Catalytic etherification of glycerol by tert-butyl alcohol to produce oxygenated additives for diesel fuel

doi:10.1016/j.apcata.2009.07.037

Applied Catalysis A: General

Volume 367, Issues 1–2, 1 October 2009, Pages 77-83

https://doi.org/10.1016/j.apcata.2009.07.037 Get rights and content

Abstract

The heterogeneous catalytic etherification of glycerol with tert-butyl alcohol was investigated in presence of lab-made silica supported acid catalysts. As reference, two commercial acid ion-exchange resins were also used. Experiments were carried out in batch mode at T_R ranging from 303 to 363 K. An increase in reaction temperature favors the formation of di-substituted ethers. The etherification reaction proceeds according to a consecutive path and the surface reaction between adsorbed glycerol and protonated tert-butanol (tertiary carbocation) can be considered as the rate determining step. Steric hindrance phenomena and water hinder the formation of tri-substituted ether (TBGE). As expected, water removal was necessary to allow the higher ethers formation. The specific activity (turnover frequency, TOF) of A-15 catalyst is significantly higher than that of the other studied acid systems, due to the wide pore diameter that allows an easier accessibility of the reagent molecules.

Graphical abstract

The heterogeneous catalytic etherification of glycerol with tert-butyl alcohol in the presence of commercial and lab-made solid acid catalysts was investigated. Experiments were carried out in batch mode at T_R ranging from 303 to 363 K. The influence of acid capacity, reaction temperature, pressure, and the presence of water on catalyst performance were investigated. Kinetic evaluations and reaction mechanism were also assessed.

Introduction

Recently, exhaust gases emitted by internal combustion engines were considered primarily responsible for environmental pollution and human diseases [1], [2], [3], [4], [5].

Biodiesel is currently used as a valuable fuel for diesel engines. In spite of a slight power loss, exhausts contain less particulate matter. Biodiesel is a mixture of methyl esters of fatty acids (FAMEs) obtained by the transesterification reaction of vegetable oils with methanol in presence of a basic catalyst [1], [2]. Such a catalytic process converts raw triglycerides into FAMEs, but produces glycerol as side product.

If biodiesel is produced on a large scale, adequate technologies capable of converting glycerol into added value chemicals are necessary [3], [4]. In particular, great attention has been already devoted to the conversion of glycerol into oxygenated additives for liquid fuels [3], [5], [6], [7]. In this context, an industrially relevant route for the conversion of glycerol into oxygenated chemicals involves the etherification to tert-butyl ethers [5], [7], [8], [9], [10].

It is well known that the addition of oxygenated additives to diesel fuels could represent a promising way enhancing the combustion efficiency in internal combustion engines with a significant reduction of pollutant emissions. Among several oxygenated additives proposed to blend with diesel, the ethers of glycerol could hold a prominent role [7], [8], [9], [10]. In particular, tert-butyl ethers of glycerol with a high content of di-ethers are considered promising as oxygenated additives for diesel fuels (diesel, biodiesel and their mixtures). However, mono-tert-butyl ethers of glycerol (MBGEs) have a low solubility in diesel fuel; therefore, in order to avoid an additional separation step, the etherification of glycerol should address the formation of di- and tri-ethers [5], [7], [8], [9], [11].

The etherification of glycerol can be carried out using heterogeneous acid catalysts like strong acid ion-exchange resins [12], [13], [14], [15], [16], [17], [18]; however, the utilization of large-pore zeolites has also been widely investigated [19], [20], [21], [22]. Usually, low surface areas and lack of thermal stability are the major drawbacks of sulfonic resins. The incorporation of organosulfonic groups over mesostructured silicas have generated effective solid acid catalysts with enhanced catalytic properties as compared with conventional homogeneous and heterogeneous acid catalysts [23]. Moreover, these type of silica materials functionalized with organosulfonic acid groups have been used previously for the conversion of biorenewable molecules [24], [25], [26], [27], showing better catalytic performances than that of the commercial sulfonated resins. Currently, these highly surface materials characterized by interconnected mesopores and high accessibility of acid sites represent the best systems for the etherification reactions [28].

The synthesis of tert-butyl ethers (GTBEs) from isobutene and glycerol on ion-exchange resins has been already investigated extensively [8], [9], [10]. Isobutene (IB) is produced by catalytic cracking and steam cracking fractions of petroleum refining and by isobutane dehydrogenation [5], [7]. What appears more attractive is to produce GTBEs by the glycerol etherification reaction in a solid–liquid catalytic process, by using tert-butyl alcohol (TBA). In fact, the use of TBA, as both reactant and solvent, instead of gaseous isobutylene, allows to overcome the technological problems arising from the need to use solvents able to dissolve glycerol (i.e., dioxane, dimethyl sulfoxide) and typical drawbacks of a complex three-phase system (mass transfer phenomena) [9], [29].

This study focused on the etherification of glycerol with tert-butyl alcohol over different solid acid systems. Attention was given primarily to investigate the main limiting factors for large scale process development.

Section snippets

Catalysts and chemicals

Two solid acid supported catalysts were prepared by the incipient wetness method using a silica carrier (S.A._BET, 250 m² g⁻¹) and two solutions containing 17 wt.% of Nafion^® ionomer (N-17) and 17 wt.% of tungstophosphoric heteropoly acid (HPW-17), respectively. An aliquot of the HPW-17 sample was mixed with a solution containing cesium to exchange a fraction of H⁺ protons with Cs⁺ (Cs-HPW). In addition, two commercial acid ion-exchange resins, Nafion^® on amorphous silica (SAC-13) and Amberlyst^® 15

Results and discussion

Physico-chemical properties of catalysts are summarized in Table 2.

Catalysts are characterized by different surface area ranging from 53 to 207 m² g⁻¹ and porosity comprised between 0.07 and 0.80 cm³ g⁻¹. In terms of SA and porosity, the data obtained with SAC-13 sample are similar to that provided by the supplier. Furthermore, all the catalytic systems show an average pore diameter (APD) increasing with the porosity, apart from A-15 that is characterized by a wide pore texture (300 Å), although it

Conclusions

The main findings of this study can be summarized as follows:

•
the etherification of glycerol with tert-butyl alcohol effectively takes place on solid acids catalysts, hence providing a promising way to transform glycerol into value added products to be used as oxygenated additives blended with diesel fuels;
•
the accessibility of acid sites plays a fundamental role in promoting catalyst activity and the systems with large pores are more indicate to perform reaction at high rate;
•
a low

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Catalytic etherification of glycerol by tert-butyl alcohol to produce oxygenated additives for diesel fuel

Abstract

Graphical abstract

Introduction

Section snippets

Catalysts and chemicals

Results and discussion

Conclusions

Fuel

Renew. Sust. Energy Rev.

Fuel Process. Technol.

Fuel

Appl. Catal. A: Gen.

Appl. Catal. A: Gen.

Appl. Catal. A: Gen.

J. Catal.

Appl. Catal. A: Gen.

J. Mol. Catal. A: Chem.

React. Funct. Polym.

React. Funct. Polym.

J. Mol. Catal. A: Chem.

Zeolites

Appl. Catal. A: Gen.

J. Catal.

J. Catal.

Appl. Catal. A: Gen.