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Title: One-step catalytic conversion of ethanol into 1,3-butadiene
Other Titles: Katalytische eenstapsomzetting van ethanol naar 1,3-butadieen
Authors: Janssens, Wout
Issue Date: 22-Apr-2015
Abstract: The interest in the production of 1,3-butadiene these days is increasing in both academic and industrial research. Researchers are looking for alternative on-purpose production routes to replace the current 1,3-butadiene supply as byproduct in the naphtha steam crackers, for ethylene production, to disconnect the supply and price from the ethylene demand. Different routes are available for such on-purpose production of 1,3‑butadiene, like the dehydrogenation of butane and butenes, the dehydration of butanediols and the conversion of ethanol to 1,3-butadiene.
The current PhD research focusses on the chemo catalytic conversion of ethanol into 1,3-butadiene. This process has been operated on an industrial scale long time ago. Despite the former success, improvements, likely at the level of the catalyst properties, are necessary for this process to become competitive in the present-day industry. In this PhD research, an extensive catalyst research was therefore conducted with the aim to increase the catalytic activity and butadiene selectivity, and to understand the complex catalytic systems. Such a catalyst research will play an important role in the improvements necessary for this process and aid the eventual assessment of the economic viability of this ethanol-based production method.
Our thermodynamic calculations proved that the conversion of ethanol into 1,3-butadiene become favorable above 150 °C and the preferred reaction temperature is between 325 and 430 °C. The calculations also illustrated that 1,3-butadiene is thermodynamically not the most favorable reaction product; ethylene and all isomers of butene are more favorable products. Therefore, a multifunctional catalyst for the 1,3-butadiene formation process should be capable of catalyzing selectively the different reactions, but avoiding the formation of the thermodynamically more stable side-products.
Inspired by old literature, an extensive catalytic research was conducted focusing on magnesia-silica type materials for the 1,3-butadiene formation from ethanol. The influences of the synthesis procedure, the Mg/Si ratio and the nature of different dopants on the catalytic activity were studied. The binary magnesia-silica catalysts didn’t prove to be very active in the 1,3-butadiene formation reaction. The modification of the magnesia-silica support with transition metal(oxide)s by means of a consecutive impregnation step significantly increased the ethanol conversion and 1,3-butadiene yield. Catalysts containing CuO, ZnO and Ag dopants obtained 1,3-butadiene yields higher than 50 % at full ethanol conversion.
In the next part of this work, a thorough investigation was conducted on the silver doped magnesia-silica catalysts. The influences of the silver content and the type of silica material on the catalytic performance were studied in depth. The prepared catalysts were characterized by different techniques such as 29Si NMR, N2 sorption, SAXS, XRD, ESEM/EDX, IR spectroscopy of adsorbed pyridine and CO2, CO2‑TPD and UV-vis DRS. It was shown that the contact with water during the impregnation of silver resulted in changes of the pore structure and in the formation of an amorphous magnesium silicate phase on the final catalyst. These changes also resulted in an increase of the Lewis acidity and basicity of the final catalyst. The catalytic results were combined with the characterization results to explain the catalytic activity of the silver doped magnesia-silica catalysts. The addition of silver increases the dehydrogenation activity of the magnesia-silica catalysts. The rate-determining step in the 1,3-butadiene formation therefore shifts from the dehydrogenation on silver-free catalysts to the aldol condensation on silver doped catalysts. These rate-determining reactions are catalyzed by, respectively, the silver species and the basic sites (MgO-pairs and basic OH groups) present on the surface of the active catalysts. The following steps in the reaction sequence, MPV reduction of crotonaldehyde and dehydration of crotyl alcohol, are catalyzed by, respectively, Lewis acidic Mg(II) atoms and mildly acidic silanol groups. The optimal silver loading is 1‑2 wt% silver for a magnesia-silica catalyst with a molar Mg/Si ratio of 2. The highest 1,3-butadiene productivity was reached at higher reaction temperatures, with a record productivity of almost 0.3 gram butadiene per gram catalyst per hour at 480 °C and a WHSV of 1.2 per hour.
In the final part, the obtained information about the silver doped magnesia-silica catalysts is used to make alterations in the preparation method to gain further insights and improvements in this complex catalytic system and increase their catalytic activity. First, the sequence of the preparation method was altered. For these types of catalysts, silver was loaded on the silica source before mixing with the Mg source. Secondly, different solvents are introduced in the preparation method, both during the mixing of the Mg and Si sources and during the impregnation of silver, to control the contact between the MgO and SiO2 phases and to control the formation of the magnesium silicate phase. Both alternative procedures are able to create active and selective catalysts for the conversion of ethanol into 1,3-butadiene, only when the magnesia and silica phase were simultaneously contacted with water. Both methods demonstrate the importance of the simultaneous contact with water, resulting in the formation of a magnesium silicate phase. The magnesium silicate phase is a vital part of the final active catalyst. The dehydrogenation and aldol condensation activity are significantly higher in the presence of the magnesium silicate phase, resulting in significantly higher 1,3-butadiene yields and productivities compared to catalysts without a magnesium silicate phase.
The combination of catalytic experiments with thorough physicochemical characterizations was the foundations for the interpretation of the catalytic activity of the magnesia-silica materials. The novel insights, gained during this PhD research, also led to the construction of a tentative reaction pathway model explaining the role of the different active sites on the multifunctional catalyst in the conversion of ethanol into 1,3‑butadiene.
Publication status: published
KU Leuven publication type: TH
Appears in Collections:Centre for Surface Chemistry and Catalysis

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