Process for making acetals

Acetals are produced from the reaction of aldehydes and alcohols, e.g. methylal by the reaction of methanol and formaldehyde, by the reaction in a reaction distillation column of the alcohol and aldehyde in the presence of a catalyst and the concurrent fractional distillation of the reaction mixture to separate the reaction products, water and acetal.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a process for the production of acetals 
from the reaction of a linear alkyl alcohol with an aldehyde. More 
particularly the invention relates to a process wherein the reaction is 
carried out simultaneously with the separation of the reaction products 
(water and methylal) from the reactants. 
2. Related Art 
It is known that acetals may be synthesized from alcohols and aldehydes in 
the presence of an acidic catalyst through a condensation reaction as 
illustrated by the following equations; 
##STR1## 
The IU names for each of the reaction products is as follows: 
(1) methanol+formaldehyde yields dimethyl acetal formaldehyde; common name 
methylal or dimethoxy methane 
(2) ethanol+formaldehyde yields diethyl acetal formaldehyde; common name 
ethylal or diethoxy ethane 
(3) methanol+acetaldehyde yields dimethyl acetal acetaldehyde; common name 
1,1 dimethoxy ethane 
(4) ethanol+acetaldehyde yields diethyl acetal acetaldehyde; common name 
1,1 diethoxy ethane or acetal. 
While "acetal" is one of the common names of diethyl acetal acetaldehyde, 
the term acetal as used herein means the reaction product of an alcohol 
with an aldehyde. 
Acetals are considerably more stable to alkalies than to acids. They are 
considered so much more stable than free aldehydes to the action of basic 
reagents and oxidizing agents that aldehydic compounds are often converted 
into the acetals to protect the aldehydic function from damage during the 
course of synthesis operations involving other parts of the molecule. 
Methylal is useful in the production of high concentration formaldehyde as 
disclosed in U.S. Pat. No. 4,967,014. Therein a two step process for the 
production of highly concentrated formaldehyde is disclosed wherein the 
first step is to react methanol with lower concentrations of formaldehyde 
to form methylal and the subsequent oxidation of the methylal to 
formaldehyde. Methylal is also useful for extracting certain sulfonated 
organic compounds from the alcohols used to prepare such compounds as 
disclosed in U.S. Pat. No. 4,096,175. 
U.S. Pat. No. 4,385,965 discloses a process for the recovery of pure 
methylal from methanol-methylal mixtures. U.S. Pat. No. 5,223,102 
discloses a process for the electrooxidation of methanol to formaldehyde 
and methylal. 
SUMMARY OF THE INVENTION 
Briefly, the present invention is a process for the production of acetals 
by contacting an alcohol and an aldehyde in a distillation column reactor 
containing an acidic catalyst in a distillation reaction zone thereby 
catalytically reacting said alcohol and aldehyde to produce acetal product 
and water and concurrently in said distillation reaction zone and 
fractionating the acetal product from said water and unreacted materials. 
The catalyst may be in the form of a catalytic distillation structure 
which provides both the catalytic sites and the distillation sites. The 
acetal product is withdrawn from the distillation column reactor as 
overheads and the water is removed as bottoms. The distillation column 
reactor is operated to keep the alcohol and aldehyde within the 
distillation reaction zone. 
The term "reactive distillation" is sometimes also used to describe the 
concurrent reaction and fractionation in a column. For the purposes of the 
present invention, the term "catalytic distillation" includes reactive 
distillation and any other process of concurrent reaction and fractional 
distillation in a column regardless of the designation applied thereto. 
More specifically the acidic catalyst is of such a nature as to allow vapor 
flow through the bed, yet provide a sufficient surface area for catalytic 
contact. The catalyst packing is preferably arranged in the middle portion 
of the distillation column reactor, more preferably occupying about 
one-third to one half of the column.

DETAILED DESCRIPTION OF THE INVENTION 
Suitable acidic catalysts are acidic cation exchange resins or molecular 
sieves, of particular interest are the Y and beta zeolites. Generally the 
molecular sieves and cation exchange resins are in much too fine a form to 
use as distillation packing since there is a very large pressure drop 
through the bed and the free flow of internal reflux and rising vapor is 
impeded. However, catalysts in the shape of conventional distillation 
structures, such as rings, saddles, and the like may be used in the 
present invention. 
The catalytic material is preferably a component of a distillation system 
functioning as both a catalyst and distillation packing, i.e., a packing 
for a distillation column having both a distillation function and a 
catalytic function, however, the present integrated refinery may also use 
such systems as described in U.S. Pat. Nos. 5,133,942; 5,368,691; 
5,308,592; 5,523,061; and European Patent Application No. EP 0 755 706 A1. 
The reaction system can be described as heterogenous since the catalyst 
remains a distinct entity. A preferred catalyst structure for the present 
reaction comprises flexible, semi-rigid open mesh tubular material, such 
as stainless steel wire mesh, filled with a particulate catalytic material 
in one of several embodiments recently developed in conjunction with the 
present process. 
Of particular interest is the structured packing disclosed and claimed in 
U.S. Pat. No. 5,730,843 which is incorporated herein in its entirety. 
Other catalyst structures useful in the present refinery scheme are 
described in U.S. Pat. Nos. 5,266,546; 4,242,530; 4,443,559; 5,348,710; 
4,731,229 and 5,073,236 which are also incorporated by reference. 
The particulate catalyst material may be a powder, small irregular chunks 
or fragments, small beads and the like. The particular form of the 
catalytic material in the structure is not critical so long as sufficient 
surface area is provided to allow a reasonable reaction rate. The sizing 
of catalyst particles can be best determined for each catalytic material 
(since the porosity or available internal surface area will vary for 
different material and, of course, affect the activity of the catalytic 
material). 
The particular reactions and products of interest are formaldehyde and 
acetaldehyde reactions with C.sub.1 -C.sub.3 alcohols to produce the 
corresponding acetal. The products are thus dimethyl acetal formaldehyde 
(methylal or dimethoxy methane), diethyl acetal formaldehyde (ethylal or 
diethoxy ethane), dimethyl acetal acetaldehyde (1,1 dimethoxy ethane), 
diethyl acetal acetaldehyde (1,1 diethoxy ethane) and dipropyl acetal 
formaldehyde (dipropoxymethane). 
The reactants are preferably linear alcohols and linear aldehydes, more 
preferably having one or two carbon atoms. The alcohol and aldehyde may be 
fed to the distillation column reactor above the catalyst section. The 
success of catalytic distillation lies in an understanding of the 
principles associated with distillation. First, because the reaction is 
occurring concurrently with distillation, the initial reaction product is 
removed from the reaction zone as quickly as it is formed. Second, because 
the reaction mixture is boiling, the temperature of the reaction is 
controlled by the boiling point of the mixture at the system pressure. The 
heat of the reaction simply creates more boil up but no increase in 
temperature. Third, the reaction has an increased driving force because 
the reaction products have been removed and cannot contribute to a reverse 
reaction (Le Chatelier's Principle). 
As a result, a great deal of control over the rate of reaction and 
distribution of products can be achieved by regulating the system 
pressure. Also, adjusting the through-put gives further control of product 
distribution and degree of olefin conversion. The temperature in the 
reactor is determined by the boiling point of the liquid mixture present 
at any given pressure. The temperature in the lower portions of the column 
will reflect the constitution of the material in that part of the column 
which will be higher than the overhead; that is, at constant pressure a 
change in the temperature of the system indicates a change in the 
composition in the column. To change the temperature the pressure is 
changed. Temperature control in the reaction zone is thus controlled by 
the pressure; by increasing the pressure, the temperature in the system is 
increased and vice versa. It can also be appreciated that in catalytic 
distillation as in any distillation there is both a liquid phase (internal 
reflux) and a vapor phase. Thus, the reactants are partially in liquid 
phase which allows for a more dense concentration of molecules for 
reaction, whereas, the concurrent fractionation separates product and 
unreacted materials, providing the benefits of a liquid phase system (and 
a vapor phase system) while avoiding the detriment of having all of the 
components of the reaction system continually in contact with the catalyst 
which would limit the conversion to the equilibrium of the reaction system 
components. 
Referring now to the figure a simplified flow diagram of the process is 
shown. The distillation column reactor 10 is shown to have a distillation 
reaction zone 12 containing the catalytic distillation structure, a 
stripping section 15 and a rectification section 16. The stripping section 
15 and rectification section 16 both contain standard distillation 
structures such as inert packing, bubble cap trays or sieve trays. 
An aqueous solution of aldehyde is fed to the distillation column via flow 
line 1 and alcohol is fed via flow line 2. The alcohol reacts with the 
aldehyde in the aqueous solution to form acetal and water. The acetal 
product is removed as overheads via flow line 5 and condensed in condenser 
13 and carried on to receiver 18 via flow line 3. Product methylal is 
removed by flow line 9 and a portion of the overheads is returned to the 
distillation column reactor 10 as reflux via flow line 6. Water is removed 
as bottoms via flow line 8. In some cases wherein the acetal product is 
higher boiling than water (1,1 diethoxy ethane b.p.=102.2.degree. C.) the 
product is primarily removed as overheads due to azeotroping with the 
alcohol and water. 
The distillation column reactor is preferably operated such that the 
aldehyde and alcohol are maintained within the distillation reaction zone 
for substantially complete reaction. If necessary a molar excess of 
alcohol may be supplied with unreacted alcohol being removed as overheads 
with the acetal product. If such is the case then a separate distillation 
column for separation of the alcohol from the acetal product would be 
required. 
EXAMPLE 
A one inch diameter distillation column is loaded with 2 feet of LZY 82 
molecular sieve catalyst prepared as a distillation structure as described 
above and supported in the column by 2 feet of standard inert packing. The 
LZY 82 is an ultra stable Y zeolite. In addition 2 feet of 1/4" saddles 
are loaded on top of the catalyst. Methanol and formalin (a 40% aqueous 
solution of formaldehyde) feed are started to the column and the pressure 
increased until the desired temperature is reached. After four hours the 
temperature in the bed is 252.degree. F. with an overhead pressure of 100 
psig. The overheads contains 77% methylal and 22% methanol as measured by 
gas chromatography. After another two hours the temperature in the bed was 
between 250.degree. F and 300.degree. F. with an overhead pressure of 95 
psig. At this point the overheads contains 73.5% methylal and 26.5% 
methanol. 
The collected overheads is further distilled in a 2" Oldershaw column to 
concentrate the product. The resultant overheads contains 0.2% dimethyl 
ether, 3.5% methanol, 0.8% unknown and 95.5% methylal. 
The activity of the LZY 82 molecular sieve was unexpectedly high in the 
presence of the water. Normally one would expect the water in the formalin 
and water product to dilute the acidity of the molecular sieve and reduce 
the activity.