Glycothermal synthesis and catalytic properties of nanosized Zn 1-x Co x Al2O4 (x = 0, 0.5, 1.0) spinels in phenol methylation

Article abstract of DOI:10.1007/s10562-011-0546-x

Zn_1-x Co _x Al₂O₄ (x = 0, 0.5, 1.0) spinels were synthesised under microwave-assisted solvothermal conditions using 1,4-butanediol as reaction medium. The results of XRD and HRTEM have indicated nanocrystallinity of prepared materials (average crystallite size in the range 6-16 nm), and N₂ adsorption-desorption measurements have revealed high feature of their surface area and mesoporous nature. Acid-base properties of prepared materials were determined using TPD-NH₃ method and cyclohexanol test. All studied spinels that underwent examination were active in phenol methylation but Co-substituted zinc aluminate (Zn _0.5Co_0.5Al₂O₄) was found to be the most efficient in the selective formation of ortho-methylated products. It could be ascribed to its dominant basic character and the presence of weak and strong acid centres, and the highest surface area as well. Graphical Abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-011-0546-x

Catal Lett (2011) 141:592–601
DOI 10.1007/s10562-011-0546-x
Glycothermal Synthesis and Catalytic Properties of Nanosized
Zn Co Al O (x 5 0, 0.5, 1.0) Spinels in Phenol Methylation
1
2x
x
2 4
Wiktoria Walerczyk
Hanna Grabowska
•
Mirosław Zawadzki
•
Received: 23 June 2010 / Accepted: 4 January 2011 / Published online: 15 January 2011
Ó The Author(s) 2011. This article is published with open access at Springerlink.com
Abstract Zn1-xCo Al O (x = 0, 0.5, 1.0) spinels were
x
occupy both octahedral and tetrahedral sites, depending on
2 4
synthesised under microwave-assisted solvothermal con-
ditions using 1,4-butanediol as reaction medium. The
results of XRD and HRTEM have indicated nanocrystal-
linity of prepared materials (average crystallite size in the
the degree of inversion. Moreover, so called random spi-
nels are also possible in which there is statistical distribu-
tion of ions between both sites. The physico-chemical
properties of spinels depend mainly on cations distribution
and their nature which may be changed or modified by the
introduction of the third metal to the spinel structure.
Various substitutions may be incorporated into it, so ade-
quate selection of substituting ion and appropriate chemical
composition are of the utmost importance in order to
achieve desired properties of material, including its cata-
lytic performances. Synthesis method is also a significant
factor influencing the properties of the obtained material
such as crystallite size, specific surface area or porosity
[1–3].
range 6–16 nm), and N adsorption–desorption measure-
2
ments have revealed high feature of their surface area and
mesoporous nature. Acid–base properties of prepared
materials were determined using TPD-NH method and
3
cyclohexanol test. All studied spinels that underwent
examination were active in phenol methylation but
Co-substituted zinc aluminate (Zn0.5Co0.5Al O ) was found
2
4
to be the most efficient in the selective formation of ortho-
methylated products. It could be ascribed to its dominant
basic character and the presence of weak and strong acid
centres, and the highest surface area as well.
Common spinel compounds known as aluminate family,
including ZnAl O , have been already studied as catalysts
2
4
Keywords Aluminate spinels Á Glycothermal method Á
and catalysts’ carriers because of their combination of
desirable properties such as high mechanical resistance,
high thermal stability, low sintering temperature, low sur-
face acidity and high ability of cations diffusion. Therefore,
they are mainly used as high temperature ceramics, but also
as useful material for optical and catalytic applications. In
our earlier studies we elaborated the high active aluminate
catalysts, which enable obtaining methylated phenols [4],
synthesis of 1-methylimidazole [5], and efficient methyla-
tion of 2- and 4-hydroxypyridine [6, 7].
Phenol methylation
1
Introduction
Spinels are mixed oxides with general formula AB O ,
2
4
where A represents a divalent metal cation that usually
occupies a tetrahedral site while B represents trivalent
metal cation that normally occupies the octahedral sites of
2
?
a cubic crystal structure. In normal spinel structure A
3
Aluminates may be obtained by many different meth-
ods, e.g. solid state reaction (ceramic, mechanochemical)
method [8] or wet chemical routs such as (co)precipitation
?
and B
occupy 8 tetrahedral and 16 octahedral sites,
3
respectively, while in the inverse structure B ions may
?
[
9], sol–gel [10], solvothermal methods [11] and some
other methodologies like molten salts synthesis [12],
combustion in various solutions [3], polymeric precursors
method [13], etc. Glycothermal method, being one type of
the solvothermal process, offers some advantages over
W. Walerczyk (&) Á M. Zawadzki Á H. Grabowska
Institute of Low Temperature and Structure Research, Polish
Academy of Sciences, Ok o´ lna 2, 50-422 Wrocław, Poland
e-mail: wiktoria.walerczyk@int.pan.wroc.pl
1
23

Zn1-xCo
x 2
Al O
4
(x = 0, 0.5, 1.0) as Phenol Methylation Catalyst
593
most other methods like short reaction time, the possibility
of using various precursors and reaction media. Different
reaction parameters such as pressure, temperature, syn-
thesis time and even heating method can affect the reaction
products [14]. Mixed oxides obtained by glycothermal
synthesis are usually nanocrystalline materials with well-
controlled chemical composition and very interesting tex-
tural properties (high surface area, mesoporosity) [15].
Phenol belongs to compounds characterised by high
reactivity which alkyl derivatives can be obtained during
alkylation with olefins or alcohols under relatively mild
conditions in the liquid as well as gas phase. Usually, the
reaction of phenol with methanol is performed in the gas
phase in the presence of catalyst. Reaction conducted in the
gas phase has many advantages especially when used cat-
alyst is highly active and selective. In the process carried
out in such a way the troublesome separation of reaction
products is eliminated.
sites as the active centres for the ortho-methylation
reactions.
Some of the present authors have studied catalytic
reaction of phenol with methanol leading to ortho-cresol
and 2,6-xylenol in the presence of ZnAl O . It was stated
2
4
that zinc aluminate was an active and selective catalyst in
the alkylation of phenol and other hydroxyarenes [4, 6, 7,
38]. The catalytic properties in methylation of phenol of
zinc aluminate with incorporation of different metals, e.g.
copper and iron were also investigated [39, 40].
The aim of the present work was to study the effect of
2?
2
?
substitution of Zn by other divalent cation, namely Co
,
in the framework of zinc aluminate spinel on phenol con-
version and ortho-methylated products selectivity during
methylation. Zn1-xCo Al O (x = 0, 0.5, 1) spinels, pre-
x
2 4
pared under microwave-assisted glycothermal conditions,
were characterized using XRD, TEM/SEM, N adsorption–
2
desorption, TPD-NH and CHOL test to find a relationship
3
The most important commercial alkylphenols are the
methyl derivatives (the cresols and xylenols); among them,
ortho-cresol and 2,6-xylenol, belong to the most crucial
because of their further applications. They are the most
sought after for the further production of agrochemicals,
antioxidants, plastics, pharmaceuticals, and very useful
polymers, etc. There is always competition between C- and
O-alkylation and the strength of acid centres profoundly
affects the product selectivity [16]. The surface of catalysts
should possess both basic and acidic (mainly Lewis)
properties to secure a high yield of C-methylated phenol
derivatives. The catalyst selectivity is attributed to a pres-
ence of acidic centres possessing weak and medium
strength [17]. For catalysts exhibiting mainly Br o¨ nsted
acidic sites, especially strong, the selectivity towards
C-methylated products decreased with temperature [16]. It
is accepted that only Br o¨ nsted acidity is responsible for
coke formation. Suitable catalysts for the gas phase
methylation of phenol with methanol have been reported in
the literature. Catalysts having basic properties such as
MgO [18–20] or Fe O -based [21–23] produce predomi-
between the structure, acid–base properties and the cata-
lytic behaviour.
2 Experimental
2.1 Samples Preparation
ZnAl O (ZAO), Zn0.5Co0.5Al O (ZCAO) and CoAl O
2 4
2
4
2
4
(CAO) were prepared from the following precursors: zinc
acetate, Zn(CH COO) Á2H O (POCH Poland); cobalt ace-
3
2
2
tate, Co(CH COO) Á2H O (POCH Poland); and aluminium
3
2
2
isopropoxide, Al[OCH(CH ) ] (Alfa Aesar) mixed in
3 2 3
molar ratio 1:0:2, 1:1:4, 0:1:2, respectively. First, appro-
priate amount of zinc and/or cobalt acetates were dissolved
in water, and aluminium isopropoxide was suspended in
99% 1,4-butanediol HO(CH ) OH (Alfa Aesar). Then
2
4
solutions of metals precursors were mixed together and put
in a Teflon vessel placed in an autoclave with microwave
heating (MW Reactor, Ertec, Poland). Synthesis time was
set up to 60 min and reaction was provided at 200 °C under
autogenic pressure of *25 atm. Reaction products were
washed with acetone and centrifuged; this procedure was
repeated several times to remove residues of organic
compounds. After that, products were extruded and
obtained wires were air dried at 70 °C, crushed and then
calcined at 550 and 1100 °C for 3 h.
2
3
nantly ring alkylated products with high ortho-selectivity.
With the increase in the acidity of catalysts, e.g. for Al O
2
3
or zeolites [24–26], the selectivity for C-alkylated products
decreases and products of O-alkylation are obtained.
Among active catalysts in the phenol methylation, mate-
rials with spinel structure are very interesting, e.g. ferrites
[
27–30] and also others [31–33]. Issues concerning the
mechanism of phenol alkylation have been presented in
many publications. Among them the works of Mathew
et al. [28, 34–37] are worth to be quoted, in which the
authors have presented a proposal for the mechanism of
phenol methylation occurring on well-characterised spinel
catalysts. They underlined the role of an acid–base pair
2.2 Characterization Methods
Phase identification of obtained materials, as-prepared and
after heat treatment were analysed using Powder X-Ray
Diffraction. XRD measurements were performed by X’Pert
Pro Panalytical diffractometr with Cu Ka radiation
123

5
94
W. Walerczyk et al.
˚
(
k = 1.5406 A). XRD data and Scherrer equation were
used to average grain size calculation.
catalytic activity in cyclohexanol decomposition were
carried out in a continuous-flow fixed bed quartz reactor
with electric heating. The catalyst (0.1 g) mixed with SiC
Samples morphology and microstructure were analysed
by High Resolution Transmission Electron Microscopy
3
(to 1 cm volume) was held on quartz wool. During reac-
3
tion total gas flow was set on 10 dm /h (dry N 99.999%
(
HRTEM) and Selected Area Electron Diffraction (SAED).
2
HRTEM images and SAED patterns were recorded on
Philips CM 20 SuperTwin microscope operated at 200 kV
with 0.25 nm resolution. Studied materials were grinded in
mortar, suspended in methanol, and dispersed ultrasoni-
cally. One drop of suspension was placed on a copper grid
covered with performed carbon and allowing the solvent to
evaporate, then specimen were dried and placed in the
microscope.
saturated with CHOL—2.6 mmol/h) and reaction temper-
ature was fixed at 300 °C. Reaction products were analysed
using gas chromatograph equipped with FID detector.
Total conversion of CHOL and both yields and selectivities
to CHEN and CHON were calculated.
2.3 Catalytic Methylation of Phenol
The morphology of samples was also observed with the
use of Scanning Electron Microscopy (SEM) and elemental
analysis was made by using Energy Dispersive X-ray
Spectroscopy (EDS). The measurements were carried out
on SEM 515 Philips Scanning Electron Microscope
equipped with EDAX.
Alkylation of phenol with methanol was performed in a
continuous manner by a conventional flow method at
atmospheric pressure without added carrier gas. The reac-
tions in the presence of ZAO, ZCAO and CAO were car-
ried out using an electrically heated standard down flow
fixed-bed vertical quartz reactor of 6 mm inner diameter
3
Textural properties including porosity and specific sur-
filled with 3 cm of catalyst (0.6–1.2 mm particle size).
face area were calculated from N adsorption–desorption
2
The space below catalyst bed was filled with quartz wool.
The temperature of the bed was continuously controlled
with a thermocouple placed inside the catalyst bed. A
solution of phenol in methanol with an addition of water
(molar ratio 1:5:1, respectively) was introduced to the
reactor from the top at pre-determined flow rate equal
isotherms obtained at liquid nitrogen temperature. The
measurements were carried out on a Autosorb-1 Quanta-
chrome Instruments automated system. Prior to measure-
ments samples were degassed under vacuum at 200 °C for
2
h. The adsorption data were interpreted by the applica-
3
-1
tion of the conventional Brunauer–Emmet–Teller (BET)
method for the determination the specific surface area
SBET. The Barret–Joyner–Halenda (BJH) method was
employed for pore size distribution calculations. The mean
pore size (D) corresponds to maximum of pore size dis-
tribution curve.
1.5 cm /h (load 0.5 h ) by a micro-feed syringe pump.
Water was added to the aromatic–alcohol mixture of
reactants to prolong the activity of the catalysts during the
reaction of methylation [28, 42–44]. It is suggested that the
addition of water into the substrates mixture lowers coke
deposition on the catalyst surface and the share of side
reactions is decreased as well [43]. The reaction conditions
were reducing, since the optimal methanol to phenol ratio
used was 5, far exceeding the nominal value of 2 from
reaction of phenol with methanol stoichiometry leading to
synthesis of 2,6-xylenol. A large excess of methanol is
used to secure high conversion of phenol and due to its lost
by decomposition during reaction of methylation and is
necessary due to weak interaction of MeOH to the catalyst
surface [36]. The alkylation reactions were studied con-
tinuously as a function of increasing temperature from 220
up to 360 °C. Before the proper experiments catalysts were
activated during 2 h in the flow of the reactants mixture at
temperature up to 200 °C. The experiments were carried
out for 1–2 h under steady conditions at chosen tempera-
tures. The reactant mixture was evaporated and preheated
to the reaction temperature in the upper part of the reactor
before it entered the catalyst bed. Liquid reaction products,
along with unreacted substrates, were collected in a trap
below the catalytic bed and analysed by a HP 6890 gas
chromatograph equipped with a FID detector and a capil-
lary column (30 m 9 0.32 mm 9 0.25 lm) filled with 5%
The total surface acidity and the acid strength distribu-
tion were investigated by the Temperature-Programmed
Desorption of ammonia (TPD-NH ) method. Before the
3
measurements, the samples (2 g) were firstly heated from
room temperature to 550 °C in argon flow for 1 h and next
cooled to 180 °C. After cooling the samples adsorption of
pure ammonia was performed for 0.5 h followed by a
purge with argon at 180 °C for 1 h to remove physically
adsorbed NH . Finally, TPD-NH measurements were
3
3
started in argon with a heating rate of 10 deg/min.
Amounts of desorbed ammonia were analysed using a gas
chromatograph with a TCD detector.
Reaction of cyclohexanol (CHOL) decomposition was
chosen to determine acid–basic properties of obtained
materials. Both acid and basic centres are responsive for
cyclohexanol dehydrogenation to cyclohexanone (CHON),
whereas cyclohexanol reacts on acid centres to cyclohex-
ene (CHEN). Literature data suggest that decomposition to
CHEN is the measure of the surface’s acid strength while
CHON/CHEN selectivity ratio gives information about
basic nature of materials [41]. The measurements of
1
23

Zn1-xCo
x 2
Al O
4
(x = 0, 0.5, 1.0) as Phenol Methylation Catalyst
595
phenyl methyl silicone. Helium was used as a carrier gas.
The identification of products was carried out by the
comparison of their retention times to the known standards.
The conversion of phenol was defined as a fraction of
reacted phenol, whereas the selectivity was obtained on the
basis of the percentage of the given alkyl phenols (ortho-
cresol and 2,6-xylenol) in the liquid reaction products. The
total ortho-selectivity was defined as a sum of the selec-
tivities towards ortho-cresol and 2,6-xylenol. Gases formed
during reaction, such as CO, CO , CH and H were not
Table 1 Structural and textural properties of studied spinels
Sample
ZAO
550
ZCAO
550
CAO
550
T
calc. (°C)
1100
21
1100
37
1100
44
av
d
XRD (nm)
6
6
16
Latice parameters
˚
a (A)
8.0799 8.0795 8.0812 8.086
90 90 90 90
527.49 527.77 527.75 530.60 527.93 531.67
8.0821 8.1012
a (°)
90 90
3
V (A )
˚
2
2
4
2
S
BET (m /g)
107
40
152
0.32
6.6
18
113
0.26
7.8
18
analysed.
3
BJH (cm /g) 0.19
V
0.08
8.1
0.12
17.3
0.12
12.2
D
pores (nm)
6.5
3
Results and Discussion
.1 Characterisation Results
sizes of samples heated at 550 °C amounted 6 nm for ZAO
as well as for ZCAO, and 16 nm for CAO sample evi-
dencing the nanometer character of the prepared spinels.
After heat treatment at 1100 °C, there are greater differ-
ences between average crystallite sizes (21, 37, 44 nm for
ZAO, ZCAO and CAO, respectively), which indicate better
thermal stability of ZAO sample. Analysis of XRD profiles
of studied samples suggests also different crystallite size
distribution for CAO sample than for others. In this case,
diffraction peaks could be approximate by Lorentz profile
what is characteristic for materials with a wide distribution
in the size of crystallites. The data included in Table 1 also
show that all prepared spinels had characteristic cubic
structure but an increase of lattice parameters (unit-cell
parameter and volume) due to Co incorporation into zinc
aluminate lattice was noticed for ZCAO and CAO samples.
3
XRD measurements confirm spinel structure of samples
heated at 550 (Fig. 1) and 1100 °C (not shown). XRD
patterns of ZAO and ZCAO samples exhibit peaks char-
acteristic only for spinel structure of ZnAl O4 (JCPD
2
0
5-0669). Data obtained from measurement of CAO sam-
ple indicate CoAl O (JCPD 38-0814) and a small amount
2
4
of the second phase presence. Additional diffraction peak
at 2H = 42.4°, unmatched to spinel structure of CoAl O ,
2
4
is very weak suggesting low content of this phase. It could
originate from (200) lattice plane of cubic system of CoO
(
JCPD 43-1004) but there is no other evidence on this
thesis because HRTEM images and SAED patterns did not
further confirm its presence. Scherrer equation and XRD
2
?
measurements were being used to calculated average
av
Considering the ionic radii for 4-coordinated Zn
2?
(0.060 nm) and Co (0.058 nm) as well as 6-coordinated
crystallite size (d
) and lattice parameters, and the
XRD
3
?
3?
results are presented in Table 1. The average crystalllite
Al (0.0535 nm) and Co (0.065 nm), one can expect
the Co substitution for tetrahedral sites resulting in
decrease of lattice parameters. However, the observed lat-
tice expansion (more pronounced for samples heated at
2
?
1
100 °C) indicates that some amount of Co should be in
octahedral sites leading to considerable level of inversion
of spinel structure. Such trend of ZnAl O lattice expan-
2
4
sion due to Co doping has already been reported in the
literature by Popovi c´ et al. [45]. Thought, the general
IV
formula for ZCAO sample can be written as [Zn0.5Co0.5-
VI
d] [Al2-dCo ]O , where d is the inversion parameter of the
spinel structure.
d
4
Representative micrographs of HRTEM images of ZAO,
ZCAO and CAO samples are shown in Fig. 2a, b and c,
respectively. For ZAO sample, there are well visible small
av
TEM
particles with narrow size range of crystallite where d
amounted 4.1 nm. ZAO particles have rather spherical
shape and lattice fringes with distance about 0.29 nm
characteristic for (220) lattice plane of zinc aluminate
spinel structure. SAED patterns obtained from this sample
show broadening of diffraction rings indicating low
Fig. 1 XRD patterns of spinels prepared by the glycothermal
method: (a) ZAO, (b) ZCAO, (c) CAO, and JCPDS data for
(
2 4 2 4
d) ZnAl O (No. 05-0699) and for (e) CoAl O (No. 38-0814)
123

5
96
W. Walerczyk et al.
crystallinity. Diffraction rings with lattice distances 0.291,
0
.244, 0.203, 0.158 and 0.143 nm could be assigned to
220), (311), (400), (511) and (440) lattice plane of cubic
ZnAl O structure, respectively. HRTEM images of ZCAO
(
2
4
are similar to the ZAO one. Small spherical crystallites of
size in the range 4–8 nm were noticed. SAED patterns of
ZCAO sample also present broad diffraction rings with
lattice distance characteristic only for ZnAl O without
2
4
impurities. The morphology of CAO sample is less uni-
form, particles with different shape and size are visible but
most of them are quasispherical with diameter in the range
av
7–18 nm and d
amounts of 8.5 nm. The presence of
TEM
sliver crystals with 30 nm of the length, 3 nm of the
breadth and lattice fringes could be noticed on some ima-
ges of CAO sample which might be assigned to Co O
3
4
(JCPDS 42–1467) as well as to CoAl O structure. SAED
2 4
patterns of CAO sample contain only rings corresponding
to the cobalt aluminate with well visible individual spots
attributed to large crystals. It confirms spinel structure of
CAO sample and higher crystallinity of this material in
comparison to ZAO and CAO sample. HRTEM images
recorded on the samples heated at 1100 °C (not shown)
reveal significant increase in particle sizes (corresponding
particle size distributions become broader) but nanocrys-
talline nature of the samples is still preserved. SAED
measurement has shown that particles are well crystallized
and diffraction rings match the corresponding XRD peaks.
SEM images (not shown) reveal that there are no sig-
nificant differences between prepared spinel samples, and
all of them consist of characteristic agglomerates of
irregular shape. For all spinels, EDS analysis confirmed
that atoms content is in a good agreement with the
scheduled one.
Taking into account the nitrogen adsorption–desorption
isotherms, displayed in Fig. 3, and the calculated values of
both the specific surface area SBET and the pore diameter
D, listed in Table 1, one can see that all prepared spinels
are typical mesoporous materials. The isotherms are of type
IV and show the hysterisis loop of H1/H2 type what is
characteristic for mesoporous materials, according to
IUPAC classification. Such a shape of isotherms is typical
for materials consisting of particles crossed by nearly
cylindrical channels or spheroidal particles formed in
agglomerates or aggregates, in which pores can have uni-
form (type H1) or nonuniform size and shape (type H2). It
should be noted that the nitrogen isotherms show a sharp
increase of adsorbed nitrogen volume at high partial pres-
sures (P/P * 0.6) which corresponds to a large mesopore
o
volume with a relatively narrow distribution of pore
Fig. 2 HRTEM images of (a) ZAO, (b) ZCAO and (c) CAO spinels
with SAED patterns shown as insets
diameters in the range 2–20 nm with a maximum at
*
6–8 nm. This type of sorption properties has been pre-
1
23

Zn1-xCo
x 2
Al O
4
(x = 0, 0.5, 1.0) as Phenol Methylation Catalyst
597
(Fig. 4). The highest acidity exhibits ZCAO (1.2 mmol
NH /g) while ZAO and CAO samples show much lower
3
and comparable total acidity, respectively 0.73 and
0.62 mmol NH /g. The acid strength distribution shows
3
that ZCAO sample has higher part of weak (Tdes \ 300 °C)
and strong acid sites (Tdes [ 500 °C) than ZAO and CAO,
but lower part of the medium ones.
CHOL test specifies the acid–base properties of the
studied materials. Results of CHOL decomposition are
collected in Table 2. Tests of CHOL conversion show that
ZCAO sample is the most active among examined mate-
rials exhibiting also the highest selectivity to CHON and
the lowest selectivity to CHEN what indicates on dominate
basic character of this sample. The lower conversion of
cyclohexanol is observed for CAO sample. Selectivity to
CHEN for ZAO and CAO is similar and confirms results
Fig. 3 N
c) CAO spinels with corresponding pore size distributions (as an
inset)
2
adsorption–desorption isotherms of (a) ZAO, (b) ZCAO,
(
obtained from TPD-NH . However, it should be noticed
3
that the selectivity to CHON indicates rather basic char-
acter of the CAO surface and rather equal content of acid
and basic centres in the case of ZAO sample. Differences
in results obtained in CHOL conversions between ZCAO
and the other studied samples are probably due to the
changing of spinel surface caused by the partial substitu-
tion of Zn by Co ions. It is well known that the acid–base
properties of spinel-type compounds depend on the extent
of polarity of the metal cation and the oxygen anion,
respectively. Besides, Jacobs et al. [48] have previously
revealed that octahedral sites are exposed almost exclu-
sively at the spinel surface: for ZnAl O and CoAl O non
viously reported for nanosized crystalline or amorphous
materials and was attributed to interparticle mesoporosity
[
46, 47]. The data listed in Table 1 show that ZCAO
sample exhibits the higher surface area and pore volume
than others, which could be a result of the numerous sur-
face defects caused by substitution Zn by Co ions. Heat
treatment at 1100 °C leads to decrease of specific surface
area and increase of pore diameter for all examined sam-
ples but ZAO maintained significant value of SBET, i.e.
2
0 m /g, and D was changed a little. N adsorption–
4
2
2
4
2 4
desorption measurements exhibited that microwave-assis-
ted glycothermal method is useful to prepare spinels with
appropriate textural properties for materials used as cata-
lysts or catalyst supports.
or only a very small amount of Zn and Co, respectively,
were detected by LEIS at the spinel surface. It may sug-
gests dominant role of octahedral sites in determining the
2?
surface acidity/basicity. Supposing that only a part of Co
2?
Total acidity and strength distribution of acid sites were
ions substitute for Zn
on tetrahedral sites, and the
3?
remaining cobalt ions substitutes for Al on octahedral
estimated from measurements of NH thermodesorption
3
3
?
sites (simultaneously removing Al ions to tetrahedral
sites) it would be expected that the nature of active centres
of ZCAO should be different. Moreover, for zinc alumi-
nate substituted by other transition metal ions, e.g. for
2
?
Zn1-xMn Al O , the oxidative transfer of Mn in the tet-
x
2 4
3
?
rahedrally coordinated sites to Mn in octahedral positions
takes place what is reflected by its catalytic activity [48].
Table 2 Results of CHOL decomposition over studied spinels
Sample
ZAO
ZCAO
CAO
a
Conversion of CHOL (%)
b
Selectivity to CHEN (%)
c
Selectivity to CHON (%)
61.5
29.3
68.4
2.3
74.9
5.2
25.8
25.3
74.7
3.0
94.8
18.2
CHON/CHEN
a
Cyclohexanol
b
Cyclohexene
c
Fig. 4 TPD-NH
3
profiles for spinel samples
Cyclohexanone
123

5
98
W. Walerczyk et al.
3
.2 Catalytic Performance
catalysts are illustrated in Fig. 6. Mainly C-methylated
products at ortho positions were obtained, however,
O-alkylation leading to anisole also occurred and its yield
reached the highest value (*26%) when as catalyst CAO
sample was used while the lowest phenol conversion to
anisole was observed over ZAO catalyst. Shape of all
curves indicate that yield of anisole declines when tem-
perature is increased suggesting that anisole is not a side
product but the intermediate which undergoes the rear-
rangement giving ortho-cresol and also phenol as a
demethylation product [49]. As seen in Fig. 6 all catalysts
are active in C-methylation at ortho positions to hydroxyl
group of phenol ring. Parallel with ortho-cresol formation
2,6-xylenol was formed. Zinc aluminate (ZAO) in the
reaction conditions exhibited the lowest conversion to
anisole, on the other hand, it was also the least effective
catalyst for ortho-alkylation at 2- and 2,6-positions. In its
case the maximal value of sum of yields of ortho-cresol
and 2,6-xylenol achieved up to 34% only at 340 °C while
for the CAO catalyst it reached 41% at temperature lower
by 60 degree. The best efficiency, in comparison to other
spinels, was exhibited by ZCAO catalyst which allowed to
obtain about 24% of ortho-cresol at 280 °C and up to 54%
of 2,6-xylenol at 360 °C. It should be noted that in the
presence of ZCAO catalyst C-methylation reaction has a
The course of the phenol methylation on studied catalysts
was examined as a function of temperature under the same
conditions. The influence of the reaction temperature upon
the conversion, main reaction products yields and ortho-
selectivity were investigated. The major methylated reac-
tion products were found to be anisole, ortho-cresol and
2
,6-xylenol. Except for these methyl derivatives, in the
reaction products other compounds were also found. It was
established that in this group traces of meta- and para-
cresols, isomeric xylenols, higher methylated methylphe-
nols, methyl substituted anisole and some amounts of other
compounds, which had not been identified, were also
present.
Figure 5 shows changes in the catalytic activity of the
spinel catalysts with process temperature. One can con-
clude that all studied materials are active in the reaction of
phenol methylation. Conversion of phenol for all catalysts
increases significantly with temperature increasing. The
higher initial activity was obtain for ZCAO and CAO, i.e.
cobalt containing aluminate spinels. These spinels among
tested catalysts exhibited an efficient activity without
decay; the phenol conversion was growing up to 320 °C
reaching in both cases higher level than 95%. After further
increase of reaction temperature the activities of ZCAO
and CAO remained on almost the same level. In the
presence of ZAO catalyst the phenol conversion reached
only about 50% at 340–350 °C, then it was visible lower.
Over all studied catalysts, after stopping the reaction,
cooling down, and starting all over again, almost the same
conversions were attained at the same temperatures, as
previously had been observed.
The yields of main methylated products obtained during
alkylation reaction in relation to temperature over studied
Fig. 5 Comparison of phenol conversions over glycothermally
synthesised spinels in methylation reaction
Fig. 6 Temperature dependence of yields of main methylation
products obtained for ZAO, ZCAO and CAO catalysts
1
23

Zn1-xCo
x 2
Al O
4
(x = 0, 0.5, 1.0) as Phenol Methylation Catalyst
599
consecutive character. First, ortho-cresol was formed and
then 2,6-xylenol as a result of subsequent reaction.
studied catalysts shows that Co-modified zinc aluminate is
the most efficient (active and ortho-selective) in studied
reaction.
Figure 7 summarizes data of ortho-selectivity for phenol
methylation over all the studied catalysts obtained as the
effect of temperature. The selectivity towards ortho-cresol
and 2,6-xylenol are shown and additionally the curves for
total ortho-selectivities were also plotted. It should be
noted that the highest selectivity to both ortho-methylated
products was obtain when ZCAO was used as a catalyst. In
this case the total ortho-selectivity increased with tem-
perature reaching a maximal value (above 62%) at 310 °C
which remained nearly on the same level with further
increases of reaction temperature. It was observed that this
catalyst possessing zinc as well as cobalt cations led to
increase in the rate of consecutive methylation of the ini-
tially formed ortho-cresol, causing higher selectivity
towards 2,6-xylenol. In the case of ZAO catalyst similar
high value of selectivity was obtained, to the ortho-meth-
ylated phenol derivatives, as in the case of ZCAO. The
catalyst with the lowest activity and ortho-selectivity has
established to be a CAO, for which significant decrease in
ortho-selectivity was observed when reaction temperature
was increased. The comparison of catalytic behaviour of
The effect of contact time on phenol conversion and the
main methylation products selectivities was studied in
order to confirm the observed reaction sequence during
alkylation. It was investigated by varying the feed rate over
ZCAO catalyst at 260 °C and the results are presented in
Fig. 8. Anisole and ortho-cresol are primary alkylation
products since both are formed at the onset of the reaction.
As the contact time increases, the selectivity of anisole
decreases while the selectivity of ortho-cresol increases
first, attains the maximum value and then decreases. The
formation of 2,6-xylenol was favored by a large contact
time and may occur by the direct C-alkylation of phenol
via ortho-cresol or from anisole. The shape of the curves in
Fig. 8 indicates that anisole is responsible to a large extent
for the secondary products: at lower contact time, anisole
initially formed undergoes an intramolecular rearrange-
ment and may act as methylation agent for phenol to give
ortho-cresol while at higher contact time, it undergoes
further methylation to give 2,6-xylenol. It has become clear
since the reaction temperature was increased, anisole
selectivity decreased indicating that secondary reactions of
anisole were suppressing. It seems that phenol methylation
over ZCAO, and probably over other studied catalysts, is
possible via both pathways, i.e. direct methylation and
through anisole as the intermediate.
Catalytic properties of materials are determined by the
type and quality of active centres. Acid–base properties of
catalysts’ surface play a crucial role in the reaction of
phenol methylation. These properties of applied catalysts
should be taken into account to explain their activity,
products distribution and selectivity in studied reaction.
Fig. 7 Comparison of selectivities towards formation of ortho-cresol
and 2,6-xylenol and total ortho-selectivity for ZAO, ZCAO and CAO
catalysts
Fig. 8 Contact time dependence of phenol conversion and selectiv-
ities to anisole, ortho-cresol and 2,6-xylenol for ZCAO catalyst at
260 °C
123

6
00
W. Walerczyk et al.
Two independent methods were adopted to evaluate the
acidic and basic properties of studied solids. TPD-NH3
measurements were used to investigate total surface acidity
and the acid strength distribution. The gas phase compet-
itive reaction of CHOL (dehydration/dehydrogenation)
over aluminate catalysts was studied in order to determine
their acid–base character. Dehydration activity of the cat-
alysts could be correlated with their surface acidity while
dehydrogenation with basicity. The results of acidic prop-
in the ortho position. This model of phenol adsorption was
proposed by Tanabe et al. [51] and widely accepted. It was
found by Mathew et al. that methylated phenols had weak
interaction with the surface of spinel-type materials in
comparison with phenol and were highly susceptible to
desorbing from the surface and just could facilitate efficient
methylation on such systems [28]. We observed that acidity
and catalytic activity were the highest when ZCAO as
catalyst had been used.
2
?
2?
erties of catalysts’ surface measured by TPD-NH indi-
3
The substitution of Co for Zn ions creates more
cated that the order of total acidity was: ZCAO [
ZAO [ CAO. ZCAO sample was mainly characterized by
higher part of weak and strong acid sites (at Tdes \ 300
and [ 500 °C, respectively), while ZAO and CAO by
medium centres. The relationship observed for the activity
in CHOL conversion was the same as mentioned above. All
aluminate spinels exhibited good conversions yielding
CHEN as the product of dehydration and CHON obtained
during dehydrogenation. Aluminate spinel containing zinc
and cobalt ions (ZCAO) showed a general increase in
acidity and visible increase in dehydrogenation activity
which meant that it was characterized by the most basic
properties. It was accepted, that CHON/CHEN selectivity
ratio gives information about basic nature of catalysts
surface (see Table 2).
acidic centres, as revealed from the NH desorption and
3
CHOL conversion studies. Stronger acid sites are required
for secondary alkylation to yield 2,6-xylenol, whereas only
weaker acid sites are needed for the formation of primary
alkylation to yield ortho-cresol. The results which we have
obtained show that on all catalysts besides C-methylated
phenol derivatives also anisole (product of O-methylation)
was simultaneously obtained. The last one was intermedi-
ate, because its yield decreased at higher temperatures.
Under the influence of studied catalysts anisole is a reactive
molecule and undergoes the rearrangement, giving ortho-
cresol, and phenol as a demethylation product. The
mechanism of this rearrangement we proposed earlier [49].
It must be stated that further work is still required in order
to understand the real role of the spinel structure and sur-
face acid–base properties on the catalytic performance of
Zn and Co containing aluminates.
It is known that ortho-selectivity of phenol alkylation is
governed by the type of phenol adsorption on the catalyst
surface, depending on its acidic strength. For better results
of ortho-alkylation toward the hydroxyl group of phenol
catalyst used apart from acidic centres should also have
basic ones because during reaction phenol molecule inter-
acts with an acid–base pair sites, dissociatively adsorbing
as a phenolate ion on the acid site while proton interacts
with the adjacent basic site. The dissociated proton from
phenol quite easy activates the molecule of methanol
forming methyl carbocation. It has been reported, among
others, by Mathew et al., that the activation of the reactant
molecule is through its association with acid–base pair
centres on metal oxide; this function plays an important
role in the catalytic activity towards phenol conversion and
desired product selectivity for any alkylation [37, 50]. On
more basic catalysts the phenol molecule adsorbs vertically
to the catalysts surface due to the repulsion between the
4 Conclusions
Microwave-assisted glycothermal method was successfully
used to synthesise nanosized spinel type mixed oxide
containing Al, Zn and/or Co ions. Nanocrystalline spinels
are characterised by high surface area and mesoporous
nature. Acid–base properties measured in the test of CHOL
conversion indicate that Zn0.5Co0.5Al
relatively more basic surface character than others
(ZnAl or CaAl ) what may explain its higher selec-
tivity to ortho-cresol formation. Simultaneously, the high-
est conversion of CHOL and the highest NH desorption
O spinel possesses
2 4
O
O
2 4
2
4
3
suggest that also acid centres are present on the surface of
this samples what was reflected in high phenol conversion
and 2,6-xylenol obtaining. Additionally, adsorption of
phenol on acid sites can provide also to O-alkylation
of phenol, but also could be responsible for ortho-cresol
formation through the rearrangement of adsorbed anisole
[49, 52]. It seems that these specific properties of
2
-
highly nucleophilic O surface anions and the aromatic
ring. This behaviour of phenol molecules gives preference
mainly to an attack of the methyl cation in the ortho-
position, because these positions are the nearest to the
catalyst surface. In this case C-methylated products can be
obtained with high ortho-selectivity. Phenol takes a posi-
tion oblique or parallel to the surface on more acidic cat-
alysts’ surface because acid centres interact with the p-
electrons of the benzene ring of phenolate formed on the
catalyst surface, which gets low probability of substitution
Zn0.5Co0.5Al O lead to the optimal ratio between acidic
2 4
and basic character of this material surface what results in
good catalytic performance for phenol methylation to
ortho-methylated derivatives.
1
23

Zn1-xCo
x 2
Al O
4
(x = 0, 0.5, 1.0) as Phenol Methylation Catalyst
601
Acknowledgments This work is supported by Polish Committee for
Scientific Research (Grant No. N N507 500738). The authors are very
grateful to Mrs. Ludwina Krajczyk for HRTEM studies.
24. Samolada MC, Grigoriadou E, Kiparissides Z, Vasalos IA (1995)
J Catal 152:52
25. Marczewski M, Perot G, Guisnet M (1996) React Kinet Catal Lett
5
7:21
Open Access This article is distributed under the terms of the
Creative Commons Attribution Noncommercial License which per-
mits any noncommercial use, distribution, and reproduction in any
medium, provided the original author(s) and source are credited.
26. Sad ME, Padr o´ CL, Apestegu ´ı a CR (2008) Catal Today
133–135:720
27. Sreekumar K, Sugunan S (2002) J Mol Catal A: Chem 185:259
28. Mathew T, Shiju NR, Sreekumar K, Rao BS, Gopinath CS (2002)
J Catal 210:405
2
3
3
9. Wolska J, Przepiera K, Grabowska H, Przepiera A, Jabło n´ ski M,
Klimkiewicz R (2008) Res Chem Intermed 34:43
0. Ballarini N, Cavani F, Passeri S, Pesaresi L, Lee AF, Wilson K
References
(
2009) Appl Catal A: Gen 366:184
1. Wang Y, Zhou Z, Jia M, Zhu X, Zhang W, Jiang D (2005) Catal
Lett 104:67
32. Reddy AS, Gopinath CS, Chilukuri S (2006) J Catal 243:278
33. Wang Y, Yang P, Liu G, Jia M, Zhang W, Jiang D (2008) Catal
Commun 9:2044
34. L a´ z a´ r K, Mathew T, Kopp a´ ny Z, Megyeri J, Samuel V, Mirajkar
SP, Rao BS, Guczi L (2002) Phys Chem Chem Phys 4:3530
35. Mathew T, Tope BB, Shiju NR, Hedge SG, Rao BS, Gopinath CS
(2002) Phys Chem Chem Phys 4:4260
36. Mathew T, Viayaraj M, Pai S, Tope BB, Hedge SG, Rae BS,
Gopinath CS (2004) J Catal 227:175
37. Mathew T, Shylesh S, Devassy BM, Vijayarai M, Satyanarayana
S, Rao BS, Gopinath CS (2004) Appl Catal A Gen 273:35
38. Grabowska H, Mi s´ ta W, Trawczy n´ ski J, Wrzyszcz J, Zawadzki M
(2001) Appl Catal A: Gen 220:207
39. Mi s´ ta W, Zawadzki M, Grabowska H (2003) Res Chem Intermed
29:137
40. Grabowska H, Mi s´ ta W, Zawadzki M, Wrzyszcz J (2003) Polish J
Chem Technol 5:32
41. Martin D, Duprez D (1997) J Mol Catal A: Chem 118:113
42. Sparks AK (1972) US Patent 3,670,030
43. Frabetti AF (1975) US Patent 4,041,085
44. Leach BE (1980) US Patent 4,227,024
45. Popovi c´ J, Tkal cˇ ec E, Gr zˇ eta B, Kurajica S, Rakvin B (2009) Am
Mineral 94:771
46. Kim SS, Shah J, Pinnavaia TJ (2003) Chem Mater 15:1664
47. Triantafyllidis KS, Lapas AA, Vasalos IA, Liu Y, Wang H,
Pinnavaia TJ (2006) Catal Today 112:33
48. Jacobs J-P, Maltha A, Reintjes JGH, Drimal J, Ponec V,
Brongersma HH (1994) J Catal 147:294
49. Grabowska H, Zawadzki M, Syper L (2004) Appl Catal A: Gen
265:221
50. Kotanigawa T (1974) Bull Chem Soc Jpn 47:950
51. Tanabe K, Nishizaki T (1977) In: Bond GC, Wells PB, Tompkins
FC (eds) Proc 6th Intern Congr Catal Chem, vol 12, London,
p 863
52. Jacobs JM, Parton RF, Boden AM, Jacobs PA (1988) Stud Surf
Sci Catal 41:221
1
2
3
4
5
6
. Valenzuela MA, Bosch P, Aguilar-Rios G, Montoya A, Schifter I
(1997) J Sol–Gel Sci Technol 8:107
. Calbo J, Tena MA, Monr o´ s G, Llusar M, Badenes JA (2006) J
Sol–Gel Sci Technol 38:167
. P a˘ curariu C, Laz a˘ u I, Ecsedi Z, Laz a˘ u R, Barvinschi P, M a˘ rgi-
nean G (2007) J Eur Ceram Soc 27:707
. Grabowska H, Mi s´ ta W, Trawczy n´ ski J, Wrzyszcz J, Zawadzki M
(2001) Res Chem Intermed 27:305
. Grabowska H, Zawadzki M, Syper L, Mi s´ ta W (2005) Appl Catal
A: Gen 292:208
. Grabowska H, Zawadzki M, Syper L (2006) Appl Catal A: Gen
3
14:226
7
8
9
. Grabowska H, Zawadzki M, Syper L (2008) Catal Lett 121:103
. Tavangarian F, Emadi R (2010) J Alloy Compd 489:600
. Zawrah MF, Hamaad H, Meky S (2007) Ceram Int 33:969
1
1
1
0. Wei X, Chen D (2006) Mater Lett 60:823
1. Staszak W, Zawadzki M, Okal J (2010) J Alloy Compd 492:500
2. Ouahdi N, Guillemet S, Demai JJ, Durand B, Er Rakho L,
Moussa R, Samdi A (2005) Mater Lett 59:334
1
1
3. Gama L, Ribeiro MA, Barros BS, Kiminami RHA, Weber IT,
Costa ACFM (2009) J Alloy Compd 483:453
4. Ifrah S, Kaddouri A, Gelin P, Leonard D (2007) C R Chimie
1
5. Inoue M (2004) J Phys-Condens Matter 16:S1291
0:1216
1
1
6. Santacesaria E, Grasso D, Gelosa D, Carr a´ S (1990) Appl Catal
6
4:83
7. Campelo JM, Garcia A, Luna D, Marinas JM, Moreno MS (1988)
Stud Surf Sci Catal 41:249
1
1
8. Rao VV, Chary KVR, Durgakumari V, Narayanan S (1990) Appl
Catal 61:89
1
2
2
9. Choi WC, Kim JS, Lee TH, Woo SI (2000) Catal Today 63:229
0. Cavani F, Maselli L, Passeri S, Lecher JA (2010) J Catal 269:340
1. Kotanigawa T, Yamamoto M, Shimokawa K, Yoshida Y (1971)
Bull Chem Soc Jpn 44:1961
2
2
2. Grabowska H, Kaczmarczyk W, Wrzyszcz J (1989) Appl Catal
4
7:351
3. Grabowska H, Jabło n´ ski J, Mi s´ ta W, Wrzyszcz J (1996) Res
Chem Intermed 22:53
123