Hydrogenation of 1-hexene by rhodium catalysts

Journal vf Moleculm- Catalysis.11 (1981) 181 - 192 @ EIsevier Sequoia S.A., Lausanne -Printed in The Netherhxk

HYDROGENATION

J. A. PAJAREF, fnstituto

OF I-HEXENE

l3Y RHODIUM

181

CATALYSTS

P. REYES

de Cal6lisLs y Petroteoquimica.

C.S.Z.C.,

Serrmo.

Z Z9. Madrid

(6)

(Strain)

L. A. OR0 and R. SARIEGO Deparfamento Saragosia

de Quimica

horgdnica,

Facdtad

de Ciencks.

Urtiuersidad

de Zaragoza.

(Spcin)

The catalytic activity for I-hexene hydrogenation (20 “C, 1 atm) of several Rh-based catalysts (an homogeneous [Rh(NBD)(ZMe-py),] ClO, catalyst; the same complex supporkd on an acrylic resin, Duolite CC3; three Rh/duolite catalysts prepared by reduction of [Rh(NBD)(2-Me-py)s] CEO,/ duolik; and a series of Rh/SiO,, Rh/Al@s and Rh/Y zeolite catalysts, prepared following conventional impregnation or exchange proceciures) has been measured. The metallic dispersion of the metal/support catalysts has been determined by several methods of selective chemisorption and by transmission electron microscopy; the Rh/duo!ite system shows a narrow distribution of metal particEe sizes between 18 and 25 A. The turnover numbers are very similar for Rh/SiO, and Rh/duolite catalysts. Samples of Rh/&03 show smaRer activity, probably due to a strong metaI/support interaction. Much higher activity was shown by the Rh/Y zeolite system. The good crtalytic activity of Rh/duolite makes this catalyst competitive with conventional rhodium supported catalysts, with the advantage of having a controlled, narrow-centered metal size distribution.

fntioduction Although a variety of organic and inorganic materials can be used as supports for heterogenizing homcgeneous catalysts, the use of ion-exchange resins has scarcely been studied [_5 - 41 . Hawever, several polymer-bound rhodium cata1yst-s have been prepar& by decomposition of osganometalk complexes [5 , 61, a method which is receiving increasing interest [7,8]_ We study in f&is paper a resin-supported metal catalyst formed by treatment with mo!ecular hydrogen of the rhodium(E) organic complex prepared

*rkthor

tu whom correspondence

should

be addressed.

182

TABLE Testud

1 characteristics

of the

supports SiOz

~203 SBET

(m

2 g-l)

149

128

d, (g cm-‘)

4.01

0.811 3.03

Pore volume, V, (cm3 g-l)

0.56

0.904

d,

(a cme3)

Mean pore dia.meter, d (.&I

1.2*

75

141

Zeolite

YNa

Duolite

786

0.0207

-

0.76 1.18

-

-

-

-

by the interaction of [Rh(NBD)(2-Me-py)z] CIO, (NBD = 2,5-norbornadiene; LMepy = 2-methyl-pyridine) with an ion-exchange resin (Duolite CC3). The ca’dyic activity for the hydrogenation of I-hexene of this rhodium/duolite system is studied, as well as that of other analogous rhodium catalysts supported on silica, alumina and Y zeolite. Some homogeneous hydrogenation data are also reported.

RhC13 aq., Rh = 40 wt.%, was obtained fro-m Fluka AG, Buchs, Switzerland. All the solvents were dried and distilled immediately before use (acetone with 4 A molecular sieve, and ethanol with C&I,). Commercial olefins were deperoxidized by filtration through grade I neutral alumina. Gases, He, Hz, Nz, 99.995 vol.% pure, used in texture characterization, were supplied by SE-O., Madrid. Before storage, they were passed through a purification train with 5 A molecular sieve from Union Carbide. The following materials were used as supports: (i) Duolite CC3, a cationic resin supplied in the acid (H+) form by DIA-PROSIM . Chemically, it is a carboxylic resin with acrylic backbone. It is fairly insoluble in water and has excellent mechanical and chemical resistance_ Some of its characteristics, corresponding to the HC form, are given in Table 1. (ii) Girdler T-126 alumina, and BASF D-11-11 silica, had both a moderate specific surface and a negligible micropore contribution. The starting cylindrical pellets were ground and sieved to -0.59 + 0.42 mm. Both supports were heated 4 h at 7OG “C in an air stream. The textural characteristics of the materials after this thermal treatment are given in Table 1. The fourth support used was NaY zeolite, Linde SK-40. Analysis of the nitrogen adsorption isotherm by the Dubinin-Raduskevich-Kaganer method [9] gave a specific surface, SDRK = 912 m2 g-l, with a micropore volume, ‘J, = 0.32 cm3 .g:-‘_ X-ray diffractograms [lOI show a small degree of crys-

183 TABLE Chemical

2 analysis

Support

and dispersion

of Rk/support

Rk content

cataysts Dispersion

Mean

particle

size

(A) m2O3

0.092 0.609 2.45

0.89 0.91 0.42

10 10 21

SiO2

0.117

0.252 G.685 1.35

0.38 0.62 0.81 0.84

24 15 11 11

0.285 0.391 0.534

O-23 0.23 0.49

39 40 18

0.052 0.091 0.170

0.49 0.35 0.35

18 25 25

Y-zeolite

Duolite

CC3

[Rh(NED)(2-Me-py)2]C10q/duolite,

Rh (%)

0.126

talhnity for the alumina, while those of Y zeoli’& correspond line compound.

to the crystal-

[Rk(lVBD)(2-Me-py),] CL!!!!. The previously reported compiex, was prepared by a simplified route: [Rh(NBD)(Z-Me-py),] ClO, [ll] [RhCl(NBD)] 2 (0.05 mmol) dissolved in acetone, was stirred for 30 min with AgClO, (0.10 mmol). After filtration of the AgCl formed, the solution was treated v&h an excess of 2methylpyridine (2Me-py), and after concentration the li#-yellow complex was precipitated by addition of ether (yield: 87%). [Rh(NBD)(B-Me-py),] CQ/dunlite. The Duolite CC3 resin, previously washed with water and methanoI, was suspended in a solution of the ]Rh(NBD)(2-Me-py)s]ClC, complex in absolute ethanol. (I - 4 X 10m3M)_ After 8 h of stirring the reddish product was filtered off and washed with ethanol. Rhodiumiduolite_ The above mentioned product of the reaction of [Rh(NBD)(2-Me-py)2]C10~ with duolite was snuspended in ethanol and shaken for 7 h in the presence of hydrogen. The materSslowly changes colour from red to green and EnaLLy to black. The rhodium concentration in these m.aterids is reported below (Table 2). Rhodium/silicu, R~~dium/aiuminu, Rhodium/Y teolite. Rh/Si02 and Rh/A&& ce_tiysts, with 0.1 - 2.5 wt_% Rh, were prepared by impregnation from an aqueous RhC13 solution folIowing the minimum volume method.

184

The operation was carried out in a rctavapor, usually over 10 g of support, heating to dryness at 70 %. The drying process was continued for 16 b. in an oven at 120 “C. The impregnated material was decomposed by a two-step reducing process in hydrogen flow, fist at 300 “C for 2 h, and then at 50 “C for 2 h. The catalysts were cooled in a hydrogen flow to room temperature. Rh/Y zeolite samples were prepared through ionic exchange, at 50 “C, of the sodium ions of the NaY zeolite with aqueous solutions of RhC& (V E 50 cm3). The zeolite had been dried at 500 “C and then water stabilized in a closed vessel in the presence of NHICI- saturated solution_ The final composition was Na56(A102)5S(Si02)136 .240H20. Although the amount of support +c be impregnated was weighed from this stabilized zeolite, the percentages of metal have bEen referred to the anhydrous zeolite. Once exchanged, the material was filtered, washed with distilled water, and dried for 16 h at 120 “C in an oven. The decomposition process was the same as described above.

Equipment arzd methods Catalytic actiuiry_ The hydrogenation of the substrate was performed in a round-bottomed flask fitted with a side-arm silicone septum to allow injection and sampling for g.1.c. analysis. The flask was connected to a gas burette that allowed pumping of the system and hydrogen dosing. The flask was shaken in 2 water thermostat. The catalyst solution (OF suspension) was degassed in the hydrogenation flask and then H, was admitted. After the prehydrogenation time the substrate solution was added. Acetone or ethanol was used as solvent. The hydrogenation rates were followed with a Perkin Elmer 3920-B gas-liquid chromatograph, using a 1/S in o.d. X 4 m column packed with 15 wt.% fl$‘-osydipropionit.rile-Chromosorb W SO/l00 mesh connected to a second l/8 in o-d. X 4 m column packed with 20 wt.% dinonyl phthalateChromosorb W SO;100 mesh. Cizemical analysis. The Rh content was determined by atomic absorption spectroscopy in a Perkin Elmer 360 apparatus, using a nitrous oxideacetylene flame, h = 399.2 nm and 0.7 mm slit. The chemical attack was made with 38% HF in a sand-bath until total dissolution of the support. The metallic component was dissolved in a sulphuric acid-hydrochloric acid mixture. The rhodium contents experimentally determined, as per cent, referred to the total metal plus anhydrous support mass, are given in Table 2. BET specific surface and active surface. Specific surfaces, SBEr N,, were determined in a Micromeritics 2100 apparatus, 16,.2 A2 being taken for the cross-section of the adsorbed nitrogen moleule. The specific surface of Duolite CC3 was determined in the same apparatus, with krypton as adsorbate, taking 21.0 .42 for the krypton molecule cross-section and using the extrapolated vapour pressure of liquid Kr. Selective chemisorption 0:’ H,, 02, CO and Hz/O2 titrations, in order +a measure the specific metallic surface, were made in an adsorption volumetric apparatus [lo] using an LMKS E aratron 170 M-6A capacitance manometer as

pressure transducer. Particle size was calcukted Tom the expression d = following Spenadel and Boudart [12j _ 5j%I,,.P Pore volume. Tot-d pore volume, V, = 1 jd,, - I jd,, was calculated from the experimentally measured real and apparent densities, determined by conventional helium and mercury volumetrics (Table 1). Pore size distribution was measured by mercury porosimetry, with an Aminco 60.000 porosimeter, and also from the total nit.rogen adsorption isotherms obtained in the Micromeritics 2100 apparatus, following the Pierce and Brunauer ‘model-less’ methods [IO, 135 . 2kM2SRkSiO~ eleceron microscopy. TEM micrographs were taken in a Siemens Eln~iskop LO2 apparatus, working at IQ0 kV, X 100 - 300 X 103. RhjSiOz and Rh/Al,03 samples were prepared for examination by an extractive replica method [X4) , the ca-Cyst being metallized with a carbon layer, and then the support being extracted on a 20% HF plus 8% acetone aqueous solution. The metallic dispersion of Rh/zeolite and Rh/duolite was better studied by an ultramicrotome procedure, the sample being embedded in a Spurr resin 1151 from which 300 - 800 A thick cuts were obtained with a diamond blade [16] . Infrared spectroscopy. IR spectra of Rh/support catalysts were obtained using the disk technique in a greaseless, vacuum-tight cell. The spectra were taken at rocm temperature with a double beam Perkin Elmer 125 apparatus.

Results and discussion Characterization of Rh/support catdysts The experimentally determined rhodium contents for t3e different Rhj support catalysts used in this work are given in Table 2, in which data on metallic dispersion and particle size are also included. Results for the Rh/A1203, Rh/SiOz and Rh/zeolite systems were calculated from the monolayer for hydrogen chemisorption at room temper&Ire, obtained by back extrapolation of the straight part of the isotherm (1 JO - 300 mmHg hydrogen pressures). Pumping under high vacuum for at kast 2 h at 530 “C, or 16 h at 500 “C, is necessary in order to reach a relativel:ir clean metal surface

Cl01 -

Other chemisorption-based experimental meth XLS for the determination of metallic dispersion present some problems by comparison with hydrogen cheFisorption. Oxygen chemisorption at room temperature gives results quite n?ar to those obtained from K2 chemisorption for ‘he Rh/AIzOs and RhjSiO= catiyst series; similar resuks have been clbtained recently by Fuentes and Figueras [ 171. Ho-never, on Rhjzeolite catalysts, the amount of chemisorbed oxygen is remarkably lower than that ,Df hydrogen, this being probably due to penetration problems_ Hydrogenjoxygen titratic?ns give ratios near 3 for the hydrogen chemisorbed bn samples pre-viously covered with oxygen, c:onfirm;Jlg the stoichiometry

I

loooh

4

(b) Fig. 1. T.E.M.

micrographs

(a) Rh 2_45R/Al,O,;

(b)

Rh 0_052%/Duolite

CC3.

187

b

Fig. 2. Particle size distributions. (a) Rh 2.45%/&03;

Rh--O

+ 3/2K2

+

Rh-K

(b) Rh O.O52%/Duolite

CC3.

+ Hz0

found by Benson and Boudart on supported platinum [IS] ; particle sizes calculated from hydrogen titration using this ratio are also very similar to those obtained from direct hydrogen chemisorption. Finally, ix. spectra of Rh/support with chemisorbed CO show wide bands at 2090 cm-r of single bonded CO, and at 1870 cm-‘, typic& of CO adsorbed to multicenters [IS, 201; a smaller band at 2030 cm-l was also observed. While Wanke and Dougharty 1211 in a previous work found CO:K ratios hetween 1 and 4, depending on particle size, in our case the stoichiometric ratio COrK was always lower than 1 (between 0.41 and 0.72) for the different samples. Therefore this method is not suitable for the determination of meftiic dispersion on supported rhodium catalysts [lo, 221. A tern. micrograph of an Rh 2_45%/Al,O, catalyst is given in Fig. l(a); the particle histogram can be seen in Fig. Z(a). The mean particle diameter, d, = CNidi3/~~?r,di2 = 24 .8c, is slightly higher than that calculated from hydrogen chemisorption (21 R), which is consistent with the known tendency to measure predominantly the larger particles when counting highdispersion catalysts by the former method. Chemisorption methods are not applicable to Rh/duoliti catiysts, as the resin decomposes at the temperatures required In the cleaning treatment. Particle sizes for three Rh/duoLite catalysts with different Rh contents, determined from t.e.m., are also collected in Table 2. A micrcgaph of an Rh 0_052%/duolite sample can be seen in Fig. I(b). The particle size distribution is quite narrowcentered (Fig. 2(b)) sh owing the potential of this preparation procedure for obtaining metal catalysts with fairly regular particle sizes CS, 71. Small crystallites, probably aggregates of unreduced precursor molecules, are sparsely seen in the t.e.m. micrographs of Rh/duolite samples. This hypothesis is supported by the t.e.m. micrograph of the [Rh(NBD) (.&Me-py)a] ClOJduolite (Fig. 3).

‘1 .._ _-.

I

Fig. 3. T.E.M.

TABLE

micrograph

,

1000d

of [Rh(NBD)(Z-Me-py)z]ClOd/Duolite

CC3.

3

Hydrogenation

of sc,me unsaturated

substrates

with [Rh(NBD)(2-Me-py)a]C10z

Substrate

Products and rare of appearance** (-01 (mol Rh)-1 (h)-I)

Rehydrogenation (min)

1-Hexene+** 1 -Hexene 2,5-Norbomadiene

Hexane, 4.2 l-Helene, 2.4; Hexane, 0.6 Norbomene, 8.4; Norbomane,

30 30 20

48.6

time

20 OC, volume *Substrate, 2.00 :nmol; ca’alyst, 0.02 mmol; PH, , L atm; temperature, of acetone solution, 15 ml. **Initial rate of a;,pearance (no decomposition of the catalyst was obsemed in this stage). ***The rate of appearance of hexane with the [Rh[NBD)(Z-Me-py)Z]CQ/duolite catalyst is 1.1 (mol (mol Rh)-’ (h)-l).

Catalytic activity Acetone soLEions of [ Rh(NBD)(Z-Me-py),] ClO, react Gith molecule hydrogen z-t atmlxpheric pressure and 20 “C 70 give species which show

189

modest activity for the hydrogenation of several unsaturated substrates (Table 3). In some cases some decomposition of the homogeneous catalyst to rhodium metal was observed. with a corresponding increase in hydrogenation rate. When acetone suspensions of the [ Rh(NBD )( 2-Me-py)s ] ClOJduolite system were similarly treated, no decomposition was observed, but the rate of hydrogenation of I-hexene was Iower, (I.1 mol (mol Rh)-‘- (h)-I). On the contrary, ethanol suspensions of the [Rh(NBD)(B-Me-py).] C:O&hzrolite system react with hydrogen with slow formation of the metallic rhodium/ duolite system, characterized as described above*. Probably the decomposition process is favoured by the higher swelling power of the ethanol as compared with that of acetone. The strong colour change, from yellow to red, upon interaction of [ Rh(NBD)(2-Me-py)& ClO, with duolite suggests that some chemical reaction occurs. In order to determine the type of rhodium complex formed in the so-called [Rh(NBD)(B-Me-py)a] ClOJduolite system, we have studied the following reactions: [ Rh(NBD)(B-Me-py),]

CIO, + Na(Me-COO)

1/2[Rh(_Me-COO)(NBD)] AgC104 Me&O

l

2 + NaCIO,

[Rh(NBD)(MesCO),]

ClO,

+

+ BMe-pyl/B[RhC1(NBD)] Na(Me-COO)

2

+

1/2[Rh(Me-COO)(NBD)]

2.

In fact, some previous studies [24] have shown :hat the suspected yellow complex Rh(MeCOO)(NBD)(py) gives, at room temperature, the red compound ]Rh(Me-COO)(NBD)], . So we believe that the carboxyiic units of the polymer should be bridging rhodium-norbomadiene groups, with the formation of [Rh(R-COO)(NBD)], dimers on the resin exchange_ Results for the catalytic activity of the supported catalysts are given in Table 4. The rates at 20 “C are given per gramme of catalyst, per gramme of rhodium metal and also as turnover number (rate per minute per surface Rh atom). It can be seen that the Rh/zeolite catalysts exhibit the highest activity. The structure insensitioe character of olefin hydrogenation reactions is firmly estiblished [25,26] _ The turnover numbers found for Rh/Si& and Rhlduolite catalysti are in agreement with this. In both pca.sesthe catalysts show a fairly good hq drogenation activity, @pical of rhodium metal, which

*A similar behaviour has beer. observed for the [Rh(KBD)(PoIy-CN)o] ClO, complex, pregmred by reaction of [Rh(NBDJ(Me&O)E]ClO~ t2.11 and Poly-CN (Polg(styren+ divinytbenzene) fr.mctionalized with CN groups). T’h~e materi& are presently being studied.

190 TABLE4 Hydrogenation of 1 hexene* Catalyst

RI?

(57,)

-/AI,03

0.092 0.609 2.45

Rate of hydrogen uptake (m01H2 g-l min-I)

0.31x10-4 1.09 x10-4 1.8-5x 1O-4

(molH2 g-l (RhO)min-I)

0.034 0.018 0.008

Mean partidesize Turnover number (min-l) 3.9

2.0 1.8

(A) 10 10 21 24 15 11 11

0.117 0.252 0.685 1.35

2.12x10-" 7.69x lo+ 9.90x10-4 11.9 x10-4

0.181 0.305 0.145 0.088

49

Rh/Y zeolite

0.285 0.391 0.534

2.1.4 x 10-4 2i.6 X 10e4 43-l x lo- 4

0.857 0.707 O-308

381 320 168

39 40 18

Rh/duolite

0.052 c.091 0.17

0.82x 1O-4 l-47 x 10-a 1.48x10-'

0.169 O-163 O-087

34 48 26

18 25 25

Rh/SiOp

50 18 11

.

*Acetone;temperature,20 'C;PH~ !latm.

is in agreement with the behaviour of the metallic component in systems -.vith a weak metal/support interaction_ Within the Rh/SiOe series it seems that the catalytic activity diminishes for the lower particle size catalysts, with d < 15 A. The decrease of activity per site of very small particles for an easy reaction has recently been found by Figueras and coworkers [ 171 for benzene hydrogenation on Rh/A1203 catalysts_ This is attributed to the formation of clusters, with lower activity, in the surface, rather than to a lower catalytic activity of the metal atoms in the edges, whose relative importance increases for the lower-sized metal particles 126, 27 J _ The lower turnover numbers of the Rh/AlaOs catalysts are probably related to a strong metal/support interaction; it is known that rhodium readily reacts with alumina, diffusing into the subsurface region [28]_ Ahmina strongly reacts with noble metals and with metal oxides [29] , this fact determining some properties - resistance to sistering and also to reduction, etc. - that makes alumina the catalytic support most used at present [30]. The higher activity of rhodium supported on Y zeolite for orefin hydrogenation and other reactions has also been observed by some authors [31, 32]_ Fcr benzene hydrogen&ion, similar results have been found on E%/Y zeo!ite by Da!.la Betta and Boudart [33] , and on Pd/Y ze&ite by Figueras et al. [ 34]_ Both authors base their explanation for the higher activity of these

catalysts, as compared with Rh/aIumm a or Rh/siEca, on a high density of election-acceptor sites in the Rh/zealite. Boudart [33] emphasizes the electron deficiency of pIatinum cIusters a.5 a result of partial electron transfer to the zeohte. Okamoto et al. ]32] assign the high cataI:rtic activity of Rh/Y zeohte to a combination of two types of centers: (i) surface rhodium metal atoms, and (ii) rhodium cations, mainly Rh(I), which are very active in hydrogenation and isomerization Et is most unlikely to be the existence of Rh(I) in our experiments, after the strong reduction treatment (2 h at 550 “C in hydrogen) undergone by the ca:alysts in the preparation procedure. It is more plausible to think of a reinforced activity in terms of Boudart’s (Zoc. cit.) ideas, based on the effect of the fraction of smaher rhodium aggregates, as could be inferred from the metal particle size &&rib&ions [lo] _ As expected, the catiytic activity of the homogeneous and heterogeneous complex is in agreement with the known fact that a mononuclear complex is able to catalyze a strucCure insenslfive reaction 1351. The satisfactory results of catalytic activity obtained for the Rh/duolite system show this catalyst to be competitive with conventional silica- or alumina-supported metal catalysts. R new advantage of these catalysts would be seen by study of structure sensitive reactions, where the preparation of catalysts, narrowly centered on a particular size (easier to obtain on a resin, because of its surface homogeneity) is essential. Finally, it is worth mentioning that duoiite resins do not seem to have particularly favourable properties as supports; perhaps it would be more promising to investigate resins with a developed macroporous system [36] _

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33 36

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