inlet valve. Aer the reaction, cooling
water was owed through the heat exchange coil in the reaction
vessel bring the temperature of the reactor to room tempera-
ture. The product was then separated from the mixture by
vacuum ltration and samples were qualitatively analyzed by
a gas chromatography-mass spectrometer (GC-MS) on an Agi-
lent 6890/5973 GC-MS equipped with a HP-5MS capillary
column (30 m ꢁ 0.25 mm i.d. ꢁ 0.25 mm lm thickness).
Quantitative analyses were carried out using GC on an HP7820
GC equipped with a ame ionization detector (FID) and an HP-5
capillary column (30 m ꢁ 0.25 mm i.d. ꢁ 0.25 mm lm thick-
ness). The temperature of the injector and the detector were
250 ꢀC. The oven temperature was programmed as follows: from
ꢀ
and calcined at 550 C for 4 h to obtain the precursor for the
catalyst, NiO/DF3C. Finally, the precursor was reduced under
ow of H2 under temperature programmed to obtain Ni/DF3C.
The IL-Ni/DF3C was prepared by a conventional impregnation
method. The acetone solution of [C2OHmim][BF4] was added
dropwise to the Ni/DF3C, mixed fully, and then transferred to
a vacuum drying oven at 60 ꢀC for 3 h. The acetone slowly
evaporated from the slurry; however, the ionic liquid remained
on the inner surface of the Ni/DF3C as a result of its charac-
teristically high viscosity and low volatility. The initial ionic
liquid loading ranged from 0 wt% to 20 wt%. The catalysts are
denoted as X-Ni/DF3C, where X refers to the ionic liquid
loading.
50 C to 65 C at a rate of 1 C minꢂ1, and nally holding at
150 ꢀC for 5 min. The qualitative analysis was conducted on the
basis of the holding time of the peak. The content of the reac-
tants and products was directly obtained by the GC chemstation
system, according to the area of each chromatograph peak.
Conversion of a-pinene (Xa-pinene) and selectivity to cis-pinane
(Scis-pinane) were calculated by eqn (1) and (2):
ꢀ
ꢀ
ꢀ
Characterization
XRD patterns were obtained with a Shimadzu XRD-600 advance
powder diffractometer operated at 45 kV and 40 mA, using Cu-
Ka radiation. The samples were scanned over Bragg angles in
the range from 10–80ꢀ at a scan rate of 10ꢀ minꢂ1. XPS was
measured with an ESCALAB 250XI+ spectrometer, and the
binding energy scales for samples were referenced by calibra-
tion to the C 1s binding energy of adventitious carbon at
284.8 eV, with peaks tted by Gaussian–Lorentzian curves with
the use of XPSPEAK. SEM imaging was performed on a Zeiss
Sigma HD with an SE detector. The specic surface areas,
average pore width, and total pore volume were calculated with
the Brunauer–Emmett–Teller (BET) equation and Barrett–Joy-
ner–Halenda (BJH) methods with the use of a Micromeritics
3Flex instrument. FTIR measurements were performed in
a Nicolet IS 10 FTIR spectrometer at a resolution of 4 cmꢂ1 with
an average of 20 background scans and 50 sample scans from
Ca-pinene;0 ꢂ Ca-pinene
Xa-pinene
¼
ꢁ 100%
(1)
Ca-pinene;0
Ccis-pinane
P
Scis-pinane
¼
ꢁ 100%
(2)
Cp
where Ca-pinane,0 is the initial concentration of a-pinene, Cp is
the concentration of the different hydrogenation products.
Results and discussion
The main goal of the present work was to achieve maximum
selectivity to cis-pinane in the hydrogenation of a-pinene. In the
present work on catalyst screening for a-pinene, Ni supported
on DF3C coated with ionic liquid layer showed about 98%
selectivity to cis-pinane by suppressing the by-product forma-
tion. In continuation, detail catalyst characterization for
4000 to 400 cmꢂ1
.
Catalyst performance testing
Catalytic hydrogenation experiments of a-pinene were per- understanding the observed highest selectivity to cis-pinane is
formed in a batch reactor comprising a 100 mL stainless steel discussed below.
autoclave (Shanghai Huotong Experimental Instrument Co.,
Ltd., China) with a maximum pressure of 25 MPa. A schematic were measured as shown in Table 1. The specic surface area of
of the experimental setup is depicted in Scheme S1.† Approxi- the DF3C without any pretreatment was 124.06 m2 gꢂ1
Nitrogen adsorption–desorption isotherms of the catalysts
,
mately 60 mL of a-pinene and 5 g of the catalyst were introduced however, aer calcination, the specic surface area of the DF3C
into the autoclave. The air in the reactor was purged with increased to 225.74 m2 gꢂ1, and the average pore width
a vacuum pump for 20 minutes. Then, hydrogen from the decreased from 9.23 nm for DF3C to 5.28 nm and the total pore
cylinder was introduced into the reaction still at a pressure of volume increased from 0.15 to 0.16 cm3 gꢂ1. These changes are
5 MPa and maintained for 20 minutes to ensure that the reactor likely caused by the layer of carbon on the surface of DF3C,
did not leak. Then the gas in the reactor was discharged to the which also contributed to catalyst deactivation. A relatively large
atmosphere through the discharge valve, and the vessel was specic surface area and specic pore volume were conrmed
recharged with hydrogen at 5 MPa. This operation was cycled 4 in the treated DF3C by SEM imaging (Fig. 1). Comparing the
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Table 1 The texture properties of the catalysts
Surface area
Average pore
size (nm)
Total pore volume
Samples
(m2 gꢂ1
)
(cm3 gꢂ1
)
DF3C
124.06
225.74
197.83
153.72
99.38
96.05
149.74
37.95
8.26
9.23
5.28
6.79
0.15
0.16
0.16
0.15
0.14
0.14
0.15
0.07
0.03
DF3C aer calcinations
Ni/DF3C
0.05-IL-Ni/DF3C
0.10-IL-Ni/DF3C
0.10-IL-Ni/DF3C (used)
0.10-IL-Ni/DF3C (20 runs)
0.15-IL-Ni/DF3C
0.20-IL-Ni/DF3C
9.06
10.15
10.69
9.33
14.87
28.39
liquid and active metal nickel were relatively evenly distributed
over the surface of the catalyst.
The X-ray diffraction (XRD) pattern of the catalysts is shown
in Fig. 2. The patterns featured 13 peaks, which corresponded to
zeolite Y, Al2O3, and ZSM-5, respectively (Fig. 2a). The diffrac-
tion peaks for the DF3C sample became more intense aer
calcination (Fig. 2b) since the carbon covering the surface of the
catalyst is burned off. For DF3C loaded nickel (Fig. 2c), in
addition to the previously mentioned diffraction peaks, three
characteristic peaks at 2q ¼ 44.5ꢀ, 51.8ꢀ, and 76.3ꢀ corre-
sponded to the (111), (200), and (220) planes of Ni, respectively.
Moreover, there was no a great difference in the diffraction
patterns of Ni/DFCC coated with various contents of IL (Fig. 2c–
f), which indicated that the crystal structure was unaffected by
the content of ionic liquid.35
Fig. 1 SEM images of DF3C (A, B), Ni/DF3C (C), IL-Ni/DF3C (D).
The FT-IR results (Fig. S2†) showed characteristic peaks of
[C2OHmim][BF4] at 3557 and 3426 cmꢂ1 resulting from tele-
scoping vibrations of hydroxyl groups. The typical peaks for the
imidazole ring were also observed at 1578, 1467, 2965, 2895,
3168, and 3122 cmꢂ1, which are attributed to stretching of
CH3(N), stretching of the ring in plane, asymmetric stretching
catalysts with different ionic liquid loadings showed that the
specic surface area of the catalyst decreased rapidly as the
ionic liquid loading increased. Comparing the catalysts before
and aer used showed that the specic surface area of the
catalysts decreased aer used, which is attributed to deposition
of reactants on the catalyst surface during the reaction.
Furthermore, the specic surface area for 0.10-IL-Ni/DF3C aer
20 runs was greater than that of 0.10-IL-Ni/DF3C (fresh), which
might be caused by the leaching ionic liquid as reported by early
literature.34
The morphologies and microstructures of the prepared
samples were characterized by SEM. The DF3C was composed of
spherical particles with a rugged surface and a particle size of
approximately 50 mm (Fig. 1A). Notably, the rich pore structure
of DF3C and its large specic surface area (Fig. 1B), are suitable
characteristics for a catalyst support. Comparing the pictures of
the Ni/DF3C coated with or without ionic liquid, the ionic liquid
layer caused the inner surface of the catalyst to appear smooth
(Fig. 1D), unlike the rough surfaces of the DF3C support. This
viscous appearance in the SEM images is attributed to the
surfaces of the nickel crystal being covered by a layer of ionic
liquid. SEM-EDX was used to analyze the dispersion of the ionic
liquid and the active metal nickel on the DF3C (Fig. S1†). Here,
nitrogen and uorine could be representative of the distribu-
tion of the ionic liquid, and the results showed that the ionic
Fig. 2 XRD patterns of the DF3C (a), DF3C after calcinations (b), Ni/
DF3C (c), 0.05-IL-Ni/DF3C (d), 0.10-IL-Ni/DF3C (e), 0.15-IL-Ni/DF3C
(f).
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of HCH on the hydroxyethyl group, symmetric stretching of with performance tuned by ionic liquid coatings, where the
HCCH, asymmetric stretching of ring HCCH, and stretching of coatings controlled the electron density of active metal sites.40
NC(H)NCH, respectively. In addition, a characteristic peak of
The inuence of the ionic liquid loading on the activity of the
[BF4]ꢂ also appeared at 846 cmꢂ1. The peaks at 3557, 3426, and catalyst in the hydrogenation of a-pinene was examined. The
1648 cmꢂ1 are attributed to bending and stretching vibrations conversions of a-pinene over the catalysts with different ionic
of physically adsorbed water on the surface of the DF3C, liquid loadings are shown in Fig. 4. In the hydrogenation of a-
whereas those at 2169, 1079, and 464 cmꢂ1 are attributed to Si– pinene, the Ni/DF3C system reached a total pinene conversion
H telescopic vibrations, RO4 (R: tetrahedral Si or Al) asymmetric within 50 min, whereas, for the SCILL system, the conversion
stretching vibrations, and RO bending vibrations, respectively. was much lower over the same reaction time. For the 0.20-IL-Ni/
There was not Si–O(H)–Ni peak at 985 cmꢂ1, which indicated DF3C catalyst, the reaction rate was very slow and the conver-
that no covalent bonds formed between Ni and DF3C.
sion of reactants was only 5.63% aer 180 min; however, for the
XPS measurements (Fig. 3) were performed on the Ni/DF3C 0.15-IL-Ni/DF3C catalysts, 38.51% conversion was achieved
with or without ionic liquid lm to better understand high within the same time. Comparing the catalytic performance on
stability and selectivity to cis-pinane of the Ni/DF3C coated with SCILL and Ni/C catalyst revealed that the active metal nickel on
ionic liquid. The binding energy of the Ni 2p3/2 in the Ni/DF3C the inner surface of the SCILL catalyst might be partially
catalyst were 852.6 eV (Ni0), and 855.6 eV (Ni2+). Notably, these poisoned by the ionic liquid as the chemical interaction
energies were 1.3 eV higher than those in the IL-Ni/DF3C cata- between ionic liquid and metal nickel as the results shown in
lytic system. Oxidization of nickel might be attributed to reac- XPS analysis. As Canova et al.41 have studied that the a-pinene
tions with oxygen in the air during the sample testing, similar to hydrogenation was carried out in the presence of a hydrogena-
previous reports.36 The differences observed here are attributed tion Ni-based catalyst, wherein to obtain increased stereo-
to interactions of ionic liquid with the active metal sites. The IL selectivity, an effective amount of the reactive surface of the
may act as a ligand directly interacting with the catalytically hydrogenation catalyst is inactivated with a catalyst modier.
active site.37 The lower binding energy of the SCILL catalyst Since the ionic liquid layer was only a few nm thick, when the
suggested that [C2OHmim][BF4] directly interacted with the ionic liquid loading was less enough, the diffusion inside lm
active metal nickel, which could be related to similar effects will play a minor role in the overall rate.42 As increasing the
reported in bimetallic catalysis.38,39 Supported metallic catalysts ionic liquid loading or even as reaction solvent, the ionic liquid
are generally highly reactive in the hydrogenation of unsatu- might be able to block the active sites and the internal pores of
rated compounds; however, the selectivity and stability of such the catalyst, at this time, the mass transfer resistance might not
catalysts are generally poor. To increase the selectivity to the be ignored in the reaction rate.31 In summary, it can be inferred
product of interest, conventional monometallic catalysts are that the mass resistance alone was not the only reason for the
typically doped with other metals or non-metal elements to reduced reaction rate in SCILL catalytic system, but also the
regulate the electronic environment of the active components.32 chemical interaction between the [C2OHmim][BF4] layer and
For SCILL-based catalysts, a thin lm of ionic liquid covers the the active site on the surface of the SCILL catalyst.
active sites of the catalyst and interacts with the active metal by
The results for selectivity as a function of a-pinene conver-
controlling the electron density at the metal sites, where the sion (Fig. 5) indicated that the selectivity toward cis-pinane
degree of electron donation correlates with the interionic remained constant throughout the reaction with only minor
interactions in the ionic liquids, which in turn controls the uctuations. However, the selectivity of the catalyst varied with
activity and selectivity of the catalyst. Schwab et al. used a sup-
ported Pd catalyst with an ionic liquid layer in the stereo-
selective hydrogenation of 2-hexyne to cis-2-hexene and
achieved an outstanding yield of 88%.30 Babucci et al. have re-
ported a class of atomically dispersed supported metal catalysts
Fig. 3 XPS spectra of the Ni 2p region of Ni/DF3C and 0.10-IL-Ni/ Fig. 4 Conversion vs. time profiles. Reaction conditions: a-pinene 60
DF3C.
mL, catalyst 5.0 g, H2 5.0 MPa, 100 ꢀC, 180 min.
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vertically arranged on top of each other,46 NA is the Avogadro
constant, Sg is the specic surface area of the Ni/DF3C without
ionic liquid (99.75 m2 gꢂ1), rIL is the density of [C2OHmim][BF4]
(1.36 g cmꢂ3, 25 ꢀC, 1 atm). The calculated MLs for the catalysts
with different ionic liquid loading are listed in Table 2. As
shown in Scheme S2,† when the ionic liquid loading was less
than 10%, incomplete monomolecular ionic liquid layer was
formed on the catalyst surface. In this case, some of the active
metal sites were not covered by the ionic liquid layer in the
hydrogenation process. Thus, reactants tended to interact with
the exposed active metal sites outside of the ionic liquid layer,
such that the effects on the catalytic selectivity decreased,
whereas the reaction rate increased. However, when the ionic
loading was greater than 10% or even as reaction solvent,
a multi-molecular ionic liquid layer formed on the catalyst
surface and the selectivity did not increase greatly; however, the
reaction rate was greatly reduced owing to an increase in the
mass transfer resistance. Although, this calculation model of
the ionic liquids thickness maybe brief, it could help us to
understand the mechanism of the SCILL. In summary, the
thickness of the ionic liquid layer plays a key role in the reaction
process and the activity and selectivity of the catalyst are both
related to the ionic liquid loading (i.e., the thickness of the ionic
liquid layer).
Fig. 5 Selectivity vs. conversion profiles. Reaction conditions: a-
pinene 60 mL, catalyst 5.0 g, H2 5.0 MPa, 100 ꢀC, 180 min.
different ionic liquid loading. When the loading of the ionic
liquid was greater than 10 wt%, the selectivity of the catalyst was
more than 98%; however, when the loading of ionic liquid was
5 wt%, the selectivity decreased to approximately 90%. The
ionic liquid-free catalyst had a selectivity of approximately 87%.
The coating of Ni/DF3C with [C2OHmim][BF4] improved the cis-
pinene selectivity further with the activity decreased. The
amount of modier should be sufficient to effect a stereo-
chemical improvement, and yet less than that amount which
substantially adversely affects reaction rate and conversion.41
The increase in the cis-pinene selectivity aer the application of
the ionic liquids could be attributed to the following reasons.
The active metal nickel was directly modied by the ligand
effects of [C2OHmim][BF4], as conrmed by XPS analysis of IL-
Ni/DF3C (Fig. 3). Additionally, a strong electrostatic interfacial
eld was created by the adsorbed ionic liquid, which generated
diffusion barriers and modied the access of different
substances into the reaction system.38 The bulky gem-dimethyl
bridge group on the a-pinene has a larger steric hindrance,
preventing the a-face of a-pinene from making contact with the
catalyst.6,43 Hence, hydrogen activated by nickel preferentially
attacks the a-face, which is less sterically hindered. Further-
more, the ionic liquid layer might increase the difference of the
steric hindrance between a-face and b-face, which improves the
cis-pinane selectivity.44
To further study the effect of the catalyst coated with ionic
liquid on the hydrogenation of a-pinene, several parameters
such as reaction temperature and H2 pressure were studied, and
the results were shown in Fig. S3.† The catalytic performances
of 0.10-IL-Ni/DF3C in a-pinene hydrogenation at different
temperatures under H2 pressure of 5 MPa are shown in
ꢀ
Fig. S3A.† At a lower temperature of 70 C, the conversion was
low at 15.77%. As the reaction temperature increased from 70 to
100 ꢀC, the conversion gradually increased and when the
temperature was 100 ꢀC, the conversion reached above 99%;
however, the cis-pinane selectivity remained at approximately
98% with a slight decrease. It's worth noting that the ionic
liquid used in our catalytic system under mild conditions (the
maximum reaction temperature was 110 ꢀC) was thermal stable
according to a literature reported by Cao et al.48 Thus, it may be
concluded that higher temperatures increased conversion but
had no notable effects on catalyst selectivity, which is consistent
with previous reports in the literature.49 Hydrogenation of a-
pinene was performed at different H2 pressures from 2 to
7 MPa, and the results of conversion of a-pinene and selectivity
to cis-pinane are shown in Fig. S3B.† The conversion increased
markedly as the H2 pressure was increased and at 5 MPa the
conversion reached 99.26%. In addition, the selectivity for cis-
pinane increased slightly with increasing H2 pressure. The
ndings presented herein and their discussion, indicate that
the reaction temperature and H2 pressure had not distinct
effects on the selectivity for cis-pinane, but played an important
role in the reaction rate and conversion.
This result is attributed to the thickness of the ionic liquid
layer, which can be estimated from the mass ratio of the ionic
liquid to Ni/DF3C. Interestingly, Cheng et al. have reported that
less than a monolayer IL coating on the surface of the catalysts
as the mIL/mcatalyst ration is below 0.10, and multilayer IL coat-
ings when the ratio is above 0.10, and his conclusions are
a good explanation for our experimental results.35 The thickness
of the ionic liquid layer was estimated by using eqn (3)45
1=3
mIL ꢁ NA
ML ¼
(3)
1=3
2=3
MIL ꢁ mcatal ꢁ Sg ꢁ rIL
Table 2 Calculated MLs for catalysts with different IL loading
where the lm thicknesses are given in monolayers (MLs) and 1
ML of ionic is considered to be one losed layer of ion pairs
IL loading (wt%)
MLs
0
0
5
0.37
10
1.15
15
4.51
20
27.60
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Table 3 Effect of catalysts on the hydrogenation of a-pinenea
Because the ionic liquid molecule could be synthesized
according to the actual needs, so far, various ionic liquids have
been prepared. The conventional ionic liquid 1-butyl-3-
methylimidazolium tetrauoroborate ([BMIM][BF4]), 1-ethyl-3-
methylimidazolium tetrauoroborate, ([EMIM][BF4]), etc. have
been always used as a stabilizer to protect the metal nano-
particles catalysts (NPs) from aggregating to prolong the life-
time of the NPs catalyst. Zhao et al. have synthesized well-
dispersed NiO nanoparticles with room temperature ionic
liquid [BMIM][BF4].50 One of the attractive features of ionic
liquids is the option to modify them to maximize their benets
in specic reactions and a variety of functionalized ILs are now
available.51 Functionalized ILs have a more unique function
than traditional ionic liquid. Xue Yang et al. has studied the
biphasic hydrogenation over Rh nanoparticles in hydroxyl
functionalized ionic liquid (OH-ILs).51 For the a-pinene hydro-
genation over SCILL catalysts, we have studied the conventional
ionic liquid [EMIM][BF4] and corresponding hydroxylated ionic
liquid [C2OHmim][BF4] as modier for the Ni/DF3C catalyst
with the same thick 1 ML, the results were listed in Table S1.†
Almost all the reactant was converted in 180 min over these
three catalysts, although, maybe there were some difference in
reaction rate which have been particularly discussed above. And
we could found that the selectivity toward cis-pinane over
[EMIM][BF4]-Ni/DF3C was similar to that of Ni/DF3C without
ionic liquid layers. However, for the [C2OHmim][BF4]-Ni/DF3C,
the selectivity toward cis-pinane increased to approximately
98%, which could be attributed to some special performance of
the functional ionic liquid such as viscosity, boiling point,
melting point, electrochemical environment and electro-
chemical interaction between metal particles and ionic liquid.
Ethanol has been used as solvent in general hydrogenation
reaction and especially for the pinene hydrogenation.1 Cocker
et al. has studied the hydrogenation of a-pinene over Pt/C
(5 wt%) catalyst with EtOH as solvent and obtained high
selectivity for cis-pinane (98.5%).44 Dorjnamjin et al. have used
the OH-ILs as a reducing agent which reduces the Ag ion to Ag0
as well as a stabilizer in forming Ag nanoparticles, and the
–CH2OH– group in OH-ILs is converted to the –CHO– group by
reducing Ag ion and protects the Ag nanoparticles at the same
time.52 For the SCILL catalytic system, the –OH functional group
was graed in the traditional ionic liquid [EMIM][BF4], making
the SCILL catalytic performance was similar to the case of using
EtOH as reaction solvent, at the same time, the lifetime of the
catalyst was prolonged as a result of the OH-ILs protection.52
The work also studied varies of catalysts for the hydrogena-
tion of a-pinene at the same conditions as shown in Table 3. It
can be seen that the hydrogenation of a-pinene was difficult to
take place without the catalyst (entry 1). Ni/DF3C and Pd/C
showed excellent catalytic efficiency, the conversion of the a-
pinene could reach over 99%, however, the selectivity toward
cis-pinane was below 90%. For 0.10-IL-Ni/DF3C, the a-pinene
conversion and cis-pinane selectivity were over 99% and 98%,
respectively. Hou et al. have studied the a-pinene hydrogenation
over the Ru nanoparticles, and the selectivity for cis-pinane
could reach above 98%.53 Yang et al. applied the Pd-based
Entry
Catalyst
X (%)
S (%)
1
2
Blank
1.70
99.47
99.06
99.31
98.10
72.36
87.94
98.26
88.91
96.0
Ni/DF3C
0.10-IL-Ni/DF3C
Pd/C
3
4b
5c
Ru/C
a
Reaction conditions: a-pinene 60 mL, catalyst 5 g, H2 5.0 MPa, 100 ꢀC,
180 min. X: conversion of a-pinene; S: selectivity toward cis-pinane.
b
Commercial Pd/C (5%), the molar ratio of the Pd to a-pinene was
c
similar to the case of Ni-based catalyst. The experimental data was
obtained from ref. 47.
conversion and selectivity could reach 99.4% and 81.3%,
respectively.49 In summary, metal Ni, Pt, Ru-based catalysts47
could achieve high a-pinene conversion and selectivity (over Ni-
SCILL and Ru-based catalyst), but the price of these three kinds
of metal catalysts was greatly different, metal Ni is very cheap
compared with other noble metal like Pt, Ru, etc. It was possible
to teach a cheap Ni-based catalyst to act like an noble Ru metal
catalyst for hydrogenation reaction by coating it with an ionic
liquid lm.32
Finally the reusability of the catalyst coated with or without
ionic liquid was also investigated for the hydrogenation of a-
pinene (Fig. 6). The recycled catalyst was easily removed from
the reaction mixture by simple vacuum ltration for the next
cycle. The conversion of a-pinene was maintained at approxi-
mately 99.0% throughout the rst 13 cycles for the SCILL
system, indicating its excellent stability; however, in the case of
the Ni/DF3C catalytic system, the conversion steadily decreased
from 99.8% to 62.5% aer fourth cycles. Moreover, aer 13
cycles, the activity of IL-Ni/DF3C began to decrease, the
conversion eventually decreased to 84.3% and the selectivity
also decreased from 98.2% to 87.5%. The decrease of activity
and selectivity might be explained by the loss of ionic liquid
from the catalyst over multiple runs owing to weak interactions
between the ionic liquid and the Ni/DF3C as the earlier
Fig. 6 Recycling of the Ni/DF3C coated with or without ionic liquid.
Reaction conditions: a-pinene 30 mL, catalyst 2.5 g, H2 5 MPa, 100 ꢀC,
catalyst for the hydrogenation of the a-pinene, and the 90 min.
5984 | RSC Adv., 2019, 9, 5978–5986
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literature reported that it should be further emphasized that
even a very low solubility of the ionic liquid in the owing liquid
media will make long term stability of the SCILL catalyst system
impossible due to slow but steady leaching.34 As shown in
Fig. S4,† The SCILL catalyst used aer 20 runs has approxi-
mately 4% weight loss, which corresponds to the residual ionic
liquid on the surface of the catalyst. The quality of the residual
ionic liquid estimated by the thermogravimetric analysis may
not be accurate enough, but it could reect the fact of the
leaching loss of ionic liquid during the multiple cycles of
reaction as reported in some literature.34,42,54 Overall, these
studies demonstrate greatly enhanced reusability and selectivity
of the IL-Ni/DF3C catalyst, which is attributed to the effects of
the ionic liquid layer.
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Conflicts of interest
There are no conicts to declare.
25 W. Zhao, Z. Xue, J. Wang, J. Jiang, X. Zhao and T. Mu, ACS
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26 Z. Li, R. Li, T. Mu and Y. Luan, Chem.–Eur. J., 2013, 19, 6005–
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Acknowledgements
The authors are grateful for the nancial support for this work
from the National Natural Science Foundation of China (Grant 27 R. A. Brown, P. Pollet, E. McKoon, C. A. Eckert, C. L. Liotta
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(Grant No. 2016Z002).
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