Y.K. Lugo-José et al. / Applied Catalysis A: General 469 (2014) 410–418
411
Table 1
Physical and chemical properties of M/SiO2.
Metal loading (wt%)
Catalyst
Mx/SiO2
Calcination
temp. (◦C)
Reduction temp. (◦C)
100% H2
Dispersion (%)
O2–H2 titr.
Particle
diameter (nm)
1.1
4.0
2.1
2.2
1.6
1.8
Pd
Pd*
Pt
Rh
Ru
Ni
300
–
–
–
300
450
350
350
300
300
300
450
3.9
55.0
11.7
21.0
11.7
4.8
31.4
2.0
9.7
5.3
11.4
22.6
*
Pd/SiO2 synthesized by SEA.
Eqs. (1)–(6) illustrate the hydrogenation, decarbonylation, and
decarboxylation pathways proposed for the HDO of organic acids
and esters. In addition, the CO2 and CO products from the decar-
boxylation (DCX) and decarbonylation (DCN) routes can undergo
further reaction through water–gas shift and methanation, con-
suming large amount of H2.
precursor and the –OH group on the support took place, allowing
a controlled impregnation with highly dispersed metal precursors
[34–36]. The catalysts prepared by SEA or incipient wetness were
dried overnight at 70 ◦C, followed by calcination in air and reduction
with 100% H2 at their respective temperatures for 2 h (see Table 1).
The exception was the 4 wt% Pd/SiO2, which was not subjected to
a calcination step due to the fact that the 1.1 wt% Pd/SiO2 catalyst
exhibited very large particle size. The commercial catalysts were
already prepared, and therefore it was decided that no calcination
step was required.
Hydrogenation : R1 − COO − R2 + 4H2 ↔ R1 − CH3
+ R2 − H + 2H2O
(1)
(2)
R − COOH + 3H2 ↔ R − CH3 + 2H2O
2.2. Catalyst synthesis (Pd/X)
Decarbonylation : R1 − COO − R2 + 2H2 ↔ R1 − H
+ R2 − H + CO + H2O
Three different palladium catalysts were investigated for the
detailed kinetics of the HDO of PAc: 4.0 wt% Pd/SiO2 (Aerosil 300,
SBET = 330 m2/g, Evonik), 5.0 wt% Pd/C (CP-97, SBET = 615 m2/g, com-
mercial BASF catalyst) and 2.3 wt% Pd/TiO2 (TiO2, SBET = 46.1 m2/g,
Evonik). The Pd/SiO2 and Pd/TiO2 catalysts were synthesized by
SEA, while the commercial Pd/C was supplied by BASF and followed
the same procedure described in the previous section.
(3)
(4)
R − COOH + H2 ↔ R − H + CO + H2O
Decarboxylation : R1 − COO − R2 + H2 ↔ R1 − H
+ R2 − H + CO2
(5)
(6)
R − COOH ↔ R − H + CO2
3. Pulsed H2 chemisorption
The present work aims to explore the catalytic chemistry of
the HDO of PAc (a model compound for aliphatic organic acids)
over supported group VIII noble metal catalysts. The monometallic
catalysts have been selected based on their potential to catalyze
different bond breaking events relevant to the above reactions. The
catalysts include Pd, Pt, Ru, Rh and Ni supported on SiO2. Addi-
tionally, Pd was screened on different supports (i.e. carbon, SiO2,
and TiO2) to investigate the influence of the support material on
activity, selectivity and activation energy. These studies provide
insight on the different reaction pathways to form paraffins/olefins
or oxygenated products from biomass derived organic acids.
The metal dispersions of the catalysts were determined by
pulsed hydrogen titration of oxygen pre-covered sites using a
Micromeritics 2920 AutoChem II Analyzer. Prior to the analysis,
the catalysts (0.1–0.2 g) were reduced in flowing 100% H2 for 2 h,
followed by purging with Ar for 2 h to remove any physisorbed
hydrogen. After cooling to 40 ◦C in flowing Ar, the catalysts were
exposed to 10% O2/He for 30 min followed by purging with Ar
(30 min). Titration with pulses of 10% H2/Ar was then employed
until no further H2 uptake occurred. This method was applied for
Pd, Pt, and Rh based catalysts. For Ru and Ni, a modified hydrogen
chemisorption method was implemented. The sample experienced
the same pretreatment described previously; however, no H2-
chemisorption at 40 ◦C was done. After being exposed to argon,
the sample was heated up to 100 ◦C by temperature programmed
oxidation, following by flushing with argon for 30 min. Then, the
sample temperature was raised up to 225 ◦C and exposed to 10%
H2/Ar. This titration temperature was confirmed based on the
results of temperature programmed oxidation/reduction described
in the Supplemental Information.
2. Experimental
2.1. Catalyst synthesis (M/SiO2)
For the screening reaction on M/SiO2 (M = Pd, Ru, Ni), the cat-
alysts were synthesized by incipient wetness utilizing aqueous
solutions of the following metal salts: palladium nitrate hydrate
(Pd(NO3)2·xH2O, 99.9 Sigma-Aldrich), ruthenium nitrosyl nitrate
(Ru(III)(NO)(NO3)3, 31.3% min Alfa Aesar) and nickel perchlorate
hexahydrate (Ni(II)(ClO4)2·6H2O, 99.99% Alfa Aesar). The SiO2 sup-
port (Silica Star, SBET = 100 m2/g) and the commercial catalysts of
2.14 wt% Pt and 2.06 wt% Rh over SiO2, were obtained from BASF.
In addition, a 4.0 wt% Pd catalyst with a different SiO2 support
(Aerosil 300, SBET = 330 m2/g, Evonik) was synthesized by strong
electrostatic adsorption (SEA), where the pH of the metal salt com-
plex solution (200 ppm of [Pd(NH3)4]2+Cl2, 99.9% Sigma Aldrich)
was controlled based on the PZC of the SiO2 [32,33]. Once the
pH of the solution is acquired, the support was impregnated and
shaken for 1 h. After the final pH was obtained, the difference (ꢀpH)
indicated that a strong electrostatic interaction between the metal
3.1. Catalyst evaluation
The activity and selectivity of each catalyst was measured for
the HDO of PAc. The experiments were carried out in a single pass,
in-situ at temperatures ranging from 200 to 400 ◦C at a total flow
of 50 sccm H2 for 2 h at 1 atm pressure. The reduction tempera-
ture was chosen depending on the different reduction treatments
required for each metal (Table 1). The feed stream for the screening
experiments consisted of 1.0% PAc (Alfa Aesar, 99%) and 20% H2,