Titania particles and a process for their production

Abstract

The invention provides a process for the production of titania particles with a desired morphology. The process comprises providing a titania sol and then drying the sol to provide dried titania particles. The process is characterized in that the morphology of the dried titania particles is controlled by applying one or more of the following criteria: (a) the titania sol is produced from a TiO.sub.2 containing slurry obtained using a precipitation step in a sulphate process, wherein the size of micelles formed during the precipitation is controlled; (b) the titania sol is produced from a TiO.sub.2 containing slurry and the pH of the slurry is controlled in order to affect the extent to which the titania sol is flocculated; (c) the titania sol is produced from a TiO.sub.2 containing slurry and the iso-electric point of the titania is adjusted in order to affect the extent to which the titania sol is flocculated; (d) the titania sol is dried by application of heat and the temperature used during the drying step is controlled.

Claims

1. A process for the production of titania particles, the process comprising: providing a titania sol; and drying the titania sol to provide dried titania particles, wherein, a morphology of the dried titania particles is controlled by applying one or more of the following criteria: (a) the titania sol is produced from a TiO.sub.2 containing slurry obtained using a precipitation step in a sulphate process, wherein the size of micelles formed during the precipitation is controlled, (b) the titania sol is produced from a TiO.sub.2 containing slurry using a precipitation step in a sulphate process, and the iso-electric point of the titania is adjusted in order to affect the extent to which the titania sol is flocculated, (c) the titania sol is produced from a TiO.sub.2 containing slurry using a precipitation step in a sulphate process, and the titania sol is dried by application of heat and the temperature used during the drying step is controlled, and wherein, the size of micelles formed during the precipitation is controlled by the use of (a) a Mecklenburg precipitation with a nucleation level in the range of from 0.1 to 15 wt %, or (b) a Blumenfeld precipitation with a drop ratio of from 50:50 to 99:1.

2. The process of claim 1, wherein the pore size of the dried titania particles is controlled by applying one or more of the following criteria: (A-i) the titania sol is produced from a TiO.sub.2 containing slurry obtained using a precipitation step in a sulphate process, and the size of micelles formed during the precipitation is controlled, (A-ii) the titania sol is produced from a TiO.sub.2 containing slurry and the iso-electric point of the titania is adjusted in order to affect the extent to which the titania sol is flocculated.

3. The process of claim 1, wherein the shape of the dried titania particles is controlled by applying the following criteria: (B-i) the titania sol is dried by application of heat and the temperature used during the drying step is controlled.

4. The process of claim 1, wherein two or more of the criteria (a) to (c) are applied.

5. The process of claim 1, wherein all three of the criteria (a) to (c) are applied.

6. The process of claim 1, wherein all three of the criteria (a) to (c) are applied and the morphology of the dried titania particles is further controlled by also applying the following criteria: (d) the titania sol is produced from a TiO.sub.2 containing slurry and the pH of the slurry is controlled in order to affect the extent to which the titania sol is flocculated.

7. The process of claim 1, wherein one or more active catalytic components are incorporated during the production of the titania particles.

8. The process of claim 1, wherein one or more thermal stabilizer components are incorporated during the production of the titania particles.

9. A process for the production of titania particles, wherein the process comprises: providing a titania sol; and spray drying the titania sol to provide dried titania particles, wherein, a morphology of the dried titania particles is controlled by: producing the titania sol from a TiO.sub.2 containing slurry obtained using a precipitation step in a sulphate process and adjusting the iso-electric point to be 3 pH units or more from the pH of the slurry, by the addition of dispersant, in order to reduce the extent to which the titania sol is flocculated, and wherein, a size of micelles formed during the precipitation is controlled by the use of (a) a Mecklenburg precipitation with a nucleation level in the range of from 0.1 to 15 wt %, or (b) a Blumenfeld precipitation with a drop ratio of from 50:50 to 99:1.

10. The process of claim 9, wherein the iso-electric point of the titania is adjusted to be 4 pH units or more from the pH of the slurry by the addition of dispersant, in order to reduce the extent to which the titania sol is flocculated.

11. The process of claim 9, wherein the morphology of the dried titania particles is further controlled by the temperature used during the spray drying step being controlled.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is an image obtained by scanning electron microscopy (SEM) of the particles of the product obtained in Example 2.

(2) FIG. 2a is an image obtained using transmission electron microscopy (TEM) of the particles of the product obtained using 6% nucleation at precipitation in Example 3.

(3) FIGS. 2b-2f are images obtained using transmission electron microscopy (TEM) of the micelles produced by sulphate precipitation at nucleation levels of 6%, 2%, 1%, 0.5% and 0.1%, respectively, in Example 3.

(4) FIGS. 3a-3e are images obtained by scanning electron microscopy (SEM) of the particles of the products obtained in Example 4 at pH values of 5.5, 4.5, 3.25, 2, and 1.5, respectively.

(5) FIG. 4a is an image obtained by scanning electron microscopy (SEM) of the particles of the product obtained in Example 6 prepared using 15% silica as a dopant.

(6) FIG. 4b is an image obtained by scanning electron microscopy (SEM) of the particles of the product obtained in Example 6 prepared using 10% WO.sub.3 as a dopant.

(7) FIGS. 5a-5d are images obtained by scanning electron microscopy (SEM) of the particles of the dried products obtained in Example 7 for a sol having 1% solids at dryer inlet temperatures of 110 C., 150 C., 200 C., 250 C., respectively.

(8) FIGS. 6a-6d are images obtained by scanning electron microscopy (SEM) of the particles of the dried products obtained in Example 7 for a sol having 10% solids at dryer inlet temperatures of 110 C., 150 C., 200 C., 250 C., respectively.

(9) FIGS. 7a-7d are images obtained by scanning electron microscopy (SEM) of the particles of the dried products obtained in Example 7 for a sol having 17% solids at dryer inlet temperatures of 110 C., 150 C., 200 C., 250 C., respectively.

EXAMPLES

Example 1

(10) A concentrated anatase titania sol was obtained by a 6% nucleated precipitation that was carried out in accordance with the method of WO2011/033286. Samples of the sol were thermally dried at (a) 105 C. and (b) 200 C.

(11) The specific surface areas of each of the dried samples were tested using the BET method.

(12) TABLE-US-00001 Thermally Dried Sample Sample 105 C. 200 C. BET specific surface area 280.9 311.2 (m.sup.2/g)

(13) When repeated with a higher drying temperature being applied, the particles were more toroidal in shape and had a higher specific surface area.

Example 2

(14) A concentrated titania sol was prepared using clean Scarlino rutile nuclei (washed free of salts, 0.5 ms/cm). The sol was produced as in method outlined in WO2011/033286. In this regard, the washed Scarlino nuclei were peptised to pH 1.5, 10% citric acid was added, MIPA was added to take the pH to 8, and then the particles were washed to <2 ms/cm.

(15) The concentrated sol was then spray dried at 17% at 110 C. using a Lab Plant Super 7 laboratory spray drier.

(16) The specific surface area of the sample was tested using the BET method. The pore size and pore volume were measured by both mercury porosimetry and nitrogen isotherms.

(17) TABLE-US-00002 BET SSA (m.sup.2/g) 87.52 Mercury Large Pore Size (m) 1.7018 Mercury Small Pore Size (nm) 23.1 Nitrogen Large Pore Size (nm) 40.65 Nitrogen Small Pore Size (nm) 1.4 Pore Volume mercury (cm.sup.3/g) 0.36 Pore Volume nitrogen (cm.sup.3/g) 0.31

(18) Scanning electron microscopy (SEM) was carried out to image the particles of the product obtained. The obtained image is shown in FIG. 1.

(19) When the experiment is repeated with a higher drying temperature being applied, the particles become more toroidal in shape and have a higher specific surface area.

(20) Thus the invention applies for rutile material as well as anatase.

Example 3

(21) Several different concentrated titania sol products were obtained by precipitation that was carried out in accordance with the method of WO2011/033286. These were obtained using different levels of nucleation at precipitation. One had a 1% nucleated precipitation, one had a 2% nucleated precipitation and one had a 6% nucleated precipitation.

(22) Samples from each product were spray dried using a LabPlant SD-05 laboratory spray drier.

(23) The specific surface areas of each of the dried samples were tested using the BET method.

(24) The pore size was measured using mercury porosimetry, using a Micromeritics AutoPore IV porosimeter.

(25) TABLE-US-00003 % Nucleation at precipitation 1 2 6 BET SSA (m.sup.2/g) 219.8 269.1 314.5 Pore Size (nm) 9.5 6.7 4.3

(26) It can be seen that by using a lower level of nucleation, the pore size (diameter) was higher.

(27) This confirms what the present inventors have determined, namely that by controlling the extent of nucleation, and therefore by controlling the micelle size, the pore size in the resultant titania particles can be controlled, with lower nucleation levels giving rise to larger pore sizes in the resultant titania particles.

(28) Accordingly, a desired set of properties in the end product can be obtained by suitable control of the parameters in the process of manufacture of the titania.

(29) Transmission electron microscopy (TEM) was also carried out to image the particles of the product obtained using 6% nucleation at precipitation. The obtained image is shown in FIG. 2a.

(30) Transmission electron microscopy (TEM) was then carried out to image the micelles produced by sulphate precipitation at nucleation levels of 6%, 2%, 1%, 0.5% and 0.1%. The obtained images are shown in FIGS. 2b-2f respectively.

(31) It can be seen that micelle sizes as large as 150 nm or more can be obtained with a nucleation level of 0.1% or lower. By increasing the nucleation level the size of the micelles decreases. Therefore control of the micelle size can be exerted. As a consequence, the pore size in the resultant titania particles can be controlled.

(32) As discussed above, the present inventors have determined that by controlling the micelle size the pore size in the resultant titania particles can be controlled, with larger micelles giving rise to larger pore sizes in the resultant titania particles.

(33) Accordingly, a desired set of properties in the end product can be obtained by suitable control of the parameters in the process of manufacture of the titania.

Example 4

(34) A range of concentrated slurries were obtained by Mecklenberg precipitation, in accordance with the method of WO2011/033286. A 6% nucleation level was used at precipitation. The titania slurries were peptised with a peptising agent to achieve various pH levels (1.5, 2, 3.25, 4.5 and 5.5). Hydrochloric acid was used as the peptising agent.

(35) The flocculation size of the slurries was determined using X-ray sedimentation method on a Brookhaven machine (BI-XDC X-ray Disc Centrifuge).

(36) TABLE-US-00004 pH from peptisation 5.5 4.5 3.25 2 1.5 Size of Flocculated 1319 962 957 33 14 product in slurry (nm)
It can be seen that at a pH close to the iso-electric point (pH 5-6) there is more flocculation and the slurry is less dispersed.

(37) This leads towards larger pore sizes. It also leads towards particles that have a rough outer surface and that appear fluffy. This was illustrated by the use of scanning electron microscopy.

(38) In this regard, scanning electron microscopy (SEM) was carried out to image the particles of the product obtained. The obtained images are shown in FIGS. 3a-e. FIG. 3a is pH 5.5, FIG. 3b is pH 4.5, FIG. 3c is pH 3.25, FIG. 3d is pH 2, and FIG. 3e is pH 1.5.

(39) It can be seen that at a pH closer to the iso-electric point (pH 5-6), larger pore sizes are obtained and the particles have a rough outer surface and appear fluffy. As the pH moves further away from the iso-electric point, smaller pore sizes are obtained and the particles have a smoother outer surface and are either toroidal or spherical.

Example 5

(40) A range of concentrated sols were prepared by Mecklenberg precipitation, in accordance with the method of WO2011/033286. A 1.8% nucleation level was used at precipitation, peptisation was effected to pH 1.5, and citric acid (dispersant) was added.

(41) The sols were prepared with various levels of citric acid (1%, 2.3%, 3% and 10%) as the dispersant, to give a range of sols with differing extents of flocculation. Subsequently, MIPA was added to take the pH to 8. The particles were then either left unwashed or were washed (to give a conductivity of <2 ms/cm). The sols were then spray dried using a LabPlant Super 7 laboratory spray dryer.

(42) The dried samples were then analysed for surface area via the BET method and porosity by both mercury porosimetry and nitrogen isotherms.

(43) TABLE-US-00005 Citric Acid Level (%) 1 2.3 3 10 Conductivity(ms/cm.sup.1) 20.6 21.1 19.8 20.6 BET SSA (m.sup.2/g) 177.8 179.9 136 75.4 Mercury Large Pore Size 2.599 1.9317 2.2885 1.5703 (m) Mercury Small Pore Size 14.2 14.2 13.1 11.2 (nm) Nitrogen Large Pore Size 39.6 42.5 33.5 26.5 (nm) Nitrogen Small Pore Size 0.67 0.67 0.66 0.64 (nm) Conductivity (ms/cm.sup.1) <2 <2 <2 <2 BET SSA (m.sup.2/g) 248.8 254.1 258 239.6 Mercury Large Pore Size 1.8142 1.6069 1.59 1.9178 (m) Mercury Small Pore Size 14.9 13.2 11.4 8.9 (nm) Nitrogen Large Pore Size 33.5 26.7 21.1 14.8 (nm) Nitrogen Small Pore Size 0.81 0.54 0.54 0.56 (nm)

(44) When lower amounts of dispersant were used, the iso-electric point was closer to the pH of the slurry. This resulted in the slurry being less dispersed.

(45) It can be seen from the results that this use of lower amounts of dispersant (1% and 2.3%) leads to large surface areas, both in the washed and unwashed products. The use of lower amounts of dispersant (1% and 2.3%) also leads to larger pore sizes in the particles, both in the washed and unwashed products.

(46) The porosity results show three distinct pore size regions: >1 um=cavities between particles 5-20 nm=the pores within the particles (between the micelles) 0.6 nm=pores within micelles.

(47) The washing of the particles reduces the level of salt and therefore the conductivity. As the salt level (and therefore conductivity) is reduced, there are fewer charges present causing repulsion between particles and therefore the particles can pack together more closely. In addition, gaps are left behind that were previously filled by salts. This means that a higher surface area can be achieved.

(48) In addition, the gelling behaviour of the sol appears to reduce when the conductivity is lowered, and higher concentrations of particles in the sol may be possible.

Example 6

(49) A range of sols were prepared using a Blumenfeld process with a 70:30 drop ratio and a 10 minute drop time to give a modal micelle size of 23 nm. One sol was prepared in the standard method outlined in WO2011/033286. Another sol was doped with 10% WO.sub.3 in the form of ammonium metatungstate at precipitation, and then processed according to the method outlined in WO2011/033286. A final sol was prepared with 10% silica added in the form of silicic acid; this was added after the peptisation stage, by passing sodium silicate through an ion exchange column to produce silicic acid, after this the sol was prepared as in WO2011/033286.

(50) The sols were then spray dried using a Lab Plant Super 7 laboratory spray dryer.

(51) Scanning electron microscopy (SEM) was carried out to image the particles of the doped products obtained. The obtained images are shown in FIGS. 4a-b. FIG. 4a is 15% silica, FIG. 4b is 10% WO.sub.3.

(52) The spray dried porous titania samples were then calcined at 500 C. for 5 hours, 1 day, 3 days and 7 days. The specific surface areas of the calcined samples were then measured via the BET method.

(53) TABLE-US-00006 BET SSA after calcination at 500 C. (m.sup.2/g) Variant Control 5 hrs 1 day 3 days 7 days Standard 301.3 84.8 77.7 70.5 65.3 Std + 10% 257.7 113.6 112.5 102.0 100.4 WO.sub.3 Std + 15% 278.1 265.2 262.8 257.2 255.4 SiO.sub.2

(54) It can be seen that the use of dopants gives rise to improved thermal stability. In particular, the use of the SiO.sub.2 dopant leads to a product where the particles are sufficiently stable to retain their large surface areas even after high temperature calcination for prolonged periods of time.

Example 7

(55) A range of sols were prepared by the Mecklenberg method with a 6% nucleation level at precipitation. The sols were prepared in the standard way and the diluted to different levels to give sols at a range of solids content (1%, 10%, 17% & 25% wt/wt % solids).

(56) The sols were then dried via a Lab Plant laboratory spray dryer, and particle size measured via the laser diffraction method using a Malvern Instruments Ltd MasterSizer instrument.

(57) TABLE-US-00007 Dryer Feed Concentration (wt/wt %) 1% 10% 17% 25% Particle Size 3.05 6.75 8.59 10.17 (m) Malvern

(58) Therefore it can be seen that particle size can be controlled by controlling the solids content of the spray dryer feed, with higher solids contents leading to larger particles.

(59) The inlet temperature to the dryer was altered (110 C., 150 C., 200 C., 250 C.) to assess the effect of this drying temperature on sols with 1% solids, 10% solids and 17% solids.

(60) Scanning electron microscopy (SEM) was carried out to image the particles of the dried products obtained. The obtained images are shown in FIGS. 5a-d (1% solids), FIGS. 6a-d (10% solids) and FIGS. 7a-d (17% solids).

(61) In each case, image a is after drying at 110 C., image b is after drying at 150 C., image c is after drying at 200 C. and image d is after drying at 250 C.

(62) Therefore it can be seen that particle shape can be controlled via the spray dryer inlet temperature. A lower inlet temperature gives more spherical particles (which can be hollow), whilst a higher temperature leads to the formation of toroidal (doughnut shaped) particles.

Example 8

(63) A concentrated titania sol was obtained by precipitation that was carried out in accordance with the method of WO2011/033286.

(64) The samples used were from a 6% nucleated Mecklenburg precipitation, that had been peptised to pH 1.5, addition with 10% citric acid, MIPA neutralised and CFF washed to <2 ms/cm.

(65) The sol was spray dried using a laboratory scale Lab Plant Super 7 spray dryer, to form porous spherical particles. The sol was at a solids concentration of 17% wt/wt, and was spray dried at a temperature of 110 C.

(66) The dried particles were then dispersed in water at a concentration of 100 g/l.

(67) The resulting dispersion was then milled for 30 minutes using a high shear Silverson mixer. The particle size was measured using a Malvern Instruments Ltd MasterSizer laser diffraction instrument. Measurements were taken prior to milling (0 minutes), during milling (at 10 minutes and 20 minutes) and after milling (at 30 minutes).

(68) TABLE-US-00008 Milling time 0 min 10 min 20 min 30 min d(v, 0.1) m 2.24 1.99 1.95 1.98 d(v, 0.5) m 6.55 6.09 5.98 5.79 d(4, 3) m 8.05 6.89 6.55 6.18 d(v, 0.9) m 14.64 12.64 11.9 10.81 Modal particle size m 7.99 7.15 7.13 7.02

(69) This shows that the particles obtained are very stable under high shear forces.

(70) The experiment was then repeated but with the spray dried particles being thermally treated at 500 C. for 7 days prior to milling, in order to assess whether the stability of the particles was still maintained after heat treatment.

(71) Again, the particle size was measured using a Malvern Instruments Ltd MasterSizer laser diffraction instrument. Measurements were taken prior to milling (0 minutes), during milling (at 10 minutes and 20 minutes) and after milling (at 30 minutes).

(72) TABLE-US-00009 Milling time 0 min 10 min 20 min 30 min d(v, 0.1) m 2.64 2.96 2.34 2.30 d(v, 0.5) m 6.64 7.46 6.01 5.78 d(4, 3) m 8.03 8.85 6.95 6.65 d(v, 0.9) m 14.70 16.02 11.85 11.32 Modal particle size m 7.92 9.16 7.05 6.94

(73) It can be seen that the particles remain very stable under high shear forces even after heat treatment.

(74) High shear mixing stability is important of a predictor of robustness and would indicate good resistance to mechanical stresses, including compressive forces such as those within catalyst installations. This robustness may, for example, be important in end uses relating to catalysis, and especially where extrusion of is required in the manufacture of the catalyst product, such as in SCR and combined SCR/DPF.

Example 9

(75) Blaine data, comparing toroidal particles obtained by the invention and spherical particles obtained by the invention, was obtained by a test carried out according to BS4359: Part 2: 1982.

(76) Both samples were obtained from example 7.

(77) The toroidal sample was one that had been spray dried at a concentration of 10% wt/wt solids and spray dried at 250 C. (i.e. the product shown in FIG. 6d).

(78) The spherical sample was one that had spray dried at a concentration of 10% wt/wt solids and spray dried at 110 C. (i.e. the product shown in FIG. 6a).

(79) TABLE-US-00010 Sample Toroidal Spherical Blaine Porosity 0.720 0.688 Blaine Test - 21180 19869 cm.sup.2/g SG (Pycnometer) - 3.08 2.98 g/cm.sup.3

(80) It can be seen that the toroidal particles obtained by the invention have improved porosity as compared to spherical particles obtained by the invention.

(81) This shows that the process of the invention can be carried out in a manner to ensure that the toroidal shaped particles are obtained when end applications are envisaged that required good permeability. This may, for example, be the case in end uses such as SCR and combined SCR/DPF.

Example 10

(82) Concentrated titania sol products were obtained by precipitation that was carried out in accordance with the method of WO2011/033286. These were obtained via the Blumenfeld method using various drop ratios. Each had a 10 minute drop time.

(83) The micelle size of the titania sols were measured by CPS disc centrifuge particle size analyser.

(84) Samples from each product were spray dried using a LabPlant SD-05 laboratory spray drier. The specific surface areas of each of the dried samples were tested using the BET method. The pore size was measured using mercury porosimetry, using a Micromeritics AutoPore IV porosimeter.

(85) It was seen in these experiments that by altering the drop ratio, the micelle size can be controlled and in turn the pore size can be controlled. In this regard, as the drop ratio was raised from 70:30 up towards 90:10 the micelle size increased, the pore size increased, and the surface area decreased.

(86) TABLE-US-00011 Drop Ratio 90:10 70:30 Micelle size (nm) 56.7 22.1 Pore Size (nm) 23.4 3.6 SSA (m2/g) 170.1 295.1

(87) The results set out in the above table clearly illustrate that altering the drop ratio has a significant effect:changing the drop ratio from 90:10 to 70:30 decreases the micelle size by a factor of over 2.5, and thus gives rise to significantly smaller pore sizes and therefore larger specific surface area values.

(88) Accordingly, a desired set of properties in the end product can be obtained by suitable control of the parameters in the process of manufacture of the titania. For example, if smaller pore sizes (and a higher SSA) is desired in the product, a lower drop ratio can be selected.