Template-free synthesis of highly selective amorphous aluminosilicate catalyst for toluene alkylation

Article abstract of DOI:10.1016/j.apcata.2018.02.029

In this paper, we report a simple template-free synthetic method that produces amorphous, nanoporous aluminosilicate and highly selective catalyst for toluene alkylation. The structure and composition of the material and its corresponding control material are characterized by various techniques. The material shows excellent catalytic selectivity for alkylation of toluene by tert-butyl alcohol in temperatures between 50–150 °C under atmospheric pressure, producing exclusively 4-tert-butlytoluene (ca. 99%).

Full text of DOI:10.1016/j.apcata.2018.02.029

Accepted Manuscript
Title: Template-free synthesis of highly selective amorphous
aluminosilicate catalyst for toluene alkylation
Authors: Sreenivasulu Peta, Tao Zhang, Viktor Dubovoy,
Katherine Koh, Maocong Hu, Xianqin Wang, Tewodros Asefa
PII:
S0926-860X(18)30096-6
DOI:
Reference:
https://doi.org/10.1016/j.apcata.2018.02.029
APCATA 16569
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
4-11-2017
10-2-2018
23-2-2018
Please cite this article as: Peta S, Zhang T, Dubovoy V, Koh K, Hu M,
Wang X, Asefa T, Template-free synthesis of highly selective amorphous
aluminosilicate catalyst for toluene alkylation, Applied Catalysis A, General (2010),
https://doi.org/10.1016/j.apcata.2018.02.029
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Template-free synthesis of highly selective amorphous aluminosilicate
catalyst for toluene alkylation
Sreenivasulu Peta,a,b,c Tao Zhang,b Viktor Dubovoy,a Katherine Koh,a Maocong Hu,d Xianqin
Wang,d Tewodros Asefaa,b,c,*
a Department of Chemistry and Chemical Biology, Rutgers, The State University of New Jersey, 610
Taylor Road, Piscataway, New Jersey 08854, USA
b
Department of Chemical and Biochemical Engineering, Rutgers, The State University of New
Jersey, 98 Brett Road, Piscataway, New Jersey 08854, USA
c Institute for Advanced Materials, Devices and Nanotechnology, Rutgers, The State University of New
Jersey, 607 Taylor Road, Piscataway, New Jersey 08854, USA
d
Department of Chemical, Biological and Pharmaceutical Engineering, New Jersey Institute of
Technology, University Heights, Newark, NJ 07102, USA
* Corresponding author: E-mail: tasefa@chem.rutgers.edu (T. Asefa)
Graphical abstract
1

Highlights
 A simple template-free synthetic method to make aluminosilicate is reported.
 Amorphous, nanoporous aluminosilicate catalyst is synthesized.
 The catalyst shows high selectivity for toluene alkylation.
 The material catalyzes alkylation of toluene by tert-butyl alcohol under mild condition.
 The catalyst gives exclusively 4-tert-butlytoluene (ca. 99%) as product.
Abstract
In this paper, we report a simple template-free synthetic method that produces amorphous,
nanoporous aluminosilicate and highly selective catalyst for toluene alkylation. The structure and
composition of the material and its corresponding control material are characterized by various
techniques. The material shows excellent catalytic selectivity for alkylation of toluene by tert-butyl
alcohol in temperatures between 50-150 °C under atmospheric pressure, producing exclusively 4-
tert-butlytoluene (ca. 99%).
Keywords: Aluminosilicate; Toluene Alkylation; Alkylation reaction; 4-Tert-butlytoluene; Selective
catalyst
1. Introduction
Aluminosilicates have long been used as catalysts in many types of reactions in the chemical
industries and refineries to produce various products [1]. Aluminosilicates have also been recognized
2

as having potential applications as adsorbents/membranes for scavenging various pollutants from the
environment and CO2 from flue gases [2]. Aluminosilicates find these diverse applications due to
their high surface areas, large pore volumes, uniform microporous channels, acidic sites, and
remarkable hydrothermal and thermal stabilities [3-5]. For aluminosilicates to serve efficiently for
many of these applications, particularly for catalysis of aromatic alkylation, isomerization and related
reactions, which are widely carried out in petrochemical refinery processes, they need to have
microporous structures and be shape/size selective. Therefore, many aluminosilicates are made using
small molecules as templates [6]. However, the microporosity of aluminosilicates can sometimes
compromise their performances in these applications, especially during catalytic reactions involving
bulkier reactants, intermediates, and products. Consequently, their mesoporous counterparts—
materials that allow better mass transport during catalysis—are sometimes sought and preferable [7,
8]. Hence, in recent years, there have been a great deal of interest to develop mesoporous and micro-
mesoporous aluminosilicate materials, whose pores can let bigger reactants and products diffuse in
and out of the catalytically active sites better and help reactants transform to products faster during
various reactions [9-11].
Traditional method of synthesis of aluminosilicates is based on co-precipitation of silica
precursors such as Na2SiO2 and aluminium salts and alkoxides. While this method has been used to
produce various catalysts, it often involves costly and sometimes toxic, organic templates and energy-
intensive solvothermal, microwave heating, and hydrothermal synthetic conditions [12, 13].
Furthermore, the materials need to be calcined at high temperature to remove the organic templates
used to create the pores in the materials, which however produces harmful gaseous substances. Thus,
facile, scalable synthetic methods that can produce mesoporous aluminosilicates are still needed. In
3

particular, synthetic methods that can produce aluminosilicates with pores that allow reactants to
undergo faster mass diffusion and catalytic transformations in the materials are desired [14, 15]. Thus,
such materials are of huge importance for catalysis of many industrially relevant reactions.
4-tert-Butyltoluene (4-TBT) is a commercially important aromatic compound that is useful for
making substances such as 4-tert-butylbenzoic acid, 4-tert-butylbenzaldehyde, etc., compounds that
serve as precursors for various pharmaceuticals, alkylated resins and fragrances and as polymerization
regulators for polyesters [16]. Currently, 4-TBT is produced in the chemical industry using
homogeneous catalysts such as phosphoric acid, sulphuric acid, and boron triflouride. However, these
catalysts have drawbacks associated with their extreme safety and handling requirements and
corrosive wastes. To overcome these issues, new synthetic methods that can involve no/less toxic,
costly organic templates and produce recyclable mesoporous aluminosilicate catalysts with better
mass diffusion properties for reactants and products alike are needed [16].
To this end, herein we report a simple template-free, hydrothermal synthetic method that yields
amorphous, mesoporous aluminosilicate material that exhibits excellent catalytic selectivity toward
toluene alkylation reaction with tert-butyl alcohol. The material is found to catalyze the reaction,
selectivity yielding (99 %) 4-TBT as product. To the best of our knowledge, this degree (99 %)
selectivity to a 4-TBT product by this reaction has never previously been achieved with laboratory-
synthesized aluminosilicate catalysts.
2. Experimental Section
2.1. Chemicals and Reagents
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Aluminium isopropoxide, tetraethyl orthosilicate (TEOS), tetrapropylammonium bromide (TPABr),
2-propanol, toluene, and tert-butanol (TBA) were purchased from Sigma-Aldrich, and they were all
used without further purification.
2.2. Synthesis of Mesoporous Aluminosilicates
In typical synthesis of the mesoporous aluminosilicate catalyst, 0.4 g (2 mmol) of aluminium
isopropoxide was mixed with 20 mL of 2-propanol, and the mixture (without any template) was then
stirred overnight at 40 °C. Into the resulting solution was added 10.4 g (50 mmol) of TEOS and 25
mL water. The solution was transferred into an autoclave, sealed, and heated at 130 °C for 3 days. It
was then filtered, and the solid product was separated and washed with copious amount of ethanol.
After letting it to dry in oven at 100 °C, it was calcined at 550 °C for 5 h. The resulting solid material
was denoted as AS-NT (to represent “aluminosilicate synthesized using no template”). By using 0.2
g TPABr as template under otherwise similar procedure, aluminosilicate, which was denoted as “AS-
T” to represent “aluminosilicate synthesized using template”, was produced. This material was used
for comparative studies.
2.3. Catalytic Toluene Butylation
In a typical procedure for catalytic toluene butylation reaction, 1 mL of toluene was mixed with
2 mol of TBA and 0.2 g of catalyst. The mixture was placed in a 25 mL Teflon-lined Parr reactor and
sealed tightly. The reaction was conducted at 50, 100 or 150 °C for 9 h. At the end of the reaction,
the catalyst was separated by filtration, and the products were analyzed with gas chromatography
(GC, Agilent 6850) instrument equipped with HP-1 capillary column (30 m length, 0.32 mm id, 0.25
5

μm film thickness) and flame ionization detector (FID). The products were identified using GC-MS
5890 Series II equipped with 5972 Series mass selective detector. After completion of the reaction,
the catalyst was recovered from the reaction mixture via centrifugation and washed thoroughly with
acetone, and its reusability was tested in multiple cycles.
3. Results and Discussions
The aluminosilicate materials obtained above were both characterized by various analytical
techniques. Elemental analysis as well as elemental mapping on the materials conducted using energy
dispersive X-ray (EDX) spectroscopy (Figure S1 in Supporting Information) showed that both
materials had Al, O and Si atoms. The morphologies of both AS-NT and AS-T materials were
analyzed using field emission scanning electron microscopy (FE-SEM) (Figures 1a and 1b). The
images showed that both materials possessed sub-micron-sized particles and nanoporous structures.
The transmission electron microscopy (TEM) images (Figures 1c and 1d) showed that both materials
had some pores that appeared to be due to the void spaces created in between the particles. Further
characterization of the materials by wide-angle X-ray diffraction (XRD) indicated that the
frameworks in the materials were amorphous (Figure 2a).
6

Figure 1. (a, b) FE-SEM of AS-T and AS-NT, respectively, and (c, d) TEM images of AS-T and AS-
NT, respectively.
Nitrogen adsorption/desorption measurements were carried out for both AS-T and AS-NT
materials, and using the data the corresponding isotherms were plotted (Figure 2b). The isotherm for
AS-T appeared to be Type II/IV with H3-type hysteresis loop, and the one for AS-NT seemed to be
Type II/III with H3-type hysteresis loop. The isotherms of both materials also indicated the presence
of monolayer-multilayer type adsorption and capillary condensation processes at higher relative
pressures. The curves qualitatively revealed the presence of, at least, some mesoporous and
macroporous structures in both materials. Pore size analysis showed that both AS-T and AS-NT had
mesoporosity, with their average diameters centered at ca. 31 nm and ca. 13 nm, respectively (see
7

Figure S1 in Supporting Information). The large mesopores in both materials may have to be due to
the inter-particle pores formed from the close packing of the particles in the materials, as discussed
earlier. Interestingly, the material made without a template showed relatively more monodisperse
pores and greater porosity than the one made using a template. The total pore volumes of AS-T and
AS-NT were found to be 0.66 cm3 g−1 and 0.92 cm3 g−1, respectively, and their corresponding BET
surface areas were found to be 100 m2 g-1 and 323 m2 g-1, respectively (Table S1, in Supporting
Information section).
!
2-Theta (deg.)
Figure 2. (a) Wide angle XRD patterns and (b) N2 adsorption-desorption isotherms of AS-T and AS-
NT materials.
The large difference in total pore volume, pore diameter and surface area between the two
materials seemed to be due to the differences in size and morphology of their particles and the types
of inter-particle void spaces the particles generated. Based on the TEM image in Figure 1c, the bigger
and different-sized particles in AS-T appeared to have packed in random manner, giving different
pore sizes (Figure S2). This is most likely related to TPABr template and the possibility that TPA+
8

cations interact differently with the silicate and the aluminate anions formed from the hydrolysis of
TEOS and aluminium isopropoxide, respectively, during the self-assembly process. On the other
hand, the smaller particles in AS-NT appeared to have packed together more closely, forming smaller
and more uniform inter-particle void spaces (Figure 1d). The higher surface area and pore volume in
AS-NT could be the result of the formation of more pores from the necking of many smaller sized
particles, as can be seen in the TEM image in Figure 1d.
The acidity of the materials measured by TPD (Figure 3) indicated the presence of two NH3
desorption peaks in both materials: a broad peak centered at ca. 100 °C and a second one at ca. 400
°C corresponding to weak and strong acid sites, respectively. The intensity of the strong acid peak is
relatively higher in the case of AS-T, which indicates the presence of strong acid sites via the
interaction between the template with Al and Si moieties in this material. The nature of acidity was
further characterized using FT-IR spectroscopy of pyridine that was adsorbed on the materials (Figure
4). The IR band between 1540–1548 cm -1 and that between 1440–1460 cm-1 in aluminosilicates are
commonly related to Brønsted (B) and Lewis (L) acid sites, respectively. The IR spectra given in
Figure 4 indicated the presence of both Brønsted and Lewis acidic sites in both AS-T and AS-NT
materials. Both Brønsted (at ~1540) and Lewis (~1440) acid peaks were higher in intensity in the
case of AS-T compared with those in AS-NT. A band at ~1490 related to both Brønsted and Lewis
acid sites [17] was also observed in both the samples.
9

Figure 3. TPD spectra of the AS-T and AS-NT materials.
Figure 4. FT-IR spectra of pyridine adsorbed on AS-T and AS-NT materials.
The catalytic properties of the two materials toward toluene alkylation were then investigated.
During acid-catalyzed alkylation of toluene with TBA using a solid acid catalyst, the reaction is
generally known to take place through the formation of a carbocation intermediate from the alcohol,
10

followed by the electrophilic substitution of a proton in the phenyl ring of toluene by the carbocation,
as in Scheme S1 in Supporting Information. So, the reaction has the potential to form at least three
possible products: 4-TBT, 3-tert-butyltoluene (3-TBT), and 2-tert-butyltoluene (2-TBT) [18, 19]. The
relative ratio of these three products often depends on the types of catalysts and their catalytic
properties as well as the specific reaction conditions employed for the reaction.
In our experiment, the reaction was conducted using AS-NT and AS-T as catalysts without any
solvent under atmospheric pressure and at different temperatures, namely 50, 100, or 150 °C. The
results, compiled in Table 1, indicated that, at 150 °C, AS-T catalyzed the reaction with ~32 %
conversion of toluene and 99% selectivity to 4-TBT product whereas AS-NT catalyzed the reaction
with ~24 % conversion of toluene and with 99% of 4-TBT product. AS-T gave a higher conversion
of toluene than AS-NT most likely because it has a higher amount of Brønsted and Lewis acid sites
[16a]. It is worth emphasizing though that both AS-NT and AS-T gave 99% selectivity to 4-TBT
despite their differences in catalytic conversion values; this is most likely because they both have
optimal amount of Brønsted and Lewis acids [16a]. Furthermore, raising the reaction temperature
from 50 to 150 °C did not alter the selectivity, except making both materials to give slightly higher
catalytic conversions (Figure S3 in Supporting Information). In fact, both materials gave only 4-TBT
as product under all the reaction conditions in which their catalytic properties were evaluated in this
work (Figure S4 in Supporting Information). The possible reasons as to why the catalysts reported
herein favored this product are discussed further below.
It is known that acid-catalyzed alkylation of toluene by TBA goes through the dehydroxylation
of TBA, whose carbocation product can be in equilibrium with isobutylene [20]. The carbocation
subsequently alkylates toluene through electrophilic substitution. Thus, the mechanism by which
11

toluene alkylation proceeds over the catalytically active aluminosilicate materials is likely to be the
same as the one illustrated in Scheme S1 in Supporting Information.
Table 1. Catalytic conversion and selectivity of AS-T and AS-NT for toluene butylation with t-
butanol (TBA) after 9 h-long reaction.
Catalyst
Conversion of Toluene (%)
Product Selectivity to 4-TBT (%)
AS-Ta
AS-NTa
AS-Tb
28
22
29
22
32
24
29
21
~99
~99
~99
~99
~99
~99
~99
~99
AS-NTb
AS-Tc
AS-NTc
AS-Td
AS-NTd
a T = 50 °C, Toluene = 1 mL, t-Butyl alcohol (TBA) = 2 mL, and Catalyst = 0.2 g; b T = 100 °C, Toluene = 1 mL,
TBA = 2 mL, and Catalyst = 0.2 g; c T = 150 °C, Toluene = 1 mL, t-Butyl alcohol = 2 mL, and Catalyst = 0.2 g.
d Used catalyst, recovered 9 h after the reaction and re-tested at T = 150 °C, Toluene = 1 mL, t-Butyl alcohol = 2
mL, and Catalyst = 0.2 g.
Notably, the selectivity of the aluminosilicate catalysts, especially the one synthesized without
using any template, to give only one product in the toluene alkylation reaction, is interesting. For this
reaction as well as other related ones (e.g., disproportionation of toluene to xylenes and benzene)
catalyzed by most aluminosilicate catalysts possessing acidic sites, the types of products forming
from reaction are often dependent on whether or not the isomerization of some of products within the
12

catalysts is possible. This, in turn, can be affected by various structural factors present in the catalyst,
such as the pore sizes of the catalyst, the diffusion coefficient of reactants, intermediates and products
within the pores of the catalysts and their thermodynamic stability, the reaction conditions, the acidity
of the active sites of catalysts, and possible coke deposition [21]. For example, shorter reaction time
during toluene alkylation has been reprted to favor 4-tert-alkylated products because isomerization
of these products to 2- and 3-tert-alkylated isomers in the pores of the catalysts is unlikely [22].
Hence, pore narrowing within aluminosilicate material by silylation of the pore walls using
tetraalkoxysilanes or other agents is sometimes carried out to favor the selectivity of the reactions to
4-tert-alkylated products [23]. The external surface area and the crystal size of the catalyst have also
been reported to favor the selectivity of the reaction to 4-tert-alkylated products [24]. This is because
an increase in the size of the catalyst particles reduces the total external surface area of the catalyst
and limits the number of available acidic sites on the external surface of the catalyst; this in turn
impedes the transformation of 4-tert-alkylated toluene to 2- or 3-tert-alkylated toluene product via
isomerization [25]. In our case, the major factor appears to be the presence of optimal density of
Brønsted and Lewis acid sites in the catalyst, rather than the selectivity of the pores. The right relative
amount of Brønsted and Lewis acid sites has been reported to favor TBT product [16a].
Finally, the recyclability and reusability of the catalysts were evaluated by recovering them after
the end of each reaction. Both AS-T and AS-NT catalysts were found to maintain their catalytic
selectivity and give 99% of 4-TBT as product (Tables S2 in Supporting Information).
4. Conclusions
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In conclusion, we have shown a simple template-free synthetic method involving only heat
treatment at moderate temperature that can give amorphous nanoporous aluminosilicate that exhibits
high catalytic selectivity for toluene alkylation with tert-butanol almost exclusively giving 4-TBT as
product. The reported synthetic method is simple and potentially applicable for making other types
porous titanium silicate and metal oxide catalysts. The finding of high catalytic selectivity observed
for the catalyst suggests that the reported material can have strong industrial relevance, given the
importance of selective toluene alkylation reaction in the chemical industry.
Acknowledgements
T. A. gratefully acknowledges the financial assistance of the National Science Foundation (NSF)
under Grant No.: NSF DMR-1508611.
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