Reactivity Differences of Octenes over Solid Phosphoric Acid

Abstract. Reactivity differences between 1-octene and 2,4,4-trimethylpentene have been studied over solid ... Industrial & Engineering Chemistry Resea...
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Ind. Eng. Chem. Res. 2006, 45, 578-584

Reactivity Differences of Octenes over Solid Phosphoric Acid Arno de Klerk Fischer-Tropsch Refinery Catalysis, Sasol Technology Research and DeVelopment, P.O. Box 1, Sasolburg 1947, South Africa

Reactivity differences between 1-octene and 2,4,4-trimethylpentene have been studied over solid phosphoric acid in the temperature range 75-220 °C. No 1-octene oligomerization occurred unless it was preceded by skeletal isomerization, while 2,4,4-trimethylpentene readily oligomerized even at 100 °C. The stability of the carbocation intermediate could explain the reactivity difference between linear and branched olefins, but not why 1-octene was almost impervious to oligomerization. This could be explained in terms of the stability of the phosphoric acid ester and the time period it is able to polarize the olefin, which implied that solid phosphoric acid followed a nonclassic carbocation mechanism. This could also account for the limited carbon number distribution of the product and the cis-favoring double-bond isomerization that was observed. Comparative classic carbocation data was obtained in the temperature range 75-125 °C over Amberlyst 15 to illustrate the difference. Introduction Solid phosphoric acid (SPA) catalysts are widely used for the oligomerization of short-chain olefins from cracker gas to produce high octane olefinic motor gasoline.1 Heavier olefinic feeds are not normally considered for oligomerization by SPA and have consequently not been studied. Nevertheless, since heavier olefins are formed during the oligomerization of the lighter olefins, SPA catalysts invariably coprocess such material. In a high-temperature Fischer-Tropsch (HTFT) refining environment, the oligomerization of heavier olefins by SPA catalysis have been applied in practice.2 The high olefin content of primary Fischer-Tropsch products (60-70% olefins in a hightemperature Fischer-Tropsch process)3 causes the motor gasoline to exceed the olefin specification of the final fuel. For example, the Euro-4 specification limits olefins to 18 vol %. It thus becomes necessary to hydrogenate some of the olefinic motor gasoline or to convert some of the naphtha range olefinic material to higher boiling distillate range products. Both of these options are practiced commercially in South African HTFT refineries. The Sasol Synfuels refineries in Secunda hydrogenate SPA-derived olefinic motor gasoline3,4 and produce distillates by oligomerization over SPA;2 the PetroSA Mossgas refinery in Mossel Bay converts Fischer-Tropsch naphtha range olefins to distillate by the H-ZSM-5 zeolite-catalyzed COD process.3,5,6 The quality of the hydrogenated motor gasoline from an SPA oligomerization process is sensitive to both feed and operating conditions.7 At higher operating temperatures, it was found that the quality of the hydrogenated motor gasoline was lower, because the most branched products, like high octane trimethylpentenes, were converted to distillate range products, but not the less branched low octane material. Most of the products from SPA oligomerization are branched and contain at least one tertiary carbon. A description of the difference in reactivity based on the stability of the carbocation intermediate only, was inadequate. The reactivity differences of olefins of the same carbon number were relevant for another reason too. If FischerTropsch naphtha range olefinic material is to be converted to distillate range products over SPA, rather than by utilizing a * To whom correspondence should be addressed. Tel: +27 16 9602549. Fax: +27 11 522-3517. E-mail: [email protected].

zeolite-based process, then the reactivity difference between linear and branched olefins becomes crucial. HTFT products are rich in linear olefins, which do not contain tertiary carbons. Preferential conversion of the most branched material over SPA suggests that it will be fairly unreactive to long-chain linear olefins, despite its high reactivity to propene and n-butenes. The conversion of linear olefins does not present a problem to ZSM-5 catalysts, and long-chain olefins are just as reactive, if not more reactive, than short-chain olefins.8,9 This is expected if reaction occurred by a classic Brønsted acid-catalyzed protonation of the olefin, as it indeed does on H-ZSM-5. However, previous descriptions of the oligomerization reaction over SPA catalysts generally show that a classic carbocation mechanism is not followed.10-13 An explanation of the reactivity differences of long-chain olefins over SPA catalysts was consequently of fundamental and industrial interest. In studying the reactivity differences of long-chain olefins, it was decided that the focus should be on the octenes as model compounds, with 1-octene as a representative of a long chain Fischer-Tropsch naphtha range olefin and 2,4,4-trimethylpentene as a representative of a highly branched butene dimerization product. The catalytic reactivity of these compounds over SPA was explored at industrially relevant conditions (150-220 °C and 3.8 MPa). To highlight the differences between SPA and pure Brønsted acid catalysis, a comparative oligomerization study over SPA and Amberlyst 15 was also done at low temperature (75-125 °C). Amberlyst 15 is an acidic resin catalyst of strong Brønsted acidity that is likely to follow a classic carbocation mechanism. Experimental Section Materials. The olefins used were analyzed to confirm the purity. The 1-octene (Aldrich, >98%) was 98.4% pure, and the 2,4,4-trimethylpentene mixture (Merck, >95%) contained 74.9% 2,4,4-trimethyl-1-pentene and 18.5% 2,4,4-trimethyl-2-pentene. For some batch reactor experiments, 2,4,4-trimethyl-1-pentene (Aldrich, >97%) was used. The solvents were pentane (Associated Chemical Enterprises, CP grade), which contained 23.9% 2-methylbutane and 75.8% n-pentane, n-hexane (Aldrich, >95% HPLC grade), n-heptane (Fluka, >99%), and n-octane (Riedel de Hae¨n, >99%). Two catalyst types were used in the laboratory studies: an acidic resin, Amberlyst 15 (dry, Fluka), and a solid phosphoric

10.1021/ie050812+ CCC: $33.50 © 2006 American Chemical Society Published on Web 11/25/2005

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acid, C84/3 (Celite FB, Su¨d-Chemie-Sasol). The SPA catalyst had a free acid content of 25% H3PO4 and an ortho/pyrophosphoric acid ratio of 330:170. In addition to these, one experiment was done with orthophosphoric acid (Aldrich, >98%). Equipment and Procedure. The first set of experiments was performed at 75-125 °C and atmospheric pressure (86 kPa barometric pressure due to altitude of our laboratory) in standard laboratory glassware. The setup consisted of a round-bottom flask connected to a reflux condenser with a cooling water temperature of 5 °C. The flask was heated and stirred with a magnetic stirrer bar. The solvent determined the temperature of the boiling reacting mixture: n-hexane (75 °C), n-heptane (100 °C), and noctane (120-125 °C). The SPA catalyst was crushed to a particle size of less than 1 mm, while the Amberlyst 15 catalyst was used in its commercial form (beads of less than 1 mm diameter). The experiments at 100 °C were repeated under pressure (0.6 MPa) with a pentane mixture as solvent. These experiments were done in small 316 stainless steel reaction vessels, 0.1 m in length and 0.01 m internal diameter. After charging the reaction vessels with the catalyst and feed, they were kept at constant temperature by submerging them in a re-circulating oil bath (Lauda Proline RP 1840). The temperature was controlled to within 1 °C. To stop the reaction before depressurizing, the reaction vessels were transferred to a 5 °C water bath (Julabo F25). The inertness of the reaction vessels was verified by loading some reaction vessels with only the 1-octene and 2,4,4-trimethylpentene mixtures and omitting the catalyst. The SPA catalyst was used in its commercial form (6 mm extrudate), as was the Amberlyst 15. The reaction mixtures used in this series of experiments were not stirred and mass transfer was expected to be limiting. The experiments at 150 °C and higher temperatures were done in stirred stainless steel batch reactors with a useful internal volume of about 150 mL. An external heater controlled the temperature and all tests were done at 3.8 MPa pressure. Pressure was maintained by nitrogen. The batch reactors were equipped with a dip-tube for sampling purposes. Analyses. An Agilent (Hewlett-Packard) 6890 gas chromatograph with flame ionization detector (GC-FID) was used for most analyses. The separation was done with a 50 m HP-PONA methyl siloxane column; 200 µm internal diameter and 0.5 µm film thickness. The following temperature program was used: 40 °C for 5 min, then ramping at 4 °C per minute to 120 °C and from 120 °C to 300 °C at 20 °C per minute, keeping it at 300 °C for 6 min. The carrier gas was hydrogen and a 100:1 split ratio was used. Conversion is based on disappearance of the product being studied, which in the case of 1-octene includes conversion by double bond isomerization. The two double bond isomers of 2,4,4-trimethylpentene are considered together due to the feed mixture used and conversion therefore excludes double bond isomerization. Selectivity to a specific product is expressed in terms of the mass fraction of that product in the material that was converted. Yield is expressed as the conversion multiplied by the selectivity value. Results Reactivity of 1-Octene at 75-125 °C and Atmospheric Pressure. The reactivity of 1-octene was evaluated at low temperature in a paraffin matrix to limit reaction and make it easier to spot differences in the way the catalysts reacted. At 75 °C and atmospheric pressure, Amberlyst 15 catalyzed only the double-bond isomerization of 1-octene (Table S1, Supporting Information), but on increasing the temperature to 100 and 125 °C, skeletal isomerization and dimerization were

Figure 1. Double-bond isomerization (9), skeletal isomerization ([), and oligomerization (b) selectivity during 1-octene conversion over Amberlyst 15 at atmospheric pressure and 100 °C (solid symbols) and 125 °C (open symbols).

also observed. Double-bond isomerization took place rapidly and at 100 and 125 °C (Table S1, Supporting Information), and 1-octene conversion was 95-100% after a contact time of 0.2 h‚gcatalyst‚golefin-1. At 100-125 °C, dimerization was not preceded by skeletal isomerization, and these two reactions took place in parallel, both increasing with increasing contact time (Figure 1). The results indicated that the ease of reaction over Amberlyst 15 was double bond isomerization > skeletal isomerization ≈ dimerization. The experiment with 1-octene was repeated with solid phosphoric acid as catalyst. At 75 °C, no reaction took place and at 100 °C only double-bond isomerization was observed (Table S2, Supporting Information). The reaction was slow, and the double-bond isomerization rate remained constant at 0.13 golefin‚gcatalyst-1‚h-1, with less than 10% of the 1-octene being converted after 0.65 h‚gcatalyst‚golefin-1. Even at 120 °C, the reaction was still slow, with a 1-octene conversion of 40% after 0.9 h‚gcatalyst‚golefin-1 (Table S2, Supporting Information). Conversion was mostly due to double-bond isomerization, although a small amount of skeletal isomers and dimers (less than 0.5% yield) were formed. The ratio of skeletal isomerization to dimerization remained constant at 5:1, but it was not clear whether these two reactions took place in parallel or whether skeletal isomerization preceded dimerization. No reaction was observed at 120 °C when the solid phosphoric acid catalyst was replaced with orthophosphoric acid. This also indirectly confirmed the inertness of the glassware for possible catalytic activity during the reaction. The results indicated that 1-octene was fairly unreactive, especially over SPA, but did not point to mechanistic differences, only to the apparent difference in acid strength of the catalysts. Reactivity of 1-Octene and 2,4,4-Trimethylpentene at 100 °C and 0.6 MPa. The reactivity of 1-octene and of 2,4,4trimethylpentene was compared over Amberlyst 15 and SPA (Figure 2). It confirmed that Amberlyst 15 was a more active catalyst than SPA for the conversion of linear and branched material. There was a significant difference in the oligomerization rate of 1-octene and 2,4,4-trimethylpentene over Amberlyst 15 (Figure 3). The oligomerization selectivity of the 2,4,4-trimethylpentene was time invariant and remained fairly constant at a selectivity of 0.8, but the oligomerization selectivity of 1-octene increased with contact time due to the formation of branched octenes by skeletal isomerization. This was as expected from the classic carbocation mechanism of the reaction. It can be added here that in one experiment with Amberlyst 15 almost no conversion was observed. On further investigation it turned

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Figure 2. Conversion of 1-octene (9) and 2,4,4-trimethylpentene (b) over Amberlyst 15 (solid symbols) and C84/3 SPA (open symbols) at 100 °C, 0.6 MPa.

Figure 4. Conversion of 1-octene at 180 °C (9), 200 °C (b), and 220 °C ([), as well as 2,4,4-trimethylpentene at 150 °C (]), 180 °C (0), and 200 °C (O) over C84/3 SPA at 3.8 MPa.

Figure 3. Oligomerization of 1-octene (9) and 2,4,4-trimethylpentene (O) over Amberlyst 15 at 100 °C, 0.6 MPa.

Figure 5. Conversion (0) during the reaction of 1-octene over SPA at 200 °C and 3.8 MPa. The change in skeletal isomerization selectivity (2) and oligomerization selectivity (b) is shown, with double-bond isomerization (not shown) constituting the remainder.

out that the reaction vessel contained some water (about 1%). The suppressed catalytic activity of the resin was not surprising, since water is known to have an inhibiting effect14 and similar observations have been reported before.15 Over SPA, the 1-octene formed almost no oligomers (0.6:1. This was initially thought to be due to a difference in conversion, but at higher conversion (Table S2, Supporting Information) the same was seen. SPA definitely displays cis-favoring double-bond isomerization. Reactivity of 1-Octene and 2,4,4-Trimethylpentene at 150-220 °C and 3.8 MPa. Commercially SPA catalysts are used in the temperature range 150-245 °C.16 This temperature region was investigated for both 1-octene and 2,4,4-trimethylpentenes. Conversion at 150 °C was slow, but in the temperature range 180-220 °C both 1-octene and the 2,4,4-trimethylpentenes were readily converted (Figure 4). Conversion of 1-octene was mostly by double-bond isomerization (Table S5, Supporting Information), and 1-octene only started dimerizing once some skeletal isomers had been formed (Figure 5). The 2,4,4-trimethylpentenes were readily converted to oligomers

Figure 6. Selectivity to C9 and heavier products during reaction of 2,4,4trimethylpentenes over SPA at 3.8 MPa and 180 °C (9), 200 °C (b), and 180 °C diluted with n-decane (0).

(Figure 6). As shown in Figure 6, it was found that oligomerization selectivity increased slightly with conversion, except at high conversion where it rapidly increased, possibly due to the formation of skeletal isomers less susceptible to cracking by β-scission.17 The selectivity profile (Table S6) was also related to the olefin concentration. Dilution with paraffins suppressed oligomerization, but it did not affect the overall reaction rate. Discussion Reactions and Mechanism. The main reactions of octenes over acid catalysts are double-bond isomerization, skeletal isomerization, oligomerization, and cracking (Figure 7). When a carbocation mechanism is followed, it involves the protonation of the olefin and the subsequent reaction of the carbocation on

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Figure 7. Reaction network of acid-catalyzed olefin reactions.

Figure 9. Difference in trans-2-octene to cis-2-octene ratio during doublebond isomerization of 1-octene over Amberlyst 15 at 100 °C (9) and 125 °C (b) and SPA at 100 °C (0), 120 °C (O), 180 °C (]), 200 °C (+), and 220 °C (×).

Figure 8. Phosphoric ester mechanism.

its own (monomolecular reactions such as double-bond isomerization, skeletal isomerization, and cracking) or with another olefin (bimolecular reaction like oligomerization).18,19 Under mild reaction conditions, typically temperatures of 200 °C and less, only acyclic mono-olefins are produced. At more severe operating conditions, paraffins, cycloparaffins, and aromatics can also be produced.10 Reactions involving hydrogen transfer are therefore possible but were not considered in the present study, since the experiments were mostly conducted under mild reaction conditions. The mechanism over SPA is not described in classic Brønsted acid-catalyzed terms, but involves the formation of a phosphoric acid ester (Figure 8).10-13 Due to the more formal interaction between the phosphoric acid and the reacting olefin, the reactivity of olefins is not only determined by the stability of the carbocation, but also by the stability of the phosphoric acid ester. Ethene requires a high temperature to oligomerize over SPA because it forms an ester that is thermally stable to about 200 °C.10,20 The same is true for propene, which forms an ester that is thermally stable to about 125 °C.10,21 The n-butenes do not form such stable esters and reactions can take place at room temperature,22 although the stability threshold for n-butene esters are less clearly defined. The ester stability therefore decreases with increasing chain length of the linear olefin. However, the reactivity sequence of linear olefins does not show a monotonic increase with chain length because n-butenes have the highest reactivity. The C5 and heavier olefins show a trend of decreasing olefin reactivity with increasing chain length. Test work at 160 °C and 3.8 MPa showed a drop in 1-pentene conversion at constant butene conversion with increasing 1-pentene concentration,7 suggesting that the butenes, rather than the 1-pentene, were more readily protonated and became the primary carbocation source. Reasonable conversion of 1-hexene could be demonstrated at 200 °C and 6 MPa over SPA.23 Oligomerization of 1-octene over various solid acid catalysts resulted in much lower conversion,23 although the catalysts tested did not include SPA. The present test work showed that 1-octene is considerably less reactive than 1-hexene, because even at 200 °C it was difficult for oligomerization to take place (Figure 5). The reactivity of linear olefins seems to be a tradeoff between the strength of the phosphoric ester being formed and the time

period the olefin has an interaction with the acid before desorbing as an olefin again. If the olefin forms an ester with the phosphoric acid that is too strong, the ester is too stable to react with another olefin. If the interaction is too weak, the time period that the olefin has an interaction with the phosphoric acid is so short that there is a low probability of interacting with another olefin while being polarized and even the probability of a monomolecular transformation is low. The time period that the olefin remains polarized is not only influenced by the strength of the ester, but also by the stability of the polarized intermediate. The stability of the intermediate is expected to increase in the same order as carbocation stability: primary < secondary < tertiary.24 A branched molecule is consequently expected to be more reactive than a linear molecule of the same carbon chain length, because a tertiary carbon results in a more stable polarized intermediate. Although this may seem to be an adequate description of the reaction mechanism and reactivity differences over SPA, other factors may play an important role in the reactivity of longer chain olefins. For example, adsorption of olefins may decrease with in increased degree of branching25 and longer chain olefins, which become increasingly apolar, is less likely to adsorb strongly on the polar catalyst surface. Longer olefins are also more bulky with a slower rate of diffusion and SPA is known to be mass transfer limited.26 A description of the mechanism on SPA is further complicated by the self-dissociative behavior of phosphoric acid (3H3PO4 / H4PO4+ + H3O+ + H2P2O72-).27,28 This is responsible for creating Brønsted acid sites on the catalyst, which causes reactions to occur via both carbocation and ester formation pathways. In the discussion of the experimental data the contribution of the various mechanisms will be highlighted. Double-Bond Isomerization. The primary products from double-bond isomerization of 1-octene are cis-2-octene and trans-2-octene. During pure Brønsted acid catalysis (classic carbocation mechanism), the olefin is protonated on the R-carbon, resulting in the β-carbon becoming a carbocation. On deprotonation the olefin is formed in accordance with energetic considerations and it is expected that the cis- and trans-isomers will be formed in their equilibrium ratio. Thermodynamically trans-2-octene is the favored product, and double-bond isomerization data over Amberlyst 15 confirmed that the reaction was likely to follow a classic carbocation mechanism in the temperature range 100-125 °C (Figure 9). Double-bond isomerization of 1-octene over SPA produced more cis-2-octene than expected from pure Brønsted acid

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Table 1. Approach to Double-Bond Isomerization Equilibrium of 2,4,4-Trimethyl-1-pentene (TMP1) and 2,4,4-Trimethyl-2-pentene during Reaction over SPA at 3.8 MPa TMP1/TMP2 activity ratio T (°C) 150 180 200 180 200

diluent used yes yes yes no no

contact time (h‚gcat‚golefin-1)

exptl

lit.29

0.34 0.10 0.34 0.25 0.23

2.85 2.67 2.48 2.92 2.49

2.86 2.68 2.58 2.68 2.58

catalysis (Figure 9). This pointed to a mechanism that involved a transition state causing the isomerization to be cis-favoring, due to a more formal interaction with the catalyst that imposed a restriction on the way in which olefin formation could take place (nonclassic carbocation). The 2-octenes are also susceptible to double-bond isomerization and with increasing time and temperature the ratio of trans-2-octene to cis-2-octene gradually increased in accordance with thermodynamic expectations. The low reactivity of SPA to n-octene conversion and cis-favoring double-bond isomerization, both suggest that the phosphoric acid ester mechanism is dominant in linear olefin conversion. Double-bond isomerization of 2,4,4-trimethylpentene over SPA was rapid. Dilution with an inert moderated oligomerization and allowed double-bond isomerization of the 2,4,4-trimethylpentenes to reach equilibrium (Table 1). The equilibrium ratios were calculated from the work by Karinen, Lylykangas, and Krause,29 and activity coefficients were calculated by the UNIFAC method.30 It was not possible to use these data to distinguish between the mechanisms. Oligomerization, Skeletal Isomerization, and Cracking. There is a significant difference in the carbon number distribution of the oligomers produced from 1-octene and 2,4,4trimethylpentene over SPA. Oligomerization of 1-octene over SPA is limited to dimerization to form some C16 olefins. No heavier material was detected, nor any intermediate carbon numbers between C8 and C16. Oligomerization of 2,4,4trimethylpentene over SPA at 100 °C was accompanied by cracking. Selective cracking (depolymerization) of C8 and C16 olefins produced an isobutene fragment, and re-oligomerization of the isobutene yielded C8, C12, and C16 olefins. This form of depolymerization is known to occur and was historically used as method to determine the trimethylpentene content of oligomerization products.31,32 At higher temperatures, the product from 2,4,4-trimethylpentene reaction over SPA contained hundreds of compounds, indicative of extensive isomerization, cracking, and oligomerization. Brønsted acid catalyzed cracking tends to yield not only isobutene fragments. This is in part due to skeletal isomerization of 2,4,4-trimethylpentene to form 2,3,4-trimethylpentene, which suppresses depolymerization to yield isobutene.33 It is important to note that the formation of 2,3,4-trimethylpentene by skeletal isomerization of the 2,4,4-trimethylpentene is likely to be Brønsted acid catalyzed and is different from the desorption of 2,3,4-trimethylpentene from the intermediate of butene dimerization by the ester based mechanism.13 The change in product spectrum is also characteristic of a change in mechanism from “true polymerization” to “conjunct polymerization”.12,34 This can also be seen as a change from phosphoric ester dominated reactivity to include more Brønsted acid catalyzed reactivity. The carbon number distribution of the product was nevertheless capped, and it contained no material heavier than C16. In this respect, oligomerization of octenes over SPA is different to oligomerization over Amberlyst 15, where heavier oligomers

were detected during reaction with both 1-octene and 2,4,4trimethylpentene. Olefin oligomerization catalyzed by SPA seldom yields more than 20% material boiling above 200 °C or a T95 boiling point higher than 300 °C.7,16,32,35-37 Since similar results are reported with liquid phosphoric acid,10,38 it is unlikely that the limitation on carbon chain length is due to diffusion limitations imposed by the catalyst. This is seen as further evidence that the ester based mechanism dominates catalysis by SPA. It can be argued that longer chain olefins have a higher propensity to crack and that the limited carbon number distribution can equally well be explained by classic Brønsted acid catalyzed cracking. If we accept this hypothesis, knowing that it is not due to diffusion limitations, it implies that the balance between cracking and oligomerization rate determines the chain length. Since cracking requires strong acid sites and Amberlyst 15 yielded a heavier product at the same temperature, it follows that SPA must have stronger acid sites than Amberlyst 15, because cracking is more dominant. Since it was difficult to convert n-octenes over SPA, but not over Amberlyst 15, this results in a conundrum. It is consequently unlikely that the limitation on carbon number distribution observed with SPA is due to Brønsted acid catalysis. The oligomerization behavior of 1-octene also suggests that the ester mechanism is dominant during SPA catalysis. If we assume that this was not the case, and that the classic Brønsted acid catalyzed mechanism was dominant, protonation should not be chain length dependent. Considering that 1-butene can be oligomerized over phosphoric acid at temperatures as low as 60 °C38 and skeletally isomerized to yield trimethylpentenes as main product of dimerization at 130 °C,13 monomolecular reactions such as skeletal isomerization of 1-octene should proceed readily. Yet, almost no 1-octene oligomerization was observed and skeletal isomerization was very slow even at 200 °C, while these reactions proceeded readily over Amberlyst 15 at 100 °C. When comparing the ability of Amberlyst 15 to convert 1-octene with that of SPA, it is tempting to ascribe the difference in reactivity to a difference in acid strength, rather than mechanism. However, the nature of the catalyst and its interaction with olefins should not be underestimated. For example, according to the data of Golombok and De Bruijn amorphous silica alumina (ASA) with 13 mass % alumina was more reactive for 1-butene conversion than Amberlyst and Nafion resin catalysts,39 but that ASA did not readily convert 1-octene at temperatures below 150 °C (even by double bond isomerization).40 This is especially relevant since it has previously been pointed out that amorphous silica alumina (ASA) have some peculiar characteristics in common with SPA and is possibly also following a nonclassic carbocation mechanism.13 For example, the product of n-butene oligomerization over ASA is sensitive to the type of n-butene used41 and double-bond isomerization of 1-butene over ASA is cis-selective.42 Evidence based on chemical shift tensor calculations indicated that SPA may actually be a stronger acid than most zeolites,21 although some zeolites readily convert 1-octene,23 while SPA does not. It can therefore be said that the phosphoric ester based mechanism is important in explaining the reactivity differences of the octenes. Commercial Implications. The production of trimethylpentene during SPA oligomerization of butene is valuable because it can be hydrogenated to high-octane motor gasoline. Loss of trimethylpentene due to further oligomerization is not desirable, yet cracking and further oligomerization to yield heavier olefins

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takes place even at 100 °C. It is therefore not possible to eliminate such reactions during commercial operation with SPA, and trimethylpentenes are effectively intermediate products from butene oligomerization that can best be retained by operating at the lowest practical temperature and shortest practical residence time. Conversely, n-octenes are poor motor-gasoline components, and it is desirable to promote their oligomerization to produce heavier olefins. In general, Fischer-Tropsch derived material contains mostly linear hydrocarbons,3 and it would be beneficial to convert the n-octenes to distillate range products if the material is to be used as transportation fuel. Judging by the present results, this would be difficult to do using SPA catalysis. As a matter of fact, based on the mechanism and its relation to the reactivity sequence, it is unlikely that SPA catalysis would be suitable for oligomerization of any linear olefins heavier than hexene. It is also clear that oligomerization by SPA has limited applicability to distillate production. The carbon number distribution is mechanistically limited, because the probability of monomolecular reactions (like cracking) increases and bimolecular reactions (like oligomerization) decreases with increasing chain length of the olefin. Conclusion It was shown that solid phosphoric acid catalyses olefin reactions mostly by a phosphoric ester mechanism, although the classic Brønsted acid catalyzed carbocation mechanism is also operative. The observed higher oligomerization, skeletal isomerization, and cracking reactivity of 2,4,4-trimethylpentene compared to 1-octene could be explained by the classic carbocation mechanism, but other differences could only be explained in terms of the stability of the phosphoric acid ester and the time period it is able to polarize the olefin: (a) the cis-favoring double-bond isomerization of 1-octene over SPA compared to thermodynamically controlled double bond isomerization over Amberlyst 15; (b) the low double-bond isomerization reactivity of 1-octene over SPA, despite SPA having sufficient acid strength to catalyze more energetically demanding reactions such as the oligomerization and cracking of 2,4,4trimethylpentene; (c) the limitation on carbon chain length formed during oligomerization, which could not be explained by cracking control or diffusion limitations; (d) the low 1-octene oligomerization activity, which could not be ascribed to the absence of a tertiary carbon, diffusion limitations or decreased polarity only (in combination with literature data a trend of decreasing olefin reactivity with increasing chain length could be shown for C5 and heavier olefins, while the opposite applied to C4 and lighter olefins); and (e) the low skeletal isomerization activity of 1-octene and 2,4,4-trimethylpentene, compared to the extensive skeletal isomerization and cracking activity of Amberlyst 15, which could not only be ascribed to a difference in acid strength. Acknowledgment Contributions by R. Barnard (FTRC 0663) and J. L. Swart (FTRC 0364) are gratefully acknowledged, as well as some insightful discussions with R. J. J. Nel. All work was done at Sasol Technology Research and Development, and permission to publish this work is appreciated. Supporting Information Available: Details on the doublebond isomerization of the linear octenes over the temperature

range 75-125 °C are given (Tables S1-S4), as well as the analysis of the n-octene isomers (Figure S1). Contact time dependent changes in the composition of the product from 1-octene (Table S5) and 2,4,4-trimethylpentene (Table S6) conversion over SPA in the temperature range 180-220 °C have been recorded too, as well as some double-bond isomerization data of the 2,4,4-trimethylpentenes in the temperature range 75125 °C (Table S7). This material is available free of charge via the Internet at http://pubs.acs.org. Literature Cited (1) Leprince, P. Oligomerization. In Petroleum Refining 3. ConVersion Processes; Leprince, P., Ed.; Editions Technip: Paris, 2001; pp 321-331. (2) Dry, M. E. High yield high quality diesel from Fischer-Tropsch process. ChemSA 1984, 10, 286. (3) Steynberg, A. P., Dry, M. E., Eds. Fischer-Tropsch technology; Elsevier: Amsterdam, 2004. (4) Swart, J. S.; Czajkowski, G. J.; Conser, R. E. Sasol upgrades Synfuels with refining technology. Oil Gas J. 1981, 79 (35), 62. (5) Terblanche, K. The Mossgas challenge. Hydrocarbon Eng. 1997, Mar-Apr, 2. (6) Knottenbelt, C. Mossgas “gas-to-liquids” diesel fuels - an environmentally friendly option. Catal. Today 2002, 71, 437. (7) De Klerk, A.; Engelbrecht, D. J.; Boikanyo, H. Oligomerization of Fischer-Tropsch olefins: Effect of feed and operating conditions on hydrogenated motor-gasoline quality. Ind. Eng. Chem. Res. 2004, 43, 7449. (8) Garwood, W. E. Conversion of C2-C10 to higher olefins over synthetic zeolite ZSM-5. ACS Symp. Ser. 1983, 218, 383. (9) Gnep, N. S.; Bouchet, F.; Guisnet, M. R. Comparative study of the oligomerization of C2-C6 olefins on HZSM-5. Prepr. Am. Chem. Soc. DiV. Pet. Chem. 1991, 36, 620. (10) Ipatieff, V. N. Catalytic polymerization of gaseous olefins by liquid phosphoric acid. I. Propylene. Ind. Eng. Chem. 1935, 27, 1067. (11) Farkas, A.; Farkas, L. Catalytic polymerization of olefins in the presence of phosphoric acid. Ind. Eng. Chem. 1942, 34, 716. (12) Schmerling, L.; Ipatieff, V. N. The mechanism of the polymerization of alkenes. In AdVances in Catalysis and related subjects; Frankenburg, W. G., Komarewsky, V. I., Rideal, E. K., Eds.; Academic Press: New York, 1950; Vol. II, p 21. (13) De Klerk, A. Isomerization of 1-butene to isobutene at low temperature. Ind. Eng. Chem. Res. 2004, 43, 6325. (14) Du Toit, E.; Nicol, W. The rate inhibiting effect of water as a product on reactions catalysed by cation-exchange resins: formation of mesityl oxide from acetone as case study. Appl. Catal. A 2004, 277, 219. (15) Bucsi, I.; Olah, G. A. Oligomerization of 2-methylpropene and transformation of 2,4,4-trimethyl-2-pentene over supported and unsupported perfluorinated resinsulfonic acid catalysts. J. Catal. 1992, 137, 12. (16) Egloff, G.; Weinert, P. C. Polymerisation with solid phosphoric acid catalyst. Proc. World Pet. Congr., 3rd, 1951, IV, 201. (17) Buchanan, J. S.; Santiesteban, J. G.; Haag, W. O. Mechanistic considerations in acid-catalyzed cracking of olefins. J. Catal. 1996, 158, 276. (18) Schmerling, L. Reactions of hydrocarbons. Ionic mechanisms. Ind. Eng. Chem. 1953, 45, 1447. (19) Corma, A. Inorganic solid acids and their use in acid-catalyzed hydrocarbon reactions. Chem. ReV. 1995, 95, 559. (20) Ipatieff, V. N.; Pines, H. Polymerization of ethylene under high pressures in the presence of phosphoric acid. Ind. Eng. Chem. 1935, 27, 1364. (21) Krawietz, T. R.; Lin, P.; Lotterhos, K. E.; Torres, P. D.; Barich, D. H.; Clearfield, A.; Haw, J. F. Solid phosphoric acid catalyst: A multinuclear NMR and theoretical study. J. Am. Chem. Soc. 1998, 120, 8502. (22) Ipatieff, V. N.; Pines, H.; Schaad, R. E. Isomerization of normal butenes. J. Am. Chem. Soc. 1934, 56, 2696. (23) De Klerk, A. Oligomerization of 1-hexene and 1-octene over solid acid catalysts. Ind. Eng. Chem. Res. 2005, 44, 3887. (24) March, J. AdVanced Organic Chemistry, 3rd ed.; Wiley: New York, 1985; pp 942-954. (25) Abbot, J.; Wojciechowski, B. W. Kinetics of reactions of C8 olefins on HY zeolite. J. Catal. 1987, 108, 346. (26) Langlois, G. E.; Walkey, J. E. Improved process polymerizes olefins for high-quality gasoline. Petroleum Refiner 1952, 31 (8), 79. (27) Munson, R. A. Self-dissociative equilibria in molten phosphoric acid. J. Phys. Chem. 1964, 68, 3374.

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ReceiVed for reView July 11, 2005 ReVised manuscript receiVed October 21, 2005 Accepted October 21, 2005 IE050812+