Effect of surfactant-induced porosity and acidity modification of MOF-808 catalyst on the glycerol acetylation reaction

Abstract

A promising approach for the catalytic acetylation of glycerol to produce high value-added chemicals such as diacetins (DAGs) and triacetin (TAG) is the use of acidic Metal-Organic Frameworks (MOFs). Particularly, MOF-808 has shown potential as catalyst for this reaction due to its tunable porosity and the ability to incorporate acid sites. Herein, we focused on modifying the porosity and acidity of MOF-808 to improve its catalytic performance in the acetylation of glycerol. Porosity changes were induced during the synthesis of MOF-808 by promoting the formation of larger pores using three different surfactants: cetyltrimethylammonium bromide (CTAB), sodium dodecyl sulfate (SDS), and pluronic P123 (PLU), in combination with propionic acid as a modulator. Results showed that the proposed modifications add either mesoporosity; pores with widths larger than 2.0 nm, or macroporosity, i.e., pores with widths larger than 50.0 nm, when it is used surfactants, to the purely microporous MOF-808. On the other hand, the acidity of porosity enhanced MOF-808 was modified by a post-synthetic acid treatment with sulfuric acid. This treatment led to the incorporation of well dispersed sulfate groups into the MOF structure which did not experienced a strong loss of porosity. Catalytic experiments demonstrated that the sulfated MOF samples exhibited higher glycerol conversion if compared to the other samples, while the non-sulfated MOF modified with pluronic P123 showed the highest selectivity towards triacetin. This suggests that the improved accessibility provided by mesopores facilitates the sequential acetylation steps, particularly favoring the formation of triacetin as the final product.