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Novel porous organocatalysts for cycloaddition of CO2 and epoxides

Three classes of organosilicas (DMO, OMOs and PMOs) containing immobilized multi-hydroxyl bis-(quaternary ammonium) iodide salts were prepared and tested in the cycloaddition of CO2 and epoxides. Owing to its higher surface area, pore volume and optimum nucleophilicity of the iodide ion, OMO-2 with two hydroxyl groups was found to be the most active catalyst. For substrates that are easy to activate such as propylene oxide, 1,2-epoxybutane and epichlorohydrin, excellent yields and selectivities were obtained under mild reaction conditions (0.5 MPa CO2, 50 °C and 10–15 h). Moreover, OMO-2 showed very good catalytic properties (yield ≥ 93% and selectivity ≥ 98%), and excellent chemical and textural stability in the synthesis of 1,2-butylene carbonate over 5 cycles.


Introduction
Among greenhouse gases (GHGs), carbon dioxide (CO 2 ) is the main anthropogenic contributor to global warming and climate change. 1,2 Carbon capture is recognized as one of the most promising strategies to mitigate the adverse effects of CO 2 emissions. 3,4 Previously viewed largely as an industrial waste product, CO 2 is increasingly being valued as a cheap, readily available and non-toxic C1 source for the synthesis of chemicals and fuels. 5,6 Accordingly, CO 2 utilization is gaining popularity as a complementing tool to CO 2 capture and sequestration towards GHG mitigation. However, the inherent thermodynamic stability of CO 2 leads to low reactivity. As such, high CO 2 pressure and reaction temperature are generally required for its transformation. Nonetheless, combining CO 2 with highly reactive substrates such as epoxides and aziridines, in the presence of suitable catalysts, provides a pathway to overcome this energy barrier. The catalytic conversion of CO 2 and epoxides to cyclic carbonates (CCs) is one of many reactions using CO 2 as an alternative C1 feedstock. Application of CCs as aprotic polar solvents, active pharmaceutical ingredients, and monomers in the production of ne chemicals makes them an important group of organic materials in the chemical industry. In the generalized mechanism for the synthesis of CCs, regioselective CO 2 insertion is achieved by a combination of epoxide activation by Y through oxygen atom coordination and ring-opening by a nucleophile, X À (Scheme 1).
A variety of homogeneous catalysts such as ionic liquids 7 and metal complexes; 8 immobilized catalysts including supported ionic liquids, 7 and traditional heterogeneous catalysts 9 were reported to be active towards CO 2 cycloaddition. On their own, the last group of catalysts showed low reactivity, and as such received less attention compared to free and immobilized homogeneous catalysts. The latter may consist of a single catalytic component, or a combination of a catalyst and a cocatalyst. 8 Examples of one-component catalyst include tributylpropylammonium iodide immobilized on silica, 10 quaternary ammonium 11 and phosphonium 12 salts. Two-component catalysts include benzylbromide/DMF, 13 tetrabutylammonium salt/ EDTA, 14 and polymer-supported quaternary onium salts/ aqueous solutions of metal salts. 15 These catalysts are not highly popular as they increase the complexity of the process, leading to tedious catalyst separation/recovery and product purication steps. Accordingly, to achieve simplied and costeffective chemical processes, one-component catalysts are preferred.
One-component catalysts can be mono-or bifunctional in nature. Mono-functional catalysts such as alkyl quaternary ammonium 16 or phosphonium 17 halides play a single role, which consists of opening the epoxide ring, leading to the insertion of CO 2 . These extensively investigated catalysts were found to exhibit good to excellent yields in the cycloaddition of CO 2 and epoxides. 18,19 Moreover, one-component bifuntional homogeneous or immobilized catalysts may contain specic functional groups, such as -COOH, -NH 2 and -OH or metals (e.g. Zn, Co and Al). Bifunctionality implies that the catalytic material plays two roles in the course of the reaction, namely; (1) activation of the epoxide by the functional groups, 20 rendering it more susceptible to undergo cycloaddition -Y in Scheme 1, and (2) opening of the epoxide ring by the halide ion, 21 similar to the mono-functional catalysts, X À in Scheme 1. The above functional groups activate the epoxides via hydrogen bonding, while epoxide activation by metal occurs through the formation of metalalkoxides via Lewis acid interactions. 22,23 Examples of metal-free one-component bifunctional catalysts include free and supported hydroxylalkyl ammonium salts 11 or phosphonium 17 salts. Regardless of the design, type or functionality of the catalyst, the ring opening of all epoxides requires a nucleophile. The iodide ion was reported as one of the most efficient owing to its exceptional leaving group ability, stemming from its bulky nature and lower electronegativity.
In terms of sustainable and low-cost processes, highly active metal-free one-component bifunctional catalysts with stable active sites and easy reusability are attractive. Many such catalysts, covalently anchored onto a variety of solid materials were reported in the literature. 24 In some cases, these hybrid solids demonstrate similar or even better activities compared to their homogeneous counterparts. 17,25 Porous inorganic solids such as mesoporous SBA-15 silica 26,27 was used as support for onecomponent porous hybrid solid (PHS) catalysts for the synthesis of cyclic carbonates. Compared to organic supports such as polystyrene, they offer better thermal and mechanical stability, enhanced chemical interaction between the support and the active organic species and high loading of active sites, thereby enhancing production. Moreover, mesoporous inorganic supports alleviate molecular traffic within the pore system, which could enhance both the rate and selectivity of the reaction.
The main preparation method for PHS entails post-synthesis surface functionalization of a silica; typically through graing of an aminosilane or phosphine-silane, followed by quaternization to afford an immobilized quaternary ammonium or phosphonium salt -Scheme 2a. Oen, such organosilicas consist of a disordered distribution of the organic moieties; hence these solids may be referred to as disordered mesoporous organosilicas (DMO). Alternatively, the co-condensation of a silica network former (e.g. TEOS or TMOS) and an organosilica precursor (organotrialkoxysilanes, bridged-or polysilsesquioxanes) over a structure directing agent is a one-pot synthesis method to obtain ordered mesoporous organosilicas (OMOs) -Scheme 2b. Furthermore, a distinctive group of mesoporous organocatalysts, with a unique homogeneous distribution and high loading of covalently linked organic functionalities within the silica framework are periodic mesoporous organosilicas (PMOs). These materials are derived from the hydrolytic sol-gel polycondensation of organic bridgedsilsesquioxanes (R[Si(OR 0 ) 3 ] n ; n $ 2) in the presence of surfactant templates without addition of silica precursors -Scheme 2c.
Signicant progress in the synthesis of CCs from CO 2 and epoxides over mesoporous organocatalysts derived by the graing of aminosilane followed by quaternization (Scheme 2a) has been reported in a number of reviews. 7,28 Although OMOs containing ammonium halides, 29,30 and PMOs containing melamine 31 and urea 32 moieties were reported as active catalysts in the synthesis of CCs, OMOs and PMOs received noticeably less attention compared to DMOs. Considering the higher surface area of OMOs compared to DMOs, and the signicantly higher organic content achievable with PMOs, organocatalysts with greater surface area and active sites can be obtained from OMOs and PMOs respectively. If such active sites can be readily accessed, a signicant enhancement in catalytic performance (i.e. higher reaction rates, selectivity and yield) can be expected. In addition, higher active site density is expected to enhance catalytic activity. In this sense, homogeneous 33 and heterogeneous 34 organocatalysts containing two active sites per molecule were found to be more effective than those containing only one. In this contribution, we present four groups of novel immobilized multi-hydroxyl bis-(quaternary ammonium) salts prepared via three routes: (1) graing of a bridged-SQ on silicas, followed by quaternization, (2) co-condensation of (i) bridged-SQ and (ii) poly-SQ with TEOS, and (3) PMOs derived from (i) bridge-SQ and (ii) poly-SQ. Furthermore, their activity in the synthesis of CCs from CO 2 and epoxides under mild reaction conditions was discussed. The reusability of the most active catalyst was also investigated to better understand the catalyst performance (yield and selectivity), chemical stability (resistance to leaching) and structural stability (textural properties) with reuse.

Preparation of mesoporous organosilica materials
Method A: preparation of DMOs via graing and quaternization of amine. Three commercial grade silicas (Q-10, P-10 and SiL) and SBA-15 silica were used as supports. SBA-15 was prepared using an established procedure. 35 Graing was carried out in toluene in the presence of water. Our group previously demonstrated that addition of water improves both the quality and loading of graed amine on MCM-41. 36 Based on literature reports, 37 it is possible to estimate the amount of water required to obtain a fully hydroxylated silica surface. This can be achieve by considering the maximum number of surface silanol groups to be 5 Â 10 18 SiOH per m 2 , and by introducing twice as many water molecules per m 2 . 38 Accordingly, for a typical silica with a surface area of 300 m 2 g À1 , the volume of water to be added is approximately 0.1 mL g À1 .
A typical procedure for graing was as follows; 1 g of thermally pretreated (at 130 C for 4 h) silica was dispersed in 30 mL of toluene at room temperature, followed by addition of an appropriate volume of water; 0.1 mL for Q-10 and P-10, 0.15 mL for SiL, and 0.3 mL for SBA-15. The temperature of the mixture was raised to 85 C and further stirred for 30 min. Finally, 2 mL of BHBPD was added and the resultant mixture was reuxed at 85 C for 16 h in an oil bath. The graed materials were then ltered, washed with toluene, methanol, then Et 2 O, and dried under vacuum at 60 C for 4 h. The obtained solids are hereaer referred to as S@amine, where S indicates the support. The amine-graed solids were then quaternized as follows: 1 g of material was dispersed in 15 mL of toluene, followed by addition of 2 mL of 2-iodoethanol under a nitrogen atmosphere. The reaction mixture was kept under reux at 70 C for 3 days. The obtained solids were ltered off, washed with toluene, methanol, then Et 2 O, and dried at 70 C under vacuum for 4 h to yield pale yellow powder materials of immobilized diammonium iodide salt. The obtained solids are hereaer referred to as DMO-Q10, DMO-P10, DMO-SiL and DMO-SBA15 aer the corresponding S@amine. The synthesis of amine-graed materials and their quaternized counterparts is represented in Scheme 3, with Q-10 as silica support.
Method B: preparation of OMOs via co-condensation of BHBPD or poly-SQ salt and TEOS. The synthesis of all porous organosilica by hydrolysis and condensation with a silica precursor were performed in the presence of a neutral surfactant (P123) under acidic condition. In a typical procedure, 2.0 g of triblock copolymer P123 and 2 g of KCl were dissolved in a mixture of 30 g distilled water and 60 g of 2 M HCl solution in a 250 mL Teon-liner at 40 C. Notice that the use of KCl was shown to enhance ordering of mesoporous organosilicas. 39 To this solution, 4.3 g of TEOS was added and the mixture stirred for 1 h to ensure the pre-hydrolysis of TEOS. Thereaer, 2 mL (2.83 mmol) of BHBPD, corresponding to a nominal amine loading of 2.0 mmol per gram of silica, was slowly added under stirring. The resultant mixture was further stirred at 40 C for 20 h. Finally, the mixture was transferred into an autoclave and heated at 100 C under static condition for 2 days. The solid products were collected by ltration, washed with water, and dried at ambient conditions. The surfactant was removed by reuxing in 100 mL of a 98 : 2 v/v% ratio of ethanol : conc. HCl mixture at 60 C for 6 h. The powder material was collected by ltration, washed with ethanol, and dried at ambient conditions. A similar preparation, but without BHBPD, was carried out for SBA-15 silica, herein referred to as SBA-15*. To make sure all P123 has been removed, 1 g of the solid ethanolextracted material was subjected to additional Soxhlet extraction with 100 mL of methanol at 70 C for 16 h. The obtained solid was dried overnight in ambient condition, then at 60 C under vacuum for 4 h.
An off-white powder material, hereaer referred to as OMO-1, was obtained from the co-condensation of TEOS and BHBPD. Because of the acidic conditions, OMO-1 was actually in the form of a quaternary ammonium chloride salt. OMO-1 was subjected to; (1) ion-exchange with ammonium iodide to obtain the corresponding ammonium iodide salt -OMO-2 and (2) dequaternization, regenerate the amine -OMO-3. To obtain a QAS with additional -OH groups, OMO-3 was quaternized with 2-iodoethanol to afford OMO-4. The structures of OMO-(1-4) are represented in Scheme 4.
Ion-exchange was carried out as follows; 1 g of OMO-1 was dissolved in 30 mL of methanol followed by addition of 1 mL of ammonium iodide under a nitrogen atmosphere. The mixture was reuxed at 65 C for 16 h. The resultant solid was recovered by ltration, washed with methanol, then Et 2 O and dried under vacuum at 70 C for 4 h to afford a yellow powder (OMO-2). To regenerate the amine from OMO-1, 1 g of material was dispersed in 50 mL of ethanol, followed by the addition of 1 mL Scheme 3 Synthesis of Q-10@amine and DMO-Q10 via grafting and quaternization.
Scheme 4 Synthesis of OMO-1 to OMO-4 via co-condensation followed by ion-exchange or quaternization. of triethylamine. Aer reux at 60 C for 2 h, the solid was recovered by hot-ltration and washed with boiling ethanol, then dried at 60 C under vacuum for 4 h to afford an off-white powder material -OMO-3. OMO-3 was quaternized as described in method A to afford a yellow powder -OMO-4.
The preparation of OMOs from BHBPD-derived salt involves two steps; the synthesis of the salt, hereaer referred to as poly-SQ salt, followed by its co-condensation with TEOS. The poly-SQ salt was synthesized as follows; to a 25 mL two-neck round bottom ask containing 10 mL of toluene, 2 mL (2.83 mmol) of BHBPD was added. Under a nitrogen atmosphere, 2.0 equivalent of 3-chloropropyltrimethoxysilane was added. The reaction ask was sealed under nitrogen and reuxed at 85 C for 24 h. Toluene and unreacted reagents were removed under reduced pressure at 90 C to afford a brownish-yellow liquid. The procedure for the co-condensation of the poly-SQ salt and TEOS was similar to that of BHBPD and TEOS (OMO-1). An amount of poly-SQ salt was added to a pre-hydrolyzed TEOS solution with a nominal amine loading of 2.0 mmol per gram of silica. The template was extracted as described for OMO-1 to afford a light grey solid denoted OMO-5. Ion-exchange of OMO-5 with ammonium iodide (as described for OMO-1) produced a yellow solid, OMO-6 (Scheme 5).
Method C: synthesis of PMOs via condensation of BHBPD and poly-SQ salt. To an aqueous solution of P123, prepared as described in method B, 2 mL of BHBPD or 2.8 mL of poly-SQ salt was added slowly under stirring. The resultant mixture was further stirred at 40 C for 20 h, and the mixture was transferred into an autoclave and heated at 100 C under static condition for 2 days. The solid products were collected by ltration, washed with water, and dried at ambient conditions. Surfactant extraction, ion-exchange, dequaternization and quaternization were carried out on the resultant solid as described for OMO-1. All PMOs are represented in Scheme 6. PMO-1 and PMO-3 were obtained as off-white solids; PMO-5 was light-grey; and PMO-2, PMO-4 and PMO-6 were yellow powder materials.

Characterization of materials
Textural properties and organic content. The specic surface area, pore volume and mean pore diameter of all materials were determined by nitrogen adsorption-desorption measurements at À196 C on a Micromeritics 2020 instrument, as reported earlier. 10 Samples were degassed under nitrogen for 4 h at 120 C prior to analysis. Specic surface areas were calculated using the BET method at relative pressures of 0.06 to 0.2. The total pore volume (V p ) was recorded at p/p 0 ¼ 0.99 to exclude possible interparticle pores. The average pore size (D p ) was determined using the density functional theory.
The organic or nitrogen content (N-content) of all organosilicas was determined using thermogravimetric analysis (TGA) on a TA Q500 instrument. Sample were heated to 120 C at 10 C min À1 under owing nitrogen and held for 30 min, to ensure the removal of physisorbed species (e.g. water and solvents). Then the temperature was raised to 800 C under owing nitrogen, then air at 800 C for 10 min. The organic content was calculated based on the weight loss beyond 200 C.
IR spectroscopy. To identity the incorporated organic species, ATR-IR measurements were recorded on a Nicolet 6700 spectrometer equipped with a liquid N 2 -cooled MTC-A (mercury-tellurium-cadmium) detector and a Fisher Scientic horizontal ATR unit with a ZnSe crystal. For this, a few mg of powder sample was homogeneously spread onto the surface of the ATR crystal, and the IR spectra were recorded from 600 to 4000 cm À1 at a resolution of 2 cm À1 using 128 scans. For each scanning, the spectrum was collected by subtracting air (background) spectrum from the original spectrum. All ATR-FTIR experiments were run in triplicate to ensure the reproducibility of the results, and average wavenumbers were reported herein. 13 C and 29 Si solid state and solution NMR. Solid-state NMR spectra were collected at room temperature using an AVANCE III 200 MHz spectrometer equipped with a MAS probe. The spectra were recorded using cross polarization with high power decoupling, with samples spinning at 4.5 kHz. The number of scans was in the range of 6000-8000. The contact time and recycling delays were set at 10 ms and 2 s for 13 C NMR respectively. For 29 Si NMR, the contact time and recycling delays were set at 2 ms and 1.5 s. For solution NMR measurements, 13 C and 1 H NMR spectra were obtained using a Bruker AV 400 MHz spectrometer. Chemical shis are reported in ppm relative to deuterated chloroform.
Where separation was needed to isolate the cyclic carbonate prior to NMR analysis, silica gel ash chromatography was used with a 3 : 1 ethylacetate : ether mixture as solvent.
Cycloaddition reaction. Catalytic activity was evaluated using the model reaction in Scheme 7.
Scheme 5 Synthesis of poly-SQ salt and OMO-5 and OMO-6 via cocondensation followed by ion-exchange. The reaction conditions were chosen such that the reaction was signicantly away from total conversion; enabling a meaningful comparison of catalytic activity and stability. In a typical procedure, a 100 mL high pressure autoclave equipped with a magnetic stirrer was charged with 2 mL (17 mmol) of styrene epoxide (SO) and 2 mol% of organocatalyst (DMO, OMO or PMO). The reactor was directly pressurized with CO 2 to 0.5 MPa and heated to 50 C under stirring for 5 h. At the end of the reaction, the reactor was cooled with an ice bath to #15 C and CO 2 was released slowly. Then, 50 mL of dodecane (internal standard for GC analysis) was added to the reaction mixture, which was then suspended in diethyl ether and ltered off to recover the catalyst. Aer removal of all volatiles using a rotavap, a solvent-free reaction mixture was obtained. The spent catalyst was further stirred in acetone for 30 min, ltered off and dried in a vacuum oven at 70 C for 2 h. TG analysis and nitrogen adsorption-desorption measurements were carried out on all spent catalysts to determine variations in N-content and textural properties.
Analysis of reaction mixture: about 0.2 mL of the solventfree reaction mixture was injected into a Varian CP-3800 GC equipped with Agilent J&W VF-1ms capillary column (15 m length, 0.25 mm inner diameter and 0.25 mm lm thickness) and a ame ionization detector (FID); with helium as carrier gas. The temperature program used for analysis was as follows: initial oven temperature 80 C (1.5 min); ramp at 30 C min À1 and held at 250 C (3 min); injector port temperature: 200 C; detector temperature: 250 C. For solid reaction mixtures, 0.1 g of the mixture was dissolved in 0.2 mL of toluene, and 0.2 mL of this solution was injected into the GC. 13 C and 1 H NMR spectra of liquid samples were obtained using a Bruker AV 400 MHz spectrometer with tetramethylsilane as the internal standard. Chemical shis are reported in ppm relative to deuterated chloroform.

Characterization of mesoporous organosilicas
As shown in Fig. 1 and S1, † the nitrogen adsorption-desorption isotherms for the organosilica materials and SBA-15 were of Type IV with H1 hysteresis loop, characteristic of solids with mesopores. The corresponding textural properties and organic content are presented in Table 1. The TGA proles of these materials may be found in Fig. 2  condensation. On the other hand, template removal by solvent extraction minimizes framework shrinkage, leading to higher surface area, pore volume and number of silanols compared to its calcined counterpart. 40,41 The capillary condensation of all DMOs and OMOs were observed to be less sharp compared to their corresponding supports and SBA-15*, respectively; an indication of decreased pore size. This provided further evidence that immobilization of the organic moieties took place on the internal surface of the supports. Compared to DMOs and OMOs, all PMOs had signicantly lower surface areas, while their pore volumes were also lower than the corresponding OMOs (Table 1, entries 10 vs. 13, 11 vs. 14 and 12 vs. 15).
Moreover, PMOs showed a broad pore size distribution (Fig. S2 †). This nding may be attributed to the higher organic content of PMOs, which adversely affects their textural properties.
Similar organic contents were measured for all amine functionalized supports and DMOs (Fig. 2a). The calculated organic content of Q-10@amine, P-10@amine, SiL@amine and SBA-15@amine were 1.59, 1.70, 1.64 and 1.80 mmol g À1 respectivelyaccounting for a graing efficiency between 80 and 90%. This nding illustrates that despite the smaller surface areas of P-10 and Q-10 silicas compared to SiL and SBA-15, their larger pore volume and pore diameter enabled them to accommodate as much BHBPDs as SiL and SBA-15.
The N-content per gram of silica support for both S@amine solids and DMOs being the same, the yield of the quaternization reactions was evaluated to be between 92 and 97%. This nding conrms the structure of DMOs, i.e. as immobilized symmetrical diammonium salts. The weight lost below 200 C of SBA-15 was <2% compared to ca. 10% for SBA-15* (Fig. 2b), suggesting the presence of some P123 surfactant (vide infra). It is known that template removal via solvent extraction does not completely remove the surfactant. 42 Hence, the N-content of all OMOs and PMOs were obtained taking into account the presence of about 8 wt% of surfactant. Relatively higher organic content, up to 80 wt%, was obtained for PMOs compared to DMOs and OMOs (Fig. 2c).
The 13 C NMR of amine@Q-10 and DMO-Q10 showed peaks around 10, 17, 51, 56, 58 and 62 ppm, which were assigned to the ve carbon nuclei of BHBPD as illustrated in Fig. 3a. Assynthesized SBA-15 exhibited resonances attributed to -CH 2 -CH-(76 and 74 ppm) and -CH 3 (18 ppm) groups of the PPO, and -CH 2 -(71 ppm) of the PEO block of P123. 43 Following template removal by solvent extraction, 13 C NMR of the resultant material, SBA-15*, showed very low intensity signals between 71 and 75 ppm, and the complete disappearance of the signal at 18 ppm. Moreover, signals at 16, 50 and 59 ppm were assigned to trapped methanol and ethanol molecules used during template removal. Hence, the 8 wt% organic content of SBA-15*, as determined by TGA was assigned to the presence of small amount of template. As with DMOs, ve carbon nuclei could be identied in the 13 C NMR of all OMOs (Fig. 3b) and PMOs (Fig. 3c).
The 29 Si CP MAS NMR spectra showed only Q n species for SBA-15*, Q n and T n for DMOs and PMOs, and only T n for PMOs (Fig. 3d). The characteristic resonances in the range of À89 to À118 ppm were ascribed to Q n units, with the peaks at about À97, À102.1 and À110.9 ppm corresponding to Q 2 [Si(HO) 2 (-OSi) 2 ], Q 3 [Si(HO)(OSi) 3 ] and Q 4 [Si(OSi) 4 ] species, respectively (Fig. 3d(i)). All DMOs, OMOs and PMOs showed two intense signals at ca. À58 and À66 ppm, attributed to Si-species covalently bonded to carbon atoms, namely T 2 [C-Si(OH)(OSi) 2 ] and T 3 [C-Si(OSi) 3 ] respectively (Fig. 3d(i)-(iv)). 44 These signals arise from immobilization of BHBPD onto the silica surface and framework of silica for DMOs and OMOs/PMOs, respectively. Furthermore, for some OMOs and PMOs, a weak shoulder at ca. 50 ppm, attributed to T 1 [C-Si(OH) 2 (OSi)] species was also observed. The presence of T 1 and T 2 -signals is an indication that the condensation process was not complete for these materials. Hence, the calculated nitrogen content based on TGA results, which assumes 100% condensation, may be slightly overestimated. All materials showed IR absorption bands ( Fig. 4 and S4 †) at approximately 640, 800 and 1080 cm À1 , which was attributed to Si-O-Si bonds of the silicate network. The broad band centered at ca. 3320 or 3440 cm À1 were assigned to the O-H stretching vibrations of silanol groups, hydroxyl groups of BHBPD and/or adsorbed water. A band at 1650 cm À1 conrmed the presence of adsorbed water, whose physisorption was enhanced by the ionic nature of the -N-I bond. Furthermore, the spectra of all organosilica materials showed -CH 2stretching vibrations of the propyl chain characterized by weak absorbance at ca. 3030 and 2980 cm À1 , -CH 2bending at 1550 cm À1 and C-N stretching at 1450 cm À1 . 45 These ndings are consistent with the presence of BHBPD on the silica supports. The absence of a band at 960 cm À1 suggests that the amount of uncondensed Si-OH groups was minimal; which is in agreement with 29 Si MAS NMR data. It is worth noting that bands at slightly different wavenumbers were observed for PMOs and OMOs/DMOs, suggesting differences in the chemical environment between these two groups of materials.

Catalytic activity
The catalytic activity of iodide-containing organosilicas was evaluated using the cycloaddition of CO 2 and styrene oxide (SO) to styrene carbonate (SC) under neat conditions. Because of its low activity, SO provides a more meaningful comparison than a highly active epoxide such as 1,2-epoxybutane (1,2-EB). The effect of temperature, time and CO 2 pressure on the selectivity and yield is presented in Table 2, entries 1-11. At 100 C, 4 h and 1 MPa CO 2 , yields and selectivity $ 97% were obtained ( Table 2, entry 1) with DMO-SiL as catalysts. In an earlier study, we reported similar ndings in the presence of N,N,N-tributyl-Npropylammonium iodide-functionalized mesoporous silicas. 10 These results are in agreement with other reports where immobilized ammonium 46 and phosphonium 17 salts were used. To obtain yields signicantly below 100%, the following conditions were; 50 C, 5 h and 0.5 MPa CO 2 . As seen in Table 2, entries 2-11, conversion was found to be <50% for all catalysts. Amongst the DMOs, the highest yield was obtained with DMO-P10 and DMO-SiL (Table 2, entries 3 and 4). This may be attributed to smaller particle size of the silicas supports P-10 (40 mm) and SiL (40-60 mm) compared to Q-10 (75-150 mm) and SBA-15 (<150 mm). 47 With respect to OMOs, a 45% yield of SC was obtained with OMO-2 (Table 2, entry 6). However, OMO-4 consisting of four CO 2 -activating hydroxyl groups, exhibited lower activity (Table 2, entry 7), suggesting that increasing the number of hydroxyl groups from two to four was not benecial to OMOs. A similar nding by Cheng et al., 48 was attributed to an enhancement in hydrogen bond formation between the halide and additional OH groups, leading to decreased nucleophilicity or epoxide ring-opening ability of the iodide ion. Alternatively, the observed decrease in activity could originate from increase diffusion-constraints with additional ethylhydroxyl groups, such that any benet in CO 2 activation due to extra hydroxyl groups was suppressed by slow reaction kinetics. This nding suggests that similar to homogeneous systems, 49 there is an optimal number of hydroxyl groups required for efficient activation of epoxides. A combination of diffusion-constrain and a high ratio of hydroxyl groups to organics may be at the origin of the low yield obtained with OMO-6 (  DMOs. This nding was attributed to the larger surface area and moderate pore volume of the latter, leading to enhanced accessibility to active sites and improved mass transport. PMOs were found to be the least active compared to DMOs and OMOs, with PMO-2 the most active of all PMOs -24% yield ( Table 2, entry 9). The inferior catalytic performance of PMOs was attributed to their low surface area, pore volume and disordered pore structure. Accordingly, OMO-2, the most active catalyst, was used to optimize the reaction conditions and material reusability.

Optimization of reaction time
Using SO as substrate, an increase in yield (44.9 to 76.9%) was observed when the reaction time was raised from 5 to 15 h ( Table 2, entry 6 vs. 12). Compared to 1,2-EB for instance, SO is signicantly more difficult to activate, and expected to take signicantly longer reaction times under mild conditions to attain high yields. However, prolonged reaction times may lead to the formation of side products, ultimately reducing product yield and selectivity. Hence, optimization of reaction time was carried out with 1,2-EB. As seen in Fig. 5, the yield of 1,2-butylene carbonate (1,2-BC) was found to peak aer 15 h (97.2%), which is 25% more  than was obtained for SC under similar conditions ( Table 2,  entry 12). Moreover, no noticeable change in selectivity ($97.5%) was observed aer 20 h. Nonetheless, a slight decrease in yield was recorded aer 20 h, suggesting the onset of the formation of side products beyond 15 h. Hence, optimum reaction conditions for the synthesis of 1,2-BC using OMO-2 were; 2 mol% catalyst, 0.5 MPa CO 2 , 50 C and 15 h.

Reusability studies
One of the most attractive attributes of heterogeneous catalysts is their reusability. In this regard, OMO-2 was subjected to recycling experiments with 1,2-EB as substrate. As shown in Table 3, no signicant changes in product yield and selectivity were observed over ve cycles. Although a 7% decrease in organic content was observed compared to the fresh catalyst ( Table 3, entry 1 vs. 2), the N-content of all spent catalyst was relatively constant, indicating the stable nature of OMO-2. This nding is consistent with the textural properties of fresh and spent catalysts being comparable.
Similar high yields were obtained in the synthesis of 1,2-BC and 4-(chloromethyl)-1,3-dioxolan-2-one under mild reaction conditions (Table 4, entries 3 and 5). As with the synthesis of PC, similar high yields of 1,2-BC and 4-CDO were obtained at 4 h, but at higher temperature and pressure (Table 4, entries 4 and 6). It is worth noting the high yield and selectively of these cyclic  carbonates under the current mild reactions condition using a metal-free catalyst. Compared to other organocatalaysts immobilized on inert support, namely; ammonium salts on nanocrystalline zeolites, 50 imidazole-based ionic liquid (IL) on graphene oxide 51 and covalent organic frameworks, 52 triphosphonate cavitand on SBA-15, 53 phosphonium-based polymeric IL, 54 phosphonium salt on polystyrene, 55 OMO-2 was signicantly more active. As noted earlier, under the current mild reaction conditions, the yield of SC was limited to 77% (Table 4, entry 7).
However, 97% yield was obtained by doubling the reaction temperature and pressure, while limiting the reaction to 4 h ( Table 4, entry 8). Furthermore, reacting CHO and GPS under the current mild reaction conditions led to very low yields (Table 4, entries 9 and 11). Nonetheless, signicantly higher yields were obtained at higher temperature and pressure (Table 4, entries 10 and 12). This is not surprising as these three epoxides are much more difficult to activate (due to steric hindrance) than the rst three in Table 4.