Dimerization of Isobutene over Sulfonic

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Ind. Eng. Chem. Res. 1997, 36, 4452-4458

Liquid-Phase Etherification/Dimerization of Isobutene over Sulfonic Acid Resins Marco Di Girolamo, Massimo Lami, Mario Marchionna, Ermanno Pescarollo, Lorenzo Tagliabue,* and Francesco Ancillotti Snamprogetti Research Laboratories, Via Maritano 26, 20097 S. Donato Milanese, Italy

Simultaneous isobutene etherification/dimerization reactions have been studied in the liquid phase using macroporous sulfonic acid resins. It has been found that, in the presence of methanol, oligomerization to species heavier than dimers may be greatly reduced with respect to data reported in the literature for isobutene dimerization. Products of the described synthesis are MTBE and a mixture of branched C8 hydrocarbons having excellent properties for gasoline blending. These two products may be obtained in a wide range of relative ratios. The influence of methanol (i.e., methanol/isobutene molar ratio), feedstock type, and reactor design on product composition and byproducts formation is discussed. Introduction In order to face serious environmental concerns, it is generally accepted (Unzelman, 1997) that the worldwide trend for gasoline formulation will be toward lower evaporative emissions and a more complete combustion by means of (a) an important and expanding role of oxygenates and branched saturated hydrocarbons (such as those found in alkylate) as gasoline components and (b) a reduction of gasoline volatility and of aromatics, lighter olefins, and sulfur contents. Besides tert-alkyl ethers (MTBE, ETBE, TAME, etc.), branched saturated hydrocarbons represent the other most important class of compounds for the gasoline reformulation. They can be made by alkylation and by olefins dimerization. Alkylate is particularly suitable for its high octane number, low volatility, and absence of sulfur and aromatics. Alkylation is a well-established process; however, a number of environmental concerns will cause increasing troubles for new plants based on the present technologies: HF should be ruled out for installations in populated areas due to its extreme toxicity, while H2SO4 is highly corrosive and produces great amounts of acid muds which are difficult to dispose of. Alternative processes with solid acid catalysts are being developed, but their commercial applicability still has to be proved (Rao and Vatcha, 1996). This work describes an effective way to perform the combined production of MTBE and highly branched C8 hydrocarbons through the simultaneous dimerization/ etherification of isobutene. 2,4,4-Trimethyl-1-pentene and 2,4,4-trimethyl-2-pentene (RON ) 100; MON ) 89) are mainly obtained among C8 species. Since they can be easily hydrogenated to 2,4,4-trimethylpentane (isooctane), which is the reference compound for rating the gasoline antiknock properties (RON ) 100; MON ) 100), the quality for gasoline blending of these low-volatile olefins can be further improved. It is well-known that isobutene dimerization may be carried out in the liquid phase with acid catalysts, but, so far, this reaction has found only a few industrial applications (Scharfe, 1973) because of its high exothermicity, which makes control of the oligomerization reactions very difficult. Diisobutenes formation as byproducts in the MTBE synthesis has been more often described due to its interest for industrial applications S0888-5885(97)00093-6 CCC: $14.00

(Rehfinger and Hoffmann, 1990; Izquierdo et al., 1993; Vila et al., 1994; Miracca et al., 1996). Now, we have found that by feeding methanol together with isobutene (methanol/isobutene ) 0.2-0.6 mol/mol) a thorough control of this fast and highly exothermic reaction is achieved, resulting in a formation of trimers and tetramers much lower than previously reported for this synthesis (Haag, 1967; Scharfe, 1973). Since methanol is fed to the system, some MTBE is formed; however, in this case, the optimal methanol/ isobutene molar ratio is sensibly lower than the stoichiometric one used in the MTBE synthesis. If needed, the produced MTBE may be easily separated from the oligomers mixture. As in the MTBE synthesis, every C4 feedstock containing isobutene (from FCC units, steam-cracking, isobutane dehydrogenation units, etc.) can be conveniently exploited. This paper addresses a number of factors influencing the joint production of MTBE and isobutene oligomers. The effects on products and byproducts formation of methanol/isobutene molar ratio, resin type, and feedstock composition are discussed, referring to results obtained both on a pilot plant and in a bench-scale reactor. Finally, in order to assess the importance of a proper reactor configuration, results from runs where the pilot reactor was operated with heat transfer are compared with ones obtained in adiabatic operations. Experimental Section Analytical Methods. The products have been analyzed with a Hewlett-Packard gas chromatograph Model 5890 Series II equipped with a flame ionization detector (FID) and a constant-volume valve for samples injection. Using this injection system, internal standards are not necessary and the analysis is extremely accurate (standard deviation < 0.1%). For a better resolution two analytical procedures have been used. Reaction products were resolved by a WCOT fused-silica capillary column CPSIL 19 CB (50 m × 0.25 mm) with a method allowing the complete separation of C4, dimethyl ether (DME), methanol, tert-butyl alcohol (TBA), methyl tertbutyl ether (MTBE), and C8, C12, and C16 hydrocarbons. The column temperature was held at 40 °C for 6 min followed by a 6 °C/min ramp up to 90 °C and by a second ramp at 8 °C/min up to 180 °C; the final temperature was held for 15 min. C4’s separation was performed with an aluminacoated KOH fused-silica capillary column (50 m × 0.25 © 1997 American Chemical Society

Ind. Eng. Chem. Res., Vol. 36, No. 11, 1997 4453

Figure 1. Pilot-plant scheme. Table 1. Physical Properties of Catalysts Employed in This Study acidity, mequiv H+/g (dry) moisture content, % apparent density, g/cm3 surface area, m2/g particle size (95% vol.) (mm) porosity, cm3/g

Amberlyst 15

Amberlyst 35

4.75 53.1 0.77 45 0.63-1.25 0.30

5.23 55.4 0.82 44 0.63-1.25 0.35

Table 2. Isobutene Dimerization and Product Composition isobutene conversion (%) product composition (%) C8 C12 C16

a

b

c

85.6

58

up to 99%

54.2 40 5.8

52 40 8

58.0 38.3 3.7

a MeOH ) 0, isobutene 53% by weight, Amberlyst 15, T max ) 98 °C, Twater ) 40 °C, LHSV ) 5.5 h-1 (this work). b MeOH ) 0, isobutene ) 30% by weight, Amberlyst 15, temperature ) 60 °C, LHSV ) 1 h-1 (Haag, 1967). c MeOH ) 0, isobutene ) 45.4% by weight, catalyst ) acid slurry, temperature = 100 °C (Scharfe, 1973).

mm) at 110 °C. Before the analysis, every sample was washed with water in order to remove methanol and MTBE, which are very harmful for this particular column. Equipment. Preliminary catalytic tests have been carried out on a laboratory scale with a downflow tubular reactor (36 × 1.4 i.d. cm) packed with measured amounts of catalyst (20 cm3) and provided with a thermocouple sliding inside a sheath to measure temperature profiles. Reactants were introduced in the reactor with a HPLC pump at liquid hourly space velocities (LHSV) in the range 3-9 h-1. The reactor effluent was collected in a cooled vessel; samples were withdrawn every hour

through a valve directly into 40 cm3 stainless steel bottles. Sampling bottles were pressurized and connected to the gas chromatograph for the analysis. The reaction temperature was controlled by circulating water at 40 °C from a thermostatic bath to the reactor jacket. This arrangement resulted in peaks of 70-110 °C, depending on the methanol/isobutene molar ratio, isobutene concentration, and LHSV used, located near the reactor inlet and in a lower temperature plateau near the outlet zone. A backpressure regulator held the whole system at 15 bar in order to maintain the reactants in the liquid phase over all the investigated temperature range. Further experimentation was carried out on the pilot plant whose scheme is presented in Figure 1. Methanol and C4 hydrocarbons taken as side streams in the ECOFUEL’s MTBE plant (Ravenna, Italy) are homogenized in a static mixer and fed to the reactor with liquid hourly space velocities of 3-9 h-1. The reactor is a 6 m × 1 in. i.d. stainless steel tube, packed with 2 L of Amberlyst 35 and provided with a jacket for water cooling. Reaction is monitored by 12 thermocouples (RT 01-RT 12). Twelve valves located in correspondence with the thermocouples allow samples collection. Temperatures of reactants and cooling water are set by two heat exchangers (E1-C, E2-C). Data acquisition and remote control are performed through the SCADA (Supervisory Control Alarm and Data Acquisition) software FIX DMACS version 5.5 (Intellution Co.). Catalysts. Commercial samples of Amberlyst 15 and Amberlyst 35 (Rohm and Haas Co.) have been used as catalysts. Both resins are macroporous styrene-divinylbenzene copolymers with sulfonic groups as the functional structure. Their physical properties are reported in Table 1. In order to avoid the loss of sulfonic groups, reaction temperatures above 120 °C have been carefully avoided.

4454 Ind. Eng. Chem. Res., Vol. 36, No. 11, 1997

Figure 2. Isobutene dimerization and codimerizations. Main reactions. Table 3. Effect of Methanol/Isobutene Molar Ratio on Selectivity and Product Compositiona isobutene (% by weight) reaction temp (max) cooling water temp (°C) isobutene conversion (%) overall to MTBE product composition (% by weight) C8 C12 C16 a

0.2

0.4

0.6

46 74 40

50 73 40

48 72 40

88 21

79 37

79 60

79.2 19.7 1.1

88.3 10.9 0.8

92.6 7.1 0.3

Catalyst ) Amberlyst 15; LHSV ) 5.5 h-1.

Every batch of resin has been washed with distilled water, dried in air, and heated up in vacuum to constant weight before use. Since the particle distribution of these resins is quite narrow, no further sieving has been performed. Chemicals. In the bench-scale experiments, reagentgrade methanol (purity > 99.9% by weight, water < 0.1% by weight) (Carlo Erba, Milan, Italy), isobutene, n-butenes, and isobutane with purity > 99.5% (SIAD, Milan, Italy) have been used. In the pilot-plant runs, methanol (water < 0.1% by

Table 4. Compositions (% by Weight) of Typical C4 Streams

isobutene 1-butene 2-butenes C4 paraffins

steam cracking

FCC

isobutane dehydrogenation

28-46 30-45 5-15 4-8

10-25 8-15 15-35 30-60

45-55 0 0 45-55

weight) and a C4 feedstock from steam cracking after butadiene extraction (Raffinate-1) were used. Chemistry The composition of the mixture of C8 hydrocarbons produced by isobutene dimerization in the presence of methanol is strictly related to the composition of the C4 cut. For instance, when feedstocks from isobutane dehydrogenation units are used, the only possible reaction for isobutene besides the etherification to MTBE is the addition to a tert-butyl ion to form a higher molecular weight carbocation; this ion may add another molecule of isobutene (oligomerization) or eliminate a proton to form two C8 isomers (DIB), namely, 2,4,4-trimethyl-1pentene and 2,4,4-trimethyl-2-pentene (Figure 2).


Figure 3. Etherification reactions.

Figure 4. Isobutene conversion (overall and to MTBE) vs catalyst type: Amberlyst 35 (squares); Amberlyst 15 (triangles). Table 5. Effect of Feedstock Type on Selectivity and Product Compositiona FCC isobutene (% by weight) 22 reaction temp (max) 70 cooling water temp (°C) 50 isobutene conversion (%) overall 90 to MTBE 41 product composition (% by weight) C8 90.3 C12 9.4 C16 0.3 C8 composition (% by weight) DIB 69.8 DMHE 8.5 TMP 21.7 a

SC

dehydrogenation

38 75 45

45 75 40

89 42

91 45

90.2 9.8 0.01

88.2 11.4 0.4

79.4 14.5 6.1

98.6 0 1.4

Table 6. Composition of a C8 Olefinic Fraction from a Steam-Cracking Feed species

ID

% by weight

2,4,4-trimethyl-1-pentene 2,4,4-trimethyl-2-pentene 2,2-dimethylhexenes 2,3-dimethylhexenes 2,3,3-trimethylpentenes 2,3,4-trimethylpentenes 3,4,4-trimethylpentenes other dimethylhexenes and methylheptenes

DIB DIB DMHE DMHE TMP TMP TMP DMHE

61.98 17.40 7.55 6.62 0.72 1.63 3.80 0.30

Catalyst ) Amberlyst 35; R ) 0.4; LHSV ) 4.4 h-1.

When linear butenes are present, e.g., for feedstocks from FCC units or steam crackers, the mechanism is more complex: in this case, linear butenes may react with isobutene to form codimers. The first step is still the formation of a tert-butyl ion, but reactions with isobutene, 2-butenes, and 1-butene are now possible (Figure 2). The addition of a linear olefin gives a primary carbocation which eventually rearranges, forming other trimethylpentenes (TMP) (from 2-butenes) and dimethylhexenes (DMHE) (from 1-butene). In every case, codimerization reactions are slower than isobutene dimerization. At high temperature, when linear butenes are present in high concentration with respect to isobutene, 1-butene

Figure 5. Etherification/dimerization of isobutene. Typical product composition.

bond isomerization to 2-butenes and etherification to 2-methoxybutane (MSBE) may be observed. The pro-


Figure 6. Temperature and products profiles. R: [, 0.3; 0, 1. Tin (reactor inlet temperature) ) 58 °C, LHSV ) 7 h-1, isobutene ) 45% by weight.

duced diisobutenes may eventually react at very small extents with methanol, forming the corresponding ethers (Figure 3). Results and Discussion Experiments of isobutene dimerization have been carried out first in the absence of methanol using Amberlyst 15 as the catalyst and a 53% by weight isobutene feed (as from isobutane dehydrogenation). A great difficulty in limiting the temperature of the watercooled bench-scale reactor below 120 °C was met since isobutene oligomerizes very quickly even at low temperatures. However, results in good agreement with previously reported data have been obtained, despite some differences in experimental setup and operating conditions (Table 2).

An extensive formation of heavy species which fall in the upper end (C12) or even outside (C16) the gasoline boiling range (35-180 °C) was observed. An impressive increase in selectivity to dimers together with a strong decrease in the formation of C16 was obtained by introducing methanol into the reacting system (methanol/isobutene ) 0.2-0.6 mol/mol) (Table 3). As a matter of fact, methanol influences the isobutene dimerization process in a number of ways. First, because of its high polarity, methanol is preferentially adsorbed on the active sites of the catalyst according to the following reaction:

SO3-H+ + MeOH a MeOH2+ + SO3Since MeOH2+ is a species less acid than SO3-H+ (Gates


Figure 7. Adiabatic runs. Temperature and products profiles. R ) 0.4, LHSV ) 7 h-1, isobutene ) 28% by weight.

and Rodriguez, 1973), the rate of oligomerization reactions is much reduced; this positively affects the selectivity to dimers. Further, since isobutene is more rapidly converted to MTBE than to dimers, the concentration of free isobutene is also reduced, resulting in a lower tendency to oligomerization. In order to select the most efficient catalyst for the synthesis, the performances of Amberlyst 15 (4.8 mequiv of H+/g) and Amberlyst 35 (5.3 mequiv of H+/g) were compared in runs carried out at different methanol/ isobutene molar ratios. As for the MTBE production (Miracca et al., 1996), Amberlyst 35 resulted in a more active catalyst than Amberlyst 15; in fact, the catalytic activity of these ion-exchange resins is strongly dependent on their acid capacity (Parra et al., 1997). For instance, using Amberlyst 35, isobutene conversions 5-10% higher than those with Amberlyst 15 were obtained under similar space velocity, temperature, and reactants concentration conditions (Figure 4). It is worth noting that this improvement is mainly related to the increased formation of dimers since MTBE has always been found close to its equilibrium concentration. Since the selectivity to dimers was almost unaffected by the catalyst type, further experimentation was carried out using Amberlyst 35 only. Feedstock composition, i.e., the relative content of linear olefins and paraffins, is one of the most important factors influencing the selectivity of the process. It is known that, depending on its source, a C4 cut may contain linear butenes besides isobutenes (Table 4). Through experiments carried out on the bench-scale reactor, it has been verified that linear butenes positively affect the selectivity. Since linear butenes are preferentially adsorbed on the catalytic sites with respect to the formed dimers, oligomerization reactions are hindered even when isobutene has been almost completely converted. On the contrary, paraffins cannot

be adsorbed on the catalyst; for such a reason, when feedstocks from isobutane dehydrogenation are used, the methanol/isobutene molar ratio (R) should be slightly increased in order to minimize the formation of heavy species (Table 5). A detailed composition of a dimeric fraction from a steam-cracking feed is reported in Table 6. The influence of R on product composition has been studied by processing a feed from steam cracking on the pilot plant. The reactor was first operated at R ) 1 (i.e., MTBE synthesis condition); afterward, the methanol feed was progressively reduced to R ) 0.2. A reduction in the selectivity to dimers was observed; however, at R ) 0.3 they still accounted for about 90% of the whole hydrocarbon fraction (Figure 5). In Figure 6 profiles obtained during pilot-plant runs under typical MTBE (R ) 1) and dimerization conditions (R ) 0.3) are compared. In the latter case the temperature profile is much sharper and, in the first part of the reactor, more isobutene is converted both to MTBE and to dimers. Although much less methanol is used with respect to the MTBE synthesis, isobutene is still quite selectively converted to MTBE in the first part of the reactor. Only when methanol is almost completely converted, i.e., when MTBE equilibrium is reached, does dimerization become the main reaction. It is worth noting that under dimerization conditions an increase in the isobutene conversion is always observed along the whole catalytic bed, while in the MTBE synthesis a steady level corresponding to the thermodynamic equilibrium is approached. This means that, per single pass, more isobutene can be converted to dimers than to MTBE since, at least under the conditions in the present study, the equilibrium for the isobutene dimerization is completely shifted toward the formation of dimers. Finally, to assess the influence on the synthesis of different reactor configurations, runs under adiabatic


conditions have also been performed on the pilot reactor. To operate as close as possible to the adiabatic regime, the reactor jacket, used to provide heat transfer through water circulation, was thorougly drained and dried by flowing nitrogen. The reactor adiabaticity was assessed by checking the energy balance (i.e., comparing the observed reactor outlet temperature with values calculated through the knowledge of the effluent composition and reaction enthalpies). The pilot reactor turned out to be satisfactorily adiabatic; in fact, the observed temperature increases were only 5-10% lower than the calculated ones. In Figure 7 typical temperature and composition profiles for adiabatic runs are reported. A steady temperature between 100 and 110 °C was approached. This behavior was rather unexpected. Actually, reactor runaway was considered likely to occur since oligomerization reactions are not equilibrium limited. Instead, through the presence of methanol, an otherwise impossible smooth control of the adiabatic dimerization process was achieved, although most of the reactor worked close to its threshold for the catalyst stability. The existence of a steady temperature proves that the heat released by the isobutene dimerization (∆Hr ) -9.5 to -10.5 kcal/mol of isobutene) (Rehfinger and Hoffmann, 1990) is completely taken up by the endothermic MTBE decomposition reaction (∆Hr ) +9.5 kcal/mol); in fact, for temperatures higher than 100 °C these two reactions proceeds at a similar rate, as can be seen by reported products profiles (Figure 7). It should be noted that more dimers can be produced in an adiabatic reactor because the higher the temperature, the lower the MTBE equilibrium concentration. However, methoxybutanes (MSBE) have been more extensively formed in the adiabatic runs with respect to runs in which the reactor was operated with water cooling (5% by weight vs 0.1-0.3% by weight). When feedstocks containing linear butenes are used, adiabatic operation gives lower selectivity to MTBE. It is worth noting that methoxybutanes have poor properties for gasoline blending (RON, MON < 80). When a feedstock from an isobutane dehydrogenation unit is processed, methoxybutanes cannot be formed but the reaction is still preferentially carried out by providing heat transfer since a too fast catalyst aging may occur due to the higher temperatures of the adiabatic operation. Conclusions The reaction described in this study may be exploited to produce a mixture of high octane branched C8 saturated hydrocarbons and MTBE in a wide range of proportions by varying the amount of methanol fed to the system. Several factors influence the synthesis, e.g., feedstock type, methanol/isobutene molar ratio, etc.; knowledge about these factors is important in order to safely run

the reactors and to obtain products having suitable properties for gasoline blending. Also a proper reactor design is important in order to carry out the synthesis in the most favorable conditions. A water-cooled tubular reactor is an optimal choice; as the reaction heat is removed as long it is released along the catalyst bed, a thorough temperature control can be achieved, resulting in higher selectivities to desired products and in a lower catalyst aging compared to other kinds of reactors. Finally, this reaction can be attractive for industrial applications; in fact, it brings flexibility both to already existing and future MTBE plants that could be run in the most effective way in order to fulfill the market needs for oxygenates and alkylate. Acknowledgment ECOFUEL SpA support is gratefully acknowledged. Authors thank all the people working on this project. Literature Cited Gates, B. C.; Rodriguez, W. General and Specific Acid Catalysis in Sulfonic Resin. J. Catal. 1973, 31, 27-31. Haag, W. O. Oligomerization of isobutylene on cation exchange resins. Chem. Eng. Prog. Symp. Ser. 1967, 63, 140-147. Izquierdo, J. F.; Vila, M.; Tejero, J.; Cunill, F.; Iborra, M. Kinetic study of isobutene dimerizations catalyzed by a macroporous sulphonic acid resin. Appl. Catal. A 1993, 106, 155-165. Miracca, I.; Tagliabue, L.; Trotta, R. Water Cooled Multitubular Reactors for Etherifications. Chem. Eng. Sci. 1996, 51 (10), 2349-2358. Parra, D.; Izquierdo, J. F.; Cunill, F.; Tejero, J.; Iborra, M.; Fite`, C. Catalytic Activity of Acidic Ion Exchange Resins in Methyl tert-Butyl Ether Liquid Phase Synthesis. Proceedings of The First European Congress on Chemical Engineering, Florence, Italy, 1997; Vol. IV, pp 3001-3004. Rao, P.; Vatcha, S. R. Solid-acid alkylation process development is at crucial stage. Oil Gas J. 1996, Sept 9, 56-61. Rehfinger, A.; Hoffmann, U. Formation of Di-isobutene, Main ByProduct of Methyl Tertiary Butyl Ether Synthesis Catalyzed by Ion Exchange Resin. Chem. Eng. Technol. 1990, 13, 150156. Scharfe, G. Convert butenes to high octane oligomers. Hydrocarbon Process. 1973, 53 (4), 171-173. Unzelman, G. H. Global TrendssImpact of Fuel Reformulation. NPRA Annual Meeting, San Antonio, TX, 1997; Paper AM-9726. Vila, M.; Cunill, F.; Izquierdo, J. F.; Gonzales, J.; Hernandez, A. The role of by-products formation in methyl tert-butyl ether synthesis catalyzed by a macroporous acid resin. Appl. Catal. A 1994, 117, L99-L108.

Received for review January 31, 1997 Revised manuscript received June 3, 1997 Accepted June 17, 1997X IE9700932

X Abstract published in Advance ACS Abstracts, September 15, 1997.

Dimerization of Isobutene over Sulfonic

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