J. Phys. Chem. C 2007, 111, 12075-12079
12075
Strong Brønsted Acidity in Amorphous Silica-Aluminas Bin Xu,† Carsten Sievers,‡ Johannes A. Lercher,‡ J. A. Rob van Veen,§ Patricia Giltay,§ Roel Prins,† and Jeroen A. van Bokhoven*,† ETH Zurich, Institute for Chemical and Bioengineering, 8093 Zurich, Switzerland, Department of Chemistry, TU Mu¨nchen, D-85747 Garching, Germany, and Shell International Chemicals B.V., 1031 CM Amsterdam, The Netherlands ReceiVed: May 14, 2007; In Final Form: June 11, 2007
Monomolecular cracking of propane was used to investigate the activity of the Brønsted acid sites in amorphous silica-aluminas with three different Si/Al ratios. The reaction rates increased with increasing aluminum content, but the apparent activation energies were identical. In comparison to zeolite ZSM5, the ASA catalysts showed much lower activity, both per weight and per total aluminum content. However, after correcting for the heat of adsorption, the intrinsic activation energies of ASA and H-ZSM5 were similar. This indicates that the ability of the active Brønsted acid sites to protonate propane is similar in amorphous and crystalline structures and that the much lower activity of ASA is due to the lower heat of adsorption and the small number of active sites. Few Brønsted acid sites were detected by means of pyridine adsorption followed by infrared spectroscopy; a broad band at around 3600 cm-1 was observed in the infrared region of the hydroxyl stretch vibrations. It is unclear whether this band was related to the catalytically active sites.
1. Introduction Amorphous silica-aluminas (ASAs) and zeolites are widely applied as solid-acid catalysts in (hydro-) cracking, dehydrogenation, isomerization, and alkylation reactions, which are very important in the petrochemical and refining industry.1,2 These catalysts consist of corner-sharing tetrahedral (AlO4)- and SiO4 units that form three-dimensional structures. Charge balance requires the presence of extraframework cations. When protons compensate the framework charges, these materials show Brønsted acidity. The framework of zeolites is ordered and crystalline and contains pores and cages from 4 to 20 Å in size. ASAs lack long-range order and are XRD amorphous. The catalytic activity of zeolites is generally much higher than that of ASA. It was proposed that the zeolite crystal readjusts the bond structure of the Brønsted acid sites to maintain long-range order, resulting in very strong Brønsted acids.3-5 This readjustment is much smaller in amorphous structures, leading to the assumption that the acid strength of the amorphous material is weaker.3 Many methods, such as Hammett titrations, solid-state nuclear magnetic resonance and infrared spectroscopy, temperature-programmed desorption, microcalorimetry, and reactivity measurements have been used to characterize the acidity of crystalline zeolites and ASAs. The lower activity of ASA compared to zeolites is often explained by lower acidity.6-8 Other reports suggest that the acid strength of ASA is between that of zeolite Y and mordenite9 or is equal to zeolite Y,10,11 to beta,12 or to dealuminated zeolite Y.12 Another study suggests that the low catalytic activity of ASA originates from its lower number of Brønsted acid sites and that the high activity of zeolites is not due to a particular property of zeolites.13 The intrinsic activity of the zeolitic Brønsted acid sites in monomolecular cracking does not depend on the zeolite * Corresponding author. Phone:
[email protected]. † ETH Zurich. ‡ TU Mu ¨ nchen. § Shell International Chemicals B.V.
+41
446334372.
E-mail:
structure, and the sorption of the reactant dominates the catalytic activity.14-20 Since the rate-limiting step in monomolecular cracking is the protonation of the alkane,21 the intrinsic activity should be conceptually related to the strength of the Brønsted acid sites in the catalysts. The aim of this study was, therefore, to explore the concentration and strength of Brønsted acid sites in ASA and to compare them to those in ZSM5 by means of the monomolecular cracking of propane and a variety of characterization methods. 2. Experimental Section The ASAs were prepared following the recipe in a Chevron patent (U.S. Patent 4988659, 1991). The appropriate amounts of AlCl3‚6H2O (23.5, 47.3, and 94.5 g, respectively) were dissolved in a mixture of water (600 g) and acetic acid (75 g), and the required amount of waterglass (Merck, ∼27 wt % silica) was added to 2 kg of water. The silica-containing solution was then added under stirring to the aluminum-containing solution, and the pH was increased to approximately 7 by adding concentrated ammonia. The product was filtered and exchanged four times with 0.1 M NH4+ acetate. After the last filtration, the product was dried at 80 °C in order to keep all the chargecompensating ammonium ions on the catalyst. The samples are referred to as ASA(M), where M represents the Si/Al ratio. Nitrogen physisorption measurements were performed at liquidnitrogen temperature in a Micromeritics ASAP 2000 apparatus. Prior to the measurements, the samples were degassed at 723 K overnight. The surface area was determined by the BET method, and the micropore volume was calculated from the intercept of the linear part of the t plot. Transmission electron microscopy was performed with a Philips CM30 electron microscope using a super twin lens with a point resolution of 0.2 nm, operated at 300 kV. Prior to the measurement, the samples were suspended in ethanol and deposited onto a holey carbon foil. MAS NMR measurements were performed at 104.287 MHz for 27Al and at 79.504 MHz for 29Si on a Bruker Avance AMX-400 spectrometer with a 4 mm probe at a spinning
10.1021/jp073677i CCC: $37.00 © 2007 American Chemical Society Published on Web 07/21/2007
12076 J. Phys. Chem. C, Vol. 111, No. 32, 2007
Xu et al.
TABLE 1: Properties of the ASAs sample
BET surface area (m2/g)a
micropore volume (cm3/g)a
mesopore volume (cm3/g)a,b
Al (mmol/g)c,d
Brønsted acid sites (mmol/g)e
Lewis acid sites (mmol/g)e
ASA(15) ASA(7) ASA(3)
590 460 310
0 0 0
0.60 0.49 0.29
1.0 2.1 4.2
0.016 n.d.f 0.011
0.106 n.d. 0.075
a Determined by N2 physisorption. b Pores with sizes from 2 to 50 nm. c Determined by atomic absorption spectroscopy. d The sodium content was below 0.1 wt % in all samples. e Pyridine infrared after desorption at 423 K. f Not determined.
rate of 12 kHz. For 27Al MAS NMR, a pulse length of π/6 was used. For 29Si MAS NMR, a high-power decoupling pulse sequence and a relaxation delay of 6 s were used. The 27Al chemical shifts were referenced to (NH4)Al(SO4)2‚12H2O and the 29Si chemical shifts to octakis(trimethyl siloxy)silsesquioxane. For the IR measurements, self-supporting pellets (2-4 mg) were positioned in a laboratory-constructed heated cell equipped with KBr windows and connected to a metal vacuum line capable of attaining a residual vacuum of
