Identification and tuning of active sites in selected mixed metal oxide catalysts for cyclic carbonate synthesis from epoxides and CO2

Journal of CO₂ Utilization 33 (2019) 434–444

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Identification and tuning of active sites in selected mixed metal oxide catalysts for cyclic carbonate synthesis from epoxides and CO2

T

Nagendra Kulala,b, Vaishnavi Vasistaa, Ganapati V. Shanbhaga,

⁎

a b

Materials Science Division, Poornaprajna Institute of Scientific Research (PPISR), Bidalur post, Devanahalli, Bengaluru, 562 164, India Graduate Studies, Manipal Academy of Higher Education (MAHE), Manipal, 576104, Karnataka, India

ARTICLE INFO

ABSTRACT

Keywords: Cyclic carbonate CO2 Mixed oxide Acidity Basicity

The reaction of CO2 with epoxides to produce cyclic carbonate is one of the important transformations of CO2 which has commercial applications. In this study, several mixed metal oxides were explored as potential catalysts for CO2 cycloaddition reaction with epoxides. The catalytic activities of these catalysts were correlated with their physicochemical properties. It is found that the catalytic activity in terms of cyclic carbonate yield has a good correlation with the number of weak and medium acid-base sites irrespective of type of metal oxides used indicating that a combination of acidity and basicity is an important factor for this reaction. Among, different mixed metal oxides, Mn-Ba and Sn-Ni mixed metal oxides were found to be effective solid catalysts for the synthesis of cyclic carbonate from epoxide and CO2. These mixed metal oxides showed higher activity compared to individual oxides and physical mixture of respective metal oxides. Different compositions of mixed metal oxides and the effect of calcination temperature of the catalyst on cycloaddition of epoxide and CO2 were studied. Mn-Ba oxide and Sn-Ni oxide catalysts with metal ratios of 4.3:1 and 1.5:1 respectively, showed better activity compared to other compositions. The effect of various reaction parameters was studied to know the influence on the conversion and yield. Under optimized reaction conditions, Mn-Ba and Sn-Ni mixed oxides gave 96.0 and 90.2% yield respectively for propylene carbonate. Thus, mixed oxides were shown to be highly efficient catalysts with good recyclability for propylene oxide and CO2 reaction and also can be applied to some other epoxides effectively to make cyclic carbonates.

1. Introduction Over the years, the concentration of CO2 in the atmosphere has significantly increased due to various human activities leading to disastrous environmental change. Currently, there is a considerable interest showed by the scientific community towards the conversion of effluent chemicals/gases to value-added products. CO2 being non-toxic and abundantly available gas, it can be utilized to produce cyclic carbonates which shows promising industrial applications such as intermediate for polycarbonate [1], polyurethanes [2] and dimethyl carbonate [3], solvent in lithium ion batteries [4], paint, personal care and personal cosmetics products [5,6]. Conventionally, toxic phosgene and isocyanate have been used as carbonyl sources for the synthesis of organic carbonates, but these reactions produce carcinogenic byproducts that affect the environment and human health [7]. Synthesis of cyclic carbonate from epoxide and CO2 reaction is one of the catalytic processes that has been commercialized [8]. Studies have revealed that the synthesis of cyclic carbonate

⁎

is thermodynamically favoured over the polymerization process of polycarbonate [9]. Catalysts such as metal halide with quaternary ammonium salt [10], metal halide with melamine [11], metal halide with ionic liquid [12], salen metal complex [13–18] and ionic liquid [19–24] have been reported as catalysts for CO2 conversion, but they pose several limitations including high cost of catalyst production and catalyst reusability problems. To overcome these issues, heterogeneous catalysts such as polymer [25,26] and MOF have been designed [27–33]. Although these catalysts show higher activity and selectivity for the reaction of CO2 and epoxide, their syntheses have proven to be time consuming and expensive. Moreover, they have low thermal stability which poses difficulty during catalyst regeneration. Supported metal complexes [34–36] gave high turnover numbers by using co-catalysts. Polymer supported metal catalysts [37–39] are active catalysts for carbonylation of epoxides, however, these catalysts didn’t show good activity without a co-catalyst or halide anion. Supported quaternary ammonium salt [40,41] and amine functionalized silica [42–48] showed high product

Corresponding author. E-mail addresses: [email protected], [email protected] (G.V. Shanbhag).

https://doi.org/10.1016/j.jcou.2019.07.018 Received 12 February 2019; Received in revised form 10 July 2019; Accepted 16 July 2019 2212-9820/ © 2019 Elsevier Ltd. All rights reserved.

Journal of CO₂ Utilization 33 (2019) 434–444

N. Kulal, et al.

yield, but they have inherent leaching issues. Supported ionic liquids [20,49–56] and halide supported mesoporous carbon nitrides [57,58] have been reported to be effective catalysts only in the presence of halide ions. Several oxide catalysts were reported for cycloaddition of CO2 to epoxides. Monometal oxides like MgO [59] and Nb2O5 [5] have been reported to give low yield for cyclic carbonate. In general, monometallic oxide catalysts with low amount of active sites did not give good performance for this reaction. Among mixed oxides, Mg-Al oxide [60] was reported to give high yield of 88% for propylene carbonate but only when high amount of catalyst was used. ZnO-SiO2 [61] gave high yield of propylene carbonate in the presence of tetrabutylammonium bromide as a co-catalyst. Cs–P–Si [62] mixed oxide was found to be highly active (94% yield) only under supercritical CO2 at 200 °C. This catalyst showed very low yield (> 10%) for cyclic carbonate at lower temperature (150 °C). Though this reaction is mostly carried out in a batch mode, Cs–P–Si oxide gave high cyclic carbonate yield in an up-flow fixed bed reactor in a continuous mode under supercritical CO2. Trimetallic oxides like Ce − Zr − La [63] and Zn-Mg-Al [64] have also been reported for cyclic carbonate synthesis. Combination of rare earth oxides with Zr resulted in about 66% yield, whereas bifunctional Zn-Mg-Al oxide with both acidity and basicity gave high yield (88.8%). Overall, the previous reports suggest that mixed oxides are more active than mono oxides and better than other types of heterogeneous catalysts in terms of simple and inexpensive preparation methods, high activity and high thermal stability. It is important to get a right combination of metal oxides to achieve high yield for cycloaddition of CO2 with epoxides. However, there are very few reports which attempted a detailed study on different combination of metal oxides for this reaction and achieved a good correlation of catalyst properties with activity. In this study, an attempt has been made to design metal oxides that are active for this reaction and the right combination of these oxides was explored to get high conversion and selectivity for the product. Mixed metal oxide catalyst was synthesized and tested for cycloaddition of CO2 to get cyclic carbonates. In general, these catalysts exhibit characteristic properties of acidity and basicity on their surface. The number of active sites on the surface is vital for cycloaddition of CO2 and epoxide because basicity activates CO2, whereas acidity helps in the activation of epoxide molecule. The idea of selecting a metal combination is based on combining basic BaO with d-block transition metal oxides from Mn to Zn which are known to give acidic property upon activation. Apart from this, an oxide of non-transition element Sn also extensively studied as acidic metal oxide directly as a catalyst or as mixed/supported oxide for catalysis. Hence, an idea of combining BaO and SnO2 with transition metal oxide from Mn to Zn separately for cycloaddition of CO2 and epoxide was germinated. This gave us a scope to test many combinations which have metals with different oxidation states, combinations with same oxidation states, basic oxide with acidic oxide etc. In this study, several mixed metal oxides were prepared by co-precipitation method and used for cycloaddition of CO2 to epoxides. The number and strength of active sites of these catalysts were compared with catalytic activity. The synthesized Mn-Ba mixed oxide and Sn-Ni mixed oxide catalysts possess both acidic and basic properties which leads to the desirable bi-functional property. The number of active sites can be tuned by varying calcination temperature and composition of oxides. The individual oxides of Mn, Ba, Ni and Sn have a considerable difference in acidity and basicity compared to Mn-Ba oxide and Sn-Ni oxide catalysts. The surface properties of these catalysts depend on the composition of oxides and their phases that are present on the surface. These catalysts have been characterized using XRD, ICP-OES, FTIR, BET, TPD, SEM and TEM and tested for different cyclic carbonate syntheses. Solvent N,N-dimethylformamide was used in all the catalytic activity studies as it was previously reported as the best solvent for this reaction. The factors such as reaction temperature, reaction time, CO2

pressure, and catalyst type and catalyst amount were studied and optimized to achieve high yield of cyclic carbonate. 2. Experimental section 2.1. Materials Propylene oxide, styrene oxide, epichlorohydrin, cyclohexene oxide and propylene carbonate were procured from Sigma-Aldrich. Stannic chloride hydrate was purchased from Loba Chemie Pvt. Ltd. Manganese (II) chloride tetrahydrate, barium chloride dihydrate, nickel(II) nitrate hexahydrate, zinc chloride, cobalt(II) nitrate hexahydrate, iron(II) chloride tetrahydrate, copper(II) nitrate trihydrate, NaOH, and N,Ndimethylformamide (hereafter DMF) were procured from Merck, India. 2.2. Catalyst preparation Mixed metal oxide catalysts were prepared by co-precipitation method similar to the procedure reported previously by our group [65]. Typical synthesis of mixed metal oxide is as follows. 2.2.1. Synthesis of Mn-Ba oxide catalyst MnCl2.4H2O (8.1 g) and BaCl2.2H2O (6.1 g) were dissolved in 50 ml of deionized water followed by dropwise addition of 3 M NaOH solution until a complete precipitation occurred. The precipitate was stirred for 4 h at RT, filtered, washed with deionized water until it was free from chloride and dried at 120 °C for 4 h. The dried catalyst was subjected to calcination at 600 °C @ 5 °C min−1 for 4 h under static air. 2.2.2. Synthesis of Sn-Ni oxide catalyst Ni(NO3)2.6H2O (7.3 g) and SnCl4.5H2O (17.5 g) were dissolved in 50 ml of deionized water followed by dropwise addition of 3 M NaOH until a complete precipitation occurred. The precipitate was stirred for 4 h at RT, filtered, washed with deionized water until it was free from chloride and dried at 120 °C for 4 h. It was then subjected to calcination at 600 °C @ 5 °C min−1 for 4 h under static air. All other mixed oxides with combinations of transition metals (Fe, Co, Cu and Zn) with alkaline earth metal (Ba) and post transition metal (Sn) were prepared with aforementioned procedure with metal: metal molar ratio 1:1. The catalysts are denoted as M1-M2 (x:y) oxide-z where M1 and M2 are metals and x:y is molar ratio of metals obtained by elemental analysis and z is calcination temperature. 2.3. Catalyst characterization Powder X-ray diffraction patterns of the catalysts were recorded with Bruker D2 phaser X-ray diffractometer using CuKα radiation (λ = 1.5418 Å) with high resolution Lynxeye detector. All the samples were scanned in the 2θ range of 5 to 80° Bragg angle with steps of 0.02 with an interval of 0.5 s. The specific surface area, pore volume and pore size of the catalysts were determined from N2 sorption measurement using Belsorb Mini (II) (BEL Japan) instrument. The isotherms were measured at 77 K after degassing samples below 10−2 kPa at 200 °C for 4 h. The amount of acidity and basicity on the catalyst surface was determined by temperature programmed desorption (TPD) using Belcat-II (BEL Japan). In a typical procedure, before the chemisorption experiments, 0.1 g of catalyst was pretreated at 600 °C under He for 1 h in a quartz U- tube. The temperature was then decreased to 50 °C and 10% of NH3 in He (or 10% of CO2 in He) was introduced to the sample for 30 min. Then physisorbed NH3 (or CO2) gas was removed by passing He over the sample for 15 min. The desorption of the gas was carried out by heating sample up to 600 °C at a rate of 10 °C/min under helium using TCD detector. Scanning electron microscope (SEM) measurements for the catalysts 435

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Fig. 1. XRD patterns of a) individual and bimetallic phase of Mn-Ba(4.3:1) oxide, b) Mn-Ba(4.3:1)-oxide calcined at different temperatures.

were conducted on JSM-6380LV instrument with accelerating energy of 20 kV. The morphology of the catalyst samples was taken by transmission electron microscope (TEM) and a high-resolution transmission electron microscopy (HRTEM) at an accelerating voltage of 200 kV using JEM-2100 (JEOL) instrument Fourier-transform infrared (FT-IR) spectra were obtained using the Bruker Alpha in the 400–4000 cm−1 range by using KBr pellets. Elemental composition present in the final materials was analyzed using ICP-OES by PerkinElmer.

YPC (%) =

Where XPO and YPC are PO conversion and PC yield respectively. XPO (i) and XPO (f ) correspond to initial and final molar concentrations of PO respectively. XPC is the molar concentration of PC formed in the reaction. Carbon balance (%) =

Cycloaddition of epoxide and CO2 reaction was performed by mechanically stirred high-pressure stainless-steel reactor with 50 ml volume (Amar Equipments Pvt. Ltd., India). In a typical procedure, propylene oxide (hereafter PO) (1.8 g), DMF (11 ml) and 20% of catalyst (with respect to epoxide) were put into the reactor. Then the gas inlet valve at the top of the reactor was connected to CO2 cylinder which allows CO2 gas into the reactor. The reactor consists of pressure gauge, thermocouple to monitor the temperature inside the reactor at a given time and furnace which is connected to controller unit for heating the reactor. Reaction mixture was mechanically stirred during the reaction. Once reaction was completed, stirring and heating were stopped and the reactor was cooled in an ice bath. Liquid product was centrifuged to separate out the catalyst and the excess CO2 gas was let out through the vent. The obtained liquid sample was analyzed by a Shimadzu GC-2014 AF with RTX-5 column and FID detector. The gas sample was collected from the outlet of the reactor and analyzed by gas chromatograph (Trace GC-700, Thermo Scientific) equipped with a packed column (Porapak Q) and TCD detector. Products are identified by GC–MS analysis and through standards. The formulae used for calculating conversion, yield and carbon balance are as follows.

XPO (i)

XPO (f )

XPO (i )

Final moles of C in [products + unreacted (PO +CO2)] X 100 Initial moles of C in reactants (PO +CO2)

A leaching test was carried out for carbonylation of PO to understand if any active species of the catalyst are leached out into the reaction medium. The reaction was stopped after 2 h, the reaction mixture was cooled to 15 °C (to condense volatile epoxide) and pressure was very slowly released. Catalyst was then filtered out and the reaction was continued without a catalyst using fresh CO2 in the autoclave. Recyclability experiments were carried out at 160 °C, 25 bar pressure (CO2: epoxide = 1.6), 6 h. After first run, catalyst was separated from reaction mixture by filtration, washed with acetone several times and recovered by centrifugation. Catalyst was dried in an oven for overnight and finally calcined at 600 °C for 4 h. Same procedure was repeated for succeeding runs.

2.4. Catalytic activity studies

XPO (%) =

XPC × 100 XPO (i)

3. Results and discussion 3.1. Catalyst characterization 3.1.1. X-ray diffraction Crystallinity and composition of Mn-Ba and Sn-Ni oxides catalysts were studied by powder X-ray diffraction (XRD) as shown in Figs. 1 and 2. XRD patterns of Mn-Ba(4.3:1) were obtained at different calcination temperatures from 400 to 700 °C (Fig. 1b). At 400 °C, calcined material exhibited a major Mn2O3 and Ba2Mn8O16 phases and minor quantity of BaO phase. At higher calcination temperature of 600 °C, peaks matched well with XRD pattern of orthorhombic Mn2O3 and monoclinic Ba2Mn8O16 (from ICSD), a crystal structure of hollandite. This indicates the transformation of BaO phase into Ba2Mn8O16 phase upon increase in temperature from 400 °C to 600 °C during the calcination process.

x 100

Fig. 2. XRD patterns of a) individual and mixed oxide of Sn-Ni(1.5:1) oxide b) Sn-Ni(1.5:1) oxide calcined at different temperatures. 436

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Fig. 3. a) SEM image of Mn-Ba (2:1). b) TEM image of Mn-Ba(4.3:1)-c) Lattice fringes of Mn-Ba(4.3:1)-catalyst calcined at 600 °C.

Fig. 4. a) SEM image of Sn-Ni(1.5:1), b) TEM image of Sn-Ni(1.5:1). c) Lattice fringes of Sn-Ni(1.5:1) catalyst calcined at 600 °C.

Fig. 2b shows XRD patterns of Sn-Ni(1.5:1) oxide catalyst calcined at different temperatures. The crystallinity of the material increases with increase in calcination temperature from 400 to 600 °C (Fig. 2b). All the peaks at 600 °C can be indexed as cubic NiO and tetragonal SnO2 phases (Fig. 2a). Furthermore, with an increase in calcination temperature to 700 °C, the phases corresponding to mixture of NiO, SnO2 and along with tetragonal SnO phase were formed (Fig. 2b).

with spherical morphology with average particle size of 5–15 nm (Fig. 4b) and well defined crystalline phases were present with d-spacing of 0.211 nm corresponding to (200) plane of NiO phase (Fig. 4c). 3.1.3. N2 sorption studies The N2 isotherms of Mn-Ba(4.3:1)-oxide catalyst showed type II isotherm with H3 type hysteresis loop, which indicated that these samples possessed mesoporous structure (Fig. 5a). The pore size distribution by BJH method was broad ranging from 4 to 170 nm and showed a multi pore system. The presence of macroporosity is also evident in the isotherm considering a lack of plateau at the end of P/P0 (near 1). On the other hand, Sn-Ni oxide showed Type IV adsorption isotherm with H1-type hysteresis loop, typical of large pore mesoporous solids. The relative pressure increased > 0.7 sharply due to capillary condensation of nitrogen within uniform pores. BJH plot (Fig. 5b) exhibited a narrow pore size distribution with an average mesoporous size of 14 nm.

3.1.2. Microscopy studies Catalyst morphology of Mn-Ba(4.3:1) oxide was investigated by SEM as shown in Fig. 3a. Mn-Ba(4.3:1) oxide particles possessed spheroidal morphology with uniform particle size. TEM images of MnBa(4.3:1) oxide catalyst (Fig. 3b) exhibited two types of particles with cuboidal and spheroidal morphologies with well-defined edges, and size ranging from 43 to 57 nm. Further investigation shows a well-defined lattice fringes for cuboid particles with d-spacing of 0.705 nm corresponding to (-1 0 1) plane of Ba2Mn8O16 (Fig. 3c). SEM image of Sn-Ni(1.5:1) mixed oxide catalyst showed two types of particles (Fig. 4a) which differed in size. Bigger sized particles are NiO particles of cubic morphology with average size of 34 μm and smaller SnO2 particles are dispersed on NiO particles [66]. TEM image of Sn-Ni(1.5:1) oxide catalyst suggests the presence of nanoparticles

3.1.4. Chemisorption measurements Temperature programmed desorption (TPD) method is used mainly to quantify acidic and basic sites, and their strength of distribution. Acidity and basicity of individual and binary mixed oxides (with 1:1 M

Fig. 5. N2 adsorption-desorption isotherm and BJH pore size distribution of a) Mn-Ba (4.3:1) and b) Sn-Ni (1.5:1) oxide catalysts calcined at 600 °C. 437

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ratio of metals in synthesis composition) are presented in Table 1. Among different mixed oxides calcined at 600 °C, total number of weak and medium active sites decreased in the order; Sn-Ni > Mn-Ba > FeBa > Sn-Cu > Sn-Fe > Sn-Co > Sn-Zn > Cu-Ba > Ni-Ba > CoBa > Zn-Ba > Sn-Mn (Fig. 6). TPD temperature profiles suggested that the mixed oxides tested contained only weak and medium active sites with peak max > 200 °C and 200–400 °C respectively except for Mn-Ba and Sn-Cu oxides (Table 1). Mn-Ba oxide contained strong (34 μmol/g) basic sites (> 400 °C), whereas Sn-Cu contained 28 μmol/g strong acidic sites in addition to low strength sites. As the molar ratio of Mn/Ba increased from 0.8 to 7.4 in the catalyst composition, there was a gradual decrease in both strong basic sites and total basicity (Table 2 and Fig. 7). This is because, the Mn2O3 phase increased with increase in the concentration of Mn in Mn-Ba oxide (Fig. S1). On the other hand, strong acidic sites are formed at Mn/Ba molar ratio 3.2 which decreased upon increase in molar ratio to 7.4 (Table 2). Individual oxides of Sn and Ni contained lower number of acid and base sites compared to Sn-Ni mixed oxide. When the concentration of Sn in Sn-Ni oxide was increased, there was an increase in both acidic and basic sites (Table 3 and Fig. 8). This might be due to increase in the dispersion of small SnO2 particles on larger NiO particles thereby generating more active sites. (Fig. 4). Both catalysts were calcined at different temperatures to tune the active sites. For Mn-Ba(4.3:1)- oxide, as calcination temperature was increased from 400 to 700 °C, the basicity decreased linearly, whereas there was not much change is total acidity (Table 2). Due to increase in calcination temperature, BaO and Mn2O3 phases transform into Ba2Mn8O16 phase (Fig. 1b) thus decreasing the total basic sites. Also, metal hydroxides were converted into respective oxides by the loss of −OH groups, thus decreasing basic sites. At higher calcination (> 600 °C), the catalyst particles get sintered to lose surface area considerably and that affects the amount of active sites present in the catalyst but it leads to the formation of strong acid and base sites (Fig. S2). Ammonia and CO2-TPD temperature profiles of Sn-Ni(1.5:1) oxide calcined from 400 to 700 °C showed weak and medium acidic + basic sites (Fig. S3). For Sn-Ni(1.5:1) oxide, acidity and basicity increased with increase in calcination temperature till 600 °C (Table 3) contrary to Mn-Ba oxide. It could be because, there was no bimetallic phase upon increase in calcination temperature for Sn-Ni oxide but acidic and basic sites generated due to the formation of coordinatively unsaturated sites and defect crystal sites [66]. Further increase in calcination temperature to 700 °C, decreased acidic sites caused by decrease in surface area considerably due to particle sintering as observed for Mn-Ba oxide. As the calcination temperature increased, there was an increase in number

Fig. 6. Correlation of active sites with catalytic activity for mixed oxides calcined at 600 °C. Reaction condition: 1.8 g PO, 40 bar CO2 pressure, 11 ml DMF, 135 °C, 10 h, 20 wt.% w. r. t. PO.

of acid-base sites which is expected considering the generation of new active sites upon increase in calcination. These active sites possess higher strength compared to samples calcined at low temperatures. 3.2. Catalytic activity studies Carbonylation (or cycloaddition) of propylene oxide with CO2 gives mainly propylene carbonate and a few side products such as acetone, propionaldehyde and 2-ethyl-4-methyl-1,3-dioxolane. The reaction in the absence of catalyst and solvent gave 0.2% of yield (Table 1). The addition of the solvent DMF increased the yield to 10.4% which could be attributed to activation of CO2 by the solvent due to its basic nature. To further check the influence of DMF solvent on the reaction in the absence of catalyst, the reactions were conducted at different temperatures and pressures. It is seen that the reaction with only DMF gave maximum of 29.7% yield at 180 °C and 10 h reaction time. A detailed literature data and the results obtained in this study on blank reactions are given in Supplementary Data (Table S1 to S4). The blank reaction studies show that there is a need to design a suitable catalyst to further increase the product yield to a desirable quantity. The catalyst with acid-base bifunctional property is suitable for this reaction as acidic sites activate epoxide, whereas basic sites activate CO2 molecule.

Table 1 Physicochemical properties and screening of various two mixed metal oxide (metal mole ratio = 1:1) catalysts calcined at 600 °C for propylene carbonate synthesis. Catalyst

a

Blank DMF Sn-Ni Mn-Ba Fe-Ba Sn-Cu Sn-Fe Sn-Co Sn-Zn Cu-Ba Ni-Ba Co-Ba Zn-Ba Sn-Mn

b

SBET (m2/g)

– – 39.22 23.30 17.92 19.28 16.32 14.21 15.70 15.84 18.80 14.32 12.92 13.60

Acidic sites (μmol/g) (A)

Basic sites (μmol/g) (B)

W

M

S

Total

W

M

S

Total

– – 83 73 45 42 49 45 48 45 48 40 37 33

– – 41 29 40 71 34 36 23 21 28 31 21 29

– – 0 0 0 28 0 0 0 0 0 0 0 0

– – 124 102 85 141 83 81 71 66 76 71 58 62

– – 80 103 73 41 54 61 74 72 44 31 52 42

– – 60 44 72 33 47 34 26 29 42 38 33 30

– – 0 34 0 0 0 0 0 0 0 0 0 0

– – 140 181 145 74 101 95 100 101 86 69 85 72

Total sites (A + B) (μmol/g)

Conv. %

Yield %

– – 264 283 230 215 184 176 171 167 162 151 143 134

0.5 17.2 78.6 80.1 77.4 74.7 67.5 67.1 66.0 65.8 66.1 66.3 64.6 63.8

0.2 10.4 73.8 69.2 62.6 62.1 59.5 59.1 58.8 58.6 56.1 54.3 53.9 50.8

Reaction condition: 1.8 g PO, 40 bar CO2 pressure, 11 ml DMF, 135 °C, 10 h, 20 wt.% catalyst w.r.t. PO, W = weak, M = medium, S = strong; a without catalyst and solvent, b without catalyst and with DMF. 438

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Table 2 Physicochemical properties and catalyst testing of Mn-Ba oxides. Catalyst

Mn2O3-600 BaO-600 Mn-Ba(0.8:1)-600 Mn-Ba(1.3:1)-600 Mn-Ba(3.2:1) -600 Mn-Ba(4.3:1)-600 Mn-Ba(7.4:1)-600 Mn-Ba(4.3:1)- 400 Mn-Ba(4.3:1)- 500 Mn-Ba(4.3:1)-600 Mn-Ba(4.3:1)-700 Mn2O3 +BaO (1.5:1)a

SBET (m2/g)

10.60 6.23 24.1 23.3 26.2 26.5 18.6 79.5 24.1 26.5 21.3 –

Acidic sites (μmol/g) (A)

Basic sites (μmol/g) (B)

W

M

S

Total

W

M

S

Total

85 15 64 73 46 43 34 72 68 43 51 –

0 0 42 29 14 28 20 19 17 28 19 –

0 0 0 0 28 21 14 0 0 21 17 –

85 15 106 102 88 92 68 91 85 92 87 –

51 62 98 103 89 67 62 68 52 67 33 –

0 24 55 44 38 21 31 99 82 21 36 –

0 0 61 34 28 23 0 0 0 23 20 –

51 86 214 181 155 111 93 167 134 111 89 –

Total sites (A + B) (μmol/g)

Conv. %

Yield %

136 101 320 283 243 203 161 258 219 203 176 –

49.4 37.7 68.5 80.1 94.2 98.1 89.8 84.7 92.7 98.1 96.5 52.7

38.9 32.1 63.1 68.4 77.5 90.6 84.2 71.1 70.8 90.6 83.0 30.8

Reaction condition: 1.8 g PO, 40 bar CO2 pressure, 11 ml DMF, 135 °C, 10 h, 20 wt.% w.r.t. PO, W = weak, M = medium, S = strong,

a

physical mixture.

Fig. 7. Chemisorption profiles of Mn-Ba oxide catalysts calcined at 600 °C a) NH3-TPD and b) CO2-TPD. Table 3 Physicochemical properties and catalyst testing of Sn-Ni oxide. Catalyst

SnO2-600 NiO-600 Sn-Ni(1.8:1)-600 Sn-Ni(1.5:1)-600 Sn-Ni(0.9:1)-600 Sn-Ni(0.5.1)-600 Sn-Ni(1.5:1)- 400 Sn-Ni(1.5:1)- 500 Sn-Ni(1.5:1)- 600 Sn-Ni(1.5:1)- 700 SnO2+ NiO (1.5:1)a

SBET (m2/g)

11.17 12.11 27.61 33.72 39.22 25.30 41.25 36.52 33.72 26.25 –

Acidity (μmol/g) (A)

Basicity(μmol/g) (B)

W

M

S

Total

W

M

S

Total

74 48 55 58 83 77 39 49 58 41 –

38 26 84 86 41 32 24 26 86 64 –

0 0 0 0 0 0 0 0 0 0 –

112 76 149 144 124 109 63 75 144 105 –

61 42 73 77 80 49 83 88 77 65 –

0 0 134 104 60 66 24 41 104 107 –

0 0 0 0 0 0 0 0 0 0 –

61 52 207 181 140 115 107 129 181 172 –

Total sites (A + B) (μmol/g)

Conv. %

Yield %

173 116 356 325 264 224 170 204 325 277 –

60.8 53.2 87.4 88.6 81.5 83.3 78.2 80.9 88.6 83.6 74.6

53.0 39.9 77.5 81.1 72.0 67.0 72.2 76.3 81.1 76.2 66.3

Reaction condition: 1.8 g PO, 40 bar CO2 pressure, 11 ml DMF, 135 °C, 10 h, 20 wt.% w. r. t. PO, W = weak, M = medium, S = strong

Different mono and binary mixed metal oxides which contained both acidic and basic sites but differ in their amount, nature and strength were screened under reaction conditions; 135 °C, 40 bar pressure, 1.8 g propylene oxide and solvent DMF. Binary mixed oxides chosen for this study were prepared with combination of transition metals (Mn, Fe, Co, Ni, Cu and Zn) with alkaline earth metal (Ba) and post transition metal (Sn). Among mixed oxides, Sn-Ni oxide gave the highest propylene carbonate (hereafter PC) yield of 73.8% followed by Mn-Ba oxide (69.2%) (Table 1). The PC yield decreased in the order; SnNi > Mn-Ba > Fe-Ba > Sn-Cu > Sn-Fe > Sn-Co > Sn-Zn > CuBa > Ni-Ba > Co-Ba > Zn-Ba > Sn-Mn. In order to investigate the reason for this trend, the activity was correlated with its properties such as strength of acidity, basicity and total active sites. Among these

a

physical mixture.

properties, the trend in order of activity in these oxides matched with total number of both weak and medium active sites as shown in Fig. 6. The PC yield decreased with decrease in weak + medium acid-base sites irrespective of the nature of the metals present in these oxides. There was no correlation observed for catalytic activity with only one type of active sites alone for this reaction. The activity of Sn-Cu indicated that high strength acid sites were not particularly helpful in increasing activity for this reaction. Therefore, the carbonylation of epoxides with CO2 could be mainly driven by the amount of acid and base sites in mixed oxides. The individual oxides and physical mixture of NiO and SnO2 as well as Mn2O3 and BaO gave lower activity compared to their respective mixed oxide (Table 2 and 3). SnO2 and NiO catalysts gave 53.0 and 39.9% PC yield 439

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Fig. 8. Chemisorption profiles of Sn-Ni oxide catalysts calcined at 600 °C a) NH3-TPD and b) CO2-TPD.

respectively which is due to the presence of lower number of active sites. Similarly, Mn2O3 and BaO gave 38.9 and 32.1% PC yield respectively which are much lower compared to Mn-Ba oxide catalyst (69.2%).

increased, −OH sites decreased, whereas generation of basicity could be credited to the imperfections on the surface in small crystallites of metal oxide and metal or oxygen vacancies present in crystallites which causes imbalance in local charge [67]. Such sites could be responsible for increase in selectivity for PC at higher calcination temperatures. The PC yield decreased to 83.0% at 700 °C which could be due to sintering of particles as it is evident in drastic decrease in surface area. Therefore, calcination temperature of 600 °C was an optimum parameter for MnBa(4.3:1) oxide catalyst. For Sn-Ni oxides with different metal compositions, Sn-Ni(1.5:1) oxide showed best performance with 88.6% PO conversion and 81.1% PC yield (Fig. S7). The conversion of PO decreased for catalyst at lower Sn/Ni ratio which could be due to lower amount of basicity in the catalysts (Table 3). At lower calcination temperature of 400 °C, though Sn-Ni oxide catalyst contained high number of total active sites, it gave lower product yield of 72.2% (Table 3). As the calcination temperature increased from 400 to 600 °C, PC yield increased from 72.2 to 81.1%. At lower calcination temperature of 400 °C, more side products were observed and PC selectivity increased with increase in calcination temperature similar to that for Mn-Ba oxide catalyst. The increase in PC yield could be attributed to increase in total weak and medium active sites due to increase in calcination temperature up to 600 °C. Higher calcination at 700 °C decreased the PC yield to 76.2% which could be attributed to decrease in total active sites due to particle sintering. Hence, a clear correlation is observed between catalytic activity and total amount of active sites generated by varying calcination temperature.

3.2.1. Optimization of molar composition of mixed oxides Mn-Ba and Sn-Ni oxides with highest activities compared to other catalysts were chosen for further detailed study. When metal composition is varied, physicochemical properties of mixed oxides change which leads to difference in catalytic activity. For optimization of MnBa oxide, Mn to Ba molar ratio was varied to obtain Mn:Ba ratios from 0.8 to 7.4 (from ICP-OES) and catalytic activity was tested for carbonylation reaction (Table 2). As the molar ratio of Mn:Ba increased from 0.8 to 4.3, the PC yield increased substantially from 63.1 to 90.6% and then decreased to 84.2% upon further increasing to 7.4. Mn-Ba(0.8:1) oxide gave low yield though it contained high number of total acid-base sites (Fig. S4). The presence of strong basic sites might have resulted in strong adsorption of CO2 which led to blockage of active sites and therefore resulted in lower conversion. Mn-Ba(4.3:1) catalyst with similar amount of acidity (92 μmol/g) and basicity (111 μmol/g) showed a better activity for propylene carbonate synthesis. Hence, Mn-Ba oxide with 4.3:1 ratio was selected for further study. To tune the active sites by varying calcination temperature, Mn-Ba (4.3:1) oxide catalyst was calcined at different temperatures from 400 to 700 °C and tested for CO2 and epoxide reaction (Table 2). As the calcination temperature increased from 400 to 600 °C, the PC yield increased from 71.1 to 90.6% (Table 2). Though 400 °C calcined catalyst contained highest active sites among the samples, it gave a low PC yield (71.1%) and gave considerable side products such as acetone and propionaldehyde. As lower calcination temperature gives predominantly −OH groups as basic sites, it can be concluded that such active sites are not favorable for this reaction which leads to undesirable side products (Fig. S5 and Fig.S6). As the calcination temperature

3.2.2. Influence of reaction conditions 3.2.2.1. Effect of catalyst loading. The effect of catalyst loading on the reaction was studied for the synthesis of PC at 160 °C and 25 bar CO2 pressure and the results are illustrated in Fig. 9. As concentration of MnBa(4.3:1) oxide catalyst was increased from 10 to 20%, both PO

Fig. 9. Effect of catalyst loading a) Mn-Ba(4.3:1) -600, b) Sn-Ni(1.5:1)-600, reaction condition: 1.8 g PO, 160 °C, 25 bar (CO2 pressure), 8 h and 11 ml DMF(solvent). 440

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Fig. 10. Effect of a) temperature, b) pressure and c) time on catalytic activity of Mn-Ba(4.3:1)-600 and Sn-Ni(1.5:1) -600 on PC Yield.Reaction conditions: 1.8 g PO, 20 wt. % of catalyst (w.r.t. PO), 8 h, 160 °C for [(b) and (c)], 25 bar(CO2 pressure)for [(a) and (c)], and 11 ml DMF (solvent.

Fig. 11. Catalyst reusability tests a) Mn-Ba(4.3:1)-600, b) Sn-Ni(1.5:1)-600, reaction condition: 1.8 g PO, 20 wt. % of catalyst (w.r.t. PO), 160 °C, 25 bar (CO2 pressure) and 6 h for Mn-Ba(4.3:1)-600and 8 h for Sn-Ni(1.5:1)-600 catalystand 11 ml DMF (solvent).

conversion and PC yield increased from 62.3 to 97.7% and 60.2 to 92.0% respectively in 8 h reaction time. Further increase in catalyst weight to 25% did not affect the product yield. For Sn-Ni(1.5:1) oxide catalyst, upon increasing catalyst concentration from 10 to 20%, there was an increase in PO conversion from 57.3 to 96.9% and yield from 54.9 to 90.2% respectively for 8 h reaction which could be mainly attributed to an increase in number of active sites accessible for the synthesis of PC. With further increase in catalyst weight to 25%, there was a marginal improvement in conversion.

30 bar (CO2: PO = 1.9) decreased PO conversion and PC yield by about 10%. Increase in the molar ratio increases the availability of CO2 for the reaction and increase in pressure brings the molecules closer to facilitate the reaction. Decrease in yield at higher pressures could be because of the blockage of basic sites of the catalyst caused due to adsorption of acidic CO2. 3.2.2.4. Effect of reaction time. For Mn-Ba(4.3:1) oxide, there was a linear increase in epoxide conversion to 98.1% (96.0% yield) in 6 h and then remained almost constant, whereas for Sn-Ni(1.5:1) oxide catalyst, the conversion reached maximum of 96.9% (with 90.2% yield) in 8 h time. The marginal decrease in the product yield after attaining > 90.2% for both the catalysts was due to the increase in formation of side products such as acetone and propionaldehyde. Under identical reaction conditions, Mn-Ba(4.3:1) oxide showed a marginally better performance compared to Sn-Ni(1.5:1) oxide (Fig. 10c).

3.2.2.2. Effect of temperature. The reaction was carried out at 25 bar CO2 pressure and 20% catalyst loading to study the effect of temperature on the reaction (Fig. 10a). It is observed that both conversion of epoxide and PC yield increased as the temperature was increased from 120 to 160 °C. At the reaction temperature of 160 °C conducted for 8 h, Mn-Ba(4.3:1) oxide catalyst exhibited 98.1% PO conversion and 95.7% PC yield, whereas Sn-Ni(1.5:1) oxide gave almost similar result (96.9% conversion and 90.2% PC yield). Further increase in temperature to 180 °C, catalyst showed a similar PO conversion, whereas selectivity for PC was slightly lower than that at 160 °C. Synthesis of cyclic carbonate from epoxide and CO2 is an exothermic reaction and therefore [58,67,68], the formation of cyclic carbonate is inhibited at higher temperatures.

3.2.3. Catalyst leaching and reusability tests The leaching test was carried out for both Mn-Ba and Sn-Ni oxide catalysts to understand if any active species are leached out into the reaction medium as explained in Section 2.4. It is seen that there is a negligible increase in conversion till 8 h reaction after removing the catalyst from the reaction mixture. This confirms the heterogeneity of the catalysts for carbonylation of epoxide with CO2. Recyclability experiments for Mn-Ba(4.3:1) and Sn-Ni(1.5:1) oxide catalysts were carried out under 160 °C, 25 bar pressure (CO2: epoxide = 1.6) and 6 h reaction. For both the catalysts, after 1st run, there was a decrease of product yield by 5–10% for the subsequent recycle which is probably due to the deactivation of those active sites which could not be regenerated with the used procedure (Fig. 11). After 2nd run, there was only a marginal decrease in activity with 0.2–1.5% decrease in the yield compared with previous cycle.

3.2.2.3. Effect of CO2 pressure. The reactions were carried out at different CO2 pressures (and thereby changing PO: CO2 molar ratios) to study the effect of pressure on catalyst performance. It can be observed from Fig. 10b that an increase in CO2 pressure from 15 bar (CO2: PO = 0.97) to 25 bar (CO2: PO = 1.6) increased the PO conversion and PC yield. At 25 bar pressure, conversion of PO and PC yield for Mn-Ba(4.3:1) catalyst were 98.1 and 95.7% respectively. Sn-Ni (1.5:1) oxide also exhibited 96.9% conversion with 90.2% yield under similar condition. However, further increasing the CO2 pressure to 441

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group seems to have positive effect on the epoxide conversion, whereas electron withdrawing –Cl and phenyl groups decreased the epoxide conversion. On the other hand, cyclohexene oxide (33–37% yield) is distinctively different from other epoxides. It is a bicyclic compound where epoxide ring is fused with 6-membered ring and therefore ring opening is more difficult compared to other epoxides.

Table 4 Importance of solvent in cycloaddition of PO with CO2 reaction. Solvent

Catalyst

Conv. (%)

Yield (%)

Nil Nil DMF DMF cyclohexane cyclohexane

Mn-Ba(4.3:1) Sn-Ni(1.5:1) Mn-Ba(4.3:1) Sn-Ni(1.5:1) Mn-Ba(4.3:1) Sn-Ni(1.5:1)

52.4 47.7 98.1 84.8 48.2 44.6

48.3 45.2 96.0 82.1 46.0 43.1

3.2.6. Plausible mechanism Based on the type of catalyst and conditions used, the mechanisms were proposed for this reaction in few literatures like for Mg-Al mixed oxide by Kaneda et al [60], Zn/SBA-15 by Zhong et al [70] and with DMF solvent by Aresta et al [5]. Based on the understanding of the catalyst and the reaction, a plausible reaction mechanism was explained for cycloaddition of propylene oxide with CO2 under two different conditions as shown in Scheme 1. In presence of DMF, DMF−CO2 adduct is formed initially, whereas propylene oxide is activated by the acid site on the catalyst surface. The activated CO2 molecule in the form of adduct then attacks propylene oxide to open the epoxide ring. Then it undergoes cyclization followed by removal of DMF to form propylene carbonate (path 1). In a second mechanism, in the absence of DMF, CO2 adsorbs on basic site of the catalyst to form carbonate species, whereas propylene oxide coordinates independently on acidic site of the catalyst. The CeO bond of the epoxide is loosened and electron rich oxygen attacks the carbonyl carbon of the CO2 to form propylene carbonate (path 2).

Reaction condition: 30.1 mmol PO, 160 °C, 25 bar CO2 pressure, 20 wt. % of catalyst (w.r.t. epoxide), time 6 h.

3.2.4. Importance of solvent in cycloaddition of PO with CO2 reaction The cycloaddition of CO2 with PO using Mn-Ba and Sn-Ni oxide catalysts in the absence of solvent gave 48.3 and 45.2% PC yield respectively (Table 4). The non-polar solvent, cyclohexane slightly decreased the product yield by ˜2% compared to solvent free reaction indicating that it does not have any positive influence on the reaction and the dilution effect might have caused a decrease in activity. However, upon adding DMF, the yields were substantially increased to 96.0% for Mn-Ba oxide and 82.1% Sn-Ni oxide catalyst showing the influence of DMF solvent in this reaction. These results indicate that an appropriate solvent can play an important role in enhancing the activity of the catalyst. These results are in accordance with the previous studies on solvent effect for this reaction. Solvent studies for the synthesis of propylene carbonate from propylene oxide and CO2 was studied by Murugan et al and Aresta et al [5,69]. Among all the polar and nonpolar solvents taken in these studies, highest yield was obtained in presence of DMF. It was concluded that, due to its basicity, it activates CO2 by forming an intermediate DMF−CO2 adduct. In another report, DMF was used as solvent for this reaction in presence of Mg-Al mixed oxide catalyst [60]. Also, Zhong et al found that synergistic effect between Zn/SBA-15 and DMF was responsible for the enhanced activity for the cycloaddition of CO2 to propylene oxide [70].

4. Conclusions Binary mixed metal oxides were studied as catalysts for cycloaddition of CO2 with epoxides to yield cyclic carbonates. Mixed metal oxide catalysts were more effective for the synthesis of cyclic carbonates than individual oxides which could be attributed to higher number of acidic and basic sites. Mn-Ba and Sn-Ni mixed oxide catalysts showed better catalytic performance compared to other mixed oxides tested for this reaction. Both these catalysts calcined at 600 °C contained mesoporosity but differed in pore structures. Sn-Ni oxide possessed a narrow pore size distribution with average pore size of 14 nm, whereas Mn-Ba oxide had a multipore structure with four peaks in the BJH plot. It is found that the catalytic activity in terms of cyclic carbonate yield has a good correlation with total weak and medium acid-base sites irrespective of type of metal oxides used indicating that a combination of acidity and basicity is an important factor for this reaction. Mn-Ba oxide and Sn-Ni oxide catalysts with molar ratio of 4.3:1 and 1.5:1 respectively, showed better activities compared to other compositions. Calcination temperature plays an important role in generating active sites and hence influences catalytic activity. At lower calcination temperature of 400 °C, more side products were observed and propylene carbonate selectivity

3.2.5. Cycloaddition of CO2 to various epoxides The blank reactions were carried out for different epoxides at 160 °C, 25 bar CO2 pressure, DMF solvent for 6 h. Propylene oxide gave 19.1% conversion, whereas other epoxides gave lower conversions in the absence of catalyst (Table 5). However, in presence of Mn-Ba(4.3:1) and Sn-Ni(1.5:1) oxide catalysts, the product yield substantially increased for all the epoxides compared to blank reactions. Among the epoxides, the product yield decreased in the order; propylene oxide > epichlorohydrin > styrene oxide > cyclohexene oxide. Propylene oxide and epichlorohydrin gave high cyclic carbonate yield and catalytic activity decreased with increase of steric hindrance caused by side chain substituent of epoxides [11]. Also, electron donating −CH3

Table 5 Cycloaddition of reaction of CO2 with various epoxides catalyzed by Mn-Ba (4.3:1) and Sn-Ni (1.5:1) oxides calcined at 600 °C. Reactant

Product

Catalyst

Time (h)

Carbon balance (%)

Conv. %

Yield %

Blank Mn-Ba(4.3:1) Sn-Ni(1.5:1)

6 6 8

96.5 95.3 97.6

20.5 98.1 96.9

19.1 96.0 90.2

Blank Mn-Ba(4.3:1) Sn-Ni(1.5:1)

6 6 8

98.3 96.1 96.9

11.9 94.2 93.5

11.6 87.7 85.4

Blank Mn-Ba(4.3:1) Sn-Ni(1.5:1)

6 6 8

97.7 95.2 96.0

9.5 89.8 87.4

9.3 82.3 75.3

Blank Mn-Ba(4.3:1) Sn-Ni(1.5:1)

6 6 8

99.1 94.7 96.1

trace 41.7 38.1

0.0 36.5 33.3

Reaction condition: 30.1 mmol of epoxides, 160 °C, 25 bar pressure, 11 ml DMF (solvent), 20 wt. % of catalyst (w.r.t. epoxide). 442

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Scheme 1. Reaction mechanism of cycloaddition of epoxide with CO2 using Mn-Ba oxide catalyst.

increased with increase in calcination temperature till the material had adverse effect like decrease in surface area due to particle sintering. Under optimized reaction conditions; 160 °C, 25 bar pressure and CO2: epoxide ratio of 1.6:1, Mn-Ba(4.3:1) and Sn-Ni(1.5:1) mixed oxides gave 96.0 and 90.2% yield respectively for propylene carbonate. Thus, it is shown that these mixed oxides are efficient catalysts with good recyclability and can be applied as catalysts for different epoxides to make cyclic carbonates.

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Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement GVS is thankful to Vision Group on Science and Technology, Govt. of Karnataka, India for sponsoring the project under Centre of Excellence in Science Engineering and Medicine (CESEM) Grant (GRD No. 307). Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jcou.2019.07.018. References [1] D.J. Darensbourg, M. Ulusoy, O. Karroonnirum, R.R. Poland, J.H. Reibenspies, B. Çetinkaya, Highly selective and reactive (salan) CrCl catalyst for the copolymerization and block copolymerization of epoxides with carbon dioxide, Macromolecules 42 (2009) 6992–6998. [2] M. North, R. Pasquale, C. Young, Synthesis of cyclic carbonates from epoxides and CO2, Green Chem. 12 (2010) 1514–1539.

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