Preparation of highly active NiW hydrotreating model catalysts with
1,2-cyclohexanediamine-N,N,NANA-tetraacetic acid (CyDTA) as a chelating
agent
G. Kishan, L. Coulier, V. H. J. de Beer, J. A. R. van Veen and J. W. Niemantsverdriet*
Schuit Institute of Catalysis, Eindhoven University of Technology, 5600 MB Eindhoven, The Netherlands.
E-mail: j.w.niemantsverdriet@tue.nl
Received (in Oxford, UK) 25th April 2000, Accepted 16th May 2000
Published on the Web 8th June 2000
Changing the order in which oxidic W and Ni convert to
sulfides by adding 1,2-cyclohexanediamine-N,N,NA,NA-tet-
raacetic acid (CyDTA) as a chelating agent for nickel in the
preparation of NiWS–SiO2 catalysts is the key ingredient in
obtaining a high activity for thiophene hydrodesulfuriza-
tion.
30 min. Next, the sample was cooled to room temperature under
helium and transported to the XPS under N2 atmosphere.
XPS spectra were obtained on a VG Escalab 200MK
spectrometer, at a constant pass energy of 20 eV. Binding
energies were corrected with reference to the Si 2p peak of SiO2
at 103.3 eV.
Thiophene hydrodesulfurization was carried out with 5 cm2
of model catalyst in a microflow reactor operating in batch
mode (1.5 bar, 400 °C, with 4% thiophene in H2), after
sulfidation at 400 °C. Gas samples were taken with a syringe for
gas chromatograph (GC) analysis of the products. All activity
results presented are the average of at least six different
measurements, which showed good reproducibility.
Fig. 1 shows W 4f and Ni 2p XPS spectra of the NiW-
CyDTA–SiO2 model catalyst after sulfidation at the indicated
temperatures. The W 4f spectrum of the fresh catalyst shows a
doublet at 35.6 eV characteristic of W(VI) oxide.15 The small
peak at 41.6 eV corresponds to the W 5p3/2 state. As Fig. 1
shows, sulfidation of W starts at 150 °C and is completed at 300
°C, where the W 4f spectrum shows a doublet at 32.6 eV,
characteristic of WS2.15 Sulfidation of a Ni-free W–SiO2
sample (not shown) proceeded similarly to the progression
shown in Fig. 1A. Addition of CyDTA to W–SiO2 in the
preparation had no measurable effect on the sulfidation rate of
tungsten.
Sulfidation of Ni, however, is greatly affected by CyDTA.
The Ni 2p spectrum of fresh NiW-CyDTA–SiO2 (Fig. 1B)
exhibits the pattern characteristic of Ni2+, with the Ni 2p3/2 peak
at 855.4 eV accompanied by shake-up features.15 The Ni 2p3/2
binding energy, however, is 1.5 eV lower than that of Ni in
CyDTA-free Ni–SiO2 and NiW–SiO2 catalysts, evidencing
complexation of nickel by CyDTA. As Fig. 1B shows, the
complexed nickel is stable in H2S–H2 upto temperatures just
below 250 °C. In contrast, NiO reacts in H2S at room
temperature, and conversion to nickel sulfide (Ni 2p3/2 binding
energy of 854.0 eV15) is complete at 100 °C. The same is true
for Ni sulfidation in NiW–SiO2 catalysts, however, the Ni 2p3/2
peak shifts to a higher binding energy of 854.5 eV after
complete sulfidation of W at sulfidation temperatures around
300 °C. We take the additional 0.5 eV shift of the Ni 2p3/2 peak
as evidence that the initially formed Ni3S2 rearranges, and
redisperses over the reactive edges of the WS2 slabs, as
proposed by Reinhoudt et al.11
Stringent future legislation for low-sulfur diesel fuel places
increasingly higher demands on the performance of hydro-
treating catalysts.1,2 Supported sulfides of molybdenum or
tungsten promoted with nickel or cobalt are widely applied to
this end.2 In the preparation, sulfidation of the oxidic precursors
in a mixture of H2S–H2 or in the sulfur-containing hydrocarbon
feed is an essential step. The sulfidation of supported MoO3 and
oxidic CoMo and NiMo catalysts has received considerable
attention.3–10 The generally accepted view is that in order to
obtain active CoMoS and NiMoS catalysts, sulfidation of
molybdenum should precede that of cobalt and nickel, such that
the reactive edges of the MoS2 slabs can serve as anchoring sites
for the promoter atoms.8–10 Chelating agents such as nitrilo-
triacetic acid (NTA)9,10 and ethylenediaminetetraacetic acid
(EDTA)4,5 assist in stabilizing nickel and cobalt, such that their
conversion to sulfides is retarded with respect to molybde-
num.
Much less is known about tungsten sulfidation. Tungsten
oxide is more difficult to convert to sulfides than molybde-
num.3,11 Shimizu et al.12 report that chelating agents improve
the activity of NiW–Al2O3 catalysts in hydrodesulfurization of
benzothiophene and hydrogenation of o-xylene. The origin of
the promotional effect is not clear, however.
Our purpose is to demonstrate that addition of 1,2-cyclohex-
anediamine-N,N,NA,NA-tetraacetic acid (CyDTA) as a chelating
agent for Ni in the preparation of NiWS–SiO2 catalysts leads to
a 2.3-fold increase in thiophene hydrodesulfurization activity,
as compared to a standard NiWS–SiO2 catalyst. We correlate
the catalytic activity with the order in which nickel and tungsten
convert to sulfides, as measured by X-ray photoelectron
spectroscopy (XPS). To measure XPS spectra at improved
resolution, we used model supports consisting of a thin
hydrophilic SiO2 layer on a silicon substrate.13
SiO2 supports were prepared by oxidizing Si(100) at 750 °C
for 24 h in air. Oxidized wafers were cleaned in H2O2–NH3(aq)
(3+2 v/v) at 65 °C and hydroxylated in boiling water for 30 min.
The sample was covered with an aqueous solution of ammo-
nium metatungstate (Merck), nickel nitrate [Ni(NO3)2·6H2O;
Merck] and 1,2-cyclohexanediamine-N,N,NA,NA-tetraacetic acid
(C14H22N2O8·H2O; Merck) and spin coated under N2 at 2800
rpm.14 Concentrations were adjusted to result in loadings of 6 W
atoms nm22 and variable Ni loading between 1 and 6 atoms
nm22. The amount of CyDTA added was equal to the amount of
Ni present. Catalysts prepared without CyDTA were calcined
(500 °C, 30 min), whereas NiW-CyDTA–SiO2 catalysts were
used without calcination. Sulfidation was carried out in a glass
tube reactor with 10% H2S in H2 at a heating rate of 5 °C min21
(2 °C min21 for NiW-CyDTA–SiO2) to the desired tem-
perature, after which samples were kept at that temperature for
Fig. 1B shows that CyDTA retards the sulfidation of Ni to ca.
250–300 °C. This temperature range coincides with the
disappearance of the N 1s signal characteristic of the CyDTA
ligand, indicating that the decomposition of the latter deter-
mines the rate of Ni sulfidation.
The activity of the catalysts for thiophene hydrodesulfuriza-
tion is shown in Fig. 2. The blank silica support in the reactor
has an order of magnitude lower activity than W–SiO2, and
shows mainly cracking products. The W–SiO2 catalyst calcined
at 500 °C is detectably active, with a product distribution
showing predominantly 1-butene as the primary product. The
Ni–SiO2 catalyst calcined at 500 °C exhibits lower activity.
Synergism is clearly observed in the standard NiW–SiO2 model
DOI: 10.1039/b003272g
Chem. Commun., 2000, 1103–1104
This journal is © The Royal Society of Chemistry 2000
1103
Kishan
