HIGH-TEMPERATURE SYNTHESIS OF HEXAALUMINATES BY FLAME SPRAYING PYROLYSIS

Abstract

The invention relates to a process for preparing aluminates of the general formula (I)

A.sub.1B.sub.xAl.sub.12-xO.sub.19-y where A is at least one element from the group consisting of Sr, Ba and La, B is at least one element from the group consisting of Mn, Fe, Co, Ni, Rh, Cu and Zn, x=0.05-1.0, y is a value determined by the oxidation states of the other elements, which comprises the steps (i) provision of one or more solutions or suspensions comprising precursor compounds of the elements A and B and also a precursor compound of aluminum in a solvent, (ii) conversion of the solutions or suspensions or the solutions into an aerosol, (iii) introduction of the aerosol into a directly or indirectly heated pyrolysis zone, (iv) carrying out of the pyrolysis and (v) separation of the resulting particles comprising hexaaluminate of the general formula (I) from the pyrolysis gas.

Claims

1. A process for preparing aluminates of formula (I):
A.sub.1B.sub.xAl.sub.12-xO.sub.19-y wherein A is at least one element from the group consisting of Sr, Ba and La, B is at least one element from the group consisting of Mn, Fe, Co, Ni, Rh, Cu and Zn, x=0.05-1.0, y is a value determined by the oxidation states of the other elements, the process comprising: (i) providing one or more solutions or suspensions comprising precursor compounds of the elements A and B and also a precursor compound of aluminum in a solvent; (ii) converting the solutions or suspensions into an aerosol; (iii) introducing the aerosol into a directly or indirectly heated pyrolysis zone; (iv) carrying out pyrolysis; and (v) separating resulting particles comprising aluminate of formula (I) from the pyrolysis gas.

2. The process according to claim 1, wherein the element A is La and the element B is Co or Ni.

3. The process according to claim 1, wherein the element A is Sr or Ba and the element B is Ni.

4. The process according to claim 1, wherein the precursor compound of the element A or B is an acetylacetonate.

5. The process according to claim 1, wherein the precursor compound of the element A or B is a carboxylate.

6. The process according to claim 5, wherein the carboxylate is 2-ethylhexanoate.

7. The process according to claim 1, wherein the precursor compound of the element A or B is an alkoxide.

8. The process according to claim 1, wherein the precursor compound of the element A or B is a nitrate.

9. The process according to claim 1, wherein the precursor compound of the element A or B is an oxide or hydroxide.

10. The process according to claim 1, wherein the precursor compound of aluminum is an alkoxide.

11. The process according to claim 7, wherein the precursor compound of aluminum is aluminum sec-butoxide.

12. The process according to claim 1, wherein the solvent is xylene.

13. The process according to claim 1, wherein the pyrolysis is carried out at a temperature of from 900 to 1500° C.

14. The process according to claim 1, wherein the pyrolysis zone is heated by a flame.

15-20. (canceled)

Description

EXAMPLES

Chemicals Used

[0076] Lanthanum 2-ethylhexanoate 10% strength in hexane (LEH)

[0077] Lanthanum acetylacetonate (LAA)

[0078] Cobalt acetylacetonate (CoAA)

[0079] Aluminum sec-butoxide (AlsB)

[0080] Xylene (Xyl)

Examples 1 to 12

[0081] The flame synthesis reactor comprises three sections: a metering unit, a high-temperature zone and a quench. By means of the metering unit, the gaseous fuel ethylene, an N.sub.2/O.sub.2 mixture and the metal-organic precursor compounds dissolved in a suitable solvent are fed via a standard two-fluid nozzle (e.g. from Schlick) into the reactor, a combustion chamber which is lined with refractory material or is water-cooled. The reaction mixture is burnt in the high-temperature zone, giving an oxidic product having nanoparticulate properties. Particle growth is stopped by a subsequent quench, in general using nitrogen. The particles are separated off from the reaction offgas by means of a Baghouse filter.

[0082] The schematic structure of the two-fluid nozzle is shown in FIGS. 1a (sectional view) and 1b (plan view).

[0083] The reference numerals have the following meanings: [0084] 1 Two-fluid nozzle [0085] 2 Ethylene/air inlet for support flame [0086] 3 Air inlet [0087] 4 Inlet for precursor solutions

[0088] The experiments were aimed at the synthesis of cobalt-based hexaaluminates or mixtures having a high content of the hexaaluminate phase. Here, numerous synthesis parameters were varied, specifically

i) the temperature of the high-temperature zone (from 1000 to 1200° C.);
ii) the mass flow of the precursor feed (320 or 400 mL/h);
iii) the molar ratio of the precursor compounds;
iv) the molality (0.2 and 0.5 mol/kg) of the precursor solution;
v) the atomization pressure of the two-phase nozzle (1.5, 2 or 3 bar);
vi) the type of lanthanum precursor (LAA or LEH).

[0089] The results show that a relatively high temperature in the reaction zone and the correct molar ratio of the precursors in the precursor solution promote the formation of the hexaaluminate phase. The mass flow, the molality, the atomization pressure of the nozzle (which influences the droplet size) and the type of lanthanum precursor have only a small influence on the formation of the hexaaluminates. However, other product properties such as the crystallite size and the degree of agglomeration are influenced.

[0090] The results of the experiments are summarized in table 1.

TABLE-US-00001 TABLE 1 Burner Quench Nozzle Mass Air C.sub.2H.sub.4 N.sub.2 air Temperature flow Molality β Mass Yield Example [m.sup.3/h] [kg/h] [m.sup.3/h] [bar] [° C.] [mL/h] Reaction mixture [mol/kg] [g] [g] 1 1.6 0.03 50 2 1100 320 87.5 g AlsB; 342.6 g LEH; 7.8 g CoAA 0.5 1400 20.5 562.1 g Xyl 2 50 2 1200 320 88.4 g AlsB; 345.9 g LEH; 6.8 g CoAA 0.5 1000 25.3 558.9 g Xyl 3 50 2 1200 400 89.1 g AlsB; 27.1 g LAA; 6.8 g CoAA 0.5 1000 14.1 877.0 g Xyl 4 50 2 1000 320 89.1 g AlsB; 27.1 g LAA; 6.8 g CoAA 0.5 1000 13.1 877.0 g Xyl 5 1.6 0.15-0.07 50 2 1200 320 93.9 g AlsB; 19.4 g LAA; 6.9 g CoAA 0.5 1000 26.6 879.9 g Xyl 6 50 2 1200 320 94.3 g AlsB; 15.0 g LAA; 6.9 g CoAA 0.5 1000 23.3 881.5 g Xyl 7 50 2 1200 400 94.3 g AlsB; 15.0 g LAA; 6.9 g CoAA 0.5 1000 17.4 881.5 g Xyl 8 50 3.5 1200 400 94.3 g AlsB; 15.0 g LAA; 6.9 g CoAA 0.5 1000 25.4 881.5 g Xyl 9 min. 2 1200 400 94.3 g AlsB; 15.0 g LAA; 6.9 g CoAA 0.5 1000 20 881.5 g Xyl 10 50 2 1200 400 50.3 g AlsB; 7.9 g LAA; 4.7 g CoAA 0.25 1000 5.8 937.0 g Xyl 11 50 1.5 1200 400 94.3 g AlsB; 15.0 g LAA; 8.8 g CoAA 0.5 1000 26.5 881.5 g Xyl 12 50 2 1100 320 88.2 g AlsB; 26.8 g LAA; 7.9 g CoAA 0.5 1000 11 877.1 g Xyl

[0091] In examples 1 to 5, the following constituents were identified qualitatively by means of XRD:

[0092] Main constituents: LaAlO.sub.3 and CoLaAl.sub.11O.sub.19,

[0093] Secondary constituents: cubic Al.sub.2O.sub.3 phase (no α-Al.sub.2O.sub.3)

[0094] Amorphous phase detectable

[0095] In the products from examples 6 to 10, the following constituents were identified qualitatively by means of XRD:

[0096] Main constituents: CoLaAl.sub.11O.sub.19 and cubic Al.sub.2O.sub.3 phase (no α-Al.sub.2O.sub.3)

[0097] Secondary constituent: LaAlO.sub.3

[0098] Amorphous phase detectable

[0099] The crystallite size of the primary particles of the hexaaluminate phase is influenced mainly by the atomization pressure of the two-phase nozzle, the mass flow of the quench and the concentration of the precursor solution used. This crystallite size can be estimated from the XRD pattern and is a few 10 nm (from 10 to 20 nm). The BET surface area is from 60 to 80 m.sup.2/g and is in agreement with the particle size determined by means of XRD.

[0100] A representative X-ray diffraction pattern is shown in FIG. 2.

[0101] In order to determine the catalytic properties, the material was pressed by means of a punch press to give pellets and the pellets were subsequently broken up and pushed through a sieve having a mesh opening of 1 mm. The pellets have a diameter of 5 mm and a height of 5 mm. The target fraction has a particle size of from 500 to 1000 μm.

[0102] Preparation of a Comparative Catalyst

[0103] The comparative catalyst was prepared as described in WO2013/118078. Cobalt nitrate (83.1 g of Co(NO.sub.3).sub.3x6H.sub.2O) and lanthanum nitrate (284.9 g of La(NO.sub.3).sub.3x6H.sub.2O) are dissolved completely in 250 ml of distilled water. The metal salt solution is admixed with 250 g of boehmite, forming a suspension (ratio of Co:La:Al=6:14:80). Disperal from SASOL is used as boehmite.

[0104] The suspension is stirred for 15 minutes by means of a mechanically driven stirrer at a stirrer speed of 2000 revolutions per minute. The dissolved nitrates are precipitated completely by adjusting the pH and separated from the solvent by filtration. After drying and washing of the product, the material is subsequently precalcined at 520° C. in a furnace. The calcined material is then pressed by means of a punch press to give pellets and the pellets are subsequently broken up and pushed through a sieve having a mesh opening of 1 mm. The pellets have a diameter of 13 mm and a thickness of 3 mm. The target fraction has a particle size of from 500 to 1000 μm.

[0105] For the high-temperature calcination, the material obtained after sieving is heated at 1100° C. for 30 hours in a muffle furnace while passing a stream of 6 liter/minute of air over the material. The furnace is heated to the temperature of 1100° C. at a heating rate of 5° C.

[0106] The specific surface area which can be determined by means of the BET method was 8 m.sup.2/g.

Catalysis Experiments

[0107] To determine the catalytic properties and the stability of catalysts, these were subjected to a test procedure consisting of six successive phases under process conditions in a laboratory catalysis apparatus. The individual phases of the test procedure differ in terms of the gas composition H.sub.2:CO.sub.2:CH.sub.4 (v/v/v, see Table 2). The reactions were carried out for all phases at 750° C. and 10 bara at a GHSV of 3000 h.sup.−1. A minimum amount of 20 ml in each case of sample was used for each test.

TABLE-US-00002 TABLE 2 Time on stream/h H2 CO2 CH4 Phase 1 50 2 1 0 Phase 2 56 3 1 0 Phase 3 24 2 1 0.5 Phase 4 28 2 1 1 Phase 5 28 1 1 0.5 Phase 6 33 2 1 0

[0108] The composition of the product fluids obtained in the reactions was determined by means of GC analysis using an Agilent GC. Evaluation of the results of phases 1, 2 and 6 make it possible to determine the activity of the catalyst for the desired RWGS reaction and for the undesirable secondary reaction of methanation of CO.sub.2 (Sabatier process). Phases 3, 4 and 5 of the test procedure make it possible to draw conclusions regarding the influence of hydrocarbons on the RWGS reaction by methane activation and also regarding the carbonization behavior and the deactivation tendency of the catalyst. Comparison of the results of phases 1 and 6 makes it possible to draw conclusions regarding the long-term and carbonization behavior.

[0109] In Table 3, the catalytic properties of the catalyst of the invention (sample 1) and of the comparative catalyst (sample 2) have been compared.

TABLE-US-00003 TABLE 3 Column (C) 1 2 3 4 5 6 Conversion Conversion Conversion Conversion Conversion Conversion of H2 of H2 of H2 of H2 of H2 of H2 in phase in phase in phase in phase in phase in phase 1/% 2/% 3/% 4/% 5/% 6/% Theoretical H2 32 24 31 31 47 31 conversion in equilibrium without CH4 formation Theoretical H2 51 48 28 12 8 51 conversion in equilibrium taking methanation (CH4 formation) into account Sample 1 32 24 31 31 44 31 Sample 2 47 45 34 24 31 45 Column (C) 8 9 Formation Formation of of CH4/ CH4/ 10 11 12 7 mmol/h mmol/h Conversion Conversion Conversion Column per g of per g of of CH4 in of CH4 in of CH4 in 6 − cat in cat in phase phase phase column 1 phase 1 phase 6 3/% 4/% 5/% Theoretical H2 conversion in equilibrium without CH4 formation Theoretical H2 conversion in equilibrium taking methanation (CH4 formation) into account Sample 1 −1% 2 0.5 −4 −2 −1 Sample 2 −2% 57 42 −8 5 12 Sample 1 = hexaaluminate produced according to the invention (flame CoLaAl.sub.11O.sub.19) as per Example 6 Sample 2 = Comparative catalyst (wet-chemically prepared CoLaAl.sub.11O.sub.19)

[0110] The results of the catalysis experiments show the following:

[0111] Column 7: Sample 1 (according to the invention) tends to display a lower carbonization tendency and thus a lower deactivation tendency than Sample 2 (comparison). Both samples display relatively good stability against deactivation.

[0112] Columns 8 and 9: Sample 1 (according to the invention) displays little/barely any methanation. Sample 2 (comparison) displays very distinct methanation.

[0113] Columns 3, 4 and 5: Sample 1 (according to the invention) displays, particularly in the presence of methane, higher or equally high H.sub.2 conversions for the reverse water gas shift reaction compared to Sample 2 (comparison). According to columns 8 and 9, Sample 2 (comparison) catalyzes methane formation to a significantly greater extent, which has to be taken into account when comparing the H.sub.2 conversions as per columns 1, 2 and 6. Owing to the formation of methane, overall higher H.sub.2 conversions are obtained for Sample 2 (comparison). For comparison, the theoretical H.sub.2 conversions with and without methane formation in thermodynamic equilibrium were calculated (rows 1 and 2, Table 3). As can clearly be seen, Sample 1 according to the invention displays no methanation activity.

[0114] Columns 10, 11 and 12: Sample 1 (according to the invention) does not convert methane present in the gas phase in the presence of CO.sub.2 and H.sub.2. The reference catalyst (Sample 2) activates methane and converts it, particularly at relatively high concentrations (see columns 11 and 12), which is disadvantageous for the desired reaction. This is also reflected in the lower H.sub.2 conversions for Sample 2 (comparison) as per columns 4 and 5. Negative conversions (methane formation) results from a slight methanation activity of the samples.