CHA type zeolitic materials and methods for their preparation using combinations of cycloalkyl and ethyltrimethylammonium compounds

Abstract

The present invention relates to a process for the preparation of a zeolitic material having a CHA-type framework structure comprising YO.sub.2 and X.sub.2O.sub.3, wherein said process comprises the steps of: (1) providing a mixture comprising one or more sources for YO.sub.2, one or more sources for X.sub.2O.sub.3, one or more optionally substituted ethyltrimethylammonium cation-containing compounds, and one or more tetraalkylammonium cation R.sup.1R.sup.2R.sup.3R.sup.4N.sup.+-containing compounds as structure directing agent; (2) crystallizing the mixture obtained in step (1) for obtaining a zeolitic material having a CHA-type framework structure; wherein Y is a tetravalent element and X is a trivalent element, wherein R.sup.1, R.sup.2, and R.sup.3 independently from one another stand for alkyl, wherein R.sup.4 stands for cycloalkyl, and wherein the YO.sub.2:X.sub.2O.sub.3 molar ratio of the mixture in (1) ranges from 2 to 1,000, as well as to zeolitic materials which may be obtained according to the inventive process and to their use.

Claims

1. A synthetic zeolitic material having a CHA-type framework structure, wherein the CHA-type framework structure comprises YO.sub.2 and X.sub.2O.sub.3, wherein Y is a tetravalent element and X is a trivalent element, and wherein the IR-spectrum of the zeolitic material comprises: a first absorption band (B1) ranging from 3,720 to 3,750 cm.sup.1; and a second absorption band (B2) ranging from 1,850 to 1,890 cm.sup.1; wherein the ratio of the maximum absorbance of the first absorption band to the second absorption band B1:B2 ranges from 1 to 2.5.

2. A process for the preparation of a zeolitic material having a CHA-type framework structure comprising YO.sub.2 and X.sub.2O.sub.3, the process comprising: (1) obtaining a mixture comprising one or more sources for YO.sub.2, one or more sources for X.sub.2O.sub.3, one or more optionally substituted ethyltrimethylammonium cation-containing compounds, and one or more tetraalkylammonium cation R.sup.1R.sup.2R.sup.3R.sup.4N.sup.+-containing compounds as structure directing agent; (2) crystallizing the mixture obtained in (1) for obtaining a zeolitic material having a CHA-type framework structure; wherein Y is a tetravalent element and X is a trivalent element, wherein R.sup.1, R.sup.2, and R.sup.3 are independently from one another alkyl, wherein R.sup.4 is cycloalkyl, and wherein the YO.sub.2:X.sub.2O.sub.3 molar ratio of the mixture in (1) ranges from 2 to 1,000.

3. The process of claim 2, wherein the mixture in (1) does not contain any substantial amount of a source for Z.sub.2O.sub.5, wherein Z is P.

4. The process of claim 2, wherein the one or more tetraalkylammonium cation R.sup.1R.sup.2R.sup.3R.sup.4N.sup.+-containing compounds comprise one or more N,N,N-tri (C.sub.1-C.sub.4)alkyl-(C.sub.5-C.sub.7)cycloalkylammonium compounds.

5. The process of claim 2, wherein Y is selected from the group consisting of Si, Sn, Ti, Zr, Ge, and mixtures of two or more thereof.

6. The process of claim 2, wherein X is selected from the group consisting of Al, B, In, Ga, and mixtures of two or more thereof.

7. The process of claim 2, wherein the molar ratio of the one or more optionally substituted ethyltrimethylammonium cations C.sub.2H.sub.5N(CH.sub.3).sub.3.sup.+:YO.sub.2 in the mixture according to (1) ranges from 0.005 to 0.5.

8. The process of claim 2, wherein the molar ratio C.sub.2H.sub.5N(CH.sub.3).sub.3:R.sup.1R.sup.2R.sup.3R.sup.4N.sup.+ of the one or more optionally substituted ethyltrimethylammonium cations to the one or more tetraalkylammonium cations R.sup.1R.sup.2R.sup.3R.sup.4N.sup.+ in the mixture according to (1) ranges from 0.01 to 5.

9. The process of claim 2 further comprising one or more of the following: (3) adjusting the pH of the crystallized mixture obtained in (2) to a pH ranging from 3 to 11, and/or (4) isolating the zeolitic material from the crystallized mixture in (2) or (3), and/or (5) washing the zeolitic material obtained in (2), (3), or (4), and/or (6) drying and/or calcining the zeolitic material obtained in (2), (3), (4), or (5), and/or (7) subjecting the zeolitic material to an ion-exchange procedure, wherein (3) and/or (4) and/or (5) and/or (6) and/or (7) can be conducted in any order.

10. The process of claim 9, wherein in (6) the zeolitic material is spray dried.

11. The process of claim 10, wherein the crystallized mixture obtained in (2) is directly subject to spray drying in (6).

12. A synthetic zeolitic material having a CHA-type framework structure obtained according to the process of claim 2.

13. The zeolitic material of claim 12, wherein the particle size D10 of the zeolitic material ranges from 150 to 300 nm, wherein the average particle size D50 of the zeolitic material ranges from 300 to 450 nm, and wherein the particle size D90 of the zeolitic material ranges from 500 to 900 nm.

14. The zeolitic material of claim 12, wherein the micropore volume of the zeolitic material determined according to DIN 66133 ranges from 0.5 to 3 cm.sup.3/g.

15. A method of converting an organic compound by contacting said compound with a catalyst containing the synthetic zeolitic material of claim 12 under suitable conversion conditions.

16. A method for selectively reducing nitrogen oxides NO.sub.x by contacting a stream containing NO.sub.x with a catalyst containing the zeolitic material of claim 12 under suitable reducing condition.

17. A method of oxidizing NH.sub.3 by contacting a stream containing NH.sub.3 with a catalyst containing the zeolitic material of claim 12 under suitable oxidizing conditions.

18. A method of decomposing of N.sub.2O by contacting a stream containing N.sub.2O with a catalyst containing the zeolitic material of claim 12 under suitable decomposition conditions.

19. A method of controlling emissions in Advanced Emission Systems by contacting an emission stream with a catalyst containing the zeolitic material of claim 12 under suitable conditions.

20. A fluid catalytic cracking FCC process comprising adding the zeolitic material of claim 12 to the components of the FCC at a suitable time and under suitable conditions for the FCC process.

Description

DESCRIPTION OF THE FIGURES

(1) FIGS. 1a, 2a, 3a, and 5a display the X-ray diffraction patterns (measured using Cu K alpha-1 radiation) of the calcined products obtained according to Examples 1-3 and Comparative Example 2, respectively. For comparative purposes, the line pattern of the CHA type structure is indicated in the diffractogram of the respective figures. In the figure, the angle 2 theta in is shown along the abscissa and the intensities are plotted along the ordinate.

(2) FIGS. 1b, 2b, 3b, 4a, and 5b display the IR-spectra obtained for the calcined products obtained according to Examples 1-3, and Comparative Examples 1 and 2, respectively. In the figures, the wavenumbers in cm.sup.1 is shown along the abscissa, and the absorbance is plotted along the ordinate.

(3) FIGS. 1c. 2c, 3c, 4b, and 5c display the .sup.27Al MAS solid-state NMR spectra obtained for the calcined products of Examples 1-3 and Comparative Examples 1 and 2, respectively. In the figures, the chemical shift in ppm is shown along the abscissa, and the relative intensity in arbitrary units is plotted along the ordinate. Furthermore, the integration of the relative peak intensities is displayed above the abscissa for the relevant peaks, of which the ppm-values for the maxima is indicated above the respective peak preceded by the indication M for maximum.

(4) FIGS. 1d, 2d, 3d, 4c, and 5d display the .sup.29Al MAS solid-state NMR spectra obtained for the calcined products of Examples 1-3 and Comparative Examples 1 and 2, respectively. In the figures, the chemical shift in ppm is shown along the abscissa, and the relative intensity in arbitrary units is plotted along the ordinate. Furthermore, the integration of the relative peak intensities is displayed above the abscissa for the relevant peaks, of which the ppm-values for the maxima is indicated above the respective peak preceded by the indication M for maximum.

EXAMPLES

(5) X-ray diffraction experiments on the powdered materials were performed using an Advance D8 Series 2 Diffractometer (Bruker/AXS) equipped with a Sol-X detector using the Cu K alpha-1 radiation.

(6) .sup.27Al MAS solid-state NMR experiments were measured by direct excitation with 15-pulse under 10 kHz Magic Angle Spinning using 250 ms recycle delay and 20 ms acquisition. The data was processed with 50 Hz exponential line broadening.

(7) .sup.29Si MAS solid-state NMR experiments were performed using a Bruker Avance spectrometer with 300 MHz .sup.1H Larmor frequency (Bruker Biospin, Germany). Spectra were processed using Bruker Topspin with 30 Hz exponential line broadening, manual phasing, and manual baseline correction over the full spectrum width. Spectra were referenced with the polymer Q8M8 as an external secondary standard, by setting the resonance of the trimethylsilyl M group to 12.5 ppm.

(8) The IR-spectra were obtained from samples free of a carrier material, wherein said sample were heated at 300 C. in high vacuum for 3 h prior to measurement. The measurements were performed using a Nicolet 6700 spectrometer in a high vacuum measurement cell with CaF.sub.2 windows. The obtained data was transformed to absorbance values, and the analysis was performed on the spectra after base line correction.

(9) The particle size distribution of the samples was performed by dispersing 0.1 g of the zeolite powder in 100 g H.sub.2O and treating by ultrasound for 10 minutes. The dynamic light scattering was performed on a Zetasizer Nano ZS with the Malvern Zeta Sizer Software. Version 6.34, applying 5 runs 10 second measurement time for each sample. The given values are the average particle size by number in nanometer.

(10) The micropore volume of the calcined samples was determined according to DIN 66133.

Example 1: Preparation of a Zeolitic Material Having the CHA Framework Structure Using Trimethylcyclohexylammonium and Ethyltrimethylammonium

(11) 530.71 g N,N,N-trimethylcyclohexylammoniumhydroxide (20 wt-% solution in H.sub.2O) were mixed with 66.74 g of aluminiumtriisopropylate and 215.66 g ethyltrimethylammoniumhydroxide (20 wt-% solution in H.sub.2O). Afterwards, 686.93 g LUDOX AS 40 (40 wt-% colloidal solution in H.sub.2O) and 11.49 g CHA seeds were added to the stirred mixture. The resulting gel was placed in a stirred autoclave with a total volume of 2.5 L. The autoclave was heated within 7 h to 170 C. The temperature was kept constant for 72 h. Afterwards the autoclave was cooled down to room temperature. Then, the solids were separated by filtration and intensive washing until the washwater had a pH of 7. Finally the solid was dried for 10 hours at 120 C. Solid yield=335 g. The material was calcined at 550 C. for 5 hours.

(12) The characterization of the calcined material via XRD is displayed in FIG. 1a and displays the CHA-type framework structure of the product and afforded an average crystal size of 101 nm and a crystallinity of 87%. The calcined material displayed a BET surface area of 665 m.sup.2/g, a pore volume of 1.29 cm.sup.3/g and a median pore width of 2.76 nm. The elemental analysis of the calcined material showed 93.4 wt-% SiO.sub.2, 6.5 wt-% Al.sub.2O.sub.3, and 0.1 wt-% Na.sub.2O in the sample, thus affording an SiO.sub.2:Al.sub.2O.sub.3 atomic ratio (SAR) of 24.5.

(13) The IR-spectrum of the calcined sample is shown in FIG. 1b, wherein amongst others absorption bands having maxima at 3,736 cm.sup.1 and 1,865 cm.sup.1 may be seen, which display a ratio of maximum absorption of the former to the latter of 1.48.

(14) The particle size distribution of the calcined sample afforded a D10 value of 0.21 m, a D50 value of 0.27 m, and a D90 value of 0.67 m.

(15) The .sup.29Si MAS NMR of the calcined zeolitic material is displayed in FIG. 1c and displays peaks at 103.87 and 110.12 ppm, wherein the integration of the peaks offers relative intensities of 0.418 and 1 for the signals, respectively.

(16) The .sup.27Al MAS NMR of the calcined zeolitic material is displayed in FIG. 1d and displays peaks at 58.44 and 0.34 ppm, wherein the integration of the peaks offers relative intensities of 1 and 0.069 for the signals, respectively.

Example 2: Preparation of a Zeolitic Material Having the CHA Framework Structure Using Trimethylcyclohexylammonium and Ethyltrimethylammonium

(17) 530.71 g N,N,N-trimethylcyclohexylammoniumhydroxide (20 wt-% solution in H.sub.2O) were mixed with 66.74 g of aluminiumtriisopropylate and 215.66 g ethyltrimethylammoniumhydroxide (20 wt-% solution in H.sub.2O). Afterwards, 686.93 g LUDOX AS 40 (40 wt-% colloidal solution in H.sub.2O) and 11.49 g CHA seeds were added to the stirred mixture. The resulting gel was placed in a stirred autoclave with a total volume of 2.5 L. The autoclave was heated within 7 h to 170 C. The temperature was kept constant for 24 h. Afterwards the autoclave was cooled down to room temperature. Then, the solids were separated by filtration and intensive washing until the washwater had a pH of 7. Finally the solid was dried for 10 hours at 120 C. Solid yield=337 g. The material was calcined at 550 C. for 5 hours.

(18) The characterization of the calcined material via XRD is displayed in FIG. 2a and displays the CHA-type framework structure of the product and afforded an average crystal size of 96 nm and a crystallinity of 86%. The calcined material displayed a BET surface area of 635 m.sup.2/g, a pore volume of 1.80 cm.sup.3/g and a median pore width of 1.31 nm. The elemental analysis of the calcined material showed 93.4 wt-% SiO.sub.2, 6.2 wt-% Al.sub.2O.sub.3, and 0.1 wt-% Na.sub.2O in the sample, thus affording an SiO.sub.2:Al.sub.2O.sub.3 atomic ratio (SAR) of 24.7.

(19) The IR-spectrum of the calcined sample is shown in FIG. 2b, wherein amongst others absorption bands having maxima at 3,736 cm.sup.1 and 1,865 cm.sup.1 may be seen, which display a ratio of maximum absorption of the former to the latter of 1.64.

(20) The particle size distribution of the calcined sample afforded a D10 value of 0.30 m, a D50 value of 0.48 m, and a D90 value of 0.70 m.

(21) The .sup.29Si MAS NMR of the calcined zeolitic material is displayed in FIG. 2c and displays peaks at 104.02 and 110.23 ppm, wherein the integration of the peaks offers relative intensities of 0.377 and 1 for the signals, respectively.

(22) The .sup.27Al MAS NMR of the calcined zeolitic material is displayed in FIG. 2d and displays peaks at 58.49 and 0.78 ppm, wherein the integration of the peaks offers relative intensities of 1 and 0.149 for the signals, respectively.

Example 3: Preparation of a Zeolitic Material Having the CHA Framework Structure Using Trimethylcyclohexylammonium and Ethyltrimethylammonium

(23) 12.38 kg N,N,N-trimethylcyclohexylammoniumhydroxide (20 wt-% solution in H.sub.2O) were mixed with 1.56 kg of aluminiumtriisopropylate and 5.03 kg ethyltrimethylammoniumhydroxide (20 wt-% solution in H.sub.2O). Afterwards, 16.03 kg LUDOX AS 40 (40 wt-% colloidal solution in H.sub.2O) and 268.12 g CHA seeds were added to the stirred mixture. The resulting gel was placed in a stirred autoclave with a total volume of 60 L. The autoclave was heated within 7 h to 170 C. The temperature was kept constant for 24 h. Afterwards the autoclave was cooled down to room temperature. Then, the solids were separated by filtration and intensive washing until the washwater had a pH of 7. Finally the solid was dried for 10 hours at 120 C. Solid yield=7.5 kg. The material was calcined at 550 C. for 5 hours.

(24) The characterization of the calcined material via XRD is displayed in FIG. 3a and displays the CHA-type framework structure of the product and afforded an average crystal size of 106 nm and a crystallinity of 90%. The calcined material displayed a BET surface area of 650 m.sup.2/g, a pore volume of 1.20 cm.sup.3/g and a median pore width of 0.51 nm. The elemental analysis of the calcined material showed 93.7 wt-% SiO.sub.2, 6.2 wt-% Al.sub.2O.sub.3, and 0.1 wt-% Na.sub.2O in the sample, thus affording an SiO.sub.2:Al.sub.2O.sub.3 atomic ratio (SAR) of 25.6.

(25) The IR-spectrum of the calcined sample is shown in FIG. 3b, wherein amongst others absorption bands having maxima at 3,734 cm.sup.1 and 1,867 cm.sup.1 may be seen, which display a ratio of maximum absorption of the former to the latter of 1.44.

(26) The particle size distribution of the calcined sample afforded a D10 value of 0.15 m, a D50 value of 0.43 m, and a D90 value of 0.70 m.

(27) The .sup.29Si MAS NMR of the calcined zeolitic material is displayed in FIG. 3c and displays peaks at 103.68 and 110.08 ppm, wherein the integration of the peaks offers relative intensities of 0.391 and 1 for the signals, respectively.

(28) The .sup.27Al MAS NMR of the calcined zeolitic material is displayed in FIG. 3d and displays peaks at 58.49 and 0.26 ppm, wherein the integration of the peaks offers relative intensities of 1 and 0.1 for the signals, respectively.

Comparative Example 1: Preparation of a Zeolitic Material Having the CHA Framework Structure Using Trimethylcyclohexylammonium and Tetramethylammonium

(29) 534.54 g N,N,N-trimethycyclohexylammoniumhydroxide (20 wt-% solution in H.sub.2O) were mixed with 56.54 g of aluminiumtriisopropylate and 150.62 g tetramethylammoniumhydroxide (25 wt-% solution in H.sub.2O). Afterwards, 692.01 g LUDOX AS 40 (40 wt-% colloidal solution in H.sub.2O) and 11 g CHA seeds were added to the stirred mixture. The resulting gel was placed in a stirred autoclave with a total volume of 2.5 L. The autoclave was heated within 7 h to 170 C. The temperature was kept constant for 15 h. Afterwards the autoclave was cooled down to room temperature. Then, the solids were separated by filtration and intensive washing until the washwater had a pH of 7. Finally the solid was dried for 10 hours at 120 C. Solid yield=327 g. The material was calcined at 550 C. for 5 hours.

(30) The characterization of the calcined material via XRD confirmed the CHA-type framework structure of the product and afforded an average crystal size of 115 nm and a crystallinity of 91%.

(31) The calcined material displayed a BET surface area of 621 m.sup.2/g, a pore volume of 1.07 cm.sup.3/g and a median pore width of 0.68 nm. The elemental analysis of the calcined material showed 93.4 wt-% SiO.sub.2, 6.4 wt-% Al.sub.2O.sub.3, and 0.2 wt-% Na.sub.2O in the sample, thus affording an SiO.sub.2:Al.sub.2O.sub.3 atomic ratio (SAR) of 25.

(32) The IR-spectrum of the calcined sample is shown in FIG. 4a, wherein amongst others absorption bands having maxima at 3,732 cm.sup.1 and 1,866 cm.sup.1 may be seen, which display a ratio of maximum absorption of the former to the latter of 1.33.

(33) The particle size distribution of the calcined sample afforded a D10 value of 0.61 m, a D50 value of 0.92 m, and a D90 value of 1.58 m.

(34) The .sup.29Si MAS NMR of the calcined zeolitic material is displayed in FIG. 4b and displays peaks at 104.1 and 110.3 ppm, wherein the integration of the peaks offers relative intensities of 0.334 and 1 for the signals, respectively.

(35) The .sup.27Al MAS NMR of the calcined zeolitic material is displayed in FIG. 4c and displays peaks at 58.3 and 6.3 ppm, wherein the integration of the peaks offers relative intensities of 1 and 0.124 for the signals, respectively.

Comparative Example 2: Preparation of a Zeolitic Material Having the CHA Framework Structure Using Trimethylcyclohexylammonium and Tetramethylammonium

(36) 276.8 kg N,N,N-trimethylcyclohexylammoniumhydroxide (20 wt-% solution in H.sub.2O) were mixed with 34.80 kg of aluminiumtriisopropylate and 77.99 kg tetramethylammoniumhydroxide (25 wt-% solution in H.sub.2O). Afterwards, 358.32 kg LUDOX AS 40 (40 wt-% colloidal solution in H.sub.2O) and 5.73 kg CHA seeds were added to the stirred mixture. The resulting gel was placed in a stirred autoclave with a total volume of 1600 L. The autoclave was heated within 7 h to 170 C. The temperature was kept constant for 18 h. Afterwards the autoclave was cooled down to room temperature. Then, the solids were separated by filtration and intensive washing until the washwater had a pH of 7. Finally the solid was dried for 10 hours at 120 C. The material was calcined at 550 C. for 5 hours.

(37) The characterization of the calcined material via XRD is displayed in FIG. 5a and displays the CHA-type framework structure of the product and afforded an average crystal size of 118 nm and a crystallinity of 92%. The calcined material displayed a BET surface area of 654 m.sup.2/g, a pore volume of 1.09 cm.sup.3/g and a median pore width of 0.68 nm. The elemental analysis of the calcined material showed 93.2 wt-% SiO.sub.2, 6.6 wt-% Al.sub.2O.sub.3, and 0.2 wt-% Na.sub.2O in the sample, thus affording an SiO.sub.2:Al.sub.2O.sub.3 atomic ratio (SAR) of 24.

(38) The IR-spectrum of the calcined sample is shown in FIG. 5b, wherein amongst others absorption bands having maxima at 3,733 cm.sup.1 and 1,866 cm.sup.1 may be seen, which display a ratio of maximum absorption of the former to the latter of 1.35.

(39) The particle size distribution of the calcined sample afforded a D10 value of 0.39 m, a D50 value of 0.58 m, and a D90 value of 1.22 m.

(40) The .sup.29Si MAS NMR of the calcined zeolitic material is displayed in FIG. 5c and displays peaks at 104.2 and 110.5 ppm, wherein the integration of the peaks offers relative intensities of 0.394 and 1 for the signals, respectively.

(41) The .sup.27Al MAS NMR of the calcined zeolitic material is displayed in FIG. 5d and displays peaks at 58.5 and 2.7 ppm, wherein the integration of the peaks offers relative intensities of 1 and 0.225 for the signals, respectively.

Example 4: Selective Catalytic Reduction Testing

(42) The samples obtained according to Example 1 and according to comparative examples 1 and 2 were tested under a selective catalytic reduction conditions relative to their NO.sub.x conversion capacity. To this effect, the respective calcined samples were ion-exchanged with copper. The copper loaded samples were then aged at 850 C. for 6 hours in an atmosphere containing 10 volume percent of water. The aged samples were then contacted at various temperatures with a gas stream containing 500 ppm nitrogen oxide, 500 ppm ammonia, 5 volume percent water, 10 volume percent oxygen and balance nitrogen. Specifically, the capacity of the samples to convert nitrogen oxide under selective catalytic reduction conditions was tested at 200 C., 300 C., and 450 C. The results of said testing are displayed in table 1 below.

(43) TABLE-US-00001 TABLE 1 Results from selective catalytic reduction testing conducted on the powder samples. Sample (wt.-% Cu) Example 1 Comp. Ex. 1 Comp. Ex. 2 Temperature (2.60 wt. %) (2.50 wt.-%) (2.20 wt. %) 200 C. 94 75 90 300 C. 100 84 93 450 C. 89 80 84

(44) For testing the samples under conditions which closely reflect the conditions experienced in the treatment of exhaust gas from automotive combustion engines, the aforementioned samples after having been ion-exchanged with copper were provided as a wash coat on a flow-through monolith substrate, wherein the coated substrate was then aged in an atmosphere containing volume percent water for 5 hours at 750 C. The coated monoliths were then contacted with the same gas stream employed for testing the powder samples at 200 C. and 600 C., respectively. The results from said core testing of the samples is displayed in table 2 below.

(45) TABLE-US-00002 TABLE 2 results from selective catalytic reduction testing conducted on the coated monolith samples. Sample Temperature Example 1 Comp. Ex. 1 Comp. Ex. 2 200 C. 74 67 65 600 C. 83 83 88

(46) Thus, as may be taken from the results from selective catalytic reduction testing, it has surprisingly been found that both in the testing runs performed on the powder samples as well as on the testing performed on the coated monolith samples, the results obtained with the inventive sample clearly outperform those obtained with the comparative examples, in particular with respect to the conversion of nitrogen oxides at lower temperatures. Consequently, it has quite unexpectedly been found that inventive method not only provides for a highly efficient process for the preparation of a zeolitic material having a CHA framework structure, but furthermore quite surprisingly affords a material displaying an unexpectedly improved performance with respect to the conversion of nitrogen oxides under selective catalytic reduction conditions.

LIST OF THE CITED PRIOR ART REFERENCES

(47) U.S. Pat. No. 4,544,538 WO-A-2008/083048 WO-A-2008/039742 WO-A-2008/033229 WO 2009/141324 A1 WO 2011/064186 A1 EP 2 325 143 A2 U.S. Pat. No. 4,610,854 US-A-2007/0043249 Zones et al. in Studies in Surface Science and Catalysis, Vol. 84, pp. 29-36 WO 2013/182974 A US 2004/253163 A1