# Conversion of Stearic Acid into Bio-Gasoline over Pd/ZSM-5 Catalysts with Enhanced Accessibility
## 3 Results and Discussion
### Reactions of Stearic Acid
#### 3.2.3 Effect of the Si/Al Atomic Ratio in Hierarchical ZSM-5  
The Si/Al atomic ratio of the zeolitic ZSM-5 materials is a key variable, since it determines the acid concentration, which is meant to play a decisive role in the stearic acid conversion and the obtained product distribution. To determine the effect of this parameter, three Pd/h-ZSM-5 samples, with Si/Al atomic ratios of 33, 51, and 122, were assessed as catalysts of this reaction. The attained conversions and selectivity by groups are shown in Figure 7, while the selectivity by atom carbon number is illustrated in Figure 8. It can be appreciated that decreasing the Si/Al ratio provoked an improvement of the stearic acid conversion, but this was not as significant as it might be expected. Thus, a three-fold increase in the aluminum content did not even double the attained conversion. Moreover, it was remarkable the high conversion of stearic acid obtained over Pd/h-ZSM-5 (122) (47%), despite its low acidity. Additionally, if the textural properties of these three catalysts are compared, the catalyst with the highest accessibility (higher mesopore surface area/ lowest micropore surface area) is also the sample with the lowest Si/Al atomic ratio (Pd/h-ZSM-5 (33)). Therefore, considering its enhanced properties, it should provide a much higher conversion than that observed (68%), if only accessibility and aluminum content were responsible for the attained conversions. This fact bears out the relevant role also played by the Pd species, contributing significantly to the decarboxylation activity. Moreover, as all the samples contained similar Pd amounts, the availability of the Pd centers  
Figure 6: Stearic acid conversion and selectivity by groups over the Pd/n-ZSM-5 (32) and Pd/h-ZSM-5 (33) samples (T = 300 \({}^{\circ}\)C).  
may become the limiting factor for the conversion of stearic acid when using catalysts with high acid sites concentration.  
Regarding the product distribution, the three catalysts gave rise to selectivities towards gasoline-range hydrocarbons above 70%, with a slight increase being observed as the acidity of the support was reduced. On the other hand, the selectivity to n-paraffins was lower than 30%, regardless of the used Si/Al atomic ratio. Regarding the selectivity by carbon atom number, it was interesting to observe how the main hydrocarbon fraction obtained at the lowest Si/Al atomic ratios (33 and 51) was C5, while for the highest Si/Al atomic ratio, it was C8, and to a lesser extent, C11. This fact suggests also that the OCA pathway is promoted by the presence of Pd, this effect becoming more evident when using zeolite catalysts with low Al contents, i.e., high Pd/Al ratios. In contrast, at low Pd/Al ratios, the end-chain cracking reactions take place in a higher extent than the OCA pathway, explaining that lighter hydrocarbons are predominant in the product distribution  
Figure 7: Stearic acid conversion and selectivity by groups over Pd/h-ZSM-5 catalysts with different Si/Al ratios (T = 300 \({}^{\circ}\)C).