| Calcium carbide(CaC2)is an important starting material for production of many organic building blocks via gaseous acetylene,such as polyvinyl chloride,vinyl acetate and polyvinyl alcohol.Due to the easily flammable and explosive properties of acetylene,much effort has been made to develop safe and economic protocols for the use of CaC2 in organic synthesis in recent years.It is well known that the alkynyl group of CaC2 has a strong nucleophilicity and can attack carbon-oxygen double bonds,carbon-nitrogen triple bonds,carbon-carbon triple bonds and other unsaturated chemical bonds,resulting in addition reaction.It is notable that the nucleophilicity of alkynyl group may also render CaC2 as a basic catalyst.This work proposes two kinds of organic reactions in the presence of CaC2 to produce acetylene-derived products,such as biofuels and chemicals,which fully utilizes the reactivity and catalytic activity of CaC2 and avoids production,transportation and storage of gaseous acetylene.One of organic reactions is reaction of ethanol with CaC2 and the other is reaction of acetone with CaC2.We choose the above two reactions because they may involve activation and cleavage of C-H bonds,and ethanol and acetone have been produced on a large scale through biomass fermentation.The reaction routes and mechanisms are also investigated using quantum chemistry calculation methods.The main conclusions are as follows:(1)The reaction of CaC2 with ethanol can achieve the coupling of reactivity and catalytic activity of CaC2,and the main products can be controlled by adjusting temperature.The liquid product is mainly ethyl vinyl ether at the temperatures lower than 190℃ with a optimal yield of 11.4%.The long-chain alcohols are the main products at the temperatures higher than 255℃,including butanol,pentanol,hexanol,heptanol and nonanol,with a optimal total yield of 56.3%.Ethyl vinyl ether is generated via a two-step route,including the extraction of the H of hydroxyl in ethanol by alkynyl of CaC2 to form acetylene,followed by the subsequent reaction of acetylene with ethanol to form vinyl ethyl ether.The formation of long-chain alcohols originates from the activation of β-H in the alcohols by the alkyne group.There are two reaction routes,self-condensation and aldol-condensation,take place simultaneously in the conversion of ethanol to 1-butanol.In a self-condensation mechanism,the β-H on ethanol is activated by a catalyst to bond with a hydroxyl group in another ethanol to form H2O,thereby promoting the formation of C-C bond between the corresponding β-C to α-C in another ethanol.The aldol-condensation route involves aldo condensation of ethanol with acetaldehyde(formed from ethanol dehydrogenation)to form 2-butenol,followde by subsequent hydrogenatation to form 1-butanol.Methanol can not condensate to form ethanol due to the absence of β-H.The main products in the reaction of methanol with CaC2 are vinyl methyl ether and 1,1-dimethoxyethane.(2)CaC2 is also effective in catalyzing condensation reaction of a wide range of aliphatic alcohols with β-C,featuring excellent substrate tolerance.The condensation reaction of alcohols mainly occurs in β-C,which contains the most hydrogen atoms in the alcohol.The condensation reaction in linear alcohols is more likely to occur than that in branched alcohols because of the lower steric hindrance in linear alcohols.(3)Calcium alkoxides(calcium methoxide and calcium ethoxide)that generated from the reaction of CaC2 and alcohols have a alkaline catalytic activity for the condensation of ethanol to long-chain alcohols.Quantitative analysis of the products reveals that the conversion of ethanol to 1-butanol over calcium alkoxides follows the aldol condensation of ethanol and acetaldehyde(generated from dehydrogenation of ethanol),and 2-butenol is an important intermediate.The Ca atom in the calcium alkoxide acts as Lewis acid sites to adsorb ethanol and acetaldehyde,thereby promoting the subsequent aldol condensation reaction.In addition,the substitution reaction between the calcium alkoxide and its non-corresponding alcohols will occurre to generate new calcium alkoxide and new alcohol,for example,calcium ethoxide react with methanol to produce calcium methoxide and ethanol.(4)Based on the quantum chemical calculation with density functional theory(DFT),it is found that the catalytic activity of calcium ethoxide originates from the adsorption of Ca atom to O atom in ethanol,which promotes the activation of ethanol to form acetaldehyde and activated H.The adsorption energy of ethanol on calcium ethoxide is-120 kJ·mol-1.1-Butanol can be generated from ethanol by three reaction pathways:pathway(1)is the aldol condensation of ethanol and acetaldehyde adsorbed on the associated calcium ethoxides,followde by the hydrogenation;pathway(2)is the aldol condensation of ethanol and acetaldehydes adsorbed on the same calcium ethoxide;pathway(3)is the directly reaction of acetaldehyde and calcium ethoxide.The calculation of the reaction energy barrier shows that pathways(1)and(2)are more likely to occur due to the lower energy barriers.The highest energy barrier of the pathway(1)lies on the aldo condensation reaction and is 319 kJ·mol-1.The highest energy barrier of the pathway(2)lies on ethanol dehydrogenation and is 288 kJ·mol-1.(5)Methyl-substituted benzenes(p-xylene,mesitylene and 2-ethyl-1,4-dimethylbenzene),methyl-substituted naphthalenes(2-methylnaphthalene,2,6-dimethylnaphthalene,2,3,6-trimethylnaphthalene)and 3,5-xylenol are successfully prepared in one pot by the reaction of acetone and CaC2 with the optimum yields of 34.1%,18.6%and 23.8%,respectively.The formation of the above products results from the coupling of the acetone condensation reaction with the subsequent aromatization reaction,and isophorone is an important intermediate in the reactions.The alkynyl moiety of CaC2 catalyzes conversion of acetone to intermediates and aromatization of intermediates to aromatic hydrocarbons and 3,5-xylenol by effectively activating a-H in acetone and isophorone.The low temperature facilitates the acetone condensation reaction,and the high temperature facilitates the aromatization reaction.The alkynyl moiety also participates in the aromatization reaction to form methyl-substituted naphthalenes,which opens us new routes for the synthesis of aromatic hydrocarbons and the application of CaC2 in organic synthesis. |
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