Hydroformylation of aqueous formaldehyde using a rhodium-tricyclohexylphosphine catalyst system

Aqueous formaldehyde is hydroformylated to glycol aldehyde in the presence of a rhodium-phosphine ligand complex catalyst in which the phosphine ligand is a trialkyl- or tricycloalkylphosphine and has a specified cone angle. A preferred phosphine ligand is tricyclohexylphosphine.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention is related to a process and accompanying catalyst for the 
preparation of glycol aldehyde and, more particularly, is related to the 
preparation of glycol aldehyde from the reaction of aqueous formaldehyde, 
carbon monoxide and hydrogen in the presence of rhodium-phosphine complex 
catalysts. 
2. Description of the Prior Art 
Glycol aldehyde is a valuable intermediate in many organic reactions, and 
is particularly useful as an intermediate in the production of ethylene 
glycol through a catalytic hydrogenation process. 
Ethylene glycol is a valuable commercial chemical with a wide variety of 
uses, e.g., as a coolant and antifreeze, monomer for polyester production, 
solvent, and an intermediate for production of commercial chemicals. 
The reaction of formaldehyde with carbon monoxide and hydrogen in the 
presence of a variety of catalysts at elevated temperatures and 
superatmospheric pressures is a well known reaction and yields glycol 
aldehyde, together with methanol, as well as lesser amounts of polyhydroxy 
compounds which can be subsequently separated by proper separation 
procedures. For example, U.S. Pat. No. 2,451,333 describes the reaction of 
formaldehyde, carbon monoxide and hydrogen over a cobalt catalyst to 
produce ethylene glycol. U.S. Pat. No. 3,920,753 discloses the production 
of glycol aldehyde by the reaction of formaldehyde, carbon monoxide and 
hydrogen in the presence of a cobalt catalyst under controlled reaction 
conditions; however, the process produces relatively low yields of 
product. European Pat. No. 002,908 describes a process for the production 
of glycol aldehyde from the reaction of formaldehyde, in the presence of a 
rhodium-triphenyl phosphine ligand catalyst, with carbon monoxide and 
hydrogen, in a tertiary amide solvent. 
European patent Application 82/200,272.1 describes a process for the 
preparation of glycol aldehyde which comprises reacting formaldehyde, 
hydrogen and carbon monoxide in the presence of either a rhodium or cobalt 
containing catalyst precursor, together with a strong protonic acid, a 
tertiary amide solvent and a triaryl phosphine. 
U.S. Pat. No. 4,200,765 describes a process of preparing a glycol aldehyde 
involving reacting formaldehyde, carbon monoxide, and hydrogen in a 
tertiary amide solvent in the presence of a catalytic amount of rhodium in 
complex combination with carbon monoxide, using triphenyl phosphine as the 
preferred catalyst promoter. The phosphine-containing catalysts can be 
prepared by employing suitable phosphine ligands other than triphenyl 
phosphine. Among a long list of such suitable phosphine ligands is 
included tricyclohexylphosphine. The sources of formaldehyde used in the 
process as disclosed in the patent are typical of those commonly used in 
the technology and include paraformaldehyde, methylal, formalin solutions 
and polyoxymethylenes. Paraformaldehyde is preferred since the best 
results are obtained therewith. Also disclosed are solutions of 
formaldehyde in solvents such as solutions of formaldehyde in aqueous 
reaction solvent, such as N-methyl pyrrolidin-2-one. 
The art relative to the hydroformylation of formaldehyde to glycol aldehyde 
has preferred to use paraformaldehyde as the formaldehyde source in view 
of the improved yields which are obtained. The use of aqueous formaldehyde 
as the formaldehyde source has not yielded sufficient conversion or 
selectivity to glycol aldehyde. Further, the hydroformylation of aqueous 
formaldehyde has resulted in deactivation of the rhodium catalyst and as 
well excessive condensation of glycol aldehyde with formaldehyde and other 
aldehydes to form higher molecular weight high-boiling sugar-like 
by-products. The condensation is an aldol-type reaction with becomes more 
severe at higher temperature and in more basic medium. 
Economically, it would be preferable to utilize aqueous formaldehyde as the 
formaldehyde source in the hydroformylation thereof to glycol aldehyde. It 
is, thus, a primary objective of the present invention to provide a 
reaction system which will allow the hydroformylation of aqueous 
formaldehyde to give glycol aldehyde with sufficient selectivity and 
without the disadvantageous catalyst deactivation and sugar by-product 
formation attendant in prior art reaction systems. 
SUMMARY OF THE INVENTION 
The foregoing and additional objects are accomplished by the present 
invention which in one of its aspects is a process for selectively 
producing glycol aldehyde by selectively hydroformylating aqueous 
formaldehyde, which process comprises contacting in a hydroformylation 
zone aqueous formaldehyde with carbon monoxide and hydrogen in the 
presence of a triorganophosphine ligand-stabilized rhodium catalyst 
wherein a specified triorganophosphine ligand is used. The rhodium complex 
catalyst has the empirical formula: RhH.sub.m (CO).sub.n L.sub.p wherein 
"Rh" is rhodium, "H" is hydrogen, "CO" is carbon monoxide and "L" is a 
triorganophosphine ligand, and wherein m is 0, 1 or 3, n is from 1 to 3, 
and p is 1 or 2, the sum of m, n and p being from 3 to 6, said 
triorganophosphine ligand being selected from the group consisting of 
trialkylphosphines, including tricycloalkylphosphines, wherein each of the 
three alkyl, including cycloalkyl groups are alike or different, and each 
contains from 1 to 10 carbon atoms, and wherein said triorganophosphine 
ligand has a cone angle within the range of 159 to 171 degrees. 
DETAILED DESCRIPTION OF THE INVENTION 
The hydroformylation of formaldehyde in the presence of a rhodium complex 
catalyst to produce glycol aldehyde is well known and there is a large 
body of prior art pertaining thereto. The description herein will be 
limited mainly to those process limitations and catalyst limitations to be 
observed in order to accomplish the high selectivity and high conversion 
desired. In other words, unless otherwise specified herein conventional 
hydroformylation conditions and procedures may be utilized. 
The source of formaldehyde for use in the present process is aqueous 
formaldehyde. Aqueous formaldehyde is typically known in the art as 
formalin which comprises aqueous 37 to 50% solutions of formaldehyde. 
In the prior art processes, the hydroformylation of aqueous formaldehyde to 
glycol aldehyde has not been readily successful. As previously explained, 
yield and selectivity to glycol aldehyde have been substantially below 
those achieved utilizing paraformaldehyde as the formaldehyde source. 
Catalyst deactivation and the formation of sugar-like by-products has also 
been the disadvantageous result of hydroformylating aqueous formaldehyde 
in the presence of rhodium-triorganophosphine complex catalysts. The 
present invention, however, is based on the discovery that aqueous 
formaldehyde can be hydroformylated to glycol aldehydes at glycol aldehyde 
selectivities of greater than 80% by utilizing a specific 
rhodium-phosphine ligand catalyst complex which, although known to the 
art, has not been specifically suggested for use in the hydroformylating 
of aqueous formaldehyde and which besides yielding the high selectivity to 
glycol aldehyde is not deactivated by the formaldehyde reactant and does 
not result in any appreciable levels of sugar by-product formation. 
The catalyst utilized in the present invention is a rhodium complex 
catalyst of the empirical formula: 
EQU RhH.sub.m (CO).sub.n L.sub.p (I) 
wherein "Rh" is rhodium, "H" is hydrogen, "CO" is carbon monoxide and "L" 
is a triorganophosphine ligand, and wherein m is 0, 1 or 3, n is from 1 to 
3, and p is 1 or 2, the sum of m, n and p being from 3 to 6. The 
triorganophosphine ligand must be one selected from the group consisting 
of trialkylphosphines, including tricycloalkylphosphines, wherein each of 
the three alkyl (including cycloalkyl) groups are alike or different (but 
preferably alike) and each contains from 1 to 10 carbon atoms. The 
triorganophosphine ligand must be one wherein the cone angle is within the 
range of 159 to 171 degrees. 
While the prior art literature discloses the general class of rhodium 
complex catalysts listed above, the prior art was generally concerned with 
preparation of glycol aldehyde from paraformaldehyde for the reasons above 
set forth, and the prior art does not teach a method for choosing a 
catalyst for obtaining glycol aldehyde from aqueous formaldehyde which 
will give a combination of high conversion rate, high selectivity to 
glycol aldehyde and good catalyst stability. Those in the art will 
recognize that high selectivity and high conversion may be all that are 
needed in a batch run; however good catalyst stability is also absolutely 
necessary in a commercial process wherein product is produced under 
continuous operating conditions. The inventor has unexpectedly discovered 
that to have the three qualities of high selectivity to glycol aldehyde, 
high conversion rate and good stability, the trialkylphosphine must have a 
cone angle within the range of 159 to 171. 
The cone angle is a measure of steric properties of the phosphine. A very 
thorough discussion of cone angle can be found in the following 
publication: Chadwick A. Tolman, "Steric Effects of Phosphorus Ligands in 
Organometallic Chemistry and Homogeneous Catalysis", Chemical Reviews, 
1977, Vol. 77, No. 3, pp. 313-348. All references in the Specification and 
the claims to "cone angle" are utilized in the Tolman article, and as such 
are measured in the Tolman article. As stated in the Tolman article the 
cone angle, in general terms, is the smallest angle of a cone (with its 
apex at a specified point in the phosphine ligand) which would contain all 
of the alkyl or cycloalkyl groups attached to the phosphorus atom. 
Suitable trialkylphosphines and tricycloalkyl phosphines which have the 
proper cone angle are set forth in Table 1. 
TABLE 1 
______________________________________ 
Cone Angle 
Phosphine (Degrees) 
______________________________________ 
Tricyclohexylphosphine 
170 
Tri-sec-butylphosphine 
160 
Tri-isopropylphosphine 
160 
______________________________________ 
The especially preferred phosphine for use in the present invention is 
tricyclohexylphosphine. 
The following Table 2 lists various triorganophosphines, and their 
respective cone angles which are not suitable for the present invention 
because their cone angle is not within the proper range. Also, two of the 
phosphines, di-tert-butylphenylphosphine and tribenzylphosphine, are not 
within the scope of the invention and not satisfactory for a catalyst 
because they include an aryl group, even though their cone angles are 
within the desired range. 
TABLE 2 
______________________________________ 
Cone Angle 
Phosphines (Degrees) 
______________________________________ 
Di-tert-butylphenylphosphine 
170 
Tribenzylphosphine 165 
Tri-tert-butylphosphine 
182 
Trimethylphosphine 118 
Tri-n-butylphosphine 
132 
Triisobutylphosphine 
143 
Triphenylphosphine 145 
Triphenylphosphite 128 
______________________________________ 
The hydroformylation reaction is preferably carried out in a solvent which 
will dissolve polar materials. Suitable solvents include a wide variety 
and are exemplified by N-substituted amides in which each hydrogen of the 
amido nitrogen is substituted by a hydrocarbon group, e.g., 
1-methylpyrrolidin-2-one, N,N-dimethylacetamide, N,N-diethylacetamide, 
N-methylpiperidone, 1,5-dimethylpyrrolidin-2-one, 
1-benzylpyrrolidin-2-one, N,N-dimethylpropionamide, hexamethylphosphoric 
triamide and similar such liquid amides; nitriles, such as acetonitrile, 
benzonitrile, propionitrile and the like; cyclic ethers such as 
tetrahydrofuran, dioxane and tetrahydropyran; ethers such as diethyl 
ether, 1,2-dimethyoxybenzene, alkyl ethers of alkylene glycols and 
polyalkylene glycols, e.g., methyl ethers of ethylene glycol, propylene 
glycol and di, tri- and tetraethylene glycols; ketones, such as acetone, 
methyl isobutyl ketone, and cyclohexanone; esters, such as ethyl acetate, 
ethyl propionate and methyl laurate; lactones of organic carboxylic acids, 
such as butyrolactone and valerolactone organic acids such as acetic acid, 
propionic acid and caproic acid; and alkanols, such as methanol, ethanol, 
propanol, 2-ethylhexanol and the like; and mixtures thereof. Many of the 
solvents are non-reactive in the medium whereas others are capable of 
functioning as ligands. The selected solvent should preferably be liquid 
under the reaction conditions. 
The rhodium may be introduced into the reaction zone in any convenient 
manner. For example the rhodium salt of an organic acid may be combined 
with the ligands in the liquid phase and then subjected in the reaction 
zone to the synthesis gas. Alternatively, the catalyst can be prepared 
from a carbon monoxide complex of rhodium, such as hexarhodium 
hexadecacarbonyl, by heating such with the ligands. Also, and the method 
of choice, is to introduce into the reaction zone as a catalyst precursor, 
a rhodium complex such as the rhodium dicarbonyl complex formed with 
acetylacetonate ligand, and then introducing separately to the reaction 
zone the triorganophosphine ligand. The general method of forming similar 
catalysts is disclosed and discussed in various literature such as U.S. 
Pat. No. 4,484,006 issued Nov. 20, 1984 to Henry R. Menapace; U.S. Pat. 
No. 4,287,370 issued Sept. 1, 1981 to Norman Harris, et al; and in British 
Patent Specification 1,243,189 of Malcolm John Lawrenson, et al published 
Aug. 18, 1971. Other references to similar catalysts are in the article by 
B. Fell, et al, Tetrahedron Letters, 1968, pages 3261-3266; U.S. Pat. No. 
4,260,828 issued Apr. 7, 1981 to Morrell, et al; U.S. Pat. No. 4,268,688 
issued May 19, 1981 to Tinker, et al; U.S. Pat. No. 4,258,214 issued Mar. 
24, 1981 to Bahrmann, et al; U.S. Pat. No. 3,965,192 issued June 22, 1976 
to Frank B. Booth; European Patent Application Publication 0-080-449-Al 
published June 1, 1983 to Monsanto Chemical Company; U.S. Pat. No. 
3,239,566 issued Mar. 8, 1966 to Slaugh et al; and U.S. Pat. No. 4,482,749 
issued Nov. 13, 1984 to Dennis et al. 
The preferred catalyst for use in the invention is one of Formula I above 
which has been prepared utilizing a precursor consisting of a rhodium 
complex with a beta-diketone, such as the rhodium dicarbonyl complex 
formed with acetylacetonate ligand. The beta-diketone utilized for forming 
the complex may be any of those generally available. Suitable 
beta-diketones include acetylacetone, dibenzoylmethane, benzoylacetone, 
diprivaloylmethane, 3-alkyl-2,4-pentanedione, and 2-acetylcyclohexanone. 
Preferably the beta-diketone will be composed only of carbon, hydrogen and 
oxygen and will be free of ethylenic and acetylenic unsaturation. The 
especially preferred beta-diketone is acetylacetone. 
The amount of catalyst employed in the hydroformylation reaction does not 
seem to be critical and may vary considerably. At least a catalytically 
effective amount of catalyst should be used, of course. In general, an 
amount of catalyst which is effective to provide a reasonable reaction 
rate is sufficient. As little as 0.001 mole of rhodium per liter of 
reaction medium can suffice while amounts in excess of 0.1 mole does not 
appear to materially affect the rate of reaction. For most purposes, the 
effective preferred amount of catalyst is in the range of from about 0.002 
to about 0.025 mole per liter. 
The selectivity for glycol aldehyde and the rate of reaction appears to 
change relative to the ratio of the amount of phosphine ligand to the 
amount of rhodium. Generally, it has been found that increasing the amount 
of phosphine ligand relative to the rhodium to greater than 1:1 
dramatically increases the rate of reaction while simultaneously 
decreasing glycol aldehyde selectivity. The rate of formaldehyde 
hydroformylation and glycol aldehyde selectivity obtained using excess 
phosphine can be moderated by incorporating an acid into the reaction 
medium. Thus, it has been found that by adding phosphoric acid to a 
catalyst which has a ratio of tricyclohexylphosphine to rhodium of 4:1, 
the glycol aldehyde selectivity can approach glycol aldehyde selectivity 
obtained from a 1:1 ratio although, a substantial reduction in the rate of 
formaldehyde hydroformylation takes place. 
The reaction conditions are not overly critical in that wide ranges of 
elevated temperatures and superatmospheric pressures are operable. The 
practical limitations of production equipment will dictate to a great 
extent the selection of temperatures and pressures in which the reaction 
is to be effected. Thus, using available production systems, the selected 
elevated temperature should be at least about 75.degree. C., and can range 
up to about 200.degree. C., and even higher. For most purposes, the 
preferred operating temperature ranges from about 75.degree. C. to about 
125.degree. C. It has been found that an increase in reaction temperature 
from 100.degree. C. to 150.degree. C. results in an increase in the 
selectivity to methanol and ethylene glycol. Thus, it appears that the 
hydrogenation pathway is highly favored at 150.degree. C. This suggests 
that the glycol aldehyde produced selectivity at lower temperatures such 
as 100.degree. C. could be hydrogenated in situ with the same catalyst 
system in another reactor at the higher temperature. The superatmospheric 
pressure should be at least about 10 atmospheres and can range up to 
almost any pressure attainable with production apparatus. Since extremely 
high pressure apparatus is quite expensive, pressures to about 500 
atmospheres are suggested. Most desirably, the pressure should be in the 
range of from about 100 to about 400 atmospheres, particularly when 
employing the aforesaid preferred temperature range. It has been found, 
however, that the glycol aldehyde selectivity decreases slightly as a 
result of decreasing the total pressure from about 225 atmospheres to 100 
atmospheres. Lowering the total pressure to 500 psig (31 atmospheres) 
results in a substantial decrease in glycol aldehyde selectivity. 
The reaction pressures represent the total pressure gases contained in the 
reactor, i.e., carbon monoxide and hydrogen, and, if present, any inert 
diluent gas such as nitrogen. As in any gaseous system, the total pressure 
is the sum of partials pressures of component gases. In the present 
reaction, the molar ratio of hydrogen to carbon monoxide can range from 
about 1:10 to about 10:1, with the preferred ratio, from about 1:5 to 
about 5:1, and the reaction pressure can be achieved by adjusting the 
pressure of these gases in the reactor. For best results, the molar ratio 
of carbon monoxide to hydrogen is maintained at high values where partial 
pressures of carbon monoxide favor production of glycol aldehyde. Thus, to 
produce glycol aldehyde, the partial pressure of carbon monoxide is 
usually adjusted to be about 3 to about 10 times that of hydrogen. 
As with any process of this kind, the present process can be conducted in 
batch, semi-continuous, and continuous operation. The reactor should be 
constructed of materials which will withstand the temperatures and 
pressure required, and the internal surfaces of the reactor are 
substantially inert. The usual controls can be provided to permit control 
of the reaction such as heat exchangers and the like. The reactor should 
be provided with adequate means for agitating the reaction mixture; mixing 
can be induced by vibration, shaking, stirring, oscillation and the like 
methods. 
The results obtained with the present process are surprising and totally 
unexpected. For the first time, substantial formaldehyde conversion and 
selectivity to glycol aldehyde can be achieved by hydroformylating aqueous 
formaldehyde. Thus, glycol aldehyde selectivities greater than 80% and 
even as high as 94% have been achieved with greater than 70% aqueous 
formaldehyde conversion. The remarkable results pertaining to the 
hydroformylation of aqueous formaldehyde are believed to be attributable 
to the catalytic complex which is utilized. Thus, the rhodium-phosphine 
ligand complex wherein the phosphine ligand is a triorgano phosphine which 
has a cone angle within the range of 159 to 171.degree. results in 
conversion rates and glycol aldehyde selectivities thought only possible 
utilizing paraformaldehyde as the formaldehyde source.