Borosilikat ve boro-aluminosilikat zeolitlerinin sentezi ve karakterizasyonu

Abstract

ZSM-5 tipi zeolitlerin katalitik uygulamalarda katı asit katalizör olarak buldukları kullanım alanları giderek artmaktadır. Bu nedenle son yılların en fazla araştırılan malzeme gruplarından biri haline gelmiş, iyon değiştirici, adsorban ve katalizör olarak çeşitli uygulama alanları bulmuşlardır. Bu çalışmanın amacı farklı kuvvette ve türde asit merkezler içeren ZSM-5 tipi borosilikat ve boro-aluıninosilikat zeoMerinin sentezlenmesi, H- formlanna dönüştürülmesi ve çeşitli yöntemlerle karakterize edilmesidir. Çalışma üç bölümden meydana gelmektedir. Birinci bölüm sentez deneylerinin gerçekleştirilmesini içermektedir. Bu bölüm farklı iki bor kaynağı (H3BO3 ve Na2B4O7.10H2O) kullanılarak ve reaksiyon kanşımındaki su miktarı kontrol edilerek farklı büyüklükte MFI tip borosilikat kristalleri sentezlenmiştir. Daha sonra borik asit ile yapılan borosilikat sentezlerine ait jel bileşiminde hiçbir parametre değiştirilmeden reaksiyon karışımına belli oranlarda artan alüminyum ilave edilerek MFI tip boro-aluminosilikatlar sentezlenmiştir. Katalizörlerin hazırlanması sürecinde zeolitlerin tanımlanması için X-Işım kırınım analizi (XRD) ve taramalı elektron mikroskobu (SEM) analiz yöntemlerinden faydalanılmıştır. İkinci bölümde sentezlenen MFI tip ZSM-5 katahzörlerinin hidrojen formlarmm hazırlanması çalışmaları yapılmıştır. Bu amaçla TP A" katyonunun kalsinasyonu için gerekli koşullar tennogravimetrik analiz (TGA) cihazı ile ayrmtılandınlarak TPA kalsinasyonu gerçeMeştirilmiştir. TPA" katyonunun kalsinasyonu öncesi ve sonrası İR analizleri yapılarak TPA'nın yapıyı terkedip terketmediği bu yöntemle kontrol edilmiştir. Örneklerin H formlarının hazırlanması için NH4CI çözeltisi ile zeolitin yapısmda bulunan Na+ ile NH41" katyonlarının yer değiştirmesi sağlanmıştır. NH/ ile tekrarlı iyon değişimlerinin ardından uygulanan Kjehdehl analizi ile Na+ ile yer değiştiren NH4+ iyonlarının konsantrasyonları kontrol edilmiştir. NH44" ile iyon değişimini tamamlandıktan sonra NH3 kalsinasyonlannm koşullan TGA ile belirlenerek büyük ölçekte NH3 kalsinasyonlan yapılmıştır. Üçüncü bölümde, H-formuna getirilen zeolitler çeşitli yöntemlerle karakterize edilmiştir. N2 gazı kullanılarak ve sıvı azot sıcaklığında (-196 °C) gerçekleştirilen adsorbsiyon deneyleri ile yüzey alanları belirlenmiş NH3-TPD yöntemlerinden faydalanılarak da asit merkezleri karakterize edilmiştir.

In the last decade zeolites have found wide use in industrial application as catalyst, molecular sieves and ion-exchanger. Zeolites have a framework which consist of linked Si and Al tetrahedra. The frameworks may also be considered as consisting of linked chains or building blocks. Because of the difference in atomic charge between Al and Si, extra- lattice cations must be present to preserve electrical neutrality; a single positive charge is 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 their "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*. Isomorphous substitution of elements such as B, Fe, Ga, Ti, Zr... for silicon and duminum in zeolitic frameworks has largely been studied. It was expected that acidic and porous properties of zeolites may be modified by incorparation of elements of different size and different chemical features. At the present time only boron seems to have resulted in actual industrial application for pentasil type zeolite in the Assoreni (methyl tertierbutylether viii into methanol and isobutane) and Amoco processes (xylene isomeration and ethylbenzene conversion). Because of its small size and chemical features, boron was expected to result in a smaller unit cell volume and in a new acidic properties when it takes the place of duminum. Acidity induced by lattice boron was shown to be much weaker than that due to lattice Aluminum on basis of temperature programmed desorption of ammonia (NH3-TPD) and proton NMR controversy still exists about catalytic properties of boron pentasil zeolites. Negligible activity of pure boron pentasil samples for acidic type reactions was reported while activity was claimed for pure silicon pentasil samples admixed with an alumina based binder. Boron could be incorparated into the framework either during synthesis in the presence of a boron compound or after synthesis by solid-solid reaction or by having BCI3 vapor contacting the sample. On the other hand, zeolites have active sites and the presence of active sites in the micropores confers to the zeolites their shape-selective activity. It has been shown that shape-selective properties of zeolite may be strongly affected by the presence of active sites at the external surface of their crystallites, the zeolites with a large external surface being less selective than those with smaller surface. The presence of molecules blocking the pores of a zeolite or the partial destruction of its structure may drastically lower its activity by dimimshing the microporous volume accessible to the reactants. Therefore, the accurate determination of the external surface area and of the microporous volume of zeolite is of prime importance to the understanding of its catalytic behavior. The external surface area and the microporous volume of a solid are usually obtained from its nitrogen adsorption isotherm. IX The purpose of this study was two series of [B]-ZSM-5 and [B-ATJ- ZSM-5 samples of different crystal sizes, to convert them to their H- forms, and to characterize the product zeolite crystal for their acid site densities and internal/external surface areas. A series of borosilicate samples were synthesized using two boron sources, namely boric acid and borax, the amount of water in the reaction mixture was also varied to control the partical size. Synthesis runs were carrried out at die temperature of 170 °C for different time periods of up to 7 days. Different amounts of aluminum was incorparated into the crystal structures in a second series of experiments by varying the amount of alumina in the synthesis mixture. Synthesis experiments were carried out in 20 ml stainless steel autoclaves, lined with teflon. X-ray diffraction (XRD) was used for phase identification with which crystalinity was also determined. Scannig electron microscopy (SEM) was used to observe the crystal size and morphology. After synthesis calcination of the organic cation, tetrapropylammonium (TPA), was fallowed in a thermogravimetric analyzer (TGA) under different conditions and bulk calcinations of the samples were carried out in a muffler furnace with air circulation at the optimum conditions determined from TGA experiments. Repeated exchanges with 1 M NH4CI solution were them applied to the samples at 85 °C to prepare the NH4 + forms. NH/ contents were followed by applying Kjedahl analysis to the samples after each ion-exchange and the procedure was discontinued when NH41" contents were seen to stay constant. The final exchange capacities were used as a quantitative estimate of the Broensted site densities. NH3 calcination conditions were determined by TGA prior to carrying out the bulk calcinations in a muffle furnace. The H-forms obtained were finally, characterized by NH3-TPD and adsorption experiments. Relative peak intensities of the weak and strong acid sites were followed by NH3-TPD. BET and t-plot de Boer models were applied to the adsorption isotherm data to calculate the BET and t-areas and the micropore volumes. The results of this study, in which [BJ-ZSM-5 and [B-A1J-ZSM-5 samples of different acid densities and crystal size were synthesized can be summarized briefly as follows. [B]-ZSM-5 crystallization with borax had high nucleation and growth rates due to the higher alkalinity of the reaction mixture. As a result larger crystal sizes were achieved with boric acid due to fewer forming initially with respect to those obtained with borax. The crystal size of the borosilicate samples could be controlled by varying the water content of the reaction mixture. Aluminum incorparation slowed down the crystallization and the final crystalinity achieved. The metastability of the products was evidenced by their decrease in crystalinity at longer reaction time. The stability region was obesrved to get narrower with increasing aliminum incorparation into the structure. Low heating ratio with O2 flow were seen to be necessary for the complete calcination of the TPA cation and approximately 10 ion-exchange treatments were sufficient for the preparation of NH/ forms. BET surface areas and t-areas of the samples were seen to vary in the ranges of 299.36 to 374.28 m2/g and 30.68 to 95.02 m2/g, respectively. The range of micropore volumes obtained were 0. 136 to 0. 181 cm3/g. Micropore volumes were run to be related to the crystalinity while BET and t-areas were in general related to the crystal size. The total Brönsted site densities for the borosilicate and boro- aluminosilicate samples were observed to vary in the range of 0.3004 to 0.4747 meq/g and 0.4419 to 0.6207 meq/g respectively. xi Presence of sites of two different strengths was evident from the NH3- TPD results. The increase in the relative amount of strong sites with increasing aluminum incorparation was in accordance with the expentation.