DEACTIVATION OF METHANOL SYNTHESIS CATALYSTS

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DEACTIVATION OF METHANOL SYNTHESIS
CATALYSTS
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Deactivation of Methanol Synthesis Catalysts
CONTENTS
1 INTRODUCTION
2 THERMAL SINTERING
3 CATALYST POISONING
4 REACTANT INDUCED DEACTIVATION
5 SUMMARY
TABLES
1 DEACTIVATION PROCESSES ON METHANOL SYNTHESIS
CATALYSTS
2 MELTING POINT, HUTTIG AND TAMMANN TEMPERATURES OF
COPPER, IRON AND NICKEL
3 SINTERING RATE CONSTANTS CALCULATED INLET AND OUTLET
SIDESTREAM UNIT FOR VULCAN VSG-M101
4 COMPARISON BETWEEN CALCULATED S∞ AND DISCHARGED
MEASUREMENTS ON VULCAN VSG-M101

5 EFFECT OF POSSIBLE CONTAMINANTS AND POISONS ON
CU/ZNO/AL2O3 CATALYSTS FOR METHANOL SYNTHESIS
6 GUARD SCREENING TEST RESULTS ON METHANOL MICRO-
REACTOR. EFFECT OF DEPOSITED METALS ON METHANOL
ACTIVITY
FIGURES
1 THE HΫTTIG AND TAMMANN TEMPERATURES OF THE
COMPONENTS OF A SYNTHESIS CATALYST
2 A SCHEMATIC REPRESENTATION OF TWO CATALYST SINTERING
MECHANISMS
3 SIDE STREAM DATA FOR VULCAN VSG-M101. INLET TEMPERATURE
242 O
C, PRESSURE 1500 PSI, GAS COMPOSITION 6% CO, 9.2% CO2,
66.9% H2, 2.5% N2 AND 15.4% CH4, SPACE VELOCITY 17,778 HR-1
.
MEAN OUTLET TEMPERATURE 280 O
C
4 TEMPERATURE DEPENDENCE OF THE RATE OF SINTERING
5 MECHANISM OF SULFUR RETENTION
6 CORRELATION OF SULFUR CAPACITY WITH TOTAL SURFACE AREA
7 EFFECT OF DEPOSITED (NI+FE) PPM ON METHANOL SYNTHESIS
CATALYST ACTIVITY
8 DISCHARGED (FE + NI) DEPOSITION LEVELS ON METHANOL
SYNTHESIS PLANT SAMPLES
9 EPMA ANALYSIS OF DISCHARGED LABORATORY SAMPLE OF
POISONED VULCAN VSG-M101
10 THE EFFECT OF CO2 ON SYNTHESIS CATALYST DEACTIVATION
REFERENCES

DEACTIVATION OF METHANOL SYNTHESIS CATALYSTS
1 INTRODUCTION
The activity of a Cu/ZnO/Al2O3 methanol synthesis catalyst is directly related to
the material’s copper surface area. Therefore catalyst manufacture requires the
preparation of phases that will give high and stable copper surface areas. In
operation in real plants, three main deactivation processes take place on
methanol synthesis catalysts, thermal sintering, catalyst poisoning and reactant
induced deactivation. The effects of these three deactivation processes are
shown in Table 1.
Deactivation
Process
Description Effect
Thermal Sintering Temperature induced loss of
copper surface area with time
on line.
Permanent loss of activity
Catalyst Poisoning Transport of catalyst poisons
into converter with process
gas.
and selectivity
Reactant Induced Deactivation caused by
composition of reactant
gases.
Table 1.Deactivation Processes on Methanol Synthesis Catalysts
It should be noted that the effects of both thermal sintering and reactant induced
poisoning reduce the catalyst activity and therefore in order to maintain
production rates, the converter operating temperatures have to be increased
which in turn increases the rate of by-product formation.
Resistance to deactivation has been a key design criteria used in the
development of the VULCAN VSG-M101 series methanol synthesis catalysts.
The ability of the VULCAN VSG-M101 series catalysts to resist deactivation is
fundamentally linked to the processes used in manufacture, in particular the
efficiency of mixing at the atomic scale between the copper, zinc oxide and
alumina components of the catalyst. The purpose of this paper is to describe the
detailed mechanisms by which the three types of deactivation processes proceed
on methanol synthesis catalysts and to detail the factors, which, critically, control
the rate of these deactivation processes.

2 THERMAL SINTERING
It is widely accepted that the methanol synthesis activity under typical industrial
conditions is directly related to the copper metal area. Catalyst manufacture
requires, therefore, the preparation of phases that will give high and stable
copper surface area. This is exceptionally difficult to achieve since the very small
(50 Å to 100 Å diameter) copper crystallites which are present in freshly reduced
catalyst have a high surface to volume ratio. This means that the energetics of
the copper crystals are dominated by the high-energy atoms which are located at
the surface of the crystallites rather than those which from the bulk of crystallites.
Thermodynamically there is a significant driving force for merging the small
crystallites into larger particles with low surface/volume ratios and
correspondingly low metal surface areas and hence the observed reduction in
methanol synthesis activity. This process is generally termed catalyst sintering.
Temperature is the dominant factor in controlling the rate of sintering of metallic
and oxidic species. Relationships have been evolved which utilize the melting
point of the material of concern to estimate the magnitude of the sintering
process at any given temperature. The Hϋttig temperature is calculated as one
third of the melting point (in absolute units) of the metal or oxide and gives an
indication of the temperature at which atoms at the very surface of crystallites
can become mobile. The Tammann temperature is one half of the melting point
(in absolute units) and is considered to be the temperature at which bulk metal or
metal oxide lattices experience mobility. Copper has a relatively low melting
point compared to other commonly used metallic catalysts such as iron and
nickel (Table 2).
Metal Melting Point
(o
C)
Hϋttig Temp
(o
C)
Tammann
Temp (o
C)
Copper 1083 179 405
Iron 1535 330 631
Nickel 1455 303 591
Table 2. Melting Point, Huttig and Tammann Temperatures of Copper, Iron
and Nickel
The Hϋttig and Tammann temperatures of the three principal components of an
activated methanol synthesis catalyst (copper, zinc oxide and alumina) are
shown in Figure 1.

Figure 1. The Hϋttig and Tammann Temperatures of the Components of
a Synthesis Catalyst
It is clear from Figure 1 that neither zinc oxide nor alumina has a Hϋttig or
Tammann temperature which is close to the operating temperature of a synthesis
convertor. However, the range of temperatures which are covered by the Huttig
and Tammann temperatures of copper metal are almost identical to those
experienced by an operating methanol synthesis catalyst which is typically
between 200 and 300°C dependent upon catalyst age and converter type.
Two mechanisms which have been suggested for the sintering of metal catalysts
are atomic migration and particle migration. Both of these mechanisms are
shown schematically in Figure 2. Since methanol synthesis catalysts operate at
temperatures below the Tammann temperature of copper it can be expected that
most of the sintering that occurs can be attributed to particle growth by the
atomic migration mechanism. However, at more elevated temperatures for
example in a poorly controlled reduction process, the particle migration model
would become operable.

Figure 2. A Schematic Representation of Two Catalyst Sintering
Mechanisms
Particle Migration
Particle Migration
Particle CollisionSurface Transfer
Surface Atom Mobility
Particle Growth
Atomic Migration
It has been shown that it is possible to describe the rate of sintering processes
which proceed by the atomic migration model by a form of rate equation which is
called the Generalized Power Rate Law equation (GPLE). The general from of
the GPLE is given below.
n
)S-(Sk
dt
dS
RateSintering ∞=−=
Where S = surface area
k = rate constant
S∞ = limiting value of surface area
At any given point the sintering rate is dependent on the surface area of the
catalyst (S) to a specified power (n) and therefore the rate of sintering of the
catalyst can be expected to be faster at the start of the catalyst life when the
surface area is highest.
The rate constant k contains an Arrenhius factor and therefore introduces
temperature dependence to the sintering process.

The magnitude of the rate constant, k, and the activation energy contained within
it, will vary among different types of catalyst and will be dependent upon the
efficiency of mixing, at the atomic scale, which has occurred between the copper,
zinc oxide and alumina components during the manufacture of the catalyst. The
factor (S∞) is a limiting value of the active surface area of the catalyst. The
physical rationale for this factor in the rate equation is that, at the end of catalyst
life, the crystallites will be large and relatively stable and little thermodynamic
drive will exist for sintering to proceed.
Data has been produced for the sintering of methanol synthesis catalysts using a
methanol sidestream unit. This unit was designed to allow exposure of reduced
catalyst samples to the plant supply, giving exposure to synthesis gas,
pressures, temperatures and space velocities identical to the plant synthesis
converter. Figure 3 shows copper surface area data at the inlet and outlet of the
sidestream unit for VULCAN VSG-M101 fitted to second order GPLE models.
Figure 3. Side stream data for VULCAN VSG-M101. Inlet temperature
242 o
C, pressure 1500 psi, gas composition 6% CO, 9.2% CO2,
66.9% H2, 2.5% N2 and 15.4% CH4, space velocity 17,778 hr-1
.
Mean outlet temperature 280 o
C
From the second order GPLE model the sintering rate constants, k, can be
derived (Table 3). As expected (k) is higher at the outlet point of the sidestream
unit, which has a mean operating temperature of 280 o
C. This clearly exemplifies
the influence of the Arrhenius component of the rate constant (k) within the GPLE
on the rate of loss of copper surface area.

Catalyst Position
in Sidestream
Temp (o
C) Sintering
constant, k
(days.m2
/gm)-1
Inlet 242 1.0 x 103
Outlet 280 1.8 x 103
Table 3. Sintering Rate Constants Calculated Inlet and Outlet
Sidestream Unit for VULCAN VSG-M101
From the model an estimate of (S∞) the limiting value of the active surface area of
VULCAN VSG-M101 can be made of 12.7 m2
/g. This is a reasonable fit to plant
data for VULCAN VSG-M101 catalyst discharged after a number of years on line
(Table 4).
Time on
Line
(years)
Copper surface
area
(m2
/g)
Predicted S∞ for
VULCAN VSG-M101
- 12.7
Plant Discharged Sample
of VULCAN VSG-M101
4 Top bed 1
11.2
Bottom bed 1
14.7
Top bed 2
11.7
Table 4. Comparison Between Calculated S∞ and Discharged
Measurements on VULCAN VSG-M101
The effect of initial copper surface on the rate of sintering is demonstrated in
Figure 4. In this sidestream experiment two copper-zinc oxide – alumina
catalysts with a 30% difference in initial copper surface area were tested. It
should be noted that the high area catalyst retained a higher copper surface area
than the other sample throughout the course of the experiment. As time
progresses the difference between the two catalysts becomes smaller. This
change can be attributes to a faster sintering rate of the high surface area
sample because of a larger value of S in the GPLE equation.

Figure 4.
Temperature Dependence of the Rate of Sintering
100 200 300
0.2
0.3
0.5
1
RelativeCopperSurfaceArea
240°C 250°C 260°C 270°C
400
Time (Days)
The Influence of Copper Surface Area on the Rate of Catalyst Sintering
In summary, all highly dispersed copper containing catalysts are likely to undergo
a process of thermal sintering at the reaction temperatures which exist in
methanol synthesis converters. It is likely that the sintering process occurs by an
atomic migration mechanism.
The rate of the sintering process is controlled by a number of factors, which
include the catalyst temperature, the catalyst’s initial copper surface area and the
limiting value of the copper surface area. The use of catalysts, which display
high copper surface areas (and therefore high activities) in the freshly reduced
state, can be expected to give activity benefits throughout the life of the catalyst.
The magnitude of the improvements which can be expected from the use of high
area catalysts is likely to be greatest at the start of catalyst life but a prolonged,
though somewhat reduced benefit will still be experienced after several years of
operation.

3 CATALYST POISONING
There is a large number of materials which could act as poisons on a copper-zinc
oxide- alumina synthesis catalyst (see Table 5), but only a few of these
substances are regularly discovered on analysis of discharged catalyst samples.
For example, silica (which would lower the synthesis activity and promote by-
product formation) and chloride (which causes very high rates of copper
crystallite sintering) are both potent poisons for copper catalysts but are rarely
transported onto the synthesis catalyst in any significant quantities in well
operated methanol plants. However iron, nickel and sulfur are often found in
significant quantities on discharged synthesis catalysts and the characteristic
behavior of these poisons will therefore be discussed in some detail.
Contaminant or Poison Possible sources Effects
Silica or other acidic oxides Transport in steam in plant
gases
Waxes, other by-products
formed
γ-alumina Catalyst manufacture Dimethyl ether formed
Alkali Catalyst manufacture Decreased activity; higher
alcohols formed
Iron Transport as Fe(CO)5 Methane, paraffins, waxes
formed
Nickel Transport in plant as Ni(CO)4 Methane formed, decreased
activity
Cobalt Catalyst manufacture Methane formed, decreased
activity
Lead, heavy metals Catalyst manufacture Decreased activity
Chlorine compounds Transport in plant gases Permanent decrease in
activity
Sulfur compounds Transport in plant gases Permanent decrease in
activity
Table 5. Effect of Possible Contaminants and Poisons on Cu/ZnO/Al2O3
Catalysts for Methanol Synthesis
Sulfur is a powerful poison for copper catalysts. Electron rich sulfur containing
compounds have a strong affinity for the surface of copper crystallites. The zinc
oxide component of the methanol synthesis catalysts plays a critical role in
resisting sulfur poisoning because the thermodynamics that are associated with
the formation of zinc sulfide are more favorable than those of copper sulfide
formation. A schematic representation of the mechanism of sulfur adsorption on
a copper/zinc/alumina is shown below.

Figure 5. Mechanism of Sulfur Retention
The final formation of bulk zinc sulfide is the rate determining part of the process
and the transfer of sulfur from the copper to the zinc will continue until all of the
zinc oxide is covered with sulfur. At this point the rate of sulfur pick up on the
catalyst will decrease and ‘slip’ of sulfur will continue through the catalyst bed.
An effective catalyst requires an intimate mixture of Cu and ZnO and a high free
ZnO surface area. Figure 6 shows the correlation between the sulfur capacity of
a catalyst and total surface area. A high surface area is achieved through careful
control of the manufacturing process.
Figure 6. Correlation of Sulfur Capacity with total Surface Area

Iron and nickel can be transported into synthesis converters as volatile carbonyl
species [Fe(CO)5 and Ni(CO)4)] which are generated by low temperature
reactions of CO rich gas with metal surfaces in other parts of the plant. However,
at more elevated temperatures, such as those found in the synthesis converter
the carbonyl species will readily decompose on contact with the high surface
area copper catalyst. Unlike sulfur there is no natural ‘sink’ for iron or nickel
within the copper-zinc oxide-alumina catalyst.
To demonstrate the effect of metal deposition on catalyst activity a 6 tube micro-
reactor unit was used to screen methanol guard systems. Small ambient
temperature guard beds were installed upstream of the methanol synthesis
catalyst. The synthesis catalysts were run under high gas flow conditions
(160,000 l/hr/kg) and high temperature (265 o
C) in order to accelerate the lay
down of poisons. Methanol synthesis activities were measured at a pressure of
50 barg and a temperature of 225°C using gas of composition 6% CO, 9.2%
CO2, 67% H2, 17.8% N2. The results are shown in Table 6 and Figure 7.
Guard
System
Deposited metals on discharged catalyst % methanol at
334 hoursFe ppm Ni ppm
No Guard 6984 2372 0.72
1 1388 1382 0.97
2 5431 3441 0.85
3 8095 3445 0.8
4 8316 3462 0.77
5 8347 3398 0.77
6 2088 2293 0.92
7 290 698 1.08
8 740 1096 1.04
9 2020 2476 0.9
10 2150 2126 0.9
11 1510 1696 0.9
12 2550 2678 0.94
13 451 1177 1.03
14 0 305 1.17
15 2430 2346 0.9
16 2210 2846 0.94
17 2070 2016 0.96
Table 6. Guard Screening Test Results on Methanol Micro-reactor. Effect of
Deposited Metals on Methanol Activity.

Results have been published that suggest that deposited nickel has twice the
potency of deposited iron as a poison for methanol synthesis catalysts1
. As both
iron and nickel are deposited in our experiments it is difficult to separate the
relative effects of the two metals. Figure 7 below plots the methanol production
at the end of the screening test against the total metals deposited. From this
data it can be seen that as little as 1000 ppm (Ni + Fe) deposited on the catalyst
can lead to a 15% drop from the initial methanol synthesis activity.
Figure 7. Effect of Deposited (Ni+Fe) ppm on Methanol Synthesis
Catalyst Activity
The amounts of iron and nickel deposited on discharged methanol synthesis
catalysts varies greatly from plant to plant. Figure 8 shows a range of iron and
nickel levels found on discharged plant samples. The highest levels of metals
depositions are usually found at the top of beds and in samples near the reactor
walls. Some of these plants will have experienced severe poisoning of the
methanol synthesis activity due to levels of iron and nickel present. The other,
very important, side-effect of metals deposition is side product formation and loss
of selectivity. Nickel contamination can lead to higher methanation. As iron is a
Fischer-Tropsch catalyst, iron deposition can lead to not only methane
production but also to the production of other long chain hydrocarbons and
waxes.
0.4
0.6
0.8
1
1.2
1.4
0 5000 10000 15000
ppm Ni + Fe
%methanolat334hrs

Figure 8. Discharged (Fe + Ni) Deposition Levels on Methanol Synthesis
Plant Samples
A crushed sample of VULCAN VSG-M101 was run in a laboratory reactor for
1,511 hrs under high gas flow conditions (160,000 l/hr/kg) and high temperature
(265 o
C) in order to accelerate the lay down of poisons. The analysis of the
discharged sample showed the following accumulation of poisons, iron 8,173
ppm, nickel 3,786 ppm and sulfur 510 ppm. The discharged sample was further
analyzed using EPMA (Electron Probe Microanalysis). This technique bombards
the sample with an electron beam and detects the element specific X-rays which
are emitted from the sample. The technique is particularly powerful in mapping
the position of poisons on the sample and giving the poison profile across
particles.
As can be seen in Figure 9 the sulfur and iron contaminants are located with high
specificity on the edge of the particles. The nickel contamination, however,
penetrates to a much higher degree throughout the particles. This suggests that
nickel may indeed be a more potent poison for methanol synthesis catalysts than
iron, as suggested by other workers1
.
0
1000
2000
3000
4000
5000
6000
Discharged Methanol Synthesis Plant Samples
DepositedFe+Nippm

Figure 9. EPMA analysis of Discharged Laboratory Sample of Poisoned
VULCAN VSG-M101
Poison Level
Units ppm (w/w)
Iron 870
Nickel 165
Sulphur 190
Chlorine 9
4 REACTANT INDUCED DEACTIVATION
The most commonly encountered form of reactant induced deactivation of
synthesis catalysts is that caused by the CO2 component of the reaction gas.
The role, which CO2 plays in deactivating synthesis catalysts, is intriguing as
although CO2 is required to form methanol at high rates it also has the potential
to cause catalyst deactivation. It is likely that the root cause of the deactivation
potential of CO2 is the ‘extra’ oxygen atom that is formed either by the interaction
of CO2 with the copper surface or on the hydrogenation of CO2 to methanol. The
oxygen atom which is produced by CO2 decomposition or by CO2 hydrogenation
has, thereafter, three clear reaction paths which it can follow. First, it can react
with another molecule of CO2 to form a strongly bound surface carbonate
species or, second and third it can react with either H2 or CO to generate H2O or
to reform a molecule of CO2.
CO2 Interactions:
CO2 + 2Cu ↔CO + Cu2O [1]
CO2 + Cu2O ↔ Cu2O-CO2 [2]
CO2 Hydrogenation:
CO2 + 2H2 + 2Cu → CH3OH + Cu2O [3]
Oxygen Removal:
Cu2O + H2 ↔ H2O + 2Cu [4]
Cu2O + CO ↔ CO2 + 2Cu [5]

All of these processes which involve the addition or removal of oxygen from the
copper surface are potentially damaging to the catalyst activity. For example, if a
catalyst shows an enhanced tendency to catalyze the decomposition of CO2,
reaction [1], then the subsequent production of high concentrations of strongly
adsorbed carbonate complexes, reaction [2], could lead to blockage of the
surface area of the copper crystallites which effectively poisons the copper
surface.
It is also possible that the combination of reactions [1] and [2], which add oxygen
to the catalyst surface, and reactions [4] and [5], which facilitate the removal of
oxygen, could enhance the rate of sintering of the catalyst. This may occur
because the interaction of oxygen with copper has been found to cause
‘reconstruction’ of the copper crystallite surface. This is a change to the physical
structure of the copper surface, which can create mobility among the copper
atoms and, in consequence, may promote catalyst sintering.
The effect of CO2 on the rate of copper catalyst deactivation has been found to
vary between different types of synthesis catalyst. This probably reflects the
disparate preparation routes of the catalysts and the different amounts of
oxidation, and surface reconstruction, which the copper surfaces within each
catalyst, will experience.
For example, the data in Figure 10 reveal the effect of CO2 partial pressure on
the relative activity of two synthesis catalysts after these catalysts (VULCAN
VSG-M101 and a competitive material) had been treated at a total pressure of 80
bar and a reaction temperature of 285 o
C for a time of 147 hours in a laboratory
reactor. After subjecting the catalysts to this deactivation treatment with a
reaction gas which contained CO2 at a total pressure of 3 bar, the relative activity
of the deactivated catalysts was fond to be identical to that of the freshly reduced
materials. However, when the catalysts were subjected to a reaction gas
containing CO2 at a pressure of 12 bar the competitive catalyst deactivated at a
considerably faster rate than VULCAN VSG-M101. These data indicate that
catalyst formulation and catalyst manufacturing procedures can play a significant
role in generating synthesis catalysts which are resistant to the permanent
deactivation which can be induced by CO2.

Figure 10. The Effect of CO2 on Synthesis Catalyst Deactivation
5 SUMMARY
There appear to be three main processes which can be expected to influence the
rate of deactivation of methanol synthesis catalysts
The first of these processes, sintering of the copper component of the catalyst is
common to all catalysts in all converters. The rate of the sintering process is
principally controlled by reaction temperature. Low reaction temperatures have
been shown to reduce the rate at which a catalyst will move towards a low, and
limiting, copper surface area. Catalysts with high surface areas will show large
activity benefits towards the start of life but a prolonged; though slightly reduced
activity benefit will also be observed throughout the life of the catalyst.
Catalyst poisoning can also occur in some synthesis converters. The most
common catalyst poisons are sulfur, iron and nickel. The impact of sulfur on the
activity can be reduced if the catalyst is formulated in such as way that the zinc
oxide component is allowed to act as a ‘sink’ for the sulfur poison.

Iron and nickel can be vicious poisons for copper catalysts leading to
deactivation and also loss of catalyst selectivity. Nickel may be a particularly
potent poison due to its ability to penetrate further into methanol catalyst
particles.
The methanol synthesis reactants can also induce permanent catalyst
deactivation. For example, CO2 can cause reconstruction of copper surfaces
and this process may enhance the rate of copper crystallite sintering. However,
the magnitude of the impact of CO2 on the catalyst deactivation rate has been
found to show extensive variation among different types of synthesis catalysts.
References
1 Roberts G.W., Brown D.M., Hsiung T.H., Lewnard J.J., ‘Deactivation of
Methanol Synthesis Catalysts’, I.E. Chem. Res. 32 pp. 1610-1621 (1993)

DEACTIVATION OF METHANOL SYNTHESIS CATALYSTS

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