Carbon nitride heterogeneous catalyst containing rhodium, method for preparing the same, and method for preparing acetic acid using the same

Abstract

A carbon nitride heterogeneous catalyst containing rhodium, a method for preparing the catalyst, and a method for preparing acetic acid using the catalyst is disclosed. The heterogeneous catalyst is characterized in that the rhodium metal is contained in carbon nitride which is a support insoluble in a liquid solvent, such as water or alcohol. Thus, the catalyst can easily be separated from a resulting product even by a simple process such as filtration. Accordingly, the carbon nitride heterogeneous catalyst exhibits excellent long-term stability and activity by being capable of overcoming the disadvantages of the method using a conventional homogeneous catalyst and minimizing the phenomenon of rhodium leaching, compared to the results of the conventional homogeneous catalytic reactions. The catalyst can thus be effectively used for the preparation of acetic acid by a carbonylation reaction between methanol and carbon monoxide.

Claims

1. A composite catalyst for carbonylation of methanol to acetic acid, comprising a carbon nitride support and rhodium dispersed in the network of the carbon nitride support to reduce the leaching level of the rhodium in the methanol carbonylation.

2. The composite catalyst of claim 1, wherein the rhodium is contained in an amount of 0.1 wt % to 10 wt % based on the total weight of the carbon nitride.

3. The composite catalyst of claim 1, wherein the composite catalyst has a specific surface area in a range of 0.5 m.sup.2/g to 100 m.sup.2/g.

4. The composite catalyst of claim 1, wherein the carbon nitride is selected from the group consisting of graphite carbon nitride, -carbon nitride, -carbon nitride, cubic carbon nitride, pseudocubic carbon nitride, and combinations thereof.

5. A method of preparing a composite catalyst comprising carbon nitride support and rhodium dispersed therein, comprising heating a rhodium precursor and melamine resin serving as a carbon source at a temperature of 500 C. to 550 C. in a nitrogen atmosphere.

6. The method of claim 5, wherein the rhodium precursor is selected from the group consisting of rhodium chloride, rhodium nitrate, dichloro tetracarbonyl dirhodium, (acetylacetonato)dicarbonyl rhodium, acetylacetonato bis(ethylene)rhodium, dicarbonyl(pentamethyl-cyclopentadienyl)rhodium, and combinations thereof.

7. The method of claim 5, wherein the carbon nitride is selected from the group consisting of graphite carbon nitride, -carbon nitride, -carbon nitride, cubic carbon nitride, pseudocubic carbon nitride, and combinations thereof.

8. A method of preparing acetic acid, comprising reacting at a temperature of 50 C. to 200 C. by injecting a carbon monoxide-containing gas into an methanol containing solution at a pressure of 10 bar to 200 bar in the presence of the composite catalyst according to claim 1.

9. The method of claim 8, wherein the methanol containing solution comprises methanol as a reactant and iodomethane as co-catalyst and water.

10. The method of claim 9, wherein the methanol:iodomethane:water mixing ratio is as follows: 10 parts by weight to 80 parts by weight of methanol; 10 parts by weight to 60 parts by weight iodomethane; and 10 parts by weight to 30 parts by weight water.

11. The method of claim 8, wherein the carbon monoxide-containing gas is a mixed gas of carbon monoxide and nitrogen.

12. The method of claim 11, wherein the mixing molar ratio between carbon monoxide and nitrogen is as follows 7 parts by mole to 9 parts by mole of carbon monoxide; and 1 part by mole to 3 parts by mole of nitrogen.

13. The method of claim 8, wherein, in the molar ratio between methanol and carbon monoxide is maintained in a range of 0.6 to 10.0 based on 1 mole of methanol.

14. A composite catalyst, prepared by the method of claim 5.

15. A method of producing acetic acid by carbonylating methanol in the presence of a composite catalyst comprising a carbon nitride support and rhodium dispersed therein.

16. The method of claim 15, wherein the rhodium is contained in an amount of 0.1 wt % to 10 wt % based on the total weight of the carbon nitride.

17. The method of claim 15, wherein the composite catalyst has a specific surface area in a range of 0.5 m.sup.2/g to 100 m.sup.2/g.

18. The method of claim 15, wherein the carbon nitride is at least one kind selected from the group consisting of graphite carbon nitride, -carbon nitride, -carbon nitride, cubic carbon nitride, pseudocubic carbon nitride, and combinations thereof.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 shows a graph illustrating the yield obtained using a catalyst according to an exemplary embodiment of the present invention.

(2) FIG. 2 shows X-ray diffraction (XRD) graph of heterogeneous Rh-g-C.sub.3N.sub.4 catalysts according to the present invention.

(3) FIG. 3 shows X-ray photoelectron spectroscopy (XPS) graph of heterogeneous Rh-g-C.sub.3N.sub.4 catalysts according to the present invention.

(4) FIG. 4 shows TEM images of Rh-g-C.sub.3N.sub.4 catalysts according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

(5) Hereinafter, the present invention will be described in more detail with reference to the following Examples. However, these Examples are for illustrative purposes only and the scope and contents of the present invention should not be reduced or limited by these Examples. Furthermore, it will be obvious that one of ordinary skill in the art can easily employ the present invention into practice with respect to the present invention in which experimental results are not specifically disclosed, based on the detail of the present invention including Examples provided herein below, and all such modifications and alterations should also belong to the scope of the appended claims.

EXAMPLES

Example 1: Preparation of a Heterogeneous Catalyst, Rh(0.5)-g-C3N4

(6) Powder form of melamine (2.985 was mixed with distilled water (50 mL) and a solution (0.421 g), in which rhodium nitrate (Rh(NO.sub.3).sub.3, 10 wt %) was dissolved in 10 wt % of nitric acid, was added thereto and the mixture was stirred at a rate of 180 rpm/min in a constant-temperature water bath maintained at room temperature. After stirring for 2 hours, the resultant was subjected to drying under reduced pressure while simultaneously increasing the temperature of the constant-temperature water bath to 60 C. The solution, upon completion of drying under reduced pressure, was added into a circulating dryer kept at 80 C. and dried for at least 12 hours to obtain a melamine-rhodium nitrate precursor in the form of reddish-brown powder. The precursor was added into a tube reactor and subjected to a thermal reaction while flowing a nitrogen gas thereinto at a rate of 50 mL/min. For the thermal reaction, the temperature of the tube reactor was increased from room temperature to 250 C. at a rate of 1.9 C./min, maintained at 250 C. for 30 minutes, increased from 250 C. to 350 C. at a rate of 1.7 C./min, and maintained at 350 C. for 30 minutes. Then, the temperature was sequentially increased from 350 C. to 550 C. at a rate of 3.3 C./min and maintained at 550 C. for 240 minutes. As a result, a rhodium-containing carbon nitride catalyst, in which rhodium metal (0.5 wt %) was positioned in a carbon nitride support, was prepared in the form of dark reddish-brown powder by the slow thermal condensation.

Example 2: Preparation of a Heterogeneous Catalyst, Rh(1)-g-C3N4

(7) A heterogeneous catalyst was prepared in the same manner as in Example 1, except that the amount of rhodium nitrate and melamine used in Example 1 was changed to 0.843 g of rhodium nitrate and 2.970 g of melamine so that the rhodium element could be contained in an amount of 1 wt % based on the total amount of the heterogeneous catalyst. The catalyst prepared by the method of Example 2 was indicated as Rh(1)-g-C.sub.3N.sub.4.

Example 3: Preparation of a Heterogeneous Catalyst, Rh(5)-g-C3N4

(8) A heterogeneous catalyst was prepared in the same manner as in Example 1, except that the amount of rhodium nitrate and melamine used in Example 1 was changed to 2.808 g of rhodium nitrate and 1.90 g of melamine so that the rhodium element could be contained in an amount of 5 wt % based on the total amount of the heterogeneous catalyst and the catalyst had a specific surface area of 7.032 m.sup.2/g. The catalyst prepared by the method of Example 3 was indicated as Rh(5)-g-C.sub.3N.sub.4.

Comparative Example 1: Preparation of a Heterogeneous Catalyst, g-C3N4

(9) A heterogeneous catalyst was prepared in the same manner as in Example 1, except that only melamine was heat-treated without adding any metal precursor at all. The catalyst prepared by the method of Comparative Example 1 was indicated as g-C.sub.3N.sub.4.

Comparative Example 2: Preparation of a Heterogeneous Catalyst, Sm-g-C3N4

(10) A heterogeneous catalyst was prepared in the same manner as in Example 1, except that rhodium nitrate used as a metal precursor in Example 1 was replaced with samarium nitrate hexahydrate (Sm(NO.sub.3).sub.3.Math.6H.sub.2O) and the amount used was changed to 1.035 g of samarium nitrate hexahydrate and 6.65 g of melamine so that the samarium element could be contained in an amount of 5 wt % based on the total amount of the heterogeneous catalyst. The catalyst prepared by the method of Comparative Example 2 was indicated as Sm(5)-g-C.sub.3N.sub.4.

Comparative Example 3: Preparation of a Heterogeneous Catalyst, Ni-g-C3N4

(11) A heterogeneous catalyst was prepared in the same manner as in Example 1, except that rhodium nitrate used as a metal precursor in Example 1 was replaced with nickel nitrate (Ni(NO.sub.3).sub.2.Math.6H.sub.2O) and the amount used was changed to 1.752 g of nickel nitrate and 6.65 g of melamine so that the nickel element could be contained in an amount of 5 wt % based on the total amount of the heterogeneous catalyst. The catalyst prepared by the method of Comparative Example 3 was indicated as Ni(5)-g-C.sub.3N.sub.4.

Comparative Example 4: Preparation of a Heterogeneous Catalyst, Cu-g-C3N4

(12) A heterogeneous catalyst was prepared in the same manner as in Example 1, except that rhodium nitrate used as a metal precursor in Example 1 was replaced with kappa nitrate (Cu(NO.sub.3).sub.2.Math.3H.sub.2O) and the amount used was changed to 1.920 g of kappa nitrate and 9.50 g of melamine while simultaneously changing the temperature of the constant-temperature water bath to 50 C. so that the copper element could be contained in an amount of 5 wt % based on the total amount of the heterogeneous catalyst and the catalyst had a specific surface area of 4.585 m.sup.2/g. The catalyst prepared by the method of Comparative Example 4 was indicated as Cu(5)-g-C.sub.3N.sub.4.

Comparative Example 5: Preparation of a Heterogeneous Catalyst, Pd-g-C3N4

(13) A heterogeneous catalyst was prepared in the same manner as in Example 1, except that rhodium nitrate used as a metal precursor in Example 1 was replaced with palladium nitrate (Pd(NO.sub.3).sub.2) and the amount used was changed to 10.829 g of palladium nitrate and 9.50 g of melamine while simultaneously changing the temperature of the constant-temperature water bath to 55 C. so that the palladium element could be contained in an amount of 5 wt % based on the total amount of the heterogeneous catalyst and the catalyst had a specific surface area of 4.486 m.sup.2/g. The catalyst prepared by the method of Comparative Example 5 was indicated as Pd(5)-g-C.sub.3N.sub.4.

Comparative Example 6: Preparation of a Heterogeneous Catalyst, Ru-g-C3N4

(14) A heterogeneous catalyst was prepared in the same manner as in Example 1, except that rhodium nitrate used as a metal precursor in Example 1 was replaced with ruthenium nitrosyl nitrate (Ru(NO)(NO.sub.3).sub.3) and the amount used was changed to 16.667 g of ruthenium nitrosyl nitrate and 4.475 g of melamine so that the ruthenium element could be contained in an amount of 5 wt % based on the total amount of the heterogeneous catalyst. The catalyst prepared by the method of Comparative Example 6 was indicated as Ru(5)-g-C.sub.3N.sub.4.

Comparative Example 7: Preparation of a Heterogeneous Catalyst, Ir-g-C3N4

(15) A heterogeneous catalyst was prepared in the same manner as in Example 1, except that rhodium nitrate used as a metal precursor in Example 1 was replaced with iridium chloride (IrCl.sub.3) and the amount used was changed to 0.388 g of iridium chloride and 4.750 g of melamine while simultaneously changing the temperature of the constant-temperature water bath to 75 C. so that the iridium element could be contained in an amount of 5 wt % based on the total amount of the heterogeneous catalyst. The catalyst prepared by the method of Comparative Example 7 was indicated as Ir(5)-g-C.sub.3N.sub.4.

Experimental Example 1: Preparation of Acetic Acid by Carbonylation Reaction of Methanol

(16) The carbonylation reactions for preparing acetic acid from methanol and carbon monoxide using the heterogeneous catalysts prepared in Examples 1 to 3 and Comparative Examples 1 to 7 were performed in a 125 mL batch-type autoclave equipped with a Teflon container. The reactants used in the carbonylation reactions were 8 mL of methanol, 10 mL of iodomethane (CH.sub.3I) as a reaction co-catalyst, 2 mL of distilled water, and 0.1 g of the prepared heterogeneous catalyst. For the preparation of the atmosphere of the reaction performed at high pressure, carbon monoxide (a reactant) was mixed with nitrogen (an internal standard material) in a 90:10 molar ratio (carbon monoxide nitrogen) and the mixed gas was injected at a pressure of up to 40 bar to prepare the reaction. Then, the heating process was performed until the internal temperature of the reactor reached 135 C. while stirring the reactants and co-catalyst at 100 rpm. When the internal temperature of the reactor reached 135 C., the heating was stopped and the carbonylation reaction was performed for 7 hours after increasing the stirring speed to 300 rpm. With regard to the reaction, the sample was collected at the time-point of 7 hours after the initiation of the reaction, when the conversion of methanol as a reactant became stabilized at a certain level, and the conversion of methanol and selectivity on products were calculated. The results are shown in Table 1 below.

(17) TABLE-US-00001 TABLE 1 Methanol Conversion Yield of Selectivity (Mol %) (Carbon Acetic Acid Acetic Methyl Leaching Category Mol %) (Mol %) Acid Acetate Others (%) Example 1 88.2 62.4 70.8 24.8 4.4 Example 2 99.4 88.1 88.6 10.6 0.8 Example 3 99.7 93.3 93.6 6.2 0.2 8.2 Comparative 19.8 5.7 29.0 2.3 68.7 Example 1 Comparative 6.5 4.2 65.1 26.7 8.2 Example 2 Comparative 22.6 1.2 5.4 6.6 88.0 Example 3 Comparative 13.8 7.6 54.7 17.6 27.7 Example 4 Comparative 65.8 39.3 59.7 35.7 4.6 1.4 Example 5 Comparative 28.9 10.6 36.8 0.0 63.2 11.4 Example 6 Comparative 51.4 9.0 17.4 0.8 81.8 23.8 Example 7 1) Others: Most was analyzed to be acetone. 2) Yield = (methanol conversion) (selectivity on acetic acid) 3) Leaching (%) represents the results confirming the amount of a metal contained in a liquid phase after a reaction compared to the amount of the metal contained in a catalyst before use by ICP analysis.

(18) The catalysts of Examples 1 to 3 are carbon nitride heterogeneous catalysts containing rhodium provided in the present invention and the amount of rhodium element positioned inside of the support was fixed in a range of 0.5 wt % to 5 wt % based on the total weight of the support. Accordingly, as can be confirmed from Table 1 above, the catalysts exhibited excellent catalytic activities having 88.2 carbon mol % or higher with regard to the conversion of methanol as a reactant and 70.8 mol % or higher with regard to the selectivity on acetic acid.

(19) In contrast, it was confirmed that the catalyst of Comparative Example 1, in which no metal as an active point was incorporated during the formation of a carbon nitride network through rearrangement by thermal modification, and the catalysts of Comparative Examples 2 to 4, in which samarium, nickel, and copper, for which precedent studies as active metals with regard to methanol carbonylation are relatively not available, were positioned inside of the carbon nitride network, showed a significantly reduced conversion of methanol and selectivity on acetic acid, compared to the catalysts of Examples 1 to 3 provided in the present invention.

(20) Additionally, comparing the carbon nitride heterogeneous catalysts of Comparative Examples 5 and 6, in which palladium, ruthenium, and iridium, which are known as active metals with regard to methanol carbonylation, are contained in the network, to the catalyst of Example 3 in which equal weight parts of rhodium are contained in the carbon nitride network, the Pd(5)-g-C.sub.3N.sub.4 catalyst of Comparative Example 5 showed a slightly lower yield compared to the catalytic activity of the rhodium carbon nitride, but the catalyst could be used in preparing acetic acid by methanol carbonylation due to little problem with regard to the leaching of metal components, whereas the Ru(5)-g-C.sub.3N.sub.4 catalyst of Comparative Example 6 showed a significant decrease of conversion of methanol and selectivity on acetic acid.

(21) Comparing to the catalyst of Example 3 in which equal weight parts of an element were contained as described above, it was confirmed that the catalyst of Comparative Example 7 in which iridium is incorporated into the network showed a trend of having extremely low conversion of methanol and selectivity on acetic acid in reaction conditions for a much easier operation of the process suggested in the present invention instead of the existing high-temperature high-pressure reaction conditions, unlike the results shown in the previous study [Coordination Chemistry Reviews, 243 (2003), 125-142].

(22) As such, the carbon nitride containing rhodium suggested in the present invention exhibited an industrial advantage in that the carbon nitride containing rhodium can prepare acetic acid with high yield in low temperature and pressure conditions, which enables less energy consumption and easy operation, compared to the existing high-temperature high-pressure conditions.

(23) With regard to the stability of catalysts, in order to quantitatively confirm the components remaining in the liquid product due to the leaching of the metal as an active material during the progress of the reaction, the experimental results obtained using the above catalysts were subjected to ICP analysis. For this purpose, only the results of the catalysts of Comparative Examples 6 and 7, in which the yields of acetic acid were 10 mol % or higher, and the catalyst of Example 3, in which equal weight parts of rhodium metal were contained, were selected and analyzed. As a result, Comparative Example 5 showed a leaching of palladium (1.4 wt %) in a liquid phase, Comparative Example 6 showed a leaching of ruthenium (11.4 wt %), and Comparative Example 7 showed a leaching of iridium (23.8 wt %).

(24) However, Example 3 showed a leaching of rhodium (8.2 wt %) and the catalytic activity and degree of leaching in the carbon nitride containing rhodium and palladium suggested in the present invention were shown to decrease, comparing with the results of the degree of leaching of active metals in other Comparative Examples.

(25) Meanwhile, in FIG. 1, the selectivity on acetic acid according to the amount of rhodium element contained inside of the carbon nitride in the reaction of the rhodium-containing carbon nitride heterogeneous catalyst (Examples 1 to 3) and the acetic acid yield in the heterogeneous catalyst reactions containing other metals inside of the carbon nitride support (Comparative Examples 1 to 7) were compared and diagrammed into a graph.

(26) According to FIG. 1, it was confirmed that the rhodium-containing carbon nitride heterogeneous catalyst, in which rhodium is positioned in the carbon nitride support in an amount of 0.5 wt % to 5 wt % has higher catalytic activity, compared to g-C.sub.3N.sub.4, Sm-g-C.sub.3N.sub.4, Ni-g-C.sub.3N.sub.4, Cu-g-C.sub.3N.sub.4, Pd-g-C.sub.3N.sub.4, Ru-g-C.sub.3N.sub.4, and Ir-g-C.sub.3N.sub.4 heterogeneous catalysts.

Experimental Example 2: Characteristics of Rh-g-C3N4

(27) The heterogeneous Rh-g-C.sub.3N.sub.4 catalysts such as Rh(1)-g-C.sub.3N.sub.4, Rh(3)-g-C.sub.3N.sub.4 and Rh(5)-g-C.sub.3N.sub.4 were characterized by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy. AS shown in FIG. 2 and FIG. 3, the Rh nanoparticles was well dispersed in the matrices of g-C.sub.3N.sub.4 and the Rh nanoparticles were mainly existing as metallic state, which were confirmed by insignificant XRD peaks of the Rh species and characteristic binding energies or Rh species from XPS analysis. In addition, the well dispersed Rh metals were confirmed by TEM images as shown in FIG. 4.

Experimental Example 3: Activity Comparison of Rh(5)-g-C3N4 with Simply Impregnated Rh(5)/g-C3N4

(28) Rh-incorporated g-C.sub.3N.sub.4 (Rh-g-C.sub.3N.sub.4) as shown in Example 3 showed a higher AA yield with the value of 93.3%, where the Rh metals were highly dispersed in the matrices of the g-C.sub.3N.sub.4. The Rh-g-C.sub.3N.sub.4 catalyst also showed a better catalytic performance than the simply impregnated Rh(5)/g-C.sub.3N.sub.4 prepared by a wet-impregnation method with the less AA yield of 63.2% as shown in Table 2. In addition, a less amount of leached Rh metal was observed on the Rh-g-C.sub.3N.sub.4 compared to the impregnated Rh(5)/g-C.sub.3N.sub.4 due to a higher dispersion of the Rh metals in the matrices of g-C.sub.3N.sub.4 on the most active and stable Rh-g-C.sub.3N.sub.4 as shown in Table 3.

(29) TABLE-US-00002 TABLE 2 Selectivity Yield Conv. (mol %) (AcOH Catalyst (MeOH mol %) AcOH MeOAc mol %) Byproduct Rh(5)/g-C.sub.3N.sub.4 99.5 63.5 36.5 63.2 Rh(5)-g-C3N.sub.4 99.7 93.6 6.2 93.3 Acetone 0.2%

(30) TABLE-US-00003 TABLE 3 Catalyst Leaching amounts (wt %) Rh(5)/g-C.sub.3N.sub.4 39.7 Rh(5)-g-C.sub.3N.sub.4 8.2