A Study on the Stability of Carbon Nanoforms–Polyimidazolium Network Hybrids in the Conversion of CO2 into Cyclic Carbonates: Increase in Catalytic Activity after Reuse

Abstract

Three different carbon nanoforms (CNFs), single-walled and multi-walled carbon nanotubes (SWCNTs, MWCNTs) and carbon nanohorns (CNHs), have been used as supports for the direct polymerization of variable amounts of a bis-vinylimidazolium salt. Transmission electron microscopy confirmed that all CNFs act as templates on the growth of the polymeric network, which perfectly covers the nanocarbons forming a cylindrical (SWCNTs, MWCNTs) or spherical (CNHs) coating. The stability of these hybrid materials was investigated in the conversion of CO2 into cyclic carbonate under high temperature and CO2 pressure. Compared with the homopolymerized monomer, nanotube-based materials display an improved catalytic activity. Beside the low catalytic loading (0.05–0.09 mol%) and the absence of Lewis acid co-catalysts, all the materials showed high TON values (up to 1154 for epichlorohydrin with SW-1:2). Interestingly, despite the loss of part of the polymeric coating for crumbling or peeling, the activity increases upon recycling of the materials, and this behaviour was ascribed to their change in morphology, which led to materials with higher surface areas and with more accessible catalytic sites. Transmission electron microscopy analysis, along with different experiments, have been carried out in order to elucidate these findings.

Imidazolium-Containing Hybrid Organic–Inorganic Materials for the Conversion of CO2: Unveiling the Key Role of the Ionic Template

“…The polymers suffer from their low surface areas and as a consequence of the partial inaccessibility from the reactant to the imidazolium sites inside the structure. Nevertheless, an article using carbon nanotubes as a template for polyimidazolium bromide was recently published . The higher specific surface area (196 m 2 /g) allowed exposure for a higher number of active sites, thus increasing the TOF compared to other polyimidazolium-based materials tested under the same conditions.…”

“… a Entries 2; 4–7: TOF and productivity calculated considering 3 h reaction. ,,,, b Entry 3: Productivity calculated considering 77% yield, 450 mg material, 24 mL SO (211 mmol), and 6 h reaction c Entry 8: TOF and productivity calculated considering the reported TON = 209, 75% conversion, 300 mg material, 209.7 mmol SO, and 3 h reaction d Entry 9: TOF and productivity calculated considering the reported TON = 206, 93% conversion, 96% selectivity, 30 mg material, 4 mmol SO, and 5 h reaction …”

“… c Entry 8: TOF and productivity calculated considering the reported TON = 209, 75% conversion, 300 mg material, 209.7 mmol SO, and 3 h reaction …”

“…The postfunctionalization by an imidazolium salt, for instance, can be performed through the functionalization of the surface by a linker (e.g., chloropropyl or bromopropyl moieties) followed by its nucleophilic substitution by an imidazole derivative to generate the corresponding imidazolium salt. − The generation of various imidazolium-containing materials by cosynthesis was reported too. Some solid catalysts were synthesized starting from a homogeneous imidazolium salt which is polymerized, , either via the vinyl extremities − or through poly nucleophilic substitution. − The modification of terephthalic acid by imidazolium was also a strategy recently developed to obtain an imidazolium-functionalized metal–organic framework (MOF). − MOFs present the advantage over polymers of possessing a high specific surface, due to their elevated degree of organization. On the other hand, MOFs can display a lack of stability, when facing humidity or higher temperature.…”

“…Nevertheless, an article using carbon nanotubes as a template for polyimidazolium bromide was recently published. 11 The higher specific surface area (196 m 2 / g) allowed exposure for a higher number of active sites, thus increasing the TOF compared to other polyimidazolium-based materials tested under the same conditions. If a decreased working temperature is considered (entries 9−11), a more straightforward comparison can be done with the test performed at 125 °C (entry 12).…”