PROCESS FOR PREPARING A POROUS INORGANIC POWDER

Abstract

The present invention is directed to a process for preparing an inorganic aerogel, the process comprising the steps of providing a composition (I) suitable to form an inorganic gel with a gelation time t.sub.G, spraying the composition (I) into supercritical carbon dioxide at a spraying time t.sub.S to obtain gel particles, and drying the gel particles obtained in step (ii) by supercritical liquid extraction, wherein the ratio t.sub.S:t.sub.G is in the range of from 0.2 to 0.99. The present invention further is directed to the inorganic aerogel as such as well as the use of the inorganic aerogel according to the invention in particular for medical and pharmaceutical applications or for thermal insulation.

Claims

1. A process for preparing an inorganic aerogel, the process comprising: (i) providing a composition (I) suitable to form an inorganic gel with a gelation time t.sub.G, (ii) spraying the composition (I) into supercritical carbon dioxide at a spraying time t.sub.S to obtain gel particles, (iii) drying the gel particles obtained in step (ii) by supercritical liquid extraction, wherein the ratio t.sub.S:t.sub.G is in the range of from 0.2 to 0.99.

2. The process according to claim 1, wherein the inorganic aerogel is a silica based aerogel.

3. The process according to claim 1, wherein composition (I) comprises at least one alkoxysilane.

4. The process according to claim 1, wherein the ratio t.sub.S:t.sub.G is in the range of from 0.4 to 0.95

5. The process according to claim 1, wherein the composition (i) further comprises an additive.

6. The process according to claim 1, wherein the carbon dioxide is used for the supercritical liquid extraction according to step (iii).

7. An inorganic aerogel obtained or obtainable by a process according to claim 1.

8. The aerogel according to claim 7, wherein the inorganic aerogel is a silica based aerogel.

9. An article, comprising the inorganic aerogel according to claim 7.

10. The article of claim 9, wherein the inorganic aerogel is a silica based aerogel.

11. The article of claim 9, which is medical article, a pharmaceutical article, an additive for food, a catalyst support, a sensor, or a thermal insulation.

Description

EXAMPLES

[0113] Process for Preparing Silica Aerogel Powder by Spraying into Supercritical CO.sub.2

[0114] 1. Acidic Hydrolysis [0115] A PP-beaker was charged with 170.4 mg hydrochloric acid, 1.36 g water, 7.68 g ethanol, tetraethoxysilane (TEOS) as well as dimethyl diethoxysilane (DMDEOS) in the amounts given in Table 1 and the solution was stirred for 30 min at room temperature. Thereafter the solution was diluted with 100 ml of acetone and homogenized by stirring.

[0116] 2. Basic Condensation [0117] In parallel a second PP-beaker was subsequently charged with 816 mg water, 3.92 g NH.sub.3 which was added in water and 7.68 g ethanol and the solution was homogenized by stirring. At the end of the above mentioned 30 min, the afore mentioned acidic solution was poured into the basic solution and the mixture was stirred for 30 sec. before the stirrer was removed. [0118] The gelling sol was placed in a syringe pump and was sprayed (with 90 ml/min) at a defined relative spray time t.sub.S/t.sub.G (=absolute spray time is (from mixing acidic and basic solution)/gelation time t.sub.G into an autoclave prefilled with superdritical CO2 (p=120-140 bar, T=50-60° C.). Waiting time, before starting supercritical extraction (=Exchange of auto-clave-atmosphere by fresh CO.sub.2) was varied but had no influence on the product quality. After supercritical extraction the autoclave was vented and the product, a white powder, was removed. [0119] As known by the specialist, hydrophobic silica aerogels can be prepared by adding hydro-phobizing agents (e.g. alkyl chlorosilanes or alkyl alkoxysilanes) to the sol. This is also transferable to the process described herein.

TABLE-US-00001 TABLE 1 Exp. 1-11 Exp. 12-22 Reagent Amount [g] Amount [g] TEOS 34.76  33.68 DMDEOS — 0.76 EtOH 7.68 7.68 Water 1.36 1.36 HCl 0.17 0.17 NH3 3.92 3.92 Water 0.82 0.82 EtOH 7.68 7.68 [0120] The reacting sol was injected into the autoclave with a flow rate of constant 90 ml/min. From the experimental data it becomes obvious, that the relative spray time t.sub.S/t.sub.G has a significant influence on the quality of the prepared silica aerogel powder, whereas the waiting time before supercritical extraction has no influence (Table 2). With increasing relative spray time t.sub.S/t.sub.G the aerogel surface area and the pore volume increase. The same trend is observed when hydrophobic aerogel powders were prepared via the process described herein. If the material is sprayed at a relative spray time of one or greater than one (after the point of gelation of the specific system), no product is obtained.

TABLE-US-00002 TABLE 2 spray conditions ambient absolute relative RESULT gelation spray spray autoclave waiting autoklave surface pore Exp. gel time time t.sub.s time pressure time temperature area volume No. t.sub.g [min] [min] t.sub.s/t.sub.g [bar] [h] [° C.] [m.sup.2/g] cc/g 1 17 2 0.12 130 1 53 128 0.34 2 17 2 0.12 134 1 55 177 0.59 3 14 2 0.14 138 15 56  76 0.32 4 20 8 0.40 139 1 55 367 0.99 5 20 8 0.40 136 1 55 362 0.76 6 13 8 0.62 131 15 55 407 1.37 7 17 14 0.82 137 1 55 535 2.2 8 18 13 0.72 133 1 55 647 2.75 9 14 11 0.79 135 1 53 554 4.4 10 18 15 0.83 134 15 55 462 2.35 11 17 20 1.18 132 1 54 — — 12 34 2 0.06 130 1 53 333 0.6 13 31 2 0.06 131 1 56 299 0.43 14 33 2 0.06 138 0.25 53 205 0.36 15 28 2 0.07 136 0.25 50 241 0.2 16 39 15 0.38 129 0.25 55 611 2.07 17 40 15 0.38 134 1 54 411 1.25 18 34 15 0.44 138 1 55 507 2.39 19 40 30 0.75 133 0.25 53 653 2.3 20 36 30 0.83 134 1 55 698 2.34 21 38 30 0.79 139 1 55 647 2.77 22 37 42 1.14 135 1 53 — —

[0121] Pore volumes and BET surface were determined in accordance with ISO 9277:2010.