In all cases of oleic acid methyl ester (OAME) cracking, between 20 to 30% of pyrogram total area was due to unidentified and products with similarity identification between 60 and 75%. This value is higher than that of unidentified products estimated when cracking myristic acid and palmitic acid as model molecules [26, 28]. This fact implies a more complex mechanistic scheme during cracking of unsaturated fatty molecules [25, 29] when compared to the scheme retained during cracking of saturated fatty acids which is attributed essentially to decarboxylation and decarbonylation reactions, in agreement with the literature data [30–33]. In the present case, a complete deoxygenation of the oleic acid methyl ester (OAME) was not obtained. But, in their majority, the remaining oxygenated compounds were susceptible to further decomposition when the contact time between catalyst and reactant and/or intermediate products was increased. Therefore, it was expected that the general trends in the product distribution observed in this work would be maintained with a higher degree of decomposition of the feed.
3.1.Cracking of pure oleic acid methyl ester in absence of catalyst
Oleic acid methyl ester C18:1 (OAME) pyrogram after cracking at 650°C in absence of catalyst is presented in Fig. 1. The pyrogram can be divided into two main domains. The left part region, below retention time close to 33 min, where peaks rather well separated of limited size appear; and the right part, above retention time of 33 min, where peaks, poorly separated, of important size are observed. The larger peak in this second region (retention time close to 50 min) is due to untransformed OAME. Product classes reported in Table 1 show that thermal cracking of OAME at 650°C produced only 4.3% of deoxygenated compounds, essentially associated to peaks below retention time of 33 min. In the right part of the pyrogram, many oxygenated and unidentified products are present. Along all the pyrogram, a large amount of carboxylic acid methyl esters was observed (75.1%), i.e. methyl esters of C2 to C18 saturated and unsaturated carboxylic acids. This result implies that under these conditions, cracking of OAME and methyl esters of lower chain length can occur not only close to the carboxylate position, but also within the hydrocarbon chain, probably in the beta position of the C = C bond. The presence of all chain lengths between the C2 and C19 methyl esters suggests also an important migration of the C = C double bond position before cracking occurs. Although the number of hydrocarbons (HC) observed from pure OAME cracking is low, the HC production is much higher after cracking in presence of catalysts studied.
3.2.Cracking of oleic acid methyl ester adsorbed on zeolites
The cracking of OAME impregnated on zeolite catalysts at 650°C presented a higher degree of reaction than the cracking of pure OAME. Figure 1 shows together with that of pure OAME, the pyrograms of OAME/HY, OAME/NaY, OAME/HZSM-5 and OAME/NaZSM-5. The cracking of OAME in contact with zeolites presented a higher degree of transformation than the cracking of pure OAME. The quantitative results are shown in Table 1. The area % of hydrocarbons varies from 24 to almost 40% in presence of catalysts while this value is only 4.3% for the cracking of OAME in absence of catalysts. Simultaneously, the number of oxygenated compounds decreased in the catalytic cracking. Therefore, both acidic and sodium zeolite catalysts helped the deoxygenation of OAME, in the same way they favored deoxygenated products as reported in the case of fatty acids cracking [24]. The number of produced hydrocarbons over HY and NaY was practically the same. However, the number of hydrocarbons over HZSM-5 was much higher than over NaZSM-5, suggesting that the acid sites in HZSM-5 are more effective in producing the deoxygenation reactions. In all cases, the number of unidentified compounds is higher when cracking is performed in presence of catalyst, suggesting more complex reaction schemes.
Table 1
Product family distribution after cracking of oleic acid methyl ester (OAME) at 650°C, pure and in presence of NaZSM-5, HZSM-5, NaY and HY catalysts.
Catalyst | Unidentified | Oxygenated | Hydrocarbons |
Pure OAME | 11.2 | 84.5 | 4.3 |
OAME/NaZSM-5 | 18.1 | 57.4 | 24.5 |
OAME/HZSM-5 | 19.1 | 41.9 | 39.0 |
OAME/NaY | 36.1 | 26.4 | 37.5 |
OAME/HY | 15.7 | 45.1 | 39.2 |
Table 2 summarizes a first association of hydrocarbon families obtained after cracking of OAME. In the case of pure OAME, cracking produced mainly linear alkenes, the majority of them being 1-alkenes. These alkenes are an important class of products in the cracking of saturated fatty acids, as the main decomposition mechanism is due to simple decarbonylation [28, 33]. The second class of important products has been observed to be polyunsaturated molecules, including dienes, trienes and alkynes. In the presence of all catalysts, the cracking of OAME produced a high number of aromatics. Protonated catalysts HZSM-5 and HY were more effective for aromatization than sodium catalysts NaZSM-5 and NaY, respectively. The second most important hydrocarbon family was that of alkenes in the case of NaZSM-5, HZSM-5 and NaY. In the case of HY, the second most important group was that of alkanes.
Table 2
Hydrocarbon family distribution after cracking at 650°C of oleic acid methyl ester (OAME), pure and in the presence of NaZSM-5, HZSM-5, NaY and HY catalysts.
Catalyst | Alkanes | Alkenes | Polyenes | Aromatics |
OAME | 0.6 | 2.3 | 1.3 | 0.1 |
OAME/NaZSM-5 | 0.0 | 10.0 | 0.0 | 14.0 |
OAME/HZSM-5 | 2.1 | 13.1 | 1.6 | 22.2 |
OAME/NaY | 7.8 | 11. 8 | 0.6 | 16.8 |
OAME/HY | 12.7 | 3.6 | 2.1 | 20.0 |
Table 3 details the distribution of saturated and unsaturated products. Whereas more linear alkanes appear in the case of OAME cracking over HZSM-5, the presence of ramified alkanes is remarkable when OAME cracking was performed over both NaY and HY zeolites. In the case of alkenes, whereas more linear alkenes appear with OAME/NaZSM-5 and OAME/HZSM-5, ramified alkenes show the highest content in the cracking with OAME/NaY, OAME/HZSM-5 and OAME/HY. Therefore, in a systematic way, Y type zeolite presents a more important alkylation property than ZSM-5 type zeolite, probably due to acid sites of stronger strength or in greater numbers [34]. In fact, although acidity measurements were difficult to obtain under the present experimental conditions, an estimation based on Si/Al ratio [35] suggests that the number of acid sites is probably two times more important with HY (with Si/Al = 6) than with HZSM-5 (with Si/Al = 15). The higher the Al content, the higher the acidity. Another explanation is the occurrence of shape selectivity. The pore size of the Y zeolite (7.4 Ǻ) is larger than that of ZSM-5 (5.1–5.6 Ǻ) zeolite, enabling alkylation reactions which require more space inside the porous structure [36–37]. It is also important to recall that 1-alkenes represent more than 50% of all alkenes in the case of thermal cracking of pure OAME and cracking in presence of NaZSM-5. On the contrary, in the other cases, small amounts of 1-alkenes are identified, when present. All three HZSM-5, Na/Y and HY catalysts favor the formation of cyclic and internal linear alkenes, eventually alkylated. These three zeolites show therefore a high capacity of isomerization, probably due to higher number of acidic sites existing on their surface. In the case of Y-based catalysts, the larger pore size compared to that of HZSM-5 may also favor molecules with higher kinetic diameter, i.e. ramified ones, as commented above.
Table 3
Distribution of alkane and alkene hydrocarbon families into linear, cyclic and ramified classes, after cracking at 650°C of oleic acid methyl ester (OAME), pure and in presence of NaZSM-5, HZSM-5, NaY and HY catalysts.
Catalyst | Linear Alkanes | Cyclo-Alkanes | Ramified Alkanes | Linear Alkenes | Cyclo-Alkenes | Ramified Alkenes |
OAME | 0.2 | 0.1 | 0.3 | 1.8 | 0.5 | 0.4 |
OAME/NaZSM-5 | 0.0 | 0.0 | 0.0 | 7.4 | 1.5 | 2.6 |
OAME/HZSM-5 | 1.8 | 0.3 | 0.3 | 5.7 | 2.3 | 7.0 |
OAME/NaY | 0.5 | 2.3 | 7.2 | 2.1 | 1.6 | 9.6 |
OAME/HY | 0.4 | 1.3 | 12.2 | 0.3 | 0.9 | 3.3 |
Table 4 presents the distribution of aromatic compounds. The formation of aromatics was not significant in the case of thermal cracking of OAME. On the contrary, as said above, the formation of aromatic molecules is very impressive when OAME is cracked over zeolite samples. HY and HZSM-5 zeolites showed a higher aromatization capacity than the sodium exchanged ones.
Whereas a high number of molecules containing a single benzene ring are present with OAME/NaZSM-5 and OAME/HZSM-5, molecules containing more than one benzene ring are identified in a significant way during the cracking of OAME adsorbed on Y type zeolites. In the case of OAME/HY, polyaromatic compounds with 3 benzene rings were detected, suggesting an important possibility of coking of this catalyst during cracking of OAME. The formation of mono aromatic hydrocarbons may be associated with the dehydrogenation of six carbon numbered cyclic alkanes and alkenes [32, 38]. Polyaromatic hydrocarbons, on the other hand, are formed through the polymerization and dehydrogenation of mono alkyl aromatics [23] and alternatively through an intramolecular radical cyclization mechanism [38]. It is known that polyaromatics are generally considered as potential precursors of coke [39–40]. This behavior, together with the hydrogenation properties shown by Y type zeolite must be linked to strong acidic properties of Y zeolite and to hydrogen transfer phenomena, leading on one hand to rather large number of saturated hydrocarbons, and on the other hand to strongly dehydrogenated polyaromatic molecules.
Table 4
Distribution of monoaromatic and polyaromatic product families after cracking at 650°C of oleic acid methyl ester (OAME), pure and in presence of NaZSM-5, HZSM-5, NaY and HY catalysts.
Catalyst | Monoaromatics | Polyaromatics |
OAME | 0.0 | 0.1 |
OAME/NaZSM-5 | 13.6 | 0.4 |
OAME/HZSM-5 | 17.4 | 4.8 |
OAME/NaY | 9.3 | 7.5 |
OAME/HY | 12.3 | 7.7 |
Figure 2 summarizes the quantities of hydrocarbons associated to the carbon chain numbers: C3-C7 (gaseous and gasoline) range, C8-C12 (kerosene) range and C12+ (diesel) range. Results show three features concerning product distribution: i) the number of products in the C12 + range is rather low, the majority of deoxygenated molecules in this range being aromatics in the case of OAME cracked over catalysts; ii) the C3-C7 range is more important in the case of ZSM-5 zeolites than in the case of Y zeolites. It can be noted that with both Na-ZSM-5 and H-ZSM-5, significant amount of propene is formed. This property has also been observed when cracking petroleum feedstock in the FCC process using ZSM-5 in the catalyst formulation [41]. These results confirm that ZSM-5, during its cracking activity, has a higher capacity to promote the formation of small unsaturated molecules than Y zeolite; iii) with both ZSM-5 and Y zeolites, the presence of sodium induces a decrease in the formation of light products. This result also confirm that protonated zeolites promote the cracking reactions of oleic acid methyl ester towards light compounds.
The cracking of OAME over HZSM-5, NaY and HY led to the production of a large number of molecules in the range C8-C12 (kerosene range), the best catalyst being NaY. As aviation transport needs to strongly decrease its carbon emissions, with a possibility of zero net carbon emission by 2050 [42] the necessity to increase the production of green kerosene is mandatory. Then, the present work indicates that one possible way to increase the production of this fuel fraction can be through the addition of some residue biodiesel to the FCC cracking cuts. Other oxygenated molecules such as bio-oil and more probably fatty acids and triglycerides, due their molecular structure close to that of OAME can probably play an additional role in this increasing market.