Iron-silicon oxide particles having an improved heating rate

Abstract

Core-shell particles containing crystalline iron oxide in the core and amorphous silicon dioxide in the shell and in which a) the shell contains from 5 to 40% by weight of silicon dioxide, b) the core contains b1) from 60 to 95% by weight of iron oxide and b2) from 0.5 to 5% by weight of at least one doping component selected from the group consisting of aluminum, calcium, copper, magnesium, silver, titanium, yttrium, zinc, tin and zirconium, c) where the % by weight indicated are based on the core-shell particles and the sum of a) and b) is at least 98% by weight of the core-shell particles, d) the core has lattice plane spacings of 0.20 nm, 0.25 nm and 0.29 nm, in each case+/0.02 nm, determined by means of HR-TEM.

Claims

1. Core-shell particles comprising crystalline iron oxide in the core and amorphous silicon dioxide in the shell, wherein the shell consists of silicon dioxide and wherein silicon dioxide makes up from 5 to 40% by weight of the total weight of the core-shell particles, the core consists of iron oxide and at least one doping component selected from the group consisting of aluminium, calcium, copper, magnesium, zinc and tin, wherein iron oxide makes up from 60 to 95% by weight of the total weight of the core-shell particles and the total amount of the at least one doping component make up from 0.5 to 5% by weight of the total weight of the core-shell particles, wherein the total weight of the silicon dioxide, the iron oxide, and the at least one doping component equals 100% by weight, the core has lattice plane spacings of 0.20 nm, 0.25 nm and 0.29 nm, in each case+/0.02 nm, determined by means of high resolution transmission electron microscopy.

2. Core-shell particles according to claim 1, wherein no lattice plane spacings which can be assigned to the doping component can be detected by X-ray diffraction or HR-TEM within a crystalline structure of the iron oxide.

3. Core-shell particles according to claim 1, wherein the doping component is aluminium or zinc.

4. Core-shell particles according to claim 3, wherein the proportion of doping component is from 1 to 2% by weight of the total weight of the core-shell particles.

5. Core-shell particles according to claim 1, wherein the ratio of (magnetite+maghemite) to haematite determined by means of X-ray diffraction is from 70:30 to 95:5 and that of magnetite to maghemite is from 50:50 to 90:10.

6. Core-shell particles according to claim 1, wherein a compound comprising the elements iron, silicon and oxygen and in the HR transmission electron micrograph have a spacing of the lattice planes of 0.31+/0.01 nm and are present between the core and the shell.

7. Core-shell particles according to claim 1, wherein the core-shell particles have been modified by adsorption, reaction on the surface or complexation of or with inorganic and organic reagents.

8. A process for producing the core-shell particles according to claim 1, comprising: igniting and reacting a mixture comprising an aerosol obtained by atomization of a solution comprising in each case an oxidizable and/or hydrolysable iron compound, and in each case at least one dopant selected from the group consisting of aluminium, calcium, copper, magnesium, silver, titanium, zinc, tin and zirconium, a hydrogen-containing fuel gas and an oxygen-containing gas in a first zone of a flow reactor; adding a hydrolysable and/or oxidizable silicon compound to the reaction mixture in a second zone of the flow reactor; and then optionally cooling the reaction mixture and separating the solid from materials in gaseous or vapour form; and then optionally subsequently treating the solid with an agent for surface modification.

9. The process according to claim 8, wherein in zone 1 the average residence time is from 10 ms to 1 ms and the temperature is from 800 to 1300 C. and in zone 2 the average residence time is from 0.1 to 10 s and the temperature is from 400 to 900 C.

10. The process according to claim 8, wherein the silicon compound is at least one selected from the group consisting of SiCl.sub.4, CH.sub.3SiCl.sub.3, (CH.sub.3).sub.2SiCl.sub.2, (CH.sub.3).sub.3SiCl, HSiCl.sub.3, (CH.sub.3).sub.2HSiCl and CH.sub.3C.sub.2H.sub.5SiCl.sub.2, H.sub.4Si, Si(OC.sub.2H.sub.5).sub.4 and Si(OCH.sub.3).sub.4.

11. The process according to claim 8, wherein water or steam is additionally introduced in zone 2.

12. The process according claim 8, wherein the agent for modifying the surface is an organosilane, a silazane or a polysiloxane.

13. Core-shell particles according to claim 1, wherein the core-shell particles are suitable as constituent of rubber mixtures, as constituent of polymer preparations, as constituent of adhesive compositions, as constituent of shaped polymer composites which can be obtained by welding in an alternating electromagnetic field, and for producing dispersions and for the immobilization of enzymes.

Description

EXAMPLES

(1) Analysis

(2) To determine the iron oxide content, the sample was homogenized in a laboratory mill and, after decomposition by fusion, determined titrimetrically. The Fe(III) content was determined and the Fe.sub.2O.sub.3 content was calculated therefrom. The content of Si is determined by means of ICP-OES and subsequently calculated as oxide. The content of doping component is determined by ICP-OES after dissolution in mineral acid and converted into oxide contents.

(3) The BET surface area is determined in accordance with DIN 66131.

(4) The determination of the core materials is carried out by X-ray diffraction (Reflexion, / diffractometer, Co-K, U=40 kV, I=35 mA; scintillation counter, adjusted graphite monochromator; angle range (2)/step width/measurement time: 10-100/0.04/6 s (4 h)). A quantitative phase analysis is carried out by the Rietveld method (errors about 10% relative). The quantitative phase analysis is carried out with the aid of set 60 of the ICDD database PDF4+ (2010). The phase analysis and the crystallite size determination are carried out using the Rietveld program SiroQuant, Version 3.0 (2005).

(5) The thickness of the shell is determined by means of high resolution transmission electron microscopy (HR-TEM).

(6) The heating time from 20 C. to 200 C. is determined in a silicone composition. The silicone composition is obtained by mixing 33 g of ELASTOSIL E50, from Momentive Performance Materials, 13 g of silicone oil type M 1000, from Momentive Performance Materials, 4 g of AEROSIL150, from Evonik and 2.5 g, corresponding to 4.76% by weight, of core-shell particles by means of a SpeedMixer for 230 s and 245 s at 3000 rpm. The silicone composition is subsequently applied in a thickness of about 1 mm to a glass microscope slide. The energy input is effected by induction by means of a water-cooled coil having a diameter of 80 mm. The frequency is 510 KHz, and the power is about 12 KW, Fives Celes GTMC 25 KW, France.

(7) Leaching test: 0.33 g of core-shell particles are stored in 20 ml of HCl (1 mol/l) or H.sub.2O.sub.2 (0.5 mol/l) or a solution of 8% by weight of NaCl and 2% by weight of CaCl.sub.2 in water at 60 C. for a period of 12 hours. Part of the solution is subsequently analysed for iron by means of suitable analytical techniques, for example ICP (inductively coupled plasma spectroscopy).

Example 1

(8) An aerosol is produced by atomization of 4500 g/h of an aqueous solution consisting of 26.1 g of iron(II) chloride, 1.3 g of zinc nitrate and 72.6 g of water, in each case per 100 g of solution, and 3.0 kg/h of nitrogen by means of a two-fluid nozzle. The resulting aerosol is reacted with 8.8 standard m.sup.3/h of hydrogen and 19 standard m.sup.3/h of air, of which 15 standard m.sup.3/h is primary air and 4 standard m.sup.3/h is secondary air, in a first zone. The average residence time of the reaction mixture in the first zone is about 540 ms. A mixture of 410 g/h of gaseous Si(OC.sub.2H.sub.5).sub.4 and 4 standard m.sup.3/h of nitrogen and separately 2.5 kg/h of steam are introduced into the stream of the reaction mixture from the first zone. The average residence time of the reaction mixture in the second zone is 1.7 s. The reaction mixture is subsequently cooled and the solid obtained is separated from the gaseous materials on a filter.

(9) Examples 2 to 10 are carried out analogously. Starting materials and conditions are reported in Table 1. The physicochemical properties of the core-shell particles are reported in Table 2.

(10) The powder of Example 6 in EP-A-2000439 is employed as Comparative Example. This is an iron-silicon mixed oxide powder doped with 1.8% by weight of manganese. The heating time from 20 C. to 200 C. is 15 s.

(11) The powder of Example 10 of WO 2012/048985 is employed as further Comparative Example. This is an iron-silicon mixed oxide powder doped with 108% by weight of phosphorus. The heating time from 20 C. to 200 C. is 17 s.

(12) The core-shell particles of the invention display significantly shorter heating times than powders according to the prior art.

(13) TABLE-US-00001 TABLE 1 Starting materials and amounts Reaction zone I Reaction zone II Dopant Iron chloride Water Throughput t.sub.RZ1 .sup.a) T.sub.RZ1 TEOS t.sub.RZ2 T.sub.RZ2 Ex. g/100 g g/h ms C. g/h s C. 1 Zn(NO.sub.3).sub.2 1.3 26.1 72.6 4500 540 979 410 1.7 808 2 2.7 26.7 70.6 4000 543 971 360 1.8 860 3 2.2 27.3 70.5 4500 519 1056 430 1.6 803 4 2.0 27.7 70.3 4400 546 950 430 1.7 850 5 Al(NO.sub.3).sub.3 7.1 21.0 71.9 4500 525 1032 430 1.6 849 6 3.7 25.3 71.0 4500 520 1053 430 1.7 847 7 3.1 26.1 70.8 4500 526 1027 400 1.8 825 8 MgCl.sub.2 8.8 19.4 71.8 4500 540 986 410 1.7 840 9 Cu(NO.sub.3).sub.2 3.4 25.7 70.9 4500 528 1003 430 1.7 837 10 CaCl.sub.2 2.2 27.6 70.2 4500 530 998 430 1.5 835 11 Y(NO.sub.3).sub.2 1.3 26.1 72.6 4500 545 989 410 1.7 810 12 Ag(NO.sub.3).sub.2 2.0 27.7 70.3 4500 560 980 410 1.8 830 13 Zr(NO.sub.3).sub.2 2.2 27.6 70.2 4500 540 980 425 1.7 845 14 TiCl.sub.4 3.4 25.7 70.9 4500 541 985 425 1.8 840 15 C.sub.10H.sub.14O.sub.5Ti 4.0 26.0 70.0 4500 530 1000 430 1.7 846

(14) TABLE-US-00002 TABLE 2 Materials parameters Example 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Doping component Zn Al Mg Cu Ca Y Ag Zr Ti Ti Silicon dioxide % by 15.0 14.1 14.5 14.3 14.0 14.6 14.3 14.5 14.1 13.6 14.5 14.0 14.6 14.5 13.9 weight Iron oxide % by 80.8 84.2 84.1 84.5 83.7 83.9 84.5 81.8 83.5 83.4 84.1 83.7 84.0 84.1 83.4 weight Doping component % by 4.2 1.8 1.4 1.2 2.3 1.5 1.3 3.8 2.4 3.0 1.4 2.3 1.6 1.4 2.7 weight Proportion of iron oxide from XRD (magn. + magh.)/haem..sup.a) 70:30 76:24 85:15 82:18 87:13 85:15 76:24 85:15 82:18 (magn./magh.) 57:43 68:32 65:35 65:35 83:17 83:17 70:30 66:34 70:30 BET surface area m.sup.2/g 9 13 13 14 11 14 14 13 13 11 17 18 19 20 19 Saturation magnetization Am.sup.2/kg 66.7 61.7 54.6 66.1 66.6 65.4 61.6 60.0 65.4 61.7 66.7 Heating time 20 C. -> 200 C. S 8.2 9.2 5.4 5.5 6.4 6.2 5.6 6.0 6.2 8.2 6.2 5.0 5.4 7.0 6.0 Fe in solution HCl ppm 60 240 130 126 131 137 120 100 50 55 60 40 60 H.sub.2O.sub.2 3 2 2.8 3 2 2 3 3 3 3 2 5 4 7 NaCl, CaCl.sub.2 4.5 16 11 17 21 20 21 23 15 21 .sup.a)magn. = magnetite; magh. = maghemite; haem. = haematite