A process for producing methacrylic acid

Abstract: A process comprising producing methacrylic acid by vapour phase oxidation of methacrolein with molecular oxygen or molecular oxygen-containing gas using a catalyst-filled, fixed bed shell-and-tube reactor, which process is characterised in that (1) the catalyst in each of the reaction tubes is divided into two or more layers in the axial direction of the tube to provide plural reaction zones and (2) the amount of catalytically active component of the catalyst per unit volume of reaction tube decreases from a gas inlet portion toward a gas outlet portion of the reactor.

Description: [Technical field to which the invention belongs] This invention relates to a process for producing methacrylic acid. More particularly, the invention relates to a process for producing methacrylic acid at high yield, through vapor phase catalytic oxidation of methacrolein. [Conventional technology] A large number of proposals have been made in the past concerning the catalysts useful in preparation of methacrylic acid through oxidation of methacrolein with molecular oxygen or molecular oxygen-containing gas, in the presence of an oxidation catalyst. Demands for improvements in activity, selectivity and life of the catalyst, however, have still long been pending. [Object of the invention] The object of the present invention is to provide a process which enables production of methacrylic acid through vapor phase catalytic oxidation of methacrolein, at high yield and with stability for a prolonged period. [Means to achieve the object] The reaction to produce methacrylic acid by vapor phase catalytic oxidation of methacrolein using fixed bed shell-and-tube reactor is apt to give rise to such undesirable side reactions as parallel reactions or sequential reactions which reduce selectivity for methacrylic acid and in consequence reduce its yield. The side reactions also provide causes for transfer of used catalyst to undesirable oxidation-reduction condition and clogging of pores in the catalyst, leading to shortening of catalyst life. We have made concentrative studies how to effectively inhibit such undesirable side-reactions and now discovered that the inhibition could be achieved to improve methacrylic acid yield, by dividing the catalyst layer in each reaction tube into at least two layers to provide plural reaction zones, and filling the catalyst in the reaction zones in such a manner that the amount of the catalytically active component per unit volume of the reaction tube decreases from the gas inlet portion toward the gas outlet portion; and also that whereby catalyst deterioration can be inhibited to allow stable methacrylic acid production at high yield over prolonged period. Thus, according to the present invention a process for producing methacrylic acid through vapor phase oxidation of methacrolein with molecular oxygen or a molecular oxygen-containing gas using a fixed bed shell-and-tube reactor filled with catalyst is provided, which is characterized in that (1) the catalyst layer in each reaction tube is divided into at least two layers in the axial direction of the tube to provide plural reaction zones, and (2) the catalyst is filled in the plural reaction zones in such a manner that the amount of the catalytically active component per unit volume of the reaction tube decreases from the gas inlet portion toward the gas outlet portion, wherein the catalyst is a complex oxide expressed by the following general formula (1) containing molybdophosphoric acid: Mo a P b A c B d C e O x wherein Mo is molybdenum; P is phosphorus; A is at least an element selected from the group consisting of arsenic, antimony, germanium, bismuth, zirconium, selenium, cerium, copper, iron, chromium, nickel, manganese, cobalt, tin, silver, zinc, palladium, rhodium and tellurium; B is at least an element selected from the group consisting of vanadium, tungsten and niobium; C is at least an element selected from the group consisting of alkali metals, alkaline earth metals and thallium; and O is oxygen; and a, b, c, d, e and x represent atomic ratios of Mo, P, A, B, C and O, respectively, where when a is 12, b is 0.5-4, c is 0.001-5, d is 0.001-4 and e is 0.001-4 and x is a numerical value determined by degree of oxidation of each of the elements and wherein the gas inlet potion contains pelletized catalyst and the gas outlet portion contains ring-fomed catalyst. [Working embodiment of the invention] According to the invention, the catalyst layer in each reaction tube in a fixed bed shell-and-tube reactor to be used in the process is divided into at least two layers to provide plural reaction zones. The more the number of reaction zones, the more effectively prevented are the side reactions, but from industrial standpoint it is economical to provide 2 to 3 reaction zones. The dividing ratio of the reaction zones is variable depending on composition and shape of catalyst to be filled in the reaction zones and cannot be unqualifiedly specified. It can be suitably determined in each individual occasion so as to secure the optimum activity and selectivity as a whole. Preparation methods and starting materials of the catalyst to be used in the present invention are not critical, but any of the methods and starting materials heretofore generally employed for preparation of this type of catalyst may be used. Examples of typical means for filling the catalyst in the reaction zones in order to achieve the reduction in the amount of the catalytically active component per unit volume of the reaction tube from the gas inlet portion toward the gas outlet portion the shape of molded catalyst, is changed so that the gas inlet portion is filled with pelletized catalyst, and the gas outlet portion, with ring-formed catalyst. The molded catalysts are obtained by molding the complex oxide which is expressed by the earlier given general formula (I) and which contains molybdophosphoric acid by any of conventionally practiced methods, such as extrusion molding, tabletting or the like. Operation conditions of the vapor phase catalytic oxidation reaction according to the present invention are not critical, but those generally used may be adopted. For example, the vapor phase oxidation reaction can be carried out by introducing a starting gas containing 1-10 volume % of methacrolein, 3-20 volume % of molecular oxygen, 0-60 volume % of steam and 20-80 volume % of inert gas such as nitrogen, carbon dioxide, etc. into the catalyst layers in each reaction tube, and reacting them under such conditions as at temperature ranging from 250-450°C, pressure of 1-10 atmospheres and space velocity (SV) of 300-5000 hr -1 . [Effect of the invention] According to the process of the present invention, undesirable side reactions can be effectively inhibited and object methacrylic acid can be produced at high yield. The inhibition of side reactions also results in reduced catalyst degradation and extension in catalyst life. Thus, object methacrylic acid can be produced at high yield stably over prolonged period. [Examples] Hereinafter the present invention is explained more specifically, referring to working Examples, in which the conversion, selectivity and one-pass yield were calculated by the following equations. Conversion (mol %) = (mol number of reacted methacrolein / mol number of supplied methacrolein) x 100 Selectivity (mol %) = (mol number of formed methacrylic acid / mol number of reacted methacrolein) x 100 One-pass yield (mol %) = (mol number of formed methacrylic acid / mol number of suppled methacrolein) x 100 Example 1 Into 40 liters of heated ion-exchange water, 8830 g of ammonium paramolybdate and 531 g of ammonium metavanadate were added and dissolved under stirring. Into this aqueous solution 625 g of orthophosphoric acid (85% by weight), an aqueous solution formed by dissolving 812 g of cesium nitrate in 9 liters of ion-exchange water and 243 g of antimony trioxide powder were added by the order stated, followed by concentration under heating and stirring. Thus obtained slurry was dried for 15 hours at 250°C and pulverized. < Preparation of Catalyst (1) > Moisture content of the powder formed by above pulverization was adjusted with water and the composition was molded into pellets of each 5 mm in diameter and 6 mm in length with an extrusion-molding machine. The pellets were dried and calcined in an air stream at 400°C for 3 hours to finish pelletized Catalyst (1). The amount of the active component of Catalyst (1) per unit volume of the reaction tube was about 1.20 g/ml. < Preparation of Catalyst (2) > Moisture content of the powder formed by above pulverization was adjusted with water and the composition was molded into rings of each 5 mm in diameter, 1.5 mm in inner diameter of the trough-hole and 6 mm in length with an extrusion-molding machine. The rings were dried and thereafter calcined in an air stream at 400°C for 3 hours to finish ring-formed Catalyst (2). The amount of the active component of Catalyst (2) per unit volume of the reaction tube was about 1.02 g/ml. The composition of Catalysts (1) and (2) was same as follows in terms of atomic ratios excepting oxygen: Mo 12 P 1.3 V 1.09 Cs 1.0 Sb 0.4 . < Oxidation reaction > The gas inlet portion of a stainless steel reactor of 25.4 mm in inner diameter was filled with 750 ml of Catalyst (1), and the gas outlet portion thereof, with 750 ml of Catalyst (2). Into the reactor which was filled with the catalyst layers as above, a gaseous mixture obtained through vapor phase catalytic oxidation of isobutylene in the presence of a multi-element catalyst formed of Mo-Bi-Co-W-Fe oxides, i.e., a gaseous mixture composed of methacrolein 3.5 vol. % isobutylene 0.04 vol. % methacrylic acid + acetic acid 0.24 vol. % steam 20 vol. % oxygen 9 vol. % and other components 67.22 vol. % was introduced and reacted at a temperature of 290°C and a space velocity of 1200 hr -1 . The result was as shown in Table 1. Comparative Example 1 The reaction was conducted in the same manner as in Example 1, except that 1500 ml of the Catalyst (1) alone, which was obtained in Example 1, was filled in the reactor. The result was as shown in Table 1. Comparative Example 2 The reaction was conducted in the same manner as in Example 1, except that 1500 ml of the Catalyst (2) alone, which was obtained in Example 1, was filled in the reactor. The result was as shown in Table 1. Comparative Example 3 The reaction was conducted in the same manner as in Example 1, except that the filling sites of the Catalysts (1) and (2) were reversed from those in Example 1, i.e., the gas outlet portion of the reactor was filled with 750 ml of Catalyst (1) and the gas inlet portion of the reactor, with 750 ml of Catalyst (2). The result was as shown in Table 1. Example 2 The gas inlet portion of the same reactor to that used in Example 1 was filled with 750 ml of Catalyst (1), and the gas outlet portion, with a mixture of 600 ml of Catalyst (2) with 150 ml of spherical α-alumina of 5 mm in diameter, and the reaction was carried out in the same manner as in Example 1. The result was as shown in Table 1. Example 3 The reaction was carried out in the same manner as in Example 1, except that the gas inlet portion of the reactor was filled with 500 ml of Catalyst (1) and the gas outlet portion, with 1000 ml of Catalyst (2). The result was as shown in Table 1. Example 4 The reaction was carried out in the same manner as in Example 1, except that the gas inlet portion of the reactor was filled with 1000 ml of Catalyst (1) and the gas outlet portion, with 500 ml of Catalyst (2). The result was as shown in Table 1. From the results of the reactions as shown in Table 1, it is understood that high selectivity is achieved according to the process of the present invention. Example 5 The reactions of Example 1 and Comparative Example 1 were each carried out continuously for 4000 hours. The results were as shown in Table 2. From the results of the reactions shown in Table 2, it is understood that catalyst degradation is inhibited according to the process of the present invention. Performance Methacrolein conversion (mol %) Methacrylic acid selectivity (mol %) One-pass yield (mol %) Example 1 82.3 77.6 63.9 Example 2 82.1 77.8 63.9 Comparative Example 1 81.0 74.0 59.9

Claims: A process comprising producing methacrylic acid by vapour phase oxidation of methacrolein with molecular oxygen or molecular oxygen-containing gas using a catalyst-filled, fixed bed shell-and-tube reactor, which process is characterised in that (1) the catalyst in each of the reaction tubes is divided into two or more layers in the axial direction of the tube to provide plural reaction zones and (2) the amount of catalytically active component of the catalyst per unit volume of reaction tube decreases from a gas inlet portion toward a gas outlet portion of the reactor wherein the catalyst is a complex oxide containing molybdophosphoric acid of formula (I): Mo a P b A c B d C e O x where Mo is molybdenum; P is phosphorus; A is one or more element selected from arsenic, antimony, germanium, bismuth, zirconium, selenium, cerium, copper, iron, chromium, nickel, lead, manganese, cobalt, tin, silver, zinc, palladium, rhodium and tellurium; B is one or more element selected from vanadium, tungsten and niobium; C is one or more element selected from alkali metals, alkaline earth metals and thallium; and O is oxygen; a, b, c, d, e and x denote the atomic ratios of Mo, P, A, B, C and O, respectively, such that when a is 12, b is from 0.5 to 4, c is from 0.001 to 5, d is from 0.001 to 4 and e is from 0.001 to 4 and x is a numerical value determined by the degree of oxidation of each of the elements, and wherein the gas inlet portion contains pelletized catalyst and the gas outlet portion contains ring-formed catalyst. A process according to claim 1 comprising (a) filling pelletised catalyst into the gas inlet portion and (b) filling ring formed catalyst into the gas outlet portion wherein the amount of catalytically active component of the catalyst per unit volume of the reaction tube decreases from the gas inlet portion to the gas outlet portion.

Inventors: KIMURA NAOMASA, ONODERA HIDEO, SHIRAISHI EIICHI, KIMURA, NAOMASA, ONODERA, HIDEO, SHIRAISHI, EIICHI
Applicants: NIPPON CATALYTIC CHEM IND, NIPPON SHOKUBAI CO., LTD.
Application year: 2000
Main classification: C07C57
IPC codes: B01J27, C07C57, C07C51, C07B61