J. CHEM. SOC. FARADAY TRANS., 1992, 88(14), 2065-2070 2065

Characterization of Microporous Amorphous Alumina-Boria

Alessandro Delmastro, Giuseppe Gozzelino, Daniele Mazza and Mario Vallino* Dipartimento di Scienza dei Materiali e lngegneria Chimica, Politecnico di Torino, C. so Duca degli Abruzzi 24,l-10129 Torino, Italy Guido Busca and Vincenzo Lorenzelli lstituto di Chimica, Facolta di lngegneria , Universita di Genova, P. Ie Kennedy, 1-16129 Genova , Italy

Alumina-boria samples with B : (B + Al) 0-50 atom% have been prepared by an unconventional procedure consisting of the reductive decomposition of the mixture of nitrates in the presence of glycerol X-ray diffraction (XRD)-amorphous samples having surface areas of 380-470 m2 g-' were obtained after calcination at 673 K. Their thermal stability has been studied by differential thermal analysis (DTA) and XRD analysis, showing that crystallization occurred in all cases above 1150 K. FTlR data show the predominance of octahedral A13+ and triangular B3+ both in the bulk and on the surface. Correspondingly, all samples show lower catalytic activity in methanol dehydration compared with y-alumina. It was concluded that boron tends to hinder crystallization of amorphous alumina, to increase its surface area and to further decrease it surface acidity.

Considerable attention has been devoted in recent years in the field of heterogeneous catalysis to amorphous aluminas. High-surface-area amorphous aluminas having zeolite-type porosity have been prepared by two different procedures by Teichner and co-workers' and by o u r ~ e l v e s . ~ ~ ~ Surface char- acterizations of amorphous alumina samples have also been

Planar layers of amorphous aluminas have been characterized r e~en t ly ,~ while amorphous Al,O, has been utilized as the support for different catalysts such as vanadia for selective oxidation6 and TiCl, for alkene poly- mer i~a t ion .~

The addition of typical glass-forming components such as phosphoric anhydride' has been shown to increase the stabil- ity of amorphous a l ~ m i n a . ~ This can be predicted, and has also been reported,','' for alumina-containing boria, another glass former. Alumina-boria materials are also of interest in the field of heterogeneous catalysis. They constitute weakly acidic solids'-' active in Beckman rearrangement of oximes12 and useful as supports for CO-MO hydro- sulfurization catalysts. ' Catalysts comprised of mixed borates of aluminium and a transition metal have also been patented. l4

In the present paper we report a solid-state, surface and morphology characterization of alumina-boria materials pre- pared by a non-conventional procedure.

Experimental

Characterization Methods

Glycerol, aluminium nitrate Al(NO,), * 9H,O, methanol and pyridine were pure products from Carlo Erba (Milano, Italy). Pyridine was further purified by multiple freeze-pumpthaw cycles before vaporization and adsorption experiments.

Differential thermal analysis (DTA) was performed with a Netzsch 404 DTA cell equipped with a computerized 434 programmer unit.

X-Ray powder diffraction (XRD) analysis was performed with computer-aided Philips PW 1710 equipment (Cu-Ka radiation, graphite monochromator).

Infrared spectra were recorded with a Nicolet MX1 Fourier-transform spectrometer equipped with a conven-

tional gas-manipulation and an evacuation ramp. The samples for pyridine adsorption study were pressed into self- supporting disks and pretreated by heating under dynamic evacuation (10- Torrt for 2 h) in the IR cell.

Surface area and pore distribution were determined from the BET adsorption isotherms of N, at 77 K obtained with a Carlo Erba Sorptomatic 1800. The pore-distribution curves were obtained from the N,-adsorption isotherm evaluating the data of the desorption cycle by means of the standard model proposed by Hasley.'

The catalytic activity in the methanol dehydration was evaluated by analysing the emuent of a tubular plug reactor (i.d. 0.25 in$) through a Carlo Erba Fractovap D gas chro- matograph equipped with TCD detector and an H P 3900 A integrator.

Preparation Procedure

Mixtures of Al(NO,), . 9 H 2 0 and boric acid in appropriate ratios were placed in a porcelain dish and 10 wt.% of an organic reducing agent, in this case glycerol and a small amount of distilled water were added. The components dis- solve at temperatures below 373 K forming a single liquid phase. Glycerol was used in place of tartaric acid, previously e m p l ~ y e d , ~ . ~ because it reacts with boric acid forming ether complexes even at room temperature. This fact prevents the non-negligible volatilization of boric acid at temperatures higher than 373 K and also increases its solubility in water which is otherwise rather low.

The clear aqueous solution was heated to 363 K, at which temperature the nitrates begin to decompose with emission of gaseous products (NO,, CO, CO, , H,O). After completion of the reactions and removal of water a spongy bulky solid was obtained that was treated in air at 450 K for 1 h. It was then transferred to an oven at 673 K for 30 min in order to achieve complete removal of nitrogen oxides and complete oxidation of the organic substances. Some characteristics of the samples obtained are summarized in Table 1. The samples will hereinafter be denoted as ABX, where X is the atom% ratio 100B/(A1 + B).

t 1 Torr = (101 325/760) Pa. $ 1 in z 2.54 cm.

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2066 J. CHEM. SOC. FARADAY TRANS., 1992, VOL. 88

Table 1 catalyst as prepared at 673 K

Surface area and pore size distribution for samples of ABx

pore size distribution (%) surface

sample area/m’ g-’ <15 A 15-30 A >30 A

ABO AB5 ABlO AB15 AB20 AB25 AB30 AB40 AB50

367 351 422 457 46 1 464 47 1 450 38 1

83.9 48.2 68.4 68.9 92.1 99.2 87.6 98.8 99.8

15.8 51.7 31.5 35.0 7.7 0.8

12.3 1.17 0.16

0.2 0.08 0.08 0.06 0.18 0 0.05 0.03 0.004

Results and Discussion

XRD and DTA study of the Thermal Evolution of Amorphous Alumina-Borias

All the solids prepared as described in the experimental section were amorphous when examined by X-ray powder diffraction, as shown in Fig. l(a). After heat treatment at 673 K, a temperature which ensures that the samples retain their amorphous state, DTA was performed on the different samples of the ABX series.

The samples AB5 and ABlO display two exothermic peaks, the first in the range 1150-1220 K and the second between 1390 and 1420 K [Fig. 2(a)]. The samples with a higher boron content (AB15 to AB50) are characterized by the dis- appearance of the second peak [Fig. 2(b)]. The XRD analyses show, after the first exotherm, the presence of a crystalline phase, with an orthorhombic pseudotetragonal cell (ao z b, x 780 pm; co x 280 pm), whose structure was investigated by some of us previously.’6 This phase has a mullite-like structure and a wide compositional range, already assessed earlier16 to be of the composition represented by AB15 and AB50. This phase was shown to be metastable: it begins indeed to evolve towards the stable equilibrium phases present on the Al,O,-B,O, diagram after prolonged heat treatments at temperatures higher than 1390 K; these further recrystallization processes are however sluggish, requiring long heat treatments and they fail therefore to produce evident DTA signals. The AB5 and ABlO compositions after the first exotherm are found to be of biphasic composition: their XRD patterns show indeed, besides the well resolved peaks of the mullite-like phase, also the broad peaks of y-Al,O, (JCPDS No. 26-39). The second exotherm on the

17 27 37 47 57 281degrees

Fig. 1 (b) heat treated at 1220 K

XRD powder diffraction spectra of AB15, (a) amorphous and

> $ 10 cn fn .-

5

1 ,

0 0 temperature/”C

Fig. 2 DTA of (a) AB5 and (b) AB25

DTA runs is attributed to the presence of the second phase. This corresponds to the transformation of y- to a-A,O,, as confirmed by the XRD patterns of the samples treated at temperatures higher than 1390 K, which show XRD patterns of well crystallized a-Al,O, together with the peaks of the mullite-like phase.

Thus, boron behaves differently to phosphorus3 in its effect on the y- to a-transformation temperature of Al,O,. It has a less marked effect at low concentrations (AB5 and AB10) than phosphorus in hindering this transformation, whilst at higher concentrations, A1 participates fully in the formation of the binary mullite-like phase, thus preventing the forma- tion of free alumina. This phase has a wide compositional range in contrast to the strict stoichiometry of AlPO,.

Surface Area and Porosity

The surface areas of the different amorphous samples heated at 673 K are reported in Table 1. The pure amorphous alumina sample ABO is completely analogous to that pre- pared previously using tartaric acid instead of glycerol as the reducing In contrast to phosph~rus ,~ B addition seems to have a positive effect on the surface area, at least in an intermediate compositional range, whilst at higher B content the trend is reversed. On the same samples the pore- size distribution curve shows a monomodal zeolitic-type shape (Fig. 3) with a maximum centred at very small pore radii (12-18 A). The position of this maximum in the distribu- tion curves seems to be slightly affected by the composition. In particular low B content, as in AB5, shifts slightly the maximum of the curve toward higher values, whilst on further increase of the B content this maximum returns grad- ually to the values typical for the pure alumina (i.e. ca. 120 nm).

FTIR Spectra in the Skeletal Region

The FTIR spectra of ABO, AB5, AB15, AB30 and AB50 in the skeletal region (KBr pressed disks) are shown in Fig. 4.

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40

0 10 20 30 40 50 pore radius/A

Fig. 3 AB50

Pore size distributions of (-) AB5, (---) AB25 and (---)

- h 2 a.. .* I

The spectrum of pure amorphous alumina (ABO) consists of a main absorption apparently having two unresolved com- ponents centred near 680 and 580 cm-' The position of this band, according to Tarte,17 is indicative of the presence of AlO, octahedra. In contrast, the absence of definite absorp- tions in the region 900-700 cm- ' would indicate the substan- tial absence of A10, tetrahedra. The spectrum agrees with previous conclusions arising from our surface character- ization study2 and from 27Al NMR data4,l8 that evidence the definite predominance of octahedral A1 cations over tetra- hedral ones in amorphous aluminas.

The spectrum is distinctly modified by addition of B. In the sample AB5, the main band is still evident near 580 cm-'. However, broad shoulders are evident near 900 cm-' and 1050 cm-', while definite maxima are detected at 1300 and 1420 cm-', as well as at 1650 cm-'. By increasing the B content, the bands in the region 1450-1250 cm-' increase regularly, while the two shoulders in the region 1100-900 cm- ' become only slightly more pronounced.

The band near 1650 cm-' is not present in the spectra recorded on pressed disks of the pure powders (without KBr) after evacuation at temperatures above 473 K. This suggests that this band is due to the scissoring-mode of water mol- ecules, probably involved in H-bonding, so justifying the

2000 1600 1200 800 400 waven umberlcm - '

Fig. 4 FTIR spectra (KBr pressed disks, skeletal region) of alumina-borias. (a) ABO, (b) AB5, (c) AB15, (d) AB30 and (e) AB5O

l l l l l l l l l l l l l l l l l

800 400 2000 1600 1200 wavenumber/cm -'

2067

Fig. 5 FTIR spectra (KBr pressed disks) of model boron-xygen compounds. (a) H3BO3, (b) B 2 0 3 and (c ) Na2B,0, . 10H,O

higher frequency with respect to gas-phase water (1595 cm- ').

Information on the structure of the borate species present on alumina-boria can be obtained from the above spectra, taking into account the characteristic spectra of the different boron-oxygen compounds. Bands in the region 1500-1200 cm-' are typically due to the asymmetric B-0 stretching modes of planar BO:- groups, as in rare-earth orthoborates having calcite-like and aragonite-like structures [v,, (BO,) in the 1300-1200 cm-' reg i~n '~] . These species can also be responsible for a symmetric stretching band near 940 cm- ', that is IR inactive in symmetrical BOZ- species, and for an in-plane and an out-of-plane bending band near 740 and 650 cm-', respectively. In the case of 'condensed' planar borate structures, such as in B 2 0 , , whose spectrum is reported in Fig. 5, an absorption due to asymmetric B-0 stretches extends from 1260 cm-' (main maximum) to 1450 cm-' (shoulder).20 Tetrahedral borate species are instead charac- terized by complex absorptions due to asymmetric and sym- metric BO, stretches in the region 1100-900 cm-1.21,22 The bands due to trigonal and tetrahedral borate species have similar absolute intensities, as is deduced from the spectrum of borax, Na2B,0, . 10H20, shown in Fig. 5. In this com- pound, in fact, half of the borate species are tetrahedral and half trigonal: the bands 1500-1300 cm-', due to BO, asym- metric stretches, and those in the region 1200-900 cm- ', due to BO, asymmetric stretches, have comparable intensities.

The spectra of our alumina-boria samples show that in these materials trigonal planar borates, responsible for the pair of bands at 1420 and 1300 cm- ', are definitely predomi- nant over tetrahedral borates, that could be responsible for the shoulder near 1050 and 900 cm- ' However, neither B20, nor its hydration product H,BO,, whose spectrum is also reported in Fig. 5, are present as separate phases because the spectra appear to be definitely different. In conclusion, alumina-boria predominantly contains partially condensed planar borate species together with a much smaller amount, if any, of tetrahedral borate species. These data are in sub- stantial agreement with those deduced by "B NMR on dif- ferent alumina-boria sample^.^*'^

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2068 J. CHEM. SOC. FARADAY TRANS., 1992, VOL. 88

Surface Characterization by FTIR Spectroscopy

In Fig. 6 the spectra of a pressed disk of pure AB5 and AB50 after activation by outgassing at 773 K are reported. The broad absorption extending from near 3800 cm-' to near 2600 cm- ' is due to OH-stretching, mainly due to H-bonded hydroxy groups possibly located in the bulk. At the higher frequency side of this absorption a sharp band is detected in all B-containing materials near 3695 cm- '. This frequency compares well with the v (OH) of free BOH groups such as those of H$O, in N, cryogenic matrices (3668.5 cm-' 23)

or on the surface of silica impregnated by boria (3703 cm- ' 24). In the case of AB5 poorly defined absorptions are also evident as shoulders in the region 3800-3700 cm- ' and could correspond to OH groups bound to aluminium ions as in pure amorphous alumina.2 The sharp band at 2343 cm- ' present in all cases is due to CO, , resulting from combustion of organic materials used in the preparation procedure, trapped in the micropores. In all cases the transmission is zero in the region 1500-1200 cm-l, due to the asymmetric BO, stretches, as discussed above. In disks produced by pressing the pure powders, containing much more material than KBr pressed disks, these absorptions are so intense as to obscure the region. The region below lo00 cm- ' is obscured, as also in amorphous and crystalline aluminas, by A1-0 stretching modes.

The use of pyridine as a spectroscopic probe molecule is possible although the spectrum of adsorbed pyridine on B- containing materials can be discussed only in the region of the v 8a vibration (1630-1570 cm-'). In fact, all other sensi- tive bands fall in regions obscured by B-0 stretches.

The spectra of pyridine adsorbed on pure amorphous alumina after evacuation at 300 K and 473 K are reported in Fig. 7. After evacuation at room temperature two v 8a modes, corresponding to two different adsorbed species, are detect- able at 1590 cm-' and near 1612 cm-'. Evacuation at 473 K causes the disappearance of the lower frequency band and the decrease of the higher frequency one that shifts also to near 1618 cm-'. The band at 1590 cm-' is very likely due to pyri- dine adsorbed on A13+ ions belonging to the octahedral coordination sphere, having a single coordinative unsatu- ration before pyridine adsorption. This band is in fact detected also on the corundum (cc-Al,03) surface25 as well as metal aluminates,26 where A13 + ions have octahedral coordi- nation. The band at 1618-1612 cm-' could be due to pyri- dine adsorbed on a similar site, but having two coordinative unsaturation, before pyridine adsorption. As discussed pre- viously,2 the band usually detected at 1625 cm-' after pyri- dine adsorption on transitional a l u m i n a ~ , ~ ~ . ~ ' due to

I \" - - / tw 0 ' 4000 3600 3200 2800 2400 2000 1600 1200

wavenumber/cm-'

Fig. 6 FTIR spectra (pressed disks of the pure powders) of (a) AB5 and (b) AB50 after outgassing at 773 K for 2 h

7700 1600 1500 1700 1600 1500 ~

wavenumber/cm -' Fig. 7 FTIR spectra of adsorbed pyridine on (a) ABO, (b) AB5, (c) AB30 and (d) AB50. Upper curves, evacuation at room temperature; lower curves, evacuation at 473 K after pyridine adsorption

pyridine interacting with A13 + ions in tetrahedral coordi- nation, is not detected at all on amorphous alumina.

The addition of boron does not alter the situation very much (Fig. 7). On all surfaces up to AB30 two Lewis-bonded pyridine species are detected characterized by v 8a vibrations at 1590 cm-' and near 1612-5 cm-', assigned to pyridine on different octahedral A13+ sites. A weak broad band is also detected on all alumina-borias, near 1650 cm-' due to a small amount of pyridinium ions. This species disappear completely on evacuation at 473 K. A small number of weak Brmsted sites are generated by B addition to alumina. In the case of AB50, where measurements are made difficult by an even higher opacity in the 1600 cm- ' region, only the lower- frequency band (1 590 cm - ') is observed.

The above data evidence that the Lewis surface acidity of alumina-borias is not stronger than that of pure amorphous alumina, due to partly coordinatively unsaturated octahedral A13+. B does not act as a Lewis site. This is probably due to the fact that trigonal borate species are definitely predomi- nant: in these species the empty orbital on B is involved in n-type bondings with lone pairs of oxygen, and is conse- quently unavailable for coordination. Tetrahedral B, as in BPO, 2 8 can act as a very strong Lewis site. However, according to the above data arising from the analysis of the spectrum in the skeletal region, these species are present in negligible amounts if at all.

In contrast to what has been shown for alumina- aluminium phosphate amorphs, the presence of B does not favour the stabilization of tetrahedral A1 cations in amorp- hous alumina. Consequently, on amorphous alumina-boria, as on amorphous pure alumina, A13+ cations are, and remain, predominantly octahedral and, when exposed on the surface, act as rather weak Lewis-acid sites. Also Brernsted acidity, generated by B addition in amorphous alumina, is very weak, in accord with the weakness of boric acid.

Catalytic Activity in Methanol Dehydration

The surface activity of samples of alumina-boria was investi- gated by evaluation of the rate of methanol dehydration at different temperatures. The experiments were carried out at temperatures between 460 and 560 K, a temperature range where the products of the methanol dehydration were only water and dimethyl ether.

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J. CHEM. SOC. FARADAY TRANS., 1992, VOL. 88 2069

Table 2 Catalytic bed temperature for typical reaction rate observed in the methanol dehydration

temperaturelK'

ABO, AB5 485-490 504-508 5 17-522 AB10, AB15 500-505 521-524 536-538 AB20, AB25, AB30, 477-479 494-497 507-5 10

AB40, AB5O

Conditions: catalyst volume = 0.62 cm3; methanol feed = 6.6 mmol h-'; gaseous diluent, helium. Min. and max. of temperature value obtained by two experimental runs for each sample. T,, T, and T3 are the temperatures of the catalytic bed when the observed reaction rates are, respectively, 10, 20 and 30 pmol h-' m-'.

In all cases the observed reaction rate increases with tem- perature until the methanol conversion reaches 67%. At higher temperatures a constant value of reaction rate is observed because of the establishment of an equilibrium. The catalytic activity measurments were carried out at tem- peratures where the system was far from the equilibrium in order to estimate the relative catalytic activities of the differ- ent samples. The temperatures required to observe a reaction rate of 10, 20 and 30 pmol h-' m-2 were evaluated and reported in Table 2. The typical patterns of the curves showing reaction rate us. reaction temperature for the differ- ent catalyst samples are reported in Fig. 8 where the curve obtained with a commercial y-A120, (Akzo 0.01-1.5 E), tested under the same conditions, is also reported for com- parison.

The data provide evidence for three different types of behaviour of our amorphous samples. The catalysts with a B content higher than 20% show the highest catalytic activity, irrespective of the boron content in the range 20-50%, although they are by far less active than crystalline y- alumina. Pure amorphous alumina and AB5 show an inter- mediate catalytic activity, while ABlO and AB15 samples show the smallest catalytic activity in methanol dehydration.

This behaviour can be interpreted, according to the above FTIR data, assuming that B first decreases the number and/or the strength of the more active Lewis-acidic A13+ surface cations of amorphous alumina but provides Br~nsted-acidic centres whose activity becomes predominant at higher B content.

Conclusions The conclusions can be summarized as follows: (a) The un- conventional procedure we adopted allows us to prepare amorphous alumina-boria samples [B/(B + Al) = 0-50 atom%] having surface area in the range 380-470 m2 g-' after calcination at 673 K. The maximum surface area is obtained for the sample AB30 (471 m2 g- '). (b) The materials are all amorphous up to ca. 1150 K. The higher B-content samples crystallize to give a metastable mullite-like phase, while lower B-content samples give a mixture of this phase and y-alumina. (c) These amorphous alumina-borias show a zeolitic type microporosity (pore radii centred near 12-18 A). (d) These amorphous solids are weakly acidic, and possess both Lewis and Brernsted sites, as shown by pyridine adsorp- tion. (e) These materials are, accordingly, less active in meth- anol dehydration than transitional aluminas. (f) The surface chemistry of these amorphous alumina-boria samples is explained by the large predominance of octahedrally coordi- nated A13+ and trigonal B3+ (thought to produce weak Lewis-acid sites when exposed at the surface) over tetra- hedrally coordinated A13+ and B3+ (well known to produce very strong Lewis sites) as well as by the presence of weak BOH Brmsted sites. These structural features derived from solid state and surface characterization by IR spectroscopy are in agreement with literature NMR data.

As a main conclusion we find the addition of boron to amorphous alumina hinders its crystallization, although less markedly than phosphorus, but, unlike phosphorus, tends to enhance the surface area. Moreover, at least at low content, boron tends to decrease further the surface acidity of amorp- hous alumina.

This research was funded by MPI (Minister0 Publica Istruzione) Fondi, 40%.

TIK

Fig. 8 Reaction rate as a function of reaction temperature for 0, amorphous alumina ABO; 0, AB10; A, AB20 and ., y-alumina (Akzo). Full line: first run; dashed line: second run

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