Methods of producing epoxides from alkenes using a two-component catalyst system

In this example, a combination of two catalysts, Au/TiO 2 and TS-1 were used to catalyze the demanding reaction of butene epoxidation in an aqueous solution using molecular oxygen under very mild reaction conditions. Peroxy initiator was not necessary but carbon monoxide as a sacrificial reductant was used.

Methods

The two different batches of Au/TiO 2 catalysts used in the reaction (denoted Au/TiO 2 -02-4 and Au/TiO 2 -02-9) were supplied by the World Gold Council. There was no observable difference in their catalytic performance. The Au loadings of Au/TiO 2 -02-4 and Au/TiO 2 -02-9 were 1.51 and 1.49 wt. % and the average Au particle sizes were 3.8±1.5 nm and 3.6±1.32 nm, respectively. TS-1 was synthesized using the method of Thangaraj et al. (See A. Thangaraj, R. Kumar, S. P. Mirajkar and P. Ratnasamy, J. of Catal., 1991, 130, 1). Diffuse reflectance UV visible spectroscopy (Perkin Elmer LAMBDA 1050) was used to verify the absence of extra framework Ti. The silicalite sample was synthesized using the same procedure as for TS-1, except that TiBuOH was omitted.

The epoxidation reaction was carried out in a high pressure glass reactor (Cole-Parmer) which contained 50 mL of ion-exchanged distilled water (DDI H 2 O, pH˜5.5). The reactor was maintained at 40° C. (except when specified). The gas feed composition, 2.5% CO, 1.25% O 2 and 1.25% 1-butene, balance He, was adjusted using 5850E Brooks flow controllers. 40 mL min −1 of the feed gas was directed into the aqueous phase through a 5 μm stainless steel porous frit (Scientific Instrument Services). The pressure in the vessel was maintained at 480 kPa with a back pressure regulator (Mighty Mite). The experiments were started with the catalysts (0.1 g Au/TiO 2 ±0.15 g TS-1) placed above the liquid level in a small polyethylene cup with a small magnet in the cup and held in place by a rare earth magnet placed outside the reactor. At a predetermined time (defined as time=0), the rare earth magnet was used to guide the cup into the water, thereby initiating the aqueous phase catalytic reaction. After the reaction, 50 mL of H 2 O was used to replace the H 2 O-catalysts mixture and the feed was again introduced at 480 kPa to calibrate the gas chromatography (GC) areas of the different components of the feed. The exit gas from the reactor was monitored with on-line GC. All liquid products were separated from the solid catalysts by filtration through a 0.2 μm polyvinylidene fluoride (PVDF) membrane (Pall) and analyzed with an Agilent 6890 GC.

Due to the high solubility of CO 2 in water, it was difficult to obtain good instantaneous carbon balance to assess the importance of the combustion pathway. This value can however be assessed by summing O xs (defined as that O beyond what is required for stoichiometric CO oxidation) over time and comparing it with the total alkene reacted or epoxide formed. O xs can be calculated from R Oxs which is the difference in the rates of oxygen atom and CO consumption (Eq. 1). The efficiency of generation of O xs from CO oxidation is defined in Eq. 2.

R Oxs =2 *R O2 used −R CO used   (1)

E Oxs =R Oxs /R CO =efficiency in O xs production  (2)

Results

Au/TiO 2

FIG. 1 a shows CO oxidation over a Au/TiO 2 catalyst at room temperature in a feed of 2.5% CO and 1.25% O 2 in He. Before t=0, the catalyst was suspended above the liquid in a small polyethylene cup. It was relatively active for CO oxidation, albeit due to diffusion limitation, the activity was noticeably lower than when the same catalyst was placed in a plug flow reactor. At t=0, the catalyst was dispersed into the aqueous phase and the CO conversion decreased sharply due to the low solubility of the gases in H 2 O. E Oxs was zero in the gas phase but increased transiently to a value of ˜0.3 before decaying to zero after around 100 min ( FIG. 1 b ). When the experiment was repeated with half the amount of catalyst, the aqueous phase CO conversions and E Oxs did not change significantly suggesting that the rate limiting factor was not the catalytic process but the dissolution of gases into the liquid. At the end of the experiment, with 0.05 g Au/TiO 2 catalyst, there were 5.2 μmoles of H 2 O 2 in the aqueous phase. The amount of H 2 O 2 detected exceeded the amount expected from the equilibrium concentration of the oxidation of H 2 O with O 2 by many orders of magnitude (K 25 =2.58*10 −22 for H 2 O oxidation with O 2 ).

FIG. 2 a shows that when the H 2 O temperature was increased to 40° C., CO conversions in the gas phase (gas phase temperature was less than 40° C.) and aqueous phase increased slightly. When 1.25% 1-butene was included in the feed, CO conversion was slightly suppressed in the gas phase but remained unaffected in the aqueous phase. The E Oxs was close to zero, with and without butene in the feed, and was much less than that observed at the room temperature reaction condition ( FIG. 2 b ). Thus, no epoxide was formed without the second component of this binary catalyst system.

Au/TiO 2 +TS-1 Mixture

FIG. 3 a shows the CO and O 2 conversions for three separate runs in a feed of CO, O 2 and 1-butene using the binary catalyst system of Au/TiO 2 +TS-1. Table 1 summarizes these results as well as results of control experiments. CO conversion in the gas phase was lower than the runs without TS-1 because diffusional effects were more severe in a deeper bed in the polyethylene cup due to the presence of TS-1. In the gas phase, within uncertainties, the O 2 and CO conversions followed the stoichiometric ratio for CO oxidation. Once the catalysts were dispersed into the aqueous phase, O 2 conversions were consistently higher than the stoichiometric amounts for the corresponding CO conversions. Both CO and O 2 conversions increased with time on stream until at the end of the experiment their conversions were similar or higher than that of the initial gas phase conversions, whereas the rate of O xs production remained constant and positive ( FIG. 3 c ).

Upon dispersing the catalysts into H 2 O (t=0), butene 1,2-epoxide was detected in the gas phase. The epoxide amount detected in the gas phase was low initially, as most of it was dissolved in H 2 O, but grew with time as its concentration in solution increased ( FIG. 4 ). Gas phase butene consumption was low initially as dissolved butene was utilized first (5.62*10 −5 moles was dissolved in 50 mL H 2 O as calculated from E. Wilhelm, R. Battino, and R. Wilcock, Chem. Rev., 1977, 77, 219) and the steady state was only reached after 150 min. In a separate extended time experiment ( FIG. 5 ), in which the catalysts were placed in H 2 O right at the beginning (thus, the data before 80 min are not steady state data), it was observed that even after 500 min, CO and O 2 conversions appeared to be continually increasing, while butene conversion appeared to be steady for a while before it declined slightly with long time on stream. The increases in CO and O 2 conversions were only observed in the binary catalytic system of Au/TiO 2 and TS-1 and were not observed if Au/TiO 2 was used without TS-1 or a mixture of Au/TiO 2 and silicalite was used (silicalite has the same crystal structure as TS-1 but without Ti incorporation), or when butene was omitted from the feed.

These results confirmed the formation of butene 1,2-epoxide by direct detection in the exit gas and in the product solution. Table 1 shows that in addition to epoxides, 1,2-butane diol was detected in the product solution as the major oxygenated hydrocarbon product, which was formed by rapid hydrolysis of the epoxide in an aqueous medium. The results also showed that combustion of butene was insignificant under these conditions because, within experimental uncertainties: (1) The sum of moles of hydrogen peroxide, epoxide and diol was close to that of the total moles of O xs , whereas the total combustion of one mole of butene to CO 2 and H 2 O would require 12 moles of O xs , and (2) The alkene consumed integrated over time was close to the sum of epoxide and diol integrated over the same time period.

The results of control experiments, runs 4 to 7 in Table 1, showed that all components are needed for epoxide formation, which include CO, O 2 , supported Au catalyst for peroxide generation, butene, and TS-1 for epoxidation.

This example illustrates the carbon monoxide-assisted epoxidation of propene using molecular oxygen and a catalyst system of Au/TiO 2 and TS-1.

Methods

Materials:

Au/TiO 2 catalysts were supplied by the World Gold Council. Two different batches labeled Au—TiO 2 -02-4 and Au—TiO 2 -02-9 were used. The Au loadings were 1.51 and 1.49 wt. % and the average Au diameters were 3.8±1.5 nm and 3.6±1.32 nm, respectively. There was no discernible difference in the catalytic performances of the two catalysts. TS-1 was synthesized using the method of Thangaraj et al. (See Thangaraj, A., Kumar, R., Mirajkar, S. P., and Ratnasamy, P., Journal of Catalysis 130 (1), 1 (1991)). The incorporation of Ti into the silicalite framework was verified using diffuse reflectance UV visible spectroscopy (Perkin Elmer LAMBDA 1050). The Ti content was 1.0 wt. %, as determined by ICP. The silicalite sample was synthesized using the same procedure as for TS-1, except that no TiBuOH was added.

Catalytic Tests:

The epoxidation reaction was carried out in a Lab-Crest pressure glass reactor which was filled with 50 mL of solvent. The solvent used was either ion-exchanged distilled water or anhydrous CH 3 OH, or their mixture. The feed gas, at a flow rate of 40 mL min −1 , was adjusted to the desired composition using mass flow controllers, and flowed into the liquid through a 5 μm stainless steel porous frit. The pressure in the vessel was maintained at 480 kPa with a back pressure regulator. The temperature of the liquid in the reactor was maintained at 40° C. with the help of an external water bath. Before the experiment began, the catalyst mixture (typically 0.1 g Au/TiO 2 and 0.15 g TS-1) was placed in a small polyethylene cup held with a small, Teflon-covered magnet above the liquid. The feed composition was adjusted, the CO oxidation activity of the catalyst was tested, and the system was allowed to reach a steady state. Then, the catalyst cup was guided by a magnet into the liquid that was stirred constantly to commence the experiment. The exit gas was sampled regularly with a gas chromatograph. At the conclusion of the experiment, the liquid was analyzed by gas chromatography and product identification was assisted by GC-MS. Isotopic labeling was determined using a GC-MS. FIGS. 6-8 and Table 2 show the mass spectra of the compounds of interest, results of the isotope labeling experiments, and GC analysis information. The peroxide content was determined by titration with cerium sulfate using a ferroin indicator.

Analysis of TS-1:

About 0.039 g of the TS-1 was dissolved completely in 2 mL distilled deionized (DDI) water and 3 mL 48% HF. The resulting solution was further diluted with 45 mL water, which was then analyzed using ICS-EAS.

Product Analysis:

The gas phase products were analyzed by on-line gas chromatography (Agilent 6890 GC) using two columns: a carbosphere packed column with a thermal conductivity detector (TCD) for analysis of CO, O 2 and CO 2 , and a Econo-cap EC-Wax capillary column with a FID detector for organics. The liquid product was also analyzed by the same capillary column by injecting a small sample of the liquid with a syringe. All liquid products were separated from the solid catalysts by filtration through a 0.2 μm an PVDF membrane (Pall) before analysis. The isotope distributions were determined using a GC-MS (Agilent 6890 GC with 5973 MSD) with the FID and MSD sharing a capillary column via a splitter.

H 2 O 2 and/or alkylhydroperoxide were titrated as follows. A ferroin indicator solution was prepared by dissolving 0.123 g iron(II) sulfate heptahydrate (FeSO 4 .7H 2 O) into 15 mL DDI water, then adding 0.259 g 1,10-phenanthroline. The titration solution was prepared by dissolving 0.114 g cerium(IV) sulfate (Ce(SO 4 ) 2 ) into dilute sulfuric acid (1/19 v/v). Into a conical flask, 5 mL sample and 10 mL diluted sulfuric acid (1/19 v/v) were added. Two drops of ferroin indicator were added to the above solution, turning it red. The titration solution, 0.6875 mM Ce(SO 4 ) 2 , was then added until a color change to pale blue was observed.

Reactor System:

The reactor was a Lab-Crest pressure glass reactor (Cole-Parmer) into which a stream of premixed gas flows and is dispersed with a 5 μm stainless steel porous frit (Scientific Instrument Services). The pressure in the reactor was maintained with a back pressure regulator (Mighty Mite). A polyethylene cup was used to hold the catalyst above the liquid by placing a teflon-coated magnetic bar in the basket and holding it in place with another magnet outside the reactor. When the external magnet was removed, the catalyst would drop into the liquid and disperse. The gas composition was adjusted using mass flow controllers (Brooks). The temperature of the reactor was maintained by placing it in a water bath.

Results

The CO oxidation-assisted epoxidation process was conducted by bubbling a gas mixture of CO, O 2 , propene, and He through a suspension of Au/TiO 2 and TS-1 catalysts. In a typical experiment, the feed gas was 2.5% CO, 1.25% O 2 , and 1.6% propene in He and the total pressure was 480 kPa. The rates of CO, O 2 , and propene consumption and formation of volatile products, including propene oxide (PO), acetone, propanediol, and 2-propanol were monitored by analyzing the gas phase products at the reactor exit periodically with an on-line GC-MS. CO 2 was also analyzed, but its rate of formation could not be calculated accurately from the exit gas due to significant dissolution into the liquid. At the conclusion of the experiment, the liquid phase was analyzed for organic products by GC-MS, and peroxides by titration. The results are presented in Table 3.

When the catalyst was above the liquid, CO was oxidized to CO 2 with stoichiometric consumption of O 2 , independent of the presence of propene in the feed or the liquid composition. There was no detectable reaction of propene when it was present. A similar stoichiometric consumption of CO and O 2 was observed when Au/TiO 2 was lowered into a water-only solvent in a feed without propene. On the other hand, in a feed that includes propene, extra O consumption (O xs ), beyond what was required for stoichiometric CO oxidation, was observed immediately when a Au/TiO 2 +silicalite catalyst mixture was lowered into a 20:80 H 2 O/methanol solvent. However, the rate of O xs consumption fell quickly to an undetectable level within an hour. At the conclusion of the experiment, about 21 μmole of peroxide were detected in the aqueous solution (Exp. 10 in Table 3). Silicalite, which has the same crystal structure as TS-1 but without Ti incorporation, is inactive for epoxidation. The quantity of peroxide detected was about ¼ that of O xs . The fate of the remaining O xs was not determined.

With propene in the feed and a mixture of Au/TiO 2 and TS-1 as catalyst in a methanol-H 2 O solvent, the rate of O xs consumption was maintained over the course of the experiment ( FIG. 9 a ). The CO and O 2 consumption rates reached a steady state in ˜1 h ( FIG. 9 b ), whereas propene consumption rate required ˜2 h, probably due to its higher solubility in the solvent. Over this time, epoxide began to appear in the gas phase. Its concentration increased continuously, consistent with the fact that it was accumulating in the liquid. At the completion of the experiment, the liquid was analyzed for the propene oxide contents. As illustrated by experiments 1 to 7 in Table 3, the product distributions and the reaction rates depended strongly on the solvent. In pure H 2 O (Exp. 1), the catalyst was active, but the major product was propanediol, indicating rapid hydrolysis of propene oxide. At the other extreme when anhydrous methanol was the solvent, there was little reaction, even for CO oxidation (Exp. 7). Very interestingly, propene oxide was practically the only product of propene reaction in experiments when the solvent was methanol with 5 or 20% H 2 O (Exp. 2 to 6). This could be a consequence of the hydrophobic nature of the TS-1 pores that partitions CH 3 OH preferentially to H 2 O. Within the uncertainties (estimated 10%) in both O 2 and propene balances over the course of the experiment, little combustion or hydration of propene had occurred under these conditions. Titration identified the presence of peroxide in the liquid (2 nd to last column, Table 3), but the technique could not distinguish H 2 O 2 from other peroxides, such as CH 3 OOH. The yield of propene oxide increased with CO reaction rate, which increased with CO concentration in the feed (compare Exp. 2 and 4). The yield also increased with increasing propene concentration (compare Exp. 2 and 3, and 5 and 6).

When methanol was replaced by ethanol (Exp. 8), the CO oxidation rate decreased by a factor of about 2.5, and propene oxide formation by over a factor of 10. The slower epoxidation rate in ethanol than in methanol has been reported, although the decrease was less than that observed here. (See S. M. Danov, A. V. Sulimov and A. V. Sulimova, Russian Journal of Applied Chemistry, 2008, 81, 1963-1966.) Importantly, selective propene oxide formation was possible.

Control experiments showed little propene oxide formation when either Au/TiO 2 (not shown) or TS-1 (Exp. 10, Table 3) was missing from the catalyst mixture, supporting the hypothesis that a stable intermediate oxidant was formed on Au/TiO 2 that diffuses to TS-1 to effect epoxidation. CO was necessary (Exp. 11, Table 3), confirming that this intermediate oxidant cannot be formed by oxidation of water or methanol directly, but by a mechanism that is coupled to CO oxidation. Finally, when methanol was replaced by acetonitrile in the solvent (95/5 acetonitrile/H 2 O, Exp. 9), no propene oxide was formed, no peroxide was detected in the liquid by titration, and the CO oxidation activity was low.

The lack of catalytic activity in an anhydrous methanol solvent is not surprising. Others have reported the importance of moisture for high CO oxidation activities. (See Costello, C. K. et al., Applied Catalysis, A: General 243 (1), 15 (2003); Daté, M., Okumura, M., Tsubota, S., and Haruta, M., Angewandte Chemie International Edition 43 (16), 2129 (2004); and Kung, M. C., Davis, R. J., and Kung, H. H., Journal of Physical Chemistry C 111 (32), 11767 (2007)). Surprising results were obtained, however, when the role of H 2 O 2 was investigated in the epoxidation reaction using H 2 18 O in a methanol-H 2 O solvent. If epoxidation proceeded via the formation of H 2 O 2 from H 2 O and O 2 , H 2 18 O could be used to form H 18 O 16 OH, and the resulting propene oxide would be 50% labeled as C 3 H 6 18 O. Table 4 shows the results of these experiments (Exps. 12-14). Only 16 O-labeled PO was detected both in the exit gas and the liquid. In contrast, a distribution of C 16 O 2 , C 16 O 18 O, and C 18 O 2 , enriched in 18 O content, was detected in the gas phase product. The formation of a distribution of isotopically labeled CO 2 indicates oxygen scrambling between CO 2 and H 2 O, most likely via a bicarbonate intermediate.

Complementary experiments were conducted using 18 O 2 in the feed. The results (Exp. 15 and 16) showed that only 18 O-labeled PO was formed until at least 1 h into the experiment, when small amounts of 16 O-labeled PO began to appear. Interestingly, the CO 2 was mostly labeled with 16 O, consistent with rapid isotope scrambling with water. A similar observation was obtained using the ethanol/H 2 O solvent (Exp. 17). Only 18 O-labelled PO was formed when using 18 O 2 . In addition to the fact that the methanol results have been repeated, other evidence also suggested that these results were not experimental artifacts due to scrambling in the detection system. Injection of a solution of unlabelled propene oxide in H 2 18 O into the GC-MS did not change the cracking pattern of the propene oxide, although a huge m/e=20 peak due to the labelled water was observed. In the experiments using H 2 18 O, trace amounts of 18 O-labelled (>90% labelled) acetone could be detected, which was likely formed by hydration of propene followed by oxidative dehydrogenation of the propanol product. Thus, labelled reactive organics can be detected accurately.

These results show that H 2 O, although necessary for CO oxidation, is not involved directly in the epoxidation reaction. It is believed that the role of moisture is to maintain the activity of the active site on Au for CO oxidation. Alcohol is needed for epoxidation, which does not occur in the acetonitrile/water solvent. Whereas the data unequivocally showed the formation of a stable intermediate oxygen carrier that is formed from O 2 on Au/TiO 2 , and diffuses to TS-1 to effect epoxidation, unfortunately, there was no direct observation on the nature of this intermediate. Two logical possibilities are: H 2 O 2 and CH 3 OOH (or C 2 H 5 OOH if ethanol is used).

H 2 O 2 could be formed according to Scheme 1 shown in FIG. 10 . O 2 is adsorbed on the active site on Au/TiO 2 as peroxy or superoxide, which reacts with CO to form CO 2 and an adsorbed O. The adsorbed O may react with another CO in a nonproductive pathway, or react with methanol to form a surface OH and a CH 2 OH radical. Two surface OH combine to form H 2 O 2 which carries the same isotope label as O 2 . This mechanism would postulate HOCH 2 CH 2 OH as a byproduct. Unfortunately, its presence was not detected, although its concentration might be too low for detection.

Alternatively, the Au-superoxide picks up a proton from water or methanol to form Au-peroxide (Scheme 2 in FIG. 10 ), which reacts with methanol or methoxide to form CH 3 OOH, in which the O of the OH would have the same isotope label as O 2 . The methylhydroperoxide is responsible for propene epoxidation. Reaction of Au-superoxide with CO would lead to nonproductive consumption of CO.

As used herein, and unless otherwise specified, “a” or “an” means “one or more.” All patents, applications, references, and publications cited herein are incorporated by reference in their entirety to the same extent as if they were individually incorporated by reference.

As will be understood by one skilled in the art, for any and all purposes, particularly in terms of providing a written description, all ranges disclosed herein also encompass any and all possible subranges and combinations of subranges thereof. Any listed range can be easily recognized as sufficiently describing and enabling the same range being broken down into at least equal halves, thirds, quarters, fifths, tenths, etc. As a non-limiting example, each range discussed herein can be readily broken down into a lower third, middle third and upper third, etc. As will also be understood by one skilled in the art, all language such as “up to,” “at least,” “greater than,” “less than,” and the like includes the number recited and refers to ranges which can be subsequently broken down into subranges as discussed above. Finally, as will be understood by one skilled in the art, a range includes each individual member.

It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein, but include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims.