Alumina grafted SBA-15 sustainable bifunctional catalysts for direct cross-coupling of benzylic alcohols to diarylmethanes

Article abstract of DOI:10.1039/d0cy00471e

AlSBA-15 catalysts possessing Br?nsted acid and Lewis acid-base bifunctionalities catalyze the direct arylation of benzyl alcohols to diarylmethanes with an 85% product yield through C-O bond activation. 2 and 4wt%AlSBA-15 catalysts have been synthesised by adopting a simple and efficient post-synthetic metal implantation route. The synthesised catalysts were characterized using XRD, N₂ adsorption and desorption, ²⁷Al MAS NMR, XPS, HR-TEM, NH₃ and CO₂-temperature-programmed desorption (TPD) and pyridine-transmission-FTIR spectroscopy techniques to confirm the existence of Br?nsted acid and Lewis acid-base bifunctionalities. Through various control experiments, it is verified that Br?nsted acid sites activate the benzyl alcohol and Lewis base sites interact with phenylboronic acid concurrently to accomplish the coupling reaction. In the recyclability study, 4wt%AlSBA-15 preserves its activity and stability up to 5 cycles. The 4wt%AlSBA-15 catalyst unlike homogeneous catalysts does not require additives, long reaction time and expensive metals.

Full text of DOI:10.1039/d0cy00471e

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Chaoqiu Chen, Yong Qin et al.
Highly dispersed Pt nanoparticles supported on carbon
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Alumina Grafted SBA-15 Sustainable Bifunctional Catalysts for
Direct Cross-Coupling of Benzylic Alcohols to Diarylmethanes
Chandran Rajendrana, Govindaswamy Satishkumara*, Charlotte Langb and Eric M. Gaigneauxb
Received 00th January 20xx,
Accepted 00th January 20xx
AlSBA-15 catalysts possessing Brønsted acid and Lewis acid-base bifunctionalities catalyze the direct arylation of benzyl
alcohol to diarylmethanes with 85% product yield through C-O bond activation. 2 and 4wt%AlSBA-15 catalysts have been
synthesised by adopting a simple and efficient post-synthetic metal implantation route. The synthesised catalysts were
characterized using XRD, N2 adsorption and desorption, 27Al MAS NMR, XPS, HR-TEM, NH3 and CO2-temperature-
programmed desorption (TPD) and pyridine-Transmission-FTIR spectroscopy techniques to confirm the existence of
Brønsted acid and Lewis acid-base bifunctionalities. Through various control experiments, it is verified that Brønsted acid
sites activate the benzyl alcohol and Lewis base sites interact with Phenylboronic acid concurrently to accomplish the
coupling reaction. In the recyclability study, 4wt%AlSBA-15 preserves its activity and stability up to 5 cycles. 4wt%AlSBA-15
catalyst unlike the homogenous catalysts does not require additives, long reaction time and expensive metals.
DOI: 10.1039/x0xx00000x
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naphthyl and quinolyl alcohols and boronic acids to produce
diarylmethane (Scheme 1c)17. Nevertheless, in the above examples,
the high reactivity of Grignard reagent as a partner, the poor
Introduction
Diarylmethanes are common structural constituents in biologically
active drugs1,2 and supramolecules3,4
functional group tolerance, the use of expensive palladium catalysts,
the need of a base and extended reaction time, are shortcoming
which limits their potential applications. Interestingly, Shi et al. have
proposed that Lewis acidity of the boronic acid derivatives and the
Lewis basicity of benzyl alcohol is strong enough to activate each
other to facilitate the desired cross-coupling through the formation
of borates16. This concept stimulated us to examine for the same
reaction mesoporous solid catalysts having immobilized
bifunctionalities like Brønsted acid sites to activate the benzyl alcohol
and Lewis basic sites to interact with Phenylboronic acid, and to
accomplish the desired cross-coupling reaction.
.
Usage of conventional
acids/Lewis acids as catalysts to synthesise diarylmethane have been
replaced by transition metal catalysts in the last few decades.
However, largely, highly reactive electrophiles such as benzyl halides
and their equivalents have been used as coupling partner despite
their environmental footprint. Among the various available
environmental benign electrophiles, the easily available benzyl
alcohol and its derivatives are preferred in the direct arylation since
it is a step- and atom-economic reaction5-8. On the other hand, there
are hurdles that have to be dealt with alcohols like high C-O bond
dissociation energy, the poor leaving tendency of its -OH group, and
the necessary protection of this -OH group while activating the C-O
bond.
Therefore,
highly
active
alcohols
such
as
allylic9-12, allenylic alcohols13 and propargylic alcohols14 have been
widely applied in cross-coupling reactions. Z.-J. Shi et al. first
reported the nickel catalyzed direct cross-coupling of benzyl alcohols
as coupling partner to synthesise diarylmethane using excess
Grignard reagent (Scheme 1a)15. Later, Shi’s group has reported
Pd(PPh3)4-catalyzed synthesis of diarylmethane using benzyl alcohol
through Suzuki–Miyaura coupling in the absence of any additives
(Scheme 1b)16. Recently, S.M. Samec et al. have reported the nickel
catalyzed Suzuki coupling reaction involving primary and secondary
a
C. Rajendran and Dr. G. Satishkumar,
Advanced Materials and Catalysis Lab, Department of Chemistry,
School of Advanced Sciences, Vellore Institute of Technology, Vellore -632014,
Tamilnadu, India. Tel: +91-416-2202714.
E-mail: satishkumar.g@vit.ac.in; satishgsamy@gmail.com.
b
Prof. Eric M. Gaigneaux and Dr. Charlotte Lang
Institute of Condensed Matter and Nanosciences, UCLouvain,
1348 Louvain-la-Neuve, Belgium.
Scheme 1. Various approach for the synthesis of diarylmethane using
benzyl alcohols and arylboronic acid.
† Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
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In the past few decades owing to its easy separation and recovery, desorption (TPD) and pyridine-Transmission-FTIR spectroscopy
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DOI: 10.1039/D0CY00471E
heterogeneous catalysis has been attracting much attention in techniques to understand the cooperative effect of the
pharmaceutical and fine chemical synthesis. The possibility of functionalities (Brønsted acid and Lewis base) to catalyze the
assembling multi-functionalities at the solid surface of reaction. Based on this understanding a plausible mechanism has
heterogeneous catalysts significantly enhances its capacity to been proposed for the derived coupled product involving the
catalyze various organic reactions18,19
. In particular, high surface Brønsted acid and Lewis base sites.
area mesoporous supports could immobilize two or more
functionalities in their channels at an appropriate distance, which is
difficult or impossible to achieve in solution, thus catalyze multistep Experimental
or consecutive reactions by cooperative effects. In recent years,
Synthesis of 2, 4wt%AlSBA-15 and γ-Al2O3
considerable efforts have been made to develop mesoporous acid-
base bifunctional catalysts with different strengths by introducing
diverse acids like sulfonic acids20, carboxylic acids21, phosphotungstic
acids22, Al-doped silica frameworks23 and bases like primary,
secondary, tertiary amines24-26, or alkaline earth metal oxides27 into
the mesoporous matrix. The widely adopted synthetic strategies to
covalently introduce the functional groups into the mesoporous silica
matrix are co-condensation or grafting routes. In case of grafting
procedure often the basic amine sites are created by the hydrolysis
of alkoxide bonds followed by condensation with the silanols in the
mesoporous matrix. Grafting of second functionality in the same
mesoporous silica matrix is highly challenging as the undesired
mutual quenching of acid and base sites is possible28. Hence one way
to avoid this quenching issue is that one of the reactive components
is protected during the grafting procedure to avoid acid-base site
interaction. Later, acid or base and thermal treatment can cleave the
protecting group. However, the grafting method has several
drawbacks like partial pore blocking, the stability of the silica matrix
and the second functional group can be affected due to essential acid
or base and thermal treatment, respectively. In the co-condensation
method, the functional groups are introduced in the synthesis gel
during the preparation of mesoporous material. Althᴏugh this
technique avoids the issues assᴏciated with the grafting procedure,
complete removal of the template is highly challenging and causes
serious problems to the attached functional groups. Hence, there is
a need for develᴏping an easy, efficient method to immobilise two or
more functionalities in the mesoporous support. Interestingly, by
varying the grafting aluminium weight % over the mesoporous silica
support, it is possible to generate Brønsted acid and Lewis acid-base
bifunctionalities through metal implantation method29. However, to
the best of our knowledge, this type of bifunctional catalysts is less
exploited for catalysis.
Mesoporous silica SBA-15 was synthesised through a hydrothermal
technique using P123 triblock copolymer structure directing agent as
reported in the literature30. The grafting of 2 and 4wt% alumina over
the SBA-15 was done by following the procedure proposed in the
literature31. In brief, 1 ml of trimethylamine and 1 g SBA-15 were
introduced to the solution containing 5.8 g of Al(O-sec-Bu)3
(correspond to 4wt% Al) and anhydrous toluene (100 ml) at 85°C. The
reaction mixture was stirred continuously for 6 h. The solid was
separated by filtration, washed with toluene, and dried. The
obtained dry solid was added to 30ml of ethanol and hydrolysed at
25°C for 24 h. Subsequently, the solid was washed with ethanol,
filtered and then dried in air at 120°C. Finally, the acid part of the
bifunctionality has been created by heating the dried sample at
550°C for 10h at a heating rate 1°C/min through metal implantation
method29. γ-Al2O3 has been synthesised by following the above
procedure without mesoporous silica support.
Synthesis of IE4wt%AlSBA-15M
AlSBA-15 having aluminium exclusively in the tetrahedral symmetry
was synthesised by following the recipes given in the literature32. In
brief, 1 g of silica SBA-15 in 50 mL of water containing 0.122g of
sodium aluminate stirred at room temperature for 12 h. The solid
material was then filtered, washed with distilled water, and dried at
room temperature in air. Further, it is calcined at 550°C for 5h and
the obtained material is designated as 4wt%AlSBA-15M (M-
Modified). Subsequently, 4wt%AlSBA-15M sample was ion-
exchanged using 1M ammonium nitrate solution refluxing at 100°C
for 3h33. The obtained material was filtered and the above ion
exchange procedure was repeated thrice and dried overnight at
100°C. Finally, to obtain IE4wt%AlSBA-15M (IE-Ion-exchanged) dried
sample was calcined at 550°C for 5h.
Herein, aluminium containing mesoporous bifunctional catalysts
possessing Brønsted acid and Lewis acid-base functionalities have
been synthesised by adopting a simple and efficient post-synthetic
metal implantation route. The synthesised catalysts were used to
catalyze the reaction between 2-naphthylmethanol and
phenylboronic acid to construct the 2-benzyl naphthalene (Scheme
1d). 2 and 4 weight % of aluminium has been grafted over the
mesoporous silica SBA-15 followed by calcination at high
temperature. During the high-temperature thermal treatment part
of the grafted aluminium ions get into the silica Framework thereby
generating Brønsted acid sites. Whereas, the remaining aluminium
ions form alumina clusters contain Lewis acid-base sites in the
mesoporous channels of SBA-15. To our delight, the best catalyst
Synthesis of CeO2
0.5g of Cerium nitrate was dissolved in 30 ml water and 1M NaOH
solution was added and stirred for 1h at room temperature. Then,
the solid material was filtered, washed with distilled water, and dried
at room temperature in air and calcined at 550°C for 1h.
Synthesis of CeO2/IE4wt%AlSBA-15M
0.027g of Ce(NO3)3.6H2O (corresponding to 2wt% ) was dissolved in
10 ml of distilled water and impregnated on 0.5g of IE4wt%AlSBA-15.
This suspension was stirred overnight at 80℃ until dried and the
sample was calcined at 550℃ for 1h.
4wt%AlSBA-15
catalyzed
the
reaction
between
2-
naphthalenemethanol and phenylboronic acid with a yield of 85% to
the coupled product. The synthesised catalysts have been thoroughly
characterized using XRD, N2 adsorption and desorption, HR-TEM,
27Al MAS NMR, XPS, NH3 and CO2- temperature-programmed
2 | J. Name., 2012, 00, 1-3
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elemental mapping confirms the uniform distribution of Al all over
Catalytic studies
View Article Online
DOI: 10.1039/D0CY00471E
the SBA-15 particles.
The reaction was carried out by addition of 100mg AlSBA-15 catalyst
to the mixture of 1 equiv. of 2-napthalenemethanol (0.3 mmol), 1
equiv. of phenylboronic acid (0.3 mmol), in 2-3ml of dichloroethane
solvent. The reaction mixture was stirred at 80°C for 6 h in N2
atmosphere. After cooling, the reaction mixture was stirred with
10ml of ethyl acetate and the catalyst was separated from the
reaction mixture by filtration. The filtrate was further concentrated
to remove the solvent and used for GC-MS analysis. For column
chromatography, 1.5 g of silica gel (100-200 mesh) was added to the
residue and purified by eluting with hexane to get the desired
isolated product.
In the interest of describing the state of distributed Al in the
mesopores of SBA-15 the synthesised materials was further
characterized using 27Al MAS NMR, and XPS techniques. 27Al MAS
NMR spectra of 2wt%AlSBA-15 shown in Fig. 1 displayed two
resonance signals centered at ca. 60 and 10 ppm endorsing the
presence of tetrahedral (framework) and octahedral (extra
framework) Aluminium species like in γ-Alumina, respectively35,36
.
High thermal treatment implemented during the grafting process
implants part of the grafted Al into silica framework which can
generate the Brønsted acid site and the remaining Al in the
extraframework exists as alumina clusters having Lewis acid-base
property (Scheme 2). It is interesting to note that 4wt%AlSBA-15
exhibited a major peak at ca. 10 ppm along with small peaks at ca. 60
Results and discussion
The low-angle XRD pattern of 2 and 4wt%AlSBA-15 shown in Fig.
S1† displayed a high intensity peak (100) beside two less
intensive reflections corresponding to the hexagonal
arrangement of the mesopores in SBA-1530. The amorphous
wide-angle XRD pattern of 2 and 4wt%AlSBA-15 (Fig. S2†)
corroborates the absence of bulk alumina crystals in the
mesopores of SBA-15. The synthesised alumina displayed
typical wide-angle XRD pattern corresponding to γ-Alumina34
(JCPDS No. 29-0063). The physicochemical properties of the
synthesised materials are summarized in Table 1. The surface
area of 2 and 4wt%AlSBA-15 decreased to 798 and 743 m2/g
respectively from its parent SBA-15’s surface area of 902 m2/g.
The micropore surface area decreases from 109 to 79 m2/g
upon 2wt% alumina grafting. In the case of 4wt%AlSBA-15,
micropore surface area remains essentially unaltered indicating
that subsequent grafting of aluminium ion proceeds into the
mesopores of the support SBA-15. The synthesised γ-Alumina
also possesses a good surface area of 241 m2/g. Interestingly,
alike parent SBA-15, 2 and 4wt%AlSBA-15 exhibit typical type IV
isotherm characteristic of ordered mesopores endorsing the
successful grafting of alumina without blocking the mesopores of the
SBA-15 (Fig. S3†). Fig. S4† (A and B) shows the HR-TEM images of 2
and 4wt%AlSBA-15 catalysts recorded at different magnifications
with corresponding EDAX data. TEM images confirm the successful
grafting of 2 and 4 wt%Al over the SBA-15 support by preserving the
mesoscopic channels. Fig. S5† displays SEM-EDAX mapping of 2 and
4wt%AlSBA-15. The obtained results suggest that the weight % of Al
in both the materials are close to the theoretical values and the
(e)
(d)
(c)
AlOh(Octahedral)
AlTd(Tetrahedral)
AlV(Pentagonal)
(b)
(a)
300
200
100
0
-100
-200
-300
27
Al  (ppm)
Fig. 1. 27Al MAS NMR spectra for Al samples: (a) γ-Al2O3, (b)
2wt%AlSBA-15 (c) 4wt%AlSBA-15 (d) 4wt%AlSBA-15M
(e) IE4wt%AlSBA-15.
Table 1 Physicochemical Characterization of Synthesised Catalysts
Entry
Material
Al weight
(%)a
Al weight
(%)b
Total Surface
area
Meso. area
(m2/g)
Micro.
area
(m2/g)
Pore
volume
(cm3/g)
Pore Diameter
(nm)
(m2/g)
1
2
3
4
SBA-15
2wt%AlSBA-15
4wt%AlSBA-15
γ-Al2O3
-
-
902
798
743
241
793
719
681
-
109
79
62
-
1.69
1.42
1.23
-
7
4
4
-
2
4
2.2
3.9
aNominal weight percentage. bWeight percentage obtained from SEM-EDAX.
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and 38 ppm which are attributed to aluminium in octahedral,
tetrahedral and pentahedral environment respectively36,37. It was
claimed that amorphous silica-alumina materials generate Brønsted
acid sites through pseudo-bridging silanol groups, having arisen due
to the interaction of terminal silanols with neighbouring
coordinatively unsaturated aluminium atoms (pentahedral), which
expedites the activation of the hydroxyl group in alcohols as leaving
group38. Further, the intense extraframework aluminium peak in
4wt%AlSBA-15 indicates that it has more alumina clusters in the
mesopores of SBA-15 compared to 2wt%AlSBA-15 in which part of
the grafted aluminium clogged in the micropores (Table 1). XPS
spectra of 4wt%AlSBA-15 in the binding energy region of Si2p, Al2p,
exhibit multiple chemical shifts owing to silica-alumina interface (Fig.
2). Successful grafting of alumina with the silanols of mesoporous
support has been confirmed through the appearance of Si2p
components at 103.2 and 102.0 eV corresponding to Si-O-Si and Al-
O-Si linkages. Likewise, the existence of alumina clusters in the
mesopores of 4wt%AlSBA-15 is verified through Al2p binding energy
peaks at 73.1 and 74.2 eV which correspond to Al-O-Si and Al-O-Al
linkage39.
Moderate
acid site
View Article Online
DOI: 10.1039/D0CY00471E
Strong acid site
Weak acid site
Total Acidity
(mmol/g)
252oC
(c)
(b)
(a)
1.623
0.504
0.185
447oC
228oC
250oC
419oC
T
-----Isothermal
600
100
200
300
400
500
T=Constant
Temperature (°C)
(450oC)
Fig. 3. NH3-TPD Analysis for (a) γ-Al2O3, (b) 2wt%AlSBA-15 and (c)
4wt%AlSBA-15.
Ammonia and pyridine-Transmission-FTIR studies on acid sites of
synthesised catalysts
Furthermore, in the catalysis perspective, it is essential to estimate
the acid sites available in the synthesised catalysts. Therefore,
synthesised catalysts γ- Alumina, 2 and 4wt%AlSBA-15 have been
investigated using ammonia-TPD (Fig. 3) and the total acidity is
recorded. Generally in NH3-TPD studies, ammonia desorb from weak
acid sites at below 200 °C; on the other hand, moderate and strong
acid sites desorb the ammonia molecule between 250 to 350°C and
350 to 450°C, respectively36,40. It can be seen that there is a clear
increase in the total acidity count from γ- Alumina to 4wt%AlSBA-15
(Fig. 3, inset). The total acidity count for γ- Alumina (0.185 mmol/g)
is solely arisen from its moderate acid sites. Moderate and strong
acid sites exist in both 2 and 4wt%AlSBA-15 catalysts as they have
displayed two ammonia desorption peaks at ca. 250 and 450°C. It is
interesting to note that substantial increase in the total acidity count
was observed for 4wt%AlSBA-15 (1.623 mmol/g) compared to
2wt%AlSBA-15 (0.504 mmol/g) (Fig. 3, inset). Higher Aluminium
loading and additional Brønsted acidity emanating from pseudo-
bridging silanol groups have contributed to the increased acidity of
4wt%AlSBA-15 catalyst. Although NH3-TPD technique has been
broadly used to quantify the total acid sites, owing to its high
reactivity with catalyst surface via hydrogen bonding it does not
provide information on the Brønsted /Lewis distribution of the
probed sites40-42. Thus, it has been decided to examine the
synthesised catalysts γ- Alumina, 2 and 4wt%AlSBA-15 by pyridine-
Transmission-FTIR spectroscopy technique. Fig. 4 illustrates the IR
spectra of adsorbed pyridine at room temperature, 100 and 200°C
on γ- Alumina, 2 and 4wt%AlSBA-15. Based on the adsorption
coefficients, in situ transmission FTIR spectroscopy can be applied to
quantify acid sites present in the catalyst 43-46. The reported molar
Scheme 2. Graphical illustration of the synthesised bifunctional
4wt%AlSBA-15 catalyst.
Survey spectrum- 4wt%
AlSBA-15
Si2p
8000
7000
6000
5000
4000
3000
2000
1000
0
700
600
500
400
300
200
100
0
102.0
Al-O-Si
Si-O-Si
103.2
-1000
110
100
90
1200 1000
Al2p
800
600
400
200
0
Binding Energy (eV)
Binding Energy (eV)
O1s
600
500
400
300
200
100
0
531.1
Al-O-Si
4000
3000
2000
1000
0
73.1
Al-O-Si
74.2
Al-O-Al
Si-O-Si
533.0
520
525
530
535
540
545
60
65
70
75
80
85
Binding Energy (eV)
Binding Energy (eV)
Fig. 2. XPS Spectra of catalyst 4wt%AlSBA-15
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extinction coefficients εL = 1.44 cm. µmol-1and εB = 0.59 cm. µmol-1 all the other peaks remain present and even after VeievwacAurtiactleioOnnlinaet
DOI: 10.1039/D0CY00471E
(B = Brønsted acid and L = Lewis acid peaks.) were used for the 200°C (Fig. 4B,C). This observation clearly suggests the availability of
estimation of acid sites present in the synthesised catalysts47. The strong Lewis and Brønsted acid sites in 2 and 4wt%AlSBA-15
spectra recorded at room temperature for all the three catalysts catalysts. The quantification of Lewis and Brønsted acid sites was
displayed peaks at 1623, 1493 and 1454 cm-1 corresponding to done based on this Pyridine-Transmission-FTIR study (Fig. 4). IR
pyridine coordinating to Lewis acid sites. In case of 2 and 4wt%AlSBA- spectra recorded after desorption at 100°C were used for the
15, the peak at 1549 cm-1 and a shoulder at 1638 cm-1 are assigned calculation of the distribution between Brønsted and Lewis acid sites
to the pyridinium ions chemisorbed on the strong Brønsted acid and acidity calculation details are given in the supporting information
sites45. As anticipated, peak corresponding to the Brønsted acid sites page S4. Percentages of Lewis (65, 52%) and Brønsted acid sites (35,
are completly missing for γ- Alumina (Fig. 4A) which complements 49%) are available in the 2 and 4wt%AlSBA-15 catalysts, respectively.
the results obtained from ammonia-TPD (Fig. 3). Interestingly, for 2 4wt%AlSBA-15 comprises 2.5 fold higher Brønsted acid sites
and 4wt%AlSBA-15 catalysts after evacuation at 100°C, except the compared to 2wt%AlSBA-15 with almost equal distribution of
peaks at 1440 and 1590 cm-1 corresponding to weakly coordinated Brønsted and Lewis acid sites. The total acidity count for all the
pyridine on Si-OH groups present in the mesoporous support SBA-15, catalysts derived from pyridine-Transmission-FTIR analysis is lesser
than ammonia-TPD acidity data. This might be understood on the
basis of the fact that unlike ammonia the bulkier aromatic pyridine
L
(1454 cm-1)
(A)
L+B
L
(1493 cm-1)
can probe only accessible surface acid sites available in the catalyst.
Hence, it was decided to use the acidity data obtained at 100°C from
pyridine-Transmission-FTIR studies for Turn over number (TON)
calculations of 2 and 4wt%AlSBA-15 catalysts.
(1618 cm-1)
Acid site (mmol/g)
(c) 100C
(d)
(c)
L = 0.142 (100%)
(d) 200C
L = 0.077 (100%)
Catalytic Activity
(b)
(a)
In an initial attempt, experiments were conducted to screen the best
catalyst towards the cross-coupling between 2-naphthylmethanol
and phenylboronic acid (Table 2. Entries 1-3). Among the catalysts,
at the given conditions 4wt%AlSBA-15 proved to be more efficient
compared to 2wt%AlSBA-15 and γ-Al2O3 and yielded after 24h 90%
of the desired coupled product with a TON of 10. γ-Al2O3 which does
not contain Brønsted acid sites exhibited poor catalytic performance
whereas 2wt%AlSBA-15 demonstrated an intermediate catalytic
activity with 60% yield to the coupled product. These results indicate
the prerequisite of Brønsted acid sites in the catalyst to catalyze the
above reaction. Subsequent optimization studies like time,
temperature and solvent were carried out over the best catalyst,
namely 4wt%AlSBA-15. The influence of time on the product yield
was investigated and it was found that within 6 hours 85% of the
product has been formed affording TON of 9.47 (Table 2, entries 6,7).
From the temperature optimization studies, it is clear that
temperature plays a critical role in the reaction. It was found that an
increase in temperature from 60 to 70°C does not improve the yield
(Table 2, entries 8, 9). However, rising the temperature to 80°C
brought a better yield (Table 2, entry 7). It was observed that at 60°C
reactants 2-naphthylmethanol and phenylboronic acid have
completely converted and the desired product 2-benzyl naphthalene
obtained as major product with 67% yield. At higher temperature
(80°C) formation of side products found diminished and the coupled
product yield has been improved to 85%. The formed byproducts
1700 1650 1600 1550 1500 1450 1400 1350 1300
Wave number (cm-1)
(B)
L
(1622 cm-1
B
)
L
B
(1454 cm-1
)
(1549 cm-1
)
(d)
L+B
(1493 cm-1
(1638 cm-1
)
)
Acid site (mmol/g)
(c) 100C
(c)
(b)
B = 0.105 (35.4%)
L = 0.192 (64.6%)
(d) 200C
B = 0.082 (31%)
L = 0.183 (69%)
(a)
1700 1650 1600 1550 1500 1450 1400 1350 1300
Wave number (cm-1)
L
(C)
L
(1623 cm-1
B
)
(1454 cm-1
)
(d)
L+B
Acid site (mmol/g)
(c) 100C
(1638 cm-1
)
(1493 cm-1
)
B
(1549 cm-1
)
(c)
(b)
B = 0.267 (48.7%)
L = 0.283 (51.3%)
(d) 200C
B = 0.225 (44.3%)
L = 0.282 (55.6%)
(a)
1700 1650 1600 1550 1500 1450 1400 1350 1300
Wave number (cm-1)
Fig. 4. Pyridine-Transmission-FTIR studies of synthesised catalysts (A) have been identified through GC-MS analysis and details are given in
γ-Al2O3.; (B) 2wt%AlSBA-15; (C) 4wt%AlSBA-15. (a) Degassed at
250°C, (b) After pyridine adsorption, (c) After pyridine desorption examined like polar protic (Ethanol, H2O), and polar aprotic
the supporting information page S11. Among the various solvents
100°C, (d) After pyridine desorption 200°C: The extinction
coefficients were used47 εL = 1.44 cm. µmol-1 and εB = 0.59 cm. µmol-
1 (B = Brønsted acid and L = Lewis acid peaks.)
(Acetonitrile, DMF, DMSO, THF, DCE), DCE appears to be the best
solvent based on the product yield (Table 2, entries 10-15).
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Table 2. Optimization of reaction conditions on 2-naphthalenemethanol with phenylboronic acid.
Entry
Catalyst (100mg)
Solvent
Temp.
(°C)
80
80
80
80
80
80
80
60
70
80
80
80
80
80
80
80
80
80
Time (h)
TON(a)
TOF(b) (h-1) Yield (%)(c)
1
2
2wt%AlSBA-15
4wt%AlSBA-15
γ-Al2O3
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
Ethanol
H2O
ACN
THF
24
24
24
24
24
12
6
17.02
10.02
2.84
1.67
60
90 (79)d
3
-
-
3
4
5 e
6
SBA-15
-
-
ND
-
-
-
ND
4wt%AlSBA-15
4wt%AlSBA-15
4wt%AlSBA-15
4wt%AlSBA-15
4wt%AlSBA-15
4wt%AlSBA-15
4wt%AlSBA-15
4wt%AlSBA-15
4wt%AlSBA-15
4wt%AlSBA-15
4wt%AlSBA-15M
IE4wt%AlSBA-15M
CeO2/IE4wt%AlSBA-15M
9.69
1.62
87 (77)d
85 (75)d
67
7
9.47
1.57
8
6
7.46
1.24
9
6
6
7.79
-
1.29
70
10
11
12
13
14
15
16
17
18
-
ND
6
-
-
ND
6
6.68
1.11
60
6
6.68
1.11
60
DMF
DMSO
DCE
DCE
DCE
6
6
-
-
-
-
-
-
-
-
-
-
ND
ND
6
ND
35 (20)d
60 (35)d
6
6
Reaction conditions: 1 (0.3 mmol), 2 (0.3 mmol) and catalyst (100mg) in solvent (2 mL) was stirred at 80°C for 6h and 24h under N2
atmosphere; (a) Calculated by dividing the moles of product formed at 6h (21600 sec) by the moles of Brønsted acid sites present in the
catalyst; (b) Calculated dividing TON by time; (c) Yield monitored by GC (n-Decane as internal standard); (d) Isolated yield; (e) Without
catalyst; ND- Not Detected.
Role of Brønsted acid and Lewis base sites in the diarylmethane reaction. To further confirm the participation of Lewis acid-base sites
construction reaction
in the arylation of 2-naphthylmethanol it is decided to vary the
composition of tetrahedral and octahedral aluminium in the
Despite the remarkable catalytic activity of 4wt%AlSBA-15 towards 4wt%AlSBA-15 which in turn varies the Brønsted and Lewis acid-base
arylation of 2-naphthylmethanol, ambiguity still remains on the role sites and which consequently, would affect the coupled product
of each active sites as the catalyst possesses both Brønsted and Lewis yield. We synthesised 4wt%AlSBA-15M (M-modified) containing
acid-base functionalities. Hence, further investigations are required aluminium exclusively in the tetrahedral symmetry through post
to gain insight on the role of each functionality to produce the grafting technique using an aqueous solution of sodium aluminate32
desired coupled product. To identify the involvement of Brønsted (Fig. 1d). As expected, 4wt%AlSBA-15M could not catalyze the
acid sites, the reaction was carried out using 2-pyridinemethanol as reaction due to the presence of Na+ ions which balance the negative
one of the reactants instead of 2-naphthylmethanol and in the other charges associated with tetrahedral aluminium (Table 2, entry 16).
trial, 0.5 ml of pyridine was used in the reaction mixture under the Hence, 4wt%AlSBA-15M catalyst was ion exchanged with H+ to
same reaction conditions. In both the experiments 4wt%AlSBA-15 replace the Na+ ion and it is designated as IE4wt%AlSBA-15M. During
did not afford the desired product. Owing to the high basicity of the ion exchange process, the alkaline environment and high-
2-pyridinemethanol and pyridine molecules they likely preferentially temperature treatment generated extraframework aluminium
adsorbed over the Brønsted acid sites of the 4wt%AlSBA-15 thus species which is evidenced through the appearance of a resonance
preventing the studied reaction to proceed. Earlier, it was noticed peak at ca 10 ppm in 27Al MAS NMR shown in Fig. 1(e). Interestingly,
that γ-Al2O3 catalyst which does not have the Brønsted acid sites contrarily to 4wt%AlSBA-15, IE4wt%AlSBA-15M possesses more
could not catalyze the coupling reaction. Hence, this observation aluminium in tetrahedral symmetry suggesting the presence of more
reinforces the involvement of Brønsted acid site in the above
Brønsted acid sites than Lewis acid-base sites. IE4wt%AlSBA-15M
6 | J. Name., 2012, 00, 1-3
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Weak
Moderate
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DOI: 10.1039/D0CY00471E
Strong
(B)
(A)
Total Basicity
(mmol/g)
A= 0.429
B =0.629
0
100
200
300
400
500
600
700
800
-----Isothermal
T=Constant
(600oC)
Temperature (°C)
Scheme 3. Plausible mechanism of diarylmethane formation via
Fig. 5 CO2-TPD Analysis of (a) 4wt%AlSBA-15; (b) CeO2/IE4wt%AlSBA-
benzylic C-O Bond Activation.
15M
benzylic carbocation was chemisorbed over the bridged oxygen
anion of aluminosilicate framework. Concurrently, phenylboronic
acid chemisorbed over the Lewis base site exposed by the alumina
clusters to form boronate ion. The released water molecule
protodeboronates the chemisorbed boronate ion to generate the
transient aryl anion50.
catalyzes the coupling reaction with 35% product yield (Table 2,
entry 17). The insignificant existence of extraframework aluminium
in IE4wt%AlSBA-15M thus severely affected the product yield. This
observation strongly supports the significant role of alumina clusters
in 4wt%AlSBA-15 which bring Lewis acid-base sites required in the
studied reaction. It is anticipated that Lewis base sites in the alumina
clusters of 4wt%AlSBA-15 alone can activate the phenylboronic acid
through acid-base interaction. Therefore it was planned to
impregnate well-known Lewis base cerium oxide (2wt%) into the
mesopores of IE4wt%AlSBA-15M and the obtained sample was
examined for the chosen coupling reaction. The wide-angle XRD
pattern of CeO2/IE4wt%AlSBA-15M catalyst displayed cerium oxide
peaks which are in good agreement with the earlier report48 (Fig.
S6†).The surface basic properties of CeO2/IE4wt%AlSBA-15M and
4wt%AlSBA-15 were studied through CO2-TPD investigation as
shown in the Fig. 5. Both the catalysts displayed desorption peak at
ca below 200 °C and above 500 °C revealing the existence of weak
and strong basic sites49. To our delight, the presence of cerium oxide
in CeO2/IE4wt%AlSBA-15M enhanced the product yield to 60% at the
same reaction conditions (Table 2, entry 18). This observation
validates the involvement of Lewis base sites in the activation of
phenylboronic acid to accomplish the coupling reaction. Although
CeO2/IE4wt%AlSBA-15M has more basic sites (0.629mmol/g) than
4wt%AlSBA-15 (0.429mmol/g), the high dispersion of alumina
clusters in 4wt%AlSBA-15 increases the accessibility of reactant
molecules to produce a higher product yield. Therefore, based on the
various characterization results and experiments attempted it is
concluded that the bifunctionality, i.e. simultaneous presence of
Brønsted acid and Lewis base sites, of 4wt%AlSBA-15 indeed
participates in the reaction to yield the desired coupled product.
Table 3. Substrate variation of Boronic acids
A plausible mechanism has thus been proposed involving the
Brønsted acid and Lewis base sites in 4wt%AlSBA-15 (Scheme 3).
Initially, 2-naphthylmethanol is protonated by the Brønsted acid
sites; subsequently a water molecule is released. The formed
Reaction conditions: The solution of 1 (0.3 mmol), 2 (0.3 mmol) and
catalyst (4wt%Al/SBA-15) (100mg) in DCE (2 mL) was stirred at 80oC
for 6h under N2 atmosphere. All are isolated yield product. (a) 130oC
for 6h.
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100
The formed transient aryl anion immediately reacts with nearby
chemisorbed benzylic carbocation to give the desired coupled
product. The released proton from the water molecule adsorbs over
the bridged oxygen atom to regenerate the Brønsted acid site.
View Article Online
DOI: 10.1039/D0CY00471E
90
80
70
60
50
40
30
20
10
0
75
75
75
74
72
71
Under the optimized conditions, the tolerance of the catalyst to
various functional groups was studied by varying functional groups
in the phenylboronic acid and variety of arylmethanols (Table 3).
Initially, variation of the boronic acid was explored. It was found that
electron donating groups –CH3 and –OCH3 are compatible with the
reaction conditions to give good yield. With the electron withdrawing
substituents–4Cl,–4Br, –3Cl, –2F, –2Br, 4wt%AlSBA-15 demonstrated
good yields. However, with the multi halo substituents (3j, 3k)
depreciation in the yield was observed. Likewise, 3–CF3 and 4–CF3
induced decrease in the yield. Substituents like –COCH3 and
–CHO were tolerated to the optimized conditions. Furthermore,
examples like 3p, 3q and 3r demanded harsher reaction conditions
to achieve a better yield. Subsequently, variation of the benzylic
alcohol was examined. Predictably, benzylic alcohol possessing
electron donating and electron withdrawing substituents were
compatible with the reaction conditions (4a-h, Table 4).
0
1
2
3
4
5
No of Cycles
Fig. 6 Recyclability study of catalyst 4wt%AlSBA-15 in the arylation of
2-naphthylmethanol at the optimized conditions.
Spent catalyst after 5 cycles was characterized using wide-angle XRD
and SEM-EDAX (Fig. S9† and S10†). The SEM-EDAX results confirm
the presence of 3.73wt% of Al in the spent catalyst after 5 cycles.
Further, amorphous XRD pattern of spent4wt%AlSBA-15 catalyst
assures that the aluminium incorporated in the framework and
grafted alumina clusters on the mesopores of the support SBA-15 are
intact.
The privilege of using heterogeneous catalyst is acknowledged
through its performance in recyclability study. Therefore,
recyclability of 4wt%AlSBA-15 catalyst has been studied towards the
cross-coupling between 2-naphthylmethanol and phenylboronic acid
at the optimized conditions. After each run the catalyst was
recovered by filtration and washed thoroughly with ethyl acetate
solvent (10ml). Further, the catalyst was dried overnight at 100 °C
and employed for the next run. The catalyst maintained its high
activity and stability up to 5 cycles (Fig. 6).
Conclusions
In this study, 4wt%AlSBA-15 conclusively proved as an efficient,
stable and sustainable heterogeneous catalyst towards the
construction of biarylmethane using 2-naphthylmethanol and
phenylboronic acid through C-O bond activation. Bifunctionality, i.e.
the simultaneous presence of Brønsted acid and Lewis acid-base
sites, is easily introduced in the mesopores of SBA-15 by
implementing simple and efficient post-synthetic metal implantation
route.The high catalytic performance of 4wt%AlSBA-15 indicates the
significance of heterogeneous catalysts having bifunctionality in
activating the reactant molecules through cooperative effect.
Detailed characterization and different control experiments revealed
that Brønsted acid and Lewis base sites both played a critical role to
deliver the desired coupled product by activating the 2-
naphthylmethanol and Phenylboronic acid, respectively. Stability of
the 4wt%AlSBA-15 catalyst is evidenced through its consistent
performance in 5 consecutive cycles. Compared to homogeneous
catalysts, 4wt%AlSBA-15 does not require additives, long reaction
time and expensive metals. This study opens a new path to employ
bifunctional heterogeneous catalyst in academic research and
chemical industries for challenging organic transformations.
Table 4. Substrate variation of Benzylic alcohols
Conflicts of interest
“There are no conflicts to declare”.
Reaction conditions: The solution of 1 (0.3 mmol), 2 (0.3 mmol) and
catalyst(4wt%Al/SBA-15) (100mg) in DCE (2 mL) was stirred at 80oC
for 6h under N2 atmosphere. All are isolated yield product.
Acknowledgements
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27. H. Wang, L. Z. Liang, W. J. Zhang, J. Y. Han, J. PVoireowuAsrticMleaOtnelirn.e,
This study was financially supported by VIT transdisciplinary
grant from VIT management. Authors thank SIF-IISC Bangalore
for Solid state NMR and VIT-SIF for 1H and 13C NMR.
DOI: 10.1039/D0CY00471E
2012, 19, 889-895.
28. P. Kasinathan, C. Lang, J. Schnee, C. D'Haese, E. M. Gaigneaux,
A. Jonas, A. Fernandes, Chemistry - A European Journal, 2019,
25, 1–11.
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Table of contents entry
Alumina Grafted SBA-15 Sustainable Bifunctional Catalysts for Direct
Cross-Coupling of Benzylic Alcohols to Diarylmethanes
C. Rajendrana, G. Satishkumara*, Charlotte Langb and Eric M. Gaigneauxb
aAdvanced Materials and Catalysis Lab, Department of Chemistry, School of Advanced Sciences,
Vellore Institute of Technology,Vellore-632014, Tamilnadu, India
bInstitute of Condensed Matter and Nanosciences, UCLouvain,
1348 Louvain-la-Neuve, Belgium.
Email address of the Corresponding author: satishkumar.g@vit.ac.in ; satishgsamy@gmail.com
AlSBA-15 (Bifunctional catalyst)
OH
LA - Lewis acid
LB - Lewis base
B
- Brønsted acid
+
R
B
OH
HO
OH
HO
OH
Al
B
26 examples (45% to 79%)
LB
LA
H
O
O
B
R
OH
HO
HO Al
OH
O
OH
O
Al
Si
O
Si
O
AlSBA-15 catalysts possessing Brønsted acid and Lewis acid-base bifunctionalities catalyze the
direct arylation of benzyl alcohol to diarylmethanes with 85% product yield through C-O bond
activation.