PROCESS FOR THE PREPARATION OF A PHENOL

Abstract: Process for the preparation of a phenol by an oxidative decarboxylation in the gas phase of a corresponding arylcarboxylic acid in the presence of a Cu-containing catalyst, characterized in that the following process steps are carried out: (a) supplying an oxidant-containing gas mixture to the catalyst bed under such conditions that tar formation is virtually absent, (b) supplying arylcarboxylic acid and steam with exclusion of oxygen, resulting in formation of gaseous phenol.

Description: PROCESS FOR THE PREPARATION OF A PHENOL The invention relates to a process for the preparation of a phenol by an oxidative decarboxylation in the gas phase of a corresponding arylcarboxylic acid in the presence of a Cu-containing catalyst. The preparation of a phenol by an oxidative decarboxylation has long been known. GB-A-762738, which is an equiavlent of NL-B-90.684 already discloses such a process, wherein the oxidation, the decarboxylation as well as the hydrolysis are effected in a single process step, at a temperature of at least 200°C, preferably 230-250°C. In relation with this process a number of patent publications have appeared over the years, aiming to suppress the principal drawback of said process, which is the formation of a considerable number of by-products, mainly in the form of tar. With the aim of preventing tar formation it has been suggested to have the reaction take place in the gas phase on a solid copper-containing catalyst; see for instance NL-B-107561 and NL-B-110374. These, as well as later publications, which are mainly aimed at optimization of the catalyst (e.g. NL-A-7810528, EP-A-52839 and EP-A-40452), do not disclose systems whereby tar formation is actually suppressed, however. In all cases it appears that after some time the catalyst is covered with a black tar-like coat, which in particular affects its activity. The process according to the invention provides a process for the preparation of a phenol from the corresponding arylcarboxylic acid in the gas phase, with virtually complete prevention of tar formation. Thus a process is obtained which combines high preparation selectivities with an economically attractive embodiment. The process according to the invention is characterized in that the following process steps are carried out: a) Supplying an oxidant-containing gas mixture to the catalyst bed under such conditions that tar formation virtually absent, b) Supplying arylcarboxylic acid and steam with exclusion of oxygen, resulting in formation of gaseous phenol. By performing the process in this way, a two-step process is obtained: first an oxidation, then formation of the phenol in combination with reduction of the catalyst. Here and below, arylcarboxylic acid is understood to be a compound having the following structure: R 5 R 2 where R 1 through R 5 may be hydrogen (on the proviso that at least R 1 or R 5 is hydrogen) or organic groups, which have a so-called Hammett constant of between -1 and +2. A description of this Hammett or σ value, which represents a measure of the influence of the group on the reactivity of the arylcarboxylic acid, can be found in J. March, Advanced Organic Chemistry 1989, pages 242-250; see in particular table 4 on page 244. The groups that can be used therefore are: C j^ -Cg alkyl, cylcoalkyl, aryl, arylalkyl, amino, halogen, nitro. Esters and anhydrides of (I) are also suitable, while the groups may also be connected with each other via ring system, as is the case for instance in naphthalene carboxylic acid (substituted or not). Multiple arylcarboxylic acids, such as trimellitic acid and pyromellitic acid, can also be used as starting material. Mixtures of the arylcarboxylic acids described above can also be used in the process according to the invention. The invention relates particularly to a process for the conversion of unsubstituted benzoic acid (R 1 through R 5 = hydrogen) into the corresponding unsubstituted phenol. The oxidation of the Cu-containing catalyst, involving conversion of copper into Cu(II), is a first reaction step in the process. It brings about an increase in the degree of oxidation of the copper. The oxidation of the copper-containing catalyst proceeds well particularly when carried out using an oxygen-containing gas. Air, whether or not enriched with oxygen or oxygen-depleted, can very well be used for this. Other oxidizing gases can also be used, e.g. N 2 0- or 0 3 -containing gases. The pressure applied is not critical, but in general an elevated pressure will be chosen so as to accelerate the oxidation process. Pressures of 0.1-2.5 MPa are therefore suitable. This oxidation step should be so performed that (virtually) no tar is formed. This can be achieved by various measures. For instance it can be ensured that no phenol is absorbed to the catalyst any more, hence that the catalyst is essentially free of phenol. This can be accomplished by stripping with steam or an inert gas at the end of the second step. Or for instance the temperature of the catalyst bed in this step can be kept below 200°C, preferably between 150-190°C (because the rate of oxidation of Cu is high and tar formation does not occur until the temperature exceeds 200°C, in the presence of phenol). It is also possible to apply a higher temperature (e.g. 191-270°C) and keep the contact time between the reaction mixture and the oxygen short (e.g. 0.1-5 minutes), so that not all the copper is converted into the Cu(II) form. The catalyst can be applied onto a carrier, consisting for instance of oxides of silicon, titanium or cadmium, of for instance of carbon. Preferably, the catalyst is used immobilized on a carrier. Silica is preferably used as carrier. The catalyst load will as a rule be 5-40 wt.% metal on carrier. A high degree of loading contributes to a high conversion per volume unit. There may be advantage in using a catalyst containing a co-catalyst besides copper. This co-catalyst can be chosen in particular from groups III through VIII as well as from the group of the lanthanides and actinides of the Periodic System of the Elements. These components have an effect on the oxidative capacity of the Cu in the catalyst. The catalyst used may for instance consist of copper, zirconium and, if desired, an alkali metal or an alkali earth metal such as sodium, potassium, lithium or magnesium. Also, rare earths (atom numbers 37 through 71) or zirconium, silver, vanadium, chromium, molybdenum, hafnium and tungsten or mixtures thereof in combination with copper can be used. Examples of suitable catalyst systems are described in EP-A-52839, EP-A-40452, NL-A-7810528, NL-B-110374 and NL-B-107561. In general, all catalysts which are suitable for the gas phase reaction can also be used in the two-step process according to the invention described in the present specification. The second step in the process according to the invention comprises a combination of a reduction and the formation of phenol, in which process carbondioxide (C0 2 ) is released. The reduction and the formation of phenol are effected in the absence of oxygen, in contrast to the process described in NL-B-283.477. Owing to the absence of oxygen, reoxidation of the Cu-containing catalyst in this process step is avoided, so that no reactions can occur between the phenol or its intermediate products and any oxidized catalyst products, so that formation of tar is prevented. It is advantageous to use such an amount of steam in the second step of the process according to the invention as to achieve at least equimolarity thereof relative to the amount of Cu(II) benzoate. Preferably, some excess is applied, a two- to four-fold excess as a rule being sufficient to ensure a high yield. The pressure under which the second process step is performed is not critical, but the advantage of raising the pressure to above atmospheric level is that it has a favourable effect on the reaction kinetics. The pressure to be applied will generally be between 0.1 and 2.5 MPa; higher pressures, though allowable, do not yield substantial improvements of the process. It can be advantageous to aftertreat the reaction mixture obtained in order to promote the formation of phenol. This can be effected very well by using an acid ion exchanger in a fixed bed reactor. The reaction products are subjected to an upgrading operation in order to separate and recover the phenol obtained. This can be done in ways known by themselves, for instance by distillation. The bottom flow of the distillation, which may contain non-converted arylcarboxylic acid, can be recycled to the process, optionally after a purification step. The catalyst can be applied in the form of a fixed or moving beds. The two-step reaction can be carried out intermittently, successively supplying for a short time an oxygen-containing gas to e.g. a fixed-bed reactor and then - with exclusion of oxygen - having the second step take place. After the catalyst has been conditioned for the first step, the oxidative gas mixture can be supplied again. The two-step process can also be carried out continuously by having the oxidation step take place in the first reactor of a multiple reactor system and then bringing the catalyst, preferably in a moving bed, to a second reactor, where the second step takes place. In that case the reactors can be fully adapted to the desired requirements. The process according to the invention is suitable in particular for the preparation of unsubstituted phenol from unsubstituted benzoic acid. This phenol can be used for instance as starting material both for phenol-formaldehyde resins and for the preparation of caprolactam, starting material for nylon-6, or for the preparation of bisphenol-A. 5 The invention will now be elucidated in the following examples, which should not be construed as limiting the invention. Examples 10 Tests were performed using a set-up as represented in the annexed diagram, in which A represents a steam-generating device (H 2 0/N 2 mixture), B a benzoic acid evaporator and C an air supply device. The three flows go to the reactor bed in D. E, F and G are coolers, cooling the 15 reaction mixture to 130°C, 8°C and -80°C, respectively. H is the analysis section for the offgas mixture. The reactor consisted of a packed bed of about 2.5 ml, filled with copperoxide on silica. The gas flows from A, B and C were adjustable, products in E through H were analyzed using gas 20 chromatography (GC ) and high-pressure liquid chromatography (HPLC) . Example I The catalyst was oxidized with air for 30 seconds. 25 Next, benzoic acid and steam were supplied for 240 seconds. The catalyst bed had a temperature of 250°C. The retention time of the acid and the steam on the columns was 4 seconds. The conversion of the benzoic acid was 20%. At the end of the cycle, only steam was supplied for 2 seconds in order to 30. strip off adsorbed phenol. The selectivity to phenol was 96%, to benzene 4%. After five cycles the catalyst was extracted with ether. HPLC analysis showed that no tar products had formed on the catalyst. 35 Example II Analogously to example I, so much steam and benzoic acid was supplied for 60 seconds at 300°C that the retention time on the column was 2 seconds. The conversion was 40%, the selectivity to phenol was 91%, to benzene 9%. No tar was 0 formed.

Claims: C L A I M S 1. Process for the preparation of a phenol by an oxidative decarboxylation in the gas phase of a corresponding arylcarboxylic acid in the presence of a Cu-containing catalyst, characterized in that the following process steps are carried out: a) Supplying an oxidant-containing gas mixture to the catalyst bed under such conditions that tar formation is virtually absent, b) Supplying arylcarboxylic acid and steam with exclusion of oxygen, resulting in formation of gaseous phenol. 2. Process according to claim 1, characterized in that the oxidation of the catalyst is carried out with oxygen-containing gas. 3. Process according to any one of the claims 1-2, characterized in that the Cu-containing catalyst also contains a co-catalyst. 4. Process according to any one of the claims 1-3, characterized in that the oxidation is carried out with a deficiency of oxidant relative to the amount of Cu in the catalyst. 5. Process according to any one of the claims 1-4, characterized in that the catalyst applied in step a) is essentially free of phenol. 6. Process according to claim 5, characterized in that at the end of step a), phenol is stripped from the catalyst. 7. Process according to any one of the claims 1-4, characterized in that the temperature of the catalyst bed is 120-190°C during step a). 8. Process according to any one of the claims 1-4, characterized in that step a) takes 0.1-5 minutes. 9. Process according to any one of the claims 1-8, characterized in that step a) is carried out with a virtually equimolar amount of steam relative to the amount of Cu(II). 10. Process according to any one of the claims 1-9, characterized in that step b) is carried out at a temperature of 250-350°C. 11. Process according to any one of the claims 1-10, characterized in that arylcarboxylic acid is used as unsubstituted benzoic acid. 12. Process as substantially described in the specification and in the examples.

Inventors: BUIJS WIM, BUIJS, WIM
Applicants: DSM NV, DSM N.V.
Application year: 1992
Main classification: C07C39
IPC codes: B01J23, C07B61, C07C37, C07C39