Methyl tert-butyl ether synthesis from tert-butanol via inorganic solid acid catalysis

Applied Catalysis A: General 183 (1999) 1±13

Methyl tert-butyl ether synthesis from tert-butanol via inorganic solid acid catalysis John F. Kniftona,*, John C. Edwardsb a

Shell Chemical Co., PO Box 1380, Houston, TX 77251-1380, USA b Texaco Inc., PO Box 509, Beacon, NY 12508, USA

Received 12 August 1998; received in revised form 28 October 1998; accepted 30 October 1998

Abstract Three classes of inorganic solid acid catalyst have been demonstrated to be effective for methyl tert-butyl ether (MTBE) syntheses from methanol/tert-butanol (TBA) feed mixtures using a continuous, plug-¯ow, reactor system. These catalysts include heteropoly acids, such as 12-tungstophosphoric acid and 12-molybdophosphoric acid, on Group III and IV oxide supports, such as titania, HF-treated montmorillonite clays, as well as mineral acid-activated clays. Changes in the structure of the 12-tungstophosphoric acid-on-titania during etheri®cation service have been investigated using 31 P and 1 H MAS NMR. An unexpected, in situ, phase separation of the desired MTBE plus isobutene products from aqueous methanol has been observed at high (>80%) tert-butanol conversion levels and operating temperatures 1608C. Milder etheri®cation conditions allow MTBE selectivites to 94 mol% and sustained etheri®cation activity for the HF/clay catalyst even with crude TBA feedstocks. # 1999 Elsevier Science B.V. All rights reserved. Keywords: MTBE; tert-Butanol; Acidic clays; Heteropoly acids

1. Introduction Methyl tert-butyl ether (MTBE) production capacity in the US now stands at ca. 240 000 barrels/day [1]. The majority of this MTBE is generated through isobutene etheri®cation with methanol Eq. (1) over an acidic (sulfonated) resin catalyst [2], the isobutene being introduced as a mixed C-4 alkene/alkane stream, such as Raf®nate-1, that typically has an isobutene content of 10±20%. Some MTBE is also produced commercially from tert-butanol ± a by-product of propylene oxide manufacture. In this case, the tertbutanol may be reacted directly with methanol in the *Corresponding author.

presence of an acid catalyst to give the desired MTBE Eq. (2), with water as a coproduct.

(1)

(2) For reaction (2) the choices of solid acid catalyst include:

0926-860X/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 8 6 0 X ( 9 8 ) 0 0 3 7 4 - 3

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J.F. Knifton, J.C. Edwards / Applied Catalysis A: General 183 (1999) 1±13



Hydrogen ion-exchange resins, such as Amberlyst 15 [3].  Inorganic solid acids. In this paper we describe the successful application of three classes of inorganic solid acid catalysts to the generation of MTBE via synthesis route (2). These classes of catalyst include:  Heteropoly acids on Group III and IV oxides [4].  Hydrogen fluoride-treated montmorillonite clays [5].  Mineral acid-activated montmorillonite clays [6]. We will focus on the use of these catalysts in continuous reactor con®gurations, where we have identi®ed certain intrinsic advantages to the use of the inorganic solid acids ± versus hydrogen ionexchange resins. These advantages of the inorganic solid acids ± based upon their greater thermal and oxidative stabilities ± include:  They allow etherification to be conducted at higher operating temperatures (>1408C) where rates of MTBE formation are extremely facile.  They allow the optional production of pure isobutene as an attractive coproduct.  They allow use of crude tert-butanol feed streams containing various oxygenated impurities, including peroxides. Prior studies by Adams et al. [3] have served to demonstrate the use of montmorillonite clay-derived catalysts for the production of MTBE from methanol/ tert-butanol mixtures under re¯ux, or batch autoclave, conditions. Kim et al. [7], have examined MTBE syntheses from methanol/tert-butanol combinations in the presence of unsupported heteropoly acids. In particular, they examined the effect of the acidity of the heteropoly compound upon the etheri®cation reaction. The limited information on reaction (2) has been reviewed [8]. 2. Experimental The heteropoly acid on Group III and IV oxide supports were prepared using the incipient wetness technique [9]. Two typical preparations are as follows: 1. In the case where 12-molybdophosphoric acid is impregnated on titania, a solution of 12-molybdophosphoric acid (10.0 g) in water (50 ml) was added, with stirring, to 125 cc of HSA titania

carrier extrudate (from Norton 1/8 in. extrudates, surface area 51 m2/g). The liquid was absorbed into the extrudates, with stirring, for 1±2 h. The mixture was then rotary evaporated to remove the excess liquid and calcined at 100±3508C in vacuo. Weight of recovered, yellow-colored, extrudates: 116 g. Analyses showed the presence of: Mo, 2.2%; P, 0.08%. 2. Where 12-tungstophosphoric acid is impregnated into titania, a solution of 12-tungstophosphoric acid (40.0 g) in water (150 ml) was added, with stirring to 125 cc of HSA titania carrier extrudates (from Norton 1/8 in. extrudates, surface area 60 m2/g). The liquid was absorbed into the extrudates, with stirring, for 1±2 h. The mixture was then rotary evaporated to remove the excess liquid and calcined at 150±3508C under slow nitrogen flow conditions. Weight of recovered, greycolored, extrudates: 151 g. Analyses showed the presence of: W, 17.0%; P, 0.33%. Samples of hydrogen ¯uoride-modi®ed montmorillonite clay were typically prepared as follows: To 100 g of neutral montmorillonite clay (Engelhard Grade F2C, powder) was added a solution of 48% hydro¯uoric acid (20 g) in distilled water (180 g). The mixture was stirred for 3±4 days at room temperature, the solids allowed to settle and the liquid decanted off. The residual solids were washed with distilled water, then methanol, and dried at 1508C in vacuo, followed by calcining at 5008C for 2 h. The recovered grey/white powder was found to comprise, by analysis: F, 1.2%. All mineral acid-treated montmorillonite clays were purchased from Engelhard and used as received. MTBE synthesis was conducted using a continuous, plug-¯ow, reactor system ®tted with temperature, pressure controls as well as equipment for monitoring temperature, pressure and ¯ow rate. In a typical experiment, the reactor was ®rst charged with 25 cc of 12-molybdophosphoric acid-on-titania extrudates, prepared as described supra. A screen of glass beads was placed at the top and bottom of the reactor to ensure the that extrudates would remain in the middle portion. The catalyst bed was ®rst conditioned overnight by washing with methanol/tert-butanol (2:1 mix) at 1008C, 20 bar back pressure and a liquid ¯ow rate of 25 cc/h. The same solution of methanol (1281.6 g, 40.0 mole) plus tert-butanol

Experiment

Catalyst

1

Mo±P/TiO2

2

W±P/TiO2

3

W±P/SiO2

4

W±P/Al2O3

a

Temperature (8C)

LHSV

100 120 150 150

1.0 1.0 1.0 4.0

100 150 150 180

Sample

Feed/product composition (wt%) MTBE

Isobutene

MeOH

TBA

H2O

TBA conversion (%)

Feed Product Product Product Product

23.2 35.6 36.2 34.8

2.2 5.0 7.6 6.5

47.5 38.5 32.5 32.1 33.5

52.4 30.2 16.1 11.7 14.4

6.0 10.6 11.6 10.7

42 69 78 73

87 82 75 77

1.0 1.0 8.0 1.0

Feed Product Product Product Product

28.6 36.9 26.1 24.0

1.4 6.7 4.0 8.4

47.1 37.0 32.5 37.5 43.4

52.7 26.1 12.2 23.4 5.8

7.7 11.6 8.8 18.2

50 77 56

90 77 75

150

1.0

Feed Product

45.9

8.7

46.4 27.9

53.5 10.0

7.5

81

89

150

1.0

Feed Product

39.7

6.2

45.9 31.8

54.0 12.8

9.4

76

81

Two-phase product liquid, analysis of heavier phase, lighter phase is rich in isobutene.

a

MTBE selectivity (mol%)

a

J.F. Knifton, J.C. Edwards / Applied Catalysis A: General 183 (1999) 1±13

Table 1 Methyl tert-butyl ether synthesis using oxide-supported heteropoly acid catalysts ± continuous studies, 20 bar

3

4

J.F. Knifton, J.C. Edwards / Applied Catalysis A: General 183 (1999) 1±13

(1482.4 g, 20.0 mole) was then pumped through the catalyst bed at 25 cc/h, while the reactor was held at 1008C, at a total pressure of 20 bar. Samples of product were taken periodically, either by trapping in a dry ice cooled container, or by collecting onstream (on line) in a 316 ss bomb. Typical analyses data for samples taken under these conditions are summarized in Table 1. Conversion of tert-butanol was estimated using the equation: …Wt% conc: of TBA in feed ÿ wt% conc: of TBA in product†  100 : wt% conc: of TBA in feed

conversions per pass, the product effluent now comprises two-phase mixtures: a lighter, isobutene±MTBE product rich phase, and a heavier, aqueous methanol rich phase. Etheri®cation presumably takes place via the intermediate formation of the corresponding C-4 tertiary carbocation. At the higher operating temperatures (e.g. >1208C), the substantial formation of the coproduct isobutene could originate either through tertbutanol dehydration, or MTBE product de-etheri®cation, as depicted in Eq. (3).

Yields of MTBE were estimated from: Moles of MTBE in product liquid  100 : moles of TBA converted 3. Results and discussion 3.1. Oxide-supported heteropoly acids Typical MTBE syntheses from tert-butanol plus methanol are illustrated in Table 1 for 12-molybdophosphoric acid (H3PMo12O40, designated Mo±P) and 12-tungstophosphoric acid (W±P) impregnated into high surface area titania, silica, and alumina supports. Each experimental series was conducted using a 25 cc capacity, plug-¯ow, continuous reactor over a range of etheri®cation conditions. Catalyst preparative procedures and MTBE syntheses conditions are detailed also in Section 2. The feedstock was a 2:1 molar mixture of methanol and tert-butanol. It may be seen from the data in Table 1 that:  MTBE may be selectively generated from methanol/tert-butanol mixtures in up to 90 molar%, at moderate temperatures, using the 12-tungstophosphoric acid-on-titania and 12-molybdophosphoric acid-on-titania catalysts (Expt. 1 and 2). These MTBE selectivities are substantially higher than those reported previously for heteropoly acids [7].  tert-Butanol conversion levels reaching 81% may be achieved at moderate feed rates (LHSV 1), and even at high throughputs (LHSV 4±8), the tertbutanol conversions exceed 70% (Expt. 1).  Increasing the etherification temperature generally leads to a greater isobutene (iso-C4) by-product make, so that by 1808C (Expt. 2), with >80% TBA

(3) Experiments such as Expt. 2 using the 12-tungstophosphoric acid-on-titania catalyst (17 wt% W loading, titania surface area 60m2/g), and those of Fig. 1, using the same 12-molybdophosphoric acid-on-titania catalyst (2.2 wt% Mo loading, titania surface area 51m2/g), illustrate the sustainable etheri®cation performance of these heteropoly acid-on-titania catalysts at very high LHSVs of ca. 8, particularly in the operating temperature range 140±1608C. Excellent MTBE selectivities and tert-butanol conversions were also realized, at lower space velocities, when the 12tungstophosphoric acid was supported on both silica and alumina (see Table 1, Expt. 3 and 4). However, our experience has been that the heteropoly acid moieties, particularly W±P, exhibit superior long-term stability in etheri®cation service when dispersed on commercially available titania supports (vide infra). In a previous publication in this series, we reported 31 P and 1 H MAS NMR spectra, as well as FT±IR data, for the fresh 12-tungstophosphoric acid-on-titania compositions [10], useful in applications such as MTBE syntheses. In that work we concluded that the adsorbed, highly acidic [11], 12-tungstophosphoric acid appeared to be present in a number of forms on the titania surface including: a bulk salt phase, [PW12O40]3ÿ (ÿ15 ppm), two weakly bound intact

J.F. Knifton, J.C. Edwards / Applied Catalysis A: General 183 (1999) 1±13

5

Fig. 1. MTBE production from MeOH:TBA (2:1 molar) as a function of operating temperature; catalyst: 12-molybdophosphoric acid-ontitania (2.2%); LHSV 8; 20 bar.

Keggin ion species (see Fig. 2), a range of partially fragmented clusters, such as the 11-``defect'' Keggin ion [12], and a range of species formed by high or complete fragmentation of the Keggin structure. The relative amounts of these species will vary depending on the form of the support. For example, adsorption is incomplete on extruded titania, resulting in a bulk salt formation, as found by others [12], whereas there was almost complete adsorption on pressed titania pellets, yielding predominantly fragmented Keggin units. Impregnation of powdered TiO2, likewise led to the complete adsorption (and fragmentation) without bulk salt formation. Solid-state 1 H high speed MAS NMR provided information regarding the acidic nature of the catalyst, as well as the degree of W±P dispersion. Adsorption of the Keggin ion onto the TiO2 surface leads to an increase in the acidity of the unit acid protons, as reported by Gao and Moffat [13], and broad resonances observed at 7±8 ppm are due to dispersion of acid sites on the surface. A second acidic resonance at about 11 ppm increased with adsorption

effectiveness and could be related to the formation of hydroxonium ions upon fragmentation of the adsorbed Keggin structure. In this work, we compared our published 31 P and 1 H MAS NMR data for the fresh W±P/TiO2 catalysts [10] with data collected after their extended use in MTBE service under the etheri®cation conditions typi®ed by Table 1. Generally, we ®nd that these oxide-supported 12-tungstophosphoric acid catalysts show excellent stability during etheri®cation service, however, there are some signi®cant changes in their 31 P and 1 H NMR spectra. In particular, we ®nd:  Extended usage in MTBE service [4] generally leads to a qualitative decrease in total phosphorus signal. Some loss of surface-bonded heteropoly acid moiety is evident as a function of time on stream, and this is most noticeable at temperatures >1608C, where there is product phase separation. These 31 P data are consistent with P-elemental analyses for the same recovered catalyst samples.

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J.F. Knifton, J.C. Edwards / Applied Catalysis A: General 183 (1999) 1±13

Fig. 2. Keggin ion structure.









In addition to the intact Keggin ion signal at ca. ÿ15 ppm, the used catalyst, after >200 h service also shows resonances for a range of partially and highly fragmented clusters in the region ÿ2 to 2 ppm. Continued MTBE service leads to brand new bands in the 4 ppm region due to organophosphate species. These spectral changes are illustrated in Fig. 3 for a typical W±P/TiO2 catalyst after 430±700 h of high-temperature service (140± 1658C). The P±OR, organophosphate, species on the titania surface were confirmed by Block decay and cross polarization experiments. 1 H solid state, Block decay, MAS data for the same series of used catalysts generally exhibit a significant shift in the remaining acidic protons, to 3.15± 3.22 ppm, and new resonances at 1.04±1.07 ppm due to the aliphatic protons of the coordinated organic, P±OR, species noted supra. A typical spectrum is illustrated in Fig. 4. There is still some evidence for the highly acidic protons [10] at 11.6 ppm. W±P/TiO2 catalyst samples recovered after protracted MTBE service (ca. 2000 h) exhibit little or no remaining proton concentration on the surface of the titania. 13 C NMR, on the other hand, revealed no evidence for measurable quantities of carbon on the used catalysts.

We have also examined the synthesis of methyl tertbutyl ether using the procedures and equipment of Table 1, but with a 12-tungstosilicic acid-on-titania catalyst. Again, the W±Si/TiO2 catalyst was prepared via the incipient wetness technique described in Section 2; the titania extrudates had a surface area of 51 m2/g. Typically, at 1208C, the estimated tert-butanol conversion level was 76% and the MTBE molar selectivity 88 mol% (see Table 2). Raising the feed space velocity to 6, TBA conversion levels of ca. 76% are sustainable for extended periods. Performance then is somewhat similar to those of the W±P/TiO2 and Mo±P/TiO2 catalysts illustrated in Table 1 and Fig. 1. Regarding the in¯uence upon etheri®cation performance of changes in the level of heteropoly acid loading on the oxide support, we conducted a series of comparative experiments, using 12-tungstophosphoric acid-on-titania catalysts, with different tungsten loadings. The data, summarized in Fig. 5, illustrate no statistically signi®cant trends in etheri®cation activity (TBA conversion), and a modest, four percentage points, drop in MTBE selectivity, as the metal loading is raised from 4 to 17 wt%. Lower tungsten loadings appear adequate for normal etheri®cation service. 3.2. HF-treated clays A clearer illustration of the isobutene±MTBE product phase separation phenomena at higher etheri®cation temperatures (Table 1) is provided here using a hydrogen ¯uoride-treated montmorillonite clay catalyst. This modi®ed clay was prepared by treating a neutral montmorillonite with 48% hydro¯uoric acid solution, under the conditions detailed in Section 2 to a ¯uoride content of 1.2%. Feeding the same plug-¯ow reactor of Table 1 a 1.1:1 molar mix of methanol and tert-butanol at a series of etheri®cation temperatures, we observe the onset of product phase separation at above 1508C and two distinct product phases in the operating temperature range 160±1808C. The compositions of each phase for two on-line samples taken at 1608C and 1808C are illustrated in Table 3. At both temperatures the lighter phase comprises isobutene and MTBE as the principal components and the heavier phase is mainly aqueous methanol. Estimated tert-butanol con-

J.F. Knifton, J.C. Edwards / Applied Catalysis A: General 183 (1999) 1±13

7

Fig. 3. 31 P NMR spectra for a series of new and used W±P/TiO2 catalysts: (a) fresh; (b) after 430 h of MTBE service; (c) after 530 h of MTBE service; and (d) after 700 h of MTBE service.

version levels at 1608C are ca. 84%; they exceed 90% at 1808C. Desired MTBE product is thereby concentrated in the lighter phase, while by-product water and recycle methanol are found primarily in the heavier phase. In general, for both the HF-treated clays and oxidesupported heteropoly acid catalysts, a methanol-totert-butanol feed mix close to molar stoichiometry

(i.e. containing very little excess MeOH), enhances the onset of MTBE product phase separation and leads to higher isobutane yields in the 160±1808C etheri®cation temperature regime (in accordance with the equilibria of Eq. (3)). The same HF-modi®ed montmorillonite clay catalyst has also proven to have excellent performance in MTBE service over extended periods using crude tert-

8

J.F. Knifton, J.C. Edwards / Applied Catalysis A: General 183 (1999) 1±13

Fig. 4. 1 H NMR spectrum of W±P/TiO2 catalyst after 530 h of MTBE service at 1658C.

butanol feedstocks. The data in Fig. 6 illustrate 60 days of MTBE service using a tert-butanol feedstock that also contained signi®cant quantities of water, acetone, isopropanol, di-tert-butylperoxide, tert-butyl formate, plus traces of MTBE recycle. Etheri®cation was conducted using a 2:1 molar mix of methanol and crude tert-butanol, conditions were 1408C, 20 bar, and LHSV 2. Over the 1200 h period, TBA conversion levels remained close to 71% with MTBE molar selectivities in the range 65±71%. The remaining coproduct is then primarily isobutene (31±24% molar selectivity). It is particularly noteworthy that neither the peroxide (di-tert-butylperoxide), nor the ester (tert-butyl formate), components in this crude TBA feedstock

affected the HF/clay performance, since it is known that these materials can lead to rapid deactivation of hydrogen ion-exchange resins [14]. Hydrogen ¯uoride treatment of the montmorillonite clays provides particularly active and stable acid sites ± more so than with the mineral acids (see Section 3.3). Related HF treatment is also very effective in generating active and stable acidic sites for alkylation [15]. 3.3. Mineral acid-activated clays The third class of inorganic solid acid catalyst effective for reaction (2) is the mineral acid-activated montmorillonite clays. These clays are commercially available and have titratable acidities up to at least

Table 2 Methyl tert-butyl ether synthesis using titania-supported 12-tungstosilicic acid catalyst ± continuous studies, 20 bar Experiment Catalyst

5

W±Si/TiO2

Temperature (8C)

LHSV

120 150 150

1.0 1.0 6.0

Sample

Feed Product Product Product

Feed/product composition (wt%) MTBE

Isobutene MeOH

42.5 36.4 37.9

3.2 7.5 5.6

46.5 31.6 34.2 33.7

TBA

H2O

TBA conversion (%)

53.5 12.9 10.4 13.0

9.6 10.6 9.7

76 81 76

MTBE selectivity (mol%) 88 77 78

J.F. Knifton, J.C. Edwards / Applied Catalysis A: General 183 (1999) 1±13

9

Fig. 5. MTBE production from MeOH:TBA (2:1 molar) as a function of tungsten loading; catalyst: 12-tungstophosphoric acid-on-titania; 1508C; LHSV 1; 20 bar.

16 mg KOH/gÿ1. Table 4 illustrates the synthesis of MTBE from methanol/tert-butanol mixtures for a series of mineral acid-activated montmorillonite clays with different levels of acidity 3±16 mg KOH/gÿ1 and differing physical forms (powders, granules, and extrudates), over a range of etheri®cation temperatures, feed rates, and compositions. Surface areas are typically in the range 275±350 m2/g. MTBE may be generated in up to 40% concentration with the powdered clay having a titratable acidity of 10 mg KOH/gÿ1 (Expt. 7) and up to 44% conc., with 94 mol% selectivity, using the more acidic granules (Expt. 9). Total MTBE plus isobutene selectivity under these conditions (2:1 MeOH/TBA feed, 1008C, LHSV 1) is essentially quantitative (Expt. 9). Etheri®cation activity drops off rapidly, however, below 1008C (e.g. TBA conversion 32% at 808C, Expt. 9). Nevertheless, the MTBE selectivities of Table 4 are substantially higher than those reported previously [3]. At higher feed throughputs (LHSV 8), the more acidic clay powders still provide up to 40% MTBE in

the product ef¯uent at 1508C, and while MTBE selectivities are now a little lower 78±81 mol%, TBA conversion levels remain in the 77±78% range at this temperature (Expt. 7 and 8). It is noteworthy that for mineral acid-treated clays there is a measurable loss in activity ± TBA conversions 77!71% ± over extended service, e.g. >300 h, 1508C, LHSV 8, 20 bar, see Expt. 8. The lower acidity extrudates (3/ 16 in. d) do not perform well in this temperature regime (Expt. 10). Another experimental series served to examine clay performance using a close to stoichiometric (1.1:1 MeOH/TBA) feed mix. Over the temperature range 100±1608C, TBA conversion levels now exceed 80% per pass (Expt. 11) and once again, as with the HFtreated clays (Table 3), at 1808C, 88% TBA conversion, we see an in situ product phase separation ± into an isobutene plus MTBE lighter phase and an aqueous methanol-rich heavier phase. The combined MTBE‡isobutene molar selectivity is once more near quantitative, but isobutene make (Eq. (3)) is consis-

10

Experiment

Catalyst

MeOH/TBA molar ratio

6

HF/clay

1.1:1

Temperature (8C)

120

Two-phase product mix.

2

Time on stream (days)

1

140

2

160

3

180

a

Feed rate LHSV

4

Sample

Product composition (wt%) H2O

TBA

MTBE

4.1 4.1 9.2 9.0

67.1 51.1 49.4 32.4 32.2

12.7 12.3 26.7 26.5

22.1 37.5

25.8 6.6

11.0 10.5

30.9 14.9

9.0 30.7

21.2 38.1

27.4 6.3

10.8 10.1

31.4 14.5

7a

0.8 29.6

7.8 52.2

68.9 5.8

2.8 5.6

19.4 6.4

8a

0.9 29.7

8.1 52.3

68.6 5.7

2.8 5.6

19.5 6.3

Feed 1 2 3 4

MeOH

3.4 3.8 7.7 8.0

32.5 28.4 30.1 23.8 24.1

5a

9.9 30.2

6a

Isobutene

J.F. Knifton, J.C. Edwards / Applied Catalysis A: General 183 (1999) 1±13

Table 3 Methyl tert-butyl ether synthesis using HF-treated clay catalysts ± continuous studies

J.F. Knifton, J.C. Edwards / Applied Catalysis A: General 183 (1999) 1±13

11

Fig. 6. MTBE production from MeOH: crude TBA mixture; catalyst: HF-on-montmorrilonite clay (1.2% F); 1408C; LHSV 2; 20 bar.

tently higher with the 1.1:1 feed composition. A similar trend was reported by Adams et al. [3] when studying MTBE production under both re¯ux and batch autoclave conditions. Predrying the clays in vacuo to a low water content (e.g. <1% as in [3]) did not appear to signi®cantly change their etheri®cation performance [6]. Pillaring of the clay with chlorohydrol plus certain phosphates may, however, lead to a further improvement in MTBE productivity [16], most likely because etheri®cation can now more readily take place in the inner layer space [17]. 4. Conclusions We have demonstrated the utility of three classes of inorganic solid acid catalysts for MTBE syntheses from methanol/tert-butanol mixtures using continuous, plug-¯ow reactors. These catalysts include heteropolyacids like 12-tungstophosphoric

acid and 12-molybdophosphoric acid on Group III and IV oxide supports such as titania, HF-treated montmorillonite clays and mineral acid activated clays. Changes in the structure of the 12-tungstophosphoric acid-on-titania during etheri®cation have been investigated via 31 P and 1 H MAS NMR. We conclude that extended usage in MTBE service does lead to measurable loss of heteropoly acids species from the surface of the titania, as well as changes in proton acidity, and the eventual build up of new organophosphate species, but no signi®cant carbon laydown. An unexpected phase separation of the desired MTBE and isobutene products from aqueous methanol has been discovered when operating the etheri®cation reaction (2) at high (>80%) tert-butanol conversion levels and operating temperatures 1608C. Under milder conditions, MTBE selectivity may reach 94 mol%, and etheri®cation activity for the HF/clay catalyst may be maintained for at least 1200 h using crude TBA feedstocks.

12

Table 4 Methyl tert-butyl ether synthesis using mineral acid ± treated montmorillonite clays ± continuous studies, 20 bar Experiment Clay catalyst

Temperature (8C)

LHSV

MeOH/TBA feed molar ratio

Product composition (wt%) TBA

H2O

2.2 5.0 6.0

35.4 32.5 31.8

25.2 11.9 12.1

7.6 11.5 9.9

54 78 78

87 75 81

31.9 36.6 37.9 35.7a

3.2 6.9 6.9 5.2

34.5 33.4 32.8 34.4

24.6 11.8 12.2 15.4

5.8 9.9 10.0 9.1a

54 78 77 71a

92 75 77 79a

2:1 2:1 2:1 2:1

18.2 44.0 44.4 38.6

1.4 2.6 3.8 6.2

40.2 30.8 30.6 33.4

36.6 14.1 12.4 11.1

3.6 8.5 8.8 10.2

32 74 77 79

91 94 91 77

1.0 1.0 6.0

2:1 2:1 2:1

15.8 33.7 15.1

3.3 8.7 4.6

40.8 33.7 40.7

36.4 15.6 35.8

3.7 8.2 3.8

32 71 33

78 75 71

2.0 2.0 2.0 4.8 2.0

1.1:1 1.1:1 1.1:1 1.1:1 1.1:1

21.4 38.1 37.2 31.6 38.5b 11.6b

3.2 12.9 16.0 19.3 27.6 8.2

24.0 18.3 23.0 20.2 19.5 43.0

47.0 19.7 11.6 17.2 8.8 8.3

3.0 10.4 11.9 11.5 4.9b 28.2b

32 71 83 75 88b

80 65 55 51 b b

Titratable acidity (mg KOH/g)

Surface area (m2/g)

Form

7

10

300

Powder

120 150 150

1.0 1.0 8.0

2:1 2:1 2:1

29.6 38.1 40.0

8

15

300

Powder

120 150 150 150a

1.0 1.0 8.0 8.0

2:1 2:1 2:1 2:1

9

16

300

Granules (20/60)

80 100 120 150

1.0 1.0 1.0 4.8

10

3

275

Extrudates 120 (3/16 in.) 150 150

11

3

350

Granules (20/60)

a b

After 340 h on stream. Two-phase product mixture.

100 140 160 160 180

MTBE

Isobutene

MTBE selectivity (mol%) J.F. Knifton, J.C. Edwards / Applied Catalysis A: General 183 (1999) 1±13

MeOH

TBA conversion (%)

J.F. Knifton, J.C. Edwards / Applied Catalysis A: General 183 (1999) 1±13

In comparison with hydrogen ion-exchange resins, the inorganic solid acids have the advantages of:  High MTBE productivity at temperatures above the thermal stability limits of acidic resins.  Stable, long-term performance with crude tertbutanol feedstocks containing peroxide and ester impurities.  Demonstrated ability to achieve in situ MTBE± isobutene product phase separation from the byproduct water (Eq. (2)) and recycle methanol at temperatures 1608C, thereby simplifying subsequent MTBE purification steps. Acknowledgements The authors wish to thank Huntsman Corporation for permission to publish this paper and Messrs. P. Lopez, R. White and M. Banks for technical assistance. References [1] Chemical Marketing Reporter, 1 October 1995. [2] Chemical Economics Handbook ± SRI International, Report 543.7500, May 1994.

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[3] J.M. Adams, K. Martin, R.W. McCabe, S. Murray, Clay and Clay Minerals 34 (1986) 597. [4] J.F. Knifton, US Patent 4 827 048 (1989) to Texaco Chemical Company. [5] J.F. Knifton, US Patent 5 157 161 (1992) to Texaco Chemical Company. [6] J.F. Knifton, US Patent 5 099 072 (1992) to Texaco Chemical Company. [7] J.-S. Kim, J.-M. Kim, G. Seo, N.-C. Park, H. Niiyama, Appl. Catal. 37 (1988) 45. [8] R.J.J. Jansen, H.M. van Veldhuizen, M.A. Schwegler, H. van Bekkum, Recl. Trav. Chim. Pays-Bas 113 (1994) 115. [9] J.F. Knifton, N.J. Grice, US Patent 4 683 335 (1987) to Texaco Inc. [10] J.C. Edwards, C.Y. Thiel, B.L. Benac, J.F. Knifton, Catal. Lett. 51 (1998) 77. [11] K. Na, T. Iizaki, T. Okuhara, M. Misono, J. Mol. Catal. A 115 (1997) 449. [12] L.R. Pizzio, C.V. Caceres, M.N. Blanco, Appl. Catal. A 167 (1998) 283. [13] S. Gao, J.B. Moffat, Catal. Lett. 42 (1996) 105. [14] J.R. Sanderson, J.F. Knifton, US Patent 5 741 952 (1998) to Huntsman Specialty Chemicals Corp., and references therein. [15] J.F. Knifton, P.R. Anantaneni, M.E. Stockton, US Patent 5 770 782 (1998) to Huntsman Petrochemical Corp. [16] J.F. Knifton, US Patent 5 352 847 (1994) to Texaco Chemical Company. [17] J.M. Adams, Appl. Clay Sci. 2 (1987) 309.