Pore-expanded SBA-15 sulfonic acid silicas for biodiesel synthesis

Article abstract of DOI:10.1039/c1cc14563k

Here we present the first application of pore-expanded SBA-15 in heterogeneous catalysis. Pore expansion over the range 6-14 nm confers a striking activity enhancement towards fatty acid methyl ester (FAME) synthesis from triglycerides (TAG), and free fatty acid (FFA), attributed to improved mass transport and acid site accessibility.

Full text of DOI:10.1039/c1cc14563k

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COMMUNICATION
Pore-expanded SBA-15 sulfonic acid silicas for biodiesel synthesisw
J. P. Dacquin,ab A. F. Lee,a C. Pireza and K. Wilson*a
Received 26th July 2011, Accepted 27th October 2011
DOI: 10.1039/c1cc14563k
Here we present the first application of pore-expanded SBA-15
in heterogeneous catalysis. Pore expansion over the range 6–14
nm confers a striking activity enhancement towards fatty acid
methyl ester (FAME) synthesis from triglycerides (TAG), and
free fatty acid (FFA), attributed to improved mass transport and
acid site accessibility.
corresponding slow in-pore diffusion and catalytic turnover.
We recently reported the benefits of hierarchical porous networks,
incorporating 300–500 nm macropores to break up conventional
mesoporous channels to act as rapid transport conduits to the
active sites.9 However, the mesopores in this work remained close
to the molecular dimensions of a typical TAG (B5 nm); hence,
there is further scope for improving catalyst performance by also
enlarging the mesopores themselves.
Around 80% of the world’s energy demands are currently met
by fossil fuels. However, diminishing oil resources and rising
concerns over the global impact of CO2 emissions on climate
change have led to a demand for sustainable energy supplies.
Biodiesel sourced from non-food feedstocks has potential as
an environmentally benign, alternative transportation fuel.
Despite its early promise, societal concerns over land use,
and political debate over ‘eco-friendly’ alternative energy
technologies, have led to decreased biodiesel uptake in recent
years. However, with ever rising crude oil costs, biodiesel
manufacture is again viewed as a financially viable renewable
fuel, and indeed recent reinstatement of tax credits by the US
government is widely expected to increase biodiesel production.1
Biodiesel comprises fatty acid methyl esters (FAME) of free
fatty acids (FFAs) derived from various non-edible oils and
animal fats, which are converted via reaction with liquid H2SO4
or soluble bases such as Na or K alkoxides to respectively esterify
FFA or transesterify TAGs to their corresponding FAME.2
Unfortunately, biodiesel purification remains an energy intensive
step, generating vast quantities of aqueous waste containing
alkali salts and impure glycerol by-product. Development of a
heterogeneously catalysed process offering impurity-free fuel
from low cost raw materials is essential to overcome these
obstacles to commercialisation.3 Although numerous solid acids
and bases have been explored for biodiesel synthesis,4 there have
been minimal efforts to tailor catalyst porosity to optimise mass-
transport of the bulky and viscous C16–C18 TAGs or FFAs
typical of plant oils. Conventional SBA-15 mesoporous silica
derived catalysts have been previously examined in biodiesel
synthesis5–8 possessing long, isolated parallel channels with
Porogens including trimethylbenzene,10,11 triethylbenzene
or triisopropylbenzene12 have been employed to swell the
Pluronic P123 micelles used to produce SBA-15, enabling
formation of mesopores with diameters 5 to 30 nm. Surprisingly,
the potential catalytic applications of the resulting pore-expanded
materials have not been exploited. The only previous reports
of expanded mesopore catalysts are limited to MCM-41,13 and
a composite of polystyrene sulfonic acid within an ultra-large
pore SBA-15.14
While sulfonic acid-functionalized mesoporous materials
have been previously tested in biodiesel synthesis,15–17 the
pore sizes of such catalysts fall r6.5 nm and the relationship
between pore diameter and mass transport properties remains
unexplored. Here we present the first systematic report on the
impact of pore diameter on the catalytic activity of sulfonic
acid derivatised, pore-expanded SBA materials towards palmitic
acid esterification, and the transesterification of tricaprylin
and triolein, the latter a key component of commercial plant
oil feedstocks e.g. palm, sunflower and jatropha.
Large pore SBA-15 variants (SBA-15-X) were obtained by
adapting the approach of Fajula and co-workers11 as described
in the ESI. Briefly, an appropriate amount of trimethylbenzene
(TMB) was incorporated into the Pluronic P123/tetraethyl-
ortho-silicate (TEOS) micelle templating precursor solution,
which was subsequently aged at different times (1–3 days) and
temperatures (80–120 1C) in order to expand the integral
micelles and silica pores following calcination. The successful
synthesis of pore-expanded silica was first verified by low angle
XRD, N2 porosimetry and HRTEM. The N2 adsorption-
desorption isotherms shown in Fig. 1 (inset) reveal that
TMB does not perturb the isotherm type of the resulting
silica, which remain Type IV. However, the hysteresis loop
progressively shifts to higher relative pressures with both
increasing aging time and temperature indicative of increasing
pore size. The steep step and narrow hysteresis loops in the
adsorption/desorption branches, evidence retention of the
parent SBA-15 cylindrical, parallel pore channels. BJH analysis
on the desorption branch (Fig. 1), shows that pores diameters
a Cardiff Catalysis Institute, School of Chemistry, Cardiff University,
Main Building, Park Place, Cardiff CF10 3AT, UK.
E-mail: wilsonk5@cardiff.ac.uk; Fax: +44 (0) 29 208 74030;
Tel: +44 (0)29 208 70827
b Present address:UCCS, University of Lille I,
59655 Villeneuve d’Ascq, France
w Electronic supplementary information (ESI) available: Experimental
details for materials synthesis and catalyst testing; full details of
catalyst characterisation. See DOI: 10.1039/c1cc14563k
c
212 Chem. Commun., 2012, 48, 212–214
This journal is The Royal Society of Chemistry 2012

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corresponding increase in mesopore volume (from 1.08 cm3gÀ1
to 2.29 cm3 gÀ1 respectively), upon pore-expansion, consistent
with the geometric changes of the mesopore network. Our
pore-expanded silicas were subsequently functionalised with
sulfonic acid by mercaptopropyl trimethoxysilane (MPTS)
grafting followed by oxidation with H2O2 (Scheme S1).
This grafting route was adopted since it enables conformal
derivatisation of the entire surface.18 XRD, porosimetry and
HRTEM confirmed retention of the SBA-15 mesostructures
during this process, while XPS showed a single S chemical
environment (S 2p binding energy = 169.9 eV), indicative
of total thiol conversion to sulfonic acid (Fig. S2, ESIw).
The bulk and surface sulfonic acid loadings were determined
via EDX and XPS respectively, revealing a decrease in bulk
S content from 1.02 to 0.46 wt% across the series with falling
surface area as expected (Table S2, ESIw). However, the
overall SO3H surface density remains B0.21 nmÀ2 across the
series, similar to values reported for grafted sulfonic acid
silicas,19,20 and consistent with a constant surface silanol density
upon pore expansion. Indeed, the S surface concentration
determined by XPS remained B0.45 wt% across our SBA-15
series, above that needed to maintain strong acid sites.18
The performance of the large pore, mesostructured sulfonic
acid silicas was evaluated in the esterification of palmitic acid,
and separate transesterification of tricaprylin and triolein, with
methanol. These represent the two key steps necessary for the
development of a heterogeneously catalysed biodiesel manu-
facturing process: the conversion of FFA impurities into
FAME (Scheme 1); and subsequent transformation of the
remaining TAGs to FAME. Steric factors and associated
diffusion limitations render tricaprylin and triolein difficult
to transesterify via conventional solid acid catalysts,21 making
them interesting targets for mesostructured solid acids in their
own right, with the C18 triolein also the major component of
olive oil, and thus an excellent model TAG with which to
assess the potential benefits of mesopore expansion.
Fig. 1 BJH pore size distribution from the desorption branch of the
isotherm and (inset) isotherms for (a) SBA-15 and expanded pore
analogues (b) SBA-15-8 and (c) SBA-15-14.
up to 14 nm can readily be achieved via this methodology.
X-ray diffraction reveals all materials have good long range
order with characteristic (100) diffraction peaks consistent
with the p6mm hexagonal symmetry of SBA-15 (Fig. 2). A
progressive shift of the d100 diffraction peak towards lower
angle occurs across the series, associated with an increased
d-spacing with aging temperature. This pore-expansion can
also be observed by HRTEM (inset and Fig. S1, ESIw) which
reveals highly-ordered periodic mesostructures, with pore
diameters B6, 8 and 14 nm. Additional physical parameters
for the expanded SBA-15 series are given in Table S1,
ESIw. The BET surface area decreased from 900 m2gÀ1
(SBA-15) to 555 m2 gÀ1 (SBA-15-14), accompanied by a
Reactions were performed under mild conditions to maintain
relatively low conversions and enable kinetic analysis and were
benchmarked against a commercial Amberlyst-15 acid catalyst.
All samples were active for palmitic acid esterification, with
B25% conversion after 6 h (Table 1). Fig. 3 shows the
corresponding Turnover Frequencies (TOFs, initial rates nor-
malised per acid site) which indicate that all our SBA-SO3H
ÀÀÀ
samples significantly outperform Amberlyst. Pore-expansion
enhances TOF from 23 hÀ1 for conventional sulfonic acid
SBA-15 to 120 hÀ1 for the analogous SBA-15-14 material.
Fig. 2 Low angle XRD of (a) SBA-15-6 and expanded pore analogues
(b) SBA-15-8 and (c) SBA-15-14. Inset shows HRTEM of (i) SBA-15-6
and (ii) SBA-15-14.
Scheme 1 (a) FFA esterification and (b) TAG transesterification to
FAME.
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 212–214 213

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Table 1 Comparison of RSO3H-SBA-15-X activity with related solid
acids in FFA esterification and TAG transesterification.
The application of pore-expanded SBA-15 silicas in hetero-
geneous catalysis, in the present instance as solid acid catalysts, is
reported for the first time. We demonstrate pore expansion
confers 43-fold increase in activity towards both C16 FFA
esterification and C8/C18 TAG transesterification versus
conventional SBA-15, likely reflecting superior mass-transport
of the bulky free fatty acid and triglycerides. We are currently
undertaking molecular dynamic and Monte Carlo simulations
to further optimise these pore-expanded silicas and assess
the potential benefits of macropore integration to generate
hierarchical pore-expanded networks. This work highlights the
significant benefits in catalytic clean technologies achievable
through careful nanoengineering of materials for the transfor-
mation of bulky biomass-derived feedstocks.
a
C8 & C18 TAG
C16 FFA
esterification
transesterification
Convb
/%
Convb
/%
TOFc
TOFc
/hÀ1
/hÀ1
23.4
Catalyst
Ref
RSO3H
SBA-15-6
RSO3H
23.2
2.1
0.1
5.9
0.25
6.7
0.3
0.4
0.4
2.5
3.3
0.04
6.6
0.9
11.7
1.1
0.04
0.04
7.8
This
work
This
32.9
33.5
31.3
55.4
55.2
120.2
2.6
SBA-15-8
RSO3H
SBA-15-14
Amberlystd
work
This
work
RSO3H
MM-SBA-4
Cs2.8H1.2SiW12O40
35.6
100.6
9
76
11
12.5
22
We gratefully acknowledge financial support from the EPSRC
under grants EP/F063423/1 and EP/G007594/2. KW thanks the
Royal Society for the award of an Industry Fellowship, and AFL
thanks the EPSRC for the award of a Leadership Fellowship.
a
b
C18Triolein in italics; After 6 hrs reaction; Calculated per S site
c
(based on bulk S content); Amberlyst acid loading 4.3 mmol gÀ1
d
Notes and references
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Fig. 3 Reactivity of Amberlyst and sulfonic acid derivatised SBA-15-6;
SBA-15-8 and SBA-15-14 catalysts in palmitic acid esterification and
transesterification of tricaprylin and triolein.
Similar enhancement is seen in tricaprylin and triolein trans-
esterification with increasing pore diameter across the series.
We attribute the improved performance in both reactions to
the greater accessibility of sulfonic acid sites resulting from
larger mesopores. Table 1 compares conversions and TOFs
of our pore-expanded SBA-15 sulfonic acids in esterification
and transesterification, revealing they even outperform
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transesterification.18 It is therefore interesting to note the
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likely possesses similar acid strength (ÀDH(NH3)ads of
120–130 kJ molÀ1 vs. 133 kJ molÀ1 for grafted propylsulfonic
acid SBA19).
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´ ´
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c
214 Chem. Commun., 2012, 48, 212–214
This journal is The Royal Society of Chemistry 2012