Cascade reactions for the continuous and selective production of isobutene from bioderived acetic acid over zinc-zirconia catalysts

Article abstract of DOI:10.1021/cs501018k

Bio-oil (obtained from biomass fast pyrolysis) contains a high concentration of acetic acid, which causes problems related to its storage and handling. Acetic acid was upgraded directly to isobutene over a Zn_xZr_yO_z binary metal oxide. The reaction proceeds via a three-step cascade involving ketonization, aldol condensation, and C-C hydrolytic bond cleavage reactions, which was corroborated by isotopic labeling studies. Separately, ZnO and ZrO₂ are incapable of producing isobutene from either acetic acid or acetone. In contrast, under optimal conditions, a Zn₂Zr₈O_z catalyst generates a ca. 50% isobutene yield, which corresponds to 75% of the theoretical maximum. Spectroscopic investigations revealed that a balanced concentration of acid and base sites is required to maximize isobutene yields.

Full text of DOI:10.1021/cs501018k

Letter
pubs.acs.org/acscatalysis
Cascade Reactions for the Continuous and Selective Production of
Isobutene from Bioderived Acetic Acid Over Zinc-Zirconia Catalysts
Anthony J. Crisci,‡ Herui Dou,‡ Teerawit Prasomsri, and Yuriy Roman-Leshkov*
́
Department of Chemical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts
02139, United States
S
* Supporting Information
ABSTRACT: Bio-oil (obtained from biomass fast pyrolysis)
contains a high concentration of acetic acid, which causes problems
related to its storage and handling. Acetic acid was upgraded directly
to isobutene over a ZnxZryOz binary metal oxide. The reaction
proceeds via a three-step cascade involving ketonization, aldol
condensation, and C−C hydrolytic bond cleavage reactions, which
was corroborated by isotopic labeling studies. Separately, ZnO and
ZrO2 are incapable of producing isobutene from either acetic acid or
acetone. In contrast, under optimal conditions, a Zn2Zr8Oz catalyst
generates a ca. 50% isobutene yield, which corresponds to 75% of the theoretical maximum. Spectroscopic investigations revealed
that a balanced concentration of acid and base sites is required to maximize isobutene yields.
KEYWORDS: pyrolysis oil, ketonization, aldol condensation, binary metal oxides, ZnO, ZrO2, acetic acid, isobutene
pgrading bio-oil obtained from biomass fast pyrolysis into
than the parent oxides.22 Reacting ethanol over Zn1Zr10Oz (the
Uchemicals that are fungible with traditional petrochem- optimal Zn/Zr ratio) selectively produced isobutene. The
icals is a major techno-economic challenge.1 Crude bio-oil is a
mechanism was hypothesized to occur through sequential aldol
and cracking reactions. However, mechanistic details for the
proposed steps involving the condensation of ethanol to
acetone and the cracking of the presumed intermediate, mesityl
oxide, into isobutene was not provided. Therefore, developing a
process that can couple ketonization, condensation, and
cracking of bio-oil derived acetic acid into isobutene over a
single catalyst is highly desirable. Furthermore, investigating the
involved reaction pathways is equally important toward
understanding and ultimately predicting the capabilities of
these catalysts.
Herein, we report the selective and direct production of
isobutene from acetic acid with an inexpensive binary oxide,
ZnxZryOz, through a three-stage cascade reaction system in a
continuous flow reactor, Scheme 1. We provide, by way of 13C
isotopic labeling and spectroscopic studies (vide infra), insight
into the reaction pathways involved in this conversion. First,
two acetic acid molecules ketonize to form acetone, water, and
CO2 over the oxide surface through an acetoacetic acid
intermediate.23 Next, acetone undergoes self-condensation
through an aldol pathway to generate the enone, mesityl
oxide, as the main product. Mesitylene and isophorone were
also formed as minor byproducts. To complete the cycle, the
enone undergoes a C−C bond cleavage step producing
isobutene and acetic acid. Under optimal conditions, a
Zn2Zr8Oz catalyst generated a maximum isobutene yield of
complex mixture of oxygenates containing a high concentration
of carboxylic acids.2 Acetic acid, the most abundant organic
acid, may compose up to 12 wt % of the bio-oil.3,4
Consequently, untreated bio-oil is corrosive to metals (e.g.,
engines, storage vessels) and promotes unwanted polymer-
ization reactions, severely hindering its utility.5,6
Ketonization, wherein two organic acid molecules undergo
C−C coupling to form a ketone, water, and carbon dioxide, is
often cited as an effective method to neutralize and upgrade
organic acids.7−12 When it is coupled with hydrodeoxygenation,
higher-value unsaturated hydrocarbons are produced from
carboxylic acids. For instance, ketonization of acetic acid
generates acetone, and hydrogenolysis of the resulting carbonyl
functional group produces propene.13 However, this tandem
process may not be economical for upgrading low-value, small
oxygenates because it requires hydrogen gas.14 Alternatively, it
has been shown that acetone can be converted to isobutene
over zeolites such as H-ZSM-5, H-Y, and H-Beta.15−18 Using
these materials at temperatures above 333 K, acetone is first
condensed to form diacetone alcohol. Next, the alcohol is
converted into isobutene by dehydration to mesityl oxide and
subsequent cleavage. Isobutene is a valuable product used to
manufacture fuel additives (e.g., ETBE, MTBE, tri-isobutene),
materials such as butyl rubber, and various other valuable
chemicals.19−21
Recently, Sun et al. reported on the ability of ZrO2 and ZnO
catalysts to selectively convert ethanol to ethylene and acetone,
respectively. Interestingly, their bifunctional Zn1Zr10Oz catalyst
(synthesized via hard templating) possessed different reactivity
Received: July 16, 2014
Revised: September 15, 2014
© XXXX American Chemical Society
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Scheme 1. Proposed Catalytic Cycle of Acetic Acid
a
Conversion to Isobutene
Figure 1. Powder X-ray diffraction patterns of various metal oxides.
(TEM) with energy-dispersive X-ray (EDX) spectroscopy (see
Figure 2). No regular structural order was apparent and
a
All energies are reported in kcal·mol−1.
50%, which corresponds to 75% of the theoretical yield (67%
on a carbon basis). These results represent a drastic increase in
isobutene production when compared to the hitherto highest
yield of 15% obtained with ZSM-5 for this reaction.17
The presence of Zn in ZnxZryOz enables the consecutive
reactions required to transform acetic acid to isobutene. In
general, heteroatom dopants alter adsorbate-binding strength
and generate unsaturated metal cations/oxygen vacancies that
affect the acid−base and redox chemistry of the oxide.24
Additionally, the acid−base properties of metal oxides can be
modulated by their degree of crystallinity, particle size,
concentration of heteroatom dopants, and thermal treat-
ment.25,26 Zirconia is already an efficient ketonization catalyst
yet selectivity enhancements are achieved by the addition of
heteroatoms.27 For example, the additions of Ce or Mg are
thought to increase both the basicity and reducibility of the
catalyst leading to higher ketone selectivities.28−30 There are
many mixed metal oxide combinations that can be obtained
with Zn or Zr; however, there are few accounts for ZnxZryOz
solid solutions.22,31−33
A series of ZnxZryOz solid solutions with various ratios of Zn
to Zr (1:9, 2:8, 3:7 and 5:5) were synthesized via sol−gel
method and characterized. Nitrogen adsorption studies
revealed the overall surface area of the mixed oxide decreased
when increasing the overall content of Zn. Specifically, for the
samples with Zn/Zr ratios of 1:9 and 5:5, the Brunauer−
Emmett−Teller (BET) surface areas were 166 and 63 m2·g−1,
respectively. The Zn2Zr8Oz solid solution and the physical-
mixed ZnO/ZrO2 (2:8; mole basis) possessed similar surface
areas of 110 and 107 m2·g−1, respectively. At all Zn/Zr ratios,
reflections corresponding to the tetragonal or monoclinic
crystal phases of ZrO2 were not observed by powder X-ray
diffraction (PXRD) (see Figure 1). For Zn/Zr ratios higher
than 3:7, weak diffractions associated with ZnO crystalline
domains were visible. In contrast, ZnxZryOz synthesized via
hard templating yielded ZrO2 crystal domains at all Zn/Zr
ratios.22 The lack of crystallinity in the sol−gel oxides suggests a
more intimate relation between Zn and Zr than what is
achieved by hard templating. As a representative sample,
Zn2Zr8Oz was imaged by transmission electron microscopy
Figure 2. Images of Zn2Zr8Oz: (a) HR-TEM; (b) EDX elemental
mapping− Zn channel; (c) EDX elemental mapping − Zr channel; (d)
EDX elemental mapping − Zn/Zr composite.
elemental mapping showed a homogeneous distribution of Zn
throughout the sample. Table S1 shows the acid−base
properties of the various metal oxides assessed by temper-
ature-programmed desorption (TPD). In general, the binary
oxides (physically mixed and solid solutions) possessed
significantly fewer acid sites and more basic sites relative to
ZrO2. Furthermore, the homogeneous distribution of Zn in
ZnxZryOz profoundly affects the acid−base properties of the
constituent oxide. For example, despite possessing similar
surface areas, Zn2Zr8Oz, ZnO/ZrO2 (2:8) and ZrO2 chemisorb
66, 246, and 355 μmol·g−1 of ammonia, and 241, 66, and 89
μmol·g−1 of carbon dioxide, respectively.
To successfully catalyze the reactions involved in the
proposed cascade (Scheme 1), both acid and base sites are
required. Although ZnO (wurtzite) and ZrO2 (tetragonal) are
both amphoteric oxides,25 ZnO is generally considered to be a
weak base, and ZrO2 is an acid. Independently, neither oxide is
active for the conversion of acetic acid to isobutene (yield
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Figure 3. Conversion of (a) acetic acid and (b) acetone on ZnxZryOz with varying Zn/Zr w/w ratios. Reaction conditions: T = 723 K, P = 1 atm,
catalyst mass = 4 g, WHSV = 6.25 gacetic acid (gcat.·h)−1 or 4.7 gacetone (gcat.·h)−1, time = 4 h. Catalyst 2:8-M refers to a physical mixture of ZnO and
ZrO2; “others” refers to the sum of hexenones and mesitylene.
<3%). Although both catalysts are active and highly selective for
the ketonization reaction of acetic acid to acetone, they are
drastically less active and selective for the aldol condensation
step of acetone to mesityl oxide. Specifically, at 723 K and a
WHSV of 25 gfeed (gcat.·h)−1, ZnO and ZrO2 generate acetone
yields from acetic acid of 55 and 70% at 67 and 95%
conversion, respectively. Note that the theoretical carbon yield
for acetone obtained through the ketonization reaction is 75%.
Conversely, reacting acetone over ZnO (20% conversion)
produces mesityl oxide and mesitylene in yields of 11 and 1.0%,
respectively; however, ZrO2 generates 15% mesityl oxide and
9% mesitylene yields at 48% conversion at the same reaction
temperature and space velocity. Consequently, negligible
isobutene yields are observed from the single oxide catalysts
under the reaction conditions investigated.
In contrast to the results obtained with the single oxides, a
2:8 physical mixture of ZnO and ZrO2 generates isobutene
from both acetic acid (6.5% yield; 100% conversion) and
acetone (10% yield; 80% conversion). The mixture appears to
promote the aldol condensation and cracking reactions, as
demonstrated by the increased isobutene yield. We observe
mesityl oxide yields (including mesityl oxide converted to
isobutene) from acetic acid and acetone of 24 and 48%,
respectively. Previous reports have shown that simply
mechanically mixing and heating ZnO and ZrO2 changes the
acid−base properties of the oxides at the grain boundaries.33
Therefore, the active sites responsible for isobutene production
may be located solely at the interface between ZnO and ZrO2.
Consequently, the frequency of interactive Zn−Zr pairs of the
ZnxZryOz solid solutions was expected to be greater than the
simple physical mixture. The isobutene yield from acetic acid
over a Zn2Zr8Oz amorphous binary metal oxide is 3-fold higher
than that obtained for the physically mixed oxide (Figure S1).
The performance of the ZnxZryOz series was revaluated at
optimized space velocities for acetic acid (6.25 gfeed (gcat.·h)−1)
and acetone (4.75 gfeed (gcat.·h)−1) at 723 K. Over Zn1Zr9Oz, a
25% isobutene yield was achieved and increasing the Zn
content to Zn2Zr8Oz, further increased the isobutene yield to
48% (Figure 3a). The use of catalysts featuring Zn/Zr ratios
higher than 2:8 resulted in decreased yields and in the
appearance ZnO reflections in the PXRD diffractograms.
Importantly, the activity of Zn2Zr8Oz was extremely stable in
the presence of the acetic acid feed (See Figure S1a). At lower
space velocities, the high yield was maintained for 4 h without
appreciable loss of selectivity. A full conversion of the acetic
acid was observed in all experiments due to the high
ketonization activity for all mixed oxide catalysts. Using acetone
as a feed resulted in 50% isobutene yield for all ZnxZryOz
catalysts (ca. 80% conversion and 90% mass balance; Figure
3b).
In comparison to ZrO2, Tanabe et al. reported that the
Zn1Zr1Oz oxide possesses lower acidity.32,34 Even at low Zn to
Zr (1:10) ratios, the concentration of Lewis and Brønsted acid
sites decreased as suggested by DRIFT spectra of pyridine on
the binary oxide.22 In agreement with these reports, the
ZnxZryOz catalysts possessed fewer acid sites and more basic
sites relative to ZrO2 (see Table S1). Such weak acid−base
pairs are capable of catalyzing both ketonization and aldol
condensation reactions.35 At low temperatures and space
velocities, ZrO2 selectively produces mesitylene from acetone,
whereas Zn2Zr8Oz generates mesityl oxide.36,37 Mesitylene is
the aldol product of mesityl oxide and acetone. Reducing or
eliminating certain acid/base sites may be responsible for this
shift in selectivity. Additionally, the stability of surface
adsorbates is proportional to acid−base strength, and
adsorbate-binding strength affects the rates and inhibition
(competitive adsorption of ketonization products) of the
ketonization reaction.38,39 Metal ratios must be balanced to
efficiently convert acetic acid directly to isobutene. Without
competing adsorbates (CO2 and acetic acid), any ratio of Zn to
Zr (up to 1:1) is capable of efficiently converting acetone to
isobutene (see Figure 3). The acid−base strength of ZnxZryOz
increases ketonization rates while enabling the cleavage of
mesityl oxide.
We hypothesized that the weak Brønsted acid sites on
ZnxZryOz catalyzes the cleavage of mesityl oxide. McAllister et
al. reported that mesityl oxide could be cleaved to isobutene
and acetic acid over phosphoric acid on silica.40 To confirm this
hypothesis, various possible intermediates were theoretically
examined to estimate their stability, as well as their activation
upon protonation at different positions (see Table S2). The
proton affinity (PA) was calculated as follows: PA = H(mesityl
oxide-H+) − [H(mesityl oxide) + H(H+)]; this equation was
used to determine the stability of protonated mesityl oxide. The
protonation at the O of mesityl oxide shows the highest
stability of the intermediates (i.e., PAMO_O = −253 kcal·mol−1),
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and at the same time, the C1−C2 bond is strengthened (i.e.,
the bond length decreased from 1.48 to 1.40 Å). As a result, the
C1−C2 bond is less likely to be cleaved due to the presence of
a conjugated system among O, C1, C2, and C3 (see MO_O
species in Table S2). On the other hand, the protonation of the
mesityl oxide carbons results in less stable intermediates than
those obtained from the protonation of the mesityl oxide
oxygen. The energetic profiles calculated with density func-
tional theory (DFT) indicate that the protonation at C2 (i.e.,
PAMO_C2 = −195 kcal·mol−1) is more favorable than the
protonation at the other carbons (i.e., PAMO_C1 = −172 kcal·
mol−1, and PAMO_C3 = −177 kcal·mol−1). In addition, the C2-
protonation appears to lead to the stabilization of the partial
positive charge at the adjacent tertiary carbon (i.e., C3) and to
the elongation of the C1−C2 bond (i.e., the bond length
increased significantly from 1.48 to 1.65 Å). Taken together,
these calculations suggest a favorable β-scission mechanism to
form the corresponding isobutene product (see Scheme S1).
The reaction pathway shown in Scheme 1 was verified by
reacting either 1-13C acetic acid, 2-13C acetic acid, 2-13C
acetone, or 1,3-13C acetone over Zn2Zr8Ox and tracking mass
distributions with gas chromatography−mass spectrometry
(GC-MS). The distribution of 13C isotopes within product
molecules is a function of the position of the 13C label within
the carbon backbone of the starting reagent (see Scheme 2).
weak acid−base pairs to the ketonization and aldol con-
densation activity, whereas Brønsted acid sites are related to the
hydrolysis activity of mesityl oxide to isobutene. However,
given the complexity of heterometallic oxides, further research
is needed to achieve precise tuning of acid−base pairs at the
molecular level. Unlike previously reported Zn−Zr oxides, our
materials do not show long-range order representative of
independent crystalline domains and thus suggest a very close
interaction between Zn and Zr. This inexpensive, robust, and
highly active catalyst shows great potential to neutralize and
upgrade carboxylic acids for bio-oil upgrading. Current aims in
our group focus on upgrading mixed feeds of carboxylic acids
commonly found in bio-oil, as well as performing thorough
analyses of surface−substrate interactions.
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental details; tables including physicochemical proper-
ties of various metal oxides, the calculated proton affinities
(PA), molecular geometries, and Mulliken charges of mesityl
oxide; a figure presenting the conversion of acetic acid over
various metal oxides at a WHSV of 25 gfeed (gcat.·h)−1. This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: yroman@mit.edu.
■
a
Scheme 2. 13C-Labelled Reaction Study
Author Contributions
‡A.J.C. and H.D. contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was funded by the Cooperative Agreement between
the Masdar Institute of Science and Technology (Masdar
Institute), Abu Dhabi, UAE, and the Massachusetts Institute of
Technology (MIT) (Reference No. 02/MI/MI/CP/11/
07633/GEN/G/00).
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