Silica-supported sulfated zirconia: A new effective acid solid for etherification

Article abstract of DOI:10.1039/b007475f

Silica-supported sulfated zirconia exhibits a comparable and even higher ether production than a reference acid resin (Amberlyst 15) giving an ether yield of 30% at 50% conversion.

Full text of DOI:10.1039/b007475f

Silica-supported sulfated zirconia: a new effective acid solid for etherification
Shaobin Wang* and James A. Guin
Department of Chemical Engineering, 230 Ross Hall, Auburn University, AL 36849, USA.
E-mail: shaobin@eng.auburn.edu
Received (in Cambridge, UK) 14th September 2000, Accepted 7th November 2000
First published as an Advance Article on the web
Silica-supported sulfated zirconia exhibits a comparable and
even higher ether production than a reference acid resin
(Amberlyst 15) giving an ether yield of 30% at 50%
conversion.
cial resins, Nafion NR50 and silica-supported Nafion SAC-13
(from Fluka and Aldrich, respectively), were also tested.
Additionally, a zeolite sample H-ZSM-5, obtained from United
Catalyst, was also tested.
The reactions were carried out at 80 °C for 2 h in a 25 ml
stainless steel batch reactor with constant agitation under a
pressure of 1.8 MPa of dried helium. The catalyst loading was
0.5 g in all the cases. Before reaction, all catalysts were dried
overnight in an oven at 60 °C under vacuum. The reactants,
consisting of an appropriate amount of alcohol (0.2 g methanol
or 0.27 g ethanol) and 0.5 g 23DM1B with an alcohol+olefin
molar ratio of ca. 1+1, were mixed with heptane (4 g) as a
solvent. The products were analyzed in a Varian gas chromato-
graph equipped with a capillary column and FID.
Table 1 shows the results of 23DM1B etherification with
methanol over the various laboratory-prepared and reference
catalysts. In the reaction, 23DM2B and the ether are the major
products. A small amount of 2,3-dimethylbutan-2-ol was also
detected, probably due to the reaction between the olefins and a
trace amount of water from the catalysts and reactant mixture.
Amberlyst 15 is an active catalyst converting 23DM1B to
23DM2B and ether at an overall rate of 90%; however, this
catalyst is non-selective for etherification, having instead a high
(undesirable) isomerization selectivity of 80% and 19% ether
yield. Nafion NR50 shows low activity and low isomerization
selectivity, but an ether yield of about 16%. The silica-
supported Nafion SAC-13 is less active than the unsupported
Nafion NR50, a behavior which is not the same as in some other
reactions, where Nafion SAC-13 exhibited higher activity than
Nafion NR50 due to a high surface area and enhanced
accessibility of the reactants to the acid sites.11,12 HZSM-5
shows a similar activity to the Nafion NR50 but the highest
isomerization selectivity (94%) among the all catalysts tested in
this investigation and thus the lowest ether yield. For the
laboratory-prepared solid acid catalysts, sulfated zirconia
exhibits higher conversion, ether selectivity and yield than
Nafion SAC-13, while tungstated zirconia is the least active
catalyst with an isomerization selectivity similar to that of
Nafion SAC-13 and SZ. SZ/SiO2-S exhibits much higher
conversion with a lower isomerization selectivity, and an ether
yield of 30%, which is substantially higher than that over the
reference Amberlyst 15. SZ/SiO2-N shows 40% conversion and
64% ether selectivity, both higher than Nafion NR50, and an
ether yield of 24%, which is also higher than that over
Methyl tert-butyl ether (MTBE) has been the major gasoline
additive in the past decade. However, its environmental
consequences to drinking water have caused intense public
attention in recent years.1 Substantial quantities of unsaturated
C5 and C6 compounds are present in the light gasoline produced
in the fluid catalytic cracking units of refineries. Some of these
compounds can be etherified with alcohols and possibly used to
meet the demand for oxygenates and as a partial replacement for
MTBE. Moreover, these higher ethers have lower vapor
pressures and lower water solubilities compared to MTBE.
Currently, ion exchange resins are the dominant catalysts for
ether production. However, new catalysts could offer improve-
ments over acid resins with respect to thermal stability, poor
selectivity under certain conditions, and lack of regeneration
ability. Several inorganic acid solids such as zeolites, sulfated
zirconia, heteropolyoxoanions (HPA) and supported HPA
catalysts have been evaluated for the production of MTBE.2–5
Essayem et al. investigated etherification/esterification over
two sulfated zirconia catalysts prepared by different methods
and acid resins, but they found that sulfated zirconia showed
faster deactivation.6 No research has been thus far reported for
the application of supported sulfated zirconia for etherification,
though several researchers have reported the application of such
catalysts in isomerization.7–9 It is believed that unsupported
sulfated zirconia suffers from the disadvantages of lower
surface area and the very limited accessibility to the acid sites in
liquid phase reactions with less polar solvents or in gas phase
reactions. Therefore, development of well dispersed sulfated
zirconia on supports with high surface areas is important for
some acid-catalyzed reactions. This study reports preliminary
results on the preparation of some inorganic acid solids, sulfated
zirconia, tungstated zirconia, and supported sulfated zirconia,
and their performance in the etherification of certain C6 olefins
with alcohols. These particular C6 olefins, 2,3-dimethylbut-
1-ene (23DM1B) or 2,3-dimethylbut-2-ene (23DM2B), were
chosen on the basis of their potential for making oxygenated
transportation fuel additives.10
A sulfated zirconia (SZ) was prepared by acid-treatment of
Zr(OH)4, obtained by precipitation of ZrO(NO3)2 solution with
NH3·H2O at pH = 10, with 0.5 M H2SO4 solution, followed by
evaporating and drying at 100 °C overnight and calcination at
600 °C for 2 h. A tungstated zirconia with 10 wt% WO3 was
prepared by impregnation of ammonium tungstate hydrate on
Zr(OH)4. Two silica-supported sulfated zirconia catalysts,
referred to as SZ/SiO2-S and SZ/SiO2-N, were prepared by
impregnation of 3 g of zirconium sulfate and 2.45 g ZrO(NO3)2
on 3 g of commercial silica gel support (SBET = 650 m2 g21) in
0.5 M sulfuric acid solution, respectively, followed by calcina-
tion at 600 °C for 2 h. The third catalyst, SZ/SiO2-NP, was
prepared by precipitation of 2.45 g ZrO(NO3)2 on SiO2 using
NH3·H2O at pH = 10 under constant stirring, followed by
sulfation in 0.5 M H2SO4 solution and calcination at 600 °C for
2 h. A commercial ion exchange resin, Amberlyst 15 (from
Aldrich), was used as a reference catalyst. Two other commer-
Table 1 Catalytic activity of various acid solids and commercial resins in
23DM1B etherification with methanol
23DM1B
conv. (%)
23DM2B
sel. (%)
Ether
sel. (%) (%)
Ether yield
Catalyst
Amberlyst 15
Nafion NR50
Nafion SAC-13
HZSM-5
ZrO2–WO3
SZ
SZ/SiO2-S
SZ/SiO2-N
SZ/SiO2-NP
91.0
29.0
3.2
23.2
0.9
78.7
44.0
21.7
93.5
19.4
23.2
41.7
34.4
28.7
20.6
53.9
58.9
1.1
64.2
66.2
57.6
63.8
67.4
18.8
15.6
1.9
0.3
0.6
4.7
3.1
51.8
38.2
21.2
29.8
24.4
14.3
DOI: 10.1039/b007475f
Chem. Commun., 2000, 2499–2500
This journal is © The Royal Society of Chemistry 2000
2499

Fig. 2 Effect of calcination temperature on catalytic activity in SZ/SiO2-S
Fig. 1 Effect of Zr(SO4)2 loading of SZ/SiO2-S catalysts on catalytic
catalysts.
activity.
effective catalyst for this reaction, exhibiting similar conversion
and selectivity to 23DM2B as for the etherification with
methanol with an ether yield of 13%. Nafion NR50 and Nafion
SAC-13 again are less active than Amberlyst 15, giving ether
yields of 9% and 3%, respectively. Sulfated zirconia shows a
lower activity than Amberlyst 15 with a conversion of 20% and
58% selectivity to ether but an ether yield of 12%, comparable
to that of Amberlyst 15. The SZ/SiO2-S exhibits 40%
conversion and 48% ether selectivity, resulting in an ether yield
of 18.4%, higher than that obtained over Amberlyst 15.
In summary, the commercial resin catalyst Amberlyst 15 is an
effective but non-selective catalyst in the etherification of C6
olefins with methanol and ethanol. Silica-supported SZ cata-
lysts exhibit comparable and even higher ether yields than
commercial resin catalysts such as Amberlyst 15 and are thus
potential alternative catalysts, with the catalytic activity influ-
enced by the zirconium precursor compounds and preparation
technique, the ratio of SZ to silica, and calcination tem-
perature.
Amberlyst 15. SZ/SiO2-NP exhibits a lower activity than the
other two SiO2-supported SZ catalysts, but the highest ether
selectivity producing an ether yield of 14%, close to those over
Amberlyst 15 and Nafion NR50. Therefore, SZ/SiO2-S is the
best catalyst in terms of ether production.
It is well known that etherification or isomerization of olefins
generally occurs on acid sites. The acid content is thus an
important factor influencing the catalytic activity. The surface
areas of catalysts also influence the catalytic activity. Amberlyst
15 has an acid amount of 4.4 meqH+ g21 vs. 0.89 and 0.12
meqH+ g21 for Nafion NR50 and Nafion SAC-13, respectively,
making an order: Amberlyst 15 > Nafion NR50 > Nafion
SAC-13, which is in agreement with the order of activity.
However, in consideration of activity per unit of acidity, the
activity of the above catalysts shows a different order; Nafion
NR50 > Nafion SAC-13 > Amberlyst 15, which suggests that
the catalytic activity also depends on the nature of the acid site.
SZ is believed to be a superacid solid, effective for acid-
catalyzed reactions such as isomerization. Tungstated zirconia
shows little activity probably due to a low acidity. Silica-
supported sulfated zirconia could have a better dispersion of
active sites for reactions and thus an improved catalytic activity.
However, the catalytic behavior of these supported SZ catalysts
depends on the preparation technique. Further research on the
catalyst characterization is being carried out to elucidate the
nature of the active sites responsible for etherification and
isomerization.
This research was supported by the U.S. Department of
Energy under contract no. DE-FC26-99FT40540 as part of the
research program of the Consortium for Fossil Fuel Liquefac-
tion Science.
Notes and references
1 A. Rhodes, Oil & Gas J., 1999, 97, 39.
2 F. Cillignon, M. Mariani, S. Moreno, M. Remy and G. Poncelet,
J. Catal., 1997, 166, 53.
3 F. Cillignon, R. Loenders, J. A. Martens, P. A. Jacobs and G. Poncelet,
J. Catal., 1999, 182, 302.
4 G. Baronetti, L. Briand, U. Sedran and H. Thomas, Appl. Catal. A, 1988,
172, 265.
5 M. E. Quiroga, N. S. Figoli and U. A. Sedran, React. Kinet. Catal. Lett.,
1998, 63, 75.
6 N. Essayem, V. Martin, A. Riondel, G. Coudurier and J. C. Vedrine,
Spec. Publ. R. Soc. Chem., 1998, 216, 91.
The effect of varying the composition of Zr(SO4)2 in SZ/
SiO2-S was further investigated and the results are presented in
Fig. 1. It is seen that the loading of Zr(SO4)2 in SZ/SiO2-S
greatly influences the catalytic activity and selectivity. Both the
23DM1B conversion and 23DM2B selectively generally in-
crease as the Zr(SO4)2 content increases. The ether yield also
increases with increasing Zr(SO4)2 loading and reaches a stable
level of ca. 30% for Zr(SO4)2 exceeding 50 wt%.
Calcination temperature also exerts an influence on the
catalytic behavior (Fig. 2). As shown, SZ/SiO2-S catalysts
calcined at 400–550 °C can give much higher conversions, over
80%, comparable to Amberlyst 15. When catalysts are calcined
at higher temperatures over 600 °C, the selectivity to ether will
be remarkably enhanced, although the overall conversion drops
off. In terms of the ether yield, 600 °C is the optimal
temperature at which the highest ether yield is achieved.
The catalytic activity of the etherification of 23DM1B with
ethanol was also investigated over the resin catalysts and the
inorganic acid solids. It was found that Amberlyst 15 is also an
7 J. M. Grau, C. R. Vera and J. M. Parera, Appl. Catal. A, 1998, 172,
311.
8 T. Lei, J. S. Xu, Y. Tang, W. M. Hua and Z. Gao, Appl. Catal. A, 2000,
192, 181.
9 D. J. Zalewsik, S. Alerasool and P. K. Doolin, Catal. Today, 1999, 53,
419.
10 D. E. Hendriksen, US Pat., 5 752 992, Exxon Chemical Patents Inc.,
May 19, 1998.
11 M. A. Harmer, W. E. Farneth and Q. Sun, J. Am. Chem. Soc., 1996, 118,
7708.
12 A. Heidekum, M. A. Harmer and W. F. Hoelderich, J. Catal., 1998, 176,
260.
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