Synthesis of aluminium triflate-grafted MCM-41 as a water-tolerant acid catalyst for the ketalization of glycerol with acetone

Article abstract of DOI:10.1039/c4cy01396d

An aluminium triflate covalently grafted over MCM-41 (Al(TF)-MS catalyst) was synthesized by a novel route, aiming to generate enhanced surface acidity as compared to aluminosilicates and water-tolerant mesoporous solid acid catalysts for reactions producing water as a co-product. The synthesis strategy involves an equimolar reaction of triflic acid and aluminium isopropoxide, followed by treatment with MCM-41. The direct treatment of triflic acid with Al-grafted MCM-41 (Al-MCM-41) was also attempted to synthesize silica-supported aluminium triflate. The MCM-41-supported aluminium triflate and triflic acid were synthesized as standard samples for comparison. The catalytic potentials of the samples were studied for the ketalization of glycerol with acetone into solketal. The TGA and ²⁷Al MAS NMR studies revealed the formation of aluminium triflate species on the silica surface in the Al(TF)-MS sample. The covalent grafting of the aluminium triflate species in the Al(TF)-MS sample was confirmed from catalysis and reusability tests, showing the stability of the aluminium triflate species against hydrolysis by water produced during the reaction. The solvent-free and selective conversion of glycerol to solketal at room temperature and the simple reusability of spent catalyst without any regeneration are some attractive features of Al(TF)-MS, making it a suitable catalyst for solketal synthesis. This journal is

Full text of DOI:10.1039/c4cy01396d

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Synthesis of Aluminium Triflate Grafted MCM-41 as Water Tolerant-Acid Catalyst for
Ketalization of Glycerol with Acetone
Kamlesh N. Tayade,a Manish Mishra,a,* Munusamy K.b and Rajesh S. Somanib
aDepartment of Chemical Engineering And ShahꢀSchulman Center for Surface Science and Nanotechnology,
Faculty of Technology, Dharmsinh Desai University, College Road, Nadiad – 387 001, Gujarat, India.
Fax: +91 268 2520501; Tel.: +91 268 2520502; Eꢀmail: manishorgch@gmail.com.
bDiscipline of Inorganic Materials and Catalysis, Central Salt and Marine Chemicals Research Institute
(CSMCRI), Council of Scientific and Industrial Research (CSIR), GB Marg, Bhavnagar 364002, Gujarat, India.
Abstract
The aluminium triflate covalently grafted over MCMꢀ41 (Al(TF)ꢀMS catalyst) was
synthesized by a novel route aiming to have enhanced surface acidity as compared to
aluminosilicates and water tolerant mesoporous solid acid catalyst for reactions producing
water as coꢀproduct. The synthesis strategy involves equimolar reaction of triflic acid and
aluminium isoꢀpropoxide followed by treatment with MCMꢀ41. The direct treatment of triflic
acid with Al grafted MCMꢀ41 (AlꢀMCMꢀ41) was also attempted to synthesize silica
supported aluminium triflate. The MCMꢀ41 supported aluminium triflate and triflic acid were
synthesized as standard samples for comparison. The catalytic potential of the samples was
studied for ketalization of glycerol with acetone into solketal. The TGA and 27Al MAS NMR
studies revealed the formation of aluminium triflate species on the silica surface in Al(TF)ꢀ
MS sample. The covalent grafting of aluminium triflate species in Al(TF)ꢀMS sample was
confirmed from catalysis and reusability tests showing stability of the aluminium triflate
species against hydrolysis by water produced during the reaction. The solvent free and
selective conversion of glycerol to solketal at room temperature and simple reusability of
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spent catalyst without any regeneration are some attractive features of Al(TF)ꢀMS, which
makes it a suitable catalyst for solketal synthesis.
Keywords: Aluminium triflate; MCMꢀ41; acid catalysis; glycerol; solketal.
1. Introduction
The metal triflates received much importance as Lewis acid catalyst after their use as catalyst
in amidine synthesis.[1] There has been significant exploration of metal triflates (especially
triflates of lanthanides and transition metals) as excellent Lewis acid catalysts in numerous
organic reactions showing their potential for replacement of conventional moisture sensitive
Lewis acid catalysts such AlCl3, BF3, SnCl4, TiCl4 etc.[2] The supported metal triflates were
synthesized for important acid catalyzed reactions[3] to make them reusable and also to
enhance their activity by dispersing their small amount over the support surface.
The aluminium grafted MCMꢀ41 (i.e., AlꢀMCMꢀ41) materials, prepared by post grafting
method, have been extensively studied as solid acid catalyst[4] showing higher acidity content
compared to aluminosilicates (AlꢀMCMꢀ41) prepared by direct hydrothermal synthesis. The
AlꢀMCMꢀ41 materials possess predominantly Brønsted acidity along with Lewis acidity
depending upon Al content in framework (which produces Brønsted acidity) and in extra
framework (which generates Lewis acidity).[5(a)] The amount of acidity increases with
increasing the framework Al content.[5(b)] However, the acidity in these aluminosilicates is
usually very less.[5(a-c)] To further enhance the acidity of AlꢀMCMꢀ41 materials and also to
have predominantly Lewis acidity looking the wide scope of Lewis acid catalysts, in the
present work, the aluminium triflate species supported over MCMꢀ41 was aimed to
synthesize. The aluminium triflate (Al(OTf)3) has received much attention as Lewis acid
catalyst among various metal triflates.[6-12] However, aluminium triflate in the supported form
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has not been studied, which also motivated us to study the catalytic potential of aluminium
triflate supported over MCMꢀ41 as solid acid catalyst.
Glycerol has become a very good starting raw material for several value added chemicals
because of increasing availability of glycerol from biodiesel production and hydrolysis of fats
and vegetable oils, which reduced glycerol cost tremendously. One of the very important
conversions of glycerol is ketalization with acetone to form solketal (4ꢀhydroxymethylꢀ2,2ꢀ
dimethylꢀ1,3ꢀdioxolane), which is useful as oxygenated fuel additive.[13, 14] The solketal has
also been found useful intermediate for synthesis of various valuable products.[15] The major
issue with solketal synthesis reaction is formation of water as coꢀproduct, which deactivates
the acidic sites and also causes the back reaction (hydrolysis). This has also been a serious
problem in various acid catalyzed reactions involving water as reactant or coꢀproduct such as
olefin hydration, dehydration of alcohols, esterification reactions, etc. The solvents such as
benzene, toluene, chloroform, dichloromethane, etc. have been used to remove the formed
water by distilling off to increase the yield of the solketal[16], which is not practicable owing
to the hazardous effect of these solvents.
The solid acid catalysts such as zeolite Beta (Si/Al = 16),[17] sulfonic acidꢀfunctionalized
mesoporous silicas,[18] mesoporous substituted silicates (HfꢀTUDꢀ1, ZrꢀTUDꢀ1 and Snꢀ
MCMꢀ41),[19] silica supported heteropoly acids,[20] acid functionalized carbon–silica
composite,[21] Ni–Zr supported on mesoporous activated carbon,[22] zirconia and modified
zirconia,[23] etc. have been studied for the solvent free synthesis of solketal from glycerol and
acetone. The ketalization of glycerol with acetone needs a highly stable (towards water) and
recyclable catalytic system for the viability of this process. It is also highly desirable,
especially in the reactions producing water as coꢀproduct, to have stable acid catalyst against
poisoning by water (weakening of acid sites) and irreversible/ nonꢀleachable loading of
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catalytic species (e.g., here metal triflate) over the support surface to avoid the loss of activity
in subsequent reaction cycles.
There is couple of reports on the water tolerant acid catalyst[17] or methodology[18] avoiding
the adverse effect of water on catalyst and product yield in the solketal synthesis. The zeolite
Beta with Si/Al=16 was found to be best acid catalyst as compared to zeolites with low Si/Al
ratio (14 & 28) for the reaction of glycerol with acetone and formaldehyde at 70°C.[17] The
waterꢀtolerance of the zeolite Beta (Si/Al=16) was explained in terms of its high siliceous
composition providing hydrophobic environment inside the pores to expel off the water from
the pores, thus preserving the integrity and strength of the acid sites and minimizing the
reverse reaction. The zeolites being microporous have serious issue of poor reactant diffusion
efficiency, especially for bulky molecules. So looking at broad range applicability of solid
acid catalyst in acetalization reactions, it would be desirable to have a mesoporous solid acid
material with high activity and water tolerance. A simple synthesis methodology involving
use of versatile (for small to bulky substrates), robust (stable; reusable) and inexpensive metal
based catalytic system, working under ambient and solvent free conditions would be highly
beneficial for the sustainability of the process.
In the present work, aiming the synthesis of a stable solid acid catalyst for reactions
producing water as coꢀproduct and with significantly high acidity, the synthesis of aluminium
triflate supported over mesoporous silica (MCMꢀ41) was attempted. We developed a twoꢀ
step singleꢀpot synthesis protocol, which gave highly stable aluminium triflate species grafted
(covalently bonded) over MCMꢀ41 catalyst having significantly high acidity with
predominantly Lewis acid sites. The aluminium triflate grafted over MCMꢀ41 showed
excellent activity and stability (water tolerance) in ketalization reaction of glycerol with
acetone. The study shows that this material can be suitable as water tolerant acid catalyst in
many reactions producing water as coꢀproduct.
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2. Experimental
2.1. Materials
The tetraethyl orthosilicate (TEOS; >99%) and aluminium triflate (99%) were purchased
from Aldrich. The toluene (99%) and sodium hydroxide (NaOH; >97%), were purchased
from Merck, India. The cetyl trimethylammonium bromide (CTAB; 98%), aluminium isoꢀ
propoxide (AIP; 97%), triflouromethanesulfonic acid (triflic acid; TfOH; TFA; 97%),
dichloromethane (99%), glycerol (98%) and acetone (99.9%) were procured from SD Fine
chemicals, India. All the chemicals were used without any further purification.
2.2. Synthesis of Samples
2.2.1. Synthesis of MCM-41 and Al-MCM-41
The mesoporous silica (MCMꢀ41; MS) was synthesized by basic hydrolysis of TEOS using
cetyltrimethylammonium bromide (CTAB) as a template and the Al grafted mesoporous
silica (AlꢀMCMꢀ41; AlꢀMS) was synthesized by the post synthesis grafting method.[5(d)] The
CTAB (7.44 g) was dissolved in double distilled water (170 mL) followed by addition of
NaOH (1.22 g) and the mixture was stirred to get clear solution. To the resulting solution,
TEOS (17.68 g) was added drop wise in 20 min. under vigorous stirring to obtain final gel
composition (0.25 TEOS : 0.09 NaOH: 27.8 H2O: 0.06 CTAB). Thus formed gel was allowed
to react for 36 h under stirring at room temperature. The solid mass was then filtered, washed
with distilled water and dried at 80°C for 8 h in oven, followed by calcination at 550°C for 6
h in static air to get MCMꢀ41 (called MS). For the synthesis of Al grafted MCMꢀ41 sample
(AlꢀMS) with Si/ Al molar ratio of 20; MCMꢀ41 (2 g) was added to the solution containing
0.338 g aluminium isoꢀpropoxide dissolved in 150 mL toluene and allowed to stir at room
temperature for 24 h. The solid product was then filtered, dried at 90°C for 10 h and calcined
at 550°C for 3h to get AlꢀMS sample.
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2.2.2. Synthesis of MCM-41 supported samples
The covalently grafted aluminium triflate over MCMꢀ41 was synthesized by a twoꢀstep single
pot synthesis method. The triflic acid (TFA; 1.65 mmol) solution in toluene (25 mL) was
slowly added to 25 mL solution of toluene containing aluminium isoꢀpropoxide (1.65 mmol)
and the resulting solution was stirred at room temperature for 2 h. In this solution, MCMꢀ41
(2 g) was added slowly under stirring and the mixture was kept under stirring at room
temperature for 24 h. The solid product was then filtered, washed (with toluene) and dried at
100°C in static air for 12 h. The sample was named as Al(TF)ꢀMS.
The direct treatment of AlꢀMCMꢀ41 with TFA was attempted to examine the possibility of
the synthesis of supported aluminium triflate species by functionalizing the grafted Al with
triflate group (TfOꢀ group). For this, 25 mL solution of toluene containing TFA (1.65 mmol)
was added to the 25 mL solution of toluene containing AlꢀMCMꢀ41 (2 g). The mixture was
kept under stirring at room temperature for 24 h, solid product was then filtered, washed
(with toluene) and dried at 100°C in static air. This sample was named as TFA/AlꢀMS.
The MCMꢀ41 supported aluminium triflate (AlTFꢀMS) sample was prepared as a standard
sample, which will have supported aluminium triflate species on the silica surface. The
characteristics of AlTFꢀMS was compared with Al(TF)ꢀMS and TFA/AlꢀMS samples to
monitor the formation of aluminium triflate species in these samples. The supported metal
triflate catalysts are generally synthesized by treating the support material with metal triflate
solution in an organic solvent.[3(a)] We adopted the same method with slight modification; the
MCMꢀ41 (2 g) was added in 25 mL solution of aluminium triflate (1.65 mmol) in toluene,
and the solution was kept under stirring at room temperature for 24 h under nitrogen. The
solid product was then filtered, washed (with toluene) and dried at 100°C in static air.
The MCMꢀ41 supported triflic acid (TFA/MS) was also prepared as a standard sample, which
will have TFA adsorbed over silica surface through silanols.[24] By comparing the
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characteristic data of TFA/MS with Al(TF)ꢀMS and TFA/AlꢀMS, the presence of TFA
adsorbed on silica surface (which is possible during synthesis) was check in Al(TF)ꢀMS and
TFA/AlꢀMS samples. For the synthesis of TFA/MS, 25 mL solution of TFA (1.65 mmol) in
toluene was added to the 25 mL solution of toluene containing MCMꢀ41 (2 g). The mixture
was kept under stirring at room temperature for 24 h. The solid product was then filtered,
washed (with toluene) and dried at 100°C in static air.
2.3. Characterization of samples
The Al content in the synthesized Al containing samples was estimated by inductively
coupled plasma atomic emission spectroscopy (ICPꢀAES; Perkin Elmer Instrument, Optima
200 DV). The sample (100 mg) was dissolved in 40% HF solution (15ꢀ 20 drops) followed by
addition of 5 wt.% boric acid solution making up to 100 mL in volumetric flask, which was
analyzed by ICP. The BET (BrunauerꢀEmmettꢀTeller) surface area (SBET) of the samples was
determined from N2 adsorptionꢀdesorption isotherm study at liquid nitrogen temperature (77
K) using Quantachrome NOVA 1000e surface area analyzer. Temperature programmed
desorption of NH3 (NH3 TPD) for the samples were carried out for acidity measurement
using Micromeritics, USA, taking 50 mg of sample. The sample was heated at 100°C for 1 h
under helium flow (30 mL hꢀ1) as carrier gas. The NH3 was adsorbed on the samples at 50°C
by exposing to a flow of helium mixed NH3 gas (10%, 20 mL hꢀ1) for 1 h. The desorption of
NH3 was performed by heating the sample up to the final temperature of 800°C (12°C minꢀ1)
and was measured by gas chromatography with thermal conductivity detector (TCD). The
solid state 27Al NMR spectra of AlꢀMS samples were recorded at room temperature using a
Bruker, Avance II (500 MHz) spectrometer and were spun at 8 kHz. Thermal analysis (TGAꢀ
DTG) of the samples was carried out using Mettler Toledo TGA/DST1 SF752 from 25°C to
800°C at temperature ramp of 10°C/min under 80mL/min air flow. FTIR of the samples were
recorded on FTꢀIR spectrophotometer (IRPrestigeꢀ21, Shimadzu) having a Diffuse
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Reflectance Scanning disc technique by mixing the sample with dried KBr (in 1/20 wt. ratio)
in the range of 400 – 4000 cmꢀ1 with a resolution of 4 cmꢀ1. The type of surface acidity
(Brønsted/ Lewis) in the samples was characterized by FTIR study of the pyridine adsorbed
samples. For this, the sample (0.2 g) was activated at 120ºC for 2 h. The activated sample was
cooled in desiccator under vacuum and was exposed to pyridine (25 mL) vapor for 12 h. The
pyridine adsorbed sample was degassed under vacuum for 15 min. to remove physically
adsorbed pyridine from sample.
2.4. Catalytic activity of synthesized samples for ketalization of glycerol with acetone
into solketal
The ketalization of glycerol with acetone into solketal was carried out using different
synthesized samples to evaluate their catalytic activity. This reaction was used as a model
reaction of acid catalyzed reaction producing water as coꢀproduct to test the stability of the
supported catalytic species in the samples. In a typical reaction, the catalyst sample was
added into the mixture of glycerol and acetone in a glass reaction tube of reaction station (12
Place Heated Carousel Reaction Station, RR99030, Radleys Discovery Technologies, UK).
The mixture was allowed to stir at room temperature for required reaction time, after which
reaction mixture was diluted with dimethyl sulfoxide (DMSO; 3 mL) and filtered to separate
the catalyst from reaction mass. The details of reaction conditions are given in the footnote of
the table. The reaction mixture was analyzed by gas chromatography (Agilent 7890 A)
having a HPꢀ5 (60 meter, 250 µm diameter) capillary column with a programmed oven
temperature from 50 to 280 °C, at 1 mL minꢀ1 flow rate of N2 as carrier gas and FID detector.
The conversion of glycerol was calculated on the basis of its weight percent as follows,
Conversion (%) of glycerol = 100 × [Initial wt% of glycerol – Final wt% of glycerol]/ Initial
wt% of glycerol.
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The product (solketal) was identified by GCꢀMS analysis using gas chromatograph mass
spectrometer (Agilent 5975C GC/MSD with 7890A GC system) having HPꢀ5 capillary
column of 60 m length and 250 µm diameter with a programmed oven temperature from 50
to 280°C, at 1 mL minꢀ1 flow rate of He as carrier gas and ion source at 230°C and MS with
inert MSD 5975C tripleꢀaxis detector.
3. Results and Discussion
3.1. Characterization of samples
The physicoꢀchemical characteristics of the synthesized samples are summarized in Table 1.
The details of characteristics of MCMꢀ41 (MS) and AlꢀMCMꢀ41 (AlꢀMS) samples have
already been reported in our earlier report.[5(d)] The Si/Al ratio of 20 was targeted to maintain
in all the samples. The grafting of Al on MCMꢀ41 by post synthesis grafting using aluminium
isoꢀpropoxide resulted to AlꢀMCMꢀ41 (i.e., AlꢀMS) with Si/Al ratio of 20.5. The Si/Al ratio
in TFA/AlꢀMS sample was not much changed (Si/Al = 20.3), which indicates no leaching of
Al during treatment of AlꢀMS with triflic acid solution. However, the Al content was found
to be slightly less in Al(TF)ꢀMS sample showing Si/Al ratio of 24.4, and greatly lowered in
AlTF/MS sample (Si/Al = 69.4). The result indicates poor loading efficiency of Al in
AlTF/MS sample prepared by wet impregnation method using aluminium triflate, which is a
conventional method to prepare silica supported metal triflate catalysts.[3(a)]
This was anticipated that the synthesis of Al(TF)ꢀMS sample (twoꢀstep singleꢀpot synthesis
method) might be involving equimolar reaction of aluminium isoꢀpropoxide (Al(iOPr)3) with
TFA (TfOH) in the solution forming probably aluminium monoꢀtriflate with two isoꢀ
propoxide groups i.e., Al(TfO)(iOPr)2, followed by its reaction with silica surface (with
silanols; ꢀOH groups) ultimately producing a covalently grafted aluminium triflate species on
the silica surface. Therefore, the Al(TF)ꢀMS sample was characterized to examine the
grafting of Al(TfO)(iOPr)2 over the silica surface as aluminium triflate species by comparing
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its characterization results with the characteristics of AlTF/MS. The characteristics of
Al(TF)ꢀMS were also compared with TFA/MS to insure that no TFA was adsorbed over
silica surface.
The TFA/AlꢀMS was characterized to investigate whether the direct treatment of AlꢀMS with
TFA could convert grafted Alꢀsites (of AlꢀMS) into aluminium triflate species. To confirm
this, the characteristics of TFA/AlꢀMS sample were compared with the characterization data
of AlTF/MS. In TFA/AlꢀMS sample, there is also possibility of TFA adsorption over Alꢀsites
or silanol sites, which was monitored by comparing the characteristics of TFA/AlꢀMS sample
with the characterization data of AlTF/MS.
Table 1. Physicoꢀchemical properties of the prepared samples.
Amount
surface
species* (mmol/g)
of
organic
Acid sites concentration
and strengthc
Si/Al ratio
[Al
SBET
B/L
Sample
Acid
Total
content in (m2/g)
mmol/g)]
ratiob
By
C,H,S
Temp. sites
acidity
By TGAa
(°C)
(mmol (mmol
NH3/g) NH3/g)
MS
ꢀ
1230
1156
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
99
94
0.01
0.01
20.5
[0.81]
AlꢀMS
1.2
0.88
0.88
82
0.75
0.08
3.18
0.02
1.34
1.64
0.04
0.60
0.09
4.37
0.11
0.05
1.01
1.22
0.11
0.36
TFA/Alꢀ 20.3
163
295
689
84
301
471
81
166
287
513
712
132
364
447
553
708
817
0.76
0.31
0.75
0.30
1.3
0.7
4.03
3.02
MS
[0.79]
Al(TF)ꢀ
MS
24.4
[0.70]
69.4
[0.25]
AlTF/MS
TFA/MS
460
580
0.34
0.40
0.32
0.41
1.2
1.2
5.22
2.70
ꢀ
SBET : BET surface area;
*The surface organic species (SOS) may be a triflate group (TfOꢀ) or surface adsorbed triflic
acid (TFA).
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aThe surface organic species (SOS) in the samples was calculated from weight losses
(attributed to decomposition of CF3SO3 species) in TGA of TFA/AlꢀMS, AlTF/MS and
̶
Al(TF)ꢀMS samples in the temperature range of 230ꢀ370°C, and in TGA of TFA/MS in the
temperature range of 350ꢀ470°C.
bRatio of IR band areas of pyridine adsorbed on Brønsted and Lewis acid sites.
cBy NH3 TPD analysis.
The synthesized samples were characterized by FTIR, TGA, 27Al MAS NMR and NH3 TPD.
The FTIR analysis of the samples was carried out to confirm the presence of CF3SO3ꢀ
functionality, which is present in both surface adsorbed TFA as well as aluminium triflate
species present in the supported samples. The TGA study can reveal the presence as well as
the amount of the surface organic species (SOS; CF3SO3ꢀ moiety) either as adsorbed TFA or
aluminium triflate in the samples. In addition, the decomposition of surface adsorbed TFA
and aluminium triflate species will occur at different temperatures, so TGA profile can also
be used to distinguish the surface adsorbed TFA (on silanol sites or Alꢀsites) or aluminium
triflate species. The 27Al MAS NMR study was carried out to investigate the structural forms
of Al (coꢀordinations) and to distinguish the Al species as functionalized with triflate (TfOꢀ)
or unꢀfunctionalized forms. The surface acidity (amount, strength and types) in the samples
was characterized by NH3 TPD analysis and FTIR analysis of pyridine adsorbed samples.
The FTIR spectra (Figure 1) of samples (except MS and AlꢀMS) show some additional bands
in the range of 1286–1176 and at ~636 cm−1, which can be assigned to S=O stretching
vibrations and deformation modes of SO2 moieties, respectively, of adsorbed TFA or triflate
species.[25,26] A band at 1027 cmꢀ1 is attributed to CꢀF stretching vibration of CF3ꢀ group. The
FTIR study clearly indicates the presence of TFA or triflate species in the supported samples.
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90
60
30
*
*
*
MS
Al(TF)-MS
TFA/Al-MS
AlTF/MS
TFA/MS
Al-MS
*
1300
1200
1100
1000
900
800
700
600
-1
Wavenumber(cm )
Figure 1. FTIR spectrum of synthesized samples (*characteristic bands for CF3SO3ꢀ moiety).
Figure 2. DTG profiles of synthesized samples.
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In DTG profiles of all the samples (Figure 2), the weight loss below 100°C can be attributed
to the loss of residual volatile organic impurities and moisture present in the samples. The
additional major weight loss in different samples, except MS and AlꢀMS, at higher
temperatures (above 200°C) is owing to decomposition of surface organic species (SOS;
adsorbed TFA or triflate species) present in the samples. The DTG profile of silica supported
TFA (i.e., TFA/MS) sample shows a weight loss centred at ~140°C probably attributed to
loss of water associated with surface adsorbed TFA and at ~375°C for decomposition of
adsorbed TFA. Angelis et al.[27] have reported the formation of TFA.H2O on SiO2 surface by
reaction of TFA with silanol groups during the synthesis of silica supported TFA catalysts.
The reaction of TFA with silanol involves the transfer of proton from TFA to silanol forming
protonated silanol, which can hold triflate ions on the silica surface (Scheme 1; structure
I)[24]. There may also be a reaction of vicinal silanol with protonated silanol forming SiꢀOꢀSi
linkage and hydronium ion (protonated water; Brønsted acid site) on the surface, which
remains associated with triflate (Scheme 1; structure II). On heating, this surface adsorbed
hydronium triflate can eliminate water molecule giving weight loss at ~140°C. The TGA
profile of TFA/MS (i.e., weight loss at ~140°C and ~375°C) can be used to ensure the
presence of TFA adsorbed on silanol sites in Al(TF)ꢀMS, TFA/AlꢀMS and AlTF/MS
samples. In DTG profile of AlTF/MS sample (Figure 2), there is a significant weight loss at
312°C, which can be attributed to decomposition of triflate group (TfOꢀ) of supported
aluminium triflate species. So, the weight loss at around 312°C in Al(TF)ꢀMS and TFA/Alꢀ
MS samples will confirm the presence of supported aluminium triflate species.
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Scheme 1. Synthesis route for preparation of TFA/MS sample and probable structure of
surface adsorbed TFA species.
Scheme 2. Synthesis route for preparation of TFA/AlꢀMS sample and probable structure of
surface adsorbed TFA species.
The DTG profile of TFA/AlꢀMS sample (Figure 2) shows the weight loss for organic moiety
at ~295°C. This weight loss cannot be attributed to either TFA adsorbed on silica surface
(which gets decomposed at ~375°C) or supported aluminium triflate species (decomposition
temperature: ~312°C). From this result, it was anticipated that the TFA might be associated
with (or adsorbed on) Alꢀsites grafted on silica surface. The adsorption of TFA over Alꢀsites
may be due to coordination bonding between Al and TFA through oxygen as shown in
Scheme 2. The coordinative bond between Al and TFA in TFA/AlꢀMS would be
comparatively weaker than TFA bonded with silanols (Scheme 1; structure I & II), which
may be the reason for lower decomposition temperature of TFA associated with Al in
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TFA/AlꢀMS (at ~295°C) than TFA adsorbed on silica surface (in MSA/MS; at ~375°C).
Similarly, the Al–triflate bond must be stronger in aluminium triflate species (in AlTF/MS) as
compared to coꢀordinatively bonded TFA over Alꢀsites (in TFA/AlꢀMS), and therefore, TFA
associated with Al in TFA/AlꢀMS would decompose at lower temperature than aluminium
triflate species (decomposition temperature: ~312°C). Furthermore, the water loss at ~140°C
was also not observed in DTG profile of TFA/AlꢀMS indicating that TFA was not adsorbed
or reacted with silanols during the synthesis. From the TGA study, it can be concluded that
TFA/AlꢀMS, prepared by direct treatment of TFA with AlꢀMS, do not have aluminium
triflate species on silica surface and TFA gets adsorbed on Alꢀsites grafted on silica surface
by making coordinative bond.
The DTG profile of Al(TF)ꢀMS sample showed significant weight loss at 315°C (Figure 2).
This weight loss can be attributed to decomposition of aluminium triflate species, which is
almost close to the decomposition temperature of supported aluminium triflate species
(~312°C) present in AlTF/MS sample. This clearly indicates that the Al(TF)ꢀMS sample
contains surface aluminium triflate species. In DTG profile of Al(TF)ꢀMS, there are no
weight losses attributed to TFA adsorbed on silanol sites or Alꢀsites, which confirms that
there is no adsorbed TFA species in the sample.
The aluminium triflate was synthesized in good yield by reaction of aluminium isoꢀpropoxide
with TFA (in 3:1 molar ratio; see ESI), which also show decomposition of triflate species in
almost same temperature range (centered at ~360°C) as pure aluminium triflate procured
from Aldrich (ESI, Figure S1). It confirms that the reaction of aluminium isoꢀpropoxide and
TFA gives aluminum triflate in the proposed method. The equimolar reaction of aluminium
isoꢀpropoxide and TFA might be producing aluminium triflate derivative (probably
aluminium mono triflate species; i.e., Al(TfO)(iOPr)2), which will react with surface silanols
of MCMꢀ41 to form surface grafted aluminium triflate species (Scheme 3). The lower
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decomposition temperature of supported aluminium triflate species in Al(TF)ꢀMS and
AlTF/MS (at 315°C and 312°C, respectively) as compared to pure aluminium triflate
(~360°C) may be due to dispersion of aluminium triflate species over silica surface and the
presence of plenty of surface hydroxyl groups (silanols) facilitating their thermal
decomposition.[3(a), 3(b)] From TGA study, it is clearly evident that the Al(TF)ꢀMS sample
contains surface aluminium triflate species.
Scheme 3. Synthesis route for preparation of Al(TF)ꢀMS sample and probable structure of
surface grafted aluminium triflate species.
The amount of supported organic species (SOS; adsorbed TFA or aluminium triflate species)
in the samples were calculated from C,H,S data as well as from their weight losses (for
decomposition of SOS) in TGA.[24,28] The amount of organic species obtained from C,H,S
data in the samples were found to be almost similar to the amount calculated from TGA
(Table 1). This was noticed that TFA/AlꢀMS has higher TFA loading (0.76 mmol/ g) than
TFA/MS (0.40 mmol/ g). In the TFA/AlꢀMS sample, the molar amounts of SOS (i.e., coꢀ
ordinatively bonded TFA) and Al were 0.76 mmol/ g and 0.79 mmol/ g, respectively
indicating that around 96% grafted Al might have reacted with TFA. It shows that almost all
Al in AlꢀMS are functionalized with coꢀordinated TFA making probably 1:1 complex, i.e.,
each Al must be having one adsorbed TFA molecule.
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There is significant difference in molar amounts of SOS and Al in Al(TF)ꢀMS sample
(Table 1; SOS and Al contents were 0.31 mmol/ g and 0.70 mmol/ g, respectively; % Al
functionalization = 44%) showing that around 44% Al of total Al content could be grafted
with triflate group or as aluminium triflate and rest of Al were in unꢀfunctionalized form. The
less functionalization might have occurred due to condensation reactions of aluminium
monoꢀtriflate isoꢀpropoxide molecules i.e., Al(TfO)(iOPr)2) with aluminium isoꢀpropoxide
molecules, followed by reaction of resulting molecules with surface silanols.
The amount of SOS and Al in AlTF/MS (Table 1; 0.34 mmol/ g and 0.25 mmol/ g,
respectively) indicates the loss of triflate groups from aluminum triflate during loading as the
molar ratio of triflate or SOS and Al was found to be less than 3 (i.e., triflate/Al molar ratio =
1.36), which may be due to hydrolysis of aluminum triflate during loading. This ratio
(triflate/Al molar ratio = 1.36) also indicates that there should be predominantly aluminium
monoꢀtriflate species along with some amount of aluminium diꢀtriflate species supported over
silica surface.
The 27Al MAS NMR study of synthesized samples revealed the presence of different types of
Al species in the samples. The 27Al NMR spectrum of AlꢀMS sample (Figure 3) shows three
resonance signals centred at around 3.8 ppm, 32.4 ppm and 56.7 ppm indicating the presence
of octahedrally, pentahedrally and tetrahedrally coꢀordinated Al species, respectively
(Scheme 4(i)).[5(c)] The 27Al NMR spectrum of AlTF/MS sample has a sharp resonance signal
at 0.50 ppm, which can for octahedral Al monoꢀ and diꢀtriflate species (Scheme 4(ii)). The
resonance signal at ꢀ2.76 ppm in 27Al NMR spectrum of TFA/AlꢀMS must be for octahedral
Al, but it cannot be for octahedral Al triflate species. The octahedral Al species having an
adsorbed TFA through coꢀordinative bond as shown in Scheme 4(iii) may give the resonance
signal at ꢀ2.76 ppm. The coꢀordinative bonding between TFA and Al will increase electron
density at Al i.e., shielding on Al, which will cause upfield shifting of signal (at ꢀ2.76 ppm)
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as compared to octahedral Al in AlꢀMS. Poh et al.,[29] have reported the signal to a more
negative chemical shift value (ꢀ5 ppm) for octahedral aluminium bonded to silica and sulfate
n+
groups as SiO–Al (OSO3H)5 in sulfated AlꢀMCMꢀ41. The 27Al NMR result shows that all
the tetrahedral, pentahedral and octahedral Al species of AlꢀMS were converted to octahedral
Al having coordinated TFA in TFA/AlꢀMS showing a single peak at ꢀ2.76 ppm. Poh et al,[29]
have explained that the disappearance of the peak corresponding to tetrahedral aluminium
after treatment with sulphuric acid was due to removal of tetrahedrally coordinated
aluminium from the framework and transformation to octahedral aluminium, also referred as
extraframework aluminium. Similar observation was also reported by Chen et al. for
sulphated AlꢀMCMꢀ41.[30]
-2.76ppm
TFA/Al-MS
0.50ppm
AlTF-MS
0.45ppm
54.4ppm
Al(TF)-MS
32.4ppm;
3.8ppm;
Pentahedral Al
56.7ppm;
Octahedral Al
Tetrahedral Al
Al-MS
100 80 60 40 20
0
-20 -40 -60 -80 -100
ppm
Figure 3. 27Al MAS NMR spectra of synthesized samples.
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O
Al
O
O
O
O
O
O
O
O
O
O
O
O
O
Al
O
Al
Octahedral Al
Pentahedral Al
Tetrahedral Al
(i) Al species in Al-MS
Tf
Tf
O
O
Tf
HO
O
OH
O
HO
O
O
O
Al
O
Al
O
Octahedral Al
mono-triflate species
Octahedral Al
di-triflate species
(ii) Al species in AlTF/MS
Tf
Tf
H
Tf
O
O
O
O
Al
O
O
O
O
O
O
Al
O
Al
O
O
O
O
O
Octahedral Al
with TfO- group
Tetrahedral Al
with TfO- group
Octahedral Al
with adsorbed TFA
(iv) Al species in Al(TF)-MS
(iii) Al species in TFA/Al-MS
Scheme 4. Different types of Al species in the samples.
The resonance signal at 0.45 ppm in 27Al NMR spectrum of Al(TF)ꢀMS sample (Figure 3)
can be attributed to octahedral Al triflate species in the sample (Scheme 4(iv)). This is also
confirmed from 27Al NMR spectrum of AlTF/MS sample showing a sharp resonance signal at
0.50 ppm for octahedral Al triflate species. The presence of an electron withdrawing group
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(i.e., TfOꢀ group) on Al must show downfield shifting of signal for octahedral Al in Al(TF)ꢀ
MS and AlTF/MS samples as compared to octahedral Al signal in AlꢀMCMꢀ41. The upfield
shifting of the signal for octahedral Al in Al(TF)ꢀMS and AlTF/MS samples is possible due
to coꢀordination of either surface hydroxyl groups (silanols) or water molecules with Alꢀ
triflate species populating more electron density at Al. The resonance signal centred at ~54.4
ppm in 27Al NMR spectrum of Al(TF)ꢀMS may be representing presence of some framework
tetrahedral Al triflate species (which is at 56.7 ppm for unꢀfunctionalized tetrahedral Al in
AlꢀMS sample). The presence of a shoulder in the signal for octahedral Al (at 0.45 ppm) in
Al(TF)ꢀMS sample may be for unꢀfunctionalized octahedral Al species. From 27Al MAS
NMR study, it was confirmed that Al(TF)ꢀMS sample contains supported aluminium triflate
species along with some unꢀfunctionalized Al, and TFA/AlꢀMS does not have aluminium
triflate species and it has octahedral Al with coꢀordinatively bonded TFA.
Al-MS
TFA/Al-MS
TFA/MS
AlTF/MS
Al(TF)-MS
B-Pyridine
L-Pyridine
1550
1500
1450
1400
Wave number (cm-1)
Figure 4. FTIR spectrum of (b) pyridine adsorbed samples (BꢀPyridine: pyridine bound to
Brønsted acid sites; LꢀPyridine: pyridine bound to Lewis acid sites).
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The FTIR study of pyridine adsorbed samples (Figure 4) reveals the presence of both
Brønsted and Lewis acidic sites in the samples showing bands at ~1546 cm−1 and ~1446 cm−1
assigned to pyridine bound to Brønsted and Lewis acid sites, respectively.[31] The band at
1490 cm−1 is representative of adsorbed pyridine at both the acidic sites. The lowest B/L ratio
(B/L = 0.7) in Al(TF)ꢀMS sample shows higher Lewis acidity in Al(TF)ꢀMS as compared to
other samples (Table 1), which may be due to the presence of extraframwork octahedral
aluminium triflate species as well as octahedral unꢀfunctionalized Al. This is reported that in
AlꢀMCMꢀ41, the octahedral Al species provide Lewis acidity and the tetrahedral Al species
are responsible for Brønsted acidity.[5(a),29] The FTIR result of pyridine adsorbed Al(TF)ꢀMS
sample supports the 27Al MAS NMR result (Figure 3) showing predominant octahedral
extraframwork Al species for Lewis acid sites along with tetrahedral octahedral framework
Al for Brønsted acid sites. The silanols and water molecules coꢀordinated with Al (aluminium
triflate species and unꢀfunctionalized Al) can also generate Brønsted acid sites (Scheme
5(a)). It can be seen that the amount of Lewis acid sites in Al(TF)ꢀMS (calculated from the
data given in Table 1) is much higher (1.78 mmol/g) than the amount of TfOꢀ moiety (0.31
mmol/g) i.e., aluminium triflate species and even total Al content (0.70 mmol/g) present in
the sample. The higher amount of Lewis acidity in Al(TF)ꢀMS in comparison of Al content
could be possible due to adsorption of more than one molecules of base (ammonia/ pyridine)
on each octahedral Alꢀsites. Each octahedral Al species (either aluminium triflate or unꢀ
functionalized Al) have three coꢀordinations with surface vicinal silanols and/ or water
molecules. The adsorption of more than one base molecule on an octahedral Alꢀsite can occur
by breaking the coꢀordinations with silanol/ water. We demonstrated by 27Al MAS NMR
study of benzyl amine adsorbed AlꢀMS sample that the octahedral Al species are converted
into tetrahedral Al species due to adsorption of amine molecule on octahedral Al by breaking
the coꢀordinations with silanols.[5(d)]
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The 27Al MAS NMR spectrum of TFA/AlꢀMS shows the presence of only octahedral Al (no
tetrahedral Al), however, FTIR result of pyridine adsorbed sample indicates the presence of
significant amount of Brønsted acid sites along with Lewis acid sites in this sample (B/L: 1.3;
Table 1). The Brønsted acid sites in TFA/AlꢀMS can be generated due to TFA adsorbed over
Alꢀsites by coordinative interaction as well as coꢀordinated surface silanols and water
molecules with these Alꢀsites (Scheme 5(b)).
(b)
(a)
*
Tf
O
Tf
O
Tf
H
O
#
#
#
Al
Al
O
Al
O
*
*
*
*
*
H
H
*
O
H
H
H
H
H
H
H
O
O
O
O
O
O
O
O
O O
O
O
O
&
Tetrahedral
Al triflate
Octahedral
Al triflate
TFA/Al-MS
Al(TF)-MS
(c)
OTf
OTf
OH
OTf
#
HO
HO
#
Al
Al
*
*
*
*
*
*
O
H
H
H
H H
H
H
H
O
H
H
O
O
O
O
O
O
O
O
&
AlTF/MS
Scheme 5. Structure of the surface Al species and Bronsted and Lewis acid sites in (a)
TFA/AlꢀMS, (b) Al(TF)ꢀMS; (c) AlTF/MS [* = Bronsted acid sites; # = Lewis acid sites].
The aluminum triflate (which is a Lewis acid) supported on MS (i.e., in AlTF/MS sample)
contain predominantly Brønsted sites (B/L: 1.2; Table 1). In contrast, the 27Al MAS NMR
spectrum of AlTF/MS shows the presence of only octahedral Al, and therefore, this sample
should have higher Lewis acidity. The higher amount of Brønsted acidity in AlTF/MS could
be generated due to coꢀordination of aluminium triflate species (in octahedral form) with
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surface silanols (Scheme 5(c)). In TFA/MS, the protonated silanols bonded with triflate ions
on the silica surface (Scheme 1) act as Brønsted acid sites.[24] The presence of significant
amount of both types of acid sites, Brønsted as well as Lewis sites, in silica supported TFA
has also been reported by Angelis et al.[27]
The MCMꢀ41 has negligible acidity (0.1 mmol/ g), which increased after grafting of Al in Alꢀ
MS sample to 0.88 mmol/ g (weak strength) (Table 1). The TFA/MS, TFA/AlꢀMS, Al(TF)ꢀ
MS and AlTF/MS samples have significantly high acidity (2.70 mmol/ g, 4.03 mmol/ g, 3.02
mmol/ g and 5.22 mmol/ g, respectively) with weak, medium and strong strength. It is to be
noted that the SOS content (adsorbed TFA or triflate amount) in these samples is in the range
of 0.31 to 0.76 mmol/ g, however, the amount of acidity in these samples is significantly
higher (2.70 to 5.22 mmol/g). The reason for very high acidity in TFA/AlꢀMS, Al(TF)ꢀMS
and AlTF/MS samples could be the dispersion of adsorbed TFA or aluminum triflate species
over large surface and also due to coꢀordinations of surface silanols and/ or water molecules
with aluminum species as shown in Scheme 5. Similarly, the high acidity in TFA/MS sample
can be due to the hydration of adsorbed TFA (on silanol sites) by vicinal silanols as well as
water molecules generating more number of acidic sites.
The BET surface area (S
BET) of the MS sample was 1230 m2/ g, which decreased to 1156 m2/
g in AlꢀMS sample (Table 1). The SBET was significantly reduced to 708 m2/ g in TFA/Alꢀ
MS sample after loading of TFA over AlꢀMS. Similarly, SBET of Al(TF)ꢀMS, AlTF/MS and
TFA/MS samples was also decreased (817 m2/ g, 460 m2/ g and 580 m2/ g, respectively). The
decrease in SBET of supported samples in comparison of MS is indicative of the loading/
grafting in the internal surface (inside the pores) of MS.
The characterization results reveal that Al(TF)ꢀMS sample, prepared by a twoꢀstep single pot
synthesis method, contains surface grafted Al with probably one triflate group (along with
unꢀfunctionalized grafted Al species) generating significant amount of acidity with
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predominantly Lewis sites. The TFA/AlꢀMS sample, prepared by treating AlꢀMS with TFA,
does not contain aluminium triflate species and it has TFA adsorbed over Alꢀsites generating
good amount of acidity. The AlTF/MS sample, prepared by using conventional procedure,
has also substantial amount of acidity due to supported aluminium triflate species (probably
as predominantly monoꢀ and few diꢀtriflate species) on silica surface.
3.2. Catalytic activity of synthesized samples for ketalization of glycerol with acetone to
solketal
The catalytic activity of all the synthesized samples was studied for the synthesis of solketal
by acid catalyzed reaction of acetone with glycerol, which is also known as ketalization
reaction (Scheme 6). Table 2 shows the activity of different samples (in terms of glycerol
conversion, in ketalization of acetone with glycerol under given reaction conditions. The
selectivity to the desired product (solketal) was ~100% with all the samples, which was
observed not to be affected on varying the various reaction variables showing that the
reaction does not produce other products such as hemiketal or six membered cyclic ketal.
However, the conversion of glycerol varies with sample’s acidity and reaction conditions
(such as catalyst amount, amount of acetone, reaction temperature, etc.).
Scheme 6. Acid catalyzed ketalization of glycerol with acetone to solketal.
Except MS and AlꢀMS, all the samples showed good activity in ketalization giving glycerol
conversion in the range of 75ꢀ89% (Table 2, entry 1ꢀ7) when 1 mL acetone was used in the
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reaction. The acidity of samples was observed to be influencing conversion giving highest
conversion (89%) with AlTF/MS having highest acidity (5.22 mmol) and lowest conversion
(75%) with TFA/MS having lowest acidity (2.7 mmol). The AlTF/MS gave 89% glycerol
conversion, which was higher than pure Al(OTf)3 used under homogeneous condition (79%
glycerol conversion). Higher activity of the supported sample (i.e., AlTF/MS) may be
attributed to dispersion of the aluminium triflate species and generation of more acidic sites
on the silica surface. The role of acid catalyst (Brønsted/ Lewis acid sites) in ketalization
reaction is to activate the acetone molecule for nucleophilic addition of glycerol (Scheme 7
)
to form hemiketal. The hemiketal (adsorbed over acid site) undergoes dehydroxylation
producing a tertiary carbonium ion intermediate,[17] which reacts (nucleophilic attack) with
the secondary hydroxyl group to form the fiveꢀmembered ring ketal (solketal).
Table 2. Conversion of glycerol in ketalization of glycerol with acetone to solketal using
different synthesized samples.a
Acetone
amount (mL)
Conversion of glycerol
(wt. %)
Entry
Catalyst
MS
AlꢀMS
Al(TF)ꢀMS
TFA/AlꢀMS
AlTF/MS
Al(OTf)3
1
1
1
1
1
1
1
3
3
3
3
0
9
1
2
3
84
89
89
79
75
92
87
93
86
(80b, 79c)
(78b, 10c)
(80b, 6c)
4
5
6d
7
TFA/MS
(0b)
Al(TF)ꢀMS
Al(TF)ꢀMS
Al(TF)ꢀMS
Al(TF)ꢀMS
(90b)
8
9e
10f
11g
a5.4 mmol glycerol, 50 mg catalyst, room temperature (30°C), 4 h.
bFirst reuse of spent catalyst under similar reaction condition.
cSecond reuse of spent catalyst under similar reaction condition.
g
e100 mg catalyst; f50°C; 100 mg catalyst, 50°C.
dReaction with pure aluminium triflate (20 mg; amount corresponds to Al present in 50mg
catalyst), procured from Aldrich, under homogeneous condition.
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For reuse study, all the spent catalyst samples were separated by filtration of reaction
mixture, washed with acetone (by stirring in 5 mL acetone) and dried at room temperature for
overnight. The spent catalysts were reused for solketal synthesis reaction under similar
reaction condition as with fresh catalyst. The spent Al(TF)ꢀMS sample showed good
reusability with almost maintained activity in subsequent reaction cycles (Table 2, entry 3),
whereas TFA/AlꢀMS, AlTF/MS spent catalysts gave significant decrease in the glycerol
conversion in second reactions cycle (Table 2, entry 4, 5). The TFA/MS spent catalyst has no
activity in even first reuse (Table 2, entry 7).
HO
HO
HO
O
HO
O
δ+
OH
H
O
δ−
O
H
O
HO
OH
*/ #
*/ #
*/ #
Hemiketal adsorbed
over acid site
Adsorbed acetone
molecule over acid site
* = Bronsted acid site
# = Lewis acid site
HO
HO
HO
H
H
O
O
O
O
O
H
H
O
Solketal
O
O
H
O
H
*/ #
*/ #
*/ #
Scheme 7. Plausible reaction mechanism of ketalization of glycerol with acetone to solketal
over solid acid catalysts.
The spent catalyst samples, after washing with acetone and drying at 110°C for 2 h, were
analyzed by TGA and compared with TGA profile of fresh catalysts to confirm the presence
and amount of SOS in the samples. Those spent samples which did not show reusability, did
not show weight loss at higher temperature (above 200°C; for decomposition of organic
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moieties) in TGA profile indicating leaching of loaded organic moieties (adsorbed TFA or
triflate species) from the samples during reaction. This clearly shows that nonꢀreusability of
the supported samples viz. TFA/AlꢀMS, AlTF/MS and TFA/MS is due to the leaching of
loaded catalytic species. The TGA of Al(TF)ꢀMS sample before and after reaction (second
cycle) did not show significant leaching (Figure 5) showing that loaded aluminium triflate
species were quite stable and therefore, spent catalyst showed almost similar activity as fresh
catalyst.
Figure 5. TGAꢀDTG profiles of Al(TF)ꢀMS (i)before and (ii)after reaction.
The Si/Al ratio in the spent TFA/AlꢀMS and Al(TF)ꢀMS samples after second reuse were
found to be unchanged (Si/Al ration were 20.6 and 24.5, respectively), and there was
negligible amount of Al in the spent AlTF/MS. This indicates that in the reaction, TFA/Alꢀ
MS lost only adsorbed TFA but grafted Al remained intact, whereas AlTF/MS sample lost
the supported aluminium triflate species. The triflate moiety (SOS content) as well as Al
content in Al(TF)ꢀMS spent catalyst were not much changed showing that the grafted
aluminium triflate species in spent catalyst remained intact.
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The water produced during the reaction may attack on Alꢀstes (i.e., ligand substitution
reaction) in TFA/AlꢀMS sample replacing coꢀordinatively bonded TFA. Similarly, the
supported aluminium triflate species in AlTF/MS sample can also be replaced from the
surface by nucleophilic reaction of water molecules on Al. Kobayashi et al reported higher
stability of metal triflates against water hydrolysis than metal halides.[32] In Al(TF)ꢀMS
sample, the triflate group seems not to be substituted from Al by nucleophilic reaction of
water molecules showing good stability of grafted aluminium triflate species. The aluminium
triflate species in Al(TF)ꢀMS sample is covalently grafted on silica surface by SiꢀOꢀAl
bonding, therefore, the grafted aluminium triflate species could not be removed from silica
surface by reaction of water. This result confirms the covalent grafting of aluminium triflate
species on silica surface through SiꢀOꢀAl linkage. Furthermore, the good reusability of
Al(TF)ꢀMS catalyst also indicates that acidic sites are not poisoned/ deactivated by
adsorption of water molecules on the sites. The reusability of Al(TF)ꢀMS was studied for five
cycles, which did not show significant change in conversion of glycerol (80ꢀ84%) and SOS
content (by TGA).
On increasing the amount of acetone from 1 mL to 3 mL in the reaction, the conversion of
glycerol with Al(TF)ꢀMS increased from 84% to 92% (Table 2, entry 3 and 8), which is due
to increased concentration of acetone in the reaction mixture for interaction with acidic sites
of catalyst and then its reaction with glycerol. The excess acetone in the reaction can also
play the role of solvent. The high molar amount of acetone (6 to 8 times to glycerol) has been
[18,20-23]
taken for higher conversion of glycerol in previous studies.
For atom efficient
process, acetone should be used in equimolar or at least in lowest possible molar amount in
the reaction. It can be seen that Al(TF)ꢀMS gave substantially good conversion of glycerol
(84%; Table 2, entry 3) when 1 mL of acetone (which corresponds to acetone : glycerol
molar ratio of ~2.5 : 1) was employed in the reaction. With mesoporous substituted silicates
28

Page 29 of 35
Catalysis Science & Technology
View Article Online
DOI: 10.1039/C4CY01396D
(ZrꢀTUDꢀ1, HfꢀTUDꢀ1 and SnꢀMCMꢀ41 Lewis acid catalysts), the highest 52% conversion
could be obtained with HfꢀTUDꢀ1 in equimolar reaction of acetone and glycerol under reflux
condition, which increased to 65% conversion at 2:1 molar ratio. The highest glycerol
conversion (~90%) was reported with zeolite Beta (Si/Al=16) using 1.2:1 molar ratio of
acetone and glycerol under reflux condition.
The spent Al(TF)ꢀMS catalyst of the reaction carried out using higher volume of acetone (3
mL) was recovered by centrifugation and was directly reused, without any solvent wash, after
room temperature drying for overnight. The conversion of glycerol with this spent catalyst
(without solvent wash) in next cycle was 90%, which showed almost similar activity of the
spent catalyst as with the fresh catalyst (92% conversion; Table 2, entry 8). This shows that
Al(TF)ꢀMS catalyst is directly reusable, without any solvent wash, after room temperature
drying. This experiment indicates that there is not poisoning/ deactivation of the active sites
by adsorption of reactant(s)/ product and water molecules, so spent catalyst does not require
any treatment (washing/ thermal) for regeneration. The spent Al(TF)ꢀMS catalyst from the
reaction mixture, in which 1 mL acetone (3 mL) was used in the reaction, could not be
recovered by centrifugation due to more viscous reaction mass, which needed to be diluted
with acetone (5 mL) to separate the catalyst by filtration or centrifugation. Therefore, the
spent Al(TF)ꢀMS catalyst of the reaction using less amount of acetone (1 mL) could not be
possible to reuse directly without solvent wash.
We noticed that as compared to the reusability procedure of the reported solid acids in
solketal synthesis, the reusability procedure of Al(TF)ꢀMS catalyst is very simple. For the
reusability of spent catalyst in solketal synthesis, the spent catalysts have been regenerated to
restore the activity either by washing with a solvent followed by drying at high temperature
(75ꢀ110°C)[20,22] or by calcination at high temperature[19]. The spent sulfonic acidꢀ
functionalized mesoporous silica[18] was reused without washing or regenerating the catalyst
29