Highly effective decarboxylation of the carboxylic acids in fast pyrolysis oil of rice husk towards ketones using CaCO₃ as a recyclable agent

Highlights

• Decarboxylation of the carboxylic acids in bio-oil investigated for the first time using CaCO₃ as a recyclable agent.

• After decarboxylation, all of the carboxylic acids and aldehydes can be converted with conversion of 100%.

• The quality of the upgraded oil was improved, its acidity decreased by 98.82% and HHV increased by 50.11%.

Abstract

The carboxylic acids in fast pyrolysis oil (bio-oil) are one of the detrimental properties that impede its straightforward application in internal combustion engines. In this paper, a novel approach is proposed to convert the carboxylic acids in bio-oil to ketones efficiently. That is, the acids in a bio-oil derived from pyrolysis of rice husk were firstly transformed to their calcium salts by neutralization with CaCO₃ and then subjected to decarboxylation at high temperature to form ketones and regenerate CaCO₃ simultaneously. Due to various carboxylic acids contained in the bio-oil, both symmetric and asymmetric ketones were formed. Although phenols, aldehydes and saccharides had almost unnoticeable effect on the decarboxylation of carboxylic acids, they, particularly saccharides, tended to undertake carbonization reaction, leading to the char formation associated with CaCO₃ regenerated. After decarboxylation, the acidity of the upgraded bio-oil decreased dramatically from 167.62 to 1.98 mg of KOH/g, whereas its HHV increased from 18.38 to 27.59 MJ/kg. Although the upgraded bio-oil yield was only 46.10%, the energy efficiency could be reached to 69.20%. Decarboxylation of the water-soluble fraction, which was obtained by water extraction of the bio-oil, was also investigated. With the aid of water extraction, most of the acids and saccharides could be enriched in the water-soluble fraction, while the phenols were concentrated into the oil-soluble fraction. After decarboxylation of the water-soluble fraction, the HHV and acidity of the upgraded bio-oil were improved to be 30.05 MJ/kg and 1.03 mg of KOH/g, respectively. The energy efficiency could be enhanced to 77.59%.

Introduction

Fast pyrolysis oil, also called bio-oil, is a liquid product obtained by fast pyrolysis of biomass. Due to the simplicity in pyrolysis equipment and operation conditions as well as flexibility and adaptability with respect to biomass resources, fast pyrolysis of biomass to bio-oil is thought to be a very promising route for exploitation of biomass resource [1]. Up to now, this technology, however, has still been confronted with many obstacles because of the detrimental properties of bio-oil, such as its high water content and oxygen content which leads to low heat value, high acidity, variable viscosity, and thermal and chemical instability. The high acidity is mainly due to the high content of carboxylic acids in bio-oil and would cause serious corrosion issues during storage and application [2]. Normally, the carboxylic acids account for 20–30 wt% of the total mass of bio-oil, depending on the pyrolysis process and biomass resources, with pH value of about 2–3 [3], [4]. Therefore, how to efficiently convert the carboxylic acids in bio-oil is of important significance either for the purpose of its straightforward application as transportation fuels in internal combustion engines or obtaining value-added chemicals.

The researchers and scientists around the world have attempted many methodologies decrease the carboxylic acids and ameliorate the bio-oil quality in the past several decades. Hydrogenation [5] is mainly expected to reduce the unsaturated compounds and oxygen content in bio-oil and thus enhance the stability and heat value. In most cases, unfortunately, this needs noble metal catalysts and high hydrogen pressure atmosphere. Furthermore, esterification [6], [7], [8], [9] is more preferential than others (e.g. catalytic reforming, hydrogenation, etc.) because it is not only capable of converting the acids into esters with alcohols but also manipulating at mild temperature so that the side reactions can be suppressed to the most extent. However, the high water content in bio-oil usually makes the esterification reaction hard to react completely because it is a reversible reaction. Moreover, bio-oil is such an unstable mixture that many side reactions are likely to happen simultaneously, especially at high temperatures, resulting in undesirable products, tar, char and coke. Therefore, by simple esterification it is difficult to achieve high quality fuels. The acidity in the upgraded oil via esterification can only be reduced to tens of mg of KOH/g. To enhance the acid conversion, coupling techniques that combined esterification with extraction [10] or hydrogenation [7], [8] simultaneously were also attempted.

It is well known that carboxylic acids can be decarboxylated into ketones under suitable conditions [11]. Early in 1858, Friedel [12] pioneered decarboxylation of calcium acetate to acetone associated with the formation of calcium carbonate under high temperature (Scheme 2), and this approach had been employed in the commercial manufacture of acetone until World War I. Noyce [13] compared the decarboxylation of calcium acetate, sodium acetate and their mixture aiming at higher yield of acetone. In an improved distillation procedure of calcium acetate, which involved removing the acetone vapor immediately from the high temperature zone by nitrogen or carbon dioxide at a temperature of 450–490 °C, more than 99% yield of acetone was obtained [14]. Subsequently, the decarboxylation of other metal salts of acetate, such as strontium, barium [15], silver and copper [16] as well as other carboxylates such as calcium formate and strontium oxalate [15] were studied. A number of transition metal oxides were also attempted to catalyze the decarboxylation of carboxylic acids, including Cr₂O₃ [17], [18], ZrO₂ [19], [20], [21], MnO₂ [21], etc.

Decarboxylation of carboxylic acids to ketones in the presence of CaCO₃ is an environmentally friendly process because there is neither byproduct formed nor additional reaction media necessary. Although CaCO₃ takes part in the reaction, it can be viewed as a recyclable agent since it can be regenerated after completing the whole process. Recently, Shanks et al. [22] demonstrated that the organic acids in biomass pyrolysis vapors can be removed by chemical adsorption over CaCO₃. The spent CaCO₃ could be regenerated by heat treatment of the postreaction adsorbent and the adsorbed acids were simultaneously converted into ketones. To our best knowledge, this was the only research paper for upgrading a real bio-oil via decarboxylation up to now. Therefore, one of our objectives in this study is to assess whether the carboxylic acids in bio-oil can be converted effectively into ketones via decarboxylation for the simple reason that ketones are not only high quality fuels but also versatile chemicals. The other objective is to investigate the effect of the reactive compounds in bio-oil on the decarboxylation reaction and their changes during the ketonization of the carboxylic acids. In addition, the quality of the upgraded bio-oils was also evaluated.