D.P. Gamliel et al.
Applied Catalysis A, General 559 (2018) 20–29
fraction [40]. The area under each peak is correlated to the con-
centration of each compound in the bio-oil. Anisole, 4-ethylphenol and
benzofuran make up a significant fraction of the bio-oil. Phenols, furans
and anisoles are often studied as bio-oil model compounds, in lieu of
actual bio-oil [14,41,42]. HDO of anisole, 4-ethylphenol and benzo-
furan will be the focus of this study. Use of model compounds simplifies
the analysis of reaction pathways, which may be convoluted by the
multitude of compounds present in the bio-oil.
2.3.2. HDO of anisole, 4-ethylphenol and benzofuran
Catalytic HDO of anisole, 4-ethylphenol and benzofuran was per-
formed in a stirred stainless steel autoclave reactor (100 mL, Parr
Instruments), outfitted with a gas entrainment impeller. HDO was
conducted using a similar protocol to our previous work [43]. Each
experiment was performed at a reaction temperature of 200 °C, pressure
of 750 psi, and a stir rate of 1100 rpm. Initially, the reactor contained
5
3.9 mL of 4 wt.% model compound diluted in decane, 0.1 g of reduced
catalyst and 1.1 mL of octane, which was used as an internal standard.
Prior to heating, the reactor was purged with Ar, and then H . The
reactor was pressurized to check for leaks, then purged and heated to
00 °C. After achieving the desired reaction temperature, it was pres-
Fig. 2. TPR profiles of Ni-USY, Ru-USY and Pd-USY.
2
3
. Results
.1. Metal impregnated USY zeolites
USY zeolite was impregnated with Ni, Ru and Pd metal precursors
and then calcined, producing metal oxide particles. TPR was performed
to determine the proper reduction temperature and to probe for sup-
port-metal interactions. The TPR profiles for Ni-USY, Pd-USY and Ru-
USY are shown in Fig. 2. Ni-USY exhibited a single TPR peak located at
90 °C. A Ni TPR temperature below 400 °C suggests that the Ni does
not interact with the support [48], and the sharp peak shape indicates
that the Ni particles are fairly uniform in size. Ru-USY reduced below
00 °C, with a high temperature peak located at 185 °C, and a lower
temperature shoulder at 150 °C. A similar reduction distribution was
observed by Qu et al. [49], who attributed the lower temperature peak
2
3
surized, the impeller was engaged and the reaction clock started. Initial
reactant concentration was measured directly after heating, such that
any conversion or product formation during heating could be accounted
2
for. H was continuously supplied to maintain reactor pressure for the
duration of the experiment. Liquid samples (∼2 mL) were collected
from the reactor using a custom-made sample entrainment system. The
sampling unit was purged with Ar prior to each sample to avoid ex-
posing the reactor contents to air. Liquid samples were collected every
3
20 min (unless otherwise noted), diluted in isopropanol and filtered to
remove catalyst. The total reaction duration was 140 min.
2
Product analysis was performed using gas chromatography
equipped with mass spectrometer (GC–MS, Agilent 6890 GC with 5673
MS), using a previously outlined method [43]. The system was cali-
brated externally using pure cyclohexane, octane, cyclohexanone, ani-
sole, phenol, 4-ethylphenol, benzofuran and guaiacol. Quantification of
compounds without direct calibration standards was accomplished by
the semi-quantification method, based on molecular weight and carbon
number of the specific compound. Model compound conversion and
product selectivity were calculated using Eqs. (1) and (2), respectively:
to highly dispersed RuO
2
nanoparticles and the higher temperature
peak to bulk RuO . Finally, Pd-USY had a broad reduction peak cen-
2
tered at a temperature of 90 °C. In the literature, this peak has been
ascribed either to Pd hydride decomposition [50,51], or reduction of
PdO nanoparticles [52]. PdO typically has a low reduction temperature,
close to room temperature [53]. Water formation was observed on the
MS during this experiment (ESI Fig. S1). Water evolution and the
concurrent weight loss of the PdO-USY sample suggests that this peak
was the result of PdO nanoparticle reduction. Nonetheless, the absence
of high temperature reduction peaks on any of the three materials
suggests that there are no strong interactions between the metals and
the USY support, but the broad Pd-USY and Ru-USY peaks indicate a
wider particle size distribution.
C
C
X
C
= 1−
C
C0
(1)
(2)
moli
molproducts
Si =
* 100
∑
Due to the large excess of hydrogen, the initial reaction rate was
Fig. 3 shows the XRD patterns of each Ni, Ru and Pd impregnated
USY zeolite in the oxide and reduced forms. Each pattern exhibits in-
tense peaks, representative of the faujisite crystal structure, and smaller
assumed to be first-order in model compound concentration and zero-
order in H
culated from the slope of a plot of ln(C
rate was calculated by multiplying the first order rate constant by the
initial concentration. Initial TOF [46,47] was then calculated by nor-
malizing the initial reaction rate to the number of metal surface sites, as
shown in Eq. (3).
2
concentration [44,45]. Overall rate constant (k) was cal-
/CC0) vs. time. Initial reaction
C
2
peaks due to metal crystallites. The patterns of NiO-USY, RuO -USY and
PdO-USY all contain features of the respective metal in the oxide form,
as indicated in Fig. 3. After reduction, the oxide peaks of all three
materials disappear in favor of strong peaks indicative of pure metallic
phases. This confirms successful reduction of the majority of metals
present, and that all three materials were not re-oxidized after exposure
to atmospheric conditions.
XPS is a technique for probing metal surface concentration and
electronic state. Fig. 4 shows the XPS spectra of the active metal region
and the metal/Si ratio of (A) Ni-USY, (B) Ru-USY and (C) Pd-USY after
reduction. The Ni 2p region, shown in Fig. 4(A) can be deconvoluted
into two doublet peaks, representing NiO and Ni° [54]. The spectrum
also contains a two satellite peaks at approximately 860 and 880 eV
Binding Energy (BE) [55]. It appears that the surface Ni in this catalyst
exists as a mixed metal and oxide, even after in-situ reduction at 450 °C
k * CC0 * Reactor Volume
TOF =
molsurface metal
(3)
Where molsurface metal is the moles of metal exposed to the surface, de-
termined by the catalyst mass, metal loading and metal dispersion.
Reproducibility of the experiment was determined by performing
HDO of each model compound using the Ni-USY catalyst in triplicate.
Reaction mechanisms for each model compound were proposed based
on the product evolution with time. Kinetic models for each mechanism
were fitted to the time-evolution data using a least-squares algorithm
written in MATLAB.
22