/ APPLIED CATALYSIS

A: GENERAL ELSEVIER Applied Catalysis A: General 165 (1997) 147-157

Hydrogenation of 1,3-butadiene on Pd/SiO2 in the presence of H2S Deactivation and reactivation of the catalyst

J.C. Rodrfguez, J. Santamarfa, A. Monz6n*

Department of Chemical and Environmental Engineering, Faculty of Science, University of Zaragoza, 50009 Zaragoza, Spain

Received 21 March 1997; received in revised form 26 May 1997: accepted 27 May 1997

Abstract

The deactivation of a Pd/SiO2 catalyst by H2S during the hydrogenation of 1,3-butadiene and its subsequent reactivation have been studied. The poisoning effect of H2S on Pd catalysts during C4H 6 hydrogenation strongly decreased when the temperature of reaction was increased from 373 K to 508 K, due to the competition of butadiene for HzS chemisorption sites. The presence of HzS on the Pd sites causes an important decrease in the selectivity of the catalyst for a given conversion level. On the other hand, the irreversible adsorption of butadiene on Pd gives rise to an additional deactivation, and also to an increase in the observed selectivity. Reactivation of pre-poisoned catalysts can be performed at temperatures starting from around 400 K. The rate of sulfur desorption leading to catalyst reactivation strongly depends on the reaction atmosphere during the reactivation period, as well as on the temperature. During the reactivation stage, a high selectivity to butenes was observed, in spite of the presence of sulfur. © 1997 Elsevier Science B.V.

Keywords: Palladium catalyst; 1,3-butadiene hydrogenation; HzS poisoning; reactivation

1. I n t r o d u c t i o n butenes when sulfur compounds were added into the 1,3-butadiene feed stream. On the other hand, Boi-

Poisoning by sulfur-containing compounds is one tiaux, et al. [4] and Borgna et al. [5,6] found the of the most serious deactivation problems associated opposite effect regarding the selectivity of Ni catalysts with the use of metallic catalysts. Usually a few ppm for the same reaction when nitrogen-containing com- of this kind of compounds are sufficient to cause pounds were added in the feed stream: the electron complete deactivation of the catalyst [1,2]. Besides, donating character of these molecules favoured a their electron attracting character very often leads to charge transfer to the metallic particles and therefore dramatic modifications in the levels of selectivity [3]. increased the selectivity to the semihydrogenated Thus, Boitiaux et al. [4] reported a decrease of the compounds. selectivity of Pt and Rh catalysts for the production of There are abundant studies that illustrate the influ-

ence of sulfur on the catalytic selectivity for different reacting systems [3,7-10]. Thus, although hydrogena-

*Corresponding author. Tel.:+34 976 76 11 57; fax:+34 976 76 tion reactions are commonly considered as structure- 21 42: E-mail: amonzon@posta.unizar.es, insensitive reactions, there are exceptions, such as the

0926-860X/97/$17.00 ~.C~ 1997 Elsevier Science B.V. All rights reserved. PII S 0 9 2 6 - 8 6 0 X ( 9 7 ) 0 0 1 97-X

148 J.C. Rodr[guez et al./Applied Catalysis A: General 165 (1997) 147-157

work of Pradier and Berthier [ 1 l], who found structure reactivation periods upon the catalyst activity and sensitivity effects when studying butadiene hydroge- selectivity. nation on different Pt planes. Further, it is well docu- mented that other steps of the reaction (e.g. adsorption of the reactants on the active sites) or secondary 2. Experimental reactions (e.g. coking) exhibit a significant depen- dence on the particle size of the metal crystallites The catalyst samples were prepared by wet impreg- [8]. Thus, the phenomenon of 'multicomplexation' nation. A solution of PdC12 (Aldrich Chem. Co.) in was proposed by Boitiaux et al. [8] as responsible of HC1 was added over commercial SiO2 (Kieselgel, the low activity of the smallest metallic particles. On Riedel-de HaEn) and stirred at 313 K for 24 h, to these particles, thestrengthofchemisorptionofhydro- obtain a catalyst with a nominal Pd content of carbon molecules (e.g. butyne, butadiene, etc...) 2 wt%. This solid was dried and calcined in air at would be such that more than one reactant molecule 383 K for 12 h and at 723 K for 4 h, respectively. The could be bonded to one metallic atom, giving rise to fraction within a particle size between 125 and 250 surface coordinated compounds, which were inactive microns was used in the reaction tests. for the main reaction. This phenomenon also explains The reaction system consisted of a tubular, 0.8 cm the frequently observed negative kinetic orders in i.d. quartz reactor containing approximately 50 mg of these reactants. Also, sulfur modifies the activity of catalyst, with automatic control of flow (Brooks catalysts for coke formation [7,9,10]. Usually, these Instruments) and temperature. Product analysis demanding reactions are affected by sulfur poisoning (unreacted butadiene, butane, 1-butene, 2-trans- in a greater extent than the main reaction, which also butene and 2-cis-butene), was carried out using on- leads to important variations in the selectivity. There- line gas chromatography (Hewlett Packard 5890). fore, in spite of the inherent structure-insensitivity of Prior to the reaction tests, catalyst samples were the main chemical reaction, the overall reaction pro- reduced in situ at 673 K for 3 h, using a 50% H2/N2 cess could, at least in some instances, be considered as mixture with a total flow rate equal to 600 ml min- J. structure-sensitive [8]. After reduction, the catalyst temperature was brought

The competitive adsorption of butadiene and H2S down to the reaction temperature in a purified N2 over noble metals has been previously studied among stream. others by Oudar et al. [ 12]. These authors also dealt Reaction tests were performed at atmospheric pres- with the possibility of restoring the activity of poi- sure and temperatures ranging from 373 K to 508 K, soned catalysts by removing sulfur adatoms using using ca. 650 ml min-I feed stream of the following displacement by the reactant molecules. Hoyos composition: H2/C4H6=98/2. Additionally, in those et al. [13,14] studied the feasibility of decontamina- experiments where poisoning took place simulta- tion/regeneration of Pd-A1203 and Pd-A1203-SiO2 neously with the main reaction (simultaneous poison- catalysts poisoned by H2S or thiophene during cyclo- ing), a certified gas mixture containing 246.1 ppm of hexane dehydrogenation, using sulfur-free feeds and H2S in N2 was added to the reaction stream in the hydrogen treatments, respectively. Ffgoli et al. [15] appropriate ratio to obtain the desired H2S concentra- have investigated the regeneration of poisoned Ni/ tion. This addition amounted to an increase in the total SiO2 catalysts used in the styrene hydrogenation, flowrate of less than 2%. Alternatively, the H2S/N2 These authors concluded that unsaturated molecules, certified mixture could be added to a pure N2 feed- such as 2-butyne, are much more effective than hydro- stream to obtain catalyst poisoning under inert atmos- gen in the removal of sulfur atoms form the metallic phere (pre-poisoning). In these cases the pre- surface, poisoning time was kept at 30 min, with a total

In this work, the poisoning of a Pd catalyst and its flowrate of 50 ml min- 1 of a N2 stream containing subsequent regeneration has been examined, using the 246 ppm of H2S. In both types of poisoning experi- hydrogenation of 1,3-butadiene as a test reaction. The ments, reactivation of the catalyst samples was study is focused on the influence of the temperature attempted by shutting off the H2S supply, and con- and the reaction atmosphere during the poisoning and tinuing (or starting, in the case of a pre-poisoned

J.C. Rodrfguez et al. /Applied Catalysis A: General 165 (1997) 147-157 149

catalyst) the reaction under the conditions of tempera- diene of 0.02 atm. Under these conditions the conver- ture and feed composition already described, sion at the exit of the reactor was less than 20% at the

highest temperature used. The values of the reaction orders for H2 and buta-

3. Results diene, and of the activation energy showed in Table 1 are similar to those previously reported for this reac-

3.1. Kinetics o f the main reaction tion. Thus, low values of activation energies and a linear dependence of the rate of reaction upon the H2

The kinetic equations proposed for structure-insen- partial pressure are typical of these processes [ 16,17]. sitive hydrogenation reactions are usually character- On the other hand, although not in all cases, negative ized by a zero order regarding the hydrocarbon orders with respect to hydrocarbon have also been also concentration, a positive order for hydrogen (often observed. By way of example, Boitiaux et al. 18] close to one) and low values of the activation energy proposedaLHHWkineticequationforbutadienehydro- (e.g. 40 kJ mol ~) [16-18]. The weak or null depen- genation, in which the main contribution to the adsorp- dence of the reaction rate on the hydrocarbon partial tion term in the denominator of the equation, is the pressure results from a permanent coverage of the irreversible adsorption of the reactant molecules on metallic surface by less saturated hydrocarbon mole- the metallic sites. Due to this self-poisoning mechan- cules [19,20]. ism the main reaction rate was lbund to be inversely

In order to compare the behaviour of our catalyst in proportional to the hydrocarbon partial pressure. the absence and in the presence of H2S in the feed, the Some additional experiments were carried out using activation energy and the partial kinetic orders for the sulfur-free feeds, to study the variation of conversion/ species taking part in the main reaction (hydrogena- selectivity with temperature. The results (not shown), tion of 1,3-butadiene) were determined. The values of indicated a nearly four-fold increase in conversion the reaction orders with respect to H2 and butadiene (11.4-41.3%), when the temperature was raised from (Table 1) were obtained at a temperature equal to 333 to 379 K, while only a limited decrease in the 298 K, working with variable proportions of H2 or selectivity to butenes (80.5-74.1%) was observed in butadiene in a S-free feed stream. The partial pressures the same temperature interval. As will be shown later, ranged from 0.48 atm to 0.96 atm for H 2 and 0.015- the introduction of sulfur has a strong influence in the 0.04 atm for butadiene respectively, with the balance selectivity/conversion behaviour of the catalyst. being N2 in all cases. In these experiments the catalyst mass was 9.7 mg and the flow rate of 650 ml rain 1. 3.2. Deactivation

These conditions gave reactant conversions below 10%, which made possible a differential reactor and- Fig. l a shows typical conversion versus time curves lysis. The value of the activation energy was obtained for different concentrations of poison in the teed. using experiments at temperatures between 333 K and Since the objective of this work was to study the 379 K, with a total flow rate of 185 ml min 1, a mass effect of sulfur poisoning, a high H2 to butadiene ratio of catalyst of 3.1 rag, and a partial pressure of buta- was used in the feed, in order to suppress coke

formation. The result can be observed in the curve corresponding to the sulfur-free feed, where a stable

Table I catalyst performance was maintained throughout the Kinetic parameters of the main reaction (hydrogenation of 1,3- butadiene). /',,,=453 K experiment: in the absence of H2S, the conversion of

1,3-butadiene at the exit of the reactor remains con- - r('~H,, = k(Pc4H~. )" (Pn,) - stant at a value close to 100%. The suppression of coke k= kT,,exp[-~ (~r ~ ) ] formation was also observed in separate experiments a -0.585+0.08 using a thermobalance unit. Under the reaction con- 13 1.175-t=0.12 ditions employed, no weight gain could be observed kr,,, 0.2184±0.006 bar ~°5 s l when the catalyst was exposed to the reaction mixture E~ 28.3±1.7 kJ tool i

at temperatures between 423 and 473 K.

150 J.C. Rodrlguez et al./Applied Catalysis A: General 165 (1997) 147-157

1 . 0 , ~ , ,× × x . . . . . . × × × 1 . 0 ~ - ~ . . - , . . , , , . , , , - - . , . , , . ,

H2S C o n c e n t r a t i o n ~ • 0 . 8 • " × 0 p p m 0 . 8 1 T e m p e r a t u r e

273 K • " ,, l p p m ~ • • 4 7 3 K

0 . 6 - * • 3 ppm ~ 0.61 - 488 K

0 . 4 • • o 12 p p m 0 . 4 1

o • ,, ° 39 p p m ~ ~ • •

' o 0 . 2 o . * , o 0 . 2 1 ~ . " •

o • v e t • ~ ~ • • •

0.0 ° ° °°o°. ~, o. o , • 0.01 ~ ~ ~

T i m e x 1 0 3 ( s ) T i m e x 1 0 3 ( s )

1 . 0

H 2 S Concen t ra t i on

0 . 8 * 1 ppm c • 3 ppm d)

• 6 ppm 23

"Q 0 . 6 ~ o 12 ppm ...o,o ~ ~ 24 p p m

._> 0 . 4 o 39 p p m

(3) o o •

0 . 2 o , v o •

0.0 i i a , n , , ,

o . 0 ' 0 . 2 ' o . 4 0.6 0.8 1.0 C o n v e r s i o n ( % )

Fig. 1. (Top left) Influence of the gas phase HeS concentration on catalyst deactivation (T-373 K) (Top right) Influence of the operating temperature on catalyst deactivation (6 ppm of H2S in the feed). (Bottom) Selectivity-conversion plot for catalysts deactivated at 373 K using different sulfur concentrations in the reaction atmosphere. (Flowrate: 650 ml min -~, catalyst weight: 50 mg, pressure: 1 atm).

Catalyst deactivation becomes evident as the poison depending on the operating conditions, the long term concentration in the feed is increased, with a very catalyst activity can be different from zero, in spite of rapid deactivation for 39 ppm of H2S in the feed. On the continued presence of H2S in the feed. The varia- the other hand, the results shown in Fig. lb clearly tion of the residual activity with temperature at a demonstrate that the poisoning effect of sulfur can be constant poison level in the feed (6 ppm of H2S), is considerably mitigated by working at higher tempera- shown in Table 2. It can be observed that within the tures. Thus, while 6 ppm of H2S in the feed would range of temperatures studied the residual activity of cause almost complete deactivation of the catalyst in approximately 80 min when working at 373 K, when the temperature was increased to 488 K, a significant Table 2

Influence of the reaction temperature on the residual activity of the level of activity (>20%) was retained after this time on catalyst.

stream, while the catalyst was still highly active (97% conversion) at a reaction temperature of 508 K. Temperature (K) Residual conversion

It is also interesting to note that for temperatures 373 0.025 above 473 K, the activity of the catalyst drops to a 433 0.025 certain point, and then seems to be stabilized, at an 473 0.056 approximately constant conversion level. This indi- 488 0.220

508 0.975 cates the existence of a residual activity level, i.e.,

J.C. Rodffguez et al./Applied Catalysis A: General 165 (1997) 147-157 151

the catalyst increases with the operating temperature. 3.3. Reactivation Taking into account the kinetics given in Table 1, the increase of residual activity is considerably higher Fig. 2 shows the conversion-time and temperature- than what could be expected from the temperature time sequences on a catalyst sample through different increase alone, at least on a sulfur-free catalyst. The stages: reaction without sulfur in the feed, deactivation results then suggest that the additional increase is due with H2S addition (39 ppm) under reaction atmos- to a lower sulfur coverage at higher temperatures, phere (simultaneous poisoning), reactivation under which indicates the reversibility of the adsorption pur i f ied N 2 at 463 K, and reactivation using a sulfur- equilibrium of HzS on the Pd atoms, free feed mixture (Hz/C4H6=98/2). Under the condi-

Finally, Fig. lc shows the selectivity-conversion tions used, in the absence of H2S (time interval a), the plot for deactivation runs (simultaneous poisoning), catalyst maintains its activity, then sulfur injection at a temperature of 373 K, when there is no residual (time interval b) leads to the complete deactivation of activity. It can be seen that all the data corresponding the catalyst. During time interval c, the reaction feed to different poison concentrations in the feed stream was switched off and the catalyst temperature was can be fit by the same selectivity-conversion curve, increased to 463 K under N 2. As can be seen in Fig. 2, Note that time is not present in Fig. lc, i.e., while at a this was not sufficient to reactivate the catalyst, and given conversion the selectivities are approximately when the reaction was restarted (time interval d), a the same, the time to reach a certain conversion (and very low activity was observed. A reactivation treat- selectivity) is considerably different for different H2S ment under HzS-free reaction atmosphere (time inter- partial pressures, val e) was more efficient. Thus, when the catalyst was

-a-:: -b- -c- ::-d-i -e- _f_ 480 1.0 o ~

I" 460 0.8

440 v E O ~-

"~ 0.6 = a) 420

. l # ° ° o o 0.4 400 E

0.2 ~ ~ , 3801-

360 , a / / / i t I ; , I i I

0.0 0.5 1.0 1.5 2.0 10 15 20 Time x 10 3 (s)

Fig. 2. Influence of the reaction atmosphere upon catalyst reactivation. (a) reaction in the absence of H2S, (b) catalyst poisoning (39 ppm of H2S), (c) treatment in N2 at 463 K, (d) restarting of the reaction in the absence of HzS, (e) increase of the reaction temperature (reactivation during reaction), (f) decrease of the temperature of reaction to the reference level of 373 K.

152 J.C. Rodr[guez et al./Applied Catalysis A." General 165 (1997) 147-157

i , i i i , i i i I

1.0 - A - • o - B - 1 .0

0 0

• 453 K O

0.8 0 .8 • 463 K 473 K • 443 K

( - o x •

,o 0.6 • 453 K 0.6 ~- O

> o 433 K o 443 K > ¢ - x x O o ¢ -

O 0 0 .4 , , x 0 .4 0 x × ° 4 3 8 K

O X • • OX

X • O • • O X •

0.2 o × • • o×× , 0 .2 • 393 K 433 K

• X • • O X • D ° • O • t3 D

D • OX x • D []

0.0 0 .0 I I I I I I I I I I

0 2 4 6 8 0 3 6 9 12 15

T i m e x 10 3 (s)

Fig. 3. Influence of temperature upon reactivation of the catalyst, (A) catalysts deactivated during reaction (simultaneous poisoning, 39 ppm of H2S/BD/I-12), (B) catalysts deactivated under an inert atmosphere (pre-poisoned catalyst, 246 ppm of H2S/N2). Flowrate: 650 ml rain ] catalyst weight: 50 mg, pressure: 1 arm.

brought down to reaction temperature (373 K, interval the hydrogenation rate both in the pre-poisoned and in 1), the reaction rate was over three times greater than the simultaneously poisoned samples. It can also be after reactivation in N2. observed that the reactivation of samples of pre-poi-

The influence of the temperature upon the rate of soned was slower than that of samples deactivated reactivation is presented in Fig. 3. In this case, the during reaction, at any of the operating temperatures catalyst samples were deactivated during reaction investigated. Furthermore, the conversion-time pro- using a reactor feed containing 6 ppm of H2S (simul- files corresponding to pre-poisoned samples show an taneous poisoning, Fig. 3a), and under a N2 atmos- initial induction period which is not observed in the phere containing 246 ppm of H2S (pre-poisoning, reactivation curves of the samples deactivated during Fig. 3b). In the second case, not only the total amount reaction. Both of these facts are consistent with the of sulfur (concentration/deactivation time) passed higher sulfur loading in Fig. 3b. over the catalyst was much greater, but also the driving The difference in the initial reactivation rate of both force for adsorption was considerably higher, given types of poisoned catalyst becomes evident in the the 40-fold increase in the gas phase concentration of Arrhenius plots of Fig. 4. In both cases, the values H2S, and the absence of species competing for adsorp- of activation energies and pre-exponential factors tion. Thus, in the catalyst used for the experiment in (inside Fig. 4) were obtained by non-linear regression, Fig. 3b, a much higher sulfur loading on the catalyst using the Arrhenius equation in its reparametrized can be expected. In both cases, reactivation was form [21] (reparametrization temperature equal to carried out under HzS-free reaction atmosphere. 453 K). It is worth noting the similarity of the values The results clearly indicate that an increase of the estimated for the activation energies in both cases. reactivation temperature caused a marked increase in This would suggest that the controlling step of the

J.C. Rodr{guez et al./Applied Catah,'sis A: General 165 (1997) 147-157 153

- - 2 I I I I I

Ea=93.5+2.7 kJ/mol %--,,., -3 ~ ko=(163+5) x10-4 sl

~ Ea=107.3+11.4 kJ/mo'l,~,

.o_ -4 ko= _ - "

._> ~6 -5

~ -6 " • • ~ , Deactivated in: • ._ c -7 o H2/C4H6/H2 s • ' ~

• N2/H2S _~ , I , I J l , I , I ,

1.9 2.0 2.1 2.2 2.3 2.4 2.5

1/T x 10 3 (K -1)

Fig. 4. Arrhenius plots for reactivation of catalyst samples deactivated during reaction (simultaneous poisoning, open symbols) and under an inert atmosphere (pre-poisoned catalyst, closed symbols).

reactivation process is the same in both cases, in spite tivity of the catalyst is strongly affected by the tern- of the different conditions used during poisoning, perature: at the same level of conversion, the which resulted in a fivefold increase in the pre-expo- selectivity to butenes increases with the reaction nential factor for the reactivation of samples after temperature. This result is in contrast with the data simultaneous poisoning, in Fig. lc, where at a fixed temperature (373 K), a

single selectivity-conversion curve could be drawn for 3.4. Selectivi~' the experiments with different partial pressure of H2S

in the gas phase. In a consecutive reaction network where there is a The behaviour observed during reactivation is con-

valuable intermediate product, such as in the case of siderably different: not only the selectivity values are partial hydrogenation reactions, it is often found that higher at any temperature and conversion level than the selectivity to the intermediate product decreases as the corresponding values during deactivation, but also reactant conversion increases. The selectivity-conver- the selectivity does not seem to be affected by the sion plot is represented in Fig, 5a as selectivity to reaction temperature. The observed 'hysteresis' (i.e., butenes (sum of selectivities to 1-butene, 2-trans- the difference between the selectivity during deactiva- butene and 2-cis-butene) versus butadiene conversion, tion and reactivation at a given conversion) becomes during catalyst deactivation by simultaneous poison- less important for higher deactivation temperatures. ing, while Fig. 5b shows the behaviour during reacti- The corresponding selectivity-conversion plot for vation under HzS-free reaction atmosphere. It can be pre-poisoned catalysts is given in Fig. 5C. Although observed that during the deactivation stage, the selec- the deactivation stages cannot be compared, at least

154 ZC. Rodrfguez et al./Applied Catalysis A: General 165 (1997) 147-157

1.0 - A - ~== . . . . . o- - B - - C - 10 • A a a A o ° ~m~x@e ×°e x

a a

• Reactivation o . o × • o

0 8 o a o x 0 8 • Reactivation

(D o • x

Deactivation o× o .~ 0 6 o m 0.6

• x O o

__ ~ 0.4 ~. × ° 0.4 x

(11 [z • × x • o (1) c] •

co 0.2 ~ D[] " [].x 0 2 D o

o 433 K o 4 7 3 K x e D 433 K o 453 K [] 433 K o 473 K

0 , 0 • 4 4 3 K • 4 8 8 K ~ • 4 3 8 K • 4 6 3 K • 4 4 3 K 0 . 0

x 453 K • 508 K x 443 K × 453 K

i , i , i , i , , , L , * I , I , I I I , [ , t , I , i I

0.0 0 2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0 6 0.8 1.0 0.0 0 2 0.4 0.6 0.8 1.0

Conversion

Fig. 5. Evolution of the catalyst for the production of butenes (sum of 1-butene, 2-trans-butene and 2-cis-butene), (A) during deactivation, (B) during reactivation of catalysts by simultaneous poisoning and (C) during reactivation of pre-poisoned catalysts. 6 ppm H2S, catalyst weight: 50 mg.

. 0 I I I I I I

09 - QT =350 cm 3/min

=~ 070"8 / / ~ - - ~ - - ~ ° ~ O ~ o , o

(I.) / QT=1850 cm3/min 3 " ~ 0 . 6 o~.-..__ ~ QT=650 cm /min

o 05 ~ , QT = 1300 C

0.4 o -- ~ 0.3 ~ , • • 0102 ~ QT=350 c m 3 / r r l i ~ - - --4 --o without H 2

• wi!h H2S | 0 . 0 , i , J , I , J ,

0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.-0 Conversion

Fig. 6. Selectivity-conversion plot. Closed symbols correspond to experiments carried out with H2S in the gas phase, (6 ppm of H2S, reaction temperature: 423 K). Open symbols correspond to experiments without H2S in the gas phase. Reaction temperature: 423 K, catalyst weight: 13 mg, pressure: 1 atm.

J.C. Rodrfguez et al./Applied Catalysis A: General 165 (1997) 147-157 155

during reactivation the behaviour of pre-poisoned and during reactivation the selectivity to butenes at a samples is similar to that observed in Fig. 5B for given conversion was considerably higher than during catalyst samples deactivated under simultaneous poi- self-poisoning, a behaviour similar to that of catalyst soning: a high selectivity to the desired products exists samples undergoing reactivation after sulfur poisoning over a wide interval of conversion, and all the selec- (see Fig. 5). tivity-conversion values at the different temperatures The selectivity-conversion curve just discussed for can be fit by the same curve, a sulfur-free catalyst can be compared to that of a

The selectivity-conversion plots for poisoned and catalyst sample that progressively deactivates in an sulfur-free catalysts are compared in Fig. 6. The open atmosphere containing 0.3 ppm of H2S under constant symbols correspond to the reaction of a HzS-free feed feed conditions (total flowrate =650 ml min- ~, T --- mixture (50 : 1 H2 : butadiene ratio), at 423 K. In this 423 K and H2/butadiene =50 : 1) (closed symbols in case, the different conversion levels were achieved Fig. 6). It seems clear that, at any level of conversion, by decreasing the space time from 0.133 and the selectivity obtained with the sulfur-free cata- 0.025 g h mol 1. It is interesting to note the progres- lyst is higher than the selectivity of the poisoned sive self-poisoning effect of butadiene. As the space sample. Further, at least from a conversion level of time decreases, the conversion decreases too, and as a 0.9 downwards, the catalyst deactivates without consequence the average partial pressure of butadiene much increase in selectivity. This indicates that, in in the reactor increases. After an increase in the feed a wide conversion interval, in addition to the loss flowrate, there is a transient period during which the of activity sulfur causes an important decrease in butadiene surface coverage increases, and as a con- the selectivity to butenes. sequence the conversion decreases further. This is shown by the drop in the conversion at a fixed feed flow rate (i.e., constant space time) that can be 4. Discussion observed for the data included inside every oval zone in Fig. 6, which were taken every 15 min. The self The observed catalytic behaviour during butadiene deactivation rate is higher for higher partial pressures hydrogenation is the result of complex interactions of butadiene in the gas phase, which correspond to low taking place on the catalyst surface, involving both values of space time. Simultaneously with the adsorbed hydrocarbons and sulfur-containing mole- decrease in butadiene conversion, an increase in selec- cules. In the absence of gas-phase butadiene, the tivity can be observed for the non-poisoned catalyst, equilibrium adsorption of H2S at the operating tern- Further, when the self-deactivated catalyst of Fig. 6 peratures used rapidly leads to high sulfur coverage, (flowrate 1850 ml rain 1, space time 0.025 g h mo1-1) 0~, at equilibrium. was brought back to the original conditions (flowrate The introduction of butadiene strongly changes the 350 ml min I, space time 0.133 g h mol-l) , the cata- interaction between sulfur and the catalytic surface. lyst did not restore its initial activity inmediately but As shown in Fig. 2, on a poisoned surface, sulfur is progressively, (see Table 3 and upper line in Fig. 6), tightly bonded to the metallic sites, and only a small

amount could be removed by treatment in N2 at a higher temperature; however, the presence of buta- diene was able to displace sulfur from the surface, and

Table 3 restore catalytic activity from temperatures as low as Transient selectivity/conversion behaviour of the catalyst used in 393 K. This is in agreement with previous results by the experiment of Fig. 6 after the space time was brought back to its initial value (0.133 g h mol 1). Oudar et al [12], who found a similar behaviour for

sulfur poisoned Pt(110) surfaces. These authors esti- Operation time (s) Conversion Selectivity to butenes mated the reduction of the sulfur-metal binding energy

30(/ 0.705 0.832 at 15-20% during butadiene hydrogenation. It could 900 0.816 0.795 be argued that the reactivation is due to the effect of

1800 0.869 0.742 hydrogen, in fact, Hoyos et al. [14] among others 2700 0.955 0.710

showed that decontamination of Pd catalysts could

156 J.C. Rodr{guez et al./Applied Catalysis A: General 165 (1997) 147-157

also be performed using H2 atmospheres; however, ad-species formed from diene molecules cover and from the results they presented, the maximum reacti- modify metal sites, resulting in an inhibition effect on vation by hydrogen at the temperatures used in this the adsorption and deep hydrogenation of semi-hydro- work (less than 480 K), would be limited (well below genated species. Our own results also seem to confirm 25%). It can therefore be concluded that the presence this effect: In Fig. 6 the improvement in selectivity as of butadiene in the reaction atmosphere is responsible the feed flow rate is increased cannot be interpreted for most of the reactivation observed, solely a result of a reduced residence time of the

The comparison of the residual activity of samples intermediate species, since upon returning to the poisoned at different temperatures under simultaneous original residence time a transient behaviour follows poisoning (Table 2) gives some insight into the com- during which the selectivity is significantly higher petition of butadiene and sulfur for adsorption sites on (Table 3). This suggests that the higher selectivity the surface: Although butadiene can remove sulfur observed is due to the greater coverage of the surface from poisoned surfaces in the absence of H2S in the by hydrocarbon species. gas phase, when H2S is present in the reactor feed its In spite of this, the evidence from the reactivation adsorption is reduced, but not completely avoided, runs suggests that sulfur poisoning is responsible for Instead, an equilibrium is reached in which the sulfur the initial increase of selectivity observed at different coverage depends on the H2S and butadiene partial temperatures in Fig. 5a. During reactivation, the cata- pressures in the gas phase, and very strongly on the lyst is initially selective (the sites active at the onset of operating temperature. At a fixed gas phase concen- the reactivation process are mainly selective ones), tration, an increase in temperature leads to a lower and a high selectivity is maintained as the catalyst is value of 0s and to a higher residual activity (Table 2). progressively reactivated by removal of the sulfur

A close examination of the selectivity-conversion adsorbed mostly on selective sites. This process con- curves reveals some interesting features of sulfur tinues until high conversions are restored, and the last poisoning in the system studied: During simultaneous portions of sulfur, adsorbed on the strongest hydro- poisoning all the deactivation curves at the different genation sites are removed, which is accompanied by a temperatures (Fig. 5A) present an initial period during sudden drop of selectivity to partial hydrogenation which the conversion is still close to 100%, but the products. The process is basically the same when selectivity presents a very significant increase. Thus reactivation is started with a hydrocarbon-free pre- for instance, at a temperature of 473 K, the conversion poisoned sample, which supports the hypothesis that is practically unchanged as the selectivity jumps from sulfur is responsible for the initial change in selectiv- nearly 0 to about 30-35%. Thus, it could be speculated ity. The reactivation mechanism in both cases prob- that the strongest hydrogenation sites are deactivated ably involves the reduction of sulfur ad-atoms and its first. While these would only represent a small fraction further desorption as H2S with the same controlling of the hydrogenation sites, as indicated by the negli- step, as shown by the similarities in activation energies gible change in conversion, they account for a very of pre-poisoned and simultaneously poisoned cata- significant proportion of the butane produced, leading lysts. to the observed increase in selectivity to the inter- After the initial deactivation, the beneficial effect of mediate hydrogenation products, sulfur poisoning ceases, as selective sites are poi-

It could be proposed that the species responsible for soned, and the increase of selectivity (full symbols the elimination of the deep hydrogenation sites it is not in Fig. 6), is mainly related to the decrease in con- sulphur, but butadiene. Indeed, the influence of car- version, i.e., sulfur poisoning on the selective hydro- bonaceous species on the catalyst surface on the genation sites tends to decrease rather than increase selectivity of partial/total hydrogenations on noble selectivity. Once the most active hydrogenation sites metals has been recognized for a long time [e.g., have been eliminated, the competition between buta- [11,23]]. Also, recent results [24,25] have provided diene and H2S for adsorption sites on the catalyst further evidence of the role of the surface hydrocarbon surface determines the selectivity achieved for a given overlayer on the semi-hydrogenation selectivity. In conversion. At the same conversion level, in the particular, Sarkany [24] has proposed that firmly held absence of sulfur the selectivity is considerably higher

J.C. Rodr(guez et al./Applied Catalysis A: General 165 (1997) 147-157 157

(open symbols in Fig. 6), This can be related to the References different selectivities shown in Fig, 5a at a given conversion level: As the temperature is increased with [1] C.H. Bartholomew, P.K. Agrawal, J.R. Katzer, Adv. fatal. 31 a given gas phase composition 0s decreases (as shown (1982) 135. by the results in Fig. lb), and a higher selectivity is [2] J.p. Boitiaux, J. Cosyns, F. Vema, 'Catalysts Deactivation

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