Zinc oxide modified by alkylsilylation as an efficient catalyst for isomerization of hydrocarbon

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 fights reserved.

2429

Zinc oxide modified by alkyisilylation as an efficient catalyst for isomerization of hydrocarbon Yuzo Imizu, Takehiro Narita, Yoh Fujito, Hirofumi Yamada Department of Materials Science, Kitami Institute of Technology, Kitami, Hokkaido 090-8507, Japan Isomerization of 1-butene, 1-pentene, and cis-l,3-pentadiene was carried out on alkylsilylated ZnO catalyst. The alkylsilylation of ZnO exerts a promoting effect in the isomerization of olefin. The extents of enhancement by the silylation were depend on the type of alkylgroup of silane, especially upon silylation with Et3SiH, the activity for cis-l,3pentadiene was remarkably enhanced by a factor of 89. The selectivity for 1-butene and 1pentene isomerization were not changed before and after silylation. The silylation also did not change the kinetics as well as the activation energy for the isomerization of cis-1,3-pentadiene, suggesting that silylation did not alter the reaction mechanism. TPD study revealed that the silyl group prevents the strong adsorption of 1,3-pentadiene. Taken together, it was concluded that the alkylsilyl group served as a template to prevent irreversible adsorption of reactant and keep active sites free from the self poisoning. I. INTRODUCTION Although the adsorbed species such as reactants, intermediates, and products have received much attention in the studies of heterogeneous catalytic reactions, few study was conducted about the adsorbed species which are not involved in the reaction mechanism because such species are often considered to be merely innocent by standards which do not influence the rate processes. Double-bond isomerization of n-butene has been extensively studied on oxide catalysts to elucidate the reaction mechanisms and characterize the acid-base properties of catalysts[l]. Zinc oxide catalyzes the double-bond isomerization of 1-butene to give 2-butene, having the initial cis to trans isomer ratio 13[2]. This selectivity data suggested that the isomerization of 1butene proceeds via rc-allylic carbanion which is formed by the abstraction of proton from 1butene. Infrared studies revealed that surface reaction was rate-controlling and conformed that adsorbed rt-allylic species were most probably intermediates[2]. During butenes isomerization at 298 K, Kokes et al. observed considerable and unavoidable self poisoning, which they attributed to the pure reactants themselves, but initial rates on freshly activated catalysts gave

2430 reproducible data[2a]. Moreover it was generally accepted that only a small fraction of Zn-O pair sites are responsible and available for g-allylic carbanion formation[3]. The observation of self poisoning of reactant itself indicates that irreversible adsorption of reactant possibly hinders the adequate formation of surface intermediates to display inherent catalytic performance of ZnO. If the process of irreversible adsorption of reactant is able to circumvented during the isomerization, the formation of reactive intermediates is facilitated, thereby the catalytic activity for the isomerization can be improved. Prompted by this prospect, we have examined surface modification of ZnO with silane coupling agent, which is frequently employed to construct functional organic surfaces by covalently attaching organic moieties and control the surface adsorption properties toward incoming organic molecules by repulsive and/or attractive interactions. We found that surface alkylsilylation of ZnO exerts a promoting effect in the isomerization of 1-butene, 1-pentene, and

cis-1,3-pentadiene, especially, the activity for cis-l,3-pentadiene was remarkably enhanced by a factor of 89. We report here a study of the scope, kinetics, and mechanism of olefin isomerization catalyzed by a series of silylated ZnO. In addition to these, the temperatureprogramed desorption (TPD) of 1,3-pentadiene was conducted to elucidate the nature of adsorbed 1,3-pentadiene and the interaction between 1,3-pentadiene and attached alkylsilane. 2. EXPERIMENTAL METHODS 2.1 Preparation of modified ZnO Zinc oxide (Kadox-25 from New Jersey Zinc Co., 16-32 mesh) were placed in a Pyrex reaction vessel and outgassed at 723 K for 3h. The modification of catalyst was carded out first by exposing the out gassed sample to the saturated amount of gaseous silanes at 77 K, followed by surface reaction of the adsorbed silanes with surface hydroxyl group by outgassing for 30 min at 523 or 623 K. During the elevation of temperatures, the excessively adsorbed silane was outgassed by heating the sample at a rate of 4 K/min. The silane coupling agents employed in this study were (Me3Si)2NH, (Me3Si)20,

n-ButylMe2SiH, Et2MeSiH, Si(OMe)4,

AIIylMe2SiH, and Et3SiH. 2.2 Reaction Procedures and TPD of 1,3-Pentadiene

All isomerization were carried out around 300 K in a recirculation reactor with a volume of ca. 582 ml. The reactant 1-butene (50 Torr) or 1-pentene (50 Torr) or cis-l,3-pentadiene(30 Torr) was admitted to the catalyst(200-500 mg). The reaction products were analyzed by gas chromatography with a 7 m column packed with VZ-7 (Gas Chro Ind.) which was operated at 273 K for butenes and at 298 K for pentenes and 1,3-pentadiene. The activity data was taken from the initial rate of isomerization. TPD experiment was performed in a vacuum system first

2431 by exposing the catalyst to the saturated amount of 1,3-pentadiene at 77 K, then outgassing at 300 K for 10 min, finally followed by heating the catalyst to the temperature at 700 K at rate of 4 K/min. The desorbed products from 300 K were periodically collected in a U-shaped trap and subjected to analysis by gas chromatography. 3. RESULTS AND DISCUSSION 3.1. Isomerization of l-Butene and l-Pentene

Upon modification of ZnO with various alkylsilanes such as (Me3Si)2NH, the activity in the isomerization were enhanced by a factor of 10 for 1-butene and 5 for 1-pentene (Table 1). The extents of enhancement were depend on the kind of silane. The cis to trans product ratios for 1butene isomerization were plotted as a function of conversion (Fig. 1). The modification did not alter the selectivity, being observed to be almost identical before and after silylation. The initial cis to trans ratios were more than 15, which values are characteristics for the base-catalyzed

isomerization. The similar selectivity feature was observed for 1-pentene isomerization. These results suggest that the modification of ZnO did not influence the relative stabilities of syn-and anti-n-allylic species or the height of the kinetic barrier between them. It was reported that

doping foreign cations into ZnO affected the selectivity, accompanying increase in catalytic activity for 1-butene isomerization[4]. The increase in catalytic activity is explained by the alteration of number of active sites by change in surface structure[5]. This could not be the case for the surface silylation, since the silylation is considered to maintain the structure near or at Table 1

30 D none o (Me3Si)2NH zx AllylMe2SiH 9 Si(OMe)4

Enhancement factors for the activity in the ~ 25 =

isomerization of olefin a) reactant modifier b) 1-butene 1-pentene none

1

1

(Me3Si)2NH AllylMe2SiH

7.8 4.8

4.0 2.7

Si(OMe) 4

9.9

4.9

Et2MeSiH

1.5

2.2

n-BuMe2SiH

2.0

-

a)Relative activity. b) Madification at 623 K.

t"q

20

9 Et2MeSiH

o 15

• Me2BuSiH

..,,a

10 O O 9,i..,a --

•

5

0

A- zx

o,,

v

lb ' 20 ' 3'0 ' 4() ' 5() ' 60 Conversion (%)

Fig. 1. The ratio of cis to trans isomer for the isomerization of 1-butene on ZnO and unmodified ZnO

2432 surface. 3.2. Isomerization

cis-l,3-pentadiene

of

cis-1,3-Pentadiene on ZnO underwent exclusively cis-trans isomerization to give trans-l,3-

pentadiene and no double-bond isomerization to give 1,4-pentadiene was observed. The extents of enhancement by the modification were depend on the type of alkyl group of silane. The observed enhancement factors were 6 for (Me3Si)2NH, 12 for (Me3Si)20, 18 for (nButylMe2Si)20, and 63 for Et2MeSiH. The best results obtained with Et3SiH, which enhanced the activity of ZnO by a factor of 89. This enhancement was sufficient to create a catalytic activity enough to isomerize 1,3-pentadiene at 373 K in a diffusion controlled fashion (Fig. 2). The data for Et3SiH-modified ZnO was obtained by deducing from the Arrhenius plot. Apparently, the attached alkylsilane on ZnO plays important roles in the observed promoting effects. To obtain mechanistic insights in the promoted process for the cis-trans isomerization of 1,3pentadiene, we measured kinetics and apparent activation energy before and after silylation. The both activities of ZnO and silylated ZnO showed Langmuir dependence on the 1,3pentadiene pressure, indicating that the surface was fully covered by the reactant during the isomerization under the reaction condition examined in this study (Fig. 3). These data were consistent with the observation that the initial rate of 1-butene was zero-order on ZnO[6, 7]. The apparent activation energy after the silylation (Fig. 4) were observed to be not significantly changed, suggesting that the silylation did not alter the reaction mechanism as suggested in the

,4[

- - (n_BuMe2Si)20 • ~

n-BuMe2SiH 52 ] n-Pr3SiH

89] 63]

Et3SiH Et2MeSiH

~

1.2

C) (Me3Si)2NH/ZnO

1.0[

I"! ZnO

O

10

20 30 40 Pressure (Torr)

0.8

51 ] EtMe2SiH - ] (Me3Si)2NH

9~ 0.4

5-] (Me3Si)20

ff~ 0.2

d

1 none 0

20

40 60 80 Promoting factor

100

Fig. 2. Enhancement factors for the activity in the isomerization of 1,3-pentadiene. Modification was carried out at 523 K.

0 0

50

Fig. 3. Activity in the isomerization of cis1,3-pentadiene on ZnO and unmodified ZnO as a function of reactant pressure.

2433 isomerization of 1-butene and 1pentene.

-11

1,3-pentadiene is more reactive than

- 12

1-butene and 1-pentene.

The most

36.8 kJ/mol

-13

probable intermediates of cis-trans ZnO were n-allylic carbanion species which are formed by the abstraction of These intermediates attach to a neighboring n-system, allyl group which is considered to stabilized the carbanion species more than that

Z n O ~

o -15

methyl proton of 1,3-pentadiene. ~

42.2 kJ/mol

O

isomerization of 1,3-pentadiene on ~ -14

8.1 ld/mol

9 42.8 kJ/mol -16 O (Me3Si)2NH [] Et3SiH ~ - ~ 3 . 2 kJ/mol -17 9 (n-BuMezSi)20 9 n-BuMe2SiH -18 3.3 3.5 3.7 3.9 3.1 1/T x 10-2 (l/K) ,

I

,

I

,

I

-

formed from l-butene and 1-pentene. This electron delocalization could Fig. 4. The activation energy for cis-1, 3-pentadiene make 1,3-pentadiene more reactive isomerization on ZnO and alkylsilane-modified ZnO than 1-butene and 1-pentene. 3.3. TPD of 1 , 3 - p e n t a d i e n e To examined the state of adsorption of 1,3-pentadiene upon modification, the TPD of 1,3-pentadiene from both ZnO and silylated ZnO were carried out. As shown in Fig. 5, 1,3-pentadiene desorbed

from

ZnO

through

a o1"4

temperature range from 300 K to 500

Sill

K, resulting in two peaks around 323 K .w,,4

and 453 K. While upon modification

MeSiH

r~

with Et3SiH, the lower temperature

e3Si)20

peak was almost remained and the

e3Si)2NH

higher temperature peak observed on ZnO was significantly disappeared. From the quantitative TPD profile (Table 2), it was estimated that the Et3SiH decreased the total amount of desorbed 1,3-pentadiene by 52% and the amount of strong desorption above

tuMe2SiH

0

|

"__ 200 ' ' 4t)O'__ ' "61~10

'

m8 ~

.

|

.

1000

Temperature (K) Fig. 5. TPD spectra of cis-1,3-pentadiene.

2434

373 K by 25%, while 85% of the amount of Table 2 weak desorption below 473 K were still Amount of desorbed 1,3-pentadiene remained. These results clearly show that the silyl group prevents the strong adsorption and

modifier

only weakly bounded 1,3-pentadiene species

Amount of desorption (Ixmol/g) below above total 373 K 373K

are present on the silylated ZnO, resulting in the

none

21.2

9.6

11.6

enhancement in the activity by 89 times.

Et3SiH

11.1

8.2

2.9

Taking the extent of the increase in activity into

Et2MeSiH

8.2

5.6

2.6

account, it was suggested that one molecule of

n-BuMe2SiH

7.5

6.0

1.5

1,3-pentadiene strongly adsorbed on the surface

(Me3Si)2NH (Me3Si)20

7.7 9.9

6.0 6.6

1.7 3.3

hindered

the

formation

of

reactive

intermediates by more than one order of magnitude. 4. S U M M A R Y

The alkylsilyl group attached on ZnO is a highly effective organic modifier for the hydrocarbon isomerization, especially for cis/trans isomerization of 1,3-pentadiene. The surface silylgroup could not alter the reaction mechanism as well as the number of active sites, rather served as a template to prevent irreversible adsorption of reactant and keep active sites free from the self poisoning. We believe that the introduction of alkylsilyl group onto oxide catalysts provides a new opportunity to develop high productivity catalyst with control of active-site architecture. REFERENCES 1. K. Tanabe, M. Misono, Y. Ono, H. Hattori, New Solid Acids and Bases, Kodansha, 1989, pp 215-210. 2. a) C. C. Chang, W. C. Conner, and R. J. Kokes, J. Phys. Chem., 1973, 77, 1957, b) A. L. Dent and R. Kokes, J. Phys. Chem, 1971, 75, 487. 3. R. J. Kokes and A. L. Dent, Adv. Catal. 1972, 22, 1. 4. T. Uematsu, K. Inamura, K. Hirai, and H. Hashomotp, J. Catal, 1976, 45, 68. 5. C. S. John, Catalysis, Vol. 3, 1980, pp 169-188. 6. S. Naito, Y. Sakurai, H. Shimizu, T. Onishi, and K. Tamaru, Trans. Faraday Sot., 1971, 67, 1529. 7. E. A. Lombardo, W. C. Conner, R. J. Madon, W. K. Hall. V.V. Kharlamov, and Kh. M. Minachev, J. Catal, 1978, 53, 135.