J Porous Mater (2007) 14:71–79 DOI 10.1007/s10934-006-9010-5
An improved route for the synthesis of Al13-pillared montmorillonite catalysts L. V. Duong Æ J. T. Kloprogge Æ R. L. Frost Æ J. A. R. van Veen
Received: 27 October 2005 / Revised: 20 March 2006 / Published online: 2 December 2006 Springer Science+Business Media, LLC 2006
Abstract The distribution of Al13 pillars and the process of intercalation in montmorillonite can be enhanced through the application of an ultrasonic treatment. This paper describes the results of ultrasonic treatment in the preparation of Al-pillared montmorillonite with and without prior exchange with Na+. The resulting materials have been characterised by X-ray diffraction (XRD), N2 adsorption/desorption, Scanning Electron Microscopy and Atomic Force Microscopy (AFM). The catalytic activity was tested with the n-heptane hydroconversion test. Optimum results were obtained after ultrasonic treatment up to 20 min without prior Na-exchange before the Al13 intercalation. Longer ultrasonic treatment resulted in partial destruction of the pillared structure. The pore size diameter also increased with increasing ultrasonic treatment up to 20 min with values in the range of 4 nm. This behaviour can be explained by the loss of the typical house of cards structure after prolonged ultrasonic treatment. AFM showed that the pillars in the interlayer of the montmorillonite resulted in a distortion of the tetrahedral sheets of the clay. At atomic scale resolution it was clear that the pillar distribution is not homogenous, confirming earlier results
L. V. Duong (&) J. T. Kloprogge R. L. Frost Inorganic Materials Research Program, School of Physical and Chemical Sciences, Queensland University of Technology, GPO Box 2434, Brisbane Q 4001, Australia e-mail:
[email protected] J. A. R. van Veen Shell Research and Technology Centre Amsterdam, Shell International Chemicals, B.V., Badhuisweg 3, 1031 CM Amsterdam, The Netherlands
using high resolution TEM. The effects of ultrasonic treatment on the catalytic activity is rather limited, although the pillared clays prepared with short ultrasonic treatments of 5 and 10 min performed slightly better. Keywords AFM Al13 Pillared clay Montmorillonite n-Heptane conversion N2 adsorption/desorption SEM Ultrasonic treatment
1 Introduction Acid-treated montmorillonite clays were the catalysts commonly initially used for cracking reactions of hydrocarbons in the 1930s [1]. These acid-treated smectites catalysts were replaced after World War II with a more stable synthetic silica–alumina type which also gave better product distribution [1]. The emergence of zeolites in the 1960s revolutionised the process mainly because of their high activity, selectivity and resistance to collapse when treated at high temperatures [2, p. 30]. Nowadays, ZSM-5 and Y-zeolite are among the most popular heterogeneous catalysts in the petrochemical industry. The interest now is in producing a catalyst with a larger pore ˚ ) so as to handle the size compared to zeolite (~8 A cracking of heavier crude oil. The use of pillared clays has received considerable attention [3] because of their ability to achieve large pore sizes, but factors such as the large volumes of water and chemicals involved in the preparation, the thermal stability and coking properties still need to be overcome. Altering the preparation of a PILC can have dramatic effects on properties such as thermal stability and acidity [3–5]. This area also has received considerable
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attention with many authors who are looking at ways to economise the process for commercial viability. Current problems in preparation are time and energy costs, water usage and preparation of the expanded clay suspensions. The preparation of a pillared clay normally starts with an cation exchange step where the hydrated interlayer cation of the smectite clay is exchanged for sodium. This way an increase in swellability is achieved, making it easier to incorporate those large metal polyoxocomplexes. This cation exchange involves the use of large amounts of water and sodium salts. It would be a significant improvement if this step could be altered into a less waste-producing step. The use of ultrasonics for the cation exchange step has been reported [6, 7]. A Ca-montmorillonite was intercalated with the Al13 complex using ultrasonic treatment over a number of time periods. The most intense and sharpest peaks in the X-ray diffraction (XRD) patterns were observed for the calcined sample that had been left in the ultrasonic bath for 20 min. The same authors, in a later study [7], described how the exchangeable cations present in the smectite affected the ultrasonic treatment. They converted the Ca-montmorillonite to Na+ and La3+ forms by ion exchange. This gave exchangeable cations with valencies of +1, +2 and +3. The optimum times for ultrasonic treatment were found to be 5 min for the Na-exchanged form, 20 min for the Ca-exchanged form and 80 min for the La-exchanged form. The increase in time was ascribed to the higher charge ions being more tightly bound to the clay layers. This method of intercalation has a number of advantages that help to make largescale production of pillared clays more viable. Firstly, it reduces the time needed from several hours to less than 30 min. It also requires no heat for the process, thus saving in costs and reducing the safety risks, although some safety issues arise with ultrasonics that would need to be addressed. Finally, the clay suspension required don’t have to be cation exchanged and can be more concentrated compared to conventional methods, thus using less water, sodium salts and space. This paper describes a detailed study on the use of ultrasonic treatment of a number of smectite clays for the intercalation with Al13 Keggin complexes.
2 Experimental 2.1 Starting materials The starting materials used for this study were £ 2 lm fractions of Cheto montmorillonite SAz-1, and Miles
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montmorillonite from Queensland, Australia. The Miles montmorillonite has a significantly higher CEC than SAz-1 (see special issue nr. 5 of Clays and Clay Minerals, volume 49, 2001 for a detailed characterisation of SAz-1). A detailed description of the Miles material and the conventional Al13-pillaring procedure have been provided by Kloprogge et al. [8]. Pillaring with Al13 of non-exchanged and Na-exchanged montmorillonites was executed in an ultrasonic bath with increasing time intervals from 0 to 30 min at room temperature. For all experiments a clay suspension of 30% (w/w) in distilled water was prepared under stirring for 30 min prior to the intercalation with Al13. After washing and drying at room temperature for 24 h the expanded clays were calcined at 450 C for 2 h (heating rate 5 C/min.). 2.2 Analytical techniques 2.2.1 X-ray diffraction (XRD) The nature of the resulting material was checked by X-ray powder diffraction (XRD). The XRD analyses were carried out on a Philips wide angle PW 1050/25 vertical goniometer equipped with a graphite diffracted beam monochromator Fig. 2. The d-values and intensity measurements were improved by application of an in-house developed computer aided divergence slit system enabling constant sampling area irradiation (20 mm long) at any angle of incidence. The goniometer radius was enlarged to 204 mm. The radiation applied was CoKa from a long fine focus Co tube operating at 35 kV and 40 mA. The samples were measured at 50% relative humidity in stepscan mode with steps of 0.02 2h and a counting time of 2 s. 2.2.2 Scanning electron microscopy (SEM) Scanning electron microscope (SEM) images were obtained on a FEI Quanta 200 Environmental SEM (FEI Company, USA) operated at an accelerating voltage of 15 kV. 2.2.3 Atomic force microscopy (AFM) Sample preparation involved a SiO2 surface for attachment of the clay sheets. An industry-standard n-type Si wafer with RMS roughness of 0.2 nm covered by a native oxide layer constituted the surface. The Si surface was exposed to ultrasonic cleaning with isopropyl-alcohol, followed by rinses in doubly distilled and deionised water (DDDW). Specimens of 1 cm2 area were sectioned and attached to standard AFM
J Porous Mater (2007) 14:71–79
a
Na-exchanged - 20min Usound Na-exchanged - 10min Usound
Counts
Fig. 1 (a) Non-calcined Al13montmorillonite SAz-1. (b) Non-calcined Na-exchanged Miles montmorillonite intercalated with Al13. (c) Non-calcined Miles montmorillonite intercalated with Al13
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Na-exchanged - 5min Usound Non-exchanged- 20min Usound Non-exchanged- 10min Usound Non-exchanged - 5min Usound
2.5
7.5
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22.5
Degrees 2θ
Counts
b
20 minutes
10 minutes
5 minutes
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7.5
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Degrees 2θ
Counts
c
20 minutes
10 minutes
5 minutes
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7.5
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Degrees 2q
mounts. The clay was diluted with DDDW (filtered) and allowed to dry on the Si surface at room temperature (~23 C). The work was carried out on a JEOL JSPM-4200 system with a 25 lm tube scanner, with a z-range of ca. 3 lm. The system is based on the detection of the tipto-surface forces through monitoring optical deflection of a laser beam incident on a force-sensing/imposing
lever. The analyses were carried out under air-ambient conditions (temperature of 23 C and 65% relative humidity). The probes were of the beam-shape variety in order to ensure that only the simple lowest-order bending modes contributed to the response. Probes were obtained from Ultrasharp NT-MDT. The characteristics of probes employed in the present study are listed below.
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Designation
RTip
Ar
Surface chem.
kN (Nm–1)
A
