G Model
CATTOD-8890; No. of Pages7
Table 1
2
materials science, and to date PMOs possessing organic bridging
(RO)3–Si–(OR)3 units with methylene [31–33], ethylene [29,33,34],
been reported. Ethyl and phenyl organic linkers are the most widely
studied in catalysis because they afford a more homogeneous dis-
tribution of organic moieties, and improved surface crystallinity
and thermal stability [38,39]. The utility of such materials in catal-
ysis is of particular interest as the inclusion of different organic
moieties within the framework offers a unique opportunity to tune
surface polarity and hydrophobicity; critical parameters in control-
ling adsorption, reactant activation and product selectivity in liquid
and vapour phase catalysis.
The development of next generation catalysts for biomass con-
version requires improved methods to quantify the impact of
properties of porous materials are often empirically defined, ham-
pering our ability to tailor catalyst surfaces in an informed manner.
Simple contact angle measurements are ill-suited to the analy-
sis of powdered solids [52] since particle morphology/porosity
can influence the shape of probe droplets and their absorption.
[59,60] and templated silicas [61]. In respect of catalysis, IGC has
been exploited to correlate reactivity with the adsorption of specific
reactants in propene partial oxidation over Pd [62], CO oxidation
over Pt and Rh [63], VOC combustion over Mn–Zr mixed oxides [64],
and benzene benzylation over Fe–ZSM-5 [65], however, there are
no previous reports applying IGC to correlate catalyst hydropho-
bicity and associated activity.
Stoichiometry of reactant mixtures employed in PMO synthesis.
Molar composition
Nominal organic
hybridisation (mol%)
TEOS
BTSE (BTSB)
P123
HCl
H2O
SBA-15
0.0414
0.031
0.0207
0.031
0
0.00069
0.00069
0.00069
0.00069
0.00069
0.0159
0.0159
0.0159
0.0159
0.0159
8
8
8
8
8
25% BTSE (E25)
50% BTSE (E50)
50% BTSB (B50)
0.0052
0.0103
0.0052
0.0103
0.0207
hybridisation was subsequently added to the surfactant solution
(Table 1), and stirred at 40 ◦C for a further 72 h. The mixture was
then aged at 130 ◦C for 24 h and the resulting solid product filtered,
washed three times with deionised water and dried at room tem-
perature. Residual P123 template was extracted via a 24 h reflux
with EtOH/1 M HCl solution and filtered and dried to afford the
final powdered PMO support.
2.3. Sulfonic acid grafting
PMO materials were subsequently sulfonic acid functionalised
via a grafting route in which 1 g of each material was added to a
solution containing 1 cm3 of MPTS in 30 cm3 of toluene. The result-
ing suspension was refluxed at 130 ◦C under stirring for 24 h, and
the thiol-functionalised solid then filtered, washed 3 times with
methanol and dried at 80 ◦C overnight. Thiol groups were oxidised
to –SO3H by mild oxidation with 30 cm3 of 30 vol% H2O2 through
continuous stirring of the thiolated PMO at room temperature for
24 h. The sulfonated product was filtered, washed three times with
methanol and dried at 80 ◦C, and the final PrSO3H-PMO stored in
air and used without further treatment.
2.4. Characterisation
Nitrogen physisorption was undertaken on a Quantachrome
Nova 2000e porosimeter using NOVAWin software. Samples were
degassed at 120 ◦C for 2 h prior to analysis by N2 adsorption at
−196 ◦C. BET surface areas were calculated over the relative pres-
sure range 0.01–0.2. Pore diameters and volumes were calculated
applying the BJH method to the desorption isotherm for relative
pressures >0.35. Low angle powder XRD patterns were recorded
on a PANalytical X’pertPro diffractometer fitted with an X’celerator
Here we report the application of IGC to extract a new param-
eter for quantifying the hydrophobic properties of mesoporous
catalysts which correlates with their turnover frequency (TOF) in
palmitic acid esterification by methanol; a prototypical reaction
pertinent to biodiesel production wherein high local concentra-
tions of reactively-formed H2O can impede the forward reaction
pathway to the desired fatty acid methyl esters [66].
´
˚
detector and Cu K␣ (1.54 A) source calibrated against a Si standard
(PANalytical). Low angle patterns were recorded for 2ꢁ = 0.3–8◦
with a step size of 0.01◦. TEM micrographs were obtained with
a Phillips CM12 transmission electron microscope operated at
100 kV, with images recorded by a SIS MegaView III digital cam-
era. Image analysis was undertaken using ImageJ software. XPS was
performed on a Kratos Axis HSi X-ray photoelectron spectrometer
fitted with a charge neutraliser and magnetic focusing lens employ-
ing Al K␣ monochromated radiation (1486.6 eV). Spectral fitting
was conducted using CasaXPS version 2.3.14, with binding ener-
gies corrected to the C 1s peak at 284.8 eV and S 2p XP spectra fitted
using a common Gaussian/Lorentzian peak shape. Errors were esti-
mated by varying the Shirley background subtraction procedure
across reasonable limits and re-calculating fits. TGA was performed
using a Stanton Redcroft STA780 thermal analyser on ∼10–20 mg
samples under a 10 vol% O2/He mixtures (20 cm3 min−1 total flow)
during heating at 20 ◦C min−1 in order to study the decomposi-
tion of organic moieties. Surface energies at infinite dilution were
determined by a fully-automated Surface Measurement Systems
Ltd inverse GC system. Samples were outgassed for 2 h at 120 ◦C to
remove physisorbed water and impurities on the surface prior to
exposure to ethyl acetate, methanol or alkane pulses. Full details of
the experimental procedure are given in the ESI. Surface properties
2. Experimental
2.1. Chemicals
Mercaptopropyl trimethoxysilane (MPTS, Alfa Aesar 95%);
toluene (Fisher 99%); TEOS (tetraethoxyorthosilane, Aldrich 99%);
HCl (Fisher 36 wt%); methanol (Fisher 99%); hydrogen peroxide
(Sigma-Aldrich 30 vol%); hexanoic acid (Aldrich 99%); palmitic acid
(Aldrich 99%); 1.4-bis(triethoxysilyl)benzene (BTSB, Aldrich 96%);
1.2-bis(triethoxysilyl)ethane (BTSE, Aldrich 96%); ammonia (BOC,
99.98%).
2.2. Synthesis of SBA-15 and PMO materials
Periodic mesoporous organosilicas with 0%, 25% and 50%
ethyl/phenyl hybridisation were synthesised following the pro-
tocol of Sanchez-Vazquez et al. [67]. Briefly, 3 g of Pluronic P123
triblock copolymer was dissolved in 112 cm3 of water and 1 cm3 of
HCl under stirring at 40 ◦C. The appropriate mixture of TEOS and
BTSE (BTSB) precursor required to achieve the desired degree of
Please cite this article in press as: C. Pirez, et al., Can surface energy measurements predict the impact of catalyst hydropho-
bicity upon fatty acid esterification over sulfonic acid functionalised periodic mesoporous organosilicas? Catal. Today (2014),