WO2021171316A1

WO2021171316A1 - Fixed bed reactor with layered dimethyl ether synthesis catalysts

Info

Publication number: WO2021171316A1
Application number: PCT/IN2021/050187
Authority: WO
Inventors: Anand Janardan APTE; Giridhar Madhukar JOSHI; Rajesh Muralidhar Badhe
Original assignee: Amol Carbons Private Limited; Indian Oil Corporation Limited
Priority date: 2020-02-27
Filing date: 2021-02-26
Publication date: 2021-09-02

Abstract

A fixed bed reactor for the production of dimethyl ether is disclosed. The reactor includes multiple layers (100) of catalysts. In the multiple layers (100), a layer (110) having a methanol synthesis catalyst alternates with a layer (120) having a methanol dehydration catalyst. At least 60 atomic percent of the metal components of the methanol synthesis catalyst composition has spinel structure.

Description

FIXED BED REACTOR WITH LAYERED DIMETHYL ETHER SYNTHESIS

CATALYSTS

FIELD OF THE INVENTION

[0001] The present disclosure relates to a fixed bed reactor having layered dimethyl ether synthesis catalysts for production of dimethyl ether. Specifically, the disclosure relates to spinel structured methanol synthesis catalyst used in alternate layers along with methanol dehydration catalyst in a mixed bed reactor.

BACKGROUND AND PRIOR ART OF THE INVENTION [0002] The dimethyl ether (DME) is being increasingly viewed as the fuel of the future on account of its many favorable attributes. It is a clean burning fuel with physicochemical properties comparable to Liquid petroleum gas (LPG) and Cetane number higher than high speed Diesel. It, therefore, can serve as an effective substitute for LPG as well as diesel. Lurthermore, it can be made from a wide variety of feedstocks such as coal, natural gas, agricultural wastes etc. and it can be blended with LPG and diesel in any proportions desired.

[0003] Presently, dimethyl ether is manufactured by a two-step process. Pirst methanol is synthesized from synthesis gas by reaction 1.

1) CO + 2 ¾ - CHsOH DH -90.29 kJ/mole

Dimethyl ether is synthesized from methanol by reaction 2. 2) 2 CH3OH -> CH3OCH3 + H2O DH -23.41 kJ/mole

[0004] Normally the two reactions 1 and 2 are carried out in two separate reactors under different operating conditions. Methanol synthesis is optimally carried out at a temperature of 200° C -300° C, and at a pressure of 50 - 80 bar normally using CuZnOAhCh catalyst, with long catalyst life of 2 to 5 years. The optimum conditions for methanol to dimethyl ether conversion are temperature of 100° C to 300° C and pressure of 20 bar, using solid acid catalysts such as heteropoly acids (temperature 100-200° C), HZSM-5 (around 200°C) or g-AhCh (above 250°C) with long catalyst life. The two-step process has severe thermodynamic limitations. The method suffers from low per pass conversion ca. 20%, poor energy efficiency and requires syngas rich in hydrogen (H₂/CO =2:l). This hydrogen rich syngas is especially difficult to obtain from feedstocks such as coal and agricultural waste.

[0005] By combining reaction 3 with reactions 1 & 2, we get: 3) CO + H2O - CO2 + H2 DH -48.46 kJ/mole

Hence, dimethyl ether can be directly manufactured from syngas deficient in hydrogen (H2/CO ~ 1). The low hydrogen syngas can be readily manufactured from coal and agricultural waste. The single step process is desirable due to multiple benefits: High conversion efficiency 60% to 77%, high energy efficiency and freedom to use lean syngas deficient in hydrogen. The overall process can be represented by a summation of 2 moles methanol formed by reaction 1 followed by reactions 2 and 3. The overall reaction 4 is represented as follows:

4)

DH -254.4 kJ/mole DME

A single pot DME process using a fixed bed reactor is carried out by combining methanol synthesis catalyst with a methanol dehydration catalyst. Both, a methanol synthesis catalyst and a methanol dehydration catalyst are needed for single pot synthesis. Much of the current efforts were directed towards a single pellet containing both functionalities of methanol synthesis and methanol dehydration.

[0006] US9295978B2 discloses catalysts and methods for their manufacture and use for the synthesis of dimethyl ether from syngas. The catalysts comprise methanol synthesis compositions designated as A, B, C containing ZnO, CuO, ZrCE, alumina and ceria. A second acidic component designated as X, Y, Z containing alumina prepared by different methods. The catalysts described herein are able to synthesize dimethyl ether directly from synthesis gas, including synthesis gas that is rich in carbon monoxide. The two components are mixed in predetermined proportions in a single pellet.

[0007] US8669295B2 provides a process for preparing methanol, dimethyl ether, and low carbon olefins from syngas, wherein the process comprises the step of contacting syngas with a catalyst under the conditions for converting the syngas into methanol, dimethyl ether, and low carbon olefins, characterized in that, the catalyst contains an amorphous alloy consisting of a first component A1 and a second component, said second component being one or more elements or oxides thereof selected from Group IA, IIIA, IVA, VA, IB, IIB, IVB, VB, VIB, VIIB, VIII, and Lanthanide series of the Periodic Table of Elements, and said second component being different from the first component Al. According to the present process, the syngas can be converted into methanol, dimethyl ether, and low carbon olefins in a high CO conversion, a high selectivity of the target product, and high carbon availability. This patent lists two components to be mixed and converted into single pellet.

[0008] JP4467675B2 relates to a catalyst capable of producing dimethyl ether in a high yield from a gas containing hydrogen and carbon dioxide and having excellent durability, and a method for synthesizing dimethyl ether using the catalyst. Composition is Cu, Zn, Al and Ga for methanol synthesis and Al, Zr for methanol dehydration catalyst. The two catalysts are mixed in forming a single pellet.

[0009] KR 100732784B1 discloses a process for the preparation of dimethyl ether from hydrocarbons, including dry-reforming a feedstock mixture comprised of hydrocarbons, carbon dioxide and water vapor in the presence of a dry-reforming catalyst, to prepare a syngas, which then undergoes gas-phase direct synthesis into dimethyl ether in one step in the presence of a hybrid catalyst. The methanol synthesis catalyst contains Cu, Zn, Zr, Al, Mn and Ga are in a weight ratio from 22.6 to 67.8: 13.5 to 40.6: 2.3 to 6.7: 10.4 to 31.2: 3.8 to 7.5: 0.9 to 2.7. Aluminum is a major ingredient of synthesis catalyst and this is mixed with dehydration catalyst to produce a single pellet with both catalytic functions.

[0010] US8552074B2 provides a process for preparing methanol, dimethyl ether, and low carbon olefins from syngas, wherein the process comprises the step of contacting syngas with a catalyst under the conditions for converting the syngas into methanol, dimethyl ether, and low carbon olefins, characterized in that, the catalyst contains an amorphous alloy consisting of components M and X wherein the component X represents an element B and/or P, the component M represents two or more elements selected from Group IIIA, IVA, VA, IB, IIB, IVB, VB, VIB, VIIB, VIII and Lanthanide series of the Periodic Table of Elements. According to the present process, the syngas can be converted into methanol, dimethyl ether, and low carbon olefins in a high CO conversion, a high selectivity of the target product, and high carbon availability. This patent also discloses methanol synthesis catalyst combined with acidic dehydration catalyst into a single pellet.

[0011] KR100810739B1 relates to a process for the preparation of a catalyst for methanol and dimethyl ether synthesis from syngas, and more particularly, CuO, ZnO and A12 O Cu-Zn-Al- based porous catalyst. In this, methanol synthesis function catalyst is precipitated in pores of alumina- dehydration catalyst, resulting in single pellet with both catalysts.

[0012] JPH09173845A discloses preparation of a catalyst for producing dimethyl ether, wherein the catalyst is prepared by precipitating oxide of copper, oxide of zinc, and alumina in the pores of alumina. And dimethyl ether is produced by passing mixed gas of carbon monoxide and hydrogen through slurry in which the catalyst is suspended, and at that moment, gas or mixed gas containing carbon dioxide and/or water vapor is led into pores of alumina to produce dimethyl ether with a high yield. The ratio between copper oxide, zinc oxide, and alumina to be precipitated in the pores of alumina is such that the ratio of copper oxide to zinc oxide is 1:(0.1- 5) and alumina is (0-0.5). In this, methanol synthesis function catalyst is precipitated in pores of alumina- dehydration catalyst, resulting in single pellet with both catalysts.

[0013] KR970069123A discloses a method of manufacturing a catalyst for dimethyl ether and a method of manufacturing the dimethyl ether. In this, methanol synthesis catalyst is deposited on periphery of alumina particle, thus creating a single pellet with MS & MD functionality.

[0014] KR100588948B1 discloses a catalyst for making dimethyl ether, method of preparing the same and method of preparing dimethyl ether using the same. This patent describes a uniformly mixed methanol synthesis (MS) and methanol dehydration (MD) catalyst.

[0015] A single pot process using a fixed bed reactor is not yet commercially practiced due to rapid deactivation of methanol synthesis catalyst with about 30% of catalyst loss over 200 hours of operation. A major hindrance to commercialization of the single pot process has been short life of the methanol synthesis catalyst. The objective of the present invention is to analyze the reasons for the premature catalyst failure and to provide effective measures to prolong the catalyst life at reasonable activity. SUMMARY OF THE INVENTION:

[0016] The present invention overcomes the limitation of the prior-art documents and provides a mixed metal oxide heterogeneous catalyst for production of dimethyl ether from syn gas in a single fixed bed reactor. [0017] In one aspect, a fixed bed reactor for the production of dimethyl ether is disclosed. The reactor includes multiple layers of catalysts. In the multiple layers, a layer having a methanol synthesis catalyst alternates with a layer having a methanol dehydration catalyst. Further, at least 60 atomic percent of the methanol synthesis catalyst composition has spinel structure.

[0018] Further advantages and other details of the present subject matter will be apparent from a reading of the following description and a review of the associated drawings. It is to be understood that the following description is explanatory only and is not restrictive of the present disclosure.

BRIEF DESCRIPTION OF THE FIGURES

[0019] To further clarify the advantages and features of the disclosure, a more particular description of the disclosure will be rendered by reference to specific embodiments thereof, which are illustrated in the appended drawings. It is appreciated that these drawings depict only typical embodiments of the disclosure and are therefore not to be considered limiting of its scope. The disclosure will be described and explained with additional specificity and detail with the accompanying drawings in which: [0020] Figure 1 illustrates a layered structure of the methanol synthesis catalyst and methanol dehydration catalyst in a fixed bed reactor, in accordance with an embodiment of the present disclosure; and

[0021] Figure 2 illustrates a ternary composition diagram of a compound comprising group I, group II and group III elements, in accordance with an embodiment of the present disclosure. [0022] It may be noted that to the extent possible like reference numerals have been used to represent like elements in the drawings. Further, those of ordinary skilled in the art will appreciate that elements in the drawings are illustrated for simplicity and may not have been necessarily drawn to scale. For example, the dimensions of some of the elements in the drawings may be exaggerated relative to other elements to help to improve understanding of aspects of the disclosure. Furthermore, the one or more elements may have been represented in the drawings by conventional symbols, and the drawings may show only those specific details that are pertinent to understanding the embodiments of the disclosure so as not to obscure the drawings with details that will be readily apparent to those of ordinary skilled in the art having the benefits of the description herein.

DETAILED DESCRIPTION OF THE DISCLOSURE

[0023] For the purpose of promoting an understanding of the principles of the disclosure, reference will now be made to the embodiment illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the disclosure is thereby intended, such alterations and further modifications in the illustrated system, and such further applications of the principles of the disclosure as illustrated therein being contemplated as would normally occur to one skilled in the art to which the disclosure relates.

[0024] It will be understood by those skilled in the art that the foregoing general description and the following detailed description are exemplary and explanatory of the disclosure and are not intended to be restrictive thereof. Throughout the patent specification, a convention employed is that in the appended drawings, like numerals denote like components. [0025] Reference throughout this specification to “an embodiment”, “another embodiment” or similar language means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the disclosure. Thus, the appearances of the phrase “in an embodiment”, “in another embodiment” and similar language throughout this specification may, but do not necessarily, all refer to the same embodiment.

[0026] The terms "comprises", "comprising", or any other variations thereof, are intended to cover a non-exclusive inclusion, such that a process or method that comprises a list of steps does not include only those steps but may include other steps not expressly listed or inherent to such process or method. Similarly, one or more devices or sub-systems or elements or structures proceeded by "comprises... a" does not, without more constraints, preclude the existence of other devices or other sub- systems.

[0027] One or more of the embodiments of the present disclosure include a fixed bed reactor for the production of dimethyl ether. The mixed bed reactor has both the methanol synthesis catalyst and the methanol dehydration catalyst. Amongst the two catalysts, the role of methanol synthesis catalyst is very important, as the methanol synthesis step is the rate determining step in the overall production of the dimethyl ether. Currently available methanol synthesis catalysts are a mixture of oxides of three or more elements at the molecular level. The three elements belong to group I, II and III of the periodic table. Copper and silver are the most used active hydrogenation centers, mostly in their oxidation state 0 or +1. Copper in oxidation state +1 is also anchoring point for CO gas.

[0028] Amongst, Group - II elements, the most important elements are Zn and to a lesser extent Mg, Cd, Ca, Ba, and Sr. Oxides of these elements act as spacers for copper atoms, reduce mobility of copper atoms, and prevent sintering of copper. Further, presence of these oxides increases the catalytic activity by substantially enlarging the surface area of the catalyst. At operating temperature of the reactor, and under reducing atmosphere of syngas, the oxygen vacancies increase, and electrons are released to the lattice of ZnO, rendering it a n- type semiconductor. About 30% of copper atoms are embedded in the ZnO matrix. A strong electronic interaction between copper atoms and ZnO is desirable for the effective functioning of the methanol synthesis catalyst.

[0029] The group-III elements form extremely stable oxides. These oxides, support the Cu and Zn active centers, improve dispersion of copper atoms and prevent sintering of copper. They also act as dopants for ZnO, increasing oxygen vacancies and increasing its electronic conductivity. Apart from Al, Cr, Mn, and V, various other elements and actinides have also been utilized to this effect. For example, elements such as Cr, Mn, V, actinides, and lanthanides despite belonging to other groups are also included in group-III, as they are present at valance state 3 in the catalyst. [0030] Methanol dehydration catalyst is the second catalyst required for the single step process in the fixed bed reactor. Reaction 2, the methanol dehydration is an exothermic reversible reaction that proceeds without change in mole number. For this reason, reaction pressure does not affect conversion equilibrium, while lower reaction temperatures have a thermodynamic advantage towards higher DME production. Methanol dehydration is an acid- catalyzed reaction and an active, selective and stable catalyst at relative low temperature is desired for the afore-mentioned reaction. Furthermore, this technology is readily available from several industrial technology suppliers. Depending on catalyst characteristics, methanol dehydration can be carried out in both vapor and liquid phase, with reaction temperature in the range 100 °C - 300 °C.

[0031] y-AhCE is a solid acid catalyst that can be used as a dehydration catalyst, considering its cost advantage, high surface area, good thermal and mechanical stability. y-AhCE shows high selectivity for DME even at high temperature (up to 400 °C) because of the presence of weak Lewis acid sites, which are unable to promote side reactions. These acid characteristics require reaction temperature higher than 250 °C to favor high methanol conversion. The catalyst activity of y-AFCh can be further improved by modifying y-AFCh surface with copper, zinc, silica, aluminum-phosphates, titanium, niobium, boron or other species.

[0032] Zeolites also have a high potential for dehydration of methanol on account of their multiple favorable characteristics, such as tunable acidity, high surface area, micro-porosity or ability to support metal and/or metal-oxide. However, the acidity of zeolite may have to be carefully tuned in order to achieve a balance between catalytic activity and resistance to deactivation by water. For example, ZSM-5 with high Si/Al ratio (e.g., 127) showed high resistance in presence of water but low capacity to dehydrate methanol, while more acidic samples (e.g., ZSM-5 with Si/Al ratio of 27) offers high methanol conversion but poor water resistance and low DME selectivity. It was noted that, for the ZSM-5 zeolite, a Si/Al ratio of 38 showed the best performance in terms of CO conversion, DME yield and water resistance. Zeolites show good activity at temperatures around 200° C.

[0033] Apart from acidity, the textural properties of zeolite crystals also strongly influence the catalytic behavior of hybrid catalysts. In this regard, the one-pot CO-to-DME reaction carried out in presence of hybrid grains prepared via co-precipitation of Cu-Zn-Zr precursors on zeolite crystals with different channel systems (i.e., MOR, FER and MFI) are investigated. The microscopy analysis of hybrid grains revealed that zeolite crystal features strongly affect the metal-oxide(s) distribution, producing a very homogenous distribution over lamellar FER-type crystals and a formation of metal clusters on the other zeolite systems. Catalytic results revealed that DME productivity followed the trend: Cu-Zn-Zr /FER > Cu-Zn-Zr /MOR > Cu-Zn-Zr /MFI, and the superior catalytic activity of FER-based catalyst was traced to lower mass transfer limitations offered by anchorage of metal-oxide clusters on the lamellar crystals typical of FER zeolite. Nevertheless, significant deactivation of the catalyst was observed during time-on-stream tests. The hybrid catalysts developed so far generally tend to suffer deactivation by either coke deposition or metal sintering or poisoning from contaminants present in the reaction stream leading to the blockage of active sites.

[0034] The reasons for catalyst deterioration were identified as reduction in the active catalytic centers that participates in the methanol synthesis. Various specific mechanisms that decrease the active centers include, but not limited to,

1) Reduction in surface area of metallic copper particles in the methanol synthesis catalyst by sintering due to high exothermic heat of combined reaction 4 (AH = - 254.4KJ/mol of DME).

2) Presence of relatively large concentration of water vapor and CO2 gas in the reactor which oxidizes copper and zinc active centers to corresponding carbonate hydroxide species, thereby gradually destroying the methanol synthesis catalyst.

3) Excessive acidity in the methanol dehydration component of the catalyst converts DME to olefins and further to aromatic hydrocarbons. Aromatic hydrocarbons trapped in the pores of dehydration catalyst get cracked leading to coke deposition that blocks the pores of dehydration catalyst. If dehydration catalyst and methanol synthesis catalyst are in intimate contact, then active centers for methanol synthesis are also blocked by coke - result being serious damage to the catalyst.

4) Migration of active specie from methanol synthesis catalyst leading to reduction in activity. [0035] Considering the above observations, an advantageous methanol synthesis catalyst and a catalyst arrangement of the methanol synthesis catalyst and methanol dehydration catalyst are developed. The catalyst system of the present disclosure is highly stable and has long life. The methanol synthesis catalyst has a mixed metal oxide of spinel structure decorated with nanoparticles of copper and cuprous oxide. Further, the peak temperature reached within catalyst pellet is lowered, thereby reducing the deactivation of the methanol synthesis catalyst.

[0036] The disclosed fixed bed reactor for the production of dimethyl ether has multiple layers 100 of catalysts as shown in Figure 1. As used herein, “multiple layers” refers to number of layers equal to or greater than 10. The number of layers may be increased further to have an intimate reaction and fast production of dimethyl ether. In some embodiments, the number of catalysts layers are more than 20. In some embodiments, the layers are more than 40 in number.

[0037] The multiple layers 100 of catalysts include layers 110 of the methanol synthesis catalyst and layers 120 of the methanol dehydration catalyst in alternate manner i.e., in the multiple layers 100 of catalysts, a methanol synthesis catalyst layer 110 is separated from another methanol synthesis catalyst layer 110 by a methanol dehydration catalyst layer 120 and vice versa. Phrases such as “a layer of the methanol synthesis catalyst” and “a layer of the methanol dehydration catalyst” are used for brevity here and does not necessarily mean that the layers contain only the respective catalysts. There may be inclusion of other materials along with the respective catalyst materials in the referred particular layers. Further, when a layer 110 comprising methanol synthesis catalyst alternates with a layer 120 comprising a methanol dehydration catalyst, there may or may not be any other layers or materials present in between the layer of the methanol synthesis catalyst and the layer of the methanol dehydration catalyst. In Figure 1, an optional layer 130 is shown between the layer 110 of comprising methanol synthesis catalyst and a layer 120 comprising a methanol dehydration catalyst. The layer 110 of the methanol synthesis catalyst and the layer 120 of the methanol dehydration catalyst, along with the optional layer 130 separating them forms a repeat unit 140 of the multiple layers 100. The thickness of each layerl 10/120/130 may be changed to suit the overall proportionality. In some embodiments, a total thickness (considering all the layers 110 present) of the methanol synthesis catalyst is about 2-3 times more than the total thickness (considering all the layers 120 present) of the methanol dehydration catalyst in the fixed bed reactor. [0038] At least some part of the material of the methanol synthesis catalyst has a spinel structure. A spinel has a general formulation of AB2X4, crystallizing in the isometric cubic crystal system, with the X anions such as oxygen and sulfur arranged in a cubic close-packed lattice and the cations A and B occupying some or all of the octahedral and tetrahedral sites in the lattice. The spinel structure specifically aids the dispersion and binding of the copper atoms of the methanol synthesis catalyst. Specifically, a major portion of the copper present in the methanol synthesis catalyst is in a bound form in the spinel structure and is not available for sintering. Further, oxidation of copper active centers by water vapor and CO2 content is reduced by binding the copper atoms in the spinel structure. In the methanol synthesis catalyst of the present disclosure, at least 60 molecular percent of the metal components in the total methanol synthesis catalyst composition has spinel structure.

[0039] A quantity of copper present in the methanol synthesis catalyst composition of the present disclosure is less than the normal quantity of the copper present in generally used methanol synthesis catalysts. In some embodiments, in the fixed bed reactor, the methanol synthesis catalyst has copper in a quantity less than 40 atomic percent of the metal components in the methanol synthesis catalyst composition. In some embodiments, the quantity of copper present in the methanol synthesis catalyst is less than 30 atomic percent. Reduction of copper content in the methanol synthesis catalyst is particularly advantageous in reducing sintering of the copper particles. Reduction in copper content increases copper dispersion in the catalyst in turn reducing the copper particles available for combining with the neighboring copper particles for sintering. Therefore, the activity of the copper particles is maintained in the catalyst composition and the useful life of the catalyst is enhanced.

[0040] The sintering of the copper particles and the loss of activity is further reduced by decreasing the copper content present in a free form in the catalyst composition. Copper may be termed to be in a free form, if the copper atoms are not bound in a specific structure in a composition. Binding the copper atoms in a specific structure, such as spinel decreases the copper atoms that are available freely for the reaction with other copper particles or any other atoms, resulting in sintering or forming other compounds respectively, and thereby decreasing the availability of active copper sites for the catalyst functioning. In some embodiments, a quantity of copper present in a free form in the methanol synthesis catalyst is less than 60 atomic percent of the total copper present in the catalyst. In some embodiments, total free copper atoms present in the methanol synthesis catalyst is less than 50 atomic percent of the total copper present in the catalyst. In some specific embodiments, the free form of copper present is in a range from about 4 atomic percent to 30 atomic percent of the total copper present in the methanol synthesis catalyst.

[0041] Catalytic activity of the copper in the methanol synthesis catalyst is further enhanced by using small-sized particles of copper in the methanol synthesis catalyst. If contacting neighboring particles and sintering is carefully avoided, the size reduction of copper particles may greatly enhance the catalytic reactivity of copper. [0042] The methanol synthesis catalyst may be primarily made of one or more oxide materials, generally combining three active metal components in different proportions. These components can be classified as mono, di and tri valent species and can be grouped into A, B, C components based on valency. Copper - mostly in monovalent Cu20 form is the most active component- designated as A and surface area of copper specie is strongly correlated with activity of the catalyst. Component B, the divalent specie, is often represented by Zinc, Zn is also active catalytic component, however often the main function attributed to B component is promoter actively lowering the size of A crystallites and causing a fine dispersion of A. Component C, trivalent specie, functions by causing even higher dispersion of components A & B.

[0043] In some embodiments, the methanol synthesis catalyst includes a mixed metal oxide composition. A mixed metal oxide is an oxide having cations of more than one chemical element or cations of a single element in more than one states of oxidation. Mixed metal oxides may be salts of weak metallic acids or two different oxides that bond together strongly. In some embodiments, the mixed metal oxides may also include solid solution of metal oxides. In some of the embodiments, the methanol synthesis catalyst includes manganese and zinc. In some embodiments, most part of the mixed metal oxides of the methanol synthesis catalyst is in one or more mixed spinel structure.

[0044] Figure 2 illustrates a ternary phase diagram using normally used group I (A), group II (B) and group III (C) elements, using the relative compositions listed in table 1 below. Table 1- Ternary compositions used as methanol synthesis catalyst

[0045] In the table 1, the normally used element A is Cu. Mg and Zn are mostly used as B element and Al, Cr, Mn,

V, and Re alternately or in combination are used as element C. In Figure 1, the line 210 represents a popular composition ratio of A/B =2 and the line 220 represents spinels, if there is a solid solution formation of A-C and B-C compositions. Selected catalysts of the table were prepared by various synthesis methods as disclosed below and the relative activity of the catalysts were tested.

[0046] One of the methods used for the synthesis of the catalysts is a co-precipitation method. When the catalysts are prepared using precipitation method, it was observed that for the same catalyst composition, changing ageing time from 1 hour to 3 hours increased the catalyst activity by factor of 1.5 to 2. Eleven shortlisted catalyst compositions were prepared by K2CO3 co precipitation method using metal nitrate and metal acetate mixture. They were further characterized by XRD analysis and methanol decomposition test for activity. The catalyst 1 prepared by the co=precipitation method was taken as a benchmark with the weightage 1 to compare other four catalysts and the catalysts prepared by other synthesis methods.

[0047] Combustion Synthesis method is another method used for synthesizing the catalyst compositions. Combustion synthesis technique involves a self-sustained reaction between an oxidizer, typically precursor metal nitrates and fuels such as glycine, urea and hydrazine oxalic acid, citric acid, metal acetates, and even palmitic acid have been used. By keeping in a moderate temperature preheated chamber, the reaction mixture turns to be viscous liquid followed by a self-ignition process. Due to the high exothermic nature of the system, the combustion temperature rapidly reaches ~800° C and converts the precursor material to fine crystallites. Moreover, the high temperature reached during the combustion process and short preparation time (few seconds) along with a large amount of gas evolution promote the nanoparticles growth of the intended phase composition. Some catalyst compositions were prepared using different fuels and metal nitrates with good results. In particular, a catalyst made by mixing 60% metal acetate and remaining metal nitrates via solution combustion route showed relative activity of 1.1.

[0048] Another method used for synthesizing the catalyst compositions is redox precipitation method. In this method, tartaric acid complex of Cu, Mg, Zn nitrate mixture were added to hot alkaline solution of glucose under vigorous stirring at 70° C to get a red precipitate of the reduced methanol synthesis catalyst. After washing the precipitate free of alkali, it was dried and directly used.

[0049] In another redox precipitation method to synthesize a catalyst, a Cu, Zn, Mn acetate mixture and KMnCE were used. pH was adjusted to 7 by adding NaOH solution. After washing and drying at 110°C, the precipitate was powdered and calcined at 300°C for one hour. This catalyst showed good relative activity of 0.94.

[0050] Yet another method used for synthesizing the catalyst compositions is double decomposition method. In this method, a 150 ml. mixture of metal nitrates (0.5 M) and 50 ml. potassium chromate (2M) solution were mixed at 70° C with constant stirring. The potassium chromate was added dropwise into metal nitrate solution. The resulting precipitate was washed free of alkali, dried and then calcined at 300° C for 1 hour. Composition of serial number 5 in table 1 synthesized by this method showed activity of 0.90. [0051] Yet one more method used for synthesizing the catalyst compositions is chemo- mechanical method. In this method, a one molar mixture of dry metal carbonates was intensively mixed with 1.2 moles of dry oxalic acid in a planetary mill using zirconia balls as a grinding medium for 1 hour. As a result, a mixed oxalate mixture was formed. When calcined at 300° C for 3 hours, the produced catalyst showed an activity of 1.17. [0052] In yet another method, one molar mixture of metal nitrates and acetates in the ratio 4/6 was intensively ground with 0.6 moles of ammonium carbonate. The resulting mixture was ignited at 200°C and then calcined at 350°C for 3 hours. The catalyst showed good activity of 0.87.

[0053] The selected catalysts prepared by different methods of catalyst preparation were tested in methanol decomposition set up, for their methanol decomposition activity at a temperature of 250°C and atmospheric pressure. A relative ranking of the catalysts prepared using various methods of preparation are as shown in Table 2.

Table 2. Catalyst Screening for Method of Preparation

[0054] From the table 2, it can be observed that the catalysts with serial numbers 6, 7, 8 and 9 have comparative performance to commercially used catalysts 1, 2, 3, 5 and 11. Observable variation in performance resulted from method of preparation. Chemo-mechanical method performed within 25% of commercially used catalyst.

[0055] In some embodiments, the methanol synthesis catalyst is free of intentionally added aluminum and is essentially aluminum-free. Aluminum is found to be reducing activity of methanol synthesis catalyst. One of the reasons for the reduction of methanol synthesis catalyst activity in the presence of aluminum may be the hydrophilic nature of aluminum. A hydrophilic aluminum may attract water and thereby promote oxidation of copper present in the methanol synthesis catalyst thereby reducing the copper available for the catalytic activity. Therefore, an aluminum-free methanol synthesis catalyst is envisaged to reduce degradation of catalytic activities of methanol synthesis catalyst by avoiding usage of hydrophilic alumina in MS catalyst composition. [0056] Cu2Mn6Zn is a non-limiting example composition of an aluminum-free methanol synthesis catalyst. (¾Mh₆Zh has a mixed spinel structure having a mixture of spinels CuMmC^ and ZnM CU. These copper and zinc manganate spinels can exist in solid solution form. Copper present in a spinel is highly dispersed thereby preventing agglomeration of the copper present within the spinel structures. A composition such as CinlVlneZn has only about 10% copper or zinc which may exist as free oxide not compounded with manganese. This composition with up to 10% free copper has exhibited higher activity compared to Cu2Mn6Zn. Any free ZnO will further disperse the free copper. Such composition of catalyst with 10-20% excess copper over Cu2Mn6Zn composition will have highly dispersed copper thereby reducing its chance of agglomerating with other copper specie to form larger less reactive copper particle. The typical composition of this methanol synthesis catalyst is Cu:Mn:Zn in 3:6:1 atomic ratio, having about 11% excess copper over all spinel composition of Cu:Mn:Zn in 2:6: 1 atomic ratio.

[0057] In an example embodiment, CinlVlneZn composition is prepared by (i) chemo- mechanical method of mixing metal salts with ammonium carbonate and (ii) co-ppt method, and (iii)KMn0₄ redox method and used along with g alumina as a methanol dehydration catalyst in a layered structure in a fixed bed reactor. Further, experiments were conducted combining alumina-free methanol synthesis catalyst with g alumina in different proportions and a range of MS:MD 2-3 was found to be beneficial in obtaining reasonably high efficiency and very good life of methanol synthesis catalyst. The methanol synthesis catalyst : methanol dehydration catalyst ratio of 2: 2.5 was found to increase the life of the methanol synthesis catalyst by 200%.

[0058] The methanol dehydration catalyst may be any commercially available composition such as g Alumina, Y Zeolite, modified H-ZSM 5 etc. g Alumina is specifically known as a commercially viable effective methanol dehydration catalyst. In an embodiment of using a methanol dehydration catalyst comprising aluminum ions, such as g Alumina, the layered structure of the methanol synthesis catalyst and methanol dehydration catalyst in the fixed bed reactor further facilitates avoiding mixing of aluminum with the methanol synthesis catalyst. The layered structure is advantageous in preventing entry of aluminum ions into the methanol synthesis catalyst and deactivating the methanol synthesis catalyst and further in reducing the water content in the methanol synthesis catalyst. The methanol synthesis catalyst is protected from water by scrupulously excluding aluminum from the methanol catalyst composition. Aluminum is a hydrophilic substance that adsorbs water. This water layer, if present in methanol synthesis catalyst, damages Cu20, ZnO present in the methanol synthesis catalyst into respective hydroxyl carbonates by reaction with carbonic acid formed by a reaction between water vapor and CO2. [0059] Water gas shift reaction occurs on methanol synthesis catalyst and results in lowering water content of the gas. An aluminum containing dehydration catalyst creates a local higher water content on dehydration layer due to the high adsorption capacity of alumina for water. By separating aluminum from the methanol synthesis catalyst, the local higher water content around aluminum facilitates decreasing the water content in the surrounding gas, thus reducing potential for oxidation of copper in the methanol synthesis catalyst. Thus, by using the layered structure for the methanol synthesis catalyst and methanol dehydration catalyst, the high-water content is isolated from the methanol synthesis catalyst.

[0060] In some embodiments, the fixed bed reactor is free of any liquid form that contacts both the methanol synthesis catalyst and the methanol dehydration catalyst. The absence of liquid-phase contacting both the methanol synthesis catalyst and methanol dehydration catalyst ensures prevention of mixing the aluminum from the methanol dehydration catalyst with the composition of the methanol synthesis catalyst, thereby preventing degradation of methanol synthesis catalyst.

[0061] Each of the measures such as layering of the methanol synthesis catalyst and methanol dehydration catalyst, having an aluminum-free methanol synthesis catalyst, and having dispersed copper particles increases the life span of the methanol synthesis catalyst. In one embodiment, where the methanol synthesis catalyst is free of aluminum, the methanol synthesis catalyst has a lifespan greater than 200% as compared to a methanol synthesis catalyst having aluminum.

[0062] Isolation of the methanol synthesis catalyst and methanol dehydration catalyst in the layered structure in the fixed bed reactor can further be enhanced by using packing materials for physical separation of the methanol synthesis catalyst from the methanol dehydration catalyst. The packing material may be used as one or more layers in between the methanol synthesis catalyst and methanol dehydration catalyst or alternately may be used as a mix along with any one or both of the methanol synthesis catalyst and methanol dehydration catalyst. In some embodiments, the fixed bed reactor has at least one packing material that physically separates the methanol dehydration catalyst from the methanol synthesis catalyst. The physical separation achieved by the packing material may or may not be complete. However, the physical separation achieved by the packing material is found to increase the life of the methanol synthesis catalyst. [0063] Any material that does not interfere with the reactants, surroundings, and catalysts of the methanol synthesis and dehydration reactions and does not hinder the synthesis and dehydration reactions can act as a packing material. In some embodiments, the packing materials include dead burnt alumina, dead burnt magnesia, silicon carbide, or combinations thereof. In some embodiments, the packing material is used as one or more layers in between the methanol synthesis catalyst and methanol dehydration catalyst. In other embodiments, the packing material is mixed with the methanol synthesis catalyst, methanol dehydration catalyst, or both. In one embodiment, the layer having the methanol synthesis catalyst includes an inert material in an amount greater than 20 atomic percent of the total content of the layer.

[0064] In an example embodiment, catalysts with serial number 5 and 8 in table 2 contain up to 10% free copper dispersed on a spinel structure and are expected to perform better in terms of stability. Catalysts 5, 8 along with a commercially used catalyst 11 were tested for synthesis of methanol and for synthesis of dimethyl ether. Both stability and activity were tested for all the three catalysts. In these tests syngas was simulated by mixing ¾ and CO at a ratio 2 for methanol synthesis tests while the ratio was set to 1.1 for the direct dimethyl ether synthesis tests. In each case about 30 gm of methanol synthesis catalyst was used g alumina was used as dehydration catalyst and the catalysts were layered as 4 layers each of MS and MD catalysts. Inert Silicon carbide powder of 60-80 mesh was used to separate the MS and MD catalysts. Activity test results at particular pressure and temperature obtained for the catalyst serial numbers 5, 8, and 11 are summarized in table 3. Table 3- Activity Test

[0065] It can be noted that, catalyst 11 is a commercial methanol synthesis catalyst containing aluminum. It showed very good activity only at high pressure of 60 bar and it showed 6 times lower activity for direct dimethyl ether synthesis. Catalysts 5 and 8 showed good activity. [0066] To test stability, tests were carried out only on catalysts 5 and 8 that are free of aluminum and have lower copper content and also of predominantly spinel structure. During direct dimethyl ether synthesis trials, 90 gm of MS catalyst and 45 gm of g alumina were used in a layered configuration with 8 layers of MS and 8 layers of MD catalyst. Silicon carbide 60-80 mesh particles were used as inert packing material separating two types of catalysts. The stability test results normalized for 100 hours duration are shown in table 4.

Table 4 -Stability Test

* Runaway condition, reactor temperature reached 600° C before aborting Run $ Catalyst performance after 2 hours at 600° C, run aborted due to run away condition encountered in 7^th hour.

+ Fresh catalyst powder ! Runaway condition encountered while raising temperature, but reaction could be controlled after a delay of 3 hours.

[0067] From the table 3 and table 4 results, it can be concluded that the catalyst 11 shows poor activity for low pressure operation but very good activity at typical methanol synthesis condition of 60 bar pressure. Catalyst 11 may not be suitable for single pot dimethyl ether synthesis process. Catalyst 5 and 8 show excellent activity and stability. Catalyst 8 showed better activity than catalyst 5 during activity testing, however showed poorer activity during stability test. The poor performance initially was suspected to be due to silica binder used. This was confirmed with fresh catalyst powder, which showed excellent activity. Catalysts 5, 6 and 8 can be considered as excellent candidates for single pot dimethyl ether synthesis. Although composition of the catalyst 6 was not specifically tested and compared in the above tables, the composition of 6 is close to the composition of 8. Both catalysts 5 and 8 showed excellent stability over 100 hour simulated test for 250° C operation. Over the test duration slight activity increase was observed. Activity after a sudden jump to higher temperature also showed same activity as before the thermal shock. [0068] From the example embodiment, it was found that catalyst 8 with CmMneZn composition is an excellent catalyst for the methanol synthesis functionality and g Alumina is a preferred catalyst for methanol dehydration functionality in a one pot, direct dimethyl ether synthesis. Chemo-mechanical method of mixing metal salts with ammonium carbonate was found to be a simple preparation method for synthesizing the catalysts. Co-ppt method and KMn0₄ redox method are also likely result in a catalyst of superior activity.

[0069] Embodiments of the disclosure have been described in detail for purposes of clarity and understanding. However, it will be appreciated that certain changes and modifications may be practiced within the scope of the present disclosure. Thus, although the disclosure is described with reference to specific embodiments and Figures thereof, the embodiments and Figures are merely illustrative, and not limiting of the disclosure.

Claims

We Claim:

1. A fixed bed reactor for the production of dimethyl ether, the reactor comprising: multiple layers (100) of catalysts, wherein a layer (110) comprising methanol synthesis catalyst alternates with a layer (120) comprising a methanol dehydration catalyst, and wherein at least 60 atomic percent of metal components in the methanol synthesis catalyst composition has spinel structure.

2. The fixed bed reactor as claimed in claim 1, wherein the methanol synthesis catalyst comprises copper in a quantity less than 40 atomic percent of the metal components in the catalyst composition.

3. The fixed bed reactor as claimed in claim 2, wherein a quantity of copper present in a free form in the methanol synthesis catalyst is less than 60 molecular percent of the total copper present in the catalyst and an average particle size of the free copper is less than 30 nm.

4. The fixed bed reactor as claimed in claim 1, wherein the methanol synthesis catalyst comprises a mixed metal oxide composition in a mixed spinel structure.

5. The fixed bed reactor as claimed in claim 4, wherein the methanol synthesis catalyst is free of aluminum and has a lifespan greater than 200% as compared to a methanol synthesis catalyst having aluminum.

6. The fixed bed reactor as claimed in claim 4, comprising manganese and zinc.

7. The fixed bed reactor as claimed in claim 1, wherein the reactor comprises a packing material physically separating the methanol dehydration catalyst from the methanol synthesis catalyst.

8. The fixed bed reactor as claimed in claim 7, wherein the packing material comprises dead burnt alumina, dead burnt magnesia, silicon carbide, or combinations thereof.

9. The fixed bed reactor as claimed in claim 1, wherein the layer (110) comprising the methanol synthesis catalyst comprises an inert material in an amount greater than 20 atomic percent of the total content of the layer.

10. The fixed bed reactor as claimed in claim 1, wherein the reactor is free of any liquid form that contacts both the methanol synthesis.