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.

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).

183

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

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.

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

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.

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.

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.

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.

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.

192

ACKNOWLEDGEMENTS

This work was supported by the GECH (Groupe

par Hydrogenation) and the GRECO Charbon CNRS.

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