ELSEVIER Applied Catalysis A: General 163 (1997) 101-109

i~ APPLIED CATALYSIS A: GENERAL

Selective hydrogenation of cyclopentadiene to cyclopentene over an amorphous NiB/SiO2 catalyst

Wei - J iang Wang, M ing-Hua Q iao , Jun Yang, Song-Ha i X ie , J ing -Fa Deng*

Department of Chemistry, Fudan Universi~, Shanghai 200433, China

Received 5 December 1996; received in revised form 6 March 1997; accepted 11 March 1997

Abstract

An amorphous NiB/SiO2 catalyst, with a large specific surface area, was prepared by a reductive-impregnation method. The selective hydrogenation of cyclopentadiene to cyclopentene was carried out in a continuous flow fix-bed reactor at atmospheric pressure and with 10 g g cat -I h -l of cyclopentadiene feed. The catalyst showed high selectivity and stability. Cyclopentene was obtained in 96--100% yield at complete conversion of cyclopentadiene at temperatures ranging from 80C to 200C and no significant decrease of the activity was observed during the reaction period of 500 h. The catalyst sample was characterized by ICP, XRD, DSC, SEM, XPS, BET and 02 adsorption. XRD measurement revealed that the amorphous state was kept after catalytic reaction. Differential kinetic study showed that the hydrogenation proceeded according to a Rideal- Eley mechanism. 1997 Elsevier Science B.V.

Keywords: Supported amorphous alloy; Nickel-boron alloy; Cyclopentadiene hydrogenation; Catalytic activity; Differential kinetics

1. Introduction

The selective hydrogenation of cyclopentadiene to cyclopentene is of great synthetic and industrial inter- est, because cyclopentene with a highly reactive dou- ble bond could be used as a basic material in industry [1-3]. The hydrogenation of cyclopentadiene is a consecutive reaction. The activation energy of the first stage is higher than that of the second one [4--6], so that it is impossible to convert cyclopentadiene to cyclopentene completely, especially under the gas-

*Corresponding author. Tel.: +86 21 65492222x3792; fax: +86 21 65341642; e-mail: knfan@fudan.ihep.ac.cn

0926-860X/97/$17.00 ~"~ ~c5) 1997 Elsevier Science B.V. All rights reserved. PI I S0926-860X(97)00 1 25-7

solid phase reaction conditions used widely in indus- try. For most hydrogenation catalysts such as Pd, Pt, Ni, Cu [7-10], at the low ratio of PH2/PD (Po: the pressure of cyclopentadiene), cyclopentene is pro- duced with 100% selectivity at low conversion of cyclopentadiene, whereas the increase in P~:/Po or W/Fo decreases the selectivity. The unconverted cyclopentadiene at low conversion is easily polymer- ized on catalysts at the reaction temperature, which leads to the deactivation of the catalysts and also to low yields of cyclopentene. These phenomena are commercially impracticable. For these reasons, it is a crucial subject to prepare special catalysts in order to obtain cyclopentene at nearly 100% selectivity at complete conversion of cyclopentadiene.

102 W.-J. Wang et al./Applied Catalysis A: General 163 (1997) 101-109

It is well known that amorphous metal alloy catalysts exhibited attractive selectivity and activity for some reactions due to their isotropic structure, devoid of any long-range ordering of the constituents [11,12]. The application of these materials in catalysis, however, is severely limited, since the amorphous state is metastable, and thus is very difficult to maintain under conditions where catalytic reactions are carried out. Furthermore, the surface area of amorphous films prepared by rapid-quenching methods is too low and the shape of amorphous alloy prepared by wet chemical reduction methods is powder, which are both inconvenient for use of the catalytic potential of these materials in technical catalysis. One of the promising routes to solve these problems has been developed by depositing the amorphous alloys in high dispersion on a suitable support. For example, the preparation of amorphous nickel-(cobalt)-phosphorus alloys deposited on silica has been reported previously [13,14]. The results showed that these supported amorphous alloy catalysts exhibited higher activity and selectivity for the hydrogenation of styrene with higher thermal stability than unsupported counterparts. Unfortunately, the phosphorus component in amorphous alloys is easily lost during catalytic runs [14] and the preparation methods are unsuitable for supporting highly reactive amorphous nickel-boron alloy on a support due to the too rapid reduction of nickel ion by potassium borohydride in water.

In this paper, the preparation of amorphous nickel-boron alloy supported on silica by a new method is reported. Its catalytic properties in the hydrogenation of cyclopentadiene and characteriza- tion results obtained by ICR XRD, XPS, SEM, DSC, BET and 02 adsorption methods are discussed in detail. The reaction mechanism is proposed on the basis of the differential kinetic results.

2. Experimental

2.1. Materials

All chemicals employed were of reagent grade and were used without further purification.

2.2. Catalyst preparation

The supported amorphous NiB/SiO2 catalyst was prepared by a reductive impregnation method. The impregnation was carried out by immersing 10 g silica gel (200 m 2 g-1 surface area, 17.5 nm pore diameter and 40-60 mesh, i.e., 0.45-0.30mm particle size) in a 1 M potassium borohydride solution (pH=13) for 2 h and then taking the silica gel out of the solution. To remove the excessive solution, the silica gel was washed with 10 ml of 95% ethanol and then dried under air at room tem- perature. Subsequently, the silica gel which had adsorbed potassium borohydride was added into 10ml of 2M nickel chloride solution at room temperature with continuous stirring for 4 h. The black particles obtained were washed with 15 ml of 0.01 M potassium borohydride aqueous solution and then washed thoroughly with a large amount of distilled water. Finally, the catalyst was dried in nitrogen at 70C for 2 h.

Elemental analysis of the catalyst determined by inductively coupled plasma (ICP) showed that the nickel content was 4.3% by weight and the elemental composition was NisoB2o (mol%).

2.3. Activity measurements and differential kinetic experiment

Hydrogenation of cyclopentadiene was carried out in a tubular glass fix-bed reactor (0.8 cm i.d.) in continuous flow conditions and under atmospheric pressure. The weight of catalyst was 1 g for each experiment. Liquid cyclopentadiene, diluted with sol- vent to keep from polymerization (commonly with 95% ethanol and 1 : 1 volume ratio), was vaporized in a bubbling-type evaporator at 95C, and then passed through the reactor using nitrogen and hydrogen mixture as the carrier gas.

Differential kinetic experiment was also carried out under the same reaction conditions as those for activ- ity measurements, but the weight of catalyst was 0.1 g for each kinetic experiment.

The effluent was analyzed by a gas chromato- graphic method. The conversion and selectivity are defined as follows:

Conversion(%) = [CPD]conv./[CPD]feed x 100, (1)

W.-J. Wang et al./Applied Catalysis A: General 163 (1997) 101-109 103

Selectivity(%) = [CPE]/[CPD]conv. x 100. (2)

Here CPD and CPE stand for cyclopentadiene and cyclopentene, respectively.

2.4. Characterization of catalyst

X-ray diffraction (XRD) patterns of support and the NiB/SiO2 catalysts were performed using a Rigaku Dmax-rA with nickel filtered Cu Ks radiation. Differential scanning calorimetry (DSC) measurements were conducted under nitrogen (99.99%) atmosphere on a Dupont 9900 computer- thermal analysis system. The morphology of the samples was determined by scanning electron microscopy (SEM) performed on a Hitachi HU- l lB STEM and H600 STEM instruments. X-ray photoelectron spectroscopy (XPS) spectra were recorded with a PHI-5000C ESCA system using A1 K,~ radiation. The base pressure of the analysis chamber was 10 -8 Pa and the energy was 46.95 eV. All binding energies were calibrated using contami- nated carbon (Cls=284.6 eV). The metallic specific surface areas of the samples were measured according to a method described in [15].

3. Results and discussion

3.1. Catalytic activity

As mentioned above, the products of cyclopenta- diene hydrogenation were cyclopentene and cyclo- pentane. To obtain cyclopentene as much as possible at complete conversion of cyclopentadiene, the cata- lytic reactivity of amorphous NiB/SiO2 catalyst was concerned about both selectivity to cyclopentene and conversion of cyclopentadiene.

This catalyst did not need any activation process before catalytic runs and immediately showed high activity and selectivity once the hydrogenation started.

Since the cyclopentadiene hydrogenation is very exothermic (AH1=-99.35 kJ tool J, AH2=-112.24 kJ mol-t), nitrogen together with 95% ethanol was also mixed as heat transfer gas into the reactant feed. Nitrogen, showing no significant effect on the cyclo- pentadiene hydrogenation (Fig. I) with its flow-rate in the range 40-160 ml min-', was used to adjust the con- tact time of the reactant with the catalyst in the reaction.

The effects of PH2/PD ratio at different Fo (cyclo- pentadiene feed) values on the catalytic activity were

..~

8

100

90.0

80.0

70.0

[ I I I J I

20 40 60 80 100 120

Feed-rate(ml.min "t)

"- ~ I00

90.0

~2 80 0

P

70.0

I 1 l

140 160 I80 200

Fig. 1. Effect of N2 feed-rate on CPD conversion and CPE selectivity (CPD feed=10 g cat-t h , H2 : CPD=I.4, temperature 120C).

104 W.-J. Wang et al./Applied Catalysis A." General 163 (1997) 101-109

=o e~ o

e'~ o

o

g

o

eq

q r.,,i II

G~

0q.

(.9

'R.

II

F

"2.

r .~

II

G)

"7

O ~ O O O O O

qq g g g g g g g g ~

0 0 0 ~ 0 gga~ggggg

o o o o o o o o q q oooooooo~

0 0 0 0 g g g g g g g ~ >

~gggggggg

ggggggg~gg 7 o

~ggg~ggggg =

Q q o q q q o q

~ g g g g g g g g

gggggggggg g m

g

I00

o ~ 90.0

0

80.0

J I ~ I L I r J

80 110 140 ]70 200

Temperature(C)

100

90.0 ~[

80.0

Fig. 2. Effect of reaction temperature on CPD conversion and CPE selectivity (CPD feed=10g cat 1 h- l , H2 :CPD=I .6 : 1 (GHSV=24 000-1).

studied, as shown in Table 1. The suitable PH2/PD ratio in order to maintain the selectivity to cyclopentene higher than 96% at complete conversion of cycio- pentadiene varied with changing the cyclopentadiene- feed. Clearly, the increase in the cyclopentadiene feed required high H 2 : cyclopentadiene ratio to maintain the 100% conversion, but the selectivity slightly decreased. At the highest cyclopentadiene feed, 16 g g cat t h-~, the selectivity was about 96%, but the optimum cyclopentadiene feed was 10-12gg cat-t h i. Thus, in the present work, the cyclopenta- diene feed was controlled at 10 g cat t h- i for most catalytic runs.

The variation of reaction temperature from 80C to 200C showed little effect on the catalytic reactivity, as shown in Fig. 2. A further increase in temperature led to a decrease in the reactivity, which was due to cyclopentadiene polymerization on the catalyst.

These results indicate that the amorphous NiB/SiO2 catalyst has high selectivity and activity compared with other commonly used hydrogenation catalysts in the present reaction conditions. For example, on Pd/ A1203 (Pd: 0.5 wt%), the selectivity and conversion are 33% and 70% in the present work; and also 90% and 92.5% at higher pressure in Ref. [7]; on Ni/SiO2 (Ni: 4.6 wt%) 84% and 100% in the present work; on NiS/SiO2 93.5% and 99% in Ref. [9] (no LHSV value was given, but according to our knowledge, it may be 0.5 h-t), respectively. If the amorphous NiB/SiO2 was

W.-J. Wang et al./Applied Catalysis A: General 163 (1997) 101-109 105

treated at 400C for 2 h in nitrogen, its activity and selectivity remained unchanged, while in the case of the treatment at 450C for 2 h in nitrogen, the best selectivity was 94% at 100% conversion.

The stability of the catalytic activity was also very excellent. For about 500 h, a 97-100% yield of cyclo- pentene was obtained at 120C and 10 g g cat -~ h -1 of cyclopentadiene was kept. This shows a possibility to apply the amorphous NiB/SiO2 catalyst to commercial cyclopentene production.

3.2. Characterization

BET nitrogen surface area of the catalyst sample was about 180 m 2 g-t. Its nickel dispersion measured by oxygen uptake at 25C using a pulse chromato- graphic technique was 0.30. These results show that the amorphous NiB/SiO2 has larger surface area and higher nickel dispersion.

Fig. 3 shows the XRD patterns of four NiB/SiO2 samples. The patterns of (b) and (c) were almost the same. After subtracting the background spectrum of SiO2, a broad peak at 20=45 was observed. This was attributed to the NiB alloy in the amorphous state. The pattern of (d) indicated that the amorphous state still remained, but on the pattern of (e), the diffraction peaks of some crystal, which was analyzed to be nickel crystallite, had appeared. DSC spectra, as

b

_=

I I I I

20 40 60 80

20( )

Fig. 3. X-ray diffraction patterns of (a) SiO2, (b) original NiB/ SiO> (c) NiB/SiO2 after 500 h of hydrogenation at 120C, (d) NiB! SiO2 treated in N2 for 2 h at 400C, and (e) NiB/SiO2 treated in N2 for 2 h at 450C.

210C

488.9 C

437.5 C~

I I I I I I

100 200 300 400 500 600

Temperature(C)

Fig. 4. DSC spectra of NiB/SiO2.

shown in Fig. 4, exhibited that there were three exothermic peaks at 412.7C, 437.5C and 488.9C, while on the unsupported counterpart at 144.1C, 253.4C and 341.4C, respectively [16]. This illus- trates that the crystalline transformation temperature of the amorphous NiB/SiO2 is higher than that of the unsupported counterpart. The SEM photograph in Fig. 5 shows the surface morphology of three NiB/ SiO2 samples. Pictures (a) and (b) revealed that the NiB alloy was cottony, which was considered to be the amorphous state, but the density of the amorphous alloy on picture (b) seemed to be larger than that on picture (a). Picture (c) indicated that NiB alloy was slightly agglomerated, thus we concluded that some crystallization had occurred. These phenomena were exactly corresponding to those in the XRD and DSC experiments. All these results indicate that the sup- ported amorphous nickel-boron alloy has much higher thermal stability. This property is considered to be mainly due to the lower metal loading and high alloy dispersion, the results are meaningful for the applica- tion of amorphous alloys in catalysis.

XPS spectra of Ni2p3/2 and Bls in the amorphous NiB/SiO2 catalyst are shown in Fig. 6. Compared with the spectra of pure nickel metal foil (852.8 eV) and nickel oxide (854.5 eV), the peak at 852.0 eV in the NiEp3/2 level is ascribed to metallic Ni and that at 855.4 eV to oxidized nickel. In the Bls level, two kinds of higher binding energy than 189.4 eV for elementary

106 W.-J. Wang et al./Applied Catalysis A: General 163 (1997) 101-109

Magni f i c~ ion :100~.~

B were assigned to boron interacting with nickel (191 eV) and to oxidized boron (194 eV), respectively [16-19]. From these results, it can be immediately concluded that the boron donates electrons to nickel. The same results were obtained on the amorphous NiB alloy powder [ 16].

The surface composition calculated from the XPS spectra using the relative peak area sensitivities was Niv0B3o (mol%). Compared with the bulk composition of NisoB2o determined by ICP spectrometry, the sur- face of the catalyst is metalloid rich.

The results in the characterization mentioned above could explain the good catalytic properties of the catalyst obtained in cyclopentadiene hydrogenation; the high activity is attributable to high dispersed metallic nickel and the active site of the amorphous nickel-boron is also metallic nickel [t6]. The higher selectivity is ascribed to the amorphous state of nickel which makes the active site of the catalyst chemically and structurally isotropic [ 11,12]. When the catalyst is crystallized, the activities of active sites at different faces would be different, which results in a decrease of the selectivity. The excellent stability of the catalytic reactivity seems to result from the high crystalline transformation temperature of the nickel-boron alloy supported on silica.

Magnii]cati~ln ~ 00(~ [11)

$q~Or~m

C Magnil]clllof~ IO00o O0

Fig. 5. SEM pictures of (a) original NiB/SiO2 sample, (b) NiB/ SiO2 sample treated in N2 for 2 h at 400C, and (c) NiB/SiO2 sample treated in N2 for 2 h at 450C.

3.3. K inet ic studies

Kinetic studies were carried out under differential conditions, i.e., the conversion was maintained at approximately 5%. When GHSV>5000 h -], the exter- nal diffusion was negligible and the intraparticle diffusion was completely eliminated, if the particle size was reduced to 40 mesh in the present work.

The dependence of the cyclopentadiene hydrogena- tion rate on the partial pressure of cyclopentadiene and hydrogen was investigated. In the present studies, the selectivity to cyclopentene was 100%. Furthermore, the addition of 10 mol% of cyclopentene or cyclopen- tane into the feed did not influence the reaction rate. Therefore, the reaction rate re) can be expressed as

m 11 rD = FDX = kDPDPH2, (3)

where FD is the cyclopentadiene feed in mol s- 1, X the conversion in percent of cyclopentadiene converted and PD is the pressure of cyclopentadiene, respec- tively. If GHSV in the reactor is fixed by adjusting the

W.-J. Wang et al./Applied Catalysis A: General 163 (1997) 101-109 107

Ni2v3t2 852.0 eV Bls

194.0 eV

Binding Energy(eV)

Fig. 6. XPS spectra of Nizp3/2 and BI~ in the NiB/SiO2 catalyst.

nitrogen flow-rate

PD oc VD :x FD, (4)

Plq2 :x v~q2 , (5)

where VD and VH2 are the flow-rate of cyclopentadiene and hydrogen, respectively. Then

rD = FDX = k ~mvn v~D H 2

or

X 1 ~m-- I n = tc~r D v.2. (6)

When vH2 remained constant, then

log X oc (m - 1 ) log FD. (7)

The logarithm of the conversion is plotted against the logarithm of the cyclopentadiene feed in Fig. 7, which exhibits a series of parallel lines depending on the reaction temperature. Their slope, -1 , indicates that the reaction rate upon the cyclopentadiene is zero order. If FD is constant, then

logX oc n log Vn~. (8)

Fig. 8 shows the plots of log X to log vIq2, which also exhibits a series of parallel lines related with the reaction temperature. The slope, 1, indicates that the reaction rate upon hydrogen velocity is the first order, thus the reaction rate ro is expressed

simply by

rD = kDPn2 = k exp( -E /RT)PH: , (9)

where k is the pre-exponential factor, and E is the activation energy. The activation energy was measured between 80C and 160C and found to be 6.93 kJ mol - l . The average pre-exponential factor k was calculated to be 2.610 -3 mol g cat - t atm -~ s Hence the rate of the reaction can be rewritten as

rD = 2.6 10 -3 exp( -835/T )Pm (10)

with E/R=835 mol - j K -1 if E is in J and R in J mol - t K -1.

Table 2 gives the rates of cyclopentadiene con- sumption calculated from Eq. (10) at several reaction temperatures, when hydrogen pressure and cyclo-

Table 2 Observed and calculated rates of reaction as a function of the reaction temperature

T(C) robs (10 6mols - l ) real (10 6mols 1)

80 3.03 3. l 8 100 3.78 3.61 120 4.17 4.05 140 4.38 4.48 160 4.78 4.92

PHi_=0.13 atm, PCVD=0.25 atm, 0.1 g of catalyst.

108 W.-J. Wang et al./Applied Catalysis A: General 163 (1997) 101-109

X 0

1.0

0.8

0.6

0.4

0.2

1.1 1.2 1.3 1.4 1.5 1.6

Log Pm

Fig. 7. The logarithm of the CPD conversion vs. the logarithm of the CPD feed plots (conditions: PH2=0.2 atm, GHSV=24000 1).

X

o

0.8

140"C NN~.%NN~eNN~Q o 160 C

1.2 1.4 1.8 2.0 2.2 I

1.6

Log FCI,D

0.6

0.4

0.2

Fig. 8. The logarithm of the CPD conversion vs. the logarithm of the PH2 plots (conditions: P0=0.25 atm, GHSV=24000 i).

pentadiene pressure are 0.13 and 0.25 atm, respec- tively. Very good agreement was found between the calculated and observed rates of reaction.

According to rate-equation (10), which is different from those obtained on NiS [5] and on Pd [20] but similar to that on Cu/A1203 [21 ], for the bimolecular

W.-J. Wang et al./Applied Catalysis A: General 163 (1997) 101-109 109

reaction studied here we propose a Redeal-Eley (R-E) type mechanism. For a Langmuir-Hinshelwood (L-H) type mechanism, we must suppose that the adsorp- tion of hydrogen and cyclopentadiene are non-com- petitive, and assume that the adsorption of hydrogen is weak and cyclopentadiene strong, thus we could obtain Eq. (10). As the active sites of the amorphous NiB/SiO2 catalyst are all elemental nickel and iso- tropic, such non-competitive adsorption is not realis- tic. Based on the R-E type mechanism, i.e., strongly adsorbed cyclopentadiene reacting with hydrogen directly from the gas phase [22], our mechanism, which is identical with that on Cu/A1203 [21], can be written as follows:

CPD + catalyst = CPD-catalyst (11)

CPD-catalyst + H2 --~ CPE + catalyst (12)

If ~D and KD are the surface coverage and adsorp- tion coefficient of cyclopentadiene, respectively, and the rate-determining step is Eq. (12), one may write

~)D = KDPD/( I + KDPD), (13)

rD = kD~DPH2 = kDKDPDPH2 / (1 + KDPD ).

Assuming that cyclopentadiene is strongly adsorbed, i.e., KDPD>>I,

rD = kDP.2. (14)

This is identical with the rate equation obtained in kinetic experiments.

4. Conclusions

It was shown that an amorphous nickel-boron alloy supported on silica was very helpful for selective hydrogenation of cyclopentadiene to cyclopentene at low temperature in gas-solid phase. The catalyst activity was very stable for a long time on stream. The results in catalyst characterization showed that the amorphous state was maintained over 80-120C. The differential kinetic study revealed that the reaction mechanism was of the Rideal-Eley type with cyclo- pentadiene strongly adsorbed on the catalyst. To our

knowledge, the present report is the first one on the behavior of amorphous alloy catalyst which may be applied to commercial catalytic processes in gas-solid phase reaction.

Acknowledgements

This work was supported by the National Natural Science Foundation of China and China Petrochem- ical Corporation.

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