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Short Communication

High-value chemicals obtained from selective photo-oxidation of glucose

in the presence of nanostructured titanium photocatalysts

 Juan C. Colmenares⇑, Agnieszka Magdziarz, Anna Bielejewska

Institute of Physical Chemistry PAS, Kasprzaka 44/52, 01-224 Warsaw, Poland

a r t i c l e i n f o

 Article history:

Received 24 June 2011Received in revised form 7 September 2011Accepted 23 September 2011Available online 2 October 2011

Keywords:

Selective photocatalysisBiomass transformationWater purificationTitaniaPlatform molecules

a b s t r a c t

Glucose was oxidized in the presence of powdered TiO2 photocatalysts synthesized by an ultrasound-pro-moted sol–gel method. The catalysts were more selective towards glucaric acid, gluconic acid and arab-itol (total selectivity approx. 70%) than the most popular photocatalyst, Degussa P-25. The photocatalyticsystems worked at mild reaction conditions: 30 °C, atmospheric pressure and very short reaction time(e.g. 5 min). Such relatively good selectivity towards high-valued molecules are attributed to the phys-ico–chemical properties (e.g. high specific surface area, nanostructured anatase phase, and visible lightabsorption) of novel TiO2 materials and the reaction conditions. The TiO2 photocatalysts have potentialfor water purification and energy production and for use in the pharmaceutical, food, perfume and fuelindustries.

Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Biomass can be utilized for the sustainable production of high-value chemicals (Werpy and Pedersen, 2004), and a number of pro-cesses has been developed for this purpose such as catalysis withZrO2 or TiO2–ZrO2 (Chareonlimkun et al., 2010; Goh et al., 2010),steam gasification (Rapagna et al., 1998), fast pyrolysis (Iwasaki,2003; Li et al., 2004), and supercritical conversion (Watanabeet al., 2002; Hao et al., 2003). In contrast to these energy intensiveprocesses, photocatalytic transformation can be driven by sunlightand performed at room temperature (Colmenares et al., 2009).

Photocatalysis can also be used in the decomposition of wastesfrom the food industry and the simultaneous production of high-value chemicals when the wastes act as electron donors (e.g. glu-cose) (Colmenares et al., 2009).

In the photochemical degradation of cellulose filter paper in the

presence of UV light and air, a broad spectrum (non-selective pro-cess) of fully reduced and oxygenated hydrocarbons were pro-duced (e.g. CH3CHO, CH3CH2CHO, HCO2CH3, (CH3)2CO, CH3OH,CH3CH2OH, CH4, and C2H6) after more than 1–2 h of irradiation(Desai and Shields, 1969). In an oxygen-free atmosphere, onlytraces of some of these products (e.g. CH3CHO, CH3CH2OH, CH4)were observed.

The photoconversion of biomass into oxygenated hydrocarbons(hydrogen carriers) can take place in aqueous media and/or alco-

hol/CH3CN (for more selective photooxidation) and the presenceof sensitizing ions (e.g. Fe3+,2+), or ‘‘hydrated’’ carbohydrates

(Greenbaum et al., 1995). Selective photo-oxidation of biomasscan provide a wide range of high value-added chemicals including1,4-diacids (succinic, fumaric and malic acids), 2,5-furandicarboxy-lic, 3-hydroxypropionic, aspartic, glucaric, glutamic, itaconic,and levulinic acid, 3-hydroxybutyrolactone, glycerol, sorbitol, andxylitol/arabitol) (Werpy and Pedersen, 2004).

Titanium dioxide is a non-toxic, cheap and versatile materialwith attractive applications in the production of electrodes, capac-itors, solar cells, catalysis and photocatalysis. Regarding the latterapplication, the possibility of carrying out selective photooxida-tions in non-aqueous media is very interesting in the context of Green Chemistry (Shimizu et al., 2004; Taylor, 2003).

The present study was carried out to determine the efficiency of heterogeneous TiO2 catalysts on the selective photocatalytic

oxidation of glucose into high-valued organic compounds.

2. Methods

 2.1. Preparation of photocatalysts

All chemicals were of analytical grade and used as received. Thecatalyst was synthesized by the sol–gel method (Brinker andScherer, 1990). A volume of 3.8 mL of 12.5 mM titanium (IV) iso-propoxide was dissolved in 80 ml of 2-propanol at room tempera-ture, 3 g of poly(ethylene glycol) (PEG, average M.W. 400) wasadded and the mixture was treated with ultrasound (35 kHz,560 W, Sonorex Digitec-RC, Bandelin) for 1 h in 5 and 10 min.

0960-8524/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.doi:10.1016/j.biortech.2011.09.101

⇑ Corresponding author. Tel.: +48 22 343 3215.

E-mail address: jcarloscolmenares@ichf.edu.pl (J.C. Colmenares). Deceased.

Bioresource Technology 102 (2011) 11254–11257

Contents lists available at SciVerse ScienceDirect

Bioresource Technology

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / b i o r t e c h

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cycles (two times/5 min. and five times/10 min., 2 min. ultrasoundoff between cycles). A mixture of 1.27 mM D-glucuronic acid and1.4 g of PEG (total volume ratio of PEG:Ti was 8:1) in water/2-pro-panol (25:47 v/v) was added dropwise during ultrasonic treatment(10 min cycles with 2 min ultrasound off between cycles, the totaltime of this step was approx. 90 min).

The final mixture was continuously exposed to ultrasound for30 min, left at room temperature for 18 h, filtered through a G5funnel, dried at 110 °C for 3 h and calcined in static air at 500 °Cfor 5 h in a furnace. The product was designated as TiO2(US).

A second catalyst, designated as TiO2(R), was synthesized in thesame manner, but under solvent reflux (82.5 °C, boiling point of 2-propanol) instead of ultrasound. For comparative purposes, themost popular photocatalyst, TiO2 Degussa P-25 was also tested.

 2.2. Photocatalytic tests

All catalytic reactions were performed in a Pyrex cylindricaldouble-walled immersion well reactor with a total volume of 450 mL (Fig. 1). The reaction system was stirred magnetically at700 rpm to obtain a homogenous suspension of the catalyst. A

medium pressure 125 W mercury lamp (kmax = 365 nm) suppliedby Photochemical Reactors Ltd. (Model RQ 3010) was placed insidethe quartz tube as a light source. The reaction temperature was30 °C. Glucose solutions (2.8 Â 10À3mol/L) were prepared in a mix-ture of Milli-Q water and acetonitrile (10:90 v/v) unless otherwisespecified. Experiments were carried out from 150 mL of mothersolution and a concentration of 1 g/L of the catalyst was used. Allreactions were carried out under ambient air (no oxygen bubblingconditions). Approx. 2 mL of samples were taken from the photore-actor at specified times and filtered through 0.2 lm, 25 mm nylonfilters in order to remove TiO2 particles before high-performanceliquid chromatograph (HPLC). Glucose conversion and organiccompounds were measured, after calibration, by HPLC (WatersHPLC Model 590 pump), equipped with a refractive index detector

(Waters 2414 Refractive Index Detector). Separation wasperformed on a XBridge™ Amide 3.5 lm 4.6 Â 150 mm columnprovided by Waters. The mobile phase was Milli-Q water/acetoni-trile (15:85 v/v) at a flow rate of 0.8 mL/min. The injection volumewas 10 lL.

All reaction products were identified by gas chromatography–mass spectrometry (GC–MS, Hewlett Packard GC 6890 Series andMS 5973, Hewlett Packard – 5 column) after silylation using N,O-bis(trimethylsilyl)trifluoroacetamide with 1% trimethylchlorosi-lane as derivatizating agent and performed at 60 °C for 1 h in aCSB/COD-Reaktor ET 108. All products identified by GC–MS wereconfirmed by liquid chromatography–mass spectrometry (LC–MS,LC Prominence Shimadzu coupled with MS 4000 Q-TRAP AppliedBiosystems).

Control experiments were performed in the dark as well as withillumination but without catalyst.

 2.3. Characterization of photocatalysts

The textural properties of photocatalysts were determined fromN2 adsorption–desorption isotherms at liquid nitrogen tempera-ture by using a Micromeritics ASAP 2020 instrument. Surface areasand pore sizes were calculated by the Brunauer–Emmett–Teller(BET) and the Barret–Joyner–Halenda (BJH) methods, respectively.Prior to measurements, all samples were degassed at 110 °C to0.1 Pa.

X-ray powder analysis was carried out using a XRD SiemensD5000 diffractometer using Ni-filtered CuKa radiation (k = 1.5406ÅA0

) at 40 kV and 30 mA. The diffraction angle 2h was scanned at arate of 2° minÀ1. The average crystallite size of anatase and rutilewas determined according to the Scherrer equation (Eq. (1)) usingthe full-width at half-maximum of the peak corresponding to 101and 110 reflections, respectively, and taking into account theinstrument broadening.

D ¼K k

bcoshð1Þ

where D is the average crystallite size of the catalyst (nm), k is thewavelength of the Cu Ka X-ray radiation (k = 1.5406 Å), K  is a coef-ficient usually taken as 0.94, b is the full width at half maximum

(FWHM) intensity of the peak observed at 2h (radian), and h is thediffraction angle.

Ultraviolet–visible light spectroscopy was performed using aUV-2501PC Shimadzu spectrophotometer. Band-gaps values werecalculated based on the Kubelka–Munk functions, F(R 1), which

Fig. 1. Photocatalytic reactor.

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are proportional to the absorption of radiation, by plotting [F(R 1)hv]2 against hv (Sakthivel and Kisch, 2003).

3. Results and discussion

The photocatalytic oxidation of glucose and selectivity towardsorganic compounds (mainly glucaric acid, gluconic acid and arabi-tol in liquid phase) are presented in Fig. 2 and the possible processpathways are shown in Scheme 1. Apart from these three productsin the liquid phase, mostly CO2 and traces of light hydrocarbons inthe gas phase were detected. Even though Degussa P-25 was thesolid for which glucose disappearance was the highest (Fig. 2), itsselectivity was poor. The best selectivity of 39.3% and 71.3% of totalorganic compounds for 10%H2O/90%ACN and 50%H2O/50%ACNused as solvents, respectively towards organic compounds in liquidphase was obtained with TiO2(US) (Tables 1 and 2).

Changes in selectivity with time can be seen in Table 1. It isnoteworthy that good selectivity with appreciable conversion,

especially with TiO2(US), was achieved within 5 min. The photocat-alyst with the lowest selectivity was P-25 (15% at 5 min. for10%H2O/90%ACN). Interestingly, after 5 min of irradiation and un-der the presence of TiO2(US), 42.2% of glucaric acid and 37.9% of 

gluconic acid (in a total 80.1% of carboxylic acids) was obtainedin the organic phase. These carboxylic acids are very importantbuilding blocks for pharmaceutical, food, perfume and fuel indus-tries. The arabitol component (20% selectivity) in the organic phaseis also a platform molecule (Colmenares et al., 2009; Werpy andPedersen, 2004).

The present study hypothesized that more selective oxidationswould occur in a mixture of water and acetonitrile (ACN) as a lowerthe amount of water might give rise to a lower concentration of highly oxidative OH radicals (16.8%, 8.5% and 0.0% of selectivelyobtained on P-25 organic products for 10%H2O/90%ACN, 50%H2O/50%ACN and 100%H2O solvents mixtures, respectively, Table 2).When working in non-aqueous media, photoconversion can bestopped when particular products have accumulated and beforecomplete mineralization has occurred. A mixture of 50:50H2O:ACN (v/v) was the best solvent composition (Table 2) since11% glucose conversion and 71.3% total organic selectivity wasachieved after 10 min of irradiation in the presence of TiO2(US).

As it would be expected, P-25 showed the highest level of glu-cose mineralization (complete oxidation to CO2 and H2O) when theamount of water in solution was increased (78.8% glucose conver-sion for 100% H2O, Table 2). It is well-known that in most of thecases in organic solvents, the mineralization rate is much lowerthan that in aqueous solution (Shiraishi et al., 2005).

The relatively high carboxylic acids selectivity (71.3%, Table 2)could be due to lower affinity of these acids for the TiO2(US)surface in the presence of acetonitrile. In the case of the DegussaP-25 photocatalyst, OH radicals react efficiently with the well-adsorbed substrate (glucose), giving mainly carbon dioxide andwater (products of complete oxidation), whereas less-adsorbedglucose on TiO2(US) yielded more selective products whichdesorbed easily from the catalyst’s surface and diffused into theprotective environment created by ACN. Acetonitrile, a weak base(pKa = 25), could stabilize the carboxylic acids by solvation,resulting in suppression of further oxidation (Gac + GuAc > 70%selectivity). Shiraishi et al. (2005) and Morishita et al. (2006) dem-

onstrated the positive influence of acetonitrile on formation of epoxides in the selective photooxidation of olefins. Additionally,it could be that, under our reaction conditions (suitable solventscomposition, temperature, etc.), TiO2(US), with a favorable point

Fig. 2. Results obtained for selective photooxidation of glucose with synthesizedsystems and commercial catalyst in terms of conversion and selectivity to organiccompounds: glucaric acid, gluconic acid and arabitol (reaction conditions:H2O:ACN = 10/90 v/v, illumination time: 5 min., catalyst 1 g/L, 30 °C, 1 atm.).

TiO2-based

photocatalyst

ACN/H2O/30o

C

HOOH

OHOH

OHOHO

O

HOOH

OH

OH

OH

OH O

CO2 + H2Ototal

mineralisation

selective

photooxidation

Glucaric acid

+

Gluconic acid

+

Arabitol

CHO

OHH

HHO

OHH

OHH

CH2OH

Glucose

OH

OHOH

OH

HO

TiO2-based

photocatalyst

ACN/H2O/30o

C

HOOH

OHOH

OHOHO

O

HOOH

OH

OH

OH

OH O

CO2 + H2Ototal

mineralisation

selective

photooxidation

Glucaric acid

+

Gluconic acid

+

Arabitol

CHO

OHH

HHO

OHH

OHH

CH2OH

Glucose

OH

OHOH

OH

HO

Scheme 1. Plausible reaction pathways.

 Table 1

Effect of photocatalysts and irradiation time on organic compounds distribution in the liquid phase (reaction conditions: Cglucose = 2.8 Â 10À3 mol/L, 150 mL of 

H2O:ACN = 10:90 v/v, catalyst 1 g/L, 30 °C, 1 atm.).

Photocatalyst Selectivity to organic compounds [%]

Glucaric acid Gluconic acid ArabitolIrradiation time Irradiation time Irradiation time

5 min 10 min 15 min 5 min 10 min 15 min 5 min 10 min 15 min

TiO2(US) 16.6 14.0 11.0 14.9 13.4 10.2 7.8 4.3 4.2TiO2(R) 14.4 13.0 11.4 13.7 14.0 11.9 5.6 5.0 3.2P-25 6.7 7.7 6.1 6.7 7.1 6.1 1.6 2.0 1.5

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of zero charge pzc (pzc describes the condition when the electricalcharge density on a surface is zero), could have a better affinity forglucose adsorption, thus minimizing carboxylic acid adsorptiononto the surface and enhancing product selectivity.

It is also noteworthy that the sine qua non condition is thesimultaneous presence of titanium dioxide and light to run thereaction. Shutting off the light stops the reaction (no further oxida-tion) within a short time (e.g. 5 min). Further studies will be fo-cused on the investigation of the mechanisms of selectivecatalytic glucose photo-oxidation through techniques such as elec-tron spin resonance spectroscopy (ESR).

A summary of the characteristics of the photocatalysts is pre-sented in Table 3. The TiO2(US) and TiO2(R) photocatalysts exhib-ited better textural, structural and optical features (high specific

surface area 65 m2/g, low band gap 3.05 eV and nanometric ana-tase phase 12.7 nm) than the commercially photocatalyst, DegussaP-25. The best features were shown by TiO2(US), synthesized withultrasound. Interestingly, TiO2(US) material absorbs light in thevisible range (absorption threshold 409 nm) possibly due the pres-ence of oxygen vacancies (electronic perturbation) on its surfacepromoted by cavitation during ultrasound-assisted synthesis(Cronemeyer, 1959; Gesenhues, 2001; Lisachenko et al., 2004).

4. Conclusions

Among the two photocatalytic systems tested, the best productselectivity was achieved with the catalyst, TiO2(US), synthesized bythe ultrasound-modified sol–gel method (the highest selectivity

for glucaric/gluconic acid and arabitol products). Solvent composi-tion and short illumination times, have a considerable effect onactivity/selectivity of the photocatalysts. Selective photocatalyticbiomass transformations hold significant promise for the develop-ment of economically and environmentally friendly synthesis pro-cesses to produce a significant number of important chemicals.

 Acknowledgements

Dr Colmenares (main author) wants to express his sincere grat-itude to the European Commission for the financial support. Thisresearch was supported by a Marie Curie International Reintegra-tion Grant within the 7th European Community FrameworkProgramme. The authors would like to thank Dr B. Mierzwa for

XRD mesurements and P. Dobosz for help during HPLC analysis.

References

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 Table 3

Summary of the most remarkable textural, structural and optical features concerning characterization of photocatalytic systems.

Photocatalyst N2 isotherms UV–vis XRD

SBETa (m2/g) Vp

a (mL/g) Band gap (eV) Absorption threshold (nm) Crystal phaseb (%) Crystallite size (nm)

Degussa-P25 51 – 3.20 387 A(88)/R(12) 17.6/24.4TiO2(US) 65 0.14 3.03 409 A(100) 12.7TiO2(R) 54 0.12 3.17 391 A(100) 12.4

a Specific surface area (S BET), cumulative pore volume (V p).b A and R denote anatase and rutile, respectively.

 Table 2

Effect of solvent composition on glucose conversion and total selectivity for organic compounds (glucaric acid, gluconic acid and arabitol) in the liquid phase after 10 min of 

illumination.

Photocatalyst Solvent composition

10%H2O/90%ACN 50%H2O/50%ACN 100%H2O

Conversion [%] R Selectivity [%] Conversion [%] R Selectivity [%] Conversion [%] R Selectivity [%]

TiO2(US) 28.8 31.7 11.0 71.3 41.2 0.0

TiO2(R) 35.0 32.0 41.5 17.2 50.3 0.0P-25 60.4 16.8 67.3 8.5 78.8 0.0

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