Surface-functionalized mesoporous gallosilicate catalysts for the efficient and sustainable upgrading of glycerol to solketal

Article abstract of DOI:10.1039/d0gc02562c

Two series of functionalized mesoporous Ga silicates were prepared in a straightforward and sustainable one-pot procedure using different alkyl silanes. The efficacy of the adopted co-synthetic approach based on aerosol processing has been proved by 29Si solid-state NMR experiments revealing a degree of functionalization close to the theoretical value. The successful incorporation of gallium as single sites within the silica framework was confirmed via71Ga solid-state magic-angle-spinning NMR measurements. These materials were tested as catalysts for the synthesis of solketal from glycerol at low temperature and under solventless conditions. A systematic study evidenced the importance of a careful tuning of surface polarity, achievable with surface functionalization as well as with different thermal treatments. The solids functionalized with a low degree of methyl groups (5%) displayed enhanced performances compared to the non-functionalized analogues, highlighting the highly beneficial role of surface hydrophobicity as well as the importance of the careful tuning of the hydrophilic/hydrophobic balance. The best functionalized catalysts proved to be easily reusable for multiple catalytic runs. With such a high-performance catalyst in hand, we propose a process which shows a favorable E-factor, indicating that the production of solketal can be envisaged in a sustainable way.

Full text of DOI:10.1039/d0gc02562c

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DOI: 10.1039/D0GC02562C
Surface-functionalized mesoporous gallosilicate catalysts for the
efficient and sustainable upgrading of glycerol to solketal
Alvise Vivian1#, Loraine Soumoy1#, Luca Fusaro1, Sonia Fiorilli2, Damien P. Debecker3,*,
Carmela Aprile1,*
1University of Namur, Department of Chemistry, Unit of Nanomaterials Chemistry, 5000 Namur,
Belgium
2Department of Applied Science and Technology, Politecnico di Torino, Institute of Chemistry, 10129
Torino, Italy
3Université catholique de Louvain (UCLouvain), Institute of Condensed Matter and Nanosciences
(IMCN), 1348 Louvain-la-Neuve, Belgium.
#These authors contributed equally to this work
*Corresponding authors: carmela.aprile@unamur.be ; damien.debecker@uclouvain.be
ABSTRACT
Two series of functionalized mesoporous Ga-silicates were prepared in a straightforward and
sustainable one-pot procedure using different alkyl-silanes. The efficacy of the adopted co-
synthetic approach based on aerosol processing has been proved by 29Si solid-state NMR
experiments revealing a degree of functionalization close to the theoretical value. The
successful incorporation of gallium as single sites within the silica framework was confirmed
via 71Ga solid-state magic-angle-spinning NMR measurements.
These materials were tested as catalysts for the synthesis of solketal from glycerol at low
temperature and under solventless conditions. A systematic study evidenced the importance
of a careful tuning of surface polarity, achievable with surface functionalization as well as with
different thermal treatments. The solids functionalized with a low degree of methyl groups
(5%) displayed enhanced performances compared to the non-functionalized analogues,
highlighting the highly beneficial role of surface hydrophobicity as well as the importance of
the careful tuning of the hydrophilic/hydrophobic balance. The best functionalized catalysts
proved to be easily reusable for multiple catalytic runs. With such high-performance catalyst
in hands, we propose a process which show favorable E-factor, indicating that the production
of solketal can be envisaged in a sustainable way.
Keywords: Hybrid materials; hydrophobicity; aerosol-assisted sol-gel process; Ga-silicates; glycerol
valorization; 71Ga NMR
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DOI: 10.1039/D0GC02562C
Introduction
In 2015, the United Nations Member States adopted their “2030 Agenda for Sustainable
Development”, accelerating the transition toward, inter alia, more sustainable mobility and
industry.1, 2 In this context, the conversion of biomass into biofuels plays a pivotal role.
Increasing the amount of biofuel in traditional fuels is now needed in order to fulfill the
requirements of the European Commission: a minimum of 14% of renewable energy is
required in road and rail transport by 2030.3 On the biofuels market, biodiesel is gaining a
growing importance due to its good compatibility with commercial diesel engines.4
Nevertheless, its production via transesterification of triglycerides coming from natural lipid
sources yields 10 wt.% of glycerol as by-product.5 The more intense production of biodiesel
over the last past years caused an oversupply of glycerol on the market.6 Its price
consequently suffered a drastic decrease and nowadays it is even considered as a waste.
However, in the context of green chemistry, glycerol represents a very interesting building
block as it directly comes from renewable feedstocks and can be produced via low energy
consumption compared to petro-chemical based reagents.7
Glycerol offers a tremendous potential to produce value-added chemicals and the
synthesis of solketal, through an acetalization reaction with acetone is a particularly relevant
pathway.8 On one side, solketal has various applications as solvent in large-scale productions
(paint, ink formulations)9, in the pharmaceutical industry (additive for injectables, vehicle for
drug)9 and even as fuel additive.10, 11 The latter provides opportunities for the biorefineries to
develop their circular economy. On the other side, this valorization pathway fulfills some of
the 12 Principles of Green Chemistry established by Anastas and Warner7: glycerol and even
acetone can be produced from renewable feedstocks (from triglycerides and hemicellulose,13
respectively), the reaction can be classified as a high atom economy process (88%) and with
a potential low E-factor.12, 13
Acetalization of glycerol is usually performed in the presence of homogeneous Brønsted
or Lewis acid catalysts such as H2SO4, HCl, SnCl4 or Sn(OAc)2.10, 14 Despite the high intrinsic
activity of these homogeneous catalysts, scientists are working hard to develop
heterogeneous catalysts, which can be recovered, reused or even implemented in continuous
flow processes, thereby improving the sustainability of the process.15 The use of zeolites,
sulfonic acid functionalized silicates, K 10 montmorillonites, zirconia-based catalysts or gallo-
silicates for the acetalization of glycerol has already shown promising results.16-21 More
recently, we proposed a new class of gallo-silicate catalysts synthesized via the aerosol-
assisted sol-gel process, which show outstanding performances in the conversion of glycerol
into solketal.22
In recent years, aerosol processes have emerged as efficient methods to synthesize
heterogeneous catalysts displaying controlled properties for their application in a wide range
of reactions.23 For example, transition metal-doped silica and titania for oxidation of volatile
organic compounds,24 metal oxides for electrocatalysis,25 titanosilicate catalysts for
epoxidation,26 or even supported metal nanoparticles for hydrogenation reactions27 have
2

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already been prepared by aerosol processes and shown excellent catalytic performances. In
DOI: 10.1039/D0GC02562C
particular, the “aerosol-assisted sol-gel process” (AASG) emerged as an efficient tool to
synthesize mesoporous silica-based solids featuring metal atoms inserted as single sites in
their framework.28 In this synthesis procedure, a sol containing a templating agent (e.g. a
surfactant), solvent, silica and a metal precursors is nebulized into droplets that are gas-
transported towards a drying chamber or tube, provoking the evaporation of the solvent. This
fast evaporation triggers the self-assembly of the template used as sacrificial agent to
generate pores and the rapid polycondensation of the inorganic precursors, yielding
homogeneous materials with spherical shape. Compared to the traditional sol-gel route, the
aerosol process presents several advantages including reduction of waste production, limited
number of steps and low environmental impact. Moreover, the possibility to produce the
solid in a continuous way makes it industrially attractive. 23, 29-31
Regarding the conversion of glycerol into solketal, the development of highly performing
heterogeneous catalysts has not yet been achieved. In order to design more efficient
heterogeneous catalysts, it is well known that multiple characteristics can be optimized such
as nature and number of active sites, mass transport of reactants and products to and from
the active sites, surface area, pore size distribution, etc. However, an important and less
studied parameter is the role played by the hydrophilic and hydrophobic properties of the
catalyst surface. The latter influences strongly the adsorption of reagents and the desorption
of products or by-products to/from the catalytic surface, thus determining the performances
of the solid. Considering the mechanism of the acetalization of glycerol, the key role played
by the hydrophilic and hydrophobic properties of the catalyst surface can be easily
understood. On the one hand, hydrophilicity of the catalytic surface will favor glycerol
adsorption in proximity of the active sites. On the other hand, more hydrophobic surfaces
would help the removal of water produced as by-product of the reaction, locally shifting the
reaction towards the formation of solketal. In fact, the elimination of water using a Dean-
Stark apparatus has already proved to have a positive effect on the solketal yield.10 As the
surface of silicates is decorated by silanol groups, its hydrophilic-hydrophobic balance can be
tuned by alkylation procedures using a co-synthetic or a post-synthetic approach.28, 32 In 2006,
Ji et al. reported on the synthesis of functionalized mesoporous silicates via an aerosol process
but their use in catalysis was not reported.33 Recently, we reported on aerosol-made methyl-
functionalized stanno-silicates catalysts and their application in the production of ethyl
lactate.28 To the best of our knowledge, the role played by the hydrophobic/hydrophilic
properties of the catalytic surface on the acetalization of glycerol to solketal was never
systematically investigated.
Here, we take advantage of the aerosol-assisted sol-gel process to develop a co-synthesis
approach and obtain, in a one-pot and green procedure, a novel class of alkyl-functionalized
gallosilicates. The specific functionalization of the catalyst surface is presented as a way to
further improve catalytic performances while taking into account the sustainability of the
process.
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DOI: 10.1039/D0GC02562C
Experimental Section
Materials
Pluronic® P-123 (Merck), Pluronic® F127 (Merck), hydrochloric acid 2M (Roth), tetraethyl
orthosilicate (TEOS 97%, TCI), methyltriethoxysilane (MTES 98%, TCI), propyltriethoxysilane
(PrTES 98%, TCI), phenyltriethoxysilane (PhTES 99%, TCI) and (Ga(NO3)3·xH2O, ROTH) .
Absolute ethanol (analytical grade) was supplied from Fisher Scientific.
Catalyst preparation
To synthesize functionalized Ga-silicates (Ga-P-x and Ga-F-x), a mixture of two silica sources,
tetraethyl orthosilicate (TEOS) and methyltriethoxysilane (MTES) for methyl functionalized
solids, propyltriethoxysilane (PrTES) for propyl functionalized solids or phenyltriethoxysilane
(PhTES) for phenyl functionalized solids was prepared. In the synthesis of methyl
functionalized solids, the quantity of MTES was chosen to reach three degrees of
functionalization: 5, 10 and 15 %mol, corresponding to the molar MTES / (MTES + TEOS) x 100
% ratio. For propyl and phenyl functionalized solids, the amount of PrTES or PhTES was
adjusted to achieve a degree of functionalization of 5 %mol. Pluronic F127 and P123 were
selected as structure directing agents. The detailed amounts of TEOS and alkylsilane used for
each material are reported in Table S1.
TEOS and MTES or PrTES or PhTES were hydrolyzed in an aqueous solution of HCl 0.02 M (20
g) to yield solution 1. Solution 2 was prepared by dissolving Pluronic® P123 or F127 (3.9 g) in
absolute ethanol (45 g). Solution 1 and 2 were respectively stirred during 15 hours and 40
minutes at room temperature. Then, the gallium precursor (Ga(NO3)3·xH2O) was added to
solution 1 in order to have a Si/Ga molar ratio equal to 74. Solutions 1 and 2 were mixed
together and then stirred for 30 minutes. The resulting solution was atomized with a 6-Jet
9306A atomizer from TSI. Then, the aerosol was passed through a tubular quartz tube heated
at 350 °C. A cellulose nitrate filter was used to collect the powders, further dried for one night
at 80 °C and calcined under static air at 250 °C for 8 h (heating rate of 1 °C/min).
Characterization
Transmission electron microscopy (TEM) images were taken using a Philips Tecnai 10
microscope operating at 80 kV. Nitrogen adsorption-desorption analyses were carried out at
77 K with a volumetric adsorption analyzer (Micromeritics ASAP 2420). Prior to the analysis,
the samples were pre-treated 8 hours at 150 °C under reduced pressure (0.1 mbar). The
Brunauer–Emmet–Teller (BET) method was applied in the 0.05-0.30 P/P0 range to calculate
the specific surface area, while the pore size distributions were calculated from the
adsorption isotherm using the Barrett–Joyner–Halenda (BJH) method. The micropore volume
and micropore surface area were calculated by the t-plot method that was applied to the
adsorption branch of the isotherms in the thickness range of 3.5–5.0 Å. The micropore
diameter was evaluated by DFT method using cylinder geometry, N2 – Cylindrical Pores –
Oxide surface model. Powder X-ray diffraction (XRD) patterns were recorded on a PANalytical
X’pert diffractometer with Cu Kα radiation (k = 1.54178 Å). Inductively coupled plasma optical
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emission spectroscopy was employed to determine the chemical composition of tDhOeI:m10.a10t3e9r/Dia0lGsC02562C
using an Optima 8000 ICP-OES Spectrometer. The silicon environment and the coordination
of the gallium atoms were studied by 29Si Magic Angle Spinning (MAS) and 71Ga MAS Nuclear
Magnetic Resonance (NMR), respectively.
29Si NMR spectra were recorded at room temperature on a Bruker Avance-500 spectrometer
operating at 11.7 T (99.3 MHz for 29Si) using a 4 mm CP-MAS Bruker probe. The sample was
packed in a 4 mm zirconia rotor and measured with a spinning frequency of 8000 Hz.
Quantitative direct excitation (DE-MAS) 29Si spectra were recorded using the following
acquisition parameters: 300s relaxation delay, 3µs (90°) excitation pulse, 52 ms acquisition
time. Cross polarization (CP-MAS) spectra were recorded using a 5 s relaxation delay and 5
ms contact time. The processing comprised exponential multiplication of the FID with a line
broadening factor of 30 Hz, zero-filling, Fourier transform, phase and baseline corrections.
The chemical shift scale was calibrated at room temperature with respect to a sample of solid
3-(Trimethylsilyl)-1-propanesulfonic acid sodium salt (DSS) (0.0 ppm).
71Ga NMR spectra were recorded at room temperature on a Bruker Avance-500 spectrometer
operating at 11.7 T using a 2.5 mm CP-MAS Bruker probe. The sample was packed in a 2.5 mm
zirconia rotor and measured with a spinning frequency of 25000 Hz. The spectra were
recorded using a Hahn echo sequence and the following acquisition parameters: a relaxation
time of 0.3 s, an excitation of 2.6 μs (90°) and 16 ms acquisition time. Data processing includes
the multiplication of the FID (Free Induction Decay) by a line broadening factor of 1000 Hz,
zero-filling, Fourier transformation and phase and baseline corrections. Fourier
transformation from the echo maximum ensured a flat baseline with only zero-order phase
correction. The chemical shift scale was calibrated at room temperature with respect to a
sample of solid gallium nitrate (0.0 ppm).
Vapor-phase water adsorption isotherms were acquired at 295 K, using a surface
characterization analyzer (Micromeritics 3Flex). Prior to the analysis, the sampled were pre-
treated 8 hours at 180°C under reduced pressure (0.07 mbar). Water was purified using
freeze-pump-thaw cycles.
Thermogravimetric analysis (TGA) were performed on a Mettler Toledo TGA/DSC 3+ STARe
System. The samples were analysed using a temperature ramp from 30 °C to 800 °C at 10 °C
min-1 under air flow (60 mL min-1).
FT-IR measurements were performed in transmission mode by using a Bruker Tensor 27
spectrometer equipped with a liquid nitrogen-cooled mercury–cadmium–telluride (MCT)
detector, operating at a 2 cm⁠−1 resolution. A pre-treatment was carried out using a standard
vacuum frame, in an IR cell equipped with KBr windows. The sample was pressed into self-
standing wafers and outgassed 1h at 400 °C before adsorption of ammonia at room
temperature.
Adsorption of ammonia was studied in the pressure range 0.01–20.0 mbar: the reversible
fraction of the adsorbed ammonia was then removed by prolonged outgassing at room
temperature.
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DOI: 10.1039/D0GC02562C
Production of solketal from glycerol and acetone
Acetalization of glycerol with acetone: glycerol (purity 99 %, 0.92 g, 0.01 mol), acetone (2.32
g, 0.04 mol) and the selected catalyst were added in a round bottom flask. The mixture was
heated at 50 °C under 800 rpm stirring during a specific time. After the reaction has reached
room temperature, 3 mL of absolute ethanol were added and the mixture was centrifuged
(4500 rpm, 10 min). 0.5 mL of the supernatant was dried under reduce pressure and analyzed
by 1H-NMR, using DMSO-d6 as solvent.
Hot filtration test: glycerol (0.92 g, 0.01 mol), absolute EtOH (0.70 mL) acetone (2.32 g, 4
equiv.) and 25 mg catalyst were added in a round-bottom flask. The mixture was heated at
50 °C under 800 rpm stirring during 1 hour. The catalyst was separated from the reaction
mixture by hot filtration (~ 50 °C) and centrifugation (4500 rpm, 10 min). 0.5 mL of the
1
supernatant was dried under reduce pressure and analyzed by H-NMR, using DMSO-d6 as
solvent. The remaining amount of supernatant was kept at 50 °C for another 5 hours under
1
800 rpm stirring. The reaction mixture was then analyzed by H-NMR, using DMSO-d6 as
solvent.
Recycling tests: glycerol (1.56 g, 0.017 mol), acetone (4 equiv.) and the catalyst (50 mg), were
added in a round-bottom flask. The mixture was heated at 50 °C under 800 rpm stirring during
1 hour. After the reaction has reached room temperature, 3 mL of absolute ethanol were
added and the catalyst was separated from the mixture by centrifugation (4500 rpm, 15 min).
0.5 mL of the supernatant was dried under reduce pressure and analyzed by 1H-NMR, using
DMSO-d6 as solvent. Then, the supernatant was carefully removed from the centrifuge tube
and the catalyst was rinsed with ethanol (3 times) and centrifuged. After this, the catalyst was
calcined in air at 300 °C for 2 h (heating rate: 1 °C/min). The subsequent catalytic tests were
carried out by repeating this procedure from the beginning (the quantities were adapted as a
function of the mass of recovered catalyst).
Results and discussion
Two series of functionalized Ga-silicates (Si/Ga atomic ratio of 74) were prepared via a co-
synthetic approach exploiting the aerosol-assisted sol-gel process and using three different
alkyl-silanes: methyl, propyl and phenyltriethoxysilane. In addition, two structure-directing
agents from the Pluronic family (F127 and P123) were selected in order to study their impact
on the morphological properties and on the catalytic activity of the materials.
Initially, Ga-based solids were synthesized selecting three degrees of methyl-
functionalization: 5, 10 and 15%mol (see experimental section for more information). These
materials were denoted as Ga-P-x-Me and Ga-F-x-Me, where P or F indicate the employed
templating agent (P123 or F127 respectively) and x indicates the nominal degree of
functionalization. In order to investigate the effect of different alkyl moieties on the textural
and surface properties of the solids, four additional functionalized materials were synthesized
using only 5%mol of propyl or phenyltriethoxysilane. The materials were denoted as Ga-P(or
6

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F)-5-Pr and Ga-P(or F)-5-Ph (where 5 indicates the degree of functionalization, Pr or Ph
DOI: 10.1039/D0GC02562C
indicates the presence of propyl or phenyl moieties respectively). All the above-mentioned
functionalized Ga-silicates were then compared to their non-functionalized analogues
synthesized under the same conditions, respectively named Ga-P-0 and Ga-F-0.
After the synthesis, particular attention should be paid to preserve the alkyl-
functionalization while removing the surfactant to release porosity. In the literature, several
methods including solvent extraction34 or calcination35 have been reported to remove
Pluronics from silica-based porous solids. However, taking inspiration from one of our
previous work, a mild calcination treatment was here selected.28 We found that the best
compromise between a total removal of the templating agent and an almost full preservation
of the functionalization can be achieved with a calcination at 250 °C for 8 hours.
Figure 1. Solid-state direct excitation 29Si MAS NMR spectra of (a) Ga-P-0 (b) Ga-P-5-Me (c) Ga-P-10-Me (d) Ga-P-15-Me
after calcination at 250 °C for 8h
Thus, all the solids were calcined at the selected temperature and analyzed performing
quantitative 29Si solid-state magic angle spinning (MAS) NMR experiments. Figure 1 shows an
example of quantitative 29Si direct excitation (DE) MAS spectra of the Ga-P-x-Me series. The
spectra of the other materials can be found in supporting information (Figure S1-S4). The
characteristic contributions typical of an amorphous silica matrix can be observed clearly: Q4
[(SiO)4Si] (around -111 ppm), Q3 [(SiO)3SiOH] (around -102 ppm) and Q2 [(SiO)2Si(OH)2]
(around -92 ppm) signals can be found in all samples. Two additional signals at −66 and −59
ppm can be seen in the 29Si NMR spectra of methyl-functionalized solids and can be assigned
to MeSi(OSi)3 (T3) and Me(HO)Si(OSi)2 (T2) species.28 These contributions can be found as well
in the whole series of functionalized solids (see Figure S1-S4) and they attest the successful
functionalization of the surface. 29Si DE-MAS NMR spectra highlight also that the theoretical
percentage of organic moieties is largely preserved for all material (see Table S2). These
results corroborate the efficiency of the synthetic procedure including the mild calcination
treatment.
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The gallium loading was evaluated by inductively coupled plasma optical emission
DOI: 10.1039/D0GC02562C
spectroscopy (ICP-OES). The experimental composition – expressed in mmol of gallium per
gram of catalyst – was very close to the nominal composition of 2.2 mmol/g (Table S2) for all
the solids. Nevertheless, the formation of catalytically active acid sites, fundamental for the
application of these solids as catalysts, is strictly related to the insertion of Ga as single sites.
The isomorphous substitution of silicon atoms by trivalent elements, as gallium, is known to
induce a combination of Brønsted and Lewis acid sites within the silica framework.21 The
coordination environment of gallium was therefore elucidated via solid-state 71Ga NMR
spectroscopy. Due to the low gallium loading replacing silicon in an amorphous environment,
the investigations were performed under magic angle spinning (MAS) at 25 kHz and at high
magnetic field (11.7 T).22
Figure 2. Solid-state 71Ga MAS NMR spectra of (a) Ga-P-0 (b) Ga-P-5-Me (c) Ga-P-10-Me (d) Ga-P-15-Me after calcination
at 250 °C for 8h. The spectra were recorded using a Hahn echo sequence (see experimental)
Figure 2 shows the MAS NMR spectra of Ga-P-0, Ga-P-5-Me, Ga-P-10-Me and Ga-P-15-Me.
The sharp peak centered at +140 ppm, observed for all the materials, is ascribed to
tetrahedral framework gallium and it constitutes a proof of the incorporation of mainly single-
36, 37
site Ga in the silica framework.
This contribution has already been observed for other
gallo-silicates reported in literature. The presence of an additional weak contribution at 0 ppm
can be assigned to the presence of minor amounts of extra-framework Ga species (see Figure
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S5 for a comparison with a pure Ga2O3 spectrum). These species are supposed to have an
DOI: 10.1039/D0GC02562C
octahedral environment and have already been reported not to be catalytically active.21
Similar results were obtained analyzing the environment of Ga in the case of Ga-F-0 (Figure
S5). 71Ga NMR measurements indicate that neither the presence of methyl-functional groups
nor their amount affects the incorporation of gallium in the silica framework of the materials.
It appears also that the chemical environment of gallium is not influenced by the nature of
the templating agent.
The samples morphology was investigated via transmission electron microscopy (TEM). All
solids are mesoporous and displays a spherical shape. The average pore diameter lies
between 6 and 7 nm, typical of aerosol solids templated with Pluronic P123 or F127.28, 38
When F127 was used as a surfactant, a well-organized structure was obtained even when the
methyl-functionalization was raised up to 15% (Figure 3e to 3h). Interestingly, a slight
deformation of the structure in the external part of the particles was visible for the materials
synthesized in the presence of Pluronic P123 (Figure 3a to 3d). This deformation was more
evident with the increasing amount of methyl-functionalization. Major modification of the
structure with the appearance of some “onion like” organizations are clearly visible raising
the functionalization to 15%. This behavior was previously observed and has been ascribed to
the different balance among the hydrophilic part (poly(ethylene oxide)) and the hydrophobic
part (poly(propylene oxide)) of the two triblock polymers.28 As indicated by their formula, the
templating agent Pluronic P123 ((PEO)20-(PPO)70-(PEO)20) has a lower number of hydrophilic
units than Pluronic F127 ((PEO)100-(PPO)65-(PEO)100).
Figure 3. TEM micrographs of (a) Ga-P-0 (b) Ga-P-5-Me (c) Ga-P-10-Me (d) Ga-P-15-Me and of (e) Ga-F-0 (f) Ga-F-5-Me (g)
Ga-F-10-Me (h) Ga-F-15-Me
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DOI: 10.1039/D0GC02562C
As consequence, the methyl groups of MTES could have more affinity with the
hydrophobic unit of P123 than F127. Since the methyl group of MTES is hydrophobic enough
to present cosurfactant properties, it will be preferentially located nearby the hydrophobic
shell of the micelle, causing a structural deformation in the final material. This behaviour may
induce a variation of the interfacial curvature favouring the transition through a lamellar
phase.
In the case of Ga-P-5-Pr and Ga-P-5-Ph, well-organized spherical particles were observed
(Figure S6). Thus, longer organic chains as in the case of propyl moieties or bulkier phenyl
groups, did not provoke a substantial deformation of the structure.
Table 1 Textural properties and composition of Ga-silicate solids
BET SSA
(m2·g-1)
BJH pore width
(nm)
Pore Volume
(cm3/g)
Material
Ga (mmol/g)1
% Funct.2
Ga-P-0
440
410
430
480
500
460
550
500
460
500
550
450
7.8
7.1
7.3
6.6
7.0
6.6
7.8
7.5
6.9
6.8
7.1
6.3
0.6
0.6
0.5
0.5
0.6
0.5
0.5
0.4
0.4
0.4
0.5
0.4
2.0
1.9
1.8
1.8
1.8
2.0
2.1
1.8
1.9
1.9
2.0
2.0
-
4
Ga-P-5-Me
Ga-P-10-Me
Ga-P-15-Me
Ga-P-5-Pr
Ga-P-5-Ph
Ga-F-0
8
13
< 3
< 3
-
Ga-F-5-Me
Ga-F-10-Me
Ga-F-15-Me
Ga-F-5-Pr
Ga-F-5-Ph
3
8
14
< 3
< 3
푇푖
1 mmol of gallium per gram of catalyst determined by ICP-OES analysis 2 % Funct. = 100 *
푖
푖
(
)
∑
푖푇 + 푄
Figure 4. Nitrogen adsorption/desorption isotherms of Ga-P-0, Ga-P-5-Me, Ga-P-10-Me and Ga-P-15-Me (left) and Ga-F-
0, Ga-F-5-Me, Ga-F-10-Me and Ga-F-15-Me (right) after calcination at 250 °C for 8h
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The low angle powder XRD patterns of Ga-silicates templated with P123 or F127 (Figure
DOI: 10.1039/D0GC02562C
S7) featured mainly first order d100 diffraction peaks, indicating well-ordered materials. The
d-spacing associated to the P123 and F127 series are respectively 9 nm and 11 nm, that is
typical of ordered mesostructured silicates assembled using a non-ionic surfactant. For both
series, N2-physisorption measurements showed type-IV isotherms characterized by
considerable hysteresis loops. P123 templated solids present H1 hysteresis loops with steeper
branches, typical of materials with narrow distribution of relatively uniform mesopores
(Figure 4 and Figure S8). F127 templated materials exhibit a H2 hysteresis loop characteristic
of cage-type mesopores with a mean pore diameter of 7 nm (Figure 4 and Figure S8). All solids
display a high specific surface area and narrow pore size distribution. An overview of the
textural properties of the two series of functionalized Ga-silicates is reported in Table 1.
The reaction selected in this work is represented by the acetalization of glycerol with
acetone to produce solketal. Based on literature data, a proposed mechanism for the target
reaction catalyzed by Brønsted or Lewis acid catalyst is shown in Figure S9.19, 39 The effective
insertion of single-site tetrahedral gallium, proved by solid-state 71Ga NMR, should give rise
to the desired acidic properties essentials in promoting the upgrade of glycerol into solketal.
In Table 2 the catalytic performances in the conversion of glycerol into solketal for the
whole set of solids are presented. Considering sustainability as a key aspect of the process,
the tests were carried out under solvent free conditions at 50 °C. A kinetic study using Ga-P-
0 or Ga-F-0 as catalyst was performed (Figure S10). The formation of solketal was followed
from 1 hour to 16 hours in order to select the adequate reaction time to compare the activity
of our catalysts in the kinetic regime (initial activity). It appears that a short reaction time of
two hours represents the best compromise between an adequate solketal yield allowing an
accurate quantification and a meaningful comparison among the catalysts far enough from
the equilibrium conversion.
To verify the reproducibility of the catalytic results, the materials were tested three times
under the same reaction conditions. The associated error calculated was around 2% in
absolute value, proving the highly reproducibility of the tests.
The comparison between functionalized and non-functionalized catalysts, synthesized
under the same conditions (compare entries 1 and 7 with 2 and 8 in table 2), revealed that
the 5% methylated solids Ga-P-5-Me and Ga-F-5-Me present improved catalytic activity in
terms of TON (defined as moles of solketal produced per moles of Ga), even when the activity
is normalized to the BET surface area (Figure S11). From this comparison emerged a positive
influence of the methylation on the performances of the catalysts. It is important to underline
that only a low degree of methylation allows to enhance the catalytic activity of the solids.
This phenomenon was already reported by our group, in the case of functionalized Sn-silicates
used in the production of ethyl lactate.28 The observed enhancement in terms of TON could
be attributed to the surface functionalization rather than to the presence of active sites
(gallium single sites) in a different coordination environment. Moreover, the presence of
bulkier functional groups in the formulation (see entries 5, 6 and 11, 12) leads to a lowering
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of the activity. To deeply investigate this point, water vapor sorption experiDmOeI: n10t.1s03w9/De0rGeC02562C
performed at 295 K on Ga-P-0, Ga-P-5-Me, Ga-P-5-Pr and Ga-P-5-Ph (Figure S12, left). In order
to meaningfully compare our solids, the amount of water adsorbed was normalized by the
BET specific surface area. The quantitative evaluation of water affinity of the catalysts surface
was featured at low relative pressure of the isotherm (in our case, 0.15), as indicated in the
literature.40 As illustrated in Figure S12 and Table S3 (compare entries 1 and 2), a reduced
affinity for water at low relative pressure is observed for Ga-P-5-Me compared to Ga-P-0.
These data suggest that the addition of methyl groups in our formulation allows to tune the
hydrophilic-hydrophobic balance of our material to more hydrophobic surfaces. The less
hydrophilic surface of Ga-P-5-Me may have a role in glycerol acetalization by locally helping
the removal of water from the active sites and therefore favoring the formation of solketal.
The quicker removal of water from the hydrophobic surface may help preserving the integrity
of the acid sites of the catalyst. In comparison with Ga-P-5-Me, the water uptake at low
relative pressure is lower for Ga-P-5-Pr and Ga-P-5-Ph (compare entries 2 with 3 and 4 in Table
S3). This result highlights that the presence of bulkier groups in our catalysts induces more
hydrophobic surfaces. Water sorption measurements evidence that the presence of too bulky
hydrophobic moieties in our materials have a detrimental effect on their catalytic activity,
hindering probably the adsorption of glycerol in proximity of the active sites.
Table 2 Catalytic activity of Ga-silicates in the conversion of glycerol to solketal. Conditions of the catalytic tests: 10 mg
of catalyst, 50°C, 2 hours, glycerol: acetone = 1 (0.01 mol) : 4.
Entry
1
Material
Ga-P-0
YSK (%)
21
25
18
18
17
13
10
16
12
9
Sel.SK (%)1
84
TOF2
532
663
506
500
462
328
242
434
311
240
243
224
TON3
1014
1326
1012
1000
923
2
Ga-P-5-Me
Ga-P-10-Me
Ga-P-15-Me
Ga-P-5-Pr
Ga-P-5-Ph
Ga-F-0
85
3
82
4
82
5
83
6
79
656
7
81
484
8
Ga-F-5-Me
Ga-F-10-Me
Ga-F-15-Me
Ga-F-5-Pr
Ga-F-5-Ph
83
867
9
82
622
10
11
12
79
479
10
9
80
485
77
447
1 Sel.SK = mol solketal / (mol solketal + mol 2,2-Dimethyl-1,3-dioxan-5-ol). The presence of the by-product
(2,2-Dimethyl-1,3-dioxan-5-ol) was identified via 1H liquid state NMR (Figure S13) 2TOF (TurnOver
Frequency) here defined as mol of solketal/mol of Ga * time. 3TON (TurnOver Number) here defined as mol
of solketal/mol of Ga after 2 hours of reaction.
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No further improvement in the catalyst performance was observed when theDmOIe: 1t0h.1y0l3a9t/Dio0GnC02562C
degree is brought to 10% (see entries 3 and 9) or to 15% (see entries 4 and 10). To go further,
the series of Ga-P-x methylated materials were analyzed via thermogravimetric analysis
(TGA). In order to evaluate the hydrophobic/hydrophilic nature of our solids, the weight loss
was quantified at 140°C and normalized by the BET specific surface area. As illustrated in
Figure S12 (right), when the percentage of methyl moieties increases, the desorption of
physiosorbed water decreases. These results indicate that an enhancement of the percentage
of methylation allows achieving a more hydrophobic surface. All these results confirm that
the fine tuning of the hydrophilic-hydrophobic balance of the catalytic surface play a crucial
role in the design of more active catalysts.
Surprisingly the solids synthesized employing P123 as surfactants display predominantly
better catalytic performances than the corresponding materials of the F127 series (compare
entries 1 to 6 with 7 to 12). Considering the 71Ga MAS NMR analysis, the insertion of Ga as
single site is very similar on Ga-P-0 and Ga-F-0 (Figure S5). Their textural properties deserve
therefore an in-depth characterization to better understand the observed inequalities in
terms of catalytic activity.
It is well-known that the presence of micropores in heterogeneous catalysts can limit the
diffusion of reagents to the active sites and/or the release of the products from the active
sites. The presence of micropores in the walls of Ga-P-0 and Ga-F-0 was here evaluated via
the t-plot method. In the case of Pluronic templated solids, the formation of micropores
occurs by aggregation of the hydrophilic blocks of two micelles within the pre-formed silica
walls. This phenomenon leads to microporosity in the final material once the templated
agents are eliminated via calcination.41 The analysis of the t-plot allows accessing to the
microporous area and micropore volume of our solids (Figure S14). The microporous surface
area of Ga-F-0 and Ga-P-0 amounts to 250 m2/g and 170 m2/g, respectively. Ga-F-0 and Ga-
P-0 display micropore volumes of 0.08 cm3/g and 0.04 cm3/g, respectively. The analysis via
the mathematical modeling using density functional theory (DFT) allows evaluating the
diameter of the micropores that is located in the 1 to 2 nm range. Hence, the decreased
activity of the Ga-F-0 could be at least partially ascribed to a limited accessibility of the active
sites located in the micropores.
However, to shed more light on the different catalytic behavior, FT-IR measurements of
ammonia adsorption on Ga-P-0 and Ga-F-0 solids were conducted as well. This analysis allows
investigating the acidic properties and providing a support for an interpretation to the
observed catalytic performances. For this study, ammonia (a basic molecular probe), which
can interact both with Brønsted (surface hydroxyls) and Lewis (unsaturated Ga (III)) sites was
selected.
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DOI: 10.1039/D0GC02562C
Figure 5. Difference FT-IR spectra of NH3 dosages on (a) Ga-P-0, (b) Ga-F-0 and (c) outgassed at room temperature
FT-IR difference spectra of ammonia adsorption on both Ga-P-0 and Ga-F-0 are shown in
Figure 5 (a and b), where the negative bands indicate species that disappear upon ammonia
dosages. In the high frequency region (3800-2000 cm-1), the adsorption of ammonia generates
the formation of several negative bands due to the various hydroxyls interacting with NH3. In
particular, both samples show a negative band at about 3745 cm-1 due to free silanols and
two negative bands at about 3720 cm-1 together with a broad shoulder at 3550 cm-1 related
to terminal and H-bonded silanols in hydroxyls chains, respectively.42 The positive signal at
around 3400 and 3316 cm-1 are ascribed to N-H stretching modes and the broad envelope in
the high frequency range is due to the hydroxyl species engaged in H-bonding with ammonia
molecules.
In the low frequency region (1800-1300 cm-1), for both samples the absorption band at
1635 cm−1 corresponds to the bending mode of NH3 molecules H-bonded to surface OH
+
groups, whereas the broad signal at around 1460 cm−1 is due to the bending mode of NH4
ions formed by proton-transfer reaction from Brønsted sites. The formation of ammonium
ions on both solids indicates that the incorporation of Ga within the silica framework provides
Brønsted sites that are enough acidic to protonate NH3, at variance with surface hydroxyls of
unsubstituted mesoporous silica which only establish hydrogen-bonding. The blue shift of the
ammonium band observed with increasing NH3 equilibrium pressure is generally ascribed to
+
the multiple lateral interactions of NH4 ions with surface oxygen atoms and/or OH groups.
As expected from 71Ga NMR both Ga-F-0 and Ga-P-0 display similar acid properties. However,
a direct comparison between the difference spectra of Ga-P-0 and Ga-F-0 outgassed at room
temperature after ammonia dosage (Figure 5c) allow evidencing some differences. It is
important to highlight that the observed residual signals correspond to species formed on the
strongest acidic sites and thus irreversible upon ammonia removal. In the high frequency
region, the fully outgassed samples show some dissimilarities, in particular for Ga-P-0 the
residual broad signal at around 2800 cm-1 is more intense than that found on Ga-F-0,
suggesting a larger population of residual H-bonded silanols in interaction with ammonia.
Furthermore, a close inspection of the residual negative hydroxyl bands of the two samples
evidences more intense residual free silanols band (3744 cm-1) for Ga-F-0 compared to Ga-P-
0, which at variance shows higher negative components at 3720 cm-1 and at 3500 cm-1.
Moreover, a negative shoulder at about 3692 cm-1 (labelled with an asterisk in the inset) can
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be discerned on Ga-P-0, which based on the literature can be ascribed to hydroDxOyIl: 1l0in.10k3e9d/D0tGoC02562C
gallium sites, in analogy to that is observed on hydroxylated gallium oxide.43
+
For both fully outgassed samples the signal of NH4 ions became less intense upon outgassing
(reversible proton transfer) and for the two samples a very similar residual intensity can be
claimed. The absorption band attributed to NH3 physisorbed on silanols (1635 cm–1)
decreased in intensity and was red-shifted up to 1621 cm–1 for the fully outgassed samples
(residual pressure < 1.10−3) ascribed to ammonia molecules adsorbed on Ga3+ acting as Lewis
sites, in analogy to amorphous aluminosilicates.42 Since this signal appears slightly more
intense for Ga-P-0 than for Ga-F-0, it can be assumed as an evidence of a little higher
concentration of exposed surface Lewis sites.
Taken together, these spectroscopic data reveal very similar surface features in term of
acidic sites for the two samples. However, some differences regarding the hydroxyl
populations and Lewis site concentration have been evidenced, suggesting an overall higher
acidity of Ga-P-0 surface, most likely due to a Ga-enriched surface compared to Ga-F-0. This
slightly higher acidity of Ga-P-0 solid together with the increased accessibility of the active
sites (lower contribution of micropores) may constitute a synergistic combination of features
contributing to the enhanced catalytic performances.
Figure 6. Evaluation of the catalytic activity over time. Comparison between Ga-P-5-Me and Ga-P-0 (left), Ga-F-5-Me and Ga-
F-0 (right). Conditions of the catalytic tests: 10 mg of catalyst, 50°C, glycerol: acetone = 1 (0.01 mol): 4
Figure 7. Comparison of the catalytic activity of Ga-P-5-Me with metal-containing silicates reported in literature for the
conversion of glycerol to solketal: (a)19, (b)21. Conditions of the catalytic test: 25 mg of catalyst, 1.48 g of tert-butanol, 80°C,
2 hours, glycerol: acetone = 1 (0.01 mol): 4.
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The catalytic activity of the best materials in each series was also evaluated over time
DOI: 10.1039/D0GC02562C
(Figure 6). Regardless of the catalyst considered, the TON increased rapidly during the first 8h
of reaction followed by a plateau (the conversion values at 8h are reported in Table S4). The
functionalized materials display a higher TON compared to non-functionalized materials.
Moreover, the difference observed between non-functionalized and functionalized materials
remains constant over time.
In order to have a meaningful notion of the performances of our best catalyst (Ga-P-5-Me),
an additional catalytic test was performed in the presence of t-butanol, a solvent commonly
used in the literature to convert glycerol into solketal. The comparison with reference
materials illustrated in Figure 7 evidence the outstanding catalytic performances of Ga-P-5-
Me. The high activity of our solid can be ascribed to a combination of aspects such as the
incorporation of gallium mainly as single site, its open mesostructure or even the
functionalization of its surface with low amount of methyl groups.
In the perspective of sustainable processes promoted by heterogeneous catalysts, a pivotal
role is played by the robustness of the catalyst under the selected reaction conditions, which
determines the possibility to reuse it in multiple catalytic runs. The reusability of our most
promising catalyst – Ga-P-5-Me – was evaluated in consecutive cycles (Figure 8, left). At the
end of each cycle, the catalyst was recovered by centrifugation, washed with ethanol and
thermally regenerated at 300°C for two hours. It is important to highlight that the selected
thermal treatment allows to preserve the initial amount of methyl groups in our catalyst as
29
confirmed by quantitative Si MAS measurement (Figure S14). The calcination treatment is
of fundamental importance since it was already reported that the activity gradually decreases
upon reuse if the catalyst is only washed with solvent. From this study emerged that the good
activity and selectivity of the catalyst was preserved at least for four catalytic cycles.
In order to investigate the stability of Ga-P-5-Me, a hot filtration experiment was
performed as well (Figure 8, right). The solketal yield was measured after one hour of reaction
after which the catalyst was removed by hot filtration (50 °C). The filtrate was then reacted
for 5 additional hours. The hot filtration experiment shows that the solketal yield remains
constant indicating the absence of active species in the filtrate.
Figure 8. Recycling test with Ga-P-5-Me (left). Conditions: 50 mg of catalyst, 50°C, 1 hour, glycerol: acetone = 1 (0.017 mol):
4. Hot filtration experiment with Ga-P-5-Me (right). Conditions: 25 mg of catalyst, 0.70 mL of ethanol, 50°C, 1 hour, glycerol:
acetone = 1 (0.01 mol): 4.
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DOI: 10.1039/D0GC02562C
To further evaluate the sustainability of our process, the associated E-Factor was
calculated, selecting a reaction time of 8 hours (to reach maximum conversion). After
reaction, the catalyst was removed by centrifugation and acetone was recovered by
distillation. The E-Factor was calculated according the following formula: E-Factor = [
(4.60 푔 퐺푙푦푐푒푟표푙 + 11.6 푔 퐴푐푒푡표푛푒 ― 2.41 푔 퐴푐푒푡표푛푒 푟푒푐표푣푒푟푒푑 푏푦 푑푖푠푡푖푙푙푎푡푖표푛 ― 3.50 푔 푆표푙푘푒푡푎푙)
= 2.9. Such low E-factor value is in the same range as typical values
/3,50 푔 푆표푙푘푒푡푎푙]
reported for the industrial production of bulk chemicals,44 and support the idea that this
catalytic process is promising. Moreover, the optimization of the distillation setup used to
recover acetone could certainly allow further improving the E-Factor of the process.
As mentioned at the beginning of this work, the importance of preserving the organic
functionalities on the surface of the solids while removing efficiently the structuring agent
determined the adoption of a mild thermal treatment at 250 °C. However, in the literature,
analogous non-functionalized metal-silicates are usually subjected directly to a thermal
treatment ranging from 500 °C to 600 °C to effectively remove the templating agent.21, 45, 46 It
is important to highlight that the calcination temperature may affect also the physicochemical
properties of the materials (e.g. surface area), but also the surface hydroxyl content,
responsible of the hydrophilic character of the surface.
For this reason, we investigated the catalytic behavior of Ga-P-0 at different calcination
temperatures. As reported in Figure 9, the increase of the calcination temperature from 250
°C to 550 °C had a beneficial impact on the catalytic performance of the non-functionalized
solid. If we compare the physico-chemical properties of the material calcined at 250 °C and
550 °C, a decrease of the BET surface area (from 440 m2/g to 350 m2/g) is observed and could
be ascribed to the increasing condensation of neighboring silanols (vide infra). The loss of BET
surface area with the increase of the calcination temperature (until 550 °C) appears
incoherent with the improvement of the catalytic activity. In fact, if the TON is normalized by
the surface area (Figure S15), this improvement is even more evident. No other major
differences were observed in terms of pore size distribution or gallium insertion (Figure S16).
Therefore, these observations could be ascribed to the beneficial role played by the
calcination temperature on the modification of the surface hydrophobicity.
As the temperature of the thermal treatment increases, the silica surface is dehydrated
causing the removal of the adsorbed water and thermal dehydration reaction, where two
adjacent silanols condense into a siloxane bridge. Experimental data reported in the literature
demonstrated that increasing the temperature of the thermal treatment the silica surface
becomes more hydrophobic due to the condensation of surface hydroxyls and the formation
of siloxane bridges.47-49 The reduction of the hydroxyl groups population, was here proved by
the higher degree of condensation. The latter was estimated by deconvolution analysis using
n
Gaussian functions of the Q contributions on quantitative direct excitation 29Si MAS NMR
spectra (Figure 10). This last procedure could represent an interesting alternative to modify
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the hydrophilic/hydrophobic balance of the surface proving again its beneficial role in
DOI: 10.1039/D0GC02562C
particular on the selected reaction.
Finally, raising the calcination temperature to 650 °C resulted in a rather remarkable loss
of catalytic activity that in this case could be ascribed to a major loss of surface area (around
200 m2/g), probably caused by the collapse of the catalyst structure.
Figure 9. Study of the influence of the calcination temperature on the catalytic activity of Ga-P-0. Conditions of the
catalytic tests: 10 mg of catalyst, 50°C, 2 hours, glycerol: acetone = 1 (0.01 mol): 4.
Figure 10. 29Si direct excitation MAS NMR spectra of Ga-P-0 calcined at 250 °C (top) and 550 °C (bottom)
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DOI: 10.1039/D0GC02562C
Conclusions
The aerosol process was used to synthesized, in a straightforward one-pot procedure and
with low environmental impact, a series of functionalized Ga-silicates. The efficient
incorporation of isolated gallium as single site within the silica framework was confirmed via
71Ga solid state NMR and the stability of the organic moieties has been assessed using 29Si
solid-state MAS NMR. The materials exhibited record activity in the acetalization of glycerol
to yield solketal at low temperature and under solventless conditions. In particular, the
materials synthesized using P123 as templating agent demonstrated the best catalytic
activity. A positive influence of the functionalization on the catalytic performances of the
materials emerged in presence of methyl moieties. In addition, it was observed that only low
degree of methylation was effective to enhance the catalytic performance of the materials,
shifting locally the reaction to the production of solketal. The best catalyst was successfully
used in multiple catalytic runs thus proving its robustness under the selected reaction
conditions. To further evaluate the sustainability of our process, the associated E-Factor was
calculated and can be compared with the E-Factor reported for the industrial production of
bulk chemicals. Moreover, a systematic study evidenced the importance of a careful tuning
of surface polarity, achievable not only with surface functionalization but also with different
thermal treatments applied to non-functionalized solids. The control of the hydroxyl group
population on the surface, possible with a control on the calcination temperature, also
allowed enhancing the catalytic activity thus further supporting the pivotal role of the surface
polarity. Nevertheless, considering the balance between sustainability and performances as
a key aspect, functionalized solids constitute the best option as they present the advantage
of a less energy-consuming thermal treatment at mild temperature.
Conflicts of interest
There are no conflicts of interest to declare.
Acknowledgements
Authors acknowledge the “Communauté française de Belgique” for the financial support –
including the PhD fellowship of A. Vivian – through the ARC programme (15/20-069). L.
Soumoy thanks the FNRS for the financial support in the context of her FRIA PhD grant.
Damien Debecker thanks the Francqui Foundation for the “Francqui Research Professor”
chair. F. Devred is thanked for the technical and logistical support. This research used
resources of the PC2 (Plateforme Technologique Physico-Chemical Characterization) and the
MORPH-IM (Morphology & Imaging) platforms located at the University of Namur.
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DOI: 10.1039/D0GC02562C
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