Catalyst which is meant for the hydrogen peroxide preparation, preparation process thereof and hydrogen peroxide preparation process

The present invention relates to a catalyst for the production of hydrogen peroxide in a process for obtaining this catalyst, as well as to a process for obtaining hydrogen peroxide by direct catalytic oxidation of hydrogen with oxygen.

Hydrogen peroxide is manufactured on an industrial scale using a process known as the RiedlPfleiderer process. According to this two-step process, anthracone dissolved in a solvent, so-called working solution, is circulated between an oxidation reactor and a hydrogenation reactor, to convert hydrogen plus oxygen into hydrogen peroxide. The variants of this process were based on the form of anthracone, the composition of the working solution and the type of catalyst used. A typical catalyst is palladium, Raney nickel or nickel boride on an inert substrate. The catalyst may be in the form of a suspension or a fixed bed. Hydrogen must be at high partial pressures in this reaction, which creates the risk of explosion. The process is characterized as complex and essentially intensive.

The process of oxidation of hydrogen with oxygen to hydrogen peroxide offers the possibility to elaborate a simpler and less expensive process. Processes of this nature have been developed, but they have not been applied industrially to date. The disadvantages of the procedures proposed so far are:

- low concentrations of the product;

- low selectivity (so high hydrogen consumption);

- low reaction rates;

- dangerous working conditions (in particular the requirements for partial hydrogen pressure within the explosive range), and

- high acid content.

Examples of such processes are given by the following patents, all of which refer to the catalytic conversion of hydrogen to oxygen in an aqueous acid medium.

US patent 4009252 belonging to Izumi and others shows good product concentrations (9 -12% H₂A₂ by weight) and worked at high acid concentrations (1 g / l KCI plus 49 g / l H₂S0₄), use palladium deposited on silicic acid and oxygen molar ratios per hydrogen of 1.5 to 20, even in the explosion range for hydrogen. The selectivity for the hydrogen peroxide ratio of hydrogen was good, with many examples in the range 80-89%. Reaction rates were generally low, ranging from less than 1 to even more than 6 g of hydrogen peroxide per liter-hour.

US patent 4611337 to Brill discloses high concentrations of hydrogen peroxide and high reaction rates using palladium deposited on coal in an aqueous solution containing 35 g / l HCl, working in a stirring reactor so as to maintain thickness of aqueous suspension at 2 mm or less. For example, 19.5% hydrogen peroxide concentrations were achieved at a flow rate of 48 g hydrogen per liter per hour using hydrogen at the partial pressure of 17 at and oxygen at the partial pressure of 51 at. but part of the advantage of the high reaction rates was lost, since most of the reaction vessel was empty. Also, the reaction conditions were in the explosive range of hydrogen.

US Patent 4772458 to Gosser et al (see also US Patent 4681751 and European Patent No. 0132294 to Gosser et al.) Refers to high concentrations and reaction rates, with moderate selectivity at low acid values ​​(less than 2, 5 g / l H₂S0₄) using metals from the Vlll group on a number of substrates, but at hydrogen concentrations of 17% or higher, making the process dangerous. The selectivities tended to be low, ranging from 30% to 70%, when the bromide ions were present in the reaction medium. If or used chloride ions, very low selectivity of about 6% was obtained. It turned out that the best results

RO 111175 Bl were obtained using a Pt / Pd catalyst in a ratio of 1:10 on an alumina support (1.10% total metal) with a hydrogen concentration of 17.8%. The concentration of hydrogen peroxide was 1 6.4% at the selectivity of 70% and the reaction rate was 52 g hydrogen peroxide per liter-hour.

The need for a direct oxidative process for the production of hydrogen peroxide appears, to obtain hydrogen peroxide in good concentrations and at high selectivity and reaction rates, while allowing the process to be conducted at low acid values ​​and below the explosive range. of hydrogen.

Also not known is the process for the preparation of Pt / carbon / polytetrafluoroethylene, Pt / styrene benzene / polystyrene and Pt / styrene benzene / teflon (RO 107824) catalysts. According to this process, hydrophobic substrates with a large specific surface type of styrene-divinyl benzene and particularly large type of activated or superactivated charcoal are used, which is impregnated with hexachloroplatinic acid dissolved in a solution consisting of two components, acetone in which hexaclorplatinic acid is also soluble benzene, wherein said acid is insoluble in a volume ratio of acetone: benzene of 1: 5. After impregnation the substrate is dried at a temperature below 65 ° C under vacuum.

This process has some disadvantages related to the fact that it is laborious and requires a vacuum drying technique, and on the other hand it leads to obtaining a platinum catalyst, which, when used to obtain hydrogen peroxide, does not ensure a very high selectivity.

The catalyst for the production of hydrogen peroxide, according to the invention, consists of:

- a partially hydrophobic, partially hydrophilic support;

- a metal in the Vlll group of the Periodic Element System and

- a source of sodium and chloride ions provided in situ in the process of obtaining the catalyst and additive.

The process for obtaining the catalyst according to the invention is that:

- the sodium citrate and the corresponding metal salt in the group III of the Periodic System of the elements in predetermined quantities are brought into contact, in aqueous solution,

- the reaction medium obtained by boiling the time required for the formation of the metal colloidal complex in the group III - sodium citrate, is heated,

- the colloidal solution is added the partially hydrophobic, partially hydrophilic, predetermined carbon support,

- the solution soaked in solid and finally evaporated

- the solid is heat treated in a hydrogen atmosphere at the appropriate temperature and sufficient time to reduce the metal complex in the group III to the metal form.

The process of obtaining hydrogen peroxide by direct oxidation of hydrogen with oxygen according to the invention consists in the following:

- an aqueous acid-containing medium containing hydrogen and oxygen is contacted with the catalyst comprising a partially hydrophobic, partially hydrophilic carbon support, a metal in the Vlll group of the Periodic Element System and a source of sodium and chloride ions, in -a pressure vessel;

- a source of sodium and chloride ions is introduced either, at the beginning of the reaction, or at the moment when a catalytic activity decrease, and

- kept in the reaction vessel:

- the pressure in the range between 3.5 and 20 MPa, at a partial pressure of the hydrogen below the explosion limit,

- the temperature within the freezing point of the aqueous medium and up to about 60 ° C.

The present invention has the following advantages:

- very selectivity is achieved

RO 111175 Bl high in the production of hydrogen peroxide, going up to 100%, under the conditions of good reaction rates and at the same time with the assurance of working at partial pressures of hydrogen below the explosive limit;

- the danger of explosion is eliminated;

- working at moderate values ​​of acidity in the reaction medium;

- the structure of the catalyst allows it to have a higher activity than those known in the prior art;

- the activity of the catalyst is maintained for a long time.

The present invention is further described in detail.

The invention is based on a series of findings made when studying the direct catalytic oxidation of hydrogen with oxygen in an aqueous medium, using a catalyst containing a metal of the Vlyl group on a support. Firstly, it was found that the nature of the catalytic support used is important. The classic supports used in prior art processes are either highly hydrophobic or highly hydrophilic.

In the present invention it has been appreciated that a hydrophilic / hydrophobic balance in the catalyst support (and thus the obtained catalyst) is desirable. The catalyst (as well as the catalyst support) must be partially hydrophobic so as to allow the gaseous reactants (hydrogen and oxygen) to contact the surface of the catalyst. however, the catalyst (and catalyst support) must also be partially hydrophilic, or partially wettable, so as to allow hydrogen peroxide formed on the surface of the catalyst to diffuse into the liquid phase. If hydrogen peroxide remains associated with the surface of the catalyst for a period of time, water is formed.

According to the invention, it has been established that this hydrophobic / hydrophilic balance is preferably achieved by using a fluorinated carbon support or a partially wettable Vulcan coal support. The degree of fluorination is preferably in the range of 10-65% F, more preferably 20-50% F.

A second finding was that the selectivity of the reaction for obtaining hydrogen peroxide could be increased by adding a source of chloride and sodium ions. This can be accomplished in the preparation phase of the catalyst, or by adding a source of these ions to the aqueous acid reaction medium. In fact, since these soluble ions are constantly removed with the aqueous reaction medium during the process, it is preferred to feed these ions into the aqueous environment throughout the process, or at least when a decrease in catalytic activity is observed. The most economical source of these ions is in the form of NaCl. Preferred quantities from 3% to 30% by weight, based on the catalyst.

By using the catalyst, it was observed that its catalytic activity, namely the preferred one, which is Pd on a fluorinated carbon support, decreased. Establishing that the degree of fluorination is important for the catalyst, an attempt was made to add a source of fluoride ions to the aqueous medium. This led to a third important finding, namely that the source of fluoride ions in the aqueous medium stabilizes the catalyst against decreasing catalytic activity. A convenient source of fluoride ions is NaF, which can be introduced in a proportion of 2 to 10% by weight based on the catalyst.

By producing a catalyst on the substrate, a fourth important finding was made. It has been established that it is preferable to suspend together the metal of the group III (preferably Pd) with sodium citrate in a solution such as aqueous. It is considered that this is how a complex Pd - sodium citrate or a colloid is formed, with two important consequences. When the catalyst support is impregnated with the Pd - sodium citrate complex, the metal is strongly retained by the support and well distributed on the surface of the support. In the preferred embodiment of the present invention, this technique for preparing the catalyst also provides sodium and chloride ions

EN 111175 Bl is desired in the catalyst, the sodium being provided by the sodium citrate and chloride ions of the metal salt of the IIIll group, which is a chloride (for example PdCl₂) which is initially suspended with sodium citrate.

The combination of all the above findings led to the process for the preparation of hydrogen peroxide according to the invention, which can be achieved with good concentrations of H₂A₂ / 5-6%), at high selectivity (up to 100%) and with a good reaction rate (5-11 g / l / hour H₂0₂), at the same time with the possibility of working at hydrogen pressures below the explosion limit and also at moderate acidities (for example 6 g / l H₂S0₄).

The preferred embodiments of the present invention are described below.

A sufficiently catalytic amount of metal of the IIIll group is used in the catalyst according to the invention. Metals such as Pd, Ru, Rh, Ir are catalytically active for the production of hydrogen peroxide, but Pd is the preferred metal. Mixtures of metals from the III group can also be used. The metal is generally used in the form of a salt, preferably a chloride, such as PdCl₂.

The metal in the group III is used as a catalyst on the support, the catalyst support being partially hydrophobic, partly hydrophilic, as described below.

The support must have a surface area of ​​between 50 m²/ g and 1500 m²/ G. It was found that a surface area of ​​130 m²/ g is appropriate. Preferably, the support is used as fine particles or granules (particle size smaller than 1 micron being adequate), but it can also be stored on other support materials, such as ceramic balls or rings, which is known in the art.

As stated above, the catalyst support (and the obtained catalyst) must have a hydrophobic / hydrophilic balance, which allows gaseous reactants (H_of + 0₂) reach the surface of the catalyst (in the aqueous environment) and at the same time allow H₂0₂ formed to be released in the aqueous environment. Highly hydrophobic catalyst supports, known in the art, are not suitable. Hydrophobicity is often defined by the contact angle according to Young's theory. A catalyst support having a contact angle of 90 ° is usually accepted as a hydrophobic catalyst support. The catalyst supports according to the present invention have a contact angle of less than 90 °.

The preferred catalyst supports according to the present invention are two, namely, prefluorinated coal and Vulcan (IG) coal.₂/ 94 p. 2391 GCA) partially wettable.

According to the above, the degree of fluorination affects the hydrophobic / hydrophilic nature of the catalyst. A fluorination degree of 10-65% is preferred. Optim is a fluoridation degree of 20-50%, asserting that it is sufficient 28% F. The partially wettable Vulcan coal is a specially treated activated carbon (Cabot company, US).

The catalyst is preferably manufactured by first preparing a complex or colloid of the metal of the group III with sodium citrate. This ensures a stronger metal grip on the catalyst support. For this purpose, the sodium citrate and the metal of the IIIll group are suspended in a solution, for example in water, and heated to form the colloid. The heating must be done at the boiling point and lasts at least 6 hours. The amount of metal in the Vlll group used must be sufficient to obtain about 0.1 -1% by weight in the final catalyst. for Pd, a proportion of 0.7% by weight in the catalyst is sufficient.

The catalyst support is impregnated with the colloidal metal solution. A reagent is preferably added to the metal colloidal suspension as a catalyst support, to decrease the density of the suspension and to decrease the tendency of the catalyst support to float to the surface. For this purpose

Methanol is suitable. After suspension, the solution is evaporated and the catalyst is reduced to a hydrogen atmosphere (preferably 14 hours at

3oo ° C [.

In accordance with the preferred embodiment described above, the catalyst normally contains the desired sodium and chloride ions, which have been found to subsequently enhance the production of H₂0₂. Sodium is given by sodium citrate, and the chloride ion is given by PdCl₂. When prepared in this manner, the catalyst can be used initially without adding NaCl to the reaction medium.

The process for producing hydrogen peroxide, according to the invention, is preferably carried out in a pressure reactor, under stirring, such as an autoclave, through which a flow of suspension is passed, at temperatures between the freezing point of the liquid medium and approx. 6O ° C, preferably at O ​​... 25 ° C. Since the reaction is strongly exothermic, it is generally necessary to cool to these temperatures.

The reactor is preferably charged with the catalyst and additives (NaCl and NaF, if desired) before adding the aqueous acid solution. As shown above, these additives can be added later during the reaction, when the catalyst activity begins The NaCl additive is preferably added in a proportion of 3-30% by weight (relative to the catalyst) and the NaF additive is preferably added in a proportion of 2-5% by weight (relative to the catalyst).

The acid solution is preferably a weak acid solution. A solution of H is convenient₂SO,₄ An acid concentration of 0.5-1% w / w H is appropriate₂S0₄. It has been found that higher concentrations of acid do not improve the process.

Thereafter, oxygen and hydrogen gas are charged to said reactor. An important advantage of the process for obtaining hydrogen peroxide, according to the invention, is that it can be carried out at a partial pressure of hydrogen, below the explosive limit. This limit is considered to be the highest percentage of hydrogen in the reaction atmosphere which will indicate an explosive range when measured with a standard MSA explosive meter. Usually a partial pressure of H is used₂ below about 4% by volume. The total pressure in the reactor is in the range from

3.5 MPa to 20 MPa, the preferred range being from 6.7 MPa to 10 MPa. The oxygen may be introduced in pure form, or preferably in admixture with nitrogen. Low oxygen content, such as air, may be used. A preferred reactor feed stream contains 3.2% H₂, 10% N₂ and 86.8% 0₂.

The reaction can be carried out continuously or in batches. Since NaCl and NaF are water-soluble additives, they must be added continuously because they are removed from the system.

Eleven concrete examples of the invention are given below.

Preparation of the catalyst

Example 1. Dissolve 8.07 g of sodium citrate in 807 ml of water, to which was added 56 ml of PdCl₂ 6.7 x 10 '³M. This mixture is then diluted with 403 ml of water. The mixture is heated to boiling for 10 hours to form a colloidal solution of Pd - sodium citrate. To this is added 2 g of fluorinated carbon (with 28% fluoride content, average particle size less than 1 micron, surface area of ​​130 m³/ g), together with 100 ml of methanol. The solution is evaporated and the solid is reduced to hydrogen for 14 hours at 300 ° C. The obtained catalyst contains about 0.7% Pd. The catalyst is partially wettable, black, in slightly sticky powder form.

Example 2. Other catalysts are prepared, according to the process described in example 1, with similar fluorinated carbon supports in all other respects, but with a content of 10% and 65% respectively.

Example 3. Another catalyst is prepared according to the process

RO 111175 Bl described in Example 1, but using a partially wettable Vulcan coal support (Cabot company, USA, Vulcan 9 A

CS-329).

Production of hydroxide peroxide 5 gen

Example 4. A 450 ml autoclave, under stirring, through which a suspension passes, is charged, as follows, with: 0.3 g catalyst (from Example 1], 0.03 g NaCl, 50 ml H₂SO_d0.6% by weight. The autoclave is placed in a cooling bath maintained at 0 ° C. Hydrogen gas and oxygen gas are introduced into the autoclave and the pressure rises to 6.7 MPa with a total gas flow rate of 300 ml / minute (3.2% by volume H₂, 10% N₂ and 86.8% O₂), under energetic stirring. The product conversion and selectivity are analyzed after 1, 3, 6 and 10 h. The gas phase is analyzed by gas chromatography on the technological flow with a thermal conductivity detector. Argon is used as a support gas for analysis. H₂, N₂ and 0₂ from the gas supply stream is separated by a stainless steel column with 80-100 mesh Porapak QS fill (9901550 mesh / cm²].

The product is titrated with potassium permanganate to determine quantitative H₂0₂ formed. The titration equation is:

5H₂0₂ + 2KMnO₄ + -------) 2MnS0₄ + KfS0₄ + 8 H₂0 + 50₂

Concentration H₂0₂ is measured directly by titration and confirmed by UV spectroscopy. Convert H₂ is calculated as the ratio of initial H content ₂ - content measured by H ₂ content initially by H ₂

Selectivity H₂0₂ is calculated by considering that if all H₂ reacted to H₂0₂, the selectivity is 100%, as follows:

, H _P measured selectivity H _P0 _P = · —— -------- x 100

H ₂0 ₂ calculated where:

3.2% x F xtx conversion H _P% in

H_P0_P calculated = ------------------------— x 34 x ^{2 2} '22.4 50 where: F = gas flow t = reaction time

The results are presented in table 1.

Table 1

<td>Reaction time, hours</td><td>HpOp concentration,% w / w</td><td>Convert H₂ %</td><td>selectivity HPO_?%</td>

<td> 1</td><td> 1,1</td><td> 70</td><td> 84</td>

<td> 3</td><td> 2,3</td><td> 61</td><td> 73</td>

<td> 6</td><td> 3,8</td><td> 58</td><td> 63</td>

<td> 10</td><td> 5,4</td><td> 52</td><td> 59</td>

RO 111175 Bl

Example 5. This example illustrates the results of the production of H₂0₂without the NaCl additive. The catalyst obtained after a few passes according to Example 4 is thoroughly washed and filtered 5 to remove NaCl. When this washed catalyst is then used to produce H₂A₂ (same conditions as in Example 4, but without addition of NaCl), the results for this catalyst after 10 hours are 1.32% w / w H₂0₂, convert H₂ of 25.5% and selectivity H₂0₂ of 30%.

Example 6 in this example illustrates the stabilizing effect of NaF. The process of producing H is repeated₂0₂as shown in Ex ample 4. Without addition of NaF, after 8 days of reaction, conversion H₂ drops to 33%. When NaF is added to the aqueous medium in the amount of 0.01 g, conversion H₂ after 8 days it is 44%.

Example 7. The importance of hydrophobic / hydrophilic balance in the catalyst support is illustrated in this example. The catalyst in example 2 containing 10% F and 65% F is subjected to reaction conditions similar to those in example 4, with the following results after 10 hours:

Table 2

<td>o / o F</td><td>Concentration H₂0₂% by weight</td><td>Convert H₂%</td><td>Selectivity H₂0₂ %</td>

<td>10% F</td><td> 2,1</td><td> 25</td><td> 66</td>

<td>65% F</td><td> 2,2</td><td> 31</td><td> 38</td>

Example 8. This example 2o illustrates the effect of the variation of the amount of NaCl added in the reaction medium. The catalyst in example 1 (0.7 wt% Pd on fluorinated carbon support) is used in reaction under 25 conditions similar to those in example 4 (□, 3 g catalyst, 50 ml H₂S0₄ 1% by weight, varying amounts of NaCl, 3.2% H₂, 10.0% N₂ and the remainder 0₂, Q ° C, 6.7 MPa, 300 ml / minute gas, reaction time 10 hours), the results are presented in table 3.

Table 3

<td>NaCl g</td><td>Concentration H₂0₂% by weight</td><td>Convert H₂ %</td><td>Selectivity H₂A₂ %</td>

<td> 0,0117</td><td> 5,83</td><td> 61</td><td> 53</td>

<td> 0,0306</td><td> 5,83</td><td> 53</td><td> 60</td>

<td> 0,0500</td><td> 5,86</td><td> 53</td><td> 61</td>

<td> 0,1008</td><td> 5,79</td><td> 48</td><td> 69</td>

Example 9. This example 35 illustrates the effect of the variation of the amount of NaF added in the reaction medium. Repeat the procedure of Example 4, but with 0.0261 g NaCl and 0.0054 g NaF. After 6 hours, 4% by weight H is obtained₂0₂, con- 4 0 version H₂ is 61% and H selectivity₂A₂ is 23.6%.

Example 10. This example shows that NaBr and KBr do not bring similar enhancements to NaCl and NaF additives according to the present invention. Repeat the procedure of Example 4, using 0.0361 g KBr instead of NaCl (the acid solution is 1% by weight H₂S0₄). After 10 hours, 1.1% by weight H is obtained₂0₂, convert H₂ is about 4% and selectivity H₂0₂

RO 111175 Bl is estimated at 100%. Repeat the same procedure with 0.0308 g NaBr instead of NaCl. After 10 hours of reaction 1.1% by weight H is obtained₂0₂, convert H₂is about 3% (below the limit of 5 gas chromatography detection) and selectivity H₂0₂ is estimated at about 100%.

Example 11. This example illustrates the production of H₂0₂ with another catalyst support, partially wettable Vulcan coal. With the catalyst from example 3, the reaction is carried out under the conditions of example 4, obtaining the results from table 4.