Molybdenum-modified mesoporous SiO2as an efficient Lewis acid catalyst for the acetylation of alcohols

Article abstract of DOI:10.1039/d1ra02134f

A suitable, expeditious and well-organized approach for the acetylation of alcohols with acetic anhydride in the presence of 5%MoO3-SiO2 as an optimum environmentally benign heterogeneous catalyst was developed. The high surface area obtained for 5%MoO3-SiO2, 101 m2 g-1 compared to other catalysts, 22, 23, and 44 m2 g-1 for 5%WO3-ZrO2, 5%WO3-SiO2, and 5%MoO3-ZrO2, respectively, appears to be the driving force for better catalytic activity. Amongst the two dopants used, molybdenum oxide is the better dopant compared to its tungsten oxide counterpart. High yields of up to 86% were obtained with MoO3 doping while WO3 containing catalysts did not show any activity. Other reaction parameters such as reactor stirring speed, and solvent variation were studied and revealed that the optimum stirring speed is 400 rpm and cyclohexane is the best solvent. Thus, the utilization of affordable and nontoxic materials, short reaction times, reusability, and producibility of excellent yields of the desired products are the advantages of this procedure.

Full text of DOI:10.1039/d1ra02134f

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Molybdenum-modified mesoporous SiO2 as an
efficient Lewis acid catalyst for the acetylation of
alcohols†
Cite this: RSC Adv., 2021, 11, 16468
*
Xolani S. Hlatshwayo, Matumuene Joe Ndolomingo, Ndzondelelo Bingwa
*
and Reinout Meijboom
A suitable, expeditious and well-organized approach for the acetylation of alcohols with acetic anhydride in
the presence of 5%MoO3–SiO2 as an optimum environmentally benign heterogeneous catalyst was
developed. The high surface area obtained for 5%MoO3–SiO2, 101 m2 gꢀ1 compared to other catalysts,
22, 23, and 44 m2 gꢀ1 for 5%WO3–ZrO2, 5%WO3–SiO2, and 5%MoO3–ZrO2, respectively, appears to be
the driving force for better catalytic activity. Amongst the two dopants used, molybdenum oxide is the
better dopant compared to its tungsten oxide counterpart. High yields of up to 86% were obtained with
MoO3 doping while WO3 containing catalysts did not show any activity. Other reaction parameters such
as reactor stirring speed, and solvent variation were studied and revealed that the optimum stirring speed
is 400 rpm and cyclohexane is the best solvent. Thus, the utilization of affordable and nontoxic
materials, short reaction times, reusability, and producibility of excellent yields of the desired products
are the advantages of this procedure.
Received 17th March 2021
Accepted 26th April 2021
DOI: 10.1039/d1ra02134f
rsc.li/rsc-advances
Numerous methods for the acetylation reactions have been
previously reported.21–23 However, countless of these reported
1. Introduction
methods suffer from several limitations such as drastic reaction
conditions, long reaction time, formation of toxic or undesir-
able byproducts, use of expensive reagents, utilization of cata-
lysts which are moisture sensitive or toxic, tedious workup
procedures and poor yields of the desired products.5,21–25
Therefore, the developments of new catalysts and mild methods
for the acetylation of alcohols are still in great demands. The
transformation of various chemicals under heterogeneous
catalysis has gained a great deal of interest over the past years
due to the following ndings; the efficiency of chemical trans-
formations is greatly improved, improvement of the desired
pure products, easier work-up, and usage of less toxic and
affordable catalysts.26,27
In the recent years, there were some developments made to
instigate solid reagents or solid reagents supports for uncom-
plicated work-ups which comprise silica gel supported TaCl5,28
NaHSO4,29 KSF,30 Sc(III) or La(III) triates.31 Nonetheless, several
of these catalysts contained some disadvantages like the
production of unwanted byproducts, expeditious catalyst deac-
tivation, and severe reaction conditions.32 There are still some
serious concerns with regards to green chemistry, hence
numerous efforts of replacing ordinary catalysts with new solid
acids that contain few advantages such as easily separable aer
the reaction, reusability, and presenting minimal toxicity are
applied worldwide.6,32 Thus, extensive exploitation of meso-
porous metal oxides could be a breakthrough for the develop-
ment of environmentally benign solid Lewis acid catalysts.33
The acetylation of hydroxyl groups is a fundamental process
that is frequently utilized in the transformation of organic
compounds as it provides an alternative route that is affordable
and efficient in organic synthesis.1,2 In organic synthesis, the
protection of functional groups is essential.3 Thus, through the
various transformations of functional groups, the protection of
such functional groups that results in the acetylated species is
highly important.4 The obtained acetylated species can oen be
utilized in a range of ways such as through protection of func-
tional groups in multistep synthetic processes.5,6 Generally, the
acetylated species are obtained from the most frequently
utilized reagent combination which uses an acid anhydride in
the presence of an acid7–15 or base.16–18 Furthermore, the acety-
lated groups are also more frequently found in pharmaceuti-
cals, solvents, cosmetics, fragrances and foodstuffs as well as in
plasticizers.19,20
Center for Synthesis and Catalysis, Department of Chemical Sciences, University of
Johannesburg, P.O. Box 524, Auckland Park, 2006 Johannesburg, South Africa.
E-mail: rmeijboom@uj.ac.za; Fax: +27 11 559 2819; Tel: +27 11 559 2367
† Electronic supplementary information (ESI) available: Scanning electron
microscopy images (a) 5%WO3–ZrO2, (b) 5%MoO3–SiO2, (c) 5%MoO3–ZrO2, and
(d) 5%WO3–SiO2 (Fig. S1). EDX spectrums of (a) 5%MoO3–SiO2, (b) 5%
MoO3–ZrO2, (c) 5%WO3–SiO2, and (d) 5%WO3–ZrO2 (Fig. S2). NH3-TPD of (a)
5%MoO3–ZrO2, (b) 5%WO3–SiO2, and (c) 5%WO3–ZrO2 (Fig. S3). TPR of the
synthesized solid Lewis acid catalysts (Fig. S4). Thermogravimetric analysis and
their derivatives: (a) 5%WO3–SiO2, (b) 5%WO3–ZrO2, and (c) 5%MoO3–ZrO2
(Fig. S5). See DOI: 10.1039/d1ra02134f
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More recently, investigations revealed that molybdenum or WO3 and Y ¼ ZrO2 or SiO2). The obtained mixture was placed
tungsten doped zirconium is an alternative efficient catalyst inside an oven set at 120 ꢁC for six hours and immediately
essential for reactions requiring vigorous acid site.34,35 Bearing calcined for 1 hour at 600 C under air. The generated brittle
ꢁ
this in mind, the development of well-organized effective green materials were crushed and marked 5%X–Y.
catalysts is still part of ongoing work.36,37 Herein, we wish to
report the use of 5%MoO3–SiO2 as an efficient solid acid catalyst
for the acetylation of alcohols (Scheme 1) under mild reaction
2.3. Instrumentation
conditions.
Prior to nitrogen sorption experiments (BET analysis) on
Micromeritics ASAP 2460 surface area and porosity analyzer at
ꢀ196 ꢁC, the acquired powderꢁsamples were degassed under
nitrogen gas for 8 hours at 90 C and further degassed under
2. Experimental
2.1. Materials
ꢁ
vacuum at 250 C for another 8 hours. A Rigaku MiniFlex-600
˚
diffractometer with Cu Ka radiation (l ¼ 1.5406 A) was
Ammonium metatungstate hydrate (H26N6O40W12$xH2O)
utilized at room temperature for carrying out analysis of wide
and low-angle powder X-ray diffraction (p-XRD). SDT Q600
thermogravimetric analyzer (TGA) was utilized to scrutinize the
thermal behavior of our prꢀe1pared catalysts under nitrogen ow,
($99.0%), ammonium molybdate tetrahydrate (H24Mo7N6O24
-
$4H2O) (81.0–83.0%), tetraethyl orthosilicate (C8H20O4Si)
($99.0%), zirconium(IV) oxynitrate hydrate (N2O7Zr$xH2O)
(99%), decane (C10H22) ($99%), dichloromethane (CH2Cl2)
($99.8%), nitric acid (HNO3) (70%), geraniol (C10H18O) (98%),
ethylene glycol (C2H6O2) (99.8%), 2-phenylethanol (C8H10O)
($99.0%), 3-phenyl-2-propyn-1-ol (C9H8O) (96%), and poly(-
ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene
glycol), nonyl acetate (C11H22O2) ($97.0%), pentyl acetate
(CH3COO(CH2)4CH3) (99.0%), 2-phenethyl acetate (CH3-
COOCH2CH2C6H5) ($99.0%), benzyl acetate (CH3COOCH2C6-
H5) ($99.0%), hexyl acetate (CH3COO(CH2)5CH3) (99.0%), sec-
butyl acetate (CH3CO2CH(CH3)C2H5) (99.0%), tert-butyl acetate
(CH3COOC(CH3)3) ($99.0%), and geranyl acetate (CH3CO2-
CH2CH]C(CH3)CH2CH2CH]C(CH3)2) ($97.0%) were all
purchased from Sigma Aldrich. Toluene (C6H5CH3) (99.8%) and
cyclohexane (C10H12) (98%) were purchased from Rochelle
Chemicals. Acetic anhydride [(CH3CO)2O] (97%) was purchased
from UniLAB. All chemicals were used as received.
ꢁ
ꢁ
with a range of 10 C min from 30 to 1000 C. Samples were
rstly carbon-coated using an Agar Turbo Carbon Coater and
then analyzed Tescan Vega 3LMH Scanning Electron Micros-
copy (SEM) for imaging, morphology and elemental mapping.
JEOL JEM-2100F electron microscope consisting of 200 kV
accelerating voltage was used to capture Transmission Electron
Microscopy (TEM) images of our synthesized catalysts for the
estimation of the particle sizes. The acid sites of the as
synthesized catalysts were veried by temperature programed
desorption method (TPD) using a Chemisorption Analyzer
Micromeritics AutoChem II 2920. The Carousel 12 plus reaction
station from Radleys TECH was utilized for monitoring the
stirring speed and the temperature of the reactions. For quali-
tative analysis, Shimadzu GC-MS equipped with the mass
selective detector and the capillary column was utilized for the
qualitative analysis. Shimadzu GC-2010 equipped with Restek-
800-356-1688 capillary column (30 m ꢂ 0.25 mm ꢂ 0.25 mm)
and ame ionization detector (FID) was utilized for quantitative
2.2. Catalyst synthesis
ꢁ
ꢁ
analysis. Temperatures of 250 C and 300 C were utilized for
FID and injection port respectively. For the separation process
during catalyst recycling, a NEYA 8 benchtop centrifuge was
successfully utilized.
The solid acid catalysts were synthesized by the sol–gel method
as described by Poyraz et al.,38 following the approach of
Corma.39 430 mmol of P123 and 0.02 mol of each metal
precursor were added into two separate 300 mL beaker, and
then 0.19 mol of n-butanol and 0.04 mol of HNO3 were added to
the mixtures. The mixtures were stirred at 25 ꢁC until trans-
parent gels were acquired. Two other mixtures of 0.001 mol of
ammonium molybdate and ammonium metatungstate were
separately prepared in 300 mL beakers. Thus, each salt was
added inside a stirring beaker which contained 2.5 g of Milli-Q
water plus 2.5 g of ethanol. As soon as the salt solution became
clear, it was transferred into the acquired clear gel under
vigorous stirring, and the mixture was further stirred vigorously
for 20 minutes. The amounts added of both molybdenum and
tungsten were 5 mol% (molX/molY ꢂ 100, where X ¼ MoO3 or
2.4. Catalytic evaluation
To a solution of alcohol (2 mmol) and acetic anhydride (3
mmol), 5%X–Y (X ¼ MoO3 or WO3, Y ¼ ZrO2 or SiO2) was added.
The reaction solution was stirred (400 rpm) at 35 ꢁC for 30
minutes and consecutively sampled aer every 5 minutes. Thus,
the reaction progress was monitored by GC. The controlled
experiments revealed that no reaction took place in the absence
of the catalyst and the tungsten promoter was inactive in this
reaction. The results showed that the reaction is favorable
towards MoO3 as the catalyst promoter. 5%MoO3–SiO2 was
revealed to be an ideally suited solid acid catalyst for the acet-
ylation of alcohols. For catalyst recyclability, the optimum
catalyst (30 mg) was transferred into a Falcon tube (500 mL) and
thoroughly washed three times with cyclohexane (15 mL). Aer
each wash, the solution was centrifuged at 4500 rpm for 15
minutes.
Scheme 1 Catalytic acetylation of alcohols with acetic anhydride.
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remarkable features like high surface areas, easily tunable,
chemical and thermal stability. Generally, materials with higher
surface areas turn to be more active and become favorable for
most reactions.41,42 Looking at 5%MoO3–Y catalysts on Table 1,
3. Results and discussions
3.1. Synthesis and characterization
Thermally stable solid Lewis acid catalysts were synthesized by
the sol–gel method described by Poyraz et al.38 Due to the
presence of nitrate ions, color changes such as molybdenum
solution turning dark yellow and tungsten solution becoming
opaque were observed. This is in agreement with the report in
the literature.40 The acquired parameters such as surface areas,
pore diameters, pore volumes, acidic sites, and crystallite sizes
are summarized in Table 1. Nitrogen sorption analysis
conrmed that the prepared materials were within mesoporous
materials range of 2–50 nm. Mesoporous metal oxides have
it was observed that SiO2 gave higher surface area (100.8 m2 gꢀ1
)
compared to ZrO2 (44.1 m2 gꢀ1). There is no clear correlation
between the surface area these materials possess and the acidic
sites on their surface.
As shown in Fig. 1(a), nitrogen sorption of the synthesized
solid Lewis acid catalysts aged at 600 ꢁC conveyed type IV
isotherms with hysteresis loop associated with capillary
condensation taking place in the mesopores, conrming the
mesoporosity of the as-synthesized materials.43,44 Fig. 1(b)
Table 1 BET, XRD and TPD characterization of the solid Lewis acid catalysts
Crystallite
size (nm)
Acidic sites
(a.u.)
Catalyst
ABET (m2 gꢀ1
)
Pv (cm3 gꢀ1
)
PD (nm)
5%MoO3–SiO2
5%MoO3–ZrO2
5%WO3–SiO2
5%WO3–ZrO2
101
44
23
0.29
0.13
0.02
0.07
11.6
11.7
3.7
104.9
14.5
8.1
0.05
0.03
0.04
0.06
22
12.8
37.7
Fig. 1 Characterization of the synthesized MMO's (a) BET N2-sorption isotherms, (b) BJH desorption pore volume distribution, (c) wide-angle p-
XRD, and (d) low-angle p-XRD.
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depicts the corresponding pore size distributions, acquired ascribed to tungsten clusters from the literature.46,47 Fig. S1†
from utilizing BJH calculation procedure. It can be observed shows the representative SEM images of the calcined meso-
that the MoO3-based catalysts have wide pore size distribution, porous metal oxides. Moreover, a closer inspection of the
while the WO3-based catalysts possess narrow pore size distri- synthesized materials revealed that they contained uneven
bution of uniform size. X-ray diffraction patterns of the surfaces with countless pores irregularly shaped. Thus, the EDX
synthesized solid Lewis acid catalysts calcined at 600 ꢁC are analysis conducted (Fig. S2†) conrmed that no impurities were
displayed in Fig. 1(c). Moreover, for MoO3 doping shaper peaks present in our synthesized mesoporous metal oxides catalysts.
were observed indicating non-crystalline characteristic of 5%
In most cases, the acidity of mesoporous metal oxides plays
MoO–SiO2 and 5%MoO3–ZrO2 catalysts. However, the WO3 an essential role in numerous reactions. Consequently, the
promoter revealed broad peaks because of the amorphous acidity of mesoporous metal oxides needs to be investigated.
nature of 5%WO3–SiO2 and 5%WO3–ZrO2 catalysts. As shown in The properties of solid acids also consist of strength, type, and
Fig. 1(d), all the synthesized mesoporous metal oxides catalysts a number of acid sites. Thus, NH3-TPD is an ideal method
except 5%MoO3–SiO2 exhibited diffraction peaks at low angles. which is utilized extensively for determining the active site on
The 5%MoO3–SiO2 depicted broad peak-like characteristic a catalyst.48,49 Fig. 3(a) depicts peak areas of 5%MoO3–SiO2. It
which was unclear due to low intensity indicating that the ob- was clearly observed that elevating the heating rate from 5 to
tained pores did not indicate the regular mesostructured.45
20 ꢁC minꢀ1 resulted in the increment of the peak areas, where
As illustrated in Fig. 2, TEM images provided some insights the peak area is equivalent to the active site. For all the
into mesopore structural features. These acquired images synthesized catalysts it was observed that the heating rates were
illustrated the arrangement of crystallite which clearly indicated directly proportional to the peak areas (see Fig. S3†). Thus, TPR
that pores were present. Thus, it was observed that upon using experiments were conducted to acquire a deep understanding
molybdenum as a promoter intensied the thermal stability of the synthesized mesoporous metal oxides.
and also enhanced the surface areas as shown in Table 1.
Fig. S4† shows the acquired proles which exhibit multiple
However, utilizing tungsten as a promoter resulted in the reduction peaks. Looking at molybdenum promoter, it was
amorphous materials which had dark spots that were previously previously reported that it had two peaks at 767 and 997 ꢁC,
Fig. 2 TEM images of (a) 5%MoO3–SiO2, (b) 5%WO3–ZrO2, (c) 5%WO3–SiO2 and (d) 5%MoO3–ZrO2.
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Fig. 3 The depiction of 5%MoO3–SiO2 via (a) NH3-TPD and (b) thermogravimetric analysis and its derivative.
respectively.49 Furthermore, it was observed that upon instead of three possible peaks which might be due to the
promoting silica and zirconium, the reduction peaks migrated limitation of temperature range on the instrument used.
to lower temperatures, demonstrating excessive reducibility.50 Fig. 3(b) depicts the thermogravimetric analysis curve of 5%
Generally, tungsten undergoes three reduction steps as previ- MoO3–SiO2 and its derivation curve. For this optimum catalyst,
ously reported.51 The obtained results of 5%WO3–Y only it was observed that three decomposition steps took place. The
revealed two reduction peaks shied to lower temperatures dehydroxylation52 from 50 up to 73 C, followed by the weight
ꢁ
Fig. 4 (a) The effect of temperature upon the acetylation of 2-phenylethanol, (b) the influence of stirring speed at 35 ꢁC, and (c) performance of
different catalysts at 35 ꢁC and 400 rpm. The reaction conditions were: acetic anhydride ¼ 3 mmol, toluene ¼ 6 mL, decane ¼ 0.132 mmol, 2-
phenylethanol ¼ 2 mmol, and catalyst amount ¼ 25 mg.
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loss at 734 ꢁC, and the third decomposition at 948 ꢁC corre- whereby toluene was utilized as a solvent. Thus, certain
sponding to the detachment of oxygen molecules.53 The total parameters such as temperatures, stirring speeds, solvents,
weight loss observed for this specic catalyst was 23 wt%. The substrates, and catalyst amount were intensely investigated in
rest of the synthesized solid Lewis acid catalysts are shown in order to enhance the activity and the producibility of the desired
Fig. S5† with 5%MoO3–ZrO2 exhibiting the average weight loss products. Fig. 4(a) demonstrate the impact of distinct temper-
of 2.75 wt%, 5%WO3–SiO2 revealing the average weight loss of atures on the acetylation of 2-phenylethanol. It was clearly
24 wt%, and lastly, 5%WO3–ZrO2 depicting the average weight illustrated that the reaction temperature is directly proportional
loss of about 18.5 wt%.
to the activity. Thus, aer establishing the optimum tempera-
ture of 35 C, the stirring speed was also investigated as illus-
ꢁ
trated in Fig. 4(b). It was clearly observed that the diffusion zone
whereby the reaction was independent of the stirring speed
appeared aer 400 rpm, hence we deduced that the optimum
stirring speed for further investigation was 400 rpm. Fig. 4(c)
depicts the activity of different catalysts with 5%MoO3–SiO2
being the most favourable catalyst for the acetylation of 2-phe-
nylethanol. This is a result of the high surface area this catalyst
possess coupled with the fairly acceptable acidity, see Table 1.
No activity was observed upon using 5%WO3–Y catalysts, indi-
cating that the tungsten promoter was inactive towards this
specic reaction. The reaction was also performed with similar
reaction parameters to investigate the formation of the product
in the absence of the catalyst and it was observed that no traces
3.2. Optimization of the catalytic variables
Generally, the exploitation of mesoporous metal oxides is
considered as one of the promising developments of the desired
solid Lewis acid catalysts which are environmentally friendly
and suitable for a variety of sustainable chemistry.33 Herein, we
delineate a mild, convenient and efficient approach for the
acetylation of various alcohols that is typically performed
utilizing acetic anhydride in the presence of solid Lewis acid
catalysts. To maximize the desired yields, the reaction condi-
tions were rstly optimized. We investigated the capability of
5%MoO3–SiO2 in the acetylation of 2-phenylethanol, in which
an insignicant activity was acquired at room temperature,
Table 2 Comparison of the effectiveness of 5%MoO3–SiO2 with other catalysts for the acetylation of 2-phenylethanola
Catalyst
Temp. (ꢁC)
Time (min)
Yield (%)
Solvent
Ref.
5%MoO3–SiO2
5%MoO3–ZrO2
5%WO3–SiO2
5%WO3–ZrO2
Catalyst-free
35
35
35
35
35
25
25
25
25
30
30
30
30
30
86
40
—
—
—
95
90
72
90
Toluene
Toluene
Toluene
Toluene
TW
TW
TW
TW
TW
54
55
56
57
Toluene
H
14[NaP5W30O110
]
90
Solvent-free
Solvent-free
Toluene
Cp2ZrCl2
Fluorous distannoxane
1,3-Dibromo-5,5-dimethylhydantoin
840
360
1800
Dichloromethane
a
TW: this work.
Fig. 5 (a) The investigation of an effective solvent, (b) deducing an optimum substrate for further investigation. The reaction conditions were:
solvents ¼ 6 mL, decane ¼ 0.132 mmol, acetic anhydride ¼ 3 mmol, substrates ¼ 2 mmol, stirring speed ¼ 400 rpm, and catalyst amount ¼
25 mg.
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of the product were acquired from the GC analysis. Further- appropriate solvent is crucial for the efficiency of the study
more, we compared these obtained results with literature as reaction. Consequently, cyclohexane which afforded the yield of
noted in Table 2. It follows that most reported catalysts such as 91.8% was deduced as the optimum solvent for further inves-
uorous distannoxane56 required longer reaction time for the tigation. Later, a variety of substrates was investigated under the
acetylation of 2-phenylethanol and gave lower yield. Thus, 5% same reaction conditions using cyclohexane as solvent. It was
MoO3–SiO2 afforded great catalytic improvement on the acety- observed that the best yield (91.8%) was obtained when 2-phe-
lation of alcohols under mild reaction conditions, hence it was nylethanol was used as substrate, followed by 3-phenyl-2-
considered as an efficient Lewis acid catalyst. Interestingly, 86% propyn-1-ol with a yield of 90.6% (see Fig. 5(b) and Table 3).
of 2-phenylethanol was converted within 30 minutes under mild Furthermore, it is noteworthy that secondary and tertiary alco-
reaction condition.
hols such as 2-butanol and tert-butanol (entry 9 and 10, Table 3)
The ability of 5%MoO3–SiO2 catalyst was investigated on can be acetylated indicating that the catalyst was not only
several solvents as illustrated in Fig. 5(a). Cyclohexane was limited to primary alcohols. Additionally, we investigated the
revealed to be the best solvent, followed by toluene and impact of the mass transfer limitation whereby a variety of
dichloromethane, while the use of N,N-dimethylacetamide as catalyst amounts were utilized as illustrated in Fig. 6(a). The
solvent did not display any conversion. Thus, the choice of an data presented in Fig. 6(a), the change in concentration of the
Table 3 Acetylation of selected alcohols with acetic anhydride over 5%MoO3–SiO2 under mild reaction conditions
Yield
(%)
Entrya
Substrate
Productb
1
91.8
2
90.6
3
4
88.6
76.4
5
50.8
6
7
8
46.4
—
—
86.9
9
65.4
31.7
10
a
Reaction conditions: substrate ¼ 2 mmol, Ac2O ¼ 3 mmol, cyclohexane ¼ 0.056 mol, 5%MoO3–SiO2 ¼ 25 mg, temperature ¼ 35 ꢁC, time ¼ 30 min.
Products: they were all conrmed with GC-MS and the standards were used as well.
b
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Fig. 6 (a) Various amount of optimum catalyst, (b) the study of mass transfer limits. The reaction conditions were: cyclohexane ¼ 6 mL, decane ¼
0.132 mmol, acetic anhydride ¼ 3 mmol, 2-phenylethanol ¼ 2 mmol, stirring speed ¼ 400 rpm, and catalyst amount ¼ 25 mg.
Fig. 7 (a) The catalytic recycle test of the optimum catalyst, (b) TEM image and (c) the normalized by mass conversion. Reaction conditions: 6 mL
of cyclohexane, temperature ¼ 35 ꢁC, decane ¼ 0.132 mmol, acetic anhydride ¼ 3 mmol, 2-phenylethanol ¼ 2 mmol, stirring speed ¼ 400 rpm.
substrate with time was used to study the kinetics of the reac- normalized by the amount of catalyst used in each reaction
tion. It is clear from Fig. 6(b) that the rate of conversion of the cycle, and it was observed that the catalyst was very stable with
substrate increases with increase in the catalysts surface area only minor loss in activity (Fig. 7(c)). TEM analysis was per-
available in the reactor. From these ndings, as depicted in formed aer the 5th catalytic cycle and it was observed that the
Fig. 6(b) it can be deduced that the mass transport limitation mesostructure of the catalyst was not affected as illustrated in
did not affect any conversion. The initial rate was directly Fig. 7(b). The stability of the catalysts could be due to the stable
proportional to the catalyst amount, and the linear increasing structure of the as-synthesized materials, and this feature could
conrmed the absence of the mass transfer limitation.
make them good catalysts for industrial application.
3.3. Catalyst recyclability
4. Conclusion
Moreover, aer deducing the kinetic zone, it follows that we
took 30 mg of the catalyst which was within the kinetic zone for In conclusion, highly stable meso-structured solid acid nano-
the catalyst recycling tests. Fig. 7 illustrates the reusability of 5% catalysts such as 5%MoO3–SiO2 can be synthesized using a sol–
MoO3–SiO2 on the acetylation of 2-phenylethanol up to the h gel method. We have demonstrated that 5%MoO3–SiO2 is
run from Fig. 7(a), it can be observed that the catalyst was a exible and very effective inexpensive heterogeneous catalyst
recyclable but the activity decreases as the number of reaction for the acetylation of alcohols with acetic anhydride. The results
cycles increases. This is due to some loss of the catalyst during show that the as-synthesized catalyst was highly selective
the separation process between each recycling test. However, to towards the desired products. The notable advantages of this
conrm the stability of the catalyst, the conversion was catalyst are the producibility of excellent yields, shorter reaction
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times, and lower reaction temperatures. Also, the catalyst was 19 H.-J. Yoon, S.-M. Lee, J.-H. Kim, H.-J. Cho, J.-W. Choi,
revealed to be environmentally friendly and could be reused
a number of times without loss of activity and selectivity.
S.-H. Lee and Y.-S. Lee, Tetrahedron Lett., 2008, 49, 3165–
3171.
20 S. A. Taghavi, M. Moghadam, I. Mohammadpoor-Baltork,
S. Tangestaninejad, V. Mirkhani and A. R. Khosropour,
Inorg. Chim. Acta, 2011, 377, 159–164.
Conflicts of interest
21 A. Kamal, M. N. A. Khan, K. S. Reddy, Y. V. V. Srikanth and
T. Krishnaji, Tetrahedron Lett., 2007, 48, 3813–3818.
22 V. Mirkhani, S. Tangestaninejad, M. Moghadam,
B. Yadollahi and L. Alipanah, Monatsh. Chem., 2004, 135,
1257–1263.
23 F. Rajabi, Tetrahedron Lett., 2009, 50, 395–397.
24 F. Shirini, M. A. Zolgol and M. Abedini, Monatsh. Chem.,
2004, 135, 279–282.
25 J. R. Satam and R. V. Jayaram, Catal. Commun., 2008, 9,
2365–2370.
26 P. Laszlo, Acc. Chem. Res., 1986, 19, 121–127.
27 F. Bigi, S. Carloni, R. Maggi, C. Muchetti and G. Sartori, J.
Org. Chem., 1997, 62, 7024–7027.
The authors declare no competing nancial interest.
Acknowledgements
We acknowledge the nancial support from South African NRF
(Grant specic number 117997 and 111710), Analytical division
(spectrum), Sasol R&D and research fund from the University of
Johannesburg. We would also like to acknowledge Mr D. Hariss
and Dr R. Meyer from Shimadzu South Africa for the usage of
their equipment throughout this study.
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