Synthesis of ethers as oxygenated additives for the gasoline pool

Abstract

Functionalized ion-exchange resins are polymer-based materials that can be used as catalysts, in a wide variety of chemical reactions, to achieve highly selective formation of desired products at cost-effective rates. Among these catalysts, sulfonic macroreticular ion-exchange resins have become critical in major industrial processes, such as the etherification of isoolefins to obtain branched ethers.

The production of branched ethers, which are oxygenated antiknocking additives used in commercial gasoline formulation, became massive after the phase out of lead-based compounds in the 1990s. Among the ethers which are used nowadays, the most relevant ones, in terms of their global market share, are methyl tert-butyl ether (MTBE) and ethyl tert-butyl ether (ETBE). These ethers are industrially obtained under mild conditions (i.e., at relatively low temperature and pressure) by means of the catalytic reaction between 2-methylpropene (isobutene) and methanol or ethanol, respectively, over sulfonic macroreticular ion-exchange resins.

On the other hand, the use of reactants obtained from renewable feedstocks to produce fuel components is convenient, since it allows reducing the carbon footprint of transportation fuels in a context in which the global vehicle fleet is growing. In this sense, the trend has been to use biomass-derived ethanol as an alternative to carbon-based alcohols. Nevertheless, this option has generated concerns regarding its competition with the food supply chain. Alternatively, the use of other alcohols, such as 1-propanol and 1-butanol, which can also be obtained through fermentative processes, is becoming more attractive, given that they would not compete with food supplies. Etherification of isobutene with these alcohols would produce heavier ethers (i.e., propyl tert-butyl ether, PTBE, and butyl tert-butyl ether, BTBE, respectively), which have potential benefits as gasoline additives. Nowadays, this is one of the alternatives at hand to achieve an actual improvement of the gasoline formulation in the short- midterm.

Even though the industrial-scale production of MTBE and ETBE is today widely spread, relevant physicochemical aspects regarding ion-exchange resins have not been completely understood. Therefore, studies aimed at providing a deeper insight into the catalytic behavior of these materials are of interest.

The present PhD thesis contains several studies related to the liquid phase syntheses of MTBE, ETBE, PTBE, and BTBE. In the first place, an investigation was carried out aimed to determine the chemical equilibrium composition and thermodynamic properties of the liquid-phase syntheses of the mentioned reactions. Secondly, a study was performed regarding the products distribution and the conditions that favor side reactions taking place along with etherification reactions. On the other hand, in order to further understanding the catalytic behavior of sulfonic macroreticular ion-exchange resins, relations were established between the catalytic activity of sixteen different resins and their most relevant properties, taking into account the reaction media properties, in the four reaction systems. Also, the viability of an integrated process to

obtain, simultaneously, ETBE and BTBE over sulfonic macroreticular ion-exchange resins was studied. In this sense, individual and simultaneous processes were compared, a catalytic screening study was carried out to determine the most suitable catalyst and the effect of temperature and initial reactants composition on the ethers formation, in terms of conversion, selectivity and reaction rate, was discussed. Finally, a kinetic study of three of the studied reaction systems is provided. These are the individual syntheses of PTBE and BTBE and the simultaneous syntheses of ETBE and BTBE over AmberlystTM 35. A mechanistic kinetic model, in terms of components activities, was proposed for each studied reaction and it was found that the most likely reaction mechanism is an Eley-Rideal mechanism in which the surface reaction step is the rate-determining step.

Les resines de bescanvi iònic funcionalitzades són materials polimèrics que s’utilitzen com a catalitzadors en una gran varietat de reaccions químiques. Entre aquests catalitzadors, les resines sulfòniques han assolit una importància cabdal en processos industrials molt rellevants, com ara l’eterificació d’isoolefines per obtenir èters ramificats, els quals, s’utilitzen com a additius oxigenats en la formulació de gasolines comercials. D’èters ramificats, se’n pot destacar el metil terc-butil èter (MTBE) o l’etil terc-butil èter (ETBE), que s’obtenen per reacció entre isobutè i metanol o etanol, respectivament, utilitzant resines de bescanvi iònic sulfòniques macroreticulades com a catalitzadors. Alternativament, l’ús d’1-propanol i 1-butanol produeix propil terc-butil èter (PTBE) i butil terc-butil èter (BTBE).

Tot i que la producció d’MTBE i ETBE està molt estesa, hi ha aspectes de la naturalesa fisicoquímica dels processos catalítics en què participen les resines que continuen sense comprendre’s completament. En conseqüència, calen estudis que aportin nous coneixements en aquest sentit. És per això, doncs, que aquesta tesi inclou diversos estudis relatius al comportament catalític de resines de bescanvi iònic sulfòniques macroreticulades en les reaccions de síntesi en fase líquida d’MTBE, ETBE, PTBE i BTBE.

Pel que fa a l’estudi sobre l’equilibri químic d’aquestes reaccions, s’han pogut determinar les propietats termodinàmiques corresponents i s’han pogut estimar els increments d’entalpia i entropia de formació dels èters resultants. En relació amb la formació de subproductes en aquests processos, s’ha pogut concloure que totes les reaccions secundàries es veuen afavorides per un increment de la temperatura d’operació. També s’han establert relacions entre les propietats dels catalitzadors i el nivell d’activitat catalítica que presenten i s’ha determinat que la capacitat àcida i el volum expansible de la fase gel de les resines determinen l’activitat catalítica que presenten. Quant a la producció simultània d’ETBE i BTBE, s’ha comprovat que el procés de síntesi simultània de tots dos èters presenta avantatges significatius respecte de la producció de cada èter individualment. Finalment, l’anàlisi cinètica de les reaccions estudiades ha permès determinar que el mecanisme de reacció més probable és del tipus Eley-Rideal i que l’etapa controlant del procés de síntesi és la de reacció en superfície.