The effect of Mo(CO)₆ as a catalyst in the carbonylation of methanol to methyl formate catalyzed by potassium methoxide under CO, syngas and H₂ atmospheres.

Abstract

In patents describing the low temperature production of methanol from syngas catalysed by the Ni(CO)₄/KOCH₃ system, Mo(CO)₆ was claimed to enhance the catalytic activity of the system. However, there has been no clarity on the effect of Mo(CO)₆ and KOCH₃ in the activation of the catalyst. Work reported in this thesis showed that most of the methyl formate is produced via a normal KOCH₃ catalyzed process under a CO atm. When the KOCH₃ system is compared with the Mo(CO)₆/KOCH₃ catalyzed system, it is noted that the amount of methyl formate increases very slightly due to the addition of molybdenum hexacarbonyl. The experiments were also performed under H₂ and synags (1:1) atm in different solvents. In all cases dimethyl ether was produced with methyl formate. Preliminary carbonylation studies performed at a syngas ratio of 1:2 showed an increase in the amount of methanol produced. Increasing the amount of Mo(CO)₆ in the Mo(CO)₆/KOCH₃ reaction under syngas (1:1) increases the production of methyl formate. High Pressure infrared (HPIR) studies for Mo(CO)₆/KOCH₃ were carried out under H₂, CO, syngas (1:1) and N₂ atmospheres. The alkoxycarbonyl complex (Mo(CO)₅(COOCH₃)⁻) was observed as an intermediate in all reactions involving Mo(CO)₆ and KOCH₃. Under a hydrogen atmosphere, the metalloester (Mo(CO)₅(COOCH₃)⁻) intermediate diminished to form a bridged molybdenum hydride (µ-HMo₂(CO)₁₀⁻) species as a stable intermediate. In contrast, under syngas atmosphere, the metallloester diminished in concentration to form the bridged hydride (µ-HMo₂(CO)₁₀⁻), which also disappeared to form the molybdenum alkoxide complex (Mo(CO)₅OCH₃⁻). The role of methanol in the formation of methyl formate is also discussed. Based on the HPIR studies, different types of metalloesters (alkoxycarbonyl complexes) were synthesized by nucleophilic reactions of alkoxides with Mo(CO)₆. Reactions of potassium alkoxides (KOR, R = -CH₃, -C(CH₃)₃, -C(CH₃)₂CH₂CH₃) with Mo(CO)₆ in THF produced water soluble alkoxycarbonyl complexes (K[Mo(CO)₅(COOR)]). The reaction of KOCPh₃ with Mo(CO)₆ yielded what is believed to be the metalloester as an insoluble compound. Attempts to improve the solubility of the formed alkoxycarbonyl complexes, K[Mo(CO)₅(COOR)], by metathesis with bulkier counter ions (PPNCl, Et₄NCl and n-Bu₄NI) was not successful. The reaction of K[Mo(CO)₅(COOCH₃)] with 18-crown-6 ether produced [K(18-crown-6)][Mo(CO)₅(COOCH₃)] which was more soluble in organic solvents. The reactions of [PPN][OCH₃] and [n-Bu₄N][OCH₃] with Mo(CO)₆ produced [PPN][Mo(CO)₅(COOCH₃)] and [n-Bu₄N][Mo(CO)₅(COOCH₃)], respectively.

The reactions of [K(18-crown-6)][OCH₃] and [K(15-crown-5)₂][OCH₃] with Mo(CO)₆ under reflux gave the [K(18-crown-6)][Mo(CO)₅(COOCH₃)] and [K(15-crown- 5)₂][Mo(CO)₅(COOCH₃)] complexes. Reactions of Ph₃PMo(CO)₅ with KOCH₃ and [PPN][OCH₃] yielded K[Ph₃PMo(CO)₄(COOCH₃)] and [PPN][Ph₃PMo(CO)₄(COOCH₃)]. Other alkoxycarbonyl complexes were synthesized by an alternative approach using alcohols as solvent. For example, [PPN][Mo(CO)₅(COOCH₂CH₃)] was synthesized by refluxing [PPN][OEt] with Mo(CO)₆ in ethanol. The isopropyl derivative [PPN][Mo(CO)₅(COOCH(CH₃)₂)] was synthesized by refluxing [PPN][OCH(CH₃)₂] with Mo(CO)₆ in isopropanol. Two methyl derivatives were also synthesized in methanol as Et₄N and PPN derivatives. A crystal structure of the [PPN]₂[Mo₆O₁₉] oxo cluster, obtained from the decomposition of [PPN][Mo(CO)₅(COOCH(CH₃)₂)] in acetonitrile was solved. The crystal crystallized in the monoclinic form with a space group of P-1. Another oxo cluster, [Et₄N]₂[Mo₄O₁₃], formed from the decomposition of the [Et₄N][Mo(CO)₅(COOCH₃)] derivative. The structure was solved in the monoclinic form with a space group of P 2₁/n. The alkoxycarbonyl complex, [PPN][Mo(CO)₅(COOCH₃)], was tested for catalytic behaviour under hydrogen and syngas to determine its role in the production of methyl formate. No methyl formate was produced under hydrogen, but methyl formate was produced under syngas (1:1). HPIR studies of [PPN][Mo(CO)₅(COOCH₃)] under syngas (1:1) showed that methyl formate is formed via the decomposition of [PPN][Mo(CO)₅(COOCH₃)] to Mo(CO)₆. Interesting results for the reaction of Mo(CO)₆ with KOCH₃ under syngas (1:1) were obtained in triglyme. Here longer carbon chain alcohols were produced and identified by GC and GC-MS. These alcohols include ethanol, 2-propanol, 2-butanol, 3-methyl-2-butanol, 3-pentanol, 2-methyl- 3-pentanol and 2,4-dimethyl-3-pentanol.