Octyl Co-grafted PrSO3H/SBA-15: Tunable Hydrophobic Solid Acid Catalysts for Acetic Acid Esterification

Article abstract of DOI:10.1002/cctc.201601370

Propylsulfonic acid (PrSO₃H) derivatised solid acid catalysts have been prepared by post-modification of mesoporous SBA-15 silica with mercaptopropyltrimethoxysilane (MPTMS), and the impact of co-derivatisation with octyltrimethoxysilane (OTMS) groups to impart hydrophobicity to the catalyst was investigated. Turnover frequencies (TOFs) for acetic acid esterification with methanol increase with PrSO₃H surface coverage across both families, suggesting a cooperative effect between adjacent acid sites at high acid site densities. Esterification activity is further promoted upon co-functionalisation with hydrophobic octyl chains, with inverse gas chromatography (IGC) measurements indicating that the increased activity correlates with decreased surface polarity or increased hydrophobicity.

Full text of DOI:10.1002/cctc.201601370

Accepted Article
Title: Octyl co-grafted PrSO3H/SBA-15: tunable hydrophobic solid acid
catalysts for acetic acid esterification.
Authors: Jinesh Manayil, Vannia dos Santos, Friederike Jentoft, Marta
Granollers Mesa, Adam F Lee, and Karen Wilson
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Octyl co-grafted PrSO3H/SBA-15: tunable hydrophobic solid acid
catalysts for acetic acid esterification
Jinesh C. Manayil,[a] Vannia C. dos Santos, [a] Friederike C. Jentoft, [b] Marta Granollers Mesa,[a] Adam
F. Lee,[a] and Karen Wilson*[a]
Abstract: Propylsulfonic acid (PrSO3H) derivatised solid acid
catalysts have been prepared by post modification of mesoporous
SBA-15 silica with mercaptopropyltrimethoxysilane (MPTMS), with
the impact of co-derivatisation with octyltrimethoxysilane (OTMS)
groups to impart hydrophobicity to the catalyst investigated. Turn
over frequencies (TOF) for acetic acid esterification with methanol
increase with PrSO3H surface coverage across both families
suggesting a cooperative effect of adjacent acid sites at high acid
site densities. Esterification activity is further promoted upon co-
functionalisation with hydrophobic octyl chains, with inverse gas
chromatography (iGC) measurements indicating increased activity
correlates with decreased surface polarity or increased
hydrophobicity.
ketonisation or esterification to reduce acidity prior to the final
hydro-deoxygenation treatment.[3b, 6]
Esterification of acetic acid, which is present in bio-oil at
around 1-10 %,[4b] is an energy efficient and atom-economical
means to improve bio-oil quality and stabilise the oil for further
upgrading.[3a] Suitable alcohols employed in esterification are
either native to the bio-oil such as phenolics (guaicol or
cresol)[3a] or externally sourced bio-derived alcohols such as
methanol, ethanol and butanol. The use of homogenous mineral
acids to catalyse esterification, while effective, requires
subsequent quenching and neutralisation of the treated bio-oils,
which results in large quantities of caustic waste streams and
associated handling problems. Solid acid catalysts are thus
sought to circumvent these problems by allowing facile
separation and opportunities for continuous operation.
Typical solid acids explored for acetic acid esterification
include sulfated zirconia, zeolites, heteropoly acids, and
functionalised mesoporous silicas.[3a, 6d, 7] Given the high water
content of bio-oils is it also critical that solid acids are developed
that exhibit excellent water tolerance or hydrophobicity. In this
respect, mesoporous sulfonic acid silicas are a particularly
interesting class of Brönsted acid catalyst[8] that are widely
explored in the context of biofuel related catalysis[9] owing to
their ability to allow both support architecture and surface
polarity to be tuned. While these materials have received some
attention for the esterification of acetic acid with methanol and
benzyl alcohol[10] in model bio-oils, the efforts to address the
impact of hydrophobicity on esterification activity are limited.
PrSO3H/SBA-15 modified with propyl groups to impart
hydrophobicity has been shown to exhibit increased water
tolerance during acetic acid esterification when compared to
H2SO4 or the conventional PrSO3H/SBA-15 catalyst.[10a] While
Introduction
The use of renewable resources for the sustainable production
of transportation fuels and chemicals is currently of great interest
due to growing concerns over the depletion of fossil fuel
reserves and associated climate change.[1] Thermochemical
processing of lignocellulosic biomass through pyrolysis or
gasification, and transesterification of non-edible and waste
plant/algal oils and fats offers a promising solution to transform
biomass for use in such applications.[1-2] Fast pyrolysis of waste
agricultural/forestry biomass for the production and subsequent
upgrading of bio-oils to liquid transportation fuels has received
considerable attention in this regard.[3] However, the direct use
of fast pyrolysis bio-oils is limited by its low heating value due to
the high oxygen content, thermal instability, strong acidity and
significant water content.[4] Typical bio-oils are a mixture of acids,
alcohols, furans, aldehydes, esters, ketones, sugars and
multifunctional compounds such as hydroxyacetic acid,
hydroxyacetaldehyde and hydroxyacetone (derived from
cellulose and hemicellulose), together with 3-hydroxy-3-
methoxybenzaldehyde, phenols, guaiacols and syringols derived
from the lignin component.[5] The production of transportation
fuels from biomass-derived pyrolysis oils is therefore only viable
if the oil is subjected to upgrading treatments including
this is
a promising approach to tailoring catalysts for
esterification reactions, our understanding of the interplay
between surface polarity and activity is limited, hindering our
ability to design improved catalyst formulations.
Inverse gas chromatography (iGC) is a powerful technique
to probe the surface apolar and polar interactions of materials at
[12]
a molecular level.[11]
IGC measurements thus allow surface–
adsorbate interactions to be investigated on porous catalysts,
with thermodynamic properties including surface energy,
hydrophobicity[13] acid-base properties,[14] heat of adsorption,[15]
and specific free energy[16] to be determined. Evaluation of
parameters such as surface polarity and hydrophobicity is critical
for understanding how adsorption processes can be controlled in
liquid and vapour phase catalysis. Here we report a study of a
series of propyl sulfonic acid functionalised SBA-15 materials in
which special attention has been paid to the effect of acid site
density on the overall acidity and catalytic performance in acetic
acid esterification with methanol. The effect of co-functionalising
PrSO3H/SBA-15 with octyltrimethoxysilane groups to impart
hydrophobic character is subsequently investigated, with the
[a]
Dr. Jinesh C. Manayil, Dr. Vannia C. dos Santos, Dr. Marta
Granollers Mesa, Prof. Adam F. Lee, Prof. Karen Wilson.
European Bioenergy Research Institute,
Aston University,
Birmingham, B4 7ET, UK
E-mail: k.wilson@aston.ac.uk
[b]
Prof F.C. Jentoft
Department of Chemical Engineering,
University of Massachusetts Amherst,
MA 01003, USA
Supporting information for this article is given via a link at the end of
the document.
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10.1002/cctc.201601370
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FULL PAPER
impact on surface properties analysed using IGC.[13] Calculated
thermodynamic parameters are correlated with esterification
activity, which in turn correlates with decreased surface polarity
or increased hydrophobicity.
respectively.[18] Following incorporation of sulfonic acid groups
new weak bands centred ~2950 and 2854 cm-1 evolve
[10c]
corresponding to CH2 vibrations from alkyl chains.[19]
which
increase in intensity with sulfonic acid loading. Further increases
are observed, along with the emergence of a new peak at 2938
cm-1 upon co-grafting with octyl chains as a result of the larger
number of –CH2 groups. Symmetric sym(CH) of methyl and
methylene groups typically overlap, but the asymmetric
stretches do not, this new band at 2938 cm-1 is thus ascribed to
as(CH2) of the methylene stretch.
CHNS analysis on both the parent PrSO3H/SBA-15 and
octyl co-functionalised samples show a steady increase in S
content with the volume of MPTMS added during grafting
process (Table 1). Fig. 1 shows that the C:SO3H molar ratio for
the Oc/PrSO3H/SBA-15 series decreases with increasing S
content, thereby demonstrating the successful synthesis of a
family of materials with tuneable surface acid site loading and
loading of inert organic groups to tune hydrophobicity. Fig. 1
(inset) and Table 1 show the acid site loading increases with S
content for both PrSO3H/SBA-15 and Oc/PrSO3H/SBA-15 series.
The surface acid site loading of the Oc/PrSO3H-SBA-15 family
Results and Discussion
The successful synthesis of the parent SBA-15, and retention of
the hexagonal close packed p6mm pore architecture upon
functionalisation with propyl sulfonic and octyltrimethoxysilane
groups, was first assessed by low angle XRD and nitrogen
adsorption isotherms (Figs. S1 and S2).[10c, 17] All functionalised
materials show similar XRD patterns with common reflections at
1.07°, 1.75°, and 2.01° consistent with the parent SBA-15,
confirmed that grafting, co-grafting and subsequent oxidation
does not alter the pore order. The type IV isotherm with H1
hysteresis loop of SBA-15 is also maintained after derivatisation
evidencing retention of mesoporosity.
The textural properties of PrSO3H/SBA-15 and
Oc/PrSO3H/SBA-15 materials are summarised in Table 1. The
BET surface area decreases slightly upon derivatisation with
PrSO3H groups, which is largely attributed to the loss of
micropore area in the pore walls as determined from t-plot
analysis, suggesting partial micropore blockage occurs during
grafting. A slight decrease in the BJH mesopore diameter is also
observed following sulfonic acid derivatisation, but the diameter
remains in the range 4.6-4.8 nm for all functionalised samples. It
is interesting to note that the micropore area associated with
these intra-wall pores decreases steadily as a function of
sulfonic acid loading, suggesting progressive filling of the
micropores. The micropore surface area of the octyl co-
derivatised samples decreases for all but the highest loaded
PrSO3H/SBA-15 sample, suggesting OTMS caps any
unfunctionalised sites remaining in the micropore channels of
the lower loaded PrSO3H/SBA-15 samples.
was lower than the parent PrSO3H/SBA-15 owing to
a
combination of the extra mass from octyl groups, and slight loss
of thiol groups during the second grafting step. DRIFTS studies
of pyridine adsorption confirm the presence of Brönsted acidic
sites[20] with bands at 1489, 1545 and 1637 cm-1 indicative of
pyridinium ion formation (Fig. S4).
NH3 calorimetry (Fig. S5) revealed the acidic strength of
PrSO3H/SBA-15 to be invariant of sulfonic acid loading, with
ΔHads(NH3) determined to be -140 kJmol-1. This was further
corroborated by using Gutmann theory of acid-base interactions
(Equation 1) which explains how the enthalpy of adsorption
(ΔHSP) of a probe molecule depends on the acceptor and donor
number (AN and DN respectively) of the adsorbate and the acid
and base constants of solid surface, (Ka and Kb respectively).
ΔHSP= AN.Kb + DN.Ka
(1)
The co-existence of sulfonic acid and octyl chains on the
SBA-15 surface is confirmed by DRIFTS (Fig. S3). The parent
SBA-15 shows characteristic bands at 700-1400 cm-1 and 3000-
3800 cm-1 indicative of framework Si-O-Si and surface silanols
Table. 1 Textural and structural properties of PrSO3H and Oc/PrSO3H functionalised SBA-15 materials.
Materials
Surface areaa
/m2 g-1
BJH pore
Total BJH
pore volume /
cm3 g-1
Unit cell
parameterb thicknessc
Wall
Micropore
area
/m2 g-1
Bulk S
contentd
/wt.%
Bulk C
contentd /
Acid site
loading
/ mmol.g-1
diameter / nm
/nm
/nm
wt%
SBA-15
759
682
637
620
4.9
4.6
4.6
4.6
0.76
0.71
0.68
0.65
10.17
10.25
10.19
10.15
5.24
5.70
5.64
5.60
305
250
228
225
-
0.80
1.03
1.39
1.58
-
0.15PrSO3H/SBA-15
0.24PrSO3H/SBA-15
0.42PrSO3H/SBA-15
0.15
0.24
0.42
0.17
0.21
0.33
0.77PrSO3H/SBA-15
663
539
545
616
587
4.7
4.6
4.6
4.7
4.8
0.77
0.64
0.61
0.75
0.64
10.19
10.13
10.15
10.19
10.09
5.52
5.57
5.58
5.51
5.30
115
123
127
149
148
0.77
0.17
0.23
0.41
0.69
1.80
1.60
1.88
2.03
2.35
0.44
0.11
0.12
0.16
0.28
Oc/0.15PrSO3H/SBA-15
Oc/0.24PrSO3H/SBA-15
Oc/0.42PrSO3H/SBA-15
Oc/0.77PrSO3H/SBA-15
aBET. bDetermined from a0 = (2d100)/√3. cDetermined from a0 - pore diameter. dBulk S and C content from CHNS.
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Calculation of acid constants from IGC measurements of the
adsorption of methanol, acetonitrile, ethyl acetate and
dichloromethane revealed similar values of Ka for low and high
loading PrSO3H/SBA-15 samples (Fig. S5).
suggesting the hydrophobic character imparted by the octyl
chain assists with inhibiting reverse ester hydrolysis as
previously reported for PrSO3H/MCM41.[19] This change in TOF
observed for both families with PrSO3H loading could be
attributed to more densely packed PrSO3H groups favouring
cooperative interactions, as previously suggested for sol-gel
prepared PrSO3H/SBA-15[10b] and grafted PrSO3H/MCM-41.[19]
High sulfonic acid site densities are also reported to favour a
Langmuir-Hinshelwood (LH) mechanism for acetic acid
esterification.[10b] Kinetic modelling determined activation
energies of 42.6 kJmol-1 for the LH mechanism, which was
slightly lower than the 45.6 kJmol-1 determined for an Eley Rideal
(ER) pathway. SAC-13 sulfonic acid resin catalysts, which follow
an ER mechanism for acetic acid esterification,[21] are also
associated with high activation energies of 63.8 kJmol-1.
Apparent activation energies could thus be used to reflect
whether there is a change in cooperativity across series of
grafted PrSO3H/SBA-15 catalysts. Activation energies for acetic
esterification for the lowest (0.15-PrSO3H/SBA-15) and highest
loaded (0.77-PrSO3H/SBA-15) materials were determined to be
63±5 kJmol-1 and 51±5 kJmol-1 respectively (Fig S9) in accord
with the hypothesis that a change in mechanism may be induced
with PrSO3H density: isolated PrSO3H groups favour an ER
pathway, while decreased acid site separation upon increasing
active site loading and PrSO3H density favours LH pathways. It
is interesting to note that literature values, which were measured
in a 1,4-dioxane solvent[10b] are slightly lower than the values
determined here. This may reflect the impact of solvent effects
influencing the reaction kinetics due to both reactant and product
solubility and competitive adsorption for surface acid sites.
 Oc/PrSO3H/SBA-15
 PrSO3H/SBA-15
0.5
28
0.4
0.3
23
0.2
0.1
18
13
8
0.1 0.3 0.5 0.7
Bulk S content / wt%
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
Bulk S content / wt %
Figure 1. Bulk C:SO3H molar ratio for Oc/PrSO3H/SBA-15 catalysts as a
function of S content. (inset) shows the evolution of acid site loading of
Oc/PrSO3H/SBA- and PrSO3H/SBA as a function of bulk S content.
The influence of sulfonic acid group density and incorporation of
hydrophobic octyl groups on esterification activity of the
PrSO3H/SBA-15 and Oc/PrSO3H-SBA-15 materials was
subsequently evaluated in acetic acid esterification with
methanol (Scheme 1).
70
60
50
40
30
20
Scheme 1 Acetic acid esterification with methanol
Reaction profiles (Fig. S6) confirm that all sulfonic acid
functionalised mesoporous silicas are active for esterification
with the conversion increasing with PrSO3H loading. Samples
co-grafted with PrSO3H and octyl groups exhibit comparable
conversions to the parent PrSO3H/SBA-15 (Fig. S6), with the
highest loaded (Oc)PrSO3H/SBA-15 catalysts achieving ~73%
acetic acid conversion after 6h with 100% methyl acetate
selectivity. In comparison, blank reactions in the absence of
catalyst exhibited < 8 % acetic acid conversion after 6 h (Fig.
S7). For both families of catalyst, initial rates of acetic acid
conversion increase with acid site density (Fig S8), reflecting the
direct impact of increased Brönsted acidity on activity. Turnover
Frequencies (TOFs) for the PrSO3H/SBA-15 series (Fig. 2) were
found to increase as a function of surface acid site density,
reaching a plateau for coverages > 0.3 H+.nm-2. Octyl post
functionalisation results in a further enhancement of the TOFs,
PrSO3H/SBA-15
10
Oc/PrSO3H/SBA-15
0
0.10
0.20
0.30
0.40
Acid site density / H+.nm-2
Fig. 2 Effect of acid site density on TOF for acetic acid esterification over
PrSO3H/SBA-15 and Oc/PrSO3H/SBA-15 catalysts. Reaction conditions: 50
mg catalyst, 60 °C, acid:alcohol ratio of 1:30
To verify the effect of octyl chains in improving water tolerance,
the esterification activity of 0.77-PrSO3H/SBA-15 and Oc/0.77-
PrSO3H/SBA-15 was compared following addition of 1 and 10
mmol of water (Fig. S10). The Oc/PrSO3H/SBA-15 catalyst
exhibited a negligible change in TOF upon addition of 1 mmol
water (1:5 molar ratio of water:acid), which was in contrast to the
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parent PrSO3H/SBA-15 sample whose TOF decreases by 20%
(Fig 3). The apparent water tolerance under these conditions
surpasses previous studies of propyl co-functionalised
PrSO3H/SBA-15 where initial rates decrease by 35% upon
addition of similar levels of water (1:6 molar ratio water:acid).[10a]
The beneficial effect of co-grafting with hydrophobic octyl chains
was particularly evident upon addition of 10 mmol water
(significantly in excess of normal reaction conditions), where
despite the expected decrease in activity under such challenging
conditions, the TOF for Oc/PrSO3H/SBA-15 was still 43% higher
than the parent PrSO3H/SBA-15.
is negligible conversion upon removal of both sulfonic and octyl
co-functionalised catalysts from reaction after 1h reaction.
The improved water tolerance of Oc/PrSO3H/SBA-15
catalysts is significant for the development of catalysts for
esterification reactions. However, despite such effects often
being claimed to relate to increased hydrophobicity there
remains a dearth of analytical studies that quantify such
properties of catalyst materials. The hydrophobicity of a surface
can originate from a change in surface polarity, which can be
determined from surface energy calculations. Recently we have
reported on the application of inverse gas chromatography (IGC)
as
a means of quantifying the specific and non-specific
70
interactions of polar and non-polar hydrocarbon adsorbates on
periodic mesoporous organo-silicas,[13] which can be related to
hydrophobicity. Building upon this work, IGC has been applied to
elucidate the surface energy and surface adsorption properties
of the current family of octyl co-grafted sulfonic acid silicas. IGC
can deliver the physical (dispersive) and chemical
(specific/polar/acid-base) surface energy associated with the
materials from adsorption measurements of alkane and acid-
base probe molecules respectively. Non-polar molecules adsorb
via non-specific London forces while polar molecules interact
through acid-base, hydrogen bonding interactions.[15, 22] The total
surface energy is thus the sum of the dispersive and specific
component from all interactions,[23] with the dispersive
component calculated as per Equation 2, from the slope of the
 0.77-PrSO3H
Oc/0.77-PrSO3H
60
50
40
30
20
10
0
plot between RTlnVN vs a(γL
)
D 1/2 (Fig. S14).[13]
RTlnVN =2NA(γS
)
D 1/2 a(γL
)
D 1/2 + constant
(2)
0
1
10
0
1
10
In this instance NA is Avagadro’s number, a is the surface area
of the probe molecule, VN is the specific retention volume of the
adsorbate, and γS and γL are dispersive components of the
solid and liquid surface energy respectively.
Water addition / mmol
D
D
Fig. 3 Impact of water addition on acetic acid esterification with methanol over
0.77-PrSO3H/SBA-15 and Oc/0.77-PrSO3H/SBA-15.
The standard free energy of adsorption can also be
calculated from the sum of dispersive and specific free energies
(Equations 3 and 4).
To further check the influence of octyl chains in driving reactively
formed water away from the acid site, the activity of
0.77PrSO3H/SBA-15 and Oc/0.77PrSO3H/SBA-15 catalysts was
subsequently evaluated under a stoichiometric acetic acid to
MeOH molar ratio. While the equilibrium acetate yield will be
reduced under such conditions, the kinetics should be more
sensitive towards accumulation of reactively formed water on the
catalyst than observed at higher alcohol concentrations. Thus,
the hydrophobic Oc/0.77PrSO3H/SBA-15 material should exhibit
sustained activity due to removal of reactively formed water,
whereas the hydrophilic 0.77PrSO3H/SBA-15 catalyst will be
expected to deactivate. This is indeed what is observed as
shown in Fig. S11 which demonstrates the hydrophobic octyl co-
functionalised Oc/0.77PrSO3H/SBA-15 catalyst exhibits over
twice the activity of the more hydrophilic counterpart
0.77PrSO3H/SBA-15, confirming the impact of octyl chains in
inhibiting the reverse ester hydrolysis.
SP
ΔGads= ΔGadsD + ΔGads
(3)
(4)
ΔGads= -RTlnVN + constant
The specific free energy due to polar interactions can be
calculated from the deviation of calculated RTlnVN values from
D 1/2
the gradient of the plot of RTlnVN v’s a(γL
)
obtained from
dispersion forces from non-polar adsorbates (Fig. S14).[13, 22c]
Measurement of adsorption isotherms under conditions of
infinite dilution (in the Henry region - when the isotherm is linear
and only interactions of the probe molecule with the material
surface exist[15]) also allows the differential isosteric heat of
adsorption to also be calculated (Fig S15) from temperature
dependent plots of volume (Vads) adsorbed vs p/p0. Surface
polarity, Xp is calculated as the ratio of the polar surface energy
to the total surface energy[23] (Equation 5) using values from
Table 2 and Fig. S14.
The stability of the Oc/0.77PrSO3H/SBA-15 and
0.77PrSO3H/SBA-15 catalysts was further evaluated by recycle
and hot filtration tests. Fig S12 demonstrates that excellent
recyclability was observed, with only a small decrease in
conversion and TOF observed on reuse. Further leaching
studies conducted via hot filtration tests (Fig. S13) reveal there
D
Xp= γSSP/(γSSP + γS
)
(5)
Fig 4 shows that the surface polarity for both Oc/PrSO3H/SBA-
15 and PrSO3H/SBA-15 samples decreases with increased
carbon content, suggesting surface polarity is
parameter to reflect the change in surface properties.
a useful
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Table. 2 Evolution of surface properties of PrSO3H/SBA-15 and Oc/PrSO3H/SBA-15 with low and high S content
Materials
IGC results
D
SP
SP
Disp. Surf. Energy (γS
)
Sp. Surf. Energy (γS
)
−ΔGads
-ΔH
methanol
/ kJ mol-1
74
/ mJ m-2
/ mJ m-2
methanol
/ kJ mol-1
16
0.15PrSO3H/SBA-15
Oc/0.15PrSO3H/SBA-15
0.77PrSO3H/SBA-15
Oc/0.77PrSO3H/SBA-15
74
50
65
42
269
130
211
89
13
14
12
61
75
64
The latter point may be particularly important in enhancing acetic
acid adsorption during an LH mechanism when methanol is in
vast excess. Given the calculated adsorption energy of acetic
acid on PrSO3H is approximately -140 kJ/mol,[10b] (higher than
both methanol and ethanol) excess alcohol will be required to
favour adsorption and open up the LH pathway.
70
60
50
40
30
20
10
0
18.0
16.0
14.0
12.0
10.0
8.0
Fig. 4 Correlation between bulk carbon content from octyl group incorporation
and the surface polarity of PrSO3H/SBA-15 and Oc/PrSO3H/SBA-15 catalysts.
ΔHads(methanol) over sol-gel PrSO3H/SBA-15,[10b] is
reported to be similar to that for water, suggesting methanol and
water binding characteristics will respond similarly and the
former can be used to asses hydrophobicity. Table 2 shows that
for methanol adsorption both -ΔGadsSP and -ΔHads decrease upon
octyl co-functionalisation, indicating methanol binding is weaker
than over the parent PrSO3H/SBA-15. Methanol adsorption
isotherms also reveal that methanol surface coverage is reduced
over octyl co-functionalised samples, further supporting the
weakened surface interactions with polar molecule (Fig. S16).
Fig. 5 shows that the increased TOFs for acetic acid
SP
Fig. 5 Correlation between -ΔGads for methanol adsorption and TOF for
acetic acid esterification with methanol over PrSO3H/SBA-15 and
Oc/PrSO3H/SBA-15 catalysts.
The activity of PrSO3H/SBA-15 and OcPrSO3H/SBA-15
with high sulfonic acid loadings is in good agreement with our
previous studies of octyl co-grafted of PrSO3H/MCM-41,[19] and
reports on the effect of PrSO3H loading on SBA-15 (achieved by
partial poisoning of acid sites with pyridine)[10a] where a two site
cooperative mechanism for esterification is proposed. Increased
surface PrSO3H density brings acid head groups in closer
proximity increasing the probability of a two-centre reaction
between acetic acid and methanol adsorbed on adjacent
PrSO3H sites. However, Molecular Dynamic simulations on
PrSO3H/MCM-41 demonstrated that at low acid site loadings,
flexing of PrSO3H chains led to hydrogen bonding interaction
with free surface silanol groups, which decreased acid strength.
esterification over PrSO3H/SBA-15 and OcPrSO3H/SBA-15 can
SP
be correlated with
a
decrease in -ΔGads
for methanol
adsorption. The impact of tuning bulk carbon content on
associated surface polarity and TOF for esterification is
correlated in Fig. 6, which reveals decreased surface polarity
(inversely related to surface hydrophobicity) leads to an
increased TOF. Thus, it can be deduced the promotional effect
of octyl groups on esterification activity can be predicted from
changes in surface polarity. Decreased surface polarity, or
increased hydrophobicity serves to both to displace the water
by-product while also weakening the interaction with methanol.
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0.8
0.7
0.6
PrSO3H Oc/ PrSO3H
60
40
20
0
1.03
1.6
1.8
2.35
Bulk C content / wt%
Fig. 6 Dependence of acetic acid esterification and surface polarity over
functionalised SBA-15 on alcohol. Reaction conditions: 50 mg catalyst, 60 °C,
acid:alcohol ratio of 1:30.
Capping of free silanol groups with OTMS, in addition to
increasing hydrophobicity, lifted this interaction thereby
increasing acid strength and favouring increased cooperativity
between sulfonic acid head groups. On SBA-15 however, the
observation that acid strength is invariant of PrSO3H loading
suggests there is a subtle difference in the surface interactions
of sulfonic acid groups at low loadings over the two supports,
which could be accounted for by a difference in surface hydroxyl
distributions or pore diameter for the two classes of material.
Scheme 2 shows a scaled illustration of sulfonic acid groups
grafted within 2.5 and 4.5 nm pores characteristic of MCM-41
and SBA-15 respectively. The high curvature of the smaller
mesopores of MCM-41 (Scheme 2a), will result in (i) PrSO3H
head-groups coming into contact with the pore wall more readily
upon flexing of the alkyl chain, and (ii) PrSO3H situated around
the pore to interact with each other more readily via cross pore
interactions as suggested previously for organically modified
silicas.[24] In contrast for the larger, low curvature pores of SBA-
15 (Scheme 2b) steric restrictions in the propyl chain flexing
means SO3H head groups cannot readily hydrogen bond to the
surface, thus acid strength will not be impaired at low loading via
such interactions.
Assuming a uniform distribution of sulfonic acid groups,
this also indicates that on geometric grounds, small mesopores
should exhibit a higher probability for cooperative interactions at
low fractional surface coverages of PrSO3H groups. Larger pore
materials should require higher loadings, or grafting to occur in
islands to facilitate cooperative interactions. This hypothesis
raises some interesting questions about the effect of pore
confinement of functional groups in sulfonic acid silicas, and the
tendency for them to promote LH or ER kinetic mechanisms.
Further studies by molecular dynamic simulations and kinetic
modelling are required to address these questions.
Scheme 2 Illustration of how cooperative effects between sulfonic acid groups
could occur (top) across the pore in 2.5 nm pores characteristic of MCM-41,
whereas in (bottom) such interactions are less favoured for 4.5 nm pores
representing SBA-15. Larger pores would require higher loadings, or grafting
to occur in islands to facilitate cooperative interactions.
Conclusions
A series of propyl sulfonic acid and octyl co-functionalised propyl
sulfonic acid SBA-15 silicas have been evaluated for potential
application in bio-oil pre-treatments using acetic acid
esterification with methanol as a model system. The turn over
frequency for acetic acid esterification with methanol is
enhanced upon both increasing surface sulfonic acid site density,
and hydrophobicity by introduction of octyl groups. Octyl
cofunctionalised catalysts also showed excellent water tolerance
suggesting their suitability for use in esterification pretreatments
of pyrolysis bio-oils. Increased surface acid site density is
believed to induce cooperative interactions between acid sites
which directs esterification via a LH mechanism. More isolated
sites present at low sulfonic acid loadings exhibit higher
activation barriers for reaction and are proposed to favour an ER
pathway. The evolution of surface properties with functional
group loading was followed using inverse gas chromatography
(IGC) which corroborates that octyl group incorporation
decreases surface polarity and increases surface hydrophobicity.
IGC measurements also indicate that in addition to increasing
the surface hydrophobicity, capping of free surface hydroxyl
groups decreases the free energy of methanol adsorption. The
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DRIFTS at 50 °C. Acid sites concentrations were measured by NH3 pulse
surface adsorption characteristics determined via IGC correlate
well with catalytic performance, suggesting this is a powerful tool
to study the effect of hydrophobicity in catalysis.
chemisorption using
a Quantachrome ChemBET 3000 instrument
interfaced to an MKS Minilab mass spectrometer (MS). Samples were
degassed at 120 °C overnight under helium prior to NH3 pulse titration at
100 °C. Temperature-programmed desorption (TPD) was subsequently
performed on ammonia saturated samples between 100-500 °C.
Ammonia adsorption calorimetry under flow conditions was performed
Experimental Section
using
a system based on a flow-through Setaram111 differential
scanning calorimeter (DSC) and an automated gas flow and switching
system, with down-stream mass spectrometer detector (Hiden HPR20)
connected via a heated capillary. In a typical experiment, the sample (5–
15 mg) was activated under dried helium (5 ml min-1) for 2 h at 100 °C.
Adsorption was monitored at 100°C, so measured enthalpies correspond
only to ammonia that binds irreversibly to the catalyst at this temperature.
Small pulses (typically 1 mL) of the probe gas (1% ammonia in helium)
were then injected at regular intervals into the carrier gas stream from a
gas sampling valve, also at 100 °C. The concentration of ammonia down-
stream of the sample was monitored with the mass spectrometer
(m/z=15), and heat evolution with the calorimeter. The net amount of
ammonia irreversibly adsorbed from each pulse was determined by
comparing the mass spectrometer signal during each pulse with a signal
recorded through a blank sample tube during a control experiment. Net
heat released for each pulse was calculated from the DSC thermal curve.
From this the molar enthalpy of ammonia adsorption ΔH◦ Ads NH3 was
obtained for the ammonia adsorbed from each pulse.
Catalyst synthesis
SBA-15 was synthesised adopting the protocol of Zhao and co-
workers.[17a] Typically 10 g of Pluronic P123 triblock copolymer was
dissolved in 75 ml of water and 250 ml of 2M HCl solution. The mixtures
were stirred at 35 °C for dissolution and then 23 ml of TEOS was added
with the synthesis maintained at 35 °C for 20 h under stirring. The
resulting gel was then aged at 80 °C for 24 h. Finally, the solid product
was filtered, washed with water and calcined under static air at 550 °C for
5 h.
A series of sulfonic acid functionalised SBA-15 and its octyl co-
derivatized forms were prepared following reported method.[10c, 19]
MPTMS in toluene was initially prepared as precursor for grafting on
SBA-15. Specific amount of MPTMS in toluene (0.01<MPTMS/SBA-
15<1) was added per gram of material to vary the thiol coverage from low
to high on SBA-15. The mixture then refluxed for 24 hours in 30 ml of
toluene. Thiol functionalised samples were then filtered washed with
methanol and dried at 80 °C. One portion of thiol functionalised samples
was oxidised with H2O2 at room temperature for 24 h (30 ml of 33 wt%
H2O2 per gram of material) to prepare sulfonic functionalised SBA-15 and
the other portion used for co-grafting with octyl groups. The series is
represented as (x)PrSO3H/SBA-15 where x gives the wt% S measured
by CHNS.
Inverse gas chromatography
IGC at infinite dilution was used to explore surface energies, heat of
sorption, adsorption isotherms and associated structure-activity
relationships[13-16]. All the measurements were performed at infinite
dilution in the Henry region (p/p0 = 0.04) to exclude interactions between
probe molecules on material surface. Measurements were performed
using a Surface Measurement System IGC with IGC controller v1.8
software. The samples (~10 mg) are packed in a 3 mm diameter column
The octyl grafted materials are synthesised from the un-oxidised
thiol grafted SBA-15 series. The un-oxidised thiol grafted samples (~1g)
from the above batch was refluxed in 30 ml of toluene for 24h with 1 ml of
octyltriethoxysilane.[19] The octyl co-grafted samples are then filtered,
washed with methanol, dried and oxidised with H2O2 at room temperature
for 24 h (30 ml of 33 wt% H2O2 per gram of material) to convert thiol
groups to sulfonic acid. S and C contents of the final octyl co-derivatised
samples were remeasured by CHNS analysis. For simplicity, this series
is denoted as Oc/(x)PrSO3H/SBA-15 where x gives the wt% S measured
by CHNS in the parent sample.
and degassed at 120 oC for
2 hours prior to analysis. Detailed
experimental procedure is given in the ESI. Surface energy
measurements were carried out with apolar (hexane, heptane, octane,
nonane, and decane) and polar (methanol, acetonitrile, ethyl acetate, and
dichloromethane) probes to calculate both dispersive (γSD) and specific
(γSSP) component of surface energy
respectively.[15] Heat of sorption
studies were performed by injecting a specific amount of methanol in a
temperature window of 90 to 100 oC. Adsorption isotherms were
recorded by injecting alcohol over the range, p/p0 0.02 to 0.1 at 80 oC.
Characterisation
Esterification
Physicochemical properties of the as-synthesised catalysts were
fully characterised. Low angle XRD patterns were recorded on a Bruker
D8 Advance diffractometer fitted with an X’celerator detector and Cu Kα
(1.54 Å) source over the range 2θ = 0.3-10°. Nitrogen porosimetry was
measured on a Quantachrome Nova 4000 porosimeter and analysed
with NovaWin software. Samples were degassed at 120 °C for 4 h prior
to analysis at -196 °C. Bulk sulphur loadings were calculated using XRF
analysis on a Bruker S8 Tiger and verified by CHNS analysis using
Thermo Scientific Flash 2000 CHNS-O analyser. DRIFTS measurements
were conducted using a Thermo Scientific Nicolet environmental cell and
smart collector accessory on a Thermo Scientific Nicolet iS50 FT-IR
Spectrometer with MCT detector. The catalysts diluted in KBr (10 wt%)
were loaded in the environmental cell and subjected to evacuation at
The reaction conditions for acetic acid esterification with methanol
employed conditions that have previously been optimised by our group.[19,
25]
Briefly, esterification reactions were performed in batch at 60 °C
employing 0.05 g of catalyst, 5 mmol of acetic acid, 150 mmol of alcohol
(acid:alcohol mole ratio 1:30), and 0.5 mmol of dihexyl ether as an
internal standard.[18-19, 25] Aliquots were withdrawn periodically from the
reaction mixture diluted with methanol and analysed by off-line GC using
a Varian 450-GC equipped with a ZB 50 15 m × 0.25 mm × 0.25 μm
capillary column. Reactions performed at a 1:1 methanol:acetic acid ratio
were performed using 0.020 g catalyst with 20 mmol alcohol and acid so
as to overcome mixing problems with small reaction volumes under
solvent free conditions. Turnover Frequencies (TOFs) were calculated by
normalising initial rates derived from the linear portion of reaction profiles
during the first hour to the acid site loadings obtained from NH3 pulse
chemisorption. Water spiking experiments were performed with addition
of 1 and 10 mmol of water. Hot filtration experiments were performed in
which the catalyst was removed from the reaction after 1 hour by hot
200 °C for 2 h to remove physisorbed water/moisture.
Analyses were
performed at 200 °C. Ex-situ pyridine adsorption studies were made by
wetting the samples with pyridine. Excess pyridine was removed
overnight in vacuo at 50 °C, with subsequent in vacuo analysis by
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filtration, with further conversion of the filtrate monitored for an additional
6 h.
[9]
aC. Pirez, A. F. Lee, J. C. Manayil, C. M. A. Parlett, K. Wilson, Green
Chemistry 2014, 16, 4506-4509; bJ. A. Melero, L. F. Bautista, G.
Morales, J. Iglesias, R. Sánchez-Vázquez, Chemical Engineering
Journal 2010, 161, 323-331.
[10] aS. Miao, B. H. Shanks, Applied Catalysis A: General 2009, 359, 113-
120; bS. Miao, B. H. Shanks, Journal of Catalysis 2011, 279, 136-143;
cJ. C. Manayil, C. V. M. Inocencio, A. F. Lee, K. Wilson, Green
Chemistry 2016.
Acknowledgements
We thank the EPSRC for financial support (EP/K000616/2,
[11] aL. Boudriche, R. Calvet, B. Hamdi, H. Balard, Colloids and Surfaces A:
Physicochemical and Engineering Aspects 2011, 392, 45-54; bJ. A. F.
Gamelas, J. Pedrosa, A. F. Lourenço, P. J. Ferreira, Colloids and
Surfaces A: Physicochemical and Engineering Aspects 2015, 469, 36-
41; cD. Gavril, Catalysis Today 2015, 244, 36-46.
EP/G007594/4 and EP/K014749/1) and the award of
a
Leadership Fellowship to AFL. KW thanks the Royal Society for
an Industry Fellowship. Support from the European Union
Seventh Framework Programme (FP7/2007-2013) under grant
agreement no. 604307 is also acknowledged. VCS
acknowledges CNPq (Conselho Nacional de Desenvolvimento
Cientifico e Tecnológico) for the award of a postdoctoral
scholarship Text. FCJ acknowledges support though NSF Award
1560519.
[12] J. M. R. C. A. Santos, J. T. Guthrie, Journal of Chromatography A 2015,
1379, 92-99.
[13] C. Pirez, A. F. Lee, C. Jones, K. Wilson, Catalysis Today 2014, 234,
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[14] T. Hamieh, M.-B. Fadlallah, J. Schultz, Journal of Chromatography A
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[15] M. Rückriem, D. Enke, T. Hahn, Microporous and Mesoporous
Materials 2015, 209, 99-104.
Keywords: Sulfonic acid • bio-fuels • esterification •
hydrophobicity • SBA-15
[16] L. Boudriche, R. Calvet, B. Hamdi, H. Balard, Colloids and Surfaces A:
Physicochemical and Engineering Aspects 2012, 399, 1-10.
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Chmelka, G. D. Stucky, Science 1998, 279, 548-552; bD. Y. Zhao, Q. S.
Huo, J. L. Feng, B. F. Chmelka, G. D. Stucky, Journal of the American
Chemical Society 1998, 120, 6024-6036.
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