Journal of Minerals & Materials Characterization & Engineering, Vol. 2, No.2, pp 101-110, 2003, printed in the USA. All rights reserved
Microwave-assisted wet chemical synthesis: advantages, significance, and
steps to industrialization
Shangzhao Shi and Jiann-Yang Hwang
Institute of Materials Processing, Department of Materials Science and Engineering
Michigan Technological University
Houghton, MI 49931
Previous research has revealed several advantages from microwave-assisted
wet chemical synthesis in reaction acceleration, yield improvement, enhanced
physicochemical properties and the evolvement of new material phases. The
study present examples that demonstrate the significance of these advantages to
industrial application. In order to achieve successful industrial application there
is a need to distinguish between the microwave athermal (not excited by heat)
effect from the microwave-induced thermal effect (temperature rise). The
optimization of this new process has to be systematically investigated, so the
advantages and benefits of this new technology can be fully exploited.
Advantages and significance
based magnetic materials
based magnetic materials have been extensively used as recording materials because
of their good magnetic properties. The emergence of nanomagnetic technology has imposed new
demands upon magnetic materials. To fabricate ultrahigh density and ultrahigh speed data storage
devices, which work on the magnetic-spintronic concept, magnetic materials with nanoscale
particles are highly preferred
Conventionally, ?-Fe
powders with monodispersed particles are synthesized first by
means of forced hydrolysis of ferric nitrate or ferric chloride in aqueous solutions, which
produces α-Fe
, and then the α-Fe
are transformed into ?-Fe
in a high-temperature
2, 3
. To effectively control the particle shape and particle size, which are essential for
achieving the desired magnetic properties, the hydrolysis solution must be very dilute. For
producing micron- and submicron-sized particles, the concentrations of the ferric irons are
generally within a range around 0.02M
2, 4, 5
and for producing nanosized powders, the ferric
concentration should be much lower. In addition to dilute solutions, the conventional hydrolysis
has to be carefully controlled and it generally requires 2-7 days. This means that, even based upon
the shortest processing time and highest concentration, the rate for producing the nanosized iron
oxide powders would be less than 0.014 gram per liter per hour. In order to use nanosized
magnetic materials for industrial applications, appreciable acceleration of the process and an
increase in the ferric concentration are required.
Several researchers have attempted the iron oxide synthesis via microwave-assisted
hydrolysis. Komarneni et al demonstrated that under otherwise identical processing conditions,
synthesis of crystalline hematite by a microwave-hydrothermal approach was 36 times faster than
by conventional hydrothermal methods
. Our study shows the capability of controlling the
particle shape and particle size with this rapid synthesis approach. Rigneau et al further shortened
the processing time to 30 minutes and also increased the ferric concentration to 0.05M
, which
could increase the production rate over one hundred times. The even more striking finding from
their results was that the microwave synthesized Fe
particles were nano scale and directly ?-
type. The significance of this finding is that it could not only remarkably simplify the synthesis
procedure, but also get rid of the difficulties involved with the calcination of nano powders, and
Author to whom correspondence should be addressed
102 Shangzhao Shi and Jiann-Yang Hwang Vol.2, No.2
avoid the detrimental microstructure changes that accompany the transformation from α-Fe
in the conventional high temperature process.
Insertion electrode materials
The role of intercalation/insertion reactions in battery electrodes was first recognized about
30 years ago. From the first prototype titanium disulfide cells, the technology has more recently
been commercialized in Li-ion cells using a cobalt oxide insertion cathode and a carbon insertion
anode. This technology has proven highly successful in small devices. Since the conduction
species intercalated in the structure exhibit very good electrical conductivity, insertion electrode
materials are also considered most promising in large device and large-scale applications
The extensive research and development on new insertion materials can be recognized from
Whittinghan’s Work. His investigation has covered a diverse group of layered and intercalated
compounds including titanium disulfide
9, 10
lithium cobalt oxide
, tetramethylammonium
intercalated vanadium oxides
layered-structure manganese dioxides
and vanadium-pillared
manganese oxide structures
. More recently, the group is investigating vanadium oxide
nanotubes, which is considered to have significant advantages due to its distinct electrical contact
regions and electrolyte-filled channels
The insertion materials are normally synthesized via conventional hydrothermal method. The
synthesis is a tedious procedure and often takes several days to a week. The use of the microwave
method has been attempted to accelerate the synthesis of layered vanadium oxide inserted with
tetramethylammonium ions
. It was found that this method could reduce the synthesis time to 30
minutes and also found that longer microwave hydrothermal treatment could lead to a new
material structure. The adoption of the microwave method to the insertion material synthesis
could not only develop a highly efficient, low cost process for synthesis of insertion materials
synthesis, but also offer chances to generate new material structures that could not be obtained
from conventional methods.
Molecular sieves
Crystalline molecular sieves have intraframework cages and channels of uniform
microporous or mesoporous size, tailor-made acidity or basicity, and high thermal stability.
Owing to these unique structures, the materials have acquired various applications in the
petroleum refining and petrochemical industry
. The advent of nanotechnology has provided a
new field for their application. The materials can be used as a high performance nanostrucutured
host for preparation of advanced materials that exhibit specific optic, optoelectronic, and
electrochemical properties suitable for molecular wire, quantum electronics and non-linear optical
devices. A number of preparations of nanoscale materials on such a nanostructured template have
been demonstrated including the synthesis of organized metal clusters, metal oxides or sulfides,
isolated conducting polymers as well as confined supermolecular compounds
The structure of the molecular sieve frameworks is of great diversity. It is sensitive to
synthesis conditions. Generally, it takes several days to several weeks for obtaining the desired
material. Raising the temperature may shorten the synthesis time but could result in different
structures. There are often several structures concurrently in development. Although each
structure may have a dominating stage, the product is more liable to mixed structures
. The
process is often complicated and time-consuming for synthesizing a pure product with tailored
Microwave-assisted synthesis has been attempted on a certain types of molecular sieves.
Significant kinetic acceleration has been demonstrated in synthesis of MgAPO-5 (20min
microwave vs 24h conventional) by Cresswell et al
and in synthesis of MCM-41 (4h microwave
vs 7-14d conventional) by U. Oberhagemann et al
. The microwave method is also produces
high surface area structures. While multi-point surface area of the conventional synthesized
MgAPO-5 is only 4.48 m
, that of the microwave synthesized MgAPO-5 attains 34.97 m
Vol.2, No.2 Microwave-assisted wet chemical synthesis 103
about 7 times surface enlargement. The microwave method further shows its effectiveness in
producing pure structures, as were the cases for the synthesis of AlPO
-11 and the synthesis of
cloverite (Park et al
). In these instances, the microwave method was able to obtain these pure
crystalline phases in 2.5h and 2.7h respectively, whereas over 1-day conventional preparation
produced AlPO
-11 mixed with some amount of AlPO
-31 and a trace of AlPO-tridymite, or
produced cloverite with unreacted reactants. Further advantage of the microwave approach was
found in the ease of processing stage control. Carmona et al
investigated the microwave-assisted
synthesis of VPI-5 along with conventional approaches. With microwave-activated refluxing
method, pure VPI-5 was the only product of the process. With other methods, however, several
phases appeared as the processing proceeded.
Organometallic compounds
Organometallic compounds are generally defined as having a metal-carbon bond with
properties distinct from those of inorganic compounds. Orgaometallic compounds cover a large
number of varieties, which contain many kinds of metal elements. Some groups have high
reactivity and reaction selectivity and are used as various kinds of catalysts or as organosynthetic
reagents. Others have chemical stability and have acquired their applications in microbiocides,
pesticides, anticancer agents, water repellants, octane number improvement, and antifoaming and
mold releasing. Organometallic compounds are also employed as precursors for preparing
ultrahigh purity metals and functional ceramics with tailored compositions and structures, such as
semiconductor elements, electroconductive materials, magnetic materials, hard materials, heat-
resistant materials and superconductive materials, etc
Preparation of organometallic compounds involves reactions of metal elements, metal salts or
other organometallic intermediates with inorganic compounds, gases or organic reagents
. The
synthesis rate varies considerably, with a few that are within one hour
but a number of others
require several hours or even days
The microwave-enhanced kinetics in synthesis of organometallic compounds was reported by
Gedye et al
in 1991, when they synthesized (C
SnCl and (C
SnOH in sealed vessels
under microwave radiation in 7 and 4 minutes respectively, whereas with conventional reflux
method, the synthesis times were correspondingly 3 hours and 1 hour. Laurent et al
microwave-assisted synthesis of alkyl- or arylhalogermanes by means of Redistribution
Reactions. With AlCl
as the catalyst, Bu
GeCl was synthesized in an open reactor in 85% yield
in 3 minutes. The kinetic enhancement was remarkable compared to conventional synthesis that
requires 5 hours at 200°C. Vanatta et al
studied the application of microwave method to the
synthesis of Group 6 (Cr, Mo, W) zerovalent organometallic carbonyl compounds. A rate-
enhancement of up to two orders of magnitude and a higher yield of microwave-assisted synthesis
were achieved compared to conventional reflux method.
Table 1. Synthesis of Group 6 (Cr, Mo, W) zerovalent organometallic carbonyl compounds:
Microwave vs reflux
Compound Time (min)
Temp (°C)
Yield (%) Rate enhancement
180/120 80/68 50
2/180 180/155 64/55 90
0.5/90 180/110 99/88 180
0.5/135 180/135 86/41 270
5/2880 180/165 62/51 576
synthesis method: microwave / reflux
104 Shangzhao Shi and Jiann-Yang Hwang Vol.2, No.2
Poly(ε-caprolactam) is an important engineering thermoplastics with a combination of useful
thermal and mechanical properties. Its synthetic fiber is widely used and has a global production
of more than 3.3 million metric tons. A commercial route to manufacture the polymer is
hydrolytic polymerization, which is conventionally carried out in a high pressure reactor at 250-
270°C for about 12-24 h. Fang et al
synthesized Poly(ε-caprolactam) by means of microwave-
assisted polymerization in nitrogen at atmospheric pressure. At 250°C for only 2 h, the synthesis
was completed with yield, purity, yield strength and tensile strength comparable to the
commercial product.
Poly(ε-caprolactone) is one of the industrial biopolymers that is used as degradable
biomedical and packaging material. It is generally prepared from catalyzed ring-opening
polymerization of ε-caprolactone in bulk or solution. Although many efforts have been made to
catalyze the process, it still takes over 10 h or several days to get the polymerization completed.
Liao et al
attempted the microwave-assisted ring-opening polymerization of ε-caprolactone in
sealed ampoules. Using 0.1% (mol/mol) stannous octanoate as catalyst, they synthesized the
polymer at 180°C in 30 min. The yield attained 90% and the average molar mass is 124000g/mol.
While in the atmospheric nitrogen environment
, Poly(ε-caprolactone) was synthesized via
microwave method at 150°C in 2 h. Besides the attainment of 92.3% yield and the comparable
yield strain and the tensile modulus, the tensile strength and the strain at break were much higher
than those of the conventionally produced polymer.
Fang et al
also conducted the microwave synthesis of Poly(ε-caprolactam-co-ε-
caprolactone), which is viewed as a engineering plastic combining the advantages of the
beneficial mechanical and biodegradable properties from its amide and ester constituents. They
found that microwave synthesis had obtained higher yields and higher amide-to-ester ratios than
conventional thermal approaches of identical conditions.
Recently, more attention has been paid to the synthesis of optically active polymers. These
polymers are important in biology and have applications in asymmetric synthesis, in
chromatographic techniques and in ferroelectric and nonlinear optical devices. Microwave-
assisted synthesis of the optically active poly(amide-imide) was attempted by Mallakpour et al
, who demonstrated that the synthesis could be completed within 10 min and the synthesized
polymers had higher inherent viscosity than those obtained from 5-h conventional synthesis.
Conjugated polyarylenes are electroluminescent polymers for display applications. To obtain
optimized emissive properties, macromolecular structures with specific functionality should be
carefully designed and synthesized. Conventionally the synthesis is by a route involving Ni(0)-
mediated coupling polymerization reactions, which is labor intensive and time-consuming (up to
24 h). Furthermore, due to the sensitivity (to oxygen and other impurities) of the reactions and the
precipitation of certain monomers and the resultant polymers, it is difficult to obtain precise
batch-to-batch reproducibility and to control the molecular weight. Carter
utilized microwave-
assisted chemistry to overcome these problems. With conditions identical to conventional
methods, the microwave approach is able to drive the polymerization to completion in 10 min.
The fast polymerization suppressed the side pathways to undesired products, oxidation of catalyst
or reactions with glassware and conversion rate of >99.5% was observed. The precipitation of
polymers from the reaction solution was eliminated even when molecular weights higher than
100,000 were reached, which ensured the control of the molecular weight by adding varying
quantities of chain end-capping reagent to the reaction mixture. The synthesis can also be
simplified to a one-step, one-pot fashion since the separate catalyst activation step is no longer
Vol.2, No.2 Microwave-assisted wet chemical synthesis 105
Steps to industrialization
Microwave-assisted wet chemical synthesis offers so many significant advantages over
conventional methods, that wide application of this technology in industry is a certainty.
Compared to solid state materials processing, microwave-assisted wet chemical synthesis is more
facile for scaling up. The fluidic reaction contents can be easily homogenized and non-uniformity
of microwave energy dissipation inside the reactor, which is a common problem in microwave-
assisted solid state processing, is therefore completely eliminated.
This situation, however, still can not guarantee successful application of the microwave
technology to industries. Much effort is needed to get a more comprehensive understanding of the
effects of microwaves on the processing operations and resultant materials, so that specified and
optimized microwave processing systems can be developed.
Identification and evaluation of microwave athermal effect
How could microwave energy have such advantages? Do the advantages come from inducing
high temperature and pressure in sealed reactors, or by super-heating or other unknown athermal
effects? Distinguishing the different advantage-producing mechanisms will lead to different
strategies for efficiently utilizing the microwave method. If there are microwave athermal effects,
our efforts should be focused on the development of microwave reactors that are able to enhance
this effect. The optimized industrial reactors should have a method to dissipate microwave energy
at an intensity level that is independent of the processing temperature but produces maximum
athermal effect. If the advantages are only from the microwave-induced high temperature and
pressure, the optimized reactors should have method to preserve heat so that desired hydrothermal
conditions could be attained and kept with minimum consumption of microwave energy.
Preliminary evidences of the microwave athermal effects appeared from some experiments
on microwave-hydrothermal synthesis of ceramic powders
34, 49-51
, in which microwave-induced
kinetic acceleration was demonstrated in comparison to conventional synthesis with nearly
identical hydrothermal (temperature and pressure) conditions. These studies, however, should be
refined since they did not give quantitative evaluations on the microwave effects. In these
experiments, microwave heated the reaction contents to a temperature and then the power was
turned down to an appropriate level so that steady-state synthesis conditions were maintained. In
laboratory scale, thermal loss could be appreciable due to the small scale of the sample volume
and the microwave energy for keeping the synthesis condition could be sufficient to produce
observable effects. On commercial scale, however, energy accumulation from heating history
becomes significant. After the system has finished the heating up stage, the accumulated thermal
energy may become almost enough to maintain the processing conditions. The microwave
energy, if required to compensate the thermal loss, would become insignificant. The microwave-
enhanced synthesis would degenerate into conventional thermal reactions and little microwave
effects could be expected.
It is worthwhile to pursue quantitative evaluation on the microwave effects. A facile
approach can be evaluation of the kinetic and yield enhancements and characterization of the
product structure features induced by different microwave radiation intensities at fixed
temperatures (or pressures). If there are certain microwave athermal effects rather than those of
microwave-induced high temperature and high pressure, they can be observed with varied
microwave radiation intensities. This approach could not only distinguish the microwave
athermal effects from the microwave-induced thermal effects, but can also find what intensity
level can produce maximum microwave effect. Furthermore, since the dissipation of microwave
energy is independent of the reaction temperature, the reaction contents could be exposed to high
intensity radiation. It is therefore possible to discover new effects that have not been found in
previous research.
The investigation can not be carried out with the currently available instrument. New
equipment should be developed. The new type of microwave-assisted reactors should have a
106 Shangzhao Shi and Jiann-Yang Hwang Vol.2, No.2
mechanism to remove the “heat waste” generated by microwave-material interactions. It could be
something like a thermowell in conventional chemical reactors, but should neither expose the
coolant to the microwave energy nor shield the reaction contents from the microwave radiation.
For research purpose, it should have sufficient heat transfer efficiency, so that microwave energy
with a wide range of intensities could be applied to the reaction contents at any reaction
temperature or pressure.
Study the microwave effects at different synthesis stages
Research on ceramic sintering has suggested that sintering conditions usually affect sintering
behavior not throughout the whole process, but primarily through the early stage
. The
mechanism is that the initial nucleation directs the sintering process. If a larger population of
nuclei has formed in the early stage, the ensuing densification will assume a higher rate and a
finer microstructure will result. The microwave-assisted wet chemical synthesis, at least in the
case of ceramic powder synthesis, may behave in a similar fashion to ceramic sintering, since it is
also a process of nucleation and growth. The microwave effects, if they exist in both the initial
and the later stages, may differently affect synthesis kinetics as well as the synthesized structures.
Comparison of the microwave effects at initial stage with that at later stages is important for
the development of the synthesis systems. If the effects at the initial stage do dominate the whole
process, we can focus the microwave radiation on that stage, leaving the later stages to
conventional heating approaches. In the situation where the investment cost of a microwave
heating facility is much higher than that of a conventional facility, such an allocation of the two
kinds of energy could be an optimal strategy.
For the initial stage, it is also necessary to discern the microwave athermal effects from the
effects induced by microwave-induced rapid heating. While conventional heating normally takes
over an hour to heat the contents up to 200°C, microwave heating needs only several minutes or
even less. Some researchers on ceramic sintering have attributed the enhanced densification
kinetics and the refined microstructures to the microwave-induced high heating rate
53, 54
Meanwhile, research on ceramic powder preparation also accounted for the responsibility of rapid
heating for the development of monodispersed fine particles
. If the athermal effect is
overwhelming, our further efforts should be devoted to the optimal microwave intensity and
frequency. Otherwise, we should consider that the fast-heating function could be performed by
other approaches, such as low frequency microwave or radio frequency heating, for which large
capacity equipment is commercially available and the equipment cost is cheaper.
A dual-function reactor could satisfy the study on the microwave effects at separate stages.
The reactor could comprise a microwave autoclave and a conventional autoclave. The two
autoclaves will be connected to each other and have a method for materials to flow in each
direction. For studies on the microwave effects at initial stage, reaction contents can be first
charged into the microwave autoclave and heated up by microwaves to the reaction temperature.
The reaction contents would be then transferred into the conventional autoclave and the
temperature would be kept constant by conventional heating. On the other hand, for studies on the
microwave effects at the later stage, reaction contents can be first charged into the conventional
autoclave and heated by conventional method to the reaction temperature. The reaction contents
would then be transferred into the microwave autoclave and the temperature kept constant by
microwave heating. Again a heat-removing device should be present in the microwave autoclave
for dissipating various levels of microwave intensity without disturbing the temperature profile.
Study the temperature influence on the microwave effects
Different hydrothermal conditions may result in different microwave effects, as was revealed
in the microwave-assisted synthesis of iron oxide powders. While α-Fe
seemed to be the
general product under moderate microwave-hydrothermal conditions
, ?-Fe
was obtained by
Vol.2, No.2 Microwave-assisted wet chemical synthesis 107
microwave method under higher temperature and pressure
. There is a debate on the microwave
acceleration effect. But a common recognition is that the high frequency electromagnetic waves
have the capability of concentrating their energy onto charged ions (or molecular dipoles) instead
of a conventional, unbiased distribution of the energy. For an aqueous system, although water is a
good microwave absorber, its dielectric loss factor drops from above 20 to below 5 as the
temperature rises from 3°C to 95°C
56, 57
. On the other hand, in a study of grape and sugar
solutions, the loss factors of low moisture grapes and high concentration sugar solutions rise as
the temperature rises
. It has found that NaCl and CuSO
solutions have different dielectric
responses to microwaves from that of pure water
. These findings have verified the different
response of the solutes to microwave radiation in aqueous solutions and indicate that the relative
microwave-responsibility of the reaction species to solvents varies with temperatures. Different
temperatures may induce different distribution of microwave energy between reaction species and
their solvent and result in different microwave effects. Studying the temperature influence could
find the optimal hydrothermal conditions for obtaining maximum microwave effects. It is also
possible to find more benefits that have not yet been revealed.
Research instruments available in most laboratories have limited temperature range for
carrying out this study. The reaction vessel is typically made of Teflon, which suffers substantial
deformation above 160°C under the autogenous pressure of aqueous solution. A strengthened
shell made of polyetherimide has raised the vessel’s working temperature to some extent
however, since the maximum working temperature of polyetherimide is around 170°C
, it is
unlikely to protect the vessel when processing in high temperature range comparable to
conventional autoclaves. There are some microwave-reactors that are able to utilize glassware for
reaction vessels
. However, when the reactor is used for microwave hydrothermal processing, the
maximum processing pressure is limited to less than 25 bar, corresponding to the autogenous
pressure of water vapor at 235°C. Also, the vessel volume is only 10 ml. There is a disclosed
patent for a microwave reactor using a ceramic vessel
, which could be used for high
temperature synthesis, but its function should be enhanced to meet the requirements for
systematic research in this regard.
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