Etherification of Glycerol with Isobutene on Amberlyst 35 Ion Exchange Resin Catalyst in Presence of a Cationic Emulsifier

Transcription

1 Etherification of Glycerol with Isobutene on Amberlyst 35 Ion Exchange Resin Catalyst in Presence of a Cationic Emulsifier VASILEIOS MANGOURILOS 1, DORIN BOMBOS 1, TRAIAN JUGANARU 1, ION BOLOCAN 1, MIHAELA BOMBOS 2, DRAGOS CIUPARU 1 * 1 Petroleum Gas University of Ploieºti, 39 Bucureºti Bld., , Ploieºti, Romania 2 National Research Institute for Chemistry and Petrochemistry, ICECHIM, 202 Spl. Independentei, , Bucharest, Romania The improvement of mass transfer between glycerol and hydrocarbon phases during glycerol etherification with isobutene can be achieved by emulsifying the reaction mixture. This aspect was evidenced by performing an experimental study on glycerol etherification with isobutene using a cross linked cationic ion exchange resin Amberlyst 35 as catalyst with strong acid sites at the surface. Etherification experiments were conducted without emulsifier and in presence of an imidazolinic cationic emulsifier. The distribution of the reaction products is significantly modified in presence of the cationic emulsifier, towards the increase of the yields to di- and tri-ethers, products with low polarity and boiling points in the range of gas oils, which makes them suitable as components for diesel fuels. Keywords: glycerol etherification, isobutene, ion exchange resin, Amberlyst 35, cationic emulsifier In todays context of global warming there is a continuous research effort to find alternative routes to replace fossil fuels with environmentally friendly, renewable bio-derived fuels. One of the most commonly used alternative fuels is the biodiesel obtained at industrial scale by transesterification of vegetable fats with methanol, process in which glycerol results as a by-product. The amount of glycerol obtained from biodiesel production is about 1 kg for each 10 kg of biodiesel and it contains impurities such as sodium hydroxide, methanol and large amounts of water. Because the costs for glycerol purification is rather high, and the pharmaceutical or fine chemicals industry require small amounts of glycerol, most of the glycerol obtained in the biodiesel production process is used as fuel in cement ovens. Most of the recent studies [1-4] are dedicated to conversion of glycerol to fine chemicals such as surfactants, additives for the food industry, cosmetics, etc., rather than to larger scale commodity chemicals. However, the increasing demand of biodiesel on the market will significantly raise the amounts of glycerol by-product available and its conversion to a higher value commodity chemical will significantly impact both the price of biodiesel and the economic efficiency of bio-refineries. Conversion of glycerol to commodity chemicals may be split in two main categories: (i) oxidation or reduction to other products with three carbon atoms [1-4]; and (ii) reaction with organic compounds to form higher value products. Among the commodity chemicals that can be synthesized from glycerol the most common are propylene glycol, propionic acid, acrylic acid, propylic alcohol, isopropyl alcohol, allyl alcohol and acrolein. For example, conversion of glycerol to propylene glycol has been recently achieved in two steps: dehydration to acetol followed by hydrogenation to propylene glycol [5]. The alternative to convert glycerol into ethers has also attracted a great deal of research effort. By reacting glycerol with olefins a wide range of ethers may be obtained. Glycerol derived ethers counting as renewable fuel components can be used in the pool of high tonnage fuels. Since the first etherification of glycerol in the presence of some acid catalysts at relatively high temperatures [6], many studies were dedicated to glycerol etherification with isobutene using different catalysts, such as ion exchange resins, zeolites and homogenous acid catalysts [7-12]. However, the two reagents are very different in nature with glycerol a dense, highly polar and hydrophilic product, and isobutene a light, nonpolar and hydrophobic compound, so they show low reciprocal solubility and form two liquid phases while in contact. For these reasons, the performance of the etherification process has been proven to be strongly dependent on the mass transfer between the two liquid phases [8]. In the attempt to improve the mass transfer between the glycerol and the isobutene phases we performed the etherification reaction in the presence of an emulsifier. Here we report the results observed in etherification experiments of glycerol with isobutene using a strongly acidic cross linked ion exchange resin Amberlyst 35 as catalyst in the presence of an imidazolinic cationic emulsifier. We have selected this ion exchange resin based on its previously reported high catalytic activity and its narrow pore size distribution specific to highly cross linked ion exchange resins [8, 11]. Experimental part The experiments were performed in a 600 ml stainless steel Berghoff autoclave equipped with mechanical stirring. The autoclave is electrically heated, with automatic temperature control. Analytical grade glycerol provided by Chimopar and 99% purity isobutene purchased from Linde were the reagents used for all the experimental runs. The macroporous ion exchange resin catalyst Amberlyst 35 (A 35) was provided by Rohm and Haas. The catalyst is strongly acidic, with a concentration of sulphonic groups higher than 5 eq./kg, 300 Å average pore diameter and 50 m 2 /g specific surface area. Prior to its use, the catalyst was washed with methanol to remove water and other impurities from the surface, then dried and subsequently swelled with glycerol. The swelled catalyst was then loaded into the reactor together with the glycerol. For all experiments performed the concentration of the catalyst with respect to the amount of glycerol loaded in the reactor was 5 wt%. * dciuparu@upg-ploiesti.ro 1338

2 For some experiments 2-alkyl, 1-polyethylenepolyamine-imidazoline (ROT 1), provided by Atica Chemicals, was added to the reaction mixture as emulsifier. The amine nitrogen content of this emulsifier is 7.52 wt%, therefore it was conditioned by adding phosphoric acid 85 vol% till the ph of the reaction mixture reached 1.5. The concentration of the emulsifier in the reaction mixture was of 0.1 wt%. All experimental runs were performed for 5 h at 80 C, at isobutene/glycerol molar ratio of 1.5/1 and the ph of the glycerol phase of 1.5 unless specified otherwise. The reactor was always loaded with the same amount of glycerol, 151 g. The stirring rate was maintained at 1300 rot/min for each experiment and the pressure in the reactor was monitored continuously during the whole duration of each experiment. The analyses of starting materials and reaction products were performed by gas-chromatography, using an instrument from Agilent Technologies with FID detector, equipped with DB-WAX polar column of 30 m length and 0.32 mm inner diameter. The chromatographic column was operated between C, with nitrogen as carrier gas. The chromatographic analyses of reaction products have demonstrated the presence of two isomers for the glycerol mono-ether (1- and 2-t-butoxi-glycerol) and the glycerol di-ether (1,2- and 1,3-di-t-butoxy-propanol) and of the tri-ether (1,2,3-tri-t-butoxi-propane). Based on the material balance for each experiment glycerol conversion, and yields to mono-ethers, di-ethers, tri-ether and isooctenes were calculated. Results and discussion The first indication of the influence of the ROT 1 emulsifier was given by the evolution of pressure in the reactor during the experimental run under identical reaction conditions. When a first run was performed in the absence of emulsifier the pressure in the reactor dropped at half of its initial value in about 190 min time that will be further referred to as half-pressure time, while in presence of ROT 1 the half-pressure time was approximately 75 min. Since the pressure in the reactor is controlled by the amount of isobutene in the reactor, this behaviour suggests an isobutene consumption rate more than two times faster in the presence of ROT 1 than in the absence of emulsifier. Under these reaction conditions isobutene can be converted either to glycerol ethers, or, through oligomerization reactions, to isooctenes. However, because the A35 catalyst has sulphonic groups at the surface and it was swelled in glycerol the catalyst particles are covered with a film of glycerol at the surface. In order to react either with glycerol molecules or other isobutene molecule, an isobutene molecule needs to reach the catalyst surface, thus dissolving in the glycerol phase. Along these lines, the higher isobutene consumption rate observed in presence of ROT 1 suggests the emulsifier increases the rate of the mass transfer of isobutene to the glycerol phase. It is most likely that the intensification of the mass transfer is due to a higher contact surface between to the liquid phases resulting from the formation of an emulsion with the dispersed phase consisting of isobutene. Indeed, in a separate experiment at room temperature, using a Berzelius beaker, we mixed glycerol with 1-octene at one to three volume ratio similar to that used in our autoclave reactor in presence and in absence of ROT 1. While in the absence of the emulsifier the phases separated immediately after ceasing the stirring, in presence of ROT 1 the mixture formed a milky emulsion that was stable at room temperature for more than a week, showing our hypothesis is valid. Whether isobutene reacts with glycerol or oligomerizes may be determined from the distribution of the reaction products. Figures 1 to 3 below show the glycerol conversion, yields to ethers and the distribution of ethers obtained in the presence and in absence of emulsifier at ph 1.5. The conversion of glycerol decreases by few percents when ROT 1 emulsifier is added to the reaction mixture, suggesting the faster increase of the concentration of isobutene dissolved in the polar phase likely favors the oligomerization reaction. If this was the case, then the isobutene yield to isooctenes should increase and the amount of isobutene found in ethers should be smaller. The yields to different ethers depicted in figure 2 for the two cases with and without emulsifier show, however, Fig. 1. The influence of the ROT 1 emulsifier on the conversion glycerol that more isobutene reacted with glycerol when the reaction was performed in presence of ROT 1 than in its absence. The yield in mono-ethers in presence of ROT 1 is 15 % smaller, the yields of di-ethers are about the same for the two cases, but the yield in tri-ether is roughly 10 % higher when the reaction is performed in presence of emulsifier, which indicates the selectivity to ethers of isobutene is about 15 % higher in presence of ROT 1. This is most likely due to a more rapid mass transfer of isobutene to the glycerol phase making isobutene molecules readily available to glycerol reaction intermediates completing the glycerol conversion to tri-ether. Indeed, the isobutene conversion is higher, but the yield to isooctenes is also higher when the reaction is performed in presence of ROT 1, leading to overall lower isobutene selectivity to ethers as it can be seen in figure 3. This is equally important as the economic efficiency of the etherification process depends essentially on the selectivity to ethers. Fig.2. The influence of ROT 1 on the distribution of ethers 1339

3 Fig.3. The influence of ROT 1 on isobutene conversion and selectivity to ethers The influence of the ph of the polar phase on glycerol etherification with isobutene in presence of ROT 1 is shown in figures 4 to 8. It should be noted in figure 4 that the time required for the pressure to achieve half of its initial value, that is the half-pressure time, decreased by almost 50 % as the ph decreased from 5 to 1.5. Fig. 5. The influence of ph on the conversion of glycerol and isobutene Fig. 6. The influence of the ph on the yield of ethers Selectivity of isobutene to the three ethers in the range of ph investigated varied similarly to the yields to ethers as seen in figure 7, with the ph increase leading to the increase of isobutene selectivity to mono-ethers and to the decrease Fig.4. The influence of the ph on the half-pressure time of the selectivity to di- and tri-ethers, likely determined by the effects of surface coverage with irreversibly adsorbed The emulsifying efficiency of cationic emulsifiers emulsifier molecules. increases with their degree of ionization. The decrease of the ph in the polar phase increases the degree of ionization of the cationic emulsifier used, likely favoring the formation of an emulsion with drops of smaller diameter, which improves the mass transfer with favorable effects on the rate of isobutene consumption. Figure 5 shows the decrease of the ph leads to about 10 % increase in glycerol conversion, with conversion achieving about 80% for a ph of 1.5. Isobutene conversion to ethers shows a similar shape of the variation, but at a lower slope, as can be observed in the same figure. The relatively low value for the slope of variation of glycerol conversion with the ph is likely due to the high efficiency of the emulsifying process at 80 C likely caused by the lower viscosity of the continuous phase at this temperature. Thus, the mass transfer was more efficient and the process performances were maintained at high values. Fig.7. The influence of the ph on the selectivity to ethers The yield to mono-ethers increased as the ph rose, and The yield to isooctene was maintained at relatively high the yields to di- and tri-ether decreased when the ph values and approximately constant (about 48%) for the increased, as shown in figure 6. The improvement of process performances, that is the increasing of yields to range of ph studied as observed in figure 8. It is possible that the accessibility of isobutene molecules to the acid di- and tri-ether at low ph values, is probably due to mass sites of the polymer grain is not influenced by the distribution transfer improvement and to a more effective contact between mono-ethers and isobutene, as a consequence of the diameters of isobutene drops in the emulsion formed. The influence of the isobutene to glycerol molar ratio on of an efficient emulsifying of the two reagents when the the performance of the etherification process is showed in cationic emulsifier was strongly ionized. 1340

4 The yields to di- and tri- ethers increased when the isobutene to glycerol molar ratio increased, but yield to mono-ethers decreased as seen in figure 11. It is likely that with increasing the isobutene concentration, the viscosity of the reaction mixture decreases and the contact between the two phases improves, favoring the formation of di- and tri-ethers. The decrease in the yield to mono-ethers with the increase of the isobutene/glycerol molar ratio is most likely due to the consumption of this reaction intermediary in the etherification reaction with isobutene available at a higher concentration. Fig.8. The influence of the ph on the yield of isooctenes Fig.11. The influence of the isobutene/glycerol molar ratio on the yields to ethers Selectivity to mono-, di- and tri-ethers vary similar to the glycerol conversion as showed in figure 12. Fig.9. The influence of the isobutene/glycerol molar ratio on the half-pressure time figures 9 to 13. The increase of the amount of isobutene in the reactor has a negative impact on the size of the isobutene drops formed in the continuous phase, diminishing the mass transfer between the two phases and, implicitly, the consumption rate of isobutene, such that the half-pressure time increases when the isobutene to glycerol molar ratio increases, as seen in figure 9. Glycerol conversion and isobutene conversion to ethers increased with the increase of the isobutene/glycerol molar ratio, with similar variation curves showed in figure 11; this increase is likely due to the improvement of mass transfer between the two phases as a consequence of the decrease in the viscosity of the emulsion when the concentration of isobutene in the reacting mixture increases. Fig.12. The influence of the isobutene/glycerol molar ratio on the selectivity to ethers Fig.10. The influence of the isobutene/glycerol molar ratio on the conversion of the reagents Fig. 13. The influence of the isobutene/glycerol molar ratio on the yield to isooctene 1341

5 The isobutene/glycerol molar ratio has little influence on the yield to isooctenes as depicted in figure 13. The yield to isooctenes dropped approximately 2% when the isobutene/glycerol molar ratio increased from 1.5 to 2.5. Increasing of isobutene concentration in the reaction mixture does not seem to have an important influence on the mass transfer and likely does not improve the contact between isobutene molecules and the acid sites of the ion exchange resin. Conclusion The performance of glycerol etherification with isobutene on a strong acid ion exchange resin such as Amberlyst 35 is significantly influenced by the presence of a cationic emulsifier in the reaction mixture. The distribution of the reaction products is shifted towards di- and tri-ethers, products with lower polarity and lower boiling point, suitable as components for diesel fuel. The increase of the efficiency of the etherification process in presence of the cationic emulsifier was also evidenced by a considerably lower value of the reactor pressure halving time. The degree of emulsifier ionization also influences the process performance. A highly ionized emulsifier, corresponding to ph 1.5, favours obtaining of a direct emulsion with a distribution of drops diameters that improves the mass transfer and impedes blocking the acid sites of the catalyst with base emulsifier molecules. The increase of the amount of isobutene leads to the increase of the reactor pressure halving time; however, when the isobutene/glycerol molar ratio increases, glycerol conversion and yields to di- and tri-ethers also increase. Acknowledgement: The authors gratefully acknowledge the financial support of this work from the National Authority for Scientific Research through grants no PNCDI2/2007 and PNCDI2/2007. References 1.CARRETTIN, S., A. CORMA, IGLESIAS, M.,SÁNCHEZ,F., Applied Catalysis A: General, 291, 1-2, 2005, p PORTA F., PRATI, L., Journal of Catalysis, 224 (2), 2004, p PRATI L., VILLA A., PORTA, F., DI WANG, DANGSHENG S., Catalysis Today, 122 (3-4), 2007, p PEROSA, A., P. TUNDO, Industrial and Engineering Chemistry Research, 44(23), 2005,p MIYAZAWA T., KOSO S., KUNIMORI K., TOMISHIGE K., Applied Catalysis A, 329, (2007), p CLACENS, J.M., POUILLOUX,Y., BARRAULT, J., Applied Catalysis A: General, 227(1-2), 2002, p KLEPÁCOVÁ, K., MRAVEC, D., HÁJEKOVÁ, E., BAJUS, M., Petroleum and Coal, 45, (2003), p KARINEN, R.S., KRAUSE, A.O.I., Applied Catalysis A. General 306, (2006), p RUPPERT, A, M., MEELDIJINK,J.D., KUIPERS,B.W.M., H.ERNE, B.,H.,WECKHUYSEN, B.M., Chemistry European Journal, 14, (2008), p MELERO, J.A., VICENTE, G., MORALES, G., PANIAGUA, M., MORENO, J.M., ROLDAN, R., EZQUERRO,A.,PEREZ, C., Applied Catalysis A: General 346, (2008), p KLEPÁCOVÁ, K., MRAVEC, D.,KASZONYI, M.BAJUS, A., Applied Catalysis A. General 328, (2007), p GU, Y., AZZOUZI,A., POUILLOUX, Y., JEROME, F., BARRAULT,J., Green Chemistry, 10, (2008), p. 164 Manuscript received: