Abstract
Kinetic studies on vapor phase pyridine synthesis, Zeolites catalyst
synthesis, Reaction feasibility studies, Process standardization, Catalyst
deactivation studies (coking, sintering, poisoning), Product analysis through
GC, Catalyst characterization studies like XRD, IR, TGA/DTA, TPD of
ammonia, SEM, BET surface area, CHNS, Pore size distribution.
Effectiveness factor calculations, Studies on diffusion and Mass transfer
effects, coke kinetic models and Reaction kinetic studies.
Pyridine and its homologues (e.g., picolines and lutidines) are commonly refered
to as pyridine bases. They are widely used as building blocks for synthesis of pharma
and agrochemicals due to their high biological activity. Drugs such as
MethpyrimileneTM (antihistamine), DipyridamolTM (vasodilator), DiodoneTM (contrast
material) and Cetylpyridinium chloride (antiseptic) and agrochemicals, such as
GramoxoneTM (herbicide), DiqurtTM (herbicide)[1-4], OmadineTM (fungicide), and
DursbanTM (insecticide) are produced from pyridine (Fig 1.1). From picolines,
pharmaceutical chemicals such as nicotinic acid amide for vitamin B complexes and
isonicotinic acid hydrazide (tuberculostatic agent), agrochemicals such as TordonTM
(herbicide) and N-serveTM (ammonia-loss prevention from fertilizers) and fine
chemicals such as 2-vinyl pyridine (adhesives for tire cord) are produced (Fig 1.2).
Furthermore, alkyl pyridines are key materials for drugs such as PentazocineTM
(painkiller), OmeprazoleTM (anti-ulcer) and AnginimTM (arteriosclerosis medical agent).
Traditionally, pyridine bases were isolated from coal tar, currently an
economically unattractive method. Ramsay synthesized pyridine in 1876 [5] by passing
a mixture of acetylene and hydrogen cyanide through a red-hot tube. Numerous other
methods for preparing pyridine bases from acyclic molecules (such as carbonyls,
dicarbonyls, dicyano alkanes, alkenes and alkynes) were reported later but only a
limited number of these processes (viz., the condensation of aldehydes with ammonia)
have achieved industrial application.
The investigations reported in chapters 1 to 5, provide the ful range of research
studies made on the catalysis and kinetics of vapor phase aminocyclization of mixed
aldehydes to form pyridine and its bases in a fixed bed reactor. They cover process
feasibility, catalyst selection, characterization and deactivation, mass transfer effects and
reaction kinetics constrained and un constrained by the coke formation and
deactivation.
Apart from providin g vital information on the chemical feasibility o f pyridine
synthesis by the reaction scheme proposed in this work and its equilibrium limitations,
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the thermodynamic analysis presented in chapter 1 has clearly established that (a)
higher pressure has no favorable influence on the reaction (b) no pyridine formation in
non catalytic reaction mode and (c) atractiveness of equilibrium conversions for large
scale production of pyridine from this process. The thermodynamic studies have also
provided vital clues for process rationalization.
A continuous tubular downflow fixed bed reactor has been employed for the
catalyst/process rationalization and kinetic studies as reported in this work. The reactor is
equipped with a w e l designed downstream section to colect gas, vapor and liquid
products and byproducts of the reaction. W e l laid out experimental procedures have
been adopted for catalyst screenin g, process parameter standardization, material
balance checkin g and, time on stream process and catalyst re generation studies. The
material balance investigations have shown the adequacy of analytical tools employed in
this work for product characterization. They have also confirmed the formation of
polymeric side products mainly due to thermal excursions in preheatin g zone of the
fixed bed reactor. The catalyst has no role in its formation. Provision of a separate
vaporizer for aldehydes with appropriate temperature control w i l dampen the
polymeric deposit formation.
The HZSM-5 catalyst, employed in this work, and the coke formed durin g the
reaction have been extensively characterized by employin g sophisticated analytical
techniques like FT-IR, DTA, TGA, XRD, SEM, TPD, CHNS and BET Surface area. The
Major highlight of the catalyst characterization studies is the rationalization o f its
activity through Al+3
ion
concentration management. It has unequivocaly established the
superior performance of HZSM-5 catalyst with Si/Al = 240 with reference process
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conversions and selectivity both under conditions constrained or non-constrained by
catalyst deactivation. Its superior performance on the physical property front is
evidenced by the minimum variations in surface area, structure, pore volume and size
distribution and crystal size inspite o f the catalyst deactivation due to coke formation.
The temperature programmed desorption (TPD) studies with ammonia have shown the
absence of very strong acid sites and the predominance of moderately strong acid sites in
this catalyst. They contribute to relatively beter selectivity and low coke formation
tendency.
The other important observations made in our studies are
1. A slight shift in XRD patterns of coked catalyst hintin g of a partial pore blocka ge.
2. Changes in surface texture and crystalite shape and size as Si/Al of the freshly
prepared catalyst has been varied from 40 to 240.
3. The thermal analysis of coked catalyst indicates that DTA peak temperatures
and coke oxidation losses are inversely proportional to Si/Al ratio.
4. The FT – IR analysis shows that optimization of OH groups on catalyst w i l l be a
key factor for minimizin g coke formation.
The process rationalization studies as reported in chapter 3, have established the
need to maintain the acetaldehyde and formaldehyde feed mix at 1:1, aldehyde to
ammonia ratio at 1:4, reaction temperature in the region of 623 to 648ºC, mole ratio of
water in the aldehyde feed mixture around 3.7% and the space velocity o f the feed at
0.137 hr -1. These studies have shown the importance of multiparameter process
rationalization for the aminocyclization process.
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The results reported in chapter 4 have provided a new dimension to the
understanding of the catalyst deactivation phenomenon due to coke formation in vapor
phase aminocyclization of aldehydes. It is interesting to note (a) the significant increase in
atomic H/C ratio of the coke and (b) variations o f coke compositions as Si/Al ratio of
catalyst is varied from 40 to 240. The modeling of coke formation has been achieved by
employing an exponential catalyst decay function. This model can be used conveniently
for predicting the time on stream behavior of aminocyclization of aldehydes to
pyridine. This function is accordingly used in the rate equation for aldehyde conversion
to pyridine and its bases.
The catalyst re generation studies have shown (a) the efficacy of methanol in
achieving effective aeration a t temperatures as low as 450 ºC within 6 hours. (b) less
crystalinity of coked catalyst as compared to its re generated counter part and (c)
gradual
loss in catalyst activity as the number of catalyst re generation cycles increase.
They have also shown the need to look at movin g or fluid bed reactor options more
seriously for smooth operation on lar ge scale aminocyclization employin g a deactivating
HZSM-5 catalyst.
It is interesting to note the catalytic vapor phase aminocyclization reaction to
produce pyridine has received very litle reaction en gineering atention as evident from the
lone research paper published by Titova et. a l as early as in 1973. The time variant
catalytic activity due to coke formation provides the main complexity in its kinetics
evaluation. After a careful analysis of the HZSM-5 catalyst deactivation behavior, a two
step kinetic evaluation strate gy has been evolved.
The external and internal mass transfer resistances have been evaluated to
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identify appropriative process conditions and catalyst morpholo gy to minimize their
effect on reaction kinetics. The effectiveness factor has been evaluated for various
particle sizes.
A series- paralel reaction scheme covering acetaldehyde and formaldehyde
reaction with ammonia to form pyridine, acetaldehyde and ammonia reaction to form a
mixture of picolines in paralel mode has been considered for the kinetic study. The coke
formation on catalyst from both the above reactions has been lumped with no interaction
assumed between the precursors for its formation. The first phase studies have been
designed to evaluate reaction kinetics unconstrained by the catalyst deactivation based
on experimental measurement of reactant and product concentrations as a function
space time during the early phase of time on stream operation wherein the catalyst
deactivation is negligible. Appropriate rate equations
have been formulated for acetaldehyde conversion to pyridine and picolines. Employing
non linear regression analysis, the fractional conversion have been related to space time.
By differentiation of the resultin g expression with reference to time, the rates of reaction,
reaction velocity constants and activation ener gies have been evaluated. The latter is
found to be half of that reported earlier for the overal conversion of acetaldehyde.
The second phase studies concentrated on the computation of time variant catalyst
decay function `a’ from experimental conversion v s space time data. The catalyst
deactivation function `ØC’ has been separately evaluated from experimental cock
deposition studies and found to confirm that exponential activity decay model. It has been
found that time variant `a’ and `ØC’ profiles perfectly match for the ful range of
acetaldehyde conversions covered in this work. Our studies also show that the catalyst
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activity decay function is non selective for the paralel reactions scheme considered in
this work and can be incorporated into the rate equations for pyridine and picoline
formations to evaluate the concentrations of reactants and products and coke deposition
as a function of time.
6.1 Avenues for future work
Moving and Fluid bed reactor O ptions
The present studies have clearly established the potential o f vapor phase pyridine
synthesis through aminocyclization of aldehydes and ammonia for commercial
application if appropriate is reactor system is chosen along with a catalyst regenerationrecycle methodology. The moving bed concept may be feasible for aminocyclization
since the time interval between successive regeneration is of the order of few hours. As
stated earlier, the decision to transport the solids become a crucial issue either in
moving or fluid bed options since solid motion introduces the attrition and blocka ge
complications. To facilitate insitu re generation and recycle of the catalyst particles, a
fluid bed reactor looks to be a viable option as compared to a fixed bed system. The
other reasons for its choice could be the desire to use smal size particles and high heat
transfer compatibility. The choice of moving bed or fluidization regimes may, however,
place a new set of process demands and limitations on catalyst shape, size, flow ability,
atrition resistance and other morpholo gical parameters. The tailorin g o f the critical
catalyst morpholo gical parameters viz., shape, size and porous structure can offer a
new avenue to catalyst activity and life mana gement. The mechanical transport
properties of catalyst particles need to be closely evaluated and tailored i f they have to
be cycled between the reactor and the re generator. The atrition forces and thermal
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shocks have to receive special atention. The fluid bed reactor studies therefore, assume
urgency.
Development of better catalysts
The present studies have adequately demonstrated the complexities of
aminocyclization reaction system and the sensitivity o f the catalyst to coke formation. A
better catalyst and deeper fundamental understanding on the reaction mechanism are
s t i l needed. Recently, crystaline alumino silicate catalyst with beter thermal stability
surface acidity and pore structure have been receiving research atention. Modification of
HZSM-5 catalyst with metals is another option being examined by basic researchers
to evolve a catalyst system with more acceptable channel structure and architecture.
Tailoring of acidity levels in Zeolite catalyst as per process requirements definitively
deserve an undivided atention. Special distribution o f activity within the catalyst
particle may have specific advanta ges in cases of partial pore blockin g and coke
deposition inside the pores.
Advanced Studies on Catalyst re generation and reuse
Catalyst regeneration and reuse have to be an integral part of a w e l conceived catalyst
management programme in a rapidly deactivatin g reaction system like aminocyclization.
The work related to catalyst re generation and reuse has been briefly reported in open and
patented literature. From economic standpoint, catalyst regeneration is always preferred
over its total replacement after deactivation. However, the success of a re generation
process largely depends on the depth of understandin g of the deactivation phenomenon
and structural integrity of the catalyst system.
In the present work, catalyst re generation is briefly studies to broadly assess the
contours of activation – deactivation – activation cycle.
Advanced scientific investi gations are recommended in the folowin g areas to work
out a viable catalyst regeneration strategy for aminocyclization of aldehydes to pyridine
synthesis.
(a) Coke oxidation mechanism involvin g poly aromatics transformation aldehydes,
ketones, acids, anhydrides etc., and decarboxylation or decarbonylation of
oxygenated compounds.
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(b) Morphological changes including sintering o f zeolite catalyst durin g oxidative
removal of coke.
(c) Removal methodolo gy applicable to intracrystaline coke present in the
deactivated catalyst and the role of chemical additives.
(d) Assessment of the hysterisis in reported reaction – regeneration cycles to work
out a viable catalyst re generation – reuse strategy. Application of micro
calorimetric techniques for quantitative assessment to be explored.
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