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Abstract
Recently, catalysts based on transition metal phosphides (TMPs) have attracted increasing interest for their use in hydrodeoxygenation (HDO) processes destined to synthesize biofuels (green or renewable diesel) from waste vegetable oils and fats (known as hydrotreated vegetable oils (HVO)), or from bio-oils. This fossil-free diesel product is produced completely from renewable raw materials with exceptional quality. These efficient HDO catalysts present electronic properties similar to noble metals, are cost-efficient, and are more stable and resistant to the presence of water than other classical catalytic formulations used for hydrotreatment reactions based on transition metal sulfides, but they do not require the continuous supply of a sulfide source. TMPs develop a bifunctional character (metallic and acidic) and present tunable catalytic properties related to the metal type, phosphorous-metal ratio, support nature, texture properties, and so on. Here, the recent progress in TMP-based catalysts for HDO of waste oils is reviewed. First, the use of TMPs in catalysis is addressed; then, the general aspects of green diesel (from bio-oils or from waste vegetable oils and fats) production by HDO of nonedible oil compounds are presented; and, finally, we attempt to describe the main advances in the development of catalysts based on TMPs for HDO, with an emphasis on the influence of the nature of active phases and effects of phosphorous, promoters, and preparation methods on reactivity.
Highlights
The production of chemicals and fuels using waste biomass as the raw material has attracted scientific and industrial interest because these processes produce zero carbon dioxide emissions, which helps to solve global warming
transition metal phosphides (TMPs) have been employed as alternative electrocatalysts to noble metals (Pt) in the electrochemical hydrogen evolution reaction (HER), with interesting results obtained in alkaline and acidic media
When the catalyst is based on a TMP, it would be the active phase for hydrogenation, this type of catalyst possesses a bifunctional nature, as TMPs present acid properties originating from P−OH and/or unreduced Ni2+ [101]
Summary
The production of chemicals and fuels using waste biomass as the raw material has attracted scientific and industrial interest because these processes produce zero carbon dioxide emissions, which helps to solve global warming. Catalysts 2019, 9, 293 are straight chain paraffinic hydrocarbons without aromatics, oxygen, and sulfur, and with high cetane numbers [4] These properties endow HVO fuels with an extraordinary quality that surpasses biodiesel (fatty acid methyl esters, FAME) and crude oil diesel [3]. Among the different thermochemical treatments to convert biomass wastes in a resource such a liquid fuel, pyrolysis, gasification, and liquefaction, the first process—fast pyrolysis, in which the waste biomass is rapidly heated in the absence of oxygen producing bio-oil or pyrolysis oil—is the one that maximizes its production [5,6] This product cannot be directly used because of its high content of oxygenates, in the form of phenols, furans, carboxylic acids, ethers, and aromatics alcohols. MoP and WP [16] and the iron group have been reported to be more active than metal sulfides
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