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Nickel silica-alumina catalysts for ethene oligomerization—control of the selectivity to 1-alkene products
Applied Catalysis A: General 245 (2003) 43–53
Nickel silica-alumina catalysts for ethene oligomerization—control of the selectivity to 1-alkene products C.P. Nicolaides, M.S. Scurrell∗ , P.M. Semano Molecular Sciences Institute, School of Chemistry, University of the Witwatersrand, P.O. WITS, 2050 Johannesburg, South Africa Received 27 June 2002; received in revised form 16 November 2002; accepted 22 November 2002
Abstract A rational approach to control the behavior of nickel(II) ion-exchanged silica-alumina oligomerization catalysts has been attempted. Emphasis has been placed on securing higher selectivities to 1-alkenes from the oligomerization of ethene, with data being presented for 1-hexene. A key challenge is to reduce the relatively high rate of double-bond shift which leads to the formation of internal alkene products. One of the most effective means of inducing favorable changes in the 1-hexene content of the hexane fraction appears to be an increase in the time-on-stream, though this aspect is not observed for all catalysts studied. Addition of nickel ions in excess of those introduced by ion-exchange, addition of potassium ions, and changes in the Si/Al ratio of the support are further variables investigated. These variable do not, for each one studied, lead to dramatic changes in the selectivity, but the combined effect of appropriate adjustments in these factors do give some improvement. In addition, the total nickel ion content influences the carbon number-based product distribution, with the highest hexene selectivity being found with catalysts containing 1.5 mass% Ni2+ . © 2002 Elsevier Science B.V. All rights reserved. Keywords: Ethene oligomerization; Nickel(II)-exchanged silica-alumina; 1-Alkene selectivity; 1-Hexene selectivity
1. Introduction Recent accounts deal with the behavior of highly active and stable catalysts for the oligomerization of ethene, based on nickel(II) ion-exchanged silica-aluminas [1,2]. These catalysts can also be successfully used for the oligomerization of propene and butene [3]. A detailed analysis of the dimer product obtained during the oligomerization of propene [4] reveals that a comparatively high selectivity (ca. 52 mass%) to linear hexenes is obtained. However, the selectivity to 1-hexene itself is low, and it seems ∗ Corresponding author. Tel.: +27-11-717-6716; fax: +27-11-717-6749. E-mail address: [email protected] (M.S. Scurrell).
likely that the double-bond shift reaction of the alkene on these catalysts is comparatively rapid so that the product distribution within a given Cn alkene product will reflect formation of the more thermodynamically favored internal alkene. Attainment of the thermodynamic equilibration of the structural isomers would result in a very low content of the 1-alkene [3] and even partial attainment would severely restrict the 1-alkene content. A remaining challenge, therefore, in the use of such catalysts concerns the selective formation of 1-alkenes, such as 1-hexene, which are highly valuable chemical feedstocks (e.g. for use as co-monomers in alkene polymerization processes). Studies of attempts to secure selective 1-hexene formation from ethene on heterogeneous nickel-based oligomerization catalysts are very rare. However, in
0926-860X/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0926-860X(02)00615-4
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a recent report by Belltrame et al. [5], it was shown that such catalysts (Ni-ZSM-5) may exhibit improved selectivities to terminal alkenes after they have presumably undergone partial deactivation due to coking. We have now attempted a rational approach applied in order to modify the behavior of Ni(II) ionexchanged silica-alumina oligomerization catalysts by means of changes in catalyst composition and/or changes in the process variables associated with their use, and have secured information on conditions favouring 1-alkene production. Our approach is illustrated by analysis of the trimer product obtained in the oligomerization of ethene.
2. Experimental Catalysts comprised nickel ion-exchanged silicaaluminas. The silica-alumina was prepared according to previously published procedures [1] by addition of nitric acid to a mixed solution of sodium aluminate and sodium silicate. After suitable aging, the sodium hydrogel was thoroughly washed to a final pH of 6. The gels were then recovered by filtration followed by drying at 100 ◦ C and calcination at 550 ◦ C. The Si/Al ratio in the supports was varied between 25 and 500. Ion-exchange of the Na+ form of silica-alumina (NaSA) with Ni2+ was carried out under reflux conditions using an aqueous solution of nickel chloride. The nickel content of the solution and mass of support used corresponded to a three-fold excess of nickel ions present based on a Ni:Al stoichiometry of 1:2. The ion-exchanged solids were filtered and washed extensively with water to remove the chloride ions and excess nickel ions, and were then dried at 70 ◦ C. The supports and catalysts are designated as NaSA-X and NiSA-X, respectively, where X represented the Si/Al ratio of the support. The catalysts used here are essentially identical to those prepared earlier and used for ethene oligomerization [1]. The NiSA-50 series of catalysts was subjected to additional subsequent modification involving the addition of further nickel or the introduction of potassium, carried out in either case by means of incipient wetness impregnation with a solution of nickel(II)
chloride or potassium carbonate, respectively. The quantity of solution used was made equal to the pore volume (0.65 cm3 g−1 ) obtained from the measurement of the textural properties of the supports, which were carried out using nitrogen adsorption/desorption procedures at 77 K. The modified NiSA-50 catalysts contained up to 4.0 mass% Ni (of which 0.8 mass% was the quantity originally present after incorporation by ion-exchange), or possessed K+ /Ni2+ ratios in the range 0.05–1.00. Catalytic oligomerization of ethene was carried out using a fixed-bed tubular reactor constructed of stainless steel. Charges of 1.0 g catalyst were used with a particle size fraction in the range 300–500 m. Catalysts were activated by heating gradually in flowing nitrogen to 300 ◦ C, and maintained at this temperature for 3 h. The catalyst was then cooled in flowing nitrogen to room temperature at which point ethene was introduced and the reactor brought to the required reaction conditions. Care had to be exercised in bringing the reactor to the desired reaction pressure and temperature in order to avoid the development of excessive temperature excursions within the catalyst bed arising from the exothermic nature of the reaction. Ethene (99.7% purity) supplied by Fedgas was fed to the reactor and most reactions were carried out under the following conditions: reaction temperature, 100 ◦ C; pressure, 1.5 MPa; MHSV, 4 h−1 ; time-on-stream up to 500 min, with samples of the exit stream being taken at regular intervals and analyzed by gas chromatography using a capillary column (50 m methyl bonded silicone) and FID detection. Some runs were conducted at higher pressures up to 3.5 MPa. 3. Results 3.1. Textural properties of the NiSA-X catalysts The NiSA-X samples showed a gradual decrease in specific surface area with increasing value of X as shown in Table 1. There was a concomitant increase in pore volume and mean diameter as shown. Despite these changes in textural properties, it is felt that the differences displayed by the resulting NiSA-X catalysts as a function of X largely reflect changes in acidity (acid site density and acid strength of sites)
C.P. Nicolaides et al. / Applied Catalysis A: General 245 (2003) 43–53 Table 1 Physical properties of the NiSA-X samples Si/Al ratio
BET surface area (m2 g−1 )
Pore volume (cm3 g−1 )
Average pore diameter (nm)
25 50 100 100a 160 200 275 275a 300 400 500 500a
509 470 465 460 430 420 391 393 387 378 353 358
0.56 0.60 0.64 0.66 0.71 0.72 0.77 0.75 0.84 0.97 0.95 0.94
5.2 5.7 6.8 7.0 7.4 7.6 8.0 8.0 8.8 9.5 10.6 10.0
a
Duplicate batch preparation.
rather than being due to the changes in the physical properties. 3.2. Effect of the Si/Al ratio of the catalyst support Under the conditions used for ethene oligomerization in the present studies, the major product was butene. The hexene content of the product obtained was typically in the range 5–20 mass% (Table 2). The ethene conversion, the percentage of 1-hexene in the exit stream and the percentage selectivity to 1-hexene, expressed as the percentage of 1-hexene in the total hexene fraction were found to vary with the Si/Al ratio of the support. The selectivity to 1-hexene increased from just over 7 to 16.8% with increasing Si/Al ratio in the range 25–500. There was a concomitant decrease in ethene conversion from about 15 to 9.4% (see also Fig. 1). As shown in Fig. 2, the dependence of 1-hexene selectivity on Si/Al ratio is most obvious for Si/Al ratios lying in the range 25–200, with almost no further change in selectivity occurring above a Si/Al ratio of 200. The carbon number-based product distributions obtained with these catalysts are depicted in Fig. 3. Attempts to secure active oligomerization catalysts by incorporating nickel into a silica support were unsuccessful. No conversion of ethene was observed with such systems. Separate experiments carried out using conventional glassware apparatus confirmed that 1-hexene
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Table 2 The effect of Si/Al on the activity and selectivity of NiSA-X catalystsa Si/Al ratio and batch numberb
Conversion (%)
1 − C6 2− in C6 2− (%)
25B 25C 25C 50B 50C 50C 100B 100C 100C 160A 160A
15.8 15.0 14.9 13.5 12.9 13.7 12.6 12.1 11.9 12.1 11.8
6.1 7.4 7.8 8.8 8.9 8.8 10.5 10.9 16.0 15.5 9.9
200A
11.9 11.7
16.2 16.3
225A
11.3 11.0
16.5 16.7
275A
12.6 11.0
16.8 16.4
300A
11.5 10.7
16.5 16.5
400A
10.9 9.7
16.8 16.6
9.4 9.4 0
16.6 16.6 0
500C 500G Ni2+ /SiO2
a T = 100 ◦ C, P = 1.5 MPa, MHSV = 4 g g−1 h−1 , and time-on-stream = 280 min. b The integer is the value of Si/Al ratio and the letter refers to the specific batches of catalyst.
readily underwent double-bond shift when contacted with NaSA-25 and NiSA-25 under reflux conditions. 3.3. Effect of increasing time-on-stream All catalysts examined showed virtually identical trends with increasing time-on-stream in that there was a steady decrease in conversion as shown in Table 3. As ethene conversion fell a steady rise in the 1-hexene selectivity was seen (Table 4). Catalysts with high Si/Al ratios showed slightly higher rates of deactivation attributed to the relatively low active site density associated with these solids. The highest 1-hexene selectivity observed was just over 18%, obtained with the NiSA-500 catalyst after 440 min on stream at an ethene conversion level of 8.6%.
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Fig. 1. Ethene conversion as a function of Si/Al ratio of the support. T = 100 ◦ C, P = 1.5 MPa, MHSV = 4 h−1 , and time-on-stream = 280 min.
3.4. Effect of additional nickel ions The incorporation of additional nickel resulted in a complex behavior (Table 5 and Fig. 4), with ethene conversion first increasing with increasing nickel con-
tent up to a maximum of about 23% conversion at a nickel content of 1.5 mass%. Above this nickel content the conversion fell fairly sharply. The 1-hexene selectivity mirrored the ethene conversion level in that the higher the conversion, the lower was the selectivity.
Fig. 2. 1-Hexene selectivity as a function of Si/Al ratio of the support. T = 100 ◦ C, P = 1.5 MPa, MHSV = 4 h−1 , and time-on-stream = 280 min.
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Fig. 3. Product distribution as a function of Si/Al ratio of the support. T = 100 ◦ C, P = 1.5 MPa, MHSV = 4 h−1 , and time-on-stream = 280 min.
Table 3 Conversion of ethane and selectivity (1 − C6 2− in C6 2− (%) fraction) (in italics) vs. time-on-stream at T = 100 ◦ C, P = 1.5 MPa, and MHSV = 4 g g−1 h−1 Time (min)
Si/Al = 25C
Si/Al = 50B
Si/Al = 100B
Si/Al = 160A
Si/Al = 200A
Si/Al = 225A
Si/Al = 275A
Si/Al = 300A
Si/Al = 400A
Si/Al = 500C
280
14.9 7.8
13.5 8.8
12.6 10.5
12.0 15.5
11.9 16.3
11.3 16.5
10.9 16.4
10.6 16.5
10.8 16.6
9.3 16.6
350
14.8 7.8
12.2 9.1
12.0 10.8
11.9 16.3
11.8 16.8
10.9 16.8
10.6 16.9
10.4 16.9
10.0 17.0
8.0 17.4
380
14.1 8.0
11.3 9.2
11.8 11.1
11.4 16.6
11.3 16.9
10.6 17.0
10.1 17.1
10.0 17.3
9.9 17.5
7.8 17.7
440
14.0 8.7
10.9 9.3
11.5 11.8
10.9 16.9
10.8 17.2
10.0 17.4
9.9 17.6
9.7 17.6
9.4 17.9
7.2 18.2
Table 4 1-Hexene selectivity (1 − C6 2− in C6 2− (%) fraction) vs. time-on-stream at T = 100 ◦ C, P = 1.5 MPa, and MHSV = 4 g g−1 h Time (min)
Si/Al = 25C
Si/Al = 50B
Si/Al = 100C
Si/Al = 160A
Si/Al = 200A
Si/Al = 225A
Si/Al = 275A
Si/Al = 300A
Si/Al = 400A
Si/Al = 500C
280 350 380 440
7.8 7.8 8.0 8.6
8.8 9.1 9.1 9.3
10.5 10.8 11.0 11.7
15.5 16.3 16.5 16.8
16.2 16.7 16.9 17.1
16.4 16.8 17.0 17.3
16.4 16.8 17.1 17.6
16.5 16.9 17.3 17.5
16.5 17.0 17.4 17.9
16.6 17.4 17.7 18.1
The highest 1-hexene selectivity obtained was 13.5%, with an ethene conversion level of just above 6% seen for the catalyst containing a total of 4 mass% Ni. The maximum ethene conversion (and minimum 1-hexene selectivity) was observed for catalyst having a Ni/Al ratio close to 1, whereas the Ni/Al ratio present in the ion-exchanged material NiSA-50 is close to 0.5. In addition to the observed ethene conversion and 1-hexene selectivity changes just described, there were noticeable changes in the overall product spectrum (Fig. 5).
The catalyst containing 1.5 mass% Ni exhibited a tendency to produce relatively less butene and more hexene than catalysts having either higher or lower nickel contents. In general, catalysts with a high nickel content exhibited relatively higher rates of deactivation. 3.5. Effect of added potassium ions The introduction of potassium ions to NiSA-50 resulted in catalysts having lower activities. As Table 6
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Fig. 4. Conversion of ethene as a function of Ni mass% (added by impregnation). T = 100 ◦ C, P = 1.5 MPa, MHSV = 4 h−1 , and time-on-stream = 280 min.
shows, there was a steady fall in ethene conversion with increase in potassium content, but a more complex effect on 1-hexene selectivity was seen in that selectivity increased and then decreased as the potassium content was increased. The maximum 1-hexene selectivity observed was 13.4%, obtained for a K+ /Ni2+ ratio of 0.5, with an ethene conversion of 10.4%. The overall product distribution was not greatly affected by the level of potassium present (Fig. 6), though a slight tendency for butene selectivity to be increased Table 5 The effect of the amount Ni2+ added by impregnation on the activity and selectivity of NiSA-50 Ni2+ (mass%)
Conversion (%)
1 − C6 2− in C6 2− (%)
C6 in stream (%)
0.8a 0.8 1.2 1.5 1.5 3.0 4.0 4.0
13.2 12.9 17.2 23.2 23.5 13.7 6.2 6.1
8.8 8.9 6.2 5.8 5.2 8.3 13.6 13.5
10.2 12.4 8.1 10.3 11.5 6.0 5.9 5.9
T = 100 ◦ C, P = 1.5 MPa, MHSV = 4 g g−1 h−1 , and time-on-stream = 280 min. a The ion-exchanged catalyst.
(at the expense of selectivity to higher oligomers) was observed at high potassium contents. Catalysts with low potassium contents showed slightly lower deactivation rates, but for higher potassium contents the deactivation rate was higher relative to that of the parent NiSA-50. 3.6. Effect of reaction pressure Our earlier work demonstrated that the NiSA catalysts could most successfully bring about ethene oligomerization at a total operating pressure of Table 6 The effect of K+ on conversion (%) and selectivity (%) after K+ impregnation K+ /Ni2+
Conversion (%)
1 − C6 2− in C6 2− (%)
0 0.1 0.2 0.5 0.7 1.0 1.9
13.5 13.1 12.8 10.3 8.0 3.9 0.3
8.8 9.2 11.6 13.4 10.8 5.5 2.5
T = 100 ◦ C, P = 1.5 MPa, MHSV = 4 g g−1 h−1 , and time-on-stream = 280 min.
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Fig. 5. Product distribution as a function of Ni mass% (added by impregnation). T = 100 ◦ C, P = 1.5 MPa, MHSV = 4 h−1 , and time-on-stream = 280 min.
Fig. 6. Product distribution as a function of K+ /Ni2+ atom ratio for NiSA-50. T = 100 ◦ C, P = 1.5 MPa, MHSV = 4 g g−1 h−1 , and time-on-stream = 280 min.
3.5 MPa [1]. The effect of total pressure on the 1-hexene selectivity was not specifically studied in that work. Such an investigation now reveals that for reaction pressures in the range 1.0–3.5 MPa, the 1-hexene selectivity (and the percentage of 1-hexene in the exit stream) is highest at the intermediate pressure of 1.5 MPa (Table 7). This pressure was therefore chosen as the standard one for conducting all other experimental work in the present work. 3.7. Summarized selectivity–conversion data From the above it is clear that attempts to effect changes in the 1-hexene selectivity by adjustment of catalyst composition or process variables almost al-
Table 7 The effect of pressure on the activity and selectivity of the catalyst Pressure (bar)
Conversion (%)
1 − C6 2− in C6 2− (%)
10 15 20 25 30 30a 35 35a
5.4 11.9 15.6 26.1 30.2 31.6 41.9 38.6
8.6 10.3 8.1 7.8 6.4 5.9 3.4 4.0
T = 100 ◦ C, MHSV = 4 g g−1 h, and time-on-stream = 280 min for NiAmSiAl-100D. a Duplicate experiments.
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ways result in concomitant changes in the ethene conversion level. It is therefore instructive to summarize the experimental data obtained in the form of selectivity–conversion plots such as those depicted in Figs. 7–9. In these plots the 1-hexene yield curves are included, where the yield, Y, is calculated as Y=
[(1 − C6 2− in C6 2− (%)) × (C2 2− conversion (%))] 100
It is stressed that the yields calculated in this way do not necessarily correspond exactly to the actual single-pass yields observed because the latter are additionally dependent on the overall hexene selectivity which varies according to the carbon number-based product distribution. However, for all practical purposes investigated here the latter is essentially invariant. Therefore, the summarized plots are useful in assessing the effects that the various variables studies have on the selectivity–conversion relationship. Fig. 7 clearly shows that the Y value is almost directly proportional to the conversion level, i.e. 1-hexene se-
lectivity is not affected to any appreciable extent by changes in reaction pressure. In addition, the incorporation of potassium ions at low K+ contents into the base catalysts, shows a favorable upward movement in the yield locus, but this trend is drastically reversed for higher K+ contents, with Y values falling dramatically as more potassium is added. In Fig. 8, the addition of nickel to the base catalyst results in a weak, virtually neutral effect on Y, but with a negative trend being shown for the highest nickel content case. The marked positive effect of increasing Si/Al ratio from 100 to 200 is clearly evident as is the more or less neutral effect of changing Si/Al ratios in the range 25–100, and also the negative effect resulting from the use of a Si/Al ratio of 500. Finally, Fig. 9 reveals that the effect of increasing time-on-stream is markedly dependent on the Si/Al ratio of the support as are the specific Y values. Thus for low Si/Al ratios, Y values are relatively low (just above 1.0%), but move in a slightly favorable manner to higher values with increasing time-on-stream. For
Fig. 7. 1-Hexene yield (mass%) as a function of pressure (MPa) (䊉) and as a function of K+ /Ni2+ atom ratio (䉱).
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Fig. 8. 1-Hexene yield (mass%) as a function of Ni mass% (added by impregnation) () and as a function of Si/Al ratio of the support (䊊).
high Si/Al ratios an opposite trend is evident with the yield loci moving to lower values with increasing time-on-stream, this negative effect being markedly more pronounced for a Si/Al ratio of 500 compared with that found with a ratio of 400. For the Si/Al = 200 case, the Y value is essentially constant at just below 2.0% for the range of times-on-stream studied.
4. Discussion The formation of hexene isomers other than 1-hexene probably result from the sequential double-bond shift of any 1-hexene formed as a primary product. We note that any tendency for the oligomerization process to become influenced by intraparticle diffusion constraints is likely to exacerbate the situation, in that the relative rate of sequential formation of positional isomers will tend to be encouraged. Applying the Weisz criterion to the highest reaction rates observed with the catalyst studied
showed that Φs has a maximum value of 0.03, an order of magnitude lower than the limiting value of 0.3 for a second order reaction. The order of the reaction is likely to be 2, and is unlikely to be of a higher order. The Φs value can rise to values >1 if the order is 1 or less, before diffusion transfer limitations are likely to influence the kinetics. Double-bond shift in alkenes is an extremely facile reaction and is one of the few heterogeneous catalyzed reaction exhibiting turnover frequencies approaching those displayed by enzyme systems. For example, on (Si, Al) zeolites turnover frequencies approaching 107 s−1 [6] have been reported for this reaction. Turnover frequencies for the oligomerization reaction studied with the present catalysts have lower values of about 0.04–0.28 s−1 calculated from the rates observed with the NiSA-50 and NiSA-500 catalysts (Table 2) based on the assumption that each Ni ion introduced to the silica-alumina support by ion-exchange constitutes a single active site. Furthermore, the double-bond isomerization reaction is one that is catalyzed by a
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Fig. 9. 1-Hexene yield (mass%) as a function of increasing time-on-stream (280–440 min) for various Si/Al ratios of the support (Si/Al in the range 25–500).
variety of active sties. Acid, base and metal catalysts can all be used for this reaction, which implies that the selective elimination of this reaction is extremely difficult. Nevertheless the current work demonstrates which strategies are the more effective. The relatively wide range of turnover frequencies calculated for oligomerization of ethene on the NiSA-X catalysts associated with different Si/Al ratios in the support suggests that the activity of the oligomerization sites are greatly affected by the Si/Al ratio, possibly reflecting a high degree of acid strength and more complete site isolation for the highly siliceous samples (we note that in our work carried out using slurry reactor operation [2] with similar catalysts, but at a higher pressure of 3.5 MPa, a turnover frequency of 0.42 s−1 was established). The oligomerization activity of similar nickel silica-alumina systems has been correlated with support acidity in other work [7]. We also note that Ni2+ /SiO2 catalysts exhibit very low oligomerization activity, and confirmed in the present
work (Table 2), in line with the low acidity of the silica support thus increasing the Ni2+ content above the ratio of Ni/Al = 1 will not have any beneficial effect, in line with our observations. The selectivity to 1-hexene within the hexene fraction does not, however, increase for Si/Al ratios above 200 (Fig. 2) and so it seems likely that isomerization reaction is also catalyzed by the fewer, but more active sites present in catalysts having higher Si/Al ratios. Indeed, it is not unlikely that the oligomerization and isomerization sites are one and the same, though we have no direct evidence that this is the case. Some additional isomerization activity might be attributed to a few residual acid sites present on the catalysts even after ion-exchange with nickel ions. The addition of relatively low amounts of potassium ions might poison such sites and be responsible for the slight improvement in 1-hexene selectivity reflected by the small increase in the 1-hexene yield locus depicted in Fig. 7 for K+ /Ni2+ ratios increasing from 0
C.P. Nicolaides et al. / Applied Catalysis A: General 245 (2003) 43–53
to 0.5. The incorporation of further amounts of potassium ions, however, results in the production of additional isomerization centres as well as a decrease in oligomerization activity, probably by a poisoning of the oligomerization centers themselves. The virtually constant 1-hexene selectivity depicted in Fig. 7 as a function of reaction pressure is more readily explained by both oligomerization and isomerization reactions having near-identical pressure dependencies. The additional nickel incorporated to the base NiSA-50 catalyst appears to increase both oligomerization and isomerization rates to an equal extent (Fig. 8) so that 1-hexene yields remain virtually unchanged, except for a catalyst containing a high nickel content (4.0 mass%) when evidence of a tendency to promote the relative isomerization/oligomerization rate is reflected in a fall in 1-hexene selectivity. It is not clear whether the additional presence of chlorine resulting from the impregnation of nickel(II) chloride plays any direct or indirect role in the catalytic activity of the resulting solids. The most effective means of inducing favorable changes in the 1-hexene content of the hexene fraction appears to be an increase in the time-on-stream (Fig. 9) which is in agreement with Belltrame et al.’s observations [5]. Clearly, the manner in which the yield loci move in a positive manner for the NiSA-25 catalyst is encouraging, but the effect is weaker for the NiSA-100 case, and both series of catalysts exhibit relatively low yield levels when fresh. The use of higher Si/Al ratios is also problematic in that although the 1-hexene content of the hexene fraction increases steadily with increasing time-on-stream conversions levels fall so that overall the yield loci move in a negative direction. The use of a Si/Al ratio of 200 offers the optimum behavior of all catalysts studied. However, even for this case, increased 1-hexene content in the hexene fraction is offset by a fall in conversion with increasing time-on-stream such that the overall yield fails to increase, but, it does not fall so that it is at least constant. Finally, concerning the overall yield of 1-hexene, the carbon number-based product distribution must be taken into account. The distribution is only weakly
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affected by the various factors examined in this work apart from Ni content, when catalysts having a Ni2+ content of 1.5 mass% is associated the highest hexene selectivity. This finding, however, was made using a support with a Si/Al ratio of 50 and the effect may not apply directly to the situation where a support has a different Si/Al ratio. Nevertheless, it is concluded that the highest 1-hexene contents in the hexene fraction and the highest hexene fraction in the oligomerization products are likely to be obtained for catalysts having Si/Al ratios in the region of 50–200, with some additional Ni present over and above that incorporated by ion-exchange and the incorporation of a small amount of potassium ions. Further, some improvement in overall 1-hexene yield might result from the use of catalysts which have been in use for some time. The realization of these predictions in the process of securing more selective catalysts is the subject of further experimental investigations.
Acknowledgements Thanks are expressed for financial support given by the National Research Foundation (NRF) and the University of the Witwatersrand. References [1] J. Heveling, C.P. Nicolaides, M.S. Scurrell, Appl. Catal. A 173 (1998) 1. [2] M.D. Heydenrych, C.P. Nicolaides, M.S. Scurrell, J. Catal. 197 (2001) 49. [3] C.P. Nicolaides, M.S. Scurrell, in: D.C. Sherrington, A.P. Kybett (Eds.), Supported Reagents and Catalysts in Chemistry, Spec. Publ. R. Soc. Chem. 266 (2001) 226. [4] D.R. Stull, E.F. Westrum Jr., G.C. Sinke, The Chemical Thermodynamics of Organic Compounds, Wiley, New York, 1969. [5] P. Belltrame, L. Forni, A. Talamini, G. Zuretti, Appl. Catal. 110 (1994) 39. [6] W.O. Haag, R.M. Lago, P.B. Weisz, Nature 309 (1984) 589. [7] R.L. Espinoza, C.J. Korf, C.P. Nicolaides, R. Snel, Appl. Catal. 29 (1987) 175.
Nickel silica-alumina catalysts for ethene oligomerization—control of the selectivity to 1-alkene products C.P. Nicolaides, M.S. Scurrell∗ , P.M. Semano Molecular Sciences Institute, School of Chemistry, University of the Witwatersrand, P.O. WITS, 2050 Johannesburg, South Africa Received 27 June 2002; received in revised form 16 November 2002; accepted 22 November 2002
Abstract A rational approach to control the behavior of nickel(II) ion-exchanged silica-alumina oligomerization catalysts has been attempted. Emphasis has been placed on securing higher selectivities to 1-alkenes from the oligomerization of ethene, with data being presented for 1-hexene. A key challenge is to reduce the relatively high rate of double-bond shift which leads to the formation of internal alkene products. One of the most effective means of inducing favorable changes in the 1-hexene content of the hexane fraction appears to be an increase in the time-on-stream, though this aspect is not observed for all catalysts studied. Addition of nickel ions in excess of those introduced by ion-exchange, addition of potassium ions, and changes in the Si/Al ratio of the support are further variables investigated. These variable do not, for each one studied, lead to dramatic changes in the selectivity, but the combined effect of appropriate adjustments in these factors do give some improvement. In addition, the total nickel ion content influences the carbon number-based product distribution, with the highest hexene selectivity being found with catalysts containing 1.5 mass% Ni2+ . © 2002 Elsevier Science B.V. All rights reserved. Keywords: Ethene oligomerization; Nickel(II)-exchanged silica-alumina; 1-Alkene selectivity; 1-Hexene selectivity
1. Introduction Recent accounts deal with the behavior of highly active and stable catalysts for the oligomerization of ethene, based on nickel(II) ion-exchanged silica-aluminas [1,2]. These catalysts can also be successfully used for the oligomerization of propene and butene [3]. A detailed analysis of the dimer product obtained during the oligomerization of propene [4] reveals that a comparatively high selectivity (ca. 52 mass%) to linear hexenes is obtained. However, the selectivity to 1-hexene itself is low, and it seems ∗ Corresponding author. Tel.: +27-11-717-6716; fax: +27-11-717-6749. E-mail address: [email protected] (M.S. Scurrell).
likely that the double-bond shift reaction of the alkene on these catalysts is comparatively rapid so that the product distribution within a given Cn alkene product will reflect formation of the more thermodynamically favored internal alkene. Attainment of the thermodynamic equilibration of the structural isomers would result in a very low content of the 1-alkene [3] and even partial attainment would severely restrict the 1-alkene content. A remaining challenge, therefore, in the use of such catalysts concerns the selective formation of 1-alkenes, such as 1-hexene, which are highly valuable chemical feedstocks (e.g. for use as co-monomers in alkene polymerization processes). Studies of attempts to secure selective 1-hexene formation from ethene on heterogeneous nickel-based oligomerization catalysts are very rare. However, in
0926-860X/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0926-860X(02)00615-4
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a recent report by Belltrame et al. [5], it was shown that such catalysts (Ni-ZSM-5) may exhibit improved selectivities to terminal alkenes after they have presumably undergone partial deactivation due to coking. We have now attempted a rational approach applied in order to modify the behavior of Ni(II) ionexchanged silica-alumina oligomerization catalysts by means of changes in catalyst composition and/or changes in the process variables associated with their use, and have secured information on conditions favouring 1-alkene production. Our approach is illustrated by analysis of the trimer product obtained in the oligomerization of ethene.
2. Experimental Catalysts comprised nickel ion-exchanged silicaaluminas. The silica-alumina was prepared according to previously published procedures [1] by addition of nitric acid to a mixed solution of sodium aluminate and sodium silicate. After suitable aging, the sodium hydrogel was thoroughly washed to a final pH of 6. The gels were then recovered by filtration followed by drying at 100 ◦ C and calcination at 550 ◦ C. The Si/Al ratio in the supports was varied between 25 and 500. Ion-exchange of the Na+ form of silica-alumina (NaSA) with Ni2+ was carried out under reflux conditions using an aqueous solution of nickel chloride. The nickel content of the solution and mass of support used corresponded to a three-fold excess of nickel ions present based on a Ni:Al stoichiometry of 1:2. The ion-exchanged solids were filtered and washed extensively with water to remove the chloride ions and excess nickel ions, and were then dried at 70 ◦ C. The supports and catalysts are designated as NaSA-X and NiSA-X, respectively, where X represented the Si/Al ratio of the support. The catalysts used here are essentially identical to those prepared earlier and used for ethene oligomerization [1]. The NiSA-50 series of catalysts was subjected to additional subsequent modification involving the addition of further nickel or the introduction of potassium, carried out in either case by means of incipient wetness impregnation with a solution of nickel(II)
chloride or potassium carbonate, respectively. The quantity of solution used was made equal to the pore volume (0.65 cm3 g−1 ) obtained from the measurement of the textural properties of the supports, which were carried out using nitrogen adsorption/desorption procedures at 77 K. The modified NiSA-50 catalysts contained up to 4.0 mass% Ni (of which 0.8 mass% was the quantity originally present after incorporation by ion-exchange), or possessed K+ /Ni2+ ratios in the range 0.05–1.00. Catalytic oligomerization of ethene was carried out using a fixed-bed tubular reactor constructed of stainless steel. Charges of 1.0 g catalyst were used with a particle size fraction in the range 300–500 m. Catalysts were activated by heating gradually in flowing nitrogen to 300 ◦ C, and maintained at this temperature for 3 h. The catalyst was then cooled in flowing nitrogen to room temperature at which point ethene was introduced and the reactor brought to the required reaction conditions. Care had to be exercised in bringing the reactor to the desired reaction pressure and temperature in order to avoid the development of excessive temperature excursions within the catalyst bed arising from the exothermic nature of the reaction. Ethene (99.7% purity) supplied by Fedgas was fed to the reactor and most reactions were carried out under the following conditions: reaction temperature, 100 ◦ C; pressure, 1.5 MPa; MHSV, 4 h−1 ; time-on-stream up to 500 min, with samples of the exit stream being taken at regular intervals and analyzed by gas chromatography using a capillary column (50 m methyl bonded silicone) and FID detection. Some runs were conducted at higher pressures up to 3.5 MPa. 3. Results 3.1. Textural properties of the NiSA-X catalysts The NiSA-X samples showed a gradual decrease in specific surface area with increasing value of X as shown in Table 1. There was a concomitant increase in pore volume and mean diameter as shown. Despite these changes in textural properties, it is felt that the differences displayed by the resulting NiSA-X catalysts as a function of X largely reflect changes in acidity (acid site density and acid strength of sites)
C.P. Nicolaides et al. / Applied Catalysis A: General 245 (2003) 43–53 Table 1 Physical properties of the NiSA-X samples Si/Al ratio
BET surface area (m2 g−1 )
Pore volume (cm3 g−1 )
Average pore diameter (nm)
25 50 100 100a 160 200 275 275a 300 400 500 500a
509 470 465 460 430 420 391 393 387 378 353 358
0.56 0.60 0.64 0.66 0.71 0.72 0.77 0.75 0.84 0.97 0.95 0.94
5.2 5.7 6.8 7.0 7.4 7.6 8.0 8.0 8.8 9.5 10.6 10.0
a
Duplicate batch preparation.
rather than being due to the changes in the physical properties. 3.2. Effect of the Si/Al ratio of the catalyst support Under the conditions used for ethene oligomerization in the present studies, the major product was butene. The hexene content of the product obtained was typically in the range 5–20 mass% (Table 2). The ethene conversion, the percentage of 1-hexene in the exit stream and the percentage selectivity to 1-hexene, expressed as the percentage of 1-hexene in the total hexene fraction were found to vary with the Si/Al ratio of the support. The selectivity to 1-hexene increased from just over 7 to 16.8% with increasing Si/Al ratio in the range 25–500. There was a concomitant decrease in ethene conversion from about 15 to 9.4% (see also Fig. 1). As shown in Fig. 2, the dependence of 1-hexene selectivity on Si/Al ratio is most obvious for Si/Al ratios lying in the range 25–200, with almost no further change in selectivity occurring above a Si/Al ratio of 200. The carbon number-based product distributions obtained with these catalysts are depicted in Fig. 3. Attempts to secure active oligomerization catalysts by incorporating nickel into a silica support were unsuccessful. No conversion of ethene was observed with such systems. Separate experiments carried out using conventional glassware apparatus confirmed that 1-hexene
45
Table 2 The effect of Si/Al on the activity and selectivity of NiSA-X catalystsa Si/Al ratio and batch numberb
Conversion (%)
1 − C6 2− in C6 2− (%)
25B 25C 25C 50B 50C 50C 100B 100C 100C 160A 160A
15.8 15.0 14.9 13.5 12.9 13.7 12.6 12.1 11.9 12.1 11.8
6.1 7.4 7.8 8.8 8.9 8.8 10.5 10.9 16.0 15.5 9.9
200A
11.9 11.7
16.2 16.3
225A
11.3 11.0
16.5 16.7
275A
12.6 11.0
16.8 16.4
300A
11.5 10.7
16.5 16.5
400A
10.9 9.7
16.8 16.6
9.4 9.4 0
16.6 16.6 0
500C 500G Ni2+ /SiO2
a T = 100 ◦ C, P = 1.5 MPa, MHSV = 4 g g−1 h−1 , and time-on-stream = 280 min. b The integer is the value of Si/Al ratio and the letter refers to the specific batches of catalyst.
readily underwent double-bond shift when contacted with NaSA-25 and NiSA-25 under reflux conditions. 3.3. Effect of increasing time-on-stream All catalysts examined showed virtually identical trends with increasing time-on-stream in that there was a steady decrease in conversion as shown in Table 3. As ethene conversion fell a steady rise in the 1-hexene selectivity was seen (Table 4). Catalysts with high Si/Al ratios showed slightly higher rates of deactivation attributed to the relatively low active site density associated with these solids. The highest 1-hexene selectivity observed was just over 18%, obtained with the NiSA-500 catalyst after 440 min on stream at an ethene conversion level of 8.6%.
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Fig. 1. Ethene conversion as a function of Si/Al ratio of the support. T = 100 ◦ C, P = 1.5 MPa, MHSV = 4 h−1 , and time-on-stream = 280 min.
3.4. Effect of additional nickel ions The incorporation of additional nickel resulted in a complex behavior (Table 5 and Fig. 4), with ethene conversion first increasing with increasing nickel con-
tent up to a maximum of about 23% conversion at a nickel content of 1.5 mass%. Above this nickel content the conversion fell fairly sharply. The 1-hexene selectivity mirrored the ethene conversion level in that the higher the conversion, the lower was the selectivity.
Fig. 2. 1-Hexene selectivity as a function of Si/Al ratio of the support. T = 100 ◦ C, P = 1.5 MPa, MHSV = 4 h−1 , and time-on-stream = 280 min.
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Fig. 3. Product distribution as a function of Si/Al ratio of the support. T = 100 ◦ C, P = 1.5 MPa, MHSV = 4 h−1 , and time-on-stream = 280 min.
Table 3 Conversion of ethane and selectivity (1 − C6 2− in C6 2− (%) fraction) (in italics) vs. time-on-stream at T = 100 ◦ C, P = 1.5 MPa, and MHSV = 4 g g−1 h−1 Time (min)
Si/Al = 25C
Si/Al = 50B
Si/Al = 100B
Si/Al = 160A
Si/Al = 200A
Si/Al = 225A
Si/Al = 275A
Si/Al = 300A
Si/Al = 400A
Si/Al = 500C
280
14.9 7.8
13.5 8.8
12.6 10.5
12.0 15.5
11.9 16.3
11.3 16.5
10.9 16.4
10.6 16.5
10.8 16.6
9.3 16.6
350
14.8 7.8
12.2 9.1
12.0 10.8
11.9 16.3
11.8 16.8
10.9 16.8
10.6 16.9
10.4 16.9
10.0 17.0
8.0 17.4
380
14.1 8.0
11.3 9.2
11.8 11.1
11.4 16.6
11.3 16.9
10.6 17.0
10.1 17.1
10.0 17.3
9.9 17.5
7.8 17.7
440
14.0 8.7
10.9 9.3
11.5 11.8
10.9 16.9
10.8 17.2
10.0 17.4
9.9 17.6
9.7 17.6
9.4 17.9
7.2 18.2
Table 4 1-Hexene selectivity (1 − C6 2− in C6 2− (%) fraction) vs. time-on-stream at T = 100 ◦ C, P = 1.5 MPa, and MHSV = 4 g g−1 h Time (min)
Si/Al = 25C
Si/Al = 50B
Si/Al = 100C
Si/Al = 160A
Si/Al = 200A
Si/Al = 225A
Si/Al = 275A
Si/Al = 300A
Si/Al = 400A
Si/Al = 500C
280 350 380 440
7.8 7.8 8.0 8.6
8.8 9.1 9.1 9.3
10.5 10.8 11.0 11.7
15.5 16.3 16.5 16.8
16.2 16.7 16.9 17.1
16.4 16.8 17.0 17.3
16.4 16.8 17.1 17.6
16.5 16.9 17.3 17.5
16.5 17.0 17.4 17.9
16.6 17.4 17.7 18.1
The highest 1-hexene selectivity obtained was 13.5%, with an ethene conversion level of just above 6% seen for the catalyst containing a total of 4 mass% Ni. The maximum ethene conversion (and minimum 1-hexene selectivity) was observed for catalyst having a Ni/Al ratio close to 1, whereas the Ni/Al ratio present in the ion-exchanged material NiSA-50 is close to 0.5. In addition to the observed ethene conversion and 1-hexene selectivity changes just described, there were noticeable changes in the overall product spectrum (Fig. 5).
The catalyst containing 1.5 mass% Ni exhibited a tendency to produce relatively less butene and more hexene than catalysts having either higher or lower nickel contents. In general, catalysts with a high nickel content exhibited relatively higher rates of deactivation. 3.5. Effect of added potassium ions The introduction of potassium ions to NiSA-50 resulted in catalysts having lower activities. As Table 6
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Fig. 4. Conversion of ethene as a function of Ni mass% (added by impregnation). T = 100 ◦ C, P = 1.5 MPa, MHSV = 4 h−1 , and time-on-stream = 280 min.
shows, there was a steady fall in ethene conversion with increase in potassium content, but a more complex effect on 1-hexene selectivity was seen in that selectivity increased and then decreased as the potassium content was increased. The maximum 1-hexene selectivity observed was 13.4%, obtained for a K+ /Ni2+ ratio of 0.5, with an ethene conversion of 10.4%. The overall product distribution was not greatly affected by the level of potassium present (Fig. 6), though a slight tendency for butene selectivity to be increased Table 5 The effect of the amount Ni2+ added by impregnation on the activity and selectivity of NiSA-50 Ni2+ (mass%)
Conversion (%)
1 − C6 2− in C6 2− (%)
C6 in stream (%)
0.8a 0.8 1.2 1.5 1.5 3.0 4.0 4.0
13.2 12.9 17.2 23.2 23.5 13.7 6.2 6.1
8.8 8.9 6.2 5.8 5.2 8.3 13.6 13.5
10.2 12.4 8.1 10.3 11.5 6.0 5.9 5.9
T = 100 ◦ C, P = 1.5 MPa, MHSV = 4 g g−1 h−1 , and time-on-stream = 280 min. a The ion-exchanged catalyst.
(at the expense of selectivity to higher oligomers) was observed at high potassium contents. Catalysts with low potassium contents showed slightly lower deactivation rates, but for higher potassium contents the deactivation rate was higher relative to that of the parent NiSA-50. 3.6. Effect of reaction pressure Our earlier work demonstrated that the NiSA catalysts could most successfully bring about ethene oligomerization at a total operating pressure of Table 6 The effect of K+ on conversion (%) and selectivity (%) after K+ impregnation K+ /Ni2+
Conversion (%)
1 − C6 2− in C6 2− (%)
0 0.1 0.2 0.5 0.7 1.0 1.9
13.5 13.1 12.8 10.3 8.0 3.9 0.3
8.8 9.2 11.6 13.4 10.8 5.5 2.5
T = 100 ◦ C, P = 1.5 MPa, MHSV = 4 g g−1 h−1 , and time-on-stream = 280 min.
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Fig. 5. Product distribution as a function of Ni mass% (added by impregnation). T = 100 ◦ C, P = 1.5 MPa, MHSV = 4 h−1 , and time-on-stream = 280 min.
Fig. 6. Product distribution as a function of K+ /Ni2+ atom ratio for NiSA-50. T = 100 ◦ C, P = 1.5 MPa, MHSV = 4 g g−1 h−1 , and time-on-stream = 280 min.
3.5 MPa [1]. The effect of total pressure on the 1-hexene selectivity was not specifically studied in that work. Such an investigation now reveals that for reaction pressures in the range 1.0–3.5 MPa, the 1-hexene selectivity (and the percentage of 1-hexene in the exit stream) is highest at the intermediate pressure of 1.5 MPa (Table 7). This pressure was therefore chosen as the standard one for conducting all other experimental work in the present work. 3.7. Summarized selectivity–conversion data From the above it is clear that attempts to effect changes in the 1-hexene selectivity by adjustment of catalyst composition or process variables almost al-
Table 7 The effect of pressure on the activity and selectivity of the catalyst Pressure (bar)
Conversion (%)
1 − C6 2− in C6 2− (%)
10 15 20 25 30 30a 35 35a
5.4 11.9 15.6 26.1 30.2 31.6 41.9 38.6
8.6 10.3 8.1 7.8 6.4 5.9 3.4 4.0
T = 100 ◦ C, MHSV = 4 g g−1 h, and time-on-stream = 280 min for NiAmSiAl-100D. a Duplicate experiments.
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ways result in concomitant changes in the ethene conversion level. It is therefore instructive to summarize the experimental data obtained in the form of selectivity–conversion plots such as those depicted in Figs. 7–9. In these plots the 1-hexene yield curves are included, where the yield, Y, is calculated as Y=
[(1 − C6 2− in C6 2− (%)) × (C2 2− conversion (%))] 100
It is stressed that the yields calculated in this way do not necessarily correspond exactly to the actual single-pass yields observed because the latter are additionally dependent on the overall hexene selectivity which varies according to the carbon number-based product distribution. However, for all practical purposes investigated here the latter is essentially invariant. Therefore, the summarized plots are useful in assessing the effects that the various variables studies have on the selectivity–conversion relationship. Fig. 7 clearly shows that the Y value is almost directly proportional to the conversion level, i.e. 1-hexene se-
lectivity is not affected to any appreciable extent by changes in reaction pressure. In addition, the incorporation of potassium ions at low K+ contents into the base catalysts, shows a favorable upward movement in the yield locus, but this trend is drastically reversed for higher K+ contents, with Y values falling dramatically as more potassium is added. In Fig. 8, the addition of nickel to the base catalyst results in a weak, virtually neutral effect on Y, but with a negative trend being shown for the highest nickel content case. The marked positive effect of increasing Si/Al ratio from 100 to 200 is clearly evident as is the more or less neutral effect of changing Si/Al ratios in the range 25–100, and also the negative effect resulting from the use of a Si/Al ratio of 500. Finally, Fig. 9 reveals that the effect of increasing time-on-stream is markedly dependent on the Si/Al ratio of the support as are the specific Y values. Thus for low Si/Al ratios, Y values are relatively low (just above 1.0%), but move in a slightly favorable manner to higher values with increasing time-on-stream. For
Fig. 7. 1-Hexene yield (mass%) as a function of pressure (MPa) (䊉) and as a function of K+ /Ni2+ atom ratio (䉱).
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Fig. 8. 1-Hexene yield (mass%) as a function of Ni mass% (added by impregnation) () and as a function of Si/Al ratio of the support (䊊).
high Si/Al ratios an opposite trend is evident with the yield loci moving to lower values with increasing time-on-stream, this negative effect being markedly more pronounced for a Si/Al ratio of 500 compared with that found with a ratio of 400. For the Si/Al = 200 case, the Y value is essentially constant at just below 2.0% for the range of times-on-stream studied.
4. Discussion The formation of hexene isomers other than 1-hexene probably result from the sequential double-bond shift of any 1-hexene formed as a primary product. We note that any tendency for the oligomerization process to become influenced by intraparticle diffusion constraints is likely to exacerbate the situation, in that the relative rate of sequential formation of positional isomers will tend to be encouraged. Applying the Weisz criterion to the highest reaction rates observed with the catalyst studied
showed that Φs has a maximum value of 0.03, an order of magnitude lower than the limiting value of 0.3 for a second order reaction. The order of the reaction is likely to be 2, and is unlikely to be of a higher order. The Φs value can rise to values >1 if the order is 1 or less, before diffusion transfer limitations are likely to influence the kinetics. Double-bond shift in alkenes is an extremely facile reaction and is one of the few heterogeneous catalyzed reaction exhibiting turnover frequencies approaching those displayed by enzyme systems. For example, on (Si, Al) zeolites turnover frequencies approaching 107 s−1 [6] have been reported for this reaction. Turnover frequencies for the oligomerization reaction studied with the present catalysts have lower values of about 0.04–0.28 s−1 calculated from the rates observed with the NiSA-50 and NiSA-500 catalysts (Table 2) based on the assumption that each Ni ion introduced to the silica-alumina support by ion-exchange constitutes a single active site. Furthermore, the double-bond isomerization reaction is one that is catalyzed by a
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Fig. 9. 1-Hexene yield (mass%) as a function of increasing time-on-stream (280–440 min) for various Si/Al ratios of the support (Si/Al in the range 25–500).
variety of active sties. Acid, base and metal catalysts can all be used for this reaction, which implies that the selective elimination of this reaction is extremely difficult. Nevertheless the current work demonstrates which strategies are the more effective. The relatively wide range of turnover frequencies calculated for oligomerization of ethene on the NiSA-X catalysts associated with different Si/Al ratios in the support suggests that the activity of the oligomerization sites are greatly affected by the Si/Al ratio, possibly reflecting a high degree of acid strength and more complete site isolation for the highly siliceous samples (we note that in our work carried out using slurry reactor operation [2] with similar catalysts, but at a higher pressure of 3.5 MPa, a turnover frequency of 0.42 s−1 was established). The oligomerization activity of similar nickel silica-alumina systems has been correlated with support acidity in other work [7]. We also note that Ni2+ /SiO2 catalysts exhibit very low oligomerization activity, and confirmed in the present
work (Table 2), in line with the low acidity of the silica support thus increasing the Ni2+ content above the ratio of Ni/Al = 1 will not have any beneficial effect, in line with our observations. The selectivity to 1-hexene within the hexene fraction does not, however, increase for Si/Al ratios above 200 (Fig. 2) and so it seems likely that isomerization reaction is also catalyzed by the fewer, but more active sites present in catalysts having higher Si/Al ratios. Indeed, it is not unlikely that the oligomerization and isomerization sites are one and the same, though we have no direct evidence that this is the case. Some additional isomerization activity might be attributed to a few residual acid sites present on the catalysts even after ion-exchange with nickel ions. The addition of relatively low amounts of potassium ions might poison such sites and be responsible for the slight improvement in 1-hexene selectivity reflected by the small increase in the 1-hexene yield locus depicted in Fig. 7 for K+ /Ni2+ ratios increasing from 0
C.P. Nicolaides et al. / Applied Catalysis A: General 245 (2003) 43–53
to 0.5. The incorporation of further amounts of potassium ions, however, results in the production of additional isomerization centres as well as a decrease in oligomerization activity, probably by a poisoning of the oligomerization centers themselves. The virtually constant 1-hexene selectivity depicted in Fig. 7 as a function of reaction pressure is more readily explained by both oligomerization and isomerization reactions having near-identical pressure dependencies. The additional nickel incorporated to the base NiSA-50 catalyst appears to increase both oligomerization and isomerization rates to an equal extent (Fig. 8) so that 1-hexene yields remain virtually unchanged, except for a catalyst containing a high nickel content (4.0 mass%) when evidence of a tendency to promote the relative isomerization/oligomerization rate is reflected in a fall in 1-hexene selectivity. It is not clear whether the additional presence of chlorine resulting from the impregnation of nickel(II) chloride plays any direct or indirect role in the catalytic activity of the resulting solids. The most effective means of inducing favorable changes in the 1-hexene content of the hexene fraction appears to be an increase in the time-on-stream (Fig. 9) which is in agreement with Belltrame et al.’s observations [5]. Clearly, the manner in which the yield loci move in a positive manner for the NiSA-25 catalyst is encouraging, but the effect is weaker for the NiSA-100 case, and both series of catalysts exhibit relatively low yield levels when fresh. The use of higher Si/Al ratios is also problematic in that although the 1-hexene content of the hexene fraction increases steadily with increasing time-on-stream conversions levels fall so that overall the yield loci move in a negative direction. The use of a Si/Al ratio of 200 offers the optimum behavior of all catalysts studied. However, even for this case, increased 1-hexene content in the hexene fraction is offset by a fall in conversion with increasing time-on-stream such that the overall yield fails to increase, but, it does not fall so that it is at least constant. Finally, concerning the overall yield of 1-hexene, the carbon number-based product distribution must be taken into account. The distribution is only weakly
53
affected by the various factors examined in this work apart from Ni content, when catalysts having a Ni2+ content of 1.5 mass% is associated the highest hexene selectivity. This finding, however, was made using a support with a Si/Al ratio of 50 and the effect may not apply directly to the situation where a support has a different Si/Al ratio. Nevertheless, it is concluded that the highest 1-hexene contents in the hexene fraction and the highest hexene fraction in the oligomerization products are likely to be obtained for catalysts having Si/Al ratios in the region of 50–200, with some additional Ni present over and above that incorporated by ion-exchange and the incorporation of a small amount of potassium ions. Further, some improvement in overall 1-hexene yield might result from the use of catalysts which have been in use for some time. The realization of these predictions in the process of securing more selective catalysts is the subject of further experimental investigations.
Acknowledgements Thanks are expressed for financial support given by the National Research Foundation (NRF) and the University of the Witwatersrand. References [1] J. Heveling, C.P. Nicolaides, M.S. Scurrell, Appl. Catal. A 173 (1998) 1. [2] M.D. Heydenrych, C.P. Nicolaides, M.S. Scurrell, J. Catal. 197 (2001) 49. [3] C.P. Nicolaides, M.S. Scurrell, in: D.C. Sherrington, A.P. Kybett (Eds.), Supported Reagents and Catalysts in Chemistry, Spec. Publ. R. Soc. Chem. 266 (2001) 226. [4] D.R. Stull, E.F. Westrum Jr., G.C. Sinke, The Chemical Thermodynamics of Organic Compounds, Wiley, New York, 1969. [5] P. Belltrame, L. Forni, A. Talamini, G. Zuretti, Appl. Catal. 110 (1994) 39. [6] W.O. Haag, R.M. Lago, P.B. Weisz, Nature 309 (1984) 589. [7] R.L. Espinoza, C.J. Korf, C.P. Nicolaides, R. Snel, Appl. Catal. 29 (1987) 175.