ZSM-5 zeolitinin sentezinde Si/Al oranı ile kristal boyut ve morfolojisinin denetlenmesi

Abstract

Doğal ve sentetik zeolitler düzenli gözenek yapıları, geniş iç yüzey alanları, değişebilen katyonları ve moleküler düzeydeki şekil ve boyut seçici özellikleri ile son yılların en fazla araştırılan malzeme gruplarından biri hali ne gelmiş, iyon değiştirici, adsorban ve katalizör olarak çeşitli uygulama alanları bulmuşlardır. Si/Al oranı 5 - oo aralığında değişebilen ZSM-5, yüksek termal, hidrotermal ve asit kararlılığı, kuvvetli asit merkezleri ve mükemmel boyut ve şe kil seçicilik özellikleri ile sentetik zeolitlerinin en önemlilerinden biridir. Ad sorban ve katalizör olarak kullanıldığı uygulamalar açısından ZSM-5' in iste nen Si/Al oranı ile boyut ve morfolojide sentezlenebilmesi çok önemidir. Bu çalışmanın amacı ZSM-5 zeolitinin kristalizasyonu sırasında Si /Al oranı ile kristal boyut ve morfolojisinin denetlenebilmesidir. Bu amaçla, 170 °C 'da, sabit H20/Si02=25 oranında, (TPA)20, Na20, Al203 ve OH miktarları ile TPA ve silika kaynakları değiştirilerek çeşitli sürelerde sentez deneyleri yapılmıştır. 100 mol Si02 başına (TPA)20 miktarı 1-12.5, Na20 miktarı 3.5-17.5, AI2Û3 miktarı 0-2, OH miktarı ise 7-46 mol aralıklarında değiştirilmiştir. TPA kaynağı olarak TPA-Br ve TPA-OH, Na20 kaynağı olarak NaOH kullanılmış, silika kaynağı olarak ise Ludoks kolloidal silika çözeltisi ile Cabosil kullanımları denenmiştir. Sentez deneyleri 18 ml'lik, içi teflon kaplı, paslanmaz çelik otoklavlarda gerçekleştirilmiş, elde edilen kristaller, X-ışını kırınım analizi, taramalı elektron mikroskopisi, atomik absorpsiyon ve yaş kimyasal analizler ile karakterize edilmiştir. Deneyler sonucunda Si/Al oranlan ile boyut ve morfolojileri çok geniş bir aralıkta değişen ZSM-5 kristalleri sentezlenebilmiş ve özellikleri sentez bileşimleri ile ilişkilendirilebilmiştir. Denenen parametrenin çoğunun iç ilişki li olduğu, bir başka deyişle etkilerinin diğer parametrelerin belli seviyelerin de önem kazandığı görülmüştür. Çalışılan bileşim aralığında genel olarak, Na arttıkça özellikle düşük Si02/AI203 oranlarında morfolojinin bozulduğu OH arttıkça ise tersine, yüksek Si02/AI203 oranlarında sentezlenmiş zeolitin çözünmeye başladığı gözlenmiştir. Çözünmenin başladığı alkalinitede TPA oranının artması çözünmeyi geciktirmektedir. Yüksek alkalinitede Na' un da yapıyı stabilize edici benzer bir işlevi vardır. OH' in tane boyutunu küçültücü, TPA'nın da belirli bir minimum seviyeye kadar kristalizasyonu hızlandırıcı etkileri olduğu görülmüştür.

The Controlling of Morphology and Crystal Size with Si/AI Ratio in Synthesizing of Zeolite ZSM-5 Zeolites are open framework aluminosilicates, widely used in industrial applications as ion exchange resins, moleculer sieves, and catalysts and catalysts supports. Formally, they can be represented as being derived from silica (SİO2) by the replacement of SiO^" tetrahedra by AIO45" tetrahedra and described by general oxide formula, where the part in square brackets represents the lattice. My/x[(AI02)y(Si02)i-y].nH20 Because of the difference in atomic charge between Al and Si, extra-lattice cations ( Mz+) must be present to preserve electrical neutrality, a single positive charge needed for each ( AIO2 ) unit. These cations are not part of the framework and may be easily exchanged. In addition, water of hydration is usually present but also is not part of the lattice structure. The catalytic activity of zeolites is generated by converting them to an M acid form " by heating the ammonium form " of zeolite. This causes decomposition of the ammonium ions and yields a material where extra- lattice cations are H+. This will now act as a very powerful acid catalytst in reactions of hydrocarbons such as isomerizations, alkylations, and hydrogen transfers. The size and shape-selective characteristics of zeolites come from their unique framework structures : In general they are formed from open VI arrangements of AIO^" and SİO44" tetrahedra linked by sharing oxygen atoms. The Al and Si atoms are often referred to as T ( tetrahedral) atoms. Although zeolites having these unique properties have been known for a long time as natural zeolites, increasing interest has only been devoted in 70's to synthetic crystalline aluminosilicate zeolites. During this period, different synthetic zeolites with SİO2/AI2O3 ratio in the range of 2:1 to 10:1 have been used in petroleum industry. For example, zeolite A and X are useful as sorbents, zeolite Y is used extensively as cracking catalyst and synthetic mordenite ( with its SİO2/AI2O3 ratio of 10:1 is among the most siliceous synthetic zeolites) is used in a few particular processes. Some years ago, the laboratories of the Mobil Oil Corporation have systhesized a novel class of highly siliceous zeolites, belonging to the Pentasil group, exhibiting very valuable catalytic properties such as low aging, shape selectivity and high activity for a variety of chemical reactions. Among these materials, zeolite ' ZSM-5 ' has been investigated in greatest detail and has become a material of high commercial importance. These ZSM-5 zeolites are synthesized from reaction mixtures that contain organic nitrogeneous cations. Because the organic molecules are incorporated into the zeolite channels, these zeolites were called ' nitrogeneous zeolites ' or ' organosilicates '. In 1977, the laboratories of Union Carbide Corporation have patented a new very high siliceous zeolite called * Silicalite ' which is a silica polymorph and which has the same structure as ZSM-5. It might be said that many of the physical properties of ZSM-5 zeolite are mainly dependent on the structure and are, therefore, essentially invariant over the entire compositional range. Hence, they later concluded that silicalite appears to be a member of the ZSM-5 substitutional series. 10-ring window zeolites such as ZSM-5, have attracted considerable interest for the catalysis of a wide range of hydrocarbon transformations. When converted into the H+ cationic form, ZSM-5 can function as a solid acid catalyst and a shape-selective matrix in hydrocarbon transformation reactions. Because of the remarkable catalytic acitivity and shape-selectivity of this material, new or improved hydrocarbon processes have been developed, such as the conversion of methanol to gasoline. Although the detail of the mechanism is still in doubt, Brönsted acidity is thought to play a dominant role. VII H-ZSM-5 zeolites get referred to as hydrophobic, implying that water is excluded from the zeolite. H-ZSM-5 series and silicalite, sorb very little water. Therefore they can extract organic molecules such as hexane from water. Zeolites can be prepared either from aluminosilicate or from clay minerals. The former process for preparing zeolites was based on the results of original laboratory synthesis using amorphous hydrogels. The gels are defined as hydrous metal aluminosilicates prepared from aluminate and silicate solutions. This silicate solution can be taken from very different silicate sources such as ludoks, Cabosil, silicic acid, QUSO etc. The gels are crystallized in a closed hydrothermal system at varying temperatures. ZSM-5 is generally synthesized from mixed alkali-organic cation base system. Quaternary ammonium cations are used as structure directing agents or templates. Tetrapropylammonium (TPA) is the most common organic template used for ZSM-5 synthesis. The preferred temperature range for synthesis is 150 to 200 °C. The range of composition of the reaction mixtures, in terms of mole ratios of oxides, yielding ZSM-5, reported in the original patent, is given in the following table Table 1. Range of composition of reaction mixtures, in terms of mole ratios of oxides, yielding ZSM-5 phase. VIII The industrial applications require the zeolites be available with certain controllable properties. Besides the Si/AI ratio which determines the number of acid catalytic centers in the zeolite, control of morphology through synthesis, is one of the most important problems in the field. There have been attempts to grow large crystals, which is important for single crystal studies and also for making true molecular sieve mem branes. Recently, crystallite size was also found to change some important properties of the molecular sieves, such as the diffusivities for various gases, sorption rates and most important of all, the selectivities for certain reactions, when used as catalysts. The purpose of this study is to investigate the synthesis parameters to control the Si/AI ratio, size and shape of the ZSM-5 crystals. Synthesis runs are carried out at the temperature of 170 °C and at different times. H20/Si02 ratio is kept constant, at 25. The amounts of (TPA)20, Na20, Al203 and OH are varied and TPA and silica sources are changed. The number of moles of (TPA)20 per 100 moles of Si02 is changed in the range 1-12.5. Similary Na20 is changed in the range 3.5- 17.5, Al203 in 0-2 and OH in 7-46 moles per 100 moles of Si02. TPABr and TPAOH are used as TPA sources in different experiments, and NaOH as the Na source. Ludox colloidal silica sol and Cabosil are tested as silica sources. Synthesis experiments are carried out in 20 ml. stainless steel Modified Morey type autoclaves, lined with Teflon, to decrease the reaction, adhesion and nucleation on the surface. The synthesized zeolite samples are analyzed by X-ray powder diffraction for phase identification and with scanning electron microscopy for morphology. Atomic Absorption Spectroscopy and wet chemical methods are used for the determination of Si/AI ratios in the samples. ZSM-5 zeolite could be synthesized with a wide range of sizes and shapes and with different Si/AI ratios, in these experiments. The changes in these properties were explained in terms of the synthesis parameters. Most of the parameters studied are interrelated. In other words the effects of one, are seen to be more pronounced at certain levels of the others. In the composition range studied, in general, morphology deteriorates as the amount of Na in the synthesis mixture increases, especially at low Si02/Al203 ratios. Increase in OH, on the other hand, causes the zeolite to dissolve, especially at high Si02/AI203 ratios. An increase in TPA content of the reaction mixture at this alkalinity, delays dissolution, hence increases the stability field. Na, also, has a stabilizing effect on the structure at high alkalinity. The effect of increasing OH to decrease the IX crystal size and the effect of TPA below a minimum level, to increase the rates of nucleation and crystallization are also recognized.