Process for preparing methacrylic acid

A process is disclosed for producing an alpha-, beta-ethylenically unsaturated monocarboxylic acid compound which comprises the aldol-type condensation of a saturated aliphatic monocarboxylic acid compound under vapor phase conditions in the presence of a hydrocarbon of 6 to 12 carbon atoms and a solid catalyst.

FIELD OF THE INVENTION 
In general, the field of this invention relates to a method of reacting in 
the vapor phase a saturated aliphatic monocarboxylic acid compound and 
formaldehyde, the reaction product stream comprising unreacted saturated 
aliphatic monocarboxylic acid compound, an alpha, betaethylenically 
unsaturated aliphatic monocarboxylic acid compound of one more carbon atom 
than the starting saturated aliphatic monocarboxylic acid compound, water, 
unreacted formaldehyde and by-products. The reaction is in the presence of 
a silica catalyst and a C.sub.6 to C.sub.12 hydrocarbon. Surprisingly, it 
has been found that introduction of a C.sub.6 to C.sub.12 hydrocarbon into 
the reactor increases yield of the alpha-beta-unsaturated aliphatic 
monocarboxylic acid compound. 
In more specific terms, the field of this invention relates to a process 
for preparing methacrylic acid from formaldehyde and propionic acid in an 
improved yield, based on propionic acid, without any increased loss of 
reactants to by-products, in the presence of a silica catalyst and a 
C.sub.6 to C.sub.12 hydrocarbon. The process is a vapor phase aldol-type 
condensation of propionic acid and formaldehyde wherein the products are 
methacrylic acid, water and by-products. Unreacted propionic acid, and 
unreacted formaldehyde are present in the process effluent. 
BACKGROUND OF THE INVENTION 
Unsaturated acids, such as methacrylic and acrylic acids, acrylonitrile, 
and the esters of such acids, such as methyl methacrylate, are widely used 
for the production of corresponding polymers, resins and the like. Various 
processes and catalysts have been proposed for the conversion of alkanoic 
acids, such as acetic acid or propionic acid, and formaldehyde to the 
corresponding unsaturated monocarboxylic acids, e.g., methacrylic acid, by 
an aldol-type reaction. Generally, the reaction of a carboxylic acid and 
formaldehyde takes place in the vapor or gas phase while in the presence 
of a basic or acidic catalyst. 
The literature is replete with disclosures of the reaction of aliphatic 
carboxylic acid compounds with formaldehyde to produce alpha, 
beta-ethylenically unsaturated aliphatic monocarboxylic acid compounds of 
one more carbon atom than in the saturated carboxylic acid. For every 
molecule of alpha, beta-ethylenically unsaturated aliphatic monocarboxylic 
acid produced there is one molecule of water by-product. It is necessary 
to separate the alpha, beta-ethylenically unsaturated carboxylic acid 
compound, formaldehyde and the starting unsaturated carboxylic acid. 
In the case of methacrylic acid, this means that the methacrylic acid must 
be separated from propionic acid, formaldehyde and water. This separation 
presents several problems since each of the components are water soluble 
and because propionic acid and methacrylic acid have boiling points that 
are so close that it is difficult to fractionate one from the other. 
Further, the separation is complicated by the fact that methacrylic acid 
has a tendency to homopolymerize, and formaldehyde, if water is removed 
from the system, also has a tendency to homopolymerize. 
Of the various a1pha. beta-ethylenically unsaturated compounds, it is 
generally recognized that methacrylic acid has one of the greatest 
tendencies to polymerize and it is extremely difficult to handle at 
elevated temperatures. In this regard, we have found that the presence of 
certain reaction by-products greatly increase the propensity of 
methacrylic acid to homopolymerize. Specifically, alpha-, beta- 
unsaturated ketones, i.e., ethylisopropenyl ketone and 
2,5-dimethylcyclopenten-1-one, have been shown to greatly increase the 
degree of methacrylic acid homopolymerization. Additionally, methacrylic 
acid, propionic acid and formaldehyde individually form binary azeotropes 
with water. The boiling points of the three binary azeotropes are within 
1.degree. F. of each other and are thus exceedingly difficult to separate. 
The following table lists boiling points and weight percentages of binary 
azeotropes of water and methacrylic acid, propionic acid and formaldehyde 
at 760 mm Hg. 
______________________________________ 
Wt % Wt % H.sub.2 O 
B.P. .degree.F. 
______________________________________ 
Methacrylic acid 
23.1 76.9 210.7 
Propionic acid 
17.8 82.2 210.4 
Formaldehyde 
18.25-21.0 79.0-81.75 
210.4 
______________________________________ 
In somewhat greater detail, the invention relates to a process for an 
aldol-type condensation of a saturated aliphatic monocarboxylic acid 
compound and an aldehyde wherein said monocarboxylic acid is propionic 
acid and said aldehyde is formaldehyde. As is well-known, an aldol-type 
condensation can be base-catalyzed and is subject to ready dehydration if 
the .beta.-hydroxyl group is adjacent to an .alpha.-hydrogen atom. The 
product is an .alpha., .beta.-unsaturated acid of one more carbon atom 
than the original unsaturated aliphatic monocarboxylic acid, when the 
reacting aldehyde is formaldehyde. The reaction using propionic acid and 
formaldehyde is: 
EQU CH.sub.3 CH.sub.2 COOH+HCHO .fwdarw.CH=C(CH.sub.3)COOH+H.sub.2 O 
While the prior art has indicated that it is possible to carry out these 
reactions utilizing various catalysts, none of these references disclose 
specifically the use of a silica catalyst comprising at least one cation 
of a Group I or Group II metal and a silica support wherein the cation is 
present in a concentration of 0.001 to 0.2 equivalents per 100 grams 
silica support on a dry solids basis in the presence of a C.sub.6 to 
C.sub.12 hydrocarbon. 
A careful review of the prior art has failed to disclose any examples 
wherein addition of a C.sub.6 to C.sub.12 hydrocarbon to the aldol-type 
condensation reaction improved the product yield. 
In the prior art a number of methods which use solvent materials have been 
taught to separate the unreacted propionic acid and unreacted formaldehyde 
from the aqueous effluent resulting from vapor phase condensation of 
propionic acid and formaldehyde. U.S. Pat. No. 3,414,485 teaches use of a 
selective organic solvent to recover methacrylic acid from an aqueous 
reaction product effluent. Suitable organic solvents include o-, m- and 
p-xylene, toluene, n-octane, mono-chlorobenzene, methylamylketone, ligroin 
and methyl methacrylate monomer. U.S. Pat. No. 3,478,093 teaches use of a 
lactam having 4 to 7 ring members and a hydrocarbon radical substituent on 
the nitrogen atom as an extraction solvent to separate methacrylic acid 
from aqueous mixtures. U.S. Pat. No. 3,781,332 teaches use of a dual 
mixture containing methyl or ethyl methacrylate and not more than 50% of 
xylene, ethyl benzene or a mixture thereof. U.S. Pat. No. 4,040,913 
teaches a decantation method wherein an organic solvent extracts 
methacrylic acid and azeotropes with propionic acid. The aqueous raffinate 
is separated by decantation. U.S. Pat. No. 4,142,058 teaches use of a 
mixed solution of methyl methacrylate and toluene to separate methacrylic 
acid from an aqueous solution containing acetic acid. U.S. Pat. No. 
4,147,721 teaches use of methyl n-propyl ketone as a solvent to recover 
methacrylic acid from an aqueous reaction product. 
However, introduction of a C.sub.6 to C.sub.12 hydrocarbon into the reactor 
to increase reaction yield has not been previously taught. Preferably the 
C.sub.6 to C.sub.12 hydrocarbon azeotropes with propionic acid to permit 
separation of propionic acid from methacrylic acid by downstream 
distillation. 
The general object of this invention is to provide an improved method of 
reacting a saturated monocarboxylic acid compound, with formaldehyde to 
obtain an increased yield of an alpha, beta-ethylenically unsaturated 
aliphatic monocarboxylic acid compound of one more carbon atom than the 
starting saturated monocarboxylic acid compound. A more specific object of 
this invention is to provide an improved method of preparing methacrylic 
acid from propionic acid and formaldehyde which results in an increased 
yield of the desired methacrylic acid. 
The general object of this invention can be attained by injecting a C.sub.6 
to C.sub.12 hydrocarbon into the reaction of a saturated aliphatic 
monocarboxylic acid compound, and a formaldehyde compound in the presence 
of a catalyst comprising a silica support and at least one cation of a 
Group I or Group II metal in a concentration of about 0.001 to 0.2 cation 
equivalents per 100 grams by weight silica support on a dry solids basis 
at a temperature of from about 280.degree. C. to about 500.degree. C. 
under vapor phase conditions. Yields have increase by about 10% (e.g., 30% 
to 33%). Suitable C.sub.6 to C.sub.12 hydrocarbons are substantially 
non-reactive, water-immiscible compounds capable of breaking a water 
azeotrope of saturated aliphatic carboxylic acid compound. In a preferred 
method of operation, upon distillation, a major proportion of the 
ethylenically unsaturated monocarboxylic acid compound remains in the 
bottom of the column, and a major portion of the water, a portion of the 
formaldehyde compound and a major portion of the C.sub.6 to C.sub.12 
hydrocarbon are removed overhead. In separation of the reaction products 
of propionic acid and formaldehyde in the production of methacrylic acid 
and water using a silica catalyst, we have found it advantageous to 
recycle the recovered C.sub.6 -C.sub.12 hydrocarbon through the reactor. 
We have also found that by removing a side stream below the top of the 
distillation column, it is possible to recycle a substantial portion of 
unreacted formaldehyde and propionic acid together with the hydrocarbon to 
the reactor and avoid the polymerization and plugging of the distillation 
column by polymerized formaldehyde as is pointed out in application Ser. 
No. 624,049, Pat. No. 4,599,144, filed on even date in the names of 
Baleiko, et al, incorporated herein by reference. In a more preferred 
method of operation, the unreacted propionic acid, formaldehyde and 
C.sub.6 to C.sub.12 hydrocarbon are recycled to the inlet ports of the 
reactor and employed to produce methacrylic acid. 
SUMMARY OF THE INVENTION 
A method is disclosed of reacting a saturated aliphatic monocarboxylic acid 
compound and a formaldehyde compound, the reaction products comprising 
saturated aliphatic monocarboxylic acid compound, alpha, 
beta-ethylenically unsaturated aliphatic monocarboxylic acid compound of 
one more carbon atom than the starting saturated aliphatic monocarboxylic 
acid compound, water and by-products. The process comprises adding to the 
reactor a C.sub.6 to C.sub.12 hydrocarbon. Preferably the C.sub.6 to 
C.sub.12 hydrocarbon is a substantially non-reactive compound capable of 
breaking or preventing the formation of a water azeotrope of said 
saturated aliphatic monocarboxylic acid compound. In a preferred method of 
operation, said reaction products are fractionally distilled together with 
said substantially non-reactive compound under conditions whereby (1) a 
major proportion of the ethylenically unsaturated monocarboxylic acid 
compound remains in the bottom of the distillation column, (2) a major 
portion of the water, a portion of the formaldehyde compound and a major 
portion of the compound capable of breaking or preventing the formation of 
said azeotrope are removed overhead. In a more preferred method of 
operation, a side stream is removed below the top of the distillation 
column comprising water, a major portion of formaldehyde, and a 
substantial proportion of saturated aliphatic monocarboxylic acid. The 
non-reactive compound is recovered and recycled through the reactor.

DESCRIPTION AND PREFERRED EMBODIMENTS OF THE INVENTION 
The process of this invention will be understood from the following 
description and examples. 
The process of this invention is a process for the preparation in improved 
yield of alpha-, beta-ethylenically unsaturated monocarboxylic compounds 
of one more carbon atom than the starting saturated aliphatic 
monocarboxylic acid compound. Particularly it is a process for preparation 
in improved yield of methacrylic acid from propionic acid and formaldehyde 
wherein a C.sub.6 to C.sub.12 hydrocarbon is added to the reactor, thereby 
increasing yield to methacrylic acid, based on propionic acid. 
It is not known at this time why addition of a C.sub.6 to C.sub.12 
hydrocarbon to the reaction of propionic acid and formaldehyde to produce 
methacrylic acid in the presence of a silica catalyst improves the yield. 
However, it is postulated that the presence of the C.sub.6 to C.sub.12 
hydrocarbon aids the reaction by desorbing one or more of the reaction 
components or products from the catalyst. It is considered that the 
success of this method of preparing methacrylic acid in increased yield is 
due primarily to the use of particular process conditions, specifically a 
silica catalyst and use of a suitable C.sub.6 to C.sub.12 hydrocarbon. 
In a preferred method of operation, addition of a suitable C.sub.6 to 
C.sub.12 hydrocarbon to the reactor breaks or prevents the formation 
downstream of a propionic/water azeotrope. Upon distillation of reactor 
effluent, the unreacted propionic acid in the distillation tower is 
capable of being removed in a sidestream. This sidestream typically 
contains 60-95 (wt) % of the unreacted formaldehyde and 10-70 (wt) % of 
the unreacted propionic acid entering the distillation column, and up to 
no more than a total about 50 (wt) % water and methacrylic acid. The 
sidestream is suitable for direct recycle to the methacrylic acid 
synthesis reactor. The recycle of large percentages of unreacted 
formaldehyde and unreacted propionic acid directly back to the synthesis 
reactor reduces need for recovery of formaldehyde and propionic acid 
downstream and downsizes recovery equipment. Immediate recovery and 
recycle of unreacted formaldehyde and unreacted propionic acid immediately 
back to the synthesis reactor is an economic advantage. 
Because the product effluent stream contains amounts of unreacted propionic 
acid and unreacted formaldehyde as well as water, selection of a suitable 
hydrocarbon in a preferred method of operation wherein reactor effluent is 
distilled to separate the components is determined by the boiling points 
of azeotropes of propionic acid and methacrylic acid. Both propionic acid 
and methacrylic acid form water azeotropes which boil at approximately 
99.degree. C. Separation by distillation of the C.sub.6 to C.sub.12 
hydrocarbon in the presence of water requires that the water:hydrocarbon 
azeotrope which forms have a boiling point below the boiling points of the 
propionic acid and methacrylic acid water azeotropes. Preferably, the 
boiling point of the water:hydrocarbon azeotrope be no more than 
95.degree. C. 
Boiling points at atmospheric pressure of preferable hydrocarbon:water 
azeotropes are: 
______________________________________ 
Hydrocarbon % Water B.P..degree.C. 
______________________________________ 
n-Hexane 5 -- 
n-Heptane 13 79 
n-Octane 23 90 
n-Nonane 40 95 
______________________________________ 
Branched C.sub.6 to C.sub.12 saturated aliphatic hydrocarbons, aromatic 
hydrocarbons of 6 to 12 carbon atoms, cycloalkanes of 6 to 12 carbon 
atoms, and mixtures thereof which form water:hydrocarbon azeotropes with 
boiling points of no more than 95.degree. C. can also be preferred. 
A large number of catalysts, both water-tolerant and water-intolerant 
types, exhibit activity in the aldoltype condensation reaction of this 
invention. Specific catalyst materials that are useful in the process 
include synthetic alkali metal aluminosilicates, natural alkali metal 
aluminosilicates, synthetic alkaline earth metal aluminosilicates, natural 
alkaline earth metal aluminosilicates, alkali metal hydroxides on 
synthetic aluminosilicates, alkali metal hydroxides on natural 
aluminosilicates, alkaline earth metal hydroxides on synthetic 
aluminosilicates, alkali metal hydroxides on silica gel, alkaline earth 
metal hydroxides on silica gel, sodium silicate on silica gel, potassium 
silicate on silica gel, molybdenum oxide on silica gel, silica gel, 
synthetic manganese aluminosilicate, natural manganese aluminosilicate, 
synthetic cobalt aluminosilicate, natural cobalt aluminosilicate, 
synthetic zinc aluminosilicate, and natural zinc aluminosilicate. 
Catalyst compositions found to be especially useful in the reaction to form 
methacrylic acid from propionic acid and formaldehyde are the subject of 
applications numbered Ser. No. 624,040 and Ser. No. 624,041 filed an even 
date in the names of Hagen, et al, and Kaduk, et al, respectively, which 
are hereby incorporated by references. 
The synthesis reactor feed stock should be composed of propionic acid, 
formaldehyde, and some water. The mole ratio of propionic acid to 
formaldehyde should be maintained within the range from about 25/1 to 
about 1/25; with a preferred range of about 2/1 to 1/2. The feed stock or 
feed mixture is obtained by adding the required amounts of propionic acid 
and formaldehyde to the recycle mixture of propionic acid and 
formaldehyde, to maintain the desired proportions. Preferred concentration 
of water in the reactor (including water during the reaction) is at least 
3 (wt) % water of the reactor contents, including the C.sub.6 to C.sub.12 
hydrocarbon. 
The reaction takes place over a wide temperature range; temperatures in the 
range of about 280.degree. C. to about 500.degree. C. are satisfactory. 
Desirable and advantageous results are obtained by operating with 
temperatures in the range of about 280.degree. C. to about 350.degree. C. 
The process is normally run at atmospheric pressure, although higher or 
lower pressures can be used. 
The space velocity of the vaporized feed mixture over the catalyst may be 
varied over wide limits. Space velocity figures in this specification are 
based on the total number of moles of materials entering the catalyst 
zone. Total moles are multiplied by the volume of a mole of an ideal gas 
at 0.degree. C. and one atmosphere (22.4 liters/mole), to obtain the total 
volume. A space velocity in the range from about 100 liters per hour per 
liter of catalyst to about 1000 liters per hour per liter of catalyst is 
preferred. 
Any of the various formaldehyde containing materials may be used, such as 
formalin, methanolic formaldehyde solution, paraformaldehyde, and 
trioxane. 
The reactor effluent stream contains water of reaction, one mole of water 
for each mole of methacrylic acid produced. In a preferred method of 
operation, to separate the water from the propionic acid and methacrylic 
acid, the C.sub.6 to C.sub.12 hydrocarbon acts as an entrainer in the 
reactor effluent to prevent or break the water-propionic acid azeotrope 
upon distillation. The resulting entrainer can be any hydrocarbon capable 
of azeotroping with water and not forming a multi-component azeotrope with 
acid, as one of the components. Suitable entrainers include aliphatic 
saturated hydrocarbons of 6 to 8 carbon atoms such as n-hexane, n-heptane 
and n-octane, including their isomers, as well as benzene, o-, m-, or p- 
xylenes and toluene. n-Heptane is preferred. 
An element of the preferred process of the invention is distillation of the 
reactor effluent stream under process conditions wherein concentration of 
60-90 (wt) % of the unreacted formaldehyde and 25-70 (wt) % of unreacted 
propionic acid entering the distillation column are removed by a side-draw 
from the central part comprising from 10% to 90% of the theoretical trays 
of the distillation column for recycle to the synthesis reactor. The 
distillation column overhead consists of water, the hydrocarbon entrainer, 
comprising a C.sub.6 to C.sub.12 hydrocarbon, a small amount of 
formaldehyde and a trace of propionic acid. The distillation column 
bottoms contain methacrylic acid, propionic acid and the heavy by-products 
of the methacrylic acid synthesis reaction. Typical distillation column 
conditions are: 
______________________________________ 
Column Temperatures 
______________________________________ 
Overhead, .degree.F. 
160.degree.-175.degree. 
Side-Draw, .degree.F. 
210.degree.-250.degree. 
Bottoms, .degree.F. 
285.degree.-315.degree. 
Column Pressure, Atm. 
1 
______________________________________ 
The primary purpose of the reactor effluent distillation tower is to remove 
water from the methacrylic acid synthesis reactor effluent. The overhead 
from this tower, consisting of formaldehyde, water, the entrainer and a 
small amount of propionic acid, is sent to a formaldehyde recovery and 
dehydration section. There, aqueous formaldehyde is reacted with an 
alcohol, preferably 2-ethyl-1-hexanol (2-EH) forming 2-ethylhexyl 
hemiformal, which is then dried. The dry hemiformal is subsequently 
thermally cracked liberating dry formaldehyde for recycle to the reaction 
section. 
Introduction of a C.sub.6 to C.sub.12 hydrocarbon into the reactor also 
serves to prevent or break the binary azeotropes which otherwise can form. 
n-Heptane, as an example, upon introduction into the reactor serves to 
prevent or break the propionic acid-water azeotrope (BP 210.degree. F.) 
with a lower boiling n-heptane-water azeotrope (BP 174.6.degree. F). Upon 
distillation with no n-heptane present, propionic acid would be carried 
overhead from the effluent distillation column in substantial amounts. The 
propionic acid-water azeotrope is 17.8 (wt) % propionic acid. 
In a preferred method of operation, the reactor effluent distillation tower 
separates water from the methacrylic acid synthesis reactor effluent and 
separates a substantial proportion of unreacted propionic acid and the 
greater proportion of unreacted formaldehyde from the synthesis reactor 
effluent wherein the resulting sidedraw stream of propionic acid and 
formaldehyde taken from the effluent contains no more than about 50 (wt) % 
water and methacrylic acid, the major portion of the water present in the 
reactor effluent being carried overhead in the form of the n-heptane-water 
azeotrope. 
In a typical example of the method of operation, the effluent distillation 
tower consisted of a 40-tray two-inch vacuum jacket Oldershaw column 
equipped with a forced convection reboiler and a downflow condenser. 
Thermowells and sample taps were provided on every fifth tray of which 
several sample taps functioned as feed or product removal taps. 
Surprisingly, it was found that under the conditions of introduction of a 
suitable hydrocarbon into the effluent distillation column at temperatures 
of from approximately 160.degree. F. to 315.degree. F. at one atmosphere 
over the length of the distillation column, high concentrations of 
unreacted propionic acid and unreacted formaldehyde occurred within the 
column at certain tray levels, permitting removal of the unreacted 
propionic acid and unreacted formaldehyde from the distillation column. 
The sidestream so removed from the distillation column can contain as much 
as 60-95 (wt) % of the unreacted formaldehyde and 10-70 (wt) % of the 
unreacted propionic acid contained in the synthesis reactor effluent. The 
sidestream can contain as much as a total of 50 (wt) % water and 
methacrylic acid. 
Surprisingly, it has also been found that control of the water content of 
the sidestream can be obtained by control of the ratio of hydrocarbon to 
water in the column feed. This concentration ratio is dependent upon 
synthesis reactor operating conditions, in particular, the propionic 
acid/formaldehyde mole feed ratio to the reactor, the extent of 
formaldehyde conversion, the resulting water make in the reactor, the 
water concentration in the reactor feed, and C.sub.6 to C.sub.12 
hydrocarbon concentration. 
Since recovery of the unreacted formaldehyde and the hydrocarbon is 
necessary for the economic aspects of the process, the unreacted 
formaldehyde/water/hydrocarbon stream taken as overhead from the sidedraw 
distillation column is further processed to recover the formaldehyde and 
hydrocarbon. The formaldehyde is recovered by forming a hemiacetal with an 
alcohol selected from the group consisting of 2-ethylhexanol, cyclohexanol 
and other commercially available heavy alcohols. 2-Ethylhexanol is 
preferred. The hemiacetal, after removal of the water, is distilled to 
break the hemiacetal by heat to obtain the formaldehyde. The alcohol is 
removed as bottoms from the distillation column and recycled to reform the 
hemiacetal. The formaldehyde and hydrocarbon are recycled to the reactor 
or can be recovered separately. 
The invention comprises a method of reacting in vapor phase a saturated 
aliphatic monocarboxylic acid compound and a formaldehyde compound, the 
reaction product stream comprising unreacted saturated aliphatic 
monocarboxylic acid compound, unreacted formaldehyde compound, alpha, 
beta-ethylenically unsaturated aliphatic monocarboxylic acid compound of 
one more carbon atom than the starting saturated aliphatic monocarboxylic 
acid compound and water. The process comprises adding to the reactor a 
C.sub.6 to C.sub.12 hydrocarbon which is a substantially non-reactive 
compound capable of breaking or preventing the formation of a water 
azeotrope of said saturated aliphatic carboxylic acid compound. In a 
preferred method, the reaction product is fractionally distilled with the 
substantially non-reactive compound under conditions whereby (1) a major 
proportion of ethylenically unsaturated monocarboxylic acid compound 
remains in the bottom of the distillation column, (2) a major portion of 
the water, a portion of the formaldehyde compound and a major portion of 
the compound capable of breaking or preventing the formation of said 
azeotrope are removed overhead and (3) a side stream is removed below the 
top of the distillation column comprising water, a major portion of 
formaldehyde and a substantial portion of saturated aliphatic 
monocarboxylic acid. 
Specifically, the invention comprises a method of reacting propionic acid 
and formaldehyde to prepare methacrylic acid and water in an improved 
yield. In a preferred mode, the reaction products are fractionally 
distilled with C.sub.6 to C.sub.12 hydrocarbon which acts as an entrainer 
and unreacted propionic acid and unreacted formaldehyde are removed in a 
sidedraw stream. 
Embodiments of the process of the present invention can be found in the 
following examples. These embodiments and examples are presented for 
purposes of illustration only and are not intended to limit the scope of 
the invention. 
EXAMPLE I 
The reactor system consisted of a 1 inch O.D..times.0.834 inch I.D..times.6 
foot heated Inconel tube equipped with a 0.25 inch O.D. thermowell. The 
catalyst zone was typically 4 feet in length and typically contains 200 
gms of catalyst. Thermocouples inserted into the thermowell measured and 
controlled temperature at 6 inch intervals. The feed system was designed 
to handle a propionic acidparaformaldehyde slurry. The slurry was pumped 
to a vaporizer in which the paraformaldehyde was thermally decomposed to 
monomeric formaldehyde at 400.degree. F. This system allowed the use of 
lower reactor temperatures than a feed system using trioxane and propionic 
acid since trioxane does not completely decompose to monomeric 
formaldehyde below a temperature of 750.degree. F. 
Catalyst, 143.1 g., was cesium phosphate (Cs.sub.3 PO.sub.4) on silica gel, 
prepared by coforming. Anion (on phosphorus) catalyst loading was 1350 
ppm. Cation catalyst loading as 17,000 ppm. The catalyst was regenerated 
after 2 days on feed at a temperature of 700.degree. F. for a period of 48 
hours. 
Details of the reaction were: days on feed 3.2; reactor temperature 
700.degree. F; reactor pressure 20.0 psig, propionic acid formaldehyde 
molar feed ratio 1.55:1. 
Three runs were made injecting n-heptane into the reactor and compared with 
a control with no heptane. Conditions of the separate runs and results are 
in Table I. Improvement in yield of methacrylic acid in the presence of 
n-heptane ranged from 4.1% (27.4-26.3/26.6.times.100) to 15.2% 
(30.3-26.3/26.3.times.100) 
TABLE I 
______________________________________ 
Run No. 184 182 183 185 
______________________________________ 
Conditions 
Contact Time, Sec. 
13.4 13.9 13.8 14.8 
WHSV, 1/hr 1.44 1.35 1.46 1.40 
Diluent: N.sub.2, Mole % 
2.3 2.8 2.4 2.7 
Diluent: H.sub.2 O, wt % 
0.9 0.6 0.8 0.8 
Hydrocarbon: C.sub.7, Mole % 
0.0 15.6 5.8 5.9 
Results 
Conversion, % 33.6 31.8 31.4 33.1 
Selectivity, Mole % 
78.4 95.3 87.2 83.0 
Yield, Mole % 26.3 30.3 27.4 27.5 
______________________________________ 
Note: Conversion, selectivity and yield are based on propionic acid. 
EXAMPLE II 
The following example illustrates that removal of water by distillation of 
reactor effluent containing a C.sub.6 to C.sub.12 hydrocarbon is 
facilitated by presence of the hydrocarbon. Over 90% of the water is 
removed as overhead. 
Into a 2" vacuum jacketed Oldershaw distillation column containing 30 trays 
and 22 inches of 0.16" Pro Pack 316 S.S. packing at the top, equivalent to 
about 10 to 20 theoretical trays, 1212.9 g/hr was fed at Tray No. 8 of 
simulated methacrylic acid (MA) reactor effluent having the following 
composition of formaldehyde (FA), water (H.sub.2 O ), propionic acid (PA) 
and n-heptane (C.sub.7), 
______________________________________ 
FA H.sub.2 O 
PA MA C.sub.7 
______________________________________ 
wt % 9.11 3.69 40.37 16.05 
30.51 
mole % 19.1 14 35 12.1 19.8 
______________________________________ 
Also incorporated into the feed to prevent MA polymerization in the tower 
bottoms was 1000 ppm p-benzoquinone and 500 ppm phenothiazine. In addition 
4500-5000 ppm oxygen (as 50 vol.% with nitrogen) was sparged into the 
reboiler of the column. All levels are based on MA in the feed. 
As a control, actual pilot-plant reactor effluent containing small amounts 
of the by-products was fed to the reactor effluent column under nearly 
identical condition. Additional inhibitor was required to keep the system 
MA polymer free. The pilot plant reactor effluent had the composition of 
by-products described below; 
______________________________________ 
(wt) % 
______________________________________ 
3-Pentanone 0.034 
Isobutyric Acid 0.05 
2,5-Dimethylcyclopentenone 
0.013 
2,2,4-Trimethylbutyrolactone 
0.003 
______________________________________ 
An inhibitor package of 1100 ppm p-benzoquinone, 1100 ppm t-butylcatechol 
and 550 ppm phenothiozine together with an O.sub.2 addition rate of 10,000 
ppm O.sub.2 (all based on MA fed to the column) allowed operation with no 
visible evidence of MA polymers. 
The column was operated at atmospheric pressure with the temperature at 
various locations in the column as follows: 
______________________________________ 
Column Feed Sidedraw Column 
Bottoms Tray No. 8 Tray No. 13 
Overhead 
______________________________________ 
296.degree. F. 
264.degree. F. 
220.degree. F. 
169.degree. F. 
______________________________________ 
The compositions and takeoff rates for the column bottoms, sidedraw (Tray 
No. 18 ) and overhead are given below. 
______________________________________ 
Composition Analysis 
Rate (wt) % 
Location 
(g/HR) FA H.sub.2 O 
PA MA C.sub.7 
______________________________________ 
Overhead 
59.9 25.69 72.60 1.71 
-- .about.0 
Aqueous 
Organic 373.8 -- -- -- -- .about.100 
Sidedraw 
238.3 41.22 3.50 52.12 
2.36 
0.8 
Bottoms 553.6 &lt;0.01 0.07 65.23 
34.69 
.about.0 
.sup. 1225.6 g 
(101.0% Theory) 
______________________________________ 
The wt. % of FA and PA in the sidedraw stream correspond to 88.9% of the FA 
fed to the column (unreacted FA) and 25.4% the PA fed to the column 
(unreacted PA).