Polymer-supported catalysts for clean preparation of n-butanol

Article abstract of DOI:10.1039/c4cy00436a

A new type of RANEY metal catalyst supported by polymer was developed for the clean preparation of n-butanol. Unlike traditional supported catalysts, the newly developed alkalescent polyamide 6 (PA6) supported RANEY nickel catalyst provided a 100.0% conversion of n-butyraldehyde without producing any detectable n-butyl ether, the main byproduct in industry. The significantly enhanced catalyst selectivity of the polymer-supported RANEY metal catalyst was attributed to the elimination of the acid-catalyzed side reaction associated with RANEY metals and traditional catalyst supports, such as Al₂O₃ and SiO₂. By eliminating acid-catalyzed side reactions, therefore, green chemistry could be achieved through reducing resources and energy consumption in chemical reactions. Furthermore, the preparation and recycling of the polymer-supported catalysts are also much more eco-friendly than for traditional Al₂O₃-/SiO ₂-supported catalysts. The methodology developed in this study to use alkalescent polymers as the catalyst support could be applied to the whole catalyst family, including a series of important RANEY metal catalysts (e.g., RANEY nickel, RANEY cobalt, RANEY copper) used routinely in the chemical industry.

Full text of DOI:10.1039/c4cy00436a

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Polymer-supported catalysts for clean preparation
of n-butanol
ab
Cite this: Catal. Sci. Technol., 2014,
, 2499
4
b
b
b
Haibin Jiang, Shuliang Lu, Xiaohong Zhang, Hui Peng,
b
ab
Wei Dai* and Jinliang Qiao*
Received 8th April 2014,
Accepted 15th May 2014
DOI: 10.1039/c4cy00436a
www.rsc.org/catalysis
A new type of RANEY® metal catalyst supported by polymer was
developed for the clean preparation of n-butanol. Unlike tradi-
tional supported catalysts, the newly developed alkalescent poly-
amide 6 (PA6) supported RANEY® nickel catalyst provided a
have faint acidity, which often leads to side reactions, such
as hydrogenation of n-butyraldehyde to n-butanol, hydrogena-
1
8
tion of adiponitrile to 1,6-hexanediamine and hydrogena-
19
tion reactions of alkenes and alkynes. Without a revolutionary
change in the catalyst support, these side reactions would be
difficult to significantly reduce or even eliminate. In order to
reduce the negative effects caused by the weak acidity of the
1
00.0% conversion of n-butyraldehyde without producing any
detectable n-butyl ether, the main byproduct in industry. The sig-
nificantly enhanced catalyst selectivity of the polymer-supported
RANEY® metal catalyst was attributed to the elimination of the
acid-catalyzed side reaction associated with RANEY® metals and
2 3 2
conventional catalyst supports (e.g., Al O and SiO ), many
investigations have been carried out to neutralize the acidity
of catalyst supports by, for example, adding alkaline agents
2 3 2
traditional catalyst supports, such as Al O and SiO . By eliminating
1
9–26
acid-catalyzed side reactions, therefore, green chemistry could be
achieved through reducing resources and energy consumption in
chemical reactions. Furthermore, the preparation and recycling of
the polymer-supported catalysts are also much more eco-friendly
into conventional supports or reactants.
However, side
reactions caused by the support acidity cannot be completely
avoided as mixing at the molecular level is hardly achieved in
these cases. Besides, the addition of alkalescent agents into
reactants often leads to increased cost, difficult operation,
more byproducts, and possibly a more complicated separa-
tion process.
2 3 2
than for traditional Al O -/SiO -supported catalysts. The method-
ology developed in this study to use alkalescent polymers as the
catalyst support could be applied to the whole catalyst family,
including
a
series of important RANEY® metal catalysts
On the other hand, polymer materials with different struc-
tures possess excellent processibility, recyclability and surface
properties. Therefore, polymer materials could be ideal alter-
natives for replacing current catalyst supports to meet differ-
ent specific demands of catalysts for different chemical
reactions. More importantly, polymer materials could be
chemically functionalized to adjust the alkalinity or acidity at
molecular level to minimize side reactions. As such, polymers
could be optimum catalyst supports for clean chemical reac-
tions as long as the specific surface area of the final catalyst
could be large enough. Fortunately, RANEY® metals with
large specific surface area could be suitable components for
polymer-supported catalysts.
(
e.g., RANEY® nickel, RANEY® cobalt, RANEY® copper) used
routinely in the chemical industry.
The chemical industry has made irreplaceable contributions
to the world, but has also caused negative impacts on our
environment and consumed a huge amount of resources and
energy. Catalysts, as the soul of the chemical industry, play a
key role in reducing these negative effects for the chemical
industry. It is well recognized that environmental pollution
and consumption of resources and energy can be reduced
significantly by improving catalyst selectivity to minimize
side reactions. It is well known that 90% of chemical
reactions in chemical industry are based on heterogeneous
In this work, polymer-supported RANEY® Ni was studied
for the clean preparation of n-butanol, which, with a world-
wide consumption of more than 3 million tons per year, is
widely used as a solvent and a raw material for other
1
catalytic processes, and the majority of industrial catalysts
are supported catalysts. While Al O (ref. 3–11) and SiO2
2
2
3
(
ref. 12–17) are the two major catalyst supports, they both
2
7
chemicals. Commercially, n-butanol is mainly obtained
from hydrogenation of n-butyraldehyde (obtained from the
oxo reaction of propylene) in the presence of a hydrogenation
a
College of Materials Science and Engineering, Beijing University of Chemical
Technology, Beijing 100029, China. E-mail: qiaojl.bjhy@sinopec.com
b
SINOPEC Beijing Research Institute of Chemical Industry, Beijing 100013,
2
8
China. E-mail: daiw.bjhy@sinopec.com
catalyst. The most widely used commercial hydrogenation
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catalyst is Al O -supported Ni catalyst. Reaction (1) shows the
2
3
hydrogenation reaction of n-butyraldehyde to n-butanol,
whereas reaction (2) represents the main side reaction, which
generates n-butyl ether. In order to separate n-butyl ether
from n-butanol, a large amount of energy is required because
an azeotrope is formed. For the reduction of pollution and
energy and resource consumption in n-butanol production, it
is important to eliminate this side reaction. It is well known
that reaction (2) is an acid-catalyzed reaction and the yield of
n-butyl ether increases with increasing the acid strength of
2
9,30
the catalyst.
Use of a non-acid support is expected to
reduce or even eliminate the side reaction associated with
the acidity of the catalyst support.
(
(
1)
2)
In this study, a neutral polymer, polypropylene (PP), was
selected as the first polymer support for RANEY® Ni catalyst.
Compared with the traditional Al O -supported Ni catalyst,
the preparation of the PP-supported catalyst is relatively
simple and energy efficient. In this experiment, PP granules
2
3
Fig. 1 (a) Schematic representation of the process of Ni–Al alloy
particles embedding in the polymer surface. (b) Photograph of Ni–Al/
PP granules. Inset shows the cross-sectional view of a cut sample.
(
F280M, Sinopec Maomin Company) were buried in a
scattered manner into a full mould of Ni–Al alloy powder
(
(
48 wt.% Ni). The mould was then compressed tightly
2 MPa) and heated to 200 °C. The heating caused the PP to
flame ionization detector (Agilent 7890, DB-WAX, FID). The
residual n-butyraldehyde content and n-butyl ether content
were determined with an external standard to indicate the
activity and selectivity of the catalysts.
The numerical results from the hydrogenation of
n-butyraldehyde at 100–140 °C with the three different cata-
lysts, RANEY® Ni/PP catalyst, RANEY® Ni/MAHPP catalyst,
and Ni/Al O catalyst, are listed in Table 1. It can be seen
2 3
from the table that n-butyl ether content over the RANEY®
Ni/PP catalyst is nearly one order of magnitude lower than
that of both the RANEY® Ni/MAHPP catalyst and Ni/Al O
2 3
melt and expand and fill the surrounding gaps amongst the
alloy powder. In other words, Ni–Al alloy powders were
embedded into the surface of the expanded PP particles (see
Fig. 1a). Thereafter, the mould was cooled down, leading to
the formation of the special granule (Ni–Al/PP), in which the
Ni–Al alloy particles were embedded into the PP granule
surface, as shown in Fig. 1b. After sieving out Ni–Al/PP
granules from the excess Ni–Al alloy powder, a PP-supported
RANEY® Ni catalyst (RANEY® Ni/PP) was obtained by
alkaline leaching of the Ni–Al/PP.
For comparison, maleic anhydride grafted PP (MAHPP)-
supported RANEY® Ni catalyst (RANEY® Ni/MAHPP) and
Al O -supported Ni catalyst (20 wt.% Ni/Al O ) were also pre-
2 3 2 3
Table 1 Hydrogenation of n-butyraldehyde with different catalysts
over 100–140 °C
pared. MAHPP (GPM200AL, with 1 wt.% MAH) was pur-
chased from Ningbo Nengzhiguang Company, and RANEY®
Ni/MAHPP catalyst was prepared through the same procedure
as that for the RANEY® Ni/PP catalyst. The Ni/Al O catalyst
Catalyst
T (°C) Conversion (%) n-Butyl ether (wt.%)
RANEY® Ni/PP
100
110
99.99
100
0.013
0.053
0.095
0.499
0.300
0.632
1.049
1.843
0.159
0.292
0.677
1.706
0.000
0.000
0.015
0.016
1
1
20
40
100
100
2
3
was prepared according to a commercial impregnation
method, using Al and Ni(NO ·6H O solution, dried at
20 °C for 12 h, calcined at 360 °C for 4 h and reduced at
RANEY® Ni/MAHPP 100
99.99
2
O
3
3
)
2
2
110
120
100
100
100
100
100
100
100
1
4
1
1
40
100
10
120
40
100
00 °C for 8 h. The catalytic reaction was performed in a
4 mm (internal diameter) tube microreactor made of stain-
Ni/Al2O3
1
less steel under a pressure of 4.0 MPa and at a temperature
ranging from 100 to 140 °C. The amount of catalyst used was
1
RANEY® Ni/PA
99.99
2
0 ml. The flow of n-butyraldehyde was controlled by a
1
1
140
10
20
100
100
100
−
1
micro-syringe pump with a flow rate of 30 ml h . The prod-
uct was analyzed by gas chromatography equipped with a
2500 | Catal. Sci. Technol., 2014, 4, 2499–2503
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catalyst at the same temperature, indicating an excellent
selectivity for the RANEY® Ni/PP catalyst. More specifically,
the n-butyl ether content is 0.053 wt.% for the RANEY® Ni/PP
catalyst at 100% conversion while the corresponding n-butyl
ether content increased significantly to 0.632 and 0.159 wt.%,
Fig. 2 Chain structure of PA6.
2 3
respectively, for the RANEY® Ni/MAHPP and Ni/Al O cata-
lysts under the same conditions. Clearly, therefore, the neu-
tral support PP did effectively reduce the acid-catalyzed side
reaction with respect to the acidic supports (i.e., MAHPP and
Al O ). However, the RANEY® Ni/PP catalyst didn't eliminate
was neutralized by the basic PA6; and 2) there was no adsorp-
tion of n-butanol by the acidic Al O associated with the
2
3
2
3
the n-butyl ether byproduct completely, though the fraction
of the byproduct is very small. To understand why a small
fraction of the byproduct (n-butyl ether) is still produced by
the RANEY® Ni/PP catalyst with a neutral support, we further
carried out element analysis by X-ray photoelectron spectrom-
etry (XPS). The element analysis results are given in Table 2,
which shows the presence of Al O in the RANEY® Ni/PP cat-
RANEY® Ni in the RANEY® Ni/PA catalyst because the
N atom in the PA6 support has a higher affinity to n-butanol
than the Al atom in Al O . However, our XPS measurements
2
3
confirmed that the basic N atom in the PA6 cannot affect the
acidity of the Al atom because of the relatively large inter-
molecular distance. Indeed, the XPS Al 2s peaks of the
RANEY® Ni/PA and Ni/Al O catalysts were located at 74.03
2
3
2
3
alyst. This is consistent with the literature report that
and 73.93 eV, respectively, which are almost the same within
the experimental error and indicate no charge-transfer (base–
acid neutralization) interaction between the basic PA6 and
RANEY® Ni usually contains a small fraction of Al O
2
3
1
3
because of the incomplete leaching of Al in Ni–Al alloy.
Therefore, it is the Al
2
O
3
in RANEY® Ni that is responsible
2 3
acidic Al O in the RANEY® Ni. On the other hand, it is
for the small fraction of n-butyl ether produced by Ni/PP cata-
lyst with a neutral polymer support.
well known that the adsorption ability of a catalyst
support to reactants and products can largely affect the cata-
1
8,23
In order to further reduce the acid-catalyzed side reaction,
an alkalescent polymer, polyamide 6 (PA6, BL2340-H, Sinopec
Baling Company), with lone pair electrons at the N atom in
every repeating unit (Fig. 2) was selected as a new support to
replace PP. The RANEY® Ni/PA catalyst was prepared through
the same procedure as the RANEY® Ni/PP catalyst except that
the compressing temperature was increased to 250 °C.
The experimental results from the RANEY® Ni/PA catalyst
are listed also in Table 1. As can be seen, the n-butyl ether
content in final product generated by RANEY® Ni/PA catalyst
is undetectable at 100 and 110 °C, whilst remaining very low
lytic reactivity.
The PA6 support possesses strong adsorp-
tion ability to n-butanol since the N atoms in PA6 can form
hydrogen bonds with the –OH groups in n-butanol. There-
fore, the following process might have occurred. Once pro-
duced from n-butyraldehyde catalyzed by Ni metal, n-butanol
was selectively adsorbed by N atoms in the PA6, rather than
the acidic Al atoms in the RANEY® Ni. Unlike the acidic Al
atoms in Al O , the basic N atoms in the PA6 support
2
3
cannot catalyze n-butanol to n-butyl ether. For the RANEY®
Ni/MAHPP catalyst, maleic anhydride in the MAHPP support
can also selectively adsorb n-butanol over Al O . However, the
2
3
(
0.015 and 0.016 wt.%) even at 120 and 140 °C, respectively.
acidic maleic anhydride can effectively catalyze n-butanol to
n-butyl ether, leading to the formation of even more n-butyl
ether by the RANEY® Ni/MAHPP catalyst with respect to the
It is worth noting that the n-butyl ether content in the final
product from the RANEY® Ni/PA catalyst at 120 °C and
above is close to one order of magnitude lower than that for
the RANEY® Ni/PP catalyst and nearly two orders of magni-
tude lower than that for both the RANEY® Ni/MAHPP and
2 3
Ni/Al O catalyst (Table 1). Clearly, therefore, it is the inter-
play of the alkalinity and strong adsorption ability to
n-butanol intrinsically associated with the N atoms in the PA
support that makes the clean preparation of n-butanol possi-
ble by the RANEY® Ni/PA catalyst. The relationship between
the alkalinity or acidity of the catalyst support and the
byproduct content (n-butyl ether) are summarized in Table 3.
We have performed SEM imaging to further elaborate the
difference between the RANEY® Ni/PA catalyst and Ni/Al O
2 3
Ni/Al O catalysts. Of particular significance, Table 1 shows
that clean preparation of n-butanol with a 100% conversion
and undetectable n-butyl ether can be achieved with the
PA-supported RANEY® Ni catalyst at a relatively low temper-
ature (110 °C).
To understand why the PA6 support can reduce the side
2
3
reaction caused by the Al
two possibilities: 1) the acidity of Al
2
O
3
in the RANEY® Ni, we consider
in the RANEY® Ni
catalyst. As can be seen in Fig. 3, they showed quite different
surface morphologies and different porosities. The BET
specific surface area of the RANEY® Ni/PA catalyst is only
2
O
3
2
−1
4
.5 m g , much lower than that of the Ni/Al
2
2
O
3
−1
catalyst,
Table 2 Element analysis results of the RANEY® Ni/PP, RANEY® Ni/PA
which is usually in several tens to hundreds m g . At first
glance, it seems strange to see that the more active polymer-
supported catalyst has a lower specific surface area than that
and Ni/Al O
2 3
catalyst surfaces by XPS
Catalyst
C (at.%) O (at.%) N (at.%) Al (at.%) Ni (at.%)
2 3 2 3
of the Ni/Al O catalyst. Unlike the Ni/Al O catalyst, however,
most of the surface area of the RANEY® Ni/PA catalyst is cov-
ered by active Ni component, as indicated by XPS data in
RANEY® Ni/PP 55.3
RANEY® Ni/PA 38.9
30.8
43.3
46.0
—
1.4
—
3.2
3.8
31.1
10.7
12.6
3.8
Ni/Al O
2 3
19.1
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Table 3 The relationship between the property of catalyst support and the byproduct content
Organic support with acid or alkaline group which
Inorganic support
Al (acidity)
Organic support
can adsorb n-butanol
Support with alkalinity or acidity
n-Butyl ether content
2
O
3
PP (neutral)
Low
PP-g-MAH (acidity)
Very high
PA6 (alkalinity)
Very low to undetectable
High
supported catalysts do not need calcination at high tempera-
tures and hydrogen reduction, as required by traditional
Al O -supported catalysts; therefore, energy and hydrogen
2
3
consumption can also be reduced. Furthermore, the recycling
of polymer supported catalysts needs only calcining in air to
remove polymer and the remaining metal alloys could be
reused directly. In contrast, recycling of the traditional
supported catalysts needs nitro-hydrochloric acid, which pro-
2 3
Fig. 3 SEM images of (a) the Ni/PA catalyst and (b) the Ni/Al O
catalyst.
duces a lot of NO and the acid-solubilized metals are diffi-
x
cult to separate. Because a series of important RANEY® metal
catalysts, including but not limited to RANEY® nickel,
RANEY® cobalt, and RANEY® copper, are routinely used in
the chemical industry, we believe that the use of polymers as
catalyst supports can be applied to the whole catalyst family
to achieve green chemistry by eliminating side reactions and
cutting down resources and energy consumption caused by
side reactions in chemical industries.
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catalyst supports. The polymer support can provide more eco-
friendly catalyst preparation and recycling processes than
those of traditional Al O and SiO2 supports. Polymer
2
3
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