High-Productivity Continuous Conversion of Glucose
to α-Hydroxy Esters over Postsynthetic and Hydrothermal
Sn-Beta Catalysts
The retro-aldol fragmentation of glucose is a complex reaction of industrial relevance, which provides a potentially sustainable route for the production of α-hydroxyester compounds of relevance to the green polymer industry, such as methyl lactate and methyl vinyl glycolate. Although the zeolite catalyst, Sn-Beta, has shown itself to be a promising catalyst for this process, important information concerning the stability of the catalyst during continuous operation is not yet known, and improvements to its yield of retro-aldol products are also essential. Here, we perform detailed spectroscopic studies of a selection of Sn-Beta catalysts and evaluate their performances for the retro-aldol fragmentation of glucose under continuous processing conditions, with the dual aims of developing new structure–activity–lifetime relationships for the reaction and maximizing the productivity and selectivity of the process. Kinetic studies are performed under both established reaction conditions and in the presence of additional promoters, including water and alkali salts. Generally, this study demonstrates that the reaction conditions and choice of catalyst cannot be optimized in isolation, since each catalyst explored in this study responds differently to each particular process perturbation. However, by evaluating each type of the Sn-Beta catalyst under each set of reaction conditions, we reveal that postsynthetic Sn-Beta catalysts exhibit the best levels of performance when activity, selectivity, and stability are taken into account. Specifically, the best levels of performance are obtained with a postsynthetic Sn-Beta catalyst that is preactivated in a flow of methanol prior to reaction, which provides α-hydroxyester yields over 90% at the early stages of continuous operation and operates at high yield and selectivity for over 60 h on stream. Space–time-yields over two orders of magnitude higher than any previously reported for this reaction are achieved, setting a new benchmark in terms of the retro-aldol fragmentation of glucose.
Introduction<!>Catalyst Synthesis and
Characterization<!><!>Catalyst Synthesis and
Characterization<!><!>Catalyst Synthesis and
Characterization<!><!>Catalyst Synthesis and
Characterization<!><!>Glucose Isomerization<!><!>Glucose Isomerization<!><!>Glucose Isomerization<!>Glucose Upgrading to α-Hydroxy Esters<!><!>Glucose Upgrading to α-Hydroxy Esters<!><!>Glucose Upgrading to α-Hydroxy Esters<!><!>Glucose Upgrading to α-Hydroxy Esters<!><!>Glucose Upgrading to α-Hydroxy Esters<!><!>Glucose Upgrading to α-Hydroxy Esters<!>Conclusions<!>Catalyst Synthesis<!>Kinetic Studies<!>Catalyst Characterization<!>
<p>Climate change and
the depletion of nonrenewable resources are
increasing the need to replace common fossil feedstock with more sustainable
alternatives.1,2 In this light, the chemical industry
is increasingly directing its attention toward biomass as a renewable
source of carbon for the production of chemical commodities.3−5 Although several approaches are undergoing intense study, a major
challenge is to develop methods to catalytically convert abundant
monosaccharide sugars into useful chemicals. Given that glucose is
the cheapest and most abundant monosaccharide sugar that can be gathered
from agricultural waste biomass, its use as a feedstock is of particular
interest.6,7</p><p>One of the most interesting catalysts
for glucose upgrading is
the Lewis acidic zeolite, Sn-Beta.8−12 At a relatively low temperature (90–110 °C),
Sn-Beta is known to catalyze the isomerization of glucose to fructose,
which is an important reaction in the food industry and for future
biorefineries.9,13,14 At higher temperatures (150 °C and above), Sn-Beta is known
to promote the retro-aldol fragmentation of mono- and di-saccharides
to α-hydroxy ester compounds, including methyl lactate (ML)
and methyl vinyl glycolate (MVG).15,16 These α-hydroxy
ester compounds have attracted significant industrial interest due
to their potential as monomers and precursors for several surfactants.17−19</p><p>However, several aspects of the retro-aldol fragmentation
of monosaccharides
are yet to be fully understood or optimized. For example, in previous
studies, the monosaccharide fructose has primarily been used as the
substrate, due to its higher solubility and reactivity compared to
glucose.20 Hence, these reports do not
directly address the retro-aldol conversion of glucose, which is the
most abundant and cheapest monosaccharide available for future biorefining.16,21 Furthermore, although studies have shown how the selectivity of
Sn-Beta for retro-aldol product formation can be enhanced by addition
of alkali salts to the system, combined yields of ML and MVG have
rarely exceeded 60% when monosaccharides are used as substrates, resulting
in an undesirable loss of valuable feedstock.12,18−22 Finally, previous studies focusing on the retro-aldol conversion
of glucose have been performed at dilute catalyst concentrations in
batch reactors, which is not conducive for achieving high levels of
reactor productivity and does not provide essential information on
the stability of the catalyst, which is one of the most important
catalyst performance indicators.23−26 In regard to this latter point,
although a small number of studies have recently started to address
the continuous performance of Sn-Beta for retro-aldol product formation,
several specific hurdles remain. For example, although Zhang et al.27 recently addressed the continuous conversion
of glucose to ML in a continuous flow reactor, their study was performed
at very low weight hourly space velocities (eq 1, WHSV, 0.1224 h–1), which
results in low volumetric productivities being achieved and could
mask intrinsic deactivation of the catalyst by operating in an "excess
catalyst" regime.24,25 Likewise, although
we recently demonstrated how the addition of small amounts of water
to the feed could increase the productivity and stability of Sn-Beta
for retro-aldol product formation during continuous operation, the
beneficial role of water was only demonstrated when using fructose
as the substrate at a WHSV of 0.45 h–1.16,26 As such, the intrinsic stability of Sn-Beta for the continuous retro-aldol
fragmentation of glucose to α-hydroxyesters is not yet known,
and high-productivity performance has not yet been established.1</p><p>In addition to developments
from the process side, further developments
are also necessary for the catalyst preparation side. Generally, the
hydrothermal synthesis of Sn-Beta is known to generate an active and
selective catalyst for retro-aldol fragmentation, due to the particular
active site speciation and the defect-free nature of its hydrophobic
framework.28−30 However, despite its typically excellent levels of
performance, the synthesis of this material has several drawbacks
that complicate its industrial use.31−33 Therefore, many studies
have focused on the synthesis of Sn-Beta materials made by alternative,
postsynthetic procedures.8,32−38 In these, a commercially available parent aluminosilicate zeolite
is converted into Sn-Beta by removal of the original aluminum atoms,
followed by reinsertion of Sn in a second synthesis step. Although
several breakthroughs have been made in this regard, postsynthetic
Sn-Beta materials have rarely exceeded the performance of hydrothermal
Sn-Beta.28,29 Consequently, the synthesis of better performing
postsynthetic Sn-Beta materials and evaluation of their performances
for retro-aldol fragmentation under continuous processing conditions
are important targets.</p><p>Recently, we reported how the performance
of postsynthetic Sn-Beta
materials prepared by solid-state incorporation (SSI) could be improved
by "preactivation" prior to reaction.39 Preactivation involved treatment of the synthesized catalyst
in a flow of methanol at 110 °C for 0.5 h, followed by calcination
in air prior to operation. This procedure resulted in substantial
improvements to the activity and stability of Sn-Beta for the isomerization
of glucose to fructose.38 In particular,
the activity of the catalyst was found to increase by a factor of
two following preactivation, and its stability was also found to be
enhanced by this procedure, making this catalyst a promising postsynthetic
alternative to hydrothermal Sn-Beta.</p><p>Herein, we fully interrogate
the catalytic performance of preactivated
Sn-Beta and compare its properties and performance to both its postsynthetic
parent material, i.e., Sn-Beta prepared by classical SSI, and also
to hydrothermally prepared Sn-Beta, for glucose isomerization and
the retro-aldol fragmentation of glucose to ML and MVG. The properties
of each catalyst are investigated by means of 119Sn MAS
NMR, diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS),
vapor sorption, and porosimetry. The performance of each catalyst
is evaluated in the absence and presence of different activity promoters,
such as alkali and water, to fully evaluate the performance of each
material, while aiding the development of structure–activity
correlations.12,16,21,22 To better represent industrial operational
conditions while also providing important details regarding the stability
of each catalyst, kinetic testing is primarily performed in the continuous
regime in plug flow reactors (PFRs).23−25,39−41 In addition to facilitating the understanding of
the properties and performance of preactivated Sn-Beta, the second
aim of this research is to improve the productivity, selectivity,
and stability of the retro-aldol process starting from glucose, as
opposed to more reactive monosaccharides, such as fructose. Through
this research, we achieve space–time-yields that are over two
orders of magnitude higher than previously reported in the open literature,
providing a new benchmark in terms of Sn-Beta-catalyzed retro-aldol
fragmentation reactions.</p><!><p>Three Sn-Beta
samples were prepared by various synthetic procedures. First, a sample
of Sn-Beta was prepared by classical hydrothermal synthesis (henceforth
denoted as Sn-Beta HDT), using fluoride as the mineralizing agent
and tetraethylammonium hydroxide as the organic structure directing
agent.41 Alongside this, a sample of postsynthetic
Sn-Beta was prepared by the SSI of tin(II) acetate into the vacant
lattice sites of dealuminated zeolite Beta (original Si/Al = 19, dealuminated
in hot HNO3 solution, see the Experimental
Details Section for details).28,32 Half of the
postsynthetic sample was subsequently preactivated according to our
recently identified procedure, which involves treatment in a flow
of methanol for 0.5 h at 110 °C, followed by calcination in air
at 550 °C for 3 h.38 These samples
were denoted as Sn-Beta SSI and Sn-Beta PA, respectively, as summarized
in Scheme 1. All samples
were prepared with a final Sn loading of 1 wt % Sn, corresponding
to an Si/Sn molar ratio of 200 in the catalyst (Table 1).</p><!><p>Surface area determined
from nitrogen
adsorption using the BET equation.</p><p>Micropore volume determined by the
t-plot method.</p><p>Determined
by ICP-MS measurements.
All other experimental details are provided in the Experimental Details Section.</p><!><p>Textural analysis of each synthesized catalyst (Sn-Beta
SSI, Sn-Beta
PA, and Sn-Beta HDT) by XRD and porosimetry showed that each catalyst
possessed a crystalline, microporous structure, characteristic of
the BEA framework, confirming the successful synthesis of each material
(Table 1 and Figure S1). The main textural differences among
these materials related to the dimensions of the crystallites.28 In particular, it has previously been demonstrated
that Sn-Beta HDT exhibits crystallite sizes of the order of 10–15
Î¼m, whereas postsynthetic Sn-Beta samples typically possess
crystal sizes of 0.5–2 μm, in line with previous reports
that have demonstrated that zeolite synthesis in fluoride media results
in the formation of larger crystallites.28 The larger crystallites of Sn-Beta HDT likely account for the slightly
lower surface area and micropore volume of Sn-Beta HDT compared to
Sn-Beta SSI and Sn-Beta PA (Table 1).</p><p>To compare the active site distributions of
the three catalyst
materials, 119Sn MAS NMR spectroscopy analysis was performed.
In line with our recent studies, analyses were performed with a Carr–Purcell–Meiboom–Gill
(CPMG) echo train acquisition method, adapted from the protocol demonstrated
for Sn-Beta samples by the Ivanova group.43,44,56 Under these conditions, the technique has
been shown to be able to effectively distinguish different species
of Sn by their chemical environment and coordination. The technique
was performed on hydrated materials to emulate the state of the catalyst
upon placement into the reactor, and a long recycle delay (d1 = 135
s) was employed, to provide a quantitative overview of the different
Sn species present in each material.39</p><p>Figure 1 presents
the 119Sn CPMG MAS NMR spectra of Sn-Beta HDT (top line),
Sn-Beta PA (middle line), and Sn-Beta SSI (bottom line). The resonance
at −600 ppm is typical of extra-framework Sn species, typically
tin oxide, whereas signals at lower chemical shift values are known
to be related to Sn species present within the zeolite lattice but
present in a hydrated form.45 In line with
previous reports, Sn-Beta HDT did not present a detectable resonance
at −600 ppm, indicating that extra-framework Sn in this material
was either absent or was present below the detectability limit of
the measurement.46 In contrast, some extra-framework
Sn was observed for both postsynthetic samples, accounting for approximately
25–30% of the total signal. The absence of extra-framework
Sn in Sn-Beta HDT can be attributed to a more effective incorporation
of tin by the hydrothermal methodology, in line with previous reports.28,43</p><!><p>119Sn CPMG MAS NMR spectrum of various Sn-Beta samples
prepared by hydrothermal synthesis (Sn-Beta HDT, top line) and postsynthetic
methods (Sn-Beta PA, middle line and Sn-Beta SSI, bottom line). (Recycle
delay d1: 135 s). Spinning side bands are marked with an asterisk.</p><!><p>Although the dominant resonance in all samples
was found in the
region attributable to isomorphously substituted Sn species (−650
to −750 ppm), differences in the chemical shift and intensity
of the signals were observed. In Sn-Beta HDT, a main resonance at
−690 ppm was observed, alongside a less intense resonance at
−737 ppm. Sn-Beta SSI, on the other hand, exhibited a main
peak at −720 ppm and a second resonance at −650 ppm.
However, preactivation of the postsynthetic catalyst caused the −650
ppm resonance to be lost, the dominant resonance to shift from −720
to −690 ppm, and a new resonance at −737 ppm to be formed.
However, the SnO2 content present in the postsynthetic
material was not impacted by preactivation, resulting in Sn-Beta PA
containing a similar quantity of SnO2 to Sn-Beta SSI.</p><p>The molecular understanding of how the difference in chemical shift
relates to the properties of hydrated Sn species is the topic of debate
in the scientific community. Although diverse and sometimes contradictory
assignments are reported in the literature, it is reasonable to assume
that the shifting of these signals are related to changes in the active
site properties such as T-site location, Sn–O–Si bond
angle, and/or degree of hydration.46,47 In this light,
it is notable how preactivation of Sn-Beta SSI modifies the active
centers of the postsynthetic catalysts in such a way that they exhibit
NMR signals comparable to those found in the hydrothermal sample.
This finding suggests that common active site properties may be found
for these catalysts, despite being prepared by very different preparation
procedures.</p><p>To determine the lattice properties of each catalyst
and hence
to identify how preactivation affected the properties of the zeolite
framework, study of the three catalysts was also undertaken by DRIFTS
(Figure 2). As can
be seen, the spectrum of Sn-Beta PA shows an increased contribution
of isolated silanol groups (≡Si–OH, 3740 cm–1), a decrease in the magnitude of H-bonding interactions (3200–3600
cm–1),48,49 and a decrease in intensity
characteristic of internal defect sites (960 cm–1), when compared to the spectrum of Sn-Beta SSI.50 Taken together, these changes indicate that the postsynthetic
material becomes less defective and more hydrophobic following preactivation.
Interestingly, Sn-Beta HDT already exhibited very low signals in these
regions, in agreement with previous reports that have shown that this
material possesses a low number of defect sites and larger crystallite
sizes.28</p><!><p>DRIFT spectra of Sn-Beta SSI (dashed line),
Sn-Beta PA (middle,
violet line), and Sn-Beta HDT (bottom, red line) recorded at 25 °C.</p><!><p>To further evaluate the differences in hydrophilic
properties between
the samples, vapor sorption studies were performed using water as
the adsorbate (Figure 3). This analysis revealed drastic differences between Sn-Beta HDT
and the postsynthetic Sn-Beta samples. In particular, Sn-Beta HDT
adsorbed up to 20 times less water than postsynthetic Sn-Beta materials,
confirming the highly hydrophobic nature of this material. Although
both postsynthetic samples adsorb much higher quantities of water,
it is notable that Sn-Beta PA demonstrated a substantial (ca. 30%)
decrease in hydrophilicity in comparison to Sn-Beta SSI. Thus, Sn-Beta
PA possesses a different active site structure from Sn-Beta SSI and
is also more hydrophobic than the parental Sn-Beta SSI material. Taken
together, catalyst characterization suggests that preactivation modifies
the active site and the textural properties of postsynthetic Sn-Beta.
In particular, the resulting sample (Sn-Beta PA) possesses active
Sn sites more comparable to Sn-Beta HDT, albeit with some residual
extra-framework Sn present from the original synthesis, but it has
textural properties (cf. hydrophilicity and defect sites) more comparable
to those of Sn-Beta SSI but with enhanced levels of hydrophobicity.</p><!><p>Water
sorption isotherms of Sn-Beta SSI (top, downward triangles),
Sn-Beta PA (middle, upward triangles), and Sn-Beta HDT (bottom, circles).
Isotherms gathered at 20 °C from 0 to 0.7 P/P0.</p><!><p>The activity of Sn-Beta for glucose
isomerization (GI) is dependent on its water affinity and active site
speciation.28,29,39,51 Therefore, due to the differing hydrophilicity
and active site environment(s) of the three Sn-Beta materials described
above, preliminary analysis of their kinetic performance was undertaken
by use of glucose isomerization. In particular, the performances of
Sn-Beta PA, Sn-Beta HDT, and Sn-Beta SSI were evaluated for continuous
GI at 110 °C using a reaction feed of 1 wt % glucose in pure
methanol, in line with previous studies (Figure 4).28,39 The initial activity
of each catalyst was evaluated considering the percentage of glucose
converted at 0.5 h of time on stream and was normalized to account
for the different WHSVs of each reaction by transformation to a turnover
frequency (TOF, eq 2).28,39 Among the three catalytic materials, Sn-Beta HDT exhibited the highest
initial activity, achieving a TOF of 175 h–1. Although
both postsynthetic samples were lower in activity than Sn-Beta HDT,
Sn-Beta PA (112 h–1) exhibited twice the activity
of Sn-Beta SSI (64 h–1), demonstrating the extent
of improvement in activity that occurs upon preactivation of the SSI
catalyst in a flow of methanol.2</p><!><p>(A) TOF value for continuous glucose isomerization performed
in
pure methanol for Sn-Beta HDT, Sn-Beta PA, and Sn-Beta SSI, calculated
at 0.5 h of time on stream. Reaction conditions: 1 wt % glucose in
methanol, flow rates of 1.5 mL min–1 (Sn-Beta HDT),
1.0 mL min–1 (Sn-Beta PA), and 0.6 mL min–1 (Sn-Beta SSI), 100 mg of catalyst, 110 °C, 15 bar. (B) Arrhenius
expression obtained for glucose isomerization for Sn-Beta HDT (red
squares), Sn-Beta SSI (black circles), and Sn-Beta PA (violet triangles).
Reaction conditions: 1 wt % glucose in methanol, 10 min reaction time,
4 mL reaction volume, mol glucose/mol Sn of 50, reaction temperatures
of 90, 105, and 120 °C.</p><!><p>In general, the best catalyst for GI possesses the lowest affinity
for water (Figure 3). However, the correlation between vapor sorption and activity is
not strong, given that Sn-Beta PA exhibited twice the activity of
Sn-Beta SSI yet was only approximately 30% more hydrophobic. Furthermore,
despite Sn-Beta HDT being approximately 20 times more hydrophobic
than Sn-Beta PA, its activity was only around 30% higher than that
of Sn-Beta PA. However, on a broader level, the general observation
is in agreement with recent findings, which revealed how more hydrophobic
Sn-Beta catalysts ensure preferential interaction of the active site
with the substrate.29,52 This preferential interaction
is reportedly achieved by the fact that more hydrophobic materials
strongly hinder access of the polar solvent molecules (such as alcohols
or water) to the active site, thus avoiding competition between the
solvent and the substrate for coordination to the Sn site. Interestingly,
Gounder et al.29 reported how this increment
in activity due to exclusion of the polar solvent was accompanied
by an increase in the activation energy. The authors hypothesized
that this increase in activation energy occurred since stabilization
of the transition state by solvent molecules in the vicinity of the
active site was lost upon exclusion of the solvent for hydrophobic
materials. Yet, despite this loss of transition state stabilization,
increased absorption on the substrate onto the Sn active site due
to the absence of the reaction solvent resulted overall in increased
activity for Sn-Beta materials that exhibited higher levels of hydrophobicity.</p><p>To further probe this hypothesis and determine a potential explanation
for the improved activity of Sn-Beta PA versus Sn-Beta SSI, the activation
energies and the pre-exponential factors for Sn-Beta HDT, Sn-Beta
PA, and Sn-Beta SSI were determined for GI between 90 and 120 °C
(Figures 4B and S2). These specific kinetic experiments were
performed in batch reactors, so that intrinsic kinetic data could
be directly determined without contribution from catalyst deactivation
or limitations that can be found by working at high levels of reactant
conversion.23 All Arrhenius parameters
were determined according to the quantity of glucose converted. As
can be seen (Table 2), Sn-Beta HDT has an activation energy 18 kJ mol–1 higher than that of Sn-Beta SSI and a pre-exponential factor (A)
two orders of magnitude larger.52 The higher
activation energy suggests a less facile mechanism for isomerization
over Sn-Beta HDT than for Sn-Beta SSI, but the accompanying increase
in the pre-exponential factor indicates that the entropic contributions
to reaction are more favorable for the hydrothermal catalyst, such
as Sn-site location, substrate accessibility, and reactivity.53 These observations are in good agreement with
the recent studies of Gounder and coworkers.29</p><!><p>Reaction conditions: 1 wt % glucose
in methanol, 10 min reaction time, 4 mL reaction volume, mol glucose/mol
Sn of 50, reaction temperatures of 90, 105, and 120°C.</p><!><p>Interestingly, preactivation of
Sn-Beta clearly increased the activation
energy of the postsynthetic catalyst, from 70 kJ mol–1 for Sn-Beta SSI to 86 kJ mol–1 for Sn-Beta PA.
Furthermore, preactivation also increased the pre-exponential factor
by two orders of magnitude (Table 2). Thus, preactivation not only results in a twofold
increase in catalytic performance but also results in the Arrhenius
parameters of Sn-Beta PA matching those of Sn-Beta HDT. This observation
strongly indicates that Sn-Beta PA and Sn-Beta HDT possess common
active site properties, which are characterized by increased activation
energies relative to Sn-Beta SSI, but improved entropic characteristics.
This finding is in excellent agreement with the findings from 119Sn MAS NMR spectroscopy, which indicated that the preactivated
sample possessed active Sn sites much more comparable to those of
Sn-Beta HDT than its postsynthetic parent material, Sn-Beta SSI. The
increment in activity and temperature dependence after preactivation,
alongside a lower affinity for water absorption, allow us to hypothesize
that preactivation results in the formation of an improved material
that experiences better interaction between the substrate and active
site of reaction, due to lower levels of competitive absorption with
the reaction solvent, unlocking in this way improved overall performances.
However, although preactivation leads to a twofold increment in activity
for GI after preactivation (Figure 4), it is notable that Sn-Beta PA still exhibits somewhat
lower activity in comparison to Sn-Beta HDT for GI, when evaluated
on a TOF basis.28,39 We hypothesize that this is likely
due to the extra-framework SnO2 content of this material,
which is present from the original SSI synthesis and does not appear
to change in abundance during preactivation (Figure 1). The presence of residual extra-framework
SnO2 would evidently decrease the apparent TOF of this
catalyst relative to the hydrothermal sample, given that such species
are known to be spectators in the context of Lewis acid catalysis.54</p><!><p>Considering
the differing properties and performance among the Sn-Beta preparations,
the potential of these catalysts for the continuous retro-aldol fragmentation
of glucose to the α-hydroxyesters ML and MVG was evaluated.
The different Sn-Beta preparations were first tested for the retro-aldol
fragmentation of glucose in methanol at 160 °C, at a WHSV of
4.752 h–1. In this first instance, reactions were
carried out without promoters to understand the intrinsic activity
and stability of each catalyst, under conditions optimized by previous
researchers.15 We note that a high WHSV
was used to avoid operating in an excess catalyst regime, which can
mask deactivation of the catalyst and hinder proper evaluation of
the stability of the reaction system.24,25 However, the
WHSV was deliberately set so as to attain a high degree of substrate
conversion (>90% initial conversion), which is especially important
toward attaining high yields of retro-aldol products, given the cascade
nature of the reaction pathway (Scheme 2) and the conversion versus selectivity relationship
of the reaction (vide infra).</p><p>The results of the kinetic experiments are shown in Figure 5 in which glucose conversion (eq 3) and individual product yields (eq 4) measured at the reactor outlet at that point
of time are plotted as a function of the time on stream. The use of
conversion and individual product yields was made to simplify interpretation
of the data in the first instance, by allowing the overall carbon
balance to be directly evident while also allowing the product distribution
to be followed with time. This is especially helpful in the case of
a complex cascade reaction, such as the retro-aldol fragmentation
of glucose to α-hydroxyesters, where even a small decrease in
substrate conversion markedly affects the calculated values of selectivity
(eq 5). Nevertheless,
conversion and selectivity dependencies are provided for further comparison
in Figure 6.345</p><!><p>Time
on stream data for glucose upgrading at 160 °C carried
out in methanol by (A) Sn-Beta HDT, (B) Sn-Beta SSI, and (C) Sn-Beta
PA. Conversion (black line) and yields of the major products (bar
charts, including fructose, ML, and MVG) are reported as a function
of time on stream. Reaction conditions: 1 wt % glucose in methanol,
1 mL min–1 flow rate, 100 mg of catalyst, 160 °C,
resulting in a WHSV of 4.752 kgglucose kg–1catalyst h–1.</p><p>(A) Percentage
of glucose converted as a function of time on stream
for Sn-Beta HDT (red squares), Sn-Beta SSI (black circles), and Sn-Beta
PA (violet triangles). (B) Conversion versus selectivity relationship
for Sn-Beta HDT (red squares), Sn-Beta SSI (black circles), and Sn-Beta
PA (violet triangles). Reaction conditions: 1 wt % glucose in methanol,
1 mL min–1 flow rate, 100 mg of catalyst, 160 °C,
resulting in a WHSV of 4.752 kgglucose kg–1catalyst h–1.</p><!><p>As can be seen (Figure 5), at 160 °C and a WHSV of 4.752 h–1, each
Sn-Beta catalyst initially converted more than 90% of the
substrate and produced significant yields of retro-aldol products
(ML + MVG, henceforth referred to as RA products). Yet, clear differences
in the overall performance of each catalyst were evident. Under these
promoter-free conditions, Sn-Beta HDT was the most stable catalyst
in terms of glucose conversion, followed by Sn-Beta PA and lastly
Sn-Beta SSI (Figure 6A). In particular, Sn-Beta HDT lost around 25% of its activity over
24 h, whereas the postsynthetic materials experienced greater losses
in performance (55 and 70% loss of activity for Sn-Beta PA and Sn-Beta
SSI, respectively). This observation is in line with previous studies
focused on glucose isomerization, which have indicated that hydrothermally
prepared Sn-Beta possessed enhanced stability compared to Sn-Beta
samples prepared by postsynthetic routes.28 This initial finding also confirms that preactivation of Sn-Beta
improved the stability of the postsynthetic catalyst, though to a
slightly lower extent than observed for glucose isomerization. Broadly
speaking, all three catalysts exhibited comparable conversion versus
selectivity relationships (Figure 6B), approaching 50% selectivity to RA products at full
conversion. However, it is interesting to note how the initial yield
of RA products was higher for Sn-Beta PA (65%) than for Sn-Beta HDT
(52%), confirming the interesting potential of this catalyst for higher
temperature operation (Figure 5). Interestingly, when Sn-Beta SSI was tested under the same
conditions, the initial yield of RA products was only 37%, further
demonstrating the superior performance of the preactivated material
to that of unmodified Sn-Beta SSI in terms of activity and stability.</p><p>Despite the interesting yield of α-hydroxy esters achieved
by these catalysts at initial time on stream, the steep deactivation
exhibited by each sample represents a challenge for process intensification.
One of the main causes for the deactivation of Sn-Beta in methanol
is known to be the strong interaction between the Sn site of the catalyst
and the reaction solvent.16 Recently, we
have shown how the stability of Sn-Beta can be enhanced through the
addition of small amounts of water to the reaction feed, since the
presence of some water minimizes alkoxylation and/or condensation
of the sites during continuous operation.16,26 However, the stabilizing role of water during retro-aldol fragmentation
has only previously been observed for a postsynthetic Sn-Beta catalyst
employing fructose as the substrate,16 and
hence, its general applicability to stabilize other types of Sn-Beta
catalysts (i.e., hydrothermal Sn-Beta catalysts) or retro-aldol reactions
using glucose as the substrate has not yet been determined.</p><p>To understand the sensitivity of these catalysts to the addition
of water as a stability promoter, the retro-aldol fragmentation of
glucose was carried out in a solution of methanol/water (99:1 wt %,
henceforth 99:1) (Figure 8). The principal effect observed upon adding a small amount
of water to each reaction feed is that the stability of each catalyst
is improved, as evidenced by the higher levels of conversion maintained
by each catalyst during continuous operation over an initial 24 h
period. Interestingly, although Sn-Beta HDT was still the most stable
catalyst under these conditions, the stability of Sn-Beta PA and Sn-Beta
SSI was enhanced to a much greater degree by the addition of water.
Given that the beneficial effect of water primarily relates to the
fact that small quantities of water limit the interaction of the active
sites with the methanol solvent,16,26 this finding
suggests that the major benefit of the hydrothermal preparation is
that it results in the synthesis of a catalyst more resistant to deactivation
by methanol in the first instance, which hence exhibits higher stability
in the absence of additional water (Figure 6A). However, when small amounts of water
are added to mitigate this primary deactivation mechanism, all Sn-Beta
catalysts broadly exhibit similar levels of stability, resulting in
all samples exhibiting broadly similar rates of deactivation when
a methanol/water solution (99:1 v/v) was used as the solvent.</p><p>Although all catalysts exhibited improved stability following the
addition of 1 wt % water, the addition of water did not change the
selectivity of the process toward the target retro-aldol products
for any catalyst material. In particular, the conversion versus selectivity
relationships were generally unchanged upon addition of 1 wt % water
to the reaction feed for all catalysts (Figure 7B–D). As such, the maximum selectivity
toward retro-aldol products was still higher for Sn-Beta HDT and Sn-Beta
PA (ca. 50–60% at >90% substrate conversion) than for Sn-Beta
SSI (<40% at >90% substrate conversion). We note that the low
level
of selectivity both reflects the cascade nature of the reaction process
(Scheme 1) and further
emphasizes the high reactivity and multifunctionality of Sn-Beta in
being able to catalyze different reaction pathways.12,30 Consequently, despite the improved level of stability achieved upon
addition of water, the selectivity of the process to retro-aldol products
in methanol/water (99:1) still has margins for improvements.</p><!><p>(A) Percentage
of glucose converted as a function of time on stream
during retro-aldol fragmentation in methanol/water, for Sn-Beta HDT
(red squares), Sn-Beta SSI (black circles), and Sn-Beta PA (violet
triangles). Conversion versus selectivity relationship for (B) Sn-Beta
HDT, (C) Sn-Beta PA, and (D) Sn-Beta SSI in the absence (black) and
presence (blue) of 1 wt % water in the reaction feed. Reaction conditions:
1 wt % glucose in methanol/water (99:1), 1 mL min–1 flow rate, 100 mg of catalyst, 160 °C, resulting in a WHSV
of 4.752 kgglucose kg–1catalyst h–1.</p><!><p>Previous studies have
shown how the addition of small quantities
of alkali salts can improve the selectivity of certain Sn-Beta catalysts
toward retro-aldol fragmentation.12,21,22,30,55 It has been proposed that the effect of alkali salts arises from
their ability to exchange protons present in the active site environment,
thereby minimizing the partial Brønsted acidity of Sn-Beta materials
and favoring the retro-aldol fragmentation pathway.21,55 However, the susceptibility of different preparations of Sn-Beta
catalysts for the alkali effect in continuous flow has not yet been
evaluated. Thus, to determine the general susceptibility of these
catalysts to alkali exchange, Sn-Beta HDT, Sn-Beta PA, and Sn-Beta
SSI were tested for glucose upgrading at 160 °C in pure methanol
and in the presence of alkali (KCl) but without added water. We note
that KCl was used as the alkali promoter, as its beneficial concentration
range is wider than that of other promoters with basic characters,
such as K2CO3, and it can therefore be introduced
into the feed in a slight excess without compromising performance,
thus facilitating continuous operation.22</p><p>Figure 8A presents the conversion of glucose achieved
over
Sn-Beta HDT, Sn-Beta SSI and Sn-Beta PA as a function of time on stream,
whereas Figure 8B–D
presents the conversion versus selectivity dependence of each catalyst
in the presence (orange symbols) and absence (black symbols) of alkali.
Upon addition of alkali to the reaction feed, it is evident that the
selectivity toward α-hydroxyester products increased dramatically
for all catalyst materials (Figure 9B–D). Although the increase in selectivity toward
retro-aldol products was the greatest for Sn-Beta HDT at lower levels
of conversion, the highest overall RA selectivity was observed for
Sn-Beta PA, which exhibited a RA selectivity greater than 80% at maximum
conversion upon addition of KCl to the feed.</p><!><p>(A) Percentage of glucose
converted as a function of time on stream
during retro-aldol fragmentation of glucose in the presence of alkali,
for Sn-Beta HDT (red squares), Sn-Beta SSI (black circles), and Sn-Beta
PA (violet triangles). Conversion versus selectivity relationship
for (B) Sn-Beta HDT, (C) Sn-Beta PA, and (D) Sn-Beta SSI in the absence
(black) and presence (orange) of KCl in the reaction feed. Reaction
conditions: 1 wt % glucose in methanol containing KCl (0.004 g L–1), 1 mL min–1 flow rate, 100 mg
of catalyst, 160 °C, resulting in a WHSV of 4.752 kgglucose kg–1catalyst h–1.</p><p>Percentage of glucose converted as a function of time
on stream
during retro-aldol fragmentation of glucose in methanol/water and
in the presence of alkali, for Sn-Beta HDT (red squares), Sn-Beta
SSI (black circles), and Sn-Beta PA (violet triangles). Conversion
versus selectivity relationship for (B) Sn-Beta HDT, (C) Sn-Beta PA,
and (D) Sn-Beta SSI in the absence (black) and presence (violet) of
water and KCl in the reaction feed. Reaction conditions: 1 wt % glucose
in methanol/water (99:1) containing KCl (0.004 g L–1), 1 mL min–1 flow rate, 100 mg of catalyst, 160
°C, resulting in a WHSV of 4.752 kgglucose kg–1catalyst h–1.</p><!><p>However, it is apparent that the addition of alkali salts
to each
reaction also leads to unexpected changes in the stability of each
catalyst, when compared to the analogous reaction run in pure methanol
without alkali salts present (comparing Figure 8A to Figure 6A). In particular, Sn-Beta SSI and Sn-Beta PA exhibited
very slight improvements in stability upon addition of alkali. However,
Sn-Beta HDT suffered a loss of stability upon inclusion of alkali
and lost over 70% of its initial activity in 24 h, in contrast to
less than 30% over the same time frame without addition of alkali.
This has not previously been observed and is indicative of compatibility
problems between the hydrothermal sample and alkali ions in the feed.
Given the comparable active site properties of Sn-Beta PA and Sn-Beta
HDT, the negative impact of alkali on the stability of Sn-Beta HDT
is likely a consequence of the different hydrophilic and defect site
density of Sn-Beta HDT relative to both postsynthetic catalysts. Since
K+ has been shown to interact with both Sn sites and silanol
groups, we hypothesize that the much lower defect site density of
Sn-Beta HDT may indirectly result in over-titration of the Sn sites
at the investigated K+/Sn ratios, since less K+ ions will be consumed via interaction with silanol groups.</p><p>The beneficial effect of alkali is thus clearly dependent on the
precise choice of catalyst used for retro-aldol fragmentation. In
particular, for both postsynthetic samples (Sn-Beta SSI and Sn-Beta
PA), the addition of alkali was beneficial both to stability and selectivity,
resulting in improved overall performance throughout the reaction
period (Figure S2). In contrast, although
the RA selectivity of Sn-Beta HDT was improved upon addition of alkali,
its stability diminished substantially upon inclusion of alkali. As
such, upon comparing the absolute RA product yield of Sn-Beta HDT
in both the absence and presence of alkali, no clear benefit of adding
alkali to the reaction after the first 8 h of the reaction can be
seen, as the steeper rate of deactivation in the presence of alkali
counteracts the improved selectivity (Figure S2).</p><p>To understand whether the stabilizing effect of water could
be
used in synergy with the effect of alkali, a final series of reactions
was performed in methanol/water (99:1) in the presence of alkali salts
(Figure 9). Although
the copresence of water lowered the rates of deactivation for all
catalysts, compared to the analogous experiments performed only with
alkali (comparing Figures 9A to Figure 8A), the stability of Sn-Beta HDT was still negatively impacted by
the presence of alkali (Figure 9A). Thus, under these conditions, Sn-Beta HDT displayed the
lowest levels of stability, losing approximately 50% of its initial
activity over the 60 h reaction period investigated (Figure 9A). In contrast, both postsynthetic
samples retained over 80% of their initial performance over the same
time period, with Sn-Beta PA retaining marginally more activity than
Sn-Beta SSI.</p><p>The selectivity of each catalyst was also improved
upon addition
of alkali and water, but to a greater degree than observed with alkali
alone. In fact, it is notable that the RA product selectivity plateaus
at a much higher level for all catalysts, when alkali and water are
employed in combination. Thus, maximum RA product selectivity greater
than 90% was observed for both postsynthetic catalysts, which was
slightly higher than recorded for Sn-Beta HDT, which reached a maximum
selectivity of 80% at the highest levels of substrate conversion.
In line with the data presented in Figure 8, the lower stability of Sn-Beta HDT in the
presence of alkali counteracts its improved selectivity, resulting
in there being no net-benefit regarding the addition of alkali to
the feed for Sn-Beta HDT beyond the first 4–8 h under these
particular conditions (Figure S3).</p><p>As a consequence of the perturbations of water and alkali, all
catalysts exhibit a large increase in overall yield to RA products,
when compared to their analogous reactions performed without water
and alkali (comparing Figure 10 to Figure 5). In particular, the initial yield of α-hydroxy esters products
was approximately 80% for Sn-Beta HDT and Sn-Beta SSI and above 95%
for Sn-Beta PA (75% yield for ML and >20% yield for MVG). As a
consequence
of their higher stability in the presence of alkali salts, the postsynthetic
catalysts outperformed the hydrothermal catalyst by some margin, especially
over longer operational periods. In particular, Sn-Beta PA was found
to produce the highest yields of retro-aldol products throughout the
reaction period, which reached a plateau of approximately 45–50%
after the first 24 h on stream (Figure 10).</p><!><p>Time on stream data for glucose upgrading at
160 °C carried
out in methanol/water (99:1) in the presence of alkali by (left) Sn-Beta
HDT, (middle) Sn-Beta SSI, and (right) Sn-Beta PA, shown as conversion
(black line) and yields (bar charts) as a function of time on stream.
Reaction conditions: 1 wt % glucose in methanol/water (99:1), (KCl
0.004 g L–1), 1 mL min–1 flow
rate, 100 mg of catalyst, 160 °C, WHSV: 4.752 kgglucose kg–1catalyst h–1.</p><!><p>When determining the optimal catalyst for a process,
it is important
to consider activity, selectivity, and stability, all of which are
dependent on the precise properties of a catalyst and the reaction
conditions used.23−25 For Sn-Beta HDT, better performance is achieved when
retro-aldol fragmentation is performed in methanol/water but without
addition of alkali to the feed, given that alkali negatively affects
the stability of the hydrothermal catalyst. In contrast, for both
postsynthetic samples, the addition of alkali is beneficial to both
stability and selectivity, and hence, a better performance is achieved
in methanol/water with alkali present. However, between these two
general selections, the latter option results in much higher RA product
yields being achieved, particularly over extended operational periods.
This is exemplified in Figure 11, which compares the combined RA product yield produced
using the best-performing postsynthetic catalyst (Sn-Beta PA) in the
presence of water and alkali, to the combined RA yield achieved using
Sn-Beta HDT in the presence of the water additive alone, under otherwise
identical reaction conditions.</p><!><p>Yield of retro-aldol (RA) products (ML
+ MVG) as a function of
time on stream during retro-aldol fragmentation of glucose using Sn-Beta
HDT (methanol/water, no alkali) and Sn-Beta PA (methanol/water and
alkali). Reaction conditions: 1 wt % glucose in methanol/water (99:1),
1 mL min–1 flow rate, 100 mg of catalyst, 160 °C.
Where applicable, i.e., for Sn-Beta PA, KCl was present at 0.004 g
L–1.</p><!><p>Although operating at
submaximal conversion is essential toward
obtaining accurate insight into the stability of a catalyst, in the
case of retro-aldol fragmentation, this approach clearly hinders the
achievement of high yields, given the steep conversion versus selectivity
relationship of the reaction. Even so, the maximum combined RA yields
achieved by each of the catalysts described above are far above those
previously reported for this reaction when using glucose as the substrate,
which have typically been in the region of 45–60% (Table S1). Although most previous studies report
only ML yields and do not explicitly report on the production of MVG,
the ML yields achieved over Sn-Beta PA are still higher than previously
reported and thus still represent a new benchmark in terms of retro-aldol
fragmentation reactions using glucose as the substrate.</p><p>In addition
to providing important opportunities to probe the stability
of heterogeneous catalysts, continuous PFRs offer several other advantages
over batch reactors, including improved mass and heat transfer, increased
safety, better scalability, and smaller reactor volumes. As a consequence
of these benefits, employing these catalysts in such an operational
mode (PFR operating at high WHSV (4.752 h–1, vide
supra)) results in substantial increases in space–time-yields.
In particular, the values of space–time-yield achieved by use
of Sn-Beta PA under its most suitable conditions (methanol/water solvent
with the alkali additive) are over 2 orders of magnitudes higher than
previously reported for this particular reaction, where most studies
have been carried out in batch reactors (Table S1).12,16,19,27,37,57,58 Alongside the increased
overall yields of retro-aldol products (>95%, registered at full
conversion
of glucose), the performances of the catalysts explored in this study
are thus substantially higher than the best results reported to date,
demonstrating the importance of optimizing the global reaction system
(i.e., catalyst and conditions) in unison.</p><!><p>In
this manuscript, postsynthetic Sn-Beta catalysts were compared
in performance to hydrothermally prepared Sn-Beta for the isomerization
of glucose to fructose at 110 °C and more thoroughly for the
retro-aldol fragmentation of glucose to α-hydroxyesters at 160
°C. Initially, the best levels of performance were observed for
the Sn-Beta catalyst prepared by hydrothermal synthesis, in line with
the majority of previous research in the open literature. However,
more thorough evaluation of the performances of these catalyst, in
the continuous regime and in the presence of activity promoters (water
and alkali), led to the observation that postsynthetic Sn-Beta samples
exhibit the best levels of performance, when activity, selectivity,
and stability are fully considered. In particular, the highest levels
of retro-aldol product yield and stability are obtained with postsynthetic
Sn-Beta catalysts preactivated in a flow of methanol prior to reaction.
The active sites of this catalyst resemble those found in hydrothermal
Sn-Beta, but its hydrophilicity and defect site density are more comparable
to other postsynthetic analogues. Until now, the best performance
for the conversion of glucose was achieved by the hydrothermal preparations
of Sn-Beta, so the preactivation protocol represents an innovative
and simple post synthetic methodology to access new Sn-Beta materials
with an improved performance while providing important guidelines
with respect to tailoring future Sn-Beta catalysts. More broadly,
this study implies that the performances of different Sn-Beta catalysts
intimately depend on the precise operational conditions employed,
since different catalysts respond to different conditions and promoters
to different degrees and in a very diverse manner. In fact, the responses
of the catalysts to promoters appear to not only depend upon the nature
of the active sites but also the active site environment and the properties
of the zeolite lattice, making these relationships nonpredictable.
Thus, along with demonstrating that postsynthetic Sn-Beta catalysts
are more favorable for α-hydroxyester formation than hydrothermal
analogues, this suggests that future development of Sn-Beta materials
should not solely focus on pure activity and selectivity measurements
under idealized conditions but should also study the susceptibility
of each catalyst to the process promoters (alkali salts and water)
under more realistic operational conditions. Thanks to this approach,
we achieve α-hydroxyester yields over 90% when employing postsynthetic
Sn-Beta catalysts that are preactivated in a flow of methanol prior
to reaction. We also demonstrate continuous operation of the catalyst
for over 60 h, providing important information regarding the stability
of these catalysts under continuous processing conditions. In doing
so, we obtained space–time-yields over two orders of magnitude
higher than previously reported for this reaction while achieving
unmatched yields of retro-aldol products at early stages of the reaction.</p><!><p>Hydrothermal synthesis of Sn-Beta
was performed following a procedure described in the literature.42 First, 30.6 g of tetraethyl orthosilicate (Sigma
Aldrich, 98%) was added to 33.1 g of tetraethylammonium hydroxide
(Sigma Aldrich, 35%) under careful stirring, forming a two-phase system.
After 60–90 min, one phase was obtained and the desired amount
of the metal source (SnCl4·5H2O, Sigma
Aldrich >99.5%) dissolved in 2.0 mL of H2O was added
dropwise.
The solution was then left for several hours under stirring until
a viscous gel was formed. The gel was finalized by the addition of
3.1 g of hydrofluoric acid (Fischer Chemicals 50%) in 1.6 g of demineralized
H2O, yielding a solid gel with the molar composition: 1.0Si,
1:0.005Sn, 1:0.02Cl–, 1:0.55TEA+, 1:0.55F–, and 1:7.5H2O. The obtained gel was transferred
in a Teflon-lined stainless steel autoclave and kept for 7 days at
140 °C to crystallize. The obtained crystals were filtered and
washed with deionized water. Calcination at 550 °C (2 °C
min– 1) for 6 h under static air was carried
out to remove the organic template.</p><p>Postsynthetic synthesis
of Sn-Beta (Sn-Beta SSI) was carried out on the commercial zeolite
Al-Βeta (Zeolyst, NH4+-form, Si/Al = 19).
This was dealuminated by treatment in HNO3 solution (13
M HNO3, (VWR, Technical Chemicals, 68%) 100 °C, 20
mL g–1 zeolite, 20 h). Solid-state incorporation
was achieved by grinding an appropriate amount of tin(II) acetate
with the necessary amount of dealuminated zeolite for 10 min in a
pestle and mortar. Following this procedure, the sample was heated
in a combustion furnace (Carbolite MTF12/38/400) to 550 °C (10
°C min–1 ramp rate) first in a flow of N2 (3 h) and subsequently air (3 h) for a total of 6 h. Gas
flow rates of 60 mL min–1 were employed at each
stage.</p><p>Sn-Beta PA was obtained by treating Sn-Beta SSI at 110
°C
in a continuous flow of methanol (VWR, methanol, >99.9 wt % purity)
in a continuous reactor, at a flow rate of 1.5 mL min–1 for 0.5 h. The methanol-treated catalyst was immediately calcined
in air at 550 °C for 3 h (10 °C min–1)
following treatment in the flow reactor.</p><!><p>Continuous glucose upgrading reactions
were performed in a plug flow, stainless steel, tubular reactor. The
catalyst was pelletized (size fraction 63 and 77 Î¼m) and densely
packed into a 1/4 inch stainless steel tube (4.1 mm internal diameter).
Two plugs of quartz wool and a frit of 0.5 Î¼m held the catalyst
in location. Temperature control was achieved with a thermostatted
oil bath at the desired reaction temperature, and pressurization was
achieved by means of a backpressure regulator. Continuous glucose
isomerization experiments were performed at 110 °C, using a reaction
feed of 1 wt % glucose in methanol, flow rates of 1.5 mL min–1 (Sn-Beta HDT), 1.0 mL min–1 (Sn-Beta PA), and
0.6 mL min–1 (Sn-Beta SSI), and 100 mg of catalyst
in each case. Retro-aldol fragmentation reactions were performed at
160 °C, using a reaction feed of 1 wt % glucose in solvent (methanol
or methanol/water (99:1)), at flow rates of 1 mL min–1. The same catalyst mass (100 mg) was used in every retro-aldol fragmentation
reaction. Aliquots of the reaction solutions were taken periodically
from a sampling valve placed after the reactor and were analyzed with
an Agilent 1260 Infinity HPLC equipped with a Hi-Plex Ca column and
ELS detector and quantified against an external standard (sorbitol)
added to the sample prior the injection. Samples were also measured
by GC-FID (Agilent 8890) for quantification of the C3 and C4 products.</p><!><p>XRD spectra were acquired
using a PANalytical X'PertPRO X-ray diffractometer, using a
CuKÎ± radiation source (40 kV and 30 mA). Diffraction patterns
were recorded between 6 and 55° 2θ (step size 0.0167°,
time/step = 150 s, and total time = 1 h). Specific surface area was
determined from nitrogen adsorption using the BET equation, and microporous
volume was determined from nitrogen adsorption isotherms using the
t-plot method. Porosimetry measurements were performed with a Quantachrome
Autosorb-iQ-MP/XR, and samples were degassed prior to use (115 °C,
6 h, N2 flow). Vapor adsorption experiments were performed
on the same instrument maintaining the sample at 20° and adsorbing
water from 0 to 0.7 P/P0. All the samples were nonenriched and were measured with a Bruker
Avance III HD spectrometer at operating frequencies of 400 and 149
MHz for 1H, and 119Sn, respectively. Typically,
between 50 and 100 mg of solid sample was packed in a 4 mm rotor,
which was spun at ±12,000 Hz. Other spinning frequencies were
employed for identification of spinning side bands. For 119Sn MAS NMR, samples were measured by the CMPG method as described
in ref (56). Spectra
were acquired both in direct excitation and cross polarization modes.
A recycle delay time of 135 s was applied, as this provides a (semi)quantitative
representation of the various Sn species present in each catalyst.
DRIFT spectroscopy analyses were performed in a Harrick praying mantis
cell. The spectra were recorded on a Bruker Tensor Spectrometer over
a range of 4000–650 cm–1 at a resolution
of 2 cm–1.</p><!><p>Supporting kinetic
data, characterization information,
and literature comparisons (PDF)</p><p>sc1c06989_si_001.pdf</p><p>The authors declare no
competing financial interest.</p>