Applied Catalysis, 52 (1989) 41-56 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

41

Influence of Sulphur Level on Hydrodeoxygenation

ANTTI VUORI”, ANNE HELENIUS and JOHAN B-SON BREDENBERG*

Department of Chemical Engineering, Helsinki University of Technology, Kemistintie I, SF-02150 Espoo (Finland)

(Received 25 November 1988, revised manuscript received 21 February 1989)

ABSTRACT

The effect of catalyst sulphidation on hydrodeoxygenation was studied under pseudo-steady- state conditions in a continuous trickle-bed reactor at 523 K. Sulphur was added to the feed in the form of carbon disulphide. The catalyst used was a commercial COO-MoO,/y-Al,03 hydrotreating catalyst which was presulphided at 523 or 623 K. Experiments were made both with a high and a low oxygen content of the feed by using o-methoxyphenol (guaiacol) as a model compound at different concentrations, and also with addition of other oxygen-containing compounds. The con- version of guaiacol increased with increasing sulphur content of the feed. The selectivity of the hydrodeoxygenation had its highest value, however, when a presulphided catalyst was used with- out sulphur added to the feed. This indicates that oxygen and sulphur compete for the same hydrogenolytically active sites. The effect on the selectivity of the formation of specific products varied with the sulphur content. This is at least partly due to steric effects. The proportion of both the 0- and ring alkylation decreased with increasing sulphur content. The selectivity of phenol formation increased significantly even at the lowest level of sulphur addition, whereas the selec- tivity of veratrole formation decreased only gradually. In the competing hydrodeoxygenation sys- tems, diphenyl ether did not have any effect, whereas isopropanol decreased the conversion of quaiacol. The sulphur content of the catalyst, measured after reaction, did not increase with in- creasing sulphur content of the feed but remained at a relatively low level. This was about 2 wt.- % for high and 3 wt.-% for low oxygen-content feeds. Presulphidation tests showed that a signif- icantly higher sulphur-content of the catalyst could easily be achieved. This gives additional proof of competition between oxygen and sulphur atoms for the same active sites on the catalyst.

INTRODUCTION

Oil and natural gas, the most important raw material for the production of fuels and organic chemicals, have shown sudden and unexpected price changes since the 1973 oil crisis. This, together with an anticipated uncertainty of fu- ture supply, has reawakened interest in raw materials such as coal and biomass.

The proportion of sulphur-, oxygen- and nitrogen-containing hetero com- pounds in the primary liquefaction products from coal and biomass is very

“Present address: Kemira Oy, Espoo Research Center, P.O. Box 44, SF-02271 Espoo, Finland.

0166-9834/89/$03.50 0 1989 Elsevier Science Publishers B.V.

42

different from that in crude oil. The most abundant heteroatom in the lique- faction products from both coal and, especially, biomass is oxygen, and the liquefaction products from biomass usually have only a very limited content of sulphur and nitrogen.

Conventional refinery processes involve substantial hydrodesulphurization (HDS). The low contents of oxygen and nitrogen are assumed to cause no problems. The catalyst used is typically COO-MOO, on alumina and is used in a sulphided form which gives the highest activity [ 1,2]. The sulphidation of the catalyst, and the maintenance of the sulphur content are achieved simply by the sulphur compounds in the feed.

Hydrodeoxygenation (HDO ) is a process analogous to HDS and the cata- lysts used are usually the same [ 31. This poses a potential problem, namely the maintenance of the sulphidation level of the catalyst in the HDO of pri- mary liquefaction products from biomass. A separate sulphiding compound in the feed is needed.

The HDO of primary liquefaction products from biomass, and especially the role of sulphidation of the catalyst, have been studied only to a limited extent [ 4-71. The main purpose of this work was to shed more light on the role of the catalyst sulphidation in HDO. The work was performed by using a simple mon- omeric lignin model compound, guaiacol (o-methoxyphenol) , with a conven- tional COO-MoO,/p-Al,O, catalyst.

EXPERIMENTAL

All chemicals were commercially pure or of analytical-reagent grade and were used without further purification. The catalyst used was Ketjenfine 124- 3E (HD ) COO-MoO.Jy-A1,03 ( Akzo Chemie), an extruded catalyst of nomi- nal diameter 2.5 mm containing 4.0 wt.-% Co0 and 12.0 wt.-% MOO,, and having a minimum surface area of 225 m’/g.

The reactions were performed in a trickle-bed reactor (Catatest Unit Model A; Geomecanique). The reactor (volume 140 ml) was packed with 20 ml of catalyst in the middle and the remainder was filled with crushed silicon carbide (nominal diameter 1.68 mm). Fresh catalyst was used for each run. In most of the runs the catalyst was presulphided for 3 h under 5.0 MPa hydrogen at 523 or 623 K.

In order to test the presulphidation procedure, some separate presulphida- tion experiments were performed at temperatures from 523 to 673 K for run times from 3 to 15 h.

The sulphidation feed was 1.0% (v/v) carbon disulphide in cyclohexane at a total LHSV of 1.0 h-l. During the experiments, sulphur was also added as carbon disulphide. The experimental runs are summarized in Table 1.

The analyses of the liquid products in runs l-10 were performed on a Perkin-

43

TABLE 1

Experimental data

All experiments were performed at a constant temperature of 523 K and a pressure of 50 bar (Hz).

Run Pre- LHSV of Feed composition/mol-% CS, in Reaction No. sulphidation” guaiacol/ feed/% time/h

temperature/K h-’ Guaiacol Diphenyl Iso- n-Hexane (v/v) ether propanol

1 _b 1.0 100 0.0 30 2 523 1.0 100 0.1 30 3 523 1.0 100 1.0 30 4 523 1.0 100 5.0 30 5 - 0.1 10 90 0.0 15 6 523 0.1 10 90 0.1 30 7 523 0.1 10 90 1.0 15 8 523 0.1 10 10 80 1.0 15 9 523 0.1 10 10 80 1.0 15

10 523 0.1 10 90 0.0 30

11 523 1.0 100 0.0 30 12' - 1.0 100 0.0 30 13 623 1.0 100 0.1 30 14 623 1.0 100 1.0 30 15 623 1.0 100 5.0 30 16 623 1.0 100 0.0 30

"3 h under 5.0 MPa hydrogen by 1.0% (v/v) CS2 in cyclohexane with a total LHSV of 1.0 h-l. bRun without presulphidation. ‘Repetition of run No. 1.

Elmer 900 gas chromatograph equipped with a flame ionization detector and a Hewlett-Packard 3390 integrator. The phenolic products were silylated [8] before analysis. The column was a 1.83-m glass column packed with 10% SP 2250 on lOO-200-mesh Supelcoport. The products were, in addition, chroma- tographed as such on a 1.83-m glass column packed with 0.1% SP 1000 on 80- 100-mesh Carbopack C.

Owing to the poor resolution of guaiacol and veratrole, and the unsuitability of the packed columns for the analysis of dimeric products, the liquid products in runs 11-16 were analysed as such with a Hewlett-Packard 5880 gas chro- matograph equipped with a flame detector and a 25-m SE-30 silica capillary column.

4-Heptanone was used in all runs as an internal standard. The identifica- tions were confirmed by gas chromatographic-mass spectrometric analysis on a Hewlett-Packard 5790-Jeol DX 303/DA 5000 instrument equipped with a 25-m SE-30 silica capillary column.

44

The sulphur content of the catalyst was determined as barium sulphate by the wet method [ 91. The carbon content of the catalyst was analysed as carbon dioxide by burning the sample in oxygen.

RESULTS AND DISCUSSION

The results of this present work may in a general sense be characterized as qualitative. The effects of the catalyst sulphidation have mainly been studied on the basis of guaiacol conversion and of the selectivity of the formation of specific product compounds. The characterization of the catalyst used has been limited to the determination of its sulphur and carbon contents after the experiments.

Effect of sulphidation on catalyst activity

The experiments were performed under reaction conditions that represent the region between transient and steady-state behaviour of the catalyst. The results clearly show that sulphidation increases the activity of the CO-MO cat- alyst used when measured both with the conversion of guaiacol (Table 2) and the molar ratio of compounds formed containing one and two oxygen atoms

TABLE 2

Conversion of guaiacol

Run No. Conversion”/% as function of run time (h)

2 3 4 5 10 15 20 25 30

1 44.3 43.0 45.3 52.8 36.3 37.8 35.9 34.7 29.1 2 54.4 42.5 39.0 37.7 36.4 34.3 36.4 32.3 31.6 3 67.7 45.2 45.2 44.4 43.2 44.3 42.5 40.1 40.1 4 65.3 60.8 62.5 61.7 57.8 56.0 54.9 55.3 54.3 5 75.8 50.2 41.5 74.7 65.9 55.2 6 99.7 99.2 100 100 99.7 96.9 7 96.1 100 98.0 98.9 98.7 98.0 8 100 100 99.0 98.4 98.4 97.5 9 100 100 97.8 96.4 100 98.8

10 99.1 99.1 97.5 99.0 97.3 92.2 89.2

11 34.0 31.4 30.2 30.2 25.7 24.9 21.4 12 32.6 36.8 30.5 30.6 24.0 17.5 13 7.9 29.5 27.5 27.1 23.3 23.1 24.1 20.8 14 33.6 33.7 33.0 30.4 28.5 26.2 25.6 24.5 26.2 15 41.8 40.8 37,l 35.8 34.4 31.7 31.6 31.3 26.5 16 28.1 28.6 26.3 26.5 23.4 20.2 17.6 22.4 22.5

‘Results of runs l-10 contain a systematic error due to the method of analysis.

45

TABLE 3

Formation of compounds containing one and two oxgyen atoms from guaiacol

Run Molar ratio of one to two oxygen atom-containing compounds’ as a function of run No. time (h)

2 3 4 5 10 15 20 25 30

1 0.08 0.04 0.02 0.02 2 0.87 0.50 0.32 0.24 3 0.92 0.54 0.24 0.19 4 0.67 0.28 0.17 0.17 5 1.13 0.38 0.44 0.13 6 5.96 4.92 7 3.80 3.81 11.56 8.12 8 3.48 4.63 9.20 10.85 9 1.52 2.42 4.75 6.86

10 8.49 7.24

11 2.37 0.70 0.47 0.42 12 0.06 0.03 0.02 0.01 13 1.30 0.61 0.48 0.47 14 1.65 0.69 0.47 0.47 15 2.34 0.66 0.49 0.45 16 2.66 0.74 0.63 0.58

0.05 0.02 0.04 0.08 0.04 0.18 0.17 0.22 0.15 0.11 0.17 0.14 0.20 0.13 0.13 0.13 0.12 0.12 0.16 0.12 0.06 0.04 6.29 5.20 5.81 4.99 2.54 6.33 5.05 6.00 4.09 1.52 1.34 4.95 5.10 3.00 2.64 2.25

0.36 0.34 0.32 0.30 0.01 0.00 0.00 0.38 0.36 0.38 0.30 0.41 0.37 0.30 0.26 0.29 0.37 0.37 0.35 0.39 0.32 0.47 0.51 0.54 0.63 0.42

“Results of runs l-10 contain a systematic error due to the method of analysis.

(Table 3 ) . This corresponds to most of the results published on the effects of catalyst sulphidation in HDO [4-7,10,11], although opposite results have also been reported [ 121. Further, it was found that an increase in sulphur content in the feed steadily increases the conversion of guaiacol. The sulphur content of the catalyst did not, however, show any corresponding increase.

Catalyst presulphidation without sulphur added to the feed did not have any significant effect on the conversion of guaiacol at high oxygen levels (runs 11 and 12, Table 2), whereas an increase in conversion took place at low oxygen levels (runs 5 and 10, Table 2). This effect only partially corresponds to the results of Krishnamurthy et al. [5] on the HDO of dibenzofuran with a NiO- MoO,/y-A1203 catalyst. According to them, presulphidation of the catalyst at 623 K for 2 h with a stoichiometric excess of a 10% H2S-H2 mixture has a considerable effect on its activity whereas the addition of CS2 after presulphi- dation shows only a marginal improvement. One of the reasons for these dif- ferent results is that Krishnamurthy et al. [5] used a closed batch reactor system, whereas an open flow reactor system requiring the addition of sulphur to the feed to obtain a steady concentration in the reactor was used in this work.

The molar ratio of the formation of compounds containing one and two ox-

46

ygen atoms (Table 3 ) is interesting. The highest selectivity of HDO, especially at a high oxygen content of the feed, is achieved when a presulphided catalyst is used without any sulphur added to the feed (Runs 11 and 16 in Table 3 ). The conversion of guaiacol (Table 2 ) in these runs was only at the same level as that in experiments with the oxidic form of the catalyst (runs 1 and 12). This can be interpreted to mean that the sulphur added to the feed competes with the oxygen in guaiacol for the same active HDO sites on the catalyst. Competition between different heteroatoms has been reported for both HDO and HDS [ 13-151, The results published on hydrodenitrogenation (HDN) are more contradictory [S-19]. This is probably related to the fact that HDN is generally assumed to require ring hydrogenation as a preceding reaction step whereas HDO and HDS do not.

The competing systems of oxygen, sulphur and nitrogen are clearly an area where more work is needed.

It is generally accepted that there are at least two different types of active sites on HDO catalysts, namely hydrogenative and hydrogenolytic [ 12,13,20- 221, although opposite opinions have also been published [23]. The hydrogen- olytic sites are believed to be anion vacancies on which the C-O bonds are cleaved. In addition, it has been found that presulphidation at a low tempera- ture produces a CO-MO catalyst having a different activity and structure than that of CO-MO catalysts presulphided at higher temperatures [24-261. The temperature limit between types I and II is about 700-800 K. Hence, the pre- sulphidation temperatures of 523 and 623 K in this work are both well under this limit, and the catalyst used was obviously of type I in all experiments. It is unclear, however, to what extent the structure of an active presulphided HDO catalyst corresponds to that of a similar presulphided HDS catalyst. It should also be noted in this connection that the support has its own effect on the HDO reaction [ 3, 27, 281.

Effect of sulphidation on product formation

The selectivity of formation of specific products was found to be highly de- pendent on both the presulphidation conditions and the sulphur content of the feed. In general, both 0- and ring alkylation decreased with increasing sulphur content of the feed.

Fig. 1. Bonding of guaiacol on the CO-MO catalyst through the oxygen atom of the hydroxy group.

47

Fig. 2. Bonding of guaiacol on the CO-MO catalyst through the oxygen atom of the methoxy group.

A _

B _

c -

D. -

5

OH

3- QOCH 3

H- 0

Q 00, - / H

/I IL s

MO+

OH

O"- S- ~~ /

Ma+

Fig. 3. Pathways of formation of the main products in the HDO of guaiacol on the CO-MO catalyst. (A) Pyrocatechol; (B) phenol; (C) veratrole; (D) anisole; (E) 3-methylpyrocatechol.

48

t lhl

I-= l I

Oa 5 10 15 20 25 30

t Ihl

Fig. 4.

0 I I )

0 5 10 15 20 25 30 t (hl

Fig. 4. Selectivity of the formation of phenol at 523 K. (A) Low oxygen content, presulphidation at 523 K; (B) high oxygen content, presulphidation at 523 K; (C) high oxygen content, presul- phidation at 623 K. Without presulphidation: ( l ) 0.0% (v/v) CS, in the feed. With presulphi- dation: (0) 0.0%; (A) 0.1%; V 1.0%; (0) 5.0% (v/v) C&in the feed.

The formation of specific products may be explained on the basis of the bonding of guaiacol on the sulphided CO-MO catalyst. In principle, guaiacol may be bound on the sulphided catalyst through the oxygen atom of either the hydroxy (Fig. 1) or the methoxy group (Fig. 2 ). Simultaneous bonding through both oxygen atoms will not be considered in this connection [ 29,301. On the catalyst, guaiacol may react via several different pathways. Fig. 3 shows the reaction pathways leading to the formation of the main products in the HDO of guaiacol. The formation of veratrole (path C ) is maximal when the oxidic form of the catalyst is used. The formation of pyrocatechol (path A) and phenol (path B) are favoured by the sulphided catalyst. The selectivity of the for- mation of anisole (path D ) is maximal when a presulphided catalyst is used without sulphur being added to the feed. The selectivity of the formation of 3- methylpyrocatechol (path E) did not show any clear trends.

A possible explanation for these changes in the selectivities of specific prod- ucts might be steric effects on the catalyst surface [3]. The sulphur atom, which has a higher molecular radius than the oxygen atom, together with the methoxy group in an ortho position, blocks the bonding of guaiacol through

I

50

s I%1 4

3b

20

10

I

(a)

0

0 5 10 15 20 25 30 t (hl

s 1%) (b) 40

0 B@-p---? +J 0 P_ 5 10 15 20 25 30 t IhJ

Fig. 5.

51

s (%I F (cl 40

301 0

10 - p4-

A v

7 v#w--- 0 I 1 I b 1 I )

0 5 10 15 20 25 30

t (h)

Fig. 5. Selectivity of the formation of veratrole at 523 K. Reaction conditions and symbols as in Fig. 4.

the oxygen atom of the hydroxy group on the catalyst (cf., Fig. 1). When a sulphided catalyst is used, an increasing proportion of guaiacol is bound through the oxygen atom of the methoxy group (Fig. 2 ) , giving pyrocatechol and phenol as products. This hypothesis may also explain why the HDO of 4-propylguaia- co1 with a CO-MO catalyst gives significantly more 3-propylphenol than 4-pro- pylphenol when an oxidic form of the catalyst is used, whereas sulphidation increases the proportion of 4-propylphenol formation [ 7,311.

It is also important in this connection to note that the changes in the selec- tivity of the formation of specific products did not take place simultaneously. For example, the selectivity of the formation of phenol increased significantly even with the slightest addition of sulphur (Fig. 4)) whereas the selectivity of the formation of veratrole decreased only gradually (Fig. 5 ) .

Effect of oxygen-containing compounds on the HDO reaction

The effect of oxygen-containing compounds on the HDO reaction was stud- ied in two different ways, namely by changing the molar fraction of guaiacol in the feed and by adding other oxygen-containing compounds to the feed,

A lower molar fraction of guaiacol in the feed (runs 5-10) led to a higher

52

conversion of guaiacol (Table 2 ) , as would be expected. Correspondingly, there were more one-oxygen compounds in the product (Table 3). No other signifi- cant effect was found. Krishnamurthy et al. [5] studied the HDO of dibenzo- furan on a Ni-MO/y-AlzOs catalyst at 638 K and found that an increase in the initial concentration of dibenzofuran favours hydrogenolysis instead of hydro- genation reactions. This is explained as being due to the strong adsorption of the oxygen-containing compounds on strongly acidic hydrogenative sites. Ni- MO catalysts are known to have a higher hydrogenation activity than CO-MO catalysts [ 13, 221, however, which together with the higher reaction temper- ature may explain the difference from our results.

Of the competing oxygen compounds added to the guaiacol feed, diphenyl ether (run 8) did not react at all, obviously owing to the low reaction temper- ature. On the other hand, isopropanol (run 9) only decreased the conversion of guaiacol to one-oxygen products (Table 3). The effect of isopropanol may be interpreted as indicating competition between it and guaiacol on the same active HDO sites on the catalyst.

The catalyst

The catalyst used was analysed only after the experiments. It has already been mentioned that an increase in the sulphur content of the feed did not increase the sulphur content of the catalyst correspondingly. The sulphur con- tent of the catalyst determined after the experiments was about 1.5-2.5 wt.-% for high and 3 wt.-% for low oxygen-content feeds. The sulphur content of 3 wt.-% was already achieved by the presulphidation method used. In addition, the presulphidation tests showed that by using a longer presulphidation time (Fig. 6) or a higher presulphidation temperature (Fig. 7), a sulphur content of 4 wt.-% was easily achieved. The sulphur content of 4 wt.-% corresponds to about 60% of the theoretical value. It corresponds fairly well with values pub- lished elsewhere [ 32, 331, although much higher values have also been pub- lished [ 11. It should also be noted in this connection that CO-MO catalysts have been found to contain, in their working state, non-stoichiometric elemen- tal sulphur in addition to the stoichiometric sulphide anions [ 341. The method used in this work does not give good results for the non-stoichiometric sulphur.

The low and relatively constant sulphur content of the catalysts used, to- gether with the fact that the highest selectivity of the HDO was achieved when a presulphided catalyst was used without sulphur being added to the feed, shows clearly that oxygen and sulphur compete for the same hydrogenolytically ac- tive sites and that sulphur already present at the beginning of a reaction is removed in part by oxygen. This is also indicated by the transient behaviour of the catalyst at the beginning of the runs (see, e.g., Fig. 4).

The sulphidation of the CO-MO catalyst also has an effect on the catalyst

53

OL , I 1 I I -

0 3 6 9 12 15 t (hi

Fig. 6. Effect of sulphidation time on the sulphur content of the CO-MO catalyst. ( q ) Fresh catalyst; (0 ) presulphidation at 523 K; ( A ) presulphidation at 623 K. Theoretical maximum as MoS2+Co9S, is 6.7 wt.-%.

0 I I I I _

250 300 350 400 T (VI

Fig. 7. Effect of sulphidation temperature on the sulphur content of the CO-MO catalyst with a constant presulphidation time (3 h) Theoretical maximum as MO& + Co,S, is 6.7 wt.-%.

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TABLE 4

Carbon contents of the CO-MO catalyst (wt.-% )

Reaction Presulphidation temperature/K

523 (low 0)”

523 (high O)Q

623 (high O)Q

Fresh catalyst 0.0 0.0 0.0 Reaction without presulphidation 17.7b 12.8 12.8 Presulphidation only 3.7 3.7 1.9

CS, in the feed/% (v/v): 0.0 0.1 1.0 5.0

18.1 8.6 7.7 8.7 9.2 12.1

lO.Ob 15.9 10.1 20.7 7.8

“Oxygen content of the feed. bReaction time 15 h instead of 30 h.

deactivation during use. The run times of 15 or 30 h used in this work are too short for any clear conclusions to be drawn about the deactivation of the cat- alyst. Some comments may still be made. The carbon contents of the CO-MO catalyst used are given in Table 4. In general, the carbon content of the sul- phided catalysts was found to be slightly lower than that of the oxidic form of the catalyst. This, together with the higher conversion, gives an indication that sulphidation of the CO-MO catalyst protects it against coke formation in the HDO.

The experiments with presulphided catalysts without sulphur being added to the feed also gave some evidence of the catalytic system moving to the oxidic form during the run.

CONCLUSIONS

The major findings of this work may be summarized as follows. The HDO activity of COO-MoO,/y-Al,O, catalyst is clearly enhanced by sulphidation. An increase in the sulphur content in the feed steadily increases the conversion of guaiacol. Oxygen and sulphur compete for the same hydrogenolytically ac- tive sites. As a result, the sulphur content of the catalyst does not increase with increasing sulphur content in the feed. The best selectivity of the HDO is achieved when a sulphided catalyst is used without sulphur being added to the feed. The activity of the catalyst measured by conversion is then, however, only at the same level as with the oxidic form of the catalyst. The selectivity of the formation of specific products is clearly dependent on the sulphur content. The effects may be at least partly explained by steric effects.

55

ACKNOWLEDGEMENTS

This work was funded in part by a grant from the Ministry of Education. We acknowledge the contributions by M. Htirkijnen, M. Liiri, M, NiemelB: and L. Pekic.

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