D1. Heterogeneous Catalyzed Polymer Hydrogenation in Oscillating Systems
This project will: develop new techniques for polymer hydrogenation by incorporating
novel reactor designs. One such design will implement oscillating conditions to
address specific limitations of current methods.
Primary Faculty Co-Advisors:
Dr. Kerry Dooley, Chemical Engineering (Heterogeneous Catalysis)
Dr. Carl Knopf, Chemical Engineering (Bubble Column Reactor)
Dr. Dmitri Nikitopoulos, Mechanical Engineering (Two-Phase Flow)
Off-campus Participant: Miguel Baltanas (University of Santa Fe and INCAPE
Research Institute)
Technical Proposal:
Some 70% of the world polymer market is devoted to just four parent polymers:
polystyrene (PS), polyethylene (PE), polypropylene (PP), and polyvinylchloride (PVC).
With the flood of new plastics applications, these four simple polymers cannot satisfy
the need for more specialized materials. Innovative uses demand elastomers that are
stronger, better able to resist chemical attack, possess a wider temperature range of
usage and better environmental stability, and are easy to produce. One way to produce a
derivative with potentially improved attributes is through a post-polymerization
reaction. Hydrogenations are just one type of modification that can “fit the bill” for
creating new plastics. It is convenient because it builds on commercially available
polymers and components, meaning no new investments are required for monomer
production. One such example of a useful polymer formed upon hydrogenation of PS is
polyvinylcyclohexane (PVCH). PVCH has a Tg that is 42 K higher than PS, meaning it
can be used at elevated temperatures where PS would pass into its melt state and
become soft. Another example is hydrogenated nitrile butadiene rubber (HNBR), which
has superior resistance than standard NBR to degradation from prolonged oil contact
and abrasion, even at elevated temperatures.
The goal of this research is to advance the field of polymer hydrogenation by
designing and executing new approaches to the catalysis/reactor design. One of the most
widely studied hydrogenations is the reduction of a diene polymer, such as polybutadiene
(Fig 1).
Fig 1: Polybutadiene
During this reaction, the olefinic double bonds in the parent polymer are
hydrogenated until saturation is reached. The exact end product of hydrogenation largely
depends on the configuration of repeating units in the polymer.
Current hydrogenation techniques are severely limited in their ability to economically
and efficiently produce hydrogenated elastomers. While diimide reduction using a
hydrazide reagent is one way to hydrogenate certain polymers, almost all reactions are
now carried out in the presence of a catalyst to improve reaction rates and make the
process economically viable.
Homogeneous Catalysis
Many polymer hydrogenation reactions are carried out using homogeneous catalysis.
The main advantages of this technique are mild reaction conditions (though not always
the case), and the ability to better realize quantitative hydrogenation. The major
disadvantages of this method are incomplete conversion to saturated polymer and the
inherent difficulty in post-polymerization separation of catalyst from product. Over the
past several decades, most hydrogenation research has been in homogeneous catalysis.
Consequently, each hydrogenation reaction has been studied with many different
transition metal complexes and solvents. The best choices for each respective reaction
are generally understood. In fact, material on homogeneous polymer catalysis
practically reads as a recipe book. Given that homogeneous methods are well known,
the future of hydrogenation research lies in heterogeneous techniques.
Heterogeneous Catalysis
The shortcomings of homogeneous catalysis can be addressed by developing
heterogeneous alternatives. Since heterogeneous catalysts do not in general impose
undue separation requirements, they are clearly the desired choice. Unfortunately,
several problems must be addressed before polymer hydrogenation by heterogeneous
catalysis can reach its potential. The main problems are:
1. Catalyst Selectivity: Obviously, the desired catalyst must selectively catalyze
hydrogenation as opposed to chain scission or side chain hydrodealkylation.
However, at the extreme temperatures and pressures commonly found in current
reactor systems, the catalyst often operates unselectively. For instance, PS
hydrogenation to PVCH may require a temperature around 200ºC and pressures
over 5 MPa while still taking about 3 h to achieve 90% conversion. Most of the
literature does not deal with the selectivity issue directly. Instead, people work
around it by finding alternative solvents or reaction conditions (e.g., low
temperatures and addition of THF solvent) to abate side reactions. Currently, the
only known sure way to limit chain scission is to keep temperatures low.
2. Leaching of Precious Metals: Because precious metals like Pt, Pd, and Ru are
required, solvent leaching can be a serious problem. Constant replacement or
regeneration of the catalysts is typically not economically viable. Therefore, the
solvent/catalyst choice must be carefully screened for any corrosive tendencies.
Despite its importance, most current research does not address the leaching issue.
3. Solvent Choice: Since the polymer must typically be dissolved in solution for the
reaction to take place, the solvent choice is important. Solvents must be relatively
cheap, noncorrosive, cannot contaminate the product, and must adequately
dissolve the polymer reactant. By far the most popular solvents are
decahydronaphthlalene (DHN) and tetrahydronaphthlalene (THN). Solvents
containing oxygen, nitrogen, or sulfur are usually avoided as these tend to be
difficult to separate from product. Certain fluorinated solvents will dissolve many
polymers, but prolonged contact with H2 at high pressures can lead to undesired
HF formation and safety concerns. Gehlsen et al. (1995) successfully conducted
hydrogenation of PS homopolymers to synthesize PVCH derivatives using dilute
cyclohexane solvent. For certain polymers such as PS and poly(α-methylstyrene)
(PαMS), 10% (vol.) THF was added to enhance miscibility and prevent the large
increase in polydispersity that was evident when DHN was used by Xu et al.
(2003). Hucul et al. (1998) suggest that other saturated hydrocarbon solvents will
work, but that cyclohexane is preferred for hydrogenating high molecular weight
aromatic polymers. They do not explain why. Further research could lead to more
discoveries on improved solvents for polymer hydrogenation.
4. High Pressure: One of the most severe problems that limit the application of
heterogeneous catalysts is that high reactor pressures must be maintained.
Pressures greater than 1 or even 10 MPa are not uncommon. For instance,
Gehlsen et al. (1995) performed PS hydrogenation at 3.4 MPa. These high
pressures are designed to overcome the mass transfer limitations encountered
when hydrogen must first diffuse from the bulk gas into the bulk liquid solvent,
then from the bulk solvent to the solid catalyst surface, and then into the catalyst
pores (Fig 2).
Fig2: Three phase mass transfer process
5. Catalyst Activity: There are two main problems related to activity. First, the
catalyst often loses activity at the low temperatures (<350ºC) required to limit chain
scissions. Second, the polymer chains are often entangled into shapes that have difficulty
diffusing in and out of the tiny pore sizes of many “off the shelf” catalysts. In addition to
entanglement phenomena, the relatively high viscosity of concentrated polymer solutions
makes the problem worse. The result is that the polymer is often only exposed to the
outer surface of the catalyst, which severely limits the area for reaction. One way to
overcome this problem was discovered by Hucul et al. (2000). By using Pt on ultra-wide
pore (UWP) silica (Fig 3), Dow created a catalyst that exhibits modest activity for PS
hydrogenation at low temperatures (~150ºC) and relatively high polymer concentrations
(~15-25% polymer by weight) with little to no chain degradation. This study can serve as
a benchmark for the future work proposed below. The activity is mainly limited by the
relatively low surface areas of such catalysts.
Fig 3: TEM (right) and SEM (two on left) of Pt/SiO2 catalyst. Note the black Pt
particles on the 0.1μm resolution TEM
Hucul et al. (2000) performed the reaction in a slurry batch reactor by grinding
the silica into a powder. The final product mixture could then still be separated by
filtration or centrifugation. However, it would ultimately be desirable to eliminate this
step. The Dow method required only a fraction of the time other catalysts needed to
almost completely saturate PS. The UWP catalyst had a surface area of 16.5 m 2/g, a pore
volume of 1.57 cm3/g, and an average pore diameter of 3800 Ǻ. Typically, catalysts with
such large pores are avoided because of the correspondingly low surface areas. It would
be beneficial if larger surface area materials could be used at different conditions. A
specific example is 5 weight% Pd/BaSO4 catalyst with physical characteristics similar to
those used by Xu et al. (2003) and Gehlsen et al. (1995). Ba and Pd can alloy, breaking
up ensembles of sites that can cause chain scission. However, other alloying metals such
as Zn and Cu can be used instead. Gehlsen et al. (1995) hydrogenated PS at 140ºC and
3.4 MPa H2. They were able to saturate PS homopolymers with only minor amounts of
chain scission, although the reactions required >12 h contact times. The reactions of Xu
et al. (2003) took less than 10 h, under conditions similar to Gehlsen et al. (except DHN
solvent vs. 10% THF/cyclohexane for Gehlsen et al.). However, Xu et al. did observe
significant scission as measured by changes in polydispersity.
Rhodium and platinum are generally the most useful active metals for hydrogenation of
polymers, although Pt is a poor catalyst for the hydrogenation of nitrile copolymers.
Much still has to be learned about the relative activities of hydrogenation vs. chain
scission, and about catalyst long-term degradation, in order to see what will really work
economically.
Outline of Proposed Research
High pressures, high temperatures, excess solvent, selectivity and catalyst recovery issues
invariably make the hydrogenation process expensive. In this project we suggest alternate
routes for getting hydrogen to the surface of a heterogeneous catalyst at lower pressures
and temperatures. A particular idea is to make use of a bubble column reactor where
H2/liquid contact takes place at conditions associated with bubble resonance. At
resonance conditions, increases in kla (the product of mass transfer coefficient and
interfacial area) and gas holdup of >600% can take place at <3mm amplitudes in the airwater system (Ma, 2003). The measured values of kla are also significantly higher than
equivalent values for a gas-liquid stirred tank at equivalent power/mass (Dooley, 2004).
See Figure 4 below. The pulsatile flow induces gas bubble breakup by two mechanisms,
one associated with induced shear at a sparger and the other associated with resonance
stabilization of small bubbles in the bulk liquid. Current research in reactor design has
tried to attain such resonant conditions by ultrasound, but not by using the mechanically
attainable (piston/cam arrangement) frequencies (<50 Hz for the air/water system) that
are proposed as part of this project. The reasons why are: (1) lack of understanding of
the existing mass transfer literature on low frequency oscillating two phase flow; (2)
failure to generate large enough amplitudes (at least ~1 mm is needed) with ultrasonic
transducers or horns. However, high amplitude pulsed flows generated by flow
modulation have been shown to greatly enhance reaction rates of gas/liquid reactions as
long as the reactions are gas mass transfer limited (Khadlikar et al., 1999). This is the
case for essentially all catalytic hydrogenations – polymeric or not.
0.03
0.025
0.02
klast k
0.015
klasp k
0.01
0.005
3.32410
4
0
0
0.032
2
4
6
PpM k
8
10
12
11.775
Figure 4. Experimental air-water kla values for oscillating bubble column (klasp), and
equivalent values for a gas-liquid stirred tank (klast), vs. power/mass in m2/s3. The stirred
tank values were computed from the Calderbank correlation.
At this time, the mass transfer/reaction behavior of a two-phase polymer/bubble
column or polymer/catalyst monolith system is not widely understood. Even a dilute
polymer solution will have a higher viscosity than the simple air/water system (~0.4 cP)
currently used in bubble column research. More concentrated polymer solutions or melts
may exhibit large dampening effects on bubble resonance at typical hydrogenation
conditions. Also, the higher viscosity will certainly increase the resistance to mass
transfer, while also increasing gas holdup in the column. This has a two-fold effect. First,
the increased resistance to mass transfer means the hydrogenation reaction is probably
operating entirely in the gas mass transfer-limited regime. In reactor design terminology,
the catalyst “wetting” by H2 is poor. Second, the increased holdup lowers the viscosity of
the system somewhat, possibly leading to a higher interfacial area/volume (a), if a way
such as bubble resonance can be found to decrease bubble size. In the air/water system,
the increased holdup does not have important rheological implications. Instead, the
increased holdup just increases “a”. But for viscous solutions, clearly, the hydrodynamic
properties of the polymer solution in reactor channels and catalyst pores, and how these
are affected by flow instabilities, will have an important impact on the behavior of the
hydrogenation reaction. For high viscosity solutions, foaming may also take place, and
the implications for reactor behavior are not clear at present.
A different route for achieving better mass transfer but in existing processing equipment
might be to conduct such reactions as part of a reactive extrusion. An extruder efficiently
generates high pressures in the melt or concentrated solution, and vigorous mixing, while
not allowing added gas to form isolated domains as is possible in an autoclave or bubble
column. Therefore it could improve gas “wetting” of the catalyst. The extruder might
also need to operate under oscillating conditions of gas flow, because the stoichiometric
requirements of most polymer modification reactions require high gas to liquid
volumetric ratios, and therefore operation in the slug-flow regime, in order to introduce
the gas to the extruder. Such slugging can be controlled by pulsatile flow of either gas or
liquid. The gas must be introduced at the “mixing” (low pressure region) of the extruder
in order to prevent the backflow of polymer. Again, operating at even 30% lower
pressures than current process practice, even with lower catalyst activities, would pay off
economically.
In general, almost all current research is locked into using either stirred tanks or trickle
bed reactors at high pressures, either of which is quite expensive for large-scale use.
While ongoing research for high molecular weight (>100,000) polymer hydrogenation
does not seem too focused on the catalyst wetting problem (probably due to the use of
homogeneous catalysts, or lack of implementation in pilot-scale plants), this is an
important issue for the future that should be addressed.
Overall, current research shows a strong relationship between the solvent/ polymer/
catalyst/ reactor design and how each of these influences polymer hydrogenation. The
Dooley and Knopf labs will work together to address the problems outlined above
associated with heterogeneous hydrogenation. We propose new methods to optimize
polymer/catalyst/H2 contacting based on recent results for application of oscillating flow
to gas-liquid contactors (Ma, 2003; Khadlikar et al., 1999). This will involve work on the
oscillating bubble column first developed in the Knopf lab and on the vented
extruder/torque rheometer and catalyst characterization equipment in the Dooley lab.
Students involved in this project will receive hands on instruction in the related fields of
polymer rheology, two-phase flows involving polymers, catalysis, and reactor design.
Analytical tools such as GC, NMR, FTIR, TGA and BET will also be an integral part of
research. By combining research ideas into a new direction, students will be able to offer
ideas and test concepts that would not be feasible otherwise.
Number of IGERT apprentices to be recruited and probable home departments:
Two, both from Chemical Engineering, or one from Chemical and one from Mechanical
Engineering.
Consistency with the Macromolecular Education, Research & Training theme:
This project requires students to understand catalyzed polymer hydrogenation
reactions and how they can be carried out in actual reactors. It covers the advanced
topic of how well different reactor systems can offer the desired reaction characteristics
and end product. It applies some of the concepts and problems of polymers presented in
the macromolecular classes to real systems, thus making this more of a “hands-on”
project.
How does the project form a vector cross-product of existing research themes by the
participants?
Existing research directions. Dr. Dooley has been researching heterogeneous catalysis
for many years. Recently, he has also researched the incorporation of polymers to
cement for improved processability. Therefore, integrating catalysis and polymers is an
obvious addition to his research. Dr. Knopf is currently working with oscillating bubble
column reactors and oscillating flows in catalyst monolith reactors. The bubble column
represents one possible solution to hydrogenating polymers, with a particulate catalyst.
The monolith reactor represents another, with a fixed bed catalyst. Other reactor
configurations such as the vented extruder are available in Dr. Dooley’s lab.
New research direction. The biggest research gain from taking a team approach comes
from the ability of each team member to specialize in a different area of research
appropriate for his background. For instance, Dr. Knopf’s knowledge of multiphase
transport phenomena is useful in designing potential oscillating reactor systems;
polymer hydrogenation is a logical extension of his past work on mass transfer in these
systems. Dr. Dooley’s experience with heterogeneous catalysis and diffusion in porous
media can be used to prepare the required catalysts and determine operating conditions.
Dr. Nikitopoulos is an expert in two-phase flow. Hopefully, the results of this research
will be applicable to industrial use in the future. Also, the IGERT team will merge ideas
from chemistry and chemical engineering, resulting in a more diversified approach to
this research.
How do students benefit from the team-oriented research, beyond what would be
available to them from either advisor separately?
Polymer hydrogenation represents an excellent application for Dr. Knopf’s oscillatory
flow multiphase contactors. Dr. Knopf’s oscillatory gas-liquid contacting research is
still in its infancy and, while two systems have been constructed and one has been used
for mass transfer and gas holdup studies, neither has been applied to any reactions of
research interest. Polymer hydrogenation would be the first such reaction to be tested in
this new type of contactor. However, this class of reaction could also be a model for
future work in biodiesel hydrotreating and hydrocracking. A student working on the
project would gain invaluable experience because he/she would be able to test the
special characteristics of these reactors on reactions important on an industrial level.
Similarly, by working as a team, students in Dr. Dooley’s group will gain access to the
bubble column and monolith reactor for polymer hydrogenation. The students involved
with the team will also be able to pass ideas between one another, and gain experience
working in a group environment such as at a job. Hopefully, each student will gain
knowledge about the different aspects of the project that they would not have gained if
they worked alone.
Briefly describe the support level available to each individual faculty or off-campus
participant (i.e., without IGERT):
All LSU faculty involved in the project are independently supported for research in
related fields. More specifically, Dr. Dooley and Dr. Knopf have received unrestricted
grants from various corporations for this research. Both professors are well funded and
this guarantees that the graduate students have sufficient resources for research
materials. Two oscillatory multiphase reactor systems (bubble column and catayst
monolith reactor) are already available for this project.
Interdisciplinary strengths of the team project:
Dr. Dooley and Dr. Knopf have different research interests. Dr. Dooley’s specialty is
catalysis, reaction engineering, and high pressure processing. Dr. Knopf works more in
the field of computer aided design, high pressure processing, and multiphase contactors
such as bubble columns. Dr. Nikitopoulos is an expert in two phase flow and is already
working with Dr. Knopf to develop flow regime maps for two-phase oscillating
systems. However, polymer hydrogenation is an area where all of their interests come
together. This is because the project demands innovative catalyst design, novel reactor
schemes, and attention to internals design in order to address the specific difficulties
associated with polymer hydrogenation.
Commitment of faculty & off-campus participants to work side-by-side with
apprentices:
Dr. Dooley, the BASF professor in the Chemical Engineering Department, is
primarily an experimentalist. He frequently meets with students to discuss laboratory
matters and answer questions. He spent the weeks over the winter holiday in the lab
explaining the many pieces of experimental hardware. Included in his hands on
demonstrations were: gas chromatography, surface area measurement,
thermogravimetric analysis, extrusion, typical catalyst minireactors, and vacuum
systems. In addition to learning use of typical lab instrumentation, many other valuable
laboratory skills such as use of fittings, pumps, gas cylinder maintenance, and calcining
furnaces were described.
References:
Dooley, K.M., personal communication, 2004.
Hucul, D.A.; Hahn, S.F. “Catalytic Hydrogenation of Polystyrene”. Adv. Mater., 2000,
12(23), 1855-1858.
Hucul, D.A. “Process for Hydrogenating Aromatic Polymers”. U.S. Patent, 1998, Patent
No. 6,090,359, 1-12.
Gehlsen, M.D.; Weimann, P.A.; Bates, F.S.; Harville, S.; Mays, J.W. “Synthesis and
Characterization of Poly(vinylcyclohexane) Derivatives”. J. Polymer Sci. B: Polymer
Phys., 1995, 33, 1527-1536.
Khadilkar, M.R.; Al-Dahhan, M.H.; Dudukovic, M.P. “Parametric Study of UnsteadyState Flow Modulation in Trickle-bed Reactors”. Chem. Eng. Sci., 1999, 54, 2585-2595.
Liu, W. “Ministructured Catalyst Bed for Gas-Liquid-Solid Multiphase Catalytic
Reaction”. AIChE J, 2002, 48(7), 1519-1531.
McManus, N.T.; Rempel, G.L. “Chemical Modification of Polymers: Catalytic
Hydrogenation and Related Reactions”. J. Macromol. Sci. - Revs. Macromol. Chem.
Phys., 1995, 35, 239-285.
Ma, J. “Forced Bubble Columns”, M.S. Thesis, Louisiana State University, Baton Rouge,
2003.
Mikkola, J.P.; Salmi, T. “Three-phase Catalytic Hydrogenation of Xylose to XylitolProlongling the Catalyst Activity by Means of On-line Ultrasonic Treatment”. Catal.
Today, 2001, 64, 271-277.
Xu, D.; Carbonell, R.G.; Kiserow, D.J.; Roberts, G.W. “Kinetic and Transport Processes
in the Heterogeneous Catalytic Hydrogenation of Polystyrene”. Ind. Eng. Chem. Res.,
2003, 42, 3509-3515.