Ind. Eng. Chem. Prod. Res.
570
Dev. 1903, 22, 578-582
Nickel-Aluminum Flame-Sprayed Catalysts and Their Catalytic Activities Ichlro Nakabayashl' Department of Chemical Engineering, Faculty of Engineering, Tokushima University, Minamijosanjima-cho, Tokushima, 770, Japan
Tomlo Yoshino Department of Radiopharmaceuticals, Faculty of Pharmaceutical Science, Tokushima University of Arts and Science, Yamashiro-cho, Tokushima, 770, Japan
Satoshl Abe Nikkl Chemical Go., Ltd., Ohtemachi, Chiyoda-ku, Tokyo, 100, Japan
Nickel catalysts for the hydrogenation of acetone were prepared by flame-spraying either aluminum onto a nickel surface or a nickel aluminum alloy onto a steel surface. In the first preparation the sprayed material was heat-treated to produce a Raney-type alloy, while in the second method NiAl and Ni3AI were formed without subsequent heating. A maximum in catalyst activity occurred for a sample alloyed at 700 "C which contained NiAI, as the active phase. The catalyst prepared from the nickel aluminum alloy had considerable hydrogenation activity. However, heating this catalyst at 700 "C destroyed its activity, but activity was partially restored following additional heating at 800 O C . On the basis of several observations of laminated Raney nickel catalysts with X-ray diffraction and a microscopic analyzer, it is concluded that the activity of the catalyst is correlated to the composition of Raney nickel alloy and that the active centers are distributed probably at random on the surface of the catalyst.
Introduction Raney nickel catalyst has often been used in hydrogenation processes in commercial plants because of its high activity. Such a process, however, has usually been carried out batchwise or noncontinuously because of the nature of the Raney nickel catalyst, which is usually in the form of a finely divided powder. From the technical or economical point of view, batch operation is not advantageous; therefore, numerous papers have described the preparation methods of various type of Raney nickel and their catalytic activities. Bag and his collaborators (1933, 1936) used granular Raney nickel alloy for this purpose. Later, Rappoport and Silchenko (1937) and Wilson (1945) tried to make a suitable catalyst from similar materials. Yasumura and Yoshino (1966,1967), Yasumura et al. (1968), and Nakabayashi (1971,1972) also reported studies concerning this matter (i.e., Raney nickel pellet catalysts and a sandwich-type Raney nickel catalyst). This paper presents a modification of the method for preparing a sandwich-type Raney nickel catalyst in which a metal and an alloy are sprayed onto the surface of other metals by an acetylene-oxygen or hydrogen-oxygen flame. Subsequently, an aluminum-nickel alloy is formed by heat treatment. The active nickel catalyst is prepared by leaching the aluminum in an aluminum-nickel alloy. Although a similar technique has been used by others (Larson, 1974; Baird and Steffgen, 1977) to prepare hydrogenation catalyst, the hydrogenation activity of the alloy containing only NiAl and Ni,Al has not bee reported. The present paper mainly deals with dependence of the activity of the catalyst upon composition of the alloys formed a t the interphase, and capability of restoration of the deactivated catalyst by heat treatment followed by leaching. In addition, distribution of the active centers
on the catalyst surface is referred to. Experimental Section We adopted two methods for preparing nickel-aluminum alloys. In one (method A) the alloy is formed when aluminum is sprayed onto the surface of a nickel plate, and then the plate is heated to an appropriate temperature in air. In the other (method B) the alloy is formed when an aluminum tube (3.2 mm diameter) filled with nickel pow der is melted and then sprayed onto the steel with a flame-spraying gun, without heat treatment of the resulting laminated steel. Nickel aluminide formed by method B is generally used as a protective coating material for chemical plants due to its higher heat stability and has the composition of A1 = 20, Ni = 80 wt %. The catalysts were activated by leaching the alloys in 20% aqueous sodium hydroxide solution a t 70 "C for 2 h. Since no hydrogen evolution was observed after 80 min, the 2-h leach was sufficient to remove the readily dissolvable aluminum. BET surface areas of catalysts were measured by N2 physisorption. The activity was measured for hydrogenation of acetone. The total geometrical catalyst surface area was adjusted to 12 cm2before leaching. The apparatus having a capacity of about 600 mL was filled with nitrogen at 1 atm and cooled to 0 "C prior to measurement. Sample pieces were quickly placed in a 260-mL glass vessel which was a part of the apparatus. Samples were introduced while wet by acetone after leaching. After 1mL of acetone was injected into the vessel, the nitrogen was replaced by hydrogen, and then the whole system was dipped into a hot bath thermostated at 60 "C. A decrease in the gas volume was measured as the conversion (Nakabayashi et al., 1979). The resulting product was confirmed to be only 2-propanol by a gas chromatograph (column, PEG-6000; column temperature, 80 "C; a flow of helium, 60 mL/min). 0 1983 American Chemical
Society
Ind. Eng. Chem. Prod. Res. Dev., Vol. 22, No. 4, 1983 579 Aluminum-coated nickel alloy preheated at 700 OC Ni
Catalyst from Ni-A1 alloy at 700 'C Ni (111) Ni (220)
Catalyst from Ni-A1 alloy at 800 'C
Aluminum-coated nickel alloy preheated at 800 'C Ni
Ni~Ala
Catalyst from Ni-Al-Fe alloy plate Nickel aluminide-coated iron plate Ni 3Al Ni 3Al
20
40
80
60 2 8 (deg)
20
40 26 (deg)
60
80
Figure 2. X-ray diffraction lines of Raney nickel catalysts.
Figure 1. X-ray diffraction lines of Raney nickel alloy.
The surface morphology and the cross section structure of the catalyst were examined by a scanning electron microscope (JEOL Ltd., Model 50-A) and the alloy phase and the crystallographic structure were observed by an X-ray diffractometer (Rigaku denki Ltd.).Active catalysts for X-ray diffraction were immersed in polystylenebenzene solution, and the resultant thin film protected the samples from air oxidation.
Results and Discussion The observed X-ray diffraction patterns of Raney nickel alloy before alkali treatment are shown in Figure 1. Aluminum-coated nickel plates were kept at various temperatures up to 800 "C in the air for 2 h. At any temperature below 600 "C, the alloy formation at the boundary was not detected by the X-ray diffraction method. The alloy phases formed at 700 and 800 "C were composed of NiA13 with a very little Ni2A13and Ni2A13with a little NiA13, respectively. This is the same as those reported in the previous papers (Nakabayashi et al., 1969) and is consistent with the detailed investigation by Baird and Steffgen (1977). Analysis of the pattern of nickel aluminide-coated iron plate (method B) indicated that this Raney nickel alloy did not contain NiA13or Ni2Al3 but did contain NiAl and Ni3A1. No Ni-Al-Fe alloys were observed. The X-ray diffraction patterns of Raney nickel catalyst after alkali treatment are shown in Figure 2. X-ray diffraction lines of h e y nickel catalyst from the alloy at 700 OC are broad, so its surface structure has some crystal distortion. The mean crystallite size of this catalyst calculated by Scherrer's equation is about 200 A, and that of Raney nickel from the alloy at 800 "C is about 370 A. When nickel aluminide-coated iron plate was treated with alkali, its X-ray peak intensity became slightly weaker. The crystal structure of nickel could not be distorted, because all nickel peaks in the X-ray diffraction pattern were not broad. BET surface areas of Raney nickel catalyst from 700 and 800 "C heat-treatment (method A) were
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20 0 0
NiAls Ni2A13
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60 90 Time (min)
120
150
Figure 3. Effect of composition of Ni-A1 alloy on activity of Raney nickel catalyst at 60 "C.
0.9 and 2.1 m2/cm2of apparent surface. The surface area of the catalyst from sprayed nickel aluminide (method B) was too small to give meaningful data because the apparatus required for the specimen to have an apparent surface area of at least 50 cm2. Broadening of X-ray diffraction peaks in Raney-type catalysts after leaching but not in coated iron plate catalyst may be related to crystal distortion and to differences in crystal size as well. This could account for the smaller surface areas (sharper X-ray peaks) in the latter case. Figure 3 shows the catalytic activities for hydrogenation of acetone. The Raney nickel catalyst that was prepared from a sample heat-treated at 700 "C was the most active. In the case prepared by flame-spraying nickel aluminide (method B), the catalyst obtained was poor in the activity of hydrogenation. I t proved that the alloy phase was mainly NiA13 when formed at 700 "C, but was mainly Ni2A13following a 800 "C heat treatment, and only NiAl and Ni3A1 were formed when nickel aluminide was flame-sprayed onto the steel surface. The fact is well known that compared with aluminum in NiA13 and Ni2Al3
500
Ind. Eng. Chem. plod. Reo. Dev.. VoI. 22. NO. 4. 1983
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30
60
90
120
150
Tlme (mln) Figure 4. Conversion vs. reaction time for the hydrogenation of acetone. Catalyst prepared from Ni-AI-Fe alloy was used.
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Figure 6. Cross section of Raney nickel alloy-iron boundary. . ..~. AIwinum-cootea n l c k e l alloy vreneatea a t 70C C' for 2 nr ~
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Composition profile
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N l c k e l o l m l n l d e - c n a t e d Iron D l o t e
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Figure 5. Cross section of Ni-AI alloy boundary (700 OC. 2 h).
phases, aluminum in NiAl and Ni,AI phases is difficult to leach using a 20% sodium hydroxide solution. This is probably a reason for the significant reduction of the catalytic activity shown in Figure 3. In general, h e y nickel catalyst is poisoned by oxidation in air or deactivated by annealing above 600 "C under the reductive atmosphere (Nakabayashi et al., 1979). We examined whether the inactive Raney nickel catalyst by oxidation in air could he activated by heat treatment in nitrogen stream, followed by alkali treatment. The inactive catalyst prepared from Ni-AI alloy (method B) consisted of NA and Nifl and w a not ~ activated by heat treatment at up to 700 "C,followed by alkali leaching. Figure 4 shows the restoration of the catalytic activities for hydrogenation of acetone. With increasingtemperature of heat treatment the inactive catalyst began to have considerable activity and the restoration ratio decreased by repeated heat and alkali treatments. The restoration of catalytic activity of coated iron plate catalyst was repeated several times. This is probably based on the alloy crystal rearrangement, because of the remaining relative amount of aluminide on the catalyst surface. The crystal rearrangement of nickel aluminide (NiAI and Ni,AI) was detected from the X-ray diffraction pattern of the catalyst heated at 800 'C in a
Sconnlna alstnnce
Figure 7. Line X-ray a n a l w on the c r m &ion alloy (sample scanning speed 0.5 rm/s).
of h e y nickel
nitrogen stream. On the contrary, the Raney nickel catalysts prepared from the alloys consisting of NiAI, and Ni,AI, were not reactivated by these heat and alkali treatments, because the majority of the aluminum had been leached away. Figures 5 and 6 show the hack scattered electron images of the phase boundary and Ni Ka, A1 Ka, and Fe K a radiation profiles showing the distribution of nickel, aluminum, and iron atoms in the same cross sections of the samples. These were prepared by flame-spraying aluminum onto nickel followed by heating at 700 "C (method A) and by flame-spraying nickel-aluminum alloy onto steel (method B) without subsequent heating, respectively. In the common range of nickel and aluminum distributions (Ni K a and AI Ka radiation profiles in Figure 5). the Ni-AI alloy phase is probably formed. Therefore, the thickness of alloy phase can easily he estimated. The flame-sprayed nickel aluminide (method B) is very difficult to be me-
Ind. Eng. Chem. Rod. Res. Dev.. VOI. 22. No. 4. 1983 581
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Composition profile
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Figure 8. Surface of poisoned Raney nickel catalyst. prepared from Ni-AI alloy plate (IO0 OC. 2 h).
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