The formation of methyl isocyanate during the reaction of nitroethane over Cu-MFI under hydrocarbon-selective catalytic reduction conditions

Article abstract of DOI:10.1006/jcat.2001.3343

The reaction of nitroethane with NO in the presence of O₂ was investigated over Cu-MFI catalyst. Nitroethane reacted readily in NO/O₂ over Cu-MFI with initial conversion to CO₂ and N₂. Deactivation due to deposited material was observed below 330°C with the eventual emergence of isocyanates, mainly CH₃NCO by dehydration, but with some HNCO, by deposit decomposition. Nitromethane reacted in a similar way but deactivation was slower. While HNCO was then the only isocyanate formed, significant amounts of HCN and NH₃ were also observed. With both systems, nitrogen rose either by hydrolysis of isocyanate to amine (or ammonia) on Broensted sites and subsequent SCR reactions involving transition metal ions or through reaction of NO₂ with deposits. The rates of these processes were sufficiently fast for nitroethane to be feasible as an intermediate during the SCR of NO by ethane on Cu-MFI.

Full text of DOI:10.1006/jcat.2001.3343

Journal of Catalysis 203, 487–494 (2001)
doi:10.1006/jcat.2001.3343, available online at http://www.idealibrary.com on
The Formation of Methyl Isocyanate during the Reaction of
Nitroethane over Cu-MFI under Hydrocarbon-Selective
Catalytic Reduction Conditions
Irene O. Y. Liu, Noel W. Cant, ,1 Brian S. Haynes,† and Peter F. Nelson‡
Department of Chemistry, Macquarie University, New South Wales 2109, Australia; †Department of Chemical Engineering, University of Sydney,
New South Wales 2006, Australia; and ‡CSIRO Division of Energy Technology, PO Box 136, North Ryde, New South Wales 1670, Australia
Received April 17, 2001; revised July 9, 2001; accepted July 16, 2001
nitroso-like compounds are key intermediates in the se-
lective catalytic reduction (SCR) of nitrogen oxides using
hydrocarbons. Subsequent work has established that nitro-
gen can indeed be formed when such compounds, or deriva-
tives of them, are adsorbed on Cu-MFI (3–5), H-MFI (4),
H-mordenite (6, 7), and Fe-MFI (8) and reacted with nitro-
gen oxides, especially NO2. The full chemistry is yet to be
resolved but facets of sequences such as nitroso → oxime
tautomer → amide (by the Beckmann rearrangement) →
acid plus nitrile → amine (or ammonia) → N2 have all been
established, principally by spectroscopic means.
The reaction of nitroethane with NO in the presence of O2
has been investigated over Cu-MFI. For short periods on-line,
conversion is complete above 250 C with CO2 and N2 as the
products. However, at temperatures below 330 C accumulation of
deposits leads to deactivation with the appearance of isocyanates
and CO. FTIR gas analysis confirms the formation of toxic methyl
isocyanate (CH3NCO), with yields approaching 25% along with
smaller amounts of isocyanic acid (HNCO). Nitromethane reacts
similarly but deactivation to give HNCO alone is an order of mag-
nitude slower. Emergence of HNCO is accompanied by significant
amounts of HCN and NH3. Nitroethane reacts in O2 alone in a man-
ner similar to that in NO/O2 but with less than half the initial N2
yield. Deactivation is faster and small amounts of HCN and NH3
appear concurrently with CH3NCO and HNCO. In the nitroethane
system, HNCO, HCN, and NH3 arise from decomposition of de-
posited material as demonstrated by the temperature–programmed
decomposition in helium. The main reaction pathway with the two
nitrocompounds appears to be rearrangement followed by dehy-
dration to the corresponding isocyanate and hydrolysis to amine
(or ammonia), with the latter steps proceeding on Brønsted acid
sites. Nitrogen is then formed by reaction with NO/O2 on the tran-
sition metal as in the ammonia-selective catalytic reaction which
is fast over Cu-MFI. A parallel route to N2 involves reaction of
NO2 with deposited material but this can occur only as long as the
catalyst retains activity for the oxidation of NO. Otherwise deac-
tivation ensues with appearance of isocyanate. Selective catalytic
reduction of NO using ethane over Cu-MFI in excess O2 yields
some NO2 in which case the various reactions involving nitroethane
would be sufficiently fast for it to be an intermediate in this
c
The situation is somewhat simpler for the steady-state
reaction of possible intermediates in the methane-SCR sys-
tem. Nitrosomethane (CH3NO) and its oxime tautomer
==
(CH2 NOH) are not easily studied but the Beckmann
rearrangement product, formamide (NH2CHO), has been
shown to react completely at temperatures as low as 200 C
(9). With Co-MFI and H-MFI, the low-temperature prod-
ucts are HCN and H2O, while NH3 and CO become domi-
nant by 300 C. Some of the NH3 and HCN is converted to
N2 if NO is added (9) and both conversions are complete
if NO2 is used instead (10). However, considerable CO re-
mains even at 360 C (9). Cu-MFI is much more active, with
complete conversion of formamide to N2 and CO2 at 300 C
in O2 alone (9). Nonetheless the sequence NH2CHO →
HCN + H2O [→ NH3 + CO] → N2 + CO2 can still be in-
ferred from the way the catalyst deactivates at lower tem-
perature (10).
system.
2001 Academic Press
Parallels exist in the reactions of nitromethane (11–13).
Co-MFI gives complete conversion to NH3 and CO2 at
300 C (12). Conversion of NH3 to N2 is complete by 360 C
when the feed includes O2 or NO/O2. Deactivation is ap-
parent at temperatures below 290 C, with isocyanic acid
(HNCO) eventually taking over as the major nitrogen-
containing product. Since HNCO is readily hydrolyzed, the
sequence appears to be CH3NO2 → HNCO + H2O →
NH3 + CO2 followed by NH3 + NOx , with each step suffi-
ciently fast to be feasible at the onset temperature for the
CH4-SCR reaction. As with the reactions of formamide,
Key Words: Cu-MFI; methyl isocyanate; nitroethane;
hydrocarbon-selective catalytic reduction.
INTRODUCTION
There have been many suggestions, starting with
Yokoyama and Misono in 1994 (1, 2), that nitro and/or
1 To whom correspondence should be addressed. Fax: 61 2 9850 8313.
E-mail: noel.cant@mq.edu.au.
487
0021-9517/01 $35.00
c
Copyright
2001 by Academic Press
All rights of reproduction in any form reserved.

488
LIU ET AL.
Cu-MFI is more active still, with complete conversion of absorbance at 90 C, as established in previous work (14),
1
nitromethane at 300 C, but the intermediate steps can still and CH3NCO from its peak at 2308 cm using the corre-
be recognized during the course of prolonged reaction at sponding relationship ppm = 3990 absorbance at 90 C,
lower temperatures (14).
calculated from spectra of an authentic sample measured
On the basis of the above, it appears that N2 can arise via for us by Infrared Analysis, Inc., Anaheim, California.
NH3 in both C1 systems, through intermediate formation
of cyanide in the nitroso (amide) path but isocyanate in the
nitro path. The aim of the present work was to establish the
RESULTS
corresponding reaction characteristics for nitroethane with
Figure 1 shows the exit gas composition for the reaction
Cu-MFI chosen as the catalyst because of its higher oxi-
of a C2H5NO2/NO/O2 stream over Cu-MFI with points ob-
dation activity and superior SCR performance with larger
tained in random order, allowing approximately 1 h at each
molecules even though it is ineffective with methane. There
temperature. The principal products at all temperatures are
have been no previousdeterminationsofthe productsofthe
CO2 and N2. Small amounts of CO are evident below 300 C
reaction of higher nitrocompounds under steady-state con-
with some NO2, formed by oxidation of residual NO, above
ditions. However Szanyi and Paffett (15) have reported the
325 C. On the time scale used here no nitroethane emerged
observation of bands assignable to adsorbed acetonitrile
at any temperature. However, although the carbon and ni-
when nitroethane is adsorbed on Cu-MFI at 400 K, and
trogen balances were complete within experimental error
above 350 C there wasan increasinglylarger deficit at lower
Chen et al. (8) have shown that 2-nitropropane deposited
on Fe-MFI is readily converted to nitrogen (largely 14N15N)
temperatures. Thus, deposited material was being built up.
by reaction with 15NO2 at 200 C. HNCO and HCN are also
If the reaction was allowed to run for some hours below
known minor products of the C2H4/NO/O2 SCR reaction
330 C then peaks due to other products eventually be-
over Cu-MFI (16–19).
came apparent in the infrared spectra of the product stream
as shown in Fig. 2. Spectrum a corresponds to that obtained
after 4 h for a reaction started at 285 C with subtraction of
EXPERIMENTAL
the contribution due to CO2. It shows two relatively broad
bands at 2284 and 2308 cm 1, with weaker ones near 2340
Cu-MFI was prepared by exchange of Na-MFI (Si/Al =
and 2266 cm 1. Superimposed fine structure is evident be-
11.9, Tosoh Inc.) with copper acetate followed by drying at
1
low 2300 cm which clearly corresponds to that of iso-
110 C and calcination in air at 500 C. The exchange level
cyanic acid (HNCO), spectrum b. The fine structure in (a)
was 80% (3.2 wt% Cu). Activity testing was carried out
can be removed by subtracting an appropriate multiple of
on 100-mg samples of 300- to 600- m particle size, con-
tained in a 4-mm-i.d. Pyrex U-tube reactor and pretreated
in 10% O2/He to 480 C before reaction. The gas mixture,
with a total flow rate of 100 cm3/min, was made up using
a set of electronic flowmeters (Brooks Model 5850TR or
5850E) and standard gas mixtures (BOC, Australia). Ni-
troethane (Fluka AG) was supplied by passing a separate
stream of He through a saturator held at a temperature such
that the nitroethane concentration in the reactant mixture
was 750 ppm as determined by complete oxidation and
analysis of the CO2 produced. Nitromethane was provided
similarly to a concentration of 1000 ppm.
The exit stream from the reactor was analyzed by a com-
bination of gas chromatographic and on-line FTIR mea-
surements in a manner similar to that described previously
(12). In essence, the stream was periodically sampled into a
micro-gas chromatograph (MTI Instruments, Model M200)
with molecular sieve and Poraplot U columns for analysis of
N2, O2, N2O, and CO2 (detection limits <5 ppm) and then
made to flow through a 16-cm-pathlength infrared cell in
heated box held at 90 C. Spectra, each comprising 64 scans
1
at 0.25 cm resolution, were processed to obtain concen-
trations based on Beer–Lambert plots for individual lines
FIG. 1. Exit gas composition and atom balances as a function of tem-
perature for the reaction of 750 ppm C2H5NO2 with 1000 ppm NO in 3%
O2/He over 100 mg of Cu-MFI with a flow rate of 100 cm3/min. Data for
1 h of reaction at each temperature.
of NO, NO2, and HCN. HNCO was determined from its
1
peak at 2284 cm using the relationship ppm = 3364

METHYL ISOCYANATE FROM NITROETHANE ON Cu-MFI
489
than nitromethane at the lower temperature of 250 C
(14). Isocyanate, now HNCO alone, does not appear until
1800 min (30 h) of reaction. (This required operation unat-
tended overnight and a change in feed saturation is respon-
sible for the step change apparent in CO2 and N2 production
between 500 and 1500 min.) The concentrations of minor
products are also different with the smaller nitroparaffin.
Notably, NO2 and N2O are present in significant amounts
from the beginning but NO2 has declined to zero by the
time deactivation starts. In addition, ammonia and hydro-
gen cyanide emerge at the same time as HNCO.
The data in Figs. 3 and 4 require that N2 is derived from
NO as well as the nitroparaffin, indeed almost equally in
both cases. This is a significant contributor to the overall
chemistry which is rather different when NO is absent. As
shown in Fig. 5 for reaction at 288 C, the initial produc-
tion of CO2 is similar when using C2H5NO2 with O2/He
alone, but the concentration of N2 is lower by a factor of 2
since it can now arise only from the nitrocompound. Deac-
tivation commences somewhat earlier and the concentra-
tions of NH3 and HCN that emerge at the same time as
CH3NCO and HNCO are considerably greater than when
NO is present as well (Fig. 3). Concentrations of the two
FIG. 2. Infrared spectra in isocyanate region after correction for CO2:
(a) product gas from C2H5NO2/NO/O2 mixture after 4 h of reaction over
Cu-MFI under conditions of Fig. 1; (b) spectrum of HNCO as determined
in Ref. (14); (c) spectrum a after subtraction of contribution from HNCO;
(d) spectrum of authentic sample of methyl isocyanate.
(b), which gives spectrum c, a close match to that of an
authentic sample of methyl isocyanate, spectrum d. The
concentrations of CH3NCO and HNCO, calculated using
the calibration factors given earlier, are 200 and 65 ppm,
respectively.
It should be noted that methyl isocyanate is the extremely
toxic substance (TLV = 0.02 ppm) that was responsible for
the disaster in Bhopal, India, in 1984. The flow system used
here was fitted with an alarm system set up to provide a
warning signal if the difference between the flow being
vented and the inlet one exceeded 1% , as this would in-
dicate a leak.
Figure 3showshowthe product distribution changed with
time during the experiment described above in relation to
Fig. 2. Carbon dioxide and nitrogen are the only significant
products for the first 50 min (apart from the rapidly falling
amounts of NO2 seen in the first few analyses). Both de-
cline thereafter as small amounts of CO appear followed
by CH3NCO, HNCO, and, finally, unreacted nitroethane.
The carbon balance is 60 to 80% throughout, with the ni-
trogen balance higher (since more than half of it is initially
present in NO). Traces (<20 ppm) of N2O, NH3, and HCN
are just measurable under some conditions.
The results of an equivalent experiment with nitro-
methane (CH3NO2) at almost the same temperature are
shown in Fig. 4. As before, CO2 and N2 are the principal
initial products. However, deactivation is much slower, 17
FIG. 3. Exit gas composition and atom balances as a function of time
during the reaction of 750 ppm C2H5NO2 with 1000 ppm NO in 3% O2/He
times relative to nitroethane, and also 10 times slower over 100 mg of Cu-MFI at 285 C with a flow rate of 100 cm3/min.

490
LIU ET AL.
FIG. 5. Exit gas composition and atom balances as a function of time
FIG. 4. Exit gas composition and atom balances as a function of time
during the reaction of 1000 ppm CH3NO2 with 1000 ppm NO in 3% O2/He
over 100 mg of Cu-MFI at 290 C with a flow rate of 100 cm3/min.
during the reaction of 750 ppm C2H5NO2 in 3% O2/He over 100 mg of Cu-
MFI at 288 C with a flow rate of 100 cm3/min (1200 ppm water introduced
where indicated).
action of methylamine was accompanied by a mass balance
deficit due to adsorption which would have been sufficient
to make its observation difficult under the conditions of the
experiment illustrated in Fig. 5 given a relatively high de-
tection limit by FTIR ( 70 ppm here for its strongest band
at 780 cm 1).
The data in Fig. 5 show that hydrolysis of product methyl
isocyanate is less than 30% when 1200 ppm water is added
at 288 C. Combustion processes typically produce 10%
or more water which would certainly result in greater
isocyanates both pass through shallow maxima and show
small stepwise drops when 1200 ppm of water is subse-
quently introduced. These drops are correlated with a rise
in production of CO2, as expected for isocyanate hydrol-
ysis. In the case of HNCO the other expected product is
NH3, and a small increase in the concentration of the lat-
ter is again discernible. The hydrolysis product expected
for methyl isocyanate is methylamine. Tests with methy-
lamine showed that it, like ammonia, was very reactive
under SCR conditions at 275 C (Table 1). However, the re-
TABLE 1
Reactions in O2/He over Cu-MFI
Product concentration (ppm)
Temperature
Conversion
(% )
Feed (all in O2/He)
( C)
CO2
N2
N2O
NO2
a
800 ppm NH3 + 1000 ppm NO
275
275
275
275
270
>95
>95
40
80
>95
—
220b
—
—
400
1100
350c
—
270
380
50
20
—
<20
—
50
50
400
<20
25
400 ppm CH3NH2 + 1000 ppm NOa
1000 ppm NOb
a
800 ppm NH3
360 ppm HCN + 1000 ppm NOa
a In 3% O2/He, using 100 mg of Cu-MFI with a total flow rate of 100 cm3/ min.
b In 5% O2/He, using 140 mg of Cu-MFI with a total flow rate of 100 cm3/ min.
c Forty percent deficit in carbon and 20% in nitrogen, possibly due to disproportionation and adsorption of di- and trimethylamines.

METHYL ISOCYANATE FROM NITROETHANE ON Cu-MFI
491
FIG. 7. Composition of gas evolved during temperature-programmed
decomposition in 100 cm3/min He of deposits formed on Cu-MFI by
C2H5NO2/O2 reaction as per Fig. 6.
lative amounts of C and N liberated during the combined
decomposition/oxidation of Figs. 7 and 8 were 2600 and
1400 mol/g, respectively, in reasonable agreement with
those estimated from the mass balance deficits in Fig. 6,
FIG. 6. Exit gas composition and atom balances as a function of time
during the reaction of 750 ppm C2H5NO2 in 3% O2/He over 100 mg of
Cu-MFI at 328 C with a flow rate of 100 cm3/min.
hydrolysis. Only further experimentation, beyond the scope
of the present work, can establish if hydrolysis would then
be complete.
Deactivation was also apparent, but slower, when ni-
troethane was reacted in O2/He alone at a higher tempera-
ture. At 328 C, as shown in Fig. 6, deactivation did not com-
mence until 170 min of reaction but then followed a course
similar to that in Fig. 5, with NH3 and HCN emerging to-
gether with CH3NCO and HNCO. After 280 min, when the
accumulated mass balance deficits in carbon and nitrogen
were, very approximately, 3600 and 2200 mol/g, respec-
tively, the sample was heated in He alone at 10 C/min.
As may be seen from Fig. 7, decomposition commenced
at 350 C. Hydrogen cyanide was generated in the largest
amount with a peak concentration of 380 ppm at 470 C.
Other products include CO2 and NH3, with peak concen-
trations at 420 and 450 C, respectively, plus HNCO, which
exhibits a peak at 380 C and is formed continuously at the
hold temperature of 480 C.
Following the experiment illustrated by Fig. 7, the sample
was subjected to temperature-programmed oxidation after
first being cooled to 330 C in helium. As shown in Fig. 8,
CO2 and N2 are formed immediately on admission of oxy-
gen with peak yields of both at 430 C. Small amounts of
N2O and CO are evident at lower temperature. The cumu-
FIG. 8. Composition of gas formed during temperature-programmed
oxidation in 100 cm3/min 3% O2/He of material remaining after decom-
position of deposits from C2H5NO2/O2 reaction as per Fig. 7.

492
LIU ET AL.
acid sites, i.e., with R = H or CH3:
–
*
)
–
==
–
R CH2 NO2
R CH NOOH
–
–
→ R CNO [ + H2O] → R NCO.
The work of Obuchi et al. (21) shows that isomerization of
N-oxides (R-CNO) to isocyanates (R-NCO) is facile, with
substantial conversion at 100 C on alumina for example.
There are two possible routes from the isocyanate to N2.
One is through hydrolysis to R-NH2 (or ammonia) followed
by an SCR reaction, the other by deposition of the iso-
cyanate and reaction with NO2. Hydrolysis is far from com-
plete when water isadded with a greatlydeactivated catalyst
(Fig. 5) but this would not preclude its occurrence at a
faster rate prior to deactivation. Hydrolyses of isocyanates,
both acid- and base-catalyzed, are well-known reactions
(22, p. 807), and tests show that the process is fast at 300 C
on both acidic and basic oxides (23, 24). Only silica is
relatively inactive. As shown in the work of Connerton
et al. (25), Cu-MFI contains similar numbers of Brønsted
acid sites whether it is made from H-MFI or, as here, from
Na-MFI.
FIG. 9. Exit gas composition and atom balances as a function of tem-
perature during the reaction of 1000 ppm C2H6 with 1000 ppm NO in 3%
O2/He over 100 mg of Cu-MFI with a flow rate of 100 cm3/min.
The NH3-SCR reaction is also fast on Cu-MFI (26). As
shown in Table 1, both NH3-SCR and the corresponding
reaction with methylamine are near complete at 275 C with
bearing in mind the considerable errors inherent in the the present catalyst. Thus the sequence
latter.
R–NCO + H2O → R–NH2 + CO2
The results of an SCR experiment carried out over the
Cu-MFI catalyst with an ethane/NO/O2 mixture are shown
in Fig. 9. The maximum yield of reduced products occurs
at 365 C, with formation of 185 ppm of N2 and 25 ppm
of N2O, a net removal of 40% which is similar to the
maximum observed by Burch and Scire (20). The total
conversion of NO is higher at 430 C but NO2 then con-
stitutes a significant fraction of the products. Some NO2
is also evident at the two lowest temperatures. Carbon
monoxide is always negligible compared with CO2 and both
C and N mass balances are always within experimental
error.
R–NH2 + NO + O2 → N2 + H2O[+CO2],
with the first proceeding on Brønsted acid sites and the
second requiring the transition metal, appears feasible until
prevented by a buildup of deposited material.
The alternative path to N2, the reaction of deposited ma-
terial with NO2, has been shown to occur at 200 C for a
variety of compounds, including isocyanates, adsorbed on
Fe-MFI (8). As may be seen from Table 1, Cu-MFI has con-
siderable activity for the oxidation of NO to NO2 at 275 C
and some NO2 is formed from residual NO in the reactions
with NH3, CH3NH2, and HCN. This is also true during the
initial stages of the reaction of nitromethane (Fig. 4) but the
NO2 declines to zero by the time deactivation commences.
With nitroethane, NO2 is observable only at the very be-
ginning of reaction and deactivation starts much earlier.
One possible implication is that NO2 can slow deactivation
by reacting with material being deposited. This would be
less effective with nitroethane since its extra carbon will in-
crease the amount of NO2 required for complete oxidation.
It is also significant here that although Co-MFI deactivates
faster than Cu-MFI during the reaction of nitromethane in
NO/O2, there is no deactivation if NO2/O2 is used instead
DISCUSSION
The data in Figs. 3 and 4 demonstrate that nitroethane
and nitromethane react similarly over Cu-MFI under SCR
conditions. At temperatures near 290 C, each yields CO2
and N2 initially but deactivation due to deposited mate-
rial results in the eventual appearance of isocyanates. The
simplest interpretation is that the reaction is sequential,
nitroparaffin → isocyanate → N2 + CO2,
with the deposited material having a larger effect on the (12). Complete reaction is possible at 200 C in the latter
rate of the second step than the first so the intermediate system. Overall it appears that NO2 is capable of generat-
eventually becomes detectable.
ing N2 by reaction with deposited material. However, this
The conversion of nitroparaffin to isocyanate is likely cannot be the sole route to N2 since nitroethane reacts in a
to occur through dehydration of its aci-form on Brønsted similar way in O2 alone (Figs. 5 and 6) where neither NO2

METHYL ISOCYANATE FROM NITROETHANE ON Cu-MFI
493
nor NO is present. The probable route to N2 in that system In this case the expected hydrolysis products are CO and
is hydrolysis followed by ammonia oxidation which is also methylamine:
quite fast at 270 C (Table 1).
In situ infrared measurements during the course of the
nitromethane/NO/O2 reaction over Co-MFI and H-MFI in-
dicate that the deposited material comprises highly stable
s-triazine compounds related to melamine (C3H6N6) and
cyanuric acid (C3H3N3O3) (9, 14, 27). This is understand-
able since melamine itself is made commercially by com-
bination of isocyanic acid and ammonia using alumina or
solid phosphate catalysts (28). The deposits formed during
the reaction of nitromethane over Cu-MFI appear to have
the same structure (29). The larger size of trimers of methyl
isocyanate may restrict equivalent trimerization reactions
within the cavities of MFI, but they would still be possible
on external surfaces, leading to blockage of pore entrances.
Isocyanates can also add to ammonia, amines, and amides
to form stable ureas (22, p. 823).
The reactions of nitromethane and nitroethane dif-
fer in the distribution of minor products as deactiva-
tion proceeds. Nitromethane gives rise to much more am-
monia which is understandable since it is an expected
hydrolysis product with HNCO compared with methy-
lamine in the C2 system. Nitromethane also produces more
HCN, the expected dehydration product of formaldoxime
–
CH3NH CHO → CH3NH2 + CO.
This can explain why nitroethane produces considerably
more CO than nitromethane. The coproduct methylamine
was not seen but that is understandable given its reactivity
and uptake (Table 1) coupled with a high detection limit
here. If the C2-oxime were to undergo dehydration rather
than rearrangement then the expected product is acetoni-
trile:
==
CH3CH NOH → CH3CN + H2O.
This was not seen in the present work although the conver-
sion of adsorbed nitroethane to adsorbed acetonitrile has
been observed to proceed on Cu-MFI at a lower tempera-
ture (127 C) (15).
It should be noted here that the dehydration, hydrolysis,
and rearrangement reactions mentioned above are all well-
known proton-catalyzed reactions (22), and hence can pro-
ceed on small numbers of Brønsted sites remaining when
the metal sites necessary to convert reduced nitrogen com-
pounds to N2 itself have been deactivated.
Unlike the above, there is no standard chemistry that
can explain the small amounts of HNCO, and even smaller
amounts of HCN, produced in the largely deactivated state
when nitroethane is reacted in NO/O2 or O2 alone (Figs. 3,
5, and 6). They probably arise through direct decomposi-
tion of deposited material since both are generated when
the deactivated catalyst is heated in He alone (Fig. 7).
HNCO appears initially at a considerably lower temper-
ature than does HCN, consistent with the larger amount
of the former produced while deactivation is proceeding.
The HCN/HNCO ratio is lower during reaction in NO/O2
(Fig. 3) than in O2 alone (Fig. 4) because the HCN it-
self reacts readily with NO/O2 to give N2 on Cu-MFI
(Table 1), whereas it undergoes deactivation with forma-
tion of cyanogen (C2N2) when reacted with O2 alone (30).
It appears likely that the small amounts of HCN and HNCO
seen during the C2H4/NO/O2 reaction on Cu-MFI in other
studies (16–19) also arise by decomposition of deposits.
The question as to whether it is feasible for nitroethane
to be a major intermediate in N2 formation in the SCR re-
action using ethane over Cu-MFI can now be considered.
As shown in Fig. 9, that reaction converts 300 ppm of NO
at 296 C with formation of 60 ppm of N2, 20 ppm of N2O,
and 65 ppm of NO2. If the 120 ppm of NO that has been re-
duced to N2 were converted to nitroethane first then, based
on Figs. 1 and 3, complete reaction to N2 and CO2 would
be expected initially. Any deposits formed would tend to
be removed by reaction with the NO2 also present so deac-
tivation should be slow if present at all. Hence it is feasible,
although not proved, for nitroethane to be a major inter-
mediate in the ethane/NO/O2 reaction. It could be formed
==
–
(CH2 NOH) or formamide (NH2 CHO). This suggests
that some nitromethane is reacting with loss of oxygen,
==
CH3NO2 → CH3NO → CH2 NOH → HCN + H2O,
and, if the Beckmann rearrangement were to proceed,
==
–
CH2 NOH → NH2 CHO → HCN + H2O.
Formamide reacts readily in O2/He over Cu-MFI at 250 C
and the reaction exhibits slow deactivation during which
time HCN can be observed (10). However, the concentra-
tions of HCN formed are much smaller than those of NH3
and CO which arise through hydrolysis:
–
NH2 CHO + H2O → NH3 + HCOOH
→ NH3 + CO + H2O.
Since the HCN/CO ratio is high, not low, during the reaction
of nitromethane it appears that the HCN observed in this
system arises from the oxime not the amide. This is under-
standable since Beckmann rearrangements do not usually
proceed with movement of a hydrogen (22, p. 1008), as is
required to convert CH2 NOH to NH2 CHO.
On the other hand, the Beckmann rearrangement of the
corresponding C2-oxime from nitroethane should be faster
since it can proceed with shift of a methyl rather than hy-
drogen:
==
–
==
–
CH3CH NOH → CH3NH CHO.

494
LIU ET AL.
within cavities by an initial hydrogen abstraction followed
by addition of NO2 to the resulting radical.
3. Beutel, T., Adelman, B., and Sachtler, W. M. H., Catal. Lett. 37, 125
(1996).
4. Wu, J., and Larsen, S. C., J. Catal. 182, 244 (1999).
5. Rebrov, E. V., Simakov, A. V., Sazonova, N. N., and Stoyanov, E. S.,
Catal. Lett. 64, 129 (2000).
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(1999).
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8. Chen, H.-Y., Voskoboinikov, T., and Sachtler, W. M. H., J. Catal. 186,
91 (1999).
It is significant that NO2 formation during the SCR re-
action at 296 C is much less than that with NO/O2 alone
at a lower temperature (Table 1). Furthermore, the NO2
concentration first declines with temperature above 300 C
during SCR to a minimum near 365 C where ethane con-
sumption is complete. The implication is that NO2 is being
consumed by a reaction involving ethane that has a higher
activation energy than NO oxidation. Thus NO2 does ap-
9. Cant, N. W., Cowan, A. D., Liu, I. O. Y., and Satsuma, A., Catal. Today
54, 473 (1999).
pear to be a key reactant in this system in agreement with 10. Liu, I. O. Y., unpublished results.
11. Cant, N. W., Cowan, A. D., Doughty, A., Haynes, B. S., and Nelson,
current views concerning hydrocarbon-SCR reactions over
transition metal zeolites.
P. F. , Catal. Lett. 46, 207 (1997).
12. Cowan, A. D., Cant, N. W., Haynes, B. H., and Nelson, P. F., J. Catal.
176, 329 (1998).
CONCLUSIONS
13. Lombardo, E. A., Sill, G. A., d’Itri, J. L., and Hall, W. K., J. Catal. 173,
440 (1998).
14. Cant, N. W., Chambers, D. C., Cowan, A. D., Liu, I. O. Y., and Satsuma,
A., Top. Catal. 10, 13 (2000).
15. Szanyi, J., and Paffett, M. T., J. Chem. Soc. Faraday Trans. 92, 5165
(1996).
16. Radtke, F., Koeppel, R. A., and Baiker, A., Appl. Catal. A 107, L125
(1994).
17. Radtke, F., Koeppel, R. A., and Baiker, A., Environ. Sci. Technol. 29,
2703 (1995).
18. Radtke, F., Koeppel, R. A., and Baiker, A., J. Chem. Soc. Chem. Com-
mun., 427 (1995).
19. Radtke, F., Koeppel, R. A., and Baiker, A., Catal. Today 26, 159
(1995).
20. Burch, R., and Scire, S., Appl. Catal. B 3, 295 (1994).
21. Obuchi, A., Wo¨gerbauer, C., Ko¨ppel, R. A., and Baiker, A., Appl.
Catal. B 19, 9 (1998).
22. March, J., “Advanced Organic Chemistry,” p. 807. Wiley, New York,
1985.
Nitroethane reacts readily in NO/O2 over Cu-MFI with
initial conversion to CO2 and N2. Deactivation due to de-
posited material occurs below 330 C with the eventual
emergence of isocyanates, principally CH3NCO by dehy-
dration, but with some HNCO, by deposit decomposition,
as well. Nitromethane reacts in a similar way but deacti-
vation is much slower. While HNCO is then the only iso-
cyanate formed, significant amounts of HCN and NH3 are
also seen. With both systems nitrogen can arise either by hy-
drolysis of isocyanate to amine (or ammonia) on Brønsted
sites and subsequent SCR reactions involving transition
metal ions or through reaction of NO2 with deposits. The
rates of these processes are sufficiently fast for nitroethane
to be feasible as an intermediate during the selective cat-
alytic reduction of NO by ethane on Cu-MFI.
23. Du¨mpelmann, R., Cant, N. W., and Trimm, D. L., J. Catal. 162, 96
(1996).
24. Chambers, D. C., Angove, D. E., and Cant, N. W., J. Catal. in press.
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Trans. 91, 1841 (1995).
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ACKNOWLEDGMENTS
This work has been supported by grants from the Australian Research
Grants Committee. We are grateful to Mr. A. Asano for provision of the
starting zeolite sample.
27. Satsuma, A., Cowan, A. D., Cant, N. W., and Trimm, D. L., J. Catal.
181, 165 (1999).
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