Synthesis of MTBE from Isobutane Using a Single Catalytic System

A new route for the synthesis of methyl tert-butyl ether (MTBE) from isobutane using a single catalytic system has been developed. The key of the proc...
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Ind. Eng. Chem. Res. 1998, 37, 4215-4221

4215

Synthesis of MTBE from Isobutane Using a Single Catalytic System Based on Titanium-Containing ZSM-5: Influence of Reaction Parameters Rafael Van Grieken,* Gabriel Ovejero, David P. Serrano, Marı´a A. Uguina, and Juan A. Melero Department of Chemical Engineering, Faculty of Chemistry, Complutense University of Madrid, 28040 Madrid, Spain

A new route for the synthesis of methyl tert-butyl ether (MTBE) from isobutane using a single catalytic system has been developed. The key of the process is the use of a bifunctional material, Al-TS-1, capable of catalyzing the liquid-phase oxidation of isobutane with H2O2 and subsequently the etherification with methanol. The procedure carried out, simultaneously or sequentially, both reactions (oxidation and acid catalysis) as well as the influence of the reaction parameters has been studied. A two-stage process at two temperature levels was selected, according to the results obtained by studying both reactions separately. In addition, the influence of other reaction parameters, such as reactant molar ratios, solvent concentration, and catalyst composition, has been investigated to establish the optimum conditions to yield MTBE. Introduction The high-quality standards imposed upon gasoline have made MTBE the additive of choice for gasoline improvement whereas the use of tetraethyllead and similar lead compounds has been phased out because of concerns about the effect of these compounds on health. MTBE is an ether with a good antiknocking performance and excellent physicochemical properties compared to those of alcohols and does not produce environmentally hazardous emissions.1 During the past few years, the use of MTBE as an acceptable gasoline additive has substantially increased its commercial production. The commercial process currently used for MTBE synthesis involves the liquid-phase reaction of methanol and isobutene at temperatures below 100 °C and pressures up to 2 MPa, over different organic resins (Amberlyst 15, LEWATIT, ...). However, the thermal instability of these organic resins and high possibility of corrosive behavior are considered to be significant drawbacks.2 Recently, several inorganic solid acid catalysts, especially zeolites, were reported for the etherification of isobutene with methanol, such as ZSM5, Y, and β zeolites,3-5 whereas Bylina et al.6 employed ion-exchanged montmorillonites such as Al3+-pillared K-10 and K-306 clays. On the other hand, a growing problem is the availability of isobutene not only for MTBE production but also for alkylation. Therefore, it will be necessary to resort to other sources such as tert-butyl alcohol7,8 and isobutane as raw materials to fulfill this expanding demand for MTBE. Most of the isobutane is now applied to alkylation processes by reacting with light olefins (propylene, butenes, and amylenes), over strong acids as catalysts, to yield alkylates having good antiknocking performance and very useful in reformulated * To whom the correspondence should be addressed. Tel: 3491-3944185. Fax: 34-91-3944114. E-mail: [email protected]. ucm.es.

gasoline. Alternatively, isobutane can be thermally cracked9 or dehydrogenated to isobutene and consequently converted into MTBE.10 A number of patented dehydrogenation processes are now in use; however, they have some drawbacks, such as high investments and operating costs. In this context, we have reported in a previous communication11 an alternative way to synthesize MTBE directly from isobutane over a single catalytic system based on aluminum titanium silicalites with an MFI structure (Al-TS-1), which has already been synthesized by different methods.12-14 The key of the process is the use of a bifunctional catalyst, capable of catalyzing both the oxidation of isobutane with H2O2 to tert-butyl alcohol and subsequently the etherification with methanol to yield MTBE. In the present work, we report a more extensive study of this interesting heterogeneous liquidphase process. Experimental Section 1. Materials and Characterization. Al-TS-1 samples were synthesized by thermal treatment of wetness-impregnated Al2O3-TiO2-SiO2 cogels with aqueous 20 wt % (TPA)OH solution, except when the starting SiO2/Al2O3 ratio was less than 160, for which a 30 wt % (TPA)OH solution was used. The amorphous solids were prepared by following a two-step sol-gel process described elsewhere.14 The TS-1 sample was synthesized using as raw material amorphous SiO2TiO2 solids prepared by the sol-gel process.15,16 Characterization of the catalyst samples was performed by different conventional techniques. Chemical analyses were carried out by X-ray fluorescence (XRF) with a Philips PW 1480 spectrometer. X-ray diffraction (XRD) patterns were collected with a Philips X’PERT diffractometer with Cu KR radiation. Crystallinity was determined from the peak area between 2θ ) 22° and 2θ ) 25° using a highly crystalline TS-1 sample as reference. Fourier transform IR (FTIR) spectra were recorded by means of a Nicolet 510P spectrophotometer

10.1021/ie980113f CCC: $15.00 © 1998 American Chemical Society Published on Web 10/08/1998

4216 Ind. Eng. Chem. Res., Vol. 37, No. 11, 1998 Table 1. One-Step Reaction: Influence of H2O2 Addition and H2O Concentration in the Isobutane Conversion over Al-TS-1 experimentsa run

T (°C)

H2Od

dropwise H2O2

1.1 1.2 1.3b 1.4b 1.5c

100 70 100 70 100

6.4 6.4 6.4 6.4 12.0

no no yes yes yes

H2O2 conv (%) 100 100 100 65 88

isobutane conv (%) rel abs 24.3 43.6 45.4 68.4 36.1

2.8 5.0 5.2 5.1 3.6

H2O2 selectivity (%) MTBE

ButOH

others from i-C4

CH2O + HCO2Me

global

7.8 0.8 11.1 0.9 6.7

4.1 32.6 16.7 57.4 11.5

20.4 14.6 30.7 14.6 30.2

7.5 3.8 13.3 5.0 10.4

39.8 51.8 71.8 77.9 58.8

a Reaction conditions: methanol/isobutane molar ratio ) 0.91; H O /isobutane molar ratio ) 0.11; reaction time ) 4 h; amount of 2 2 catalyst ) 0.6 g, Si/Ti molar ratio ) 76, Si/Al molar ratio ) 80. b Addition of H2O2 (50 mass %) ) 6.5 mg/min for 210 min. c Addition of H2O2 (27.3 mass %) ) 11.4 mg/min for 210 min. d Mass % after total addition and conversion of H2O2.

using the KBr wafer technique. Diffuse reflectance UV-vis spectra (DR UV-vis) were obtained under ambient conditions on a Cary-1 spectrophotometer equipped with a diffuse reflectance accessory. Ammonia temperature programmed desorption (TPD) was carried out with a Micromeritics TPD/TPR 2900. Morphology and size of the crystallites were determined by scanning electronic microscopy (SEM) images taken with a JEOL JSM-6400 microscope. 29Si and 27Al magic angle spinning nuclear magnetic resonance (MAS NMR) spectra were recorded on a Bruker spectrometer, model MSL400. Competitive adsorption measurements of water and n-hexane vapor mixtures for the determination of the modified hydrophobicity index (HI) were carried out according to a method described in the literature.17 The final adsorbed loading (Li) was calculated directly from the measured breakthrough curves, and the modified hydrophobicity index was calculated as the mass ratio between n-hexane and water adsorbed on the sample (Ln-hexane/Lwater) in equilibrium with the gas stream containing both components. The characterization results show that all the materials obtained present an MFI structure and high crystallinity as concluded from XRD measurements. DR UVvis results confirm the presence of Ti atoms in tetrahedral positions in the zeolite framework whereas Ti extraframework species are not detected. 29Si and 27Al MAS NMR spectroscopy confirms the presence of Al atoms in tetrahedral coordination sites corresponding to the zeolite lattice. 2. Apparatus and Procedure. The experiments were carried out in a magnetically stirred batch reactor equipped with a temperature controller and pressure gauge. The reactants, except H2O2, and the catalyst were placed in the Teflon-lined reactor, which was pressurized with nitrogen (initial pressure of nitrogen: 10 kg/cm2) to keep the mixture in the liquid phase at the reaction temperature. Once the temperature reached the set point value, the oxidant was added slowly, around 5.0-6.5 mg/min of 50 wt % aqueous H2O2 solution, while the temperature was fixed for a predetermined time (total pressure: 20-30 kg/cm2). When a two-stage process was selected, once the first step was complete, the temperature was increased to 100 °C and fixed for 3 h to carry out the second step (total pressure: 30-40 kg/cm2). Complete consumption of H2O2 was achieved for all experiments, unless indicated otherwise. The reaction products were analyzed by gas chromatography (Varian 3400) on a capillary DV-1 column (60 m × 0.02 mm) using a flame ionization detector. The reaction results are presented using the following definitions:

relative isobutane conversion ) reacted isobutane moles × 100 reacted H2O2 moles absolute isobutane conversion ) reacted isobutane moles × 100 fed isobutane moles H2O2 conversion )

reacted H2O2 moles × 100 fed H2O2 moles

H2O2 selectivity for i product ) H2O2 moles reacted to yield i product × 100 reacted H2O2 moles global H2O2 selectivity ) H2O2 moles reacted to yield any product × 100 reacted H2O2 moles Results and Discussion 1. One-Step Reaction. The key of the process described in this work is the use of a bifunctional material, capable of catalyzing both isobutane oxidation with H2O2 into tert-butyl alcohol and subsequently etherification with methanol to yield MTBE. Since the temperature for the acid-catalyzed MTBE synthesis from isobutene over zeolites is usually around 100 °C4 and not higher due to thermodynamic limitations5 and this temperature is also frequently used for hydrocarbon oxidation reactions over TS-1,18 the first attempts to obtain MTBE from isobutane, methanol, and hydrogen peroxide were carried out at 100 °C (Table 1, run 1.1). In this first experiment, the entire amount of H2O2 was added to the reaction mixture at room temperature and then the system was heated to 100 °C. However, the MTBE yield was only 7.8%; hence, we considered lowering the reaction temperature. As shown in Table 1 (run 1.2), an experiment was carried out at 70 °C under conditions similar to those at 100 °C. According to the results obtained, MTBE formation was greatly reduced in comparison with that of the reaction at 100 °C whereas tert-butyl alcohol became the main product, decreasing the reaction of H2O2 in the formation of other secondary products, such as the oxidation of methanol to formaldehyde and methyl formate. Additionally, the global H2O2 selectivity, which is related to the extent of hydrogen peroxide decomposition to water and oxygen, is higher for the reaction at 70 °C, indicating that the oxidant is being more efficiently used for the hydrocarbon oxidation. At both temperatures, the global H2O2 selectivity is low when the entire amount of the oxidant agent is added initially; hence, it was considered necessary to investigate a way of contacting

Ind. Eng. Chem. Res., Vol. 37, No. 11, 1998 4217

the H2O2 with the isobutane + solvent + catalyst mixture to improve oxidant efficiency. Table 1 (runs 1.3 and 1.4) shows the results of the reaction carried out with H2O2 dropwise addition at both temperatures. According to the results published by Thangaraj et al.,19 a better use of oxidant could be achieved by using dropwise addition of hydrogen peroxide since thermal decomposition is minimized. This effect is more pronounced at higher temperature (100 °C). Additionally, the dropwise addition of H2O2 allows one to work under safer conditions as consequence of the low concentration of peroxide in the organic medium. The presence of Al atoms in the zeolite framework could increase its hydrophilic character in comparison with that of TS-1, and thus, the effect of water in the oxidation medium should be studied. The water influence in this process was studied by a comparison of the reaction results of two experiments (Table 1, runs 1.3 and 1.5) carried out by varying the concentration of water in the reaction medium. According to the results obtained, the oxidation capacity of the catalyst decreases when a higher water concentration is present, probably due to the competitive blockage of the catalytic sites or the surroundings by water molecules, hindering or delaying the oxidation of the hydrocarbon. As a result, the competitive reaction, H2O2 decomposition, is enhanced, lowering H2O2 selectivity toward oxidation products and therefore the relative and absolute isobutane conversion. These experimental results are in good agreement with those recently reported by Corma et al.20 for alcohol oxidation with H2O2 over Al- and Ticontaining materials such as Ti-Al-β zeolite. Therefore, the yield of oxidation products is reduced although the decrease in MTBE is even more drastic due to the role of water in the equilibrium of the ether formation and in the kinetics of the catalytic process.21 As can be seen from runs 1.3 and 1.4 in Table 1, the reaction temperature plays an important role in the isobutane conversion, peroxide selectivity, and product distribution for this one-step process for the MTBE synthesis. Therefore, we decided to study the influence of temperature in the range between 70 and 110 °C. These temperatures are usually used for hydrocarbon oxidation using TS-1 and H2O2. The results obtained in the reactions carried out at different temperatures with dropwise H2O2 addition are given in Figure 1. All the runs were carried out to achieve complete consumption of the oxidant agent. As shown in Figure 1a, the higher the temperature, the lower the absolute isobutane conversion. Under the reaction conditions tested, H2O2 is clearly the limiting reactant and the use of higher temperatures (∼100 °C) promotes secondary reactions of hydrogen peroxide, decreasing its use in the oxidation of isobutane. According to the variation of the product distribution with the temperature (Figure 1b) and the reaction scheme proposed by Sato et al.22 for 1-octene oxidation, the reaction network can be described as in Figure 2. The oxidation of isobutane by H2O2 using Al-TS-1 as catalyst yields tert-butyl alcohol as the primary product, which could continue the reaction at higher temperatures either to form MTBE directly or, upon dehydration, to form isobutene, both reactions derived from the acid properties of the catalyst. The only alcohol formed from isobutane oxidation is tert-butyl alcohol due to the reactivity of the tertiary carbon present in this molecule

Figure 1. One-step reaction. Influence of reaction temperature: (a) reaction parameters; (b) molar product distribution. Reaction conditions were the same as those for runs 1.3 and 1.4 described in Table 1 up to total consumption of H2O2.

and the lack of oxidation activity in primary carbons shown by Ti-containing zeolitic materials.18,23 TBHP formation could be the consequence of a further oxidation of tert-butyl alcohol catalyzed by the acid centers of the zeolite, although a free radical chain autoxidation of isobutane seems also to be an alternative reaction mechanism for TBHP formation initiated by hydroxyl radicals arising from H2O2 cleavage. This second mechanism would be supported by the increase of TBHP formation as the temperature increases as shown in Figure 1b since the formation of free radicals from H2O2 decomposition is favored at high temperatures. As the main secondary reaction, the oxidation of methanol leads to formaldehyde, present in the reaction medium as methylene glycol, and consequently to formic acid, which readily reacts with methanol to yield methyl formate. This process is catalyzed preferentially at high

4218 Ind. Eng. Chem. Res., Vol. 37, No. 11, 1998

Figure 2. Reaction scheme.

temperatures, as shown in the molar product distribution in Figure 1b. Once isobutene is formed in the presence of hydrogen peroxide, isobutene oxide is postulated to be an intermediate for other reactions. The nucleophilic attack of the methoxy or hydroxy group leads to the formation of 2-methoxyisobutanol, as was reported by Neri et al.,24 or 2-hydroxyisobutanol, although the latter was not detected. Further reaction of isobutene oxide yields acetone and formaldehyde (epoxide oxidative cleavage) and isobutyraldehyde (epoxide isomerization). Thus the increase in the relative conversion of isobutane shown in Figure 1a at low temperatures is a consequence of the lower extent of methanol oxidation and secondary reactions. From the experiments carried out in the one-step oxidation of isobutane to yield MTBE, it can be concluded that temperature and water concentration have a significant influence not only in the isobutane oxidation but also in the alcohol etherification. In this way, we have carried out a detailed investigation on the methanol and tert-butyl alcohol etherification using AlTS-1 as catalyst. Figure 3a illustrates the results obtained for the methanol and tert-butyl alcohol etherification reaction at different temperatures, showing a negligible MTBE formation at temperatures below 60 °C. However, as the temperature increases, the MTBE formation becomes significant, reaching the higher ether formation at 100 °C. Figure 3a also shows that MTBE selectivity after 2 h of reaction is around 98% for the different temperatures tested. Etherifications, conceptionally one of the simplest reactions to catalyze, occur over most acid zeolites. Additionally this kind of reaction is experimentally and theoretically well studied and understood. At low temperature (in the liquid phase), the reaction proceeds via an Eley-Rideal type mechanism. This mechanism involves a complex transition state in which four reactions have to proceed in a synchronous manner: (i) formation of an alkoxonium ion by proton donation to one alcohol molecule from the zeolite, (ii) cleavage of water from the alkoxonium ion and formation of a carbenium ion, (iii) binding of the carbenium ion with a second alcohol molecule to form protonated ether, and (iv) donation of the proton back to the zeolite and ether desorption. According to this reaction mechanism, very low temperatures are not able to form the tert-butoxo-

Figure 3. Etherification reaction: (a) influence of temperature in the absence of water with 2 h reaction time; (b) influence of H2O concentration at 100 °C (H2O concentration in wt %). Reaction conditions: tert-butyl alcohol/methanol mass ratio ) 0.1; tert-butyl alcohol/catalyst mass ratio ) 2.5; amount of catalyst ) 0.4 g, Si/ Ti molar ratio ) 76, Si/Al molar ratio ) 80.

nium ion, and therefore the etherification reaction does not proceed. As the temperature increases, the kinetics of intermediate formation is faster and thus the equilibrium conversion is accomplished in a shorter time. On the other hand, alcohol etherification is a catalytic process where the H2O content plays an important role not only in the thermodynamic equilibrium but also in the kinetics.21 Figure 3b shows the results of tert-butyl alcohol and methanol etherification with different H2O contents in the reaction medium and different reaction times. According to the experiments carried out during 2 h of reaction, there is a significant difference in regard to tert-butyl alcohol conversion, the latter decreasing as initial water concentration increases. This difference almost disappears after 3 h of reaction. The water presence affects not only the kinetics of etherification

Ind. Eng. Chem. Res., Vol. 37, No. 11, 1998 4219 Table 2. Two-Step Reaction: Influence of the H2O2/Isobutane Molar Ratio isobutane conv (%)

experimentsa We

H2O2 selectivity (%) MTBE ButOH H2CdCMe2 Me2C(OMe)CH2OH ButOOH CH2O + HCO2Me global

run H2Ob t (min)c

Zd

2.1 2.2 2.3

6.9 8.8 14.9

210 300 300

6.7 0.40 58.7 23.5 6.7 0.52 60.6 31.3 6.7 0.97 57.5 55.5

35.3 33.0 32.3

6.1 8.9 4.5

4.0 7.1 4.9

0.1 0.5 4.0

1.3 1.4 2.7

22.4 18.5 18.6

86.0 83.0 84.6

2.4 2.5 2.6

3.6 4.8 6.9

150 210 300

1.7 0.08 65.4 5.4 1.7 0.11 69.7 7.7 1.7 0.17 67.6 11.2

38.7 39.8 39.4

10.0 11.3 13.6

8.8 11.9 6.3

2.5 2.1 5.8

2.7 1.9 2.7

16.3 12.4 11.4

87.0 86.4 85.8

rel

abs

a Reaction conditions: temperature ) 80 °C for 6 h to H O conversion)100% and 100 °C for 3 h (except run 2.3, 5 h); amount of 2 2 catalyst ) 1 g, Si/Ti molar ratio ) 76, Si/Al molar ratio ) 80. b Mass % after total addition and conversion of H2O2. c Addition time of H2O2 (50 mass %) (5 mg/min; except run 2.3, 8 mg/min). d Methanol/isobutane molar ratio. e H2O2/isobutane molar ratio.

reaction but also the equilibrium composition clearly shown by the changes induced in the MTBE selectivity. 2. Two-Step Reaction. According to the results obtained for the one-step synthesis of MTBE by means of the present process, we decided to carry out the reaction at two temperature levels, such as in a temperature gradient reactor: 80 °C for the oxidation step with dropwise H2O2 addition and 100 °C for the acidcatalyzed step. The influence of the different reaction variables is described below. 2.1. Influence of the H2O2/Isobutane Molar Ratio. Table 2 shows the isobutane conversion and hydrogen peroxide selectivity to form different products for several experiments carried out at different conversion levels of isobutane. The reactions were performed at two different methanol/isobutane molar ratios (6.7 for runs 2.1-2.3 and 1.7 for runs 2.4-2.6), and conversion levels of isobutane were changed by varying the amount of fed H2O2. From the experimental results, a similar trend is observed at both levels of methanol concentration in the reaction medium. An increase in the amount of H2O2 fed to the reaction leads to a higher absolute isobutane conversion with a similar efficiency in the use of H2O2 to yield products from isobutane oxidation. On the other hand, the increase of absolute isobutane conversion does not appreciably change the H2O2 selectivity toward the different products and therefore the product distribution. According to these results, the H2O2 utilization and product distribution do not depend on the amount of H2O2 added but are dependent on the method of addition as was seen for the one-step process. It must be noted that a high amount of fed H2O2 leads to a high concentration of water in the reaction medium, and so longer times for the second step (mainly) are necessary in order to achieve an appreciable yield of MTBE (Table 2, run 2.3). 2.2. Influence of the Methanol/Isobutane Molar Ratio. Figure 4 shows the experimental results for the two-step process where the methanol/isobutane molar ratio is varied under the reaction conditions previously described. Since water concentration has a strong influence on the activity not only for oxidation but also for etherification reaction, we decided to maintain the water concentration in all the experiments by adding the same amount of aqueous solution of H2O2 to same initial mass of the reaction mixture (isobutane + methanol). Therefore, the H2O2/isobutane and H2O2/ methanol molar ratios vary in the different experiments although, as shown in the previous section, these variations have no significant influence on the relative parameters of reaction (H2O2 selectivity toward the different reaction products and relative isobutane conversion).

Figure 4. Two-step reaction. Influence of the methanol/isobutane molar ratio: (a) reaction parameters; (b) molar product distribution. Reaction conditions: (isobutane + methanol)/H2O2 mass ratio ) 20; temperature ) 80 °C for 6 h to H2O2 conversion ) 100% and 100 °C for 3 h; amount of catalyst ) 1 g, Si/Ti molar ratio ) 76, Si/Al molar ratio ) 80; addition of H2O2 (50% w/w) ) 5 mg/ min for 300 min.

Methanol plays an important role in this process since it is not only one of the reactants in the second step but also the solvent in the oxidation step. As solvent, it allows the appropriate mixing of the two-phase system,

4220 Ind. Eng. Chem. Res., Vol. 37, No. 11, 1998 Table 3. Two-Step Reaction: Influence of Catalyst Composition catalystb runa 3.1 3.2 3.3 3.4 3.5

molar ratio Al/Ti Si/Ti

TPDc

I960/I800d

HIe

isobutane conv (%) rel abs

0 0.36 0.61 0.95 1.50

0 0.09 0.12 0.17 0.26

1.80 1.55 1.44 1.36 1.10

8.4 11.2 11.9 9.2 6.9

48.7 64.7 62.8 58.7 56.0

49 57 73 76 89

19.5 25.9 25.2 23.5 22.4

H2O2 selectivity (%) MTBE

ButOH

H2CdCMe2

Me2C(OMe) CH2OH

ButOOH

CH2O + HCO2Me

global

0.5 40.4 42.0 35.3 33.7

41.9 12.9 10.0 6.1 7.5

0.6 5.1 3.4 4.0 5.0

0.2 0.5 0.1 0.1 0.0

0.0 0.0 1.1 1.3 2.3

5.0 27.8 27.1 22.4 25.7

55.0 96.0 93.8 86.0 88.2

a Reaction conditions: methanol/isobutane molar ratio ) 6.7, H O /isobutane molar ratio ) 0.40; temperature ) 80 °C for 6 h to H O 2 2 2 2 conversion ) 100% and 100 °C for 3 h; amount of catalyst ) 1 g, addition of H2O2 (50 mass %) ) 5 mg/min for 210 min, H2O % in mass b c after total addition and conversion of H2O2 ) 6.9. Catalyst molar composition and properties, SiO2 mass % ) 97-97.5 NH3 adsorption capacity in mmol/g. d Ratio between the intensities, in absorbance units, of the 960 and 800 cm-1 IR bands. e Modified hydrophobicity index (Ln-hexane/Lwater).

containing separately the substrate (isobutane) and the oxidant (H2O2), and the diffusion into the zeolite pores. Among the different reaction parameters, the H2O2 selectivities toward formaldehyde + methyl formate and toward the mixture MTBE + tert-butyl alcohol + isobutene have been chosen to establish the optimum methanol concentration in the reaction mixture. The former indicates the extent of the methanol oxidation, and the latter shows the H2O2 efficiency to yield valuable products in regard to the MTBE synthesis. Additionally, Figure 4a also shows the global H2O2 selectivity and the relative isobutane conversion as a measurement of the thermal decomposition of the oxidant agent and the H2O2 conversion to yield isobutane derivatives, respectively. According to the results exposed, as the methanol concentration begins to increase, the global H2O2 selectivity, the H2O2 selectivity toward MTBE + tert-butyl alcohol + isobutene, and the relative isobutane conversion increase to maxima as a consequence of a better mixing of the two-phase system, which allows an increase in the accessibility of the hydrocarbon to the Ti sites without significant changes in the extent of the methanol oxidation. Beyond these maxima, a progressive increase of solvent enhances the competitive oxidation of methanol instead of isobutane, and hence, there is a decrease in H2O2 selectivity toward valuable products in the MTBE synthesis. Both trends show clearly that an optimum methanol concentration will be necessary to allow a reasonable isobutane oxidation with a minor presence of methanol oxidation. Under this optimum methanol/isobutane molar ratio, the H2O2 yields of MTBE and the mixture MTBE + tert-butyl alcohol + isobutene are 40% and 60%, respectively. 2.3. Influence of the Catalyst Composition. Table 3 shows the results of the experiments conducted under similar reaction conditions for five catalysts with different Al/Ti molar ratios in order to determine the optimum Al and Ti contents in the zeolitic framework. For all the samples, a material with an MFI structure and high crystallinity was obtained as concluded from XRD measurements. As shown in Table 3, the relative intensity of the 960 and 800 cm-1 IR bands and NH3 adsorption capacity are in good agreement with the chemical composition (Si/Ti and Al/Ti molar ratios) determined by XRF. Additionally, a reaction in the absence of catalyst was carried out. Products arising from isobutane or methanol oxidation were not detected, and the only reaction was hydrogen peroxide decomposition to oxygen and water.

An experiment was carried out using TS-1 (run 3.1) as catalyst. This material does not present any activity to catalyze the etherification process due to the lack of Al, which should provide the Bro¨nsted acidity needed for such reaction. This result is in good agreement with its negligible NH3 adsorption capacity (TPD). In addition, low methanol oxidation using TS-125 and a large increase of this secondary reaction rate in the presence of acid sites, suggest that the incorporation of Al atoms into the lattice favors alcohol oxidation. On the other hand, the presence of Al in the MFI structure where there are Al/Ti molar ratios of 0.4-0.6 (runs 3.2 and 3.3 in Table 3) allows a better utilization of the oxidant, due not only to an enhancement of methanol oxidation but also to an increase in isobutane oxidation. At this point, it is important to mention the similarity among the crystal sizes of all the catalyst samples tested. Other experiments to be published by Ovejero et al.26 reveal interesting results regarding the synergism between Ti and Al atoms in the MFI structure for n-hexane oxidation with H2O2 in methanol. These studies show that the presence of Al provides the Al-TS-1 samples with a different activity per Ti atom for hydrocarbon oxidation with H2O2 compared to that of TS-1, with an optimum Al/Ti molar ratio around 0.7. Additionally, this catalytic behavior for oxidation processes seems to be related to the different hydrophilic/ hydrophobic characters of the samples, which have been measured according to a method described in the literature.17 From these adsorption measurements we have calculated the modified hydrophobicity indices (HI’s) for the different samples which are shown in Table 3. These values of HI correlate fairly well with the activity of Ti sites in the hydrocarbon liquid-phase oxidation with H2O2. In particular, it was observed26 that as the HI increases, the catalytic activity per Ti atom is enhanced. This surprising result could be attributed to the modification of the electronic density around Ti sites induced by the close presence of Al, changing its hydrophilic character and hence providing an increase in its intrinsic activity for oxidation. However, when the Al content is high (Al/Ti molar ratio > 0.6; runs 3.4 and 3.5 in Table 3), this synergic effect is overcome by the higher adsorption of H2O instead of hydrocarbon, which leads to a decrease in the catalytic activity to analogous values of TS-1. Regarding the TBHP detected in the reaction product, its formation seems to be catalyzed by the presence of Al in the catalyst, in accordance with the lack of this product in the presence of TS-1 (at least in the 80 °C oxidation step).

Ind. Eng. Chem. Res., Vol. 37, No. 11, 1998 4221

Under the reaction conditions described in Table 3 and using an Al-TS-1 sample with an Al/Ti molar ratio around 0.6 as catalyst (run 3.3), the H2O2 yield of MTBE is around 42%. Conclusions MTBE synthesis from isobutane using a single catalytic system based on titanium-containing ZSM-5 is a promising alternative to MTBE synthesis from isobutene obtained by isobutane dehydrogenation. The relative high cost of H2O2 can be compensated with the use of a bifunctional catalytic system that allows MTBE synthesis from isobutane in a single reactor. The experimental results show that low temperatures and continuous H2O2 addition to the reaction mixture are desirable to obtain a high oxidant agent efficiency and tert-butyl alcohol as the main product, minimizing secondary reactions and allowing safe working conditions. However, at low temperatures the acid-catalyzed reactions do not take place and then MTBE formation is reduced. On the other hand, high temperatures lead to secondary reactions, yielding undesirable products from methanol and tert-butyl alcohol oxidation and derivatives of isobutene oxide reactions. Additionally, the reaction system here studied requires the minimum presence of water in the reaction medium not only for isobutane oxidation but also for the alcohol etherification reaction. The methanol concentration in the reaction medium plays an important role, since it is not only the reactant in the etherification reaction but also the solvent in the first step of the process, and therefore an optimum value facilitates isobutane oxidation with relatively low methanol oxidation. However, the H2O2/ isobutane molar ratio does not seem to have an influence on the relative isobutane conversion or the product distribution, which is an interesting experimental feature for a hypothetical application of this process. TS-1 does not present any activity for MTBE synthesis. Al-TS-1 samples with low Al/Ti molar ratios (0.40.6) in the MFI structure are desirable to improve isobutane oxidation, although high Al/Ti molar ratios decrease the oxidizing activity of the catalyst. The best H2O2 yield of MTBE is over 40% with appreciable yields of other interesting products in the MTBE synthesis (tert-butyl alcohol and isobutene). It is noteworthy that isobutane selectivity is around 65% and 90% for MTBE and MTBE + tert-butyl alcohol + isobutene, respectively. Acknowledgment This work was partially funded by the CICYT (Comisio´n Interministerial de Ciencia y Tecnologı´a), Project MAT 96/0924. Literature Cited (1) Unzelman, G. H. Oxygenates for the Future. 1. U.S. Clean Air Act Expands Role for Oxygenates. Oil Gas J. 1991, 89, 44. (2) Takesono, T.; Fujiwara, Y. Method for Producing MTBE and Fuel Composition Containing the Same. U.S. Patent 4,182,913, 1980. (3) Chu, P.; Kuhl, G. H. Preparation of MTBE over Zeolite Catalysts. Ind. Eng. Chem. Res. 1987, 26, 365. (4) Pien, S. I.; Hatcher, W. J. Synthesis of MTBE on HZSM-5 Zeolite. Chem. Eng. Commun. 1990, 93, 257. (5) Nikolopoulos, A. A.; Oukaci, R.; Goodwin, J. G., Jr.; Marcelin, G. Selectivity Behaviour during the Equilibrium-Limited

High Temperature Formation of MTBE on Acid Zeolites. Catal. Lett. 1994, 27, 149. (6) Bylina, A.; Adams, J. M.; Graham, S. H.; Thomas, J. M. Chemical Conversions Using Sheet Silicates: Method for Producing Methyl tert-Butyl Ether. J. Chem. Soc., Chem. Commun. 1980, 1003. (7) Nelson, E. C.; Storm, D. A.; Patel, M. S. Preparation of MTBE from TBA and Methanol. U.S. Patent 4,918,244, 1990. (8) Knifton, J. F.; Sanderson, J. R. One Step Synthesis of MTBE from t-Butanol Using Fluorophosphoric Acid-Modified Zeolite Catalysts. U.S. Patent 5,220,078, 1993. (9) Monfils, J. L.; Barendregt, S.; Kapur S. K.; Woerde, H. M. Upgrade Isobutane to Isobutylene. Hydrocarbon Process. 1992, 2, 47. (10) Sarathy, P. R.; Suffridge, G. S. Etherify Field Butanes. Part 2. Hydrocarbon Process. 1993, 2, 43. (11) Van Grieken, R.; Ovejero, G.; Serrano, D. P.; Uguina, M. A.; Melero, J. A. Synthesis of MTBE from Isobutane Using a Single Catalytic System Based on Titanium-Containing ZSM5 Zeolite. J. Chem. Soc., Chem. Commun. 1996, 1145. (12) Bellusi, G.; Carati, A.; Clerici, M. G.; Esposito, A. Double Substitution in Silicate by Direct Synthesis: a New Route to Crystalline Porous Bifunctional Catalysts. Stud. Surf. Sci. Catal. 1991, 63, 421. (13) Thangaraj, A.; Kumar, R.; Sivasanker, S. Evidence for the Simultaneous Incorporation of Aluminum and Titanium in MFI Structure. Zeolites 1992, 12, 135. (14) Ovejero, G.; Van Grieken, R.; Uguina, M. A.; Serrano, D. P.; Melero, J. A. Bifunctional Properties of Al-TS-1 Synthesized by Wetness Impregnation of Amorphous Al2O3-TiO2-SiO2 Solids Prepared by the Sol-Gel Method. Catal. Lett. 1996, 41, 69. (15) Uguina, M. A.; Ovejero, G.; Van Grieken, R.; Serrano, D. P.; Camacho, M. Synthesis of Titanium Silicalite-1 from an SiO2TiO2 Cogel Using a Wetness Impregnation Method. J. Chem. Soc., Chem. Commun. 1994, 27. (16) Serrano, D. P.; Uguina, M. A.; Ovejero, G.; Van Grieken, R.; Camacho, M. A. Synthesis of TS-1 by Wetness Impregnation of Amorphous SiO2-TiO2 Solids Prepared by the Sol-Gel Method. Microporous Mater. 1995, 4, 273. (17) Weitkamp, J.; Kleinschmit, P.; Kiss, A.; Berke, C. H. The Hydrophobicity Index: a Valuable Test for Probing the Surface Properties of Zeolitic Adsorbents or Catalysts. In Proceedings of the 9th International Zeolite Congress; Butterworth-Heinemann: Boston, MA, 1992; Vol. 2, p 79. (18) Huybrechts, D. R. C.; De Bruycker, L.; Jacobs, P. A. Oxyfunctionalization of Alkanes by H2O2 on Titanium Silicate. Nature 1990, 345, 240. (19) Thangaraj, A.; Kumar, R.; Ratnasamy, P. Catalytic Properties of Crystalline Titanium Silicalites. II. Hydroxylation of phenol with hydrogen peroxide over TS-1 zeolites. J. Catal. 1991, 131, 294. (20) Corma, A.; Esteve, P.; Martı´nez, A. Kinetics of the Oxidation of Alcohols by Hydrogen Peroxide on Ti-Beta Zeolite: the Influence of Alcohol Structure on Catalyst Reactivity. Appl. Catal. 1996, 143, 87. (21) Cunill, F. Tertiary Alkyl Ether Synthesis; Barcelona University: Spain, 1993. (22) Sato, T.; Dakka, J.; Sheldon, R. A. Titanium Substituted Zeolite Beta Catalyzed Selective Epoxidation of 1-Octene with Hydrogen Peroxide. Stud. Surf. Sci. Catal. 1994, 84, 1853. (23) Clerici, M. G. Oxidation of Saturated Hydrocarbons with Hydrogen Peroxide, Catalyzed by Titanium Silicate. Appl. Catal. 1991, 68, 249. (24) Neri, C.; Esposito, A.; Buonuomo, F.; Anfossi, B. Process for the Synthesis of Glycol Monomethylethers. Eur. Pat. Appl. 100,118, 1984. (25) Maspero, F.; Romano, U. Oxidation of Alcohols with H2O2 Catalyzed by Titanium Silicate-1. J. Catal. 1994, 146, 476. (26) Ovejero, G.; Van Grieken, R.; Uguina, M. A.; Serrano, D. P.; Melero, J. A. Study on the Titanium and Aluminium Coincorporation into the MFI Zeolitic Structure. Accepted for publication in J. Mater. Chem.

Received for review February 20, 1998 Revised manuscript received July 14, 1998 Accepted August 6, 1998 IE980113F