Ind. Eng. Chem. Res. 2010, 49, 3561–3570
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Isoamylene Trimerization in Liquid-Phase over Ion Exchange Resins and Zeolites Marta Granollers, Jose´ F. Izquierdo, Javier Tejero, Montserrat Iborra, Carles Fite´, Roger Bringue´, and Fidel Cunill* Chemical Engineering Department, Faculty of Chemistry, UniVersity of Barcelona, C/ Martı´ i Franque´s 1-11, 08028-Barcelona, Spain
Liquid-phase trimerization of 2-methyl-1-butene and 2-methyl-2-butene mixtures over solid acid catalysts was carried out in a batch-stirred tank reactor in the temperature range 333-373 K. Diisoamylenes and triisoamylenes were the main products. Cracking products among C6-C9 and C11-C14 were also obtained under the assayed conditions. The catalityc performance of five different acidic ion exchange resins (Amberlyst 15, Amberlyst 35, Amberlyst 70, Purolite CT-252, Purolite CT-276) and four zeolites (H-BEA-25, H-FAU30, H-FAU-6 and H-MOR-20) was assessed. Experimental results showed that Amberlyst 15 and H-FAU-30 were the best catalysts for isoamylene trimerization, with selectivities above 40% at 373 K. The most influencing physical properties of the catalysts on the selectivity toward dimers and trimers were acid capacity, acid strength, and specific surface area for resins, and microporous surface area for zeolites. Isoamylene trimerization extent was larger at higher temperature. The most probable mechanism of formation of these compounds involves the reaction between one molecule of dimer with one molecule of isoamylenes, although trimers could also be formed directly from three molecules of isoamylenes. 1. Introduction A high content of large hydrocarbons in fuels is required to meet the growing demand for middle distillate products, such as aviation turbine fuels and diesel fuel. Diesel fuel generally provides a higher energy efficiency in compression ignition engines than automotive gasoline provides in spark combustion engines, and it presents a higher rate of demand growth than automotive gasoline, especially in Europe.1 Improved fuel compositions are needed to meet the stringent quality specifications for aviation fuel and the ever tightening quality specifications for diesel fuel as established by industry requirements and governmental regulations. New diesel fuels will involve deeper hydrodesulfurization and more severe hydroprocessing operations to reduce aromatics content. Reduced gasoline aromatics, olefins, and benzene contents and lower gasoline vapor pressure would lead to modifications in the main gasoline producing processes, such as fluid catalytic cracking (FCC) and catalytic reforming. Other processes, such as alkylation, polymerization, and isomerization could also be affected. Reformulated gasoline and diesel fuel would stretch existing refining processing configurations to the limit, and new processes would be required. As a result, some refineries’ processes could substantially differ in the coming decades. One of the most important problems of gasoline use is the emission of organic volatile compounds, mainly olefins, because they are involved in the tropospheric ozone formation. The distribution and use of fuels contribute about 40% in the emission of organic volatile compounds of anthropogenic origin, about 25% from exhaust pipe, and 15% from evaporative emission.2 For this reason, the content of olefin compounds in European reformulated gasolines are now limited to 18% volume maximum.3 C5 olefins obtained from fluid catalytic cracking are the main contributors to the total olefins content in gasoline (40%). Presently, they are added directly to the pool of gasoline, and * Towhomcorrespondenceshouldbeaddressed.E-mail:
[email protected]. Tel.: +34 934021304. Fax: +34934021291.
they are responsible for more than 90% of the total of gasoline ozone formation potential.4 As a result, it is interesting to reduce their content in gasoline, taking advantage of their reactivity to be converted to other more suitable compounds for reformulated fuels. From this scenario, alternatives to C5 streams with lower Reid vapor pressure and higher octane (to gasoline pool) or cetane (to diesel pool) are needed. C5 oligomerization is one of the process alternatives. Dimerization has proved to be a promising way to obtain olefins for gasoline and trimerization for premium quality diesel.5-8 In particular, trimerization of 2-methyl-1butene (2M1B) and 2-methyl-2-butene (2M2B), which are the most reactive compounds of the C5 olefin fraction, obtains C15 olefins with low cetane numbers about 15 (experimental determination) and boiling points in the range 523-543 K.9 Therefore, these olefins could be included in the diesel composition to increase its production capacity. Studies on isoamylene trimerization are scarce. Some data are available in the open literature from the oligomerization of C4 olefins (mainly isobutene)10-19 and dimerization of isoamylenes.7,20-27 In the study of oligomerization of isobutene in the absence of polar components, many authors concluded that triisobutenes are formed in consecutive reactions via diisobutenes.10,11,13,14 The use of heterogeneous catalysts, especially macroporous ion exchange resins, have been proven to yield the best results in the isoamylene dimerization.7,21 Some data are available in the open literature from the oligomerization of C5 linear olefins in continuous mode over zeolites,5,8,28,29 but from isoamylenes only one reference has been found.8 Olefin dimerization is exothermic and trends to oligomerization. The presence of alcohol in the reacting mixture favors the dimerization and reduces the appearance of higher oligomers.26,27 Therefore, dimerization has been studied in the presence of different alcohols to determine their influence in the reaction and the best alcohol-olefins proportion for enhancing dimerization and minimizing etherification and oligomerization.26 One of the most important conclusions was that, in order to promote trimerization, alcohol should not be present in the reaction medium.27
10.1021/ie901382p 2010 American Chemical Society Published on Web 03/18/2010
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In the study of 2M2B oligomerization,20 a few amounts of C7-C9 were formed and were attributed to intramolecular cracking. In a study of isoamylene dimerization in the presence of three different acidic resins, C6-C9 compounds were detected. It was suggested that the formation of cracking products diminishes when the catalytic active centers of resins are located mainly on the surface.22 Other studies on isoamylene oligomerization showed that C6-C9 and C11-C14 products were formed as a result of cracking processes.8,23,24 Given the antecedents, the aim of the present work is to determine the best catalyst for enhancing isoamylene trimerization between several macroporous ion exchange resins and zeolites in the absence of alcohol. The effect of the temperature on the main and side reactions will be evaluated. Finally, a possible oligomerization reaction network will be proposed.
Table 1. Physical and Structural Properties of Sulfonic Macroporous Ion Exchange Resins catalyst
A-15
A-35
A-70a
CT-252
CT-276
% cross-linking capacity (eq H+ · kg-1) specific surface area (m2 · g-1)c pore volume (cm3 · g-1), Vgd mean pore diameter, dpore (nm) mean particle j p (mm) diameter, D density, Fs (g · cm-3)e porosity, θ (%)f temperature limit (K)
high 4.81 42
high 5.32 34
low 3.01 31b
medium 5.40 22
medium-high 5.20 23
0.33
0.21
0.15
0.22
0.21
34.3
32.9
19.5
39.4
35.7
0.74
0.51
0.57
0.78
0.78
1.42 31.7 393
1.54 24.6 423
1.51 18.8 463
1.49 24.7 423
1.49 23.8 423
a
2. Materials, Apparatus, Analysis, and Methods 2.1. Materials. Reactants consisted of an isoamylene mixture of 2M1B (6 wt %) and 2M2B (94 wt %) (Fluka, Buchs). As catalysts, five macroporous ion exchange resins: Amberlyst 15, Amberlyst 35, and Amberlyst 70 (Rohm and Haas, Chauny, France) and Purolite CT-252, Purolite CT-276 (Purolite, Wales, GB), and four zeolites: H-beta (Su¨dchemie, Bruckmu¨hlHeufeld, Germany) with SiO2/Al2O3 ≈ 25, two US-Y (Grace, Worms, Germany) with SiO2/Al2O3 ≈ 6 and 30, and CBV21A mordenite (Zeolyst, Kansas, U.S.) with SiO2/Al2O3 ≈ 20, have been tested. The nine catalysts will be named A-15, A-35, A-70, CT-252, CT-276, H-BEA-25, H-FAU-6, H-FAU-30, and HMOR-20, respectively. Microporous resins were not considered, because most likely they collapse and present a low catalytic activity due to the apolar character of the reactants and products. Among the tested resins, A-15 and A-35 were chosen because they presented the best selectivity to trimers (>20%) in a previous work,9 compared with Purolite CT-175 and CT-275.9 In addition to CT-175 and CT-275, it was thought that it would be interesting to test the Purolite CT-252 and CT-276 resins, because they are more affordable and they present slightly different structural properties in the dry state: low surface area and larger porous diameter. In addition, CT-252 has a larger porous diameter and acid capacity than CT-276. Finally, A-70 was also chosen for its thermal resistance, since it can effectively operate up to 200 °C, where the maximum operating temperature of the others is 120-150 °C. On the other hand, zeolites have been selected because of the scarce references about isoamylene oligomerization available in the open literature. Some of the structural and physical properties of resins and zeolites are summarized in Tables 1 and 2, respectively. 2.2. Apparatus. All experiments were carried out in a 200 cm3 stainless-steel jacketed batch reactor equipped with a sixblade magnetic stirrer (Autoclave Engineers, USA). The reaction temperature range was 333-373 K, controlled within ( 0.1 K by a 1,2-propanediol-water thermostatic mixture. The system pressure was kept at 2.0 MPa to ensure the liquid phase over the reaction. A detailed description of the experimental apparatus can be found elsewhere.7 The experimental device is shown in Figure 1. 2.3. Analysis Method. Samples were taken online from the reaction medium through a sampling valve (Valco A2CI4WE.2) that injected 0.2 µL of pressurized liquid into an Agilent gas-liquid chromatograph 6890 equipped with a capillary column (HP 19091S-433; 5% phenyl methyl siloxane, 30.0 m × 250 µm × 0.25 µm nominal). A mass selective detector HP5973N was used to identify and quantify the reaction system
Chlorinated resin and low cross-linking. b After rinsing with methanol, then with toluene and finally with isooctane. c BET method (N2 for Sg g 1 m2/g). d Determined by adsorption-desorption of N2 at 77 K. e Skeletal density, measured by helium displacement. f θ in dry state was estimated as 100 Vg/(Vg + (1/Fs)). Table 2. Physical and Structural Properties of Zeolites zeolites
SiO2/Al2O3 Brønsted acid site concentration (Mmol · g-1) BET surface area (m2 · g-1), Sga pore volume (cm3 · g-1), Vgb mesopore volume (cm3 · g-1), Vmeso micropore volume (cm3 · g-1), Vmicro external surface area (m2 · g-1), Sext mesporous surface area (m2 · g-1), Smeso microporous surface area (m2 · g-1), Smicro
H-BEA-25 H-FAU-6 H-FAU-30 H-MOR-20
25.1 1.2
5.7 4.2
29.3 1.1
19.5 1.4
483
458
693
388
0.663
0.315
0.517
0.260
0.527
0.098
0.253
0.062
0.123
0.202
0.280
0.179
246
108
122
29.9
219
82.2
181
37.5
265.1
420
511.9
350.1
a BET method (N2 for Sg g 1 m2 · g-1). adsorption-desorption of N2 at 77 K.
b
Determined by
components. The injector temperature was set to 523 K, the electron source of the mass detector was set to 503 K, and the quadrupole was set to 423 K. The oven temperature was programmed with a 1.5 min hold at 308 K, followed by a 35 K min-1 ramp to 348 K, maintained for 3 min. A second temperature ramp of 25 K min-1 heated the oven to 403 K; this temperature was held for 1 min. Helium (Abello´-Linde, Barcelona, Spain), with a minimum purity of 99.998%, was used as the carrier gas. Owing to the large amount of different formed products, the chromatographic peaks were grouped into five fractions on the basis of their respective gas chromatograph retention times and molecular weight, corresponding to isoamylenes (IA), cracking products from C6 to C9, diisoamylenes lumped together (DIA), cracking products from C11 to C14, and triisoamylenes lumped together (TIA). These groups were calibrated by means of an absolute calibration of the compound portions obtained by distillation of the reaction products, which were produced in preliminary runs from isoamylene oligomerization at 353 K by using 4 g of A-35. No significant amounts of tetramers or higher oligomers were detected in the runs. This fact was confirmed by means of the simulated distillation curves according to the ASTM-2887 method. Also, no significant amounts of cracking compounds
Ind. Eng. Chem. Res., Vol. 49, No. 8, 2010
with less than five carbon atom number and higher than 15 carbon atom number were detected. A standard chromatogram and the simulated distillation curve are shown in the Supporting Information. 2.4. Procedure. The search for the best catalyst, the temperature effect, and the other steps studied in this work was done through different catalyst loads within 0.5-6 g of commercial dried catalyst. Amberlyst 15 (A-15), Amberlyst 35 (A-35), and Purolite resins were dried at 383 K for 3 h at atmospheric pressure and subsequently during 5 h at 373 K under vacuum. This treatment assured a residual amount of water less than 3% w/w.7 Amberlyst 70 (A-70) was rinsed with methanol, then with toluene, and finally with isooctane to remove the residual water in the pore structure of the resin. Next, it was dried at 373 K for 1 h. By using this procedure, BET area was about 30 m2 · g-1, which is the same order than for the other resins. The aim of this pretreatment is extensively explained in experimental section 3.4. In the case of zeolites, they were activated at 773 K for 3 h in an atmospheric oven. For each experiment, the reactor was loaded with fresh dried and previously weighted catalyst and heated up to the desired reaction temperature. A weighted amount of isoamylene mixture (about 130 g) was previously introduced in a calibrated buret. Once the reactor reached the experiment temperature, the reacting mixture was shifted into the reactor with nitrogen by pressure difference, and the stirring was activated and kept at 500 rpm to avoid the effect of the external mass transfer resistance.7 That instant was considered the starting time of the reaction. To follow the evolution of the composition of the reaction medium, 10 liquid samples were taken out every 25 min and the following ones every 50 min. The length of the experiment was set to 8 h. Samples were taken out by pressure difference through a pipe that connected the reactor to the chromatograph (see Figure 1). After the sampling injection valve had taken the 0.2 µL, the remaining liquid mixture retained on the sampling pipe (about 2 mL) was returned to the reactor by shifting with nitrogen. 2.5. Calculations. Percentatges of isoamylene conversion, and selectivity toward dimers, trimers, and cracking products among C6-C9 and C11-C14 were respectively calculated as follows:
Figure 1. Scheme of the experimental setup.
XIA )
o nIA
- nIA o nIA
100
3563
(1)
SIAfC6-C9 ) nIAfC6-C9 100 (2) nIAfC6-C9 + nIAfDIA + nIAfC11-C14 + nIAfTIA SIAfDIA )
nIAfC6-C9
nIAfDIA 100 + nIAfDIA + nIAfC11-C14 + nIAfTIA (3)
SIAfC11-C14 ) nIAfC11-C14 100 (4) nIAfC6-C9 + nIAfDIA + nIAfC11-C14 + nIAfTIA SIAfTIA )
nIAfC6-C9
nIAfTIA 100 + nIAfDIA + nIAfC11-C14 + nIAfTIA (5)
where nIAfC6-C9 and nIAfC11-C14 correspond to the cracking compounds resulting from isoamylene oligomers. Global compsumption rates for isoamylenes were obtained from the slope of the curves nIA versus time. Some experiments were repeated to estimate the experimental uncertainty, which was found to be lower than 5%. For all the experiments, differences in the material balance were lower than 5%. 3. Results and Discussion 3.1. Effect of External Mass Transfer Resistance. The effect of external mass transfer resistance was studied in previous works in which the same setup was used.7,25-27 For dimerization of isoamylenes at 373 K and 2.0 MPa, no significant influence of external mass transfer resistance was observed in the stirring speed range of 100-700 rpm over the same type of catalysts. As a consequence, it was assumed that resistance could be also considered negligible for the oligomerization reaction system, and the stirrer speed of 500 rpm was selected to perform further experiments. 3.2. Effect of the Internal Mass Transfer Resistance. The effect of internal mass transfer resistance was studied over 2 g of Amberlyst 15 (a macroporous conventionally sulfonated ion
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Figure 2. Conversion and selectivity versus 1/Dp: (A) 6 g of A-35, 373 K; (B) 2 g of A-15 373 K.
exchange resins) and 6 g of Amberlyst 35 (a macroporous oversulfonated one). In addition, A-15 and A-35 have the smallest porous diameters and the highest cross-linking degrees among the resins chosen (see Table 1). Therefore, the conditions for which the internal mass transfer resistance is negligible for both resins would be the same as for the remainder of the resins. Bead sizes of 1.0-0.8, 0.8-0.63, 0.63-0.4, 0.4-0.25, and 0.25-0.16 mm of both catalysts were tested at 373 K and 2.0 MPa. The smaller fractions were obtained by grinding and sieving of the bigger particles. To check if the procedure of obtaining different size fractions influences catalytic activity, three previous experiments were done with A-15. The first experiment was performed with only sieved original beads, the second one was performed with grinded and sieved resin, and the last one was done with a mixture of the two first fractions. The same results were obtained, within the experimental error, and it can be inferred that there was no effect from the methods of preparing catalyst fractions of different bead sizes. Figure 2 depicts isoamylene conversion and selectivity toward products for different particle size after 8 h for A-35 and A-15. It can be seen that, after 8 h, the conversion was very similar for all the particles sizes and near to 100%. However, it has to be noted that for larger beads, the selectivity toward trimerization was 5-10% higher than for small particle sizes and, on the contrary, for dimers the selectivity was 5-10% lower. A possible explanation of this fact is that in the presence of internal mass transfer resistances in bigger beads, the dimers molecules can be retained longer in the gel-phase and can produce trimers easily before their desorption. Besides, as both reactions are very exothermic, thermal effects, which accelerate the reaction rates, cannot be discarded. On the other hand, no significant influence of the bead size on selectivities toward cracking products was detected. The internal mass transfer resistance has not been checked with zeolites because the particle size of these catalysts is very small (1-10 µm). In general, for these zeolites crystal sizes, the criteria of Weisz-Prater for observing chemically controlled reaction rates is extensively fulfilled.30 One of the main goals of the paper is the selection of the best catalyst for oligomerization (dimers + trimers), not to do a kinetic study. The industrial bead distribution mean diameter j p ) 0.74 mm for A-15 and 0.51 mm for A-35) is very close (D to the threshold of no influence of the size diameter, particularly for A-15 (see Figure 2B). The variation for A-15 is less than 3%, and for A-35 it is below 6%. So, these differences are of the same order of magnitude than the experimental error (5%).
For this reason, it was assumed that the use of commercial particle sizes did not affect the results. 3.3. Effect of the Catalyst Load. The effect of catalyst load (from 0.5 to 6 g) was studied for two different bead size distributions of A-15 at 373 K: commercial size and 0.25-0.16 mm range. To allow the comparison for the different catalyst loads, the group WCATt(nIAo)-1 [gCAT · min · mol-1] has been used instead of time [min]. Results are shown in Figures 3 and 4. For both bead size distributions, no significant effect of the catalyst load was observed within the experimental error. In addition, the effect the slight internal mass transfer resistance has on how the total conversion is achieved can be observed, because for the same WCATt(nIAo)-1, 0.16-0.25 mm bead size experiments show less conversion values than commercial size (0.74 mm of medium particle size). This could be explained by the fact that both dimerization and trimerization take place simultaneously in the presence of internal mass transfer resistances, which leads to a higher consumption of isoamylenes than occurs only by dimerization. Considering that there is no significant effect of the catalyst load between 0.5 and 6 g, in the next steps of the study catalyst loads within 0.5-6 g have been used. 3.4. Effect of the Catalyst Nature. Additional experiments were carried out at 373 K during 8 h over 6 g of dried resins and activated zeolites. This catalyst load was used to ensure total conversion above 98% after 8 h for all catalysts. A particular issue regarding the BET area of resin A-70 was considered. This resin apart from the presence of chlorine in its structure of the polymer, has a low cross-linking degree (C10) at this stage of reaction in which cracking and copolymerization are the main reactions. Finally, it is generally accepted that the oversulfonation not only increases the concentration of acid sites on poly(styrenedivinylbenzene) resins, but also increases the acid site strengths and thermal stability.31 In addition, cracking formation requires stronger acid centers than oligomerization or isomerization.28 Therefore, these oversulfonated resins as A-35 and Purolites are more active to crack than to oligomerize. This fact would explain that, at the end of the second period, A-15 shows the higher selectivity toward trimers, because this resin presents the largest specific surface area, porous diameter, and weaker acid strength as well. As for zeolites, a similar table is shown in order to check the properties involved in the reaction process (see Table 4). The selectivity toward C6-C9 is very similar within the experimental error for all the zeolites in both periods. It can be seen that H-FAU-30 is the zeolite with highest activity and selectivity to
microporous surface area (m2 · g-1) time zero
-rIA,0 (mol · g-1 · h-1)
1st period SC6-C9,90% (0-100 min) SDIA,90% SC11-C14,90% STIA,90% 2nd period
SC6-C9,98% SDIA,98% SC11-C14,98% STIA,98%
265.1
350.1
420
511.9
0.5
1.4
2.2
2.7
2.9
2.6
4.0
2.3
75.6 2.4 19.0
70.9 2.2 24.3
56.9 5.1 33.9
51.3 3.1 43.3
3.1 65.1 2.9 28.9
3.08 62.7 3.2 31.1
4.1 48.3 7.3 40.3
5.0 32.8 17.5 44.7
trimers in both periods and H-BEA-25 is the zeolite with lower activity and highest selectivity toward dimers. By considering the physical and structural properties of zeolites, the initial reaction rates and selectivities toward dimers and trimers can be correlated with the microporous surface, which indicates that the higher are the microporous surface areas, the higher are the isoamylene reaction rates and selectivities toward trimers. In addition, when the reactions over zeolites occur in the microporous area, the activity and selectivity of all reactions depend on the shape and size of cages and on the channels and their aperture.32 It seems that oligomerization of isoamylenes could be located at the intersections of the channels because H-BEA-25 and H-MOR-20 show lower selectivity toward trimers than H-FAU zeolites. The formation of trimers in H-BEA-25 and H-MOR-20 is more sterically hindered by the space available in their channel intersections (cages about 1 nm). Hence, dimers formation is promoted in these zeolites. For H-FAU zeolites, the size of cage channels (1.3 nm) seems large enough to reduce the steric constraints influences, and trimers formation is not so hindered. The difference between H-FAU-6 and H-FAU-30 can be explained by the fact that H-FAU-30 has a higher microporous surface area than does H-FAU-6, and consequently H-FAU-30 has a larger number of cages where trimerization reaction can take place. Selectivity toward light cracking products at the end of the second period is very low (between 3-5%), but the selectivity
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Figure 6. Variation of mol of DIA with time for different temperatures (4 g of A-35).
Figure 7. Variation of mol of TIA with time for different temperatures (4 g of A-35).
toward C11-C14 cracking products increases with a larger microporous surface area, from 3% for H-BEA-25 to 18% for H-FAU-30. This increase can be related to the formation of trimers, which crack easily and promote the formation of these cracking compounds. Finally, by comparing the results with ion exchange resins and zeolites at a similar isoamylene conversion (98-99.5%), it can be seen that, when trimerization is the target reaction, the best catalysts are H-FAU-30 and A-15, obtaining a selectivitiy of 44-50%. These values are higher than those obtained for dimers (25% for A-15 and 33% for H-FAU-30). Resins, as a group, show the highest potential formation of cracking byproducts, with a C11-C14 distribution larger than C6-C9. On the other side, zeolites form proportionally less cracking byproducts with a more similar distribution between both cracking families. 3.5. Effect of Temperature. Experiments were carried out over 4 g of A-35 for a temperature range from 333 to 373 K. Such catalyst and load were chosen in order to compare the experimental results with those of previous works carried out by Cruz et al. in the same setup and catalyst conditions but in the presence of alcohol.7,25-27 In addition, as all the tested resins have sulfonic active centers, the behavior of A-35 at different temperatures could represent qualitatively the entire group of used resins. In the case of zeolites, as the same products as with resins have been observed, it has been assumed that the temperature effect would be similar. In the present work total isoamylene conversion for all temperatures was obtained after 8 h. In the study of Cruz et al., total isoamylene conversion was also observed only when no alcohol was present.27 Besides, at the initial stage of reaction a fast isomerization between 2M1B and 2M2B was observed from 2M2B/2M1B molar ratio of 9.5-5. It is well-known in the literature33,34 that the double bond isomerization reaction takes place readily at the initial stages. For this reason, the reaction with pure isoamylenes was not considered in the present study. Subsequently, when dimerization prevails over isomerization, an increase from 5 to 10 at 60 °C and from 5 to 16 at 100 °C of the 2M2B/2M1B molar ratio during the experiment was detected. As well, Shah and Sharma21 stated in their study of isoamylene dimerization over ion exchange resins that the isoamylene isomerization was fast, with a rate of disappearance of 2M1B being about 1.5 times higher than that of 2M2B. Figure 6 shows the dimers mole variation obtained at 333-373 K. At the initial stages, it can be seen that dimers are rapidly formed at all temperatures. Thereafter, the amount of
dimers levels off at 333 K, whereas it decreases smoothly at 343-373 K (see Figure 6). This decrease is more noticeable at higher temperature, which indicates that subsequent reactions, like trimerization and cracking, gain importance. This fact is confirmed in Figures 7-9, where it can be seen that the increase of temperature enhances the formation of trimers and cracking products. Trimers formation is always increasing, and the trimers are mainly produced at the initial stage of the reaction (Figure 7). In the study carried out by Cruz et al., no cracking compounds were observed using alcohol, and the detected amounts of trimers were
