**1. Introduction**

Hydrogenation of edible oils is a process used since its development in the early 1900s, to convert ats [1]. Principal products obtained by hydrogenation include oleomargarine stock, shortening, salad and cooking oils [2]. Hydrogenation changes the melting and solidification characteristics of the oils and is usually employed to reduce the degree of unsaturation of the naturally occurring triacylglycerides (TAGs). Most of the unsaturated fatty acids in TAGs contain 18 carbon atoms and the unsaturated fatty acids are almost completely in *cis*-configuration. The degree of hydrogenation which leads to hardening of the oil depends on the application, but it is always desired to reduce the level of polyunsaturated fatty acids such as linolenic (C18:3) and linoleic (C18:2) acids due to their high sensitivity to oxidation [3, 4]. This process is commonly used for vegetable oils with significant levels of linoleic acid, such as soybean and sunflower oils [5].

The hydrogenation process is usually carried out in a three-phase slurry reactor in a semibatch mode where hydrogen gas is bubbled with pressure in hot vegetable oil in the presence of a catalyst [6]. In the industrial practice, hydrogenation process is typically carried out using nickel-based catalysts, either in the form of nickels Raney, or supported on different materials [7–10]. Economic price, high activity and easy availability of nickel make it superior over the other metals. High nickel loading is usual in commercial supported catalysts [11]. Normal operating conditions in commercial batch reactors are temperature range: 120–200°C, pressure: 1–5 bar and catalyst loading ranging from 0.01 to 0.2 wt% depending on the properties of the final product [12, 13]. It is desired to maximize the amount of oleic acid (C18:1) in the final product, as well as to eliminate linolenic acid and to reduce the content of linoleic acid to a substantial extent, without going too far towards producing the fully saturated stearic acid (C18:0), since these are not easily digested as foodstuffs [14].

During partial hydrogenation, some of double bonds of unsaturated fatty acids in TAGs can be isomerized into *trans* fatty acids (TFAs). Their intake was convincingly associated with risk of coronary heart disease (CHD) based on epidemiologic and clinical studies and has been shown to be harmful to human health [15–17]. In the last three decades, numerous research works were published in order to explain the effect of TFAs on the cellular metabolism. The consumption of foods high in TFAs has been shown to increase LDL-C and decrease HDL-C levels, which increases the risk of developing CHD [18]. For this reason, the demand for smaller levels of TFAs content in hydrogenated oils has increased and the search for alternatives is important to improve the hydrogenation process. Improvements are required to find new types of hydrogenation catalysts [19–25], new technological solutions [26] and new directions in edible oil modification processes, involving interesterification, fractionation, or blending [27].

The major advances in finding solutions leading to the reduction of TFAs in hydrogenated oils have been achieved in the field of hydrogenation catalysts. In the last two decades, there has been a growing interest for nanosized structures in the range 1–20 nm in different fields of research [28]. This is also the size of metal particles in supported metal catalyst of new generation of nickel [19, 25, 29–31] and precious metal hydrogenation catalysts [20, 32–34]. In general, such nanoparticles of metals such as nickel, palladium, ruthenium, or platinum are used for hydrogenation, since the dissociatively adsorbed hydrogen is easily accessible on these group VIII metals. Supported metal catalysts containing both a group VIII and a group II metal [32, 35, 36], or a case where the catalysts containing both a group VIII and a group IB metal [22, 29, 37–40] although insufficiently studied, can be found in the literature. In these catalysts, the metal of group II or group IB is added as the modifier with the purpose of promoting the *cis*-isomer selectivity. Recently, systematic investigation has been performed over Pd-Mg, Pt-Mg and Ni-Mg supported on silica [36] and Ni-Mg-Ag supported on diatomite catalysts [29]. Regarding to *cis*/*trans*-selectivity, these catalysts produce less *trans* isomers, promoting the *cis*-selectivity. The results have been interpreted by implying electronic effect—modifying the local electron density of the transition metal either directly or through the support [36] or geometrical effect (two separate metal phases)—blocking or masking the effect on the active metal particles without forming the chemical bond [29].

process. The developed kinetic models of the hydrogenation of soybean and sunflower oils over studied catalytic systems were found useful in the prediction of the rate of reac-

**Keywords:** hydrogenation, soybean oil, sunflower oil, *trans* fatty acids, nickel catalysts,

Hydrogenation of edible oils is a process used since its development in the early 1900s, to convert ats [1]. Principal products obtained by hydrogenation include oleomargarine stock, shortening, salad and cooking oils [2]. Hydrogenation changes the melting and solidification characteristics of the oils and is usually employed to reduce the degree of unsaturation of the naturally occurring triacylglycerides (TAGs). Most of the unsaturated fatty acids in TAGs contain 18 carbon atoms and the unsaturated fatty acids are almost completely in *cis*-configuration. The degree of hydrogenation which leads to hardening of the oil depends on the application, but it is always desired to reduce the level of polyunsaturated fatty acids such as linolenic (C18:3) and linoleic (C18:2) acids due to their high sensitivity to oxidation [3, 4]. This process is commonly used for vegetable oils with significant levels of linoleic acid, such as soybean and sunflower oils [5].

The hydrogenation process is usually carried out in a three-phase slurry reactor in a semibatch mode where hydrogen gas is bubbled with pressure in hot vegetable oil in the presence of a catalyst [6]. In the industrial practice, hydrogenation process is typically carried out using nickel-based catalysts, either in the form of nickels Raney, or supported on different materials [7–10]. Economic price, high activity and easy availability of nickel make it superior over the other metals. High nickel loading is usual in commercial supported catalysts [11]. Normal operating conditions in commercial batch reactors are temperature range: 120–200°C, pressure: 1–5 bar and catalyst loading ranging from 0.01 to 0.2 wt% depending on the properties of the final product [12, 13]. It is desired to maximize the amount of oleic acid (C18:1) in the final product, as well as to eliminate linolenic acid and to reduce the content of linoleic acid to a substantial extent, without going too far towards producing the fully saturated stearic acid

During partial hydrogenation, some of double bonds of unsaturated fatty acids in TAGs can be isomerized into *trans* fatty acids (TFAs). Their intake was convincingly associated with risk of coronary heart disease (CHD) based on epidemiologic and clinical studies and has been shown to be harmful to human health [15–17]. In the last three decades, numerous research works were published in order to explain the effect of TFAs on the cellular metabolism. The consumption of foods high in TFAs has been shown to increase LDL-C and decrease HDL-C levels, which increases the risk of developing CHD [18]. For this reason, the demand for smaller levels of TFAs content in hydrogenated oils has increased and the search for alternatives is important to improve the hydrogenation process. Improvements are required to find new types of hydrogenation catalysts [19–25], new technological solutions [26] and new directions in edible

oil modification processes, involving interesterification, fractionation, or blending [27].

tions, product selectivity and catalytic activity.

132 New Advances in Hydrogenation Processes - Fundamentals and Applications

**1. Introduction**

diatomite, silica gel, perlite, activity, selectivity, kinetics

(C18:0), since these are not easily digested as foodstuffs [14].

Although many preparation methods have been developed for synthesis of a well-defined supported metal nanocatalyst, traditional precipitation methods remain widely used, especially for industrial applications, due to their relatively low cost and simplicity. These approaches typically involve precipitating of metal salts with an alkaline precipitant in the presence of suspended supports and then thermal decomposition of salt to yield a dispersion of metal particles on the support. It is often challenging to generate uniform metal dispersions, especially in the case of high metal loading in supported metal catalysts. The desired metal dispersion depends on different factors, including synthesis method, nature of the support, identity of metal precursor salt, concentration of the metal, prereduction treatment and reduction conditions [41–44].

The most common methods used for preparation of supported nickel catalysts include impregnation, co-precipitation and precipitation-deposition (PD). Among these methods, to prepare catalysts with high nickel loading, the most suitable is the PD method. However, in the synthesis of supported catalysts by this method, the interaction of the precipitating precursor with the support such as silica or alumina plays a dominant role. Nickel ions (Ni2+) can react with hydroxyl ions and silica to form a bulk compound, nickel hydrosilicate, which is more stable than the bulk hydroxide or hydroxyl-carbonate and nuclei stabilized by interaction with silica surface [45, 46]. It is undoubtedly proven that the reason for the difficulty of reduction of the active phase on the supported metal catalyst lies in the strong mutual interaction between precipitating nickel precursors and the silica support, with at least partial formation of nickel hydrosilicates [46–49].

In the partial hydrogenation process of edible oils, a catalyst with the high activity and selectivity is required [50–52]. To meet these requirements, the catalyst support should provide sufficient surface area for the metal to disperse and there must be adequate metalsupport interaction [35, 43, 46, 48, 49, 53–57]. The nickel phase on different support surfaces exhibits different extents of metal-support effects. This implies that the surface properties of the catalyst could be changed by the nature of the supported Ni2+ phase, thus acquiring different characteristics and exhibiting different performances toward activity and selectivity, which are known to vary considerably with changes in the preparation conditions [41].

To control the fatty acid composition through temperature, pressure, catalyst and reaction time it is necessary to have a kinetic equation. The kinetic equations based on complex mechanisms as Horiuti-Polanyi [58] obtained from an extensive experimental work, give good results for predicting the reaction products, but in practice, simple mechanisms are employed with approximate results. An alternative is to use empirical modeling approach, which includes mathematical and statistical techniques for chosen empirical model [59–66].

The present work contains a part of our comprehensive research that we conducted on different nickel-based supported hydrogenation catalysts for their use in partial hydrogenation of edible oils. In this work, the characteristics and the structure of high loading nickel-based catalysts supported on diatomite, silica gel and perlite of different properties are related to their activity and selectivity in the hydrogenation of sunflower and soybean oils. Nitrogen physisorption and mercury porosimetry measurements, infrared and X-ray diffraction spectroscopy analyses, temperature programmed reduction studies, quantitative hydrogen chemisorption measurements and X-ray photoelectron spectroscopy were used. The kinetic models for hydrogenation of soybean and sunflower oils were developed to obtain the related kinetic parameters. The partial results derived of each one, treated together, have allowed us to present an overall picture of the nickel-based supported catalysts and some conclusions concerning the relationship in the triad—synthesis, structure and properties.
