TECHNICAL FIELD

[0001] Embodiments are generally related to the methods and processes for utilization of carbon-di-oxide (CO ₂ ) and fixation of excess carbon-di-oxide (CO ₂ ) in organic molecules using heterogeneous catalysis in ambient condition. Embodiments are also related to production of cyclic carbonates by the sustainable chemical fixation of CO ₂ into epoxides in presence of spinel bimetallic oxide hollow microspheres as robust heterogeneous catalyst under solvent free and mild reaction conditions with excellent conversion of reactant and selectivity of desired product.

BACKGROUND OF THE INVENTION [0002] The alarming repercussion of the industrial revolution has escalated the release of towering amounts of CO ₂ emission in the atmosphere. The ascending amount of anthropogenic CO ₂ has ripened into an important scientific, industrial and technological priority as it is the major contributor for global warming and climatic change. The organization of this primary greenhouse gas as a renewable feedstock for chemical methodologies is a stimulating challenge as a response for a long-term sustainable society without compromising on the economic growth.

[0003] CO ₂ is being used a state-of-the-art raw material for the production of a wide range of value-added products as it is abundantly and easily available, inexpensive, renewable, and non-toxic C-1 source. The selective catalytic fixation of CO ₂ to form energy-rich products such as cyclic carbonates and polycarbonates is being done by exploiting CO ₂ as a C-1 source. The formation of five-membered cyclic carbonates by a phosgene-free method is one of the most encouraging ways for CO ₂ exploitation as it is a 100% atom efficient reaction. Moreover, cyclic carbonates serve a wide range of applications such as its use as aprotic solvents, as electrolytes in Li-ion batteries due to their high dielectric constant, plastics, cosmetics and adhesives in chemical processes. Meanwhile, they are also explored as intermediates for polymer and drug synthesis in respective industries.

[0004] Use of homogenous and heterogeneous catalysts such as ionic liquids, metal complexes, metal oxides, metal organic frameworks (MOFs) have been reported for the selective fixation of cycloaddition of CO ₂ and epoxides to form cyclic carbonates. Undoubtedly, homogenous catalysts have proven to give excellent yields of cyclic carbonates; however, they suffer complexities of catalyst separation and reusability. The use of heterogeneous catalysts can easily overcome these limitations especially at large scale in industrial level.

[0005] In one embodiment of prior art, Zirconium- and hafnium-based metal- organic frameworks as epoxide ring-opening catalysts (US10233145B2) which proposes synthesis and characterization of hafnium (Hf) and zirconium-based metal- organic frameworks (MOFs) and method of using the MOFs to catalyze the fixation reactions of epoxides and CO ₂ to form cyclic carbonates. In another embodiment of prior art, US7365214B2, a process for the preparation of cyclic carbonates discloses the synthesis of cyclic carbonates by the reaction of an epoxide and CO2 using a zeolite (zeolite- Y) based catalyst in presence of Lewis bases such as pyridine or pyridine derivatives, alkyl or aryl phosphene, alkyl ammonium salts and phosphonium salts as co-catalyst. In another embodiment of prior art, CN101318949B, a process for synthesizing cyclic carbonate with catalysis of solid carried ion liquid catalyst which reveals a method for synthesis of immobilized ionic liquids for cycloaddition of epoxides and CO2.

[0006] The US20080033185A1 , discloses a process for the preparation of alkylene carbonate for the synthesis of alkylene carbonate by the catalytic carboxylation of CO2 and alkylene oxide by employing metal salt immobilized on a solid support as a heterogeneous catalyst. Also, EP2146977B1, proposes synthesis of cyclic carbonates in the presence of dimeric aluminium (salen) catalysts discloses dimeric aluminium (salen) catalyst and its uses thereof for the conversion of epoxide and CO ₂ to cyclic carbonates in presence of tetrabutyl ammonium bromide as a co catalyst. The WO2018083357A1 discloses organocatalysts for the production of cyclic carbonates for the preparation of cyclic carbonate by employing imidazole derivatives based organocatalysts. The JP2003251189A, discloses alkylene carbonate synthesizing catalyst for the production of alkylene carbonate with high selectivity by the cycloaddition reaction of an alkylene oxide with CO2.

[0007] An ideal cycloaddition reaction should be performed at ambient, eco- friendly and mild reaction conditions which can be favourable for industries according to technical and economic aspects along with cost-effective and easier scaling-up process. However, the available catalytic approaches and techniques with homogenous and heterogeneous catalytic systems for this reaction are too harsh. Meanwhile, some new developments have been reported for synthesis of metal oxide based catalytic approaches for the same reaction but suffer the disadvantage of extreme reaction conditions.

[0008] Based on the foregoing a need therefore exists for a process for fixation of greenhouse gas CO ₂ into epoxide to form industrially important cyclic carbonates using efficient and recyclable spinel bimetallic oxide hollow microspheres as catalyst in presence of a base under solvent free and mild reaction conditions, as discussed in greater detail herein.

SUMMARY OF THE INVENTION

[0009] The following summary is provided to facilitate an understanding of some of the innovative features unique to the disclosed embodiment and is not intended to be a full description. A full appreciation of the various aspects of the embodiments disclosed herein can be gained by taking the entire specification, claims, drawings, and abstract as a whole.

[0010] The synthetic protocol for the catalytic fixation of CO ₂ into epoxides to form cyclic carbonates catalyzed by spinel bimetallic oxide hollow microspheres in presence of a base are provided.

[0011] One embodiment of a spinel bimetallic oxide may be a copper cobalt bimetallic spinel comprising of Cu2+ in the octahedral voids and Co2+ and Co3+ are distributed over the tetrahedral and other half of the octahedral sites which are prepared solvo-thermally and they might serve as Lewis acidic sites for catalytic fixation of CO2 in to epoxides. The catalyst preparation describes that at solvothermal time of between 1 -6 h, the spinel bimetallic oxide precursor reveals well-defined morphology and growth of marigold petals on the parental microspheres.

[0012] It is a feature of the present invention that the structural examination based on XRD and microscopic analysis of spinel bimetallic oxide suggests the presence of porous marigold morphology with a hollow interior architecture. Spinel bimetallic oxide precursor and final spinel bimetallic oxide consists of well-defined hollow microspheres with marigold flower like configurations having well decorated nano petals. After calcination, the textural surface of the spinel bimetallic oxide appears rough and grainy with retention in marigold morphology. [0013] Next embodiment of a method of catalyzing an organic transformation wherein the as-synthesized spinel bimetallic oxide hollow microspheres is applied as a catalyst in presence of a base for solvent-free conversion of CO2 and epoxide to form cyclic carbonates in the temperature range of 25 °C-120 °C using CO2 pressures between 1-25 bar for 1-5 h. Similarly, the discovery reveals the outstanding base is selected from a group of bases namely pyridine, triethylamine, l,8-diazabicyclo[5.4.0]-undec-7-ene and tetrabutylammonium halides. Additionally, different reaction parameters such as effect of reaction time, temperature, pressure and catalyst amounts are also determined and discussed.

[0014] In yet another embodiment of the discovery, a range of terminal and internal epoxides thereof are explored for sustainable chemical fixation of CO2. The catalyst spinel bimetallic oxide hollow microsphere can be separated and recovered by simple filtration method and reused for further cycles. The catalyst is sustainable in catalytic activity and morphology after recycling process.

[0015] It is a feature of the current discovery that the aforementioned process of catalytic CO2 fixation is solvent-free, phosgene-free and is performed under mild reaction conditions of 80 °C, 20 bar CO2 pressure for 3 h with modest conversion of the epoxides greater than or equal to 94% and selectivity of the cyclic carbonates greater than or equal to 87%. [0016] It is a feature of the current discovery that spinel bimetallic oxide hollow microspheres have been synthesized by facile solvothermal method under mild reactions conditions with good yield and high purity.

[0017] It is a feature of the current discovery that the combined properties of hollow porous interior design with strong Lewis acidic character of the as- synthesized spinel bimetallic oxide hollow microsphere is responsible for the high catalytic activity.

[0018] It is a feature of the current discovery that there is synergistic effect between the Lewis acidic hollow microspheres as it provides a greater number of active sites belonging to the octahedral and tetrahedral sites of spinel bimetallic oxide hollow micro sphere combined with base.

[0019] It is a feature of the current discovery that this methodology is a step- forward towards newer opportunities in the field of CO2 chemistry using bimetallic spinel oxides for fixation of industrially evolved excess CO2 in to organic molecules for formation of different important components using heterogeneous catalysis research approach.

[0020] The aforementioned aspects and other objectives and advantages can now be achieved as described herein. The present invention reports sustainable chemical fixation of industrially evolved excess CO ₂ into epoxide to form cyclic carbonates using spinel bimetallic oxide hollow microspheres as catalyst in presence of a base under solvent free ambient conditions. As a result, the spinel bimetallic oxide hollow microspheres catalyst with unique morphological characteristics revealed enhanced activity with 94% conversion and 94% selectivity with 88% yield in presence of base in mild conditions.

[0021] The present invention reports the 94% selectivity of cyclic carbonate at 80°C using 20 bar pressure in 3 h reaction time. Hence, it is worth to mention here that, this is the first report where spinel bimetallic oxide hollow microspheres as catalyst is applied for the fixation of CO ₂ and obtained efficient results at mild condition. The efficient to good activity is attributed to the exposed Lewis acidic sites of spinel bimetallic oxide on the surface due to the solvothermally obtained robust hollow mesoporous architecture. Furthermore, effects of different reaction parameters such as effect of reaction temperature, effect of pressure, effect of catalyst loading, effect of reaction time with the synthesised spinel bimetallic oxide catalyst are studied and discussed in-depth. Substrate scopes with various industrially important epoxides with CO ₂ to cyclic carbonates are performed and analysis of their yield and selectivity is done using spectroscopic techniques. Recyclability of spinel bimetallic oxide catalyst is experimentally determined and discussed. Recyclability results reveal that, catalyst can be recycled for several cycles without losing noticeable catalytic activity as well as morphological characteristics. Based on the activity, characterization data and with the help of literature, an effort is made to predict a plausible mechanism to form cyclic carbonates via fixation of CO ₂ in to organic epoxide.

BRIEF DESCRIPTION OF DRAWINGS [0022] The accompanying figures, in which like reference numerals refer to identical or functionally- similar elements throughout the separate views and which are incorporated in and form a part of the specification, further illustrate the present invention and, together with the detailed description of the invention, serve to explain the principles of the present invention.

[0023] FIG. 1 illustrates a graphical representation of XRD, TGA, FE-SEM and TEM results of as- synthesized spinel bimetallic oxide precursors and spinel bimetallic oxide hollow microspheres, in accordance with the disclosed embodiment;

[0024] FIG. 2 illustrates a graphical representation influenced by effect of multiple reaction parameters such as time, temperature, pressure and reusability as demonstrated through use of spinel bimetallic oxide hollow microspheres as catalyst, in accordance with the disclosed embodiments; and

[0025] FIG. 3 illustrates the pleasurable reaction mechanism of catalytic fixation of CO2 in epoxide to form cyclic carbonate in presence of spinel bimetallic oxide hollow microspheres, in accordance with the disclosed embodiments.

DETAILED DESCRIPTION [0026] The particular values and configurations discussed in these non-limiting examples can be varied and are cited merely to illustrate at least one embodiment and are not intended to limit the scope thereof.

[0027] The embodiments now will be described more fully hereinafter with reference to the accompanying drawings, in which illustrative embodiments of the invention are shown. The embodiments disclosed herein can be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout. As used herein, the term "and/or" includes any and all combinations of one or more of the associated listed items.

[0028] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms "comprises" and/or "comprising," when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. [0029] Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.

[0030] The synthesis of cyclic carbonates by phosgene free method deals with the cycloaddition reaction of an epoxide with CO ₂ . In a typical trend, the reaction between an epoxide and CO ₂ is given in Scheme 1.

Scheme 1 Catalytic fixation of CO2 in to epoxide to form cyclic carbonate [0031] The source of CO2 gas may be purified carbon dioxide obtained in a pressurized cylinder or impure carbon dioxide gas containing other gases like nitrogen, carbon monoxide and trace amounts of other gases. The pressure of CO2 gas can be altered as required to attain maximum conversion of epoxide and selectivity towards cyclic carbonate in minimum time. [0032] The present invention deals with the synthesis of spinel bimetallic oxide or a mixed metal oxide and its use thereof for the sustainable chemical fixation of CO2 into epoxides. The detailed synthesis procedure is described in later sections of the invention. It has been characterized very well using various analytical, spectroscopic, and microscopic methods. Obtained results of XRD, TGA, SEM and TEM analysis are summarized in FIG. 1.

[0033] FIG. 1 illustrates the graphical representation 100 of XRD analysis of a) un-calcined as-synthesized precursor of spinel bimetallic oxide hollow microsphere and calcined spinel bimetallic oxide hollow microsphere catalyst, b) TGA analysis of un-calcined as- synthesized precursor and calcined spinel bimetallic oxide hollow microsphere catalyst. SEM analysis (c, d, and e) of un-calcined as- synthesized precursor of spinel bimetallic oxide hollow microsphere and SEM analysis (f, g, and h) of calcined spinel bimetallic oxide hollow microsphere catalyst. TEM analysis (i, j and k) of calcined spinel bimetallic oxide hollow microsphere catalyst.

[0034] The present invention teaches a phosgene-free process for the fixation of CO2 into epoxides in presence of a base, where the reaction pressure is selected from a minimum pressure of 1 atm and maximum pressure of 20 bar and heated in the temperature range of 25 °C to 120 °C for 1- 5 h of reaction time. The epoxide could be an aryl substituted epoxide such as styrene oxide, cyclohexene oxide or an alkyl substituted epoxide such as propylene oxide, butylene oxide, epichlorohydrin and derivatives thereof. Based on the collective properties of hollow and porous interior design, multiple valence states, good surface area and strong acidic nature, the spinel bimetallic oxide was applied as a catalyst for chemical fixation of CO2 and epoxide to synthesize cyclic carbonates. The spinel bimetallic oxide hollow microspheres presented in this invention can be separated from the reaction mixture by simple filtration. It is also stable for temperatures up to 600 °C. It has a crystalline mesoporous structure comprising of marigold morphology and hollow interior architecture.

[0035] To determine the catalytic activity, a neat reaction was performed under optimized reaction conditions, in absence of catalyst which did not yield the desired product. Different catalyst loading incorporated for conversion of epoxide and CO2, was an essential factor on which the catalytic activity was dependent. The spinel bimetallic oxide hollow microspheres demonstrates high catalytic activity using 5 wt% of the above stated material as catalyst for the conversion of styrene oxide and CO2 at 80 °C, 20 bar CO2 pressure. With this present invention, it was indicated that the highest conversion and selectivity of 94% was obtained after a reaction time of 3 h. Further increase in time and temperature promoted decrease in selectivity due to the formation of by-products. To the best of the inventors’ knowledge, it can be said based on the structural characteristics that the results obtained under eco friendly and mild reaction conditions are greatly distinct as compared to previous reported literature with pure metal oxides. On performing the same reaction at elevated pressure, the epoxide underwent dilution in excess CO2 amount, as a result of which concentration of epoxide became comparatively low. This was not favourable for the cycloaddition reaction considering that CO2 acts as a solvent at higher pressures. The obtained result for various reaction parameters and its effect of cycloaddition reaction as well as recyclability of the spinel bimetallic oxide hollow microspheres is shown in FIG. 2.

[0036] FIG. 2 illustrates a graphical representation 200 illustrating a) Effect of reaction time with spinel bimetallic oxide hollow microspheres catalyst on formation of cyclic carbonate, b) Effect of reaction temperature with spinel bimetallic oxide hollow microspheres catalyst on formation of cyclic carbonate, c) Effect of CO2 pressure time with spinel bimetallic oxide hollow microspheres catalyst on formation of cyclic carbonate, d) Recyclability study of spinel bimetallic oxide hollow microspheres catalyst, in accordance with the disclosed embodiment.

[0037] The performances of various bases were tested under the optimized reaction conditions for the cycloaddition reaction of epoxide (styrene oxide) and CO2 for the production of cyclic carbonates (styrene carbonate). There was negligible amount of yield obtained in the absence of base. The yield of the cyclic carbonate could be increased by adding 8 mol% of base along with the optimum amount of catalyst. The use of bases with lower pKa values such as TEA, DBU and pyridine gave very low yields. Tetrabutylammonium salts showed low to moderate conversions and selectivity towards the formation of cyclic carbonate product. Tetrabutylammonium iodide (TBAI) showed the highest catalytic activity under optimized reaction conditions compared to its bromide and chloride counterparts due to its better nucleophilicity and leaving ability.

[0038] To evaluate the efficiency and versatility of spinel bimetallic oxide hollow microspheres catalyst, a broad range of other demonstrative epoxides were explored under optimized reaction conditions (80 °C, 20 bar CO2 pressure, 3 h). The catalytic system revealed applicability for both terminal and internal epoxides delivering smooth selective transformation to their corresponding cyclic carbonates as shown in Table 1. Propylene oxide was found to be the most active epoxide towards the cycloaddition reaction followed by halogenated epoxides. The epoxide containing electron donating group (butylene oxide) was found to be the least active epoxide. Further, in order to assay the durability and reusability of the spinel bimetallic oxide hollow microspheres, it was recovered via simple filtration method, washed with ethanol and dried for further reuse. The recovered catalyst was robust and could be recycled for five times and the catalytic activity was maintained throughout without any quantitative loss. The hollow parent morphology was well-retained even after five recycles which proves the ideality of the spinel bimetallic oxide hollow microspheres as catalyst. Additionally, the catalyst in the present invention is stable in atmospheric environment, easy to synthesize, easily separable and highly active for various substrates.

Table 1 Substrate scope using spinel bimetallic oxide hollow microspheres as catalyst for the chemical fixation of CO ₂ into various industrially important epoxides. [0039] FIG. 3 illustrates a reaction mechanism for catalytic fixation of CO ₂ into epoxide to form cyclic carbonate in presence of spinel bimetallic oxide, inset predicted crystal structure of spinel bimetallic oxide, in accordance with the disclosed embodiments. Based on experimental indications and prior art search, a plausible reaction mechanism was proposed (FIG. 3). The Lewis acidic catalyst interacts with the oxygen of the epoxide, resulting in its activation for ring opening due to polarization of the C-0 bond. Subsequently, an iodide (I-) anion from TBAI allows nucleophilic attack on the b- carbon of the activated epoxide to form a C-I bond which enables ring opening of the epoxide to form an iodo-alkoxide intermediate. In the meantime, a second molecule of TBAI dissociates to form in- situ tributylamine and iodobutane by SN2 mechanism or E2 mechanism, which further reacts with electrophilic CO2 to form a carbamate salt. The carbamate salt further coordinates with the Lewis acidic catalyst coordinated epoxide and generates a metal coordinated carbonate intermediate. Finally, the O- of the carbonate attacks the carbon atom and it undergoes ring closure to give the desired cyclic carbonate as product with regeneration of catalyst and TBAI. Hence, the inventors conclude that the synergism between spinel bimetallic oxide hollow microspheres and base guides the reaction to proceed smoothly to form cyclic carbonate as the desired product. [0040] It will be appreciated that variations of the above-disclosed and other features and functions, or alternatives thereof, may be desirably combined into many other different systems or applications. Also that various presently unforeseen or unanticipated alternatives, modifications, variations or improvements therein may be subsequently made by those skilled in the field.