Description:TECHNICAL FIELD OF THE INVENTION:
The present invention relates to the field of catalyst. Particularly, the present invention relates to the design of an active bifunctional catalyst system for syngas to dimethyl ether (DME) conversion in a single step process through thermocatalytic pathway.
BACKGROUND OF THE INVENTION:
DME is a colourless gas at an ambient condition and easily liquefied under light pressure. Since its physical and chemical characteristics are very similar to those of Liquefied Petroleum gas (LPG), it is an easy substitute for LPG. DME is not only an easy substitute for LPG, but also a very clean substitute for diesel fuel because a DME fuelled diesel car emits neither soot nor particle matters. According to International DME Association, currently more than 65% of the DME produced worldwide is blended with LPG. Having remarkable fuel properties such as quiet, soothless combustion, ultra-low emissions, LPG compatible with high energy density, DME offers pathways to sustainable and zero-emission energy for LPG blending applications.
The focus of research nowadays has shifted to finding alternative fuels mainly due to ecological and economical considerations. Among the alternative fuels, dimethyl ether (DME), which has been recently discovered as a clean fuel, can be synthesized from synthetic gas which can be generated from different primary sources. These primary sources can be natural gas, coal, heavy oil, and also biomass. In prior art, mostly two DME synthesis methods from synthesis gas have been discussed, one of these being the two-step traditional synthesis method where methanol synthesis is followed by a dehydration step, and the other being a direct single step conversion of synthesis gas to DME.
The reaction commercially operates in two steps. First step involves methanol synthesis from syngas (CO +H2) and in the second step methanol undergoes dehydration to make dimethyl ether (DME)
CO + H2 CH3OH
CH3OH CH3OCH3 + H2O
Recently, the single step method has been gaining attention where a single catalyst system is used for both synthesis gas to methanol conversion followed by dehydration to make dimethyl ether.
WO2019122078A1 discloses a catalyst system and process for preparing dimethyl ether from synthesis gas as well as the use of the catalyst system in this process. But this prior art uses two separate catalysts for two steps and the first step being a reversible reaction the yield of methanol is very less resulting in overall reduced yield of dimethyl ether in the second step.
Indian patent application number 202027027453 a catalyst system and process for preparing dimethyl ether from synthesis gas as well as the use of the catalyst system in this process.
The references available in the prior art shows that the catalysts employed in the synthesis gas-to-dimethyl ether (DME) process undergo rapid deactivation. The known processes are often not satisfying with regard to the long-term stability of the catalyst system employed. The present invention has overcome the shortcomings of the prior art by providing a bifunctional catalyst with multiple components for single step conversion of synthesis gas to DME with improved stability and yield of DME.
OBJECTS OF THE INVENTION:
The main object of the present invention is to provide a bifunctional catalyst for the single step continuous process for producing DME from synthesis gas in a single step.
It is another object of the present invention to provide a bifunctional catalyst which has a significantly improved stability in the direct synthesis of dimethyl ether from synthesis gas.
It is a yet another object of the present invention to provide a bifunctional catalyst which has a significantly improved yield in the direct synthesis of dimethyl ether from synthesis gas.
SUMMARY OF THE INVENTION:
The present invention provides a bifunctional catalyst system for the single step conversion of syngas to dimethyl ether, the catalyst comprises: methanol synthesis catalyst; and methanol dehydration catalyst,
wherein the methanol synthesis catalyst comprises active metals, support or structural promoters and binder, and wherein the methanol dehydration catalyst comprises ?-Al2O3.
The methanol synthesis catalyst is 30-70 wt.% and the methanol dehydration catalyst is 30-70 wt.%.
The methanol synthesis catalyst comprises:70-90 wt% of active metals and their oxides; 5-20 wt% of a support or structural promoters; and 5-10 wt% of a binder.
The active metals and their oxides are selected from the group of Cu, Co, La, Pd, ZnO, In2O3, Ga2O3. NiO and Cr2O3.
The active metals and their oxides are Cu, ZnO and Cr2O3 and wherein Cu is in a range of 40 to 60 wt%, ZnO is in a range of 15 to 30 wt%, and Cr2O3 is in a range of 5 to 10 wt% .
The support is selected from the group of Silica (SiO2), amorphous Alumina-Silica (Al2O3-SiO2), Alumina (Al2O3), Magnesium silicate (Talc), Ceria (CeO2). Titania (TiO2) or a combination thereof.
The support comprises Ceria (CeO2), titania (TiO2) and alumina (Al2O3) and wherein CeO2 is present in a range of 5 to 10 wt%; Titania (TiO2) is present in a range of 5 to 10 wt%; Al2O3 is present in a range of 5 to 10 wt%.
The binder is selected from the group consisting of Pseudo boehmite, ludox silica, precipitated silica and Hydroxy propyl methyl cellulose; preferably Pseudo boehmite.
The Cu is 55 wt%, ZnO is 20 wt%, Cr2O3 is 5 wt%; Al2O3 is 5 wt%, Ceria (CeO2) is 5 wt% and Titania (TiO2) is 5 wt %; Pseudo boehmite is 5 wt%; methanol dehydration catalyst (?-Al2O3) is 100 wt%; Methanol synthesis and methanol dehydration catalysts are maintained in 50:50 weight ratio.
In addition, the present invention provides a method of preparation of the bifunctional catalyst system for the single step conversion of syngas to dimethyl ether, comprising: mixing an aqueous solution of metal salts and support or structural promoters with constant stirring to form a homogeneous solution; adding an aqueous solution of precipitating agent to above aqueous solution at temperature in a range of 60 to 80°C till pH in a range of 7 to 11 is attained; aging of the above solution at temperature in a range of 60 to 80°C for time period in a range of 10 to 20 hours followed by washing to obtain a precipitate; drying of precipitate at temperature in a range of 80 to 120°C followed by calcination at temperature in a range of 350 to 500°C for a time period in a range of 2 to 6 hours to obtain a catalyst powder; and extruding the catalyst powder using a binder and 0.5 to 0.75 M peptizing agent.
The precipitating agent is selected from a group consisting of Na2CO3, NaOH, CaO, NH4OH, Urea and CaCO3 or a mixture thereof.
The peptizing agent is nitric acid or citric acid.
The plasticizer is selected from a group consisting of Hydroxy propyl methyl cellulose, sugar and starch.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS:
These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:
Figure 1 depicts testing of Catalyst in three different configurations.
Figure 2 depicts a stability study for different configurations of the catalyst (a). Uniform mixing of methanol synthesis and dehydration catalyst in 50:50 wt ratio (Case 2) (b). Mixing of methanol synthesis and dehydration catalyst (Case 3).
DETAILED DESCRIPTION OF THE INVENTION:
For convenience, before further description of the present disclosure, certain terms employed in the specification, and examples are delineated here. These definitions should be read in light of the remainder of the disclosure and understood as by a person of skill in the art.
The terms used herein have the meanings recognized and known to those of skill in the art, however, for convenience and completeness, particular terms and their meanings are set forth below. The articles “a”, “an” and “the” are used to refer to one or to more than one (i.e., to at least one) of the grammatical object of the article.
The terms “comprise” and “comprising” are used in the inclusive, open sense, meaning that additional elements may be included. It is not intended to be construed as “consists of only”. The term "at least one" is used to mean one or more and thus includes individual components as well as mixtures/combinations. Throughout this specification, unless the context requires otherwise the word “comprise”, and variations such as “comprises” and “comprising”, will be understood to imply the inclusion of a stated element or step or group of element or steps but not the exclusion of any other element or step or group of element or steps. The term “including” is used to mean “including but not limited to”. “including” and “including but not limited to” are used interchangeably.
Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. Although any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the disclosure, the preferred methods and materials are now described.
The present disclosure is not to be limited in scope by the specific embodiments described herein, which are intended for the purposes of exemplification only. Functionally equivalent products, compositions, and methods are clearly within the scope of the disclosure, as described herein.
In an aspect, the present invention provides a bifunctional catalyst system for the single step conversion of syngas to dimethyl ether, the catalyst comprises:
a. methanol synthesis catalyst; and
b. methanol dehydration catalyst,
wherein the methanol synthesis catalyst comprises active metals, support or structural promoters and binder, and wherein the methanol dehydration catalyst comprises ?-Al2O3.
In one of the embodiments of the present invention, the methanol synthesis catalyst is 30-70 wt.% and the methanol dehydration catalyst is 30-70 wt.%.
In an embodiment of the present invention, the methanol synthesis catalyst comprises:
a. 70-90 wt% of active metals and their oxides;
b. 5-20 wt% of a support or structural promoters; and
c. 5-10 wt% of a binder.
In an embodiment of the present invention, the active metals and their oxides are selected from the group of Cu, Co, La, Pd, ZnO, In2O3, Ga2O3. NiO and Cr2O3.
In an embodiment of the present invention, the active metals and their oxides are Cu, ZnO and Cr2O3 and wherein Cu is in a range of 40 to 60 wt%, ZnO is in a range of 15 to 30 wt%, and Cr2O3 is in a range of 5 to 10 wt%.
In an embodiment of the present invention, the support or structural promoters is selected from the group of Silica (SiO2), amorphous Alumina-Silica (Al2O3-SiO2), Alumina (Al2O3), Magnesium silicate (Talc), Ceria (CeO2). Titania (TiO2) or a combination thereof.
In an embodiment of the present invention, the support or structural promoters comprises Ceria (CeO2), titania (TiO2) and alumina (Al2O3) and wherein CeO2 is present in a range of 5 to 10 wt%; Titania (TiO2) is present in a range of 5 to 10 wt%; Al2O3 is present in a range of 5 to 10 wt%.
In an embodiment of the present invention, the binder is selected from the group consisting of Pseudo boehmite, ludox silica, precipitated silica and Hydroxy propyl methyl cellulose; preferably Pseudo boehmite.
In an embodiment of the present invention, the Cu is 55 wt%, ZnO is 20 wt%, Cr2O3 is 5 wt% ; Al2O3 is 5 wt%, Ceria (CeO2) is 5 wt% and Titania (TiO2) is 5 wt %; Pseudo boehmite is 5 wt%; methanol dehydration catalyst (?-Al2O3) is 100 wt%; Methanol synthesis and methanol dehydration catalysts are maintained in 50:50 weight ratio.
In another aspect, the present invention provides a method of preparation of the bifunctional catalyst system for the single step conversion of syngas to dimethyl ether, comprising:
a. mixing an aqueous solution of metal salts and support or structural promoters with constant stirring to form a homogeneous solution;
b. adding an aqueous solution of precipitating agent to above aqueous solution at temperature in a range of 60 to 80°C till pH in a range of 7 to 11 is attained;
c. aging of the above solution at temperature in a range of 60 to 80°C for time period in a range of 10 to 20 hours followed by washing to obtain a precipitate;
d. drying of precipitate at temperature in a range of 80 to 120°C followed by calcination at temperature in a range of 350 to 500°C for a time period in a range of 2 to 6 hours to obtain a catalyst powder; and
e. extruding the catalyst powder using a binder and 0.5 to 0.75 M peptizing agent.

In another embodiment of the present invention, the precipitating agent is selected from a group consisting of Na2CO3, NaOH, CaO, NH4OH, Urea and CaCO3 or a mixture thereof.
In another embodiment of the present invention, the peptizing agent is nitric acid or citric acid.
In another embodiment of the present invention, the plasticizer is selected from a group consisting of Hydroxy propyl methyl cellulose, sugar and starch.
EXAMPLES:
Example 1: List of materials used for the preparation of bifunctional Catalyst in the extrude form.
Table 1. List of materials used for the preparation of bifunctional Catalyst in the extrude form
S.No. List of Materials wt% ranges
Methanol Synthesis Catalyst
1 Metals and oxides:
Copper (Cu), Copper oxide, Zn, Zinc Oxide (ZnO), In, Indium oxide (In2O3), Zr, Zirconia (ZrO2), Ga, Gallium Oxide (Ga2O3), nano Zirconia (nano-ZrO2), Cobalt (Co), Cobalt oxide (CoO), Lanthanum (La), La2O3. Ni,
Nickel oxide (NiO), Palladium (Pd), PdO, Cr, Chromium oxide (Cr2O3) 70 – 90
2 Support/Structural Promoters:
Silica (SiO2), amorphous Alumina-Silica (Al2O3-SiO2), Alumina (Al2O3), Magnesium silicate (Talc), Titania (TiO2), ceria 5 – 20
3 Binders: 5 – 10
Pseudo boehmite, ludox silica, precipitated silica, Hydroxy propyl methyl cellulose
4 Other chemicals required for precipitation and making extrudates:
Peptizing agents (nitric acid, citric acid), plasticizer (Hydroxy propyl methyl cellulose, sugar, starch), precipitating agent (sodium carbonate Na2CO3, NaOH, CaO, NH4OH, Urea and CaCO3), Cetyl Trimethyl Ammonium Bromide (CTAB), Ethylene diamine.
Methanol Dehydration Catalyst
1 ?-Al2O3 100%

For the single step reaction, both the catalysts are used in 50:50 wt% ratio.
Example 2: Bifunctional Catalyst system components.
Methanol synthesis catalyst: 55 wt % Cu+5 wt % TiO2+20 wt % ZnO+ 5 wt % Cr2O3+ 10wt % Al2O3+5 wt % CeO2
Table 2. List of materials forming the bifunctional catalyst system
S.No. List of Materials (wt% ranges)
1 Metals and their oxides:
Copper (Cu, 55wt%), Zinc Oxide (ZnO, 20 wt%), Chromium oxide (Cr2O3, 5 wt%)
2 Support/Structural Promoters:
Alumina (Al2O3, 5 wt%), Titania (TiO2, 5wt%), ceria (CeO2, 5wt%)
3 Binders:
Pseudo boehmite (5 wt%)

Active metals and their oxides are Cu, ZnO and Cr2O3. Al2O3 act as structural support, whereas CeO2 and TiO2 act as support and provide active metal-support interactions.
Methanol Dehydration catalyst: ?-Al2O3
Example 3: Method of preparation of the best bifunctional catalyst system.
The catalyst was prepared by simple co-precipitation method with Na2CO3 as precipitating agent. Under vigorous stirring, a mixed aqueous solution of Copper nitrate trihydrate (0.5 M), Zinc nitrate hexahydrate (0.5 M), (0.5), Chromium nitrate (0.5 M), Aluminium nitrate nonahydrate (0.5M), CeO2 and TiO2 supports were added to form a homogenous solution. Then, aqueous solution of Na2CO3 (1.2 M) was added drop wise to the solution at 75 °C till pH=8. The solution was aged at same temperature for 15 h and later washed thoroughly with hot distilled water to remove all the Na+ ions. The obtained material was dried overnight at 80 °C and calcined in static air at 350 °C for 4h (rate 2°C/min). The catalyst powder is subsequently extruded using pseudo boehmite and 0.75 M nitric acid.
Example 4: Catalyst characterization.
BET surface area, pore volume, pore size, X-Ray Diffraction Spectrophotometer (XRD), X-Ray Fluorescence Spectrophotometer (XRF), Crushing strength, FE-SEM-EDX, CO2 desorption, ammonia temperature programmed desorption (NH3-TPD), H2 temperature programmed reduction (H2 – TPR).
Textural properties of the catalyst along with its functionality plays a key role in converting the syngas to DME. The DME production through methanol involves hydrogenation of CO to methanol, which further undergo dehydration to DME on acidic sites. The table 3 shows the results of comparative study of some key parameters between bifunctional catalyst and methanol synthesis catalyst. The results of table 3 provides information on acidity, which is important to drive this dehydration reaction. Addition of dehydration catalysts shown to improve overall acidity of the catalyst, which is important for just dehydration of the methanol to DME. The acidity is maintained such that, the DME does not undergo further dehydration to hydrocarbons. The addition of dehydration catalyst also ensures improved surface area, while maintaining required pore volume and pore radius required to handle small molecules such as CO, CH3OH and DME.
Table 3. Results of Comparative study
Catalyst Acidity (NH3 uptake) µmol/g SA Pore Volume Pore Radius
Methanol Synthesis Catalyst 365 105 0.23 4.3
Methanol synthesis and methanol dehydration catalyst (50:50 wt ratio) 420 192 0.24 4.6

Example 5: Catalyst activity testing.
Catalyst has been tested in three different configurations as shown in table 4. The bifunctional catalyst of the present invention (case 2) was tested in comparison with two bed catalyst (case 1) and single bed catalyst (case 3).
Case 1: Methanol synthesis and dehydration catalysts are separated by inert layer in a two-bed single reactor.
Case 2: Uniform mixing of methanol synthesis and dehydration catalyst in (50:50 wt%) to form a single bifunctional catalyst system in a single bed.
Case 3: Mixing of two catalysts as separate grains in a single bed reactor.
The activity of the catalysts is tested under different temperature conditions from 230 – 300 °C, Pressure 30 – 40 bar, Gas Hourly Space Velocity (GHSV) 4000 – 15000 h-1 while maintaining CO:H2 ratio of 1:2, in a tubular fixed bed reactor. During the entire reaction, N2 was used as internal standard for data analysis. Liquid and gas samples were analyzed by gas chromatography (GC) and refinery gas analyzer (RGA).
Best results are achieved when the catalyst is prepared by 50:50 wt% mixing of the methanol synthesis and the dehydration catalysts as a single catalyst particle, rather testing them separately to form a single bifunctional catalyst system. The results of the study also showed that the activity of the catalyst is stable when tested for ˜100 hrs.
Table 4. Results of catalyst activity testing
Activity test* Feed
CO:H2 % CO Conversion % Product Selectivity % DME Yields
CO2 Methanol DME
Case-1: Two Bed Catalyst 1:2 24 4 0.0070 96 23
Case-2: Single Bed Catalyst 1:2 76 18 6x10-4 82 63
Case-3: Single Bed Catalyst 1:2 54 22 3x10-7 78 42 , Claims:1. A bifunctional catalyst system for the single step conversion of syngas to dimethyl ether, the catalyst comprises:
a. methanol synthesis catalyst; and
b. methanol dehydration catalyst,
wherein the methanol synthesis catalyst comprises active metals, support or structural promoters and binder, and wherein the methanol dehydration catalyst comprises ?-Al2O3.

2. The catalyst system as claimed in claim 1, wherein the methanol synthesis catalyst is 30-70 wt.% and the methanol dehydration catalyst is 30-70 wt.%.

3. The catalyst system as claimed in claim 1, wherein the methanol synthesis catalyst comprises:
a. 70-90 wt% of active metals and their oxides;
b. 5-20 wt% of a support or structural promoters; and
c. 5-10 wt% of a binder.

4. The catalyst system as claimed in claim 3, wherein the active metals and their oxides are selected from the group of Cu, Co, La, Pd, ZnO, In2O3, Ga2O3. NiO and Cr2O3.

5. The catalyst system as claimed in claim 4, wherein the active metals and their oxides are Cu, ZnO and Cr2O3 and wherein Cu is in a range of 40 to 60 wt%, ZnO is in a range of 15 to 30 wt%, and Cr2O3 is in a range of 5 to 10 wt% .

6. The catalyst system as claimed in claim 3, wherein the support or structural promoters is selected from the group of Silica (SiO2), amorphous Alumina-Silica (Al2O3-SiO2), Alumina (Al2O3), Magnesium silicate (Talc), Ceria (CeO2). Titania (TiO2) or a combination thereof.

7. The catalyst system as claimed in claim 6, wherein the support or structural promoters comprises Ceria (CeO2), titania (TiO2) and alumina (Al2O3) and wherein CeO2 is present in a range of 5 to 10 wt%; Titania (TiO2) is present in a range of 5 to 10 wt%; Al2O3 is present in a range of 5 to 10 wt%.

8. The catalyst system as claimed in claim 3, wherein the binder is selected from the group consisting of Pseudo boehmite, ludox silica, precipitated silica and Hydroxy propyl methyl cellulose; preferably Pseudo boehmite.

9. The catalyst system as claimed in claim 1, wherein the Cu is 55 wt%, ZnO is 20 wt%, Cr2O3 is 5 wt% ; Al2O3 is 5 wt%, Ceria (CeO2) is 5 wt% and Titania (TiO2) is 5 wt %; Pseudo boehmite is 5 wt%; methanol dehydration catalyst (?-Al2O3) is 100 wt%; Methanol synthesis and methanol dehydration catalysts are maintained in 50:50 weight ratio.

10. A method of preparation of the bifunctional catalyst system for the single step conversion of syngas to dimethyl ether as defined in claim 1, comprising:
a. mixing an aqueous solution of metal salts and support or structural promoters with constant stirring to form a homogeneous solution;
b. adding an aqueous solution of precipitating agent to above aqueous solution at temperature in a range of 60 to 80°C till pH in a range of 7 to 11 is attained;
c. aging of the above solution at temperature in a range of 60 to 80°C for time period in a range of 10 to 20 hours followed by washing to obtain a precipitate;
d. drying of precipitate at temperature in a range of 80 to 120°C followed by calcination at temperature in a range of 350 to 500°C for a time period in a range of 2 to 6 hours to obtain a catalyst powder; and
e. extruding the catalyst powder using a binder and 0.5 to 0.75 M peptizing agent.

11. The method as claimed in claim 10, wherein the precipitating agent is selected from a group consisting of Na2CO3, NaOH, CaO, NH4OH, Urea and CaCO3 or a mixture thereof.

12. The method as claimed in claim 10, wherein the peptizing agent is nitric acid or citric acid.

13. The method as claimed in claim 10, wherein the plasticizer is selected from a group consisting of Hydroxy propyl methyl cellulose, sugar and starch.