Product Distributions in Ethyl tert-Butyl Ether Synthesis over Different

Jan 26, 2009 - Product Distributions in Ethyl tert-Butyl Ether Synthesis over Different Solid Acid Catalysts. Levent Degirmenci, Nuray Oktar and Gulse...
0 downloads 0 Views 2MB Size
Download PDF
2566

Ind. Eng. Chem. Res. 2009, 48, 2566–2576

Product Distributions in Ethyl tert-Butyl Ether Synthesis over Different Solid Acid Catalysts Levent Degirmenci, Nuray Oktar, and Gulsen Dogu* Department of Chemical Engineering, Gazi UniVersity, Maltepe, 06570 Ankara, Turkey

Vapor-phase ethyl tert-butyl ether (ETBE) synthesis was carried out in a fixed-bed flow reactor in the presence of three different kinds of heteropolyacid catalysts and four different kinds of ion-exchange resins. An activity change in the order of silicotungstic acid > tungstophosphoric acid (Dawson) > tungstophosphoric acid (Keggin) was observed with heteropolyacids and Amberlyst 15 > Lewatit K-2629 > Amberlyst 35 > Amberlyst 16 with ion-exchange resins. In ETBE synthesis over heteropolyacid catalysts, diethyl ether and ethylene (valuable petrochemical feedstocks) formation was also observed at temperatures above 368 K. DRIFTS analysis of pyridine-adsorbed samples revealed changes in acidity in accordance with activity. The significance of Brønsted acidity on the catalyst activity was illustrated. The inhibiting effect of water on ethanol adsorption and hence the catalyst activity was demonstrated by interpretation of experiments conducted using Amberlyst15. Conversion of diluted ethanol to ETBE in the presence of Amberlyst 15 was found to be higher than the results obtained with pure ethanol using Amberlyst 16 and Amberlyst 35. 1. Introduction An increase in the population, especially in the last century, resulted in excessive use of energy and depletion of oil reserves. Consumption of oil resources, especially petroleum, was significantly increased mainly because of excessive use of transportation fuels. Up to 60% of oil consumption was reported to be due to transportation.1 In order to decrease the amount of petroleum used in transportation, some measures were applied to automobiles such as bioethanol usage as a substitute for gasoline. Bioethanol employment as a gasoline substitute seemed to be the finest precaution in preventing the depletion of oil resources. However, replacement of gasoline completely with bioethanol was reported to have the potential to cause a shortage in sugar and corn products.2 Consequently, improvements in the burning characteristics of gasoline appeared to be very important for efficient utilization of fuels. An increase in energy consumption has also created enviromental problems. In the last 30 years, regulations and legislative measures by governments had been applied to decrease the harmful impact of oil-based products on the environment. Gasoline was reformulated by the removal of lead compounds that had been employed to increase the octane number. Oxygenates as alternatives to lead compounds were used as octane enhancers. Besides increasing the octane number, oxygenates also increased the combustion efficiency by decreasing exhaust CO and hydrocarbon emissions.1,3-5 Alcohols and tertiary ethers are the main compounds proposed as gasoline blending oxygenates. Ethanol and methanol have very high octane numbers and are being used as additives or transportation fuels, in certain countries. However, their high Reid vapor pressure (RVP) in the blend causes evaporation from gasoline, which is quite undesired. Also, phase separation in the fuel tanks in the presence of a small amount of water is another limitation of using ethanol-blended gasoline. As a consequence, the main concern in oxygenate production had been the synthesis of oxygenates with high octane number and low RVP, which led to investigations on tertiary ethers. Methyl tert-butyl ether (MTBE) is the most widely produced tertiary ether worldwide * To whom correspondence should be addressed. Fax: 90 (312) 2308434. E-mail: [email protected].

because of the availability of methanol.5 However, objections arising from environmental and health problems against the use of MTBE in gasoline reformulation limited its application. MTBE, following the discovery of being an underground water pollutant, has been banned in certain countries.2 Its water solubility is quite high (∼42 000 mg/L at 25 °C).6 Alternatives to MTBE, such as ethyl tert-butyl ether (ETBE), tert-amyl methyl ether (TAME), and tert-amyl ethyl ether (TAEE) were reported to cost more in production. However, in countries with large areas suitable for agriculture, ethanol-based oxygenates and especially ETBE are considered to be a good alternative to MTBE.7,8 ETBE as an alternative has also emerged with its superior properties such as higher octane number, lower RVP, and much lower water solubility compared to MTBE.1,9 In a recent study, it was found that MTBE and ethanol, when added to gasoline, increased the RVP of the mixture.10 On the other hand, ETBE was revealed to have a decreasing effect on the RVP of the mixture.10 Tertiary ether synthesis, such as TAME and TAEE, in the presence of an acidic ion-exchange resin was investigated earlier by the authors of this work.11-15 Reaction mechanisms of TAEE and TAME production were determined from DRIFTS analysis, and reaction and adsorption rate parameters were reported in these publications. Mechanisms of etherification reactions were also investigated in the earlier kinetic studies of Tejero et al.,16 Iborra et al.,17 Linnekoski et al.,18 and Rihko et al.19 Equilibrium limitations were also reported to become quite significant in ETBE synthesis, with an increase in temperature.20 There are a variety of catalysts used in tertiary ether synthesis. Etherification reactions were initially conducted in the presence of mineral acids, like sulfuric acid. However, because of corrosion, environmental concerns, and difficulties encountered in the separation of the catalyst from the reaction mixture, mineral acids were replaced by acidic ion-exchange resins and heteropolyacid catalysts.21 Acidic ion-exchange resins constitute a large part of etherification studies, with many investigations reported in the literature. Acidic ion-exhange resins are known to be polymeric macroporous structures with -SO3H groups, which determine the activity of the catalyst.13 Etherification of isoamylenes was

10.1021/ie801508r CCC: $40.75  2009 American Chemical Society Published on Web 01/26/2009

Ind. Eng. Chem. Res., Vol. 48, No. 5, 2009 2567 Table 1. Physical Properties of Ion-Exchange Resins property structure matrix form cross-linking level exchange capacity (mequiv/g) surface area (m2/g) porosity (%) avarage pore diameter (nm) particle diameter (mm) a

Amberlyst A-15 a

MR styrene-divinylbenzene H+ H 5.2b 53c 33e 30c 0.74g

Amberlyst A-35

Amberlyst A-16

Lewatit K-2629

MR styrene-divinylbenzene H+ H 5.2c 50c 29e 30c 0.15-0.25h

MR styrene-divinylbenzene H+ L 4.8c 30c 25f 25c 0.38-0.45h

MR styrene-divinylbenzene H+ 4.8d 40d

Macroreticular. b Reference 14. c Reference 31. d Reference 32. e Reference 29. f Reference 30. g Reference 22. h Reference 33.

Figure 1. Effect of the temperature on the fractional conversion of (a) IB and (b) EtOH over heteropolyacid catalysts (IB/EtOH: 0.17).

investigated with Amberlyst 15, Amberlyst 35, Amberjet 1500H, Purolite CT-175, and Purolite CT-275 catalysts in a study conducted by Cruz et al., in which Amberlyst 35 was found to exhibit the highest activity among other catalysts.22 Yang et al. conducted ETBE synthesis in the presence of Amberlyst 15 with tert-butyl alcohol (TBA) and ethanol used as the reactants. It was found that conversion of TBA to ETBE had decreased in the presence of water in the feed mixture. Results also revealed that the temperature had an increasing effect on conversion.23 Heteropolyacids have gained increasing attention in recent years, exhibiting good activity in vapor-phase ETBE production. They are known as strong acid catalysts and used in the solid state as acid and oxidation catalysts.24 Keggin- and WellsDawson-type heteropolyacids are the most well-known molecular configurations. Dawson-type heteropolyacids exhibit a loose structure, enabling the transport of polar molecules inside the

catalyst, which has an increasing effect on redox- and acidtype catalytic reactions.25,26 There are a limited number of studies for the synthesis of oxygenates using heteropolyacid catalysts. MTBE production in the presence of tungstophosphoric acid-Dawson (TPA-D) was accomplished by Baronetti et al.27 Their results indicated that the activity of TPA-D was higher than that of Amberlyst 15 at 100 °C. Dehydration of ethanol to produce diethyl ether (DEE) using different heteropolyacid catalysts was investigated by Varıs¸lı et al.28 That work showed that silicotungstic acid (STA) was more active than tungstophosphoric acid-Keggin (TPA-K) and molybdophosphoric acid (MPA) in the etherification of ethanol. These results were explained by the higher number of protons present in the STA structure.28 Besides, STA was reported to be more stable than TPA and MPA.

2568 Ind. Eng. Chem. Res., Vol. 48, No. 5, 2009

353-368 K for acidic ion-exchange resins and of 353-523 K for heteropolyacid catalysts. Effects of the temperature and isobutene/ethanol (IB/EtOH) molar ratios on the product distributions were investigated in detail. The effect of the presence of water on the activity of Amberlyst 15, which gave the highest ETBE yields at 353 K, was also investigated by using a diluted EtOH (95%, v/v) feed composition. Activities of catalysts were compared by evaluating the results of reaction experiments in accordance with pyridine adsorption diffuse reflectance FT-IR spectroscopy (DRIFTS) analyses, by which information on the acid sites of catalysts was obtained and interpreted. 2. Experimental Section Figure 2. Effect of the temperature on the fractional conversion of IB to ETBE in the presence of acidic resin catalysts (IB/EtOH: 0.14-0.17).

The major objective of the present study is to investigate the best operating conditios of vapor-phase ETBE synthesis using a wide variety of heteropolyacids and acidic resin catalysts. A detailed comparison of the activities of various acidic ionexchange resins (Amberlyst 15, Amberlyst 16, Amberlyst 35, and Lewatit K2629) and also heteropolyacid catalysts (STA, TPA-K, and TPA-D) was made, and also the effects of operating conditions on product distributions were investigated. Etherification reactions were conducted in a temperature interval of

2.1. Chemicals and Catalysts. Ethanol (EtOH; absolute, 99.99%) supplied from Merck and isobutene (IB) supplied from Air Products were used as reactants in ETBE synthesis. Acidic ion-exchange resins, Amberlyst 16 and Lewatit K-2629 used in etherification reactions, were purchased from Fluka, and Amberlyst 35 and Amberlyst 15 were purchased from Rohm & Haas and Sigma-Aldrich, respectively. The physical properties of ion-exchange resins are given in Table 1. Heteropolyacid catalysts, tungstophosphoric acid-Keggin (TPA-K), tungstophosphoric acid-Dawson (TPA-D), and silicotungstic acid (STA) were obtained from Acros Organics, Spectrum Chemical, and Sigma-Aldrich, respectively.

Figure 3. Effect of the temperature on product distribution using STA (IB/EtOH: 0.17): (a) ETBE, DEE, and ethylene selectivities with respect to EtOH; (b) fractional conversion of IB to ETBE and fractional conversion of EtOH to DEE and ethylene.34

Ind. Eng. Chem. Res., Vol. 48, No. 5, 2009 2569

Figure 4. Effect of the temperature on product distrubution using TPA-K (IB/EtOH: 0.17): (a) ETBE, DEE, and ethylene selectivities with respect to EtOH; (b) fractional conversion of IB to ETBE and fractional conversion of EtOH to DEE and ethylene.34

2.2. Catalytic Reactions. Etherification of EtOH with IB was carried out in a fixed-bed flow reactor, in the vapor phase in the presence of acidic ion-exchange resins (Amberlyst 15, Amberlyst 16, Amberlyst 35, and Lewatit K2629) and heteropolyacid catalysts (TPA-K, TPA-D, and STA). A catalyst amount of 0.1 g was used in all experiments. This corresponds to a space time of 8.85 × 10-4 g · min/cm3, which is defined as the ratio of the catalyst mass to the total flow rate measured at room temperature. In the experiments conducted in the presence of heteropolyacid catalysts, the temperature interval was selected as 353-523 K and the IB/EtOH molar ratio was varied between 0.17 and 0.5. In the experiments conducted in the presence of acidic ion-exchange resin catalysts, stability was taken into account, the temperature interval was kept lower (353-368 K), and the IB/EtOH molar ratio was varied between 0.05 and 0.5. The catalysts were thermally treated at 110 °C for 2 h in order to achieve desorption of physically adsorbed water. Thermally treated catalyst samples were used in the reaction experiments and characterization analysis. EtOH was introduced into the reaction system with the aid of an evaporator and transferred to a reactor with helium employed as the inert carrier. IB was fed from a separate line to the reactor. The total volumetric flow rate of the reactants and helium was kept constant at 113 mL/min in all experiments. The helium flow rate was changed between 53 and 71 cm3/min to adjust the feed composition. All of the connection lines were held at 120 °C in order to maintain vapor-phase conditions. The effect of water on the ETBE synthesis was also investigated by reaction experiments con-

ducted in the presence of Amberlyst 15. The feed composition was altered by diluting EtOH to 95% (v/v) in these experiments. Reactant and product analyses were conducted using a gas chromotograph (Agilent 6890N) equipped with flame ionization detector with a Chromosorb AW (15% FFAB) column. 2.3. Characterization of the Acidic Nature of Catalysts. Thermally treated catalysts were first treated with pyridine and then analyzed by diffuse reflectance FT-IR (DRIFTS) in order to obtain data on acid sites. These results were evaluated in accordance with reaction experiments to clarify differences in catalyst activities. Analyses were carried out at room temperature by a Perkin-Elmer Spectrum One instrument with samples prepared by mixing 0.02 g of catalyst and 0.07 g of KBr in each analysis. Acid sites of the catalysts were determined by analysis in an interval of 400-4500 cm-1 wavelength. 3. Results and Discussion 3.1. Activity Comparison of Heteropolyacid and Acidic Resin Catalysts at Different Temperatures. The effect of the temperature on the conversion of IB over heteropolyacid catalysts is demonstrated in Figure 1a, in a temperature interval of 353-523 K. Conversion of IB to ETBE increased with an increase in the temperature to 368 K. IB conversion decreased at temperatures higher than 368 K, which is mainly due to the approach to equilibrium conversion of ETBE synthesis. Equilibrium conversions evaluated using the equilibrium constant relation given by Jensen and Datta20 are also shown in Figure

2570 Ind. Eng. Chem. Res., Vol. 48, No. 5, 2009

Figure 5. Effect of the temperature on product distrubution using TPA-D (IB/EtOH: 0.17): (a) ETBE, DEE, and ethylene selectivities with respect to EtOH; (b) fractional conversion of IB to ETBE and fractional conversion of EtOH to DEE and ethylene.

1a. Figure 1a revealed the change in catalyst activities in an order of STA > TPA-D > TPA-K for conversion of IB to ETBE. According to a model proposed by Shikata et al.,25 the etherification reaction between polar EtOH and apolar IB was thought to occur in a pseudoliquid state by which sorption of IB inside the catalyst structure was enhanced in the presence of EtOH. The elliptical shape of TPA-D was more flexible than that of TPA-K, enabling efficient adsorption of polar molecules such as EtOH. The activity of STA was found to be higher than that of TPA-D, which can be explained by higher pKa values determined in recent studies.25,26 The number of protons of STA (four protons per mole) is higher than that of the TPAtype heteropolyacid catalysts (three protons per mole). A higher activity of STA than the other heteropolyacids was also reported by Varıs¸lı et al.28 in the dehydration reaction of EtOH to produce DEE, and this result was explained by the presence of more protons in the STA structure than the others. The variation of the overall EtOH conversion in the same temperature range is given in Figure 1b. The qualitative behavior of EtOH conversion is quite similar to the conversion trend of IB in the temperature range of 350-400 K. In this temperature range, the main product is ETBE. The lower values of EtOH conversion are simply due to the excess presence of EtOH (IB/EtOH molar ratio was 0.17 in these experiments). A sharp increase in the overall EtOH conversion over 400 K is due to the formation of side products, DEE and ethylene, which were formed as a result of dehydration of EtOH at such high temperatures. The overall EtOH conversion was again in the order of STA > TPA-D > TPA-K,

supporting the statements regarding the activity comparison mentioned above. The effect of the temperature on the conversion of IB in the presence of acidic ion-exchange resin-type catalysts is also demonstrated in Figure 2. A comparison of the activities of ionexchange resins revealed that Amberlyst 15 had exhibited the highest activity. The activity order of ion-exchange resins is Amberlyst15 > Lewatit K-2629 > Amberlyst 35 > Amberlyst 16 (Figure 2). Amberlyst 16, which showed the lowest activity, has the lowest surface area, and also its hydrogen ion-exchange capacity is low (Table 1). A comparison of the results given in Figure 2 with the results reported in Figure 1a revealed that, at temperatures lower than 368 K, Amberlyst 15 was also more active than heteropolyacids. At 368 K, the IB conversions on Amberlyst 15 and STA were comparable. Although heteropolyacids are more acidic than resin catalysts, they have very low surface area values (on the order of 1-5 m2/g), which limits their activity especially at lower temperatures. Reaction experiments conducted in the presence of heteropolyacid catalysts revealed high conversions to side products over 423 K, which were determined as DEE and ethylene. These side products were formed as a result of the dehydration reaction of EtOH. Besides DEE and ethylene, the formation of trace amounts of diisobutene, triisobutene, and acetaldehyde was also observed over 423 K. Product distributions were investigated in terms of ETBE, DEE, and ethylene selectivities with respect to EtOH, together with reactant conversions to different products for STA, TPA-K, and TPA-D type catalysts in Figures 3-5,

Ind. Eng. Chem. Res., Vol. 48, No. 5, 2009 2571

Figure 6. DRIFTS analysis for pyridine-adsorbed (a) heteropolyacid catalysts and (b) acidic ion-exchange resins.

respectively. Figure 3a revealed a stable 100% ETBE selectivity with STA, in a temperature interval of 353-368 K. At temperatures higher than 368 K, the selectivity of ETBE decreased as a result of DEE formation. DEE formation with EtOH dehydration reaction over heteropolyacid catalysts was also reported by Varıs¸lı et al.28 and also in our recent publication.34 DEE selectivity passed through a maximum at 423 K; at higher temperatures, some ethylene formation along with DEE was observed, decreasing its selectivity (Figure 3a). DEE selectivity approached 100% at 423 K. Above this temperature, ethylene formation increased while DEE selectivity decreased. This is due to the thermodynamic limitations of DEE formation reaction and decomposition of DEE to ethylene at higher temperatures. An ethylene selectivity value of over 0.7 observed at 523 K was highly promising for the production of ethylene from EtOH. Fractional conversion of IB to ETBE obtained in the presence of STA passed through a maximum at 368 K, while formation of DEE from EtOH was observed at higher temperatures (Figure 3b). The fractional conversion value of EtOH to DEE reached its highest value at 473 K. Ethylene formation at higher temperatures is also illustrated in Figure

3b. Similar product distributions were obtained in the presence of TPA-K and TPA-D (Figures 4 and 5). A comparison of the results given in Figures 3-5 also indcated that STA was more active than the other two heteropolyacid catalysts. 3.2. Diffuse Reflectance FT-IR Spectroscopy (DRIFTS) Experiments. In order to support the activity test results obtained with different solid acid catalysts, characterization experiments were conducted with DRIFTS analyses of pyridineadsorbed samples (Figure 6). Heteropolyacid catalysts revealed the presence of both Lewis and Brønsted acid sites. The band observed at 1536 cm-1 corresponds to the pyridinium ion adsorbed on the Brønsted acid sites and the band observed at 1439 cm-1 corresponds to the CN vibration for coordinatively bonded pyridine on the Lewis acid sites.36,37 The band observed at 1486 cm-1 corresponds to a mode of physically adsorbed pyridine. A comparison of the acid sites indicated that the highest relative Brønsted acidity had been obtained with the STA sample, which strongly supported its highest activity in ETBE synthesis. Brønsted acid sites of TPA-K and TPA-D were close in intensity. However, the relative intensity of the Lewis

2572 Ind. Eng. Chem. Res., Vol. 48, No. 5, 2009

Figure 8. Effect of the feed composition on DEE selectivity (based on EtOH) in the presence of (a) STA, (b) TPA-K, and (c) TPA-D.

Figure 7. Effect of the IB/EtOH molar ratio on the fractional conversion of IB to ETBE in the presence of (a) STA, (b) TPA-K, and (c) TPA-D.

acid sites of TPA-D (at 1439 cm-1) was determined to be higher than that of TPA-K. In fact, as illustrated in Figure 1a, the activity of TPA-D was also higher than that of TPA-K. This result indicated that Lewis acid sites also contribute to the formation of ETBE. On the basis of studies conducted by Shikata et al.25 and the interpretation of reaction experiments, it was thought that higher EtOH adsorption had been achieved with TPA-D because of higher acidity enhancing higher sorption of IB. Our conclusion is that the higher activity of STA is probably mainly due to the presence of a higher number of protons (four protons per mole) in this material than TPA-K and TPA-D (three protons per mole). DRIFTS analysis of pyridine-adsorbed ion-exchange resins exhibited acid sites in the wavelength range between 1550 and 1624 cm-1 (Figure 6b). These peaks correspond to the Brønsted

acid sites, due to the -SO3H groups. No Lewis acid sites were observed in these resin-type catalysts. The highest peak intensity was obtained with Amberlyst 15, explaining its highest activity. Relative peak intensites changed as Amberlyst15 > Lewatit K-2629 > Amberlyst 35 > Amberlyst 16, in the same order as the catalyst activity, as expected. 3.3. Effect of the Feed Composition on Product Distributions. Effect of the IB/EtOH molar ratio for the conversion of IB to ETBE in the presence of heteropolyacids is given in Figure 7. Quite high IB conversion values were observed in the presence of excess EtOH with a IB/EtOH molar ratio of 0.17. This is especially quite significant with STA. This may be due to the penetration of EtOH into the pseudoliquid structure of heteropolyacids, which might enhance sorption of IB into the solid acid lattice. It was interesting to observe that IB conversion passed through a minimum with an increase in the IB/EtOH mole ratio in the feed. This behavior was observed for all heteropolyacid catalysts and especially at lower temperatures. An increase in the conversion of IB to ETBE with the IB/EtOH

Ind. Eng. Chem. Res., Vol. 48, No. 5, 2009 2573

Figure 9. Effect of the feed composition on ethylene selectivity (based on EtOH) in the presence of (a) STA, (b) TPA-K, and (c) TPA-D.

molar ratio was observed in a range of 0.3-0.5 IB/EtOH molar ratio, as seen from the figure. This increase is especially significant at lower temperatures, at which the surface reaction contribution may be more important than reaction within the bulk of the pseudoliquid lattice of the catalyst. A decrease of the EtOH concentration may allow more sites to become available for the adsorption of IB on the catalyst surface. An increase in the conversion of IB to ETBE for all IB/EtOH molar ratios has been detected in a temperature range of 353-368 K. Conversion of IB to ETBE decreased at temperatures above 368 K due to side-product formation. The effect of the feed composition on DEE selectivity based on EtOH in the presence of heteropolyacid catalysts is given in Figure 8. At the temperature of maximum DEE selectivity (423 K), a decrease due to an increase in the IB amount in the feed composition was observed for all catalysts. The effect of the feed composition on EtOH-based ethylene selectivity was also determined and shown in Figure 9. The change in selectivity with an increase in the IB concentration in the feed was found to be negligible, which indicated minute interaction of IB with EtOH at temperatures higher than 423 K. ETBE selectivity values with respect to EtOH were also determined in the presence of

Figure 10. Effect of the feed composition on ETBE selectivity (based on EtOH) in the presence of (a) STA, (b) TPA-K, and (c) TPA-D.

heteropolyacid catalysts and are given in Figure 10. ETBE selectivity increased with an increase of the IB concentration in the feed, as expected. The effect of the IB/EtOH molar ratio on the conversion of IB to ETBE in the presence of Amberlyst 15 is given in Figure 11. Some increase of the conversion of IB to ETBE was observed with an increase in the IB/EtOH molar ratio, especially at lower temperatures. The decreasing trend observed with increasing temperature in the conversion of IB to ETBE is essentially due to equilibrium limitation. The effects of the temperature and IB/EtOH molar ratios on IB conversion to ETBE are given in Figure 12 for reaction experiments conducted in the presence of Lewatit K-2629. Investigation of the figure revealed a quite similar behavior to that of Amberlyst 15. On the other hand, no significant effect of the feed composition was observed with Amberlyst 16 and Amberlyst 35, which were much less active than Amberlyst 15 (Figure 13) and Lewatit K-2629 (Figure 14).

2574 Ind. Eng. Chem. Res., Vol. 48, No. 5, 2009

Figure 11. Effect of the IB/EtOH molar ratio on ETBE production in the presence of Amberlyst 15.

Figure 12. Effect of the IB/EtOH molar ratio on ETBE production in the presence of Lewatit K2629.

Figure 13. Effect of the IB/EtOH molar ratio on ETBE production in the presence of Amberlyst 16.

3.4. Effect of the Water Presence on the Catalyst Performance. The effect of the water presence in the feed stream on the activity of Amberlyst 15 was investigated for a IB/EtOH molar ratio of 0.14 (Figure 15). The decrease of observed conversions of IB to ETBE in the presence of water exhibited the inhibiting effect of water in the reaction. This inhibiting effect is more significant at lower temperatures. This behavior can be explained by competitive adsorption of EtOH and water on the catalyst surface. Water is known to have a higher affinity than EtOH to adsorb on acid sites of Amberlyst 15. By the adsorption of water, some of the active sites are blocked for the adsorption of reactants, namely, EtOH and IB. The effect

Figure 14. Effect of the IB/EtOH molar ratio on ETBE production in the presence of Amberlyst 35.

Figure 15. Effect of the water presence in the feed stream on the activity of Amberlyst 15 (IB/EtOH: 0.14).

Figure 16. Effect of the feed composition in the presence of water on the activity of Amberlyst 15 (H2O/EtOH: 5/95).

of the feed composition in the presence of water on the activity of Amberlyst 15 is given in Figure 16. Reaction results demonstrated an increase in the conversion of IB to ETBE with increasing IB/EtOH molar ratio and temperature. Fractional conversions in the presence of Amberlyst 15 with 95% (v/v) EtOH in the feed composition were found to be higher than conversions obtained in the presence of Amberlyst 16 and Amberlyst 35 with 100% EtOH in the feed composition. Results showed that Amberlyst 15 was the most suitable ion-exchange resin, giving the highest conversions with pure EtOH. Moreover, conversion values obtained for the experiments with water in the feed mixture were higher than those of Amberlyst 16 and Amberlyst 35.

Ind. Eng. Chem. Res., Vol. 48, No. 5, 2009 2575

4. Concluding Remarks

Acknowledgment

The activities of different heteropolyacid catalysts were compared by reaction experiments conducted in a temperature interval of 353-523 K at a IB/EtOH molar ratio of 0.17. Fractional conversions of IB to ETBE revealed that the highest activity had been obtained in the presence of STA. An increase in the conversion of IB to ETBE was observed with increasing temperature up to 368 K. Above this temperature, equilibrium limitations become significant. DEE was observed as a side product at temperatures higher than 368 K, which had a decreasing effect on conversions to ETBE, along with equilibrium limitations. At temperatures higher than 423 K, the formation of ethylene was also observed, giving a maximum at 523 K. Quite high ethylene yield values obtained at this temperature indicated the possibility of the production of this petrochemical feedstock from bioethanol. Conversion of IB to ETBE in the presence of acidic ion-exchange resins was also investigated, and the highest activity was achieved in the presence of Amberlyst 15. Product distribution in the presence of heteropolyacids showed very high ETBE selectivities, approaching 100% at temperatures as high as 368 K. At higher temperatures, sideproduct formation occurred. DEE remained the main side product up to a temperature of 423 K; at higher temperatures, ethylene formation was also observed, which decreased EtOH selectivity to DEE. Similar selectivity and conversion patterns were observed with heteropolyacid catalysts. DRIFTS analysis with pyridine-adsorbed catalysts indicated that the Brønsted acidity of STA was highest, which also showed the highest activity. This result indicated the importance of the Brønsted acidity on the activity of these catalysts in ETBE synthesis. In the case of acidic resin catalysts, all of the acidity is due to Brønsted acid sites. Results obtained with the heteropolyacid catalysts indicated that Lewis acid sites also contributed to the etherification reaction. An increase in the conversion of IB to ETBE was observed with the heteropolyacid catalysts by an increase in the IB/EtOH mole ratio between 0.30 and 0.50, especially at lower temperatures. This was concluded to be due to the high coverage of the surface by EtOH at high EtOH feed ratios, causing a blocking effect for adsorption of IB. On the other hand, at much lower IB/EtOH ratios, penetration of EtOH into the pseudoliquid structure of heteropolyacids which might enhance sorption of IB into the solid acid lattice, causes higher conversion values. IB conversion to ETBE in the presence of Amberlyst 15 decreased with an increase in the temperature for all IB/EtOH molar ratios due to equilibrium limitation at temperatures higher than 353 K. Among the resins investigated, Lewatit K2629 exhibited higher activity than Amberlyst 16 and Amberlyst 35 and emerged as an alternate catalyst for ETBE production. The highest ETBE yield (about 35%) was obtained with Amberlyst 15 at a IB/EtOH mole ratio of 0.5 at 353 K, which was close to the equilibrium conversion. The effect of the water presence in the feed was investigated in the presence of Amberlyst 15. Because of the strong adsorption of water on the active sites, the catalyst activity decreased in the presence of water. However, the results revealed a higher activity of Amberlyst 15 (in the presence of water) than Amberlyst 16 and Amberlyst 35 in experimental conditions where 100% EtOH was used. Despite the fact that water had an inhibiting effect in EtOH adsorption, and hence the activity, diluted EtOH had still set a good alternative to pure EtOH to reduce EtOH costs in ETBE production with Amberlyst 15.

Financial support from TUBITAK Grant 106M287 and a Gazi University Research Fund is gratefully acknowledged. The authors also thank Prof. Timur Dogu of METU for his valuable contributions. Literature Cited (1) Dogu, T.; Varıs¸lı, D. Alcohols as Alternatives to Petroleum for Environmentally Clean Fuels and Petrochemicals. Turk. J. Chem. 2007, 31, 551–567. (2) Manzini, F. Inserting renewable fuels and technologies for transport in Mexico City Metropolitan Area. Int. J. Hydrogen Energy 2006, 31, 327– 335. (3) Obali, Z., Dogu, T. Activated carbon-tungstophosphoric acid catalysts for the synthesis of tert-amyl ethyl ether (TAEE). Chem. Eng. J. 2007in press. (4) Brockwell, H. L.; Sarathy, P. R.; Trottav, R. Synthesize ethers. Hydrocarbon Process. 1991, 133–141. (5) Schmidt, R. J.; Bogdan, P.; Gilsdorf, N. L. Meeting the challenge of reformulated gasoline. CHEMTECH 1993, 41–46. (6) http://www.efoa.org/mtbe/pdf/appendix1.pdf (7) Ahmed, F. E. Toxicology and human health effects following oxygenated or reformulated gasoline. Toxicol. Lett. 2001, 123, 89–113. (8) Malc¸a, J.; Freire, F. Renewability and life-cycle energy efficiency of bioethanol and bio-ethyl tertiary-butyl ether (bioETBE): Assessing the implications of allocation. Energy 2006, 31, 3362–3380. (9) Ancillotti, F.; Fattore, V. Oxygenate fuels: market expansion and catalytic aspect of synthesis. Fuel Process. Technol. 1998, 57, 163–194. (10) Da Silva, R.; Cataluna, R.; Menezes, E. W.; Samios, D.; Clarisse, M.; Piatnicki, S. Effect of additives on the antiknock properties and Reid vapor pressure of gasoline. Fuel 2005, 84, 951–959. (11) Oktar, N.; Murtezaoglu, K.; Dogu, G.; Gonderten, I.; Dogu, T. Etherification Rates of 2-Methyl 2-Butane and 2-Methyl-1- Butene with Ethanol for Environmentally Clean Gasoline Production. J. Chem. Technol. Biotechnol. 1999, 74, 155–161. (12) Oktar, N.; Murtezaoglu, K.; Dogu, T.; Dogu, G. Dynamic analysis of adsorption equilibrium and rate parameters of reactants and products in MTBE, ETBE and TAME production. Can. J. Chem. Eng. 1999, 77, 406– 412. (13) Boz, N.; Dogu, T.; Murtezaoglu, K.; Dogu, G. Appl. Catal. A 2004, 268, 175–182. (14) Dogu, T.; Boz, N.; Aydin, E.; Oktar, N.; Murtezaoglu, K.; Dogu, G. Ind. Eng. Chem. Res. 2001, 40, 5044–5051. (15) Boz, N.; Dogu, T.; Murtezaoglu, K.; Dogu, G. Catal. Today 2005, 100, 419–424. (16) Tejero, J.; Cunill, F.; Izkuierdo, F.; Iborra, M.; Perra, D. Scope and limitations of mechanistic inferences from kinetic studies on acidic macroporous resins. Appl. Catal. A 1996, 134, 21. (17) Iborra, M.; Izquierdo, J. F.; Cunill, F.; Tejero, J. Application of the response surface methodology to the kinetic study of the gas phase addition of ethanol to isobutene on a sulfonated styrene divinylbenzene resin. Ind. Eng. Chem. Res. 1992, 31, 1840. (18) Linnekoski, J. A.; Krause, A. O.; Rihko, L. K. Kinetics of the heterogenously catalyzed formation of tert-amyl-ethyl ether. Ind. Eng. Chem. Res. 1992, 31, 1840. (19) Rihko, L. K.; Paakonen, P. K.; Krause, A. O. Kinetic model for the etherification of isoamylenes with methanol. Ind. Eng. Chem. Res. 1997, 36, 614–621. (20) Jensen, K. L.; Datta, R. Ethers from ethanol. 1. Equilibrium thermodynamic analysis of the liquid phase ethyl tert-butyl ether reaction. Ind. Eng. Chem. Res. 1995, 34, 392–399. (21) Heese, F. P.; Dry, M. E.; Mo¨ller, K. P. Single stage synthesis of diisopropyl ethersan alternative octane enhancher for lead-free petrol. Catal. Today 1999, 49, 327–335. (22) Cruz, V. J.; Izquierdo, J. F.; Cunill, F.; Tejero, J.; Iborra, M.; Fite, C. Acid ion-exchange resins catalysts for the liquid-phase dimerization/ etherification of isoamylenes in methanol or ethanol presence. React. Funct. Polym. 2005, 65, 149–160. (23) Yang, B. L.; Yang, S. B.; Yao, R. Synthesis of ethyl tert-butyl ether from tert-butyl alcohol and ethanol on strong cation-exchange resins. React. Funct. Polym. 2000, 44, 167–175. (24) Mizuno, N.; Misono, M. Heterogeneous Catalysis. Chem. ReV. 1998, 98, 199–217. (25) Shikata, S.; Nakata, S.; Okuhara, T.; Misono, M. Catalysis by Heteropoly Compounds. 32. Synthesis of Methyl tert-Butyl Ether Catalyzed by Heteropolyacids Supported on Silica. J. Catal. 1997, 166, 263–271.

2576 Ind. Eng. Chem. Res., Vol. 48, No. 5, 2009 (26) Bielanski, A.; Lubanska Micek-Ilnicka, A.; Pozniczek, J. Polyoxometalates as the catalysts for tertiary ethers MTBE and ETBE synthesis. Coord. Chem. ReV. 2005, 249, 2222–2231. (27) Baronetti, G.; Briand, L.; Sedran, U.; Thomas, H. Heteropoly-acid based catalysis. Dawson acid for MTBE synthesis in gas phase. Appl. Catal. A 1998, 172, 265–272. (28) Varisli, D.; Dogu, T.; Dogu, G. Ethylene and diethyl-ether production by dehydration reaction of ethanol over different heteropolyacid catalysts. Chem. Eng. Sci. 2007, 62, 5349–5352. (29) Reis, S. C. M.; Lachter, E. R.; Nascimento, R. S. V.; Rodrigues, J. A.; Reid, M. G. Transesterification of Brazilian vegatable oils with methanol over ion-exchange resins. J. Am. Oil Chem. Soc. 2005, 82, 661– 665. (30) Paako¨nen, P. K.; Krause, A. O. I. Diffusion and chemical reaction in isoamylene etherification within a cation-exchange resin. Appl. Catal. A 2003, 245, 289–301. (31) Honkela, M. L.; Root, A.; Lindblad, M.; Krause, A. O. I. Comparison of ion Exchange resin catalysts in the dimerisation of isobutene. Appl. Catal. A 2005, 295, 216–223. (32) Fite, C.; Tejero, J.; Iborra, M.; Cunill, F.; Izquierdo, J. F.; Parra, D. The effect of the reaction medium on the kinetics of the liquidphase addition of methanol to isobutene. Appl. Catal. A 1998, 169, 165– 177.

(33) Paakkonen, P. K.; Krause, A. O. I. Comparative study of TAME synthesis on ion-exchange resin beads and a fibrous ion-exchange catalyst. React. Funct. Polym. 2003, 55, 139–150. (34) Degirmenci, L.; Oktar, N.; Dogu, G. Effect of Heat Treatment on the Activity of Heteropoly Acid Catalysts in ETBE Synthesis. Turk. J. Chem. 2008, in press. (35) Chen, L. F.; Norena, L. E.; Wang, J. A.; Zhou, X. L.; Navarrete, J.; Hernandez, I.; Montoya, A.; Perez Romo, P. P.; Salas, C. P. S. A study of n-hexane hydroisomerization catalyzed with the Pt/H3PW12O40/Zr-MCM41 catalysts. Catal. Today 2008, in press. (36) Sawant, D. P.; Vinu, A.; Jacob, N. E.; Lefebvre, F.; Halligudi, S. B. Formation of nanosized zirconia-supported 12-tungstophosphoric acid in mesoporous silica SBA-15: A stable and versatile solid acid catalyst for benzylation of phenol. J. Catal. 2005, 235, 341–352. (37) Sunita, G.; Devassy, B. M.; Vinu, A.; Sawant, D. P.; Balasubramanian, V. V.; Halligudi, S. B. Synthesis of biodiesel over zirconiasupported isopoly and heteropoly tungstate catalysts. Catal. Commun. 2008, 9, 696–702.

ReceiVed for reView October 6, 2008 ReVised manuscript receiVed December 16, 2008 Accepted December 20, 2008 IE801508R