Aqueous-phase catalytic oxidation of furfural with H2O2: High yield of maleic acid by using titanium silicalite-1

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Aqueous-phase catalytic oxidation of furfural with

H O : high yield of maleic acid by using titanium

2

Cite this: RSC Adv., 2014, 4, 54960

silicalite-1†

a

b

a

N. Alonso-Fagundez, I. Agirrezabal-Telleria, P. L. Arias, J. L. G. Fierro, R. Mariscal

´

a

and M. Lopez Granados*

´

This investigation explores the selective liquid-phase oxidation of furfural to maleic acid (MA) using

hydrogen peroxide as an oxidant and titanium silicalite (TS-1) as a catalyst. The eﬀect of temperature and

of the concentration of H

8 mol%, was obtained under the following reaction conditions: 4.6 wt% of furfural, 4.6 wt% of catalyst,

a H /furfural mol ratio of 7.5, corresponding to 12.3 wt% of H , 323 K and 24 hours of reaction. To

reduce the amount of H employed, a two-step sequence of reactions was conducted using TS-1 and

2

Amberlyst 70 consecutively as catalysts in the ﬁrst and second steps, respectively. In this case, a H O /

2 2

O , furfural and catalyst on the MA yield was studied. The highest yield,

7

2

O

2

2 2

O

2 2

O

2

furfural mol ratio ¼ 4.4 was required, which is quite close to the stoichiometric ratio (3.0), and a maleic

acid yield close to 80% was obtained under 4.6 wt% of furfural, 4.6 wt% of catalyst and 28 h of reaction

at 323 K; after 52 h of reaction, the MA yield reached 92%. Fresh and used catalysts were characterised

by X-ray diﬀraction (XRD), Raman spectroscopy, total reﬂection X-ray ﬂuorescence (TXRF), X-ray

photoelectron spectroscopy (XPS), N

2

adsorption–desorption isotherms and thermogravimetric analysis.

anatase was

Ti was largely incorporated within the silicalite framework, but the presence of some TiO

2

also conﬁrmed. Ti leaching was observed, especially during the ﬁrst run but became much less

important in successive cycles. Leaching aﬀects both anatase and Ti species within the silicalite

framework. Notwithstanding the leaching, when using pure furfural, TS-1 could be reused for six runs

without noticeable deactivation, whereas when using furfural directly derived from biomass, weak but

visible deactivation occurred upon reutilisation; this deterioration must be related to the presence of

some organic products other than furfural.

Received 31st July 2014

Accepted 13th October 2014

DOI: 10.1039/c4ra11563e

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and fumaric acid, both of which have multiple additional

applications, are also synthesised from MA and/or MAN.

Introduction

1,2

Maleic anhydride (MAN) and its hydrated form, maleic acid Currently, maleic anhydride is industrially produced primarily

MA), are important C

chemical intermediates with applica- by selective oxidation of n-butane (it can also be produced from

(

4

tions in multiple elds of the chemical industry. They are benzene, but this obsolete route has been almost fully displaced

important raw materials used in the manufacture of unsatu- for environmental and economic reasons by butane tech-

3

rated polyester resins, vinyl copolymers, tetra- and hexahy- nology). Both MA and MAN are interconverted into one other

1,2

drophthalic anhydride (curing agents in epoxy resins), surface by hydration–dehydration processes.

coatings, lubricant additives, plasticisers, copolymers, agro-

Alternative routes based on the oxidation of furfural to MAN

chemicals, and pharmaceuticals, as well as in the production of and MA, either in gas or liquid phase, have been reported by

g-butyrolactone, butanediol and tetrahydrofuran. Succinic acid diﬀerent research groups. Furfural is a chemical currently

obtained from the pentoses contained in lignocellulosic agro-

residues; approximately 250–400 ktons of furfural are syn-

thesised annually from this carbohydrate feedstock.

Therefore, these routes from furfural constitute renewable

a

Group of Sustainable Energy and Chemistry (EQS), Institute of Catalysis and

4

–6

Petrochemistry (ICP-CSIC), C/ Marie Curie 2, Campus de Cantoblanco, 28049

Madrid, Spain

b

Department of Chemical and Environmental Engineering, Engineering School of The pathways for the manufacture from biomass of MA, MAN and

University of The Basque Country (UPV/EHU), Alameda Urquijo s/n, Bilbao 48013, their derived products. Thus, a renewable route for phthalic

Spain. E-mail: mlgranados@icp.csic.es

anhydride formation by Diels–Alder condensation with furan

has very recently been demonstrated.

†

Electronic supplementary information (ESI) available: Additional information

7

on experimental details, catalytic activity, other proposals of mechanism of

Gas-phase oxidation of furfural to maleic anhydride is a well-

known route, and yields up to 75% can be achieved by using

reaction and characterisation of catalysts by TG analyses, XRD, and

adsorption. See DOI: 10.1039/c4ra11563e

N

2

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vanadium-oxide-based catalysts; cheap air O

2

is used as an to activate hydrogen peroxide is linked to the particular envi-

8–12

oxidising agent.

However, dilute furfural streams (below 1% ronment of Ti(IV) cations incorporated into the silicalite

v/v) are required to achieve high MAN selectivity. The reaction is framework and located within the hydrophobic pores of the

conducted at high temperatures (higher than 523 K), and the zeolite. This allows simultaneous adsorption of the hydro-

21,26,27

vaporisation of furfural is required. Both of these features have phobic substrate and the oxidant.

a negative impact on the energy balance and consequently the

economic viability of this process.

The latter results using furan encouraged us to explore the

utilisation of TS-1 in the selective oxidation of furfural to MA by

Liquid oxidation with O

been explored using homogeneous catalysts like phosphomo- maleic acid (a C

lybdic acid in a water/tetrachloroethane biphasic system, key question is whether, besides the epoxidation and oxidation

2

at milder temperatures has also using H

2

O

2

5

. Because the oxidation of furfural (a C aldehyde) to

4

diacid) implies the loss of a carbon atom, the

1

3,14

achieving MA yields not larger than 50%.

although milder temperatures are required (383 K), high O₂of a C atom.

pressure (2 MPa) is needed. The reusability of these catalysts In principle the oxidation of furfural with H

has not been proved, which is crucial to evaluate the feasibility advantageous than the furan oxidizing route previously

Moreover, steps, the TS-1 catalyst can also promote in practice the removal

2 2

O is more

21,22

of possible industrial applications.

demonstrated.

Furan is currently produced by decarbon-

1,2

Another liquid-phase alternative that has recently been ylation of furfural and therefore the production of MA from

investigated is furfural oxidation with hydrogen peroxide. So furan would imply two steps: rst decarbonylation and subse-

far, the best results using this strategy have been achieved by quently the oxidation of furan. A rst drawback of this furan

using either a soluble acid catalyst like H SO and p-tol- route is that the decarbonylation of furfural is conducted at

2

4

uensulphonic acid or by using solid sulphonic-based catalysts relatively high temperatures (423–453 K) and in the presence of

1

5–19

such as sulphonic resins or poly-(styrene sulphonic acid).

Oxidation with H seems attractive for industrial application, periods are required to regenerate the initial activity that

rst because the reaction proceeds under very mild conditions complicates the viability of the furan route. On the contrary the

low temperatures and atmospheric pressure), and second route here proposed implies the direct conversion of furfural in

because aqueous furfural solutions can be processed, which is a one-step and at mild temperatures.

noble metal catalysts that unavoidably deactivate. Reactivation

2 2

O



(

key advantage. When pure furfural is produced from lignocel-

In this work, we demonstrate for the rst time that TS-1 is an

lulosic agroresidues, the primary raw product is an aqueous excellent catalyst for the transformation of furfural to MA. We

furfural solution that must be further puried by an energy- have also conducted the necessary study examining the eﬀect of

intensive double distillation. Processing primary raw aqueous operating variables like temperature, H O /furfural ratio and

2 2

furfural solutions would imply that distillation steps are not catalyst and furfural concentration on the catalytic properties of

required, which positively impacts the energy and economic TS-1. Specic attention was paid to the deactivation of catalyst

balance of the overall process.

and the reutilisation behaviour of TS-1 under the optimum

However, the main drawback for oxidation with hydrogen reaction conditions, using both pure furfural provided by a

peroxide using acid catalysts is the control of selectivity. A wide commercial supplier and aqueous solutions of furfural

distribution of products such as MA, succinic acid (SA), fumaric obtained directly from corncob residues. A commercial TS-1

acid (FumA), b-formylacrylic acid or 2(5H)-furanone is obtained. with a Ti/Si at. ratio above the threshold required to fully

This lack of selectivity is a consequence of the very complex saturate the silicalite framework with Ti cations was used in this

reaction network that starts with the Baeyer–Villiger oxidation investigation. This means that a TS-1 with the largest possible

15,16,19

of furfural to the corresponding ester.

2 2

Large H O /furfural incorporation of Ti within silicalite (active sites) was used.

ratios (much larger than 7) are required to improve the MA

yield, whereas for lower ratios succinic acid is the main product.

Maleic acid could be obtained by dehydrogenation of succinic

acid, but this would mean an additional step. The identica-

tion of a robust solid catalyst that can selectively oxidise furfural

Experimental

Materials

20

to MA in one step and that can be easily recovered from the Titanium silicalite 1 was purchased from Tricat Zeolites,

reaction mixture and reutilised for a large number of runs is still H-ZSM-5 (Si/Al ¼ 19) was purchased from Akzo Nobel, anatase

a challenge.

catalyst support was purchased from Alfa Aesar and Amberlyst-

The selective oxidation of furan with aqueous H

2

O

2

using 70 was purchased from The Dow Chemical Company. Furfural

21,22

TS-1 has been already investigated.

The furan ring possesses (99 wt%), hydrogen peroxide (30 wt%), furan (99 wt%) and

a similar chemical structure to furfural, except for the absence furan-2(5H)-one (98 wt%) were supplied by Sigma-Aldrich.

of the aldehydic functional group. Maleic dialdehyde and Furoic acid (98 wt%) was provided by Fluka, and oxalic acid

5

-hydroxy-furan-2(5H)-one were obtained selectively by the (99.5 wt%), used as internal standard, was purchased from Alfa

oxidation of furan using H /furan with approximate mol Aesar. 5-Hydroxy-furan-2(5H)-one, which was used as a starting

ratios of 1 and 2, respectively. These products are closely related reactant, was synthesised according to the ndings of Kumar

2 2

O

22

to maleic acid, as the latter can be obtained from them by et al., comprising the oxidation of furan with H O using TS-1

2

further oxidation. Titanium silicalite 1 (TS-1) is frequently used as catalyst. Further details of the preparation are given in the

23–25

in the epoxidation of numerous alkenes.

The ability of TS-1 ESI section.†

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Paper

Catalytic activity measurements

mixture was then titrated with a ca. 0.1 M sodium thiosulfate

solution until the brown triiodide colour was reduced to a light

The oxidation of furfural was carried out in a glass ask reactor

with three necks: one for the thermocouple, one for the addition

of reactants through an addition funnel and the third for

sampling through a septum. The required amounts of TS-1 and

ꢀ

1

straw colour. A few drops of starch solution (10 g L ) were

added, and the solution became dark blue. Titration was

continued until the colour of the solution changed sharply from

blue to colourless. A titration of a blank solution (pure water)

was also carried out following the same procedure. The actual

titer of the sodium thiosulfate solution was determined by

titration with potassium iodate solution (0.1 N).

2 2

of H O solution were loaded into the reactor and heated up to

the reaction temperature by immersion in an oil bath. Aer the

reaction mixture reached the selected temperature, the reaction

started when a known amount of aqueous furfural solution was

added to the reactor. To prevent the mixture from cooling down

because of the incorporation of the aqueous furfural, the latter

solution was preheated to the reaction temperature and rapidly

added to the reactor. The resulting reaction mixture (approxi-

mately 40 mL) was vigorously stirred by a magnetic bar at

The amount of H O present in solution was calculated as

2

follows:

H O present in reaction mixture ðmolÞ

2

1

mol H

2

O

2

34

¼

^ð^A^ꢀ^B^Þ^ð^M^Þ2

mol Na

2

S

2

O

3

2 2

1 mol H O

800 rpm.

where A is the titration volume of Na

the titration volume of Na consumed for the blank and M

is the molarity of the Na S O solution.

2 2 3

S O for the sample, B is

A known amount of reaction mixture (ca. 0.5 g) was sampled

2 2 3

S O

at diﬀerent reaction times and was analysed by HPLC, using a

ꢀ1

2

2 3

known amount of a 30 mg g aqueous solution of oxalic acid as

an internal standard (ca. 15 mg of oxalic acid was added to each

aliquot). Subsequently, the aliquot was diluted 10–20 times with

water. This diluted aliquot, aer being ltered through a 0.22

mm syringe lter, was then analysed in an Agilent 1200 HPLC

chromatograph equipped with a refraction index detector and a

Bio-Rad Aminex HPX-87H column (300 mm ꢁ 7.8 mm). A 0.005

M H SO mobile phase was employed as the eluent at 328 K and

For the reutilisation tests, catalyst was ltered out of the

reaction medium by using a ltration cell and a ceramic

membrane. The catalyst was washed twice with water (ca. 200

mL) to remove reaction products that could interfere in

successive analysis, carefully removed from the membrane to

prevent losses during handling, and dried at 333 K overnight

before performing a new reaction run. The ltered solution

from the catalyst wash steps (including the ltrate from the

reaction medium) was analysed by TXRF for the quantitative

determination of Ti and Si.

2

4

ꢀ

1

a 0.4 mL min ow rate. In addition to oxalic acid and furfural,

the following products were detected by HPLC: MA, malic acid,

SA, formic acid (FA), fumaric acid (FumA), 5-hydroxy-furan-

2(5H)-one (hydroxyfuranone), furan-2(5H)-one (furanone) and

furoic acid (FurA). Deeper details of the analyses are given in

Fig. SI1 of ESI section.†

Furfural conversion and product yields were calculated

according to the following formulas:

Catalyst characterisation

Powder X-ray diﬀraction (XRD) measurements were performed

ꢂ

at room temperature in the 5–50 range in scan mode (0.02 , 1 s)

with a Siemens D5000 automated diﬀractometer, over a 2q

Bragg–Brentano geometry using Cu Ka radiation and a graphite

monochromator.

0

furf

m

ꢀ mfurf

furf

Furfural conversion ðmol%Þ ¼

ꢁ 100

0

m

Nitrogen adsorption–desorption isotherms were recorded at

the temperature of liquid nitrogen (77 K) using a Micromeritics

m

prod

0

furf

Product yield ðmol%Þ ¼

ꢁ 100

TriStar 3000 apparatus. The sample was previously outgassed at

m

ꢂ

1

40 C for several hours. The specic area was calculated by

0

where m

refers to the mole quantity initially loaded into the applying the BET method to the range of relative pressures

furf

0

reactor, and mfurf and mprod refer to the number of moles of the (P/P ) of the isotherms between 0.03 and 0.3 and by assuming a

furfural and products, respectively, in the reaction mixture at a value of 0.162 nm for the cross-section of adsorbed nitrogen

2

given time. The HPLC chromatographic factor of the organic molecules at 77 K.

products was calculated by analysing solutions with known

TXRF (total reection X-ray uorescence) analysis was per-

concentrations of the diﬀerent organic products.

formed in a S2 PICOFOX instrument equipped with two X-ray

The hydrogen peroxide concentration at a given reaction ne focus lines, Mo and W anodes, and a Co(Li) detector with

2

time was determined by iodometric titration. Approximately an active area of 80 mm and a resolution of 157 eV at 5.9 keV

0

.5 g of reaction sample was diluted 10–20 times with water. (Mn Ka). The acquisition time was 300 s and 500 s for qualita-

Then, 0.5 g of this diluted solution was poured into an Erlen- tive and quantitative analysis, respectively. To carry out the

meyer ask containing 20 mL of water. Aer adding 2 mL of TXRF analysis, a Mo X-ray source was used for Si and Ti deter-

potassium iodide solution (10%) and 2.5 mL of an acid mixture mination. For the analysis of solid samples, 10–15 mg of sample

consisting of a solution of ammonium molybdate and H

2

SO

4

,

was ground in an agate mortar to a powder with a particle size

the ask was closed, gently agitated and allowed to rest for ve less than 10 mm. Subsequently, 10 mL of high-purity water was

minutes to form triiodide species by oxidation of the iodide added to the powder. The sample was homogenised for 15–20

with H O , which gives a brown colour to the solution. The min by ultrasonic desegregation in order to disperse any

2

possible agglomeration of particles. A 5 ml aliquot of the

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suspension was removed and placed on a at carrier of plastic

aer which the water was evaporated by vacuum.

Raman spectra were recorded on a single-monochromator

Renishaw 1000 spectrophotometer equipped with a CCD

cool detector (200 K), with an argon laser as the exciting source

(

l ¼ 532 nm), a supernotch lter, and an in situ cell that enables

sample treatment under a ow of gas. Raman spectra were

recorded at room temperature with a laser power of 2 mW aer

an in situ calcination of the samples in synthetic air with a

ꢀ1

heating rate of 10 K min up to 833 K for 30 min.

X-ray photoelectron spectra (XPS) were acquired with a VG

Escalab 200 R spectrometer equipped with a hemispherical

electron analyser and a Mg Ka (1253.6 eV) X-ray source. The

ꢀ5

solids were outgassed at 393 K for 1 h at 10 mbar to remove

water before transfer to the ion-pumped analysis chamber. Ti

Fig. 1 Eﬀect of reaction temperature on the catalytic properties of TS-

1

at three diﬀerent temperatures: (a) 323 K, (b) 333 K and (c) 343 K.

2p, C 1 s, O 1 s and Si 2p core level were scanned a suﬃcient

Reaction conditions: 4.6 wt% of furfural, 2.3 wt% of catalyst, 12.3 wt%

number of times to obtain high signal-to-noise ratios. The static of H O , equivalent to a H O /F mol ratio ¼ 7.5. Symbols: furfural

2

charge of the samples was corrected by referencing all binding conversion (-); 5-hydroxy-furan-2(5H)-one yield (B); maleic acid

yield (C); formic acid yield (;); and malic acid yield (:).

energies (BEs) to the Si 2p peak (103.4 eV). The accuracy of the

BE was ꢃ0.1 eV. The areas of the peaks were computed by tting

the experimental spectra to Gaussian/Lorentzian curves aer

removing the background (using the Shirley function). Surface are used, the reaction initiates with a Bayer–Villiger step to form

atom ratios were calculated from peak area ratios normalised by 2-hydroxyfuran, which is in equilibrium with furan-2(5H)-one

28

using the corresponding atomic sensitivity factors.

Thermogravimetric analysis (TGA) under

and furan-2(3H)-one (see Scheme SI1†). The eventual oxidation

controlled of these furanones gives rise to SA, hydroxyfuranone and MA.

a

atmosphere was carried out on a Mettler Toledo TGA/SDTA Therefore, it appears that when reaction initiates with a Baeyer–

e

ꢀ1

8

51 , using a heating rate of 10 K min up to a maximum Villiger oxidation,

a

wider distribution of products is

15–19

temperature of 973 K under a synthetic air atmosphere.

obtained.

This is not the case with TS-1. We will return to

these points when discussing the reaction mechanism below.

It was also assumed that formic acid arises both from the

Results and discussion

Parametric study of the reaction conditions

deep oxidation of the organics and from the loss of formic acid

19

5 4

required to convert a C chemical into a C compound.

A study of the eﬀect of diﬀerent operating variables on catalytic Accordingly, the FA yield was calculated by subtracting the FA

behaviour was performed rst. The aim was to nd the best supposedly released by the formation of C products (1 mol of

reaction conditions to obtain the highest MA yield. The FA per mol of C product) from the total molar quantity of FA

following operation variables were modied: reaction temper- detected (the molar quantity of FA formed by deep oxidation

ature, H and furfural concentrations and catalyst loading.

was divided by 5 to express the value on a furfural basis). As

4

2 2

O

Reaction temperature eﬀect. The eﬀect of reaction temper- shown in further gures, the carbon balance can approach

ature on the catalytic properties was examined rst. Tempera- 100% under optimum reaction conditions, but for the reaction

15–19

tures in the range used for other catalysts were explored.

Fig. 1 compares the catalytic activity of commercial TS-1 at three 100%. In principle, this incomplete carbon balance may be due

diﬀerent temperatures (323, 333 and 343 K) while keeping the to the total oxidation to CO (not quantiable by HPLC anal-

rest of the reaction conditions constant (4.6 wt% of furfural, ysis). However, this is not the only possibility, and the formation

conditions used in Fig. 1, the carbon balance was always below

2

2.3 wt% of catalyst and 12.3 wt% H

2

O

2

, equivalent to an of HPLC-silent products can also contribute to the incomplete

2 2

H O

/furfural mol ratio of 7.5). Yields are based on the initial carbon balance. The carbon balance improved as the reaction

amount of furfural loaded into the reactor.

progressed, suggesting that these unidentied HPLC products

5

-Hydroxy-furan-2(5H)-one (hydroxyfuranone), MA and FA may be (partially oxidised) intermediates of the reaction.

were the main reaction products, malic acid was also detected As expected, the rate of furfural conversion increased upon

but with a yield less than 5%. The yield for the remaining raising the reaction temperature. Thus, at 323 K, full furfural

possible reaction products (SA, furanones, FumA, furoic acid) conversion was reached aer 6 h, whereas only 3 h was required

was well below 2.4%. The low yield of these products, especially for 333 K and 1 h was enough at 343 K. A rapid perusal of the

SA and furanones, was rather unexpected: previous results gures also indicates that at short reaction times, hydroxyfur-

obtained in the liquid phase oxidation of furfural with H

2

O

2

anone predominates among the detected products, but for

yielded a wider variety of products including SA and fur- longer periods, MA is the major product. It has been previously

15–19

anones.

It is concluded that the reaction mechanism when suggested that MA is formed at the expense of hydroxyfuranone

using TS-1 is diﬀerent to that of the conventional acid catalysts oxidation; in our experiments at 323, 333 and 343 K, the

used previously to conduct this reaction. When acid catalysts hydroxyfuranone yield reached a maximum (close to 40%) at 3,

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and 1 h, respectively, while MA yield continuously rose,

indicating that the latter is not a primary product. A remark- the major detected products were hydroxyfuranone and maleic

able result is that the maximum yield of MA (57%) was achieved acid; the combined yield of FA and malic acid was smaller than

at 323 K aer 24 h of reaction, whereas at higher reaction 9%. The yields of the remaining products was smaller than 2%.

Paper

2

4

As regards the transformation to oxidised C compounds,

19

temperatures, the yield decreased. Thus, the maximum yield at For H

33 K was 52% (for 24 h), and the maximum at 343 K was 49% slower than that at 7.5; thus, aer 24 h of reaction, only a 28%

for 6 h). For longer reaction times at 343 K, the MA yield visibly MA yield was reached, whereas for a ratio of 7.5, the yield was

decreased. 57%. In principle, one might assume that a H /furfural mol

2

O

2

/furfural mol ratio ¼ 4, the MA formation rate was

3

(

2 2

O

In a reaction pathway compatible with the results of Fig. 1, ratio > 7.5 would result in a better MA yield, and at short reac-

furfural is rapidly oxidised through unknown intermediates to tion times (time < 7.5 h) in these conditions, the formation rate

hydroxyfuranone: a C species (formic acid) is released to form of MA is substantially faster (at the expense of a faster disap-

1

the C product. Hydroxyfuranone is subsequently oxidised to pearance of hydroxyfuranone). However, the MA yield attains a

4

MA. In addition, furfural and all the other organic products can maximum yield of 44% at 7.5 h of reaction, and aer 24 h of

undergo deep oxidation to formic acid and to CO

In summary, for the reaction temperatures tested, 323 K concentration results in an overoxidation of MA.

results in the maximum MA yield. This was the reaction In summary, an excess of H with respect to the stoi-

2

.

reaction it decreases to 36%. The high hydrogen peroxide

2 2

O

temperature selected for further studies. We discarded lower chiometric amount for furfural oxidation to MA is needed to

temperatures because the reaction rates would be very slow. improve the MA yield. The optimum H O /furfural mol ratio is

2

Similar conclusions can be reached by working at lower H O

approximately 7.5; lower ratios result in lower reaction rate, and

concentrations at the three diﬀerent temperatures tested larger ratios result in overoxidation. Similar conclusions were

results included in Fig. SI2 of the ESI†). obtained when conducting similar experiments to those pre-

concentration eﬀect. Fig. 2 shows the eﬀect of sented in Fig. 2, but with a larger catalyst concentration (4.6

hydrogen peroxide concentration on the catalytic properties. wt%). The results are presented in Fig. SI3 of the ESI section.†

Three diﬀerent H concentrations were tested (6.6, 12.3 and Catalyst loading eﬀect. The eﬀect of catalyst loading on

4.6 wt%), equivalent to H /furfural mol ratio of 4, 7.5 and 15, catalytic properties is depicted in Fig. 3: experiments were

2

(

2 2

H O

2 2

O

2

2 2

O

respectively. The remaining reaction parameters were kept conducted by changing the catalyst concentration and keeping

constant (4.6 wt% of furfural, 2.3 wt% of catalyst and 323 K). constant the reaction temperature (323 K), the furfural

The hydrogen peroxide concentration was in excess of the concentration (4.6 wt%), and the H O concentration (12.3 wt%,

2

stoichiometric amount required for the oxidation of furfural to

H

2

O

2

/furfural mol ratio ¼ 7.5). When no catalyst was incorpo-

rated, the furfural conversion rate was lower than when cata-

concentration, the faster the furfural conversion rate. For the lysed with TS-1; SA and furan-2(5H)-one were the major

highest H /furfural mol ratio tested, which was 15, full products detected; FumA and FurA were also observed but with

furfural conversion was attained in just 2 hours of reaction, a yield lower than 4 and 6%, respectively; and the MA yield

ꢀ1

MA, which is H

2

O

2

/MA (mol mol ) ¼ 3. The larger the H

2 2

O

2 2

O

whereas 3 h was needed for H O /furfural mol ratio ¼ 4.

was negligible except at 24 h of reaction. This behaviour is

compatible with a self-catalysed reaction, following a reaction

mechanism in which the rst step is the Baeyer–Villiger oxida-

tion of furfural to 2-hydroxyfuran. A similar situation was found

for other experiments conducted without TS-1 catalyst but with

2

2 2

lower and higher H O concentrations: SA and furan-2(5H)-one

were the major products (see Fig. SI4 in the ESI†). The addition

of 1.2 wt% of catalyst substantially accelerated the furfural

conversion rate and, more importantly, remarkably modied

the yield pattern of the obtained products: the yields of SA and

furanone signicantly decreased and became negligible,

whereas MA and hydroxyfuranone were the major products.

The incorporation of more catalyst intensies this eﬀect, and

thus, for a catalyst concentration of 4.6 wt% (TS-1/furfural wt.

ratio ¼ 1), a MA selectivity close to 80% was attained for 24 h of

reaction, with residual formation of malic and formic acids. It

must be stressed that, for the latter specic reaction conditions,

the carbon balance was close to 100% for all aliquots analysed

throughout the kinetic experiments. The latter suggests that for

a catalyst concentration lower than 4.6% a carbon balance

smaller than 100% would be very likely due to the formation of

undetectable intermediates. On the other hand, a larger catalyst

concentration (9.2 wt%) resulted in the overoxidation of MA,

Fig. 2 Eﬀect of H

for three diﬀerent H

sponding to (a) 6.6, (b) 12.3 and (c) 24.6 wt% of H

conditions: 4.6 wt% furfural, 323 K, 2.3 wt% catalyst. Symbols: furfural

conversion (-); 5-hydroxy-furan-2(5H)-one yield (B); maleic acid

yield (C); formic acid yield (;); and malic acid yield (:).

2

O

2

concentration on the catalytic properties of TS-1

/furfural mol ratios (4, 7.5 and 15), corre-

. Reaction

2

O

2

2 2

O

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Fig. 4 Eﬀect of furfural concentration (wt%) on the catalytic behaviour

of TS-1: (a) 2 wt%; (b) 4.6 wt%; (c) 7.5 wt%; and (d) 9.2 wt% of furfural.

Reaction conditions: 323 K, TS-1/F wt ratio ¼ 1; H

Symbols: furfural conversion (-); 5-hydroxy-furan-2(5H)-one yield

B); maleic acid yield (C); formic acid yield (;); malic acid yield (:);

conversion (>); H selectivity (A).

2

O /F mol ratio ¼ 7.5.

(

2

H O

2

2 2

O

Thus, when 4.6 wt% of furfural is used (Fig. 4b), for 1 h of

reaction, the selectivity of the H devoted to produce C

4

2

O

2

compounds is close to 80%, and as the reaction progresses, the

selectivity continuously decreases down to a value of 40% aer

Fig. 3 Eﬀect of catalyst loading (wt%) on the catalytic properties of TS-

2

4 h of reaction. Similar trends can be observed for the

remaining experiments in Fig. 4. Formic acid formation partly

O

2

. Symbols: furfural conversion (-); 5-hydroxy- explains the unselective transformation of H O , but there

1

: (a) 0 wt%; (b) 1.2 wt%; (c) 2.3 wt%; (d) 4.6 wt%; (e) 9.2 wt%. Reaction

conditions: 4.6 wt% furfural, 323 K and H

/F ¼ 7.5, which corre-

spond to 12.3 wt% H

2

O

2

furan-2(5H)-one yield (B); maleic acid yield (C); formic acid yield (;);

malic acid yield (:); furan-2(5H)-one yield (P); succinic acid yield (A).

are other unselective routes; we will address this point in

Section 3.3.

Regarding MA selectivity, a very remarkable result is that for

reaction times shorter than 7.5 h, the furfural concentration

very likely to CO

larger FA yield and incomplete carbon balance.

2

, as can be deduced from the lower MA yield, clearly has a very positive eﬀect on the MA yield. Thus, the MA

yield aer 7.5 h of reaction is 25, 53, 68 and 69% for 2, 4.6, 7.5

Eﬀect of concentration of furfural in aqueous solution. Fig. 4 and 9.2 wt% of furfural, respectively. The intrinsic MA forma-

shows the eﬀect of furfural concentration on conversion and tion rate was calculated aer 1 h of reaction to take into account

yields; the reaction temperature, H O /furfural mol ratio and the fact that the catalyst concentration also increases

2

TS-1/furfural wt ratio were kept constant at 323 K, 7.5 and 1, throughout the series of experiments (furfural/TS-1 ratio was

respectively. In this gure, the conversion and selectivity of kept constant). The intrinsic rate was found to be 0.14, 0.44,

ꢀ1

H

2

O

2

during the course of the reaction is also included. H

conversion % indicates the ratio of the total H consumed to wt% of catalyst, respectively. Therefore, the intrinsic MA reac-

the initial H loaded, whereas the selectivity is the ratio of the tion rate also rises with furfural concentration, which means

mol% of H consumed to produce the C organic compounds that the eﬀect is due to the increase of furfural concentration.

detected by HPLC analysis to the mol% of H O actually An explanation could be that as the furfural concentration rises,

2

O

2

0.82 and 0.87 mmol of furfural gcat

s

for 2, 4.6, 7.5 and 9.2

2 2

O

2 2

O

2

O

2

4

2

consumed (the amount theoretically consumed to produce FA is so does the relative surface furfural coverage with respect to

not included in this quantity). Except for the experiment con- other reactants. In any case, the increase in both the MA yield

ducted with 9.2 wt% of furfural, the carbon balance was and formation rate levels oﬀ for furfural concentrations larger

essentially well above 90% in all experiments. This indicates than 7.5 wt%. Moreover, when 9.2 wt% of furfural is used,

that most of the organic products were detected by HPLC in this overoxidation of MA is evident for longer reaction times; thus,

series of experiments, and consequently, the deviation of the for 24 h of reaction, the MA yield (71% yield) is smaller than that

H

2

O

2

selectivity from 100% is an indication of the unselective for the 4.6 and 7.5 wt% experiments (78 and 80% yields,

conversion (decomposition) of H O . respectively). The overoxidation of products in the 9.2 wt%

H O consumption is very selective at the beginning of the experiment is also evident by the stronger deviation of the

2

reaction when hydroxyfuranone predominates. However, as the carbon balance, to a value notably lower than 100%.

reaction proceeds, selectivity worsens, indicating that a large In summary, we can conclude that among all the reaction

fraction of H is consumed in unselective transformations. conditions tested, the optimum reaction conditions to obtain

2 2

O

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the highest MA yield (80%) are those used in Fig. 4b and c: 323 but the Ti sites in anatase are not as selective as framework Ti in

K, between 4.6–7.5 wt% of TS-1, TS-1/furfural wt ratio ¼ 1; H O / TS-1: the H O eﬃciency is lower, the carbon balance is

2

2 2

furfural mol ratio ¼ 7.5 and 24 h of reaction.

incomplete and the MA yield is lower (MA yield of 31% was

observed aer 24 h of reaction). These latter results suggest that

Ti sites in anatase are not responsible for the good behavior of

TS-1. In summary, among the family of catalysts tested, TS-1

features a high activity and selectivity to MA.

A proposal of the reaction mechanism for oxidation using

TS-1 catalyst. The rst characteristic of the performance of TS-1

is the low selectivity for succinic acid and furan-2(3H)-one with

Comparison with other solid catalysts and reaction

mechanisms

Fig. 5 compares the catalytic behaviour of TS-1 with other

selected catalysts. Reaction conditions, indicated in the

caption, were selected to obtain a high MA yield with TS-1.

Ambelyst 70, a sulphonic resin, was chosen because of its

respect to values obtained with sulphuric acid, sulphonic resins

15,16,19

proven capacity to catalyse furfural oxidation to C diacids.

15–18,29

4

or poly(-styrene sulphonic acid).

This strongly suggests

ZSM-5, a Ti-free zeolite with acid sites, was tested because it

presents the same MFI-like silica framework as TS-1. Finally,

anatase, which has also been detected to be present in the TS-1

as a result of decient insertion of Ti within the MFI-like

framework, was also investigated.

that the reaction with TS-1 goes through a diﬀerent mechanism.

It is accepted that, when using catalysts possessing sulphonic

acid sites, the reaction starts with a Baeyer–Villiger step (see

Scheme SI1†). Fig. SI6† and the discussion therein demonstrate

that, when using TS-1, furfural oxidation does not start with a

Baeyer–Villiger oxidation.

Amberlyst 70 displayed an oxidation activity pattern typically

catalysed by strong acids and a wide number of products can be

It has been reported that furan can be oxidised very selec-

15,16,19

obtained.

Succinic acid, furan-2(5H)-one and maleic acid

21

tively with hydrogen peroxide to maleic dialdehyde or to

are mainly formed, hydroxyfuranone, and malic, fumaric and

formic acids are also detected at non-negligible yields, espe-

cially at shorter reaction times. Aer 24 h of reaction, the yield

of SA, furan-2(5H)-one and MA was approximately 50, 30 and

22

2 2

hydroxyfuranone by using a H O /furfural mol ratio of 1 and 2,

respectively. The latter can eventually be oxidised to MA.

Therefore, if furan could be formed from furfural, the oxidation

of furan would yield MA, provided that a H

is used.

2 2

O /furfural ratio > 2

20%, respectively. Although the ZSM-5 zeolite presents acid

sites on its surface, these sites are not as active as those of TS-1

or as the sulphonic sites of Amberlyst 70. The latter presents a

comparatively large strong-acid-site concentration (2.55 mmol

One possibility is that the reaction starts with the oxidation

of furfural to furoic acid, which can subsequently be trans-

formed to furan by releasing formic acid (see Scheme SI2 in the

ESI† for a description of this mechanism proposal). However,

this possibility can be discarded because when the oxidation of

ꢀ

1

19

g_c_a_t), which explains its relatively high activity. ZSM-5

displays an activity pattern resembling that of a catalyst-free

run (see Fig. 3a). Aer 24 h of reaction, an MA yield of 15%

and SA yield of 30% was observed for ZSM-5. Accordingly, the

catalytic activity of TS-1 cannot be associated to acid sites

present in the MFI-framework of TS-1, but rather to the pres-

ence of Ti(IV) sites in the TS-1. Anatase presents very active sites,

furoic acid with H

wt% of furoic acid, 4.6 wt% of TS-1 and a H

2

O

2

was conducted by using TS-1 (at 323 K, 4.6

/furoic acid mol

2 2

O

ratio of 7.5, equivalent to 12.3 wt%) the MA yield was much

lower than that obtained with furfural (see Fig. SI7 in the ESI†

for further details).

Another plausible possibility is that furan can be formed by

the decarbonylation of furfural. However, the decarbonylation

of furfural requires diﬀerent catalysts and reaction conditions

30–32

to proceed,

and therefore this route can also be discarded.

We propose a diﬀerent mechanism based on the results and

on the proposal of Jacobs et al. when oxidising furan to maleic

21

dialdehyde. They proposed that one of the double bonds in the

furan ring is epoxidised and the thus-formed epoxide then

rapidly rearranges, yielding unsaturated 1,4-dicarbonyl

compounds. The same epoxidation and rearrangement steps

can be applied to furfural (see Scheme 1): the double bond with

less steric hindrance is selectively epoxidised, and the furfural-

epoxide rapidly undergoes a rearrangement to yield the Z-4-

oxopent-2-enedial. The loss of a carbon atom is still needed for

the formation of the corresponding C derivative. We propose

4

that the formation of a C intermediate occurs via Baeyer–Vil-

4

Fig. 5 Catalytic activity of the diﬀerent catalysts tested: (a) Amberlyst liger oxidation of the aldehyde functionality at 1-position of the

7

0; (b) ZSM-5; (c) Anatase; and (d) TS-1. Reaction conditions: 4.6 wt%

furfural, 4.6 wt% catalyst, 323 K, H

/F ¼ 7.5, which corresponds to

2.3 wt% H . Symbols: (-) furfural conversion; 5-hydroxy-furan-

(5H)-one yield (B); maleic acid yield (C); formic acid yield (;); malic

acid yield (:); furan-2(5H)-one yield (P); succinic acid yield (A);

fumaric acid yield (,); H conversion (>); H selectivity (A).

Z-4-oxopent-2-enedial. An ester is formed, which is rapidly

hydrolysed to form b-formylacrylic acid ((Z)-4-oxobuten-2-enoic

acid), releasing formic acid. The electron withdrawing nature of

the acyl group directly bonded to the reacting aldehyde at C1

strongly increases its electrophilicity favoring the Baeyer–

2 2

O

1

2

2 2

O

2 2

O

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Scheme 1 Proposal of mechanism via epoxidation of the furan ring and subsequent release of formic acid.

Villiger reaction and justifying the regioselecctivity of the

process. Formylacrylic acid is in equilibrium with the hydrox- short reaction times, hydroxyfuranone is primarily produced,

yfuranone. Experimentally, the formation of MA parallels the with a relatively selective utilisation of H . Kinetic data

On the other hand, when TS-1 is used as catalyst and at

2 2

O

consumption of hydroxyfuranone. This suggests that hydrox- show that furfural oxidation to hydroxyfuranone is very rapid

yfuranone can be directly oxidised to MA. However, it cannot be and selective, even when lower H O /furfural mol ratios are

2

excluded that MA is formed by the oxidation of b-formylacrylic, used (see Fig. SI3a†). Therefore, a good strategy would be to

in equilibrium with hydroxyfuranone. We could not identify any conduct the oxidation in two steps; in the rst step, to oxidise

HPLC peak as b-formylacrylic, very likely because the equilib- rapidly furfural to hydroxyfuranone using TS-1 and a H

rium is strongly shied to hydroxyfuranone. It is also possible furfural mol ratio close to stoichiometry. For short reaction

that the former is present at very low concentration, and times, the unselective catalytic decomposition of H is

2 2

O /

2 2

O

therefore diﬃcult to detect by HPLC, but still involved as an prevented. In a subsequent step, the oxidation of hydroxy-

intermediate of the reaction. If so, whereas b-formylacrylic is furanone to MA will be conducted with another catalyst

consumed by oxidation to MA, so must be hydroxyfuranone.

because TS-1 unselectively decomposes H O . Sulphonic-

2 2

based catalysts have demonstrated a capacity to transform

1

5,16,19

hydroxyfuranone to MA.

In practice, we proceeded as follows: rst, the oxidation of

furfural to hydroxyfuranone was conducted by using TS-1 as

catalyst with a H O /furfural mol ratio of 2.4 (2 mol of H O are

2 2 2 2

Improvement in selective utilisation of H

process

2

O

2

using a two-step

: conversion

2 2

In principle, there are three routes to convert H O

needed for the stoichiometric oxidation of furfural to hydroxy-

furanone). Previous reactions with a low H O concentration

to organic products (including formic acid), catalytic decom-

position and uncatalysed thermal decomposition. Blank

experiments (see Fig. SI5 in the ESI† and discussion therein)

allowed us to conclude that (i) although TS-1 catalytically

2

(

Fig. SI3a† (H O /F mol ratio ¼ 2.15)) showed that aer 4 hours

2

of reaction, the hydroxyfuranone concentration did not

increase, most likely because of the total consumption of the

2 2

decomposes H O , the decomposition is remarkably inhibited

H

4

2

O

2

. For that reason, this rst reaction step was carried out for

by the presence of furfural, especially at short reaction times

and (ii) thermal decomposition is less than 20% aer 24 h of

reaction. This has practical implications when searching for

further improvements in the selective utilisation of hydrogen

peroxide: rst, catalytic decomposition is very low at short

h. Aer that, the TS-1 catalyst was ltered oﬀ and the second

oxidation step was conducted by using an Amberlyst 70 sul-

phonic resin as a catalyst. Further addition of H was per-

formed to obtain a H O /furfural mol ratio of 2 with respect to

2 2

O

2

the initial furfural concentration. One mol of H O is needed for

2

reaction times, and second, the thermal decomposition of H O

is as important as catalytic decomposition.

2

the stoichiometric oxidation of hydroxyfuranone to maleic acid,

so a small excess of H was incorporated to account for non-

selective decomposition. To prevent the dilution of the reaction

media, a 70 wt% H solution was used to incorporate H in

2 2

O

2 2

The stoichiometric H O /furfural mol ratio required to

selectively oxidise furfural to MA is 3. The maximum yield to MA

was achieved by using a mol ratio ¼ 7.5, which is more than

double the stoichiometric ratio. The unselective utilisation of

2

O

2

2 2

O

this second step. Fig. 6 displays the results obtained by taking

samples at diﬀerent reaction times. Products yields are calcu-

lated with respect to the initially loaded furfural.

2 2

H O comes from thermal and catalytic decomposition. Inter-

estingly, the results presented in Fig. SI5† indicate that the

catalytic decomposition is seriously inhibited by the presence of

furfural. At the beginning of the reaction, decomposition is

practically negligible: only aer 5 hours is the catalytic

decomposition detectable.

Aer 4 h, the oxidation of furfural with TS-1 resulted in a

7

0% hydroxyfuranone yield, while the yield of maleic acid was

less than 10%. However, once the TS-1 was ltered oﬀ and

Amberlyst 70 and further H were incorporated, the selective

2 2

O

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Fig. 7 TS-1 catalyst reutilisation tests for aqueous solution of furfural

Fig. 6 Consecutive two-step oxidation of furfural with TS-1 and

Amberlyst 70. Reaction conditions: 4.6 wt% furfural, 323 K and 4.6 wt%

of catalyst for each step. Symbols: furfural conversion (-); 5-hydroxy-

furan-2(5H)-one yield (B); maleic acid yield (C); formic acid yield (;);

(

4.6 wt%), 323 K, H

2

O

2

/F mol ratio ¼ 7.5, corresponding to 6.6 wt% of

2 2

H O , 4.6 wt% catalyst and 7 h reaction.

2 2

malic acid yield (:); and H O conversion (>).

hydroxyfuranone between 40–50%. Attending to these results,

we can conclude that when commercial furfural is used, TS-1 is

quite stable, and no sign of deactivation could be observed for

at least 6 cycles.

Reutilisation runs were also conducted by using aqueous

furfural solutions obtained from lignocellulosic biomass.

Briey, furfural was obtained from corncobs by using Amberlyst

oxidation of hydroxyfuranone to maleic proceeded continu-

ously, and aer 24 h of reaction with Amberlyst (equivalent to 28

h of total reaction time), the MA yield was close to 76%, very

close to that obtained aer 24 h with TS-1 (78%). The MA yield

reached almost 90% aer 2 days with Amberlyst. These results

therefore demonstrate that MA can be produced from furfural

by a two-step catalyst process with high selective utilisation of

7

0 as catalyst, at 453 K, 19 bar and by using a N

2

ow of 150 mL

ꢀ1

min (STP) to strip the water-furfural vapour stream. Stripping

2 2 2 2

both furfural and H O . The overall H O /furfural mol ratio

of furfural by N results in the rapid extraction of furfural from

2

was 4.4.

the reaction mixture, which prevents furfural from being

consumed in further secondary reactions in the liquid phase.

This furfural stream is condensed to separate gases from the

Stability of the TS-1 catalyst

To evaluate the stability of the TS-1 catalyst in the oxidation of aqueous furfural solution. Further details of the procedure can

33,34

furfural, two diﬀerent experiments were conducted. First, be found elsewhere.

The furfural concentration of this

successive reutilisation runs were performed with aqueous aqueous solution was determined to be 2.5% (GC analysis).

solutions of pure furfural distributed by laboratory chemical Other impurities like acetic acid and 5-methyl-furfural were also

suppliers. In another set of experiments, the stability of the detected by GC-MS. The results obtained aer successive

catalysts was tested with furfural aqueous solutions obtained reutilisation runs at 323 K, 24 h of reaction, H O /furfural mol.

2

from biomass (corncobs).

ratio ¼ 7.5 and catalyst/furfural wt ratio ¼ 1 are displayed in

Regarding the aqueous solution of pure furfural, successive Fig. 8. The actual furfural concentration in the reaction media

reutilisation runs were carried out aer 7 h of reaction, with a was 2 wt%, and at this relatively low concentration of furfural,

catalyst and furfural content of 4.6 wt% and a H

2 2

O /furfural the rates of furfural conversion and MA formation are lower

molar ratio of 7.5, corresponding to 12.3 wt% H . The catalyst than those found when the optimum reaction conditions are set

2 2

O

was ltered oﬀ and dried before each new run (see Experimental (see Fig. SI8 in the ESI† for the kinetics of oxidation when this

section for details). Medium-term reaction times (7 h) are more biomass furfural solution was used, and see also Fig. 4a for the

sensitive to detect the deactivation processes. Longer reaction kinetic curve of commercial furfural at similar reaction condi-

times may miss the deactivation if the deactivation is not very tions to those in Fig. 8). To obtain a measurable and relatively

intense. Fig. 7 shows the results obtained aer 6 reutilisation high MA yield (larger than 30%), a long reaction time is needed

cycles. The conversion and the yields of the main products are (24 h).

represented (MA, 5-hydroxyfuranone and FA). The results of a

When aqueous furfural obtained directly from biomass is

blank experiment without TS-1 catalyst are also included (rst used, a slight but continuous deactivation of TS-1 can be

column): much lower values for the conversion and yields to observed. The deactivation is more visible in the MA yield, as

selective oxidation products were obtained in this blank test.

the hydroxyfuranone yield remained above 40% for the rst six

When TS-1 was reused, the conversion values remained at runs (in the seventh run the yield dropped to 31%). It is also

00% throughout all the runs. Moreover, the yields of MA, noteworthy that the formic acid yield slightly but inexorably

1

hydroxyfuranone and formic acid remained quite similar for all increased upon reutilisation runs, which conrms that the

the runs investigated; MA yields were between 43–48% and selective oxidation sites of TS-1 are being deteriorated by the

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identifying the possible physico-chemical changes occurring

during utilisation. Laser Raman spectra of fresh and used

catalyst samples using a 523 nm laser are represented in Fig. 9.

The used catalyst aer 6 runs with pure furfural is represented

in Fig. 7. Samples were previously calcined in situ in the cell by



owing synthetic air at 823 K for 30 min. This treatment is

required because hydrocarbonaceous species remain adsorbed

on the catalyst aer utilisation, which precludes the recording

of good quality Raman spectra due to the uorescence

phenomenon associated with these C-containing residues (see

Fig. SI9 in the ESI†).

Raman bands at 290, 380, 437, 469, 815, 837, 960 and 1125

ꢀ

1

cm that have been previously reported for TS-1 are clearly

observed in the Raman spectra of Fig. 9; the features at 960 and

Fig.

obtained from biomass. Reaction conditions: 2 wt% furfural, 323 K,

/F ¼ 7.5, which corresponds to 5.3 wt% H , and 2 wt% catalyst,

8

Reutilisation experiments for aqueous furfural solution

ꢀ1

1125 cm

demonstrate the substitutional Ti(IV) insertion

within the silica framework: they have been assigned to the

stretching Ti–O–Si vibrations of inserted TiO tetrahedra

respectively, in-phase and out of phase antisymmetric

H

2

O

2

2 2

O

24 h.

4

(

35–38

stretching of the four connected Ti–O–Si bridges).

The

bands detected at 144, 515 and 637 cm correspond to Raman

features of anatase (TiO ); there must be another anatase-

utilisation of real furfural. All these results strongly suggest that

the impurities present in the real aqueous furfural derived from

biomass have a detrimental eﬀect on the deterioration of TS-1.

The removal of these impurities from biomass-derived furfural

aqueous solutions is recommended to prevent the deterioration

of TS-1.

Aer the 7th run, the used TS-1 was ltered oﬀ, dried at 323 K

overnight and nally calcined at 823 K in an oven under air for 3

hours. This calcined catalyst was again used for an additional

ꢀ

1

2

1

ꢀ

derived band at 390 cm , but it is overshadowed by the most

ꢀ

1

intense band of TS-1 at 380 cm . Visible Raman spectroscopy is

very sensitive to extraframework titanium oxide species and

therefore is an excellent probe for the detection of very small

TiO domains. Therefore, these bands demonstrate the pres-

2

ence of extraframework Ti and that not all the Ti cations are

inserted within the zeolite framework.

8th run, and the MA yield was partially restored. This indicates

No large diﬀerences can be detected between the spectra

from fresh and used catalysts, which suggests that TS-1 is

essentially unaltered by the utilisation of the catalyst in the

reaction medium. Nevertheless, a closer examination (see inset

that the deactivation of TS-1 is partially due to the deposition of

organic products derived from these impurities, which are

removed by calcination. Other sources of deactivation will be

discussed below.

ꢀ

1

2

in the gure) indicates that the TiO band at 144 cm is

These contaminants must be removed when considering an

industrial application. As indicated in the Introduction, a

apparently less intense, which could suggest that there may be

some leaching during the utilisation as catalyst.

Chemical analyses by TXRF were conducted to conrm the

leaching. Ti and Si were analysed in both the fresh and used

solids and in the liquid phase aer each reaction run, the latter

1

double distillation is required to purify furfural. The rst

distillation removes impurities and an azeotrope with 35% of

furfural is yielded. The second distillation column further

removes water and results in pure and very concentrated

furfural.

The double distillation may compromise seriously the

economic viability of the process here described. Possible

solutions to reduce the cost and energy consumption can be the

following. The azeotrope free of contaminants can be used for

the oxidation to MA, preventing the use of the second distilla-

tion column. In practice the furfural–water azeotrope is

condensed before entering the second vacuum distillation

column and two phases are formed: a rich furfural phase (which

is that readily further distilled) and a poorer aqueous furfural

stream. Another solution could be that the latter diluted stream,

free of contaminants, could be withdrawn and used in the

oxidation to MA, leaving the concentrated furfural for processes

requiring very concentrated and free of contaminants furfural.

Characterisation of fresh and used catalyst

Fig.

9 Visible laser Raman spectra of fresh (dashed) and used

Fresh and used catalysts aer several reaction cycles were

(continuous) catalysts after in situ calcination at 823 K for 30 min in

characterised by diﬀerent techniques with the intention of synthetic air. A denotes Raman bands from the anatase phase.

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Table 1 Summary of TXRF results of fresh and used catalysts and in the liquid reaction mixtures after successive reaction cycles

Amount of metal leached to the reaction mixture (mg)

Si/Ti at. ratio

b

Catalyst

in solid

Run 1

Run 2

Run 3

Run 4

Run 5

Run 6

Run 7

a

Used with comm.

furfural

31 (25.5)

Ti 3.51 (11.2%) 0.96 (3%)

Si 0.31 (0.03%)

Ti 2.29 (15.4%) 0.46 (3.1%)

Si 0.48 (0.13%) 0.52 (0.14%)

0.41 (1.3%) 0.47 (1.5%)

0.56 (1.8%)

0

0.21 (1.4%)

0.36 (1.1%)

0.27 (0.03%)

0.24 (1.6%)

0

—

0

a

Used with biomass- 29.4 (25.5)

derived furfural

0.28 (1.9%) 0.28 (1.9%)

0.20 (1.3%)

0

0.31 (0.81%) 0.16 (0.04%)

a

b

Si/Ti at. ratio of fresh sample. Values between brackets represent the metal leached during the run cycle with respect to the total amount of metal

initially present in the solid (expressed as wt%).

case for runs conducted with commercial and biomass-derived main chemical parameters deduced from the XPS analyses. The

furfural. Table 1 summarises the results of TXRF.

Ti 2p core level was deconvoluted into two contributions; one is

The Si/Ti at. ratio of the catalyst used with the commercial represented by the Ti 2p3/2 peak at 459.8 ꢃ 0.1 eV, and other is

furfural is clearly higher than that of fresh catalyst, which represented by the Ti 2p3/2 contribution at 458.6 ꢃ 0.2 eV (the

indicates that Ti was leached during the six utilisation runs. The corresponding Ti 2p1/2 levels are at located at 5.7 eV higher BE).

same conclusion may be reached by examining the Ti concen- The rst contribution was assigned to tetrahedral Ti(IV) incor-

tration in the liquid reaction mixture. Most of the Ti is leached porated in the silicalite framework (Ti

t

), whereas that repre-

during the rst run (ca. 56% of the total Ti leached), and the sented by the peak at 458.6 was assigned to octahedral Ti(IV) in

39–41

impact of lixiviation seems to decline upon reutilisation. Si anatase TiO (Ti ).

For very good tting, a small contribution

2

o

leaching also takes place, although to a much smaller degree, represented by the Ti 2p₃_/₂contribution at ca. 457.2 eV was

and in some cases below the detection limit of the technique: Ti required; this has been assigned to Ti(III) produced by the

leaching represents ca. 20% of the initial Ti content, whereas Si photoreduction of Ti(IV) within the XPS chamber, driven by the

leaching accounts for less than 0.1% of the initial Si.

X-ray impact and the ultrahigh vacuum required to record the

40,41

The former TXRF analyses also suggest that the contribution spectra.

of the leached species to the overall activity is not relevant. In

The rst evidence of leaching arises from the comparison

the rst run 3.5 mg of Ti was leached to the reaction medium, between the XPS Ti/Si ratios of fresh and used samples. The XPS

whereas for the 3rd and successive runs the amount of Ti Ti/Si ratio of used samples is 0.027, half the value of the fresh

leached was much smaller (around 0.4–0.6 mg). If the leached samples (0.052). Moreover, a comparison between the XPS and

homogeneous Ti species would have a signicant contribution TXRF Ti/Si ratios of fresh and used samples indicates that the

to the overall activity, the total activity of the latter runs would leaching process is very intense at the surface of the TS-1 crys-

have been appreciably smaller than that of the rst run, and not tallites. Thus, the XPS value of the fresh sample (0.052) is larger

similar.

The same analyses were carried out with furfural from

biomass. As with commercial furfural, Ti is leached in all

reaction cycles, especially aer the rst one. When consid-

ering the relative amount of Ti losses in successive runs, we

have to take into account that the loading of catalyst for the

experiments with biomass-derived furfural was smaller

(

2 wt%) than that used with commercial furfural (4.6 wt%). In

this case, the aggregate loss of Ti represents ca. 27% of the

total Ti. This slight increase in the leached Ti may be due

either to the longer contact time between the furfural and

catalyst (when using furfural from biomass, each run takes 24

hours) or to some of the products detected with the furfural

from biomass (dihydroxyacetone, acetic acid, and 5-methyl-2-

furancarboxaldehyde) that could promote Ti leaching. As for

pure furfural, most of the Ti is leached (ca. 58%) during the



rst run. Si leaching also takes place to a slightly higher extent

than with “commercial” furfural: Ti leaching represents ca.

7% of the initial Ti content, whereas the leached Si accounts

2

for less than 0.4% of the initial Si.

The leaching of Ti is also evidenced by the comparison of the

XPS spectra of fresh and used TS-1 catalyst. Fig. 10 compares the

Fig. 10 Ti 2p core level of fresh and used TS-1 (with commercial

Ti 2p core level of both samples, and Table 2 summarises the furfural).

54970 | RSC Adv., 2014, 4, 54960–54972

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Table 2 Summary of XPS results in the fresh and used catalysts

biomass, weak but visible deactivation occurs upon reutilisa-

tion; this deterioration must be related to the presence of other

organic products than furfural.

Ti 2p 3/2

Ti/Si

t

Ti /Si

o

Ti /Si

a

The amount of H O required when using TS-1 is 2.5 times

2 2

Fresh

Used

459.9 (67.4%)

0.052 (0.039)

0.035

0.015

0.008

4

58.8 (28.5%)

57.2 (4.1%)

the value that is stoichiometrically needed. A very selective two-

step process has also been demonstrated, which needs a H

furfural mol ratio of 4.4, an amount of H much closer to the

2 2

O /

a

459.8 (57.0%)

0.027 (0.032)

2 2

O

4

58.5 (29.3%)

57.2 (13.7%)

stoichiometric value (which is 3). This process involves the

consecutive utilisation of TS-1 to selectively oxidise furfural to

hydroxyfuranone, and consequently, once TS-1 is removed,

Amberlyst 70 to catalyse the selective oxidation of hydroxyfur-

anone to MA.

a

Ti/Si determined from TXRF analysis.

than that determined by TXRF analysis (0.039), perhaps because

anatase-like domains are present at the external surface of TS-1.

However, the XPS Ti/Si value of the used sample (0.027) is

smaller than that determined by TXRF (0.032), indicating the

surface is no longer enriched in Ti and therefore that Ti

leaching at the surface is very intense. Finally, the Ti leaching

Acronym list

FA

FumA

Hydroxyfuranone

MA

Formic acid

Fumaric acid

5-Hydroxy-furan-2(5H)-one

Maleic acid

process aﬀects both anatase Ti (Ti ) and Ti inserted in the sili-

o

calite (Ti

t

). Thus, the XPS Ti

o

/Si ratio of the used sample is 0.008, MAN

/Si ratio of the SA

Maleic anhydride

Succinic acid

vs. 0.015 for the fresh sample, and the XPS Ti

t

used sample is 0.015, vs. 0.035 for the fresh sample.

Taking into account that 20% of Ti was leached upon

reutilization, it is quite remarkable that no deactivation was Acknowledgements

observed when pure furfural is used for the reutilization

Financial support from Spanish Ministry of Economy and

Competitiveness (CTQ2012-38204-C03-01) is gratefully

acknowledged.

experiments. A likely hypothesis is that outermost Ti is being

leached and innermost active Ti sites remain to conduct the

reaction. In this sense, the decrease observed upon reutilization

of the surface at. Ti/Si by XPS (from 0.052 to 0.027, ca. 50%

decrease) is larger than that observed for the bulk TXRF ratio

Notes and references

(from 0.039 to 0.032, 18% decrease) indicating that leaching

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54972 | RSC Adv., 2014, 4, 54960–54972

Article Doi

Aqueous-phase catalytic oxidation of furfural with H2O2: High yield of maleic acid by using titanium silicalite-1

DOI: 10.1039/c4ra11563e

Source and publish data:

Authors:

Article abstract of DOI:10.1039/c4ra11563e

Full text of DOI:10.1039/c4ra11563e

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