Increasing energy demand, especially in the transportation sector, and soaring CO2 emissions necessitate the exploitation of renewable sources of energy. Despite the large variety of new energy carriers, liquid hydrocarbon still appears to be the most attractive and feasible form of transportation fuel taking into account the energy density, stability and existing infrastructure. Biomass is an abundant, renewable source of energy; however, utilizing it in a cost-effective way is still a substantial challenge. Lignocellulose is composed of three major biopolymers, namely cellulose, hemicellulose and lignin. Fast pyrolysis of biomass is recognized as an efficient and feasible process to selectively convert lignocellulose into a liquid fuel—bio-oil. However bio-oil from fast pyrolysis contains a large amount of oxygen, distributed in hundreds of oxygenates. These oxygenates are the cause of many negative properties, such as low heating value, high corrosiveness, high viscosity, and instability; they also greatly limit the application of bio-oil particularly as transportation fuel. Hydrocarbons derived from biomass are most attractive because of their high energy density and compatibility with the existing infrastructure. Thus, converting lignocellulose into transportation fuels via catalytic fast pyrolysis has attracted much attention. Many studies related to catalytic fast pyrolysis of biomass have been published. The main challenge of this process is the development of active and stable catalysts that can deal with a large variety of decomposition intermediates from lignocellulose. This review starts with the current understanding of the chemistry in fast pyrolysis of lignocellulose and focuses on the development of catalysts in catalytic fast pyrolysis. Recent progress in the experimental studies on catalytic fast pyrolysis of biomass is also summarized with the emphasis on bio-oil yields and quality.

Catalytic fast pyrolysis of lignocellulosic biomass

Ceria has been the subject of thorough investigations, mainly because of its use as an active component of catalytic converters for the treatment of exhaust gases. However, ceria-based catalysts have also been developed for different applications in organic chemistry. The redox and acid-base properties of ceria, either alone or in the presence of transition metals, are important parameters that allow to activate complex organic molecules and to selectively orient their transformation. Pure ceria is used in several organic reactions, such as the dehydration of alcohols, the alkylation of aromatic compounds, ketone formation, and aldolization, and in redox reactions. Ceria-supported metal catalysts allow the hydrogenation of many unsaturated compounds. They can also be used for coupling or ring-opening reactions. Cerium atoms can be added as dopants to catalytic system or impregnated onto zeolites and mesoporous catalyst materials to improve their performances. This Review demonstrates that the exceptional surface (and sometimes bulk) properties of ceria make cerium-based catalysts very effective for a broad range of organic reactions.

https://www.onlinelibrary.wiley.com/doi/pdf/10.1002/cssc.201000054

Ceria‐Based Solid Catalysts for Organic Chemistry

Alcohol coupling, also known as the Guerbet reaction, is a potentially important process to increase the value of short chain alcohols. Metal oxides, metal phosphates, and supported transition metals, such as copper, are important components of heterogeneous catalysts for the reaction. However, the wide variety of catalyst compositions, reaction conditions, and reactor configurations used to study the reaction complicate a direct comparison of various catalysts. In this review, rates over different catalysts will be compared, the influences of the acid and base properties of the catalyst on product selectivity will be presented, and possible reaction paths to accomplish alcohol coupling will be discussed.

Heterogeneous Catalysts for the Guerbet Coupling of Alcohols

On the Structure of Activated Hydrotalcites as Solid Base Catalysts for Liquid-Phase Aldol Condensation

The vapor-phase aldol condensation of acetone was studied over MgO promoted with 0.7–1.0 wt.-% of alkali (Li, Na, K and Cs) or alkaline earth (Ca, Sr and Ba) metal ions. The basic properties of the samples were characterized by chemisorption of carbon dioxide. The basicity of MgO increased on addition of the promoter following the basicity order of the promoter oxide: the stronger the electron donor properties of the promoter, the greater the generation of surface basic sites. Major reaction products were mesityl oxide (MO), isomesityl oxide (IMO) and isophorone (IP). The selectivity to (MO + IMO + IP) over unpromoted MgO was practically 100%, thereby showing that magnesium oxide is suitable for selectively obtaining α,β-unsaturated ketones. The reaction was totally inhibited by co-feeding acetic acid along with acetone whereas the co-injection of pyridine did not affect the acetone conversion. This indicated that the self-condensation of acetone over MgO-based catalysts is catalyzed by basic sites. The promoter addition increased the activity of the MgO catalyst and a good correlation was obtained between catalyst activity and the concentration of basic sites. Such a proportionality between activity and surface basicity was an additional evidence that the rate-determining step in the aldol condensation mechanism is controlled by the surface base property. All the catalysts exhibited similar IP/(IMO+MO) selectivity ratio, except the Li/MgO sample which produced substantially larger amounts of isophorone. Because the tricondensation of acetone to give isophorone requires strong basic sites, the higher selectivity toward isophorone was indicative of the presence of stronger surface basic sites in the Li/MgO sample. Results from carbon dioxide chemisorption confirmed that Li/MgO exhibited the strongest basic properties. The generation of high-strength basic sites was explained by assuming that the addition of lithium causes a structural promotion of the MgO sample by replacing the Mg 2+ ions by Li + in the MgO lattice. The replacement would result in strained Mg O bonds and formation of [Li + O − ] species, which causes the generation of stronger basic sites.

Sigrid Lippert

Papers

Secondary reactions of the base-catalyzed aldol condensation of acetone