The influence of the support on the properties of nickel

catalysts for edible oil hydrogenation

M. Gabrovska a,*, J. Krstic b, R. Edreva-Kardjieva a, M. Stankovic b, D. Jovanovic b

a Institute of Catalysis, Bulgarian Academy of Sciences, Acad. G. Bonchev str. bl. 11, 1113 Sofia, Bulgariab Institute of Chemisty, Technology and Metallurgy, Department of Catalysis and Chemical Engineering,

Njegoseva 12, 11000 Belgrade, Serbia and Montenegro

Received 1 July 2005; received in revised form 27 September 2005; accepted 4 October 2005

Available online 18 November 2005

Abstract

Two silica-containing materials, diatomite and waterglass, have been used as supports of nickel catalysts for edible oil hydrogenation. The

active phase has been deposited following the precipitation–deposition method. The textural and structural characteristics of the supports and

precursors have been studied by nitrogen sorption, scanning electron microscopy (SEM), infrared (IR) spectroscopy and X-ray powder diffraction

(XRD). The thermogravimetric (TG) analyses have been carried out under hydrogen flow at different heating rates. Chemisorption of hydrogen has

also been applied for precursor characterization and the metal particle size has been calculated from hydrogen adsorption isotherms at 25 8C. Thecatalysts have been tested in the soybean oil hydrogenation reaction. The differences in the textural and structural properties of the catalysts under

study have not been found to reflect on their activity and selectivity. However, the use of water glass as the support is considered to be preferable,

because of the lower economic cost of the catalyst, related to the elimination of the mechanical, chemical and thermal treatment of the crude

diatomite.

# 2005 Elsevier B.V. All rights reserved.

Keywords: Water glass; Diatomite; Precipitation–deposition; Textural and structural characterization; Reduction; Edible oil hydrogenation; Selectivity

www.elsevier.com/locate/apcata

Applied Catalysis A: General 299 (2006) 73–83

1. Introduction

The hydrogenation of vegetable oils is of special interest in

the process of modification of edible fats and oils. The first aim

of this process is to change the normal liquid oil into a semisolid

product with a desired consistency in a certain temperature

range. The second one is to reduce the diene and triene contents

of the product to a minimum, thus contributing to the stability

of the product against oxidative rancidity [1–3].

Nickel, copper, copper-chromite, platinum and palladium

are the most common metals used as active species in

heterogeneous catalysts for partial hydrogenation of poly-

unsaturated triglycerides [4,5]. The use of monolithic catalysts

in the hydrogenation of edible oils, as an alternative to slurry

systems, has received increasing interest throughout the last

* Corresponding author. Tel.: +359 2 9793578; fax: +359 2 9712967.

E-mail addresses: margo@ic.bas.bg (M. Gabrovska),

jkrstic@nanosys.ihtm.bg.ac.yu (J. Krstic), rumedkar@ic.bas.bg

(R. Edreva-Kardjieva), mstankov@nanosys.ihtm.bg.ac.yu (M. Stankovic),

dusanmj@nanosys.ihtm.bg.ac.yu (D. Jovanovic).

0926-860X/$ – see front matter # 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.apcata.2005.10.011

years [6]. However, hydrogenation with nickel catalysts has

been the choice of industry, largely due to the availability, low

cost and inert nature of the metal relative to the oil. The typical

catalyst system for the hydrogenation of edible oils represents

reduced nickel in average concentration of 22 wt.%, deposited

on a suitable support and dispersed in the absence of air into

highly hydrogenated fat to stabilize it [4]. The choice and the

chemical preparation of the support, the technique of nickel and

promoter deposition, particle size, pore size and pore size

distribution of the precursor, as well as the activation procedure,

are known to be important for the activity and selectivity of the

catalyst. Kieselguhr (a diatomaceous earth), clay, silica, silica-

alumina, bentonite and palygorskite, as well as LTA-type

zeolites, have been used as supports of the nickel catalysts

[2–4,7–17].

Nickel supported on kieselguhr is the most widely used

catalyst in the industrial hydrogenation process. This support,

produced mainly from amorphous SiO2, has adequate textural,

thermal and chemical characteristics for the reaction under

study [4,12]. The crude diatomite, bentonite or palygorskite, as

natural silicon sources, contain large quantities of impurities.

M. Gabrovska et al. / Applied Catalysis A: General 299 (2006) 73–8374

Series of mechanical, chemical and thermal treatments of these

materials are necessary in order to obtain an activated support.

It is obvious that the establishment of a low-cost route for

producing the suitable support is opening the way for its

cheaper manufacture.

Water glass represents a viscous solution of sodium silicate

having a SiO2/Na2O molar ratio ranging from 3.75 to 1.5. Its

usage as a catalyst support for edible oil hydrogenation has

attracted our attention because a great number of laborious

steps would be avoided by its application as the source of silica,

instead of natural materials.

The number of papers about the application of water glass as

a catalyst support are rather scanty. For example, Tebben and

Weterings have reported that the silica carrier formed from

alkali water glass after neutralization and spray drying of the

material has no tendency to coalesce when used in processes

requiring precipitation of an insoluble nickel compound onto

the support [18].

The main support function is to keep up a fine dispersion of

nickel crystallites, making them available for reaction and also

to retard the growth or sintering of the small metal particles into

large agglomerates with a lower surface area and lower activity.

MgO is often used as a promoter in commercial nickel

catalysts, owing to its ability to stabilize the metal state and to

prevent the sintering of small nickel particles [19].

In this paper, we present the results from the comparative

study of two magnesium-promoted nickel catalysts. One of

them is supported on diatomite and the other one, on water

glass, both having been prepared by precipitation–deposition

technique. The aim was to elucidate the support influence on

the structure and catalytic properties of the catalysts in the

soybean oil hydrogenation reaction.

2. Experimental procedures

2.1. Supports

The diatomite, designated as D, is obtained from

Barosevac (the ‘‘Kolubara’’ Coal Basin—field B, Lazarevac,

Serbia and Montenegro). The crude diatomite is treated

mechanically, chemically (with an aqueous solution of HCl)

and thermally (at 800 8C) in order to obtain an activated

support [15].

The water glass material, designated as W, is a commercial

product with module (molar ratio) SiO2/Na2O = 3.0 (GALE-

NIKA—Magmasil A.D., Zemun (Belgrade, Serbia and

Montenegro).

2.2. Preparation of the precursors

All the chemicals used in the synthesis of the precursors

described in this paper, are of ‘‘pro analysis’’ purity grade.

The precursors, based on a diatomite (Ni–D) and on a

water glass (Ni-W), are obtained by the precipitation–dep-

osition method. Aqueous solutions of Ni(NO3)2�6H2O

(35.0 g dm�3 Ni), Mg(NO3)2�6H2O (1.4 g dm�3 Mg), Na2CO3

(110.0 g dm�3), an aqueous suspension containing 2.0 wt.% of

diatomite and an aqueous solution containing 2.0 wt.% of water

glass are prepared.

AvolumeofNi–Mgnitrate solution (molar ratioMg/Ni = 0.1)

is poured into a reaction vessel, equipped with a stirrer,

thermometer andpHelectrode. The solution is heated up to 90 8Cand co-precipitated with a ‘‘cold’’ Na2CO3 solution (20 8C)under vigorous stirring at constant value of pH 9.0� 0.2. A

peristaltic pump with defined flow rate is controlling the drop

wise addition of the Na2CO3 solution to the reaction vessel. The

obtained precipitate is aged for 30 min at 90 8C. The temperature

control and the constant stirring are performed with the

instrumentMR3001KandEKT3001, ‘‘Heidolph’’, respectively.

For the pH monitoring, a pH-meter 3320 ‘‘Jenway’’ with ATC

probe and pH Electrode C2401-8 ‘‘Radiometer’’ is used.

The water glass aqueous solution (20 8C) is added to the

aged precipitate in a quantity to satisfy the molar ratio SiO2/Ni

of 1.0 (Precursor Ni-W). The diatomite aqueous suspension

(20 8C) is added to the aged precipitate in a quantity to achievethe same SiO2/Ni molar ratio (Precursor Ni-D). The resulting

materials are aged again for 30 min at 90 8C under constant

stirring and than filtered and thoroughly washed with hot

(�90 8C) distilled water until absence of NO3� and Na+ ions is

obtained. The precursors are dried for 24 h in an oven at 105 8Cand ground to a powder.

2.3. Characterization of the supports and precursors

The silicate analysis is done by a classical chemical

procedure applicable in the analysis of sediment rocks. The

nickel content is determined gravimetrically by the dimethyl-

glyoxime method, while magnesium and other elements in

supports and precursors are determined on a Varian AA 775

spectrophotometer.

The X-ray diffraction (XRD) measurements are carried out

on a Philips PW 1710 powder diffractometer employing Cu Ka

radiation (l = 0.15148 nm), operated at U = 45 kV and 40

I = mA.

The infrared (IR) spectra of the supports and precursors are

recorded with a Perkin-Elmer 983 G spectrometer. The spectra

are performed using the KBr pellets technique: 1 mg of each

sample is mixed with 200 mg of KBr (KBr for spectroscopy

Uvasol, Merck, Germany). The spectral range of 4000–

250 cm�1 with a resolution of a 4 cm�1 is investigated. The

samples are dried at 105 8C before measurement.

The scanning electron microscopy (SEM) observations are

carried out on a JEOL Superprobe 733microscopewith a 25 kV

beam.

The nitrogen adsorption–desorption isotherms are deter-

mined at �196 8C on a Sorptomatic 1990 (Thermo Finnigan)

apparatus. The samples are out-gassed for 12 h at 110 8C and

degassed prior to the adsorption measurements. The specific

surface area, the total pore volume (Vp) at p/po = 0.998 and the

mean pore diameter are estimated with Win ADP (CE

Instruments) using the BET method [20].

The thermal analyses are performed using a LINSEIS device

(System 2000). The dried finely powdered precursors are

decomposed at heating rates of 2, 5, 10 and 20 8C min�1 under

M. Gabrovska et al. / Applied Catalysis A: General 299 (2006) 73–83 75

Table 1

The chemical composition of the supports

Support Chemical composition (wt.%)

SiO2 Al2O3 Fe2O3 CaO MgO Na2O K2O

Water glassa 74.51 – 0.50 – – 24.80 0.18

Diatomiteb 93.07 3.87 0.56 0.59 0.80 0.05 0.56

a Dry mass.b After activation.

hydrogen atmosphere. The H2 flow is introduced with a rate of 5

l h�1 to the thermogravimetric (TG) system after deoxygenat-

ing purifying.

The hydrogen chemisorption measurements are carried out

in a laboratory set-up using volumetric device. The samples are

previously reduced in the equipment for 5 h at 430 8C with a

gas mixture of H2/N2 (1:1, v/v, flow rate of 5 l h�1) at a heating

rate of 2 8C min�1. The adsorption isotherms are obtained

at 25 8C and pressures in the range of 0�100 Torr

(1 Torr = 133.3 Pa) after reduction. The hydrogen monolayer

coverage (adsorption capacity) is determined by extrapolation

of the linear part of the isotherm. The total amount of adsorbed

hydrogen is used to determine the metal nickel surface area and

metal nickel crystallite size. A ratio of 2 atoms of nickel per

molecule of chemisorbed hydrogen is assumed and the surface

density of a Ni atom is taken as 1.54 � 1019 atoms m�2Ni . The

crystallite size (dNi) is calculated assuming that the metallic

nickel presents in a cubic form and has metal density (rNi) of

8.9 � 106 g m�3 [21].

2.4. Reduction of the precursors

The activation of the precursors is performed in a laboratory

set-up by a dry reduction method. The reduction process is

accomplished with a gas mixture of H2/N2 (1:1, v/v) at a flow

rate of 5 l h�1. The reduction temperature is raised up to 430 8Cat a heating rate of 2 8C min�1 and held constant for 5 h. After

cooling down to room temperature, the reduced precursors,

denoted RNi-D and RNi-W, respectively, are passivated with a

mixture of 350 ppm O2 in nitrogen in order to reduce the

exceptional pyrophority of the metal nickel. The passivated

materials are impregnated with the pure paraffin oil.

2.5. Catalytic activity test

The partial hydrogenation of the edible soybean oil is

performed in a 7.5 dm3 reactor under the following conditions:

oil mass—5000 g; catalyst concentration—0.10 or 0.022 wt.%

Ni with respect to the amount of oil; initial hydrogenation

temperature—145 8C; final hydrogenation temperature—

180 8C; initial H2 pressure—0.08 MPa; final H2 pressure—

0.16 MPa; stirring rate—720 rpm.

A refined soybean oil with a composition of the fatty acids as

follows: palmitic (C16:0)—10.1%, stearic (C18:0)—4.1%,

oleic (C18:1)—23.2%, linoleic (C18:2)—52.8%, linolenic

(C18:3)—7.2%, is used as the starting material. The remaining

fatty acids: myristic (C14:0), palmitoleic (C16:1), arachidic

(C20:0), eicosenoic (C20:1) and behenic (C22:0) are in the total

concentration of 2.6%.

Taking a probe each 10 min follows the change in the fatty

acid composition of the soybean oil during the hydrogenation

reaction.

The hydrogenated products (triglycerides) are converted

into methyl esters using amethod described in detail by Christie

[22]. The analyses of the fatty acid methyl esters of the crude

soybean oil and hydrogenated products are performed by a

Hewlett–Packard HP 5890 gas chromatograph.

3. Results and discussion

3.1. Supports

The results of the chemical analysis of both supports are

summarized in Table 1.

The XRD pattern of diatomite (Fig. 1a) shows reflections

characteristic for amorphous silica (the broad peak centred at

2Q = 208) with a small amount of well-crystallized quartz

(2Q = 26.68; JCPDS file 46-1045). The XRD pattern of the

water glass reveals that the material is badly crystallized

(Fig. 1b).

The IR spectrum of diatomite (Fig. 2a) displays character-

istic bands of the amorphous hydrated SiO2. The broad

absorption band centred at 1100 cm�1 is attributed to the nasmode of Si–O stretching vibrations, while the sharp bands

centred at 800 and 470 cm�1 are attributed to the ns mode of Si–

O stretching vibrations. The peak at 1625 cm�1 is due to the

deformation mode (d) of H–O–H angle of the water molecules.

Some of the IR bands, characteristic of the amorphous hydrated

SiO2, are registered in the IR spectra of water glass, too

(Fig. 2b). They are of relatively low intensity and shifted to

lower wave numbers (the bands centred at 1050, 770 and

450 cm�1). A new band centred at 1450 cm�1 appears proving

the presence of CO32� ions [23,24].

SEM is used to investigate the morphology and the

uniformity of the particle shape, as well as to determine the

chemical homogeneity of the supports. The SEM image of

diatomite (Fig. 3) shows that this support consists of different

natural inorganic and biological products, differing in kind,

shape and size. In contrast, the SEM image of water glass

reveals very well pronounced habit of the angulated particles,

randomly distributed on the surface, ranging in effective

diameter from 8 to 170 mm.

The observed differences in the XRD and IR spectra and

SEM photography of the supports could be ascribed to their

different nature and different preliminary thermal treatments.

3.2. Precursors

3.2.1. Unreduced precursors

The results from the silicate and chemical analysis of both

precursors are summarized in the Table 2. An atomic ratio of

Mg/Ni about 0.1 is found in the both precursors that being

nearly equal to the Mg/Ni ratio in the synthesis solutions. The

obtained results demonstrate that the Ni2+ and Mg2+ ions are

quantitative precipitated.

M. Gabrovska et al. / Applied Catalysis A: General 299 (2006) 73–8376

Fig. 1. XRD patterns of the supports: (a) diatomite and (b) water glass.

The X-ray diffraction patterns of the precursors (Fig. 4) are

considerably differing from the spectrum of both supports. The

registered reflections are very broad and weak and indicate the

presence of poorly crystalline samples. The high background

below 2Q = 108 of the precursor Ni-W (Fig. 4c) indicates

advanced amorphization than in Ni-D. The XRD patterns of the

unsupported basic nickel carbonate (BNC) (Fig. 4a) disappear

in the patterns of both precursors (Fig. 4b and c). The observed

phenomenon proves that an interaction occurs between the

Fig. 2. IR profiles of the supports: (a) diatomite and (b) water glass.

basic nickel carbonate and the silica from the supports. As is

well known, when the nickel salt is precipitated with sodium

carbonate in the presence of silica, the synthesized compound is

the silica-supported basic nickel carbonate with a composition

of Ni(OH)x(CO3)y/SiO2�zH2O that varies considerably with the

changes in the precipitation conditions [23,25]. The tendency of

the silica support to react strongly with the Ni-containing

species has been reported in several studies [24,26,27]. This

strong interaction has been suggested to result in the formation

of nickel hydrosilicate layers on the surface of the silica

particles. The formed phases have been identified as nickel

antigorite [26,10,27,28], nickel chrysotiles [29], nickel

montomorillonite [26,28], nickel polygorskite [24], serpentine

[30] or even orthosilicate-type [24]. The poorly crystallized

nickel hydrosilicate compounds with imperfect nickel anti-

gorite and/or nickel montmorillonite-like structure are always

formed during the co-precipitation of nickel nitrate and alkali

silicate solutions at temperature under 100 8C [28].

The reflections characteristics of nickel silicate hydroxide

(2Q = 12�378; Ni3Si2O5(OH)4, JCPDS file #22-0754) and

antigorite-like phase appeared (2Q = 35�388; Ni3Si2O5(OH)4,

JCPDS file #21-0963). Theymay be observed as very broad and

weak reflections in both precursors (Fig. 4).

IR studies are used to prove the interactions of nickel species

with the silica from the supports. The IR spectrum of the

unsupported basic nickel carbonate exhibits characteristic

bands at 1640, 1380, 520, 465 and 360 cm�1 (Fig. 5a). The

band at 1625 cm�1, characteristic of a deformation mode (d) ofthe water molecules, and at 1380 cm�1 typical of the presence

of CO32� ions are still preserved in the spectra of the precursors

(Fig. 5), however with higher intensity. On the other hand, the

bands observed in the region 520–360 cm�1, characteristic of

lattice and stretching vibrations of Ni–O– in nickel basic

carbonate, disappeared. The IR spectra of both precursors do

not display the existence of free silica (bands at 1097 and

800 cm�1). The shift of the diatomite band at 1120 cm�1

(Fig. 2a) toward 1067 cm�1 in the Ni-D precursor (Fig. 5b), as

well as the shift of the water glass support from 1050 cm�1

(Fig. 2b) toward 1015 cm�1 in the Ni-W precursor (Fig. 5c), is

assigned to the vibrations of silicate species having interacted

with the basic nickel carbonate.

The SEM images of both precursors illustrate a change in the

morphology of the original supports (Fig. 3). It may be seen that

the preparation procedure leads to the formation of precursors

with smaller particles than the corresponding supports. The

SEM image of precursor Ni-D displays well-pronounced habit.

The surface consists of fragments different in shape and size.

The larger ones have an effective diameter of 3�7 mm. These

are covered with numerous fine particles with an effective

diameter of 0.5�3 mm. In contrast to Ni-D, the surface of the

Ni-W precursor appears to be composed of identical, smaller

and randomly distributed particles, similar to those of the

support structure. The particles are very fine with an effective

diameter of 0.25–1.5 mm, separated by craters.

The BET surface area and the textural characteristics of the

diatomite support and both precursors are represented in

Table 3. The table does not contain data for the water glass

M. Gabrovska et al. / Applied Catalysis A: General 299 (2006) 73–83 77

Fig. 3. SEM images of the samples.

support, because the dry material still contains a large amount

of water. Its presence makes the BET surface area and pore

volume measurements nonsense.

The increase in surface area from 17 m2 g�1 in fresh

diatomite (Table 3) up to 98 m2 g�1 in Ni-D may be explained

by the reaction between Ni- and Si-containing species. The

Table 2

The chemical composition of the precursors

Precursor The chemical composition of the precursors

Ni (wt.%) Mg (wt.%) Mg/Ni ratio S

Ni-D 41.93 1.80 0.10 4

Ni-W 42.21 1.70 0.09 4

formation of nickel hydrosilicate-type linkages (–Si–O–Ni–)

lead to a highly porous network [24]. This phenomenon is

more pronounced in precursor Ni-W and confirms the results

from XRD (Fig. 4c) and SEM (Fig. 3) analyses, revealing

respectively more poorly crystallized sample and smaller

particles.

iO2 (wt.%) NiO (wt.%) MgO (wt.%) SiO2/Ni ratio

3.70 53.32 2.98 1.02

3.48 53.70 2.82 1.01

M. Gabrovska et al. / Applied Catalysis A: General 299 (2006) 73–8378

Fig. 4. XRD patterns of the samples: (a) BNC; (b) Ni-D; and (c) Ni-W.

The nitrogen adsorption–desorption isotherms of the

diatomite and the precursors are represented in Fig. 6. The

isotherm of diatomite is close to type II characteristic for non-

porous or macroporous adsorbents according to the IUPAC

1985 classification [31]. The lack of full reversibility of the

adsorption–desorption isotherm, i.e. the presence of a hyster-

esis loop, indicates adsorption on an open and macroporous

Fig. 5. IR spectra of the samples: (a) BNC; (b) Ni-D; and (c) Ni-W.

stable surface. The isotherms of the precursor Ni-D changed in

shape, transforming into type IV characteristic for the

mesopores materials. The isotherm exhibits a hysteresis loop

close to type H4 [31]. Obviously, the modification of the

macroporous diatomite texture into the mesoporous one is

provoked by the reaction of nickel species with the support. The

isotherm of the Ni-W precursor is similar in shape to the Ni-D,

with more pronounced hysteresis loop, close to type H3.

Comparing the values of Vp and Vmes of both precursors, it

may be seen that they are six times higher in Ni-W than in Ni-D,

indicating higher access of the nitrogen molecule to the

corresponding pores of mesopore region (Table 3). The ratio

4Vp/SBET is used as a simple mode of the mean pore diameter

(dp) characterization [31]. The diatomite support possesses

large pores (dp = 60 nm). The formation of nickel silicate-like

phases reduces the pore size to 4.5 nm in the Ni-D sample. A

similar dp value is found in Ni-W.

In order to obtain a more comprehensive representation of

the texture of the diatomite and the precursors, the pore size

distribution (PSD) is estimated. The desorption branch of the

nitrogen isotherms at p/po from 0.2 to 0.998 is used, applying

the BHJ model and the standard isotherm of de Boer and co-

workers [32].

The PSD curve of the diatomite (Fig. 7) demonstrates a

texture with polydisperse type of distribution. The mean part of

the pores is situated at dp maximum equal to 47 nm. The PSD

curves of both precursors reveal a transformation from the

polydisperse to the monodisperse type of distribution in the

range of 3�5 nm with a dp maximum about 4 nm.

It may be concluded that the studied precursors are

composed of small particles but Ni-W sample possesses more

developed porous structure in the mesopore region.

It presents some interest to study whether the similarities in

the structure and texture of the precursors reflect on the

decomposition of the precursors under hydrogen atmosphere.

This activation procedure represents one of the most important

operations in the course of the catalyst synthesis, definitely

determining the characteristics and performance of the final

catalyst.

The differential thermal gravimetric (DTG) profiles of both

precursors, i.e. the first derivative of the weight losses,

measured at heating rates of 2, 5, 10 and 20 8C min�1 under

hydrogen flow, are studied.

The precursor Ni-D does not exhibit appreciable weight

losses in the low temperature region undergoing the same

weight losses (about 40%) at all heating rates studied. This is

easily explained by the high temperature (800 8C) calcinationof the support, prior to the precursor preparation. An analogous

behaviour of the precursor Ni-W is registered. This sample also

manifests the weight losses (about 30%) at all heating rates.

However, low temperature weight losses are observed with this

material. These peaks are not related to hydrogen consumption

but to the weight loss due to desorption of the adsorbed and

crystallization water in the samples. The higher temperature

weight loss peaks present broad minimum centered at 400 8Cfor the lowest heating rate (2 8C min�1) and at 500 8C for the

highest one (20 8C min�1).

M. Gabrovska et al. / Applied Catalysis A: General 299 (2006) 73–83 79

Table 3

The BET surface area, the textural characteristics and the type of isotherms of the samples

Sample Characteristics of the samples

SBET (m2 g�1) Vp (cm3 g�1) Vmes (cm

3 g�1) Max pore width (nm) dp (nm) Type of isotherm

D 17 0.25 0.10 2.20 58.8 II-like

Ni-D 98 0.11 0.10 3.82 4.49 IV-like

Ni-W 505 0.66 0.61 3.86 5.23 IV-like

Vp is total pore volume at p/p0 = 0.998; Vmes is cumulative pore volume of mesopore region; dp is mean pore diameter, dp = 4Vp/SBET.

Fig. 6. The nitrogen adsorption–desorption isotherms of the samples: (D)

adsorption and (~) desorption.

The temperature range of the intensive decomposition-

reduction is lower for the precursor Ni-D than Ni-W at all

heating rates. However, both precursors are reduced at a

considerably lower temperature at the heating rate of

2 8C min�1. The use the lowest heating rate in the activation

of the precursors by reduction has to be preferred.

Fig. 7. Pore size distribution of the samples.

M. Gabrovska et al. / Applied Catalysis A: General 299 (2006) 73–8380

Fig. 8. DTG profiles of the samples obtained at the heating rate of 2 8C min�1:

(a) Ni-D; (b) Ni-W; and (c) BNC.

Fig. 9. XRD patterns of the reduced precursors: (a) RNi-D and (b) RNi-W.

Table 4

The results from the measurements of H2 chemisorption

Precursor SNi ðm2Ni g

�1cat Þ SspNi ðm2

Ni g�1Ni Þ DNi (nm)

Ni-D 21.4 51.1 11.0

Ni-W 20.7 49.1 11.4

Basic nickel carbonate is used as a reference compound for

the TG experiments. The DTG profiles of the precursors

together with the profile of BNC determined at the heating rate

of 2 8C min�1 are shown in Fig. 8. The reduction of Ni2+ ions

to Ni0 in the reference compound starts at about 210 8C and

reaches a maximum at 255 8C (Fig. 8c). The DTG profile of the

Ni-D precursor (Fig. 8a) resembles that of the unsupported

basic nickel carbonate. The performance of the Ni-D sample

may be attributed to the reduction of the Ni2+ ions to Ni0

representing weak support interactions. The DTG profile of

the Ni-W precursor (Fig. 8b) demonstrates that the reduction

peak is shifted to a higher temperature, indicating that the

reduction of the Ni2+ ions to Ni0 is more difficult and exhibit

stronger interaction with the support. Consequently, it is clear

that stronger interaction between basic nickel carbonate and

SiO2 of the Ni-W precursor hampers the reduction of the Ni2+

ions to Ni0 due to the formation of nickel phyllosilicate

structures.

3.2.2. Reduced precursors

It is well known that the active phase in the vegetable oil

catalytic hydrogenation reaction is metallic nickel. The X-ray

diffraction patterns of the reduced and passivated precursors are

examined to determine the state of nickel in the reduced

materials. The XRD phase analysis shows that both reduced

samples exhibit similar reflections and consist of following

phases: metallic nickel, quartz and nickel silicate. However,

they are in different proportions (Fig. 9). The diffractogram of

RNi-D shows well-resolved reflections mainly of crystallized

metal nickel phase at d = 0.2036 and 0.1758 nm (JCPDS file

#4-850) and a reflection corresponding to quartz at

d = 0.3354 nm (JCPDS file #46-1045) (Fig. 9a). The XRD

patterns of RNi-W register one reflection characteristic to the

metal nickel phase at d = 0.2045 nm and a small signal

corresponding to the quartz phase (Fig. 9b). All reflections of

RNi-Wmaterial are broader and with lower intensity compared

to the RNi-D precursor. These results are in accordancewith the

total specific surface area of the reduced materials. The sample

RNi-W possesses a surface area of 132 m2 g�1 while the RNi-D

one – 11 m2 g�1.

The SEM images of both precursors demonstrate visible

changes in their morphology after reduction and passivating

(Fig. 3). The samples undergo significant packing in result of

water and CO2 removing. The large aggregates are covered

with small particles of similar size and shape in both reduced

precursors.

These results are confirmed by the hydrogen chemisorption

experiments after reduction of the samples at 430 8C. The tworeduced precursors demonstrate similar values of the metallic

nickel surface (SNi), specific nickel surface (SspNi) and nano-

sized nickel particles (DNi) (Table 4).

The observed similarity in the mean particle size of the

reduced precursors suggests identical dispersion of the metal

nickel crystallites on the surface. It may be concluded that the

reduction procedure of the precursors eliminates the differences

between the natural and synthetic siliceous supports.

M. Gabrovska et al. / Applied Catalysis A: General 299 (2006) 73–83 81

3.3. Catalytic activity and selectivity

The hydrogenation of soybean oil is represented by the

variation of the fatty acids content as a function of time

(Figs. 10 and 11). The data shows that the hydrogenation of

linolenic (C18:3) and linoleic (C18:2) acids on both catalysts

turns them almost entirely into the oleic acid (C18:1). Stearic

acid (C18:0) remains below 12 wt.%. If it exceeds this limit, the

edible oils acquire unpleasant taste. The final content of the

linoleic (C18:2) acid is less than 5 wt.%, which that is in

agreement with the general requirements for edible hydro-

genated oils. This low content also prevents the air oxidation

process. On the other hand, the linoleic acid is one of the

‘‘essential’’ fatty acids, e.g. its presence is necessary for human

nutrition. Therefore, small amounts of it are allowed in the

edible hydrogenated oils.

It may be concluded that both catalysts manifest similar

activity and the hydrogenated products fulfill all requirements

for the conventional partly hydrogenated edible oils.

Vegetable oil hydrogenation includes a series of side

processes (like the geometrical and position isomerization

reactions). However, evaluation of the catalyst selectivity by

first approximation indicates a reaction mechanism composed

by a series of consecutive reduction steps:

C3�!k3 C2�!k2 C1�!k1 C0;

Fig. 10. The fatty acids content as a function of time during the soybean

hydrogenation in the presence of the catalysts Ni-D, where (&) C18:0; (*)

C18:1; (D) C18:2; and (^) C18:3.

where C3, C2, C1 and C0 are the concentrations of linolenic,

linoleic, oleic and stearic acids, and k1, k2 and k3 are the

corresponding rate constants.

The rate constants k1, k2 and k3 are the cumulative constants

as previously emphasized [33]. All the geometric and positional

isomerization processes have been neglected. Solution of the

differential equations system, determining the rate of the above

process, may be explicitly expressed as the time (t) function of

all the system components concentration:

C3 ¼ A3 expð�k3tÞ (1)

C2 ¼�A2þ A3

k3k3 � k2

�expð�k2tÞ þ A3

k3k2 � k3

expð�k3tÞ

(2)

C1 ¼�A1þ A2

k2k2 � k1

þ A3k2k3

ðk2 � k1Þðk3 � k1Þ

�expð�k1tÞ

þ k2k1 � k2

�A2þ A3

k3k3 � k2

�expð�k2tÞ

þ A3k2k3

ðk1 � k3Þðk2 � k3Þexpð�k3tÞ

(3)

Fig. 11. The fatty acids content as a function of time during the soybean

hydrogenation in the presence of the catalysts Ni-W, where (&) C18:0; (*)

C18:1; (D) C18:2; and (^) C18:3.

M. Gabrovska et al. / Applied Catalysis A: General 299 (2006) 73–8382

Table 5

The rate constants and the selectivity of the studied catalysts

Catalyst Rate constants Selectivity

k1 � 10�4

(min�1)

k2 � 10�2

(min�1)

k3 � 10�2

(min�1)

SLn SLo

Ni-D 1.97 1.17 2.93 2.5 59.4

Ni-W 2.06 1.21 2.90 2.4 58.6

C ¼ C3t¼0 þC2t¼0 þC1t¼0 þC0t¼0 �C3 � C2 � C1 (4)

Relation (4) is actually representing the law of substance

preservation within this model. Since, based on the relation

(4), the system components are not all linearly independent, it is

sufficient to find the equations (1)–(3) by integrating, with the

fourth component being a linear combination of the first three.

The selectivity towards linolenic acid (C18:3) is denoted

SLn and it represents the relation k3/k2, while the selectivity

towards linoleic acid (C18:2) is denoted SLo = k2/k1. The rate

constants of the studied catalysts are determined by a numeric

minimization (Neadler–Mead method) of the square of

corresponding functions deviation from the experimental

values by subsequent fitting method [33]. The calculated

values of the rate constants are represented in Table 5.

It must be emphasized that the calculated selectivity,

regarding the mono- and polyunsaturated fatty acid hydro-

genation, is similar for both catalysts and corresponds to the

published data [34–40].

4. Conclusions

The use of siliceous supports considerably diverse in origin,

preliminary thermal treatments and morphology, lead to

preparation of precursors, also differing in morphology, texture

and structural properties, in spite of their identical composition.

The preparation procedure causes the formation of

precursors with small particles and poorly crystallized nickel

silicate hydroxide and antigorite-like structures. The establish-

ment of nickel hydrosilicate-type linkages is a precondition for

a highly porous material. This phenomenon is more pro-

nounced in the precursor supported on water glass, where more

developed mesopore region appears.

The reduction procedure eliminates the differences between

the natural and synthetic siliceous supports: both reduced

precursors demonstrate similar values of the metallic nickel

surface, specific nickel surface and nano-sized nickel particles.

Both catalysts manifest similar activity and selectivity under

the chosen reaction parameters when the observed differences

on the catalyst performance become negligible.

The advantage of water glass use as support is the lower

economic cost of the catalyst, as the procedures of mechanical,

chemical and thermal treatment of the crude diatomite may be

eliminated. The present study claims that water glass is a

suitable silica-containing support for the edible oil hydrogena-

tion catalyst.

Acknowledgements

The authors (M.G. and R.K.) are grateful to the National

Science Fund at the Ministry of Education and Science of

Bulgaria, for the partial financial support (Project X-1411). The

authors (J.K., M.S. and D.J.) greatly acknowledge the support

of the Serbian Ministry of Science and Environmental

Protection through the Project—TR 6712 B.

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