Cyclic carbonates synthesised from CO2:Applications, challenges and recent research trends

Cyclic carbonates are a class of versatile compounds that can be prepared using the greenhouse gas carbon dioxide as building block and that can find applications as green solvents and as sustainable alternative to toxic reactants currently employed in the chemical industry. The current trends in research related to cyclic carbonates are reviewed in this contribution

The Role of Water Revisited and Enhanced:A Sustainable Catalytic System for the Conversion of CO2 into Cyclic Carbonates under Mild Conditions

The role of water as highly effective hydrogen bond donor (HBD) for promoting the coupling reaction of CO2 with a variety of epoxides was demonstrated under very mild conditions (25 °C ≤ T ≤ 60 °C, 2 bar ≤ p(CO2) ≤ 10 bar). Water led to a dramatic increase in the cyclic carbonate yield when employed in combination with Bu4NI, while it had a detrimental effect with the corresponding bromide and chloride salts. The efficiency of water in promoting the activity of the organic halide was compared with state-of-the-art HBDs. Although water requires higher molar loadings to achieve similar degree of conversion of CO2 and styrene oxide into the corresponding cyclic carbonate under the same, mild reaction conditions, its environmental friendliness and much lower cost make it a very attractive alternative as HBD. The effect of different parameters such as the amount of water, CO2 pressure, reaction temperature, and nature of the organic halide used as catalyst was investigated using a high-throughput reactor unit. The highest catalytic activity was achieved with either Bu4NI or PPNI: with both systems, the cyclic carbonate yield at 45 °C with different epoxide substrates could be increased by a factor two or more by adding water as a promoter, while retaining high selectivity. Water is an effective hydrogen bond donor even at room temperature, allowing to reach 85% conversion of propylene oxide with full selectivity towards propylene carbonate in combination with Bu4NI (3 mol%)

Efficient and Selective Oxidation of Aromatic Amines to Azoxy Derivatives over Aluminium and Gallium Oxide Catalysts with Nanorod Morphology

Aluminium oxide and gallium oxide nanorods were identified as highly efficient heterogeneous catalysts for the selective oxidation of aromatic amines to azoxy compounds using hydrogen peroxide as environmentally friendly oxidant. This is the first report of the selective oxidation of aromatic amines to their azoxy derivatives without using transition metal catalysts. Among the tested transition-metal-free oxides, gallium oxide nanorods with small dimensions (9-52 nm length and 3-5 nm width) and fully accessible, high surface area (225 m(2) g(-1)) displayed the best catalytic performance in terms of substrate versatility, activity and azoxybenzene selectivity. Furthermore, the catalyst loading, hydrogen peroxide type (aqueous or anhydrous), and the amount of solvent were tuned to optimise the catalytic performance, which allowed reaching almost full selectivity (98 %) towards azoxybenzene at high aniline conversion (94 %). Reusability tests showed that the gallium oxide nanorod catalyst can be recycled in consecutive runs with complete retention of the original activity and selectivity

Efficient and Easily Reusable Metal-Free Heterogeneous Catalyst Beads for the Conversion of CO2 into Cyclic Carbonates in the Presence of Water as Hydrogen-Bond Donor

Two porous Amberlite resin beads consisting of ammonium-functionalized polystyrene cross-linked with divinylbenzene were demonstrated to be efficient, easily recyclable, and viable metal-free heterogeneous catalysts for the reaction of CO2 with epoxides to yield cyclic carbonates. The catalysts were prepared from two affordable, commercially available resin beads, which differ in the nature of their functional groups, i.e., trimethylammonium chloride or dimethylethanolammonium chloride. These materials were converted through a straightforward ion-exchange step into their iodide counterparts (Amb-I-900 and Amb-OH-I-910). The ion-exchanged resin beads were tested as heterogeneous catalysts for the reaction of CO2 with styrene oxide at different reaction conditions (45-150 °C, 2-60 bar of CO2, 3-18 h). The effect of the presence of water as a hydrogen-bond donor in combination with a heterogeneous catalyst was systematically investigated here for the first time. With both catalysts, the presence of water led to higher yields of cyclic carbonate (from 12% to 58% with Amb-I-900 and from 59% to 66% with Amb-OH-I-910; ≥98% selectivity). The highest catalytic activity was observed with Amb-OH-I-910, due to the presence of -OH groups in its active site, which together with water enhanced the activity through hydrogen-bonding interactions. This catalytic system attained higher turnover numbers and turnover frequencies (TON = 505, TOF = 168 for reaction at 150 °C) and improved cyclic carbonate productivity compared to the state-of-the-art supported polymeric bead catalysts and was active in catalyzing the synthesis of styrene carbonate also at low temperature (33% yield at 45 °C and 10 bar of CO2). Additionally, the Amb-OH-I-910 proved to be a versatile catalyst for the conversion of a variety of epoxides into their corresponding cyclic carbonates with good to excellent yields and very high selectivity (≥98%). The two polymeric bead catalysts could be easily recovered and reused without significant loss in their activity and thus represent an easily accessible, environmentally friendly, cost-effective catalytic system for the synthesis of cyclic carbonates from CO2

Novel elastic rubbers from CO₂-based polycarbonates

We report the fixation of carbon dioxide (CO2) into novel rubber polymers based on polycarbonate domains. Our strategy consisted in the atom-efficient terpolymerisation of CO2 with a long-alkyl-chain epoxide and an unsaturated epoxide to obtain polycarbonates with a glass transition temperature (Tg) below room temperature and with pendant double bonds along the backbone to enable peroxide-promoted cross-linking, thus generating an elastic rubber. First, a wide range of epoxides with long alkyl chains (C6-C12) were coupled with CO2 to give polycarbonates with high yields, using a binary catalytic system consisting of an aluminium amino-tris(phenolate) complex and bis(triphenylphosphoranylidene)ammonium chloride (PPNCl). The synthesised polycarbonates were characterised using FTIR and NMR spectroscopy to determine yields and selectivity, using DSC to measure the Tg, and using GPC to obtain the molecular weight distribution. Next, the terpolymerisation was carried out by including allyl glycidyl ether (AGE) in the reaction mixture together with a long-alkyl-chain epoxide and CO2. Almost complete epoxide conversions (81-100%) and extremely high selectivity (&gt;97%) towards the desired polycarbonates were achieved, with only traces of the cyclic carbonate side-products. The obtained polycarbonates displayed a Tg &lt; 0 °C and thus behave as low-viscosity fluids at room temperature. The pendant unsaturated groups introduced with the AGE monomers allowed cross-linking of the terpolymers with dicumyl peroxide, leading to an elastic rubber-like behaviour as witnessed by their markedly decreased solubility in gel-content tests and by their storage modulus, loss modulus, and Tg, which were determined by dynamic mechanical analysis (DMA). In summary, we have successfully demonstrated that the terpolymerisation of long-chain epoxides, AGE and CO2 yields polycarbonates that can be cross-linked to obtain elastic rubber properties, thus opening the prospects for a new range of applications for CO2-based green polycarbonates.</p

Nanostructured oxides synthesised via scCO₂-assisted sol-gel methods and their application in catalysis

Nanostructured metal oxides and silicates are increasingly applied in catalysis, either as supports or as active species in heterogeneous catalysts, owing to the physicochemical properties that typically distinguish them from bulk oxides, such as higher surface area and a larger fraction of coordinatively unsaturated sites at their surface. Among the different synthetic routes for preparing these oxides, sol-gel is a relatively facile and efficient method. The use of supercritical CO2 (scCO2) in the sol-gel process can be functional to the formation of nanostructured materials. The physical properties of the scCO2 medium can be controlled by adjusting the processing temperature and the pressure of CO2, thus enabling the synthesis conditions to be tuned. This paper provides a review of the studies on the synthesis of oxide nanomaterials via scCO2-assisted sol-gel methods and their catalytic applications. The advantages brought about by scCO2 in the synthesis of oxides are described, and the performance of oxide-based catalysts prepared by scCO2 routes is compared to their counterparts prepared via non-scCO2-assisted methods

Binderless zeolite LTA beads with hierarchical porosity for selective CO₂ adsorption in biogas upgrading

In the context of CO2 removal from biogas, a series of binderless zeolite LTA adsorbents with a macroscopic bead format (0.5–1.0 mm) and with hierarchical porosity (i.e. with the zeolitic micropores being accessible through meso- and macropores mainly in the 10–100 nm range) was synthesized with a variety of Si/Al ratios (1.2–3.9) using Amberlite IRA-900 anion-exchange resin beads as a hard template. The CO2 and CH4 adsorption capacity of the beads in Na-form with different Si/Al ratios were measured, reaching higher CO2/CH4 selectivity and similar, yet slightly higher CO2 adsorption compared to commercial zeolite LTA pellets containing a binder. Subsequently, one the zeolitic beads was subjected to different degrees of ion-exchange (0–96%) with KCl and then tested in the adsorption of CO2 and CH4. The best performance among all the ion-exchanged beads was achieved with Na58K42-LTA beads, which gave very high CO2/CH4 selectivity (1540). Although essentially no CH4 was adsorbed on these beads, the CO2 adsorption capacity was still substantial (1.9 mmol g−1 at 0.4 bar CO2, i.e. the partial pressure of CO2 in biogas)

Ti and Zr amino-tris(phenolate) catalysts for the synthesis of cyclic carbonates from CO2 and epoxides

Herein, we report the application of four amino-tris(phenolate)-based metal complexes incorporating Ti(IV) or Zr(IV) centres (2a-3b) as homogeneous catalysts for the conversion of CO2 and epoxides into cyclic carbonates. The four complexes were synthesised, characterised and then evaluated in combination with tetrabutylammonium iodide, bromide or chloride as binary catalytic systems for the reaction of CO2 with 1,2-epoxyhexane as epoxide substrate at 12 bar CO2 pressure and 90 °C for 2 h. The catalytic systems comprising the two Ti(IV) complexes (2a and 2b) showed similar performance. One notable exception was the catalytic system consisting of titanium complex 2b, bearing an axial Cl-ligand, and tetrabutylammonium chloride, which displayed higher catalytic activity compared to other titanium-based systems. Even higher activity was achieved with Zr(IV) complex 3a, bearing an axial isopropoxide ligand, which reached turnover numbers (TONmetal) up to 1920 for the reaction of CO2 with 1,2-epoxyhexane at 12 bar CO2 pressure and 90 °C for 2 h. This performance is comparable with that of state-of-the-art catalysts for this reaction. The catalytic system consisting of complex 3a and tetrabutylammonium bromide was explored further by investigating its applicability with a broad substrate scope, achieving quantitative conversion of several epoxides with CO2 into cyclic carbonate products at 90 °C and 12 bar CO2 pressure for 18 h. The selectivity towards the cyclic carbonate products was ≥ 98% for all studied terminal epoxides and ≥ 80% for all examined cyclohexene-type epoxides

Highly-accessible, doped TiO₂ nanoparticles embedded at the surface of SiO₂ as photocatalysts for the degradation of pollutants under visible and UV radiation

A series of photocatalysts consisting of C- and N-doped titanium dioxide (TiO2) nanoparticles highly dispersed and firmly embedded at the surface of a silica matrix were prepared using a novel synthesis method in which activated carbon has a double role: it acts as support for depositing the TiO2 nanoparticles and as hard template for generating a silica matrix that embeds them. Additionally, the use of activated carbon in combination with ammonia during the synthesis led to carbon and nitrogen doping of the TiO2 domains, which enhanced their absorption of radiation in the visible range. The combination of these features led to higher activity (i.e. higher removal % and TON) in the photocatalytic degradation of probe pollutants (phenol and rhodamine B) compared to the benchmark P25 TiO2 under UV and, even more markedly, under visible radiation. Particularly, the photocatalyst prepared with 10 wt% of TiO2 nanoparticles (10%TiO2NP@SiO2) displayed much enhanced TON values under visible radiation compared to P25 TiO2 (a 12 times higher TON with rhodamine B, and an 8 times higher TON with phenol). The TON values are also significantly higher compared to any previously reported TiO2-SiO2 photocatalyst. The TiO2NP@SiO2 photocatalysts can be effectively reused in consecutive runs. The photocatalytic activity of the prepared materials was correlated to their physicochemical properties by means of a thorough characterisation using a combination of techniques (XRD, ICP-OES, N2 physisorption, TEM, UV–vis, FT-IR and XPS)