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J. Phys. Chem. C 2009, 113, 16476–16484
Sulfated Nanocapsular Aluminas: Controlling their Bro¨nsted and Lewis Acidity Jaime S. Valente,*,†,‡ Esteban López-Salinas,† Xim Bokhimi,§ Jorge Flores,‡ Ana M. Maubert,‡ and Enrique Lima| Instituto Mexicano del Petróleo, Eje Central No. 152, 07730 México D. F., México, UniVersidad Autonoma Metropolitana-A, Química de Materiales, AVenida San Pablo No. 180, 02200 México D. F., México, Instituto de Física, UNAM, A.P. 20-364, 01000 México D. F., México, and Instituto de InVestigaciones en Materiales, UNAM, A. P. 70-360, 04510 México D.F., México ReceiVed: June 15, 2009; ReVised Manuscript ReceiVed: August 1, 2009
Sulfated aluminas were prepared by a sol-gel method, using sulfuric acid as both hydrolysis catalyst and sulfate source. The nominal amount of sulfate was varied between ∼2 and 17% wt. Solids thus obtained present unique nanocapsular morphology with capsules’ shells made up of Al-O polynuclear species, which crystallized into boehmite when aged for 30 days. Upon calcination, small γ-alumina crystals were obtained, which present large surface areas and pore volumes. The presence of sulfate ions generated strong Bro¨nsted acid sites, while Lewis acid sites were present in the coordinatively unsaturated sites of tetracoordinated and pentacoordinated aluminum atoms, as was demonstrated by both pyridine adsorption followed by FTIR and 27 Al MAS NMR. Catalytic performance was tested using 2-propanol conversion as a model reaction, showing a marked dependence of activity and selectivity with sulfate loading. 1. Introduction Aluminas are perhaps the transition metal oxides most widely used as catalysts and catalyst supports. They have been employed in a variety of chemical, petrochemical and petroleum refining processes.1 Their main advantages are their low cost, good thermal stability, high specific surface area, surface Lewis acidity, and the important interaction that they exhibit with deposited transition metals.2-4 Aluminas are generally produced industrially by precipitation, drying, and calcination of aluminum oxy-hydroxides. The catalytic properties of aluminas largely depend on their crystalline structure and texture. Therefore, a great effort has been devoted to master these physicochemical properties.4-10 Recently, several research groups worldwide have driven their efforts to synthesize nanostructured aluminas to control their textural properties. Hence, many strategies to obtain such characteristics have been proposed; for instance, the use of hypercritical drying conditions of xerogels11,12 and the employment of surfactant assemblies.13-16 The activity and selectivity of aluminas as catalysts are also intimately related to the amount, strength, and nature of acid sites.17-19 Additionally, control over acid sites type and population is highly desirable, as Bro¨nsted acid sites are necessary in certain reactions to achieve skeletal rearrangements. Thus, with the idea of changing and adjusting alumina acidity, several strategies have been proposed, that is, doping it with sulfate or phosphate ions to create Bro¨nsted acid sites.19-22 Furthermore, the Lewis acidity is related to aluminum coordination and to the chemical nature of its neighbors. The creation of 4-fold (AlIV) and 5-fold (AlV) species of aluminum coordinately unsaturated sites (CUS), their amount, vicinity, and accessibility could be * To whom correspondence should be addressed. Phone: + 52 (55) 91758444. E-mail:
[email protected]. † Instituto Mexicano del Petróleo. ‡ Universidad Autonoma Metropolitana-A. § Instituto de Física. | Instituto de Investigaciones en Materiales.
the key to controlling the acidity.17 Conventionally, aluminas are dehydrated during preparation, producing CUS that are accompanied by a redistribution of the charge on the aluminum and oxygen atoms, and thus creating Lewis acidity. In the scientific literature, it is claimed that when doping the alumina surface with sulfate ions, the overall Lewis acidity should decrease, as part of the surface is occupied by thermally stable sulfate groups and thus, a lower amount of CUS cations can be produced upon activation. The residual Lewis acidity has sometimes been observed to increase in strength due to inductive effects from the charge-withdrawing sulfate groups.22,23 Previously, the authors have reported the synthesis of sulfated aluminas by a simple sol-gel procedure, using different amounts of sulfuric acid as both hydrolysis catalyst and as sulfate source.20,21 It was found that the as-prepared powders were made up of hollow spheres or capsules with diameters between 10 and 50 nm, a morphology that has not been obtained when using other synthetic procedures.20,21 Upon aging at room temperature for a variable number of days (up to 30), these nanocapsules were found to interact with each other and aggregate.20 Aggregation and coalescence of the capsules were hindered when peptizing ions are present in the solvent,24 since these anions surround the colloidal-sized nanocapsules and obstruct the capsule-capsule interaction. This peptizing effect was observed with sulfuric acid; it has been demonstrated that the interaction between the nanocapsules in the sol decreased as the sulfate ion concentration in it increased. This interaction induces partial crystallization of the atomic local distribution in the shells from the noncrystalline state into the atomic distribution of crystalline boehmite.20 The X-ray diffraction patterns of the aged samples with different sulfate amounts showed boehmite peaks along with the noncrystalline phase obtained in the unaged samples. XRD patterns show that boehmite concentration and crystallinity decreased when the SO42-/Al molar ratio was increased. Thus, crystallization of the nanocapsules’ shells into boehmite depended on the interaction between capsules and on sample aging, which induced atom
10.1021/jp905603j CCC: $40.75 2009 American Chemical Society Published on Web 08/25/2009
Sulfated Nanocapsular Aluminas reordering.20 After annealing at 550 °C, the boehmite was pseudomorphically transformed into γ-alumina. In the present work, a method for controlling the Bro¨nsted and Lewis acidity over sulfated γ-aluminas obtained by calcination of these sol-gel boehmite samples is proposed. The effect of the sulfate amount, introduced as sulfuric acid during boehmite synthesis, on the type of acidity was studied, as well as the textural and catalytic properties of aluminas. The sol-gel method was used to prepare sulfated aluminas because it provides an attractive and convenient route to manipulate the structural and textural properties and purity of product.12,19,25,26 Pyridine adsorption followed by FTIR was used to determine the type and strength of the different acid sites. Also, the acidic function properties of sulfated aluminas have been investigated in the conversion of 2-propanol, because depending on the acidic or basic nature of the solids three different reactions can occur, dehydrogenation, dehydration, and/or coupling.27
J. Phys. Chem. C, Vol. 113, No. 37, 2009 16477 The concentration of Bro¨nsted and Lewis acid sites was obtained using a standard procedure based on the Lambert-Beer law.28 The absorbency (AI) is determined as the integrated area under the curve as follows
AI ) B · C ·
(1)
Where ∫ev dv is the extinction coefficient and it is proportional to 0.4343 · Iv. It is generally accepted that Iv ) 3.03 cm µmol-1 and 3.26 cm µmol-1 for the bands at 1545 and 1450 cm-1, respectively.29 B is related to weight /area (g cm-2) wafer ratio, and C is the concentration of Bro¨nsted or Lewis acid sites, which can be calculated as
CBrönsted ) 2. Experimental Section 2.1. Synthesis of Materials. The synthesis procedure of sulfated aluminas has been described previously.21 Briefly, aluminum trisec-butoxide (ATB) was used as aluminum source and anhydrous 2-propanol as solvent. ATB was dissolved and refluxed for 1 h. Sulfuric acid was added dropwise, and the solution was refluxed and stirred at 85 °C for 1 h. Afterward, deionized water was added. The gel was placed in a glass vessel and aged for 30 days at room temperature. Afterward, the product was dried at 120 °C. The molar ratios 2-propanol/ATB ) 60 and H2O/ATB ) 1 were constant. The molar ratio H2SO4/ ATB was 0.01, 0.03, 0.06, or 0.09, resulting in nominal 1.92, 5.77, 11.54, and 17.32% wt H2SO4, respectively. The samples were labeled as ASO4-0.01, ASO4-0.03, ASO4-0.06, ASO40.09, respectively. Dried solids were calcined at two different temperatures from room temperature up to 550 or 700 °C for 4 h at 4 °C min-1 heating ramp. Calcined samples were labeled ASO4-X-CY, where X is the H2SO4/ATB molar ratio, and Y indicates calcination temperature (550 or 700 °C). In thermal analyses and FTIR (sulfate studies) only dried materials were used, so samples were referred to only as ASO4-X. 2.2. Experimental Techniques. Textural Analysis. Specific surface area and pore size distribution were obtained from nitrogen adsorption-desorption isotherms measured in a Micromeritics ASAP-2100 analyzer. The adsorption was carried out on calcined solids (550 °C) after outgassing at 400 °C under a 10-5 Torr residual pressure during 4 h. The surface areas were calculated by the Brunauer-Emmett-Teller (BET) method, and the pore size distribution and total pore volume were determined by the Barret-Joyner-Halenda (BJH) method. Thermal Analysis. Weight loss of sulfated aluminas (dried at 120 °C) as a function of temperature was recorded in a PerkinElmer TGA-7 using a linear 10 °C min-1 heating rate and 20 cm3 min-1 flow of dry air from room temperature to 1000 °C. Infrared Spectroscopy. For pyridine adsorption measurements, IR spectra were obtained at a resolution of 4 cm-1 in the range of 1700-1400 cm-1 using a Perkin-Elmer model 170SX FTIR spectrometer. A wafer of sample was mounted in a pyrex vacuum cell fitted with CaF2 windows. The samples were pretreated at 550 °C for 1 h in a stream of O2 followed by evacuation at 550 °C, then cooled to room temperature to obtain the background IR spectra. The sample was evacuated to 7.5 × 10-4 Torr, followed by exposure (2.14 L h-1) to pyridinesaturated N2 for 15 min, then outgassed 1 h at room temperature (pressure 7.5 × 10-4 Torr), and finally linearly heated up to the desire temperature.
∫ evdv
CLewis )
AI(1545 cm-1) weight/area · (0.4343 · 3.03) AI(1450 cm-1) weight/area · (0.4343 · 3.26)
For the measurements concerning the sulfate vibration bands, the activation of the samples was conducted in a quartz IR cell. A wafer was heat-treated in flowing O2 at a given temperature (100-500 °C) for 1 h, the sample was then cooled to room temperature and an FTIR spectrum was recorded, in the range of 1700-1000 cm-1. 27 Al MAS NMR Measurements. The 27Al MAS NMR spectra were acquired on a Bruker ASX 300 Spectrometer operating at resonance frequency of 78.2 MHz. 27Al MAS NMR spectra were obtained after short single pulses (π/12) with a repetition time of 500 ms. The samples were spun at 10 kHz and the chemical shifts were referenced to 1 N aqueous solution of AlCl3. A rough estimation of the number of (AlOx) species was attempted by fitting Gaussian/Lorentzian lines to the peaks, calculating the area under the peaks and dividing it by the total area in order to obtain each species’ percent; the Bruker software WIN NMR was used for this end. However and given the second-order quadrupolar broadening included in these peaks due to the use of conventional (
