Biomedical Engineering Reference
In-Depth Information
are substances that accelerate the rate of chemical reactions without being consumed. Cata-
lyst has been employed in industry today to selectively speed up desired reactions. The most
common and preferred catalysts are solids as solids can be separated and recovered more
easily from reaction mixture.
It is thus rightly so that the single case of reactions most important in bioprocess engi-
neering application are those which occur on solid surfaces when either an heterogeneous
catalyst is used to promote the rate of reaction (catalysis) or one main reactant is in solid
phase. Solid catalysts are easy to be recovered and reused for carrying out reactions
otherwise involving only fluids (gaseous and/or liquid substances). Solid feedstock and/
or products are also common in bioprocesses where renewable biomass is being converted.
Reactions occurring on surfaces are remarkably similar kinetically. Catalysts for various reac-
tions are found among a wide variety of metals, metal oxides, and metals on various support
materials such as carbon and metal oxides. One property of most catalysts is that they
provide a large amount of surface per unit volume on which reaction can occur, which nor-
mally requires the effective surface to be contained within a porous matrix of some sort. This
particular characteristic leads to a number of interesting and important problems arising
from the interaction of the rates of transport of mass and energy through such porous
matrices, which we shall discuss in detail later.
Common catalytic reactions are
1.
Alkylation and dealkylation reactions. Alkylation is the addition of an alkyl group to an
organic compound. It is commonly carried out with Friedel
e
Crafts catalysts, AlCl
3
along
with trace of HCl. For example,
AlCl
3
CH
3
CH
2
CH
]
CH
2
þ
C
4
H
10
/
C
8
H
18
SiO
2
e
Al
2
O
3
, SiO
2
e
MgO, and montmorillonite clay are common dealkylation catalysts.
2.
Isomerization reactions. Acid-promoted Al
2
O
3
is a catalyst used in conversion of normal
(straight chain) hydrocarbons to branched hydrocarbons. Acid and base catalysts are
both found suited in most isomerization reactions.
3.
Hydrogenation and dehydrogenation. The most ingredients involving hydrogen are
d
-orbital containing metals Co, Ni, Rh, Ru, Os, Pd, Ir, Pt, and metal oxides MoO
2
and
Cr
2
O
3
. Dehydrogenation is favored at high temperatures (
600
C) and hydrogenation is
>
favored at low temperatures. For example,
CH
3
CH
]
CHCH
3
/
CH
2
]
CHCH
]
CH
2
þ
H
2
is catalyzed by calcium nickel phosphate, Cr
2
O
3
, etc.
4.
Oxidation reactions. The transition group (IUPAC group 8, 10, 11) elements are
commonly used. Ag, Cu, Pt, Fe, Ni, their oxides, and V
2
O
5
and MnO
2
are good oxidation
catalysts. For example
Ag
2
C
2
H
4
þ
O
2
!
2
C
2
H
4
O
V
2
O
5
2SO
2
þ
O
2
!
2
SO
3
Search WWH ::
Custom Search