Adsorptive separation of cresol isomers

Abstract: Para-cresol and meta-cresol are coextracted from a feed mixture comprising p-cresol, o-cresol and m-cresol and at least one other alkyl phenol with an adsorbent containing at least 5 wt.% water (LOI) and comprising an X zeolite exchanged with barium ions or with a mixture of barium and potassium ions so that both para- and meta-cresol are preferentially adsorbed. The coextracted cresol isomers are thereafter removed from the adsorbent by contacting it with a desorbent material including an aliphatic alcohol and recovering said coextracted cresols as a product stream. Para-cresol and meta-cresol can thereafter be recovered as purified individual isomers by a second stage adsorptive separation using a second adsorbent selective for para-cresol, said second adsorbent being dried to an LOI of less than about 4 wt.%, preferably about 2 wt.%, water.

Description: The field of art to which the claimed invention pertains is the separation of phenolic isomers. More specifically, the invention relates to a process for coextracting the para- and meta-cresol isomers from a feed mixture comprising ortho-, meta- and para-cresol and at least one other alkyl phenol with a particular zeolitic adsorbent. BACKGROUND OF THE INVENTION The cresol isomers are in substantial demand for various applications in the manufacture of phenolic resins, plasticizers, inhibitors, agricultural chemicals, ore flotation chemicals, germicides, antiseptics and disinfectants. The use varies with the particular isomer. For example, para-cresol is particularly useful in disinfectants or fumigating compositions, in production of cresotimic acid, dye stuffs and organic intermediates. Meta-cresol is particularly useful as a disinfectant, fumigating compositions, photographic chemicals, ore flotation chemicals, paint and varnish removers, in the production of synthetic resins, explosives and organic chemicals. Furthermore, applications for intermediates or final products utilizing a purified mixture of para- and meta-cresol exist, especially in the commercially important butylation of mixed m- and p-cresols. Previous adsorptive separation processes were suitable for extracting the para-cresol isomer from meta- and ortho-cresol, but effective processes for separating both para- and meta-cresol were not available. Adsorptive processes for separating cresol isomers have been previously known, e.g., from U.S.-A-3,014,078; U.S.-A-3,969,422 and U.S.-A-4,356,331. Also the separation of xylenols from cresols with adsorbents is known, for example, from U.S.-A-4,124,770 and U.S.-A-4,386,225. U.S.-A-3,014,078 teaches the separation of cresol isomers by employing an adsorbent consisting of an X-type zeolite exchanged with calcium, barium, etc., to selectively adsorb an isomer of cresol from a cresol isomer feed mixture thereby producing a rich adsorbent. U.S.-A-4,356,331 discloses a process for selectively adsorbing an alkyl phenol isomer onto a potassium exchanged Y zeolite, preferably also containing an additional cation, e.g., strontium or barium and subsequently desorbing the selectively adsorbed isomer with an aliphatic ketone desorbent. This patent disclosure is directed specifically to the separation of p-cresol from m-cresol. U.S.-A-3,969,422 to Neuzil et al discloses a process for separating para-cresol from at least one other cresol isomer, especially meta-cresol, with the preferred adsorbent being barium-potassium exchanged X zeolite and the preferred desorbent being a saturated alcohol from 1 to 7 carbon atoms per molecule. The separation of xylenols from cresols with X or Y zeolites exchanged with various cations or mixtures thereof, e.g., potassium and barium, and alcohols as desorbents was discused in U.S.-A-4,386,225 and U.S.-A-4,124,770. In both patents containing similar disclosures all cresol isomers are extracted to substantially exclude the xylenol, which is collected in the raffinate. The problem addressed by the present invention relates to the need for an improved process for separating the cresol isomers, by which high purity products of both para-cresol and meta-cresol can be obtained. This problem has now been solved by employing a particular adsorbent material and conditions, to co-extract meta- and para-cresol which can be subsequently separated into pure fractions of the individual isomers, meta-cresol and para-cresol. Both products are valuable and therefore the ability to obtain highly pure fractions of each of the isomers of cresol is desirable. SUMMARY OF THE INVENTION The present invention is a process for coextracting para- and meta-cresol from a feed mixture comprising para-, ortho- and meta-cresol and at least one other alkyl phenol, which process comprises contacting, at adsorption conditions, the feed mixture with an adsorbent comprising an X type zeolite exchanged with barium ions or a mixture of barium and potassium ions, the adsorbent containing at least 5 weight % water as measured by loss on ignition at 900°C (LOI method), to effect the selective adsorption of both the para- and meta-cresol isomers and thereafter recovering the para- and meta-cresol isomers by contacting the resulting adsorbent with a desorbent, at desorption conditions effective to produce an extract stream enriched in para- and meta-cresol relative to the feed. The desorbent may be an aliphatic alcohol with C 5 and C 6 aliphatic alcohols being preferred. In a subsequent step para-cresol can be separated from the meta-cresol by further contacting the extract stream with a second adsorbent, having less than about 4 wt.% and preferably about 2 wt.% water (LOI method) and subsequently desorbing the para-cresol from the resulting rich second adsorbent at desorption conditions with a desorbent, which may be the same as in the first desorption step. Other embodiments of the present invention are disclosed in the following discussion. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a chromatographic separation (pulse test) of a feed mixture containing m- and p-cresol, 2,6-dimethyl phenol and o-ethyl phenol with a Ba/K-X zeolite containing 5.48% water (LOI) and n-pentanol as the desorbent. Figure 2 is a summary of pulse test data obtained in the separation of Example I using Ba/K-X in which net retention volume is plotted vs. water content (LOI) of the adsorbent. Figure 3 is similar to Figure 1, except that the adsorbent was Ba-X having a water content (LOI) of 5.66%. Figure 4 is a summary of the pulse test data of Example II, using Ba-X, in which net retention volume (NRV) is plotted vs. water content (LOI). DESCRIPTION OF THE INVENTION Adsorbents to be used in the process of this invention comprise specific crystalline aluminosilicates or molecular sieves, namely X zeolites, exchanged at exchangeable cationic sites with barium and potassium ions. The zeolites have known cage structures in which the alumina and silica tetrahedra are intimately connected in an open three-dimensional network to form cage-like structures with window-like pores. The tetrahedra are cross-linked by the sharing of oxygen atoms with spaces between the tetrahedra occupied by water molecules prior to partial or total dehydration of this zeolite. The dehydration of the zeolite results in crystals interlaced with cells having molecular dimensions and thus, the crystalline aluminosilicates are often referred to as "molecular sieves" when the separation which they effect is dependent essentially upon differences between the sizes of the feed molecules as, for instance, when smaller normal paraffin molecules are separated from larger isoparaffin molecules by using a particular molecular sieve. In the process of this invention, however, the term "molecular sieves", although widely used, is not strictly suitable since the separation of specific aromatic isomers is apparently dependent on differences in electrochemical attraction of the different isomers and the adsorbent rather than on pure physical size differences in the isomer molecules. In hydrated or partially hydrated form the preferred type X crystalline aluminosilicates encompass those zeolites represented, in terms of moles of metal oxides, by the formula 1 below: where "M" is a cation which balances the electrovalence of the tetrahedra and is generally referred to as an exchangeable cationic site, "n" represents the valence of the cation and "y" represents the moles of water. Adsorbents comprising the type X zeolites are useful for the adsorptive process for co-extracting m- and p-cresol from mixtures of alkyl phenols herein described. These zeolites are described and defined in U.S.-A-2,882,244. The term "type X structured" zeolites as used herein shall include all zeolites which have general structures as represented in the above cited patent. Typically, the type X structured zeolites, as initially prepared, are predominantly in the sodium form. The term "exchanged cationic site" generally refers to the site in the zeolite occupied by the cation "M". This cation, usually sodium, can be replaced or exchanged with other specific cations, dependent on the type of the zeolite to modify characteristics of the zeolite. The preferred zeolites for use in this invention are type X zeolites exchanged with barium ions or a mixture of barium and potassium ions. Cations occupying exchangeable cationic sites in the zeolite are exchanged with other cations by ion exchange methods well known to those having ordinary skill in the field of crystalline aluminosilicates. Such methods are generally performed by contacting the zeolite or an adsorbent material containing the zeolite with an aqueous solution of the soluble salt of the cation or cations desired to be placed upon the zeolite. After the desired degree of exchange takes place, the sieves are removed from the aqueous solution, washed, then dried to a desired water content. By such methods, the sodium cations and any non-sodium cations which might be occupying exchangeable sites as impurities in a sodium-X zeolite can be essentially completely replaced with other cations. It is preferred that the zeolite used in the process of this invention contain barium or a combination of barium and potassium cations at exchangeable cationic sites prepared as disclosed in the above mentioned U.S.-A-3,969,422. The weight ratio of barium oxide to potassium oxide in one of the preferred adsorbents can be from about 15.9 to 6.5. It has been found that an adsorbent comprising an X zeolite containing at exchangeable cationic sites barium cations or a mixture of barium and potassium cations satisfies the selectivity requirements and other adsorbent requirements previously discussed and are therefore preferred for use in this process. Adsorbents for this process may be prepared by partially (for mixed cations) or essentially completely ion exchanging potassium-type X base material, in a particle size range of from 20 to 40 U.S. Mesh (0.42 to 0.84 mm), with barium cations. Typically the ion exchanges will be done with aqueous solutions of the soluble salts, such as the chlorides, of the respective metal. The term "essentially complete" shall mean that the residual potassium content of the adsorbent after the ion exchange of the base material shall be less than 2 wt.% K 2 O. After ion-exchange and water wash to remove excess ion exchange solution, the adsorbent will be dried to reduce the water content as measured by loss on ignition (LOI) at 900°C to not less than 5 wt.% and more preferably within a range of from 5 to 7 wt.%, and most preferably 5.5 to 6 wt.% (LOI). Maintaining adsorbent water content above 5% is critical to process performance and water may be added to the process during operations as necessary to maintain this minimum level. Water may be added to the adsorbent, if necessary, either on an intermittent or more preferably on a continuous basis by itself or in admixture with feed or desorbent material to maintain the desired concentration of water on the adsorbent. Typically, adsorbents used in separative processes contain the crystalline material dispersed in an amorphous inorganic matrix or binder, having channels and cavities therein which enable liquid access to the crystalline material. Amorphous material such as silica, or silica-alumina mixtures or compounds, such as clays, are typical of such inorganic matrix materials. The binder aids in forming or agglomerating the crystalline particles of the zeolite which otherwise would comprise a fine powder. The adsorbent may thus be in the form of particles such as extrudates, aggregates, tablets, macrospheres or granules having a desired particle size range, from 16 to 40 mesh (Standard U.S. Mesh) (0.42 to 1.19mm). Feed mixtures which can be used in the process of the invention include complex mixtures of cresol isomers and other alkyl phenols, obtained from coal tar, coal gasification, or synthetic processes, including p-cresol, m-cresol, o-cresol, m-ethyl phenol, o-ethyl phenol, p-ethyl phenol, dimethyl phenols, diethyl phenols, methyl ethyl phenols and isopropyl phenols. A typical feed derived from coal tar was analyzed as follows: Component Wt. % m-Cresol 50 p-Cresol 45.3 2,6 Dimethyl Phenol (DMP) 1.0 o-Ethyl Phenol 3.0 o-Cresol 0.4 2,4 + 2,5 DMP 0.3 100.0 In the preferred isothermal, isobaric, liquid-phase operation of the process of the present invention, desorbent materials comprising aliphatic alcohols (C 5 -C 6 ), selected to differ in boiling point by at least 5°C from the boiling range of the feedstock so the desorbent may be recovered for reuse, result in high selectivity for the coextracted product when used with the above discussed adsorbent. Although both liquid and vapor phase operations can be used in many adsorptive separation processes, liquid-phase operation is preferred for this process because of the lower temperature requirements and because of the higher yields of extract product that can be obtained with liquid-phase operation over those obtained with vapor-phase operation. Adsorption conditions will include a temperature range of from 20 to 200°C and a pressure sufficient to maintain liquid phase, ranging from atmospheric to 500 psig (101.3 to 3550 kPa). Desorption conditions will include the same range of temperatures and pressures as used for adsorption conditions. At least a portion of the extract stream, and preferably at least a portion of the raffinate stream, from the separation process, are passed to separation means, typically fractionators or evaporators, where at least a portion of the desorbent material is separated to produce a co-extract product and a raffinate product, respectively. The raffinate will contain the less strongly adsorbed components of the feed, e.g., o-cresol, the dimethyl phenols, the ethyl phenols, etc. The coextract product taken from the first stage contains a highly purified mixture of para-cresol and meta-cresol. These two components may be separated in a second stage adsorptive separation, similar to the first, in which an adsorbent selection for para-cresol is used. Part or all of the desorbent from the first stage may be removed from the first extract, but if the same desorbent is used in the second stage, some of the desorbent from the first stage will not be detrimental if left in the feed to the second stage. The adsorbent may be any para-selective adsorbent, such as disclosed in U.S.-A-4,356,331 or U.S.-A-3,969,422, but it is preferred to use the same adsorbent as the first stage, but having a water content below 4% (wt.) whereby the adsorbent is selective for para-cresol. The desorbent may be selected from the desorbents disclosed in said patents, e.g. an aliphatic alcohol or a mixture thereof with an aliphatic ketone, but it is preferred to use the same desorbent as the first stage. A dynamic testing apparatus is employed to test various adsorbents with a particular feed mixture and desorbent material to measure the adsorbent characteristics of adsorptive capacity, selectivity and exchange rate. The apparatus consists of an adsorbent chamber of approximately 70 cc volume having inlet and outlet portions at opposite ends of the chamber. The chamber is contained within a temperature control means and, in addition, pressure control equipment is used to operate the chamber at a constant predetermined pressure. Quantitative and qualitative analytical equipment such as refractometers, polarimeters and chromatographs can be attached to the outlet line of the chamber and used to detect quantitatively or determine qualitatively one or more components in the effluent stream leaving the adsorbent chamber. A pulse test, performed using this apparatus and the following general procedure, is used to determine selectivities and other data for various adsorbent systems. The adsorbent is filled to equilibrium with a particular desorbent material by passing the desorbent material through the adsorbent chamber. At a convenient time, a pulse of the feed mixture is injected for a duration of several minutes. Desorbent flow is resumed, and the m- and p-cresol coextract and other components are eluted as in a liquid-solid chromatographic operation. The effluent can be analyzed on stream or alternatively, effluent samples can be collected periodically and later analyzed separately by analytical equipment and traces of the envelopes of corresponding component peaks developed. From information derived from the test, adsorbent performance can be rated in terms of void volume, retention volume for an extract or a raffinate component, selectivity for one component with respect to the other, and the rate of desorption of an extract component by the desorbent. The retention volume of an extract or a raffinate component may be characterized by the distance between the center of the peak envelope of an extract or a raffinate component and the peak envelope of the tracer component or some other known reference point. It is expressed in terms of the volume in cubic centimeters of desorbent pumped during this time interval represented by the distance between the peak envelopes. Selectivity, (β), for an extract component with respect to a raffinate component may be characterized by the ratio of the distance between the center of the extract component peak envelope and the tracer peak envelope (or other reference point) to the corresponding distance between the center of the raffinate component peak envelope and the tracer peak envelope. The rate of exchange of an extract component with the desorbent can generally be characterized by the width of the peak envelopes at half intensity. The narrower the peak width the faster the desorption rate. The desorption rate can also be characterized by the distance between the center of the tracer peak envelope and the disappearance of an extract component which has just been desorbed. This distance is again the volume of desorbent pumped during this time interval. The invention herein can be practiced in fixed or moving adsorbent bed systems or in cocurrent, pulsed batch systems described in U.S.-A-4,159,284 or in cocurrent continuous simulated moving bed systems like that disclosed in U.S.-A-4,402,832 and U.S.-A-4,478,721, but the preferred system for this separation is a countercurrent simulated moving bed system, such as described in U.S.-A-2,985,589, incorporated herein by reference. Cyclic advancement of the input and output streams can be accomplished by a manifolding system, which are also known, e.g., by rotary disc valves shown in U.S.-A-3,040,777 and U.S.-A-3,422,848. The following examples are presented to illustrate the selectivity relationship that makes the process of this invention possible. EXAMPLE I The purpose of this example is to present the results of a pulse test obtained from the above described pulse test apparatus packed with barium and potassium exchanged X (BaK) zeolite with 1-pentanol desorbent to separate and recover para-cresol and meta-cresol from a feed mixture of 0.3g of 2,6-dimethyl phenol (DMP), 0.3 cc each of o-ethyl phenol, m-cresol, p-cresol, 0.25 cc n-C 16 (tracer) and 2.0 cc 1-pentanol (desorbent). A feed pulse consisting of 2.0 cc of the above mixture was introduced to the pulse test column at 135°C. The desorbent flow rate was 1.17 cc/min. The water content of the adsorbent was 5.48% (LOI at 900°C). From the plot of relative concentration (by gas chromatographic area counts) vs. net retention volume (NRV) in ml. shown in Figure 1, selectivities calculated among the various compounds are set forth in the following Table 2 (under the headings gross retention volume, net retention volume and selectivity factor (β) of each component with respect to the reference component, p-cresol): Component Gross Retention Volume Net Retention Volume Peak Width At Half Height Selectivity Factor β n-C 16 37 0 9.2 Tracer 2,6-DMP 41.4 4.40 8.6 4.18 o-Ethyl Phenol 41.8 4.8 8.4 3.83 p-Cresol 55.4 18.4 13.7 Refer. m-Cresol 55.3 18.3 13.1 1.005 Several other pulse tests were conducted as described above except that the water content was varied. The results of all these pulse tests are shown in the graph of Figure 2, wherein the net retention volumes of para-cresol and meta-cresol were plotted against the water content (LOI) of the adsorbent. As will be seen from Figure 1 and the data points in Figure 2, which have been connected by the lines drawn from a cubic spline fit, para-cresol and meta-cresol are virtually coextracted at an LOI of 5.48%. This is also evident from the selectivity factor of meta-cresol, in Table 2, of 1.005. This can be contrasted to the same separation where the adsorbent contains 3.42% of water (LOI) and the selectivity factor of meta-cresol to para-cresol is 1.94. In the latter case, the selectivity of ortho-cresol is so close to that of meta-cresol that meta-cresol could not be coextracted with para-cresol. EXAMPLE II The pulse test of Example I was repeated except that a 77 cc column of Ba-X, dried to a water content of 5.66% (LOI at 900°C), was used as the adsorbent. The feed was 2 cc of a solution containing 0.5 cc each of o-cresol, m-cresol, and p-cresol, 0.3 cc of n-C 16 (tracer), and 2 cc of the desorbent, 1-pentanol. The temperature was 100°C and the flow rate was 1.26 cc/min. The results are shown in Figure 3 and set forth in the following Table 3. Component Gross Retention Volume Net Retention Volume Peak Width At Half Height Selectivity Factor β n-C 16 48.0 0 14.3 Tracer p-Cresol 65.1 17.1 23.6 Ref. m-Cresol 65.6 17.6 23.5 0.97 o-Cresol 58.1 10.1 17.3 1.69 The results of several other pulse tests similar to the above tests, except at different water content of the adsorbent, Ba-X, are shown in Figure 4. There it is noted that, at an LOI of 5.66%, the net retention volumes are virtually the same and hence no separation between the para- and meta-isomers takes place and are consequently coextracted. However, at 4.5% (LOI) water, there is substantial separation and, due to similar retention volumes for ortho-cresol and meta-cresol, meta- and para-cresol cannot be coextracted. EXAMPLE III The pulse test of Example I was repeated except that the adsorbent, Ba-X xeolite, was dried to a water content of 6.2% (LOI at 500°C). The feed was 2.0 cc of a solution containing 2 g. each of o-cresol, m-cresol, p-cresol, 2,6-dimethyl phenol, o-ethyl phenol and 1.5 g. n-C 16 . The temperature was 150°C and the desorbent was 1-pentanol. The results are shown in the following Table 4. Component Gross Retention Volume (cc) Net Retention Volume (cc) Selectivity Factor (β) n-C 16 41.9 0.0 Tracer 2,6 dimethyl phenol 50.0 8.1 2.82 o-ethyl phenol 50.2 8.3 2.75 o-cresol 61.0 19.1 1.20 p-cresol 64.8 22.9 Refer. m-cresol 65.2 23.3 0.98 In pilot plant runs, using the above adsorbent and desorbent, a commercial feedstock containing the above components was separated and an extract product containing p-cresol and m-cresol at 99.0 - 99.9% purity and 99.0 - 99.6% recovery was consistantly obtained.

Claims: A process for coextracting para-cresol and meta-cresol from a feed mixture comprising para-cresol, ortho-cresol and meta-cresol and at least one other alkyl phenol, comprising contacting the feed mixture with an adsorbent comprising an X zeolite exchanged with barium ions or with a mixture of barium and potassium ions, the adsorbent containing at least 5 wt. water as measured by loss on ignition (LOI) at 900°C, whereby para- and meta-cresol are selectively adsorbed, and thereafter contacting the resulting adsorbent with a desorbent comprising an aliphatic alcohol, whereby para- and meta-cresol are extracted and an extract stream enriched in para- and meta-cresol relative to the feed mixture is produced, the adsorption and desorption being carried out a temperature within the range of from 20°C to 250°C and a pressure sufficient to maintain liquid phase. A process as claimed in claim 1, wherein the feed mixture contains at least one other alkyl phenol selected from m-ethyl phenol, o-ethyl phenol, p-ethyl phenol, dimethyl phenols, diethyl phenols, methyl ethyl phenols, isopropyl phenols and mixtures thereof. A process as claimed in claim 1 or 2, wherein the aliphatic alcohol used for extracting para- and meta-cresol has 5 or 6 carbon atoms per molecule. A process as claimed in any of claims 1 to 3, wherein the extract stream is contacted with a second adsorbent comprising an X or Y zeolite exchanged with barium, potassium, or barium and potassium ions, the second adsorbent containing less than 4 wt.% water (LOI), whereby para-cresol is selectively adsorbed, thereafter the resulting adsorbent is contacted with a second desorbent comprising an aliphatic alcohol or a mixture thereof with an aliphatic ketone, whereby purified meta-cresol is eluted from the adsorption step, and purified para-cresol is recovered from the desorption step, the adsorption and desorption being carried out at a temperature within the range of from 20°C to 250°C and a pressure sufficient to maintain liquid phase. A process as claimed in claim 4, wherein the second adsorbent contains 2 wt.% water (LOI). A process as claimed in claim 4 or 5, wherein the second adsorbent is the same apart from its water content as the adsorbent used for contacting the feed mixture. A process as claimed in any of claims 4 to 6, wherein the same aliphatic alcohol is used for the second desorbent as for the desorbent used for extracting para- and meta-cresol. A process as claimed in any of claims 1 to 7, carried out in a countercurrent simulated moving bed system.

Inventors: ZINNEN HERMANN ALBERT, ZINNEN, HERMANN ALBERT
Applicants: UOP INC, UOP
Application year: 1992
Main classification: C07C37
IPC codes: C07C37, B01D15, C07C39, B01J20