One-pot two-step process for direct propylene oxide production catalyzed by bi-functional Pd(Au)@TS-1 materials

Accepted Manuscript

Title: One-pot two-step process for direct propylene oxide

production catalyzed by bi-functional Pd(Au)@TS-1 materials

´

Author: Alejandro Prieto Miguel Palomino Urbano Dıaz

Avelino Corma

PII:

S0926-860X(16)30272-1

DOI:

Reference:

http://dx.doi.org/doi:10.1016/j.apcata.2016.05.019

APCATA 15883

To appear in:

Applied Catalysis A: General

Received date:

Revised date:

Accepted date:

21-3-2016

10-5-2016

21-5-2016

´

Please cite this article as: Alejandro Prieto, Miguel Palomino, Urbano Dıaz,

Avelino Corma, One-pot two-step process for direct propylene oxide production

catalyzed by bi-functional Pd(Au)@TS-1 materials, Applied Catalysis A, General

http://dx.doi.org/10.1016/j.apcata.2016.05.019

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Highlights



Bi-functional catalysts based on palladium and gold nanoparticles supported onto

active nanocrystalline TS-1 zeolite are prepared and characterized.

=

Active and reusable catalysts for the direct PO production from H₂, O₂and C₃with the

in-situ generation of intermediate H₂O₂are evaluated.

One-pot two-step consecutive process is carried out using only one stable catalyst with

high selectivity for PO production.

Modification of suitable reaction parameters, working under safety supercritical CO₂

conditions, allowed the optimization of catalytic results.

1

One-pot two-step process for direct propylene oxide production catalyzed by bi-functional Pd(Au)@TS-1 materials

Alejandro Prieto, Miguel Palomino, Urbano Díaz, Avelino Corma

H₂

H₂O₂

O₂

H₂O

TS-1

scCO₂+ MeOH

Pd/Au

Reusable bi-functional catalysts based on Pd and Au nanoparticles supported onto active TS-1 nanocrystalline matrix for

direct propylene oxide production by one-pot two-step consecutive reaction under safety CO₂supercritical conditions.

2

One-pot two-step process for direct propylene oxide production

catalyzed by bi-functional Pd(Au)@TS-1 materials

Alejandro Prieto^a, Miguel Palomino^a, Urbano Díaz^a*, Avelino Corma^a

^aInstituto de Tecnología Química (UPV-CSIC), Universitat Politècnica de Valéncia - Consejo

Superior de Investigaciones Científicas, Avenida de los Naranjos s/n., 46022 Valencia, Spain.

E-mail: udiaz@itq.upv.es

Abstract

Different bi-functional materials (Pd(Au)@TS-1) based on metallic nanoparticles supported

onto active nanocrystalline titanium silicalite (TS-1) zeolites were synthesized, characterized

and used as recyclable heterogeneous catalysts for direct propylene oxide production from

hydrogen, oxygen and propylene through one-pot two-step consecutive process. These

catalysts allowed carrying out the combined reaction where metallic nanoparticles catalyzed

the formation of in situ H₂O₂that was the necessary intermediate for propylene epoxidation

catalyzed by active TS-1 nanocrystalline support. Several variables were considered such as use

of supercritical CO₂conditions, modifiable content of metallic species, and presence of

additional co-solvents, surface acidity inhibitors and H₂O₂stabilizers. Reusability and stability of

the bi-functional catalyst was showed through consecutive catalytic cycles.

Keywords: Propylene oxide; Bi-functional catalysts; Supercritical CO₂conditions; Palladium

nanoparticles; Gold nanoparticles; Titanium silicalite.

3

Introduction

Propylene oxide (PO) is an important monomer in chemical industry due to the

presence of highly reactive oxyranic group, which lead to many different organic products

under the appropriate conditions. For this reason, PO is currently being used mainly for the

synthesis of polyurethane foams, resins and propylene glycol, being its worldwide production

about 7.5 million tons per year, and this market is annually growing ~ 4-5% [1, 2]. However,

although the most convenient production process for PO synthesis would be the direct

oxidation of propylene with oxygen [3], results in this direction have been poor [4] and the

development of safe and low contaminating propylene oxide synthesis process is still a matter

of study. The nowadays production processes are mainly chlorohydrins and organic

hydroperoxide processes, but both have important drawbacks. In the case of the chlorohydrins

process, stoichiometric amounts of CaCl₂are produced together with a large amount of waste

water. On the other hand, hydroperoxide process produces styrene or isobutylene together

with PO in stoichiometric ratio, two compounds severely affected by market trends [5, 6].

Moreover, although heterogeneous catalysts can be alternatively used in the

hydroperoxide method [7], PO production is being carried out in presence of molybdenum-

type homogeneous catalyst [8]. In this sense, the use of homogeneous catalysts implies

difficult and expensive catalyst recoveries, being affected by poor thermal stability. As

alternative, heterogeneous catalysts have important advantages, including easy and cheap

catalyst separation and low costs associated to catalyst losses, factors which enhance the

catalyst reusability [9], making hence desirable the development of highly active and selective

heterogeneous catalysts [10] for effective direct PO production.

In this way, titanium silicalite-1 (TS-1), firstly synthesized by Taramasso et al. [11] and

optimized by different synthesis methods, has been an excellent solid catalyst for propylene

epoxidation, using hydrogen peroxide (H₂O₂) as external oxidizing agent. However, H₂O₂is

currently obtained at industrial level from anthraquinone, using heterogeneous catalysts

containing Pd or Ni [12], favoring high waste generation due to the use of toxic solvents and

extensive purification methods, besides the high initial required budget and its high operation

costs [13]. All these drawbacks increase the H₂O₂market value up to the PO value, making this

process non-viable [14] unless H₂O₂is formed in-situ and non-expensive intermediate and

purification steps are required. Advances in this sense were achieved with the HPPO process

[15], where H₂O₂is produced during the reaction in one separated reactor, through the

anthraquinone route, and then is fed to an additional reactor, in which the propylene

epoxidation occurs.

However, although HPPO approach represents an improvement of previous PO

synthesis methodologies, the process requires the use of two different catalysts and two

different individual reactors. A further improvement of the current methodology would be the

development of a process in which both the H₂O₂synthesis from H₂and O₂and the propylene

epoxidation reactions are catalyzed by heterogeneous bi-functional catalysts, in a one-pot

direct reaction. Considering that catalysts containing Pd [16] or Au [17] nanoparticles (NPs)

favor the H₂O₂formation, Hayasahi et al. [18], showed that bifunctional materials formed by

Au NPs supported onto titania matrix were able to catalyze the reaction between propylene

4

and in-situ generated H₂O₂from H₂and O₂with high selectivities in gaseous phase and

atmospheric pressure, especially when Au NPs with averages sizes between 2 and 5 nm were

incorporated [19]. However, it has also been observed that Au@TiO₂catalysts suffer

deactivation phenomenon and its catalytic activity decreases with time [20]. For this reason,

taking into account the high catalytic performance of TS-1 catalysts, would be interesting to

use this kind of catalysts instead of TiO₂as supports for the Au NPs deposition. In this regard,

Yap et al. [21] showed that Au@TS-1 catalysts, containing Au NPs with average sizes between

1 and 2 nm, were the most active when TS-1 is used as supporting matrix, allowing to achieve

higher selectivities, being these results consistent with those reported by Lee et al. [22], who

also reported that 1 nm Au NPs were the most active NPs when TS-1 catalysts were used as

support. Furthermore, Tsubaki et al. showed that catalysts containing both Pd and Au were

also able to generate PO from in-situ generated H₂O₂in gaseous phase, showing the benefits of

combining them [23]. However, though results obtained up to now are highly promising due to

the high PO selectivities, propylene conversions under 8% were obtained, being observed a

decrease in selectivity at higher conversions [21, 24, 25, 26, 27]. Moreover, in order to improve

the catalytic results, it would be desirable to increase the hydrogen and oxygen amounts in the

mixture. Howbeit, the broad flammability range of hydrogen-oxygen mixtures, which could be

explosive with hydrogen concentrations between 3.8 and 95.5% [28], requires to take greater

precautions.

Taking into account all these issues, there are several pathways to increase the

amounts of H₂and O₂at the same time that the reaction is kept in non-explosive regime,

including the use of capillary micro-channels [29] or supercritical conditions [30]. In the first

approach, the use of a miniaturized reactor allows working with hydrogen/oxygen mixtures

that would be explosive under standard conditions, leading to an enhancement in the

intermediate H₂O₂production step. On the second approach, supercritical carbon dioxide

(scCO₂) could be a possible alternative, due to its interesting properties, including the

complete miscibility between CO₂, H₂and O₂in all proportions at supercritical conditions,

much higher miscibility even at sub-critical conditions [31] and the reduction of the H₂/O₂

flammability range in presence of CO₂, both at atmospheric and at high pressure conditions

[30, 32]. Making use of all these advantages, H₂O₂synthesis from H₂and O₂in supercritical

media was reported by Chen et al. [33], obtaining high yields when Pd and PdPt@TS-1

catalysts were used. On the other hand, the use of scCO₂as solvent for the epoxidation of

propylene with external H₂O₂in presence of TS-1 catalyst has also been reported [34], being

even more selective than in conventional reaction conditions.

Considering both the high H₂O₂production and the high selectivity of the epoxidation

in scCO₂media together with the inherent safety benefits, the next step would be to carry out

the propylene epoxidation with in-situ generated H₂O₂in these conditions, in order to increase

the PO yield. In that sense, promising results have been obtained with PdPt@TS-1 catalysts in

presence of scCO₂as solvent [35, 36], showing the benefits of this approach. Furthermore,

PdAu@TS-1 catalysts have also shown a high catalytic activity on the H₂O₂direct synthesis

reaction in the presence of a mixture of CO₂and methanol as solvents [37]. On the other hand,

the addition of small amounts of gold to Pd@TS-1 appears to be disadvantageous for the PO

production from in-situ generated H₂O₂in liquid phase [38], increasing the selectivity towards

propane. However, taking into account the high selectivity of PdAu@TS-1 catalysts for the

5

synthesis of H₂O₂, it would be interesting to study the best reaction conditions for PO direct

synthesis in batch conditions, under scCO₂media, using this type of catalysts.

In the present work, several bi-functional materials based on Au and Pd(Au) NPs

deposited over TS-1 have been prepared and their catalytic activity has been studied on the

propylene epoxidation reaction with in-situ generated H₂O₂from H₂and O₂. The catalysts were

able to carry out the one-pot two-step consecutive process where the metallic NPs catalyze

the formation of H₂O₂which was used as oxidizing reagent for propylene epoxidation catalyzed

by TS-1 nanocrystals. The effect of supercritical CO₂as solvent, during the reaction process,

together with the use of additional co-solvents was considered, seeking to enhance and

optimize the performances and selectivity for direct PO synthesis. Physico-chemical

modification of the bi-functional catalysts through the variable content in Pd and Au NPs, use

of surface acidity inhibitors and addition of low amounts of H₂O₂stabilizers were also

considered, together with catalyst stability studies through consecutive regeneration

processes and catalytic reuses.

6

Experimental

Catalysts preparation

Synthesis of nanocrystalline TS-1

The preparation of nanocrystalline TS-1 zeolite, used as inorganic support, was carried

out by combination of micellar and hydrothermal synthesis methodologies [39].

Specifically, 2.0 g of Tween 20 (s.d. FINE CHEM.) were dissolved in 32 g of distilled

water. This surfactant solution was added to 19.2 g of tetrapropylammonium hydroxide (TPA-

OH, 32% aqueous solution, Alfa Aesar) under mild stirring, resulting in the formation of a clear

transparent solution. 36 g of tetraethyl orthosilicate (TEOS, Aldrich) were added to the above

micellar solution in a dropwise fashion under vigorous stirring. The stirring was continued for 1

h. After, 1.8 g of tetra n-butyl titanate (TNBT, Aldrich) in 9.1 g of isopropyl alcohol (IPA, s.d.

FINE CHEM.) were added dropwise under vigorous stirring, being continued for additional 1 h.

The resulting mixture still remained clear, exhibiting a molar composition of 0.03 TiO₂: SiO₂:

0.12 TPAOH : 0.009 Tween 20 : 0.88 IPA : 14.45 H₂O. The mixture was then crystallized at

160ºC for 18 h under autogeneous pressure. The product was recovered by centrifugation,

washed with distilled water and dried (100ºC, 12 h). The calcination was carried out in a muffle

furnace at 350ºC for 2h (heating rate of 2.7ºC/min) and finally at 550ºC for 3h (heating rate

3.3ºC/min).

Metallic nanoparticles deposition

Bi-functional catalysts were prepared by sol-immobilization method following the

methodology described by Dimitratos et al. [40]. In detail, for the preparation of Pd(Au)@TS-1

catalysts, containing 1% Pd and 1% Au, 16.6 mg of PdCl₂(ABCR) and 20 mg of HAuCl₄.3H₂O

(Aldrich) were dissolved in 0.5 g and 1.0 g of water, respectively. Then, both solutions were

added to a freshly prepared 1% wt polyvinyl alcohol water solution (Aldrich, PVA/(Au + Pd)

(wt/wt) = 1.2), followed by the addition of 0.1M NaBH₄water solution (Aldrich, NaBH₄/(Au +

Pd) (mol/mol) = 5). After 30 min under vigorous agitation, the solution was acidified to pH=1

with sulfuric acid, and 1 g of previously prepared and calcined TS-1 was added. After 2 h of

vigorous stirring, the mixture was filtered, and the recovered catalyst was washed with

distilled water until pH=7 and dried at 100ºC overnight. The catalyst was calcined at 200ºC

under air for 3h. It is important to remark that the catalysts used without previous reduction

treatment did not show any catalytic activity on PO production (results not shown). For this

reason, and unless otherwise stated, all the catalysts were reduced for 3 hours in the presence

of H₂at room temperature just before each catalytic test, as previously reported [36].

The general acronym of the synthesized bi-functional catalysts was PdxxAuyy@TS-1,

being xx and yy indicative of the metallic amount supported onto TS-1 support, i. e., 02, 04, 05

and 10 would correspond to 0.2 % wt, 0.4 % wt, 0.5 % wt and 1.0 % wt of metal, respectively.

7

Catalysts characterization

XRD analysis was carried out with a PANalytical X’Pert PRO diffractometer in the Bragg-

Brentano geometry using Cu Kα radiation. C, N and H contents were determined with a Carlo

Erba 1106 elemental analyzer, while Pd and Au contents were obtained by means of atomic

absorption spectroscopy (Spectra AA 10 Plus, Varian). A Cary 5 spectrometer equipped with a

diffuse reflectance accessory was used for UV measurements. Volumetric analyses were

performed by nitrogen adsorption isotherms at 77K with a Micromeritics ASAP2010. Before

the measurements, the samples were outgassed for 12 hours at 400^ºC. The BET specific

surface area [41] was calculated from the nitrogen adsorption data in a relative pressure range

from 0.04 to 0.2. The total pore volume [42] was obtained from the amount of N₂adsorbed at

a relative pressure of ~ 0.95. External surface area and micropore volume were estimated with

the t-plot method in the t range from 3.5 to 5. The pore diameter and the pore size

distribution were obtained following the Barret–Joyner–Halenda (BJH) method [43] on the

adsorption branch of the isotherms. Scanning and transmission electron micrographs (SEM and

TEM) were obtained in a JEOL6300 and Philips CM10 (100kV) microscopes, respectively. Before

TEM observation, the samples were prepared by suspending the solid in water, ultrasonicating

for 30 min and placing one drop for evaporation on carbon-coated copper grid (300 mesh).

Catalytic tests

PO synthesis reaction from propylene in presence of ex situ H₂O₂, as oxidizing agent,

were performed in a 3 mL round bottom glass reactor in which the stirring was driven by a

Teflon-coated magnetic stirrer. Known amounts of TS-1 catalyst (2.5 mg), methanol (1.2 g),

and propylene (9 mmol) were added, followed by the addition of the oxidant, 35% wt H₂O₂in

water (0.4 mmol). Then, the mixture was heated at 60ºC, and the reaction was monitored for 5

h.

The experiments for the direct synthesis of PO with in situ generated H₂O₂were carried

out in a 15 mL stainless steel reactor which contained a relief valve, for safety. The stirring was

driven by a Teflon-coated magnetic stirrer. Known amounts of catalyst (15 mg), acidity

inhibitor (ammonium acetate, 0.01 g) and co-solvent (0.2 g of different co-solvents) were

added to the reactor, followed by the addition of propylene (2 mmol) and CO₂, reaching

carbon dioxide vapor pressure (>55 bar) [44]. Oxygen and hydrogen were added to the reactor

by means of high pressure burettes, and then the reactor was heated up to desired

temperature (ranging from room temperature to 80ºC, according each experiment). The

reaction experiments were carried out for 5h, unless otherwise stated. At the end of the

reaction, the reactor was cooled down and the pressure was slowly released by venting,

accumulating the gaseous mixture in an inert gas sampling bag. 3-pentanone was used for

recovering any product that could be retained on the reactor walls.

The amount of formed products, i.e., propylene oxide, acetone, propionaldehyde,

acrolein, isopropanol, 1-methoxy-2-propanol (MP1), 2-methoxy-1-propanol (MP2), propylene

glycol (PG) and propylene carbonate were analyzed using a Shimadzu Gas chromatograph GC-

2010 Plus provided with FID detector and 20 m length, 0.10 mm ID, 0.10 μm df. Permabond

FFAP column. The amounts of propane and un-reacted propylene, oxygen and hydrogen were

analyzed using a Bruker 450-GC which contains two different independent channels. The first

one is provided with a thermal conductivity detector (TCD) and three different columns:

8

Hayesep N (0.5 m length), Hayesep Q (1.5 m length) and molsieve 13X (1.2 m length), using

argon as carrier. The second one is provided with two different flame ionization detectors (FID)

and three different columns: capillary column CP-Wax (1 m length and 0.32 mm ID), CP-

Porabond Q (25 m length and 0.32 mm ID) and CP-Wax (5 m length and 0.32 mm ID).

Due to the extensive list of possible by-products that have been reported in literature

for this reaction, all the products have been calibrated using mixtures with known

composition. All the products in the liquid and in the gaseous phases have been identified by

comparison with known standards.

9

Results and discussion

Synthesis and characterization of catalysts

The bi-functional Au@TS-1 catalysts were based on gold nanoparticles supported onto

nanocrystalline titanium silicalite TS-1 which exhibited a double function, as inorganic matrix

and epoxidation catalyst of olefins. The modification with metallic promoters, such as

palladium and gold species was also considered to improve the catalytic activity of the solids

(PdAu@TS-1). In this sense, several materials consisting on PdAu nanoparticles were prepared

in order to study the dependence of their physico-chemical characteristics on the catalytic

activity [40].

The preparation of TS-1 nanocrystals was carried out by combination of micellar and

hydrothermal routes in presence of surfactants which facilitated the formation of small

titanium silicalite crystallites during the synthesis process [39]. XRD diffractograms clearly

evidenced that the as-synthesized and calcined phases, removing organic structural directing

agents, corresponded to crystalline TS-1 zeolite, showing the characteristic diffraction bands of

MFI-type materials (Figure 1). The alkaline media present in the synthesis gel favored the

formation of charged small nuclei which were probably surrounded by surfactant molecules,

avoiding crystal growth. In fact, TS-1 nanocrystals with ~600 nm of size and containing very

elevated titanium content (Si/Ti=35, ~2.0 % wt) were obtained (Table 1). In these samples, at

least most of the titanium was tetrahedrally coordinated into the zeolitic framework, this fact

being important because it was reported that when high amounts of titanium are

incorporated, significant octahedral extra-framework titanium was obtained, which is not

active for epoxidation processes [45, 46]. UV-vis spectra and SEM micrographs confirmed the

coordination state of the titanium and the size distribution of the TS-1 crystals respectively, as

shown in Figure 2. The textural properties obtained from nitrogen adsorption showed that the

nanocrystalline TS-1 was microporous, exhibiting type I plate isotherm characteristic of

conventional 3D microporous zeolites (Figure 3 and Table 1).

(c)

(b)

(a)

5

10

15

20

25

30

35

40

2



Figure 1. XRD patterns of (a) purely siliceous MFI-type zeolite, (b) as-synthesized TS-1 and (c) calcined

TS-1.

10

Table 1. Chemical analyses and textural properties of Pd(Au)@TS-1 samples.

b

Ti

Au

Pd

S_BET

S_MICRO

V_TOT

V_MICRO

Samples^a

(%wt) (%wt) (%wt) (m²g^-1) (m²g^-1) (cm³g^-1) (cm³g^-1)

TS-1

Pd10

Au10

1.89

1.88

1.86

1.99

1.92

1.91

1.90

-

416

400

407

435

412

424

398

327

408

402

266

243

255

272

257

267

245

218

258

261

252

0.240

0.235

0.252

0.239

0.242

0.231

0.201

0.246

0.227

0.228

0.130

0.119

0.175

0.130

0.125

0.128

0.120

0.107

0.123

0.124

0.120

0.86

-

0.96

0.21

0.18

0.54

1.41

-

0.12

0.30

0.48

Pd01Au01

Pd02Au02

Pd05Au05

Pd10Au10

Pd20Au20^c

Pd10Au01

Pd10Au02

Pd10Au05

0.15

0.21

0.42

0.97

-

0.83

0.77

0.76

1.89

1.88

1.74

^aNumber after Pd and Au indicates the theoretical Pd and Au loading (0.2 %wt, 0.4 %wt , 0.5 %wt, 1.0 %wt

and 2.0 %wt). All samples were supported onto TS-1 zeolite. ^bFrom t-plots curves. ^cChemical analysis was

not performed because the sample Pd20Au20@TS-1 was not possible to be dispersed.

Figure 2. UV-Visible spectrum (a) and SEM micrographs (b) of calcined TS-1 obtained through combined

hydrothermal and micellar routes.

Figure 3. N₂isotherms of TS-1 and Pd(Au)@TS-1 samples: (a) TS-1, (b) Pd10@TS-1, (c) Au10@TS-1, (d)

Pd02Au02@TS-1, (e) Pd05Au05@TS-1, (f) Pd10Au10@TS-1, (g)Pd20Au20@TS-1.

11

Bi-functional Au@TS-1 materials were obtained through the deposition of gold species

(from 0.2 % to 2.0 % in weight) by impregnation methodologies onto nanocrystalline TS-1

zeolite, used as supporting matrix. The incorporation of additional metallic promoters was

similarly carried out, merging gold and palladium salts. Following this procedure, several

Au@TS-1, Pd@TS-1 and PdAu@TS-1 materials were prepared with different loading of metallic

species in function of metallic salts amount used during the impregnation process. The

chemical content of the different samples prepared is shown in Table 1, being observed that

the titanium content of TS-1 support (~2% wt) was not modified by the presence of metallic

species.

Furthermore, the crystallinity of the zeolitic phase remained unmodified after the

palladium and/or gold deposition, such as it is confirmed by XRD patterns of the different

samples which maintain the diffraction peaks characteristics of MFI-type zeolites without

formation of silica amorphous phases (Figures 4 and 5). Interestingly, the peak corresponding

to (111) metallic gold diffraction can be observed at 38.2º 2 in the case of the Au@TS-1

sample, as previously reported in literature [47]. However, the (111) diffraction peak of

metallic palladium, in the case of the Pd@TS-1 sample, which commonly appears at 40.1º 2

[48], was not observed. Remarkably, the absence of this (111) diffraction peak from palladium

was also confirmed in our previous studies [36], although the presence of palladium

nanoparticles was confirmed through TEM and ICP analyses. This behavior has also been

reported by Zhao et al. [49], who only observed the characteristic palladium (111) diffraction

peak when samples with metallic contents greater than 6% were measured. Nevertheless, the

presence of a broad signal in the PdAu samples was observed between the theoretical 2

position of the (111) diffraction peaks of gold and palladium, specially at high metallic loadings,

being this broad signal indicative of the presence of PdAu alloys as it was reported by Venezia

et al. [50].

Figure 4. XRD of different samples: (a) TS-1, (b) Pd10@TS-1, (c) Au10@TS-1 and (d) Pd10Au10@TS-1

samples.

12

Figure 5. XRD of different samples: (a) TS-1, (b) Pd01Au01@TS-1, (c) Pd02Au02@TS-1, (d)

Pd05Au05@TS-1, (e) Pd10Au10@TS-1 and (f) Pd20Au20@TS-1 samples.

On the other hand, it was confirmed that the followed deposition methodology to

incorporate metallic species did not imply the generation of extra-framework titanium species.

In this sense, UV-vis analysis showed not only the absence of extra-framework titanium species

which are usually observed at ~ 330 nm [51], but also an intense absorption band centered at

~ 210 nm corresponding to charge transfer transition [O^2- Ti⁺⁴] of isolated tetrahedral

titanium species present in the zeolitic framework (Figure 6) [52]. However, a shoulder in the

absorption band in Pd10Au10@TS-1 sample was also observed, which may be indicative of the

presence of small amounts of titanium in octahedral coordination at ~ 230 nm on the samples

with higher metallic content [53].

Figure 6. UV of PdAu@TS-1 samples: (a) TS-1, (b) Pd10@TS-1, (c) Au10@TS-1, (d) Pd01Au01@TS-1, (e)

Pd02Au02@TS-1, (f) Pd05Au05@TS-1, (g) Pd10Au10@TS-1 and (h) Pd20Au20@TS-1 samples.

Moreover, a reduction in the total surface area and microporous volume was not

detected in the samples with gold and/or palladium loadings lower than 1% wt. However, a

decrease in the total surface area was detected in samples with higher loadings, especially in

13

the Au20Pd20@TS-1 sample (Table 1), due to the partial blockage of the pores by the metallic

nanoparticles. Nevertheless, nanoparticle deposition up to 2% wt did not lead to the complete

blockage of the pores, exhibiting in all cases type I isotherms characteristic of microporous

zeolitic materials (Figure 3).

14

Figure 7. TEM micrographs and histograms of (a) Pd10@TS-1, (b) Au10@TS-1, (c) Pd10Au02@TS-1, (d)

Pd10Au05@TS-1, (e) Pd05Au05@TS-1 and (f) Pd10Au10@TS-1 samples.

The TEM micrographs of different PdAu@TS-1 catalysts are shown in Figure 7, being

observed that, regardless the metallic loading, most of the metallic nanoparticles exhibited

sizes between 3-8 nm, although some nanoparticles larger than 20 nm were also measured

(see histograms in Figure 7). This fact was also corroborated by previous studies where

PdAu@TS-1 catalysts (2.5% wt Pd and 2.5% wt Au) containing PdAu nanoparticles of ~130 nm

were formed through similar methodologies [37]. However, although it is well-known that

small nanoparticles are usually more active, it was confirmed that these catalysts with large

nanoparticles were also able to generate H₂O₂, due to the high catalytic performance showed

for these materials for H₂O₂direct synthesis [37].

15

Catalytic activity

Preliminary batch catalytic tests, using nanocrystalline TS-1 zeolite as catalyst for the

propylene epoxidation, in presence of external H₂O₂, were performed to produce PO. The

results obtained clearly confirmed the high reactivity of the titanium silicalite TS-1 zeolite

support to catalyze the epoxidation of propylene and the high efficiency of the process to

obtain elevated PO yields (~80-85%, with respect to the oxidizing agent) with high selectivity

(~95-98%). This fact evidenced the validity of nanocrystalline TS-1 materials to be used as

active supports to generate effective bi-functional catalysts after the incorporation of different

metallic nanoparticles. Moreover, the hydrothermal stability of the nanocrystalline TS-1 zeolite

support was additionally studied through several catalytic runs carried out with the same

sample, without regeneration. Specifically, in Figure 8, it was observed that the PO yield

obtained after the first reuse was similar compared with the results led by the activated fresh

TS-1 catalyst. During the consecutive reuses, it was observed that PO selectivity remained

stable, although the catalytic activity at short times decreased. However, even if the activity

slightly decreased, longer reaction times led to complete conversion of H₂O₂in presence of

propylene to produce PO. This behavior is related with the formation of coke [10], which

partially blocks zeolitic channels and pores. Although the initial activity can be completely

restored through calcination at 550ºC, it is important to consider that the aim of this work is

the use of PdAu@TS-1 catalysts for PO direct synthesis from H₂O₂, and under this high

temperature reactivation treatment, the metallic nanoparticles could suffer sintering

phenomena, negatively affecting to their catalytic performance. This issue will be further

discussed on the stability tests. Definitively, these results corroborated that the TS-1 zeolite,

used as active supporting matrix in the following one-pot two-step reactions, was catalytically

effective, stable and active after several catalytic runs, without observing a substantial activity

loss.

Figure 8. Catalytic activity and reuses of TS-1 zeolite support prepared by micellar route. The amount of

the H₂O₂was externally added according to the amount of the catalyst weight used in each reuse. On

average, R_H2O2/Ti=120. Propylene is added in excess, using H₂O₂as limiting reactant. Reaction conditions:

14 bar of pressure, 60ºC, 5 h reaction time.

16

Considering these results, the next step was the catalytic study for the direct

epoxidation of propylene with in-situ generated H₂O₂, during one-pot two-step reaction

process. With this purpose, batch catalytic tests were carried out for the direct PO production

from hydrogen, oxygen and propylene, using previously characterized bi-functional PdAu@TS-

1 nanocrystalline materials as catalysts. It is important to point out that when high yields to PO

are obtained, also an increase in the amount of by-products formed during the reaction is

observed, and high selectivities are only obtained at low propylene conversions, especially

when gold is used as metal for the H₂O₂synthesis from H₂and O₂, in gaseous phase [21, 25].

Due to the high number of possible products that could be obtained in the reaction, it

was decisive to find the most favorable reaction conditions, using bi-functional solid catalysts,

for both the in situ generation of H₂O₂and the consecutive epoxidation of propylene, in order

to obtain high PO yield and selectivity, avoiding the by-products formation. Supercritical CO₂

(scCO₂) has been claimed to be a good solvent for the direct production of PO, enhancing also

the H₂O₂efficiency for the epoxidation reaction [34]. For this reason, in the first set of

reactions, CO₂has been used as reaction medium together with several co-solvents in order to

find out the optimal reaction conditions, taking the bi-functional Pd10Au10@TS-1 catalyst as

reference.

Table 2. Catalytic tests carried out with in-situ generated H₂O₂using supercritical CO₂conditions in

presence of different catalysts. ^a

=

b

c

d

e

Catalyst

@TS-1

Entry

Co-Solvent

C₃/H₂/O₂Χ_C3=S_POS_MPS_PG

S_C3

Y_PO

1

2

Pd10Au10

-

1/0.9/0.9

1/1.2/0.8 64.9 2.2 1.1

1/1.0/1.0 37.0 1.3

1/1.1/1.2 52.4 1.5 4.9 0.5 93.1 0.8

1.2

-

>99

-

Pd10Au10

MeOH

H₂O

-

96.7 1.4

3

-

0.4 94.4 0.5

4

MeOH+H₂O

5

Pd10Au10 MeOH+H₂O+AI^f1/1.2/1.3 17.9 57.8 3.0 1.3 37.9 10.3

6

Pd10Au10

H₂O+AI

1/0.8/1.3 15.9 33.0

-

1.2 63.4 5.2

7

MeOH+AI

1/1.0/1.0 15.8 75.5 2.8 1.1 16.3 11.9

8

Pd10Au10 MeOH+AI+NaBr 1/0.9/1.0 12.9 83.4 2.7 0.3 13.5 10.8

9

Pd10

Au10

Pd10

Au10

MeOH+AI

MeOH

1/1.2/1.2 11.5 75.0 3.9 1.1 19.9 8.6

1/1.0/1.0 2.6 9.0 91.0 0.2

1/1.1/1.0 36.9 25.9 8.6 0.6 64.9 9.5

1/1.2/1.1 29.0 1.7 1.3 97.0 0.5

10

11

12

-

MeOH

-

^aReactions were carried out for 5 h with pressures around 75 bar and 60ºC, feeding 2 mmol of propylene and 15 mg

catalyst. ^bGaseous molar ratios. ^cPropylene conversion. ^dPropylene oxide, methyl propanols, propylene glycol and

propane selectivity. ^ePropylene oxide yield. ^fAcidity Inhibitor (CH₃COONH₄). Conversion, selectivity and yield values

are given in %.

The catalytic results showed that the bi-functional Pd10Au10@TS-1 catalyst was not able to

produce PO from in-situ generated H₂O₂from H₂and O₂using CO₂as sole solvent (Table 2,

entry 1), the only detected product being propane.

17

In that sense, it is nowadays accepted that the formation of metal peroxo compounds

(Ti-OOH) through the reaction between Ti and H₂O₂is a key intermediate step on the

epoxidation reaction with TS-1 catalysts. Bellusi et al. [54] reported that Ti-OOH species have a

five-membered cyclic structure with a donor hydroxyl moiety coordinated to titanium which

reacts with propylene to yield propylene oxide (Figure 9).

Figure 9. Mechanism for the formation of five-membered cyclic structure over TS-1 catalyst and its

reaction with propylene to yield propylene oxide.

Following this approach, donor hydroxyl groups might come from the solvent, being

obtained the best results on the propylene epoxidation reaction in presence of H₂O₂as

oxidizing agent and methanol as solvent, which efficiently helped to obtain the five-membered

cyclic structure. Moreover, not only methanol, but also others protic solvents such as ethanol,

t-butanol or even water, are able to help in the formation of these intermediate species.

Taking this into account, we also considered the incorporation of additional co-

solvents. The results obtained showed that the use of methanol, water or a mixture of both

solvents result in an improvement of the propylene oxide selectivity, but propane was the

main reaction product in all experiments (Table 2, entries 2-4). These results are consistent

with previous study of Hölderich et al. who already pointed out that the co-deposition of gold

together with palladium lead not only to worse PO yields compared with bare palladium

catalysts, but also to higher propane selectivity [38].

However, it has already been proved [35] that the hydrogenation product selectivity

using PdPt based catalysts can be effectively reduced in presence of acidity inhibitors,

enhancing PO selectivity. The reason for this behavior remains unclear, although the positive

effect of alkalis or NH₄NO₃on PO productivity and selectivity, using Au based bifunctional

catalysts, has been previously reported [55, 56], being effectively suppressed propylene

hydrogenation over gold clusters [57, 58]. According to this, the higher activity could be

related with two main reasons: i) the formation of Au-amine complexes which are more active

than single Au species for H₂O₂synthesis and ii) the more efficient removal of residual Cl^-ions

+

(from the precursor HAuCl₄) by the NH₄species. In that sense, it is important to consider that

chlorine is a poisoning agent for Au catalysts, due to the formation of Au-Cl species, and should

be removed efficiently after the catalyst preparation. Taking this account, we have also tested

the effect of ammonium acetate in presence of water, methanol and mixtures of these

solvents in order to find out the best reaction medium (Table 2, entries 5-7). Best results were

achieved when methanol and ammonium acetate were used in absence of water, leading to

PO selectivities up to 75.5% and conversions of 15.8% (Table 2, entry 7). Moreover, the use of

ammonium acetate seems to have not only an effect in the propane formation, but also in the

18

reduction of non-desired ring-opening reactions, leading to an improvement of the propylene

oxide selectivity. In this sense, it has already been published that the pre-treatment of TS-1

with different salts, such as sodium acetate and ammonium acetate among others, neutralizes

the weak acid Si-OH groups of the catalyst, avoiding non desired ring-opening reactions [10,

59].

Furthermore, these tests also showed that the use of water as co-solvent substantially

worsened the catalytic results, both in absence and in the presence of ammonium acetate

(Table 2, entries 3 and 6). As we previously discussed, water can also help in the formation of

the five-membered cyclic structure, promoting PO formation, but methanol is the most

preferred solvent for the formation of such intermediate. For this main reason, the results

obtained in the presence of water are not as good as the ones obtained with methanol.

Besides, it is well-known [60, 61, 62] that halide ions have a beneficial effect in the

oxidation of H₂to H₂O₂over Pd-based catalysts, due to its inhibitor effect in the water

formation reaction, increasing H₂O₂selectivity. For this, the reaction was also tested in the

presence of ammonium acetate and sodium bromide, seeking to reduce both the H₂O₂

degradation to water and the formation of propane. The results showed an improvement in

propylene oxide selectivity (83.4%), with a slightly lower propylene conversion (Table 2, entry

8).

On the other hand, the effect of the addition of gold as promoter was also analyzed

(Table 2, entries 9 and 10). The results showed the bare Pd10@TS-1 catalyst exhibited a lower

catalytic performance than Pd10Au10@TS-1 catalyst when the same reaction conditions were

used (Table 2, entries 7 and 9). Interestingly, the use of bare Au10@TS-1 catalyst led to very

low catalytic activity, being propane the main reaction product, showing the beneficial effect

of gold and palladium metallic species coexisting in the same solid catalyst. In this sense, the

results were consistent with those previously reported by Moreno et al. [37] who also

observed that bare Au nanoparticles deposited over TS-1 were not able to produce any H₂O₂,

meanwhile bare Pd nanoparticles and especially Au-Pd alloy nanoparticles deposited over TS-1

were indeed able to produce H₂O₂when H₂and O₂are fed in a CO₂atmosphere. These

experimental results clearly suggest a synergetic effect between palladium and gold. This

behaviour has also been observed in H₂O₂direct synthesis [63, 64] and the reason remains still

as matter of study. However, the role of gold seems to be mainly the modification of the

characteristics of palladium, acting as an activity promoter. The enhanced activity could be

related with two main reasons: i) Pd/Au alloys are more active and its structure is more stable

than bare palladium [65] or ii) gold inhibits the O₂dissociation on palladium, reducing the

water formation reaction and enhancing the selectivity towards H₂O₂[66]. So, the synergetic

effect between palladium and gold leads to a higher H₂O₂production efficiency, resulting in a

higher PO production.

Interestingly, the results here obtained using methanol as co-solvent in the absence of

the acidity inhibitor (Table 2, entry 2) showed that the presence of methoxypropanols is barely

observed, being propane the main product. In order to elucidate the reason for this behavior,

the reaction was also tested in presence of Pd10@TS-1 and Au10@TS-1 catalysts without

acidity inhibitor. The results (Table 2, entries 11 and 12) showed that, meanwhile Pd10@TS-1

catalyst led to higher amounts of methoxypropanols and lower of propane than

19

Pd10Au10@TS-1 catalyst (Table 2, entry 2), the Au10@TS-1 catalyst leads mainly to propane,

being methoxypropanols barely observed. These results clearly show that the formation of

propane is strongly favored by the presence of single gold nanoparticles on the solid catalyst,

being important that gold is present together with palladium in order to obtain satisfactory

catalytic results.

Besides, it is well-known that H₂O₂synthesis in batch conditions is promoted when low

reaction temperatures, close to 0ºC, are used, meanwhile propylene epoxidation in batch

conditions is normally promoted at higher temperatures, in the range of 40-60ºC [10, 67, 68].

However, gold catalysts have shown their best catalytic performance for propylene

epoxidation in gaseous phase with in-situ generated H₂O₂at higher temperatures, up to 200ºC

[22, 25, 69], and even at 300ºC [70, 71]. Taking into account the high catalytic activity of gold

species in gaseous phase at high temperatures, it was important to study the effect of gold on

the most preferred reaction temperature in batch conditions. The results obtained showed a

slight decrease on selectivity when reaction is carried out at 70ºC, and the increase from 70ºC

to 80ºC resulted in a sharp decrease on selectivity, being observed the presence of higher

amounts of ring-opening products. The results presented on Table 3 proved that 60ºC was the

optimal reaction temperature to carry out the one-pot two-step process under our reaction

conditions, even in presence of gold nanoparticles as catalytic promoters.

Table 3. Catalytic tests carried out with in-situ generated H₂O₂using supercritical CO₂conditions in

presence of Pd10Au10@TS-1 catalyst at different temperatures.^a

=

c

d

e

f

Entry

C₃/H₂/O₂Χ_C3=

S_PO

S_MP

S_PG

S_C3

Y_PO

Temperature

1

2

3

4

5

6

RT^b

1/0.9/1.0

4.0 77.1 1.5

9.3 77.5 0.7

-

18.9 3.1

40

50

60

70

80

0.4 14.4 7.2

1.3 23.5 7.4

1.1 16.3 11.9

1/1.0/1.1 10.7 69.2 1.8

1/1.0/1.0 15.8 75.5 2.8

1/0.9/1.1 16.9 70.5 12.3 2.2

8.1 11.9

1/1.2/1.0 20.1 45.2 26.2 10.8 14.3 9.1

a

Reactions were carried out for 5 h at different reaction temperatures with pressures around 75 bar, feeding 2

mmol of propylene and 15 mg catalyst. Room temperature (ºC). ^cGaseous molar ratios. Propylene conversion. ^e

Propylene oxide, methyl propanols, propylene glycol and propane selectivities. ^fPropylene oxide yield. Conversion,

selectivity and yield values are given in %.

b

d

On the other hand, metallic nanoparticles are not only able to generate H₂O₂, but also

can decompose H₂O₂to form water, catalyzing the propylene hydrogenation. Besides, metallic

nanoparticles can also lead to the blockage of the zeolitic pores, favoring diffusion problems

and decreasing the catalytic performance. For this reason, the amount of metals deposited

onto the TS-1 support was studied, in order to obtain as much H₂O₂as possible, at the same

time that selectivity to PO remained high. Moreover, considering that the presence of gold had

a beneficial effect in our catalytic system, although could favor the hydrogenation reaction, the

most favorable Pd/Au ratios in our catalysts were also investigated. Thus, several bi-functional

catalysts with different Au and Pd loadings (0.2%, 0.5%, 1.0% and 2.0% in weight) and different

Pd/Au ratios (R_Pd/Au=1, 2, 5, 10) were compared (Table 4).

20

Table 4. Catalytic tests carried out with in-situ generated H₂O₂using supercritical CO₂conditions in

presence of different Pd(Au)@TS-1 catalysts.^a

=

b

c

d

e

Catalyst

@TS-1

Entry

Co-Solvent C₃/H₂/O₂Χ_C3=

S_PO

S_MP

S_PG

S_C3

Y_PO

1

2

3

4

5

6

7

Pd02Au02

MeOH+AI^f

MeOH+AI

1/1.1/1.1 12.1 77.9 3.3

1/1.0/1.1 12.3 77.6 7.3

1/1.0/1.0 15.8 75.5 2.8

1/1.0/1.0 10.2 71.3 1.3

1/0.9/1.1 12.9 69.7 5.1

1/1.0/1.1 15.6 82.9 2.8

1/0.9/1.1 18.2 57.1 2.4

1.7 15.2 9.4

2.1 13.0 9.5

1.1 16.3 11.9

0.1 25.3 7.3

3.4 16.6 9.0

1.5 12.2 12.9

1.3 38.1 10.4

Pd05Au05

Pd10Au10

Pd20Au20

Pd10Au01

Pd10Au02

Pd10Au05

^aReactions were carried out for 5 h with pressures around 75 bar and 60ºC, feeding 2 mmol of propylene and 15 mg

catalyst. ^bGaseous molar ratios. ^cPropylene conversion. ^dPropylene oxide, methyl propanols, propylene glycol and

propane selectivity. ^ePropylene oxide yield. ^fAcidity Inhibitor (CH₃COONH₄). Conversion, selectivity and yield values

are given in %.

In the first place, we studied the activity of materials with similar loading of Pd and Au

(Table 4, Entries 1-4), observing that the best catalytic result was obtained with Pd10Au10

catalyst. It is important to point out that, in general, the propane selectivity increased with the

metallic loading, meanwhile the selectivity to PO and ring-opening products decreased, owing

to the ability of the metallic nanoparticles to carry out hydrogenation reactions. This behavior

can be attributed mainly to two reasons: i) higher hydrogenation capacity at higher metallic

loadings and ii) pore and channels partial blocking due to the deposition of increasing amounts

of metallic nanoparticles, which led to diffusional problems.

With regard to the metallic ratio (Table 4, Entries 5-7), we observed that best results

were obtained with Pd10Au02 catalyst, with a Pd/Au ratio 5, which yielded 12.9% PO, with a

selectivity of 82.9%, being observed an increase of propane production when gold ratio was

increased. In this way, although it is clear that the presence of gold effectively enhances the

bifunctional catalyst performance, gold amount has to be controlled, due to its high activity for

the propylene hydrogenation in presence of methanol as co-solvent [38].

On the other hand, the most adequate reaction time for the propylene oxide

production was also evaluated. Considering the previous results, reaction was carried out using

Pd10Au02 catalyst and in presence of methanol together with small amounts of ammonium

acetate as buffer from 1 to 12 hours at 60ºC.

21

Table 5. Catalytic tests carried out with in-situ generated H₂O₂using supercritical CO₂conditions in

presence of different Pd10Au02@TS-1 catalysts at different reaction times.^a

=

b

c

d

e

Entry

C₃/H₂/O₂Χ_C3=

S_PO

S_MP

S_PG

S_C3

Y_POTOF^fPRO^g

Time (h)

1

2

3

4

5

1

1/0.8/1.0

1/0.9/1

2.9 72.5 0.8

10.9 83.7 1.8

-

20.9 2.1

6.3

52

3

5

1.2 13.3 9.1 27.8 228

1.5 12.2 12.9 39.5 323

1/1.0/1.1 15.6 82.9 2.8

1/1.0/1.0 21.2 83.1 6.3

7

3.3

7.3 17.6 49.8 407

12

1/1.0/1.0 26.1 45.4 24.2 7.5 22.9 11.8 35.1 287

a

Reactions were carried out at different reaction times with pressures around 75 bar and 60ºC, feeding 2 mmol of

propylene and 15 mg catalyst. ^bGaseous molar ratios. ^cPropylene conversion. ^dPropylene oxide, methyl propanols,

propylene glycol and propane selectivities. Propylene oxide yield. ^fTurnover frequency (h^-1). ^gProductivity (mol_PO

e

kg_cat^-1h^-1). Conversion, selectivity and yield values are given in %.

The results showed that higher propylene conversion and propylene oxide selectivity

were obtained when reaction time was increased, being observed a strong decrease on

propylene oxide selectivity after 12h. This behavior could be explained both by the formation

of ring-opening products at longer times and by the deactivation of PdAu species, which are

highly active for H₂O₂production during short times [72], producing only water afterwards.

This last behavior has been assigned to the Pd oxidation during the reaction process [73]

favored by the H₂O₂formed, leading to a decrease not only in the catalytic activity but also in

the selectivity of the Pd nanoparticles.

Finally, the stability of the bi-functional Pd(Au)@TS-1 catalysts was evaluated through

several consecutive catalytic cycles. After each reuse, the catalyst solid was recovered,

exhaustively washed with methanol, dried at 100ºC overnight and reduced in presence of

hydrogen at room temperature during 3 h. After the 2^ndreuse, the catalyst was regenerated

through a temperature treatment at 200ºC followed by reduction with H₂, as detailed on the

experimental part.

Figure 10. Stability tests carried out with in-situ generated H₂O₂using supercritical CO₂conditions in

presence of Pd10Au02@TS-1 catalyst. Reactions were carried out for 5 h with pressures around 75 bar

and 60ºC, feeding 2 mmol of propylene, 15 mg catalyst and in presence of acidity inhibitor. Conversion,

selectivity and yield values are given in %.

22

The results showed a decrease in the catalyst performance during the successive

catalytic reuses. This trend could be related with two main reasons: i) nanoparticle aggregation

during reaction and ii) partial blockage of the active sites with associated coke formation and

adsorbed organic compounds produced during the reaction process.

(b)

(a)

(d)

(c)

Figure 11. TEM micrographs and histograms of Pd10Au02@TS-1 sample: (a) fresh catalyst, (b) after 1^st

use, (c) after 2^nduse and (d) after 3^rduse.

In fact, TEM micrographs of the catalyst prior and after several catalytic cycles showed

that effectively the nanoparticles were agglomerated during reaction process, favoring lower

catalytic activity (Figure 11). Moreover, elemental analysis of reused samples showed the

23

presence up to 5% in weight of carbon, which would explain the drop in activity, due to the

partial blockage of active sites together with possible diffusional problems.

It is also worth to mention that the regeneration of the catalyst through temperature

treatment at 200ºC facilitated only the partial recovering of the initial catalytic activity. As we

already discussed, TS-1 catalysts require higher temperatures, close to 550ºC, in order to

efficiently restore the initial catalytic activity [10]. However, such harsh temperature

treatments cause structural alterations on the PdAu bimetallic nanoparticles, promoting

sintering phenomena [74] which negatively affect both the formation of H₂O₂and the PO

production. For this reason, high temperature treatments were avoided, taking into account

that H₂O₂synthesis is enhanced with small nanoparticles [75] and the most preferable gold

nanoparticles size for PO production is 2-5 nm [57, 76, 77].

Conclusions

Promising results have been achieved in the propylene oxide (PO) direct formation

through in-situ generated H₂O₂from hydrogen, oxygen and propylene, using high pressure

conditions in batch reactor in presence of optimal bi-functional catalysts, based on gold and

palladium nanoparticles supported onto nanocrystalline titanium silicalite zeolite (Pd(Au)@TS-

1). These catalysts were prepared and characterized, being active and reusable to carry out the

one-pot two-step process in which metallic nanoparticles catalyzed the generation of in-situ

H₂O₂as intermediate product for propylene epoxidation catalyzed by TS-1 active matrix. In this

way, propylene oxide can be obtained using only one bi-functional recyclable catalyst, allowing

to carry out a consecutive reaction and avoiding additional intermediate purification steps. As

consequence, higher efficiency of catalytic route would be achieved inside of more sustainable

and environmental combined process. Optimization of catalytic results was achieved through

the modification of different reaction parameters, including amount of metallic nanoparticles

in the solid catalysts, reaction temperature and time, and effect of solvents nature on the

reaction process, working always under safety supercritical CO₂conditions.

Acknowledgements

The research leading to these results has received funding from European Community’s Seventh

Framework Programme, through the Collaborative Project INCAS, Contract Nr. NMP2-LA-2010-245988.

Authors thank additional funds from Spanish Government (MAT2014-52085-C2-1-P and Severo Ochoa

Excellence Program SEV-2012-0267).

24

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One-pot two-step process for direct propylene oxide production catalyzed by bi-functional Pd(Au)@TS-1 materials

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