ZrO2 particles with different size and HCBZ zeolite have been used as supports for Ni to synthesise ethanol steam reforming catalysts. Catalyst acidity was tuned by addition of CaO and the redox properties by adding CeO2. Intrinsic acidic properties of the supports were also exploited for the ethanol dehydration to ethylene. In both cases we explored the possibility to use diluted bioethanol solutions, a less expensive raw material, and low reactor temperature with the aim of process intensification. 1. Scope Bioethanol is an increasingly important renewable fuel, in view of bioerefinery development, which can improve the sustainability of the production of some important chemicals, such as hydrogen and ethylene. Ethanol dehydration is promoted by acidic catalysts (typically alumina), whereas hydrogen can be produced by ethanol steam reforming over supported metal catalysts. Catalyst acidity in the latter case should be avoided in order to improve process selectivity. On the other hand, acidic supports are among the most widely used for steam reforming catalysts (e.g. Ni/Al2O3). In this work we have selected two different materials, commercial ZrO2 and HCBZ zeolite, characterized by different acidity. The latter parameter was tuned by addition of CaO and the oxygen exchange/redox properties by addition of CeO2. Ni was selected as active phase for ethanol steam reforming. Water/ethanol ratio and reaction temperature were optimized for both steam reforming and dehydration reactions in order to improve the energy efficiency and economic sustainability of the processes. Product distribution and catalysts resistance to coking were interpreted on the basis of the redox and acidic properties of the materials for both reactions. 2. Results and discussion Two commercial preformed zirconia particles (3 and 4.5 mm) and a HCBZ zeolite were used as such after grinding into 0.15-0.25 mm particles for ethanol dehydration to ethylene. For ethanol steam reforming all the supports were impregnated with Ni 10 wt%, and in case added with 9 wt% CaO or 9 wt% CaO + 3 wt% CeO2. The catalysts were characterized by N2 adsorption-desorption, X-rays diffraction, Temperature Programmed Reduction and FT-IR of adsorbed bases to account for surface acidity. Spent catalysts were characterized by Temperature Programmed Oxidation to quantify coke deposition. Activity testing was carried out in a continuous bench scale plant, by feeding water/ethanol solutions with ratio 3:1 mol/mol for steam reforming and 3:1 or 0:1 mol/mol for ethylene production. The molar ratio 3:1 was selected because it is the stoichiometric one for ethanol steam reforming, but also because it corresponds to a ca. 50 wt% ethanol solution, which is easily and cheaply obtainable by flash distillation from the fermentation broth. This allows the use of a less expensive raw material than pure ethanol, improving the economic sustainability and energy efficiency of the process. Activity testing was carried out at 400, 500 and 625°C at atmospheric pressure for ethanol steam reforming. The aim was to decrease as much as possible the operating temperature for process intensification. However, despite possible activity or selectivity problems, the main issue at low temperature is coke accumulation. Coking may occur over Ni particles in form of nanotubes and this phenomenon can be effectively prevented by improving metal dispersion and oxygen transport on catalyst surface. Both CaO and CeO2 addition can be effective from this point of view. On the other hand, ethanol dehydration is favoured by surface acidity, which was effectively tuned by the addition of the basic oxide, so limiting the selectivity to ethylene at low temperature. Full ethanol conversion was obtained for T500°C, with stable performance of the catalyst with time on stream. The only byproduct obtained was CH4, only at 500°C, with max selectivity 13%. The catalysts were active also at 400°C, but H2 productivity was not satisfactory and the spectrum of byproducts enlarged, including higher methane selectivity (max 22%), acetaldehyde and acetone (max 6% each). Ethylene was and important byproduct for the undoped ZrO2 and HCBZ samples at 400°C. However, while CaO doping depressed ethylene selectivity from 14.4% to <1% for the former support, for the latter ethylene remained the main product at 400°C, poorly affected by doping. Surface acidity was instead the desired feature for ethylene production. The intrinsic acidity of the ZrO2 catalyst was insufficient to fully convert ethanol, leading to 81% ethanol conversion, 88% selectivity to ethylene and 99% C balance at 400°C. Much better performance was achieved with the HCBZ zeolite. In this case, we were able to attain full ethanol conversion at 400°C with 97.5 1 % selectivity to ethylene and 99% when feeding pure ethanol. C balance and ethylene selectivity increased during the test (lasting 8 h-on-stream), indicating fast coking of the strongest acidic sites, which were ruled out then leading to stable catalyst performance. Furthermore, in order to valorize diluted and cheaper ethanol solutions we also tested a 50 wt% ethanol solution as feed, with practically unchanged results for conversion and selectivity. At difference with anhydrous ethanol, much more stable selectivity and C balance were achieved using water/ethanol = 3 mol/mol since the beginning of the test, confirming the interesting possibility to exploit diluted bioethanol solutions (Figure 1). Figure 1. Ethanol dehydration to ethylene over HCBZ catalyst at 400°C. Comparison between different water/ethanol ratio in the feed (3/1 or 0/1 mol/mol) . 3. Conclusions Different catalysts were compared for two processes, aiming at valorizing diluted ethanol solutions, less expensive raw materials than pure ethanol. A 50 wt% ethanol feed leads to full conversion and stable performance during ethanol steam reforming at T 500°C. Surface acidity should be limited for this application. By contrast, HCBZ zeolites allowed to dehydrate ethanol to ethylene with full conversion, satisfactory selectivity and stable performance, particularly when using diluted ethanol solutions.
Valorization of diluted bioethanol streams catalyzed by ZrO2- and HCBZ zeolite-based materials / G. Ramis, I.G. Rossetti, C. Resini, Y.V. Kolen’Ko, M. Cortés Reyes, M. Angeles Larrubia Vargas. ((Intervento presentato al convegno Europacat tenutosi a Firenze nel 2017.
|Titolo:||Valorization of diluted bioethanol streams catalyzed by ZrO2- and HCBZ zeolite-based materials|
|Data di pubblicazione:||2017|
|Settore Scientifico Disciplinare:||Settore ING-IND/25 - Impianti Chimici|
|Citazione:||Valorization of diluted bioethanol streams catalyzed by ZrO2- and HCBZ zeolite-based materials / G. Ramis, I.G. Rossetti, C. Resini, Y.V. Kolen’Ko, M. Cortés Reyes, M. Angeles Larrubia Vargas. ((Intervento presentato al convegno Europacat tenutosi a Firenze nel 2017.|
|Appare nelle tipologie:||14 - Intervento a convegno non pubblicato|