Hybrid catalysts for CO 2 conversion into cyclic carbonates

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Hybrid catalysts for CO

2 conversion into cyclic carbonates

Calabrese, Carla; Giacalone, Francesco; Aprile, Carmela

Published in: Catalysts DOI: 10.3390/catal9040325 Publication date: 2019 Document Version

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Calabrese, C, Giacalone, F & Aprile, C 2019, 'Hybrid catalysts for CO 2conversion into cyclic carbonates', Catalysts, vol. 9, no. 4, 325. https://doi.org/10.3390/catal9040325

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catalysts

Review

Hybrid Catalysts for CO

2 Conversion into

Cyclic Carbonates

Carla Calabrese1,2 , Francesco Giacalone1,* and Carmela Aprile2,*

1 _{Department of Biological Chemical and Pharmaceutical Sciences and Technologies, University of Palermo,}

Viale delle Scienze, Ed. 17, 90128 Palermo, Italy; carla.calabrese@unipa.it

2 _{Laboratory of Applied Materials Chemistry (CMA), University of Namur, 61 rue de Bruxelles,}

5000 Namur, Belgium

* Correspondence: francesco.giacalone@unipa.it (F.G.); carmela.aprile@unamur.be (C.A.); Tel.: +39-091-2389-7530 (F.G.); +32-(0)81-724-514 (C.A.)

Received: 3 March 2019; Accepted: 25 March 2019; Published: 2 April 2019 

Abstract: The conversion of carbon dioxide into valuable chemicals such as cyclic carbonates is an appealing topic for the scientific community due to the possibility of valorizing waste into an inexpensive, available, nontoxic, and renewable carbon feedstock. In this regard, last-generation heterogeneous catalysts are of great interest owing to their high catalytic activity, robustness,

and easy recovery and recycling. In the present review, recent advances on CO2cycloaddition to

epoxide mediated by hybrid catalysts through organometallic or organo-catalytic species supported onto silica-, nanocarbon-, and metal–organic framework (MOF)-based heterogeneous materials, are highlighted and discussed.

Keywords:carbon dioxide; cyclic carbonate; heterogeneous catalysis; ionic liquids; carbon nanotubes; fullerene; graphene; MOF

1. Introduction

1.1. Carbon Dioxide: From Waste to Feedstock

The design of new technologies able to mitigate the environmental impact of CO2is an issue of

growing interest from both academic and industrial standpoints. CO2is a common product of biological

processes such as aerobic respiration or the alcoholic fermentation of sugars. However, carbon dioxide emission is also associated with the combustion of fossil fuels arising from anthropogenic activities for the production of energy, transportation, and industrial processes.

Since the Industrial Revolution, the concentration of carbon dioxide in the atmosphere steadily increased, becoming an environmental issue to be addressed. In this scenario, carbon dioxide capture,

utilization, and storage emerged as tangible processes moving toward sustainable development [1].

Focusing on its utilization, carbon dioxide found application in separation processes, dry-cleaning, refrigerators, fire extinguishers, in the food or agrochemical industry, or as a solvent for several reactions under supercritical conditions [2].

The conversion of carbon dioxide into valuable chemicals is attracting the attention of the scientific community due to the possibility of valorizing industrial waste into an inexpensive,

available, nontoxic, and renewable carbon feedstock [3–5]. Moreover, carbon dioxide was tested

as a one-carbon (C1) building block in organic synthesis for the preparation of several chemicals including methanol, urea, lactones, various heterocycles, biodegradable polymers, and carboxylated structures, among others [6,7].

Scheme1summarizes some industrial organic syntheses using carbon dioxide as a C1 synthon.

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Catalysts 2019, 9, 325_{Catalysts 2019, 9, x FOR PEER REVIEW} _{2 of 30}2 of 30

Scheme 1. Industrialized organic syntheses using CO2 as a one-carbon (C1) feedstock.

All these processes make CO2 a key pillar for the sustainable and resource-efficient production

of chemicals [8]. Independently of the specific application, the drawback of the conversion of carbon dioxide is mainly represented by its elevated thermodynamic stability. The carbon atom in CO2 is

present in its most oxidized state, resulting in a low molecular reactivity. To overcome this problem, highly energetic starting materials such as hydrogen, epoxides, and amine, among others, are usually employed together with a catalyst able to properly decrease the activation energy of the selected reaction (Figure 1). For doing so, in the last few decades, several catalytic systems able to work under both homogeneous and heterogeneous conditions were developed in the perspective of greener chemical processes.

Figure 1. Organic synthesis using CO2 as a building block.

The activation of carbon dioxide is pivotal for its effective transformation. The basic features of CO2 reactivity must be considered to reach a good conversion of this molecule into useful chemicals.

Scheme 1.Industrialized organic syntheses using CO2as a one-carbon (C1) feedstock.

All these processes make CO2a key pillar for the sustainable and resource-efficient production

of chemicals [8]. Independently of the specific application, the drawback of the conversion of carbon

dioxide is mainly represented by its elevated thermodynamic stability. The carbon atom in CO2is

present in its most oxidized state, resulting in a low molecular reactivity. To overcome this problem, highly energetic starting materials such as hydrogen, epoxides, and amine, among others, are usually employed together with a catalyst able to properly decrease the activation energy of the selected

reaction (Figure1). For doing so, in the last few decades, several catalytic systems able to work

under both homogeneous and heterogeneous conditions were developed in the perspective of greener chemical processes.

Scheme 1. Industrialized organic syntheses using CO2 as a one-carbon (C1) feedstock.

All these processes make CO2 a key pillar for the sustainable and resource-efficient production

of chemicals [8]. Independently of the specific application, the drawback of the conversion of carbon dioxide is mainly represented by its elevated thermodynamic stability. The carbon atom in CO2 is

present in its most oxidized state, resulting in a low molecular reactivity. To overcome this problem, highly energetic starting materials such as hydrogen, epoxides, and amine, among others, are usually employed together with a catalyst able to properly decrease the activation energy of the selected reaction (Figure 1). For doing so, in the last few decades, several catalytic systems able to work under both homogeneous and heterogeneous conditions were developed in the perspective of greener chemical processes.

Figure 1. Organic synthesis using CO2 as a building block.

The activation of carbon dioxide is pivotal for its effective transformation. The basic features of CO2 reactivity must be considered to reach a good conversion of this molecule into useful chemicals.

Figure 1.Organic synthesis using CO2as a building block.

The activation of carbon dioxide is pivotal for its effective transformation. The basic features of

CO2reactivity must be considered to reach a good conversion of this molecule into useful chemicals.

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Catalysts 2019, 9, 325 3 of 30

charge on carbon and oxygen atoms. Therefore, the carbon atom, bearing a partial positive charge, can behave as an electrophile. On the other hand, the oxygen atoms, with a partial negative charge, can act as nucleophiles. Owing to the linear geometry of the molecule, with the two dipole moments

opposite each other, the overall molecule is apolar. From the above, the amphoteric CO2behavior

leads to two possible activation pathways: the oxygen atoms can exhibit a Lewis base characteristic, while the carbon atom can play the role of a Lewis acid center. As the electrophilic characteristic of carbon is higher than the nucleophilicity of the oxygen atoms, carbon dioxide is a better acceptor than donor of electron density.

Herein, the conversion of carbon dioxide into cyclic carbonates via reactions with epoxides is discussed, focusing on heterogeneous hybrid materials based on catalytic active species supported onto silica, metal–organic frameworks (MOFs), and carbon nanostructures. Several reviews on the production of cyclic carbonates are proposed for a detailed overview covering a broad scope of catalytic

systems working under both homogeneous and heterogeneous conditions [9–16].

1.2. Carbon Dioxide Conversion into Cyclic Carbonates

One of the most interesting pathways to valorize CO2is represented by its fixation into epoxides

for the production of cyclic carbonates [9].

Cyclic carbonates are organic compounds with interesting properties, such as low vapor pressure, high boiling point, low toxicity, and biodegradability. Based on these features, cyclic carbonates find widespread applications as aprotic high-boiling polar solvents, electrolytes for batteries, precursors for polymeric materials, fuel additives, plastic materials, and intermediates for the synthesis of fine

chemicals such as dialkyl carbonates, glycols, carbamates, and pyrimidines, among others [17–19].

In industry, cyclic carbonates are traditionally prepared via synthetic methodologies involving the use of phosgene. However, such procedures allow the production of equimolar amounts of carbonate and chlorinated salts and large volumes of chlorinated solvents. Moreover, even if phosgene is a versatile building block widely used in the production of plastics and pesticides, it is also hazardous, toxic, corrosive, and difficult to handle. Exposure to phosgene may cause collateral health effects. Therefore, its application as feedstock in industrial large-scale synthesis should be replaced with sustainable alternative routes having a lower environmental impact. Among them, the synthesis of cyclic carbonates from carbon dioxide and epoxide emerged as a low-toxicity alternative. For instance,

Scheme2shows three possible synthetic routes for the conversion of ethene into the corresponding

cyclic carbonate: (i) hydrochlorination of ethylene, followed by hydrolysis, and conversion with COCl2;

(ii) formation of ethylene oxide, subsequent hydrolysis, and reaction with COCl2; (iii) formation of

ethylene oxide and CO2fixation. Life-cycle assessment (LCA) methodology was applied to evaluate

the ratio between the amount of CO2emitted and ethene carbonate produced [3]. The obtained results

clearly identified the reaction between ethylene oxide and CO2as the most sustainable route for the

production of ethylene carbonate.

Catalysts 2019, 9, x FOR PEER REVIEW 3 of 30

Carbon dioxide is a linear molecule, in which the carbon–oxygen bonds are polar, with a net partial charge on carbon and oxygen atoms. Therefore, the carbon atom, bearing a partial positive charge, can behave as an electrophile. On the other hand, the oxygen atoms, with a partial negative charge, can act as nucleophiles. Owing to the linear geometry of the molecule, with the two dipole moments opposite each other, the overall molecule is apolar. From the above, the amphoteric CO2 behavior

leads to two possible activation pathways: the oxygen atoms can exhibit a Lewis base characteristic, while the carbon atom can play the role of a Lewis acid center. As the electrophilic characteristic of carbon is higher than the nucleophilicity of the oxygen atoms, carbon dioxide is a better acceptor than donor of electron density.

Herein, the conversion of carbon dioxide into cyclic carbonates via reactions with epoxides is discussed, focusing on heterogeneous hybrid materials based oncatalytic active species supported onto silica, metal–organic frameworks (MOFs), and carbon nanostructures. Several reviews on the production of cyclic carbonates are proposed for a detailed overview covering a broad scope of catalytic systems working under both homogeneous and heterogeneous conditions [9–16].

1.2. Carbon Dioxide Conversion into Cyclic Carbonates

One of the most interesting pathways to valorize CO2 is represented by its fixation into epoxides

for the production of cyclic carbonates [9].

Cyclic carbonates are organic compounds with interesting properties, such as low vapor pressure, high boiling point, low toxicity, and biodegradability. Based on these features, cyclic carbonates find widespread applications as aprotic high-boiling polar solvents, electrolytes for batteries, precursors for polymeric materials, fuel additives, plastic materials, and intermediates for the synthesis of fine chemicals such as dialkyl carbonates, glycols, carbamates, and pyrimidines, among others [17–19]. In industry, cyclic carbonates are traditionally prepared via synthetic methodologies involving the use of phosgene. However, such procedures allow the production of equimolar amounts of carbonate and chlorinated salts and large volumes of chlorinated solvents. Moreover, even if phosgene is a versatile building block widely used in the production of plastics and pesticides, it is also hazardous, toxic, corrosive, and difficult to handle. Exposure to phosgene may cause collateral health effects. Therefore, its application as feedstock in industrial large-scale synthesis should be replaced with sustainable alternative routes having a lower environmental impact. Among them, the synthesis of cyclic carbonates from carbon dioxide and epoxide emerged as a low-toxicity alternative. For instance, Scheme 2 shows three possible synthetic routes for the conversion of ethene into the corresponding cyclic carbonate: (i) hydrochlorination of ethylene, followed by hydrolysis, and conversion with COCl2; (ii) formation of ethylene oxide, subsequent

hydrolysis, and reaction with COCl2; (iii) formation of ethylene oxide and CO2 fixation. Life-cycle

assessment (LCA) methodology was applied to evaluate the ratio between the amount of CO2 emitted

and ethene carbonate produced [3]. The obtained results clearly identified the reaction between ethylene oxide and CO2 as the most sustainable route for the production of ethylene carbonate.

Scheme 2. Synthetic routes for the production of ethene carbonate [3].

According to green chemistry principles [20], carbon dioxide fixation into epoxides for the synthesis of five-membered cyclic carbonates is a productive catalytic process displaying an atom

Scheme 2.Synthetic routes for the production of ethene carbonate [3].

According to green chemistry principles [20], carbon dioxide fixation into epoxides for the

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economy of 100%. In order to face the challenging thermodynamic stability of carbon dioxide, epoxides have to be used combined with a catalyst to reduce the activation energy of the process.

1.3. Catalytic Systems for the Synthesis of Cyclic Carbonates

In this context, a growing interest toward the synthesis of cyclic carbonates via reaction of CO2

with epoxides led to the design of catalytic systems bearing Lewis acid sites for the electrophilic activation of epoxide and/or carbon dioxide and Lewis base sites as nucleophilic species. Both sites can be included into two different systems (e.g., the metal of a complex as a Lewis acid and the anion of a salt as a Lewis base) or can belong to a sole bifunctional catalyst (e.g., a complex with a cationic metal center and a labile anionic ligand). Several catalysts, working under both

homogeneous and heterogeneous conditions, were developed for the conversion of CO2into cyclic

carbonates via reaction with epoxides. In particular, metal oxides [21], metal–organic frameworks

(MOFs) [22–24], metal salts [25], metal complexes [12,26–28], Lewis base systems [29], ionic liquids (ILs) [30–35], and organic polymers [32,36,37] were proposed as catalysts for this reaction. Based on the environmental impact and the cost efficiency, the overall sustainability of this process has to be evaluated and improved according to some key criteria such as (i) the presence of solvents, (ii) the use of metal species, (iii) the achieved yields and selectivity, and (iv) the required reaction conditions

(temperature, pressure, reaction time). The choice between metal-based or organo-catalysts for CO2

fixation brings strengths and weaknesses. Metal-based systems lead to milder reaction conditions (e.g., lower temperature, reduced catalytic loading) owing to their ability to activate and/or stabilize substrates or intermediates via coordination interactions. On the other hand, organo-catalysts can be considered as suitable and safer alternative for the design of sustainable processes because of their low cost, non-toxic nature, and good stability and inertness toward moisture and air. With the aim to increase the catalyst lifetime, several heterogeneous organo-catalysts were developed, providing simplified work-up procedures combined with the possibility to use them under continuous flow conditions. It is well known that the design of heterogeneous catalysts is particularly envisaged from industrial parties because of their simple recovery from the reaction medium and the possibility of using them in fixed-bed reactors.

2. Hybrid Catalysts for the Synthesis of Cyclic Carbonates

2.1. Silica-Based Hybrid Catalysts

Silica gel and ordered mesoporous silicas display useful features, e.g., high surface area, narrow range of pore sizes (microporous to mesoporous), good thermal and mechanical stability, widespread availability, and easy covalent functionalization strategies covering a broad range of organic or organo-metallic moieties. The huge variety of chemical modifications onto the silica surface is linked to the presence of silanol groups. The covalent functionalization of the silica surface is usually

performed via condensation reactions between the silanol groups and a selected organo-silane [38,39].

Therefore, silylating coupling agents such as chlorosilanes, alkoxy-silanes, and silyl-amines found widespread application. Their reaction with surface silanol functionalities usually occurs on free and geminal silanol groups, whereas hydrogen-bonded silanol moieties are less reactive owing to the local hydrophilic networks. Silicas are employed as supports for the covalent grafting of organic salts in order to improve their applicability and reusability the conversion of carbon dioxide into cyclic carbonates. In this scenario, several heterogeneous catalytic systems based on alkylammonium, imidazolium, pyridinium, and phosphonium salts were developed.

2.1.1. Ammonium-Functionalized Silica

In recent years, ammonium salts were grafted onto silica-based supports in order to be tested as organo-catalysts, as well as bifunctional catalytic systems endowed with Lewis acids sites, for the synthesis of cyclic carbonates via reaction of carbon dioxide with epoxides.

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Catalysts 2019, 9, 325 5 of 30

In 2015, Werner and Kort [40] reported silica-supported ammonium iodide salts as a recyclable

heterogeneous organo-catalyst able to work even at reaction temperatures lower than 100 ◦C.

As reported in Scheme 3, the synthesis of the above material was carried out by reacting

aminopropyl-functionalized silica gel 1 with 2-iodoethanol under argon at 60 ◦C. The catalytic

tests were run under solvent-free reaction conditions at 90 ◦C with a CO2 pressure of 1 MPa.

The solid 2 showed much higher activity if compared with tetrabutylammonium iodide as

a homogeneous catalyst. The co-catalytic role of the hydroxyl functionalities in the epoxide

activation was highlighted by comparing the performance of tetrabutylammonium iodide with that of tri-n-butyl-(2-hydroxyethyl)ammonium iodide in the conversion of butylene oxide into the corresponding carbonate (19% vs. 96%). Catalyst 2 was tested at 2 mol.% for 13 cycles affording

full conversion and excellent isolated yields of butylene carbonate (≥93%). Under the same reaction

conditions, a broad range of epoxides were converted with selectivity toward the corresponding

carbonates of≥98% and isolated yields in the range 67–99%.

Catalysts 2019, 9, x FOR PEER REVIEW 5 of 30 reported in Scheme 3, the synthesis of the above material was carried out by reacting aminopropyl-functionalized silica gel 1 with 2-iodoethanol under argon at 60 °C. The catalytic tests were run under solvent-free reaction conditions at 90 °C with a CO2 pressure of 1 MPa. The solid 2 showed much

higher activity if compared with tetrabutylammonium iodide as a homogeneous catalyst. The co-catalytic role of the hydroxyl functionalities in the epoxide activation was highlighted by comparing the performance of tetrabutylammonium iodide with that of tri-n-butyl-(2-hydroxyethyl)ammonium iodide in the conversion of butylene oxide into the corresponding carbonate (19% vs. 96%). Catalyst 2 was tested at 2 mol.% for 13 cycles affording full conversion and excellent isolated yields of butylene carbonate (≥93%). Under the same reaction conditions, a broad range of epoxides were converted with selectivity toward the corresponding carbonates of ≥98% and isolated yields in the range 67– 99%.

Scheme 3. Silica-supported ammonium iodide salts as a recyclable heterogeneous organo-catalyst.

In the same year, Hajipour and co-workers [41] grafted onto a pre-functionalized silica 3 1,4-diazabicyclo[2.2.2]octane (DABCO)-based ammonium salts (4, Scheme 4). The reaction of styrene oxide with CO2 was chosen to investigate the influence of the anionic species leading to the order of

activity of Br− _{> Cl}−_{> CrO}₃_Cl−_{> Cr}₂_O₇₂₋_.

Scheme 4. Preparation of SiO2-supported ammonium salts based on 1,4-diazabicyclo[2.2.2]octane

(DABCO).

Carbon dioxide conversion was carried out at 100 °C, under solvent-free conditions for 24 h, using 0.6 g of catalyst for 10 mmol of epoxide with 0.4 MPa CO2. Styrene carbonate was obtained

with yields from 19% to 91% and selectivity in the range 25–99%. The most active catalyst was tested for four consecutive cycles showing a decreased efficiency from 94% styrene carbonate yield to approximately 72% upon the fourth use due to the leaching of the catalyst under the heating regimes.

A couple of years later, SBA-15 and MCM-41 were used by Fontaine et al. as supports for the grafting of an active ammonium salt 5 modified with a trimethoxysilane as the linker [42]. The surface of the obtained hybrids was passivated with 1,1,3,3-tetramethyldisiloxane (TMDS) leading to the solids 6 and 7 (Scheme 5).

Scheme 3.Silica-supported ammonium iodide salts as a recyclable heterogeneous organo-catalyst.

In the same year, Hajipour and co-workers [41] grafted onto a pre-functionalized silica 3

1,4-diazabicyclo[2.2.2]octane (DABCO)-based ammonium salts (4, Scheme4). The reaction of styrene

oxide with CO2was chosen to investigate the influence of the anionic species leading to the order of

activity of Br−> Cl−> CrO3Cl−> Cr2O72−.

Catalysts 2019, 9, x FOR PEER REVIEW 5 of 30 reported in Scheme 3, the synthesis of the above material was carried out by reacting aminopropyl-functionalized silica gel 1 with 2-iodoethanol under argon at 60 °C. The catalytic tests were run under solvent-free reaction conditions at 90 °C with a CO2 pressure of 1 MPa. The solid 2 showed much

higher activity if compared with tetrabutylammonium iodide as a homogeneous catalyst. The co-catalytic role of the hydroxyl functionalities in the epoxide activation was highlighted by comparing the performance of tetrabutylammonium iodide with that of tri-n-butyl-(2-hydroxyethyl)ammonium iodide in the conversion of butylene oxide into the corresponding carbonate (19% vs. 96%). Catalyst 2 was tested at 2 mol.% for 13 cycles affording full conversion and excellent isolated yields of butylene carbonate (≥93%). Under the same reaction conditions, a broad range of epoxides were converted with selectivity toward the corresponding carbonates of ≥98% and isolated yields in the range 67– 99%.

Scheme 3. Silica-supported ammonium iodide salts as a recyclable heterogeneous organo-catalyst.

In the same year, Hajipour and co-workers [41] grafted onto a pre-functionalized silica 3 1,4-diazabicyclo[2.2.2]octane (DABCO)-based ammonium salts (4, Scheme 4). The reaction of styrene oxide with CO2 was chosen to investigate the influence of the anionic species leading to the order of

activity of Br− _{> Cl}−_{> CrO}₃_Cl−_{> Cr}₂_O₇₂₋_.

Scheme 4. Preparation of SiO2-supported ammonium salts based on 1,4-diazabicyclo[2.2.2]octane

(DABCO).

Carbon dioxide conversion was carried out at 100 °C, under solvent-free conditions for 24 h, using 0.6 g of catalyst for 10 mmol of epoxide with 0.4 MPa CO2. Styrene carbonate was obtained

with yields from 19% to 91% and selectivity in the range 25–99%. The most active catalyst was tested for four consecutive cycles showing a decreased efficiency from 94% styrene carbonate yield to approximately 72% upon the fourth use due to the leaching of the catalyst under the heating regimes.

A couple of years later, SBA-15 and MCM-41 were used by Fontaine et al. as supports for the grafting of an active ammonium salt 5 modified with a trimethoxysilane as the linker [42]. The surface of the obtained hybrids was passivated with 1,1,3,3-tetramethyldisiloxane (TMDS) leading to the solids 6 and 7 (Scheme 5).

Scheme 4.Preparation of SiO2-supported ammonium salts based on 1,4-diazabicyclo[2.2.2]octane (DABCO).

Carbon dioxide conversion was carried out at 100◦C, under solvent-free conditions for 24 h,

using 0.6 g of catalyst for 10 mmol of epoxide with 0.4 MPa CO2. Styrene carbonate was obtained with

yields from 19% to 91% and selectivity in the range 25–99%. The most active catalyst was tested for four consecutive cycles showing a decreased efficiency from 94% styrene carbonate yield to approximately 72% upon the fourth use due to the leaching of the catalyst under the heating regimes.

A couple of years later, SBA-15 and MCM-41 were used by Fontaine et al. as supports for the

grafting of an active ammonium salt 5 modified with a trimethoxysilane as the linker [42]. The surface

of the obtained hybrids was passivated with 1,1,3,3-tetramethyldisiloxane (TMDS) leading to the solids

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Scheme 5. Grafting of ammonium iodide salts 5 onto silica supports.

Both materials were employed for the conversion of styrene oxide and hexene oxide, whereby the MCM-41-supported catalyst was more active than the analogous ammonium salt supported onto SBA-15. In particular, the conversion of carbon dioxide was performed using a catalyst loading of 10 mol.%, at room temperature for 24 h and with a CO2 pressure in the range 0.1–0.3 MPa. Furthermore,

the reusability of 6 and 7 was investigated over three catalytic cycles of styrene oxide. The catalyst 7 displayed higher recyclability over four cycles with preservation of high styrene carbonate yield (99– 92%) whereas the efficiency of the ammonium salt grafted onto SBA-15 decreased after three cycles providing from 86% to 20% yield of the final product owing to some leaching of the catalyst from the support.

More recently N,N,N-tributyl-N-propylammonium iodide-functionalized mesoporous silicas were proposed as catalysts for the solvent-free synthesis of cyclic carbonates from epoxides and CO2

[43]. SBA-15 and a silica gel were used as support for the synthesis of the solids 8 and 9, respectively (Scheme 6). Styrene oxide and 1,2-butylene oxide were selected as target substrates to evaluate the performance of the solids, using a catalytic loading of 2 mol.% under mild conditions (1 MPa CO2,

100 °C and 4–6 h).

Scheme 6. Preparation of N,N,N-tributyl-N-propylammonium iodide-functionalized mesoporous

silicas and their catalytic performances.

The reusability of the catalysts was found to be dependent on the nature of the support and the substrate. In the presence of 1,2-epoxybutane as substrate, the SBA-15-based catalyst (8) exhibited a constant catalytic activity for five reaction cycles (yield > 96%). Conversely, in the presence of styrene oxide, a gradual decrease in yield was observed. Based on textural properties and 13_C

cross-polarization magic-angle spinning (CP-MAS) NMR measurements, such decline was ascribed to the adsorption of solid reaction product (styrene carbonate) onto the catalyst surface. Using a silica gel as support and an improved catalyst recovery step gave rise to a more efficient and reusable catalyst. Furthermore, by comparison with the homogeneous tetrabutylammonium iodide salt, both 8 and 9 showed improved catalytic activity owing to the synergistic effect of the silanol groups of the solid supports.

Turning to bifunctional catalytic systems based on the presence of Lewis acid sites, in 2012, silica-supported bimetallic aluminum(salen) complexes bearing pendant quaternary ammonium groups

Scheme 5.Grafting of ammonium iodide salts 5 onto silica supports.

Both materials were employed for the conversion of styrene oxide and hexene oxide, whereby the MCM-41-supported catalyst was more active than the analogous ammonium salt supported onto SBA-15. In particular, the conversion of carbon dioxide was performed using a catalyst loading of

10 mol.%, at room temperature for 24 h and with a CO2pressure in the range 0.1–0.3 MPa. Furthermore,

the reusability of 6 and 7 was investigated over three catalytic cycles of styrene oxide. The catalyst

7displayed higher recyclability over four cycles with preservation of high styrene carbonate yield

(99–92%) whereas the efficiency of the ammonium salt grafted onto SBA-15 decreased after three cycles providing from 86% to 20% yield of the final product owing to some leaching of the catalyst from the support.

More recently N,N,N-tributyl-N-propylammonium iodide-functionalized mesoporous silicas were

proposed as catalysts for the solvent-free synthesis of cyclic carbonates from epoxides and CO2[43].

SBA-15 and a silica gel were used as support for the synthesis of the solids 8 and 9, respectively

(Scheme6). Styrene oxide and 1,2-butylene oxide were selected as target substrates to evaluate the

performance of the solids, using a catalytic loading of 2 mol.% under mild conditions (1 MPa CO2,

100◦C and 4–6 h).

Scheme 5. Grafting of ammonium iodide salts 5 onto silica supports.

Both materials were employed for the conversion of styrene oxide and hexene oxide, whereby the MCM-41-supported catalyst was more active than the analogous ammonium salt supported onto SBA-15. In particular, the conversion of carbon dioxide was performed using a catalyst loading of 10 mol.%, at room temperature for 24 h and with a CO2 pressure in the range 0.1–0.3 MPa. Furthermore,

the reusability of 6 and 7 was investigated over three catalytic cycles of styrene oxide. The catalyst 7 displayed higher recyclability over four cycles with preservation of high styrene carbonate yield (99– 92%) whereas the efficiency of the ammonium salt grafted onto SBA-15 decreased after three cycles providing from 86% to 20% yield of the final product owing to some leaching of the catalyst from the support.

More recently N,N,N-tributyl-N-propylammonium iodide-functionalized mesoporous silicas were proposed as catalysts for the solvent-free synthesis of cyclic carbonates from epoxides and CO2

[43]. SBA-15 and a silica gel were used as support for the synthesis of the solids 8 and 9, respectively (Scheme 6). Styrene oxide and 1,2-butylene oxide were selected as target substrates to evaluate the performance of the solids, using a catalytic loading of 2 mol.% under mild conditions (1 MPa CO2,

100 °C and 4–6 h).

Scheme 6. Preparation of N,N,N-tributyl-N-propylammonium iodide-functionalized mesoporous

silicas and their catalytic performances.

The reusability of the catalysts was found to be dependent on the nature of the support and the substrate. In the presence of 1,2-epoxybutane as substrate, the SBA-15-based catalyst (8) exhibited a constant catalytic activity for five reaction cycles (yield > 96%). Conversely, in the presence of styrene oxide, a gradual decrease in yield was observed. Based on textural properties and 13_C

cross-polarization magic-angle spinning (CP-MAS) NMR measurements, such decline was ascribed to the adsorption of solid reaction product (styrene carbonate) onto the catalyst surface. Using a silica gel as support and an improved catalyst recovery step gave rise to a more efficient and reusable catalyst. Furthermore, by comparison with the homogeneous tetrabutylammonium iodide salt, both 8 and 9 showed improved catalytic activity owing to the synergistic effect of the silanol groups of the solid supports.

Turning to bifunctional catalytic systems based on the presence of Lewis acid sites, in 2012, silica-supported bimetallic aluminum(salen) complexes bearing pendant quaternary ammonium groups

Scheme 6. Preparation of N,N,N-tributyl-N-propylammonium iodide-functionalized mesoporous silicas and their catalytic performances.

The reusability of the catalysts was found to be dependent on the nature of the support and the substrate. In the presence of 1,2-epoxybutane as substrate, the SBA-15-based catalyst (8) exhibited a constant catalytic activity for five reaction cycles (yield > 96%). Conversely, in the presence of

styrene oxide, a gradual decrease in yield was observed. Based on textural properties and13C

cross-polarization magic-angle spinning (CP-MAS) NMR measurements, such decline was ascribed to the adsorption of solid reaction product (styrene carbonate) onto the catalyst surface. Using a silica gel as support and an improved catalyst recovery step gave rise to a more efficient and reusable catalyst. Furthermore, by comparison with the homogeneous tetrabutylammonium iodide salt, both 8 and 9 showed improved catalytic activity owing to the synergistic effect of the silanol groups of the solid supports.

Turning to bifunctional catalytic systems based on the presence of Lewis acid sites, in 2012, silica-supported bimetallic aluminum(salen) complexes bearing pendant quaternary ammonium

groups were used by North [44] in the synthesis of cyclic carbonates from epoxides and carbon dioxide

(8)

Catalysts 2019, 9, 325 7 of 30

were used by North [44] in the synthesis of cyclic carbonates from epoxides and carbon dioxide in both batch and gas-phase flow reactors (Scheme 7).

Scheme 7. Synthesis of silica-supported bimetallic aluminum(salen) complexes.

The influence of particle and pore size of the silica support on catalyst activity was investigated with smaller particle sizes (<80 mm) being advantageous to catalyst activity, whereas the silica pore size had little effect on the catalytic performance. The nature of the silica support also affected catalyst stability, and the Fluorochem LC301 silica gave a catalyst with both the highest intrinsic activity and the lowest rate of deactivation. The catalysts were attached to the silica through a linker containing three or eleven carbon atoms (10, 11). The longer linker allowed obtaining a catalyst loading three times higher than the shorter linker, but this was offset by a threefold lower activity for the catalyst with the longer linker. After a reaction time of 24 h, the ethylene carbonate production in a flow reactor at 100 °C gave rise to turnover frequency values up to 1.38 h−1_{. In a batch reactor at 26 °C, the}

most active catalyst (2.35 mol.%) led to a 60% conversion of styrene oxide after 24 h, corresponding to a turnover number of 25.5.

Jianmin Sun et al. [45] developed a series of bifunctional Zn/SBA-15-supported ammonium salts as single-component heterogeneous catalysts for the coupling of CO2 with epoxides. The catalytic

activities of bifunctional solids 12a–d were tested on the reaction of propylene oxide and carbon dioxide at 150 °C and 3 MPa CO2 for 12 h (Scheme 8). Based on the nature of the nucleophilic species,

the catalytic activity increased in the order I−_{> Br}− _{> Cl}−_{> CH}₃_COO−_{. In particular, the most performing}

catalyst 12a led to almost quantitative conversion and 99% propylene carbonate yield with a turnover frequency (TOF) of 326 h−1_{. It is worth mentioning that TOF values were calculated as moles of}

propylene carbonate produced per mole of zinc ion per hour, whereas zinc content was based on X-ray photoelectron spectroscopy (XPS) elemental analysis. The overall versatility of 12a was verified with five substrates bearing both electron-withdrawing and electron-donating substituents. The recyclability of 12a was studied for five runs showing a loss in the catalytic conversion of about 20% probably due to a partial leaching of alkyl ammonium moieties, as well as a structural collapse.

Scheme 8. Bifunctional NH3I-Zn/SBA-15 catalyst for CO2 conversion into cyclic carbonates.

2.1.2. Imidazolium-Functionalized Silica

Imidazolium salts represent one of the most investigated active species for the design of heterogeneous catalysts based on silica supports for the synthesis of cyclic carbonates from CO2 and

12a A-_{= I} -12b A-_{= Br} -12c A-_{= Cl} -12d A-_{= CH} 3COO - Zn-SBA-15 Si O O O NH3 A CO2 + O O O R Catalyst 150 °C, 3 MPa 12 h O Isolated yields: 62-99% TOF(Zn): 47-326 h-1

Scheme 7.Synthesis of silica-supported bimetallic aluminum(salen) complexes.

The influence of particle and pore size of the silica support on catalyst activity was investigated with smaller particle sizes (<80 mm) being advantageous to catalyst activity, whereas the silica pore size had little effect on the catalytic performance. The nature of the silica support also affected catalyst stability, and the Fluorochem LC301 silica gave a catalyst with both the highest intrinsic activity and the lowest rate of deactivation. The catalysts were attached to the silica through a linker containing three or eleven carbon atoms (10, 11). The longer linker allowed obtaining a catalyst loading three times higher than the shorter linker, but this was offset by a threefold lower activity for the catalyst with the longer linker. After a reaction time of 24 h, the ethylene carbonate production in a flow reactor

at 100◦C gave rise to turnover frequency values up to 1.38 h−1. In a batch reactor at 26◦C, the most

active catalyst (2.35 mol.%) led to a 60% conversion of styrene oxide after 24 h, corresponding to a turnover number of 25.5.

Jianmin Sun et al. [45] developed a series of bifunctional Zn/SBA-15-supported ammonium salts

as single-component heterogeneous catalysts for the coupling of CO2with epoxides. The catalytic

activities of bifunctional solids 12a–d were tested on the reaction of propylene oxide and carbon

dioxide at 150◦C and 3 MPa CO2for 12 h (Scheme8). Based on the nature of the nucleophilic species,

the catalytic activity increased in the order I− > Br− > Cl− > CH3COO−. In particular, the most

performing catalyst 12a led to almost quantitative conversion and 99% propylene carbonate yield

with a turnover frequency (TOF) of 326 h−1. It is worth mentioning that TOF values were calculated

as moles of propylene carbonate produced per mole of zinc ion per hour, whereas zinc content was based on X-ray photoelectron spectroscopy (XPS) elemental analysis. The overall versatility of 12a was verified with five substrates bearing both electron-withdrawing and electron-donating substituents. The recyclability of 12a was studied for five runs showing a loss in the catalytic conversion of about 20% probably due to a partial leaching of alkyl ammonium moieties, as well as a structural collapse.