CHE 410: Heterogeneous CatalysisPrepared by: K. Bjorkman, A. Korinda, N. Mashayekhi, D. Petrone, P. Ryan, C. Wilmer and W. C. Wong
Presented by: Chris WilmerJune 6, 2008
Methanol-to-olefin synthesis is a commercially valuable process, particularly because of the high-demand for propylene and ethylene. Today these compounds are created mainly through the non-catalytic cracking of naphtha under steam. The methanol-to-olefin (MTO) process, however, uses a novel molecular-sieve catalyst that efficiently converts methanol into propylene and ethylene. The molecular-sieve under consideration is silicon, aluminum, phosphate and oxygen based, and is hence called “SAPO-34”.
Our literature review of the MTO process seeks to investigate: the mechanisms leading to the creation of large organic compounds inside the pores of the catalyst, the role of these organic compounds in the formation of propylene and ethylene, the discrepancy between observed product ratios of propylene and
ethylene and corresponding thermodynamic predictions, and finally, the reactivity of the products under MTO conditions. We discovered that the large organic compounds are typically N-methyl-benzenes formed after a kinetic induction period, and only in the presence of impurities in the methanol feed. The successive methlyation
of N-methyl-benzene is followed by de-ethylation
and de-propylation
to produce ethylene and propylene respectively; the more methyl substituted the benzene is, the higher the selectivity for propylene. Under typical reaction temperatures (~650 K), thermodynamics predicts that the ratio of
propylene to ethylene is between 5-10, but experimentally the ratio is observed to be between 0.5 and 1. Feeding ethylene back
into the catalyst lowers selectivity for ethylene, indicating that it can act as both a product and a reactant. The same holds true
for propylene.
Brief Introduction
•
What is SAPO-34?•
What is the MTO reaction?•
What is interesting about this reaction?
Brief Introduction
•
What is SAPO-34?▫
It is a porous mineral
A crystalline silicon aluminum phosphate molecular sieve (zeolite is a crystalline aluminum silicate)
Brief Introduction
•
What is SAPO-34?▫
It is a porous mineral
A crystalline silicon aluminum phosphate molecular sieve (zeolite is a crystalline aluminum silicate)
▫
The pores are 0.38nm across (about 3-4 atoms)It has a chabazite framework (CHA). It is a 3-dimensional 8-membered-ring molecular sieve. The cage size 0.63 nm
Brief Introduction
•
What is SAPO-34?▫
It is like aluminum-phosphate with some of the phosphorous (P+5) atoms substituted by silicon (S+4) atoms. Hydrogen (H+) are added to balance the framework charge.
Brief Introduction
•
What is SAPO-34?▫
It is like aluminum-phosphate with some of the phosphorous (P+5) atoms substituted by silicon (S+4) atoms. Hydrogen (H+) are added to balance the framework charge
▫
It is a catalyst for the methanol-to-olefin reaction
Brief Introduction
•
What is the methanol-to-olefin (MTO) reaction?▫
In general, it is the conversion of the alcohol, methanol, into a light
alkene
for example:
3 2 4 2
3 3 6 2
2 23 3CH OH C H H OCH OH C H H O
→ +→ +
Brief Introduction
•
What is interesting about this reaction?▫
The cages allow for the trapping of organic molecules which act as organic reaction centers
that catalyze the reaction in cooperation with active sites over the surface of the catalyst. This is in contrast to more typical catalysis that merely involves active sites on the framework
Brief Introduction
•
What is interesting about this reaction?▫
The cages allow for the trapping of organic molecules which act as organic reaction centers
that catalyze the reaction in cooperation with active sites over the surface of the catalyst. This is in contrast to more typical catalysis that merely involves active sites on the framework
▫
The reaction intermediates are long-lived (and this is not typical) to the extent that they can be characterized after being trapped in the catalyst
Presentation Outline
How are the organic reaction centers formed?
How are the reaction products, ethylene and propylene, formed?
How does the observed ratio of propylene to ethylene compare to the thermodynamic prediction?
Are the primary products, ethylene and propylene, reactive under MTO conditions?
We are going to try to answer the following questions:
Question One: How are the organic reaction centers formed?
Light Olefins Are Made from Methylbenzenes as Feed over Zeolite
Beta
•
The yield of olefins and the selectivity to propylene increases with the number of methyl groups on benzene
•
38% of ethylene made by co-
feeding 5 moles 13C-methanol: 1 mole 12C-hexamethylbenzene was 12C, indicating that HMB continued to be source of light olefins even in the presence of methanol
GC-MS of volatile products over Si/Al2=75 Beta at 450°C using various
methylbenzenes as feed
J. Phys. Chem. B 106 (2002) 2294, Sassi
Χ
44
Χ
54
Χ
104
Χ
204
24 25211815
Retention time (min)
13C CP/MAS NMR spectra of products retainedin SAPO-34 after various reaction times, followingeach time, a pulse of methanol (0.053 g/g cat) at 673K
GC analyses of volatile products formedafter 1 and 2 pulses of methanol at 673K
J. Am. Chem. Soc. 122 (2000) 10726, Song
methanolmethoxymethyl-substitutedaromatics 4 sec of reaction
following 1st methanol pulse (0.053 g/g cat) at t=0 sec leads to 14% conversion
4 sec of reactionfollowing 2nd methanolpulse (0.053 g/g cat) at t=360sec leads to 100% conversion
8 Si/100T, ~2 micron crystals
Methylbenzenes Are Suggested to be the Trapped Reaction Centers for MTO over SAPO-34
Haw, J.; Marcus, D. “Well-defined (supra)molecular
structures in zeolite
MTO catalysis”. Top. In Cat. 34. (2005). 41-48.
T=673o
K
The Induction Period during MTO
•
Initial Pulse of unpurified solvent-grade MeOH
–
14% conversion
•
Second pulse after 6 minutes –
nearly 100% conversion with much greater product seen
Proposed Function of Organic Reaction Centers
Haw, J.; Marcus, D. “Well-defined (supra)molecular
structures in zeolite
MTO catalysis”. Top. In Cat. 34. (2005). 41-48.
•
Schematic of how the organic reaction center facilitates the reaction of methanol to ethylene or propylene•
Here, the organic reaction center is formed by the trimerization
of propylene ,
however, organic reaction centers “appear”
even when
only methanol is fed to a fresh catalyst
Input
Output
3 C3 H8
3 C3 H6
3H2 O
3 CH3 OH3 C3 H6
CH3 OH
H2 O C3 H6
C2 H4n=2
n=1
nCH3 OH nH2 O
Output
UOP 4813C-34
Formation of Organic Reaction Centers
•
Hypothetically, the organic reaction centers can be formed from:•
Organic impurities•
In the catalyst•
In the feed
•
Or from methanol
•
Once the initial organic reaction centers are formed, products can create more reaction centers (autocatalytic)
•
Methanol does not react with the rigorously calcined
catalyst at typical operating temperatures.
•
Gas chromatography with a ‘rigorously calcined’
catalyst showed very little MTO conversion
Organic Impurities in Catalyst
Haw, J.; Song, W.; Marcus, D.; Nicholas, J. “The Mechanism of Methanol to Hydrocarbon Catalysis.”
Am. Chem. Soc. 36. (2003). 317-326.
T=648o K
Regularly calcined
Rigorously calcined
The ‘Feed Impurities’
mechanismConcept
•
Organic impurities, that are very difficult to remove from a methanol feed, initiate the formation of organic reaction centers
•
However, large hydrocarbons (such as benzene) cannot come directly from the impurities, because they would not enter the catalyst:
“Aromatics, even benzene, cannot pass through the eight- ring windows that interconnect the cages of HSAPO-34, but
must be prepared in place by ship-in-a-bottle routes from olefins or other precursors.”
-
Sassi, 2002
The ‘Feed Impurities’
mechanismPossible Mechanism
•
Acetone, perhaps present as an impurity (ethanol has also been suggested), is converted to acetic acid inside the catalyst cage, and then to isobutylene.
•
Isobutylene is the first component to get ‘stuck’
in the
cage; it is too big.•
Isobutylene, and possibly 2-butene, trimerize
and
rearrange to form a heptamethylcyclopentenyl
cation:
Question Two: How are the reaction products, ethylene and
propylene, formed?
Product Mechanism Comparison
Ethylene Propylene
Arstad, B., et. al. J. Am. Chem. Soc. (2004) 126: 2991-3001.
∆E=20.8 kcal/mol ∆E=10.8 kcal/mol
•
The more methyl branches exist on the benzene ring, the more energetically favorable is the detachment of an alkene
fragment•
The number of methyl branches has a stronger influence on the reaction energies when forming propylene versus ethylene
∆E=26.4 kcal/mol ∆E=22.5 kcal/mol
Ethylene
•
Few methyl groups on organic reaction center
•
Reaction energy▫
20.8-26.4 kcal/mol•
Occurs over weak acid sites▫
Better selectivity with coking
Propylene
•
Many methyl groups on organic reaction center
•
Reaction energy▫
10.8-22.5 kcal/mol•
Requires stronger acid sites•
Decreasing selectivity with “coking”▫
Reduces strong acid sites▫
Makes pores smaller▫
Napthalenes
rather than benzenes become ORC’s
Arstad, B., et. al. J. Am. Chem. Soc. (2004) 126: 2991-3001.Haw, J. F., et. al. Acc. Chem. Res. (2003) 36: 317-326.Haw et. al. US Patent 2006.
Product Mechanism Comparison
•
Meave
≡
avg. number of methyls
per benzene
•
Increasing space velocity of methanol, increased Meave
and decreased ethylene selectivity
•
Increasing substituted number of methyls
on benzene, Meave
, increases selectivity towards propene
•
Addition of water increased ethylene selectivity
•
When benzenes approximately have 2 methyls, then transition occurs for making more propylene
Weiguo
Song, Hui Fu, and James F. Haw, 2000 Supramolecular
Origins of Product Selectivity for Methanol-to-Olefin Catalysis on HSAPO-34
How are the reaction products formed?
How are the reaction products formed?
•
Fresh catalyst subjected to 13C-
methanol at 400°C
•
Methanol feed then cut off and products retained on the catalyst examined after various durations
•
13C CP/MAS NMR spectra indicate number of methyl groups decrease with time (due to dealkylation)
•
Examination of volatile products show increase in ethylene selectivity (higher ethylene with decreased ethyl groups is consistent with Beta)
Weiguo
Song, Hui Fu, and James F. Haw, 2000 Supramolecular
Origins of Product Selectivity for Methanol-to-Olefin Catalysis on HSAPO-34
Question Three: How does the observed ratio of propylene to
ethylene compare to the thermodynamic prediction?
0
0.5
1
1.5
2
2.5
645 665 685 705 725
Mol
ar ra
tio o
f Eth
ylen
e/Pr
opyl
ene
Temperature (K)
Experimental, SAPO-34, Wilson 1998Thermodynamic Equilbrium
•
From experiment, SAPO-34 is more selective towards ethylene at elevated temperatures
•
Both correlations increase ethylene content with increasing temperature because they are endothermic
•
SAPO-34 has a higher molar ratio because of its selective characteristics described earlier
How does the observed ratio of propylene to ethylene compare to the thermodynamic prediction?
2 C3 H6 <-> 3 C2 H4ΔH°
= 117 kJ/mol (Endothermic)
Question Four: Are the primary products, ethylene and propylene,
reactive under MTO conditions?
Ethylene in Feed Propylene in Feed
Wu, X. Applied Catalysis A. (2001) 218: 241-250.
Product Reactivity
Time on Stream 10 hours Time on Stream 10 hours
•
Selectivity for ethylene decreases with additional ethylene in the feed
•
Indicates that ethylene is being consumed to form other products•
ethylene to propylene
•
Selectivity for propylene decreases with additional propylene in the feed
•
Indicates that propylene is being consumed to form other products•
propylene to ethylene
Acknowledgements•
CHE-410 classmates▫
K. Bjorkman, A. Korinda, N. Mashayekhi, D. Petrone, P. Ryan, W. C. Wong
•
Prof. Hayim Abrevaya