OSDA-Free Synthesis and Characterization of Faujasite Zeolites to Investigate the Effects of Varying Silicon-to-Aluminum Ratio and Added Alcohol on Resultant Structure(s)

Methane is a prominent greenhouse gas which can trap ~25x more heat than carbon dioxide. It is not feasible to significantly lower societal methane emissions; however, it is possible to oxidize methane to methanol, a more manageable and useful chemical. Soluble-form methane monooxygenase (sMMO) is a naturally occurring enzyme that performs this conversion. Unfortunately, sMMO is slow to react and difficult to implement at scale. sMMO consists of square planar diiron active sites surrounded by a protein structure. Iron phthalocyanines (FePC) have a similar square planar iron active site to sMMO and can be encapsulated in faujasite (FAU) zeolite (FePC@FAU), restricting access to the primary binding site, similar to the function of the protein structure. The primary issue with synthesizing FePC@FAU is that the crystallization must be performed without using organic structure directing agents (OSDAs) to allow the phthalocyanines to be encapsulated within the FAU supercages. Additionally, crystallization of phase-pure FAU is very sensitive to procedural changes because there are many other similar zeolite phases which can be created instead of, or in addition to, FAU (e.g., GIS, LTA, SOD). These phases are undesirable because they cannot properly restrict phthalocyanines in their supercages while allowing access to organic molecules. Previous literature has reported the use of aluminum isopropoxide as the aluminum during FePC@FAU syntheses. However, our lab has not been able to consistently replicate these results. Instead, we developed a modified procedure which utilizes aluminum hydroxide and colloidal silica (LUDOX HS-30) at high gel Si:Al ratios (i.e., >2:1) that has consistently produced FAU and FePC@FAU zeolites. In this work, we repeated our attempts to perform the syntheses using aluminum isopropoxide to compare our results with those in previous literature, as well as with our developed synthesis procedures using aluminum hydroxide. We varied the silicon-to-aluminum ratios to explore how the amount of aluminum in the gel impacts the resultant crystal topology and performed syntheses using aluminum hydroxide and added isopropanol to simulate the gel composition resulting from use of aluminum isopropoxide in the existing literature. These samples were analyzed using their gravimetric yields, X-ray diffraction patterns, and micropore volumes (obtained through collection nitrogen adsorption isotherms). Our ongoing work aims to verify the results of previous literature, compare our successful recipes with the previously reported analogues, and to determine the optimal silicon-to-aluminum ratio for the consistent synthesis of pure faujasite.