Furan hydrogenation over Palladium catalysts: Deactivation and regeneration

Catalyst Deactivation 1999 B. Dehnon and G.F. Froment (Editors) 9 1999 Elsevier Science B.V. All rights reserved.

453

F u r a n h y d r o g e n a t i o n over P a l l a d i u m catalysts: D e a c t i v a t i o n and regeneration.

S. D. Jackson, I. J. Huntingdon, N. A. Hussain, and S. R. Watson. Synetix, R T & E Group, P O Box 1, Belasis Ave., Billingham, Cleveland TS23 1LB, U.K.

Abstract Palladium catalysts used for furan hydrogenation were shown to deactivate due to carbon deposition. The deposit, which was unique to the given catalyst, was characterised by TPO, TPR, and FTIR. Although the catalyst could be regenerated in either dioxygen or dihydrogen, the initial catalytic behaviour could not be regained. 1. INTRODUCTION Furan hydrogenation can be performed over most of the group VIII metals. However the literature on the hydrogenation of furan is not extensive. Therefore we set out to investigate the fundamentals of furan hydrogenation over palladium catalysts. From the limited information in the literature [1 ] it appeared that there may be a problem of catalyst deactivation as unsupported palladium deactivated rapidly. Given this problem we decided to study the deactivation of the system using a pulse-flow reactor and to examine the surface deposit by infra-red spectroscopy, isotope exchange, and temperature programmed oxidation and reduction (TPO/TPR). However we have found that the carbonaceous deposit is fundamental to the catalysis and its nature, form, and reactivity are defined by the catalyst. 2. EXPERIMENTAL Pulsed reaction studies were performed in a dynamic mode using a pulse-flow microreactor system with on-line GC-MS. Using this system the catalysts could be reduced in situ. Following reduction, furan was admitted by injecting pulses of known size into the dihydrogen/dinitrogen carrier-gas stream, with resulting vaporisation, and hence to the catalyst. Typical reaction conditions were 0.101 MPa and 373 K. After passage through the catalyst bed the total contents of the pulse were analysed by GC-MS. TPO was performed after the reaction testing by heating the sample in 2% dioxygen/helium. Infra-red spectra were obtained using a commercial FTIR spectrometer (Nicolet 5DXC). The studies were performed in transmission mode, with the catalyst in the form of a pressed disc, and using an environmental cell the catalysts could be reduced in situ; furan admitted with or without dihydrogen. The palladium catalysts used in this study were prepared by impregnation and had a nominal weight loading of 1%. Three supports were used, alumina, silica, and zirconia.

454

3. R E S U L T S Under reaction conditions all three catalysts showed considerable carbon laydown. Initially this resulted in an increase in activity, however as time on line increased and the amount of retained material increased activity began to decrease (Table 1). Table 1. Activity of palladium/alumina at 373 K and a GHSV of 9000 h "~. Catalyst

Pulse No.

% Conversion

Rate (gmol.g~.s-~) a

Pd/alumina

1

94

0

2

82

0

3

75

14.82

4

62

51.35

5

71

46.14

10

68

27.13

a) Rate of formation of THF.

FTIR analysis of the carbonaceous deposit revealed that the deposit was unique to each catalyst and was made up of THF, non-cyclic aldehydes and ketones. Table 2. Infra-red analysis of surface residue (cm~). i

,

,,

....

JP~zirconia i Pal/alumina

Furan

I

3142w

i i

THF

.

.

.

.

.

i

3000mw

2985s 2966s

i

2945mw 2925mw

,

2950vw

3125mw 2992s 3068mw 2 9 4 9 m s.... 2987ms , ....

!

,

Butyraldeh.a

i

, 2875mw

i

i

2889vw

't

I

2914ms i

--

2956ms

.....

,

287ims

|

.

2900m

,

2800w i /

i I

'

1738w

1863mw 1750m

.

.

.

.

I

Ir

'

1714m

i742w

1735S

t

1769ms

,

'

1688mw'

1688m

1558W ' 1461w

1712s

I

,.I

1461row

|

1361w 1250w 1187w

1578ms 1478ms

1464mw

l~164ms

1364mw

135;/ms

,I

1370w

1371m " 1243mw ' l194vw i l193ms

l l'85mw

I164m

I175w .....

i 121ms

1150m

"'

1281mw

i

I

. . . . . . .

i

1086w

1475mw

I

I

i

1124w ,

, . . 'i

!

, ,

,

,

|

1085s

a) DEK, diethyl ketone; Butyraldeh., butyraldehyde.

. . . . .

i088mw . . . . . .

455 Differences in the catalysts' behaviour to treatment with deuterium were also observed by FTIR, reinforcing the different nature of the deposits on the catalysts [2]. TPO of the catalysts after use confirmed the different reactivities of the carbonaceous deposits. TPO of the Pd/silica catalyst (Figure) gave rise to evolution of CO and CO2 in a single event centred at 423 K, whereas with the Pd/zirconia CO2 was evolved in a number of events between 423 K and 673 K.

f

TPO of Carbonaceous Deposit Catalyst' Pdlsilica 1.2 1

c

I

-

/ ~ Carbon dioxide

= "~ 0.8

~,,,, < T;~o.6 9

Carbon Mono "

~o.4 & -

I i

W

i~

0.2 ,~

200

300

:

,.

400

1

500

......

~

I

600'

700

__

_

800

Temperature (K)

After testing, the catalysts could be regenerated by either a f treatment in dioxygen FURAN H Y D R O G E N A T I O N or in dihydrogen, CATALYST: PdlZlRCONIA, 373 K. however the first I1600~ reaction/regeneration z 1400 FRESH cycle was found to be ~" 1200 different from 1ooo subsequent ones AFTER 0 2 800 (Figure).

~

--II----

~

4. DISCUSSION

400

AFTER H2

200

-----4,-----

0

0 500 1000 1500 2000 2500 All catalysts were O CUMMULATIVE FURAN INJECTED active and selective for (Times 1E17 ) the hydrogenation of furan to THF once a carbonaceous deposit was present on the surface. Indeed a fresh Pd/alumina had no intrinsic activity for furan hydrogenation, indicating the catalytic site was generated in situ and involved the carbonaceous deposit. Similar results were obtained from the silica and zirconia supported samples but the amount of carbon deposited varied considerably. As well as the amount varying, the TPO and FTIR results revealed that the nature and reactivity of the carbonaceous deposit were a function of the catalyst. It was also clear that the initial carbon deposit resulted in a change to the catalyst that could not be reversed by an oxygen regeneration or a

456 hydrogen regeneration, whereas subsequently it was possible to regenerate the catalysts back to an identical point. It is worth noting that the TPO obtained at the end of the first cycle of reaction was identical to that found after the third, indicating that the carbonaceous deposit that is removable is consistent between cycles. However the TPO and TPR do reveal differences in the carbonaceous deposit between catalysts in terms of reactivity. The nature of carbonaceous deposit can be ascertained from the FTIR data. At least three surface residues could be identified, adsorbed THF, a non-cyclic ketonic/aldehydic species, and an alkane species, however not all species were present on all catalysts. Gas phase bands were unambiguously assigned to furan and THF. Analysis of the deposit on the Pd/zirconia suggested that the main species was adsorbed THF with a non-cyclic ketonic/aldehydic species also being present. The deposit on the Pd/alumina catalyst was significantly different from that on the Pd/zirconia, with changes in band shapes and the absence some C-H and C-O bands, but the most telling difference was observed when deuterium was added to the system. When deuterium is added to a catalyst that has a hydrocarbonaceous residue it is usual for there to be exchange [3], as long as there are free metal sites where the dihydrogen can adsorb/dissociate. The exchange between the support hydroxyls and deuterium confirm that this process is occurring on the Pd/alumina. Therefore it is likely that the initial contact between the surface and the residue on the Pd/alumina is through at least one quaternary carbon that cannot take up hydrogen. The formation of aldehydic/ketonic species was not unexpected as it is known that in some hydrogenation systems butanol has been detected as a minor product [4], while under more forcing conditions [5] the furan molecule will fracture to carbon monoxide and C-3. However it would appear that a ring-opening has taken place rather than a complete fracture as adsorbed carbon monoxide was not detected. 5. REFERENCES 1. G. Godawa, A. Gastet, P. Kalck, and Y. Maire, J. Molec. Catal., 34, 199 (1986); SU Patent 417150, assigned to T. M. Beloslyudova (1974). 2. S.D. Jackson, I. J. Huntingdon, and N. A. Hussain, Catalysis of Organic Reactions, Frank E. Herkes (ed.), p.559, Marcel Dekker, New York., 1998. 3.

e.g.S.D. Jackson and N. J. Casey, Jr. Chem. Soc. Faraday Trans. 1, 91, 3269 (1995).

4.

SU Patent 438648, assigned to D. Z. Zavelskii (1975).

5.

K.C. Pratt and V. Christoverson, Fuel Processing Tech., 8, 43 (1983).