UC Santa Cruz

SECTION ONE

Applications of Metal Functionalized Mesoporous Silicate Nanomaterials

Abstract

Presented here are two projects highlighting the metallization of mesoporous silica nanoparticles (MSN) for specific applications in adsorption and heterogeneous catalysis.

The intra-framework incorporation of aluminum during synthesis gives rise to mesoporous aluminosilicate nanoparticles (MASN), which are the primary scaffold presented throughout this work. Because aluminum is trivalent, its tetrahedral coordination by oxygen upon substitution into the SiO2 framework gives rise to negatively charged sites. This negative surface charge can be taken advantage of in the anchoring of positively charged species such as metal cations and metal-organic complexes.

In the first project, presented in chapter 2, the further metallization of MASN was explored for application in the adsorptive desulfurization of military logistics fuels (i.e. JP8 and JP5), as a portable alternative to the conventional process of industrial hydrodesulfurization (HDS). It was found that the addition of 20w% Ag+ to MASN by wet impregnation resulted in an adsorbent with good desulfurization capacity (12.7 mgS·g–1) for the removal of the refractory pollutant dibenzothiophene (DBT) from model fuel (n-decane). Reuse of the material upon removal of DBT from Ag-MASN with appropriate solvents, however, was not possible, and ICP elemental analysis revealed Ag+ leached from the MASN in the process. To stabilize the reusability of this Ag-MASN in model fuel tests, the reduction of extra-framework Ag+ to Ag0 was performed via glow discharge plasma in a low-pressure argon atmosphere. An average initial desulfurization capacity of 15 mgS·g–1, and retention of 79% desulfurization capacity after six reuse cycles, was achieved using this plasma treated Ag-MASN (PT-Ag-MASN), although some silver was still observed to leach from the adsorbent upon cycling.

In another study (chapter 3), MASN was used as a solid support for the heterogeneous catalyst referred commonly referred to as amorphous nickel boride. Sodium borohydride (NaBH4) reduction of NiCl2 impregnated MASN resulted in a thin coating of a highly dispersed nickel-boron composite (NBC). Compared to unsupported bulk NBC, the supported NBC-MASN catalyst was more active for the selective reduction of the nitro group on a variety of substituted nitroarenes reduced with hydrazine hydrate (N2H4�H2O). Up to nine reuse cycles of NBC-MASN for the reduction of p-nitrotoluene to p-toluidine could be achieved when equimolar NaBH4 (catalyst regenerating agent) was present in-situ with N2H4�H2O. Fundamental questions arose concerning the difference in physical character between supported and unsupported NBC. These questions are discussed in light of considerable uncertainty in the literature as to the actual composition of such amorphous material. Preliminary evidence suggests the presence of residual borohydrides within bulk unsupported NBC is responsible for the materials partial conversion to crystalline nickel boride upon heating in inert atmosphere, and the long history of referring to these materials as amorphous borides.

Finally, chapter 4 reports on the attempted substitution of transition metals such as Cu and Ni into the SiO2 framework, in the context of creating robust and reusable adsorbents for desulfurization. These adsorbent materials did not perform well as desulfurizing adsorbents, compared to MSN or MASN. Even upon impregnation with AgNO3 or subsequent plasma treatment, these materials did not perform better than Ag- MASN or PT-Ag-MASN (AgNO3 wet impregnated MASN and the subsequent plasma treated material, respectively). Additionally, the location of Ni and Cu in these materials was not confirmed beyond the general association of the transition elements with the MSN framework, as shown by energy-dispersive x-ray spectroscopy (EDS). Because of the lack of performance as desulfurization adsorbents, and ambiguity of the incorporation/substitution of the transition elements into the SiO2 framework, investigation into these materials was discontinued. An attempt was also made to create an Ag/Cu binary metallic extra-framework metallized sorbent by galvanic a galvanic exchange method. The product GE-Ag/Cu-MASN shows some promise for enhanced cyclability in the desulfurization of model fuel. The data for this mixed metal species work is briefly discussed in chapter 4.

SECTION TWO

Synthesis Routes Toward Metal Organic Framework Thin Films on Conductive Substrates

Abstract

Presented here are two facile methods for fabricating metal organic framework (MOF) thin films on the transparent conductive substrate indium tin oxide coated glass (ITO). The first method, presented in chapter 7, illustrates the successful electrochemical deposition of several MOFs on ITO which was previously deposited with metallic films composed of discrete or overlapping metallic nanostructures. The partial anodic degredation of these metal deposites served as the metal ion source for the growth of MOF films, with relatively low surface roughness and good lateral coverage. The second method, presented in chapter 8, details the successful hydrothermal growth of several isoreticular MOF (IRMOF) films grown on ITO previously hydrothermally deposited with oriented zinc oxide nanowire arrays. The ZnO nanowires serve as nucleation sites for the zinc based MOFs (IRMOF-1, -3, -8, and -9). Both the anodically electrodeposited MOF films and hydrothermally grown IRMOF films were characterized by grazing incidence X-ray diffraction (GIXRD) and electron microscopy. Additionally the electrodeposited films were characterized by in-situ elemental X-ray dispersive spectroscopy (EDS) to give some understanding of the state of the metal underlayer remaining after complete MOF film growth.