P.K. Rakshit et al.
MolecularCatalysis448(2018)78–90
wetness technique over commercial silica-alumina support.
Tetraamineplatinum (II) nitrate (Alfa Aesar) was used as the precursor
for platinum and tin (II) acetate (Sigma Aldrich) as the precursor for tin.
Commercially available ultra – pure Silica-alumina (SA1) extrudates
containing 99% SiO2 and 1% alumina were procured from Alfa Aesar
and used as support. The SA1 was crushed and sieved for the desired
particle size of 0.4–0.6 mm. The Pt salt was dissolved in 1 mol/L HNO3
and tin salt was dissolved in 1:1 solution of glacial acetic acid and
water. The resulting solutions were co-impregnated on the support by
adding drop wise in a rotating round bottom flask. After impregnation,
the catalyst was aged for 4 h at atmospheric condition and then vacuum
dried in a Rota-vapor at 65 °C. The catalyst was further dried in an oven
at 85 °C for 12 h. The dried catalyst was calcined at 500 °C for 5 h to
obtain it in the final impregnated form. The catalysts prepared were
labeled as xPt-ySn-SAz where x and y are the weight percentages of Pt
and Sn respectively, with respect to weight of silica-alumina support
and z = 1,2 or 3 depending upon the support chosen. Six catalysts were
prepared namely, 1Sn-SA1, 1Pt-SA1, 3Pt-SA1, 3Pt-1.5Sn- SA1, 3Pt-3Sn-
SA1 and 3Pt-4.5Sn-SA1 to study the effect of Pt and Sn content on
catalyst performance. Reported work on similar catalysts was done with
total metal loading of 1–2% weight [24–26]. At low concentration le-
vels, the XRD profiles reported were not conclusive enough to identify
the phases of the metals. Hence, in the present work higher con-
centration of metals was impregnated on the support. In order to study
the effect of catalyst acidity on acetic acid hydrogenation, Pt-Sn bi-
metallic catalyst was also prepared using two more commercially
available SiO2-Al2O3 supports: SA2 (containing 95% SiO2 and 5% Al2O3
sourced from Sigma Aldrich) and SA3(containing 85% SiO2, 15% Al2O3
sourced from Saint Gobain). Supports of varying alumina content were
procured purposely to vary the total acidity of the catalyst and check
their effect on conversion and selectivity.
CH3COOH (g) + H2 → CH3CHO (g) + H2O (g) ΔRH0 = 24.5 kJ/mol
CH3COOH (g) + 4H2 → 2CH4 + 2H2O (g) ΔRH0 = −20 kJ/mol
(3)
(4)
Catalyst for acetic acid hydrogenation to ethanol is commonly
prepared by wet impregnation of noble metals on support materials
[20]. The support adsorbs the acetic acid molecules and hydrogenates it
with the dissociated hydrogen made available by the impregnated
metals [20,21]. Hence, the conversion of acetic acid to ethanol would
strongly depend on the ability of the metal to dissociate hydrogen,
ability of support to adsorb acetic acid, spillover of hydrogen to the
adsorbed acetic acid, acidity of support, and metal dispersion. Side
reactions (reactions (2)–(4)) to produce ethyl acetate, acetaldehyde and
methane are also reported to be feasible [20–23]. The selectivity to
these side products tend to increase with increase in temperature and
decrease in pressure [26]. Therefore, the catalyst must have the ability
to suppress these side reactions and improve ethanol selectivity at
lowest possible operating temperature and pressure.
Conversion of carboxylic acids to alcohols using noble metal based
heterogeneous catalyst system is known since long period of time [20].
Pt on oxide supports dissociates hydrogen and transfers it to adjacent
support sites via spillover mechanism, wherein, adsorbed acetic acid
molecule gets hydrogenated to produce aldehydes or alcohol [20,21].
Accordingly, noble metal catalysts, mainly Pt, emerged as an active
metal for acetic acid hydrogenation because of their ability to dissociate
and transfer hydrogen by spillover mechanism. Subsequently, literature
reported that bimetallic Pt-Sn catalysts tested at 270–350 °C tempera-
ture, 2–2.6 MPa pressure, WHSV = 0.6–3 h−1 and H2/acetic acid molar
ratio of 10–20 gave improved activity and selectivity for acetic acid
hydrogenation reaction as compared to monometallic catalysts owing
to improved dispersion of Pt and formation of Pt-Sn alloy. However,
even then, the activity and ethanol selectivity varied significantly with
variation in support material [24,26]. Catalytic performance of Pt-Sn
catalyst supported on carbon nanotubes (CNT), silicon carbide (SiC)
and few other oxide supports such as SiO2 and Al2O3 showed variable
yields [24–26]. It is evident that the support plays a key role in the
activity and selectivity of ethanol formation. Hence, the mechanism of
chemical interaction between support and reactants needs to be thor-
oughly examined. It has been reported that oxide supports with proton
donation ability can contribute towards esterification reaction of acetic
acid and ethanol leading to ethyl acetate production via reaction (2)
[27]. It is also reported that the lewis acid sites of the support can lead
to ethanol or ethyl acetate production competitively [25]. Hence, the
effect of support acidity on activity of acetic acid hydrogenation and
selectivity of ethanol needs to be studied. SiO2-Al2O3 is a well-accepted
support in industrial catalytic systems with tunable surface properties
for different applications and to our knowledge SiO2-Al2O3 supported
bimetallic Pt-Sn catalyst has not yet been studied for acetic acid hy-
drogenation.
2.2. Catalyst characterization
2.2.1. Powder X-ray diffraction (XRD)
The calcined forms of catalysts were characterized by powder X-ray
diffraction (XRD) analysis using a Rigaku miniflex X-ray diffractometer
using Ni filtered Cu Kα radiation (λ = 0.15406 nm) from 2θ = 6 to 85°,
at a scan rate of 2 min−1 with the beam voltage and a beam current of
30 kV and 15 mA respectively. Sample preparation for the X-ray ana-
lysis involved packing of approximately 0.3–0.5 g of powder into the
sample holder with light compression to make it flat and tight.
2.2.2. Temperature programmed desorption
NH3-TPD measurement of the catalyst samples was performed using
Thermo Fischer Scientific TPDRO (1100 series) analyzer to determine
the surface acidity. Approximately 150 mg of the sample was loaded in
a quartz cell and initially flushed with a He gas flow at 400 °C for 2 h,
cooled to 150 °C and then saturated with ammonia at the same tem-
perature for 30 min at a flow rate of 20 mL/min. After exposure to
ammonia, the catalyst samples were subsequently degassed with the He
flow and the temperature was raised up to 800 °C at a linear heating
rate of 10 °C/ min for measuring desorbed NH3 by using a thermal
conductivity detector (TCD). For TPD experiments with Isopropylamine
(IPA), degassed samples were first saturated with IPA using IPA pulses
coming from vapor generator kit operating at ambient temperature.
After saturation with IPA, the catalyst samples were subsequently de-
gassed with the He flow and the temperature was raised up to 800 °C at
a linear heating rate of 10 °C/ min for measuring the adsorbed IPA
decompose as propylene using thermal conductivity detector (TCD).
Hence, for the present work, SiO2-Al2O3 has been chosen as the
preferred support to study the performance of bimetallic Pt-Sn catalysts
for conversion of acetic acid to ethanol. The Pt-Sn catalysts were im-
pregnated on SiO2-Al2O3 support with varying silica and alumina
content, characterized and tested for activity and selectivity. A sys-
tematic study was carried out to study the effect of catalyst acidity and
reduction temperature on acetic acid conversion and ethanol se-
lectivity. Based on the results, an effort was made to establish the
structure – activity relationship to achieve high ethanol yield over
supported Pt-Sn catalysts.
2. Experimental section
2.2.3. Temperature programmed reduction study (TPR)
Reduction studies of the catalyst samples were performed on the
same instrument used for TPD. In this analysis, 80 mg of the sample was
loaded in an annular quartz reactor and was flushed with Ar gas at a
flow rate of 20 mL/min and at 200 °C for 2 h, following which the
2.1. Catalyst preparation
Supported bimetallic Pt–Sn catalysts were prepared using incipient
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