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Solid phosphoric acid catalysts: The effect of free acid composition on selectivity and activity for 1-hexene dimerisation
Applied Catalysis A: General 369 (2009) 83–89
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Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata
Solid phosphoric acid catalysts: The effect of free acid composition on selectivity and activity for 1-hexene dimerisation R. Schwarzer, E. du Toit *, W. Nicol University of Pretoria, Department of Chemical Engineering, Lynnwood Road, Pretoria, South Africa
A R T I C L E I N F O
A B S T R A C T
Article history: Received 7 July 2009 Received in revised form 1 September 2009 Accepted 1 September 2009 Available online 8 September 2009
The rate of dimerisation of 1-hexene in a batch reactor at 200 8C was used to compare the activity and dimerisation route selectivity of solid phosphoric acid (SPA) at various phosphoric acid strengths and subsequently free acid compositions. The initial reaction rates, calculated from an elementary kinetic rate model, showed an increase in catalyst activity with increased acid strength. This trend becomes more significant at acid strengths where the more condensed pyrophosphoric acid started becoming present in the free acid. At low acid strengths, a reaction pathway where linear hexenes were directly oligomerised dominated the reaction progression. An increase in acid strength favoured a two-step reaction pathway, where linear hexenes were first isomerised to skeletal hexene isomers before dimerisation. This sequential pathway becomes especially dominant where pyrophosphoric acid became present in the free acid. However, analysis of the final product with proton NMR showed no significant difference in the degree of branching with a variation in dimerisation route selectivity as a function of acid strength. ß 2009 Elsevier B.V. All rights reserved.
Keywords: Oligomerisation Catalysis Phosphoric acid Acid strength
1. Introduction The oligomerisation of olefins has allowed the petrochemical industry greater manoeuvrability in the production of gasoline from olefins. These reactions are acid-catalysed and several solid acid catalysts have been investigated for the oligomerisation of shorter-chain olefins: ZSM-5 [1,2], Amberlyst 35 [3], Beidellite and Ultrastable Y Zeolite catalysts [4], sol–gel aluminosilicates [5] and solid phosphoric acid [6,7]. Solid phosphoric acid is a relatively affordable acid catalyst and has the advantage that the spent catalyst can be used as fertiliser [8], which makes it environmentally friendly and cost effective. SPA consists of a layer of different phosphoric acid species, i.e. ortho-, pyro-, tri- and polyphosphoric acid, supported on a mixture of silica and silicon phosphates [9]. It is therefore known as a liquid-supported phosphoric acid catalyst. The liquid acid layer is referred to as the ‘‘free acid’’ content of the catalyst [10]. Although the silicon phosphates affect the crushing strength of the catalyst, it is accepted that they have no influence on the catalytic activity of SPA [11]. It is accepted that water inhibits the activity of solid acid catalysts [12]. This is also true for solid phosphoric acid. Cavani et al. [10] state that an increased amount of water in the feed
* Corresponding author. Tel.: +27 12 420 3796; fax: +27 12 420 5048. E-mail address: [email protected] (E. du Toit). 0926-860X/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2009.09.002
significantly inhibits the oligomerisation rate of propene over SPA. The inhibiting effect of increased hydration levels is believed to be twofold. Firstly, SPA is extremely hydrophilic, flooding the pores when excess water is present in the reaction mixture and restricting access to the catalytically active sites. In addition, the acid strength of phosphoric acid is controlled by the absorption of water [13]. Furthermore, at lower acid strengths (higher water content) the acid distribution in the liquid-free acid layer is shifted towards the less condensed orthophosphoric acid. Although Zhirong et al. [14] report no definite trend between the total amount of free phosphoric acid and the catalytic activity towards propene oligomerisation, they suggest that a higher pyrophosphoric acid content in the free acid is important for the catalytic activity. Other researchers also report a strong dependence between phosphoric acid strength and the activity of SPA towards the oligomerisation of shorter-chain olefins [7,10,13,15]. The hydration state of SPA is also reported to have an effect on the composition and quality of the produced fuel. While investigating the effect of the acid strength of liquid phosphoric acid on the gasoline yield for the oligomerisation of propylene, Bethea and Karchmer [15] found that when acid strength was increased, longer-chain oligomers were formed, resulting in a decrease in the gasoline yield. Prinsloo [13] reports an increase in the diesel selectivity of propene oligomerisation when pyrophosphoric is at a maximum in the free acid – therefore at lower hydration levels. Most of the studies on oligomerisation reactions
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Nomenclature AARE absolute average relative error concentration of component i (mol/L) Ci ki rate constant for reaction i (L g1 min1) mass catalyst loaded (g) mcat SPA base phosphoric acid strength (W%) P2 Oo5 reaction rate of component i (mol/min g) ri initial reaction rate of component i (mol/min g) ri0 rate selectivity parameter for direct dimerisation of rx linear hexenes WFree acid weight free acid determined from titration (g) weight H2O in the reaction mixture (g) W H2 O W% Weight percentage in water t time (min) Reaction A AB B C, C2 C1 C3 C* D
components subscripts 1-hexene all linear hexenes (A + B) linear hexenes tetra- and trisubstituted olefins beta disubstituted olefins monosubstituted and alfa disubstituted olefins mono- and disubstituted olefins oligomerised product
focus on shorter-chain olefins. Specifically, no quantitative study on the effect of acid strength and subsequently free acid distribution on the oligomerisation of longer-chain olefins are reported in the literature. With longer-chain olefins, a significantly larger amount of skeletal and linear isomers are possible – increasing the complexity of the reaction system. When Schwarzer et al. [16] investigated the effect of temperature on the oligomerisation of 1-hexene with SPA, no oligomerisation products heavier that the dimer was formed. They subsequently concluded that only the dimerisation of 1-hexene needs to be considered. Furthermore a sequential reaction pathway where only branched hexene isomers were dimerised was used to model the reaction rate. However it was suggested that the direct dimerisation of linear hexene isomers may form a significant parallel reaction pathway at lower temperatures. The purpose of this work was therefore to investigate the effect of acid strength and acid distribution of the free acid layer of solid phosphoric acid not only on its activity towards the dimerisation of 1-hexene, but also on the route selectivity of the reaction. Batch kinetic experiments at 200 8C with various catalyst acid strengths and free acid distributions were performed and a first-order rate model was fitted to the data for quantification purposes. In addition, the degree of branching of the oligomerised product was tested to determine whether a change in the reaction pathway would result in a variation in the composition of the final product. 2. Experimental
transfer, the catalyst was ground to less than 150 mm [16]. The catalyst was heated with the solvent to 200 8C (temperature control within 1 8C) after which the 1-hexene was charged to the reactor. The reactor pressure was maintained at 6 bar by means of a nitrogen blanket in order to ensure a liquid phase reaction mixture. The equipment and experimental procedure are described in more detail by Schwarzer et al. [16]. The progression of the reaction rate was measured by intermittent samples analysed with an Agilent Technologies 6890 gas chromatograph (GC) fitted with a flame ionisation detector (FID). Elutriation was established on a 50-m long Pona column with a 0.2 mm inner diameter and a 0.5 mm film thickness, with N2 as carrier gas at a flow rate of 25 ml/min. A split ratio of 100:1 was used. The initial column temperature was 40 8C, where it was held for 5 min, after which the temperature was ramped to 300 8C at 8 8C/min where it was finally held constant for 5 min. The residence time of various hexene isomers was established by the injection of analytical grade standards from Sigma–Aldrich: cis and trans 2-hexene (85%), 3,3-dimethyl-1-butene (95%), 2methyl-2-pentene (98%), 2,3-dimethyl-1-butene (97%), 2,3dimethyl-2-butene (98%) and 2-ethyl-1-butene (95%). The rest of the hexene isomers were identified from the retention indices given by Paca´kova´ and Ladislav [17]. A wide range of cracked products (C4–C13) was detected. However, these products only started to form after some dimerisation products (C12) were already present in the reaction mixture. For simplification, the dimerisation and subsequent cracking were therefore quantified as the depletion of the total hexene isomer fraction. Similarly to the previous investigation [16], no trimerisation or higher oligomerised products were formed at any of the temperatures investigated. As reported in the previous work, this seems to be linked to the equilibrium distribution of the different cracked and dimerised products and catalyst deactivation is not suspected. All the components were elutriated before the tetradecane solvent. The consistency of the mass balance was verified by the mass fraction of the solvent in the reaction mixture as the reaction progressed. Only a slight increase (average of 3.6%) in the solvent fraction was observed from start to finish of all the runs. This was assumed to be due to the flashing of cracked products during sampling and was therefore accounted for under the formation of oligomerised products. 2.2. Manipulation and measurement of catalyst acid strength The acid strength of the free acid layer determines the condensed state or equilibrium distribution of the different phosphoric acid species, e.g. ortho, pyro-, triphosphoric acid, etc. (from less condensed to more condensed acids). The equilibrium acid distribution given by Jameson [18], reproduced here in Fig. 1, was used to estimate the free acid composition for each experimental run. Here the acid strength is given as the percentage phosphoric acid (P2O5, W%) in water and this was therefore also used as the basis for acid strength in this work. The acid strength of the catalyst in the reaction mixture is a function not only of the weight of free acid (WFree acid) and its original acid strength ðP2 Oo5 Þ – i.e. before addition to the reaction mixture – but also of the reaction mixture water content. It was calculated for each experimental run using Eq. (1).
2.1. Batch kinetic dimerisation of 1-hexene The dimerisation of 1-hexene (Sigma–Aldrich, 230545-1 L, 97%) was completed in a 200-ml batch reactor with tetradecane (Sigma– Aldrich, 172456-500 ml, 99%) as solvent. Solid phosphoric acid (C84/3), obtained from Su¨d-chemie Sasolburg South Africa, was used as catalyst. To eliminate the possibility of internal mass
P2 O5 ðW%Þ ¼
P2 Oo5 W Free acid W Free acid þ W H2 O
(1)
The acid strength of the catalyst in the reaction mixture can therefore be varied by manipulating the free acid weight fraction of the catalyst charged to the reaction vessel, the original acid
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Table 2 Three reactions completed for H NMR.
P2O5 (W%) mcat (g) 1-Hexene (g) ppm H2O Hexene conversion (%)
Sample 1
Sample 2
Sample 3
64.3% 12.34 272.2 1461 88.9%
72.8% 12.29 267.3 150 80.3%
73.6% 28.54 263.7 80 85.1%
layer determined from the equilibrium acid distribution data given by Jameson [18], according to which only ortho- and pyrophosphoric acid were present at the acid strengths investigated. 2.3. Degree of branching of final product
Fig. 1. Equilibrium distribution of different phosphoric acid species in the free acid layer as a function of P2O5% acid strength as reproduced from the results of Jameson [18].
strength of the free acid layer and the water content of the reaction mixture. The original acid strength of the free acid layer was manipulated by baking the catalyst at either 200 8C or 600 8C. According to vapour liquid equilibrium data given by Brown and Whitt [19], this would result in a P2 Oo5 acid strength of 73.9% and 86%, respectively. The free acid weight fraction of the catalyst charged to the reaction mixture was determined according to the technique described by Cavani et al. [10]. The catalyst is washed in water at room temperature for 10 min – leaching off the supported liquid phosphoric acid. The free acid concentration is then calculated from the amount of acid neutralised by the titration of a 0.2 M solution NaOH to a pH of 4.62 [9]. In some of the experiments the catalyst was washed prior to baking in order to reduce the free acid weight fraction. Finally the water content of the reaction mixture was manipulated by either drying the solvent and 1-hexene with a molecular sieve (to <80 ppm water in the total mixture), or by the addition of water (up to 2000 ppm in the total mixture). Adsorption experiments at 100 8C where >10 000 ppm of water was added to the solvent with comparable catalyst amounts to those used in the hydrated runs, showed no phase separation in the liquid mixture and complete adsorption of the water. It was therefore assumed that all the water added to the reaction mixture was adsorbed by the catalyst when the resultant catalyst acid strength was calculated. The water content of the tetradecane and 1-hexene was measured with a Karl-Fischer, Metrohm 787 KF titrino. All the experimental runs with a description of the catalyst pre-treatment technique, measured free acid weight fraction, catalyst weight, mixture water content and subsequent free acid strength in the reaction mixture (calculated with Eq. (1)) are summarised in Table 1. Also given in Table 1 is the percentage pyrophosphoric acid in the free acid
The branching of the oligomerised product was tested with proton NMR for three different phosphoric acid strengths. The reaction conditions for these three runs are shown in Table 2. To ensure that a clear indication of the product branching could be observed with the H NMR analysis, the dimerisation was completed without the addition of solvent. No difference in the reaction profiles of the various product groups were observed between similar experiments with or without solvent in the reaction mixture. It was therefore concluded that the solvent does not influence the reaction mechanism. Since olefin content interferes with the branching measurement, the product was hydrogenated before fractionation and H NMR analysis. The hydrogenation was done in a Parr autoclave reactor at 60 8C at a hydrogen pressure of 50 bar. A 0.3% Pd Al2O3 catalyst with a particle size of 3 mm from Heraeus was used. The reaction was left to run for 24 h. The bromine number was used to determine the extent of the hydrogenation. A Mettler and Toledo DL 58, with the solvent and titrant prepared as specified by the manufacturer, was used for this purpose. The hydrogenation was found to be at least 97.8% complete, which was sufficient for H NMR testing. The hydrogenated product was distilled into two fractions for H NMR testing: (1) the gasoline fraction, boiling higher than C6 paraffin’s (75–174 8C); and (2) the distillate, which was taken as the product boiling after n-decane (>174 8C) [8]. The product distribution of each fraction was determined by GC–MS. 3. Results and discussion In this work it is proposed that the dimerisation of 1-hexene (A) over SPA occurs via two parallel pathways. The first route follows the sequential reaction scheme described by Schwarzer et al. [16], where isomerisation to linear hexenes (B) is followed by the formation of branched hexene isomers (C) with subsequent dimerisation to D, where this product group includes all the oligomerised as well as cracked products – excluding hexene. The second route, not included in the previously described model,
Table 1 The reactions completed for this study, indicating the catalyst loading, water content, free acid content, resulting free acid strength and composition.
Washed and dried (600 8C) Dried (600 8C) Hydrated (addition of water to reaction mixture) Washed (10 min, dried 200 8C) Washed (10 min, dried 200 8C) Untreated catalyst Dried reaction mixture Dried reaction mixture with excess catalyst
mcat (g)
ppm (H2O)
Free acid (W%)
Acid strength (P2O5, W%)
Pyrophosphoric acid (%)
3.0 5.5 5.0 6.3 3.3 4.3 5.1 15.1
145 149 2045 148 146 151 72 80
0.6 0.4 21.6 5.7 5.7 21.6 21.6 21.6
34.1 47.0 61.2 70.7 68.1 72.7 73.4 73.7
0.0 0.0 0.0 5.0 0.0 16.1 21.1 23.6
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Fig. 2. Reaction progression at 200 8C and P2O5 = 73.7%. (*) A (1-hexene); (*) B (linear hexene isomers); (5) C1 (beta disubstituted olefins); (&) C2 (tetra- and trisubstituted olefins); (^) C3 (monosubstituted and alfa disubstituted olefins); (~) D (overall hexene depletion).
Fig. 3. Simplified reaction scheme.
between selectivity trends as a function of acid distribution. occurs via direct dimerisation of linear hexene isomers. In addition, the branched hexene isomers (C) were subdivided into groups, based on the position and degree of substitution of the double bond: C1: beta disubstituted olefins (double bond between two secondary carbons) e.g. trans-4-methyl-2-pentene. C2: tetra- and trisubstituted olefins (with the double bond between a secondary and tertiary carbon or two tertiary carbons) e.g. 2-methyl-2-pentene; cis- and trans-3-methyl-2-pentene; 2,3-dimethyl-2-butene. C3: monosubstituted and alfa disubstituted olefins (double bond situated between a primary and tertiary carbon) e.g. 2,3dimethyl-1-butene; 2-ethyl-1-butene, 3-methyl-1-pentene. Fig. 2 shows the concentration profiles of the various product groups identified in the reaction scheme (for an experimental run with an acid strength of 73.7%). For all the experimental data sets it was obvious that C1 and C3 form steadily throughout the course of the reaction but do not deplete to oligomerised product. In an attempt to simplify the reaction scheme, these two isomers were subsequently grouped together as a non-reactive hexene isomer, C*. For both pathways the isomerisation of 1-hexene to linear hexene isomers (A–B) always precedes any further reactions and this step is always much faster than the subsequent steps. The linear hexene isomers (B) were therefore grouped together with 1-hexene as AB for modelling and quantification purposes. Furthermore, only the C2 isomer was measured in significant amounts in the reaction mixture after the reaction was terminated. This step is therefore described as reversible in the final reaction scheme. The equilibrium constant was calculated from experimental runs which were left to run for an extended period of time. The heat of reaction (HR) calculated for step C–D in this manner is 7.1 kJ/mol. The above factors were taken into account to produce a simplified reaction scheme, illustrated in Fig. 3. A rate model was developed where each of the steps in the reaction scheme was described by a first-order rate expression (Eqs. (2)–(5)). A separate set of rate constants was determined for each of the experimental runs, since the purpose of the work was not to provide a detailed kinetic model, but rather to distinguish
dC AB ¼ mcat ðkABC kABC kABD ÞC AB dt
(2)
dC C ¼ mcat kABC C AB dt
(3)
dC C kCD ¼ mcat kABC C AB kCD C C þ CD dt K CD
(4)
dC D kCD ¼ mcat kABD C AB þ kCD C C CD dt K CD
(5)
The kinetic parameters were determined from the optimisation of the sum of the absolute average relative error (AARE) between the predicted and experimentally determined concentration (Eq. (6)) for all the reagents. Experimental 4 X C Predicted C X n;t n;t AARE ¼ (6) Experimental Cn;t n t The ability of the model to describe the concentration profiles of the isomer groups in the reaction scheme is illustrated in Fig. 4(a– f). Since a first-order rate model for all the reactions in the sequence resulted in an adequate description of the experimental data, it was assumed that the initial rates of the two parallel 0 0 reaction routes, rABD and rABC , respectively, would give a good quantitative indication of the relative contribution of the different pathways throughout the course of the reaction. The route selectivity was subsequently expressed as the initial rate of the direct depletion of AB towards oligomerised product (D) as a fraction of the total initial depletion rate of AB towards D (i.e. 0 jrABD j r C ) as shown in Eq. (7). rx ¼
0 rABD 0 j r0 jrAB C
(7)
The route selectivity, (rx), calculated in this manner is shown as a function of acid strength in Fig. 5. At low acid strengths the direct dimerisation of linear hexenes is prominent, but as the acid strength increases the dimerisation tends towards the two-step dimerisation of branched hexenes. In fact, at acid strengths above 71% the model shows that the dimerisation almost exclusively follows the sequential pathway where all the linear hexenes are
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Fig. 4. Oligomerisation of 1-hexene for various acid strengths of the catalyst (a–f). (*) AB; (&) C; (^) C*; (~) D. (a) P2O5 = 29.3%, (b) P2O5 = 40.4%, (c) P2O5 = 61.6%, (d) P2O5 = 70.7%, (e) P2O5 = 72.7%, (f) P2O5 = 73.7%.
Fig. 5. The reaction rate selectivity, rx, for various acid strengths together with the phosphoric acid content of the free acid.
Fig. 6. Initial rate of depletion of AB for various acid strengths.
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Table 3 Mole percentage carbon distribution from GC–MS for the gasoline and distillate cut. Acid Strength
C5 C6 C7 C8 C9 C10 C11 C12 C13
Gasoline cut
Acid Strength
73.6%
72.8%
64.3%
0% 1% 15% 15% 18% 37% 14% 0% 0%
0% 0% 5% 12% 16% 43% 22% 1% 0%
0% 0% 16% 17% 17% 35% 15% 0% 0%
first isomerised to branched hexenes before dimerisation. The rate selectivity of the linear pathway therefore goes to zero. Also shown on the secondary y-axis of Fig. 5, is the estimated pyrophosphoric acid content of the free acid layer. Increased selectivity towards the dimerisation of branched hexenes is observed when the pyrophosphoric acid content in the free acid becomes significant. In fact, as the pyrophosphoric acid in the free acid layer is increased, the linear dimerisation pathway seems to disappear in comparison to the two-step dimerisation route. The shape of the concentration profile of C2 at higher acid strengths (e.g. Fig. 4e and f) confirms the dominance of a sequential reaction pathway. Here C2 forms a distinct maximum from where it readily depletes whereas at lower acid strengths this feature is less prominent. The total initial depletion rate of AB as a function of the P2O5 acid strength is given in Fig. 6. As expected, the rate of AB depletion increases with increased acid strength. However, the increase in catalyst activity is much more significant at acid strengths where the more condensed pyrophosphoric acid starts forming part of the free acid layer, >68% according to Jameson [18]. Both the concentration profiles as well as the initial rate analysis indicate a clear difference in route selectivity as a function of acid strength and subsequently free acid layer composition. However, when the hydrogenated final products of the runs indicated in Table 2 were analysed, no meaningful difference in product quality was observed. This was confirmed by the similarity in the product distribution of both the gasoline and distillate fraction of the three products (given in Table 3) as well as in the degree of branching (given in Table 4). This can be explained if a different catalytic mechanism is assumed for orthophosphoric versus the more condensed acids such as pyrophosphoric acid. It is generally accepted that dimerisation over SPA does not occur via the normal carbocation mechanism when acid catalysts such as Zeolites, for instance, are used as catalysts. Instead, phosphoric acid reacts with an olefin resulting in a phosphoric acid ester intermediate [20,21]. This phosphoric acid ester mechanism is specifically associated with orthophosphoric acid. The stability of the ester determines the ability of the catalyst to oligomerise the olefin [22]. For example, butene is known to be more reactive than both shorter- and longerolefin chains over SPA; this is connected to the stability of the butene ester with SPA [7]. The stability of the ester is also connected to the linearity of the reacting olefin – where a more branched olefin results in a more stable ester intermediate [8,16]. This mechanism can be used to explain the apparent one-step Table 4 Fraction branched. Acid strength
Gasoline cut
Distillate cut
73.6% 72.8% 64.3%
2.35 2.58 2.35
2.95 2.97 3.00
C9 C10 C11 C12 C13 C14 C15 C16 C17
Distillate cut 73.6%
72.8%
64.3%
0% 3% 53% 35% 7% 1% 0% 1% 0%
0% 4% 53% 37% 4% 1% 0% 1% 0%
0% 5% 56% 35% 4% 0% 0% 0% 0%
dimerisation of linear hexenes. Here the skeletal isomerisation of linear hexenes may occur in the free acid layer of the catalyst. The resultant branched isomer immediately forms a stable phosphoric ester intermediate, which is oligomerised before any branched hexene isomers are detected in the bulk reaction mixture. Although this dimerisation route may therefore appear to occur in a single step, it actually also follows a two-step mechanism via skeletal hexene isomers. However, a similar mechanism cannot explain the observed sequential nature of the AB–C–D pathway at higher acid strengths. Since a higher acid strength implies a greater pyrophosphoric acid (or even a more condensed acid) content in the free acid, it is proposed that pyrophosphoric acid may actually follow the classic carbocation mechanism. The relatively low pKa value of pyrophosphoric acid compared to orthophosphoric acid, 0.85 versus 2.1 for the first dissociation constant [23], implies that pyrophosphoric acid will be more likely to act as a proton donor (Brønsted acid) than orthophosphoric acid. Furthermore, in a study by De Klerk [24] on the reactivity of octenes over Amberlyst 15 and SPA respectively, it was concluded that Amberlyst 15 was a more catalytically active catalyst toward the oligomerisation of longerchain olefins than SPA. Amberlyst 15 is known to be a strong Brønsted acid. This may also explain the significantly increased catalytic activity when more pyrophosphoric acid is present in the reaction mixture. 4. Conclusions Batch kinetic experiments for the dimerisation of 1-hexene at 200 8C were done at various SPA acid strength levels and subsequently for different free acid distributions. Based on the modelled initial reaction rates of the various possible reaction steps, the rate of 1-hexene dimerisation increased dramatically as the acid strength increased to where the more condensed pyrophosphoric acid became present in the free acid layer. The initial reaction rate analysis also shows that the direct dimerisation of linear hexenes is favoured at lower SPA acid strengths. A twostep dimerisation pathway, where the skeletal isomerisation of linear hexenes precedes dimerisation, becomes dominant as the acid strength is increased. At acid strengths that favour the presence of pyrophosphoric acid in the free acid layer, the direct dimerisation pathway disappears when compared to the two-step route. However, H NMR and fractionation of the hydrogenated fuels that result from the dimerisation of 1-hexene at different SPA strengths and free acid distributions, show no significant variation in either product distribution or the degree of branching of the different fractions. This can be explained if a different catalytic mechanism is assumed for orthophosphoric versus the more condensed acids such as pyrophosphoric acid. It is therefore proposed that dimerisation with orthophosphoric acid would catalyse the reaction via a phosphoric acid ester intermediate whereas pyrophosphoric acid would act as a Brønsted acid via the classic carbocation mechanism. With the phosphoric ester
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Contents lists available at ScienceDirect
Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata
Solid phosphoric acid catalysts: The effect of free acid composition on selectivity and activity for 1-hexene dimerisation R. Schwarzer, E. du Toit *, W. Nicol University of Pretoria, Department of Chemical Engineering, Lynnwood Road, Pretoria, South Africa
A R T I C L E I N F O
A B S T R A C T
Article history: Received 7 July 2009 Received in revised form 1 September 2009 Accepted 1 September 2009 Available online 8 September 2009
The rate of dimerisation of 1-hexene in a batch reactor at 200 8C was used to compare the activity and dimerisation route selectivity of solid phosphoric acid (SPA) at various phosphoric acid strengths and subsequently free acid compositions. The initial reaction rates, calculated from an elementary kinetic rate model, showed an increase in catalyst activity with increased acid strength. This trend becomes more significant at acid strengths where the more condensed pyrophosphoric acid started becoming present in the free acid. At low acid strengths, a reaction pathway where linear hexenes were directly oligomerised dominated the reaction progression. An increase in acid strength favoured a two-step reaction pathway, where linear hexenes were first isomerised to skeletal hexene isomers before dimerisation. This sequential pathway becomes especially dominant where pyrophosphoric acid became present in the free acid. However, analysis of the final product with proton NMR showed no significant difference in the degree of branching with a variation in dimerisation route selectivity as a function of acid strength. ß 2009 Elsevier B.V. All rights reserved.
Keywords: Oligomerisation Catalysis Phosphoric acid Acid strength
1. Introduction The oligomerisation of olefins has allowed the petrochemical industry greater manoeuvrability in the production of gasoline from olefins. These reactions are acid-catalysed and several solid acid catalysts have been investigated for the oligomerisation of shorter-chain olefins: ZSM-5 [1,2], Amberlyst 35 [3], Beidellite and Ultrastable Y Zeolite catalysts [4], sol–gel aluminosilicates [5] and solid phosphoric acid [6,7]. Solid phosphoric acid is a relatively affordable acid catalyst and has the advantage that the spent catalyst can be used as fertiliser [8], which makes it environmentally friendly and cost effective. SPA consists of a layer of different phosphoric acid species, i.e. ortho-, pyro-, tri- and polyphosphoric acid, supported on a mixture of silica and silicon phosphates [9]. It is therefore known as a liquid-supported phosphoric acid catalyst. The liquid acid layer is referred to as the ‘‘free acid’’ content of the catalyst [10]. Although the silicon phosphates affect the crushing strength of the catalyst, it is accepted that they have no influence on the catalytic activity of SPA [11]. It is accepted that water inhibits the activity of solid acid catalysts [12]. This is also true for solid phosphoric acid. Cavani et al. [10] state that an increased amount of water in the feed
* Corresponding author. Tel.: +27 12 420 3796; fax: +27 12 420 5048. E-mail address: [email protected] (E. du Toit). 0926-860X/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2009.09.002
significantly inhibits the oligomerisation rate of propene over SPA. The inhibiting effect of increased hydration levels is believed to be twofold. Firstly, SPA is extremely hydrophilic, flooding the pores when excess water is present in the reaction mixture and restricting access to the catalytically active sites. In addition, the acid strength of phosphoric acid is controlled by the absorption of water [13]. Furthermore, at lower acid strengths (higher water content) the acid distribution in the liquid-free acid layer is shifted towards the less condensed orthophosphoric acid. Although Zhirong et al. [14] report no definite trend between the total amount of free phosphoric acid and the catalytic activity towards propene oligomerisation, they suggest that a higher pyrophosphoric acid content in the free acid is important for the catalytic activity. Other researchers also report a strong dependence between phosphoric acid strength and the activity of SPA towards the oligomerisation of shorter-chain olefins [7,10,13,15]. The hydration state of SPA is also reported to have an effect on the composition and quality of the produced fuel. While investigating the effect of the acid strength of liquid phosphoric acid on the gasoline yield for the oligomerisation of propylene, Bethea and Karchmer [15] found that when acid strength was increased, longer-chain oligomers were formed, resulting in a decrease in the gasoline yield. Prinsloo [13] reports an increase in the diesel selectivity of propene oligomerisation when pyrophosphoric is at a maximum in the free acid – therefore at lower hydration levels. Most of the studies on oligomerisation reactions
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Nomenclature AARE absolute average relative error concentration of component i (mol/L) Ci ki rate constant for reaction i (L g1 min1) mass catalyst loaded (g) mcat SPA base phosphoric acid strength (W%) P2 Oo5 reaction rate of component i (mol/min g) ri initial reaction rate of component i (mol/min g) ri0 rate selectivity parameter for direct dimerisation of rx linear hexenes WFree acid weight free acid determined from titration (g) weight H2O in the reaction mixture (g) W H2 O W% Weight percentage in water t time (min) Reaction A AB B C, C2 C1 C3 C* D
components subscripts 1-hexene all linear hexenes (A + B) linear hexenes tetra- and trisubstituted olefins beta disubstituted olefins monosubstituted and alfa disubstituted olefins mono- and disubstituted olefins oligomerised product
focus on shorter-chain olefins. Specifically, no quantitative study on the effect of acid strength and subsequently free acid distribution on the oligomerisation of longer-chain olefins are reported in the literature. With longer-chain olefins, a significantly larger amount of skeletal and linear isomers are possible – increasing the complexity of the reaction system. When Schwarzer et al. [16] investigated the effect of temperature on the oligomerisation of 1-hexene with SPA, no oligomerisation products heavier that the dimer was formed. They subsequently concluded that only the dimerisation of 1-hexene needs to be considered. Furthermore a sequential reaction pathway where only branched hexene isomers were dimerised was used to model the reaction rate. However it was suggested that the direct dimerisation of linear hexene isomers may form a significant parallel reaction pathway at lower temperatures. The purpose of this work was therefore to investigate the effect of acid strength and acid distribution of the free acid layer of solid phosphoric acid not only on its activity towards the dimerisation of 1-hexene, but also on the route selectivity of the reaction. Batch kinetic experiments at 200 8C with various catalyst acid strengths and free acid distributions were performed and a first-order rate model was fitted to the data for quantification purposes. In addition, the degree of branching of the oligomerised product was tested to determine whether a change in the reaction pathway would result in a variation in the composition of the final product. 2. Experimental
transfer, the catalyst was ground to less than 150 mm [16]. The catalyst was heated with the solvent to 200 8C (temperature control within 1 8C) after which the 1-hexene was charged to the reactor. The reactor pressure was maintained at 6 bar by means of a nitrogen blanket in order to ensure a liquid phase reaction mixture. The equipment and experimental procedure are described in more detail by Schwarzer et al. [16]. The progression of the reaction rate was measured by intermittent samples analysed with an Agilent Technologies 6890 gas chromatograph (GC) fitted with a flame ionisation detector (FID). Elutriation was established on a 50-m long Pona column with a 0.2 mm inner diameter and a 0.5 mm film thickness, with N2 as carrier gas at a flow rate of 25 ml/min. A split ratio of 100:1 was used. The initial column temperature was 40 8C, where it was held for 5 min, after which the temperature was ramped to 300 8C at 8 8C/min where it was finally held constant for 5 min. The residence time of various hexene isomers was established by the injection of analytical grade standards from Sigma–Aldrich: cis and trans 2-hexene (85%), 3,3-dimethyl-1-butene (95%), 2methyl-2-pentene (98%), 2,3-dimethyl-1-butene (97%), 2,3dimethyl-2-butene (98%) and 2-ethyl-1-butene (95%). The rest of the hexene isomers were identified from the retention indices given by Paca´kova´ and Ladislav [17]. A wide range of cracked products (C4–C13) was detected. However, these products only started to form after some dimerisation products (C12) were already present in the reaction mixture. For simplification, the dimerisation and subsequent cracking were therefore quantified as the depletion of the total hexene isomer fraction. Similarly to the previous investigation [16], no trimerisation or higher oligomerised products were formed at any of the temperatures investigated. As reported in the previous work, this seems to be linked to the equilibrium distribution of the different cracked and dimerised products and catalyst deactivation is not suspected. All the components were elutriated before the tetradecane solvent. The consistency of the mass balance was verified by the mass fraction of the solvent in the reaction mixture as the reaction progressed. Only a slight increase (average of 3.6%) in the solvent fraction was observed from start to finish of all the runs. This was assumed to be due to the flashing of cracked products during sampling and was therefore accounted for under the formation of oligomerised products. 2.2. Manipulation and measurement of catalyst acid strength The acid strength of the free acid layer determines the condensed state or equilibrium distribution of the different phosphoric acid species, e.g. ortho, pyro-, triphosphoric acid, etc. (from less condensed to more condensed acids). The equilibrium acid distribution given by Jameson [18], reproduced here in Fig. 1, was used to estimate the free acid composition for each experimental run. Here the acid strength is given as the percentage phosphoric acid (P2O5, W%) in water and this was therefore also used as the basis for acid strength in this work. The acid strength of the catalyst in the reaction mixture is a function not only of the weight of free acid (WFree acid) and its original acid strength ðP2 Oo5 Þ – i.e. before addition to the reaction mixture – but also of the reaction mixture water content. It was calculated for each experimental run using Eq. (1).
2.1. Batch kinetic dimerisation of 1-hexene The dimerisation of 1-hexene (Sigma–Aldrich, 230545-1 L, 97%) was completed in a 200-ml batch reactor with tetradecane (Sigma– Aldrich, 172456-500 ml, 99%) as solvent. Solid phosphoric acid (C84/3), obtained from Su¨d-chemie Sasolburg South Africa, was used as catalyst. To eliminate the possibility of internal mass
P2 O5 ðW%Þ ¼
P2 Oo5 W Free acid W Free acid þ W H2 O
(1)
The acid strength of the catalyst in the reaction mixture can therefore be varied by manipulating the free acid weight fraction of the catalyst charged to the reaction vessel, the original acid
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Table 2 Three reactions completed for H NMR.
P2O5 (W%) mcat (g) 1-Hexene (g) ppm H2O Hexene conversion (%)
Sample 1
Sample 2
Sample 3
64.3% 12.34 272.2 1461 88.9%
72.8% 12.29 267.3 150 80.3%
73.6% 28.54 263.7 80 85.1%
layer determined from the equilibrium acid distribution data given by Jameson [18], according to which only ortho- and pyrophosphoric acid were present at the acid strengths investigated. 2.3. Degree of branching of final product
Fig. 1. Equilibrium distribution of different phosphoric acid species in the free acid layer as a function of P2O5% acid strength as reproduced from the results of Jameson [18].
strength of the free acid layer and the water content of the reaction mixture. The original acid strength of the free acid layer was manipulated by baking the catalyst at either 200 8C or 600 8C. According to vapour liquid equilibrium data given by Brown and Whitt [19], this would result in a P2 Oo5 acid strength of 73.9% and 86%, respectively. The free acid weight fraction of the catalyst charged to the reaction mixture was determined according to the technique described by Cavani et al. [10]. The catalyst is washed in water at room temperature for 10 min – leaching off the supported liquid phosphoric acid. The free acid concentration is then calculated from the amount of acid neutralised by the titration of a 0.2 M solution NaOH to a pH of 4.62 [9]. In some of the experiments the catalyst was washed prior to baking in order to reduce the free acid weight fraction. Finally the water content of the reaction mixture was manipulated by either drying the solvent and 1-hexene with a molecular sieve (to <80 ppm water in the total mixture), or by the addition of water (up to 2000 ppm in the total mixture). Adsorption experiments at 100 8C where >10 000 ppm of water was added to the solvent with comparable catalyst amounts to those used in the hydrated runs, showed no phase separation in the liquid mixture and complete adsorption of the water. It was therefore assumed that all the water added to the reaction mixture was adsorbed by the catalyst when the resultant catalyst acid strength was calculated. The water content of the tetradecane and 1-hexene was measured with a Karl-Fischer, Metrohm 787 KF titrino. All the experimental runs with a description of the catalyst pre-treatment technique, measured free acid weight fraction, catalyst weight, mixture water content and subsequent free acid strength in the reaction mixture (calculated with Eq. (1)) are summarised in Table 1. Also given in Table 1 is the percentage pyrophosphoric acid in the free acid
The branching of the oligomerised product was tested with proton NMR for three different phosphoric acid strengths. The reaction conditions for these three runs are shown in Table 2. To ensure that a clear indication of the product branching could be observed with the H NMR analysis, the dimerisation was completed without the addition of solvent. No difference in the reaction profiles of the various product groups were observed between similar experiments with or without solvent in the reaction mixture. It was therefore concluded that the solvent does not influence the reaction mechanism. Since olefin content interferes with the branching measurement, the product was hydrogenated before fractionation and H NMR analysis. The hydrogenation was done in a Parr autoclave reactor at 60 8C at a hydrogen pressure of 50 bar. A 0.3% Pd Al2O3 catalyst with a particle size of 3 mm from Heraeus was used. The reaction was left to run for 24 h. The bromine number was used to determine the extent of the hydrogenation. A Mettler and Toledo DL 58, with the solvent and titrant prepared as specified by the manufacturer, was used for this purpose. The hydrogenation was found to be at least 97.8% complete, which was sufficient for H NMR testing. The hydrogenated product was distilled into two fractions for H NMR testing: (1) the gasoline fraction, boiling higher than C6 paraffin’s (75–174 8C); and (2) the distillate, which was taken as the product boiling after n-decane (>174 8C) [8]. The product distribution of each fraction was determined by GC–MS. 3. Results and discussion In this work it is proposed that the dimerisation of 1-hexene (A) over SPA occurs via two parallel pathways. The first route follows the sequential reaction scheme described by Schwarzer et al. [16], where isomerisation to linear hexenes (B) is followed by the formation of branched hexene isomers (C) with subsequent dimerisation to D, where this product group includes all the oligomerised as well as cracked products – excluding hexene. The second route, not included in the previously described model,
Table 1 The reactions completed for this study, indicating the catalyst loading, water content, free acid content, resulting free acid strength and composition.
Washed and dried (600 8C) Dried (600 8C) Hydrated (addition of water to reaction mixture) Washed (10 min, dried 200 8C) Washed (10 min, dried 200 8C) Untreated catalyst Dried reaction mixture Dried reaction mixture with excess catalyst
mcat (g)
ppm (H2O)
Free acid (W%)
Acid strength (P2O5, W%)
Pyrophosphoric acid (%)
3.0 5.5 5.0 6.3 3.3 4.3 5.1 15.1
145 149 2045 148 146 151 72 80
0.6 0.4 21.6 5.7 5.7 21.6 21.6 21.6
34.1 47.0 61.2 70.7 68.1 72.7 73.4 73.7
0.0 0.0 0.0 5.0 0.0 16.1 21.1 23.6
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Fig. 2. Reaction progression at 200 8C and P2O5 = 73.7%. (*) A (1-hexene); (*) B (linear hexene isomers); (5) C1 (beta disubstituted olefins); (&) C2 (tetra- and trisubstituted olefins); (^) C3 (monosubstituted and alfa disubstituted olefins); (~) D (overall hexene depletion).
Fig. 3. Simplified reaction scheme.
between selectivity trends as a function of acid distribution. occurs via direct dimerisation of linear hexene isomers. In addition, the branched hexene isomers (C) were subdivided into groups, based on the position and degree of substitution of the double bond: C1: beta disubstituted olefins (double bond between two secondary carbons) e.g. trans-4-methyl-2-pentene. C2: tetra- and trisubstituted olefins (with the double bond between a secondary and tertiary carbon or two tertiary carbons) e.g. 2-methyl-2-pentene; cis- and trans-3-methyl-2-pentene; 2,3-dimethyl-2-butene. C3: monosubstituted and alfa disubstituted olefins (double bond situated between a primary and tertiary carbon) e.g. 2,3dimethyl-1-butene; 2-ethyl-1-butene, 3-methyl-1-pentene. Fig. 2 shows the concentration profiles of the various product groups identified in the reaction scheme (for an experimental run with an acid strength of 73.7%). For all the experimental data sets it was obvious that C1 and C3 form steadily throughout the course of the reaction but do not deplete to oligomerised product. In an attempt to simplify the reaction scheme, these two isomers were subsequently grouped together as a non-reactive hexene isomer, C*. For both pathways the isomerisation of 1-hexene to linear hexene isomers (A–B) always precedes any further reactions and this step is always much faster than the subsequent steps. The linear hexene isomers (B) were therefore grouped together with 1-hexene as AB for modelling and quantification purposes. Furthermore, only the C2 isomer was measured in significant amounts in the reaction mixture after the reaction was terminated. This step is therefore described as reversible in the final reaction scheme. The equilibrium constant was calculated from experimental runs which were left to run for an extended period of time. The heat of reaction (HR) calculated for step C–D in this manner is 7.1 kJ/mol. The above factors were taken into account to produce a simplified reaction scheme, illustrated in Fig. 3. A rate model was developed where each of the steps in the reaction scheme was described by a first-order rate expression (Eqs. (2)–(5)). A separate set of rate constants was determined for each of the experimental runs, since the purpose of the work was not to provide a detailed kinetic model, but rather to distinguish
dC AB ¼ mcat ðkABC kABC kABD ÞC AB dt
(2)
dC C ¼ mcat kABC C AB dt
(3)
dC C kCD ¼ mcat kABC C AB kCD C C þ CD dt K CD
(4)
dC D kCD ¼ mcat kABD C AB þ kCD C C CD dt K CD
(5)
The kinetic parameters were determined from the optimisation of the sum of the absolute average relative error (AARE) between the predicted and experimentally determined concentration (Eq. (6)) for all the reagents. Experimental 4 X C Predicted C X n;t n;t AARE ¼ (6) Experimental Cn;t n t The ability of the model to describe the concentration profiles of the isomer groups in the reaction scheme is illustrated in Fig. 4(a– f). Since a first-order rate model for all the reactions in the sequence resulted in an adequate description of the experimental data, it was assumed that the initial rates of the two parallel 0 0 reaction routes, rABD and rABC , respectively, would give a good quantitative indication of the relative contribution of the different pathways throughout the course of the reaction. The route selectivity was subsequently expressed as the initial rate of the direct depletion of AB towards oligomerised product (D) as a fraction of the total initial depletion rate of AB towards D (i.e. 0 jrABD j r C ) as shown in Eq. (7). rx ¼
0 rABD 0 j r0 jrAB C
(7)
The route selectivity, (rx), calculated in this manner is shown as a function of acid strength in Fig. 5. At low acid strengths the direct dimerisation of linear hexenes is prominent, but as the acid strength increases the dimerisation tends towards the two-step dimerisation of branched hexenes. In fact, at acid strengths above 71% the model shows that the dimerisation almost exclusively follows the sequential pathway where all the linear hexenes are
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Fig. 4. Oligomerisation of 1-hexene for various acid strengths of the catalyst (a–f). (*) AB; (&) C; (^) C*; (~) D. (a) P2O5 = 29.3%, (b) P2O5 = 40.4%, (c) P2O5 = 61.6%, (d) P2O5 = 70.7%, (e) P2O5 = 72.7%, (f) P2O5 = 73.7%.
Fig. 5. The reaction rate selectivity, rx, for various acid strengths together with the phosphoric acid content of the free acid.
Fig. 6. Initial rate of depletion of AB for various acid strengths.
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Table 3 Mole percentage carbon distribution from GC–MS for the gasoline and distillate cut. Acid Strength
C5 C6 C7 C8 C9 C10 C11 C12 C13
Gasoline cut
Acid Strength
73.6%
72.8%
64.3%
0% 1% 15% 15% 18% 37% 14% 0% 0%
0% 0% 5% 12% 16% 43% 22% 1% 0%
0% 0% 16% 17% 17% 35% 15% 0% 0%
first isomerised to branched hexenes before dimerisation. The rate selectivity of the linear pathway therefore goes to zero. Also shown on the secondary y-axis of Fig. 5, is the estimated pyrophosphoric acid content of the free acid layer. Increased selectivity towards the dimerisation of branched hexenes is observed when the pyrophosphoric acid content in the free acid becomes significant. In fact, as the pyrophosphoric acid in the free acid layer is increased, the linear dimerisation pathway seems to disappear in comparison to the two-step dimerisation route. The shape of the concentration profile of C2 at higher acid strengths (e.g. Fig. 4e and f) confirms the dominance of a sequential reaction pathway. Here C2 forms a distinct maximum from where it readily depletes whereas at lower acid strengths this feature is less prominent. The total initial depletion rate of AB as a function of the P2O5 acid strength is given in Fig. 6. As expected, the rate of AB depletion increases with increased acid strength. However, the increase in catalyst activity is much more significant at acid strengths where the more condensed pyrophosphoric acid starts forming part of the free acid layer, >68% according to Jameson [18]. Both the concentration profiles as well as the initial rate analysis indicate a clear difference in route selectivity as a function of acid strength and subsequently free acid layer composition. However, when the hydrogenated final products of the runs indicated in Table 2 were analysed, no meaningful difference in product quality was observed. This was confirmed by the similarity in the product distribution of both the gasoline and distillate fraction of the three products (given in Table 3) as well as in the degree of branching (given in Table 4). This can be explained if a different catalytic mechanism is assumed for orthophosphoric versus the more condensed acids such as pyrophosphoric acid. It is generally accepted that dimerisation over SPA does not occur via the normal carbocation mechanism when acid catalysts such as Zeolites, for instance, are used as catalysts. Instead, phosphoric acid reacts with an olefin resulting in a phosphoric acid ester intermediate [20,21]. This phosphoric acid ester mechanism is specifically associated with orthophosphoric acid. The stability of the ester determines the ability of the catalyst to oligomerise the olefin [22]. For example, butene is known to be more reactive than both shorter- and longerolefin chains over SPA; this is connected to the stability of the butene ester with SPA [7]. The stability of the ester is also connected to the linearity of the reacting olefin – where a more branched olefin results in a more stable ester intermediate [8,16]. This mechanism can be used to explain the apparent one-step Table 4 Fraction branched. Acid strength
Gasoline cut
Distillate cut
73.6% 72.8% 64.3%
2.35 2.58 2.35
2.95 2.97 3.00
C9 C10 C11 C12 C13 C14 C15 C16 C17
Distillate cut 73.6%
72.8%
64.3%
0% 3% 53% 35% 7% 1% 0% 1% 0%
0% 4% 53% 37% 4% 1% 0% 1% 0%
0% 5% 56% 35% 4% 0% 0% 0% 0%
dimerisation of linear hexenes. Here the skeletal isomerisation of linear hexenes may occur in the free acid layer of the catalyst. The resultant branched isomer immediately forms a stable phosphoric ester intermediate, which is oligomerised before any branched hexene isomers are detected in the bulk reaction mixture. Although this dimerisation route may therefore appear to occur in a single step, it actually also follows a two-step mechanism via skeletal hexene isomers. However, a similar mechanism cannot explain the observed sequential nature of the AB–C–D pathway at higher acid strengths. Since a higher acid strength implies a greater pyrophosphoric acid (or even a more condensed acid) content in the free acid, it is proposed that pyrophosphoric acid may actually follow the classic carbocation mechanism. The relatively low pKa value of pyrophosphoric acid compared to orthophosphoric acid, 0.85 versus 2.1 for the first dissociation constant [23], implies that pyrophosphoric acid will be more likely to act as a proton donor (Brønsted acid) than orthophosphoric acid. Furthermore, in a study by De Klerk [24] on the reactivity of octenes over Amberlyst 15 and SPA respectively, it was concluded that Amberlyst 15 was a more catalytically active catalyst toward the oligomerisation of longerchain olefins than SPA. Amberlyst 15 is known to be a strong Brønsted acid. This may also explain the significantly increased catalytic activity when more pyrophosphoric acid is present in the reaction mixture. 4. Conclusions Batch kinetic experiments for the dimerisation of 1-hexene at 200 8C were done at various SPA acid strength levels and subsequently for different free acid distributions. Based on the modelled initial reaction rates of the various possible reaction steps, the rate of 1-hexene dimerisation increased dramatically as the acid strength increased to where the more condensed pyrophosphoric acid became present in the free acid layer. The initial reaction rate analysis also shows that the direct dimerisation of linear hexenes is favoured at lower SPA acid strengths. A twostep dimerisation pathway, where the skeletal isomerisation of linear hexenes precedes dimerisation, becomes dominant as the acid strength is increased. At acid strengths that favour the presence of pyrophosphoric acid in the free acid layer, the direct dimerisation pathway disappears when compared to the two-step route. However, H NMR and fractionation of the hydrogenated fuels that result from the dimerisation of 1-hexene at different SPA strengths and free acid distributions, show no significant variation in either product distribution or the degree of branching of the different fractions. This can be explained if a different catalytic mechanism is assumed for orthophosphoric versus the more condensed acids such as pyrophosphoric acid. It is therefore proposed that dimerisation with orthophosphoric acid would catalyse the reaction via a phosphoric acid ester intermediate whereas pyrophosphoric acid would act as a Brønsted acid via the classic carbocation mechanism. With the phosphoric ester
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