Process for producing dialkyl succinates

Abstract

The present invention provides a process for producing dialkyl succinates, comprising hydrogenating dialkyl maleate in a multi-stage process comprising at least two hydrogenation stages, wherein in a first hydrogenation stage, a feed stream comprising dialkyl maleate is exposed to hydrogenation conditions over a catalyst at a temperature T1 to produce a first stream comprising dialkyl succinate and unsaturated compounds, and in a second hydrogenation stage, the first stream is exposed to hydrogenation conditions at a temperature T2 to produce a second stream comprising dialkyl succinate and a lower concentration of unsaturated compounds than the first stream, wherein T2 is less than T1.

Description

[Technical Field]

[0001] The present invention relates to a process for the preparation of dialkyl succinates from dialkyl maleates by multi-step hydrogenation. [Background technology]

[0002] Dimethyl succinate is widely used in a variety of applications, including as a solvent and as a chemical intermediate. Recently, there has been growing interest in its use in the polymerization of dimethyl succinate with 1,4-butanediol to produce polybutylene succinate (PBS), which is gaining attention as a potential biodegradable plastic.

[0003] Various methods for making dimethyl succinate are known, including the esterification of succinic acid with methanol using an acid catalyst such as sulfuric acid, and the esterification of maleic anhydride to form dimethyl maleate, followed by hydrogenation to form dimethyl succinate. The second process benefits from fewer side reactions and a more environmentally friendly profile in terms of by-product generation and overall efficiency. The second process also benefits from its ability to be integrated with 1,4-butanediol production processes, which can also use dimethyl maleate as a starting material. Dimethyl maleate is subjected to hydrogenation and hydrogenolysis, proceeding via dimethyl succinate to produce 1,4-butanediol, along with tetrahydrofuran and γ-butyrolactone, which also have commercial value.

[0004] Processes for the esterification of maleic anhydride to form dimethyl maleate are disclosed, for example, in U.S. Pat. No. 4,795,824 and WO 90 / 08127. Dimethyl maleate can be hydrogenated to form dimethyl succinate by various means, typically using molecular hydrogen and a platinum group metal catalyst, such as palladium supported on carbon. The reaction is highly exothermic, and it can be beneficial to operate the hydrogenation reactor at higher temperatures to recover energy as useful heat that can generate steam for use elsewhere in the plant. Operating the reactor at higher temperatures increases the reaction rate and reduces the catalyst bed size.

[0005] The inventors have found that a drawback of operating at high temperatures using a platinum group metal catalyst, such as palladium on carbon, is that relatively high levels of gamma-butyrolactone can be formed when the reaction is operated at high conversion. The reaction is operated at high conversion to minimize the concentration of unconverted unsaturated compounds, i.e., dimethyl maleate and dimethyl fumarate.

[0006] Therefore, it is not currently feasible to achieve low levels of unsaturated compounds along with γ-butyrolactone while operating at a temperature high enough to generate useful heat that can be used elsewhere in the plant. Furthermore, the inventors have found that dimethyl maleate, dimethyl fumarate, and especially γ-butyrolactone cannot be easily separated from dimethyl succinate, e.g., without high energy input and multiple distillation columns. Both the unsaturated compounds and γ-butyrolactone in the dimethyl succinate product affect the quality of the PBS formed from the polymerization of dimethyl succinate with 1,4-butanediol. Summary of the Invention

[0007] Accordingly, the present invention provides a process for producing dialkyl succinate, the process comprising hydrogenating dialkyl maleate in a multi-stage process comprising at least two hydrogenation stages, In a first hydrogenation stage, a feed stream comprising dialkyl maleate is exposed to hydrogenation conditions over a catalyst at a temperature T1 to produce a first stream comprising dialkyl succinate and unsaturated compounds; In a second hydrogenation stage, the first stream is exposed to hydrogenation conditions at a temperature T2 to produce a second stream comprising dialkyl succinates and a lower concentration of unsaturated compounds than the first stream; A process is provided in which T2 is smaller than T1.

[0008] The unsaturated compounds are dialkyl maleates and optionally dialkyl fumarates, typically dialkyl maleates and dialkyl fumarates. The dialkyl maleates are unconverted from the feed stream. The dialkyl fumarates typically result from the isomerization of dialkyl maleates.

[0009] The inventors have found that the process advantageously allows for the preparation of dialkyl succinates with minimal γ-butyrolactone, resulting in a highly pure dialkyl succinate product after distillative purification. This is possible while maximizing heat recovery from the process, which reduces plant operating costs and environmental impact. By optionally utilizing a base metal catalyst in the second hydrogenation stage, a further advantage can be seen in that the second hydrogenation stage can be made more active at lower temperatures without affecting product purity. Thus, reactor size and catalyst volume can additionally be minimized, which has further advantages in terms of operating costs and environmental impact.

[0010] Also provided is a process for producing polybutylene succinate, comprising producing a dialkyl succinate by the process of the present disclosure and then polymerizing the dialkyl succinate with 1,4-butanediol in a subsequent polymerization stage. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is a schematic diagram of the experimental apparatus used in the examples. [Figure 2] 1 is a schematic diagram of the process of the present invention. [Figure 3] 1 is a chart showing gamma-butyrolactone formation versus overall conversion of unsaturated compounds at various temperatures. DETAILED DESCRIPTION OF THE INVENTION

[0012] The dialkyl maleate may be a C1-C5 dialkyl maleate, such as dimethyl maleate, diethyl maleate, dipropyl maleate, dibutyl maleate, or dipentyl maleate. Preferably, the dialkyl maleate is dimethyl maleate or diethyl maleate, more preferably dimethyl maleate. Thus, alkyl as used herein may be methyl, ethyl, propyl, butyl, or pentyl, preferably methyl or ethyl, more preferably methyl.

[0013] The feed stream for the first hydrogenation stage containing dialkyl maleates can be obtained by esterification of maleic anhydride with the corresponding alkyl alcohol. Preferably, the esterification is a two-stage esterification in which maleic anhydride or maleic acid is esterified in the first stage to produce monoalkyl maleates, and the monoalkyl maleates are esterified in the second stage to produce dialkyl maleates. The second stage can be a reactive distillation using an acidic resin catalyst, preferably a sulfonic acid-based ion exchange resin. The feed stream containing dialkyl maleates preferably contains at least about 90% by weight, preferably at least about 95% by weight, of dialkyl maleates. The feed stream containing dialkyl maleates preferably contains less than about 1% by weight of monoalkyl maleates, preferably less than about 0.5% by weight of monoalkyl maleates. The feed stream containing dialkyl maleates preferably contains less than about 5% by weight of alkyl alcohol. Suitable processes for the esterification of maleic anhydride to form dialkyl maleates are disclosed, for example, in US Pat. No. 4,795,824 and WO 90 / 08127, which are incorporated herein by reference.

[0014] Suitably, the feed stream containing dialkyl maleates is treated to reduce the level of sulfur to less than about 0.2 ppmw, preferably by passing the stream through a guard bed. The feed stream containing dialkyl maleates may be treated to neutralize any monoalkyl maleates present, preferably by treatment with a base selected from NaOH, Na2CO3, NaHCO3, or an alkylamine.

[0015] Typically, the first hydrogenation stage is carried out in a first hydrogenation reactor, and the second hydrogenation stage is carried out in a second hydrogenation reactor. As will be apparent to those skilled in the art, such reactors are operated in series. However, the two stages can be carried out in a single reactor, for example, having two or more zones. Preferably, there are two hydrogenation stages. The first and second hydrogenation stages can typically be liquid-phase or mixed vapor / liquid-phase reaction stages. In other words, conditions are such that a liquid or mixed liquid / vapor phase is maintained. This can be achieved by controlling one or more conditions, such as the feed ratio of the hydrogen to dialkyl maleate-containing feed stream, pressure, and temperature. The first hydrogenation stage can be carried out in a trickle-bed reactor, but the type of reactor is not particularly limited, and any reactor suitable for the desired reaction can be used.

[0016] A hydrogen source, typically hydrogen gas, is added to the first hydrogenation stage. The hydrogen gas is preferably supplied to the stage in molar excess relative to the dialkyl maleate. The hydrogen partial pressure in the first hydrogenation stage is typically in the range of, inclusive, from about 5 to about 150 barg, preferably from about 20 to about 100 barg, e.g., about 60 barg.

[0017] In the first hydrogenation stage, dialkyl maleate is hydrogenated over a catalyst to produce dialkyl succinate, along with unreacted dialkyl maleate and, optionally, its isomeric dialkyl fumarate, and, optionally, γ-butyrolactone. T1 is preferably at least about 140°C, and preferably at least about 150°C. At lower temperatures, the reaction becomes less practical to carry out and recover useful heat. T1 is preferably not higher than about 220°C, and preferably not higher than about 200°C. At higher temperatures, the production of undesirable by-products may become too high, which may lead to thermal runaway. Running the first hydrogenation stage at such a temperature T1 means that useful heat can be extracted, e.g., low-pressure steam can be generated and used as a heat transfer fluid elsewhere in the plant, eliminating the need for additional steam generation means in the plant and thus saving energy in the plant. Preferably, the catalyst in the first hydrogenation stage is in a catalyst bed, and temperature T1 is the temperature at the inlet of the catalyst bed. Those skilled in the art will understand how to control the temperature T1, including, for example, by controlling the amount and temperature of liquid recirculation. The inventors have found that running the first hydrogenation stage at a temperature T1 results in the formation of γ-butyrolactone, and the amount of γ-butyrolactone produced at a particular temperature T1 depends on the dialkyl maleate conversion. The lower the T1, the higher the dialkyl maleate conversion can be before γ-butyrolactone becomes impractical. The higher the T1, the lower the dialkyl maleate conversion can be before γ-butyrolactone becomes impractical. As will be understood by those skilled in the art, conversion can be controlled by factors such as catalyst loading (i.e., the amount of catalyst per unit of fresh feed), temperature, pressure, and the rate of liquid recirculation. For example, the first hydrogenation stage can be operated so that the dialkyl maleate conversion is 99.95% or less. Typically, the first hydrogenation stage is operated so that the dialkyl maleate conversion is at least about 25%, preferably at least about 50%, and more preferably at least about 75%.

[0018] The amount of unsaturated compounds, i.e., total dialkyl maleate and dialkyl fumarate, will depend on the dialkyl maleate conversion carried out in the first hydrogenation stage. The first stream typically contains less than about 1000 ppmw, preferably less than about 250 ppmw, and more preferably less than about 100 ppmw of gamma-butyrolactone. The first stream may be substantially free of gamma-butyrolactone. This, of course, may require a relatively low dialkyl maleate conversion in the first hydrogenation stage.

[0019] The catalyst in the first hydrogenation stage comprises an active metal for hydrogenating dialkyl maleate to dialkyl succinate. The active metal may comprise a platinum group metal, particularly palladium, platinum, rhodium, or ruthenium. Preferably, the catalyst comprises a support, i.e., the catalyst is a supported catalyst. Suitably, the support comprises alumina, silica, zirconia, zinc oxide, chromate, carbon, or a mixture thereof. The liquid hourly space velocity in the first hydrogenation stage is typically up to about 50 h , based on the dialkyl maleate in the feed stream to the first hydrogenation stage. -1 Those skilled in the art can determine the liquid hourly space velocity required for a particular system to achieve the desired conversion of the dialkyl maleate.

[0020] The first hydrogenation stage typically includes a liquid recycle in which the dialkyl maleate feed is diluted with dialkyl succinate from the first stream. The liquid recycle is typically operated to maintain the molar ratio of dialkyl maleate to dialkyl succinate fed to the hydrogenation stage in the range of 10:1 to 100:1, preferably 20:1 to 30:1. Such liquid recycle can help regulate the temperature in the first hydrogenation stage. Typically, the liquid recycle will proceed through a heat exchanger to reduce the temperature, and optionally a pump. This heat exchanger can be used to provide heat to another stream in the process or the plant in which the process is operated, or to generate steam, for example, to generate low-pressure steam that can be used as a heat transfer fluid elsewhere in the plant, eliminating the need for additional steam generation means in the plant and thus saving energy in the plant.

[0021] In the second hydrogenation stage, which can be considered a finishing stage, the unconverted dialkyl maleate in the first stream, together with the dialkyl fumarate, is hydrogenated over a catalyst to provide a dialkyl succinate. T2 is suitably at least about 40°C, preferably at least about 50°C. T2 is suitably less than about 140°C, preferably less than about 120°C. Optionally, the first stream can be cooled to a temperature T2 in a cooling stage, for example, by using cooling water or by heat exchange with a stream from another part of the process or the plant in which the process is operated. Advantageously, operating at a temperature T2 results in the completion of the hydrogenation of the unsaturated compounds to dialkyl succinate in the second stage without increasing the amount of γ-butyrolactone relative to the first stream. As will be apparent to those skilled in the art, 1,4-butanediol, which is produced via hydrogenolysis of γ-butyrolactone, is typically not produced at detectable levels in the second hydrogenation stage. As such, the process maximizes the yield of dialkyl succinate. Thus, the impurity levels in the second stream are low enough to allow effective purification by distillation to provide very high purity dialkyl succinate. Thus, T2 can be defined as the temperature at which no γ-butyrolactone is produced under hydrogenation conditions in the second hydrogenation stage.

[0022] The second stream, i.e., the product stream from the second hydrogenation stage, typically contains less than about 1000 ppmw, preferably less than about 250 ppmw, and more preferably less than about 100 ppmw, of unsaturated compounds, i.e., total dialkyl maleates and dialkyl fumarates. The second stream typically contains less than about 1000 ppmw, preferably less than about 250 ppmw, and more preferably less than about 100 ppmw of γ-butyrolactone. The amount of other minor impurities in the second stream that are not unsaturated compounds or γ-butyrolactone, such as alkyl alcohols, monoalkyl succinates, and dialkyl-2-alkoxy succinates (derived from the reaction of alkyl alcohols with unsaturated compounds), will depend substantially on the composition of the feed to the first hydrogenation zone, including the dialkyl maleates. Such impurities pass through the hydrogenation stage but can be easily removed by distillation. The second stream is primarily a dialkyl succinate-rich stream, for example, containing greater than about 80% by weight dialkyl succinate.

[0023] The second stage is suitably operated so that the conversion of the unsaturated compounds in the first stream is maximized, i.e., greater than about 99%, preferably greater than 99.99%. As will be appreciated by those skilled in the art, conversion can be controlled by factors such as catalyst loading (i.e., the amount of catalyst per unit of fresh feed), temperature, pressure, and liquid recirculation rate.

[0024] A hydrogen source, typically hydrogen gas, is added to the second hydrogenation stage. The hydrogen gas is preferably supplied to the stage in molar excess relative to the dialkyl maleate. The hydrogen partial pressure in the second hydrogenation stage is typically in the range of about 5 to about 150 barg, preferably about 20 to 100 barg, inclusive, for example, about 60 barg.

[0025] The catalyst in the second hydrogenation stage contains an active metal for hydrogenating dialkyl maleate to dialkyl succinate. The active metal may be a platinum group metal, particularly palladium, platinum, rhodium, or ruthenium, or a base metal. Advantageously, the active metal comprises a base metal, preferably nickel or copper, more preferably nickel. In this case, an additional advantage is seen in that the second hydrogenation stage can be made more active at a lower temperature without affecting the purity of the product. Therefore, it is additionally possible to minimize the reactor size and catalyst volume, which has further advantages in terms of operating costs and environmental impact. Preferably, the catalyst contains a support, i.e., the catalyst is a supported catalyst. Suitably, the support comprises alumina, silica, zirconia, zinc oxide, chromate, carbon, or a mixture thereof. Those skilled in the art can determine the liquid hourly space velocity required for a particular system to achieve maximum conversion of unsaturated compounds.

[0026] The second hydrogenation stage may include a liquid recycle in which the first stream is diluted with the dialkyl succinate from the second stream. Typically, the liquid recycle will proceed through a pump and then a heat exchanger, utilizing a cooling medium such as air or water, to reduce the temperature.

[0027] The second stream is typically fed to a purification zone, where dialkyl succinate can be advantageously provided at very high purity, particularly with very low levels of γ-butyrolactone. The purification zone suitably comprises a distillation column operated to produce a dialkyl succinate stream containing greater than about 99.5 wt.% dialkyl succinate. Those skilled in the art can determine what packing to use and, for example, how many trays are required. Furthermore, those skilled in the art can determine the operating conditions required, for example, with respect to pressure, temperature, and residence time. The high purity dialkyl succinate that can be produced does not originate from the specific nature of the purification zone, but rather from the low concentration of γ-butyrolactone in the feed to the purification zone, which is an advantageous effect of the present invention, since it is impractical to separate γ-butyrolactone by distilling the product due to the low relative volatility of γ-butyrolactone. Preferably, the distillation column is operated to produce an overhead stream comprising alkyl alcohols, i.e., alkyl alcohols that may be present in the initial feed stream to the first hydrogenation zone, and a bottoms stream comprising heavies.

[0028] The dialkyl succinate produced by this process can be used for any purpose, particularly for producing polybutylene succinate. Accordingly, a process for producing polybutylene succinate is also provided herein, which comprises producing a dialkyl succinate by the process disclosed herein and then polymerizing the dialkyl succinate with 1,4-butanediol in a subsequent polymerization step. Preferably, the 1,4-butanediol is produced from dialkyl maleate, i.e., by hydrogenation and hydrogenolysis. Advantageously, the dialkyl succinate, 1,4-butanediol, and polybutylene succinate are produced in the same plant.

[0029] The invention will now be described, by way of example, with reference to the accompanying drawings. It will be understood by those skilled in the art that the drawings are schematic and that in a commercial plant additional items of equipment may be required, such as reflux drums, pumps, vacuum pumps, compressors, gas recycle compressors, temperature sensors, pressure relief valves, control valves, flow controllers, level controllers, etc. The provision of such ancillary equipment does not form part of the present invention and is in accordance with conventional chemical engineering practice.

[0030] In Figure 2, dialkyl maleate feed 1 from the maleic anhydride esterification stage (not shown) is fed to guard bed 3 to reduce the sulfur level to less than about 0.2 ppmw. Feed stream 5 from the guard bed is then fed to a first hydrogenation stage comprising hydrogenation reactor 7. The feed contains less than 0.5 wt.% monoalkyl maleate, less than 5 wt.% alkyl alcohol (carried over from the esterification reaction), and less than 2 wt.% dialkyl fumarate. Hydrogenation reactor 7 is a trickle-bed reactor containing a catalyst bed comprising a palladium-on-carbon hydrogenation catalyst. Hydrogen gas is fed to reactor 7 via line 9 in molar excess relative to the dialkyl maleate. Reactor 7 is operated such that the temperature at the inlet to the catalyst bed is T1 and the dialkyl maleate conversion is such that acceptable amounts of unsaturated compounds and gamma-butyrolactone are present, e.g., less than 100 ppmw unsaturated compounds and less than 100 ppmw gamma-butyrolactone. The reactor provides product stream 11, which primarily contains the desired reduction product dialkyl succinate, along with unsaturated compounds and gamma-butyrolactone, as well as any impurities present in feed stream 5, such as alkyl alcohol. The first hydrogenation stage includes liquid recycle 17, in which dialkyl maleate feed 5 is diluted with dialkyl succinate from product stream 11 from reactor 7. The liquid recycle is operated to maintain a molar ratio of dialkyl maleate to dialkyl succinate in the feed of approximately 20:1. The liquid recycle serves to regulate the temperature in reactor 7. Additionally, because reactor 7 is operated at temperature T1, useful heat can be extracted via heat exchanger 25 and used to generate steam for use elsewhere in the plant. Product stream 11 from reactor 7 is used as a feed stream for hydrogenation reactor 13 in the second hydrogenation stage. Hydrogenation reactor 13 is a trickle-bed reactor containing a catalyst bed containing a catalyst composed of nickel on an alumina-based support. Hydrogen gas is fed in molar excess to the dialkyl maleate via line 15 to reactor 13. Reactor 13 is operated so that the temperature at the inlet of the catalyst bed is T2. The conversion of unsaturated compounds is greater than 99.99% in this reactor.Thus, reactor 13 can be considered a finishing reactor that reduces unsaturated compounds and does not increase the concentration of gamma-butyrolactone. Product stream 17 from reactor 13 contains primarily dialkyl succinate and contains less than 100 ppmw of unsaturated compounds and less than 100 ppmw of gamma-butyrolactone. The second hydrogenation stage optionally includes a liquid recycle 19 in which dialkyl maleate feed 11 is diluted with dialkyl succinate from crude product stream 17 from reactor 13. The liquid recycle serves to regulate the temperature in reactor 13. The recycle is typically cooled with water in heat exchanger 23. Product stream 17 is fed to purification zone 21, which produces a purified dialkyl succinate product stream 27 containing greater than 99.5 wt.% dialkyl succinate. Purification zone 21 includes at least one distillation column operated according to common sense. The extremely high purity of the dialkyl succinate in product stream 27 is only possible due to the low content of γ-butyrolactone in crude product stream 17. [Example]

[0031] A trickle-bed liquid recirculating pilot plant unit shown in Figure 1 was charged with 1.5 wt% Pd / C hydrogenation catalyst (50 mL).

[0032] In FIG. 1, 2 is the hydrogen feed, 4 is the nitrogen feed, 6 is the dimethyl maleate feed, 8 is the feed pump, 10 is a heat exchanger to remove heat from the liquid recycle 12, 14 is a trickle bed reactor containing the required catalyst, 16 is a gear pump, 18 is the crude product stream, 20 is the high pressure liquid recycle vessel, 22 is the crude product stream, 24 is the high pressure product letdown vessel, 26 is the exit gas, and 28 is the degassed crude dimethyl succinate product.

[0033] The catalyst was activated by heating under a continuous flow of hydrogen prior to the introduction of the dimethyl succinate feed via a reciprocating pump. Once dimethyl succinate product was observed, liquid recycle was initiated (20:1 wt:wt) and the feed was changed to a process stream containing primarily dimethyl maleate (Table 1).

[0034] [Table 1]

[0035] Feed and product samples were analyzed by GC-FID to determine the concentrations of unsaturated compounds (dimethyl maleate + dimethyl fumarate) and γ-butyrolactone in the crude product. Tests were conducted at constant temperature (140°C inlet), pressure (880 psig), and recirculation (20:1). The liquid feed rate, and hence the liquid hourly space velocity (LHSV), feed rate (mL h -1 ) / catalyst volume (mL) was adjusted as necessary.

[0036] Table 2 shows the results from a run at a reactor temperature of 140 °C, where the conversion was calculated as a function of the catalyst deactivation factor or LHSV (feed rate (mL h)). -1 ) / change in catalyst volume (ml).

[0037] [Table 2]

[0038] Effect of higher temperature (215°C inlet) on γ-butyrolactone formation Experiment 1 A trickle-bed liquid recirculating pilot plant unit (Figure 1) was packed with Pd / C catalyst.

[0039] The catalyst was activated by heating under a continuous flow of hydrogen prior to the introduction of the dimethyl succinate feed via a reciprocating pump. Once product was observed, liquid recirculation was initiated (20:1 wt:wt) and the feed was changed to the process stream listed in Table 1.

[0040] The unit was operated under the conditions shown in Table 3.

[0041] [Table 3]

[0042] The unit was operated under these conditions for 220 hours, during which time the total conversion stabilized at about 99.37 wt % with the following levels of major components: dimethyl maleate 4390 ppmwt, dimethyl fumarate 1536 ppmwt, and gamma-butyrolactone 4161 ppmwt.

[0043] Experiment 2 Run 1 was repeated with an aged catalyst which reduced the conversion. Upon stabilization, this resulted in a total conversion of 97.93 wt % with the following major component levels: dimethyl maleate 13768 ppmw, dimethyl fumarate 5669 ppmwt, and γ-butyrolactone 550 ppmwt.

[0044] Experiment 3 Run 1 was repeated with a more aged catalyst that had undergone a high temperature regeneration step. Upon stabilization, this resulted in a total conversion of 99.28 wt % with the following levels of major components: dimethyl maleate 5117 ppmwt, dimethyl fumarate 1694 ppmwt, and γ-butyrolactone 1674 ppmwt.

[0045] Demonstration of low γ-butyrolactone formation / high conversion when operated at low temperature - Ni catalyst A trickle-bed reactor as described in Example 1 was loaded with Ni / Al2O3 / SiO2 catalyst (62.5 mL).

[0046] The catalyst was activated by heating under a continuous flow of hydrogen prior to the introduction of the dimethyl succinate feed via a reciprocating pump. Once product was observed, liquid recirculation was initiated (20:1 wt:wt) and the feed was changed to that shown in Table 1.

[0047] The liquid recirculation unit was operated under the conditions shown in Table 4 using the same feed as shown in Table 1.

[0048] [Table 4]

[0049] The unit was operated under these conditions for 49 hours, and dimethyl maleate stabilized at 41 ppmwt in the crude product. No dimethyl fumarate or gamma-butyrolactone was observed.

[0050] FIG. 3 shows a comparison of γ-butyrolactone formation versus total conversion of unsaturated compounds at 75° C. (Ni catalyst), 140° C., and 215° C. (Pd catalyst).

[0051] Second Hydrogenation Stage - Finishing Reactor Demonstration The unit was operated using Ni on Al2O3 / SiO2 catalyst under the conditions shown in Table 5. The feed for this run was crude product from a previous run, as shown in Table 6, and the reactor was operated as a second hydrogenation stage in accordance with the present invention.

[0052] [Table 5]

[0053] [Table 6]

[0054] The unit was operated under these conditions for 218 hours, during which time the dimethyl maleate in the crude product stabilized at 15-22 ppmwt (>99.99 wt% conversion) and there was no evidence of dimethyl fumarate or gamma-butyrolactone in the crude product. Thus, unsaturated compounds were converted and gamma-butyrolactone was not made.

[0055] These data demonstrate that when the hydrogenation is operated at temperatures above 140°C and at high conversions of dialkyl maleate, γ-butyrolactone production increases to levels that can cause problems for downstream purification (e.g., as shown in Figure 3). Thus, when operating at temperatures that facilitate the recovery of energy as useful heat, γ-butyrolactone production is suboptimal. Therefore, the hydrogenation must be run at lower conversions, e.g., less than 99.9%. While this is not optimal in itself, the problem can be ameliorated by using a second hydrogenation stage according to the present invention. Such a second hydrogenation stage acts as a finishing stage, allowing for conversion of >99.99 wt.% of residual unsaturated compounds without additional γ-butyrolactone production. In the specific case of nickel catalysts, finishing hydrogenation can be made more active at lower temperatures without affecting product purity.

Claims

1. 1. A process for producing dialkyl succinate, said process comprising hydrogenating dialkyl maleate in a multi-stage process comprising at least two hydrogenation stages; In the first hydrogenation stage, the feed stream containing the dialkyl maleate is heated to a temperature T １ to hydrogenation conditions over a catalyst at a temperature of 1000° C. to produce a first stream comprising dialkyl succinate and unsaturated compounds; In the second hydrogenation stage, the first stream is heated to a temperature T ２ to produce a second stream comprising dialkyl succinate and a lower concentration of said unsaturated compounds than said first stream; T ２ But, T １ Smaller than,process.

2. T １ 2. The process of claim 1, wherein the temperature is at least about 140°C.

3. T ２ 3. The process of claim 1 or 2, wherein the temperature is less than about 140°C.

4. 4. The process of any one of claims 1 to 3, wherein in the first hydrogenation stage, the feed stream is exposed to hydrogenation conditions over a platinum group metal catalyst, optionally a supported catalyst.

5. 5. The process of any one of claims 1 to 4, wherein in the second hydrogenation stage, the first stream is exposed to hydrogenation conditions over a platinum group metal catalyst, optionally a supported catalyst.

6. 5. The process of any one of claims 1 to 4, wherein in the second hydrogenation stage, the first stream is exposed to hydrogenation conditions over a base metal catalyst, optionally a supported catalyst.

7. 7. The process of claim 6, wherein the base metal is nickel.

8. 8. The process of any one of claims 1 to 7, wherein the first hydrogenation stage comprises a liquid recycle in which a dialkyl maleate feed is diluted with dialkyl succinate from the first stream.

9. 9. The process of claim 8, wherein the liquid recycle is operated to maintain the molar ratio of dialkyl maleate to dialkyl succinate fed to the hydrogenation stage in the range of from 10:1 to 100:1, preferably from 20:1 to 30:

1.

10. 10. The process of any one of claims 1 to 9, wherein the first stream comprises less than about 1000 ppmw, preferably less than about 250 ppmw, more preferably less than about 100 ppmw gamma-butyrolactone.

11. The process of any one of claims 1 to 10, wherein the second stream comprises less than about 1000 ppmw, preferably less than about 250 ppmw, more preferably less than about 100 ppmw of the unsaturated compounds.

12. The process of any one of claims 1 to 11, wherein the second stream comprises less than about 1000 ppmw, preferably less than about 250 ppmw, more preferably less than about 100 ppmw gamma-butyrolactone.

13. The process of any one of claims 1 to 12, wherein the second stream is fed to a purification zone.

14. 14. The process of claim 13, wherein the purification zone comprises a distillation column operated to produce a dialkyl succinate stream comprising greater than about 99.5 wt.% dialkyl succinate.

15. 15. The process of any one of claims 1 to 14, wherein the feed stream comprising dialkyl maleate is obtained by esterification of maleic anhydride with the corresponding alkyl alcohol.

16. 16. The process of claim 15, wherein the dialkyl maleate-containing feed stream contains less than about 1 wt. % monoalkyl maleate, preferably less than about 0.5 wt. % monoalkyl maleate.

17. 17. The process of claim 15 or 16, wherein the dialkyl maleate-containing feed stream contains up to about 5 wt.% alkyl alcohol.

18. 18. A process for producing polybutylene succinate, comprising producing a dialkyl succinate by the process of any one of claims 1 to 17, and then polymerizing the dialkyl succinate with 1,4-butanediol in a subsequent polymerization stage.

19. 19. The process of claim 18, wherein the 1,4-butanediol is produced from a dialkyl maleate.

20. 20. The process of claim 18 or 19, wherein the dialkyl succinate, 1,4-butanediol, and polybutylene succinate are produced in the same plant.

21. The process of any one of claims 1 to 20, wherein the alkyl is methyl.

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