Continuous Catalytic Deoxygenation of Waste Free Fatty Acid-Based Feeds to Fuel-Like Hydrocarbons Over a Supported Ni-Cu Catalyst

Abstract

While commercial hydrodeoxygenation (HDO) processes convert fats, oils, and grease (FOG) to fuel-like hydrocarbons, alternative processes based on decarboxylation/decarbonylation (deCOx) continue to attract interest. In this contribution, the activity of 20% Ni-5% Cu/Al2O3 in the deCOx of waste free fatty acid (FFA)-based feeds—including brown grease (BG) and an FFA feed obtained by steam stripping a biodiesel feedstock—was investigated, along with the structure-activity relationships responsible for Ni promotion by Cu and the structural evolution of catalysts during use and regeneration. In eight-hour experiments, near quantitative conversion of the aforementioned feeds to diesel-like hydrocarbons was achieved. Moreover, yields of diesel-like hydrocarbons in excess of 80% were obtained at all reaction times during a BG upgrading experiment lasting 100 h, after which the catalyst was successfully regenerated in situ and found to display improved performance during a second 100 h cycle. Insights into this improved performance were obtained through characterization of the fresh and spent catalyst, which indicated that metal particle sintering, alloying of Ni with Cu, and particle enrichment with Cu occur during reaction and/or catalyst regeneration.

Promotional Effect of Cu, Fe and Pt on the Performance of Ni/Al2O3 in the Deoxygenation of Used Cooking Oil to Fuel-Like Hydrocarbons

“…Notably, the spent catalyst comprises particles slightly more enriched in Cu relative to those in the fresh formulation, all particles in the spent catalyst containing between 65 and 80% Ni. This indicates that particles not only grow in size but also become Cu-rich during the reaction, which is also in line with previously reported results [11]. This conclusion is clearly illustrated by the TEM micrographs and the TEM-EDS elemental maps included in Figure A9; the elemental maps also showing that Ni and Cu are present in close association on both the fresh and spent catalysts.…”

“…All of these trends indicate that while the Ni-Cu and Ni-Fe catalysts retain their deoxygenation activity within the time period investigated, cracking activity either remains constant or declines with TOS, consistent with results reported in other studies [11,17,23]. Although the Ni-Pt catalyst also retains its deoxygenation activity throughout the entire experiment, cracking reactions remain prevalent during the entirety of the run.…”

“…The Cu map of the spent Ni-Cu catalyst shown in Figure A9 also provides an example of the Cu-hollow space not observed in the fresh formulation, as Ni-Cu particles likely undergo Cu-hollowing through a mechanism based on the Kirkendall effect [35]. These observations are consistent with the widely reported bulk and surface enrichment of Ni-Cu nanoparticles with Cu [11,36,37]. In the case of the Fe-promoted catalyst, TEM results indicate that a similar particle size change takes place during reaction.…”

“…Remarkably, complete deoxygenation occurs over all catalysts tested (see Figure 1 and Tables A2-A4), the concentration of diesel-like (C10-C20) hydrocarbons in the reaction products being >82% irrespective of both catalyst and TOS, heavier (C21-C35) hydrocarbons comprising the remainder of the product mixtures. Whereas heavier hydrocarbons stem from the deoxygenation of long-chain ester intermediates [21], diesel-like hydrocarbons are the result of the deoxygenation of the triglycerides and the fatty acids constituting the UCO feed (as well as of the cracking of heavier hydrocarbons) [11,18]. Remarkably, complete deoxygenation occurs over all catalysts tested (see Figure 1 and Tables A2-A4), the concentration of diesel-like (C10-C20) hydrocarbons in the reaction products being >82% irrespective of both catalyst and TOS, heavier (C21-C35) hydrocarbons comprising the remainder of the product mixtures.…”

“…Against this backdrop, the present work investigated the conversion of UCO to renewable diesel via deCO x over supported Ni catalysts promoted with Cu, Fe or Pt. The performance of these formulations was tested in a fixed bed reactor using industrially-relevant reaction conditions for 76 h of time on stream (TOS), as previous work has shown that catalysts of this type require >48 h of TOS to attain a steady state [11]. In addition, the analysis of the fresh, spent, and regenerated catalysts was undertaken in an effort to understand the distinct performance displayed by these formulations.…”