Hydrogelator as growth-controlling agent for enhancing the catalytic activity of NiB amorphous alloy catalyst

Article abstract of DOI:10.1007/s11164-018-3591-x

A series of NiB-x (x denotes as the molar ratio of hydrogelator:Ni) amorphous alloys were prepared using a hydrogelator, 2,4-(3,4-dichlorobenzylidene)-d-sorbitol (DCBS), as the controlling agent and were tested in the selective hydrogenation of benzophenone to benzhydrol. The effect of different amounts of DCBS addition on the activity of the NiB amorphous alloys was investigated. The addition of small amounts of DCBS during NiB preparation could markedly improve its activity. The conversion of benzophenone was 96.6% and the yield of benzhydrol can reach 82.0% under the optimal reaction conditions over the NiB-0.015 catalyst. The high activity of NiB catalysts is attributed to the regulation effect of three-dimensional networks formed by DCBS, which could inhibit the agglomeration of NiB particles during preparation and make them more dispersed, resulting in larger surface areas and more active Ni sites. The Brunauer–Emmett–Teller method, transmission electron microscopy, H₂-chemisorption and temperature-programmed desorption of hydrogen measurements indicated these results.

Full text of DOI:10.1007/s11164-018-3591-x

Research on Chemical Intermediates
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Hydrogelator as growth-controlling agent for enhancing
the catalytic activity of NiB amorphous alloy catalyst
Yuanyuan Ma1 Jie Dong1 Xiaowei Yang1 Libo Niu1 Huiling Zhang1
Guoyi Bai1
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Received: 3 May 2018 / Accepted: 10 September 2018
Ó Springer Nature B.V. 2018
Abstract
A series of NiB-x (x denotes as the molar ratio of hydrogelator:Ni) amorphous
alloys were prepared using a hydrogelator, 2,4-(3,4-dichlorobenzylidene)-D-sorbitol
(DCBS), as the controlling agent and were tested in the selective hydrogenation of
benzophenone to benzhydrol. The effect of different amounts of DCBS addition on
the activity of the NiB amorphous alloys was investigated. The addition of small
amounts of DCBS during NiB preparation could markedly improve its activity. The
conversion of benzophenone was 96.6% and the yield of benzhydrol can reach
82.0% under the optimal reaction conditions over the NiB-0.015 catalyst. The high
activity of NiB catalysts is attributed to the regulation effect of three-dimensional
networks formed by DCBS, which could inhibit the agglomeration of NiB particles
during preparation and make them more dispersed, resulting in larger surface areas
and more active Ni sites. The Brunauer–Emmett–Teller method, transmission
electron microscopy, H2-chemisorption and temperature-programmed desorption of
hydrogen measurements indicated these results.
Keywords Amorphous alloy Á Hydrogelator Á Controlling agent Á Hydrogenation Á
Benzophenone
Electronic supplementary material The online version of this article (https://doi.org/10.1007/s11164-
018-3591-x) contains supplementary material, which is available to authorized users.
&
&
1
Libo Niu
libo_niu@126.com
Guoyi Bai
baiguoyi@hotmail.com
Key Laboratory of Chemical Biology of Hebei Province, College of Chemistry and
Environmental Science, Hebei University, Baoding 071002, People’s Republic of China
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Y. Ma et al.
Introduction
Benzhydrol and its derivatives are important chemical intermediates which have
attracted much attention of chemical and pharmaceutical researchers [1]. For
example, benzhydrol is an important precursor for many bioactive compounds, such
as drugs for pollenosis, allergic conjunctivitis and hives. Considering that traditional
chemical reduction or synthesis routes are not environmentally friendly, catalytic
hydrogenation of benzophenone to produce benzhydrol over metal catalysts has
attracted much interest due to the green nature process and the ready availability of
suitable starting materials. Presently, noble metal catalysts, such as palladium [2, 3],
platinum [4, 5], silver [6–8] and rhodium [9], have been used extensively for
benzophenone hydrogenation and catalytic hydrogenation of other unsaturated
organic compounds, but the high prices have limited their application for large-scale
production. In addition, Raney Ni is another common catalyst used in this
conversion, but its pyrophoric nature has limited its further application [10].
Therefore, it is necessary to find efficient non-noble metal catalysts for the selective
hydrogenation of benzophenone.
Amorphous alloy catalysts have also attracted much attention especially in the
hydrogenation of unsaturated compounds containing C=C and C=O groups, due to
their unique isotropic nature and high concentration of coordinative unsaturated
sites [11–13]. Unfortunately, the small nanoparticles prepared by the chemical
reduction method are unstable and tend to aggregate into large particles, resulting in
catalysts with broad size distributions and lower catalytic activity. In order to
overcome this drawback, various techniques such as ultrasound, metal modifiers and
polymers have been used to improve the NiB preparation and then enhance the
performance of the catalysts. For instance, Li and co-workers [14] have developed a
simple approach for synthesizing a hollow structured Ni–Co–B amorphous alloy
catalyst for 2-ethyl-2-hexenaldehyde hydrogenation, and these hollow sphere
catalysts have exhibited higher activities than a Ni–Co–B solid catalyst. Further-
more, the use of the three-dimensional networks formed by gelator as a growth
controller has been demonstrated to be a potentially useful method to prepare metal
nanoparticles with a more dispersed morphology [15]. For example, Begum et al.
[16] have synthesized poly(N-isopropylacrylamide-co-acrylic acid) [p(NiPAco-
AAc)] microgel to dispersed Ag nanoparticles by free radical precipitation
polymerization in aqueous medium. Liu and co-workers [17] have synthesized Se
and Ag nanoparticles in agarose hydrogel and have found that the morphology of
the nanoparticles could be controlled by adjusting the concentration of the gelator
and metals salts. Moreover, Farooqi and co-workers have prepared functionalized
polymer microgels to study their pH sensitivity, thermal stability and catalysis
mechanism in relevant catalytic reactions [18–21]. Naseem and co-workers have
described in detail the stability of Pt NPs within responsive microgel networks [5].
However, the use of a low molecular weight gelator in amorphous alloy
preparations to enhance their catalytic performance has rarely been reported.
In a conclusion of our previous works on NiB preparation [22–24], a
hydrogelator, 2,4-(3,4-dichlorobenzylidene)-D-sorbitol (DCBS; Scheme 1), was
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Hydrogelator as growth-controlling agent for enhancing…
Scheme 1 Schematic representation for the synthesis of a NiB amorphous alloy catalyst and the
hydrogenation of benzophenone
selected as a new additive in the preparation of NiB amorphous alloys to achieve the
activity improvement of NiB catalysts. DCBS can be dissolved in water and self-
assembled into three-dimensional networks. Then, NiB nanoparticles were fabri-
cated inside the networks by the chemical reduction of nickel ions using KBH4 as
the reduction agent. It was found that NiB exhibited good activity in benzophenone
hydrogenation with a small amount of DCBS addition during the preparation. The
relationship between the catalytic performance and the hydrogelator addition of the
catalysts has been investigated and clarified based on the reaction and character-
ization results.
Experimental
Catalyst preparation
All chemicals were analytical or technical grades purchased from Baoding Huaxin
Reagent and Apparatus and used without further purification. Raney Ni was
obtained from Aladdin. NiB amorphous alloys were prepared by the process of
chemical reduction using KBH4 as the reducing agent. A typical procedure for the
preparation of NiB amorphous alloys is as follows [22–25]: 1.18 g NiCl2Á6H2O was
dissolved in 10 mL distilled water, then, 25 mL KBH4 (aq) (25 mmol,) containing
NaOH (0.20 M) was added dropwise to the above solution with vigorous stirring in
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an ice bath. The obtained black precipitate was washed with distilled water until
neutral. Then, the catalyst was further washed with absolute ethanol three times to
replace residual water and stored in absolute ethanol for use. Upon the addition of
DCBS during preparation, a certain amount of additive was dissolved in distilled
water to form a transparent solution. The solution was used as a solvent for
NiCl2Á6H2O and KBH4 instead of distilled water, and the preparation procedure was
the same as described above. The obtained catalysts were denoted as NiB-x (x is the
molar ratio of hydrogelator:Ni).
Catalyst activity test
Catalyst activity testing was carried out in a 100-mL stainless steel autoclave
equipped with electric heating and a mechanical stirrer system. The catalyst
(0.05 g), benzophenone (2 g) and ethanol (50 mL) were successively added into the
autoclave and the reactor was filled with H2 three times followed by evacuation,
pressurized with H2 to 2.5 MPa and heated to 373 K. Hydrogenation was then
started by stirring the reaction mixture (400 r/min) when the temperature reached
373 K and was kept for 60 min. The reaction mixtures were analyzed by gas
chromatography on a 30-m HP-5 capillary column. The product structures were
confirmed by using GC–MS on a thermo Trace1300-ISQ instrument.
Catalyst characterization
The nickel contents of the catalysts were analyzed by inductively coupled plasma
spectrometry (ICP) on an Agilent 7500a spectrometer. The Brunauer–Emmett–
Teller (BET) surface area was measured using a Micromeritics Tristar II 3020
surface area and pore analyzer. X-ray diffraction (XRD) patterns were acquired on a
Bruker D8 diffractometer using CuKa radiation. Transmission electron microscopy
(TEM) images of the catalysts were obtained on a FEI Tecnai G2 F20 S-TWIN
microscope. H2-chemisorption and temperature-programmed desorption of hydro-
gen (H2-TPD) were performed using a Micromeritics Autochem II 2920 instrument
to measure the active nickel of the catalysts. FT-IR spectra of catalysts were taken
on a Bruker VERTEX 70 Fourier-transform spectrophotometer.
Results and discussion
Catalyst selection
Firstly, a series of NiB amorphous alloys were prepared by adding different amounts
of DCBS and were tested in the selective hydrogenation of benzophenone to
investigate the effect of DCBS addition. As shown in Table 1, for the NiB catalyst,
the conversion of benzophenone was 73.9% and the selectivity for benzhydrol was
89.0%. It is apparent that the NiB showed lower activity in this transformation
compared to Raney Ni, which is a frequently-used commercial catalyst in
hydrogenation. In particular, with the addition of a small quantity of DCBS during
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Y. Ma et al.
preparation, the catalytic performance of the NiB catalysts obviously improved. For
instance, when the amount of DCBS increased from 0.005:1 to 0.015:1 (molar ratio,
gelator:Ni), the conversion of benzophenone increased from 77.4 to 96.6%. We
assumed that the aggregates formed by DCBS induced good dispersion of active Ni
species, hence accounting for the good activity of the NiB catalysts. Meanwhile, the
selectivity of benzhydrol was decreased slightly from 87.9 to 84.9%. On the other
hand, when NiB-0.02 used as catalyst, the conversion of benzophenone was not
obviously increased, but the selectivity of benzhydrol decreased from 84.9 to
80.9%. The experimental results demonstrated that the three-dimensional networks
were efficient enough to avoid the agglomeration of NiB when the molar ratio of
gelator:Ni is 0.015, while more addition of DCBS may cause a decrease of
benzhydrol selectivity. We ascribed the decrease of benzhydrol selectivity as
attributed to the adsorbed residue DCBS molecules on the surface of catalyst. The
FT-IR spectra of the NiB-x catalysts showed a broad peak centered at 3445 cm-1
corresponding to the asymmetric stretching vibration of –OH groups [26], which
had not been detected in the traditionally prepared NiB sample, indicating a small
amount of residual DCBS molecules on the surface of the catalysts (Fig. S1). In
addition, the peak became larger with an increasing amount of DCBS during
preparation, indicating that more DCBS molecules were adsorbed on the surface of
the catalysts as more DCBS was added to the preparation system. Then, the –OH
group in the DCBS molecules may accelerate the adsorption of benzhydrol through
the hydrogen bond, which will induce dehydration or further hydroganation of the
aromatic ring of the obtained benzhydrol to diphenylmethane or cyclo-
hexyl(phenyl)methanol. GC–MS results demonstrated that the main by-products
were diphenylmethane and cyclohexyl(phenyl)methanol, which is in good agree-
ment with our assumption (Fig. S2). The same result can also be obtained from the
experimental data of selectivity in Table 1. When the amount of DCBS increased
from 0.005:1 to 0.020:1, the selectivity of diphenylmethane increased from 5.9 to
11.1%, and the selectivity of cyclohexyl(phenyl)methanol also increased, from 4.8
to 7.2%, and meanwhile the selectivity of benzhydrol decreased from 87.9 to 80.9%.
The increase of by-product selectivity was almost equal to the decrease of
benzhydrol selectivity, which is also consistent with the assumption. Finally, NiB-
0.015 is proven to have the best performance in benzophenone hydrogenation
among the DCBS self-assembly-assisted NiB catalysts.
Optimization of reaction conditions
The effect of the temperature on the hydrogenation of benzophenone over NiB-
0.015 is illustrated in Fig. 1. The conversion of benzophenone was only 4.8%, with
the selectivity as high as 100% at a lower reaction temperature (313 K). However,
the activity was improved rapidly with the increase of temperature with a slight loss
of the selectivity. The conversion of benzophenone can reach up to 96.6% with the
selectivity of benzhydrol at 84.9% and the temperature was 373 K. In addition, the
selectivity to benzhydrol obviously decreased (dropped to 9.3%) when the
temperature further increased to 393 K. The conversion of benzophenone increased
with the increasing reaction temperature, but the selectivity decreased. The reason
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Hydrogelator as growth-controlling agent for enhancing…
Fig. 1 Effect of temperature on the hydrogenation of benzophenone to benzhydrol (reaction conditions:
benzophenone 2.0 g, catalysts 0.05 g, ethanol 50 ml, initial P (H2) 2.5 MPa, reaction time 60 min,
stirring rate 400 r/min)
for the decrease of selectivity may be that the high temperature promotes
dehydration or further hydrogenation of the benzhydrol. However, the curve about
the yield of benzhydrol showed a trend of decreasing after increasing, and the
maximum value was found at 373 K. During this temperature, the yield of
benzhydrol could reach 82.0%. Thus, 373 K is chosen as the proper reaction
temperature.
The influence of reaction time on the benzophenone hydrogenation is shown in
Fig. 2. It was found that the conversion of benzophenone increased considerably,
Fig. 2 Effect of reaction time on hydrogenation of benzophenone to benzhydrol (reaction conditions:
benzophenone 2.0 g, catalysts 0.05 g, ethanol 50 ml, reaction temperature 373 K, initial P (H2) 2.5 MPa,
stirring rate 400 r/min)
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Y. Ma et al.
from 38.7 to 96.6%, when the reaction time increased from 15 to 60 min, and then
increased slightly to 100% with a further increase of the reaction time to 75 min.
Furthermore, the selectivity of benzhydrol decreased to 44.0% after 75 min.
Prolonging the reaction time promoted the adsorption of substrates on the surface of
the catalyst then increased the conversion of benzophenone. On the other hand, a
longer reaction time also accelerated the adsorption of benzhydrol, which resulted in
the gradual decrease of benzhydrol selectivity. Similar to the discussion of
temperature above, the yield of benzhydrol showed a similar trend of decreasing
after increasing and reached the maximum value at 60 min. Thus, in the view of
yield tendency and transformation efficiency, 60 min has been chosen as the proper
reaction time.
Catalyst characterization
XRD patterns of NiB and NiB-0.015 are shown in Fig. 3. Both of the catalysts
presented one broad peak at around 2h = 45°, which belongs to the characteristic
diffraction peak of metal boride, but no diffraction peaks corresponding to the
crystalline of Ni2B were detected in either sample [27, 28]. The XRD results
confirmed the typical amorphous structure of the prepared catalysts [29, 30],
indicating that the addition of DCBS had not changed the amorphous structure of
the NiB catalysts [31]. Meanwhile, no peaks related to Ni crystal were observed in
the catalysts [32], further indicating the amorphous nature of the Ni.
The nickel content, BET surface area and H2-chemisorption values of NiB, and
NiB-0.015 were measured and the results are listed in Table 2. It was found that the
NiB-0.015 has a larger surface area compared to NiB. With respect to H2-
chemisorption, NiB-0.015 also shows a higher value (0.022 cm2/g) than the
traditional NiB catalyst (0.009 cm2/g), which indicated that the surface of the
Fig. 3 XRD patterns of the amorphous alloy catalysts: a NiB and b NiB-0.015
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Hydrogelator as growth-controlling agent for enhancing…
Table 2 Physicochemical properties of the amorphous alloy catalysts
Sample
Nickel content (wt%) Substrate:metal Surface area (m2/g) H2-chemisorption (cm2/g)
NiB
85.6a
15:1
15:1
13.9
21.3
0.009
0.022
NiB-0.015 85.5a
aBased on ICP results
catalyst became larger with the addition of DCBS during preparation. Furthermore,
both catalysts have the similar nickel content (85.6% for NiB and 85.5% for NiB-
0.015, based on the ICP result). The above results illustrated that, with the addition
of DCBS during preparation, the obtained NiB-0.015 catalyst should have more
active sites compared to the NiB catalyst with similar nickel contents and larger
surface area.
The surface morphology of the NiB and NiB-0.015 catalysts were recorded using
TEM. As shown in Fig. 4, the particles of both catalysts presented an approximately
spherical morphology, which is consistent with the previously reported Ni-based
amorphous alloy catalysts [31]. It should be noted that a cotton-like structure was
obtained from the NiB sample, which resulted from obvious agglomeration of the
catalyst particles. The agglomeration may lead to coverage of some active centers
and thus decreases the hydrogenation activity. Fortunately, this agglomeration could
be inhibited with the addition of DCBS. The particles of NiB-0.015 become well
dispersed and the particle size distribution decreased to around 10 nm. It is worth
noting that, in aqueous medium, DCBS can self-assemble into three-dimensional
networks with the fibers of a few hundred nanometers [26]. The hydrogelator,
DCBS, significantly influenced the morphology and particle size of the NiB
amorphous catalysts, subsequently affecting their surface areas. This is mainly
because the hydrogelator self-assembled in water and formed a three-dimensional
network structure, which could effectively inhibit the agglomeration of NiB
nanoparticles [26]. This feature may be of benefit to the exposure of more active
sites and the rapid transportation of hydrogenation-relevant substrates, which
explains the higher hydrogenation activity and H2-chemisorption value for this
Fig. 4 TEM images of a NiB and b NiB-0.015
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Y. Ma et al.
catalyst. In addition, no evidence for the existence of fibers formed by DCBS was
obtained on the surface of NiB-0.015 from the corresponding TEM image.
Therefore, we propose that most of the DCBS may be removed by washing during
the preparation which avoids the agglomeration of the particles mainly in the
reduction process.
H2-TPD curves of NiB and NiB-0.015 samples are shown in Fig. 5. NiB exhibits
three main peaks at around 347 K, 411 K, and 510 K and a shoulder peak at 551 K,
indicating the presence of four types of absorbing sites on the surface of NiB
catalysts. The NiB-0.015 also exhibited three main desorption peaks centered at
348 K, 421 K and 498 K, while the shoulder peak centered at 551 K in the NiB
sample could not be detected in the NiB-0.015 sample. Furthermore, the two lower-
temperature desorption peaks in NiB-0.015 become smaller and have the tendency
to shift to a higher temperature, with the higher temperature desorption peak also
decreasing to a lower temperature. It can be concluded that the active sites in NiB-
0.015 have a trend to become uniform with the regulating effect of DCBS during the
preparation [33].
Kinetics of catalytic reduction of benzophenone
In order to investigate the mechanism of benzophenone hydrogenation over NiB
catalysts, we have studied the effect of different benzophenone to catalyst molar
ratios and hydrogen pressure on the reaction. As shown in Table S1, when the
catalyst dosage increased under the hydrogen pressure of 2.5 MPa (other reaction
conditions unchanged), the conversion of benzophenone increased from 9.4 to
96.6%, indicating that, with more catalyst added to the reaction system, more active
sites are provided for adsorption of benzophenone thus accelerating the hydro-
genation reaction. According to the Langmuir–Hinshelwood mechanism,
Fig. 5 H2-TPD profiles of a NiB and b NiB-0.015
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Hydrogelator as growth-controlling agent for enhancing…
benzophenone and H2 should both adsorb on the surface of the nanoparticles. When
the concentration of benzophenone is high, more substrates were absorbed on the
surface of the catalysts, which inhibited the absorption of H2, resulting in lowering
the reaction rate. In addition, we also raised the hydrogen pressure from 2.5 to
5.0 MPa under the same benzophenone to catalyst ratio (30:1), and the conversion
of benzophenone increased from 51.3 to 70.9%. These results indicated that higher
concentrations of H2 facilitate its adsorption on the active site, thus facilitating the
benzophenone hydrogenation. Therefore, we conclude that the catalytic hydro-
genation of benzophenone obeys the Langmuir–Hinshelwood mechanism [34, 35].
Conclusion
In conclusion, a novel chemical reduction preparation method was developed for
NiB catalysts by using a hydrogelator as the growth controlling agent. The catalysts
prepared with DCBS showed relatively good activity for benzophenone hydro-
genation, much better than traditional NiB and Raney Ni catalysts. Under the
optimized reaction conditions, the conversion of benzophenone and the selectivity
to benzhydrol were 96.6% and 84.9%, respectively. Based on the results of catalyst
characterizations, it was found that the addition of a hydrogelator during preparation
could make the NiB exhibit larger surface areas, more active centers and smaller
particles, thus enhancing the activity of the corresponding catalysts.
Acknowledgements Financial support from the National Natural Science Foundation of China
(21676068), the Natural Science Foundation of Hebei Province (B2016201167), and Science and
Technology Research Project of Hebei higher education institutions (QN2015037) is gratefully
acknowledged.
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