This journal is c The Royal Society of Chemistry 2012 Catal. Sci. Technol., 2012, 2, 1173–1179 1173

Cite this: Catal. Sci. Technol., 2012, 2, 1173–1179

Glycerol utilization: solvent-free acetalisation over niobia catalysts

G. S. Nair,aE. Adrijanto,

aA. Alsalme,

bcI. V. Kozhevnikov,

bD. J. Cooke,

a

D. R. Brownaand N. R. Shiju*

d

Received 23rd August 2011, Accepted 16th January 2012

DOI: 10.1039/c2cy00335j

With increasing biodiesel production, availability of glycerol is expected to increase. New

processes are needed for converting this surplus glycerol to value-added chemicals. In this work,

we used niobia catalysts for the liquid-phase acetalisation of glycerol without using any solvent.

High conversions were achieved (B80%), though water was present in the reaction system. The

calcination temperature changed the strength as well as the nature of acidity of the samples.

Samples with higher Brønsted acid strength exhibited higher catalytic performance. The results

show that niobia is a water tolerant, reusable catalyst for glycerol acetalisation.

1. Introduction

Biodiesel production is accompanied by the formation of

glycerol as a by-product. Its availability has tripled within

the last ten years due to the increased biodiesel production.1

Several countries target increased biofuel usage in future,

hence the glycerol availability will still increase.2 With the

oversupply and the consequent drop in price, glycerol is

expected to become a major platform chemical.3 The use of

glycerol is limited now, being mainly confined to pharmaceuticals

and cosmetics. Hence new processes are needed that can convert

the surplus glycerol to value-added chemicals. In addition to

taking advantage of the relatively low price of glycerol, this will

also improve the economic viability of biodiesel manufacture.

Research efforts are gaining momentum for the synthesis of

value added products from glycerol. A few studies on glycerol

reactions have been reported that involve catalytic processes

such as reforming, oxidation, hydrogenolysis, etherification,

dehydration and esterification reactions.1,4,5 A possible way of

utilizing glycerol is through its condensation with aldehydes

and ketones to acetals and ketals respectively.6–12 Glycerol

acetals and ketals may be used as fuel additives, surfactants

and flavours.6–12 When incorporated into standard diesel fuel,

they have led to a decrease in particles, hydrocarbons, carbon

monoxide and unregulated aldehyde emissions.12,13 These

products can act as cold flow improvers for use in biodiesel, also

reducing its viscosity. This is important because there is a growing

demand for new additives specifically for biodiesel that are

biodegradable, non-toxic and renewable. The addition of these

compounds to biodiesel improved the viscosity and also met the

established requirements for flash point and oxidation stability.14

The main drawback of the glycerol acetalisation is the produc-

tion of water. This weakens the acid strength of the catalyst and

limits the glycerol conversion. The use of solvents such as benzene,

toluene, petroleum ether or chloroform to increase the glycerol

conversion into acetals or ketals has been described.7 However, this

method is not very efficient and presents environmental problems.

We report solvent-free liquid-phase glycerol acetalisation with

acetone using niobia catalysts. Niobia (Nb2O5) has been used as

a water-tolerant solid acid catalyst for various water-involving

reactions.15–17 The effect of calcination/pretreatment temperature

of hydrated niobium oxide, acidity and other reaction parameters

on the catalytic performance was investigated. The results of this

study show that niobia acts as a stable active catalyst for glycerol

acetalisation (Scheme 1), forming 2,2-dimethyl 4-hydroxymethyl-

1,3-dioxolane (solketal) predominantly.

2. Experimental

Niobia sample (Niobia HY-340) was kindly provided by

CBMM, Brazil. The composition of this batch is as follows:

Nb2O5 80.5 � 5%, Fe 200 ppm max., free Cl 400 ppm max.

and loss on ignition 20.5 � 5%. The samples were calcined at

different temperatures for 4 h in a muffle furnace in static air

before use. The catalysts are represented as Nb2O5-xxx, where

Scheme 1 Acetalisation of glycerol.

aMaterials and Catalysis Research Centre, Dept. of Chemical andBiological Sciences, University of Huddersfield,Queensgate HD1 3DH, UK

bDepartment of Chemistry, University of Liverpool,Liverpool L69 7ZD, UK

cKing Saud University, PO Box 2455, Riyadh 11451, Saudi ArabiadVan’ t Hoff Institute for Molecular Sciences, University ofAmsterdam, Science Park 904, 1098 XH Amsterdam,The Netherlands. E-mail: n.r.shiju@uva.nl; Fax: +31 (0)20 525 5604

CatalysisScience & Technology

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1174 Catal. Sci. Technol., 2012, 2, 1173–1179 This journal is c The Royal Society of Chemistry 2012

xxx denotes the calcination temperature in K (Table 1). The

reactions were carried out in the liquid phase in a 50 ml glass

reactor equipped with a condenser and a magnetic stirrer.

Required amounts of glycerol (99+%, Alfa Aesar) and

acetone (99+%, Alfa Aesar) were stirred with a specific mass

of the catalyst in powder form. The molar ratio of glycerol to

acetone was 1 : 1.5 unless otherwise stated. To monitor the

reaction, samples of the reaction mixture were taken periodically

and analysed by gas chromatography (Perkin Elmer Clarus 500)

using a 50 m BP5 capillary column and an FID detector. In a

typical reaction, 34 mmol of glycerol was reacted with 50 mmol

acetone and 0.2 g catalyst. The N2 adsorption–desorption

isotherms were measured at 77 K on a Micromeritics ASAP-

2000 after evacuation at 473 K for 5 h. Surface areas were

calculated by the BETmethod. Powder X-ray diffraction patterns

were collected on a Bruker powder X-ray Diffractometer (D5000)

operated at 40 kV and 20 mA using Nickel filtered Cu Karadiation (1.5406 A). The system used for ammonia adsorption

flow calorimetry has been described previously.18,19 It is based on

a Setaram 111 DSC with an automated gas flow and switching

system, with a mass spectrometer (Hiden HPR20) to sample the

downstream gas flow. The sample (20–30 mg) was held on a glass

frit in a vertical silica sample tube and activated at 423 K under a

dried nitrogen flow (5 ml min�1) for five hours. After activation,

the sample temperature was maintained at 423 K and 1 ml pulses

of the probe gas (1% ammonia in nitrogen) at atmospheric

pressure were injected at regular intervals into the carrier gas

stream from a gas sampling valve. The ammonia concentration

downstream of the sample was monitored continuously by mass

spectroscopy. The pulse interval was chosen to ensure that the

ammonia concentration in the carrier gas returned to zero to

allow the DSC baseline to stabilise. The net amount of

ammonia irreversibly adsorbed from each pulse was determined

by comparing the MS signal with that recorded through a control

experiment with a blank sample tube. The net heat released by each

pulse was calculated from the thermalDSC curve (Table 1). Diffuse

reflectance infrared Fourier transform (DRIFT) spectra of

adsorbed pyridine were obtained on a Nicolet NEXUS FTIR

spectrometer equipped with a controlled environment chamber

(Spectra-Tech Inc., model 0030-101). Catalyst samples were diluted

with KBr powder (10 wt% in KBr) and pre-treated at

150 1C/0.1 Torr for 1 h. The samples were then exposed

to pyridine vapour at room temperature for 1 h, followed

by pumping out at 150 1C/0.1 Torr for 1 h to remove

the physisorbed pyridine. Then the DRIFT spectra of the

adsorbed pyridine were recorded at room temperature.

Atomistic simulation techniques used in this work are

formulated within the framework of the Born model of the

solids.20 This comprises of long-range electrostatic and short-range

interactions to describe the attractive van der Waals interactions

and the dispersive London forces acting between the ions. The

long-range Coulombic interactions converge slowly as a function

of increasing ion separation. This is overcome by using the

mathematical transformation of Ewald21 when the system is

periodic in three dimensions and that of Parry22 when the system

is only periodic in two dimensions (for example when studying

surfaces). The short-range forces are described using simple,

parameterised, pairwise potentials such as the Buckingham

potential of the form F(rij) = Aijexp(�rij/rij) � (Cij/r6ij), where

F(rij) is the short range interaction energy and Aij, rij and Cij are

parameters specific to each pairwise interaction and rij is the

separation between atom i and atom j. Additionally, the effect of

polarisability on the anion is incorporated using the shell model of

Table 1 Textural properties of niobia samples

Catalyst calcinationtemperature/K

BET surfacearea/m2 g�1

Pore volume/cm3 g�1

Average porediametera/nm

Average porediameterb/nm

Acid site concentrationc/mmol g�1

Acid strengthd/kJ mol�1

473 121.0 0.18 5.8 5.5 0.28 92.8573 105.3 0.17 6.5 6.3 0.20 98.3773 59.4 0.16 9.0 8.2 0.13 87.3973 10.6 0.05 — — — —

a Calculated from BJH adsorption. b From BJH desorption. c NH3 adsorbed with �DHoads Z 80 kJ mol�1. d Average heat of adsorption

above 80 kJ mol�1.

Table 2 Potential parameters used in this work

A/eV P/A C/eV A6 C/eV A�2 Charges, e

Nb5+(core)� � �O2�(shell) 3023.184 0.300 0.0 — Nb 5+O2�(shell)� � �O2�(shell) 22 764 0.149 27.89 — O (core) 0.077O2�(core)� � �O2� (shell) — — — 27.29 O (shell) �2.077

Fig. 1 X-Ray diffraction patterns of niobia samples as a function of

calcination temperature.

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Dick and Overhauser.23 Here the ion is divided into a core

containing the mass and the core electron charge and a shell

containing the valence electron charge. These are connected by a

harmonic spring. The polarisability of the model ion is then

determined by the spring constant (k) and the charges of the core

and the shell. We used the values of the potential parameters

reported by Jackson et al.24 and are shown in Table 2.

All simulations were performed using the METADISE

(minimum energy techniques applied to dislocation interface and

surface energies) code which enables us to perform static, energy

minimisations on a solid-state system.25 In the METADISE

program the crystal is regarded as a series of charged planes

parallel to the surface and periodic in two dimensions. The basis

of this approach is to calculate the total interaction energy, often

called the lattice energy.

3. Results and discussion

Fig. 1 shows the XRD patterns of the Nb2O5 catalysts calcined

at different temperatures. The samples heated to 473 and

573 K were essentially amorphous, with very broad peaks.

For the sample calcined at 773 K, peaks corresponding to the

orthorhombic (T) phase of niobium pentoxide were observed.

Some amorphous material was still present in this sample

(Fig. 1). At 973 K, well defined peaks corresponding to the

characteristic reflections of T-Nb2O5 were observed, indicating

complete crystallization. The niobia structure was simulated

starting from the bulk T-Nb2O5 previously described by Kato

and Tamura.26 This structure has a space group symmetry of

PBAM. From this, a pseudo cell was generated with no

symmetry constraints where Nb lattice sites with less than

10% occupancy were removed. The lattice disorder of the

remaining Nb, described by 50% occupied sites less than 1 A

were replaced by a singular occupied site representing an

average of the two sites. The structure of the crystal was then

optimised with no applied pressure in order to find the

minimum energy configuration. During the optimisation, no

symmetry constraints were applied. The relaxed structure

has lattice parameters of a = 6.00, b = 27.58, c = 3.63 and

a= b= g= 901, which differs from the original experimental

Fig. 2 Simulated X-ray diffraction pattern (left) using the relaxed T-Nb2O5 structure (right).

Fig. 3 (a) Low angle XRD of Nb2O5-473 and (b) N2 adsorption–desorption isotherms of Nb2O5-473 and Nb2O5-573.

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1176 Catal. Sci. Technol., 2012, 2, 1173–1179 This journal is c The Royal Society of Chemistry 2012

structure by 3–8%. This is in good agreement, especially

considering that the structure of T-Nb2O5 was not used in

the derivation of the potential model and that no symmetry

restraints were applied during the simulation. Fig. 2 shows the

relaxed T-Nb2O5 structure, in which 8 Nb ions are present in

distorted octahedra and the remaining 8 Nb ions occupy

pentagonal bipyramidal sites. The oxide ions form a large open

channel structure throughout the unit cell. Using this relaxed

structure, a simulated X-ray powder diffraction pattern (Fig. 2)

was generated using the methodology of Gaussian function as

implemented in the GDIS-Package.27 This pattern matches well

with the experimental pattern, demonstrating that the experimental

sample and the simulated niobia lattice have the same structure.

Table 1 shows the textural properties of the Nb2O5 samples

derived from the nitrogen physisorption isotherms. Both surface

area (from 121 to 10.6 m2 g�1) and pore volume (from 0.18 to

0.06 cm3 g�1) decreased with increasing calcination temperature.

The average pore diameter increased with calcination. These

changes may be correlated with amorphous to crystalline

transformation. In general, surface area decreased with

increasing crystallinity. The most crystalline orthorhombic

(T) phase obtained by calcination at 973 K had the lowest

surface area. The low angle XRD pattern of Nb2O5-473

(Fig. 3a) shows only one peak at around a 2y value of 1.51.

The pattern resembles more that of disordered mesoporous

materials than that of regular mesoporous materials.28–31 The

mesoporosity in these samples is due to the extra framework

void space resulting from the intergrowth of small primary

particles. The mesoporosity is confirmed by the N2 adsorption

isotherm (Fig. 3b), which can be classified as type IV.

The total acidity of the samples was evaluated by ammonia

adsorption microcalorimetry (Fig. 4). The initial differential

heat of NH3 adsorption was the highest for Nb2O5-573

(the sample calcined at 573 K), indicating stronger acid sites

than other samples. The heat of adsorption decreased with

increasing calcination temperature. We have quantified acidity

by estimating the total ammonia adsorbed with heat greater

than 80 kJ mol�1 and averaged the enthalpies of points above

80 kJ mol�1 for average acid strength (Table 1). It is an arbitrary

method, but provides an indication of concentration and

strength.32,33 Nb2O5-973 not only had weaker acidity but also

the concentration of sites was lower (Fig. 4). Note that this sample

is highly crystalline with a low surface area. So, as calcination

temperature was increased, the sample transformed from the

amorphous to the crystalline phase, accompanied by a loss in

surface area and a decrease in number and strength of acid sites.

The nature of acid sites (Brønsted or Lewis) was determined

by DRIFT spectroscopy of adsorbed pyridine (Fig. 5). The

presence of Brønsted acid sites is indicated by the band at

B1540 cm�1 characteristic of the pyridinium ion.34 Lewis acid

sites are detected by observing the band at 1450–1460 cm�1

attributed to the complex between pyridine and the Lewis site.

At low calcination temperatures, samples had both Lewis and

Brønsted acid sites. In contrast, Nb2O5-973 exhibited only

Lewis acidity.

Fig. 4 NH3 adsorption microcalorimetry data of niobia samples

calcined at different temperatures.

Fig. 5 DRIFT spectra of adsorbed pyridine: (a) Nb2O5-973,

(b) Nb2O5-473 and (c) Nb2O5-773.

Fig. 6 Glycerol conversion (left) and selectivity to 1a (right) using

niobia samples calcined at different temperatures. 6.4 wt% catalyst wrt

glycerol; reaction temperature = 343 K.

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Fig. 6 shows the catalytic performance of niobia samples.

These acetalisation reactions were conducted at 343 K, with a

glycerol : acetone molar ratio of 1 : 1.5 using 6.4 wt% of

catalyst (wrt glycerol) without any solvent. The activities were

influenced significantly by the calcination temperature. The

samples pre-treated at lower temperatures were most active

while the sample calcined at 973 K was least active. For the

samples pre-treated at 473 and 573 K, more than 70% glycerol

was converted after 2 h. For the samples calcined at higher

temperatures, initial conversions were lower but increased

over extended reaction time. Initial activities (TOF, s�1) were

calculated from the glycerol kinetic curve and taking ammonia

adsorbed with heat greater than 80 kJ mol�1 for acid site

concentration. The values are 4.7, 8.0 and 5.7 for niobia

calcined at 473, 573 and 773 K. Comparison with NH3

adsorption microcalorimetry data (Fig. 4) shows that the

activity of the catalysts is in accordance with their acidity,

indicating that higher acidity is favourable for higher catalytic

activity. A reaction without any catalyst gave B15% glycerol

conversion at a temperature of 343 K after 6 h.

Glycerol acetalisation can produce two isomers, 5- and

6-membered cyclic acetals 1a and 1b, respectively

(Scheme 1), with the simultaneous liberation of water. In

our experiments, all catalysts exhibited higher selectivity to

the 5-membered cyclic acetal 1a. Selectivity preference towards

5-membered acetal was observed previously.6 It is proposed

that dehydration of the acetone/glycerol hemiketal yields a

tertiary carbenium ion, followed by a rapid nucleophilic attack

of the secondary hydroxyl group to form the five-membered

ring ketal (Scheme 2).6 The acid strength had an obvious effect

on the selectivity. The selectivity to 5-membered cyclic acetal

was higher when the catalyst acidity was stronger. The sample

pre-treated at 573 K, which possesses stronger acid sites,

converted around 80% of glycerol with a selectivity towards

1a of B92% after 6 h at 343 K, indicating the usefulness of

Scheme 2 Tentative mechanism for the formation of 5-membered

cyclic acetal from glycerol and acetone.

Fig. 7 Glycerol conversion and selectivity to 1a during initial period

of reaction; catalyst = Nb2O5-473; reaction temperature = 323 K;

glycerol : acetone = 1 : 3 (molar); 6.4 wt% catalyst wrt glycerol.

Fig. 8 Glycerol conversion at different reaction temperatures. Catalyst = Nb2O5-573; 6.4 wt% catalyst wrt glycerol. The dark and open symbols

indicate conversion of glycerol and selectivity to 1a respectively.

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1178 Catal. Sci. Technol., 2012, 2, 1173–1179 This journal is c The Royal Society of Chemistry 2012

this material for glycerol acetalisation (Fig. 6). Though water

is produced during the reaction, high conversions were

achieved indicating the tolerance of niobia acid sites towards

water. Based on the present results, it is not so straightforward

to correlate the nature of acid sites (Brønsted/Lewis) with the

activity and selectivity.

The reaction occurs significantly during the first few hours

itself as illustrated by the data obtained for Nb2O5-473 at

323 K (Fig. 7). Glycerol conversion wasB15% after 15 minutes

of reaction, increased rapidly for around 2 h, and then more or

less remained constant with reaction time. The selectivity to

5-membered acetal was high from the beginning of the reaction

and remained so with reaction time.

The catalytic performance was influenced by the reaction

temperature (Fig. 8). 20% glycerol was converted after 1 h at

303 K using sample calcined at 573 K, which increased to

B70% after 20 h of reaction. Upon increasing the reaction

temperature to 323 K, the rate increased and B68% glycerol

conversion was observed at 1 h. However, further increase in

reaction temperature to 343 K did not seem to increase the rate

a great deal. If the higher reaction times are affordable, the

reaction may be conducted at temperatures even close to room

temperature. Higher temperatures favour 5-membered acetal

(1a) (Fig. 8). Nevertheless, 1a was the major product even

at 303 K.

Fig. 9 shows the catalytic performance as a function of

catalyst amount. Higher catalyst amounts increased the

conversion, as expected. It is particularly interesting that low

catalyst amounts are effective for the reaction even at low

temperature, indicating the promising performance of niobia

for glycerol acetalisation.

To check the reusability, four consecutive reaction cycles

were performed using Nb2O5-573 K. After each cycle, the

catalyst was filtered, washed with acetone and water and

calcined at 573 K. The results were similar to the original

reaction and no significant deactivation was observed, confirming

the reusability of the catalyst (Fig. 10).

4. Conclusions

In summary, our results show that high conversions can be

achieved for glycerol acetalisation using niobia catalysts in rela-

tively short times under suitable experimental conditions. High

selectivity to solketal was observed. No solvent is necessary which

makes the system environmentally benign. Acid strength of

niobia depends on the calcination temperature and higher acid

strength favours higher catalytic performance. The catalyst can

be reused without deactivation, hence making it an ideal choice

for the acetalisation.

Fig. 9 Catalytic performance as a function of catalyst amount. The percentage weights of catalyst wrt glycerol are indicated in the figure. The

dark and open symbols indicate conversion of glycerol and selectivity to 1a respectively. Catalyst = Nb2O5-573; reaction temperature = 323 K.

Fig. 10 Catalyst reuse studies. After each cycle, catalyst was filtered,

washed with acetone, water and calcined at 573 K. Catalyst =

Nb2O5-573; reaction temperature = 343 K, time = 6 h.

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This journal is c The Royal Society of Chemistry 2012 Catal. Sci. Technol., 2012, 2, 1173–1179 1179

Acknowledgements

We acknowledge CBMM, Brazil for providing niobium oxide.

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