Hydrodeoxygenation (HDO) reaction is a route with much to offer in the conversion and upgrading of bio-oils into fuels; the latter can potentially replace fossil fuels. The catalyst’s design and the feedstock play a critical role in the process metrics (activity, selectivity). Among the different classes of catalysts for the HDO reaction, the transition metal phosphides (TMP), e.g., binary (Ni2P, CoP, WP, MoP) and ternary Fe-Co-P, Fe-Ru-P, are chosen to be discussed in the present review article due to their chameleon type of structural and electronic features giving them superiority compared to the pure metals, apart from their cost advantage. Their active catalytic sites for the HDO reaction are discussed, while particular aspects of their structural, morphological, electronic, and bonding features are presented along with the corresponding characterization technique/tool. The HDO reaction is critically discussed for representative compounds on the TMP surfaces; model compounds from the lignin-derivatives, cellulose derivatives, and fatty acids, such as phenols and furans, are presented, and their reaction mechanisms are explained in terms of TMPs structure, stoichiometry, and reaction conditions. The deactivation of the TMP’s catalysts under HDO conditions is discussed. Insights of the HDO reaction from computational aspects over the TMPs are also presented. Future challenges and directions are proposed to understand the TMP-probe molecule interaction under HDO process conditions and advance the process to a mature level.
Achieving ultradeep desulfurization of transportation fuels dictates the removal of the last fragments of sulfur present in bulky refractory molecules such as dibenzothiophene (DBT). Improving the hydrodesulfurization (HDS) performance of the next-generation Ni 2 P catalyst is crucial; however, it is still unclear how a DBT direct desulfurization (DDS) reaction proceeds on different surface terminations of this material. This work aims at elucidating the influence of Ni 3 P-, Ni 3 P 2 -, and partially sulfided Ni 3 P 2 (Ni 3 PS)-terminated surfaces of the Ni 2 P(001) crystal on the DDS reaction of DBT using density functional theory (DFT) computations. The scission of the first C−S bond in DBT was found to be kinetically hindered compared to the second C−S bond cleavage over the three surfaces owing to the highly stable structure of adsorbed DBT compared to the C 12 H 9 SH intermediate. Notably, the overall DBT desulfurization process is thermodynamically favorable only on the Ni 3 PS surface, due to the presence of the active phosphosulfide (Ni−P−S) phase. Microkinetic modeling was conducted to probe the reaction orders, apparent activation energy, and rate-controlling steps as a function of temperature. Notably, the ring-opening step of DBT, via the first C−S bond cleavage, is the rate-controlling step on Ni 3 P and Ni 3 PS surfaces, which is rationalized by the high energy barrier of this step. The activation energy of the reaction over the surfaces followed the order Ni 3 P < Ni 3 PS < Ni 3 P 2 . Compared to the bare Ni 3 P 2 surface, the lower activation energy on the Ni 3 PS surface is explained by the destabilization effect of the reaction intermediates on the latter. Thus, the partially sulfided Ni 3 P 2 provides a better model for the DBT desulfurization process than the fresh Ni 3 P 2 surface.
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