Synthesis and Catalytic Applications of Transition Metal Incorporated Mesoporous Materials

Abstract

The work presented in the thesis sheds light on the synthesis of SBA-15 supported transition metal nanocomposites and their importance as improved catalytic systems for the reduction of nitroaromatics. The main emphasis has been laid on the synthesis of varying morphologies of transition metallic nano species within the mesoporous host as a function of increased metal loading and studying its effect on the various physicochemical and catalytic properties of nanocomposites. The whole work has been divided into five chapters. Chapter 1 Introduction, Literature survey and Experimental procedures This chapter provides a brief introduction to the historical background of the porous materials, different pathways of synthesis of mesoporous materials like SBA-15, importance of SBA-15 as a preferred mesoporous host. Moreover, reports based on the recent developments in the field of transition metal (Au, Ag, Cu/CuO as well as bimetallic Au-Ag) supported SBA-15 nanocomposites have also been discussed. A brief description of the methodology and various techniques used for the characterization of optical, thermal, surface structural morphology and catalytic properties of prepared nanocomposites have also been incorporated. Chapter 2 Homogeneous dispersion of Au nanoparticles into mesoporous SBA-15 exhibiting improved catalytic activity for nitroaromatic reduction This chapter demonstrates the synthesis of homogeneously dispersed Au loaded SBA-15 nanocomposites by varying the Au loading from 1-5 and 10 wt. % by post modified method. It was found that with an increase in the amount of Au loading, followed by calcination at 350 oC, a change of morphology from Au nanospheres (5-6 nm) for 4 wt. % Au loading to rod-like Au nanostructures (width 6-9 nm and length 90-180 nm) beyond 5 wt. % loading was observed. The surface area (664 m2/g) and pore volume (1.33 cm3/g) of bare SBA-15 were significantly reduced to 292 m2/g and 0.6031 cm3/g for Au nanosphere, and 457 m2/g and 0.7677 cm3/g for Au nanorod dispersed SBA-15, respectively, due to partial filling of mesopores. Moreover, the quantum sized, non-aggregated Au nanostructures stabilized within the mesochannels of SBA-15 exhibited improved catalytic activity over catalytically inactive bare SBA-15 with 89 % selectivity for m-phenylenediamine and 81% for p-nitroaniline formation for the reduction of m- and p-dinitrobenzene respectively. vi Chapter 3 Preparation, surface structural morphology and catalytic properties of uniformly dispersed Ag nanoparticle loaded mesoporous SBA-15 This chapter deals with the preparation of varying morphologies of Ag nanoparticles within the channels of APTMS modified SBA-15 and study of the changes in physicochemical parameters and catalytic properties with increased Ag impregnation. The prepared materials were characterized by XRD, TEM, FTIR and solid-state UV-Visible spectroscopy. It was found that gray colored samples were formed with Ag impregnation in comparison to white colored bare SBA-15, exhibiting broad absorption band in the range of 450-500 nm and a weak absorption band at 700-900 nm indicating the formation of long anisotropic Ag nanostructures (Ag nanorods) within the mesoporous matrix. XRD studies revealed the retention of mesoporous structure, even after surface modification with APTMS and Ag loading. TEM micrographs depicted well dispersed Ag nanostructures (7-8 nm) fixed within the mesoporous matrix. Moreover, incorporation of Ag led to a significant decrease from 680 m2/g of SBA-15 to 385 m2/g due to plugging of mesopores with Ag. The Ag-impregnated SBA-15 catalyst displayed superior catalytic activity than bare SBA-15 with 4 wt. % Ag-loaded catalyst exhibiting optimum activity for selective reduction of p-nitrophenol to p-aminophenol (100%) and p-dinitrobenzene to p-nitroaniline (87%) along with small amount of p- phenylenediamine formation. Moreover, comparative studies of various physicochemical and catalytic parameters pertaining to 4 wt. % Au/Ag/CuO loaded SBA-15 nanocomposites have also been discussed. Chapter 4 Fine CuO anisotropic nanoparticles supported on mesoporous SBA-15 for selective hydrogenation of nitroaromatics This chapter illustrated the formation of well dispersed CuO nanospheres (~5-6 nm) and nanorods (aspect ratio ~11-20 nm) having monoclinic crystal phase within the mesoporous channels of SBA-15. Moreover, the effect of calcination temperature, changes in the size, shape, dispersion ability and catalytic activity of the metal oxide NPs present within the mesoporous host as a function of increased metal loading have also been discussed. It has been supported by various characterization techniques. Elemental mapping studies confirmed uniform distribution of CuO nanoparticles on the surface of SBA-15. An increase in surface area was also observed from 694 m2g-1 for SBA-15 to 762 m2g-1for 10 wt. % Cu loading probably due to the deposition of an excess of CuO nanoparticles on the outer siliceous surface. The high surface area of the 10 wt. % CuO/ap-SBA-15 also contributed to the efficient adsorption of nitro groups on the vii active sites of the CuO/ap-SBA-15 resulting in enhanced catalytic activity. The catalytic activity also increased with Cu loading and 10 wt. % CuO/ap-SBA-15 catalyst displayed the highest catalytic activity for the reduction of m-chloronitrobenzene and m-nitrotoluene with 83 % and 100 % selectivity for m-chloroaniline and m-aminotoluene respectively. The reaction rate showed dependence on the electron withdrawing ability of the substituents present on nitrobenzene and was found to be maximum for m-CNB. Chapter 5 Uniform dispersion of a bimetallic/binary mixture of Au-Ag supported SBA-15 nanocomposites for selective reduction of nitroaromatics This chapter comprises the formation of a binary mixture of Au-Ag nanoparticles with controlled shape and size supported on mesoporous SBA-15 by post modified method involving variation in the amount of Au loading w.r.t fixed Ag loading. DRS studies showed the presence of single plasmon band for various bimetallic Au:Ag loadings illustrating that the NPs formed an alloy structure with homogeneous composition.TEM micrographs depicted discrete bimetallic nanostructures uniformly distributed and stabilized within/on the surface of the mesoporous host. Among all bimetallic nanocomposites, Au:Ag(5:1)/m-SBA-15 nanocomposites exhibited the best catalytic activity (k= 2.12×10-2 min-1 and 3.99×10-2 min-1) in comparison to monometallic Au/m-SBA-15 and Ag/m-SBA-15 nanocomposites for the selective reduction of nitrobenzene (NB) to aniline (AN) and p-nitroacetophenone (p-NAP) to p-aminoacetophenone (p-AAP)respectively. The strong synergism between Au and Ag resulted in the improved catalytic activity of the bimetallic Au:Ag nanocomposites in comparison to the monometallic Au/Ag counterparts and bare SBA-15 for the reduction of NB and p-NAP respectively. The catalytic activity was greatly influenced by metal loading, surface area and distribution of bimetallic nanospheres within/on the surface of the mesoporous host.