Selective production of bio-based: Para -xylene over an FeOx -modified Pd/Al2O3catalyst

Article abstract of DOI:10.1039/d0gc00944j

para-Xylene (PX) is a basic building block of polyethylene terephthalate, which is currently produced from petroleum resources. Developing a renewable route to PX is highly desirable to address both economic and environmental concerns. Several attempts used noble metal catalysts, e.g. Pd/Al2O3, to synthesize PX from biomass-derived 4-methyl-3-cyclohexene-1-carboxaldehyde (4-MCHCA), but suffered from a severe decarbonylation reaction, resulting in toluene as the main product. In this paper, we report an FeOx modification strategy to suppress the decarbonylation reaction on a Pd/Al2O3 catalyst, leading to a drastic shift in selectivity towards PX with a yield up to 81percent via a cascade dehydroaromatization-hydrodeoxygenation (DHA-HDO) pathway. Characterization and control experiments revealed that the electron density of Pd sites decreased in an FeOx-modified Pd/Al2O3 catalyst compared to Pd/Al2O3, thus tuning the preferential adsorption mode of the substrate from η2-(C,O), the key transition state of the decarbonylation reaction, to the η1-(O) mode that favors the hydrodeoxygenation process. Notably, this designed catalyst is highly stable and is readily applicable in the selective synthesis of a broad range of desired aromatic chemicals via the same DHA-HDO pathway from cyclohex-3-enecarbaldehyde derivatives. Overall, this work develops a controllable catalyst modification strategy that tailors an efficient catalyst for petroleum-independent bio-PX synthesis.

Full text of DOI:10.1039/d0gc00944j

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DOI: 10.1039/D0GC00944J
ARTICLE
Selective production of bio-based para-xylene over FeOx-
modified Pd/Al2O3 catalyst
Yuxue Xiao,a,b† Qingwei Meng,b† Xiaoli Pan,b Chao Zhang,*a Zaihui Fu,a Changzhi Li*b,c
Received 00th January 20xx,
Accepted 00th January 20xx
Para-xylene (PX) is a basic building block of polyethylene terephthalate, which is currently produced from petroleum
resources. Developing renewable route to PX is highly desirable for both economic and environmental concerns. Several
attempts used noble metal catalysts, e.g. Pd/Al2O3 to synthesize PX from biomass-derived 4-methyl-3-cyclohexene-1-
carboxaldehyde (4-MCHCA), but suffered severe decarbonylation reaction, resulting in toluene as the main product. In this
paper, we report a FeOx modification strategy to suppress decarbonylation reaction on Pd/Al2O3 catalyst, leading to a
drastically shift in selectivity towards PX with the yield up to 81% via a cascade dehydroaromatization-hydrodeoxygenation
(DHA-HDO) pathway. Characterization and control experiments revealed that electron density of Pd sites decreased in FeOx
modified Pd/Al2O3 catalyst compared to Pd/Al2O3, thus tuning the preferential adsorption mode of the substrate from η2-(C,
O), the key transition state of decarbonylation reaction, to η1-(O) mode that favors hydrodeoxygenation process. Notably,
this designed catalyst is highly stable and is readily applicable in selectively synthesis of a broad range of desired aromatic
chemicals via the same DHA-HDO pathway from cyclohex-3-enecarbaldehyde derivatives. Overall, this work develops a
controllable catalyst modification strategy that tailors an efficient catalysis for petroleum-independent bio-PX synthesis.
DOI: 10.1039/x0xx00000x
selective strategy with sustainable feedstock in replace of fossil
resources for PX production is highly desirable.
Considerable efforts have been paid in seeking advanced
1. Introduction
Aromatic compounds are indispensable chemicals in industry,
which account for almost one-third of the commodity
petrochemical market and the demand is steadily growing due
to the rapid development of the polymer industry. para-xylene
(PX) is one special aromatic building block for terephthalic acid
and ultimately polyethylene terephthalate production. The
market for PX has grown by five to ten percent annually over
the last several years, and reached 45 million tons in 2016.1
Currently, PX is produced via catalytic reforming of naphtha and
thermal catalytic cracking of hydrocarbons, both of which
heavily reliant on fossil resource and suffer energy-intensive
separation/purification process (account for ca. 60% of the total
energy consumption) owing to the ignorable boiling point
differences among xylene isomers (138.3 oC, 139.1 oC and 144.4
oC for PX, m-xylene and o-xylene, respectively). Development of
technologies to convert renewable biomass into PX. The first
report for the production of aromatics from biomass have been
documented in the early 1980s2, through non-catalytic pyrolysis
of e.g. woods and following upgrading of the pyrolysis vapors
with zeolite catalyst.3 In virtue of the aromatic feature of lignin,
recent advances have also demonstrated the potential for the
cleavage of lignin to a spectrum of aromatic compounds.4
However, due to the complicated structure of lignin and raw
lignocellulose biomass, the products of the above two methods
are usually a mixture of aromatic compounds with low PX
selectivity, along with olefins, CO, CO2, H2O and other small
molecules. Brookhart and co-workers recently reported an
elegant route for the synthesis of PX using ethylene as the sole
feedstock through four steps,5 higher selectivity was achieved
in comparison with the pyrolysis method. Processes for
converting methanol/isobutyl alcohol to aromatics over zeolites
were reported afterwards, providing PX yield of ca. 24% with
total aromatics yields of about 60%6. Particularly, Dauenhauer7,
Davis8 and other groups9 reported an interesting route for the
synthesis of PX from sugar-derived furans such as 2,5-
dimethylfuran, and ethylene (or its derivatives) through
cycloaddition and subsequent dehydration reactions. Using H-
BEA7a and Zr-BEA7b as catalysts, Dauenhauer et al. achieved high PX
yield of ca. 90%. Farmer10 group demonstrated a solvent-free route
to biobased diethyl terephthalate from 2,5-furandicarboxylic acid
diethyl ester with a yield of 59%. At the premise of cost-effective
commercialization of sugar-derived furan compounds, this route
a.National & Local Joint Engineering Laboratory for New Petro-chemical Materials
and Fine Utilization of Resources and Key Laboratory of Chemical Biology and
Traditional Chinese Medicine Research (Ministry of Education), College of
Chemistry and Chemical Engineering, Hunan Normal University, Chang sha
410081, Hunan
E-mail: chaozhang2006@126.com
b.CAS Key Laboratory of Science and Technology on Applied Catalysis, Dalian
Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China
Email: licz@dicp.ac.cn
c. Dalian National Laboratory for Clean Energy, 457 Zhongshan Road, Dalian,
116023, China
† These authors contributed equally to this work.
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
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would have potential in application. Shanks11 and Delcroix12 groups
have also reported excellent works producing toluic acid from
coumalic acid and sorbic acid respectively, driving the progress of
bio-PX path further.
View Article Online
DOI: 10.1039/D0GC00944J
4-Methylcyclohex-3-enecarbaldehyde (4-MCHCA) can be
readily produced from biomass-derived isoprene (generates
from fermentation of inedible lignocellulose or by biosynthetic
of CO213) and acrolein (glycerol dehydration product14) via Diels-
Alder reaction. Our recent progress demonstrated that 30 wt%
W2C/C showed excellent selectivity for the sequential
dehydroaromatization-hydrodeoxygenation (DHA-HDO) of 4-
MCHCA to afford a high PX yield in one step at 350-375 oC
(Route 1 in Scheme 1).15 While promising, the stability of
W2C/AC is a big problem due to carbon deposition, crystallite
growth, and oxidation of W2C species. Noting that noble metals
show excellent catalytic activity in a variety of reactions
including dehydroaromatization and hydrodeoxygenation.16 Pd,
Pt and Rh supported on active carbon and Al2O3 were employed
in this transformation.15,17 Unfortunately, despite the key
Scheme 1. Catalytic conversion of biobased 4-MCHCA into toluene and PX.
monometallic Pd catalyst, whilst the competitive
decarbonylation of aldehyde group (C-C scissor) is remarkably
suppressed. As a result, a high PX yield of 81.1% is obtained. A
series of characterizations indicate that FeOx decreased the
electron density of Pd sites and hence suppressed the carbonyl
adsorption mode of η2-(C, O), which is the necessary mode for
decarbonylation. Comparing with previous reported W2C/AC
catalyst (Route 1, scheme 1),15 this bimetallic catalyst features
with much more stability, lower reaction temperature, and
lower metal loading. The above advantages render it an
effecient route for PX production from bio-based 4-MCHCA in a
single reactor.
intermediate
p-tolualdehyde
was
formed
through
dehydroaromatization of 4-MCHCA, toluene instead of PX, was
the dominant product in all these one-pot transformation cases
due to the competitive decarbonylation reaction after
dehydroaromatization step.17a Despite an ingenious alternative
route involving Wolff-Kishner-Huang reduction following by
dehydroaromatization reaction was developed by Tong
group17b (Route 2 in Scheme 1). Such a two-step process suffers
relatively low PX yield, and the use of corrosive hydrazine
hydrate (1 molar equiv.) and potassium hydroxide (2.5 molar
equiv.). Therefore, robust metal catalysts feature with high
stability and enable low temperature one-step conversion of 4-
MCHCA into PX is highly desirable.
The use of bimetallic catalysts is a promising strategy to modify
the properties, in comparison with their parent metals, in
relation with the activity, stability, and particularly the
selectivity, mainly through electronic and morphology effects.18
For example, in biomass conversion, Pd-Fe catalyst was
observed to be active and selective for vapor-phase
hydrodeoxygenation of guaiacol,19 and aqueous-phase
reforming of ethylene glycol.20 Bimetallic catalysts were also
observed able to change the reaction path and thus altered the
main product. The incorporation of Cu onto Pd/SiO2 catalyst
greatly altered furfural reaction pathway from breaking C-C
bonds to C=O bonds scission21. Besides, NiFe and PdFe alloys
were also used for furfural hydrogenation, turning out to
suppress decarbonylation by changing the adsorption mode of
the formyl group.21,22 These above progresses enlighten us the
possibility to modify the surface properties of noble metal-
based catalysts for the conversion of 4-MCHCA, so as to tuning
the selectivity to PX product.
2. Results and discussion
2.1 Transformation of 4-MCHCA to para-xylene
All the catalysts were prepared by incipient wetness
impregnation method and were reduced at 350 oC before using
for the conversion of 4-MCHCA. FeOx modified 1Pd/Al2O3
catalysts were labeled as 1PdnFeOx/Al2O3 (n=1, 3, 5, 7, 10, 15).
The nominal Pd loading is 1 wt%, and n means n% weight
loading of Fe in PdnFeOx/Al2O3 catalyst. Table 1 shows the
product distribution for the transformation of 4-MCHCA on
different FeOx modified Pd/Al2O3 catalysts. A very low
conversion of 1.7% was obtained on Al2O3 at 300 oC with trace
PX formation (entry 1). While toluene was generated with a high
yield of 85.5% on 1Pd/Al2O3 catalyst, indicating
dehydroaromatization-decarbonylation (DHA-DCO) cascade
reaction occurred dominantly by catalysis of Pd sites.17a
Interestingly, FeOx modified Pd/Al2O3 catalyst leads to reversed
product distribution. When Fe loading increased from 0 to 1%,
3%, 5%, 7% and finally to 10%, the yield of PX showed an
increase from 8.7% to 70.1% gradually; in response, the yield of
toluene decreased from 85.5% to 18.9% (entries 2-7). These
results indicate that the FeOx modification strategy on Pd-based
catalyst could direct convert 4-MCHCA into PX at 300 oC, a
temperature 50 oC-75 oC lower than that over W2C/AC.15 Excess
of FeOx loading did not increase PX yield anymore (entry 8),
while increasing the reaction temperature to 325 oC could
further enhance PX yield to a highest datum of 81.1% (entry 9).
Herein, FeOx-modified Pd/Al2O3 catalysts are prepared for the
direct conversion of 4-MCHCA. These Al2O3 supported
bimetallic catalysts allow for great improving of
hydrodeoxygenation (C=O scissor) activity as compared to the
2 | J. Name., 2012, 00, 1-3
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2.2 Catalyst characterization
BET, Temperature-programmed-reduction
View Article Online
DOI: 10.10(3T9P/DR0),GC00X9-4ra4Jy
diffraction (XRD), Scanning transmission electron microscope
(STEM), Transmission electron microscope (TEM), High-
resolution TEM (HRTEM) were conducted to understand the
physical/chemical state of the Pd/Fe metals. Table S3 shows
that with the increase of Fe loading, the surface area of the
catalysts decreased from 206 cm3/g of 1Pd/Al2O3 to 178 cm3/g
of 1Pd10FeOx/Al2O3. Generally, high surface area is benefit for
the reaction in heterogeneous catalysis. However, for the
transformation of 4-MCHCA, PX selectivity increased with the
rising of Fe loading, while the conversion was not clearly
affected (Table 1). Thus, it could be assumed that the surface
area of the catalysts did not show a clear influence on the
catalytic performance. This may due to that the electronic effect
of FeOx has greater impact on the selectivity compared to the
impact of surface area of Al2O3. Another possible reason is, the
surface area of Al2O3 is high enough to expose the active cites
even after loading 10 wt% Fe. As shown in Figure 1 (A), TPR
Table 1. Catalytic conversion of 4-MCHCA to PX over FeOx modified Pd/Al2O3
catalysts.
Yield/%
PhMe MCE MCA
Conv.
/%
T/
oC
Entry Catalyst
PX
1
Al2O3
1.7
300
300
300
300
300
300
300
300
325
300
1.0
0
0.2
0.6
1.0
1.1
4
0
2
1Pd/Al2O3
100
100
100
100
100
100
100
100
13.3
8.7
85.5
66.6
42.3
27.8
26.5
18.9
11.0
16.1
-
5.0
8.5
6.8
4.3
4.4
4.1
3.6
0.9
-
3
1Pd1FeOx/Al2O3
1Pd3FeOx/Al2O3
1Pd5FeOx/Al2O3
1Pd7FeOx/Al2O3
1Pd10FeOx/Al2O3
1Pd15FeOx/Al2O3
1Pd10FeOx/Al2O3
10FeOx/Al2O3
1Pd/Al2O3 +
23.8
49.4
58.9
65.8
70.1
70.0
81.1
2.5
4
5
6
2.2
4.0
4.5
1.0
-
7
8
9
10
11a
100
300
13.8
81.0
0
5.2
10FeOx/Al2O3
Reaction condition: 4-MCHCA concentration: 20 mg/mL; Solvent: n-hexane;
Catalyst: 0.25 g; WHSV: 0.2 h-1; Carrier gas: H2 (10ml·min-1); aPhysical mixture
of 1Pd/Al2O3 and 10FeOx/Al2O3 catalyst.
o
profile of Pd/Al2O3 exhibits one peak at 77 C, which can be
PhMe: Toluene; MCE: 4-Methyl-cyclohexene; MCA: Methyl cyclohexane.
attributed to reduction of Pd2+ to Pd0.23 The reduction
temperature increased to about 110~130 oC on
1PdnFeOx/Al2O3 (n=3, 7, 10) catalysts, probably due to the
interaction between Pd and Fe species. TPR profile of
10FeOx/Al2O3 shows two peaks at T max of 389 °C and 543 °C
respectively, implying the stepwise reduction of surface Fe3+
species to Fe0 species in two steps, viz. the reduction of Fe2O3
to Fe3O4 species (389 °C) and further reduction of Fe3O4 species
to Fe0 species (543 °C).24 For 1PdnFeOx/Al2O3 catalysts, the
corresponding two peaks decreased to 280~290 °C and
420~520 °C, respectively, indicating that Pd species promoted
the reduction of Fe species. It was also found from TPR profile
that PdFe alloy was not formed in 1PdnFeOx/Al2O3 because no
reduction peak attributing to PdFe alloys was observed.
According to the above TPR results, it can be inferred that, the
1PdnFeOx/Al2O3 catalysts after reduction at 350 oC could
contain Fe2+ and Fe3+ species (e.g. in Fe3O4) and Pd0, while Fe0
would not form. Figure 1 (B) shows the XRD patterns of the
catalysts after reduction at 350 oC. No diffraction peak ascribing
to Pd or PdO was observed, suggesting the high dispersion of
Pd. STEM characterization illustrates that the average Pd
nanoparticle sizes of the 1PdnFeOx/Al2O3 (n=1, 3, 5, 7, 10)
catalysts are similar and in the range of ca. 3-4 nm (Figure 2 and
To probe the role of FeOx, the reaction with 10FeOx/Al2O3 was
investigated. Unexpected, it only resulted in a poor conversion
(13.3%) with PX yield of 2.5% (entry 10). By contrast, the
reaction with a physical mixture of 1Pd/Al2O3 and 10FeOx/Al2O3
as catalyst provided a similar performance to that over
monometallic catalyst 1Pd/Al2O3 (entry 11 vs entry 2), indicating
that consecutive DHA–DCO still dominated the transformation.
The above comparative experiments (entries 2, 9-11)
demonstrate that, in 1PdnFeOx/Al2O3 catalyzed 4-MCHCA
transformation, Pd species plays the major role in initiating first
dehydroaromatization step; while in the second step, the
change in products selectivity is likely due to the chemical
interaction between FeOx and Pd, which tuned the pathway
from decarbonylation on monometallic 1Pd/Al2O3 towards
hydrodeoxygenation on 1PdnFeOx/Al2O3. Taken together,
through FeOx modification strategy, decarbonylation reaction
on Pd/Al2O3 catalyst is greatly suppressed in the conversion of
4-MCHCA, leading to a drastically shift in selectivity from
toluene towards more valuable PX. In comparison with previous
developed W2C/AC catalyst (30 wt% loading),15 this work
features with lower reaction temperature, and far less metal
loading (1 wt% Pd).
Figure 1. (A) TPR profiles of the mono and bimetallic catalysts; (B) The XRD patterns of the catalysts after reduction at 350 oC.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
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Figure S1). Apparently, addition of FeOx did not affect the Pd
nanoparticle size, indicating PdFe alloys were not formed and
further excluded the influence of Pd nanoparticle size on
decarbonylation performance in this work. Diffraction peaks at
36.8º and 44.7º attributing to Fe3O4 (311) and (400)25 were
observed on 1PdnFeOx/Al2O3 (n=5, 10) catalysts. This is in good
agreement with TPR results that Fe2O3 in the calcined catalysts
was reduced to Fe3O4 at 350 oC.
View Article Online
DOI: 10.1039/D0GC00944J
As shown in Figure 2, B & E, only Pd nanoparticles with a d-
spacing of 0.224 nm according to Pd (111) was observed in
HRTEM images and the electron diffraction pattern of
1Pd/Al2O3 and 1Pd5FeOx/Al2O3, indicating no PdFe alloys were
formed. This is consistent with the report that PdFe alloy was
hard to form on Al2O3 support.25 TEM and the EDS mapping
could help to probe the properties and distribution of the Fe/Pd
species. As shown in the TEM image of 1Pd10FeOx/Al2O3 (Figure
2, H), FeOx exists mainly in the form of floccules. The mappings
of Fe and Pd overlapped very well (Figure 2, J, K, L for
1Pd5FeOx/Al2O3 and M, N, O for 1Pd10FeOx/Al2O3), indicating
Fe species tends to distribute around Pd species. Thus, despite
alloys were not formed, FeOx species could interact with Pd in
a weaker way, which was also demonstrated by the TPR results.
To understand the interaction between FeOx and Pd, and the
effect mechanism of the FeOx on Pd to suppress the
decarbonylation reaction, XPS, in situ CO-DRIFTS and Propanal-
DRIFTS of the catalysts after 350 oC reduction were conducted.
The XPS spectra in Pd 3d region (Figure 3 (A)) for 1Pd/Al2O3,
1PdnFeOx/Al2O3 (n=1, 5, 10) samples (334.9-335.6 eV) show the
typical Pd 3d5/2 binding energy value (~335.1 eV) for metallic
Pd,26, 27 consistent with the TPR and HRTEM results. In detail,
the binding energy of Pd 3d5/2 for 1PdnFeOx/Al2O3 (n=1, 5, 10)
catalysts gradually shifts to higher value (335.6 eV) compared
Figure 2. STEM images for (A) 1Pd/Al2O3, (D) 1Pd5FeOx/Al2O3, (G) 1Pd10FeOx/Al2O3;
HRTEM images and electron diffraction patterns (inset) for (B) 1Pd/Al2O3, (E)
1Pd5FeOx/Al2O3; The TEM image of (H) 1Pd10FeOx/Al2O3; Pd nanoparticle size
distribution determined by STEM of (C) 1Pd/Al2O3, (F) 1Pd5FeOx/Al2O3, (I)
1Pd10FeOx/Al2O3; The EDS mapping images of Pd and Fe in 1Pd5FeOx/Al2O3 (J, K, L) and
1Pd10FeOx/Al2O3 (M, N, O) catalysts. All the samples have been reduced at 350 oC.
Figure 3. (A) The Pd 3d XPS spectra of 1Pd/Al2O3, 1PdnFeOx/Al2O3 (n=1, 5, 10) catalysts; (B) The Fe 2p XPS spectra of 1PdnFeOx/Al2O3 (n=1, 5, 10) and 10FeOx/Al2O3
catalysts; FTIR spectra of CO (C) and propanal (D) adsorption on the catalysts. All the samples have been reduced at 350 oC before characterization.
4 | J. Name., 2012, 00, 1-3
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with that of 1Pd/Al2O3 catalyst (334.9 eV), suggesting obvious of C–O linkage. Figure 3 (C) shows the in situ FTVIRiewsAprteiccletrOanlionef
DOI: 10.1039/D0GC00944J
electron transfer from Pd to FeOx. The Fe 2p3/2 spectra are propanal adsorption, by which the important role of FeOx on
shown in Figure 3 (B). After reduction of catalysts, Fe2+ and Fe3+ changing the adsorption mode of aldehyde group on Pd sites
species were co-existed without formation of Fe0 phase. Semi- was studied. All the catalysts show the doublet C=O adsorption
quantitative analysis was conducted by deconvolution of Fe2+ peaks,30 at around 1735 and 1760 cm-1. While there are also two
and Fe3+ 2p3/2. The ratio of Fe2+/(Fe2++Fe3+) for 10FeOx/Al2O3 is clear differences between monometallic Pd/Al2O3 and
less than 10%, indicating even on the catalyst surface, quite low bimetallic catalysts: 1) the amount of the aldehyde adsorbed on
amount of Fe3+ was reduced to Fe2+. While for 1Pd1FeOx/Al2O3 1Pd10FeOx/Al2O3 decreased remarkably compared with that on
catalyst, the ratio increased to 56.9% and it decreased gradually 1Pd/Al2O3; 2) obvious red shift of the aldehyde adsorption
with increase of Fe loading, indicating the electron transfer peaks occurred from 1734 cm-1 of 1Pd/Al2O3 to 1743 cm-1 of
from Pd to FeOx promoted reduction of Fe3+ to Fe2+, which is in 1Pd10FeOx/Al2O3. These differences are similar to that
good agreement with the TPR results. Reduction peak lower observed in CO-FTIR characterization, and hence demonstrated
o
than 350 C for 10FeOx/Al2O3 was not observed (Figure 1A), the electronic interaction between FeOx and Pd as observed by
probably because the amount of surface Fe3+ reduce to Fe2+ was XPS and its effect on adsorption mode of aldehyde.
too small to be observed. Similar electron interaction between As
a result, the restraining effect of FeOx on the
Pd0 and Fe2O3 was observed previously and was verified by decarbonylation over Pd/Al2O3 catalyst can be explained by the
theoretical calculation.28 The reduced electron density of Pd changed adsorption mode of aldehyde group of the key
may change the adsorption mode of aldehyde group of the intermediate p-tolualdehyde on the surface of the catalyst.
substrate on the catalyst and further change the According to previous report,21 there are two kinds of
decarbonylation performance. To verify this assumption, CO adsorption modes of aldehyde on Pd sites:21 one is the η1-(O)
and propanal were used as probe molecules to study the surface species, by which the carbonyl group is bonded to one
adsorption modes of the 1PdnFeOx/Al2O3 catalyst by FTIR. Pd site through the lone pair of oxygen atom; another is the η2-
Figure 3 (B) shows the in situ FTIR spectra of CO adsorption on (C, O) mode, by which the C and O atoms of the carbonyl group
1Pd/Al2O3, 1PdnFeOx/Al2O3 (n=1, 5, 10) and 10FeOx/Al2O3 interact with two adjacent Pd sites. In the work of Resasco
catalysts. Two bands at the wavenumbers around 2079 cm−1 group,21 it was found that the aldehydes tend to adsorb on Pd
and 1919 cm−1 attributing to CO linear and bridge adsorption, surface in form of η2-(C, O). The adsorbed aldehydes would
respectively,29 were observed on 1Pd/Al2O3 catalyst. After the firstly be dehydrogenated to form a more stable and
modification of iron species, two different phenomena were electropositive acyl surface species, and then decompose into
observed. 1) The intensities of the two bands for the linear and alkanes and CO by decarbonylation reaction (Scheme 2 A).
bridge adsorptions of CO on Pd sites were both decreased. As Similar discoveries were also reported by Tsuji31, Roberge32
the bimetallic catalysts were prepared by sequential groups. After modification of FeOx, electron density of Pd was
impregnation of Pd(NO3)2 and Fe(NO3)3 (see in ESI, 2. Catalyst decreased. The resulting Pd nanoparticles with electropositivity
preparation), one can assume that more Pd atoms would be in would repel with the electropositive acyl surface species, which
contact with and covered by FeOx with the increase of Fe makes the acyl intermediate, the key intermediate for
loading. To verify this assumption, CO-chemisorption was decarbonylation, hard to form. Thus, decarbonylation on
conducted to probe the number of Pd atoms exposed. As shown bimetallic
PdFeOx/Al2O3
catalyst
was
suppressed.
in Table S2, CO chemisorption of 1Pd/Al2O3 was 17.49 umol/g. Hydrogenation of aldehyde to alcohol was reported by
However, with increase of Fe loading, the value decreased adsoption mode η2-(C, O)33 or η1-(O)21. However, no matter
gradually to 4.64 umol/g for 1Pd10FeOx/Al2O3, demonstrating which adsorption mode was on 1Pd10FeOx/Al2O3 catalyst,
the cover of FeOx on Pd atoms. 2) Red shifts of the CO electropositive Pd would interact stronger with electronegative
adsorption bands to higher wavenumbers, which further O than with C of the aldehyde. DFT studies34 has demonstrated
verified the decreased electron density of Pd after modification that the first step for hydrogenation of aldehyde to alcohol was
by FeOx, as discussed in XPS characterization. The decrease of hydrogenation on O to OH. Thus, it is likely that O of aldehyde
Pd electron density reduced back-donation of electron density group would be preferentially hydrogenated on bimetallic
from Pd-3d orbits to the anti-bonding π molecular orbital of 1Pd10FeOx/Al2O3, following by hydrogenation of C of aldehyde
coordinated carbonyls, thus resulting in increased bond order group and then hydrogenolysis of the formed C-OH according
O
O
C+
O
H
C
O
C H
C H
Pd/Al2O3
A
B
+
CO
Pd
Pd
Pd
FeOx
FeOx
η2-(C, O)
acyl species
Pd Pd
H
H
O
O
H
CH
H
H
OH
C H
C
O
CH OH
C H
H2
H2
PdFeOx/Al2O3
+
H2O
FeOx
Pd
Pd
Pd
Pd
Pd
e-
FeOx
FeOx
FeOx
FeOx
η1-(O)
Scheme 2. The conversion of 4-MCMCA and adsorption mode of p-tolualdehyde on Pd/Al2O3 and PdFeOx/Al2O3 catalysts.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 5
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Journal Name
to the literature35 (see scheme 2 B). Of which, modification of
FeOx on Pd species plays a key role in suppression of
decarbonylation and promoting hydrodeoxygenation reaction
that tuned the selectivity to PX.
View Article Online
DOI: 10.1039/D0GC00944J
2.3 Versatility of the catalyst
To investigate the versatility of the bimetallic PdFeOx/Al2O3-
catalyzed DHA-HDO cascade reactions, four biomass-derived
cyclohex-3-enecarbaldehyde derivatives were prepared from
bio-based diene (2,3-dimethyl-1,3-butadiene, 2,4-hexadiene
and isoprene)36 and dienophiles (acrolein) using [Bmim]Zn2Cl5
as the catalyst with good yields(See Supplementary Information
5.1) according to our previous method.17a These compounds, as
well as 3,4-dimethyl-benzaldehyde, were then employed in the
developed gaseous-phase reaction over 1Pd/Al2O3 and
1Pd10FeOx/Al2O3. All the substrates underwent mainly DHA-
DCO cascade reaction over 1Pd/Al2O3 to afford one-carbon
losing compounds as the main products (yields range from 73%
to 96%). It was pleasant to find that all the substrates were
dominated by a consecutive DHA-HDO reaction pathway over
1Pd10FeOx/Al2O3, providing the corresponding methylated
benzene derivatives with excellent yields (73%-85%, see in
Table 2). In contrast, the yields of the corresponding
decarbonylation products were remarkably decreased to less
than 20%. Obviously, the selectivity and conversion efficiency of
this transformation over 1Pd10FeOx/Al2O3 held almost
constant regardless of the presence of the methyl group with
different numbers and at different positions. It should be noted
that all these products are important bulk chemicals currently
produced from petroleum resource. Thus, our strategy
represents a highly selective and atom-economic route to
valuable aromatic chemicals from biomass-derived building
blocks.
Figure 4. 1Pd10FeOX/Al2O3 stability tests. Reaction conditions: 4-MCHCA
(WHSV: 0.2 h-1), H2 as the carrier gas (10 mL·min-1), 325 oC.
2.4 Stability of 1Pd10FeOX/Al2O3
Catalyst stability is a major concern for the development of
high-performance catalysts and the application potential of the
catalytic process. Hence, the stability of the bimetallic catalyst
1Pd10FeOX/Al2O3 in the gaseous-phase conversion of 4-MCHCA
was assessed. As shown in Figure 4, 4-MCHCA could be
completely converted for up to 24 h, with the yield of PX
remained nearly constant at approximately 80%, indicating the
high stability of the bimetallic catalyst. XRD pattern of the used
catalyst did not show diffraction peaks assigned to Pd (Figure
S2), indicating metallic Pd remained high dispersion. Comparing
with previous developed W2C/AC catalyst, this is another
attractive feature because the activity of W2C/AC clearly
decreased with the extension of reaction time.
Table 2: Substrate scope test: versatility of 1Pd10FeOx/Al2O3 for the synthesis of
various aromatic compounds through decarbonylation-suppression strategy.
3. Conclusions
In summary, this work provides an efficient strategy for the
direct conversion of biomass-derived 4-MCHCA to PX by a FeOx
modified Pd/Al2O3 catalyst. Using the catalyst with Pd loading
of 1 wt% and Fe loading of 10 wt%, complete conversion of the
feedstock and high molar yields of PX (~81%) can be achieved
DHA-DCO
Product
DHA-HDO
Product
Entry
Catalyst
Substrate
Yield
%
Yield
%
1
2
3
4
5
6
7
8
9
1Pd/Al2O3
75.5
15.6
96.4
16.4
73
22.4
84.2
3.6
O
o
via the consecutive DHA-HDO process at 325 C. Comparing
1Pd10FeOx/Al2O3
1Pd/Al2O3
with monometallic Pd/Al2O3 catalyst, FeOx modification
decreases the electron density of Pd species, which in turn, shift
the adsorption mode of carbonyl group from η2-(C, O), the
necessary mode for decarbonylation, to η1-(O), a mode that
liable for hydrodeoxygenation reaction. As a result, this
bimetallic catalyst not only improves hydrodeoxygenation (C=O
scissor) activity, but also suppresses competitive
decarbonylation of aldehyde group (C-C session); accordingly, a
highly selective reaction pathway towards PX is foreseeable.
This bimetallic catalyst is much more stable and allows for a
lower reaction temperature than previous developed W2C/AC
catalyst (Route 1, Scheme 1). Moreover, this is a continuous
transformation process that is readily applicable to the
synthesis of various (multi) methylated arenes from bio-based
O
1Pd10FeOx/Al2O3
1Pd/Al2O3
83.4
27
O
1Pd10FeOx/Al2O3
1Pd/Al2O3
17.5
96.2
15.9
84.8
80.9
3.8
O
1Pd10FeOx/Al2O3
1Pd/Al2O3
84.2
6.5
O
10
1Pd10FeOx/Al2O3
19.5
73.2
Reaction conditions: Substrate concentration: 20mg/mL; Solvent: hexane; WHSV: 0.2
h-1; Reaction temperature: 325 oC; Catalyst: 0.25g; Carrier gas: H2, 10 ml·min-1.
DHA: dehydroaromatization; HDO: hydrodeoxygenation; DCO: decarbonylation.
6 | J. Name., 2012, 00, 1-3
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building blocks, thus providing a potentially petroleum-
independent solution to valuable aromatic compounds.
Notes and references
View Article Online
DOI: 10.1039/D0GC00944J
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Support from the National Key R&D Program of China
(2019YFC1905300), the National Natural Science Foundation of
China (21902152, 21878288, 21721004, 21690083), the China
Postdoctoral Science Foundation (2019M651160), the Strategic
Priority Research Program of the Chinese Academy of Sciences
(XDB17020100), and DNL Cooperation Fund CAS (DNL180302)
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Table of contents
Selective production of bio-based para-xylene by catalysis of FeOx modified Pd/Al2O3
is presented.