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The effects of alkali metal hydroxide pretreatment on the hydrogenation activity of nickel-loaded Y zeolites
Journal
of Molecular
Catalysis,
63 (1990)
193-199
193
The effects of alkali metal hydroxide pretreatment on the hydrogenation activity of nickel-loaded Y zeolites Brendan Physical
Coughlan and Mark A. Keane*
Chemistry
Laimratories,
University
College,
Galway
(3%~)
(Received March 12, 1990; revised May 22, 1990)
Abstract A range of NiKY and NiNaY zeolites prepared by ion exchange were treated with NaOH and KOH solutions of varying pH at room temperature and reduced in a hydrogen atmosphere at 723 K. By a combination of iodometric titrations and X-ray line broadening it is shown that reduction of the hydroxide treated samples generates a greater mass of nickel metal, which is in the form of smaller crystallites compared with the untreated zeolites. The pretreated samples exhibit a uniquely high steady state catalytic activity in the hydrogenation of benzene to cyclohexane.
Introduction The unique structural characteristics of zeolites can be made use of in the preparation of metal-loaded catalysts; the support increases the efficiency of the metal by physically separating the small metal crystallites. Agglomeration of the metal particles results in a decrease in the number of surface metal atoms per unit mass of metal, with a resultant lowering of catalytic activity. The factors which influence the extent of reduction of the exchanged nickel
cations ultimately affect the dispersion of the nickel metal. A series of systematic studies conducted in these laboratories on the effects of sample preparation, pretreatment conditions and the nature of the support on the level of Ni2+ reduction have been reported [ 11. Indeed, the redox processes occurring during the reduction of nickel-exchanged zeolites have been the subject of multilateral studies [2-81. The diihculty in obtaining a homogeneous, well-dispersed metal phase in zeolites is well recognized [g-13]. Although the use of a support stabilizes the small metal particles, sintering or the growth of metal crystal&es occurs if the catalyst is used at elevated temperatures. Suzuki et a.?. [ 14, 151 have reported the preparation of a highly dispersed nickel Y zeolite by treating NiNaY with NaOH followed by calcination and reduction steps. The authors state that the calcination step results in the formation of oligomeric nickel oxide located in the supercage. These unique and highly dispersed oligomeric *Author to whom correspondence should be addressed at: Chemistry Department, The University, Glasgow G12 8&Q (U.K.)
0304-5102/90/$3.50
0 Elsevier Sequoia/Printedin The Netherlands
194
clusters of nickel oxide are converted into fine nickel metal particles by hydrogen reduction. Suzuki et al. concentrated their investigations on treatments with NaOH solutions in the pH range 9-11 and intimated that the resultant generation of finely dispersed metal particles should have far-ranging catalytic implications. It was decided to develop Suzuki’s iindings and extend the study to incorporate the treatment of NiKY with KOH solutions and include hydroxide solutions of pH 12 and 13. The effect of the hydroxide treatment on the extent of nickel cation reduction and the catalytic activity of the resultant metal phase were considered. The catalytic hydrogenation of benzene to cyclohexane was chosen as a model reaction to monitor the activity of the metal phase. This study constitutes a natural continuation of the work conducted in these laboratories [ 161 directed at probing the role of the alkali metal co-cation in the genesis of a catalytically active supported metal phase.
Experimental
The starting or parent zeolite was Linde Molecular Sieve [formula (LZY52): Na&102)58(Si02)134(H20)2601. KY was prepared by refluxing 250 g NaY with 400 cm3 1 M KN03 for 24 h, after which the zeolite was vacuum filtered, thoroughly washed with hot deionised water to remove the occluded salt and oven dried at 373 K for a further 24 h. The partially exchanged KNaY samples were exchanged a further nine times and stored over saturated NH&l solutions. Samples containing up to cu. 7 Ni2+/U.C. (i.e. seven nickel ions per unit cell) were prepared by taking 100 g NaY or KY and refhrxing with 400 cm3 Ni(N03)2 solution, the concentration of which was chosen on the basis that the exchange procedure was cu. 80% efficient in obtaining the desired exchange level [ 161. In order to prevent hydrolytic Na+ or K+ loss or exchange of hydroxylic nickel species, i.e. to ensure stoichiometric exchange, the pH of the nitrate solution was kept at cu. 6. In preparing samples of loading greater than cu. 7 Ni2+/U.C., repeated exchange was necessary. The resulting nickel-exchanged zeolites were vigorously stirred for 1 h at room temperature with the NaOH/KOH solutions, filtered, washed and dried as before. Atomic absorption for Ni2+ concentrations, and flame emission spectroscopy for Na+ and K+ concentrations, using a Perkln Elmer 360 atomic absorption spectrophotometer, were employed to determine the cation contents to within f 2%. Analysis of the hydroxide filtrates revealed trace amounts of nickel ( < 1% of the total metal loading) which suggests that the nickel content of the catalysts remains largely unaffected by the modiilcation process. Thermal analyses were also conducted on all the prepared samples using a Perkin Elmer thermobalance operated in the TG mode to measure mass loss as a function of temperature. The percent reduction in flowing hydrogen of Ni2+ ions, supported on both NaY and KY carriers, was obtained by an iodometric technique. A 3 g hydrated sample, in pellet form (1.18-l .70 mm diameter), was heated (at
195
10 K min-‘) in a fixed bed catalytic reactor (of internal diameter 3 cm) [16], in a 120 cm3 min-’ stream of hydrogen at 723 K for 18 h. The sample was stored over saturated NH&l and the water content was determined before degree of reduction measurements. The hydrated reduced sample (1 g), in powder form, was then refluxed at 373 K for 16 h with 20 cm3 0.1 M K2Cr207, the suspension was filtered and the filtrate made up to 250 cm’; 50 cm3 of this solution was transferred to a 500 cm3 volumetric flask with 100 cm3 of recently boiled deionized water, 6 cm’ cont. HCI, 2 g NaHCOa and 3 g KI; the flask was left to develop in the dark for 15 min after which the volume was made up to 500 cm3 with deionized water. A 50 cm3 aliquot of this solution was titrated against standardized 0.01 M Na$&Oa using starch as indicator. The unreduced sample was also carried through the procedure as a blank and the amount of nickel metal present was determined from the difference in reading between the blank and the sample. Examination of the samples for the presence of nickel crystallites was performed by studying X-ray line broadening (Jeol JDX-85 diffractometer) around the nickel line at a 28 angle of 52.2” under the following conditions: time constant=4; voltage=30 kV; current=20 ti, scan time=0.5 min-‘; counts/s = 8 x 102; radiation = Co K,. A correction for instrument broadening under identical conditions was made using Ni powder in a Nay/KY matrix. All catalytic reactions were carried out under atmospheric pressure in a fixed bed tubular glass reactor [ 161 at 473 K. The catalysts (3 g for each reaction) were pelletized without binder using a pressure of 4000 kg cm- ’ and sieved in the mesh range 1.18-l .70 mm. Catalyst activation was the same as for the degree of reduction measurements. A hydrogen flow of 120 cm3 min-’ was maintained during ‘each reaction. Benzene was fed into the reactor from a precision motor-driven syringe at a flow rate of 2.0 cm3 h-l, yielding a W/F value of 134.1 g mol- ’ h where W is the mass of hydrated catalyst (g) and F represents the feed rate of benzene (mol h-l). Product analysis was carried out using a Bye Unicam GCV chromatograph with a flame ionization detector. Diffraction patterns of all the catalysts, before and after use in catalysis, were obtained to ensure maintenance of sample crystallimQ. Infrared spectroscopy in the range 1200-350 cm- ’ was also used as a check on crystallinity. The band at ca. 395 cm-’ has been assigned to a breathing of the pore opening in zeolites [ 171 and is thus the most sensitive to changes in crystallinity.
Results and discussion The chemical compositions of the ion exchanged samples are given in Table 1. The names of the samples are followed by a number showing the exchange level of Ni2+ as a percentage of the total cation exchange capacity, for example in the case of nickel-exchanged KY (NiKY), NiKY-23.5 indicates a 23.5% exchange of the 58 indigenous K+ ions per unit cell, resulting in a nickel loading of 6.8 Ni’+/U.C. By and large, the exchange process was
196 TABLE 1 Chemical composition of NiNaYiNiKY samples prepared by ion exchange Zeolite sample
Na+K+/U.C.
Ni2+AJ.C.
H+/U.C.
NaY NiNaY-6.8 NiNaY-22.8 NiNaY-35.7 NiNaY-63.1
58.0 53.7 44.0 36.0 22.4
2.0 6.6 10.4 18.3
0.3 0.8 1.2
KY NiKY-8.0 Nii-23.5 NiKY-35.6 NiKY-62.5
58.0 53.3 44.2 36.9 22.3
2.3 6.8 10.3 18.1
0.2 0.5
Water content (wt.%) 25.1 25.3 26.6 27.6 29.1 22.4 22.3 23.5 24.8 27.6
TABLE 2 Variation in % Ni2’ reduction with NaOH/KOH treatment Zeolite sample
Ni2+ reduction (%) Pretreatment with NaOH/KOH
Reduced without pretreatment
pH= 10.6
pH=12
pH= 13
NiNaY-6.8 NiNaY-22.8 NiNaY-35.7 NiNaY-63.1
96.2
97.0 90.0
98.5 95.8
95.7 79.3
83.0 80.1
90.1 89.3
63.3 54.1
NiKY-8.0 Nii-23.5 NiKY-35.6 NiKY-62.5
95.4 70.4 57.2 58.9
97.1 84.7 75.1 72.1
98.6 93.1 89.4 86.8
94.1 68.8 53.2 56.9
82.1 68.1 60.7
stoichiometric. The extent of hydrolysis, as evidenced by the number of protons present in the structure, was minimal and occurred only for the lower exchanged samples. The number of protons present was inferred from the total number of metal ions, assuming an overall charge of 58 positive ions per unit cell. Sample crystallinity, monitored by X-ray diffraction and IR spectroscopy, was maintained after the preparation and modification of all the tabulated samples. Treatment of the samples with NaOH/KOH results in a marked enhancement in Ni2+ reducibility, which increases with the pH of the hydroxide solution, Table 2. The supported nickel hydroxide species generated during the modil?cation step must therefore be reduced with a greater relative ease than are the Ni2+ cations coordinated to the framework oxygens in the untreated samples. Calcining the samples prior to reduction resulted in a
197
slight suppression of the NaOH/KOH promoting effect, e.g. in the case of NiKY-62.52 (pH of KOH = 13) precalcination in nitrogen at 723 K lowered the percent Ni2+ reduction from 86.8 to 80.1. The metal phase generated was characterized by X-ray line broadening using the Scherrer formula: d -K/V@
cos 19,
which relates the mean particle diameter (d) to the X-ray broadening (p) of the diffraction lines. Metal dispersion can be related to crystallite size according to: D = 0.505/d
where D is the dispersion or fraction of metal atoms exposed to the surface. The nickel particle sizes are listed in Table 3. The lower limit of detection for this method is cu. 5 run, with the result that it gives no information on the state of metal in the internal pore structure. The latter statement follows from the f t that the dimensions of the largest cage (CCL1.3 nm) restricts the form ion of intracrystalline particles greater than cu. 1.3 run without some sf ctural breakdown. X-ray line broadening is therefore diagnostic of the presence of nickel metal located on the external surface. Furthermore, this technique does not differentiate between metal particles exceeding supercage dimensions, which have been reported to form within the zeolite framework [ 18,191. Although the NaOH/KOH treatments promoted the extent of Ni2+ reduction, the nickel particles generated are in the form of smaller crystallites, i.e. dispersion is enhanced. This is in complete contrast to NH8 pretreatments [ 161 where the promotion of Ni2+ reduction resulted in the formation of larger metal particles. Hydrogen reduction of the highly dispersed oligomeric clusters of NiO therefore generates a greater volume of intracrystalline nickel metal. The alkali metal hydroxide pretreatment must serve TABLE 3 Variation of nickel crystallite size with NaOH/KOH treatment Zeolite sample
Nickel crystsUite size (d> (m) Pretreatment with NaOH/KOH
Reduced without pretreatment
pH= 10.6
pH= 12
pH=13
NiNaY-6.8 NiNaY-22.8 NiNaY-35.7 NiNaY-63.1
21.8 33.8 40.3 53.0
18.7 27.1 29.1 35.1
13.7 20.1 21.2 26.1
24.5 38.7 46.1 68.9
NiKY-8.0 NiKY-23.5 NiKY-35.6 Nii-62.5
21.3 30.8 35.4 46.5
17.5 22.1 26.0 33.0
12.2 14.1 17.8 22.2
25.1 35.1 41.3 50.0
198 TABLE 4 Variation of benzene hydrogenation rate with NaOH/KOH treatment; W/F= T=473 K Zeolite sample
134.1 g mol-’
h;
R (mol g-l h-r) Pretreatment with NaOH/KOH
Reduced without pretreatment
pH = 10.6
pH= 12
pH=13
NiNaY-6.8 NiNaY-22.8 NiiaY-35.7 NiNaY-63.1
0.30 0.34 0.06 0.02
0.38 0.43 0.11 0.10
0.45 0.49 0.16 0.17
0.28 0.31 0.03 0.01
Nii-8.0 Nii-23.5 NiKY-35.6 NiKY-62.5
0.32 0.41 0.23 0.44
0.42 0.46 0.31 0.49
0.50 0.51 0.43 0.52
0.29 0.40 0.20 0.42
to retard the agglomeration of metal particles on the external surface, and will presumably also inhibit the growth of intracrystalline metal clusters. This effect is further promoted by increasing the pH of the hydroxide solution. The transition metal catalysis of benzene hydrogenation is a well-known reaction and exhibits the general features of unsaturate-aromatic interactions. The metal function promotes the hydrogenation reaction, while the acid function (also generated during reduction) catalyzes skeletal isomerization and allqlation reactions which lead to coke formation and the ultimate suppression of catalytic activity (161. The higher levels of Ni” reduction with hydroxide pretreatment and the overall improved dispersion of (both internal and external) active metal has resulted in a marked improvement in the rate of benzene conversion, Table 4. The treatment proved more effective for the potassium-based samples, which also exhibited much higher conversion in the untreated form due to lower levels of surface acidity compared to the NiNaY system [20]. More importantly, these extremely high yields of cyclohexane were maintained over a 24 h period. Residual carbon measurements of the spent NiKY catalysts were negligible, indicating the virtual absence of coking. Reduction of the NaOH/KOH treated samples must therefore result in the generation of a lower level of zeolite acidity compared with the untreated samples, as evidenced by higher conversion levels and suppressed coke formation.
Conclusions
The degree of reduction of Ni2’ cations supported on NaY and KY was promoted with NaOHJKOH treatments. The resultant metal phase is in the form of much smaller crystallites which exhibit markedly enhanced levels
199
of benzene conversion to cyclohexane. These effects are promoted by increasing the pH of the hydroxide solution and are more pronounced for the potassium-based zeolites.
References 1 B. CoughIan and M. A. Keane, J. Cutal. (1990), in press. 2 Ch. Minchev, V. Kanazirev, L. Kosova, V. Penchev, W. Gunsser and F. Schmidt, in L. V. C. Rees (ed.), Proc. 5th Int. Coqf Zeolites, London, 1980, Heyden, London, 1980, p. 355. 3 H. S&rubbers, G. Schulz-EkIoff and H. Wddeboer, in Metal Microstructures in Zeolites, Stud. Surf Sci. Catal., 12 (1982) 261. 4 M. Suzuki, K. Tsutsumi and H. Takahasi, Zeolites, 2 (1982) 51. 5 N. P. Davidova, N. V. VaIchev and D. M. Shopov, React. Kin&. Catal. I.&t., 16 (1981) 201. 6 J. C. Carru, D. Delafosse and M. Btiend, in: Structure and Reactivity of ModQied Zeolites, Stud. Surf Sci. Catal., 18 (1984) 337. 7 J. Jeanjean, S. Djemel, M. F. GuiIIeux and D. DeIafosse, J. Phys. Chem., 85 (1981) 4145. 8 L. Zheng, G. Wang and X. Bai, in Proc. 7th Int. Co& Zeolites, Tokyo, 1986, Stud. Sud Sci. Cutal., 28 (1986) 965. 9 N. Jaeger, P. Plath and G. Schulz-Ekloff,Acta Phys. Chem. (Szeged), 31 (1985) 189. 10 D. Delafosse, in Catalysis by Zeolites, Stud. Sue Sci. Cutal., 5 (1980) 237. 11 D. Delafosse, J. Chim. Phys., 83 (1986) 791. 12 N. I. Jaeger, P. Ryder and G. Schulz-Ekloff, in Structure and Reactivity of Modified Zeolites, Stud. Su?_-f Sci. CataL, 18 (1984) 299. 13 S. Narayanan, J. Sci. Ind. Res., 44 (1985) 319. 14 M. Sano, T. Maruo, H. Yamatera, M. Suxuki and Y. Saito, J. Am. C/rem.Sot., 109 (1987) 52. 15 M. Suzuki, K. Tsutsumi, H. Takahashi and Y. Saito, Zeolites, 8 (1988) 284. 16 M. A. Keane, Ph.D. Thesis (Vols. 1 and 2), National University of Ireland, 1988. 17 E. M. Flanigen, H. Khatami and H. A. Szymauski,in Molecular Sieve Zeolites, Adv. Chem. ser., 101 (1971) 201. 18 D. Exner, N. Jaeger, R. Nowak, H. S&rubbers and G. Schulz-EkIoff,J. Catal., 74 (1982) 188. 19 P. A. Jacobs, H. N&s, J. Verdonck, E. G. Derouane, J. P. GiIson and A. J. Simoens, Z?a?z.s. Faraday Sot. 1, 5 (1979) 1196. 20 B. CoughIan and M. A. Keane, J. CoUoid. Inte@ace Sci. (1990), in press.
of Molecular
Catalysis,
63 (1990)
193-199
193
The effects of alkali metal hydroxide pretreatment on the hydrogenation activity of nickel-loaded Y zeolites Brendan Physical
Coughlan and Mark A. Keane*
Chemistry
Laimratories,
University
College,
Galway
(3%~)
(Received March 12, 1990; revised May 22, 1990)
Abstract A range of NiKY and NiNaY zeolites prepared by ion exchange were treated with NaOH and KOH solutions of varying pH at room temperature and reduced in a hydrogen atmosphere at 723 K. By a combination of iodometric titrations and X-ray line broadening it is shown that reduction of the hydroxide treated samples generates a greater mass of nickel metal, which is in the form of smaller crystallites compared with the untreated zeolites. The pretreated samples exhibit a uniquely high steady state catalytic activity in the hydrogenation of benzene to cyclohexane.
Introduction The unique structural characteristics of zeolites can be made use of in the preparation of metal-loaded catalysts; the support increases the efficiency of the metal by physically separating the small metal crystallites. Agglomeration of the metal particles results in a decrease in the number of surface metal atoms per unit mass of metal, with a resultant lowering of catalytic activity. The factors which influence the extent of reduction of the exchanged nickel
cations ultimately affect the dispersion of the nickel metal. A series of systematic studies conducted in these laboratories on the effects of sample preparation, pretreatment conditions and the nature of the support on the level of Ni2+ reduction have been reported [ 11. Indeed, the redox processes occurring during the reduction of nickel-exchanged zeolites have been the subject of multilateral studies [2-81. The diihculty in obtaining a homogeneous, well-dispersed metal phase in zeolites is well recognized [g-13]. Although the use of a support stabilizes the small metal particles, sintering or the growth of metal crystal&es occurs if the catalyst is used at elevated temperatures. Suzuki et a.?. [ 14, 151 have reported the preparation of a highly dispersed nickel Y zeolite by treating NiNaY with NaOH followed by calcination and reduction steps. The authors state that the calcination step results in the formation of oligomeric nickel oxide located in the supercage. These unique and highly dispersed oligomeric *Author to whom correspondence should be addressed at: Chemistry Department, The University, Glasgow G12 8&Q (U.K.)
0304-5102/90/$3.50
0 Elsevier Sequoia/Printedin The Netherlands
194
clusters of nickel oxide are converted into fine nickel metal particles by hydrogen reduction. Suzuki et al. concentrated their investigations on treatments with NaOH solutions in the pH range 9-11 and intimated that the resultant generation of finely dispersed metal particles should have far-ranging catalytic implications. It was decided to develop Suzuki’s iindings and extend the study to incorporate the treatment of NiKY with KOH solutions and include hydroxide solutions of pH 12 and 13. The effect of the hydroxide treatment on the extent of nickel cation reduction and the catalytic activity of the resultant metal phase were considered. The catalytic hydrogenation of benzene to cyclohexane was chosen as a model reaction to monitor the activity of the metal phase. This study constitutes a natural continuation of the work conducted in these laboratories [ 161 directed at probing the role of the alkali metal co-cation in the genesis of a catalytically active supported metal phase.
Experimental
The starting or parent zeolite was Linde Molecular Sieve [formula (LZY52): Na&102)58(Si02)134(H20)2601. KY was prepared by refluxing 250 g NaY with 400 cm3 1 M KN03 for 24 h, after which the zeolite was vacuum filtered, thoroughly washed with hot deionised water to remove the occluded salt and oven dried at 373 K for a further 24 h. The partially exchanged KNaY samples were exchanged a further nine times and stored over saturated NH&l solutions. Samples containing up to cu. 7 Ni2+/U.C. (i.e. seven nickel ions per unit cell) were prepared by taking 100 g NaY or KY and refhrxing with 400 cm3 Ni(N03)2 solution, the concentration of which was chosen on the basis that the exchange procedure was cu. 80% efficient in obtaining the desired exchange level [ 161. In order to prevent hydrolytic Na+ or K+ loss or exchange of hydroxylic nickel species, i.e. to ensure stoichiometric exchange, the pH of the nitrate solution was kept at cu. 6. In preparing samples of loading greater than cu. 7 Ni2+/U.C., repeated exchange was necessary. The resulting nickel-exchanged zeolites were vigorously stirred for 1 h at room temperature with the NaOH/KOH solutions, filtered, washed and dried as before. Atomic absorption for Ni2+ concentrations, and flame emission spectroscopy for Na+ and K+ concentrations, using a Perkln Elmer 360 atomic absorption spectrophotometer, were employed to determine the cation contents to within f 2%. Analysis of the hydroxide filtrates revealed trace amounts of nickel ( < 1% of the total metal loading) which suggests that the nickel content of the catalysts remains largely unaffected by the modiilcation process. Thermal analyses were also conducted on all the prepared samples using a Perkin Elmer thermobalance operated in the TG mode to measure mass loss as a function of temperature. The percent reduction in flowing hydrogen of Ni2+ ions, supported on both NaY and KY carriers, was obtained by an iodometric technique. A 3 g hydrated sample, in pellet form (1.18-l .70 mm diameter), was heated (at
195
10 K min-‘) in a fixed bed catalytic reactor (of internal diameter 3 cm) [16], in a 120 cm3 min-’ stream of hydrogen at 723 K for 18 h. The sample was stored over saturated NH&l and the water content was determined before degree of reduction measurements. The hydrated reduced sample (1 g), in powder form, was then refluxed at 373 K for 16 h with 20 cm3 0.1 M K2Cr207, the suspension was filtered and the filtrate made up to 250 cm’; 50 cm3 of this solution was transferred to a 500 cm3 volumetric flask with 100 cm3 of recently boiled deionized water, 6 cm’ cont. HCI, 2 g NaHCOa and 3 g KI; the flask was left to develop in the dark for 15 min after which the volume was made up to 500 cm3 with deionized water. A 50 cm3 aliquot of this solution was titrated against standardized 0.01 M Na$&Oa using starch as indicator. The unreduced sample was also carried through the procedure as a blank and the amount of nickel metal present was determined from the difference in reading between the blank and the sample. Examination of the samples for the presence of nickel crystallites was performed by studying X-ray line broadening (Jeol JDX-85 diffractometer) around the nickel line at a 28 angle of 52.2” under the following conditions: time constant=4; voltage=30 kV; current=20 ti, scan time=0.5 min-‘; counts/s = 8 x 102; radiation = Co K,. A correction for instrument broadening under identical conditions was made using Ni powder in a Nay/KY matrix. All catalytic reactions were carried out under atmospheric pressure in a fixed bed tubular glass reactor [ 161 at 473 K. The catalysts (3 g for each reaction) were pelletized without binder using a pressure of 4000 kg cm- ’ and sieved in the mesh range 1.18-l .70 mm. Catalyst activation was the same as for the degree of reduction measurements. A hydrogen flow of 120 cm3 min-’ was maintained during ‘each reaction. Benzene was fed into the reactor from a precision motor-driven syringe at a flow rate of 2.0 cm3 h-l, yielding a W/F value of 134.1 g mol- ’ h where W is the mass of hydrated catalyst (g) and F represents the feed rate of benzene (mol h-l). Product analysis was carried out using a Bye Unicam GCV chromatograph with a flame ionization detector. Diffraction patterns of all the catalysts, before and after use in catalysis, were obtained to ensure maintenance of sample crystallimQ. Infrared spectroscopy in the range 1200-350 cm- ’ was also used as a check on crystallinity. The band at ca. 395 cm-’ has been assigned to a breathing of the pore opening in zeolites [ 171 and is thus the most sensitive to changes in crystallinity.
Results and discussion The chemical compositions of the ion exchanged samples are given in Table 1. The names of the samples are followed by a number showing the exchange level of Ni2+ as a percentage of the total cation exchange capacity, for example in the case of nickel-exchanged KY (NiKY), NiKY-23.5 indicates a 23.5% exchange of the 58 indigenous K+ ions per unit cell, resulting in a nickel loading of 6.8 Ni’+/U.C. By and large, the exchange process was
196 TABLE 1 Chemical composition of NiNaYiNiKY samples prepared by ion exchange Zeolite sample
Na+K+/U.C.
Ni2+AJ.C.
H+/U.C.
NaY NiNaY-6.8 NiNaY-22.8 NiNaY-35.7 NiNaY-63.1
58.0 53.7 44.0 36.0 22.4
2.0 6.6 10.4 18.3
0.3 0.8 1.2
KY NiKY-8.0 Nii-23.5 NiKY-35.6 NiKY-62.5
58.0 53.3 44.2 36.9 22.3
2.3 6.8 10.3 18.1
0.2 0.5
Water content (wt.%) 25.1 25.3 26.6 27.6 29.1 22.4 22.3 23.5 24.8 27.6
TABLE 2 Variation in % Ni2’ reduction with NaOH/KOH treatment Zeolite sample
Ni2+ reduction (%) Pretreatment with NaOH/KOH
Reduced without pretreatment
pH= 10.6
pH=12
pH= 13
NiNaY-6.8 NiNaY-22.8 NiNaY-35.7 NiNaY-63.1
96.2
97.0 90.0
98.5 95.8
95.7 79.3
83.0 80.1
90.1 89.3
63.3 54.1
NiKY-8.0 Nii-23.5 NiKY-35.6 NiKY-62.5
95.4 70.4 57.2 58.9
97.1 84.7 75.1 72.1
98.6 93.1 89.4 86.8
94.1 68.8 53.2 56.9
82.1 68.1 60.7
stoichiometric. The extent of hydrolysis, as evidenced by the number of protons present in the structure, was minimal and occurred only for the lower exchanged samples. The number of protons present was inferred from the total number of metal ions, assuming an overall charge of 58 positive ions per unit cell. Sample crystallinity, monitored by X-ray diffraction and IR spectroscopy, was maintained after the preparation and modification of all the tabulated samples. Treatment of the samples with NaOH/KOH results in a marked enhancement in Ni2+ reducibility, which increases with the pH of the hydroxide solution, Table 2. The supported nickel hydroxide species generated during the modil?cation step must therefore be reduced with a greater relative ease than are the Ni2+ cations coordinated to the framework oxygens in the untreated samples. Calcining the samples prior to reduction resulted in a
197
slight suppression of the NaOH/KOH promoting effect, e.g. in the case of NiKY-62.52 (pH of KOH = 13) precalcination in nitrogen at 723 K lowered the percent Ni2+ reduction from 86.8 to 80.1. The metal phase generated was characterized by X-ray line broadening using the Scherrer formula: d -K/V@
cos 19,
which relates the mean particle diameter (d) to the X-ray broadening (p) of the diffraction lines. Metal dispersion can be related to crystallite size according to: D = 0.505/d
where D is the dispersion or fraction of metal atoms exposed to the surface. The nickel particle sizes are listed in Table 3. The lower limit of detection for this method is cu. 5 run, with the result that it gives no information on the state of metal in the internal pore structure. The latter statement follows from the f t that the dimensions of the largest cage (CCL1.3 nm) restricts the form ion of intracrystalline particles greater than cu. 1.3 run without some sf ctural breakdown. X-ray line broadening is therefore diagnostic of the presence of nickel metal located on the external surface. Furthermore, this technique does not differentiate between metal particles exceeding supercage dimensions, which have been reported to form within the zeolite framework [ 18,191. Although the NaOH/KOH treatments promoted the extent of Ni2+ reduction, the nickel particles generated are in the form of smaller crystallites, i.e. dispersion is enhanced. This is in complete contrast to NH8 pretreatments [ 161 where the promotion of Ni2+ reduction resulted in the formation of larger metal particles. Hydrogen reduction of the highly dispersed oligomeric clusters of NiO therefore generates a greater volume of intracrystalline nickel metal. The alkali metal hydroxide pretreatment must serve TABLE 3 Variation of nickel crystallite size with NaOH/KOH treatment Zeolite sample
Nickel crystsUite size (d> (m) Pretreatment with NaOH/KOH
Reduced without pretreatment
pH= 10.6
pH= 12
pH=13
NiNaY-6.8 NiNaY-22.8 NiNaY-35.7 NiNaY-63.1
21.8 33.8 40.3 53.0
18.7 27.1 29.1 35.1
13.7 20.1 21.2 26.1
24.5 38.7 46.1 68.9
NiKY-8.0 NiKY-23.5 NiKY-35.6 Nii-62.5
21.3 30.8 35.4 46.5
17.5 22.1 26.0 33.0
12.2 14.1 17.8 22.2
25.1 35.1 41.3 50.0
198 TABLE 4 Variation of benzene hydrogenation rate with NaOH/KOH treatment; W/F= T=473 K Zeolite sample
134.1 g mol-’
h;
R (mol g-l h-r) Pretreatment with NaOH/KOH
Reduced without pretreatment
pH = 10.6
pH= 12
pH=13
NiNaY-6.8 NiNaY-22.8 NiiaY-35.7 NiNaY-63.1
0.30 0.34 0.06 0.02
0.38 0.43 0.11 0.10
0.45 0.49 0.16 0.17
0.28 0.31 0.03 0.01
Nii-8.0 Nii-23.5 NiKY-35.6 NiKY-62.5
0.32 0.41 0.23 0.44
0.42 0.46 0.31 0.49
0.50 0.51 0.43 0.52
0.29 0.40 0.20 0.42
to retard the agglomeration of metal particles on the external surface, and will presumably also inhibit the growth of intracrystalline metal clusters. This effect is further promoted by increasing the pH of the hydroxide solution. The transition metal catalysis of benzene hydrogenation is a well-known reaction and exhibits the general features of unsaturate-aromatic interactions. The metal function promotes the hydrogenation reaction, while the acid function (also generated during reduction) catalyzes skeletal isomerization and allqlation reactions which lead to coke formation and the ultimate suppression of catalytic activity (161. The higher levels of Ni” reduction with hydroxide pretreatment and the overall improved dispersion of (both internal and external) active metal has resulted in a marked improvement in the rate of benzene conversion, Table 4. The treatment proved more effective for the potassium-based samples, which also exhibited much higher conversion in the untreated form due to lower levels of surface acidity compared to the NiNaY system [20]. More importantly, these extremely high yields of cyclohexane were maintained over a 24 h period. Residual carbon measurements of the spent NiKY catalysts were negligible, indicating the virtual absence of coking. Reduction of the NaOH/KOH treated samples must therefore result in the generation of a lower level of zeolite acidity compared with the untreated samples, as evidenced by higher conversion levels and suppressed coke formation.
Conclusions
The degree of reduction of Ni2’ cations supported on NaY and KY was promoted with NaOHJKOH treatments. The resultant metal phase is in the form of much smaller crystallites which exhibit markedly enhanced levels
199
of benzene conversion to cyclohexane. These effects are promoted by increasing the pH of the hydroxide solution and are more pronounced for the potassium-based zeolites.
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