WO2001074485A1

WO2001074485A1 - Phosphorous-aluminium-mixed oxide catalyst, process for its preparation and use thereof

Info

Publication number: WO2001074485A1
Application number: PCT/EP2001/003442
Authority: WO
Inventors: Fabrizio Cavani; Tiziana Monti; Pietro Panseri; Giambattista Castelli; Vittorio Messori
Original assignee: Borregaard Italia S.P.A.
Priority date: 2000-03-31
Filing date: 2001-03-27
Publication date: 2001-10-11
Also published as: AU2001256223A1; CN1274418C; CN1212192C; CN1583262A; HK1052477A1; CN1420805A; HK1052477B

Abstract

An A1 and P mixed oxide, with surface area 50-100 m2/g, prepared by gradually heating up to 400-800 °C a mixed compound in its turn obtained by coprecipitation from an aqueous solution of A1 and P compounds, is advantageously used as a catalyst for the monoalkylation of diphenol compounds.

Description

PHOSPHORUS -ALUMINIUM-MIXED OXIDE CATALYST , PROCESS FOR ITS PREPARATION AND USE THEREOF

Background of the invention

Diphenol monoalkylethers such as guaiacol (l-methoxy-2-hydroxybenzene) and guetol (l-ethoxy-2-hydroxybenzene) as well as other diphenol monoalkylethers can be prepared by a condensation reaction (etherification) of catechol or of the other diphenols with an alkylating agent, catalyzing such transformation by use of a suitable material, having characteristics promoting monoalkylation at the oxygen atom to form the monoalkylether. The reaction can be carried out in gas, liquid or mixed phase, and the preferred alkylating agent is the corresponding alcohol. Catalytic systems disclosed in patent literature are systems based on boron and phosphorous mixed oxides, as claimed in EP 420 756 (1990), in the name of UBE Industries. This material is characterized by remarkable catalytic activity, which however gradually decreases in time, since the active phase is slowly but gradually lost under the reaction conditions. In order to avoid such problem, co- feeding of B and/or P volatile compounds is necessary to prevent the loss of the active components through sublimation under the reaction conditions. Alternatively, the activity decrease may be obviated by increasing the reaction temperature, which however results in a decrease in selectivity to the desired product. Furthermore, the gradual disgregation of the active phase reduces the mechanical strength of the catalyst extrudate or pellet.

On the other hand, FR 2,303,784 (in the name of UBE Industries, 1976) discloses the use of a catalytic system based on Al/B/P/O, which still requires feeding the B and/or P compounds to keep performances concerning reagent conversion unchanged in time. Finally, EP 509,927 (in the name of UBE Industries, 1992), discloses the use of a catalyst consisting of an Al_aP_DTi_cSidO_e mixed oxide, in which the atomic ratios of the claimed components are (when a = 1), b = 1.0-1.6, c = 0.05- 0.5, d = 0.05-0.2. An alkali metal ion (typically potassium) can further be added to the composition (in a 0-0.9 ratio). Compared with the prior catalytic systems, this system has advantages in that its reactivity characteristics are much more constant in time, and therefore no co-feeding of volatile compounds is necessary. The claimed catalyst is characterized by a specific surface area ranging from 30 to 50 m^/g after the final thermal treatment carried out at a temperature ranging from 300 to 600°C, preferably at 400°C. This catalyst is used for the etherification reaction at a claimed temperature ranging from 200°C to 400°C. It should be stressed that, according to said patent, the absence of Ti or Si in the catalyst composition results in a decrease in selectivity to guaiacol (for the etherification of catechol with methanol) and, above all, in a remarkable decrease in the catalytic activity. Conversion at a temperature of 280°C decreases, in fact, from 64.7% to 46.3% (catalyst without Ti), to 55.7% (catalyst without Si); selectivity decreases from 98.3% to 95.8% (catalyst without Ti), to 97.4% (catalyst without Si).

In view of what stated above, it is highly desirable to provide a catalyst more active than the claimed or known ones, so as to operate under milder reaction conditions hence providing acceptable conversions with high selectivities to monoethers (for example guaiacol). An ideal system should operate at lower reaction temperatures than those described in patent literature, namely under conditions providing high selectivity to monoethers while minimizing the formation of undesired products. The present invention relates to a catalytic system based on Al/P/O, prepared according to a technique different from that described in patent literature, and which has the following advantages compared with the prior art systems: 1) The specific surface area is high, ranging from 50 to 100 m²/g after calcination at 400-800°C (preferably 400-650°C), due to the preparation procedure used, and is therefore remarkably higher than that of the systems described in literature and calcinated at the same temperature. 2) Thanks to the features of point 1), the solid can be subjected to a thermal sintering - treatment at temperatures higher than those described in literature, for example at 600°C, while maintaining high specific surface area, ranging from 50 to 80 m²/g, and hence high catalytic activity. Treatment at higher temperatures provides materials characterized by higher structural and morphological stabilities, resulting in catalytic performances more stable in time.

3) Thanks to the features of points 1 and 2, a catalytic system active even when no Si and Ti components are present can be obtained. As a consequence, lower-cost catalysts may be prepared by means of simpler procedures having better reproducibility, in that the preparative and composition parameters are substantially lower.

4) The catalysts prepared by the procedure of the present invention can operate at temperatures lower than those described in literature, and therefore under more favorable conditions in terms of both selectivity to the desired product and life time of the catalyst itself. Lower temperatures, in fact, slow down the ever present hydrolysis of the phosphate while decreasing the formation of heavy products which results in deactivation problems. For example, catechol conversion from 20 to 30% can be attained at a temperature of 190°C, with selectivity to guaiacol above 99% for more than 1.300 hours of reaction in flow, without decreasing reactivity. The unconverted reagent can then be easily recycled to the reaction medium. Disclosure of the invention

The present invention relates to a catalyst for the oxygen monoalkylation (monoetherification) of polyhydroxybenzenes by reaction between these compounds and an alcohol, more particularly for the reaction between dihydroxybenzenes and straight or branched, saturated or unsaturated, C_rC₆, preferably C_rC₄, alcohols. The reaction can be carried out in gas phase, or in mixed phase, with high conversion of the limiting reagent, and high selectivity to the oxygen monoalkylation product (l-alkoxy-2-hydroxybenzene, for example 1- methoxy-2-hydroxybenzene or guaiacol), and with low selectivity to other, undesired products, such as the complete etherification products (for example veratrole). A further advantage of the catalyst of the invention is that the reaction can be carried out for very long life times without deactivating the catalyst, both in terms of activity and selectivity, and that there is no need for co-feeding, together with the reagents, compounds to keep unchanged in time the performances of the catalyst itself. Typically, in case of the catalytic systems known in literature, such as systems based on boron, phosphorous and oxygen, co-feeding compounds such as alkylborates, alkylphosphates or other volatile boron and phosphorous compounds is necessary, in that the catalyst gradually loses activity as the B/P/O mixed compound is slowly, but continuously hydrolyzed in the reaction medium, and boron and phosphorous are released in the form of volatile compounds. Formation of vapor pressure by continuously feeding B and P compounds prevents or at least slows down the deactivation process. Continuous feeding of said compounds, however, involves additional burdens for the process, in terms of reagent costs as well as operations for separating said compounds from the unconverted reagents and from the products. Furthermore, the presence of B and/or P compounds, which might be hydrolyzed in the presence of water vapor, can result in undesired corrosion downstream the reactor itself.

The catalyst of the present invention can operate in the absence of doping compounds continuously fed to the reactor, and no decrease in the catalytic activity and general performances (in terms of selectivity and yield in the desired product) is observed during at least 1.300 hours of reaction. The most important advantage of the catalyst system of the invention is that it is possible to operate at reaction temperatures lower than those used in literature; in fact the catalyst system of the invention effectively operates at temperatures ranging from 170° to 200°C, preferably at a temperature of 190°C, which is outside the temperature range claimed for example in EP 509,927.

The catalyst of the invention consists of aluminum oxide and phosphorous oxide present in the form of aluminum and phosphorous mixed oxide, Al/P/O, wherein the aluminum to phosphorous atomic ratio ranges from 0.40 to 1.0, preferably from 0.60 to 0.80. Contrary to the teaching of other patents disclosing materials based on phosphates, no further doping components have to be added to the catalyst composition to increase the stability and the activity of the catalyst itself.

The preparation of the catalytic material of the invention consists in coprecipitating an Al and P compound, precursor of the final catalyst. In general the preparation is carried out starting from a water-soluble aluminum compound and a water-soluble phosphorous compound. The two compounds are dissolved, then the elements are coprecipitated by changing the solution pH, thereby obtaining a precursor which is then filtered, dried and finally treated at high temperature in the air. The following procedure is given by way of example of the procedure used for the preparation of the catalyst of present invention, but is not intended to limit the scope of this invention. The most important conditions for the preparation of a suitable precursor for the preparation of a catalyst having useful characteristics for the monoetherification reaction under the conditions described in the present invention are: i) preparation of a homogeneous solution, and ii) coprecipitation of Al and P in the form of oxohydrates by suitably adjusting the solution pH. Any type of Al and P soluble compound fulfilling the above mentioned criteria can therefore be used for the purpose of the present invention.

Aluminum trichloride hexahydrate and 85% o-phosphoric acid are dissolved in water; in particular, the Al compound is dissolved in an HC1 aqueous acidic solution, keeping the solution at about room temperature (avoiding the hydrolysis of the compound and hence the precipitation of aluminum hydroxide). Said solution is then added with phosphoric acid and the mixed compound is coprecipitated by addition of ammonium hydroxide to pH about 7, then keeping this value. Alternatively, the solution containing Al and P can be added dropwise to a solution at pH 7 (for example containing ammonium acetate), keeping this pH by addition of ammonia or other alkali compounds.

The precipitate, which is generally (but not necessarily) in the form of gel, is preferably dried at 120-150°C, and finally calcinated in the air for some hours

(alternatively, the coprecipitate can be directly calcinated). Calcination is carried out at temperatures ranging from 400 to 800°C, preferably at 400-650°C, most preferably according to the following timetable: heating from room temperature to 280-320°C at an about 10°C / minute rate; staying at 280-320°C for 3-5 hours; further heating to 580-620°C at an about 10°C / minute rate; - staying at 580-620°C for 3-5 hours.

The catalyst final form can be any one suitable for use in a fixed bed reactor, such as extrudates, pellets or granules, prepared according to known technologies.

The described procedure is an example of thermal treatment, but these conditions do not limit the invention in any way. Intermediates temperatures, heating gradients and isotherm times may be suitably changed in order to optimize the thermal treatment depending on the prepared amount of catalyst.

Thermal treatment is necessary to promote removal of the volatile compounds, according to the known industrial practice. This is mandatory for the preparation of a catalyst characterized by high surface area.

Alternatively to ammonium hydroxide (or to gas ammonia), primary, secondary or tertiary amines with C_rC₄ alkyls, or also tetra-(C_r C₄)alkylammonium hydroxides, can be used as a precipitating agents.

This procedure provides a solid having specific surface area higher than 50 m²/g.

The main difference between the preparations described in literature is evident in the different procedure. The catalyst disclosed in EP 509,927 is prepared by dispersing aluminum hydroxide Al(OH)₃ in water, and refluxing the suspension (T 100°C) while stirring. The suspension is then added with a titanium oxide sol and a silica sol in the suitable amounts to reach the desired atomic ratio, and finally 85% phosphoric acid is added. The resulting suspension is stirred and heated for some hours, and the obtained paste is dried, molded and finally calcinated in the air at a temperature of 400°C. Thus, the procedure includes no complete dissolution of the Al reagent, and therefore the final compound is affected by the original characteristics of the used aluminum hydroxide. The surface area disclosed in the above mentioned patent ranges, after treatment at 400°C, from 30 to 50 m²/g, which is lower than that obtained according to the process of the invention.

In the catalyst of the present invention, in particular in the final compound after thermal treatment, the Al/P atomic ratio ranges from 0.5 to 1.0, preferably from 0.6 to 0.8. Lower or higher ratios worsen the catalyst performances, mainly in terms of selectivity to the desired product. Different ratios also result in remarkably lower life times, meant as times in which the catalytic action between two successive regenerations remains sufficiently stable. This is due to the disgregation of the catalyst particles (extrudate or pellet) and to deposition of carbon products or pitches on the surface of the catalyst itself. In the case of the catalyst of the present invention, a thermal regeneration treatment may be carried out when deactivation due to deposition of carbon products or pitches on the catalyst occurs. A typical regeneration treatment can be effected at temperatures from 450° to 650°C, more conveniently at 650°C, in air stream. At this temperature, the products which caused deactivation can be completely removed, whereas lower temperatures would not provide such complete removal thereof. A further advantage of the system of the invention is therefore that an effective regeneration can be carried out whenever needed, at a temperature similar to those used for calcination and sintering the catalyst itself. The advantage is that such regeneration treatment induces no changes in the characteristics compared with the fresh catalyst.

The reactivity of the catalyst prepared by etherification of diphenols or generally of polyhydroxybenzenes is analyzed with a laboratory flow reactor containing the catalyst in the desired form. In particular, reagents (diphenol and alcohol) are fed either together or separately to the reactor, optionally after evaporation in an evaporator. A carrier gas, preferably nitrogen, may be used to transport the reagents on the catalyst bed inside the reactor. The reactor can be made of stainless steel, glass or quartz. The temperature of the catalytic bed is kept at the desired level by means of a suitable heating system. The reaction temperature can range from 170 to 220°C, more conveniently from 180 to 200°C.

Reagents can be: i) as for polyhydroxybenzenes, catechol, hydroquinone or resorcinol (diphenols), or such reagents can have substituents such as methyl groups or halogen atoms on the ring, and ii) as for alcohols, methanol, ethanol, n- propanol, isopropanol, n-butanol, isobutanol and other alcohols of up to 6 carbon atoms, optionally unsaturated.

Tests were carried out loading 7.5 ml of catalyst in a laboratory micro- reactor, feeding 3 g/h of a liquid mixture consisting of 50% by weight of catechol and 50% of methanol, and at a gas flow rate of 35 Nml/min. The reaction temperature was varied from 190 to 200°C (Examples) and 275°C (Comparative Examples).

EXAMPLES Example 1 59.2 g of A1C1₃.6H₂0 are dissolved in 200 ml of 0.5 N HC1 (190 ml H₂0 +

10 ml 37% HC1) with stirring. 21 ml of 85% H₃P0₄ by weight are subsequently added. The still clear solution is then added dropwise with cone. NH₄OH (about 90 ml) to adjust pH to about 7. The formed precipitate is subsequently dried by suction on a filter to obtain a paste. The paste is then dried in a static dryer at 120°C for 12 hours to obtain a dry white mass, which is then treated in air stream at high temperature, using the following timetable: from room temperature to 300°C at 10°/min rate, keeping the temperature of 300°C for 4 hours, subsequent heating from 300 to 600°C at 10°/min rate, keeping the temperature of 600°C for 4 hours. The surface area of the resulting sample is 74 + 5 m² /g. The solid is then subjected to compression, granulation and sieving to obtain granules having average size ranging from 0.5 to 1 mm. The Al/P analytic ratio in the final catalyst, determined by plasma spectrometry, is 0.64 + 0.04. Example 2

The same procedure as in Example 1 is followed, but the amounts of A1C1₃.6H₂0 and H₃P0₄ are 20 g and 8.1 ml, respectively. The Al/P analytic ratio in the final catalyst, after thermal treatment, is 0.67 ± 0.04. Comparative example 3

The same procedure as in Example 1 is followed, but the amounts of A1C1₃.6H₂0 and H₃P0₄ are 90.0 g and 21.0 ml, respectively. The Al/P ratio in the final catalyst, after thermal treatment, is 0.95 ± 0.04. Example 4

The catalyst prepared as described in Example 1 was tested in the monoetherification reaction of catechol with methanol, under the reaction conditions described above. In particular, the reaction temperature was 190°C. The test results obtained over a life time of 1300 hours of reaction, are summarized in the Table.

Example 5 The catalytic tests are carried out on the catalyst prepared as described in

Example 1, under the same reaction conditions as in Example 4, except that the reaction temperature is 200°C.

Comparative example 6

The catalytic tests are carried out on the catalyst prepared as described in Example 1, under the same reaction conditions as in Example 4, except that the reaction temperature is 275°C.

Comparative example 7

The catalytic tests are carried out on the catalyst prepared as described in Example 2, under the same reaction conditions as in Example 4, except that the reaction temperature is 275°C.

Comparative example 8

The catalytic tests are carried out on the catalyst prepared as described in Example 3, under the same reaction conditions as in Example 4, except that the reaction temperature is 275°C.

TABLE

Catalytic reactivity tests in catechol monoalkylation

*Reaction time means the time elapsed from the starting of the catalytic tests. Example 9

The same procedure as in Example 5 is followed, using ethanol in place of methanol as the etherifying agent. Test results during a catalyst life time of 600 hours are: catechol conversion 24% selectivity to guetol 96%

From the above showed tests, it can be evinced that:

1) The catalyst described in Example 1 keeps its reactivity characteristics for at least 1300 reaction hours, with a conversion above 20% and selectivity to guaiacol of 99.1-99.2% (reactivity example 4). 2) The catalyst described in Example 1 shows a loss of selectivity to guaiacol when tested at temperatures above 200°C; more particularly, selectivity drops to 96.54%o (reactivity comparative example 6) at a temperature of 275°C (also reported in the examples of EP 509,927). 3) The catalyst described in Example 2, prepared with slightly different atomic ratio than that in Example 1, shows comparable performances, under the same experimental conditions, with those of the catalyst described in Example 1 (reactivity example 7). 4) The catalyst described in comparative example 3, characterized by

Al/P ratio substantially different from that of the catalysts of Examples 1 and 2, has remarkable worse catalytic performances than the catalysts of the present invention, mainly in terms of selectivity to guaiacol (reactivity comparative example 8). Example 10

The procedure of Example 4 is followed, using isopropanol in place of methanol as the etherifying agent, and operating under the following reaction conditions:

8.5 ml of catalyst loaded, feeding 5 g / h of a liquid mixture consisting of 6% by weight of catechol and 94% of isopropanol, transport gas flow rate of 25 N ml/min., reaction temperature 140°C.

Test results during a catalyst lifetime of 90 hours are: catechol conversion: 25% selectivity to monoether: 75% Example 11

The catalyst prepared as described in Example 1 was tested in the hydroquinone monoetherification reaction with methanol, under the same reaction conditions as in Example 4, except that the reaction temperature was 200°C, and feeding the hydroquinone/methanol mixture at a 1.2 g/hour rate. The test results during a catalyst life time of 15 hours of reaction, are: hydroquinone conversion: 17%> selectivity to hydroquinone monomethyl ether: 96%

Claims

1. Aluminum and phosphorous mixed oxide characterized by an Al : P atomic ratio ranging from 0.4:1.0 to 1: 1 and surface area ranging from 40 to 100 m²/g.

2. Aluminum and phosphorous mixed oxide as claimed in claim 1, characterized by an Al : P atomic ratio ranging from 0.6 : 1 to 0.8 : 1, and surface area higher than 50 m²/g.

3. A process for the preparation of an aluminum and phosphorous mixed oxide as claimed in claims l-2,comprising the following steps: a) coprecipitating an Al and P mixed compound from a solution of a water- soluble aluminum compound and a water-soluble phosphorous compound; b) drying the precipitate from step a) at a temperature ranging from 80 to 180°C; c) calcination in air stream the product from step b) by gradual heating from room temperature to a final temperature ranging from 400°C to 800°C.

4. A process as claimed in claim 3, characterized in that the precipitate from step a) is directly subjected to calcination.

5. A process as claimed in claims 3 and 4, characterized in that aluminum chloride and phosphoric acid are used as water-soluble Al and P compounds.

6. A process as claimed in claims 3 to 5, characterized in that coprecipitation is carried out by adjusting pH to about 7.

7. A process as claimed in claims 3 to 6, characterized in that coprecipitation is carried out by addition of a base selected from the group consisting of ammonia, primary, secondary and tertiary ( -C^alkylamines, the hydroxides thereof and (C_rC₄)tetralkylammonium hydroxides.

8. A process as claimed in claims 3, 5, 6 and 7, characterized in that the precipitate is dried at a temperature ranging from 100 to 150°C.

9. A process as claimed in claims 3 to 8, characterized in that calcination is carried out by gradually heating the product from step b) of claim 3 from room temperature to a final temperature ranging from 400 to 650°C.

10. A process as claimed in claims 3-7, characterized in that calcination is carried out according to the following timetable: - heating from room temperature to 280-320°C at an about 10°C / minute rate; staying at 280-320°C for 3-5 hours; further heating to 580-620°C at an about 10°C / minute rate; staying at 580-620°C for 3-5 hours.

11. A process for the preparation of polyhydroxybenzenes monoalkylethers optionally having alkyl groups or halogen atoms on the aromatic ring, by reaction with straight or branched, saturated or unsarurated, - alcohols, in gas, liquid or mixed phase, said reaction being catalyzed by an aluminum and phosphorous mixed oxide as claimed in claims 1-2.

12. A process as claimed in claim 11, characterized in that it is carried out at temperatures ranging from 170°C to 220°C.

13. A process as claimed in claim 12, characterized in that it is carried out at temperatures not exceeding 200°C.

14. A process as claimed in claims 11-13, characterized in that polyhydroxybenzene is selected from the group consisting of 1,2-, 1,3- and 1,4- dihydroxybenzene and the alcohol is selected from the group consisting of methanol, ethanol, propanol and isopropanol.