The present invention relates to a process for producing ethene by the vapour phase
dehydration of ethanol using a heteropolyacid catalyst. In particular, the process of the
present invention involves an initiation procedure comprising drying of the heteropolyacid
catalyst at a specific range of temperature prior to use in an ethanol dehydration reaction.
Ethene is an important commodity chemical and monomer which has traditionally
been produced industrially by the steam or catalytic cracking of hydrocarbons derived from
crude oil. However there remains an increasing need to find alternative economically
viable methods of making this product. By virtue of its ready availability from the
fermentation of biomass and synthesis gas based technologies, ethanol is emerging as an
important potential feedstock from which ethene can be made in the future.
The production of ethene by the vapour phase chemical dehydration of ethanol is a
well-known chemical reaction which has been operated industrially for many years (see for
example Kirk Othmer Encyclopaedia of Chemical Technology (third edition), Volume 9,
pages 4 11 to 413). Traditionally this reaction has been carried out in the presence of an
acid catalyst such as activated alumina or supported phosphoric acid.
In recent years attention has turned to finding alternative catalysts having improved
performance. This has led to the use of supported heteropolyacid catalysts, such as those
disclosed in EP1 925363, which have the benefit of improved selectivity, productivity and
reduced ethane formation following the dehydration of a feedstock comprising ethanol and
ethoxyethane for the production of ethene. This is desirable because firstly ethane is an
undesirable by-product and secondly its separation from ethene on a large scale is both
difficult and energy intensive. Related documents WO 2007/063281 and WO
2007/003899 also disclose modes of carrying out dehydration of oxygenate feedstocks
with supported heteropolyacid catalysts. Supported heteropolyacid catalysts may be readily
prepared using wet impregnation techniques by dissolving a heteropolyacid in a suitable
solvent to form a heteropolyacid solution and then impregnating a suitable catalyst support
with the heteropolyacid solution.
In the dehydration process, a feed typically comprising ethanol, and optionally water
and other components, is continuously fed to a reactor containing a bed of heteropolyacid
catalyst and the products continuously removed. Under steady state conditions, the feed
entering the reactor is rapidly converted near the inlet into an equilibrium mixture of water,
ethanol and ethoxyethane (the product of a rapid first stage dehydration of the ethanol).
Such processes are typically conducted at elevated temperature and pressure.
Due to i) the nature of heteropolyacids; ii) the process for preparing supported
heteropolyacid catalysts; and iii) the loading of said catalysts into a reaction zone, the
heteropolyacid component will almost certainly be exposed to water (such as moisture in
the atmosphere) under conditions at which it may become bound to the heteropolyacid
component. Thus the hydration state of the heteropolyacid component of the supported
heteropolyacid catalyst prior to heating the supported heteropolyacid catalyst will be above
zero (i.e. the heteropolyacid component of the supported heteropolyacid catalyst has water
molecules chemically bound thereto). Typically, the hydration state of a heteropolyacid
decreases on exposure to increasing temperature; that is, the number of water molecules
bound to the heteropolyacid decreases with increasing temperature. The degree of
hydration of the heteropolytungstic acid may affect the acidity of the supported catalyst
and hence its activity and selectivity.
WO 201 1/104495 discloses a dehydration process for the preparation of alkene using
a supported heteropolyacid catalyst. That document teaches an initial catalyst drying step
conducted at a temperature of at least 220 °C, so as to remove bound water such that at
least part of the heteropolyacid component of the catalyst has a hydration state of zero,
followed by a reduction in temperature under anhydrous atmosphere, before the catalyst is
contacted with the reactant feedstream. WO 201 1/104495 teaches that the drying step
advantageously leads to improved ethane selectivity of the catalyst in the subsequent
ethanol dehydration reaction.
It has thus now become desirable to dry the supported heteropolyacid catalyst at high
temperatures, typically around 240 °C, as part of the start-up procedure preceding ethanol
dehydration. However, one problem that has hitherto not been acknowledged relates to
catalyst deactivation. It has been found that when the supported heteropolyacid catalyst is
dried at such high temperatures, deactivation of the heteropolyacid catalyst is exacerbated.
Without being bound by any particular theory, this is believed to occur as a result of
undesirable side reactions with high activation energies, which contribute to deactivation,
becoming more prevalent as a result of the higher temperatures being used.
In particular, the number of heteropolyacid decomposition sites formed on the
surface of the catalyst support is believed to increase during the high-temperature drying
step, or such decomposition sites are 'seeded' during the drying step and subsequently
develop into decomposition sites during the dehydration reaction. Mobility of
heteropolyacid at the surface of the support toward such 'seed' sites is also believed to
compound the problem.
Replacement of the catalyst in a dehydration system is labour intensive, has
significant materials costs and involves temporarily shutting down what is likely to be a
continuous process, which has detrimental impact on product output. Thus, a problem of
catalyst deactivation poses a serious issue to the economic viability of ethanol dehydration
processes.
It has now surprisingly been found that catalyst lifetime in an ethanol dehydration
reaction can be extended by performing a drying step, before committing the supported
heteropolyacid catalyst to the ethanol dehydration reaction, at lower temperatures than
taught in the prior art. By performing the drying step over a specific range of intermediate
temperatures immediately followed by contacting with an ethanol-containing vapour
stream over another specific range of temperatures as part of the start-up procedure for
ethanol dehydration, productivity loss over time as a result of catalyst deactivation is
significantly reduced. Furthermore, at least in some embodiments, the maximum ethene
productivity (mole/kg catalyst/hr) may also be increased.
According to the present invention, there is provided a process for the preparation of
ethene by vapour phase chemical dehydration of a feed-stream comprising ethanol and
optionally water and/or ethoxyethane, said process comprising contacting a dried
supported heteropolyacid catalyst in a reactor with the feed-stream having a feed
temperature of at least 200 °C; and wherein before the supported heteropolyacid catalyst is
contacted with the feed-stream having a feed temperature of at least 200 °C, the process is
initiated by:
(i) drying a supported heteropolyacid catalyst in a reactor under a stream of
inert gas having a feed temperature of from above 100 °C to 200 °C; and
(ii) contacting the dried and supported heteropolyacid catalyst with an ethanolcontaining
vapour stream having a feed temperature of from above 100 °C
to 160 °C.
Preferably, the initiation of the ethanol dehydration process further comprises: (iii)
ramping the feed temperature of the ethanol-containing vapour stream to at least 200 °C,
preferably over the course of 10 minutes to 8 hours, more preferably over the course of 20
minutes to 4 hours.
It has been surprisingly found that an ethanol dehydration process is particularly
advantageous when initiated by drying the supported heteropolyacid catalyst and
contacting with an ethanol-containing vapour stream at the above temperature ranges. In
particular, this initiation procedure reduces the level of heteropolyacid catalyst deactivation
observed in the ethanol dehydration reaction and, at least in some embodiments, increases
ethene maximum productivity.
Moreover, it has been found that productivity in a process for producing ethene by
the vapour phase dehydration of ethanol using a heteropolyacid catalyst is improved by
operating at high temperature; in particular at temperatures higher than those exemplified
in the prior art, for example higher than 240 °C. However, it has been found that higher
operating temperatures also exacerbate catalyst deactivation, much like the high
temperature drying steps used in start-up procedures in the prior art. Nevertheless, it has
also been surprisingly found that by drying the supported heteropolyacid catalyst and
contacting with an ethanol-containing vapour stream at the temperatures recited
hereinbefore, catalyst deactivation associated with subsequently operating the ethanol
dehydration reaction at higher than conventional temperatures is reduced. Thus, the drying
and contacting steps (i) and (ii) which precede the ethanol dehydration reaction in
accordance with the process of the present invention, allows the full benefit of higher
ethene productivity to be realised whilst avoiding significant catalyst deactivation.
Mechanisms by which the supported heteropolyacids are believed to undergo
deactivation during operation include: i) neutralisation by inorganic cations, such as
ammonia / ammonium cations, and organic nitrogen-containing compounds; ii) carbon
deposition; and iii) decomposition of the heteropolyacid to its constituent oxides.
Deactivation as a result of neutralisation by inorganic cations and organic nitrogencontaining
compounds may be mitigated by committing the ethanol based raw materials to
a clean-up procedure to remove the neutralising species. In contrast, the features of the
initiation procedure according to the present invention are believed to largely eliminate
deactivation of the heteropolyacid catalyst as a result of decomposition of the
heteropolyacid to its constituent oxides.
During the preparation of a supported heteropolyacid catalyst, the catalyst may be
optionally dried. However, as the skilled person will appreciate, the supported catalyst will
inevitably be exposed to moisture upon transport and introduction into a reactor. Drying
removes condensed water vapour from the surface of the support which can negatively
impact the surface chemistry of the support, for instance, the acidity of the heteropolyacid
component. However, drying inside the reactor in accordance with the present invention
has also been surprisingly found to reduce the formation of heteropolyacid decomposition
sites on the catalyst.
Without being bound by any particular theory, it is believed that the low temperature
drying according to step (i) of the process reduces the possibility of forming 'seed' sites
which may subsequently lead to decomposition. 'Seed' sites may be produced following,
for example, changes in the oxidation state of the heteropolyacid on exposure to heat; the
formation of carbon residues; and/or formation of defect structures. Where exposure to
heat has given rise to chemical changes in the heteropolyacid at a specific surface location
to form such 'seed' sites, the likelihood of full decomposition of heteropolyacid to its
constituent oxides at these sites is substantially increased. Moreover, mobility of
heteropolyacid at the surface of the support at elevated temperatures towards such 'seed'
sites exacerbates the rate of decomposition. As a consequence, the catalyst lifetime is
significantly extended, which has clear economic benefits relating to re-use and
replacement of the catalyst, as well as the reduction of waste. The operating conditions of
the present invention thus correspond to a narrow window within which significant catalyst
deactivation is avoided, whilst ethylene productivity is promoted.
In accordance with the present invention, the supported heteropolyacid catalyst is
dried in step (i) of the initiation under a stream of inert gas having a feed temperature of
from above 100 °C to 200 °C. Preferably, the supported heteropolyacid catalyst is dried in
step (i) under a stream of inert gas having a feed temperature of from 100 °C to 180 °C;
more preferably from 110 °C to 170 °C; most preferably from 120 °C to 160 °C; for
example 150 °C.
Reference herein to "initiated" or "initiation" in regard to steps (i) and (ii) of the
process of the present invention is intended to mean that these steps precede exposure of
the catalyst to the feed-stream at a feed temperature of at least 200 °C. Furthermore,
"initiated" or "initiation" is also intended to mean that no other steps materially affecting
the composition or nature of the supported heteropolyacid catalyst are undertaken before
step (i), after the supported heteropolyacid catalyst is positioned within the reactor.
Reference herein to "inert gas" is intended to mean a gas that is not consumed in the
reaction of the process of the present invention, and is not consumed by any other process
which may be catalysed by the supported heteropolyacid catalyst. Examples of suitable
inert gases are nitrogen, argon, helium, methane and carbon dioxide. Preferably, the inert
gas is selected from nitrogen, argon and helium, more preferably, the inert gas is nitrogen.
By the term "stream of inert gas" as used herein, it is meant that the atmosphere under
which the drying step takes place is an inert gas that is constantly being removed and
replenished with fresh (or recycled) inert gas (i.e. a gas flow). For example, the "stream of
inert gas" is preferably a stream of nitrogen gas.
Reference herein to "drying" is intended to mean exposing the supported
heteropolyacid catalyst to heat such that the dew point of water vapour in the reactor, and
any other vapour that may be present, is exceeded under the pressure at which the reactor
is operated. The low temperature drying of the supported heteropolyacid as part of an
initiation procedure for an ethanol dehydration reaction in accordance with the process of
the present invention has been found to have numerous benefits with regard to the
dominant surface chemistry of the supported heteropolyacid catalyst.
Drying of the supported heteropolyacid catalyst in accordance with the process of the
invention is undertaken for a period of at least one hour. Preferably, drying is undertaken
for a period of from 1 to 48 hours, more preferably from 2 to 16 hours, most preferably 2
to 12 hours. In some embodiments, the drying time includes a period of time in which the
feed temperature of the inert gas for drying is ramped up to match a higher feed
temperature used for the subsequent step of contacting with an ethanol-containing vapour
stream. Whilst not wishing to be bound by any theory, it is thought that minimizing the
period of constant temperature, following a ramped increase of the feed temperature of the
inert gas temperature and prior to contact with the ethanol-containing vapour stream, is
commercially advantageous.
In accordance with the present invention, the dried supported heteropolyacid catalyst
is contacted in step (ii) of the initiation with an ethanol-containing vapour stream having a
feed temperature of from above 100 °C to 160 °C. Preferably, the dried supported
heteropolyacid catalyst is contacted in step (ii) with an ethanol-containing vapour stream
having a feed temperature of from 120 °C to 158 °C, more preferably from 130 °C to 156
°C, even more preferably from 140 °C to 154 °C, most preferably from 148 °C to 152 °C,
for example 150 °C.
Reference herein to an "ethanol-containing vapour stream" is intended to mean a
gaseous stream comprising at least 50 wt.% ethanol and the balance being made up of
diluents. Preferably, the ethanol-containing vapour stream comprises 80 wt.% or more
ethanol, more preferably 90 wt.% or more; most preferably 95 wt.% or more; with the
balance preferably being made up of inert gas diluents. Suitable inert gas diluents are
nitrogen, argon, helium, methane and carbon dioxide. Preferably, the inert gas diluents are
selected from nitrogen, argon and helium, more preferably, the inert gas diluent is nitrogen.
The amount of water in the ethanol-containing vapour stream is at most 10 wt.%,
preferably at most 7 wt.%, more preferably at most 5 wt.%, even more preferably at most 3
wt .%, and still more preferably at most 2 wt.%, based on the total weight of ethanolcontaining
vapour stream. The amount of ethoxyethane in the ethanol-containing vapour
stream is at most 5 wt.%, preferably at most 3 wt.%, and more preferably at most 2 wt.%,
based on the total weight of ethanol-containing vapour stream. Most preferably, the
ethanol-containing vapour stream is anhydrous or the ethanol-containing vapour stream
comprises or consists essentially of ethanol and any balance is made up of inert gas
diluents. As will be appreciated, in some embodiments, the ethanol-containing vapour
stream may be identical in composition to the feed-stream containing ethanol which
undergoes ethanol dehydration. However, in preferred embodiments, the ethanolcontaining
vapour stream is different from the feed-stream containing ethanol.
Step (ii) of contacting the dried supported heteropolyacid catalyst with an ethanolcontaining
vapour stream has been found to be of particular benefit in obtaining steady
state conditions for the ethanol dehydration reaction and enhancing catalyst performance.
Furthermore, contacting the catalyst with an ethanol-containing vapour stream at a
temperature of from above 100 °C to 160 °C ensures that detrimental exotherms are
avoided, which can lead to undesirable competing oligomerisation reactions during the
subsequent ethanol dehydration of the feed-stream. In a particularly preferred embodiment,
the inert gas stream which is used for drying the supported heteropolyacid in step (i) is
converted to an ethanol-containing vapour stream for contacting step (ii) by addition of
ethanol vapour to the inert gas stream.
The dehydration of the feed-stream according to the present invention is believed
(Chem. Eng Comm. 1990, 95, 27 to 39) to proceed by either the direct dehydration to
olefm(s) and water (Equation 1); or via an ether intermediate (Equations 2 and 3).
(1) EtOH = + H20
(2) 2EtOH - - Et20 + H20
(3) Et20 = + EtOH
The direct conversion of the ether to two moles of olefin and water has also been
reported (Chem. Eng. Res. and Design 1984, 62, 8 1 to 91). All of the reactions shown
above are typically catalysed by Lewis and/or Bronsted acids. Equation 1 shows the
endothermic direct elimination of ethanol to ethene and water; competing with Equation 1
are Equations 2 and 3 i.e. the exothermic etherification reaction (Equation 2), and the
endothermic elimination of ethoxyethane to produce ethene and ethanol (Equation 3).
However, the dehydration reaction of ethanol to ethene is overall said to be endothermic.
The present invention provides a process for the preparation of ethene by vapour
phase chemical dehydration of a feed-stream comprising ethanol and optionally water
and/or ethoxyethane, said process comprising contacting a dried supported heteropolyacid
catalyst in a reactor with the feed-stream having a feed temperature of at least 200 °C;
wherein before the dried supported heteropolyacid catalyst is contacted with the feedstream
having a feed temperature of at least 200 °C, the process is initiated by: (i) drying a
supported heteropolyacid catalyst in a reactor under a stream of inert gas having a feed
temperature of from above 100 °C to 200 °C; and (ii) contacting the dried supported
heteropolyacid catalyst with an ethanol-containing vapour stream having a feed
temperature of from above 100 °C to 160 °C.
Preferably, the feed-stream comprises water and/or ethoxyethane, more preferably
the feed-stream comprises water and ethoxyethane. When both ethoxyethane and water are
present in the feed-stream, it is preferred that the molar ratio of ethoxyethane to water is
from 3:1 to 1:3, preferably from 3:1 to 1:1, more preferably 2:1 to 1:1.
Suitably, the amount of water in the feed-stream of the process of the present
invention is at most 50 wt.%, more preferably at most 20 wt.%, most preferably at most 10
wt .%, or even at most 7 wt.%>, based on the total weight of water, ethanol and ethoxyethane
in the feed-stream. Preferably, the amount of water in the feed-stream is at least 0.1 wt.%,
more preferably at least 0.5 wt.% and most preferably at least 1 wt.%, based on the total
weight of water, ethanol and ethoxyethane in the feed-stream.
Suitably, the amount of ethoxyethane in the feed-stream of the process of the present
invention is at most 50 wt.%, more preferably at most 40 wt.%, most preferably at most 35
wt .% based on the total weight of water, ethanol and ethoxyethane in the feed-stream.
Preferably, the amount of ethoxyethane in the feed-stream is at least 0.1 wt.%, more
preferably at least 0.5 wt.% and most preferably at least 1 wt.%, based on the total weight
of water, ethanol and ethoxyethane in the feed-stream.
The liquid product stream following olefin removal comprises mostly unreacted
ethanol, ethoxyethane and water. The applicants have found that it is particularly
preferable to recycle the major portion of the alcohols and ethers back to the vapour phase
dehydration reactor after water removal.
In some embodiments of the invention, the feed-stream comprises an inert gas
diluent. In other embodiments, an inert gas diluent is added down the catalyst bed, or
between multiple catalyst beds arranged in series or in parallel, if used. Preferred diluents
for the feed-stream include nitrogen, helium, ethene and/or saturated hydrocarbons, for
example hexanes, 2-methylpropane or n-butane. More preferably, the feed-stream diluent
is selected from nitrogen and/or helium.
As described above, it has now been found that higher temperatures used for the
dehydration reaction give greater ethene productivity. Since the present invention
diminishes the negative effects of high operating temperatures on catalyst deactivation, it is
preferred that the dried supported heteropolyacid is contacted with the feed-stream when it
has a feed temperature of at least 220 °C, more preferably at least 240 °C. In particular
preferred embodiments, the feed temperature is at least 252 °C, at least 255 °C, at least 260
°C, at least 280 °C or even at least 300 °C. The upper limit of the feed temperature of the
feed-stream is below the temperature at which selectivity for ethene is negatively impacted
and/or one which is overly energy intensive. Preferably, the upper limit of the feed
temperature of the feed-stream is 350 °C, more preferably 325 °C. Reference to "feed
temperature" herein is intended to refer to the temperature of a particular stream at the
point at which it is fed to the reactor.
The pressure inside the reactor during the dehydration reaction when the supported
heteropolyacid catalyst is contacted with the feed-stream is preferably in the range of from
0.1 MPa to 4.5 MPa, more preferably at a pressure in the range of from 0.5 MPa to 3.5
MPa, and most preferably at a pressure in the range of from 1.0 MPa to 2.8 MPa.
Reference herein to the pressure inside the reactor corresponds to the sum of the
partial pressures of the reactants, namely those of ethanol, water and ethoxyethane, as well
as the partial pressure of the ethene product. Unless otherwise indicated herein, partial
pressures of inert gas diluents, such as helium and nitrogen, or other inert components are
excluded from the total stated pressure. Thus, reference to reactor pressure herein is in
accordance with the formula: Pre actor = Pwater + Pethanoi + Pethoxyethane + Pethene- Furthermore,
unless otherwise indicated, reference to reactor pressures herein is to absolute pressures,
and not gauge pressures.
As will be appreciated by the skilled person, there is often a pressure drop that occurs
in a dehydration reactor between the point where the feed stream enters the reactor and that
where the effluent stream emerges from the reactor. As a consequence, there is, to a
varying extent, an internal pressure gradient which exists inside the reactor itself. It is
therefore to be understood that reference herein to the "pressure inside the reactor" means
any pressure falling within the pressure range defined by the above-mentioned internal
pressure gradient. The pressure inside the reactor itself therefore lies between the feedstream
pressure and the effluent stream pressure.
The supported heteropolyacid may suitably be provided in the reactor in the form of
one or more catalyst beds in the reactor, preferably multiple catalyst beds which may be
arranged in series or in parallel. In preferred embodiments, the catalyst bed(s) is/are
selected from adiabatic packed beds, tubular fixed beds or fluid beds. Most preferably the
catalyst bed(s) in the reactor is/are selected from adiabatic packed beds.
Desirably, the reactor is configured such that the temperature differential across the
one or more catalyst beds during drying of the supported hetereopolyacid catalyst in step
(i) is minimal, since this assists with uniform drying of the supported heteropolyacid
catalyst. Preferably, the temperature differential across the catalyst bed(s) during drying of
the supported hetereopolyacid catalyst in step (i) is no more than 20 °C, preferably no more
than 15 °C, more preferably no more than 10 °C, most preferably no more than 5 °C. The
temperature differential can be readily determined by means of multiple temperature
sensors positioned at different locations across the catalyst bed.
The term "heteropolyacid", as used herein and throughout the description of the
present invention, is deemed to include inter alia; alkali, alkali earth, ammonium, free
acids, bulky cation salts, and/or metal salts (where the salts may be either full or partial
salts) of heteropolyacids. Hence, the heteropolyacids used in the present invention are
complex, high molecular weight anions comprising oxygen-linked polyvalent metal atoms.
Typically, each anion comprises 12-18, oxygen-linked polyvalent metal atoms. The
polyvalent metal atoms, known as peripheral atoms, surround one or more central atoms in
a symmetrical manner. The peripheral atoms may be one or more of molybdenum,
tungsten, vanadium, niobium, tantalum, or any other polyvalent metal. The central atoms
are preferably silicon or phosphorus, but may alternatively comprise any one of a large
variety of atoms from Groups I- VIII in the Periodic Table of elements. These include
copper, beryllium, zinc, cobalt, nickel, boron, aluminium, gallium, iron, cerium, arsenic,
antimony, bismuth, chromium, rhodium, silicon, germanium, tin, titanium, zirconium,
vanadium, sulphur, tellurium, manganese nickel, platinum, thorium, hafnium, cerium,
arsenic, vanadium, antimony ions, tellurium and iodine. Suitable heteropolyacids include
Keggin, Wells-Dawson and Anderson-Evans-Perloff heteropolyacids. Specific examples
of suitable heteropolyacids are as follows:
18-tungstophosphoric acid H6[P2W18062].xH20
12-tungstophosphoric acid H3[PW12O40].xH2O
12-tungstosilicic acid H4[SiW12O40].xH2O
Cesium hydrogen tungstosilicate Cs3H[SiW12O40].xH2O
and the free acid or partial salts of the following heteropolyacids acids:
Monopotassium tungstophosphate KH5[P2W18062].xH20
Monosodium 12-tungstosilicic acid NaK3[SiW12O40].xH2O
Potassium tungstophosphate K6[P2W18062].xH20
Ammonium molybdodiphosphate (NH4)6 [P2Mol8O62].xH20
Potassium molybdodivanado phosphate K5[PMoV2O40].xH2O
In addition, mixtures of different heteropolyacids and salts can be employed. The
preferred heteropolyacids for use in the process described by the present invention is any
one or more heteropolyacid that is based on the Keggin or Wells-Dawson structures; more
preferably the chosen heteropolyacid for use in the process described by the present
invention is any one or more of the following: heteropolytungstic acid (such as
silicotungstic acid and phosphotungstic acid), silicomolybdic acid and phosphomolybdic
acid. Most preferably, the chosen heteropolyacid for use in the process described by the
present invention is any one or more silicotungstic acid, for example 12-tungstosilicic acid
(H [SiW12O40].xH2O).
Preferably, the heteropolyacids employed according to the present invention may
have molecular weights of more than 700 and less than 8500, preferably more than 2800
and less than 6000. Such heteropolyacids also include dimeric complexes.
The supported catalyst may be conveniently prepared by dissolving the chosen
heteropolyacid in a suitable solvent, where suitable solvents include polar solvents such as
water, ethers, alcohols, carboxylic acids, ketones and aldehydes; distilled water and/or
ethanol being the most preferable solvents. The resulting acidic solution has a
heteropolyacid concentration that is preferably comprised between 10 to 80 wt %, more
preferably 20 to 70 wt% and most preferably 30 to 60 wt%. This said solution is then
added to the chosen support (or alternatively the support is immersed in the solution). The
actual volume of acidic solution added to the support is not restricted, and hence may be
enough to achieve incipient wetness or wet impregnation, where wet impregnation (i.e.
preparation using an excess acidic solution volume relative to pore volume of support), is
the preferred method for the purposes of the present invention.
The resulting supported heteropolyacid may be modified, and various salts of
heteropolyacid may then be formed in the aqueous solution either prior to, or during,
impregnation of the acidic solution onto the support, by subjecting the supported
heteropolyacid to a prolonged contact with a solution of a suitable metallic salt or by
addition of phosphoric acid and/or other mineral acids.
When using a soluble metallic salt to modify the support, the salt is taken in the
desired concentration, with the heteropolyacid solution. The support is then left to soak in
the said acidic solution for a suitable duration (e.g. a few hours), optionally with periodic
agitation or circulation, after which time it is filtered, using suitable means, in order to
remove any excess acid.
When the salt is insoluble it is preferred to impregnate the catalyst with the HPA and
then titrate with the salt precursor. This method can improve the dispersion of the HPA
salt. Other techniques such as vacuum impregnation may also be employed.
The amount of heteropolyacid impregnated on the resulting support is suitably in the
range of 10 wt % to 80 wt % and preferably 20 wt % to 50 wt % based on the total weight
of the heteropolyacid and the support. The weight of the catalyst on drying and the weight
of the support used, may be used to obtain the weight of the acid on the support by
deducting the latter from the former, giving the catalyst loading as 'g heteropolyacid/kg
catalyst'. The catalyst loading in 'g heteropolyacid /litre support' can also be calculated by
using the known or measured bulk density of the support. The preferred catalytic loading
of heteropolyacid is 150 to 600g heteropolyacid / kg Catalyst.
According to a preferred embodiment of the present invention the average
heteropolyacid loading per surface area of the dried supported heteropolyacid catalyst is
more than 0.1 micro moles/m2.
It should be noted that the polyvalent oxidation states and hydration states of the
heteropolyacids stated previously and as represented in the typical formulae of some
specific compounds only apply to the fresh acid before it is impregnated onto the support,
and especially before it is subjected to the dehydration process conditions. The degree of
hydration of the heteropolyacid may affect the acidity of the supported catalyst and hence
its activity and selectivity. Thus, either or both of these actions of impregnation and
dehydration process may change the hydration and oxidation state of the metals in the
heteropolyacids, i.e. the actual catalytic species used, under the process conditions given,
may not yield the hydration/oxidation states of the metals in the heteropolyacids used to
impregnate the support. Naturally therefore it is to be expected that such hydration and
oxidation states may also be different in the spent catalysts after reaction
According to a preferred embodiment of the present invention, the amount of
chloride present in/on the said heteropolyacid supported catalyst is less than 40 ppm,
preferably less than 25 ppm and most preferably less than 20 ppm.
The supported heteropolyacid catalyst used in the process of the present invention
may be a fresh catalyst or a previously used catalyst. Thus, in one embodiment, at least a
portion of the supported heteropolyacid catalyst has previously been employed in a process
for the preparation of an ethene from a feed comprising ethanol, water and ethoxyethane.
For example, at least a portion of the supported heteropolyacid may derive from an extract
of heteropolyacid from a previously used catalyst i.e. from a partially deactivated material.
According to a further preferred embodiment of the present invention, the
heteropolyacid supported catalyst is a heteropolytungstic acid supported catalyst having the
following characteristic:
PV > 0.6 - 0.3 x [HPA loading/Surface Area of Catalyst]
wherein PV is the pore volume of the dried supported heteropolytungstic acid catalyst
(measured in ml/g catalyst); HPA loading is the amount of heteropolyacid present in the
dried supported heteropolyacid catalyst (measured in micro moles per gram of catalyst)
and Surface Area of Catalyst is the surface area of the dried supported heteropolytungstic
acid catalyst (measured in m2 per gram of catalyst).
Suitable catalyst supports may be in a powder form or alternatively may be in a
granular form, or in a pelletised form, a spherical form or as extrudates (including shaped
particles) and include, but are not limited to, clays, bentonite, diatomous earth, titania,
activated carbon, aluminosilicates e.g. montmorillonite, alumina, silica-alumina, silicatitania
cogels, silica-zirconia cogels, carbon coated alumina, zeolites, zinc oxide, flame
pyrolysed oxides. Supports can be mixed oxides, neutral or weakly basic oxides. Silica
supports are preferred, such as silica gel supports and supports produced by the flame
hydrolysis of SiCl . Preferred supports are substantially free of extraneous metals or
elements which might adversely affect the catalytic activity of the system. Thus, suitable
silica supports are typically at least 99% w/w pure. Impurities amount to less than 1%
w/w, preferably less than 0.60% w/w and most preferably less than 0.30% w/w. The pore
volume of the support is preferably more than 0.50ml/g and preferably more than 0.8 ml/g.
Suitable silica supports include, but are not limited to any of the following: Grace
Davison Davicat® Grade 57, Grace Davison Davicat® 1252, Grace Davison Davicat® SI
1254, Fuji Silysia CariAct® Q15, Fuji Silysia CariAct® QIO, Degussa Aerolyst® 3045
and Degussa Aerolyst® 3043.
The average diameter of the supported heteropolyacid particles is preferably from
500 m to 8,000 mh ; more preferably from 1,000 mhi to 7,000 mh ; even more preferably
from 2,000 mhi to 6,000 mh , most preferably from 3,000 m to 5,000 mh . It has been
surprisingly found that the effects of the present invention are enhanced with supported
heteropolyacid particles of larger size (i.e. falling within the above ranges). However, in
some embodiments, these particles may be crushed and sieved to smaller sizes of, for
example, 50-2,000 mhi, if desired.
The average pore radius (prior to impregnation with the heteropolyacid) of the
support is 10 to 500A, preferably 30 to 350A, more preferably 50 to 300 A and most
preferably 60 to 250A. The BET surface area is preferably between 50 and 600 m2/g and
is most preferably between 130 and 400 m /g.
The BET surface area, pore volume, pore size distribution and average pore radius
were determined from the nitrogen adsorption isotherm determined at 77K using a
Micromeritics TRISTAR 3000 static volumetric adsorption analyser. The procedure used
was an application of British Standard methods BS4359:Part 1:1984 'Recommendations
for gas adsorption (BET) methods' and BS7591:Part 2:1992, 'Porosity and pore size
distribution of materials' - Method of evaluation by gas adsorption. The resulting data
were reduced using the BET method (over the pressure range 0.05-0.20 P/Po) and the
Barrett, Joyner & Halenda (BJH) method (for pore diameters of 20-1000 A) to yield the
surface area and pore size distribution respectively.
Suitable references for the above data reduction methods are Brunauer, S, Emmett, P
H, & Teller, E, J . Amer. Chem. Soc. 60, 309, (1938) and Barrett, E P, Joyner, LG &
Halenda P P, J. Am Chem. Soc. , 1951 73 373-380.
Samples of the supports and catalysts were out gassed for 16 hours at 120 °C under a
vacuum of 5x10 Torr (0.6666 Pa) prior to analysis.
In another aspect, the present invention also provides a use of a dried supported
heteropolyacid catalyst prepared by the initiation procedure described hereinbefore for
improving ethene productivity and/or for extending catalyst lifetime in a process for
producing ethene by the vapour phase chemical dehydration of a feed-stream comprising
ethanol and optionally water and/or ethoxyethane, wherein said process comprises
contacting a supported heteropolyacid catalyst with the feed-stream having a feed
temperature of at least 200 °C.
In other aspects, the present invention also provides a composition comprising (or
consisting of) a product obtained by a process according to the present invention and/or
derivatives thereof, including a product obtained by a process according to the present
invention per se, and/or derivatives thereof. As used herein, a derivative is a composition
comprising or consisting of a product arising from a further process, said further process
having utilised the product of the present invention as a feedstock at any stage. By way of
non-limiting example, polyethylene may be such a derivative. As the composition/product
described here arises from a process as described above, any features of the process
described above are also applicable to these aspects, either individually or in any
combination.
The present invention will now be illustrated by way of the following examples and
with reference to the following figures:
FIGURE 1: Graphical representation of ethene productivity against time of catalyst
exposure to a feed stream at 260 °C for Example 1 and Comparative Examples 1 to 3; and
FIGURE 2 : Graphical representation of ethene productivity against time of catalyst
exposure to a feed stream at 260 °C for Example 2 and Comparative Example 4.
Catalyst Preparation
A silicotungstic acid (STA) catalyst was used for conducting the dehydration
reactions according to the following examples.
A high purity silica support with a surface area of 147m2/g, pore volume of 0.84 ml/g
and a mean pore diameter of 230 A was used for preparation of the STA catalyst. The
catalyst was prepared by adding silica (512 g) to a solution of silicotungstic acid (508 g) in
water (1249 g). Once the silicotungstic acid solution had fully impregnated the pores of
the support the excess solution was drained, under gravity, from the support and this was
then dried.
The STA loading on the catalyst support as STA.6H20 , on a dry weight basis, was
estimated to be 24.5% w/w, based on the weight gained by the silica during the catalyst
preparation.
For Example 1 and Comparative Examples 1 to 3 below, the catalyst was crushed to
a particle size of 100 to 200mpi before being loaded into the reactor tube.
For Example 2 and Comparative Example 4 below, the catalyst was crushed to a
particle size of 850 to IOOOmhi before being loaded into the reactor tube.
Vapour Phase Dehydration Reactions for Example 1 and Comparative Examples 1 to 3
A mass of STA catalyst (as indicated in Table 1 below) prepared in accordance with
the above method was loaded into a reactor tube having an isothermal bed and pressurised
to 0.501 MPa under inert gas (nitrogen and helium) flow. The catalyst was heated at 2
°C/min to either 150 °C (Example 1) or 240 °C (Comparative Examples 1 to 3) under a
combined nitrogen (0.01500 mol/hr) and helium flow (0.00107 mol/hr) and held at this
temperature for 8 hours before being cooled to 150 °C, if not already at this temperature.
Ethanol (0.04084 mol/hr) was then added to the nitrogen / helium flow and the
temperature was increased at 2 °C/min to 225 °C. Once at 225 °C the feed pressure was
increased at a rate of 0.1 MPa/min such that the pressure inside the reactor was increased
to the value of 2.858 MPa. The diethyl ether and water reagents were added to the ethanol,
helium and nitrogen flow. At this point the flows of the feed components were adjusted to
give ethanol (0.02677 mol/hr), diethyl ether (0.00776 mol/hr), water (0.00297 mol/hr),
helium (0.00106 mol/hr) and nitrogen (0.01479 mol/hr).
Once the catalyst performance had stabilised at 225 °C, typically after around lOOhrs,
the catalyst temperature, which is the same as the feed temperature in this particular
reactor, was increased to 260 °C and the ethene productivity monitored versus time by online
GC analysis. The results of dehydration experiments at varying pressure are presented
in Table 1 below.
Table 1
The results in Table 1, which are represented graphically in Figure 1, illustrate the
benefits of the process of the invention with regard to catalyst lifetime. It is clear from
Figure 1 that ethene productivity remains high with Example 1, which benefits from
having had a catalyst drying step at a temperature of 150 °C in accordance with the
invention, for a significantly longer period of time compared with Comparative Examples
1 to 3, which have had a catalyst drying step at high temperature (240 °C) not in
accordance with the present invention. Figure 1 also illustrates that the maximum ethene
productivity in the ethanol dehydration reaction may also be increased by virtue of a drying
step according to the present invention, in comparison with a high temperature drying step
not in accordance with the invention as in the case of Comparative Examples 1 to 3. The
maximum ethene productivity observed for Example 1 was 478 mole/kg catalyst/hr, whilst
the maximum ethene productivity observed for Comparative Examples 1 to 3 was only 4 11
mole/kg catalyst/hr (Comparative Example 1).
Vapour Phase Dehydration Reactions for Example 2 and Comparative Example 4
A mass of STA catalyst (as indicated in Table 2 below) prepared in accordance with
the above method described above was loaded into a reactor tube and pressurised to 0.5
MPa under nitrogen flow. The catalyst was heated to either 150 °C or 240 °C under
nitrogen (0.4957 mol/hr) flow and held at this temperature for 2 hours before being cooled
to 150 °C, if not already at this temperature.
Ethanol ( 1.3228 mol/hr) was then added to the nitrogen flow and the temperature of
the feed to the catalyst bed was increased to 225 °C. Once at 225 °C the feed pressure was
increased to the value of 2.857 MPa. The diethyl ether and water reagents were then added
to the ethanol and nitrogen flow. At this point the flows of the feed components were
adjusted to give ethanol (0.8544 mol/hr), diethyl ether (0.2476 mol/hr), water (0.0949
mol/hr) and nitrogen (0.4957 mol/hr).
After 24 hrs the temperature of the feed to the catalyst bed was increased to 260 °C
and the ethylene productivity monitored versus time by on-line GC analysis. The results of
dehydration experiments at varying pressure are presented in Table 2 below.
Table 2
Time
Max.
on Ethylene
Temp, Mass of Process Total
Stream Productivity
Example under catalyst Temp. Pressure
at (mole/kg
N2 flow (g) (°C) (MPa)
260°C catalyst/hr)
(°C) (hrs)
Example 2 150 0.507 0 260 2.857 444
Example 2 150 0.507 15 260 2.857 427
Example 2 150 0.507 29 260 2.857 424
Example 2 150 0.507 45 260 2.857 413
Example 2 150 0.507 59 260 2.857 415
Example 2 150 0.507 89 260 2.857 402
Example 2 150 0.507 115 260 2.857 4 11
Example 2 150 0.507 130 260 2.857 415
Example 2 150 0.507 145 260 2.857 410
Example 2 150 0.507 160 260 2.857 395
Example 2 150 0.507 175 260 2.857 410
Example 2 150 0.507 190 260 2.857 404
Example 2 150 0.507 206 260 2.857 401
Comparative Example 4 240 0.508 0 260 2.857 436
Comparative Example 4 240 0.508 16 260 2.857 402
Comparative Example 4 240 0.508 30 260 2.857 398
Comparative Example 4 240 0.508 45 260 2.857 383
Comparative Example 4 240 0.508 60 260 2.857 376
Comparative Example 4 240 0.508 115 260 2.857 335
Comparative Example 4 240 0.508 130 260 2.857 334
Comparative Example 4 240 0.508 145 260 2.857 330
Comparative Example 4 240 0.508 160 260 2.857 319
Comparative Example 4 240 0.508 176 260 2.857 316
Comparative Example 4 240 0.508 191 260 2.857 300
Comparative Example 4 240 0.508 206 260 2.857 287
The results in Table 2, which are represented graphically in Figure 2, illustrate the
benefits of the process of the invention with regard to catalyst lifetime. It is clear from
Figure 2 that ethene productivity remains high with Example 2, which benefits from
having had a catalyst drying step at a temperature of 150 °C in accordance with the
invention, for a significantly longer period of time compared with Comparative Example 4,
which has had a catalyst drying step at high temperature (240 °C) not in accordance with
the present invention. It will also be appreciated that ethene productivity is generally
higher for Example 2 than for Example 1. For instance, even after 206 hours on stream, the
ethene productivity for Example 2 is above 401 mole/kg catalyst/hr, whereas after 207
hours on stream the ethene productivity for Example 2 is above 263 mole/kg catalyst/hr.
The dimensions and values disclosed herein are not to be understood as being strictly
limited to the exact numerical values recited. Instead, unless otherwise specified, each such
dimension is intended to mean both the recited value and a functionally equivalent range
surrounding that value. For example, a dimension disclosed as "40 mm" is intended to
mean "about 40 mm."
Every document cited herein, including any cross referenced or related patent or
application, is hereby incorporated herein by reference in its entirety unless expressly
excluded or otherwise limited. The citation of any document is not an admission that it is
prior art with respect to any invention disclosed or claimed herein or that it alone, or in any
combination with any other reference or references, teaches, suggests or discloses any such
invention. Further, to the extent that any meaning or definition of a term in this document
conflicts with any meaning or definition of the same term in a document incorporated by
reference, the meaning or definition assigned to that term in this document shall govern.
While particular embodiments of the present invention have been illustrated and
described, it would be obvious to those skilled in the art that various other changes and
modifications can be made without departing from the spirit and scope of the invention. It
is therefore intended to cover in the appended claims all such changes and modifications
that are within the scope and spirit of this invention.

Claims
1. A process for the preparation of ethene by vapour phase chemical dehydration of a
feed-stream comprising ethanol and optionally water and/or ethoxyethane, said process
comprising contacting a dried supported heteropolyacid catalyst in a reactor with the feedstream
having a feed temperature of at least 200 °C; wherein before the supported
heteropolyacid catalyst is contacted with the feed-stream having a feed temperature of at
least 200 °C, the process is initiated by:
(i) drying a supported heteropolyacid catalyst in a reactor under a
stream of inert gas having a feed temperature of from above 100 °C to 200
°C; and
(ii) contacting the dried supported heteropolyacid catalyst with an
ethanol-containing vapour stream having a feed temperature of from above
100 °C to 160 °C.
2. A process according to Claim 1, wherein the supported heteropolyacid catalyst is
dried in step (i) under a stream of inert gas having a feed temperature of from 100 °C to
180 °C, preferably from 110 °C to 170 °C, more preferably from 120 °C to 160 °C, for
example 150 °C.
3. A process according to Claim 1 or Claim 2, wherein the dried supported
heteropolyacid catalyst is contacted in step (ii) with an ethanol-containing vapour stream
having a feed temperature of from 120 °C to 158 °C, preferably from 130 °C to 156 °C,
more preferably from 140 °C to 154 °C, most preferably from 148 °C to 152 °C, for
example 150 °C.
4. A process according to any of Claims 1 to 3, wherein the feed temperature of the
feed-stream is at least 220 °C, preferably where the feed-temperature is at least 240 °C.
5. A process according to any of the preceding claims, wherein the upper limit of the
feed temperature of the feed-stream is 350 °C; preferably wherein the upper limit of the
feed temperature of the feed-stream is 325 °C.
6. A process according to any of the preceding claims, wherein the pressure inside the
reactor when the supported heteropolyacid catalyst is contacted with the feed-stream is
from 0.1 MPa to 4.5 MPa; preferably wherein the pressure inside the reactor is from 0.5
MPa to 3.5 MPa; and most preferably wherein the pressure inside the reactor is from 1.0
MPa to 2.8 MPa.
7. A process according to any of the preceding claims, wherein the initiation of the
ethanol dehydration process further comprises: (iii) ramping the feed temperature of the
ethanol-containing vapour stream to at least 200 °C, preferably over the course of 10
minutes to 8 hours, more preferably over the course of 20 minutes to 4 hours.
8. A process according to any of the preceding claims, wherein the feed-stream
comprises water and/or ethoxyethane, preferably wherein the feed-stream comprises water
and ethoxyethane.
9. A process according to any of the preceding claims, wherein the ethanol-containing
vapour stream comprises or consists of ethanol, any balance being made up of inert gas
diluents.
10. A process according to any of the preceding claims, wherein drying in step (i) is
undertaken for a period of from 1 to 48 hours; preferably a period of from 2 to 16 hours;
more preferably over a period of from 2 to 12 hours.
11. A process according to any of the preceding claims, wherein the catalyst is
provided in the form of one or more catalyst beds in the reactor.
12. A process according to Claim 11 wherein the catalyst is provided in the form of
multiple catalyst beds; preferably arranged in series or in parallel.
13. A process according to Claim 11 or Claim 12, wherein the catalyst bed(s) is/are
selected from adiabatic packed beds, tubular fixed beds or fluid beds, preferably adiabatic
packed beds.
14. A process according to any of Claims 11 to 13, wherein the temperature differential
across the bed of supported hetereopolyacid catalyst in the reactor during drying of the
supported hetereopolyacid catalyst in step (i) is no more than 20 °C, preferably no more
than 15 °C, more preferably no more than 10 °C, most preferably no more than 5 °C.
15. A process according to any of the preceding claims, wherein the average diameter
of the supported heteropolyacid catalyst particles is from 500 mih to 8,000 mh ; preferably
from 1,000 m to 7,000 m ; more preferably from 2,000 mhi to 6,000 mhi, most preferably
from 3,000 mi to 5,000 m h.
16. A process according to any of the preceding claims, wherein the amount of
heteropolyacid in the supported heteropolyacid catalyst is in the range of from 10 wt.% to
50 wt.% based on the total weight of the supported heteropolyacid catalyst.
17. A process according to any of the preceding claims, wherein at least a portion of
the supported heteropolyacid catalyst has previously been employed in a process for the
preparation of an ethene from a feed-stream comprising ethanol, water and ethoxyethane.
18. A process according to any of the preceding claims, wherein the supported
heteropolyacid catalyst is a supported heteropolytungstic acid catalyst, preferably a
supported silicotungstic acid catalyst, for example 12-tungstosilicic acid
(H4[SiW12O40].xH2O).
19. A composition comprising a product obtained by a process according to any
preceding claim, and/or derivatives thereof.