Conversion of Stearic Acid into Bio-Gasoline over Pd/ZSM-5 Catalysts with Enhanced Accessibility

Abstract

Palladium supported on nanocrystalline ZSM-5 (n-ZSM-5, Si/Al = 32) and hierarchical ZSM-5 (h-ZSM-5) with different acidity (Si/Al = 33, 51, 122) were tested in the liquid-phase conversion of stearic acid under nitrogen atmosphere (6 bar). The incorporation of Pd into ZSM-5 zeolite increased significantly the share of gasoline in the reaction products due to the promotion by this metal of both decarboxylation and hydrogen transfer reactions. Likewise, the Pd nanoparticles dispersed over the zeolitic support favored the conversion of light olefins formed by end-chain cracking reactions into gasoline-range hydrocarbons according to an oligomerization/cyclization/aromatization pathway. Additionally, Pd/h-ZSM-5 gave rise to higher conversion and selectivity towards gasoline than Pd/n-ZSM-5, due mainly to the enhanced accessibility and improved Pd dispersion achieved when using the hierarchical zeolite. The decrease in the Si/Al atomic ratio in Pd/h-ZSM-5 samples resulted in a rise in the stearic acid conversion, although it was lower than expected. This finding denotes that, for supports with high acid sites concentration, the Pd availability became the limiting factor as the metal was loaded in similar amounts in all catalysts. Finally, the increase of the reaction temperature with the Pd/h-ZSM-5 (122) catalyst augmented both stearic acid conversion and gasoline selectivity, since it enhanced the conversion of the light olefins, formed as primary cracking products, into liquid hydrocarbons. Therefore, it can be concluded that Pd supported on hierarchical ZSM-5 zeolite is a convenient catalyst for obtaining bio-gasoline from oleaginous feedstock.

Selective Decarboxylation of Fatty Acids Catalyzed by Pd-Supported Hierarchical ZSM-5 Zeolite

“…It is also noteworthy that, in an experiment carried out at 250 °C under the same experimental conditions and catalyst [Pd/h-ZSM-5 (122)] but adding 6 bar of nitrogen instead of hydrogen, the conversion decreased abruptly from 61 to 6%. 72 This result bears out the role of hydrogen in activating palladium sites for DC. On the basis of these results, a Energy & Fuels temperature of 250 °C was selected for further studies to limit the extension of cracking reactions and favor the stearic acid DC while still keeping a significant catalytic activity (conversion over 60%).…”

“…Taking into account that the three catalysts show a similar Pd content (∼1 wt %), it is concluded that the transformation of the fatty acid also depends heavily upon the acidity of the MFI support. Additionally, the same experiment carried out using Pd supported over a commercial ZSM-5 (Pd/n-ZSM-5), which shows a Si/Al atomic ratio of 32 and whose physicochemical properties may be found elsewhere, 72 pointed out a distinctly lower conversion (33%) than the equivalent Pd/h-ZSM-5 (33) (71%), while its selectivity toward C 13 −C 18 hydrocarbons was 94%, slightly higher than the 82% value attained over Pd/h-ZSM-5 (33). These results prove the advantage of using a hierarchical ZSM-5 support, because its better accessibility facilitates not only cracking but DC reactions as well, leading to a rather enhanced catalytic activity.…”

“…On the basis of the results obtained in this work, the reaction scheme shown in Figure 10 is proposed, which includes the three major pathways for the conversion of fatty acids into liquid hydrocarbons: (a) Under an inert atmosphere (nitrogen) and temperatures above 275 °C, cracking reactions that occur over the accessible strong acid sites of hierarchical ZSM-5 prevail, leading mostly to gasoline and to a lesser extent gaseous hydrocarbons. 72 (b) DC reactions are predominant when operating at low hydrogen pressures (6 bar) and mild temperatures (225−275 °C). Under these conditions, the combination of Pd and acid sites in the catalyst leads to high conversion of the fatty acid via DC, producing C n−1 long-chain hydrocarbons.…”

“…The three supports were impregnated with the same amount of Pd (1 wt %). Details about the properties of these materials can be found elsewhere, − as summarized in Table .…”

Effect of zeolite morphology on charge separated states: ZSM-5-type nanocrystals, nanosheets and nanosponges

“…32,33,34 In that context of emergence of new zeolitetype materials such as hierarchical zeolites it appears very important to identify the influence of the morphology on the formation and evolution of CSS. 25,35,36,37,38 MFI-type zeolite is particularly interesting due to its three-dimensional channel system with 10-membered ring (MR) openings (0.55-0.56 nm). Thanks to new synthesis methods, [27][28][29][30][39][40][41][42][43] it is now possible to control the morphology of zeolites to eventually make their properties "tunable" not only by preparing materials in the form of microcrystals but also with more complex architectures.…”