Catalytic partial oxidation of cyclohexane in a single-gauze reactor

Article abstract of DOI:10.1006/jcat.1999.2806

C₆H₁₂ catalytic partial oxidation in a single-gauze reactor produced ~ 85% selectivity to olefins and oxygenates at 25% C₆H₁₂ conversion and 100% oxygen conversion, with cyclohexene and 5-hexenal as the dominant products. Experiments were performed with a 90% platinum-10% rhodium single gauze (~ 90-μm wire diameter) at C₆H₁2/O₂ molar ratios of 0.4-5, flow rates of 1-3 std L/min, preheat temperatures of 100°-300°C, N₂ dilution from 5% to the air composition, and 1.2-2 atm. The C₆H₁₂/O₂ ratio was the most important variable for operation of the single-gauze reactor because temperatures, reactant conversions, and product selectivities all changed significantly as C₆H₁₂/O₂ was varied. Low dilution favored olefin production while high dilution suppressed the homogeneous reactions necessary for oxygenate formation. Oxygenates were also favored by high flow rates and low inlet temperatures. Higher reactor pressures (≤ 2 atm) increased the yield of cyclohexene and 5-hexenal and allowed complete oxygen conversion. Cyclohexanone was produced with 5% selectivity at C₆H₁₂/O₂ ~ 4. Reaction pathways for C₆H₁₂ partial oxidation were hypothesized, and the products were consistent with the proposed surface-assisted gas-phase sequences.

Full text of DOI:10.1006/jcat.1999.2806

Journal of Catalysis 191, 245–256 (2000)
doi:10.1006/jcat.1999.2806, available online at http://www.idealibrary.com on
Catalytic Partial Oxidation of Cyclohexane in a Single-Gauze Reactor
R. P. O’Connor and L. D. Schmidt1
Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455-0132
Received September 27, 1999; revised December 18, 1999; accepted December 22, 1999
over single gauzes has been studied, and it has been found
that yields of oxygenates increase with carbon number. This
work extends the use of the single-gauze reactor to cyclo-
hexane partial oxidation. Cyclohexane is the lightest cyclic
alkane whose C–C bond strength is equal to that of its cor-
responding linear alkane, and the high symmetry of cyclo-
hexane suggests a small number of possible product species.
The most important commercial reaction of cyclohex-
ane is its liquid-phase oxidation with air in the presence
of soluble cobalt catalyst to produce cyclohexanol and cy-
clohexanone. Cyclohexanol can be converted into adipic
acid (for manufacture of Nylon-6,6) with a strong oxidizing
agent or dehydrogenated with zinc or copper catalysts to cy-
clohexanone (5). Cyclohexanone is an intermediate in the
production of caprolactam (for manufacture of Nylon-6) as
well as adipic acid (6). Conventional technology (7) uses a
liquid-phase bubble column or autoclave typically operat-
ing at 10 atm, 150 C, and a residence time exceeding 1 h.
Selectivities to cyclohexanone and cyclohexanol of 75% can
be achieved but only at <5% conversion of cyclohexane.
This paper describes an autothermal method to pro-
duce close to 60% selectivity to cyclohexene and the
1,6-difunctionalized oxygenated species 5-hexenal, at 25%
cyclohexane conversion and a residence time six orders of
magnitude smaller than that for comparable liquid-phase
industrial processes.
Cyclohexane oxidation in a single-gauze reactor can produce
85% selectivity to olefins and oxygenates at 25% cyclohexane
conversion and 100% oxygen conversion, with cyclohexene and
5-hexenal as the dominant products. Experiments are performed
with a 90% platinum–10% rhodium single gauze ( 90- m wire
diameter) at cyclohexane/oxygen (C6H12/O2) molar ratios of 0.4 to
5.0, flow rates of 1 to 3 standard liters per minute, preheat temper-
atures of 100 to 300 C, N2 dilution from 5% to the air composition,
and pressures of 1.2 to 2 atm. The cyclohexane/oxygen ratio is the
most important variable for operation of the single-gauze reactor
because temperatures, reactant conversions, and product selectiv-
ities all change significantly as C6H12/O2 is varied. Low dilution
favors olefin production while high dilution suppresses the homoge-
neous reactions necessary for oxygenate formation. Oxygenates are
also favored by high flow rates and low inlet temperatures. Higher
reactor pressures (up to 2 atm) increase the yield of cyclohexene
and 5-hexenal and allow complete oxygen conversion. Cyclohex-
anone can be produced with 5% selectivity at C6H12/O2 4. Reac-
tion pathways for cyclohexane partial oxidation are hypothesized,
and the products are consistent with the proposed surface-assisted
c
gas-phase sequences.
2000 Academic Press
Key Words: cyclohexane; partial oxidation; single-gauze reactor;
oxygenates; short contact time.
1. INTRODUCTION
Oxygen-containing hydrocarbons, or oxygenates, are
typically produced at low temperature in the liquid phase
through multistage processes involving expensive separa-
tions. The current methods for production of oxygenates
require expensive catalysts, large residence times, and care-
ful temperature control. Practical selectivities of desired
oxygenated compounds can be obtained only at low conver-
sions. A direct method that selectively produces oxygenates
would be industrially significant for the manufacture of
specialty chemicals.
2. EXPERIMENTAL
2.1. The Single-Gauze Catalyst
The catalyst in all experiments was a single 40-mesh (77%
transparency) woven gauze with 90% Pt (bymass), 10% Rh,
and traces (<10 ppm) of Pd. Alloying Pt with 10% Rh in-
creases the mechanical strength of the wires and reduces
Pt losses during high-temperature operation (8). Multiple-
layer gauze catalysts woven from fine 90% Pt–10% Rh wire
have long been used in the industrial Ostwald process for
the partial oxidation of ammonia to produce nitric acid and
in the closely related Andrussow process in which a mixture
of ammonia, air, and natural gas is converted into hydro-
cyanic acid (9). This research uses a single layer of the 90%
Pt–10% Rh gauze. New gauze catalysts require activation
In recent years, short-contact-time reactors employing
Pt–10% Rh single-gauze catalysts have shown promise for
the high-throughput and autothermal production of oxy-
genates (1–4). Partial oxidation of linear C1–C5 alkanes
1 To whom correspondence should be addressed. E-mail: schmi001@
tc.umn.edu.
245
0021-9517/00 $35.00
c
Copyright
2000 by Academic Press
All rights of reproduction in any form reserved.

246
O’CONNOR AND SCHMIDT
before they achieve reproducible results. During the acti- rate to 0.1 SLPM (standard liters per minute) for all gases.
vation process, extensive surface facetting occurs [see, e.g., Cyclohexane (Aldrich, HPLC-grade 99 +% purity) is liq-
(4)]. While the nominal wire diameter is initially 76 m, the uid at room temperature and was introduced as vapor with
wire diameter increases to 90 m on catalyst activation.
a syringe pump, fluidized-bed heater, and superheater in
Species residence times over the catalyst are very short series.
(<1 ms) and mass-transfer rates to and from the catalytic
The reactor pressure was maintained with a downstream
surface are high. It is believed (1–4) that the heat from valve and indicated by a standard regulator. For analysis,
the exothermic surface reactions along with desorbed free a valve sending product gas to the gas chromatograph was
radicals initiate a gas-phase reaction sequence that leads opened, and the pressure of the reactor and sample lines
to the formation of unstable oxygenates. The large trans- was adjusted prior to steady-state sampling. Pressures of
parency of the single gauze facilitates rapid mixing of the 1.2 to 2 atm (121.6–202.7 kPa) were possible with the ex-
colder gases passing between the catalyst wires with the perimental setup. All product gases were incinerated and
hot gases and radicals leaving the gauze surface, resulting vented in a fume hood.
in fast quenching of the homogeneous reactions so that oxy-
genates cannot decompose. Oxygenates are more reactive
2.3. Catalyst Activation
than alkanes or olefins and are not predicted by equilib-
rium calculations. Furthermore, oxygenates are not pro-
duced significantly from C5 and C6 alkanes in Pt-coated
foam-monolith reactors (10). The quenching action in
the wake region of the single gauze, however, kinetically
freezes the product mixture so that valuable nonequilib-
rium species can be obtained.
To activate a fresh gauze for these experiments, n-butane
was reacted over the catalyst for 10 h at C4H10/O2 =
1.4–2.0 (molar ratio in feed). Activation was confirmed by
the reproducibility of temperature, conversion, and selec-
tivities. Activation with cyclohexane alone was not suc-
cessful after 10 h. All results reported in this paper were
obtained with a catalyst fully activated by n-butane oxi-
dation. No signs of catalyst deactivation (metal loss, coke
formation, etc.) were observed after many hours of reactor
operation.
2.2. The Single-Gauze Reactor
The single-gauze reactor for these experiments consisted
of a quartz tube with 19-mm inner diameter and 40-cm
length. Two 1-cm-long and 1-mm-thick quartz-tube inserts
(15-mm inner diameter) held the gauze in place. The quartz
inserts were wrapped with thin Fiberfrax paper (amorphous
Al2O3–SiO2 fibers) to prevent bypassing of gases between
the insert and the reactor wall and also to ensure rigid-
ity. The reactor was configured vertically and the feed was
introduced from the bottom of the reactor. In the region
of the catalyst, approximately 1 in. of external insulation
was placed around the reactor tube to minimize radial heat
losses.
A chromel–alumel thermocouple inside a closed quartz
thermowell was used to measure the temperature 5 mm
downstream of the single gauze. The thermowell elimi-
nates the possibility of catalytic chemistry occurring on
the chromel–alumel wire. The catalyst surface tempera-
ture cannot be directly detected with this probe due to the
quartz thermowell which would dissipate heat and inter-
fere with the downstream flow pattern, thus altering the
chemistry. Experiments where the 5-mm-downstream tem-
perature was 500 C corresponded to a surface temper-
2.4. Startup and Shutdown
To light off the gauze catalyst, the cyclohexane flow was
first allowed to stabilize in the nitrogen (diluent) flow.
Next, oxygen flow was increased until the desired cyclo-
hexane/oxygen ratio was achieved. The gauze catalyst was
directly heated until light-off, which occurred at roughly
200–250 C for cyclohexane/oxygen feed ratios (C6H12/O2)
of 0.4–5.0, respectively. For cases in which the preheat tem-
perature exceeded the ignition temperature, no external
ignition source was necessary. At ignition, it was found that
the temperature 5 mm downstream increased at a rate of
10 C/s. The reaction attained steady state within 15 min
of ignition, and no significant transients were observed in
these experiments after the onset of steady state. The feed
ratios were fuel-rich, so the reaction was shut down by stop-
ping oxygen first and then cyclohexane while maintaining
the nitrogen flow.
2.5. Reactor Safety
Most experiments were performed outside the flamma-
ature of 800 C (measured with a pyrometer), indicating bility limits, always on the fuel-rich side. For cyclohexane
large axial temperature gradients in the wake region of the oxidation, the stoichiometric molar ratio for combustion
gauze. The accuracy of the measured downstream temper- is C6H12/O2 = 0.11. In these experiments, the cyclohex-
ature was 5 C.
ane/oxygen ratio was varied between 0.4 and 5.0, never
The flow rates of the high-purity (99.9 +% ) gases (oxy- close to the dangerous total-oxidation ratio. The flamma-
gen, nitrogen, and occasionally n-butane for catalyst acti- bility limits in a static system with air are C6H12/O2 =
vation) entering the system from high-pressure cylinders 0.06–0.44, and flames are generally more favorable for
were adjusted using mass-flow controllers which are accu- higher pressure, lower dilution, and lower flow rate.

CATALYTIC PARTIAL OXIDATION OF CYCLOHEXANE
247
TABLE 1
Experiments at C6H12/O2 = 0.4 with a flow rate of
2.5 SLPM (30% N2 dilution), preheat temperature of
T0 = 200 C, and reactor pressure of P = 1.2 atm developed
flames originating near the catalyst. The flame velocity was
lower than the gas velocity, so the flames extinguished near
the reactor entrance. The reactor walls were deposited with
solid carbon in this case. At C6H12/O2 = 0.55, less intense
flames emerged but also extinguished. At C6H12/O2 = 0.65,
small pulsing flames were observed steadily shooting up-
stream of the gauze. These observations were confirmed in
several experiments.
Product Distribution for Cyclohexane
Partial Oxidation at C6H12/O2 = 2.0, 2.5
SLPM, 30% N2 Dilution, T0 = 200 C, and
P = 1.2 atm
% Carbon-atom
Product
selectivity
c-C6H10
5-C6H10
31.3
25.0
10.3
8.5
8.4
4.5
2.1
1.8
1.5
1.4
1.3
1.0
0.7
0.6
0.6
0.5
O
C5H10
CO2
CO
O
Under no reactor conditions did flames develop experi-
mentally when operating at C6H12/O2 > 0.7. Because most
experiments were conducted at C6H12/O2 > 1 and low pres-
sure, the possibility of dangerous flames or explosions was
minimal. In addition to avoiding flames and the possibil-
ity of explosions, fuel-rich operation was necessary for sig-
nificant production of carbon-containing compounds other
than CO and CO2.
c-C6H6
2-C6H10
C2H4
O
CH3CHO
C7+ oxygenates
CH2CHCHO
C2H5CHO
C3H6
c-C6H10
C4H8
O
2.6. Product Analysis
C5H10
Reaction products were filtered by a 2- m sintered ce-
ramicelement and sent to a Hewlett–Packard 6890gaschro-
matograph with a thermal-conductivity detector (TCD).
Combining gas chromatography (GC) with mass spectrom-
etry (MS) allowed qualitative determination of several un-
known product species. A Finnigan Mat 95 instrument was
employed for the GC–MS product analysis. Nitrogen was
the calibration gas for mass balances since it was an inert
diluent in all experiments. The carbon and hydrogen bal-
ances typically closed to within 5% . All data were repro-
duced on several catalysts with results consistent with those
shown.
The product selectivities were calculated on a carbon-
atom basis. Carbon-atom selectivities are calculated as the
ratio of the moles of a specific product to the total moles
of all products, scaled by the number of carbon atoms in
the species. This definition accounts for the mole-number
increase due to reaction and satisfies the requirement that
all selectivities sum to unity.
Note. Only significant product species
0.5% selectivity) are shown.
(
temperature, and reactor pressure. Operating variables of
2.5 SLPM, 30% N2, T0 = 200 C, and P = 1.2 atm (121.6 kPa)
promote the production of oxygenates from n-pentane (4)
and thus should be adequate starting values for cyclohex-
ane. The stoichiometric cyclohexane/oxygen ratio for com-
bustion of cyclohexane to CO2 and H2O is 0.11, while
the ratio for the partial oxidation to C6 oxygenates (con-
taining one oxygen atom) or cyclohexene is C6H12/O2 = 2,
approximately 18-fold richer than the combustion ratio.
The product distribution for C6H12/O2 = 2 is summarized in
Table 1.
Figure 1a indicates the temperature measured 5 mm
downstream of the catalyst as well as cyclohexane and
oxygen conversions. The temperature ranged widely, from
700 to 300 C, as C6H12/O2 was varied from 0.65 to 5.0.
The drop in temperature was sharp initially but began to
level off with high C6H12/O2. The curves drawn through the
data are intended to be an aid to the eye. Oxygen conver-
sion was 95% for C6H12/O2 < 1, but there appears to be
an abrupt transition where the O2 conversion fell to 85%
for C6H12/O2 = 1, after which it gradually increased to 90%
for C6H12/O2 = 3. For C6H12/O2 > 3, oxygen conversion de-
creased drastically and then leveled off at approximately
40% . The transition at C6H12/O2 1 was accompanied by
a change in the cyclohexane-conversion trend. Dropping
sharply until a cyclohexane/oxygen ratio of about unity, the
cyclohexane conversion roughly leveled out at 20% be-
Adiabatic reaction temperatures for experimental prod-
uct distributions were calculated from species enthalpies to
be consistent (within 50 C) with temperatures measured
by the thermocouple 5 mm downstream. Although the sur-
face temperatures can greatly exceed these downstream
temperatures, the overall energy balance (after hot and
colder gases mix) shows that the reactor is nearly adiabatic.
3. RESULTS
3.1. Effect of Cyclohexane/Oxygen Ratio
For most partial-oxidation systems, a keyparameter isthe fore eventually decreasing to 4% at C6H12/O2 = 5. Opera-
fuel/oxygen ratio. The cyclohexane/oxygen molar feed ra- tion beyond C6H12/O2 = 5 was not possible at a 2.5-SLPM
tio C6H12/O2 was varied at constant flow rate, dilution, inlet flow rate due to pump limitations. The reaction was nearly

248
O’CONNOR AND SCHMIDT
FIG. 1. Effect of cyclohexane/oxygen ratio for 2.5 SPLM, 30% N2, T0 = 200 C, and P = 1.2 atm.
extinguished under these conditions, as indicated by the low were very significant (>60% selectivity) for C6H12/O2 < 1
temperature and cyclohexane conversion.
and were produced with 35–40% selectivity for C6H12/O2 =
Figure 1b shows the overall selectivities (on a carbon- 1–5. Oxygenates, on the other hand, were 10% selec-
atom basis) to alkanes, COx, olefins, and oxygenates. Alka- tive for C6H12/O2 < 1 but increased drastically to nearly
nes were only significant (5–10% selectivity) for C6H12/ 45% maximum selectivity at C6H12/O2 = 2.5. Oxygenates
O2 < 1. Methane and ethane were the dominant alkanes, overtook olefins as the dominant product at a cyclohex-
although propane, n-butane, and n-pentane were present ane/oxygen ratio of about 1.75, and the yield of oxygenates
in small amounts (<1% selectivity). The selectivity to the was optimized (at 10% ) for C6H12/O2 2.0.
total-oxidation products CO and CO2 was roughly 20%
Figure 1c indicates the major individual olefins. For
throughout the entire range of cyclohexane concentrations. low cyclohexane/oxygen ratios, ethylene and 1,3-butadiene
This result is consistent with the postulate that CO and were abundant products, giving evidence of cracking reac-
CO2 are produced primarily on the catalyst surface. Olefins tions. C2H4 and 1,3-C4H6 followed the same trendsand were

CATALYTIC PARTIAL OXIDATION OF CYCLOHEXANE
249
both minimal for C6H12/O2 > 1.5. Propylene (not shown), partial-oxidation reactions, by at least an order of magni-
another cracked product, was >5% selective at low ra- tude. Higher flow rates translated to lower temperatures
tios but was not produced for C6H12/O2 > 1.5. Cyclohex- because reactant contact times were shorter and less het-
ene is an important olefin at all ratios, and it was maximum erogeneous chemistry occurred (compared with homoge-
(>30% selectivity) at C6H12/O2 2.0. Benzene was about neous reactions). The increase in overall conversions with
5% selective for C6H12/O2 < 3 and then increased to >10% flow rate is consistent with the relative increase in gas-phase
selectivity for cyclohexane/oxygen ratios exceeding 3. Cy- chemistry initiated by the catalyst.
clohexadiene, the intermediate between cyclohexene and
If surface chemistry is favored at low flow rates, the selec-
benzene, was just a few percent selective at low ratios tivity to CO + CO2 should decrease with flow rate. Indeed,
(C6H12/O2 < 1) and wasnot present under richer conditions. COx selectivity fell at least 5% as the flow rate was raised
Figure 1d shows the major oxygenates. At low cyclo- from 1 to 3 SLPM (Fig. 2b). Alkanes were negligible at all
hexane/oxygen ratios (C6H12/O2 < 0.8), the predominant flow rates examined. For flow rates <2 SLPM, olefins were
oxygen-containing species were C2–C3 compounds and favored over oxygenates. Oxygenates surpassed olefins at
small amounts of methanol (not shown). C2HxO includes 2 SLPM, and for a 2.5-SLPM flow rate, oxygenates ( 43%
acetaldehyde (CH3CHO) and ethanol (CH3CH2OH), and selectivity) were more selective than olefins (<40% se-
C3HxO includes propionaldehyde (CH3CH2CHO), pro- lectivity) and exhibited a maximum in yield ( 10% ). For
panol (C3H7OH), acetone (CH3COCH3), and 2-propenal higher velocities (3 SLPM), olefins slightly increased at the
(CH2CHCHO). For C6H12/O2 > 1, pentanal (C5H10O) expense of oxygenates, although the effect was small.
and especially 5-hexenal (5-C6H10O) became the sig-
Figure 2c shows that the flow rate did not greatly influ-
nificant oxygenated products. Selectivities of 25% to ence cyclohexene selectivity (30–35% ). Benzene exhibited
5-hexenal and 15% to pentanal were achieved at cyclo- a weak decline of a few percent, and cyclohexadiene was
hexane/oxygen ratios of 2 and 3, respectively. 5-Hexenal se- not produced. Ethylene increased slightly with flow rate,
lectivity decreased for cyclohexane/oxygen ratios from 2 to but 1,3-butadiene was negligible at all flow rates here, so
3but then increased significantlyto >30% for C6H12/O2 = 5. some ethylene must be formed by a pathway different than
Cyclohexanone (c-C6H10O) was produced most selectively that which explains the high C2H4 + 1,3-C4H6 selectivity at
( 5% ) for C6H12/O2 4. The corresponding alcohol, cy- low cyclohexane/oxygen ratios (Fig. 1c). Propylene, buty-
clohexanol, was not detected (<0.1% selectivity) in these lene, and pentene were all <1% selective.
experiments. Operation at C6H12/O2 = 4 was desirable in
Figure 2d reveals that 5-hexenal may have increased
terms of oxygenate formation: 5-hexenal was at least as moderately with flow rate, although the scatter suggests
selective as cyclohexene, and formation of cyclohexanone that the selectivity was practically constant. Pentanal did
was optimum. However, cyclohexane and oxygen conver- increase >5% in selectivity from 1 to 3 SLPM, and the C2
sions both suffered at this ratio (Fig. 1a), causing low yields and C3 oxygenates were favored somewhat by higher flow
of C6 oxygenates and high oxygen breakthrough.
rates. Although produced in very small amounts, 2-hexenal
followed the same trend as 5-hexenal, indicating similar
pathways. Cyclohexanone selectivity was negligible at flow
rates from 1 to 3 SLPM under these conditions (in particu-
lar, C6H12/O2 = 2).
3.2. Effect of Flow Rate
The total flow rate of reactants passing through the sin-
gle gauze should affect the surface chemistry because of
variation in contact time on the catalyst. In Fig. 2, experi-
mental data are shown for C6H12/O2 = 2.0, 30% N2 dilution,
T0 = 200 C, and P = 1.2 atm. This cyclohexane/oxygen ra-
3.3. Effect of Preheat
The temperature of the feed gases was varied between
tio was chosen because it provided the maximum yield of T0 = 100 and 300 C for C6H12/O2 = 2.0, 2.5 SLPM, 30% ni-
total oxygenates (Fig. 1) and because it is the theoretical trogen dilution, and P = 1.2 atm. The experimental results
stoichiometric ratio for C6 oxygenates. The total flow rate are shown in Fig. 3. Note that T0 corresponds to the temper-
was varied between 1 and 3 SLPM, which correspond to ature at the catalyst entrance and not the reactor entrance.
superficial catalyst contact times of 1.0 and 0.33 ms, re- The normal boiling point of cyclohexane is 81 C so that
spectively, under these conditions.
to avoid cyclohexane condensation the inlet temperature
Figure 2a shows that oxygen conversion (75–90% ) and could not be much less than 100 C.
cyclohexane conversion ( 20% ) both increased slightly
The conversions and catalyst exit temperature (at 5 mm
with flow rate. The downstream temperature fell 50 C downstream) are plotted in Fig. 3a. The reactor temperature
as the flow rate was varied from 1 to 3 SLPM, perhaps a was essentially linear with preheat, increasing 100 C for
counterintuitive result because conversions increased and a change in amount of preheat of 200 C. Cyclohexane and
most reactions taking place are exothermic. On the con- oxygen conversions were practically constant at roughly 25
trary, the surface reactions (such as complete combus- and 90% , respectively. With conversions constant, it could
tion) are more strongly exothermic than are the gas-phase be expected that selectivities also would not be strongly

250
O’CONNOR AND SCHMIDT
FIG. 2. Effect of flow rate for C6H12/O2 = 2.0, 30% N2, T0 = 200 C, and P = 1.2 atm.
influenced by preheat. Figure 3b agrees with this assertion. and C3HxO were constant, and pentanal decreased with
Alkanes were nonsignificant and olefins were 42% selec- increasing preheat. These opposing trends of C5H10O and
tive for feed temperatures of 100–300 C. Olefin selectivity 5-C6H10O should be considered in any mechanism that at-
was enhanced slightly for higher preheat.
tempts to explain the data.
Figure 3c shows the major olefins. Cyclohexene was not
greatly influenced by preheat, although the data do indi-
cate a weak optimum (>30% selectivity) near T0 = 200 C.
All other olefinic species were basically constant except
ethylene, which was slightly more abundant at higher inlet
temperatures. Figure 3d indicates that the increase in oxy-
genate selectivity with preheat was due to higher produc-
tion of 5-hexenal (maximum of 25% selectivity). C2HxO
3.4. Effect of Dilution
So far, all experiments were performed with 30% nitro-
gen diluent present. In many reactions, dilution simply low-
ers the temperature and thus decreases gas-phase reaction
rates. In these instances the dilution effect would essentially
be the opposite of the preheat effect. However, the single-
gauze reactor should be more sensitive to dilution because

CATALYTIC PARTIAL OXIDATION OF CYCLOHEXANE
251
FIG. 3. Effect of preheat for C6H12/O2 = 2.0, 2.5 SPLM, 30% N2, and P = 1.2 atm.
of the unique quenching effect downstream of the cata- conversion decreased from 20 to 10% . In terms of moles
lyst. In Fig. 4, dilution was varied between 5% (diluent por- converted, the decrease in conversion with dilution was the
tion of total feed) and 55.6% at C6H12/O2 = 2.0, 2.5 SLPM, same for both C6H12 and O2: about half of the moles were
T0 = 200 C, and P = 1.2 atm. At this cyclohexane/oxygen converted in air versus (roughly) pure oxygen.
ratio, 55.6% N2 corresponds to an air feed, as noted on the
axes. Because some N2 was needed for GC calibration, a
It may be surprising that the temperature dropped only
30 C when conversions dropped drastically. As noted pre-
pure-oxygen feed was not tested, but the 5% dilution case viously, the highly exothermic production of CO and CO2
should closely resemble reactor operation with pure O2. on the catalyst surface plays a large role in the measured
Figure 4a illustrates the temperature and conversions. reactor temperature. Figure 4b shows that the CO + CO2
Temperature, oxygen conversion, and cyclohexane conver- selectivity increased from 20 to 40% as dilution was
sion all fell off with increasing diluent concentrations. At increased from 5 to 56% . In this span the COx carbon-
low dilution the O2 conversion was 95% , but it decreased atom yield was constant at 4% with varying dilution. The
sharply, falling to <50% with an air feed. The cyclohexane decrease in reactor temperature must therefore have been

252
O’CONNOR AND SCHMIDT
FIG. 4. Effect of dilution for C6H12/O2 = 2.0, 2.5 SPLM, T0 = 200 C, and P = 1.2 atm.
due to lower gas-phase conversion. From Fig. 4b, alkanes ene decreased about 10% in selectivity. Benzene slightly
were nonsignificant at all nitrogen concentrations. Olefins increased and ethylene decreased somewhat. Propylene,
decreased with dilution, becoming >10% less selective butylene, and pentene were not produced significantly, re-
with an air feed. Oxygenates exhibited an optimum in se- gardless of dilution. The individual oxygenated species are
lectivity ( 40% ) and yield ( 8% ) at approximately 25% shown in Fig. 4d. C2 and C3 oxygenateswere most significant
N2. At this point, oxygenates also overtook olefins as the for lower dilution. 5-Hexenal and pentanal showed similar
most selective reaction product, until COx dominated at trends, contrary to the opposing behavior for varying pre-
very high dilution when much less homogeneous chemistry heat, emphasizing that dilution and preheat were distinct
occurred.
phenomena for this system. The optima in C5H10O and 5-
Cyclohexene (Fig. 4c) basically accounts for the decline C6H10O were at 30% N2, while 2-C6H10O selectivity was
in olefin selectivity with increasing dilution, as cyclohex- actually maximized ( 4% ) at the lowest dilution.

CATALYTIC PARTIAL OXIDATION OF CYCLOHEXANE
253
3.5. Effect of Pressure
oxygen both increased with pressure, as indicated in Fig. 5a.
The cyclohexane conversion increased from <20 to 25% ,
The pressure of the single-gauze reactor was varied be- while the oxygen conversion was enhanced from 90% at
tween 1.2 and 2 atm for C6H12/O2 = 2.0, T0 = 200 C, and P = 1.2 atm to 100% at P = 2 atm. The exit temperature in-
30% nitrogen dilution. For these experiments we chose a creased approximately 20 C throughout this range of pres-
constant catalyst contact time of 0.7 10 3 s. Because su- sures.
perficial contact time decreases linearly with flow rate and
Figure 5b shows that the selectivity to CO and CO2 de-
increases linearly with pressure, the flow rate was increased creased with pressure, suggesting that surface reactions be-
as the pressure was raised so that the contact time was main- came lessimportant than gas-phase reactionsat higher pres-
tained at 0.7 ms. The experimental results are shown in sure, as expected. As the pressure was raised to 2 atm, the
Fig. 5.
selectivity to oxygenates decreased nearly 5% while the
In principle, higher pressure should cause higher reac- selectivity to olefins increased about 10% . At P = 2 atm
tion rates in the gas phase. Conversions of cyclohexane and and the operatingconditionsindicated, olefinswere favored
FIG. 5. Effect of pressure for C6H12/O2 = 2.0, T0 = 200 C, 30% N2 dilution, and a catalyst contact time of 0.7 ms.

254
O’CONNOR AND SCHMIDT
over oxygenates by almost 15% in selectivity. Production tion of the major species is hypothesized to occur in the
of alkanes (mostly ethane) also began to become important gas phase once the reaction is heterogeneously initiated.
at higher pressures. Alkanes are expected predominantly Figure 6 shows a simplified surface-assisted homogeneous
from homogeneous pathways because surface steps should reaction scheme consistent with the major species observed
lead to unsaturated products from cyclohexane.
experimentally.
Figures 5c and 5d present the influence of pressure
The initial step is H-atom abstraction of cyclohexane to
on the selectivities to individual olefins and oxygenates, form the cyclohexyl radical. This step can occur oxidatively
respectively. Cyclohexene, ethylene, and propylene selec- whereby an oxygen molecule reacts directly with cyclohex-
tivities increased with reactor pressure, while benzene pro- ane and strips off a hydrogen atom, forming HOO and
duction was cut in half as the pressure was raised to 2 atm. C6H11 . Alternatively, a radical R (e.g., C4H7 or OH ) des-
The trend in benzene resembles the trend in COx forma- orbing from the gauze surface into the gas phase could
tion, suggesting that benzene may be a surface-generated collide with cyclohexane and abstract H, forming RH and
species. The selectivity to 5-hexenal (Fig. 5d) does not ap- C6H11 . The direct reaction with O2 has a large activation
pear to be greatly influenced by the pressure. The decrease energy, although the intense heat provided by the surface
in oxygenates was due mainly to a decrease in the produc- reactions could help overcome the energy barrier for the
tion of pentanal, which declined in selectivity from 10% at oxidative step. Because the system is lean in oxygen, the
1.2 atm to <2% at 2.0 atm. At P = 2 atm, the selectivity to radical-assisted initiation step is likely to be as important as
5-hexenal and cyclohexene was 58% .
the oxidative route for abstracting a hydrogen atom. Fur-
thermore, plug-flowcalculationsusinga homogeneous-only
reaction mechanism (11) with >150speciesand 3000reac-
tions suggest that surface generation of radicals is necessary
to account for the experimental cyclohexane conversions at
millisecond time scales.
4. DISCUSSION
4.1. Overall Process
The cyclohexyl radical can dehydrogenate oxidatively to
form cyclohexene, undergo -decomposition reactions to
form smaller products (especially at higher temperatures),
or directly add an oxygen molecule to form the important
cyclohexylperoxy radical C6H11OO .
The cyclohexylperoxy radical can go to cyclohexene by
loss of HOO , but it can also rearrange by a back-biting re-
action in which the radical abstracts an internal H atom. The
abstraction generally occurs at least two carbons away from
the site of the peroxy radical. The cyclohexylperoxy radical
should closely follow this rule due to the close proximity
of the peroxy group (at the position) with the two ax-
ial -H atoms which are therefore more readily abstracted.
Interactions with -, -, and especially -H atoms are not
spatially preferred. The C6H10OOH species with the free
radical to the peroxy carbon should be the favored cyclo-
hexylperoxy isomer, as depicted in Fig. 6.
Loss of a hydroxy radical from -C6H10OOH , which
should occur with low activation energy (12), leads to a
species that strongly favors ring rupture and the formation
of the olefinic aldehyde 5-hexenal. If -C6H10OOH instead
ejects a formyl radical (CHO ), pentanal is produced. The
data in Fig. 1d suggest a competition between these C5 and
C6 oxygenates, consistent with the parallel consumption of
-C6H10OOH hypothesized in Fig. 6.
Within this scheme, cyclohexanone formation should not
be favored because it requires the cyclohexylperoxy radi-
cal with the unpaired electron on the peroxy carbon. Re-
arrangement to this isomer from the originally formed
The cyclohexane/oxygen ratio was the most important
variable for operation of the single-gauze reactor, as seen
in Fig. 1. Temperatures, reactant conversions, and product
selectivities all changed significantly as C6H12/O2 was varied
from 0.65 to 5.0. Oxygenates were produced at >40% selec-
tivity for C6H12/O2 2. Nearly 70% selectivity to three par-
ticular products—5-hexenal, pentanal, and cyclohexene—
could be achieved with this single-gauze reactor. Cyclo-
hexanone was made with 5% selectivity at C6H12/O2 4,
although oxygen and cyclohexane conversions were both
relatively low.
The flow rate (or contact time) of reactant gases (Fig. 2)
was somewhat important. Oxygenates were favored by
higher flow rates, showing an optimum in selectivity at
roughly 2.5 SLPM for the range examined. From Fig. 3,
the effect of preheat was smaller and different than that
of dilution. Selective production of oxygenates over olefins
was promoted by lower inlet temperatures. Small amounts
of nitrogen diluent (<10% N2) favored olefin production,
while high dilution (>40% N2) suppressed the homoge-
neous reactions necessary for oxygenate formation (Fig. 4).
The constant yield ( 4% ) of CO + CO2 suggests that the
surface chemistry was largely unaffected by dilution. A re-
actor pressure of 2 atm (Fig. 5) promoted higher yields of
the C6 products cyclohexene and 5-hexenal while decreas-
ing the formation of pentanal.
4.2. Reaction Mechanism
Although a full explanation of the data requires both C6H11OO species is spatially difficult, as it requires in-
heterogeneous and homogeneous mechanisms, the forma- teraction of the axial peroxy group with the equatorial

CATALYTIC PARTIAL OXIDATION OF CYCLOHEXANE
255
FIG. 6. Proposed surface-assisted homogeneous reaction pathways for cyclohexane partial oxidation.
-hydrogen. The 5% maximum selectivity to cyclohex- indeed produce 1,3-C4H6 + C2H4, as drawn in the scheme
anone was apparently made possible by internal abstrac- in Fig. 6. Some ethylene is formed by another pathway
tion of the -hydrogen with transfer of a radical from the (Figs. 1–5), perhaps through cyclohexyl -decompositions
-carbon to the -carbon, as indicated in Fig. 6. There may (on the surface or in the gas phase).
be alternate pathways; Figure 6 is just one possibility and
probably a partial explanation at best.
To address potential arguments that the entire process is
strictlyhomogeneous, the Pt–10% Rh catalyst must be lit off
The high selectivities of cyclohexene over wide ranges for the single-gauze reactor to produce valuable species. Ex-
in conditions are consistent with the proposed mechanism. periments at C6H12/O2 = 2, 2.5 SLPM, 30% N2, T0 = 200 C,
The decline in cyclohexene selectivity with dilution (Fig. 4c) and P = 1.2 atm but without catalyst ignition showed ab-
suggests predominantly gas-phase production of cyclohex- solutely no reaction. The presence of an unignited gauze
ene. Benzene, which along with COx is predicted by equi- would interfere with gas-phase reactions if free radicals
librium at the high temperatures on the surface, must pro- were terminated on the surface. Experiments with an empty
ceed through cyclohexene. Benzene is probably produced tube (no catalyst present) heated to 200 C also showed no
by both heterogeneous and homogeneous reactions. Des- reaction, verifying that the essential reactions here require
orption from the catalyst after a single dehydrogenation an ignited catalyst.
is unfavorable, implying that most of the cyclohexene is
a gas-phase product. It is attractive that while benzene is
a thermodynamic sink for the oxidative dehydrogenation
of cyclohexane, the cyclohexene/benzene production ratio
was typically >5 in these experiments.
For higher temperatures (lower cyclohexane/oxygen ra-
tios), large amounts of ethylene and 1,3-butadiene resulted
from cracking reactions. One mechanistic possibility is that
cyclohexene (which is equivalent to cyclohexane with adja-
cent free radicals) undergoes a double -scission, rupturing
the 3,4 and 5,6 C–C bonds. This concerted reaction would
5. CONCLUSIONS
Coupled heterogeneous and homogeneous chemistry us-
ing a 90% Pt–10% Rh single-gauze catalyst allows the se-
lective production of nonequilibrium species from the par-
tial oxidation of cyclohexane. A portion of the cyclohexane
feed reacts completely on the surface, generating heat and
free radicals which initiate a gas-phase reaction sequence.
The homogeneous chain reactions that produce desired
oxygenated hydrocarbons and olefins are then thermally

256
O’CONNOR AND SCHMIDT
quenched by cold gases passing between the wires. Much ably that separation of the product stream would be energy-
higher selectivities to oxygenates can be achieved with a intensive.
Pt–10% Rh single gauze versus a Pt-coated foam monolith
which does not permit quenching.
Efforts are underway to computationally model the
single-gauze reactor using detailed chemistry for the pro-
Two main regimes of operation exist, with a transition duction of oxygenates and olefins from the partial oxidation
near a cyclohexane/oxygen ratio of unity. Lean operation of cyclohexane. Numerical simulations will aid in further
(C6H12/O2 < 1) allows >60% selectivity to olefins (mostly understanding the surface-assisted gas-phase process and
cyclohexene, 1,3-butadiene, and ethylene). For richer oper- should help adjust reactor operation for desired product
ation (C6H12/O2 > 1), 5-hexenal, cyclohexene, and pentanal distributions.
dominate, with 70% maximum selectivity to these three
products. A reactor pressure of 2 atm is desirable because
it inhibits the formation of COx and pentanal and increases
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Our single-stage autothermal reactor can process 8 kg/
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