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.
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Date
2010
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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.
Description
Ph. D. University of KwaZulu-Natal, Westville 2010
Keywords
Methanol., Alcohols., Potassium--Chemical warfare., Theses--Chemistry.