Alkylphenols to phenol and olefins by zeolite catalysis: A pathway to valorize raw and fossilized lignocellulose

Article abstract of DOI:10.1039/c5gc01868d

Selective conversion of alkylphenols to phenol and olefins is presented as a challenging key step in upgrading raw and fossilized lignocellulose. An exceptional and stable dealkylation performance is achieved by application of an acidic ZSM-5 zeolite, in which co-feeding of water is crucial to maintain catalytic activity. The role of water is attributed to competitive adsorption of water and phenol. The lignin-first pathway towards phenol yields a tenfold improvement of phenol compared to the state-of-the-art single-step lignocellullosic depolymerization techniques.

Full text of DOI:10.1039/c5gc01868d

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Alkylphenols to phenol and olefins by zeolite catalysis: a pathway
to valorize raw and fossilized lignocellulose
D. Verboekend,a* Y. Liao,a W. Schutyser,a B. F. Selsa*
Received 00th January 20xx,
Accepted 00th January 20xx
Selective conversion of alkylphenols to phenol and olefins is presented as a challenging key step in upgrading raw and
fossilized lignocellulose. An exceptional and stable dealkylation performance is achieved by application of an acidic ZSM-5
zeolite, in which co-feeding of water is crucial to maintain catalytic activity. The role of water is attributed to competitive
adsorption of water and phenol. The lignin-first pathway towards phenol yields a tenfold of phenol compared to the state-
of-the-art single-step lignocellullosic depolymerization techniques.
DOI: 10.1039/x0xx00000x
www.rsc.org/
challenge of importance for today’s and future industry.15-19
Moreover, in order to ensure the economic feasibility of future
wood-fueled biorefineries, the valorisation of lignin is
Introduction
The contemporary large-scale consumption of fossil fuels will
eventually lead to the depletion of the world’s (cheap and
clean) oil reserves. The energy supplies should on the long
term be replaced by alternative energy sources as wind, water,
and solar. However, as the oil runs out, also the main source of
everyday base-chemicals required in the production of
polymers, pharmaceuticals, and other specialty chemicals may
be lost. Of the different types of base chemicals, aromatics
represent an important building block for the chemical
industry in the manufacture of plastics, rubbers, and other
synthetic fibers.1 Ultimately, these aromatics should be
derived from a sustainable source, such as woody biomass. On
the middle-long term the shortage of oil may, at least partially,
be compensated by the increased valorization of solid
fossilized lignocellulosic resources, such as peat, lignite, and
predominately coals. Hence, the most likely important future
sources of aromatics will be derived from solid lignocellulose in
its raw unprocessed form (wood) or its fossilized form (coal).
The main constituents of raw lignocellulose are cellulose,
hemi-cellulose, and lignin. While today many useful chemicals
can be derived from (hemi-)cellulose,2-11 the valorization of
lignin is of high relevance as it is the only of the three types
that comprises a substantial amount of aromatics (ca. 40 wt.%
benzene units).12 In addition, lignin is a common by-product of
the paper industry and of the second generation bioethanol
manufacture. In these processes, the surplus lignin is often
simply burnt off for heat and/or electricity recovery.13,14 The
extraction of aromatics from lignin therefore represents a
prerequisite.17-21
Several pathways to depolymerize lignin have been
reported.16-19 A particularly interesting lignin-first technology
simultaneously extracts and dissembles protolignin from the
lignocellulose matrix.22-26 Of the latter, a highly efficient
process was devised that manages to, without exhaustive
preparative steps, highly selectively convert protolignin into
methoxylated and alkylated phenols and oligomers thereof
Scheme 1: Illustration highlighting the relatively large steams of alkylphenols to
be derived from fossilized (coal) or raw (wood) lignocellulose. The values
represent streams in wt.%. *cresols and xylenols are not included. The dashed
box highlights the desired products in the dealkylation of alkylphenols (catalytic
conversion D). The details regarding the conversions A-D are summarized in
Table 1.
a.Dr. D. Verboekend, Y. Liao, W. Schutyser, Prof. B. F. Sels.
Department M2S, K.U. Leuven, Kasteelpark Arenberg 23, 3001 Heverlee
(Belgium), E-mail: danny.verboekend@biw.kuleuven.be, E-mail:
bert.sels@biw.kuleuven.be here.
Electronic Supplementary Information (ESI) available: [Thermo-gravimetric
analysis, catalytic tests, X-ray diffraction patterns, 27Al MAS NMR spectra, and N2
adsorption isotherms]. See DOI: 10.1039/x0xx00000x
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Table 1: Steps required to convert raw or fossilized lignocellulose into (among other chemicals) phenols and olefins.
phenol yielda
conversion
A
reagents
catalyst/process
products
step
(wt.%)
-
overall
(wt.%)
-
Source
45-47
fossilized
lignocellulose: coal
(+ O2 and H2O)
raw lignocellulose:
birch
(+ methanol and H2)
methoxylated
alkylphenols (+H2)
alkylphenols
Lurgi Phenoraffin
syngas, coal tar oil (rich in alkylphenols)
B
e.g. Ru/C, Ni/C
cellulose, hemicellulose, small lignin
oligomers and lignin monomers
(methoxylated alkylphenols)
alkylphenols, (methanol/methane not
mentioned)
50b
50
27,48
C
D
e.g. NiMo/Al2O3,
Pd/C, Ru/TiO2
H-ZSM-5
95
98
48
47
28-38
phenol and olefins
this work
(+H2O)
aMaximum phenol yields with respect to phenol present in protolignin. bClose to the expected theoretical maximum yield of birch wood lignin.24,27
(conversion B, Table 1, Scheme 1). About 50% of the wood
protolignin is for instance converted into phenolic monomers,7
approaching the theoretical maximum. The state of the art also
provides the knowledge to demethoxylate such methoxylated
(alkyl)phenols, yielding selectively the alkylphenol at high
conversions (and methane or methanol, conversion C).28-38
The high selectivity to lignin monomers using the lignin-
first strategy stands in great contrast to many other state-of-
the-art depolymerization techniques, such as pyrolysis,39,40
base-catalyzed depolymerisation,41,42 and hydrogenolysis.43,44
The latter works yield, besides often substantial unwanted side
products as char and light gases, a wide variety of up to 20 or
more products, and a phenol yield limited to ca. 5 %, with
respect to the phenol present in protolignin (Table S1). Such a
broad product slate not only requires dedicated separation
efforts, the upgrading of all the products into useful chemicals
requires a wide variety of catalytic reforming. In contrast, the
lignin-first strategy (Scheme
2) yields essentially three
products, facilitating their separation and catalytic upgrading.
Alternatively, the technology to isolate aromatics from coal
was developed decades ago.45-47 A good example is the Lurgi
process, which results primarily into syngas, but also yields a
residue (coal tar), which is rich in ethyl and propyl phenols
(conversion A, Table 1,
Scheme 1).45-48 Hence, using the
pathway from either coal or wood, for every ton of (dry)
lignocellulosic matter processed, roughly 100 kg of
alkylphenols are made available (scheme 1). Hence, to fuel
tomorrow’s chemical industry, the valorization of novel
intermediates, such as alkylphenols is highly relevant.
However, in contrast to the commonly known opposite
reaction, viz. the alkylation of phenols,49-52 the knowledge on
the dealkylation of alkylphenols is not very well established.
Herein, we stress the need to process alkylated phenols in
future wood- or coal-fuelled biorefineries. A highly efficient
zeolite-based process that enables to convert (mixtures of)
ethyl and propylphenols into high-value products, that is,
phenol and the associated olefins, ethylene and propylene
Scheme 2: The catalytic upgrading of alkylphenols derived from coal and lignin.
The details of the reaction are summarized in Table 1. The structure of lignin and
coal were based on refs. 18 and 41, respectively. The dashed box highlights the
challenging reaction of interest within this contribution. The overall phenol yield
(47%) on lignin basis (steps B to D, Table 1) exceeds the yields of state-of-the-art
single step lignin conversion processes which do not typically attain phenolic
monomer yields exceeding 5 % (Table S1). The exclamation signs highlight that
the selective demethoxylation of propylsyringol remains challenging and that,
like is the case for oil, the presence of sulphur and nitrogen could cause catalyst
poisoning, which needs to be addressed, for example, by hydrotreating.53
(Scheme 2), is pioneered. The developed pathway represents
an elegant example of the appeal of harvesting the
functionality present within biomass. Combined with the
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lignin-first depolymerization pathway, the overall process mixture of ethylphenol and propylphenol was achieved using
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enables to obtain a tenfold higher yield of phenol compared to an additional saturator and a molar DfOloI:w10.1c0o3m9/pCo5GsiCti0o1n868oDf
the state-of the art single-step processes. Whereas large pore 0.81/0.11/0.06/0.02 (N2/H2O/ethylphenol/propylphenol).
zeolites (H-USY) are less selective and deactivate rapidly, a
medium pore H-ZSM-5 is established as a superior catalyst,
Results and Discussion
enabling a high selectivity and long catalysts life-time.
Additionally, the results open the door to a unique type of
zeolite-based catalysis, in which water plays a pivotal role.
Alkylbenzene versus alkylphenol
Solutions to challenges of future bio-refineries may often be
inspired by the currently-available technologies.54 For
example, an obvious approach to successfully dealkylate
alkylphenols to valuable chemicals may be the evaluation and
utilization of the knowledge on similar existing processes in
Experimental
Materials
The catalyst used were commercial ZSM-5 zeolites (code the petrochemical industry. A relevant comparison to the
‘ZSM5’) of distinct nominal Si/Al ratios (Si/Al = 15: CBV 3024E, dealkylation of alkylphenols may be the dealkylation of
Si/Al = 40: CBV 8014, Si/Al = 140: CBV 28014) and a USY zeolite alkylbenzenes. However, phenols behave markedly different
(code ‘USY’, Si/Al = 40: CBV 780), all supplied by Zeolyst. The compared benzenes,55,56 and parallels of their chemistry are
NH4-ZSM-5 zeolites were calcined in static air (550°C, 5.5 h, mostly based on the similar carbon skeletons. The dealkylation
ramp rate 5°C min-1) to obtain the protonic form. The number of alkylbenzenes is not heavily studied as oil-derived aromatics
following the zeolite code indicates the Si/Al ratio of the are usually not strongly alkylated,53 and dealkylation is merely
sample.
performed to valorize side products from petrochemical
processes, such as in the catalytic reforming of naphta.57,58 The
dealkylation of ethyl and propylbenzenes is most commonly
performed in the gas phase using catalysts based on acidic
zeolites, like H-ZSM-5.57,58 This reaction was termed shape-
selective as a very high selectivity to benzene was obtained.
However, the reaction is only selective with a zeolite modified
with rare-earth cations, MgO, or most commonly with noble
metals, like Pt. In addition, the reaction is performed under a
(costly) H2 atmosphere, yielding instead of alkenes, lower-
value alkanes in addition to benzene. These adaptions are
required as the unmodified H-ZSM-5 originates severe
(deactivation by) coking.59-62
Characterization
Thermo-gravimetric analysis (TGA) was performed by using a
TGA-Q500 analyzer. The measurements were conducted in air
(30 cm3 STP min1) ramping the temperature from 30 to 800°C
at 10°C min1. Carbon content was determined by combustion
analysis using a Thermo Finnigan Flash EA 1112. 27Al MAS NMR
was performed using Bruker Avance 400 spectrometer
(B0 = 9.4 T) on fully hydrated samples, packed into 4 mm
zirconia rotors. Scans (18000) were accumulated with a recycle
delay of 200 ms. Powder XRD measurements were performed
on a STOE Stadi P instrument in transmission mode by using
CuKa radiation. Nitrogen-sorption measurements were
executed at -196°C with a Micromeritics TriStar 3000
instrument.
4-n-propylphenol is the most abundant alkylphenol to be
derived from lignin and is also very abundant in tar oil.16,17,48
This molecule was therefore selected as a prime substrate to
study within this contribution. Similar to the above-described
experiences on alkylbenzenes, H-ZSM-5 was first studied
(sample ZSM5-40, Figure 1a). The result shows an initial highly-
Catalytic testing
Dealkylation experiments were performed in a custom-built selective conversion to phenol and propylene. However, within
plug-flow fixed-bed reactor equipped with 4 parallel quartz 1 h the conversion was reduced from 80% to ca. 20%. Inspired
reactors (inner diameter of 3 mm). Typically, the quartz by patent literature63 and the need for catalysts, which process
reactors were filled with in total 120 mg of catalysts (sieve lignocellulose, to cope with moist streams,64 the influence of
fraction: 0.125-0.250 mm), yielding a catalyst bed height of ca. co-feeding water was tested. Remarkably, the addition of
13 mm. The alkylphenols (Sigma-Aldrich, >97%) and water water to the stream enabled to maintain the catalytic activity.
were brought to the gas phase using a nitrogen flow (20 mL Moreover, the water-free deactivation was completely
min-1) passed through a saturator. Afterwards these flows reversible (Figure 1b). Importantly, during the addition or
were mixed, yielding a gas mixture of molar composition: removal of water, no influence on the selectivity was
0.86/0.12/0.02 (N2/H2O/alkylphenol), and passed through the observed. This means that the ZSM-5 catalysts yields, unlike in
reactors. In the absence of water (only in Figures 1 and 2) the the dealkylation of alkylbenzenes, highly selectively phenol
molar flow composition was 0.98/0/0.02 (N2/H2O/alkylphenol). and olefin without the need for noble metals and associated
Effluent gasses were characterized using an in-line gas H2 atmosphere or other modifications. This outstanding
chromatograph equipped with a HP1 column and a FID performance stems from the unique role that water plays.
detector. The WHSV was varied by reducing the (total) catalyst
amount from 120 mg to 7.5 mg, while maintaining the
catalysts bed height by diluting the catalyst bed with quartz of
the same sieve fraction (0.125-0.25 mm). The dealkylation of a
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that it might shorten the lifetime of carbenium ion
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intermediates.50 Similarly, the water could actively participate
DOI: 10.1039/C5GC01868D
by moderating the zeolite’s acidity by the formation of water
clusters.66 On another account, effects have been attributed to
confinement effects or nest effects caused by water within the
zeolite pores.51 Conversely, water could also act as a sweeping
agent, forcing formed products to diffuse away from the
zeolite,67 and rendering the role of water to be mostly
adsorptive, hence physical, rather than chemical. However,
clear evidence of the role of water has thus far remained
absent.
Although further study may be required to unravel the role
of water in closer detail, the results in this contribution enable
to conclude that the role of the water is mostly sorptive. First
of all, should water be required to generate the active site, no
activity at all should be observed in the water-free
dealkylation. In contrast, Figure 1a clearly demonstrates a
significant activity during the first 2 h TOS in the absence of
water. Moreover, the deactivation in the absence of water
(Figure 1b) is fully reversible and should therefore not be
related to the presence of excessive coking. To study this in
more detail, characterization of the spent catalysts in Figure 1a
was executed. Thermo-gravimetric analysis (TGA) was
performed to assess the deposits on the catalysts after
reaction. The results (Figure SI1) reveal that in the range
where coke species may be expected (350-700°C)69 both
catalysts show exactly the same weight loss centered around
500°C. However, the sample that was tested in the absence of
water displayed an additional weight loss around 150°C, which
could very well be due to the presence of (alkyl)phenol. The
carbon content spent of the catalysts was quantified using
elemental analysis. These results show that the ZSM-5
catalytically evaluated in the presence of water comprised 4.4
wt.% of carbon, whereas the one tested the absence of water
comprised 6.0 wt.% of carbon. Based on the TGA, the
difference 1.6 wt.% likely constitutes (alkyl)phenol. The
presence of easily removable species on the ZSM-5 zeolite in
the absence of water was further supported by the carbon
content of the sample tested in Figure 1b. This ZSM5-40 was
treated in the absence of water for over 4 h followed by the
(re)introduction of water for another 4 h. The resulting
sample, although catalytically evaluated for 3 times longer,
showed carbon content of 4.8 wt.%. This is substantially lower
than that of the sample treated for only 4 h in the absence of
water in Figure 1a (6.0 wt.%). The difference in carbon content
(1.2 wt.%) indicates that during the absence of water, species
are deposited on the solid, which are removed in presence of
water. The non-chemical role of water is further corroborated
by the similar selectivities in Figure 1. Should the role of water
be chemical, then its presence may change the nature of the
active site, which likely cause an increase in side products. In
contrast, no significant change in selectivity is observed at any
stage of the reaction.
Figure 1: (a) Catalytic conversion (X) of propylphenol and selectivity (S) to phenol
and propylene over ZSM5-40 in the presence (solid symbols) and absence of
water (empty symbols). (b) Influence of the temporal removal of water during
the dealkylation of propylphenol performed over ZSM5-40. The absence of water
was compensated with nitrogen to maintain a constant flow of alkylphenol. T =
305°C and WHSV = 3.7 h-1.
On the role of water
In general, the role of water in zeolite-catalysed reactions is
only partially understood. Though the role of water has been
recognized, the underlying fundamentals were not
systematically studied. In a variety of reactions, mostly dealing
with condensation reactions of alcohols and aldehydes,49-
52,56,65-68
the water effect is even completely neglected or only
scarcely mentioned. More specifically for phenols, a study on
the zeolite-catalyzed alkylation of phenol with propylene
evidenced a clear influence of water.49 It enhanced activity by
25%, but the nature of the enhancement remains unclear.
Similarly, the liquid-phase condensation of formaldehyde with
phenol performed using beta zeolites requires the use of
externally added water.65 Other recent works evidenced that
the alkylation of phenol proceeds well in water with beta
zeolites.50,51 Alternatively, it was demonstrated that in the
alkylation of cresol with propanol the zeolite hydrophobicity
plays a key role.52 However, in these reactions the exact role of
water was not clarified. The latter may be due to that
alkylations are, on an academic scale, often executed in batch
reactors, which inhibits the study of deactivation phenomena
(as can be observed in Figure 1). The origin of the water effect
in zeolite catalysis has been tentatively attributed to a variety
of mostly co-catalytic reasons. For example, it was reported
For a more direct demonstration of the origin of the water-
free deactivation, we analyzed the products that are released
by a spent catalysts after exposure to water (Figure 2). To this
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Figure 2: Peak area of phenol as analysed by GC over a ZSM5-40 catalyst as a
function of TOS in different conditions (the labels indicate the flows present).
The absence of propylphenol or water was compensated with nitrogen to
maintain a constant flow. T = 305°C and (for TOS 0-5 h) WHSV = 3.7 h-1.
end, a ZSM5-40 catalyst was evaluated in the dealkylation of
propylphenol in the absence of water. As in Figure 1a, a rapid
deactivation was observed during the first hour. After 5 h the
reactor was flushed with nitrogen for 15 h. During this period,
the phenol concentration reduced almost to zero confirming
that all weakly-adsorbed phenol was removed. After ca. 20 h
TOS, water was introduced into the stream, which induced an
immediate increase in the concentration of phenol. On the
other hand, no such effect was observed for propylphenol
Figure 3: Yields of propylphenol, phenol + olefin (dealkylated products), and
other products as a function of temperature over ZSM5-40 (a) and USY-40 (b)
zeolite catalysts. The temperature was raised using a ramp rate of 1°C min-1.
WHSV = 3.7 h-1.
(Figure SI2). These results demonstrate that water enables to
maintain catalytic activity by means of sweeping phenol from
the catalyst.
Shape selectivity
In order to assess the nature of the high selectivity of ZSM-5,
additional experiments were performed using a USY zeolite
with the FAU topology. This zeolites features larger micropores
(0.74 nm) compared to ZSM-5 (0.55 nm). Nevertheless, the
acidity and the dimensionality (3D) of the micropore network
are the same. The USY has as additional advantage that, unlike
ZSM-5, its synthesis does not involve costly organic
templates.70,71 The catalytic performance of the H-USY is
compared to that of H-ZSM-5 in Figure 3. The H-ZSM-5 displays
an exceptional catalytic behavior showing a clear activity lift
off around 275°C and an almost constant selectivity to phenol
and olefin of ca. 95%. The latter performance becomes even
more impressive when compared to the USY data. The
faujasite catalyst displayed a higher activity compared to ZSM-
5, as conversion starts to occur around 225°C. However, in the
225-350°C-range the formation of byproducts, mostly cresols,
ethylphenols, and dipropylphenols, account for almost 70% of
the converted molecules. The superiority of the ZSM-5
catalysts is confirmed with stability test at constant
temperature (Figure 4a). These reveal a lower selectivity for
the USY and a more pronounced deactivation. Importantly,
this deactivation should not be related to any structural
collapse as both the ZSM-5 and USY zeolite displayed a
Figure 4: Conversion (X) of propylphenol (a) and ethylphenol (b) and selectivity
to dealkylated products (phenol + olefin) (S) as a function of time on stream
(TOS) over ZSM5-40 (solid symbols) and USY-40 (open symbols). For
propylphenol (a), the temperatures were 305°C (ZSM5-40) and 365°C (USY-40).
In the case of ethylphenol (b), the used temperatures were 385°C (ZSM5-40) and
445°C (USY-40). WHSV for ethylphenol and propylphenol were 3.3 h-1 and 3.7 h-1,
respectively.
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preserved crystallinity, porosity, and Al coordination before
and after reaction (Figures SI3-SI5). This deactivation is more
likely related to coking, which is corroborated by the higher C
content on USY (5.3 wt.%) compared to ZSM-5 (4.4 wt.%) after
reaction.
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DOI: 10.1039/C5GC01868D
Hence, the faujasite catalyst is noticeably less selective
than ZSM-5. Accordingly, it is clear that, in addition to water,
the micropore dimensionality is of paramount importance to
attain an active and selective performance. Since the n-
alkylphenols and some side products (cresols and
ethylphenols) fit into the ZSM-5 micropores, the high phenol
and olefin yields may be attributed to a transition-state shape
selectivity. Herein, the tight fit of the linear alkylphenols,
olefins, and phenol inside zeolite micropores inhibits the
formation of side products. However, more than in the case of
alkylbenzenes, strong chemical effects, based on the unique
interaction of phenols in the zeolite micropore, may occur. It
should be noted that an accurate determination of the nature
of a shape-selectivity requires dedicated study.72-75 The latter
lies beyond this contribution.
Acidity and contact time
In order to gain more insight in the influence of the relative
abundance of the acid sites, ZSM-5 zeolites of different Si/Al
ratios were evaluated in the dealkylation of 4-n-propylphenol
(
Figure 5). By variation of the Si/Al ratio, the total number of
Figure 5: The dealkylation of propylphenol over ZSM-5 zeolites with distinct Si/Al
ratios: 15 (ZSM5-15), 40 (ZSM5-40), and 140 (ZSM5-140). (a) Conversion of
propylphenol (X) as a function of temperature. The temperature was raised using
a ramp rate of 1°C min-1. (b) Conversion (X) of propylphenol and selectivity to
Brønsted acid sites was varied, which clearly influenced the
activity: by increasing the Si/Al from 15 to 140 the conversion
lift off shifted from ca. 225°C to 325°C. This activity trend could
be suitably related by plotting the temperature of 40%
conversion (T40) to the total acidity (Figure 6a). Besides the
activity affect, the Si/Al ratio also displayed clear influence on
the selectivity and stability of the catalysts. Figure 5b, shows
that the selectivity for dealkylated products and the stability
increases with higher Si/Al ratio, hence lower total acidity. In
addition, the carbon content of the spent catalysts is lower for
the less-acidic zeolites. The latter was related in Figure 6b, and
may be explained by the closer proximity of acid sites at low
Si/Al ratio, enhancing chance for undesired secondary
reactions.
dealkylated products (phenol
+ olefin) (S) as a function of time. The
temperatures in (b) were 270°C (ZSM5-15), 305°C (ZSM5-40), and 365°C (ZSM5-
140). WHSV in (a) and (b) was 3.7 h-1.
In any conversion where secondary reactions are to be
suppressed, a crucial parameter is the contact time.
Accordingly, we have varied the weight hourly space velocity
(WHSV) from ca. 4 to 60 h-1 at 385°C (Figure 7). At low WHSV
the conversion, as well as the selectivity, are approaching
100%. Only at the WSHV’s exceeding ca. 12 h-1, the conversion
starts to gradually drop down to 50% at WHSV of 60 h-1.
Moreover, an additional attractive feature is that with
increasing WHSV the selectivity gradually increases to 98%.
This implies that, in line with the results from Figure 6, the
formation of byproducts is the result of secondary reactions.
Hence, the highest selectivity can be obtained by coupling a
relatively high reaction temperature to low contact times
and/or low zeolitic acidity.
Figure 6: (a) Temperature at 40% conversion of propylphenol (T40) from the
experiments in Figure 5a as a function of the total acidity. (b) Carbon content of
spent catalysts (solid symbols) and selectivity to dealkylated products (phenol +
olefin) (S, open symbols) as a function of the total acidity. S and the carbon
content were determined for catalysts that were exposed to the conditions in
Figure 5b for 4 h. The total acidities were adapted from Ref. 76.
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DOI: 10.1039/C5GC01868D
Figure 9: Conversion (X) of propylphenol and selectivity (S) to dealkylated
products (phenol + olefin) as a function of time over a ZSM5-140 catalyst.
T = 365°C, WHSV = 3.7 h-1.
Figure 7: Conversion (X) of propylphenol and selectivity (S) to dealkylated
products (phenol + olefin) as a function of WHSV (T = 385°C) over a ZSM5-40
catalyst. X and S were taken after 4 h TOS.
compared to propylphenol. Nevertheless, the side reactions, in
the case of USY40, occur at the same temperature. In addition,
the same deactivation trend was observed in the conversion of
4-ethylphenol (Figure 4b). Besides activity and selectivity
concerns, the life time of the catalyst is of crucial
importance. Therefore, ZSM5-140 was tested for the duration
Versatility and stability
To get a better idea of the potential of the water-assisted
dealkylation process, its effectivity was also tested in the
conversion of 4-ethylphenol, one of the most abundant
alkylphenols in coal tar oils.48 Figure 8 illustrates that, even
more clearly like in the case of propylphenol (Figure 3), ZSM-5
is superior compared to USY based a higher selectivity to
phenol and olefins. As expected, the dealkylation of
ethylphenol occurs at a higher temperature (ca. 70°C higher)
of 72 h (Figure 9). The resulting performance highlights the
remarkable stability of the ZSM-5 catalysts, attaining, after the
initial deactivation in the first 5 h TOS, a constant conversion
of ca. 70% with S > 95% to phenol and olefin. After 72 h, the
spent catalysts comprised 1.6 wt.% of coke. This is only 0.3
wt.% more than the coke content after 5 hours of TOS (1.3
wt.%, see Figure 6b), which suggests that the coking occurs
primarily during the early stages of the reaction.
As coal tars usually yields a mixture of alkylphenols,48 the
capacity of the ZSM-5 catalyst to handle a feed containing both
ethylphenol and propylphenol was tested (Figure 10). The
used molar ratio of ethylphenol/propylphenol was equal to
three, which is the typical ratio in coal tars.48 The temperature-
ramped evaluation of the conversion shows, as can be
expected, that the propylphenol is first converted followed by
the conversion of ethylphenol around 70°C higher. Like in the
dealkylation of the individual components, the selectivity to
phenol and olefins exceeded 95% at all stages of the testing.
The similar behavior of the mixture and the individual
components was corroborated by the stable performance in
the evaluation of the ZSM-5 catalysts as a function of time on
stream.
It should be stressed that, whereas in deriving phenol from
lignin a negligible amount of poisons is to be encountered, coal
tar may contain substantial S- and N-containing compounds
(Scheme 2). These have a reputation to deactivate catalysts
rapidly, which is why they are often removed from refinery
streams by hydrotreating.53 However, based on the absence of
a metal function on the acidic zeolite catalysts, the
deactivation by poisons is limited to N-containing organic
bases, such alkyl pyridines.48 However, since the majority of
these bulky molecules cannot enter the MFI micropores, their
Figure 8: Yields of ethylphenol, phenol + olefin (dealkylated products), and other
products as a function of temperature over ZSM5-40 (a) and USY-40 (b) zeolite
catalysts. The temperature was raised using a ramp rate of 1°C min-1. WHSV =
3.3 h-1.
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the catalytic and adsorptive behaviour of (alkyl)phenols in
View Article Online
DOI: 10.1039/C5GC01868D
zeolites, paving the way for further study on the fundamental
origin of the shape-selective behaviour and the competitive
adsorptive of water and (alkyl)phenol.
Acknowledgements
W.S. acknowledges funding from the Research Foundation
Flanders (FWO) for a doctoral fellowship. The authors are
grateful to W. Vermandel for technical assistance during the
catalytic testing, I. Cuppens for elemental analysis, and Dr. K.
Houthoofd for MAS NMR experiments. FISCH-ARBOREF
financial support by the Flemish government is acknowledged.
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