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1. WO2005063620 - PROCEDE DESTINE A ELIMINER UN ACIDE SULFURIQUE PRESENT DANS DES SOLUTIONS PRINCIPALEMENT A BASE D'ALCOOL DE H2O2 PROVENANT D'UNE SYNTHESE DIRECTE

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PROCESS FOR THE REMOVAL OF SULFURIC ACID PRESENT IN ESSENTIALLY ALCOHOLIC SOLUTIONS OF H202 COMING FROM DIRECT SYNTHESIS
The present invention relates to a process for producing a solution of hydrogen peroxide purified to a high purity in a prevalently alcohol solvent.
More specifically, the present invention relates to a process for the removal of sulfuric acid and metal impuri-ties, present in essentially alcohol solutions of H202 coming from direct synthesis, by means of precipitation.
Hydrogen peroxide is an extremely effective oxidizing agent with the advantage of not having an environmental impact as the only by-product due to its use is water. The demand for H202 is predicted to increase in the next few years, mainly for its use in the synthesis of base chemical products or for new applications such as, for example, the desulfuration of gas oils .
In the synthesis of a base product such as propylene oxide via H202, described in international patent applica- tions WO 02/14297 and WO 02/14298, there is the advantage, for example, of eliminating the chlorine derivatives or the coproduction of other products such as styrene or ter-butyl alcohol, which, on the other hand, characterize the current synthesis of this product.
In the ammoximation of cyclo-hexanone with H0 , the coproduction of salts is avoided, as described in U.S. patent 4,794,198, whereas in the hydroxylation of benzene to phenol, as described in U.S. patent 6,133,487, the process is simplified, i.e. the coproduction of acetone in the synthesis of phenol from cumene, is avoided.
The production on an industrial scale of aqueous solutions of H20 by means of a complex two-step process, is known.
In this process, a solution of an anthraquinone, such as butyl anthraquinone or ethyl anthraquinone, in an organic medium immiscible with water, is first hydrogenated and then oxidized with air to produce H202 which is subsequently extracted in aqueous phase, concentrated by distil-lation and purified.
This process, however, suffers from substantial disadvantages deriving from the necessity of operating with large volumes of reagents, the numerous steps required, the relatively high cost of the intermediates and production of inactive by-products.

An alternative method to the synthesis of H202 via anthraquinone is the method defined direct synthesis from H2 and 02. The reaction takes place in the presence of a catalyst, consisting of palladium and platinum, optionally sup-ported on an inert solid and a solvent mainly based on alcohol and water. Examples of direct synthesis processes of H02 are cited, for example, in international patent application WO 02/14217 and in US patents 6,630,118 and 6,649,140.
The solution of H202 produced by direct synthesis can be used directly in the integrated processes mentioned above, when the solvent and levels of impurities, in particular the hydrogen peroxide acid reaction promoters , are compatible with the downstream processes. This means se-lecting suitable operating conditions during the synthesis, as described in U.S. patent 6,284,213, or the solution produced by direct synthesis must be subjected to a purification treatment. If a concentrated aqueous solution is to be obtained starting from the solution obtained from direct synthesis, a separation of the solvent and concentration of H20 must be combined with the purification treatment, as described in WO 02/14217.
The impurities present in the solutions produced by direct synthesis can be classified on the basis of their origin. Noble metals, in particular Pd and Pt, derive from the dissolution, by the action of acid promoters present in solution, of the active phase of the hydrogen peroxide synthesis catalysts. Acids are present in the H202 solution as they are added in the synthesis phase as reaction promot-ers. The main representative of this group is sulfuric acid or nitric, hydrobromic, hydrochloric and phosphoric acids. Heavy metals can derive from the chemical attack of the acid solution of H202 on the surfaces of metallic materials with which the apparatus is constructed. In this group of metals, Fe, Cr, Ni, Mn, Cu, Zn, Mo and W, can be mentioned.

The Applicant has now found that it is possible to proceed in a simple and convenient way for the removal of impurities and inorganic acids, particularly H2S04, of the

H20 solution produced in an essentially alcoholic solvent, with direct synthesis from hydrogen and oxygen, by treating the latter with carbonates and/or bicarbonates and/or oxides and/or hydroxides of alkaline and/or alkaline earth metals. It has been observed that, thanks to this addition, there is not only the almost complete precipitation (resid-ual H2S04 < 30 ppm, generally < 10 ppm) of the acids present in the form of salts corresponding to the alkaline and/or alkaline earth metals, but, surprisingly, the impurities of a metallic nature (heavy metals) present in the solution are also removed from the solution.
Solutions purified by precipitation are more stable than the original ones .
An object of the present invention therefore relates to a process for the removal of impurities, and in particular sulfuric acid, present in H202 solutions coming from direct synthesis starting from hydrogen and oxygen fed to a reactor containing a palladium and platinum catalyst in dispersion in an essentially alcoholic liquid reaction medium containing a promoter consisting of sulfuric acid and a halogen, which comprises:
a) adding to the solution of H202 an at least stoichiometric quantity, with respect to the sulfuric acid present, of carbonates and/or bicarbonates and/or oxides and/or hydroxides of alkaline and/or alkaline earth metals;
b) removing the solid precipitated from the liquid phase.
According to the present invention, the removal of the impurities can be effected in a specific apparatus starting from solutions obtained by direct synthesis through any process using at least one inorganic acid, such as sulfuric acid, as reaction promoting agent and in the presence of a solvent selected from C1-C4 alcohols, preferably ethanol . Examples of processes of this type are described in international patent applications WO 02/14217, WO 02/92501, WO 02/92502 WO 03/14014 and in U.S. patents 6,630,118 and 6,649,140.
For removing the sulfate ion, introduced in the syn- thesis phase, from the solutions of H202, it has been found that it is sufficient to put the solution in contact with carbonates and/or bicarbonates and/or oxides and/ hydroxides of alkaline metals, such as sodium or potassium, alka-line earth metals, such as calcium, strontium, barium. In this way there is the precipitation of the sulfate ion in the form of sulfates of the alkaline and/or earth alkaline metals added. With this addition, other ions present also precipitate, in relation to the relative solubility prod-ucts , and in addition the impurities of a metallic nature such as, for example, one or more of the following metals: Pd, Pt, Fe, Cr, Ni , Mn, Cu, Zn, Mo and W, present in the solution, are also removed from the solution.
The precipitating reagent, on the basis of its phys-ico-chemical characteristics (solubility) , can be added dissolved in water, in an alcohol solvent or directly as a finely subdivided solid. In the case of K2C03, which has a high solubility in water and is commercially available as an aqueous solution at 47% by weight, the addition is pref-erably effected by means of an aqueous solution.
In the case of precipitating reagents which are only slightly soluble or insoluble in water, the contact can take place directly between the finely dispersed solid and the solution to be purified which, being acid, allows the attack of the precipitating agent and its consumption in the neutralization reaction and precipitation of sulfates . This is the case, for example, of barium oxide or calcium carbonate .
The quantity of precipitating reagent is evaluated on the basis of the stoichiometry of the main reaction and the desired degree of removal. The dosage is preferably stoichiometric to avoid having an excess basicity in the purified solution, which could cause instability of the hydrogen peroxide. In the case of insoluble precipitating re-agents, however, the dosage can also be higher than the stoichiometric value.
The time and contact procedure between the precipitating reagent and solution to be purified are also linked to the type of precipitating reagent. The time can be short, in the order of minutes, in the case of the dosage of a dissolved reagent, for example in aqueous phase, or longer, in the order of several hours, in the case of solid precipitating reagents .
The time necessary can be calculated from a continuous measurement of the pH of the solution/suspension which, from initial characteristic values of high acidity, generally pH < 2, tends to become stable at values close to neutrality, pH = 5-7, indexes of the practically complete removal of the free acidity. With the stabilization of the pH, the precipitation can be considered as being complete.

The separation of the precipitate from the purified solution can take place by decanting, and removal of the supernatant, or by filtration or centrifugation.
The removal of the sulfuric acid is also correlated, in addition to the pH of the solution and its composition, to the temperature. Suitable temperatures are those lower than 50°C, preferably from 10 to 40°C.
Contemporaneously with the removal of the sulfuric acid by precipitation, the process, object of the present invention, allows an almost total purification of the solution from heavy metals as these metals can be removed to overall residual values in the purified effluent lower than 20 ppb. The purification therefore proves to be surprisingly effective for several groups of contaminants forming solutions of H202 with an improved stability with respect to the original solutions .
A purification step on a bed of resins, with anionic and/or cationic exchange, can also be added to the precipitation process, on the basis of the necessities of the op-erations which can be effected downstream on the purified solution, for example to remove the minimum quantities of alkaline or alkaline earth ions, such as Na+, K+ or Ca++, which are introduced into the solution with the precipitating reagent .
The solution of hydrogen peroxide purified according to the process object of the present invention can be used as such in integrated processes, for example for the production of epoxides or in ammoximation processes, or treated, for example by distillation, to recover an aqueous solution of H02 at a concentration ranging from 15 to 60% by weight .
Some illustrative and non-limiting examples are provided hereunder for a better -understanding of the 'present invention and for its embodiment.
REFERENCE EXAMPLE
A hydro-alcohol solution of H202 at 7% by weight is prepared according to the process described in Example 5 of U.S. patent 6,649,140. The reaction solvent consists of MeOH/H20 in a ratio of 95/5 and contains H2S04 and HBr as reaction promoters . The direct synthesis effluent of hydrogen peroxide from hydrogen and oxygen has the following macroscopic composition (Table 1) .
Table 1



Analysis by means of ionic chromatography showed the presence of the following acids (Table 2) Table 2



Analysis via ICP-MS showed the presence of the elements indicated in the following table (Table 3)
Table 3



The stability of the solution of H202 was characterized by monitoring its concentration with time, by periodically removing aliquots of solution preserved at 25°C in a Pyrex container, in the dark and subjecting them to poten-tiometric titration via permanganate. Over a period of 3500 hours of observation, the decomposition rate of H202 expressed as follows :
v[%/h] = (initial H202 [weight %] - final H202 [weight

%] /initial H202 [weight %] ) • 100/time [h]
proved to be 0.0037 %/h.

EXAMPLE 1 (precipitation as K2S04)
An aqueous solution at 16% by weight of K2C03 in a stoichiometric quantity with respect to the H2S04 present in the solution, is added dropwise, under stirring in about 5 minutes, to 200 g of the solution described in the reference example, placed in a Pyrex flask. The mixture is left under stirring for a further 10 minutes, the suspension is left to decant and is filtered after 16 hours. The following results were observed on the liquid separated (Table 4) .
Table 4



The decomposition rate of the hydrogen peroxide in the pu-rified solution, evaluated as in Example 1, proved to be 0 . 00006%/h .
EXAMPLE 2 (precipitation as BaS04)
BaO in powder form is added, under stirring, in a quantity equal to 1.2 times the stoichiometric quantity with respect to the H2S0 present in the solution, to 80 g of the solution described in the reference example, placed in a Pyrex flask. The suspension is left under stirring for about 30 minutes, after which it is left to decant and is then filtered on paper. The following results were observed on the liquid separated (Table 5) .
Table 5



The decomposition rate of the hydrogen peroxide in the purified solution, evaluated as in Example 1, proved to be 0.00005%/h.

EXAMPLE 3 (precipitation as CaS04)
CaC03 in powder form is added, under stirring, in a quantity equal to 2 times the stoichiometric quantity with respect to the H2S04 present in the solution, to 80 g of the solution described in the reference example, placed in a Pyrex flask. The suspension is left under stirring for about 60 minutes, after which it is left to decant and is then filtered on paper. The following results were observed on the liquid separated (Table 6) .
Table 6



The decomposition rate of the hydrogen peroxide in the purified solution, evaluated as in Example 1, proved to be 0.00005%/h.