(739g) Shining Light on Hydrotreating Catalyst Activation

Summary. The activation of Co-Mo hydrotreating
catalysts was followed by in situ XAS under conditions close to industrial
practice. We demonstrate that active phase genesis strongly depends on
activation parameters and precursor composition. The structure of the active
phase is correlated to activity trends to obtain structure-performance
relations in the hydrodesulfurzation of gas-oil.

Introduction

Two-dimensional
transition metal sulfides are chemically versatile and have a wide range of
applications in catalysis, opto-electronics and batteries [1]. MoS₂
can provide a low cost alternative to Pt as hydrogen evolution catalyst and has
been applied successfully for many years in oil-refining to catalytically
remove sulfur from oil. In hydrodesulfurization (HDS) catalysts, the electronic
properties of MoS₂ are tuned by the addition of Co/Ni, improving the
intrinsic activity by about one order of magnitude. Studies on model systems,
often performed under reaction conditions deviating substantially from those in
industrial practice, have led to the Co-Mo-S model with Co/Ni atoms located at
the edges of MoS₂ nanosheets [2]. A drawback of these model studies
is that trends in model HDS reactions using thiophene or dibenzothiophene (DBT)
usually do not match with those observed in industrial gas-oil HDS tests.
Consequently, structure-performance relations at the lab scale are not well
suited to predict performance under real HDS conditions (20-60 bar, 350-400°C,
gas-oil feed).

In
the present work, we follow the activation of Co-Mo HDS catalysts by in situ
X-ray absorption spectroscopy (XAS) under conditions close to industrial
practice (20 bar, 350°C, model diesel feed). First, strategies for measuring in
situ XAS in gas-liquid-solid systems are discussed. Then, by applying this
methodology, we demonstrate that the temperature of the oxide-to-sulfide phase
transition in HDT catalysts strongly depends on activation parameters
(pressure, sulfiding agent) and the composition of the catalyst precursor. The
influence of a range of organic additives with varying chelating propensity for
the precursor metals on the catalytic performance in model and gas-oil HDS
reactions is determined. By correlating these activity trends to the structure of
the active phase, we obtain deeper insight into structure-performance relations
in real gas-oil HDS.

Experimental

High
metal loading catalyst precursors (~20 wt%), representative of commercial ones,
were prepared via impregnation of γ-alumina with an aqueous solution of Co
and Mo salts and various additives such as phosphoric acid (P), polyethylene
glycol (PEG), citric acid (Cit) and nitrilotriacetic acid (NTA). The dried or
calcined precursors were activated in H₂/H₂S (10%) or H₂
and an organosulfide dissolved in n-hexadecane at 1 or 20 bar pressure. The
sulfided catalysts were characterized by XAS, XPS and TEM; their performance
was evaluated in thiophene, DBT and gas-oil HDS reactions. XAS experiments were
performed at the Co K and Mo K edge with a homemade in situ cell at
BM26A of the ESRF.

Results and discussion

Figure
1 shows that the Co sulfide phase forms in a very short timespan (< 5 min)
when Co-Mo HDS catalysts are activated under conditions similar to industry (20
bar H₂, model diesel feed).  This ?ignition' is very different from
the gradual sulfide phase formation, which is usually observed under model
laboratory conditions (1 bar H₂/H₂S); it  emphasizes that
active phase genesis should be monitored under conditions
as close as possible to commercial practice [3]. The rapid phase transition can be ascribed to in situ generation of H₂S by the
decomposition of organosulfides in the feed at elevated temperatures, where Co
would already be sulfided in a conventional H₂/H₂S
sulfidation procedure.

Fig.1 a) in situ Co K
XANES of a Co-Mo catalyst heated in a mixture of
n-hexadecane/tert-nonylpolysulfide(5%) and H₂ gas (20 ml/min) at 20
bar. The time per spectrum was 5 minutes. b) composition of the catalyst as
determined by LCF. c) picture of the experimental setup.

In dried and
calcined catalyst precursors prepared without additives, a significant portion
of the Mo remains oxidic (20-25%) and Co preferentially forms Co₉S₈,
which is undesired because of its low HDS activity. Catalytic performance was
strongly improved by incorporating additives in the impregnation step, which
facilitated the formation of MoS₂. The use of weakly chelating
additives led to the highest HDS activity in model compound reactions (DBT, thiophene),
which can be ascribed to optimal Co-Mo interaction. These catalysts were found
to form sulfides at temperatures as low as 50°C; this finding contradicts the
general notion that delayed sulfidation of the promoter leads to catalysts with
optimal Co-Mo interaction [3]. We propose that the reducibility of Mo in the
precursor is the key parameter that determines the efficient incorporation of
the promoter ions into the active ?Co-Mo-S' phase.

Conclusions

We have for the
first time characterized the structure of the active phase for a suite of Co-Mo
catalysts under real HDS conditions via in situ XAS. Activation in a
model diesel feed led to rapid active phase genesis , initiated by thermal
decomposition of organosulfides in the feed. An optimum interaction of the
promoter and the active phase was achieved for catalyst precursors prepared
with weakly chelating ligands. It is proposed that the reducibility of Mo in
the precursor is the key parameter that determines the efficient formation of
the active ?Co-Mo-S' phase.

References

[1]          M. Chhowalla et al., Nature
Chem. 5 (2013) 263.

[2]          Y. Zhu et al., Angew. Chem. Int. Ed. 53 (2014) 10723.

[3]          R. Cattaneo et al., J.
Catal. 191 (2000) 225.