750
S. A. I. Barri, D. Chadwick
CO , CO2, H2
Syngas
Sulfolane was used as solvent (100 g) and paraformalde-
hyde (7.5 g) was the source of formaldehyde. After adding
the appropriate catalyst, the mixture was stirred at 950 rpm
with a gas entrainment stirrer to enhance mixing and gas–
liquid mass transfer. The resins were dried at 90 °C,
whereas the phosphotungstic acid was used as supplied. The
zeolites were calcined at 500 °C. At the end of the run, the
products were refluxed with methanol (50 g) and 70% nitric
acid (2 g) for 4 h to form methyl glycolate. The reaction
product mixture was analysed by GC-FID using a HP-FFAP
30 m 9 0.32 mm capillary column with a temperature
programme from 60 to 220 °C to elute all the products and
solvent. A blank run without catalyst showed no reaction.
Biomass
gasification
- H2
CO (+H2O)
H2CO HOCH2CO2H
CH3OH
CH3OH
HOCH2CO2CH3
CH3OH + HOCH2CH2OH
+H2
Scheme 1 Ethylene glycol from syngas via carbonylation of
formaldehyde
technology because of their relatively low thermal stability.
Separation and/or thermal regeneration of the catalyst is
complicated, if not impractical. Scope for increasing reac-
tion rate by raising temperature is limited. In contrast,
inorganic solid acid catalysts such as zeolites are known for
their thermal stability and offer the potential for higher
reaction temperatures with high acidity. Zeolites have been
demonstrated as catalysts for carbonylation of dimethyl
ether [14–16] and dimethoxymethane [17]. Furthermore,
carbonylation of 1,3,5 trioxane (a precursor for formalde-
hyde) to 1,3-trioxlan-4-one over zeolites has been reported
[18–20]. In this paper, we report on the catalytic perfor-
mance of zeolites ZSM-5, Y, and mordenite for the car-
bonylation of formaldehyde to produce methyl glycolate
(after esterification with methanol), and in particular dem-
onstrate their high selectivity and intrinsic activity. Com-
parison is made with two resins (Amberlyst 15 and 70) and
phosphotungestic acid. Amberlyst 70, which has not been
studied previously in this context, has higher thermal sta-
bility than Amberlyst 15. The effect of temperature on
catalytic performance is explored up to 220 °C, beyond the
level reported previously (150 °C).
3 Results and Discussion
The present results are concerned with the carbonylation
step and esterification shown in Scheme 1. ZSM-5 and Y
have been studied because of their high acidity and dif-
ferent pore structures which present not only varying dif-
fusional paths to the reactants and products, but also
influence the acidity of the framework. ZSM-5, with its
medium pore size channels, has one of the strongest acidity
offered by zeolites in general because of its acid site iso-
lation, as the Si/Al ratio is normally high. Zeolite Y, on the
other hand, has large pore size channels with supercages at
the centre of the channel intersections and is normally
synthesised at low Si/Al ratio, and then undergoes a dea-
lumination process to increase its acid site strength. The
acid site concentrations of the zeolites used were calculated
based on their Si/Al ratios assuming all the aluminium
were in the framework. Phosphotungstic acid and the resins
were titrated with standard solution of sodium hydroxide to
determine the concentration of acid sites. The values in
mmol H?/g of dry catalyst are given in Table 1.
2 Experimental
Methyl glycolate (MG) in high yields was produced
over all the catalysts. Table 1 gives maximum yields of
MG after 2 h reaction time and the corresponding tem-
peratures. SiO2/Al2O3 ratios are given in parentheses.
Dimethyl diglycolate (DMDG) and methoxy methyl ace-
tate (MMAc) were produced as side products, Table 1.
Minor amounts of methyl polyglycolate were also formed.
In addition there were traces of unidentified products.
Unreacted formaldehyde/paraformaldehyde was substan-
tially converted by self reaction in the acid catalysed
esterification step to methyl formate and small amounts of
acetaldehyde and methyl acetate. Based on the major
products, the carbon mass balance of all catalytic runs
reported here was greater than 90%; the reproducibility of
the methyl glycolate yield was within 5%.
Phosphotungstic acid, Sulfolane, and methanol were sup-
plied by Sigma-Aldrich. Paraformaldehyde was supplied
¨
by Riedel-de-Haen. Amberlyst-15 and Amberlyst-70 were
supplied by Rohm and Haas, France (code 69286 and
44355 respectively). Zeolites HY, ZSM-5 and mordenite
were obtained from Zeolyst International (CBV 760, CBV
5524G, and CBV 21A, respectively).
Both Amberlyst-15 and Amberlyst-70 were ion
exchanged with 0.1 M nitric acid solution several times at
room temperature. The resins were then dried at 100 °C
overnight before use. Phosphotungstic acid was used as
supplied. The zeolites were calcined at 500 °C (slow
heating rate 1 °C/min) immediately prior to use.
Catalytic tests were carried out in a stirred autoclave
The heteropolyacid and the resin catalysts produced a
significant amount of DMDG, whereas the zeolite catalysts
reactor at constant CO pressure of 80 bar and 100–220 °C.
123