Methanol to Olefins Class Presentation v2
31
CHE 410: Heterogeneous Catalysis Prepared by: K. Bjorkman, A. Korinda, N. Mashayekhi, D. Petrone, P. Ryan, C. Wilmer and W. C. Wong Presented by: Chris Wilmer June 6, 2008
Transcript of Methanol to Olefins Class Presentation v2
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
Presenter
Presentation Notes
Hello, my name is Chris. This talk is about an up and coming catalytic reaction for the very important methanol-to-olefin synthesis reaction (which, if you don’t know what that is, I’ll explain in the next few slides). These slides were put together by me and my classmates from CHE-410 by Prof. Hayim Abrevaya.
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?
Presenter
Presentation Notes
Before I begin, I’d like to go over a few terms and main ideas behind 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)
Presenter
Presentation Notes
Just what is SAPO-34? SAPO is a abbreviation for silicon aluminum phosphate oxygen, in reference to the more commonly know material, a zeolite, which has just silicon and oxygen atoms. In SAPO, many of the silicon atoms are replaced by aluminum and phosphate atoms.
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
Presenter
Presentation Notes
Just what is SAPO-34? SAPO is a abbreviation for silicon aluminum phosphate oxygen, in reference to the more commonly know material, a zeolite, which has just silicon and oxygen atoms. In SAPO, many of the silicon atoms are replaced by aluminum and phosphate atoms. Ref: Acta Cryst. (1985) C41 1698-1700 slide 4: at the very end you may verbally state that large cage size vs small pore aperture/opening (as is the case with SAPO-34) implies if large molecules are formed inside the cages, then they can not get out.
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.
Presenter
Presentation Notes
Just what is SAPO-34? SAPO is a abbreviation for silicon aluminum phosphate oxygen, in reference to the more commonly know material, a zeolite, which has just silicon and oxygen atoms. In SAPO, many of the silicon atoms are replaced by aluminum and phosphate atoms.
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
Presenter
Presentation Notes
Just what is SAPO-34? SAPO is a abbreviation for silicon aluminum phosphate oxygen, in reference to the more commonly know material, a zeolite, which has just silicon and oxygen atoms. In SAPO, many of the silicon atoms are replaced by aluminum and phosphate atoms.
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
→ +→ +
Presenter
Presentation Notes
My undergraduate degree is not in chemical engineering, and so I sympathize with the few of you in the audience who may not be chemical engineers. Before I started this course, I did not know what an olefin was, and so I’ve included here the definition. Turns out, that it’s an alkene.
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
Presenter
Presentation Notes
Why has it taken over twenty years to get only half-way to understanding this reaction? From the experimental perspective, it is difficult to observe what is going on inside a molecular sieve. From the computational perspective, accuracy requires taking into account a significant volume of the catalyst (several cages as opposed to just one) and sufficient computational power has not existed until recently (REF). From the theoretical/conceptual perspective, the influence of the topology of the pore on the chemistry may be very strong, and hence this is unfamiliar to chemists and chemical engineers from ‘textbook’ chemistry.
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
Presenter
Presentation Notes
Why has it taken over twenty years to get only half-way to understanding this reaction? From the experimental perspective, it is difficult to observe what is going on inside a molecular sieve. From the computational perspective, accuracy requires taking into account a significant volume of the catalyst (several cages as opposed to just one) and sufficient computational power has not existed until recently (REF). From the theoretical/conceptual perspective, the influence of the topology of the pore on the chemistry may be very strong, and hence this is unfamiliar to chemists and chemical engineers from ‘textbook’ chemistry.
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:
Presenter
Presentation Notes
In this presentation, I will try to address these questions, using the latest literature and my opinion (and the opinion of my peers), about the methanol-to-olefin synthesis reaction.
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)
Presenter
Presentation Notes
What is beta? It is also a 3-dimensional molecular sieve but with larger pores. This Beta slide proves that if you feed methylbenzenes you can make ethylene and propylene. You can fortunately do this experiment over Beta, since the pores are large enough to accommodate methylbenzenes but not over SAPO-34 (too small for methylebenzenes). . Finally coming back to your slide 20, state in the slide that this reaction is occurring over 0.3 g catalyst so that the stated volumes of methanol used for various experiments now become more meaningful. Also indicate that temperature is 400C. Furthermore state that these are successive and independent experiments. Indicate that 134 ppm is the peak for methylbenzenes.
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
Presenter
Presentation Notes
These are GC plots taken over SAPO-34.
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
Presenter
Presentation Notes
Propene -> Trimethylcyclohexane -> Trimethylbenzene -> Hexamethylbenzene ->varying side chain alkylations result in ethylbenzene or isopropylbenzene, which then lose ethylene and propylene -> remethylation of methylbenzene by MeOH or DME Weakly reactive on SAPO-34, so it can be used to create ORC
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
Presenter
Presentation Notes
Catalysts sometimes contain aromatics formed through incomplete calcination of templating agents.
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
Presenter
Presentation Notes
Shows one set of results from our efforts to reduce the contributions of organic impurities to the initiation of MTO catalysis. methanol containing only ca. 11 ppm total organic impurities onto highly purified HSAPO-34 at 648 K and measured product yields after each successive pulse. The total yield of volatile hydrocarbons after the first pulse was an astonishingly low 0.0026%,
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
Presenter
Presentation Notes
Coking takes place over strong acid sites, which explains decreased propylene selectivity (TALK MORE ABOUT THIS LATER!!) Correlation for ethylene selectivity over weak acid sites
•
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?
Presenter
Presentation Notes
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 methyl groups is consistent with Beta)
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)
Presenter
Presentation Notes
Emphasize shape selectivity
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
Presenter
Presentation Notes
1,2, 3 : refer to 1: no ethylene, 2: ~3% ethylene, 3: ~4-10% ethylene Co-feed reacts less over time shown by converging selectivity and this also shows a decreasing selectivity over time. Parallel means the co-feed does not react to form the other.
Acknowledgements•
CHE-410 classmates▫
K. Bjorkman, A. Korinda, N. Mashayekhi, D. Petrone, P. Ryan, W. C. Wong
•
Prof. Hayim Abrevaya
