Hydrogenation of levulinic acid to Γ-valerolactone over anatase-supported Ru catalysts: Effect of catalyst synthesis protocols on activity

Hydrogenation of levulinic acid to γ-valerolactone over anatase-supported Ru

catalysts: Effect of catalyst synthesis protocols on activity

1

2

1

2

1

A.S. Piskun , J. Ftouni , Z. Tang , B. M. Weckhuysen , P.C.A. Bruijnincx and H. J. Heeres

1. Department of Chemical Engineering, ENTEG, University of Groningen, Nijenborgh 4, 9747 AG Groningen,

The Netherlands

2. Inorganic Chemistry and Catalysis, Debye Institute for Nanomaterials Science, Utrecht University,

Universiteitsweg 99, 3584 CG Utrecht, The Netherlands.

*

corresponding h.j.heeres@rug.nl

Graphical abstract

Highlights



Catalyst preparation protocols strongly affect catalyst performance of Ru on

titania (anatase) for the catalytic hydrogenation of levulinic acid to

valerolactone

Best results regarding activity were obtained when using a RuNO(NO

3 3

)

precursor (when considering TOF) without an intermediate calcination step

and 10% hydrogen in the reduction gas.

The information can be used to prepare optimum Ru based catalysts for the

catalytic hydrogenation of levulinic acid

1

Abstract

γ-Valerolactone (GVL) is a value-added renewable chemical with great potential and can be

obtained from biomass by the hydrogenation of levulinic acid (LA) using metal-based

catalysts, such as Ru/TiO

2

. We here report an in depth study of the effect of catalyst

(anatase), varying the nature of the Ru-

synthesis parameters on the performance of Ru/TiO

2

precursor and the conditions of the calcination and/or reduction step. Catalyst performance

was evaluated under batch conditions at a hydrogen pressure of 45 bar and using either

water (90 °C) or dioxane (150 °C) as solvent. The experiments showed that catalyst activity

depends greatly on the Ru precursor used (RuCl

when considering the turn-over frequencies (TOF) of the catalysts were obtained using the

RuNO(NO precursor, whereas RuCl performed better when considering the initial rate

3

, RuNO(NO

3

)

3

, Ru(NH

3

)

6

Cl

3

). Best results

3

)

3

based on Ru intake. An intermediate calcination step and the use of a hydrogen-rich sweep

gas during the final reduction step were shown to have a negative impact on catalyst

activity. Characterization of the fresh catalysts by BET and TEM provided valuable insight in

the relation between the catalyst structure and its activity.

Keywords: biobased chemicals, hydrogenation, Ru catalysts, titania

1. Introduction

Levulinic acid (LA) is a major product of the controlled dehydration of the C6-sugars

D-glucose, D-mannose and D-galactose) in lignocellulosic biomass. It is recognized as an

(

important carbohydrate-derived renewable platform molecule and has attracted

considerable interest from a number of chemical companies [1],[2]. The family of LA

derivatives is large and some have much potential for commercialization. For instance, LA

can be converted to 2-methyltetrahydrofuran (MTHF) and vario us levulinate esters, which

may be used as gasoline and biodiesel additives, respectively [3]-[8]. δ-Aminolevulinic acid is

a well-known herbicide and β-acetylacrylic acid has been proposed as a (co-)monomer for

novel acrylate polymers [1]. Arguably, most attention has been devoted to the conversion of

LA to γ-valerolactone (GVL), as GVL is considered as an important platform chemical in its

own right and can be used as food additive, solvent and as precursor for fuel additives and

2

bulk polymers [9]-[13]. The conventional way to produce GVL involves the hydrogenation of

LA or its esters [14]-[16] with molecular hydrogen or an alternative hydrogen donor (e.g.

formic acid), preferably using heterogeneous catalysts (Scheme 1). The intermediate 4-

hydroxypentanoic acid (4-HPA) may be found in relatively high amounts when using water as

the solvent, its exact amount depending on the relative rate of hydrogenation versus the

rate of the intramolecular esterification reaction.

Recently, Liguori and Barbaro [17] published a comprehensive review on the direct

catalytic conversion of renewable sources to GVL with an emphasis on the heterogeneous

catalysts that have been used for this reaction. Noble metal-based catalysts are most

commonly and successfully emp loyed, with Ru-based ones in particular showing high activity

and selectivity to GVL [14],[18]-[21]. In addition to the nature of the active metal phase, the

choice of support also has a large effect on catalyst performance, in particular on catalyst

stability.

Activated carbons are the most widely used support for Ru in LA hydrogenation [22]-

[

24], mainly due to their good performance and availability [24]-[26], and as such Ru/C

catalysts can be regarded as one of the benchmark catalysts for this reaction. Under

continuous flow conditions and using water as the solvent, however, slow though

irreversible deactivation of the Ru/C catalyst was observed, presumably due to Ru sintering

and a reduct ion in sp ecific surface area as a result of the deposition of carbonaceous

deposits [15],[22],[25],[27].

Various metal oxides, including SiO

2

, Al

2

O

3

, Nb

2

5

O , ZrO

2

and TiO , h ave b een tested as

2

a support for LA hydrogenation with Ru as the active metal [16],[19],[24],[25]. A major

advantage of such metal oxides over carbon supports is their mechanical and thermal

stability, which allows for repetitive regeneration and coke removal at elevated

temperature. However, for some of these oxides, e.g. Al

2

O

3

and SiO , the stability under

2

hydrothermal, acidic conditions is known to be limited, which is of relevance for LA

hydrogenations carried out in water or at high LA loadings [25],[28]. Estimations of oxide

o

support stability in pressurized water at 200 C were recently provided by Lange using

Pourbaix diagrams [2]. SiO

whereas Al is well-known to convert into boehmite (AlO(OH)) at pH values between 4.5

and 11.5. TiO

2

has the tendency to be converted into silica gel at all pH values,

O

2 3

2

, calculated to be stable over the entire pH range, and ZrO

3

2

are attractive

alternatives [28], even though hydrated Zr(OH)

most stable phase in water. Indeed, a screening study on LA hydrogenation involving 50

catalysts in a flow reactor system revealed that Pt on TiO (P25 from Degussa) and ZrO

4

was found to be the thermodynamically

2

performed best, with constant GVL yields for runtimes exceeding 100 h (> 95 mol.% at 200

^oC and 40 bar H2, 11 wt.% LA in GVL) [28]. LA hydrogenation studies in water as solvent

comparing Ru/TiO

revealed that quantitative LA conversion could be achieved within 4 h with the Ru/TiO

catalysts, while Ru/ZrO was less active and 92% LA conversion was observed after 6 h [29].

This enhanced hydrogenation activity of Ru/TiO was attributed to a better Ru dispersion on

TiO . Recently, some of us reported on a catalyst screening study for LA hydrogenation in

water (90°C, 45 bar H ) using a wide range of supported Ru-catalysts (1 wt. % Ru on C, CNT,

Al , TiO , ZrO , Nb and H-Beta-12.5) and again found that TiO (anatase form, A100)

performed better than for Ru/ZrO in this solvent [30]. The results listed above thus show

the potential of TiO as support for this reaction and an overview of titania-supported Ru

2

(P25) and Ru/ZrO

2

catalysts (both 1 wt.% Ru) in a batch set-up at 70 ^oC

2

O

3

2

O

5

2

catalysts for LA hydrogenation is given in Table 1.

Some of these studies have shown that the phase composition of the TiO support

2

(

anatase, rutile and combinations thereof) influences catalyst performance. For example, Al-

Shaal et al. investigated the hydrogenation of LA using Ru supported on rutile and P25 TiO

a 75:25 mixture of anatase-rutile) supports in ethanol and ethanol–water mixtures [25]. The

rutile-supported catalyst gave no LA conversion in neither ethanol nor ethanol–water,

whereas Ru/TiO (P25) showed much better performance. Furthermore, a comparison of the

catalytic transfer hydrogenation of levulinate esters using Ru(OH) /TiO [31] with anatase,

rutile and anatase-rutile titanias, showed the anatase-rutile-based catalyst to perform best

86% for TiO (A); > 99% for TiO (R) and TiO (A75:R25)). Very recently, Ruppert et al. reported

a detailed study on the influence of various TiO supports (anatase, rutile and mixtures

thereof) on the Ru-catalyzed LA hydrogenation [32]. A Ru/TiO (AR) catalyst (10-20% rutile)

2

(

2

x

2

(

2

again proved to be more active than fully anatase-based ones. The lower activity of the

latter was attributed to the presence of larger Ru particles and agglomerates on the surface

and relatively small Ru nanoclusters predominantly found in micropores, thought to be

inactive in the hydrogenation reaction. Activity of the anatase-based Ru catalyst could be

improved by implementation of either a calcination step of the anatase support before

4

impregnation to reduce the micropore volume or by avoiding a high temperature reduction

step during catalyst synthesis by using a mild chemical reduction step [32].

Besides variations in support phase composition, many different synthetic

procedures for titania-supported Ru catalyst preparation have been reported varying in the

choice of, e.g., metal precursor, impregnation method and activation procedure (Table 1),

which all may affect catalyst performance. Wet impre gnat ion is most commonly used

method for the synthesis of these Ru/TiO

such as RuCl , Ru(NO)(NO , Ru(acac) , Ru(NH

Limited information is available on the effects of these synthesis parameters on LA

hydrogenation activity for Ru/TiO catalysts. For a series of Ni/γ-Al catalysts, the effect of

2

catalysts [19],[20],[25],[29], usin g Ru p recurso rs

3

)

3

)

6

Cl or Ru (CO)12 [19],[20],[25],[29]-[35].

3

2

O

2 3

catalyst preparation method on catalytic activity for LA hydrogenation to GVL has been

investigated [36]. Four different Ni catalysts were prepared via wet impregnation, incipient

wetness impregnation, precipitation and flame spray pyrolysis, leading to different textural

and, as a result, catalytic properties with wet impregnation giving the best results. Another

example concerns the hydrogenation of LA to pentanoic acid using Ru supported on H-ZSM-5

[

37], for which the use of Ru(NH

3

)

6

3

Cl as precursor resulted in much better yields than when

using RuCl

3

.

To the best of our knowledge, a systematic study on the influence of several

important synthesis variables (Ru precursor, calcination and/or reduction step) in the

catalyst preparation procedure for the hydrogenation of LA with supported Ru catalysts has

not been reported to date. We here report such an approach using Ru supported on pure

anatase TiO

during the catalyst synthesis procedure. In addition, we have recently shown good catalytic

results for Ru on anatase TiO [30]. Perusal of the literature on Ru/TiO catalyzed LA

2

. The support choice was fixed to reduce the complexity of possible variations

2

hydrogenation (Table 1) finally shows that catalyst performance not only depends on

support choice and catalyst synthesis parameters, but that a strong dependence on the

choice of solvent can also be expected. Therefore, catalyst activity and selectivity to GVL

were determined in both water and dioxane, the latter being a typical example of an organic

solvent. It is well known that higher activities are attainable for reactions run in water

compared to dioxane at otherwise similar conditions. As such, an optimum catalyst

preparation procedure for catalysis in water not necessarily be the best for dioxane. Catalyst

characterization (N

2

physisorption, Transmission Electron Microscopy (TEM) and Scanning

5

Electron Microscopy (SEM) were carried out to rationalize the results and provided insight

into the relation between catalyst performance and catalyst structure.

2. Experimental section

2

.1. Materials

Levulinic acid (purity > 98%, less than 1 wt.% H

2

O) and dioxane (purity > 99%) were

purchased from Acros Organics, deuterium oxide (purity 99.9%) was purchased from Sigma-

Aldrich. Hydrogen and nitrogen gas were from Linde Gas (purity > 99.9%). The ruthenium

precursors, RuCl

Sigma-Aldrich; RuNO(NO

Organics. All chemicals were used as received. TiO

3

·xH

2

O (35-40 wt.% Ru) and Ru(NH

(in dilute nitric acid, 1.5 wt.% Ru) was supplied from Acros

support (≥ 99%, powder, average

3

)

6

Cl

3

(98 wt.% Ru) were supplied by

)

3 3

2

diameter of 156 nm (see Figure S1, Supplementary information)) was obtained from Sigma-

o

Aldrich and dried at 120 C for 4 h in air before use. XRD analysis only shows peaks of the

anatase formand clear peaks of the rutile phase are absent (Figure S2, Supplementary

information).

2

.2. Analytical equipment

X-Ray Diffraction (XRD): The crystal form of the TiO support was determined using X-

2

ray diffraction (D8 Advance, Bruker) with filtered Cu K

α

radiation and a wavelength of 1.5404

Å. The tube voltage was 40 kV, and the current was 30 mA. The 2θ scanning rate was

0

.02 /min in a range from 5 to 80 .

Transmission Electronic Microscopy (TEM): TEM measurements in bright field mode

were conducted with a CM12 microscope (Philips), operating at 120 keV. Samples were

made by ultra-sonication in ethanol and dropping the suspension onto carbon coated 400

mesh copper grids. Images were taken on a slow scanning CCD camera. The metal particle

distribution is calculated by measuring at least 100 particles with the software Nano

Measurer 1.2.

Scanning Electronic Microscopy (SEM): The average size of the TiO particles was

2

determined using Scanning Electron Microscope Jeol JSM-7000F (Field Emission Scanning

Electron Microscope). For this purpose, the size of about 300 particles was measured.

6

Nitrogen physisorption experiments: Nitrogen physisorption experiments were

o

carried out in a Micromeritics ASAP 2020 at 196.2 C. The samples were degassed in

o

vacuum at 200 C for 10 h. The surface area was calculated using the standard BET method

(

SBET). The single point gas adsorption pore volume (V ) was calculated from the amount of

T

gas adsorbed at a relative pressure of 0.98 in the desorption branch. The pore size

distributions (PSD) were obtained from the BJH method using the adsorption branch of the

isotherms. The mean pore size was taken as the position of the PSD maximum. The t-plot

method was employed to quantify the micropore volume (V ).

M

2

.3. Catalysts preparation

A total of fifteen Ru/TiO

2

catalysts were synthesized via a standardized wet

o

impregnation procedure. The anatase titania support was dried (120 C for 4 h in air) before

use. The ruthenium loading was kept constant at 1 wt.% Ru for all catalysts. A number of

parameters were varied: Ru precursor (RuCl

step (with or without) and the amount of hydrogen in the gas phase during the reduction

step (5, 10, 50 or 100 vol.% H ). Throughout this paper, the catalysts are abbreviated based

on the precursor used for its preparation and the calcination/reduction procedure. For

instance, Ru/TiO (Cl,N,10) is a catalyst made using RuCl as the precursor, without

calcination and reduced in an atmosphere with 10 vol.% H in nitrogen.

3

·xH

2

O; RuNO(NO

3

)

3

or Ru(NH

3

)

6

3

Cl ), calcination

2

3

2

A typical catalyst synthesis entailed the dissolution of a required amount of the

o

precursor in 25 mL of water while stirring (1100 rpm) for 30 min at 30 C to obtain a

homogeneous solution. The titania support (1 g) was added gradually to the precursor

o

solution under stirring. Subsequently, the temperature was increased to 85 C and kept until

complete evaporation of the water (after about 19 h). After the impregnation, the catalyst

was calcined and/or reduced. Catalysts prepared without a calcination step were directly

reduced after the impregnation. When the catalysts were calcined, this was done after

impregnation, followed by the reduction step. Calcination was performed using a

o

Micrometrics AutoChem II 2920 system at 450 C (heating rate 2 C/min) in a nitrogen

atmosphere (N flow of 100 mL/min) for 4 h. The reduction step was carried out in the same

2

o

system at 400 C for 4 h. Either a N

2

/H

2

mixture (with variable composition) or pure H was

2

used, the total gas flow rate was maintained at a flow rate of 100 mL/min.

7

2

.4. Catalytic hydrogenation of LA

LA hydrogenation reaction were performed according previously published standard

conditions for water [38] and dioxane [39]. The hydrogenation reactions using water as the

solvent were performed in a stainless steel batch autoclave (100 mL, Parr Instrument

Company). The heating mantel of the autoclave was equipped with electric heating rods and

a cooling coil (using water) for proper temperature control. The reactor content was agitated

o

using an overhead stirrer (Heidolph, RZR 2102 control). The temperature (90 C) and

hydrogen pressure (45 bar) were measured online with a Eurotherm 2208e. For liquid

sampling during the reactions, the reactor was equipped with a dip-tube. LA (2.9 g) was

dissolved in water (40 mL, 0.6 M) and catalyst (0.06 g, LA/Ru molar ratio of 4350) were

introduced in the autoclave. The stirrer was started (2000 rpm) and the system was flushed

with nitrogen for 5 min. The mixture was heated to the desired temperature and

subsequently hydrogen was admitted to the reactor to 45 bar. This moment is set as t = 0

min. During the reaction, hydrogen was regularly admitted to the reactor to keep the

pressure constant at 45 bar. The information has been added in the experimental section in

the manuscript. 16 catalysts were prepared and tested for the hydrogenation reaction in

water and dioxane. The hydrogenation result for one of the catalysts (Ru/TiO2(NO,N,100,

particle size 4.7 nm) in water was discarded from the dataset as hardly any reactivity was

observed, likely due to an unknown experimental error.

The reactions in dioxane were performed in a 50 mL Parr batch autoclave at a

o

temperature of 150 C for 4 h using a hydrogen pressure of 45 bar and a stirring speed of

1

250 rpm. The tests were typically performed using 27.8 g of dioxane, 7.37 wt.% of levulinic

acid (2.2 g, 0.63 M) with 0.06 g of catalyst (LA/Ru molar ratio of 3200). Then the autoclave

was purged three times with argon after which the reaction mixture was heated to reaction

temperature and charged with H . This was taken as the starting point of the reaction;

2

during the reaction samples were taken regularly, filtered and 1 wt.% of anisole was added

as internal standard to the samples. At the end of the reaction, the autoclave was cooled

rapidly to room temperature in an ice bath, after which the remaining H was released.

2

An overview of reaction conditions is given in Table 2.

8

2.5. Determination of the concentrations of LA, GVL and 4-HPA in the liquid

phase

Analyses of the reaction products from LA hydrogenation in water:

The composition of the aqueous reaction mixtures (LA, 4-HPA and GVL) was

1

determined quantitatively by H-NMR. NMR analysis proved to be the most reliable method

for the quantification of 4-HPA, which is difficult to determine by GC or HPLC [35]. A sample

(

approximately 200 µL) was weighed, dissolved in D O and dioxane (internal standard, 10 µL)

2

was added. All spectra were integrated using MestReNova software. The number of moles of

a component in the sample was calculated using eq. 1:

퐼푛푡푒푔푟ꢀ푙ꢁꢀ푛ꢀ푙푦푡푒)

푁_푠

(1)

푚표푙 = 푚표푙 × (

ꢄ ×

푎

푠

퐼푛푡푒푔푟ꢀ푙ꢁꢂ푛푡. ꢃ푡ꢀ푛푑. )

푁_푎

where N

δ 2.1 ppm for LA (3 hydrogen atoms), δ 1.03 ppm for 4-HPA (3 hydrogen atoms), δ 1.3 ppm

for GVL (3 hydrogen atoms)) and N is 8, being the number of hydrogen atoms of dioxane at

a

is the number of hydrogen atoms of the NMR resonances used for the calculation

(

S

δ 3.6 ppm. The concentrations of LA, GVL and 4-HPA in the samples were calculated using

eq. 2.

푚표푙_푎

푉_ꢅ

(2)

퐶_푎=

× 퐷_푓

Where V

t

is the volume of the mixture in NMR tube and D

f

the dilution factor, which was

calculated as follows (eq. 3):

푚ꢀꢃꢃ 표ꢆ 푡ℎ푒 ꢃꢀ푚푝푙푒 ꢂ푛 푁푀푅 푡푢푏푒 + 푚ꢀꢃꢃ 표ꢆ 퐷 푂 + 푚ꢀꢃꢃ 표ꢆ ꢂ푛푡푒푟푛ꢀ푙 ꢃ푡ꢀ푛푑ꢀ푟푑

2

(3)

퐷_푓=

푚ꢀꢃꢃ 표ꢆ 푡ℎ푒 ꢃꢀ푚푝푙푒 ꢂ푛 푁푀푅 푡푢푏푒

Analyses of the reaction products from LA hydrogenation in dioxane:

Reaction products were analyzed using a Shimadzu GC-2010A gas chromatograph

equipped with a CPWAX 57-CB column (25 m × 0.2 mm × 0.2 µm) and FID detector, using

authentic samples for calibration.

2

.6. Definitions

The conversion of LA and the yield and selectivity for the products (4-HPA and GVL)

were calculated according to eq. 4-6.

9

^퐶퐿퐴,0 ⁻^퐶퐿퐴

^퐶퐿퐴,0

(4)

(5)

푋_퐿_퐴

=

× 1ꢇꢇ%

^퐶퐺ꢈ퐿

푌

=

× 1ꢇꢇ%

퐺ꢈ퐿

^퐶퐿퐴,0

^퐶⁴ꢉ퐻푃퐴

^퐶퐿퐴,0

푌

4

ꢉ퐻푃퐴

푌

푖

(6)

푆_푖=

× 1ꢇꢇ%

^푋퐿퐴

Where XLA is the conversion of LA (mol.%); CLA,0 the initial concentration of LA (mol/L);

LA the concentration of LA at a certain time t (mol/L); YGVL the yield of GVL (mol.%); Y4-HPA

C

the yield of 4-HPA (mol.%) and S

i

the selectivity to GVL or 4-HPA (mol.%).

, mol/molRu∙s) was determined from the experimentally

The initial reaction rate (R

0

obtained concentration-time profiles using a procedure given by Fogler [40]. For this

purpose, the concentration time profile was modeled using a higher order polynome. The

initial rate was determined by differentiation of the polynome and setting the value for the

time at zero.

-

1

The turnover frequency (TOF, s ) was calculated according to eq. 7.

^푅0

(7)

푇푂퐹 =

^퐷ꢊꢋ

Where R

0

is the initial reaction rate (mol/molRu∙s) and DRu is the dispersion of

ruthenium, which was calculated as follows (eq. 8):

6

^×^푉ꢊꢋ

(8)

퐷_ꢊ_ꢋ

=

^ꢀꢊꢋ × 푑_ꢊ_ꢋ

3

where VRu is the volume occupied by a bulk Ru atom (0.01365 nm ), aRu is an area per Ru

2

atom (0.0635 nm ) and dRu is the average diameter of a Ru particle (nm, taken from the TEM

data).

1

0

3. Results and discussion

3

.1. Catalyst characterization

Fifteen Ru/TiO catalysts were synthesized using RuCl

Cl as Ru precursors, in/excluding a calcination step, and with different hydrogen

). Relevant properties of

catalysts were determined (BET surface area, micropore volume and the

2

3

·xH

2

O, RuNO(NO ) ,

3

Ru(NH

3

)

6

3

partial pressures during the reduction step (5, 10, 50 or 100 vol.% H

the various Ru/TiO

average Ru nanoparticle size) and the results are provided in Table 3. N

2

-physisorption

2

analysis of fresh catalysts show that the surface areas are between 13 and 9 m /g, which are

2

lower than for the bare anatase TiO

2

support (17 m /g). The average Ru particle size was

determined with TEM (Table 3) and two representative TEM pictures are shown in Figure 1.

The average Ru nanoparticle size varied considerably over the series, ranging from 1.1 to

1

4.2 nm. In some cases, especially when the Ru nanoparticle sizes are larger than 6 nm,

considerable particle agglomeration was observed.

The observation on the basis of TEM that well-dispersed Ru on TiO (A100) with small

2

average Ru nanoparticles sizes are attainable for some of the catalysts is surprising

considering a recent study on Ru/TiO

Ru(acac) on large surface-area TiO

2

(A100) [32]. Here, the wet impregnation of ruthenium

2

(

3

2

(A100, 83-336 m /g), followed by calcination at 200°C in

air and a reduction at similar conditions with hydrogen resulted in an inhomogeneous metal

distribution with very large Ru nanoparticles (as seen by TEM) and small ones in the

micropores. A Ru/TiO (A100) catalyst for which the support was calcined at high

2

temperature before impregnation to reduce microporosity showed no particle agglomerates

and a rather homogeneous distribution was obtained with an average Ru particle size of 4.5

nm. Comparison between our results and these given above is difficult as the anatase

2

support used in our study has a far lower BET surface area (17 m /g versus 83-336 m /g in

32]). However, it shows that the catalyst preparation procedure is of high importance and

under optimum conditions, highly dispersed Ru/TiO (A100) can be obtained.

[

2

1

3

.2. Hydrogenation experiments using the anatase-supported Ru catalysts

Initial hydrogenation experiments were performed with Ru/TiO (Cl,N,10) in both

water an d dioxane. Standard reaction conditions were selected based our previous results

19],[38],[39] and are given in Table 2. The lower reaction temperature selected for the

2

[

o

reaction in water (90 C) compared to dioxane (150 C) reflects the surprisingly high activity

of Ru-based hydrogenation catalysts under aqueo us condit io ns, for which the underlying

reasons have been recently discussed [15],[17],[19],[38],[41],[42]. The hydrogen pressure

was set at 45 bar for all experiments. Concentration-time profile for the main components

(LA, 4-HPA and GVL) under these standard reaction conditions are shown in Figure 2.

In the case of water as the reaction medium, substantial amoun ts of 4-HPA are

formed, in line with our own observations and literature data [23],[38],[41]. Other

intermediates (e.g. α-angelicalactone) or consecutive hydrogenation products of GVL (e.g.,

1

,4-pentanediol or 2-methyltetrahydrofuran) were not detected. Full conversion of LA was

achieved within 6 h with 89% selectivity to GVL, the remainder being 4-HPA (Figure 2). The

intramolecular esterification of 4-HPA is known to be an equilibrium reaction with an

equilibrium constant of about 16 [38]. The ratio of GVL/4-HPA obtained here after 6 h of

reaction is 5, indicating that the two components are not yet at equilibrium.

o

The reaction in dioxane at 150 C gave almost quantitative LA conversion after 4 h

(Figure 2), with full selectivity to GVL. No 4-HPA was detected in dioxane, as the

intramolecular esterification reaction to GVL is much faster in organic solvents than in water

due to equilibrium considerations (Scheme 1) [38].

3.3. Effect of variation of the Ru-precursor on catalytic activity

Variation of the Ru precursor at otherwise fixed synthesis conditions (i.e., calcination

and direct reduction with 10 vol.% H

and Ru/TiO (NH ,N,10) for which LA hydrogenation activity is shown in Figure 3 and a

compilation of the data is given in Table 4.

2

) gave the catalysts Ru/TiO

2

(Cl,N,10), Ru/TiO (NO,N,10)

2

3

1

2

Remarkable differences in catalyst activity were observed for the three precursors,

with the trends in activity being the same in both solvents.The catalyst prepared with RuCl

was the most active in both cases (Table 4), intermediate activity was found for RuNO(NO

and lowest LA conversions was seen after 4 h for Ru(NH Cl

3

)

3

)

6

3

. In all cases, GVL was the

major product, with, as expected, minor amounts of the intermediate 4-HPA for the

reactions performed in water.

However, the general trend regarding LA conversion after 4 h is not exactly the one

as described above for catalyst prepared with different precurors by a calcination step

followed by a direct reduction with 10 vol.% H

and RuNO(NO gave the highest LA conversion after 4 h (Table 4). The highest activity for a

catalyst prepared using the RuCl precursor in water was observed for catalyst 1 (86% LA

conversion), compared to 77% LA conversion when using the RuNO(NO precursor (catalyst

0). These highest conversions were obtained for the uncalcined catalysts which were

reduced at relatively low hydrogen partial pressures. For catalysts made with the Ru(NH Cl

2

. In general, the catalysts prepared with RuCl

3

)

3 3

3

)

3 3

1

3

)

6

3

precursor, LA conversions were in general by far lower than for the other two precursors and

best performance was found for catalyst 12 (24% LA conversion). The choice of Ru

precursors will influence the average Ru particle size, which is expected to play a large role in

determining catalyst activity. In this study, the highest activity is observed for the Ru catalyst

with an intermediate Ru particle size (Ru/TiO (Cl,N,10), 2.2 nm). In addition, the Ru oxidation

2

0

state might play a role as well, with Ru nanoparticles assumed to be the catalytically active

species [43], even though RuO species may also play a role [41],[44],[45].

2

3

.4. Effect of reduction procedure on catalyst activity

For a number of catalysts, prepared with RuCl as the precursor, the partial pressure

of hydrogen was varied in the reduction step (10, 50 and 100 vol.% H in N /H mixture, 1

bar). The results obtained with the catalysts (Ru/TiO (Cl,N,10), Ru/TiO (Cl,N,50) and

Ru/TiO (Cl,N,100) in Table 4) under standard conditions in both solvents are presented in

Figure 4.

3

2

The partial pressure of hydrogen during the catalyst reduction step has a profound

influence on activity. Again the order of activity is the same for both solvents, with the best

results obtained with the catalyst reduced at 10 vol.% of H . The TEM results show the same

2

1

3

particle sizes of around 2.2 nm for the reduction with 10 and 50 vol.% H

leads to agglomeration and an average particle size of 8.4 nm. The catalytic results obtained

with Ru/TiO (Cl,N,10) and Ru/TiO (Cl,N,50) differ considerably, despite the same TEM Ru

particle size and indicate that factors other than mere particle size affect catalytic activity

vide infra).

2

, while 100 vol.% H

2

(

3.5. Effect of an intermediate calcination step on catalyst activity

The effect of a calcination step for Ru-catalyzed reactions has been reported in a

number of studies. For instance, Luo et al. reported a negative effect on activity for the

hydrogenation of LA in dioxane using a Ru/H-ZSM-5 catalyst [37]. The formation of highly

mobile RuO species during the calcination step and subsequent sintering, was mentioned as

x

the most likely reason. Ruppert et al. showed that avoiding a thermal treatment step in the

catalyst preparation protocol was beneficial for catalytic activity for LA hydrogenations using

Ru/TiO (A100) in water [32]. This finding was attributed to a lowering of the average Ru

2

particle size for the mild chemical treatment compared to calcination, leading to higher

catalyst activities. The negative effect of a high temperature calcination step on the average

Ru particle diameter (and catalytic activity) was also observed for the hydrogenation of o-

xylene over Ru/TiO

2

, Ru/Al

2

O

3

and Ru/SiO

2

catalysts [46], again rationalized by

agglomeration of Ru particles as a consequence of the high migration rates of RuO

at elevated temperature [47],[48].

2

species

We tested the effect of an intermediate calcination step during catalyst preparation

on the catalytic activity in water for the catalysts prepared with the RuCl precursors and the

3

results are given in Figure 5.

An intermediate calcination step negatively affects catalyst performance, as clearly

demonstrated by the lower activity seen for all calcined catalysts when using RuCl , in line

with the literature data reported above. Calcination indeed led to an increase in average Ru

nanoparticle size when using RuCl as precursor for catalyst synthesis, see Figure 6 for

details. However, for catalyst prepared with the Ru(NH Cl precursor, the calcined sample

catalyst 15) has a lower average Ru nanoparticle size than the uncalcined one (catalyst 14).

Whether this is specific for catalysts perpaerd with the Ru(NH Cl precursor cannot be

concluded unequivocally as the dataset is too limited, with only two samples for Ru(NH Cl

compared to seven for RuCl

3

)

6

3

(

3

)

6

3

)

6

3

.

1

4

The effect of the partial pressure of hydrogen during the reduction step on catalyst

activity of the calcined catalysts reveals the presence of an optimum value for intermediate

hydrogen levels (Figure 5). This might be the result of a balance in the average Ru particle

diameter and extent of Ru reduction, which are expected to oppositely affect catalyst

activity. TEM analysis (Figure 6) shows that higher amounts of H in the reduction step for

2

the calcined samples led to sintering, which is expected to have a negative effect on catalytic

activity.

3

.6. Discussion

3

.6.1. Effect of ruthenium particle size on catalytic activity for LA hydrogenation in water

The effect of the Ru nanoparticle size on catalytic activity for the hydrogenation of LA

in water was investigated by comparing the initial rate of all reactions (mol/molRu.s) and TOF

-

1

(

s ) versus the average Ru nanoparticle diameter as determined by TEM, see Figure 7. The

TOF values are a better activity performance indicator as these are normalized on the

surface Ru atoms (based on TEM particle size data).

For catalysts, prepared with the RuCl precursor, a volcano type plot is found for the

3

initial rate as function of Ru particle size, with the highest activity for Ru nanoparticles with

an average diameter of about 2 nm (Figure 7, left). A similar trend can be seen for the

catalysts prepared with the other precursors.

A number of papers have reported the effects of Ru nanoparticle size on catalytic

activity for hydrogenation reactions with Ru on TiO

that the optimum Ru particle size for LA hydrogenation (in terms of LA conversion after a

fixed batch time) for Ru/TiO (A100) is in the range of 3-4 nm. The authors proposed that the

2

catalysts. Ruppert et al. [32] reported

2

activity for smaller Ru particles (< 1 nm) is lower as these are present in the micropores and

thus less available for the hydrogenation reaction amongst others due to diffusion

limitations [32]. Primo et al. examined the influence of metal nanoparticle size of Ru/TiO

2

(0.64 wt.% Ru) catalysts for the hydrogenation of lactic acid to 1,2-propanediol [49]. When

increasing the Ru particle diameter from 2 to 10 nm by subjecting the catalysts to a

o

calcination step at 400 C, the product yield decreased from 99 to 38%.

1

5

TOF values, expressed as mol LA converted per mol of surface Ru per s, as a function

of the Ru nanoparticle size (Figure 7) show volcano type plots with a maximum TOF within

the range. This implies that LA hydrogenation is structure sensitive for these catalysts.

Surface sites of different configuration (e.g. ensembles of surface atoms), of which the

concentration is a function of particle size and shape, then contribute differently to the

observed, averaged reaction rate. Detailed overviews on st ructure sensitive (hydrogenation)

reactions are given by Murzin and van Santen et al. [50],[51]. Recent research by Cao et al.

on the hydrogenation of LA using both Ru/Al

2

3

O and Ru/C catalysts also revealed the

presence of typical volcano-type activity plots when the TOF was plotted as a function of the

average Ru particle size [52]. A maximum TOF was found for catalyst with an average Ru

particle diameter of about 1.5 nm for both supports.

Our dataset is too limited to draw definite conclusions regarding the optimal Ru

nanoparticle size for the hydrogenation of LA, but seems to be between 1 and 3 nm for

catalysts prepared with the two chloride precursors, RuCl

nm for RuNO(NO , in line with the data from Cao et al. for Ru/Al

Of interest is the observation that the highest TOF value in the range is observed for

the catalyst prepared with the RuNO(NO precursor. One of the possible reasons for a

3

, Ru(NH

3

)

6

Cl

3

and between 1 and 4

3

)

3

O

2 3

and Ru/C catalysts.

)

3 3

better performance of the catalysts prepared with this precursor is the absence of chloride

residues, which may be present when using the other two chlorine containing precursors.

Analyses of the reduced catalysts by XPS shows the presence of only minor amounts of

precursor-related elements. The content of Cl in Ru/TiO

2

(Cl,N,10) catalyst is below 0.6 at.%,

(NH ,N,10). As such,

whereas it is below the detection limit for Ru/TiO (NO,N,10) and Ru/TiO

2

3

there is not a clear relation between the activity of the catalysts and the chlorine content of

the samples. The nitrogen content in the three samples was also below the detection limit,

indicating that differences in residual nitrogen content do not play a role.

Catalyst activity thus depends on Ru nanoparticle size; the observation that the

activity versus Ru-nanoparticle size plots are also a function of the precursor used during

synthesis, also suggests that other factors play a role as well, however. These can include the

ratio of various charged and non-charged Ru species, or the presence of minor amounts of

residues from the catalyst precursors.

1

6

3

.6.2. Solvent effect

The hydrogenation reactions as reported in this study were carried out both in water

and dioxane (see Table 2 for experimental details). Comparison of the LA conversion after 4

h reaction time for both water and dioxane in the form of a parity plot is provided in Figure

8

. A reasonable fit was obtained, showing that the performance in water and dioxane follow

a similar trend. As such, the effect of Ru particle size distribution on catalytic activity as

shown for water, seems to be valid for dioxane as well. In addition, it suggests that the

molecular mechanism for the hydrogenation reactions in both solvents are similar.

4. Conclusions

We have shown that the performance of Ru/TiO catalysts for the hydrogenation of

2

LA in water and dioxane strongly depends on the preparation protocol (nature of the Ru-

precursor, calcination and/or reduction step and the amount of hydrogen present in the

sweep gas during reduction). Best results were obtained when using the RuCl

3

precursor

(

when considering initial rate on Ru intake) and the RuNO(NO precursor (when

3 3

)

considering TOF) without an intermediate calcination step and 10% hydrogen in the

reduction gas, whereas calcination and the use of a hydrogen rich sweep gas gave rise to

lower catalytic activity. Catalyst characterization studies showed that the different

preparation protocols lead to catalysts with differences in average Ru particle sizes. It was

shown that catalyst activity (TOF and initial rates) in both water and dioxane is strongly

correlated with the average Ru particle size, and an optimum activity was found for average

Ru particle sizes of about 1-5 nm. This implies that LA hydrogenation in water is structure

sensitive. The fact that catalysts with similar particle sizes though prepared with different

precursors show different TOF values suggest that additional factors than particle size-

dependent structure sensitivity must play a role. A possible factor is the difference in

amounts of charged and neutral ruthenium species, which are expected to have different

activity profiles. However, the influence of minor amounts of precursor residues such as Cl,

as found for Ru/TiO (Cl,N,10), cannot be ruled out.

2

5. Acknowledgement

This research was conducted in the framework of the Dutch CatchBio program.

Financial support from the Smart Mix Program of the Netherlands Ministry of Economic

1

7

Affairs and The Netherlands Ministry of Education, Culture and Science is gratefully

acknowledged. The authors thank the members of the CatchBio User Committee for

valuable suggestions and discussions. A. Goryachev J.P. Hofmann and E. J. M. Hensen from

the Laboratory of Inorganic Materials Chemistry, Department of Chemical Engineering and

Chemistry, Eindhoven University of Technology, The Netherlands are gratefully

acknowledged for the XPS analyses.

1

8

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2

1

Figure 1. TEM image of Ru/TiO

2

(Cl,Y,0) (left,) and Ru/TiO (Cl,Y,50) (right).

2

0

,8

,7

,6

,5

,4

,3

,2

,1

,0

0

,8

,7

LA

GVL

HPA

LA

GVL

0,6

0,5

0,4

0,3

0,2

0,1

0,0

0

1

2

3

4

5

6

0

1

2

3

4

Time, hour

Figure 2. Concentration-time reaction profiles for the hydrogenation of LA using

Ru/TiO (Cl,N,10) catalyst (#1) in water (left) and dioxane (right) under standard reaction

conditions (Table 2).

2

1

00

100

90

80

70

60

50

40

30

20

10

0

Ru/TiO (Cl,N,10) (Catalyst #1)

2

Ru/TiO (NO,N,10) (Catalyst #9)

9

8

7

6

5

4

3

2

1

0

2

Ru/TiO (NH ,N,10) (Catalyst #11)

2

3

2

3

0

1

2

3

4

0

1

2

3

4

Time, hour

Figure 3. LA conversion versus time using Ru/TiO

2

catalysts from three different precursors

in water (left) and in dioxane (right) under standard reaction conditions (Table 2).

1

00

Ru/TiO (Cl,N,10) (Catalyst #1)

2

100

2

Ru/TiO (Cl,N,50) (Catalyst #2)

9

8

7

6

5

4

3

2

1

0

Ru/TiO (Cl,N,50) (Catalyst #2)

2

9

8

7

6

5

4

0

Ru/TiO (Cl,N,100) (Catalyst #3)

2

30

20

10

0

1

2

3

4

0

1

2

3

4

Time, hour

Figure 4. Effect of hydrogen amount in the reduction gas during activation on LA conversion

over Ru/TiO (1 wt.% Ru) catalysts in water (left) and in dioxane (right) under standard

reaction conditions (Table 2).

2

3

-

1

3

2

,5x10

,0x10

,5x10

1

00

#

1 (Ru/TiO₂(Cl,N,10))

2 (Ru/TiO₂(Cl,N,50))

3 (Ru/TiO₂(Cl,N,100))

⁴⁽^R^u^/^Tⁱ^O2 (Cl,Y,100))

⁵⁽^R^u^/^Tⁱ^O2 (Cl,Y,50))

6 (Ru/TiO₂(Cl,Y,10))

With calcination

Without calcination

#1

9

8

7

6

5

4

3

2

1

0

-1

#2

-

1

#7 (Ru/TIO₂(Cl,Y,5))

⁸⁽^R^u^/^Tⁱ^O2 (Cl,Y,0))

2,0x10

,5x10

1,0x10

#

1

#5

#

6

-2

#8

#

3

5

,0x10

#7

#

4

0

,0

0

10 20 30 40 50 60 70 80 90 100

0

1

2

3

4

%

H in the reduction gas

2

Time, hour

Figure 5. Effect of the composition of the reduction gas and the use of an intermediate

calcination step on the initial rate for the catalytic hydrogenation of LA in water for various

Ru/TiO

2

catalysts (prepared using RuCl

3

as the precursor) under standard reactions

conditions (Table 2).

1

5

4

3

2

1

0

9

8

7

6

5

4

3

2

1

0

#

4

with calcination

without calcination

#

5

#3

#6

#7

#8

#

2

#

1

0

10 20 30 40 50 60 70 80 90 100

%

H in the reduction gas

2

Figure 6. Effect of the hydrogen amount in the reduction gas on the Ru particle size diameter

for Ru/TiO catalysts prepared with RuCl precursor with and without a calcination step.

2

3

2

4

Figure 7. Initial rate and TOF for LA hydrogenation in water versus the average Ru-

nanoparticle for the Ru/TiO

conditions: see Table 2.

2

catalysts prepared in this study according to TEM. Reaction

0

10 20 30 40 50 60 70 80 90 100

1

00

100

90

80

70

60

50

40

30

20

10

0

RuCl₃

#1

9

8

7

6

5

4

3

2

1

0

^R^u^N^O⁽^N^O3⁾

3

#10

#9

Ru(NH ) Cl

3

6

3

#2

#15

#

12

#

14

#4

13

#

3

#11

#

0

10 20 30 40 50 60 70 80 90 100

LA conversion in water, %

Figure 8. Parity plot with the LA conversion after 4 h reaction time in water and dioxane.

2

5

O

OH

O

OH

cat.

OH

+ H O

+

H₂

2

O

levulinic acid

LA)

4-hydroxypentanoic acid

(4-HPA)

-valerolactone

(

(GVL)

Scheme 1. Proposed reaction scheme for the catalytic hydrogenation of LA to GVL.

2

6

Table 1. Literature overview on LA hydrogenation using Ru/TiO catalysts in batch and

2

continuous flow set-ups.

H

2

Reaction

time

LA

conversion

(%)

GVL

Selectivity

(%)

Ruthenium

precursor

o

Catalyst

TiO

2

support

Solvent

T ( C)

pressure

bar)

Ref.

(

(h)

Batch

Ru1%/TiO

Ru5%/TiO

Ru2%/TiO

Ru1%/TiO

2

P25

A75:R25)^a

P25

A75:R25)

Tronox

R100)

P25

A75:R25)

P25

A75:R25)

P25

A75:R25)

P25

A75:R25)

RuNO(NO

)

3 3

dioxane

200

130

40

4

100

92

99

[19]

[20]

[25]

(

RuCl

3

40

12

40

0.6

2.7

0.5

3

(

ethanol

n.r.^d-

68

-

Ru(acac)

3

(

ethanol+H

ethanol

2

O

-

92

88

Ru(acac)

(

ethanol+H

2

81

RuCl

3

H

2

O

100

95

99

(

RuCl

2

[29]

[30]

[31]

(

Ru0.5%/TiO

2

RuCl

3

H

2

O

70

90

40

45

4

5

100

93

99

85

76

(

Ru1%/TiO

2

(A100)

ST-01^b

2

Ru(OH)

x

/TiO

x

/TiO

x

/TiO

2

86

(

A100)

TTO-55^c

R100)

P25

A75:R25)

P25

A75:R25)

P90

A90:R10)

RuCl

3

2-propanol

90

-

24

> 99

49

80

(

RuCl

(

Ru1%/TiO

2

30

70

30

70

30

64

99

62

95

Ru(acac)

3

H

2

O

2

O

2

O

50

1

[32]

(

60

(

100

40

100

40

(R100)

7

0

95

83

ST-01

Ru1%/TiO

2

30

54

99

48

93

(A100)

Ru(acac)

3

H

2

O

50

1

[32]

7

0

Continuous flow

Ru0.4%/TiO

2

P25

A75:R25)

P25

A75:R25)

P25

A75:R25)

RuCl

3

H

2

O

2

O

2

O

50

270

90

24

n.r.^d-

45

-

n.r.^d-

6

7

36

44.3

54

[33]

[34]

[35]

(

Ru5%/TiO

2

52.6

(

Ru1%/TiO

a

2

25

(

b

2

c

A: Anatase, P: rutile. ST-01: anatase TiO

2

, SBET = 339 m /g (Ishihara Sangyo Co., Ltd.). TTO-55: rutile TiO

2

, SBET

2

d

=

47 m /g (Ishihara Sangyo Co., Ltd.) not reported

2

7

Table 2. Standard reactions conditions for LA hydrogenation with the Ru/TiO catalysts in

2

batch.

Solvent

water [38]

0.63

40

dioxane [39]

LA initial concentration (mol/L)

Solvent intake (mL)

0.64

28

o

Temperature ( C)

90

150

45

Hydrogen pressure (bar)

Catalyst intake (g)

LA/Ru molar ratio

Stirring rate (rpm)

Reaction time (h)

45

0.060

4350

2000

4

0.060

3200

1250

4

Table 3. Catalyst characterization data for the different Ru/TiO catalysts, including catalyst

2

labeling scheme.

#

Ru precursor

Hydrogen

vol.%) in

Calcination^bBET Surface

Micropore

volume

dRu (nm)

2

(

area (m /g)

(TEM)

3

reduction

gas^a

10

(cm /g)

1

2

3

4

5

6

7

8

9

Ru/TiO

2

(Cl,N,10)

(Cl,N,50)

RuCl

3

No

Yes

No

Yes

No

13

12

2.4x10^-⁴

4.4x10^-⁴

≈0

2.2±0.7

2.1±0.7

8.4±4.5

14.2±6.6

10.1±4.6

5.9±2.9

5.2±3.7

4.1±1.8

4.7±2.1

1.1±0.1

7.7±7.0

1.5±0.5

3.1±0.8

4.2±1.1

1.4±0.6

2

50

Ru/TiO

2

(Cl,N,100)

100

50

10

2

(Cl,Y,100)

12

≈0

(Cl,Y,50)

(Cl,Y,10)

n.d.^c

n.d.

12

n.d.

2

10

n.d.

Ru/TiO

2

(Cl,Y,5)

(Cl,Y,0)

5

n.d.

2

No

n.d.

Ru/TiO

2

(NO,N,10)

(NO,N,50)

RuNO(NO

3

)

3

10

3.9x10^-⁴

2.1x10^-⁴

≈0

1

0

1

2

3

4

5

2

50

11

1

Ru/TiO

2

(NO,Y,100)

100

10

13

2

(NH

2

(NH

3

,N,10)

,N,50)

Ru(NH

3

)

3

)

3

)

3

)

6

Cl

3

12

3.7x10^-⁴

≈0

3

50

9

Ru/TiO

2

(NH

3

,N,100)

100

13

≈0

1

Ru/TiO

2

(NH

3

,Y,100)

Ru(NH

Yes

13

≈0

a

0

b

0

Reductions always performed at 400 C, make up gas is nitrogen. Calcination performed at 450 C under nitrogen

c

atmosphere. n.d.: not determined

2

8

Table 4. Overview of results for the hydrogenation of LA using various Ru on titania catalysts

in water and dioxane

#

Catalyst name

Solvent

LA conv.

Initial reaction

rate

Selectivity GVL

Selectivity 4-HPA

Yield GVL

(mol.%)

a

(

mol.%)

(mol.%)

(

mol/molRu·s)

-1

1

2

3

4

Ru/TiO

2

(Cl,N,10)

(Cl,N,50)

(Cl,N,100)

(Cl,Y,100)

H

2

O

86

96

69

5

7

1

4

15

16

15

9

55

78

77

89

18

2

24

19

3

7

5

3.20×10

2.12×10

2.63×10

92

100

93

97

83

100

99

100

87

70

91

41

91

92

94

92

87

100

50

80

76

86

90

50

87

78

7

-

7

-

17

-

79

96

64

67

4

7

1

5

13

11

14

4

50

71

72

82

16

29

12

16

2

Dioxane

H

Dioxane

H

Dioxane

H

Dioxane

H

Dioxane

H

Dioxane

H

Dioxane

H

Dioxane

H

Dioxane

H

Dioxane

H

2

O

-

1

1.94 ×10

-

2

O

4.57×10

5.31 ×10

-1

-

2

O

1.14×10

2.30 ×10

1

-

-1

-

2

1

5

6

7

8

9

Ru/TiO

2

(Cl,Y,50)

(Cl,Y,10)

(Cl,Y,5)

(Cl,Y,0)

(NO,N,10)

2

O

2

O

2

O

2

O

2

O

9.14×10

6.85×10

2.28×10

4.57×10

2.97×10

12

29

9

59

8

-

6

-

13

-

48

-

23

-

9

-

-1

2.48 ×10

-

1

10

11

12

13

14

15

Ru/TiO

2

(NO,N,50)

(NO,Y,100)

2

O

2.29×10

2.83×10

1.14×10

-

2

O

-1

3.18 ×10

-

1

(NH

3

,N,10)

3

,N,50)

3

,N,100)

3

,Y,100)

2

O

1.94×10

7.08 ×10

-2

-

3

2

O

1.14×10

5.31 ×10

-3

6

5

16

21

-

2

O

1.37×10

-

2

10

19

27

4.6 ×10

5.71×10

-

2

O

13

-

-1

Dioxane

4.07 ×10

^aAfter 4 h batch time

2

9

Article Doi

Hydrogenation of levulinic acid to Γ-valerolactone over anatase-supported Ru catalysts: Effect of catalyst synthesis protocols on activity

DOI: 10.1016/j.apcata.2017.09.032

Source and publish data:

Authors:

Article abstract of DOI:10.1016/j.apcata.2017.09.032

Full text of DOI:10.1016/j.apcata.2017.09.032

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