lembert on 1984
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Lembert on 1984TRANSCRIPT
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Applied Catcalysis, 13 (1984) 181-192 Elsevier Science Publishers B.V., Amsterdam -Printed in The Netherlands
181
PHENANTHRENE HYDROCONVERSION AS A POTENTIAL TEST REACTION FOR THE
HYDROGENATING AND CRACKING PROPERTIES OF COAL HYDROLIQUEFACTION CATALYSTS
Jean-Louis LEMBERTON and Michel GUISNET
Laboratoire Associe au CNRS 350, UER Sciences, 40 Avenue du Recteur Pineau,
86022 Poitiers Cedex, France
(Received 18 June 1984, accepted 21 August 1984)
ABSTRACT
The hydroconversion of phenanthrene was performed over nickel and molybdenum
sulfides supported on alumina under the conditions of the liquefaction of coal.
The reaction proceeds through a multistep mechanism of hydrogenation, isomeri-
zation and cracking reactions. The initial products of the reaction are dihydro
and tetrahydrophenanthrenes, and the cracking reactions are limited to the
opening of their saturated rings : the former product yields Z-ethyl biphenyl
and the latter mainly n-butyl naphtalene. The cracking of the saturated central
ring of dihydrophenanthrene probably occurs through an acid mechanism ; the
saturated terminal ring of tetrahydrophenanthrene first isomerizes by a bifunc-
tional mechanism into a methyl cyclopentanic ring which in turn cracks into
n-butyl naphtalene by an acid mechanism.
INTRODUCTION
Many reactions, catalytic or not, occur during coal hydroliquefaction ;
the most significant seems to be the hydrogen transfer from solvent to coal [l-4].
The main role of the catalysts would be the replenishment of the hydrogen donor
species in the solvent [51 ; consequently, the catalysts possess necessarily a
hydrogenating function. However, other reactions can be catalyzed during coal
liquefaction, namely the hydrogen transfer from solvent to coal, the alkyl transfer
from coal to solvent, the cleavage of C-N, C-S, C-O and C-C bonds in coal.
The purpose of the present work was to find a model reaction which would allow
to compare the activities of different catalysts in the hydrogenation and the
cleavage of aromatic carbon bonds, very numerous in coal. We opted for the hydro-
conversion of phenanthrene, since this molecule, in the aromatic or in a partially
0166-9834/84/$03.00 0 1984 Elsevier Science Publishers B.V.
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182
hydrogenated form, is typical of coal : effectively, a 9,10-dihydrophcnanthrene
type of linkage between the aromatic units of coal has been proved [6]. Moreover,
the hydrocracking of polynuclear aromatic hydrocarbons is also very significant in
the processing of coal oils, since there are large amounts of aromatic compounds in
the liquefaction products [7].
We report herein the data on the mechanisms of phenanthrene hydroconversion over
nickel and molybdenum sulfides supported on alumina ; similar classical hydrodesul-
furization catalysts have been already used for coal hydroliquefaction (Synthoil
and H-Coal processes). The reaction was performed under similar conditions to those
of coal hydroliquefaction (temperature, hydrogen pressure, sulfur in the reactant).
EXPERIMENTAL
Catalyst
The NiMo/A1203 catalyst was Ketjen I53-I,5 E,
and a 3.2 weight % of NiO ; its specific surface
containing a 14.2 weight % of Moo3
area was 210 m2 g-I. The sulfida-
tion was carried out in a flow reactor at 320C under a 3.5 MPa hydrogen pressure,
using a 2 weight % solution of dimethyl disulfide in gasoil. The catalyst obtained,
which will be called NiMoS, contained 5.6 weight % of S. It was then crushed and
sieved to obtain particles smaller than 60 pm.
Apparatus and procedure
Phenanthrene hydrocracking was performed in a 300 ml stainless steel autoclave
(Sotelem, France) comprising a fixed head, a removable lower unit and a magnetical
stirrer. In a typical run, 100 g of phenanthrene (Aldrich, 98+ % purity) were
heated in the lower unit to melting point (1OOOC). The catalyst and the sulfiding
agent (CS2, weight of CS2 = weight of catalyst) were quickly added and the auto-
clave sealed. Then it was flushed twice with H2, pressure - tested with H2 at the
intended reaction pressure and vented to 1 MPa. Heating without stirring was then
begun. At the desired operating temperature (430C), generally reached after 45
minutes, the H2 pressure was increased to 10 MPa and stirring begun. The H2 pressure
was maintained at 10 MPa during the experiment. To end the run, the stirring was
stopped, the heater removed and the unit allowed to cool down to ambient temperature
(a 100C decrease in temperature generally took 3 minutes). Samples of the gas phase
were taken for GLC analysis (50 meter squalan capillary column, 2O"C), and the
autoclave vented and opened. Using acetone as a solvent, the contents were removed
with care and weighed. The difference between the weight of the starting material
(100 g) and that of the products recovered (corrected for catalyst and acetone)
represented the weight of phenanthrene converted into gases. Finally the liquid
products were analyzed by GLC using a 25 meter CP Sil capillary column (Chrompack),
with a temperature programming from 40 to 160C (3C mn-I).
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Identification of the products
Figure 1 shows the chromatogram of the liquids obtained at a 59.5 % phenanthrene
conversion.
FIGURE 1 Gas Chromatogram of products from phenanthrene hydroconversion over
NiMoS at 430C.
l- and Z-methyl naphtalenes, l- and Z-ethyl naphtalenes, biphenyl and 9,10-
dihydrophenanthrene (P2H, 17 in figure 1) were identified using commercial stan-
dards. The other products were analyzed by GLC-MS. Actually we do not know
whether methyl propyl naphtalene is a n-propyl or an isopropyl, nor whether it
is a l-methyl 2-propyl or a l-propyl 2-methyl. Likewise, n-butyl naphtalene
could be a 1-butyl or a 2-butyl. Finally, several products present in the liquids
were not totally identified. Two of them (15 and 16), of the same molecular
weight as tetrahydrophenanthrene (P4H, 19), with a P-15 ion as base peak in the
MS analysis, are probably methyl cyclopentanic isomers (iP4H). Another product
(14), of the same molecular weight as octahydrophenanthrene (PSH, 18), with a
P-15 ion as base peak in the MS analysis, was identified as a methyl cyclopenta-
nit isomer (iP8H); however we do not know whether one or two saturated rings of
octahydrophenanthrene were isomerised. Other molecules, like 1,2-diethyl naphta-
lene, were not found in the liquids; since we have a l- or 2-ethyl naphtalene
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184
peak, we can presume that diethyl naphtalene was formed, but probably not sepa-
rated from n-butyl naphtalene in the GLC analyses.
RESULTS
Table 1 gives the results of hydrotreating phenanthrene at 430C over presulfi-
ded NiMo/A1203 (NiMoS), or with no catalyst (run 0).
TABLE 1
Phenanthrene hydroconversion over NiMoS (43O"C, 10 MPa H2).
Run no 0 1 2 3 4 5 6 7
Catalyst weight (g) 0 0.2 0.2 0.2 0.5 0.5 1.5 2.5
Reaction time (h) 5 2 3 5 3 5 5 5
Conversion (%) 17.5 43.2 50.4 59.5 66.9 67.9 69.1 69.9
Hydrogenation products (wt.%) 8.5 34.7 36.0 38.3 38.8 38.1 33.2 27.3
PZH 6.7 11.3 11.0 9.9 7.6 7.5 6.1 5.8
P4H 1.8 17.5 18.1 20.1 21.1 20.5 18.6 16.1
P8H 0 5.9 6.9 8.3 10.1 10.1 8.5 5.4
Isomerization products (wt.%) o 2.3 2.8 5.3 7.8 8.0 8.4 8.7
iP4H 1.5 1.8 3.0 4.5 4.6 4.8 5.0
iP8H 0.8 1.0 2.3 3.3 3.4 3.6 3.7
Cracking products
Cl4 (wt.%;
0 3.7 8.6 12.4 17.8 19.3 25.5 30.9
3.7 7.3 9.3 13.6 14.4 18.8 21.5
'13-'12 1.1 2.3 3.1 3.3 3.6 3.8
c11 0.2 0.6 0.9 1.3 2.0 3.2
c1O 0.2 0.2 0.2 0.7 1.6
c9-c5 0.1 0.4 0.8
Gases (wt.%) 9.0 2.5 3.0 3.5 2.5 2.5 2.0 3.0
c4 0.7 0.1 0.3 0.4 0.2 0.2 0.1 0.3
c3 2.4 0.7 1.1 1.3 1.2 0.9 0.9 1.2
c2-c1 5.9 1.7 1.6 1.8 1.1 1.4 1.0 1.5
Phenanthrene conversion is much more significant with NiMoS than with no cata-
lyst. Without a catalyst, the only products of the reaction are hydrogenated ones,
i.e. dihydro, tetrahydro and octahydrophenanthrene (P2H, P4H and P8H, respectively).
More significant amounts of gases are obtained without a catalyst than with NiMoS.
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185
However the distribution in the gases formed - mainly methane, ethane and propane -
is roughly the same with or without a catalyst ; the amount of n-butane is small,
and no isobutane is detected. This distribution is typical of a hydrogenolysis
reaction. Moreover the formation of gases with NiMoS does not depend on the cata-
lyst weight nor on the reaction time. Thus it could be supposed that gases are
formed on the walls of the reactor through hydrogenolysis of hydrogenated species.
The fact that a smaller amount of gases is obtained with NiMoS could indicate that
there is competition for the adsorption of the hydrogenated species between the
reactor walls and the catalyst.
With NiMoS, the initial products of the reaction are mainly P2H, P4H and P8H
(run 1). As phenanthrene conversion increases (runs 1-4) an increase in the amount
of hydrogenation, isomerization and cracking products is observed ; then (runs 5-7)
the amount of hydrogenation products decreases, that of isomerization products
stabilizes and that of cracking products continues to increase. In every case, no
perhydrophenanthrene was detected, neither was any product heavier than phenan-
threne.
NO CATALYST
FIGURE 2 Phenanthrene hydroconversion over NiMoS at 430C : product distribution.
CRACKING * PRODUCTS
I
o/ FMTHRFNF CONVERTFO
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186
In figure 2 are plotted versus the percentage of phenanthrene converted - minus
the gases - the amounts of :
a) each hydrogenated species (PZH, P4H, P8H) ;
b) each isomerized species (iP4H, iP8H) ;
c) the cracking products,
PZH reaches its maximum before P4H and P8H, which suggests that P2H hydrogenates
into P4H and P8H. When the conversion reaches a practically stable level (runs 4-7,
table 1> the decrease in P4K and PBHcoin~iides with a stabi\iration in the isomeriza-
tion product yield and with an increase in the cracking product yield.
Table 1 also indicates that not only the amount but also the extent of the cracking
increase when the conversion of phenanthrene increases ; nevertheless, C14 molecules
are always the main products. In figure 3 the amounts of these products are plotted
versus m.t., where m is the catalyst weight (grams) and t the reaction time (hours).
10
0 1 5 10 mt (gh)
FIGURE 3 Phenanthrene hydroconversion over NiMoS at 430C : distribution of
cracking products.
It can be seen that NiMoS catalyses the opening of a saturated terminal ring,
yielding n-butyl naphtalene and methyl propyl naphtalene. Diethyl naphtalene,
although not observed, is probably formed since small quantities of ethyl naphta-
lene are obtained (figure 1). NiMoS also catalyses the opening of the saturated
central ring by cleavage of the 8a-9 C-C bond, yielding Z-ethyl biphenyl.
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187
When m.t.>2.5 g.h, the amounts of n-butyl naphtalene and of methyl propyl naphta-
lene remain constant, whereas that of 2-ethyl biphenyl continues to increase.
Further cracking will yield products with less than 14 carbon atoms. However only
small amounts of secondary cracking products are obtained : 1 % l- and 2-methyl
naphtalene and 0.1 % biphenyl are formed when m.t. = 12.5 g.h (run 7).
DISCUSSION
Hydrogenation reaction
The results reported herein clearly show that hydrogenation is the first step of
phenanthrene hydroconversion. As is generally supposed [8,91, polynuclear aromatic
hydrogenation occurs ring-by-ring in a series fashion : dihydrophenanthrene (P2H)
appears first, followed by tetrahydro and octahydrophenanthrenes (P4H and P8H).
No perhydrophenanthrene is detected, which is consistent with the observation that
hydrogenation of the last ring of a polycondensed-ring aromatic, compared with the
initial hydrogenation steps, proceeds with considerable difficulty [lo].
We compare in table 2 the experimental phenanthrene-hydrophenanthrenes ratios
to those of theoritical equilibrium at 430C and 1OMPa calculated form previously
reported equations [11,121.
TABLE 2
Phenanthrene-hydrophenanthrenes ratios at 430C and 10 MPa.
run no 1 2 3 4 5 6 7 equilibrium
P2H/Phen. 0.21 0.22 0.24 0.23 0.23 0.20 0.19 0.40
P4H/Phen. 0.31 0.36 0.50 0.64 0.64 0.60 0.53 1.31
P8H/Phen. 0.10 0.14 0.20 0.30 0.31 0.27 0.18 1.68
P4H/P2H 1.55 1.64 2.08 2.78 2.78 3.00 2.79 3.27
P8H/P4H 0.32 0.39 0.40 0.47 0.48 0.45 0.34 1.28
PSH/P2H 0.50 0.63 0.83 1.30 1.35 1.35 0.95 4.20
The data in table 2 indicate that none of the equilibria appears to be established.
Only the P4H/P2H ratio approaches its equilibrium value at the highest conversion
levels (runs 4-7). These results confirm that the reaction P2H+P4H is rapid, while
the hydrogenation of P4H to P8H is much slower. Tetrahydrophenanthrene has been
shown to be the main primary product during the hydrogenation of phenanthrene II31.
Cracking reaction
Since the catalyst exhibits a hydrogenating function (Ni and MO sulfides) as
well as an acid one (sulfides and alumina support), three mechanisms can be pro-
posed to explain the cracking of the hydrogenation products on NiMoS.
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188
- Bifunctional mechanism : it involves the hydrogenating function for both hydro-
genation and dehydrogenation, and the acid function for the formation and the
scission of carbocations. The first step is the dehydrogenation of a saturated
carbon-carbon bond (SI). The olefin formed (01) leads by protonation to a carboca-
tion (C,') which gives an olefin (OS) and another carbocation (C,+) by B-scission ;
the (C,+) carbocation is then deprotonated into an olefin (02). The olefins (OB) and
(OS) are finally hydrogenated into saturated compounds (S2) and (SS). This mechanism
can also explain the isomerization of the saturated compound (SI), Effectively, the
carbocation (CI+) can rearrange into a (Cl,+), either through an alkyl transfer, or
through a protonated cyclopropane [14]. By loss of H+, the carbocation (C'I+) leads
to an olefin (O,) which in turn hydrogenates into a saturated compound (S'I). It
must be noted that the carbocation (C'I '), like the (C,+), can crack into an olefin
and another carbocation.
sj -H20, +H+_C{ -H+
I7 q - o* +y s* 03 A syj
-H+ CT- 0; +H2 ) si
- acid mechanism : the reaction can occur through two different paths, depending on
the nature of the C-C bond to be broken.
i) saturated C-C bonds : the carbocation (C,+) is formed by transfer of a hydride
ion H- from a saturated C atom (SI) to an adsorbed carbocation. This reaction is
very slow compared with the formation of the (C,+) carbocation via an olefin
(bifunctional mechanism). Thus, the participation of this mechanism in the cracking
reaction is probably not very significant when the catalyst exhibits hydrogenation
properties, as is the case with NiMoS.
Sl -H- +H- - q ---y Cf __3 s2
ii) aromatic C- saturated C bonds : we describe below the cleavage of such bonds
during the dealkylation of an alkyl benzene. This reaction involves a benzenium ion
intermediate.
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189
- hydrogenolysis of C-C bonds (scission of saturated C-C bonds or dealkylation of
aromatics) : this type of reaction, which can occur on badly sulfided Ni and MO,
proceeds through highly dehydrogenated species [15,16].
The main cracking reactions observed are the following :
- opening of the saturated central ring of phenanthrene, yielding 2-ethyl biphenyl ;
- opening of a saturated terminal ring, yielding butyl naphtalene, methyl propyl
naphtalene and probably diethyl naphtalene.
Both the bifunctional and the acid mechanisms involve carbonium ions interme-
diates : consequently the more stable the carbonium ion intermediate, the faster the
rates of these reactions. The discussion will then be limited to those reactions
involving stable carbocations (tertiary, secondary or conjugated benzylic primary
carbocations) since the reactions involving non conjugated primary carbocations are
considered to be infinitely slow.
Cracking of the central ring. This reaction cannot occur through a bifunctional ___
mechanism, but it can by acid cracking or even hydrogenolysis. Acid cracking involves
the formation of an unstable primary carbonium ion :
However this ion can isomerize rapidly by hydride shift to yield a very stable
conjugated carbocation 0 0 . 9p
Such a mechanism could thus explain the formation
of 2-ethyl biphenyl. +
This reaction could also be accounted for by the hydrogenolysis of the 8a-9
(or lo-10a) C-C bond in P2H. However, hydrogenolysis should also lead to 2,2'-
dimethyl biphenyl by cleavage of the 9-10 C-C bond ; actually this reaction is not
observed. Consequently, if 2-ethyl biphenyl resulted from hydrogenolysis, then the
absence of 2,2'-dimethyl biphenyl would imply that the cleavage of a saturated C-C
bond is very slow compared with that of a saturated C-aromatic C bond, which seems
highly improbable [17,18].
Cracking of the terminal ring. P4H cracking yields mainly butyl naphtalene,
methyl propyl (or isopropyl) naphtalenes and probably diethyl naphtalene. The
formation of butyl naphtalene could be accounted for by an acid cracking mechanism
such as :
-
___- _____ -------- _ _ _ _ _ - _ _ - _________----_-------- -________-____ 8 0 C L 1-c
&p :;>c& a@ P &@ 4@&
FIGURE 4 Mechanism of tetrahydrophenanthrene hydrocracking.
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191
However this reaction involves a primary carbonium ion and consequently can be
neglected. Thus, butyl naphtalene is most likely formed by cracking of the methyl
cyclopentanic isomers of P4H present in the reaction products. It has been shown
that other hydroaromatics, like tetralin of tetrahydroanthracene, isomerize before
cracking [8,191. P4H isomerization occurs through protonated cyclopropane interme-
diates whose scission gives stable secondary carbocations (figure 4A). By loss of H+
(figure 4B), two of these carbocations lead to olefins which in turn hydrogenate into
methyl cyclopentanic isomers of P4H ; these latter lead by acid cracking to secondary
carbocations having a butyl naphtalene skeleton (figure 4B). The cracking of the P4H
isomers is probably more rapid than their formation, since butyl naphtalene is
obtained in bigger quantities than iP4H.
Methyl propyl naphtalenes could be formed from P4H through a bifunctional process.
This mechanism, shown in figure 4C, involves the 8-scission (ES) of P4H secondary
carbocations into primary benzylic carbocations stabilized by resonance. Methyl
propyl naphtalenes are formed from these cations by a hydride ion transfer from
another saturated molecule and by hydrogenation of the olefinic C-C bond.
Furthermore, figure 4A indicates that methyl propyl (or isopropyl) naphtalenes,
as well as 1,2-diethyl naphtalene, can be formed through the same procedure as above
from the methyl cyclopentanic isomers of P4H. To explain the slow formation rate of
these products compared with that of butyl naphtalene, it must be assumed that a-
scission and most likely hydride transfer occur very slowly.
Hydrogenolysis reactions can also explain the formation of butyl, methyl propyl
and diethyl naphtalenes. However the formation of the two latter requires the scis-
sion of a saturated C-C bond. This scission was not observed in P2H cracking, which
suggests that methyl propyl and diethyl naphtalenes, and consequently butyl naphta-
lene too, result mainly from the reactions reported in figure 4.
CONCLUSION
On NiMoS, phenanthrene hydroconversion proceeds through a multi-step mechanism
of hydrogenation, isomerization and cracking reactions ; the initial product of the
reaction is dihydrophenanthrene which in turn hydrogenates into tetrahydro and
octahydrophenanthrenes. On this weak acid catalyst, the cracking reactions are
practically limited to the opening of the saturated ring of dihydro and tetrahydro-
phenanthrenes ; the fastest reaction is the scission of the terminal saturated ring
which leads, in two steps, to butyl naphtalene : bifunctional isomerization of
tetrahydrophenanthrene into methyl cyclopentanic isomers, followed by the opening
of the saturated isomer ring by an acid mechanism.
Phenanthrene hydroconversion seems to be a very suitable reaction for comparing
the hydrogenation and cracking activities of catalysts under the conditions of
coal liquefaction. We are currently applying this reaction to other catalysts
exhibiting acid and/or hydrogenating properties.
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192
ACKNOWLEDGEMENTS
This work was supported by the GECH (Groupe
par Hydrogenation) and the GRECO Charbon CNRS.
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