COMMUNICATION
Versatile mesoporous carbonaceous materials for acid catalysis
Vitaly L. Budarin, James H. Clark,* Rafael Luque and Duncan J. Macquarrie
Received (in Cambridge, UK) 6th October 2006, Accepted 25th October 2006
First published as an Advance Article on the web 14th November 2006
DOI: 10.1039/b614537j
The SO3H loading in sulfonated Starbon1-400 was found to be
Starbon mesoporous materials were synthesized after pyrolysis
0.5 mmol g21 and 0.3 mmol g21 for the SO3H sulfonated
of expanded starch and subsequently functionalised with
commercial DARCO1 as measured by TG-IR.
We first tested our sulfonated Starbon1 in the esterification of
sulfonated groups, providing highly active and reusable
materials in various acid catalysed reactions.
different organic acids in water (Table 1). Esterification reactions
are one of the most useful transformations for organic acids,4
A new family of mesoporous carbonaceous materials named
Starbons1 has been recently discovered in our group.1 These
especially for a dicarboxylic acid since the diester can be used as an
materials are directly produced from high surface area forms of
starch and other expanded polysaccharides after pyrolysis at
different temperatures. Mesoporous forms of the polysaccharide
are needed to generate the Starbons1 and high surface area
starches have been found to be more stable upon storage for
extended periods of time than any other high surface area
polysaccharides (e.g. cellulose). Moreover, Starbons1 can be
directly prepared from the parent material (mesoporous expanded
starch2) in a simpler and less wasteful process than other
mesoporous forms of carbon.3 The most extraordinary character-
istics of these materials are a high degree of mesoporosity as well as
easily tunable surface properties, from more starch-like to more
graphite like, depending on the pyrolysis temperature. Materials
have been found to be stable in a wide range of conditions,
including boiling toluene and water and even in acid and basic
solutions (high temperature Starbons1). Starbons1 can be
produced in different forms such as powders and monoliths, thus
extending their range of useful applications. We have functiona-
lised the Starbons1 with acidic groups and tested the solid acid’s
activity in various acid catalysed reactions under thermal or
microwave conditions.
Starbons1 were synthesized as recently reported in the
literature.1 Material carbonised at 400 uC (herein after referred
to as Starbon1-400) was chosen for its ideal hydrophilicity/
hydrophobicity ratio and was subsequently functionalised.
Starbon1-400 was suspended in H2SO4 (99.999% purity, 10 mL
acid per g material) and heated for 4 h at 80 uC. After sulfonation,
the solid acid was washed with distilled water until the washings
were neutral, conditioned in boiling toluene (150 uC, 4 h) and
water (100 uC, 3 h) and finally oven dried overnight (100 uC)
before being tested in the catalytic reactions. Sulfonated material
was denoted as Starbon1-400-SO3H. Products were analysed by
GC using an Agilent 6890 N GC model equipped with a 7683B
series autosampler. Response factors of the different reaction
products were determined with respect to the starting material in
each reaction using known compounds in calibration mixtures of
specified compositions.
intermediate in the manufacture of polymers, fine chemicals,
perfumes, plasticizers and solvents.5,6
The drive towards greener process chemistry has resulted in
some research reports of the partial or even complete replacement
of organic solvents with water in a few organic reactions but to our
knowledge there are no reports on esterification of organic acids in
water. Indeed, esterifications have been reported as very sensitive
to water environments due to different equilibrium steps in which
water is involved.7 However, we can expect to increase the reaction
rates by altering the local water concentration in the active centre
of the solid acids through careful adjustment of the local surface
properties.
Table 1 summarises the conversion and selectivity values
obtained for the esterification reaction of different diacids,
under similar conditions, employing various solid acids. Four
different substrates (succinic, fumaric, levulinic and itaconic
acid) were screened in the esterification reaction in aqueous
ethanol, yielding a very high conversion and selectivity to their
respective esters. The rates of esterification of diacids (succinic,
fumaric and itaconic) for our Starbon1 material were found to be
between 5 and 10 times higher than any commercial alternative
solid acid catalyst (zeolites, sulfated zirconias, acidic clays, etc.). A
diester selectivity improvement (from 35–50% range for the
majority of the solid acids to .90% for Starbon1-400 at
conversion levels of ca. 90%) was also obtained. Starbon1 acid
also exhibited a superior performance in the esterification reaction
of the different acids screened compared to microporous
commercial sulfonated carbons (DARCO1 and NORIT1,
Table 1), not only in terms of conversion but also in terms of
selectivity to the diester (at conversion values of ca. 90%, selectivity
to the diester was 35% for sulfonated DARCO1 compared to the
90% of Starbon1) .
We then moved on to another acid catalysed reaction, the
liquid-phase acylation of alcohols under microwave conditions.
Acylation of aromatic alcohols is one of the most frequently used
transformations for the synthesis of aromatic ketones, important
intermediates in the production of pharmaceuticals, dyes, agro-
chemicals and fragrances.8,9 Furthermore, the use of acetic acid
(HAc) instead of the corrosive acetyl chloride or the lachrymator
acetic anhydride provides many advantages from both economical
and environmental points of view. Data is summarised in Table 2
and Fig. 1.
Green Chemistry Centre of Excellence, Department of Chemistry, The
University of York, York, UK YO10 5DD. E-mail: jhc1@york.ac.uk;
Fax: +44 1904 432705; Tel: +44 1904 432567
634 | Chem. Commun., 2007, 634–636
This journal is ß The Royal Society of Chemistry 2007