Crystal structure, thermal decomposition mechanism and catalytic performance of hexaaquaaluminum methanesulfonate

Article abstract of DOI:10.1080/00958972.2017.1287907

Hexaaquaaluminum methanesulfonate crystals, [Al(H₂O)₆][CH₃SO₃]₃ were synthesized by a hydrothermal reaction of Al(OH)₃ with methanesulfonic acid. Single-crystal diffraction determination revealed that Al³⁺ was coordinated by six water molecules in octahedral geometry, while the CH₃SO₃ ^– anion connected with Al³⁺ through coordinated water molecules by hydrogen bonds. The six-coordinate environment of Al was also determined by ²⁷Al MAS NMR measurement. Thermogravimetric analysis and Fourier transform infrared spectroscopy showed that the decomposition intermediate at 265–365?°C was Al₂(μ-OH)(CH₃SO₃)₅ and the final product was amorphous Al₂O₃ residue with about 0.8 wt% SO₃ at 520–800?°C. A pure phase of [Al(H₂O)₆][CH₃SO₃]₃ was confirmed by powder X-ray diffraction analysis. Esterification of n-butyric acid with n-butanol and ketalization of cyclohexanone with glycol catalyzed by [Al(H₂O)₆][CH₃SO₃]₃ and Al₂(μ-OH)(CH₃SO₃)₅, respectively, proceeded in 100% yield by continuously removing the produced water. In the case of tetrahydropyranylation of n-butanol at room temperature in dichloromethane, the catalytic activity of [Al(H₂O)₆][CH₃SO₃]₃ was much lower than that of Al₂(μ-OH)(CH₃SO₃)₅. Furthermore, both [Al(H₂O)₆][CH₃SO₃]₃ precursor and Al₂(μ-OH)(CH₃SO₃)₅ catalysts could be recycled.

Full text of DOI:10.1080/00958972.2017.1287907

Journal of Coordination Chemistry
ISSN: 0095-8972 (Print) 1029-0389 (Online) Journal homepage: http://www.tandfonline.com/loi/gcoo20
Crystal structure, thermal decomposition
mechanism and catalytic performance of
hexaaquaaluminum methanesulfonate
Rui Wang, Rongrong Li, Heng Jiang, Hong Gong & Yanfeng Bi
To cite this article: Rui Wang, Rongrong Li, Heng Jiang, Hong Gong & Yanfeng Bi (2017):
Crystal structure, thermal decomposition mechanism and catalytic performance
of hexaaquaaluminum methanesulfonate, Journal of Coordination Chemistry, DOI:
10.1080/00958972.2017.1287907
To link to this article: http://dx.doi.org/10.1080/00958972.2017.1287907
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Date: 03 February 2017, At: 06:44

Publisher: Taylor & Francis
Journal: Journal of Coordination Chemistry
DOI: http://dx.doi.org/10.1080/00958972.2017.1287907
Crystal structure, thermal decomposition mechanism and catalytic
performance of hexaaquaaluminum methanesulfonate
RUI WANG, RONGRONG LI, HENG JIANG*, HONG GONG and YANFENG BI*
School of Chemistry and Materials Science, Liaoning Shihua University, Fushun 113001, China
Hexaaquaaluminum methanesulfonate crystals, [Al(H2O)6][CH3SO3]3 were synthesized by a
hydrothermal reaction of Al(OH)3 with methanesulfonic acid. Single crystal diffraction
determination revealed that Al3+ was coordinated by six water molecules in octahedral geometry,
–
while the CH3SO3 anion connected with Al3+ through coordinated water molecules by hydrogen
bonds. The six-coordinate environment of Al was also determined by 27Al MAS NMR
measurement. Thermogravimetric analysis and Fourier transform infrared spectroscopy showed
that the decomposition intermediate at 265-365 °C was Al2(μ-OH)(CH3SO3)5 and the final
product was amorphous Al2O3 residue with about 0.8 wt% SO3 at 520-800 °C. A pure phase of
[Al(H2O)6][CH3SO3]3 was confirmed by powder X-ray diffraction analysis. Esterification of
n-butyric acid with n-butanol and ketalization of cyclohexanone with glycol catalyzed by
[Al(H2O)6][CH3SO3]3 and Al2(μ-OH)(CH3SO3)5, respectively, proceeded in 100% yield by
continuously removing the produced water. In the case of tetrahydropyranylation of n-butanol at
room temperature in dichloromethane, the catalytic activity of [Al(H2O)6][CH3SO3]3 was much
lower than that of Al2(μ-OH)(CH3SO3)5. Furthermore, both [Al(H2O)6][CH3SO3]3 precursor and
Al2(μ-OH)(CH3SO3)5 catalysts could be recycled.
Keywords: Hexaaquaaluminum methanesulfonate; Crystal structure; Thermal decomposition;
Catalytic activity; Lewis acid
*Corresponding authors. Email: hjiang78@hotmail.com (H. Jian); biyanfeng@lnpu.edu.cn (Y.F. Bi)
1

1. Introduction
Transition-metal trifluoromethanesulfonates as Lewis acid catalysts have received attention in
organic synthesis since they are stable to water (water-tolerant) [1-6]. Metal methanesulfonates
as less expensive, water-tolerant and reusable Lewis acid catalysts were also reported in catalytic
reactions such as esterification [7-9], acylation [10], condensation [11-16],
tetrahydropyranylation [17] and diacetylation of aldehydes [18, 19].
The hydrated aluminum methanesulfonate (Al(CH3SO3)3, abbreviated as Al(MS)3) is a
good candidate for its acid catalytic performance due to strong Lewis acidity of Al3+; for
example, Al(MS)3·4H2O could be designed to prepare a promising industrial organic-inorganic
hybrid silica catalyst [20] and employed as a promoter for styrene hydroesterification catalyzed
by a triphenylphosphine-palladium (PPh3–Pd) complex [21]. Song et al. reported that
Al(MS)3·4H2O could be a reusable catalyst for the Biginelli reaction [15] and three-component
condensation of 2-naphthol, aldehydes and carbamates [16]. However, the synthesis and
characterization of hydrated Al(MS)3 was obscure and structure-utility relationships as well as
catalysis mechanisms were seldom considered. This prompted us to determine the structure of
hydrous and anhydrous aluminum methanesulfonates in order to understand their catalytic
mechanism.
Herein hexaaquaaluminum methanesulfonate ([Al(H2O)6][MS]3) with a well-defined
structure is reported and general strategies for preparing water-tolerant catalyst from the hydrate
precursor with high catalytic performance are also discussed. Crystalline [Al(H2O)6][MS]3 was
synthesized by the hydrothermal reaction of Al(OH)3 with methanesulfonic acid and crystal
structures were determined by single crystal diffraction. The thermal decomposition mechanism
of [Al(H2O)6][MS]3 was investigated by Thermogravimetric/Differential thermogravimetric
(TG/DTG) analysis and Fourier transform infrared (FTIR) spectra. The catalytic activity and
reusability of [Al(H2O)6][MS]3 and its decomposition intermediate Al2(μ-OH)(MS)5 for the
esterification, ketalization and tetrahydropyranylation of alcohol reactions are reported. Their
catalytic mechanisms are also proposed.
2

2. Experimental
2.1. General information
Methanesulfonic acid (99%, extra pure, MSH) was purchased from Acros. Analytical grade
Al(OH)3 was purchased from Shanghai Meixing Chemical Co., Ltd., China. All other reagents
were purchased from Shanghai Chemical Reagent Co., Ltd., Shanghai, China and used without
further purifications.
2.2. Synthesis of [Al(H2O)6][MS]3
3.0 g of Al(OH)3, 11.1 g CH3SO3H and 20 mL double distilled water were charged into a 45 mL
PTFE bottle and sealed in a stainless steel autoclave. The reaction was carried out at 140 °C for
24 h in a dynamic hydrothermal process. After cooling slowly to ambient temperature, the
mixture was filtered and the filtrate was evaporated with a rotary evaporator at 60 °C for 2 h at
8.0 KPa. The product slowly crystallized when cooled to room temperature; then it was
recrystallized from 95 vol% isopropanol (5 vol% water) at -20 °C. Finally, the solid was dried
under high vacuum at RT for 12 h. Aluminum analysis was carried out by complexometric
titration with EDTA using xylenol orange as an indicator. Calculated for [Al(H2O)6][MS]3: Al,
6.42%. Found: Al, 6.40%.
2.3. Preparation of Al2(μ-OH)(MS)5
2.5000 g of crystalline [Al(H2O)6][MS]3 was put into an alumina crucible with a cover and then
heated to 265 °C at a rate of 5.5 °C/min in a muffle furnace and maintained at this temperature
for 5 h. After calcination, it was cooled to room temperature in a desiccator and 1.635 g product
was obtained. Calculated for Al2(μ-OH)(MS)5: Al, 9.88%. Found: Al, 9.81%.
2.4. Hygroscopic property of Al2(μ-OH)(MS)5
Using 3.12 g of Al2(μ-OH)(MS)5 scattered on a watch glass and exposing in the air at a relative
humidity 17-37%, the weight increase was monitored by analytical balance for every 5-12 h
interval times. At ambient relatively humidity, the moisture sorption weight change versus time
is shown in figure S4.
3

2.5. Characterization
TG/DTG analysis was carried out in air on a Perkin-Elmer Pyris 1 with a flow rate of 20 mL
min-1 at 20 K min-1 from 30 to 800 °C. Ceramic crucibles for TG/DTG were used and the sample
powder mass was 7-8 mg. FTIR spectra were obtained in Nujol mulls on a Perkin-Elmer
Spectrum GX. Sixty-four scans were co-added with a resolution of 4 cm-1 from 4000-400 cm-1.
1H NMR and 13C NMR spectra were recorded on a Bruker AVANCE III 500 MHz spectrometer.
1
All samples were dissolved in D2O. H NMR chemical shifts () were provided in ppm relative
to TMS as the internal reference with a resonant frequency of 500 MHz. The resonant frequency
13
27
of C NMR was 125.8 MHz and there was no internal standard. Al MAS NMR spectra were
recorded on a Bruker AVANCE III 500 solid state spectrometer (11.7 T) at a Larmor frequency
of 130.3 MHz with pulse length of 0.22 ms, relaxation delay of 2 s and rotor spinning speed of
10 kHz. The spectra were referenced to a 1.0 M AlCl3 solution. The chemical composition of the
decomposition product of [Al(H2O)6][MS]3 calcinated at 800 °C for 3 h was measured by an
X-ray fluorescence (XRF) spectrometer (ZSX-100E, Rigaku). XRF generator was operated at
200 V and 65 A. Powder X-ray diffraction (PXRD) data were collected with a D/max-RB X-ray
diffractometer (Rigaku) using Cu-Kα radiation (λ = 1.5418 Å) in the 2θ range of 3-50° with a
step size of 0.02° and a scanning rate of 6° min-1. For analyzing products of esterification,
ketalization and tetrahydropyranylation, gas chromatograph (GC) determinations were
performed using a Perkin Elmer Autosystem XL equipped with FID detector. The separation
column was an Elite-5 (30 m×0.53 mm ID×1.5µm df, 5% phenyl-95% dimethyl polysiloxane).
The temperatures of the injector and detector were set to 280 °C and 260 °C, respectively. The
oven temperature was held at 80 °C for 2 min and then increased to 240 °C at 20 °C min-1. The
carrier gas was highly pure nitrogen with a flow rate of 4 mL min-1. The split ratio was 20:1.
2.6. Single crystal X-ray crystallography
Single-crystal X-ray diffraction data measurements for [Al(H2O)6][MS]3 were performed on a
Bruker D8 QUEST COMS diffractometer operating at 292(2) K using graphite monochromated
Mo-Kα radiation (λ = 0.71073 Å). The crystal structures were solved with direct methods and
refined by full-matrix least squares on F2 using the SHELXL-2014/7 program package [22]. All
non-hydrogen atoms were refined with anisotropic displacement parameters. Hydrogens were
refined as riding with isotropic displacement parameters 1.2-1.5 times the Ueq of the attached
4

atoms. Structure refinement details, selected bond lengths and angles, and hydrogen bonds for
[Al(H2O)6][MS]3 are given in tables 1, S1 and S2, respectively. The crystallographic data has
been deposited in the Cambridge Crystallographic Data Center with CCDC reference number
1486614.
2.7. Typical procedure catalytic reaction
Esterification: In a 100 mL RB flask equipped with a Dean-Stark trap attached to a water cooled
reflux condenser, 1.0 mmol of [Al(H2O)6][MS]3 or 0.5 mmol of Al2(μ-OH)(MS)5, 0.24 mol of
n-butanol, 0.2 mol of butyric acid and 15 mL of cyclohexane were added. The mixture was
refluxed for 60 min to continuously remove water in a magnetic stirrer equipped with an oil bath
at 110-115 °C. The catalyst was insoluble in the mixture and can be separated by decantation for
reuse.
Ketalization: In a 100 mL RB flask equipped with a Dean-Stark trap attached to a water
cooled reflux condenser, 1.0 mmol of [Al(H2O)6][MS]3 or 0.5 mmol of Al2(μ-OH)(MS)5, 0.2 mol
of glycol, 0.2 mol of cyclohexanone and 15 mL of cyclohexane were added. The mixture was
refluxed for 50 min to continuously remove water in a magnetic stirrer equipped with an oil bath
at 110-115 °C. The catalyst was insoluble in the mixture and can be separated by decantation for
reuse.
Tetrahydropyranylation: In a 25 mL RB flask, 15 mmol of n-butanol, 16.5 mmol of
3,4-dihydro-2H-pyran (DHP), 1.0 mmol of [Al(H2O)6][MS]3 or 0.5 mmol of Al2(μ-OH)(MS)5
and 10 mL of dichloromethane were charged. The mixture was stirred at room temperature for
45 min. The catalyst was insoluble in the mixture and can be separated by decantation for reuse.
3. Results and discussion
3.1. Single-crystal X-ray determination
The structure of [Al(H2O)6][MS]3 was determined by X-ray single crystal structure analysis.
Crystallographic data and structure refinement (table 1) indicate that the compound crystallized
in the monoclinic system, space group P21/c and Z=4. The asymmetric unit includes one Al ion
coordinated with six water molecules and three methanesulfonate groups (figure S1). Figure 1
presents the local coordination environment around the Al1 and hydrogen bonds between
Al(H2O)6 group and ten methanesulfonates. The charges on the Al3+ cation are counter-balanced
5

by the methanesulfonates [23]. Al has a slightly distorted octahedral geometry. The bond length
of Al–O is 1.8521(14) to 1.8980(14) Å. The angle of O5–Al–O6 is 178.86(7)° and the angle O5–
Al–O4 is 89.50(7)° (table S1). Hydrogen bonds are formed between hydrogens of coordinated
water and O of methanesulfonate with average distance of 1.7936(2) Å and 2.5995(18) Å for
–
H···O and O–H···O, respectively (table S2). The [Al(H2O)6]3+ layers and CH3SO3 layers
stacking along the crystallographic b axis are shown in figure S2. The methanesulfonates have
the same orientation, with the –SO3 groups alternating up or down towards the nearest cation
[24, 25].
3.2. Thermal decomposition mechanism
The dehydration of [Al(H2O)6][MS]3 is investigated in air by TG/DTG. Figure 2 shows that the
thermal decomposition of [Al(H2O)6][MS]3 takes place in two stages. [Al(H2O)6][MS]3 is stable
up to 100 °C. The intermediate product at the first stage between 265 °C and 350 °C is assumed
as Al2(μ-OH)(MS)5. The calculated weight loss for Al2(μ-OH)(MS)5 is 35.0% according to
scheme 1, which agrees with the observed value 34.5% as shown in figure 2. The experimental
result of the calcinating [Al(H2O)6][MS]3 at 265 °C for 3 h shows that the weight loss is 34.6%.
The calculated aluminum content for Al2(μ-OH)(MS)5 intermediate is in agreement with the
experimental value of the complexometric titration analysis. These results of instrumental,
titration analysis and gravimetric analysis strongly support the assumption of Al2(μ-OH)(MS)5 as
the first stage decomposition intermediate. The thermal decomposition mechanism of the first
stage may be loss of five coordinated water molecules and one MSH by dehydration and
hydrolysis dimerization of [Al(H2O)6][MS]3.
After the first stage decomposition (from the TG curve in figure 2) the assumed
Al2(μ-OH)(MS)5 decomposes rapidly at 365 °C to 520 °C and the weight percent of final
decomposition product is 13.9%, which is slightly higher than the theoretical value of 12.1% for
the formation of pure Al2O3. The XRF determination of the decomposition product of
[Al(H2O)6][MS]3 calcinated at 800 °C for 3 h shows that the mass percent of Al2O3 and SO3 is
99.2% and 0.8%, respectively. Compared with pure Al2O3, the FTIR spectrum of the
decomposition product of [Al(H2O)6][MS]3 calcinated at 800 °C for 3 h clearly shows a weak
2-
broad peak at 1150 cm-1 (figure S6), which is the characteristic vibrational frequency of SO4
[26]. These results confirm the second stage decomposition path as described in scheme 1. The
6

PXRD result shows that the powder is an amorphous solid (figure S7). The residual SO42- delays
the conversion of amorphous Al2O3 to its crystalline form [27].
The assumed Al2(μ-OH)(MS)5 is verified by FTIR (figure 3). From figure 3(a) the
bending vibrational mode of H–O–H is 1689 cm-1 and the vibrational frequencies of free
–
CH3SO3 is at 1208 cm-1, 1155 cm-1 and 1045 cm-1. However, when [Al(H2O)6][MS]3 was
calcinated at 265 °C for 5 h, the FTIR spectrum of Al2(μ-OH)(MS)5 in figure 3(b) clearly shows
a sharp peak at 3393 cm-1, indicating that the –OH forms a bridge between two Al3+ ions [28]. In
hydroxyl containing polynuclear metal complexes, the hydroxyl group is usually bridged
between two metals [29-33]. For example, [Cu(HL)(OH–)]n shows a strong band close to 3500
cm-1 [32], (R3R’N)[Ru(OH)2Cl2]∙0.5H2O exists in a polymeric form with bridging hydroxyl
groups evidenced by ν-OH at 3460 cm-1 [31]. The X-ray structure determination results of some
of these hydroxyl containing polynuclear metal complexes directly supported the FTIR analysis
[29, 30]. The peak at 789 cm-1 can be attributed to the bridging –OH bending mode [34]. The
–
vibration frequencies of CH3SO3 at 1288 cm-1 and 1090 cm-1 in figure 3(b) are completely
–
different from those of [Al(H2O)6][MS]3 in figure 3(a), indicating that CH3SO3 coordinates
bidentate with Al3+ in a bridged binuclear complex. Indeed, as indicated by packing diagram of
[Al(H2O)6][MS]3 in figure S3, the nearest distance between two adjacent Al centers is 6.394 Å,
which is favorable to an Al2(μ-OH)(MS)5 unit in the dehydration process.
Al2(μ-OH)(MS)5 is strongly hygroscopic. The moisture sorption curve in figure S4 shows
that the weight increases rapidly in 48 h. It is calculated that [Al2(μ-OH)(H2O)10][MS]5 is formed
according to the average saturated adsorption weight as shown in figure S4. The FTIR spectra of
moisture sorption products for 12 h and 36 h in figure S5 clearly show that the peaks at
–
1288 cm-1 and 1090 cm-1 gradually disappear and the characteristic peaks of free CH3SO3 are
observed at 1208 cm-1, 1155 cm-1 and 1045 cm-1.
3.3. NMR analysis
1
The H and 13C NMR chemical shifts of [Al(H2O)6][MS]3 in D2O are 2.659 and 38.36 ppm,
respectively (figure S8 and figure S9), consistent with that of In(MS)3 and Bi(MS)3 in D2O [10].
The 27Al MAS NMR study was carried out to investigate the coordination state of Al. The
resonance signal at -3.911 ppm of [Al(H2O)6][MS]3 (figure S10(a)) indicates six-coordinate
octahedral Al [35]. However, the chemical shift of Al2(μ-OH)(MS)5 changes to higher field
7

at -25.680 ppm (figure S10(b)), which may arise from the electronegativity effect by coordinated
–
CH3SO3 toward Al3+ [36]. Kotsarenko et al. reported the 27Al NMR of anhydrous Al2(SO4)3 [37]
at -25 ppm, which was close to that of Al2(μ-OH)(MS)5 at -25.68 ppm. The crystal structure of
2-
anhydrous Al2(SO4)3 (ICSD #38034) clearly showed that six oxygens from SO4 were
-
coordinated with Al3+ [38]. Therefore, it is reasonable to infer that oxygens from CH3SO3 are
coordinated with Al3+ and the coordination number is six. The satellite resonance peaks in
figure S10 are spinning sidebands [39], which are denoted as asterisks. The spinning sidebands
are different between [Al(H2O)6][MS]3 and Al2(μ-OH)(MS)5, however, the spinning sidebands of
partially moisture sorption product of Al2(μ-OH)(MS)5 in figure S10(c) agree well with those of
[Al(H2O)6][MS]3 and Al2(μ-OH)(MS)5, respectively. The 27Al NMR spectrum of
Al2(μ-OH)(MS)5 moisture sorption product for 24 h clearly shows that a new peak at -4.016 ppm
appears (figure S10(c)), which is closer to that of [Al(H2O)6][MS]3, indicating part of
Al2(μ-OH)(MS)5 is transformed to [Al2(μ-OH)(H2O)10][MS]5 as described in scheme S1.
3.4. PXRD analysis
The results of the PXRD pattern and a simulated pattern calculated from the single crystal data
for [Al(H2O)6][MS]3 are shown in figure S11. The experimental PXRD patterns are coincident
with those simulated from X-ray single-crystal data, which indicates the presence of a pure phase
of as synthesized [Al(H2O)6][MS]3. The differences in diffraction intensities are probably due to
the variation of the preferred orientation of crystallites in the powdered sample.
3.5. Catalytic esterification by [Al(H2O)6][MS]3 and Al2(μ-OH)(MS)5
The catalytic activity of various aluminum salt catalysts is summarized in table S3. It can be seen
from table S3 that the yields are 100% for the [Al(H2O)6][MS]3 precursor and Al2(μ-OH)(MS)5
catalyzed esterification of n-butyric acid with n-butanol, while the yields of other aluminum salt
catalysts such as Al2(SO4)318H2O, AlCl36H2O, Al(NO3)39H2O and KAl(SO4)212H2O are
fairly low. It is presumed that the anions of the aluminum salt play an important role in the
catalytic activity. In case of [Al(H2O)6][MS]3 precursor, the FTIR characteristic peaks of –SO3 of
the separated catalyst after reaction appear at 1284 cm-1 and 1087 cm-1 (figure S12(b)), which are
analogous to those of Al2(μ-OH)(MS)5 as illustrated in figure 3(b), indicating that the active
species may be Al2(μ-OH)(MS)5.
8

Although the precise mechanism of the esterification is unclear, the following steps may
occur in the present reaction: (1) Aluminum salt catalysts act as Lewis acids in systems, which
coordinate the carbonyl of n-butyric acid to boost the electropositive nature of carbon of
–
carbonyl; (2) Alcoholic hydroxyl and CH3SO3 groups coordinate with Al3+; (3) Due to the
activation of acid and alcohol by coordinating unsaturated Al3+, water can be more easily
produced and removed from the reaction system. The aluminum salt with the less-nucleophilic
–
CH3SO3 anion is more electrophilic, which promotes the desired reaction through higher Lewis
acidity.
After reaction, the catalyst separated by decantation without any treatment for recycling
purposes. It can be seen from figure 4 that the catalytic activity of the recovered
[Al(H2O)6][MS]3 is practically the same as that of the fresh catalyst after recycling five times.
The reusability of [Al(H2O)6][MS]3 precursor and Al2(μ-OH)(MS)5 is excellent and the
esterification yield is very high.
3.6. Catalytic ketalization by [Al(H2O)6][MS]3 and Al2(μ-OH)(MS)5
The ketalizations of cyclohexanone with glycol catalyzed by [Al(H2O)6][MS]3 precursor and
Al2(μ-OH)(MS)5, respectively, were investigated. The catalytic activity is excellent and the
ketalization yield is 100%. [Al(H2O)6][MS]3 precursor is soluble in the reaction mixture at the
initial stage of the reaction, however, the catalyst is gradually precipitated as a solid along with
water-separating. In case of the Al2(μ-OH)(MS)5, the catalyst is insoluble in the reaction mixture
throughout the reaction stage. The reusabilities of [Al(H2O)6][MS]3 precursor and
Al2(μ-OH)(MS)5 were carried out for four cycles, which did not show significant loss of catalytic
activity (figure 5).
3.7. Catalytic tetrahydropyranylation of alcohol by Al2(μ-OH)(MS)5
For the catalytic tetrahydropyranylation of n-butyl alcohol with DHP in dichloromethane at room
temperature, Al2(μ-OH)(MS)5 exhibits the highest catalytic activity (100% yield) among various
aluminum salts (table S4). Compared with Al2(μ-OH)(MS)5, the yield by [Al(H2O)6][MS]3 is
only 40%, demonstrating that the alcohol and DHP can be easily activated by anhydrous Al2(μ-
–
OH)(MS)5 at room temperature [40]. The aluminum salt with the less-nucleophilic CH3SO3
anion is more electrophilic, which promotes the desired reaction through stronger Lewis acidity.
9

After reaction, the Al2(μ-OH)(MS)5 catalyst can be separated by decantation without any
treatment for recycling. The reusability of Al2(μ-OH)(MS)5 was investigated in catalysis of
tetrahydropyranylation under the same condition. Figure 6 shows that the catalytic activity of
Al2(μ-OH)(MS)5 decreases slightly during five runs, which may be due to the hygroscopicity of
Al2(μ-OH)(MS)5 at ambient conditions.
4. Conclusion
The crystal structure, thermal decomposition mechanism and catalytic properties of
[Al(H2O)6][MS]3 have been studied, and catalytic properties of its decomposition intermediate
Al2(μ-OH)(MS)5 have also been determined. A pure phase of [Al(H2O)6][CH3SO3]3 was well
defined by single-crystal X-ray diffraction measurements, TG, IR, NMR and PXRD. The
intermediate dehydration product between 265-365°C is Al2(μ-OH)(MS)5, which contains two
–
Al ions, five CH3SO3 and a bridging OH group. Al2(μ-OH)(MS)5 is strongly hygroscopic,
transformed to moisture sorption product [Al2(μ-OH)(H2O)10][MS]5. The catalytic activity and
reusability of both [Al(H2O)6][MS]3 precursor and Al2(μ-OH)(MS)5 are excellent for the
esterification and ketalization reactions by continuously removing the produced water. As for the
tetrahydropyranylation of alcohols in dichloromethane, Al2(μ-OH)(MS)5 exhibits very high
catalytic activity compared with [Al(H2O)6][MS]3 under the same reaction conditions. It can be
inferred that CH3SO3 group is easier to replace by other ligands due to weaker coordination with
Al than water molecules. These observations clearly show that Al3+ is the active site for the
catalytic reactions. As a consequence, the hexaaquaaluminum methanesulfonate, which can be
converted into an active Al2(μ-OH)(MS)5 catalyst, may be a promising Lewis acid catalyst
precursor in acid catalyzed organic reactions.
Acknowledgement
This work was supported by the National Natural Science Foundation of China (No. 21301170).
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Table 1. Crystal data and structure refinement of [Al(H2O)6][MS]3.
Empirical formula
C3H21AlO15S3
420.36
Formula weight
Crystal system
Monoclinic
P21/c
Space group
a/Å, α/°
16.5119(15), 90
6.4294(6), 118.629(6)
17.7145(14), 90
1650.7(3)
b/Å, β/°
c/Å, γ/°
Volume/Å3
Z
4
ρcalc/g·cm-3
1.691
Absorption coefficient (μ/mm-1)
0.572
F(000)
880
Crystal size/mm3
2θ range for data collection/°
Index ranges
0.130×0.130×0.120
2.304-26.087
-20≤h≤20, -7≤k≤7, -21≤l≤21
26322
Reflections collected
Independent reflections
Data / restraints / parameters
Rint
3253
3253 / 0 / 203
0.1115
Goodness-of-fit on F2
Final R indexes [I>2 sigma(I)]a
Final R indexes [all data]
Extinction coefficient
1.061
R1=0.0332 wR2=0.0886
R1=0.0378 wR2=0.0923
0.0066(6)
a
2
2 2 1/2
R=∑|F0-Fc|/∑|F0|, wR2=[∑w(F0 -Fc2)2/∑w(F0 ) ]
where F0, Fc are the
experimental structure factor and structure factor; S=[∑w(F0-Fc)2/(m-n)]1/2
where ‘m’ is the number of reflections and ‘n’ is the total number of parameters
refined.
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Figure captions
Figure 1. The view of local coordination environment of Al3+ cation and hydrogen bond of the
compound [Al(H2O)6][MS]3 (dotted lines represent hydrogen bond, hydrogen in CH3SO3 is
omitted).
Figure 2. The TG/DTG curves of [Al(H2O)6][MS]3.
Figure 3. FTIR spectra of the [Al(H2O)6][MS]3 (a) and its calcination product at 265 °C for
5 h (b).
Figure 4. Reusability of [Al(H2O)6][MS]3 precursor and Al2(μ-OH)(MS)5 on esterification.
Figure 5. Reusability of [Al(H2O)6][MS]3 precursor and Al2(μ-OH)(MS)5 catalyzed ketalization
reaction.
Figure 6. Reusability of Al2(μ-OH)(MS)5 catalyzed tetrahydropyranylation of n-butanol with
DHP.
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