Synthesis, characterisation and catalytic studies of ZSM-5 and TS-1 prepared by a new method

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Studies in Surface Science and Catalysis, volume 154 E. van Steen, L.H. Callanan and M. Claeys (Editors) © 2004 Elsevier B.V. All rights reserved.

SYNTHESIS, CHARACTERISATION AND CATALYTIC STUDIES OF ZSM-5 AND TS-I PREPARED BY A NEW METHOD Sankar, G . \ Zenonos, C . \ Beale, A . M . \ Sanchez-Sanchez, M . \ Franklin, I.L.^ and Garcia-Martinez, J.^ ^Davy Faraday Research Laboratory, The Royal Institution of GB, 21 Albemarle Street, London WIS 4BS, UK. ^Departamento de Quimica Inorganica. Universidad de Alicante. Ap. 99, E-03080, Alicante, Spain. ABSTRACT Aluminosilicate ZSM-5 is produced directly from high-silica zeolite Y or zeolite (3 by a simple hydrothermal treatment of the alkali hydroxide treated starting zeolite material and without using any structure directing organic agent. NMR and FTIR results clearly suggest that majority of the Al(III) species is present in the framew^ork yielding Bronsted acid sites. Addition of appropriate template such as tetrapropylammonium bromide or tetrabutylammonium bromide directs the formation of ZSM-5 and ZSM-11, respectively. The protonated form of all the ZSM-5 catalysts shows very good catalytic activity for the conversion of methanol to hydrocarbon. Similarly, by taking a titanium grafted zeolite b as a starting material, TS-1 has been prepared. Both Ti K-edge XANES and epoxidation of 1-octene confirms the presence of active Ti(IV) centres in this catalyst. Keywords: ZSM-5, TS-1, zeolite, titanosilicate, XANES and epoxidation INTRODUCTION Microporous aluminosilicates, titanosilicates, silicoaluminophosphates and transition metal ions substituted aluminophosphates have found a variety of applications over the last two decades.[l-13] By appropriate choice of the chemical composition, it is possible to design and synthesize environmentally benign microporous catalysts that can be employed to perform highly shape selective reactions. The most challenging step in producing these materials is the synthesis of a microporous solid with a specific structure for a given catalytic application. Although a number of synthetic approaches for the production of microporous aluminosilicates have been considered in the literature, since the early work by Barrer[14], one of the most commonly used procedure in the recent decade is the use of an organic template molecule, preferably an amine or a quaternary ammonium hydroxide as the structure directing agent (SDA). In addition, in a few recent studies, zeolites were also used as the source for silica or alumina to produce other microporous architectures with the aid of an organic SDA and furthermore, it was also shown that certain aluminosilicates such as zeolite Y or zeolite A transform to other microporous (more dense form) aluminosilicates, but these examples were restricted to only low silica materials.[15] Although, the use of these organics has certain advantages, in particular directing the formation of a specific structure, considerable effort has, however, been directed towards the synthesis of microporous solids without the use of organic template. The advantages of this approach include reduction in the production costs and in addition, it is possible to avoid the release of toxic carbon or nitrogen products, produced when the templated microporous materials are subjected to heat-treatment, in air, to remove the occluded organic species from the zeolitic pores during the activation process. A number of zeolites (eg. zeolite A, zeolite Y or X or Chabazite) have been prepared without using any organic templates. [16,17] Amongst the various microporous aluminosilicates, ZSM-5 has found an important role in many shape selective reactions, since this material is the most acidic in the aluminosilicate family of catalytic materials. [18] In addition, since the discovery of TS-1,[6] a titanium substituted analogue of ZSM-5 which efficiently oxidises alkenes in the presence of hydrogen peroxide, considerable attention has been paid towards the understanding of the synthesis, structure and catalytic properties of metallosilicates having the silicalite structure.[6,13] The most commonly employed procedure of preparing aluminium (or titanium) containing materials with an MFI structure is by subjecting a gel mixture containing appropriate amounts of aluminium oxide (or titanium oxide), silicon oxide, sodium hydroxide and more importantly tetrapropylammonium bromide (TPABr) as

759 the SDA;[ 19,20] there are few reports that deal with the preparation of ZSM-5 without the use of an organic agent. [21-24] Here we describe the production of ZSM-5 and TS-1 without the aid of a SDA, which we have achieved by the use of commercially available zeolite Y or zeolite (3 (as the silicon and aluminium source) which were subjected to hydrothermal treatment after reacting with sodium hydroxide solution [25]. EXPERIMENTAL In a typical synthesis of ZSM-5, 1 gram of zeolite Y or zeolite (3 having Si/Al ratio of ca 80 (in the case of zeolite (3, materials containing Si/Al ratio of 150 and 300 was also used) was reacted with 0.3M solution of NaOH. The resulting mixture was stirred for ca 1 hour before transferring to a Teflon lined autoclave for hydrothermal treatment at temperatures between 150 and 175°C for about 24 hours. In the preparation of TS-1, we employed the following procedures. First we dehydrated dealuminated zeolite (3 and to which we grafted titanocene dichloride. This material was calcined first and then subsequently reacted directly with tetrapropyl ammonium hydroxide or NaOH or tetrapropylammonium bromide and NaOH. This system was subjected to hydrothermal treatment at 150°C for ca 2 days. Ti K-edge X-ray absorption spectra (XAS) were recorded, in situ, at the station 8.1 of Daresbury Synchrotron Radiation source, which operates at 2GeV with a typical current between 150-250mA. Si(l 11) double crystal monochromator was used in this work and fluorescence mode of detection was employed to record XAS data. Data were processed using the EXCALIB and EXBROOK programme. RESULTS AND DISCUSSION From the XRD patterns shown in figure 1, it is clear that irrespective of the starting zeolite material, they dissolve in NaOH solution forming a slightly turbid mixture and that when subjected to hydrothermal treatment for few hours only yields an amorphous solid. Highly crystalline ZSM-5 material is formed after 24 hours of hydrothermal treatment. However, prolonged (beyond 36 hours) hydrothermal treatment or at elevated temperature (above 150''C) resulted in the further conversion of ZSM-5 to dense phases such as tridymite or quartz. In a separate experiment, we mixed appropriate amounts of the SDA to the turbid solution of zeolite Y or |3 dissolved in NaOH, before introducing the solution to the Teflon lined autoclaves. When the SDA, tetrapropylammonium bromide or tetra butyl ammonium bromide, is added to a mixture of the starting zeolite in the alkaline solution and after hydrothermal treatment for 24 hours at 150°C we form respectively, highly crystalline ZSM-5 or

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29degiees Figure 1. Typical XRD patterns of (bottom) zeolite Y (Si/Al=80), (middle) zeolite p dissolved in NaOH and hydrothermally treated for 8 hours at 150oC. In (top) we show the typical ZSM-5 material obtained after hydrothermal treatment, for 24 hours, of the NaOH reacted zeolite starting material. ZSM-11 materials; it should be noted that by using the high silica form of zeolite Y or |3, it was not necessary to add excess silica to the gel mixture. [26] It is also interesting to note that when tetraethylammonium hydroxide is used as the SDA (typical structure directing agent used for the synthesis of zeolite (3), zeolite (3 was produced only when zeolite (3 was used as the starting material.

760 ZSM-5 prepared by this new method was also characterized by NMR study and IR techniques. The silicon and aluminium environment in both the initial solution (starting zeolite material dissolved in NaOH) and the final solid obtained from both the templated and non-templated synthesis procedure were studied by NMR. The solution ^^Si NMR shows clearly the presence of Q, Ql, Q2, Q3 species,[27] suggesting that there is a complete dissolution of the starting crystalline zeolite. In figure 2 we show the typical ^^Si and ^^Al NMR of two of the various ZSM-5 materials (final products, converted to proton form) prepared by different methods. Although we do not see appreciable differences in the spectra of the various materials, it is noteworthy that a decrease in the ^^Si signal associated with silicon with one aluminium neighbour and a corresponding increase in the signal associated with that for an octahedral Al(III) species in ^^Al NMR spectra. However, in situ IR data (see figure 3) clearly shows the presence of Bronsted acid sites, as seen by the intense band at 3610 cm'^ characteristic of the OH stretching frequency associated with the Si-O-Al bridges. This bridging hydroxyl band is seen in all the transformed, protonated zeolites, irrespective of whether the preparation was carried out with or without a template molecule. Catalytic studies of the ZSM-5 indeed confirm that the protonated form of the transformed materials converts more than 90% of the methanol to olefins at typical temperatures above 250°C and demonstrates that the materials prepared by this method are highly active and selective for acid catalysed reactions.

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Figure 2. Typical NMR spectra of ZSM-5 prepared from(I) and (III) H-zeolite Y and (II) and (IV) from H-zeolite (3 (Si/Al=75). We present the ^^Si on the left and ^^Al NMR spectra on the right.

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wurenaiiihir # cm Figure 3. Typical IR spectra of the dehydrated, protonated form of ZSM-5 prepared without the aid of organic template from zeolite Y (Si/Al=80). For clarity only the spectral region corresponding to the hydroxyl groups are shown here. In order to extend the above finding, of converting other zeolites to ZSM-5 with and without the assistance of a SDA, to the preparation of other metallosilicates having silicalite structure, we carried out a

761 systematic investigation using a dealuminated zeolite (3 as the starting material to prepare TS-1. To do so, we adopted the same procedure, described in our earlier work on MCM-41,[5] of grafting titanocene dichloride onto a dehydrated, dealuminated sample of zeolite p.

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Figure 4. On the left we show the Ti K-edge XANES of calcined Ti(lV) grafted zeolite p. On the right calcined and dehydrated TS-1 prepared using this titanium zeolite p as the starting material (1) prepared without an organic template molecule and (2) prepared using TAPOH as the structure directing agent. The pre-edge peak is marked as A. This grafted sample (referred as Tifzeolite P) was first calcined in air to obtain the starting material for the preparation of TS-1. Investigation of the pre-edge peak of the Ti K-edge X-ray absorption near edge structure (XANES),[ 11,28,29] see figure 4, of this calcined Tifzeolite p material suggests that we have tetrahedral titanium centres. This calcined Ti^zeolite p was subjected to three types of treatments. In the first procedure, Titzeolite p was used as the starting material to prepare silicalite with the aid of tetrapropylammonium bromide as the template molecule. In a typical synthesis, one gram Ti|Zeolite-P was dissolved in 30ml 0.3M NaOH solution and stirred for 30 minutes. 0.21 grams of TPABr was added and stirred for a further 1-2 hours. This solution was subjected to standard hydrothermal conditions for ca 24 hours at 175°C. XRD pattern of the dried sample suggests that it was possible to prepare pure TS-1 by this method and more importantly, the pre-edge intensity in the Ti K-edge XANES spectra of (see figure 4) the calcined sample indeed suggest that we have a high quality TS-1 sample. In a second method, we mixed the calcined Tifzeolite p with tetrapropylammonium hydroxide, (without NaOH) as the dissolving and templating agent. In a typical synthesis. One gram TifZeolite-P was dissolved in 2 grams of TPAOH (40% solution) and was stirred for a further 1-2 hours. This solution was subjected to standard hydrothermal conditions for ca 24 hours at 175°C. The resufting solid was indeed TS-1, as seen from the XRD pattern. Once again, from the pre-edge intensity of the Ti K-edge XANES data (see figure 4) of the calcined material, it can be inferred that the majority of the titanium centres are present in tetrahedral coordination. The third and most important procedure for the preparation of TS-1 is the simple hydrothermal treatment of the calcined Titzeolite P reacted with NaOH solution. In a typical synthesis. One gram TifZeolite-p is dissolved in 30ml 0.3M NaOH solution and stirred for 3-4 hours. This solution was subjected to standard hydrothermal conditions for ca 4 days at 150°C. The XRD pattern of the resulting solid produced by this non-templated method indeed resembles that of the XRD patterns of TS-1 produced with template. Comparison of the pre-edge of the Ti K-edge XANES data of the calcined TS-1 (prepared without template) with the

762 as-prepared (See figure 4) indeed suggest there are significant number of tetrahedral titanium species present in the material. Although the pre-edge intensity of the calcined TS-1 material, prepared without the use of an organic template molecule, is not as high as of the ones prepared with template, this new preparative approach is a significant step forward, which can be fine tuned to produce high-quality TS-1 catalysts. A detailed investigation using scanning electron microscopy and ED AX revealed that the titanium is distributed evenly over all the crystals. Catalytic conversion of 1-octene to octene oxide in the presence of H2O2 indeed suggested that the catalysts are active for oxidation reaction. Once again the TS-1 prepared in presence of an organic template molecule appears to be more active compared to the one synthesised without a template molecule, in line with the XANES results. In summary, the results of the present study clearly shows that by appropriate choice of Si/Al ratio, it is possible to prepare ZSM-5 without the aid of either seeding or templating agents; a novel approach which is likely to be useful for the production of zeolitic membranes and environmentally benign catalytic materials. Furthermore, our study indicated that it is possible to extend this approach successfully for the preparation of the important metallosilicate, TS-1. ACKNOWLEDGEMENTS We thank EPSRC for financial support and CLRC for the facilities at the SRS Daresbury laboratory. GS thank Leverhulme trust for a senior research fellowship. JG-M thanks the Ministerio de Educacion, Cultura y Deportes de Espana for an FPI fellowship. We also thank Dr D. Gleeson and Dr Abil Aliev for useful discussions. MSS thanks the EU for a Marie-Curie fellowship.

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