Cage-defining Ring: A Molecular Sieve Structural Indicator for Light Olefin Product Distribution from the Methanol-to-Olefins Reaction

The methanol-to-olefins (MTO) process produces high-value-added light olefins from nonpetroleum sources. Acidic zeotypes containing cages bounded by 8-ring (small-pore) windows can effectively catalyze the MTO reaction, since their cages can accommodate the necessary aromatic intermediates that produce the light olefin products that escape. While progress on the mechanisms of the MTO reaction continues, zeotype structure–reaction property relationships have yet to be elucidated. Here, we report MTO reaction results from various small-pore, cage-containing silicoaluminophosphate/metalloaluminophosphates (SAPO/MAPOs) and zeolites under the same reaction conditions. The MTO behaviors of microporous materials having the following topologies are investigated: LEV, ERI, CHA, AFX, SFW, AEI, DDR, RTH, ITE, SAV, LTA, RHO, KFI, and UFI. The previous observation that light olefin product distributions from a series of small-pore, cage-containing zeolites can be classified into four structural categories is further supported by the results shown here from zeolite structures not investigated in the previous study and SAPO and MAPO materials with isostructural frameworks to all the zeolites. Additionally, these data reveal that light olefin product distributions are very similar over a given topology independent of framework composition. To develop a structure–property relationship between the framework topology and the MTO light olefin product distribution, the concept of the cage-defining ring size is introduced. The cage-defining ring size is defined as the minimum number of tetrahedral atoms of the ring encircling the center of the framework cages in the molecular sieve topology. It is shown that the cage-defining ring size correlates with MTO light olefin product distribution

Enhancing the Ethylene and Propylene Selectivities in the Methanol-to-Olefins Reaction by Exploiting the Intricate Relationship between Framework Topology and Acidity

This thesis describes and presents results from several related projects within the theme of molecular sieve synthesis and catalysis. The early part of the thesis focuses on understanding the link between cage size/dimension and acidity (i.e., acid site density and strength) in the methanol-to-olefins (MTO) reaction. This relationship between cage size and acidity, once identified and investigated, is exploited in the latter parts of the thesis to rationally design materials that are able to steer the light olefins product distribution toward either more ethylene or propylene in a significant improvement over SAPO-34 (CHA), the commercial catalyst. In Chapters 2, 44 zeolites and silicoaluminophosphates (SAPOs) belonging to five frameworks (AEI, CHE, LEV, SWY, and ERI) with a wide range of Si/Al=4-31 and Si/(Al+P)=0.04-0.3, are synthesized and characterized using a myriad of techniques. Their MTO behavior is then systematically investigated to rationalize the effect of cage dimensions on the olefins product distribution as a function of acid site density and strength. The results from this study show that changes in acid site density and strength play a secondary role to the dominating influence of cage architecture on product distribution in AEI- and CHA-type molecular sieves. Decreasing the cage size, in going from AEI and CHA to LEV, SWY, and ERI, however, results in substantial changes in the ethylene-to-propylene ratio (E/P) as a function of acidity. These changes are attributed to differences in the identity and concentration of the hydrocarbon-pool (HP) species that form, particularly in early stages of the reaction. In Chapters 3 and 4, ERI-type molecular sieves (e.g., SSZ-98, UZM-12, ERI-type zeolites, and SAPO-17) are thoroughly investigated as promising methanol-to-ethylene materials due to their narrow cage size. Specifically, numerous ERI-type molecular sieves are synthesized using several organic structure-directing agents (OSDAs) with varied Si/Al or Si/T-atoms ratios. The list of ERI-related materials synthesized and tested in MTO included a new disordered SAPO, denoted as CIT-16P, which upon thermal treatment in air transforms to SAPO-17 (ERI). The reaction results show that decreasing the Si/Al (or increasing the Si/T) ratio, irrespective of other material properties, improves the E/P of ERI-type molecular sieves (E/P=1.1-1.9) over CHA-type molecular sieves (E/P=0.82-0.85) in MTO. Dissolution-extraction experiments reveal that the rapid formation of cyclic intermediates and the shift in their composition toward less-methylated methylbenzenes and methylnaphthalenes are found to be key to enhancing the ethylene selectivity in ERI-type molecular sieves. In Chapter 5, several SAT-type molecular sieves are investigated as promising methanol-to-propylene catalysts. This effort entails the synthesis of CIT-17, an SAT SAPO-type molecular sieve, which is isostructural to STA-2 (MgAPO-SAT). Following the successful synthesis of CIT-17, the MTO behavior of several SAT-type molecular sieves (MgAPO, CoAPO, and SAPO) are investigated in MTO. The combination of low acidity of CIT-17 and unique structural features of the narrow SAT-cage lead to a catalytic pathway and mechanism that predominantly favors propylene (propylene-to-ethylene ratios (P/E) of 2-4.2; propylene selectivity of 40-50%). Indeed, CIT-17 achieves one of the highest P/E ratio values reported for this class of materials.</p

Targeted morphology of copper oxide based electrospun nanofibers

In this work, CuO-based nanofibers were synthesized via electrospinning. Smooth, defect-free fibers with a diameter of 261 ± 63 nm were fabricated and characterized. Thermal treatment under air at 823 K transformed the smooth nanofibers to a network of segmented, macroporous CuO nanoparticles with an average fiber diameter of 160 ± 41 nm and a crystallite size of 55.4 nm. The effects of solution properties (polymer molecular weight, polymer/metal concentration, and solvent identity) and processing conditions (voltage, tip-to-collector distance, extrusion rate, and humidity) were also investigated. Solution properties were found to strongly influence viscosity, conductivity, dielectric constant, density, and surface tension, which invariably affected fiber dimension, morphology and surface structure. Fibers as thick as 536 nm and as thin as 70 nm with cylindrical and fused structures were produced by manipulating the solution properties. Processing conditions were found to moderately affect fiber uniformity and fiber diameter

Targeted morphology of copper oxide based electrospun nanofibers

In this work, CuO-based nanofibers were synthesized via electrospinning. Smooth, defect-free fibers with a diameter of 261 ± 63 nm were fabricated and characterized. Thermal treatment under air at 823 K transformed the smooth nanofibers to a network of segmented, macroporous CuO nanoparticles with an average fiber diameter of 160 ± 41 nm and a crystallite size of 55.4 nm. The effects of solution properties (polymer molecular weight, polymer/metal concentration, and solvent identity) and processing conditions (voltage, tip-to-collector distance, extrusion rate, and humidity) were also investigated. Solution properties were found to strongly influence viscosity, conductivity, dielectric constant, density, and surface tension, which invariably affected fiber dimension, morphology and surface structure. Fibers as thick as 536 nm and as thin as 70 nm with cylindrical and fused structures were produced by manipulating the solution properties. Processing conditions were found to moderately affect fiber uniformity and fiber diameter

Synthesis and Characterization of Silicoaluminophosphate CIT-16P and Its Transformation to SAPO-17

A silicoaluminophosphate molecular sieve, CIT-16P, is synthesized using butane-1,4-bis(quinuclidinium) [(C7H13N)-(CH2)4-(NC7H13)]2+ dihydroxide (DiQ-C4-(OH)2) as an organic structure-directing agent (OSDA). Upon the removal of the OSDA, either by thermal treatment in air at temperatures exceeding 490 °C or by extended ozone treatment at 150 °C, CIT-16P transforms to SAPO-17 (ERI topology). The structure solution of CIT-16P in its as-synthesized form is obtained using a Rietveld refinement of the powder X-ray diffraction pattern. The primary composite building units (CBUs) of CIT-16P are highly distorted cancrinite (can) CBUs that transform into stable can units of the ERI-type framework as a result of the OSDA removal. The distortion of can CBU is maintained without transformation by the presence of tightly bound DiQ-C4 dications in the as-synthesized form of CIT-16P. The transformed material is characterized and evaluated as a catalyst in the methanol-to-olefins (MTO) reaction. The catalytic behavior of the formed SAPO-17 (Si/T-atom = 0.022) (T = Si + Al + P) at 400 °C and WHSV of 1.3 h–1 produces elevated C3+ olefin products (i.e., propylene, butenes, and pentenes) in early stages of the reaction. However, as the reaction proceeds, the C3+ fraction decreases in favor of more ethylene