Method for producing dialkyl succinate

Abstract

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

Description

Technical Field

[0001] The present invention relates to a process for preparing dialkyl succinates from dialkyl maleates by multi-stage hydrogenation. Background Art

[0002] Dimethyl succinate is widely used in various applications, including solvents, and as a chemical intermediate. Recently, there has been growing interest in its use in the production of polybutylene succinate (PBS) by polymerization with 1,4-butanediol. PBS has emerged as a leading biodegradable plastic.

[0003] A variety of methods for preparing dimethyl succinate are known, including esterifying succinic acid with methanol using an acid catalyst such as sulfuric acid, and esterifying maleic anhydride to form dimethyl maleate, followed by hydrogenation to form dimethyl succinate. The second method benefits from fewer side reactions and is more environmentally friendly in terms of by-product generation and overall efficiency. The second method also benefits from the ability to be integrated with a 1,4-butanediol production process, which can also use dimethyl maleate as a starting material. Dimethyl maleate is exposed to hydrogenation and hydrogenolysis, and proceeds via dimethyl succinate to produce 1,4-butanediol, as well as tetrahydrofuran and γ-butyrolactone (which are also commercially valuable).

[0004] The method for esterifying maleic anhydride to form dimethyl maleate is disclosed in for example US 4,795,824 and WO90 / 08127. Dimethyl maleate can be hydrogenated to form dimethyl succinate by various means, usually using molecular hydrogen and platinum group metal catalyst (for example, palladium supported on carbon). Reaction is highly exothermic, and operating hydrogenation reactor at higher temperatures to recycle energy as useful heat may be beneficial, which allows to generate steam for use in other places on equipment. Operating reactor at high temperature increases reaction kinetics, thereby reduces catalyst bed size.

[0005] The present inventors have discovered that a disadvantage 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 conversions. The reaction will be operated at high conversions to minimize the concentration of unconverted unsaturated compounds, i.e., dimethyl maleate and dimethyl fumarate.

[0006] Therefore, it is currently impossible to achieve low levels of unsaturated compounds and gamma-butyrolactone while also operating at sufficiently high temperatures to generate useful heat that can be used elsewhere in the plant. Furthermore, the inventors have discovered that dimethyl maleate, dimethyl fumarate, and especially gamma-butyrolactone cannot be easily separated from dimethyl succinate, for example, without high energy input and multiple distillation columns. Both the unsaturated compounds and gamma-butyrolactone in the dimethyl succinate product affect the quality of the PBS formed by the polymerization of dimethyl succinate with 1,4-butanediol. Summary of the Invention

[0007] The present invention therefore provides a process for producing dialkyl succinate, which process comprises hydrogenating a dialkyl maleate in a multistage process comprising at least two hydrogenation stages, wherein:

[0008] In a first hydrogenation stage, a feed stream comprising the 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

[0009] exposing the first stream to hydrogenation conditions at a temperature T2 in a second hydrogenation stage to produce a second stream comprising dialkyl succinate and a lower concentration of the unsaturated compound than the first stream;

[0010] Among them, T2 is smaller than T1.

[0011] The unsaturated compounds are dialkyl maleates and optional dialkyl fumarates, typically dialkyl maleates and dialkyl fumarates. Dialkyl maleates are not converted from the feed stream. Dialkyl fumarates are typically produced by the isomerization of dialkyl maleates.

[0012] The present inventors have advantageously discovered that this process enables the production of dialkyl succinates with minimal γ-butyrolactone, resulting in a very high-purity dialkyl succinate product after purification by distillation. This is possible while also maximizing the heat recovered from the process, which reduces equipment operating costs and environmental impact. Further benefits can be seen by optionally utilizing a base metal catalyst in the second hydrogenation stage, as the second hydrogenation stage can be made more active at lower temperatures without affecting product purity. Consequently, reactor size and catalyst volume can also be minimized, which has further benefits in terms of operating costs and environmental impact.

[0013] 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 DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 is a schematic diagram of the experimental apparatus used in the examples.

[0015] Figure 2 Schematic diagram of the method of the present invention.

[0016] Figure 3 is a graph showing the relationship between γ-butyrolactone formation and the total conversion of unsaturated compounds at different temperatures. DETAILED DESCRIPTION

[0017] The dialkyl maleate can be a C1 to 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. Therefore, the alkyl group used herein can be methyl, ethyl, propyl, butyl or pentyl, preferably methyl or ethyl, more preferably methyl.

[0018] The feed stream comprising dialkyl maleate for the first hydrogenation stage can be obtained by the esterification of maleic anhydride and corresponding alkyl alcohol. Suitably, esterification is a two-stage esterification, in which maleic anhydride or maleic acid are esterified to produce monoalkyl maleate in the first stage, and monoalkyl maleate is esterified to produce dialkyl maleate in the second stage. The second stage can be a reactive distillation using an acidic resin catalyst, preferably a sulfonic acid ion exchange resin. The feed stream comprising dialkyl maleate preferably comprises at least about 90% by weight of dialkyl maleate, preferably at least about 95% by weight. The feed stream comprising dialkyl maleate preferably comprises less than about 1% by weight of monoalkyl maleate, preferably less than about 0.5% by weight of monoalkyl maleate. The feed stream comprising dialkyl maleate preferably comprises less than or equal to about 5% by weight of alkyl alcohol. Suitable methods for esterifying maleic anhydride to form dialkyl maleates are disclosed, for example, in US 4795824 and WO 90 / 08127, which are incorporated herein by reference.

[0019] Suitably, the feed stream comprising dialkyl maleates is treated to reduce the sulfur level to less than about 0.2 ppmw, preferably by passing the stream through a guard bed. The feed stream comprising dialkyl maleates may be treated to neutralize any monoalkyl maleates present, preferably by base treatment with a stream selected from NaOH, Na2CO3, NaHCO3 or an alkylamine.

[0020] Typically, the first hydrogenation stage is carried out in the first hydrogenation reactor, and the second hydrogenation stage is carried out in the second hydrogenation reactor. As will be apparent to the technician, such reactors are operated in series. However, these two stages can be carried out in a single reactor (for example, with two or more zones). Preferably, there are two hydrogenation stages. The first hydrogenation stage and the second hydrogenation stage can generally be a liquid or mixed gas / liquid phase reaction stage. In other words, the conditions are such that a liquid or mixed liquid / vapor stage is maintained. This can be achieved, for example, by controlling one or more conditions (such as the feed ratio, pressure and temperature of hydrogen and the feed stream comprising dialkyl maleate). 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.

[0021] A source of hydrogen (typically hydrogen) is added to the first hydrogenation stage. Hydrogen is preferably fed to this stage in a molar excess relative to the dialkyl maleate. The hydrogen partial pressure in the first hydrogenation stage is typically within the range of about 5 barg to about 150 barg, inclusive, preferably about 20 barg to about 100 barg, for example about 60 barg.

[0022] In the first hydrogenation stage, dialkyl maleate is hydrogenated over a catalyst to provide dialkyl succinate, along with unreacted dialkyl maleate and, optionally, its isomer, dialkyl fumarate, and, optionally, gamma-butyrolactone. T1 is suitably at least about 140°C, preferably at least about 150°C. At lower temperatures, operating the reaction and recovering useful heat becomes less practical. T1 is suitably no more than about 220°C, preferably no more than about 200°C. At higher temperatures, the production of unwanted by-products may become too high, which may lead to thermal runaway. Operating the first hydrogenation stage at such a temperature T1 means that useful heat can be extracted, for example, by generating low-pressure steam and using it as a heat transfer fluid elsewhere on the apparatus, thereby eliminating the need for additional steam generation equipment on the apparatus and thus saving energy on the apparatus. Suitably, 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. A skilled person will understand how to control temperature T1, including, for example, by controlling the amount and temperature of liquid recirculation. The present inventors have found that operating the first hydrogenation stage at temperature T1 leads to the formation of gamma-butyrolactone, wherein the amount of gamma-butyrolactone prepared at a certain temperature T1 depends on the dialkyl maleate conversion. The lower T1 is, the higher the dialkyl maleate conversion may be, and then gamma-butyrolactone becomes impractical. The higher T1 is, the lower the dialkyl maleate conversion may be, and then gamma-butyrolactone becomes impractical. As will be understood by the technician, the conversion can be controlled by factors such as catalyst loading (i.e. the amount of catalyst per unit fresh feed), temperature, pressure and liquid recirculation rate. For example, the first hydrogenation stage can be operated so that the dialkyl maleate conversion is 99.95% or lower. 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%.

[0023] The amount of unsaturated compounds (i.e., total dialkyl maleate and dialkyl fumarate) will depend on the conversion of dialkyl maleate operated 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 conversion of dialkyl maleate in the first hydrogenation stage.

[0024] 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 carrier, i.e., the catalyst is a supported catalyst. Suitably, the carrier comprises aluminum oxide, silicon dioxide, zirconium oxide, zinc oxide, chromate, carbon or a mixture thereof. Based on the dialkyl maleate in the feed stream of the first hydrogenation stage, the liquid hourly space velocity in the first hydrogenation stage is typically at most about 50 h -1 The skilled artisan can determine the liquid hourly space velocity required for a particular system to achieve the desired conversion of dialkyl maleate.

[0025] The first hydrogenation stage generally comprises liquid recirculation, in which the dialkyl maleate feed is diluted with the dialkyl succinate from the first stream. Usually liquid recirculation is operated so that the mol ratio of the dialkyl maleate and the dialkyl succinate fed into the hydrogenation stage is maintained in the range of 10:1 to 100:1, preferably 20:1 to 30:1. This liquid recirculation can help regulate the temperature in the first hydrogenation stage. Usually, liquid recirculation will be carried out to reduce the temperature via a heat exchanger, and optionally carried out via a pump. This heat exchanger can be used for providing heat to another stream in the equipment in which the method or the method are operated, or produces steam, and for example low-pressure steam can be produced and used as a heat transfer fluid elsewhere on the equipment, thereby eliminating the need for the add-on device generating steam on the equipment, and therefore saving energy on the equipment.

[0026] In the second hydrogenation stage, which can be considered a refining stage, the unconverted dialkyl maleate and any dialkyl fumarate in the first stream are hydrogenated over a catalyst to provide 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 not more than about 120°C. If desired, the first stream can be cooled to a temperature T2 in a cooling stage, for example, using cooling water or by heat exchange with a stream from another part of the process or the equipment in which the process is operated. Advantageously, operating at temperature T2 results in the second stage completing the hydrogenation of unsaturated compounds to dialkyl succinate 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 produced by hydrogenolysis of γ-butyrolactone is generally not produced at detectable levels in the second hydrogenation stage. Therefore, this process maximizes the yield of dialkyl succinate. Thus, the impurity levels in the second stream are low enough to allow efficient purification by distillation, thereby providing dialkyl succinate of very high purity.Thus, T2 can be defined as the temperature at which no gamma-butyrolactone is produced under the hydrogenation conditions of the second hydrogenation stage.

[0027] 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 gamma-butyrolactone. Other minor impurities in the second stream that are not unsaturated compounds or gamma-butyrolactone (such as alkyl alcohols, monoalkyl succinates, and dialkyl-2-alkoxy succinates (derived from the reaction of alkyl alcohols with unsaturated compounds)) will essentially depend on the composition of the feed to the first hydrogenation zone containing dialkyl maleates. These impurities will pass through the hydrogenation stage but can be easily removed by distillation. The second stream is a stream rich in dialkyl succinate, which primarily contains dialkyl succinate, for example, greater than about 80 weight %.

[0028] The second stage is suitably operated so as to maximize the conversion of the unsaturated compounds in the first stream, i.e., such conversion is greater than about 99%, preferably greater than 99.99%. As will be appreciated by the skilled artisan, the 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 recycle rate.

[0029] A source of hydrogen (typically hydrogen) is added to the second hydrogenation stage. Hydrogen is preferably fed to this stage in an excess molar amount relative to the dialkyl maleate. The hydrogen partial pressure in the second hydrogenation stage is typically within the range of about 5 barg to about 150 barg, inclusive, preferably about 20 barg to about 100 barg, for example about 60 barg.

[0030] The catalyst in the second 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 or a base metal. Advantageously, the active metal comprises a base metal, preferably nickel or copper, more preferably nickel. In this case, further benefits can be seen because the second hydrogenation stage can be made more active at lower temperatures without affecting product purity. Therefore, reactor size and catalyst volume can be minimized in addition, which has further benefits in terms of operating cost and environmental impact. Preferably, the catalyst comprises a carrier, i.e., the catalyst is a supported catalyst. Suitably, the carrier comprises aluminum oxide, silicon dioxide, zirconium oxide, zinc oxide, chromate, carbon or a mixture thereof. Technicians can determine that a particular system achieves the required liquid hourly space velocity for the maximum conversion of unsaturated compounds.

[0031] The second hydrogenation stage may comprise a liquid recycle in which the first stream is diluted with dialkyl succinate from the second stream. Typically, the liquid recycle will be via a pump and then via a heat exchanger, for example using a cooling medium such as air or water to reduce the temperature.

[0032] The second stream is typically fed to a refining zone where dialkyl succinate can be advantageously provided in very high purity, particularly with very low levels of gamma-butyrolactone. The refining zone suitably comprises a distillation column that is operated to produce a dialkyl succinate stream comprising greater than about 99.5% by weight of dialkyl succinate. A skilled person can determine what packing to use and, for example, how many trays are needed. In addition, a skilled person can determine the required operating conditions based on, for example, pressure, temperature, and residence time. The high purity of the producible dialkyl succinate is not due to the details of the refining zone, but rather to the low concentration of gamma-butyrolactone in the feed to the refining zone, a beneficial effect of the present invention, because due to the relatively low volatility of gamma-butyrolactone, it is impractical to separate gamma-butyrolactone by distilling the product. Suitably, the distillation column is operated to produce an overhead stream comprising alkyl alcohol (i.e., alkyl alcohol) and a bottoms stream comprising heavies, which overhead stream may be present in the initial feed stream to the first hydrogenation zone.

[0033] The dialkyl succinate produced by the method of the present invention can be used in any manner, in particular for the production of polybutylene succinate. Therefore, a method for producing polybutylene succinate is provided herein, comprising producing a dialkyl succinate by the method disclosed herein, and then polymerizing the dialkyl succinate with 1,4-butanediol in a subsequent polymerization stage. Suitably, 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 on the same equipment.

[0034] The present invention will now be described by way of example with reference to the accompanying drawings. Those skilled in the art will appreciate that the drawings are illustrative and that other equipment items may be required in a commercial plant, such as a reflux drum, pumps, vacuum pumps, compressors, gas recirculation compressors, temperature sensors, pressure relief valves, control valves, flow controllers, and level controllers. The provision of such ancillary equipment items does not form part of the present invention and is consistent with conventional chemical engineering practice.

[0035] exist Figure 2In the embodiment of the present invention, the feed 1 of dialkyl maleate from maleic anhydride esterification stage (not shown) is supplied to guard bed 3, so that sulfur level is reduced to less than about 0.2ppmw. Then, the feed stream 5 from guard bed is supplied to the first hydrogenation stage including hydrogenation reactor 7. The feed stream comprises less than 0.5% by weight of monoalkyl maleate, less than 5% by weight of alkyl alcohol (carried into by esterification reaction) and less than 2% by weight of dialkyl fumarate. Hydrogenation reactor 7 is a trickle bed reactor accommodating a catalyst bed, which contains a carbon-supported palladium hydrogenation catalyst. Hydrogen is fed to reactor 7 via line 9 in an amount in excess relative to dialkyl maleate moles. Operation reactor 7 is such that the temperature at the inlet of catalyst bed is T1, and dialkyl maleate conversion is such that an acceptable amount of unsaturated compounds and gamma-butyrolactone is present, for example, less than 100ppmw of unsaturated compounds and less than 100ppmw of gamma-butyrolactone. The reactor provides a product stream 11, which contains primarily the desired reduction product (dialkyl succinate) as well as unsaturated compounds and gamma-butyrolactone and any impurities, such as alkyl alcohols present in the feed stream 5. The first hydrogenation stage includes a liquid recycle 17 in which the dialkyl maleate feed 5 is diluted with dialkyl succinate from the product stream 11 of the reactor 7. The liquid recycle is operated so that the molar ratio of dialkyl maleate to dialkyl succinate in the feed is maintained at approximately 20:1. The liquid recycle helps regulate the temperature in the reactor 7. In addition, because the reactor 7 is operated at a temperature T1, useful heat can be extracted via a heat exchanger 25 and used to generate steam for use elsewhere on the plant. The product stream 11 from the reactor 7 is used as the feed stream for the hydrogenation reactor 13 in the second hydrogenation stage. The hydrogenation reactor 13 is a trickle bed reactor containing a catalyst bed containing a catalyst composed of nickel on an alumina-based support. Hydrogen is fed to the reactor 13 via line 15 in an amount in excess relative to the molar amount of dialkyl maleate. The reactor 13 is operated so that the temperature at the inlet of the catalyst bed is T2. In this reactor, the conversion of unsaturated compounds is greater than 99.99%. Therefore, the reactor 13 can be considered as a finishing reactor that reduces unsaturated compounds and does not increase the concentration of gamma-butyrolactone. The product stream 17 from the reactor 13 mainly comprises dialkyl succinate and 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 the dialkyl maleate feed 11 is diluted with the dialkyl succinate of the crude product stream 17 from the reactor 13. The liquid recycle helps to regulate the temperature in the reactor 13. The water cooling recycle in the heat exchanger 23 is usually used.Product stream 17 is fed to refining zone 21, which produces a refined dialkyl succinate product stream 27 containing greater than 99.5% by weight of dialkyl succinate. Refining zone 21 contains at least one distillation column, which is operated using common knowledge. The extremely high purity of the dialkyl succinate in product stream 27 is entirely possible due to the low content of gamma-butyrolactone in crude product stream 17.

[0036] Example

[0037] 1.5 wt% Pd / C hydrogenation catalyst (50 mL) was charged into Figure 1 A trickle bed liquid recirculation pilot plant unit is shown.

[0038] exist Figure 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 that takes heat from the liquid recycle 12, 14 is a trickle bed reactor containing the desired catalyst, 16 is a gear pump, 18 is the crude product stream, 20 is a high pressure liquid recycle vessel, 22 is the crude product stream, 24 is a high pressure product discharge vessel, 26 is the outlet gas, and 28 is the degassed crude dimethyl succinate product.

[0039] The catalyst was activated by heating under a continuous flow of hydrogen before the dimethyl succinate feed was introduced via a reciprocating pump. Once dimethyl succinate product was observed, liquid recycle (20:1 (wt:wt)) was started and the feed was changed to a process stream containing primarily dimethyl maleate (Table 1).

[0040] Components Concentration, weight % Methanol 4.40 γ-Butyrolactone 0.00 Dimethyl maleate 92.22 Dimethyl fumarate 1.84 Dimethyl 2-methoxysuccinate 0.79 Monomethyl maleate 0.44 water 0.10 other 0.21

[0041] Table 1 - Process feed composition

[0042] 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. The test was performed at a constant temperature (140°C inlet), pressure (880 psig) and recirculation (20:1). The liquid feed rate and thus the liquid hourly space velocity (LHSV, calculated as the feed rate (mL h -1 ) / catalyst volume (mL)).

[0043] Table 2 shows the results for operation at a reactor temperature of 140°C, where conversion is the catalyst deactivation or LHSV change (measured as feed rate (mL h -1 ) / catalyst volume (ml)).

[0044]

[0045]

[0046] Table 2 - Test data at 140°C reactor inlet temperature

[0047] DMM = dimethyl maleate, DMF = dimethyl fumarate

[0048] Effect of higher temperature (215℃ inlet) on the formation of γ-butyrolactone

[0049] Experiment 1

[0050] The C-supported Pd catalyst was loaded into a trickle bed liquid recirculation pilot plant unit ( Figure 1 ).

[0051] The catalyst was activated by heating under a continuous flow of hydrogen before the dimethyl succinate feed was introduced via a reciprocating pump. Once product was observed, liquid recirculation (20:1 (wt:wt)) was started and the feed was changed to the process stream as described in Table 1.

[0052] The unit was operated under the conditions shown in Table 3.

[0053] parameter Target Reactor inlet temperature, °C 215 Reactor pressure, psig 880 <![CDATA[LHSV,h -1 ]]> 3.0 Recycle ratio, weight:weight 20:1

[0054] Table 3 - High temperature operating conditions

[0055] Under these conditions, the unit was operated for 220 hours, during which time the overall conversion stabilized at approximately 99.37% by weight with the following levels of key components: dimethyl maleate 4390 ppmwt, dimethyl fumarate 1536 ppmwt and gamma-butyrolactone 4161 ppmwt.

[0056] Experiment 2

[0057] Experiment 1 was repeated but using an aged catalyst to reduce conversion. Once stabilized, this provided an overall conversion of 97.93 wt% with the following key components at the following levels: dimethyl maleate 13768 ppmw, dimethyl fumarate 5669 ppmwt and gamma-butyrolactone 550 ppmwt.

[0058] Experiment 3

[0059] Experiment 1 was repeated using a further aged catalyst that had undergone a high temperature regeneration step. Once stabilized, this provided an overall conversion of 99.28 wt % with the following key components: dimethyl maleate 5117 ppmw, dimethyl fumarate 1694 ppmwt, and gamma-butyrolactone 1674 ppmwt.

[0060] Demonstration of low γ-butyrolactone formation / high conversion when operating at low temperatures - Ni catalyst

[0061] Al2O3 / SiO2 supported Ni catalyst (62.5 mL) was loaded into the trickle bed reactor as described in Example 1.

[0062] 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 (20:1 (wt:wt)) was started and the feed was changed to the feed stream shown in Table 1.

[0063] The liquid recycle unit was operated under the conditions shown in Table 4 using the same feed shown in Table 1.

[0064] parameter Target Reactor inlet temperature, °C 75 Reactor pressure, psig 880 <![CDATA[LHSV,h -1 ]]> 3.5 Recycle ratio, weight:weight 20:1

[0065] Table 4 - Low temperature operating conditions

[0066] Under these conditions, the unit was operated for 49 hours with dimethyl maleate stabilizing at 41 ppmwt in the crude product. No dimethyl fumarate or gamma-butyrolactone was observed.

[0067] Figure 3 Shown is a comparison of gamma-butyrolactone formation and overall conversion of unsaturated compounds at 75°C (Ni catalyst), 140°C, and 215°C (Pd catalyst).

[0068] Demonstration of the second hydrogenation stage - finishing reactor

[0069] The unit was operated using an Al2O3 / SiO2 supported Ni catalyst under the conditions shown in Table 5. The feed for this run was the crude product from the previous run (as shown in Table 6) such that the reactor was operating as a second hydrogenation stage according to the present invention.

[0070] parameter Target Reactor inlet temperature, °C 85 Reactor pressure, psig 880 <![CDATA[LHSV,h -1 ]]> 3.5 Recycle ratio, weight:weight 20:1

[0071] Table 5 - Second Reactor Operating Conditions

[0072] Components Concentration, weight % Methanol 5.982 γ-Butyrolactone 0.000 Dimethyl maleate 0.366 Dimethyl fumarate 0.013 Dimethyl succinate 89.594 Dimethyl 2-methoxysuccinate 0.835 Monomethyl succinate 0.400 water 0.540 other 2.70

[0073] Table 6 - Feed to finishing reactor

[0074] Under these conditions, the unit was operated for 218 hours, during which time the dimethyl maleate in the crude product stabilized at 15 to 22 ppmwt (>99.99% conversion by weight), with no evidence of any dimethyl fumarate or gamma-butyrolactone in the crude product. Thus, the unsaturated compounds were converted, and no gamma-butyrolactone was produced.

[0075] These data indicate that at temperatures of 140°C and above and when operating the hydrogenation at high conversions of dialkyl maleate, gamma-butyrolactone increases to levels that can be problematic for downstream refining (e.g., Figure 3 (as shown). Therefore, when operating at these temperatures to promote energy recovery as useful heat, the production of gamma-butyrolactone is suboptimal. Consequently, the hydrogenation must be run at lower conversions (e.g., less than 99.9% conversion). This is not optimal per se, but the problem can be solved by using a second hydrogenation stage according to the present invention. This second hydrogenation stage acts as a finishing stage and can achieve conversions of >99.99% by weight of the remaining unsaturated compounds without producing additional gamma-butyrolactone. In the specific case of nickel catalysts, the finishing hydrogenation can be made more active at lower temperatures without affecting product purity.

Claims

1. A method for producing dialkyl succinate, comprising: Dialkyl maleates are hydrogenated in a multistage process comprising at least two hydrogenation stages, wherein: In a first hydrogenation stage, a feed stream comprising said 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 exposing the first stream to hydrogenation conditions at a temperature T2 in a second hydrogenation stage to produce a second stream comprising dialkyl succinate and a lower concentration of the unsaturated compounds than in the first stream; Among them, T2 is smaller than T1.

2. The method of claim 1, wherein T1 is at least about 140°C.

3. The method of claim 1 or claim 2, wherein T2 is less than about 140°C.

4. A process according to any preceding claim, 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. The process according to any preceding claim, 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. The process according to 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. A method according to claim 6, wherein the base metal is nickel.

8. The process according to any preceding claim, wherein the first hydrogenation stage comprises a liquid recycle in which the dialkyl maleate feed is diluted with dialkyl succinate from the first stream.

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

1.

10. The process of any preceding claim, wherein the first stream comprises less than about 1000 ppmw, preferably less than about 250 ppmw, more preferably less than about 100 ppmw of gamma-butyrolactone.

11. The process of any preceding claim, 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 preceding claim, 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. A process according to any preceding claim, wherein the second stream is fed to a refining zone.

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

15. The process according to any preceding claim, wherein the feed stream comprising dialkyl maleate is obtained by esterification of maleic anhydride with the corresponding alkyl alcohol.

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

17. The process of claim 15 or claim 16, wherein the feed stream comprising dialkyl maleate comprises less than or equal to about 5 weight percent alkyl alcohol.

18. A method for producing polybutylene succinate, the method comprising: A dialkyl succinate is produced by a process according to any preceding claim and then polymerised with 1,4-butanediol in a subsequent polymerisation stage.

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

20. A process according to claim 18 or claim 19 wherein the dialkyl succinate, 1,4-butanediol and polybutylene succinate are produced on the same equipment.

21. A method according to any preceding claim, wherein the alkyl group is methyl.

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