Note: Basic and acid strength tests were performed using TPD-CO2 and TPD-NH3, respectively.
4. Recyclability and Stability of Supported Catalysts
4.1. Recyclability
To produce renewable diesel on a commercial scale, the performance of catalysts involved in the reaction is of utmost important. The determination in the recyclability of supported catalysts is often conducted by researchers from the view of economics and sustainability as more cost can be saved and the process is greener if the efficiency of catalysts remains after multiple runs of production. The catalysts utilized for renewable diesel production without support are difficult to recycle, especially for those that are nanosized. Therefore, supported catalysts are more advantageous than unsupported catalysts for their stability and ease of recovery. As the catalysts take up a considerable amount of operating cost, the loss of catalyst has to be minimized and it is favourable to have long lifetime catalysts to reuse and recycle for several runs.
To examine the recyclability of used catalysts, the catalysts can be recovered through filtration and being washed with solvent to remove the traces of contaminants on the catalyst surface or recalcination or re-reduction method as the treatment method. The study by Malins reported the catalyst activity of recovered spent catalyst was gradually decreased after each run, and the treatment by washing spent catalyst with acetone and drying led to lower product yield compared to the spent catalyst treated with recalcination and re-reduction. Through the later method, a total amount of 53.6 wt% hydrocarbon content was obtained in the fifth run of the experiment using the spent NiMo/SiO
2-Al
2O
3 catalyst as recalcination eliminates the organic contaminants (carbon deposits). However, the ratio of
n-C
18/
n-C
17 content was altered after each treatment and experiment cycle due to the change in catalyst structure
[141][51]. For the derivation of renewable diesel from waste cooking oil, the catalyst, CaO-La
2O
3/nanorode AC also exhibited excellent reusability as it was able to yield >63% for 5 cycles of studies with high selectivity of
n-C
15 by washing the spent catalyst with hexane multiple time prior reusing it
[142][52]. The reusability of NiO-Fe
2O
3 with MWCNT as support has a higher yield (70–89%), even used for six consecutive runs, and only showed noticeably decreased deoxygenation activity after the fifth run of the experiment
[139][53]. However, in the study by Khalit et al., the selectivity of n-(C
15+C
17) was significantly reduced without remarkable influence on the hydrocarbon yield after reusing the catalyst for four consecutive runs of the experiment
[1].
The recyclability of the spent catalyst may perform well from the aspect of products yield or selectivity, but the coking activity of the catalyst is subjected to different operating conditions. Coke deposition on the catalyst is studied as it is believed the occurrence leads to catalyst deactivation through the coke deposition on the active sites of the catalyst. The application of La
2O
3 as catalyst supported by HZSM5 has the issue of coking activity as the catalyst active sites were reduced in each run, leading to catalyst deactivation. The coking activity was analyzed through TGA analysis, where soft and hard coke (12 wt% and 7 wt%, respectively) was detected in the La
2O
3/HZSM5
[143][54]. A total coke formation of 16% using Ni/AC was reported and the addition of other catalysts into Ni/AC was suggested to reduce the coke activity of catalysts
[1]. However, the application of a bimetallic catalyst, CaO-La
2O
3, with similar support (AC) resulted in negligible coke formation due to the anti-coking properties of the catalyst
[142][52]. In other words, the occurrence of coke deposition is related to the combination of the type of catalyst and catalyst support used for renewable diesel production.
4.2. Stability
The lifespan of supported catalysts not only depends on their reusability; their stability is one of the important characteristics. The interaction between the catalyst and the support defines the stability of supported catalysts, as higher stability of supported catalysts is achieved with stronger interaction. The catalytic stability of supported catalysts can be determined with longer reaction time under optimum processing conditions of other parameters (e.g., pressure or H
2/feed ratio etc.), or multiple runs (related to recyclability); while the stability of supported catalysts can be analyzed using characterization tests such as transmission electron microscopy (TEM), high resolution-transmission electron microscopy (HR-TEM), Raman analysis, x-ray diffraction (XRD), thermogravimetric analysis (TGA), and more. It was reported that the catalytic stability of Ni with a different type of support, such as ZrO
2, Al
2O
3, and SiO
2, has been studied for an on-stream experiment lasting up to 20 h at the optimum conditions. The observed outcome from the mentioned supported catalysts was a significant reduction of conversion (32–45%) in the first few hours (6–8 h), with less decrement observed in Ni/ZrO
2 compared to others
[118][55]. Another reported incredible performance of Ni-WC/AC was that the change of catalytic activity and carbon activity was insignificant during the 24 h continuous hydrodeoxygenation, implying its potential to substitute noble catalysts like Pt/AC and Pd/AC with comparable catalytic activity
[144][56]. Other than Ni with different supports, the stability of Pt/SAPO11R was reported after 4 re-uses in hydro-isomerization of deoxygenated palm oil
[145][57].
In addition, the study by Papageridis et al. reported characterization tests and carbon analysis for spent catalysts, Ni/Al and Ni/LaAl, where the spent Ni/LaAl had better stability characteristics than the other ones due to less sintering as compared to Ni/Al. One of the positive results was that the coke formation on the catalyst surface was very thin and no clear carbon structures were viewable using TEM
[127][58]. However, another study conducted by Papageridis et al. further investigated the spent catalysts: Ni supported on SiO
2, ZrO
2 and Al
2O
3 using HR-TEM after the first attempt using TEM due to undetectable carbon deposits. The results of HR-TEM showed that a thin graphitic carbon shell covered the catalysts with a thickness ranging from 2.2 to 4.2 nm
[118][55]. From the EDX study, the differences between fresh and spent catalysts were increment of carbon, reduction of oxygen, and sulfur and nickel content for Ni/AC. The decrement of Ni was due to the coke coverage and the obtained higher carbon content was ascribed to coke formation
[1]. Other than the analysis mentioned above, the XRD study was used to examine the quality of catalysts’ structure and chemical and thermal stability. Nur Azreena et al. reported outstanding properties of La
2O
3/HZSM5 as no diffraction peaks were observed due to its good chemical stability and structural qualities. This supported catalyst also exhibited excellent thermal stability because only a minor peak increase was observed after usage
[143][54]. The loss of acid and base sites in the spent catalyst due to coking and metal phase transformation can be further confirmed using XRD. It was reported that the active sites of Ni
0 were dominant in fresh catalysts, which would be easily transformed into inactive Ni
3C after exposure to CO environment
[1].