Study On The Catalytic And Templating Role Of Cu₃（BTC）₂ Framework And Its Pore Engineering

Metal-organic frameworks(MOFs)field has witnessed extraordinary exploration and development during the last thirty years.MOFs outperform the previously prepared porous materials(e.g.,zeolites and porous silica)due to their tailorable porous structures.The frameworks’versatility enabled MOFs to present great performance when used for gas adsorption and separation,catalysis,drug delivery,templating,etc.Due to their rigid structure and uniform porosity,MOFs were successfully used as a template to precisely control the size of metal/metal oxide/semiconductor nanoparticles.During templating,MOFs acted as chemically inert templates without affecting the reaction thermodynamics.In this thesis,we are presenting a novel trend called dual-function MOF for utilizing the catalytic and templating functions of MOFs to facilitate the synthesis of uniform nanoparticles under the MOF thermal stability requirements.Graphitic carbon nitride quantum dots(g-CNQDs)are one of the promising photoresponsive materials;however,the synthesis of uniform QDs is challenging.In the second chapter of this thesis,we are presenting MOF[Cu3(BTC)2](BTC=1,3,5-benzenetricarboxylate,HKUST-1)as a dual-function framework to prepare monodispersed g-CNQDs under mild conditions.HKUST-1 was used to catalyze the condensation of g-CNQDs precursor(cyanamide)at low temperature and as a template to control the particle size growth inside its cavities.Cyanamide(CA)was loaded inside HKUST-1 cavities using the chemical vapor infiltration technique at 80℃.Due to the catalytic role of the open metal sites(OMSs)inside HKUST-1 large cavity,CA dimerization started at 90℃ while condensation was optimized at 120℃.Under these mild conditions,HKUST-1 framework was kept intact thus,the g-CNQDs size was controlled by the dimensions of the framework micropores.The produced g-CNQDs were extracted and characterized to investigate their composition.The results confirmed the formation of well-condensed and partially crystalline g-CNQDs which composed of a few number of carbon nitride sheets and containing tri-s-triazene units in their structure.The prepared g-CNQDs possessed a uniform particle size averaged around 2.22±0.68 nm.The QDs showed photoluminescence(PL)emission with a quantum yield of 3.1%.They were successfully utilized as a PL probe for quantitative detection of mercury in water with a limit of detection of 5.8×10-8M.During the catalytic condensation of CA inside HKUST-1 cavities,ammonia gas was produced as a byproduct which induced defect formation inside the structure.These defects led to an expansion of a few portions of the framework micropores into mesopores.By exploring this observation in-depth,we found that ammonia could be used as an etching agent to cut the metal-carboxylate bonds off and hance,introduce higher order of porosity in carboxylate microporous MOF(HKUST-1).The gas-phase etching strategy led to formation of uniform mesopores inside the framework without affecting the morphology or particle size.The pore features were estimated using the N2 sorption isotherm analysis which confirmed the mesopore formation after washing out the etched linkers and structure debris.The etching temperature(ranged from 80 to 200℃)was used to control the mesopore size(ranged from 9.2 to 38 nm)while the mesopore volume(0.059 to 0.303 cm3 g-1)was tuned by the amount of etchant(5 to 25 ml NH3 in a fixed reactor volume).Due to the anisotropic nature of the framework lattice planes,the micropore expansion was found to follow specific lattice directions.The lattice planes induced the etching process with preferential inner structure etching in the<100>directions.Consequently,mesopore formation on the surface of the {111} exposed facets of octahedral HKUST-1 crystals with depth inside the crystal.This plane-oriented etching led to formation of mesopores with characteristic shapes(triangle and rectangle shapes depending on the etching directions).Because the pore expansion originated from oriented etching in the<100>directions,pore closure was found to be directed in opposite orientation if the framework soaked in its precursors,solutions.This advantage of the established etching process enabled the formation of reversible mesopores which is mimicking surgery operation but at a molecular level.The framework partial regeneration was observed after soaking in its precursors,solutions for 12 h at room temperature.Methylene blue dye(a relatively large-size molecule)was used as a visual probe molecule to show the successful pore closure.Furthermore,the concept was successfully applied to the M-MOF-74 family(M=Cu and Ni),demonstrating the generality of the strategy.The two isotypic MOFs showed different responses toward the etching process due to the varied metal oxophilicity.Cu-MOF-74 was found to be more susceptible where the mesopore diameter was larger(51.5 nm)comparing with the Ni-MOF-74(3.3 nm)when treated under similar conditions(20 ml NH3,160℃/12 h).The generated mesopores had elongated oval shape due to selective etching of the structure components arranged in{2-10} lattice planes while the {300} planes which represent the exposed crystal facets did not affect.Taking the advantages of the MOF crystallinity together with the variation in the stability of the lattice planes and homogeneity benefiting from gas-phase etchant,we achieve to manipulate microporous MOFs for mesopore formation and renovation at a molecular scale.Hence,precisely and rationally design and control the pore features for specific applications such as shape and size-dependent molecular sieving/separation and encapsulation as well.Molecular surgery strategy may provide us a powerful tool for tailoring and tuning the properties of MOF materials.