Abstract
In recent decades, the use of biomass as alternative resources to produce renewable and sustainable biofuels such as biodiesel has gained attention given the situation of the progressive exhaustion of easily accessible fossil fuels, increasing environmental concerns, and a dramatically growing global population. The conventional transesterification of edible, nonedible, or waste cooking oils to produce biodiesel is always accompanied by the formation of glycerol as the by-product. Undeniably, it is essential to economically use this by-product to produce a range of valuable fuels and chemicals to ensure the sustainability of the transesterification process. Therefore, recently, glycerol has been used as a feedstock for the production of value-added H2 and chemicals. In this review, the recent advances in the catalytic conversion of glycerol to H2 and high-value chemicals are thoroughly discussed. Specifically, the activity, stability, and recyclability of the catalysts used in the steam reforming of glycerol for H2 production are covered. In addition, the behavior and performance of heterogeneous catalysts in terms of the roles of active metal and support toward the formation of acrolein, lactic acid, 1,3-propanediol, and 1,2-propanediol from glycerol are reviewed. Recommendations for future research and main conclusions are provided. Overall, this review offers guidance and directions for the sufficient and economical utilization of glycerol to generate fuels and high value chemicals, which will ultimately benefit industry, environment, and economy.
Highlights
Until now, considerable effort has been dedicated toward developing renewable resources to completely or partially replace with fossil fuels, including wind, solar, geothermal, nuclear, tidal power, and biomass, among which biomass is regarded as the best energy precursor, especially given the introduction of the concept of biofuels [1]
Owing to the changes in the energy and environmental landscapes, stringent environmental regulations have been imposed by governments, e.g., B-5, which is composed of 5% of biodiesel and 95% of diesel fuel, is primarily used in Canada to reduce greenhouse gas (GHG) emissions and other toxic gas emissions caused by burning fossil fuels, which has boosted biodiesel production in recent decades [2]
Similar results were reported by Wu et al [167], where Ru-Cu/carbon nanotubes (CNTs) catalysts were applied in glycerol hydrogenolysis to enhance 1,2-propanediol production, and the results showed that the bimetallic catalysts showed a higher selectivity toward 1,2-propanediol formation than single-metal catalysts due to the hydrogen spillover effect resulting from the presence of highly dispersed tiny Ru clusters on the surface of Cu particles
Summary
Considerable effort has been dedicated toward developing renewable resources to completely or partially replace with fossil fuels, including wind, solar, geothermal, nuclear, tidal power, and biomass, among which biomass is regarded as the best energy precursor, especially given the introduction of the concept of biofuels [1]. Most published review articles are restricted to one specific glycerol valorization technique, e.g., 1,3-propanediol by Wang et al [24] and Eokum et al [25]; lactic acid by Arcanjo et al [26]; fuel additives by Smirnov et al [27], Cornejo et al [28], and Nanda et al [29]; acrolein by Galadima and Muraza [30]; and H2 and syngas by He et al [4], Lin [31], and Macedo et al [32].On the contrary, in this review, we discuss the latest developments and advances in heterogeneous catalysts and reactor configurations for some of the most common conversion pathways including steam reforming to H2, dehydration to acrolein, oxidation to lactic acid, and selective hydrogenolysis to 1,3-propanediol and 1,2-propanediol, followed by a discussion on the directions for future research and major conclusions
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