Asphaltene based Oxidized Carbons: New Catalysts for Synthetic Transformations

Abstract

Since the terminology of catalysis was introduced in the 1800s, catalysts have been considered the essential materials to facilitate chemical reactions. Predominantly, metal-based catalysis has driven various synthetic reactions until now. Although metal-based catalysts are commonplace and have been well-studied in the context of a wide variety of transformations, increasing costs, high toxicity, and limited resources have warranted the development of alternatives. Among potential candidates, carbocatalysts, materials mostly comprising carbon elements, have attracted attention to promoting a broad range of transformations. For example, graphene oxide (GO) has been broadly utilized as a catalyst for various organic transformations. However, the high oxygen content and large particle size prevented further utilization under harsh synthetic conditions (e.g., microwave irradiation or light) where unexpected side reactions occurred. In this dissertation, we introduced asphaltene oxide (AO) as a new class of carbocatalyst for various types of organic synthesis and overcame the limitations mentioned above. Asphaltene, as a precursor material, often thrown away from petroleum refining processes, was composed of polycyclic aromatic hydrocarbons (PAHs). Initially, we hypothesized that AO's finite surface area compared to GO might be used in applications inaccessible to other carbocatalyst, such as microwave-assisted reactions. Initially, AO was synthesized using a modified Hummers' method, a standard protocol for preparing GO from graphite. In the following research, it was found that AO successfully promoted various synthetic applications, including etherifications, condensations, C-C cross-couplings, and Fischer indole syntheses from the utilization of microwave reactors. In an effort to expand usage in organic synthesis, we focused on contemporary drawbacks that carbocatalysts encountered. Carbocatalysts are commonly known as heterogeneous catalysts. Even if carbocatalyst has been well-studied and substituted metal-based catalysts in various organic synthesis, these heterogeneous features have inherent limitations such as catalysis deactivation. For this reason, we hypothesized that increasing the oxygen content of AO would enhance its solubility in a manner that improves catalytic activity when compared to heterogeneous counterparts. Indeed, refluxing asphaltene in 60% nitric acid afforded a derivative that exhibited good solubility in both organic solvents and aqueous media; as such, hereafter, the material was termed 'soluble AO' (sAO). These solubility features made sAO accessible to a relatively in-depth analysis of the structure and molecular weight compared to other carbocatalysts. Furthermore, sAO effectively functioned as a catalyst for esterifications, Fischer indole condensations, multicomponent reactions, and cationic polymerizations with remarkable catalytic. Furthermore, it was possible to be utilized in both microwave reactions and aqueous media synthesis.