Luiz P. Ramos*, Claudiney S. Cordeiro, Maria Aparecida F. Cesar-Oliveira,

Fernando Wypych, Shirley Nakagaki

Research Center in Applied Chemistry, Department of Chemistry, Federal University of Parana, Curitiba, Parana, Brazil

Corresponding author email: luiz. ramos@ufpr. br

OUTLINE

Introduction 255

Heteropolyacids 257

Zeolites 258

Clay Minerals 260

Clay Minerals Improving Acidity 262

Acid-Activated Clay Minerals in Biodiesel Production 263

Case 1 263

Case 2 263

Case 3 264

Layered Materials 265

Layered Double Hydroxides 265

Layered Hydroxide Salts 265

Layered Carboxylates 266

Layered Materials as Heterogeneous Catalysts in (Trans)Esterification Reactions 266

Polymeric Catalysts 269

Concluding Remarks 272

References 272

INTRODUCTION

It is well known that most of the products derived from the chemical industry involve a catalyst in at least one step of synthesis (Figueiredo and Ribeiro, 1987). However, traditional processes for chemical conversion have numerous inconveniences such as the generation of undesirable by-products and environmental pollu­tion. For this reason, civil groups as well as govern­mental agencies are pressing the industrial sector to overcome these problems by developing alternative processes in which waste generation is minimized or even eliminated. This concept is also part of the atom
economy theory proposed by Trost (1991), in which the majority of the atoms present in chemical reagents must be incorporated into useful products.

Many traditional processes based on homogeneous catalysis have been reviewed in order to minimize waste generation. In addition, many researchers have shown that heterogeneous catalysts are excellent alternatives to generate lower amounts of waste streams and also to improve the quality of coproducts, which may contribute with additional revenue for the overall pro­duction process.

The biodiesel industry is another important sector that is following the same strategic pathway (Cordeiro

Bioenergy Research: Advances and Applications

http://dx. doi. org/10.1016/B978-0-444-59561-4.00016-4

et al., 2012; Di Serio et al., 2008). Biodiesel is a biodegrad­able fuel derived from renewable sources that can be obtained by different routes such transesterification and esterification. The traditional transesterification process of oils and fats is based on the use of a homoge­neous system, with methanol as the transesterification agent and a base catalyst, usually an alkoxide or a hydroxide (NaOH or KOH) that generates the corre­sponding alkoxide in situ (Van Gerpen and Knothe,

2009) . Then, the synthetic mono alkyl esters can be used as biodiesel after suitable purification. The main problem of these processes is related to the required pu­rification steps of the mono alkyl esters as well as glyc­erin, which must be recovered in good condition due to their high commercial value.

Ideally, the biodiesel fuel must be free of residues formed in the chemical process like free and bonded glycerin, soaps and water, which are normally used in washing stages for purification. The presence of glycerin in the resulting biofuel is problematic because this pol­yol may undergo dehydration during combustion, pro­ducing a toxic unsaturated aldehyde named acrolein that is not only a dangerous atmospheric pollutant but also a reactive chemical that can be easily involved in condensation reactions, producing an accumulation of carbon deposits that may block filters and compromise the engine performance (Mittelbach and Tritthart,

1988) . Soaps and free fatty acids (FFAs) cause degrada­tion of engine components and free water can interfere with the biodiesel acid number and induce hydrolysis and biological contamination under nonadequate stor­age conditions (Ramos et al., 2003).

The traditional fatty acid esterification processes in homogeneous media uses Bransted acids such as sulfu­ric or hydrochloric as reaction catalysts (Ilgen et al., 2007). However, the extensive use of these catalysts may induce corrosion in reactor components and pipe­lines. Also, the purification of the monoesters produced in this way is also expensive and may require additional washing steps and distillation (Altiparmak et al., 2007).

A traditional sequence for biodiesel production in­volves (1) the recovery of vegetable oil by pressing and/or solvent extraction, (2) the oil pretreatment to adjust its properties for transesterification, (3) the trans­esterification process, (4) the purification by several stages of water washing and (5) the recovery of reaction coproducts, especially glycerin. Each of these steps adds costs to the overall process. Thus, the introduction of operation units that are able to reduce the contamination degree of mono alkyl esters and glycerin may be an important measure to make biodiesel more competitive from an economic and environmental point of view.

For these reasons, many researches had focused their efforts to substitute homogeneous catalysts for heteroge­neous ones. The biodiesel produced in a heterogeneous system is easily purified and glycerin is of high purity, diminishing the investment to achieve a suitable market specification (Ramos et al., 2003). Many classes of chem­ical compounds have been tested as solids for hetero­geneous catalysis to produce biodiesel by either esterification or transesterification processes. Among these, zeolites, ionic exchange resins, inorganic oxides, layered compounds, guanidines and metal complexes have been already used (Cordeiro et al., 2011).

In order to have a truly heterogeneous catalytic pro­cess, the solid catalyst must not leach into the reaction medium and it also needs to be stable under the reaction conditions and reusable. While using solid catalysts for biodiesel preparation, whether by esterification or trans­esterification, the most common catalyst classifications are solid Bronsted acids, Bransted bases or Lewis acids. The same solid catalyst, however, may present more than one of these sites and depending on the acidity or basicity of the solid, the catalytic performance can vary considerably (Sharma et al., 2011).

Recently, in addition to this primary classification, a number of other factors have been considered while developing solid catalysts for esterification or transester­ification reactions. The solids hydrophobicity, for once, is used to unveil the water tolerance. The knowledge of the pore and channel system is used to improve the mass transfer of the catalytic substrate, which for this kind of reaction presents a relatively high viscosity (Wilson and Lee, 2012).

Metal oxides, mainly calcium (CaO), magnesium (MgO) and strontium (SrO) oxides, are among the most extensively studied solid bases for heterogeneous cata­lytic processes (Sharma et al., 2011). Among all alkaline and alkaline earth metal oxides, CaO is the most widely studied. Many are the reasons to explain this fact, including its low cost, low toxicity and low solubility in methanol, which is the most commonly used primary alcohol for the catalytic transesterification of oils and fats (Sharma et al., 2011; Kusdiana and Saka, 2001).

CaO also has a long catalytic life, with high activity in many recycling processes under moderate reaction condi­tions (Lopez et al., 2007). Besides these advantages, CaO can be obtained from various and sometimes unusual natural sources. Naturally occurring minerals such as cal — cite (CaCOs) and several calcium salts (Lopez-Granados et al., 2010) as well as mollusk shells and egg shells (Cho and Seo, 2010) can be used as a source of CaO by calcination.

The impregnation of different alkaline salts in zeolites followed by appropriate thermal treatment can produce basic zeolites and the resulting solids have shown good activity as heterogeneous catalysts for transesterifica­tion. Studies have shown that the basicity of the resulted zeolite can be related to the electropositive nature of the exchanged alkaline cation (Philippou et al., 2000).

The infrequent use of acid catalysis in transesterifica­tion reactions, in comparison to the base catalysis, is in part justified by the lower catalytic activities of the acid compounds. On the other hand, acid catalysts are less sensitive to several contaminants such as water and FFAs, which in many cases can deactivate the base catalyst or drive the catalytic reaction to other products (Van Gerpen and Knothe, 2009).

Notwithstanding this apparent disadvantage of the acid catalyst, solid catalysts with Bransted or Lewis acid properties have been recently investigated in het­erogeneous processes. These solids are promising solid catalysts for the replacement of strong inorganic acids that, although effective in both esterification and trans­esterification homogeneous catalytic systems, have serious adverse factors such as corrosion of the reaction vessels. Furthermore, the use of strong inorganic acids leads to medium — and long-term environmental prob­lems (Helwani et al., 2009a). Thus, the possibility of using solids with acid properties, rather than highly corrosive liquids, therefore replacing homogeneous pro­cesses by heterogeneous ones, may be advantageous since higher catalytic efficiencies may be obtained in more sustainable conversion processes. These are likely to outweigh the higher costs that are often associated with the rational synthesis and use of suitable solid acids.

Furthermore, the research of acid catalysts has also been driven by the possible use of waste cooking oil and other cheap and widely available raw materials for biodiesel production. For such materials, the catalyst must be suitable for acting in the presence of high water and acid content, properties that are often found in low cost feedstocks. In general, solids with high acid proper­ties usually meet these prerequisites (Oliveira et al., 2010; Zhao et al., 2012).

The present work presents a discussion about the most important classes of inorganic solids and poly­meric materials that have been applied in the synthesis of (m)ethyl monoesters through esterification or transes­terification. However, biological systems such as immo­bilized lipases are not treated in this book chapter. Luckily, highly qualified reviews have been published recently on this specific subject (Di Serio et al., 2008; Fjer- baek et al., 2009; Tana et al., 2010).