Development of a novel mesoporous catalyst UDCaT-6: Kinetics of synthesis of tert-amyl methyl ether (TAME) from tert-amyl alcohol and methanol

Article abstract of DOI:10.1021/jp0482060

UDCaT-6, a novel active mesoporous and stable catalyst, was synthesized by generating in situ nanosized acidic centers of chlorosulfonic acid treated zirconia in the pores of highly ordered hexagonal mesoporous silica (HMS). For the first time, we have used chlorosulfonic acid as a source of sulfating agent to treat zirconia in pores of the HMS. The catalyst is characterized by XRD, FTIR, EDAX, SEM, and BET surface area and pore size analysis, and probe reactions. The structural integrity of HMS is maintained in UDCaT-6. The activity and stability of UDCaT-6 was tested in liquid phase alkylation of toluene with benzyl chloride and vapor phase synthesis of tert-amyl methyl ether (TAME) from ferf-amyl alcohol (TAA) and methanol where corrosive acid HCl and water are generated as biproducts. A complete theoretical and experimental analysis is presented and kinetics are evaluated. The model explains the experimental data very well.

Full text of DOI:10.1021/jp0482060

J. Phys. Chem. A 2004, 108, 9557-9566
9557
Development of a Novel Mesoporous Catalyst UDCaT-6: Kinetics of Synthesis of tert-Amyl
Methyl Ether (TAME) from tert-Amyl Alcohol and Methanol
Ganapati D. Yadav* and Ambareesh D. Murkute
Department of Chemical Engineering, UniVersity Institute of Chemical Technology (UICT), UniVersity of
Mumbai, Matunga, Mumbai - 400 019, India
ReceiVed: April 24, 2004; In Final Form: August 10, 2004
UDCaT-6, a novel active mesoporous and stable catalyst, was synthesized by generating in situ nanosized
acidic centers of chlorosulfonic acid treated zirconia in the pores of highly ordered hexagonal mesoporous
silica (HMS). For the first time, we have used chlorosulfonic acid as a source of sulfating agent to treat
zirconia in pores of the HMS. The catalyst is characterized by XRD, FTIR, EDAX, SEM, and BET surface
area and pore size analysis, and probe reactions. The structural integrity of HMS is maintained in UDCaT-6.
The activity and stability of UDCaT-6 was tested in liquid phase alkylation of toluene with benzyl chloride
and vapor phase synthesis of tert-amyl methyl ether (TAME) from tert-amyl alcohol (TAA) and methanol
where corrosive acid HCl and water are generated as biproducts. A complete theoretical and experimental
analysis is presented and kinetics are evaluated. The model explains the experimental data very well.
1. Introduction
number of reactions. However, the surface area of UDCaT-5
(∼83 m2/g) is lower than that of S-ZrO2 (∼100 m2/g). We
thought to synthesize a mesoporous material possessing higher
acidity and greater surface area. Thus, the current work deals
with synthesis and characterization of a novel material, named
UDCaT-6, and its application in the synthesis of tert-amyl
methyl ether (TAME), a fuel oxygenate, from tert-amyl alcohol
and methanol. This work covers studies in kinetics of the
reaction.
Production of oxygenated fuel additives such as methyl tert-
butyl ether (MTBE), ethyl tert-butyl ether (ETBE), and tert-
amyl methyl ether (TAME) grew due to their role in making
ecofriendly and cleaner burning gasolines. However, MTBE,
the fastest growing petrochemical during 1980s, received a
setback due to its leakage into groundwater and particularly due
to the strong lobbying of ethanol producers in the United States,
which ultimately resulted in a ban on its use as a fuel additive
in several Western countries. The focus is now shifted to other
additives such as TAME and ETBE. The blending Reid vapor
pressure (RVP) of TAME (1 psi) is lower than that of MTBE
(8 psi) and ETBE (4 psi). TAME has an octane number of 106,
which is closer to MTBE’s 109. Thus, TAME shows a lot of
promise as an alternative to MTBE. The synthesis of TAME
has been attempted by several researchers by using a variety of
solid acid catalysts.
TAME is synthesized in the liquid phase from isoamylenes
and methanol over strongly acidic macroporous ion-exchange
resin catalysts in continuous and semibatch mechanically
agitated slurry reactors. A single site mechanism for isomer-
ization and a dual site mechanism for etherification of isoam-
ylenes with methanol over ion-exchange fiber SMOPEX-101
have been proposed.4 Oost and Hoffmann5 determined the
microkinetics of the synthesis of TAME from methanol and a
mixture of isoamylenes over Lewatit SPC 118, a macroporous
sulfonic acid resin in a continuous flow recycle reactor in the
range 50-70 °C at 1.6 MPa. Reactive distillation combined with
evaporation has been used to synthesize MTBE and ETBE by
a condensation of methanol and ethanol, respectively, with tert-
The limitations of microporous zeolites as catalysts in a
variety of reactions involving bulky reactants are well docu-
mented. During the past decade, a plethora of work has dealt
with the synthesis of mesoporous materials with the incorpora-
tion of different functionalities. Among many solid acids other
than zeolites, incorporation of superacidity into metal oxides
has received considerable attention. Sulfated zirconia is the most
extensively studied material due to its superacidic nature.1
However, some of the major problems associated with sulfated
zirconia in the bulk form are its low efficiency due to low
surface area (∼100 m2/g) and its rapid deactivation and relatively
poor stability in reactions where water is generated as a
coproduct. As an alternative, we synthesized a novel highly
acidic mesoporous solid catalyst named UDCaT-1, which is a
synergistic combination of S-ZrO2 and hexagonal mesoporous
silica (HMS).2 (The acronym UDCaT stands for UniVersity
Department of Chemical Technology (UDCT), by which our
institute was known until recently). We have brought out the
novelties of UDCaT-1 with reference to sulfated zirconia
(S-ZrO2) in a number of industrially relevant reactions.1a-f
Similarly, UDCaT-2 was a superacidic shape selective catalyst.1g-j
Sulfuric acid is the most frequently used sulfating agent for the
treatment of zirconia. UDCaT-4 was prepared as a novel
mesoporous solid acid catalyst via synergism of persulfated
alumina and zirconia into HMS.3a-b Very recently, we synthe-
sized a novel stable superacidic catalyst UDCaT-5,3c for the
preparation of which chlorosulfonic acid was used for the first
time as a new source for the incorporation of sulfate ions into
zirconia. UDCaT-5 contains more sulfate ions without any
chloride ions and is more active and stable in comparison
with S-ZrO2. This novel material was fully characterized and
it was found to possess a remarkable activity and extraordi-
nary stability in comparison with conventional S-ZrO2 in a
* To whom correspondence should be addressed. Tel.: ++91-22-2410
2121; 2414 5616. Fax: ++91-22-2410 2121; 2414 5614. Email:
gdyadav@yahoo.com, gdyadav@udct.org.
10.1021/jp0482060 CCC: $27.50 © 2004 American Chemical Society
Published on Web 10/09/2004

9558 J. Phys. Chem. A, Vol. 108, No. 44, 2004
Yadav and Murkute
butyl alcohol at atmospheric pressure.6-8 We have brought
out the novelties of ion-exchange resins with reference to other
solid acid catalysts such as clays and their modified forms,
zeolites, sulfated zirconia, mesoporous solid acid, UDCaT-1,
and UDCaT-2 for condensation reactions of alcohols to get
different ethers such as MTBE, ETBE, and TAME.9-14
Synthesis of TAME from tert-amyl alcohol and methanol was
studied over K-10 clay, 20% w/w dodecatungstatophos-
phoric acid (DTP) supported on K-10 clay, and several cation
exchange resins; Amberlyst-36 was found to be a better cat-
alyst and the reaction follows pseudo-first-order kinetics.9
Despite their good activity, ion-exchange resins have severe
limitations of temperature instability and loss of acid sites due
to leaching.15-19 This has clearly limited their potential applica-
tions in high temperature reactions. Many acidic catalysts,
such as microporous zeolites, sulfated zirconia (S-ZrO2), and
heteropoly acids (HPAs), have been employed to catalyze
similar types of etherification reactions such as MTBE from
methanol and tert-butanol.20-24 Zeolites have been investi-
gated as potential alternative catalysts for the synthesis of
MTBE. However, most of the zeolites are less efficient than
Amberlyst-1519,25-27 and only â-zeolite is nearly as active as
the resin.28
decanted and the precipitate of HMS was dried on a glass plate.
The template was removed by calcining the resulting material
at 500 °C in air for 3 h.29
UDCaT-6 was prepared by adding an aqueous solution of
2.5 g zirconium oxychloride to 5 g precalcined HMS by incipient
wetness technique, and this was dried in an oven at 120 °C for
3 h. The dried material was hydrolyzed by ammonia gas and
washed with distilled water until no chloride ions were detected,
which was confirmed by AgNO3 test. It was further dried in
an oven for 2 h at 120 °C. Zr(OH)4/HMS was immersed in
15 cm3/g of 0.5 M chlorosulfonic acid in ethylene dichloride
for 5 min and then, without allowing moisture absorption, it
was oven dried to evaporate the solvent at 120 °C. The sam-
ple was kept in the oven at 120 °C for a further 24 h and cal-
cined thereafter at 650 °C for 3 h to get the active catalyst
UDCaT-6.
2.3. Characterization of Catalysts. Infrared spectra of the
samples pressed in KBr pellets were obtained with a Perkin-
Elmer instrument at a resolution of 2 cm-1 between 4000 and
350 cm-1, and in each case the sample was referenced against
a blank KBr pellet.
Powder X-ray diffraction (XRD) patterns were obtained by
using Cu KR radiation (λ ) 1.540 562). Samples were step
scanned from 1 to 40 in 0.045 steps, with a stepping time of
0.5 s.
The foregoing suggests that there still exists tremendous scope
to design and synthesize better catalysts than cation exchange
resins for the synthesis of oxygenated fuels. The current research
is in that direction.
Surface area measurements and pore size distributions
analysis were done, after pretreating the sample under high
vacuum at 300 °C for 4 h, by nitrogen adsorption on a
Micromeritics ASAP 2010 instrument at an adsorption temper-
ature of 77 K.
2. Experimental Section
The elemental compositions of HMS and UDCaT-6 were
obtained by Energy Dispersive X-ray Spectroscopy (EDXS) on
a KEVEX X-ray spectrometer.
Scanning electron micrographs of HMS and UDCaT-6 were
taken on a Cameca SU 30 microscope. The dried samples were
mounted on specimen studs and sputter coated with a thin film
of gold to prevent charging. The gold coated surface was then
scanned at various magnifications using the SEM.
2.1. Chemicals. Zirconium oxychloride, aqueous ammonia
solution, and methanol (all AR grade), 95% w/w ethanol, benzyl
chloride, and toluene were procured from M/s. s.d. Fine
Chemicals Ltd., Mumbai. The tert-amyl alcohol and tetraethyl
orthosilicate (TEOS) were procured from Fluka. Chlorosulfonic
acid, dodecylamine, and hexadecylamine were obtained from
Spectrochem Ltd., Mumbai. Amberlyst-15 was obtained from
Rohm and Haas.
2.4. Reaction Procedure and Analysis. Liquid phase alky-
lation of toluene with benzyl chloride was conducted in a
cylindrical glass reactor of 5 cm i.d. and 10 cm height. The
reactor had four glass baffles and a four bladed disk turbine
impeller located at a height of 0.5 cm from the bottom of the
vessel and mechanically agitated with an electric motor at 1000
rpm. Samples were withdrawn periodically and analyzed by a
Chemito GC equipped with a 3.8 mm × 4m stainless steel
column packed with 10% OV-17 on Chromosorb WHP, as well
as with a FID detector.
Vapor phase synthesis of TAME was conducted in a down
flow fixed bed Hastelloy HC-276 reactor with a 25.4 mm i.d.
and a 300 mm length at atmospheric pressure, equipped with
an upstream vaporizer and downstream condenser (Figure 1).
The liquid feed containing tert-amyl alcohol and methanol was
fed by a double piston (Well Chrom HPLC-pump K-120)
pump to the vaporizer by using N2 as a carrier gas at a WHSV
of 20 h-1, and the effluent was analyzed online by GC
(GC1000 Chemito, Toshniwal Instruments, India) equip-
ped with a 0.22 mm × 25 m capillary column and a FID
detector.
2.2. Catalyst Preparation. Zirconium hydroxide was pre-
pared by hydrolysis of aqueous zirconium oxychloride (ZrOCl2‚
8H2O) with aqueous ammonia at room temperature in the pH
range 9-10. The precipitated zirconium hydroxide was washed
with deionized water until a neutral filtrate was obtained; the
absence of chlorine ion was detected by phenolphthalein and
AgNO3 tests. Zirconium hydroxide was dried in an oven for 24
h at 100 °C and crushed to 100 mesh size.
Sulfated zirconia was prepared by immersing Zr(OH)4 in 15
cm3/g of 0.5 M sulfuric acid followed by drying at 120 °C for
24 h and calcination at 650 °C for 3h.1a
UDCaT-5 was prepared by immersing Zr(OH)4 in 15 cm3/g
of 0.5 M solution of chlorosulfonic acid in ethylene dichloride
and keeping the Zr(OH)4 in the solution for 5 min. Without
allowing moisture absorption, the sample was kept in an oven
and heating was started to evaporate the solvent. After 30 min,
the temperature was raised to 120 °C, where the sample was
maintained for a further 24 h. It was then calcined at 650 °C
for 3 h to get the final catalyst, UDCaT-5.3c
Hexagonal mesoporous silica (HMS) was prepared as follows.
Dodecylamine (5 g) was dissolved in 41.8 g of ethanol and 29.6
g of distilled water. Tetraethyl orthosilicate (20.8 g) was added
to it under vigorous stirring. The addition of ethanol improved
the solubility of the template. The reaction mixture was aged
for 18 h at 30 °C. Clear liquid above the white precipitate was
In a typical run, 2 g of catalyst was charged to the reactor
and stacked between glass beads and ceramic wool. The reactor
was maintained under isothermal conditions during all runs and
the effects of various parameters such as mole ratio, WHSV,
temperature, and W/FA (weight of catalyst per unit molar flow
0

Development of a Novel Mesoporous Catalyst
J. Phys. Chem. A, Vol. 108, No. 44, 2004 9559
Figure 1. Schematic flow diagram of fixed bed catalytic reactor. (1) Pump; (2) Vaporizor; (3) Reactor; (4) Condenser; (5) Phase separator; (6)
Receiver.
SCHEME 1: Condensation of tert-amyl alcohol with methanol over solid acid (H+) over UDCaT-6
rate of reactant) were studied. A material balance was done to
find the amount of water formed from the stoichiometry of the
reaction. Water results from the dehydration of alcohol (reactions
a, b, and c in Scheme 1).

9560 J. Phys. Chem. A, Vol. 108, No. 44, 2004
Yadav and Murkute
Figure 3. (A) XRD of (a) HMS and (b) UDCaT-6; (B) (a) HMS, (b)
UDCaT-5, (c) S-ZrO2, (d) UDCaT-6.
TABLE 1: BET Surface Area and Pore Size Analysis of
HMS, S-ZrO2, UDCaT-5, and UDCaT-6
Figure 2. FTIR spectra: (a) HMS; (b) UDCaT-6; (c) S-ZrO2; (d)
UDCaT-5.
BET surface area pore volume average pore diameter
catalysts
(m2/g-1
)
(cm3/g-1
)
(Å)
3. Results and Discussion
HMS
1120
100
83
1.2
33
20
20
32
S-ZrO2
UDCaT-5
UDCaT-6
0.11
0.11
0.70
3.1. Characterization. 3.1.1. IR Spectroscopy. It was ex-
pected that, during the calcinations of UDCaT-6, chlorosulfonic
acid would decompose to sulfuryl chloride, pyrosulfuryl dichlo-
ride, sulfuric acid, sulfur dioxide, chlorine, and water,30 and
sulfur must be retained on the surface of the UDCaT-6. Similar
types of results were obtained during synthesis of UDCaT-5.3c
Thus, FTIR spectroscopy was used to elucidate the nature of
the sulfur ion retained on UDCaT-6. The FTIR spectra depict
the presence of the characteristic covalent SdO band in the
region of 1000-1200 cm-1 on UDCaT-6 (Figure 2.). The IR
spectra of S-ZrO2 and UDCaT-5 show a similar pattern to that
of UDCaT-6, indicating the presence of the bidentated chelating
sulfate group coordinated to zirconia. This band is often
responsible for the characteristic superacidity in S-ZrO2 and
UDCaT-5.1-3 A similar nature of the sulfated group was
observed in UDCaT-6. Furthermore, the band at 1631-1642
cm-1 is attributed to the δO-H bending frequency of water
molecules associated with the sulfate group,2,3,31 and it suggests
that chlorosulfonic acid decomposes during the calcination of
UDCaT-6 and the sulfate ion is retained on the surface of
UDCaT-6, which was further corroborated by elemental analysis
that will be discussed later. An additional broad band at 3400
cm-1 corresponds to the stretching vibration δOH of the hydroxyl
group,3,31 and it reveals that during calcination at 650 °C,
condensation of hydroxyl groups of Zr(OH)4 must be occuring
in HMS, leading to a crystalline zirconia without any change
of the nature of sulfate species. The retention of sulfur in
UDCaT-6 was further confirmed by EDAX and the formation
of crystalline zirconia in HMS was further supported by XRD
diffraction.
870
pores of HMS. The stabilization of the tetragonal phase of
zirconia by sulfate ions using sulfuric acid is well known for
S-ZrO2. The case with UDCaT-5 for which chlorosulfonic acid
was used as a sulfating agent was similar.3c The same
phenomenon was also observed in the case of UDCaT-6; it
would mean that there was an introduction of sulfate ions
through chlorosulfonic acid treatment, which must have stabi-
lized the tetragonal phase of zirconia. FTIR analysis further
supports the introduction of sulfate ions in UDCaT-6.
3.1.3. BET Surface Area and Pore Size Analysis. The textural
characterization of HMS and UDCaT-6 was performed by
nitrogen BET surface area and pore size analysis (Table 1). Both
HMS and UDCaT-6 display characteristic type IV adsorption
isotherms (Figure 4) with well defined steps in N2 adsorption
and desorption isotherms and hysteresis in the desorption
isotherm at relative pressures (p/p0) in the range 0.45-0.8. This
behavior is indicative of uniform meso pore sizes in both HMS
and UDCaT-6 that are filled spontaneously due to capillary
condensation. HMS retains its mesoporosity even after having
been converted to UDCaT-6, and the explanation is in ac-
cordance with IUPAC classification.32 Furthermore, UDCaT-6
displays fairly uniform pore size distribution centered at 32 Å
and the average pore diameter of UDCaT-6 is reduced margin-
ally to 32 Å from 33 Å of HMS (Table 1 and Figure 5);
however, there was no broadening of the size distribution,
indicating that the active centers of chlorosulfonic acid treated
zirconia are uniformly dispersed inside the mesopores of HMS.
The reduction in the BET surface area and pore volume of
UDCaT-6 vis-a`-vis HMS is much more remarkable in com-
parison to their respective pore sizes, indicating that nanopar-
ticles of zirconia are generated in situ after treatment of
chlorosulfonic acid and that some large particles can block a
few pore junctions, thereby reducing accessibility of some
channels of HMS. Thus, there is a reduction in surface area
and pore volume of UDCaT-6. However, all of these values
are far greater than those of bulk unsupported S-ZrO2 and of
UDCaT-5 (Table 1). Our results are in consonance with earlier
literature reports.33
3.1.2. X-ray Diffraction. The structural integrity of HMS, as
well as of UDCaT-6, was determined with X-ray diffraction.
One diffraction peak in the low angle region (2θ ) 1-10°) is
visible, indicating that HMS has a long range hexagonal ordering
and is also visible in UDCaT-6 (Figure 3A). Both bulk S-ZrO2
and UDCaT-5 show the presence of the crystalline tetragonal
phase of zirconia in the ordinary region (2θ ) 30 and 50°) due
to incorporation of sulfate ion.1-3 However, UDCaT-6 shows
the reflection of zirconia at 2θ ) 30° and 2θ ) 50° both having
very low and broad intensities (Figure 3B). These diffraction
peaks for UDCaT-6 can be attributed to the tetragonal crystalline
phase rather than the monoclinic phase of zirconia inside the

Development of a Novel Mesoporous Catalyst
J. Phys. Chem. A, Vol. 108, No. 44, 2004 9561
Figure 4. Adsorption isotherms: (A) HMS, (B) UDCaT-6.
comparison with the uncalcined material. These results are in
accordance with those of Marler et al.,35 who have provided
experimental evidence for a decrease in the X-ray Bragg
intensities after filling the pores of calcined boron-substituted
MCM-41 silica with various organic liquids. Pore filling by 1,2-
dibromoethane, bromoform, and diiodomethane almost com-
pletely eliminated the (100) Bragg reflection. Recalcination to
remove the organic adsorbent restored the (100) reflection to
nearly its original intensity. Glinka et al.36 have carried out
neutron diffraction contrast studies on MCM-41 and observed
that Bragg diffraction disappeared when the pores were filled
with D2O/H2O mixtures that have the same scattering of cross-
section as SiO2. Edler and White37 reported large changes in
the Bragg intensities when MCM-41 silicas were dried for long
periods. Furthermore, introduction of scattering material into
the pores of HMS leads to an increased phase cancellation
between scattering from the wall and the pore regions and
therefore leads to reduced scattering intensities for the Bragg
reflections at low angle (2θ ) 1-7°). The degree of cancellation
is mainly determined by the scattering contrast between the
framework walls and the pores.38 Earlier literature suggests that
the Bragg’s intensity of mesoporous material below low angle
is remarkably dependent on the scattering material present
inside. Figure 3A shows the reduction of the typical (100) Bragg
reflections of UDCaT-6 in comparison with HMS. Our results
are in consonance with the past literature. This led us to
corroborate that in UDCaT-6 nanoparticles of zirconia (,3.3
nm) must have formed inside the pores of HMS, and some of
them are trapped at the pore junctions. Further, XRD patterns
of both unsupported S-ZrO2 and UDCaT-5 show the pure
tetragonal phase of zirconia in the ordinary region. This lends
credence to the conclusion that the tetragonal phase of zirconia
must be trapped inside the pores of UDCaT-6. Impregnation of
the aqueous salt of the zirconia precursor by incipient wetness
technique into the pore space of HMS and subsequent hydrolysis
Figure 5. Pore size distributions: (A) HMS, (B) UDCaT-6.
XRD, BET surface area, and pore size analysis provide an
explanation for entrapment of zirconia in mesoporous HMS.
During the synthesis of UDCaT-6, the growth of crystalline
zirconia is very unlikely because XRD data as well as BET
surface area measurements indicate that the characteristic
hexagonal structure of HMS and its mesoporosity are not
perturbed in UDCaT-6, thus indicating framework stability.
Growth of zirconia inside the HMS framework would have
resulted in extensive damage of the framework and broadening
of pore size distribution, which were not observed. Previous
reports suggest that the intensity of the Bragg X-ray peak of
mesoporous material at low angle is significantly dependent on
the scattering material present inside the pores. Anderson et al.34
found that the removal of surfactant by calcination was
accompanied by a very large increase (by a factor of almost
five) in the intensity of the most intense Bragg X-ray peak, in

9562 J. Phys. Chem. A, Vol. 108, No. 44, 2004
Yadav and Murkute
Figure 6. SEM and EDAX of HMS and UDCaT-6.
TABLE 2: EDAX of HMS and UDCaT-6
catalysts
HMS
elements
weight %
atomic %
O K
Si K
54.49
45.51
100.00
41.23
42.59
0.38
67.76
32.24
total
UDCaT-6
O K
Si K
S K
60.23
35.44
0.28
Figure 7. Conceptual mechanism for the formation of acid sites into
UDCaT-6.
Zr L
15.80
100.00
4.05
total
by ammonia gas result in precipitation of Zr(OH)4 in mesopores
of the HMS. As a result, once it is precipitated, it is very difficult
for Zr(OH)4 to come out from the pores of HMS during washing
and drying. After treatment with chlorosulfonic acid and
calcination at 650 °C, condensation of the hydroxyl group of
Zr(OH)4 occurs, leading to a crystalline zirconia without a
change of nature of the sulfate species. FTIR and XRD spectra
provide an explanation for these assumptions. Furthermore, the
pore volume of UDCaT-6 is much less than that of pure HMS,
indicating that large amounts of crystalline nanoparticles of
zirconia are present inside the pores of UDCaT-6.
La1 distribution spectra determined by SEM-EDXS analysis
have clearly shown homogeneous distribution of S and Zr atoms
in UDCaT-6. These results are in accordance with earlier
literature reports.33 SEM and EDXS analysis further supports
the argument that active centers of chlorosulfonic acid treated
zirconia are successfully embedded inside pores of HMS and
that the structural integrity of the HMS is not altered even after
it is converted to UDCaT-6.
We believe that this is a novel method for generation of active
sites inside pores of catalytically inactive mesoporous material
HMS by impregnating zirconia and treating it with chlorosul-
fonic acid as new source for sulfate ions, resulting in a novel
catalyst UDCaT-6 (Figure 7).
3.1.4. SEM and EDXS. SEM of HMS and UDCaT-6 (Figure
6 and Table 2) reveal that HMS is made up of sub-micrometer
sized free-standing or aggregated sphere shaped particles.39
A
3.2. Catalytic Activity and Stability of UDCaT-6. The
activity, robustness, and reusability of solid acids should be
tested in the presence of HCl and water in liquid and vapor
phase reactions. The liquid phase benzylation of toluene with
similar type of morphology is observed in UDCaT-6. The EDXS
analysis shows the incorporation of zirconia and sulfur in
UDCaT-6. It should be noted that sulfur Ka1 and zirconium

Development of a Novel Mesoporous Catalyst
J. Phys. Chem. A, Vol. 108, No. 44, 2004 9563
TABLE 3: Activity of HMS, S-ZrO2, UDCaT-5, and
UDCaT-6 in Liquid Phase Alkylation of Toluene with Benzyl
Chloridea
catalysts
parameter
HMS S-ZrO2
UDCaT-5
185
UDCaT-6
933
TON for benzylation
of toluene with
benzyl chloridea
selectivity
0
96
0
98%, mixture of benzylated toluene
a Reaction Conditions: mole ratio of toluene/benzyl chloride, 10:1;
speed of agitation, 1000 rpm; temperature, 90 °C; catalyst loading, 0.018
g/cc; reaction time, 1 h. TON (turnover number) ) moles of product/
moles of zirconia present in the catalyst.
TABLE 4: Activity of UDCaT-6 and Amberlyst-15 for
Synthesis of TAMEa
Figure 8. Effect of mole ratio of methanol to TAA on conversion
and yield. W/FA : 259.74 kg h mol-1; Temp: 120 °C.
product distribution
0
yield of %
Yield of %
TAMEc
catalysts
HMS
Amberlyst-15
S-ZrO2
UDCaT-5
UDCaT-6
conversion %
(2MB2+2MB1)b
0
98
94
94
95
0
96
92
88
82
0
4
8
12
18
a Reaction conditions: mole ratio of tert-amyl alcohol/methanol, 1:5;
WHSV ) 20 h-1; carrier gas, N2; temperature, 120 °C. b Yield of 2MB2
+ 2MB1 ) amount of 2MB1 and 2MB2 formed/amount of tert-amyl
alcohol reacted. c Yield of TAME ) amount of TAME formed/amount
of tert-amyl alcohol reacted.
benzyl chloride was used to evaluate the reactivity and stability
of UDCaT-6, where corrosive HCl is evolved as a coproduct;
it will be a good probe reaction to study the robustness of the
catalyst.3 Table 3 shows that UDCaT-6 gives an excellent
turnover number in comparison with S-ZrO2 and UDCaT-5 for
the same reaction. The reusability of the catalyst was also tested
thrice for benzylation of toluene to establish that there was no
deactivation. These results confirmed that there was no leaching
of the sulfate ion in the presence of HCl; thus, UDCaT-6 is a
stable catalyst.
Figure 9. Assessment of mass transfer resistance. W/FA : 259.74 kg
0
h mol-1; Temp: 120 °C.
as well as the etherification of isoamylenes with methanol.
UDCaT-6 gives excellent per pass yield of TAME compared
to other catalysts (Table 4). Further, there was no formation of
dimers of the alkenes. With both catalysts, tert-amyl alcohol
dehydrates to give 2MB1 and 2MB2 and then reacts with
methanol to give TAME; thus, equivalent moles of water is
formed.
3.3. Effect of the Mole Ratio of TAA to Methanol. The
effect of the mole ratio of tert-amyl alcohol to methanol was
studied in the range 1:1-1:5 (Figure 8). Both the conversion
of tert-amyl alcohol and the yield toward TAME increase with
the increasing mole ratio of tert-amyl alcohol to methanol. As
the concentration of methanol increases, the number of available
sites for adsorption of tert-amyl alcohol decreases, and there is
no further increase in conversion and yield above a mole ratio
of 1:5.
Vapor phase synthesis of TAME from tert-amyl alcohol and
methanol was studied and compared with Amberlyst-15, S-ZrO2,
and UDCaT-5. As mentioned earlier, the stability of sulfated
zirconia is susceptible where water is generated as a coproduct.
When fresh sulfated zirconia is mixed with water, the pH of
the suspension decreases quickly as a result of sulfate group
hydrolysis that is confirmed by the identification of sulfate ions
in the aqueous phase. A test for sulfate leaching was conducted
for both fresh and reused UDCaT-6 by pH measurement,40 and
it was found that there was no leaching of sulfate ions. A similar
phenomenon was observed in the case of UDCaT-5.3c The
stability of UDCaT-6 on time-on-stream (TOS) was tested at
120 °C for 70 h. It was found that the conversion of TAA, yield
of TAME, and selectivity toward TAME remained practically
the same. The catalyst was thus stable. This further bolsters
the earlier observations on the stability of UDCaT-6.
3.4. Effect of External Mass Transfer. The influence of
mass transfer resistance on conversion of mesitylene was
assessed by increasing the carrier gas flow rate under otherwise
similar conditions of temperature, space time of tert-amyl
Additionally, in the case of the etherification of the tert-amyl
alcohol with methanol, there is in situ generation of isoamylenes,
namely, 2-methyl-1-butene (2MB1) and 2-methyl-2-butene
(2MB2), via cracking of tert-amyl alcohol. Possible reactions
are depicted in Scheme 1. A good cracking catalyst and a poor
etherification catalyst would result in loss of the generated
isoamylenes. A reverse phenomenon would result in poor yields
of TAME. It was therefore necessary that the best catalyst should
give a reasonable activity for the cracking of tert-amyl alcohol
alcohol (W/FA ), and catalyst quantity at 120° C, 259.74 kg h
o
mol-1, and 2 g, respectively. Figure 9 indicates that there is no
external mass transfer limitation.
A theoretical calculation was also done based on the Wiesz-
Prater modulus to asses the influence of intraparticle diffusion
2
resistance, according to which the value of (-robsFpRp /De[As])

9564 J. Phys. Chem. A, Vol. 108, No. 44, 2004
Yadav and Murkute
Figure 11. Effect of space time on conversion of TAA and yield of
TAME at different temperatures. Dashed lines: TAME yield (one pass).
Solid lines: conversion (one pass). TAME yield ) amt TAME formed/
amt tert-amyl alcohol reacted.
Figure 10. Effect of temperature on conversion of TAA and yield of
TAME at different space times. Dashed lines: TAME yield (one pass).
Solid lines: conversion (one pass). TAME yield ) amt TAME formed/
amt tert-amyl alcohol reacted.
temperature at a fixed molar ratio of reactants; however, the
yields of 2MB1 and 2MB2 decrease and the ratio of their yields
in the outgoing stream is practically unaffected at all temper-
atures studied.
has to be far less than unity for the reaction to be intrinsically
kinetically controlled. The calculated value of 2.5 × 10-8
demonstrates the absence of intraparticle resistance giving a
catalyst effectiveness factor of unity. Thus, a WHSV of 20 h-1
was maintained constant during further experiments.
(v) Preliminary analysis of rate data showed that the reactants
and products have lower boiling points and were weakly
adsorbed at 120 °C and 1 atm pressure.
3.5. Effects of Temperature and Space Time. The effects
of temperature on the conversion of tert-amyl alcohol (TAA)
and yield of TAME were studied at a WHSV of 20 h-1. With
increasing temperature and space time, both conversion and yield
were found to increase (Figures 10 and 11), which was due to
increased contact time of the substrate on the catalyst surface.
It also supported the earlier observations that the reaction was
free from external mass transfer and intraparticle diffusion
effects. The range of space velocities suggests that 259.74 kg h
mol-1 is the optimum space time that gives 95% conversion of
TAA and 18% yield toward TAME. There was no effect of
temperature and space time on the selectivity of TAME, which
remained the same at all temperatures and space times.
Based on the above observations, Scheme 1 was conceived
and only reactions (a)-(d) were found to be relevant. The
chemisorption of tert-amyl alcohol (A) and methanol (B) on a
vacant site (S) gives surface species AS and BS. The chemi-
sorbed species AS undergoes three parallel reactions to produce
TAME (T) (from AS and BS), and 2MB1 and 2MB2 (from
AS). The isomerization of 2MB1 to 2MB2 reaction is also likely
to occur but is of no consequence. The reaction was carried out
at 120 °C in the vapor phase, and it was found that through
preliminary analysis of the data the adsorption terms were
insignificant. Thus, the overall rate of reaction of tert-amyl
alcohol (A) (mol/s‚g-cat), assuming parallel reactions (a), (b),
and (c) as rate controlling, is given by
4. Development of Kinetic Model
-r′ ) (k1CBS + k2 + k3)CAS
(1)
The experimental observations, in the absence of external
mass transfer limitations, show the following trends for the rate
of reaction and selectivity of TAME:
(i) Rate of reaction is proportional to the partial pressure of
TAA and the yield and selectivity of TAME remain constant
for a given TAA feed rate and temperature.
A
The concentration CS of the vacant sites (S) is calculated from
the total sites (Ct), adsorption equilibrium constants (Ki), and
concentration of the relevant species (Ci). Thus,
(ii) Rate of reaction is proportional to partial pressure of
methanol up to a certain value, beyond which it remains
practically the same. The yield of TAME increases with the
increasing partial pressure of methanol in the feed. It remains
practically the same for a mole ratio of 5:1 of methanol to tert-
amyl alcohol. There was no formation of dimethyl ether, even
with increasing partial pressure of methanol.
-r′ ) (k1KBCB + k2 + k3)KACA
(2)
(3)
A
-r′ ) k′CA when CB . CA
A
0
0
where the overall pseudoconstant for the reaction of A (k′) is
given by
(iii) Yields of the olefins, 2MB1 and 2MB2, which are
dehydration products of TAA, are also proportional to the partial
pressure of TAA in the feed. There was no dimerization or
further oligomerization of 2MB1 and 2MB2 at all partial
pressures of TAA or methanol.
k′ ) (k1KBCB + k2 + k3)KA
(4)
0
For a fixed bed reactor with plug flow of vapors, where the
mole ratio of methanol (B) is taken in far excess over tert-
amyl alcohol (A), the total overall fractional yield φ of TAME
(iv) Initial rate of reaction of TAA, and hence conversion of
TAA and the yield of TAME, increases with increasing

Development of a Novel Mesoporous Catalyst
J. Phys. Chem. A, Vol. 108, No. 44, 2004 9565
reaction is very high and the reaction no longer remains a
pseudo-first-order reaction. Since there are substantial changes
in the concentration of methanol at 120 °C due to much higher
rates of reaction, the first term in eq 2 (that is, k1KBCBKACA) is
more predominant oVer others and the rate follows ap-
proximately second-order kinetics.
The values of the rate constant (k′) were calculated at different
temperatures up to 115 °C and then an Arrhenius plot was made
(not shown) to estimate the frequency factor and energy of
activation. The values of the frequency factor and the apparent
energy of activation were calculated to be 1.33 × 1019
cm3mol-1s-1 and 39 kcal mol-1, respectively. This high value
of activation energy also confirmed that the overall rate of
reaction was not influenced by either external mass transfer or
by intraparticle diffusion resistance, and that it was an intrinsi-
cally kinetically controlled reaction.
Figure 12. Pseudo-first-order plot: -ln(1 - XA) vs W/FA0 for synthesis
of TAME. [ 100 °C; 9 110 °C; 2 115 °C; b 120 °C.
5. Conclusion
with reference to the amount of A reacted is obtained from the
instantaneous yield as follows:
The current work has addressed for the first time the use of
chlorosulfonic acid as a source of sulfate ion to treat zirconia
on HMS to synthesize a novel catalyst UDCaT-6 that exhibits
excellent catalytic activity and stability in both vapor phase
synthesis of TAME and liquid phase alkylation reactions. A
kinetic model was built up for synthesis of TAME that suggested
that the reaction is intrinsically kinetically controlled. The model
explains the observations very well. A time-on-stream study
was conducted up to 70 h, and no drop in catalytic activity was
observed. Thus, UDCaT-6 is a robust and novel catalytic
material, and it can be used for a prolonged period of time
without deactivation. A number of other reactions are being
studied with this novel catalyst.
k1KBCB
1
ψ ) φ ) )
)
(5)
k1KBCB + k2 + k3
k2 + k3
k1KBCB
1 +
When k1KBCB , k2 + k3 then eq 5 reduces to
k1KBCB
ψ ) φ ) )
(6)
k2 + k3
Eq 6 shows that the yield of TAME is independent of the
concentration of A and should increase with the concentration
of B.
On the contrary, when
Acknowledgment. G.D.Y. acknowledges funding from the
Darbari Seth Professorship Endowment and NMITLI program
of the CSIR, New Delhi (2001-03).
k1KBCB . k2 + k3, ψ ) φ ) 1
(7)
it will become independent of the concentration of B.
Nomenclature
In the current studies, the yield of TAME was found to be
independent of the concentration of A (tert-amyl alcohol), and
it increased with methanol concentration at low values and was
independent of concentration of methanol at higher concentra-
tions. The yield was found to increase with increasing temper-
ature for a fixed mole ratio of A and B, which is also according
to the above eq 6.
A
tert-amyl alcohol (TAA)
B
methanol
[As]
[A0]
Ci
catalyst surface concentration of TAA (mol cm-3
)
initial concentration of TAA (mol cm-3
concentration of species i (mol cm-3); can be expressed
)
as partial pressure (atm)
The instantaneous combined yield of olefins 2MB1 and 2MB2
is given by
CA
initial concentration of A, mol/cm3
initial concentration of B, mol/cm3
0
CB
0
De
effective diffusivity of A (cm2 s-1
)
k2 + k3
k1KBCB + k2 + k3
molar flow rate of TAA (mol h-1
)
ψMB ) φMB
)
(8)
FA
0
-1
k′
pseudo rate constant (cm3 g s-1
)
k1
k2
surface reaction rate constant for reaction of A with B
The combined yield of the olefins was found to be unaffected
by the concentration of A and was independent at higher
concentrations of B. These observations were taken as a valid
proof of this kinetic model.
Now, eq 2 is integrated to get the following for a fixed bed
vapor phase catalytic reactor:
surface reaction rate constant for dehydration of TAA
to 2MB1
k3
surface reaction rate constant for dehydration of TAA
to 2MB2
K4
isomerization reaction equilibrium constant for 2MB1
to 2MB2
Ki
adsorption equilibrium constant for species i
W
FA
-ln(1 - XA) ) k′CA
(9)
0
-ri
-robs
Rp
rate of reaction of species i (mol/s‚g-cat)
0
-1
observed rate of reaction (mol g s-1
radius of catalyst particle (cm)
weight of catalyst (g)
)
Thus, a plot of -ln(1 - XA) versus W/FA was made at different
temperatures (Figure 12) to get a good fit up to 115 °C.
However, the fit is not so good at 120 °C because the rate of
0
W
Xi
fractional conversion of species i

9566 J. Phys. Chem. A, Vol. 108, No. 44, 2004
Yadav and Murkute
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