Ind. Eng. Chem. Res. 2003, 42, 2783-2790
2783
MATERIALS AND INTERFACES Characterization and Catalytic Properties of Titanium-Pillared Clays Prepared at Laboratory and Pilot Scales: A Comparative Study Jose´ L. Valverde,† Antonio de Lucas,† Fernando Dorado,*,† Rosario Sun-Kou,‡ Paula Sa´ nchez,† Isaac Asencio,† Agustı´n Garrido,† and Amaya Romero† Departamento de Ingenierı´a Quı´mica, Facultad de Quı´micas, Universidad de CastillasLa Mancha, Campus Universitario s/n, 13004 Ciudad Real, Spain, and Departamento de Ciencias, Seccio´ n de Quı´mica, Pontificia Universidad Cato´ lica del Peru´ , Lima, Peru´
The textural and structural characteristics and the acid properties of Ti-pillared montmorillonites prepared at bench scale (1 kg per batch level) have been compared with those prepared at laboratory scale (a few grams). The pillared clays have been examined by X-ray diffraction and characterized by different techniques and methods including nitrogen sorption isotherms, temperature-programmed desorption/reduction, and atomic absorption. The catalytic performance was evaluated by means of the selective reduction of NO by propylene over Cu2+ ion-exchanged samples. The differences of the textural characteristics between the laboratory and pilot samples did not significantly affect the catalytic results. 1. Introduction The proven applications of pillared clays in the fields of heterogeneous catalysis and adsorption, coupled with the low price of the starting raw clays, have prompted interesting perspectives for their production on an industrial scale. However, it is necessary to understand the successive steps involved in the preparation of these materials in order to successfully scale-up the process. Al-,1-3 Cr-,4,5 Zr-,6,7 and Fe-pillared8,9 clays are among the principal pillared intercalated clays reported in the literature. Despite the interesting catalytic properties of TiO2, Ti-pillared clays (TiPILCs) have received less attention than the other oxide-pillared clays. Two different procedures for the preparation of titaniumpillared clays have been performed: the first method uses a TiCl4 solution in hydrochloric acid,10-13 whereas the second route employs hydrolyzed titanium alkoxide in HCl as the pillaring agent.14,15 Both methods have proven to be suitable for the preparation of TiPILCs, although the first one requires careful handling of TiCl4. Among the pillared clays, TiPILCs have the following remarkable characteristics: (a) they have a high thermal stability;11,16 (b) their large pore sizes allow further incorporation of active species without hindering pore diffusion;17 (c) intercalating TiO2 between the SiO2 tetrahedral layers is a unique way of increasing the surface area and acidity of the TiO2 support;18 (d) TiPILC-based catalysts have shown to be excellent for the selective catalytic reduction (SCR) of NO by NH3 or hydrocarbons,19-22 and they have been found to be * To whom correspondence should be addressed. Tel.: +34926295300. Fax: +34-926295318. E-mail: fernando.dorado@ uclm.es. † Universidad de CastillasLa Mancha. ‡ Pontificia Universidad Cato´lica del Peru´.
highly resistant to SO2 poisoning and also possess durability.17,18,23 On the other hand, most of the works reported in the literature have been performed over pillared clays prepared at laboratory scale, generally a few grams per batch. Studies on samples produced by scale-up preparation methods at the pilot level are almost nonexistent in the open literature. Intercalation is generally carried out in dilute systems wherein a dilute homoionic clay suspension is brought into contact with a dilute intercalating solution. This requires handling of large suspension volumes during the preparation and washing steps. Vaughan24 indicated the importance of three prerequisites for the manufacturing of commercial catalysts: (1) use of the whole clay, with minimal refining or nonrefining; (2) pillaring of the clay without previous cationic exchange; (3) use of high clay and polycation concentration. These prerequisites have been considered by several authors.25-27 Problems encountered in the scaling up of a small pilot production of Al- and Al/Fe-pillared clays have been addressed.28 The catalytic properties of these pilot samples have been evaluated under several reactions, e.g., terpene conversion,29 SCR of NO,30 or hydroconversion of heptane.31 The main objective of this work was to compare the textural and structural characteristics and the acid and catalytic properties of titanium-pillared clays prepared in amounts of a few grams, a typical quantity for laboratory syntheses, with pilot-produced materials (about 1 kg), obtained by scaling up pillaring procedures commonly used in laboratory preparations. This information is of interest from the perspective of the potential uses of such materials in industrial catalytic processes. SCR of NO by propylene in the presence of water
10.1021/ie0208772 CCC: $25.00 © 2003 American Chemical Society Published on Web 05/08/2003
2784
Ind. Eng. Chem. Res., Vol. 42, No. 12, 2003
Figure 1. Pilot plant: (1) osmosis plant; (2) osmosized water tank; (3) pillaring solution tank; (4) synthesis tank; (5) slurry stream; (6) basket centrifuge; (7) filtrated stream; (8) filtrated stream recirculation; (9) waste stream; (10) centrifuge pump; (11) periltastic pump.
and oxygen, over Cu ion-exchanged samples, was used as a test reaction. 2. Experimental Section 2.1. Pillaring Procedures. (a) Laboratory Samples. The starting clay was a purified montmorillonite (purified-grade bentonite powder from Fisher Co.), which had a particle size of 2 µm or less and a cationexchange capacity (CEC) of 97 mequiv/100 g of dry clay. The pillaring solution of partially hydrolyzed Ti polycations was prepared by first adding titanium methoxide to a 5 M HCl solution to obtain a HCl/Ti molar ratio of 2.5. The resulting solution was aged for 3 h at room temperature. Then, 1 g of starting clay was dispersed in 0.75 L of deionized water for 3 h under stirring. The pillaring solution was then slowly added with vigorous stirring into the suspension of clay until the amount of pillaring solution reached that required to obtain a Ti/ clay ratio of 15 mmol of Ti/g of clay. The intercalation step took about 16 h. Subsequently, the mixture was separated by vacuum filtration or centrifugation and washed with deionized water until the liquid phase was chloride-free. The sample was dried at 120 °C for 12 h and calcined for 2 h at different temperatures. (b) Pilot Samples. The pillared samples prepared at pilot scale were essentially obtained in conditions similar to those of the laboratory-scale ones (HCl/Ti ) 2.5 mol/mol; Ti/clay ) 15 mol/kg; H2O/clay ) 0.75 kg/ g). The main differences were the volumes of both the pillaring solution and the clay suspension. Typically, 1 kg of raw clay was pillared in every batch. The pilot plant designed to prepare different types of PILCs (Figure 1) consisted of three areas: pillaring solution area, reaction area, and filtration area. It was equipped with almost all of the utilities typically needed for the synthesis of industrial catalysts. The pillaring solution area consisted of a tank with a capacity of 25 L and working temperatures up to 100 °C, an agitation system with variable speed (40-2000 rpm), and a dosage system of the pillaring solution that comprised a peristaltic pump and acid solutions resistant hoses. The reaction area was equipped with a tank where the ion exchange took place, with 1000 L capacity and working temperatures up to 200 °C, a controlled agitation system, and a centrifuge pump which supplied deionized water to the tank. Finally, the filtration area was equipped with a continuous centrifugation system
based on a peristaltic pump that supplied the suspension to a basket centrifuge. A recirculation system to the tank that is placed in the centrifuge outlet allows one to avoid sample loss. Wet cakes were further processed with tray dryers and calciners. The total product capacity of the pilot plant was around 1 kg of clay per batch. Laboratory-scale pillared clays and pilot-scale samples will be distinguished by adding respectively “lab” and “ps” to the name of the sample. 2.2. Catalyst Characterization. The X-ray diffraction (XRD) patterns were obtained using a Philips model PW 1710 diffractometer with Ni-filtered Cu KR radiation. To maximize the (001) reflection intensities, oriented clay-aggregate specimens were prepared by drying clay suspensions on glass slides. The XRD pattern of the parent clay exhibits a main XRD peak at around 9°. This peak is commonly assigned to the basal (001) reflection [d(001)]. In pillared clays, the d(001) peak was found to shift toward the lower 2θ region, which is a clear indication of the enlargement of the basal spacing of the clay. The residual CECs were determined by successive ion-exchange steps with KCl (three times) followed by measurement of the K content on the clay. The samples were calcined at 400 °C. The residual CECs were determined because they provided an evaluation of the layer charge, which was not compensated for by the positively charged pillaring species. Surface area and pore size distribution were determined by using nitrogen as the sorbate at 77 K in a static volumetric apparatus (Micromeritics ASAP 2010 sorptometer). Pillared clays were previously outgassed at 180 °C for 16 h under a vacuum of 6.6 × 10-9 bar. Specific total surface areas were calculated by using the Brunauer-Emmett-Teller (BET) equation, whereas specific total pore volumes were evaluated from nitrogen uptake at a relative pressure of N2 (P/P0 equal to 0.99). The Horvath-Kawazoe method was used to determine the microporous surface area and micropore volume. To quantify the total amount of metals incorporated into the catalyst, atomic absorption measurements, with an error of (1%, were conducted by using a Spectraa 220FS analyzer with simple beam and background correction. The samples were previously dissolved in hydrofluoric acid and diluted to the interval measurement. The total acid site density of the catalysts was measured by temperature-programmed desorption of ammonia (TPDA), using a Micromeritics TPD/TPR 2900 analyzer. The samples were housed in a quartz tubular reactor and pretreated in flowing helium (g99.9990% purity) while heating at 15 °C/min up to the calcination temperature of the sample (500 °C). After a period of 30 min at this temperature, the samples were cooled to 180 °C and saturated for 15 min in an ammonia stream (g99.9990% purity). The catalyst was then allowed to equilibrate in a helium flow at 180 °C for 1 h. Next, the ammonia was desorbed by using a linear heating rate of 15 °C/min up to 500 °C. Temperature and thermal conductivity detector signals were simultaneously recorded. The area under the curve was integrated to determine the relative total acidity of the catalyst. The average relative error in the acidity determination was lower than 3%. TPR measurements were carried out with the same apparatus as that described above. After loading, the
Ind. Eng. Chem. Res., Vol. 42, No. 12, 2003 2785
Figure 2. XRD patterns of the original clay and the pillared samples prepared both at laboratory and pilot scale calcined at (a) 200 °C and (b) 400 °C.
sample was outgassed by heating at 15 °C/min in an argon flow up to the calcination temperature of the sample and kept constant at this temperature for 30 min. Next, it was cooled to room temperature and stabilized under an argon/hydrogen flow (g99.9990% purity; 83/17 volumetric ratio). The temperature and thermal conductivity detector signals were then continuously recorded while heating at 15 °C/min up to 500 °C. The liquids formed during the reduction process were retained by a cooling trap placed between the sample and the detector. It was previously found that montmorillonite consumes some hydrogen at temperatures >500 °C because of the reduction of the metals in its structure. This consumption is negligible at temperatures
