Lactic acid–magnesium oxide nanocrystal interactions: how nanoparticle size and shape affect chemistry and template oligomerization

Abstract

L-Lactic acid was allowed to react with small amounts of commercial MgO, Nanoactive s , and Nanoactive Magnesium Oxide Plus s particles, each of which differs in surface area, shape, and reactivity. The reactions were carried out by refluxing the nanoparticles in a solvent suspension of methanol or propanol. Upon addition of the lactic acid monomer, at reflux temperature, two reactions competed with each other: (1) acid-base to yield magnesium lactate salt, and (2) oligomerization to yield a nanocomposite prepolymer. The products were characterized for thermal, chemical, and morphological properties. Additionally, titrations were performed to determine how much MgO was consumed by the acid, and how this changed with nanoparticle size and shape. Polymerization appears to initiate on the surface of the magnesium oxide particles, the results of which are physically unique composites of lactic acid and magnesium oxide, and final properties depend on MgO nanoparticle characteristics. One of the most interesting results was the finding that larger, less reactive MgO favored the acid-base neutralization reaction, while smaller, more reactive MgO particles favored the MgO induced oligomerization pathway.

Controlled Synthesis of Different Morphologies of MgO and Their Use as Solid Base Catalysts

“…Magnesium oxides (MgO's), a typical wide band gap (7.2 eV) semiconductor, represent an important class of functional metal oxides with a broad range of properties. They also find tremendous application in catalysis, refractory industries, electronics, cosmetics, and toxic wastewater treatment remediation. − To extend the application of MgO's as catalysts, it is essential to tailor their surface chemistry as well as morphologies, as the catalytic process occurs only over the surface. MgO being a weak base, its pH in aqueous solution is highly dependent on the morphology and surface area, the sludge formed during the water treatment process is easier to precipitate and filter than that formed by other alkalis .…”

Theoretical Investigation of Surface Reactions of Lactic Acid on MgO Clusters

“…MgO acts as base as well as a surface to enhance polymerization reactions. The surface reaction is found to be more effective at high surface area particles at the nanoscale level . Although bulk MgO is relatively inert, nanocrystalline MgO with surface areas on the order of 250–500 m 2 /g exhibit enhanced reactivity .…”

“…12 High surface area MgO has recently been used as a catalyst for the polymerization of lactic acid, and the structural properties of the resulting polymer differ from those of conventionally prepared PLA. 11 Because of the number of edge, corner, and defect sites available in nanocrystalline MgO, multiple reactions are possible in the initial reaction stages. An understanding of LA−MgO surface reactions could elucidate the species that are involved in the polymerization.…”

“…Although bulk MgO is relatively inert, nanocrystalline MgO with surface areas on the order of 250–500 m 2 /g exhibit enhanced reactivity . High surface area MgO has recently been used as a catalyst for the polymerization of lactic acid, and the structural properties of the resulting polymer differ from those of conventionally prepared PLA . Because of the number of edge, corner, and defect sites available in nanocrystalline MgO, multiple reactions are possible in the initial reaction stages.…”

“…The surface reaction is found to be more effective at high surface area particles at the nanoscale level. 11 Although bulk MgO is relatively inert, nanocrystalline MgO with surface areas on the order of 250−500 m 2 /g exhibit enhanced reactivity. 12 High surface area MgO has recently been used as a catalyst for the polymerization of lactic acid, and the structural properties of the resulting polymer differ from those of conventionally prepared PLA.…”