First studies dealing with the catalytic cracking of triglycerides molecules date from 1979 over ZSM-5 catalyst (Weisz et al, 1979). In this pioneering work, the authors performed the catalytic cracking of several vegetable oils achieving complete conversions of them in a mixture of paraffinic, olefinic and, above all, aromatic hydrocarbons (ca. 42-78%). After this initial work, a huge amount of work dealing with this topic has been reported in the literature over different acid catalysts: zeolitic molecular sieves (such as HZSM-5, H-Y and H-mordenite) (Bhatia et al., 1998; Idem et al, 1997; Katikaneni et al, 1995b, 1995c, 1996; Leng et al., 1999; Milne et al, 1990; Ooi et al, 2005; Prasad and Bakhshi, 1985; Prasad et al., 1986a, 1986b; Twaiq et al, 1999), Al-containing mesostructured materials (Al-MCM-41 and Al-SBA-15) (Bhatia et al., 2009; Demirbas, 2009; Idem et al., 1997; Ooi and Bhatia, 2007; Ooi et al., 2004, 2005; Twaiq et al., 2003a, 2003b) and amorphous materials (alumino-silicates, pillared clays and alumina) (Boocock et al., 1992; Katikaneni et al., 1995b, 1995c; Idem et al., 1997; Vonghia et al., 1995).

Products usually obtained by means of the catalytic cracking of vegetable oils and animal fats are depicted in Fig. 15.3. They are usually grouped in an ‘organic liquid product’ (gasoline, kerosene and diesel fractions), gaseous products (hydrocarbons C1-C5, CO, CO2), water and coke. The oxygen initially present in the feedstock is removed as water (which is easily isolated), CO and CO2. Therefore, there is not a remarkable presence of oxygenated hydrocarbons in the final organic cracking products.

The catalyst properties (e. g. crystalline nature, shape selective effect), the reaction conditions (temperature, pressure, space velocity, presence of steam, type of reactor. . .) and the nature of feedstocks, dramatically influence the conversion and yield towards the different reaction products. Generally, the presence of zeolites increases the yields towards the OLP fraction, whereas amorphous catalysts predominantly produce high amount of gases (Idem et al., 1997; Katikaneni et al, 1995c). Co-feeding steam during the reaction process helps to increase both the olefinic compounds formation and the durability of the catalyst. This fact takes place because the presence of steam diminishes the coke formation and thus the catalyst deactivation (Katikaneni et al., 1995b). The use of a fluidized bed instead a fixed bed reduces generally the selectivity towards the OLP fraction

due to the shorter contact time that diminishes the possibility of forming liquid hydrocarbons from the olefins C2-C5 oligomerization reactions (Katikaneni et al., 1997). In all the different studies, an OLP with a high concentration of aromatics has been obtained (over 50%) as well as a high triglyceride conversion (> 80%). Furthermore, the almost null presence of oxygenated hydrocarbons in the final cracking products is confirmed by the different performed studies (Katikaneni et al., 1995c, 1997; Leng et al., 1999; Twaiq et al., 2003a). The different authors have shown that although the initial decomposition of triglyceride molecule is mainly a thermal process, in the subsequent secondary cracking reactions (hydrogen transfer, isomerization, oligomerization, b-scission, aromatization), the acid catalyst has a crucial role (Twaiq et al., 2003a). Table 15.1 summarizes the most relevant work dealing with the catalytic cracking of triglyceride molecules indicating type of feedstock, reaction conditions and catalyst. As observed, most of the studies have been performed in fixed bed reactors, in a range of temperatures generally between 300 and 500°C and with liquid space velocities ranging from 2 to 4/hour.

The general reaction pathway of the acid-catalyzed cracking of a triglyceride molecule is depicted in Fig. 15.4. Once the triglyceride molecule has been primarily decomposed to heavy oxygenated hydrocarbons such as fatty acids, ketones, aldehydes and esters, their reactions to reach other products start by means of the breaking of the C-O and C-C bonds by b-scission reactions. The breaking of the bonds C-O and C-C follows two competitive routes: (1)

Reference Bhatia et al. (2009)

Boocock et al. (1992) Chew et al. (2009) Dandik et al. (1998) Haag et al. (1980)

Idem et al. (1997)

Katikaneni et al. (1995b)

Katikaneni et al. (1997) Leng et al. (1999)

Table 15.1 Continued Reference Feedstock Type of reactor Reaction conditions Catalyst Ooi et al. (2004) Fatty acids from palm oil Fixed bed Atmospheric pressure, T = 400-450°C, WVSH: 2.5-4.5/h HZSM-5, MCM-41 Ooi et al. (2005) Palm oil Fixed bed Atmospheric pressure, T = 450°C, WVSH: 2.5/h HZSM-5, MCM-41, SBA-15 Prasad et al. (1986a) Rapeseed oil Fixed bed Atmospheric pressure, T = 340-400°C, WVSH: 2-4/h HZSM-5 Prasad et al. (1986b) Rapeseed oil Fixed bed Atmospheric pressure, T = 340-400°C, WVSH: 2-4/h HZSM-5 Twaiq et al. (1999) Palm oil Fixed bed Atmospheric pressure, T = 350-400°C, WVSH: 1-4/h HZSM-5, beta, USY Twaiq et al. (2003b) Palm oil Fixed bed Atmospheric pressure, T = 450°C, WVSH: 2.5/h AI-MCM-41 Weisz et al. (1979) Corn oil, peanut oil Fixed bed Atmospheric pressure, T = 400-500°C, QFeed = 2 ml/h HZSM-5

CO

____ >. Thermal cracking

——- >■ Catalytic cracking

Oligomerization

Light olefins C2—C5 —————- ► Olefins C2—C1 g

15.4 General reaction mechanism for the catalytic cracking of triglyceride molecules over acid catalysts.

decarboxylation (CO2) and decarbonylation (CO) reactions followed by C-C bond cleavage of the resulting hydrocarbon radicals or (2) C-C bond cleavage within the hydrocarbon section of the oxygenated hydrocarbon molecule followed by decarboxylation and decarbonylation of the resulting short-chain molecule (Idem et al, 1996). The occurrence of these different reaction routes depends on the double bonds in the initial oxygenated hydrocarbon. Whereas C-C bond breaking in the a and b positions is favoured in the presence of unsaturated hydrocarbon molecules, decarboxylation and decarbonylation reactions take place before C-C bond cleavage for saturated oxygenated hydrocarbons because, in a saturated hydrocarbon chain, the less endothermic bonding is the one associated with the b position of the carbonyl group (Osmont et al., 2007). Different subsequent cracking reactions finally yield CO, CO2 and water, as the main oxygenated compounds, and a mixture of hydrocarbons produced by different reactions such as b-scission, hydrogen transfer, isomerization, cyclization
or aromatization, some of them possible because there is an acid catalyst present in the reaction system. Furthermore, coke is formed by means of polymerization reactions (Maher and Bressler, 2007).