Process for producing ether

Abstract

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

Description

[Technical Field]

[0001] The present invention relates to a process for producing ether compounds, and in particular, the present invention relates to a process for producing ether compounds directly from alkyl esters using molecular hydrogen over a heterogeneous catalyst. [Background technology]

[0002] Ethers are used as solvents in a variety of applications. Ethers are particularly desirable for use as solvents in various applications due to their excellent solvency, chemical stability, and compatibility with other organic solvents and formulation products. Known methods for synthesizing ethers include the following three methods: (1) alkyl halides treated with alkoxides (the so-called "Williamson ether synthesis"), (2) the addition of alcohols to olefins, and (3) the acid-catalyzed coupling of alcohols. However, the above three methods have undesirable drawbacks, including: (1) the use of strongly acidic or basic conditions that can lead to competing elimination reactions that produce undesired olefins; (2) limited options for biologically derived raw materials due to lack of reactivity with the above reactions, which limits the structural diversity of the products; and (3) the use of toxic raw materials and the generation of waste streams in the production process. Therefore, it is desirable to provide a viable method for producing ethers that can be successfully scaled up commercially without the drawbacks of the above known methods.

[0003] For example, previously known methods for producing ethers include: (1) J. Org. Chem., 2007, 72, 5920-5922; Tetrahedron (1) a process for producing thionates (salts or esters of thioic acid), such as thioethers (sulfides, which are compounds in which sulfur and two organic residues are combined), as disclosed in J. Org. Chem., 1981, 46, 831-832; (2) a process for the catalytic reduction of α-monoglycerides at approximately 700 psi (4.8 megapascals [MPa]) and 120 degrees Celsius (°C) using a 5% Pd / C catalyst mixed with an acid promoter, as disclosed in U.S. Pat. No. 8,912,365; and (3) a process for the catalytic reduction of α-monoglycerides using a 5% Pd / C catalyst mixed with an acid promoter at approximately 700 psi (4.8 megapascals [MPa]) and 120 degrees Celsius (°C), as disclosed in Russian Chemical Bulletin 1988, 37(1), 15-19 and Russian Chemical Bulletin (5) processes for the hydrogenation of lactones to cyclic ethers to produce, for example, tetrahydrofuran, at high selectivities, such as greater than 90%, using various metal catalysts on various supports, as disclosed in U.S. Pat. Nos. 3,370,067, 3,894,054, and 4,973,717; and (6) processes using homogeneous metal complex catalysts (e.g., ruthenium / triphos complexes) that require impractical separation of the catalyst from the product. Angewandte Chemie, International Edition, 2015, 54, 5196-5200; ChemSusChem, 2016, 9, 1442-1448. It would be desirable to have alternative processes for producing ethers that can be produced commercially and that offer advantages over existing processes. Summary of the Invention

[0004] The present invention relates to a process for producing an ether product from an ester starting material.

[0005] In a broad embodiment, the process of the present invention involves producing ethers by hydrogenating esters in the presence of a heterogeneous catalyst.

[0006] In one embodiment, the process of the present invention involves the selective reduction of carboxylic acid derivatives directly to ethers using molecular hydrogen and an appropriate catalyst formulation to achieve high (e.g., greater than 5%) absolute ether selectivity and high (e.g., greater than 85%) direct ether selectivity.

[0007] Absolute ether selectivity is the proportion of total products formed in the reaction, and direct ether product selectivity is the proportion of direct ether products relative to total ether products.

[0008] In another embodiment, the process of the present invention for producing an ether comprises combining (a) at least one ester with (b) hydrogen in the presence of (c) a heterogeneous catalyst to reduce the ester by hydrogenation to form the ether.

[0009] In yet another embodiment, the present invention comprises a solvent comprising the above ether product produced by the above process.

[0010] Some of the advantageous features that may be provided by one or more embodiments of the process of the present invention include, for example:

[0011] (1) An active catalyst is used in a one-step process: the catalyst is active for reducing the ester and directly hydrogenating the ester to form the ether, rather than going through the known two-step ether-forming process, such as (i) ester hydrogenolysis to form the alcohol, followed by (ii) alcohol dehydration.

[0012] (2) Use of a relatively inexpensive method. This process uses inexpensive molecular hydrogen as the reducing agent, rather than using expensive hydrosilanes, metal hydrides, or metal hydride / Lewis acid complexes as hydride donors. The highly reactive hydrosilanes or metal hydrides used in the prior art processes also require complex and expensive process designs to ensure the safety of operators performing the prior art processes.

[0013] (3) Use of a heterogeneous catalyst. The fact that the catalyst is heterogeneous rather than homogeneous can contribute to reducing production costs due to the reusability of the catalyst.

[0014] (4) Use efficient processes.

[0015] (5) Use of a flexible process: The process is applicable to general ester compounds (cyclic or acyclic) as raw materials, and the process is not limited to a specific ester compound. DETAILED DESCRIPTION OF THE INVENTION

[0016] In one embodiment, the present invention comprises a unique and novel method for synthesizing ethers from esters using heterogeneous catalysts. Reactions of esters include a variety of chemical reaction techniques or routes, such as hydrogenolysis, hydrolysis, dehydration, hydrogenation, and transesterification. In a general embodiment, the process of the present invention comprises producing ethers by hydrogenation of esters, such as propyl acetate, in the presence of a heterogeneous catalyst. The novel hydrogenation reaction route or scheme of the present invention, for example, the hydrogenation of propyl acetate reduction reaction scheme where R1 is -CH3 and R2 is -CH2CH3, is generally shown below as Reaction Scheme (I). [ka]

[0017] In the above Reaction Scheme (I), water is produced by the reduction process, and the produced water can be separated by conventional processes such as distillation or other procedures known in the art. The functional groups R1 and R2 can be alkyl functional groups, including linear 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. The desired ether product obtained from the above Reaction Scheme (I) can be a symmetrical ether when R1 is equivalent to R2, or an asymmetrical ether when R1 is not equivalent to R2, for example, an asymmetrical ether can be ethyl propyl ether.

[0018] By "symmetric ether" herein is meant an ether containing two identical functional groups, where R1 is equal to R2. By "unsymmetric ether" herein is meant an ether containing two different functional groups, where R1 is not equal to R2.

[0019] In the present 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 an ester (R1COOCH2R2) by hydrogenation to form an ether (R1CH2OCH2R2) while maintaining the alkoxy group intact. The present process differs from known processes because it does not follow the typical route of ester (R1COOCH2R2) hydrogenation. The ester is first decomposed into two alcohols (R1CH2OH + R2CH2OH molecules) via hydrogenolysis, and then a mixture of ethers (R1CH2OCH2R1 + R1CH2OCH2R2 + R2CH2OCH2R2) is formed via dehydration. Direct hydrogenation can maintain the ether structure from the ester by simply removing the carbonyl oxygen. Therefore, the asymmetric ester provided in this process advantageously results in the direct production of the asymmetric ether, since this process does not decompose the ester into two alcohol molecules via hydrogenolysis. The selectivities of ethyl propyl ether in the reaction examples are listed in Tables III and V described in the Examples below.

[0020] The term "direct ether product" as used herein means an ether formed by a one-step reduction process of an ether from an ester.

[0021] As used herein, the term "indirect ether product" refers to an ether formed by a two-step reduction process of an ether from an ester, the two-step reduction process comprising: (i) hydrogenolysis and (ii) A dehydration step is included.

[0022] The term "direct ether product selectivity" herein means the ratio of direct ether products (eg, ethyl propyl ether) to total ether products (eg, ethyl propyl ether + dipropyl ether + diethyl ether).

[0023] As used herein, the term "absolute ether product selectivity" refers to the proportion of ether products among all products (e.g., ethers, alcohols, and alkanes) formed in a reaction.

[0024] Advantageously, one unique factor of the present invention involves increased direct ether selectivity using the one-step process of the present invention over known two-step processes.

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

[0026] In one desirable embodiment, as shown in Reaction Scheme (I) above, the process of the present invention for producing an ether includes the steps of (A) feeding an ester compound, such as propyl acetate, component (a), to a reactor; (B) feeding hydrogen, component (b), to the reactor to form a hydrogen atmosphere within the reactor; (C) charging the reactor with a heterogeneous catalyst system, such as a combination of a transition metal on an acidic support, sufficient to cause a hydrogenation reaction within 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, the heating step (D) can be carried out at a temperature between 350 Kelvin (K) and 650 K.

[0027] In some embodiments, the catalyst system of the present invention is a combination of, for example, a transition metal and an acidic support. For example, the catalyst used in some embodiments of the present invention can include 0.1 weight percent (wt%) to 20 wt% of a transition metal supported on an acidic support element. The synergistic effect from both the transition metal component and the acidic support component promotes the direct ether reduction route, as depicted in reaction pathway Reaction Scheme (I). Without the combination of the transition metal and the acidic support element as disclosed herein, the undesirable two-step ether formation route would occur. Steady-state rates and product selectivities for the competing reaction pathways of the prototype ester compound can be achieved, for example, in packed bed and / or trickle bed reactors, depending on reactant pressure, temperature, and ester conversion, controlled by superficial residence time.

[0028] The ester compound to be reduced to the ether, component (a), can include, for example, one or more ester compounds containing carboxylic acid derivatives, esters containing linear or branched alkyl groups, and cyclic or acyclic alkyl groups, and mixtures thereof. In some embodiments, the asymmetric ether comprises reaction scheme (I) in which the R group is not equal to the R group. In some embodiments, the ester useful in the present invention can be, for example, ethyl acetate (available from Sigma-Aldrich), propyl acetate (available from Sigma-Aldrich), pentyl acetate (available from Sigma-Aldrich), γ-butyrolactone, butyl butyrate (available from Sigma-Aldrich), and mixtures thereof.

[0029] The concentration of component (a), the 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 a desired production rate and / or avoid increased separation costs. In some embodiments, the concentration of the ester is from 1 wt. % to 100 wt. The concentration of the ester is based on the total weight of the ester compounds in the liquid feedstock.

[0030] The concentration of hydrogen, component (b), useful in the process of the present invention is, 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, for example, less than (<) 3 wt. %, may reduce reactivity or ether selectivity and, therefore, may require an undesirable increase in reaction pressure. The concentration of hydrogen is based on the total weight of hydrogen in the gaseous feedstock.

[0031] In a broad embodiment, the catalyst, component (c), used in the process of the present invention can comprise one or more heterogeneous catalyst compounds. The catalyst used in the process of the present invention can comprise, for example, a combination of (ci) transition metals supported on (cii) acidic support (carrier) elements. For example, the transition metal (component (ci)) can include palladium (Pd), platinum (Pt), nickel (Ni), ruthenium (Ru), cobalt (Co), rhodium (Rh), and mixtures thereof. The acidic support (component (cii)) can include, for example, niobium oxide (NbO) supports, tungsten oxide (WO) supports, and mixtures thereof. In one preferred embodiment, the heterogeneous catalyst useful in the present invention can be Pd supported on a NbO support, Pd supported on a WO support, and mixtures thereof.

[0032] The heterogeneous catalysts of the present invention exhibit several advantageous properties. For example, the heterogeneous catalysts useful in the present invention provide a synergistic effect between the Lewis acidity and / or Bronsted acidity from the catalyst's metal compound (e.g., Pd or Pt) and the catalyst's support (e.g., NbO) to catalyze direct ester hydrogenation, which may otherwise result in reduced ether selectivity.

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

[0034] The process equipment used to carry out the reduction process can be any conventional reactor, such as a packed bed reactor or a trickle bed reactor, and the ester conversion and ether selectivity can be controlled by reactor pressure, temperature, and superficial residence time.

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

[0036] For example, the temperature of the process 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 aforementioned ranges may result in lower reactivity than disclosed herein. Temperatures above the aforementioned ranges may result in undesirable alkane and alcohol by-products, which may reduce ether selectivity.

[0037] For example, the ester conversion rate of the process 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 aforementioned conversion ranges may result in side reaction products.

[0038] The process of the present invention can be carried out as a batch process or a continuous process. In some embodiments, when a batch process is used, the residence time of the process of the present invention is, for example, from 0.1 hours (hours) to 24 hours in one embodiment, from 0.1 hours to 8 hours in another embodiment, and from 1 hour to 24 hours in yet another embodiment. In some embodiments, residence times below the aforementioned residence time ranges may result in lower ester conversion, and in some embodiments, residence times above the aforementioned residence time ranges may result in undesirable side reaction products.

[0039] In some embodiments, when a continuous process is used, the residence time for the process of the present invention is, for example, from 0.1 seconds (seconds) to 100 seconds in one embodiment, from 1 second to 10 seconds in another embodiment, and from 10 seconds to 100 seconds in yet another embodiment. Residence times below the aforementioned residence time ranges may result in lower ester conversions, and in some embodiments, residence times above the aforementioned residence time ranges may result in undesirable side reaction products.

[0040] Some advantageous properties and / or benefits of using the reduction process of the present invention include, for example, that the process of the present invention can achieve a steady-state rate and that the process can provide better selectivity of the product over the competing reaction pathway of the ester compound. Also, conventional processes for producing ethers also produce salts, whereas the process of the present invention does not produce salts.

[0041] After the ester compound undergoes a reduction process, the resulting ether product is formed. The turnover rate from ester to ether product is 10 -8 moles of ether / gram of catalyst·sec (mol / g cat ·seconds)~10 -5 mol / g cat seconds, and in one typical embodiment, 5 x 10 -8 mol / g cat ·Seconds~5×10 -6 mol / g catEster turnover rates below the aforementioned rate ranges may result in lower ether production rates, and in some embodiments, ester turnover rates above the aforementioned residence time ranges may result in undesirable side reaction products.

[0042] The selectivity to the ether product may depend on whether a vapor or liquid process is used to form the ether, and whether a batch or continuous process is used. Generally, the selectivity to the ether product is greater than 5% in one embodiment, between 5% and 16% in another embodiment, and between 5% and 8% in yet another embodiment.

[0043] The ether products produced by the process of the present invention can be symmetrical or asymmetrical ethers, but for purposes of illustration and not limitation, the process of the present invention will be described with reference to asymmetrical ethers. Surprisingly, it has been discovered that the process of the present invention is selective for asymmetrical ethers because the ester is converted directly to an ether without undergoing ester hydrogenolysis and alcohol dehydration. Ester hydrogenolysis and alcohol dehydration are two processes known to be unselective for certain ethers.

[0044] When an asymmetric ether is required for a particular process or end use, use of the process of the present invention is advantageous over conventional processes for the following reasons.

[0045] (1) The ether products are more stable than the corresponding ester products under basic and acidic conditions, and the ether products of the present invention are not subject to hydrolysis, which can occur at high humidity and / or high temperature.

[0046] (2) The hydrogenation chemistry of the present process keeps the backbone of the ester product intact. During hydrogenation, only the oxygen molecule is cleaved from the backbone, leaving the carbon molecule and backbone oxygen intact. In conventional reaction processes, the reaction breaks the backbone and recombines parts together under different reaction conditions. Therefore, direct hydrogenation / reduction of the ester to an ether does not occur.

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

[0048] Because the ether products of the present invention are derived from organic and renewable sources, they have minimal environmental impact. For example, the ether products can be advantageously used as green and bio-based solvents on a global scale to address stringent regulations imposed on chemical industry solvents regarding toxicity, non-biodegradability, volatile organic compound (VOC) emissions, etc. Green and bio-based solvents are typically 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 cosmetics and personal care applications.

[0049] The present invention provides bio-based solvents with cost and performance advantages over solvents known in industry. In addition, the chemical transformations provided by the process of the present invention may be useful in production processes for, for example, producing bio-based surfactants, antifoams, and lubricants in an economical and environmentally friendly manner.

[0050] The ether production process of the present invention can also be used to develop (1) a more robust capping process to overcome the problem of limited reactant alkyl chloride type and final product impurities, (2) novel capped low viscosity-low volatility lubricants, and (3) novel surfactants and novel bio-based antifoams for food and pharmaceutical applications, metalworking fluid applications, and other applications utilizing ether solvents. [Example]

[0051] The following inventive and comparative examples (collectively "Examples") are presented herein to further illustrate the present invention, but should not be construed as limiting the scope of the claims. Unless otherwise indicated, all parts and percentages are by weight.

[0052] catalyst The catalyst ("Catalyst") formulations used in the examples are set forth in Tables I and II. The examples using Catalyst 1 through Catalyst 18 below were tested in a gas phase reactor, while the examples using Catalyst 19 through Catalyst 34 were tested in a liquid phase reactor.

[0053] Examples using Catalysts 1-5 and 19-22 are representative examples of the present invention, while examples using Catalysts 6-18 and 23-34 are comparative examples. The formulations of the examples of the present invention include Pd as the transition metal and NbO or WO as the catalyst support. The Pd loading on the catalysts useful in the present invention is between 0.1% and 5% by weight.

[0054] The process of the present invention (1) uses molecular hydrogen (H) as a reducing agent, (2) is carried out in the gas or liquid phase without the addition of a solvent, and (3) includes processes using a heterogeneous catalyst for the reduction of esters to ethers, where the heterogeneous catalyst is a transition metal, e.g., a Pd-based catalyst, and the catalyst support (support) is an acid support, e.g., a WO-based catalyst support or a NbO-based catalyst support. [Table 1] [Table 2]

[0055] General procedure for the synthesis of supported catalysts The heterogeneous catalysts of the present invention can be prepared by incorporating the transition metal catalyst onto the catalyst support by conventional methods such as impregnation, precipitation, co-precipitation, impregnation with colloidal metal nanoparticles, strong electrostatic adsorption, ion exchange, mechanical mixing, etc. To prepare the heterogeneous catalysts for use in the examples, the transition metal particles were deposited onto the support using the incipient wetness impregnation method described below.

[0056] A calculated amount of metal precursor was dissolved in deionized water with a volume equal to the support pore volume to form a solution. This solution was added dropwise to the support to achieve incipient wetness. The wet solid resulting from incipient wetness impregnation was dried in stagnant air for 12 hours. The dried solid was then calcined in flowing air at 573-973 K for 3-4 hours. The resulting calcined solid was then reduced in flowing hydrogen / helium (H / 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 exposing the sample to ambient air. See Tables I and II above for the metal support, catalyst support, and precursors used for each of the catalysts prepared for use in the examples.

[0057] Test Measurement Part A: Catalytic rate measurements in gas-phase reactors Catalysts 1 through 23 were tested in a gas-phase reactor. The gas-phase reactor was a tubular packed-bed reactor held in a stainless steel tube (outer diameter [OD] 9.5 millimeters [mm]) containing 10 milligrams (mg) to 500 mg of catalyst. The catalyst was held in the center of the reactor using a glass rod and packed glass wool. 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). Catalyst temperature was measured by a K-type thermocouple contained in a 1.6 mm stainless steel sheath (available from Omega) aligned coaxially within the reactor and submerged in the catalyst bed. The catalyst bed volume was adjusted to 1.4 cubic centimeters (cm) by mixing excess silicon carbide (SiC) (available from Washington Mills, Carborex green 36) with the desired amount of catalyst. 3 The reactor pressure was kept constant with a constant flow rate of 1000 psi (1000 psi). The system was pressurized using a back-pressure regulator (BPR, Equilibar LF series available from Equilibar Precision Pressure Control) controlled by an electronic pressure regulator (EPR, Equilibar GP1 available from Equilibar Precision Pressure Control). The reactor pressure was monitored upstream and downstream of the catalyst bed using a digital pressure gauge (available from Omega) and an EPR, respectively.

[0058] The gases used in the examples were H2 (available from Airgas Inc. as "Ultra High Purity 5.0") and He (available from Airgas Inc. as "Ultra High Purity 5.0"). Gas flow rates were controlled using mass flow controllers (available from Bronkhorst as "EL-FLOW High Pressure"). Liquid propyl acetate (CH 10O2, supplied by Sigma Aldrich, 537438, 99.5% or more [≥]) flow rate was controlled by C5H 10 O2 was passed through polyetheretherketone (PEEK) polymer tubing (1.6 mm outer diameter and 0.25 mm inner diameter [ID]), the outlet of which was positioned within a small bed of nonporous sand (SiO2 50-70 mesh particle size, Sigma Aldrich, 274739) in crossflow with H2, and was controlled using a stainless steel syringe pump with a Hastelloy cylinder (100DX with a D-series controller, available from Teledyne Isco). To avoid condensation, the transfer lines surrounding the liquid inlet were maintained at 373 K using heating tape (available from Omega). All transfer lines downstream of the liquid inlet were heated above 373 K using heating tape, and line temperatures were monitored with K-type thermocouples (available from Omega) displayed on a digital reader (available from Omega).

[0059] The catalyst was heated to the desired temperature at 0.05 Kelvin / s (K s) before all catalyst measurements. -1 ), and the catalyst is held at that temperature for the desired time within 101 kilopascals (kPa) and H is supplied at 100 cubic centimeters per minute (cm 3 minutes -1The reactor effluent was pretreated in situ by flowing it through a gas chromatograph (HP6890, available from Agilent). The reactor effluent was characterized using an online gas chromatograph (HP6890, available from Agilent). The gas chromatograph (GC) was equipped with a capillary column (DB-624UI, 30 meters (m) long, 0.25 mm internal diameter, 1.40 micrometers [μm]) connected to a flame ionization detector to quantify the concentration of combustible species. Sensitivity factors and retention times for all components were determined using gas and liquid standards. Control of reactor pressure and temperature, reactant flow rates, and GC sampling were automated to allow continuous measurements. Conversion was calculated on a carbon basis based on the amount of carbon appearing in the product. Carbon and oxygen balances were within ±20%. Reactor conditions during rate and selectivity measurements were varied by successively decreasing and then increasing the reactant pressure over the entire range of 1 MPa to 10 MPa, with one or more conditions measured at least twice throughout the experiment to ensure that any trends measured were not the result of systematic deactivation.

[0060] Part B: Catalytic rate measurements in liquid-phase reactors Catalysts 24 through 39 were tested in a liquid-phase reactor. Rate and selectivity measurements were performed in a trickle-bed reactor containing a stainless steel tube (1.6 mm outer diameter) containing 1,000 mg to 4,000 mg of catalyst (30 mesh to 60 mesh). The catalyst was held in the center of the reactor using a Pyrex glass rod and packed glass wool. The reactor was heated in an aluminum clamshell containing two heating cartridges controlled by an electronic temperature controller (EZ-Zone, available from Watlow). The reaction temperature was measured by a K-type thermocouple contained within a 3.2 mm stainless steel sheath (available from Omega) coaxially aligned within the reactor and submerged within the aluminum clamshell. The system was pressurized to 6.5 MPa using a dome-type backpressure regulator (BPR) (Equilibar LF series, available from Equilibar Precision Pressure Control) controlled by an electronic pressure regulator (EPR) (Equilibar GP1, available from Equilibar Precision Pressure Control). The reactor pressure was monitored using a digital pressure gauge (available from Omega) and EPR.

[0061] The gas flow rates of H2 (available from Airgas) and He (available from Airgas) were controlled using mass flow controllers (EL-FLOW high pressure controllers available from Bronkhorst). Liquid propyl acetate (CH 10 O2) (Sigma Aldrich, 537438, ≥ 99.5%) flow rate was 10 O2 was supplied through stainless steel tubing (1.6 mm outer diameter and 0.15 mm inner diameter) in cross-flow with H2 and He and was controlled using a high performance liquid chromatography (HPLC) pump (P-LST40B available from Chromtech).

[0062] The catalyst was heated for 0.08 K·s before all catalytic measurements. -1 Heat to 503K and 50cm 3 ·minute -1The reactor was pretreated in situ by holding it in flowing He (20 kPa) and H2 (81 kPa) for 1 h. The reactor effluent passed through a stainless steel cooling chamber containing cold water (temperature approximately 377 K), and then the gaseous and liquid products were separated in a gas-liquid separator (GLS). The liquid products collected in the GLS were delivered by an HPLC pump to a high-pressure liquid sampling valve (LSV, Transcendent Enterprise Inc., PLIS-6890, 1 μL injection volume) attached to an online gas chromatograph (Agilent, HP7890B). A manual BPR (Swagelok) was installed at the outlet of the LSV to maintain the liquid pressure at 1,380 kPa to prevent product evaporation within the sampling system. The gaseous and liquid products were characterized using online gas chromatograph (Agilent, HP7890B). The GC was equipped with two capillary columns (DB-Wax UI, 60 m long, 0.25 mm internal diameter, 0.25 μm) for the liquid products. A GS-GASPRO (GC column, 60 m long, 0.32 mm internal diameter, available from Agilent) was connected to a flame ionization detector to quantify species concentrations. Sensitivity factors and retention times for all gaseous and liquid products were determined using gas standards and a methanizer (Polyarc System, PA-SYC-411, available from Activated Research Company), respectively. Control of reaction pressure and temperature, reactant and product flow rates, and GC sampling were automated to allow continuous measurements. Conversion was calculated on a carbon basis based on the amount of carbon appearing in the products. Carbon balances were within ±10%.

[0063] Test results Reaction pathway using propyl acetate as the ester starting compound A kinetic study was conducted on the catalysts (Catalysts 1 to 39) used in the examples to directly and selectively reduce propyl acetate (a representative ester) with molecular hydrogen to determine the reaction pathway for converting propyl acetate to ether. GC analysis revealed several products in the product stream exiting the fixed-bed reactor. The various products included light hydrocarbons such as C2-C3 alkanes, ethanol, propanol, dipropyl ether, ethyl propyl ether, acetic acid, and ethyl acetate. Based on this observation, it can be concluded that the reaction occurred via several routes within the reactor, including the following: For example, (1) hydrogenolysis of propyl acetate to one ethanol and one propanol; (2) hydrolysis of the propyl acetate to form propanol and acetic acid; (3) subsequent dehydration of the alcohol to form light hydrocarbons and ether products such as dipropyl ether and ethyl propyl ether; (4) transesterification of propyl acetate with ethanol to form ethyl acetate and propanol; and (5) the present invention's route, Reaction Scheme (I), i.e., direct hydrogenation of propyl acetate with hydrogen to form ethyl propyl ether. Because the desired product is ethyl propyl ether from direct hydrogenation of propyl acetate, the route of Reaction Scheme (I) of the present invention is the desired reaction pathway. It should be noted that ethyl propyl ether can also be formed via Route (3) above. However, Route (3) above is undesirable because it involves an alcohol dehydration reaction, which is not selective for the formation of asymmetric ethers over symmetric ethers such as dipropyl ether or diethyl ether. The present invention's process is not limited to the production of either symmetric or asymmetric ethers. Advantageously, the process of the present invention provides unsymmetrical ethers when desired or required in a selective and straightforward manner.

[0064] Part A: Results of gas-phase ester reduction Examples 1 to 5 and Comparative Examples A to M For Inventive Examples 1-5 and Comparative Examples A-R, the ether selectivity results using Catalysts 1-23 are listed in Table III. Hydrogenation was carried out under the following reaction conditions: a temperature range of 433 K to 504 K, an H pressure of 6.2 MPa, and an ester pressure of 10 kPa. For Inventive Example 2, Inventive Example 5, and Comparative Examples L-R, the catalyst was diluted with support powder. The purpose of dilution with support powder was to reduce the ester conversion to less than 10%. [Table 3]

[0065] In Example 1 of the present invention, propyl acetate reduction was carried out in a gas-phase reactor using a 0.8 wt% Pd / NbO catalyst (Catalyst 1). The absolute ethyl propyl ether selectivity was 8.5%, and the absolute dipropyl ether selectivity was 0.1%. The direct ether selectivity relative to ethyl propyl ether was 98.8%.

[0066] In support of Inventive Example 1, selectivities for the inventive ether products were measured at various ester feed rates as listed in Table IV. To demonstrate the reaction pathway, reactions were run at low conversion ranges of 1% to 48%. Table IV lists the conversions of esters to ether products and the selectivities for the direct selective reduction of esters to the desired ether products in a gas-phase reactor over a catalyst containing 0.8 wt. % Pd supported on Nb2O5. As previously described in Inventive Example 1, selectivities were measured under the following conditions: 10 kPa C5H 10 Based on a function of conversion at O2, 6.2 MPa H2 and 503K. [Table 4] Notes for Table IV: *Amount of ether produced directly from esters relative to the total amount of ethers. In Table IV, for example, "direct ether selectivity" = absolute selectivity of ethyl propyl ether / (ethyl propyl ether + dipropyl ether) * 100.

[0067] In inventive Example 2, Catalyst 2 contained 4.3 wt. % Pd metal on a NbO support. The absolute ethyl propyl ether selectivity was about 5%, and the absolute dipropyl ether selectivity was less than 0.1%. The direct ethyl propyl ether selectivity was 98.4%. The results of this inventive Example 2 were similar to inventive Example 1, which contained a lower Pd loading of 0.8 wt. %.

[0068] In Inventive Example 3, Catalyst 3 was prepared on WO. The ester conversion in Inventive Example 3 was significantly higher than that of NbO (Catalyst 1 and Catalyst 2) with approximately the same metal loading. The absolute ethyl propyl ether selectivity was 13.6%, and the absolute dipropyl ether selectivity was 0.5%. The direct ethyl propyl ether selectivity was 96.5%. The results indicate that ethyl propyl ether was mainly formed via the direct route of Reaction Scheme (I).

[0069] In inventive Examples 4 and 5, boehmite was used as the catalyst support in Catalyst 4 and Catalyst 5, respectively. In inventive Examples 4 and 5, the absolute ethyl propyl ether selectivities reached levels of 7.5% and 6.5%, respectively, and the direct ether selectivities of ethyl propyl ether were 93.8% and 94.2%, respectively.

[0070] In Comparative Examples A through D, various oxide supports were used, including TiO2, CeO2, ZrO2, and activated carbon. Neither TiO2, CeO2, ZrO2, nor activated carbon supports (Catalysts 6 through 9) showed any improvement in ethyl propyl ether selectivity. In Comparative Examples E through K, various metal components were used, including Pt, Ni, Ru, Co, Rh, Fe, and Mn (Catalysts 10 through 16) on NO5 supports. Using Pt (Catalyst 10), the conversion was up to 17.1%, with an absolute ethyl propyl ether selectivity of 2.5% and a direct ethyl propyl ether selectivity of 98.0%. Using Ni (Catalyst 11), the conversion was up to 1.3%, with an absolute ethyl propyl ether selectivity of 2.9%, and a direct ethyl propyl ether selectivity of 99.3%. Using Ru (Catalyst 12), the conversion was up to 12%, with an absolute ethyl propyl ether selectivity of 1.4%, and a direct ethyl propyl ether selectivity of 100%. Using Co (catalyst 13), the conversion was up to 5.0%, with an absolute ethyl propyl ether selectivity of 0.8% and a direct ethyl propyl ether selectivity of 95.2%. Using Ru (catalyst 14), the conversion was up to 9.8%, with an absolute ethyl propyl ether selectivity of 2.8% and a direct ethyl propyl ether selectivity of 99.3%. There was no evidence of direct hydrogenation of esters when Fe (catalyst 15) and Mn (catalyst 16) were used as the metal component on an NO support.

[0071] In Comparative Examples L to R, Re was used as catalysts 17 to 23, which were γ-Al2O3, CeO2-Al2O3, Al2O3-MgO, and SiO 2、 It was used as the metal component with various oxide supports such as TiO2. There was no evidence of direct ester hydrogenation with Re-based catalysts.

[0072] Part B: Results of liquid phase ester reduction Examples 6 to 9 and Comparative Examples S to DD The selectivities for ether products for Inventive Examples 6-9 and Comparative Examples S-DD using Catalysts 24-39 are listed in Table V. The reactions were carried out in a liquid-phase reactor using various catalysts. Hydrogenation was carried out under the following reaction conditions: temperature 503 K, H pressure 4,977 kPa, and propyl acetate pressure 1,573 kPa. No dilution was performed in Inventive Examples 6-9 and Comparative Examples S-DD. [Table 5]

[0073] In Example 6 of the present invention, the absolute selectivity for propyl acetate reduction was 6.0%, and the direct selectivity to ethyl propyl ether was 96.8%.

[0074] Inventive Example 7 used a catalyst containing 5 wt. % Pd on a Nb2O5 support. The results of Inventive Example 7 were similar to those of Inventive Example 6 under the same conditions.

[0075] In Example 8 of the present 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. The hydrogenation was carried out under the following reaction conditions: CH4 at 1,595.2 kPa. 10 The reaction was carried out under O2, H2 at 4,976.9 kPa and 503 K. The absolute ethyl propyl ether selectivity reached exactly 13%, and the direct ether selectivity of ethyl propyl ether reached 86.1%.

[0076] In support of Example 8 of the present invention, the selectivities for ether products of the present invention were measured at various ester feed rates as set forth in Table VI. Table VI sets forth the conversion of esters to ether products and the selectivity of the direct selective reduction of esters to the desired ether products in a liquid phase reactor over a catalyst comprising 1% Pd supported on NbO. The selectivities were measured under the following conditions: CH at 1,595.2 kPa. 10Based on a function of O2, H2 at 4,976.9 kPa and conversion at 503 K. The selectivities for ether products of the present invention at various ester feed rates are listed in Table VI. [Table 6] Notes for Table VI: *Amount of ether produced directly from esters relative to the total amount of ethers. In Table VI, for example, "direct ether selectivity" = absolute selectivity of ethyl propyl ether / (ethyl propyl ether + dipropyl ether) * 100.

[0077] In inventive Example 9, catalyst 22 was prepared on a WO support with a Pd loading of 5 wt. The absolute ethyl propyl ether selectivity in inventive Example 9 reached 16.1%, and the absolute dipropyl ether selectivity was 2.3%. The direct ether selectivity relative to ethyl propyl ether reached 87.5%.

[0078] In Comparative Examples S-Z, several supports, including activated carbon, Al2O3, MoO3, CeO2, ZrO2, SiO2, and TiO2, were used in Catalysts 28-35. Catalysts with activated carbon supports exhibited lower ester conversion rates. Catalysts with Al2O3 supports promoted more dehydrated products. None of the supports used in Comparative Examples S-Z exceeded both absolute and direct ethyl propyl ether selectivities compared to the WO3 or Nb2O5 supports used in Examples 6-9 of the present invention.

[0079] In Comparative Examples AA-DD, Ni, Ru, and Pt metal components were tested using Catalysts 36-39. Ni and Pt showed some absolute ethyl propyl ether selectivity, but the dipropyl ether selectivity was higher than desired for Comparative Examples AA-DD.

[0080] Discussion of results Part A: Vapor-phase ester reduction Table IV shows the distribution of ester reduction products at various conversion rates. In the vapor-phase process, the selectivity of dipropyl ether and diethyl ether approaches 0, indicating that substantially all ethyl propyl ether is formed via the direct hydrogenation reaction pathway, since alcohol dehydration does not have a preference for the selectivity of symmetric or asymmetric ethers. Therefore, Example 1 of the present invention suggests that Pd on NbO can catalyze the direct hydrogenation of esters to ethers.

[0081] The results of Inventive Example 3 show that the WO3 support provides direct hydrogenation as well.

[0082] The results of Inventive Example 4 and Inventive Example 5 show that the boehmite support is capable of direct hydrogenation.

[0083] The results of Comparative Examples A-D show that supports such as TiO2, CeO2, ZrO2 and activated carbon used as catalysts in the process of the present invention are not very effective in providing a direct hydrogenation route from esters to ethers.

[0084] The results of Comparative Examples E to I demonstrate that single transition metal catalysts, such as Pt, Ni, Ru, Co, and Rh, used in the process of the present invention, can catalyze the direct hydrogenation of esters to ethers, providing direct ether selectivities of greater than 95%. However, absolute ether selectivities are less than 5% for catalysts containing these metal components. These catalysts require further optimization to achieve high absolute ether selectivities.

[0085] The results of Comparative Examples J-R show that the monometallic transition metals such as Fe, Mn, and Re used as catalysts in the process of the present invention are not very effective in providing a direct hydrogenation route from esters to ethers.

[0086] Part B: Solution-phase ester reduction The results of Examples 6-9 of the present invention show that Pd catalysts on Nb2O5 or WO3 supports can catalyze esters to ethers by direct hydrogenation, as also observed from the results in the gas-phase reduction process above.

[0087] The results of Comparative Examples S-Z show that supports such as activated carbon, Al2O3, MoO3, CeO2, ZrO2, SiO2, and TiO2 used as catalysts in the process of the present invention are not very effective in providing a direct hydrogenation route from esters to ethers.

[0088] The results of Comparative Examples AA-DD show that transition metals such as Ni, Ru and Pt used as catalysts in the process of the present invention are not very effective in providing a direct hydrogenation route from esters to ethers.

[0089] Based on the above results of the gas-phase reduction process and the liquid-phase reduction process, The process of the present invention, which uses a Pd catalyst on a WO3 support, allows the direct conversion of esters to ethers. Catalytic hydrogenation can be advantageously provided. The present application also relates to the following aspects: (1) 1. A process for producing ether products by the direct selective reduction of esters, said process comprising treating (a) at least one ester with (b) hydrogen and (c) in the presence of a heterogeneous catalyst to directly and selectively reduce said at least one ester by hydrogenation to form at least one ether, wherein the absolute ether product selectivity is greater than 5% and the direct ether product selectivity is greater than 85%. (2) The process according to (1) above, wherein the ester is an ester containing a linear or branched alkyl group, a cyclic or acyclic alkyl group, or a mixture thereof. (3) The process according to (1), wherein the heterogeneous catalyst comprises a transition metal. (4) The process according to (3), wherein the transition metal comprises at least one of palladium, platinum, nickel, and mixtures thereof. (5) 2. The process according to claim 1, wherein the heterogeneous catalyst is a transition metal supported on a support element. (6) 6. The process of claim 5, wherein the supported support element comprises at least one of a niobium oxide support, a tungsten oxide support, a boehmite support, and mixtures thereof. (7) The process according to (1) above, wherein the temperature of the process is 350K to 650K and the pressure of the process is 0.1MPa to 10MPa. (8) The process according to (1) above, wherein the conversion of the ester is 1% to 70%, and the absolute selectivity to the ether product is 5% to 20%. (9) The process according to (1) above, wherein the ether is an asymmetric ether. (10) The process according to (1) above, wherein the process is a gas-phase reduction process carried out under gas-phase process conditions. (11) The process according to (1) above, wherein the process is a liquid-phase reduction process carried out under liquid-phase process conditions. (12) A solvent containing the ether described in (1) above.

Claims

1. 1. A process for producing an ether product by the direct selective reduction of an ester, said process comprising: (a) treating at least one ester with (b) hydrogen, and (c) in the presence of a heterogeneous catalyst to directly and selectively reduce said at least one ester by hydrogenation to form at least one ether; the heterogeneous catalyst comprises a transition metal selected from at least one of palladium, platinum, nickel, and mixtures thereof, the transition metal being supported on a support element selected from at least one of a niobium oxide support, a tungsten oxide support, a boehmite support, and mixtures thereof; The process has an absolute ether product selectivity of greater than 5% and a direct ether product selectivity of greater than 85%.

2. 10. The process of claim 1, wherein the ester is an ester containing a straight or branched chain alkyl group, a cyclic or acyclic alkyl group, and mixtures thereof.

3. 2. The process of claim 1, wherein the process temperature is between 350K and 650K and the process pressure is between 0.1 MPa and 10 MPa.

4. 2. The process of claim 1, wherein the conversion of the ester is from 1% to 70% and the absolute selectivity to the ether product is from 5% to 20%.

5. The process of claim 1 , wherein the ether is an asymmetric ether.

6. 10. The process of claim 1, wherein the process is a gas phase reduction process conducted under gas phase process conditions.

7. 10. The process of claim 1, wherein the process is a liquid phase reduction process carried out under liquid phase process conditions.

8. A method for preparing a solvent containing an ether, comprising the process described in claim 1.

9. 10. The process of claim 1, wherein the heterogeneous catalyst comprises a transition metal in a weight percent (wt%) of 0.1 wt% to 20 wt%.

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