Molecular sieve abstract
Large zeolite and zeolite-like molecular sieve crystals of from
3 to 500 .mu.m are fixed on moldings by a process in which an adhesion
promoter is used or the moldings are softened.
Molecular sieve claims
We claim:
1. A process for the manufacture of a supported zeolite, aluminum
phosphate or silicoaluminophosphate catalyst in which molecular
sieve crystals of the catalyst are affixed to an inert, amorphous
and non-porous molding composed of glass, ceramic, metal, polymeric
materials or glass- or carbon-containing polymeric composites which
has been preshaped as a structural element to act as a carrier for
the catalyst, said process comprising:
preheating said structural element to its softening point without
melting it, and coating the surface thereof with a free-flowing
powder of said catalyst crystals for adherence to said surface in
its softened state, said crystals having a particle size of from
3 .mu.m to about 500 .mu.m and a transformation or melting point
which is above the softening point of said structural element: and
then cooling said structural element to permanently affix at least
a single layer of said catalyst crystals thereto.
2. A process as claimed in claim 1 wherein glass is used as the
preshaped support.
3. A process as claimed in claim 2 wherein a mineral thermal cement
as an adhesion promoter is applied to the glass surface to be coated
prior to the preheating step.
4. A process as claimed in claim 1 wherein said catalyst crystals
have a molecular sieve structure of the zeolite or pentasil zeolite
types identified as AFI, FAU, DOH, DDR, mixed DOH/DDR, MFI or MFL.
5. A process as claimed in claim 1 wherein said catalyst crystals
are of the pentasil type in the acidic H form, in the ammonium form
or in an alkali-free form still containing an amine or a quaternary
ammonium salt.
6. A process as claimed in claim 1 wherein the catalyst crystals
are irregularly shaped with the smallest dimension of size being
at least 3 .mu.m.
7. A process as claimed in claim 1 wherein the catalyst crystals
are of the pentasil zeolite type which have been prepared in the
absence of an alkali but in the presence of aqueous ammonia, an
amine, a tetraalkylammonium compound or mixtures thereof so as to
retain ammonium, amine or tetraalkylammonium components in the pores
of the catalyst, and said crystals are coated on a glass as the
preshaped structural element having a softening point above 400.degree.
C.
8. A process as claimed in claim 7 wherein the catalyst crystals
are a borosilicate of the pentasil zeolite type.
9. The supported catalyst structure obtained by the process of
claim 1.
10. The supported catalyst structure obtained by the process of
claim 9.
11. The supported catalyst structure obtained by the process of
claim 8.
Molecular sieve description
The present invention relates to a process for fixing large zeolite
and zeolite-like molecular sieve crystals of from 3 to 500 .mu.m
on moldings by means of an adhesion promoter or by softening a molding.
Processes catalyzed by heterogeneous catalysts are widely used
in the chemical industry. The catalysts are preferably used in the
form of moldings, for example as strands, extrudates, pellets, wagon
wheels or in the form of stars or clover leaves, in order to avoid
the pressure build-up in the reactor. On the other hand, mass transfer
and acute transport problems in the catalyst bed can greatly influence
the course of the reaction.
Various forms of reactors and catalyst have been tested in order
to overcome these problems. "Chemische Reaktionstechnik",
Baerns, Hofmann and Renken, Thieme-Verlag, Stuttgart, pages 163
to 193 (1987), discloses, for gas-phase reactions catalyzed by heterogeneous
catalysts, the use of reactors in which the catalyst is present
in the immediate vicinity of the reactor wall and free space is
present longitudinally in the middle of the reactor. In these types
of reactor, the catalyst is present in a cage specially mounted
for this purpose (Ind. Eng. Chem. 56 (1964), 39 et seq.) or is
fixed to the reactor wall by impregnation, for example by means
of an aluminum wash coat. This procedure is known for zeolite catalysts,
for example from JP 60/106 535 or for the preparation of thin films
also from JP 60/028 826. EP-A-55 044 and U.S. Pat. No. 3730910
mention possibilities for the further production of zeolite composites.
Especially for pentasil zeolites, there is also the possibility
of fixing by crystallization onto a preshaped carrier by the method
stated in EP-A-304 852. However, this gives rise to disadvantages
as a result of the large number of expensive operations. For kinetic
reasons, a very small catalyst particle size is desirable, particularly
in dehydration and oxidation reactions, since good results are obtained
only with short residence times in the catalyst pellet, ie. with
small diffusion distances of the starting materials and the products
to the active catalyst surface and back to the gas phase. Kinetically
more advantageous shapes of the catalyst are therefore desirable.
Catalysts in the form of monoliths are therefore used, for example
in SCR technology. This has the advantage that the pressure loss
in the reactor bed is substantially reduced.
The preparation of such monoliths is difficult and requires special
knowledge with regard to the addition of additives to the active
material in order to achieve good strengths of the molding, and
is very labor-intensive, for example when carrying out stepwise
drying and calcination in long shaft furnaces.
The processes described above for the production of heterogeneous
catalysts have the disadvantage that they comprise many process
steps. Particularly where use in catalysis or the separation of
substances is envisaged, this can however have an adverse effect
on the reproducibility. In addition, the binder materials to date
have been restricted to a small number, such as aluminas, clay minerals,
cordierite or silica, which may in turn have, for example, undesirable
catalytic activity.
If exothermic reactions occur in the catalytic reaction, it is
difficult to remove the liberated heat rapidly with a conventional
fixed-bed reactor, particularly in the case of reactions at low
pressure. This may contribute to the generation of temperature peaks
in the reactor, which may lead to irreversible damage to the catalyst.
If, on the other hand, catalytic reactions are endothermic, expensive
process engineering measures must be taken in order, for example,
to preheat the product stream with the aid of an exactly controlable
heating zone.
It is an object of the present invention to provide, for the use
of molecular sieve crystals, a possibility of remedying these abovementioned
disadvantages and of fixing these materials permanently so that
they can advantageously be used, for example, in catalysis and for
the separation of substances.
BRIEF DESCRIPTION OF THE DRAWINGS
The drawings illustrate the invention with reference to Examples
5 and 9 below wherein:
FIG. 1 is a scanning electron micrograph of a cross-section through
a glass absorber sheet coated with aluminum pentasil crystals; and
FIG. 2 is a graph showing the measured isotherms for both the glass
absorber sheet and the pure aluminum pentasil crystals of FIG. 1.
We have found that this object is achieved by a novel and improved
process for fixing large zeolite and zeolite-like molecular sieve
crystals of from 3 to 500 .mu.m on moldings, wherein an adhesion
promoter is used or the molding is softened.
Compared with existing methods, the novel process has the advantage
that it is no longer necessary to mold the crystals with the aid
of a binder material to give catalysts or adsorbents; instead, said
crystals are applied directly to a molding and this coated molding
is then used as a reactor or adsorber.
Advantages of the novel process are in particular that a smaller
amount of active catalyst or adsorbent material is required than
in the conventional procedure, that high gas velocities in conjunction
with good heat transfer between reactor wall and catalyst can be
realized and that simple regeneration can be effected.
The novel process can be carried out as follows:
The large primary crystals of zeolites or of zeolite-like molecular
sieves having edge lengths of from 3 to 500 .mu.m can be treated
in the presence of, and in contact with, a corresponding molding
at a temperature which on the one hand is below the transformation
or melting point of the molecular sieve crystals and on the other
hand is chosen sufficiently high that the corresponding molding
merely softens at the surface without however melting.
Zeolites are understood as being crystalline aluminosilicates which
have a highly ordered structure with a three-dimensional network
of SiO.sub.4 and AlO.sub.4 tetrahedra which are linked by common
oxygen atoms. The ratio of the Si and Al atoms to oxygen is 1:2
(cf. Ullmans Encyklopadie d. Techn. Chemie, 4th Edition, Volume
24 page 575 (1983)). The electrovalency of the aluminum-containing
tetrahedra is balanced by the inclusion of cations in the crystal,
for example of an alkali metal or hydrogen ion. Cation exchange
is possible. The voids between the tetrahedra are occupied by water
molecules prior to the dehydration by drying or calcination.
The zeolites are generally used in the acidic H form or neutral
alkali form. In the zeolites, other elements, such as B, Ga, Fe,
Cr, V, As, Sb, Bi or Be or mixtures thereof may be incorporated
in the framework instead of aluminum, or the silicon can be replaced
by another tetravalent element, such as Ge, Ti, Zr, or Hf.
Zeolites are divided into various groups according to their structure.
A list of such structures is given in the literature (W. M. Meier
and D. H. Olson, "Atlas of Zeolite Structure Types", 2nd
Edition, Butterworths, London 1987).
Molecular sieves suitable for the novel process are zeolites from
the mordenite group and zeolites of the faujasite type, for example
Y, X or L zeolites. This group of zeolites includes the "ultrastable"
zeolites of the faujasite type, ie. dealuminated zeolites. Processes
for the preparation of such zeolites are described in "Catalysis
by zeolites" Volume 5 from "Studies in Surface Science
and Catalysis", ed. B. Imelik et al. Elsevier Scientific Publishing
Comp. 1980 page 203 and "Crystal Structures of Ultra-stable
Faujasites", Advances in Chemistry Series No. 101 American
Chemical Society Washington, D.C., page 226 et seq. (1971), and
in U.S. Pat. No. 4512961.
Zeolites and zeolite-like molecular sieves having the structure
types AFI, FAU, LTA, DOH, DDR or mixed DOH/DDR structures are advantageously
used, and zeolites of the pentasil type having an MFI or MEL structure
are particularly advantageously employed.
Such pentasils have, as a common building block, a five-numbered
ring composed of SiO.sub.4 tetrahedra. They possess a high and variable
SiO.sub.2 /Al.sub.2 O.sub.3 3 ratio and pore sizes which are between
those of the zeolites of type A and those of type X or Y.
These zeolites may have different chemical compositions. They are
aluminosilicate, borosilicate, ferrosilicate, titanosilicate, vanadosilicate,
beryllosilicate, gallosilicate, chromosilicate, arsenosilicate,
antimonosilicate and bismuthosilicate zeolites or mixtures thereof
and aluminogermanate, borogermanate, gallogermanate and ferrogermanate
zeolites or mixtures thereof.
The aluminosilicate zeolite is prepared, for example, from an aluminum
compound, preferably Al(OH).sub.3 Al.sub.2 (SO.sub.4).sub.3 or
a hydrolyzable organic aluminum compound, as an intermediate and
a silicon component, preferably finely divided silica or colloidal
silica, in aqueous amine solution, in particular in a polyamine,
such as 16-hexane diamine or 13-propane diamine or triethylene
tetramine solution, with or, in particular, without the addition
of an alkali metal or alkaline earth metal compound, at from 100
to 220.degree. C. under autogenous pressure.
These include the isotactic zeolites according to EP-A-34 727 or
EP-A-4 504. The aluminosilicate zeolites obtained have an SiO.sub.2
/Al.sub.2 O.sub.3 ratio of from 10 to 40000 depending on the choice
of the amounts of starting materials. Such aluminosilicate zeolites
can also be synthesized in an ether medium, such as diethylene glycol
dimethyl ether, triglyme or tetraglyme, in an alcoholic medium,
such as methanol or 14-butanediol or 16-hexanediol or polyols,
or in water.
The preparation of large monodisperse aluminosilicate zeolites
prepared in the absence of alkali is described by Muller et al.,
A.C.S. Symp. Ser. Vol. 398 (1989), 346-359. According to this method
of preparation, zeolites up to 300 .mu.m can be obtained with high
yields of solid.
The synthesis of large borosilicate zeolite crystals having an
MFI structure is disclosed in DE-A-41 20 918. The size measured
over the longitudinal crystallite axis is from 2 to 150 .mu.m. In
the preparation, an SiO.sub.2 -containing mixture of water, a tetraalkylammonium
compound, a boron component and ammonia is subjected to a hydrothermal
reaction with a molar ratio of ammonia/tetraalkylammonium of from
3:1 to 150:1 in the absence of a metal hydroxide or of a metal
salt.
This process also provides the possibility of influencing the size
of the resulting crystals and their size distribution in a simple
manner by means of the composition of the reaction mixture by using,
for example, a template mixture composed of tetrapropylammonium
salts and tetrapropylammonium hydroxide, and requires only small
amounts of tetraalkylammonium corresponding to the tetraalkylammonium/SiO.sub.2
ratio of from 0.042:1 to 0.2:1 preferably from 0.042:1 to 0.15:1.
In particular, it is possible to obtain the resulting borosilicate
zeolites predominantly in the form of uniform large particles which
occur without significant agglomeration if, in the reaction mixture,
molar ratios of SiO.sub.2 /B.sub.2 O.sub.3 of from 10:1 to 500:1
preferably from 10:1 to 250:1 particularly preferably from 10:1
to 100:1 are used and/or a dilution of SiO.sub.2 /H.sub.2 O of
from 0.07:1 to 0.025:1 preferably from 0.05:1 to 0.04:1 is maintained.
This alkali-free process furthermore makes it possible for the material
to be obtained directly in an active H form after heat treatment
at from 350.degree.to 600.degree. C., preferably from 400.degree.to
550.degree. C., particularly preferably from 450.degree.to 500.degree.
C. and without additional ion exchange and, in particular owing
to the crystallite size of more than 2 .mu.m, ie. from 2 to 150
.mu.m, preferably from 3 to 120.mu.m, particularly preferably from
5 to 80 .mu.m, without further shaping, and to be used as a catalyst.
Since zeolites prepared in this manner have a very monodisperse
particle size distribution and furthermore scarcely agglomerate
as primary crystallites, they are particularly suitable for the
fixing described according to the invention.
The similar preparation Of large titanium zeolite crystals having
an MFI structure suitable for catalytic oxidation and ammoxidation
reactions is mentioned in DE-A-41 38 155.
Ferrosilicate zeolites are obtained, for example, from an iron
compound, preferably Fe.sub.2 (SO.sub.4).sub.3 and a silicon compound,
preferably finely divided silica in aqueous amine solution, and
in particular 16-hexanediamine, with or without the addition of
an alkali metal or alkaline earth metal compound, at from 100.degree.to
220.degree. C. under autogenous pressure.
The silicon-rich zeolites which can be used (SiO.sub.2 /Al.sub.2
O.sub.3 >10) include the ZSM types (from Mobil), Ferrierite,
NU-1 NU-2 Silicalit.RTM. (from Union Carbide/UOP), zeolite-.beta.
or silicon-rich, dealuminated Y-zeolites.
Crystals having the structure type DOH or DDR or mixed crystals
having DOH/DDR structures can also be used for the novel process.
The preparation of such crystals is described by Muller et al.,
"Recent Advances in Zeolite Science" (Editors Klinowski
and Barrie) Elsevier, Amsterdam (1989), 241 to 252. The dimensions
of the primary particles may be up to 200 .mu.m.
If, owing to the method of preparation, the zeolites are not in
the acidic H form but, for example, in the alkali metal or alkaline
earth metal form, these can be converted completely or partly into
the desired H form by ion exchange, for example with ammonium ions
and subsequent calcination or by treatment with acids.
Microporous phosphates as mentioned by E. M. Flanigen et al. in
"Aluminophosphate molecular sieves and the periodic table",
Pure & Appl. Chem. 58 No. 10 (1986), 1351-1358 are also suitable
for the novel process.
U.S. Pat. No. 4310440 discloses many such microporous aluminophosphates
(abbreviation: AlPOs). According to this publication, they are prepared
by using templates (structure formers) as starting components in
addition to phosphorus- and aluminum-containing compounds. Suitable
templates are a large number of organic nitrogen compounds (for
example amines, ammonium compounds and nitrogen heterocycles). Depending
on the type of template and the reaction conditions, it is possible
to prepare many different aluminophosphate structures which are
characterized by their different X-ray diffraction data.
The abovementioned aluminophosphates differ in the structure of
their cavity system, ie. in pore size, pore volume and surface area.
These differences result in different properties, such as adsorption
capacities, the ability to separate certain substances or catalytic
activity. Aluminophosphates consist of a crystal lattice which is
composed of TO.sub.4 -tetrahedra, phosphorus and aluminium occuring
as tetrahedron atoms T. In general, the numbers of AlO.sub.4 --and
PO.sub.4 -tetrahedra are equal so that the crystal lattice does
not carry any charge and hence no charge-balancing cations are present.
Microporous aluminophosphates are used as adsorbents and also as
catalysts or as catalyst carriers (Review article: E. M. Flanigen
et al.: "Structural, synthetic and physicochemical concepts
in alumino phosphate-based molecular sieves" in "Innovation
in Zeolite Materials Science" P. J. Grobet et al. (Ed.), Elsevier,
1988 pages 13 to 27).
The aluminophosphates prepared under hydrothermal conditions are,
for example, AlPO-5 AlPO-8 AlPO-9 AlPO-11 AlPO-12 AlPO-14
AlPO-21 AlPO-25 AlPO-31 and AlPO-33 and MCM 9. Syntheses of these
compounds are described in EP-A-132 708 U.S. Pat. No. 4310440
and Wilson et al., J. Am. Chem. Soc. 104 (1982), 1146.
For example, AlPO.sub.4 -5 (AFI) is synthesized by homogeneously
mixing orthophosphoric acid with pseudoboehmite (Catapal.RTM. SB)
in water, adding tetrapropylammonium hydroxide to this mixture and
then carrying out the reaction at about 150.degree. C. for from
20 to 60 hours under autogenous pressure in an autoclave. The AlPO-5
filtered off is dried at from 100.degree.to 160.degree. C. and calcined
at from 450.degree.to 550.degree. C.
The synthesis of particularly large crystals of AlPO-5 having an
AFI structure is described by Muller et. al. in A.C.S. Symp. Ser.
398 (1989), 346-359. According to this publication, AFI aluminophosphate
crystals having lengths of up to 500 .mu.m can be prepared.
The preparation of a microporous aluminophosphate defined as AlPO.sub.4
-11 is described in U.S. Pat. No. 4310440 Examples 32 to 36.
According to this publication, the compound is synthesized by using,
as starting components, phosphoric acid, an aluminum compound and
a dialkylamine, eg. di-n-propylamine or diisopropylamine. The mixture
is treated hydrothermally. The product initially formed is a microporous
aluminophosphate whose pores are filled with the template. The template
can be removed by calcining this compound.
According to U.S. Pat. No. 4440871 it is possible additionally
to incorporate silicon into the crystal lattice of these aluminophosphates,
with the result that the silicoaluminophosphates (abbreviation:
SAPOs) are obtained.
The synthesis of the SAPOs is effected in a similar manner to that
of the AlPOs, a silicon source being additionally introduced into
the synthesis mixture. The charges of the AlO4--,PO.sub.4 -and SiO.sub.4
-tetrahedra present in the crystal lattice of the SAPOs do not in
general balance, so that a charged crystal lattice whose charge
must be balanced by opposite ions is obtained.
For this reason, SAPOs can also be used as ion exchangers, in addition
to the applications already mentioned in the case of the AlPOs.
SAPOs in their H form are also solid acid and can accordingly be
used, for example, as acidic catalysts.
The synthesis of these compounds is described, for example, in
EP-A-103 117 or U.S. Pat. No. 4440871. These silicoaluminophosphates
have a zeolite structure. SAPOs are prepared by crystallization
from an aqueous mixture at from 100 to 250.degree. C. and autogenous
pressure in the course of from 2 hours to 2 weeks, the reaction
mixture of a silicon component, an aluminum component and a phosphorus
component being reacted in aqueous organic amine solutions.
The preparation of a microporous silicoaluminophosphate defined
as SAPO-11 is described in U.S. Pat. No. 4440871 Examples 15
to 22 and is carried out in a manner similar to that described
for AlPO-11 the only difference being that a reactive silicon source
is additionally introduced into the reaction mixture.
The AlPO.sub.4 -11 and SAPO-11 have an identical structure, which
was described, for example, by Bennett et al. (Zeolites, 7 (1987),
160). The structure is classified as a crystal structure under the
designation AEL by Meier and Olson ("Atlas of Zeolite Structure
Types" 2nd Ed., Butterworths, London, 1987).
A new possibility for the synthesis of AlPO.sub.4 -11 and SAPO-11
using 12-bis(4-pyridyl)ethane is mentioned in DE-A-41 31 68.
For example, ZYT-5 ZYT-6 ZYT-7 ZYT-9 ZYT-11 and ZYT-12 (JP
59/217 619) are also suitable silicoaluminophosphates. The reactors
produced according to the novel process using the abovementioned
zeolites and zeolite-like molecular sieve crystals can be used in
heterogeneous catalysis for nucleophilic and electrophilic substitutions,
for addition and elimination reactions, for double bond and skeletal
isomerizations, including rearrangement reactions, and for redox
reactions, for example for alkylation, isomerization, disproportionation,
acylation, cyclization, hydration, dehydration, amination, hydrogenation,
dehydrogenation, dehydrocyclization, hydroxylation, oxidation, epoxidation
and skeletalisomerization reactions and combinations of these reactions,
for the selective reaction of organic molecules.
Particular examples are the alkylation and isomerization of xylenes,
phenols, cresols, ethylbenzenes, anilines substituted in the nucleus,
dihydroxybenzenes and similar halogenated isomeric aromatics.
Industrially important reactions are, for example, the alkylation
and isomerization reaction to give p-xylene, stated in U.S. Pat.
No. 4670616 or EP-A-54 385.
The reactors produced by the novel process can moreover be used
in the conversion of methanol into a mixture of C.sub.2 --to C.sub.4
-olefins. Reactions of this type are described, for example, in
W. Holderich et al., "Aluminosilicate and Borosilicate Zeolites
and their use in the conversion of methanol to olefins" in
D. Olson and A. Bisio (eds.) Proceedings of 6th YZC, Reno, Nev.
1983 Butterworths (1984), pages 545 to 555 or alternatively in
W. H/olderich, "Zeolites: Catalysis for the synthesis of organic
compounds", Elsevier, Studies Surf. Sci., Catal., Vol. 49
Amsterdam (1989), pages 69 to 93 and W. Holderich and H. van Bekkum,
Stud. Surf. Sci., Catal. 58 (1991), 631-726.
Rearrangement reaction which can advantageously be carried out
using the reactors produced according to the invention are, for
example, syntheses of phenylacetaldehydes or phenylethanols, as
stated in DE-A-37 40 270 and in DE-A-38 01 106.
In the production of reactors or adsorbers by the novel process,
selected moldings can be preheated to the softening point in a furnace
or a movable heating zone. A free-flowing powder of large zeolite
or zeolite-like molecular sieve crystals is continuously dusted
onto the molding surface preheated in this manner. The crystals
remain adhering to the softened molding surface in a thin layer
comprising about 1 to 5 crystal layers and are permanently fixed
thereon after the coated molding has been cooled. If the crystallite
size is too small, there is a danger that the majority of the crystals
will sink into the softened molding surface and will no longer be
available for separation of substances or catalytic reactions during
subsequent use. Crystals which do not adhere can be separated off
for further coatings and reused.
In order to produce particularly hard coatings, the molding surface
may furthermore be pretreated with an adhesion promoter prior to
dusting with crystals. If the moldings are intended to be used subsequently
at high temperatures, it has proven useful to use a mineral thermal
cement for the coating in the case of glass surfaces.
Crystals having a size of at least 3 .mu.m have proven suitable,
this relating to the smallest dimension in the case of irregularly
shaped crystals.
For pentasil crystals prepared in the absence of an alkali, the
amine-containing form of the crystals can be used for the novel
coating method if moldings whose softening point is above 400.degree.
C., in particular above 500.degree. C., are coated. The exothermic
combustion of the amine, ammonium or tetraalkylammonium forms still
present in the zeolite pores supports the fixing of the zeolite
crystals to the surface of the solid. This makes it possible to
prepare, directly from the `as made` form of crystals prepared in
the absence of an alkali, a molding which can be used for catalytic
reactions and has zeolite crystals in the acidic H form. In contrast
to a conventional method for working up zeolite powders and further
processing to give a supported catalyst, the novel procedure makes
it possible to dispense with energy-consumptive calcination and
molding steps while simultaneously reducing the amount of expensive
zeolite active material and dispensing with additional binder material.
The novel process is advantageous in that inert materials which
are easy to mold, for example glass in the form of spheres, porous
grit, tubes, coiled tubes, sheets, rods or bulbs, are coated in
a single operation so that all that has been done is that the chemically
inert carrier has been coated permanently on the surface with a
small amount of zeolite active material for subsequent use. Particularly
in the case of glass tubes, the inner wall can be coated according
to the invention so that, when used as a wall reactor, it is possible
to pass the reaction medium through the remaining free tube cross-section
during the catalytic reaction at high velocity and substantially
without a pressure drop along the reactor. If high selectivities
are required in a zeolite-catalyzed reaction, in the wall reactor
produced according to the invention unconverted starting material
can be recycled to the inlet stream and the conversion based on
end product can thus be increased.
Since, in this type of wall reactor, the active component is firmly
anchored to the surface of the moulding, there is very good heat
transfer between the catalyst and the reactor wall surface, so that
the occurrence of temperature peaks is avoided where it is intended
to effect isothermal operation of a highly exothermic reaction.
Similarly, when using a novel tube or a sheet in the case of substance
separation problems, the rapid heat transfer can be advantageously
used to permit virtually isobaric switching between adsorption and
desorption step by means of rapid temperature variation. With regard
to use as a wall reactor or as an adsorber, there is the additional
advantage that components conforming to standards and coated according
to the invention can be used in such a way that they can be built
up in a modular manner for the particular application. This is advantageous
in particular with regard to use as a wall reactor, since deactivated
modules can be changed rapidly and, in contrast to conventionally
operated reactors, without giving rise to highly contaminated mixtures
of abraded catalyst material and product.
Owing to the small amount of active material, brief flushing of
the modules with air at from 300.degree.to 550.degree. C. is generally
sufficient for regeneration.
The examples which follow illustrate the novel process and its
advantages in catalytic reactions and adsorptive separations of
substances.
EXAMPLES
Example 1
This example illustrates the synthesis of uniform large boron pentasil
crystals.
2.22 kg of tetrapropylammonium bromide, 7.96 kg of silicasol LUDOX.RTM.
AS-40 8.9 kg of demineralized water and 386 g of trimethyl borate
were initially taken in a steel autoclave having a stirring apparatus.
7.2 kg of an aqueous ammonia solution (25% by weight) were added
to this suspension. The resulting suspension was heated to 185.degree.
C. in the closed autoclave while stirring (100 rpm) and was crystallized
in the course of 168 hours. After cooling, the solid was filtered
off, washed neutral, dried at 120.degree. C. and finally subjected
to a heat treatment at 500.degree. C.
The yield of borosilicate zeolite was 97%, based on SiO.sub.2.
The chemical analysis of the product gave a composition of 96.5%
by weight of SiO.sub.2 1.48% by weight of B.sub.2 O.sub.3 and traces
of sodium (140 ppm) and A.sub.l.sub.2 O.sub.3 (90 ppm). The product
was completely crystalline and showed the powder diffraction pattern
typical of a boron pentasil having the MFI structure.
The crystals had a mean size of 12 .mu.m and a narrow size distribution.
Example 2
This example illustrates the synthesis of uniform large crystals
over a long crystallization time in a static experiment.
6.9 g of tetrapropylammonium bromide, 75.3 g of silicasol LUDOX.RTM.
AS-408 83.1 g of demineralized water and 3.66 g of trimethyl borate
were initially taken in a teflon-lined steel autoclave. 68.8 g of
an aqueous ammonia solution (25% by weight) were added to this suspension.
The resulting suspension was homogenized, heated to 185.degree.
C. in the closed autoclave and crystallized in the course of 168
hours. After cooling, the solid was filtered off, washed neutral,
dried at 120.degree. C. and finally subjected to a heat treatment
at 500.degree. C. The yield of borosilicate zeolite was 98%, based
on the SiO.sub.2 used.
The chemical analysis of the product gave a composition of 97.6%
by weight of SiO.sub.2 1.7% by weight of B.sub.2 O.sub.3 and traces
of sodium (120 ppm). The product was completely crystalline and
showed the powder diffraction pattern typical for a boronpentasil
having the MFI structure. The crystals had a mean size of 50 .mu.m
with a narrow particle size distribution.
Example 3
This example illustrates the synthesis of a borosilicate zeolite
using a mixture of tetrapropylammonium hydroxide and tetrapropylammonium
bromide, the synthesis time furthermore being shorter.
34.7 g of tetrapropylammonium bromide, 468 g of silica-sol LUDOX.RTM.
AS-40 524 g of demineralized water and 22.8 g of trimethyl borate
were initially taken in a steel autoclave having a stirring apparatus.
423 g of an aqueous ammonia solution (25% by weight) and 38.8 g
of an aqueous tetrapropylammonium hydroxide solution (20% by weight)
were added to this suspension.
The resulting suspension was heated to 185.degree. C. in the closed
autoclave while stirring (100 rpm) and was crystallized in the course
of 46 hours. After cooling, the solid was filtered off, washed neutral,
dried at 120.degree. C. and finally subjected to a heat treatment
at 500.degree. C.
The yield of borosilicate zeolite was 96%, based on SiO.sub.2 used.
The chemical analysis of the product gave a composition of 97.1%
by weight of SiO.sub.2 1.69% by weight of B.sub.2 O.sub.3 and traces
of sodium (130 ppm) and Al.sub.2 O.sub.3 (1200 ppm). The product
was completely crystalline and showed the powder diffraction pattern
typical for a boron pentasil having the MFI structure.
The crystals had a size distribution of from 5 to 22 .mu.m.
Example 4
This example illustrates the alkali-free synthesis of uniformly
large aluminum pentasil crystals.
0.043 kg of tetrapropylammonium bromide, 0.446 kg of silicasol
LUDOX.RTM. AS-40 and 0.658 kg of demineralized water were initially
taken in a steel autoclave. 26.5 g of aluminum triisopropylate and
0.426 kg of an aqueous ammonia solution (25% by weight) were added
to this suspension. The resulting suspension was heated to 185.degree.
C. in the closed autoclave while stirring (100 rpm) and crystallized
in the course of 120 hours. After cooling, the solid was filtered
off, washed neutral, dried at 120.degree. C. and finally subjected
to a heat treatment at 500.degree. C.
The yield of aluminosilicate zeolite was 95%, based on SiO.sub.2
used. The chemical analysis of the product gave an Si/Al molar ratio
of 23.
The product was completely crystalline and showed the powder diffraction
pattern typical for an aluminum pentasil having the MFI structure.
The crystals had a mean size of from 30 to 50 .mu.m.
Example 5
This example serves as a comparison and describes the use of aluminumpentasil
zeolite crystals prepared in Example 4 as heterogeneous catalysts
in a conventional fixed-bed procedure for the conversion of ethylbenzene
into diethylbenzenes.
The aluminumpentasil crystals synthesized, dried and calcined in
Example 4 were converted into 2 mm pellets with the aid of a press
(pressure 10N/m.sup.2), and a particle fraction of from 1 to 1.6
mm was separated off using a sieve. About 2.5 g of the material
thus obtained was accurately weighed, introduced into a coil-tube
reactor (length 600 mm, internal diameter 6 mm) and used in a through-circulation
oven.
The catalytic reaction was carried out at 250.degree. C. The starting
material used was ethylbenzene which was introduced via a saturator
at 20.degree. C. in a nitrogen stream having a flow rate of 10 l/hour.
The total reaction time was 8 hours. The resulting product mixture
was analyzed by gas chromatography.
The resulting conversion of ethylbenzene was 3.4%, the selectivity
of p-diethylbenzene being 67% and that of m-diethylbenzene being
33%, based in each case on the product.
Example 6
This example describes the possibility for fixing, according to
the invention, aluminumpentasil zeolites and thus controlling the
reaction stated in Example 5 in a wall reactor to give a high selectivity
with respect to the desired product p-diethylbenzene.
Glass tubes having a length of 300 mm and an internal diameter
of 6 mm were filled with the zeolite crystals prepared in Example
4 and were heated to 600.degree.to 650.degree. C. for from 10 to
30 minutes while being passed horizontally and with rotation through
an annular heating zone.
After cooling, the crystals which did not adhere to the inner wall
of the tube were removed, and the inside of the tube was cleaned
by treatment with compressed air. The coating with zeolite crystals
was determined gravimetrically as 167 g/m.sup.2.
The individual tubes were then coupled together by means of glass
adapters to form a module arranged in series. The module was placed
in a through-circulation oven and heated to the reaction temperature
of 250.degree. C.
The catalytic reaction took place at 250.degree. C. in a nitrogen
stream saturated at 20.degree. C. with ethylbenzene, the flow rate
being 10 l/h.
The resulting reaction products were characterized by gas chromatography.
At conversion of ethylbenzene of from 2 to 2.5%, a selectivity of
100% with respect to p-diethylbenzene was found. No m- or o-diethylbenzenes
were observed in the product stream even after a 3-fold reaction
time of 6.7 hours in each case.
Example 7
The rearrangement of styrene oxide to give p-phenylacetaldehyde
in a conventional fixed-bed reactor over a boron zeolite catalyst
obtained from the boronpentasil zeolite crystal powders prepared
in Example 1 is described as a Comparative Example.
The zeolite powder obtained from the boronpentasil crystals prepared
in Example 1 was compressed to give 2 mm pellets using a mechanical
press tool. The pellets were then separated to give a sieve fraction
of from 1 to 1.6 mm. 3.5 g of catalyst were installed in a coil-tube
reactor.
The reactor was brought to the reaction temperature of 240.degree.
C. At 12 mbar, from 9.6 to 37.8 g/h of styrene oxide (heated to
110.degree. C. in an upstream evaporator) were passed over the catalyst.
The reacted mixture was condensed and was analyzed by gas chromatography.
At conversion of 100% of styrene oxide used, a yield of phenylacetyldehyde
of more than 99.8% was found after reaction for six hours.
The temperature in the catalyst bed increased from 240.degree.
C. to 315.degree. C. The spent catalyst was blackish brown and caked
and was difficult to remove from the reactor.
Example 8
The reaction stated in Example 7 was carried out isothermally over
boronzeolite crystals fixed according to the invention in a tubular
wall reactor having a total length of 600 mm.
At 15 g/h of styrene oxide and 15 mbar, the reacted mixture was
condensed after a reaction time of three days and was analyzed by
gas chromatography.
The conversion was 100%, based on styrene oxide used, and the yield
of phenylacetaldehyde was more than 99.5%. A temperature increase
in the tubular wall reactor above the reactor temperature of 240.degree.
C. was not observed.
After the reaction, the cooled glass tube was a pale beige color.
The regeneration was carried out by passing air over at 450.degree.
C. in the course of 30 minutes.
Example 9
This example illustrates the novel application and permanent fixing
of large aluminumpentasil zeolite crystals on a glass layer and
the use of this element as a sheet adsorber for separating off n-hexane.
In a procedure similar to that in Example 6 a 2 mm thick glass
sheet was coated with the aluminumpentasil crystals prepared in
Example 5 Thermokitt.RTM. additionally being used as an adhesion
promote. Thermokitt.RTM. is a readily available thermal cement for
metal, glass, porcelain and similar materials. FIG. 1 shows a scanning
electron micrograph of a cross-section through the adsorber sheet.
The adsorptivity of the adsorber sheet produced according to the
invention for n-hexane was tested in comparison with the pure aluminumpentasil
crystals as powder.
The adsorption was carried out after activation at 200.degree.
C. and under greatly reduced pressure and after cooling at 20.degree.
C. in a Sartorius.RTM. microbalance. FIG. 2 shows the measured isotherms
both for the adsorber sheet and for the pure aluminumpentasil crystals.
It shows the typical curve of a type I form characteristic for microporous
adsorbents. Under the conditions of measurement, the pure aluminumpentasil
crystals have an adsorption capacity of 104 mg n-hexane/g of zeolite
and the adsorber sheet with the fixed crystals has an adsorption
capacity of 70 mg n-hexane/g of zeolite.
This shows that the large pentasil crystals fixed according to
the invention can be used for adsorption and retain their typical
micropore properties. |