Electrocatalytic oxidation of n-propanol to produce propionic acid using an electrocatalytic membrane reactor

Article abstract of DOI:10.1039/c3cc41181h

An electrocatalytic membrane reactor assembled using a nano-MnO₂ loading microporous Ti membrane as an anode and a tubular stainless steel as a cathode was used to oxidize n-propanol to produce propionic acid. The high efficiency and selectivity obtained is related to the synergistic effect between the reaction and separation in the reactor.

Full text of DOI:10.1039/c3cc41181h

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Electrocatalytic oxidation of n-propanol to produce
propionic acid using an electrocatalytic membrane
reactor†
Jiao Li,a Jianxin Li,*a Hong Wang,a Bowen Cheng,a Benqiao He,a Feng Yan,a
Yang Yang,a Wenshan Guob and Huu Hao Ngob
Cite this: Chem. Commun., 2013,
49, 4501
Received 13th February 2013,
Accepted 28th March 2013
DOI: 10.1039/c3cc41181h
www.rsc.org/chemcomm
An electrocatalytic membrane reactor assembled using a nano- conductivity and mass transfer on the surface of the electrode as
MnO2 loading microporous Ti membrane as an anode and a well as the separation of reactants and products.4 How to enhance
tubular stainless steel as a cathode was used to oxidize n-propanol the efficiency and selectivity of electrochemical reaction has
to produce propionic acid. The high efficiency and selectivity become the most critical problem in the field of organic electro-
obtained is related to the synergistic effect between the reaction chemical synthesis. A variety of new strategies such as reaction
and separation in the reactor.
media, reaction conditions, and electrochemical devices have been
employed to tackle this challenging issue.5 For example, Bianchini
et al.6 reported selective conversion of ethanol into potassium
acetate in both passive and active direct fuel cells containing
platinum-free electrodes and an anion-exchange polymer
membrane. The yield of potassium acetate was 46.51%. Despite
all the efforts, only a limited number of organic electrochemical
processes have been developed in the industry so far.
As an important chemical, propionic acid is broadly applied as food
and feed preservatives, herbicides, therapeutic agents, perfumes,
inhibitors, liquid crystal mixtures etc.1 Industrial manufacture of
propionic acid is either by chemical routes or by biological fermen-
tation. Chemical routes include carbonylation of ethylene, oxidation
of propanal and direct oxidation of hydrocarbons. However, these
techniques have various disadvantages such as drastic conditions
(high temperature and pressures), more than one synthetic stages
and/or production of mixture that requires a further complex work-
up.1 Propionic acid and its salts can also be obtained by bio-
synthesis with the advantage of the method being environmentally
friendly and having low disposal costs. However, the severe inhibi-
tion of end-products during cell growth and the formation of
by-products make its production far from satisfactory.2
With the ever-increasing need for a sustainable chemical pro-
cess, tremendous attention has been paid to efficient, environ-
mentally friendly methods for production of organic molecules.3 It
is noteworthy that the electrochemical method, involving electron
transfer without the use of any oxidizing or reducing reagents,
serves as an alternative method for the formation of useful organic
compounds under mild conditions.4 Nevertheless, the application
of the electrochemical method in organic synthesis still remains an
underdeveloped field, because the efficiency and selectivity of
electrochemical reactions are limited as a result of weak cell
Apart from the above-mentioned methods, the membrane-based
separation and transport of the products and/or the reagents from or
toward the reactor increase the yield and/or the selectivity of some
processes and maximize the synergistic effects between the reaction
and separation.7 In addition, the membrane used to immobilize a
homogeneous catalyst could realize the catalyst recovery, regenera-
tion, and reuse in successive catalytic runs.8 Recently, a simple
strategy employing an electrocatalytic membrane reactor (ECMR)
with a self-cleaning function has been designed for industrial waste-
water purification.9 A nano-TiO2 loading porous conductive carbon
membrane functioned both as a filter and an anode to treat oily
wastewater in an ECMR. The TiO2/carbon membrane generated
microflows and reactive intermediates that alleviated concentration
polarization and decomposed organic foulants into CO2 and H2O or
small biodegradable products on the membrane surface or in pores.
To solve the challenging problem of the efficiency and selectivity
in the electrochemical reaction, we propose an ECMR with a func-
tional tubular microporous Ti membrane as both a separation barrier
and an anode and a tubular stainless steel as a cathode connected by
a DC regulated power supply to be used to produce propionic acid
from n-propanol under atmospheric pressure and mild temperature
(Fig. 1). In the reactor, n-propanol permeates the membrane from
outside-in and propionic acid is obtained from the inside.
a State Key Laboratory of Hollow Fiber Membrane Materials and Processes,
School of Materials Science and Engineering, Tianjin Polytechnic University,
Tianjin 300387, P. R. China. E-mail: jxli0288@yahoo.com.cn;
Fax: +86-22-8395 5055; Tel: +86-22-8395 5798
b Centre for Technology in Water and Wastewater, School of Civil and
Environmental Engineering, University of Technology Sydney, PO Box 123,
Broadway, NSW 2007, Australia
A tubular microporous Ti membrane was employed as a con-
ductive substrate because of its significant mechanical strength, good
† Electronic supplementary information (ESI) available: Experimental details, charac-
terization data, GC-MS analyses and reaction mechanism. See DOI: 10.1039/c3cc41181h chemical stability and specific conductivity.10 Manganese dioxide was
c
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Fig. 1 Schematic diagram of the ECMR system.
used as a catalytic layer of the electrode as it is excellent in oxidation
and reduction and chemically stable.11 A novel functional nano-
MnO2 loading porous conductive Ti membrane (MnO2/Ti) with a
MnO2 loading of 3.6 wt% was prepared via a sol–gel approach i.e.
thermal decomposition of manganese(II) nitrate on the original
titanium membrane (Experimental section in the ESI†).12
Fig. 2 Concentration of feed and permeate compositions versus reaction time
at different residence times and temperatures (a–h: the reaction temperature at
25 1C; i: 50 1C).
The morphology and compositions of the MnO2/Ti membrane
Further, the concentration of n-propanol in the reactor
were characterized by a HRTEM image taken from the edge of the decreased slightly from 165 to 156 mmol LÀ1 at 0.54 min of
MnO2 bipyramids as shown in Fig. S1 (ESI†). The marked lattice the residence time and the concentration of propionic acid was
fringes observed with an interplanar spacing of 0.31 nm are kept at about 0.01 mmol LÀ1 (Fig. 2b). It implied that the
ascribed to the interplanar spacing (110) plane for the tetragonal concentration of the reactant – n-propanol was maintained at a
b-MnO2.13 Moreover, X-ray diffraction analysis and X-ray photo- high level and the concentration of the product – propionic acid –
electron spectroscopy revealed that the manganese oxide particles was quite low during the ECMR operation. In the permeate, the
(most of b-MnO2 and a little of Mn2O3 crystallites) were well concentration of n-propanol was 135.66 mmol LÀ1, and about
distributed on the membrane surface (Fig. S2 and S3, ESI†). With 9.30 mmol LÀ1 propanal and 6.50 mmol LÀ1 propionic acid
respect to oxygen transfer, MnO2 is more active than Mn2O3.14 The were produced by the oxidation of n-propanol, respectively.
mean diameter of MnO2 crystallites was approximately 22.7 nm
Upon further increasing the residence time from 3.22 to
(Fig. S2-1, ESI†). In addition, the well-resolved lattice fringe 22.55 min at 25 1C, the concentration of n-propanol in the reactor
obtained from the MnO2 power represented the high crystallinity remained quite stable in the range of 167.29 to 150.91 mmol LÀ1
of the MnO2 bipyramid, which was strongly related to the electro- (Fig. 2c–h), and only traces of propionic acid (0–2.64 mmol LÀ1) were
chemical performance of the MnO2/Ti membrane (Fig. S4, ESI†).
detected. It suggested that the produced propionic acid might flow
In the reactor, 160 Æ 5 mmol LÀ1 n-propanol solution through the membrane into the permeate. In the permeate, the con-
(500 mL) and 15 g LÀ1 Na2SO4 were used as the reactant and centration of n-propanol dropped from 114.32 to 42.99 mmol LÀ1
,
the aqueous electrolyte, respectively. The ECMR was operated in whereas the concentration of propionic acid increased from
the dead-end mode and the operating voltage was 2.8 V, which 12.94 to 104.09 mmol LÀ1. Besides, an interesting pheno-
was the optimum value (Table S1, ESI†). A suction pump (BT100-2J) menon was that only a small amount of propanal in the range
was used to control the residence time (RT). The residence time of 12.55–22.71 mmol LÀ1 remained in the permeate.
(min) was defined as the ratio of the volume of the membrane pore
(cm3) to the flow rate of solution (mL minÀ1).15
When the reaction temperature increased from 25 to 50 1C at a
residence time of 22.55 min, the concentration of propionic acid in
In general, oxidation of n-propanol leads to the production the permeate increased from 104.09 to 152.5 mmol LÀ1, while the
of propanal and propionic acid (Fig. S5 and Table S2 in the concentration of n-propanol decreased from 42.99 to 2.59 mmol LÀ1
ESI†), which were determined using GC.16 Changes in the (Fig. 2i). In the meantime, it was found from the carbon molar
concentration of feed and permeate compositions with RT from mass balance before and after the electrocatalytic reaction that only
0 min to 22.55 min are illustrated in Fig. 2.
a little amount of n-propanol (0.85–21.83 mmol) might be com-
As shown in Fig. 2a, when the residence time was 0 min, namely pletely mineralized into CO2 and H2O or reacted with the generated
the ECMR is similar to a conventional electrocatalytic reactor acid to produce propyl propionate by esterification during the
without any permeation, the concentration of n-propanol in the ECMR operation (Table S3, ESI†).
membrane reactor decreased from 157 to 143 mmol LÀ1, and the
Changes in the conversion of n-propanol and selectivity to
concentration of propanal and propionic acid increased from 0 to propionic acid versus residence time and reaction temperature in
0.13 mmol LÀ1 and 0.26 mmol LÀ1, respectively, during the 75 min the ECMR are plotted in Fig. 3. 60.77% and 56.82% improvements
operation at 25 1C. Thus, the corresponding conversion and in the n-propanol conversion (from 16.46% to 77.23%) and the
selectivity in the conventional electrocatalytic reactor were very selectivity to propionic acid (from 12.42% to 69.24%) were achieved
low, approximately 7% and 5%, respectively (Fig. S6, ESI†). The with an increase in the residence time from 0 min to 22.55 min,
reason was that the initial electrocatalytic oxidation of n-propanol respectively. The results also confirmed that the longer residence
on the surface of the membrane electrode constrained the diffu- time the reaction was carried out for in the ECMR, the more
sion in the reactor, thereby preventing a new oxidation reaction.17 amount of n-propanol was oxidized to propionic acid (Fig. 2b–h).
c
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represents a pair of adjacent Mn and O atoms at the surface
of the MnO2 matrix. Electrocatalysis takes place by successive
generation and consumption of a Mn(V) species (Fig. S8, ESI†).
Normally, an electrocatalytic membrane reactor is a piece of
chemical equipment that combines a catalyst-filled reaction
chamber with a membrane in order to add reactants or remove
products or by-products of the reaction. In ECMR operation,
the final product propionic acid and low concentration of the
intermediate product proponal were removed through the
membrane in the reaction zone so as to enhance the conversion
and selectivity due to the forced across flow-mode.7 The max-
imum filtration rate was 5.00 ml minÀ1 at a residence time of
0.54 min (Table S4, ESI†). Meanwhile, as the larger surface area
Fig. 3 Conversion of n-propanol and selectivity to propionic acid versus resi-
dence time and reaction temperature.
With the reaction temperature rising from 25 to 50 1C, the
conversion of n-propanol and the selectivity to propionic acid
reached the highest value of 98.44% and 79.33%, respectively. It
revealed that a higher reaction temperature could promote the
collision of the reactants, thereby enhancing the reaction rate.18
Therefore, the results revealed that the residence time and reaction
temperature play the key role in the oxidation process of n-propanol
as well as the production of propionic acid in the ECMR. Besides,
the production efficiency of propionic acid is governed by the
synergistic effect between membrane separation and electrocatalytic
oxidation in the ECMR.9
Since the selection of electrode material was essential to electro-
catalytic oxidation processes, oxide electrodes are likely to be
suitable for carbohydrate oxidation because these molecules are
expected to get adsorbed on the oxide layer easily through formation
of hydrogen bonds via the OH groups, and facilitate electron
transfer. Thus, MnO2 can be considered a good electrocatalyst
owing to the fact that it possesses a number of higher valent oxo-
manganese species such as Mn(IV) and Mn(V), which are generally
strong chemical oxidants.19 During the ECMR operation, the TiO2
catalyst formed during the preparation processes of Ti and MnO2/Ti
membranes (Fig. S2 and S3, ESI†) was electrified to generate the
electron–hole pair, which interacted with H2O to produce absorbed
hydroxyl radicals on the membrane surface [eqn (1)].20 Then the
absorbed hydroxyl radicals interacted with the oxygen of the MnO2
lattice in the MnO2/Ti anode to generate a higher valent oxo-
manganese species (MnVQO) oxide [eqn (2)].21 As a strong chemical
oxidant, the higher valent oxo-manganese species reacted with the
absorbed n-propanol molecule to produce propanal or propionic
acid by a redox reaction or electron transfer [eqn. (3 and 4)].22 At the
same time, the higher valent oxo-manganese species could be
consumed chemically by organics and reduced to MnO2 (MnIVQO),
which continuously interacts with the absorbed hydroxyl radicals to
form a higher valent oxo-manganese species.21
of the MnO2/Ti membrane (230 m2 À1) could intensify the
g
contact between reactants and catalyst, the forced convection
and diffusion promoted the n-propanol molecule to transport
to the surface of the MnO2 catalyst during the ECMR operation.7
As a result, the synergy between electrocatalytic oxidation and
membrane separation led to the high effectivity and selectivity of
propionic acid production from n-propanol.
In conclusion, a novel functional nano-MnO2 loading porous Ti
membrane was prepared and employed as the anode to produce
propionic acid from n-propanol in an ECMR. Propionic acid could
be produced effectively by electrocatalytic oxidization of n-propanol
by controlling the residence time and reaction temperature. Such
an ECMR described here would have a wide range of potential
applications in the field of electrochemical organic synthesis.
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(3)
(MnVQO) + CH3CH2CHO + H2O
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c
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Chem. Commun., 2013, 49, 4501--4503 4503