4.2 Experimental
4.3.7 Spent catalysts characterization
All fresh and spent catalysts obtained from the 300 °C and 350 °C HDO tests were characterized for their crystalline structure by powder XRD measurement. Figure 4-12
illustrated XRD patterns for three representative catalysts (CoMo/MCM-41, CoMo/-
Al2O3 and Ru/C) before and after the bio-oil hydro-treatment tests at 300 °C and 350 °C.
Figure 4-12(A) and Figure 4-12(B) are the XRD patterns of fresh/regenerated/spent
CoMo/MCM-41 and fresh/spent CoMo/-Al2O3, respectively. For each fresh/regenerated
CoMo catalyst there are characteristic peaks detected which correspond to cobalt and
0 20 40 60 80 100 0 100 200 300 400 500 600 700 800 CoMo/SBA-15 CoMo/MCM-41 CoMo/Al2O3 CoMo/HZSM-5 CoMo/MCM-41-350C CoMo/Al2O3-350C (A) Temperature (C) M as s % 0 20 40 60 80 100 0 100 200 300 400 500 600 700 800 Fresh Ru/C Used Ru/C-300C Used Ru/C -350C Temperature (C) M ass % (B)
molybdenum oxides, as labeled in the Figure 4-12(A). For instance, the XRD peaks at 2 of 12.5° (0 2 0), 23.5° (1 1 0), 26.7° (0 4 0), 33.7° (1 1 1) and 39.0 (0 6 0) can be
attributed to the diffraction of the MoO3 phase (Ali et al., 2012; Cauzzi et al., 1999;
Mora et al., 2014; Nava et al., 2009a; Peña et al., 2014a, 2014b; Salerno, 2004; Thanabodeekij et al., 2007; Valencia and Klimova, 2011; Wang et al., 2001; Zepeda,
2008). The diffraction at 2 of 23.3°, 26.8° and 27.5° may also related to β-CoMoO4
phase (Nava et al., 2009b; Zepeda et al., 2006, 2005). From Figure 4-12(A), in the regenerated CoMo/MCM-41 catalyst, the MoO3 peaks are detected in much stronger signals than its fresh catalyst. The sharper and stronger XRD signals imply sintering of the species during the regeneration process, which would lead to the growth of the
crystalline sizes, increased crystallinity and poor dispersion of the MoO3 species in the
support. This poor catalyst dispersion for the regenerated CoMo/MCM-41 catalyst is
evidenced by its significantly reduced BET surface area (39 m2/g) compared with that of
the fresh CoMo/MCM-41 (140 m2/g), as shown in Table 4-5. In the spent catalysts of
both CoMo/MCM-41 and CoMo/-Al2O3, two broad diffraction peaks at 25.4° and 43.5°
were detected, which are corresponding to the (002) and (100) diffraction of carbon which are indication of the carbon/coke deposits on the spent catalysts (Poh et al., 2012; Tsubouchi et al., 2003). From the obtained XRD patterns for all spent catalysts, it can be seen that the diffraction lines of the Co and Mo oxides peaks are remarkably weakened or disappear, which could be covered and masked by the carbon/coke deposition during the upgrading process, which could deactivate the catalyst in the process. The severe
carbon/coke deposition on the spent catalysts of both CoMo/MCM-41 and CoMo/-Al2O3
is evidenced previously by TGA analysis (Figure 4-11(A)).
Figure 4-12(C) displays the XRD patterns of the fresh and spent Ru/C catalysts. In all
XRD plots, two broad and strong diffraction carbon peaks at 2 of 25.4° (002) and 43.5°
(100) were detected, which come from the diffraction of carbon support materials. In
both the fresh and spent Ru/C catalysts, a strong XRD peak at 2 of 28° and a weak
XRD peak at 2 of 59° were detected, ascribing to the diffraction of RuO2 (Hyun et al.,
2010; Okal, 2009). It is also observed that the XRD patterns of the fresh and spent Ru/C catalysts are very similar, and the diffraction lines of RuO2 species are still detectable in
the spent catalysts, implying that the coke deposits in the spent Ru/C catalysts (300C
and 350C) are not severe, which can be evidenced by the TGA results discussed
previously in Figure 4-11(B).
Figure 4-13 shows FESEM micrographs of fresh and regenerated CoMo/MCM-41 in
three different magnifications. By comparing fresh CoMo/MCM-41 catalyst
(Figure 4-13(A)) with regenerated CoMo/MCM-41 images (Figure 4-13(B)), it is clear
that the size and shape of MCM-41 crystal are almost of no change. The similar morphology of regenerated catalyst as that the fresh one, suggesting that the crystalline
structure of catalyst was retained after the bio-oil HDO process at 300C in supercritical
ethanol followed by regeneration, or the MCM-41 catalyst support can resist in
supercritical ethanol condition. Figure 4-14 represents FESEM micrographs of fresh and
spent CoMo/SBA-15 catalysts in three different magnifications. In the spent CoMo/SBA-
15 catalyst as illustrated in Figure 4-14(B) the surfaces of the SBA-15 crystals are more
rough and some spherical particles can be observed, which may represent carbon/coke deposits formed during the bio-oil HDO process (Hu et al., 2011; Wang et al., 2012). Comparing the FESEM images of fresh and spent CoMo/SBA-15 catalysts, the morphology of crystals is very similar, suggesting that the crystalline structure of
CoMo/SBA-15 was retained after the bio-oil HDO process at 300C in supercritical
ethanol. Thus, the mesoporous catalyst support materials SBA-15 or MCM-41 (as per
Figure 4-13) can resist in supercritical ethanol condition at 300 or 350C without
collapsing of their crystalline structure.
Comparing the FESEM images of the calcined mesoporous catalyst support materials
MCM-41 and SBA-15 (Figure 4-4) and the FESEM images of the CoMo loaded
fresh/regenerated/spent catalysts (Figure 4-13 and Figure 4-14), the particle sizes of the
metal loaded fresh/regenerated/spent catalysts are generally bigger than those of the calcined support materials. This suggests that the metal loading, calcination, and HDO tests could increase the particle sizes of the MCM-41 and SBA-15 crystals, as expected.
Figure 4-12: Powder XRD patterns of fresh and spent catalysts: (a) CoMo/MCM-41, (b)
CoMo/-Al2O3 and (c) Ru/C.
I n ten sity ( a. u .) In ten sity ( a. u .) In ten sity ( a. u .) (1): Fresh (b) (2): Spent at 300 °C (3): Spent at 350 °C (1): Fresh (a) (2): Regenerated (3): Reg - Spent at 300 °C (4): spent at 350 °C (1): Fresh (c) (2): Spent at 300 °C (3): Spent at 350 °C 10 20 30 40 50 60 70 80 90 10 20 30 40 50 60 70 80 90 50 2Ɵ (deg) 10 20 30 40 50 60 70 80 90 50 2Ɵ (deg) 50 2Ɵ (deg) Ru RuO2 Carbon MoO3 β-CoMoO4 -Al2O3 MoO3 β-CoMoO4
Table 4-5: Textural properties of fresh and regenerated CoMo/MCM-41
Figure 4-13: FESEM micrographs of fresh CoMo/MCM-41 (A) and regenerated
CoMo/MCM-41 (B) in three different magnifications. Sample Name BET surface area
(m²/g) Langmuir surface area (m²/g) Micropore volume (cm3/g) CoMo/MCM-41-fresh 140 194 0.005 CoMo/MCM-41-reg 39 53 0.005 (A) (A) (A) (B) (B) (B)
Figure 4-14: FESEM micrographs of (A) fresh CoMo/SBA-15 and (B) spent CoMo/SBA-15
catalysts (300C HDO test) in three different magnifications.
4.4
Conclusions
A catalyst screening study for the hydro-de-oxygenation of fast pyrolysis oil in supercritical ethanol solvent was accomplished using CoMo-supported on mesoporous
materials (SBA-15, MCM-41), commercial materials (HZSM-5, -Al2O3 and activated
carbon) as catalysts in comparison with a commercial catalyst of Ru/C. The HDO of the
(A) (B)
(A)
(A) (B)
pyrolysis oil (PO) with the mesoporous materials-supported CoMo catalysts at 300 and
350C effectively converted the PO into light oil (LO) fraction and a heavy oil (HO)
fraction. The following conclusions were obtained.
(1) The highest oil yield obtained was with Ru/C catalyst at both temperatures (66.6 wt% at 300 °C and 61.0 wt% at 350 °C) with the lowest and negligible coke formation (<1 wt%). Different from the performance of other catalysts that produced approx. 8-20 wt% HO in the process, Ru/C catalyst generated no HO fraction at both temperatures.
(2) The use of all mesoporous materials-supported CoMo catalysts produced almost similar yields of total oil products (HO+LO) as the Ru/C catalyst did. The total oil yield in all tests (although with different catalysts and temperatures) varies in a very narrow range of 55-65 wt%.
(3) Among all the supported CoMo catalysts, CoMo/MCM-41 produced the highest oil fraction OF (= LO + HO) yield at both temperatures (61.9 wt% at 300 °C and 57.8wt% at 350 °C). The spent CoMo/MCM-41 can be regenerated and the regenerated CoMo/MCM-41, produced similar oil yields as the fresh catalyst. Furthermore, the CoMo/MCM-41 catalyst produced HO and LO of the highest H/C, although of a similar O/C ratio.
(4) A higher hydro-treatment temperature (350 °C) remarkably increased the gas, coke and LO yields, accompanied by reducing the HO and AF yields, suggesting that a higher temperature promotes the gasification/hydro-cracking reactions (leading to more gas formation) and conversion of HO into LO and coke. With any catalyst, hydro-treatment at a higher temperature in supercritical ethanol although reduced the oil yields, but it led to LO products of increased H/C ratio and reduced O/C ratio and increased heating value HHV
(5) Compared with the elemental composition of PO feed, the HDO treatment produced LO and HO products of increased carbon and hydrogen contents
and decreased oxygen content. The HHV of the LO and HOs was improved to 31- 33 MJ/kg and 36-37 MJ/kg, respectively, compared with ~ 25 MJ/kg for the PO. The HO has very low O/C ratio <0.1.
(6) HDO of the PO at 300 ºC with all catalysts except Ru/C could not effectively reduce the molecular weight distribution of the bio-oil. However, at a higher temperature, 350 ºC, all catalysts were effective for reducing the molecular weight of the pyrolysis oil by HDO treatment.
(7) Among all catalysts tested, Ru/C catalyst produced the highest CH4 and CO2
yields, and consumed the largest amount of H2 at both temperatures, suggesting the highest activity of Ru/C for bio-oil HDO. The H2 consumption for all in-house prepared catalysts is similar, which might explain their similar performance in HDO of the PO according to the O/C ratios.
(8) The Ru/C catalyst has superb resistance to coking. However, for all CoMo- based catalysts, the amount of coke deposits during the HDO tests was significant, depending on the type of support. Two catalysts supported on mesoporous support (SBA and MCM-41) produced less coke in the bio-oil HDO process, compared with the catalysts supported on HZSM-5 and Al2O3. Moreover, the mesoporous catalyst support materials SBA-15 or MCM-41
and can resist in supercritical ethanol condition at 300 or 350C without
collapsing of their crystalline structure.
4.5
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