Regeneration of NiAl2O4 spinel type catalysts used in the reforming of raw bio-oil

Abstract

The regenerability of Ni catalysts in reforming reactions is a key factor for process viability. Accordingly, this study addresses the regeneration of two spinel NiAl 2 O 4 type catalysts by reaction-regeneration cycles in the oxidative steam reforming (OSR) of raw bio-oil. The spinel type catalysts were prepared by different methods including a supported Ni/La 2 O 3 -αAl 2 O 3 catalyst and a bulk NiAl 2 O 4 catalyst. The experimental setup consists of two units connected in series for i) the thermal treatment of bio-oil at 500 ºC, in order to control the deposition of pyrolytic lignin, followed by; ii) the oxidative steam reforming (OSR) of the remaining oxygenates in a fluidized bed catalytic reactor.The conditions in the OSR reaction step were: 700 ºC; oxygen/steam/carbon ratio (O/S/C), 0.34/6/1; space time, 0.75 g catalyst h/g bio-oil (for supported catalyst) and 0.15 g catalyst h/g bio-oil (for bulk catalyst). Three different strategies have been studied in the regeneration step by coke combustion, including the in situ regeneration inside the reactor at 650 ºC and 850 ºC, and the ex situ regeneration in an external oven at 850 ºC, for 4 h in all the cases. The behavior of the fresh and regenerated catalysts has been explained according to their metallic properties, determined by different characterization techniques (temperature programmed reduction (TPR), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and transmission electronic microscopy (TEM)). According to these results, the combustion ex situ of the catalyst at 850 °C is able to completely regenerate the bulk catalyst, since these

Oxidative Steam Reforming of Raw Bio-Oil over Supported and Bulk Ni Catalysts for Hydrogen Production

“…As observed in Figure 10, there are differences between the fresh and the regenerated catalysts; in the latter, the presence of LaNiO3 is observed (whose lower activity compared to La2NiO4 has been proven in Section 2. The results in Figure 8 are in agreement with the results previously obtained by Remiro et al [66] for the bulk spinel catalyst prepared by the co-precipitation method, and consequently, it can be deduced that the total activity recovery subsequent to the regeneration by coke combustion at 850 • C in the external oven (in an air atmosphere, without gas flow) is a general property of the bulk NiAl 2 O 4 spinel catalysts, regardless of the method for synthetizing the fresh catalyst.…”

“…Remiro et al [66] have proven the relevance of the operating conditions in the regeneration step by coke combustion on the metallic properties of two NiAl 2 O 4 spinel type catalysts (one of them obtained by calcination at 850 • C of a Ni/La 2 O 3 -Al 2 O 3 supported catalyst and the other was a bulk catalyst prepared by co-precipitation and also calcined at 850 • C) used in the OSR of raw bio-oil. It was proven that the temperature and the gas-solid contact in the regeneration by coke combustion have a significant effect in the formation of different oxidized Ni species (NiO and NiAl 2 O 4 ), which is a key factor for the redispersion of the Ni 0 active sites after the reduction and, consequently, for the recovery of the catalyst's activity.…”

“…The results in Section 2.3 prove that the total removal of coke is not enough for the recovery of the activity of the Ni catalysts used in the OSR of raw bio-oil, which should be attributed to the existence of other deactivation causes besides coke deposition, such as metal sintering or changes in the metal species. The sintering of Ni at 700 • C has been previously reported for Ni/LaAl catalyst used in the SR of bio-oil [15,61] for the commercial G90 catalysts used in the SR of biomass pyrolysis volatiles [73] and for spinel NiAl 2 O 4 catalysts prepared by different methods and used in the OSR of bio-oil [66]. Besides the changes in the metallic structure originated in the reaction step, the effect of the operating conditions in the regeneration step upon the physical-chemical properties of the catalyst (especially the metal properties) should be also considered, as they would be responsible for the activity of the regenerated catalyst.…”

“…On the one hand, the existence of an additional deactivation cause, besides coke deposition, such as Ni sintering due to the high temperature and water content in the reaction medium [61,66]; on the other hand, there could be metal loss due to the detachment during coke combustion of some Ni 0 particles located at the edges of the coke filaments. Consequently, the regeneration treatment should be able not only to remove coke but also to re-disperse the Ni particles, and moreover, the loss of Ni during the regeneration should be avoided.…”

“…Recently, Remiro et al [66] have reported that a bulk NiAl 2 O 4 spinel catalyst prepared by co-precipitation completely recovered the activity of the fresh catalyst after a regeneration treatment consisting of coke combustion in an air atmosphere at 850 • C in an external oven. The activity recovery was attributed to the total recovery of the spinel structure of the fresh catalyst under these severe regeneration conditions, which favoured the reconstruction of the NiAl 2 O 4 spinel by reaction of NiO with Al 2 O 3 in the support.…”