Catalyst synthesis

27 Pages·2007·3.16 MB·English

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„It is in the preparationof catalysts that the chemistis
most likely to revert to type and to employalchemical
methods. From all evidence, it seems that the work
should be approached with humility and supplication,
and the productionof a good catalystreceived with
rejoicing and thanksgiving.“
Murray Raney, Ind. Eng. Chem. 1940, 32, 1199.
Seite 1
Types of Catalysts
Terminology: J. Haber, Pure&Appl.Che. 63, 1227 (1991)
Bulk Catalysts Supported Catalysts
Metal oxides Often noble metals
Metals (skeletal catalysts) Systems which cannot be
dispersed by other methods
Noble metal gauze
number of surface atoms
Dispersion =
total number of atoms
Rule of thumb for specific surface area
with respect to particle size:
¤ 1 µm ˜ 1 m2/g
¤ one magnitude higher/lower particle size corresponds to
one magnitude lower/higher specific surface area
Targets for Catalyst Preparation
(cid:216) Activity
(cid:216) Selectivity
(cid:216) Stability
-coking
-poisoning by reactands
-sintering
-poisoning by impurities
(cid:216) Morphology
(cid:216) Mechanical strength/resistance
(cid:216) Thermal properties
(cid:216) Regeneration
(cid:216) Reproducibility
(cid:216) No patent / patentable
(cid:216) Costs
Seite 2
Bulk Catalysts: Fused Catalysts
(cid:216)Important for preparation of metal alloys for gauzes
(cid:216)High dispersion of elements (in melt)
(cid:216)Ostwaldprocess (Pt/Rh)
(cid:216)Andrussovprocess (Pt or Pt/Rh)
(cid:216)Sulfuric acid catalyst (V O /MS O)
2 5 2 2 7
(cid:216)Typically high costs of energy, process control difficult
Pt/Rh gauzeunactived, partiallyactivated, well- Installation gauzepad
activated, and withRhO crystalson surface
2 3
fromCatalystHandbook, 2nd Ed., M.V. Twygg(ed), Wolfe PublishingLtd., 1989
Bulk Catalysts: Raney Catalysts
(cid:216)Principle: Formation of alloy in a melt, then extraction of one
component
(cid:216)usually: Ni/Al alloy, extraction of Al with alkali hydroxide
solution
(cid:216)Other systems known, technically Raney-Cu is used for the
reduction of organic nitro compounds and nitriles
Seite 3
Bulk Catalysts: Precipitated Catalysts
Source
Crystals
(cid:216) Precipitated Catalysts
Nucleation on
is decisive ntrati Nucleii
nce Nucleation threshold
o
C Saturation concentration
(cid:216) Problems:
Time
(cid:216)Quality varies because of changing
concentrations (simplified!)
continuous precipitation
(cid:216)Inclusion of ions
decomposable couterions, NO-, NH+, oxalate, citrate...
3 4
(cid:216)Local supersaturation
homogeneous precipitation, e.g. with urea
(cid:216)component with low K precipitates first
sp
in co-precipitation
Precipitation at high supersaturationor continiuously
Bulk Catalysts: Precipitated Catalysts
» Nowadays increasing use of organic solvents, e.g. in production
of VPO ((VO)P O) catalysts or Ziegler (zirconocene/MAO/SiO-
2 2 7 2
catalyst) catalysts
» Precipitation of defined precursor compounds followed by
thermal decomposition or transformation into final catalyst
Examples:
Ni/Al O
2 3
Hydrotalcites, such as Ni Al (OH) CO .4 H O, have different
6 2 16 3 2
calcinationand reduction behavior than mixtures of
components due to dispersion of elements on atomic level
Methanol catalyst (Cu/ZnO)
Depending on precipitation conditions many different phases
with strongly varying catalytic properties can be obtained in
dependence of conditions during transformation into catalyst
Seite 4
Bulk Catalysts: Sol-Gel Materials
(cid:216)Sol-Gel process produces typically highly porous materials with high
specific surface areas
(cid:216)Precursors mostly metal alkoxides, but could as well be simply salts
Hydrolysis: M-OR + H O MOH + ROH
2
Condensation: MOH + XO-M M-O-M + XOH X=H,R
(cid:216)Both steps proceed simultaneously, relative reaction rate important
for product properties
if hydrolysis faster than condensation less branched material
if hydrolysis slower than condensation highly branched material
J. Brinker, W. Scherer, Sol-Gel-Science, Academic Press 1990;
J. Livage, M. Henry, C. Sanchez, Progr. Solid St. Chem. 18, 259 (1988)
Sol-Gel Formation
1. Sol Formation
2. Gelation
Advantage: High degree of control over product properties
easy preparation of mixed oxides with atomic
dispersion of elements
Seite 5
Gelation of Silica
Gel Drying / Aerogels
P
critical
liquid point
solid
normal 1
drying
2 supercritical
gas drying
T
Seite 6
Bulk Catalysts:
Hydrothermal Synthesis
(cid:216)Discrimination against precipitation sometimes difficult; typically higher
temperature, often pressure, longer reaction time than for precipitation
(cid:216)Crucial for zeolitesynthesis
(crystalline, microporous aluminosilicates)
Lab autoclaves Stirred autoclave Industrial stirred reactors
50 -250 ml 1.5 -2.5 l 50 -5000 l
Zeolites
Mx+ [Al Si O ] . p H O
m/x m n 2(m+n) 2
Seite 7
Zeolites
LTA (4.1 Å) BEA (6.6 x 6.7 Å)
LTL (7.7 Å)
8MR 12MR
12MR
GIS (4.5 x 3.1)
MFI (5.5 x 5.1 Å)
MOR (7.0 x 6.5 Å)
8MR 10MR 12MR
Zeolite Synthesis
Si4+, Ti4+, salt,
Al3+, B3+, Ga3+ Oxide source oxide
P5+ hydroxide
T = 80 –200°C
p = 1 –50 bar
t = 1h -20 days
Template GEL H O
2 autoclaves
amine,
alkylammonium,
alcohol
heating
seeds Mineralizer OH-, F- unit
temperaturecontrol
sometimes
Seite 8
Bulk Catalysts:
Hydrothermal Synthesis
(cid:216)Discrimination against precipitation sometimes difficult;
typically higher temperature, often pressure, longer reaction
time than for precipitation
(cid:216)Crucial for zeolitesynthesis
(crystalline, microporous aluminosilicates)
(cid:216)Cristallizationfrom silicionand aluminum containing solutions,
mainly under alkaline conditions
(cid:216)minimum Si/Al ratio is 1
(cid:216)Often “template”required (organic additive, e.g.
tetraalkylammoniumions), mainly for high silica zeolites
(cid:216)Many parameters affect zeolitesynthesis
-reagents
- pH
-mixing sequence
-temperature
- heating rate
-concentrations LTA
-template MFI
LindeType A (0.2 -2 µm)
- ...
ZSM-5, Silicalite-1 (150µm)
Bulk Catalysts: Solid State Reactions
(cid:216)Transformation of definiedPrecursors s.o.
(cid:216)also reaction of mixtures of oxides obtained by milling
example: Styrene catalyst
milling of KOH + FeO + CrO , then calcinationat 900 -1300 K
x x
(cid:216)Solid state ion exchange
example: zeolites, 600-700 K)
Seite 9
Bulk Catalysts: Other Methods
(cid:216)Flame hydrolysis (Aerosil...)
(cid:216)Sputter processes
(cid:216)Spezialprocesses for Carbides, Nitrides
(cid:216) ...
TiO
2
http://www.engin.umich.edu/~cre/web_mod/aerosol/frame08_pics/pict.htm
Supported Catalysts: Supports
l g-,h-or a -Al O , SiO , TiO , ZrO , MgO, C
2 3 2 2 2
l Surface functionality typically -OH for Oxides,
oxo-functionality for Carbons
l Oxidic surfaces in elektrolytes generally charged
M-OH M-O- + H+
M-OH + H+ MOH+
2
=> decisive for loading, since no adsorption if wrongly charged!
PZC(Point of zero charge) = particle is not charged at this pH
G.A. Parks, Chem. Rev. 65, 177 (1965)
Rule of thumb: MO PZC > pH 11.5
2
MO 8.5 < PZC < 12.5
M O 6.5 < PZC < 10.4
2 3
MO 0 < PZC < 7.5
2
M O PZC < 0.5
2 5
Seite 10