Molecular sieve abstract
Alpha, beta-unsaturated aldehydes are prepared by reacting formaldehyde
and a reactant aldehyde of formula RCH.sub.2 CHO wherein R is a
member of the class consisting of -H, -alkyl, -aryl, -aralkyl, -cycloalkyl,
and -alkylaryl radicals in the presence of AMS-1B borosilicate crystalline
molecular sieve catalyst. Metacrolein is prepared from propionaldehyde
and formaldehyde.
Molecular sieve claims
What is claimed is:
1. A process for the preparation of alpha, beta-unsaturated aldehydes
by reacting formaldehyde with a reactant aldehyde of formula RCH.sub.2
CHO wherein R is a member of the class consisting of --H, --alkyl,
--aryl, --aralkyl, --cycloalkyl, and --alkylaryl radicals, in the
presence of AMS-1B borosilicate crystalline molecular sieve catalyst
under reaction conditions wherein the reactant aldehyde:formaldehyde
mole ratio is from about 1:1 to 20:1 at a temperature within the
range of from about 250.degree. C. to about 430.degree. C.
2. The process of claim 1 wherein said AMS-1B catalyst composition
is the hydrogen form AMS-1B.
3. The process of claim 2 wherein hydrogen of hydrogen form of
AMS-1B is replaced by a member of the class consisting of a rare
earth metal, lanthanum and sodium.
4. The process of claim 1 wherein said formaldehyde is selected
from the group consisting of dry formaldehyde monomer in a gaseous
state, paraformaldehyde, methanolic formaldehyde and trioxane.
5. The process of claim 1 wherein said formaldehyde is trioxane.
6. The process of claim 1 wherein said mole ratio of said reactant
aldehyde to formaldehyde is within the range of 10:1 to 1:1 reactant
aldehyde to formaldehyde.
7. The process of claim 1 wherein mole ratio of said reactant aldehyde
to formaldehyde is 1:1.
8. The process of claim 1 wherein R of said formula RCH.sub.2 CHO
contains from 1 to 18 carbon atoms.
9. The process of claim 1 wherein said reactant aldehyde is selected
from the group consisting of acetaldehyde, propionaldehyde, n-butyraldehyde,
n-valeraldehyde, isovaleraldehyde, n-caproaldehyde, n-heptaldehyde,
capric aldehyde, laurel aldehyde, 2-phenylpropanol, 2-p-tolylethanal,
2-cyclopentylethanal and 2-phenylethanal.
10. The process of claim 1 wherein said reactant aldehyde is acetaldehyde.
11. The process of claim 1 wherein said reactant aldehyde is propionaldehyde.
12. The process of claim 1 wherein said temperature is within the
range of from about 275.degree. C. to 350.degree. C.
13. The process of claim 1 wherein water content of said formaldehyde
and said reactant aldehyde is no greater than about 8% by weight.
14. The process of claim 1 wherein water content of said formaldehyde
and said reactant aldehyde is no greater than about 4% by weight.
15. The process of claim 1 wherein said AMS-1B borosilicate crystalline
molecular sieve composition is incorporated within an alumina or
silica-alumina matrix.
16. The process of claim 15 wherein said AMS-1B borosilicate crystalline
content in said matrix ranges from about 10 to 80 wt.%.
17. The process of claim 15 wherein said AMS-1B borosilicate crystalline
content in the matrix ranges from about 30 to 65 wt.%.
18. The process of claim 1 wherein said AMS-1B borosilicate crystalline
molecular sieve composition is unsupported.
Molecular sieve description
BACKGROUND OF THE INVENTION
This invention relates to the preparation of alpha, beta-unsaturated
aldehydes by reacting formaldehyde with a reactant aldehyde of the
formula RCH.sub.2 CHO wherein R is a member of the class consisting
of --H, --alkyl, --aryl, --aralkyl, --cycloalkyl, and --alkylaryl
radicals, the number of carbon atoms in R being preferably from
1 to 18.
It is well-known that unsaturated aldehydes can be prepared by
condensing two aldehydes over a suitable catalyst. This invention
relates to a process for preparing unsaturated aldehydes, e.g.,
acrolein, methacrolein, ethacrolein and the like, by condensing
two aldehydes, one of which is formaldehyde, the other aldehyde
of formula RCH.sub.2 CHO containing two hydrogens on the alpha carbon,
in the presence of a catalyst comprising a borosilicate crystalline
molecular sieve, designated as AMS-1B. The catalyst has the following
composition in terms of mole ratios of oxides:
wherein M is at least one cation, n is the valence of the cation,
Y is a value within the range of 4 to about 600 and Z is a value
within the range of 0 to about 160 and providing a specific X-ray
diffraction pattern.
Unsaturated aldehydes, such as acrolein, methacrolein, ethacrolein
and the like, are widely used for the production of glycerol, polymers
and copolymers, pharmaceuticals, herbicides and other compounds
of considerable utility. Various processes and catalysts have been
proposed for the preparation of unsaturated aldehydes by an aldol-type
reaction. Generally, the reaction of the two aldehydes takes place
in the vapor or gas phase in the presence of a basic catalyst.
Various catalysts have been proposed for such reaction. For example,
U.S. Pat. No. 2639295 to Hagemeyer teaches the preparation of
unsaturated aldehydes such as acrolein, methacrolein and the like
by condensing formaldehyde with aliphatic aldehydes in the presence
of an ammonium salt or the salt of a primary or secondary amine.
Preferred catalysts are secondary amine hydrogen halides. A molar
excess of a second aldehyde in the ratio of usually 1:5 is taught
wherein formaldehyde or acetaldehyde is reacted with the second
aldehyde to obtain conversions of formaldehyde of 34.0 to 92.5%.
U.S. Pat. Nos. 3573702 and 3701798 to Snapp, et al. teach a
process for producing alpha, beta-unsaturated aldehydes which comprises
contacting formaldehyde and a saturated aldehyde in the vapor phase
in the presence of a solid catalyst comprising a supported rare
earth metal oxide of the lanthanide series, the support being any
inert material such as alumina or kieselguhr but which is preferably
silica gel. A molar ratio of formaldehyde to an excess of the second
aldehyde is taught, up to 1:25 in order to ensure maximum conversion
of the formaldehyde. Example 10 of Snapp U.S. Pat. Nos. 3573702
and 3701798 teaches that a 1:3 molar ratio gave formaldehyde conversions
of 34 to 45%. U.S. Pat. Nos. 3574703; 3845106; 3928458 to
Hagemeyer, et al. teach the preparation of alpha, beta-unsaturated
aldehydes by the vapor phase condensation of saturated aldehydes
with at least two hydrogen atoms attached to the alpha carbon with
formaldehyde in the presence of an unmodified silica gel catalyst.
The activity and effectiveness of the catalysts are taught as functions
of their pore volume and surface area. A 3:1 ratio of saturated
aldehyde to formaldehyde is taught to obtain formaldehyde conversions
of 35 to 45%, and selective yields based on formaldehyde consumed
ranged from 88 to 94%.
Olefin oxidation processes for preparation of unsaturated aliphatic
aldehydes are known. U.S. Pat. No. 3437690 to Young, et al. teaches
a process for preparing acrolein which comprises reacting in the
vapor phase propylene and oxygen in the presence of a catalyst comprising
a calcined mixture of an oxide of arsenic, a molybdochromic heteropoly
acid and a carrier. The oxide of arsenic can be alone, or together
with an oxide of chromium, manganese, iron or boron. Mole ratio
of olefin to oxygen can range from 1:0.2 to 1:10 preferably from
1:0.3 to 1:8. Conversions of propylene to acrolein are taught as
within the range of from 3.4 to 16.4% with yields based on propylene
within the range of 9.4 to 45.2%. U.S. Pat. No. 3359309 also
to Young, teaches a similar process for olefin oxidation to acrolein
using a catalyst comprising an arsenic oxide, a heteropoly acid
of molybdenum containing manganese on a carrier. Conversions based
on propylene ranged from 5.2 to 18.4%, and yields based on propylene
consumed ranged from 14.4 to 61%.
Accordingly, a number of processes using basic catalysts for the
condensation of two aldehydes have been taught heretofore. Other
processes have been taught for the oxidation of olefins using an
oxide of arsenic. However, the processes and catalysts taught heretofore
suffer from disadvantages which are greatly minimized in the process
of the instant invention. For instance, the processes taught in
U.S. Pat. Nos. 2639295; 3574703; 3845106; and 3928458 are
inferior to the present invented process in that formaldehyde conversions
are low when the second aldehyde concentration is low, that a molar
ratio of at least 3:1 is required for conversions of 35 to 45%,
based on formaldehyde consumed. The processes taught in U.S. Pat.
Nos. 3359309 and 3437690 are also inferior to the process of
the instant invention. Conversions of olefin taught in U.S. Pat.
Nos. 3359309 and 3437690 are within the range of from 3.4 to
18.4%.
An object of the present invention is to provide a process for
making unsaturated aldehydes from formaldehyde and other aldehydes.
A further object is to provide a process for making acrolein. Another
object is to provide a process for making methacrolein. Other objects
will appear hereinafter.
Quite unexpectedly, it has been found that a catalyst comprising
an AMS-1B borosilicate crystalline molecular sieve having the following
composition in terms of mole ratios of oxides:
where M is at least one cation, preferably hydrogen, n is the valence
of the cation, Y is a value within the range of 4 to about 600
and Z is a value within the range of 0 to about 160 and providing
a specific X-ray diffraction pattern, performs in a much superior
manner for the present process with respect to conversion and selectivity
relative to previously taught catalysts. Whereas previously taught
catalyst formulations usually require a basic metal on silica or
alumina substrates, the catalyst of the instant invented process
is a borosilicate crystalline molecular sieve catalyst. Yield and
selectivity are also improved over previously taught catalysts.
The improved process has several unexpected results. Whereas previously
taught processes result in low formaldehyde-based yields of alpha,
beta-unsaturated aldehydes when the ratio of aldehyde to formaldehyde
is low, such as 1:1 the aldehyde:formaldehyde ratio for the process
of the present invention is 1:1 to 20:1 preferred is 1:1 to 10:1
more preferred is 1:1 with consequent economic advantage. Also,
in the olefin process, substantial amounts of other products, mainly
acrylic acid, often are formed from the olefin when the olefin oxidation
process is used.
SUMMARY OF THE INVENTION
Disclosed is a process for preparing alpha, beta-unsaturated aldehydes
by reaction between formaldehyde and a reactant aldehyde of formula
RCH.sub.2 CHO wherein R is a member of the class consisting of --H,
--alkyl, --aryl, --aralkyl, --cycloalkyl, and --alkylaryl radicals,
the number of carbon atoms in R being preferably from 1 to 18 in
the presence of an AMS-1B borosilicate crystalline molecular sieve
catalyst under reactant conditions wherein the reactant aldehyde:formaldehyde
ratio is from about 1:1 to 20:1 at a temperature within the range
of from about 250.degree. C. to about 430.degree. C.
DETAILS OF THE INVENTION
The process of the instant invention relates to a process for preparing
alpha, beta-unsaturated aldehydes by reaction between formaldehyde
and a reactant aldehyde of formula RCH.sub.2 CHO wherein R is a
member of the class consisting of --H, --alkyl, --aryl, --aralkyl,
--cycloalkyl, and --alkylaryl radicals, the number of carbon atoms
in R being preferably from 1 to 18 in the presence of AMS-1B borosilicate
crystalline molecular sieve catalyst. Yield of alpha, beta-unsaturated
aldehydes is increased over previously taught processes and production
of by-products is minimized. The general method requires the presence
of AMS-1B borosilicate crystalline molecular sieve catalyst. Dry
formaldehyde, paraformaldehyde, methanolic formaldehyde or trioxane
is reacted with an aldehyde of formula RCH.sub.2 CHO wherein R is
defined as above in the gas phase at a temperature within the range
of from about 250.degree. C. to about 430.degree. C.
The present invention relates to a process using a synthetic crystalline
molecular sieve material, a crystalline borosilicate, as a catalyst.
The family of such crystalline borosilicate materials, which are
identified as AMS-1B borosilicates, and which are taught in commonly-assigned
U.S. Pat. No. 4269813 incorporated herein by reference, has a
particular X-ray diffraction pattern. Such crystalline borosilicate
can generally be characterized, in terms of the mole ratios of oxides,
as follows in Equation I:
wherein M is at least one cation, n is the valence of the cation,
Y is between 4 and about 600 and Z representing the water present
in such material is between 0 and about 160 or more.
In another instance, the claimed crystalline borosilicate can be
represented in terms of mole ratios of oxides for the crystalline
material not yet activated or calcined at high temperatures as follows
in Equation II:
wherein R is an alkylammonium cation, M is at least one cation,
n is the valence of the cation, Y is a value between 4 and 600
Z is a value between 0 and about 160 and W is a value greater than
0 and less than 1.
In Equation I, M can represent an alkali-metal cation, an alkaline-earth-metal
cation, an ammonium cation, an alkylammonium cation, a hydrogen
cation, a catalytically-active-metal cation, or mixtures thereof.
In Equation II, M can represent an alkali-metal cation, an alkaline-earth-metal
cation, an ammonium cation, a hydrogen cation, a catalytically-active-metal
cation, or mixtures thereof.
Advantageously, the value for Y falls within the range of 4 to
about 500. Suitably, Y is 4 to about 300; preferably, about 50 to
about 160; and more preferably, about 80 to about 120.
Suitably, Z is within the range of 0 to about 40.
The original cation M in the above formulations can be replaced
in accordance with techniques well-known in the art, at least in
part by ion exchange with other cations. Preferred replacing cations
include tetraalkylammonium cations, metal ions, ammonium ions, hydrogen
ions, and mixtures of the above. Particularly preferred cations
are those which render the AMS-1B crystalline borosilicate catalytically
active, especially for hydrocarbon conversion. These materials include
hydrogen, rare earth metals of Group IIIB, lanthanum, aluminum,
metals of Groups IA, i.e., sodium, potassium, lithium, etc., IIA,
i.e., calcium, strontium, barium, etc., and VIII, i.e., iron, cobalt,
nickel, etc., of the Periodic Table of Elements found in the 46th
Edition of the Handbook of Chemistry and Physics published by the
Chemical Rubber Company; noble metals, manganese, and other catalytically
active materials and metals known to the art. Rare earth metals,
lanthanum, sodium and hydrogen are considered especially useful.
The catalytically active components can be present anywhere from
about 0.05 to about 25 weight percent of the AMS-1B crystalline
borosilicate. The form wherein hydrogen replaces the original cation
M and n is 1 in the above formulations is designated HAMS-1B. The
hydrogen form of the AMS-1B crystalline borosilicate catalyst imparts
an acidic character to the catalyst to improve yields of unsaturated
aliphatic aldehydes. Divalent or trivalent cations are generally
recognized to impart acidic character to molecular sieves, but the
hydrogen ion is considered to impart more acidic character.
Embodiments of these borosilicates are prepared by the method which
comprises: (1) preparing a mixture containing an oxide of silicon,
an oxide of boron, a hydroxide of an alkali metal or an alkaline
earth metal, an alkylammonium cation or a precursor of an alkylammonium
cation, and water; and (2) maintaining said mixture at suitable
reaction conditions to effect formation of said borosilicate, said
reaction conditions comprising a reaction temperature within the
range of about 25.degree. C. to about 300.degree. C., a pressure
of at least the vapor pressure of water at the reaction temperature,
and a reaction time that is sufficient to effect crystallization.
The hydrogen form can be obtained by ion exchange.
The AMS-1B crystalline borosilicate useful in this invention can
be in an unsupported form for use either in a fixed bed or a fluidized
bed reactor. The AMS-1B crystalline borosilicate can be combined
with active or inactive materials, synthetic or naturally-occurring
zeolites, as well as inorganic or organic materials which would
be useful for binding the borosilicate. Well-known materials include
silica, silica-alumina, alumina, magnesia, titania, zirconia, alumina
sols, hydrated aluminas, clays such as bentonite or kaolin, or other
binders well-known in the art. Typically, the borosilicate is incorporated
within a matrix material by blending with a sol of the matrix material
and gelling the resulting mixture. Also, solid particles of the
borosilicate and matrix material can be physically admixed. Typically,
such borosilicate compositions can be pelletized or extruded into
useful shapes. Catalytic compositions can contain about 0.1 wt.%
to about 100 wt.% crystalline borosilicate material and preferably
contain about 10 wt.% to about 80 wt.% of such material and most
preferably contain about 30 wt.% to about 65 wt.% of such material.
Catalytic compositions comprising the crystalline borosilicate
material of this invention and a suitable matrix material can be
formed by adding a finely-divided crystalline borosilicate and a
catalytically active metal compound to an aqueous sol or gel of
the matrix material. The resulting mixture is thoroughly blended
and gelled, typically by adding a material such as ammonium hydroxide.
The resulting gel can be dried and calcined to form a composition
in which the crystalline borosilicate and catalytically active metal
compond are distributed throughout the matrix material.
Specific details of catalyst preparation are described in U.S.
Pat. No. 4269813.
It has been found that borosilicate catalysts prepared by the above
method are effective in catalyzing the reaction of aldehydes of
the formula RCH.sub.2 CHO wherein R is defined as hereinbefore and
formaldehyde wherein the reactant aldehyde:formaldehyde ratio is
from about 1:1 to about 20:1 at a temperature within the range of
from about 250.degree. to about 430.degree. C. and contact time
is from about 0.1 to about 20 seconds.
The reactant aldehyde is of the formula RCH.sub.2 CHO, and has
at least two hydrogens on the alpha carbon, the number of carbon
atoms in R being preferably from 1 to 18. Examples of acetaldehyde,
propionaldehyde, n-butyraldehyde, n-valeraldehyde, isovaleraldehyde,
n-caproaldehyde, n-heptaldehyde capric and laurel aldehydes, 2-phenylpropanal,
2-p-tolylethanal, 2-cyctopentylethanal, and 2-phenylethanal. For
example, acetaldehyde and formaldehyde are reacted to form acrolein,
propionaldehyde and formaldehyde to form methacrolein, n-butyraldehyde
and formaldehyde to form ethacrolein, etc.
It is essential for the process and catalyst of the instant invention
that water in the reactant aldehyde-formaldehyde feed, preferably
an aldehyde-trioxane (or gaseous formaldehyde monomer) feed, and
in the reactor under operating conditions be maintained at low levels,
no greater than a maximum of 8% by weight of the combined weight
of the reactant aldehyde-formaldehyde feed, preferably no greater
than 4% by weight. Since water is produced as a by-product of the
instant reaction, the reaction can be self-deactivating to the extent
that higher conversions of the reactant aldehyde-formaldehyde feed
to alpha, beta-unsaturated aldehyde cause higher gas phase concentrations
of water in the catalyst bed, thus requiring an increased operating
temperature which in turn decreases selectivity to unsaturated aldehyde
product. Formaldehyde can be used in any suitable dry form such
as dry formaldehyde monomer in a gaseous state, paraformaldehyde,
methanolic formaldehyde and trioxane. Trioxane pyrolyzes into gaseous
formaldehyde in the presence of the acidic form of AMS-1B catalyst.
As indicated in the examples, the novel process of the present
invention is carried out to synthesize alpha, beta-unsaturated aldehydes
from reactant aldehydes and formaldehyde. The instant invented process
is useful in synthesis of methacrolein by the vapor phase reaction
of propionaldehyde and formaldehyde. The instant invented process
is also useful in synthesizing other unsaturated aldehydes such
as acrolein, ethacrolein, etc.
The instant invented process is a single step process for the synthesis
of methacrolein which is catalyzed effectively by an AMS-1B borosilicate
crystalline molecular sieve catalyst as described herein.
The invented process for synthesis of methacrolein involves the
condensation of formaldehyde, preferably as trioxane, with propionaldehyde.
Although the mechanism is unknown, the mechanism probably involves
initial attack of hydroxy-methyl carbonium ion or its reactive equivalent
upon the enol form of the aldehyde.
The reaction occurs at atmospheric pressure in the gas phase when
the reactants are passed through the catalyst in the presence of
a nitrogen carrier gas at a temperature of 250.degree. C. to 430.degree.
C. Reactant pressures of from about 0.5 to 10 atmospheres can be
used. A broad range of reactant ratios can be successfully used
for this process. For example, when propionaldehyde and trioxane
(in mole ratios varying from 2:1 to 1:1 propionaldehyde:available
formaldehyde) are allowed to react at a temperature of 300.degree.
C. (or 325.degree. C.), yields of methacrolein obtained vary, respectively,
from 68-88% based on formaldehyde and from 44-57% based on propionaldehyde.
Other ratios, i.e., 20:1 up to 2:1 propionaldehyde to available
formaldehyde, can be used but with consequent loss in propionaldehyde-based
yields and in propionaldehyde selectivities.
Yield calculations can be based upon either the reactant aldehyde
or formaldehyde. For example, propionaldehyde-based yields are calculated
as follows: ##EQU1## Formaldehyde-based yields are calculated as
follows: ##EQU2## Propionaldehyde selectivity is calculated as follows:
##EQU3## Formaldehyde selectivity is calculated similarly.
In the following examples the percent of total aldehyde observed
in the product mixture (either as unreacted propionaldehyde, as
methacrolein or as 2-methyl-2-pentenal, a by-product of the reaction)
varies from 94-100%, depending on the reactant ratios, reaction
conditions, and age of the catalyst. The compound, 2-methyl-2-pentenal,
is the aldol condensation-dehydration product of propionaldehyde,
and it is formed in highest yields (up to 7%) under conditions of
high temperature, high contact times, and high mole ratios of propionaldehyde
to formaldehyde in the feed. Under opposing conditions, however,
methacrolein is formed in very high yield and with high selectivity.
For example, when a feed containing a 2:1 mole ratio of propionaldehyde
to formaldehyde (as trioxane) was passed through the reactor at
300.degree. C. (SPR=0.108 ml/min, N.sub.2 carrier rate=6 ml/min)
at 44.2% yield (based on propionaldehyde, 50%=theoretical maximum)
of methacrolein was obtained at 50.9% conversion of propionaldehyde
(50%=theoretical maximum) with 86.8% selectivity based on propionaldehyde
and at least 88.4% selectivity based on formaldehyde (88.4=% yield
of methacrolein based on trioxane).
Catalytic efficiency was calculated at 1.32 gms methacrolein/gm.
cat-hr. The selectivity for 2-methyl-2-pentenal was 4.2% based on
propionaldehyde. A significant quantity of this component (about
8% yield) is also formed when a propionaldehyde blank is run through
the reactor under the experimental conditions described above. Although
aldol condensations are typically best catalyzed by base in homogeneous
systems, they are also well-known to occur via catalysis with acid.
Further improved selectivity based on propionaldehyde, as described
in the following, was obtained with feed containing a 1:1 mole ratio
of propionaldehyde to formaldehyde. The example was run under conditions
as described above except a solution containing a 1:1 mole ratio
of propionaldehyde to formaldehye was utilized as feed. The yield
of methacrolein was 56.6% at 57.3% conversion of propionaldehyde
with 98.4% selectivity for methacrolein based on propionaldehyde.
Selectivity for 2-methyl-2-pentenal was only 1.9%.
Under these conditions a small amount (1.1% yield based on propionaldehyde)
of this by-product was formed. The crossed aldol reaction competes
against the homoaldol reaction for catalytically active sites, possibly
due to the steric constraints placed on the latter process by the
relatively greater bulk of the transition state leading to the formation
of 3-hydroxy-2-methyl-2-pentenal and/or by the size of the micropores
within the molecular sieve framework.
The invention will be illustrated by reference to the following
specific examples.
EXAMPLE I
The reactor consisted of a quartz tube fitted with a thermocouple
through the center of the tube to measure and control temperatures.
Inlets were provided at the top of the reactor for the carrier gas
stream and feed materials. The catalyst bed was positioned in the
reactor by an inert support material. Product was removed at the
bottom of the quartz tube. Heat was supplied by an electric tube
furnace.
A solution of propionaldehyde (10.0 ml, 8.05 gms, 0.1386 moles)
and trioxane (2.081 gms, 0.0693 moles) was prepared, and the total
solution volume was measured at 12.0 ml. The solution was drawn
into a syringe which was then attached to a syringe pump and connected
to a septum mounted near the top of the reactor with a long stainless
steel needle. The reactor was loaded with 1.00 gms of alumina-supported
HAMS-1B catalyst (50 wt. % HAMS-1B and 50 wt. % alumina), and the
catalyst bed was then brought to a temperature of 300.degree. C.
under a stream of nitrogen gas flowing at a rate of 6.0 ml/min.
After a 1 ml pre-run was collected and drained, a 4.0 ml portion
of the solution was allowed to pass through the reactor at a rate
of 0.108 ml/min. The clear colorless product was collected in a
receiver and analyzed by quantitative G-C analysis (SP 1200 column).
It was found to contain 1.43 gms (88% yield based on trioxane) of
methacrolein and 1.32 gms of unreacted propionic acid.
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