Chapter 8
Preparation of Ethylene Glycol Esters from Synthesis Gas Use of Promoted Homogeneous Ruthenium-Rhodium Catalysts R. Whyman, K. Gilhooley, S. Rigby, and D. Winstanley Imperial Chemical Industries plc, New Science Group, P.O. Box 11, The Heath, Downloaded by CORNELL UNIV on July 26, 2016 | http://pubs.acs.org Publication Date: December 16, 1987 | doi: 10.1021/bk-1987-0328.ch008
Runcorn, Cheshire, WA7 4QE United Kingdom
C2-oxygenated products, particularly the acetate esters of ethylene glycol, may be prepared in good selectivities directly from synthesis gas in the presence of composite bimetallic homogeneous catalysts which contain mixtures of ruthenium and rhodium, as
major and minor components respectively, together with promoters in the form of nitrogen-containing bases or alkali metal cations, in acetic acid as solvent/co-reactant. These are the first catalyst
systems containing ruthenium as the major metallic component which have been demonstrated (i) to provide good molar selectivities (e.g. C2/C1 ~ 1-2) to specific
C2-oxygenates, with no hydrocarbon formation, and (ii) to match the previously-documented behavior of mono-metallic rhodium catalysts in respect of both activity and selectivity.
A feature which is a key to any wider utilisation of chemistry based on synthesis gas is an understanding of, and more
particularly, an ability to control, those factors which determine the selectivity of the Cx to C2 transformation during the hydrogénation of carbon monoxide. With the exception of the rhodium-catalysed conversion of carbon monoxide and hydrogen into ethylene glycol and methanol, in which molar ethylene
glycol/methanol selectivities of ca 2/1 may be achieved, other catalyst systems containing metals such as cobalt or ruthenium exhibit relatively poor selectivities to ethylene glycol (1). Our initial studies in this area were based on the reasoning that, since the reduction of carbon monoxide to C2 products is a
complex, multi-step process, the use of appropriate combinations of metals could generate synergistic effects which might prove more
effective (in terms of both catalytic activity and selectivity) than simply the sum of the individual metal components. In 0097-6156/87/0328-0108$06.00/0
© 1987 American Chemical Society Fahey; Industrial Chemicals via C1 Processes ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
8. WHYMAN ET AL.
Production of Ethylene Glycol Esters
1 09
particular, the concept of the combination of a good hydrogénation catalyst with a good carbonylation, or "CO insertion", catalyst seemed particularly germane. As a result of this approach we
discovered an unprecedented example of the effect of catalyst promoters, particularly in the enhancement of C2iC1 selectivity, and one which has led to the development of composite mixed-metal homogeneous catalyst systems for the conversion of C0/H2 into
ethylene glycol esters (2,3) (Equation 1) and ethanol (£). Ru/Rh (10:1)
CO + H2
>.
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HOAc
CH20Ac
MeOAc + EtOAc + I
CH20H
+
CH20Ac
I
(1)
CH20Ac
Other recent reports have also indicated that mixed-metal systems, particularly those containing combinations of ruthenium and rhodium complexes, can provide effective catalysts for the production of ethylene glycol or its carboxylic acid esters (5~9). However, the systems described in this paper are the first in which it has been demonstrated that composite ruthenium-rhodium catalysts, in which
rhodium comprises only a minor proportion of the total metallic component, can match, in terms of both activity and selectivity, the
previously documented behavior (_p of mono-metallic rhodium catalysts containing significantly higher concentrations of rhodium. Some details of the chemistry of these bimetallic promoted catalysts are described here.
Experimental
Chemicals.
Synthesis gas, as an equimolar mixture of carbon monoxide
and hydrogen, was purchased from either Air Products Ltd or British
Oxygen Company Ltd. Ru(acac)3 was purchased from Johnson Matthey Chemicals Ltd and used without further purification; Ru3(C0)12, Rh6(C0)16 and Rh(C0)2acac were prepared according to literature procedures (10-12).
Glacial acetic acid and the various
additives/promoters were purchased from BDH Ltd and used without further purification. Tetraglyme (ex-Aldrich Chemical Company Ltd) was dried over activated 3A molecular sieves before use.
High Pressure Experimentation. A typical autoclave experiment was carried out as follows. Ru(acac)3 (0.80g, 2.0 mmol), Rh(C0)2 (acac)
(0.052g, 0.2 mmol), Cs2C03.2H20 (0.362g, 2.0 mmol Cs) and glacial
acetic acid (52. 5g, 50 ml), were charged into a 100ml capacity silver-lined stainless steel autoclave fitted with a flip-flop stirrer. The autoclave was sealed, purged four times with a C0/H2 mixture, pressurised to ca 500 atm C0/H2 (1:1), stirred and heated to 230°C whereupon the pressure was increased to 1000 atm with further C0/H2. After 2 hr at 230°C, during which a further 210 atm C0/H2 was added (in 25-30 atm increments) to maintain the total pressure at 1000 (±25) atm, the autoclave was allowed to cool to ambient temperature and carefully vented. After discharge from the autoclave the initially clear yellow-orange product solution rapidly became cloudy as excess dissolved carbon monoxide escaped. The organic reaction products were analysed by gas chromatography using a 1 5% Carbowax 20M on Chromosorb W HP column. In situ spectroscopic measurements were carried out using a Hastelloy C-276 high pressure
Fahey; Industrial Chemicals via C1 Processes ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
1 10
INDUSTRIAL CHEMICALS VIA C, PROCESSES
infrared cell, the details of which have been described previously
(J_3).
Very narrow pathlengths, oa 20-30u, between the cell windows
were used in order to minimise the very strong background absorptions due to glacial acetic acid itself.
Even under these conditions the
maximum useful window only covered the range 2200-1900 cm""1 and thus did not allow the detection of absorptions in the typical "bridging" carbonyl region of the infrared spectrum. Results and Discussion
Preliminary Studies.
Initial work carried out at 1500 atm pressure
and 230-21J5°C using bulky steel autoclaves with the reactants Downloaded by CORNELL UNIV on July 26, 2016 | http://pubs.acs.org Publication Date: December 16, 1987 | doi: 10.1021/bk-1987-0328.ch008
contained in glass liners indicated that combinations of ruthenium
and rhodium, in which rhodium comprised the minor component, could give good selectivities to C2 products (with molar ethylene glycol/methanol selectivities frequently greater than 1.5/1) particularly when the reaction was carried out in the presence of
alkali metal cations as promoters. Subsequently these systems have been studied in greater detail at lower pressures (ca 1000 atm) in silver-lined autoclaves, the use of which has facilitated a high degree of temperature control and has ensured the generation of reliable, highly reproducible data under these severe reaction conditions.
The Ru/Rh/Cs/HOAc Catalyst Composition. Table I illustrates the effect, on product distribution and catalytic activity, of the incremental addition of cesium ions to a catalyst precursor composition containing ruthenium and rhodium, in the molar ratio of
10:1, dissolved in glacial acetic acid. The results of control experiments, in which no cesium is present, are also included. Several points emerge from this Table. First, a small but experimentally significant synergism is observed on the addition of ruthenium to rhodium in the absence of cesium.
However, the
selectivity to C2 products is very poor and typical of those reported in other investigations, particularly those in which ruthenium has
been used as a catalyst (J_).
Secondly, the addition of only a very
small amount of cesium to this ruthenium/rhodium combination has a
profound effect upon the course of the reaction - not only is the selectivity to ethylene glycol acetates dramatically enhanced, partly
at the expense of methyl acetate production, but the overall catalytic activity, based on carbon monoxide conversion, almost doubles.
The addition of cesium has therefore resulted in the
generation of a true promotional effect. Thirdly, both the selectivity to ethylene glycol and the total CO conversion approach a maximum at approximately stoichiometric ratios of cesium to metal;
further addition of cesium results in a gradual decrease in both selectivity and activity. Finally, replacement of acetic acid by tetraglyme as solvent at the optimum Ru/Rh/Cs ratio results in a
dramatic reduction in both the selectivity to C2 products and the overall catalyst productivity, the catalyst becoming inferior to that of the unpromoted Ru/Rh/H0Ac system. Clearly the combination of metals, acetic acid solvent and promoter are all essential constituents of the composite catalyst for the selective production of C2 esters.
Fahey; Industrial Chemicals via C1 Processes ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
8. WHYMANETAL.
Production of Ethylene Glycol Esters
111
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