Sulfated alumina catalysts: Consequences of sulfate content and source

Article abstract of DOI:10.1007/s00706-005-0374-z

Consequences of the loading level of sulfate ions (3, 6, and 10-wt%) as well as the source of sulfate (H₂SO₄ or (NH ₄)₂SO₄) on the structural, textural, and surface acid-base properties as well as the impacts on catalytic activity towards 2-propanol conversions on γ-Al₂O₃ and on aluminum hydroxide gel is described. Structural investigations of the catalysts by XRD revealed that the sulfation processes do not remarkably affect the γ-phase of alumina irrespective of the sulfate content or source. N ₂-adsorption at 77∈K indicated that sulfated gel catalysts exhibit the highest S _BET areas and, in general, S _BET for all catalysts were found to decrease with the increase of sulfate content, such a decrease is more pronounced for the 10% loaded catalysts. Pyridine adsorption as followed by FTIR indicated that sulfation of alumina increases the strength of its Lewis acid sites and creates Bronsted acidity in the case of highly loaded catalysts. The catalytic decomposition of 2-propanol in the gas phase indicated that, amongst all the catalysts investigated, the 6% loaded ones exhibited 100% activity (2-propanol conversion) and the highest propene (dehydration product) selectivity. Springer-Verlag 2005.

Full text of DOI:10.1007/s00706-005-0374-z

Monatshefte f€ur Chemie 136, 2007–2016 (2005)
DOI 10.1007/s00706-005-0374-z
Sulfated Alumina Catalysts: Consequences
of Sulfate Content and Source
Gamal A. H. Mekhemer, Hussein A. Khalaf, Seham A. A. Mansour,
ꢀ
and Ahmed K. H. Nohman
Chemistry Department, Faculty of Science, Minia University, El-Minia 61519, Egypt
Received January 11, 2005; accepted (revised) April 13, 2005
Published online November 11, 2005 # Springer-Verlag 2005
Summary. Consequences of the loading level of sulfate ions (3, 6, and 10-wt%) as well as the source
of sulfate (H2SO4 or (NH4)2SO4) on the structural, textural, and surface acid–base properties as well as
the impacts on catalytic activity towards 2-propanol conversions on ꢀ-Al2O3 and on aluminum hydro-
xide gel is described. Structural investigations of the catalysts by XRD revealed that the sulfation
processes do not remarkably affect the ꢀ-phase of alumina irrespective of the sulfate content or source.
N2-adsorption at 77 K indicated that sulfated gel catalysts exhibit the highest SBET areas and, in general,
SBET for all catalysts were found to decrease with the increase of sulfate content, such a decrease is
more pronounced for the 10% loaded catalysts. Pyridine adsorption as followed by FTIR indicated that
sulfation of alumina increases the strength of its Lewis acid sites and creates Brønsted acidity in the
case of highly loaded catalysts. The catalytic decomposition of 2-propanol in the gas phase indicated
that, amongst all the catalysts investigated, the 6% loaded ones exhibited 100% activity (2-propanol
conversion) and the highest propene (dehydration product) selectivity.
Keywords. Alumina; Dehydration; Pyridine; Sulfate; 2-Propanol.
Introduction
Because many catalytic materials can not easily be prepared in the form of small
particles or suitably robust high-surface area particles, they may be dispersed as
minute particles on a sturdy high-surface area support, e.g., ꢀ-Al2O3 [1]. It is one
of the transition aluminas most widely applied as catalyst support. This is due to
reasons of commercial as well as preparation availability. Furthermore, it is stable
at relatively high temperatures, mechanically stable, exposing Lewis acid sites,
easily formed with high surface area, and in a variety of pore structures [2].
Sulfated metal oxides (particularly sulfated zirconia) have attracted the atten-
tion of many researchers in the last few years. This is actually due to their higher
activity in reactions for which acid catalysis plays a key role in many important
ꢀ
Corresponding author. E-mail: nohmana@yahoo.com

2008
G. A. H. Mekhemer et al.
catalytic processes in the chemical and petroleum industries. Some metal oxides
doped with sulfate give rise to new solid acid catalysts which have high acidity.
The doping effect of sulphur on specific surface areas and crystallographic phase
transitions are now well established. The use of sulfate as acidity promoting
additives for catalytic aluminas has been reported [3] and this effect has been
suggested to arise at least partly from changes in acidity, and to be twofold [4].
Sulfates play an important role as support stabilizer [5, 6], the stabilizing effect
seems to delay the most undesired phase transition of ꢀ-Al2O3. The stability and
the surface acidity of sulfated catalysts are indeed directly dependent on the
sulfation process as well as on the textural properties of the parent alumina gel
[7]. The introduction of a small amount of sulphur compounds onto metal oxides
enhanced remarkably the acidic properties of the oxides, regardless of the types of
the introduced sulphur compounds, for example (NH4)2SO4, H2S, or H2SO4, pro-
vided that the optimal calcination temperature is applied and that sulphur species
are fully oxidized [8–10]. The structure of the acidic species has been proposed to
be a surface complex which is comprised of a metal cation and sulfate ions with
two covalent S¼O bonds [9]. Transformations of the introduced sulphur com-
pounds to acidic species were also proposed [9, 10]. Most of the previous studies
have applied the same catalyst preparation techniques [11, 12]. Namely alumina
gel formed by the precipitation with ammonium hydroxide was impregnated with a
sulfate source of variable concentrations.
In this work we aimed at exploring the effect of different sulfate loads and
sources on: i) the bulk structural and textural characteristics of ꢀ-alumina (AlO
series) in comparison to aluminium hydroxide (dried gel, AlH series), ii) the surface
acidity of these catalysts as revealed by FTIR spectra of adsorbed pyridine vapour,
and iii) the decomposition of 2-propanol vapour, as a model catalytic reaction, in
order to characterize and evaluate the acidity of the tested sulfated aluminas.
Results and Discussions
Characterization of the Catalysts
Crystalline Phases (X-Ray Diffraction, XRD)
Examination of XRD diffractograms, Figs. 1a and 1b, indicates that the impreg-
nation of alumina with sulfate compounds and then calcination at 870 K for 1 h
doesn’t modify detectably the phase structure of the ꢀ-Al2O3. Inspecting the dif-
fractograms and matching with the relevant ASTM standards helps indicating that
all sulfate modified catalysts take on the ꢀ-alumina structure (ASTM 29–1486).
In fact, the presence of sulfate ions, in such relatively low loading level (ꢁ10%) is
not capable of changing the bulk structure of ꢀ-alumina. It seems, however, to act
as a crystalline phase stabilizer. It is also acceptable to propose that the sulfate ions
are likely to be present as dispersed surface species rather than clusters or multi-
layers at such relatively low loading. Also, XRD revealed that the introduction of
sulfate ions, either from the salt or the acid, onto the AlO series (Fig. 1a) renders it
slightly more crystalline than the AlH series (Fig. 1b), as indicated by the slightly
higher XRD peak intensities of the AlO series.

Sulfated Alumina Catalysts
2009
Fig. 1a. X-Ray powder diffractograms of yxAlO series of catalysts
Fig. 1b. X-Ray powder diffractograms of yxSlH series of catalysts
Nitrogen Adsorption
The nitrogen adsorption–desorption isotherms at 77 K of sulfate modified alumi-
nas in comparison with pure oxide (ꢀ-Al2O3) are illustrated in Figs. 2a and 2b. It
can be concluded that all isotherms belong to type IV according to the BET
classification [13], and the hysteresis loops are of type H3. Textural data for all
the investigated catalysts are compiled in Table 1. The AlH series exhibits the
higher SBET areas as compared to the AlO series. The addition of sulfate, either
from the salt or from the acid, onto AlO and onto AlH results in an SBET decrease
as the loading level of sulfate increases from 3 to 10%. This is most likely
attributable to elimination of available surfaces either external or (probably inter-
nal, noticeable decrease in Vp(total) with the increase in sulfate loading for both

2010
G. A. H. Mekhemer et al.
Fig. 2a. N2 adsorption–desorption isotherms of yxAlO series of catalysts
Fig. 2b. N2 adsorption–desorption isotherms of yxAlH series of catalysts
AlO and AlH series can be perceived, Table 1) upon sulfation. The differences
between SBET and Scum for most catalysts indicate that considerable surfaces are
contained in narrow pores. This is also supported by the higher values of Vp(total)
as compared to Vp(cum). Data cited in Table 1 also indicate that the values of
Vp(cum) decrease with the increase in sulfate content particularly for the 10%
loaded catalysts. The modifications observed for the surface texture were further
predicted by pore analysis using the t-method [14, 15]. The analysis facilitated
detection and differentiation between the micro- and meso-pores for all the sul-
fated catalysts. The close agreement of SBET and St values for most of the cata-
lysts, thereby fulfilling the main criteria [16] for the correct choice of the t-curves
used in the analysis.

Sulfated Alumina Catalysts
2011
Table 1. Texture data of alumina and sulfated alumina catalysts
Catalyst
SBET=m2 ꢄ gꢂ1
Sta=m2 ꢄ gꢂ1
Scum=m2 ꢄ gꢂ1
Pore volume=
cm3 ꢄ gꢂ1
Particle
sized=nm
b
c
Vp(cum)
Vp(total)
AlO
187
190
160
130
185
184
179
226
215
187
250
237
210
193
198
172
134
189
176
172
220
217
179
258
240
201
116
117
103
87
0.18
0.18
0.15
0.13
0.16
0.15
0.14
0.18
0.18
0.15
0.18
0.18
0.15
0.34
0.34
0.30
0.26
0.34
0.33
0.31
0.31
0.29
0.24
0.29
0.28
0.24
33
35
37
50
40
41
39
31
31
40
31
31
32
3SAlO
6SAlO
10SAlO
3xSAlO
6xSAlO
10xSAlO
3SAlH
111
110
103
112
111
91
6SAlH
10SAlH
3xSAlH
6xSAlH
10xSAlH
116
110
94
a
b
Surface area derived from t-method (Refs. [30–32]); cumulative pore volume (Refs. [30–32]);
c
d
total pore volume determined at P=P0 ¼ 0.99 (Refs. [30–32]); particle size derived from Debye
equation (see Ref. [33])
IR Spectroscopy
Hydroxyl Stretching and Sulfate Regions (3900–3300 and 1500–1000 cmꢂ1)
The IR results of AlO (not shown) indicated the presence of four bands at
ꢁꢀ ¼ 3780, 3730, 3690, and 3580 cmꢂ1. The first three bands are due to the isolated
hydroxyl (OH) groups [17]; whereas the fourth weak broad band at ꢁꢀ ꢃ 3580 cmꢂ1
is due to hydrogen bonded OH [18]. Types of alumina OH surface groups have
been extensively investigated and discussed [18–20] and do not need here any
further discussion. The spectrum of 3xSAlO is found to display bands similar to
those found for AlO. The other sulfated catalysts (not shown) were found to be
largely similar amongst themselves. They were found to display a new band at
ꢁꢀ ꢃ 3640 cmꢂ1 – in addition to the bands found for AlO – mostly due to bridged
S–OH groups [9].
The IR spectra of sulfated alumina catalysts in the sulfate region display bands
at 1370 cmꢂ1 for the 3% loaded catalysts. Increased sulfate content enhances the
intensity of the bands and shifts the characteristic ꢁꢀ vibration of S¼O groups
towards higher values (6%: ꢁꢀ ꢃ 1383 and 10%: ꢁꢀ ꢃ 1388 cmꢂ1). These shifts are
mostly owing to formation of new sulfate species in the case of such sulfate loads.
The bands appearing near ꢁꢀ ¼ 1100 cmꢂ1 are not well resolved, undoubtedly due to
overlapping. These results point to exclude bulk-like sulfate species [21].
IR Spectra of Adsorbed Pyridine
The IR spectrum of pyridine (Py) adsorption on AlO is shown to display five
bands at ꢁꢀ ¼ 1613, 1596, 1578, 1490, and 1445 cmꢂ1. These bands are assigned

2012
G. A. H. Mekhemer et al.
to hydrogen bonded and coordinately bonded Py. The band at ꢁꢀ ꢃ 1613 cmꢂ1 (8a Py)
is due to Py coordinatively bound to Lewis acid sites of moderate strength [22].
Some authors [22, 23] assigned such sites to tetrahedral aluminum vacancies; the
band at 1596 cmꢂ1 is due to Py coordinatively bonded to octahedral aluminum
Lewis acid sites [22]. The other bands ꢁꢀ ¼ 1579, 1491, and 1445 cmꢂ1 are due
to Py species coordinated to Lewis acid sites. No bands around 1540–1550 cmꢂ1
due to Brønsted acid sites were observed, this indicates the absence of such sites on
the alumina surface. As to the sulfated samples, the IR spectrum of 3xSAlO is
shown to display a similar spectrum to that of pure alumina, a weak shoulder is
observed only at 1620 cmꢂ1. This band (shoulder) is mostly due to strong Lewis
acid sites. Thus, such sulfate load (3-wt%) is not capable of generating sites
responsible for formation of pyridinium ion species. Increasing the sulfate load
(6 and 10-wt%) leads to generation of Brønsted acid sites (bands at ꢁꢀ ¼ 1543 and
1648 cmꢂ1). Moreover, the spectra of 6xSAlO and 10xSAlO display a band at
ꢁꢀ ꢃ 1626 cmꢂ1 which is assigned to strong Lewis acid sites. Py ring modes (8a)
vibrations (bands at 1626 and 1596 cmꢂ1) indicate the presence of at least two
different Lewis acid sites differing in acidity strength. The IR spectra of Py adsorp-
tion on sulfated AlH series at room temperature are shown in Fig. 3. The spectra
are largely similar to those aforementioned of sulfated AlO series. The difference
lies only in that; the spectrum of 3xSAlH displays a weak band at ꢁꢀ ꢃ 1543 cmꢂ1,
which could be actually assigned to Brønsted acid sites. The adsorption of Py clear-
ly demonstrates that the sulfation of alumina leads to the appearance of two different
types of Lewis acid sites and also to the generation of Brønsted acid sites.
Spectra taken from Py adsorption on pure and sulfated AlH series (yxSAlH)
at 520 K were shown to exhibit a gradual weakness in the intensity of the bands
characteristic of the room-temperature adsorbed species of Py, besides slight shifts
towards higher frequencies. The spectrum of pure alumina displays in addition to
Fig. 3. In situ IR spectra recorded at RT for irreversibly adsorbed Py at RT on yxSAlH series of
catalysts

Sulfated Alumina Catalysts
2013
the bands which appeared at RT, a new band at ꢁꢀ ꢃ 1623 cmꢂ1. This is most
probably due to the presence of strong Lewis acid sites, indicating at least two
different Lewis acid sites. Spectra of 3xSAlH, 6xSAlH, and 10xSAlH display bands
characteristic of 8a mode at ꢁꢀ ¼ 1628 and 1616 cmꢂ1 for 3xSAlH, 1632 and
1620 cmꢂ1 for 6xSAlH, and 1633 and 1621 cmꢂ1 for 10xSAlH. These bands are
generally assigned to Py coordinatively bound to strong Lewis acid sites, at least
more than two different Lewis acid sites are present (due most likely to different
degrees of coordination unsaturation of Al-cations) [23, 24]. The spectra of sul-
phated alumina yxSAlO samples at 520 K (not shown) are quite similar to those
obtained for yxSAlH samples.
2ꢂ
These results indicate that the presence of SO4 species >3-wt% generates
Brønsted acid sites and at least two different (in acidic strength) types of Lewis acid
sites. Accordingly, Lewis acid sites exposed on sulfated samples are stronger than
those exposed on pure alumina. The presence of strong Lewis acid sites on the
surface of sulfated samples is due to the sulfate groups, which are assumed to
withdraw electrons from the metal ion thus enhancing the ability of Al3þ ion sites
to interact directly with Py [25]. On the other hand, Brønsted acid sites may be
generated by the interaction of Lewis acid sites with water molecules.
Catalytic Activity towards 2-Propanol Decomposition
Preliminary experiments for temperature scanning showed that the decomposition
of 2-propanol (2-PrOH) on metal oxide catalysts proceeds through simultaneous
dehydration and dehydrogenation reactions (at ꢅ520 K as reaction temperature).
2-PrOH dehydrates at 520 K producing propene. Therefore, the results of 2-PrOH
decomposition at 520 K on test catalysts are listed in Table 2. Since the alumina
surface is dominated by acid sites (Py adsorption), it is clear that it exhibits higher
Table 2. 2-Propanol and its decomposition products on alumina and sulfated alumina catalysts at
520 K
Catalyst
Conversion=%
Isopropanol
Percentage selectivity
Propene=%
Acetone=%
AlO
79.2
89.1
94.2
95.3
97.6
96.5
95.5
97.9
96.7
95.4
98.2
96.8
95.8
98.4
97.0
5.8
4.7
2.4
3.5
4.5
2.1
3.3
4.6
1.8
3.2
4.2
1.6
3.0
3SAlO
6SAlO
10SAlO
3xSAlO
6xSAlO
10xSAlO
3SAlH
100.0
97.3
92.0
100.0
97.5
90.7
6SAlH
100.0
97.6
10SAlH
3xSAlH
6xSAlH
10xSAlH
92.2
100.0
97.9

2014
G. A. H. Mekhemer et al.
dehydration (propene) selectivity 94.2%, Table 2. All sulfated catalysts exhibit
higher decomposition activity than pure alumina. Also, the propene selectivities
are slightly higher than on pure alumina. The results cited in Table 2 also indicate
that the decomposition of 2-PrOH increases with increasing the sulfate loading
level from 3 to 6-wt%. The 6-wt% loaded catalysts show the highest (100%)
decomposition activity and the highest propene selectivity. These results are in fair
agreement with the results observed by Waqif et al. [26] for the conversion of
methanol on sulfated alumina.
The high 2-PrOH dehydration on alumina is due to the presence of strong
surface Lewis acid sites [26]. Hence, the increase in the dehydration activity of
the sulfated catalysts can be related to the increase of the acidity of Brønsted acid
sites and=or to the increase of the number of Lewis acid sites. This seems to be a
maximum for the 6-wt% loaded catalysts, this series also is shown to withstand
losses in surface area and pore volume better than the 10-wt% loaded catalysts
(Table 1). Hence better dispersion and exposure of active sites can be expected for
the 6-wt% loaded catalysts.
Experimental
Materials
Pure alumina, denoted in the text as AlO, was obtained from alumina gel by calcination at 870 K for
3 h. The gel denoted in the text as AlH, was prepared according to Lippens [27] by a dropwise addition
of a (1:1) solution of NH4OH (AR-grade, Prolabo product) to a 0.1M solution of Al(NO3)3 ꢄ 9H2O
(AR-grade, BDH product).
Two series of sulfated alumina catalysts were obtained by impregnation of AlO and AlH with an
aqueous solution of (NH4)2SO4 or H2SO4; which has been held stirred for 1 h. The impregnating
2-
solutions were adjusted to give 3, 6, and 10-wt% SO4 content. The catalysts were obtained by
calcination of the dried samples at 870 K for 1 h. The amount loaded (6-wt%) was calculated to be
similar to that needed to cover the entire support surface in the case of a homogenous dispersion (area
2
2ꢂ
˚
of SO4 ¼ 25 A ) [28]. Two other sulfate loads were taken; a lower one with 3-wt% and a higher one
with 10-wt% SO42ꢂ. For convenience, the calcination products obtained are designated in the text as
ySAlO, yxSAlO, ySAlH, and yxSAlH; where y is the loading level and x signifies that the impregnating
sulfate solution is H2SO4.
Catalyst Characterization Methods and Techniques
X-Ray Diffraction (XRD)
XRD diffractograms were recorded for all samples using a model JSX-60PA JEOL diffractometer
˚
(Japan) equipped with a source of Ni-filtered, CuKꢂ radiation (ꢃ ¼ 1.5418A). The generator was
operated at 35 kV and 20mA. For identification purposes, diffraction patterns (I=Io) versus d spacing
˚
(A) were matched with the relevant ASTM standards [29].
Nitrogen Adsorption
Full nitrogen adsorption=desorption isotherms at ꢂ196ꢆC were obtained using a NOVA 2000, version
6.10 high-speed gas sorption analyzer (Quantachrome Corporation USA). The calcined samples were
first outgassed at 470 K for 1 h prior to adsorption measurements. Various specific surface areas (SBET
,
Scum, and St) were determined by adopting appropriate methods [30, 31] for the analysis of the N2-
adsorption isotherms. Surface texture, surface area analysis, and porosity estimations were performed
using standard and well-established methods.

Sulfated Alumina Catalysts
2015
Infrared Spectroscopy
IR spectra were obtained at a resolution of 4 cmꢂ1, in the range of 4000–500 cmꢂ1, using a Genisis-II
FT-IR spectrophotometer, Mathson (USA). For pyridine (Py) adsorption on test samples, a wafer of
15–20 mg=cm2 was mounted in a Pyrex vacuum cell fitted with CaF2 windows. The samples were
pretreated at 700 K for 1 h in a stream of O2 followed by evacuation at 700K for 1 h, then cooled to
room temperature to obtain the background IR spectra. Then, 5 Torr Py (1 Torr ¼ 133.32 Pa) were
admitted at various temperatures (300–570 K) for 5 min, degassed for 5 min at each temperature in
order to remove physisorbed Py, and the spectra were then taken at room temperature.
Catalytic Activity (2-Propanol Decomposition)
The catalytic activity experiments for 2-propanol (2-PrOH) decomposition were carried out in a
fluidized bed quartz flow reactor at atmospheric pressure. Portions of 0.2 g of the catalyst were
activated in-situ at 670 K for 1 h in N2. 2-PrOH (Merck, 99.9%) was introduced at a flow rate of
14.8cm3 ꢄ minꢂ1 into the carrier gas flow of N2. The reaction products were analyzed by gas chroma-
tography on a 2 m long 1=800 column packed with 10% Carbowax and Chrom WHP 80=100 using a
model 3400 Varian Gas Chromatograph equipped with a flame ionization detector (FID).
Acknowledgements
We acknowledge with appreciation the GC equipment donation (V-8151=87039; c=o Prof. Mohamed
I. Zaki of Minia University) by the Alexander-von-Humboldt-Foundation, Bonn=Germany.
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