Metal-doped hydroxyapatite catalyst

Abstract

The present invention provides the use of a metal-doped hydroxyapatite catalyst for highly selective conversion of an alcohol to an aldehyde at low temperatures. More specifically, the invention provides the use of a silver-doped hydroxyapatite catalyst for the highly selective oxidative dehydrogenation of ethanol to acetaldehyde. The present invention also provides the method for converting ethanol to acetaldehyde using a silver-doped hydroxyapatite catalyst.

Claims

1. A method for converting an alcohol to an aldehyde, the method comprising: contacting an alcohol with a doped hydroxyapatite as a catalyst to form an aldehyde, wherein the doped hydroxyapatite has been doped with a dopant selected from the group consisting of a metal, a metal oxide, and mixtures thereof, wherein the metal is a transition metal selected from the group consisting of group 5 transition metals, group 11 transition metals, and chromium.

2. The method according to claim 1, wherein the transition metal comprises at least one of silver, gold, vanadium or chromium.

3. The method according to claim 1, wherein the metal oxide comprises an oxide of a transition metal, and optionally an oxide of a group 3, group 4, group 5, group 6, group 10, group 11 or group 12 transition metal.

4. The method according to claim 1, wherein the metal oxide comprises at least one of silver oxide, gold oxide, vanadium oxide, or chromium oxide.

5. The method according to claim 1, wherein the doped hydroxyapatite is doped with the dopant at an atomic percentage up to about 10 at % and optionally up to 6 at %.

6. The method according to claim 1, wherein the doped hydroxyapatite comprises a stoichiometric hydroxyapatite or a non-stoichiometric hydroxyapatite.

7. The method according to claim 1, wherein the doped hydroxyapatite comprises a non-stoichiometric hydroxyapatite having a Ca/P molar ratio of from about 1.45 to about 1.70 and optionally from about 1.50 to about 1.65.

8. The method according to claim 1, wherein the converting comprises oxidizing the alcohol to the aldehyde via an oxidative dehydrogenation reaction of the alcohol to the aldehyde.

9. The method according to claim 1, wherein the alcohol comprises a lower alcohol of 1 to 6 carbon atoms and optionally ethanol.

10. The method according to claim 1, wherein the aldehyde comprises a lower aldehyde of 1 to 6 carbon atoms and optionally acetaldehyde.

11. The method according to claim 1, wherein the contacting is performed at a temperature of from about 150 C. to about 350 C. and optionally from about 200 C. to about 275 C.

12. The method according to claim 1, wherein the contacting is performed at a pressure of from about 1 atm to about 20 atm and optionally at about 1 atm.

13. The method according to claim 1, wherein the contacting is performed at a weight hourly space velocity of from about 1 h.sup.1 to about 10 h.sup.1 and optionally from about 4 h.sup.1 to about 7 h.sup.1.

14. The method according to claim 1, wherein the doped hydroxyapatite is doped with the dopant at an atomic percentage of up to about 10 at %, and wherein the contacting is performed at a temperature of from about 150 C. to about 350 C.

15. The method according to claim 14, wherein the atomic percentage of the dopant is from about 1 at % to about 6 at % of the doped hydroxyapatite.

16. A method comprising: contacting an alcohol with a doped hydroxyapatite as a catalyst to form an aldehyde at a temperature of from about 150 C. to about 350 C., wherein the doped hydroxyapatite has been doped with from about 0.1 at % to about 10 at % of a dopant selected from the group consisting of silver, gold, vanadium, chromium, silver oxide, gold oxide, vanadium oxide, chromium oxide, and mixtures thereof.

17. The method according to claim 16, wherein the doped hydroxyapatite is doped with about 1 at % to about 6 at % of the dopant.

18. The method according to claim 15, wherein the contacting is performed at a temperature below 300 C.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) The accompanying drawings illustrate disclosed embodiments and serve to explain the principles of the disclosed embodiments. It is to be understood, however, that the drawings are designed for purposes of illustration only, and not as a definition of the limits of the invention.

(2) FIG. 1 is a graph showing the acetaldehyde selectivity of the ethanol conversion over various HAP catalysts with different Ag content (0, 1, 3 and 6 at %) at reaction temperatures between 200 C. and 350 C.

(3) FIG. 2 is a bar chart showing the product selectivity of the ethanol conversion over pure stoichiometric HAP catalyst at reaction temperatures between 175 C. and 350 C.

(4) FIG. 3 is a bar graph showing ethanol conversion and acetaldehyde yield over pure stoichiometric HAP catalyst at reaction temperatures between 175 C. and 350 C.

(5) FIG. 4 is a bar graph showing ethanol conversion and acetaldehyde yield over 1 at % Ag-doped HAP at reaction temperatures between 175 and 350 C.

(6) FIG. 5 is a bar chart showing product selectivity of the ethanol conversion over 1 at % Ag-doped HAP at reaction temperatures between 175 C. and 350 C.

(7) FIG. 6 is a bar graph showing ethanol conversion and acetaldehyde yield over 3 at % Ag-doped HAP at reaction temperatures between 175 and 350 C.

(8) FIG. 7 is a bar chart showing product selectivity of the ethanol conversion over 3 at % Ag-doped HAP at reaction temperatures between 175 C. and 350 C.

(9) FIG. 8 is a bar graph showing product selectivity of the ethanol conversion over 6 at % Ag-doped HAP at reaction temperatures between 175 C. and 350 C.

(10) FIG. 9 is a bar chart showing ethanol conversion and acetaldehyde yield over 6 at Ag-doped HAP at reaction temperatures between 175 C. and 350 C.

(11) FIG. 10 is a graph showing the stability of the 6 at % Ag-doped HAP at 15 hours.

EXAMPLES

(12) Non-limiting examples of the invention will be further described in greater detail by reference to specific Examples, which should not be construed as in any way limiting the scope of the invention. Based on the foregoing disclosure, it should be clear that by the method, the objectives set forth herein can be fulfilled. It is, therefore, to be understood that any variations evidently falling within the scope of the claimed invention and thus, the selection of specific device or apparatus, and specific metals modified on the HAP support can be determined without departing from the scope of the disclosure. Thus, the scope of the disclosure should include all modification and variation that may fall within the scope of the claims.

Example 1

Preparation of the Ag-Doped HAP

(13) Materials:

(14) Analytical grade calcium hydroxide (Ca(OH).sub.2), analytical grade silver nitrate (AgNO.sub.3) and analytical grade phosphoric acid (H.sub.3PO.sub.4 (85 wt %)) were obtained from commercial sources and used without further purification.

(15) Preparation Procedure:

(16) Samples of pure HAP and Ag-doped HAP were prepared by a co-precipitation route. The general formula of calcium hydroxyapatite can be expressed as:
Ca.sub.10-xAg.sub.x(PO.sub.4).sub.6(OH).sub.2, where 0x0.6.

(17) In a typical preparation method for pure HAP, Ca(OH).sub.2 powder was suspended in deionised water followed by addition of AgNO.sub.3 into the suspension in stoichiometric amount. In a typical preparation method for Ag-doped HAP, Ca(OH).sub.2 powder was suspended in deionised water followed by addition of AgNO.sub.3 into the suspension in stoichiometric amount. For both pure and Ag-doped HAP, 0.1 M H.sub.3PO.sub.4 was dispensed from a burette into the suspension containing the metal ions at room temperature with constant magnetic stirring. After all the H.sub.3PO.sub.4 was added, the mixture (precursor solution) was warmed and maintained at 70 C. for 90 minutes. The mixture was subsequently transferred to an oven at 100 C. for 2 hours. The mixture was then removed from the oven and aged overnight at room temperature. The precipitate from this mixture was washed repeatedly with deionized water and filtered under vacuum. The final precipitate was obtained after drying the wet precipitate in an oven at 100 C. overnight and grinding with an agate mortar and pestle to obtain a fine homogeneous pure or Ag-doped HAP powder. The resultant pre or Ag-doped HAP powder was loaded in an alumina crucible and calcined in a gas tube furnace under flowing oxygen at 600 C. for 1 hour.

(18) TABLE-US-00001 TABLE 1 The physical characterisation of pure HAP and Ag-doped HAP. Ag content BET specific surface Catalyst (at %) area (m.sup.2/g) Pure HAP 0 75 1 at % Ag-HAP 1 65 3 at % Ag-HAP 3 70 6 at % Ag-HAP 6 41

(19) Table 1 shows some of the physical characteristics of the prepared pure and Ag-doped HAP. The Brunauer-Emmett-Teller (BET) surface areas were measured using the N.sub.2 physisorption method, and shows that the pure HAP has a BET surface area of 75 m.sup.2/g which decreases with increasing Ag-content. The HAP sample with 6 at % Ag showed a BET surface area of only 41 m.sup.2/g. The decrease in surface area with increasing Ag content can be attributed to the progressive blocking of the HAP pores by the Ag metal.

Example 2

Overview of Catalytic Testing

(20) The reaction was carried out at reaction temperatures between 175 C. and 350 C., more preferably between 200 C. and 275 C., and at atmospheric pressure using 200 mg catalyst. The method described in the present disclosure allows for the production of acetaldehyde selectively from ethanol at appropriate reaction temperatures.

(21) The Reactor:

(22) The reactor used for this process had three zones. The first zone was loaded with 3 mm diameter glass beads, acting as a pre-heater as well as a mixing zone for the ethanol and air feedstock. The second zone was loaded with the catalyst, which contacted with the vaporized feedstock from the first zone. The second zone was also the reaction zone in which the ethanol was converted to aldehyde in the presence of a catalyst. The third zone was the post-reaction zone. The reaction temperatures of the three zones were kept constant by three-heating zone heaters and the catalyst temperature was monitored using a thermocouple inside the catalytic bed.

(23) Priming of the Catalyst:

(24) Approximately 200 mg of catalyst (sieve size 400 m to 250 m) was diluted with equal amount of quartz and loaded into a down flow fixed bed stainless steel reactor. Prior to the reaction, the catalysts were in situ treated under N.sub.2 gas at 175 C. for 1 hour.

(25) Testing Procedure:

(26) The disclosed experiments were carried out in a fixed bed reactor using 200 mg of catalyst (sieve size 400 m to 250 m). Ethanol (Fischer Scientific, HPLC grade) was fed into the reactor with a fixed flow rate of 0.025 mL/min at a weight hourly space velocity (WHSV) of 5.9 h.sup.1, with a simulated air mixture (60 mL/min) and preheated at 175 C. before entering into the reactor. All the gas flows were supplied into the system by employing pre-calibrated mass flow controllers. The reaction was carried out in the temperature range of 175 C. and 350 C., at atmospheric pressure. During the reaction, the ethanol flow rate was set at 0.025 mL/min with a corresponding weight hourly space velocity (WHSV) of 5.9 h.sup.1. Synthetic air with a flow rate of 60.0 mL/min was also introduced. The products were analysed by an online Gas Chromatography (GC) equipped with both a Flame Ionisation Detector (FID) and Thermal Conductivity Detector (TCD).

(27) Data Analysis:

(28) The analysis of the reaction products along with the reactants was performed using an online gas chromatograph (Agilent 6890) equipped with a flame ionization detector using a HP-5 capillary column- and thermal conductivity detector using a Hayesep D column. The GC was pre-calibrated using standards (reactant and products). The conversions of ethanol and selectivity to forming acetaldehyde were calculated as follows:

(29) $\begin{array}{cc}{X}_{\mathrm{EtOH}}\left(\%\right)=\frac{N_{\mathrm{EtOH}}^{in}-N_{\mathrm{EtOH}}^{\mathrm{out}}}{N_{\mathrm{EtOH}}^{in}}*100& \left(1\right)\\ S_{\mathrm{acetaldehyde}}\left(\%\right)=\frac{N_{\mathrm{acetaldehyde}}}{\underset{i-1}{\overset{n}{.Math.}}{N}_{{\mathrm{product}}_i}}*100& \left(2\right)\\ Y_{\mathrm{acetaldehyde}}\left(\%\right)=\frac{X_{\mathrm{EtOH}}*S_{\mathrm{acetaldehyde}}}{100}& \left(3\right)\end{array}$

(30) where X.sub.EtOH, is the conversion in mole percentage of ethanol, N.sup.in.sub.EtOH is the number of moles of ethanol fed into the reactor and N.sup.out.sub.EtOH is the number of moles of ethanol observed in the products. S.sub.acetaldehyde, is the selectivity towards acetaldehyde product in mol %, N.sub.acetaldehyde is the number of moles of acetaldehyde product observed in the reaction products and .sub.i=1.sup.n N product.sub.i is the total number of moles of reaction products. Y.sub.acetaldehyde is the yield of acetaldehyde in mold.

Example 3

Pure HAP and Ag-Doped HAP

(31) Under similar reaction conditions as outlined in Example 2, HAP samples with various amounts of Ag-doping (0, 1, 3 and 6 at %) were tested for selective partial oxidation of ethanol at reaction temperatures between 200 C. and 350 C. This particular temperature range was selected, as the optimum temperature range for selective conversion of ethanol to aldehyde is between 200 C. and 300 C. 200 mg of catalyst, WHSV of 5.9 h.sup.1, ethanol flow rate of 0.025 mL/min and an oxygen flow rate of 12.0 mL/min was used. As seen in FIG. 1, of all the catalysts screened, the 3 at Ag-doped HAP and the 6 at % Ag-doped HAP exhibited the highest catalytic activity. It is also worth noting that the light off temperature, or the temperature at which total combustion and CO.sub.2 production begins to occur, was observed to decrease with increasing Ag content of the HAP. It appears that high Ag content favours selective production of acetaldehyde at low reaction temperatures. Further, it can be seen that Ag-doped HAP catalysts invariably showed better acetaldehyde selectivity than pure HAP at temperatures up to 325 C.

(32) Pure HAP (Control)

(33) Under similar reaction conditions as outlined in Example 2, the catalytic activity of pure HAP was tested, as shown in FIG. 2 and FIG. 3. In both FIG. 2 and FIG. 3, 200 mg of pure HAP catalyst, WHSV of 5.9 h.sup.1, ethanol flow rate of 0.025 mL/min and an oxygen flow rate of 12.0 mL/min was used. The pure HAP was shown to have maximum ethanol conversion of 28% at 350 C. Ethanol conversion was observed to increase with increasing temperature, but acetaldehyde selectivity decreased sharply in the temperature range of 200 C. to 275 C., and continued to decrease at higher temperatures. The pure HAP catalyst showed maximum acetaldehyde selectivity of approximately 65% at 200 C., but with a very low conversion of ethanol, at approximately 6%. Therefore, higher selectivity for acetaldehyde was observed at lower reaction temperatures. These results clearly indicate that pure HAP is not a suitable catalyst for selective conversion of ethanol to acetaldehyde.

(34) 1 at % Ag-Doped HAP

(35) Under similar reaction conditions as outlined in Example 2, the catalytic activity of 1 at Ag-doped HAP was tested, as shown in FIG. 4, and FIG. 5. In both FIG. 4 and FIG. 5, 200 mg of pure HAP catalyst, WHSV of 5.9 h.sup.1, ethanol flow rate of 0.025 mL/min and an oxygen flow rate of 12.0 mL/min was used. The 1 at % Ag-doped HAP was shown to have ethanol conversion of 49% at 300 C., which is significantly higher in comparison to pure HAP which was shown to have ethanol conversion of 15% at the same temperature. It is important to note that acetaldehyde selectivity did not reach 100% throughout the temperature range tested for both pure and 1 at % Ag-doped HAP. The maximum acetaldehyde selectivity observed for 1 at % Ag-doped HAP was 90% with an ethanol conversion of 30% at 275 C.

(36) 3 at % Ag-Doped HAP

(37) Under similar reaction conditions as outlined in Example 2, the catalytic activity of 3 at % Ag-doped HAP was tested, as shown in FIG. 6 and FIG. 7. In both FIG. 6 and FIG. 7, 200 mg of pure HAP catalyst, WHSV of 5.9 h.sup.1, ethanol flow rate of 0.025 mL/min and an oxygen flow rate of 12.0 mL/min was used. The 3 at % Ag-doped HAP was shown to increase ethanol conversion as reaction temperature increased, but acetaldehyde selectivity decreased at temperatures greater than 250 C. Above this temperature, acetaldehyde yield decreased significantly. In contrast, CO.sub.2 selectivity significantly increased at temperatures greater than 250 C. Ethanol conversion was observed to increase with increasing temperature, the maximum being at 87.4% at 350 C. However, acetaldehyde selectivity decreased with temperature due to considerable formation of CO.sub.2 above 250 C. The maximum acetaldehyde yield was observed at 350 C.

(38) 6 at % Ag-Doped HAP

(39) Under similar reaction conditions as outlined in Example 2, the catalytic activity of 3 at % Ag-doped HAP was tested, as shown in FIG. 8 and FIG. 9. In both FIG. 8 and FIG. 9, 200 mg of pure HAP catalyst, WHSV of 5.9 h.sup.1, ethanol flow rate of 0.025 mL/min and an oxygen flow rate of 12.0 mL/min was used. The 6 at % Ag-doped HAP was shown to have similar catalytic activity to the 3 at Ag-doped HAP, with 100% acetaldehyde selectivity at a reaction temperature up to 250 C. However, there is less formation of CO.sub.2 than with the 3 at Ag-doped HAP.

(40) The Pure HAP catalyst exhibited ethanol conversion below 30% even at relatively high temperature (350 with a significant amount of acetaldehyde formation. In contrast, Ag-doped HAP catalysts exhibited high ethanol conversion and higher selectivity for acetaldehyde. In general, Ag-doped HAP showed better ethanol conversion and acetaldehyde selectivity compared to pure HAP, even at lower temperatures of reactions. It is clear that the selective conversion of ethanol to acetaldehyde can be improved by adding metals such as Ag to an HAP catalyst.

Example 4

Catalytic Stability of Ag-Doped HAP

(41) The 6 at % Ag-doped-HAP was tested for catalytic stability at reaction temperatures in the range of 150 C. to 350 C. Instead of using simulated air mixture, purified air was used for this experiment. An ethanol flow rate of 0.050 mL/min, WHSV of 11.8 h1, and a purified air flow rate of 40 mL/min was used. The catalyst was observed to have stable activity over a 15 hours reaction time (data not shown). As shown in FIG. 10, at 225 C., acetaldehyde yield was 79% with an ethanol conversion of 87% and acetaldehyde selectivity of 91%. At a slightly higher temperature of 250 C., acetaldehyde yield was 89% with an ethanol conversion of 97% and acetaldehyde selectivity of 92%. At temperatures greater than 250 C., CO.sub.2 began to form, decreasing the acetaldehyde yield.

Applications

(42) The disclosed use of a metal-doped hydroxyapatite as a catalyst for converting an alcohol to an aldehyde may improve the conversion and selectivity of the reaction.

(43) The disclosed use may be a useful alternative for converting ethanol to acetaldehyde.

(44) The disclosed use may facilitate an oxidative dehydrogenation reaction of an alcohol to an aldehyde, improving the conversion, yield and selectivity of the reaction.

(45) The disclosed use may allow the use of low-value feedstock such as ethanol to be converted to high-value chemicals such as acetaldehyde.

(46) The disclosed use may contribute to more cost-effective production of high-value chemicals that use acetaldehyde as a precursor.

(47) The disclosed use may allow the use of bio-ethanol as a feedstock, making the bulk production of acetaldehyde cost-effective and environmentally friendly.

(48) The disclosed use may allow the use of inexpensive materials such as hydroxyapatite to be used as a catalyst, therefore making the bulk production of acetaldehyde more economical.

(49) The disclosed use may be applied to developing a use for other metals and metal oxides supported on hydroxyapatite to catalyse other oxidation reactions.

(50) The disclosed method may allow the reaction to be carried out at low temperatures below 300 C., decreasing the formation of by-products.

(51) It will be apparent that various other modifications and adaptations of the invention will be apparent to the person skilled in the art after reading the foregoing disclosure without departing from the spirit and scope of the invention and it is intended that all such modifications and adaptations come within the scope of the appended claims.