Mechanistic Insight to C–C Bond Formation and Predictive Models for Cascade Reactions among Alcohols on Ca- and Sr-Hydroxyapatites

Abstract

starch or sugar-rich biomass (corn (maize), other cereals, sugar cane, etc.) into sugar, fermentation, and distillation. Advanced process: hydrolysis of ligno-cellulosic biomass, fermentation and distillation. Biodiesel production: extraction and esterification of vegetable oils, used cooking oils and animal fats using alcohols. Advanced processes: hydrogenation of oil and fat; gasification and catalytic conversion to liquid fuels (biomass to liquid, BTL). Biomethane: biogas from anaerobic digestors and landfills used as compressed gas in natural gas vehicles. ENERGY INPUT AND EMISSIONS -Because of the variety of feedstocks and processes, figures vary widely and make it difficult to identify indicative values. Sugar-cane ethanol: fossil fuel input some 10%-12% of final energy and up to 90% CO 2 reduction compared with gasoline. Corn ethanol: high energy input and much smaller CO 2 reduction (15-25%). Ligno-cellulosic ethanol: total energy input may be higher than for corn ethanol, but most such energy could be provided from biomass itself, with CO 2 reduction up to 70% (100% with power co-generation). Biodiesel: about 30% energy input and up to 60% CO 2 reduction. COSTS -High sensitivity to feedstock, process, land type and crop yield. BARRIERS -Competition with food and fibre production for use of arable land; cost; regional market structure; biomass transport; lack of well managed agricultural practices in emerging economies; water and fertiliser use; conservation of bio-diversity; logistics and distribution networks. PROCESS -Bioethanol conventional production -Bioethanol is the most common biofuel, accounting for more than 90% of total biofuel usage. Conventional production is a well known process based on enzymatic conversion of starchy biomass into sugars, and/or fermentation of 6-carbon sugars with final distillation of ethanol to fuel grade. Ethanol can be produced from many feedstocks, including cereal crops, corn (maize), sugar cane, sugar beets, potatoes, sorghum, cassava. Coproducts (e.g animal feed) help reduce production cost. If sugar cane is used, conversion into sugar is easier. Crushed stalk (bagasse) can be used to provide heat and power for the process and for other energy applications. The world's largest producers of bio-ethanol are Brazil (sugar-cane ethanol) and the United States (corn ethanol). Ethanol is used in low 5%-10% blends with gasoline (E5, E10) but also as E-85 in flex-fuel vehicles. In Brazil, gasoline must contain a minimum of 22% bioethanol.Bioethanol advanced productionWhile conventional processes use only the sugar and starch biomass components, R&D focuses on advanced processes that utilise the all available ligno-cellulosic materials. These processes hold the potential to increase variety and quantity of suitable feedstock including cellulosic wastes, maize stover, cereal straw, foodprocessing wastes, as well as dedicated fast-growing plants such as poplar trees and switch-grass. Cellulosic feedstock could be grown on non arable land or be produced from integrated crops,...

Formation Pathways toward 2‐ and 4‐Methylbenzaldehyde via Sequential Reactions from Acetaldehyde over Hydroxyapatite Catalyst

“…Besides the 2‐MB and 4‐MB products and their intermediates, other C 4 –C 6 alcohols and aldehydes (e.g., 1‐butanol, 1‐hexanol, butyraldehyde) form as side products via direct (e.g., the Meerwein‐Ponndorf‐Verley (MPV) reaction) or indirect, surface‐mediated H‐transfer from ethanol . Notably, the Ca‐HAP catalyst and reaction conditions used here do not dehydrogenate C−C bonds of small oxygenates, and therefore, saturated aldehydes and alcohols do not reform the enes or enals needed to create aromatic products …”

“…Figure shows apparent reaction rate of aldol‐type reaction between C 2 −C 6 enals and acetaldehyde (0.1 kPa reactant, 0.35 kPa C 2 H 4 O, 1 kPa C 2 H 5 OH, 100 kPa H 2 ) and that of DA reaction between 2,4‐hexadienal (C 6 ) and ethylene (0.1 kPa C 6 H 8 O, 2 kPa C 2 H 4 , 99 kPa H 2 ) over HAP catalyst at 548 K. The apparent rate for DA reaction with ethylene is two orders of magnitude lower than that for aldol condensation with acetaldehyde, even at an ethylene co‐reactant pressure 50–100 times greater than that obtained in situ when feeding pure ethanol streams (5 kPa C 2 H 5 OH, 96 kPa H 2 , 573 K) . DA reactions frequently use high reactant pressures (3–7 MPa) to drive the reaction forward.…”

“…1‐butanol, 1,3‐butadiene, and diethylether) . Acetaldehyde, which is readily formed from ethanol via dehydrogenation, participates in multiple types of coupling reactions (e.g., aldol reaction, Guerbet reaction) that give larger chemical products, and the mechanisms and active surface species of these reactions have been investigated in detail over catalysts such as hydroxyapatite (HAP), metal oxides, and mixed metal oxides …”

“…Such coupling reactions continue in a series of steps that consume primary products and form larger species . While the cascades of ethanol coupling (Guerbet reaction) processes produce broad distributions of linear and branched alcohols by numerous self‐ and cross‐condensation reactions, narrower product distributions that provide selectivity to specific species emerge if unsaturated aldehydes (and enals) mainly participate in condensation reactions.…”

Catalytic Upgrading of Ethanol to n‐Butanol: Progress in Catalyst Development

“…Hydrogen transfer may occur through one of two mechanisms: surface‐mediated hydrogenation or direct intermolecular hydride transfer, also known as the Meerwein–Ponndorf–Verley (MPV) reaction. In the upgrading of ethanol, with ethanol as the sole source of hydrogen, surface‐mediated hydrogenation, which is common on metals, involves activation and transfer of hydrogen atoms from either gaseous H 2 derived from ethanol or ethanol itself . The product selectivity is highly dependent on the nature of the metal for the preferential hydrogenation of C=C or C=O bonds.…”