Preparation, characterization and catalytic hydroformylation properties of carbon nanotubes-supported Rh–phosphine catalyst

Applied Catalysis A: General 187 (1999) 213–224

Preparation, characterization and catalytic hydroformylation properties of carbon nanotubes-supported Rh–phosphine catalyst Yu Zhang, Hong-Bin Zhang ∗ , Guo-Dong Lin, Ping Chen, You-Zhu Yuan, K.R. Tsai Department of Chemistry & State Key Lab of Phys. Chem. for the Solid Surfaces, Xiamen University, Xiamen 361005, China Received 9 April 1999; received in revised form 31 May 1999; accepted 31 May 1999

Abstract Two kinds of carbon nanotubes grown catalytically, as a novel material for catalyst carrier, were prepared and characterized. Propene hydroformylation catalyzed by the Rh–phosphine complex catalysts supported by carbon nanotubes was investigated, and compared to that catalyzed by the Rh–phosphine complex catalysts supported by SiO2 (a silica gel), TDX-601 (a carbon molecular sieve), AC (an active carbon), and GDX-102 (a polymer carrier). Activity assay of the catalysts showed that the carbon nanotubes-supported Rh–phosphine complex catalysts displayed not only high activity of propene conversion but also excellent regioselectivity to the product butylaldehyde. Under the reaction conditions of 393 K, 1.0 MPa, C3 H6 /CO/H2 = 1/1/1 (v/v), GHSV = 9000 ml (STP) h−1 (g catal.)−1 , P/Rh = 9–12 (molar ratio), and Rh-loading at 0.1 mmol Rh (g carrier)−1 , the molar ratio of normal/branched (n/i) aldehydes reached 12–13 at a turnover frequency (TOF) of 0.12 s−1 , corresponding to propene conversion of ∼32%. The characterization by using TEM, HRTEM, XRD, Raman, XPS, BET and temperature programmed desorption (TPD) methods indicated that the carbon nanofibers prepared were quite even nanotubes with the outer diameters at 15–20 nm and the inner diameters (i.e., pore diameters) at ∼3 nm. Each tube wall was constructed of many layers of carbon with graphite-like platelets in a cross-section orientation of ‘parallel type’ or ‘fishbone type’; their C (1 s) electron binding energy was about 0.5 eV lower than that of graphite. These results, together with the results of comparative studies of the Rh–phosphine complex catalysts supported by several other carriers, implied strongly that the tubular channels with the inner diameter of ∼3 nm in the carbon nanostructures and its hydrophobic surface consisting of six-membered C-rings played important roles in enhancing the activity of propene hydroformylation, especially the regioselectivity of butylaldehyde on the Rh–phosphine complex catalysts supported by them. ©1999 Elsevier Science B.V. All rights reserved. Keywords: Carbon nanotubes grown catalytically; Carbon nanotubes carrier; Propene; Supported Rh–phosphine complex catalyst; Hydroformylation; Spatio-selective catalysis

1. Introduction Hydroformylation of olefins in the presence of a homogeneous catalyst to form aldehydes containing an additional carbon atom has been applied commer∗ Corresponding author. Tel.: +86-592-2086580; fax: +86-592-2086116 E-mail address: [email protected] (H.-B. Zhang)

cially for producing higher aldehydes from olefins and syngas for years [1]. However, a homogeneous catalyst system has disadvantages, such as discontinuous operation and difficulties in the separation of catalyst from the reaction mixture. In recent years, a considerable effort has been directed towards the development of immobilization of homogeneous catalysts, and special attention has been given to the supported aqueous phase catalysts (SAPCs) [2,3] and the supported

0926-860X/99/$ – see front matter ©1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 8 6 0 X ( 9 9 ) 0 0 2 2 9 - X

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liquid phase catalysts (SLPCs) [4]. These supported catalyst systems allow the separation of catalyst from the reaction mixture to be greatly simplified, thus facilitating successive operation, but are still faced with the problem of how to improve the low regioselectivity to the product aldehydes (i.e., molar ratio of normal/branched (n/i) aldehydes). Carbon nanotubes, one type of tubular carbon nanofibers, as a novel material for catalyst carrier, are attracting increasing attention recently [5]. Scientists are surprised by the tubular nanochannel (2–3 nm for inner diameter) together with the tube wall with graphite-like structure and the surface constructed of six-membered carbon rings. Noticeable also is its large specific surface area (100–700 m2 g−1 ); the hydrophobic or hydrophilic character of the surface can be controlled by chemical treatment and/or modification. All these peculiarities make carbon nanotubes a novel material for catalyst carrier, with a range of properties suitable for supporting many types and amounts of metal or active complex species. In the present work, two kinds of carbon nanotubes grown catalytically and the Rh–phosphine complex catalysts supported by them were prepared and characterized. Their property of catalytic propene hydroformylation was investigated and compared to those of the Rh–phosphine complex catalysts supported by SiO2 (a silica gel), TDX-601 (a carbon molecular sieve), AC (an active carbon), and GDX-102 (a copolymer of styrene with divinylbenzene). The results have potential significance for better understanding of the peculiarities of the carbon nanotubes as a novel material for catalyst carrier and for the development of new supported catalyst systems based on the carbon nanotubes.

2. Experimental 2.1. Preparation of the carbon nanotubes grown catalytically A Ni–MgO catalyst used for catalytic growth of the carbon nanotubes was prepared by our previously reported method [6]; i.e., 2.91 g of Ni(NO3 )2 ·6H2 O (purity in AR grade) and 2.56 g of Mg(NO3 )2 ·6H2 O (in AR grade) powders were mixed thoroughly, followed by addition of 2 g of citric acid (in AR

grade) and 20 ml of deionized water to form a solution; subsequently, the solution was evaporated by evacuation from room temperature to 353 K in a programmed way. The solid material obtained was dried at 373 K in air, and subsequently, calcined at 973 K in air for 5 h, and finally, a black and fluffy sample of the catalyst precursor was obtained. The result of XRD measurements indicated that the NiO and MgO components formed a Nix Mg1−x O solid solution in the catalyst precursor after calcination. The growth of carbon nanotubes was performed according to our method described previously [6] by catalytic decomposition of CH4 or CO on the Ni–MgO catalyst. The catalyst precursor of 40 mg was reduced in a flow of purified hydrogen at 973 K for 30 min, followed by introducing the feedgas of CH4 or CO at 873 K, with a flow rate of 2400 ml h−1 for CH4 and 1200 ml h−1 for CO. After 60 min of the reaction operation, about 150 and 90 mg of the raw products were obtained from CH4 and CO, respectively. The raw products were further purified by means of immersion in a certain concentration (∼4 M) of nitric acid solution so as to dissolve the catalyst particles (containing metal Ni0 , NiO, and MgO) attached at the extremities of the nanotubes, followed by washing with deionized water, then drying at 473 K in a flow of nitrogen, and finally, hydrogen-treating at 673 K for 1 h. Thus, the carbon nanotubes products with hydrophobic surface were obtained. From the TEM observation, it could be roughly estimated that more than 90% of the carbon deposit products were in the form of nanotubes. 2.2. Preparation of supported Rh–phosphine complex catalysts The complex HRh(CO)(PPh3 )3 was prepared by the known method [7], with the observed infrared absorption bands at 2010 cm−1 for Rh–H stretching and 1920 cm−1 for C–O stretching of carbonyl, and with 31 P(1 H)-NMR double peaks at 43–41 ppm, which were in good agreement with values reported earlier [8]. 31 P(1 H)-NMR measurement showed that the purity of the Rh–phosphine complex synthesized reached above 98%. The supported Rh–phosphine complex catalysts were prepared by an incipient wetness technique. A solution containing desired amounts of the

Y. Zhang et al. / Applied Catalysis A: General 187 (1999) 213–224

HRh(CO)(PPh3 )3 complex in benzene was impregnated onto the purified and hydrogen-treated carbon nanotubes carrier; after 30 min, the solvent benzene was evaporated by evacuation at room temperature, and if necessary, a solution containing additional amounts of PPh3 in benzene was impregnated again onto the above sample, followed by another 30 min wait, and then evacuation at room temperature to remove the solvent benzene from the solid material. The sample of catalyst precursor obtained was further dried and preserved in an atmosphere of purified nitrogen. With SiO2 (80–100 mesh, product from the Ocean-chemical Plant of Qingdao), or TDX-601 (80–100 mesh, produced by the Institute of Inorg. Chem. of Shanghai), or AC (80–100 mesh, supplied by the Active Carbon Plant of Shanghai), or GDX-102 (40–60 mesh, produced by the Chem. Reagent Plant of Tianjin), to replace the carbon nanotubes, the Rh–phosphine complex catalysts supported by each material were prepared, following the same procedure as that described above. 2.3. Evaluation of catalyst activity for propene hydroformylation The evaluation of the catalyst activity was performed in a fixed-bed continuous flow reactor–GC combination system, operating under a pressure of 1.0 MPa. The hydroformylation reaction of propene over the catalysts was carried out at a stationary state and under the following reaction conditions: 393 K, 1.0 MPa, feedgas composition C3 H6 /CO/H2 = 1/1/1 (v/v), GHSV = 9000 ml (STP) h−1 (g catal.)−1 . A catalyst sample of 100 mg was used each time for testing. The reactants and products were analyzed by an on-line SQ-206 Model gas chromatograph (GC, made by Beijing Analytic Instruments Co.), equipped with a thermal conductivity detector and a 6 m long polyethyleneglycol (PEG)/#102 white support column, with hydrogen as carrier gas. All data were taken at 2 h after the start of the reaction, unless otherwise noted.

215

microscope and a Hitachi H-9000 machine, respectively. BET surface area, pore volume and pore diameter of the carriers were determined by using a Sorptomatic-1900 Surface Area Analyzer (Carlo Erba Instruments, Italy). X-ray diffraction measurements were carried out by using a Rigaku D/Max-C X-ray Diffractometer with Cu K␣ radiation at a scanning rate of 8◦ min−1 . Laser Raman spectra were taken by using a highly sensitive confocal microprobe Raman system (LabRam I from Dilor, France) with an air-cooled 1024 × 256 pixels CCD (Wright, England). The 514.5 nm line from a Coherent-Innova Model 200 argon ion laser was used for excitation. 31 P(1 H)-NMR spectra of solution and supported liquid-film were taken by using a Varian Unity +500 NMR spectrometer at room temperature and 200 MHz, calibrated externally by 85% phosphoric acid in aqueous solution. Temperature-programmed desorption (TPD) of ammonia and benzene on the carbon nanotubes and SiO2 was conducted on a fixed-bed continuous-flow reactor–GC combination system. Purified gaseous helium was used as carrier gas; the rate of elevation of temperature was 10 K min−1 ; 50 mg of adsorbent sample was used each time for testing. NH3 -pre-adsorption was carried out by treating the adsorbent samples by passing a stream of purified gaseous ammonia with the flow rate of 30 m min−1 at room temperature (RT) for 10 min; and pre-adsorption of benzene was performed by treating the adsorbent samples by passing a stream of purified benzene-saturated helium at RT and the flow rate of 10 ml min−1 for 10 min, followed by elevating temperature at a rate of 10 K min−1 from RT to 373 K in a stream of purified helium as carrier gas at the flow rate of 30 ml min−1 and keeping at 373 K in the stream of helium for 30 min. This would remove the liquid benzene formed due to coagulation condensation in the micropores of the adsorbents. Then the temperature was lowered to RT for TPD measurement.

3. Results and discussion

2.4. Spectroscopic measurements

3.1. Catalytic propene hydroformylation behavior of the supported Rh–phosphine complex catalysts

TEM and HRTEM observations were performed by using a JEOL JEM-100CX transmission electron

Activities of propene hydroformylation over the supported Rh–phosphine complex catalysts based

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Table 1 Results of the activity assays of the Rh-catalysts supported by the carbon nanotubes for hydroformylation of propenea Carrier

Rh-loading (mmol Rh (g carrier−1 ))

P/Rh (mol/mol)

Conversion of C3 H6 (%)

TOF (s−1 )

STY (mol h−1 (g Rh−1 ))

Selectivity (n/i)

Carbon nanotubes (derived from CH4 )

0.025

12

10.7

0.16

5.57

10.5

0.05

6 9 12 15

17.0 20.8 18.8 16.8

0.13 0.15 0.14 0.12

4.42 5.41 4.91 4.37

6.2 10.5 11.4 12.0

0.1

6 9 12 15

26.1 30.8 30.6 28.9

0.10 0.12 0.12 0.11

3.40 4.01 3.98 3.76

9.0 12.0 13.5 13.5

Ends-unopened carbon nanotubes (derived from CH4 )b

0.1

6

17.2

0.06

2.10

6.0

Carbon nanotubes (derived from CO)

0.025

12

16.1

0.24

8.38

6.9

0.05

6 9 12 15

21.6 24.8 25.4 22.4

0.16 0.18 0.19 0.17

5.62 6.45 6.61 5.83

5.1 9.0 11.2 12.4

0.1

6 9 12 15

31.0 34.1 32.5 32.0

0.12 0.13 0.12 0.12

4.03 4.44 4.23 4.16

7.2 10.3 11.2 13.7

a Reaction conditions: 393 K, 1.0 MPa, feedgas composition C H /CO/H = 1/1/1 (v/v), GHSV = 9000 ml (STP) h−1 (g catal.)−1 ; the 3 6 2 by-products formed from side reactions of hydrogenation (i.e., propane, n-butanol and i-butanol) were below 1% (molar fraction) of the total carbon-based products. b The catalyst particles attached at the extremities of those nanotubes were not removed due to their being free from an acid-immersion treatment; thus, their ends were not opened.

on the carbon nanotubes and the other four carriers selected were evaluated; parts of the results are shown in Tables 1 and 2. The Rh–phosphine complex catalysts supported by the two kinds of carbon nanotubes displayed not only high activity of propene conversion but also excellent regioselectivity (represented by n/i, a molar ratio of n-butylaldehyde to its isomer, i-butylaldehyde, in the products), in comparison with those for the Rh–phosphine complex catalysts supported by the other carriers, such as SiO2 , TDX-601, AC, and GDX-102. Under the reaction conditions of 393 K, 1.0 MPa, C3 H6 /CO/H2 = 1/1/1 (v/v), GHSV = 9000 ml (STP) h−1 (g catal.)−1 and the molar ratio of P/Rh at 12, the average turnover frequencies (TOFs) of propene at each Rh site were 0.14 and 0.19 s−1 , respectively, over the two carbon nanotubes-supported Rh–phosphine complex

catalysts with the Rh-loading at 0.05 mmol Rh (g carrier)−1 . Fig. 1(a) and (b) illustrate the results of the stability test of the two carbon nanotubes-supported Rh–phosphine complex catalysts with the Rh-loading at 0.1 mmol Rh (g carrier)−1 and P/Rh = 9 (molar ratio) during 30 h of hydroformylation operation: under the reaction conditions mentioned above, the conversion of propene reached ∼30% and ∼36% (the corresponding TOF at 0.11 and 0.14 s−1 , respectively) at the initial stage of the reaction, and subsequently descended slowly and came down to 19% and 24% (the corresponding TOF at 0.07 and 0.09 s−1 , respectively) after 30 h of the reaction operation; however, the descent tendencies in the regioselectivity were much more gentle, with the corresponding n/i ratios still maintained at a level of 12–10.

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Table 2 Results of the activity assays of the Rh-catalysts supported by the other four carriers for hydroformylation of propenea Carrier

Rh-loading (mmol Rh (g carrier−1 ))

P/Rh (molar ratio)

Conversion of C3 H6 (%)

TOF (s−1 )

STY (mol h−1 (g Rh−1 ))

SiO2

0.05

0.15

6 9 12 15 6 9 12 15 6

19.3 23.9 22.5 19.6 30.3 32.8 31.7 30.5 28.5

0.14 0.18 0.17 0.15 0.11 0.12 0.12 0.11 0.07

5.02 6.22 5.85 5.10 3.94 4.27 4.12 3.97 2.47

6.4 7.3 7.5 9.1 7.0 7.9 8.2 9.9 9.1

0.05

6

11.3

0.08

2.94

15.3

0.1 0.15

9 12 15 6 6

11.4 10.2 10.4 9.1 10.9

0.08 0.08 0.08 0.034 0.027

2.97 2.65 2.71 1.18 0.95

19.4 21.2 23.8 15.3 17.8

AC (active carbon)

0.05 0.1

6 6

2.9 5.3

0.02 0.02

0.76 0.69

2.8 2.5

GDX-102b

0.05 0.1

6 6

22.3 38.5

0.17 0.14

5.80 5.01

3.4 5.6

0.1

TDX-601 (carbon molecular sieve)

Selectivity (n/i)

a Reaction conditions: 393 K, 1.0 MPa, feedgas composition C H /CO/H = 1/1/1 (v/v), GHSV = 9000 ml (STP) h−1 (g catal.)−1 ; the 3 6 2 by-products formed from side reactions of hydrogenation (i.e., propane, n-butanol and i-butanol) were below 1% (molar fraction) of the total carbon-based products. b A carrier of co-polymer of styrene with divinylbenzene.

Over the Rh–phosphine complex catalysts supported by the other two carriers with large surface area and large pore diameters, SiO2 and GDX-102, the conversions of propene were close to those of the carbon nanotube-supported catalysts, but their regioselectivities were relatively low, even in the case of high P/Rh molar ratios. It is worth noting that the TDX-601-supported system displayed the highest regioselectivity among these catalyst systems investigated in the present work, but the corresponding conversion activity of propene reached only about a half of those for the carbon nanotubes-based systems. As for the AC-supported catalyst system, its activity and regioselectivity were both extraordinarily low. It should be mentioned that the same deactivation phenomenon was also observed on other Rh–phosphine complex catalysts in liquid phase or supported aqueous phase or supported liquid phase. The SiO2 -supported HRh(CO)(PPh3 )3 catalyst also displayed the deactivation characteristic analogous to that of the carbon nanotubes-supported systems.

The stability test of the SiO2 -supported catalyst for 30 h of reaction under comparable conditions (i.e., 1.0 MPa, C3 H6 /CO/H2 = 1/1/1 (v/v), GHSV = 9000 ml (STP) h−1 (g catal.)−1 , Rh-loading at 0.1 mmol (g carrier)−1 and P/Rh = 9 (molar ratio)) showed that propene conversion descended slowly from the initial 32.8% (corresponding to a TOF of 0.12 s−1 ) down to 20.3% (the TOF at 0.07 s−1 ) after 30 h of reaction; correspondingly, the n/i ratio descended from the initial 7.9 down to 6.4 after 30 h of reaction. As is generally accepted now, the loss of activity with time may be due to degradation reactions of the ligand [1] and/or to the oxidation of the PPh3 -ligand to triphenylphosphine oxide (OPPh3 ) [9]. In order to get better information about the location of the supported Rh–phosphine complex species on the carbon nanotubes carrier, an ends-unopened carbon nanotube sample (i.e., the catalyst particles attached at the extremities of those nanotubes were not removed due to being free from an acid-immersion treatment; thus their ends were not opened) was used as

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Fig. 1. Stability testing for 30 h of the supported Rh-catalysts based on the carbon nanotubes produced from CH4 (a) and CO (b). Reaction conditions: 393 K, 1.0 MPa, C3 H6 /CO/H2 = 1/1/1 (v/v), GHSV = 9000 ml (STP) h−1 (g catal.)−1 , Rh-loading at 0.1 mmol Rh (g carrier)−1 , and P/Rh = 9 (molar ratio).

the carrier. The Rh–phosphine complex species were not able to get into the inner channel of the carbon nanotubes in the catalyst supported by it, and were only dispersed on the outer surface. Under the comparable conditions described in Table 1, propene conversion and the n/i ratio on the ends-unopened carbon nanotubes-supported catalyst reached only 17.2% (corresponding to a TOF of 0.06 s−1 ) and 6.0, respectively, which were obviously lower than those of the catalyst supported by the ends-opened carbon nanotubes (i.e., 26.1% of propene conversion and 9.0 of the n/i ratio). This result provides an important indication that the high propene conversion and the excellent regioselectivity (n/i ratio) on the ends-opened carbon nanotubes-supported catalysts were mainly due

to the contributions made by the catalytically active Rh–phosphine complex species located on the inner surface of the tubular nanochannel. The molar ratio P/Rh has a pronounced effect on the catalytic hydroformylation behavior of the supported Rh–phosphine complex catalysts. It is shown by the data in Tables 1 and 2 that the TOF of propene and the molar ratio of normal/branched aldehydes (n/i) both increased initially as addition of PPh3 was increased, and tended towards a stable level with the P/Rh molar ratio reaching 12–15. It seems that a P/Rh molar ratio of 9–12 would be appropriate for the Rh–phosphine complex catalysts supported by the two kinds of carbon nanotubes. The results of the investigation about effects of Rh-loading on the hydroformylation performance of the carbon nanotubes-supported Rh–phosphine complex catalysts are also shown in Table 1. The experimental results showed that the TOF of propene and the selectivity to n-butylaldehyde (n/i) both tended towards stable levels at a Rh-loading of 0.1 mmol Rh (g carrier)−1 . It seems that the Rh-loading of ∼0.1 mmol Rh (g carrier)−1 was proper for the two kinds of carbon nanotube carriers. Table 3 shows the effect of temperature on the hydroformylation catalyzed by the Rh–phosphine complex catalysts supported by the carbon nanotubes. The TOF of propene and the STY of butylaldehyde were both enhanced, but the n/i (molar ratio) descended somewhat, with elevating reaction temperature. The experimental results indicated that the by-products formed from side reactions of hydrogenation (i.e., propane, n-butanol and i-butanol) were below 1% (molar fraction) of the total carbon-based products for reaction temperatures not over 393 K. It was also found that a reaction temperature higher than that which is enough (e.g., above 403 K) would easily bring about an increase in the by-products of hydrogenation and condensation, and easily lead to the stability of catalytically active complex species descending, thus speeding up deactivation of the catalysts, which is consistent with the report by Pelt et al. [9]. 3.2. Characterization of the carbon nanotubes carriers It is evident that the exceedingly good performance that the carbon nanotubes-supported Rh–phosphine

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219

Table 3 Effect of temperature on propene hydroformylation over the carbon nanotubes-supported Rh-catalystsa Carrier

Temperature (K)

Conversion of C3 H6 (%)

TOF (s−1 )

STY (mol C3 H7 CHO h−1 (g Rh−1 ))

Selectivity (n/i)

Carbon nanotubes derived from CH4

383

24.6

0.09

3.20

14.1

393 403

30.6 39.1

0.12 0.15

3.98 5.09

13.5 13.5

383

27.7

0.10

3.60

13.9

393 403

32.5 39.3

0.12 0.15

4.23 5.11

11.2 11.2

Carbon nanotubes derived from CO

Reaction conditions: 1.0 MPa, feedgas composition C3 H6 /CO/H2 = 1/1/1 (v/v), GHSV = 9000 ml (STP) h−1 (g catal.)−1 , Rh-loading at 0.1 mmol (g carrier)−1 , and P/Rh = 12 (molar ratio). a

Fig. 2. TEM images of the carbon nanotubes prepared by catalytic decomposition of CO (a) and CH4 (b).

Fig. 3. HRTEM images of one of the carbon nanotubes produced from catalytic decomposition of CO (a) and CH4 (b).

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Table 4 BET surface area, pore volume, and pore diameter of the various carriers Carrier

BET surface area (m2 g−1 )

Pore volume (cm3 g−1 )

Pore diameters(nm)

Carbon nanotubes (derived from CH4 ) Carbon nanotubes (derived from CO) SiO2 TDX-601 (carbon molecular sieve) AC (active carbon) GDX-102a

155 237 380 1210 592 501

0.46 1.33 1.03 0.43 0.21 1.66

3.2–3.6 2.4–3.2 6–12 1.6–2.4 1.4–2.0 20–100

a

A carrier of co-polymer of styrene with divinylbenzene.

complex catalysts displayed in catalytic hydroformylation is closely related to the peculiar structure and properties of the carbon nanotubes. Fig. 2 gives the TEM images of the raw products of the two kinds of carbon nanotubes prepared from the decomposition of methane and the disproportionation of carbon monoxide, respectively, on the Ni–MgO catalyst. These carbon nanotubes are small and even in diameter size, and their morphology is more or less twisted. The catalyst particles attached at the extremities of the nanotubes can be eliminated by dissolution by the nitric acid solution, leading to the end of the tubes to be open. From the TEM images, it could be roughly estimated that their outer diameters were 15–20 nm and the inner diameters were ca. 3 nm, and the tube lengths were probably as long as 10 ␮m. In a general way, the carbon nanotubes derived from CH4 have thicker trunks than those derived from CO. The HRTEM observation further reveals that the wall of the nanotube is constructed by many layers of carbon with graphite-like platelets in an extremely ordered arrangement: the orientation of the cylindrical graphite-like platelets is parallel to the tube axis (so-called ‘parallel type’) for the nanotube produced from CO and the conical graphite-like platelets are inclined to the central axis (so-called ‘fishbone type’ [10]) for that derived from CH4 , respectively, as shown in Fig. 3. The surface area, pore volume, and pore diameters of the carbon nanotubes and the other four carriers were measured by the BET method; the results are shown in Table 4. The surface area of the carbon nanotubes produced from CO was as high as ∼237 m2 g−1 , and higher than that (∼155 m2 g−1 ) of those produced from CH4 . However, the sizes of the pore diameters (i.e., inner diameter) of the two kinds of carbon nanotubes were quite close to each other: 2.4–3.2 nm for those derived from CO and 3.2–3.6 nm

for those from CH4 . This is consistent with the results estimated from the TEM observation mentioned above. Fig. 4 shows the XRD patterns of the carbon nanotubes and graphite. The results show that the main XRD feature of the carbon nanotubes at 2θ = 26.2◦ is close to that of graphite at 2θ = 26.6◦ , but somewhat broadened, indicating that the degree of long-range order of these nanostructures is relatively low in comparison with that of graphite. In addition, the other two weak peaks at 2θ = 21.0◦ and 2θ = 23.9◦ are also observed; these are ascribed to chaoite [11]. The Raman spectra shown in Fig. 5 indicate that the Raman spectral features of the carbon nanotubes are rather different from those of graphite, but much closer to those of a low-order carbon, indicating that

Fig. 4. XRD patterns of graphite (a), and the carbon nanotubes produced from catalytic decomposition of CH4 (b) and CO (c). * peaks due to graphite phase; ** peaks due to chaoite phase.

Y. Zhang et al. / Applied Catalysis A: General 187 (1999) 213–224

Fig. 5. Laser Raman spectra of graphite (a), a low-ordering carbon (b), the carbon nanotubes produced from catalytic decomposition of CO (c) and CH4 (d).

Fig. 6. C (1s)-XPS spectra of the carbon nanotubes produced from the catalytic decomposition of CH4 (a) and CO (b).

the long-range-order degree of the arrangement of carbon atoms on the surface of the carbon nanotubes is not as high as that of graphite. The results of the XPS measurement shown in Fig. 6 reveal that the binding energy of C (1 s) electron in these carbon nanotubes was 283.8–284.0 eV, and 0.6–0.4 eV lower than that (284.4 eV) of graphite, implying that the valence electrons on the carbon atom, especially on C-rings, in the carbon nanotubes could escape somewhat more easily or become delocalized in comparison with those in graphite. The comparative investigation of adsorption of NH3 and C6 H6 on the carbon nanotubes and SiO2 may be

221

Fig. 7. TPD spectra of NH3 (I) and C6 H6 (II) adsorbed on the carbon nanotubes derived from CH4 (a), CO (b) and SiO2 (c).

expected to provide useful information about the nature of the surface of the carbon nanotubes. The TPD spectra of NH3 and C6 H6 shown in Fig. 7 revealed that an obvious difference in the adsorption/desorption behavior towards NH3 and C6 H6 existed between the carbon nanotubes and SiO2 . For ammonia adsorption on SiO2 , a strong NH3 -TPD peak at 364 K (Fig. 7Ic) was observed; while, on the carbon nanotubes, almost all NH3 -TPD peaks were quite weak and ambiguous beyond recognition (Fig. 7Ia and b), indicating that the interaction of the surface of the carbon nanotubes with basic and/or hydrophilic molecules such as NH3 was weak. But for benzene adsorption and desorption on these carriers, the situation was completely different: strong C6 H6 -TPD peaks at 431 and 410 K were observed on the two kinds of carbon nanotubes, respectively (Fig. 7IIa and b); while there was not any C6 H6 -TPD signal detected on the silica-gel (Fig. 7IIc). These results demonstrate distinctly that the surface of the carbon nanotubes is indeed markedly hydrophobic; it is worth noting that their interaction with an aromatic compound molecule such as benzene is strong. 3.3. Nature of the promoting action of the carbon nanotubes carrier The BET measurements have shown that the surface areas of the two kinds of carbon nanotubes were ∼237 and ∼155 m2 g−1 , respectively; such values were much lower than those of the other four carriers (i.e., SiO2 , TDX-601, AC, and GDX-102). It is

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Table 5 Relation of the pressure boundary of n-butylaldehyde in gas-phase for capillary condensation in the carbon nanotubes with their pore diameters at different temperatures T (K) T (K)

P0 (C3 H7 CHO) (atm)

P (C3 H7 CHO) (atm)

Pore diameter (nm)

383

2.796

2.1 1.8 1.5

3.9 2.6 1.8

393

3.606

2.5 2.1 1.8 1.5 1.2 0.50 0.30

3.0 2.0 1.6 1.2 1.0 0.56 0.44

403

4.586

2.5 1.8 1.2

1.8 1.2 0.8

thus evident that, in comparison with the catalysts supported by the other four carriers, the high propene conversion activity, especially the excellent regioselectivity (n/i) on the carbon nanotubes-supported Rh–phosphine complex catalysts were not due to the difference in their surface areas. On the other hand, it can be estimated according to the well-known Kelvin equation based upon the capillary coagulation theory that, under the actual reaction condition (i.e., 1.0 MPa, 393 K, C3 H6 /CO/H2 = 1/1/1 (v/v)) and even in the case of propene conversion attaining to 45% (corresponding to a C3 H7 CHO partial pressure of 0.21 MPa in the reaction exit gas), the capillary coagulation of C3 H7 CHO occurred only in the hydrophobic micropores with the pore diameters of ≤2.0 nm. According to the pressure boundary of gas-phase for the capillary condensation of n-butylaldehyde shown in Table 5 and the pore diameter data in Table 4, as well as the actual propene conversion levels attained in the present work, one can conclude that the capillary coagulation of n-butylaldehyde could not occur on the carbon nanotubes carriers with pore diameters of ca. 3.0 nm as well as on the other four carriers. The possibility that the 3 nm pores act merely to capillary-condense a carbon nanotubes-supported liquid phase catalyst may be ruled out. As a matter of fact, the n/i ratio on the liquid phase Rh–phosphine complex catalyst system was not as

high as that on the corresponding complex catalyst supported by carbon nanotubes under comparable conditions. It was earlier reported by Wilkinson [12] that, when RhH(CO)(PPh3 )3 was used as catalyst precursor, a n/i ratio of 2.33 was obtained under conditions of 373 K, ∼3.5 MPa, CO/H2 = 1/1 (v/v) and P/Rh = 10 (mole ratio); when PPh3 was used as solvent for the catalytic reaction system, corresponding to P/Rh ∼ = 600 (mole ratio), the n/i ratio attained to 15.3 under the conditions of 398 K, 1.25 MPa, CO/H2 = 1/1. Thus, it seems to us that the excellent catalytic performance of the carbon nanotubes-supported Rh–phosphine complex catalysts is most probably associated closely with the nanochannels of the carbon nanotubes carrier and its surface constructed of the graphite-like six-membered C-rings. The tubular nanochannels with pore diameters of ∼3 nm are quite suited to accommodating the ∼1.8 nm RhH(CO)(PPh3 )3 complex (the catalyst precursor) [13] and/or its fragment RhH(CO)(PPh3 )2 (the functioning catalytically active species), leaving an appropriate space for diffusion and reactions of the reactant molecules. This would be in favor of enhancing the regioselectivity to reaction intermediates and the product butylaldehyde molecules by means of spatiospecific selective catalysis by rigorous spatio-restraint. Moreover, as shown by the above TPD measurements, a strong interaction may exist between the phenyl of the phosphine ligands and the hydrophobic carbon nanotubes surface constructed of the graphite-like six-membered C-rings, due to their similarities in structural and electronic properties. This, on one hand, would be beneficial to even dispersion of the Rh–phosphine complex species on the surface; but, on the other hand, it would also be easy to bring about decomplexation of coordinated PPh3 ligands, thus leading to lowering of the activity and the regioselectivity, unless it is compensated by adding a proper excess of the PPh3 ligand. The experimental results indicated that a P/Rh molar ratio of 9–12 would be appropriate. The mobility of the excess phosphine ligands on the PPh3 -modified surface of the carbon nanotubes was probably also in favor of maintaining the Rh–phosphine complex species with higher probability in their catalytically active configurations via complexation–decomplexation–recomplexation of mobile PPh3 ligands. Moreover, one does not rule out

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the possibility that plenty of delocalizable valence electrons on the graphite-like six-membered C-rings on the surface of the carbon nanotubes might delocalize partially onto the phenyl-ring of coordinated PPh3 ligands, thus favoring the donation of electron from coordinated PPh3 ligands to the Rh central atom. The two factors would conduce to enhancing TOF of the reactant molecules on the Rh active sites. Most probably, these factors make the carbon nanotubes-supported Rh–phosphine complex catalysts display the excellent performance of catalytic hydroformylation. The results of activity assay of the Rh–phosphine complex catalysts supported by the other carriers provide a set of distinct contrasts. As mentioned in Section 3.1, propene conversion and the n/i ratio on the ends-unopened carbon nanotubes (surface area: ∼115 m2 g−1 )-supported catalyst were obviously low due to the inner surface and the nanochannels of the nanotube carrier being unable to get utilized. The surface of the inorganic oxide carrier SiO2 is hydrophilic; its interaction with the phenyl-ring of the phosphine ligands can be expected to be weak, probably due to a lack of similarities in their structural and electronic properties. This can get support from the contrastive experiments described above of adsorption/desorption of ammonia and benzene, respectively, on the carbon nanotubes and SiO2 . Moreover, the pore diameters of the SiO2 carrier is 2–4 times as large as those for the carbon nanotubes, and its spatio-restraint is smaller than that of the carbon nanotubes. These are probably the reasons why the observed n/i ratio of the product butylaldehyde on the SiO2 -supported Rh–phosphine complex catalyst was lower than that on the carbon nanotubes-supported catalysts. Both pore diameter (1.4–2.0 nm) and pore volume (0.21 cm3 g−1 ) of the active carbon carrier are comparatively small, and probably too small to accommodate the catalytically active Rh–phosphine complex, so that most of the inner surface could not be utilized, resulting in quite a low TOF and poor regioselectivity. The pore diameter of TDX-601 carbon molecular sieve is somewhat larger than that of active carbon. Though not large enough to accommodate the RhH(CO)(PPh3 )3 complex of ∼1.8 nm size, it may accommodate its catalytically active fragment, such as RhH(CO)(PPh3 )2 , and leave a certain space for diffusion and reaction of the reaction molecules. As a result, the n/i ratio

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of butylaldehyde reached was extraordinarily high due to the pronounced spatioselective catalysis by the more rigorous spatio-restraint, but the conversion of propene was still quite low. Over the GDX-102 supported Rh–phosphine complex catalyst, the TOF of propene was high, but the corresponding regioselectivity of butylaldehyde was relatively low, probably due to the pore diameter being too large to give rise to the micro-environments needed for the spatioselective catalysis.

4. Conclusions 1. The structure and properties of the carbon nanotubes, as a novel material of catalyst carrier, produced catalytically from the decomposition of CH4 or CO, have been characterized by means of TEM, HRTEM, XRD, Raman, XPS, BET and TPD methods. The results demonstrated that the prepared carbon nanofibers were even nanotubes with the outer diameters of 15–20 nm and an inner diameter of ca. 3 nm, and that their tube walls were constructed of many layers of carbon with graphite-like platelets in an extremely ordered arrangement: with the orientation of the cylindrical graphite-like platelets parallel to the tube axis for those derived from CO, and the conical graphite-like platelets inclined to the central axis of the tube for those produced from CH4 , respectively. The degree of long-range order of these carbon nanostructures was relatively low in comparison with that of graphite. Their surface possesses strong hydrophobicity, especially strong adsorption ability towards benzene. 2. Such carbon nanotubes grown catalytically have been first employed as a novel carrier material for preparation of the supported Rh–phosphine complex catalysts. The carbon nanotubes-supported Rh–phosphine complex catalysts prepared for propene hydroformylation displayed not only a high activity for propene conversion but also excellent regioselectivity to the product butylaldehyde. These, together with the results of comparative study of the Rh–phosphine complex catalysts supported by the ends-unopened carbon-nanotubes, SiO2 , TDX-601, AC, and GDX-102, are in favor of the viewpoint that the

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tubular nanochannels of the carbon nanostructures and their hydrophobic surface consisting of six-membered C-rings play important roles in enhancing the propene hydroformylation activity, and especially regioselectivity to the product butylaldehyde. But this is a preliminary suggestion. For better understanding of the promoting action of the carbon nanotube carriers, more detailed knowledge about the interaction of the supported Rh–phosphine complex species with the carrier surface and the reaction chemistry of the reactant molecules in the tubular nanochannels is needed. 3. The present work also provides a successful example for the application of the carbon nanotubes as an alternative to middle-pore molecular sieves under certain circumstances.

Acknowledgements The authors gratefully acknowledge the financial supports from the Fujian Provincial Natural Science Foundation and the National Natural Science Foundation of China.

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