Mesoporous sulfonic acid silicas for pyrolysis bio-oil upgrading via acetic acid esterification

Article abstract of DOI:10.1039/c5gc01889g

Propylsulfonic acid derivatised SBA-15 catalysts have been prepared by post modification of SBA-15 with mercaptopropyltrimethoxysilane (MPTMS) for the upgrading of a model pyrolysis bio-oil via acetic acid esterification with benzyl alcohol in toluene. Acetic acid conversion and the rate of benzyl acetate production was proportional to the PrSO₃H surface coverage, reaching a maximum for a saturation adlayer. Turnover frequencies for esterification increase with sulfonic acid surface density, suggesting a cooperative effect of adjacent PrSO₃H groups. Maximal acetic acid conversion was attained under acid-rich conditions with aromatic alcohols, outperforming Amberlyst or USY zeolites, with additional excellent water tolerance.

Full text of DOI:10.1039/c5gc01889g

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Mesoporous sulfonic acid silicas for pyrolysis bio-oil
upgrading via acetic acid esterification
Jinesh C. Manayil,a Carlos V. M. Inocencio,a Adam F. Leea* and Karen Wilsona*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Propylsulfonic acid derivatised SBAꢀ15 catalysts have been prepared by post modification of SBAꢀ15 with
mercaptopropyltrimethoxysilane (MPTS) for the upgrading of a model pyrolysis bioꢀoil via acetic acid
esterification with benzyl alcohol in toluene. Acetic acid conversion and the rate of benzyl acetate production
was proportional to the PrSO3H surface coverage, reaching a maximum for a saturation adlayer. Turnover
frequencies for esterification increase with sulfonic acid surface density, suggesting a cooperative effect of
adjacent PrSO3H groups. Maximal acetic acid conversion was attained under acidꢀrich conditions with
aromatic alcohols, outperforming Amberlyst or USY zeolites, with additional excellent water tolerance.
www.rsc.org/
method to raise pH and improve bioꢀoil stability.4 Strong
mineral acids such as H2SO4 are effective at catalysing low
temperature esterification, but their corrosive and hazardous
1. Introduction
Technologies to utilise renewable resources for the sustainable
production of transportation fuels and chemicals is of great
current interest due to growing concerns over the depletion of
fossil fuel reserves and associated climate change.1 Promising
solutions are offered through the thermochemical processing of
lignocellulosic biomass through pyrolysis or gasification, and
transesterification of nonꢀedible and waste plant/algal oils and
fats.1ꢀ3 Fast pyrolysis of waste agricultural/forestry biomass for
the production and subsequent upgrading of bioꢀoils to liquid
transportation fuels has received considerable attention in this
nature hinders handling and storage, while as soluble catalysts,
resulting upgraded bioꢀoils require subsequent neutralisation
generating large quantities of aqueous waste for cleanꢀup. Solid
acid catalysts can circumvent these disadvantages, however
new tailored solid acids are desirable with superior acid site
accessibility and stability in aqueous environments, and
compatible with high molecular weight hydrocarbons present in
bioꢀoil that may lead to poreꢀblockage of microporous solid
acids such as zeolites.15, 16
Catalytic esterification of acetic acid has been previously
investigated with a range of alcohols, notably methanol and
ethanol which do not occur at any significant concentration in
pyrolysis bioꢀoils, and must therefore derive from an additional
external carbon source, impacting upon the overall process
sustainability.14, 17ꢀ19 If a continuous process were to operate via
reactive distillation, it would be desirable for both water and the
ester is removed continuously to drive the equilibria to
completion. For this purpose alcohols with boiling points
higher than water are required to avoid alcohol loss by
evaporation.20 Aromatic esters find application in a wide range
of areas spanning fine, pharmaceutical and agrochemicals
sectors,21 thus in the context of a biorefinery, in addition to
improving bioꢀoil stability, the production of benzyl acetate
5
regard.4 However, the direct use of fast pyrolysis bioꢀoils is
limited by its low heating value due to the high oxygen content,
thermally instability, and strong acidity with associated high
water content.6ꢀ8 Typical bioꢀoils 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 3ꢀhydroxyꢀ 3ꢀ
methoxy benzaldehyde, phenols, guaiacols and syringols
derived from the lignin component.9,
routes to transportation fuels are therefore only economically
viable if the bioꢀoil is subjected to upgrading treatments to
improve their physicochemical properties.5, 11ꢀ14
There are several promising catalytic processes for bioꢀoil
upgrading/preꢀtreatment,
condensation and hydroꢀdeoxygenation. Acetic acid is one of
the major impurities present in bioꢀoils (around 1ꢀ10 %),7 and
its removal via esterification affords a simple and effective
10
Biomass pyrolysis
from esterification with benzyl alcohol could add value to the
including
ketonisation,
aldol
23
overall process.22,
There has been previous interest in
catalysing acetic acid esterification with benzyl alcohol and
cresol over zeolites, zirconia, alumina, and silica,22, 24 however
such microporous catalysts are illꢀsuited to the liquid phase
transformation of viscous bioꢀoils containing sterically
challenging components. Sulfonated mesoporous carbons25 and
silicotungstic/zirconia functionalised mesoporous SBAꢀ15
silica18 have also shown promise for acetic acid esterification
a.European Bioenergy Research Institute, Aston University, Birmingham B4 7ET, UK.
Electronic Supplementary Information (ESI) available: [DRIFT spectra, porosimetry
data, TGA, Pyridine adsorption spectra, esterification reaction results]. See
DOI: 10.1039/x0xx00000x
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with benzyl alcohol, but of idealised mixtures unrepresentative Quantachrome Nova 4000 porosimeter and analysed with
of pyrolysis bioꢀoils and under inert atmospheres. Sulfonic acid NovaWin software. Samples were degassed at 120 °C for 4 h
silicas are an important class of solid acid catalysts26ꢀ30 well prior to analysis at ꢀ196 °C. Thermogravimetric analysis (TGA)
known for their applications in biodiesel production, but have was performed on a Mettler Toledo, TGA/DSC2 Stare system
rarely been exploited for bioꢀoil upgrading and even then only under flowing nitrogen during heating at 10 °C.minꢀ1 from 40ꢀ
31
for acetic acid esterification with pure ethanol,17, or in the 800 °C. Total sulfur loadings were calculated from the TGA
presence of light aldehyde components present in pyrolysis bioꢀ weight loss between 200ꢀ600 °C, and verified by XRF analysis
oils which hinder ester formation at low reaction on a Bruker S8 Tiger. DRIFTS measurements were conducted
temperatures.32 Real pyrolysis oils, whether from fast, using a Thermo Scientific Nicolet environmental cell and smart
intermediate or catalytic routes, contain significant (~25 wt%) collector accessory on a Thermo Scientific Nicolet iS50 FTꢀIR
nonꢀpolar lignin derivatives,8 for which toluene may be Spectrometer with MCT detector. The catalysts diluted in KBr
considered a representative solvent. Here we report the first (10 wt%) were loaded in the environmental cell and subjected
investigation of acetic acid esterification with benzyl alcohol in to evacuation at 200 °C for 2 h to remove physisorbed
toluene as
a simulated bioꢀoil over propylsulfonic acid water/moisture. Analyses were performed at 200 °C. Exꢀsitu
functionalised SBAꢀ15 (PrSO3H/SBAꢀ15), which exhibits pyridine adsorption studies were made by wetting the samples
outstanding water tolerance. Catalyst formulation and the with pyridine. Excess pyridine was removed overnight in vacuo
impact of reaction conditions are optimised for maximal at 50 °C, with subsequent in vacuo analysis by DRIFTS at 50
activity and ester yield and significantly outperform °C. Acid sites concentrations were measured by NH3 pulse
commercial solid acid catalysts.
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. Temperatureꢀ
programmed desorption (TPD) was subsequently performed on
ammonia saturated samples between 100ꢀ500 °C.
2. Experimental
2.1 Catalyst synthesis
SBAꢀ15 was synthesised adopting the protocol of Zhao and
2.3 Esterification
coꢀworkers.33 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 were added, 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 statically in air at 550 °C for 5 h.
Esterifications were performed in batch at atmospheric
pressure using a Radley carousel reaction station.35 Reactions
were performed at 100 °C employing 0.05 g of catalyst, 10
mmol of acetic acid, 5 mmol of benzyl alcohol (acid:alcohol
mole ratio 2:1), and 1.28 mmol of dihexyl ether as an internal
standard in 10 ml toluene. Aliquots were withdrawn
periodically from the reaction mixture and analysed by offꢀline
GC after dilution with dichloromethane using a Varian 450ꢀGC
equipped with a CPꢀSil 5 CB 15 m × 0.25 mm × 0.25 ꢁm
capillary column. The impact of acid:alcohol molar ratio was
investigated for a fixed 10 mmol of acetic acid, adjusting the
toluene accordingly to maintain a constant reaction volume.
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. Esterification was also explored with
other common bioꢀoil alcohol components, anisyl alcohol, pꢀ
cresol and mꢀcresol for acid:alcohol molar ratios of 2:1. Water
tolerance was tested by deliberately spiking a reaction with 40
mol% water relative to benzyl alcohol prior to catalyst addition.
Recycle experiments were performed via hot filtration of spent
catalyst in toluene and reactivation via boiling in water for 1 h
prior to reꢀuse to remove residual ester. Catalyst stability was
assessed via C/H/N/S elemental analysis of asꢀprepared and
hotꢀfiltered spent catalysts, and removal of the catalyst from the
reaction medium during the initial stage of reaction and
subsequent analysis of the filtrate for acetic acid conversion and
ester after 5 h.
A series of sulfonic acid functionalised SBAꢀ15 were
prepared following the method reported elsewhere.34 The thiol
coverage was varied from low to saturation (based on hydroxyl
density). A stock solution of MPTMS in toluene was initially
prepared as precursor for grafting on SBAꢀ15. Specific amount
of MPTMS in toluene solution (0.01<MPTMS/SBAꢀ15<1) was
added per gram of material. The overall volume of toluene was
kept constant as 30 ml with further addition of toluene and the
mixture refluxed for 24 hours. The resulting thiol functionalised
samples were then filtered washed with methanol and dried at
80 °C. Oxidation of thiol groups was carried out with H2O2 at
room temperature for 24 h (30 mL of 33 wt% H2O2 per gram of
material). The samples are denoted as PrSO3H(xML)/SBAꢀ15,
x gives the volume of MPTMS per g of SBAꢀ15, ML for
saturation (monolayer coverage).
2.2 Characterisation
Physicochemical properties of the asꢀsynthesised 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 = 0.3ꢀ10°. Nitrogen porosimetry was measured on a
2 | J. Name., 2012, 00, 1-3
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3. Results and Discussion
20
3.1 Characterisation
PrSO3H(1ML)/SBA-15
Successful synthesis of the parent SBAꢀ15 mesoporous silica
support, which possesses a characteristic p6mm space group,
and its retention following sulfonate grafting was first
15
10
5
PrSO3H(0.5ML)/SBA-15
confirmed by low angle XRD and nitrogen adsorption
PrSO3H(0.1ML)/SBA-15
PrSO3H(0.01ML)/SBA-15
36
isotherms shown in Figs. 1 and S1 respectively.33,
XRD
patterns for the PrSO3H(xML)/SBAꢀ15 materials were
essentially unchanged after the grafting procedure, with
common reflections and peak intensities as a function of S
coverage, evidencing hexagonally closeꢀpacked mesopore
channels that were stable during the thiol functionalization and
subsequent oxidation. Porosimetry showed type IV isotherms
with H1 hysteresis for SBAꢀ15 and all PrSO3H(xML)/SBAꢀ15
samples, highlighting the presence of mesopores. Textural
properties of PrSO3H/SBAꢀ15 materials are summarised in
Table 1, wherein a continual decrease in BET surface area was
observed with increasing sulfonic acid loading, accompanied by
a slight narrowing in the BJH mesopore diameter (Fig. 2),
consistent with uniform grafting of sulfonic acid groups
throughout the pore network which retain a common unit cell.
Thermogravimetric analysis of PrSO3H/SBAꢀ15 revealed
SBA-15
0
3
4
5
6
7
8
9
10 11
Pore diameter / nm
Fig. 2 BJH pore size distributions of PrSO3H/SBAꢀ15 as a function of nominal S
loading.
SBAꢀ15 and PrSO3H/SBAꢀ15 materials postꢀoxidation. SBAꢀ
15 exhibited bands at 700ꢀ1400 cmꢀ1 and 3000ꢀ3800 cmꢀ1
indicative of framework SiꢀOꢀSi and surface silanols
respectively.37 Sulfonic acid grafting attenuated the silanol
features, coincident with the appearance of weak new bands
centred around 2900 cmꢀ1 and 1370 cmꢀ1, attributed to CꢀH and
CH2ꢀSi vibrations of the propyl backbone (Fig. S3), whose
intensities increase with increase S loading. Acidic properties
of the PrSO3H/SBAꢀ15 family were evaluated via pyridine
adsorption studies and NH3 chemisorption to quantify their
Brönsted/Lewis character38 and acid site densities. As expected,
DRIFT spectra (Fig. S4) of the sulfonated materials exhibited
bands at 1489, 1545 and 1637 cmꢀ1 indicative of pyridinium
ions coordinated to Brönsted acidic sites. The KubelkaꢀMonk
Brönsted peak intensity increased linearly with SO3H surface
density (via XRF), in excellent correlation with the acid site
density determined by NH3 titration (Fig. 4). It should be noted
that the maximum sulfonic acidic density obtained in this work
over SBAꢀ15 of 0.3 SO3H molecule nmꢀ2 is comparable to that
reported for MCMꢀ41 (0.5 SO3H molecule nmꢀ2).34
two regimes: loss of physisorbed water <150
decomposition of propylsulfonate groups >350 C (Fig. S2).
Quantification of the second loss between 400ꢀ600 C enables
°C; and
°
°
calculation of the bulk S content.35 As anticipated, the resulting
S loading increased with the volume of MPTMS employed
during synthesis (Table. 1), which is confirmed by XRF
analysis.
The presence of sulfonic acid functions was confirmed via
DRIFTS. Fig. S3 shows DRIFT spectra of the MPTS
functionalised SBAꢀ15 materials, and Fig. 3 those of the parent
(100)
(110) (200)
PrSO3H(1ML)/SBA-15
PrSO3H(0.5ML)/SBA-15
PrSO3H(0.1ML)/SBA-15
Silanols
PrSO3H(1ML)/SBA-15
PrSO3H(0.5ML)/SBA-15
CH2-Si
PrSO3H(0.01ML)/SBA-15
SBA-15
PrSO3H(0.1ML)/SBA-15
PrSO3H(0.01ML)/SBA-15
υCH
SBA-15
0.5
1.5
2.5
3.5
3700
3400
3100
2800
215900 1700 1500 1300 1100 900 700
Wavenumber / cm-1
Wavenumber / cm-1
2θ / °
Fig. 1 Low angle powder XRD pattern of PrSO3H/SBAꢀ15 as a function of nominal S
Fig. 3 DRIFT spectra of PrSO3H/SBAꢀ15 as a function of nominal S loading.
loading.
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Scheme 1 Bioꢀoil upgrading via acetic acid esterification with benzyl alcohol.
3.2 Acetic acid esterification
The utility of our PrSO3H/SBAꢀ15 family of materials for
Since the majority of reactions were conducted under excess
acid, wherein complete acetic acid conversion is not possible,
the esterification of acetic acid with benzyl alcohol (Scheme 1
)
was subsequently explored in toluene as a simulated bioꢀoil catalyst performance is reported with respect to either the ester
matrix.
yield or the maximum possible acid conversion. Under our
reaction conditions, only 4 % and 10 % benzyl alcohol
conversions were observed in blank reactions after 6 h and 24 h
respectively (Fig. S5), confirming the need for catalytic
Table. 1 Textural and structural properties of PrSO3H/SBAꢀ15 materials.
Materials
Surface
areaa
/ m2 g-1
BJH pore
Total BJH
Unit cell
Wall
Micropore
volume
/cc g-1
Micropore
area
/ m2 g-1
Bulk S
contentd
/ wt.%
Bulk S
contente
/ wt.%
diameter / pore volume parameterb thicknessc
nm
/ cm3 g-1
/nm
/nm
SBAꢀ15
741
5.358
0.742
9.76
4.4
0.143
330
ꢀ
ꢀ
PrSO3H(0.01ML)/SBAꢀ15
675
5.059
5.060
5.060
5.072
0.771
0.766
0.732
0.737
9.58
9.76
9.80
9.97
4.52
4.7
0.108
248
0.27
0.13
PrSO3H(0.1ML)/SBAꢀ15
PrSO3H(0.5ML)/SBAꢀ15
PrSO3H(1ML)/SBAꢀ15
671
628
619
0.109
0.099
0.092
250
226
211
0.29
0.49
0.62
0.19
0.82
1.03
4.74
4.9
aBET. b Determined from a0 = (2d100)/√3. c Determined from a0 ꢀ pore diameter. dTGA weight loss between 450ꢀ600 °C. e Bulk S content from XRF.
for temperatures
≥80 °C, reaching 100 % within 6 h at 100 °C,
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
3.5
3
accompanied by a selectivity to benzyl acetate of 80 %, with
biphenyl ether as the major byꢀproduct through acidꢀcatalysed
etherification of the alcohol. The apparent activation energy of
68 kJ.molꢀ1
(Fig. S8) is comparable to Amberlystꢀ15 sulfonic
acid resins (73.3 kJ mol−1)39, but higher than that reported over
HY and HꢀZSMꢀ5 zeolites (~40 kJ.molꢀ1),22 which operated
over a significantly higher temperature range. These compares
favourably with values for acetic acid esterification with
2.5
2
methanol over sulfonic acid functionalised SBAꢀ15 of 42ꢀ52
1.5
1
40
kJ.molꢀ1,17,
wherein there are no mass transfer or internal
diffusion limitations for the lighter alcohol.
The impact of sulfonic acid loading on acetic acid
esterification over PrSO3H/SBAꢀ15 was subsequently explored
under these optimal conditions, and the resulting reaction
profiles for ester production and benzyl alcohol conversion
shown in Figs. 5 and S9 respectively. Both benzyl alcohol and
acetic acid (Fig. 6) conversions increased monotonically with
sulfonic acid loading, with PrSO3H(1ML)/SBAꢀ15 showing the
best performance as anticipated for a Brönsted acid catalysed
transformation. Biphenylether was the principal byꢀproduct
from etherification. Initial rates of benzyl acetate formation
shown in Fig. 6 mirror the trends in 5 h acid conversion,
indicating that acid site density/strength were the predominant
factors controlling esterification with the small decreases in
surface area and mean pore diameter with S loading having no
detrimental impact on reactivity.
0.5
0
0
0.05 0.1 0.15 0.2 0.25 0.3 0.35
SO3H surface density / molecules.nm-2
Fig. 4 Evolution of surface acid properties of PrSO3H/SBAꢀ15 as a function of S
loading.
promotion at the low reaction temperatures necessary to prevent
bioꢀoil degradation. Initial reaction conditions for acetic acid
esterification catalysed by PrSO3H/SBAꢀ15 were optimised
with respect to catalyst charge and temperature (Fig. S6-7) for
the most acidic PrSO3H(1ML)/SBAꢀ15 material. Esterification
activity was approximately first order in catalyst up to a 50 mg
charge, which was subsequently adopted for all reactions.
Conversions increased dramatically above the background rate
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4
3.5
3
60
50
40
30
20
10
PrSO3H(1ML)/SBA-15
PrSO3H(0.5ML)/SBA-15
PrSO3H(0.1ML)/SBA-15
PrSO3H(0.01ML)/SBA-15
2.5
2
1.5
1
0.5
0
0
0
0.2
0.4
0.6
0.8
1
50 100 150 200 250 300
Surface acid density / mmol.g-1
Fig. 7 Sulfonic acid loading dependence of TOF for benzyl alcohol conversion in
acetic acid esterification over PrSO3H/SBAꢀ15. Reaction conditions: 50 mg catalyst, 100
°C, acid:alcohol ratio of 2:1, toluene solvent.
Reaction time / min
Fig. 5 Benzyl acetate formation over PrSO3H/SBAꢀ15 series. Reaction conditions: 50
mg catalyst, 100 °C, acid:alcohol ratio of 2:1, toluene solvent.
mode of action, with negligible benzyl alcohol conversion
observed following catalyst removal after 30 min reaction (Fig.
S10).
100
20
16
12
8
90
80
70
60
50
40
30
20
10
0
An important objective of this work was to understand the
impact of (simulated) bioꢀoil composition upon its upgrading
via acetic acid esterification, and hence the performance of our
most promising PrSO3H(1ML)/SBAꢀ15 catalyst was
subsequently explored as a function of acid:alcohol ratio, which
varies widely in real pyrolysis bioꢀoils.41 The resulting impact
upon acid conversion is shown in Fig. 8, revealing a significant
increase in conversion under acidꢀrich conditions, as previously
reported for zeolites,22 and indicating that benzyl alcohol is able
compete effectively for adsorption sites with acetic acid, in
contrast to methanol.31 This is an important observation since
acetic acid is commonly in excess of phenolic components (~6
versus 2 wt%)7 suggesting that our sulfonic acid catalysts
should be efficacious against upgrading of real bioꢀoils via
esterification under mild reaction conditions.
Bioꢀoils contain a range of ligninꢀderived phenolics, hence
our PrSO3H(1ML)/SBAꢀ15 catalyst was also screened for
acetic acid esterification against representative examples, and
methanol (Fig. 9). While it proved equally active towards acetic
acid esterification with anisyl alcohol (4ꢀmethoxybenzyl
alcohol), reactivity was far poorer employing cresols and
methanol, the latter presumably reflecting the inability of
methanol to compete effectively for adsorption sites in the
presence of excess acid.17 This is also a reflection of the
aromatic alcohols being better nucleophiles than phenol, and
that lower boiling point of methanol leads to lower reaction
temperatures at ambient pressure.
4
0
0.13
0.19
0.82
1.03
Bulk S loading / wt%
Fig. 6 Acetic acid conversion and initial rate of ester production of PrSO3H/SBAꢀ15 as a
function of S loading. Reaction conditions: 50 mg catalyst, 100 °C, acid:alcohol ratio of
2:1, toluene solvent. Conversion after 5 h.
Turnover Frequencies (TOFs) per acid site show a strong
variation between low and high S loadings (Fig. 7), attributed
to a difference in acid strength, with high sulfonic acid loadings
possessing stronger acidity as previously observed for MCMꢀ41
mesoporous silica due to steric repulsion orienting RSO3H
groups inꢀpore.34 The resulting TOFs of the high loading
PrSO3H/SBAꢀ15 catalysts are superior to those of microporous
Hβ, HY, HꢀZSMꢀ5 zeolites (<10 hꢀ1) and silicotungstic
acid/ZrO2/SBAꢀ15 (36 hꢀ1):22 this is especially impressive since
the latter were determined under more favourable reaction
conditions.
Leaching
studies
conducted
with
the
PrSO3H(1ML)/SBAꢀ15 catalyst confirmed its heterogeneous
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100
80
60
40
20
0
100
80
60
40
20
0
Equilibrium
displacement
Control
40 mol% water addition
5:1
2:1
1:1
1:2
1:4
1:5
0
60
120
180
240
300
Acid to alcohol mole ratio
Reaction time / min
Fig. 8. Dependence of acetic acid esterification over PrSO3H(1ML)/SBA-15 on
Fig. 10. Impact of 40 mol% water addition on acetic acid esterification over
PrSO3H(1ML)/SBAꢀ15. Reaction conditions: 50 mg catalyst, 100 °C, acid:alcohol ratio
of 2:1, toluene solvent.
acid:alcohol ratio. Reaction conditions: 50 mg catalyst, 100 °C, toluene solvent.
6 h reaction, i.e. quantitative in the amount water added,
evidencing negligible catalyst deactivation, but simple
displacement of the reaction equilibrium towards the reverse
hydrolysis reaction as expected. Recycle experiments, and
elemental analysis of the reaction mixture and isolated spent
catalyst also confirmed that the PrSO3H(1ML)/SBAꢀ15 catalyst
was relatively stable to sulfur leaching under our reaction
conditions, with only a small (<20 %) decrease observed after
three esterification cycles (Fig. S11). In contrast, significant
surface carbon accumulated over spent catalysts following each
reꢀuse, likely due to the mild regeneration treatments employed
between each cycle (room temperature washing with toluene
followed by methanol). This surface carbon, which likely arose
from strongly adsorbed aromatic oligomers (as observed over
sulfonated carbons47) has a pronounced negative impact on
100
80
60
40
20
0
conversion
(Fig. S12), and we are currently exploring
regeneration protocols to remove these residues
The excellent performance of our PrSO3H(1ML)/SBAꢀ15
catalyst was evidenced by benchmarking against widely used
commercial solid acid catalysts, specifically an Amberlystꢀ15
sulfonic acid resin and two USY zeolites with different Si:Al.
molar ratios of 30 and 2.6. Fig. 11 shows that both zeolites
offer significantly lower benzyl alcohol conversion than both
sulfonic acid materials, with the poorer activity of the more
acidic USYꢀ2.6 suggesting that this may be accounted for by
selfꢀpoisoning through strong adsorption of acetic acid and/or
benzyl alcohol. However, despite comparable acetic acid
conversion over PrSO3H(1ML)/SBAꢀ15 and Amberlystꢀ15, the
benzyl acetate yield is an order of magnitude lower over the
latter commercial material due to preferential formation of the
undesired diphenyl ether condensation product.
Fig. 9. Dependence of acetic acid esterification over PrSO3H(1ML)/SBAꢀ15 on alcohol.
Reaction conditions: 50 mg catalyst, 100 °C, acid:alcohol ratio of 2:1, toluene solvent.
Pyrolysis bioꢀoils, whether derived via thermal or catalytic
routes, all contain significant water (spanning 10ꢀ60 wt%42),
and hence water tolerance is a key requirement for any practical
bioꢀoil upgrading catalyst, and hence we conducted a spiking
experiment in which a 40 mol% relative to benzyl alcohol was
deliberately introduced to the reaction media at the start of
reaction. Such a high water concentration is far in excess of that
reported to strongly suppress (trans)esterification activity over
44
heteropolyacids,43,
sulphated zirconiaꢀtitania,45 and
Amberlystꢀ15,46 where values around 1 mol% effectively
stopped reaction. Fig. 10 shows that this high level of water had
no impact on the initial rate of benzyl alcohol esterification,
while overall conversion only decreased from 100 to 60 % after
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Seventh Framework Programme (FP7/2007ꢀ2013) under grant
agreement no. 604307 is also acknowledged.
3.5
3
100
80
60
40
20
0
Notes and references
2.5
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Propyl sulfonic acid functionalised SBAꢀ15 (PrSO3H/SBAꢀ
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aromatic alcohols in toluene as a simulated bioꢀoil medium.
Massꢀnormalised activities increase with S loading, reflecting
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