Regioselective Baeyer-Villiger oxidation of lignin model compounds with tin beta zeolite catalyst and hydrogen peroxide

Article abstract of DOI:10.1039/c7ra03830e

Lignin depolymerization represents a promising approach to the sustainable production of aromatic molecules. One potential approach to the stepwise depolymerization of lignin involves oxidation of the benzylic alcohol group in β-O-4 and β-1 linkages, followed by Baeyer-Villiger oxidation (BVO) of the resulting ketones and subsequent ester hydrolysis. Towards this goal, BVO reactions were performed on 2-adamantanone, a series of acetophenone derivatives, and lignin model compounds using a tin beta zeolite/hydrogen peroxide biphasic system. XRD, ¹¹⁹Sn MAS NMR spectroscopy, DRUVS and XPS were used to determine tin speciation in the catalyst, the presence of both framework Sn and extra framework SnO₂ being inferred. Conversion of ketones to BVO products was affected by electron donation as well as steric hindrance, 4′-methoxyacetophenone affording the highest yield of ester (81%). As the size and complexity of the ketone increased, excess hydrogen peroxide was typically needed for successful BVO. Yields of ester products derived from β-O-4 and β-1 lignin models were modest due to the formation of polymeric material stemming from direct ring hydroxylation.

Full text of DOI:10.1039/c7ra03830e

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Regioselective Baeyer–Villiger oxidation of lignin
model compounds with tin beta zeolite catalyst
and hydrogen peroxide†
Cite this: RSC Adv., 2017, 7, 25987
John A. Jennings,ab Sean Parkin,a Eric Munson,c Sean P. Delaney,c Julie L. Calahan,c
d
ab
Kunlun Honge and Mark Crocker
*
Mark Isaacs,
Lignin depolymerization represents a promising approach to the sustainable production of aromatic
molecules. One potential approach to the stepwise depolymerization of lignin involves oxidation of the
benzylic alcohol group in b-O-4 and b-1 linkages, followed by Baeyer–Villiger oxidation (BVO) of the
resulting ketones and subsequent ester hydrolysis. Towards this goal, BVO reactions were performed on
2-adamantanone, a series of acetophenone derivatives, and lignin model compounds using a tin beta
zeolite/hydrogen peroxide biphasic system. XRD, 119Sn MAS NMR spectroscopy, DRUVS and XPS were
used to determine tin speciation in the catalyst, the presence of both framework Sn and extra framework
SnO2 being inferred. Conversion of ketones to BVO products was affected by electron donation as well
as steric hindrance, 40-methoxyacetophenone affording the highest yield of ester (81%). As the size and
complexity of the ketone increased, excess hydrogen peroxide was typically needed for successful BVO.
Yields of ester products derived from b-O-4 and b-1 lignin models were modest due to the formation of
polymeric material stemming from direct ring hydroxylation.
Received 3rd April 2017
Accepted 8th May 2017
DOI: 10.1039/c7ra03830e
rsc.li/rsc-advances
cleaving the bridging ether groups using thermal and/or
reductive techniques, requiring high temperatures and pres-
sures. As an alternative, Stahl and coworkers3 performed an
1. Introduction
Lignin is biosynthesized to defend plant-life against chemical
and biological attack.1 As the largest natural source of
aromatics, products of lignin depolymerization have potential
as both renewable fuel (e.g., benzene, toluene, and xylenes
(BTX)) and ne chemicals (e.g., vanillin). However, most lignin
produced by the pulp and paper industries is currently burned
on-site as a low-grade fuel. Due to lignin's characteristic irreg-
ularity and poor solubility, lignin is challenging to utilize on an
industrial scale.
In lieu of using lignin, which is difficult to analyze, models of
various linkage motifs are commonly used to investigate
depolymerization strategies. The most abundant of these link-
ages is the b-O-4 structure, which represents up to 60% of the
linkages found in lignin.2 A benzylic alcohol moiety and
a bridging aryl ether bond are characteristic of the b-O-4
linkage. Many lignin depolymerization strategies focus on
extensive study of stoichiometric oxidants, metal-catalyzed
aerobic oxidations, and metal-free catalytic aerobic oxidations.
The authors found the most selective and efficient benzylic
oxidation system to be 4-acetamido-2,2,6,6-tetramethylpiper-
idine-N-oxyl/oxygen, yielding almost quantitative benzylic
oxidation for “dimeric” model compounds (i.e., compounds
containing two aromatic groups linked in b-O-4 fashion).3 The
authors also reported that in some less selective oxidation
systems, aer g-oxidation, models were converted to
substituted aldehydes via retro aldol reactions. According to
Stahl and coworkers,3 the utility of this retro aldol reaction is
limited due to formation of unidentied products, presumably
phenolic radical coupling products. Retro aldol reactions have
been reported for both a and g-ketones.3–5
Aer benzylic oxidation, Stahl and coworkers cleaved the b-
O-4 linkage via Dakin oxidation, affording an 88% yield of 3,4-
dimethoxybenzoic acid and 42% yield of guaiacol (Scheme 1).
The low yield of guaiacol is derived from phenolic radical
coupling initiated by hydrogen peroxide, which is capable of
both one and two-electron oxidations. Indeed, poor isolated
yields of phenolic compounds is a common problem in the
oxidation of aromatic molecules.6
aDepartment of Chemistry, University of Kentucky, Lexington, KY, USA. E-mail: mark.
crocker@uky.edu; Tel: +859-257-0295
bUniversity of Kentucky Center for Applied Energy Research, Lexington, KY, USA
cDepartment of Pharmaceutical Sciences, University of Kentucky, Lexington, KY, USA
dEuropean Bioenergy Research Institute, Aston University, Birmingham, UK
eCenter for Nanophase Materials Sciences and Chemical Science Division, Oak Ridge
National Laboratory, Oak Ridge, TN, USA
Westwood and coworkers7 also reported almost quantitative
benzylic oxidation of lignin model compounds using a 2,3-
dichloro-5,6-dicyano-1,4-benzoquinone (DDQ)/oxygen system.
† Electronic supplementary information (ESI) available. CCDC 1541264 and
1541265. For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c7ra03830e
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Scheme 1 Dakin oxidation of a lignin dimer model compound performed by Stahl and coworkers.3
Scheme 2 Benzylic oxidation performed by Wang et al.5
Aer zinc-based reduction/cleavage the authors obtained up to converted directly to esters via Baeyer–Villiger Oxidation (BVO),
92% isolated yields of substituted propylarenes, although the and subsequently hydrolyzed in a one-electron free medium. In
yields of phenolic moieties were not reported.
another publication concerning benzylic oxidation of lignin
Wang and coworkers5 also observed almost quantitative model compounds, Patil et al.9 reported the rst well-
benzylic oxidation of lignin model compounds using vanadyl characterized homogeneous BVO of an aromatic ketone using
sulfate/TEMPO/oxygen (Scheme 2). In addition to benzylic an oxidation system generating performic acid in situ
ketones, the reaction yielded an unexpected a-retro aldol (Scheme 4).
product (20% yield). Benzylic (Ca) retro aldol reactions simplify
As a consequence of the acidic nature of the reaction, a high
models by elimination of formaldehyde, increasing the reac- yield of 3,4-dimethoxybenzoic acid from hydrolysis of the b-O-4
tivity in oxidizing systems.4,5 Indeed, Wang et al.5 found that model was reported (Scheme 4, entry (a)). However, the
models with no g-carbon were more active to further oxidation. complete absence of the phenol co-product was attributed to
Following benzylic oxidation, Wang et al.5 depolymerized the polymerization. In the same report a BVO product (not hydro-
ketone with a copper/phenanthroline/superoxide system to lyzed in situ) was reported in 10% yield (>99% selectivity)
form a benzoic acid and a phenol, the authors noting a signi- (Scheme 4, entry (b)).9
cant decrease in phenol yields in models with electron-donating
methoxy groups.
Recently, several heterogeneous systems for the oxidation of
lignin model compounds have been identied.10–13 However,
In one of the few reports resulting in high yields of phenols, these catalysts do not yield ester products resulting from oxygen
Stahl and coworkers8 used a redox-neutral system (excess formic insertion. Rather, reports of heterogeneous oxidation result in
acid and sodium formate) to effectively depolymerize oxidized small molecules resulting directly from Ca–Cb or Cb–O4 bond
lignin and lignin model compounds to discrete products. cleavage. In the present study an oxidation system consisting of
Depolymerization products of models included phenols, which hydrogen peroxide (a weaker oxidant than performic acid) and
were obtained in excellent yields (Scheme 3). The absence of a heterogeneous tin beta zeolite catalyst was explored in an
a one-electron reactant supports the hypothesis that phenols effort to avoid phenolic polymerization. The nature of this
produced in a peroxide-free environment can be isolated in reaction system was anticipated to facilitate ready isolation of
excellent yields. However, this very successful example of b-O-4 the desired products.
cleavage suffers from a large excess of formic acid and is
homogeneous in nature.
Most reports of heterogeneously catalyzed BVO concern
cyclic ketones,14 with few reports of acyclic ketones, and even
Although selective benzylic oxidation is a rather facile rst fewer of benzylic ketones. BVO of aromatic aldehydes to formate
step, there are few methods for which the phenolic portions of esters using hydrothermally synthesized tin beta zeolite has
models produced by Cb–O4 bond cleavage are isolated in high
yield. To avoid the production of phenolate radicals, which
readily repolymerize, benzylic ketones can in principle be
Scheme 3 Depolymerization of a lignin model compound performed Scheme 4 Lignin model depolymerization strategy reported by Patil
by Stahl and coworkers.8
et al.9
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been reported,15 but employed highly active peroxyimidic acids. ammonia, consistent with near complete removal of Al from the
Panchgalle et al.16 reported BVO of acetophenone derivatives framework, while tin beta zeolite adsorbed 28.4 mmol of
using an ionic liquid as co-oxidant, but provided no details of ammonia per gram of catalyst. Most of the ammonia desorbed
the catalyst synthesis, characterization, or role of the ionic below 525 K during TPD, indicating that tin incorporation
liquid. Hydrothermally synthesized tin beta zeolite has also resulted in weakly acidic sites. Pore-size distribution measure-
been reported as being active in several other Lewis acid cata- ments on the tin beta zeolite showed that 55% of the total pore
lyzed reactions such as Meerwein–Ponndorf–Verley (MPV) volume corresponded to the mesopore range. This is note-
reductions.17 However, synthetic challenges such as long crys- worthy given that the presence of mesopores is critical to the
tallization times (e.g., 20 days), and hazardous synthetic reaction of bulky lignin model compounds, and ultimately
reagents (hydrouoric acid) make large-scale hydrothermal lignin itself.
synthesis of tin beta zeolite problematic.18
The preservation of beta zeolite structure was veried via
As an alternative to hydrothermal synthesis, tin beta zeolite XRD (Fig. 1). Additionally, subtle changes in the d302 interlayer
has recently been synthesized using post-synthetic (PS) modi- spacing conrmed that rst dealumination had occurred, with
cation strategies.18–22 Indeed, PS incorporation of tin into beta subsequent inclusion of tin. Indeed, the interlayer spacing of H-
ꢀ
˚
˚
zeolite has presented numerous advantages over hydrothermal beta zeolite was reduced from 3.864 A (2q ¼ 22.54 ) to 3.817 A
ꢀ
˚
synthesis, e.g., higher tin loading, introduction of multiple (2q ¼ 22.82 ) upon dealumination but increased to 3.856 A (2q ¼
active sites, and smaller beta particle size.18 PS tin beta zeolite 22.59ꢀ) aer incorporation of tin, consistent with the ndings of
has been used for many of the same heterogeneously catalyzed Tang et al.20 In addition to incorporation of tin into the beta
reactions as its hydrothermally synthesized analogue, including zeolite framework, the presence of crystalline extra framework
BVO,14,18,20–22 MPV reductions,14 Oppenauer oxidations (OPO),14 (EFW) tin was also observed (2q ¼ 26.7ꢀ and 34.0ꢀ).19 Applica-
hydration of epoxides20 and glucose isomerization.19 In this tion of the Scherrer equation indicated the average tin dioxide
work, we applied a PS tin beta zeolite/hydrogen peroxide particle size to be 48 nm. EFW species are incorporated as
oxidation system to acetophenone derivatives and dimeric a consequence of tin hydrolysis by adventitious water during
lignin model compounds.
graing, or from calcination due to the high tin loading (3.7
wt%). The presence of water before calcination hydrolyzes Sn–
Cl bonds, forming low-coordinate Sn–OH species; upon calci-
nation, Sn–OH bonds in close spatial proximity dehydrate
forming catalytically inactive SnO2 crystals as opposed to
2. Results and discussion
2.1. Tin beta synthesis and characterization
Beta zeolite (Si : Al, 25 : 1) was rst treated with 13 M nitric acid
to afford dealuminated beta zeolite (De-Al beta) with a Si/Al ratio
of >1800 (Table 1). The De-Al beta was treated with butyltin
trichloride in toluene, followed by triethylamine to promote
formation of Sn–O–Si bonds via hydrochloric acid elimination
(procedure modied from Corma and coworkers).23 The catalyst
was then calcined at 500 ꢀC for 3 h to remove remaining ligands,
yielding tin beta zeolite with a Si/Sn ratio of 12 (Sn-beta, 3.7 wt%
Sn).
The surface area and micropore volumes measured at each
step of tin beta zeolite synthesis (Table 1) showed little varia-
tion, indicating that removal of framework aluminum did not
lead to collapse of the zeolite lattice. Ammonia temperature-
programmed desorption (NH3-TPD) was performed to
measure the relative acidity of De-Al beta zeolite and tin beta
Fig. 1 X-ray diffractograms of tin dioxide, H-beta, De-Al-beta, and Sn-
zeolite. De-Al beta zeolite adsorbed a minimal amount of beta.
Table 1 Si/Al ratio, surface area and pore-size distribution of zeolite samples. Tin loading is 3.7 wt%a,b
Pore volumee (cm3 gꢁ1
)
Zeolite
Si/Al ratio
Surface aread (m2 gꢁ1
)
Micro
Meso
Macro
NH3 ads. (mmol gꢁ1
)
H-beta
De-Al beta
Sn-beta
25
480
472
479
0.142
0.135
0.142
0.253
0.227
0.158
0.161
—
—
—
>1800b
>1800b,c
ꢂ1
28.4
a
b
c
d
Micropore (2 nm), mesopore (2 nm–50 nm) macropore (>50 nm). Determined by ICP analysis. Determined by PIXE. Determined by BET
method. e Micropore volume calculated from t-plot.
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framework Sn–O–Si bonds. Dijkmans et al.21 found the
maximum tin loading for the PS tin beta zeolite used in their
report without formation of EFW species to be approximately
2%.
Fig. 2 shows SEM images of De-Al beta and Sn-beta. SEM
images of the dealuminated beta zeolite show highly textured
particles characteristic of beta zeolite. Images of beta zeolite
following tin incorporation show textured particles as well as
a smaller (ꢂ40 nm) secondary phase that is structurally
different and which can be assigned to EFW tin dioxide.
In an effort to evaluate tin speciation, 119Sn magic-angle-
spinning (MAS) nuclear magnetic resonance (NMR) spectros-
copy was employed. 119Sn MAS NMR spectroscopy has been
reported to effectively distinguish between EFW and framework
tin species based on coordination number. Removal of water
aids in the elucidation of open tetrahedral, closed tetrahedral
and octahedral tin species present in the catalyst. To remove
adsorbed water, tin beta zeolite was heated at 200 ꢀC overnight
in vacuo prior to data collection. Spectra of both the hydrated
and dehydrated samples were collected, effectively monitoring
the dehydration process.
Fig. 3 Comparison of 119Sn MAS NMR spectra of tin dioxide, dehy-
drated tin beta zeolite, and hydrated tin beta zeolite.
spectrum of hydrated tin beta zeolite resonate at ꢁ579 ppm,
and are absent aer dehydration. In a recent report, Yakimov
et al.24 observed penta-hydrated tin species at ꢁ581 ppm.
Crystalline tin dioxide resonating at ꢁ606 ppm was also
observed.
In the spectrum of the dehydrated tin beta zeolite (Fig. 3),
a broad resonance at ꢁ437 ppm indicates the presence of both
open and closed tetrahedral tin species, which were previously
hydrated.20,25 Tetrahedrally coordinated tin species reported by
Davis and coworkers19 were centered at ꢁ424 ppm and
ꢁ443 ppm aer dehydration, the authors assigning the down-
eld resonance to open tin sites (strongly acidic, SnOH(OSi)3)
As shown in Fig. 3, 119Sn MAS NMR spectra for hydrated and
dehydrated tin beta zeolite contain resonances for both octa-
hedral and tetrahedral tin. The observed resonances in the
spectrum of hydrated tin beta zeolite at ꢁ630 ppm to ꢁ720 ppm
are assigned to the presence of hexa-coordinate hydrated
tetrahedral tin species. Davis and coworkers19 observed similar
resonances centered at ꢁ688 ppm and ꢁ700 ppm for hydrated
species. Penta-coordinate hydrous tin species present in the
Fig. 2 SEM images of De-Al beta zeolite (left) and Sn-beta (right).
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and the upeld resonance to closed tin sites (weakly acidic, this work with those reported in the literature, 2-adamantanone
Sn(OSi)4). Following dehydration, crystalline tin dioxide was was oxidized using the tin beta zeolite/H2O2 system (Scheme 5).
still present as indicated by a resonance centered at ꢁ604 ppm 2-Adamantanone is less susceptible to the hydroxylation side-
corresponding to EFW tin (Fig. 3).
reactions discussed below, which makes 2-adamantanone
Diffuse reectance UV-Vis spectroscopy is a sensitive tech- a valuable probe molecule to aid in discerning reactivity trends.
ꢀ
nique that can distinguish between octahedral and tetrahedral Aer 24 h at 80 C the corresponding lactone was obtained in
Sn4+ to O2ꢁ ligand to metal charge transfer.20 A local intensity 83% yield (>99% selectivity), comparable to previous reports for
maximum at 200 nm suggests incorporation of tin into tetra- tin beta zeolite/H2O2.18,22
hedral sites (ESI Fig. S1†), consistent with the nding of Davis
The tin beta zeolite/hydrogen peroxide oxidation system was
et al.19 who reported an intensity maximum at 203 nm. However, then used to investigate BVO of acetophenone 1 (Table 2). Low
a broad intensity maximum at 281 nm arises from the presence yields of phenyl acetate (1a, 33%) were observed as a result of
of hexa-coordinate polymeric tin (EFW tin), which largely poor electron donation to the carbonyl oxygen, slowing nucle-
dominates the spectrum. Consequently, the spectrum of tin ophilic addition of hydrogen peroxide.26,27 Alkyl migration from
beta zeolite is similar to that of bulk tin dioxide (maximum at the Criegee intermediate is considered to be the rate-limiting
270 nm).
step, however, when using weak oxidants such as hydrogen
Tin speciation was also investigated by X-ray photoelectron peroxide, the rate-limiting step can become nucleophilic addi-
spectroscopy (XPS). The spectrum of tin beta zeolite contained tion.28 While this result is disappointing, angiosperm and
broad signals centered around 486.0 eV and 494.6 eV corre- gymnosperm lignins are the result of coniferyl (G) and sinapyl
sponding to tin 3d3/2 and 3d5/2 photoelectrons (Fig. 4). In (S) alcohol polymerization, these monolignols containing one
comparison, the 3d3/2 and 3d5/2 photoelectrons of crystalline tin and two methoxy group(s), respectively. Therefore, addition of
dioxide produced signal maxima at 487.0 eV and 495.5 eV, the an electron-donating group (EDG) was investigated to deter-
signals being much narrower (FWHM ¼ 1.6 eV) than the cor- mine if it positively affects catalyst activity.26,27 Indeed, when
responding signals for dehydrated tin beta (FHWM ¼ 3.7 eV). EDGs were present (i.e., methyl and methoxy groups, corre-
We propose that line broadening in the spectrum of dehydrated sponding to 2 and 3, respectively), a higher yield of the corre-
tin beta is due to the presence of overlapping signals from sponding phenyl acetate (2a, 3a) was observed. Due to an EDG
a mixture of octahedral and tetrahedral Sn4+ species, consistent inuenced increase in the basicity of the carbonyl oxygen,27
with the data presented above.
was converted in 81% yield to 3a at 45 ꢀC aer 24 h. When the
3
ꢀ
In summary, characterization of the synthesized tin beta temperature was increased to 80 C selectivity decreased from
zeolite conrmed the presence of tetrahedral Sn4+ species. 90% to 83%, and a notable darkening of the reaction mixture
Given that Lewis acid sites are believed to be responsible for was observed, implying formation of phenolic resins. To
Baeyer–Villiger catalysis,14 such low coordinate Sn species demonstrate facile cleavage of the ester, aer BVO 40-methox-
should function as active sites. These correspond to the “open” yphenyl acetate (3a) was hydrolyzed to 4-methoxyphenol (9b,
and “closed” tetrahedral tin sites observed by 119Sn MAS NMR 98% yield) using potassium carbonate and methanol at room
spectroscopy, although other coordinatively unsaturated temperature for 30 min.29
amorphous tin species on the catalyst surface could also
contribute to catalysis.
Considering that 4 contains an EDG in the ortho-position,
surprisingly low yields of 4a were observed (Table 2). This could
be due to hindrance of the ortho-substituent to both coordina-
tion of 4 to the catalyst surface, and alkyl migration. A similar
effect was observed for 5, which contains both meta and para-
methoxy groups, and which gave 5a in 58% yield. In reactions
with poor selectivity, such as the BVO of 1 and 4, water-soluble
resins were also obtained. Likely the product of direct hydrox-
ylation (discussed below) or phenolic radical coupling following
ester hydrolysis, resin formation was much more prevalent at
80 ꢀC than 45 ꢀC (as in the case of model 3), presumably due to
the increased rate of hydrogen peroxide homolysis. Addition-
ally, mequinol was detected in 9% yield from hydrolysis of
model 3a at 45 ꢀC (Table 2). Hydrolysis could have occurred
during the reaction, during reaction work up, or in the gas
chromatograph inlet.
2.2. BVO of simple cyclic and acyclic ketones
In nearly all cases, literature reports of heterogeneous BVO
concern cyclic ketones. Thus, in order to compare the catalyst in
Fig. 4 XPS spectra of tin beta zeolite and tin dioxide in the Sn 3d
region.
Scheme 5 BVO of 2-adamantanone.
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Table 2 BVO of acetophenone derivatives using tin beta zeolite/hydrogen peroxide oxidation systema
Model number
Model
Conv. (%)
Selectivity (%)
Yield (%)
Product number
Product
1
54
41
33
1a
2
99
90
58
60
90
47
59
81
27
2a
3a
4a
3b
4
5
91
64
58
5a
a
All reactions were performed in 1,2-dichloroethane at 80 ꢀC for 24 h unless otherwise noted. b Reaction was conducted at 45 ꢀC. Control reactions
were run with crystalline tin oxide (2% conversion) and dealuminated beta zeolite (1% conversion), yielding only trace amounts of 40-methoxyphenyl
acetate, 3a, at 80 ꢀC. Mequinol, 9b, was observed in 9% yield in the reaction of model 3. Phenol was observed in 23% yield in the reaction of model 1.
Given the high yield of 3a, compound 3 was selected for due to the low solubility of lignin in non-polar solvents, but
a catalyst reusability study (Fig. 5). Aer each sequential cycle provide potential for other heterogeneous BVO applications.
ꢀ
the catalyst was thermally regenerated at 500 C in air for 3 h.
Of the solvents evaluated, 1,2-dichloroethane (which is water
Minimal loss in activity (3% yield) was observed aer the 3rd immiscible) gave the highest yield of phenyl acetate. The
catalytic cycle. resistance to oxidation and immiscibility of chlorinated
Using 3 as a probe molecule, solvent effects in the tin beta solvents has previously been reported to yield high conversions
zeolite/H2O2 oxidation system were explored at 45 ꢀC (Table 3). in BVO reactions as demonstrated with chlorobenzene.18 The
Initially, water-miscible solvents were evaluated (e.g., ethanol), use of a biphasic system limits the solubility of hydrogen
however they generally afforded modest yields. Acetonitrile was peroxide in the organic solvent, decreasing the prevalence of
also tested, since in the presence of H2O2 it forms peroxyimidic side reactions such as direct ring hydroxylation and ester
acid, a more powerful oxidant/nucleophile than H2O2. However, hydrolysis, which occur readily at temperatures above 45 ꢀC. In
results were disappointing, a 24% yield of 4-methoxyphenol BVO, hydrogen peroxide acts as a two-electron oxidant,
being obtained from ester hydrolysis. Other solvents (e.g.,
toluene) were also determined to be compatible with this
oxidation system. They were not considered further in this study
Table 3 BVO of 3 using tin beta zeolite/hydrogen peroxide in different
solventsa
Conversion
(%)
Selectivity
(%)
Yield 3a
(%)
Solvent
1,4-Dioxane
Acetonitrileb
Toluene
48
40
72
55
99
90
99
95
3
45
1
95
>99
49
90
83
68
55
49
81
83
Ethanol
Ethanolc,d
1,2-Dichloroethane
1,2-Dichloroethanec
a
Selectivity and yields are expressed in terms of 40-methoxyphenyl
acetate, 3a. Reactions were conducted at 45 ꢀC unless otherwise
Fig. 5 Results of catalyst reusability study. Cycles one and two were
run in duplicate and yields are reported as averages of the two runs.
Cycle three is reported as a single run due to limited catalyst
availability.
b
noted. In the case of BVO with acetonitrile, ester hydrolysis resulted
c
in 24% yield of 4-methoxyphenol, 9b. Reaction conducted at 80 ꢀC.
d
9b was observed in 31% yield.
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performing nucleophilic addition to form the Criegee inter-
Small amounts of the corresponding phenol were detected in
mediate. However, as the temperature is increased hydrogen the reactions of 6 and 7, likely resulting from hydrolysis of the
peroxide decomposition is accelerated. Homolysis of the ester during aqueous workup or thermal cleavage upon injec-
peroxide bond forms hydroxyl radicals that are very active one- tion during gas chromatography. In the case of compounds 6–8,
electron oxidants. Hydroxyl radicals can perform direct ring no 4-methoxybenzoic acid resulting from alkyl migration was
hydroxylation as well as many other side-reactions.26 In present.
competition with BVO, direct ring-hydroxylation reactions can
The crystal structure of the BVO product of lignin dimer 6 is
involve both starting material and BVO products. Without the shown below (Fig. 6a). The crystal structure conrms that the
presence of an organic layer (i.e., using a single aqueous phase product, 6a, is the result of aryl migration. 6a is over twice the
ꢀ
˚
˚
at 80 C), reactions resulted in the production of a dark insol- length (15.3 A) of the pore openings in zeolite beta (7 A),
uble phenolic resin or tar, as has been reported for similar inhibiting its diffusion through the micropore system. Although
reactions (i.e., hydrogen peroxide mediated oxidation of in some conrmations (via rotation about the C8–C9 bond) 6a
benzene to phenol).6 Interestingly, at low temperatures the may be able to enter the pore system, conformational require-
highest selectivity was obtained when ethanol was the solvent. ments of the reaction intermediates (specically the Criegee
However when the reaction temperature was increased, lower intermediate) make the occurrence of BVO in micropores
selectivity and an increased yield of unidentiable products was unlikely. Consequently, catalysis likely takes place at tin centers
observed. In the case of ethanol at 80 ꢀC, mequinol (9b) was located in mesopores. The crystal structure of BVO product 7a
obtained in 31% yield, whereas 9b was not observed when the was also determined (Fig. 6b; see also Tables S1–S9 in the ESI†
solvent was DCE, consistent with decreased ester hydrolysis in for additional details of the structures of 6a and 7a).
the biphasic system.
The occurrence of selective aryl migration is contrary to the
ndings of Patil and coworkers9 who found that alkyl migration
was preferred over aryl migration for all lignin model
compounds in their homogeneous oxidation system. In our
work the p-methoxybenzene migrating group was presumably
better able to stabilize positive charge accumulation in the
Criegee intermediate, consistent with DFT studies of
acetophenones.27
2.3. BVO of lignin model compounds
Using the tin beta zeolite/hydrogen peroxide oxidation system,
selected lignin dimer model compounds, similar to the retro
aldol products of a-position oxidation observed by Wang et al.,5
were oxidized in good to moderate yields. In the reaction of
these lignin model compounds, increased amounts of hydrogen
peroxide were used due to their lower reactivity as compared to
acetophenone derivatives, which can be attributed to their
increased steric bulk. Ketone compounds representing the
product of benzylic alcohol oxidation in b-O-4 and b-1 linkages
were converted to their respective BVO products as shown in
Table 4.
Regioselective aryl migration was observed for all cases
excluding BVO of anisoin 9, where mixed selectivity was
observed. Anisoin, 9, was converted almost quantitatively with 7
ꢀ
eq. of hydrogen peroxide at 80 C to products consistent with
BVO, the reaction resulting primarily in alkyl migration. The
observed change in selectivity is a result of polarization of the
Cb-OH group that stabilizes the partial positive charge during Cb
Table 4 BVO of lignin dimer model compounds using tin beta zeolite/hydrogen peroxide oxidation systema
Selectivity
(%)
Product
number
Model Number
Model
Conv. (%)
86
Yield (%)
32
Product
6
37
26
6a
7a
7
8
81
41
21
9
22
23
8a
9
94
22b, 15
9a, 9b
a
Product yields were determined by 1H-NMR spectroscopy using an internal standard. Mequinol, 9b was observed in 3% and 2% yields in the
b
reaction of models 6 and 7, respectively. Isolated product yield.
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size of 0.02ꢀ. X-ray diffractograms were referenced to the
International Centre for Diffractogram Data (ICDD) database.
Elemental concentrations were determined by Proton-Induced
X-ray Emission (PIXE) (Elemental Analysis Inc., Lexington, KY).
Room temperature solid state NMR spectra were acquired
using a Tecmag Redstone spectrometer (Tecmag, Inc., Houston,
TX) operating at 111.917 MHz for 119Sn (7.05 T static magnetic
eld). Samples were packed into 7.5 mm zirconia rotors and
sealed with Teon or Kel-F end caps (Revolution NMR, LLC,
Fort Collins, CO). Experiments were performed using a 7.5 mm
double resonance MAS probe (Varian, Palo Alto, CA). All 119Sn
spectra were acquired under MAS at 5 kHz at ambient condi-
tions. 119Sn chemical shis are reported relative to samarium
stannate at ꢁ102.6 ppm with an accuracy of ꢃ0.4 ppm. 119Sn
spectra were collected with a single pulse on 119Sn followed by
acquisition. All spectra were acquired with a 50 second recycle
delay and a 2.56 ms acquisition time. Dehydrated samples were
heated to 473 K in vacuo overnight, prior to data collection.
Scanning electron microscopy (SEM) was performed on
a Hitachi S-2700 microscope equipped with a LaB6 gun and
a PGT EDS analyzer with thin window detector. Samples were
gold-coated prior to imaging. Brunauer–Emmett–Teller (BET)
surface area and micropore volume measurements were deter-
mined by nitrogen adsorption at 77 K using a Micromeritics
Gemini VII analyzer. Prior to measurement, samples were
degassed at 423 K under nitrogen. X-ray Photoelectron Spec-
troscopy (XPS) was performed on a Kratos Axis HSi X-ray
photoelectron spectrometer equipped with a charge neutral-
izer and magnetic focusing lens, using a monochromated Mg
Ka X-ray source (hn ¼ 1253.6 eV). Spectra were referenced to the
adventitious C 1s peak at 284.6 eV. Prior to analysis, XPS
samples were dried under high vacuum at 303 K and then
transferred to an in situ stage where they were dried at 673 K in
vacuo for 1 h. UV-Vis Diffuse Reectance spectra were collected
on a Varian Cary 5000 spectrometer using barium sulfate as
a reference. NH3-TPD experiments were performed on a Micro-
meritics AutoChem II analyzer using 0.500 g of sample. In each
case the sample was rst dehydrated at 673 K under argon for
1 h, cooled to 363 K and saturated with NH3 (1% in helium, 50
sccm) for 1 h. Next, the sample was purged with He (120 sccm)
Fig. 6 (a) Crystal structure of the Baeyer–Villiger oxidation product 6a
of 2-(4-methoxyphenoxy)-1-(4-methoxyphenyl)-ethanone (6). (b)
Crystal structure of the Baeyer–Villiger oxidation product 7a of 2-(4-
methoxyphenoxy)-1-(4-methoxyphenyl)-ethanone (7). Crystals of 7a
were twinned by non-merohedry, and there were two molecules in
the asymmetric unit. For the sake of clarity, only one of the indepen-
dent molecules is shown.
alkyl migration in the Criegee intermediate. Hemiacetal
decomposition followed by aldehyde oxidation aer BVO
resulted in 40-methoxybenzoic acid, 9a (22%). Mequinol, 9b, was
also detected (15%), resulting from aryl migration.
Patil et al.9 rst reported homogenous BVO of lignin model
dimer compounds under a formic acid/hydrogen peroxide envi-
ronment, achieving a maximum yield of 10% of the BVO product
of alkyl migration (Scheme 4). In the same report, using a more
complex model they reported a 78% yield of benzoic acid from
cleavage of the ester but observed polymerization of the phenolic
co-product. In this work we obtained ester yields of respectively
32% and 21% in the BVO of b-O-4 model compounds 6 and 7
using tin beta zeolite/H2O2, which can be cleaved in a second step
to recover phenolic moieties. Furthermore, b-1 model
compounds were also successfully oxidized. Moreover, the
heterogeneous nature of this catalytic system makes this reaction
more industrially applicable due to generation of water as a co-
product and separation of the catalyst by ltration.
On the other hand, the effects of direct hydroxylation of the
aromatic groups are reected in the moderate yields. Ester yields
were found to be affected by reaction time, temperature and
hydrogen peroxide concentration. Low yields can be caused by
hydrolysis of BVO products to form phenols that are susceptible
to multiple hydroxylations, as well as H2O2 homolysis, which
limits the conversion of the ketone reactant. In an effort to
improve product yields, staging of the H2O2 addition was inves-
tigated. However, the addition of aliquots of H2O2 over a three-day
period (14 eq. initially and 7 eq. aer 24 h) led to mass balances as
low as 14%, due to formation of insoluble phenolic resins.
for 1 h. The sample was then heated to 1023 K at 10 K minꢁ1
.
Effluent gas was analyzed using a mass spectrometer (Pfeiffer
Thermostar GSD301), the signal at m/z ¼ 15 being used to
monitor NH3.
Single crystal X-ray diffraction data were collected at 90.0(2)
K on a Bruker-Nonius X8 Proteum diffractometer with graded-
multilayer focused Cu Ka X-rays. Raw data were integrated,
scaled, merged and corrected for Lorentz-polarization effects
using the APEX2 package.30 Corrections for absorption were
applied using SADABS.31 The structure was solved by direct
methods (SHELXT)32 and rened against F2 by weighted full-
matrix least-squares (SHELXL-2014).33 Hydrogen atoms were
found in difference maps but subsequently placed at calculated
positions and rened using a riding model. Non-hydrogen
atoms were rened with anisotropic displacement parameters.
3. Experimental methods
3.1. Catalyst characterization
X-ray diffractograms were collected on a Phillips PW 3040 X-ray The nal structure model was checked using an R-tensor34 and
diffractometer using Cu Ka radiation (l ¼ 1.54184 A) and a step by Platon/checkCIF.35 Atomic scattering factors were taken from
˚
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the International Tables for Crystallography.36 Crystal structure 107.1 (24%) and 1H-NMR (400 MHz, CDCl3) d: 7.05–7.00 (m,
data are deposited at the Cambridge Crystallographic Data 2H), 6.95–6.92, (m, 2H), 6.90–6.85 (m, 2H), 4.80 (s, 2H), 3.80 (s,
Center (CCDC deposition numbers 1541264 and 1541265).†
3H), 3.78 (s, 3H). 13C-NMR (100 MHz, CDCl3) d: 168.3, 157.7,
155.0, 152.2, 143.8, 122.3, 116.3, 115.0, 114.9, 66.7, 55.9, 55.8
(see Fig. S2 and S3 in the ESI†). Ferrocene (10 mg mLꢁ1) was
used as an internal standard to quantify the yield of products in
the crude product mixture. HRMS (ESI) m/z [M+H] calculated for
3.2. Synthesis of graed tin beta zeolite (Si : Al ratio ¼ 25 : 1)
Commercial H-beta-zeolite (25 g), obtained from Clariant®
(Si : Al ratio ¼ 25 : 1), was treated with 13 M nitric acid and
heated at 353 K overnight (solid/liquid ratio ¼ 1 g/20 mL). The
mixture was ltered and washed with deionized water until
neutral. This dealumination procedure was repeated a second
time. Two batches were mixed and dried in vacuo overnight at
343 K, yielding De-Al beta (42.8 g). De-Al beta was added to
a solution of butyltin trichloride (7.6 mL, 45.6 mmol) in anhy-
drous toluene (150 mL) under nitrogen and stirred at room
temperature for 1 h. The solution was neutralized by addition of
triethylamine (17.6 mL, 126.2 mmol) and stirred for 1 h. The
suspension was then ltered, washed with toluene (500 mL),
and dried overnight in a vacuum oven. The catalyst was heated
in air to 623 K for 30 min followed by calcination at 773 K for
3 h, yielding 37.3 g of Sn-beta zeolite.
C
16H16O5 ¼ 289.1076, experimental ¼ 289.1072.
3.4.2 Baeyer–Villiger oxidation of 2-(2-methoxyphenoxy)-1-
(4-methoxyphenyl)-ethanone (7). Tin beta zeolite (0.150 g), 2-
(2-methoxyphenoxy)-1-(4-methoxyphenyl)-ethanone (0.41 g, 1.5
mmol), 1,2-dichloroethane (3 g) and 30% hydrogen peroxide (2
mL, 20 mmol) were added to a 50 mL round-bottomed ask
equipped with a jacketed water-cooled condenser, and heated at
the desired temperature with stirring (80 ꢀC, 24 h). BVO product
7a was isolated as pale yellow crystals. GCMS: m/z 288.1 (16%),
260.1 (17%), 137.1 (100%), 122.1 (49%), 109.1 (15%) and 1H-
NMR (400 MHz, CDCl3) d: 7.04–7.00 (m, 2H), 6.98–6.86 (m,
6H), 4.92 (s, 2H), 3.90 (s, 3H), 3.79 (s, 3H). 13C-NMR (100 MHz,
CDCl3) d: 168.3, 157.7, 150.1, 147.4, 143.9, 123.2, 122.3, 121.0,
115.4, 114.7, 112.5, 67.0, 56.1, 55.8 (see Fig. S4 and S5 in the
ESI†). Ferrocene (10 mg mLꢁ1) was used as an internal standard
to quantify the yield of products in the crude product mixture.
HRMS (ESI) m/z [M+H] calculated for C16H16O5 ¼ 289.1076,
experimental ¼ 289.1070.
3.3. Synthesis of lignin model compounds
Model compounds 6 (ref. 37) and 7 (ref. 38) were synthesized
according to literature procedures which are included in the
ESI.†
3.4.3 Baeyer–Villiger oxidation of desoxyanisoin (8). Tin
beta zeolite (0.15 g), desoxyanisoin (0.39 g, 1.5 mmol), 1,2-
dichloroethane (3 g) and 30% hydrogen peroxide (2 mL, 20
3.4. Procedure for Baeyer–Villiger oxidations
All reagents were purchased from Fisher Scientic or Sigma mmol) were added to a 50 mL round-bottomed ask equipped
Aldrich and were used without further purication, unless with a jacketed water-cooled condenser, and heated at the
stated otherwise. In a typical reaction, tin beta zeolite (0.150 g), desired temperature with stirring (80 ꢀC, 24 h). BVO product 8a
4-methoxyacetophenone, 3 (0.451 g, 3 mmol), 1,2-dichloro- was isolated as a white solid. GCMS: m/z 272.1 (4%), 148.1
ethane (3 g) and 30% hydrogen peroxide (2 mL, 20 mmol) were (100%), 121.1 (95%) and 1H-NMR (400 MHz, CDCl3) d: 8.06–
added to a 50 mL round-bottomed ask equipped with a jack- 8.03, 7.31–7.29 (m, 2H), 6.98–6.96 (m, 2H), 6.89–6.87 (m, 2H),
eted water-cooled condenser, and heated at the desired 3.88 (s, 2H), 3.81 (s, 3H), 3.71 (s, 3H). 13C-NMR (100 MHz,
temperature with stirring for 24 h. Aer completion of the CDCl3) d: 170.9, 159.1, 144.5, 131.1, 130.6, 128.8, 122.4, 114.6,
reaction, the suspension was ltered through a Nylon® 114.4, 114.0, 55.8, 55.5, 40.7 (see Fig. S6 and S7 in the ESI†).
membrane (0.45 mm pore size). The catalyst was rinsed with Ferrocene (10 mg mLꢁ1) was used as an internal standard to
dichloroethane (5 mL ꢄ 2) and water (5 mL ꢄ 2). The biphasic quantify the yield of products in the crude product mixture.
mixture was separated and the aqueous layer was extracted with HRMS (ESI) m/z [M+H] calculated for C16H16O4 ¼ 273.1127,
dichloroethane (2 mL ꢄ 3) and dried over magnesium sulfate. experimental ¼ 273.1122.
In the case of reusability experiments, the catalyst was calcined
3.4.4 Baeyer–Villiger oxidation of anisoin (9). Tin beta
at 500 ꢀC for 3 h and allowed to cool to room temperature prior zeolite (0.15 g), anisoin (0.41 g, 1.5 mmol), 1,2-dichloroethane (3
to being reused. For the hydrolysis of 3a, potassium carbonate g) and 30% hydrogen peroxide (1 mL, 10 mmol) were added to
(0.455 g, 3.3 mmol) in methanol (3 g) was added to the reaction a 50 mL round-bottomed ask equipped with a jacketed water-
mixture. The mixture was stirred for 30 min at room tempera- cooled condenser, and heated at the desired temperature with
ture and was then neutralized with hydrochloric acid to afford 4- stirring (80 ꢀC, 24 h). 4-Methoxybenzoic acid (9b)39 was isolated
methoxyphenol 9b (0.307 g, 98% yield).
as colorless crystals. Ferrocene (10 mg mLꢁ1) was used as an
3.4.1 Baeyer–Villiger oxidation of 2-(4-methoxyphenoxy)-1- internal standard to quantify the yield of products 9a (ref. 40) in
(4-methoxyphenyl)-ethanone (6). Tin beta zeolite (0.15 g), 2-(4- the crude product mixture.
methoxyphenoxy)-1-(4-methoxyphenyl)-ethanone (0.41 g, 1.5
mmol), 1,2-dichloroethane (3 g) and 30% hydrogen peroxide (2
mL, 20 mmol) were added to a 50 mL round-bottomed ask
equipped with a jacketed water-cooled condenser, and heated at
3.5. Determination of phenyl acetate yields using gas
chromatography mass spectrometry (GCMS)
the desired temperature with stirring (80 ꢀC, 24 h). BVO product GCMS analyses were performed using an Agilent 7890 GC with
6a was isolated as described above as orange-brown crystals. a tandem Agilent 5975C MS detector. The column used was
GCMS: m/z 288.1 (24%), 260.1 (10%), 137.1 (100%), 123.1 (17%), a DB-1701 (60 m ꢄ 0.25 mm ꢄ 0.25 mm) and the temperature
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program was as follows: 45 ꢀC for 3 min, ramp to 280 ꢀC at 4 ꢀC
minꢁ1, and hold for 10 min. The ow rate was set to 1 mL minꢁ1
using helium as the carrier gas. The inlet was maintained at
5 M. Wang, J. Lu, X. Zhang, L. Li, H. Li, N. Luo and F. Wang,
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8 A. Rahimi, A. Ulbrich, J. J. Coon and S. S. Stahl, Nature, 2014,
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ꢀ
260 C, and the MS source was set at 70 eV. An external cali-
bration curve of anisole or dodecane was used to calibrate BVO
products in the reaction mixture. Analysis of lignin model
dimer compounds proved difficult (due to the low response
factors observed for oxidized dimer model compounds). In lieu
1
of calibrated GCMS yields, H-NMR spectroscopy with a ferro-
cene internal standard was used to determine product yields.
¨
10 J. Mottweiler, M. Puche, C. Rauber, T. Schmidt,
´
P. Concepcion, A. Corma and C. Bolm, ChemSusChem,
4. Conclusions
2015, 8, 2106.
The tin beta zeolite/H2O2 oxidation system was applied to 2- 11 Y. Gao, J. Zhang, X. Chen, D. Ma and N. Yan, ChemPlusChem,
adamantanone, several acetophenone derivatives and oxidized 2014, 79, 825.
lignin b-O-4 and b-1 linkage models. Selective aryl migration 12 M. R. Sturgeon, M. H. O'Brien, P. N. Ciesielski, R. Katahira,
was observed in all cases excluding anisoin, where both aryl and
alkyl migration were observed. The oxidation system presented
in this work yields esters that can be cleaved in a simple
J. S. Kruger, S. C. Chmely, J. Hamlin, K. Lawrence,
G. B. Hunsinger, T. D. Foust, R. M. Baldwin, M. J. Biddy
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hydrolysis reaction, yielding phenolic moieties that are typically 13 J. S. Kruger, N. S. Cleveland, S. Zhang, R. Katahira,
difficult to isolate from b-O-4 oxidation reactions. Yields of ester
products derived from b-O-4 and b-1 lignin models were
B. A. Black, G. M. Chupka, T. Lammens, P. G. Hamilton,
M. J. Biddy and G. T. Beckham, ACS Catal., 2016, 6, 1316.
generally modest due to the formation of polymeric material 14 J. Dijkmans, W. Schutyser, M. Dusselier and B. Sels, Chem.
stemming from direct ring hydroxyl. While preventing the Commun., 2016, 52, 6712–6715.
formation of byproducts is challenging, if the reaction were run 15 A. Corma, V. Fornes, S. Iborra, M. A. Mifsud and M. Renz, J.
at low conversion (shorter residence time and/or lower
Catal., 2004, 221, 67–76.
temperature) then the selectivity should increase due to 16 S. P. Panchgalle, U. R. Kalkote, P. S. Niphadkar, P. N. Joshi,
´
decreased ring hydroxylation and resin formation. Naturally,
this would require a means for separating the products and
S. P. Chavan and G. M. Chaphekar, Green Chem., 2004, 6,
308–309.
starting material, so that the latter could be recycled. To our 17 A. Corma, M. E. Domine, L. Nemeth and S. Valencia, J. Am.
knowledge, this is the rst report of heterogeneous BVO of
Chem. Soc., 2002, 124, 3194–3195.
lignin model dimer compounds.
18 P. Li, G. Liu, H. Wu, Y. Liu, J.-G. Jiang and P. Wu, J. Phys.
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19 R. Bermejo-Deval, R. Gounder and M. E. Davis, ACS Catal.,
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Acknowledgements
This material is based on work supported by the National 20 B. Tang, W. Dai, G. Wu, N. Guan, L. Li and M. Hunger, ACS
Science Foundation under Cooperative Agreement No. 1355438. Catal., 2014, 4, 2801–2810.
Funding from the British Council under the Global Innovation 21 J. Dijkmans, J. Demol, K. Houthoofd, S. Huang, Y. Pontikes
Initiative for the GB3-Net project is also gratefully acknowl-
and B. Sels, J. Catal., 2015, 330, 545–557.
edged. The single-crystal diffractometer was funded by the NSF 22 J. Jin, X. Ye, Y. Li, Y. Wang, L. Li, J. Gu, W. Zhao and J. Shi,
(MRI CHE-0319176). Diffuse reectance UV-Vis spectra were Dalton Trans., 2014, 43, 8196–8204.
measured at the Center for Nanophase Materials Sciences, 23 A. Corma, M. T. Navarro and M. Renz, J. Catal., 2003, 219,
which is a DOE Office of Science User Facility. The authors
242–246.
would also like to thank Shelley Hopps, Dr Mark Meier, Dr 24 A. V. Yakimov, Y. G. Kolyagin, S. Tolborg, P. N. Vennestrøm
Justin Mobley, Yang Song, Gerald Thomas, and Soledad Yao for
their help in the preparation of this manuscript.
and I. I. Ivanova, J. Phys. Chem. C, 2016, 120, 28083–28092.
25 R. Bermejo-Deval, R. S. Assary, E. Nikolla, M. Moliner,
´
Y.
Roman-Leshkov,
S.-J.
Hwang,
A.
Palsdottir,
D. Silverman, R. F. Lobo and L. A. Curtiss, Proc. Natl. Acad.
Sci. U. S. A., 2012, 109, 9727–9732.
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