Adding Crystals to Minimize Clogging in Continuous Flow Synthesis

Article abstract of DOI:10.1021/acs.cgd.8b00999

For many carbon-carbon and carbon-nitrogen bond-forming reactions, the stoichiometric formation of inorganic salt crystals limits continuous flow synthesis by causing precipitation, agglomeration, and ultimately clogging of the reactor. We describe a new method of mitigating uncontrolled clogging by adding crystals into the reactant stream. Adding seed crystals of the inorganic salt byproduct shifts the crystallization profile from uncontrolled nucleation to controlled heterogeneous growth. Reactions with the addition of high quality crystals show less agglomeration compared to the unseeded reaction. We realize the formation of high quality seeds using multiple methods including filtration and settling. Modeling enables balancing of the reaction rate and deposition of the salt onto seed crystals to prolong reactor operating times without clogging.

Full text of DOI:10.1021/acs.cgd.8b00999

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Article
Adding Crystals to Minimize Clogging in Continuous Flow Synthesis
Gaurav Giri, Lu Yang, Yiming Mo, and Klavs F. Jensen
Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00999 • Publication Date (Web): 27 Nov 2018
Downloaded from http://pubs.acs.org on December 5, 2018
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Adding Crystals to Minimize Clogging in Continuous Flow Synthesis
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Gaurav Giri1,2, Lu Yang1, Yiming Mo1 and Klavs F. Jensen*1
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1 Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, MA
02139
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2 Department of Chemical Engineering, University of Virginia, Charlottesville, VA 22901
Corresponding Author Email Address: kfjensen@mit.edu
Abstract:
For many carbon-carbon and carbon-nitrogen bond-forming reactions, the stoichiometric
formation of inorganic salt crystals limits continuous flow synthesis by causing precipitation,
agglomeration, and ultimately clogging of the reactor. We describe a new method of mitigating
uncontrolled clogging by adding crystals into the reactant stream. Adding seed crystals of the
inorganic salt byproduct shifts the crystallization from uncontrolled nucleation to controlled
heterogeneous growth. Reactions with the addition of high quality crystals show less
agglomeration compared to the unseeded reaction. We realize the formation of high quality seeds
using multiple methods including filtration and settling. Modelling enables balancing of the
reaction rate and deposition of the salt onto seed crystals to prolong reactor operating times without
clogging.
Introduction:
Continuous flow synthesis is well known in production of commodity chemicals, and small scale
analogs are finding increasing use in fine chemicals, pharmaceuticals, and nanomaterials under the
term, flow chemistry. The enhanced heat and mass transfer rates present in small scale continuous
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flow systems enable efficient and safe chemical synthesis.
However, reactions that involve
solids present challenges for continuous flow methods. Solids may be present in the reaction as
reagents, intermediates, byproducts or as the product.8-21 In batch systems, forced convective
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mixing allows the use of solids during reactions. However, lack of convective mixing and the small
reactor cross sectional area in continuous flow systems increases the probability that solids clog
the channel, leading to an early termination of the process. In organic synthesis, numerous
reactions that are pharmaceutically and industrially relevant involve the formation of
stoichiometric amounts of inorganic crystalline salts,12, 22 which precipitate in organic solvents and
ultimately clog small-scale reactors (with millimeter or submillimeter diameters), sometimes
irreversibly. Minimizing clogging will help open up additional opportunities for continuous flow
synthesis.
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Numerous methods have been explored to mitigate solid clogging in microreactors, such as
addition of solvents that dissolve salts, use of pressure pulses, application of ultrasonication, or
changing the synthesis to not create solids.12, 17, 19, 23-25 For example, Buchwald et al. have used a
fluid-fluid biphasic packed bed reactor for C-N cross coupling, where the aqueous portion
dissolved the salt byproduct.23 However, using secondary fluids only to dissolve salts can reduce
the throughput of the reaction, and care has to be taken to ensure adequate mass transfer rates to
the secondary fluid phase. Changing synthetic conditions can also remove the solid formation
issue, but is harder to generalize across different chemistries. The use of different reactors can also
reduce the impact of solid formation.26, 27 For example, the use of miniature CSTR cascades can
be used to realize continuous flow production with solid byproducts.26 Ultrasonication can cause
a rise in the temperature in the reactor, changing the chemical reaction rate.12 Moreover, these
methods require additional processing equipment, and increase the system complexity. In this
work, we approach the issue of clogging from a conceptually and technically different perspective,
namely, that of crystal engineering.
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It is well known that the nucleation and growth of crystals is dependent on the supersaturation of
the molecules or ions, and the classical crystallization mechanism has been verified for KCl salt
crystals.28 The low solubility of KCl in solvents generally used for C-N bond formation reactions
(like tetrahydrofuran) causes a large nucleation rate of KCl crystallites, through homogeneous or
heterogeneous nucleation pathways. However, a competition between nucleation and growth
exists in this system. We hypothesize a proportion of the KCl formed during the reaction can grow
on the KCl seed crystals. This ‘seeding’ technique has been previously utilized to shift
crystallization systems from nucleation to growth in multiple materials, including pharmaceutical
compounds, and inorganic materials.29-32
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The research outlined in this paper involves using this hypothesis to increase reactor lifetime.
Seeded crystal growth to prevent clogging in continuous flow systems has not been explored
before. Instead of avoiding the formation of crystalline solids, we add more solids in the form of
seed crystals to suppress uncontrolled nucleation, and allow for the KCl formed during the reaction
to grow onto the seed crystals. The seed crystals acts as a sink for the inorganic salt produced in
the reaction, resulting in a decreased supersaturation profile, reducing secondary nucleation and
fractal-like crystallites, thereby reducing agglomeration and clogging in the reaction region. We
show the effectiveness of seeded crystallization strategy to reduce clogging during the
pharmaceutically relevant palladium (Pd) catalyzed C-N bond formation reactions. This
effectiveness is shown by both reactor performance, and the morphology of the crystals that are
obtained in typical reactor conditions with and without seeds. Finally, to study the possibility of
whether modelling can be utilized to predict the size and amount of seed crystals that need to be
added into the reactor, a model is developed to balance the salt formation and deposition onto the
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seeds as a function of reaction conditions and seed crystal number and morphology. Experimental
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Section:
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Materials:
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Aniline, 4-chloroanisole, sublimed grade potassium tert-butoxide (KOtBu), potassium chloride
(KCl), and 2-methyltetrahydrofuran (mTHF) (Sigma-Aldrich Corporation) were used as
purchased. Methylated Brett Phos Palladium catalyst (N-Me-BP-Pd) were prepared according to
literature procedures.33
Potassium Chloride Seed Formation:
KCl crystallites with upper size of 35 μm were obtained by grinding KCl powder using a mortar
and pestle, followed by dry sieving using a 35 μm stainless steel mesh sieve (WS Tyler).
Crystallites were suspended in mTHF by continuous stirring at 1600 rpm (Dyla-Dual Hot Plate
Stirrer, VWR International).
KCl crystallites with an upper size of 5 μm were obtained by first grinding the KCl as received,
then dry sieving with a 35 μm stainless steel mesh sieve. These crystallites were then suspended
in mTHF. A 5 μm syringe filter (VWR International) was used to isolate KCl crystallites with an
upper size limit of 5 μm. The crystal stock solution was enriched by evaporating mTHF. The
concentration and average size of the crystal seeds were measured by using a Countess II cell
counter (Beckman Coulter Incorporated).
KCl crystallites with an upper bound of 7 μm were prepared by using the settling method. The
received KCl were ground using a mortar and pestle and then dry sieved with a 35 μm stainless
steel mesh sieve. These crystallites were suspended in mTHF. After stirring the crystallites at 1600
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rpm, the crystals were allowed to settle. The settling time was determined such that the top half of
the 40 mL vial would contain crystallites smaller than 7 μm. The top portion of the suspension was
removed gently without disturbing the settled crystals. The crystal solution was enriched by
evaporating mTHF. The crystal size was measured using scanning electron microscopy (SEM).
Sodium Bromide (NaBr) seed crystals were created and characterized using the same method.
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General procedure for Pd-catalyzed amination reactions:
A 20 mL vial with a PTFE screw top cap (VWR International), equipped with a magnetic stirbar,
was charged with dodecane (51.1 mg, 0.1 mmol), aniline (279.4 mg, 3 mmol), 4-chloroanisole
(513.2 mg, 3.6 mmol) and the stock solution of KCl seed crystals (0-1 mL). This solution was
sparged with Argon for 30 min to ensure removal of dissolved Oxygen. Argon-sparged mTHF was
added to bring the final solution volume to 6.0 mL. In another 20 mL vial with a PTFE screw top
cap, the N-Me-BP-Pd catalyst (27.6 mg, 0.03 mmol) was added along with a magnetic stirbar. This
vial was evacuated and backfilled with Argon a total of 3 times. Reagent solution from the first
vial was added to this vial under positive Argon pressure to make up solution 1. This solution was
connected to the reactor using a flow pump (Asia Flow Pump, Syrris).
An oven dried vial with a PTFE screw top cap was evacuated and backfilled with Argon, and then
charged with KOtBu (0.505 g, 45 mmol). The vessel was evacuated and backfilled with Argon,
and mTHF was added to bring the final volume of the solution to 6 mL (solution 2). After vigorous
mixing, this solution was loaded into a Normject plastic syringe (10 mL) and filtered through a
PTFE membrane filter (0.4 μm porosity, VWR International) into an oven dried 20 mL vial with
a PTFE screw top cap that had been purged with argon. Solution 2 was then loaded into a stainless
steel syringe (8 mL, Harvard Apparatus). This system was connected to the reactor.
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A Teflon reactor (perfluoroalkoxy alkane (PFA), 1/16” OD x 0.04” ID x 3’- 6’ Idex Corporation)
was inserted into a spiral aluminum frame (Proto Labs) and heated to the specified temperature
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(60 - 90 C)on a hot plate (VWR International). All fluidic connects were made using ¼ in.-28
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PTFE fittings. Flowrate and pressures were allowed to stabilize to a steady state (3 reactor
volumes) and a sample was collected for the duration of the experiment. The product stream was
collected under argon, into water or mTHF. An aliquot of the organic layer was mixed in a vial
containing water, KCl and ethyl acetate. The organic layer from this vial was isolated and run in a
gas chromatography (GC) system (6890 Series, Agilent Technologies). Sample yields were
measured by GC analysis of the organic layer, relative to the dodecane internal standard. Mass
spectroscopy of the product showed the product to be 4-methoxy-N-phenylaniline.12
Optical characterization and scanning electronic microscopy (SEM):
Optical characterization of the crystal seeds were performed using the Countess II Cell Counter
(Beckman Coulter Incorporated). 10 μL of the stock solution of KCl crystal seeds were loaded
onto a cell counter chip. SEM characterization of the crystal seeds required loading and drying 100
μL of the crystal seed stock solution as a uniform layer onto 1 cm2 carbon tape attached on a sample
pin. The samples were measured on the Zeiss Merlin High Resolution SEM (Zeiss). To
characterize the KCl byproduct after the reaction, 100 μL of the reaction solution was diluted in
900 μL mTHF, and 100 μL of the diluted solution was loaded onto 1 cm2 carbon tape attached on
a sample pin. Evaporation of Au (5-10 nm) on the samples ensured a conductive layer for ease of
imaging.
Results and Discussion:
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The Pd-catalyzed C-N bond forming reaction is widely pursued in the production of intermediates,
active pharmaceutical ingredients (API), and natural products.34-37 We study the coupling of
aniline and 4-chloroanisole using a Pd catalyst, with the N-Me-Brett Phos ligand, to form 4-
methoxy-N-phenylaniline as the product (Scheme 1).38 This reaction produces a stoichiometric
amount of salt as byproduct, in this case, potassium chloride (KCl). KCl is insoluble in 2-
methyltetrahydrofuran (mTHF), the solvent used to carry out the reaction. The reaction is complete
in minutes, resulting in fast rates of salt formation, which can easily clog microfluidic or
millifluidic systems utilized in continuous flow synthesis.13, 17, 26
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Scheme 1: Pd-catalyzed amination of an aryl chloride with aniline.
A single-phase tube-based continuous flow system is employed to study clogging of this reaction.
In a typical experiment, the reagents aniline and 4-chloroanisole, along with the catalyst N-Me-
BP-Pd, are delivered to the reactor using a syringe pump, and the base KOtBu is delivered from a
stainless steel syringe driven by a syringe pump (Figure 1). The reagent streams are mixed in a T-
junction and are delivered to a heated reactor. The sample is then collected in a vial downstream.
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Figure 1: Experimental setup used to carry out the flow chemistry and clogging experiments.
In our strategy, adding crystal seeds of the salt increases the rate of crystal growth due to the
increased surface area, while having the additional benefit of lowering supersaturation, and thus
lowering the rate of crystal nucleation.39 SEM of a representative product stream after dilution and
drying shows different salt crystal morphologies with and without crystal seeds present in the
reaction. In the control reaction without seeds, fractal-like KCl crystal morphology is observed
(Figure 2a). We hypothesize that the high degree of supersaturation that occurs during the reaction
causes uncontrolled nucleation of numerous KCl crystallites, which then aggregate to form fractal
structures.40, 41 These structures have a larger surface area per unit mass compared to compact KCl
crystals, leading to more inter-particle collisions and interactions. The combination of numerous
secondary crystallites and increased interactions leads to larger agglomerates42, which causes flow
resistance in the continuous flow system, eventually resulting in clogging.
In contrast, the initial seeds added in the reaction stream serve as a sink for the KCl produced
during the reaction, and lead to a shift of the KCl crystal morphology from a fractal-like system to
a more compact structure (Figure 2b). As the KCl growth onto the seed crystals creates relatively
compact structures, aggregation of KCl is reduced compared to the control reaction, even if the
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total mass of KCl produced is the same. Lowered aggregation leads to minimized clogging events
in the small scale reactor. Real-time optical observation of the reactor also shows the different
crystal morphologies with and without the addition of initial crystal seeds (Supplementary Video
1, 2). In the control reaction, collisions between crystallites cause the formation of larger
agglomerates (Figure 2c). In contrast, the reaction with seed crystals show smaller crystals and
aggregated particles, leading to less collisions and agglomeration (Figure 2d).
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Figure 2: Scanning electron microscope (SEM) and in situ optical microscope images
showing KCl crystal morphology from the reaction stream. (a) Fractal-like morphology
observed in reaction stream without crystal seeds. Scale bar is 50 μm. 5-10 nm gold was
evaporated on the substrates to obtain contrast and reduce charging. (b) Compact crystallite
morphology observed in reaction stream with crystal seeds. Scale bar is 50 μm. 5-10 nm gold
was evaporated on the substrates to obtain good contrast. (a),(b) 0.25 M reagents (80 oC, 400
μL flowrate and residence time (tR) = 2 min). (c) In situ optical images show the progression
of agglomeration in the reactor. Image was processed for clarity by enhancing the contrast
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between the solution and the salt crystals, and removing the solvent color. (d) In-situ optical
image of reaction with seeds, no agglomeration is observed. Small crystals are outlined in the
picture. Image was processed for clarity by enhancing the contrast between the solution and
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the salt crystals, and removing the solvent color. (c),(d) 0.25 M reagents (80 C, 200 μL
flowrate and tR = 2 min).
Initially, 35 µm KCl seed crystals are created using dry sieving. These sieved seeds are weighed,
suspended in the solvent, and delivered into the reactor with the base. The effect of increasing the
crystal seed amount on reactor clogging is determined by the pressure increase inside the reactor.
A rapid increase in pressure is indicative of clogging and the interruption of flow. As an initial
study, we perform an acid-base reaction with and without seed crystals, with benzoyl chloride as
the acid and potassium tert-pentoxide as the base (Table 1). Without any seed crystals, the pressure
drop increases rapidly and the reactor clogs within 5 reactor volumes (Supplementary Figure
S1a). Adding 0.05 and 0.10 mg/mL seed crystals extends the reactor lifetime to 8 and 15+ reactor
volumes, respectively. However, when the initial seed loading is increased to 0.2 mg/mL, reactor
clogging occurs within 8 reactor volumes. These data suggest that an optimal seed loading
concentration is necessary to extend reactor lifetime.
KCl Seed Concentration Reactor Volumes Until Clog
(mg/mL)
0
Average
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Std. Dev.
0.3
0.05
0.1
0.2
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7.2
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Table 1: Reactor volumes until clogging occurs during 0.1 M acid-base reaction in a small
scale reactor (at 62.5 µL/min and room temperature, tR = 4 min). Addition of 35 µm KCl
seeds extends reactor lifetime. An optimal amount of seed addition exists at 0.1 mg/mL.
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The Pd-catalyzed amination of aniline and 4-chloroanisole is carried out next to study the effect
of KCl seed loading on reactor clogging behavior on a pharmaceutically relevant reaction, at a
concentration of 0.25 M and 70 oC, at a flowrate of 90 µL/min (Residence time (ΤR) = 4 min). In
the control reaction, without the seeds, the reactor clogs within 4 residence volumes (Table 2,
Supplementary Figure S1b). A seed loading of 0.05 mg/mL is found to extend the lifetime of the
reactor by a factor of 2.5. However, when the seed loading is increased to 0.1 mg/mL, the reactor
clogs within 5 reactor volumes. These results indicate that an optimal seed loading is required to
prevent clogging and extend the reactor lifetime. It is possible that because of the large initial seed
crystal size, a dense initial suspension can promote clogging.
KCl Seed Concentration Reactor Volumes Until Clog
(mg/mL)
0
Average
3.5
Std. Dev.
0.7
0.05
0.1
9.6
4.8
5.3
Table 2: Reactor volumes until clogging occurs during a Pd-catalyzed amination reaction,
with reaction conditions of 90 µL/min and 70 oC. Seeds were 35 µm or smaller. Addition of
35 µm KCl seeds extends reactor lifetime. An optimal amount of seed addition exists at 0.05
mg/mL.
Next, we use smaller seeds so that the KCl formed during the reaction would be presented with a
larger surface area for crystal growth, while keeping the seed mass (and volume) constant. In order
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to reduce the size of seed crystals added into the reagent, we use a 5 µm syringe filter to obtain
seed crystals with an upper size of 5 µm. The mass of the crystallites present in the filtered
suspension cannot be easily measured as it is impossible to resuspend the seed crystallites after
drying the sample completely. Therefore, an optical counting method is used to measure the
concentration of the seed crystallites. Crystals smaller than 2 µm cannot be counted using this
method, leading to an undercounting of seed loading. Using this lower bound of the seed crystal
concentration, we study the effect of the seed crystals on the Pd-catalyzed amination of aniline and
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4-chloroanisole, at a concentration of 0.25 M and 90 C, with a flowrate of 200 µL/min (tR = 2
min). Table 3 and Supplementary Figure S1c shows the result of adding smaller seed crystals.
The control reaction clogs the reactor within 5 reactor volumes, while the reaction with 100,000
seed crystals/mL shows no clogging for the duration of the reaction in all cases. This result further
validates our hypothesis that adding seed crystals causes the KCl formed during reaction to grow
compactly on the seed crystals, resulting in fewer clogging events. However, syringe filtration of
KCl crystals is a slow and cumbersome process and cannot be easily scaled for industrial
production.
KCl Seed Concentration Reactor Volumes Until Clog
(Crystals/mL)
0
Average
5.0
Std. Dev.
0.6
Control
Filtration
Settling
1x105
14.4*
12.2
5.3
9.3x106
4.0
Table 3: Reactor volumes until clogging occurs during a Pd-catalyzed amination reaction at
reaction conditions of 200 µL/min and 80 oC. Seeds were 5 µm or smaller when isolated by
filtration and counted using optical techniques, or 7 µm or smaller, when isolated by settling
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and counted using scanning electron microscopy. *No clogging occurred when the filtered
seeds were used, and the reaction was run until starting materials were used completely.
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Settling is an easier method of creating seed crystals of a similar size range as the filtered crystals.
After dry sieving the crystallites with a mesh filter of 35 µm, the KCl seeds are suspended in the
solvent, and allowed to settle. The settling velocity of different sized KCl crystallites can be
calculated as
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(1)
Here w is the settling velocity, g is gravitational acceleration, ρp is the KCl density, ρ is the solvent
density, Dp is the KCl seed crystal diameter, and μ is the solvent viscosity. Crystallites smaller than
7 µm are obtained and concentrated by drying the mTHF using an argon stream. The seed crystal
size distribution with a lower limit of 100 nm is measured using scanning electron microscopy
(SEM) (Figure 3b). The size distribution of the KCl seed crystals show the majority of the
crystallites are smaller than 1 µm , below the optical counting limit of conventional optics based
particle counters (Supplementary Figure S2).
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Figure 3: (a) Pressure drop data showing clogging behavior. Reaction with < 7 µm sized
seeds, 9.3x106 seeds/mL, counted using SEM (at 200 µL/min and 80 oC). The mixing region
before the reagents reach the reactor are sonicated to ensure that seeds do not settle at the
start of the reaction. However, clogging occurs at 20 reactor volume (80 min) in the system
without reaction. Legend: -■- Control, -▲- Control with sonication, -♦- 9.3x106 Seed
crystals/mL, -●- Reactants without catalyst (i.e., no reaction). (b) SEM images of KCl seed
crystals obtained by using the settling method. Scale bar is 2 µm.
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Model predictions provide insight into the impact of different seed crystals on the surface area
available for KCl deposition, and the possible maximum amount of KCl deposition onto the
crystals (Figure 4). Dry sieved crystals have a larger initial size (< 35 µm), which results in a low
count of seeds (~ 600 crystals/mL) that can be added into the reagent stream before the seeds
promote faster clogging, which also means a low initial surface area for KCl deposition
(Supplementary Table S1). The low initial surface area and the low crystal count limits the total
KCl that can be deposited onto the seed crystals, as the crystal deposition rate scales linearly with
both crystal count and surface area.
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Figure 4: (a) Seed crystal surface area evolution assuming 0.1 µm/s growth rate. (b) Potential
KCl deposition possible on seed crystals assuming 0.1 µm/s crystal growth rate. Legend: □
Dry Sieved Seeds, ◊ Filtered Seeds, Δ Settled Seeds.
When the seed crystals are created using filtration and are counted optically, the initial surface area
available for KCl deposition increases by a factor of three (Supplementary Table S1). More
importantly, ~100,000 crystals/mL filtered seed crystals can be added without clogging, instead of
~1,000 crystals/mL of dry sieved seed crystals. However, if the reaction is completed in a short
residence time (tR = 2 min), only 25 % of the KCl produced during a 0.25 M reaction can be
deposited onto the seed crystals due to a lack of surface area.
If the settling method is used and the seed crystals are accurately counted using SEM, the seed
crystal concentration can be increased to 6 - 10 million crystals/mL, with an average initial size of
800 nm. The full amount of KCl formed during the reaction can be potentially deposited onto these
seed crystals, even at short residence times (tR = 2 min). The relation between surface area present
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for crystal growth and the uptake potential of the seed crystals in the 3 methods of seed preparation
is represented in Figure 4, assuming a 0.1 µm/s KCl crystal growth rate. This growth rate was
chosen because it theoretically yields crystals of similar size to the final crystallites observed in
SEM images.
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The seed crystals obtained through settling are added to the Pd-catalyzed amination of aniline and
o
4-chloroanisole at a concentration of 0.25 M and 80 C, with a flowrate of 200 µL/min (tR = 4
min). Figure 3 shows that with ~9×106 seed crystals/mL clogging occurs at 17 reactor volumes
(68 min), while the control systems without or with sonication at the mixing region clogs at 5 and
10 reactor volumes, respectively. Importantly, the clogging of the seeded reactor occurs at the
mixing region, and not the reactor itself. To explore why the mixing region clogs, a control was
run without any seeds or catalyst. This reaction stream still clogs at the mixing region within 20
reactor volumes (Figure 3). X-ray diffraction (XRD) of the unreacted, mixed reagent stream
showed the existence of potassium hydroxide, which indicates that there is water present in the
system that reacts with the potassium tert-butoxide and forms KOH in the mixer (Supplementary
Figure S3).
We hypothesize that without seed addition, the supersaturation profile that develops during the
reaction causes the formation of numerous small crystallites, due to the formation of large amounts
of the byproduct in a solvent with a low solubility. Due to the rapid formation of these crystallites,
we observe dendritic growth. These uncontrolled crystal nucleation events give rise to a non-
compact morphology. Although both compact and non-compact crystal morphologies
(Supplementary Figure S4) are created in the unseeded reactor, we hypothesize that the larger
surface area of the dendritic morphologies enable interactions between separate particles, and
allow for the formation of larger multi-crystalline aggregates.43 These aggregates, in turn, have a
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slower velocity compared to the surrounding fluid due to their mass, enabling more particle
aggregation until a clog is created in the reactor.
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On the other hand, when an optimized amount of crystalline seeds are added into the reactor, the
crystal surface allows for crystal growth, instead of just nucleation of numerous crystallites as in
the unseeded reactor. As the nucleation rate is a strong function of supersaturation, a small decrease
in supersaturation can have a strong impact on the nucleation rate.44 Thus, the reduction of new
crystallites and the growth of seeds, lead to the formation of more compact structures than the
dendritic aggregates seen in the unseeded case. The compact structures reduce clogging events, as
multi particle aggregation becomes less likely.
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The high concentrations of salt produced during the reaction allows for roughened growth on the
seed surface. Thus, it is possible for these surface to serve as secondary nucleation sites as well.
However, we show that even less compact seed morphologies, like that of NaBr (Supplementary
Figure S6), can reduce the clogging issues when a NaBr producing C-N bond formation reaction
is used (Supplementary Figure S7). Therefore, we also show that the seed crystal method is
effective on other Pd-catalyzed amination reactions that produce chemically different salt
byproducts such as NaBr (Supplementary Figures S5-S7).
A priori prediction of the optimum concentration is difficult, as many factors can influence
the crystallization kinetics, including the reactor type, temperature, flow rate, crystal structure, and
any morphological modification that the reactants or products/byproducts may cause. However,
the modelling performed here balances the formation and deposition rates for a given reaction and
byproduct, and can thus be used to optimize seed size and loadings for different concentrations
and temperatures for the same reaction.
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The kinetics of KCl production and deposition on seed crystals were modelled to obtain insights
into the clogging behavior. The KCl production rate was quantified based on experiments
(Supplementary Figure S8). The KCl deposition rate is the rate at which the produced KCl is
absorbed onto the surface of the seed crystals, modeled by assuming a linear growth rate of the
KCl crystals (0.1 µm/s) at full KCl saturation (more details in S1 of Supplementary Information).
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Figure 5a shows the relationship between the KCl production rate and the deposition rate during
the residence time of the fluid in the reactor. The KCl production rate decreases over time, as
reactants are consumed. On the other hand, the KCl deposition rate increases over time, as the seed
crystals grow larger (leading to a larger surface area), while the linear growth rate remains
constant. This creates a temporal mismatch between the production and deposition rate, and the
difference leads to an initial accumulation of KCl in the reactor. The KCl accumulation peaks at
25 seconds, when KCl production rate equals the deposition rate (Figure 5b,c). Beyond 25
seconds, the excess amount of KCl gradually dwindles as the KCl deposition rate outweighs the
KCl generation rate, until the accumulated KCl is completely consumed at 60 seconds. After 60
seconds, the KCl deposition rate should simply match the KCl generation rate, so that the
incremental amount of KCl generated at this point is instantaneously absorbed by the seed crystals.
In summary, before 60 seconds, the KCl deposition is the rate-limiting step for crystal growth, and
after 60 seconds, the KCl generation becomes the rate-limiting step for crystal growth.
Our simulation results also indicate how the seed crystals can be utilized to achieve better results
to avoid clogging. Namely, it is best to match the KCl production rates with the KCl deposition
rates so that the amount of excess KCl built up in the interim is minimized. Rate matching can be
accomplished by several methods: (1) We could slow down the reaction rate, but it is not advisable
to do so as the production of product is stoichiometric with the KCl production; (2) We could
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increase the total amount of seed crystals, but there is also an upper limit, as too much seed crystals
alone can cause clogging; (3) It is also possible to optimize the seed crystal addition by injecting
progressively into the reaction stream at different concentrations; (4) Finally, external additives
that limit crystal aggregation can be investigated.45
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Figure 5: (a) Modelling KCl production rate and deposition rate. Production rate matches
the deposition rate at 25 s. Legend: -◊- KCl production rate, -□- KCl deposition rate (b)
Difference of production and deposition rates. All accumulated KCl can be deposited on the
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crystals by 60 s. (c) KCl accumulation in the reactor based on the production and deposition
rates.
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Conclusion:
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In this work, we show that crystal growth control in flow chemistry could extend the continuous
flow reactor lifetime compared to the unseeded reactors. Preliminary experiments with an acid
base salt formation reaction showed extended reactor lifetimes without clogging. For the C-N bond
forming reaction, as higher reagent concentrations were desired, larger seed crystals surface area
was required while still minimizing the total volume of seed crystals added. In order to do this,
better techniques of obtaining smaller seed crystals (sieving and settling) as well as better
techniques for counting seeds (SEM based particle counting) were utilized.
It was observed that reactions that form stoichiometric amounts of solids could have their crystal
morphology shifted to a compact form instead of fractal-like agglomerates. Different methods of
seed crystal formation and seed crystal counting were explored. Settling the crystal seeds and
counting using SEM were most useful in measuring seed crystal concentration accurately.
Theoretical calculations on solid production and deposition rates showed that clogging could be
mitigated by the use of smaller crystals, such that each crystallite could grow to have a larger
surface area without an initial increase in solid mass, or by slowing the reaction rate such that the
crystals could grow to have a larger surface area for KCl deposition.
Ultrasonication is a more general method of limiting clogging, since it can be utilized when non-
crystalline materials (such as polymers) cause clogging, as well as when multiple crystalline
byproducts are created within one reactor. However, ultrasonication based de-clogging systems
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have their own difficulty. In particular, continuous sonication has been shown to change the
kinetics of reactions due to localized temperature increases.12
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The proposed crystal engineering technique can work in conjunction with sonication techniques
to extend reactor lifetimes. As clogging is most likely to occur in narrow junctions, it is possible
to sonicate bends and narrowed regions of the reactor to ensure no clogging occurs.46 On the other
hand, over long, continuous reactors of constant diameter, it is less feasible to sonicate the entire
reactor. However, as crystallization and aggregation is a stochastic mechanism, there is a
possibility of clogging occurring in this region. We envision the crystal engineering technique
described here to reduce clogging in this reactor region.
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Crystal engineering can also be useful if further modification of the byproduct or the product is
necessary. For example, the concepts developed here can be utilized to crystallize the product
itself, providing an easy in-line purification and separation technique. With the ease of forming
and adding seed crystals, this method could become a tool alongside sonication, oscillating flows,
and pressure pulses to mitigate clogging effects of salt forming reactions in microfluidic and small-
scale reactors.
Acknowledgement
This work was supported by the Novartis-MIT Center for continuous manufacturing
Conflict of Interest Disclosure:
The authors declare no competing financial interest.
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Supplementary information description:
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S1. Methodology of KCl Simulation
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Figure S1: Pressure drop traces showing clogging behavior.
Figure S2: Crystal size distribution of settled seeds measured using SEM.
Figure S3: X-ray diffraction of reagents and products.
Figure S4: SEM of unseeded reactor final crystal morphology.
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Figure S5: Second Pd-catalyzed C-N bond formation reaction scheme with sodium bromide as
salt byproduct.
Figure S6: Sodium bromide (NaBr) seed crystals made through sieving, imaged using SEM.
Figure S7: Pressure drop for NaBr producing reaction.
Figure S8: Conversion of reaction used to simulate reaction rate.
Table S1: Seed crystal characteristics.
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Crystal Growth & Design
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Adding seed crystals of crystalline byproducts during a continuous flow synthesis promotes the
growth of compact, high quality crystals instead of agglomerates. These compact crystals are less
likely to clog the reactor, extending the reactor lifetime for the carbon-nitrogen bond forming
reaction.
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