Hierarchical macroporous-mesoporous SBA-15 sulfonic acid catalysts for biodiesel synthesis

Article abstract of DOI:10.1039/b919341c

Hierarchical macroporous-mesoporous SBA-15 silicas have been synthesised via dual-templating routes employing liquid crystalline surfactants and polystyrene beads. These offer high surface areas and well-defined, interconnecting macro- and mesopore networks with respective narrow size distributions around 300 nm and 3-5 nm for polystyrene:tetraethoxysilane ratios ≥2:1. Subsequent functionalisation with propylsulfonic acid yields the first organized, macro-mesoporous solid acid catalyst. The enhanced mass transport properties of these new bi-modal solid acid architectures confer significant rate enhancements in the transesterification of bulky glyceryl trioctanoate, and esterification of long chain palmitic acid, over pure mesoporous analogues. This paves the way to the wider application of hierarchical catalysts in biofuel synthesis and biomass conversion.

Full text of DOI:10.1039/b919341c

View Online / Journal Homepage / Table of Contents for this issue
PAPER
www.rsc.org/greenchem | Green Chemistry
Hierarchical macroporous–mesoporous SBA-15 sulfonic acid catalysts for
biodiesel synthesis†
Je´re´my Dhainaut, Jean-Philippe Dacquin, Adam F. Lee* and Karen Wilson*
Received 17th September 2009, Accepted 10th November 2009
First published as an Advance Article on the web 16th December 2009
DOI: 10.1039/b919341c
Hierarchical macroporous–mesoporous SBA-15 silicas have been synthesised via dual-templating
routes employing liquid crystalline surfactants and polystyrene beads. These offer high surface
areas and well-defined, interconnecting macro- and mesopore networks with respective narrow size
distributions around 300 nm and 3–5 nm for polystyrene:tetraethoxysilane ratios ≥2:1. Subsequent
functionalisation with propylsulfonic acid yields the first organized, macro-mesoporous solid acid
catalyst. The enhanced mass transport properties of these new bi-modal solid acid architectures
confer significant rate enhancements in the transesterification of bulky glyceryl trioctanoate, and
esterification of long chain palmitic acid, over pure mesoporous analogues. This paves the way to
the wider application of hierarchical catalysts in biofuel synthesis and biomass conversion.
are sensitive to water and FFA impurities which are always
Introduction
present in trace amounts even in refined plant oils. FFA removal
The International Energy Agency predicts a 55% increase in
from biodiesel is essential in order to prevent corrosion of
global fuel consumption by the year 2030,1 principally the result
vehicle fuel tanks and engine blocks, and is currently achieved
of the growing needs of emerging nations. The combination of
through an acid catalyzed esterification pretreatment step prior
dwindling oil reserves and growing concerns over carbon dioxide
to transesterification. Simultaneous FFA esterification and TAG
emissions and associated climate change is driving the urgent
transesterification using acid catalysts provides an alternative
development of clean, sustainable energy supplies. Biomass
single step (albeit high temperature) process for poor quality
oils containing high FFA levels.5,6,8-10 However, in both scenar-
ios reactor corrosion and removal of soluble catalyst species
from the resulting biofuel mixture is particularly problematic,
requiring aqueous quench and neutralisation steps which them-
selves generate undesired emulsions and soap formation.6,11 The
resultant E-factor (defined as the [total waste]/[product] ratio) is
concomitantly poor due to catalyst losses after each reaction and
the production of copious contaminated water. Furthermore,
the major by-product of biodiesel synthesis, glycerol, is heavily
contaminated by homogeneous catalyst residues and/or water
and therefore of little re-sale value to potential consumers in the
cosmetic, food and pharmaceutical industries.
Non-corrosive heterogeneous catalysts that are easily re-
covered and recyclable could greatly improve the environ-
mental impact of commercial biodiesel processing, eliminat-
ing the need for aqueous quench cycles while offering the
opportunity for continuous operation in flow reactors.5,6,11,12
Diverse solid acids and bases including zeolites,13 resins,14
alkali earth oxides,15-17 hydrotalcites18 and calcined dolomitic
rock19 have been investigated for TAG transesterification and
their application recently reviewed.4,5,6,12 Unfortunately, the
micro and mesoporous catalyst systems investigated to date
are not optimal for the bulky and viscous C16–C18 TAGs
typical of plant oils.20 Even tailored, large pore (>4–15 nm)
mesoporous catalysts based upon SBA-1521 that are active for
esterification and transesterification,9,22-25 often possess long,
isolated parallel channels resulting in slow in-pore diffusion and
correspondingly slow turnover. Current material syntheses have
focused solely on generating and tuning mesoporosity, largely
offers a partial solution to the replacement of fossil fuels for
the production of key chemical feedstocks and transportation
fuels such as ethanol and biodiesel. First generation alternative
biofuels have proven controversial due to their synthesis from
important food crops including corn, sugarcane and palm or
rapeseed oils, and consequent impact on global food prices.
Recent focus has therefore shifted towards so-called second
generation non-food biomass sources like cellulose, algae or
non-edible plant oils from the Euphorbiaceae family2 (notably
Jatropha or Castor oil), which do not compete with traditional
agronomies.
Biodiesel sourced from renewable second generation crops
is non-toxic and biodegradable, with the potential for closed
CO2 cycles3 and thus vastly reduced carbon footprints compared
with petroleum fuels.3,4 It comprises long chain fatty acid methyl
esters (FAMEs), typically derived from vegetable oils or animal
fats rich in triacyl glycerides (TAGs) and free fatty acids (FFAs).5
The transesterification of oil seed TAGs to methyl (or ethyl)
esters can be catalyzed by acids or bases, and the vast majority of
biodiesel is currently produced via homogeneous catalysis, using
soluble bases such as NaOH/NaOMe or KOH.6,7 Although
liquid base catalysts offer high transesterification rates, they
School of Chemistry, Cardiff University, Cardiff, UK CF10 3AT.
E-mail: leeaf@cardiff.ac.uk, wilsonk5@cardiff.ac.uk; Fax: 44 2920
870827; Tel: 44 2920 874030
† Electronic supplementary information (ESI) available: PS bead char-
acterization and comparative textural properties of pre- and post-
functionalized silicas. See DOI: 10.1039/b919341c
296 | Green Chem., 2010, 12, 296–303
This journal is
The Royal Society of Chemistry 2010
©

View Online
◦
ignoring the benefits of hierarchical porous networks containing
macropores to act as rapid transport conduits to the active
sites. Simulations show that in the Knudsen diffusion regime,26
where reactants/products are able to enter/exit mesopores
but experience attendant diffusion limitations, such bi-modal
pore structures could significantly improve catalyst activity.
Catalytically active materials possessing such interconnecting
macropore–mesopore networks27 with well-defined dimensions
would be particularly suited to boosting mass-transport in
viscous liquid phase reactions such as biodiesel synthesis. They
can be prepared using microemulsion,28 colloidal polystyrene
microspheres29 or co-surfactants30 routes in conjunction with
mesoporous templates, and are particularly attractive for appli-
cation in liquid continuous flow reactors wherein rapid pore
diffusion is required. Polystyrene microspheres allow great
control over the final macropore properties. SBA-15 silicas have
been recently synthesised incorporating macropores of a few
hundred nanometres by the use of these templating polymers,31-33
offering high specific surface areas and improved diffusion
characteristics, but to our knowledge have yet to be applied
in heterogeneous catalysis.
(TGA) revealed their complete decomposition in air at 550 C
(Figure S3†).
SBA-15 synthesis
Macroporous–mesoporous SBA-15 silicas were synthesized
following the approach of Zhao et al.,39 with the addi-
tion of polystyrene beads during the synthesis to create
macropores.31,32,33 Typically, 1.0 g of Pluronic P123 triblock co-
polymer was dissolved in 7.5 ml of water and 25 ml of 2 M HCl
◦
solution (Fisher) while stirring at 35 C. The required amount
of polystyrene beads for the appropriate PS:tetraethoxysilane
(TEOS 98%, Sigma-Aldrich) mass ratio were then added to
the solution and stirred for an hour. Once a good dispersion
of PS beads was achieved, 2.3 ml of TEOS were added to
the solution, which was maintained at 35 ◦C for 24 h while
◦
stirring. The mixture was then aged at 80 C for 24 h and the
solid product filtered, washed 3 ◦times with deionised water, and
◦
calcined statically in air at 550 C (ramp rate of 0.5 C min-1)
for 6 h. The final solids are termed MM-SBA15-X, where X =
PS:TEOS weight ratio.
Surface grafting of alkoxysilanes offers a convenient method
to covalently attach organic functional groups to porous silica
frameworks without the risk of pore blockage or leaching of the
grafted component.34 Sulfonic acid silicas are a class of solid
Brønsted acid catalyst comprising tethered organo-sulfonic acid
groups35-37 that provide alternatives to commercially available
sulfonated polymer resins, such as Amberlyst-15 and Nafion-
H. In this paper we explore the use of grafting to attach
pure Brønsted acid sites to macroporous–mesoporous SBA-15
supports and thereby generate a new class of ordered, hierarchi-
cal sulfonic acid functionalised silicas, designed specifically for
bulky plant oils. Macropore inclusion significantly enhances the
rate of both C8 TAG transesterification to FAME and C16 FFA
esterification with methanol.
Sulfonic acid functionalization
Sulfonic acid functionalization was performed by post-synthesis
grafting as described previously.9,40,41 A mixture of 0.5 g of MM-
SBA15-X and 0.50 ml of 3-mercaptopropyl trimethoxysilane
(MPTMS 95%, Alfa Aesar) in 15 ml of toluene (Fisher) was
refluxed at 130 ◦C for 24 h, and the resulting thiol-functionalized
solid filtered, washed 3 times with methanol (Fisher) and dried at
80 ◦C overnight. Thiol groups were converted to –SO3H by mild
oxidation with 10 ml of 30% hydrogen peroxide (Sigma-Aldrich)
by continuous stirring at room temperature for 24 h. Sulfonated
MM-SBA15-X solid was subsequently filtered, washed 3 times
with methanol, and dried at 80 ◦C overnight.
Glyceryl trioctanoate transesterification
Experimental
Transesterification was performed at 60 ◦C under stirring using
0.05 g of catalyst, 4.92 ml of glyceryl trioctanoate (commercial
name tricaprylin 99%, Sigma-Aldrich), 0.59 ml of dihexylether
(Fisher) as an internal standard, and 12.50 ml of methanol,
giving a MeOH:TAG ratio of 30 : 1. Aliquots of 0.1 ml were
regularly sampled and analyzed by gas chromatography using
a Varian 450-GC equipped with a VF-1ms 25 m ¥ 0.25 mm
capillary column. Product yields were calculated using response
factors derived for methyl octanoate (99%, Fluka).
Polystyrene bead synthesis
The emulsion polymerisation approach of Vaudreuil and co-
workers was adopted to generate monodisperse polystyrene
(PS) beads.38 0.083 g of potassium persulfate (99%, Sigma-
Aldrich) was dissolved in 6 ml of deionized water and dried
at 70 ◦C. Separately, 239.5 ml of deionized water was added to
a 500 ml three-neck round-bottom flask and heated to 70 ◦C
under N2. 25 ml of styrene (99%, Sigma-Aldrich) and 4.75 ml
of divinylbenzene (80%, Sigma-Aldrich) was washed three times
in a separating funnel with a 0.1 M NaOH solution (pellets,
Sigma-Aldrich) and deionized water to remove polymerization
inhibitors. The styrene and divinylbenzene were transferred to
the 500 ml temperature stabilized flask, along with 26 ml of
deionized water used to wash out the funnel. The potassium
persulfate was then added to initiate styrene polymerization, and
15 h later the resultant white solution was filtered, then washed
three times with deionized water and three times with ethanol.
The final beads possessed a narrow size distribution of ~320 nm
diameter (Figures S1 and 2†), while thermo-gravimetric analysis
Palmitic acid esterification
Esterification was performed at 60 ◦C under stirring using 0.05 g
of catalyst, 2.564 g (10 mmol) of palmitic acid (Sigma-Aldrich),
0.59 ml (2.5 mmol) of dihexylether and 12.50 ml (300 mmol)
of methanol. Aliquots of 0.1 ml were regularly sampled and
analyzed using a Varian 3900 GC equipped with a CP-Sil 5CB,
15 m ¥ 0.25 mm capillary column. Product yields were calculated
using response factors derived for methyl palmitate (99%, Sigma-
Aldrich).
This journal is
The Royal Society of Chemistry 2010
Green Chem., 2010, 12, 296–303 | 297
©

View Online
Fig. 1 SEM images of: a) SBA-15; b) MM-SBA15-1; c) MM-SBA15-2 and d) MM-SBA15-4 hierarchical macroporous–mesoporous silicas.
Material characterization
Results and discussion
Powder X-ray diffraction patterns were collected on a Bruker
D8 Advance diffractometer fitted with a Lynx eye high-speed
strip detector and Cu-Ka radiation source. Small-angle patterns
were recorded from 0.65◦ to 3◦ with 0.02◦ steps at 10 s per
point. Sulfur content was determined using an Horiba XGT-
7000 X-Ray Fluorescence spectrometer. Nitrogen porosimetry
was performed on a Quantasorb Nova 1200 instrument, after
sample evacuation at 200 ◦C for 4 h. Surface areas were
calculated using the Brunauer–Emmet–Teller (BET) method
over the range P/P0 = 0.075–0.35, where a linear relationship
is maintained. Pore size distributions were calculated using the
Barrett–Joyner–Halenda (BJH) model applied to the desorption
isotherm branch. Mesopore volumes were evaluated at P/P0 =
0.98. SEM was conducted on a JEOL JSM 7500F system;
samples were gold coated prior to analysis. High-resolution
TEM was carried out on a JEOL 2011 operating at 200 kV;
samples were deposited from ethanolic solution onto holey-
carbon copper grids. XPS measurements were performed using a
Kratos AXIS HSi instrument equipped with a charge neutraliser
and Mg-Ka X-ray source. TGA was perf◦ormed using a Stanton
Redcroft STA 780 thermal analyser at 10 C min-1 under flowing
He/O2 (80 : 20 v/v) or He (20 ml min-1 total) for template
removal and sulfonic acid decomposition studies respectively.
Materials characterization
A series of macro–mesostructured derived silicas were first
prepared using different PS:TEOS ratios to incorporate macro-
pores within the conventional SBA-15 mesopore framework.
These are denoted MM-SBA15-1, MM-SBA15-2 and MM-
SBA15-4 corresponding to 1 : 1, 2 : 1 and 4 : 1 weight ratios
of PS beads to TEOS. A pure mesoporous SBA-15 silica
was also synthesised for comparison. The morphology and
porosity of these materials were explored via electron microscopy
to confirm successful macropore incorporation. SEM of the
undoped SBA-15 reveals a bead-like morphology characteristic
of this class of material (Fig. 1).42 The addition of PS beads
progressively modifies the SBA-15-like organization, creating
a new morphology comprising two distinct structures for the
intermediate MM-SBA15-1 and MM-SBA15-2 samples, which
evolve into a more uniform macroporous network for MM-
SBA15-4. The macropores in MM-SBA15-2 and MM-SBA15-4
are very regular, with average diameters around 300 nm, close
to the parent PS bead dimensions. This contrasts with previous
reports33 wherein notable macropore shrinkage was observed
after calcination, and likely reflects the slower heating rate used
in the present study. It is also interesting to note that the
298 | Green Chem., 2010, 12, 296–303
This journal is
The Royal Society of Chemistry 2010
©

View Online
macropores are open and interconnected and thus well-suited
to promote rapid through particle diffusion.
for the slightly contracted mesoporous channels observed in the
hierarchical materials (Figure S4†).
Mesopore character was evaluated by TEM which shows
that the typical 2D periodic hexagonal structure of SBA-15
was retained in all cases, while the introduction of macropores,
as anticipated (and desired) breaks up extended mesoporous
channels. Fig. 2a demonstrates how the macropore framework
silica is composed of highly-organized concentric mesoporous
channels, resulting from self-assembly of the block copolymer
solution around polystyrene beads. Hexagonal ordering of the
mesopore channels is also clearly visible at higher magnification
(Fig. 2b and 2c), with a mean lattice parameter (a0) of 11.2 nm
measured for the purely mesoporous SBA-15, and a slight
decrease to 10.0 nm observed as the degree of macroporosity
increases.† The high curvature of mesoporous channels in
MM-SBA-4 suggests that strong electrostatic and hydrogen-
bonding interactions31 between polystyrene beads, the block
co-polymer and silica precursor promote self assembly of
templated, mesostructured channels around the physical PS
bead template. This curvature, presumably arising from strain
on the block co-polymer layers of the liquid crystal template
phase in conforming to the PS bead morphology, may account
Long range ordering and porosity were subsequently investi-
gated by XRD and N2 porosimetry. Fig. 3 presents low angle
diffraction patterns which evidence a major reflection at 2q ~0.9◦
for all samples, characteristic of a periodic mesopore framework.
Calcined SBA-15 material exhibits well defined peaks at 0.98,
1.57 and 1.81◦, associated with the (100), (110) and (200) planes
of the P6mm space group for the hexagonal arrangement of
mesoporous channels.39,43,44 Macropore incorporation slightly
shifts diffraction peaks to higher angles, corresponding to a con-
traction in the mesopore lattice parameter from 11.2 to 10 nm.
Increasing PS loading is also accompanied by attenuation and
broadening of the d100 peak; line broadening analysis (Table 1)
indicates a progressive decrease in the average mesopore domain
size from 98 to 35 nm upon macropore inclusion, consistent
with localisation of mesopore domains solely in the walls of
the macropore framework as observed by HRTEM (Fig. 2a).
Porosimetry shows all materials exhibit Type IV adsorption
isotherms with type-H1 hysteresis loops characteristic of bottle-
necked pore openings evident in Fig. 4. The loop size decreases
with increasing macropore character, which may be accounted
for by disruption of extended mesopore networks45 that typically
limit diffusion and impart hysteresis in the desorption isotherm
branch. Additional prominent hysteresis observed above P/P0 =
0.9 for MM-SBA15-4 is attributed to the high concentration of
Fig. 3 Low angle powder XRD patterns of: a) SBA-15; b) MM-SBA15-
1; c) MM-SBA15-2 and d) MM-SBA15-4 hierarchical macroporous–
mesoporous silicas.
Fig. 2 HRTEM of: a) hierarchical macro and mesopore structures
in MM-SBA15-4; b) high resolution image of hexagonal mesoporous
arrays in MM-SBA15-4 and c) mesoporous channels in MM-SBA15-2.
Fig. 4 Nitrogen adsorption–desorption isotherms of a) SBA-15; b)
MM-SBA15-1; c) MM-SBA15-2 and d) MM-SBA15-4 hierarchical
macroporous–mesoporous silicas.
This journal is
The Royal Society of Chemistry 2010
Green Chem., 2010, 12, 296–303 | 299
©

View Online
Table 1 Physical and structural properties of selected meso and macro-mesoporous silica
Surface
Mesopore
BJH pore
Mesopore domain
sizef/nm
Sample
areaa/m2g-1
volumeb/cm3g-1
diameterc/nm
d100d/nm
Wall thicknesse/nm
SBA-15
974
976
971
938
1.08 (1.18)
0.82 (1.07)
0.87 (0.99)
0.84 (1.15)
5.8
5.0
4.5
3.9
9.73
9.17
8.68
8.68
5.4
5.6
5.0
6.1
98.2
74.3
55.2
34.6
MM-SBA15-1
MM-SBA15-2
MM-SBA15-4
a Evaluated by multi-point BET method. b BJH mesopore volumes from the desorption isotherm, and total pore volume shown in brackets. c BJH
√
average pore diameters from desorption isotherm. d Interlayer spacing derived from Bragg’s Law. e Wall thickness = (2d100
/
3) - pore diameter.
f Calculated from Scherrer equation.
macropores interconnected via bottlenecks.45 Porosimetry also
confirms shrinkage of the mesopore diameter with increased
macropore character, shown in Fig. 5.
Table 2 Sulfur and sulfonic acid content of functionalized materials
evaluated by different techniques
Sample
S content awt %
RSO3H site densityb/nm2
RSO3-SBA-15
1.8 (1.90)
0.35
0.22
0.20
0.24
RSO3-MM-SBA15-1 1.2 (1.14)
RSO3-MM-SBA15-2 1.0 (0.98)
RSO3-MM-SBA15-4 1.2 (1.12)
a TGA values shown in brackets. b Based on mean S content from TGA
and XRF.
energy of 169.9 eV following peroxide treatment, indicative of
complete thiol oxidation to sulfonic acid (Figure S5†).
TGA was also performed to quantify the sulfonic acid
loading and thermal stability of untreated and functionalized
silicas (Fig. 6). SBA-15 tethered mercaptopropyl groups are
◦
known to decompose by 350 C,◦47 whereas propylsulfonic
acid moieties are stable until 450 C,43 at which point they
decompose evolving C3H6, SO2 and H2O.48 No major weight loss
occurred below 450 ◦C for any of the functionalized materials,
confirming complete conversion of grafted thiols into the desired
sulfonic acid groups. The loss between 400 and 500 ◦C, and
corresponding calculated total S content shown in Table 2, is
in good agreement with the degree of sulfonation predicted by
XRF. Although the absolute S content is expected to reflect
changes in the total surface area across these different porous
solids, if the inherent properties of the exposed silica surface
remain identical then one would anticipate a constant surface
density of grafted groups. The fall in sulfonic acid group sur-
face density on transition from mesoporous to dual-templated
SBA-15 silicas is therefore likely associated with the longer
Fig. 5 BJH pore size distributions of a) SBA-15; b) MM-SBA15-
1; c) MM-SBA15-2 and d) MM-SBA15-4 hierarchical macroporous–
mesoporous silicas.
These textural properties are summarized in Table 1. Conven-
tional mesoporous SBA-15 has a high specific surface area of
974 m2g-1 and pore volume of 1 cm3g-1, in good agreement with
the literature.9,39,40 Comparable high surface areas are achieved
for all the bi-modal MM-SBA15 materials, although the relative
mesopore volume decreases from 92 to 73% with increasing
macropore density, in line with expectations. The mesopore
walls thicken slightly following co-templating with PS beads,
demonstrating that the mesostructure is not degraded during
removal of the macropore template, and may even possess
greater mechanical stability than their pure mesoporous SBA-15
counterpart.
Sulfonic acid functionalization
Successful grafting of mercaptopropyl thiol groups on these
macroporous–mesoporous SBA-15 analogues, and their sub-
sequent oxidation to yield tethered sulfonic acid centres, was
subsequently assessed via XRF, TGA and XPS (Table 2).
Elemental analysis reveals the bulk S content ranges between 1.2
and 1.8 wt% across the series, increasing with total surface area.
These values are typical for grafted sulfonic acid silicas,40,46 and
equate to a sulfonic acid surface site density of ~0.24–0.35 nm-2.
In every case, surface sensitive S 2p XP spectra reveal the
presence of a single sulfur chemical environment with a binding
Fig. 6 TGA profiles of sulfonic acid functionalized SBA-15 and MM-
SBA15-4 hierarchical macroporous–mesoporous silicas.
300 | Green Chem., 2010, 12, 296–303
This journal is
The Royal Society of Chemistry 2010
©

View Online
calcination protocol required to synthesise the MM-SBA-15
family, and concomitant lower surface hydroxyl density for
subsequent thiol grafting.
It is important to note that the SBA-15 and MM-SBA15 ma-
terials retained their structural integrity post-functionalization.
Porosimetry and XRD (Figures S6 and S7†) demonstrate the
mesopore network is essentially unchanged by sulfonic acid
grafting, while SEM confirms no impact on the longer range
macroporosity (Fig. 7).
Scheme 1 Transesterification of tricaprylin to fatty acid methyl esters
and glycerol using methanol.
Steric factors render tricaprylin difficult to transesterify using
conventional solid acid catalysts,10,49 wherein diffusion-limited
kinetics are observed, making it an ideal model TAG with
which to assess potential benefits of the hierarchical solid
acids designed in this work. All samples proved active for
tricaprylin transesterification, albeit at a low level under these
mild conditions with only ~0.2 mmol converted after 6 h. Similar
low conversions are common in solid acid catalysed transesterifi-
cation at low temperature.50,51 Higher conversions were attained
at elevated temperature (which necessitated autoclave operation
under autogeneous conditions to prevent solvent loss), however
the primary goal of this work was not to optimise FAME yields,
hence of greater interest is the impact of macropores upon the
initial reaction rate and concomitant turnover frequency (TOF,
calculated on a per SO3H site basis). Fig. 8 reveals a striking
enhancement in the rate of transesterification as a function
of increasing macropore density. The sulfonated MM-SBA15-
4 material is more than twice as active as the corresponding
sulfonated mesoporous SBA-15, despite its lower total surface
area and pore volume. We attribute this enhanced reactivity
to the greater accessibility of sulfonic acid sites within the
mesopores of MM-SBA15-X materials. This improved mass-
transport throughout the pore network could arise in two
different ways. First, the interpenetrating macropores could
simply act as large conduits to boost tricaprylin bulk diffusion
throughout catalyst particles. Alternatively, by breaking up the
mesopore domain size, macropore incorporation may increase
the density of accessible sulfonic acid active sites which are likely
located at the entrances to these mesopores. Smaller mesopore
domains arising in the latter scenario would result in a higher
local density of such accessible active sites per mesopore volume.
In light of the strong interdependence between macropore
content and mesopore domain size (the latter accounting for
the majority of active surface acid sites) we cannot easily
discriminate these two possible mechanisms and are therefore
Fig. 7 SEM images of (a) as-prepared and (b) sulfonic acid function-
alized MM-SBA15-4 hierarchical macroporous–mesoporous silicas.
Catalytic reactivity
The efficacy of SBA-15 and MM-SBA15 sulfonic acid silicas
was subsequently evaluated towards the transesterification of
tricaprylin, a bulky C8 TAG, and separate esterification of
palmitic acid (a C16 saturated FFA), with methanol (Scheme 1).
These chemistries represent the two key steps necessary for
the development of a heterogeneous biodiesel manufacturing
process, namely the removal of FFA impurities and subsequent
transformation of remaining TAGs through to fatty acid methyl
esters.
Fig. 8 Tricaprylin transesterification with MeOH over hierarchical
macro–mesoporous SBA-15 sulfonic acid silicas. Conversions after 6 h
at 60 ◦C.
This journal is
The Royal Society of Chemistry 2010
Green Chem., 2010, 12, 296–303 | 301
©

View Online
currently undertaking molecular dynamics simulations to better
understand and optimise TAG diffusion via tuning the relative
macropore versus mesopore diameters.
2 S. Pinzi, I. L. Garcia, F. J. Lopez-Gimenez, M. D. Luque de Castro,
G. Dorado and M. P. Dorado, Energy Fuels, 2009, 23, 2325.
3 M. Lapuerta, O. Armas and J. Rodr´ıguez-Ferna´ndez, Prog. Energy
Combust. Sci., 2008, 34, 198.
Similar benefits were conferred by macropores in the ester-
ification of palmitic acid over these hierarchical sulfonic acid
silicas (Fig. 9). For all materials, conversions were far superior
to those observed during the transesterification, as is well-
established in the literature, reaching 55% after 6 h using the
sulfonated MM-SBA15-4 catalyst. The associated TOFs reveal
a similar progressive increase with rising macroporosity, with
esterification proceeding 50% faster over the acid modified MM-
SBA15-4 than its SBA-15 countepart.
4 A. Demirbas, Energy Convers. Manage., 2009, 50, 14.
5 K. Narasimharao, A. F. Lee and K. Wilson, J. Bio. Mater. Bioenergy,
2007, 1, 19.
6 A. Sivasamy, K. Y. Cheah, P. Fornasiero, F. Kemausuor, S. Zinoviev
and S. Miertus, ChemSusChem, 2009, 2, 278.
7 Z. X. Wang, J. Zhuge, H. Fang and B. A. Prior, Biotechnol. Adv.,
2001, 19, 201.
8 N. R. Soriano Jr, R. Venditti and D. S. Argyropoulos, Fuel, 2009, 88,
560.
9 S. Miao and B. H. Shanks, Appl. Catal., A, 2009, 359, 113.
10 E. Lotero, Y. Liu, D. E. Lo´pez, K. Suwannakarn, D. A. Bruce and
J. G. Jr Goodwin, Ind. Eng. Chem. Res., 2005, 44, 5353.
11 M. Di Serio, R. Tesser, L. Pengmei and E. Santacesaria, Energy Fuels,
2008, 22, 207.
12 M. Zabeti, W. M. A. W. Daud and M. K. Aroua, Fuel Process.
Technol., 2009, 90, 770.
13 E. Leclercq, A. Finiels and C. Moreau, J. Am. Oil Chem. Soc., 2001,
78, 1161.
14 Y. Liu, E. Lotero, J. G. Jr Goodwin and C. Lu, J. Catal., 2007, 246,
428.
15 M. Kouzu, T. Kasuno, M. Tajika, S. Yamanaka and J. Hidaka, Appl.
Catal., A, 2008, 334, 357; M. Verziu, B. Cojocaru, J. Hu, R. Richards,
C. Ciuculescu, P. Filip and V. I. Parvulescu, Green Chem., 2008, 10,
373.
16 R. S. Watkins, A. F. Lee and K. Wilson, Green Chem., 2004, 6,
335.
17 J. M. Montero, P. L. Gai, K. Wilson and A. F. Lee, Green Chem.,
2009, 11, 265.
18 Y. Liu, E. Lotero, J. G. Jr Goodwin and X. Mo, Appl. Catal., A, 2007,
331, 138; D. G. Cantrell, L. J. Gillie, A. F. Lee and K. Wilson, Appl.
Catal., A, 2005, 287, 183.
19 K. Wilson, C. Hardacre, A. F. Lee, J. M. Montero and L. Shellard,
Green Chem., 2008, 10, 654.
20 D. E. De Vos and P. A. Jacobs, Microporous Mesoporous Mater.,
2005, 82, 293.
21 D. Zhao, Q. Huo, J. Feng, B. F. Chmelka and G. D. Stucky, J. Am.
Chem. Soc., 1998, 120, 6024.
22 Y. Chen, J. Han and H. Zhang, Appl. Surf. Sci., 2007, 253, 9400.
23 S. Garg, K. Soni, G. M. Kumaran, R. Bal, K. Gora-Marek, J. K.
Gupta, L. D. Sharma and G. M. Dhar, Catal. Today, 2009, 141, 125.
24 I. K. Mbaraka, D. R. Radu, V. S. Y. Lin and B. H. Shanks, J. Catal.,
2003, 219, 329.
Fig. 9 Palmitic acid esterification with MeOH over hierarchical macro–
mesoporous SBA-15 sulfonic acid silicas. Conversions shown after 6 h
at 60 ◦C.
We are also investigating functionalization of this
macroporous–mesoporous SBA-15 family with more active
surface acid and base species to further promote plant oil
transesterification and esterification.
Conclusions
Macroporous SBA-15 were synthesized via a simple protocol,
using polystyrene bead templates, in order to improve diffusion
of bulk molecules such as long-chain triglycerides and free
fatty acids. PS/TEOS ratios of 2/1 and 4/1 lead to a meso-
macroporous interconnected network of high specific surface
area. TOFs evaluated from tricaprylin methanolysis catalysed by
these sulfonic-acid functionalized silicas show that macropores
enhance reactivity. These materials could provide a solution
to mass-transport limitations encountered in industry when
producing biodiesel from bulky plant oils via heterogeneous
catalysis. Our preliminary study provides a new illustration of
how tailoring the physical properties of solids can improve their
catalytic application.
25 E. Li and V. Rudolph, Energy Fuels, 2008, 22, 145.
26 S. Gheorghiu and M. O. Coppens, AIChE J., 2004, 50, 812.
27 A. Imhof and D. J. Pine, Nature, 1997, 389, 948; W. Deng, M. W.
Toepke and B. H. Shanks, Adv. Funct. Mater., 2003, 13, 61; Z. Y.
Yuan and B. L. Su, J. Mater. Chem., 2006, 16, 663.
28 X. Zhang, F. Zhang and K. Y. Chan, Mater. Lett., 2004, 58, 2872.
29 O. Sel, D. Kuang, M. Thommes and B. Smarsly, Langmuir, 2006, 22,
2311.
30 J. H. Sun, Z. Shan, T. Maschmeyer and M. O. Coppens, Langmuir,
2003, 19, 8395.
31 J. S. Yun and S. K. Ihm, J. Phys. Chem. Sol., 2008, 69, 1133.
32 C. G. Oh, Y. Baek and S. K. Ihm, Adv. Mater., 2005, 17, 270.
33 T. Sen, G. J. T. Tiddy, J. L. Casci and M. W. Anderson, Chem. Mater.,
2004, 16, 2044.
34 J. P. Blitz, and C. P. Little, Fundamentals and Applied Aspects of
Chemically Modified Surfaces, Royal Society of Chemistry, 1999,
235.
35 W. M. Van Rhijn, D. E. De Vos, B. F. Sels, W. D. Bossaert and P. A.
Jacobs, Chem. Comm., 1998, 317.
36 W. D. Bossaert, D. E. De Vos, W. M. Van Rhijn, J. Bullen, P. J. Grobet
and P. A. Jacobs, J. Catal., 1999, 182, 156.
37 I. D`ıaz, C. Marquez-Alvarez, F. Mohino, K. Prez-Pariente and E.
Sastre, J. Catal., 2000, 193, 283.
38 S. Vaudreuil, M Bousmina, S. Kaliaguine and L. Bonneviot, Adv.
Mater., 2001, 13, 1311.
Acknowledgements
We thank the Engineering and Physical Sciences Research
Council for financial support (EP/F063423/1; EP/G007594/1)
and the award of a Leadership Fellowship (A.F.L.).
39 D. Zhao, J. Feng, Q. Huo, N. Melosh, G. H. Fredrickson, B. F.
Chmelka and G. D. Stucky, Science, 1998, 279, 548.
40 P. F. Siril, N. R. Shiju, D. R. Brown and K. Wilson, Appl. Catal. A,
2009, 364, 95.
Notes and references
1 International Energy Agency, World Energy Outlook 2007 - Executive
Summary, IEA publications, 2007.
302 | Green Chem., 2010, 12, 296–303
This journal is
The Royal Society of Chemistry 2010
©

View Online
41 R. I. Kureshy, I. Ahmad, K. Pathak, N. H. Khan, S. H. R. Abdi and
R. V. Jasra, Catal. Commun., 2009, 10, 572.
42 P. Lanzafame, S. Perathoner and G. Centi, J. Porous Mater., 2007,
14, 305.
43 D. Margolese, J. A. Melero, S. C. Christiansen, B. F. Chmelka and
G. D. Stucky, Chem. Mater., 2000, 12, 2448.
44 J. J. Chiu, D. J. Pine, S. T. Bishop and B. F. Chmelka, J. Catal., 2004,
221, 400.
45 I. Nowak, Col. Surfa. A, 2004, 241, 103.
46 A. F. Lee, and K. Wilson, ‘Chapter 2: Sol–gel sulfonic acid silicas
as catalysts’ in Handbook of Green Chemistry - Green Catalysis,
Edited by P. T. Anastas & R. H. Crabtree, Wiley-VCH, 2009,
2.
47 D. Das, J. F. Lee and S. Cheng, J. Catal., 2004, 223, 152.
48 K. Wilson, A. F. Lee, D. J. Macquarrie and J. H. Clark, Appl. Catal.
A, 2001, 5913, 1.
49 X. Mo, D. E. Lo´pez, K. Suwannakarn, Y. Liu, E. Lotero, J. G. Jr
Goodwin and C. Lu, J. Catal., 2008, 254, 332.
50 L. Guerreiro, J. E. Castanheiro, I. M. Fonseca, R. M. Martin-Aranda,
A. M. Ramos and J. Vital, Catal. Today, 2006, 118, 166.
51 D. E. Lo´pez, J. G. Jr Goodwin, D. A. Bruce and E. Lotero, Appl.
Catal. A, 2005, 295, 97.
This journal is
The Royal Society of Chemistry 2010
Green Chem., 2010, 12, 296–303 | 303
©