Propane oxidation by vanadium supported on activated
carbon from sugarcane straw
Virgílio J. M F. Netoa, Thiago de S. Belana, André L. L. Magalhãesa, Alexandre B. Gaspara, Paulo G. Pries
de Oliveiraa, Fabiana M. T. Mendes a*.
a Nacional
Institute of Technology, Catalysis and Chemical Process Division- DCAP, Rio de Janeiro, 20081-312, Brazil
*Corresponding author: [email protected] *
Keywords: activated carbon, sugarcane straw, propane oxidation, support acid–base character.
1. Introduction
In Brazil, the sugar and alcohol industry generates
around 597 million tons of waste per year, such as
bagasse and straw from sugarcane [1]. The valuation
of these wastes using them as a renewable raw
material is extremely important. In a near future, the
sugarcane harvest will be completely converted from
manual to mechanized, which will increase the
amount of this waste [2]. The use of biomass to
produce activated carbon and its market is well
established and allows its widespread use in the
environmental, industrial and other sectors for
removing, recovering, separating and modifying a
variety of species in liquid- and gas-phase
applications. The physicochemical characteristics of
the activated carbon material has motivated recent
studies involving synthesis and application also in
different catalytic processes, especially, in Propane
Oxidation Reaction [3, 4]. These researchers used
for the first time an activated carbon obtained from
orange skin residue to support vanadium active sites
and got promising catalyst activity, although little is
still known. New efforts, but using sugarcane straw
instead to obtain a new activated carbon support has
been conducted [5]. Besides, when using an
activated carbon as support, several physical
chemical properties, such as surface area, pore
distribution and surface acidity, out to be
investigated. The structure of activated carbon
supported vanadia catalyst have not been fully
explored, as for alumina support [6] and there are no
reports at all, when using it prepared from sugarcane
straw. Propane Oxidation Reaction has been studied
for many decades [7] and still is a challenge
regarding feasible and economic innovation, which
goes through the search for a higher yield of
propylene or oxy-compounds. Vanadium based
catalysts are recognized to provide sites that activate
the CH bond, form oxygen intermediates and avoid
the formation of CO and CO2. However, the
structure and redox properties of VOx surface
species can govern the reaction mechanism, being
more active at submonolayer coverage [7], where
isolated tetrahedral VOx species are present. On the
other hand, the acid–base character of the support
can
control
the
vanadium
catalyst
reactivity/selectivity and influence both the reactants
and product adsorption/desorption. A support with
low acidity, with dominant Brøns andted acid sites,
usually facilitate rapid propylene desorption and
prevents its further oxidation to COx. As activated
carbon surface acid groups, can significantly vary
per the biomass source used in its preparation, so,
the support acid–base character and strength, must
be investigated. The present work aims to evaluate
the new prepared activated carbon (from sugarcane
straw) as a support for vanadium sites towards
propane oxidation reaction. Furthermore, compare
some of important properties of the new support
with that presented by industrial produced activated
carbon. The discussion is conducted in terms of their
textural properties and nature of surface functional
groups.
2. Experimental
Sugar cane straw was chemically activated with a
solution 85% (w/w) H3PO4 accordingly with the
methodology previously described [5], obtaining
("SAC") activated carbon. A commercial activated
carbon purchased from Vetec ("VAC") and other,
from Alphacarbo Company, ("AAC"), were also
used as support. A solution containing vanadium
precursor (NH4VO3) and oxalic acid was used to
prepare 3wt% VOx supported catalysts. The
materials were calcined under synthetic air flow (40
ml/min) up to 500°C (5°C/min), and kept for 5h. N2
adsorption-desorption experiments ware done with
ASAP 2020 equipment (Micromeritics). Boehm
titration [8] was performed to verify the activated
carbon surface acid groups. Thermal desorption of
NH3 was done with He flow (30mL/min) up to
500°C, at 10 °C/min. The total number of acid sites
were determined using a thermo conductive detector
(TCD). Chemical analysis and micrographs were
obtained with a scanning electron microscope with
energy dispersive analysis (SEM-EDS- Inspect S50
FEI). Propane oxidation reaction was conducted in a
conventional fixed bed reactor at 380 °C, coupled
with a gas chromatograph with FID and TCD
detectors. Reaction conditions were 13% C3H6, 18%
O2, 69% N2, volume), at a total flow of 35 ml/min,
with the O/C3 ratio of 1.3.
3. Results and discussion
Table 1 shows the activated carbons characterization.
The isotherm of all materials exhibits a modified
type II isotherm, with a hysteresis loop, suggesting a
broad pore size distribution. A high specific surface
area activated carbon can be obtained from
sugarcane straw (SAC), which has a quite similar
surface area than the one from Alphacarbo, both
being higher the one from Vetec. SAC and AAC
activated carbon materials exhibits comparable
surface area and particle size, but SAC with a little
greater pore volume than the commercial materials.
Vanadium catalyst presents a drop down in surface
area (around 800m2/g), but no major changes in pore
volume. Bohem titration revels the major presence
of carboxylic groups for SAC support, while
commercial support have mainly phenolic surface
groups. TPD-NH3 analysis, reinforces the higher
acidic character of the activated carbon (SAC and
AAC) surface. The activated carbon from Vetec is
the one with which has the less acid character
(nature and quantity).
Table 1. Texture properties, Boehm titration and NH3-TPD
Boehm
NH3-TPD
SBET
Vp
(m Eq/g C)
(mmols/g)
Carbon
2
3
(m /g) (cm /g)
Ph/L/C*
SAC
1279
1.3
0.19/0.15/0.97
0,57
AAC
1343
1.0
0.80/0.14/0.36
0,85
VAC
785
0.2
0.34/0.05/0.03
0,24
*Ph: phenolic, L: lactonic, C: carboxylic
The low vanadium loading, in present paper, was
intended to keep vanadium species far from
theoretical monolayer coverage (2.9 Vatoms/nm2)
aiming to understand support surface influence on
reaction activity/selectivity. Ghamdi et al [6] reports
that vanadia species are more selective towards
propylene, but less active and upon an increasing
amount, polyvanadates dominates the surface
species, enhancing activity and decreasing
selectivity. Figure 1 shows Propane conversion as a
function of time. All catalysts were capable to
convert propane molecule to propylene. However,
the obtained low propylene and high COx selectivity,
can be related to the support surface acidity, as well
as the amount and type of surface oxygen available.
At a very low vanadium surface density (around 0.1
V atoms/nm2), support acidity plays the main role,
controlling the catalyst activity and selectivity.
Catalysts with stronger surface acid groups favor
propylene further oxidation into combustion
products, but these acidic sites are needed to
maintain activity.
Figure 1 – Propane conversion versus reaction time
4. Conclusions.
In this work, for the first time, activated carbon
obtained from sugarcane straw is used as vanadium
support. The obtained catalysts are active for
propane conversion. Surface properties of these
activated carbon materials controls selectivity,
especially at very low vanadium density. Even
though vanadium sites are capable to convert
propane into propylene, the strong acid sites from
the support surface seems to retain propylene,
converting it to fully oxidation COx compounds.
Furthermore, activated carbon obtained from
sugarcane straw has quite similar texture properties
than the industrial one from Alphacarbo, but distinct
properties with respect to the one from Vetec,
especially surface and surface acidity. All these
findings stimulate its application as a catalysts
support in oxidation catalysis. The possibility of
waste (sugarcane straw) valorization opens a range
of opportunities.
Acknowledgments
We acknowledge the financial support from our sponsors,
CNPq-PCI and CIEE and The Alphacarbo Co and Biocatalyis
Laboratory (LABIC-DCAP).
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