Promoting effect of Ni in semi-hydrogenation of 1,3-butadiene over Ni-Pd catalysts

Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

2081

P r o m o t i n g effect of Ni in semi-hydrogenation of 1,3-butadiene over N i - P d catalysts

A. Sarkany Institute of Isotope and Surface Chemistry, Chemical Research Center, Hungarian Academy of Sciences, P.O. Box 77, H-1525 Budapest, Hungary Catalytic behaviour of Pd2NisoNb48 amorphous ribbon and Pd-Ni/SiO2 catalysts has been investigated in hydrogenation of 1,3-butadiene. Pd-Ni/SiO2 catalysts were prepared using vapor phase adsorption and decomposition of Pd(acac)2 over Ni/SiO2 and Ni(CO)4 over Pd/SiO2. Presence of Ni has been observed to increase the selectivity of competitive hydrogenation of 1,3-butadiene in propene or I-butene. The high selectivity of Pd-Ni in competitive hydrogenation has been interpreted by geometric effects caused by dilution of Pd with Ni and and partial poisoning of Pd-Ni ensembles by firmly held adspecies. 1. INTRODUCTION Alkene streams produced either by steam or catalytic cracking contain highly unsaturated hydrocarbons which cause problems in downstream applications. Selective hydrogenation of diene and acetylene impurities over Pd is an effective and economic way of removing these impurities by transforming them to valuable alkenes [ i]. In order to improve the selectivity of competitive hydrogenation Pd catalysts with different promotors are employed to influence the intrinsic selectivity of partial hydrogenation. The bimetallic catalysts allow a larger degree of freedom to tune the reaction sites for optimum activity, selectivity and lifetime. In fact, investigations with Pb-Pd [2-3], CrPd [4], Cu-Pd [5, 6], Co-Pd [7] have indicated that for several substrates the bimetallic clusters are superior to monometallic Pd catalyst. The present paper deals with hydrogenation of 1,3-butadiene on Ni-Pd catalysts prepared from PdzNisoNb4s amorphous ribbon [8] and by vapor phase adsorption and decomposition of Pd(acac)2 on Ni/SiO2 and Ni(CO)4 on Pd/SiO2. The preparation mode of these catalysts [8-10] is likely to control the Ni-Pd interaction and influence the catalytic behaviour. The effect of mixed ensembles on the complexation of diene and alkenes over Ni-Pd is another question to be answered [10]. 2. EXPERIMENTAL

2.1. Catalyst Preparation Amorphous Pd2NisoNb48 alloy with 1.5 mm width and 30 I.tm thickness was prepared in Ar atmosphere by the single roller melt quenching method (cooling rate about 106 K/see). The amorphous state was checked by DSC and XRD. The sample in as received

2082 form was catalytically inactive therefore it was activated in different athmospheres. The catalytic activity was tested over activated ribbon cut to 3-4 mm pieces. Pd-Ni/SiO~ catalysts were prepared in two ways: i) Ni(CO)4 was adsorbed and decomposed over Pd/SiOz and ii) Pd(acach was adsorbed and decomposed over Ni/SiO~ catalyst [12]. Both group of catalysts were prepared with the same SiOz using 0.05 mm fraction only for preparation ( BET surface area 220 mZ/g, pore size 15 nm). The small reactor and evaporator [11, 12] was part of a flow system used for TPR and TPD and the system allowed the use of different atmospheres and different sublimation and decomposition temperatures. The Pd and Ni content of the samples were measured by E D X R F equipped with Si(Li) detector and 1~ as X-ray source. The catalysts are listed in Table I. A I was prepared by adsorbing Pd(acac)2 at 405 K for 30 min on SiOz degassed at 723 K, next the sample was cooled to 298 K and then it was reduced in I%H~/Ar. A2 was prepared by anchoring Pd(NH3)4(OH)z to SiOz at pH=10.5 for 6 hr, dried at 363 K for 16 hr and reduced at 573 K for 6 hr. B I is 3.3 wt% Ni/SiOz prepared by impregnation with Ni(NO3h, dried at 393 K and then reduced at 673 K. B l, B2 and B3 were treated with Pd(acac)2 at 403 K [12]. C I and C2 was prepared by Ni(CO)4 decomposition over A I and A2, respectively. Table I Preparation of Pd-Ni/SiO~ samples Cat Niwt% Pdwt%

aCOads

bdva

D%

AI 0 A2 0 BI 3.3 B2 3.3 B3 3.3 CI 0.93 C2 1.22

0.72 7.08 67.1 72.4 96.6 18.2 18.5

8.5 8.5 9 12

6.5 54 12.7 13.2 10.5 I I.l 8.9

0.12 0.14 0.03 0.11 0.32 0.12 0.14

aCO irreversible adsorption in Ixmol/gcat; bmean diameter in nm

2.1. Catalyst characterization PdzNis0Nb4s ribbon following O5 and H2 treatments was characterised by TPR XRD and surface area measurements. Depth profile were analysed by AES after preselected Ar § bombardment (4.2 keV, 25 laA/cm z ). Pd-Ni/SiO2 catalysts were characterised by TPO, FTIR, and XRD. CO adsorption was measured in a gravimetric system. XPS measurements were performed by means of a VG ESCA III instrument equipped with AIK~ source (1486.6 eV). Treatments were done in an atmospheric chamber attached directly to the ESCA machine. The Pd/Ni ratio on the surface was estimated using XPS peak areas for Ni (852.8 eV) and Pd (335.2 eV) taking Scofield's sensitivity factors [I 3]. 2.3. Catalytic tests The catalysts were tested in gas phase hydrogenation of 1,3-butadiene (BD) in a small circulation batch reactor (0.187 dm 3) with a recirculation rate of 2.2 dm3/min at atmospheric pressure. Most of these hydrogenations were performed in 2.6 kPa BD (BD:Hz=4) and with a mixture containing 0.65 kPa BD, 5 kPa propene (Pr) and 5.2 kPa

2083 H2. Over Pd-Ni/SiO2 catalysts activity and selectivity tests were also run in a recirculation flow reactor (recirculation rate 17 dm3/min) with a mixture of I-butene (IB) and BD (typically 9.1 tool% BD in I-butene, H2/BD=0.9-3.6) [5]. As in earlier publications [5-7] product selectivities were calculated as the cumulative moles of product formed divided by the total moles of butadiene consumed. 3. RESULTS 3.1. Surface composition and activity of Pd2NisoNb4. ribbon In the as quenched form the ribbon is covered with a compact NbOx layer which is inactive in catalytic hydrogenations. O2 treatment of the ribbon at 773 and 923 K breaks up the original structure as indicated by the increase of the BET surface area. O2 treatment has been observed to increase the Ni/Nb ratio suggesting NiO segregation. After hydrogen treatment at 573 K Pd segregates on the surface. XRD measurements (Cu K,,t, 7t=0.15405 rim) confirm the presence of a crystalline phase consisting of Nb2Os, NbO2 and Ni supported on a still amorphous ribbon.

!

23

:>., I--,

I

r

2

Z t--, 2:

5

!

4 oO '-

2

j

A

A

3

A

'I

2 1~

I

0

020

40

60

80

2xTHETA Fig. I. X-ray diffraction patterns of Pd2Nis0Nb41 after (I): Hz at 573 K for I hr; (2): Oz at 773 K for 30 rain, Hz at 573 K for i hr; (3): Oz at 923 K for 30 min, Hz at 645 K for I hr. ( I , NbzO~; 9 Ni; A, NbOz.

0

5

10

15

20

~5

Ar SPUTTERING (rain) Fig. 2. Ni/Nb ratio (n) after 02 at 373 K 30 rain; (4,) 776 K 30 rain. Pd/Ni ratio after 02 and H2 treatment" (~) 02 377 K 30 rain, Hz 577 K 30 rain; (x) 02 796 K 30 rain, Hz 553 K I hr.

For catalytic measurements the amorphous precursor was treated in three different ways (see Table 2): A, the NbO~ layer was removed by 40% HF etching which treatment does not attact metals during short treatment. Next we used 02 treatments (97 kPa Oz) at 673 K and 873 K for 30 rain followed by reduction in H2 at 573 K for I hr; B, after H F treatment the sample was treated with steam (3.8 kPa) at 773 K for 16 hr and hydrogenated at 573 K for 4 hr; C, CC14 (6.6 kPa) and 02 (I 2 kPa) was circulated over H F treated sample at 753 K for 2 hr, afterwards it was reduced in H2 at 573 K. The above treatment results in the formation of NbCis, NbOCI~ and NiCI~ (Pd was not detected) as measured by PIXE analysis of the sublimated residues on the cold part of the reactor tube.

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The initial catalytic activity and product selectivity together with BET surface area and CO chemisorption is presented in Table 2. It is a characteristic feature of O2/H2 treated samples that the alkene hydrogenation activity decreases quickly due to selfpoisoning. Self-poisoning affects however hydrogenation of BD to a lesser extent. Treatment B and C is less effective in activation of the ribbon: (B) has increased the BET surface area but apparently thick NbOx layer covers the whole surface whereas treatment C due to wery acidic nature of the sample resulted immediate polymerisation of BD and the low initial hydrogenation activity ceased within 10-20 min. Table 2 Activation of Pd2NisoNb48 and hydrogenation of 1,3-butadiene and propene at 298 K aTreatment

bS(BET)

cCO

Run

R(BD)xI07

R(IB)xl07 dR(BD)/R(Pr)

A: O2/673K O2/873K

0.12 0.88

n.m. 0.12

B: steam 0.09 C: CC14+O2 0.21

n.o. 0.08

I st Ist 2nd e 3rd r I st Ist 2ndg

I. I 2.95 2.17 1.34 0.006 0.65 0.01

0.4 I. 1 0.09 0.03 0.02 n.o.

I 15 I 123 2376 3456 -

aDescribed in text; bS(BET) in m2/g; cCO in I.tmol/g; dlnitial diene consumption divided by the rate of n-butane formation measured after depletion of BD. n.m. not measured; n.o. not observed. Initial rates in mol gca,~ S'J, e, f and g were measured after 120, 300 and 10 min reaction time.

3.2. Characterization and reactivity of Pd-Ni/SiO2 In I%Ha/Ar both unsupported and silica supported Pd(acac)2 complex decomposed with a maximum rate at 373-412 K (20 K/rain ramp) suggesting only weak interaction of Pd(acac)2 vapour with silanol groups. Due to weak interaction with SiO2 large particles were formed as indicated by the 6% dispersion of A2 sample. TPD measurements indicated that the carbonyl adsorbed decompose at about 356 K producing mostly CO (376 K), CO2 (445 K) and CH4 (488 K) in 1% HE/Ar. Repeated adsorption and decomposition of Ni(CO)4 leads also to fast growing of Ni particles (see C l and C2). Another feature of the samples is that the reduced sample retains carbonaceous materials. Over Al the C/Pds was 1.5-2, but over Pd-Ni, B2 and C2, C/Ms ratio reached values of 4.5 and 6.9, respectively. The deposits were removed by 02 treatment (1% 02 in He) and reduced in 1%H2 for 4 hr at 773 K. The hydrogenation behaviour of the samples was tested both in static recirculation and recirculation flow reactor2 Typical selectivity after 300 min on stream and activity on BD consumption (TOF after 30 and 300 min) is given in Table 3. Due to self poisoning BD consuption decelerated with time over Pd-Ni. Over B2 sample after 300 min high selectivity of l-butene formation (95.1% residual) was observed even at high 99.6% conversion. In H2/BD=3.6 over A l rapid decrease of l-butene consuption (kink point) appeared already at 83% BD conversion. Over B2 kink point was not observed and as indicated in Table 3 the consumption rate of I-butene is the lowest on this sample after self-poisoning.

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Table 3 Pd-Ni/SiO2 catalysts Cat. Pd~/Ni~p

~

TOF3oobSI B

et/c

AI A2 BI B2 B3 CI C2

3.7 I.I 0.23 0.33 0.85 0.3 0.14

3.1 0.5 0.08 0.1 i 0.35 0.21 0.07

5.6 4.7 1.4 1.4 3.3 3.3 1.8

I I 0 n.m. 0.22 0.08 0.04

0.63 0.55 0.76 0.67 0.63 0.67 0.65

dR(BD)/R(Pr)

R(BD)/R(nB)

1563 365 2215 3680 2655 2975 2155

0.81 0.45 6.4 43 26 27 12

"TOF in IIs after 30 and 300 min on stream; bSIB, selectivity of I-butene; ct/c, ratio of trans/cis-2-butene after 300 min on stream, SnB=0; dFrom competitive hydrogenation, 100 100 8O

80

:>., 60 t--, 1,-,4 ;> '--, t--, 40 L) u2 ,-a 20

60 4o

O'2

0 0

50

100 0

50 100 TIME (min)

150

200

250

300

Fig. 3. Hydrogenation of BD over Cat B2 at 313 K in 2.66 kPa BD, H2/BD=4.7. A: after 25 min ageing; B: after 323 min on stream. Selectivity and BD in gas phase, x-BD concentration (%), Selectivity: II, IB; A,trans; • cis;O, n-butane. 4. D I S C U S S I O N Catalytic measurements over Pd2Nis0Nb4s activated by 02 and H2 treatment, and PdNi/SiOz catalysts prepared by vapor phase deposition of a second metal (Ni or Pd) on Pd/SiO~ or Ni/SiOz, respectively, have confirmed the positive effect of Ni on competitive hydrogenation of 1,3-butadiene in propene or l-butene. The amorphous ribbon in the as quenched form is inactive and requires removal of NbOx overlayer: HF and O5 treatment at T>573 K breaks up the amorphous structure and results in the segregation of NiO. Hydrogen treatment has increased Pd concentration on the surface and Pd is alloyed with Ni as indicated by the cease of 13-hydride formation in repeated TPR runs. Due to high stability of the amorphous structure the catalyst prepared by the above treatment is essentially Pd-Ni/NbzOs overlayer supported on a still amorphous ribbon. Formation of Pd-Ni alloys and segregation of Pd over Ni seems to regulate the hydrogenation activity and selectivity of competitive hydrogenation over Pd-Ni/SiO2

2086 samples. Both the ribbon and Pd-Ni/SiO2 have shown similar hydrogenation characteristics indicating that similar Ni-Pd ensembles operate on these catalysts. For the interpretation of catalytic results one has to consider particle size effect (AI and A2) and formation of bimetallic ensembles (B2-B3 and CI-C2). The lower activity of A2 in comparison to A I can be interpreted by the too strong complexation of diene to small Pd particles [14]. In fact, dispersion of A i and A2 are 0.06 and 0.56, respectively. Another consequence of weaker diene and aikene complexation over large Pd particles is that over A I both the R(BD)/R(nB) ratio (initial BD consuption divided by initial nbutane formation after depletion of BD) and the R(BD)/R(Pr) ratio measured in competitive hydrogenation are significantly higher than over A2. Presence of Ni greatly decreases the hydrogenation activity which is the consequence of dilution of Pd with Ni and the self-poisoning of Ni rich ensembles. Following hydrogen treatment of the oxidized precursor Pd segregates on Ni surface as evidenced by XPS measurements shown in Table 3. These data cannot be used for calculation of Pds since XPS underestimates the real Pds concentration as observed by LEIS [10]. Formation of Pd-Ni ensembles significanly increases the selectivity of BD hydrogenation in I-butene or propene. This effect can be tentatively interpreted by weaker complexation of the reactants. This suggestion is apparently in contradiction with rapid self-poisoning which however can be attributed to Ni rich sites. The complexation of alkenes is affected both by Ni and also by the rapid formation of firmly held adspecies. Poisoning of Ni rich ensembles with adspecies held firmly exerts geometric/electronic effects on the operation of the working sites as far as ensemble size is concerned. The self-poisoned sample by switching off some of the wery reactive "hot" sites ensures weaker BD complexation and lower availability of hydrogen on the diluted Pd-Ni working centres. The results can be interpreted by "compression" model [8]: BD due to its high strength of complexation is able to find sites by competing with firmly held adspecies while n-butenes cannot get access to catalytic sites. REFERENCES 1. 2. 3. 4. 5. 6.

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