Conversion of
C
2
& C
3
alkanes to aromatics
Starting points for discussion
•
existing technology (UOP Cyclar
TMprocess)
•
catalysts
reported to be
metal-containing zeolites
•substantial
research reported
for such conversions
•
related
issue topic covered yesterday:
Thermodynamically, conversion of
propane to aromatics
less uphill than
methane to aromatics
reaction
Δ
G
◦
400(kJ/mol)
6 CH
4→
C
6H
6+
9 H
2399
REPORTED CATALYSTS
acidic zeolite (e.g., HZSM-5) or, better,
zeolite-supported metals such as
Zn, Ga,
Mo
(also used for methane conversion)
less than fully characterized
metals possibly present as carbides (or oxycarbides);
not in metallic state
•
catalysts rapidly coked
•
require frequent regeneration
•
regeneration might contribute to
catalyst deactivation
Source: www.uop.com/ cyclar- process- produces- high- quality - aromatic- products
UOP CyclarTM Process, developed by BP & UOP endot hermic
Sabic process operat ing in Saudi Arabia Light gas product s Feed propane
Source of figure: www.uop.com/cyclar-process-produces-high-quality -aromatic-products
SUGGESTED REACTION NETWORK
Numerous authors suggest dehydrogenation catalyzed by
metal-containing function (e.g., molybdenum carbide)—slow reaction—
Feed: propane and/or butane
Aromatic
(benzene, toluene, xylenes)yields said to be
58-60%
H
2product (may be ~5% yield)
Rapid
catalyst
deactivation
requires
“continuous” regeneration,
moving bed reactors
(catalyst residence time presumably of days)
Catalyst lifetime
said to be
not short
Source: www.uop.com/ cyclar- process- produces- high- quality - aromatic- products
Possible opportunities for discovery of improved catalysts
Metal-containing molecular sieves
Active for reactions including (de)hydrogenation
large & growing class of catalytic materials
many structurally nonuniform & less than well characterized
Catalytic performance depends strongly on structure of
metal-containing species
Synthetic routes allow some tuning of structure & catalytic properties
less common routes include
Example: Catalysts made by ALD with
dimethyl zinc
(among others)
Characterized by IR, NMR, XAS, TPR, …..
Reaction at atmospheric pressure, 823 K
Zinc species not simple (not molecular)
Are there good opportunities to make well-defined
species containing metals such as Zn, Ga, &/or Mo in
zeolites?
Single-site catalysts?
Candidate research directions
Vary
metal or combination of metals in molecular sieve
molecular sieve (& pore structure)
Attempt to tailor metal-containing catalytic sites
single sites on/in molecular sieve framework
multi-atom sites (clusters)
Understand chemistry of synthesis & catalysis
Relate catalytic activity, selectivity, stability to structure
(use theory, spectroscopy of functioning catalysts)
Deeper characterization of catalysts
Comparison process: Chevron Aromax
TMFeed: alkanes such as n-hexane, n-heptane
Products: aromatics such as benzene & toluene
Catalyst: Pt clusters in LTL zeolite with exchange ions such as Ba2+ & promoters such as halides
This is naphtha reforming without the acidic function in the catalyst.