Special catalyst systems for the synthesis and conversion of some oxygenates

Special catalyst systems for the

synthesis and conversion of some

oxygenates

A.A. Wismeijer

TRdiss ^

1532

Special catalyst systems for the

synthesis and conversion of some

oxygenates

Special catalyst systems for the

synthesis and conversion of some

oxygenates

PROEFSCHRIFT

ter verkrijging van de graad van doctor in de

technische wetenschappen aan de Technische

Universiteit Delft, op gezag van de Rector Magnificus,

prof.dr. J.M. Dirken, in het openbaar te verdedigen

ten overstaan van het College van Dekanen op

dinsdag 31 maart 1987 te 16.00 uur

door

Arie Albert Wismeijer

geboren te Amsterdam

scheikundig doctorandus

TR diss

1532

Prof.dr.ir. H. van Bekkum, eerste promotor, en

Prof.dr.ir. A.P.G. Kieboom, tweede promotor

STELLINGEN

1. De mogelijkheid dat de door Ungar en Baird gevolgde activeringsprocedure van Co(CO)„NO in zeoliet NaY leidt tot migratie van Co naar het uitwen­ dig zeolietoppervlak, is door hen ten onrechte niet onderkend.

R.K. Ungar en M.C. Baird, J. Chem. Soc. Chem. Commun., 643 (1986).

2. De conclusie van Komiyama en Hirai, dat bij een Rh-suspensie, gestabili­ seerd door p-cyclodextrine, hydrogenering van o.a. 3,4-dimethyl-3-pen-teen-2-on plaatsvindt via de insluitverbinding, is aanvechtbaar.

M. Komiyama en H. Hirai, Buil. Chem. Soc. Jpn., 56, 2833 (1983). Dit proefschrift, Hoofdstuk 5.

3. Het is de vraag of de hoge Cu(I)/Cu(II) verhouding, zoals gevonden bij de door Pieters en medeauteurs beschreven CuCl-KCl-LaCl„/SiO„ katalysa­ toren uitsluitend toegeschreven kan worden aan stabilisatie van Cu(I) door het silica-oppervlak.

W.J.M. Pieters, W.C. Conner en E.J. Carson, Appl. Catal., JJ, 35 (1984).

W.C. Conner, W.J.M. Pieters en A.J. Signorelli, Appl. Catal., 11, 59 (1984).

4. Zur Loye en medeauteurs zien bij hun conclusie, dat bij Ni/TiO„, bereid door ionwisseling, het zgn. SMSI effect niet optreedt, de mogelijkheid over het hoofd dat hun waarnemingen gerelateerd kunnen zijn aan een slechtere reduceerbaarheid van hun monsters c.q. verschillen in Ni-kris-tallietgrootte.

H.-C. zur Loye, T.A. Faltens en A.M. Stacy, J. Am. Chem. S o c , 108, 8104 (1986).

gelijking met Fe in de katalytische hydrogenering van benzeen geeft aanleiding tot misverstanden, omdat geen rekening is gehouden met de waarde van het specifieke oppervlak van de geteste katalysatoren.

M.J. Phillips en P.H. Emmett, J. Catal., 101, 268 (1986).

M.C. Schoenmaker-Stolk, dissertatie Technische Universiteit Delft, 1986.

6. Het verdient aanbeveling om bij de beschrijving van de activeringsproce­ dure van katalysatoren ook de condities waaronder men afkoelt, te ver­ melden .

S.J. Uhm en T.J. Lee, J. Catal., 100, 489 (1986).

7. Als karakterisingsprocedure voor katalysatoren verdient temperatuur ge­ programmeerde desorptie veelal de voorkeur boven chemisorptie-metingen.

8. Wanneer men ten behoeve van een onderzoeksproject "Militarisering van de Ruimte" erkend gewetensbezwaarden als medewerkers zoekt, hecht men ver­ moedelijk meer waarde aan de voorspelbaarheid van de conclusies, dan aan op wetenschappelijke wijze verkregen resultaten.

Delta (periodiek van de TU Delft), 18 juni 1985, advertentie van de TU Twente.

9. Gezien haar specifieke eigenschappen, dient de zeilplank in het vaar-reglement in een aparte categorie te worden ingedeeld. Zo is het uit veiligheidsoogpunt bijvoorbeeld wenselijk om windsurfen in de directe aanvaarroute van een jachthaven te verbieden.

10. Omdat in de huidige en toekomstige maatschappij basiskennis van de chemie voor iedere burger gewenst is, valt het te betreuren, dat in het WRH-rapport "Basisvorming in het onderwijs" wordt voorgesteld het vak scheikunde in de eerste drie jaren van het voortgezet onderwijs geen aparte status meer toe te kennen.

11. De standaardlengte van een rol behang (10,05 m) en de hoogte van kamers in nieuwbouwwoningen (2,65 m) doen vermoeden, dat behangproducenten de kamer lager schatten dan de consument hoog.

A.A. Wismeijer 31 maart 1987

I

The investigation described in this thesis has been supported by the Netherlands Foundation for Chemical Research (SON) with financial aid from the Netherlands Organization for the Advancement of Pure Research (ZWO).

Typing : Mrs. M.A.A. van der Kooij-van Leeuwen Drawings: Mr. W.J. Jongeleen

1. INTRODUCTION 1

2. SOLVENT-REACTANT-SUPPORT INTERACTIONS IN LIQUID PHASE HYDROGENATION. SUPPORTED METAL CATALYSTS MODIFIED BY A

LIQUID FILM 5 Abstract 5 Introduction 6 Experimental 7 Materials 7 Procedure 7 Results and discussion 8

Acid films 8 Catalyst deactivation 8

Rate measurements 9 Selectivity 12 Films of aqueous base 13

Rate measurements 13 Selectivity 18 Conclusions 22 Acknowledgements 23 References 23

3. SOLVENT-REACTANT-SUPPORT INTERACTIONS IN LIQUID PHASE HYDROGENATION II. ADDITIONAL INVESTIGATIONS ON SUPPORTED

METAL CATALYSTS MODIFIED BY A LIQUID FILM 25

Abstract 25 Introduction 25 Experimental 26 Materials 26 Procedure 26

Results and discussion

Hydrogenation of 1-naphthol 26 Effect of liquid films in the hydrogenation of phenol

over Pt/C as compared to Pd/C 28 Comparison of silica- and carbon-supported platinum

catalysts with an aqueous film in the hydrogenation

of benzene and cyclohexene 30 Comparison of reaction order in cyclohexanone using

Pd and Pd/C with different film and solvent systems 32 Expansion of the liquid film in the presence of a

surfactant 35 Acknowledgements 36 References 37

ONE-STEP OXIDATION OF BENZENE TO HYDROQUINONE IN THE

PRESENCE OF COPPER(I)CHLORIDE 39

Abstract 39 Introduction 39 Experimental 40 Results and discussion 41

Oxygen concentration 41 Temperature dependence 43 Effect of CH„CN 43 pH dependence 44 Hydroquinone formation 45 Mechanistic considerations 46 Conclusions 51 Acknowledgements 51 References 52

THE HYDROGENATION OF CITRONELLAL TO CITRONELLOL OVER

Ru CATALYSTS. MISCELLANEOUS EXPERIMENTS 53

Abstract 53 Introduction 53 Experimental 58 Materials 58 Procedure

Selectivity and activity definition 58

Catalyst preparation 60 Catalyst support 61

Solvent 61 Effect of a water film 62

Metal ion additives 63 Cyclodextrins as additives 66

Conclusions 72 Acknowledgement 72 References 72

6. IMPROVED ACTIVITY AND SELECTIVITY IN CARBON-OXYGEN DOUBLE

BOND HYDROGENATIONS WITH Ru/Ti02 75

Abstract 75 Introduction 75 Experimental 75 Results and discussion 76

References 79

7. SELECTIVE HYDROGENATION OF CITRONELLAL TO CITRONELLOL OVER

Ru/TiO„ AS COMPARED TO Ru/SiO„ 81

Abstract 81 Introduction 81 Experimental 82 Chemicals 82 Catalysts 83 Procedure 83 Results and discussion 84

Catalyst characterization 84 Surface area of support 84 Metal surface area 84 Metal crystallite size 85

TPR 87 XPS 89 Catalytic behaviour 89

Concluding remarks 95 Acknowledgements 97 References 97

8. CONDITIONING OF Y-AlgOg CATALYST IN THE TRANSFER

HYDROGENA-TION OF 4- rert-BUTYLCYCLOHBXANONE BY 2-PROPANOL 99

Abstract 99 Introduction 99 Experimental 100

Materials 100 Procedure 100 Results and discussion 101

Conclusions 108 References 108 SUMMARY 111 SAMENVATTING 113 CURRICULUM VITAE 117 NAWOORD

₁₁₉

CHAPTKH 1

INTRODUCTION

The introduction of oxygen into hydrocarbons offers an important route to 1 2

many valuable chemicals ' . In many cases the oxygen handle is used for further modification of the molecule, e.g. by addition, condensation, acylation or hydrogenation.

This latter type of reaction is of importance in e.g. the synthesis of e-caprolactam, the precursor molecule of nylon-6, starting from phenol

3

(Scheme 1) . Here, the selectivity of the formation of cyclohexanone from phenol should be as high as possible. This is achieved by the use of Pd

4

catalysts . The cyclohexanone yield can be increased by variation of the alkalinity of the support or the liquid medium. Frequently mentioned additives are C a O5 - 7, MgO7 and Na-CO-8 , 9, NaOH or NaOPh9. Addition of 10-300 ppm alkali metal to liquid phenol not only reduces the amount of

cyclo-g hexanol in the reaction mixture, but also increases the reaction rate .

« 3 y - O H ^—•- / \ = 0 N H' °H - ( \ = N O H »- / NH NH

O

OH

Scheme 1. Route from phenol to e-caprolactam.

A part of the present thesis is devoted to the Pd catalyzed liquid phase hydrogenation of phenol with catalysts modified with a liquid film immiscible with the bulk solvent. Such systems are able to alter the relative concentrations of reactants and products at the catalyst metal and the selectivity of the reaction can be enhanced by choosing conditions where the parent compound is present at the catalyst surface in large excess to

19

species present in such a film may interact with the other molecules at the catalyst surface, thereby creating another possibility of catalyst modifica­ tion or of bifunctional catalysis.

Chapters 2 and 3 describe the use of both alkaline and acidic liquid films in the hydrogenation of phenol as well as of benzene. This latter reaction was investigated with the aim to capture the intermediately formed cyclohexene by acid catalysis as cyclohexylbenzene (Scheme 2) as was previously described by Slaugh and Leonard, who used acidic supports . In principle, alternative routes to t-caprolactam would consist of oxidizing cyclohexene, obtained by cracking of cyclohexylbenzene, to cyclohexanone or of oxidation of cyclohexylbenzene to the hydroperoxide compound and

decom-3

position into cyclohexanone and phenol (Scheme 2 ) .

O

\ OOH

Scheme 2. Capture of cyclohexene in benzene hydrogenation by its addition to benzene as a possible route to cyclohexanone.

Phenol, which is an intermediate for several chemicals and polymeric materials is nowadays largely obtained by the cumene route, which yields

1-3

acetone as a byproduct . As a one-step oxidation of benzene to phenol is an attractive alternative, the Cu(I) mediated aerial hydroxylation of

12

benzene was investigated (Chapter 4) . The interesting observation was made that, besides phenol, hydroquinone is obtained by direct dihydroxylation of benzene (Scheme 3 ) .

— <^>-

OH

*

H

° - ® - °

H

Scheme 3. Copper(I) mediated hydroxylation of benzene into phenol and hydroquinone.

Another valuable oxygen containing compound is the fragrance compound citronellol (3,7-dimethyl-6-octen-l-ol), which is industrially obtained by

catalytic hydrogenation of synthetically prepared geraniol and nerol

((E)-13

and (2)-3,7-dimethyl-2,6-octadien-l-ol respectively) . However, citronellol can also be obtained by hydrogenation of citronellal (3,7-dimethyl-6-octen--1-al). This reaction is of general scientific interest because of the presence of two different reducible groups in the same molecule (Scheme 4 ) .

O H

citronellol

Scheme 4. Hydrogenation of citronellal to citronellol.

Several parameters, which influence the selectivity and activity of the Ru catalyzed hydrogenation of citronellal, were investigated (Chapter 5 ) . The selectivity and activity of the hydrogenation depends on the method of catalyst preparation, the support, the solvent and on the presence of metal additives. Preliminary experiments employing cyclodextrins as additives are also reported. It was furthermore found that Hu/TiO_ hydrogenates the carbonyl function with better selectivity and higher activity than an

14 olefinic bond or phenyl group as compared to Bu/SiO„ (Chapter 6) .This support effect depends on the activation temperature and was interpreted in

15 terms of Cl removal and metal-support interaction (Chapter 7)

The amount of citronellol from citronellal, as obtained by the described methods, never exceeded 90*. However, the carbonyl group can be hydrogenated

with 100* selectivity with A190„ as hydrogen transfer catalyst and

16 17

2-propanol instead of H, as hydrogen source ' . In Chapter 8 experiments on the activation and conditioning of y—A1„0_ using 4- £er*-butylcyclo-hexanone as a model ketone compound are reported

-4-References

1. P. Wiseman, 'An Introduction to Industrial Organic Chemistry', Applied Science Publishers, London, 1979.

2. H.A. Wittcoff and B.G. Reuben, 'Industrial Organic Chemicals in Perspective', Wiley Interscience, New York, 1980.

3. K. Weissermel and H.-J. Arpe, 'Industrielle Organische Chemie, Verlag Chemie, Weinheim, 1976.

4. P.N. Rylander, 'Catalytic Hydrogenation in Organic Syntheses', Academie Press, New York, 1979, p. 192.

5. K. Smeykal, H.J. Naumann, H. Schaefer, J Veit, K. Becker and A. Block, Germ. Pat., 1298098 (1969); Chem. Abstr., 71, 60843c (1969).

6. E. Grasshoff, H. Meye, H.J. Naumann, G. Pohl, M. Prag, H. Schaefer and R. Schubert, East. Germ. Pat., 150999 (1981); Chem. Abstr., 96, 144913e

(1982).

7. H. Oberender, H. Schaefer, D. Timm, H. Baltz, H. Blume, J. Lunau and H. Meye, Brit. Pat. 1332211 (1973); Chem. Abstr., 80, 36753p (1974). 8. R.J. Duggan, E.J. Murray and L.0. Winstrom, U.S. Pat. 3076810 (1963);

Chem. Abstr., 59, 2671b. (1963).

9. W.B. Fisher and J.F. Van Peppen, U.S. Pat. 4162267 (1979).

10. A.A. Wismeijer, A.P.G. Kieboom and H. van Bekkum, Reel. Trav. Chim. Pays-Bas, 105, 129 (1986).

11. H. Slaugh and J.A. Leonard, J. Catal., 13, 385 (1969).

12. J. van Gent, A.A. Wismeijer, A.W.P.G. Peters Rit and H. van Bekkum, Tetrahedron Lett., 27, 1059 (1986).

13. A.J.A. van der Weerdt, private communication.

14. A.A. Wismeijer, A.P.G. Kieboom and H. van Bekkum, React. Kinet. Catal. Lett., 29, 311 (1985).

15. A.A. Wismeijer, A.P.G. Kieboom and H. van Bekkum, Appl. Catal., 25, 181 (1986).

16. D.V. Ramana and C.N. Pillai, Can. J. Chem., 47, 3705 (1969).

17. G.H. Posner, A.W. Runquist and M.J. Chapdelaine, J. Org. Chem., 42, 1202 (1977).

18. A.A. Wismeijer, A.P.G. Kieboom and H. van Bekkum, Appl. Catal., submitted.

19. P.G.J. Koopman, H.M.A. Buurmans, A.P.G. Kieboom and H. van Bekkum, Reel. Trav. Chim. Pays-Bas, 100, 156 (1981).

CHAPTER 2

SOLVENT-HEACTANT-SUPPORT INTERACTIONS IN LIQUID PHASE HVDROGENATION. SUPPORTED METAL CATALYSTS MODIFIED BY A LIQUID FILM

Abstract

The effect of impregnating metal on carbon catalysts with liquid films immiscible with the bulk solvent on rate and selectivity in liquid phase hydrogenations has been investigated. Both acid and alkaline films have been used with benzene and phenol as the reactant. The bifunctional character of such catalysts has been demonstrated and a substantial improvement in selectivity for the hydrogenation of phenol to cyclohexanone has been achieved. The factors contributing to the observed phenomena in such multi­ phase catalytic reactions are discussed.

Introduction

Reaction rate and selectivity in heterogeneously catalyzed liquid phase hydrogenation can be influenced by the composition of the reaction medium in the vicinity of the metal catalyst. Factors affecting the microenvironment of a given metal are the composition of the bulk liquid and the nature of the support.

Solvent effects in, particularly competitive, liquid phase hydrogenation have been extensively studied, although much work in this field remains to be done . Reaction rate and selectivity are not only directed by the nature and composition of the solvent, but also by the presence of active dissolved species like acids, bases and salts. Numerous examples of their effects as additives on both reaction rate and selectivity can be found in the litera-. 2-5

ture

The pronounced influence of the bulk solvent has been described in a preceding publication . Especially striking are the large effects when hydrogenations performed in strongly polar solvents are compared with similar experiments in apolar solvents. In line with this phenomenon is the

-6-observation that the rate of hydrogenation of a particular compound can be accelerated by the choice of the support. The rate of hydrogenation of benzene in cyclohexane with Ru catalysts, for instance, was raised over six times by applying polar silica instead of trimethylsilylated silica as the support .

The above mentioned effects become even more distinct in competitive hydrogenations. It proved possible to direct the selectivity of such a reaction by a proper choice of both solvent and support. The reason for this becomes clear when the partition of the reactants between solvent and catalyst surface is taken into account .

A fine example of a chemical action of the support is provided by the 7

work of Slaugh and Leonard . These authors were able to convert benzene into cyclohexylbenzene by the use of an acidic support in the hydrogenation reaction. The support clearly acts as a co-catalyst in this system, since it catalyzes the reaction between benzene and the intermediate product cyclo-hexene.

An extension of the above principles can be found in the idea to equip the catalyst carrier by direct impregnation with a distinct liquid film, immiscible with the bulk liquid. The essential aspects of the behaviour of such systems are schematically depicted in Fig. 1. Attention should be focussed in particular on the reactivity of the intermediate product. The following phenomena may be of importance:

(i) the rate and selectivity of the hydrogenation reaction are altered due to changes in the transition state caused by the change of the solvent composition around the catalyst;

(ii) the concentration of reactants and products in the vicinity of the catalyst relative to the bulk solution is changed, which may affect rate and selectivity by a change in the adsorption equilibria;

(iii) the liquid film itself acts as a co-catalyst.

The present paper focusses on the hydrogenation of benzene with platinum and of phenol with palladium on carbon catalysts with acid or alkaline liquid films. Carbon was chosen as the support, because it is inert towards both acids and bases. Benzene and phenol were chosen as the substrates since both compounds are liable to multi-step hydrogenation, yielding intermediate products, which are susceptible to reaction with the parent compound in the

7 8

presence of an acidic co-catalyst ' . The intermediate product from benzene, cyclohexene, is known to be much more prone to further hydrogenation than the first product from phenol, cyclohexanone. With phenol, the use of films of aqueous base is expected to enhance the concentration of phenol with

respect to cyclohexanone at the catalyst interface, thus possibly improving the selectivity of phenol hydrogenation towards cyclohexanone.

Fig. 1. Schematic representation of the action of a supported metal catalyst covered by a liquid film in a two-step hydrogenation.

Experimental

Materials

The 5* Pt/C, 10* Pd/C, 5* Rh/C and 5* Ru/C catalysts were purchased from Drijfhout, Amsterdam, palladium black from Strem Chemicals. The B.E.T.

2 -1

surface area of the 5% Pt/C was 1000 m .g and that of the 10% Pd/C 673 m2. g- 1. The 10* Pd/C contained 0.080 mmol H+.g_ 1.

Another batch of 5* Pt on carbon, designated as Pt/C (A), was prepared by impregnation to incipient wetness of carbon (Carbopuron 4N, obtained from Degussa) with an aqueous solution of H„PtCl6.6H20 (Drijfhout). The product was dried in air at 393 K.

All catalysts were reduced in a stream of hydrogen for two hours at 473 K.

Benzene and cyclohexene were purified as previously described . Cyclo­ hexanone was distilled prior to use. Other chemicals were reagent grade.

Procedure

The catalysts were provided with a liquid film by adding dropwise the impregnation solution to 100-500 mg of the dry catalyst. The exact loading was found from the increase in weight. As a rule the impregnated catalysts were freshly prepared before use.

-8-The hydrogenations were performed in the liquid phase (25 ml) in a thermostatted and well stirred vessel at 303 K and 101 kPa H„. Catalyst concentrations ranged from 1.0 to 7.5 g. 1 unless otherwise stated and are expressed as grams of dry material per litre. Reactant concentrations were 0.01-0.1 M except in some hydrogenations in aqueous media (0.5 M ) . The hydrogen consumption was monitored with an automatic hydrogenation

appara-g

tus . Rates of hydrogenation are expressed as moles of hydrogen consumed per mole of platinum or palladium per second. In the phenol hydrogenations in aqueous solvents the reaction mixture was analyzed by means of HPLC (Nucleosil C1 R) using a mixture of 28% methanol, 7% 1,4-dioxane, 0.1*

tri-fluoroacetic acid and 65% water as the eluent. The products of the other reactions were identified by means of gas chromatography and mass spec-trometry.

The adsorption of phenol and cyclohexanone was determined by adding a solution of the proper concentration to the catalyst, equilibrating while stirring overnight and analyzing the solution against an internal standard by gas chromatography.

Results and discussion

Acid films

Catalyst deactivation. Catalysts with a film of 96% sulfuric acid were subject to deactivation under the conditions applied. This is probably caused by the reduction of the acid to hydrogen sulfide, which has recently been shown to be a product of the hydrogenation of sulfuric acid with various metal on carbon catalysts '

Deactivation was also observed with more dilute solutions of sulfuric acid and with other oxyacids like chlorosulfonic acid, methanesulfonic acid and, to a lesser extent, phosphoric acid, as examined with 5% Pt/C as the catalyst and benzene as a test compound.

In the absence of an organic substrate, the initial rate of reduction of the H„S0. film itself (Fig. 3) increased with the load of acid, together with a further deactivation of the Pt/C catalysts (Fig. 2 . ) . The deactiva­ tion of the acid modified catalysts is somewhat less severe when benzene is present. This is caused by the competition of benzene and H„S0. for the Pt surface. The deactivation is less retarded by 1,3,5-trimethylbenzene, which is less strongly adsorbed on Pt than benzene (Fig. 2 ) . As the result of the

deactivation it was not possible to measure initial rates beyond an acid load of approximately 0.5 ml.g . log t V, 2.0-1-6 1-2 0 8 0 4 0 0 01 02 0-3 0 4 05 load (ml.g-1)

Fig. 2. Effect of the 96* H 2 S 04 f i l m l o a d o n t n e deactivation of 5* Pt/C

under hydrogenation conditions (expressed as half-time in min). 298 K; 101 kPa Hg.

(x) neat benzene;

-2

(V) 1,3,5-trimethylbenzene (7.2.10 M) in cyclohexane; (+) cyclohexane, i.e. only H„S0. reduction.

Kate measurements. Despite the above mentioned deactivation phenomena, the initial rate of benzene hydrogenation with 5* Pt/C impregnated with 96* sulfuric acid increases linearly with the load of acid (Fig. 3 ) . The rate of hydrogenation of neat benzene equals that of a 0.1 M solution of benzene in cyclohexane, whereas the plots for 5 0 * sulfuric acid and water films coincide and show only at first a small rise in activity.

The recently reported rate enhancements when aromatics were hydrogenated in liquid hydrogen fluoride or superacids were attributed to the removal of the aromatic resonance stabilization energy because of protonation of the

12 13 14 substrate ' . In the case of 96* H„S0., however, the basicity of benzene

is too low to validate our results with this theory. Moreover, the hydro­ genation of the more basic toluene and 1,3,5-trimethylbenzene was hardly accelerated by the acid film.

In connection with the possible hydroalkylation of benzene to cyclohexyl-benzene, the behaviour of the intermediate product cyclohexene was studied

-10-0 -10-05

0 6 0-8 10 1.2 1 4

load (ml.g-1)

Fig. 3. Effect of H-SO. and H?0 films on the initial rate of hydrogenation of benzene over 5% Pt/C. 298 K; 101 kPa.

2

M benzene in cyclohexane; (®) neat benzene. _2

and H„0 (o): 1.1.10 M benzene in cyclohexane. 96% H-SO.: (x) 1.1.10

50* H2S 04 (•)

Initial rate of reduction of the 96* H-SO. film (+).

observed when the catalyst is loaded with a relatively small amount of sulfuric acid or water. Increasing the load results in a decrease in rate of hydrogenation which is more pronounced with the more acidic films. When the catalyst is impregnated with aqueous solutions of hydroxyethyl cellulose with viscosities exceeding the viscosity of 9635 sulfuric acid by several orders of magnitude, a decrease in hydrogenation rate is observed but not to the extent as shown by the 963! sulfuric acid film. The large decrease in initial rate as observed with the acid films can be explained when the esterification of cyclohexene with sulfuric acid is taken into account. Since the unsaturated character of cyclohexene is removed, the cyclohexene present in the film becomes in part inactive towards hydrogenation which results in a decrease of the hydrogenation rate.

Fig. 4. Effect of 96% HgSO. (£), 5 0 * H„S04 (A) and HJO (A) films on the

hydrogenation of cyclohexene (9.9.10 M) in cyclohexane over 5S> Pt/C. 298 K; 101 kPa.

With Pt/C catalysts with water films the activity towards benzene (Fig. 3) and 1,3,5-trimethylbenzene (Fig. 5) is hardly changed with respect to the unmodified catalyst, whereas for cyclohexene a substantial decrease in rate is observed. This seems surprising in view of the low solubilities of these compounds in water. Perhaps the adsorption of cyclohexene at the catalyst surface is more weakened by water than that of the more strongly adsorbed aromatic compounds. This statement is supported by the observation that in the gas phase hydrogenation of benzene over platinum black at 318 K small amounts of cyclohexene could be detected when water was added to the

15

feed . Further evidence for this theory was recently reported by Van der 1 fi

Steen and S c h o l t e n w h o studied t h e g a s p h a s e h y d r o g e n a t i o n o f b e n z e n e over ruthenium b l a c k at 2 9 8 K in the p r e s e n c e o f s e v e r a l p o l a r adsorbates i n c l u d i n g w a t e r .

In c o n t r a s t to b e n z e n e , p h e n o l w i l l b e p r o t o n a t e d in the s u l f u r i c acid layer. R e c e n t l y the p l a u s i b i l i t y o f p r e d o m i n a n t p r o t o n a t i o n on t h e oxygen

13 17

atom of phenol was demonstrated by C NMR . Since this is accompanied with some decrease of resonance stabilization energy, a speeding up of the hydrogenation is expected. Indeed this is observed with Pd/C, 96% H„S0. (Fig. 6 ) . However, on the basis of the present experiments it is not clear whether the hydrogenation is accelerated due to a priori protonation of phenol or by protonation of alkyl-palladium surface complexes.

-12-0 2 -12-0 4 -12-0-6 -12-0-8 1 -12-0 load (ml.g-1)

Fig. 5. Effect of a H„0 film on the hydrogenation of 1,3,5-trimethylbenzene (2.9.10-2 M) in cyclohexane over 5* Pt/C. 298 K; 101 kPa.

Cyclohexanone, the intermediate product of the hydrogenation of phenol, was also hydrogenated over Pd/C, 98X H„S0.. The initial rate of hydrogena­ tion increases linearly with the amount of sulfuric acid present on the catalyst (Fig. 6 ) .

18

Brewster suggested that protonation of alkanones is responsible for the increased rate of hydrogenation in acid media. This acceleration was later

19

on also observed by Peterson and Casey . We also suppose that the increased rate of hydrogenation of cyclohexanone in the case of a catalyst with a 96% sulfuric acid film, is caused by protonation of the cyclohexanone. In addi­ tion it is noted that protonation of cyclohexanone is also required for the

formation of dicyclohexyl ether ( vide infra).

7

Selectivity. Slaugh and Leonard reported high yields of cyclohexylben-zene in the high pressure liquid phase hydrogenation of bencyclohexylben-zene at 473 K over platinum and other catalysts on acidic carriers. A small amount of cyclohexylbenzene was found when the catalytic hydrogenation of benzene was

13

performed in liquid hydrogen fluoride at 303 K . With our Pt/C, 9 6 * H-SO. catalysts, however, neither cyclohexylbenzene nor methylcyclopentane nor any other product except cyclohexane could be detected during the hydrogenation of benzene at 298 K. Although the acidic film increases the rate of benzene hydrogenation and decreases that of cyclohexene, the rate of cyclohexene hydrogenation still exceeds that of benzene several times (Figs. 3 and 4 ) . Furthermore, by stirring a solution containing a mixture of benzene and cyclohexene in the presence of Pt/C, 96* H„S0- without hydrogen at 298 K, the rate of cyclohexylbenzene formation proved to be several orders of magnitude lower than the hydrogenation rate of benzene.

The synthesis of cyclohexylphenol from phenol with the aid of 5* Ni

D

supported on silica-alumina at 493 K has been reported in the literature . Although this product was not found in our reactions, hydrogenation of phenol over Pd/C containing 0.22 ml 96* H„S0. per gram catalyst gave up to

kinit(10-3s-1) 25 20 15 10 5 0 0 0-2 0-4 0-6 0-8 10 load (ml.g-1)

Fig. 6. Hydrogenation of phenol and cyclohexanone in cyclohexane over 10% Pd/C covered by a liquid film. 298 K; 101 kPa.

Phenol: 3.1.10~2M; (■) 96* H2S04; (D) HgO.

Cyclohexanone: 1.1.10-1 M; (•) 96% HgSC^. (o) HgO.

21% dicyclohexyl ether together with traces of bicyclohexyl and 2-cyclo-hexylcyclohexanone. Hydrogenation of cyclohexanone with Pd/C, 96* H„S0. resulted also in the formation of dicyclohexyl ether.

Similar results have been reported when cyclohexanone was hydrogenated over platinum in the presence of cyclohexanol in ethanol acidified with H C 12 0.

Films of aqueous base

Rate measurements. With catalysts equipped with a basic film the rate of hydrogenation of benzene remains constant during a single run in contrast with the acid film catalysts where deactivation due to reduction of the acid occurs.

When Pt/C (A) is impregnated with aqueous solutions of KOH or Bu.NOH, a decline in rate of hydrogenation of benzene on increasing the KOH concentra­ tion is observed (Fig. 7). Comparing this result with the behaviour of Pt/C, H20 (Fig. 3 ) , it seems unlikely that this decline in rate is caused by the

repulsion of benzene from the catalyst surface by a salting out effect of the film. As a plausible explanation we assume that the Pt sites are blocked

-14-by adsorption of OH . Further evidence for this assumption will be presented in the course of this discussion.

1-3 .-1

1 2

[OH

-

] (M)

Fig. 7. Effect of the hydroxide concentration on the hydrogenation of benzene (1.1.10~ M) in cyclohexane over 5* Pt/C (A) (3.0 g. 1~ with a KOH (x) or Bu.NOH (V) film (0.80 m l . g- 1) . 298 K; 101 kPa.

In the hydrogenation of phenol over Pd/C with films of aqueous base both rate and selectivity of the reaction were expected to be affected because of the ionisation of phenol in the liquid film.

At a high immersion of the catalyst surface (0.80 ml.g ) , raising the concentration of KOH increases the rate at first, but at high KOH concentra­ tions the catalyst becomes totally inactivated (Fig. 8 ) . Again it is noted that a cation effect is not present since the same effect is observed when LiOH instead of KOH is used. In all experiments phenol was present in excess to KOH (cf. legend of Fig. 8 ) . Although all OH was expected to be consumed by the conversion of phenol in the film to potassium phenolate, this is, as will be shown in the next paragraph, not always the case. Consequently the decline in rate is probably caused by OH adsorption as in the case of benzene.

6 8 10

[öf-r] (M)

Fig. 8. Effect of the hydroxide concentration on the hydrogenation of phenol and cyclohexanone

101 kPa. Phenol

in cyclohexane over 10% Pd/C (3.0 g.1 l). 298 K;

3.1.10 2 M; (■) KOH or (D) LiOH film (0.80 ml.g * ) .

From experiments in bulk KOH solutions, the rate of hydrogenation of potassium phenolate as such was calculated (Fig. 9, Table 1 ) . Since this rate was found to exceed that of phenol in water, it was concluded that the initial rise in activity of Pd/C, KOH at low KOH concentrations must be attributed to the larger reactivity of potassium phenolate. An increased rate of hydrogenation of sodium phenolate in molten phenol as compared to

21 phenol has been reported previously

ki n i t- l (ks) 10 8 6 4 2 0 0 2 4 6 [phenolate]"1 (M~1)

Fig. 9. Hydrogenation of potassium phenolate over 10* Pd/C (6.0 g.1 ) in water in the presence of excess potassium hydroxide: (■) 0.50 M;

(D) 0.10 M. 298 K; 101 kPa.

From the higher reactivity of phenolate it has to be expected that increasing the coverage of the catalyst by an aqueous alkaline film results in a higher rate of the phenol hydrogenation, since the amount of phenolate available for the palladium metal increases. This, indeed, has been found for a 0.5 M KOH film as shown in Fig. 10. On the other hand, the use of a 2 M KOH film first increases the rate of hydrogenation substantially but has no favourable effect at higher loads, whereas a 5 M KOH film shows a maximum in rate at a load of 0.2 ml.g . These phenomena cannot be completely under­ stood if phenolate hydrogenation only takes place at film-covered palladium sites. In particular for intermediate film loads (up to 0.5 ml.g ) we have to take into account the occurrence of partially covered palladium crystal­ lites (Fig. 11). As the benzene moiety of phenolate will be preferentially solvated by the apolar bulk solvent, the use of more concentrated KOH films will induce an increase of phenolate concentration at the bulk-film inter­ face. This will generally result in a decreased phenolate concentration at the palladium surface except for intermediate loads which allow a special

1 6

-Table 1. Hydrogenation rates of phenol and cyclohexanone in d i f f e r e n t s o l v e n t - c a t a l y s t - f i l m systems8. Substrate phenol cyclohexanone a Phenol, 3.1. Solvent cyclohexane water excess KOH cyclohexane water water, pH = 11 Film (0.80 ml.g" water 2.0 M KOH 96% H2S04 water 2.0 M KOH 96% H-SO. — -2 .10 M; cyclohexanone, 1.1 *> .10" Rate of hydrogenation (10-3 s_1)

-

1

M

; 0.90 0.83 1.38 35.2b 0.93c 2.9d 0.06 0.10 0.81 5.4b 6.3 2.3 10% Pd/C, 3.0 g.1 - 1 . H2, 101 kPa; 298 K.

Extrapolated from Fig. 6. 0.1 M phenol.

From Fig. 9.

way of adsorption of phenolate in the interface onto palladium as depicted in Fig. 11. As 4-alkylphenolates show a weaker adsorption on the catalyst metal in combination with a stronger tendency for solvation by the apolar bulk liquid the above-mentioned phenomena are expected to be of more importance than for phenolate. This agrees with the experimentally observed rate maxima for 4-methyl- and 4-tert-butylphenol at intermediate 2 M KOH film loads (Fig. 12).

Of course, part of the observed decline in rate must be explained by OH adsorption on palladium, as discussed above.

In contrast to the phenols cyclohexanone hydrogenation is not inhibited at high hydroxyl anion concentrations. Apparently stabilization of the transition state of the carbonyl hydrogenation in an alkaline medium plays a role.

F i g . 10. E f f e c t of s e v e r a l KOH f i l m s on t h e r a t e of h y d r o g e n a t i o n of phenol and cyclohexanone in cyclohexane over 10% Pd/C ( 3 . 0 g . 1 ) . 298 K; 101 kPa.

P h e n o l : 3 . 1 . 1 0 "2 M. (■) 0.50 M; (o) 2 . 0 M; (+) 5 . 0 M KOH.

Cyclohexanone: 1 . 1 . 1 0- 1 M. (•) 0.50 M KOH.

bulk liquid

Fig. 11. Schematic representation of the action of a supported metal catalyst, partially covered by a liquid alkaline film, in the hydrogenation of phenol.

Micr3.s-1)

0 2 0 4 0 6 0 8 10

load (ml/g-1)

Fig. 12. Effect of a 2.0 M KOH film on the rate of hydrogenation of

4-methylphenol (A) and 4-terM>utylphenol (?) (3.0.10~2 M) in

-18-For purposes of comparison, the behaviour of the system Pd/C-phenol c.q. cyclohexanone was studied in aqueous solutions. As can be seen from Table 1 the rate of hydrogenation is determined by the composition of the film as well as the solvent. Especially striking is the 40-fold increase in rate when cyclohexanone is hydrogenated in aqueous KOH instead of in cyclohexane. The effect of a KOH film is smaller but still substantial (Figs. 8, 10). Analogous effects are found with water as the film or solvent. The fact that with films containing up to 8 M KOH cyclohexanone shows still an increase in rate of hydrogenation (Fig. 8 ) , whereas the hydrogenation in aqueous media at high pH is inhibited (Fig. 13), is rather surprising. The question as to whether reactive enolate anions or ionized hydrated cyclohexanone species are formed by adsorbed OH species on the catalyst metal, has to be answered by a detailed kinetic study using metal blacks as catalysts.

When phenol is hydrogenated in aqueous media, a maximum in the rate of hydrogenation on increasing the pH is observed (Fig. 13), because of the higher rate of hydrogenation of potassium phenolate with respect to phenol. This effect is counteracted by the adsorption of OH ions on Pd when the pH is further increased. The stronger decrease in hydrogenation rate at high pH when Pd/C is used instead of Pd black can be attributed to repulsion of the phenolate ion from the negatively charged carbon surface due to ionisation of surface COOH and aryl-OH groups.

k(10"3.s-1) k(10-5.s"1) 8 4 0 8 4 _ 0 0 6 8 10 12 14 PH

Fig. 13. Influence of pH on the rate of hydrogenation of phenol (■, Q ) and cyclohexanone (•, o) over 10% Pd/C (8.0 g.1 ) or Pd black (6.0 g.1 ) in water. Phenol, cyclohexanone: 0.50 M. 298 K; 101 kPa.

Selectivity. The effects on hydrogenation rate and reactant adsorption, as discussed in the previous section, should also become apparent, when hydrogenation selectivities are considered.

When the different catalyst-film systems are compared, by far the best results for the hydrogenation of phenol to cyclohexanone with heptane as solvent are obtained with Pd/C, 2 M KOH (Fig. 14). A high selectivity towards cyclohexanone (97%) is observed as long as phenol is present. When

selectivity (%) 1 0 0 1

-60 80 100 conversion CM

Fig. 14. The selectivity towards cyclohexanone during the hydrogenation of

„-2 -1,

phenol (1.0.10 M) in heptane over 10% Pd/C (3.0 g.1 ) with various films (0.80 m l . g_ 1) . (a) HgO film; (+) 2 M KOH film; (■)

without film. 298 K; 101 kPa.

Pd/C as such is used as the catalyst in phenol hydrogenation, the selec­ tivity towards cyclohexanone decreases at higher conversion due to accumulation of cyclohexanone, leading to a higher coverage of Pd with cyclohexanone. A similar picture is observed with Pd/C, H20 , although the

overall selectivity is somewhat better.

A plausible explanation for these observations can be found when the distribution coefficients of cyclohexanone between an apolar bulk solvent like cyclohexane and some aqueous phases are considered (Table 2 ) . These data show that with Pd/C, H„0 cyclohexanone prefers the organic phase, while phenol will be concentrated in the H„0 film with respect to the bulk solvent. Thus the phenol/cyclohexanone ratio in the vicinity of Pd is raised, which results in an improved selectivity for cyclohexanone. As conversion proceeds, the phenol/cyclohexanone ratio in both bulk solvent and film decreases and the selectivity therefore concomitantly decreases. On the other hand with Pd/C, KOH at first a film with a stationary high concentra­ tion of potassium phenolate is generated, while the cyclohexanone that is formed is extracted from the aqueous film into the bulk solvent. As con­ version proceeds and the total amount of phenol becomes less than the amount of KOH present, all phenol will be present in the aqueous film. As a result

-20-even at 973s conversion the phenol/cyclohexanone ratio in the film exceeds the value of 50. This explains that a high selectivity towards cyclohexanone is maintained until essentially all phenol is consumed.

Table 2. Distribution coefficients of cyclohexanone and phenol between aqueous phases and cyclohexane at 298 K.

Compound Aqueous phase D = C /C , . aq cyclohexane Cyclohexanone H„0 0.41 2.0 M KOC„Hc a 0.28 o o 2.0 M KOH 0.16 Phenol Ho0 6.4 pH = 12.

However, data from actual distribution measurements (see experimental) for phenol and cyclohexanone between bulk solvent and the catalyst with film do not agree with those expected from this distribution model (Fig. 1 5 ) . The relative amount of phenol with respect to cyclohexanone on Pd/C, KOH is lower than on Pd/C, H„0 and in both systems the amount of cyclohexanone removed from the organic phase is much too high. The apparent film concen­ trations found are often much higher than the solubility of phenol in H„0 (0.99 M) and of cyclohexanone in 2 M KOH (0.2 M ) . Therefore, adsorption of both phenol and cyclohexanone on the carbon support will contribute to the experimental film concentrations. For example, a monolayer coverage of

2 -1

phenol on carbon, as estimated from the support area (673 m .g ) and the -19 2

molecular dimensions of phenol (6.10 m ) , corresponds with an apparent film concentration of 2.4 M in the case of a film load of 0.8 m l . g- . This value is in fair agreement with the experimental result (Fig. 15) and with the actual adsorption of phenol from cyclohexane on Pd/C. From these considerations we conclude that the observed apparent film concentrations for cyclohexanone must be fully ascribed to adsorption on the carbon support. The observed selectivity can be understood when it is considered that the cyclohexanone that is adsorbed on carbon does not contribute to the cyclohexanone concentration in the film which determines the phenol/cyclo-hexane ratio at Pd.

It is therefore concluded that the observed improvement in selectivity can be satisfactorily described by the selective extraction mechanism and

that a change in the adsorption equilibria on the carbon support is of no importance with respect to this point.

C( i l m( M ) 3 2 1 0 0 50 100 conversion (%>)

Fig. 15. Apparent film concentrations during the hydrogenation of phenol (1.10.10-2 M) in cyclohexane over 10% Pd/C (3.0 g.l_ 1) with film

loads of 0.80 ml.g"1. 298 K.

2 M K0H film: (■) phenol, (•) cyclohexanone and , - - - as calculated from a simple distribution model for phenol and cyclohexanone, respectively.

H„0 film: (D) phenol, (o) cyclohexanone.

The impact of the use of a catalyst modified with a liquid film becomes clear when the hydrogenations are performed in the corresponding bulk liquids. With water as solvent, at pH = 11 phenol is hydrogenated to cyclo-hexanol without appreciable cyclohexanone formation, whereas at pH = 7 a high cyclohexanone selectivity is observed (Fig. 16). This enormous difference in selectivity can be largely explained when the substrate concentrations at the catalyst surface are considered. At pH = 7 phenol shows a strong monolayer adsorption on the carbon support , which will result in a relatively high concentration of phenol in the subsequent solvent layers around the catalyst particle. At pH = 11 all phenol is present as potassium phenolate, which is preferably found in the aqueous phase because of the electrostatic repulsion between phenolate and the negatively charged carbon support. The adsorption of cyclohexanone on the carbon support on the other hand, will be influenced to a lesser extent upon variation of pH.

2 2

-concentration (•/•)

-2

Fig. 16. Hydrogenation of phenol (x; 8.5.10 M) into cyclohexanone (+) and cyclohexanol (o) over 10* Pd/C (2.0 g.1 ) in aqueous solutions at 298 K. A: pH = 7; B: pH = 11.

Conclusions

Coverage of a supported metal catalyst by a thin liquid film that is immiscible with the bulk solvent affects both rate and selectivity of the reaction. This is due to changes in the microenvironment of the metal crystallites by both film-support and substrate-support interactions as well as a change in solvation of the reacting substrate. However, alterations in the concentration pattern at the metal crystallites due to the distribution of reactants and products between film and bulk solvent, are the main cause for changes in the selectivity of a reaction.

Since the physical properties of thin adsorbed water films may deviate 22

considerably from those of the bulk solvent, the present explanations are tentative and further experiments are required to describe hydrogenation catalysts impregnated with a liquid film more adequately.

The potential importance of catalysts modified by a liquid film is demonstrated by the hydrogenation of phenol into cyclohexanone with Pd/C in either bulk alkaline aqueous solution or with catalysts with an alkaline film in combination with an apolar solvent. Whereas in bulk aqueous solution at pH = 11 phenol, in contrast to cyclohexanone, will be found preferen­ tially in the aqueous phase, the KOH film will consist, during the reaction, essentially of water solvated potassium phenolate thus generating a high

phenolate concentration in the vicinity of Pd. As a result the selectivity to cyclohexanone is raised from 5* in bulk KOH to 97% with the film system.

Acknowledgements

The experiments with substituted phenols were carried out by Mr. H. van Stralen. Dr. F. van Rantwijk is acknowledged for assistance with the HPLC analyses.

References

1. L. Cerveny and V. Ruzicka, Adv. Catal., 30, 335 (1981).

2. M. Freifelder, 'Practical Catalytic Hydrogenation', Wiley Interscience, N.Y., 1971.

3. R.L. Augustine, 'Catalytic Hydrogenation', Marcel Dekker, N.Y., 1965. 4. A.P.G. Kieboom and F. van Rantwijk, 'Hydrogenation and Hydrogenolysis in

Synthetic Organic Chemistry', Delft University Press, Delft, 1977. 5. P.N. Rylander, 'Catalytic Hydrogenation in Organic Syntheses', Academic

Press, N.Y., 1979.

6. P.G.J. Koopman, H.M.A. Buurmans, A.P.G. Kieboom and H. van Bekkum, Reel. Trav. Chim. Pays-Bas, 100, 156 (1981).

7. L.H. Slaugh and J.A. Leonard, J. Catal., 13, 385 (1969).

8. Y. Yamazaki, T. Kawai and S. Kimura, Sekiyu Gakkai Shi, 20, 481 (1977); Chem. Abstr., 88, 120713y (1978).

9. G.W.H.A. Mansveld, A.P.G. Kieboom, W.Th.M. de Groot and H. van Bekkum, Anal. Chem., 42, 813 (1970).

10. A. Katagiri, K. Watanabe and S. Yoshizawa, Bull. Chem. Soc. Jpn., 54, 5 (1981).

11. I.L. Mador, A.M. Rosan and R.K. Crissey, J. Catal., 87, 276 (1984). 12. J. Wristers, J. Chem. S o c , Chem. Commun., 575 (1977).

13. A.E. Feiring, J. Org. Chem., 42, 3255 (1977).

14. E.L. Mackor, A. Hofstra and J.H. van der Waals, Trans. Faraday S o c , 54, 186 (1958).

15. J.A. Don, Thesis, Delft University of Technology, 1981.

16. P.J. van der Steen and J.J.F. Scholten, Proceedings of the 8th International Congress on Catalysis, Vol. II, p. 659, Verlag Chemie, Weinheim, 1984.

-24-17. A. Koeberg-Telder, H.J.A. Lambrechts and H. Cerfontain, Reel. Trav. Chim. Pays-Bas, 102, 293 (1983).

18. J.H. Brewster, J. Am. Chem. S o c , 76, 6361 (1954).

19. P.E. Peterson and C. Casey, J. Org. Chem., 29, 2325 (1964). 20. M. Verzele, M. Acke and M. Anteunis, J. Chem. S o c , 5598 (1963). 21. H.E. Ungnade and D.V. Nightingale, J. Am. Chem. Soc, 66, 1218 (1944). 22. J. Clifford, 'Water, a Comprehensive Treatise', Vol. V, Chapter 2, 1.

CHAPTER 3

SOLVENT-HEACTANT-SUPPORT INTERACTIONS IN LIQUID PHASE HYDROGENATION II. ADDITIONAL INVESTIGATIONS ON SUPPORTED METAL CATALYSTS MODIFIED BY A LIQUID FILM

Abstract

The selectivity of the hydrogenation of 1-naphthol over Pd/C changes from mainly hydrogenation of the unsubstituted ring, towards highly selective hydrogenation of the phenolic ring by employing Pd/C modified with an aqueous KOH film. In contrast to the results previously obtained with Pd/C, the hydrogenation of phenol over Pt/C, modified with an aqueous KOH film, is not controlled by the increased concentration of phenol at the metal sur­ face, but rather by the large difference between the individual rates of hydrogenation of phenol and cyclohexanone. Differences in behaviour when silica and carbon with H20 films are compared as supports are demonstrated

and the influence of the pore size distribution is mentioned. Reaction order measurements reveal the role of the support in concentrating the reactants at the metal surface. The positive effect of a surfactant in hydrogenations employing two immiscible liquid phases is demonstrated. Points for future research are mentioned.

Introduction

In previous work we studied Pt and Pd hydrogenation catalysts modified by a liquid film, immiscible with the bulk solvent . It was shown that the film may increase both rate and selectivity of a hydrogenation reaction. The com­ plex behaviour of such catalysts led to a tentative explanation of their way of operation. Nevertheless, additional research is required in order to describe the observed phenomena adequately.

In this chapter some additional experiments with other catalysts or substrates are reported which enlarge our knowledge of the field of multi­ phase catalysis and aim to set the lines for future research.

-26-Experimental

Materials

4.5% Rh/C, 5* Pt/C and 1% Pt/SiO„ were prepared by impregnation to

incipient wetness of the support with a solution of RhCl„.3H„0 (Janssen) in acetone or H„PtClR.6H„0 (Drijfhout) in water respectively. The product was

dried in air at 393 K. The 10* Pd/C and 5* Ru/C were purchased from Drijfhout, Pd black from Strem Chemicals. All catalysts were activated in a stream of hydrogen for two hours at 473 K, except Ru/C, which was activated

2 -1 at 673 K. SiO„ was Grace silica gel 1952 (B.E.T. area 290 m .g ) , C was

2 - 1

Degussa Carbopuron 4N (B.E.T. area 770 m .g ) . Akyporox RLM 40, tetraoxyethylene lauryl ether, was obtained from Chem-Y. Benzene and

2

cyclohexene were purified as previously described . Cyclohexanone and 1-naphthol were distilled prior to use. Other chemicals were reagent grade.

Procedure

The preparation of film modified catalysts has been described previous­ ly . 1-Naphthol was hydrogenated in a thermostatted and well stirred autoclave, equipped with a sampling device, at 313 K and 13.1 MPa HL. The other hydrogenations were performed in the liquid phase (25 ml) in a thermostatted and well stirred vessel at 303 K and 101 kPa H„. Catalyst concentrations are expressed as grams of dry material per litre. The hydrogen consumption was monitored with an automatic hydrogenation

3

apparatus . Rates of hydrogenation are expressed as moles of hydrogen consumed per mole of catalyst metal per second. Products were analyzed by gas chromatography.

Results and discussion

Hydrogenation of 1-naphthol

Previous work showed, that the use of Pd/C modified with an aqueous KOH film, prevented further hydrogenation of cyclohexanone, which was formed in the liquid phase hydrogenation of phenol . This was attributed to selective concentration of phenolate anions at the catalyst metal surface.

4

The fragrance compound 5,6,7,8-tetrahydro-l-naphthol can be obtained by 5 catalytic hydrogenation of 1-naphthol over Rh catalysts with 70% yield (Scheme 1 ) . As 1-naphthol (pK = 9.3) is a stronger acid than 5,6,7,8-tetrahydro-l-naphthol (pK = 10.3), it was attempted to increase the

a

selectivity to this product by using Rh/C impregnated with aqueous KOH. It was expected that in this way further hydrogenation could be prevented because of selective concentration of 1-naphthol at the Rh surface. Although this did not happen, the observed increase in selectivity towards hydrogena­ tion of the phenolic ring is noteworthy (Table 1 ) . This selectivity shift was thereupon further investigated with Pd/C as the catalyst, which is well known for its tendency to hydrogenate phenols to the corresponding cycloalkanones . OH

h

H, Ha. _OH

CQO

-H» O OH ">/

Scheme 1. Hydrogenation scheme of 1-naphthol.

With 1-naphthol, the presence of an ÜJ3 film on Pd/C lowers the rate of

hydrogenation and when KOH is present, the rate is further decreased to an approximately constant value (Table 1). This behaviour is hard to explain on the basis of previous findings from the Pd/C,KOH catalyzed phenol hydrogen-ation .

Increasing the amount of KOH in the film raises the selectivity to hydrogenation of the substituted ring markedly (Table 1 ) . Furthermore, further hydrogenation of 1-tetralone to 1,2,3,4-tetrahydro-l-naphtol can be, at least at low conversions, largely prevented by the use of more concen­ trated KOH films.

-28-Table 1. The effect of aqueous KOH films on the rate and selectivity of the hydrogenation of 1-naphthol over Rh/C and Pd/Ca.

catalyst 4.5* Rh/C 10* Pd/C [KOH], film M c 2.0 c 0 0.5 1.0 2.0 3.0 5.0 initial 1 0 ~4 s 214 125 52 33 9.2 12 9.4 14 9.7 rate -1 conversion % 17 20 8 4 5 7 10 7 8 OH 90 78 73 65 38 26 22 8 5 selectivi

©5

i 3 13 20 31 42 69 75 75 tyb OH 4 7 11 12 27 29 7 17 19

a 313 K; 13.1 MPa H2; 5.5.10 2 M 1-naphthol in cyclohexane; catalyst 0.8

g.1- 1; film 0.80 ml.g_ 1.

Determined at conversion as mentioned. No film.

Effect of liquid films in the hydrogeaation of phenol over Pt/C as compared to Pd/C

Experiments, in which Pt/C was used as catalyst, show that the results obtained with the hydrogenation of phenol with Pd/C , cannot b e used to predict the behaviour of Pt/C modified with a liquid film.

A 96* H„SO. film increases the initial rate of phenol hydrogenation over Pd/C, whereas the rate of hydrogenation over Pt/C decreases (Fig. 1 ) . On the other hand, an H„0 film does not affect the reaction rate for both Pt/C and Pd/C (Fig. 2 ) .

As found earlier for benzene , an aqueous KOH film strongly inhibits the hydrogenation of phenol over Pt/C, whereas the rate of cyclohexanone hydrogenation increases after a sharp minimum at low KOH concentration in the film (Fig. 3 ) . Again these results are quite different from those found with Pd/C.

k(10"3s-1)

O 0 2 0 4 load (mlg-1)

Fig. 1. Effect of catalyst impregnation with 96* H9SO. on the hydrogenation -2

of phenol (10 M) in cyclohexane over 5* Pt/C (x) as compared to 10* Pd/C ( ) . Pt/C 3.0 g.1 1; 298 K; 101 kPa. Mio_J-s~')

,L

0

r

X i i i i 0 0 2 0 4 0 6 0 8 load (ml.g-1)

Fig. 2. Effect of catalyst impregnation with H_0 on the hydrogenation of -2

phenol (10 M ) in cyclohexane over 5* Pt/C (x) as compared to 10% Pd/C ( ) .

Pt/C 3.0 g . 1- 1; 298 K; 101 kPa.

Judging from the differences in relative reaction rates of cyclohexanone and phenol over Pt/C,KOH (Fig. 3 ) , the selectivity of the hydrogenation of phenol towards cyclohexanone is expected to be strongly dependent on the KOH concentration in the film. Indeed, the selectivity of the reaction could be adjusted to both lower and (slightly) higher values than found with Pt/C without film by choosing a proper KOH film concentration (Fig. 4 ) . Clearly,

the hydrogenation of phenol over Pt/C is mainly controlled by the individual rates of hydrogenation of phenol and cyclohexanone, whereas with Pd/C,K0H the substrate concentration in the film is thought to be decisive .

-30-In conclusion, the experiments described in this section show that the effect of an acidic or alkaline film depends on the catalytic metal involved. Therefore, for a more detailed discussion, fundamental mechanistic knowledge on phenol and cyclohexanone hydrogenation as well as metal-film interactions is required.

k(1CT3 s"1)

0 2 4 6 [KOHI(M)

Fig. 3. Effect of the concentration of aqueous KOH on the hydrogenation of phenol and cyclohexanone in cyclohexane over impregnated 5* Pt/C (0.80 ml KOH.g"1) as compared to 10% Pd/C.

Phenol (10_ 1 M ) : Pt/C (x); Pd/C ( )

Cyclohexanone (10_ 1 M ) : Pt/C (o); Pd/C ( )

Pt/C 3.0 g.l_ 1; 298 K; 101 kPa.

Comparison of silica- and carbon-supported platinum catalysts with an aqueous film in the hydrogenation of benzene and cyclohexene

Regarding the difference in polarity of SiO„ and C as support material, a different effect of a water film can be expected. Firstly, the relative wettability of the support and catalyst metal is altered. Secondly, the adsorption of the substrate on the support, which is of importance at low bulk-concentrations, will be affected.

When the behaviour of Pt/Si02,H„0 and Pt/C.H-O in the hydrogenation of

benzene and cyclohexene is compared, some differences become apparent. With benzene and Pt/SiO„,H„0 a minor decrease in rate is found, while with Pt/C,H„0 the rate remained constant after a rise at low loadings (Fig. 5 ) .

selectlvlty WJ 100

60 80 100 total conversion (•/.)

Fig. 4. Effect of different films on the selectivity towards cyclohexanone in the hydrogenation of phenol (10 M) in cyclohexane over 5* Pt/C catalysts (3.0 g . 1_ 1) .

Pt/C (■); Pt/C,H„0 (D); Pt/C,0.5 M KOH (x); Pt/C,2 M KOH (+) Film: 0.80 m l . g ; 298 K; 101 kPa.

With cyclohexene and Pt/SiO„,H„0 a maximum in rate at low loadings, as was found with Pt/C.HgO, is absent (Fig. 6 ) . Although at present we are not able to explain these results, the fact, that with both benzene and cyclohexene the differences are located in the region of low H„0 loadings, is remarkable. Mi<r3-s-1) 4 0 20 O - • / / i i i i i i D 0 2 0-4 0 6 0 8 1 0 12 load (mlg"1)

Fig. 5. Comparison of 1* Pt/SiOg.HgO (x) and 5* Pt/C,H20 ( ) in the

hydrogenation of benzene .

-32-Fig. 6. Comparison of 1% Pt/SiO-.H-O (x) and 5% Pt/C.H-O ( ) in the

hydrogenation of cyclohexene (10 M) in cyclohexane.

The pore size distribution of the SiO„ and C supports, which shows marked differences (Figs. 7 and 8), is noteworthy. For SiO„ 32 c.q. 62* of the pore radius is found in the range 3-5.5 and 5.5-120 run respectively, whereas the porevolume of C is for 84* attributed to pores with radii between 500 and 1500 run. These results show, that especially at higher film loadings, part of the Pt crystallites will not be covered by a thin liquid film but is immersed in a pool of H„0.

Further work on this topic should therefore focus on (i) influence of the pore size

7 (ii) location and size of the metal crystallites on the support (iii) influence of the polarity (wettability) of the support.

oarison of reaction order in cyclohexanone using Pd and Pd/C with different film and solvent systems

In the discussion of solvent-reactant-support interactions, the importance of adsorption of reactants on the support, thereby creating a high effective concentration at the metal crystallites, was repeatedly

1 2 emphasized ' .

Such effects should also become apparent from the order of the hydrogena­ tion reaction with respect to the organic reactant. Therefore reaction rates and orders for the cyclohexanone hydrogenation have been determined as a

r e l a t i v e volume ("/o) 2 0 1 15 -10 5 -10

mfmhr

rHTTTflrTfirMTTTfh

Y

100 1 0 0 0 1 0 0 0 0 p o r e r a d i u s ( n m )

Fig. 7. Pore s i z e d i s t r i b u t i o n of SiO„ as determined by Hg-porosimetry.

r e l o t i v e v o l u m e (%) 2 0 r 15 10 rTTTÏÏÏTflrfn n_n_ru.rurrfl

mini

10 1 0 0 1 0 0 0 1 0 0 0 0 p o r e radius ( n m )

-34-function of both pH of the bulk aqueous phase and the aqueous film coverage of the catalyst in an apolar bulk solvent.

The results (Table 2 ) , obtained under conditions in which the active metal surface area of Pd/C, as judged from the absolute rates of hydrogenation, exceeded that of unsupported Pd, show that in aqueous bulk

solvent the presence of the carbon support dramatically decreases the

reaction order, which again points to an increase in cyclohexanone concentration at Pd due to adsorption on the support. An increase in pH apparently causes either some "salting out" of cyclohexanone onto Pd or conversion of cyclohexanone to a stronger adsorbing species (e.g. ionized hydrate form).

The fact that an H_0 film on Pd/C does not increase the reaction order, as might be expected from the results with Pd in H„0 solvent and the distribution coefficient of cyclohexanone is in agreement with adsorption of cyclohexanone onto carbon through the H„0 film. This causes a film con-centration which exceeds that calculated from the distribution coefficient . The results obtained with Pd/C,2 M KOH might be related to competition of enolate anions and OH for the metal surface.

In general, at sufficiently low substrate concentration, adsorption on the support creates a higher effective concentration at the metal crystal-Table 2. The reaction order of cyclohexanone in the hydrogenation over Pd

catalysts . catalyst

Pd

Pd/C

Pd

Pd/C solvent

H

2

0

H

2

0

C6H12 C6H12

pH

7.0

11.9

6.7

9.4

11.2

—

—

—

—

filmb

—

—

—

—

—

—

—

HgO

2M KOH activity, s 1.2-7.2.10"5 1.1-3.5.10~5 6.3.10~3 4.5-5.4.10~3 2.3.10"3 5.2.10-5 1.0.10~3 1.0.10~3 1.3-2.7.10~3 order

1.0

0.74-0.52

0

0.09-0

0

0

0

0

0.40 a 298 K; 101 kPa H2; cyclohexanone 0.1-0.6 M; Pd 6.0 g.1 1; Pd/C 3.0 g.1 1. b 0.80 rnl.g"1.

lites, provided that ad- and desorption onto/from the support are fast with respect to the hydrogenation reaction (Fig. 9 ).

Fig. 9. Schematic representation of substrate concentration gradient at unsupported (A) and supported (B) Pd in the case of rapid ad- and desorption of a substrate, which is strongly adsorbed on the support.

Expansion of the liquid film in the presence of a surfactant

Our previous work shows that addition of a second solvent phase (as a film) offers interesting possibilities. It seemed worthwhile to investigate if expansion of the "film" to a catalyst/water emulsion in an organic bulk solvent was possible and if stabilisation of such a system by addition of a surfactant was feasible.

The competitive hydrogenation of cyclohexene and cyclohexanone over Ru/C was chosen as test reaction. Akyporox HLM 40, tetraoxyethylene lauryl ether, was employed as surfactant.

In decane as the apolar solvent, cyclohexene is hydrogenated preferably over cyclohexanone (Fig. 10A). Impregnation of the catalyst by H„0 alters the selectivity due to better solubility of cyclohexanone in H„0, but cyclohexene is still preferably hydrogenated (Fig. 10B). Adding H„0 as a cosolvent results in an initially higher selectivity for cyclohexanone than cyclohexene as long as Ru/C is found in the H„0 phase, to which it was originally added. As time proceeds, the catalyst is stirred out of the H„0 phase and enters the organic phase as a water impregnated catalyst and the selectivity is inversed, which results in a propellor shaped hydrogenation course (Fig. IOC). In the presence of Akyporox an emulsion is formed,

-36-provided that stirring is continued, and cyclohexene and cyclohexanone are hydrogenated at almost equal rates (Fig. 10D).

Thus, these preliminary experiments show that the effect of a water film on the selectivity of the catalyst can be enhanced by designing a system that comprises two bulk solvent phases and a surfactant.

conversion (•/•) OÜ 8 0 60 4 0 20 0

r -^

x

'—7

-

A

/' 7

- / /

- / /

£ - - l 1 1 1 1 20 40 60 SO 100 20 40 60 80 100 total conversion (•/.)

Fig. 10. Competitive hydrogenation of a 1:1 mixture of cyclohexene (x) and

-2 -1 cyclohexanone (•) (5.0 .10 M) over 5* Ru/C (4.0 g.1 ) in decane

(25.0 m l ) . 298K; 101 kPa.

A. no additives. B. HgO, 0.92 m l . g- 1 Ru/C. C. HgO, 5.0 m l . g- 1 Ru/C.

D. H-O, 5.0 ml.g" Ru/C; Akyporox 40 g.1 decane.

Acknowledgements

The experiments on 1-naphthol hydrogenation were carried out by Mr. W.T. Loos with technical assistance of Mr. E. Wurtz. Hg-porosimetry was performed by Mr. N. van Westen from the Laboratory of Chemical Technology.

References

1. A.A. Wismeijer, A.P.G. Kieboom and H.van Bekkum, Reel. Trav. Chin. Pays-Bas, 105, 129 (1986); this thesis, Chapter 2.

2. P.G.J. Koopman, H.M.A. Buurmans, A.P.G. Kieboom and H. van Bekkum, Reel. Trav. Chim. Pays-Bas, 100, 156 (1981).

3. G.W.H.A. Mansveld, A.P.G. Kieboom, W.Th.M. de Groot and H. van Bekkum, Anal. Chem., 42, 813 (1970).

4. Dutch Patent 8400657 (1985); Chem. Abstr. 103, 200713s (1985). 5. M. Freifelder and G.R. Stone, J. Pharm. Sci., 53, 1134 (1964).

6. P.N. Rylander, 'Catalytic Hydrogenation over Platinum Metals', Academic Press, London, 1967, p. 337.

7. Work by H.E. van Dam from our laboratory indicates, that for Pt/C the Pt crystallites are stuck in the micropores of the carbon and directly adjacent to the macropores. For SiO„ a homogeneous distribution is expected.

-39-CHAPTER 4

ONE-STEP OXIDATION OF BENZENE TO HYDROQUINONE IN THE PRESENCE OF COPPER(I)CHXORIDE

Abstract

Aerial oxidation of benzene in a mixture of copper(I)chloride, sulfuric acid and acetonitrile-water yields phenol and through a direct route hydro-quinone under ambient conditions. The maximum hydroxylation efficiency is 42* at pH = 5.

Introduction

The single step synthesis of phenol from benzene has been a chemist's challenge for many years and undoubtedly will remain so in the near future. Numerous methods and widely spread conditions have been investigated . Of these, hydroxylation by Fenton's reagent, which uses H „ 09 as the oxidant and

2 3

proceeds at ambient temperature, is well known ' . A disadvantage of the method is the production of biphenyl from the intermediate

hydroxycyclohexa-2+ dienyl radicals, but this can be overcome by the addition of Cu , which

4 rapidly oxidizes the intermediate .

From an economic point of view, hydroxylation of aromatics mediated by metal salts with 0„ as the oxidant at ambient temperature is of interest. Nofre et al. oxidized benzoic acid in the presence of Fe(II), Cu(I) and

5 Sn(II) salts and studied the influence of some complexing agents . Cu(I) proved to be inactive at 310 K in the absence of any coordinating compounds. Benzene, toluene and naphthalene could be hydroxylated in a solution of copper(I) acetate in acetic acid under 200 kPa 0_ . Dearden et al. reported that toluene and anisole were hydroxylated in water in the presence of salts

7

of Fe(II), Ti(III), Cu(I) or Sn(II) . From the absence of dimeric products and from the observed isomer distributions it was concluded that not the

2+

hydroxyl radical but rather some metal-oxygen complex, e.g. FeO_ , was the active species. A similar conclusion was drawn by Ullrich et al. from their