J. Phys. Chem. B 2000, 104, 5511-5518
5511
Strong Acidity of MFI-Type Ferrisilicate Determined by Temperature-Programmed Desorption of Ammonia Naonobu Katada,* Tetsuo Miyamoto, Hosne Ara Begum, Norihiro Naito, and Miki Niwa Department of Materials Science, Faculty of Engineering, Tottori UniVersity, 4-101 Koyama-cho Minami, Tottori 680-8552, Japan
Akihiko Matsumoto and Kazuo Tsutsumi Department of Materials Science, Faculty of Engineering, Toyohashi UniVersity of Technology, Tempaku-cho, Toyohashi, 441-8580, Japan ReceiVed: December 13, 1999; In Final Form: March 27, 2000
Ammonia temperature-programmed desorption (TPD) with water vapor treatment method was applied to MFI-type ferrisilicate, and theoretical analysis was carried out to determine the acid amount and strength exactly. At the low iron content where isomorphous substitution of iron into the silicate matrix was predominant, a quite strong acid site with 150-180 kJ mol-1 adsorption heat of ammonia, which was stronger than that on aluminosilicate, was observed. The determined acidic property was in good agreement with microcalorimetry. Therefore, it is concluded that the isomorphous substitution of iron into the MFI structure generates a stronger acid site than that by aluminum.
Introduction Since early studies1 of ferrisilicate molecular sieve, it has been believed that isomorphous substitution of silicon by iron generates an acid site weaker than that by aluminum.2 Frequency of O-H stretching1 and its shift by adsorption of nitrogen, carbon monoxide,3 ethene, and benzene4 were smaller on the MFI ferrisilicate than those on aluminosilicate analogue (ZSM5), suggesting a stronger O-H bond and weaker interaction with these bases. Temperature-programmed desorption (TPD) of ammonia on the ferrisilicate showed lower temperature of desorption peak maximum than that on aluminosilicate.1 Many researchers have accepted the acid strength sequence B < Fe < Ga < Al in corresponding metallosilicates because this order agrees well with the catalytic activity for a reaction that is believed to be a typical acid-catalyzed one such as cracking of alkane. The sequence of acid strength is interesting from the viewpoint of theoretical chemistry, and therefore quantum chemical calculations according to this sequence have been published.5-9 However, there appears some controversy. Careful measurements of calorimetry showed that the adsorption heat of ammonia was too similar to distinguish the acid strength of the MFI type ferri-, gallo-, and aluminosilicates.10 Choudhary reported quite high catalytic activity of the ferrisilicate for liquidphase Friedel-Crafts type alkylation of benzene with benzyl chloride,11 which is usually catalyzed by solid superacid. These phenomena are possibly correlated with strong acidity of ferrisilicate. In the chemistry of clay minerals, iron-containing montmorillonite is known to possess stronger acidity than aluminum-containing analogue.12 Hence the early estimation of the acid strength becomes questionable. Because the acid strength is defined as the equilibrium constant with respect to the reaction between the * Corresponding author. E-mail:
[email protected].
acid and a base such as ammonia and amine,13 only a method using an ammonia (or amine) probe such as TPD and calorimetric measurement of adsorption heat can determine the absolute scale of acid strength. In other words, the IR technique and test reaction cannot determine the absolute acid strength. Especially the alkane cracking on the MFI silicates has complicated mechanisms, as discussed,14 and is affected by the slow diffusion of the reactant and intermediates; the shape selectivity is evidence of the diffusion control, and therefore it is difficult to consider the activity for this reaction as a measure of acid strength. On the other hand, the conventional method of TPD of ammonia has two problems: unnecessary ammonia weakly held on nonacidic site,15 e.g., hydrogen-bonded on ammonium cation,16 confuses the TPD spectrum, and the readsorption of ammonia affects the peak maximum temperature.15 We have pointed out that the difference of measurement conditions of TPD varies the peak maximum temperature widely in the range of some 10 K because of the latter problem.17 Difference of the peak maximum temperature between ferriand aluminosilicate is usually observed to be within 100 K,1,5,18-21 and in some cases the observed peak has a too broad and complicated shape to exactly determine the peak maximum.5,18,20,22 Moreover, most of these studies were based on measurements on only a few samples; hence the difference between the framework and extraframework species of iron seemed unclear. Therefore the conclusion obtained from the conventional measurements of the acidic property of ferrisilicate becomes doubtful. It has been pointed out that the conventional ammonia TPD method is not useful because of the above-mentioned disadvantages.23,24 We have improved the experimental and analytical methods in order to overcome them. First, the water vapor treatment technique was established to remove the unnecessary ammonia species based on the nature of the chemical bond between ammonia and solid,16 based on the pioneering works
10.1021/jp994375m CCC: $19.00 © 2000 American Chemical Society Published on Web 05/18/2000
5512 J. Phys. Chem. B, Vol. 104, No. 23, 2000 by Bagnasco25 and researchers of Mobil Oil.26 Second, a method to calculate the adsorption heat of ammonia as a measure of acid strength from the peak area, position, and shape was proposed on the basis of the thermodynamics.17,27 It should be noted that the proposed method does not ignore the readsorption of ammonia, and on this point it is different from the conventional energy calculation28,29 from the TPD spectrum which assumes kinetic control. Already we have applied these methods to various zeolites16,27,30-32 and nonzeolitic materials.33-35 The investigation on the MFI-type gallosilicate changed the conventional interpretation of acidity on this material: it has been believed to have weaker acidity than aluminosilicate, but it was clarified that the isomorphous substitution of gallium in silicate generated the acid site quite similar in strength to that of aluminosilicate.36 Here we report the investigation on the acidic property of the MFI-type ferrisilicate with various compositions by the developed techniques of ammonia TPD. Experimental Section Synthesis of Ferrisilicate. Ferrisilicate was synthesized principally according to literature.2 Iron nitrate and tetra-npropylammonium bromide were solved into an aqueous solution, and this was dropped slowly into a Ludox HS40 silicate solution. Sodium carbonate solution was then added. Sodium hydroxide or sulfuric acid was added to adjust the pH to 9.7. The mixed solution was stirred and heated at 423 K for 72 h in an autoclave. The yielded solid was filtered, washed with water, and finally calcined at 813 K for 20 h in flowing oxygen. The thus obtained Na-type ferrisilicate was converted into H-type by ion exchange in an ammonium nitrate solution at 343 K for 24 h, followed by calcination in flowing oxygen at 813 K for 20 h. The iron and sodium contents were measured by an inductively coupled plasma (ICP) photoemission spectrometer (Shimadzu ICPS5000). The contamination of aluminum was confirmed to be less than 10-2 mol kg-1. Impregnation of iron on ferrisilicate was carried out in order to clarify the acidic property of extraframework iron species. The ferrisilicate powder was put into an aqueous solution of iron(III) nitrate, heated on a hot plate, dried, and calcined at 813 K for 20 h in flowing oxygen. Iron oxide (BET surface area, 12 m2 g-1) was also prepared as a comparison by hydrolysis of iron nitrate with ammonia followed by calcination at 773 K in air. Characterization. To show the crystal structure, powder X-ray diffraction (XRD) was recorded by a Rigaku Miniflex Plus diffractometer with 0.45 kW Cu KR X-ray source (30 kV, 15 mA). 29Si NMR (nuclear magnetic resonance) was measured by a JEOL GSX 270 spectrometer with 53.67 MHz magnetic field frequency, 1.8 µs of 40° pulse width, 15 s observation time and 10 kHz spinning rate. ESR (electron spin resonance) spectrum was collected by JEOL JES-FE2GX spectrometer with 0.08 g of the sample at 113 K. Adsorption isotherm of nitrogen was measured at 77 K under 1 Pa to 90 kPa of the nitrogen pressure (P/P° ) 10-4-0.9). The surface area was calculated from the isotherm according to the Langmuir equation. A scanning electron microscopy (SEM) photograph was taken with a JEOL JSM-5800 scanning microscope after the deposition of thin gold film by a JEOL FINE COAT JFC-1100 ion sputter. The infrared (IR) spectrum was collected on the self-supporting disk with 10 mm diameter molded from 10-20 mg of silicate powder in an in situ cell by a JASCO FT/IR-5300 spectrometer.
Katada et al. Temperature-Programmed Desorption of Ammonia. The equipment [homemade or Japan Bel Inc. TPD-1-AT-(NH3)] and procedure were detailed in our review.31 The sample (0.1 g) was set in a quartz cell and evacuated at 773 K for 1 h under
