Title: Design of Superbasic Solid Catalysts an their use in Alkenes Isomerization and Triglycerides Transesterification Reaction.
Other Titles: Ontwerp van Nieuwe Superbasische Katalysatoren voor Alkeen Isomerisatie en Triglyceride Transesterificatie
Authors: Bota, Roxana Melania
Issue Date: 1-Jun-2012
Abstract: Since 1970 heterogeneous base catalysis received significant attention after the discovery of many materials which if properly pretreated act as solid base catalysts. In addition, compared with homogenous base catalysts, the heterogeneous base catalysts have several advantages, viz. high catalytic activity and selectivity for many reactions, easy separation from the products with possible regeneration and reuse, the generation of a non-corrosive medium, reduced waste and environmental impact, mild operation temperatures and reduced maintenance costs. As a result, substitution of new solid base catalysts for homogeneous base catalysts is a timely aim for the development of sustainable chemical technology.More particularly, in this general frame, the present research project aims at developing new solid superbase catalysts allowing activating industrially relevant but rather inert chemical (apolar) feedstocks. Moreover, preparation procedures yielding not easily reprodubible solids were replaced with more sustainable preparation routes.In a first part, sodium on gamma-alumina (superbase) catalysts were generated by controlled thermal decomposition of adsorbed sodium azide and compared with the behavior of the traditional superbase catalysts obtained by sodium metal impregnation methods. In a second part the crystalline alumina catalyst support material was replaced by a mesoporous form. The catalytic materials were evaluated at room temperature in batch and liquid phase continuous flow reactors using vinyl cyclohexane or vinylnorbornene double bond shift as test for superbasicity. Biodiesel formation ex triglycerides and methanol was used in the search for a sustainable new technology. In situ 23Na and 27Al MAS NMR on the catalysts allowed determining the physico-chemical characteristics, the catalytically active species and their interaction with the alumina support.Occasionally, XPS and TOF SIMS spectroscopy has been used to complement this information. The activity of sodium ex azide on support catalysts was found to be governed by different factors such as support nature, preparation method and activation temperature of the catalyst. It is crucial to control the thermal decomposition of the impregnated sodium azide to obtain stable and reproducible formulations. The in situ characterization of several activated catalysts allowed shedding light on the nature of the active sites and the working principle of the catalyst. 23Na MAS NMR shows that the presence of sodium metal as well as of sodium oxide particles is a prerequisite for having a good activity. Enhanced sodium metal dispersion into the structure of gamma-alumina occurs in parallel with the development of superbasic activity, which is evidence for the existence of increased electron pair donating ability of surface oxygen atoms of the gamma-alumina support. 27Al MAS NMR of activated catalysts demonstrates formation of (distorted) sodium aluminate species, while Al(IV) line broadening is indicative of interactionbetween sodium metal and surface oxygens of the support.XPS O 1s peak shifts in the catalyst relative to that of the support point to the active role of the support nature on the generation of superbase strength, gamma-alumina being a favored support.Catalyst deactivation was found to be affected by the amount of feed added rather than by thereaction time. A guard layer of pre-activated gamma-alumina preceeding the catalyst bed, was found protecting catalyst activity and extending the life-time of the catalyst. This observation relates catalyst deactivation to an adsorption phenomenon (of as such undefined feed impurity molecules).A sodium ex azide on mesoporous alumina superbasic catalyst was shown to contain Na metal and Na oxide species and showed superbasicity in the double bond shift reactions. When the effect of the azide loading on mesoporous oxide was compared with that shown for gamma-alumina, similar sodium metal to oxide transformations were seen.In the biodiesel formation reaction the two types of catalyst were found to perform in milder conditions than the traditional homogeneous as well as homogeneous currently used systems.<w:latentstyles deflockedstate="false" defunhidewhenused="true"  <w:lsdexception="" locked="false" priority="0" semihidden="false"  <object<w:latentstyles=""  <objectsince="" 1970="" heterogeneous="" base="" catalysis="" received="" significant="" attention="" after="" the="" discovery="" of="" many="" materials="" which="" act="" as="" solid="" catalysts="" if="" they="" are="" properly="" pretreated.="" in="" addition,="" compared="" with="" homogenous="" catalysts,="" have="" several="" advantages, viz.  high="" catalytic="" activity="" and="" selectivity="" for="" reactions,="" easy="" separation="" from="" products="" possible="" regeneration="" reuse,="" non-corrosive="" medium,="" reduced="" waste="" environmental="" impact,="" mild="" operation="" temperature="" maintenance="" costs.="" a="" result,="" replacement="" the<="" object
Publication status: published
KU Leuven publication type: TH
Appears in Collections:Centre for Surface Chemistry and Catalysis

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