Process for the catalytic trans-halogenation of a polyiodo-benzene and, in particular, of a di-iodo-benzene

Abstract: The invention concerns a process for the catalytic trans-halo­ genation of a poly-iodo-benzene, wherein said polyiodo-benzene is reacted with benzene and with oxygen, in the presence of a zeolite of the PENTASIL type, exchanged with a metal of the groups II,III or IV of the periodic system, and anyway present in a form different from the acidic form.

Description: EP-A-181790 and 183579, in the applicant's name, disclose a oxy-iodination of benzene whereby mono-iodo-benzene and small amounts of di-iodo-benzene are formed; while the mono-iodo derivative can be advantageously used on an industrial scale, e.g., for the manufacture of phenol, the di-iodo-benzenes have not yet found a sufficiently wide field of application. It has now surprisingly been found that particular and suitable operative conditions allow the di-iodo-benzenes to be easily trans-iodinated to mono-iodo-benzene, in the presence of oxygen and of benzene, according to the equation In its broadest aspect, the present invention concerns a process for the catalytic trans-iodination of a poly-iodo-benzene, and in particular of a di-iodo-benzene, characterized in that said poly-iodo-benzene is reacted in the gas phase with benzene and oxygen - or an oxygen-containing gas - in the presence of a PENTASIL® zeolite which is at least partially exchanged with a metal of the groups II, III or IV of the periodic system and is at any rate present in a form different from the acidic form, i.e., from the H form, the benzene to poly-iodo-benzene molar ratio ranging from 100:1 to 1:1 and said zeolite being optionally admixed with an inert binder. Best results were obtained by using zeolites of the ZSM5 or ZSM11 type, as well as zeolites of the ZSM12 type (mentioned in EP-A-166,513) and zeolites of the MB28 type (mentioned in EP-A-21,445, in the applicant's name), exchanged with alkaline-earth metals, and in particular with calcium. The above process can be very advantageously carried out parallel to an oxidative iodination of benzene, as disclosed in the above European patent applications, wherein, however, undesired amounts of di-iodo- (and poly-iodo-) benzenes are formed; i.e., at the same time, and inside the same reaction zone, higher yields of mono-iodo-benzene can be obtained (as compared to the simple synthesis from C₆H₆ and iodine), because the undesired poly-iodo-benzene by-products can be recycled to the oxy-iodination reaction, thus providing an additional source of iodine. According to a preferred embodiment of the present invention, the space velocity is from 0.1 to 100 kg/hour of (poly-iodo-benzene + benzene) mixture per kg of pure zeolite (any binder excluded) and the di-iodo-benzenes (para-, ortho-, or meta-di-iodo-benzene, or their mixtures) are fed to the reactor in the form of a solution in benzene. One can use either a zeolite in the mono-metal form (e.g., in the calcium form) or a zeolite exchanged with two or more different cations. The present catalyst can be prepared from the acidic form of the zeolite by first partially exchanging the proton with the cation of one of the desired metals (using the solution of a water-soluble salt thereof) and subsequently neutralizing all of the residual acidic sites with the diluted solution of the hydroxide of a metal different from the preceding one; by using this latter technique, a completely exchanged catalyst is obtained, and all of the Brönsted's acidic sites responsible for the decay of the catalytic activity are eliminated. The PENTASIL zeolites, exchanged, before being used, with at least one divalent or trivalent cation (preferably selected from Zn⁺⁺ cation and the cations of the alkaline-earth metals) can be used as such, or admixed with a suitable amount of an inert agent, e.g., SiO₂, which acts as a binder and/or carrier. When a thus exchanged zeolite is used, a stabilization is observed; when, on the contrary, the same PENTASIL zeolites are used in their acidic form, or in their alkaline form, a decay is observed in their activity. In the preparation of the exchanged PENTASIL zeolites one can start either from their sodium form or from their acidic form. In the first case, the sodium is exchanged with the desired cation, by using known techniques. In the second case, the preparation is carried out starting from the acidic form. Although the exchange technique may be used, the neutralization technique is more convenient. In such a case there is used an aqueous solution of a salt of the cation which yields, by hydrolysis, a basic pH (or, still better, there is used a diluted aqueous solution of the hydroxide of the metal cation). This latter method gives the certainty that a zeolite without Brönsted's acidic sites is obtained. The catalytic system may also consist of zeolites exchanged with two or more cations. The results of the invention are very surprising. In fact, the same PENTASIL zeolites, when in their acidic form or exchanged with cations different from the ones according to the invention, promote a reaction in the opposite direction, i.e. the conversion of mono-iodo-benzene into poly-iodo-benzenes. The trans-iodination can be carried out in many different ways which, however, are all within the scope of the present invention. According to a very advantageous embodiment, the reaction temperature is from 350 to 450°C; the benzene/poly-iodo-benzene molar ratio is from from 20:1 to 1:1; the poly-iodo-benzene:O₂ molar ratio is from 10:1 to 0.05:1 (preferably from 5:1 to 0.5:1) and the reaction is carried out in a fluidized bed or above a fixed catalyst bed. Further optional operative details are reported hereinafter. A solution of di-iodo-benzenes in benzene is evaporated and admixed with oxygen or air (oxygen prevents the formation of iodine) and the mixture is fed to a fixed-bed reactor loaded with the catalyst. An inert diluent, e.g. nitrogen, may optionally be used. The products can be recovered by cooling the stream leaving the reactor and resorting to usual treatments. In the case of a distillation, benzene is distilled as the overhead fraction and can be recycled to the reactor. The total pressure is usually not much higher than atmospheric pressure; however, lower or higher pressures may be used. The catalyst maintains its activity for a long time; when the catalytic activity decreases under the acceptable level, a regeneration is started; said regeneration may consist of a heat treatment in air, for some hours, at 300-550°C. According to an alternative and very efficient regeneration process a benzene stream, optionally admixed with air or any other oxygen-containing gas, is passed over the exhausted catalyst at 300-550°C. The initial activation of the catalyst is also an important step; in general, an activation in air at 450-550°C or the methods disclosed in EP-A-168,978; 169,026; and 169,027 can be used. The invention is described also with the aid of some figures: Figure 1 concerns a simple trans-iodination: Figure 2 deals with parallel oxidative and catalytic mono-iodination reactions of benzene with iodine. Following the flow diagram of Figure 1, benzene, admixed with oxygen (1) and a benzene solution containing one or more di-iodo-benzenes (2) enter, in the gas phase, reactor A loaded with the catalyst; the crude reaction effluent (3) is cooled (inside heat exchangers and heat recovery units, not shown in the Figure) and transferred to separator B from the bottom of which mono-iodo-benzene and residual di-iodo-benzenes are withdrawn (4), while most of the unreacted benzene and oxygen (5) are recycled to the trans-iodination reaction. Distillation tower C allows the fractionation of the mono- and polyiodinated compounds; mono-iodo-benzene (6) leaves the tower as the overhead fraction and the residual di-iodo-benzenes (7), admixed with a portion of mono-iodo-benzene and with a small amount of benzene, are recycled to the reaction zone. According to Figure 2, a benzene solution containing iodine in an excess with respect to benzene (1) and a benzene solution containing one or more di-iodo-benzenes (2) enter, in the gas phase, reactor A, together with a pre-heated stream of air (or of any other oxidizing gas) (3); the crude effluent from the simultaneous iodination and trans-iodination reactions (4) is cooled (inside facilities not shown in the Figure) and transferred to separator B, from the bottom of which a solution containing mono-iodo-benzene and residual di-iodo-benzenes is withdrawn together with a small amount of benzene (5), while most of the unreacted benzene, admixed with nitrogen-enriched air (6), is cooled in the recovery unit D and in cooler E before entering the separation and benzene recovery tower F (two towers in series are preferable), where the scrubbing liquid (7) may consist of benzene, mono-iodo-benzene, di-iodo-benzenes or their mixtures. The nitrogen-enriched air (8) can be vented (or transferred to other units) and the excess benzene (9) is recycled. Distillation tower C allows the fractionation of the iodinated compounds; mono-iodo-benzene (10) emanates as the overhead fraction, and the residual di-iodo-benzenes (11) leave the tower bottom as tail products and are combined with recycle stream (9). All of the iodine feed is thus completely used, the iodine dispersed in the undesired by-products (poly-iodo-benzenes) being completely recovered (by means of trans-iodination). The following Examples illustrate the invention. Example 1 (Comparative; H-ZSM5) A ZSM5 zeolite was prepared in a crude form, according to Example No. 24 of US-A-3,702,886, and subsequently exchanged at 80°C with a 1 M solution of HCl, in order to obtain the sodium-free (H-ZSM5) form. The zeolite crystallites had an average size lower than 0.5 µm; 1 g of the thus obtained zeolite was admixed with 0.3 g of binder (SiO₂) and the whole mixture was activated in air for 2 hours at 540°C; the resulting catalyst was introduced into a quartz microreactor, kept at 400°C and continuously fed with a mixture, in the gas phase, of benzene, p-di-iodo-benzene (p-DIB) and air, with a benzene:p-DIB: air molar ratio of 20:1:20. The pressure was slightly higher than 0.1 MPa (760 mmHg) and the weight hourly space velocity (WHSV) was 6 kg/h of (benzene + di-iodo-benzene) mixture per kg of pure zeolite (binder excluded). The reaction continued for 1 hour and the reaction products were collected by condensation; the conversion of p-DIB was very low (10%) and the selectivity to mono-iodo-benzene (based on converted p-DIB) was 99 %. Example 2 (Comparative; Na-ZSM5) Example 1 was repeated using a Na-ZSM5 zeolite, obtained by neutralization of the acidic form H-ZSM5 with diluted aqueous NaOH, followed by washing with de-ionized water and by an activation at 540°C. The conversion of di-iodo-benzene was lower than 5 %. Example 3 Example 1 was repeated, using a Ca-ZSM5 zeolite obtained by neutralization of the acidic form H-ZSM5 with a saturated solution of Ca(OH)₂ (at 80°C for 2 hours), followed by washing with deionized water and by activation at 540°C for 2 hours. After a 6-hour reaction time, the conversion of di-iodo-benzene was 60 %, and the selectivity to mono-iodo-benzene was higher than 99 %.

Claims: A process for the trans-iodination of poly-iodo-benzene, characterized in that the poly-iodo-benzene is reacted in the gas phase with benzene and oxygen or an oxygen-containing gas in the presence of a zeolite of the Pentasil® type, at least partially exchanged with a metal of the groups II, III or IV of the Periodic System of the Elements and present in a form different from the acidic form, the benzene to poly-iodo-benzene molar ratio ranging from 100:1 to 1:1. A process according to claim 1, characterized by recycling of the poly-iodo-benzenes formed during the oxidative catalytic reaction of benzene with iodine and oxygen or an oxygen-containing gas in a reaction zone to said reaction zone. A process according to claim 1 or 2, wherein the temperature is from 350 to 450°C. A process according to any of claims 1 to 3, wherein the benzene:poly-iodo-benzene molar ratio is from 20:1 to 1:1. A process according to any of claims 1 to 4, wherein the poly-iodo-benzene:oxygen molar ratio is from 10:1 to 0.01:1. A process according to any of claims 1 to 5, wherein the space velocity is from 0.1 to 100 kg/h of (poly-iodo-benzene + benzene) mixture per kg of pure zeolite, any binder excluded. A process according to any of claims 1 to 6, wherein the poly-iodo-benzene is fed as a solution in benzene. A process according to any of claims 1 to 7, wherein the zeolite catalyst is a zeolite of the ZSM5 type, exchanged with an alkaline-earth metal and, in particular, with calcium. A process according to any one of claims 1 to 8, wherein the poly-iodo-benzene:O₂ molar ratio is from 5:1 to 0.5:1. A process according to any of claims 1 to 9, wherein the reaction takes place in a fluidized catalyst bed. A process according to any of claims 1 to 9, wherein the reaction takes place above a fixed catalyst bed. A process according to any of claims 1 to 11, wherein the catalyst is regenerated at 300 to 550°C, by means of a benzene stream, optionally admixed with air or any other oxygen-containing gas. A process according to any of claims 1 to 12, wherein said poly-iodo-benzene is a di-iodo-benzene.

Inventors: PAPARATTO GIUSEPPE, PAPARATTO, GIUSEPPE
Applicants: ENICHEM POLIMERI, ECP ENICHEM POLIMERI S.R.L.
Application year: 1987
Main classification: C07C25
IPC codes: C07C25, C07B61, C07C67, C07C17, B01J29