Etherification of tert-amyl alcohol with methanol over ion-exchange resin

Article abstract of DOI:10.1021/op010018+

tert-Amyl methyl ether (TAME) is a proven high octane additive. The synthesis of tert-amyl methyl ether from tert-amyl alcohol and methanol has been carried out in the presence of a variety of solid acid catalysts. Amberlyst-36 was found to be very effective in comparison with other solid acids. A complete theoretical and experimental analysis is presented for the model studies of tert-amyl alcohol with methanol. The parallel reactions of tert-amyl alcohol adsorbed on the sites were found to control the overall rate of reaction, which led to the formation of TAME, 2-methyl-1-butene (2MB1), and 2-methyl-2-butene (2MB2). The reaction follows pseudo-first-order kinetics at a fixed catalyst loading. The individual rate constants for the formation of TAME, 2MB1, and 2MB2 were also evaluated from the same data.

Full text of DOI:10.1021/op010018+

Organic Process Research & Development 2001, 5, 408−414
Etherification of tert-Amyl Alcohol with Methanol over Ion-Exchange Resin
G. D. Yadav* and A. V. Joshi
Chemical Engineering DiVision, UniVersity Department of Chemical Technology (UDCT),
Matunga, Mumbai - 400 019, India
Abstract:
and sheet silicate.10 Linnekoski et al.11 have reported simul-
taneous isomerization and etherification of isoamylenes with
alkanols. Safronov et al.12 have studied thermodynamics of
the synthesis of TAME. Rihko and Krause13 determined the
reaction rates in liquid phase in a continuous stirred tank
reactor. Oost and Hoffmann14 studied the influence of internal
and external mass-transfer resistance in the synthesis of
TAME in a continuous-flow recycle reactor. Goto et al.15-17
synthesized MTBE and ETBE at atmospheric conditions by
a condensation reaction of methanol or ethanol with tert-
butyl alcohol. To achieve continuous production of MTBE
and ETBE, Goto et al.18,19 employed reactive distillation
combined with pervaporation.
Etherification of tert-amyl alcohol to produce TAME was
thus considered as an important problem necessitating the
use of different solid acid catalysts. Synthesis of MTBE from
tert-butyl alcohol and methanol has been studied in this
laboratory by using a variety of solid acids. Heteropoly acids
(HPA) supported on clays have shown superior activity as
catalysts in comparison to others in the alkylation and
etherification reactions.20
tert-Amyl methyl ether (TAME) is a proven high octane
additive. The synthesis of tert-amyl methyl ether from tert-amyl
alcohol and methanol has been carried out in the presence of a
variety of solid acid catalysts. Amberlyst-36 was found to be
very effective in comparison with other solid acids. A complete
theoretical and experimental analysis is presented for the model
studies of tert-amyl alcohol with methanol. The parallel reac-
tions of tert-amyl alcohol adsorbed on the sites were found to
control the overall rate of reaction, which led to the formation
of TAME, 2-methyl-1-butene (2MB1), and 2-methyl-2-butene
(2MB2). The reaction follows pseudo-first-order kinetics at a
fixed catalyst loading. The individual rate constants for the
formation of TAME, 2MB1, and 2MB2 were also evaluated
from the same data.
Introduction
Synthesis of alkyl-tert-alkyl ethers by reactions of iso-
olefins with alcohols is an efficient process, and these ethers
are increasingly being used as ecologically clean additives
to motor oils. tert-Alkyl ethers, namely, methyl-tert-butyl
ether (MTBE) and ethyl-tert-butyl ether (ETBE) are produced
on industrial scale as nontoxic and high-octane gasoline
additives. Production of tert-amyl methyl ether (TAME) has
been investigated in recent years.1 tert-Amyl methyl ether
(TAME) with an octane number of 106 which is almost the
same as that of MTBE (109) can be employed as a possible
high-octane additive to motor fuels. The blending Reid
vapour pressure (RVP) of TAME is 1 psi, which is much
lower than that of MTBE (8 psi) and ETBE (4 psi). Thus,
TAME shows a lot of promise as a fuel additive particularly
since MTBE is under close scrutiny and California has
already banned the usage of MTBE as an octane booster.
Synthesis of tert-alkyl ethers by etherification process
using cation-exchange resins,2-6 acid catalysts,7,8 zeolites,9
Experimental Section
Chemicals. K-10 clay was obtained from Aldrich, U.S.A.,
and Filtrol-24 from Fluka, Germany. Zirconium oxychloride,
dodecatungstophosphoric acid, 1,4-dioxane, and methanol
were obtained from M/s s.d. Fine Chemicals Pvt. Ltd.,
Mumbai, India. tert-Amyl alcohol was obtained from Acros,
U.S.A. Amberlyst-15 and Amberlyst-36 were procured from
Rohm and Haas, U.S.A. All chemicals were of analytical
grade and used without further purification.
Catalysts. Dodecatungstophosphoric acid (DTP) sup-
ported on K-10 was prepared by a well-established procedure
in our laboratory.21 A desired quantity of montmorillonite
clay (K10) was taken and dried in an oven for 2 h, and
dodecatungstophosphoric acid dissolved in methanol was
added dropwise, with stirring to prepare the catalyst by the
* To whom correspondence should be addressed.
(1) Ignatius, J.; Jarvelin, H.; Lindqvist, P. Hydrocarbon Process 1995, 74(2),
51.
(10) Fleitas, D. H.; Macanio, H. R.; Perez, C. F.; Orio, O. A. React. Kinet. Catal.
Lett. 1991, 43, 183.
(2) Adams, J. R.; Smith, L. A., Jr.; Hearn, D.; Jones, E. M., Jr.; Arganbright,
R. P. U.S. Patent 5,792,891, 1998; Chem. Abstr. 1998, 129, 150345.
(3) Jarvelin, H.; Lindqvist P.; Tamminen, E. U.S. Patent 5,852,220, 1998; Chem.
Abstr. 1999, 130, 53953.
(11) Linnekoski, J. A.; Kiviranta, P. P.; Krause, A. O.; Rihko-Struckmann, L.
K. Ind. Eng. Chem. Res. 1999, 38(12), 4563.
(12) Safronov, V. V.; Sharonov, K. G.; Rozhnov, A. M.; Alenin, V. I.; Sidorov,
S. A. J. Appl. Chem. USSR 1989, Part 1, 62(4), 763.
(4) Yang, B. L.; Maeda, M.; Goto, S. Int. J. Chem. Kinet. 1998, 30(2), 137.
(5) Chodorge, J. A.; Commereuc, D.; Cosyns, J.; Duee D.; Jorck, B. Eur. Patent
742,195, 1996; Chem. Abstr. 1997, 126, 46874.
(6) Piccoli, R. L.; Lovisi, H. R. Ind. Eng. Chem. Res. 1995, 34, 510.
(7) Marion, M. C.; Viltard, J. C.; Travers, P.; Harter, I.; Forestiere, A. Eur.
Patent 755,706, 1997; Chem. Abstr. 1997, 126, 200887.
(8) Hoffmann, U.; Krummradt, H.; Rapmund, P.; Sundmacher, K. Chem. Ing.
Technol. 1997, 69(4), 483; Chem. Abstr. 1997, 127, 6854.
(9) Miller, S. J. U.S. Patent 5,801,293, 1998; Chem. Abstr. 1998, 129, 177197.
(13) Rihko, L. K.; Krause, A. Outi I. Ind. Chem. Eng. Res. 1995, 34, 1172.
(14) Oost, C.; Hoffmann, U. Chem. Eng. Technol. 1995, 18, 203.
(15) Matouq, M. H.; Goto, S. Int. J. Chem. Kinet. 1993, 25, 825.
(16) Matouq, M. H.; Tagawa, T.; Goto, S. J. Chem. Eng. Jpn. 1993, 26, 254.
(17) Yin, X. D.; Yang, B. L.; Goto, S. Int. J. Chem. Kinet. 1995, 27, 1065.
(18) Matouq, M.; Tagawa, T.; Goto, S. J. Chem. Eng. Jpn. 1993, 27, 302.
(19) Yang, B. L.; Goto, S. Sci. Sep. Technol. 1995, 32(5), 971.
(20) Yadav, G. D.; Kirthivasan, N. J. Chem. Soc., Chem. Commun. 1995, 203.
(21) Yadav, G. D.; Kirthivasan, N. Appl. Catal., A 1997, 154, 29.
408
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Vol. 5, No. 4, 2001 / Organic Process Research & Development
10.1021/op010018+ CCC: $20.00 © 2001 American Chemical Society and The Royal Society of Chemistry
Published on Web 06/08/2001

Table 1. Efficacies of various catalysts on conversion of
tert-amyl alcohola
catalyst
% conversion % yieldb of TAME selectivityc
Amberlyst-15
Amberlyst-36
DTP
Filtrol-24
K-10
82
44.0
44.7
9.4
25.0
6.1
0.644
0.701
0.0762
0.123
0.0325
0.0346
86.8
74.3
37
51.2
58.3
20% DTP/K-10
5.6
a tert-Amyl alcohol:methanol: 1:2; catalyst loading: 0.02 g/cm3; time: 4 h;
temperature: 70 °C; speed of agitation: 800 rpm; solvent: 1,4-dioxane. b Yield
) amount of TAME formed/amount of tert-amyl alcohol reacted. c Selectivity
) rate of TAME formation/rate of formation of 2MB1 and 2MB2.
methyl ether. Identification of products was done by GC-
MS.
Reaction Scheme. There are six possible reactions that
can take place in the presence of acid catalyst as shown in
Figure 1. However, the dimers of 2-methyl-1-butene (2MB1)
and 2-methyl-2-butene (2MB2) were not detected. This
observation was helpful in devising a reaction mechanism.
Results and Discussion
Efficacies of Various Catalysts. Various solid acid
catalysts were employed to evaluate their effectiveness in
this reaction. A 0.02 g/cm3 loading of catalyst based on the
organic volume of the reaction mixture was taken at 70 °C.
The mole ratio of tert-amyl alcohol to methanol was kept at
1:2, with an agitation speed of 800 rpm. The catalysts used
were K-10 montmorillonite, dodecatungstophosphoric acid,
20% DTP/K-10, sulphated zirconia, Filtrol-24, Amberlyst-
15, and Amberlyst-36. Table 1 shows conversion of tert-
amyl alcohol, the limiting reactant, for the various catalysts.
Amberlyst-36 was the most active for the conversion of tert-
amyl alcohol compared to other catalysts and had better
selectivity towards tert-amyl methyl ether. Amberlyst-15
gave lesser conversion than Amberlyst-36 with almost the
same selectivity towards TAME. Amberlyst-36 has an ion-
exchange capacity of 5.45 mequiv, which is greater than that
of Amberlyst-15 of 4.9 mequiv. Being a soluble catalyst,
DTP gave good conversion but poor selectivity for TAME.
It should be recognized that the homogeneous catalyst DTP
was almost 5 times of 20% DTP/K10 and also not reusable
at the same catalyst loading. K-10 and 20% DTP/K-10 gave
less conversion than Amberlyst-36, which may be because
they have lesser ion-exchange capacity. S-ZrO2 is a Lewis
acid and hence is less effective. One of the reasons for the
high conversion of tert-amyl alcohol is the formation of the
gaseous 2MB1 and 2MB2, which have boiling points of 304
and 311 K, respectively, and do not reach equilibrium. In
all further experiments Amberlyst-36 was used as the
catalyst.
Figure 1. Reaction scheme for etherification of tert-amyl
alcohol with methanol.
so-called incipient wetness technique. Sulphated zirconia was
also prepared by an established procedure.22
Reaction Procedure. All experiments were carried out
in a 100 mL stainless steel autoclave manufactured by Parr
Instruments Co., U,S.A. A four-bladed-pitched turbine
impeller was used for agitation. The temperature was
maintained at (1 °C of the desired value with the help of
an in-built proportional-integral-derivative (PID) controller.
Predetermined quantities of reactants and the catalyst were
charged into the autoclave, and the temperature was raised
to the desired value.
In a typical experiment, 0.1 mol of tert-amyl alcohol and
0.5 mol of methanol were used, and 1,4-dioxane was added
to make the total volume to 50 cm3. A standard catalyst
loading of 1.0 g was used at 70 °C. The reaction mixture
was allowed to reach the desired temperature, and the initial/
zero time sample was collected. Agitation was commenced
and maintained at a particular speed. Samples were with-
drawn periodically for analysis.
Analysis. Analysis of the reaction mixture was done on
a G. C. (Chemito 8510) with 10% SE-30 column (4 m ×
3.18 mm) and FID.
Samples were analysed for methanol, tert-amyl alcohol,
and 2-methyl-1-butene, 2-methyl-2-butene, and tert-amyl
Effect of Speed of Agitation. To evaluate the role of
external mass transfer on the reaction rate, the effect of the
speed of agitation was studied. The speed of agitation was
varied from 800 to 1200 rpm (Figure 2). It was observed
that the conversion of tert-amyl alcohol was practically the
same in all of the cases without any change in selectivity.
Thus, it was ascertained that the external mass-transfer effects
(22) Yadav, G. D.; Thorat, T. S. Ind. Eng. Chem. Res. 1996, 35, 721.
Vol. 5, No. 4, 2001 / Organic Process Research & Development
•
409

mechanisms have been put forward.23 When the external
mass-transfer resistance is small, then the following inequal-
ity holds,
1/robs . 1/kSL-Aap[Ao] and 1/kSL-Bap[Bo]
(5)
The observed rate robs could be given by three types of
models wherein the contribution of intraparticle diffusional
resistance could be accounted for by incorporating the
effectiveness factor η. These models are:
(a) The Power Law model if there is very weak adsorption
of reactant species,
(b)Langmuir-Hinselwood-Hougen-Watson model,
(c) Eley-Rideal model.
It was therefore necessary to study the effects of speed
of agitation and catalyst loading to ascertain the absence of
external and intraparticle resistance so that a true intrinsic
kinetic equation could be used. Since the conversion was
found to remain practically the same in the range of 800-
1200 rpm, it indicated the absence of external solid-liquid
mass-transfer resistance. Theoretical analysis was also done
to ensure that the external mass-transfer resistance was indeed
absent as delineated below.
Figure 2. Effect of speed of agitation on conversion of tert-
amyl alcohol.
According to eq 5, it is necessary to calculate the rates
of external mass transfer of tert-amyl alcohol (A) and
methanol (B) and compare them with the rate of reaction.
For a typical spherical particle, the particle surface area
per unit liquid volume is given by
did not influence the rate of reaction. All further reactions
were carried out at 800 rpm. A theoretical analysis of the
assessment of external mass-transfer resistance is given to
support this observation. Details of this theory for general
slurry reactions are given elsewhere.22
This is a typical solid-liquid slurry reaction involving
the transfer of tert-amyl alcohol, the limiting reactant (A),
and methanol (B) from the bulk liquid phase to the catalyst
wherein external mass transfer of reactants to the surface of
the catalyst particle takes place, followed by intraparticle
diffusion, adsorption, surface reactions, and desorption. The
resistance due to external solid-liquid mass transfer and
intraparticle diffusion limitation should be eliminated before
a true kinetic model can be developed.
ap ) 6w/(Fpdp)
(6)
where w ) catalyst loading g/cm3 of liquid phase, Fp )
density of particle g/cm3, and dP ) particle diameter, cm.
For tert-amyl alcohol etherification, at a catalyst loading
of 0.02 g/cm3 and the particle size (dp) of 0.03 cm, the
calculated value of ap ) 4.21 cm2/cm3 of the liquid phase.
The liquid-phase diffusivity values of the reactants A (tert-
amyl alcohol) and B (methanol), denoted by DAB and DBA
,
were calculated using the Wilke-Chang equation24 at 70 °C
as 3.494 × 10-5 and 2.204 × 10-5 cm2/s, respectively. The
solid-liquid mass transfer coefficients for both A and B were
calculated from the limiting value of the Sherwood number
(e.g. ShA ) kSL-Adp/DAB) of 2. The actual Sherwood numbers
are typically higher by order of magnitude in well-agitated
systems but for conservative estimations a value of 2 is
taken.22-23 The solid-liquid mass-transfer coefficients kSL-A
and kSL-B values were obtained as 2.329 × 10-3 and 1.469
× 10-3 cm/s, respectively. The initial rate of reaction was
calculated from the conversion profiles. A typical calculation
shows that for a standard reaction the initial rate was
calculated as 3.33 × 10-7 gmol/cm3s. Therefore, putting the
appropriate values in eq 5:
A + zB solid catalyst8 products
(1)
At steady state, the rate of mass transfer per unit volume of
the liquid phase (gmol‚cm-3‚s-1) is given by:
RA) kSL-Aap{[Ao] - [As]}
(2)
(rate of transfer of A from bulk liquid to external surface
of the catalyst particle)
(3)
) zkSL-Bap{[Bo] - [Bs]}
(rate of transfer of B from the bulk liquid phase to the
external surface of the catalyst particle) (z ) 1 in this case)
) robs (observed rate of reaction within the catalyst particle)
(4)
1/robs . 1/kSL-Aap[Bo] and 1/kSL-Bap[Bo]
Here the subscripts “o” and “s” denote the concentrations
in bulk liquid phase and external surface of catalyst,
respectively.
Depending on the relative magnitudes of external resist-
ance to mass transfer and reaction rates, different controlling
i.e, 3 × 106 . 5.098 × 104 and 1.617 × 104
(7)
(23) Kumbhar, P. S.; Yadav, G. D. Chem. Eng. Sci. 1989, 44, 2535.
(24) Reid, R. C.; Prausnitz, M. J.; Sherwood, T. K. The Properties of Gases
and Liquids, 3rd ed.; McGraw-Hill Book Company: New York, 1977.
410
•
Vol. 5, No. 4, 2001 / Organic Process Research & Development

Figure 4. Effect of catalyst loading on conversion of tert-amyl
alcohol.
Figure 3. Effect of mole ratio on conversion of tert-amyl
alcohol.
Table 3. Effect of catalyst loading on conversion of tert-amyl
alcohol
Table 2. Effect of mole ratio on conversion of tert-amyl
alcohola
catalyst loading
(g/cm3)
% yield
of TAME
% conversion
tert-amyl
% yield of
TAME
alcohol:methanol
% conversion
selectivity
0.005
0.01
0.015
0.02
60.2
79.6
84.2
90.71
66.41
67.05
67.79
68.2
1:1
1:2
1:3
1:5
82.9
86.8
88.7
90.7
36.17
44.7
60.8
68.2
0.469
0.701
1.375
1.945
a tert-Amyl alcohol:methanol: 1:5; temperature: 70 °C; solvent: 1,4-dioxane;
speed of agitation: 800 rpm; time: 4 h; catalyst: Amberlyst-36.
a Catalyst: Amberlyst-36; temperature: 70 °C; speed of agitation: 800 rpm;
time: 4 h; catalyst loading: 0.02 g/cm3; solvent: 1,4-dioxane.
the reaction mixture. Figure 4 shows the effect of catalyst
loading on the conversion of tert-amyl alcohol. The conver-
sion increases appreciably with an increase in catalyst loading
from 0.005 to 0.01 g/cm3, which is obviously due to the
proportional increase in the number of active sites. With
further increase in the catalyst loading from 0.01 to 0.02
g/cm3, the conversion increases marginally (Table 3). All
further experiments were carried out at 0.02 g/cm3 of catalyst
loading.
As shown by eqs 1 and 2, at steady state, the rate of
external mass transfer (i.e., from the bulk liquid phase in
which A and B are located with concentration [Ao] and [Bo],
respectively) to the exterior surface of the catalyst is
proportional to aP, the exterior surface area of the catalyst
where the concentrations of A and B are [AS] and [BS],
respectively. For a spherical particle, aP is also proportional
to w, the catalyst loading per unit liquid volume as shown
by eq 6. It is possible to calculate the values of [AS] and
[BS]. For instance,
The above inequality demonstrates that there is an absence
of resistance due to the solid-liquid external mass transfer
for both the species A and B and the rate may be either sur-
face reaction-controlled or intraparticle diffusion-controlled.
Therefore, the effects of catalyst loading at a fixed particle
size and temperature were studied to evaluate the influence
of intraparticle resistance.
Effect of Mole Ratio. The effect of the variation of the
mole ratio of tert-amyl alcohol to methanol was studied from
1:1 to 1:5 mol under otherwise similar conditions. The
conversion of tert-amyl alcohol increased from 82.9 to 90.7%
as the mole ratio of tert-amyl alcohol to methanol was
increased from 1:1 to 1:5 (Figure 3). With an increase in
the mole ratio of tert-amyl alcohol to methanol from 1:1 to
1:5, the dehydration rate of tert-amyl alcohol decreases, and
hence a significant increase in selectivity towards tert-amyl
methyl ether from 36.2 to 68.2% was observed4 (Table 2).
All further experiments were carried out at tert-amyl alcohol-
to-methanol mole ratio of 1:5.
kSL-Aap{[Ao] - [AS]} ) robs at steady state
Effect of Catalyst Loading. In the absence of external
mass-transfer resistance, the rate of reaction is directly
proportional to catalyst loading based on the entire liquid-
phase volume. The loading of catalyst was varied over a
range of 0.005-0.02 g/cm3 on the basis of total volume of
) 3.33 × 10-7 gmol‚cm-3‚s-1
(8)
Thus, putting the appropriate values, it is seen that [AS] ≈
[Ao]. Similarly [BS] ≈ [Bo]. Thus, any further addition of
Vol. 5, No. 4, 2001 / Organic Process Research & Development
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411

Table 4. Effect of temperature on conversion of tert-amyl
alcohola
% yield
of TAME
temperature (°C)
% conversion
50
60
70
80
39.5
72.3
90.7
92.5
61.36
67.7
68.2
71.37
a Catalyst: Amberlyst-36; tert-amyl alcohol:methanol: 1:5; solvent: 1,4-
dioxane; time: 4 h; speed of agitation: 800 rpm; catalyst loading: 0.02 g/cm3.
Figure 5. Effect of temperature on conversion of tert-amyl
alcohol.
catalyst is not going to be of any consequence for changing
the rate of external mass transfer.
Proof of Absence of Intraparticle Resistance. The
average particle diameter of the catalyst used in the reactions
was 0.03 cm, and thus a theoretical calculation was done
based on the Wiesz-Prater criterion to assess the influence
of intraparticle diffusional resistance.25
According to the Wiesz-Prater criterion, the dimension-
less parameter Cwp, which represents the ratio of the intrinsic
reaction rate to intraparticle diffusion rate, can be evaluated
from the observed rate of reaction, the particle radius (Rp),
effective diffusivity of the limiting reactant (De), and
concentration of the reactant at the external surface of the
particle.
Figure 6. Effect of reusability of catalyst.
in the temperature range of 50-80 °C (Figure 5). The
conversion was found to increase substantially with an
increase in temperature from 50 to 70 °C. With further
increase in temperature from 70 to 80 °C, the conversion
increases only slightly as shown in (Table 4). The selectivity
to TAME also increases with temperature although margin-
ally. This is likely to be due to different activation energies
for the three parallel reactions of TAA. It appears that
reaction (a) has higher activation energy (Figure 1). All
further experiments were carried out at 70 °C.
2
(i) If Cwp ) robsFpRP /De [As] . 1
then the reaction is limited by severe internal diffusional
resistance.
(ii) If Cwp , 1,
then the reaction is intrinsically kinetically controlled.
Reusability of Amberlyst-36. The reusability of Am-
berlyst-36 was verified by employing it three times. After
each run the catalyst was washed thoroughly with methanol,
dried in an oven at 120 °C for 2 h, and weighed. The results
are shown in Figure 6. In the presence of fresh catalyst, the
conversion of tert-amyl alcohol was 90.7%. During the third
run, the conversion of tert-amyl alcohol was 89.5%, which
is almost the same as in the first run, without any significant
change in the selectivity of TAME (Table 5). Thus, the
catalyst was reusable.
The effective diffusivity of tert-amyl alcohol (De-A) inside
the pores of the catalyst was obtained from the bulk
diffusivity (DAB), porosity (ꢀ), and tortuosity (τ), where De-A
) DAB‚ꢀ/τ. In the present case, the value of Cwp was
calculated as 0.0079 for the initial observed rate, and
therefore the reaction is intrinsically kinetically controlled.
A further proof of the absence of the intraparticle diffusion
resistance was obtained through the study of the effect of
temperature, and it will be discussed later.
Effect of Temperature. The effect of temperature on
conversion under otherwise similar conditions was studied
Mechanism and Reaction Kinetics. Analysis of the
effect of various parameters and the product profile suggests
that the chemisorption of TAA should be the first step
followed by other reactions.
(25) Fogler, H. S. Elements of Chemical Reaction Engineering; Prentice-Hall:
New Delhi, 1995.
412
•
Vol. 5, No. 4, 2001 / Organic Process Research & Development

Table 5. Effect of Reusability of Amberlyst-36a
run number
% conversion
% yield of TAME
1
2
3
90.7
90
89.5
68.2
72.5
71.6
a tert-Amyl alcohol:methanol:1:5; temperature: 70 °C; solvent: 1,4-dioxane;
speed of agitation: 800 rpm; time: 4 h; catalyst loading: 0.02 g/cm3.
Chemisorption of TAA:
TAA + S {K
\
A} TAA-S
A} AS)
(9)
(A + S {K
\
where S is the vacant site
The chemisorbed species reacts according to the following
three paths
k1
TAA-S + MeOH
98 TAME-S + H2O
(10)
Figure 7. Plot of -ln(1 - XA) vs time at different catalyst
loadings.
In the above reaction, the Eley-Rideal mechanism is
assumed to be operative since no dimethyl ether was formed.
However, CB . CA , the initial concentration, and if ∑KiCi
o
o
k2
, 1, then the above equation becomes
TAA-S
TAA-S
9
8 2MB1-S + H2O
(11)
(12)
k3
-dC
A ) k′CAw
98 2MB2-S + H2O
1
dt
The following isomerisation reaction is also likely to occur
2MB1-S f 2MB2-S (13)
where k′ ) pseudo-first-order rate constant based on the
catalyst loading
1
All of the surface species desorp into the liquid phase in the
pore space followed by diffusion out of the particle.
The adsorption equilibrium constants for various species
can be written accordingly.
If the parallel forward reactions 10, 11, and 13 of TAA
are assumed to control the overall rate of the reaction, then:
) (k1CB + k2 + k3) KA (20)
(20)
o
k′ has units of (cm3/g‚mol‚sec)(cm3/g-cat).
1
The above equation can be integrated to get a typical first
order form
-ln(1 - XA) ) k′wt ) k′′
(21)
1
1
-dC
dt
-rA )
A ) k1CTAA-SCMeOH + k2CTAA-S + k3CTAA-S (14)
Thus plots can be made of LHS against t for a fixed catalyst
value of w to get the rate of constant k′′. Once again plots of
) (k1CB + k2 + k3)CAS (15)
(15)
1
k′′versus w will give k′. The Arrhenius plot of k′ versus 1/T
1
1
1
The concentration CS of the vacant sites
can be made under otherwise similar conditions to get
activation energy, which will be an apparent value.
Equation 20 can also be written as follows:
Ct ) CS + CAS + CBS + C2MB1-S + C2MB2-S
) CS[1 + KACA + KBCB + K2MB1C2MB1 + K2MB2C2MB2
]
(16)
(17)
k1′ ) k1KA C + k2 KA+ k3 KA
Bo
Ct
) kc1CB + kc2 + kc3, respectively
(22)
o
CS )
1 + KiCi
∑
In fact, the individual combined rate constants kc1, kc2, and
c3, which include KA, could be calculated from the rates of
formations of TAME, 2MB1, and 2MB2 since the product
k
Cs can be written in terms of the concentration of catalyst,
typically expressed as w in g/cm3, without loss of generality:
distribution was known. These values were calculated as kc1
w
) 0.508 cm-6‚gmol-1‚s-1‚g-cat-1 and kc2 ) 3.92 × 10-4
;
CS )
(18)
(19)
and kc3 ) 2.84 × 10-3 cm-3‚s-1‚g-cat-1
In the above theory, it is assumed that methanol is not
strongly adsorbed to generate the dehydrated product di-
methyl ether.
1 + KiCi
∑
-dCA (k1CB + k2 + k3)KACAw
rA )
)
dt
1 + KiCi
∑
Vol. 5, No. 4, 2001 / Organic Process Research & Development
•
413

Figure 9. Arrhenius plot.
Figure 8. Plot of -ln(1 - XA) vs time at different temperatures.
Plots of -ln(1 - XA) versus time for tert-amyl alcohol
were made for different catalyst loading as shown in Figure
7. The equation fits the data quite well because the R2 values
are very high, >0.98. Similar plots were made at different
temperatures, and it was found that the data fit well and the
reaction conforms to the proposed model (Figure 8). The
Arrhenius plots of ln k′ versus T-1 were made to get the
1
apparent energy of activation as 17.94 kcal/mol (Figure 9),
thereby also suggesting the reaction was intrinsically kineti-
cally controlled. The reaction follows a pseudo first-order
kinetics at fixed catalyst loading.
Selectivity of TAME. The point selectivity values for the
formation of TAME vis-a`-vis 2MB1 and 2MB2 are given by
dCTAME
dt
STAME/(2MB1+2MB2)
)
)
dC2MB1 dC2MB2
+
Figure 10. Selectivity of TAME as a Function of methanol
dt
dt
concentration after 4 h.
k1CMeOHCTAA-S
(23)
to be the most efficient catalyst. The effects of various
parameters on the rates of reaction were studied systemati-
cally to establish that there were no mass-transfer effects
and that the overall reaction was found to be intrinsically
kinetically controlled. The parallel reactions of tert-amyl
alcohol adsorbed on the sites were found to control the
overall rate of reaction which led to the formation of TAME,
2MB1, and 2MB2. The reaction follows pseudo first-order
kinetics at a fixed catalyst loading. The selectivity of TAME
over 2MB1 and 2MB2 is also found to be consistent with
the model.
k2CTAA-S + k3CTAA-S
dCTAME
k1
STAME/(2MB1+2MB2)
)
)
)
CMeOH (24)
(
)
dC2MB1 + dC2MB2
k2 + k3
∆CTAME
STAME/(2MB1+2MB2)
)
∆C2MB1 + ∆C2MB2
CTAME-0 - CTAME
C2MB1 + C2MB2
XACTAME-0
)
(25)
(26)
C2MB1 + C2MB2
k1
STAME/(2MB1+2MB2)
)
CMeOH
(
)
k2 + k3
Acknowledgment
A.V.J. thanks AICTE for awarding Junior Research
Fellowship. G.D.Y. thanks the Darbari Seth Professorship
Endowment.
Note Added after ASAP: There were errors in the
headings of Tables 3, 4, and 5 in the version posted ASAP
June 8, 2001; the corrected version was posted July 20, 2001.
Thus, point selectivity values, STAME, were plotted against
concentration of methanol as shown in Figure 10 to observe
that the relation is very well valid. This suggests that the
model is valid.
Conclusions
The synthesis of tert-amyl methyl ether by etherification
of tert-amyl alcohol with methanol is addressed in this paper.
Among the variety of catalysts used Amberlyst-36 was found
Received for review February 13, 2001.
OP010018+
414
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