Liquid phase synthesis of MTBE from methanol and isobutene over acid zeolites and Amberlyst-15

Article abstract of DOI:10.1006/jcat.1998.2366

The liquid phase synthesis of methyl tert-butyl ether (MTBE) from methanol and isobutene over H-Beta and US-Y zeolite catalysts was studied in the temperature range 30-120°C. Up to 100°C, commercial H-Beta zeolite samples with small crystal size were more active than acid Amberlyst-15 (reference catalyst) and noticeably more active than US-Y, confirming results obtained under vapour phase conditions. The influence of methanol/isobutene (MeOH/IB) molar ratio, pressure, and space time on the conversion and MTBE selectivity was investigated. At optimized reaction conditions, MTBE yields of 85-90% can be reached with zeolite H-Beta as well as Amberlyst-15. On zeolites, side reactions of isobutene are more important than on Amberlyst-15, necessitating operation at MeOH/IB ratios higher than 1:1. For the same reason, at high conversion on H-Beta, the MTBE yields are more sensitive to contact time compared to Amberlyst-15. On H-Beta zeolite, no deactivation was observed during a period of more than 50 h on stream at 65°C, 1.4 MPa pressure, and a WHSV of 14 h^-1. The catalytic activity of the zeolites is related to the external specific surface area, and to the concentration of bridging hydroxyls and silanol groups in the mesopores. A zeolite H-Beta sample with a Si/Al ratio of 36 has an optimum silanol and bridging hydroxyl content leading to stoichiometric methanol and isobutene adsorption, highest activity and MTBE yields.

Full text of DOI:10.1006/jcat.1998.2366

Journal of Catalysis 182, 302–312 (1999)
Article ID jcat.1998.2366, available online at http://www.idealibrary.com on
Liquid Phase Synthesis of MTBE from Methanol and Isobutene
over Acid Zeolites and Amberlyst-15
1
∗
∗
∗
F. Collignon, R. Loenders, J. A. Martens, P. A. Jacobs, and G. Poncelet
Unite´ de Catalyse et Chimie des Mate´riaux Divise´s, Universite´ Catholique de Louvain, Place Croix du Sud 2/17, B-1348 Louvain-la-Neuve, Belgium;
∗
and Centrum voor Oppervlaktechemie en Katalyse, Katholieke Universiteit Leuven, Kardinaal Mercierlaan, 92, B-3001 Heverlee, Belgium
Received April 15, 1998; revised September 20, 1998; accepted November 19, 1998
the catalyst, causing a loss of activity and giving rise to cor-
rosion problems (2, 3). The MeOH/IB molar ratios above
stoichiometry employed in order to ensure complete con-
version of the isobutene necessitate a recycling operation.
The use of alternative catalysts for MTBE synthesis has
been reviewed by Hutchings et al. (4). In recent years, many
catalytic systems have been evaluated under vapour phase
conditions as potential substitutes to resins, such as zir-
conium phosphates (5), sulfated zirconia (6, 7), titanium
silicalite (8), gallophosphate cloverite (9, 10), heteropoly-
acidseitherunsupported(11, 12)orsupportedonsilica(13),
clays(14), andactivecarbon(15). Hydrogenzeolites, steam-
dealuminated zeolites, and triflic acid-loaded zeolites have
been considered as well (16–25).
In a previous study, several acidic zeolites, including
H-ZSM-5, large- and small-port H-Mordenites, H-Omega,
US-Y, H-Beta, and SAPO-5, were evaluated as catalysts for
the vapour phase synthesis of MTBE (26). It was found that
zeolite H-Beta and acid Amberlyst-15 (Amb.-15) had sim-
ilar activities, and were more active than the other zeolites
tested, a result which was recently confirmed by Hunger
et al. (27). In the present work, we extended the compari-
son of H-Beta and Amberlyst-15 to the liquid phase, which
are the conditions applied in current industrial processes.
The liquid phase synthesis of methyl tert-butyl ether (MTBE)
from methanol and isobutene over H-Beta and US-Y zeolite cata-
lysts was studied in the temperature range 30–120◦C. Up to 100◦C,
commercial H-Beta zeolite samples with small crystal size were
more active than acid Amberlyst-15 (reference catalyst) and no-
ticeably more active than US-Y, confirming results obtained un-
der vapour phase conditions. The influence of methanol/isobutene
(MeOH/IB) molar ratio, pressure, and space time on the conversion
and MTBE selectivity was investigated. At optimized reaction con-
ditions, MTBE yields of 85–90% can be reached with zeolite H-Beta
as well as Amberlyst-15. On zeolites, side reactions of isobutene are
more important than on Amberlyst-15, necessitating operation at
MeOH/IB ratios higher than 1 : 1. For the same reason, at high con-
version on H-Beta, the MTBE yields are more sensitive to contact
time compared to Amberlyst-15. On H-Beta zeolite, no deactiva-
tion was observed during a period of more than 50 h on stream at
65◦C, 1.4 MPa pressure, and a WHSV of 14 h−1. The catalytic ac-
tivity of the zeolites is related to the external specific surface area,
and to the concentration of bridging hydroxyls and silanol groups
in the mesopores. A zeolite H-Beta sample with a Si/Al ratio of 36
has an optimum silanol and bridging hydroxyl content leading to
stoichiometric methanol and isobutene adsorption, highest activity
c
and MTBE yields.
ꢀ 1999 Academic Press
Key Words: MTBE; liquid phase synthesis; beta zeolites.
INTRODUCTION
EXPERIMENTAL
Methyl tert-butyl ether (MTBE), the most widely used
octane booster for reformulated gasolines, is produced in
the liquid phase over sulfonic acid resins (e.g., Amberlyst-
15) at temperatures in the range 50–70◦C, pressures bet-
ween1.0and1.5MPa, andmethanol/isobutene(MeOH/IB)
molar ratios higher than 1 : 1 (1). Di-isobutenes (2,4,4 tri-
methyl-1- and -2-pentenes), tert-butyl alcohol, and dimeth-
lether are the main side products. The reaction is exother-
mic (ꢀH = −37 kJ mol−1), and, therefore, careful control
of temperature is required in order to avoid local overheat-
ing and release of sulfonic groups and sulfuric acid from
Catalysts
Four samples of zeolite H-Beta and one sample of US-Y
were selected for this study. Some physicochemical and tex-
tural properties of these samples are given in Table 1. The
reference catalyst was Amberlyst-15 (sulfonic acid resin
from Aldrich).
Zeolite US-Y (CBV 760) and zeolite H-Beta (ZB25 and
ZB75) were commercial samples supplied by PQ. ZBSC,
another H-Beta zeolite, was provided by Su¨d-Chemie. ZBF
was a zeolite Beta sample synthesized according to the pro-
cedure of Caullet et al. (28) and having a much larger crystal
size (smaller external surface area). Sample CBV 760, an
ultrastable Y from PQ, was selected because it was the most
1 To whom correspondence should be addressed.
302
0021-9517/99 $30.00
c
Copyright ꢀ 1999 by Academic Press
All rights of reproduction in any form reserved.

LIQUID PHASE SYNTHESIS OF MTBE
303
TABLE 1
+
Crystal Size, Relative Crystallinity, Si/Al Ratio, Al Bulk, Cation Exchange Capacity (NH4 -CEC), Thermodesorption
of Ammonia (NH3-TPD), and Ammonia Desorption Enthalpy (ꢀH )
Crystal size Rel. cryst.
Al
NH+4 -CEC
NH3-TPD
ꢀH
b
c
Sample
(µm)
(%)
Si/Ala
Si/Alb (mmol g−1
)
(meq g−1
)
(mmol g−1
)
TPD/CEC (kJ mol−1
)
ZB25
ZB75
ZBSC
ZBF
CBV 760
0.1–0.7d
0.1–0.7d
n.d.
100
96
97
72
40
12.2
36.0
24.6
6.7
15.3
39.8
25.8
19.5
36.7
1.26
0.45
0.65
2.17
0.52
1.02
0.41
0.62
0.81
0.44
0.90
0.41
0.45
0.41
0.20
0.88
1.00
0.73
0.51
0.45
121
123
123
n.d.
n.d.
1–3
0.4–0.6d
30.0
Note. n.d., not determined.
a From ICP+S analysis.
b From NH4 -CEC.
c Calculated according to Ref. (31).
d Data from PQ.
active sample among a series of US-Y zeolites tested under the zeolites was obtained by treating typically 1 g of zeolite
vapour phase conditions (26).
overnight at 80◦C under reflux conditions with 200 ml of
2 M ammonium acetate solution. The slurry was filtered
hot, washed with demineralized water until the conductiv-
ity of the wash water was reduced to it+s initial value, and
dried at 60◦C for 1 h. Using these NH4 -CEC values, the
Si/Al ratios were recalculated (Si/Alc in Table 1), assuming
that each exchanged NH+4 cation site is associated with one
tetrahedral (framework) Al.
Temperature-progammed desorption of ammonia (NH3-
TPD) was carried out following the experimental condi-
tions proposed by Niwa et al. (31) and Katada et al. (32),
which allow one to determine the desorption enthalpies
of ammonia from the high-temperature desorptions and,
hence, to provide an evaluation of the average acid strength
of the zeolites. TPD was monitored with a TC detector and
theammoniainthegaseouseffluentattheoutletofthedete-
ctor continuously trapped in 0.2 M H3BO3 solution. The
total amount of ammonia evolved was determined by
Characterization
The relative crystallinity of the H-Beta zeolites (Table 1)
was evaluated from the X-ray diffraction patterns recorded
with a Siemens D-5000 diffractometer (with copper anti-
cathode), referring the area of the [302] reflection at 22.43◦
2θ to that of sample ZB25. For CBV 760, the crystallinity
was related to that of the parent CBV 500 (26). The Si/Al ra-
tios determined from the elemental analyses performed by
ICPS are indicated in Table 1. The X-ray diffraction pattern
of ZBF showed the presence of a small amount of pseudo-
boehmite (with interplanar distances of 0.615, 0.317, and
0.240 nm), and, therefore, the Si/Al ratio of 6.7 of this sam-
ple derived from the bulk analysis is underestimated.
The textural characteristics were derived from the nitro-
gen adsorption isotherms recorded at the boiling point of
nitrogenwithanASAP2000sorptometerfromMicromerit-
ics, after outgassing the samples for 4 h at 200◦C. The ex-
ternal surface areas and micropore volumes, Sext and Vµ,
respectively, were established using an approach recently
proposed (29). The mesopore distributions were calculated
using the Barrett–Joyner–Halenda method based on Kelvin
equation (30). The experimental values are given in Table 2.
The pore size distribution in the meso- and macropore re-
gionofthedifferentzeoliteswasexamined. AsseeninFig. 1,
distinct distribution curves are observed: ZB25, ZB75, and
TABLE 2
External Surface Area (Sext), Micropore Volume (Vµ), Total Pore
Volume (Vtot), Mesoporous Volume (Vmeso), and Relative Silanol
Concentration ([SiOH])
Sext.
Vµ
Vtot
Vmeso
[SiOH]
a
a
b
c
d
Sample
(m2 g−1
)
(cm3 g−1
)
(cm3 g−1
)
(cm3 g−1
)
(a.u. g−1
)
ZB25
ZB75
ZBSC
ZBF
CBV 760
219
218
124
30
0.23
0.21
0.21
0.17
0.25
1.22
1.17
1.08
0.24
0.47
0.52
0.49
0.18
0.04
0.16
100
143
110
n.d.
n.d.
˚
ZBSC have pores in the mesopore range 20–500 A. ZBF,
with the smallest external surface area among the Beta ze-
olites and the largest particles, is almost exempt of meso-
pores (0.04 cm3 g−1). The data for the US-Y zeolites were
addedforcomparison. US-YCBV760hasfewermesopores
(Vmeso = 0.19 cm3 g−1).
143
Note. n.d., not determined.
a Calculated according to Ref. (29).
The cation exchange capacity (NH+4 -CEC) of the sam-
ples was determined by the micro-Kjeldahl method on
ammonium-exchanged samples. The ammonium form of
b Calculated with N2 sorption at p/p0 = 0.99.
c Calculated according to Ref. (30).
d From FTIR (normalized integrated area of the band at 3740 cm−1).

304
COLLIGNON ET AL.
FIG. 1. Pore size distribution curves of H-Beta zeolites (ZB25, ZB75, ZBF, and ZBSC) and US-Y zeolites (CBV 500, CBV 600, and CBV 760).
backtitration with 0.005 M H2SO4. The desorption en- in the reactor via a mass flow controller for liquids (from
thalpies are indicated in Table 1.
Brooks Instrument B.V.). Pressurisation was ensured by ni-
Infrared spectra were recorded with a Bruker FTIR trogen. The reaction was studied in the temperature range
IFS-88 spectrometer on wafers obtained by compression 30–120◦C. A thermocouple placed in the catalyst bed (1 g)
(15 mg cm−2). In this study, only the OH stretching region allowed continuous monitoring of the reaction tempera-
will be considered. The samples were outgassed at 400◦C ture. Alltubings, valves, andconnectionsbetweentheoutlet
for 4 h under a residual pressure of 10−3 Pa. The relative in- of the reactor and the gas chromatograph were heated.
tegrated areas of the Si–OH band at 3740 cm−1 normalized
to the sample weight are indicated in Table 2.
On-line analysis of the reaction products was done in
a HP-5890 gas chromatograph (from Hewlett–Packard)
The adsorption capacities of MeOH and IB of the zeo- equipped with flame ionization detector and 50 m capillary
lites and Amberlyst-15 were evaluated under precatalytic column (CPSil-5-CB, film thickness: 5.49 µm). Sampling
dynamic conditions in the installation used for the vapour of the liquid effluent was done every hour by means of
phase synthesis of MTBE (26) and equipped with on-line an automated six-way sampling valve with internal volume
gas chromatographic analysis. The conditions were as fol- of 0.1 µl (Valco Instrument Co.). The temperature of the
lows: weight of catalyst = 200 mg; MeOH : IB : He = 1 : 1 : 5;
P = 0.1 MPa; WHSV = 3.25 h−1; temperature = 27◦C. The
TABLE 3
amounts of isobutene and methanol retained by the dif-
Amounts of Adsorbed Methanol and Isobutene (mmol g−1
and Molar Ratios
)
ferent zeolites and the resin determined from the break-
through curves of the two compounds are indicated in
Table 3. The saturation capacities are accurate within 5%.
Methanol
(MeOH)
Isobutene
(IB)
Catalyst
MeOH/IB
Catalytic Experiments
Amb.-15
ZB25
ZB75
ZBSC
ZBF
CBV 760
7.48
4.50
4.83
3.78
1.64
6.93
2.64
3.08
4.58
2.54
1.24
4.80
2.83
1.46
1.05
1.49
1.32
1.44
The reactions were carried out in the liquid phase in a
fixed bed stainless-steel reactor operated, unless otherwise
specified, at 2 MPa, using a MeOH : IB molar ratio of 1, and
a WHSV of 14 h−1. Methanol was blended with isobutene
in a pressure vessel, and the stirred mixture was injected

LIQUID PHASE SYNTHESIS OF MTBE
305
reactor was increased by 10◦C after each sampling of the re- for ZBSC at 65◦C, the H-Beta zeolites were less selective
action products. Prior to the catalytic evaluation, the zeolite than the resin. US-Y was very selective at low conversions.
was activated in situ at 400◦C for 3 h under a stream of nitro-
Influence of Pressure
gen. The reference Amberlyst-15 was first washed (water
and alcohol) using the procedure of Volosch et al. (33) and
pretreated in situ at 90◦C for 3 h in flowing nitrogen.
The influence of pressure (0.6, 1, 1.4, and 2 MPa) was ex-
aminedforthezeoliteZB75andtheresin, withaMeOH : IB
ratio of 1 and WHSV of 14 h−1. For both catalysts, pressure
only had a limited influence on the conversion of isobutene,
as expected under liquid phase conditions. In the case of the
zeolite, beyond the maximum conversion of IB to MTBE,
the undesired oligomerisation of IB was relatively more en-
hancedathighpressure. Asimilarobservationwasreported
by Obenaus and Droste (35).
RESULTS
Screening Tests and Influence of Temperature
on the Catalytic Activity
For the different zeolites and the acid resin (Amb.-15)
catalysts, the variations of the conversion of isobutene (IB)
and of the isobutene based yield of MTBE with reaction
temperature are compared in Figs. 2a and 2b, respectively.
The IB conversions (Fig. 2a) increased with increasing re-
Influence of MeOH : IB Molar Ratio
Experiments were performed on ZB75 and Amberlyst-
action temperature to reach a maximum of 95% at around 15 with three MeOH : IB ratios, 1, 1.2, and 1.5, at a pressure
100◦C for ZB25, ZB75, and ZBSC, and of about 92–93% of 2.0 MPa and WHSV of 14 h−1. The variations of the con-
for Amb.-15. These H-Beta zeolites were more active than version of IB and of the yield of MTBE with temperature
Amb.-15. The ZB75 catalyst was most active in terms of are shown in Figs. 3a and 3b, and 4a and 4b for ZB75 and
IB conversion, with an isobutene conversion above 90% the resin, respectively. On the zeolite, in the temperature
at 60◦C. ZBF and, especially, US-Y were less active, with range of increasing conversions, similar conversion curves
conversions of 85 and 75%, respectively, at 100◦C.
are obtained for MeOH/IB ratios of 1 and 1.2 (up to about
On Amb.-15, ZBF, and US-Y, the IB-based yield of 70◦C, Fig. 3a) but higher maximum yields to MTBE are
MTBE (Fig. 2b) increased continuously with reaction tem- reached for the ratio 1.2 compared to 1 (Fig. 3b). For the ra-
perature. On the more active ZB25, ZB75, and ZBSC, the tio 1.5, the IB conversion curve is shifted to higher tempera-
MTBE yield passed through a maximum at around 60, 70, tures and nearly similar IB conversions are achieved above
and 75◦C, respectively. The decreasing yields of MTBE no- 80◦C. However, at 70◦C and above, the MTBE yield is im-
ticed above 60–80◦C for ZB25, ZB75, and ZBSC are due proved with respect to the lower MeOH/IB ratios (Fig. 3b).
to a loss of MTBE selectivity because of increasing dimeri- On the resin, the IB conversion increases with increasing
sation of isobutene.
MeOH/IB ratio (Fig. 4a). The yields of MTBE improve
According to the thermodynamics of the reaction, at when increasing this ratio (Fig. 4b). The maximum yields of
65◦C and 1.6 MPa, and for a MeOH : IB ratio of 1 : 1, the MTBE are less influenced by the MeOH/IB ratio, but the
MTBE yield at equilibrium amounts to 91.5% (34). In our maximum value is reached at lower temperatures as the
experiments, with none of the zeolites the thermodynamic MeOH/IB ratio increases.
equilibrium represented in Fig. 2b was reached. The con-
The catalytic results obtained at 60 and 90◦C (2 MPa,
version of IB, the yield of MTBE, and the selectivities ob- WHSV of 14 h−1) on H-Beta ZB75 and on the resin for
served at 50, 65, and 90◦C are detailed in Table 4. Except the different MeOH/IB ratios are compared in Table 5. For
TABLE 4
Isobutene Conversion (XIB); Yield IB to MTBE (YMTBE); Selectivity to MTBE (SMTBE) at 50, 65,
and 90◦C; and Apparent MTBE Formation Rate at 50◦C (mol h−1 g−1
)
XIB (%)
YMTBE (%)a
SMTBE (%)
Rate
50
Catalyst
T (◦C): 50
65
90
50
65
90
50
65
90
Amb.-15
ZB25
ZB75
ZBSC
ZBF
CBV 760
26
52
68
33
7
72
85
94
85
30
7
90
95
96
95
82
71
25
49
62
32
7
68
76
82
81
28
7
87
65
65
83
74
63
98
93
92
97
97
94
89
87
95
93
97
68
68
87
90
89
0.039
0.077
0.099
0.051
0.011
0.005
3
3
100
100
Note. Reaction conditions: P = 2.0 MPa; MeOH/IB = 1; WHSV = 14 h−1
.
a For the yields at equilibrium, multiply the values at 50, 65, and 90◦C by 1.03, 1.04, and 1.07, respectively.

306
COLLIGNON ET AL.
FIG. 2. Isobutene conversion (a) and IB to MTBE yields (b) vs reaction temperature on Amberlyst-15, H-Beta zeolites (ZB25, ZB75, ZBF, and
ZBSC), and US-Y (CBV 760). Reaction conditions: P = 2 MPa; MeOH/IB = 1; WHSV = 14 h−1
.
both catalysts, higher conversions of isobutene are achieved Influence of WHSV
at 90◦C than at 60◦C. The influence of the MeOH/IB ratio
on the conversion is more marked for the resin than for
Results of experiments performed at different WHSV
the zeolite. The temperature dependence of the selectivity over ZB75 (MeOH : IB ratio of 1; 2 MPa pressure; 60◦C)
of isobutene to MTBE is more pronounced for the zeolite are presented in Table 6. Increasing WHSV had a bene-
compared with the resin and, as expected, the methanol ficial effect on the yields of MTBE and on the selectiv-
conversion decreases as the MeOH/IB ratio increases.
ity, whereas the total conversion of IB decreased. These

LIQUID PHASE SYNTHESIS OF MTBE
307
FIG. 3. Influence of MeOH/IB molar ratio: isobutene conversion (a) and IB to MTBE yield (b) vs temperature on ZB75. Reaction conditions:
P = 2 MPa; WHSV = 14 h−1
.
data show that long contact times with the zeolite catalyst isobutene oligomers. This effect should be attenuated at
at high conversion should be avoided. This could be ex- higher MeOH/IB ratios, as may be inferred from Fig. 3.
plained by lower methanol concentration in the reactant
Durability Test
stream, making dimerisation more likely. Alternatively, the
losses of MTBE yield at long contact times could be due
to a secondary conversion of the MTBE into methanol and the following conditions: pressure of 1.4 MPa, WHSV of
A prolonged test was performed at 65◦C on ZB75, under

308
COLLIGNON ET AL.
FIG. 4. Influence of MeOH/IB molar ratio: isobutene conversion (a) and IB to MTBE yield (b) vs temperature on Amberlyst-15. Reaction
conditions: same as in Fig. 2.
14 h−1, and MeOH : IB of 1. As shown in Fig. 5, the con- of MTBE of 638 g per g catalyst was produced without no-
version of IB remained stable during 53 h on stream. The ticeable loss of activity. The resin operating under industrial
transformation of IB to MTBE stabilised at a value of 82% conditions has a lifetime of at most 2 years. Considering a
with a selectivity of 87%. No tert-butyl alcohol was formed feed containing 20–30% C4, with half of it being isobutene,
and the yields of oligomerisation products remained con- a rough estimate of the production of MTBE would amount
stant. Under these experimental conditions, a total weight to 4000 g MTBE per g resin assuming 16 × 103 h lifetime of

LIQUID PHASE SYNTHESIS OF MTBE
309
TABLE 5
TABLE 6
Isobutene Conversion (XIB), Yield of MTBE (YMTBE), and Selecti-
vity to MTBE (SMTBE) on ZB75 for Different WHSV at 60◦C
Isobutene and Methanol Conversion (XIB; XMeOH), Yield of
MTBE (YMTBE), and Selectivity (SIB,MTBE; SMeOH,MTBE) on ZB75
and Amb.-15 for Different MeOH/IB Molar Ratios at 60 and 90◦C
WHSV (h−1
)
XIB (%)
YMTBE (%)a
SMTBE (%)
a
T
MeOH/ XIB YMTBE SIB,MTBE XMeOH SMeOH,MTBE
Catalyst (◦C)
IB
(%)
(%)
(%)
(%)
(%)
9
14
28
95
92
90
77
82
83
81
89
92
ZB75
60
90
1.0
1.2
1.5
90
90
88
82
87
75
91
97
85
82
72
50
100
100
100
Note. Reaction conditions: P = 2.0 MPa; MeOH/IB = 1.
a For the yields at equilibrium, multiply the values by 1.04.
1.0
1.2
1.5
95
97
95
64
74
83
67
76
87
66
62
51
100
100
100
DISCUSSION
Amb.-15 60
90
1.0
1.2
1.5
60
70
80
55
67
79
92
96
99
62
54
53
100
100
100
The activity sequence of the H-Beta and US-Y zeolites
tested in the liquid phase synthesis of MTBE is similar
to that found under vapour phase conditions, where the
resin and ZB25 and ZB75 samples reached MTBE yields
of 50–51% (26), significantly higher than for US-Y (29%),
the next most active zeolite. In the vapour phase synthe-
sis, a correlation was found between the maximum yield
of MTBE and the external surface area of the zeolite (26).
1.0
1.2
1.5
90
94
94
87
90
87
97
96
93
89
78
51
100
100
100
Note. Reaction conditions: P = 2.0 MPa; WHSV = 14 h−1
.
a
For the yields at equilibrium, multiply the values at 60 and 90◦C by
1.04 and 1.07, respectively.
the resin and WHSV of 1 h−1. Of course, in the laboratory In the liquid phase synthesis, it is now also found that the
a prolonged activity test could not be envisaged. However, apparent MTBE formation rate at 50◦C is proportional to
the present data suggest that a productivity figure similar the external surface area of the zeolite (Fig. 6). ZB75 and
to the one obtained on the resin might be within reach with ZB25 having the highest external surface area are the most
a H-Beta zeolite.
active samples. It may be inferred from this dependence
FIG. 5. Isobutene conversion, yield of IB to MTBE, and of the by-products vs time on stream on ZB75. Reaction conditions: P = 1.4 MPa,
WHSV = 14 h−1; MeOH/IB = 1; T = 65◦C.

310
COLLIGNON ET AL.
FIG. 6. Variation of MTBE formation rate at 50◦C (in mol g−1 h−1) vs external surface area (Sext, in m2 g−1) of H-Beta zeolites (ZB25, ZB75, and
ZBSC) (full symbols), and US-Y (CBV 760) (open symbol).
that the reaction is strongly diffusion limited and that only
For US-Y zeolites, extraframework aluminum (with 27Al
the active sites near the external surface contribute to the NMR signal at 30 ppm) had a negative influence on the re-
catalytic turnovers, while the bulk of the zeolite crystals is action rate in the vapour phase synthesis of MTBE (26).
inactive. A similar situation was encountered in the liquid Kiricsi et al. (38) did not find such species in acid-deal-
phasealkylationofisobutanewith1-butene(36)andphenol uminated H-Beta zeolite. For the present H-Beta zeolites,
alkylation with tert-butyl alcohol (37), both on Beta zeolite. the presence of extraframework aluminum can be esti-
The difference in activity of ZB25 and ZB75 (Fig. 6), having mated from the NH3-TPD/NH+-CEC ratios (Table 1). The
the same external surface area of ca. 220 m2 g−1 (Table 1), ratio of 1 obtained for ZB75 in4dicates, within the precision
remains to be explained.
of the measurements, the absence of a significant amount
Dealuminated zeolites have been shown to be more ac- of extraframework Al species. In sample ZB25, the ratio
tive than their nondealuminated forms in a large number is equal to 0.88, which points to the presence of some ex-
of proton-catalyzed reactions (19–23). A similar observa- traframework aluminum. Since this aluminum is mostly lo-
tion was made in the vapour phase synthesis of MTBE (26). cated inside the micropores, and the catalytic activity is not
It is not known whether ZB75 is a dealuminated form of located in this type of pores, it seems unlikely that these
ZB25 or the result of direct synthesis, but its higher con- species play a significant role in this catalyst.
tent of silanol groups (Table 1), absence of extraframework
The methanol and isobutene adsorption capacities under
Al (inferred from the NH3-TPD/NH+4 -CEC ratio of 1), precatalytic conditions (Table 3) shed more light on the ac-
lower crystallinity compared with ZB25 (Table 1), and sim- tivity difference of ZB25 and ZB75. The saturation capacity
ilar texture (Fig. 1, Table 1) suggest an acid dealuminated for isobutene is significantly larger for ZB75 (4.6 compared
form.
to 3.1 mmol g−1). Dealumination of a zeolite enhances its
The desorption enthalpies of ammonia given in Table 1 hydrophobic character and can explain the enhanced ad-
indicate that the acid sites have nearly equivalent average sorption of isobutene in ZB75. On the other hand, the ad-
strengths. For ZB25 from the same source, Katada et al. sorption capacity for methanol is very similar in these two
(32) reported a ꢀH of 122 kJ mol−1. Thus, based on the am- zeolites (4.8 and 4.5 mmol g−1).
monia desorption enthalpies, differences in acid strengths
In a recent study by in situ H-MAS NMR spectroscopy of
can hardly be invoked to account for activity differences methanol sorption on H-Beta (with Si/Al = 16), H-ZSM-5
of ZB75 and ZB25. Moreover, the most active ZB75 has (Si/Al = 22), and H-Y (Si/Al = 2.6) zeolites, Hunger et al.
a higher Si/Al ratio and contains a lower number of acid (27) and Hunger and Horvath (39) showed that the pre-
sites.
ferential adsorption sites in H-ZSM-5 and H-Y are the

LIQUID PHASE SYNTHESIS OF MTBE
311
bridging hydroxyls (SiOHAl), whereas in H-Beta, in ad-
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