Process for the preparation of ethers

Abstract

A method for producing ethers, comprising treating (a) an ester with (b) hydrogen in the presence of (c) a heterogeneous catalyst to reduce the ester by hydrogenation to form an ether product.

Description

Technical Field

[0001] This invention relates to a method for producing ether compounds. More specifically, this invention relates to a method for the direct preparation of ether compounds from alkyl esters using molecular hydrogen on a heterogeneous catalyst. Background Technology

[0002] Ethers are used as solvents in a variety of applications. Ethers are particularly well-suited for use as solvents in applications due to their excellent solubility, chemical stability, and compatibility with other organic solvents and formulations. Known routes for the synthesis of ethers include three main routes: (1) alkyl halides treated with alkoxides (the so-called "Williamson ether synthesis"); (2) alcohol addition to olefins; and (3) acid-catalyzed coupling of alcohols. However, these three routes have undesirable limitations, including: (1) the use of strongly acidic or basic conditions, which can lead to competitive elimination reactions that produce undesirable olefins; (2) a limited selection of bio-based feedstocks due to a lack of reactivity with the above reactions, which limits the structural diversity of the products; and (3) the use of toxic feedstocks and the generation of waste streams during the manufacturing process. Therefore, it is desirable to provide a feasible route for the production of ethers that can be successfully scaled up commercially without the limitations of the aforementioned known routes.

[0003] For example, known methods for producing ethers to date include: (1) methods using metal hydrides / Lewis acid complexes or hydrosilanes as stoichiometric hydride donors and noble metal catalysts, such as J. Org. Chem., 2007, 72, 5920-5922; Tetrahedron (1) Methods for producing thiosulfates (thiocarbonates or esters) such as thioethers (sulfides, which are compounds of sulfur with two organic residues), as disclosed in J. Org. Chem., 1981, 46, 831-832; (2) Methods for catalyzing garden α-glycerol monoesters with a mixture of 5 percent (%) Pd / C and an acid-co-catalyst at about 700 psi (4.8 MPa) and 120 °C, as disclosed in U.S. Patent No. 8,912,365; (3) Indirect methods for hydrogenating ethyl acetate to an ethanol intermediate, which is subsequently coupled to form a symmetrical ether byproduct on Re / (γ-Al2O3) or Re / (θ-Al2O3) with up to 4.6 percent conversion and 57 percent selectivity, as disclosed in Russian Chemical Bulletin. The following methods are disclosed: (1) 1988, 37(1), 15-19 and Russian Chemical Bulletin 1986, 35, 280-283; (5) methods for hydrogenating lactones to cyclic ethers (e.g., for the production of tetrahydrofuran) with high selectivity (e.g., greater than 90%) using various metal catalysts on various supports, as disclosed in U.S. Patent Nos. 3,370,067; 3,894,054; and 4,973,717; and (6) methods using homogeneous metal complex catalysts (e.g., ruthenium / triphos complexes), which require impractical separation of the catalyst from the product. Angewandte Chemie, International Edition, 2015, 54, 5196-5200; ChemSusChem, 2016, 9, 1442-1448. An alternative method for the production of commercially manufactured ethers is desired, and it offers advantages over existing methods. Summary of the Invention

[0004] This invention relates to a method for producing ether products from ester starting materials.

[0005] In a wide range of embodiments, the method of the present invention includes the production of ethers by hydrogenation of esters in the presence of a heterogeneous catalyst.

[0006] In one embodiment, the method of the present invention includes the direct selective reduction of a carboxylic acid derivative to an ether using molecular hydrogen and a suitable catalyst formulation to achieve high (e.g., >5%) absolute ether selectivity and high (e.g., >85%) direct ether selectivity. Absolute ether selectivity is the percentage of total product formed in the reaction, while direct ether product selectivity is the percentage of direct ether product relative to total ether product.

[0007] In another embodiment, the method of producing ethers according to the present invention includes mixing (a) at least one ester with (b) hydrogen in the presence of (c) a heterogeneous catalyst to reduce the ester by hydrogenation to form an ether.

[0008] In another embodiment, the present invention includes a solvent containing the ether product prepared by the above method.

[0009] Some advantageous features that may be provided by one or more embodiments of the method of the present invention include, for example:

[0010] (1) An active catalyst is used in a one-step process. The catalyst is active for the direct hydrogenation of the ester to reduce the ester to form an ether, rather than through a known two-step ether formation process, such as (i) hydrogenolysis of the ester to form an alcohol, followed by (ii) dehydration of the alcohol.

[0011] (2) Use of a relatively inexpensive route. This method uses inexpensive molecular hydrogen as a reducing agent, rather than expensive hydrosilanes, metal hydrides, or metal hydride / Lewis acid complexes as hydride donors. The highly reactive hydrosilanes or metal hydrides used in existing methods also require the design of complex and expensive methods to ensure the safety of operators running existing methods.

[0012] (3) Use heterogeneous catalysts. Due to the recyclability of catalysts, heterogeneous catalysts, rather than homogeneous ones, can help reduce manufacturing costs.

[0013] (4) Use effective methods.

[0014] (5) Use of flexible methods. This method is applicable to common ester compounds (cyclic or acyclic) used as feed materials; and the method is not limited to specific ester compounds. Detailed Implementation

[0015] In one embodiment, the present invention includes a unique and novel method for synthesizing ethers from esters using a heterogeneous catalyst. Ester reactions include various chemical reaction routes or pathways, such as hydrogenolysis, hydrolysis, dehydration, hydrogenation, and transesterification. In a general embodiment, the method of the present invention includes producing ethers by hydrogenation of an ester such as propyl acetate in the presence of a heterogeneous catalyst. Novel hydrogenation reaction pathways or schemes of the present invention, such as the hydrogenation reduction scheme of propyl acetate wherein R1 is -CH3 and R2 is -CH2CH3, are generally as shown in the following reaction scheme (I):

[0016]

[0017] In the above reaction scheme (I), water is produced by a reduction process; and the produced water can be separated by conventional methods such as distillation or other procedures known in the art. Functional groups R1 and R2 can be alkyl functional groups, including straight-chain or branched alkyl groups, cyclic or acyclic alkyl groups; and mixtures thereof. Examples of esters herein include, but are not limited to, ethyl acetate, propyl acetate, butyl acetate, ethyl propionate, butyl propionate, and mixtures thereof. When R1 is equivalent to R2, the desired ether product obtained by the above reaction scheme (I) can be a symmetrical ether; or when R1 is not equivalent to R2, the desired ether product obtained by the above reaction scheme (I) can be an asymmetrical ether, for example, an ethylpropyl ether.

[0018] In this article, "symmetric ether" refers to an ether containing two identical functional groups, where R1 equals R2. In this article, "asymmetric ether" refers to an ether containing two different functional groups, where R1 does not equal R2.

[0019] In this invention, the desired reaction scheme (reaction scheme (I)) is a direct hydrogenation route to obtain the desired ether product. "Direct hydrogenation" refers to the removal of the carbonyl oxygen from the ester (R1COOCH2R2) by hydrogenation to form the ether (R1CH2OCH2R2) while preserving the integrity of the alkoxy group. The method of this invention differs from known methods because it does not follow the typical route of ester (R1COOCH2R2) hydrogenation, in which the ester is first cleaved into two alcohols (R1CH2OH + R2CH2OH molecules via hydrogenolysis, and subsequently a mixture of ethers (R1CH2OCH2R1 + R1CH2OCH2R2 + R2CH2OCH2R2) via dehydration). Direct hydrogenation preserves the ether structure by eliminating only the carbonyl oxygen from the ester. Therefore, the asymmetric ester provided to this method advantageously leads to the direct production of the asymmetric ether because the method does not cleave the ester into two alcohol molecules via hydrogenolysis. The selectivity of ethylpropyl ethers in the reaction examples is listed in Tables III and V as described in the following examples.

[0020] In this article, the term "direct ether product" refers to an ether formed by the one-step reduction of an ester to an ether.

[0021] The term “indirect ether product” in this article refers to an ether formed by a two-step reduction of an ester to an ether, the reduction method comprising the steps of (i) hydrogenolysis and (ii) dehydration.

[0022] The term “direct ether product selectivity” in this article refers to the percentage of direct ether products (e.g., ethyl propyl ether) relative to total ether products (e.g., ethyl propyl ether + dipropyl ether + diethyl ether).

[0023] The term "absolute selectivity of ether products" in this article refers to the percentage of ether products in the total products (e.g., ethers, alcohols, and alkanes) formed in the reaction.

[0024] Advantageously, a unique feature of the present invention is the increased direct ether selectivity using the one-step method of the present invention compared to known two-step methods.

[0025] In one embodiment, the method of the present invention for producing ethers includes treating (a) an ester with (b) hydrogen in the presence of (c) a heterogeneous catalyst to reduce the ester by hydrogenation to form an ether.

[0026] In a desired embodiment, as shown in reaction formula (I) above, the method of the present invention for producing ethers comprises the following steps: (A) feeding component (a) an ester compound, such as propyl acetate, into a reactor; (B) feeding component (b) hydrogen into the reactor to create a hydrogen atmosphere in the reactor; and (C) charging the reactor with component (c) a heterogeneous catalyst system, such as a combination of transition metals on an acidic support, sufficient to produce a hydrogenation reaction in the reactor; and (D) heating the reactor contents components (a)-(c) at a temperature sufficient to reduce the ester compound to form an ether compound. For example, step (D) heating can be carried out at a temperature from 350 Kelvin (K) to 650 K.

[0027] In some embodiments, the catalytic system of the present invention is, for example, a combination of a transition metal and an acidic support. For example, the catalyst used in some embodiments of the invention may comprise 0.1 wt% to 20 wt% of a transition metal supported on an acidic support member. The synergistic effect from both the transition metal component and the acidic support component facilitates a direct ether reduction route, as described in reaction pathway scheme (I). Without the combination of transition metal and acidic support components as disclosed herein, an undesirable two-step ether formation route will occur. Steady-state rates and product selectivity of competing reaction pathways for obtaining model ester compounds in, for example, packed bed reactors and / or trickle bed reactors, are described as functions of reactant pressure, temperature, and ester conversion controlled via surface residence time.

[0028] The component (a) ester compound reduced to an ether may include one or more ester compounds, including, for example, carboxylic acid derivatives; esters containing straight-chain or branched alkyl groups and cyclic or acyclic alkyl groups; and mixtures thereof. In some embodiments, the asymmetric ether comprises reaction scheme (I), wherein the R1 group is not equal to the R2 group. In some embodiments, the esters used in the present invention may be, for example, ethyl acetate (available from Sigma-Aldrich); propyl acetate (available from Sigma-Aldrich); amyl acetate (available from Sigma-Aldrich); γ-butyrolactone, butyl butyrate (available from Sigma-Aldrich); and mixtures thereof.

[0029] The concentration of component (a) ester is not particularly critical. However, in some embodiments, it may be advantageous for the ester to be present in an amount of at least 1 wt% to provide the desired productivity and / or avoid increased separation costs. In some embodiments, the ester concentration is from 1 wt% to 100 wt%. The ester concentration is based on the total weight of the ester compounds in the liquid feedstock.

[0030] The concentration of component (b) hydrogen used in the methods of the present invention includes, for example, 3 wt% to 100 wt% in one embodiment, 10 wt% to 100 wt% in another embodiment, and 50 wt% to 100 wt% in yet another embodiment. Low concentrations of hydrogen (e.g., less than 3 wt%) can reduce reactivity or ether selectivity; and therefore, in this case, an undesirable increase in reaction pressure is required. The hydrogen concentration is based on the total weight of hydrogen in the gaseous feedstock.

[0031] In a wide range of embodiments, the catalyst component (c) used in the method of the present invention may comprise one or more heterogeneous catalyst compounds. The catalyst used in the method of the present invention comprises, for example, combinations of transition metals (ci) supported on an acidic support (support) member. For example, the transition metal (component (ci)) may include palladium (Pd); platinum (Pt); nickel (Ni); ruthenium (Ru); cobalt (Co); rhodium (Rh); and mixtures thereof. The acidic support member (component (cii)) may include, for example, niobium oxide (Nb₂O₅) support; tungsten oxide (WO₃) support; and mixtures thereof. In a preferred embodiment, the heterogeneous catalyst that can be used in the present invention may be Pd supported on a Nb₂O₅ support; Pd supported on a WO₃ support; and mixtures thereof.

[0032] The heterogeneous catalysts of the present invention can possess advantageous properties. For example, the heterogeneous catalysts used in the present invention provide a synergistic effect between the metal compound of the catalyst (e.g., Pd or Pt) and the Lewis acidity and / or Brønsted acidity from the catalyst support (e.g., Nb₂O₅) to catalyze direct esterification. Otherwise, ether selectivity may be reduced.

[0033] Component (c) heterogeneous catalyst includes, for example, 0.01 wt% to 20 wt% of a metal compound based on the total weight of the heterogeneous catalyst in one embodiment, 0.1 wt% to 20 wt% of a metal compound based on the total weight of the heterogeneous catalyst in another embodiment, and 1 wt% to 20 wt% of a metal compound based on the total weight of the heterogeneous catalyst in yet another embodiment.

[0034] The apparatus used to perform the reduction method can be any conventional reactor, such as a packed bed reactor or a trickle bed reactor. Furthermore, the ester conversion and ether selectivity can be controlled by reactor pressure, temperature, and surface residence time.

[0035] For example, the pressure of the method of the present invention is 0.1 MPa to 10 MPa in one embodiment, 2 MPa to 6 MPa in another embodiment, and 6 MPa to 10 MPa in yet another embodiment. Pressures below these ranges may result in lower reactivity or lower ether selectivity than disclosed herein. Pressures above these ranges may be sufficient for the present invention; however, higher costs may be required in reactor construction and operation.

[0036] For example, the temperature of the method of the present invention is 350 K to 650 K in one embodiment, 400 K to 500 K in another embodiment, and 500 K to 650 K in yet another embodiment. Temperatures below the above-mentioned temperature ranges may result in lower reactivity than disclosed herein. Temperatures above the above-mentioned temperature ranges may produce undesirable alkane and alcohol byproducts; and therefore, in such cases, the selectivity of the ether may be reduced.

[0037] For example, the ester conversion rate of the method of the present invention is 1% to 70% in one embodiment, 1% to 50% in another embodiment, and 15% to 50% in yet another embodiment. In some embodiments, ester conversion rates higher than the above-mentioned conversion rate ranges may produce side reaction products.

[0038] The method of the present invention can be carried out in a batch or continuous manner. When using a batch method, in some embodiments, the residence time of the method of the present invention is, for example, 0.1 hours (hr) to 24 hours in one embodiment, 0.1 hours to 8 hours in another embodiment, and 1 hour to 24 hours in yet another embodiment. In some embodiments, residence times below the above-mentioned residence time range may result in lower ester conversion. In some embodiments, residence times above the above-mentioned residence time range may produce undesirable side reaction products.

[0039] When using a continuous method, in some embodiments, the residence time of the method of the present invention is, for example, 0.1 seconds (s) to 100 s in one embodiment, 1 s to 10 s in another embodiment, and 10 s to 100 s in yet another embodiment. Residence times below the above-mentioned residence time range may result in lower ester conversion; in some embodiments, residence times above the above-mentioned residence time range may produce undesirable side reaction products.

[0040] Some advantageous properties and / or benefits of using the reduction method of the present invention include, for example, the method of the present invention can achieve a steady-state rate; and the method can provide better product selectivity for competing reaction pathways of ester compounds. Additionally, conventional methods for producing ethers also produce salts, while the method of the present invention does not produce salts.

[0041] After the ester compound undergoes a reduction process, the resulting ether product is formed. The conversion rate from ester to ether product can be 10%. -8 molar ether per gram of catalyst per second (mol / g) cat ·s)10 -5 mol / g cat ·s, in a typical implementation, is 5 × 10 - 8 mol / g cat ·s to 5×10 -6 mol / g cat • Esters with conversion rates below the above-mentioned range may result in low ether production; and in some embodiments, ester conversion rates above the above-mentioned residence time range may produce undesirable side reaction products.

[0042] The selectivity of the ether product can depend on whether a gas-phase or liquid-phase process is used to form the ether and whether a batch or continuous process is used. Typically, the selectivity of the ether product is >5% in one embodiment, 5% to 16% in another embodiment, and 5% to 8% in yet another embodiment.

[0043] Although the ether product produced by the method of the present invention can be a symmetrical ether or an asymmetrical ether, the method of the present invention is described with reference to asymmetrical ethers by way of illustration and not limitation. It has been surprisingly found that the method of the present invention is selective for asymmetrical ethers because, in the method of the present invention, the ester is directly converted to the ether without undergoing ester hydrogenolysis and alcohol dehydration. Ester hydrogenolysis and alcohol dehydration are two processes known to be non-selective for specific ethers.

[0044] If a particular method or end use requires asymmetric ethers, the method of this invention is more advantageous than conventional methods because:

[0045] (1) Under alkaline and acidic conditions, the ether products are more stable than the corresponding ester products. In addition, the ether products of the present invention do not undergo hydrolysis that may occur under high humidity and / or high temperature.

[0046] (2) The hydrogen reaction chemistry of the method of the present invention preserves the integrity of the ester product's main chain. During hydrogenation, only oxygen molecules detach from the main chain, leaving intact carbon molecules and main chain oxygen. In conventional reaction methods, the reaction destroys the main chain and the parts are bound together under different reaction conditions. Therefore, direct hydrogenation / reduction from ester to ether does not occur.

[0047] (3) The method of the present invention minimizes undesirable side reactions that may adversely affect the selectivity of the desired ether product.

[0048] The ether products of this invention have minimal environmental impact because they are derived from organic and renewable sources. For example, the ether products can be advantageously used globally as green and bio-based solvents to address stringent regulations imposed on chemical-based industrial solvents regarding toxicity, non-biodegradability, volatile organic compound (VOC) emissions, etc. Green and bio-based solvents are commonly used in paint and coating applications. Other applications include adhesives, pharmaceuticals, and printing inks. In some embodiments, the ether products can be used as foam control agents and flavor additives. In other embodiments, the ether products can be used in cosmetic and personal care applications.

[0049] This invention provides bio-based solvents at a cost and performance superior to those of known industrial solvents. Furthermore, the chemical transformations provided by the methods of this invention can be used in product processes to produce, for example, bio-based surfactants, defoamers, and lubricants in an economically and environmentally advantageous manner.

[0050] The ether generation method of the present invention can also be used to develop: (1) more robust end-capping methods to overcome the problems of limited reactant alkyl chloride types and end-product impurities; (2) newly end-capped low-viscosity, low-volatility lubricants; and (3) novel surfactants and novel bio-based defoamers for food and pharmaceutical applications, metal processing fluid applications and other applications utilizing ether solvents.

[0051] Example

[0052] The following embodiments (Inv.Ex.) and comparative embodiments (Comp.Ex.) (collectively referred to as "Embodiments") are provided to further illustrate the invention in detail, but should not be construed as limiting the scope of the claims. Unless otherwise specified, all parts and percentages are by weight.

[0053] catalyst

[0054] The catalyst ("Cat.") formulations used in the examples are described in Tables I and II. Examples Cat.1–Cat.18 below were tested in a gas-phase reactor, while examples Cat.19–Cat.34 were tested in a liquid-phase reactor.

[0055] The examples using Cat.1-Cat.5 and Cat.19-Cat.22 are representative embodiments of the present invention (Inv.Ex.), while the examples using Cat.6-Cat.18 and Cat.23-Cat.34 are comparative embodiments (Comp.Ex.). The formulations in Inv.Ex. comprise Pd as a transition metal and Nb₂O₅ or WO₃ as a catalyst support. The Pd weight loading that can be used on the catalyst of the present invention is from 0.1 wt% to 5 wt%.

[0056] The method of the present invention includes the following steps: (1) using molecular hydrogen (H2) as a reducing agent; (2) carrying out the process in a gas phase or liquid phase without additional solvent; and (3) reducing the ester to an ether using a heterogeneous catalyst, wherein the heterogeneous catalyst is a transition metal, such as a Pd-based catalyst, and wherein the catalyst support is an acid support, such as a WO3-based catalyst support or a Nb2O5-based catalyst support.

[0057] Table I - Catalyst Formulations Used in Gas-Phase Processes

[0058]

[0059] Table II - Catalyst Formulations Used in Liquid Phase Processes

[0060]

[0061]

[0062] General Procedure for Synthesizing Supported Catalysts

[0063] The heterogeneous catalyst of the present invention can be prepared by introducing a transition metal catalyst onto a catalyst support using conventional methods such as impregnation, precipitation, co-precipitation, impregnation of colloidal metal nanoparticles, strong electrostatic adsorption, ion exchange, and mechanical mixing. To prepare the heterogeneous catalyst used in the examples, transition metal particles were deposited onto the support using the initial wet impregnation method described below:

[0064] The calculated amount of metal precursor was dissolved in deionized water with an equal volume of support pores to form a solution. This solution was added dropwise to the support to achieve initial wetting. The wet solid obtained from the initial wetting impregnation was dried in stagnant air for more than 12 hours. Subsequently, the dried solid was calcined in flowing air at 573-973 K for 3-4 hours. The resulting calcined solid was then reduced in flowing hydrogen / helium (H2 / He) at 673 K for 4 hours. The resulting catalyst sample was cooled to ambient temperature (approximately 25°C [approximately 298 K]) and then passivated in a flowing air / He mixture before being exposed to ambient air. For the metal support, catalyst support, and precursor used to prepare each catalyst used in the examples, see Tables I and II above.

[0065] Test and measurement

[0066] Part A: Catalytic rate measurement in gas-phase reactors

[0067] Cat.1-Cat.23 were tested in a gas-phase reactor. The gas-phase reactor was a tubular packed bed reactor containing 10 mg to 500 mg of catalyst within stainless steel tubes (9.5 mm outer diameter [OD]). The catalyst was held centered in the reactor using glass rods and glass wool filling. The tubular reactor was placed in a three-zone furnace (available from Applied Test Systems, 3210) controlled by an electronic temperature controller (available from Watlow, EZ-Zone). The catalyst temperature was measured using a type K thermocouple contained within a 1.6 mm stainless steel sheath (available from Omega), coaxially aligned and immersed in the catalyst bed within the reactor. The catalyst bed volume was approximately 1.4 cubic centimeters (em). 3The catalyst bed was kept constant by mixing an excess of silicon carbide (SiC) (available at Washington Mills, Carborex Green 36) with the required amount of catalyst. The system was pressurized using a back pressure regulator (BPR, available at Equilibar Precision Pressure, Equilibar GP1) controlled by an electronic pressure regulator (EPR). Reactor pressures upstream and downstream of the catalyst bed were monitored using a digital pressure gauge (Omega) and the EPR, respectively.

[0068] The gases used in this embodiment were: H2 (purchased from Airgas Inc. in "Ultra-High Purity 5.0") and He (purchased from Airgas Inc. in "Ultra-High Purity 5.0"). A mass flow controller (purchased from Bronkhorst in "EL-FLOW HighPressure") was used to control the gas flow rate. When C5H 10 When O2 is fed through a polyether ether ketone (PEEK) polymer tube (1.6 mm OD and 0.25 mm inner diameter [ID]), the flow rate of liquid propyl acetate is controlled using a stainless steel syringe pump with a Hastelloy cylinder (100DX with D-series controller, available from Teledyne Isco). 10 O2 (supplied by Sigma-Aldrich, 537438, ≥99.5%), the outlet of the polymer tube is located within a small bed of non-porous sand (SiO2 50-70 mesh, supplied by Sigma-Aldrich, 274739) within the H2 crossflow. Heating tape (available from Omega) is used to maintain the delivery line around the liquid inlet at 373K to prevent condensation. All delivery lines downstream of the liquid inlet are heated to above 373K using heating tape; and line temperatures are monitored using K-type thermocouples (available from Omega) displayed on a digital reader (available from Omega).

[0069] Prior to all catalytic measurements, the catalyst was heated at 0.05 Kelvin per second (Ks). -1 The catalyst was brought to the desired temperature and flowed at 101 kPa and H2 at 100 cm³ / min. 3 min -1The catalyst was pretreated in situ for the required time to maintain its position. Reactor effluent was characterized using online gas chromatography (HP 6890, Agilent Technologies). The gas chromatograph (GC) was equipped with a capillary column (DB-624UI, 30 m long, 0.25 mm ID, 1.40 μm) connected to a flame ionization detector to quantify the concentration of combustible substances. Sensitivity factors and retention times for all components were determined using gas and liquid standards. Reactor pressure and temperature, reactant flow rate, and GC sampling were automatically controlled to allow for continuous measurements. Conversion was calculated on a carbon-based basis based on the amount of carbon present in the products. Carbon and oxygen balances were maintained within ±20%. Reactor conditions during rate and selectivity measurements were varied by sequentially decreasing and then increasing the reactant pressure over the entire range of 1 MPa to 10 MPa, ensuring that one or more conditions were measured at least twice throughout the experiment to ensure that the trend of the measurements was not a result of system deactivation.

[0070] Part B: Measurement of catalytic rate in liquid-phase reactors

[0071] Cat.24-Cat.39 were tested in a liquid-phase reactor. Rate and selectivity measurements were performed in a trickle-bed reactor comprising a 1.6 mm OD stainless steel tube containing 1,000 mg to 4,000 mg of catalyst (30 to 60 mesh), held centered in the reactor using Pyrex glass rods and filled with glass wool. The reactor was heated by an aluminum clamshell containing two heating cylinders controlled by an electronic temperature controller (available from Wattlon's EZ-Zone). Reaction temperatures were measured by a type K thermocouple contained within a 3.2 mm stainless steel sheath (available from Omega), coaxially aligned and submerged within the aluminum clamshell. The system was pressurized to 6.5 MPa using a dome-loaded back pressure regulator (BPR, available from Equilibar LF series, available from Equilibar Precision Pressure) controlled by an electronic pressure regulator (EPR, available from Equilibar GP1, available from Equilibar Precision Pressure). Reactor pressure was monitored using a digital pressure gauge (available from Omega) and the EPR.

[0072] The gas flow rates of H2 (available from Egas) and He (available from Egas) were controlled using a mass flow controller (available from Brownhurst's EL-FLOW high-pressure controller). A high-performance liquid chromatography (HPLC) pump (available from Chromtech's P-LST40B) was used as the feed for C5H through stainless steel tubing (1.6 mm OD and 0.15 mm I.D.) within the cross-flow of H2 and He. 10 O2, controlling liquid propyl acetate (C5H) 10 The flow rate of O2 (supplied by Sigma-Aldrich, 537438, ≥99.5%).

[0073] Prior to all catalytic measurements, by using 0.08 K s -1 Heat to 503K and at 50cm 3 min- 1 The catalyst was pretreated in situ by holding it in flowing He (20 kPa) and H2 (81 kPa) for 1 hour. The reactor effluent was then passed through a stainless steel cooling chamber containing cold water (at approximately 377 K) and the gaseous and liquid products were separated in a gas-liquid separator (GLS). The liquid product collected in the GLS was delivered via an HPLC pump to a high-pressure liquid sampling valve (LSV, Transcendent Enterprise Inc., PLIS-6890, injection volume 1 μL), which was connected to an online gas chromatograph (Agilent Technologies, HP 7890B). At the outlet of the LSV, a manual BPR (Swagelok) was installed to maintain the liquid pressure at 1,380 kPa to prevent product evaporation in the sampling system. Gas and liquid products were characterized using an online gas chromatograph (Agilent, HP 7890B). The GC was equipped with two capillary columns (DB-Wax UI, 60 m long, 0.25 mm ID, 0.25 μm) for the liquid products, and a GS-GASPRO (GC column, 60 m long, 0.32 mm ID, available from Agilent) was connected to a flame ionization detector to quantify the concentration of substances. Sensitivity factors and retention times for all gaseous and liquid products were determined using gaseous standards and a methanator (Polyarc system, PA-SYC-411, available from Activated Research Company), respectively. Reaction pressure and temperature, reactant and product flow rates, and GC sampling were automatically controlled to allow for continuous measurements. Conversion was calculated on a carbon-based basis based on the amount of carbon present in the products. Carbon balance was maintained within ±10%.

[0074] Test Results

[0075] Reaction pathway using propyl acetate as the ester starting compound

[0076] Kinetic studies were conducted on the catalysts (Cat.1-Cat.39) used in the examples, involving the direct selective reduction of propyl acetate (an example of a representative ester) with molecular hydrogen to determine the reaction pathway for the conversion of propyl acetate to ethers. Several products were observed in the product stream exiting the fixed-bed reactor by GC analysis. These products included, for example, light hydrocarbons such as C2-C3 alkanes and alkenes, ethanol, propanol, dipropyl ether, ethylpropyl ether, acetic acid, and ethyl acetate. Based on these observations, it can be concluded that the reaction occurs in the reactor via several routes, including, for example: (1) hydrogenolysis of propyl acetate to an ethanol and a propanol; (2) hydrolysis of propyl acetate to propanol and acetic acid; (3) the alcohol can then undergo dehydration to form light hydrocarbons, and further dehydration to form ether products such as dipropyl ether and ethylpropyl ether; (4) transesterification of propyl acetate with ethanol to form ethyl acetate and propanol; and (5) the route of the present invention, reaction scheme (I), namely, the direct hydrogenation of propyl acetate with hydrogen to form ethylpropyl ether. Since the desired product is ethylpropyl ether obtained by the direct hydrogenation of propyl acetate, the reaction route (I) of the present invention is the desired reaction pathway. It should be noted that ethylpropyl ether can also be formed via the route (3) described above. However, route (3) is not desirable because it involves an alcohol dehydration reaction, and such an alcohol dehydration reaction is not selective for the formation of asymmetric ethers relative to symmetric ethers such as dipropyl ether or diethyl ether. The method of the present invention is not limited to the production of symmetric or asymmetric ethers. Advantageously, the method of the present invention selectively and directly provides asymmetric ethers when desired or required.

[0077] Part A: Results of gas-phase ester reduction

[0078] Examples 1-5 and Comparative Example AM

[0079] In Examples 1-5 and Comparative Example AR of the present invention, the ether selectivity results using Cat. 1-Cat. 23 are described in Table III. Hydrogenation was carried out under the following reaction conditions: a temperature range of 433 K to 504 K, an H2 pressure of 6.2 MPa, and an ester pressure of 10 kPa. In Examples 2, 5, and Comparative Example LR of the present invention, the catalyst was diluted with a support powder. The purpose of using a support powder for dilution was to reduce the ester conversion to below 10%. Table III - Selectivity of Gas-Phase Ester Reduction

[0080]

[0081]

[0082] In Example 1 of this invention, propyl acetate reduction was carried out in a gas-phase reactor using a 0.8 wt% Pd / Nb₂O₅ catalyst (Cat. 1). The absolute selectivity of ethylpropyl ether was 8.5%, and the absolute selectivity of dipropyl ether was 0.1%. The direct ether selectivity of ethylpropyl ether was 98.8%.

[0083] Supporting Example 1 of the present invention, the selectivity of the ether product of the present invention was measured at various ester feed rates as described in Table IV. Reactions were conducted in a low conversion range of 1% to 48% to demonstrate the reaction pathway. Table IV describes the percentage conversion of the ester to the ether product and the percentage selectivity of the direct selective reduction of the ester to the desired ether product in a gas-phase reactor on a catalyst containing 0.8 wt% Pd supported on Nb₂O₅. As described in Example 1 of the present invention, selectivity is based on conversion under the following conditions: 10 kPa C₅H 10 O2, 6.2 MPa H2 and 503 K.

[0084] Table IV - Conversion Rate and Product Selectivity

[0085]

[0086] Notes to Table IV: *Amount of ethers directly produced from esters relative to the total amount of ethers. In Table IV, for example, "direct ether selectivity" = absolute selectivity of ethylpropyl ether / (ethylpropyl ether + dipropyl ether)*100.

[0087] In Example 2 of this invention, Cat.2 contains 4.3 wt% Pd metal on a Nb2O5 support. The absolute ethyl propyl ether selectivity is 5%, and the absolute dipropyl ether selectivity is <0.1%. The direct ethyl propyl ether selectivity is 98.4%. The results in Example 2 of this invention are similar to those in Example 1 of this invention, which contains a lower Pd loading of 0.8 wt%.

[0088] In Example 3 of this invention, Cat.3 was prepared on WO3. The ester conversion in Example 3 of this invention was significantly higher than that of catalysts (Cat.1 and Cat.2) on Nb2O5 with almost the same metal loading. The absolute ethylpropyl ether selectivity was 13.6%, and the absolute dipropyl ether selectivity was 0.5%. The direct ethylpropyl ether selectivity was 96.5%. The results indicate that ethylpropyl ethers are mainly formed via the direct route of reaction scheme (I).

[0089] In Examples 4 and 5 of the present invention, boehmite was used as the catalyst support for Cat.4 and Cat.5, respectively. In Examples 4 and 5 of the present invention, the absolute ethyl propyl ether selectivity reached 7.5% and 6.5%, respectively; and the direct ether selectivity of ethyl propyl ether was 93.8% and 94.2%, respectively.

[0090] In Comparative Examples A-D, various oxide supports, such as TiO2, CeO2, ZrO2, and activated carbon, were used. TiO2, CeO2, ZrO2, and activated carbon supports (Cat. 6-Cat. 9) showed improved selectivity for ethylpropyl ether. In Comparative Examples E-K, various metal components, such as Pt, Ni, Ru, Co, Rh, Fe, and Mn (Cat. 10-16), were used on N2O5 supports. Using Pt (Cat. 10), the conversion was up to 17.1%, with an absolute ethylpropyl ether selectivity of 2.5% and a direct ethylpropyl ether selectivity of 98.0%. Using Ni (Cat. 11), the conversion was up to 1.3%, with an absolute ethylpropyl ether selectivity of 2.9% and a direct ethylpropyl ether selectivity of 99.3%. Using Ru (Cat. 12), the conversion was up to 12%, with an absolute ethylpropyl ether selectivity of 1.4% and a direct ethylpropyl ether selectivity of 100%. Using Co (Cat. 13), the conversion rate was as high as 5.0%, with an absolute ethyl propyl ether selectivity of 0.8% and a direct ethyl propyl ether selectivity of 95.2%. Using Rh (Cat. 14), the conversion rate was as high as 9.8%, with an absolute ethyl propyl ether selectivity of 2.8% and a direct ethyl propyl ether selectivity of 99.3%. Using Fe (Cat. 15) and Mn (Cat. 16) as metal components on N₂O₅ supports, there was no evidence of direct hydrogenation of esters.

[0091] In Comparative Examples L-R, Re was used as the metal component, and for Cat.17-Cat.23, various oxide supports such as γ-Al₂O₃, CeO₂-Al₂O₃, Al₂O₃-MgO, SiO₂, and TiO₂ were used. There was no evidence of direct hydrogenation of esters occurring on Re-based catalysts.

[0092] Part B: Results of liquid-phase ester reduction

[0093] Examples 6-9 and Comparative Examples S-DD

[0094] The selectivity of the ether products in Examples 6-9 and Comparative Examples of the present invention is described in Table V, which describes the use of catalysts Cat.24-Cat.39. The reactions were carried out in a liquid-phase reactor using various catalysts. Hydrogenation was performed under the following reaction conditions: a temperature of 503 K, an H2 pressure of 4,977 kPa, and a propyl acetate pressure of 1,573 kPa. No dilution was performed in any of Examples 6-9 and Comparative Examples S-DD.

[0095] Table V - Liquid Phase Ester Reduction Selectivity

[0096]

[0097]

[0098] The absolute selectivity for the reduction of propyl acetate in Example 6 of this invention is 6.0%, while the direct selectivity for ethylpropyl ether is 96.8%.

[0099] In Example 7 of this invention, the catalyst used contained 5 wt% Pd on a Nb₂O₅ support. The results of Example 7 of this invention are similar to those of Example 6 of this invention under the same conditions.

[0100] In Example 8 of this invention, the catalyst used was prepared on a WO3 support. The reaction was carried out in a liquid-phase reactor using a 1 wt% Pd / WO3 catalyst. Hydrogenation was performed under the following reaction conditions: 1,595.2 kPa C5H 10 O2; 4,976.9 kPa; H2; and 503 K. The absolute selectivity of ethylpropyl ether is precisely 13%, and the direct ether selectivity of ethylpropyl ether is 86.1%.

[0101] Supporting Example 8 of the present invention, the selectivity of the ether product of the present invention was measured at various ester feed rates as described in Table VI. Table VI describes the percentage conversion of the ester to the ether product and the percentage selectivity of the direct selective reduction of the ester to the desired ether product in a liquid-phase reactor on a catalyst containing 1% Pd supported on Nb₂O₅. Selectivity is based on the conversion under the following conditions: 1,595.2 kPa C₅H 10 O2, 4,976.9 kPa, H2, and 503 K. The selectivity of the ether products of the present invention at various ester feed rates is described in Table VI.

[0102] Table VI - Conversion Rate and Product Selectivity

[0103]

[0104] Notes to Table VI: *Amount of ethers directly produced from esters relative to the total amount of ethers. In Table VI, for example, "direct ether selectivity" = absolute selectivity of ethylpropyl ether / (ethylpropyl ether + dipropyl ether)*100.

[0105] In Example 9 of this invention, Cat.22 was prepared on a WO3 support having a Pd loading of 5 wt%. The absolute ethylpropyl ether selectivity in Example 9 of this invention reached 16.1%, and the absolute dipropyl ether selectivity was 2.3%. The direct ether selectivity of ethylpropyl ether reached 87.5%.

[0106] In Comparative Example SZ, several supports, such as activated carbon, Al2O3, MoO3, CeO2, ZrO2, SiO2, and TiO2, were used in Cat. 28 to Cat. 35. The catalyst with the activated carbon support exhibited low ester conversion. The catalyst with the Al2O3 support promoted more dehydration products. Comparative Example SZ, without a support, showed superior absolute and direct ethylpropyl ether selectivity compared to the WO3 or Nb2O5 supports used in Examples 6-9 of this invention.

[0107] In Comparative Examples AA-DD, the metal components AA-DD, Ni, Ru, and Pt were used and tested in Cat.36-Cat.39. Ni and Pt showed some absolute ethylpropyl ether selectivity; however, the dipropyl ether selectivity was higher than that expected for Comparative Examples AA-DD.

[0108] Discussion of Results

[0109] Part A: Gas-phase ester reduction

[0110] Table IV shows the distribution of ester reduction products at different conversion rates. In the gas-phase method, the selectivity for dipropyl ether and diethyl ether is almost zero, indicating that essentially all ethylpropyl ethers are formed via a direct hydrogenation pathway, since alcohol dehydration shows no selectivity for symmetrical or asymmetrical ethers. Therefore, the results of Example 1 of this invention demonstrate that Pd on Nb₂O₅ can catalyze the direct hydrogenation of esters to ethers.

[0111] The results of Example 3 of this invention show that the WO3 support also provides direct hydrogenation.

[0112] The results of Examples 4 and 5 of the present invention show that boehmite supports can be directly hydrogenated.

[0113] The results of Comparative Example AD indicate that supports such as TiO2, CeO2, ZrO2, and activated carbon used as catalysts in the methods of the present invention are not very effective in providing a direct hydrogenation route from ester to ether.

[0114] The results of the comparative example EI show that single-metal transition metals such as Pt, Ni, Ru, Co, and Rh, used as catalysts in the method of the present invention, can catalyze the direct hydrogenation of esters to ethers, providing 95% direct ether selectivity. However, the absolute ether selectivity of catalysts with these metal components is <5%. These catalysts require further optimization to achieve high absolute ether selectivity.

[0115] The results of Comparative Example JR indicate that single-metal transition metals such as Fe, Mn, and Re, used as catalysts in the method of the present invention, are not very effective in providing a direct hydrogenation route from ester to ether.

[0116] Part B: Liquid-phase ester reduction

[0117] The results of Examples 6-9 of this invention show that the Pd catalyst supported on Nb2O5 or WO3 can catalyze the conversion of esters to ethers by direct hydrogenation, which is similar to the results observed from the above-mentioned gas-phase reduction methods.

[0118] The results of Comparative Example SZ indicate that supports such as activated carbon, Al2O3, MoO3, CeO2, ZrO2, SiO2, and TiO2 used as catalysts in the method of the present invention are not very effective in providing a direct hydrogenation route from ester to ether.

[0119] The results of the comparative examples AA-DD indicate that transition metals such as Ni, Ru, and Pt, used as catalysts in the methods of the present invention, are not very effective in providing a direct hydrogenation route from ester to ether.

[0120] Based on the above results of the gas-phase reduction method and the liquid-phase reduction method, the method of the present invention using a Pd catalyst on a Nb2O5 support or a WO3 support can advantageously provide direct hydrogenation of esters to ethers.

Claims

1. A method for producing an ether product by direct selective reduction of an ester, comprising treating (a) at least one ester with (b) hydrogen in the presence of (c) a heterogeneous catalyst to directly and selectively reduce said at least one ester by hydrogenation to form said at least one ether, said method following reaction scheme (I): Reaction scheme (I) Wherein R1 and R2 are alkyl functional groups, including straight-chain or branched alkyl groups, cyclic or acyclic alkyl groups; and mixtures thereof, wherein the ether is an asymmetric ether and R1 is not equivalent to R2; The heterogeneous catalyst comprises at least one transition metal selected from palladium, platinum, nickel, and mixtures thereof, the transition metal being supported on at least one support member selected from niobium oxide support, tungsten oxide support, boehmite support, and mixtures thereof; and The absolute selectivity of the ether product is greater than 5%, while the selectivity of the direct ether product is greater than 85%. The absolute selectivity of the ether product refers to the percentage of the ether product in the total product formed in the reaction, and the selectivity of the direct ether product refers to the percentage of the direct ether product relative to the total ether product.

2. The method according to claim 1, wherein the at least one ester is selected from ethyl acetate, propyl acetate, butyl acetate, ethyl propionate, butyl propionate, and mixtures thereof.

3. The method according to claim 1, wherein the temperature of the method is 350K to 650K; and wherein the pressure of the method is 0.1MPa to 10MPa.

4. The method according to claim 1, wherein the conversion rate of the ester is 1% to 70%, and the absolute selectivity of the ether product is greater than 5% to 20%; and / or the at least one ester is selected from ethyl acetate, propyl acetate, amyl acetate, γ-butyrolactone, butyl butyrate, and mixtures thereof.

5. The method according to claim 1, wherein the method is a gas-phase reduction method performed under gas-phase process conditions.

6. The method according to claim 1, wherein the method is a liquid-phase reduction method performed under liquid-phase process conditions.

7. The method of claim 1, wherein the heterogeneous catalyst comprises 0.1 wt% to 20 wt% of a transition metal.

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