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Mechanism of Methanol and Higher Oxygenate Synthesis
D.M. Bibby, C.D. Chang, R.F. Howe and S. Yurchak (Editors), Methane Conuersion 1988 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
109
MECHANISM OF MBTHANOL AND HIGHER OXYGENATE SYNTHESIS I(.
Klier, with R. G. Herman, J. G. Nunan, K. J. Smith, C. E. Bogdan, C.-W.
Young, and J. G. Santiesteban Department of Chemistry and Zettlemoyer Center for Surface Studies Lehigh University Bethlehem, Pennsylvania 18015 U.S.A. ABSTRACT The mechanism of methanol synthesis is discussed with account taken of reported isotope labeling and chemical trapping experiments. Recent studies of alkali promotion of methanol synthesis over the Cu/ZnO catalyst revealed the ion specificity Cs>Rb>K>Na,Li. Alkali are also essential for the development of methanol synthesis activity in the recently discovered alkali/MoS2 heterogeneous catalysts and reported Cut-based homogeneous catalysts. The catalysts are bifunctional and when the synthesis takes place from CO/H2, the basic component activates CO and the Cu/ZnO, MoS2, or Cut soluble complex activates hydrogen. Over the ICI Cu/ZnO/A1203 catalyst, 14C labeling experiments show that C02 is hydrogenated preferentially under the low pressure and high H2/(COtCO2) ratio industrial conditions. Side products are oxygenates, such a s higher alcohols, aldehydes, ketones, esters, and ethers, and hydrocarbons in varying amounts. The product distribution is determined by the catalyst used. The synthesis patterns have been successfully modeled over the Cu/ZnO, Cu/ZnO/Cr2%, Ce/Cu/ZnO, Cs/Cu/ZnO/Cr2Og, Cs/MoS2, Cs/Co/MoS2 and K/Co/MoS2 catalysts based on a few fundamental mechanistic steps involving Cn--Un+l linear growth, Cn--Un+l (nb2) 8-addition, and methyl ester forming reactions. Differences in the mechanisms of linear growth and in the ratio of rates of the linear growth to the 8-addition result in two substantially different distributions: over the Cu/ZnO-based catalysts, 2-methyl-1-propanol and 1-propanol are the dominant Czt products, while over the alkali/(Co)/MoSg catalyst ethanol prevails among the C2+ oxygenates. While methanol synthesis may be operated with selectivity &99%, particularly over the (Cs)/Cu/ZnO catalyst, the higher alcohol content may be increased up to 40% of oxygenated products over the alkali-doped copper-based catalysts and to 80% over the alkali/Co/MoS2 catalyst by the choice of reaction conditions. The kinetic models presented here and in the quoted literature permit the prediction of various oxygenated side product compositions a t different synthesis conditions from the temperature coefficients of the kinetic constants for linear growth, @-addition, and methyl ester forming reactions. INTRODUCTION Methanol synthesis Occurs by the reactions CO
+ 2Hz
# C H S H , A H ~ O O K= -100.46 kJ/mol and
(1)
A G ~ O O K= +45.36 kJ/mol
C02
+ 3H2 @ CHQOH + H20, AH&OK = -61.59 kJ/mol and A G ~ O O K= +61.80 kJ/mol
Simultaneously Occurring with catalytic methanol synthesis is the water gas
(2)
110
shift (WGS) reaction co + H20 e C02 + H2, AH&)OK Although
= -38.7 kJ/mol and
(3)
= -16.5 kJ/mol
AG&K
the current industrial methanol
synthesis is highly
selective to
methanol, side products are formed in quantities determined by the specific catalyst used and by the reaction conditions.
These side reactions involve
methyl formate synthesis 2CO + 2H2
--$
HCOOCH3,
(4)
the synthesis of saturated higher alcohols nCO + 2nH2 + CnHzn+lOH + (n-l)H20
(5)
and their dehydrogenation products aldehydes and ketones, the synthesis of higher, primarily methyl, esters, (n+l)CO + 2nHZ + Cn-1H2n-1COOCH3
+ (n-1)H20J
(6)
the formation of dimethylether (DME), 2CH3OH + CH3OCH3 + H20,
(7)
and the formation of higher ethers and hydrocarbons.
A t extreme reaction
conditions, acetals, ketals, and carboxylic acids are also significant products over less active older generation catalysts such as ZnO and Cr203 but not over the copper-based and alkali/MoSz catalysts discussed in the present paper. Reactions (5)
-
(7) combined with the WGS reaction (3) yield alcohols,
esters, and DME with C02, in excess to water, as co-product. generating
reactions
(4)
-
(6) follow regular
patterns,
The side-product and
the
1-3)
and
alkali-MoS2 (ref. 4) catalyst within a wide range of reaction conditions.
The
distributions DME-forming
have
been
modeled
for
the
copper-baaed
(refs.
product
reaction (7) can be minimized by suppressing acidity of the
catalyst or maximized by enhancing acidity of the catalyst (refs. 5, 6). Although DME and methanol are the primary feedstocks for the ZSM-5 catalyzed MTG o r MTO process (ref. 7), higher alcohols that can be produced over methanol catalysts by reactions (5), o r esters by reactions ( 6 ) , have been demonstrated to be suitable feedstocks for conversion to aromatic gasoline or olefins over the ZSM class of acid catalysts (ref. 8 ) .
Hence, crude methanol
containing from <1X to a large fraction of higher oxygenates may be used in the MTG and MTO processes. The catalysts used in alcohol synthesis hold the key to selectivity for methanol, for higher oxygenates, and to the control of hydrocarbon formation. Of interest are the mechanisms and the structure-function relationships in the catalysis of the C-H bond formation in reactions (1) and (2), C-C bond formation in reaction (5), and C-0 bond formation in reactions (4), (6) and (7), as well as of reactions utilizing the synthesis intermediates as building blocks for organic syntheses such as amine (refs. 9-11) and aldol (refs. 12-14) syntheses.
Further,
the mechanistic roles of Co;! and w a t e r a r e of importance to understanding the
111
kinetic behavior of oxygenate synthesis, particularly of methanol synthesis (1) and (2) in a variety of reaction conditions, with the WGS reaction (3) occurring simultaneously. METHANOL SYNTHESIS AND THE C-H BOND FORMATION
After the discovery of a number of methanol catalysts based on oxides, salts and metals by Patart in 1921 (ref. 15), commercialization of the ZnO/Cr2Og catalysts by BASF in the 1920’s (ref. 16), and systematic studies of the binary Cu/ZnO catalysts by FrBhlich and coworkers in the late 1920’s (ref. 17), the new generation low pressure (
steam
reforming,. methanol
synthesis,
and
the
MTG
process.
the process for methanol synthesis is well established
and the
catalysts well developed and performing over years of service, mechanistic work still continues in an effort to understand the reaction and to rationalize the complex kinetic behavior of reactions (1)
-
(3).
The early, primarily kinetic studies of methanol synthesis have been reviewed by Natta in 1955 (ref. 20), the mechanistic work by Kung in 1980 (ref. 21) and the characteristics of the Cu/ZnO/Mx+
catalysts along with the
available mechanistic information by the author in 1982 (ref. 22). Subsequently, a large number of papers emerged that indicate that the mechanism and kinetics of methanol synthesis a r e complex, may not be identical for different catalysts, and vary considerably with reaction conditions. The reactions that result in the first C-H bond formation have been proposed to be CO +
fi + HCOe
(8)
formyl
CO + O@ + HCOOe
(9)
formate
C02 + fi + HCOOe formate
(10)
Formyl may be formed directly from CO/H2 or by hydrogenation of the formate. Both the formate and formyl may be hydrogenated to methoxide HC@
+ 2H2 + CH30° + Hfl
which is then hydrogenated or hydrolyzed to form methanol.
(11) Formate and
methoxide are readily detected under reaction conditions by I R spectroscopy (refs. 23-25) and formyl has been reported to form on co-adsorption of CO and H2 over the Cu/ZnO catalysts (ref. 26). Initially, a hydroxycarbene route CO
+ H2
+ HtOH
hydroxycarbene
(12)
was postulated (ref. 27), but later it was pointed out that the catalyst would
have
to
lower
the
200
kJ/mol
thermodynamic
barrier
of
hydroxycarbene
formation for this path to be effective (ref. 22). The proportion with which the different mechanisms operate has been attempted to be resolved with the help of labeled compounds.
Takeuchi and
Katzer (ref. 27) used a mixture of 13C16O and 12C180 that produced 13CH3160H and
but not
12CH3180H,
catalyst.
13CH3180H and
12CH3160H,
methanol over Rh/TiOZ
This result favors the formyl path (8) and rules out the formate paths
(9) and (10) for the Rh/Ti02 catalyst under the conditions employed.
However,
the Cu/ZnO catalysts promote a rapid scrambling of 13C160 and 12C180 that is accelerated by preadsorbed w a t e r (ref. 28).
This isotope flow is consistent with
a reversible course of the formate mechanism (9).
To establish the kinetic role
of w a t e r in methanol synthesis via route (9), Vedage et al. (ref. 28) injected D 2 0 into the CO/&
mixture to obtain methanol singly deuterated on the CIQ group, Quantitative evaluation of the isotope flow led to the conclusion
CHzDO(H,D).
that reaction (9) accounted for a t least 65% of the methanol synthesis from CO/Hz
+
HzO, again under the conditions employed in ref. 28.
Evidence for path
as the primary reactant has been obtained by hydrogenating 12CO/14CO2 and 14CO/12CO2 mixtures to methanol (refs. 29-31). that
(10)
utilizes
CO2
For example, with 12CO/14CO2 mixtures, the 14C label appeared in the product methanol for a large range of CO/COz ratios, and a quantitative analysis of 14C as a function of the flow rate of the reactants over the catalyst led to the conclusion that CO2 hydrogenation is the exclusive primary path to methanol under the industrial conditions (temperature 25OOC, pressure 40-50 atmospheres, and GHSV range of 10,000 catalyst
(ref.
31).
In
-
a
120,000 hr-I), utilized with the I C I Cu/ZnO/A1203
prior
paper
(ref.
32),
the
first step of
hydrogenation was proposed to be the formate forming reaction (10). based on the evidence utilizing the
13C16O
+
12Cl80
mixtures,
14CO2/12CO and 12CO2/14CO mixtures as reactants, paths (8)
-
DzO,
C02
Thus, and
(10) are all
feasible but their dominance will be dictated by the catalyst and the reaction conditions. Reaction (9) is well-known to occur under mild conditions even in aqueous solutions of alkali hydroxides (ref. 33).
The details of this reaction have
recently been investigated by reaction path calculations (refs. 34, 35) with the result that a facile nucleophilic attack of CO
Hoe + CO + H
8
m
(1) is followed by a n activated hydrogen transfer 8
H - O Z + HCOOe (1)
(11)
113
a s represented
in Figure 1, where T is the transition
state.
The stable
structures of the metalloformate (I), formate (TI), and the transition state (T)
I\
a r e shown in Figure 1.
O -100
-
. 0
E
co+oHo
,
\
-200 -300
W
z
\
-/
\
/
\
\
0T.C
t
0.Y
-400 -
-500 -600
\
/
\
-
\
/
\
7 Y
r
/
\
\
0-H
\
1 REACTION COORDINATE
Fig. 1. MNDO energy diagram for the reaction of carbon monoxide with hydroxide to form formate. The reaction represented by equation (9) has been documented by Bogdan
(ref. 36) using the Cu/ZnO and CsOH-doped Cu/ZnO catalysts.
The I R spectrum
of the formate formed from a surface hydroxyl and CO on the Cu/ZnO catalyst is shown in Figure 2a and that of formate on CsOH/Cu/ZnO catalyst in Figure 2b. A formate specifically bonded
to the Cs+ ions is documented by the
comparison of the spectrum in Fig. 2h with reference spectra of HCOOCs (ref.
36).
The facile formation of surface HCOOCs from CsOH and CO led the author
and
coworkers
to
the
probing
of
CsOII/Cu/ZnO
and
later
HCOOCs/Cu/ZnO
catalysts for methanol synthesis (ref. 37) and the WGS reaction (ref. 38).
The
promotion by C s of the Cu/ZnO catalyst for methanol is shown in Figure 3. Data a r e given here for methanol formation from CO
+
H2, but the
simultaneous promotion of the WGS reaction in the additional presence of If20 has been documented in ref. 38.
Further, the promotion of the Cu/ZnO catalysts
for methanol is ion specific a s Cs>Rb>K>Na,L.i(ref. 39), in the same order as the basic strength of the counterion of the surface alkali cation such as O H .
The
methanol activity dependence on the concentration of the alkali surface dopant shown in Figure 3 has been explained a s follows.
The catalyst is bifunctional
and contains a basic component (e.g. CsOH) that enhances activation of CO by
114
NT
2753*(660
3.00
3500
4000
,
,
3000
2500
,
I
2000
1500
Wavenumbere (cm-’)
Pig. 2. Infrared spectra of a ) Cu/ZnO = 5/95 mol% and b) Cs/Cu/ZnO (50% surface coverage with cesium) obtained a t 20WC and ambient pressure after carrying out methanol syntheds for 2 h r a t 50 atm with H2/CO = 0.50 synthesis gas.
=.
Reduced
8 = Calcined
300’
- doped -
\
doped
I
Nominal Cs Conc. Mol.%
Fig. 3. Yield of methanol a s a function of cesium loading of the binary (Cu/ZnO = 30/70) catalyst obtained a t 250OC and ‘75 atm with H2/CO = 2.33 synthesis gas a t GHSV = 6120 t(STP)/kg catal/hr. reaction (9) and a hydrogenation component (Cu/ZnO) that activates hydrogen for the conversion HCOOe + cH3Oe.
The maximum methanol yield is obtained
when the CO and €I2 activating components are balanced. A similar pattern is obtained with the alkali/MoSp catalyst a s exemplified in
Figure 4.
The steady state activities for methanol from CO/H2 displayed in
115
I
I
I 300
T
I
a. 275OC
0
”
n
I
0
I
10
I
I
20
I
\
I
30 0
10
20
30
CSOOCH LOADING ON MoS, CATALYST, w t %
Fig. 4. Yields of methanol ( m ) , ethanol (A), total alcohols (O), and hydrocarbons (0)a s a function of cesium loading of the MoS2 catalyst obtained a t a) 275OC and b) 295°C and 81.6 atm with H2/CO = 0.96 synthesis gas a t GHSV = 7750 1 (STP)/kg catal/hr. Figure 4 have been obtained in the author’s laboratory (ref. 40) following the announcement by Dow Chemical scientists of their discovery of this new alcohol synthesis catalyst (ref. 41).
Only methanol, ethanol, and hydrocarbon yields a r e
shown although the catalyst also makes appreciable amounts of higher alcohols. The catalyst requires simultaneous presence af the alkali component and the MoS2 component for developing alcohol synthesis activity.
Consistent with the
picture obtained with the Cs/Cu/ZnO catalysts, the Cs/MoS2 catalyst appears to be a combination of basic (CsOH) and hydrogenation (MoS2) components.
The
amount of the alkali compound necessary to develop the maximum activity is significantly larger than that in the Cs/Cu/ZnO catalyet because the alkali compounds agglomerate into ca. 20 nm particles in contact with the low energy non-polar
MoS2 surface (ref. 42), while they a r e molecularly dispersed in a
submonolayer on the polar Cu/ZnO surface (ref. 43). A
methanol
further example of a bifunctional base-hydrogenation has
recently
been
reported
by
Union
Carbide
(ref.
catalyst for 44).
Their
homogeneous catalyst consists of a Cut compound and an alkali methoxide, and the hydrogenation component is believed to be the copper hydride CuH.
The
alkali methoxide may then serve as a base that activates CO by a nucleophilic attack analogous to reaction (9)
CH3@
+ CO
-+
CH30COe (111)
116
followed by hydrogenation of the metallocarboxylate 111. In summary, several new successful synthesis catalysts for methanol synthesis from CO and H2 appear to be bifunctional and consist of a basic component and a hydrogenation component.
The Cu/ZnO/A1203 catalysts appear
to hydrogenate preferentially CO2 under industrial conditions. C-H
(8) -
forming reactions
(10) have
catalysts and s e t s of conditions. mechanistic
One major remaining task is to translate the
input into kinetic equations that
synthesis reactions (1) of
conditions and
-
describe the behavior of t h e
(3) in a wide range of conditions.
CO/H2
All three initial
been found plausible f o r different
For a limited range
synthesis gas only, methanol synthesis has been
modeled as a function of surface C s concentration for the Cs/Cu/ZnO Cs/Cu/ZnO/Cr203 catalysts (ref. 45).
and
The differential equation describing the
cesium concentration dependence of the synthesis is + ko'WeHZn(l-eCs) 1 ( 1 W K e q ) S (16) 2 Find the theoretical curves obtained by the best fit to the experimental methanol
= (kl@CseH ( 1 % ~ )
synthesis rates a t 250OC and 75 atmospheres a t Hz/CO Figure 5.
=
2.33/1 a r e shown in
The key term in equation (16) proportional to BCs(l-BCe)
reflects the
bifunctionality of the catalyst, the rate of activation of CO being proportional to eCs and that of hydrogen to the free Cu/ZnO surface through
2.oq
(l-ecs).
I 0 : CslCulZnOlCr.0,
: CslCulZnO
X "
1.5
0.015
0.03 0.045
C s %/(rn'/g
0.06
0.075
1
of catalyst)
Fig. 5. Correlation of specific methanol activity as a function of normalized cesium surface concentration of Cs/Cu/ZnO (a) and Cs/Cu/ZnO/Cr~03 ( 0 ) catalysts tested at 250°C and 75 atm with Hz/CO = 2.33 synthesis gas at GHSV = 6120 (unsupported) and 10,000 (Cr203-supported) t(STP)/kg catal/hr.
117
THE ALDEHYDIC C1 INTERMEDWTE The synthesis patterns of
higher
alcohols, esters, and amines a r e
consistent with reactions of an aldehydic C1 intermediate as the building block for the
c-c,
C-0, and C-N bond forming reaction.
Formy1 HCO has already been mentioned.
Other forms of an aldehydic
intermediate that have been proposed include n-bonded formaldehyde, its H. C ,which, i f bonded t o a isomer hydroxycarbene, and “dioxymethylene“ H,C,o-, cation(s) is an anion of hydrated formaldehyde H z C ( 0 H ) z .
-
IR spectra in the 2700
3000 cm-1 region have been interpreted a s vibrational transitions of the CH2
group of dioxymethylene (ref. 46) or adsorbed formaldehyde (ref. 47), but the evidence for hydroxycarbene is lacking. A number of chemical trapping reactions provide support for the aldehydic
C 1 intermediate.
RlRZNH
+
CO/H2
Vedage e t al. (ref. 11) utilized the reaction
--$
RiRzNCH3
-
+ H20
(17)
in which the CH3 group of the product amine RlRzNCH3 w a s synthesized via
RlRZNH amine-Ci aldehyde coupling. Deluzarche e t al. (ref. 48) used methyl iodide to trap formyl with the result CH3CHo ( + @&), CH3I + CO/Hz (18) and Young e t al. (ref. 49) used various alcohols and ketones, e.g. a 3
a 3
(32 I + CHOH
I
a 3
CHzOH
I
CH3 \ ,C=O a 3
+ CO/& -+
I
(19)
I
a 2
a 2 I
CHS
to demonstrate that the addition of the C 1 intermediate formed from CO/Hz, occurred preferentially in the fl position of the Cn alcohol or ketone.
Such a
reaction is typical of aldol condensation followed by hydrogenation, with some specific features regarding oxygen retention that a r e discussed in detail below. The high rates with which all of these reactions occur over the copper-based catalysts
under
the
synthesis
conditions
indicate
that
intermediate is a kinetically important reactive species.
the
C1
aldehydic
However, there a r e
differences in the extent of the aldol-type reactions and other pathways for the C-C bond formation over different catalysts as is demonstrated next. C-C BOND FORMING REACTIONS
These reactions give rise to C2+ alcohols, aldehydes, and ketones.
Four
different types of catalysts, Cu/ZnO, Cs/Cu/ZnO, Cs/MoS2, and Cs(K)/Co/MoSz,
118
will be compared here.
Over the copper-based catalysts the C2+ oxygenates a r e
favored by low Hz/CO (1-0.5) ratios and high temperatures (>280OC)(refs. 1, 39). The main products aside from methanol a r e ethanol, I-propanol, and 2-methyli-propanol (isobutanol), and the alkali dopants enhance the rates of the chain growth.
Over the alkali/MoSZ catalysts, C2” oxygenate synthesis has been
demonstrated (ref. 41) a t Hz/CO
1 and temperatures of 250-330°C
to yield
mainly C2+ linear alcohols, and t h e presence of cobalt in the catalyst has been found to greatly enhance t h e methanol hornologation C1-C2
(ref. 50).
Isotope
and mechanistic studies have been conducted with these catalyst systems in the author’s laboratory on the Cl+Cz,
Cz--’C3,
C 3 4 C 4 , and C4+C5
reaction
steps with the following results:
clz2Czr
-
(i) Injection of 13CH30N into t h e CO/H2 synthesis gas stream yields doubly
labeled ethanol over the Cu/ZnO and Cs/Cu/ZnO catalysts (ref. 51) I3CH30H + CO/H2
(Cs)/Cu/ZnO
13CH313CH20H
This w a s interpreted a s the c1-C~
(.t
(21)
CO/Hz).
step occurring by coupling of two C1
aldehydic species by a mechnnism similar to that proposed by Fox et al. (ref. 52).
This outcome (21) rules out any
12C0
insertion
mechanism such as
-
proposed before (refs. 39, 53-55).
(ii) Injection of 13CH30H yields only P-labeled ethanol over the Cs/MoS2 and Cs/Co/MoS2 catalysts (ref. 56), a s represented by equation (22). 13CH30H + l2C0/H2
Cs/(Co)/MoSz
13CH312CH20H
(22)
This outcome (22) is opposite to that (21) observed over the copper-based catalysts and indicates a CO insertion path for linear alcohol growth over the MoS2 catalysts.
This path i s enhanced by the presence of cobalt and accounts
for t h e dominance of linear alcohols over alkali/MoS2 catalysts. (2-2:
Injection of C H Q ~ ~ C H Z Oyields H different isotopic 1-propanols over t h e Cu/ZnO, Cs/Cu/ZnO, and Cs/MoS2 catalysts. (i)
Over Cs/Cu/ZnO catalysts a t high temperatures, path (23) occurs
selectively (refs. 14, 57).
This outcome (23) is consistent with aldol-type &addition with oxygen retention reversal (ref. 14), a s shown in reaction sequence (24).
CH213CHOe + H2CO
-----)
[eOCH2CH213CHO] + % C H Z C H ~ ~ ~ C H ~
*I
@-addition
HOCH$H213CH3 The retention of
the anionic oxygen in the [-OCH2CH13CHO] intermediate is
specific to the C s promoter which prevents the dehydration of the alcoholate oxygen and favors hydrogenation of constitutes
a
reversal
of
the
the free 13CHO group.
normal
aldol
synthesis
Such a path
pattern
in
which
C B ~ C H Z ~ ~ C Hpropanol ~OH would be formed in the presence of hydrogen. (ii) Over alkali/MoS2 and alkali/Co/MoS2 catalysts, the C p C 3 s t e p occurs
-
by the same type linear growth through CO insertion a s in the C1-2
step a s
evidenced by the isotope reaction 13CH3CH2OH + CO/H2
13CH3CHzCHzOH
Cs/(Co)/MoS2
(25)
C . m A :
(i) Over the Cu/ZnO and Cs/Cu/ZnO catalysts, injection of I-propanol yields
dominantly 2-methyl-I-propanol
and 1-butanol a s a minor product, and the C s
promoter enhances t h e rate of t h e @-branching (ref. 49),
minor The dominant 8-addition
to form 2-methyl-3-propanol
occurs via a mechanistic
path analogous to (24) a s indicated by t h e 1% isotope experiments of Nunan et al.
(ref.
57).
This aldol path
with
oxygen retention
reversal
is further
corroborated by t h e outcome of 2-propanol injection into the synthesis gas (ref. 49) which results in the dominance of 1-butanol in the C4 product,
I
CH3
I CH3 enolate
(ii) Over t h e alkali-MoS2 and alkali/Co/MoS2 catalysts, the C F C 4 growth
step occurs mainly b y linear CO insertion, giving rise to the dominance of I-butanol in the C4 product (ref. 56).
120
C . 4_-Y?-C& The patterns of steps C1-4
continue over the different catalysts a s
shown above with the exception that 2-methyl-1-propanol any C5 products over the copper based catalysts.
does not give rise to
The high rate of &addition
a t C3 and the termination of t h e synthesis a t the branched C4 alcohol a r e the major factors determining the high selectivity for 2-methyl-I-propanol. C-0 BOND FORMING REACTIONS New C-0 bonds a r e formed in the CO/H2 synthesis when CO is converted to
C 0 2 by the WGS reaction (3) and in the synthesis of esters. be discussed here.
Only the latter will
Primarily methyl esters are formed, and they a r e significant.
side products over the (Cs)/Cu/ZnO catalysts but not over the alkali/(Co)/MoS2 catalysts.
The mechanism for methyl ester formation has been suggested (ref.
39) to occur via a coupling of a Cn aldehyde with a C1 aldehyde by the Cannizzaro reaction or by a nucleophilic attack of a Cn aldehyde by methoxide (Tischenko reaction).
The exception is the formation of methyl formate that
occurs via a nucleophilic attack of CO by adsorbed methoxide CH30* + CO + cH30
n "
-C - 0
_.)
IP
HCOOCHQ
(28)
The source of protons for reaction (28) could be water, surface hydroxyls, or alcohols.
-
The mechanism of reaction (28) is consistent with the location of the
isotopic label in the reaction l3CH30H + 12CO/H2
H12C0013CH3
(29)
which clearly traces the carbonyl carbon of methyl formate to CO and t h e methyl carbon to methanol (ref. 51).
Theoretical calculations similar to that for
the formate reaction (14) show that the nucleophilic attack (28) is facile but the subsequent methyl transfer that would form the acetate anion CH3COOel an isomer of t h e metallocarboxylate cH3080 of reaction [28], would have to go over
a large energy barrier.
The calculated energy
levels of
the
stable and
transient states of the pathway (28) followed by methyl transfer a r e shown in Figure 6. Unlike in the hydrogen transfer in HCOOe formation by reaction (14) the methyl transfer in Figure 6 is rendered difficult by t h e large barrier, and this explains the observation that the acetate formation is not effective as a C-C bond forming step.
On the other hand, the CH30°
C-0 bond forming step.
+
CO reaction is an effective
Nunan e t al. (ref. 51) further noted that the methyl
formate concentration in the product is below the equilibrium of the reaction CH30H t CO @ HCOOCH3 mi
tnmn-ratiiraa nf 250-2AIWr! and annrnarhaa -niiilihriiim
nt. t.nrnnnrnt.iirnn
(30) >%?IWr!.
121
higher methyl esters, explains the different rates with which methyl formate and higher methyl e s t e r s a r e formed, as will be demonstrated by the kinetic model for t h e overall Cn oxygenate synthesis pattern.
-100
t
W
z
\ /
/
\
/ /
\
>
0 LT
/
\ \
-300
-/
\
\
/
\ \
e
W
/
CH3OCO
-400
\
\-
C H C 0 Oe
-500 REACTION COORDINATE
Fig. 6. MNDO energy diagram for the reaction of carbon monoxide with methoxide to form methyl formate. KINETIC
MODEL
Cs/Cu/ZnO/C-%
FOR
OXYGENATE
SYNTHESIS
OVER
THE
Cs/Cu/ZnO
AND
CATALYSTS
The C2+ oxygenate synthesis has been succesBfully modeled for the Cs/Cu/ZnO/(Cr203) catalysts by Smith et al. (refs. 2, 3) following the pioneering work on modeling of isobutanol synthesis over K/Cu/ZnO/A1203 Smith and Anderson (ref. 1). and
ester synthesis and
catalysts by
The scheme used to account for the c 1 - C ~ alcohol
based
on
the
mechanistic
steps outlined
in
the
preceding section on the C-C and C-0 bond formation is presented in Figure 7. The model has been applied, and the rate constants 1 , @, determined, in the temperature
range of
220-33OOC a t pressures
atmospheres with synthesis gas having H2/CO
and %
of 41-75
ratios 0.5 to 2.3 for Cu/ZnO,
Cs/Cu/ZnO, and Cs/Cu/ZnO/Cr203 catalysts (refs. 2, 3).
A comparison of the
calculated and observed oxygenate product composition over the Cs/Cu/ZnO
=
0.4/30/70 catalyst at 310% is given in Figure 8. The mechanistic features stemming from the rapid aldol-type @-addition, the slow C1-+C2 aldehyde coupling step, and t h e lack of growth of the @-branched alcohols give rise to t h e selectivity pattern in which ethanol is at minimum and
122
COIH? a
-
.1
HCOOC 0 C-OH C-COOC C-C-COOC C-C-C-COOC C-C-C-C-COOC
ao' a,'
ao'
a 4
C-C-OH C-C-C-OH
1l
-
C-C-C-C-OH
I'
C-C-C-C-C-OH
1'
P1'
PI'
P1'
F:
C-C-C-OH
C C-C-C-C-OH
7
C-C-C-C-C-OH
C - C- C - C- C- C - 0 H Fig. 7. Scheme of carbon chain growth to form linear alcohols, branched alcohols, and methyl esters over alkali-promoted Cu/ZnO-based catalysts.
Fig. 8. Predicted oxygenate yields, based upon the reaction scheme shown in Fig. 7 but also including C2 (82) and C3 (83) in addition to C1 (81') carbon addition, compared to measured yields obtained with a 0.4% CsOOCH/Cu/ZnO catalyst at 310OC and 75 atm with H2/CO 0.45 synthesis gas at GHSV = 3265 #(STP)/kg catal/hr. Estimated kinetic parameter values are 8 = 0.254, 81 = 3.299, 81' = 1.138, 82 = 0.121, 83 = 0.152, a = 0.007, and ao' 0.221.
123
methanol
plus
2-methyl-1-propanol
is a t
maximum.
Methyl formate is in
equilibrium (30) with methanol and all the remaining methyl esters a r e accounted for by a single value of KINETIC MODEL
FOR
0~0.
OXYGENATE
SYNTHESIS OVER
THE ALKALI/MoS2
AND
ALKALI/Co/MoS2 CATALYSTS Smith et al. (ref. 4) have modeled the Czt oxygenate synthesis over the MoSz-based
catalysts taking into account the mechanistic features that the
linear growth I #-branching.
now proceeds by CO insertion and is more efficient than
The main kinetic feature is that the C1-C2
s t e p is faster than
(na2) steps particularly in the presence of cobalt in the catalyst.
Cn-vn+i
This kinetic pattern results in product. distribution that can be maximized at ethanol, opposite to that obtained in the synthesis over the copper based catalysts. CONCLUSIONS Methanol and higher oxygenate syntheses follow different mechanistic and kinetic patterns over the various catalysts discussed here.
Each such pattern
is regular, however, and can be modeled with a few kinetic parameters based on fundamental mechanistic steps involved in the C-H, C-C, and C-0 bond forming reactions.
Alkali
co-catalysts
play
an
important
role
by
promoting
base-catalyzed reaction steps that appear important not only in the formation of Czt oxygenates but also in methanol synthesis.
The rapid 8-addition over the
(Cs)/Cu/ZnO catalysts gives rise to the dominance of 2-methyl-1-propanol 1-propanol
in
the
Czt
coproducts of
methanol
synthesis.
Cobalt
and
greatly
enhances the rate of methanol homologation over alkali/MoS2 catalysts by a CO insertion mechanism, and the rapid linear growth particularly a t the (21-2 step over the alkali/Co/MoS~ catalysts gives rise to the dominance of ethanol in the Czt products.
Aldehydic intermediates appear significant in all alcohol and
ester forming reactions over the (Ce)/Cu/ZnO catalysts, while CO insertion appears
to
be
particularly
important
in
the
growth
pattern
over
the
dkali/(Co)/MoS2 catalysts. ACKNOWLEDGEMENTS This work was supported in part by U.S. Department of Energy Contracts DE-FG22-83PC60786,
DE-AC22-84PC70021,
and
National Science Foundation Grant INT-8612603.
DE-AC22-85PC80014
and
by
the
124
REFERENCES
1.
2. 3.
4. 5. 6. 7. 8. 9. 10.
11. 12.
13.
14. 15. 16. 17. 18. 19. 20.
K. J. S m i t h and R. €3. A n d e r s o n , C a n a d . J. Chem. Eng., 6 l , 40 (1983) and J. Catal,, 85 (1984)428. K. J. S m i t h , C. W. Y o u n g , R. G. H e r m a n , and K. Klier, Ind. E n g . Chem. Res., to be s u b m i t t e d . K. J. S m i t h , C. W. Y o u n g , R. G. H e r m a n , and K. Klier, Ind. E n g . Chem. Res., to be s u b m i t t e d . K. J. S m i t h , R. G. H e r m a n , and K. Klier, Ind. Eng. Chem. Res., to be s ub m i t t e d . L. H. S l a u g h , U. S. Patent 4,375,424 (Mar. 1, 1983); assigned to Shell Oil Co. J. T o p p - J o r g e n s e n , E u r . P a t e n t Appl. E P 148,626 ( J u l y 17, 1985); assigned to H a l d o r Topsoe A/S. C. D. C h a n g , W. H. L a n g , and A. J. Silvestri, U. S. Patent 3,894,106 ( J u l y 8, 1975) and C. D. C h a n g , A. J. Silvestri, and R. L. S m i t h , U. S. Patent 3,928,483 (Dec. 23, 1975); assigned to Mobil Oil C o r p . C. D. C h a n g and A. J. Silvestri, J. Catal., 51 (1977) 249. K. Klier, R. G. H e r m a n , and G. A. V e d a g e , U. S. Patent 4,480,131 (Oct. 30, 1984); assigned to L e h i g h U n i v e r s i t y . K. Klier, R. G. H e r m a n , and G. A. V e d a g e , U. S. Patent 4,642,381 ( F e b . 10, 1987); assigned to L e h i g h U n i v e r s i t y . ' G. A. V e d a g e , R. G. H e r m a n , and K. Klier, J. Catal., 95 (1985) 423 and in "catalysis of O r g a n i c R e a c t i o n s 11," ed. by P. N. R y l a n d e r and H. G r e e n f i e l d , M a r c e l D e k k e r , New York, i n press. W. T. Reichle, S. Y. Kang, and D. S. E v e r h a r d t , J. Catal., 101 (1986) 352. D. J. E l l i o t t and F. Pcnnella, Praprints, Div. Pet. Chem., ACS, (1986)
39.
J. G. N u n a n , C. E. B o g d a n , and K. Klier, J. Chem. SOC., Chem. Commun., submitted. M. Patart, French Patent 540,343 (Aug. 1921). C. L o r m a n d , Ind. E n g . Chem., 17 (1925) 430. P. K. Frolich, M. R. Fenske, and D. Q u i g g l e , Ind. E n g . Chem., 20 (1928) 694 and P. K. Frolich, M. R. Fenske, P. S. Taylor, and C. A. S o u t h w i c h , Jr., Ind. E n g . Chem., 20 (1928) 1327. B. M. Collins, G e r m a n Patent 2,302,658 (Aug. 2, 1973); assigned to I m p e r i a l C h e m i c a l Ind., L t d . P. D a v i e s and F. F. S n o w d o n , U. S. Patent 3,326,956 ( J u n e 20, 1967); assigned to I m p e r i a l C h e m i c a l Ind., L t d . G. N a t t a , in P. H. E m m e t t (Ed.), C a t a l y s i s , Vol. 111, R e i n h o l d , New Y o r k , 1955,
p. 349. 21. H. H. K u n g , Catal. Rev.-Sci. Eng., 22 (1980) 235. 22. K. Klier, Adv. Catal., 1 (1982) 243. 23. A. U e n o , T. Onishi, and K. T a m a r u , Trans. Faraday SOC.,fl (1971) 3585. 24. Y. Amenomiya and T. T a g a w a , Proc. 8th Intern. C o n g r . Catal., Vol. 111, (1984) 557. 25. J. F. E d w a r d s and G. L. Schrader, J. Phys. Chem., &3 (1985) 782. 26. J. Saussey, J . 4 . L a v a l l e y , J. L a m o t t e , a n d T. Rais, J. Chem. SOC., Chem. Commun., (1982) 278. 27. A. Takeuchi and J. R. K a t e e r , J. P h y s . Chem., 85 (1981) 937. 28. G. A. V e d a g e , R. Pitchai, R. G. H e r m a n , and K. Klier, Proc. 8th Intern. C o n g r . Catal, Vol. 11, (1984) 47. 29. A. Y. Rozovskii, G. I. Lin, L. G. L i b e r o v , E. V. Sliminskii, S. M. L o k t e v , Y. B. K a g a n , and A. N. B a s h k i r o v , Kinet. Katal., 18 (1977) 691. 30. V. D. K u z n e t s o v , F. S. Shub, and M. J. T e m k i n , Kinet. Katal., 23 (1982) 932. 31. G. C. C h i n c h e n , P. J. D e n n y , D. G. Parker, M. S. Spencer, and D. A. Whan, Appl. Catal., 30 (1987) 333. 32. G. C. C h i n c h e n , P. J. D e n n y , D. G. Parker, G. D. Short, M. S. Spencer, K. C. Waugh, and D. A. Whan, Preprints, Div. Fuel. Chem., ACS, (1984) 178 and G. C. C h i n c h e n , K. C. Waugh, and D. A. Whan, Appl. Catal., 25 (1986) 101.
125
G. Thomas, Ann. Chim., S (1951) 367. K. Klier, D. Zeroka, and D. Bybell, 189th National Meeting of the American Chemical Society, Miami Beach, FL Abstract NO. COLL-0033 (April 1985). 35. C. E. Bogdan et al., to be submitted. C. E. Bogdan, Ph.D. Dissertation, Department of Chemistry, Lehigh Uni36. versity, Bethlehem, PA (1988). 37. J. G. Nunan, K. Klier, C. W. Young, P. R. Himelfarb, and R. G. Herman, J. Chem. Soc., Chem. Commun., (1986) 193. 5 (1986) 38. K. Klier, C. W. Young, and J. G. Nunan, Ind. Eng. Chem., Fundam., 2 36. 39. G. A. Vedage, P. B. Himelfarb, G. W. Simmons, and K. Klier, ACS Symp. Ser., 279 (1985)-295. ier. ~~, R. G. Herman, G. W. Simmons. C. E. Lyman. and J. G. Santiesteban, 40. K-Kl_--"Direct Synthesis of Alcohol Fuels. over Molybdenum-based Catalysts," Quarterly Technical Progress Report DOE/PC/80014-3 to U.S. DOE/PETC (July 1986). G. A. Cochran, M. M. Conway, C. B. Murchison, B. W. Pynnonen, G. J. 41. Quarderer, R. F. Stevens, R. A. Stowe, and E. D. Weihl, Intern. Chem. Congr. Pac. Basin SOC., Honolulu, HI, Abstr. No. 03G35 (Dec. 1984). 42. K. Klier, R. G. Herman, G. W. Simmons, M. Najbar, and J. G. Santiesteban, "Direct Synthesis of Alcohol Fuels over Molybdenum-based Catalysts," Quarterly Technical Progress Report DOE/PC/80014-5 to U.S. DOE/PETC (March 1987). P. B. Himelfarb, Ph.D. Dissertation, Department of Materials Science and 43. Engineering, Lehigh University, Bethlehem, PA (1986). 44. B. D. Dombek , Final Technical Report DE-AC22-84PC70022 from Union Carbide Corp. to the U.S. Department of Energy (Jan. 1987). 45. C. W. Young, Ph.D. Dissertation, Department of Chemical Engineering, Lehigh University, Bethlehem, PA (1987). 46. J. C. Lavelley, J. Lamotte, G. Busca, and V. Lorenzelli, J. Chem. SOC., Chem. Commun., (1985) 1006. 47. J. F. Edwards and G. L. Schrader, J. Phys. Chem., 88 (1984) 5620; arid J. Catal., 94 (1985) 175. 48. A. Deluzarche, J. P. Hindermann, and R. Kieffer, Tetrahedron Lett., (1978) 2787. 49. C. W. Young, R. G. Herman, and K. Klier, to be published. 50. G. J. Quarderer, G. A. Cochran, R. R. Stevens, and C. B. Murchison, Eur. Patent Appl. 85109213.0 (July 23, 1985); assigned t o Dow Chemical Co. 51. J. G. Nunan, C. E. Bogdan, K. Klier, K. J. Smith, C. W. Young, and R. G. Herman, J. Catal., to be submitted. 52. J. R. Fox, F. A. Pesa, and B. S. Curetolo, J. Catal., 90 (1984) 127. 53. F. Fischer, Ind. Eng. Chem., 11 (1925) 576, and Conversion of Coal into O;ls, Van Nostrand, New York, 1925, p. 251. 54. G. N a t t a , U. Colombo, and I. Pasquon, in P. H. Emmett (Ed.), Catalysis, Vol. V, Reinhold, New York, 1957, p. 131. 55. T. J. Mazanec, J. Catal., 98 (1986) 115. J. G. Santiesteban, C. E. Bogdan, R. G. Herman, and K. Klier, Proc. 9th 56. Intern. Congr. Catal., to be published. 57. J. G. Nunan, C. E. Bogdan, K. Klier, K, J. Smith, C, W. Young, and R. G. Herman, J. Catal., t o be submitted. 33. 34.
___
109
MECHANISM OF MBTHANOL AND HIGHER OXYGENATE SYNTHESIS I(.
Klier, with R. G. Herman, J. G. Nunan, K. J. Smith, C. E. Bogdan, C.-W.
Young, and J. G. Santiesteban Department of Chemistry and Zettlemoyer Center for Surface Studies Lehigh University Bethlehem, Pennsylvania 18015 U.S.A. ABSTRACT The mechanism of methanol synthesis is discussed with account taken of reported isotope labeling and chemical trapping experiments. Recent studies of alkali promotion of methanol synthesis over the Cu/ZnO catalyst revealed the ion specificity Cs>Rb>K>Na,Li. Alkali are also essential for the development of methanol synthesis activity in the recently discovered alkali/MoS2 heterogeneous catalysts and reported Cut-based homogeneous catalysts. The catalysts are bifunctional and when the synthesis takes place from CO/H2, the basic component activates CO and the Cu/ZnO, MoS2, or Cut soluble complex activates hydrogen. Over the ICI Cu/ZnO/A1203 catalyst, 14C labeling experiments show that C02 is hydrogenated preferentially under the low pressure and high H2/(COtCO2) ratio industrial conditions. Side products are oxygenates, such a s higher alcohols, aldehydes, ketones, esters, and ethers, and hydrocarbons in varying amounts. The product distribution is determined by the catalyst used. The synthesis patterns have been successfully modeled over the Cu/ZnO, Cu/ZnO/Cr2%, Ce/Cu/ZnO, Cs/Cu/ZnO/Cr2Og, Cs/MoS2, Cs/Co/MoS2 and K/Co/MoS2 catalysts based on a few fundamental mechanistic steps involving Cn--Un+l linear growth, Cn--Un+l (nb2) 8-addition, and methyl ester forming reactions. Differences in the mechanisms of linear growth and in the ratio of rates of the linear growth to the 8-addition result in two substantially different distributions: over the Cu/ZnO-based catalysts, 2-methyl-1-propanol and 1-propanol are the dominant Czt products, while over the alkali/(Co)/MoSg catalyst ethanol prevails among the C2+ oxygenates. While methanol synthesis may be operated with selectivity &99%, particularly over the (Cs)/Cu/ZnO catalyst, the higher alcohol content may be increased up to 40% of oxygenated products over the alkali-doped copper-based catalysts and to 80% over the alkali/Co/MoS2 catalyst by the choice of reaction conditions. The kinetic models presented here and in the quoted literature permit the prediction of various oxygenated side product compositions a t different synthesis conditions from the temperature coefficients of the kinetic constants for linear growth, @-addition, and methyl ester forming reactions. INTRODUCTION Methanol synthesis Occurs by the reactions CO
+ 2Hz
# C H S H , A H ~ O O K= -100.46 kJ/mol and
(1)
A G ~ O O K= +45.36 kJ/mol
C02
+ 3H2 @ CHQOH + H20, AH&OK = -61.59 kJ/mol and A G ~ O O K= +61.80 kJ/mol
Simultaneously Occurring with catalytic methanol synthesis is the water gas
(2)
110
shift (WGS) reaction co + H20 e C02 + H2, AH&)OK Although
= -38.7 kJ/mol and
(3)
= -16.5 kJ/mol
AG&K
the current industrial methanol
synthesis is highly
selective to
methanol, side products are formed in quantities determined by the specific catalyst used and by the reaction conditions.
These side reactions involve
methyl formate synthesis 2CO + 2H2
--$
HCOOCH3,
(4)
the synthesis of saturated higher alcohols nCO + 2nH2 + CnHzn+lOH + (n-l)H20
(5)
and their dehydrogenation products aldehydes and ketones, the synthesis of higher, primarily methyl, esters, (n+l)CO + 2nHZ + Cn-1H2n-1COOCH3
+ (n-1)H20J
(6)
the formation of dimethylether (DME), 2CH3OH + CH3OCH3 + H20,
(7)
and the formation of higher ethers and hydrocarbons.
A t extreme reaction
conditions, acetals, ketals, and carboxylic acids are also significant products over less active older generation catalysts such as ZnO and Cr203 but not over the copper-based and alkali/MoSz catalysts discussed in the present paper. Reactions (5)
-
(7) combined with the WGS reaction (3) yield alcohols,
esters, and DME with C02, in excess to water, as co-product. generating
reactions
(4)
-
(6) follow regular
patterns,
The side-product and
the
1-3)
and
alkali-MoS2 (ref. 4) catalyst within a wide range of reaction conditions.
The
distributions DME-forming
have
been
modeled
for
the
copper-baaed
(refs.
product
reaction (7) can be minimized by suppressing acidity of the
catalyst or maximized by enhancing acidity of the catalyst (refs. 5, 6). Although DME and methanol are the primary feedstocks for the ZSM-5 catalyzed MTG o r MTO process (ref. 7), higher alcohols that can be produced over methanol catalysts by reactions (5), o r esters by reactions ( 6 ) , have been demonstrated to be suitable feedstocks for conversion to aromatic gasoline or olefins over the ZSM class of acid catalysts (ref. 8 ) .
Hence, crude methanol
containing from <1X to a large fraction of higher oxygenates may be used in the MTG and MTO processes. The catalysts used in alcohol synthesis hold the key to selectivity for methanol, for higher oxygenates, and to the control of hydrocarbon formation. Of interest are the mechanisms and the structure-function relationships in the catalysis of the C-H bond formation in reactions (1) and (2), C-C bond formation in reaction (5), and C-0 bond formation in reactions (4), (6) and (7), as well as of reactions utilizing the synthesis intermediates as building blocks for organic syntheses such as amine (refs. 9-11) and aldol (refs. 12-14) syntheses.
Further,
the mechanistic roles of Co;! and w a t e r a r e of importance to understanding the
111
kinetic behavior of oxygenate synthesis, particularly of methanol synthesis (1) and (2) in a variety of reaction conditions, with the WGS reaction (3) occurring simultaneously. METHANOL SYNTHESIS AND THE C-H BOND FORMATION
After the discovery of a number of methanol catalysts based on oxides, salts and metals by Patart in 1921 (ref. 15), commercialization of the ZnO/Cr2Og catalysts by BASF in the 1920’s (ref. 16), and systematic studies of the binary Cu/ZnO catalysts by FrBhlich and coworkers in the late 1920’s (ref. 17), the new generation low pressure (
steam
reforming,. methanol
synthesis,
and
the
MTG
process.
the process for methanol synthesis is well established
and the
catalysts well developed and performing over years of service, mechanistic work still continues in an effort to understand the reaction and to rationalize the complex kinetic behavior of reactions (1)
-
(3).
The early, primarily kinetic studies of methanol synthesis have been reviewed by Natta in 1955 (ref. 20), the mechanistic work by Kung in 1980 (ref. 21) and the characteristics of the Cu/ZnO/Mx+
catalysts along with the
available mechanistic information by the author in 1982 (ref. 22). Subsequently, a large number of papers emerged that indicate that the mechanism and kinetics of methanol synthesis a r e complex, may not be identical for different catalysts, and vary considerably with reaction conditions. The reactions that result in the first C-H bond formation have been proposed to be CO +
fi + HCOe
(8)
formyl
CO + O@ + HCOOe
(9)
formate
C02 + fi + HCOOe formate
(10)
Formyl may be formed directly from CO/H2 or by hydrogenation of the formate. Both the formate and formyl may be hydrogenated to methoxide HC@
+ 2H2 + CH30° + Hfl
which is then hydrogenated or hydrolyzed to form methanol.
(11) Formate and
methoxide are readily detected under reaction conditions by I R spectroscopy (refs. 23-25) and formyl has been reported to form on co-adsorption of CO and H2 over the Cu/ZnO catalysts (ref. 26). Initially, a hydroxycarbene route CO
+ H2
+ HtOH
hydroxycarbene
(12)
was postulated (ref. 27), but later it was pointed out that the catalyst would
have
to
lower
the
200
kJ/mol
thermodynamic
barrier
of
hydroxycarbene
formation for this path to be effective (ref. 22). The proportion with which the different mechanisms operate has been attempted to be resolved with the help of labeled compounds.
Takeuchi and
Katzer (ref. 27) used a mixture of 13C16O and 12C180 that produced 13CH3160H and
but not
12CH3180H,
catalyst.
13CH3180H and
12CH3160H,
methanol over Rh/TiOZ
This result favors the formyl path (8) and rules out the formate paths
(9) and (10) for the Rh/Ti02 catalyst under the conditions employed.
However,
the Cu/ZnO catalysts promote a rapid scrambling of 13C160 and 12C180 that is accelerated by preadsorbed w a t e r (ref. 28).
This isotope flow is consistent with
a reversible course of the formate mechanism (9).
To establish the kinetic role
of w a t e r in methanol synthesis via route (9), Vedage et al. (ref. 28) injected D 2 0 into the CO/&
mixture to obtain methanol singly deuterated on the CIQ group, Quantitative evaluation of the isotope flow led to the conclusion
CHzDO(H,D).
that reaction (9) accounted for a t least 65% of the methanol synthesis from CO/Hz
+
HzO, again under the conditions employed in ref. 28.
Evidence for path
as the primary reactant has been obtained by hydrogenating 12CO/14CO2 and 14CO/12CO2 mixtures to methanol (refs. 29-31). that
(10)
utilizes
CO2
For example, with 12CO/14CO2 mixtures, the 14C label appeared in the product methanol for a large range of CO/COz ratios, and a quantitative analysis of 14C as a function of the flow rate of the reactants over the catalyst led to the conclusion that CO2 hydrogenation is the exclusive primary path to methanol under the industrial conditions (temperature 25OOC, pressure 40-50 atmospheres, and GHSV range of 10,000 catalyst
(ref.
31).
In
-
a
120,000 hr-I), utilized with the I C I Cu/ZnO/A1203
prior
paper
(ref.
32),
the
first step of
hydrogenation was proposed to be the formate forming reaction (10). based on the evidence utilizing the
13C16O
+
12Cl80
mixtures,
14CO2/12CO and 12CO2/14CO mixtures as reactants, paths (8)
-
DzO,
C02
Thus, and
(10) are all
feasible but their dominance will be dictated by the catalyst and the reaction conditions. Reaction (9) is well-known to occur under mild conditions even in aqueous solutions of alkali hydroxides (ref. 33).
The details of this reaction have
recently been investigated by reaction path calculations (refs. 34, 35) with the result that a facile nucleophilic attack of CO
Hoe + CO + H
8
m
(1) is followed by a n activated hydrogen transfer 8
H - O Z + HCOOe (1)
(11)
113
a s represented
in Figure 1, where T is the transition
state.
The stable
structures of the metalloformate (I), formate (TI), and the transition state (T)
I\
a r e shown in Figure 1.
O -100
-
. 0
E
co+oHo
,
\
-200 -300
W
z
\
-/
\
/
\
\
0T.C
t
0.Y
-400 -
-500 -600
\
/
\
-
\
/
\
7 Y
r
/
\
\
0-H
\
1 REACTION COORDINATE
Fig. 1. MNDO energy diagram for the reaction of carbon monoxide with hydroxide to form formate. The reaction represented by equation (9) has been documented by Bogdan
(ref. 36) using the Cu/ZnO and CsOH-doped Cu/ZnO catalysts.
The I R spectrum
of the formate formed from a surface hydroxyl and CO on the Cu/ZnO catalyst is shown in Figure 2a and that of formate on CsOH/Cu/ZnO catalyst in Figure 2b. A formate specifically bonded
to the Cs+ ions is documented by the
comparison of the spectrum in Fig. 2h with reference spectra of HCOOCs (ref.
36).
The facile formation of surface HCOOCs from CsOH and CO led the author
and
coworkers
to
the
probing
of
CsOII/Cu/ZnO
and
later
HCOOCs/Cu/ZnO
catalysts for methanol synthesis (ref. 37) and the WGS reaction (ref. 38).
The
promotion by C s of the Cu/ZnO catalyst for methanol is shown in Figure 3. Data a r e given here for methanol formation from CO
+
H2, but the
simultaneous promotion of the WGS reaction in the additional presence of If20 has been documented in ref. 38.
Further, the promotion of the Cu/ZnO catalysts
for methanol is ion specific a s Cs>Rb>K>Na,L.i(ref. 39), in the same order as the basic strength of the counterion of the surface alkali cation such as O H .
The
methanol activity dependence on the concentration of the alkali surface dopant shown in Figure 3 has been explained a s follows.
The catalyst is bifunctional
and contains a basic component (e.g. CsOH) that enhances activation of CO by
114
NT
2753*(660
3.00
3500
4000
,
,
3000
2500
,
I
2000
1500
Wavenumbere (cm-’)
Pig. 2. Infrared spectra of a ) Cu/ZnO = 5/95 mol% and b) Cs/Cu/ZnO (50% surface coverage with cesium) obtained a t 20WC and ambient pressure after carrying out methanol syntheds for 2 h r a t 50 atm with H2/CO = 0.50 synthesis gas.
=.
Reduced
8 = Calcined
300’
- doped -
\
doped
I
Nominal Cs Conc. Mol.%
Fig. 3. Yield of methanol a s a function of cesium loading of the binary (Cu/ZnO = 30/70) catalyst obtained a t 250OC and ‘75 atm with H2/CO = 2.33 synthesis gas a t GHSV = 6120 t(STP)/kg catal/hr. reaction (9) and a hydrogenation component (Cu/ZnO) that activates hydrogen for the conversion HCOOe + cH3Oe.
The maximum methanol yield is obtained
when the CO and €I2 activating components are balanced. A similar pattern is obtained with the alkali/MoSp catalyst a s exemplified in
Figure 4.
The steady state activities for methanol from CO/H2 displayed in
115
I
I
I 300
T
I
a. 275OC
0
”
n
I
0
I
10
I
I
20
I
\
I
30 0
10
20
30
CSOOCH LOADING ON MoS, CATALYST, w t %
Fig. 4. Yields of methanol ( m ) , ethanol (A), total alcohols (O), and hydrocarbons (0)a s a function of cesium loading of the MoS2 catalyst obtained a t a) 275OC and b) 295°C and 81.6 atm with H2/CO = 0.96 synthesis gas a t GHSV = 7750 1 (STP)/kg catal/hr. Figure 4 have been obtained in the author’s laboratory (ref. 40) following the announcement by Dow Chemical scientists of their discovery of this new alcohol synthesis catalyst (ref. 41).
Only methanol, ethanol, and hydrocarbon yields a r e
shown although the catalyst also makes appreciable amounts of higher alcohols. The catalyst requires simultaneous presence af the alkali component and the MoS2 component for developing alcohol synthesis activity.
Consistent with the
picture obtained with the Cs/Cu/ZnO catalysts, the Cs/MoS2 catalyst appears to be a combination of basic (CsOH) and hydrogenation (MoS2) components.
The
amount of the alkali compound necessary to develop the maximum activity is significantly larger than that in the Cs/Cu/ZnO catalyet because the alkali compounds agglomerate into ca. 20 nm particles in contact with the low energy non-polar
MoS2 surface (ref. 42), while they a r e molecularly dispersed in a
submonolayer on the polar Cu/ZnO surface (ref. 43). A
methanol
further example of a bifunctional base-hydrogenation has
recently
been
reported
by
Union
Carbide
(ref.
catalyst for 44).
Their
homogeneous catalyst consists of a Cut compound and an alkali methoxide, and the hydrogenation component is believed to be the copper hydride CuH.
The
alkali methoxide may then serve as a base that activates CO by a nucleophilic attack analogous to reaction (9)
CH3@
+ CO
-+
CH30COe (111)
116
followed by hydrogenation of the metallocarboxylate 111. In summary, several new successful synthesis catalysts for methanol synthesis from CO and H2 appear to be bifunctional and consist of a basic component and a hydrogenation component.
The Cu/ZnO/A1203 catalysts appear
to hydrogenate preferentially CO2 under industrial conditions. C-H
(8) -
forming reactions
(10) have
catalysts and s e t s of conditions. mechanistic
One major remaining task is to translate the
input into kinetic equations that
synthesis reactions (1) of
conditions and
-
describe the behavior of t h e
(3) in a wide range of conditions.
CO/H2
All three initial
been found plausible f o r different
For a limited range
synthesis gas only, methanol synthesis has been
modeled as a function of surface C s concentration for the Cs/Cu/ZnO Cs/Cu/ZnO/Cr203 catalysts (ref. 45).
and
The differential equation describing the
cesium concentration dependence of the synthesis is + ko'WeHZn(l-eCs) 1 ( 1 W K e q ) S (16) 2 Find the theoretical curves obtained by the best fit to the experimental methanol
= (kl@CseH ( 1 % ~ )
synthesis rates a t 250OC and 75 atmospheres a t Hz/CO Figure 5.
=
2.33/1 a r e shown in
The key term in equation (16) proportional to BCs(l-BCe)
reflects the
bifunctionality of the catalyst, the rate of activation of CO being proportional to eCs and that of hydrogen to the free Cu/ZnO surface through
2.oq
(l-ecs).
I 0 : CslCulZnOlCr.0,
: CslCulZnO
X "
1.5
0.015
0.03 0.045
C s %/(rn'/g
0.06
0.075
1
of catalyst)
Fig. 5. Correlation of specific methanol activity as a function of normalized cesium surface concentration of Cs/Cu/ZnO (a) and Cs/Cu/ZnO/Cr~03 ( 0 ) catalysts tested at 250°C and 75 atm with Hz/CO = 2.33 synthesis gas at GHSV = 6120 (unsupported) and 10,000 (Cr203-supported) t(STP)/kg catal/hr.
117
THE ALDEHYDIC C1 INTERMEDWTE The synthesis patterns of
higher
alcohols, esters, and amines a r e
consistent with reactions of an aldehydic C1 intermediate as the building block for the
c-c,
C-0, and C-N bond forming reaction.
Formy1 HCO has already been mentioned.
Other forms of an aldehydic
intermediate that have been proposed include n-bonded formaldehyde, its H. C ,which, i f bonded t o a isomer hydroxycarbene, and “dioxymethylene“ H,C,o-, cation(s) is an anion of hydrated formaldehyde H z C ( 0 H ) z .
-
IR spectra in the 2700
3000 cm-1 region have been interpreted a s vibrational transitions of the CH2
group of dioxymethylene (ref. 46) or adsorbed formaldehyde (ref. 47), but the evidence for hydroxycarbene is lacking. A number of chemical trapping reactions provide support for the aldehydic
C 1 intermediate.
RlRZNH
+
CO/H2
Vedage e t al. (ref. 11) utilized the reaction
--$
RiRzNCH3
-
+ H20
(17)
in which the CH3 group of the product amine RlRzNCH3 w a s synthesized via
RlRZNH amine-Ci aldehyde coupling. Deluzarche e t al. (ref. 48) used methyl iodide to trap formyl with the result CH3CHo ( + @&), CH3I + CO/Hz (18) and Young e t al. (ref. 49) used various alcohols and ketones, e.g. a 3
a 3
(32 I + CHOH
I
a 3
CHzOH
I
CH3 \ ,C=O a 3
+ CO/& -+
I
(19)
I
a 2
a 2 I
CHS
to demonstrate that the addition of the C 1 intermediate formed from CO/Hz, occurred preferentially in the fl position of the Cn alcohol or ketone.
Such a
reaction is typical of aldol condensation followed by hydrogenation, with some specific features regarding oxygen retention that a r e discussed in detail below. The high rates with which all of these reactions occur over the copper-based catalysts
under
the
synthesis
conditions
indicate
that
intermediate is a kinetically important reactive species.
the
C1
aldehydic
However, there a r e
differences in the extent of the aldol-type reactions and other pathways for the C-C bond formation over different catalysts as is demonstrated next. C-C BOND FORMING REACTIONS
These reactions give rise to C2+ alcohols, aldehydes, and ketones.
Four
different types of catalysts, Cu/ZnO, Cs/Cu/ZnO, Cs/MoS2, and Cs(K)/Co/MoSz,
118
will be compared here.
Over the copper-based catalysts the C2+ oxygenates a r e
favored by low Hz/CO (1-0.5) ratios and high temperatures (>280OC)(refs. 1, 39). The main products aside from methanol a r e ethanol, I-propanol, and 2-methyli-propanol (isobutanol), and the alkali dopants enhance the rates of the chain growth.
Over the alkali/MoSZ catalysts, C2” oxygenate synthesis has been
demonstrated (ref. 41) a t Hz/CO
1 and temperatures of 250-330°C
to yield
mainly C2+ linear alcohols, and t h e presence of cobalt in the catalyst has been found to greatly enhance t h e methanol hornologation C1-C2
(ref. 50).
Isotope
and mechanistic studies have been conducted with these catalyst systems in the author’s laboratory on the Cl+Cz,
Cz--’C3,
C 3 4 C 4 , and C4+C5
reaction
steps with the following results:
clz2Czr
-
(i) Injection of 13CH30N into t h e CO/H2 synthesis gas stream yields doubly
labeled ethanol over the Cu/ZnO and Cs/Cu/ZnO catalysts (ref. 51) I3CH30H + CO/H2
(Cs)/Cu/ZnO
13CH313CH20H
This w a s interpreted a s the c1-C~
(.t
(21)
CO/Hz).
step occurring by coupling of two C1
aldehydic species by a mechnnism similar to that proposed by Fox et al. (ref. 52).
This outcome (21) rules out any
12C0
insertion
mechanism such as
-
proposed before (refs. 39, 53-55).
(ii) Injection of 13CH30H yields only P-labeled ethanol over the Cs/MoS2 and Cs/Co/MoS2 catalysts (ref. 56), a s represented by equation (22). 13CH30H + l2C0/H2
Cs/(Co)/MoSz
13CH312CH20H
(22)
This outcome (22) is opposite to that (21) observed over the copper-based catalysts and indicates a CO insertion path for linear alcohol growth over the MoS2 catalysts.
This path i s enhanced by the presence of cobalt and accounts
for t h e dominance of linear alcohols over alkali/MoS2 catalysts. (2-2:
Injection of C H Q ~ ~ C H Z Oyields H different isotopic 1-propanols over t h e Cu/ZnO, Cs/Cu/ZnO, and Cs/MoS2 catalysts. (i)
Over Cs/Cu/ZnO catalysts a t high temperatures, path (23) occurs
selectively (refs. 14, 57).
This outcome (23) is consistent with aldol-type &addition with oxygen retention reversal (ref. 14), a s shown in reaction sequence (24).
CH213CHOe + H2CO
-----)
[eOCH2CH213CHO] + % C H Z C H ~ ~ ~ C H ~
*I
@-addition
HOCH$H213CH3 The retention of
the anionic oxygen in the [-OCH2CH13CHO] intermediate is
specific to the C s promoter which prevents the dehydration of the alcoholate oxygen and favors hydrogenation of constitutes
a
reversal
of
the
the free 13CHO group.
normal
aldol
synthesis
Such a path
pattern
in
which
C B ~ C H Z ~ ~ C Hpropanol ~OH would be formed in the presence of hydrogen. (ii) Over alkali/MoS2 and alkali/Co/MoS2 catalysts, the C p C 3 s t e p occurs
-
by the same type linear growth through CO insertion a s in the C1-2
step a s
evidenced by the isotope reaction 13CH3CH2OH + CO/H2
13CH3CHzCHzOH
Cs/(Co)/MoS2
(25)
C . m A :
(i) Over the Cu/ZnO and Cs/Cu/ZnO catalysts, injection of I-propanol yields
dominantly 2-methyl-I-propanol
and 1-butanol a s a minor product, and the C s
promoter enhances t h e rate of t h e @-branching (ref. 49),
minor The dominant 8-addition
to form 2-methyl-3-propanol
occurs via a mechanistic
path analogous to (24) a s indicated by t h e 1% isotope experiments of Nunan et al.
(ref.
57).
This aldol path
with
oxygen retention
reversal
is further
corroborated by t h e outcome of 2-propanol injection into the synthesis gas (ref. 49) which results in the dominance of 1-butanol in the C4 product,
I
CH3
I CH3 enolate
(ii) Over t h e alkali-MoS2 and alkali/Co/MoS2 catalysts, the C F C 4 growth
step occurs mainly b y linear CO insertion, giving rise to the dominance of I-butanol in the C4 product (ref. 56).
120
C . 4_-Y?-C& The patterns of steps C1-4
continue over the different catalysts a s
shown above with the exception that 2-methyl-1-propanol any C5 products over the copper based catalysts.
does not give rise to
The high rate of &addition
a t C3 and the termination of t h e synthesis a t the branched C4 alcohol a r e the major factors determining the high selectivity for 2-methyl-I-propanol. C-0 BOND FORMING REACTIONS New C-0 bonds a r e formed in the CO/H2 synthesis when CO is converted to
C 0 2 by the WGS reaction (3) and in the synthesis of esters. be discussed here.
Only the latter will
Primarily methyl esters are formed, and they a r e significant.
side products over the (Cs)/Cu/ZnO catalysts but not over the alkali/(Co)/MoS2 catalysts.
The mechanism for methyl ester formation has been suggested (ref.
39) to occur via a coupling of a Cn aldehyde with a C1 aldehyde by the Cannizzaro reaction or by a nucleophilic attack of a Cn aldehyde by methoxide (Tischenko reaction).
The exception is the formation of methyl formate that
occurs via a nucleophilic attack of CO by adsorbed methoxide CH30* + CO + cH30
n "
-C - 0
_.)
IP
HCOOCHQ
(28)
The source of protons for reaction (28) could be water, surface hydroxyls, or alcohols.
-
The mechanism of reaction (28) is consistent with the location of the
isotopic label in the reaction l3CH30H + 12CO/H2
H12C0013CH3
(29)
which clearly traces the carbonyl carbon of methyl formate to CO and t h e methyl carbon to methanol (ref. 51).
Theoretical calculations similar to that for
the formate reaction (14) show that the nucleophilic attack (28) is facile but the subsequent methyl transfer that would form the acetate anion CH3COOel an isomer of t h e metallocarboxylate cH3080 of reaction [28], would have to go over
a large energy barrier.
The calculated energy
levels of
the
stable and
transient states of the pathway (28) followed by methyl transfer a r e shown in Figure 6. Unlike in the hydrogen transfer in HCOOe formation by reaction (14) the methyl transfer in Figure 6 is rendered difficult by t h e large barrier, and this explains the observation that the acetate formation is not effective as a C-C bond forming step.
On the other hand, the CH30°
C-0 bond forming step.
+
CO reaction is an effective
Nunan e t al. (ref. 51) further noted that the methyl
formate concentration in the product is below the equilibrium of the reaction CH30H t CO @ HCOOCH3 mi
tnmn-ratiiraa nf 250-2AIWr! and annrnarhaa -niiilihriiim
nt. t.nrnnnrnt.iirnn
(30) >%?IWr!.
121
higher methyl esters, explains the different rates with which methyl formate and higher methyl e s t e r s a r e formed, as will be demonstrated by the kinetic model for t h e overall Cn oxygenate synthesis pattern.
-100
t
W
z
\ /
/
\
/ /
\
>
0 LT
/
\ \
-300
-/
\
\
/
\ \
e
W
/
CH3OCO
-400
\
\-
C H C 0 Oe
-500 REACTION COORDINATE
Fig. 6. MNDO energy diagram for the reaction of carbon monoxide with methoxide to form methyl formate. KINETIC
MODEL
Cs/Cu/ZnO/C-%
FOR
OXYGENATE
SYNTHESIS
OVER
THE
Cs/Cu/ZnO
AND
CATALYSTS
The C2+ oxygenate synthesis has been succesBfully modeled for the Cs/Cu/ZnO/(Cr203) catalysts by Smith et al. (refs. 2, 3) following the pioneering work on modeling of isobutanol synthesis over K/Cu/ZnO/A1203 Smith and Anderson (ref. 1). and
ester synthesis and
catalysts by
The scheme used to account for the c 1 - C ~ alcohol
based
on
the
mechanistic
steps outlined
in
the
preceding section on the C-C and C-0 bond formation is presented in Figure 7. The model has been applied, and the rate constants 1 , @, determined, in the temperature
range of
220-33OOC a t pressures
atmospheres with synthesis gas having H2/CO
and %
of 41-75
ratios 0.5 to 2.3 for Cu/ZnO,
Cs/Cu/ZnO, and Cs/Cu/ZnO/Cr203 catalysts (refs. 2, 3).
A comparison of the
calculated and observed oxygenate product composition over the Cs/Cu/ZnO
=
0.4/30/70 catalyst at 310% is given in Figure 8. The mechanistic features stemming from the rapid aldol-type @-addition, the slow C1-+C2 aldehyde coupling step, and t h e lack of growth of the @-branched alcohols give rise to t h e selectivity pattern in which ethanol is at minimum and
122
COIH? a
-
.1
HCOOC 0 C-OH C-COOC C-C-COOC C-C-C-COOC C-C-C-C-COOC
ao' a,'
ao'
a 4
C-C-OH C-C-C-OH
1l
-
C-C-C-C-OH
I'
C-C-C-C-C-OH
1'
P1'
PI'
P1'
F:
C-C-C-OH
C C-C-C-C-OH
7
C-C-C-C-C-OH
C - C- C - C- C- C - 0 H Fig. 7. Scheme of carbon chain growth to form linear alcohols, branched alcohols, and methyl esters over alkali-promoted Cu/ZnO-based catalysts.
Fig. 8. Predicted oxygenate yields, based upon the reaction scheme shown in Fig. 7 but also including C2 (82) and C3 (83) in addition to C1 (81') carbon addition, compared to measured yields obtained with a 0.4% CsOOCH/Cu/ZnO catalyst at 310OC and 75 atm with H2/CO 0.45 synthesis gas at GHSV = 3265 #(STP)/kg catal/hr. Estimated kinetic parameter values are 8 = 0.254, 81 = 3.299, 81' = 1.138, 82 = 0.121, 83 = 0.152, a = 0.007, and ao' 0.221.
123
methanol
plus
2-methyl-1-propanol
is a t
maximum.
Methyl formate is in
equilibrium (30) with methanol and all the remaining methyl esters a r e accounted for by a single value of KINETIC MODEL
FOR
0~0.
OXYGENATE
SYNTHESIS OVER
THE ALKALI/MoS2
AND
ALKALI/Co/MoS2 CATALYSTS Smith et al. (ref. 4) have modeled the Czt oxygenate synthesis over the MoSz-based
catalysts taking into account the mechanistic features that the
linear growth I #-branching.
now proceeds by CO insertion and is more efficient than
The main kinetic feature is that the C1-C2
s t e p is faster than
(na2) steps particularly in the presence of cobalt in the catalyst.
Cn-vn+i
This kinetic pattern results in product. distribution that can be maximized at ethanol, opposite to that obtained in the synthesis over the copper based catalysts. CONCLUSIONS Methanol and higher oxygenate syntheses follow different mechanistic and kinetic patterns over the various catalysts discussed here.
Each such pattern
is regular, however, and can be modeled with a few kinetic parameters based on fundamental mechanistic steps involved in the C-H, C-C, and C-0 bond forming reactions.
Alkali
co-catalysts
play
an
important
role
by
promoting
base-catalyzed reaction steps that appear important not only in the formation of Czt oxygenates but also in methanol synthesis.
The rapid 8-addition over the
(Cs)/Cu/ZnO catalysts gives rise to the dominance of 2-methyl-1-propanol 1-propanol
in
the
Czt
coproducts of
methanol
synthesis.
Cobalt
and
greatly
enhances the rate of methanol homologation over alkali/MoS2 catalysts by a CO insertion mechanism, and the rapid linear growth particularly a t the (21-2 step over the alkali/Co/MoS~ catalysts gives rise to the dominance of ethanol in the Czt products.
Aldehydic intermediates appear significant in all alcohol and
ester forming reactions over the (Ce)/Cu/ZnO catalysts, while CO insertion appears
to
be
particularly
important
in
the
growth
pattern
over
the
dkali/(Co)/MoS2 catalysts. ACKNOWLEDGEMENTS This work was supported in part by U.S. Department of Energy Contracts DE-FG22-83PC60786,
DE-AC22-84PC70021,
and
National Science Foundation Grant INT-8612603.
DE-AC22-85PC80014
and
by
the
124
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