Process for the preparation of ethylene glycol by catalytic hydrogenation

A process for the preparation of ethylene glycol by the vapor phase catalytic hydrogenation of at least one of di(lower alkyl) oxalate and lower alkyl glycolate in the presence of a hydrogenation catalyst comprising a carrier, which catalyst is suitable for the hydrogenation of alkyl oxalate and alkyl glycolate to ethylene glycol, the improvement lies within the preparation of the catalyst by employment of catalysts comprising carriers having specific ranges of physical parameters, including average pore diameter and pore volume, which parameters are interrelated by a relative activity index.

This invention relates to an improved process for the preparation of 
ethylene glycol by the vapor phase catalytic hydrogenation of at least one 
of di(lower alkyl) oxalate and alkyl glycolate in the presence of a 
hydrogenation catalyst comprising a carrier, which catalyst is suitable 
for the hydrogenation of alkyl oxalate and alkyl glycolate to ethylene 
glycol. More particularly, this invention relates to the catalytic 
hydrogenation of di(lower alkyl) oxalate to produce ethylene glycol using 
catalysts comprising carriers having specific ranges of physical 
parameters, including average pore diameter and pore volume, which 
parameters are interrelated by a relative activity index. 
INTRODUCTION TO ETHYLENE GLYCOL 
Ethylene glycol is a valuable commercial chemical and finds application in 
deicing fluids, antifreeze, hydraulic fluids, manufacture of alkyd resins, 
solvents and the manufacture of polyesters. As disclosed in Kirk-Othmer, 
Encyclopedia of Chemical Technology, 3rd Edition, ethylene glycol is 
commercially made by the hydrolysis of ethylene oxide which in turn is 
made by the catalytic epoxidation of ethylene using air or oxygen. 
However, several problems, particularly raw material supply, are 
associated with these commercial processes. 
First, ethylene is made commercially from natural gas liquids or naphthas. 
Second, in the catalytic epoxidation of ethylene in commercial facilities, 
the selectivity to ethylene oxide is usually less than 80 percent, with 
carbon dioxide being the primary by-product. Finally, the hydrolysis of 
ethylene oxide to ethylene glycol in conventional processes coproduces 
diethylene glycol and triethylene glycol. 
It has been proposed to use synthesis gas, i.e., mixtures of carbon 
monoxide and hydrogen, as alternative starting materials for the 
preparation of ethylene glycol, thus reducing dependency on ethylene and 
in turn the feed stocks required to produce ethylene. In some of these 
processes, the synthesis gas is reacted to form di(alkyl) oxalates which 
are then hydrogenated to form the desired ethylene glycol. This 
hydrogenation is especially difficult since the hydrogenation must be 
sufficient to reduce the ester radical, yet avoid over hydrogenation of 
glycol and/or intermediate glycolates to ethanol and other by-products. 
Moreover, it can be readily appreciated that hydrogenation reactions can 
yield a spectrum of products, due to both under and over hydrogenation. 
These by-products not only reduce the efficiency to ethylene glycol, but 
also can present troublesome impurities that must be removed from the 
ethylene glycol. 
U.S. Pat. No. 4,112,245 to Zehner, et al., discloses the preparation of 
ethylene glycol by the vapor phase catalytic hydrogenation of dialkyl 
oxalate in the presence of a copper-containing catalyst. However, this 
patent does not disclose any significance to the physical parameters of 
the carrier material. In fact, this patent only refers to supported 
catalysts in vague, general terms without explaining what is meant by the 
term supported catalyst. The patent gives no examples of supported 
catalysts, with the possible exception of nickel on kieselguhr, in which 
kieselguhr is active in the reaction and not an inert carrier or support 
material. 
INTRODUCTION TO CATALYST CARRIERS 
It is often desired to employ catalysts that comprise carriers. Among the 
benefits that are provided by catalyst carriers are reducing the amount of 
the catalytically-active species required, providing the catalyst in a 
more easily handled form, and facilitating the use of the catalyst in 
commerical-sized reactors without, for example, undue pressure drops or 
poor distribution of reactants throughout the reaction bed. 
Carriers are available in many sizes, shapes and compositions. Moreover, 
the surface and internal structures of carriers can vary widely. 
Unfortunately, with many reactions the nature of the carrier can affect 
the performance of the catalyst. The selection of suitable carriers has 
thus proven to be an empirical and complex task. The literature relating 
to carrier selection is often couched in generalities because of the 
empiric characteristic of the art. 
For instance, Rhone-Poulenc Chemical Company Catalog (p. 4), Technical 
Documentation, SC-MIN. S-81-1-3, entitled "Spherulite Catalyst Carriers" 
discloses that an increase in carrier surface area may lead to an increase 
in reaction velocity; however, diffusional limitations can occur when too 
small pores are used. The presence of large pores enables a more rapid 
distribution of the reactants and are often used in combination with 
smaller pores. While this publication generally discloses some physical 
parameters of carriers, it does not specifically disclose processes that 
would benefit from these physical parameters. 
A. Wheeler, "Reaction Rates and Selectivity in Catalyst Pores", in 
Catalysis Vol. II, p. 105, P. H. Emmett, (Ed.) (1955), Rheinhold, N.Y.; 
discloses that the carrier contans a network of interconnecting very fine 
pores and provides the seat of catalytic activity. The total surface area, 
distribution of pore sizes and total pore volume can be determined by 
routine methods. Wheeler states that pore size, pore volume and carrier 
size determine the degree to which diffusion affects reaction rates. This 
reference does not, however, disclose any specific physical parameters for 
carriers utilized in the hydrogenation of di(lower alkyl) oxalate to 
ethylene glycol. 
Harshaw Catalyst Catalog, (p. 43-45), (1980), entitled "Transport in Solid 
Catalysts", discloses that the rate of physical transport of reactants to 
the catalyst surface must keep up with the rate of the chemical reaction 
at the surface in order to effectively use the surface area of the 
catalyst. Since transport of reactants through narrow pores is of 
necessity low, there is a limit to the size of the pores, and, hence, to 
the size of the particles and the surface area per unit volume, unless 
wide pores can be realized together with very small particles. However, 
this publication states that the "initial selection of a catalyst is still 
based on trial and error." 
Thus, while the prior art has in general terms recognized the importance of 
the physical parameters of carriers, there has been a general failure in 
the prior art to interrelate these physical parameters, especially in 
reference to particular catalytic reactions, and certainly no guidance has 
been provided toward selecting advantageous catalysts for the 
hydrogenation of di(lower alkyl) oxalates to prepare ethylene glycol. 
SUMMARY OF THE INVENTION 
This invention relates to a process for the preparation of ethylene glycol 
comprising the steps of contacting, in the vapor phase and under 
glycol-forming hydrogenation conditions, hydrogen with at least one of 
di(lower alkyl) oxalate and lower alkyl glycolate in the presence of a 
catalytically-effective amount of a hydrogenation catalyst comprising a 
carrier, which catalyst is suitable for the hydrogenation of alkyl oxalate 
and alkyl glycolate to ethylene glycol, wherein the carrier is 
characterized by a relative activity index of at least about 1.0, said 
relative activity index being defined by the formula, relative activity 
index=1.38+0.39a+0.76b+0.001c+0.35d-0.39ab+0.012bc+0.003cd, wherein a is 
defined as the nominal external surface area of a typical carrier particle 
(S), expressed in square millimeters per particle units, divided by the 
volume (V) of the same carrier particle, expressed in cubic millimeters 
per particle units, minus 1.96 ((S/V)-1.96); b is defined as the pore 
volume (P) of the carrier, expressed in cc/gram units, minus 0.84 
(P-0.84); c is defined as the average pore diameter (D), expressed in 
Angstrom units, minus 169 (D-169); and d is defined as the macroporosity 
variable (M) minus 0.24 (M-0.24), wherein the macroporosity variable is 
assigned a value of 1.0 if said carrier has at least about 20% of its pore 
volume associated with pores having a diameter of at least about 1000 
Angstroms, and a value of zero if said carrier has less than about 20% of 
its pore volume associated with pores having a diameter of at least about 
1000 Angstroms. 
In addition, this invention relates to a hydrogenation catalyst comprising 
a carrier, said catalyst being suitable for the hydrogenation of alkyl 
oxalate and alkyl glycolate to ethylene glycol, wherein the carrier is 
characterized by a relative activity index of at least about 1.0. 
Catalysts having carriers with a relative activity index of at least about 
1.0 can exhibit significantly more activity toward ethylene glycol than 
those produced from carriers whose relative activity index is 
substantially below about 1.0. 
DISCUSSION OF THE HYDROGENATION PROCESS 
Ethylene glycol can be prepared by the vapor phase catalytic hydrogenation 
of a di(lower alkyl) ester of oxalic acid at elevated temperature and 
pressure. 
An overall equation for the reaction is believed to be represented as 
follows: 
##STR1## 
The hydrogenation of di(alkyl) oxalates is believed to proceed stepwise 
according to the following equations: 
##STR2## 
The first step involves the hydrogenation of one of the alkoxycarbonyl 
groups of a di(alkyl) oxalate to form an alkyl glycolate and the 
corresponding alkanol. In the second step, the remaining alkoxycarbonyl 
group is hydrogenated to produce ethylene glycol plus the corresponding 
alkanol. 
The oxalate esters which may be hydrogenated in accordance with the 
processes of this invention conform to the general formula: 
##STR3## 
wherein R is a lower alkyl group. The preferred esters for use in the 
hydrogenation process for the preparation of ethylene glycol are those 
esters wherein R is an alkyl group containing from 1 to 4 carbon atoms. 
Especially preferred are dimethyl oxalate and diethyl oxalate. 
In carrying out the hydrogenation reaction, the di(lower alkyl) ester of 
oxalic acid is generally preheated and vaporized, with the conditions of 
the hydrogenation being selected to ensure that essentially all of the 
ester is in the vapor state when passed over the catalyst bed. Thus, the 
reaction zone is maintained at an elevated temperature and pressure 
sufficient for hydrogenation to ethylene glycol and for preventing 
condensation of the oxalate ester and the product ethylene glycol. 
The processes, in accordance with the present invention, are carried out by 
passing vaporized oxalate ester, together with hydrogen, over the catalyst 
maintained at a reaction zone temperature typically between about 
150.degree. C. and about 300.degree. C. and preferably between about 
180.degree. C. and about 240.degree. C. The molar ratio of hydrogen to 
oxalate ester passed to the reaction zone is usually at least sufficient 
on a stoichiometric basis for complete hydrogenation of the oxalate ester 
to ethylene glycol and is often between about 4:1 and 200:1 and preferably 
between about 10:1 and 100:1. A hydrogen pressure between about 1 bar and 
about 350 bars is frequently used and preferably the hydrogen pressure is 
between about 10 bars and about 100 bars. In advantageous aspects of the 
processes, the gas hourly space velocity (the total volume of the vaporous 
oxalate and hydrogen gaseous mixture, as calculated at ambient temperature 
and pressure, passed over a unit volume of hydrogenation catalyst bed per 
hour) is between about 2,000 hr..sup.-1 and about 25,000 hr..sup.-1 and 
preferably between about 5,000 hr..sup.-1 and about 15,000 hr..sup.-1. The 
liquid hourly space velocity of oxalate ester (calculated as the liquid 
volume of oxalate, expressed in liquid form per unit volume of 
hydrogenation catalyst which is passed over the catalyst) is typically 
maintained between about 0.1 hr..sup.-1 and about 3.0 hr..sup.-1 and 
preferably between about 0.5 hr..sup. -1 and about 2.0 hr..sup.-1. For 
convenience, as used herein, the oxalate liquid hourly space velocity is 
calculated prior to mixing with hydrogen and is based on a liquid rather 
than a gaseous volume. 
In particularly attractive aspects of this invention, the percent 
conversion, calculated as the moles of oxalate in the feed minus the moles 
of oxalate recovered in the feed mixture after reaction divided by the 
moles of oxalate in the feed multiplied by 100, is maintained at greater 
than about 80% and preferably greater than about 95%. The percent 
conversion is a dependent variable, as the reaction temperature, the 
liquid hourly space velocity and other reaction variables are provided at 
sufficient interrelated values to obtain the desired conversion percent. 
CATALYST AND ITS PREATION 
The catalytically-active moieties deposited on the carrier may, in the 
broadest sense, include any moiety or mixture of moieties, capable of 
selectively hydrogenating esters to form hydroxyl substituted carbons such 
as the hydrogenation of di(alkyl) oxalate to form ethylene glycol. 
Therefore, a moiety exhibiting a relatively weak hydrogenation activity is 
preferred in order to maximize ethylene glycol production and minimize 
hydrogenolysis of the ethylene glycol. Most often, the hydrogenation 
catalyst comprises copper, either in the elemental form or combined with 
oxygen. Other representative moieties may include, for example, nickel, 
cobalt, ruthenium, palladium, platinum, rhodium, rhenium and combinations 
thereof. Preferred catalysts are the copper-containing catalysts, both 
unpromoted and promoted with components (e.g., metal oxide) containing 
chromium, manganese and/or zinc. The amount of catalytically-active 
moiety, based on total weight of the catalyst, is generally from about 1 
to 50%, while a range of about 2 to 20% is preferred, and about 5 to 15% 
being more preferred. 
Carriers are usually porous substances on which the catalytically-active 
component is deposited. Most preferably, the carriers are substantially 
inactive or inert. Suitable carriers may comprise one or more of silica, 
alumina, titania, molecular sieves, diatomaceous earth, activated carbon, 
silicon carbide, pumice, zeolite and the like. The silica, titania and 
alumina carriers are preferred, and the silica carrier is especially 
preferred. 
Carriers can be categorized by their chemical composition and physical 
properties. The carriers used in accordance with this invention are often 
characterized by physical properties such as (a) pore volume, (b) average 
pore diameter, (c) carrier geometry (the nominal external surface area and 
volume of the carrier), and (d) pore-size distribution. 
The terms surface area and nominal external surface area are distinct and 
should not be confused. Surface area constitutes the total surface area of 
the carrier including the surface area of the carrier's interconnecting 
pores. The nominal external surface area is the calculated superficial 
surface area of the carrier, i.e., a smooth surface devoid of pores. The 
nominal external area directly affects the relative activity index. 
Although the internal surface area is important to catalytic activity, 
this parameter does not appear in the relative activity index formula. 
However, the magnitude of the internal surface area is determined by the 
pore volume and average pore diameter terms, which are included in the 
relative activity index formula. 
In general, a carrier having a high surface area is desired to obtain a 
high dispersion of the catalytically-active moiety on the surface of the 
carrier. Typically, the reaction rate increases as the dispersion of the 
catalytically-active moiety increases. However, a high surface area is 
obtained at the sacrifice of pore size (average pore diameter), and this 
sacrifice may be disadvantageous to activity if the reaction is 
diffusionally limited. Therefore, with surface area, a compromise or upper 
limit is typically established. Surface areas desirably range from at 
least about 50 m.sup.2 /gram to about 600 m.sup.2 /gram. Preferred surface 
areas typically range from about 75 m.sup.2 /gram to about 400 m.sup.2 
/gram, with surface areas ranging from about 100 m.sup.2 /gram to about 
300 m.sup.2 /gram being even more preferred. Several other carrier 
physical parameters have optimum ranges because they are similarly 
influenced or limited by other physical parameters. The relative activity 
index interrelates the effects of these physical parameters on oxalate 
hydrogenation activity. 
The pore volume of the carrier, i.e., the total volume of all pores, can 
affect both the catalyst preparation and the subsequent hydrogenation 
reaction. Typically, high pore volumes permit higher metal loadings to be 
achieved without the necessity of multiple impregnation treatments. In 
preparing impregnated catalysts, the volume of solvent employed often is 
chosen to be just sufficient to fill the pores. The larger this pore 
volume, the greater the quantity of catalyst precursor which can be 
introduced. For the same reason, a high pore volume is useful when the 
catalyst precursor has a relatively low solubility in the impregnation 
medium. Generally, a high pore volume results in higher catalyst 
productivities than otherwise will be obtained. In accordance with this 
invention, pore volumes, expressed in cc/gram units, desirably range from 
about 0.4 cc/gram to about 1.5 cc/gram. Preferred pore volumes typically 
range from about 0.7 cc/gram to about 1.5 cc/gram, with pore volumes 
ranging from about 0.9 cc/gram to about 1.5 cc/gram being more preferred. 
Average pore diameter is inversely related to the total surface area of the 
carrier, i.e., for a given porosity, the smaller the average pore 
diameter, the higher the surface area. In accordance with this invention, 
average pore diameters, expressed in Angstrom units, desirably range from 
about 25 to about 600 Angstroms. Preferred average pore diameters 
typically range from about 50 to about 600 Angstroms, with average pore 
diameters ranging from about 125 to about 600 Angstroms being more 
preferred. 
The geometry of the carrier is also important for catalysts used in 
processes in accordance with this invention. Advantageously, the carriers 
are as small as practical. However, when supported catalysts are used in a 
tubular reactor, the carriers are generally restricted to a minimum 
diameter to avoid an unacceptably high pressure drop across the catalyst 
bed. The minimum acceptable diameter will, in general, be a function of 
the diameter of the tubular reactor and the gas flow rate among other 
things. 
The effect of the carrier on the hydrogenation processes of this invention 
can be influenced by the shape of the carrier as well as its size. These 
geometric shapes include spheres, cylinders, cored tablets (also referred 
to as annular shapes), stars, ribbed extrudates and saddles. 
As noted above, the carriers useful in accordance with this invention can 
be defined in terms of a relative activity index which interrelates the 
previously discussed physical parameters of pore volume, average pore 
diameter, pore size distribution, nominal external surface area and volume 
of the carrier. In accordance with this invention, the hydrogenation 
catalyst activity may be substantially increased by employing a carrier 
characterized by a relative activity index of at least about 1.0. A 
relative activity index greater than about 1.25 is preferred, with an 
index greater than about 1.50 being more preferred. 
In this relationship, the nominal external surface area of the carrier (S) 
does not include the surface area of internal pores, which contribute most 
to a carrier's active surface area, e.g., where the catalyst precursor is 
deposited during impregnation. This nominal external surface area is the 
calculated superficial surface area, i.e., a smooth surface devoid of any 
pores. The volume of the carrier (V) is the calculated total volume, and 
is not to be confused with the pore volume or porosity of the carrier. The 
nominal external surface area of the carrier, expressed in square 
millimeters per particle units, to carrier volume, expressed in cubic 
millimeters per particle units, ratio desirably ranges from about 0.5 to 
5.0 mm.sup.-1. Preferred ratios typically range from about 0.75 mm.sup.-1 
to about 5.0 mm.sup.-1, with ratios ranging from about 1.0 mm.sup.-1 to 
about 4.0 mm.sup.-1 being more preferred. 
The nominal external carrier surface area to carrier volume ratio, (a) in 
the above-stated relative activity index formula, may be calculated for 
carriers of varying sizes and shapes. For example, this ratio (a) can be 
determined by geometric calculation techniques. 
The macroporosity variable typically indicates whether a carrier has a 
significant distribution of macropores. In accordance with this invention, 
macropores are generally defined as pores having a diameter of at least 
about 1000 Angstroms. The macroporosity variable, in accordance with this 
invention, is assigned a value of 1.0 if the carrier has at least about 
20% of its pore volume associated with pores in the macropore range. A 
value of zero is assigned to those carriers having less than about 20% of 
its pore volume associated with pores in the macropore range. 
Preparation of the supported catalyst, in accordance with this invention, 
typically involves several steps: (1) washing the carrier, (2) 
impregnating/coating the precursor(s) of the catalytically-active moieties 
on the carrier, (3) drying and/or calcining the impregnated carrier and 
(4) reducing the precursor of the catalytically-active moiety to its 
active form. 
Frequently, it is desirable to pretreat the carrier, e.g., by washing to 
remove significant amounts of extraneous leachable components that may be 
deleterious to the performance of the catalyst. Conveniently, the washing 
may be with an acid solution. Any suitable acid treatment (washing) 
technique may be utilized. An especially preferred acid for the treatment 
is oxalic acid. Variations of this treatment may be used to accomplish 
this purpose. The washing is generally sufficient to enhance the 
performance of the catalyst. It is thought that the washing effects the 
removal of at least a portion of the leachable iron and/or sulfur from the 
carrier. See, for example, co-pending U.S. application Ser. No. 697,927, 
filed on even date herewith by W. J. Bartley, which is herein incorporated 
by reference. That application discloses a hydrogenation process for the 
preparation of ethylene glycol from oxalate ester, in which the carrier 
has a leachable iron (Fe.sup.+2 and/or Fe.sup.+3) content not greater than 
about 0.03%. 
Any means of depositing the catalytic metal components on the carrier, 
e.g., impregnating or coating techniques, may be used. Any effective 
impregnation treatment or solute may be utilized to impregnate the 
carrier. Typically, the carrier is impregnated with a medium containing a 
precursor which is decomposable to the catalytically-active moiety in the 
final catalyst. Thus, for example, where the desired active moiety is 
copper, a copper salt, such as copper nitrate, or a copper complex, such 
as a copper ammonia complex, may conveniently be used as the solute, and 
then the precursor can be converted to the desired active moiety. Where 
the desired active material is a material other than copper, a 
decomposable salt of the desired metal is chosen as the solute of the 
impregnating solution. In general, the desired active moiety is a metal, 
or a mixture of metals, and the solute or decomposable compound is 
correspondingly a metal salt, metal oxide or metal hydroxide or a mixture 
of metal salts, oxides or hydroxides. 
Performance of the supported catalyst may be affected by the nature of the 
impregnating solution. As described in copending U.S. application Ser. No. 
697,928, filed on even date herewith by W. J. Bartley, which is herein 
incorporated by reference, a catalyst for the preparation of ethylene 
glycol with a copper-containing catalyst is prepared by contacting said 
carrier with a copper ammonium carbonate complex medium and reducing the 
catalytically-active copper moiety to its active copper form. 
After impregnation, the carrier with deposited catalytically-active moiety 
or precursor can be dried, or, if a decomposable precursor is used, it can 
be converted to the desired catalytically-active form. Usually, drying and 
decomposition are separate operations, since most decomposable precursors 
will not be decomposed under normal drying conditions. Drying typically 
can be accomplished by exposure to drying conditions including elevated 
temperatures ranging from about 50.degree. C. to about 200.degree. C. for 
several hours, e.g., 0.5 to 30 hours, with temperatures ranging from about 
75.degree. C. to about 150.degree. C. being preferred. 
In some instances, when the decomposable precursor is a salt, it may be 
desired to form the oxide by calcination to facilitate the formation of 
the metal (should that be the desired catalytically-active moiety) through 
reduction. Calcination involves high temperature heating under oxidizing 
conditions so that any hydrates, carbonates, or the like are decomposed 
and volatile material is expelled. Calcination in an air atmosphere is a 
preferred means of converting most decomposable precursors to the oxide of 
the metal. The calcination treatment will usually depend on the 
decomposable precursor. For example, a copper salt, such as copper 
nitrate, begins decomposing to the copper oxide at about 170.degree. C. 
Copper carbonate, on the other hand, does not begin to decompose until a 
temperature of about 200.degree. C. In general, calcination typically can 
be carried out by exposure to temperatures ranging from about 170.degree. 
C. to about 600.degree. C., depending on the catalyst precursor, for a 
time sufficient to allow substantial conversion to the metal oxide form, 
with temperatures in the range of about 200.degree. C. to about 
500.degree. C., being preferred. 
Where the desired catalyst has a metal rather than a metal oxide as its 
catalytically-active moiety, the catalyst may then be reduced to the metal 
form by treatment with hydrogen prior to hydrogenation or during the 
hydrogenation reaction. Other reducing agents, e.g., carbon monoxide and 
metal hydrides, can also be employed. Reduction prior to the hydrogenation 
reaction typically involves purging the catalyst with an inert gas to 
remove oxygen and reducing under conditions that include the presence of 
reducing agent and elevated temperatures. 
Actual reduction procedures will vary depending on the catalyst and 
catalytically-active moiety. Hydrogen reductions of copper oxide to copper 
metal are typically carried out at temperatures ranging from about 
100.degree. C. to about 300.degree. C. with hydrogen partial pressures 
ranging from about 0.001 to about 100 bars in the substantial absence of 
oxygen. A slow reduction time is preferred and therefore preferred 
temperatures range from about 150.degree. C. to about 250.degree. C. with 
preferred hydrogen partial pressures ranging from about 0.01 to about 10 
bars. Conditions sufficient to convert at least a major portion of the 
oxide to the metal are preferred, with a conversion of 90% or greater 
being more preferred. 
The following examples are provided to illustrate the present invention in 
accordance with the principles of this invention, but are not to be 
construed as limiting the invention.