Ammoxidation of ethane to acetonitrile over Co-beta zeolite
Yuejin Li and John N. Armor*
Air Products and Chemicals, Inc., 7201 Hamilton Boulevard, Allentown, PA 18195-1501, USA
Co-beta zeolite selectively catalyzes the ammoxidation of
ethane to acetonitrile with a rate that is 1–2 orders of
magnitude higher than those over typical metal oxide
catalysts.
2.2% Co by mass. The reactions were conducted in a
microreactor system operating in a steady-state plug-flow mode
at atmospheric pressure. The reactor is a U-shaped quartz tube
with 0.25 in od at the inlet section and 0.375 in od at the outlet
section. The catalyst was located in the outlet section at the
center of the electrical furnace which surrounds the reactor tube.
Quartz wool plugs were used to support and secure the catalyst
bed. A typical feed consisted of 5% C2H6, 10% NH3 and 6.5%
O2 in He with a total flow rate of 100 cm3 mol21. A 0.2 g
catalyst sample sieved between 20/40 mesh was used for each
run, which renders a GHSV of 15000 or a contact time of 0.24
s. The reactor effluent was analyzed by two on-line gas
chromatograghs in series, each equipped with a thermal
conductivity detector.
As shown in Fig. 1, C2H6 was selectively converted to
acetonitrile over a Co-beta catalyst with a selectivity as high as
76%. The selectivity moderately decreased with increasing
temperature. Major by-products for the ethane conversion are
CO2 and C2H4. The selectivity of CO2 was ca. 20%, and the
selectivity of C2H4 was generally low but increased linearly
with increasing temperature. The conversion of C2H6 was
strongly dependent on the reaction temperature and the
availability of O2. The highest conversion was 47% obtained at
475 °C.
An interesting observation for this reaction is that the total C2
selectivity (C2H4 + CH3CN) is relatively constant, 73 ± 3%. The
mutually compensating effect between the formations of C2H4
and CH3CN suggests that C2H4 is the primary product of this
reaction. Indeed, a reference reaction with C2H4–NH3–O2 under
similar conditions tripled the nitrile yield. We believe that C2H6
is first converted to C2H4 via oxidative dehydrogenation on
NH3 moderated Co2+ sites. Ammonia is strongly adsorbed on
most of the Co2+ sites as a Lewis base. The ethene molecule then
adds on top of the adsorbed NH3 forming an adsorbed
There is considerable interest in converting light alkanes
directly to higher value organic chemicals because of the low
cost and abundance of the alkanes. On the other hand, alkanes
are chemically stable, which poses a tremendous challenge to
achieve a high selectivity in their conversions. A commercially
successful example of alkane activation is the manufacture of
maleic anhydride by selective oxidation of butane1 using V–P–
O based catalyst. Another example is the recently announced
production of acrylonitrile by reacting propane with NH3 and
O2 (ammoxidation) over an Sb–V–Al mixed oxide catalyst.2 In
spite of these commercial advances, limited progress has been
made for conversion of other alkanes. There were a few
attempts to apply the propane ammoxidation catalysis on ethane
to make acetonitrile. However, the propane ammoxidation
catalyst, V–Sb–Al, was found ineffective for the ethane
ammoxidation to acetonitrile. As postulated by Catani and
Centi,3 different mechanistic pathways are required for these
two reactions because of the fundamental difference between
propene, an intermediate for propane ammoxidation, and
ethene, an intermediate for ethane ammoxidation.
Using Al2O3 supported Nb–Sb oxides, Catani and Centi
investigated ethane ammoxidation between 480 and 540 °C
with a contact time of 2.6 s.3 They obtained ethane to
acetonitrile selectivity of 50%, CO selectivity of > 20% and
variable selectivities for CO2 formation. HCN was also formed
during this reaction with a constant selectivity of 5%. Earlier, a
USSR patent4 disclosed that ethane was converted to aceto-
nitrile with a maximum yield of 10% over a Cr–Nb–Mo oxide
catalyst at 350–500 °C with a contact time of 19 s. By-products
of this reaction were not specified. Here, we report our recent
discovery5 using a metal exchanged zeolite as a superior
catalyst for ethane ammoxidation with a very high reaction rate
and selectivity to acetonitrile.
Some metal exchanged zeolites are known to activate small
alkanes for selective reduction of NOx.6 In the case of NOx
reduction with methane over Co-zeolites, CH4 was activated by
abstracting a proton with a NO2 species which was adsorbed on
Co2+ sites.7 Metal cations exchanged in zeolites are atomically
dispersed on the coordinately unsaturated sites to balance the
negative changes of the zeolite framework. These cations
provide unique catalytic centers, as Lewis-acid sites, with a high
site density which is usually not achievable by bulk or supported
metal oxides.8 We sought to utilize the unusual properties of
metal-zeolite systems to activate ethane for its ammoxidation
reaction [eqn. (1)].
80
MeCN Selectivity
70
60
50
C H Conversion
2
6
40
30
20
10
0
CO Selectivity
2
C2H6 + NH3 + 3/2O2 ? CH3CN + 3H2O
(1)
C H Selectivity
2
4
NH4-beta zeolite (Si/Al = 14) (20 g) was exchanged with a 2 l,
0.02 m cobalt acetate aqueous solution at 70–80 °C for 24 h.
After washing with 1 l of deionized water, the zeolite was dried
overnight at 110 °C. Elemental analysis showed that the Co/Al
atomic ratio was 0.35 (70% of the cation exchange capacity) or
360
380
400
420
T / °C
440
460
480
500
Fig. 1 Ethane ammoxidation to acetonitrile over Co-beta catalyst as a
function of temperature
Chem. Commun., 1997
2013
Li, Yuejin
