Oxygen-free conversion of methane to higher alkanes through an isothermal two-step reaction on ruthenium

Article abstract of DOI:10.1006/jcat.1996.0186

Ruthenium dispersed on silica is able to chemisorb CH₄ at temperatures significantly lower than EUROPT-1 (see the previous articles). At the temperatures used (≥80°C), H₂ desorption parallels CH₄ chemisorption but no C₂H₆ is observed. During the following temperature programmed desorption under flowing argon, CH₄ is removed through a wide range of temperature (from room temperature to 300°C) with a first contribution peaking at less than 100°C. Very small amounts of CH₄ are desorbed after an adsorption carried out at T ≥ 180°C, due to strong dehydrogenation of the adspecies. Subsequent temperature programmed surface reaction of the remaining adspecies with hydrogen displays upto four CH₄ peaks at well defined temperatures (ranging from ≈ 60 to ≈ 340°C), accompanied by a negligible formation of ethane. No Cγ was formed. The total amount of adsorbed CH₄ and the average H/C ratio of the corresponding adspecies can be derived from these experiments. In a separate set of experiments, CH₄ is switched to H₂ at the end of the exposure step, the temperature being fixed. An immediate formation of alkanes ranging from C₁ to C₆ is then evidenced. A sizeable fraction of the chemisorbed layer can so be homologated to higher alkanes. The influences of the various operating factors are reported. In particular a neat maximum of the C₂₊ production versus temperature (at 160°C) is evidenced and is clearly due to the adverse hydrogenolysis reactions, efficiently catalyzed by Ru. All the results can be interpreted in complete similarity with the Pt case.

Full text of DOI:10.1006/jcat.1996.0186

JOURNAL OF CATALYSIS 161, 282–291 (1996)
ARTICLE NO. 0186
Oxygen-Free Conversion of Methane to Higher Alkanes through an
Isothermal Two-Step Reaction on Ruthenium
1
Mohamed Belgued, Annie Amariglio, Laurent Lefort, Pierre Par e´ ja, and Henri Amariglio
Laboratoire de Catalyse H e´ t e´ rog e` ne, Universit e´ Henri Poincar e´ , Nancy I, BP 239, 54506 Vandœuvre Cedex, France
Received April 17, 1995; revised December 19, 1995; accepted January 30, 1996
RuCl3 aqueous solution to incipient wetness. After drying,
Ruthenium dispersed on silica is able to chemisorb CH4 at tem- reduction was carried out in the reactor fed with a flow
3
� 1
� 1
peratures significantly lower than EUROPT-1 (see the previous ar- of H (50 cm min ), while the temperature was slowly
2
�
�
ticles). At the temperatures used (� 80 C), H2 desorption parallels
ramped (1 K min ) to 500 C and maintained constant for
4 h. Between two successive experiments, the catalyst was
submitted to a flow of H2 (50 cm min ) at 400 C for 2 h
in order to remove any carbonaceous residue. It was then
fed with a flow of Ar or He (50 cm min ) to remove H2
and finally cooled in Ar (or He) at the temperature de-
sired for the experiment. The metal loading of the catalyst
CH4 chemisorption but no C2H6 is observed. During the following
temperature programmed desorption under flowing argon, CH4 is
removed through a wide range of temperature (from room tempera-
2
3
� 1
�
�
�
ture to 300 C) with a first contribution peaking at less than 100 C.
Very small amounts of CH4 are desorbed after an adsorption carried
3
� 1
�
out at T � 180 C, due to strong dehydrogenation of the adspecies.
Subsequent temperature programmed surface reaction of the re-
(
4.7 � 0.1 wt%) was obtained from the Laboratoire Cen-
maining adspecies with hydrogen displays upto four CH4 peaks at
�
well defined temperatures (ranging from � 60 to � 340 C), accom- tral d’Analyse (CNRS) by plasma atomic emission spec-
panied by a negligible formation of ethane. No C� was formed. The troscopy.
total amount of adsorbed CH4 and the average H/C ratio of the
corresponding adspecies can be derived from these experiments. In
a separate set of experiments, CH4 is switched to H2 at the end
All the following experiments were carried out on a sam-
ple of 100 mg.
of the exposure step, the temperature being fixed. An immediate 2. Apparatus and Procedures
formation of alkanes ranging from C1 to C6 is then evidenced. A
sizeable fraction of the chemisorbed layer can so be homologated
to higher alkanes. The influences of the various operating factors the same as previously described (1).
The experimental setup and the procedures are exactly
are reported. In particular a neat maximum of the C2+ production
versus temperature (at 160 C) is evidenced and is clearly due to
�
RESULTS
the adverse hydrogenolysis reactions, efficiently catalyzed by Ru.
All the results can be interpreted in complete similarity with the Pt
1
. Catalyst Characterization
.1. Dispersion evaluation. Dispersion of the catalyst
could be determined by measurement of the quantities of
i) H2, (ii) O2, or (iii) CO, which the catalyst was able
case.
�c 1996 Academic Press, Inc.
1
(
INTRODUCTION
to chemisorb at room temperature (rt). To this end the
chemisorption was effected by feeding the catalyst with a
flow of the concerned species in the following conditions:
In the present article we report the results of a study
similar to that of our preceding contributions (1, 2) but
relative to the case of Ru instead of Pt. We have chosen
Ru because, in addition to being a noble metal, it is also an
efficient FT (Fischer–Tropsch) catalyst.
3
� 1
(
i) pure H2 at ordinary pressure, 50 cm min , 15 min;
(
ii) Ar at ordinary pressure loaded with 100 ppm O2
until no more O2 consumption could be detected with an O2
trace analyzer (Engelhard Mark II electrochemical meter);
3
� 1
EXPERIMENTAL
(
iii) pure CO at ordinary pressure, 50 cm min , 1 min.
The amount of H2 chemisorbed was deduced from the
spectrum of TPD (temperature programmed desorption)
which resulted from feeding the catalyst with an Ar flow
1
. Catalyst
The catalyst was prepared by impregnation of a Sphero-
2
� 1
sil sample (Interchim XOB075 silica, 100 m g ) with an
3
� 1
(
50 cm min ) while the temperature was ramped at 8
�
1
K min (Fig. 1). The corresponding amount of CO was
attained through the temperature programmed surface
1
To whom correspondence should be addressed.
282
0021-9517/96 $18.00
Copyright �c 1996 by Academic Press, Inc.
All rights of reproduction in any form reserved.

OXYGEN-FREE HOMOLOGATION OF METHANE ON RUTHENIUM
283
O2). The sample could then be removed from the reactor
without suffering any damage. It was lightly ground in
a small mortar and subsequently dispersed in alcohol by
ultrasonic treatment. A drop of the suspension so ob-
tained was deposited on a copper grid covered with car-
bon. The samples were examined with a Philips CM20 high-
resolution transmission microscope equipped with a micro-
probe system (EDAX microanalysis). Magnifications rang-
ing from 88,000 � to 380,000 � were used with the micro-
scope operated at 200 kV. Subsequent enlargements could
be obtained by using thin photographic paper.
Figure 3a presents a typical representation of the obser-
vations repeated on numerous parts of the sample. Most
of the particles display linear dimensions between 15 and
8
0 A˚ , which is quite consistent with the dispersion previ-
Ru/SiO2. Adsorption: H2 (1 bar, 50 cm min , 15 min, rt). TPD (8 K ously determined.
FIG. 1. TPD spectrum of H2 preadsorbed on 100 mg of 4.7 wt%
3
� 1
�
1
3
� 1
min ) under Ar (1 bar, 50 cm min ).
We have also observed a sample which had been used in
numerous experiments, consisting in chemisorption of CH4
effected under various conditions and followed either by
TPD and TPSR with H2 or by hydrogenation at the same
temperature. After this set of experiments, the catalyst had
3
reaction (TPSR) of the CO monolayer with H2 (50 cm
�
1
� 1
min , 8 K min ). Figure 2 shows the resulting CH4 peak.
Assuming H/RuS and O/RuS ratios equal to 1 we could so
estimate that the surface Ru atoms of the sample amounted
to 17.0 and 17.8 �mol, respectively. The adsorbed CO was
converted to 16.0 �mol of CH4, which may be considered
in good agreement with the preceding results if one con-
siders that some CO molecules can occupy two metal sites
�
been submitted to H2 at 400 C for 1 h, fed with a flow of Ar
in order to remove H2, and passivated like the fresh sample
specially prepared for the electron microscopy observation.
Figure 3b presents a typical view of the observations which
do not differ from that on the fresh sample.
(
bridged adsorption). On the basis of 17 �mol of surface
2
. Evolution of H2 during Exposure to CH4
Ru atoms, the dispersion of our catalyst can be estimated
as equal to ca. 36%.
When Ru was exposed to flowing CH4 at various fixed
temperatures, TCH , release of H2 was evidenced from
4
1
.2. Electron microscopy examination. A sample, in-
�
�
TCH � 80 C. At TCH � 100 C the rate of H2 evolution dis-
4
4
tended to be submitted to electron microscopy examina-
tion, was prepared as previously described. After reduc-
tion, it was fed with a flow of Ar in order to remove the
H2 and cooled to rt. O2 was then chemisorbed on the Ru
particles by feeding them with a flow of (Ar + 100 ppm
played a clear maximum versus time on stream but, at
higher temperatures, only monotonic decreases were ob-
served (Fig. 4). Figures 5 and 6 show how the amount of
H2 released during 5-min exposures to CH4 increased as a
function of TCH and of the flow rate, respectively. Contrary
4
to the case of Pt, there was no C2H6 formed during exposure
to CH4 at any temperature.
3
. Amount of Adsorbed Methane and Its Variation with
Operating Factors
In order to determine the amount of adsorbed methane
at the end of the exposure step, we carried out a TPD inter-
�
rupted at 300 C and followed by a TPSR of the remaining
adspecies with H2.
3
.1. TPD spectra. After exposure to CH4 under given
conditions of temperature, duration, and flow rate, the Ru
sample was cooled to rt as quickly as possible (1 min or
so) while being kept in static CH4. The reactor was then
purged with flowing He and the temperature was linearly
�
ramped up to 300 C. Only CH4 was formed during heat-
FIG. 2. H2 -TPSR of CO preadsorbed on 100 mg of Ru/SiO2. Adsorp-
3
� 1
� 1
ing. Figure 7 collects TPD spectra corresponding to various
temperatures of exposure ranging between 80 and 200 C,
tion: CO (1 bar, 50 cm min , 1 min, rt). TPSR (8 K min ) with H2 (1 bar,
3
� 1
�
5
0 cm min ).

2
84
BELGUED ET AL.
FIG. 3. Electron micrographs of the catalyst; (a) fresh, (b) used.
other operating conditions being fixed. They display a low amount of CH4 produced during the TPSR much higher
�
temperature peak (at T < 100 C), followed by more or less than the corresponding one obtained after any exposure at
�
distinguishable peaks and a progressive extinction of the TCH � 160 C.
4
�
CH4 desorption at about 300 C. The dominant feature of
3
.3. Variation in the amount of CH4 adsorbed versus tem-
these spectra is the very strong decrease of the CH4 evolved
throughout the entire range of TPD when the prelimi-
nary chemisorption of CH4 was carried out at temperatures
perature. The quantities of CH4 evolved during the differ-
ent steps previously described as well as that of C2H6 re-
leased during the TPSR with H2 can be found in Table 1.
The corresponding amount of CH4 chemisorbed in each
case is easily derived and its variation with temperature is
represented in Fig. 5 where we have already plotted the
variations with temperature of the amount of H2 evolved
during the exposure to CH4. The parallelism between these
two curves is obvious. Moreover, their comparison shows
that the carbonaceous species left on the surface after the
exposure to CH4 are deeply dehydrogenated.
�
above 160 C.
No H2 and no hydrocarbon other than CH4 were evi-
denced during the TPD’s.
3
.2. TPSR Spectra. Cooling the catalyst to rt at the end
of each TPD and exposing it to H2 immediately resulted
in a CH4 production with a small amount of ethane and
propane production. All these productions reached negli-
gible levels beyond a few minutes. Thereafter, the tempera-
�
ture was ramped up to ca. 450 C, which caused the removal
3.4. Influence of the duration of the exposure to CH4. In
of CH4 accompanied by only traces of C2H6. Figure 8 col- order to illustrate the influence of the duration of the ex-
lects the TPSR spectra related to CH4 which were obtained posure to the flow of CH4, only one set of other operating
�
after the corresponding TPD spectra of Fig. 7. When CH4 conditions was chosen. The choice of 160 C as tempera-
�
was chemisorbed at TCH � 160 C, mainly two peaks ap- ture of exposure resulted from the fact that the production
4
�
�
peared (at 70 and 140 C). For TCH > 160 C, two other CH4 of higher alkanes by hydrogenation of the chemisorbed
4
�
peaks appeared at � 230 and � 320 C, which made the total species was the largest at that temperature (see Section

OXYGEN-FREE HOMOLOGATION OF METHANE ON RUTHENIUM
285
FIG. 6. Molar amounts of H2 evolved during 5-min exposures to CH4
�
at 160 C as a function of the flow rate of CH4. Same catalyst sample as for
Figs. 4 and 5.
4
. Formation of Higher Alkanes by Hydrogenation
of the Surface Species
At the end of the preliminary exposure of Ru to CH4,
when CH4 was switched to H2, significant amounts of higher
alkanes ranging up to C6 (and traces of C7) were immedi-
ately released, as in the case of Pt. The total production and
its distribution depended upon the same factors as for Pt,
i.e., temperature and duration of the exposure to CH4, flow
rate and pressure of CH4, and pressure of H2 during the
second step. Ethane was always the most abundant prod-
uct even if, under certain conditions, most of the CH4 was
converted into pentanes. No cyclo-alkanes (or only traces)
were formed and that was different from what happened
on platinum. At least in the upper part of the temperature
range explored, the alkanes were rapidly formed upon ad-
�
�
FIG. 4. Molar flow rates of H2 evolved at 80 C (a) or 160 C (b) during
exposure of Ru to CH4 as a function of time on stream. Conditions: 100 mg
3
� 1
of Ru/SiO2 exposed to 375 cm min of CH4.
4
.1). Every exposure was followed by the same succession
mission of H2 since most of the production was obtained in
of experiments as in the previous cases. Figures 9 and 10 col-
lect the corresponding TPD and TPSR spectra and Fig. 11
gives the variation of the amount of CH4 adsorbed versus
the duration of exposure.
�
1
0 s at 160 C. Only this fast production is here taken into
account and, accordingly, attention has been paid only to
FIG. 5. Molar amounts of H2 evolved and of CH4 adsorbed during
FIG. 7. TPD spectra following exposures of Ru to CH4 at various
3
5
1
-min exposures of Ru to CH4 as a function of temperature. Conditions:
temperatures. Conditions: Exposures as for Fig. 5; flow rate of He, 200 cm
3
� 1
� 1
� 1
00 mg of Ru/SiO2 exposed to 375 cm min of CH4.
min ; ramp of temperature, 8 K min .

2
86
BELGUED ET AL.
�
FIG. 9. TPD spectra following exposures of Ru to CH4 at 160 C for
different durations. Other conditions as for Fig. 5.
�
seen to be at a maximum at 160 C. However, the temper-
ature causing the maximum of the heavier alkanes (C4 + )
�
�
was slightly lower (140 C instead of 160 C, except for i-C4).
The fractions of homologated CH4 which were converted
to each of the C2+ products can be seen in Fig. 13.
For a number of experiments, the quantities of CH4
chemisorbed during the exposure step were determined
in a separate set of experiments. To this end, after
FIG. 8. Profiles of H2-TPSR of the adspecies remaining on the Ru
surface after the TPD corresponding to Fig. 7. Conditions: flow rate of H2,
3
� 1
� 1
5
0 cm min ; ramp of temperature, 6 K min .
this total production without any attempt at rate measure-
ments.
4.1. Influence of the temperature. Figure 12 shows the
variation of the molar quantities of each product versus
temperature, every other operating condition being fixed.
The overall C2+ production is also shown and is clearly
TABLE 1
Variation of the Total Amounts of the Methane Adsorbed and of
the Products Collected during TPD and H2-TPSR, as a Function of
Temperature
CH4
Temperature
�
a
b
c
C2H6d
(
C)
CH4 ads
8
20
60
80
00
0
0.65
1.25
1.80
0.35
0.25
0.66
0.60
1.90
3.60
3.10
0.60
2.15
4.80
10.20
11.60
0.00
0.00
0.31
0.38
0.48
1.91
4.00
9.12
14.9
15.9
1
1
1
2
Note. Theamounts areexpressed in�mol. Conditions of the chemisorp-
3
� 1
tion of CH4: 1 atm, 375 cm min , 5 min.
a
Amount of methane collected during the TPD.
Amount of methane collected in the flow of H2 at rt.
Amount of methane collected during the H2-TPSR.
Amount of ethane collected during the H2-TPSR.
b
FIG. 10. Profiles of H2-TPSR of the adspecies remaining on the Ru
surface after the TPD corresponding to Fig. 9. Conditions of the TPSR as
for Fig. 8.
c
d

OXYGEN-FREE HOMOLOGATION OF METHANE ON RUTHENIUM
287
�
FIG. 11. Molar amounts of CH4 adsorbed at 160 C as a function of
the exposure duration. Data are deduced from Figs. 9 and 10.
having been exposed to CH4, the catalyst was cooled to
rt. The amount of chemisorbed CH4 was determined from
subsequent TPD and H2-TPSR, as explained in (1). The
fraction of chemisorbed methane which was homologated
could so be determined. The result is given in Fig. 14.
4
.2. Influence of the duration of exposure. The effect of
�
the duration of CH4 exposure was examined at 160 C and is
illustrated in Fig. 15. Unlike the case of Pt, all the products
were formed in increasing amounts when the exposure was
prolonged. However, the increase was much stronger for
the heavier than for the lighter alkanes. In this respect it
can be seen that the overall production increased from 3.36
to 4.08 �mol when the exposure was lengthened from 5 to
FIG. 12. Molar amounts of the total homologated methane and of the
2
0 min, whereas the amount of hexanes was multiplied by various C2+ products resulting from exposures of Ru to methane at various
temperatures (step 1) and subsequent hydrogenations at the same tem-
a factor of three.
perature (step 2). Conditions: 100 mg of Ru/SiO2. Step 1: exposure to CH4
Two additional experiments clearly showed how the
continuous removal of H2 during the exposure to CH4
matters in relation with the subsequent alkane production.
After exposure of the Ru to the flow of CH4 for either 1 or
3
�
1
3
�
1
(
1 bar, 375 cm min , 5 min); Step 2: exposure to H2 (1 bar, 50 cm min ).
of the alkane is the greater. These results are consistent with
those obtained on Pt in similar experiments (2).
5
min the reactor was closed and the exposure to static CH4
was prolonged for 4 or 15 min, respectively. The corre-
4.4. Repetition of uninterrupted CH /H cycles. Ten
4
2
sponding subsequent productions equaled those obtained successive isothermal CH /H cycles were applied to the
4
2
after 1 or 5-min exposures. Further useful chemisorption of Ru sample. Each of them comprised a 5-min exposure to
3
� 1
�
CH4 did not take place during CH4 exposure under static CH (375 cm min ) at 160 C and a 1-min exposure to H
4
2
3
� 1
conditions. There was no evidence for a modification of (50 cm min ) at the same temperature. No decrease of
the previously adsorbed species either. That observation the total C2 production per cycle was evidenced.
+
strongly underlines that the removal of H2 by flowing CH4
is a requirement for obtaining adspecies in a state allowing
their mutual bonding.
4
.5. Reproducibility of the results. Two kinds of repro-
ducibility may be distinguished, according to the scale of
time. The numerous experiments which were necessary to
4.3. Influence of the flow rate of CH4. The preceding obtain such results as those given in Figs. 12 or 15 took
comment is exemplified by the results presented in Fig. 16 several days and their uncertainty did not exceed a few
showing how the total production and how the production percent. In contrast, long use of the catalyst during several
of each family of alkanes varied versus the flow rate of weeks could result in a loss of the total C2 + production of
CH4, every other operating condition being fixed. If the a given CH4/H2 cycle which could reach up to 25%. In such
C2 production tends to level off when the flow rate is over cases we preferred to replace the used sample by a fresh
3
� 1
3
00 cm min , a continuous increase of the production of one, instead of trying to rejuvenate the used sample by a
the other alkanes can be noticed and the more so as the rank prolonged subjection of H2 at an elevated temperature.

2
88
BELGUED ET AL.
FIG. 13. Distribution of the homologated CH4 in the C2+ products at various temperatures. Data are deduced from Fig. 12.
In case of interruption of the experiments for a period of consisting of C3, n- and i-C4, n- and i-C5, and n-C6 diluted in
time exceeding 1 week, a very efficient way of maintaining H2. The concentrations of alkanes in this mixture (C3, 2360
the catalyst properties was to cover its surface with oxygen ppm; i-C4, 1940 ppm; n-C4, 2060 ppm; i-C5, 1980 ppm; n-C5,
chemisorbed under mild conditions, by submitting the cata- 1970 ppm; n-C6, 240 ppm) were similar to those found in the
lyst at rt to a flow of He loaded with 1‰ O2. Removal of the reaction products. At each temperature, the hydrogenolysis
�
oxygenwithH2 at300 Callowedustorecoverthecatalystin products were analyzed after a 5-min feed of the catalyst
3
� 1
a state which was always the same as that prevailing before by this mixture flowing at 50 cm min .
�
this passivation.
Consumptionofn-hexanestartedatabout100 Candthat
�
of i- and n-pentane at 140 C. The concentrations of propane
and butanes first increased up to 140–160 C, as they were
formed from pentane and hexane, before sharply decreas-
ing at higher temperatures. Methane and ethane increased
4
.6. Evidence for the hydrogenolysis activity of the cata-
�
�
lyst. The decrease of the C2 + production beyond 160 C,
with other operating conditions being fixed (Fig. 12), can be
explained by the interference of the homologation process
with the hydrogenolysis of the alkane surface precursors.
Hydrogenolysis activity of Ru was tested versus tempera-
ture by using a mixture purchased from Air Liquide and
�
�
from 100 to 120 C and ethane began to decrease from 210 C
only. Interferencewithhydrogenolysiswasallthemorecon-
firmed as the alkanes submitted to hydrogenolysis in our
experiments are already adsorbed on ruthenium when H2
is admitted.
DISCUSSION
1
. Methane Chemisorption
As in the case of Pt (1), evolution of H2 parallels the
chemisorption of CH4 on Ru but, in contrast with this pre-
vious case, no formation of C2H6 is observed. However, it
must be noted that no H2 desorption from Pt could be mea-
�
sured under our conditions below 150 C, whereas Fig. 4
shows that H2 desorption accompanying CH4 adsorption
on Ru could easily be monitored from as low a temper-
�
ature as 80 C. Therefore CH4 chemisorption and H2 re-
moval occur faster on Ru than on Pt, so that H-deficient
adspecies can build up on the surface at lower tempera-
tures. Such a situation must favor C–C bonding rather than
FIG. 14. Yield of conversion of the adsorbed methane as a function
of the temperature. Data are deduced from Figs. 5 and 12.
ethane desorption. Except for the desorption of ethane, the

OXYGEN-FREE HOMOLOGATION OF METHANE ON RUTHENIUM
289
QH S means the molar amount of superficial hydrogen,
assumed to be essentially bonded to C,
QH des and QC H des are, respectively, the total amounts
2
2
6
of H2 and of C2H6 desorbed during the first step,
QCH TPD represents the total amount of CH4 removed
4
during the TPD.
The results corresponding to exposures either of equal
durations but carried out at different temperatures or car-
ried out at a constant temperature but of increasing dura-
tions are presented in Tables 2 and 3. The same comments
as for Pt can also be made here. x2 (more reliable than x1)
shows that most of the exposures to CH4 gave rise to highly
(
although not completely) dehydrogenated adspecies.
2
. TPD and TPSR Spectra
Tendencies very similar to those exhibited by the TPDs
carried out on Pt (1) are also present in the TPD spectra
of Fig. 7 but at much lower temperatures of exposure to
FIG. 15. Variation with the duration of the exposure to methane of
the molar amounts of the total homologated methane and of the various
�
C2 + products resulting from exposures of the Ru catalyst to CH4 at 160 C
�
and subsequent hydrogenations at 160 C. Other conditions as for Fig. 12.
same elementary processes as on Pt can be imagined to take
place on Ru, although to likely different extents for a given
set of operating conditions.
As in the case of Pt (1), approximate averages of the
H/C ratio of the CHx adspecies can be evaluated in two
ways giving rise to x1 and x2 (using the same symbols and
considerationsasintheprecedingcase). Herewerecallonly
that x1 and x2 are limiting values of x and are such as
QH S
QC H des � QH des
2
6
2
x � x1 =
= 4 + 2
QCH ads
QCH ads
4
4
and
QCH TPD
4
x � x2 = 4
,
QCH ads
4
where
FIG. 16. Variation with the flow rate of CH4 of the molar amounts of
the total homologated methane and of the various C2 + products result-
QCH ads is the molar quantity of C1 units adsorbed at the
end of the first step,
�
4
ing from exposures of the Ru catalyst to CH4 at 160 C and subsequent
�
hydrogenations at 160 C. Other conditions as for Fig. 12.

2
90
BELGUED ET AL.
TABLE 2
one on Ru. This agrees with our interpretation of the CH4
releasedathightemperatureasbeingduetothedecomposi-
Evaluation of x1 and x2 (See Text) for Exposures
tion of C2 + chemisorbed precursors. Similar comments can
at Different Temperatures
�
be done regarding Fig. 9. Even a 10-s exposure at 160 C
QH2 des
(�mol)
QCH4 TPD
(�mol)
QCH4 ads
(�mol)
allows a sizeable amount of CH4 to be chemisorbed but
mainly light and relatively little dehydrogenated adspecies
are formed. Longer exposures result in a strongly increased
evolution of CH4 in the high temperature part of the TPD,
whereas the reverse tendency is shown in the low temper-
ature one.
�
T ( C)
x1
x2
8
0
3.00
6.40
15.5
25.6
30.0
0.65
1.25
1.80
0.35
0.25
1.90
4.10
9.10
14.9
15.9
0.84
0.87
0.59
0.56
0.22
1.36
1.22
0.79
0.09
0.06
1
1
1
2
20
60
80
00
SeverelyH-deficientadspeciesareexpectedtobepresent
on the Ru surface at the end of each TPD. Their subsequent
subjection to H2, after recooling of the sample, causes im-
mediate formation of CH4 and that is not surprising in view
of what is already known concerning the reactivity of fresh
carbon deposits with H2 (3). This low temperature produc-
tion of CH4 after TPD is not observed in the case of Pt (1),
most probably because no pure carbon deposits are formed
on this metal even during TPD. Unlike on Pt also, CH4 is
the only product of the following TPSR (except for traces of
ethane) and up to four clearly distinct peaks of CH4 can be
exhibited by the spectra. Only two peaks are present when
the preliminary exposure to CH4 is effected at a tempera-
ture too low to allow a sufficient amount of chemisorbed
species to build up on the surface. A striking difference be-
tween the TPD and TPSR spectra concerns the changes ex-
hibited by them as a function of the conditions of exposure
to CH4. Whereas the TPD spectra undergo modifications in
the position and extent of their different parts, the positions
of the different peaks of which the TPSR spectra are made
remain quite constant, whereas their heights are liable to
vary. Such was also the case on Pt not only for the methane
peak but also for the ethane and propane peaks which are
absent here. That behavior strongly suggests that only a
small number of well defined carbonaceous adspecies are
present on the surface at the end of the TPD and that the
quantity of each of them depends upon the conditions of
the preliminary exposure to CH4. The first TPSR peak can
reasonably be interpreted as due to the removal of the C1
adspecies remaining after hydrogenation at rt. We suggest
that the other peaks originate in the hydrogenation and/or
in the hydrogenolysis of the aforementioned carbonaceous
adspecies.
Note. Conditions: 5-min exposures; 375 cm3 min� 1
atm.
;
1
CH4 than in the case of Pt. These spectra mainly consist
of two contributions, the first one interpretable as resulting
from the associative desorption of C1 and H adspecies (low
temperature peak) and the second one resulting from the
progressive decomposition of heavier adspecies leading to
departure of some CH4 into the gaseous phase. Undistin-
guishable from that first contribution when the exposure is
�
effected at a low temperature (80 C), the second contribu-
�
tion strengthens greatly for an exposure at 120 C and even
�
more for an exposure at 160 C. Above these temperatures
�
�
(
for TCH = 180 C and TCH = 200 C), only very small quan-
4
4
tities of CH4 can be removed throughout the entire range of
the following TPD. These results are striking in light of the
fact that the total amount of CH4 adsorbed on the surface
continued to increase with adsorption temperature (cf. Sec-
tion 3.3 and Fig. 5). However the adsorbed CHx species are
more depleted in H2 as adsorption temperature increases
(
see in Fig. 5 the curve related to H2 evolved during the
exposure), so that there is insufficient hydrogen to allow
CH4 to be formed from its possibly different precursors. In
case of Pt, adsorption had to be carried out at more than
00 C to lead to a similar observation, although to a lesser
extent (1). The lesser dehydrogenation undergone by the
adspecies on Pt, as compared with that on Ru, can also be
viewed through the larger amount of CH4 involved in the
second TPD peak on Pt, as compared to the corresponding
�
3
TABLE 3
Remarkably, no C� was formed in our experiments (no
�
Evaluation of x1 and x2 (See Text) for Exposures
reincrease of CH formation beyond 400 C), despite the
4
of Different Durations
heating to which the adsorbed species were subjected dur-
ing TPD. That is in sharp contrast to what was observed
when Ru and Co were made to react with CH4 at much
higher temperatures (4).
QH2 des
QCH4 TPD
QCH4 ads
Duration
0 s
min
min
0 min
(�mol)
(�mol)
(�mol)
x1
x2
1
1.80
7.10
15.5
18.9
1.50
1.75
1.80
1.63
4.10
6.60
9.10
11.5
3.12
2.15
0.59
0.71
1.46
1.06
0.79
0.56
1
5
2
3
. Thermodynamic and Mechanistic Considerations
What has already been stated concerning the thermo-
�
dynamic analysis of the whole process (2) obviously also
holds in the present case. Chemisorption of CH4 with par-
Note. Conditions: Exposures carried out at 160 C;
75 cm min ; 1 atm.
3
� 1
3

OXYGEN-FREE HOMOLOGATION OF METHANE ON RUTHENIUM
291
allel release of H2 at a low partial pressure, C–C bonding of exposure. Hydrogenolysis is the most reasonable expla-
between neighboring H-deficient C1 adspecies, removal of nation of that and we have put the stress on the occurrence
the C2 + surface precursors by H2 at a pressure higher than of hydrogenolysis in Section 4.6.
that at which it was released during the first step, and the
An interesting feature of the product distribution, which
necessary corresponding expenditure of energy remain the
might also have been commented in the case of Pt, is the
key notions explaining why CH4 can be homologated on
neat increased selectivity to the heavier alkanes, resulting
adequate metal surfaces in reductive conditions by the use
from increases of either the duration of exposure or the flow
of an isothermal two-step reaction sequence.
rate of CH4, in such conditions that the total amount of ad-
Concerning the mechanistic considerations, little new has
sorbed methane does not much increase. For instance one
to be added here to our reports on Pt (1, 2) except for the
can see that when the duration of exposure to CH4 increases
fact, already mentioned, that adsorption of CH4—and the
resulting C–C bonding—can take place at much lower tem-
�
from 5 to 20 min at 160 C, the quantity of CH4 chemisorbed
increases by no more than 25% (Fig. 11), whereas that of
the C6 produced is approximately multiplied by a factor of
(Fig. 15). Two effects of exposure lengthening can explain
peratures on Ru (2) than on Pt. As Ru is a very efficient
catalyst of hydrocarbon hydrogenolysis (5), it is not sur-
prising that the maximum of homologation ability versus
temperature be observed sooner than on Pt. It is this ability
of the metal to catalyze the C–C bond scissioning by H2
3
the results. The first one is obvious since more adspecies
build up on the surface, which leads to an increased prob-
ability of C–C bonding. The second effect lies in the fact
that H2 is only progressively removed from the adspecies
by the flowing CH4, so that the prolonging of the exposure
does not only cause increased adsorption but also it allows
the initially adsorbed species to lose more H2 and so in-
creases their ability to give rise to C–C bonding. Moreover,
which also allows one to understand the variation of the
product distribution versus temperature (see Fig. 13). For
example, we can see that the contribution of C2 decreases
�
when TCH increases from 80 to 120 C, due to increasing
4
coverages making C–C bonds more easily formed, whereas
its contribution reincreases for TCH > 120 C. The reverse is
�
4
additional sites become available for increased adsorption
of CH4.
of course true for C5 which turns out to be the most favored
�
homologated product at 120 C. In contrast with what was
observed on Pt, no cyclo-pentane or cyclo-hexane is formed
on Ru, which likely results from a worse fitting of their pre-
cursors to the surface geometry and to the interatomic dis-
tances of Ru, as compared to Pt. Worse fitting may result
in lesser amount of concerned precursors and/or in their
easier hydrogenolysis during the second step.
CONCLUSION
The main results of this contribution constitute mere ex-
tensions to the case of Ru of those obtained with Pt. Possible
CH4 adsorption at lower temperatures, no release of C2H6
during adsorption of CH4, lower H/C ratios of the result-
ing adspecies, and stronger contribution of hydrogenolysis
during the hydrogenation step are the main differences dis-
played by Ru. The reason that the thermodynamic barrier
of CH4 homologation can be overcome still obviously lies
in the amount of energy which has to be supplied to the
H2 released at a low partial pressure during the first step in
order to compress it and make it usable in the second step.
The results shown in Figs. 12 and 14 lead to the following
remarks:
(i) Although the methane produced during hydrogena-
tion escapes direct measurement since it cannot be distin-
guished from the CH4 already present in the reactor at the
onset of the second step, we can state that a great part of
the CH4 chemisorbed in the first step reverts to CH4 during
the second step or remains on the surface, involved in CnHy
adspecies which react with H2 only at higher temperatures
ACKNOWLEDGMENT
(
see the H2-TPSR spectra).
Gaz de France is gratefully acknowledged for having supported this
work.
�
(ii) The lowering of the heavier alkanes at T < 120 C is
not solely due to the lowering of the surface coverage since
the conversion of the chemisorbed species is also decreas-
ing. Increased average distances between adspecies and,
still more, insufficient dehydrogenation have also to be in-
voked to explain this feature.
REFERENCES
1
. Belgued, M., Amariglio, A., Par e´ ja, P., and Amariglio, H., J. Catal. 159,
441 (1996).
. Belgued, M., Amariglio, A., Par e´ ja, P., and Amariglio, H., J. Catal. 159,
2
4
49 (1996).
(
iii) The lowering of the total C2 + production above
3
4
. Rabo, J. A., Risch, A. P., Poutsma, M. M., J. Catal. 53, 295 (1978).
. Koerts, T., Deelen, M. J. A. G., and van Santen, R. A., J. Catal. 138,
�
1
60 C as well as that of the selectivity to the heavier alka-
nes above 140 C have another origin since the quantity of
chemisorbed CH4 goes on increasing with the temperature
�
101 (1992).
5. Sinfelt, J. H., Adv. Catal. 23, 91 (1973).