Methyl tert-Butyl Ether and Ethyl tert-Butyl Ether - ACS Publications

Chapter 16

Methyl tert-Butyl Ether and Ethyl tert-Butyl Ether Synthesis over Triflic Acid Modified Y-Zeolite

Downloaded by UNIV OF PITTSBURGH on May 4, 2015 | http://pubs.acs.org Publication Date: May 5, 1993 | doi: 10.1021/bk-1993-0517.ch016

R. Le Vanmao, H. Ahlafi, and T. S. Le Department of Chemistry and Biochemistry, Catalysis Research Laboratory, and Laboratories for Inorganic Materials, Concordia University, 1455 De Maisonneuve West, Montreal, Quebec H3G 1M8, Canada

The apparent activation energy for the synthesis reaction of methyl tert-butyl ether or MTBE, was found to be 64 KJ/mole. The best activity and selectivity for MTBE were observed at temperatures of 85 - 90 °C, and contact times of circa 2.5 h when the methanol / isobutene molar ratio was kept within the 1.2 - 1.5 range. There was a fierce competition between the ethyl tert-butyl ether formation and that of diethylether at reaction temperatures higher than 85 °C.

As lead antiknock additives in gasoline will be banned in most industrialized countries by the end of this decade, octane boosters for fuels such as light alcohols, methyl-tert-butyl ether (MTBE) have been increasingly used in gasoline blend. When compared to aromatic hydrocarbons which can be used to upgrade gasoline, M T B E evolves no toxic products from the incomplete combustion in engines. Furthermore, M T B E does not provoke demixing when blended with gasoline, which is not the case with methanol, for instance. M T B E , which is characterized with specifications close to those of gasoline, does not require dramatic modifications in engine technology. M T B E being an oxygenate and having a fairly high octane number, accordingly favors the "clean" combustion of gasolines and other fuels. M T B E is currently synthesized industrially from methanol and isobutene over an acidic ion-exchange resin, mostly Amberlyst 15 which is in fact a macroreticular cation-exchange resin [1,2]. ETBE which is obtained by reaction of isobutene with ethanol, is also an attractive octane enhancer for gasoline [3]. Although the commercial catalyst is very efficient, it suffers from several drawbacks such as thermal instability, acid leaching from the resin

0097-6156/93/0517-0233$06.00/0 © 1993 American Chemical Society

In Selectivity in Catalysis; Davis, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1993.

SELECTIVITY


234

IN CATALYSIS

surface and high methanol/isobutene ratio requirement. ZSM-5 zeolite was thus proposed as a remedy to such inconveniences [4]. However, the activity of the ZSM-5 zeolite catalyst is relatively low if compared to the Amberlyst 15 resin [3]. Increasing the surface acidity of the ZSM-5 zeolite by coating with triflic acid, a well-known organic superacid, results in a catalytic system which is strongly limited by diffusion phenomena due to pore narrowing [5]. However, with the larger pore sized Y-type zeolite, coating with triflic acid has provided an acid catalyst which shows catalytic activity and product selectivities comparable to those of the Amberlyst 15 in the synthesis of MTBE [5]. One of the advantages in the use of zeolite based catalysts is the significantly reduced production of (undesired) diisobutene and other oligomers of isobutene. This is due to the shape selectivity of the zeolite pore system. In this work, the triflic acid modified Y-zeolite catalyst has been investigated for the atmospheric synthesis of MTBE and ETBE. In particular, the apparent activation energy for MTBE was determined, and this value is compared with those reported in the literature [1,6]. In addition, for both syntheses, the product selectivities are reported as functions of the contact time at the temperature where the catalyst activity is the highest. The catalyst stability for the MTBE synthesis was also examined. EXPERIMENTAL CATALYST PREPARATION The acid form of the Y zeolite was obtained by activating the LZY-82 sample (Y-type zeolite, ammonium-form, powder, supplied by Linde) in air at 550 °C for 10 h. The triflic acid loading (3 wt %) was done according to the following procedure. Triflic acid (0.75 g; trifluoromethane-sulfonic acid, CF S0 H, 8 % from Fluka Chemie AG, hereafter called TFA)was dissolved in 40 ml of pure acetone. This solution was added to 25 g of zeolite (powder, acid form) contained in a small beaker. The resulting suspension was allowed to settle in the beaker, which was covered with a glass watch, and dried overnight at room temperature. The solid obtained was heated at 120 °C in air for 12 h. The final catalyst extrudates were prepared according to the procedure described elsewhere [5]. This catalyst is hereafter referred to as H-Y/TFA. 3

3

CATALYST CHARACTERIZATION The catalyst powder was characterized by atomic absorption (chemical composition), X-ray powder diffraction (structure identification and degree of crystallinity) and nitrogen adsorption/desorption. For the latter method, an automatic Micromeretics ASAP 2000 apparatus was used, which also allowed the determination of the pore size distribution in the mesopore and macropore region (2 nm to 300 nm).


16.

LEVANMAOETAL

Methyl tert-Butyl Ether and Ethyl tert-Butyl Ether


CATALYST TESTING AND ANALYSIS OF REACTION PRODUCTS The experimental set-up and the procedures for the analysis of the reaction products were identical to those described elsewhere [5]. The yield in product i was calculated by the relationship: N, Y, (C atom %) = χ 100 N(iso) where Nj and N(iso) are the numbers of carbon atoms of, respectively, product i which can be MTBE, ETBE or C8 products (diisobutene and its isomer), and feed isobutene. In particular, in the synthesis of ETBE, the yield in diethyl ether is expressed as: N(DEE) Y (C atom %)= χ 100 N(ETOH) where N(DEE) and N(ETOH) are the numbers of carbon atoms of product diethyl ether and feed ethanol, respectively. In addition, the yield in hydrocarbons other than oligomers of isobutene is expressed as : N(HC) Y (C atom %)= χ 100 N(ETOH) where N(HC) is the number of carbon atoms of such hydrocarbons. Product yieds are expressed in C atom % rather than in the usual moles % . This allows us to visualize the numbers of C atoms of isobutene or the alcohol converted into carbon containing products, independently from the molecular length or chemical composition of such products. D E E

H C

The reagent contact time is defined as : W T (expressed inh)= F c

r

where W and F are the weight of catalyst (in g) and the total flow-rate of the reagents (in g/h), respectively. The molar ratio (methanol/isobutene) was set at 1.2 - 1.5. The determination of the apparent activation energy derived from the Arrhenius formula was done using the initial rates of MTBE formation at several reaction temperatures ranging from 345 Κ (72 °C) to 371 Κ (98 °C). Higher temperatures are not recommended because of MTBE decomposition [6]. The apparent activation energy was determined using the linear regression fitting of the curves of MTBE yield as a function of reagent contact time. Such a technique is applicable only for MTBE yields corresponding to contact times lower than 1.7 h, i.e. for MTBE yields lower than 10 (C-atom) %. r


235

236

SELECTIVITY IN CATALYSIS


RESULTS AND DISCUSSION There are two values of surface area and volume of nitrogen adsorbed (BJH method), obtained with the parent H - Y zeolite and the H - Y / T F A sample (Table 1): the first corresponds to the zeolite-type micropores and the other, to the mesopores. Figure 1 shows the pore size distribution of the H - Y / T F A catalyst; there is a sharp peak (not shown here) in the micropore region and another peak at 4nm in the mesopore region. Such a bimodal pore size distribution was also observed with the parent zeolite. Although there was no significant loss in the degree of crystallinity upon loading of triflic acid onto the Y-zeolite, there was an important decrease in the surface area and the volume of nitrogen adsorbed (Table 1). These decreases are mainly related to the micropore region of the zeolite. Previous works have evidenced that: i) triflic acid species could form chemical-type bonds with the zeolite surface and thus become more thermally stable than free triflic acid [5, 8-11]; ii) serious steric hindrance was observed for the zeolite pore system upon triflic acid incorporation [5, 7-11]. Therefore, the lower volume of nitrogen adsorbed by the H - Y / T F A sample might be due to a lower accessibility to the micropores whose surface was covered with triflic acid species. While methanol and ethanol had already some difficulty to enter such reduced-sized micropores [10], the much bulkier isobutene molecule was probably even more constrained. Therefore, it is probable that only the mesopores and a portion of the micropores of the H - Y / T F A catalyst were involved in the catalytic reaction. These problems notwithstanding, the yield of M T B E obtained with the H - Y / T F A was twice as high as that obtained with the parent H - Y [5] and was practically equal to that of the Amberlyst 15. Such an enhanced activity is due to the enhanced acidity upon incorporation of triflic acid [9-11]. The presence of relatively narrow mesopores in the H - Y zeolite is advantageous because it contributes to solve the problem of diffusion limitation within the modified zeolite particles without losing the shape selectivity of such a porous system. Figure 2 shows the experimental data which were used to determine the apparent activation energy. The value found was 64 i 6 KJ/mol. The corresponding literature values range from 71 KJ/mol [1] to 82 KJ/mol [6] for the Amberlyst 15 resin, and are greater than 91 KJ/mol for an acid in solution (methyl sulfuric acid [12] and paratoluene sulfonic acid [1]). As pointed out by Gicquel and Torek [6], lower values of the apparent activation energy are partly due to diffusion limitations. The other possible cause of such variations stems from the saturation of the reaction sites by methanol [6]. Figure 3 shows the yields of M T B E plotted against the contact time, observed at 345 K , 360 Κ and 371 K , respectively. The yield in C8 products at 360 K , which is also reported in this figure, did not exceed 1.5 C-atom % at high contact times. At the reaction temperatures tested, the production of dimethyl ether (DME) and other hydrocarbons was practically negligible (less than 1 C atom %). Lower production of D M E was also observed by Chang ,,


M

16.

LEVANMAOETAL.

Methyl tert-Butyl Ether and Ethyl tert-Butyl Ether 237

Table 1 Textural properties of the H - Y and H - Y / T F A catalysts


Sample Degree of B.E.T. crystallinity surface area

BJH [7] cumulative desorption method Surface area (m /g) volume o f N adsorbed (cc/g) mi(l) mes+ma m i ( l ) mes+ma (2) (2) 2

(%>

(m /g)

H-Y

100

428

348 (79%)

92 (21%)

0.165 (52%)

0.150 (48%)

H-Y/ TFA

99

222

154 (67%)

75 (33%)

0.073 (35%)

0.136 (65%)

2

(1) micropores having diameter less than 2 nm (2) mesopores and macropores having diameters in the range of 2 - 300 nm. d V d D

J

0.40H

0.30J

0.201

0.10

-

Figure 1 Pore size distribution (mesopore region) of the H - Y / T F A catalyst. V (Volume of nitrogen adsorbed) and D (pore diameter) are expressed in cc/g and nm (10 m), respectively. 9




238

Figure 2 A) Yield of M T B E versus contact time. (O) = 345 K , ( · ) = 360 Κ and ( Δ ) = 371 Κ at short contact times. B) Arrhenius plot.


LE VANMAO ET AL.



16.

Figure 3 Yield of M T B E versus contact time. ( O ) = 345 K , ( · ) = 360 Κ and ( • ) = 371 K . (*) = yield in C8 products at 360 K .


239


240


et al [13]. At 360 Κ the production of M T B E reached a maximal value at a contact time of ca 2.5 h. At higher reaction temperatures, there was probably competition between formation and M T B E decomposition which might significantly reduce the final yield of M T B E [6]. Oligomers of isobutene higher than C8 (actually C12) were observed through G C and GC-MSD at reaction temperatures higher than 363 K . In terms of catalyst stability, runs totalizing more than 50 hours were performed (temperature = 360 K , contact time = 2.6 h and methanol/isobutene ratio = 1.2). There was no significant loss of activity. The same trends (activity maximum with respect to temperature and contact time) were obtained with the ETBE synthesis at 355 K , 363 Κ and 378 K , respectively (Figure 4). However, the yield of ETBE was much lower because of a parallel competitive reaction with the ETBE formation: the dehydration of ethanol to diethyl ether (Figure 5). The effect of this reaction was to decrease the concentration of ethanol adsorbed on the catalyst sites. The ethanol molecules which underwent dehydration to diethyl ether were no longer available to react with isobutene, since diethyl ether did not seem to be a precursor of ETBE (Figures 4 and 5). At reaction temperatures above 353 K , some hydrocarbons (C, - C ) other than C8 and higher oligomers of isobutene are produced, mostly at longer contact times (Figure 5). As also shown in Figure 5, these hydrocarbons were derived from diethylether, in accord with earlier results obtained in a study of ethanol dehydration [14]. It is well known that the dehydration of ethanol to diethyl ether (activation energy = 56 KJ/mol [14]) is much easier than that of methanol (higher activation energy [15]). On the other hand, it is worth noting that for both M T B E and ETBE syntheses, the yields of C8 hydrocarbons which derived from isobutene by acid-catalyzed oligomerization, were - as expected practically equal (Figures 3 and 4). Some attempts were made to calculate the apparent activation energy for the E T B E synthesis. However, the value obtained (133 ± 15 KJ/mol) is tenuous owing to the presence of competitive reactions such as the ethanol dehydration to diethyl ether (already significant at 363 K) and E T B E decomposition at higher temperatures (above 373 K ) . The lower concentration of ethanol, which is normally available for the ether synthesis, was probably the cause for such a high value of the apparent activation energy [6]. Finally, dilution of the ethanol feed with water reduced significantly the production of diethylether. However, at high water dilution, the yield in E T B E decreased markedly (Table 2) because of the strong competition for adsorption of water with ethanol on the acidic reaction sites. n

CONCLUSION The gas phase syntheses of M T B E and ETBE over triflic acid loaded Y zeolite catalyst are temperature and contact time sensitive reactions. The activity maxima are observed at 358 Κ (85 °C) / 363 Κ (90 °C) and contact




LE VANMAO ET AL.

Figure 4 Yield of ETBE versus contact time. ( • ) = 355 K , ( # ) = Κ and ( • ) = 378 K . (* ) = yield in C8 products at 363 K .




242

0

1

>Tp

2

3

Figure 5 Yield of D E E ( Ο = 363 Κ and • = 378 Κ), and yield of hydrocarbons (oligomers of isobutene excluded, # = 363 Κ and • = 378 K), versus contact time.

Table 2 Effect of the dilution of feed ethanol with water (Reaction conditions: temperature = 363 K , contact time = 1.5 h and ethanol/isobutene ratio = 1.2). Feed

Yc8

YHC(D

YETBE

YDEE

100 % Ethanol

20

11

0.6

50 vol % Ethanol/ 50 vol% water

22

8

0.5

8(2)

16

1

0.2

0.5 (2)

10 vol % Ethanol/ 90 vol % water

10

(1) yield in CT - C hydrocarbons excluding diisobutene and other isobutene oligomers (2) mostly ethylene N


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Methyl tert-Butyl Ether and Ethyl tert-Butyl Ether 243

time of 2.5 ± . 0.3 h. The presence of mesopores within the H - Y zeolite particles seems to favor the yield of M T B E . The lower yields of E T B E are due to the competitive dehydration of ethanol which alters the concentration of ethanol normally available for the ether synthesis. ACKNOWLEDGEMENTS


We thank the following agencies: NSERC of Canada and Quebec's Action Structurante Program for their financial support. We also thank Prof. Nick Serpone, Mrs J. Yao and M r . B. Sjiariel for their technical assistance. REFERENCES [1] F. Ancillotti,M.M.Mauri and E. Pescarollo, J. Catal. (1977), 46, 49. [2] G. Pecci and T. Floris, Hydrocarbon Process. (1997), 56, 98. [3] L.M.Tau and B.H. Davis, Appl. Catal. (1989), 53, 263. [4] P. Chu and G.H. Kuhl, Ind. Eng. Chem. Res. (1987), 26, 365. [5] R. Le Van Mao, R. Carli, H. Ahlafi and V. Ragaini, Catal. Lett. (1990), 6, 321. [6] A. Gicquel and B. Torck, J. Catal. (1983), 83, 9. [7] E.P. Barrett, L.G.Joyner and P.P. Halenda, J.Am. Chem. Soc. (1951), 73, 373. [8] R. Le Van Mao, T.M. Nguyen and G.P. McLaughlin, Appl. Catal. (1989), 48, 267. [9] R. Le Van Mao, D. Ly and J. Yao, Novel Production Methods for Ethylene, Light Hydrocarbons and Aromatics (ACS 1990 Natl Meeting, Boston), L. Albright, B.L. Crynes and S. Nowak (Ed.), Marcel Dekker (Publ.), New York (1992), p 409 - 424. [10] R. Le Van Mao and L. Huang, Novel Production Methods for Ethylene, Light Hydrocarbons and Aromatics (ACS 1990 Natl Meeting, Boston), L . Albright, B.L. Crynes and S. Nowak (Ed.), Marcel Dekker (Publ.), New York (1992), 425 - 442. [11] R. Le Van Mao, T.M.Nguyen and G.P. McLaughlin, Appl. Catal. (1989), 48, 267. [12] J.P. Beaufils and M . Hellin, Internal Report, Institut Francais du Petrole (1963), mentioned in reference 6. [13] K.H. Chang, G. J. Kim and W.S. Ahn, Ind. Eng. Chem. Res. (1992), 31, 125. [14] T.M. Nguyen and R. Le Van Mao, Appl. Catal. (1990), 58, 119. [15] F. Figueras Roca, L. De Mourgues and Y. Trambouze, J. Catal. (1969), 14, 107. RECEIVED July 21, 1992


Methyl tert-Butyl Ether and Ethyl tert-Butyl Ether - ACS Publications

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