Chromium Species as Captors of Sulfur Molecules on Nickel-Based Hydrotreating Catalysts

Abstract

Three nickel−chromium catalysts supported on aluminated mesoporous silica (SBA-15) were prepared with a fixed nickel loading of 15 wt % and differing chromium contents, which ranged between 2.6 and 5.2 wt %. They were tested in the hydrotreating of tetralin in the presence of dibenzothiophene (DBT). Moreover, a monometallic nickel catalyst was also prepared and tested, for comparison. Information regarding the structure of the calcined, reduced, and used catalysts was obtained by several physical−chemical techniques such as X-ray diffraction (XRD), hydrogen temperature-programmed reduction (H2-TPR), temperature-programmed desorption of ammonia (NH3-TPD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and elemental chemical analysis. The presence of small amounts of Cr3+ ions not only modifies the nickel dispersion, leading to the formation of a more active catalyst in the hydrogenation of tetralin, but also has a strong influence on the stability of these catalysts in the presence of 425 ppm DBT in the feed. The Cr3+ ions seem to act as a sulfur trap, thus preventing sulfur poisoning of nickel particles.

Hydrotreatment of Fast Pyrolysis Bio-oil Fractions Over Nickel-Based Catalyst

“…The characteristic XPS peaks for Ni, Cr, O, and Si are indicated. For all samples, the Cr 2p 3/2 peak was observed at 276 eV, as expected for Cr 2 O 3 [39,42,43]. Figure 4b shows a zoom into the Ni 2p region.…”

“…3. The passivated/fresh catalyst shows broad reflections which could indicate a highly dispersed metal distribution [39]. Reflections attributed to metallic nickel could be observed in the passivated/fresh catalyst (2θ = 44.5°, 51.8°, 76.4°, 92.95° and 98.45°) as well as small broad reflections attributed to nickel oxide (2θ = 37.2°, 42.6° and 62.8°).…”

“…Reflections attributed to metallic nickel could be observed in the passivated/fresh catalyst (2θ = 44.5°, 51.8°, 76.4°, 92.95° and 98.45°) as well as small broad reflections attributed to nickel oxide (2θ = 37.2°, 42.6° and 62.8°). Although present in high concentration (Cr: 12.8 wt%), reflections attributed to Cr 2 O 3 (JCPDS 381479) could not be observed, indicating a high dispersion of chromium oxide or an amorphous chromium phase [17,39,40]. After the reaction, the catalysts show sharper reflections attributed to metallic nickel for both conditions tested, indicating the reduction of the nickel oxide under the reductive conditions of the upgrading reactions.…”

Evaluation of High-Loaded Ni-Based Catalysts for Upgrading Fast Pyrolysis Bio-Oil

“…Ethane, propane, n-butane, and i-butane were also produced in higher concentrations in comparison to 1-step reactions, whereas CO 2 was minimized in comparison to 1-step reactions, in agreement with previous studies [69]. The highest cracking activity observed for the Ni-Cr catalyst may be attributed to strong acid sites, reported to be very active for cracking reactions [41].…”

“…The Ni-based catalysts were characterized in detail by a variety of analytical techniques in order to correlate the catalytic activity and their characteristics. Considering that both Ni-based catalysts consist of metallic nickel and nickel oxide, the oxidation state transformation over reductive atmosphere was followed by in-situ X-ray absorption spectroscopy (XAS), as depicted in Figure 4 for the Ni-Cr catalyst may be attributed to strong acid sites, reported to be very active for cracking reactions [41]. Some assumptions arise with regard to the high amount of gaseous hydrocarbon formed; the high temperature is reportedly attributed to the increase in yields of gaseous products [37], which is in agreement to the highest gas yield (3.9%) at 325 °C among all reactions.…”