Versatile PdTe/C catalyst for liquid-phase oxidations of 1,3-butadiene

Article abstract of DOI:10.1016/j.apcata.2015.12.024

A commercial Pd catalyst based on Sibunit carbon support was treated with H₆TeO₆ in a reducing media to obtain a Te coating on the surface of Pd particles. The PdTe/C catalyst prepared in this way showed the ability to control the radical chain oxidation of 1,3-butadiene by promoting the selective formation of 2-butene-1,4-diol, 4-hydroxybut-2-enal and furan in DMA (total selectivity of 61% and yield of 7%). At the same time, the catalyst induced oxidation of 1,3-butadiene by a non-radical heterolytic mechanism involving the formation of two groups of primary products: (1) crotonaldehyde and methyl vinyl ketone and (2) the products of oxygenation at the 1,4-positions. The compounds of the second group including 1,4-dimethoxy-2-butene and maleic acid dimethyl ester were formed on PdTe centers in MeOH. Increasing the Te concentration in the PdTe/C catalyst forced the conversion of 1,3-butadiene toward 1,4-oxygenation and simultaneously decreased the intensity of secondary oxidation, resulting in the selective formation of derivatives of the 1,4-oxygenation - 1,4-dimethoxy-2-butene and allenic alcohol methyl ether (total selectivity of 84% and yield of 48%).

Full text of DOI:10.1016/j.apcata.2015.12.024

Accepted Manuscript
Title: Versatile PdTe/C catalyst for liquid-phase oxidations of
1,3-butadiene
Author: N.I. Kuznetsova V.N. Zudin L.I. Kuznetsova V.I.
Zaikovskii H. Kajitani M. Utsunomiya K. Takahashi
PII:
S0926-860X(15)30298-2
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.apcata.2015.12.024
APCATA 15697
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
28-9-2015
18-12-2015
20-12-2015
Please cite this article as: N.I.Kuznetsova, V.N.Zudin, L.I.Kuznetsova,
V.I.Zaikovskii, H.Kajitani, M.Utsunomiya, K.Takahashi, Versatile PdTe/C catalyst
for liquid-phase oxidations of 1,3-butadiene, Applied Catalysis A, General
http://dx.doi.org/10.1016/j.apcata.2015.12.024
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Versatile PdTe/C catalyst for liquid-phase oxidations of 1,3-butadiene
N.I. Kuznetsova1, V.N. Zudin1, L.I. Kuznetsova1, V.I. Zaikovskii1,2 ,H. Kajitani3, M.
Utsunomiya3, K. Takahashi3
1Boreskov Institute of Catalysis SB RAS, Prosp. Akad. Lavrentieva 5, Novosibirsk, Russia
2Novosibirsk State University, Pirogova 2, Novosibirsk, Russia
3Mitsubishi Chemical Corporation, Kurashiki, Okayama, Japan
_____________________________
* Corresponding author. Tel.:+7 383 3269720; fax: +7 383 3269529.
E-mail address: kuznina@catalysis.ru (N.I.Kuznetsova).

Graphical abstract

Highlights
► PdTe/C catalyst can participate competing radical chain and heterolytic route in liquid-phase
oxidation of 1,3-butadiene.
►Specific location of tellurium on the surface of palladium particles in the catalyst.
►High selectivity to 1,4-butenediol derivatives in DMA (61%) and MeOH (84%).
Abstract
A commercial Pd catalyst based on Sibunit carbon support was treated with H6TeO6 in a
reducing media to obtain a Te coating on the surface of Pd particles. The PdTe/C catalyst
prepared in this way showed the ability to control the radical chain oxidation of 1,3-butadiene by
promoting the selective formation of 2-butene-1,4-diol, 4-hydroxybut-2-enal and furan in DMA
(total selectivity of 61% and yield of 7% ). At the same time, the catalyst induced oxidation of
1,3-butadiene by a non-radical heterolytic mechanism involving the formation of two groups of
primary products: (1) сrotonaldehyde and methyl vinyl ketone and (2) the products of
oxygenation at the 1,4-positions. The compounds of the second group including 1,4-dimethoxy-
2-butene and maleic acid dimethyl ester were formed on PdTe centers in MeOH. Increasing the
Te concentration in the PdTe/C catalyst forced the conversion of 1,3-butadiene toward 1,4-
oxygenation and simultaneously decreased the intensity of secondary oxidation, resulting in the
selective formation of derivatives of the 1,4-oxygenation ― 1,4-dimethoxy-2-butene and allenic
alcohol methyl ether (total selectivity of 84% and yield of 48% ).
Keywords Butadiene · Liquid-Phase Oxidation · Heterogeneous Catalysis · PdTe/C Catalyst

1. Introduction
1,3-Butadiene (BD) is readily oxidized and can be a feedstock in numerous catalytic reactions,
such as epoxidation [1], obtaining furan, maleic anhydride, and crotonaldehyde [2–4]. Under
liquid-phase oxidation conditions, there may be different routes for BD conversion. Interacting
with oxygen at a temperature above 50 оС, individual BD and BD in a nonpolar solvent form
peroxides with a polymeric structure consisting of 1,2- and 1,4-units [5], whose thermal
decomposition [6] yields 1,4- and 1,2-butenediols and products of further conversion, such as
furan and acrolein [7, 8]. In aqueous solutions of PdCl2 and CuCl2, BD is oxygenated at one of
the double bonds to form crotonaldehyde [9]. In a solution of a Pd catalyst and P–Mo–V
heteropolyacid as an oxidizer, BD was converted to furan [10]. Furan formation has also been
observed in the presence of iodides and a Pd/C catalyst [11], in the presence of NaJ and a
mixture of CuCl2–NaCl–HCl chlorides [12], and other transition metal complexes [13]. On
PdTe/C catalysts in acetic acid, BD undergoes diacetoxylation with predominant formation (98%
selectivity) of 1,4-diacetoxybutene [14, 15]. This process has been implemented on a commercial
scale by Mitsubishi Chemical Corporation. Other compositions of platinum-group metals with
oxides of transition metal different from Te have been tested in this reaction [16, 17], where the
presence of acetic acid is necessary to stabilize the product in the form of diether [18, 19]. BD
oxidation on a Pd–Te catalyst was carried out in alcohol to obtain alcoholates of 1,2- and 1,4-
diols [20]; a PdTe/C catalyst was also used in this study.
Palladium on carbon catalysts are often used in oxidation reactions due to the ability of metallic
Pd to activate oxygen and due to the high absorption capacity of carbon supports for Pd0 and
Pd2+ ions. The active component is not removed from the catalysts, and in the case of oxidation,
they can be regenerated in a reducing atmosphere. Promoting effect of Te has been noted in
several Pd catalyzed oxidations. In addition to the synthesis of 1,4-diacetoxybutene , several
other applications of Pd–Te catalysts to aerobic oxidation of multiple bonds are known. For
example, PdTe/C catalysts have been patented for oxidation of 1-butene in vapor to

methylethylketone [21], and isobutylene in a water-t-butanol solvent to methacrolein and
methacrylic acid [22]. Promotion of Pd catalysts with tellurium raises the activity and
particularly selectivity of the synthesis of acetic acid from ethylene [23, 24], the oxidation of
alcohols to carbonyl compounds ([25] and references therein), glucose to gluconic acid [26],
sodium lactate to pyruvate [27, 28], para-alkoxytoluenes to alkoxybenzaldehyde and
alkoxybenzyl acetate [29], and lactic acid to pyruvic acid [30]. The nature of the promoting
effect of Te, and Se, Bi, Sb, Tl, etc. in supported bimetallic catalysts has been discussed in the
literature [16, 30–32]. It is suggested that in the case of tellurium, the effect is due to the fairly
strong interaction between Pd and Te, which leads to a charge redistribution between the atoms
[18]. Attempts have been made to explain the promoting effect by the formation of palladium
tellurides, which acts as a coupled redox system. In several studies, telluride phases of the
compositions Pd3Te/C [30], PdTe/C and PdTe2/C [32], PdTen/SiO2(Al2O3), n = 1 and 2 [26], and
PdTe/SiO2 [16] were detected by XRD. However, it has not been possible to relate the activity of
the catalysts to the presence of a particular phase; in other cases, Pd–Te phases have not been
detected, perhaps because of the high dispersion of the active components on the surface of the
carbon support [18].
In the present study, we investigated the catalytic properties of PdTe/C catalysts prepared from a
commercial palladium catalyst by precipitation of tellurium on a palladium surface in a reducing
media. The catalyst performance was studied in the oxidation of 1,3-butadiene involving several
competing mechanisms. Parameters of the catalytic system that can force the oxidation to follow
each route, in particular, oxygenation at the 1,4-positions of BD were determined

2. Experimental
The commercial catalyst IKT-3-23 4.8%Pd/C (Pd/C) was prepared by hydrolytic deposition of
palladium from H2PdCl4 solution on carbon support (Sibunit, S0 400 m2 g-1, powder with a
particle size of 100 µm) in the presence of sodium formate reducing agent. To obtain the Pd-Te
samples, the Pd/C sample was suspended in H6TeO6 solution sufficient to obtain 0.5, 2.5 and 5%
Te in the samples. Sixfold molar excess of formalin was added, and the mixture was kept 1 h at
60 °C. The sample was filtered from solution and dried at 60 °C under reduced pressure. The
Te/C sample was prepared by addition of telluric acid H6TeO6 to carbon suspended in water.
Reducing and other procedures were made as previously described. The Pd and Te contents in
the catalysts were determined by XRF (X-Ray Fluorescence) analysis. The catalysts obtained
had the following composition: 4.7% Pd+0.5% Te/C (Pd0.5Te/C), 4.7% Pd + 1.8%Te)/C
(Pd1.8Te/C), 4.7% Pd + 2.7% Te/C (Pd2.7Te/C) and 5%Te/C (TeO2/C). The catalysts were in
contact with air during storage.
CO adsorption measurements were made on AutoChemII 2920 (Micromeritics) instrument. Pd
dispersion in the Pd/C catalyst was determined based on CO/Pd = 1 stoichiometry, and Pd
particle size (nm) was calculated as 1.13/dispersion [33].
XRD (X-Ray Diffraction) patterns of the catalysts were recorded on ARL X’TRA diffractometer
using CuK radiation (1.54184Å). The definition of phase composition was performed by
comparing the positions and intensities of the diffraction peaks with data cards PC-PDF ICSD.
The mean sizes of Pd crystallites were determined from the full widths on a half of maximum of
the (220) peak, using the Scherrer equation.
HAADF-STEM (High Angle Annular Dark Field Scanning Transmission Electron Microscopy),
EDX (Energy Dispersive X-ray spectroscopy) and elemental mapping were made on JEM
2200FS (JEOL, Ltd.) microscope. Suspension of the catalyst in ethanol was deposited on carbon-

film-coated copper grids. EDX mapping and line analysis were obtained on С K (E= 0.266 keV),
Pd L (E = 2.836 keV) and Te L (E = 4.424 keV) edges. Locality of line analysis was about 1 nm.
Catalytic experiments were carried out in a 300 mL steel reactor (Parr Instrument). The reactor
with a catalyst and solvent was sealed, air was removed by pump, the reactor was loaded with a
fixed volume of BD under stirring and heated to reaction temperature. The reactor was
pressurized with a gas of composition О2 : N2 = 10 : 90 to P= 40 atm, and stirring was started.
After some time, the reactor was cooled to 15 °C and depressurized. The liquid was centrifuged
to remove the catalyst and analyzed by gas chromatography (GC), instrument Kristall 2000,
SPB-1000, 30 m x 0.53 mm capillary column. Identification of the products was based on
retention times and data of GC-MS, HP-5 column.
Pd and Te in solution after catalysis were determined by ICP (Inductively Coupled Plasma
Atomic Emission Spectroscopy) technique, and peroxide content was determined by iodometric
titration.
3. Results and discussion
3.1. The state of Pd and Te in the catalysts
The chemisorption measurements of the Pd/C sample showed adsorption of 0.25 mol CO per 1
g-atom of Pd. Based on the adsorption stoichiometry CO/Pd=1, this corresponds to Pd dispersion
of 25% and an average palladium particle size of 4.5 nm. The adsorption on the Pd2.7Te/C
catalyst resulted in uptake of 0.024 mol CO per 1 g-atom Pd, that indicated a significant
reduction in available Pd surface after deposition of Te.
Data of XPS (X-Ray Photoelectron Spectroscopy) of the catalysts were described in a previous
paper [34]. Whereas Pd/C sample contained Pd0 on the surface, the PdTe/C samples contained
both Pd and Te in metallic and ion forms (Pd 3d5/2 binding energy 335.5 eV for Pd0 and 337.6
eV for Pd2+; Te 3d5/2 binding energy 573.6 eV for Te0 and 575.9 eV for Te4+). The Pd0 : Pd2+

ratio of 2.7 and Te0 : Te4+ ratio of 1.2 for fresh samples were changed to 1.3 and 0.7, respectively,
after contact with air.
The XRD pattern of the Pd/C sample (Fig.1, curve 1) indicates the presence of metallic
palladium (PDF #46-1043) in the form of particles with an average size of 15 nm. The presence
of palladium oxide PdO is possible in trace amount, since there is a low-intensity diffraction
halos in the area of 33-34 ° (2θ) coinciding with the location of the most intense peaks of this
phase (PdO, PDF # 41-1107). The fact that the particle size determined by XRD is larger than
that determined by the adsorption method shows that part of the ultra-small Pd particles makes
no contribution to the intensity of the diffraction peaks. The addition of Te to the catalyst leads
to an increase in relative amount of the PdO phase and, hence, to a decrease in the average
particle size of metallic Pd to 12 nm in Pd1.8Te/C and Pd2.7Te/C samples (spectra 3, 4 in Fig.1).
Tellurium-containing phases were not detected, probably, because of a high dispersion of Te.
The X-ray diffraction pattern of the Pd2.7Te/C catalyst contains low-intensity reflections around
63-65° and 71-75° (2θ) (spectrum 4 in Fig.1) which can be hypothetically attributed to the
presence of Pd–Te intermetallic compounds (PdTe (PDF#29-971) and PdTe2 (PDF#29-970)).
HAADF-STEM analysis of Pd0.5Te/C samples suggests the predominance of small supported
particles of the metal (Fig.2 a). These particles are uniformly distributed on the surface of the
carbon support without evidence of aggregation and have a narrow size distribution of 2–6 nm
(Fig.2 b) (mean size 3.52 nm, standard deviation sd=1.16 nm). At the same time, the metal
contains a fraction of large particles, as shown in Fig.3. Elemental mapping for Pd L, Te L and C
K edges (Fig.3) shows that a considerable amount of tellurium is contained in the dispersed Pd–
Te particles distributed on the carbon surface. The relative atomic concentrations of Pd and Te
for the small and large particles located in the 001 and 002 regions in Fig.3 are given in the
Table. Small particles have a bimetallic composition with a ratio Te/Pd ≈ 1/3. Large particles
also contain Te, though in a smaller proportion to Pd.

To determine the localization of palladium and tellurium for the regions with small and large
particles, we performed a detailed line EDX analysis of the particle composition in the
Pd0.5Te/C sample. The scan lines are shown in Fig.4 a and b. Analysis along the line in the
region of small particles (Fig.4 с) shows that the distribution of Te and Pd is nonuniform for
different locations in the image and the Te/Pd ratio varies, with an average Te/Pd ratio in the
intermetallic particles of about 1/3 (Table in Fig.3).
Similar measurements of the relative atomic concentrations of Te and Pd were made for the edge
of a large particle along the scan line (Fig.4 d). At the edge of this particle, the ratio Te/Pd ≈ 1/6,
whereas in the interior, this ratio decreases to almost zero. This pattern shows the localization of
Te near the Pd surface without penetration into its volume. The Te coverage on Pd surface
explains the data of CO adsorption on PdTe/C sample and may correspond to the formation of
Pd–Te intermetallic compound in a very thin surface layer of Pd.
Thus, the analysis of the catalysts shows that most of Pd is in a highly dispersed state. Tellurium
does not form an individual crystalline phase but is present in highly dispersed form on the
surface of Pd particles. This type of bimetallic particles is produced by reductive deposition of
Te catalyzed by the Pd particles present in the commercial Pd/C catalyst. Previously, the absence
of the Pd–Te phase and the localization of tellurium in the outer layer of active component
particles was noted in [18] for Pd–Te–C catalysts prepared by co-deposition of components. In
contrast, H. Hayashi et al. detected a Pd3Te intermetallic phase in Te/Pd/C catalysts prepared by
impregnation of a 5%Pd/C sample with a solution of TeO2 in HCl. The formation of the
bimetallic phase may be due to the partial dissolution of palladium during impregnation of the
sample in the acid solution. In our case, dissolution of palladium during the synthesis is ruled out.
Therefore, in the prepared catalyst, the interaction of Te with large particles of Pd takes place in
a narrow localized near- surface layer (less than 1 nm thick). The highly dispersed active fraction
consists of bimetallic particles, whose composition can vary according to the tellurium
concentration on the palladium particle. The formation of dispersed particles of Pd-Te

intermetallic compounds and a surface intermetallic compound on the surface of large particles is
also possible.
3.2. Oxidation of BD on Pd and Pd–Te catalysts in DMA
The reaction conditions and products of BD oxidation in the presence of Pd-containing catalysts
in DMA (dimethylacetamide CH3C(O)N(CH3)2) are given in Table 1. Composition of the
products was different from that observed before in absence of catalyst [34]. Scheme 1 shows
two types of oxidation reactions responsible for the formation of the main products. The products
of oxidative addition at the 1,2- and 1,4-positions 3-butene-1,2-diol (1,2-BDOL), 2-butene-1,4-
diol (1,4-BDOL) and 4-hydroxybut-2-enal (HBAL) are formed from BD by radical chain
oxidation (route 1). It is known that they are produced by thermal or catalytic decomposition of
intermediate polymeric peroxide [5-8]. Decomposition of peroxide may involve rupture of the
С–С bond, resulting in partial conversion of the peroxide to acrolein (Ac). Furan (F) is formed
from the products of the chain oxidation by dehydration of HBAL or by oxidative dehydration of
1,4-BDOL. It has been verified that the conversion of 1,4-BDOL under the conditions of BD
oxidation on a heterogeneous Pd catalyst yielded a small amount of furan. Crotonaldehyde (CA)
and methyl vinyl ketone (MVK) are formed by non-radical oxidation route 2 catalyzed by Pd2+
specied. CA has been previously obtained in an aqueous solution of palladium chloride [9]. The
formation of CA from BD in water and the formation of aldehydes from different olefins in
DMA have not been associated with radical reactions, but have been explained by the catalytic
activity of Pd2+ complexes in heterolytic oxidation of olefins at the double bond [35]. The
presence of some byproducts has been attributed to the cyclodimerization reaction to form 4-
vinylcyclohexene. Evidence for this reaction was the detection of C6-cycle oxygenates in
impurity amounts.
Under our conditions, the contribution of the two main oxidation routes to the total conversion of
BD was controlled by varying the composition and amount of the catalyst and the BD load. At a

high load of BD (molar ratio BD/Pd =40), oxidation on the Pd/C catalyst proceeded rapidly to
BD conversion of 40%. The products of the chain radical oxidation were mainly formed. The
yield of the 1,2- and 1,4-oxygenates amounted to 83.6% of the sum of all the products detected
by gas chromatography (No. 1 in Table 1). The GC detected compounds were 23% of BD
consumed. Additional consumption of BD can be due to loss in the gaseous phase, adsorption on
the carbon and formation of high-boiling secondary products. Similarly, the 1,2- and 1,4-
oxygenates were formed by BD oxidation in a solution of PdCl2 (No. 2). CA was also produced
in a significant amount. Addition of H6TeO6 to the PdCl2 solution had an inhibiting effect on the
chain radical oxidation. In the presence of H6TeO6, the total yield of oxidation products (No. 3)
was reduced, and the typical autoxidation product 1,2-BDOL almost completely disappeared.
A similar function of a radical inhibitor was performed by tellurium in the heterogeneous
catalyst. For the development of a rapid chain process in the presence of the Pd–Te catalyst, it
was required to use a low catalyst load, a high BD concentration in the reaction mixture, and
high temperature. At the BD/Pd ratio of 320 in run No. 4, conversion of BD was 13%, the yield
of GC detected products was 11.7%. The products were identical to those obtained on the Pd/C
catalyst, with the only minor difference in the selectivity to 1,2- and 1,4-oxygenates, which for
the Pd1.8Te/C catalysts was 88%. Thus, Pd proved to be an effective catalyst of the chain
process, causing decomposition of intermediate hydroperoxide, whereas tellurium, in contrast,
operates as a radical suppressor and slows down the chain process.
In the next run No. 5, the ratio of the catalyst and BD load (molar ratio BD/Pd=100) and
temperature were chosen so that, along with the chain oxidation reaction, the catalytic oxidation
reaction producing CA, MVK, and F proceeded at the same rate. Subsequent increase in the
catalyst load (molar ratio BD/Pd=20 in Nos. 6 and 7, and BD/Pd=10 in No. 8) led to complete
suppression of the radical process. Under these conditions, only the products of non-radical
catalytic oxidation of BD were formed. However, the yield of catalytic oxidation products was
low and did not increase with increasing contact time, despite the fact that the conversion of BD

reached 80% in Nos. 7 and 8. Apparently, CA and MVK undergo deeper oxidation under the
reaction conditions, presumably, with the formation of carboxylic acids, which cannot be
analyzed by the GC method used in the study.
The solution separated from catalyst after the reaction was found to contain peroxide type
compounds. The amount of peroxide compounds was not directly related to the amount of
oxidation products. Most of the peroxide was found in the system containing H6TeO6 dissolved
in DMA. Apparently, in this case, peroxide was intensively accumulated in the form of
pertelluric acid. In the other cases, the presence of peroxide compounds could not be attributed to
the formation of pertelluric acid in the solution since the metals were only slightly leached. Table
2 gives the amounts of Pd and Te in the solution after the reaction. The molar amount of
dissolved Te was lower than the determined amount of peroxide. By analogy with the telluric
acid solution, peroxide particles could be present on the catalyst in the form of surface
peroxytellurates. It is probable that the ability of Te-oxide centers to bind radical intermediates to
form peroxide species is responsible for the inhibition of chain oxidation by the tellurium present
in the Pd–Те catalyst.
The peroxide compound found in the solution could be incompletely decomposed polymeric
peroxide of BD. In addition to butadiene, peroxide compounds could result from the interaction
of the solvent with oxygen. Indeed, gas chromatographic analysis confirmed the formation of
unidentified compounds from the solvent during BD oxidation. Oxidation of the solvent was
intense when the reaction of BD autoxidation proceeded rapidly in the system. Much less
products of oxidation of the solvent were accumulated under the conditions of non-radical BD
oxidation and in the absence of BD. Thus, DMA was oxidized under conditions of BD oxidation,
predominantly, under conditions of radical chain process. The peroxides detected in the solution
are, in part or in whole, products of oxidation of the solvent [36].
Among the products of BD oxidation in DMA, of greatest practical interest are 1,4-oxygenates,
which are formed in the presence of Pd and Pd–Te catalysts by the autoxidation mechanism

predominantly at a high concentration of BD. On the Pd1.8Te/C catalyst, the total selectivity to
1,4-oxygenates 1,4-BDOL, HBAL and F was 61.8%. Although this process is less selective than
the currently employed method of producing 1,4-BDOL via acetoxylation of BD, it is valuable in
that it eliminates the need for the time-consuming hydrolysis of 1,4-diol diacetates and
utilization of the hydrolysis waste products. At the same time, when using the Pd–Te catalyst, it
is possible to choose oxidation conditions under which the chain process is inhibited and non-
radical catalytic oxidation is dominant [34]. Next the oxidation reaction was studied under
conditions favorable for the non-radical heterolytic catalytic mechanism. As a solvent we used
methanol, which allowed obtaining oxidation products in the form of stable alcoholates.
3.3.
Oxidation of BD in the presence of Pd and Pd–Te catalysts in MeOH
The reaction conditions and products of BD oxidation in aqueous methanol are given in Table 3.
No reaction was observed in the absence of catalyst, whereas the presence of the Pd/C catalyst
led to oxygenation of one of the double bonds of BD with retention of the second bond (No.1 in
Table 3). As in the process in DMA, in methanol we observed the formation of CA and MVK
(route 2 in Scheme 2). Furan formation was not detected. СA was further oxidized to the acid, as
suggested by the occurrence of crotonic acid methyl ester (CAME). Since the initial reaction
medium was neutral, the resulting crotonic acid was apparently only partially esterified. 4,4-
Dimethoxy-2-butenoic acid methyl ester (DMBAME) was detected as an impurity, which could
be a product of the secondary reaction of crotonic acid or its ester. MVK was further oxidized to
form 3-oxobutyric acid methyl ester (OBAME). The products contained a small amount of 1,4-
dimethoxy-2-butene (DMB) formed by reaction route 3. Its isomer 3,4-dimethoxy-1-butene was
not found as a product. Deeper oxidation of DMB yields maleic acid, which is observed in the
form of maleic acid dimethyl ester (MADME). The latter was not formed on the Pd/C catalyst
but was found in the next experiments with the Pd–Te catalyst. In the oxidation with the Pd/C
catalyst, the BD conversion was nearly 100%, and the yield of the oxidation products was 34%.

The reasons of the imbalance between the BD conversion and yield of GC detected products
have been specified above for the oxidation in DMA, and, apparently, are the same for the
reaction in aqueous methanol. In particular, high-boiling carboxylic acids were not detected by
GC. It can hardly be assumed that the increased consumption was associated with
polymerization of BD because of its low concentration in the solvent.
In the presence of the Pd0.5Te/C catalyst, the yield of BD oxidation products was higher due to
the less rapid secondary oxidation reactions (No. 2). The total amount of CA, CAME, and
DMBAME did not change with the addition of tellurium. In contrast, the amount of MVK and
OBAME increased. Simultaneously, there was an increase in the amount DMB, which was then
partially converted to MADME. Tellurium in the catalyst not only decreased the secondary
oxidation of the reaction products, but also affected the mechanism by making the oxygenation
of BD at the 1,4-positions preferred.
In [37], it is noted that the yield of oxidation products of 1,3-dienes with p-benzoquinone in a
homogeneous medium is increased by the addition of methanesulfonic acid. This may be due to
the conversion of oxygenates to methyl esters, which increases their stability against secondary
oxidation. We attempted to add sulfuric acid to the reaction mixture (No. 3). Indeed, in the
presence of the acid, all oxidation products were obtained in the form of methyl esters.
Conversion of BD was over 90%, and the total yield of the products increased due to 1,4-
oxygenates MADME and DMB, whose total yield doubled and amounted to 50% of the total
oxidation products determined by GC. When water was increased in No. 4 to 30% instead of
10% in all the other experiments, the main product became MADME. It should be noted that this
oxidation in CH3OH (route 3 in Scheme 2) differs from autoxidation (route 1 in Scheme 1). The
composition of the products obtained in the presence of the Pd0.5Te/C catalyst and H2SO4
differed from the composition of the products of the radical chain oxidation. In the presence of
Pd0.5Te/C, Ac and 1,2-BDOL were absent, whereas in autoxidation runs they were formed

together with 1,4-oxygenates. Peroxide compounds were also not detected in the solution after
the reaction.
The sequential steps of BD conversion to oxygenates can be examined by varying the
temperature of the oxidation reaction. It was found that even at 70 °С (No. 5), a high conversion
of BD (75%) was achieved, and secondary oxidation of MVK occurred. CA and DMB were only
slightly oxidized to the methyl esters of the corresponding acids. At this low temperature, we
detected the formation of allenic alcohol methyl ether (AME), which is formally a product of
DMB dealkoxylation. At 100 °С, the formation of this product was not detected. As the
temperature was increased to 120 °С (No. 6), the percentage of the products differed little from
that observed at 100 °С. Oxidation of CA to CAME proceeded in the same way as at 100 °С but
the oxidation of DMB to MADME was more rapid. AME was absent at 100 and at 120 °С,
perhaps because it was converted to 1,4-oxygenates.
The variation in the product composition over time was examined by varying the reaction time
from 1 to 3 h under the same conditions (No. 7). It was found that BD oxidation was close to
completion after 1 h. During this time, CA were formed together with CAME, AME, MVK, and
DMB. In the next 2 h, the conversions of CA and MVK were completed. AME was converted to
DMB and MADME, the latter being a more stable 1,4-oxygenate.
In DMA, one of the main products of BD oxidation in the autoxidation regime was 1,4-BDOL.
As noted above, the composition of the reaction products in methanol is inconsistent with
autoxidation mechanism. Nevertheless, route 3 can include the formation of 1,4-BDOL as the
primary product, which would then be converted to AME, DMB, and MADME. To clarify the
sequence of conversions of the products, we performed additional experiments to examine the
conversion of 1,4-BDOL under the conditions of BD oxidation in methanol. It was found that
1,4-BDOL is extremely unstable under the oxidation reaction conditions and forms three main
products: AME, methoxytetrahydrofuran, and MADME. The first two underwent further
conversion, so that, ultimately, MADME was the only product of the conversion of 1,4-BDOL.

In BD oxidation on the Pd0.5Te/C catalyst in methanol, neither 1,4-BDOL nor
methoxytetrahydrofuran were detected. Therefore, it is assumed that DMB and АМЕ are formed
immediately by BD oxygenation at the 1,4-positions, the process being accompanied by
dealkoxylation in the case of АМЕ. Subsequently, both are converted to MADME, with DMB
being fairly stable under the reaction conditions.
Katsuomi Takehira et al. have suggested that the reaction of BD on heterogeneous Pd–Te
catalysts in acetic acid proceeds through the formation of Pd-adsorbed adducts such as allyl
complexes [18]. (In later paper by Jan-E. Backvall et al., π-allyl complexes of Pd2+ have been
considered as intermediates in the oxidation of substituted 1,3-dienes under homogeneous
conditions [38].) It is assumed that tellurium produces a small positive charge on Pd [18] that is
necessary for formation of π-allyl complexes. Though Pd0 is detected by XPS on the surface of
the Pd/C catalyst [34], the charge required on palladium for adsorption of BD in the form of allyl
radicals could occur under the reaction conditions as a result of adsorption of molecular oxygen
δ-
in the form of [Pdδ+O2 ] without the participation of tellurium. In accordance with this, our
results show that 1,4-oxygenates can be formed, though with lower selectivity, on the Pd/C
catalyst in the absence of Te and other promoters in methanol. As shown by XPS analysis [34],
the presence of Te obviously facilitated oxidation of Pd, as the PdTe/C catalysts contained nearly
half of surface palladium in the charged state in the samples before catalysis. More oxidized state
of the surface Pd in presence of Te corresponded to enhanced production of the 1,4-oxygenates
on Pd-Te versus Pd sample (No 1 and 2 in Table 3). At the same time, it was found that the
formation of AME and DMB also took place on the Те/С catalyst not containing palladium (No.
8, Table 3). The reaction proceeded with low conversion of BD and gave a low yield of products.
Consequently, the mutual influence of Pd and Te (probably, TeO2) causes the specific activity of
Pd–Te bimetallic centers different from the activity of individual components. The formation of
bimetallic centers on the surface of the PdTe/C catalysts can be understood with consideration of

the Te localization on the surface of Pd particles, although not evident in occurrence of binary
Pd-Te phases or shift of XPS signals of Pd and Te from typical for Pd0, PdO, Te0 and TeO2.
It could be suggested that the Te content in the Pd-Te catalysts affects not only the yield of
oxygenation products, as observed before in the case of diacetoxylation in acetic acid [18], but
also the complex composition of the products formed in methanol in our case. Indeed, the
Pd2.7Te/C catalyst with a high Te concentration (No 9) showed higher activity than the Te/C
sample free of Pd, but lower conversion of BD (64%) and yield of the products (57%) than
Pd0.5Te/C catalyst. In comparison with the Pd0.5Te/C catalyst, the percentage of 1,4-oxygenate
derivatives increased in the products at the expense of a decrease in the proportion of the
products typical for unpromoted Pd: CA, MVK, and their derivatives. At the same time, Te made
the Pd surface less accessible to the reactants, which slowed down the secondary oxidation and,
in particular, eliminated the formation of MADME. As a result, the major products DMB and
AME were derived from BD oxygenation at the 1,4-positions. The amount of these products
further increased with increasing BD load in the next experiment No. 10. Thus, the high content
of Te, and addition of sulfuric acid to the solvent provided good results in 1,4-oxygenation of
BD. The yield of DMB and AME appeared from 1,4-oxygenation was 48% (No 9 in Table 3)
versus 30% yield of DMB in previously patented oxidation of BD on Pd-Te catalyst in alcohol
medium [20].
4.
Conclusion
Important characteristics of PdTe/C catalysts is the presence of both reduced Pd(0) and Te(0)
and oxidized Pd(II) and Te(IV) forms of the active components and the specific localization of
fine tellurium predominantly on the surface of palladium particles. This composition of the
surface allows the catalysts to participate in several types of oxidative conversion of BD, and the
predominant mechanism of the reaction and the composition of the products of BD oxidation are
controlled by the conditions of the process. In particular, the properties of the solvent are an

important factor affecting the realization of any mechanism. DMA favors the chain route which
leads to the formation of 3-butene-1,2-diol, 2-butene-1,4-diol and 4-hydroxybut-2-enal. In this
case, the action of Te(IV) oxide is manifested in quenching of radical intermediates in DMA.
Under the same conditions but at a low concentration of BD, the non-radical oxidation catalyzed
by Pd2+ species and leading to oxygenation of one of the double bonds is dominant. In methanol,
the second route of non-radical conversion of BD also proceeds, resulting in oxidative addition
to the 1,4-positions. This oxidation route occurs selectively on Pd–Te centers. High Te
concentration does not contribute to the high activity of the catalyst, but has a positive effect on
the selective formation of 1,4-oxygenate derivatives. This makes it possible to produce 1,4-
dimethoxy-2-butene and allenic alcohol methyl ether with high selectivity on the Pd2.7Te/C
catalyst.
Acknowledgments The authors thank colleagues from BIC: Dr. Zyuzin D.A. (XRD analysis)
and Dr. Lisitsyn A.S. (CO adsorption and surface properties).

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Figures captions:
1600
1400
1200
1000
800
600
400
200
0
C
Pd
Pd-Te
Pd
Pd
C
Pd
PdO
1
2
3
4
15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
2 Theta (degree)
Fig.1. Diffraction patterns of catalysts Pd/C (1), Pd0.5Te/C (2), Pd1.8Te/C (3), Pd2.7Te/C (4).
Reflections of observed phases and positions of suggested Pd-Te phases are indicated.

b
а
Fig.2. HAADF-STEM image (a) and particle size distribution (b) for Pd0.5Te/C catalyst.

Memo
Atom%
Pd L Te L
96.14 3.86
77.45 22.55
001
002
Fig.3.
Fig.3. Elemental mapping for Pd L, Te L and C K.

Fig.4. Line EDX-analysis for Pd and Te (C is not shown).

HO
O
OH
OH
O
F
1,4-BDOL
HBAL
.
O
OH
[HOCH2C CH = CH2]
H2C
OH
.
[HOCH2CH = CHCHO ]
1,2-BDOL
1
O
H2C
H
2
Ac
O
O
H3C
H
H3C
MVK
CA
Scheme 1. Oxidation routes (1) and (2) in DMA.
O
O
O
H3CO
H3CO
OCH3
OCH3
H3C
H3C
H3C
H
CAME
CA
DMBAME
O
O
O
2
3
OCH3
H3C
MVK
OBAME
OCH3
H3CO
H3CO
OCH3
C
C
C
OCH3
- CH3OH
O
O
MADME
AME
DMB
Scheme 2. Oxidation routes (2) and (3) in CH3OH.

Table 1. Results of oxidation of BD in DMA(1)
Run No
1
2
3
4
5
6 (2)
7 (2)
8
Catalyst
Weight, g
Pd/C
2
90
40
9.3
PdCl2
0.125
90
40
10.4
PdCl2,H6TeO6 Pd1.8Te/C Pd1.8Te/C Pd1.8Te/C Pd1.8Te/C Pd1.8Te/C
0.125, 0.8
1
0.4
90
2
1
100
10
2
100
10
T °C
90
40
100
160
18.7
90
20
3.4
BD, mmol
Products, mmol
%
20
1.8
0.7
1.8
1.9
1,4-BDOL
HBAL
F
1,2-BDOL
CA
35.3
20.1
2.1
26.1
0.5
0.2
12.0
3.6
28.4
8.5
3.4
30.1
15.0
2.4
10.4
1.8
22.0
4.7
10.6
2.7
23.6
1.9
31.3
2.8
35.4
23.1
3.3
26.2
3.6
1.6
6.2
0.7
0.8
4.8
34.7
4.0
9.4
21.2
21.5
4.4
0
1.1
11.1
5.1
0
33.1
48.1
1.4
0
0.7
3.5
6.3
0.6
36.0
51.2
1.7
0
0.3
0.3
3.1
0
40.4
52.4
1.2
0
MVK
Ac
C6-cycle
Peroxide (O2)2-
0.3
Not det.
1.6
0.2
0.3
0.6
0.4
(1)
30 ml solvent (10% H2O in DMA), 3 h contact time
6 h contact time
(2)

Table 2. Pd and Te content in the solution after catalysis. Composition of starting mixtures and
conditions are given in Table 1.
Run
No
Pd, g
In solution
Te, g
In solution
In starting
catalyst
In starting
catalyst
after
after
catalysis
0.7 10-4
3 10-4
0.1 10-4
2.2 10-4
catalysis
0.001
4
6
7
8
0.047
0.094
0.047
0.094
0.018
0.036
0.018
0.036
1 10-4
2 10-4
<1 10-4

Table 3. Results of BD oxidation in CH3OH solution (1)
Run No
1
2
3
4 (2)
5
6
7
2h
3h
8
9 (3)
10 (3)
1h
Catalyst
Pd/ Pd0.5Te Pd0.5Te Pd0.5Te Pd0.5Te Pd0.5Te
Pd0.5Te/C
+H2SO4
TeO2/ Pd2.7Te Pd2.7Te
/C /C
C
/C
/C
/C
/C
/C
C
+H2SO4 +H2SO4 +H2SO4 +H2SO4
+H2S +H2SO4 +H2SO4
O4
Weight, g
T °C
1
1
1
1
1
1
1
1
1
1
100
10
100
10
100
10
100
10
70
10
120
10
100
10
100
10
120
10
120
40
BD, mmol
Products,
mmol
%
3.4
6.0
8.2
8.5
7.5
7.5
7.2 7.8 7.9
0.06
5.7
11.7
CA
crotonaldeh
yde
27.
1
8.9
5.5
3.4
15.1
6.8
4.8
14. 9.1 4.2
5
5.0
7.0
4.2
1.1
2.0
0.5
CAME
31.
8
19.4
24.6
23.8
29.5
17. 22. 23.
crotonic
acid methyl
ester
AME
allenic
5
7
0
9
0
0
0
0
0
18
0
28.
5
29.0
35.0
33.0
alcohol
methyl
ether
MVK
methyl
vinyl
ketone
OBAME3-
oxobutyric
acid methyl
ester
27.
1
26.3
2.9
1.5
0.6
2.8
1.0
12.
8
1
0.8
8.0
0
1.0
0.6
6.7
0
17.3
22.3
30.7
17.7
2.7 17. 19.
11.6
7
8
DMB 1,4-
dimethoxy-
2-butene
MADME
maleic acid
dimethyl
ester
12.
9
24.6
12.2
28.0
22.1
17.0
32.6
26.6
0
13.3
33.7
16. 24. 22.
51.0
0
49.0
0
57.2
0
8
5
8
0
2.0 23. 28.
6
5
0
DMBAME
4,4-
1.1
5.8
1.0
0.3
0
0
5.3 1.5
0
0
0
dimethoxy-
2-butenoic
acid methyl
ester
(1)
30 ml solvent (10% H2O in MeOH), 0.1 mmol H2SO4 (where indicated), 3 h contact time
30% H2O
2 h contact time
(2)
(3)