SYNTHESIS AND CATALYTIC ACTIVITIES OF ZEOLITE ENCAPSULATED METAL COMPLEXES

Abstract

Catalysis is known to play a key role in modern chemical technologies. It has now been well established that the key role in catalytic reactions is played by intermediate chemical interaction of reactive molecules with definite functional groups (active sites) of homogeneous and heterogeneous or biological (enzyme) catalysts. Modern catalytic science faces even more complicated and challenging problems. These are connected with the urgent necessity to create new highly effective industrial processes, which are selective, ecologically pure and consume minimum energy. At least creating organized molecular systems, which are designed to have an optimum space arrangement, energy and orbital correspondence of the catalytic system components and substrates, can solve some of these problems. Zeolites are one of the most important and fascinating classes of catalysts being used in chemical industries on large scale due to their inherent capabilities of product selectivity. Zeolites are even compared with the catalytic antibodies and metalloenzymes and hence they are often referred to as "zeoenzymes'*. Hetrogenization of homogeneous catalysts by encapsulation of the metal complexes in zeolite provide ideal solution, as large size of the encapsulated metal complexes and their rigidity make them difficult to escape out of the zeolite cage. Encapsulation of metal complexes in the super cages (a-cages) of zeolite matrix possess the advantages of solid heterogeneous catalysts like easy separation and handling, ruggedness, thermostability, reusability (regeneration of the deactivated catalysts) as well as share many advantageous features of homogeneous catalysts. Zeolite encapsulated metal complexes have provided opportunity to develop catalytic process in the synthesis of fine chemicals and being used in various types of catalytic reactions like alkylation, hydrogenation, dehydrogenation, hydrocracking, cyclization, amination, acylation, isomerization, rearrangement Ill and oxidation. The catalytic oxidation of organic substrate has been studied well due to the commercial and synthetic importance of resulted functionalized molecules. The hydroxylation of phenol is a field of great challenge and is one of the industrially important reactions. Hydroxylation of phenol usually gives catechol and hydroquinone as two major products. Catechol finds applications in fine chemicals for pest control, pharmaceuticals, flavours and aroma. On the other hand, hydroquinone has mainly been used as photographic developer, polymerization, inhibitor, antioxidant and intermediate for numerous dyes. The applicability of these products have encouraged us to study hydroxylation of phenol using various types of zeolite-Y encapsulated metal complexes. The present thesis describes the synthesis of metal complexes with potentially tetradentate / pentadentate ligands encapsulated in zeolite-Y and their characterization by various physicochemical techniques. Hydroxylation of phenol using these encapsulated complexes as catalyst has been carried out and suitable reaction conditions have been optimized to achieve maximum reaction product(s). For convenience the work reported in the thesis has been divided in the following chapters. First chapter is the introductory one and describes zeolite, in general, the origin, framework structure, general formula, properties, types of natural zeolites and their applications for various industrial processes. The later part of this chapter includes a brief review of different reactions catalyzed by various types of zeolite encapsulated complexes. The reactions included in this part are oxyhalogenation, oxidation of alkane, cycloalkane, benzene, cyclohexanol and related compounds, as well as catalytic oxidations of alkene, cyclic ethers and benzyl alcohol. Extensive details on the catalytic phenol hydroxylation using various types of metal complexes encapsulated in a variety of zeolites with different oxidants (H202, 02, TBHP) as well as using other catalysts are presented. IV Second chapter describes the detailed instrumental techniques, such as, infrared spectroscopy, electronic spectroscopy, H NMR spectroscopy, atomic absorption spectrometry, thermal analysis, powder X-ray diffraction (XRD), scanning electron microscopy (SEM) etc. used to characterize the metal complexes encapsulated in the super cages of zeolite-Y as well neat complexes. Preparation of ligand A7,7Y-bis(salicylidene)propane-l,3-diamine (H2salpn), its Cu(II), Ni(II), Zn(II), Cr(III), Fe(III) and Bi(III) complexes encapsulated in the zeolite-Y is described in third chapter. The preparation of related neat complexes have also been described in this chapter. These encapsulated complexes have been characterized by various physico-chemical techniques to ensure their encapsulation in the cavities of zeolite-Y. Catalytic activity of these encapsulated as well neat complexes have been studied towards the oxidation of phenol and a suitable reaction condition has been optimized considering [Cu(salpn)]-Y as a representative catalyst. Other derivative(s) of phenol and 1-naphthol have been considered to examine the leaching of metal ion in solution during catalytic reaction and the effect of size, branching and the presence of activating and deactivating groups on the phenol oxidation. It is observed that encapsulated catalysts perform better than neat ones and are reusable after activating. Fourth chapter deals with the syntheses of tribasic pentadentate ligand N,Nbis( salicylidene)diethylenetriamine (H2saldien) , its Cu(II), Ni(II), Zn(II), Cr(III), Fe(III) and Bi(III) complexes encapsulated in the zeolite-Y as well as neat complexes. All these catalysts have been characterized by spectroscopic (IR and UV-Vis) studies and thermal as well as powder X-ray diffraction patterns. Catalytic activities of these complexes for the decomposition of H202 and for the oxidation of phenol to a mixture of catechol and hydroquinone Using H202 as an oxidant have been studied. Considering one representative example each from catalysts having metal complexes in II and III oxidation states has optimized a best-suited reaction condition. Other catalysts of the series were studied under similar reaction conditions and results were compared. Finally results of both the series were compared to generalize the catalytic performance of the catalysts prepared from the ligand H2saldien. It is concluded from the catalytic reaction data that the encapsulated complexes are better catalysts than neat ones. Under bestsuited reaction conditions, catechol formation is always higher than hydroquinone formation. However, at the expense of phenol conversion selectivity of the formation of catechol decreases, while selectivity of the hydroquinone increases in the beginning, but their formation stabilize after ca. 6 h of reaction time. Comparable IR spectral patterns of fresh and used encapsulated catalysts suggest that these can be used further for catalytic activity as not much spectral change in the catalyst is observed. Cu(II), Ni(II), Zn(II), Cr(III), Fe(III) and Bi(III) complexes of l,2-bis(2- hydroxybenzamido)ethane (H2hybe) encapsulated in the zeolite-Y by flexible ligand method are described in chapter fifth. The isolated catalysts have compositions [M(hybe)]-Y [M = Cu(II), Ni(II) or Zn(II)] and [M(hybe).2H20]-Cl- Y [ M= Cr(III), Fe(III) or Bi(III)]. The ligand coordinates through the deprotonated phenolic oxygen and amido nitrogen atoms. Spectroscopic studies and thermal analysis, XRD as well as scanning electron micrographs confirmed their encapsulation in the cavities of zeolite-Y. Catalytic performance of these catalysts towards the oxidation of phenol have been tested in two sets: first set deals with catalysts [M(hybe)]-Y (M= Cu(II), Ni(II) and Zn(II)] while second set considers the catalysts [M(hybe).2H20]Cl-Y(M = Cr(III), Fe(III) and Bi(III)]. In order to find suitable reaction conditions to get maximumoxidized products, effect of various parameters, such as, amount of phenol, catalyst, H202 concentration, temperature, different solvents and volume of solvents have been taken into consideration. Finally the catalytic performance of catalysts of both the series have been compared. No leaching of metal ions or metal complexes was detected in solution when blank reaction was carried out. It is observed from the catalytic data that under the best-suited reaction conditions, the selectivity of catechol formation ranges between 85-91 % with these catalysts. In order to see the

reusability of the catalysts, some of these have been used twice after recovering from reaction mixture and activating. The results reflect the good reusability of the catalysts as not much loss in their catalytic activity was noticed. The studies in this thesis, therefore, conclude about the effectiveness of various metal complexes encapsulated in Y-zeolite for the selective hydroxylation of phenol. The findings are quite encouraging from academic as well as industrial point of view.