I. Synthesis, Characterization, and Base Catalysis of Organic-Functionalized Molecular Sieves. II. Selective Oxidation of Ethane via Heteropolyanion-Containing Solid Catalysts

Citation

Wight, Andrea Palmisano (2004) I. Synthesis, Characterization, and Base Catalysis of Organic-Functionalized Molecular Sieves. II. Selective Oxidation of Ethane via Heteropolyanion-Containing Solid Catalysts. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/DCKV-0007. https://resolver.caltech.edu/CaltechETD:etd-05252004-102344

Abstract

This thesis is composed of two separate and unrelated projects. The first project examines the preparation of functional groups that can serve as immobilized bases in molecular sieves. Many heterogeneous, base catalysts are not able to promote diverse reaction types that require strong bases as catalysts. Additionally, some of the stronger solid base catalysts are sensitive to carbon dioxide and moisture in air and therefore are not easily suitable for recycling. Organic-functionalized molecular sieves possess an organic moiety within the pore space of a molecular sieve by incorporation of an organosilane directly into the synthesis gel of the molecular sieve. Previous work reported by Davis and co-workers demonstrated the incorporation of an acid site in zeolite Beta (*BEA type) and its use in shape-selective acid catalysis.

Here, a phosphonium functionality is prepared from halogen-containing alkyl groups in *BEA to allow the incorporation of a strong base (OH-) within the molecular sieve for base catalysis. Characterization of the phosphonium-containing material prepared is accomplished. Shape-selective chemical reactions and ion-exchanges are presented, and the results of these experiments suggest that the functional groups are located within the molecular sieve pore space, although the exact structure of these moieties is not conclusively obtained.

The second part of this thesis examines the use of niobium- and pyridine-exchanged heteropolyanions as catalyst precursors for the selective oxidation of light alkanes with dioxygen. The versatility of many oxidation catalysts is limited, thereby restricting potential usefulness. Alkenes, typically used as feedstock, are becoming costly as their demand for use in many other industrial processes increases. The use of light alkanes as reactants for selective oxidation would allow one to take advantage of an under-utilized and relatively inexpensive feedstock for selective oxidation.

Niobium- and pyridine-exchanged heteropolyanions (HPAs) have been shown to produce highly active and selective catalysts for the oxidation of propane and n-butane to acrylic acid and maleic acid, respectively, by Davis and co-workers. Specifically, molybdophosphoric acid and molybdovanadophosphoric acid were exchanged with niobium oxalate and pyridine to produce the exchanged HPAs (denoted NbPMo12pyr and NbPMo11Vpyr, respectively). Preliminary work in these studies indicates that the exchanged HPAs may also be effective for the oxidation of ethane to acetic acid. The application of this catalyst system to the selective oxidation of ethane to acetic acid and ethylene is explored here.

The exchanged heteropolyanions give higher ethane conversion at elevated pressures (230 psig and 280oC) but better yields at atmospheric pressure and 380oC. Variations of steam flow rates or reaction temperatures are not observed to improve acetic acid space-time-yield (STY). Lower gas-hourly-space-velocity (GHSV) causes the ethylene and acetic acid to over-oxidize to COx. The maximum STY of acetic acid using NbPMo12pyr is 0.021 mmol/min/g catalyst at 380oC, 0 psig, and flows of 16: 8: 16: 20 mL/min of ethane: oxygen: helium: steam.

At elevated pressure (230 psig) the addition of vanadium into the Keggin ion precursor is shown to decrease conversion (from 6.0% to 2.2%) but improve selectivity to ethylene (from 23.2% to 46.8%). The formation of acetic acid is not affected (0.002 mmol/min/g catalyst). At atmospheric pressure the addition of vanadium into the Keggin precursor does have a favorable affect on the acetic acid formation. NbPMo11Vpyr is shown to have a maximum acetic acid production of 0.062 mmol/min/g catalyst at 380oC, 0 psig, and flows of 16: 8: 16: 20 mL/min of ethane: oxygen: helium: steam.

The addition of both Nb and pyridine to the HPA is crucial for active catalyst formation, for reactions both at atmospheric pressure and 230 psig. Substitution of other metals for Nb does not yield materials that give significant ethane conversion.

Higher ethane/oxygen ratios increase the selectivity to acetic acid over NbPMo12pyr at atmospheric pressure. The oxidation of ethylene over NbPMo12pyr is accomplished, and the results indicate that acetic acid is formed from ethylene during the oxidation of ethane. D2O is substituted as the source of steam, and the observation that acetic acid contains deuterium shows that the steam in the feed is involved in its formation.

The data obtained from NbPMo11Vpyr suggest that this precursor can give a catalyst that is active and selective for producing ethylene and acetic acid from ethane and dioxygen. Further experimentation is necessary to optimize performance.

Thesis Files