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Structural characteristics and catalytic performance of nickel catalysts for selective hydrogenation of 1,6-hexanedinitrile
Journal of Molecular Elsevier
Science
Catalysis, 81 (1993)
Publishers
363-371
363
B.V.. Amsterdam
MO98
Structural characteristics and catalytic performance of nickel catalysts for selective hydrogenation of 1,6_hexanedinitrile F. Medina, P. Salagre and J.E. Sueiras* Dept. de Quimica, Facultat de Quimica de Tarragona, Tarraco, I,43005
Uniuersitat
de Barcelona,
PI. Imperial
Tarragona (Spain)
J.L.G. Fierro Instituto
de
Catcilisisy Petroleoquimica,
C.S.I.C., Campus UAM. Cantoblanco,
28049 Madrid
(Spain) (Received
June 26,1992;
accepted
January
10,1993)
Abstract Several nickel catalysts have been characterized on the basis of their Brunauer-EmmettTeller (BET) surface areas, X-ray diffraction, X-ray photoelectron spectra, field-emission scanning electron microscopy and temperature-programmed reduction properties. The catalysts were used for the heterogeneous hydrogenation of 1,6_hexanedinitrile at 443 K and atmospheric pressure, with no ammonia in the feed. Highly reduced nickel with a low BET area is obtained above 498 K. XPS and field-emission scanning electron micrographs of reduced catalysts revealed the appearance responsible
of incipiently reduced nickel on top of NiO crystals at 463-473 K. This structure is for the high hydrogenation activity: however, only 100% selectivities toward 6-ami-
nohexanenitrile a specific
were obtained
arrangement
by reduction
at temperatures
as high as 623-673
of nickel atoms is required to obtain selective
Key words: hydrogenation;
adiponitrile;
6-aminohexanenitrile;
K. It seems that
catalysts.
nickel; characterization
Introduction 1,6_Hexanedinitrile (adiponitrile) is used in the manufacture of Nylon-6 and Nylon-6,6. The compound is first partially hydrogenated to 6-aminohexanenitrile and then to 1,6_hexanediamine [ 11. The industrial preparation of these primary amines is usually accomplished in the liquid phase at elevated hydrogen pressures (270-600 bar) and at 30 bar in some recent processes, where the catalyst represents a very important factor in determining the selectivity with respect to primary amines [ 21. An ordering of metallic catalysts according to the increasing production (besides primary amines) of secondary and tertiary amines was reported to be Co, Ni, Ru, Cu, Rh, Pd, Pt [ 3,4]. Raney *Corresponding
0304-5102/93/$06.00
author.
0 1993 - Elsevier
Science
Publishers
B.V.
All. rights reserved.
364
F. Medina et al./J. Mol. Catal. 81 (1993) 363-371
nickel and Raney cobalt are probably the most frequently used catalysts for primary amine production [ 5-161. Other catalysts based on Rh, Mn and Fe have also been reported [ 17-191. An excess of ammonia is found to be essential to suppress secondary and tertiary amine formation, presumably due to the presence of a diimine intermediate [20]. Unfortunately, since most of the available literature comprises patents, the results of the characterization and detailed activities of the heterogeneous catalysts concerned often are absent. Here we report studies involving the preparation, characterization and catalytic properties of several nickel catalysts which exhibited activity in the hydrogenation of 1,6-hexanedinitrile at 1 atm pressure. Some of them display a remarkable primary amine selectivity in the absence of ammonia.
Experimental
Catalyst preparation Bulk nickel precursors were prepared by air calcination of nickel nitrate hexahydrate samples. These preparations will be designated hereafter as precursors 1-8. All samples were dried at 393 K, calcined at 623 K and reduced with hydrogen/argon at a l/20 volume ratio and temperatures shown in Table All reagents were reagent-grade, and pure gases were previously dried and deoxygenated. BET areas BET surface areas were calculated from the nitrogen adsorption isotherms at 77 K using a Micromeritics ASAP 2000 surface analyzer, and a value of 0.164 nm’ for the cross-section of the nitrogen molecule. X-ray diffraction (XRD) Powder X-ray diffraction patterns of the catalysts were obtained with a Philips PW 1010 diffractometer using nickel-filtered Cu Ka radiation. Samples were dusted on double-sided adhesive tape and mounted on glass microTABLE
1
XRD and XPS characterization
of several nickel catalysts vs. reduction temperatures
catalyst red. temp. (K) tryst. phases (XRD)
1 443 NiO
2 463 NiO
5
6
7
6
473 NiO
498 Ni NiO traces
573 Ni
598 Ni
623 Ni
673 Ni
binding energies from (XPS) Ni 2p3,2 level (eV)
855.8
855.7
855.6
854.5
854.4
3
4
854.2
F. Medina et al./J. Mol. Catal. 81 (1993) 363-371
365
scope slides. The patterns were recorded over a range of 20 angles from 5” to 85” and compared with the X-ray powder diffraction files to confirm phase identities. X-ray photoelectron spectroscopy (XPS) X-ray photoelectron spectra were recorded on a Leybold LHS 10 spectrometer provided with a hemispherical energy analyzer and a Mg Ka X-radiation source. Powdered samples were pressed into small, stainless steel cylinders and mounted on a standard sample probe, placed in a pre-evacuation chamber where they were pumped down to ca. lop5 Torr, before they were moved into the main vacuum chamber. The residual pressure in the turbopumped analysis chamber was kept below 7 x lo-’ Torr during data collection. Each spectral region was signal-averaged for a given number of scans to obtain good signal-to-noise ratios. Although surface charging was observed on all the samples, accurate binding energies (BE) were determined by charge referencing with the C1, line at 284.6 eV. Peak areas were computed by a program which assumed Gaussian lines and flat background subtraction. Scanning electron microscopy (FE-SEM and SEM) Scanning electron micrographs were obtained in a Leica Stereoscan 360 FE microscope equipped with the field emission system, operating at EHT = 5.00 kV, WD = 5 mm and magnification values of 50,000 x . Scanning electron micrographs were also taken at a voltage of 20 kV. Temperature-programmed reduction (TPR) Temperature-programmed reductions were carried out in a Perkin-Elmer TGA 7 microbalance with an accuracy of 1 ,ug, equipped with a 273-1273 K programmable temperature furnace. Samples (10 mg) were first heated at 4 K min-’ up to 673 K in a stream of He (80 cm3 min-l ) in order to release moisture. After cooling to room temperature under helium, they were heated again in a 5 vol.-% H,/Ar flow (80 cm3 min-l) up to reduction temperatures. The weight change in the samples is a measure of the oxygen released by the solid as water. Catalytic activity determination In a typical experiment, the catalyst (1 g) was placed in a fixed-bed flow reactor for hydrogenation of adiponitrile vapour at 443 K and 1 atm pressure, with a molar ratio adiponitrile/hydrogen of l/300 at a space velocity of 1500 h-l. The catalysts showed no diffusion restrictions. Reaction products were analyzed by means of an on-line gas chromatograph (HP 5840A) equipped with a 25 m phenylmethylsilicone capillary column, and a 343-573 K oven temperature program.
F. Medina et al./J. Mol. Catal. 81 (1993) 363-371
Results and discussion X-ray diffraction Powder diffraction patterns of the reduced catalysts showed diffraction peaks at 28 angles and relative intensities which can be indexed to the NiO and Ni phases. The 28 angles (with the relative intensities in parentheses) may be summarized as follows: 44.51 (loo), 51.85 (42), 76.36 (21), for the Ni phase; 37.29 (91), 43.30 (loo), 62.91 (57), 75.43 (16), for the NiO phase. At reduction temperatures of 473-498 K metallic Ni was observed with the simultaneous disappearance of NiO phase (see Table 1) . This process was complete at 498 K. Furthermore, previous particle size calculations, using the Scherrer equation, indicate the presence of larger particle sizes with increasing temperatures at which the NiO precursors had been reduced, in line with the sharp decrease of BET areas in that range of reduction temperatures. The fact that pure NiO samples were totally inactive for nitrile hydrogenation under the experimental conditions used in this study indicates that reduced nickel is the only active phase involved in the reaction. It must also be stressed that catalysts 2 and 3, which do not show diffraction lines of metallic nickel, are catalytically active (Table 2). This behaviour may reasonably be explained by assuming the formation of some kind of incipiently reduced surface nickel, probably with very small crystallite sizes, below the detection limits of current powder X-ray diffractometers. In order to confirm this interpretation, XPS and SEM surface characterizations of several such catalysts have been carried out. BET surface areas The BET surface areas of the catalysts are plotted against temperature at which their precursors were reduced in Fig. 1. As expected for bulk catalysts, the surface areas are rather low. The most important features to be noted are the linear decrease in surface area of metallic nickel with increasing reduction temperatures, in the range of temperatures tested, below 673 K. Such a decrease is due to a progressive increase of reduced particle sizes with temperaTABLE 2 Activity of several nickel catalysts for the adiponitrile hydrogenation continuous process. Temperature 443 K, pressure 1 atm, space velocity 1500 h-’ catalysts conversion (% ) selectivity (% )” monoamine diamine others
1 0
2 18
3 80
4 100
5 100
6 75
7 55
8 33
63 27 10
50 30 20
0 30 70
15 20 65
51 9 40
95 0 5
100 0 0
*Monoamine = 6-aminohexanenitrile; diamine = 1,6-hexanediamine; others = azacycloheptane, mainly.
F. Medina et al./J. Mol. Catal. 81 (1993) 363-371
367
Reduction Temperature (K)
Fig. 1. Plot of surface area vs. reduction temperature of several nickel catalysts. Highly reduced samples follow a straight line.
ture. The BET areas of catalysts are substantially higher only if NiO phase is also present; however, when metallic nickel is the only phase present, the BET area decreases sharply, much more at the highest reduction temperatures, where a strong sintering may be expected to occur. Conversely, this graph can be exploited to determine the intensity of reduction for a given nickel catalyst if only its BET area is measured.
X-ray photoelectron spectroscopy The Ni 2psj2 core level spectra of hydrogen-reduced catalysts are shown in Fig. 2 and the respective binding energies (BE) of the Ni 2p,,, peak are summarized in Table 1. Catalysts l-3 show the principal Ni 2psj2 peak at BE ca. 855.7 eV. These values, together with the presence of shake-up satellites slightly above 862 eV, clearly indicate the presence of Ni2+ ion in the catalysts. However, catalysts 4-8 show BE values of Ni 2p3,2 in the region 854.5-854.2 eV and a complete disappearance of satellite structures, which confirms the presence of metallic Ni in the catalysts. Calculation of the satellite-to-principal Ni 2ps12ratio, for catalysts l-3 also revealed that it was a maximum for catalyst 1, slightly smaller for catalyst 2 and substantially smaller for catalyst 3. This fact together with the tendency of BE to decrease when going from catalyst 1 to 3, can be taken as indicating the formation of incipiently reduced nickel on the NiO surface, probably having such small particle sizes that the metallic nickel phase remains undetectable by XRD. These results are in agreement with those of scanning electron microscopy, temperature-programmed reduction and catalytic activities, as shown below. Finally, we note that even though the absolute values of the binding energies, from the XPS literature, show a spread of several eV, we should always consider, as a general rule, the relative behavior between the nickel species in any particular spectrum.
F. Medina et al/J. Mol. Catal. 81 (1993) 363-371
r
Fig. 2. X-ray
Ni 23
photoelectron
spectra of several nickel catalysts
for the Ni 2pz12 level.
Field emission-scanning electron microscopy Stereographic pictures of catalysts 2 and 4 taken from a scanning electron microscope are shown in Figs. 3 and 4, respectively. In order to obtain better resolution and contrast, the micrographs were obtained at lower acceleration voltages. For catalyst 2 reduced at 463 K one observes almost clean surfaces of NiO octahedra, sometimes containing small aggregations of reduced nickel on the edges of the octahedra. In contrast, catalyst 4 reduced at 498 K does not show clear NiO octahedra as before, but masses of surface reduced nickel. The backscattering image of catalyst 2 confirms the different densities of both phases, being higher for the aggregations and lower for the octahedra, which corresponds to metallic nickel and NiO, respectively. The FE-SEM results therefore confirm the presence of incipiently reduced nickel in the active catalysts prepared at reduction temperatures below 473 K, in complete agreement with the XRD and XPS results above.
F. Medina et al./J. Mol. Catal. 81 (1993) 363-371
369
Fig. 3. Field emission-scanning electron micrograph taken from the surface of catalyst 2.
Fig. 4. Scanning electron micrograph taken from the surface of catalyst 4.
Temperature-programmed reductions The reduction degrees, a, of the catalysts obtained by temperature-programmed reduction are shown in Fig. 5. Catalyst 2 showed a reduction degree of 4% at equilibrium, and this value was reproducible in duplicate experiments, which contrasts with the Ni phase which was undetected in the XRD patterns. The extent of reduction progressed markedly for catalyst 3 (ca. 20% ) and dramatically for catalyst 4 (ca. 91% ). Higher reduction temperatures for catalysts
F. Medina et al./J. Mol. Catal. 81 (1993)
370
363-371
loo90-
/--*-*--
SO-
70-
i$ 60 d
50
.$
40
2
30
S s
20 ; 10 0
400
1 !i ,=:‘, I, I. I, 450 500 550 600 650 Reduction Temperature (K)
Fig. 5. Plot of percentage
reduction
I
700
degree, CY,vs. temperature
for several NiO samples.
5-8 indicate a complete reduction degree of nickel oxide. Again, the incipient reduction of nickel oxide at 463 K is clearly demonstrated by reduction experiments, thus confirming the previous findings of XRD, XPS and FE-SEM. Catalytic activity
Conversions and product distributions for the hydrogenation reaction of 1,6_hexanedinitrile on catalysts 1-8, under the experimental conditions given above, are summarized in Table 2.1,6-Hexanedinitrile conversion increases in parallel with the extent of reduction of the NiO phase up to 573 K, but it decreases at higher NiO reduction temperatures. This can mainly be due to the decrease of surface area due to sintering of the metallic nickel phase, but probably also due to a significant decrease of surface active sites, all except those responsible for the production of 6_aminohexanenitrile, which can be obtained with 100% selectivity. Conversely, selectivities toward 1,6_hexanediamine and azacycloheptane follow the opposite trend, i.e. they are high (or low) for low (or high) 6-aminohexanenitrile selectivities, respectively, suggesting different active sites in this case. Further studies are in progress in order to obtain a better characterization of the surface active sites for this type of catalyst and hydrogenation reaction.
Conclusions
From the structural and catalytic data reported in this work, it seems clear that NiO precursors become completely reduced at temperatures close to 500 K; however, the resulting Ni phase is not well suited for the selective production of 6-aminohexanenitrile. Reduction temperatures as high as 623-673 K are required to obtain selective catalysts for the catalytic hydrogenation of 1,6hexanedinitrile at 443 K and 1 atm pressure on an industrial scale. Specific
F. Medina et al./J. Mol. Catal. 81 (1993) 363-371
371
arrangements of Ni atoms might be formed on nickel crystals, being responsible for the selectivities of this hydrogenation reaction.
Acknowledgement
We gratefully acknowledge the support of this work by the Comisi6n Interministerial de Ciencia y Tecnologia (project No PB89-0240).
References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
K. Weissermel and H.J. Arpe, Industrial Organic Chemistry, Verlag Chemie, Berlin, 1978. Ger. Pat. 3 402734 Al (1984) to G. Frank and G. Neubauer, BASF AG. H. Greenfield, Znd. Eng. Chem. Prod. Res. Deu., 6 (1967) 142. J. Pasec, N. Kostova and B. Dvorak, Collect. Czech. Chem. Commun., 46 (1981) 1011. Fr. Pat., 2 063 378 (1971) to J. Bolle. Jpn. Kokai Tokkyo Koho, 75 47 909 (1975) to K. Hioki, K. Hirokawa, Y. Kuka and A. Shimade. Pol. Pat., 115 999 (1983) to W. Jerzykiewicz, Z. Krasnodebski and G. Bekierz. U.S. Pat., 4 429 159 (1984) to Ch.E. Kuthens and L.M. Lanier. Jpn. Pat., 70 32 410 (1970) to S. Taira. Fr. Pat., 1530 809 (1968) to Toyo Rayon Co. Fr. Pat., 2 149 987 (1973) to R.A. Diffenback. Jpn. Pat. 69 05 844 (1969) to S. Taira. U.S. Pat., 4 375 003 (1976) to R.J. Alain. Jpn. Kokai Z’okkyo Koho, 76 78 795 (1976) to R. Uehara, T. Horri, T. Imai, Y. Tomita and K. Yamano. Fr. Pat. 2 248 265 (1975) to Societk des Usines Chimiques Rh8ne Poulenc. Jpn. Pat. 72 31833 (1972) to A. Kuroda and S. Taira. F. Mares, J.E. Galle, S.E. Diamond and F.J. Regina, J. Catal., 112 (1988) 145. Fr. Pat. 1483 300 (1967) to K. Adam and E. Haarer, BASF, AG. J.L.G. Fierro, F. Medina, P. Salagre and J.E. Sueiras, J. Mol. Catal., 61 (1990) 197. R.L. Augustine, Catal. Reu., 13 (1976) 285.
Science
Catalysis, 81 (1993)
Publishers
363-371
363
B.V.. Amsterdam
MO98
Structural characteristics and catalytic performance of nickel catalysts for selective hydrogenation of 1,6_hexanedinitrile F. Medina, P. Salagre and J.E. Sueiras* Dept. de Quimica, Facultat de Quimica de Tarragona, Tarraco, I,43005
Uniuersitat
de Barcelona,
PI. Imperial
Tarragona (Spain)
J.L.G. Fierro Instituto
de
Catcilisisy Petroleoquimica,
C.S.I.C., Campus UAM. Cantoblanco,
28049 Madrid
(Spain) (Received
June 26,1992;
accepted
January
10,1993)
Abstract Several nickel catalysts have been characterized on the basis of their Brunauer-EmmettTeller (BET) surface areas, X-ray diffraction, X-ray photoelectron spectra, field-emission scanning electron microscopy and temperature-programmed reduction properties. The catalysts were used for the heterogeneous hydrogenation of 1,6_hexanedinitrile at 443 K and atmospheric pressure, with no ammonia in the feed. Highly reduced nickel with a low BET area is obtained above 498 K. XPS and field-emission scanning electron micrographs of reduced catalysts revealed the appearance responsible
of incipiently reduced nickel on top of NiO crystals at 463-473 K. This structure is for the high hydrogenation activity: however, only 100% selectivities toward 6-ami-
nohexanenitrile a specific
were obtained
arrangement
by reduction
at temperatures
as high as 623-673
of nickel atoms is required to obtain selective
Key words: hydrogenation;
adiponitrile;
6-aminohexanenitrile;
K. It seems that
catalysts.
nickel; characterization
Introduction 1,6_Hexanedinitrile (adiponitrile) is used in the manufacture of Nylon-6 and Nylon-6,6. The compound is first partially hydrogenated to 6-aminohexanenitrile and then to 1,6_hexanediamine [ 11. The industrial preparation of these primary amines is usually accomplished in the liquid phase at elevated hydrogen pressures (270-600 bar) and at 30 bar in some recent processes, where the catalyst represents a very important factor in determining the selectivity with respect to primary amines [ 21. An ordering of metallic catalysts according to the increasing production (besides primary amines) of secondary and tertiary amines was reported to be Co, Ni, Ru, Cu, Rh, Pd, Pt [ 3,4]. Raney *Corresponding
0304-5102/93/$06.00
author.
0 1993 - Elsevier
Science
Publishers
B.V.
All. rights reserved.
364
F. Medina et al./J. Mol. Catal. 81 (1993) 363-371
nickel and Raney cobalt are probably the most frequently used catalysts for primary amine production [ 5-161. Other catalysts based on Rh, Mn and Fe have also been reported [ 17-191. An excess of ammonia is found to be essential to suppress secondary and tertiary amine formation, presumably due to the presence of a diimine intermediate [20]. Unfortunately, since most of the available literature comprises patents, the results of the characterization and detailed activities of the heterogeneous catalysts concerned often are absent. Here we report studies involving the preparation, characterization and catalytic properties of several nickel catalysts which exhibited activity in the hydrogenation of 1,6-hexanedinitrile at 1 atm pressure. Some of them display a remarkable primary amine selectivity in the absence of ammonia.
Experimental
Catalyst preparation Bulk nickel precursors were prepared by air calcination of nickel nitrate hexahydrate samples. These preparations will be designated hereafter as precursors 1-8. All samples were dried at 393 K, calcined at 623 K and reduced with hydrogen/argon at a l/20 volume ratio and temperatures shown in Table All reagents were reagent-grade, and pure gases were previously dried and deoxygenated. BET areas BET surface areas were calculated from the nitrogen adsorption isotherms at 77 K using a Micromeritics ASAP 2000 surface analyzer, and a value of 0.164 nm’ for the cross-section of the nitrogen molecule. X-ray diffraction (XRD) Powder X-ray diffraction patterns of the catalysts were obtained with a Philips PW 1010 diffractometer using nickel-filtered Cu Ka radiation. Samples were dusted on double-sided adhesive tape and mounted on glass microTABLE
1
XRD and XPS characterization
of several nickel catalysts vs. reduction temperatures
catalyst red. temp. (K) tryst. phases (XRD)
1 443 NiO
2 463 NiO
5
6
7
6
473 NiO
498 Ni NiO traces
573 Ni
598 Ni
623 Ni
673 Ni
binding energies from (XPS) Ni 2p3,2 level (eV)
855.8
855.7
855.6
854.5
854.4
3
4
854.2
F. Medina et al./J. Mol. Catal. 81 (1993) 363-371
365
scope slides. The patterns were recorded over a range of 20 angles from 5” to 85” and compared with the X-ray powder diffraction files to confirm phase identities. X-ray photoelectron spectroscopy (XPS) X-ray photoelectron spectra were recorded on a Leybold LHS 10 spectrometer provided with a hemispherical energy analyzer and a Mg Ka X-radiation source. Powdered samples were pressed into small, stainless steel cylinders and mounted on a standard sample probe, placed in a pre-evacuation chamber where they were pumped down to ca. lop5 Torr, before they were moved into the main vacuum chamber. The residual pressure in the turbopumped analysis chamber was kept below 7 x lo-’ Torr during data collection. Each spectral region was signal-averaged for a given number of scans to obtain good signal-to-noise ratios. Although surface charging was observed on all the samples, accurate binding energies (BE) were determined by charge referencing with the C1, line at 284.6 eV. Peak areas were computed by a program which assumed Gaussian lines and flat background subtraction. Scanning electron microscopy (FE-SEM and SEM) Scanning electron micrographs were obtained in a Leica Stereoscan 360 FE microscope equipped with the field emission system, operating at EHT = 5.00 kV, WD = 5 mm and magnification values of 50,000 x . Scanning electron micrographs were also taken at a voltage of 20 kV. Temperature-programmed reduction (TPR) Temperature-programmed reductions were carried out in a Perkin-Elmer TGA 7 microbalance with an accuracy of 1 ,ug, equipped with a 273-1273 K programmable temperature furnace. Samples (10 mg) were first heated at 4 K min-’ up to 673 K in a stream of He (80 cm3 min-l ) in order to release moisture. After cooling to room temperature under helium, they were heated again in a 5 vol.-% H,/Ar flow (80 cm3 min-l) up to reduction temperatures. The weight change in the samples is a measure of the oxygen released by the solid as water. Catalytic activity determination In a typical experiment, the catalyst (1 g) was placed in a fixed-bed flow reactor for hydrogenation of adiponitrile vapour at 443 K and 1 atm pressure, with a molar ratio adiponitrile/hydrogen of l/300 at a space velocity of 1500 h-l. The catalysts showed no diffusion restrictions. Reaction products were analyzed by means of an on-line gas chromatograph (HP 5840A) equipped with a 25 m phenylmethylsilicone capillary column, and a 343-573 K oven temperature program.
F. Medina et al./J. Mol. Catal. 81 (1993) 363-371
Results and discussion X-ray diffraction Powder diffraction patterns of the reduced catalysts showed diffraction peaks at 28 angles and relative intensities which can be indexed to the NiO and Ni phases. The 28 angles (with the relative intensities in parentheses) may be summarized as follows: 44.51 (loo), 51.85 (42), 76.36 (21), for the Ni phase; 37.29 (91), 43.30 (loo), 62.91 (57), 75.43 (16), for the NiO phase. At reduction temperatures of 473-498 K metallic Ni was observed with the simultaneous disappearance of NiO phase (see Table 1) . This process was complete at 498 K. Furthermore, previous particle size calculations, using the Scherrer equation, indicate the presence of larger particle sizes with increasing temperatures at which the NiO precursors had been reduced, in line with the sharp decrease of BET areas in that range of reduction temperatures. The fact that pure NiO samples were totally inactive for nitrile hydrogenation under the experimental conditions used in this study indicates that reduced nickel is the only active phase involved in the reaction. It must also be stressed that catalysts 2 and 3, which do not show diffraction lines of metallic nickel, are catalytically active (Table 2). This behaviour may reasonably be explained by assuming the formation of some kind of incipiently reduced surface nickel, probably with very small crystallite sizes, below the detection limits of current powder X-ray diffractometers. In order to confirm this interpretation, XPS and SEM surface characterizations of several such catalysts have been carried out. BET surface areas The BET surface areas of the catalysts are plotted against temperature at which their precursors were reduced in Fig. 1. As expected for bulk catalysts, the surface areas are rather low. The most important features to be noted are the linear decrease in surface area of metallic nickel with increasing reduction temperatures, in the range of temperatures tested, below 673 K. Such a decrease is due to a progressive increase of reduced particle sizes with temperaTABLE 2 Activity of several nickel catalysts for the adiponitrile hydrogenation continuous process. Temperature 443 K, pressure 1 atm, space velocity 1500 h-’ catalysts conversion (% ) selectivity (% )” monoamine diamine others
1 0
2 18
3 80
4 100
5 100
6 75
7 55
8 33
63 27 10
50 30 20
0 30 70
15 20 65
51 9 40
95 0 5
100 0 0
*Monoamine = 6-aminohexanenitrile; diamine = 1,6-hexanediamine; others = azacycloheptane, mainly.
F. Medina et al./J. Mol. Catal. 81 (1993) 363-371
367
Reduction Temperature (K)
Fig. 1. Plot of surface area vs. reduction temperature of several nickel catalysts. Highly reduced samples follow a straight line.
ture. The BET areas of catalysts are substantially higher only if NiO phase is also present; however, when metallic nickel is the only phase present, the BET area decreases sharply, much more at the highest reduction temperatures, where a strong sintering may be expected to occur. Conversely, this graph can be exploited to determine the intensity of reduction for a given nickel catalyst if only its BET area is measured.
X-ray photoelectron spectroscopy The Ni 2psj2 core level spectra of hydrogen-reduced catalysts are shown in Fig. 2 and the respective binding energies (BE) of the Ni 2p,,, peak are summarized in Table 1. Catalysts l-3 show the principal Ni 2psj2 peak at BE ca. 855.7 eV. These values, together with the presence of shake-up satellites slightly above 862 eV, clearly indicate the presence of Ni2+ ion in the catalysts. However, catalysts 4-8 show BE values of Ni 2p3,2 in the region 854.5-854.2 eV and a complete disappearance of satellite structures, which confirms the presence of metallic Ni in the catalysts. Calculation of the satellite-to-principal Ni 2ps12ratio, for catalysts l-3 also revealed that it was a maximum for catalyst 1, slightly smaller for catalyst 2 and substantially smaller for catalyst 3. This fact together with the tendency of BE to decrease when going from catalyst 1 to 3, can be taken as indicating the formation of incipiently reduced nickel on the NiO surface, probably having such small particle sizes that the metallic nickel phase remains undetectable by XRD. These results are in agreement with those of scanning electron microscopy, temperature-programmed reduction and catalytic activities, as shown below. Finally, we note that even though the absolute values of the binding energies, from the XPS literature, show a spread of several eV, we should always consider, as a general rule, the relative behavior between the nickel species in any particular spectrum.
F. Medina et al/J. Mol. Catal. 81 (1993) 363-371
r
Fig. 2. X-ray
Ni 23
photoelectron
spectra of several nickel catalysts
for the Ni 2pz12 level.
Field emission-scanning electron microscopy Stereographic pictures of catalysts 2 and 4 taken from a scanning electron microscope are shown in Figs. 3 and 4, respectively. In order to obtain better resolution and contrast, the micrographs were obtained at lower acceleration voltages. For catalyst 2 reduced at 463 K one observes almost clean surfaces of NiO octahedra, sometimes containing small aggregations of reduced nickel on the edges of the octahedra. In contrast, catalyst 4 reduced at 498 K does not show clear NiO octahedra as before, but masses of surface reduced nickel. The backscattering image of catalyst 2 confirms the different densities of both phases, being higher for the aggregations and lower for the octahedra, which corresponds to metallic nickel and NiO, respectively. The FE-SEM results therefore confirm the presence of incipiently reduced nickel in the active catalysts prepared at reduction temperatures below 473 K, in complete agreement with the XRD and XPS results above.
F. Medina et al./J. Mol. Catal. 81 (1993) 363-371
369
Fig. 3. Field emission-scanning electron micrograph taken from the surface of catalyst 2.
Fig. 4. Scanning electron micrograph taken from the surface of catalyst 4.
Temperature-programmed reductions The reduction degrees, a, of the catalysts obtained by temperature-programmed reduction are shown in Fig. 5. Catalyst 2 showed a reduction degree of 4% at equilibrium, and this value was reproducible in duplicate experiments, which contrasts with the Ni phase which was undetected in the XRD patterns. The extent of reduction progressed markedly for catalyst 3 (ca. 20% ) and dramatically for catalyst 4 (ca. 91% ). Higher reduction temperatures for catalysts
F. Medina et al./J. Mol. Catal. 81 (1993)
370
363-371
loo90-
/--*-*--
SO-
70-
i$ 60 d
50
.$
40
2
30
S s
20 ; 10 0
400
1 !i ,=:‘, I, I. I, 450 500 550 600 650 Reduction Temperature (K)
Fig. 5. Plot of percentage
reduction
I
700
degree, CY,vs. temperature
for several NiO samples.
5-8 indicate a complete reduction degree of nickel oxide. Again, the incipient reduction of nickel oxide at 463 K is clearly demonstrated by reduction experiments, thus confirming the previous findings of XRD, XPS and FE-SEM. Catalytic activity
Conversions and product distributions for the hydrogenation reaction of 1,6_hexanedinitrile on catalysts 1-8, under the experimental conditions given above, are summarized in Table 2.1,6-Hexanedinitrile conversion increases in parallel with the extent of reduction of the NiO phase up to 573 K, but it decreases at higher NiO reduction temperatures. This can mainly be due to the decrease of surface area due to sintering of the metallic nickel phase, but probably also due to a significant decrease of surface active sites, all except those responsible for the production of 6_aminohexanenitrile, which can be obtained with 100% selectivity. Conversely, selectivities toward 1,6_hexanediamine and azacycloheptane follow the opposite trend, i.e. they are high (or low) for low (or high) 6-aminohexanenitrile selectivities, respectively, suggesting different active sites in this case. Further studies are in progress in order to obtain a better characterization of the surface active sites for this type of catalyst and hydrogenation reaction.
Conclusions
From the structural and catalytic data reported in this work, it seems clear that NiO precursors become completely reduced at temperatures close to 500 K; however, the resulting Ni phase is not well suited for the selective production of 6-aminohexanenitrile. Reduction temperatures as high as 623-673 K are required to obtain selective catalysts for the catalytic hydrogenation of 1,6hexanedinitrile at 443 K and 1 atm pressure on an industrial scale. Specific
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arrangements of Ni atoms might be formed on nickel crystals, being responsible for the selectivities of this hydrogenation reaction.
Acknowledgement
We gratefully acknowledge the support of this work by the Comisi6n Interministerial de Ciencia y Tecnologia (project No PB89-0240).
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