Synthesis of C10‐C16 linear symmetrical ethers from n‐alcohols over heterogeneous acid catalysts

Abstract

[cat] La reformulació del combustible dièsel amb compostos oxigenats com els èters lineals de més de 9 carbonis permet augmentar la qualitat ambiental del combustible als motors dièsel. Els èters lineals simètrics com el di-n-pentil èter (DNPE), di-n-hexil èter (DNHE) i di-n-octil èter (DNOE) es presenten com tres compostos amb propietats excel·lents com additius pel dièsel ja que n’augmenten l’índex de cetà i en milloren les propietats fluidodinàmiques del dièsel. Aquest treball demostra que la deshidratació de 1-octanol a DNOE és possible sobre catalitzadors sòlids àcids (resines de bescanvi iònic de poliestirè reticulades amb divinilbenzè (PE-DVB) i zeolites) i a la vegada continua amb la sèrie de síntesis d’èters lineals simètrics a partir de n-alcohols començada amb el DNPE i DNHE en treballs previs. Les resines de bescanvi iònic PE-DVB macroreticulars amb baixos continguts de DVB i les resines tipus gel foren les resines que mes rendiment a DNPE, DNHE i DNOE van obtenir. Amb zeolites es va observar que la obtenció d’èters lineals simètrics amb estava fortament lligada a la superfície mesoporosa i al nombre de centres àcids de la zeolita. En aquest treball es va determinar la constant d’equilibri de la reacció de deshidratació de 1-octanol a DNOE i es va demostrar que la reacció mencionada és una reacció exotèrmica. Fruit de l’estudi d’equilibri es van poder corregir els mètodes actuals per estimar l’entropia molar del DNOE, proposant-ne un valor nou. Finalment es va desenvolupar un estudi cinètic sobre la resina termoestable Amberlyst 70 i sobre zeolites. A l’estudi de la deshidratació de 1-octanol a DNOE sobre Amberlyst 70 es va proposar un model cinètic basat en un mecanisme Rideal-Eley (RE) on una molècula de 1-octanol adsorbida al catalitzador reacciona amb una altra molècula de 1-octanol de la fase líquida. Així mateix es va constatar que la presència d’aigua exercia un fort efecte desactivador de la reacció i es va quantificar al model cinètic mitjançant un terme corrector basat en la isoterma Freundlich. Sobre les zeolites es van proposar dos tipus de models cinètics basats en mecanismes Langmuir-Hinselwood-Hougen-Watson (LHHW) i RE per les síntesis de DNOE i DNHE.

[eng] Combustion of diesel fuel in a diesel engine is a complex process where several pollutants are emitted in the engine exhausts. To solve this problem and at the same time accomplish with the increasingly stringent regulations for diesel composition, diesel reformulation with oxygenate compounds is proposed. Linear symmetrical ethers with more than 9 carbon atoms represent a group of oxygenate compounds with excellent properties as diesel additives. Its addition in small quantities to diesel enhances the blending cetane number, improves cold start performance of diesel engine and dilutes undesired components. C10-C16 linear ethers such as di-n-pentyl ether (DNPE), di-n-hexyl ether (DNHE) and di-n-octyl ether (DNOE) are three linear symmetrical ethers with excellent properties to be added to commercial diesel. This work proves that the synthesis of DNOE from 1-octanol dehydration over acid catalysts (acidic zeolites and polystyrene-divinylbenzene (PS-DVB) ion exchange resins) is possible at 413-473 K. At the same time, the present work continues with the series of synthesis of linear symmetrical ethers from n-alcohols started with DNPE and DNHE in previous works. A catalyst screening of acidic PS-DVB ion exchange resins showed best performances to yield DNPE, DNHE and DNOE selectively on low-crosslinked macroreticular and gel­type resins with high acid capacity. The ability to swell in the reaction media of low­crosslinked and gel-type resins contributes to create porosity ,allowing the diffusion of alcohols and ethers almost equally regardless the length of the molecules. On medium and large pore zeolites synthesis of linear symmetrical ether was found to be strongly related to the zeolites mesoporous surface and the acid sites number. Selectivity to linear ether decreased as the chain length increased which revealed a great steric hindrance for bulky molecules. The large pore zeolite H-BEA-25 showed the best yield of linear ether due to the combination of high conversion of alcohol and high selectivity to linear ether. In this work, the thermodynamic equilibrium constant of 1-octanol dehydration to DNOE was determined, proving that dehydration of 1-octanol to DNOE is an exothermic reaction. The study, allowed at the same time improving current methods to estimate molar entropy of DNOE. A kinetic study of the alcohol dehydration to linear ether was performed over zeolites and the thermostable resin Amberlyst 70. Over Amberlyst 70 a series of kinetic equations based on LHHW-RE mechanisms were fitted to rate data of 1-octanol dehydration to DNOE. Pure 1-octanol experiments revealed that kinetic models where 1-octanol and water are the adsorbed compounds in the resin and the fraction of free active sites is negligible fit the best. Influence of products (water and DNOE) and solvent (1,4-dioxane) was evaluated which revealed a strong inhibition effect of water. Inhibition effect of water was quantified by means of correction factors which described the fraction of active sites not blocked by water according to different adsorption isotherms. Freundlich isotherm was found to better describe the water inhibition effect in an scenario where one molecule of 1-octanol adsorbed reacts with another molecule of 1-octanol of the liquid phase in a Rideal-Eley mechanism. Activation energy determined accounting water deactivation was 110 ± 5 kJ·mol-1. Over zeolites, LHHW-RE kinetic models were fitted to rate data for DNHE and DNOE synthesis showing that the best kinetic models assume that the fraction of active sites is negligible. Within these models, two type of equations were found to best represent rate data, one assumes that alcohol and linear ether are adsorbed in the zeolite surface. The other equation assumes that alcohol and water are preferably adsorbed through the reaction. Apparent activation energy of 100-120 kJ·mol-1 were estimated for the reactions on tested zeolites.