**An Overview of the Production of Oxygenated Fuel Additives by Glycerol Etherification, Either with Isobutene or** *tert***-Butyl Alcohol, over Heterogeneous Catalysts**

#### **Rafael Estevez \*, Laura Aguado-Deblas, Diego Luna and Felipa M. Bautista \***

Departamento de Química Orgánica, Ed. Marie Curie, Campus de Rabanales, Instituto Universitario de Nanoquímica (IUNAN), Universidad de Córdoba, 14071 Córdoba, Spain; aguadolaura8@gmail.com (L.A.-D.); qo1lumad@uco.es (D.L.)

**\*** Correspondence: q72estor@uco.es (R.E.); qo1baruf@uco.es (F.M.B.); Tel.: +34-957-212-065 (F.M.B.)

Received: 21 May 2019; Accepted: 18 June 2019; Published: 19 June 2019

**Abstract:** Biodiesel production has considerably increased in recent decades, generating a surplus of crude glycerol, which is the main drawback for the economy of the process. To overcome this, many scientists have directed their efforts to transform glycerol, which has grea<sup>t</sup> potential as a platform molecule, into value-added products. A promising option is the preparation of oxygenate additives for fuel, in particular those obtained by the etherification reaction of glycerol with alcohols or olefins, mainly using heterogeneous catalysis. This review collects up-to-date research findings in the etherification of glycerol, either with isobutene (IB) or *tert*-Butyl alcohol (TBA), highlighting the best catalytic performances reported. Furthermore, the experimental sets employed for these reactions have been included in the present manuscript. Likewise, the characteristics of the glycerol ethers–(bio)fuel blends as well as their performances (e.g., quality of emissions, technical advantages or disadvantages, etc.) have been also compiled and discussed.

**Keywords:** glycerol; heterogeneous catalysis; etherification; isobutene; *tert*-Butyl alcohol; oxygenated fuel additives
