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Applied Catalysis A: General 483 (2014) 41–51

Contents lists available at ScienceDirect

Applied Catalysis A: General

jou rn al hom ep age: www.elsev ier .com/ locate /apcata

urface and catalytic properties of some �-Al2O3 powders

hanh Khoa Phunga, Concepción Herrerab,∗, María Ángeles Larrubiab,ónica García-Diéguezb, Elisabetta Finocchioa, Luis J. Alemanyb, Guido Buscaa,∗

University of Genova, Department of Civil, Chemical and Environmental Engineering, p. J.F. Kennedy 1, 16129 Genova, ItalyDepartamento de Ingeniería Química, Facultad de Ciencias, Universidad de Málaga, Campus Teatinos, E-29071 Málaga, Spain

r t i c l e i n f o

rticle history:eceived 27 March 2014eceived in revised form 17 June 2014ccepted 18 June 2014vailable online 25 June 2014

eywords:

a b s t r a c t

The physicochemical and catalytic properties of some �-Al2O3 samples have been investigated by XRD,SBET, porosity measurements, TEM, chemical analysis (ICP-MS) and IR spectroscopy of the surface OHgroups and of adsorbed CO and pyridine. Catalytic activities in the ethanol dehydration reaction werealso studied in gas phase in a tubular flow reactor from 423 K to 723 K at atmospheric pressure with1.44 h−1 WHSV in nitrogen as well as by ethanol temperature programmed reaction (Ethanol-TPR) in aTG/DTA/MS thermal analyzer. Even small amounts of alkali (300 ppm) modify significantly the spectra in

luminascidity

mpuritiesorphology

thanol dehydrationnfrared spectroscopy

the OH stretching region and kill part of the alumina strongest Lewis acid sites with a clear decrease ofcatalytic activity, while 1% Na completely modifies the surface behaviour. Differences are also observedfor clean samples showing slightly different morphology (lamellar versus bulky). The data showed thatthe catalytic activity of �-Al2O3 is determined by a small number of sites, which are likely located onedges, corners and defects of the crystals more than on plain surfaces.

© 2014 Elsevier B.V. All rights reserved.

. Introduction

Transitional aluminas have very large application in the fieldf catalysis and adsorption technologies. In particular, gammalumina (�-Al2O3) is one of the most important oxide materialssed in heterogeneous catalysis because of its high surface area50–300 m2 g−1) coupled with high thermal stability (up to 873 K1–4]) as well as surface acidity and dispersing activity for surfacepecies [5]. It has been widely used as the support of metal [6] andulphide catalysts [7]. However, pure or “modified” aluminas rep-esent today also important bulk catalysts, such as those of mostatalytic steps of the Claus process [8,9] and those for the synthesisf dimethyl ether from methanol [10].

A large number of investigations have been carried out in ordero study the surface and physicochemical properties of aluminasn relation to their catalytic behaviour [11–13]. However, indus-

rial aluminas may be quite impure, from the point view of bothhe crystal phase composition and the presence of impurities. Inarticular it is well-known that alkali metal impurities modify the

∗ Corresponding authors at: University of Genova, Department of Civil, Chemicalnd Environmental Engineering, p. J.F. Kennedy 1, 16129 Genova, Italy.el.: +39 010 353 6024; fax: +39 010 353 6028.

E-mail addresses: concepcionhd@uma.es (C. Herrera), Guido.Busca@unige.itG. Busca).

ttp://dx.doi.org/10.1016/j.apcata.2014.06.020926-860X/© 2014 Elsevier B.V. All rights reserved.

acido-basicity properties and the reactivity of transitional aluminas[14–16]. The effects of impurities have been studied by differentmethods such as infrared (IR) spectroscopy [16,17], X-ray photo-electron spectroscopy (XPS) [18], 1H MAS and 1H{23Na} doubleresonance NMR [19], carbon dioxide adsorption, followed by FT-IRand temperature-programmed desorption (TPD) [20] and simulta-neous TPD and thermogravimetric (TG) analysis measurements ofalcohol [21]. Most authors report that the total surface acidity ofaluminas decreases as a results of sodium doping [22,23], othersreveal that sodium ions react essentially with the most acidic sur-face hydroxyl groups [18,24,25] or react with the Lewis centers, andeven increase the acidity of the surface [26].

Different �-Al2O3 preparations can also significantly differin terms of particle morphologies. Among the most commonnanocrystal habits, �-Al2O3 can have platelet, fibrous or sphericalshapes [5]. The crystal habit certainly depends on the preparationtechnique: in particular for samples obtained by thermal decom-position of precipitated boehmite, �-AlOOH, the morphology of theoxy-hydroxide precursor, which determines the morphology of thefinal alumina powder, may depend on the amount of alkali presentin the precipitation medium.

In this paper we report the results of our studies on the

physicochemical properties of various �-aluminas, using severaltechniques such as TEM, XRD, FT-IR, ethanol temperature pro-grammed reaction (Ethanol-TPR) as well as catalytic dehydrationof ethanol. The aim of this work is to go deeper on the effect of

4 alysis

ip

2

2

paagtapaiparTpf

P((aabaeV9Ipdi

2

mtuaemmmhtw3icmtwama

3t

2 T.K. Phung et al. / Applied Cat

mpurities and different morphologies on the surface and catalyticroperties of �-Al2O3.

. Experimental

.1. Preparation of catalysts

Three self-prepared alumina catalysts were prepared. Two sam-les denoted as Al2O3 (D) (i.e. dry alumina) and Al2O3 (W) (wetlumina) were prepared from Al(OH)3 (Riedel de Haen) calcinedt 1073 K (10 K min−1) during 2 h in dry and wet air (H2O 1% v/v,enerated by saturation at room temperature), respectively. Nanos-ructured �-Al2O3, denoted as Al2O3 (N), was prepared according to

procedure previously reported, intended to produce nanofibrousarticles [27,28]. An aqueous NaAlO2 (Riedel de Haen) solution wasdded dropwise to a 5 N acetic acid solution. The obtained precip-tate was decanted, filtered and washed with water. The resultingowder was dried overnight at 373 K and subsequently mixed with

non-ionic surfactant, Tergitol (15-TS-5, Sigma), using a Tergitol/Alatio of 0.5. The mixture was kept in an autoclave for 72 h at 373 K.he autoclaved mixture was calcined at 623 K for 2 h and the tem-erature was then increased at 5 K min−1 to 773 K and maintainedor 18 h. After that it was calcined at 1073 K during 2 h in dry air.

Three commercial gamma alumina samples were also used.uralox SCCa-5/200, and Puralox TH 100/150, denoted by Al2O3S) and Al2O3 (TH), respectively, that were treated at 1073 K10 K min−1) in air during 2 h. The original samples, both from Sasol,re prepared from aluminum trialkyls giving rise to aluminum tri-lkoxides that can be hydrolyzed to alcohol and boehmite. Afteroehmite decomposition, high purity �-Al2O3 is obtained. Theyre expected to have similar compositional impurities but differ-nt morphological properties, according to the producers data [29].ersal alumina, denoted as Al2O3 (V), is produced from calcining at73 K for 5 h of UOP-Versal fibrous pseudobeohmite V-250 [30].t was reported that the procedure to prepare alumina Versal wasrecipitation of sodium aluminate, allowing the production of well-efined fibrous pseudoboehmite that was converted by calcination

nto high surface �-Al2O3 [31].

.2. Characterization of samples

The porous structures of the samples were characterized byeans of adsorption/desorption of N2 at 77 K using a Quan-

achrome apparatus (Autosorb-1 model) and mercury porosimetrysing a Carlo Erba mercury porosimeter 4000. The specific surfacerea was obtained by the Brunauer–Emmett–Teller (BET) isothermquation, the pore volume by the Barrett–Joyner–Halenda (BJH)ethod from the desorption branch of the isotherm and t-Plotethod (Harkins and Jura equation) were applied to obtain theicropore volume and internal area [32]. Ca, K and Na contents

ave been determined by Inductively coupled plasma mass spec-rometry (ICP-MS) quadrupole by using a Nexion 300D. Samplesere previously treated in a microwave oven digestor Multiwave

000 from Anton Paar. The TEM (Transmission electron microscopy)mages of powder samples deposited onto a copper mesh gridoated with a carbon film were taken with a Philips CM200 (200 kV)icroscope equipped with an EDX (energy dispersive X-ray) detec-

or. The XRD (X-ray diffraction) patterns of the powder samplesere recorded with a Siemens D-501 goniometer equipped with

Johansson type monochromator with a Ge (1 1 1) crystal. Duringeasurement, Cu K 1 radiation and a fixed power source (45 kV

nd 35 mA) were used.IR (Infrared spectroscopy) spectra were recorded using Nicolet

80 FT-IR (Fourier transform infrared spectroscopy) spectrome-ers. Acidity measurements were done using the pure powders

A: General 483 (2014) 41–51

pressed into thin wafers (∼30 mg, surface area of 1.31 cm2)and activated in the IR cell connected with a conventionaloutgassing/gas-manipulation apparatus at 773 K under vaccum(10−5 Torr). The activated samples were contacted with pyridinevapor (pPy ∼ 1 Torr) at room temperature for 15 min; after whichthe IR spectra of the surface species were collected in continuousevacuation at room temperature with increasing temperature. COadsorption was performed at 130 K (real sample temperature mea-sured by a thermocouple) by the introduction of a known dose of COgas inside the low temperature infrared cell containing the previ-ously activated wafers. The sample was saturated with CO (∼4 Torr)until the intensity of the bands of the adsorbed species has reachedthe maximum. IR spectra were later collected under evacuation atincreasing temperatures between 130 and 273 K.

2.3. Catalytic experiments

Catalytic experiments were performed at atmospheric pressurein a tubular flow reactor (i.d. 6 mm) using 0.2 g catalyst (60–70 meshsieved, to have a ratio between the particle diameter and the inter-nal reactor diameter near 25) and feeding 7.9% v/v ethanol with1.44 h−1 WHSV (weight hourly space velocity) in nitrogen, totalflow rate of 32.25 cm3 min−1. The carrier gas (nitrogen) was passedthrough a bubbler containing ethanol (96%) maintained at con-stant temperature (298 K). The temperature in the experiment wasvaried stepwise from 423 K to 723 K.

Ethanol conversion is defined as usual:

XEtOH =(

nEtOH(in) − nEtOH(out)

)

nEtOH(in)

While selectivity to product i is defined as follows:

Si = ni(vi

(nEtOH(in) − nEtOH(out)

))

where ni is the moles number of compound i, and �i is the ratio ofstoichiometric reaction coefficients.

The outlet gases were analyzed by a gas chromatograph (GC)Agilent 4890 equipped with a Varian capillary column “Molsieve5A/Porabond A Tandem” and TCD (Thermal conductivity detector)and FID (Flame ionization detector) detectors in series. In order toidentify the compounds of the outlet gases, a gas chromatographycoupled with mass spectroscopy (GC–MS) Thermo Scientific withTG-SQC column (15 m × 0.25 mm × 0.25 �m) was used.

Ethanol temperature programmed reaction (Ethanol-TPR) onthe alumina samples was performed in a TG/DTA (thermogravi-metric/differential thermal analysis) thermal analyzer Q600-TAInstrument, at the temperature range of 333–873 K with a heatingrate of 10 K min−1 under 20% ethanol/He atmosphere. The gasesat the outlet of the thermobalance were analyzed by MS (massspectrometry) with a Pfeiffer Prisma TM QM200 Quadrupole MassSpectrometer. The signals of H2O (m/z = 18), ethylene (m/z = 27),ethanol (m/z = 46), propylene (m/z = 42) and diethyl ether (m/z = 74)were followed. The signals were normalized with respect to themaximum intensity of each signal during each experiment. The TPRexperiments were carried out exposing samples to an ethanol/Hestream at 333 K until saturation (maximum ethanol signal reached),then the ethanol was switched off and the sample was heated from333 to 873 K.

3. Results

3.1. Physicochemical characteristics of Al2O3 catalysts

The XRD patterns of the samples are presented in Fig. 1. The pat-terns reveal that the materials consists mainly of �-Al2O3 (peaks at

T.K. Phung et al. / Applied Catalysis

3h�aa((atss

stoccwgioaospchb

TP

NtB

Fig. 1. XRD patterns of fresh alumina samples.

7.5, 39.6, 45.9, 60.9 and 66.9◦ (2�), JCPDS 75-0921) in spite of theigh temperature used in calcination which should more produce-Al2O3. It must however be noted that the difference between �-nd �-polymorphs is associated only to a slight cell deformationnd is not always easily detected. We can remark that sample Nhighly impure, see below) is largely amorphous. For sample Al2O3W), also multiple peaks at 28.2 and 49.0◦ (2�), 38.4 and 44.1◦ (2�)re clearly observed, implying that crystallographic phases otherhan �-Al2O3 are also present, boehmite (JCPDS 74-1895) and gibb-ite (JCPDS 74-1775), respectively, which are components of thetarting material.

The data on surface area and porosity characterization of allamples are summarized in Fig. 2 and Table 1. The nitrogen adsorp-ion/desorption isotherms (Fig. 2a) present features characteristicf the type IV isotherm according to the IUPAC isotherm classifi-ation [33] with hysteresis loop which is associated with capillaryondensation taking place in mesopores. N presents H3 hysteresishich is usually observed with aggregates of plate-like particles

iving rise to slit-shaped pores. D and W exhibit an hysteresis loopntermediate between H2 and H4 due to containing both microp-res and mesopores. TH and V present H1 hysteresis which is oftenssociated with porous materials exhibiting a narrow distributionf relatively uniform (cylindrical-like) pores, while isotherm of Shows H2 hysteresis possibly due to its containing a more complex

ore structure (see Fig. 2b). Fig. 2b shows the pore size distributionurves for the samples. N, D and, to a minor extent, W, exhibit anomogeneous pore size distribution with pore diameters locatedetween 10 and 100 A. S and V exhibit also an homogeneous pore

able 1reparation method, BET surface area, pore volume and pore size of the alumina samples

Surfacearea(m2 g−1)

Microporearea(m2 g−1)

Es(m

Al2O3 (N) Nanostructured,home-made

220 11 2

Al2O3 (D) Dry calcination ofAl(OH)3

130 – 1

Al2O3 (W) Wet calcination ofAl(OH)3

224 13 2

Al2O3 (V) Versal-UOP calcined 221 – 2Al2O3 (TH) TH 100/150

puralox-sasol calcined166 18 1

Al2O3 (S) SCCa-5/200puralox-sasol calcined

184 11 1

2 Isotherm: BET area.-Plot Method: micropore volume, internal and external area (ec. Harkins and Jura).JH: porosity distribution, pore size.

A: General 483 (2014) 41–51 43

size distribution but with pore diameters between 100 and 1000 A.TH presents a wider distribution of pore sizes.

The specific surface areas vary from 130 m2 g−1 for D to224 m2 g−1 for W (Table 1). Among the studied samples, TH exhibitsthe highest pore volume and the highest average pore diameter.The pore size distribution indicates a micro-mesoporous structurefor the samples with an average pore width between 10 and 100 Afor W and D, while larger pores are measured for N, TH, V and S(Table 1). The W and D samples, but specially that denoted with Dexhibit two inflection points in the isotherm, which implies non-uniform pores’ shape and lower porosity (Fig. 2b). The calculatedmean diameter of the pores is also smaller for these two aluminas(Table 1).

Fig. 3 shows the TEM images of the alumina samples. TH sampleshows a quite pronounced lamellar morphology, usually supposedto arise from the topotactic decomposition of (pseudo) boehmites,which have a layered structure and typically show a lamellar habit[34,35]. N and V show a fibrous appearing morphology: the aver-age particle diameter in N sample is less than 2 nm with respectto an average length of 60 nm while the particles in V are bigger,(∼5 × 70–80 nm average size). D, W and S samples, instead, exhibita quite homogeneous globular or spherical morphology.

The metal impurities contents are presented in Table 2. Sodiumcontent of alumina strongly depends on the preparation methodand indeed, home-made alumina obtained from sodium aluminate,N, or by decomposition of Al(OH)3, D and W, presented significantlyhigher sodium content (9600, 1900 and 1500 ppm, respectively)than the other samples.

3.2. Surface acidity characterization by IR spectroscopy ofadsorbed probe molecules

3.2.1. Surface hydroxyl groupsThe IR spectra of alumina samples in the region of the surface

hydroxyl groups, recorded after outgassing at 773 K, are reportedin Fig. 4. The spectra of the surface hydroxyl groups of S, TH and V,are typical of �-Al2O3, showing essentially five bands at ca. 3790,3770, 3725, 3685, 3590 cm−1 [5,11]. In particular, all these spectrashow well evident the band at ca. 3770 cm−1, in agreement with therelatively low Na content [16,17]. In contrast, the spectra of thosesamples characterized by higher Na content, N, D and W do notshow this band well resolved, showing however a peak broader athigher frequency, 3785 cm−1. The sample N, which has the highest

Na and K content, does not show any absorption above 3750 cm−1.Interestingly, the samples which have relevant contents of alkaliimpurities show the strongest peak shifted from near 3725 to near3740 cm−1, and the band at ca. 3685 cm−1 relatively more intense

.

xternalurface

2 g−1)

Porevolume(cm3 g−1)

Microporevolume(cm3 g−1)

Mesoporevolume(cm3 g−1)

Pore size(Å)

09 0.711 0.002 0.708 129

30 0.254 – – 78

11 0.347 0.013 0.333 31

21 0.952 – – 17248 1.198 0.005 1.193 288

73 0.538 0.003 0.535 117

44 T.K. Phung et al. / Applied Catalysis A: General 483 (2014) 41–51

Fig. 2. (A) N2 adsorption/desorption isotherms for alumina samples, and (B) pore size distribution curves of aluminas.

T.K. Phung et al. / Applied Catalysis A: General 483 (2014) 41–51 45

of fres

th

3

opo

TA

SR

Fig. 3. TEM Images

han the other samples. The spectra of samples D and W, whichave similar impurity contents, are similar.

.2.2. CO adsorptionThe spectra of CO adsorbed on Al2O3 at 133–273 K after previous

utgassing activation at 773 K in the region 2040–2240 cm−1, areresented in Fig. 5. Two main bands at ca. 2185 and 2150 cm−1, arebserved after CO adsorption at 133 K. The higher frequency band

able 2lkali metal content on aluminas.

Conc. (ppm) SD %RSD

Al2O3(N)Ca 103.9 4.0 3.9K 2100 0.1 2.9Na 9600 0.9 9.5

Al2O3(D)Ca 84.8 4.9 5.8K 130.8 2.3 1.8Na 1900 0.1 3.2

Al2O3(W)Ca 101.0 4.4 4.4K 238.1 5.7 2.4Na 1500 0.1 4.7

Al2O3 (V)Ca 188.3 0.3 0.1K 345.2 2.8 0.8Na 301.4 2.8 0.9

Al2O3(TH)Ca 76.7 3.8 4.9K 135.9 1.1 0.8Na 26.2 0.7 2.6

Al2O3 (S)Ca 29.3 3.0 10.2K 13.9 0.3 2.3Na 14.0 0.2 1.7

D: standard deviation.SD: Relative standard deviation.

h alumina samples.

is assigned to CO species interacting with Lewis acidic Al3+ cations,while the lower frequency band is attributed to less perturbed CO.

In the case of the “purer” alumina samples, S, TH and V, the for-mation of the lower frequency band at ca. 2150 cm−1 is clearly andreversibly associated to the perturbation of the band of hydroxylgroups, mainly those absorbing near 3725 cm−1, and, to a lesserextent, those absorbing at 3770 and 3790 cm−1 (Fig. 6, left), asalready shown [5]. In contrast, in the case of N sample (Fig. 6,right) this band, which is relatively much stronger, does not cor-respond to a perturbation of the OH band observed at 3754 cm−1

at 133 K, which corresponds to that observed at ca. 3742 cm−1

at room temperature (Fig. 4). This band is not perturbed at all

upon CO adsorption, in parallel to what has previously found onK+-doped alumina [36], where a band fully insensitive to CO adsorp-tion was found at 3720–3712 cm−1. The band located between3754 and 3742 cm−1 on the N samples is consequently assigned

TH

V

S

W

N

D

Abs

orba

nce

3200340036003800Wavenumber s (cm-1)

0.1

37423680

37373686

3737 3689

3785

37853727

3690

3772

3771

3726

3727

3694

3683

3792 3771

Fig. 4. FT-IR spectra of aluminas after activation at 773 K (�OH region).

46 T.K. Phung et al. / Applied Catalysis A: General 483 (2014) 41–51

2120214021602180220022202240

2154

21602190 133 K

133 K173 K153 K

V0.15

2120214021602180220022202240aven

0.1

2182

21882162

2157

TH

133 K133 K153 K173 K

2154

21602185 133 K

0.05

133 K 153 K 173 K

DA

bsor

banc

e2156

2164

2184

0.1N

133 K 133 K 153 K 173 K

133 K

153 K

2155

2188

2179

2162

0.05

133 K

173 K

W

2120 214021602180 22002220 2240-1

2155

2187 2163

0.05

133 K133 K153 K

173 K

S

t line)

t3a

NsbwC

rfTtac2(ibni

Wavenumber s (cm-1) W

Fig. 5. FT-IR spectra of CO adsorbed on aluminas at 133 K (do

o the OH stretching of a Na–OH group [37,38], as the band at720–3712 cm−1 on K+-doped alumina has been assigned to non-cidic KOH groups [36].

On the other hand, during low temperature CO adsorption on (Fig. 6, right), the OH stretching band at 3680 cm−1 is clearly

hifted to ca. 3615 cm−1, in parallel to the formation of the strongand at ca. 2156 cm−1. The band at 3680 cm−1 is thus assigned toeakly acidic Al–OH group, likely of bridging type, interacting withO species responsible for the C O stretching band at 2156 cm−1.

In addition, the lower frequency �CO band is relatively moreesistant to outgassing and shifts by outgassing at slightly higherrequencies (until ca. 2170 cm−1) on N than on “pure” aluminas.his is shown in the right section of Fig. 7, where the spec-ra observed on the samples N and TH after outgassing at 153 Kre compared. On the TH the low frequency �CO band is almostompletely disappeared, the residual maximum is observed at164 cm−1, while on N this band is relatively much more intensein absolute as well as relative to the band at ca. 2190 cm−1) ands at higher frequency (2168 cm−1), shifting further to 2170 cm−1

y further outgassing. This is in favour of assigning a compo-ent of this band (2165–2170 cm−1) to CO adsorbed on Na+

ons, overlapped to the band of CO interacting with hydroxyl

Abs

orba

nce

3300 3400 3500 3600 3700 3800 3900 4000 Wavenumbers (cm-1)

0.02

3735

3690

3568

3792

3612

Al2O3 (V)

Fig. 6. FT-IR spectra of aluminas V (left) and N (right) before

umbers (cm-1) Wavenumb ers (cm )

and evacuated from 133 K to room temperature (solid line).

groups (2155–2165 cm−1). In fact, bands at 2177–2164 cm−1 wereattributed to CO adsorbed on Na+ ions on different Na zeolites aswell as on Na–silica–alumina [39]. In parallel, bands observed at2165–2160 cm−1 on K-zeolites and at ca. 2150 cm−1 on K-Al2O3were assigned to CO adsorbed on K+ ions [36]. Thus the carefulanalysis of the position and the behaviour of this band on aluminaallows to reveal the presence of Na+ impurities on alumina. In fact,looking at the spectra reported in the other samples, it is shownthat the position and the resistance to outgassing of this band arehigher for the samples richer in sodium and lower for the purersamples.

The higher frequency band whose maximum is at ca. 2180 cm−1

at the higher coverage that decreases in intensity by outgassingwhile its maximum shifts progressively to ca. 2200 cm−1, is dueto CO interacting with Al3+ ions. At the lowest coverage this bandreduces to a very weak maximum at 2194 cm−1 for N, 2198 cm−1

for D, 2202 cm−1 for W, and near 2220 cm−1 for V, TH, and S (Fig. 7,left side).

Indeed the spectra of CO adsorbed on TH and S appear simi-

lar to those observed on pure aluminas and discussed elsewhere[5,40,41], showing the presence of Lewis sites with different acid-ity. Instead, the spectra of CO adsorbed on W, D and N samples

3300 3400 3500 3600 3700 3800 Wavenumbers (cm-1)

37543680

36150.02 Al2O3 (N)

(dot line) and after (solid line) adsorbed CO at 133 K.

T.K. Phung et al. / Applied Catalysis A: General 483 (2014) 41–51 47

Abs

orba

nce

2120216022002240venu -1

D

VTHS

W

2194

22192234

N

2168

2190

2164

2193

0.1

2120216022002240

TH

0.01

, 213

diw

3

baiatpnu1raicors

dtit1opalnbtlaitsN

d

high ethylene selectivity at high temperature. However, sampleS is definitely more active than sample V, the former showingalmost total yield to ethylene at 623 K and the latter at 673 K.Ethanol conversion is definitely lower over sample N, which also

1444

1490

157716

22

1448

1593

1613

Abs

orba

nce

140014501500155016001650

N

TH

Wa

Fig. 7. FT-IR outgassing spectra of CO adsorbed at 153 K for D, 173 K for N and W

o not show residual peaks above 2200 cm−1: this indicates thatn these cases the strongest Lewis sites are either absent or

eakened.

.2.3. Pyridine adsorptionThe surface acidity of alumina samples has also been studied

y IR spectroscopy, using pyridine as a probe molecule. The maindsorbed species of pyridine on their surfaces are typically featuresn the region from 1400 to 1700 cm−1: in fact the bands observedt wavenumbers > 1600, 1577, ca. 1490 and 1465-40 cm−1 are dueo the 8a, 8b, 19a and 19b ring vibrational modes of molecularyridine [42,43]. According to most authors [17,44], the compo-ent at ca. 1592 cm−1 is due to pyridine bonded to coordinativelynsaturated octahedral Al cations, while the component at ca.622 cm−1 is due to pyridine bonded to coordinatively unsatu-ated tetrahedral Al cations. The band at 1616–2 cm−1 is due to

species interacting with Al ions having intermediate Lewis acid-ty. Over the TH and S alumina samples, three 8a components ata. 1622, 1616–2 and ca. 1595 cm−1 are observed in the presencef the gas (whose spectrum is subtracted) and after outgassing atoom temperature (r.t.), revealing at least three different adsorbedpecies.

By increasing outgassing temperature, the band at 1592 cm−1

ecreases the intensity and almost disappears at 423 K, whilehe band at 1612 cm−1 shifts up to 1616 cm−1 and attenuates itsntensity slowly. Looking with care, it is quite evident that onhe N sample the intensity of the lower frequency 8a band at ca.592 cm−1 is definitely higher, relatively to the others, than thatbserved on the spectra of all other alumina samples. As for exam-le, in Fig. 8 the spectra of pyridine adsorbed on TH and on Nfter outgassing at 323 K are compared. It is evident that on N theower frequency 8a band at 1593 cm−1 and also the 19b compo-ent at 1444 cm−1 are still definitely strong while on TH they areoth disappeared. The position of both these bands is similar tohose observed for pyridine adsorbed on Na+ cations of NaX zeo-ite [45,46]. It is suggested that actually the absorptions of pyridinedsorbed on Na+ ions superimpose with those of pyridine interact-ng with coordinatively unsaturated octahedral Al3+ ions and thathe higher intensity of these bands and the corresponding higher

tability upon outgassing reveals the interaction of pyridine witha+ ions on N catalyst.

In most catalysts the 8a band at ca. 1593 cm−1 is already fullyisappeared after outgassing at 373 K and shifts to ca. 1598 cm−1

mber s (cm )

K for V and 253 K for TH and S (left), and on N and TH samples at 153 K (right).

during outgassing (Fig. 9), except for N where it is still observedafter outgassing at 373 K without any shift, at 1593 cm−1. A similarresult is observed for V sample, while for the W sample this bandappears to be split into two components at ca. 1598 and 1590 cm−1.

The component at ca. 1622 cm−1, usually observed andattributed to pyridine adsorbed on the strongest Lewis sites,becomes more and more evident in samples TH and S while is muchless in the other samples, if any.

3.3. Catalytic activity in ethanol dehydration

In Table 3 and Fig. 10 the data concerning the catalytic activityof three chosen catalysts, with “high”-, “medium”- and “low”-impurity contents, in ethanol dehydration are summarized. Overall the catalysts, the selectivity to diethyl ether is high at lowtemperature, while the selectivity to ethylene increases further byrising temperature. Very small amounts of ethane (not shows inTable 3) and propene are also formed. Both V and S samples show

Wavenu mber s (cm-1)

Fig. 8. FT-IR spectra of surface species arising from pyridine adsorbed on TH and onN after outgassing at 323 K.

48 T.K. Phung et al. / Applied Catalysis A: General 483 (2014) 41–51

15701580 15901600 16101620 16301640

1623

1596 ← 1591

S

15701580159016001610162016301640

1593V

15701580159016001610162016301640Wavenu mber s (cm-1)

16231598 ← 1593

TH

1593

N

Abs

orba

nce

1597 ← 159 3

D W1591

Fig. 9. FT-IR spectra of surface species arising from pyridine adsorbed on aluminas at room temperature and evacuated from room temperature to 573 K following up todown: adsorption at r.t., outgassing at r.t., after 30 min. evacuation, at 323 K, 473 K and 573 K.

Table 3Ethanol conversion and product selectivities (S) upon ethanol dehydration with WHSV 1.44 h−1 in nitrogen.

Catalyst Temperature (K) TCa Sdiethylether Sethylene Spropene

Al2O3 (N)

423 0.0 – – –473 1.5 100 0 0523 6.2 100 0 0573 10.5 86.2 13.8 0623 33.1 70.8 29.2 0673 63.9 39.2 60.7 0.1723 82.7 11.9 87.2 0.8

Al2O3 (V)

423 0.0 – – –473 0.4 100 0 0523 19.7 98.5 1.5 0573 77.8 86.5 13.5 0623 88.1 8.2 91.7 0.0673 100.0 0.1 99.8 0.1723 100.0 0 99.7 0.3

Al2O3 (S)

423 0.0 – – –473 4.1 100 0 0523 58.6 94.3 5.7 0573 84.5 28.4 71.6 0623 100.0 0.1 99.9 0673 100.0 0 99.9 0.1

F

723 100.0

a

TC = total conversion.

Temperature (K)

400 450 500 550 600 650 700 750

Etha

nol C

onve

rsio

n (%

)

0

20

40

60

80

100

Al2O3 (N)Al2O3 (V)Al2O3 (S)

ig. 10. Conversion of ethanol over aluminas as a function of reaction temperature.

0 99.8 0.2

gives rise to a high amount of diethyl ether even at 723 K.Sample N also produces more propene at high temperaturecompared to the others alumina tested. The apparent activationenergies measured assuming a differential reactor, are above110 kJ mol−1, thus relevant diffusion limitations can be excluded.

Temperature-programmed reaction (TPR) with ethanol on alu-minas were performed in a thermobalance. No change of aluminaphase was observed until 873 K.

From the TG curves and gas MS analyses during ethanol satura-tion as well as ethanol desorption and decomposition, the amountof ethanol adsorbed and ethylene evolved were calculated. Verysmall amounts of diethyl ether appear at temperatures just belowthe formation of ethylene in all cases. These data are compared withthe amount of sodium present in the samples in Table 4. The amountof ethanol adsorbed on N is significantly less than that adsorbed onV and S. Correspondingly, lesser amount of ethylene formation isdetected on N, and this occurs at higher temperature than on V and

S. The amount of ethanol adsorbed and the temperature of ethyleneevolution show a clear relation with the Na content on the catalystsurfaces.

T.K. Phung et al. / Applied Catalysis A: General 483 (2014) 41–51 49

Table 4The amount of sodium impurities, of ethanol adsorbed and ethylene evolved during temperature programmed reactions (TPR) with ethanol on aluminas.

Sample Na impurity (atom nm−2) Total adsorbed ethanol (atom EtOH nm−2) Ethylene evolved (atom C2H4 nm−2) Temperature at C2H4 evolved (K)

Al2O3 (N) 1.143 0.24 0.002 598–713Al2O3 (V) 0.036 1.03 0.010 553–648Al2O3 (S) 0.002 1.17 0.009 553–613

Table 5The different kinds of Lewis sites are present on aluminas.

Al3+III Al3+

IV Al3+V Na+

Corner Edge

25

4

tfopmpaps(wl

eaa3tenaagfwattsdsai

rsafcaattcws

Py bands 1622 1616–1CO 2234 2220 2170–6

. Discussion

The data reported here allow to provide further evidence ofhe potentiality of the technique of IR spectroscopy of the sur-ace hydroxyl groups, of adsorbed CO at low temperature andf adsorbed pyridine in revealing details concerning the surfaceroperties of commercial and home-made aluminas. In fact, com-ercial and home-made �-aluminas differ significantly from the

oint of view of purity (in particular in relation to the alkali content)nd of morphology. Home-made samples may differ even more inarticular from the purity point of view. In this paper we consideramples with different levels of alkali and alkali earth impuritiesfrom ca. 1% in sample N to near 20 ppm of alkali in sample S) andith different morphologies, such as fibrous (samples N and V),

amellar (sample TH) and globular or spherical (S, D and W).The presence of relevant amount of Na in sample N is actually

videnced by several spectroscopic data: the absence of the bandst 3790 and 3770 cm−1 (associated to weakly acidic OH’s in “purer”luminas) and the presence of a band at 3742 cm−1 (shifting at754 cm−1 at 133 K) characterizing a new non acidic OH is assumedo be associated to the presence of Na+ ions, in fact deposition of Naliminates to a high extent the OH groups responsible for the bandear 3727 cm−1 of ‘purer’ aluminas (TH and S) and forms the bandt high frequency at 3742 cm−1 [37,38]. In parallel, the bands ofdsorbed pyridine at 1593 and 1444 cm−1, more resistant to out-assing and non-shifting with coverage, is also attributed to theormation of pyridine-Na+ complexes. This also corresponds to theeakness, if any, of the high frequency shoulder in the 8a modes of

dsorbed pyridine at 1622 cm−1, showing that the concentration ofhe strongest Lewis acid sites is very small in this sample. In parallel,he CO stretching mode of adsorbed CO at 2168 cm−1, particularlytrong and resistant to outgassing in this sample, provides evi-ence of the direct Na–CO interaction. Also CO as a probe moleculehows that in sample N the strongest Lewis acid sites (�CO bandsbove 2200 cm−1) are less abundant than in the other samples,f any.

Also samples D and W contain significant amounts of Na impu-ities, although much less than N. In fact, also in these samples thetrongest OH stretching band is at higher frequency than on pureluminas (3737 cm−1 vs 3727 cm−1). In parallel, a weak band is alsoound in these samples at ca. 3785 cm−1, different from the veryharged sample N (where bands above 3750 cm−1 are fully absent)nd the purer alumina samples TH and S where two componentsre evident at ca. 3790 and 3770 cm−1. The features observed inhe spectra of D and W may be taken as indicative of Na impuri-

ies at the level of 0.1%. Also these samples show the 8a pyridineomponent at 1622 cm−1 and the �CO bands above 2200 cm−1 veryeak or absent, thus showing that also at so low a Na content, the

trongest Lewis acid sites of alumina are nearly killed. This is not

1598–5 15932170–65

the case of the V sample, whose Na loading is even lower (0.03%).However it is clear that, although the presence of Na in this sampleis almost not reflected in changes in the spectra of the hydroxylgroups. The population of Lewis acid sites in the case of V is differ-ent than on TH and S, being absent any �CO feature at 2234 cm−1,in spite of the presence of that at 2220 cm−1, in parallel to a weakpresence of the 8a pyridine band at ca. 1622 cm−1. Thus it is con-cluded that the presence of 0.03% of Na does indeed interfere witha part, only, of the strongest Lewis sites of alumina. These data sug-gest that, indeed, four different kinds of Lewis sites are present onaluminas, as reported in Table 5.

It seems reasonable to suppose that the amount of sodium isreflected in the catalytic activity of the aluminas in ethanol dehy-dration. In fact, in spite of the relatively poor sensitivity of thisreaction, it is evident that the pure sample S is definitely more activethan the sample V which contains ca. 20 times more Na, while thesample N is by far less active. This is in agreement with our previ-ous data showing that sample V is less active than purer aluminas[41] as well as expected that alkali impurities on the surface of �-alumina will increase the dehydration barriers of the alcohols [47]and that the catalytic activity in ethanol dehydration involves theformation of ethoxy groups on the strongest Lewis acidic sites. Wepreviously found, in agreement with the data reported by Kwaket al. [48], that ethanol can either adsorb dissociatively on Lewisacid/base couples or displace hydroxyl groups producing ethoxygroups which are the surface intermediates in ethanol dehydrationmechanism. Reactivity of surface ethoxy groups would depend onthe Lewis acid strength of cations to which they are bonded. Indeed,sodium impurities may both weaken or poison the strongest Lewisacid sites and exchange surface hydroxyl groups, thus reducingthe amount and the reactivity of ethoxy groups, finally reducingcatalytic activity.

According to our previous suggestions [5], it seems likely thatmost of the observed features are located on the corners and edgesof the alumina surface. To check these hypotheses we have toconsider the amount of alkali present on our samples as well as theamount of surface species usually evaluated to exist on aluminasurfaces. The amount of hydroxyl groups still at the surface of�,�-Al2O3 is evaluated to be in the range 12–16 OH nm−2 afteroutgassing at 373 K and to be reduced to 2.6 OH nm−2 [49] or0.7 OH nm−2 [50] by outgassing at 973–1023 K. On the other hand,the density of the very strong adsorption sites responsible forammonia adsorption heat of more than 200 kJ mol−1 is reported tobe near 0.1 sites nm−2 [51–53] for aluminas outgassed in normalconditions (673–823 K). In addition, the total number of Lewis

sites is of the order of 0.5 sites nm−2. Taking into account the bulkdensity of �-Al2O3 (ca. 3.65 g cm−3), the crystal sizes and shapesfound in TEM microscopies, it is possible to calculate the numberof active sites theoretically exposed on faces and edges of dry

5 alysis

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lumina particles, using the most commonly assumed exposedaces and their edges. From these calculations, it results that at

ost one surface cationic site every 50–100 acts as a strong Lewisite on �-alumina outgassed at 673–823 K, the large majority beingtill hydroxylated or not highly exposed at the surface.

We can indeed calculate that the amount of Na ions on the and D samples (0.15–0.19 wt%, corresponding to 0.17 and

.38 atomNa nm−2, respectively) is higher than, but of the samerder of magnitude of, the amount of the strongest Lewis acid sitesf alumina, which are in fact killed. As proposed previously, evaluat-ng the number of ions located at edges, the Lewis sites can be easilyllocated on the corners of alumina nanoparticles. It is consequentlyroposed, in agreement with previous suggestions concerning theddition of K+ [36] and Ba2+ [54] to alumina, that Na+ ions andther cations when present in small amounts of the alumina sur-ace interfere with the most energetic sites of the alumina surface,.e. those located on the corners and on the edges of the crystals.lready at the content of 300 ppm (V sample, 0.036 atomNa nm−2)art of the strongest Lewis sites are killed.

On the basis of the above data, also the approach to the effect ofarticle size and shape to surface properties can be attempted. This

s done by comparing the features of samples TH and S. These sam-les have both high purity (alkali less than 50 ppm), similar surfacerea but significantly different morphology. TH is clearly lamellar,ith thinner crystals than S, which has a more bulky aspect. Theorosity is significantly different too. In spite of the similar surfacerea, TH has much bigger pore volume and pore size (Table 1). The2 adsorption hysteresis of TH is clearly type H1 usually attributed

o homogeneous pores while that of S is H2, characterizing ink-ottle porosity. The IR spectra show actually similar spectrum ofhe surface hydroxyl groups but the band at 3770 cm−1 is certainlyess intense in the spectrum of S than in that of TH. The spectrumf adsorbed CO and pyridine of the two solids are similar but theeatures at 2234 cm−1 (�CO of CO adsorbed on the strongest Lewiscid sites) and at 1622 cm−1 (8a band of pyridine adsorbed on thetrongest Lewis acid sites) are more pronounced on TH than on S.aking into account that the amount of alkali impurities is evenower on S than on TH, these small differences are to be attributedo morphological differences: it may be supposed that these speciesre indeed exposed on very strained situations such as cornersr surface defects, which are more abundant on the thinner andefinitely more lamellar particles of TH than on the more bulkyarticles of S.

We can take into account that the alkali loading in our pure S andH samples is less than 0.005 atoms nm−2, i.e. well lower than bothhe Lewis sites and the OH amounts usually measured on aluminas.lso for V sample the alkali amount is 0.036 atoms nm−2 is lowerut it is already a significant fraction (>1/3) of the stronger Lewiscid sites of pure aluminas, in agreement with our data that show

partial poisoning of the strongest Lewis sites in this sample. Themount of Na in W and D (0.17 and 0.38 atoms nm−2, respectively),gree with the full poisoning of the strong Lewis sites of aluminahile the amount of Na on N (>1 atom nm−2) is evidently sufficient

o completely change the nature of the surface. However, takingnto account the size of a sodium ion (ionic radius 0.10–0.15 nm)nd the needed co-presence of a balancing anion, we may calculatehat in principle a full monolayer of Na ions covering the entire alu-

ina surface (200 m2 g−1) can be obtained with <0.05 nm2 atom−1,.e 20 atoms nm−2. Thus, the full poisoning of the strong aluminaewis acid sites is obtained with less than 1/50 of the surface cov-rage and a strong modification of the OH groups (N sample) isbtained with less than 1/15 coverage of the surface. This is further

n agreement with the idea that both Lewis acid sites and residualydroxyl groups on alumina are concentrated in a small portion ofhe surface that can be that involved in edges and corners of theanocrystal surface.

[

[[

A: General 483 (2014) 41–51

5. Conclusions

The data reported in the present paper show that the surfaceproperties of aluminas are very sensitive to even moderate amountsof alkali, namely sodium. Thus, the alkali content in the sampleshould be known when characterization studies are performed.

In particular, a careful analysis of the Infrared spectra of thehydroxyl groups, of adsorbed CO and of adsorbed pyridine mayallow to reveal the presence of Na ions or at least to reveal theeffects of the presence of Na ions on Lewis acid sites.

A careful analysis of IR spectra coupled with details on parti-cle size, shape and porosity can also reveal the effects of surfacemorphology on the surface chemical behaviour of �-aluminas. Inparticular a lamellar sample seems to have more strongest Lewissites than a more bulky sample.

This study supports the previous proposal that most of the sur-face chemistry of alumina, included the interaction of the surface ofalumina with supported cationic species, occurs on edges, cornersand defects more than on plain surfaces. Modelling of such moredefective sites is actually needed to model the surface chemistry of�-Al2O3 as a catalyst and a catalyst support.

Acknowledgement

TKP acknowledges funding by EMMA in the framework of theEU Erasmus Mundus Action 2.

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