Hydrothermal saline promoted grafting: A route to sulfonic acid SBA-15 silica with ultra-high acid site loading for biodiesel synthesis

Article abstract of DOI:10.1039/c4gc01139b

A simple grafting protocol is reported which affords a ten-fold enhancement in acid site density of mesoporous sulfonic acid silicas compared to conventional syntheses, offering improved process efficiency and new opportunities for tailored supported solid acids in sustainable chemistry. This journal is

Full text of DOI:10.1039/c4gc01139b

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Hydrothermal saline promoted grafting: a route to
sulfonic acid SBA-15 silica with ultra-high acid site
loading for biodiesel synthesis†
Cite this: Green Chem., 2014, 16,
506
4
Received 17th June 2014,
Accepted 22nd July 2014
a
b
b
b
b
C. Pirez,*‡ A. F. Lee, J. C. Manayil, C. M. A. Parlett and K. Wilson*
DOI: 10.1039/c4gc01139b
www.rsc.org/greenchem
A simple grafting protocol is reported which affords a ten-fold the waste water generated from these quench steps and also
enhancement in acid site density of mesoporous sulfonic acid offering possibilities for continuous rather than batch pro-
8
silicas compared to conventional syntheses, offering improved cesses; both core principles of green chemistry.
process efficiency and new opportunities for tailored supported
Numerous solid acid materials have been investigated for
biodiesel synthesis spanning ion-exchange resins, sulphated
and tungstated zirconia, ion-exchanged heteropolyacids, zeo-
solid acids in sustainable chemistry.
Heterogeneous catalysis will play a critical role in the efficient
conversion of biomass to fuels and chemicals, with new acid
catalysts required for a range of transformations spanning
dehydration, etherification, esterification and transesterifica-
tion processes. The microporous nature of zeolites makes
them unsuitable for such transformations with poor mass
5
lites and sulfonic acid silicas. Practical application of such
solid acid catalysts is however limited by poor acid site accessi-
bility to bulky FFA and TAG molecules and low acid site load-
ings; necessitating use of high catalyst quantities often >20 wt%
9
for resins. Sulfonic acid functionalised mesoporous and
hierarchical meso–macroporous templated silicas are attractive
solid acids designed to improve diffusion of bulky TAG and
1
transport limiting their performance. There is thus an urgent
need for tailored solid acids with well-defined mesopore archi-
tectures and high surface acid site loadings for biomass
FFA molecules and active site accessibility in acid catalysed
10–12
biodiesel synthesis.
While existing materials offer
2
utilisation.
12,13
superior TOFs to Amberlyst-15,
0.4–0.7 mmol g limit the ultimate process yields. Further-
low acid site loadings of
One such process whose commercial viability depends on
the availability of efficient and robust solid acid catalysts is
commercial biodiesel production from the transesterification
−1
∼
more, use of one-pot co-condensation routes to achieve higher
−1
sulfonic acid loadings approaching the 5 mmol g of Amber-
and esterification of plant, algae or waste oil triglycerides
14
lyst lead to loss of mesoscopic order of the pore architecture.
3
(
TAG) and fatty acids (FFA). Biodiesel production from high
Thus a post-grafting method to increase sulphonic acid load-
ings while retaining pore order would offer greater versatility
to prepare nano-porous solid acid catalysts for biomass
transformation.
The concentration of active Si–OH, particularly on calcined
silicas, limits the grafting efficiency of mercaptopropyl silane
FFA containing waste oils cannot be produced directly using
base catalysts, and requires an acid catalysed pre-esterification
step to remove FFA as FAME, prior to base catalysed TAG trans-
esterification. The energy intensive fuel purification via
aqueous quench step associated with these processes is detri-
mental to the economic viability of biodiesel production.
Thus, the availability of efficient solid acid catalysts capable of
simultaneous TAG and FFA conversion in waste oils would
(
MPTMS) precursor used to form sulfonic acid groups on silica
surfaces. However, as silica surfaces are sensitive to hydrother-
mal treatment, it was rationalised that mild hydrothermal
treatment could be used to activate the surface of calcined
SBA-15 and increase its capacity for grafting. NaCl is used to
moderate the wall thickness and the degree of polymerization
4
–7
greatly improve biodiesel process efficiency
by eliminating
1
5
a
School of Chemistry, Cardiff University, Park Place, Cardiff, CF10 3AT, UK
during post-synthesis hydrothermal treatments of SBA-15,
b
16
European Bioenergy Research Institute, School of Engineering and Applied Sciences,
and also aid the dissolution of silica, thus we hypothesised
that addition of NaCl under mild hydrothermal conditions
could promote the activation of SBA-15 surfaces for grafting.
Here we report on a new hydrothermal saline promoted graft-
Aston University, Birmingham, B4 7ET, UK. E-mail: k.wilson@aston.ac.uk
Electronic supplementary information (ESI) available: Porosimetry data, XRD,
†
XPS, MAS-NMR, TGA, DRIFT spectra and FFA esterification reaction profiles. See
DOI: 10.1039/c4gc01139b
‡
Present address: UCCS, University of Lille I, 59655 Villeneuve d’Ascq, France.
2
ing (HSPG) method, in which the effect of [H O]–[NaCl] ratio
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during MPTMS grafting on SBA-15 on acid site loading and the range 5–7 ppm as a function of loading. This feature is
associated catalyst performance is investigated.
The impact of [NaCl] on sulfonic acid loading was first acid groups,
investigated by porosimetry, XRD, XPS and NH
associated with protons from H O bound to hydrated sulfonic
2
1
9–21
with fast proton exchange between water and
2
1,22
3
pulse titration the sulfonic acid group leading to a single peak.
The strik-
1
(Table S1†). Acid site loading and sulfur density varies in a ing correlation between δ H and acid site density is thus
1
9
linear fashion with acid site density (Fig. S3†), suggesting the indicative of increased acid strength, indicating enhanced
2
3,24
organosilane is uniformly dispersed with minimal agglomera- cooperative effects,
tion in all samples. Porosimetry and low angle XRD (Fig. S1†) from 0.7 to 2.2 mmol (H ) g . Furthermore NH3 TPD peak
evidenced that all materials retain their mesoporous structure maxima shown in Fig. S8† increase from Tol-PrSO H/SBA-15 <
upon sulfonic acid functionalization, with pore spacing ∼10 nm. PrSO H-SBA-15-OP < PrSO H/SBA-15-200, confirming enhanced
BJH analysis reveals grafting from H O–NaCl produces a acid strength.
increasing the surface acid site density
+
−1
3
3
3
2
2
9
bimodal pore size distribution with maxima ∼3.6 and 4.6 nm.
Si CP-MAS NMR (Fig. S9†) was employed to quantify the
The relative intensity of these two components passed through relative concentrations of Qn and Tm modes of the grafted
−
1
18,22,25
a maximum for [NaCl] = 3.4 mmol g
for PrSO H/ organosilane
and thus explore the mode of attachment
(
SiO2)
3
SBA-15-200, corresponding to a molar NaCl–hydroxyl ∼1 and (Table S2†). A slight decrease in the Q3 species was observed
maximum incorporation of sulfonic acid groups as shown in upon grafting in toluene indicative of attachment at isolated
Fig. 1. TEM (inset) shows the hexagonal pore structure of hydroxyl sites, while the intense Q3 band for the
2 3
SBA-15 is retained following grafting in H O–NaCl, which PrSO H-SBA-15-OP is consistent with an uncalcined material
coupled with the observed constant pore spacing from XRD, with less cross-linking and Si–O–Si bridges. In contrast the Q2
suggests the decrease in BJH pore diameter of ∼1 nm upon species of HSPG samples decreased strongly, consistent with
grafting is consistent with the formation of a compact mono- preferential grafting at geminal silanols. The increased graft-
layer of PrSO
Insight into the nature of the grafted sulfonic acid species partial ‘dissolution’ of the SBA-15 surface activating Si–O–Si
formed upon grafting in H O–NaCl as compared to con- groups towards grafting. Enhanced surface cross-linking fol-
3
H groups as illustrated in Fig. 1.
ing efficiency under HSPG conditions is thus attributed to
2
ventional toluene grafting or one-pot methods was further lowing HSPG is also evident from the increase in Tm co-
studied by MAS-NMR, TGA and XPS. The absence of any di- ordinated species which would lead to a more stable coating.
1
3
17
sulphide species peaks at 41 and 23 ppm by C MAS-NMR,
Changes in the Si–OH region observed by DRIFTS (Fig. S10
(II)
14
-S
peak at 162 eV in S 2p XPS and weight loss <350 °C in and Scheme 1a†) further support these observations, with
TGA (Fig. S3–6†) confirmed the total oxidation of grafted HSPG showing a complete loss of isolated, germinal and
MPTS to sulfonic acid groups. H MAS-NMR (Fig. S7†) revealed vicinal modes and thus reaction of these groups with the
1
8
1
1
δ H of the hydrated surface SO
2
OH
x
(H
2
O) group shifts over silane. A possible mechanism for the role of NaCl in promot-
ing surface grafting is outlined in Scheme S1b† which illus-
trates how hydrothermal treatment in H O–NaCl could open
2
Si–O–Si bridges, with the resulting Si–OH stabilised by ionic
−
interactions with Cl prior to grafting of MPTS, akin to the way
that ionic liquids are proposed to weaken hydrogen bonding in
2
6
cellulose. The decreased grafting efficiency above a 1 : 1 ratio
of NaCl–OH presumably reflects reduced silanol accessibility
from excess surface adsorbed salt. The physical properties of
HSPG and conventional sulfonic acid silicas are compared in
Table S3 and Fig. S11–12,† with HRTEM and XRD measure-
ments showing the hexagonal pore structure is maintained.
3
The optimum HSPG prepared catalyst PrSO H/SBA-15-200
was subsequently evaluated in FFA esterification and TAG
transesterification and compared to conventional grafted or
one-pot materials. The conversion of tributyrin and tripalmitin
during transesterification was observed to follow the order
PrSO
3
H/SBA-15-200 > PrSO
3 3
H/SBA-15-OP > PrSO H/SBA-15-Tol,
with PrSO
3
H/SBA-15-200 increasing the FAME yield from tri-
palmitin by a factor of three over conventional grafted
materials (Fig. 2). Despite having an inferior pore diameter,
the corresponding turnover number (TON) for PrSO H/
3
Fig. 1 Effect of [NaCl]–hydroxyl ratio during –PrSO
3
H grafting in
H
2
O–NaCl on resulting acid site loading, as compared to conventional
SBA-15-200 in transesterification exceeds conventional
toluene grafted and one-pot co-condensed materials. Inset shows
materials (Table S4†). Transesterification is rate-limited by pro-
HRTEM for [NaCl]–hydroxyl = 1.1, confirming ordered mesoporosity is
retained. (Hydroxyl density for parent SBA-15 calculated to be 3 mmol g−1
2
7
tonation of the ester and thus sensitive to catalyst acid
strength, hence enhanced performance can be attributed to
from TGA over the range 200–1000 °C.)
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Fellowship (KW). We also acknowledge the kind assistance of
Mark Isaacs for XPS analysis and D.C. Apperley at the EPSRC
UK National Solid-state NMR Service at Durham.
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9
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We thank the EPSRC (EP/K000616/2, EP/K000616/2 and EP/ 24 J.-P. Dacquin, H. E. Cross, D. R. Brown, T. Duren,
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