Facile route to conformal hydrotalcite coatings over complex architectures: A hierarchically ordered nanoporous base catalyst for FAME production

Article abstract of DOI:10.1039/c4gc01689k

An alkali- and nitrate-free hydrotalcite coating has been grafted onto the surface of a hierarchically ordered macroporous-mesoporous SBA-15 template via stepwise growth of conformal alumina adlayers and their subsequent reaction with magnesium methoxide. The resulting low dimensional hydrotalcite crystallites exhibit excellent per site activity for the base catalysed transesterification of glyceryl triolein with methanol for FAME production.

Full text of DOI:10.1039/c4gc01689k

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Facile route to conformal hydrotalcite coatings over
complex architectures: a hierarchically ordered
nanoporous base catalyst for FAME production
Cite this: DOI: 10.1039/x0xx00000x
Julia J.Creasey,a Christopher M.A. Parlett,b Jinesh C. Manayil,b Mark Isaacs,b K.
Wilsonb and Adam F. Lee*b
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
www.rsc.org/
in Europe,15 Asia,16 the Americas17 and Africa.18 Commercial
An alkali- and nitrate-free hydrotalcite coating has been
grafted onto the surface of hierarchically ordered
biodiesel is produced almost entirely via the liquid base
catalysed transesterification of C14ꢀC20 triacylglyceride (TAG)
components of lipids with C1ꢀC2 alcohols19 into fatty acid
methyl esters (FAMEs) which constitute biodiesel. Higher
alcohols have also been exploited20 as they offer a less
corrosive FAME with improved physical characteristics.21
Isolation of the desired biodiesel product from homogeneous
base catalysts (and unreacted monoꢀ and diꢀalkyl glycerides and
glycerol byꢀproduct) is necessary to circumvent saponification
and emulsification side reactions and produce a high quality
biofuel.6 Heterogeneous, solid acid22ꢀ24 and base catalysts offer
facile FAME separation, eliminating the requirement for
quenching steps and permitting continuous biodiesel
production,25 and a purer glycerol byꢀproduct stream for use as
a commodity chemical in the food and cosmetics industry.
Among solid base catalysts, hydrotalcites,26ꢀ28 alkaline earth
oxides29ꢀ33 and alkaliꢀdoped mesoporous silicas34 are good
potential candidates for biodiesel formation under mild
a
macroporous-mesoporous SBA-15 template via stepwise
growth of conformal alumina adlayers and their subsequent
reaction with magnesium methoxide. The resulting low
dimensional hydrotalcite crystallites exhibit excellent per site
activity for the base catalysed transesterification of glyceryl
triolein with methanol for FAME production
.
Rising global energy demand over the next 25 years, notably
among emergent economies,1 is driving the quest for
sustainable routes to low cost, liquid transportation fuels from
biomass feedstocks.2 Around 9 % of transportation energy
needs are predicted to be met via liquid biofuels by 2030.3 The
past decade has seen much criticism of firstꢀgeneration bioꢀ
fuels derived from edible plant materials which are attributed to
significant land use changes and deforestation in South East
Asia.4 In order for advanced bioꢀfuels to be considered truly
sustainable, they must be sourced from nonꢀedible crop
components, forestry waste, alternative nonꢀfood plants such as
switchgrass, Miscanthus or Jatropha curcas5 which require
minimal cultivation and do not compete with traditional arable
land or drive deforestation, algal sources or the lignocellulosic
components of municipal waste such as packaging materials.
Although there is burgeoning interest in extracting bioꢀoils
from aquatic biomass, which can yield 80ꢀ180 times the annual
volume of oil per hectare than plants,6 process scaleꢀup and the
availability of nutrient resources remains challenging.7 The
biorefinery concept affords biomass the simplest and most
popular approach to dropꢀin transportation fuels,8 based upon
carbohydrate pyrolysis and subsequent hydrodeoxygenation
(HDO)9 of the resulting bioꢀoils or their gasification and
subsequent FischerꢀTropsch processing10 to gasoline and
diesel,11 or lipid transesterification to biodiesel.12 Catalytic
depolymerisation of lignin may also unlock opportunities for
the coꢀproduction of phenolics and related aromatic compounds
via biorefineries for fine chemical and pharmaceutical
applications13 improving their costꢀeffectiveness.
conditions.
Hydrotalcites
(of
general
formula
subset of
[M(II)1−xM(III)x(OH)2]x+(An−x/n)·mH2O) are
a
microporous layered double hydroxides,35 conventionally
synthesised via coꢀprecipitation from nitrates36 in the presence
of alkalis which are problematic due to NOx emissions37 and in
situ alkali leaching and consequent FAME contamination.38ꢀ40
We recently reported an alkali/nitrateꢀfree route to tunable Mgꢀ
Al hydrotalcite coatings via the direct reaction of Mg(OCH3)2
with a conventional, bulk alumina support.41 While the
resulting materials exhibited excellent Turnover Frequencies
(TOFs) towards TAG transesterification, they suffer from a
number of important drawbacks, namely: poor specific activity
per unit mass towards bulky C18 substrates (0.042 mmol.minꢀ
1.gꢀ1), low surface areas, and restricted (and disordered) pore
architectures available through the use of pure alumina
templates. In contrast, hydrothermally stable silica frameworks
can be readily synthesised, offering diverse pore
interconnectivities and bi- or tri-modal pore networks 42ꢀ44
.
Here we extend our previous methodology to create
crystalline, catalytically active hydrotalcite coatings via a
versatile twoꢀstep methodology, permitting (i) the first genesis
of an ultrathin alumina adlayer over a complex (hierarchically
Biodiesel is a clean burning and biodegradable fuel14 which
remains popular for meeting transportation energy requirements
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DOI: 10.1039/C4GC01689K
ordered) template, and (ii) facilitating its subsequent reaction prepared via our alkaliꢀfree, coꢀprecipitation method from
with Mg(OCH3)2 to form a stoichiometric HT/MMꢀSBAꢀ15 Mg(NO3)2·6H2O and Al(NO3)3·9H2O precursors,26 with the
hydrotalcite catalyst. This novel methodology opens the way to Mg: Al atomic ratio tuned to match that of the MMꢀSBAꢀ15.
a
new class of solid bases built upon the tunable
interconnectivity and porosity afforded by underlying silica
architectures. The resulting nanocomposite combines the high
surface area and excellent massꢀtransport characteristics of the
parent silica, and solid basicity and transesterification
performance of a pure hydrotalcite.
Materials characterisation
Nitrogen porosimetry was undertaken on a Quantachrome Nova
1200 porosimeter. Multiꢀpoint BET surface areas were
calculated over the relative pressure range 0.01–0.3. Pore
diameters and volumes were calculated applying either the tꢀ
plot or BJH methods to the desorption isotherm. Powder XRD
Catalyst synthesis
patterns were recorded on
a
PANalytical X'pertPro
diffractometer fitted with an X'celerator detector and Cu Kα
source; the Scherrer equation was used to calculate HT
crystallite sizes. XPS was performed on a Kratos Axis HSi Xꢀ
ray photoelectron spectrometer fitted with a charge neutraliser
and magnetic focusing lens employing Al Kα monochromated
radiation (1486.7 eV). Spectral fitting was performed using
CasaXPS version 2.3.15. Base site densities were measured via
CO2 pulse chemisorption and subsequent temperature
programmed desorption (TPD) on a Quantachrome ChemBET
3000 system coupled to an MKS Minilab QMS. SEM analysis
was carried out on a Carl Zeiss EVO SEM fitted with an
Oxford Instruments energy dispersive Xꢀray (EDX) analyser
employing Oxford Instruments Inca Software. TGA was
performed using a Stanton Redcroft STA780 thermal analyser.
Synthesis of macroporous-mesoporous SBA-15 (MM-SBA-15)
An hierarchical macroporousꢀmeosporous SBAꢀ15 silica was
prepared following the method of Dhainaut et al.22 Briefly
polystyrene beads synthesised via the emulsion polymerisation
approach of Vaudreuil and coꢀworkers45 were added to an
acidified, aqueous solution of Pluronic® P123 surfactant prior
to the addition of tetramethoxysilane. The resulting gel was
hydrothermally aged without agitation, and the solid obtained
filtered, washed and dried at room temperature before
calcination at 550 °C for 6 h in air.
Alumina grafting onto MM-SBA-15 (Al-MM-SBA-15)
The AlꢀMMꢀSBAꢀ15 hierarchical framework employed the
method of Landau and coꢀworkers developed for MCMꢀ41.46
Aluminiumꢀtriꢀsecꢀbutoxide (14.5 g) was dissolved in
anhydrous toluene (100 cm3) at 85 °C with stirring.
Triethylamine (2.1 cm3) was added to this solution, followed by
dried MMꢀSBAꢀ15 (1 g). After 6 h stirring at 85 °C the solution
was filtered under vacuum (~0.1 bar), with the recovered solid
Transesterification
The HT/MMꢀSBAꢀ15 and conventional HT materials were
tested as catalysts in the transesterification of triolein to form
methyl trioleate (FAME) using a Radleys Starfish parallel
washed three times in toluene (100 cm3). The alumina surface reactor. Briefly, 50 mg of catalyst was added to 10 mmol of
was then hydrolysed in ethanol (318 cm3) containing water (1.6
triolein using a 30:14:1 methanol:butanol:oil ratio; butanol was
cm3) for 24 h at 25 °C with stirring, and the resulting solid
added as a coꢀsolvent to help solubilise the triglyceride. In light
of the significant differences in HT content between our
conventional and SBAꢀ15 coated materials, a common total
mass of catalyst (rather than mass of hydrotalcite) was
employed to ensure identical mixing characteristics within the
reaction vessel. Reactions were carried out at 90 °C in a
recovered by vacuum filtration and washed with ethanol (300
cm3) before drying at 80 ºC in a vacuum oven overnight. A
threeꢀstep calcination sequence was utilised to form an alumina
monolayer: the material was first heated to 250 °C for 1 h, then
400 °C for 1 h and finally 500 °C for 4 h (each ramp rate 1
°C.minꢀ1). Consecutive grafting cycles were undertaken
employing an identical protocol in order to progressively buildꢀ
up alumina monolayers over the silica surface, adjusting the
quantities to maintain the initial Al:Si stoichiometry.
modified ACE
aliquots removed periodically from the reaction mixture for
analysis on Varian 450 GC with 8400 autosampler
(programmable onꢀcolumn injection onto a Phenomenex ZBꢀ
1HT column (15 m x 0.53 mm x 0.15 m film thickness).

50 cm3 round bottom pressure flask, with
a
ꢀ
Synthesis of hydrotalcite-coated MM-SBA-15 (HT/MM-
SBA-15)
Initial rates were calculated from the linear portion of the
reaction profile during the first 60 min of the reaction. Turnover
frequencies (TOFs) were determined by normalising rates to the
total base site density from CO2 chemisorption.
Magnesium methoxide solution (8ꢀ10 wt% in methanol) was
added to AlꢀMMꢀSBAꢀ15 (400 mg, dried for 1 h at 80 °C), at
the minimum quantity to form a homogeneous paste on mixing.
After stirring for 15 min, the mixture was dried under vacuum
at 80 °C for 1 h to remove excess methanol. The surface Mg:Al
atomic ratio was tuned by adjusting the volume of magnesium
methoxide (10.8 cm3 for the MMꢀSBAꢀ15). The resulting
material was calcined at 450 °C for 15 h under 20 cm3.minꢀ1 O2
(ramp rate 1 °C.minꢀ1). After cooling to room temperature
under N2 (20 cm3.minꢀ1), the powder was added to distilled
water (50 cm3 for every 300 mg of powder) in a 100 cm3 roundꢀ
bottomed pressure vessel and heated to 125 °C with stirring for
21 h. After cooling to room temperature, the final HT/MMꢀ
SBAꢀ15 sample was filtered, washed with deionised water and
dried in a vacuum oven overnight at 80 °C, before storage in a
desiccator. This synthesis proved successful on the multiꢀgram
scale. A conventional hydrotalcite reference material was
Results and Discussion
Characterisation of Al-MM-SBA-15
Alumina grafted silica (AlꢀMMꢀSBAꢀ15) was first prepared as
support for subsequent conversion to a high area, hierarchically
ordered hydrotalcite coating. The alumina grafting process was
repeated four times to obtain a uniform multilayer interface,
with textural properties characterised after each grafting in
order to examine the evolution of the aluminaꢀsilica interface.
Low angle XRD and TEM of the parent MMꢀSBAꢀ15 and the
sequentially alumina grafted variants (Figures S1-2) confirmed
the presence of ordered mesopores indicative of SBAꢀ15.47
Characteristic (100), (110) and (200) reflections were observed
for all materials, indicative of the p6mm space group expected
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for hexagonally arranged mesoporous channels.48 Macropore
An estimate of the alumina film thickness may be obtained
incorporation shifted these reflections to higher angle relative to by comparing the experimentally determined Al surface density
conventional mesoporous SBAꢀ15, associated with a small
contraction in the mesopore lattice parameter.22 This
contraction is attributed to curvature of the mesopore channels
as they coalesce around the polystyrene bead template due to
strong electrostatic interactions between the beads, block coꢀ
polymer and silica precursor. Long range, hexagonally ordered
mesopores remained present for AlꢀMMꢀSBAꢀ15 even
following four consecutive grafting cycles.
Nitrogen porosimetry of the parent MMꢀSBAꢀ15 and
alumina grafted analogues confirmed that the mesoporosity
intrinsic to the SBAꢀ15 framework is maintained after each
grafting cycle (Figures S3a-b). However, the BET surface area
(and interconnecting micropore area from tꢀplot analysis), mean
mesopore diameter, and total mesopore and micropore volumes
decreased progressively with each grafting cycle (Table 1),
consistent with an increasing thickness of conformal alumina
overlayer uniformly distributed throughout the pore network.
Table 1. Porosimetry of AlꢀMMꢀSBAꢀ15 as a function of grafting cycle
Material
Surface Mesopore Mesopore Micropore Micropore
areaa
/ m2.gꢀ1
473
volumeb diameterb volumec
areac
/ m2.gꢀ1
166
/ cm3.gꢀ1
0.66
/ nm
3.9
/ cm3.gꢀ1
0.079
MMꢀSBAꢀ
15
AlꢀMMꢀ
SBAꢀ15ꢀ1
AlꢀMMꢀ
SBAꢀ15ꢀ2
AlꢀMMꢀ
SBAꢀ15ꢀ3
AlꢀMMꢀ
SBAꢀ15ꢀ4
332
271
240
201
0.48
0.47
0.46
0.45
4.0
4.0
3.7
3.7
0.024
0.013
0.007
0.004
58
33
19.3
13.5
aMultiꢀpoint BET method. bBJH mesopore volume and pore diameter
from the desorption isotherm. cCalculated by t ꢀplot method
.
The surface of AlꢀMMꢀSBAꢀ15 was subsequently
investigated by XPS. Successful alumina grafting was
confirmed by the presence of surface Al, with the Al:Si atomic
ratio increasing monotonically with each cycle, reaching 22.3
wt% Al after four cycles. Figure 1 compares the Al and Si 2p
chemical environments for the parent and alumina grafted
mesoporous silicas. The pure MMꢀSBAꢀ15 exhibited a single
Si 2p spinꢀorbit split doublet centred with Si 2p3/2 component at
103.1 eV binding energy associated with pure silica. Alumina
grafting significantly attenuated the substrate signal consistent
with a conformal (Frankꢀvan der Merwe) growth mode, rather
than the formation of 3ꢀdimensional alumina islands. This
attenuation was accompanied by the emergence of a second low
binding energy doublet at 102.2 eV for AlꢀMMꢀSBAꢀ15, which
can only be associated with a new, interfacial silicon species.
This hypothesis is supported by the observation of two distinct
Al 2p spinꢀorbit doublets, at 73.8 eV and 74.7 eV. The former
is consistent with pure alumina, and its intensity increases
monotonically with grafting cycle relative to the high binding
energy state, precisely as expected if the latter was associated
with an interfacial alumina species. The opposing binding
energy shifts for the interfacial Al and Si species are similar to
those previously observed for alumina grafted SBAꢀ15,49 and
can be understood in terms of their different Pauling
electronegativities and associated induced dipoles mediated via
the AlꢀOꢀSi bridges which increase and decrease the local
initial state charge on interfacial Si and Al atoms respectively.
Figure 1. (a) Si and (b) Al 2p XP spectra of parent MM-SBA-15 and Al-MM-SBA-15
following four alumina grafting cycles.
(derived from porosimetry and XPS) with that for a crystalline
alumina phase such as αꢀAl2O3, which exhibits a rhombohedral
(4.75 Å x 4.75 Å) surface unit mesh containing three Al atoms
within the (006) plane as shown in Scheme 1 50
With a total
.
surface area of 473 m2.gꢀ1, a single αꢀAl2O3 monolayer covering
the entire silica pore network would contain 0.0009 mol Al,
equating to an Al loading of 24.3 wt%. This is close to the
observed value of 22.3 wt%, and indicates that an alumina film
approximately 0.7 monolayers thick (~0.17 nm) is formed
following four grafting cycles, which would constrict the
mesopores by 0.34 nm relative to the parent MMꢀSBAꢀ15, in
excellent agreement with the observed pore diameter decrease
of 0.3 nm seen in Table 1
.
Characterisation of HT/MM-SBA-15
Powder XRD diffraction patterns for the methoxide
functionalised AlꢀMMꢀSBAꢀ15 material prepared via four
alumina grafting cycles (HT/MMꢀSBAꢀ15), and a reference
bulk HT sample prepared by conventional alkaliꢀfree coꢀ
precipitation (ConvHT) are shown in Figure 2. The HT/MMꢀ
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SBAꢀ15 sample shows a diffraction pattern characteristic of a reaction of the alumina adlayer with Mg(OCH3)2. This suggests
pure HT phase, very similar to that observed for the ConvHT
that either extremely thin HT crystallites are formed throughout
Table 2. Crystallographic properties of HT/MMꢀSBAꢀ15 and ConvHT
Material
Crystallite
size
Interlayer
spacing
/ nma
Lattice
parameter
/ nma
Mg:Al
ratiob
/ nma
HT/MMꢀ
SBAꢀ15
ConvHT
1.18
0.76
0.305
2.21:1
1.95:1
6.2
0.77
0.305
aCalculated using Scherrer equation; bDetermined from lattice parameter
using Vegard's Law.
Scheme 1. Crystal structure of α-alumina with primitive cell highlighted.50
the bimodal pore network (consistent with XRD), or that
hydrotalcite formation is confined to the macropores. The latter
would be expected to hinder accessibility of the mesopores (for
which macropores serve as the principal conduits), and hence
reduce both the mesopore volume and total surface area, in
standard, but with broader reflections indicative of significantly
smaller crystallite sizes (as anticipated in light of the highly
dispersed alumina substrate, which by inference appears to
undergo little restructuring during the crystallisation process)
and turbostratic disorder.51 There was no evidence for brucite52
and only a single weak reflection likely associated with trace
MgO. This confirms the successful synthesis of a hydrotalcite
phase through direct reaction of a preꢀformed, ultrathin alumina
film and magnesium methoxide from solution.
contrast to the observed values reported in Table S1
.
350
a
Adsorption
300
Desorption
250
* HT
• MgO
*
*
*
HT/MM-SBA-15
200
*
*
*
*
•
*
*
*
*
*
*
*
*
*
150
*
HT/MM-SBA-15
ConvHT
*
*
100
*
50
Brucite
75
Al-MM-SBA-15
5
15
25
35
45
2θ / °
55
65
0
0
0.2
0.4
0.6
0.8
1
Figure 2. Powder XRD patterns for HT/MM-SBA-15 and a conventional HT
standard (offset for clarity)
Relative pressure / P/P0
3.5
3
Crystallite sizes determined using the Scherrer equation,
interlayer spacings, lattice parameters and Mg:Al ratios
determined using Vegard's law (Figure S4) are reported in
Table 2. The composition, lattice parameter and interlayer
spacings of the HT/MMꢀSBAꢀ15 material were almost identical
to that of the ConvHT, confirming that the hydrotalcite phase
formed at the surface of the hierarchical silica support was
essentially indistinguishable from that of obtained by traditional
synthetic methods, but with a surface area around five times
higher. However, the significant difference in microporous
crystallites size is expected to hinder accessibility of reactatns
to active sites within the ConvHT interlayers relative to the
b
2.5
2
HT/MM-SBA-15
Al-MM-SBA-15
1.5
1
HT/MMꢀSBAꢀ15 sample whose dimensions suggest
a
hydrotalcite bilayer wherein a far greater proportion of base
sites reside on exposed surfaces.
Textural properties of the HT/MMꢀSBAꢀ15 material are
compared with those of the AlꢀMMꢀSBAꢀ15 precursor in
0.5
0
1
3
5
7
9
Figure
mesopore
3
.
Nitrogen porosimetry evidences retention of
and macropore character within the
Pore diameter / nm
adsorption/desorption isotherms following HT crystallisation,
although their demarcation is not as clear as for AlꢀMMꢀSBAꢀ
15 (Figure S3), while Table S1 shows virtually no change in
either the mesopore volume or mean mesopore diameter upon
Figure 3. (a) N2 adsorption-desorption isotherms and (b) BJH pore size
distributions for Al-MM-SBA-15 precursor and HT/MM-SBA-15 (offset for clarity).
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SEM of the HT/MMꢀSBAꢀ15 (Figure 4) confirms the shown in Fig. 5a reveal that hydrotalcite formation was
macropore network present within the parent MMꢀSBAꢀ15 accompanied by attenuation of the interfacial alumina species,
support is retained throughout the material after hydrothermal and concomitant appearance of a new low binding energy
treatment, a measure of the excellent stability of silica frameꢀ
works towards high temperature water, and conditions that a
comparable pure hierarchical alumina structure would be
unlikely to survive. TEM shows macropores are decorated with
high aspect ratio hydrotalcite nanocrystallites.
a
Data
Fit
HT-SiO2
Al-SiO2
SiO2
Al Mg Al
O
O
Si Si Si
Thermogravimetric analysis confirms the excellent thermal
stability of the HT/MMSBAꢀ15 (Figure S5), with only a small
10 % weight loss between 70 and 220 °C, associated with the
desorption of physisorbed water from the HT surface and water
from within the interlayers,53 and a 5 % loss between 250 and
350 °C attributed to hydroxide anions in the bruciteꢀlike
layers.54 EDX elemental analysis of the HT/MMꢀSBAꢀ15 yields
an overall Mg:Al atomic ratio of 2.2:1, in good agreement with
that derived from Vegard’s law in Table 2, and a total Mg
content of 19.8 wt% i.e. a quarter that of a bulk hydrotalcite of
comparable Mg:Al ratio,26 consistent with the formation of
hydrotalcite nanocrystals approximately 1 nm thick relative to
silica walls around 4ꢀ5 nm thick in the MMꢀSBAꢀ15 support.49
O
O
Si Si Si
HT/MM-SBA-15
Al Al Al
O
O
Al Al Al
O
O
Si Si Si
O
O
Al-MM-SBA15
108
Si Si Si
103
98
Binding energy / eV
b
HT/MM-SBA-15
Data
Fit
HT-SiO2
Al2O3
Al-SiO2
Al-MM-SBA-15
Figure 4. SEM and TEM micrographs of HT/MM-SBA-15. Insets highlight
macropore network and hydrotalcite crystallites decorating macropores.
77
75
73
71
Binding energy / eV
c
Data
Fit
HT-SiO2
Surface base properties of the HT/MMꢀSBAꢀ15 and
ConvHT bulk reference materials were assessed by
temperatureꢀprogrammed desorption of CO2ꢀsaturated samples,
presented in Figure S6. The desorption profile of HT/MMꢀ
SBAꢀ15 mirrors that of the bulk hydrotalcite, both exhibiting a
single desorption feature around 340 °C indicative of moderate
strength base sites, albeit spanning a broader temperature range
and hence distribution of basicity for the HT/MMꢀSBAꢀ
15.Total base site densities for the HT/MMꢀSBAꢀ15 and
ConvHT materials were 6.4x1018 and 8.6x1019 mꢀ2 respectively,
although one should recall that a large proportion of sites
present within the interlayers of the bulk hydrotalcite structure
may be inaccessible to stericallyꢀdemanding substrates. These
results confirm that high aspect ratio hydrotalcite crystallites
formed over the hierarchical silica support possess similar
intrinsic basicity to conventional coꢀprecipitated analogues.
Surface analysis of HT/MMꢀSBAꢀ15 yielded a Mg:Al
atomic ratio of 2.21 and Mg content of 16.7 wt%, both very
similar to values determined by EDX, evidencing uniform
incorporation of Mg into the alumina film throughout the pore
network of the AlꢀMMꢀSBAꢀ15 precursor. Si 2p XP spectra
ConvHT
HT/MM-SBA-15
HT/MM-SBA-15
93
88
83
Binding energy / eV
Figure 5. (a) Si 2p and (b) Al 2p, and (c) Mg 2s XP spectra of Al-MM-SBA-15
precursor and HT/MM-SBA-15.
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chemical environment at 101.5 eV. The latter suggests that enhancement per base site of the coated material is higher still,
interfacial silicon atoms are now bound (through oxygen translating to TOFs of 7.6 minꢀ1 for the bulk ConvHT standard
bridges) to a less polarising adlayer relative to alumina, versus 66 minꢀ1 for HT/MMꢀSBAꢀ15. Hydrotalcites prepared
consistent with the exchange of Al3+ for Mg2+ cations. Fig. 5b via conventional coꢀprecipitation are among the most widelyꢀ
shows analogous changes in the Al chemical environment, with used catalysts for triglyceride transesterification to FAME,
attenuation of the pure (and interfacial) alumina adlayer, and hence the nineꢀfold rate enhancement observed for our
emergence of a high energy Al state ~74 eV, consistent with the HT/MMꢀSBAꢀ15 material provides a striking benchmark of its
introduction of Mg2+ cations into the grafted alumina film exceptional performance. While the magnitude of this
during hydrotalcite formation. The corresponding Mg 2s XP enhancement does fall at longer reaction times, likely due to
spectrum of HT/MMꢀSBAꢀ15 presents a single chemical partial deactivation of the coating, the HT/MMꢀSBAꢀ15
environment around 88.5 eV. In summary, over 75 % of the remains three times as active per base site as the bulk
MMꢀSBAꢀ15 silica surface is contacted with a hydrotalcite hydrotalcite, even after 1400 min reaction.
phase, and a similar proportion of the initially grafted alumina
adlayer in AlꢀMMꢀSBAꢀ15 is converted into hydrotalcite.
Since the intrinsic base strength of active sites within the
conventional and hierarchical hydrotalcite catalysts is the same
(common CO2 desorption temperatures, Figure S6), we
attribute this nineꢀfold rate enhancement of HT/MMꢀSBAꢀ15 to
superior massꢀtransport characteristics of the macroporousꢀ
mesoporous architecture. Indeed, the magnitude of the
HT/MMꢀSBAꢀ15 enhancement with respect to the ConvHT
standard is comparable to that previously reported for a
macroporous pure HT material,28 but affords a far more flexible
and hydrothermally stable framework than the latter synthesis.
Transesterification of triglycerides
In order to establish the catalytic efficacy of the HT film
encapsulating the hierarchical silica template, the HT/MMꢀ
SBAꢀ15 material was screened in the transesterification of
glyeryl triolein, a bulky C18 triglyceride that is a major
component on oilseed feedstocks, with methanol for FAME
(biodiesel) production. The resulting reaction profiles for
HT/MMꢀSBAꢀ15 and the coꢀprecipitated ConvHT analogue are
shown in Figure 6. Transesterification proceeded rapidly over
both catalysts during the first hour of reaction before slowing
dramatically, to give limiting conversions of 34 % and 64 % for
HT/MMꢀSBAꢀ15 and ConvHT respectively. While the absolute
FAME productivity of the bulk hydrotalcite is clearly superior,
Conclusions
Sequential,
wetꢀchemical
surface
modification
of
nanostructured silicas with Al and Mg precursors offers a
versatile route to the preparation of high area, tailored solid
base hydrotalcite catalysts. Stepwise grafting and thermal
processing of aluminiumꢀtriꢀsecꢀbutoxide results in a uniform
alumina monolayer throughout the bimodal macroporeꢀ
mesopore network. Subsequent reaction with Mg(OCH3)2
affords stoichiometric incorporation of aluminium from the
alumina adlayer into ~1 nm Mg2Al hydrotalcite crystallites,
which possess identical basicity as a coꢀprecipitated, bulk
hydrotalcite. In contrast to bulk (monomodal) alumina
templates, the development of a silica based methodology
results in HT/MMꢀSBAꢀ15 catalyst exhibits similar specific
mass activity in the transesterification of glyceryl triolein with
methanol as a bulk hydrotalcite, despite containing only a small
fraction of the number of active sites, indicating far greater
active site accessibility to the bulky TAG reactant. The latter
conclusion is supported by a nineꢀfold enhancement in the TOF
per base site for the hierarchical hydrotalcite, indicating the
majority of base sites in HT/MMꢀSBAꢀ15 reside at the external
surface of nanoscale crystallites within the mesoꢀ and
macropores, rather than within the microporous interlayers of
conventional hydrotalcites. Our methodology is readily
70
60
ConvHT
50
HT/MM-SBA-15
40
30
20
10
0
0
200 400 600 800 100012001400
Reaction time / min
Figure 6. Triolein conversion to methyl trioleate via transesterification with
methanol (1:30 molar ratio) at 90 °C over HT-MM-SBA-15 and a conventional HT extendable to diverse silica architectures and other metal
standard.
oxides, opening up opportunities for the facile introduction of
hydrotalcite solid basicity into complex twoꢀ or threeꢀ
dimensional materials, e.g. membranes and monoliths, for
catalysis and sorption applications.
it is important to recall that the HT/MMꢀSBAꢀ15 only contains
a thin hydrotalcite coating and the majority of this catalyst is
composed of inert silica. A fairer comparison of the relative
catalytic performance is obtained from their initial rates of
triolein conversion and Turnover Frequencies (TOFs)
Acknowledgements
normalised per base site utilising the CO2 TPD measurements. We thank the EPSRC (EP/G007594/4) for financial support and
This reveals a common initial rate of 1 mmol.gcatalystꢀ1.minꢀ1, a Leadership Fellowship (AFL) and studentship (JJC), and the
however one must recall that the HT/MMꢀSBAꢀ15 material Royal Society for the award of an Industry.
only contains one quarter of the amount of hydrotalcite present
within the bulk ConvHT material, hence the rate normalised per
mass of hydrotalcite is four times higher for HT/MMꢀSBAꢀ15
Notes and references
a School of Chemistry, Cardiff University, Cardiff CF10 3AT, UK.
catalyst. Since the base site density of the coated hydrotalcite is
also ~34 % lower than that of its bulk counterpart, the rate
6 | J. Name., 2012, 00, 1-3
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