Scrap waste automotive converters as efficient catalysts for the continuous-flow hydrogenations of biomass derived chemicals

Article abstract of DOI:10.1039/c9gc04091a

The catalytic activity of scrap ceramic-cores of automotive catalytic converters (SCATs) was investigated in the continuous-flow hydrogenation of different biomass-derived chemicals. The waste SCAT powders were deeply characterized by ICP-MS, TGA, MP-AES, XRD, N₂ physisorption, TPR, HRTEM and EDS before and after utilization as a catalyst. The hydrogenation reactions of isopulegol to menthol, cinnamyl alcohol to hydrocinnamyl alcohol, isoeugenol to dihydroeugenol, vanillin to vanillyl alcohol and benzaldehyde to benzyl alcohol were performed studying the influence of various reaction parameters (temperature, pressure, flow rate and concentration of the starting material) on the final yields. The outstanding performance and stability obtained for the low metal content of waste-derived catalysts can be attributed to the co-presence of different noble metals as well as to the composite structure itself.

Full text of DOI:10.1039/c9gc04091a

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DOI: 10.1039/C9GC04091A
ARTICLE
Scrap waste automotive converters as efficient catalysts for the
continuous-flow hydrogenations of biomass derived chemicals
a
a
b
b,c,d
and Rafael Luque*a,e
Camilla Maria Cova, Alessio Zuliani, Roberta Manno, Victor Sebastian
Received 00th January 20xx,
Accepted 00th January 20xx
The catalytic activity of the scrap ceramic-core of automotive catalytic converters (SCATs) was investigated in the
continuous-flow hydrogenation of different biomass-derived chemicals. The waste SCATs powders were deeply
DOI: 10.1039/x0xx00000x
2
characterized by ICP-MS, TGA, MP-AES, XRD, N physisorption, TPR, HRTEM and EDS before and after the utilization as
catalyst. The hydrogenation reactions of isopulegol to menthol; cinnamyl alcohol to hydrocinnamyl alcohol; isoeugenol to
dihydroeugenol; vanillin to vanillyl alcohol and benzaldehyde to benzyl alcohol were performed studying the influence on
of various reaction parameters (temperature, pressure, flow rate and concentration of the starting material) on the final
yields.The outstanding performance and stability obtained for the low metal content of waste-derived catalysts can be
attributed to the co-presence of different noble metals as well as to the composite structure itself.
ceramic-cores of catalytic converters (CATs) derived from
end-of-life (ELV) vehicles offers a promising potential to be
employed as catalyst. More in detail, the direct utilization of
scrap ceramic-cores of CATs (denoted SCATs) as catalysts avoids
the production of additional waste or the requirement of
time-consuming processes. Furthermore, SCATs are largely
available and cheap, making them potentially commercially
competitive with traditional catalysts. In fact, every year,
end-of-life vehicles (ELVs) generate ca. 8-9 million tons of
valuable wastes in the European Union alone. The Directive
1
. Introduction
The design of inexpensive, selective and efficient catalytic
materials with low-environmental impact is of primary
importance for the development of sustainable industrial
processes.1 As a result, research has recently focused on the
synthesis of alternative environmentally friendly catalysts
having catalytic activity comparable to that of commercially
available systems. For example, Fe-Ni supported on a natural
zeolite has been proposed as alternative to noble metals for the
, 2
2
000/53/EC, otherwise referred to as "ELV Directive", seeks to
3
cyclization of levulinic acid to γ-valerolactone. Also, Sun et al.
dismantle and recycle end-of-life vehicles through more
designed a material based on CuNi nanoparticles assembled on
graphene, as valid catalyst for the hydrogenation of nitro/nitrile
compounds.4 More recently, Ni/ZrO
performances as catalyst for the selective hydrogenation of
fatty acids to alkanes and alcohols.
However, despite these promising results, the preparation of
sustainable catalysts frequently has certain limitations including
long preparation procedures or high production costs as well as
the side production of high amount of waste (high E-Factor).
Due to the content of noble metals and the presence of
different inorganic composites, the peculiar structure of
6
-7
sustainable and environmentally friendly procedures. The
directive fixes specific targets for recovery, recycling and reuse
of ELVs and their components. Between all ELVs valuable waste,
around 6 thousand tons of ceramic cores of scrap automotive
catalytic converters (SCATs) are yearly produced.
2
has shown good
5
The catalytic activity of SCATs rises from the strong stability of
the noble metals supported on the inorganic support. The
honeycomb structure of SCATs is composed of four principal
coats: the catalyst substrate, made of cordierite
(
2MgO·2Al
SiO -Al
material, a coat made of CeO
2
O
3
·5SiO
mixture), functioning as a carrier for the catalytic
or CeO /ZrO and a coat covered
2 2 3 2 2
), the wash coat (Al O , TiO , SiO or
2
2 3
O
2
2
2
with noble metals in low loadings including platinum and
palladium, and other metals, such as Fe, Mn, Ni and Ce.
Currently, SCATs are normally processed in order to extract and
recover the noble metals (Platinum group metals - PGMs).
However, these methodologies require the use of large
a. Departamento de Quımica Organica, Universidad de Cordoba, Edificio Marie-
8-10
Curie (C-3), Ctra Nnal IV-A, Km 396, Cordoba, Spain. E-mail: rafael.luque@uco.es
Nanoscience Institute of Aragon and Chemical and Environmental Engineering
b.
Department, University of Zaragoza, 50018 Zaragoza, Spain
Networking Research Center CIBER-BBN, 28029 Madrid, Spain
Aragón Materials Science Institute, ICMA, CSIC – University of Zaragoza, Pedro
c.
d.
amounts of highly toxic chemicals (i.e. inorganic acids) and long
Cerbuna 12, 50009 Zaragoza, Spain
Peoples Friendship University of Russia (RUDN University), 6 Miklukho Maklaya
e.
treatment times.
11-15
Moreover, since only the noble metals are
str., 117198, Moscow, Russia
recovered, these processes generate extensive volumes of
waste, as the SCATs matrix is not recycled. According to
literature, R&D is nowadays generally limited on diminishing the
utilization of inorganic acids or on enhancing the efficiency of
†Electronic Supplementary Information (ESI) available: GC chromatogram graph of
fluxed toluene in freshly made SCATs cartridge, TPR analysis of SCATs, ICP-MS
analysis, BET isotherm and XRD patterns of SCATs, photos of the heating units of
TM
the H-Cube® and X-Cube , GC-MS chromatogram graph of fluxed toluene in SCATs
cartridge after stability test.
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Journal Name
the extraction processes adding for example peroxides or using
2
. Experimental Sections
View Article Online
DOI: 10.1039/C9GC04091A
chelating agents.1
6-19
Only a few studies described SCATs as
) but
Ethanol (≥99.8%), acetone (≥99.5%), (−)-isopulegol (≥99%,
enantiomeric ratio: ≥99.5:0.5), (−)-menthol (≥99%, enantiomeric
ratio: ≥99.5:0.5), cinnamyl alcohol (98%), hydrocinnamyl
alcohol (≥98%), isoeugenol (98%), dihydroeugenol (≥99%),
vanillin (99%), vanillyl alcohol (≥98%), benzaldehyde (≥99%),
benzyl alcohol (99.8%) were purchased from Sigma-Aldrich Inc.,
St. Louis, MO, USA. All reagents were used without any further
purification. Scrap ceramic-cores of automotive catalytic
converters (SCATs) were collected from Provaluta España
Reciclaje de Metales, S.L., Córdoba (ES). SCATs were previously
smashed in Provaluta S.L. via a grinding process and provided in
the form of powders.
catalysts (e.g., for the reduction of nitroarenes with NaBH
4
the reactions to date have been strictly limited to batch
conditions.2
0-22
In the present work, SCATs were exploited as cheap, efficient
and environmentally friendly catalyst in a continuous flow
hydrogenation system. To the best of our knowledge, no similar
work has been previously described. In order to make SCATs
suitable for continuous flow equipment, a first cleaning
approach was necessary. Prior to the utilization, SCATs were in
fact washed under ultrasonication and subjected to a thermal
treatment in order to remove all surface residuals (gasoline,
oils, etc.) and clean. Remarkably, the direct reutilization of
SCATs as catalysts doesn’t generate substantial additional
waste (apart from the small amount of solvents used for the
2
.1 SCATs preparation
washing procedures), in significant contrast with metal Prior to the utilization as catalysts, the powders of scrap
extraction processes or costly metal/metal oxide catalyst ceramic-cores of automotive catalytic converters (SCATs),
preparations.
kindly donated by the company PROVALUTA S.L., were washed
The continuous flow system was selected to explore the activity and dried. Firstly, the powders were subjected to a sequence of
and particularly stability of SCATs in sustainable and washing cycles using hot water, ethanol, and acetone for 60’ in
cost-effective chemical synthesis. Indeed, flow-chemistry is one a US bath. The washed powders were filtrated, dried and
of the tools to achieve the United Nations Sustainable weighted after each cycle. When no loss of weight was
Development Goals (SDGs), whose target is reaching a greener observed, the washing cycles were stopped (5 cycles). A thermal
2
3
treatment was sequentially performed in order to clean the
and better future by 2030. Also, the IUPAC has recently
included flow chemistry between the top ten chemical surface of the metals and remove residuals. More in details,
innovations that will change our world in the future, with the the powders were heated for 1 h under N flux and for 3 h under
potential to make our planet sustainable. Finally, continuous- 10%vol H flux at 500 °C in a tubular furnace. Finally, the
4
1
2
2
flow reactors offer numerous advantages counting the powders were cooled down to room temperature under N
reduction of consumption by using small volumes of solvents, stream.
2
the reduction of the risk of handling dangerous chemicals and
the increased reaction selectivity and yields altogether with 2.2 SCATs characterization
easy scaling-up, responding to some of the key principles of
Green Chemistry (prevention of waste formation, less
hazardous reactions, safer chemicals, etc. ).
In order to prove the practical utilization of SCATs as catalysts,
different hydrogenations of biomass derived chemicals were
performed.
The thermogravimetric analyses (TGA) were carried out with a
Mettler Toledo TGA/SDTA 851 analyzer. The catalysts were
2
4-29
heated from 25 °C up to 900 °C at 5 °C min-1 in nitrogen
atmosphere (50 mL min-1).
TPR (Temperature programmed reduction) analysis was
performed with a ChemBET Pulsar TPR/TPD_Automated
Chemisorption Analyzer (Quantachrome Instruments). The
In particular, the hydrogenation of (−)-isopulegol to (−)-
menthol, widely used industrial synthesis, was chosen as the
initial test reaction.3 (−)-menthol (cooling minty flavor) is in
fact one of the most important chemicals, with a large range of
uses in pharmaceuticals, flavorings, cosmetics, agrochemicals
2
mesurment was performed operating under a 10% H flux (in
0-31
-1
-1
N
2
) of 20 ml min , heating the sample at 10 °C min up to
9
00 °C.
3
2-37
Powder X-ray diffraction (XRD) pattern was recorded with a
Bruker D8 DISCOVER A25 diffractometer (PanAnalytic/Philips,
Lelyweg, Almelo, The Netherlands) using CuKa (λ = 1.5418Å)
radiation. Wide angle scanning patterns were collected over a
and personal-care applications.
Nowadays, (−)-menthol is
principally synthesized through the myrcene process (Takasago
process), which last two steps include the cyclization of
3
8-40
citronellal and hydrogenation of isopulegol.
Sequentially, the selective hydrogenations of other important
2
θ range from 10° to 80° with a step size of 0.018° and counting
biomass-derived chemical for the food and fragrances time of 5 s per step.
industries were performed, highlighting the operative Textural properties of the sample were determined by N
2
conditions in order to maximise the final yields. More in details, physisorption using a Micromeritics ASAP 2020 automated
the hydrogenation of cinnamyl alcohol to hydrocinnamyl system (Micromeritics Instrument Corporation, Norcross, GA,
alcohol (sweet, balsamic and spicy flavour); isoeugenol to
dihydroeugenol (clove, spicy and peppery flavour); vanillin to
vanillyl alcohol (sweet, vanilla and caramelly flavour) and
benzaldehyde to benzyl alcohol (sweet, floral and fruity flavour)
were explored.
USA) using the Brunauer-Emmet-Teller (BET) and the Barret-
Joyner-Halenda (BJH) methods. The sample was outgassed for
= 10−2 Pa) and subsequently
2
4 h at 100 °C under vacuum (P
0
analysed.
2
| J. Name., 2012, 00, 1-3
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The metallic composition of the catalysts adopted was Further hydrogenation trials were performed in a combined
View Article Online
determined by Microwave Plasma Atomic Emission apparatus set up by linking an H-Cube®
Spectroscopy (MP-AES) (Agilent 4100 MP-AES).
Inc.) to an X-Cube™ (Thalesnano Inc.). More in details, the
representative analysis was carried out by digesting 20 mg of heating unit and the Cartrige holder of the instrument X-Cube™
D
M
O
i
I
n
: 1
i
0
P
.1lu03
s
9
(
/
T
C
h
9Gal
C
e
0
s
4
n
0
a
9
n
1
o
A
A
were used, while the hydrogen was generated in the H-Cube®
3
the catalyst with the addition of 5 mL of nitric acid (HNO ) and
equipment. This set up allows to set and control the
hydrochloric acid (HCl) in a volume ratio of 1:3, the mixture was
heated at 200 °C for 20 minutes in a microwave oven (Milestone
Ethos Plus). The digested sample was diluted with Milli-Q water
to obtaine a final volume of 30 mL, after filtration by hydrophilic
syringe filters of 0.2 µm to discard any fragmented particles.
Aberration corrected scanning transmission electron
microscopy (Cs-corrected STEM) images were acquired in a FEI
XFEG TITAN electron microscope operated at 300 kV equipped
with a CETCOR Cs-probe corrector from CEOS Company
allowing formation of an electron probe of 0.08 nm. A 10 μL
suspension of the sample was pipetted onto a TEM copper grid
having a continuous carbon film. After complete evaporation,
the sample was analyzed by High-angle annular dark-field
scanning transmission electron microscopy (STEM-HAADF).
2
temperature by the X-Cube™ and the H pressure and flow rate
by the H-Cube®.
The conversion and selectivity were analyzed by gas
chromatography (GC) in an Agilent 6890N gas chromatograph
−1
(
2
60 mL min N carrier flow, 20 psi column top head pressure)
using a flame ionization detector (FID). A capillary column
Agilent Technologies Inc. HP-5 (30 m × 0.32 mm × 0.25 μm) was
employed. In the case of the hydrogenation of (−)-isopulegol,
the formation of (−) menthol a capillary column Restek Rt®-
yDEXsa (30 m x 0.25 mm × 0.25 μm) was employed. The
retention times were confirmed using standards of the desired
products. Calibration curve was obtained with an internal
standard method using octane as standard. Standard solutions
of (−)-isopulegol / cinnamyl alcohol / isoeugenol / vanillin /
Elemental analysis was carried out with an EDS (EDAX) detector benzaldehyde (from 0.005 to 2 M) and octane in toluene were
2
which allows performing EDS experiments in the scanning analyzed by GC to give linear regressions with R > 0.999. In
mode. This analysis was conducted at the Laboratory of addition, the collected liquid fractions were analyzed by
Advanced Microscopies, LMA-INA-University of Zaragoza.
GC-MS—using the Agilent 7820A GC/5977B High Efficiency
Source (HES) MSD—in order to identify and confirm the
obtained products.
2
.3 Preparation of the cartridges
3
. Results and discussion
In order to perform the continuous flow tests, washed and
thermally-treated SCATs were charged in 30 mm-long 3.1 Characterization of the catalysts
ThalesNano CatCarts® or in 70 mm-long ThalesNano CatCarts®
In order to evaluate the washing/thermal cleaning procedure
sealed on both sides with sealings systems, made of graphite
filled PTFE sealing rings, stainless steel filters and PTFE
membranes (CatCarts® Thalesnano Inc.). The cartridges were
filled with 200 mg of SCATs (in 30 mm-long ThalesNano
CatCarts®) or 450 mg of SCATs (in 70 mm-long ThalesNano
CatCarts®). The cartridges were sequentially charge in the
hydrogenation systems and subjected to washing cycles with
toluene. After the washing cycles, fresh fluxed toluene was
analysed by GC (please see Fig. S1 in the ESI for a GC
chromatogram graph) and ICP-MS analysis in order to confirm
the cleanliness of the system and that no leaching of the
catalysts occurred.
and prove that no thermal leaching could occur during the
hydrogenation reactions, thermal gravimetric analysis were
performed on both the starting scrap core catalytic converter
powder (SCATs) and on the washed and thermally treated SCAT,
before and after utilization in the hydrogenation reactions. As
shown in Fig. 1, the loss in weight of the starting SCATs heated
up to 900 °C could be approximated to ~20%.
2
.4 Catalytic experiments
Catalytic performances of the catalyst were evaluated under
liquid phase continuous-flow conditions in an H-Cube® Mini
Plus flow hydrogenation reactor. The material was packed
(200 mg of SCATs per cartridg) in 30 mm-long ThalesNano
CatCarts® or (450 mg of SCATs per cartridge) in 70 mm-long
ThalesNano CatCarts®. The reaction solution in toluene was
pumped through and the reaction conditions were setted. The
required hydrogen was generated in situ during the reaction by
water electrolysis in the H-Cube equipment. The reactions were
performed for 120’, collecting samples every 15’ for further
analysis.
Fig. 1 TGA of starting SCATs, washed and thermally treated SCATs and washed
and thermally treated SCATs after hydrogenation reactions.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
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On the other hand, after the washing and thermal cleaning In the images reported in Fig. 2A, 2B and 2C, it is possible to
View Article Online
DOI: 10.1039/C9GC04091A
procedure, the SCATs didn’t show any relevant loss of weight observe the smashed laminar structures of SiO and Al O
2 2 3
(˂4% of weight loss above 600 °C, not relevant for the operative derived from the honeycomb structure of the catalytic
hydrogenation conditions). Furthermore, the used SCATs had converters. Bright contrast is observed for those atoms with
the same TGA profile of the freshly washed-thermal treated high atomic number (Z contrast). Energy-dispersive X-ray
SCATS, proving that no relevant changes in the structure of the spectroscopy analysis of selected surface areas, L1, L2 and L3 in
SCATs occurred.
Fig. 2C, can be associated with the presence of cerium oxide and
Additional TPR analysis of both washed and thermally treated Pt nanoparticles, as illustrated in Fig. 2D.
SCAT evidenced that the activation of the supported metals can
be centered at ca. 500 °C (please see ESI Fig. S2 for details). 3.2 Hydrogenation tests
Remarkably, this result demonstrated that is not possible to
Preliminary studies: hydrogenation of (−)-isopulegol. The
achieve an in-situ reduction of the metals under the operative
flow reaction conditions.
catalytic activity of SCATs was initially evaluated in an H-Cube®
Mini Plus flow hydrogenation reactor. Washed and thermally
treated SCATs were firstly packed (200 mg of powder per
cartridge) in 30 mm-long cartridge ThalesNano CatCarts®. The
hydrogenation of (−)-isopulegol to (−)-menthol was carried out
using a solution 20 mM of (−)-isopulegol in toluene. The upper
value of 100 °C was forced due to operative limitations of the
equipment.
The metallic composition of SCATs was determined by
Inductively coupled plasma mass spectrometry (ICP-MS) and
Microwave Plasma Atomic Emission Spectroscopy (MP-AES).
Qualitative analysis of SCATs by ICP-MS revealed Al, Si and Mg
as main components while also minor components such as Ce
and Fe and traces of Pt (0.1-0.2 wt.%) were detected (for a
complete qualitative analysis, please see ESI Table S1). A
quantitative analysis by MP-AES allowed to specifically identify
Firstly, a set of trials was performed fixing the reaction
temperature at 100 °C and varying the H
0 bar). A higher pressure delivered a higher yield. As the yield
obtained (Table 1, entry 5) at 40 bar of H pressure was almost
the same obtained at 30 bar (~91% of yield), as showed in Table
, entry 4, this last value was chosen as reaction pressure,
2
pressure (from 10 to
0
.68 wt.% of Fe, 3.01 wt.% of Ce and 0.10 wt.% of Pt in the
4
freshly ground SCATs. Additionally, SCATs were also analysed
after the catalytic tests. Considering the heterogeneity of the
substrate, no relevant metallic leaching was observed.
Due to the low concentration, no specific peaks of the metals
were detectable in XRD analysis, while most intense peaks were
associated to SiO . No difference between XRD patterns before
2
and after utilization were noticed (please see ESI Fig. S3 for XRD
patterns).
2
1
lowering the energy consumption.
Sequentially, a range of experiments were conducted operating
at 30 bar H pressure at different reaction temperature (75, 90
2
and 100 °C). Higher temperature provided higher yields,
showing a linear trend. Indeed, at stationary state, (−)-menthol
yields were 55.5%, 73.1% and 91.3% operating at 75, 90 and
Fig. 2 shows STEM-HAADF images and EDS analysis of SCATs
before utilization.
1
00 °C, respectively (Table 1, entries 6-8). Based on these
results, 100 °C was selected as optimum temperature.
Finally, different experiments were performed at different flow
-
1
rates (0.1, 0.2 and 0.3 mL min ) fixing the H
and the temperature at 100 °C. The obtained yields to
−)-menthol were 91.3%, 57.9% and 48.0% using 0.1, 0.2, 0.3 mL
2
pressure at 30 bar
(
-1
min as flow rate (Table 1, entries 9-11).
Summarizing, 91.3% of yield was obtained using SCATs under
optimum conditions (30 bar H
2
pressure, 100 °C and
-1
0
.1 ml min ), demonstrating the efficient activity of this system
as catalyst.
The catalytic performance of SCATs was subsequently
compared with commonly employed commercially available 30
mm-long cartridge ThalesNano CatCarts® containing 1%Pd/C
(
Table 1, entry 12). Surprisingly, SCATs showed almost
comparable performances respect to commercially available
%Pd/C (~94% of yield).
1
In a second phase, SCATs were charged in longer cartridges.
Notably, hydrogenation reactions under continuous-flow are
normally carried using 30 mm-long cartridge, as most of the
4
2-47
catalysts are expensive or need time-consuming synthesis.
However, the H-Cube® Mini Plus allows to use 70-mm long
cartridge, less reported in literature.
Fig. 2 (A), (B) and (C) STEM-HAADF images of SCATs. (D) EDS analysis of
selected locations (L1-L3).
48, 49
4
| J. Name., 2012, 00, 1-3
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Table 1 Temperature (T), pressure (p), flow rate, residence time (τ) § and Table 2 Temperature (T), pressure (p), flow rate, residenceViteiwmAert(icτl)e
yields at stationary state conditions (after 15‘) for the hydrogenations of yields at stationary state conditions (after 15‘).
§
Onalinnde
DOI: 10.1039/C9GC04091A
(−)-isopulegol (1) to (−)-menthol (2).
p
T
Flow rate
/ mL min-1
0.1
τ
/ min
8.8
8.8
Yield
/ %
Entry
Catalyst
/
bar
30
30
30
30
30
/ °C
100
100
100
100
100
1
2
3
4
5
BLANK
SCATs
SCATs
SCATs
SCATs
<1
0.1
95.5
90.9
89.3
57.5
0.3
2.9
0.5
1.8
1.0
0.9
6
*
1%Pd/C
30
100
0.1
8.8
91.6
Reaction parameters: (−)-isopulegol (20 mM in toluene). Yields to
−)-menthol were determined by GC using octane as internal standard.
p
T
Flow rate
/ mL min-1
0.1
τ
/ min
3.8
3.8
Yield
/ %
(
Entry
Catalyst
/
bar
30
10
20
30
40
30
30
30
30
30
30
/°C
100
100
100
100
100
75
None of the tests led to the formation of by-products, achieving in all
experiments 100% selectivity to menthol except for * where selectivity
of 8.4% to sub products was detected by GC and GC-MS analysis.
1
2
3
4
5
6
7
8
9
BLANK
SCATs
SCATs
SCATs
SCATs
SCATs
SCATs
SCATs
SCATs
SCATs
SCATs
<1
0.1
40.2
78.9
91.3
91.1
0.1
3.8
0.1
3.8
Lastly, the long-term stability of SCATs was investigated under
0.1
3.8
2
optimum continuous-flow conditions (p = 30 bar H ; T = 100 °C;
0.1
3.8
3.8
3.8
3.8
1.9
1.3
55.5
73.1
91.3
91.3
57.9
48.0
-1
flow rate = 0.1 mL min ). The hydrogenation reaction was
performed for a 6 h time-on-stream as illustrated in Fig. 3. After
90
0.1
100
100
100
100
0.1
2
h, a reduction of catalyst activity down to 85% of yield could
0.1
be observed, while selectivity remained constant at 100%. No
significant changes in the conversion of (−)-isopulegol were
further observed for the successive 4-8 h. After a washing cycle,
the activity was completely restored, proving that the decrease
in the yield was most likely due to adsorption of substances on
the active sites of the SCATs.
1
1
0
1
0.2
0.3
1
2*
1%Pd/C
30
100
0.1
3.8
94.2
Reaction parameters: (−)-isopulegol (20 mM in toluene). Yields to
−)-menthol were determined by GC using octane as internal standard.
(
None of the tests led to the formation of by-products, achieving in all
experiments 100% selectivity to (−)-menthol except for * where
selectivity of 5.8% to sub products was detected by GC and GC-MS
analysis.
Since SCATs are widely-available and cheap, it’s possible to use
them in larger quantities in order to implement and scale-up the
catalytic performances by increasing the residence time.
For the reactions, 450 mg of SCATs were packed in 70 mm-long
cartridge ThalesNano CatCarts®.
The tests were carried out operating at 30 bar H
00 °C and varying the flow rates (Table 2, entries 2-5). At
stationary state, (−)-menthol yields were 95.5%, 90.9%, 89.3%
2
pressure and
1
-1
and 57.5% for 0.1, 0.3, 0.5 and 1.0 mL min flow rates,
respectively. It is worth to point out that also the tests carried
-1
out at higher flow rates (0.3-0.5 mL min ) gave remarkable
results, as showed in Table 2, entry 3 and 4, while for 30 mm
long cartridges (less catalyst content) the yield sensibly already
Fig. 3 Stability test of SCATs. Reaction conditions: p = 30 bar H
2
; T = 100 °C;
-1
nd
flow rate = 0.1 mL min . The 2 cycle of stability was performed after washing
st
-1
the SCATs of the 1 cycle with toluene for 10’ with a flow rate = 2 mL min .
-1
decreased at flow rates of 0.2 mL min .
The catalytic performances of SCATs were also compared to the
those of commercially available 70 mm-long cartridges
ThalesNano CatCarts® containing 1%Pd/C (Table 2, entry 6)
under previously optimized conditions. Remarkably, SCATs
showed equal catalytic activities (95.5% yield) as compared to
commercial 1%Pd/C (91.6% yield).
Hydrogenation of (−)-isopulegol with the combined apparatus.
In order to study the effect of the temperature on the previous
reaction, further hydrogenation experiments were performed
in the combined apparatus (H-Cube®–X-Cube ). Indeed, the
temperature of the H-Cube® equipment has operative
TM
limitations (maximum temperature 100 °C), while the utilization
TM
of the heating unit of the X-Cube
hydrogenations up to higher temperature.
allows to perform
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Journal Name
Furthermore, the heat distribution inside the heating unit of the As expected, a higher temperature delivered a higher yield, with
View Article Online
DOI: 10.1039/C9GC04091A
H-Cube® is not homogeneous, while the heating system of the a maximum at 140 °C (Table 3, entry 11). Considering that the
TM
X-Cube permits a more uniform heat distribution (please see yields obtained (Table 3, entries 12-13) at 160 and 170 °C were
Fig. S4 in the ESI for the photos of the two different heating almost the same obtained at 140 °C (~75%) (Table 3, entry 11),
units).
this last value was chosen as optimal temperature, in order to
Washed and thermally treated SCATs were packed (450 mg of diminish the energy consumption.
powder per cartridge) in 70 mm-long cartridge ThalesNano Finally, the catalytic activity of SCATs was compared with that
CatCarts®. Firstly, the hydrogenation of (−)-isopulegol was of commercial catalyst 1%Pd/C (Table 3, entry 14) under the
performed in the previous optimized conditions (30 bar H
2
optimized conditions. Despite a complete conversion of
-1
pressure, 100 °C, 0.1 mL min , 20 mM (−)-isopulegol in toluene) isopulegol was observed, a lower yield to menthol, in
with the combined apparatus (Table 3, entry 1). The almost comparison with the reaction carried out at 100 °C (Table 2,
complete yield to menthol (>99.9%) in comparison with the entry 6) was noticed. Indeed, the higher temperature provided
previous obtained yield of 95% (Table 2, entry 2) suggested that a higher production of sub products (e.g. menthone), less
TM
the design of heating unit of the X-Cube
homogeneously heat the entire cartridge.
allowed to favored at lower temperature. However, the final yield was only
6% higher than the yield obtained with SCATs.
Based on this encouraging result, a set of trials was conducted
increasing the concentration of the starting material (Table 3, Hydrogenation of cinnamyl alcohol with the combined
entries 1-3). The experiments were performed using 20, 50 and apparatus. The hydrogenation of cinnamyl alcohol was initially
1
00 mM solutions of (−)-isopulegol in toluene and all the tests investigated performing a sequence of experiments using
provided challenging results, reaching almost complete different concentrations of the starting material (from 20 mM
conversions. Therefore, the starting solution of 100 mM of to 100 mM cinnamyl alcohol in toluene) (Table 4, entries 1-3).
(
−)-isopulegol was selected to accomplish further experiments The trials conducted at 20 and 50 mM gave ~100% of yield
for the optimization of other reaction parameters. (Table 4, entries 1-2), while a dramatic drop in yield (~46%) was
In details, a sequence of hydrogenations was performed obtained with the 100 mM solution (Table 4, entry 3). As a
-1
increasing the flow rate from 0.1 up to 2 mL min (Table 3, result, 50 mM solution was selected to conduct further
entries 4-8). A drop in the yields was observed. Since the experiments.
-1
reactions at 0.1, 0.3, 0.5 mL min (Table 3, entry 4-6) were
-
1
almost complete, while 1 mL min of flow rate provided ~50%
Table 4 Temperature (T), pressure (p), flow rate, residence time (τ) §,
concentration (c) and yields at stationary state conditions (after 15‘) for the
hydrogenations of cinnamyl alcohol (3) to hydrocinnamyl alcohol (4).
-1
of yield (Table 3, entry 7), 1 mL min was chosen for the
following experiments carried out increasing the temperature
from 100 to 170 °C (Table 3, entries 9-13).
Table 3 Temperature (T), pressure (p), flow rate, residence time (τ) §,
concentration (c) and yields at stationary state conditions (after 15‘).
p
T
Flow rate
/ mL min-1
0.1
τ
/ min
8.8
8.8
8.8
8.8
2.9
1.8
0.9
0.45
0.9
0.9
0.9
0.9
0.9
c
Yield
/ %
Entry
p
T
Flow rate
/ mL min-1
0.1
τ
/ min
8.8
8.8
8.8
8.8
2.9
1.8
0.9
0.45
1.8
1.8
1.8
1.8
1.8
c
Yield
/ %
/
bar
30
30
30
30
30
30
30
30
30
30
30
30
30
/ °C
100
100
100
100
100
100
100
100
100
120
140
160
170
/ mM
Entry
/
bar
30
30
30
30
30
30
30
30
30
30
30
30
30
/ °C
100
100
100
100
100
100
100
100
100
120
140
160
170
/ mM
1
2
3
4
5
6
7
8
9
20
50
>99.9
>99.9
99.1
1
2
3
4
5
6
7
8
9
20
50
>99.9
>99.9
46.1
0.1
0.1
0.1
100
0.1
100
0.1
100
100
100
100
100
100
100
100
100
100
99.1
0.1
50
50
50
50
50
50
50
50
50
50
>99.9
0.3
93.3
81.3
51.7
29.1
51.7
62.6
75.0
72.5
72.2
0.3
64.8
63.5
32.8
12.5
63.5
70.0
77.2
78.8
77.4
0.5
0.5
1.0
1.0
2.0
2.0
1.0
0.5
1
1
1
1
0
1
2
3
1.0
1
1
1
1
0
1
2
3
0.5
1.0
0.5
1.0
0.5
1.0
0.5
1
4*
30
140
1.0
0.9
100
81.0
1
4 *
30
140
0.5
1.8
50
98.8
None of the tests led to the formation of by-products, achieving in
all experiments 100% selectivity to menthol, except for * (1%Pd/C)
where selectivity of ~19% to sub products was detected by GC and
GC-MS analysis.
None of the tests led to the formation of by-products, achieving in
all experiments 100% selectivity to hydrocinnamyl alcohol; *test
performed with commercial catalyst 1%Pd/C.
6
| J. Name., 2012, 00, 1-3
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Journal Name
ARTICLE
-
1
Successively, the hydrogenations were carried out varying the An initial flow rate of 0.1 mL min (Table 5, entry 4) delivered a
View Article Online
-1
DOI: 10.1039/C9GC04091A
-1
flow rate from 0.1 to 2 mL min (Table 4, entries 4-8). An yield of ~100% while an increase in the flow up to 2 mL min
increase in the flow rate delivered a lower yield. As the yield produced a remarkable fall in yield (Table 5, entries 5-8). The
obtained at 0.1 mL min (Table 4, entry 4) was ~100%, while flow rate of 0.3 mL min (yield to dihydroeugenol of ~60%) was
almost the same value of yields (~65%) were obtained operating chosen as optimum flow rate in order to investigate the
with 0.3 and 0.5 mL min flow rate (Table 4, entries 5-6), 0.5 mL influence of the temperature (from 100 to 170 °C, reported in
min was chosen for the following experiments at higher Table 5, entries 9-13). A higher temperature provided a higher
temperature (from 100 to 170 °C as reported in Table 4, entries yield, with a maximum at 140 °C (Table 5, entry 11) and a
-1
-1
-1
-1
9
-13). A higher temperature provided a higher yield, reaching a plateau up to 170°C (Table 5, entries 12-13).
plateau at 140 °C and above (Table 4, entries 11-13). Under optimized reaction conditions, the reaction conducted
Commercially available 1%Pd/C was sequentially tested under using 1%Pd/C (Table 5, entry 14) provided 82% yield to final
the best operative conditions (Table 4, entry 14). In this case, product. The commercially available catalyst showed only ~10%
the commercially available catalyst provided almost 20% higher higher yield to dihydroeugenol, compared to SCATs.
yield to desired product.
Hydrogenation of vanillin with the combined apparatus. The
Hydrogenation of isoeugenol with the combined apparatus. A first tests for the hydrogenation of vanillin to vanillyl alcohol
set of trials for the hydrogenation of isoeugenol was initially were conducted varying the concentration of the starting
performed using different concentrations of isoeugenol (from material from 10 mM to 100 mM in toluene fixing the operative
-1
2
2
0 mM to 100 mM in toluene) (Table 5, entries 1-3). The conditions at 30 bar H pressure, 100 °C and 0.1 mL min flow
hydrogenations conducted at 20 and 50 mM gave almost rate (Table 6, entries 1-3). Due to the low activity, the flow rate
complete conversions of isoeugenol (Table 5, entries 1-2), while was set to 0.1 mL min- for all the following experiments, as
1
~93% of yield was obtained using the 100 mM solution (Table 5, trials at higher flux were considered unnecessary. A range of
entry 3).
experiments were then conducted incrementing the
temperature (Table 6, entries 4-8). As expected, a higher
temperature delivered higher yields, reaching a maximum at
Successively, the flow rate was increased from 0.1 to 2 mL min-
1
(Table 5, entries 4-8).
1
60 °C (Table 6, entry 7). Encouragingly, the reaction carried out
under the best reaction conditions using commercial 1%Pd/C
Table 6, entry 9) showed almost a comparable yield to the one
Table 5 Temperature (T), pressure (p), flow rate, residence time (τ) §,
concentration (c) and yields at stationary state conditions (after 15‘) for the
hydrogenations of isoeugenol (5) to dihydroeugenol (6).
(
obtained with SCATs (57.3% and 50.1% respectively).
Table 6 Temperature (T), pressure (p), flow rate, residence time (τ) §,
concentration (c) and yields at stationary state conditions (after 15‘) for the
hydrogenations of vanillin (7) to vanillyl alcohol (8).
p
T
Flow rate
/ mL min-1
0.1
τ
/ min
8.8
8.8
8.8
8.8
2.9
1.8
0.9
0.45
2.9
2.9
2.9
2.9
2.9
c
Yield
/ %
Entry
/
bar
30
30
30
30
30
30
30
30
30
30
30
30
30
/ °C
100
100
100
100
100
100
100
100
100
120
140
160
170
/ mM
1
2
3
4
5
6
7
8
9
20
50
>99.9
97.4
0.1
0.1
100
92.7
100
100
100
100
100
100
100
100
100
100
9
5
2.7
9.6
p
/ bar
30
T
Flow rate
/ mL min-1
τ
/ min
8.8
8.8
8.8
8.8
8.8
8.8
8.8
8.8
c
Yield
/ %
0.1
Entry
0.3
/ °C
100
100
100
100
120
140
160
170
/ mM
0.5
35.6
22.7
17.0
59.6
61.4
69.6
71.2
71.5
1
2
3
4
5
6
7
8
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
10
20
3.4
0
1.0
30
2.0
30
100
0
10
10
10
10
10
3
8
.4
.7
0.3
30
1
1
1
1
0
1
2
3
0.3
30
0.3
30
26.7
50.1
47.0
0.3
30
0.3
30
1
4 *
30
140
0.3
2.9
100
80.8
9 *
30
160
0.1
8.8
10
57.3
None of the tests led to the formation of by-products, achieving in
all experiments 100% selectivity to dihydroeugenol; *test
performed with commercial catalyst 1%Pd/C.
None of the tests led to the formation of by-products, achieving in
all experiments 100% selectivity to vanillyl alcohol; *test performed
with commercial catalyst 1%Pd/C.
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Hydrogenation of benzaldehyde with the combined
apparatus. The hydrogenation of benzaldehyde to benzyl
alcohol was initially studied using a concentration of the starting
material of 10 mM and 20 mM in toluene and fixing the reaction
View Article Online
DOI: 10.1039/C9GC04091A
-1
2
conditions at 30 bar H pressure, 100 °C and 0.1 mL min flow
rate (Table 7, entries 1-2). Sequentially, operating with the
lower concentration, the temperature was increased up to
1
70 °C (Table 7, entries 3-7). Neither of the experiments showed
appreciable yields. On the other hand, commercial 1%Pd/C was
highly active for the reaction (Table 7, entry 8). The obtained
results clearly demonstrated a low applicability of the SCATs for
the hydrogenation of this biomass derived chemical.
Table 7 Temperature (T), pressure (p), flow rate, residence time (τ) §,
concentration (c) and yields at stationary state conditions (after 15‘) for the
hydrogenations of benzaldehyde (9) to benzyl alcohol (10).
Fig. 4 Stability test of SCATs in the combined apparatus. Reaction conditions:
-1
p = 30 bar H
2
; T = 140 °C; flow rate = 0.5 mL min , 50 mM cinnamyl alcohol in
nd
toluene. The 2 cycle of stability was performed after washing the SCATs of
st
-1
the 1 cycle with toluene for 10’ with a flow rate = 2 mL min .
ICP-MS analysis of the collected outcome toluene was also
performed detecting no traces of metals, proving that no metal
leaching occurred even under long term trials.
p
T
Flow rate
/ mL min-1
0.1
τ
/ min
8.8
8.8
8.8
8.8
8.8
8.8
8.8
c
Yield
/ %
Entry
/
bar
30
30
30
30
30
30
30
/ °C
100
100
100
120
140
160
170
/ mM
1
2
3
4
5
6
7
10
20
10.9
3.2
Almost identical yields were observed in the second cycle.
0.1
10
10
10
10
10
1
1
0.9
0.3
Characterization after stability tests. Importantly, SCATs after
long run experiment were fully characterised and compared to
fresh washed and thermally treated SCATs.
Fig.5 shows STEM-HAADF analysis of SCATs after the catalytic
tests. As reported in Fig. 5A and Fig. 5B, the metallic
nanoparticles were still homogeneously distributed over the
0.1
0.1
0.1
10.7
9.5
0.1
0.1
10.4
8
*
30
100
0.1
8.8
10
94.2
2 3 2
Al O and SiO surface. Fig. 5E shows the EDS analysis at four
None of the tests led to the formation of by-products, achieving in
all experiments 100% selectivity to benzyl alcohol. *test performed
with commercial catalyst 1%Pd/C
different locations, L4, L5, L6 and L7 in Fig. 5C, highlighting the
presence of Ce and Pt nanoparticles. Remarkably, no relevant
variations in morphology were observed. BET analysis showed
2
-1
a non-porous material with surface area of ~13 m g before
and after utilization (please see ESI Fig. S6 for isotherm graph of
SCATs before utilization).
Stability tests. The long-term stability test of SCATs was
performed in order to prove the stability of SCATs in the
combined apparatus. Specifically, the hydrogenation of
cinnamyl alcohol (77% yield in Table 4, entry 11) was selected
as model reaction. The hydrogenation reaction was performed
for a 15 h time-on-stream, as illustrated in Fig. 4.
Despite almost no changing in the yields to hydrocinnamyl
alcohol was detected in the first 2 h, a reduction of catalyst
activity of ~12% was observed after 3 h, reaching a plateau up
to 15 h of reaction. A washing cycle was subsequently
performed by pumping toluene in the continuous flow
combined apparatus in order to check if the drop in the activity
of the SCATs was due to the adsorption of materials on the
surface of the catalysts rather than a deactivation. GC and
GC-MS analysis of the fluxed and concentrated toluene showed
the presence of the starting cinnamyl alcohol, confirming that
the slight fall in reactivity after 15 h could be due to adsorbed
starting compound on the surface of SCATs (please see Fig.S5 in
the ESI for the graph reporting the GC-MS signal).
Fig. 5 SCATs after catalytic utilization (A), (B), (C) and (D) STEM-HAADF images,
(E) EDS analysis of selected areas.
8
| J. Name., 2012, 00, 1-3
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Neither leaching nor morphological variations were observed in
the analysis of SCATs after reaction, showing a highly stable
performance of the waste catalyst under continuous flow
conditions. The observed slight initial deactivation is certainly
due to the adsorption of organic compounds on the catalyst
surface (as found by DRIFTs, results not shown). This hypothesis
was confirmed by the fact that after a washing cycle, SCATs
View Article Online
1
M. Selva and R. Luque, Curr. Opin. Green Sust. Chem., 2019,
DOI: 10.1039/C9GC04091A
1
5, 98-102.
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R. Luque, Curr. Opin. Green Sust. Chem., 2016, 2, 6-9.
B. S. Kadu, A. M. Hengne, N. S. Biradar, C. V. Rode and R. C.
Chikate, Ind. Eng. Chem. Res., 2016, 55, 13032-13039.
C. Yu, J. J. Fu, M. Muzzio, T. L. Shen, D. Su, J. J. Zhu and S. H.
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J. Ni, W. H. Leng, J. Mao, J. G. Wang, J. Y. Lin, D. H. Jiang and
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Directive 2000/53/EC of the European Parliament and of the
Council of 18 September 2000, Official Journal of the
European Communities, L269/34. Published: 21st October,
2000.
4
5
6
nd
displayed the original catalytic activity, as shown in Fig. 5 “2
cycle”.
4
. Conclusions
7
8
9
C. Hageluken, Platin. Met. Rev. 2012, 56(1), 29-35.
J. F. Weng, X. X. Lu and P. X. Gao, Catalysts, 2017, 7(9), 253.
J. Kaspar, P. Fornasiero and N. Hickey, Catal. Today, 2003, 77,
This work has demonstrated the applicability of a waste-derived
material, scrap automotive catalytic converters (SCATs), in the
continuous-flow hydrogenation of different biomass-derived
chemicals. Remarkably, the SCATs showed comparable activity
4
19-449.
1
1
0 A. Fornalczyk and M. Saternus, Acta Metall. Sin-Engl., 2013,
2
6, 247-256.
(only 6-10% less activity) to commercially available 1%Pd/C in
1 D. J. de Aberasturi, R. Pinedo, I. R. de Larramendi, R. de
Larramendi and T. Rojo, Miner. Eng., 2011, 24, 505-513.
the hydrogenations of isopulegol to menthol, isoeugenol to
dihydroeugenol as well as in the hydrogenation of vanillin to 12 M. A. Barakat and M. H. H. Mahmoud, Hydrometallurgy, 2004,
vanillyl alcohol. Due to the easy preparation procedure
7
2, 179-184.
1
1
3 C. H. Kim, S. I. Woo and S. H. Jeon, Ind. Eng. Chem. Res., 2000,
(
washing and thermal treatment) low-production of waste
3
9, 1185-1192.
5
0
(
calculated E-factor: ~20 §§§) , high activity, low price (up to
50 times less than commercial 1%Pd/C §§§§) SCATs have been
proved to be efficient,
4 M. K. Jha, J. C. Lee, M. S. Kim, J. Jeong, B. S. Kim and V. Kumar,
1
Hydrometallurgy, 2013, 133, 23-32.
environmentally-friendly, 15 M. Baghalha, H. K. Gh and H. R. Mortaheb, Hydrometallurgy,
2
009, 95, 247-253.
largely-available and cheap catalysts for continuous-flow
hydrogenation reactions, paving the way to consider their
application not only for the recovery of PGMs but also for the
direct use as active components of catalytic system.
1
1
1
6 A. Fornalczyk, J. Willner, B. Gajda and J. Sedlakova-Kadukova,
Arch. Metall. Mater., 2018, 63, 963-968.
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2
018, 182, 44-56.
8 N. Hodnik, C. Baldizzone, G. Polymeros, S. Geiger, J. P. Grote,
S. Cherevko, A. Mingers, A. Zeradjanin and K. J. J. Mayrhofer,
Nat. Commun., 2016, 7, 13164.
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Begum, T. Maki and S. Mizutani, Water Air Soil Poll., 2014,
Conflicts of interest
The authors declare no conflicts of interest.
1
2
25(9), 2112.
2
2
0 M. Zengin, H. Genc, T. Demirci, M. Arslan and M.
Kucukislamoglu, Tetrahedron Lett., 2011, 52, 2333-2335.
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Acknowledgements
The authors thank Mr. Rafael Ángel Gómez Haro and Provaluta 22 E. Mieczynska, A. Gniewek and A. M. Trzeciak, Appl. Catal. a-
Gen., 2012, 421, 148-153.
España Reciclaje de Metales, S.L., Córdoba (ES), for the supply
of scrap automotive catalytic converters. Rafael Luque
gratefully acknowledges MINECO for funding under project
CTQ2016-78289-P, co-financed with FEDER funds and the
contract for Camilla Maria Cova associated to this project. This
project has received funding from the European Union’s
Horizon 2020 research and innovation programme under the
Marie Skłodowska-Curie grant agreement No 721290. This
publication reflects only the author’s view, exempting the
2
2
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etn.eu/. The publication has been prepared with support from
RUDN University program 5-100.
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§
§
§
residence time (τ) = Catcarts volume / flow rate
3
3
§ TOS: time on stream
§§ E-Factor = total waste (kg) / product (kg). In details E-Factor=
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.0 kg/ 0.1 kg= 20
§§§§ According to prices available on
https://www.sigmaaldrich.com/
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