Trimerization of formaldehyde in the gas phase

Details Trimerization of formaldehyde in the gas phase Trimerization of formaldehyde in the gas phase

2000-05-04

2001-11-06

Solola, T. A. (Department: 1626)

Organic compounds -- part of the class 532-570 series

Organic compounds

Heterocyclic carbon compounds containing a hetero ring...

Reexamination Certificate

active

06313323

ABSTRACT:

The present invention relates to a process for producing trioxane from formaldehyde having a low water content in the gas phase, wherein a solid phosphoric acid catalyst is used in the trimerization of formaldehyde to give trioxane.
Trioxane is the most widely used starting material for the preparation of polyoxymethylene (POM). POM belongs to the group of polyacetals and is an industrially very valuable polymer with excellent properties. It displays high strength, stiffness, toughness (even at low temperatures) and dimensional stability and also good heat resistance, low water absorption capability, good electrical properties, good sliding and wear behavior and good processability.
In the industrial processes currently used for preparing formaldehyde (silver catalyst process or Formox process), formaldehyde is obtained as an aqueous solution. Trioxane is therefore usually prepared in the liquid phase from aqueous formaldehyde solutions (formalin) using mineral acids (e.g. sulfuric acid), or acid ion-exchange resins (e.g. sulfonated polystyrene).
A process based on formaldehyde having a low water content, i.e. completely or substantially anhydrous formaldehyde, is of great economic interest since considerable costs incurred in the preparation of trioxane according to the prior art as a result of numerous energy-intensive distillation steps and/or extraction steps can be avoided. Due to the lack of industrial availability of formaldehyde having a low water content, such a process for preparing trioxane in the gas phase has not yet been implemented in industry.
The industrial availability of formaldehyde having a low water content represents the foundation for processes for preparing trioxane in the gas phase. A possible process which has been described is, for example, the nonoxidative dehydrogenation of methanol to formaldehyde using sodium-containing catalysts (DE 37 19055 A1, DD 264209 A1, JP 63079850, DE 3920811 A1, JP 02040237 A, JP 61130252 A, JP 59048429 A and DE 19644188 A1). The catalytically active species is presumed to be, inter alia, atomic sodium in the gas phase, which accelerates the dehydrogenation of methanol by a free radical mechanism (S. Ruf, G. Emig, Appl. Catal. A 1997, 161, 19-24).
As regards the conversion of formaldehyde having a low water content into trioxane, from AT 252913 and SV 20944 for example, disclose the preparation of trioxane from gaseous formaldehyde containing not more than 10% by weight of water using a Lewis acid (FeCl
3
, ZnCl
2
, SnCl
4
, BF
3
, H
3
PO
4
, H
2
SO
4
, ion exchangers, zeolites) applied to an inert support material (SiO
2
, Al
2
O
3
or wooden charcoal). The best catalyst was found to be silica gel impregnated with 10% by weight of sulfuric acid. The best values for the conversion of gaseous formaldehyde (62.5% in inert gas) were obtained at a temperature of 90° C. However, these conditions are, on the basis of today's state of the art, far away from industrial conditions: the reaction temperature is in the region of the polymerization limits of formaldehyde and the formaldehyde concentration is far from formaldehyde concentrations which can be achieved industrially by the nonoxidative dehydrogenation and/or further process steps. The nonoxidative dehydrogenation also achieves a water content in the formaldehyde of far below 10% by weight.
Significant industrial disadvantages of the catalyst system described result from its nature: catalysts which are prepared by impregnation of supports with liquid acids have a very low catalytically active surface area, since the pores of the support are filled with liquid. In addition, the acid is not chemically anchored to the support and is carried out from a fixed bed. This leads to progressive deactivation of the catalyst and to contamination of the product. There are also corrosion problems which make it necessary to use expensive, corrosion-resistant materials for plant construction.
EP 604884 A1 describes the trimerization of anhydrous formaldehyde over a vanadyl phosphate hemihydrate unsupported catalyst with a selectivity of almost 100% and a low activity (22.1% of the maximum achievable equilibrium conversion, space-time yield: 35.3 g/lh). At good selectivity, the activity of the catalyst is thus only moderate. As an unsupported catalyst, the catalyst is very expensive due to the high vanadium content and a complicated method of production (two wet chemical steps to get the active composition, drying, shaping, activation). In addition, the very low mechanical stability of the granulated vanadyl phosphate is a great disadvantage for industrial use.
EP 691338 A describes the use of the heteropolyacid H
4
PVMo
11
O
40
* n H
20
(n=0-32) on an inert support material or pressed together with an inert filler for the trimerization of formaldehyde having a low water content. This catalyst displays a higher activity compared to the vanadyl phosphate hemihydrate (conversion close to the equilibrium conversion). Disadvantages such as rapid deactivation and high production costs stand in the way of the use of this catalyst.
EP 0691338 A1 describes a process for preparing trioxane in which a class of heteropolyacids having the composition H
3
PMo
m
W
n
O
40
*xH
20
; n, m=4-8; n+m=12; x=0-32 in the form of a supported catalyst is used. Thus, a catalyst based on silicon carbide and the heteropolyacid H
3
PMo
6
W
6
O
40
*xH
2
O is said to convert anhydrous formaldehyde into trioxane at a selectivity of 99%, while 93% of the equilibrium conversion is achieved (space-time yield: 85.9 g/lh). Here too, rapid deactivation and the high price stand in the way of the use of this catalyst system.
Further disadvantages are that expensive support materials such as silicon carbide are required for the use of heteropolyacids as catalyst and the preparation of suitable heteropolyacids is costly, since they are not commercially available compounds and the synthesis of the heteropolyacids includes steps which cannot readily be scaled up (e.g. ether extraction).
Unpublished long-term tests carried out by the Applicant using supported heteropolyacids also indicated that use of these catalysts is not feasible under industrial conditions (high formaldehyde partial pressure): deactivation was observed and this led in the best case to an activity loss of 80% in a few days, but in some cases even to complete loss of the catalytic activity. The reason for this is very probably overreduction of the redox-active heteropolyacids. Regeneration experiments were only partly successful, since the mechanical stability of the supported catalysts also decreased significantly in the long-term test. As a result of this, abraded material was formed and led to blockage of the test reactors.
Phosphoric acid catalysts, in particular solid phosphorous acid catalysts (SPA catalysts) are used in petrochemical processes such as the preparation of cumene from benzene and propene or in the oligomerization of alkenes to form polygasoline. Their use for the trimerization of formaldehyde has not been described hitherto. For commercial applications, use is made predominantly of phosphoric acid catalysts based on silicon phosphate.
It is an object of the present invention to provide a catalyst system which has long-term stability, particularly under industrial conditions too, and can be used for the gas-phase trimerization of formaldehyde having a low water content. This catalyst system should also have a significantly improved operating life compared to the prior art, since a sufficient long-term stability is a decisive prerequisite for industrial implementation of the process. As regards the operating life problem, there has hitherto been no solution.
It has now surprisingly been found that SPA catalysts can advantageously be used for the gas-phase trimerization of formaldehyde to give trioxane, with the operating life problem being solved and the realization of an industrial process thus being made possible.
The invention accordingly provides a process for preparing trioxane, which comprises trime

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