In Situ Generation of Br?nsted Acidity in the Pd-I Bifunctional Catalysts for Selective Reductive Etherification of Carbonyl Compounds under Mild Conditions

Article abstract of DOI:10.1021/acscatal.8b04925

Selective synthesis of ethers from biomass-derived carbonyl compounds is an important academic and industrial challenge. The existing processes based on strong acid or metallic catalysts cannot provide high selectivity to ethers due to the occurrence of side reactions. Hereby we propose a Pd-I bifunctional heterogeneous catalyst for the selective reductive etherification of aldehydes with alcohols. Extensive catalyst characterizations uncovered the presence of iodine species on the surface of Pd nanoparticles. Heterolytic dissociation of hydrogen on the I-Pd surface sites leads to the "in situ" generation of a Br?nsted acid, which promotes the reaction toward the corresponding ethers with extremely high selectivity under very mild reaction conditions.

Full text of DOI:10.1021/acscatal.8b04925

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Article
In-Situ Generation of Brönsted Acidity in the Pd-I Bifunctional Catalysts for
Selective Reductive Etherification of Carbonyl Compounds at Mild Conditions
Dan WU, Willinton Yesid Hernandez, Songwei Zhang, Evgeny I. Vovk,
Xiaohong Zhou, Yong Yang, Andrei Y Khodakov, and Vitaly V. Ordomsky
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b04925 • Publication Date (Web): 21 Feb 2019
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In-Situ Generation of Brönsted Acidity in the Pd-I
Bifunctional Catalysts for Selective Reductive
Etherification of Carbonyl Compounds at Mild
Conditions
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Dan Wu,a,b Willinton Y. Hernández,a Songwei Zhang,c Evgeny I. Vovk, c Xiaohong Zhou,
c Yong Yang,c Andrei Y. Khodakov, b* and Vitaly V. Ordomsky a, b
*
a
Eco-Efficient Products and Processes Laboratory (E2P2L), UMI 3464 CNRS-Solvay,
201108 Shanghai, People’s Republic of China, E-mail : vitaly.ordomsky-ext@solvay.com;
b Univ. Lille, CNRS, Centrale Lille, ENSCL, Univ. Artois, UMR 8181 – UCCS – Unité de
Catalyse et Chimie du Solide, F-59000 Lille, France, E-mail : andrei.khodakov@univ-
lille.fr;
c
School of Physical Science and Technology, Shanghai Tech University, Shanghai
201210, People’s Republic of China.
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ABSTRACT. Selective synthesis of ethers from biomass-derived carbonyl compounds is
an important academic and industrial challenge. The existing processes based on strong
acid or metallic catalysts cannot provide high selectivity to ethers due to the occurrence
of side reactions. Hereby we propose a Pd-I bifunctional heterogeneous catalyst for
selective reductive etherification of aldehydes with alcohols. Extensive catalyst
characterizations uncovered the presence of iodine species on the surface of Pd
nanoparticles. Heterolytic dissociation of hydrogen on the I-Pd surface sites leads to the
“in-situ” generation of Brönsted acidity, which promotes reaction toward corresponding
ethers with extremely high selectivity at very mild reaction conditions.
KEYWORDS. Acidity generation, Surface modification, Palladium and iodine, Reductive
etherification, Biomass upgrading.
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Introduction
Ethers and its derivatives are widely used as solvent, surfactants, pharmaceuticals,
polymers and liquid fuels [1-3]. Traditional routes for the synthesis of ethers normally
involve dehydration of alcohols over mineral acids or heterogeneous polymer Nafion-H at
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elevated temperatures (>150 °C) [4-6]. The Williamson ether synthesis by reaction of
alcohol with an alkyl halide is another possible approach for the ether production [7-8].
However, this reaction requires synthesis of environmentally harmful halides as
intermediates.
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The selectivity is the most important challenge in the synthesis of ethers from
biomass-derived feedstocks. The reaction selectivity is usually insufficient in the
traditional synthesis routes, because of complex structure of the biomass-derived
substrates, which simultaneously contain several functional groups like carbonyls and
hydroxyls. The reductive condensation of alcohols with aldehydes and ketones yielding
ethers has been recently proposed as an innovative route to valorize bio-sourced platform
molecules [9-11]. The reaction mechanism involves intermediate acetalization of carbonyl
group over an acid catalyst with subsequent hydrogenolysis of the reaction intermediates
to ethers over metal sites. For example, combination of Pt and Amberlyst-15 has been
used for transformation of 5-(hydroxymethyl)furfural (HMF) to 2,5-bis(alkoxymethyl)furan
with an yield close to 60 % [9]. The proximity of acid and metal sites in the bifunctional
metal-acid catalysts is commonly enhanced by using liquid acids or their solutions (H2SO4,
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HCl etc) [12]. Recently metal chlorides have been proposed for this reaction as catalysts
combining both hydrogenation and acidic properties [13]. Beside acetalization and
hydrogenolysis reaction pathway, another option was using Lewis acid zeolite such as
Sn-Beta or Zr-Beta, where reaction involves transfer hydrogenation of HMF to 2,5-
bis(hydroxymethyl)furan (BHMF) followed by etherification [14-15]. However, poor
selectivity, high temperature, low catalytic activity and difficult catalyst separation from
the reaction products have been so far serious drawbacks for the implementation of
reductive etherification in an industrial scale. In another report, with generation of PdH
species as acid sites during hydrogenation reaction, Pd on carbon with 0.7% loading is
able to catalyze furfural reductive etherification to furfuryl ethyl ether in ethanol with 81%
yield [16]. However, this process suffers from narrow reaction temperature windows,
limitation of substrates and catalyst degradation.
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Molecular iodine has been reported as an effective catalyst for the acetalization
reactions, which has been widely used as a protection method for the carbonyl groups in
organic synthesis [17-18]. One of possible catalytic effects of iodine is related to the
generation of hydrogen iodide, which acts in liquid solutions as a Brönsted acid and
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activates the carbonyl groups by hydrogen bonding effects [19-20]. Recently, surface
modification of heterogeneous catalysts, which often results in major enhancement in
chemoselectivities in numerous reactions, has gained significant interests [21-23].
Modification of metal-supported catalysts with iodine could be therefore, favorable for the
selective synthesis of ethers from aldehydes and alcohols, since both acetalization and
hydrogenolysis functions will be combined in proximity over a single catalyst. However,
this strategy of catalyst design has not yet been explored given the well-known strong
poisoning effect of iodine on metal catalysts such as Pt, Au and Pd [24-25].
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Herein, we developed an extremely efficient Pd-I catalyst (Figure 1) for reductive
condensation of aldehydes and ketones with alcohols with high ether productivity and
operating at very mild reaction conditions. The catalyst described in this work can be
prepared by “in situ” modification of a commercially available Pd-supported catalyst with
organic iodide. This substantially simplifies its preparation process.
Experimental section
Catalyst Preparation
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5 wt. % of Pd on alumina or carbon supports (Pd/Al2O3 or Pd/C, Johnson Matthey) has been
used as a parent catalyst. Deposition of iodine on the surface of Pd has been performed by addition
of 0.2 g of Pd/Al2O3 or Pd/C together with 50 mg of ethyl iodide (EtI) in 6 g of isopropanol into
40 ml stainless-steel reactor. The reactor was pressurized with 10 bar of H2 and stirred at 60 ºC for
1 h. Afterwards, the catalyst has been separated and thoroughly washed with isopropanol and dried
in the vacuum oven at 80 ºC overnight. The synthesized samples were denoted as I-Pd/C and I-
Pd/Al2O3. Preparation methods of other iodine modified catalysts like I-Pd/Al2O3 (N2), NaI-
Pd/Al2O3, I2-Pd/Al2O3 and I2/Al2O3 are specified in Supporting Information (SI).
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Characterization
For TEM analysis, a JEOL-2011F having an acceleration voltage of 200 kV was used. Prior
to TEM characterization, the samples were dispersed in ethanol solution with ultrasonic treatment
for 30 min and then dropped onto a carbon film on copper grid. The content of iodine in I-Pd/Al2O3
catalyst was determined by Energy Dispersive Spectroscopy (EDS) analysis.
XPS analysis has been performed in a ThermoFischer ESCALAB 250Xi photoelectron
spectrometer using monochromated X-ray irradiation Al Kα (hv = 1486.7 eV) and 180° double
focusing hemispherical analyzer with a six-channel detector. The BE (binding energy) of the
photoemission spectra was calibrated to Al 2p peak with BE 74.5 eV for Al containing samples
and to adventitious carbon C 1s peak with BE 284.8 eV for PdI2.
The CO pulse adsorption was performed using AutoChem II 2920 apparatus from
Micromeritics. 50 mg of sample was put in a quartz reactor, and then the samples were reduced in
a flow of 5% H2/Ar flow (60 ml/min) with heating rate 10 ºC/min at 60 ºC for 0.5 h. After cooling
down to 45 ºC the catalyst has been treated by CO pulses in He flow till full saturation. H2-TPR
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has been performed in the same unit using CryoCooler for sub-ambient (-70 ºC) measurements
with heating rate 10 ºC/min in 5% H2/Ar flow.
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FTIR spectra were recorded using a Thermo Fisher Scientific Nicolet 6700 FTIR (32 scans
at a resolution of 4 cm−1) equipped with a mercury cadmium telluride (MCT) detector. Pyridine
and CO-FTIR experiments were performed in a vacuum cell (less than 10-5 torr). The catalyst
samples for analysis were pressed in a 40 ~ 50 mg/cm2 (D=13 mm) self-supporting discs. Before
analysis, all samples were reduced at 60 ºC for 1 h with subsequent vacuum treatment for 3 h. CO
adsorption has been performed by addition of CO doses in the cell at r.t. till full saturation of the
signal. Pyridine (Py) adsorption has been performed in the same way by addition of Py doses. For
some experiments 500 torr of hydrogen has been introduced in the cell after Py adsorption.
Catalysis
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The etherification reaction has been conducted in a 40 ml stainless-steel autoclave equipped
with a magnetic stirrer, pressure gauge and an automatic temperature controller. In a typical
experiment, 2 g of isopropanol, 0.1 g of furfural and 50 mg of catalyst with or without 15 mg of
organic iodide were loaded into reactor. Afterwards, the reactor was sealed and pressurized by 20
bar of H2, followed by heating up to the target temperature with continuous magnetic stirring. After
reaction, the autoclave was cooled down, the pressure was released and the solution was separated
by filtration and analyzed by gas chromatography (GC) with biphenyl as the internal standard. In
some experiments, other alcohols and carbonyl compounds have been used instead of furfural and
isopropanol.
The products of the reaction have been analyzed by GC (Agilent Technologies 7820A)
equipped with a polar column of HP-5 and flame ionization detector (FID), with inlet temperature
of 250 ºC. The products have been separated by starting from 75 °C and hold for 5 min with
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subsequent heating to 260 ºC with the rate 10 ºC/min. GC-MS analysis (Agilent Technologies
5977A MSD with Agilent Technologies 7890B GC system equipped with HP-5 capillary column)
was used to identify the organic compounds.
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The conversion of carbonyl compound, selectivity and yield to corresponding products were
defined as follows:
푛퐴
Conversion (%) =
1 ― 푛표
퐴
푛푝
Selectivity to the product p (%) = 푛표 ― 푛
퐴
퐴
Yield (%) = Conversion
Selectivity
×
푛퐴 푛표퐴
,
and 푝 refers to the final, initial moles of carbonyl compound and final moles of product,
푛
respectively. Biphenyl was used as internal standard for the GC analysis.
Results and discussion
Etherification over the Pd-I catalysts
The commercial catalyst Pd/Al2O3 (5 wt. %) has been used for hydrogenation of
furfural in isopropanol solution. The catalyst demonstrated high activity with formation of
tetrahydrofurfuryl alcohol (THFA) as the main product (Table 1, Entry 1). Addition of
traces of ethyl iodine (EtI) in the reactor leads to a total change of the reaction route (Table
1, Entry 2), causing a favored formation of ether (2-(isopropoxymethyl)furan) as the main
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product (91.8 % selectivity) and traces of acetal (2-(diisopropoxymethyl)furan). Note that
this dramatic change in the selectivity could be explained either by the presence of EtI as
co-catalyst during the reaction, for example, by generation of HI or as modifier of the Pd-
supported catalyst.
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In order to identify the role of EtI, we did the treatment of Pd/Al2O3 with EtI in
isopropanol solution, at the reaction conditions, but without furfural. Afterwards, the solid
catalyst was washed thoroughly with isopropanol and dried in vacuum oven overnight to
remove the physically adsorbed iodine species. The modified I-Pd/Al2O3 catalyst
demonstrated lower catalytic activity compared with the mixed system Pd/Al2O3 and EtI
but showed very similar selectivity towards formation of the ether (Table 1, Entry 3).
Another indication on modification of the catalyst by iodine species is in its stable
catalytic performance after 5 cycles with intermediate separation by centrifugation and
addition of fresh substrate without EtI (Figure S1, SI). Comparable reaction rates were
observed in several consecutive reaction cycles (Figure 2). The decline of selectivity after
15 min of test for 2 and 3 cycles could be explained by accumulation of water in the
catalyst leading to higher formation of hemiacetal at the initial stage of the reaction.
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In order to check the role of hydrogen in the genesis of active sites, we have
performed the treatment of Pd/Al2O3 by EtI in inert atmosphere of nitrogen. The catalyst
demonstrates the catalytic performance similar to initial Pd/Al2O3 catalyst with THFA as
the main product of the reaction (92.6 %, Table 1, Entry 4). Thus, hydrogen atmosphere
is necessary for the effective modification of the palladium catalyst.
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In order to verify the role of iodine species on the catalytic performance we have
performed modification of the catalyst by the NaI salt which resulted only in partial catalyst
deactivation without affecting the selectivity observed for non-promoted Pd/Al2O3 catalyst
(Table 1, Entry 5). Thus, the modification of the catalyst might be explained by reaction
of Pd with organic iodide in the presence of hydrogen with formation of atomic iodine on
the surface. Numerous previous studies demonstrated that alkyl iodides could dissociate on
metal surface with generation of iodine atoms and alkanes species in the presence of H2 [26]. To
confirm the dissociation of alkyl iodides, isobutyl iodide (iBuI) and iodobenzene (IBn) were also
used as iodine source in the reaction mixture. Notably, high yield of ether (Table 1, Entry 6 & 7)
was observed in both experiments. Moreover, the analysis of the products from Pd/Al2O3 promoted
with IBn had shown the presence of benzene in the products (Figure S2, SI). This indicates on the
dissociation of IBn with formation of iodine species, which might modify the Pd catalyst.
Moreover, the iodine molecule has been also used as an additive (Table 1, Entry 8). Although the
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formation of ether was observed in the products, conversion of furfural was quite low. The main
side product was acetal due to the presence of I2 in the reaction mixture, which is well known as a
catalyst for acetalization reaction. However, the catalyst pretreated by molecular I2 in isopropanol
solution has demonstrated relatively high activity and selectivity to ether(Table 1, Entry 9). It has
been reported that I2 can dissociate on the metal surface with formation of adsorbed iodine atoms
[27]. Thus, organic iodides as well as molecular iodine can be used for modification of the surface
of Pd.
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Note that iodine could modify both the alumina support and Pd. In order to verify the role of
support in this reaction we have tested Pd/C (5 wt. % Pd) in hydrogenation of furfural in the
presence of EtI (Table 1, Entry 10-11). The non-promoted Pd/C catalyst demonstrated high
selectivity to THFA similar to Pd/Al2O3. However, additionally the catalyst yields 2-
methyltetrahydrofuran as the product of deeper hydrogenation and tetrahydropyran derivatives as
the products of ring opening and re-cyclization (Figure S3 & Table S2, SI). High activity of Pd/C
catalyst in deep hydrogenation reactions has been explained earlier by inert support and presence
of unsaturated sites over Pd nanoparticles [28-29]. Similar to the alumina supported catalyst,
promotion of Pd/C with EtI also shifts the selectivity of the reaction towards the ether. The same
effect is also observed after pre-treatment of the Pd/C catalyst by EtI (Table 1, Entry 12). It
means that the pre-treatment of alumina with iodide does not play a key role in the modification
of the selectivity of the reaction.
Non-supported Pd powder was tested in this reaction as well (Table 1, Entry 13 & 14). In
the presence of traces of EtI, the selectivity to ether increased from 0 to 33.3 %. The low conversion
of furfural on non-supported Pd resulted from the low metal surface area (Table S1, SI). These
results indicate that iodine most probably modifies Pd, leading to the new active species for
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selective synthesis of ether. In this case, the formation of PdI2 could be responsible for change of
the reaction route. Test of PdI2 salt in the reaction has shown formation of acetal as the main
product of the reaction without ether in the products (Table 1, Entry 15). The absence of ether
could be explained by absence of metallic sites for acetal hydrogenolysis.
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Iodine molecule itself has been shown earlier as an active catalyst in transformation of
aldehydes [17]. We have checked the presence of I2 in the solution after reaction over I-
Pd/Al2O3 by a test with addition of starch. The solution was clear and colorless which
means absence of I2 formation (Figure S4, SI). Additionally, I2 on Al2O3 catalyst has been
prepared by impregnation of I2 over alumina which has been used for several reactions
[30-33]. However, only acetal was observed as the product of the reaction without ether
formation (Table 1, Entry 16).
The potential application of other halogens for pretreatment of Pd has been
investigated by using chlorobutane (BuCl) and bromobutane (BuBr) as additives.
However, the presence of BuCl and BuBr only decreased the hydrogenation ability of
Pd/Al2O3. Notably, no ether was produced over these catalysts (Table 1, Entry 17 & 18).
CO adsorption over the catalysts after pretreatment shows similar adsorption capacity in
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comparison with the parent catalyst, which indicates low adsorption of Cl and Br over Pd
metal surface (Table S1, SI).
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Thus, I-Pd/Al2O3 is an efficient and stable catalyst for reductive etherification of
furfural. Moreover, we have performed the etherification reaction at even lower
temperature (35 °C) for 6 h (Table 1, Entry 19) with similar activity and selectivity.
Consistently, an approximate 80 % selectivity to ether product was identified by NMR
analysis after the removal of non-reacted isopropanol by rotary evaporation and filtration
1
of catalyst. H and 13C NMR spectra of the ether are displayed in Figures S5 and S6, SI.
The further purification of the product by extraction of furfural and furfuryl alcohol by water
shows high purity of formed ether according to NMR analysis (Figures S7 and S8, SI)
with isolated yield of 76 %. This demonstrates the practicality of this route.
Characterization of the catalyst
Deep characterization of the catalysts was performed in order to gain insights on the
structure-activity relationships. HRTEM and EDS mapping analysis were carried out to
identify the changes after modification of EtI and preferred location of the iodine species
on the I-Pd/Al2O3 catalyst (Figure 3, Figure S9, SI). The particle size distribution of Pd in
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I-Pd/Al2O3 catalyst was in the range of 3 ~ 5 nm, which is similar to the parent catalyst.
According to HRTEM images, the distance between crystallographic planes was 0.225
nm, in a good agreement with the (111) lattice spacing (0.226 nm) of the face-centered
cubic (fcc) Pd, which is in consistent with parent Pd/Al2O3 [34]. In addition, according to
Figure 3d, most intense iodine EDS signals come from the same locations as Pd and only
negligible iodine is detected on the Al2O3 support. This observation suggests that most of
iodine is adsorbed on the Pd surface and not on the support. Moreover, from EDS
elemental analysis (Table 2), the loading of iodine on the I-Pd/Al2O3 catalyst is estimated
to be 0.39 wt. %, which corresponds to 0.03 mmol of iodine per 1 g of catalyst.
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CO chemisorption analysis of I-Pd/Al2O3 catalyst also suggests interaction of I
species with the Pd surface atoms (Table 2). The CO uptake decreases from 0.14 mmol/g
for initial Pd/Al2O3 to 0.0063 mmol/g for I-Pd/Al2O3. The dramatic decrease in the CO
uptake indicates on deactivation of several Pd atoms by adsorbed I atoms. Interestingly,
for the I-Pd/Al2O3 (N2) and NaI-Pd/Al2O3 samples, the CO-adsorption uptakes did not
change a lot after treatment with the iodine compounds indicating that the iodine is not
effectively bonded to the Pd surface (Table S1, SI). CO-FTIR analysis was performed to
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confirm the interaction of CO and Pd. Figure S10, SI shows a set of CO absorption band
on initial Pd/Al2O3 with linear at 2090 cm-1 and bridged and multiple species at 1976 cm-
1 and 1913 cm-1, respectively [35]. However, there are no peaks of CO over I-Pd/Al2O3,
which indicates electron withdrawing and hindrance effect of I species over Pd surface.
The electronic state of palladium and iodine in the I-Pd/Al2O3 catalyst was
characterized by XPS. Figure 4 demonstrates Pd 3d and I 3d core level spectra of I-
Pd/Al2O3 before and after reaction compared with spectra of Pd/Al2O3 and PdI2. A Pd 3d
spectrum of one chemical state has 3d5/2 and 3d3/2 components with 5.26 eV spin-orbit
splitting. Pd 3d spectrum of Pd/Al2O3 catalyst, as shown in Figure 4a, has two doublets
with BE (Pd 3d5/2) 335.3 eV and 336.7 eV attributed to metallic Pd0 and Pd2+ (PdO)
respectively [36-37]. Both of these states coexist on the surface with domination of the
metallic state. A minor shift of Pd 3d peaks to higher BE for I-Pd/Al2O3 catalysts (both
before and after reaction) can be attributed to interaction of Pd with iodine. This interaction
leads to the electron withdrawal from Pd to I. In I-Pd/Al2O3 catalyst, Iodine has access to
electrons from Pd nanoparticles resulting in lower electron density and partial positive
charge of the Pd nanoparticles [38-39]. It is interesting that Pd2+/Pd0 ratio is lower for I-
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Pd/Al2O3 sample (before reaction) comparing to that of Pd/Al2O3 (16% vs. 32%, Table 2).
Additional information about Pd oxidation state has been obtained from H2-TPR analysis
(Figure S11, SI). Broad peak of hydrogen consumption over Pd/Al2O3 is observed at 15 °C.
For the I-Pd/Al2O3 catalyst, a very weak reduction peak is observed at approximately
50 °C. The consumption of H2 for Pd/Al2O3 and I-Pd/Al2O3 is 0.41 and 0.07 mmol/g,
respectively (Table 2). It also points on a lower fraction of PdO phase in I-Pd/Al2O3
catalyst. This might suggest that I atoms coordinate over Pd surface and protect them
from oxidation, preventing formation of PdO in I-Pd/Al2O3 catalyst.
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I 3d spectra of the considered samples are shown in Figure 4b, demonstrating 3d5/2
and 3d3/2 spin-orbit doublets with 11.5 eV splitting. For I-Pd/Al2O3 catalyst the I 3d5/2 peak
BE is 619.5 eV. The I 3d5/2 peak of PdI2 demonstrates the same BE of 619.5 eV. The I
3d5/2 BE for I atomically located on metal surfaces is expected between 618.5 and 619.7
eV [40]. An observed value within this region suggests dissociation of EtI over palladium.
The relative broadening of I 3d peaks of I-Pd/Al2O3 catalyst is probably a result of irregular
localization of I on the planes, edges and corners of Pd nanoparticles. I-Pd/Al2O3 after
reaction shows similar Pd 3d and I 3d spectra suggesting high stability of the catalyst.
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Mechanism of the reaction
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Acetalization of carbonyl groups is usually considered as an acid catalyzed reaction
and takes place in the presence of Brönsted acid sites [19-20]. Therefore, we deduce that
the generation of Brönsted acid sites in the Pd-I catalysts maybe a key step for the
carbonyl group activation.
To confirm the presence of Brönsted acid sites, an in-situ pyridine-FTIR analysis was
carried out on Pd/Al2O3 and I-Pd/Al2O3. As shown in Figure 5, after adsorption of pyridine,
both initial Pd/Al2O3 and the I-Pd/Al2O3 show a peak at 1450 cm-1 assigned to the Lewis
acid sites probably present on the Al2O3 support. No peak related to the adsorption of Py
over Brönsted acid sites (1540 cm-1) was detected for both catalysts [41]. Then, we added
hydrogen at 60 °C to check the acidity of I-Pd/Al2O3 close to the reaction conditions.
Surprisingly, we observed appearance of a Py-IR peak at 1540 cm-1. This might indicate
generation of Brönsted acidity on the catalyst under hydrogen atmosphere. Removal of
hydrogen from the cell led to disappearance of this peak. However, it could be recovered
by dossing again hydrogen. This result indicates that the acidity generation on the Pd-I
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catalyst is reversible and depends on partial pressure of hydrogen. The same experiment
over parent Pd/Al2O3 did not show any effect of hydrogen on the catalyst acidity (Figure
5). Thus, the remarkable catalytic behavior of the I-Pd/Al2O3 catalysts could be explained
by considering the electronic interactions between the Pd and iodine species. After
dissociation of hydrogen on the surface of Pd, a strong electron withdrawing effect of the
neighboring iodine species could cause polarization of hydrogen over the Pd-I pairs.
Because of the high negative charge, the adsorbed iodine atoms would favor the
stabilization of H+ produced during hydrogen dissociation resulting in the generation of
Brönsted acidity. Hydrogen spillover over the Pd catalyst has been demonstrated
previously [42-45]. Highly mobile hydrogen can generate Brönsted acid sites on the
reducible supports such as WO3 and CeO2 [45]. Iglesia [46] et al in late nineties observed
generation of Brönsted acidity in the presence of H2 on WOx−ZrO2 catalysts with
polytungstate domains. The exposure to hydrogen results in a slight reduction of tungsten
and delocalization of an electron from an H atom resulting in H+δ (Brönsted acid site). The
catalysts developed by Iglesia are obviously rather different from our Pd-I systems. The
interest and advantage of the catalytic system presented in this work is related to the
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generation of acidity in-situ during the reaction directly on the metal sites. This Pd-I
catalyst results in the intimacy of metal and acid functions and thus, very efficient
bifunctional catalysts.
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In order to confirm generation of acidity over Pd in the presence of hydrogen, we
have performed two model reactions: hydrogenation of acetone and dehydration of tert-
amyl alcohol (TAA) over Pd/Al2O3 before and after modification by EtI. Table 3 shows that
at 200 °C the non-promoted Pd/Al2O3 produces isopropanol as the main product (63 %)
with small amount of methyl isobutyl ketone (MIBK), which is a product of aldol
condensation of acetone. At the same time, I-Pd/Al2O3 demonstrates totally different
catalytic performance with high selectivity to MIBK (93 %). The aldol condensation of
acetone proceeds over acid or basic sites. Dehydration of the product of condensation
leads to mesityl oxide, which is hydrogenated to MIBK [47-48]. The presence of both
Brönsted acid and metal sites in bifunctional catalysts such as Pd/Amberlyst-15 or
Pd/zeolite is essential for this reaction. I-Pd/Al2O3 provides both types of sites in close
proximity to each other, which leads to the selective synthesis of MIBK.
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Dehydration of tert-amyl alcohol (TAA) was tested to confirm the generation of
Brönsted acidity (Table 3). Notably, I-Pd/Al2O3 exhibited high activity and selectivity to
isoamylene (Conv. 95.3% & Sel. 96.2%), while initial Pd/Al2O3 has low activity with
formation of isopentane as the main product. This significant difference could be ascribed
to the in-situ generation of strong Brönsted acid over I-Pd active sites in the presence of
hydrogen [49-50].
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A feasible reaction pathway for the etherification of furfural and isopropanol on the
bifunctional I-Pd/Al2O3 catalyst is presented in Figure 1. First, Brönsted acid sites (H+) are
produced on the sites of adsorbed iodine atoms via hydrogen dissociation on Pd-I sites.
Then, the acetal intermediates are produced via nucleophilic addition of alcohol to
aldehyde catalyzed by the Brönsted acid sites. The highest selectivity to acetal is
observed at the beginning of the reaction and decreases with the reaction time (Figure
6). Finally, the C-O bond dissociates with formation of water and addition of H from Pd
surface to form ether. This mechanism has been confirmed by isotope tracing experiment
using deuterated isopropanol-d8 as reagent (Figure S12, SI). According to the literature,
one of the possible routes for the reductive etherification could be catalyzed by acid sites
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via catalytic transfer hydrogenation of furfural by the MPV (Meerwein–Ponndorf–Verley)
reduction with isopropanol as hydrogen donor with subsequent etherification of formed
alcohol to the ether [10]. The presence of only 1H hydrogen atom on the carbon atom in
carbonyl group in furfural confirms the hydrogenolysis of grafted C-O bond and excludes
the catalytic transfer hydrogenation involving deuterated isopropanol-d8 and etherification
pathway.
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Other substrates
The I-Pd/Al2O3 bifunctional heterogeneous catalyst was evaluated in the reductive
etherification of different substrates. First, etherification of furfural with octanol also shows
high selectivity to the relevant ether. This suggests that primary alcohols might be also
used for the synthesis of surfactants and fuels (Table 4). Tertiary alcohol like tert-amyl
alcohol has been also tested in reductive etherification of furfural (Table 4). Although the
catalytic activity is relatively low in comparison with other alcohols we could observe
selective formation of tert-amyl furfuryl ether. This low activity in the reaction with tertiary
alcohols might be explained by difficulties in the formation of acetal-like intermediates due
to steric hindrance. HMF is another valuable biomass based product. It might be easily
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produced by dehydration of glucose and fructose. Reaction of HMF with isopropanol
leads to the synthesis of the ethers with the selectivity close to 83 %. The reductive
etherification of an aromatic aldehyde might also be performed with an aromatic alcohol.
Thus, reaction of benzyl aldehyde with benzyl alcohol leads to 80 % selectivity to benzyl
ether. The possibility of self-etherification reaction has been verified by conducting the
reaction without benzyl aldehyde. However, only traces of benzyl ether have been
produced in this case (Figure S13, SI). Substitution of an aromatic aldehyde by aliphatic
1-octanal also results in selective synthesis of ether (92 %). Acetophenone also
selectively reacts with methanol (Figure S14, SI). All these results demonstrate versatility
of the Pd-I catalyst, which can produce corresponding ethers from a variety of aldehydes
and ketones.
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Conclusion
To conclude, modification of Pd catalysts by treatment with organic iodine
compounds in hydrogen leads to surface modification of Pd. The resulting Pd-I
bifunctional heterogeneous catalyst generates Brönsted acidity in-situ in the presence of
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hydrogen. The acid sites associated with adsorbed I species and Pd metal active sites
are located in close proximity over the supported Pd nanoparticles. The newly designed
bifunctional heterogeneous Pd-I catalysts showed high reductive etherification activity
and selectivity even at ambient temperature. Numerous aldehydes and ketones reacted
on the Pd-I bifunctional catalysts with alcohols at very mild reaction conditions yielding
corresponding ethers with extremely high selectivity.
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Table 1. Etherification reaction of furfural over Pd catalysts treated and non-treated by iodine
compounds (T=60 °C, 20 bar H2, 0.1 g furfural, 2 g isopropanol, 50 mg catalyst, 1h)
Selectivity, %
Furfural
Conv.,
%
Entry
Catalytic system
FA THFA
Ether Acetal Hemiacetal Others f
1
2
Pd/Al2O3
Pd/Al2O3 + EtI a
I-Pd/Al2O3
87.8
72.1
35.8
59.7
34.5
92.5
55.4
38.2
68.7
99.9
0
3.9
0
99.1
0.2
0
0
91.8
83
0
1.3
12
0
0
1.7
1.4
0
0.9
1.1
3
3.6
4
I-Pd/Al2O3 (N2) b
0
92.6
83.4
0
0
7.3
c
5
NaI-Pd/Al2O3
2
0
0
0
14.6
4.7
6
Pd/Al2O3 + iBuI a
Pd/Al2O3 + IBn a
7.8
3.4
0
86.2
84.6
65.6
90
1.3
1.4
32.7
2.1
0
0
7
0
2.8
0.5
1.7
0
7.8
a
8
Pd/Al2O3 + I2
0
1.2
9
I2-Pd/Al2O3
Pd/C
3.8
0
0
2.3
10
43.3
0
56.7
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Pd/C + EtI a
I-Pd/C
88.1
16.8
52.4
24.3
54.8
36.6
53.5
46
19.7
0
1.5
69.4
39.7
0
0.3
54.5
0
2.8
0
6.3
5.8
0
90.6
0
Pd powder
Pd powder + EtI a
PdI2
8.9
0.3
0
0
0.5
33.3
0
64.3
99.3
99.9
0
0
2.1
0
0
0.7
d
I2/Al2O3
0
0
0
0
0.1
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Pd/Al2O3 + BuCl a
Pd/Al2O3 + BuBr a
Pd/Al2O3 + EtI e
0
72.3
41.1
0
0
0
27.7
58.9
2.7
0
0
0
0
99.6
7
82.5
0.8
6.9
a 15 mg of EtI, iBuI, IBn, I2, BuCl or BuBr have been added in the reaction mixture before reaction
b Pd/Al2O3 was pre-treated with EtI in isopropanol under N2 atmosphere
c Pd/Al2O3 was pre-treated with NaI in water in H2 atmosphere
d The I2/Al2O3 was prepared by impregnation method according to ref [32]
This experiment was performed at 35oC for 6 h
e
f
Others normally include tetrahydrogenfurfural (THFF), 2-methyltetrahydrofuran (MTHF), open rings derivatives and
unknown products. Others products in some reactions (Sel. > 5%) are illustrated in Table S2, SI.
Table 2. Characterizations of Pd/Al2O3 and I-Pd/Al2O3
Elemental analysis,
Pd2+/Pd, CO adsorption, H2 consumption,
PdCO/Pd,
%
wt%
Catalyst
%
mmol/g
mmol/g
Pd
5
I
0
Pd/Al2O3
32
16
0.14
0.41
0.07
29
I-Pd/Al2O3
5.7
0.39
0.0063
1.3
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Table 3. Model reactions of acetone to methylbutyl ketone (2 g acetone, 50 mg catalyst, 20 bar
H2, 200 °C, 3 h) and tert-amyl alcohol dehydration (TAA) (2 g TAA, 50 mg catalyst, 10 bar H2,
200 ºC, 3 h)
7
8
9
OH
O
O
O
H2
+
+
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Cat.
Acetone
Isopropanol Mesityl oxide
MIBK
Selectivity (%)
Mesityl oxide
Catalyst
Pd/Al O
Conversion of acetone (%)
Isopropanol
MIBK
31.0
46.8
16.1
63.1
0
0
2
3
I-Pd/Al O
2.0
93.3
2
3
H2
HO
+
Cat.
TAA
Isopentane
Isoamylene
Selectivity (%)
Isopentane
Catalyst
Pd/Al O
Conversion of TAA (%)
Isoamylene
7.1
99.9
3.7
0
2
3
I-Pd/Al O
95.3
96.2
2
3
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Table 4. Reductive etherification of various substrates (0.1 g of aldehydes or ketones, 2 g
alcohols, 50 mg Pd/Al2O3 catalyst, 15 mg EtI were added in the reactor)
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Reaction conditions,
T(oC)/t (h)/H2 (bar)
Conv. a
(%)
Sel. b
(%)
Aldehyde
Alcohol
60 / 15 / 20
95
95
60/15/20
60 / 15 / 20
60 / 1 / 30
60 / 15 / 20
80 / 3 / 30
15
52
96
96
61
91
80
83 c
92
90
a Conversion of the aldehydes or ketones
b Selectivity to corresponding ethers
c Summary selectivity of (5-(isopropoxymethyl)furan-2-yl)methanol and 2,5-bis(isopropoxymethyl)furan
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Figure 1. Scheme of the synthesis of ether from aldehyde and alcohol involving in-situ Brönsted
acidity generation in the presence of hydrogen over heterogeneous Pd-I catalyst.
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Figure 2. Evolution of conversion (a) and selectivity (b) for the first 3 cycles with reaction
time. Reaction conditions: 50 mg I-Pd/Al2O3, 100 mg furfural, 2 g isopropanol, 60oC, 20
bar H2.
ACS Paragon Plus Environment
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