Solid acid catalysts for biodiesel production - Towards sustainable energy

Article abstract of DOI:10.1002/adsc.200505160

The advantages of biodiesel as an alternative fuel and the problems involved in its manufacturing are outlined. The pros and cons of making biodiesel via fatty acid esterification using solid acid catalysts are examined. The main problem is finding a suitable catalyst that is active, selective, and stable under the process conditions. Various solid acids (zeolites, ion-exchange resins, and mixed metal oxides) are screened as catalysts in the esterification of dodecanoic acid with 2-ethylhexanol, 1-propanol, and methanol at 130-180°C. The most promising candidate is found to be sulphated zirconia. The catalyst's stability towards thermal decomposition and leaching is tested and the effects of the surface composition and structure on the catalytic activity are discussed.

Full text of DOI:10.1002/adsc.200505160

FULL PAPERS
DOI: 10.1002/adsc.200505160
Solid Acid Catalysts for Biodiesel Production --- Towards
Sustainable Energy
Anton A. Kiss, Alexandre C. Dimian, Gadi Rothenberg*
vanꢀt Hoff Institute for Molecular Sciences, University of Amsterdam, Nieuwe Achtergracht 166, 1018 WV Amsterdam,
The Netherlands
Fax: (þ31)-20-525-5604, e-mail: gadi@science.uva.nl
Received: April 18, 2005; Accepted: October 20, 2005
Abstract: The advantages of biodiesel as an alterna- decanoic acid with 2-ethylhexanol, 1-propanol, and
tive fuel and the problems involved in its manufactur- methanol at 130–1808C. The most promising candi-
ing are outlined. The pros and cons of making biodie- date is found to be sulphated zirconia. The catalystꢁs
sel via fatty acid esterification using solid acid cata- stability towards thermal decomposition and leaching
lysts are examined. The main problem is finding a is tested and the effects of the surface composition
suitable catalyst that is active, selective, and stable un- and structure on the catalytic activity are discussed.
der the process conditions. Various solid acids (zeo-
lites, ion-exchange resins, and mixed metal oxides) Keywords: alternative fuels; esterification; green
are screened as catalysts in the esterification of do- chemistry; heterogeneous catalysis; sulphated zirconia
Introduction
Table 1. Average biodiesel emissions compared to conven-
tional diesel.
Sustainable energy management has become a high pri-
ority for many countries. A large percentage of our glob-
al energy expenditure is for automotive purposes, mak-
Emission type
B20
B100
Total unburned hydrocarbons
À20% À67%
À12% À48%
À16% À79%
À12% À47%
þ2% þ10%
À20% À100%
ing the implementation of sustainable automotive fuels CO
an urgent issue worldwide.[1] Biodiesel is a renewable
fuel comprised of monoalkyl esters of fatty acids. It
can be manufactured from vegetable oils, animal fat,
and even from recycled grease from the food industry.[2]
Remarkably, it is the only alternative fuel currently
available that has an overall positive life cycle energy
balance – it yields as much as 3.2 units of fuel product en-
ergy for every unit of fossil energy consumed in its life
CO2
Particulate matter
NOx
SOx
Polycyclic aromatic hydrocarbons (PAHs) À13% À80%
Nitrated PAHs
À50% À90%
cycle, compared to only 0.83 units for petroleum die- recognized as a “green fuel” that has several advantages
sel.[3] Biodiesel contains no petroleum products, but it over conventional diesel. It is safe, renewable, non-toxic,
may be blended with conventional diesel to provide an and biodegradable in water (ca. 98% biodegrades in just
alternative to the latter. A blend of 80% petroleum die- a few weeks).[2] It contains less sulphur compounds,[7]
sel and 20% biodiesel (known as B20) can be used in un- and has a high flash point (>1308C). Furthermore, it
modified diesel engines (in fact, using B20 in buses and is almost neutral with regard to carbon dioxide emis-
trucks reduces the black smoke emitted during acceler- sions, and emits 80% fewer hydrocarbons and ~50%
ation[4]). Biodiesel can also be used in its pure form less particles. Finally, biodiesel production enjoys a pos-
(B100), but this requires minor engine modifications itive social impact, by enhancing rural revitalisation.[3,6]
to avoid maintenance problems.[5] Table 1 shows the
average biodiesel emissions compared to conventional employ transesterification of triglycerides with metha-
diesel.[3,6] nol using NaOH as a base catalyst.[8] This catalyst is cor-
Current biodiesel manufacturing processes primarily
Interest in biodiesel as an alternative fuel has acceler- rosive to equipment and also reacts with free fatty acids
ated tremendously as a result of recent legislations that toform unwantedsoapby-products, requiring expensive
require a major reduction of vehicle emissions, as well as separation. Another route to fatty esters, the main com-
the increasing price of petroleum.[2,4,6,7] Biodiesel is now ponents of biodiesel, is batch esterification catalysed by
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Anton A. Kiss et al.
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homogeneous sulphuricacid. This is followed by a distil-
lation process for water removal, recycling of excess al-
cohol and ester purification. The problem with this proc-
ess is the discontinuous operation mode that also in-
volves a costly separation of homogeneous catalyst.[9,10]
During the last decade many industrial processes
shifted towards using solid acid catalysts.[11,12] In contrast
to liquid acids that possess well-defined acid properties,
solid acids may contain a variety of acid sites.[13] Gener-
ally they are categorised by their Brønsted or Lewis
acidity, the strength and number of sites, and the textural
properties of the support (porosity and surface area). composite materials, carbons-based solid acids and
Recently, one of us proposed a continuous process for mixed metal oxides (in particular, sulphated zirconia).
fatty esters production, based on catalytic reactive distil-
In the figures that follow, the conversion is defined as:
lation.[14,15] The non-ideal nature of the mixtures, howev- X [%]¼100ꢀ[Acid]final/[Acid]initial, and the amount of
er, leads often to phase segregation, and the aqueous catalyst used is normalised, i.e., Wcat [%]¼Mcat/
phase may easily cause deactivation of the solid acid cat- (Macid þMalcohol).
alyst, so a water-tolerant catalyst is required.[16] To solve
these problems, we are investigating solid acid catalysts
for fatty acids esterification. In this paper, we report the Catalysis using Zeolites and Ion-Exchange Resins
results of a screening of catalyst candidates and discuss
the possible applications to biodiesel production.
Figure 1 shows the degree of conversion vs. time for dif-
ferent catalyst types. We tested three types of zeolites:
H-ZSM-5, Yand Beta. The zeolites showed only a small
increase of conversion (1–4%), compared to the non-
catalysed reaction (Figure 1, left). This is in agreement
Results and Discussion
On the large scale of automotive fuel applications, a with previous findings,[16] suggesting that the reaction
good esterification catalyst must fulfil several conditions may be suppressed by the limited diffusion of the bulky
that may not seem so crucial in the laboratory. First, the reactant into the zeolite pores. The catalysis probably
catalyst should be very active and selective, as by-prod- takes place only on the external surface. Zeolites con-
ucts other than water are likely to render the process un- tain silicon, aluminium and oxygen in their framework,
economical. Second, it should be water-tolerant and sta- and cations, water and/or other molecules within their
ble at relatively high temperatures. Finally, it should be pores. The SiO2/Al2O3 ratio can be used to control the
an inexpensive material that is also readily available on acid strength and hydrophobicity of the zeolites.[17]
an industrial scale. Bearing the above conditions in While acid strength increases at lower SiO2/Al2O3 ratio,
mind, we envisaged a strong Brønsted acid with in- hydrophobicity increases at higher SiO2/Al2O3 ratios.
creased hydrophobicity, which is stable at temperatures This means a trade-off is required for optimal perform-
up to 200–2508C (hydrophobicsurfaces are preferable
for conducting organic reactions in water, because water
ance.[16]
Note that the maximum achievable conversion in the
covers the surface of the solid acids and prevents the ad- case of esterification is limited by the chemical equilibri-
sorption of organicmaterials). For the first set of experi-
ments, we chose to screen four solid acid families: zeo-
um.
We then tested two representatives of the main classes
lites, mixed metal oxides, composite materials and car- of ion-exchange organic resins: Amberlyst-15, a styrene-
bon-based polysulphonic acids. All these catalysts based sulphonic acid and Nafion-NR50, a copolymer of
were benchmarked against H2SO4, as the reference ho- tetrafluoroethene and perfluoro-2-(fluorosulphonyle-
mogeneous catalyst.
thoxy)propyl vinyl ether.[18–20] Both catalysts showed
In a typical reaction, equivalent amounts of dodeca- high initial activity (Figure 1, right). However, the Am-
noic (lauric) acid (1) and 2-ethylhexanol (2) were react- berlyst catalyst deactivated after 2 h, and the Nafion af-
ed at 1608C in the presence of 1 wt % solid acid catalyst, ter 4.5 h, making them unsuitable for continuous indus-
to give 2-ethylhexanol dodecanoate (3) [Eq. (1)]. Reac- trial processes, where a long catalyst lifetime is essential.
tion progress was monitored by GC. Time-resolved pro-
Carbon-based solid acid catalysts were only recently
files were then measured for both catalysed and non-cat- reported as promising esterification catalysts.[21] These
alysed reactions, at various temperatures exceeding compounds are allegedly easily prepared by heating ar-
1008C (below 1008C, at normal pressure and equimolar omatics such as naphthalene in concentrated sulphuric
ratio of reactants, the liquids separate before equilibri- acid under nitrogen at 473–573 K. The excess sulphuric
um conversion is reached). The following results are div- acid is then removed by vacuum distillation, resulting in
ided according to the type of catalyst families: zeolites, a black solid. This solid was reported to be insoluble in
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Figure 1. Esterification of dodecanoic acid with 2-ethylhexanol: (left) reactions at 1308C using homogeneous and heterogene-
ous acid catalysts; (right) reactions at 1508C, in the presence of Amberlyst, sulphated zirconia (SZ) and Nafion, 3 wt %. The
conversion is defined as: X [%]¼100ꢀ[Acid]final/[Acid]initial, and the amount of catalyst used is normalised, i.e., Wcat [%]¼Mcat/
(Macid þMalcohol).
many solvents (water, methanol, ethanol, benzene, and
hexane). The last parts of the synthesis consist of grind-
ing the solid and washing it repeatedly in boiling water.
When attempting to perform this last part of the syn-
thesis the resulting black solid was always soluble in wa-
ter, much to our frustration. Despite repeated efforts, we
could not obtain an insoluble compound as reported in
the literature.[21]
Catalysis using Metal Oxides
We chose sulphated zirconia as a representative of the
metal oxides family, after an initial screening of various
metal oxides. Zirconia can be modified with sulphate
ions to form a highly acidic or superacidic catalyst, de-
pending on the treatment conditions.[22–26] This type of
catalyst is well known in the industry for a variety of
Figure 2. Esterification of dodecanoic acid (1) with 2-ethyl-
processes.[27–30] We found that sulphated zirconia
hexanol (2): activity comparison at 1608C and 1808C using
showed high activity and selectivity for the esterification
2 wt % sulphated zirconia catalyst. The initial rates are given
of fatty acids with a variety of alcohols ranging from 2-
as the percentage of conversion per min over the first 20 min.
ethylhexanol to methanol. Figure 2 shows the reaction
profiles for fatty acid conversion in time, at tempera-
tures of 1608C and 1808C, for the catalysed and non-cat- much higher in the case of the catalysed reaction. More-
alysed systems. The initial rate of the catalysed reaction over, high conversions can be reached even at 1408C (re-
is about three times higher compared to that of the non- sults not shown), providing that an increased amount of
catalysed reaction (after 20 min, the conversion of the catalyst is used.
catalysed reaction was double that of the non-catalysed
one).
In a separate set of experiments, we tested the catalyst
reusability and robustness. In five consecutive runs, with
Seeing these promising results with 2-ethylhexanol, no treatment between the runs, the activity dropped to
we tested the applicability of sulphated zirconia also 90% of the original value, and remained constant there-
for 1-propanol and methanol (Figures 3 and 4). As after. Re-calcination of this (slightly) deactivated cata-
shown in Figure 3, in the absence of a catalyst, high lyst restored it to the original activity.
acid conversion can be achieved only above 1808C.
Figure 4 shows the reaction profiles for the esterifica-
The increase of conversion with the temperature is tion of dodecanoic acid with methanol. As expected, the
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Anton A. Kiss et al.
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Figure 3. Esterification of dodecanoic acid with propanol, using an alcohol:acid ratio of 5:1 and sulphated zirconia as catalyst;
(left) non-catalysed reaction at 1208C, 1408C, 1608C and 1808C; (centre) 1 wt % catalyst at 1208C, 1408C and 1608C; (right)
uncatalysed, 1 wt % catalyst and 3 wt % catalyst at 1408C.
Figure 4. Esterification of dodecanoic acid with methanol, using an alcohol:acid ratio of 3:1 and sulphated zirconia as catalyst;
(left) non-catalysed reaction at 1208C, 1408C, and 1608C; (centre) 1 wt % catalyst at 1208C, 1408C and 1508C; (right) unca-
talysed, 1 wt % and 3 wt % catalyst at 1408C.
fatty acid conversion increases at higher amounts of cat- phate ions were found. Control experiments showed
alyst and higher temperatures. Comparing the reactions that when an aqueous suspension of sulphated zirconia
with the three alcohols, one sees that the esterification of was shaken at 258C sulphate ions did leach out. From
dodecanoic acid with methanol takes place at higher these tests we can conclude that sulphated zirconia is
rates compared to propanol or 2-ethylhexanol. This not deactivated by leaching of sulphate groups when a
can be explained by the relative sizes of the alcohols.
small amount of water is present in the organicphase,
but is easily hydrolysed in an aqueous phase.[14]
Testing for Leaching of Sulphonic Groups
Product Selectivity and Side Reactions
In the case of sulphated zirconia, we also tested the cat-
alyst to see if sulphonicgroups are leaching into the re-
Typically, the alcohol-to-acid ratio inside an industrial
action mixture. This is important, because water can, in reactive distillation unit may vary over several orders
principle, capture sulphonic groups from the surface and of magnitude.[14,15] Especially for stages where an excess
hydrolyse them to give homogeneous H2SO4 (if you mix of alcohol is present, the use of an acid catalyst may lead
sulphated zirconia in water, the pH of the suspension de- to side reactions such as ether formation or alcohol
creases quickly, as water accumulates on hydrophilic dehydration. For all of the catalysts described here,
acidic sites and sulphate groups are hydrolysed). The we assessed the selectivity by testing the formation
presence of sulphate ions in solution was checked with of side products in a suspension of, e.g., sulphated zirco-
BaCl2 and KOH titration. Following reaction at 403 K nia in pure 2-ethylhexanol under reflux for 24 h. No
and catalyst recovery, the resulting mixture was washed ethers or dehydration products were detected by GC
with water for the extraction of sulphate ions. No sul- analysis.
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The Influence of Surface Hydrophobicity
ity (the water layer would prevent the access of dodeca-
noic acid and 2-ethylhexanol to the catalyst, Figure 5,
Although the reaction mechanism for the heterogene- bottom).
ous acid-catalysed esterification was shown to be in prin-
The potential phase separation into an aqueous and an
ciple similar to the homogeneously catalysed one,[31] organic phase has important catalytic implications. Ho-
there is an important difference that concerns the rela- mogeneous acid catalysts transfer preferentially to the
tionship between the surface hydrophobicity and the aqueous phase, where they lose their effectiveness.
catalystꢁs activity. This is especially true for dodecanoic Some heterogeneous catalysts may be similarly inhibit-
acid and 2-ethylhexanol, that are both very lipophilic ed by water sorption, if they have too many hydrophilic
compounds. One can envisage three possible cases. First, acid sites. In fatty acid esterification, it is usually the al-
if there is but one isolated Brønsted acid site, surround- cohol, rather than the acid, that mixes better with water.
ed by a hydrophobicenvironment, it is likely that the hy-
drophobicꢀtailꢁ of the acid would be adsorbed parallel to
Accordingly, when there is an excess of alcohol, the re-
action mixture remains monophasic until equilibrium
the hydrophobicsurface (Figure 5, top). Second, if there conversion is reached. In these cases, e.g., when using a
are a few acid sites in the vicinity, the fatty acid mole- 3:1 methanol:dodecanoic acid ratio, no catalyst deacti-
cules could adsorb perpendicular to the surface, with vation was observed.
the tails forming a local hydrophobic environment. Fi-
In the case of zeolites, the SiO2/Al2O3 ratio represents
nally, in the case of a very acidic and/or hydrophilic ma- a trade-off between the hydrophobic character of a zeo-
terial (many adjacent acid sites and/or hydroxy groups), lite and its acidity. The zeolite must be hydrophobic in
the by-product water from the esterification would ad- order to avoid the absorption of the water by-product
sorb on the surface, and the catalyst would lose its activ- that will lead to deactivation. However, if the SiO2/
Al2O3 ratio is too high, the zeolite may lose its acidic
properties. At low SiO2/Al2O3 ratio, water is easily ab-
sorbed to the surface, blocking the access of the fatty
acid. By increasing the SiO2/Al2O3 ratio, ꢀreaction pock-
etsꢁ are created inside a hydrophobic environment. The
fatty acid can then absorb to acid sites on the hydropho-
bic surface and react further (water molecules are un-
likely to be absorbed on sites surrounded by hydropho-
bicareas). The literature reports that high-silica zeolites
were efficient catalysts for esterifying AcOH with
EtOH, but showed a decreased activity for 2-BuOH.[16]
Accordingly, no significant differences were observed
in our case for the various SiO2/Al2O3 ratios, probably
because the reactants are too large to fit in the pores.
The ion-exchange resin Nafion is non-porous, and
Amberlyst has large pores. The hydrophobicity in this
case is governed by the polymer chain, and the polymer
backbone itself is likely to have a high affinity towards to
the tails of the fatty acid and alcohol. Moreover, the sul-
phonic acid groups grafted on the chain are strong acids
compared to the hydroxy groups on zeolites or metal ox-
ides. All these make the resins good esterification cata-
lyst candidates, but they fail on the thermal stability
tests.
Of the mixed oxides family, sulphated zirconia is
shown to be a good catalyst with high thermal stability
and strong acid sites. It has large pores that improve
the diffusion of the fatty acid molecules. It does not leach
under the reaction conditions (although it does leach in
water) and does not give rise to side reactions such as
etherification or dehydration. This makes sulphated zir-
conia an interesting candidate for catalytic biodiesel
production.
Figure 5. Cartoon of the influence of the surface hydropho-
bicity on the catalytic activity; (top) adsorption of a fatty
acid on a hydrophobic catalyst surface with an isolated acid
site; (middle) adsorption of three fatty acid molecules on ad-
jacent acid sites, with the tails forming a local hydrophobic
environment; (bottom) formation of water layer on hydro-
philic acid catalyst.
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Anton A. Kiss et al.
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Conclusion
of the catalyst was 0.05 mm. The kinetics of dodecanoic acid es-
terification with different alcohols were studied at 608C,
1008C, 1408C, 1608C, and 1808C. The catalyst concentration
in the reaction mixture was varied from 0 to 5 wt %. The initial
reactant molar ratio used was varied from alcohol:dodecanoic
acid¼1:1 up to 5:1.
The systematicstudy of initial reaction rates and de-
tailed conversion profiles under controlled process con-
ditions (temperature, pressure, and composition) is a
suitable method for comparing solid acid catalyst candi-
dates for fatty acid esterification. Catalysts with small
pores (microporous), such as zeolites, are not suitable
for biodiesel manufacture because of the diffusion limi-
tations of the large fatty acid molecules. Ion-exchange
resins are active strong acids, but have a low thermal sta-
bility. This is problematic as the reaction must be carried
out at high temperatures. Sulphated zirconia, on the oth-
er hand, is found to be a good candidate as it is active, se-
lective, and stable under the process conditions. The hy-
drophobicity of the catalyst surface and the density of
the acid sites are of paramount importance in determin-
ing the catalystꢁs activity and selectivity.
Preparation of Sulphated Zirconia Catalyst
This is a modification of the two-step synthesis procedure.[22]
ZrOCl2 ·8 H2O (50 g) was dissolved in water (500 mL), fol-
lowed by precipitation of zirconium hydroxide at pH¼9 using
a 25 wt % NH3 solution. The resulting Zr(OH)4 was washed
with water (3ꢀ500 mL) to remove the chloride salts [ClÀ
ions were determined with 0.5 N AgNO3). In the second step,
Zr(OH)4 was dried for 16 h at 1208C, impregnated with 1 N
H2SO4 (15 mL H2SO4 per 1 g Zr(OH)4], and calcined in air
for 3 h at 6508C.
Catalyst Characterisation
Experimental Section
The textural properties were determined from the nitrogen ad-
sorption isotherm determined after degassing at 2008C under
vacuum at 5–10 mbar. The surface area was calculated using
the BETequation and the pore volume was determined at a rel-
ative pressure of 0.98. The pore size was calculated using the
Materials and Instrumentation
Reactions were performed using a system of six parallel reac-
tors (Omni-Reacto Station 6100) as well as a higher pressure
autoclave. These are batch reactors of 100 mL, equipped with
magneticstirrers, heating/cooling and partial/total reflux sys-
tem. GC analysis was performed using an InterScience GC-
8000 gas chromatograph with a 100% dimethylpolysiloxane ca-
pillary column (DB-1, 30 mꢀ0.21 mm). GC conditions: iso-
therm at 408C (2 min), ramp at 208C minÀ1 to 2008C, isotherm
at 2008C (4 min). The injector and detector temperatures were
set at 2408C. The helium flow rate was 1.9 cm3 minÀ1. Charac-
terisation of sulphated zirconia was performed by atomic emis-
sion spectroscopy with inductively coupled plasma atomisation
(ICP-AES) on a CE Instruments Sorptomatic1990. NH 3-TPD
was used for the characterisation of acidic site distribution.
Sulphated zirconia (0.3 g) was heated up to 6008C using He
(30 mL minÀ1) to remove adsorbed components. Then, the
sample was cooled at room temperature and saturated for 2 h
with 100 mL minÀ1 of 8200 ppm NH3 in He as carrier gas. Sub-
sequently, the system was flushed with He at a flow rate of
30 mL minÀ1 for 2 h. The temperature was ramped up to
6008C at a rate of 108C minÀ1. A thermal conductivity detector
(TCD) was used to measure the desorption profile of NH3. Un-
less noted otherwise, chemicals were purchased from commer-
cial firms and were used as received. Double distilled water was
used in all experiments.
À
Barrett–Joyner Halenda (BJH) method. The most important
results are given in Table 2. In principle, the specific surface
area depends on the calcination temperature. Values are in
very good agreement with literature data.[32,33]
Table 2. Sulphated zirconia (SZ) catalyst characterisation.
Parameter
Value
Surface area (m2 gÀ1
)
118
0.099
3.0
2.5
Specific pore volume (cm3 gÀ1
Average pore size (nm)
S/SZ (wt %)
)
Zr/SZ (wt %)
69.4
28.1
282
1150
O/SZ (wt %)
Acidity by NH3-TPD (meq gÀ1
)
Acidity by titration (meq gÀ1
)
Procedure for Testing Catalyst Leaching
The catalyst leaching was studied in an organic and an aqueous
phase. First, a sample of fresh sulphated zirconia catalyst
(0.33 g) was stirred with H2O (50 mL) while measuring the de-
velopment of the pH in time. After 24 h, the acidity was meas-
ured by titration with KOH (0.002 N). The suspension was then
filtered and treated with a BaCl2 solution to test for SO42À ions.
In a second experiment, the catalyst was added to an equimolar
mixture of reactants. After 3 h at 1408C, the catalyst was recov-
ered from the reaction mixture, dried at 1208C and finally stir-
red in 50 mL H2O. The pH was measured and the suspension
titrated with a diluted solution of KOH after 24 h. Sulphate
ions in the suspension were determined qualitatively with
BaCl2 (at 1408C the reaction mixture does not split into two liq-
Procedure for Acid-Catalyzed Esterification
Dodecanoic (lauric) acid (1; 20.0 g, 0.10 mol) and 2-ethylhexa-
nol (2; 15.64 mL, 13.00 g, 0.1 mol) were reacted at 1608C in the
presence of 1 wt % solid acid catalyst, to give 2-ethylhexanol
dodecanoate (3). The catalytic activity was measured as the dif-
ference between the reaction rate in the presence of catalyst
and that of the thermal (non-catalysed) reaction, using the
same initial composition and temperature. The particle size
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Solid Acid Catalysts for Biodiesel Production
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uid phases, because the water evaporates). In a third experi-
ment, the same procedure was repeated at 1008C when the re-
action mixture segregates and a separate aqueous phase is
formed.
[14] F. Omota, A. C. Dimian, A. Bliek, Chem. Eng. Sci. 2003,
58, 3175.
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A. P. Kybett), Royal Society of Chemistry, Cambridge,
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Acknowledgements
We thank M. C. Mittelmeijer-Hazeleger and J. Beckers for ex-
cellent technical support, as well as the Dutch Technology Foun-
dation STW (NWO/CW Project Nr. 700.54.653) and the compa-
nies Cognis, Oleon, Sulzer, Engelhard and Uniquema for finan-
cial support.
[20] M. A. Harmer, Q. Sun, A. J. Vega, W. E. Farneth, A.
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