Alkylation of 2-methoxynaphthalene with propylene oxide using titanium and zirconium containing molecular sieves

Applied Catalysis A: General 204 (2000) 117–127

Alkylation of 2-methoxynaphthalene with propylene oxide using titanium and zirconium containing molecular sieves Axel Brait, Mark E. Davis∗ Chemical Engineering, California Institute of Technology, Pasadena, CA 91125, USA Received 13 December 1999; received in revised form 1 March 2000; accepted 3 March 2000

Abstract Titanium and zirconium containing molecular sieves are active catalysts for the alkylation of 2-methoxynaphthalene (2-MN) with propylene oxide (PO). Temperatures above 423 K are necessary in order to promote the alkylation reaction. A major competing reaction that deactivates the catalyst is the oligomerization of PO that provides for deposition of these oligomers on the catalyst surface. A high 2-MN to PO ratio as well as the addition of PO in a semi-batch mode of operation help minimize the oligomerization reaction. The main reaction products are an O-alkylated product (I) and four C-alkylated products: 1-(2-methoxy-1-naphthyl)-2-propanol (II), 2-(2-methoxy-1-naphthyl)propanol (III), 1-(6-methoxy-2-naphthyl)-2-propanol (IV) and 2-(6-methoxy-2-naphthyl)propanol (V). The conversion of the limiting compound (PO) can be as high as 50% with selectivities towards the desired product V ranging from 12 to 20%. The ratio of 2,6- to 1,2-product is 1.6 for Ti-BEA, while Zr-BEA shows a value of 3.0. The shape-selective effect of the molecular sieve catalysts can be enhanced by passivation or poisoning of the outer surface of the catalyst by treatment with tetraethylorthosilicate (TEOS), tris[2-(diphenylphosphino)ethyl]phosphine (TETRAPHOS-II), ethylenediaminetetraacetic acid (EDTA) or hydrogen peroxide (H2 O2 ). For these cases, the 2,6- to 1,2-products ratios can reach values of up to 4. No leaching of Ti or Zr from the molecular sieve materials is observed. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Alkylation; Lewis acid; Aromatics; Propylene oxide; Naproxen; Molecular sieves

1. Introduction Previously, we have shown that Ti-BEA is an active catalyst for the alkylation of 2-methoxynaphthalene (2-MN) with propylene oxide (PO) at 353–423 K when treated with electron withdrawing ligands such as perfluoro-tert-butanol or pentafluorophenol [1]. However, leaching of Ti into solution due to the interaction of these ligands with the Ti-sites in the molecular sieve is one of the major problems ob∗ Corresponding author. Tel.: +1-626-395-4251; fax: +1-626-568-8743. E-mail address: [email protected] (M.E. Davis).

served. Additionally, pentafluorophenol itself, when used as solvent, is active in the alkylation of 2-MN and produces a significant background reaction. The formation of Zr containing molecular sieves has been described by various research groups [2–4]. The reported results suggest that zirconium is not isomorphically substituted into the framework but rather grafted onto the framework. Like Ti-BEA, Zr-BEA may be able to catalyze the alkylation of 2-MN with PO. Here, we report the use of titanium and zirconium containing molecular sieve materials in the alkylation of 2-MN with PO in the absence of electron withdrawing ligands at 473 K. Additionally, the influence of

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surface passivation on the reaction activity and product selectivities will be presented. If accomplished, the alkylation of 2-MN with PO would be a preferred method for the production of a precursor for the non-steroidal, anti-inflammatory agent naproxen ([1] and references therein). 2. Experimental 2.1. Catalyst preparation 4,40 - Trismethylenebis(N-methyl-N -benzylpiperidinium)hydroxide was used as the structure-directing agent (SDA) for the synthesis of Ti-BEA [5]. The SDA was prepared by treating a solution of 4,40 trismethylenebis(N-methylpiperidine) (Aldrich) in ethyl acetate with 2.5 equivalents of benzylbromide (Aldrich) at room temperature for 12 h. The precipitated 4,40 -trismethylenebis(N-benzyl-N-methylpiperidinium)bromide was filtered, washed with acetone and dissolved in boiling ethanol. After cooling to room temperature, acetone was added to initiate the recrystallization, while the solution was kept in the refrigerator. The purified 4,40 -trismethylenebis(N-benzyl-Nmethylpiperidinium)-bromide was ion-exchanged to the hydroxide form with AG1-X8 resin (Biorad). Ti-BEA was prepared by partially hydrolyzing 0.28 mol of tetraethylorthosilicate (TEOS, Aldrich) with a solution of 0.056 mol SDA in 8.4 mol of water. Within ca. 1 h, the initially biphasic mixture became homogeneous. Next, the reaction flask was flushed with argon and a solution of 0.0053 mol titanium(IV) isopropoxide (Aldrich) in 16 ml isopropanol (anhydrous, 8 vol.%) was added dropwise with stirring. This solution was stirred for 1 h and then the alcohols formed were evaporated off (evaporated water was added back). The solution was stirred overnight to ensure complete reaction and evaporation of all alcohols. The solution was charged into Teflon-lined autoclaves and Ti-BEA was formed after 5 days of heating at 413 K with rotation under autogeneous pressure. The crystallized Ti-BEA was centrifuged (very small particle size did not allow filtration), washed with water, air-dried and the product calcined at 823 K under an oxygen atmosphere. The Ti-BEA formed had an X-ray diffraction pattern indicative of pure *BEA and a diffuse reflectance UV–VIS spectrum

showing a single absorption centered at 210 nm [5]. The Si/Ti-ratio of the catalysts prepared was ∼40. Zr-BEA was prepared in a manner similar to Ti-BEA by using zirconium propoxide in anhydrous isopropanol rather than the titanium alkoxide. During the addition of zirconium alkoxide, the solution became very cloudy. As for the case of Ti-BEA, the calcined Zr-BEA sample showed an X-ray diffraction pattern indicative of pure *BEA and the Si/Zr-ratio was ∼40. To evaluate the influence of heteroatoms in the *BEA topology, pure Si-BEA was prepared. The synthesis was the same as for Ti-BEA except that no Ti containing solution was added. Again, the X-ray diffraction pattern was indicative of pure *BEA. To study the influence of particle size, and hence, diffusional limitations, a batch of Ti-BEA was prepared via a fluoride method to obtain larger zeolite crystals. In this case, 0.1 mol TEOS was mixed with 0.00167 mol titanium(IV) isopropoxide and 6 ml isopropanol. This mixture was added to a solution of 0.056 mol tetraethylammonium fluoride in 34 ml water and stirred overnight. The alcohols formed were then evaporated off (evaporated water was added back). The mixture was charged into Teflon-lined autoclaves and Ti-BEA was formed after 7 days of heating at 413 K with rotation under autogeneous pressure. Electron microscopy analysis showed crystal sizes in the range of 3–6 ␮m for this Ti-BEA sample, while the samples of Ti-BEA and Zr-BEA prepared via a fluoride-free route had crystal sizes below 1 ␮m. The Si/Ti-ratio for the large crystal Ti-BEA was ∼100, 2.5 times higher than with Ti-BEA prepared via a non-fluoride route. For comparison, the mixed oxides SiO2 /TiO2 and SiO2 /ZrO2 were prepared by prehydrolyzing TEOS with water and subsequently adding the respective metal alkoxide via a syringe pump. The solutions were dried and the powders calcined at 823 K under an oxygen atmosphere. The ratios of Si/Ti and Si/Zr of these mixed oxides were ∼35 and ∼30, respectively. Passivation of the outer surface of Ti-BEA was performed in various ways, e.g. by treatment with TEOS, treatment with tris[2-(diphenyl-phosphino)ethyl]phosphine (TETRAPHOS-II) or treatment with an aqueous solution of ethylenediaminetetraacetic acid (EDTA) or hydrogen peroxide (H2 O2 ). For the TEOS treatment, 1.0 g of calcined Ti-BEA was stirred in

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10 ml of hexane with 0.33 ml TEOS under argon for 3 h. The catalyst was then filtered off and rinsed with hexane, dried overnight and calcined in air at 823 K. TETRAPHOS-II treatment was performed by mixing 1.0 g of calcined Ti-BEA with 0.15 g of TETRAPHOS-II and holding this mixture overnight at 473 K. Then, the Ti-BEA was refluxed for 4 h in dichloroethane, filtered, washed thoroughly with dichloroethane and dried at 473 K. For the EDTA treatment, 1 g of H4 -EDTA was dissolved in 0.1 N NaOH (∼50 ml) to yield a clear solution, the solution was brought to reflux and 2.0 g as-synthesized Ti-BEA was added. After refluxing for 3 h, the catalyst was filtered off, washed with water, dried and calcined in air at 823 K. Treatment with H2 O2 was carried out by refluxing 2.0 g as-synthesized Ti-BEA in 50 ml water and slow addition of 12 ml H2 O2 (30 wt.% in H2 O). After refluxing for 3 h, the catalyst was separated, rinsed with water, dried and calcined at 823 K. The EDTA and H2 O2 treatments were also combined with one as-synthesized Ti-BEA sample.

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more conventional mol% which may be confusing as it can be related to the total amount of 2-MN present or the maximal achievable conversion determined by the amount of PO. TGA analyses were carried out on a DuPont 951 Thermogravimetric Analyzer. The catalyst samples used were washed with THF and dried at 373 K in air before measurement. Approximately 13 mg of sample were used and heated in air with 10 K/min from room temperature to 1073 K. Elemental analyses of Ti, Zr, and Si in catalytic materials and reaction solution were performed by Galbraith Laboratories, Inc. The estimated amount of Ti and Zr was used as a measure of the catalytically active sites for the calculation of turnover numbers (TONs, see Fig. 5). 13 C-solid state MAS NMR measurements were carried out on a Bruker AM 300 spectrometer at a frequency of 75.6 MHz. The spinning speed of the rotor was 4 kHz and between 6000 and 25 000 scans were accumulated for a spectrum. 3. Results and discussion

2.2. Catalytic reactions 3.1. Reaction products of 2-MN and PO The reactions were conducted at 473 K in a PARR autoclave. 2-MN was melted at 373 K in the autoclave and the catalyst (held at 473 K for >24 h in drying oven), in amounts varying between 100 and 2000 mg, was added to the liquid. The autoclave was closed and flushed with argon. After heating the autoclave to 473 K and adjusting the stirrer speed (600 rpm) and the reactor pressure (1.0 MPa Ar-atmosphere), PO, diluted with 1,2-dichloroethane, was added via a syringe pump over a time of generally 3 h. The reaction was performed for 3–7 h, after which the reaction was quenched to room temperature. After the reaction, the catalyst (if a solid was used) was separated from the reaction solution and washed with tetrahydrofuran (THF) to remove all physically adsorbed products. The reaction solution was combined with the THF extract and tri-tert-butylbenzene was added as an external standard. The reaction was analyzed by gas chromatography using an HP-5 column with a temperature program from 423 to 553 K. As the limiting compound in this reaction is PO and 2-MN is used in large excess, we have chosen to report the conversion in millimoles of 2-MN rather than the

The reaction of 2-MN with PO leads to one O-alkylated product (I) and four C-alkylated products (II–V). In C-alkylation, the kinetically favored products are the 1,2-alkylated 2-MN compounds, while the less sterically hindered 2,6-alkylated 2-MN compounds are the thermodynamically more stable products [6]. The reaction products are depicted in Scheme 1. The C-alkylated products are 1-(2-methoxy1-naphthyl)-2-propanol (II), 2-(2-methoxy-1-naphthyl)propanol (III), 1-(6-methoxy-2-naphthyl)-2-propanol (IV) and 2-(6-methoxy-2-naphthyl)propanol (V). Additionally, significant amounts of a product resulting from alkylation on the oxygen were observed (although not conclusively proven, the best assignment is denoted as I in Scheme 1) as were small amounts of higher alkylated products. This O-alkylated product may be due to an ether cleavage caused by the Lewis acid catalysts and a subsequent alkylation of the naphthol formed. An experiment using 2-naphthol instead of 2-MN showed no C-alkylated products but only the O-alkylated product. The O-alkylated product was generally formed with the highest

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Scheme 1. Reaction products in the alkylation of 2-methoxynaphthalene with propylene oxide.

selectivity (>28%). The selectivity towards the desired C-alkylated product (V) varied between 10 and 20%. The catalysts used in the alkylation of 2-MN with PO deactivate, probably due to the deposition of PO oligomerization products (vide infra). TGA measurements on the catalysts used in a reaction with PO alone (without 2-MN) show a loss of mass between 473 and 873 K that is not observed when 2-MN alone is contacted with the catalyst (see Fig. 1). Additionally, 13 C MAS NMR spectra show resonances at the same positions for the reaction of PO alone and the reaction of PO with 2-MN, while these peaks are absent in the reaction of 2-MN alone (see Fig. 2). The sharp resonances seen in all three experiments are due to residual solvent (THF), while the broad peaks at ∼26 and ∼75 ppm are due to the reaction products of PO. Opposite to the results obtained when fluorinated ligands were used to enhance the Lewis acidity of the active sites [1], there was no leaching of Ti or Zr into the reaction solution. The Ti and Zr concentrations in solution for experiments with or without catalysts did not show any significant differences (the longest reaction period was 62 h).

3.1.1. Alkylation of 2-MN The catalysts studied here for the alkylation of 2-MN are Ti-BEA, Zr-BEA, Si-BEA, and the mixed oxides TiO2 /SiO2 and ZrO2 /SiO2 . Two different Ti-BEA samples were used, one prepared via a fluoride route to provide larger crystals (3–6 ␮m) and another prepared via a hydroxide route to give a smaller crystal size (<1 ␮m). To ascertain whether regeneration of the Ti-BEA catalyst is possible, a deactivated Ti-BEA sample was recalcined after the alkylation reaction in oxygen at 823 K and used again in the alkylation of 2-MN. Results of the alkylation reaction are shown in Table 1. No reaction was observed at the conditions listed in the absence of a solid. Si-BEA shows a negligible conversion of 2-MN to any O- or C-alkylated products. However, the amount of PO deposited on Si-BEA is 8.9 wt.% or 0.38 mmol of PO (5.4% of the added PO). This result suggests that silanol groups on the Si-BEA can catalyze the PO oligomerization. Ti-BEA and Zr-BEA show activity for the alkylation of 2-MN in the absence of electron withdrawing groups at 473 K. For both cases, the product formed

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Fig. 1. TGA measurements of used catalysts after reaction: (a) reaction of propylene oxide alone; (b) reaction of 2-methoxynaphthalene alone; (c) reaction of 2-methoxynaphthalene with propylene oxide.

in highest amount is the O-alkylated product (38% selectivity). The main differences between the Ti-BEA and Zr-BEA samples lie in the amount of 2-MN converted to alkylated products (2.2 versus 1.3 mmol) and the ratio of 2,6/1,2-C-alkylated products (1.6 versus. 3.0). While titanium is located within framework sites, this is still a matter of debate for zirconium [2,4]. If Zr were grafted onto the internal surface of the molecular sieve, an additional space restriction could exist within the micropore structure and may be a cause of the higher 2,6/1,2-ratio. Both catalysts have a comparable crystal size excluding the possibility of a higher diffusional constraint due to larger crystals for Zr-BEA (as shown with Ti-BEA prepared via the fluoride route). However, the lower conversion of Zr-BEA could also be due to a lower Lewis acidity of the zirconium sites, as a faster deactivation by PO oligomerization does not seem to be the cause; there is a lower amount of deposit on the Zr-BEA catalyst (8.5% for Zr-BEA

compared to 11% for Ti-BEA). In this case, the porous system of Zr-BEA will be more effectively used before deactivation than Ti-BEA which would lead to a higher 2,6/1,2-ratio in the C-alkylated products (as a higher proportion of the reaction is within the porous system). This would correspond to a higher effectiveness factor η for the Zr-BEA than the Ti-BEA catalyst. To estimate the reactivity of the deactivated catalyst, additional PO was added via the syringe pump after the first run. No additional alkylation activity was observed (see below). The regeneration of Ti-BEA by calcination at 823 K in oxygen completely restored the catalytic activity (Table 1). A comparison of Ti-BEA prepared by two different synthesis routes (OH versus F) shows that larger crystals are less active (although it has to be kept in mind that the larger crystal Ti-BEA has roughly half the amount of Ti). The conversion of 2-MN drops to 0.28 mmol of converted 2-MN, eight times lower than

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Fig. 2. 13 C CPMAS NMR spectra of used catalysts after reaction: (a) reaction of propylene oxide alone; (b) reaction of 2-methoxynaphthalene alone; (c) reaction of 2-methoxynaphthalene with propylene oxide. Sharp resonances are from tetrahydrofuran.

Table 1 Alkylation of 2-methoxynaphthalene with propylene oxidea Catalyst

Ti-BEA Ti-BEA(F− ) Ti-BEAd Zr-BEA ZrO2 /SiO2 TiO2 /SiO2 Si-BEA

Conversion of 2-MN (mmol)

Selectivity (%)

2,6/1,2-ratio

Weight loss at 473–873 Kb (%)

O-alkylated product

V

Higher alkylated product

2.2 0.28 2.1 1.3 0.31 0.59 0.039

38 37 38 38 28 30 77

14 20 14 17 17 19

6.4 n.d.c 5.1 7.4 2.0 4.7

1.6 4.3 1.5 3.0 1.1 1.3

e

e

11 13 11 8.5 9.5 20 8.9

e

a Reaction conditions: 473 K, 1.0 MPa Ar-pressure, 112 mmol 2-MN, 250 mg catalyst, addition of 7.1 mmol PO in 5 ml dichloroethane via a syringe pump, 7 h reaction time. b From TGA data. c Not detected. d Deactivated Ti-BEA calcined at 823 K in O . 2 e Only 1,2-product observed.

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with the smaller crystals. This is significantly lower than the conversion for the small crystal Ti-BEA even when the lower Ti-content is taken into account. However, the ratio of 2,6- to 1,2-C-alkylated products was 4.3, nearly three times higher than with the smaller crystals. The larger crystals can impose strong pore diffusion limitations on the catalytic reaction of 2-MN and lead to a preferential formation of the 2,6-product. As discussed below, the higher 2,6- to 1,2-C-alkylated ratio may be due to an increased isomerization activity because of the longer diffusion path through the crystal. The mixed oxides TiO2 /SiO2 and ZrO2 /SiO2 reveal a lower conversion of 2-MN than the small *BEA crystals. For the case of TiO2 /SiO2 , the amount of PO oligomers deposited on the catalyst surface was twice as much as for the case with Ti-BEA. On the contrary, with ZrO2 /SiO2 , the amount of deposited PO oligomers was comparable to the amount formed with Zr-BEA and slightly lower than with Ti-BEA. The lower conversion of 2-MN with ZrO2 /SiO2 suggests a lower acidity of this catalyst compared to TiO2 /SiO2 , which is in line with the results from the *BEA samples containing Ti or Zr. The difference in the 2,6/1,2-C-alkylated product ratio was very distinct for the Zr containing catalysts, namely 1.1 for the mixed oxide and 3.0 for the molecular sieve. For the Ti containing catalysts, this ratio did not vary very much, from 1.3 for the mixed oxide to 1.6 for the molecular sieve. These results seem to suggest that, at 473 K, external surface reactivity may be dominating the observed catalytic behavior for the Ti containing sample. On the other hand, for the Zr containing samples, a shape selectivity effect exists and may be due to the grafting of Zr onto the surface rather than its incorporation into the framework which could increase the space restrictions within the pores of Zr-BEA. However, as mentioned above, the higher 2,6/1,2-ratio could also be due to a more effective usage of the catalyst interior with Zr-BEA since the activity of surface Zr should be lower than for surface Ti on Ti-BEA. A comparison of the conversion over Ti-BEA (first entry in Tables 1 and 4) showed a difference in conversion of as much as 40% (2.2 versus 3.1 mmol 2-MN). At this time, this can only be explained by various minute amounts of water present in the system. Water seems to decrease the activity of these catalysts. Additionally, the difference in conversion

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can be due to different batches of Ti-BEA. UV–VIS spectroscopy showed slightly different spectra that indicate small differences in the Ti environment of the different Ti-BEA catalyst batches. 3.1.2. Influence of pretreatments of Ti-BEA on alkylation of 2-MN 3.1.2.1. Passivation/poisoning of outer surface acidity. As there are no space restrictions on the reactants in the alkylation reaction on the outer surface, no preferential product formation due to shape selective effects can be observed if the reaction is occurring on the surface. Various methods of passivation of the outer surface of the Ti-BEA catalysts, including treatment with TEOS, TETRAPHOS-II, EDTA or hydrogen peroxide (H2 O2 ) were explored. The reaction results using these catalysts are shown in Table 2. The passivation of Ti-BEA with TEOS decreased the conversion of 2-MN from 2.2 mmol (Table 1) to 1.1 mmol 2-MN, but did not significantly change any of the product selectivities. Small crystals of *BEA are very hard to passivate with a layer of silica, and the results shown here are consistent with previous reports [7]. The passivation of the large crystal Ti-BEA(F− ) does not significantly alter its reaction behavior. Since silica passivation is much more effective for large crystals [7], the reaction over this material suggests that it is intrazeolitic. Poisoning the outer surface with a large molecule like TETRAPHOS-II leads to a catalyst with a five-times lower activity than the original Ti-BEA. In this case, the selectivity towards the O-alkylated product is decreased to 28%, while the selectivity towards product V is increased to 18%. Additionally, the reaction results are similar to those of the large crystal Ti-BEA(F− ). The PO oligomers deposited on the catalyst are half the amount of that on the original Ti-BEA. This result points towards a strong diffusional limitation or restricted accessibility, may be due to a pore blockage by the large phosphine molecule. Additionally, the ratio of the primary to the secondary alcohol formed in the 2,6-products (IV, V) changed from ∼1 for most of the reactions studied to 1.5. The treatment of Ti-BEA with EDTA, H2 O2 , or both did show a slight increase in the 2,6 to 1,2-ratios of the C-alkylated products. The Ti analyses after the treatment did not show a significant difference in the

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Table 2 Alkylation of 2-methoxynaphthalene with propylene oxidea Catalyst

A B C D E F

Conversion of 2-MN (mmol)

Selectivity (%) O-alkylated product

V

Higher alkylated product

1.1 0.30 0.46 2.5 2.4 2.6

38 44 28 37 34 33

17 16 18 15 16 16

n.d.b n.d.b 6.9 9.1 9.1 9.2

2,6/1,2-ratio

Weight loss at 473–873 K (%)

1.5 4.2 3.9 1.9 1.8 1.9

7.0 14 4.8 14 12 16

a

Reaction conditions: 473 K, 1.0 MPa Ar-pressure, 112 mmol 2-MN, 250 mg catalyst, addition of 7.1 mmol PO in 5 ml dichloroethane via a syringe pump, 7 h reaction time; A: Ti-BEA passivated by treatment with tetraethylorthosilicate, B: Ti-BEA(F− ) passivated as in A, C: Ti-BEA treated with TETRAPHOS-II, D: Ti-BEA treated with EDTA, E: Ti-BEA treated with H2 O2 /H2 O (30 wt.%); F: Ti-BEA treated with EDTA and H2 O2 /H2 O (30 wt.%). b Not detected.

Ti-content of the solids. Obviously, very little titanium is extracted by these methods. UV–VIS spectra obtained from the treated catalysts did not indicate the formation of octahedrally coordinated Ti in/on the solid.

The lower the amount of PO added, the higher the relative amount of PO converted to alkylation products. For example, 55% of the PO is converted to alkylated products when 3.6 mmol of PO are added over 3 h, whereas 5.2% of PO is converted to alkylated products when 42 mmol of PO are used. When an additional 3.6 mmol of PO are added over another 3 h (Entry 2 in Table 3), no increase in the conversion of 2-MN is observed. However, the amount of deposited oligomer on the catalyst surface increased from 7.5 to 9.6%. This suggests that the alkylation reaction of 2-MN with PO is quickly deactivated even though the catalyst is still active for PO oligomerization. Increasing amounts of added PO lead to a decrease in the selectivity towards the O-alkylated product (from 35% down to 29%). However, the amount of higher alkylated compounds increased strongly (from

3.1.3. Influence of reaction conditions 3.1.3.1. Concentration of reactants. To evaluate the influence of the amount of PO added, different amounts of PO were fed via the syringe pump. The amount of PO added was between 3.6 and 42 mmol of PO, corresponding to a 2-MN to PO ratio of 31 and 2.7, respectively. The reaction results are shown in Table 3. An increase in the amount of PO fed into the reactor did not result in a linear increase in the conversion of 2-MN; the conversion rather goes through a maximum with increasing amounts of PO. Table 3 Alkylation of 2-methoxynaphthalene with propylene oxidea Added PO (mmol)

3.6 7.2b 7.1 14 21 42c

Conversion of 2-MN (mmol)

Selectivity (%) O-alkylated product

V

Higher alkylated product

1.9 1.9 2.4 2.5 2.3 1.8

35 36 33 32 32 29

15 16 14 13 13 12

4.8 4.7 10 15 15 21

2,6/1,2-ratio

Weight loss at 473–873 K (%)

1.5 1.6 1.2 1.1 1.1 1.1

7.5 9.6 11 17 20 23

a Reaction conditions: 473 K, 1.0 MPa Ar-pressure, 112 mmol 2-MN, 0.25 g of the catalyst Ti-BEA, addition of PO diluted in dichloroethane via a syringe pump (addition of a volume of 3 ml), 3 h reaction time. b 3.6 mmol PO for 3 h; afterwards, addition of another 3.6 mmol. c Pure PO without added dichloroethane.

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Fig. 3. Reaction of 2-methoxynaphthalene (112 mmol) and propylene oxide at 473 K and 1.0 MPa Ar-pressure, with 250 mg Ti-BEA as the catalyst. Influence of different amounts of propylene oxide added.

4.8 to 21%). Additionally, the deposition of oligomers increased with increasing PO, while the conversion of PO to alkylated products goes through a slight maximum (see Fig. 3). The results presented above show that the concentration of PO influences the deactivation of the catalyst by PO oligomerization. Thus, a low PO concentration is the most desirable. On the other hand, the concentration of 2-MN should be as high as possible as there exists a linear relation between the amount of 2-MN in the reaction solution and the conversion of 2-MN (Fig. 4). The concentration of 2-MN in the reaction does not have an influence on the amount of deposited oligomers on the catalyst, as these values were 18, 15 and 17 wt.% of the catalyst for the amounts of 6.4, 65 and 112 mmol 2-MN, respectively. 3.1.3.2. Influence of catalyst mass. When the amount of catalyst used in the alkylation of 2-MN was varied between 125 and 750 mg, surprisingly, the conversion to alkylated products varied only from 1.3 to 1.9 mmol 2-MN and did not increase linearly with catalyst mass. As depicted in Fig. 5, the turnover numbers for the alkylation of 2-MN decrease with catalyst mass from 35 to 7.5. Turnover numbers were calculated by dividing the amount of converted 2-MN by the amount of Ti measured by elemental analysis.

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Fig. 4. Reaction of 2-methoxynaphthalene and propylene oxide (14 mmol) at 473 K and 1.0 MPa Ar-pressure, with 250 mg of Ti-BEA as the catalyst. Influence of different amounts of the reactant 2-methoxynaphthalene.

Though the actual values my not be correct (different contributions of Ti-sites inside and outside the microporous systems), the trend will be as the same sample of catalyst was used. The oligomerization and deposition of PO on the catalyst surface on the other hand increases linearly with catalyst mass, i.e. the percentage of PO oligomers deposited on the catalyst is always the same (Fig. 5). The amount of PO

Fig. 5. Reaction of 2-methoxynaphthalene (112 mmol) and propylene oxide (3.6 mmol) at 473 K and 1.0 MPa Ar-pressure. Influence of catalyst mass on the activity for alkylation.

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Table 4 Isomerization activity of the C-alkylated productsa Catalyst

Ti-BEAb Ti-BEAc Ti-BEAd

Conversion of 2-MN (mmol)

3.1 3.3 2.9

Selectivity (%)

2,6/1,2-ratio

O-alkylated product

V

Higher alkylated product

34 35 50

15 15 11

8.8 8.8 5.8

1.6 2.1 5.1

a

Reaction conditions: 473 K, 1.0 MPa Ar-pressure, 112 mmol 2-MN, 250 mg catalyst. Initial experiment after which the solid is removed and the liquid-phase used as the starting solution for further reaction. c 7 h of reaction time with a fresh Ti-BEA sample. d 48 h of reaction time with a fresh Ti-BEA sample. b

oligomerized and deposited on the catalyst increased from 6.0 to 34% of the PO available. This may very well be the reason for the decrease in turnover numbers for the alkylation, as more and more PO is used in oligomerization and deposition and less of it is available for the alkylation reaction. 3.1.4. Isomerization of the C-alkylated products We have shown that Ti-BEA is active for the isomerization of the 1,2-C-alkylated product when pentafluorophenol is used as a Lewis acidity enhancing ligand [1]. In the experiments performed with pure Ti-BEA, the 2,6/1,2-C-alkylated ratio showed an increase with longer reaction time, i.e. if the reaction was performed over 7 h instead of 3 h. To study a possible isomerization activity of the Ti-BEA catalyst, the catalyst was separated from the reactants after the first reaction run. The reactants were melted at 473 K, new Ti-BEA was added and the reaction performed for another 7 or 48 h. The results are shown in Table 4. Essentially, no change occurred in the amount of converted 2-MN after 7 h. However, the 2,6- to 1,2-C-alkylated product ratio increased from 1.6 to 2.1 (the loss of the 1,2-product matches the increase in the 2,6-product). When the reaction was performed for 48 h, the amount of O-alkylated product increased to 50% selectivity, while both the 2,6- and 1,2-C-alkylated products decreased. The 1,2-products, however, decreased more than the 2,6-products to give a 2,6/1,2-ratio of 5.1. These results point to the conclusion that, at the high temperatures employed in the alkylation reaction, isomerization occurs, though the rate of isomerization is very small and the O-alkylated product is formed preferentially. Whether the isomerization is bimolecular or monomolecular in nature could not be established.

4. Conclusions Titanium and zirconium containing *BEA molecular sieves are able to catalyze the alkylation reaction of 2-MN with PO. However, high temperatures are required (473 K) for this reaction to proceed in the absence of added electron withdrawing groups. No leaching of Ti or Zr from the catalytic material was observed at these temperatures. The main products formed are an O-alkylated product, that shows the highest product selectivity, i.e. 30–40%, and four C-alkylated products: II, III, IV and V (Scheme 1). The ratio of the 2,6- to the 1,2-alkylated product is 1.5 for Ti-BEA and 3 for Zr-BEA. This difference may be due to the fact that Ti is incorporated into the BEA framework, while Zr is rather grafted onto the catalyst surface. This increases the space restriction with Zr-BEA, and hence, the ratio of 2,6- to 1,2-products. However, the higher 2,6/1,2-ratio may also be due to a more effective usage of the catalyst interior surface due to a lower reactivity. The highest yields are achieved with high 2-MN concentrations (2-MN as solvent) and very low concentrations of PO. The catalyst is strongly deactivated after the alkylation reaction due to the oligomerization of PO and the deposition of these oligomers on the catalyst surface. Further PO addition does not contribute to the alkylation reaction, but the amount of PO deposited is increased. Calcination of the catalyst at 823 K in oxygen restores the catalytic activity for 2-MN alkylation. Passivation of the outer surface with TEOS did not show promising results with respect to an increased 2,6/1,2-C-alkylated ratio. Poisoning of the outer surface with a bulky Lewis acid poison (TETRAPHOS-II)

A. Brait, M.E. Davis / Applied Catalysis A: General 204 (2000) 117–127

decreased the activity of the catalyst to roughly one-fifth, but showed an increased 2,6/1,2-C ratio of 4. Treatment of Ti-BEA with EDTA, H2 O2 , or both only slightly increased this ratio. The Ti-BEA catalyst showed isomerization activity, but this reaction is very slow and both products, 1,2and 2,6-C-alkylated, are converted. The 1,2-product, however, is isomerized faster, which leads to an increased 2,6- to 1,2-product ratio. The preferentially formed product after long times, however, is the O-alkylated 2-MN. Acknowledgements We thank Albemarle Corporation for financial support, Dr. H. Yamashita for the synthesis of Si-BEA

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and Ti-BEA(F− ) and Dr. S. Hwang for the 13 C NMR measurements. References [1] A. Brait, H. Gonzalez, P. Andy, M.E. Davis, Appl. Catal. A 194–195 (2000) 265. [2] B. Rakshe, V. Ramaswamy, S.G. Hegde, R. Vetrivel, A.V. Ramaswamy, Catal. Lett. 45 (1997) 41. [3] Z. Lin, J. Rocha, P. Ferreira, A. Thursfield, J.R. Agger, M.W. Anderson, J. Phys. Chem. B 103 (1999) 957. [4] G.G. Juttu, R.F. Lobo, in: Abstracts of the North American Catalysis Society Meeting, Boston, 1999. [5] C.B. Dartt, M.E. Davis, Appl. Catal. A: Gen. 143 (1996) 53. [6] S. Pivsa-Art, K. Okuro, M. Miura, S. Murata, M. Nomura, J. Chem. Soc., Perkin Trans. (1994) 1703. [7] P.J. Kunkeler, D. Moeskops, H. van Bekkum, Microporous Mater. 11 (1997) 313.