A Review on Catalysts Development for Steam Reforming of Biodiesel Derived Glycerol; Promoters and Supports

Glycerol and Catalysis by Waste/Low-Cost Materials—A Review

The growing global demand for renewable energy sources can be reached using biofuels such as biodiesel, for example. The most used route to produce biodiesel is the transesterification reaction of oils or fats with short-chain alcohols, generating fatty acid esters (biodiesel) and a very important by-product, glycerol (Gly). Gly is widely used in different sectors of the industry, and in order to add value to this by-product, heterogeneous catalysis becomes a relevant tool, whether to transform glycerol into other chemical products of interest or even use it in the production of catalysts. Among the several studies found in the literature, the use of low-cost materials and/or wastes from the most diverse activities to prepare active catalytic materials for the transformation of Gly has been increasingly reported due to its valuable advantages, especially related to the cost of raw materials and environmental aspects. Thus, this brief review article presents the relationship between catalysis, low-cost materials, waste, and glycerol, through different studies that show glycerol being transformed through reactions catalyzed by materials produced from low-cost sources/waste or with the glycerol itself used as a catalyst.

Mechanistic Kinetic Modelling Framework for the Conversion of Waste Crude Glycerol to Value-Added Hydrogen-Rich Gas

The kinetics for crude glycerol autothermal reforming was studied over S/C ratio of 2.6 and O2/C ratio of 0.125 using 5% Ni/CeZrCa catalyst. Both power law and mechanistic kinetic models were studied. The overall power law model for crude glycerol autothermal reforming was investigated with a pre-exponential factor of 4.3 × 1010 mol/gcat.min and activation energy of 8.78 × 104 J/mol. The reaction orders with respect to crude glycerol, water and oxygen are 1.04, 0.54 and 1.78 respectively. The power law model presented an absolute average deviation of 5.84%, which showed a good correlation between the predicted and experimental rate. Mechanistic models were developed for crude glycerol autothermal reforming. For steam reforming, the Eley–Rideal approach best described the reaction rate with the surface reaction being the rate-determining step (AAD < 10%). The kinetics of the total oxidation reaction was best described by the power law model with an AAD of less than 1%, whereas for the TOR process, the molecular adsorption of crude glycerol with an AAD of 14.6% via Langmuir Hinshelwood Hougen-Watson approach was best. CO2 methanation resulted in an AAD of 5.8% for the adsorption of carbon dioxide (CO2) by the Eley–Rideal mechanism.

Microalgal Hydrogen Production in Relation to Other Biomass-Based Technologies—A Review

Hydrogen is an environmentally friendly biofuel which, if widely used, could reduce atmospheric carbon dioxide emissions. The main barrier to the widespread use of hydrogen for power generation is the lack of technologically feasible and—more importantly—cost-effective methods of production and storage. So far, hydrogen has been produced using thermochemical methods (such as gasification, pyrolysis or water electrolysis) and biological methods (most of which involve anaerobic digestion and photofermentation), with conventional fuels, waste or dedicated crop biomass used as a feedstock. Microalgae possess very high photosynthetic efficiency, can rapidly build biomass, and possess other beneficial properties, which is why they are considered to be one of the strongest contenders among biohydrogen production technologies. This review gives an account of present knowledge on microalgal hydrogen production and compares it with the other available biofuel production technologies.

Main Hydrogen Production Processes: An Overview

Due to its characteristics, hydrogen is considered the energy carrier of the future. Its use as a fuel generates reduced pollution, as if burned it almost exclusively produces water vapor. Hydrogen can be produced from numerous sources, both of fossil and renewable origin, and with as many production processes, which can use renewable or non-renewable energy sources. To achieve carbon neutrality, the sources must necessarily be renewable, and the production processes themselves must use renewable energy sources. In this review article the main characteristics of the most used hydrogen production methods are summarized, mainly focusing on renewable feedstocks, furthermore a series of relevant articles published in the last year, are reviewed. The production methods are grouped according to the type of energy they use; and at the end of each section the strengths and limitations of the processes are highlighted. The conclusions compare the main characteristics of the production processes studied and contextualize their possible use.

Flame Pyrolysis Synthesis of Mixed Oxides for Glycerol Steam Reforming

Flame spray pyrolysis was used to produce nanosized Ni-based catalysts starting from different mixed oxides. LaNiO₃ and CeNiO₃ were used as base materials and the formulation was varied by mixing them or incorporating variable amounts of ZrO₂ or SrO during the synthesis. The catalysts were tested for the steam reforming of glycerol. One of the key problems for this application is the resistance to deactivation by sintering and coking, which may be increased by (1) improving Ni dispersion through the production of a Ni-La or Ni-Ce mixed oxide precursor, and then reduced; (2) using an oxide as ZrO₂, which established a strong interaction with Ni and possesses high thermal resistance; (3) decreasing the surface acidity of ZrO₂ through a basic promoter/support, such as La₂O₃; and (4) adding a promoter/support with very high oxygen mobility such as CeO₂. A further key feature is the use of a high temperature synthesis, such as flame spray pyrolysis, to improve the overall thermal resistance of the oxides. These strategies proved effective to obtain active and stable catalysts at least for 20 h on stream with very limited coke formation.

Preparation and Characterization of Ni/ZrTiAlOx Catalyst via Sol-Gel and Impregnation Methods for Low Temperature Dry Reforming of Methane

Recently, the dry reforming of methane (DRM) has received much attention as a conversion technology of greenhouse gases. Ni-based catalysts supported on ternary metal oxide composite (ZrTiAlOx) were prepared to improve the coke resistance properties in the DRM (CH4:CO2 = 1) at low temperature. The ZrTiAlOx supports with different ratios of Zr/Ti were prepared through the modified Pechini sol-gel method, and then the Ni was impregnated on the synthesized support via the incipient wetness impregnation method. Considering the Zr/Ti ratios, different catalytic activity and durability in the DRM were identified. The Ni/ZrTiAlOx catalyst with Zr/Ti of 2 exhibited enhanced coke inhibition property compared to the others at low temperature DRM for 50 h. The catalysts with a high Zr/Ti ratio under the same condition were rapidly deactivated, while the catalyst with a low Zr/Ti ratio showed deficient activity. It was found from temperature-programmed surface reactions (TPSR) and DRIFTS (Diffuse Reflectance Infrared Fourier Transform Spectroscopy) analysis that the addition of Ti has led in to higher catalytic stability at Zr/Ti = 2, which could be as a result of oxygen vacancies generated by the ternary metal oxides. Ni/ZrTiAlOx catalyst with ratio of Zr/Ti = 2 showed high stability and good catalytic activity towards DRM for the production of syngas.

Recent Advances in Glycerol Catalytic Valorization: A Review

Once a biorefinery is ready to operate, the main processed materials need to be completely evaluated in terms of many different factors, including disposal regulations, technological limitations of installation, the market, and other societal considerations. In biorefinery, glycerol is the main by-product, representing around 10% of biodiesel production. In the last few decades, the large-scale production of biodiesel and glycerol has promoted research on a wide range of strategies in an attempt to valorize this by-product, with its transformation into added value chemicals being the strategy that exhibits the most promising route. Among them, C3 compounds obtained from routes such as hydrogenation, oxidation, esterification, etc. represent an alternative to petroleum-based routes for chemicals such as acrolein, propanediols, or carboxylic acids of interest for the polymer industry. Another widely studied and developed strategy includes processes such as reforming or pyrolysis for energy, clean fuels, and materials such as activated carbon. This review covers recent advances in catalysts used in the most promising strategies considering both chemicals and energy or fuel obtention. Due to the large variety in biorefinery industries, several potential emergent valorization routes are briefly summarized.