FR2884817A1 - Preparing acrylic acid comprises oxydehydration reaction of glycerol in the presence of molecular oxygen - Google Patents

Abstract

Preparing acrylic acid comprises oxydehydration reaction of glycerol in the presence of molecular oxygen.

Description

The present invention relates to a new route of synthesis of the acid

  acrylic, and more particularly relates to the preparation of acrylic acid from glycerol in the presence of molecular oxygen.

  The main conventional acrylic acid synthesis process utilizes a catalytic reaction of propylene with an oxygen-containing mixture. This reaction is generally carried out in the vapor phase, and most often in two stages: the first step carries out the substantially quantitative oxidation of propylene to a mixture rich in acrolein, in which the acrylic acid is a minority, the second stage carries out the selective oxidation of acrolein to acrylic acid.

  The reaction conditions of these two steps, carried out in two reactors in series, are different and require catalysts adapted to the reaction. It is not necessary to isolate acrolein in this two-step process. The reaction can also be conducted in a single reactor, but in this case it is necessary to separate and recycle large amounts of acrolein to the oxidation step.

  It has long been known that acrolein can be obtained from glycerol. Glycerol (also called glycerine) is derived from the methanolysis of vegetable oils at the same time as the methyl esters which are used in particular as fuels or fuels in diesel and heating oil. It is a natural product that has a "green" aura, is available in large quantities and can be stored and transported without difficulty. Many studies are devoted to the valorisation of glycerol according to its degree of purity, and the dehydration of glycerol to acrolein is one of the ways envisaged.

  The reaction involved in obtaining acrolein from glycerol is: CH2OH-CHOH-CH2OH a CH2 = CH-CHO + 2H2 O Generally the hydration reaction is favored at low temperatures, and dehydration is favored at high temperatures. high. To obtain acrolein, it is therefore necessary to implement a sufficient temperature, and / or a partial vacuum to move the reaction. The reaction can be carried out in the liquid phase or in the gas phase. This type of reaction is known to be catalyzed by acids. Various methods for synthesizing acrolein from glycerol are described in the literature: FR 695931; US 2, 558.520; WO 99/05085; US 5,387,720.

  The dehydration reaction of glycerol to acrolein is generally accompanied by side reactions resulting in the formation of byproducts such as hydroxypropanone, propanaldehyde, acetaldehyde, acetone, adducts of acrolein on glycerol, polycondensation products of glycerol, cyclic glycerol ethers ..., but none of the documents of the prior art suggests the formation of acrylic acid from glycerol.

  Surprisingly, the Applicant Company has discovered that it is possible to obtain acrylic acid directly from glycerol, when carrying out the dehydration reaction of glycerol in the presence of oxygen. The presence of oxygen makes it possible, in addition to the dehydration reaction, to carry out an oxidation reaction, offering the possibility of obtaining in a single step the formation of acrylic acid from glycerol. The supply of oxygen in the reaction medium also has a beneficial effect to reduce the formation of coke on the catalyst used and inhibit its deactivation, and limit the formation of by-products such as propanaldehyde, acetone and sodium hydroxide. 'hydroxypropanone.

  A process for producing acrylic acid directly from glycerol is particularly advantageous since it makes it possible to synthesize in a single reactor and thus to reduce the necessary investments.

  The principle of the process according to the invention is based on the 2 consecutive reactions of dehydration and oxidation: ## STR1 ## in which CH 2 OH -CHOH-CH 2 OH CH 2 = CHCHO + 2H 2 O CH 2 = CH-CHO + this reaction scheme, called oxidation hydration, the exothermicity of the oxidation reaction is combined with the endothermicity of the dehydration reaction, which contributes to a better thermal equilibrium of the process.

  Another advantage of this method lies in the fact that it is not dependent on a raw material of fossil origin, such as propylene, but that it uses a renewable raw material; such a process therefore meets the criteria associated with the new concept of "green chemistry", within a broader framework of sustainable development.

  The subject of the present invention is therefore a process for the manufacture of acrylic acid in one step by the reaction of oxidation of glycerol in the presence of molecular oxygen.

  The molecular oxygen may be present as air or as a mixture of gases containing molecular oxygen. The amount of oxygen is chosen to be outside the flammability range at any point in the installation. The oxygen content in the process according to the invention will generally be chosen so as not to exceed 20% relative to the reaction gas mixture (glycerol / H2O / oxygen / inert gas mixture).

  The dehydration reaction of glycerol to acrolein is generally carried out on acidic solid catalysts. These catalysts may be chosen from natural or synthetic siliceous materials or acidic zeolites; inorganic carriers, such as oxides, coated with inorganic acids, mono, di, tri or polyacids; oxides or mixed oxides or heteropolyacids. The preferred catalysts are sulphated zirconias, zirconium phosphates, tungsten zirconias, silicified zirconias, sulphated titanium or tin oxides, phosphated aluminas or silicas. These acidic catalysts, suitable for the glycerol dehydration reaction, facilitate the desorption of the acrylic acid produced, but are not optimized for oxidation reactions; the yield of acrylic acid is then limited with this type of catalyst.

  It is therefore preferable, in order to obtain a good selectivity for acrylic acid, that the solid used is also capable of catalyzing the oxidation reaction. As solids containing at least one element selected from the list Mo, V, W, Re, Cr, Mn, Fe, Co, Ni, Cu, Zn, Sn, Te, Sb, Bi, Pt will be chosen as oxidation catalyst. , Pd, Ru, Rh, present in the metal form or in the form of oxide, sulphate or phosphate. It is possible to envisage using a single catalyst capable of catalyzing both the dehydration reaction and the oxidation reaction, but also of having a mixture of catalysts, each being capable of carrying out one of the two reactions optimally. This catalyst mixture is then either an intimate mixture of catalysts or is in the form of two catalyst layers stacked in the reactor, the first layer being then capable of dehydrating the glycerol to acrolein and the second being more adapted to the oxidation of acrolein thus produced in acrylic acid. One or other of these configurations will be adapted according to the reactor technology used.

  The reaction according to the invention can be carried out in the gas phase or in the liquid phase, and preferably in the gas phase. When the reaction is carried out in the gas phase, various process technologies can be used, namely fixed bed process, fluidized bed process or circulating fluidized bed process. In the first two processes, in a fixed bed or in a fluidized bed, the regeneration of the catalyst can be separated from the reaction. It can be ex situ, for example by extraction of the catalyst and combustion in air or with a gaseous mixture containing molecular oxygen.

  In this case, the temperature and pressure at which the regeneration is effected need not be the same as those at which the reaction is carried out. According to the process of the invention, it can be carried out continuously in situ, at the same time as the reaction taking into account the presence of molecular oxygen or a gas containing molecular oxygen in the reactor. In this case, the regeneration is similar to an inhibition of the deactivation and is done at the temperature and pressure of the reaction.

  In the circulating fluidized bed process, the catalyst circulates in two capacities, a reactor and a regenerator. Since the dehydration reaction is endothermic and the oxidation reaction and the regeneration of the coke combustion catalyst are exothermic, the two systems compensate each other, which contributes to a good thermal equilibrium of the process.

  The selection of the optimal process is made according to different criteria. The fixed bed process has the advantage of simplicity. The fluidized bed processes continuously discharge the used catalyst and continuously recharge the fresh catalyst without stopping production with the possibility of being isothermal. The advantage of the circulating fluidized bed method is that it makes it possible to optimize the selectivity of the reaction by continuously returning a freshly regenerated catalyst to the reactor, while offsetting the energy exchange between the reactor and the regenerator.

  The separation / purification steps must be suitable for the production of acrylic acid, acrylic acid having a higher boiling point than water and acrolein. The co-produced acrolein may be either isolated or recycled to increase yields of acrylic acid.

  The experimental conditions of the reaction in the gas phase are preferably a temperature of between 250 ° C. and 350 ° C. and a pressure of between 1 and 5 bars. To avoid consecutive reactions and the formation of undesirable products, it is important to limit the residence time in the reactor; Moreover, by increasing the residence time, it is also possible to have stronger conversions. In particular, it is desirable to increase the contact time (residence time) of the reactants in the vicinity of the catalyst to compensate for a decrease in the conversion rate when a lower reaction temperature is used. When a weakly oxidizing catalyst is used, an increase in temperature makes it possible to improve the yield of acrylic acid.

  Glycerol is available in concentrated form, but also in the form of more economical aqueous solutions. Advantageously, an aqueous solution of glycerol with a concentration of between 10% and 50% by weight in the reactor is used, preferably between 15% and 30%. The concentration should not be too high to avoid side reactions such as the formation of glycerol ethers or reactions between acrolein or acrylic acid produced and glycerol. In addition, the glycerol solution should not be diluted too much because of the energy cost induced by the evaporation of the aqueous glycerol solution. In any case, the concentration of the glycerol solution can be adjusted by recycling the water produced by the reaction. In order to reduce the costs of transporting and storing glycerol, the reactor can be supplied with a concentrated solution of 40 to 100% by weight, the dilution at the optimum content being carried out by recycling part of the water vapor produced by the reaction and dilution water. Similarly, the heat recovery at the outlet of the reactor can also make it possible to vaporize the glycerol solution feeding the reactor.

  Glycerol derived from the methanolysis of vegetable oils in a basic medium may contain certain impurities such as sodium chloride or sulfate, non-glycerinous organic materials, and methanol. The presence of sodium salts is particularly detrimental to the catalytic dehydration reaction because these salts are capable of poisoning the acidic sites. Pre-treatment of glycerol by ion exchange may be considered.

  Compared with the conventional process for preparing acrylic acid by selective oxidation of propylene, the acrylic acid produced according to the process of the invention may contain impurities of different nature or in different amounts. Depending on the use envisaged, it may be envisaged to purify the acrylic acid according to the techniques known to those skilled in the art.

  The following Examples illustrate the present invention without however limiting its scope.

Examples

  In the examples, a tubular reactor consisting of a tube of length 85 cm and inner diameter of 6 mm is used to effect the dehydration reaction of glycerol in the gas phase at atmospheric pressure. This reactor is placed in a heated enclosure maintained at the chosen reaction temperature. The catalysts used are crushed and / or pelletized to obtain particles of 0.5 to 1.0 mm. 10 ml of catalyst are loaded into the reactor to form a catalyst bed 35 cm long. This is brought to the reaction temperature for 5 to 10 minutes before the introduction of the reagents. The reactor is fed with a 20% by weight aqueous solution of glycerol with an average feed rate of 12 ml / h, and with a flow rate of 0.8 l / h (13.7 ml / min) of oxygen. molecular for the examples according to the invention, unless otherwise indicated. In this case, the relative proportion O 2 / vaporized glycerol / water vapor is 6 / 4.5 / 89.5. The aqueous glycerol solution is vaporized in the heated chamber and then passes over the catalyst. The calculated contact time is of the order of 2.9 s.

  After reaction, the products are condensed in a trap refrigerated with crushed ice.

  Effluent samples are taken periodically. For each sample, the flow is interrupted and a slight flow of nitrogen is sent into the reactor to purge it. The trap at the outlet of the reactor is then replaced, the nitrogen flow is stopped and the reactor is returned to a stream of reagent.

  For each experiment, the total mass of input and output products is measured, which makes it possible to perform a mass balance. Similarly, the products formed are analyzed by chromatography. Two types of analysis are carried out: - a column chromatography analysis (column FFAP 2m * 1/8 ") on a Carlo Erba chromatograph, equipped with a TCD detector, the quantitative analysis is carried out with an external standard ( 2-butanone) - analysis by capillary column chromatography (FFAP column 50 m * 0.25 mm) on an HP6890 chromatograph equipped with an FID detector with the same samples stored at -15 C.

  The first method is particularly suitable for having a rapid analysis of the products, and in particular the yield of acrolein. The second method is used to have a more precise analysis of some byproducts of the reaction. On the other hand, GC-MS or silylation chromatography analyzes were performed to confirm these results.

  The products thus quantified are unreacted glycerol, acrylic acid formed and acrolein.

  In the examples which follow, the glycerol conversion and the yields are defined as follows: Conversion of glycerol (%) = 100 number of moles of glycerol remaining / number of moles of glycerol introduced.

  Yield of acrylic acid (%) = number of moles of acrylic acid produced / number of moles of glycerol introduced.

  Yield of acrolein (%) =. number of moles of acrolein produced / number of moles of glycerol introduced.

  All the results are expressed in molar percentages with respect to the incoming glycerol.

Example

  The catalyst used (10 ml): (1) a tungstated zirconia (90.7% ZrO 2 9.3% WO 3) from Daiichi Kigenso (supplier reference H1417), ie 17 g (2) a sulphated zirconia (90% ZrO 2) -10% SO4) of Daiichi Kigenso (supplier reference H1416), ie 16.5 g (3) a phosphated zirconia (91.5% ZrO2-8.5% PO4) of Daiichi Kigenso (Ref H1418), ie 12, The reaction is carried out in the absence of oxygen or in the presence of oxygen. The reaction temperature is 300 ° C. or 350 ° C. The results are shown in the table below.

  Catalyst 1 1 1 2 2 3 3 3 3 Oxygen (ml / min) 0 13.7 13.7 0 13.7 0 5.6 13, 7 13.7 Temperature of 300 300 350 300 300 300 300 300 350 reaction ( C) Glycerol mass 32 32 42 42 25 25 25 42 50 Cumulative feed (g) Conversion of 100 100 100 100 100 100 100 glycerol (%) Yield in 0 4.5 7.6 - 0.9 0, 3 * 0.7 1.1 0.7 acrylic acid (%) Yield 72.1 53 19.2 40.6 52.5 46.7 43 38 14.4 acrolein (%) -: Not determined / *: Attribution uncertain (presence of multiple nearby peaks)

Claims (8)

  1- A process for producing acrylic acid in one step by reacting glycerol with oxidation of the presence of molecular oxygen.   2. Process according to claim 1, characterized in that the molecular oxygen is in the form of air or in the form of a mixture of gases containing molecular oxygen.   3- A method according to claim 1 or 2, characterized in that the glycerol is in the form of an aqueous solution of concentration of between 10% and 50%, preferably between 15% and 30% by weight in the reactor.   4- Method according to one of claims 1 to 3, characterized in that the reaction is carried out in the presence of a single catalyst capable of catalyzing both the dehydration reaction and the oxidation reaction, or in the presence of a mixture of catalysts, each of which is capable of carrying out one of these two reactions.   5 - Process according to claim 4, characterized in that the single catalyst or the oxidation catalyst is a solid containing at least one element selected from the list Mo, V, W, Re, Cr, Mn, Fe, Co, Ni , Cu, Zn, Sn, Te, Sb, Bi, Pt, Pd, Ru, Rh, present in the metal form or in the form of oxide, sulphate or phosphate.   6. Process according to one of claims 1 to 5, characterized in that the reaction is carried out in the gas phase.   7- Process according to claim 6, characterized in that the reaction is carried out in a fixed bed reactor, fluidized bed or circulating fluidized bed.   8- Process according to claim 6 or 7, characterized in that the reaction is carried out at a temperature between 250 C and 350 C.