Sol-gel process for synthesis of nanocrystalline oxides

Abstract

A Continuous flow synthesis of nanocrystalline metal oxides by rapid sol-gel process is disclosed. The process disclosed uses an impinging microjet micromixer device to obtain the nano crystalline metal oxides. A method of fabricating and assembling the impinging microjet micromixer is also disclosed herewith.

Claims

1. A sol-gel process for continuous flow synthesis of a nanocrystalline metal oxide using an impinging jet micromixer, wherein the impinging jet micromixer comprises an inlet for reactant 1 and an inlet for reactant 2 each connected to a metallic block having a microscopic bore, the metallic block is connected to a support plate using a support tension spring and a screw for adjusting an angle of impinging jets, wherein a mixing zone is formed by the impinging jets coming out of the microscopic bore, further wherein an angle between the impinging jets is 70-120 degrees and a ratio of a length of the mixing zone to a thickness of the mixing zone is 0.6-1.2, the sol-gel process comprises the steps of: i. pumping a water solution in a solvent and a metal alkoxide solution in the solvent continuously through the inlet for reactant 1 and the inlet for reactant 2, respectively, followed by mixing, in the mixing zone: ii. synthesizing a wet gel at a flow rate of 10 to 20 ml/min with a jet diameter of 100-1000 micron and at the angle 70-120 degrees; iii. ageing the wet gel obtained in step (ii) to provide an aged gel, and iv. vacuum drying the aged gel at a temperature of 70 to 90 C. for a period of 8 to 12 hours, followed by calcination at a temperature of 350-600 C. to obtain the nanocrystalline metal oxide.

2. The process according to claim 1, wherein the solvent used is a mixture of methanol and toluene in a toluene to methanol volume ratio of 1.60.

3. The process according to claim 1, wherein a rate of mixing is faster than a rate of reaction to achieve a homogeneous nucleation.

4. The process according to claim 1, wherein the metal alkoxide is selected from the group consisting of Zirconium alkoxide, Strontium alkoxide, and Magnesium alkoxide.

5. The process according to claim 1, wherein the nanocrystalline metal oxide is selected from the group consisting of Zirconium oxide, Strontium oxide, and magnesium oxide.

6. The process according to claim 1, wherein a molar ratio of metal alkoxide to water in step (i) is 1:2 to 1:5.

7. The process according to claim 1, wherein the ratio of the length of the mixing zone to the thickness of the mixing zone is 1.

8. The process according to claim 1, wherein the thickness of the mixing zone is 7.2 to 20.7 m.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1: Schematic of the impinging jet micromixer having (1) and (2) Inlets for reactant, (3) support plates, (4) support tension springs, (5) screw for adjusting angle of the impinging sections, (6) metallic blocks having microscopic bore and (7) impingement/mixing zone.

(2) FIG. 2: (A) XRD (B) N.sub.2 sorption isotherms of the NCMgO samples and inset of (B) shows the pore size distribution of the samples which were synthesized at different angle between the jets.

(3) FIG. 3: (A) Schematic of the mixing zone (B) High speed camera images of the mixing zone at different angle between the jets.

(4) FIG. 4: (A) XRD (B) N.sub.2 sorption isotherms of the NCMgO sample and inset of (B) shows the pore size distribution of the samples which were synthesized at different flow rates, the angle between the jets was kept constant (120).

(5) FIG. 5: (A) Schematic of the mixing zone (B) High speed camera images of the mixing zone at different flow rates and the angle between the jets is 120.

DETAILED DESCRIPTION OF THE INVENTION

(6) The invention discloses a continuous flow approach for the synthesis of nano-crystalline oxides. The invention further discloses a transformed rapid sol gel process to continuous flow process using an impinging jet micromixer wherein high surface area is achieved for an impingement angle of 120, flow rates for individual reactants (jet) in the range of 10 to 20 mL/min (for the jet diameter 300 micron). The invention further provides a device wherein the reaction may be carried out to obtain metal nanocrystals of desired parameters preferably NCMgO.

(7) In accordance with the current invention the nanocrystalline metal oxides which may be prepared may be picked from Zirconium, Strontium, and MgO (NCMgO), preferably NCMgO.

(8) Present invention provides a continuous flow synthesis of metal oxides by the rapid sol-gel process for the synthesis of nanocrystalline MgO (NCMgO) wherein the process comprises: Preparing solution of 0.5-1.0 M H.sub.2O and 0.1 to 0.6M Mg(OCH.sub.3).sub.2 using methanol and toluene mixtures such that the toluene to methanol volume ratio becomes 1.60 upon the addition of equal amounts of both the reactants. pumping of these water and solutions of metal alkoxides in solvent continuously through inlets (1) and (2) followed by mixing in a mixing zone formed by the impinging jets coming out of the bores in section (6) of the device; Wet gel samples were synthesized at flow rates in the range of 10 to 2 mL/min (for the jet diameter 100 to 1000 micron) and at angles (between jet)s varying in the range of 70-140 degree. The gel was collected in samples vials and allowed to age for 1 day, then vacuum dried at 70-90 C. for 8-12 hours followed by calcination at range 350-600 C. for 4-6 hours. The gels were dried to yield NCMgO of BET surface area ranging from 220-520 m.sup.2/g and subjected to characterization.

(9) The BET surface area of the nanocrystalline metal oxide obtained is preferably in the range of 250-350 m.sup.2/g.

(10) The invention the angle between the jets (impingement) is between the range of 70-120 degrees.

(11) The average aspect ratio between the jets is between the range of 0.6-1.2 and preferably 1.

(12) The average thickness of the mixing zone in the micromixer is in the range of 7.2 to 20.7 m.

(13) The flow rates for individual reactants (jet) are is in the range of 10 to 20 mL/min.

(14) The average crystallite size of nanocrystalline metal oxides is in the range of 4.5-6.0 m.

(15) The microjet is fabricated and assembled such that the method comprises: attaching two micro-machined segments on a backbone structure that allows changing the angle between the microchannels in the same plane. A detailed schematic is shown in FIG. 1.

(16) The microchannels of 0.3 mm diameter were machined in SS316 segments.

(17) The two segments can be adjusted simultaneously to get equal angular distance from the point of jet interaction. Two reactant fluids were pumped in the individual segments.

(18) The injected fluids exit the segments at high velocity and intersect to yield a thin sheet of mixing zone followed by a thread.

(19) The velocity of the jets was adjusted to get stable mixing zone. The images of the mixing zone at different jet velocities and at different angle between the jets were recorded by using high speed camera with a frame rate of 500 frames per second (Red lake, USA). The images were analyzed using Image-Pro Plus (version 5.1) software.

(20) Effect of Jet Impingement Angle (.,)

(21) Considerable change in the area and thickness of the mixing zone was observed by varying the angle of impingement between the jets. The volumetric flow rate of the individual jets was maintained as 15 mL/min to obtain strong and stable jets followed by a stable mixing zone. FIG. 2 shows the x-ray diffraction and N.sub.2-sorption isotherms of the gel samples collected. The summary of the characterization details of the sample which were synthesized at different angle between the jets is presented in Table 1.

(22) TABLE-US-00001 TABLE 1 Angle BET Crystallite between Surface Pore size (nm) from Sample the jets area (m.sup.2/g) volume (cc/g) XRD (nm) 1 70 299 0.69 4.7 2 90 321 0.72 5.5 3 120 339 0.77 4.7 4 140 228 0.59 5.8

(23) There was a small but gradual increase in the BET surface area of the samples obtained as the angle of jet impingement varied from 70 to 120. Maximum BET surface area (350 m.sup.2/g) was observed at 120. Upon increasing the angle further from 120 to 140, BET surface area of the sample decreased from 350 to 228 m.sup.2/g. This reduction in the surface area is expected to be a strong function of the nature of mixing and the local homogeneity in the mixing zone. In view of this, experiments were carried out to understand the nature of mixing zone using high speed imaging (Typical residence time in the mixing zone was in the range of 0.5 to 20 ms). FIG. 3A shows the schematic of the mixing zone and 3B represents the high speed camera images of the mixing zones at different impingement angles between the jets (.sub.J). The aspect ratio (a/b) of the mixing zone and thickness of the mixing zone at different angles between the jets, analyzed using high speed camera images. It can be noticed that very thin mixing zone with aspect ratio close to unity (i.e. ab) was obtained when .sub.J was 120. Since the volume of the reagents pumped was constant and equal flow rates were maintained for both the jets, the enhancement in the mixing was purely due to a thinner mixing zone with larger area. This enhanced mixing helps to achieve uniformity in the concentration and thereby have uniform reaction rate (more homogeneous nucleation) which is mandatory and favorable for fast reactions. Table 2 summarizes the comparison between thickness of the mixing zone and surface area of the sample at different jet impingement angles. Since the impingement region for and beyond 140 no more remains planar, it was difficult to measure the aspect ratio and average thickness of the same.

(24) TABLE-US-00002 TABLE 2 Effect of Reynolds Number (R.sub.e) Average thickness of BET surface Angle between Average the mixing area the jets aspect ratio zone (m) (m.sup.2/g) ~70 2.1 14.3 299 ~90 1.8 13.4 320 ~120 1.5 8.7 340 ~140 228

(25) Apart from the dimensions of the impinging region, its structure and the residence time in the mixing zone also affects the extent of mixing in these domains. Based on our observations that high surface area is achieved for an impingement angle of 120, further studies were carried out to understand the effect of residence time and structure of mixing zone on the properties of dry gel at this angle. The jet Reynolds number (Re.sub.j=Dup/) was varied by changing the superficial jet velocity (u) while keeping the jet diameter (300 m) and angle (.sub.j=120 constant. With increasing velocity or flow rate of the reactants, the shape of the impingement zone (mixing zone) changed considerably thereby changing its and thickness. This would affect the surface area of the material significantly. The flow rates for individual reactants (jet) were varied in the range of 10 to 20 mL/min, beyond which it was difficult to get stable mixing zone. Table 3 summarizes the analysis of the gel synthesized at different Re.

(26) TABLE-US-00003 TABLE 3 BET Total flow Surface Crystallite rate area Pore size (nm) Sample (mL/min) Re (m.sup.2/g) volume (cc/g) from XRD 1 20 1040 327 0.73 4.9 2 30 1560 322 0.85 4.6 3 40 2080 250 0.59 4.7 batch 191 0.85 5.6

(27) BET Surface area of the samples which were synthesized in continuous flow method was higher than the sample synthesized in batch mode. For all the samples synthesized in continuous flow methods using jet micromixer, H1 type adsorption isotherm was observed, indicating the porous nature of the sample is due to aggregation of spherical particles. While in batch sample, H3 type adsorption isotherm (slit like pores) was observed. Very narrow pore size distribution was observed in the sample synthesized in continuous flow methods than the batch sample. When the total flow rate increased from 30 mL/min to 40 mL/min, the surface area of the synthesized NCMgO decreased from 322 m.sup.2/g to 250 m.sup.2/g. We verified these observations a few times and the results were reproducible within a range of 4%. With increasing liquid flow rate the mixing zone was found to deviate from planar topology, which is also captured in the high speed camera images of the mixing zone.

(28) At the low flow rates, although both the reactant fluids were in contact for sufficiently longer times the thickness of the mixing zone did not affect the final surface area of the material greatly.

EXAMPLES

(29) Following examples are given by way of illustration therefore should not be construed to limit the scope of the invention.

Example 1

(30) A microjet device was fabricated such that it comprises of two micro machined segments attached on a backbone structure that allows changing the angle between the microchannels in the same plane. The microchannels of 0.3 mm diameter were machined in SS316 segments. Individual segments have an inclination of 60 to the abscissa. The two segments can be adjusted simultaneously to get equal angular distance from the point of jet interaction.

Example 2

(31) 0.4 M Mg(OCH.sub.3).sub.2 in methanol and toluene mixtures, such that the toluene to methanol volume ratio becomes 1.60 and water were taken. Both the reactants were pumped at equal flow rates using pumps, Wet gel samples were synthesized at different flow rates and at different angles between jets (table 1, 2 and 3). Subsequently, the gel was collected in samples vials. Gels were allowed to age for 1 day, then vacuum dried at 90 C. for 12 hours followed by calcination at 500 C. for 1 hour. The dried gels were subjected to characterization.

(32) Synthesis of NCMgO was also carried out in batch process. In batch process, to 0.8 M water, equal amount of 0.4 M Mg(OCH.sub.3).sub.2 solution (prepared in toluenemethanol mixtures) was added in a beaker at 27 C. and solution turned to rigid gel within 30 sec. The wet gel was dried as explained above to get NCMgO.

Example 3

(33) Synthesis of Zr02 Gel using Alkoxide as Precursor

(34) 20 ml solution of toluene and n-propanol was prepared (equal volumes of Toluene and n-propanol). Different amounts of concentrated nitric acid (70%) were used as catalyst for this process. Upon mixing the solution of 2 ml of Zirconium propoxide in toluene and n-propanol and the catalyst in aqueous medium it yields gels. With 0.235 ml HNO3 gel is formed within 20 s while with 0.110 ml of HNO3 is formed within 5 s. Dried gel the sample in vacuum drying oven at 383 K. The surface area of gels with catalyst was in the range of 266-278 m.sup.2/g. In the absence of catalyst it takes 2 hours for the formation of gel and the surface area is below 200 m.sup.2/g.

(35) Characterization

(36) FT-IR spectra of the samples were recorded using Perkin Elmer FT-IR spectrophotometer, in the wave number range of 4000-450 cm.sup.1 with a resolution of 4 cm.sup.1. (1) X-ray diffraction patterns of the dried and calcinated samples were recorded on the PanalyticalXpert instrument operated at 40 kV and 30 mA using Cu K radiation. X-ray diffraction pattern of the sample were recorded in the 2e range of 10-80 with scan rate of 2.3/min. (2) N.sub.2 adsorption and desorption isotherms of the samples were recorded by using Quantachrome-Autosorb instrument. Surface area of the sample was calculated by applying BET method to adsorption isotherm (relative pressure in the range of 0.05-0.03). Poresize distribution was calculated by applying BJH method to desorption isotherm. Total pore volume was calculated at the maximum relative pressure value in the isotherm. (3) Small quantity of the sample dispersed in ethanol and the dispersed sample was coated on TEM grid and allowed to dry. Transmission electron micrographs of the sample were recorded using Technai-T20 transmission electron microscopy, operated at 300 kV.

ADVANTAGES OF THE INVENTION

(37) The surface area of the sample, synthesized through continuous flow methods was comparable to market standards with high reproducibility and consistency. The process will have no issues like charging, discharging, cooling/heating of batch etc. So, the overall process time will be significantly smaller than a batch. The synthesis capacity of the device can be increased to any level using multiple impinging jets (anywhere between 10 to 1000, depending upon the requirement) or using a 2D jet of any width to compensate for multiple 1D jets. Post processing time is highly reduced as compared to conventional processes.