Development and application of a simple capillary-microreactor for oxidation of glucose with a porous gold catalyst

Article abstract of DOI:10.1039/b414429e

An efficient oxidation of glucose to gluconic acid was performed using a porous gold(0) catalyst in a low-cost microreactor designed from Pyrex glass capillary tubing; compared with the conventional synthesis procedure this novel approach of using a capillary-microreactor offers a convenient and highly efficient means to optimise reaction conditions and catalytic activities.

Full text of DOI:10.1039/b414429e

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Development and application of a simple capillary-microreactor for
oxidation of glucose with a porous gold catalyst{
Chanbasha Basheer,a Sindhu Swaminathan,b Hian Kee Lee*a and Suresh Valiyaveettil*ab
Received (in Cambridge, UK) 21st September 2004, Accepted 21st October 2004
First published as an Advance Article on the web 1st December 2004
DOI: 10.1039/b414429e
An efficient oxidation of glucose to gluconic acid was
performed using a porous gold(0) catalyst in a low-cost
microreactor designed from Pyrex glass capillary tubing;
compared with the conventional synthesis procedure this novel
approach of using a capillary-microreactor offers a convenient
and highly efficient means to optimise reaction conditions and
catalytic activities.
Miniaturised microreactors have many advantages such as
excellent mass and heat transfer properties over conventional
large-scale synthetic methodologies.1,2 Other specific advantages
include inherent safety, high efficiency and rate as well as
opportunities to discover or optimize new reactions, in particular,
reactions which are explosive in nature, and screening of catalysts.3
The microreactor or lab-on-a-chip technology using photolitho-
graphic procedures has been well established for carrying out
reactions on a small scale.1 However, (for heterogeneous catalysis)
Fig. 1 (a) Schematic design of capillary-microreactor and (b) reaction
scheme of D-glucose oxidation to gluconic acid.
the technology is still not well developed due to the requirement of
catalyst immobilization in the channels (either chemical or
Fig. 2 shows the SEM micrographs of the seastar scaffold, the
mechanical bonding of the catalyst)4 and it is prone to failure
scaffold coated with gold and the porous gold sponge after the
from clogging and solvent incompatibilities.
removal of natural template. EDAX analysis was performed to
Ecofriendly catalytic methods for the oxidation of organic
confirm the complete removal of scaffold and no calcium existed
molecules are of growing interest to develop eco-sustainable
on the surface (Fig. 2(d)). These freshly prepared thin metallic
chemical processes.5 The use of gold as a relatively new catalyst for
sponges were used as catalysts for further reaction.
chemical oxidation is interesting owing to its resistance to oxygen
In the capillary-microreactor, the poly(propylene) pipette tips
deactivation.6 Owing to the importance in many applications,
were inserted at both ends of the 5 cm capillary tube and used as
oxidation of glucose to gluconic acid using heterogeneous catalysis
reservoirs. The capillary tube (0.4 mm, internal diameter) and
is important in chemical industries.7
reservoirs A and B were filled with phosphate buffer solution
In this context, we have designed and developed a new capillary
(50 mmol, pH 10) mixed with porous Au(0) catalyst. A few pieces
microfluidic system for oxidation of glucose (Fig. 1(a)). Our design
is focused on a low-cost disposable glass capillary tube (5 cm 6
0.4 mm) reactor with high efficiency for chemical transformations.
A porous gold(0) sponge catalyst was synthesized and utilized for
the oxidation of D-glucose to D-gluconic acid (Fig. 1(b)) in an
aqueous solution at room temperature (ESI{). The reaction yield
was compared with a batch scale oxidation procedure.8
Metallic foams or metallic sponges possess different properties
depending on their manufacturing techniques.9 Various templating
methods are available for the production of macroporous
materials, which includes microemulsions and colloidal crystals.10
Here we employed porous seastar skeleton as a template for
fabricating porous (pore diameter of 10–20 mm) gold catalyst.
Details on the preparation are provided as ESI.{
{ Electronic supplementary information (ESI) available: Details on
experimental procedures and characterization. See http://www.rsc.org/
suppdata/cc/b4/b414429e/
*chmleehk@nus.edu.sg (Hian Kee Lee)
chmsv@nus.edu.sg (Suresh Valiyaveettil)
Fig. 2 SEM micrographs of (a) seastar skeleton, (b) gold coated seastar
template, (c) porous gold after removal of the template and (d) EDX
spectra of the gold sponge.
This journal is ß The Royal Society of Chemistry 2005
Chem. Commun., 2005, 409–410 | 409

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of the porous and low-density catalysts were introduced into the
capillary tube and the reagents were passed through them under an
applied potential. Extreme care was taken to ensure that air bubbles
were absent inside the capillary tube by applying a constant current.
For demonstrating the capability of the reactor, 5 ml (1 mmol) of
glucose was introduced in to the reservoir. Platinum wires were used
as electrodes and a reaction potential of 5 kV and 100 mA current
was applied to reservoir A. Reservoir B was connected to the
ground. Glucose was pumped through the channels using
electrokinetic pumping.11 The reaction was monitored for 40 min
at room temperature and yields were quantified using HPLC with a
refractive index detector. The bulk-scale oxidation of glucose was
performed using a previously reported procedure.8
reaction products obtained for the oxidation of glucose using
porous Au(0) catalyst in the capillary-microreactor and conven-
tional routes are shown in Fig. 3(d). A reaction yield of 99% was
achieved in the capillary-microreactor, whereas the conventional
method yielded only 48% of gluconic acid. In the capillary-
microreactor, it is believed that active sites were generated on the
surface of Au catalyst under an applied voltage and the higher
surface area of the porous sponge catalyst also helped to obtain a
high reaction yield.14
Oxidation of glucose in our capillary-microreactor has proven
to be an attractive alternative to the conventional synthetic
procedures.15 No isomerization of glucose to fructose was
observed during the reaction and total selectivity to D-gluconic acid
was obtained. Owing to the large size and porous nature of
the catalyst, no immobilization of catalyst to the capillary tube
is required.
Analytical parameters affecting the reaction yield such as
reaction time, applied potential and pH were optimized to get a
good yield of 99% for the oxidation. Keeping a constant current
of 100 mA, the reaction yield was determined with respect to time
within the range of 10–50 min. In general, the reaction yield
increases with increase in reaction time up to 40 min and no
further increase in yield was measured at 50 min (Fig. 3(a)). After
optimization of the reaction time, we evaluated the influence of
the applied potential (from 1 to 5 kV) on the yield of the
oxidation. As can be seen, a maximum product yield was
obtained at a potential of 5 kV (Fig. 3(b)).
The low cost of fabrication, disposable nature of the capillary
tube and availability of a wide range of porous catalysts make our
reactor highly efficient, versatile and cost effective. In addition, the
observed high reaction yield, ready applicability of external stimuli
such as voltage or temperature, and the possibility for scale up,
increase the potential of our reactor design.
We gratefully acknowledge the financial support of this research
by the Agency for Science, Technology and Research (ASTAR) of
Singapore and the National University of Singapore.
The influence of pH within the range 2–12 was investigated to
maximize the yield of the reaction (Fig. 3(c)). A pH range of 6–10
gave maximum yield of gluconic acid and slightly alkaline pH was
suitable to increase the reaction yield and to avoid drastic
deactivation of the catalyst and to reduce side reactions.12,13
In order to evaluate the stability and reactivity of the catalyst,
oxidation was repeated five times with the same catalyst under the
optimum reaction conditions (40 min, 5 kV, 100 mA) with 50 mmol
phosphate buffer at pH 10. No significant loss of reactivity was
observed. Compared with conventional synthesis, the microreactor
provided higher reaction yields. The liquid chromatograms of
Chanbasha Basheer,a Sindhu Swaminathan,b Hian Kee Lee*a and
Suresh Valiyaveettil*ab
aDepartment of Chemistry, National University of Singapore, 3 Science
Drive 3, Singapore, 117543. E-mail: chmleehk@nus.edu.sg;
chmsv@nus.edu.sg; Fax: 65 6779 1691; Tel: 65 6874 2995
bNUS-Nanoscience and Nanotechnology Initiative, National University
of Singapore, 2 Science Drive 3, Singapore, 117542
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Fig. 3 Effect of (a) reaction time, (b) applied potential and (c) reaction
pH on D-glucose oxidation in the capillary-microreactor ( , D-glucose; %,
D-gluconic acid), (d) liquid chromatogram of reaction products (1) at 1 kV,
(2) 5 kV and (3) bulk scale. Peak identification: (i) D-gluconic acid and (ii)
D-glucose.
410 | Chem. Commun., 2005, 409–410
This journal is ß The Royal Society of Chemistry 2005