Process for the oxidation of organic carbonyl compounds

Abstract

A process for the oxidation of an organic carbonyl compound comprising reacting the compound, optionally in the presence of a solvent, with hydrogen peroxide in the presence of a catalyst comprising a tin-containing zeolitic material and at least one potassium salt.

Claims

1. A catalytic system comprising a catalyst comprising a tin-containing zeolitic material having a BEA structure and at least one potassium salt, wherein the at least one potassium salt is selected from the group consisting of at least one inorganic potassium salt, at least one organic potassium salt, and combinations of at least one inorganic potassium salt and at least one organic potassium salt wherein the at least one inorganic potassium salt is selected from the group consisting of potassium hydroxide, potassium halides, potassium nitrate, potassium sulfate, potassium hydrogen sulfate, potassium hydrogen phosphate, potassium dihydrogen phosphate, and potassium perchlorate, the at least one organic potassium salt is selected from the group consisting of potassium carbonate, potassium hydrogen carbonate, potassium salts of aliphatic saturated carboxylic acids having from 1 to 6 carbon atoms tricarboxylic acids having from 4 to 10 carbon atoms or tetracarboxylic acids, and wherein the tin-containing zeolitic material has a tin content in the range from 0.2 to 20 weight-%, based on the total weight of the tin-containing zeolitic material.

2. The catalytic system of claim 1, wherein the tin-containing zeolitic material has a tin content in the range of from 0.1 to 25 weight-% based on the total weight of the tin-containing zeolitic material.

3. The catalytic system of claim 1, wherein the tin-containing zeolitic material has a tin content in the range of from 0.4 to 15 weight-%, based on the total weight of the tin-containing zeolitic material.

4. The catalytic system of claim 1, wherein at least 95 weight of the zeolitic framework structure consist of SiO.sub.2, B.sub.2O.sub.3 and Sn.

5. The catalytic system of claim 4, wherein at least 99.9 weight-% of the zeolitic framework structure consist of SiO.sub.2, B.sub.2O.sub.3 and Sn.

6. The catalytic system of claim 1, wherein the catalyst is the tin-containing zeolitic material in the form of a powder or spray-powder, or is a molding comprising the tin-containing zeolitic material, wherein the tin-containing zeolitic material is comprised in the molding, and if a powder then at least 90 weight-% of the powder or spray-powder consist of the tin-containing zeolitic material.

7. The catalytic system of claim 1, wherein the catalyst is the tin-containing zeolitic material in the form of a powder or spray-powder, wherein at least 99.9 weight-% of the powder or spray-powder consist of the tin-containing zeolitic material.

8. A process for the oxidation of an organic carbonyl compound of formula (I) ##STR00017## wherein R.sub.1 and R.sub.2 are independently from one another a linear or branched alkyl residue, a linear or branched alkenyl residue, an aryl or heteroaryl residue, or a hydrogen atom with the proviso that R.sub.1 and R.sub.2 are not simultaneously a hydrogen atom, said process comprising (i) providing a liquid mixture comprising the compound of formula (I), hydrogen peroxide, at least one at least partially dissolved potassium salt, and optionally a solvent; (ii) reacting the compound of formula (I) with the hydrogen peroxide in the liquid mixture in the presence of the catalyst comprising tin containing zeolite material wherein the catalyst and the potassium salt form the catalytic system according to claim 1, to provide a compound of formula (II) ##STR00018## wherein, if neither R.sub.1 nor R.sub.2 is a hydrogen atom, R.sub.1 and R.sub.2 may form, together with the carbonyl group or the carboxyl group, a ring and the compound of formula (I) is ##STR00019## and the compound of formula (II) is ##STR00020##

9. The catalytic system of claim 1, wherein at least 98% by weight of the zeolitic framework structure consist of SiO.sub.2, B.sub.2O.sub.3 and Sn.

10. The catalytic system of claim 1, wherein the catalytic system is obtained by contacting the zeolitic material with a liquid feed stream comprising said potassium salt.

11. The catalytic system of claim 1, wherein at least 99% by weight of the zeolitic framework structure consist of SiO.sub.2, B.sub.2O.sub.3 and Sn.

12. The catalytic system of claim 1, wherein at least 99.5% by weight of the zeolitic framework structure consist of SiO.sub.2, B.sub.2O.sub.3 and Sn.

13. The catalytic system of claim 1, wherein at least 99.8% by weight of the zeolitic framework structure consist of SiO.sub.2, B.sub.2O.sub.3 and Sn.

Description

EXAMPLES

Reference Example A

Determination of the Water Uptake

(1) The water uptake of the zeolitic materials is determined by water adsorption/desorption isotherms which were performed on a VTI SA instrument from TA Instruments following a step-isotherm program. The experiment consisted of a run or a series of runs performed on a sample material that has been placed on the microbalance pan inside of the instrument. Before the measurement was started, the residual moisture of the sample was removed by heating the sample to 100 C. (heating ramp of 5 C./min) and holding it for 6 h under a nitrogen flow. After the drying program, the temperature in the cell was decreased to 25 C. and kept isothermal during the measurement. The microbalance was calibrated, and the weight of the dried sample was balanced (maximum mass deviation 0.01 weight-%). Water uptake by the sample was measured as the increase in weight over that of the dry sample. First, as adsorption curve was measured by increasing the relative humidity (RH) (expressed as weight-% water in the atmosphere inside of the cell) to which the sample was exposed and measuring the water uptake by the sample as equilibrium. The RH was increased with a step of 10 weight-% from 5% to 85% and at each step the system controlled the RH and monitored the sample weight until reaching the equilibrium conditions after the sample was exposed from 85 weight-% to 5 weight-% with a step of 10% and the change in the weight of the sample (water uptake) was monitored and recorded.

Reference Example B

Determination of the Crystallinity

(2) B.1 Crystallinity of Zeolitic Materials Having MWW Framework Structure

(3) The crystallinity of the zeolitic materials according to the present invention was determined by XRD analysis. The data are collected using a standard Bragg-Brentano diffractometer with a Cu-X-ray source and an energy dispersive point detector. The angular range of 2 to 70 (2 theta) is scanned with a step size of 0.02, while the variable divergence slit is set to a constant illuminated sample length of 20 mm. The data are then analyzed using TOPAS V4 software, wherein the sharp diffraction peaks are modeled using a Pawley fit containing a unit cell with the following starting parameters: a=14.4 Angstrom and c=25.2 Angstrom in the space group P6/mmm. These are refined to fit the data. A linear background is modelled. Independent peaks are inserted at the following positions. 8.4, 22.4, 28.2 and 43. These are used to describe the amorphous content. The crystalline content describes the intensity of the crystalline signal to the intensity to the amorphous content.

(4) B.2 Crystallinity of Zeolitic Materials Having BEA Framework Structure

(5) The crystallinity of the zeolitic materials according to the present invention was determined by XRD analysis using the EVA method as described in the User Manual DIF-FRAC.EVA Version 3, page 105, from Bruker AXS GmbH, Karlsruhe. The respective data were collected on a standard Bruker D8 Advance Diffractometer Series II using a Sol-X detector, from 2 to 50 2theta, using variable slits (V20), a step size of 0.02 2theta and a scan speed of 2.4 s/step. Default parameters were used for estimating the background/amorphous content (Curvature=1, Threshold=1).

Reference Example C

FT-IR Measurements

(6) The FT-IR (Fourier-Transformed-Infrared) measurements were performed on a Nicolet 6700 spectrometer. The powdered material was pressed into a self-supporting pellet without the use of any additives. The pellet was introduced into a high vacuum (HV) cell placed into the FT-IR instrument. Prior to the measurement the sample was pretreated in high vacuum (10.sup.5 mbar) for 3 h at 300 C. The spectra were collected after cooling the cell to 50 C. The spectra were recorded in the range of 4000 to 800 cm.sup.1 at a resolution of 2 cm.sup.1. The obtained spectra are represented in a plot having on the x axis the wave-number (cm.sup.1) and on the y axis the absorbance (arbitrary units, a.u.). For the quantitative determination of the peak heights and the ratio between these peaks a baseline correction was carried out. Changes in the 3000-3900 cm.sup.1 region were analyzed and for comparing multiple samples, as reference the band at 18805 cm.sup.1 was taken.

Reference Example 1

Preparation of Tin-Containing Zeolitic Materials Having MWW Framework Structure

Reference Example 1.1

Preparation of Tin-Containing Zeolitic Materials Having MWW Framework Structure Via Incorporation of Tin by Hydrothermal Synthesis

Reference Example 1.1.1

Preparation of a Tin-Containing Zeolitic Material Having MWW Framework Structure and Having a Sn Content of 0.46 Weight-% Via Incorporation of Tin by Hydrothermal Synthesis

(7) (i) Preparation of a B-MWW

(8) 480 kg de-ionized water were provided in a vessel. Under stirring at 70 rpm (rounds per minute), 166 kg boric acid were suspended in the water at room temperature. The suspension was stirred for another 3 h at room temperature. Subsequently, 278 kg piperidine were added, and the mixture was stirred for another hour. To the resulting solution, 400 kg Ludox AS-40 were added, and the resulting mixture was stirred at 70 rpm for another hour at room temperature. The finally obtained mixture was transferred to a crystallization vessel and heated to 170 C. within 5 h under autogenous pressure and under stirring (50 rpm). The temperature of 170 C. was kept essentially constant for 120 h. During these 120 h, the mixture was stirred at 50 rpm. Subsequently, the mixture was cooled to a temperature of from 50-60 C. The aqueous suspension containing B-MWW precursor had a pH of 11.3 as determined via measurement with a pH-sensitive electrode. From said suspension, the B-MWW precursor was separated by filtration. The filter cake was then washed with de-ionized water at room temperature until the washing water had a conductivity of less than 700 microSiemens/cm. The thus obtained filter cake was subjected to spray-drying in a spray-tower with the following spray-drying conditions:

(9) TABLE-US-00001 drying gas, nozzle gas: technical nitrogen temperature drying gas: temperature spray tower (in): 235 C. temperature spray tower (out): 140 C. nozzle: top-component nozzle: supplier Gerig; size 0 nozzle gas temperature: room temperature nozzle gas pressure: 1 bar operation mode: nitrogen straight apparatus used: spray tower with one nozzle configuration: spray tower - filter - scrubber gas flow: 1,500 kg/h filter material: Nomex needle-felt 20 m.sup.2 dosage via flexible tube pump: SP VF 15 (supplier: Verder)

(10) The spray tower was comprised of a vertically arranged cylinder having a length of 2,650 mm, a diameter of 1,200 mm, which cylinder was conically narrowed at the bottom. The length of the conus was 600 mm. At the head of the cylinder, the atomizing means (a two-component nozzle) were arranged. The spray-dried material was separated from the drying gas in a filter downstream of the spray tower, and the drying gas was then passed through a scrubber. The suspension was passed through the inner opening of the nozzle, and the nozzle gas was passed through the ring-shaped slit encircling the opening. The spray-dried material was then subjected to calcination at 600 C. for 10 h. The calcined material had a molar ratio B.sub.2O.sub.3:SiO.sub.2 of 0.06:1.

(11) (ii) Deboronation

(12) 9 kg of de-ionized water and 600 g of the calcined zeolitic material obtained according to Example 1 (i) were refluxed at 100 C. under stirring at 250 r.p.m. for 10 h. The resulting deboronated zeolitic material was separated from the suspension by filtration and washed with 4 l deionized water at room temperature. After the filtration, the filter cake was dried at a temperature of 120 C. for 16 h. The dried zeolitic material having an MWW framework structure had a B.sub.2O.sub.3:SiO.sub.2 molar ratio of 0.0020:1.

(13) (iii) Incorporation of Sn

(14) 776.25 g deionized water were provided in a glass beaker and 375 g piperidine were added under stirring. To this suspension 1.45 g of Sn(OAc).sub.2 (Sn(II) acetate) was added and the suspension stirred for another 10 minutes. 172.4 g zeolitic material obtained according to (ii) were added to the mixture and stirred for 20 min (200 r.p.m.) at room temperature. The obtained suspension was than filled in an autoclave. The mixture was treated for 48 h at a temperature of 170 C. under stirring (100 r.p.m.). Afterwards the autoclave was cooled down to room temperature and the resulting zeolitic material was separated from the suspension by filtration at room temperature and washed with deionized water until the washing water had a conductivity of less than 200 microSiemens/cm. After the filtration, the filter cake was dried at a temperature of 120 C. for 16 h. The dried zeolitic material had a Si content of 40 weight-% and a Sn content of 0.42 weight-%.

(15) (iv) Acid Treatment

(16) 173.4 g zeolitic material obtained according to (iii) were provided in a round bottom flask and 5,202 g of a 30 weight-% HNO.sub.3 aqueous solution, having a pH in the range from 0 to 1, were added. The mixture was stirred at a temperature of 100 C. for a period of 20 h (200 r.p.m.) under reflux. The suspension was filtered and the filter cake was then washed with de-ionized water at room temperature until the washing water had a pH of approximately 7. The obtained zeolitic material was dried at 120 C. for 16 h and calcined by heating to 550 C. (2 C. per minute) and subsequent heating at 550 C. for 10 h. The dried and calcined zeolitic material hat a Si content of 47 weight-% and a Sn content of 0.46 weight-% and a c parameter as determined via XRD of 26.91 Angstrom. The crystallinity of the zeolitic material determined according to XRD was 89%. Further, the zeolitic material had a BET surface area, determined according to DIN 66131 of 520 m.sup.2/g, and a Langmuir surface, determined according to DIN 66131 of 713 m.sup.2/g. Furthermore, the obtained zeolitic material had a X-ray diffraction pattern comprising peaks at 2 theta diffraction angles of (6.60.1), (7.10.1), (7.90.1), (9.60.1), (12.80.1), (14.40.1), (14.70.1), (15.80.1), (19.30.1), (20.10.1), (21.70.1), (21.90.1), (22.60.1), (22.90.1), (23.60.1), (25.10.1), (26.1OA), (26.90.1), (28.6OA), and (29.10.1).

Reference Example 1.1.2

Preparation of a Tin-Containing Zeolitic Material Having MWW Framework Structure and Having a Sn Content of 0.46 Weight-% Via Incorporation of Tin by Hydrothermal Synthesis, and Preparation of a Molding

(17) (i) Preparation of B-MWW

(18) 480 kg de-ionized water were provided in a vessel. Under stirring at 70 rpm (rounds per minute), 166 kg boric acid were suspended in the water at room temperature. The suspension was stirred for another 3 h at room temperature. Subsequently, 278 kg piperidine were added, and the mixture was stirred for another hour. To the resulting solution, 400 kg Ludox AS-40 were added, and the resulting mixture was stirred at 70 rpm for another hour at room temperature. The finally obtained mixture was transferred to a crystallization vessel and heated to 170 C. within 5 h under autogenous pressure and under stirring (50 rpm). The temperature of 170 C. was kept essentially constant for 120 h. During these 120 h, the mixture was stirred at 50 rpm. Subsequently, the mixture was cooled to a temperature of from 50-60 C. The aqueous suspension containing B-MWW had a pH of 11.3 as determined via measurement with a pH-sensitive electrode. From said suspension, the B-MWW was separated by filtration. The filter cake was then washed with de-ionized water at room temperature until the washing water had a conductivity of less than 500 microSiemens/cm. The thus obtained filter cake was subjected to spray-drying in a spray-tower with the following spray-drying conditions:

(19) TABLE-US-00002 drying gas, nozzle gas: technical nitrogen temperature drying gas: temperature spray tower (in): 235 C. temperature spray tower (out): 140 C. nozzle: top-component nozzle supplier Gerig; size 0 nozzle gas temperature: room temperature nozzle gas pressure: 1 bar operation mode: nitrogen straight apparatus used: spray tower with one nozzle configuration: spray tower - filter - scrubber gas flow: 1,500 kg/h filter material: Nomex needle-felt 20 m.sup.2 dosage via flexible tube pump: SP VF 15 (supplier: Verder)

(20) The spray tower was comprised of a vertically arranged cylinder having a length of 2,650 mm, a diameter of 1,200 mm, which cylinder was conically narrowed at the bottom. The length of the conus was 600 mm. At the head of the cylinder, the atomizing means (a two-component nozzle) were arranged. The spray-dried material was separated from the drying gas in a filter downstream of the spray tower, and the drying gas was then passed through a scrubber. The suspension was passed through the inner opening of the nozzle, and the nozzle gas was passed through the ring-shaped slit encircling the opening. The spray-dried material was then subjected to calcination at 650 C. in a rotary oven in contra current flow (0.8-1 kg/h). The calcined material had a B content of 1.4 weight-%, a Si content of 43 wt. %, and TOC of less than 0.1 wt. %. The material had a BET specific surface area, measured according to DIN 66131, of 468 m.sup.2/g.

(21) (ii) Deboronation

(22) 1,590 kg of de-ionized water and 106 kg of the calcined material obtained from (i) were refluxed at 100 C. under stirring at 70 r.p.m. for 10 h. The resulting deboronated zeolitic material was separated from the suspension by filtration and washed 4 times with 150 l deionized water at room temperature. After the filtration, the filter cake was dried at a temperature of 120 C. for 16 h. The dried zeolitic material having an MWW framework structure had a B content of 0.04 weight-%, a Si content of 42 weight-%, and a BET specific surface area, measured according to DIN 66131, of 462 m.sup.2/g.

(23) (iii) Incorporation of Sn

(24) 776.25 g deionized water were provided in a glass beaker and 375 g piperidine were added under stirring. To this suspension 2.5 g of Sn(IV)butoxyde pre-dissolved in 25 g piperidine were added and the suspension was stirred for another 10 minutes. 172.4 g deboronated zeolitic material obtained according to (ii) above were added to the mixture and stirred for 60 min (200 r.p.m.) at room temperature. The obtained suspension was than filled in an autoclave. The mixture was treated for 120 h at a temperature of 170 C. under stirring (100 r.p.m.). Afterwards the autoclave was cooled down to room temperature and the resulting zeolitic material was separated from the suspension by filtration at room temperature and washed with deionized water until the washing water had a conductivity of less than 200 microSiemens/cm. After the filtration, the filter cake was dried at a temperature of 120 C. for 16 h. The dried zeolitic material had a Si content of 40 weight-% and a Sn content of 0.42 weight-%.

(25) (iv) Acid Treatment

(26) 174 g tin containing zeolitic material obtained from (iii) above were provided in a round bottom flask and 5,220 kg of a 30 weight-% HNO.sub.3 aqueous solution, having a pH in the range from 0 to 1, were added. The mixture was stirred at a temperature of 100 C. for a period of 20 h (200 r.p.m.). The suspension was filtered and the filter cake was then washed with de-ionized water at room temperature until the washing water had a pH of approximately 7. The obtained zeolitic material was dried at 120 C. for 16 h and calcined by heating to 550 C. (2 K/min) and subsequent heating at 550 C. for 10 h. The dried and calcined zeolitic material had a Si content of 49 weight-% and a Sn content of 0.46 weight-% and a c parameter as determined via XRD of 27.1 Angstrom. The crystallinity of the zeolitic material determined according to XRD was 86%. Further, the zeolitic material had a BET surface area, determined according to DIN 66131, of 521 m.sup.2/g, a Langmuir surface, determined according to DIN 66131 of 695 m.sup.2/g.

(27) (v) Preparation of a Molding

(28) 140 g of the zeolitic calcined zeolitic material obtained from (iv) and 8.4 g Walocel were kneaded for 5 min in an edge mill. During kneading, 82.6 g Ludox AS-40 were added continuously. After 10 min, the addition of 150 ml de-ionized water was started. After another 30 min, die kneading mass was adjusted by addition of 30 ml de-ionized water. After a total kneading time of 50 min, the mass is extrudable, and the mass was extruded at a pressure of from 100 to 150 bar during 1 min. The obtained strands were dried at 120 C. for 8 h in an oven and calcined at 500 C. for 5 h. 137.2 g of white strands were obtained, having a diameter of 1.7 mm. The dried and calcined material in the form of said strands had a Si content of 46 weight-%, a Sn content of 0.41 weight-% and TOC of 0.01 weight-%. The crystallinity of the zeolitic material determined according to XRD was 78%. Further, the strands had a BET surface area, determined according to DIN 66131, of 412 m.sup.2/g, and a pore volume determined by Hg porosimetry of 0.91 ml/g.

Reference Example 1.2

Preparation of Tin-Containing Zeolitic Materials Having MWW Framework Structure Via Incorporation of Tin by Solid-State Ion Exchange

Reference Example 1.2.1

Preparation of a Tin-Containing Zeolitic Material Having MWW Framework Structure and Having a Sn Content of 12.8 Weight-% Via Incorporation of Tin by Solid-State Ion Exchange

(29) (i) Preparation of B-MWW

(30) 480 kg de-ionized water were provided in a vessel. Under stirring at 70 rpm (rounds per minute), 166 kg boric acid were suspended in the water at room temperature. The suspension was stirred for another 3 h at room temperature. Subsequently, 278 kg piperidine were added, and the mixture was stirred for another hour. To the resulting solution, 400 kg Ludox AS-40 were added, and the resulting mixture was stirred at 70 rpm for another hour at room temperature. The finally obtained mixture was transferred to a crystallization vessel and heated to 170 C. within 5 h under autogenous pressure and under stirring (50 r.p.m.). The temperature of 170 C. was kept essentially constant for 120 h. During these 120 h, the mixture was stirred at 50 rpm. Subsequently, the mixture was cooled to a temperature of from 50-60 C. The aqueous suspension containing B-MWW had a pH of 11.3 as determined via measurement with a pH-sensitive electrode. From said suspension, the B-MWW was separated by filtration. The filter cake was then washed with de-ionized water at room temperature until the washing water had a conductivity of less than 500 microSiemens/cm. The thus obtained filter cake was subjected to spray-drying in a spray-tower with the following spray-drying conditions:

(31) TABLE-US-00003 drying gas, nozzle gas: technical nitrogen temperature drying gas: temperature spray tower (in): 235 C. temperature spray tower (out): 140 C. nozzle: top-component nozzle supplier Gerig; size 0 nozzle gas temperature: room temperature nozzle gas pressure: 1 bar operation mode: nitrogen straight apparatus used: spray tower with one nozzle configuration: spray tower - filter - scrubber gas flow: 1,500 kg/h filter material: Nomex needle-felt 20 m.sup.2 dosage via flexible tube pump: SP VF 15 (supplier: Verder)

(32) The spray tower was comprised of a vertically arranged cylinder having a length of 2,650 mm, a diameter of 1,200 mm, which cylinder was conically narrowed at the bottom. The length of the conus was 600 mm. At the head of the cylinder, the atomizing means (a two-component nozzle) were arranged. The spray-dried material was separated from the drying gas in a filter downstream of the spray tower, and the drying gas was then passed through a scrubber. The suspension was passed through the inner opening of the nozzle, and the nozzle gas was passed through the ring-shaped slit encircling the opening. The spray-dried material was then subjected to calcination at 650 C. in a rotary oven in countercurrent flow (0.8-1 kg/h). The calcined material had a B content of 1.4 weight-%, a Si content of 43 weight-%, and a TOC (total organic carbon) of less than 0.1 weight-%. The crystallinity of the material, as determined via XRD, was 88%, and the BET specific surface area measured according to DIN 66131 was 468 m.sup.2/g.

(33) (ii) Deboronation

(34) 1590 kg of de-ionized water and 106 kg of the calcined material obtained according 2.1 above were refluxed at 100 C. under stirring at 70 r.p.m. for 10 h. The resulting deboronated zeolitic material was separated from the suspension by filtration and washed 4 times with 150 l deionized water at room temperature. After the filtration, the filter cake was dried at a temperature of 120 C. for 16 h. The dried zeolitic material having an MWW-type framework structure had a B content of 0.04 weight-%, a Si content of 42 weight-%, a crystallinity determined via XRD of 82% and a BET specific surface area of 462 m.sup.2/g.

(35) (iii) Incorporation of Sn

(36) 30 g of the deboronated zeolitic material obtained according to (ii) were added in a Mixer (mill type Microton MB550) together with 8.9 g Sn(OAc).sub.2 (tin(II) acetate, CAS-Nr:638-39-1, Sigma-Aldrich). The two components were milled together for 15 minutes with a stirring rate of 14,000 r.p.m. (rounds per minute). Afterwards, 10.8 g of the thus obtained powder were transferred to a porcelain holder and dried at 120 C. for 10 h.

(37) (iv) Acid Treatment

(38) 330 g of nitric acid (30 weight-%) and 11 g of the dried zeolitic material obtained from (iii) were added under stirring in a 0.5 l glass round bottom flask. The mixture in the vessel was heated to 100 C. and kept at this temperature under autogenous pressure for 20 h under stirring (200 r.p.m.). The thus obtained mixture was then cooled within 1 h to a temperature of less than 50 C. The cooled mixture was subjected to filtration, and the filter cake was washed with deionized water until a pH of 7 was reached. The filter cake was dried for 10 h at 120 C. and calcined at 550 C. for 10 h (heating ramp 2 K/min). A zeolitic material was obtained having a Sn content of 12.6 weight-%, a Si content of 36.5 weight-% and a TOC of less than 0.1 weight-%. The BET specific surface area determined according to DIN 66131 was 385 m.sup.2/g, and the crystallinity determined according to XRD was 87%.

Reference Example 1.2.2

Preparation of a Tin-Containing Zeolitic Material Having MWW Framework Structure and Having a Sn Content of 12.3 Weight-% Via Incorporation of Tin by Solid-State Ion Exchange

(39) (i) Preparation of B-MWW

(40) 480 kg de-ionized water were provided in a vessel. Under stirring at 70 rpm (rounds per minute), 166 kg boric acid were suspended in the water at room temperature. The suspension was stirred for another 3 h at room temperature. Subsequently, 278 kg piperidine were added, and the mixture was stirred for another hour. To the resulting solution, 400 kg Ludox AS-40 were added, and the resulting mixture was stirred at 70 rpm for another hour at room temperature. The finally obtained mixture was transferred to a crystallization vessel and heated to 170 C. within 5 h under autogenous pressure and under stirring (50 r.p.m.). The temperature of 170 C. was kept essentially constant for 120 h. During these 120 h, the mixture was stirred at 50 rpm. Subsequently, the mixture was cooled to a temperature of from 50-60 C. The aqueous suspension containing B-MWW had a pH of 11.3 as determined via measurement with a pH-sensitive electrode. From said suspension, the B-MWW was separated by filtration. The filter cake was then washed with de-ionized water at room temperature until the washing water had a conductivity of less than 500 microSiemens/cm. The thus obtained filter cake was subjected to spray-drying in a spray-tower with the following spray-drying conditions:

(41) TABLE-US-00004 drying gas, nozzle gas: technical nitrogen temperature drying gas: temperature spray tower (in): 235 C. temperature spray tower (out): 140 C. nozzle: top-component nozzle supplier Gerig; size 0 nozzle gas temperature: room temperature nozzle gas pressure: 1 bar operation mode: nitrogen straight apparatus used: spray tower with one nozzle configuration: spray tower - filter - scrubber gas flow: 1,500 kg/h filter material: Nomex needle-felt 20 m.sup.2 dosage via flexible tube pump: SP VF 15 (supplier: Verder)

(42) The spray tower was comprised of a vertically arranged cylinder having a length of 2,650 mm, a diameter of 1,200 mm, which cylinder was conically narrowed at the bottom. The length of the conus was 600 mm. At the head of the cylinder, the atomizing means (a two-component nozzle) were arranged. The spray-dried material was separated from the drying gas in a filter downstream of the spray tower, and the drying gas was then passed through a scrubber. The suspension was passed through the inner opening of the nozzle, and the nozzle gas was passed through the ring-shaped slit encircling the opening. The spray-dried material was then subjected to calcination at 650 C. in a rotary oven in countercurrent flow (0.8-1 kg/h). The calcined material had a B content of 1.4 weight-%, a Si content of 43 weight-%, and a TOC (total organic carbon) of less than 0.1 weight-%. The crystallinity of the material, as determined via XRD, was 88%, and the BET specific surface area measured according to DIN 66131 was 468 m.sup.2/g.

(43) (ii) Deboronation

(44) 1590 kg of de-ionized water and 106 kg of the calcined material obtained according to (i) were refluxed at 100 C. under stirring at 70 r.p.m. for 10 h. The resulting deboronated zeolitic material was separated from the suspension by filtration and washed 4 times with 150 l deionized water at room temperature. After the filtration, the filter cake was dried at a temperature of 120 C. for 16 h. The dried zeolitic material having an MWW-type framework structure had a B content of 0.04 weight-%, a Si content of 42 weight-%, a crystallinity determined via XRD of 82% and a BET specific surface area of 462 m.sup.2/g.

(45) (iii) Incorporation of Sn

(46) 120 g of the deboronated zeolitic material obtained according to (ii) were added in a Mixer (mill type Microton MB550) together with 34 g Sn(OAc).sub.2 (tin(II) acetate, CAS-Nr:638-39-1, Sigma-Aldrich). The two components were milled together for 15 minutes with a stirring rate of 14,000 r.p.m. (rounds per minute). Afterwards, 28 g of the thus obtained powder were transferred to a porcelain holder and calcined in a static oven for 3 h at 500 C., heating rate 2 K/min. The calcined powder had the following elemental composition: Sn 11.5 weight-%, Si 35 weight-% and TOC of less than 0.1 weight-%. The BET specific surface area determined according to DIN 66131 was 392 m.sup.2/g, and the crystallinity determined via XRD was 79%.

(47) (iv) Acid Treatment

(48) 1800 g of nitric acid (30 weight-%) and 60 g of the calcined zeolitic material obtained from (iii) were added under stirring in a 2.0 l glass round bottom flask. The mixture in the vessel was heated to 100 C. and kept at this temperature under autogenous pressure for 20 h under stirring (200 r.p.m.). The thus obtained mixture was then cooled within 1 h to a temperature of less than 50 C. The cooled mixture was subjected to filtration, and the filter cake was washed with deionized water until a pH of 7 was reached. The filter cake was dried for 10 h at 120 C. and calcined at 550 C. for 5 h (heating ramp 2 K/min). A material with a Sn content of 12.3 weight-%, a Si content of 37 weight-%, and a TOC of less than 0.1 weight-% was obtained. The BET specific surface area determined according to DIN 66131 was 400 m.sup.2/g, and the crystallinity determined via XRD was 84%.

Reference Example 2

Preparation of a Tin-Containing Zeolitic Material Having BEA Framework Structure and a Sn Content of 9.6 Weight-% Via Incorporation of Tin by Solid-State Ion Exchange

(49) (i) Preparation of B-BEA

(50) 209 kg de-ionized water were provided in a vessel. Under stirring at 120 rpm (rounds per minute), 355 kg tetraethylammonium hydroxide were added and the suspension was stirred for 10 minutes at room temperature. Thereafter, 61 kg boric acid were suspended in the water and the suspension was stirred for another 30 minutes at room temperature. Subsequently, 555 kg Ludox AS-40 were added, and the resulting mixture was stirred at 70 rpm for another hour at room temperature. The liquid gel had a pH of 11.8 as determined via measurement with a pH electrode. The finally obtained mixture was transferred to a crystallization vessel and heated to 160 C. within 6 h under a pressure of 7.2 bar and under stirring (140 rpm). Subsequently, the mixture was cooled to room temperature. The mixture was again heated to 160 C. within 6 h and stirred at 140 rpm for additional 55 h. The mixture was cooled to room temperature and subsequently, the mixture was heated for additional 45 h at a temperature of 160 C. under stirring at 140 rpm. 7800 kg de ionized water were added to 380 kg of this suspension. The suspension was stirred at 70 rpm and 100 kg of a 10 weight-% HNO.sub.3 aqueous solution was added. From this suspension the boron containing zeolitic material having a BEA framework structure was separated by filtration. The filter cake was then washed with de-ionized water at room temperature until the washing water had a conductivity of less than 150 microSiemens/cm. The thus obtained filter cake was subjected to drying in a nitrogen stream.

(51) The thus obtained zeolitic material was subjected to spray-drying in a spray-tower with the following spray-drying conditions:

(52) TABLE-US-00005 drying gas, nozzle gas: technical nitrogen temperature drying gas: temperature spray tower (in): 235 C. temperature spray tower (out): 140 C. nozzle: top-component nozzle supplier Gerig; size 0 nozzle gas temperature: room temperature nozzle gas pressure: 1 bar operation mode: nitrogen straight apparatus used: spray tower with one nozzle configuration: spray tower - filter - scrubber gas flow: 1,500 kg/h filter material: Nomex needle-felt 20 m.sup.2 dosage via flexible tube pump: SP VF 15 (supplier: Verder)

(53) The spray tower was comprised of a vertically arranged cylinder having a length of 2,650 mm, a diameter of 1,200 mm, which cylinder was conically narrowed at the bottom. The length of the conus was 600 mm. At the head of the cylinder, the atomizing means (a two-component nozzle) were arranged. The spray-dried material was separated from the drying gas in a filter downstream of the spray tower, and the drying gas was then passed through a scrubber. The suspension was passed through the inner opening of the nozzle, and the nozzle gas was passed through the ring-shaped slit encircling the opening. The spray-dried material was then subjected to calcination at 500 C. for 5 h. The calcined material had a B.sub.2O.sub.3:SiO.sub.2 molar ratio of 0.045, a total carbon content of (TOC) 0.08 weight-%, a crystallinity determined by XRD of 56%, ad BET specific surface area determined by DIN 66131 of 498 m.sup.2/g.

(54) (ii) Deboronation

(55) 840 kg de-ionized water were provided in a vessel equipped with a reflux condenser. Under stirring at 40 rpm, 28 kg of the spray-dried and calcined zeolitic material obtained from (i) were employed. Subsequently, the vessel was closed and the reflux condenser put into operation. The stirring rate was increased to 70 rpm. Under stirring at 70 rpm, the content of the vessel was heated to 100 C. within 1 h and kept at this temperature for 20 h. Then, the content of the vessel was cooled to a temperature of less than 50 C. The resulting deboronated zeolitic material having a BEA framework structure was separated from the suspension by filtration under a nitrogen pressure of 2.5 bar and washed four times with deionized water at room temperature. After the filtration, the filter cake was dried in a nitrogen stream for 6 h. The obtained deboronated zeolitic material was subjected to spray-drying under the conditions as described in 5.1. The obtained zeolitic material had a B.sub.2O.sub.3:SiO.sub.2 molar ratio of less than 0.002, a water uptake of 15 weight-%, a crystallinity determined by XRD of 48% and a BET specific surface area determined by DIN 66131 of 489 m.sup.2/g.

(56) (iii) Incorporation of Sn

(57) 25 g of the deboronated zeolitic material having a BEA framework structure obtained from (ii), were added to a mixer (mill type Microton MB550) together with 5.5 g of tin(II) acetate (Sn(OAc).sub.2 [CAS-Nr:638-39-1]), and the mixture was milled for 15 minutes with 14,000 r.p.m. (rounds per minute). After the milling, the mixture was transferred to a porcelain basket and calcined in air at 550 C. for 5 h, with a heating ramp of 2 K/min. The obtained powder material had a Sn content of 9.6 weight-%, a silicon (Si) content of 38 weight-%, and a TOC of less than 0.1 weight-%. The BET specific surface area measured by DIN 66131 was 423 m.sup.2/g, the crystallinity determined by XRD 51%, and the water uptake 18 weight-%. The UV/Vis spectrum showed two maxima, one at a wavelength of 200 nm and a second around 250 nm. In the FT-IR spectrum the intensity ratio between a first adsorption band with a maximum between 3701 to 3741 cm.sup.1 and a second adsorption with the maximum between 3600 to 3690 cm.sup.1 was 1.49.

(58) (iv) Acid Treatment

(59) 10 g zeolitic material obtained from (iii) were provided in a round bottom flask and 300 g of a 30 weight-% HNO.sub.3 aqueous solution, having a pH in the range of from 0 to 1, were added. The mixture was stirred at a temperature of 100 C. for a period of 20 h (200 r.p.m.). The suspension was filtered and the filter cake was then washed with de-ionized water at room temperature until the washing water had a pH of approximately 7. The obtained zeolitic material was dried at 120 C. for 10 h and calcined by heating to 550 C. (2 K/min) and subsequent heating at 550 C. for 10 h. The dried and calcined zeolitic material had a Si content of 36 weight-%, a Sn content of 9.3 weight-% and a crystallinity determined via XRD of 53%. Further, the zeolitic material had a BET specific surface area, determined according to DIN 66131, of 380 m.sup.2/g and a water uptake of 6 weight-%. The UV/Vis spectrum showed two maxima, one at a wavelength of 208 nm and a second around 250 nm. In the FT-IR spectrum the intensity ratio between a first adsorption band with a maximum between 3701 to 3741 cm.sup.1 and a second adsorption with the maximum between 3600 to 3690 cm.sup.1 was 0.93.

Example 1

Baeyer-Villiger Oxidation of Cyclohexanone to Epsilon-Caprolactone Using a Sn-MWW Having a Sn Content of 0.46 Weight-% and 1,2-Dichloroethane as Solvent with Addition of Potassium Salts

(60) General Procedure

(61) A 100 ml glass flask vessel was charged with 1.5 g cyclohexanone, 1.2 g zeolitic material obtained according to Reference Example 1.1.1, having a Sn content of 0.46 weight-% and 45 g dichloroethane. The mixture was heated to reflux (95 C.). An aqueous solution of 0.5 g H.sub.2O.sub.2 (70 weight-%) was added and the reaction was stirred for 4 h. After cooling down to room temperature, the solution was filtrated and analyzed with respect to the selectivities to epsilon-caprolactone, based on cyclohexanone and based on hydrogen peroxide, by quantitative GC analysis using di-n-butyl ether as internal standard.

Example 1.1

Baeyer-Villiger Oxidation with Addition of Potassium Dihydrogen Phosphate (KH2PO4) as Potassium Salt

(62) Example 1.1 was carried out according to the General Procedure above, wherein the aqueous solution of hydrogen peroxide additionally contained 360 micromol KH.sub.2PO.sub.4 per mol H.sub.2O.sub.2. The selectivities to epsilon-caprolactone, based on cyclohexanone and based on hydrogen peroxide, are shown in Table 1 below.

Example 1.2

Baeyer-Villiger Oxidation with Addition of Potassium Dihydrogen Phosphate (KNO3) as Potassium Salt

(63) Example 1.2 was carried out according to the General Procedure above, wherein the aqueous solution of hydrogen peroxide additionally contained 360 micromol KNO.sub.3 per mol H.sub.2O.sub.2. The selectivities to epsilon-caprolactone, based on cyclohexanone and based on hydrogen peroxide, are shown in Table 1 below.

Example 1.3

Baeyer-Villiger Oxidation with Addition of Potassium Formate (KCO2H) as Potassium Salt

(64) Example 1.3 was carried out according to the General Procedure above, wherein the aqueous solution of hydrogen peroxide additionally contained 360 micromol KCO.sub.2H per mol H.sub.2O.sub.2. The selectivities to epsilon-caprolactone, based on cyclohexanone and based on hydrogen peroxide, are shown in Table 1 below.

Comparative Example 1

Baeyer-Villiger Oxidation without Addition of a Potassium Salt

(65) Comparative Example 1.3 was carried out as described in the General Procedure in Example 1 above. The selectivities to epsilon-caprolactone, based on cyclohexanone and based on hydrogen peroxide, are shown in Table 1 below.

Example 2

Continuous-Type Baeyer-Villiger Oxidation of Cyclohexanone to Epsilon-Caprolactone Using a Shaped Sn-MWW Having a Sn Content of 0.46 Weight-% and Acetonitrile as Solvent with Addition of KH2PO4

(66) General Procedure

(67) A tubular reactor (length: 1.4 m, internal diameter: 7 mm) equipped with a jacket for thermostatization was charged with 15 g of the catalyst obtained according to reference example 1.1.2 above in the form of strands with a diameter of 1.7 mm. The remaining reactor volume was filled with inert material (steatite spheres, 2 mm in diameter, to a height of about 5 cm at the lower end of the reactor and the remainder at the top end of the reactor). The reactor was thermostatized by passing a heat transfer medium, a mixture of water and ethylene glycol, through the jacket. The heat transfer medium was fed at the lower end of the jacket so that it flew in concurrent mode to the reactor contents. The temperature of the heat transfer medium at the entrance of the jacket is defined as the reaction temperature. The flow rate of the heat transfer medium was adjusted on that the difference between entrance and exit temperature was at most 1 K. The pressure in the reactor was controlled by a suitable pressure control valve and maintained constant at 20 bar (abs). The reactor feed stream was metered by using a metering pump. The stream consisted of a mixture of acetonitrile (93.6 weight-%), cyclohexanone (2.5 weight-%), an aqueous hydrogen peroxide solution with a concentration of 40 weight-% (3.9 weight-%) (flow rate: 40 g/h). Under the conditions used the feed was liquid and only one liquid phase was present. The experiment was performed in a continuous manner. At the start of the run (t=0 is defined at which the metering pump was started) the reaction temperature was set to 90 C. After a certain period of time (usually within 4 hours on stream) a stationary state was reached. The reactor effluent after the pressure control valve was collected, weighed and analyzed by GC using di-n-butylether as internal standard.

(68) Specific Procedure

(69) Example 2 was carried out as described in the General Procedure above, wherein the hydrogen peroxide solution used for the preparation of the feed stream additionally contained potassium dihydrogen phosphate in an amount of 360 micromol KH.sub.2PO.sub.4 per 1 mol hydrogen peroxide. The selectivities to epsilon-caprolactone, based on cyclohexanone and based on hydrogen peroxide, are shown in Table 1 below.

Comparative Example 2

Baeyer-Villiger Oxidation without Addition of a Potassium Salt

(70) Comparative Example 2 was carried out as described in the General Procedure in Example 2 above. The selectivities to epsilon-caprolactone, based on cyclohexanone and based on hydrogen peroxide, are shown in Table 1 below.

Example 3

Baeyer-Villiger Oxidation of Cyclohexanone to Epsilon-Caprolactone Using a Sn-MWW Having a Sn Content of 12.8 Weight-% and 1,2-Dichloroethane as Solvent with Addition of KH2PO4

(71) General Procedure

(72) A 100 ml glass flask vessel was charged with 3 g cyclohexanone, 0.1 g zeolitic material obtained according to Reference Example 1.2.1, having a Sn content of 12.8 weight-% and 90 g dichloroethane. The mixture was heated to reflux (95 C.). An aqueous solution of 0.98 g H.sub.2O.sub.2 (70 weight-%) was added and the reaction was stirred for 4 h. After cooling down to room temperature, the solution was filtrated and analyzed with respect to the selectivities to epsilon-caprolactone, based on cyclohexanone and based on hydrogen peroxide, by quantitative GC analysis using di-n-butyl ether as internal standard.

(73) Specific Procedure

(74) Example 3 was carried out as described in the General Procedure above, wherein the aqueous solution of hydrogen peroxide additionally contained 360 micromol KH.sub.2PO.sub.4 per mol H.sub.2O.sub.2. The selectivities to epsilon-caprolactone, based on cyclohexanone and based on hydrogen peroxide, are shown in Table 1 below.

Comparative Example 3

Baeyer-Villiger Oxidation without Addition of a Potassium Salt

(75) Comparative Example 3 was carried out as described in the General Procedure in Example 3 above. The selectivities to epsilon-caprolactone, based on cyclohexanone and based on hydrogen peroxide, are shown in Table 1 below.

Example 4

Baeyer-Villiger Oxidation of Cyclohexanone to Epsilon-Caprolactone Using a Sn-MWW Having a Sn Content of 12.3 Weight-% and 1,2-Dichloroethane as Solvent with Addition of KCO2H

(76) General Procedure

(77) A 100 ml glass flask vessel was charged with 1.5 g cyclohexanone, 0.1 g zeolitic material obtained according to Reference Example 1.2.2, having a Sn content of 12.3 weight-% and 45 g 1,2-dichloroethane. The mixture was heated to reflux (95 C.). An aqueous solution of 0.49 g H.sub.2O.sub.2 (70 weight-%) was added and the reaction was stirred for 4 h. After cooling down to room temperature, the solution was filtrated and analyzed with respect to the selectivities to epsilon-caprolactone, based on cyclohexanone and based on hydrogen peroxide, by quantitative GC analysis using di-n-butyl ether as internal standard.

(78) Specific Procedure

(79) Example 4 was carried out as described in the General Procedure above, wherein the aqueous solution of hydrogen peroxide additionally contained 360 micromol KCO.sub.2H per mol H.sub.2O.sub.2. The selectivities to epsilon-caprolactone, based on cyclohexanone and based on hydrogen peroxide, are shown in Table 1 below.

Comparative Example 4

Baeyer-Villiger Oxidation without Addition of a Potassium Salt

(80) Comparative Example 4 was carried out as described in the General Procedure in Example 4 above. The selectivities to epsilon-caprolactone, based on cyclohexanone and based on hydrogen peroxide, are shown in Table 1 below.

Example 5

Baeyer-Villiger Oxidation of Cyclohexanone to Epsilon-Caprolactone Using a Sn-BEA Having a Sn Content of 9.6 Weight-% and 1,4-Dioxane as Solvent with Addition of KH2PO4

(81) General Procedure

(82) A 100 ml glass flask vessel was charged with 1.5 g cyclohexanone, 1 g zeolitic material obtained according to Reference Example 2, having a Sn content of 9.6 weight-% and 45 g 1,4-dioxane. The mixture was heated to reflux (95 C.). An aqueous solution of 0.5 g H.sub.2O.sub.2 (70 weight-%) was added and the reaction was stirred for 4 h. After cooling down to room temperature, the solution was filtrated and analyzed with respect to the selectivities to epsilon-caprolactone, based on cyclohexanone and based on hydrogen peroxide, by quantitative GC analysis using di-n-butyl ether as internal standard.

(83) Specific Procedure

(84) Example 5 was carried out as described in the General Procedure above, wherein the aqueous solution of hydrogen peroxide additionally contained 360 micromol KH.sub.2PO.sub.4 per mol H.sub.2O.sub.2. The selectivities to epsilon-caprolactone, based on cyclohexanone and based on hydrogen peroxide, are shown in Table 1 below.

Comparative Example 5

Baeyer-Villiger Oxidation without Addition of a Potassium Salt

(85) Comparative Example 5 was carried out as described in the General Procedure in Example 5 above. The selectivities to epsilon-caprolactone, based on cyclohexanone and based on hydrogen peroxide, are shown in Table 1 below.

(86) TABLE-US-00006 TABLE 1 Results of the Examples and Comparative Examples Example (E) Sn zeolite catalyst Selectivities.sup.4) Comparative framework Sn to product.sup.2) based on Example structure Sn incorporation content/ Potassium hydrogen starting (CE) type via weight-% additive peroxide/% material.sup.3) E1.1 MWW HS 0.46 KH.sub.2PO.sub.4 59 66 E1.2 KNO.sub.3 52 58 E1.3 KCO.sub.2H 59 69 CE1 51 54 E2 MWW HS 0.46 KH.sub.2PO.sub.4 13 50 CE2 12 40 E3 MWW SSIE 12.8 KH.sub.2PO.sub.4 13 18 CE3 10 15 E4 MWW SSIE 12.3 KCO.sub.2H 32 37 CE4 26 33 E5 BEA SSIE 9.6 KH.sub.2PO.sub.4 95 82 CE5 80 76 .sup.1)HS = hydrothermal synthesis; SSIE = solid-state ion exchange .sup.2)epsilon-caprolactone .sup.3)cyclohexanone .sup.4)The selectivities were calculated based on the concentrations of the starting material and the product in the product mixture determined by quantitative GC analysis using di-n-butylether, and the known amount of H.sub.2O.sub.2 and starting material at the beginning of the reaction

(87) The examples and the comparative examples clearly show that irrespective of the zeolitic framework structure type, irrespective of the tin content of the zeolitic material, and irrespective of the chemical nature of the potassium salts employed, the addition of a potassium salt to the oxidation reaction and, thus, the catalytic system of the tin containing zeolitic material and the potassium salt, leads to improved selectivities to the product, based on the starting material to be oxidized as well as based on hydrogen peroxide. Therefore, since it is these selectivities which have the most important impact on whether or not a process is interesting for industrial purposes, the process and the catalytic system according to the invention are especially suitable for medium and large scale processes. This is also illustrated by example E2 and comparative example CE2 which represent a continuous-type process where the catalyst consists of strands comprising the tin-containing zeolitic material and the catalytic system of the zeolitic material and the potassium salt additive is realized as fixed-bed catalytic system which is of particular relevance in industrial scale processes. Still further, it is shown that the improved characteristics are obtained irrespective of how the tin-containing zeolitic material is prepared since the advantageous selectivity values are obtained for zeolitic materials prepared by solid-state tin-ion exchange as well as by the incorporation of tin via hydrothermal synthesis. Yet further, it is also shown the advantageous selectivity values are obtained irrespective of the solvent employed since in the examples and comparative examples above, different solvents were used, and for each solvent, the advantageous effects were obtained.

(88) Summarized it is shown that the inventive process and the inventive catalytic system represent an overarching conceptual framework realized by the combination of a potassium salt additive and a tin-containing zeolitic material used in Baeyer-Villiger-type oxidation reactions.

CITED LITERATURE

(89) Nature 412 (2001), pages 423-425 Journal of Catalysis 234 (2005), pages 96-100 U.S. Pat. No. 5,968,473 U.S. Pat. No. 6,306,364 Microporous and Mesoporous Materials 165 (2013), pages 210-218 WO 03/074422 A1 U.S. Pat. No. 7,326,401 B2 M. A. Camblor, A. Corma, M.-J. Diaz-Cabanas and Ch. Baerlocher, J. Phys. Chem. B 102 (1998) pages 44-51