Investigation of the Reaction Pathways of Biomass-Derived Oxygenate Conversion into Monoalcohols in Supercritical Methanol with CuMgAl-Mixed-Metal Oxide

Article abstract of DOI:10.1002/cssc.201801816

Reaction pathways for the conversion of cellulose into C₂–C₆ monoalcohols by supercritical methanol depolymerization and hydrodeoxygenation (SCM-DHDO) over a CuMgAl oxide catalyst have been elucidated using a range of model compounds. SCM-DHDO of intermediate oxygenates including glycerol, methyl lactate, and 1,2-ethanediol produces similar products as those produced from the SCM-DHDO of cellulose. The pathway to C₂–C₆ monoalcohols occurs through rapid C?C coupling reactions between methanol and diols followed by C?C scission between vicinal alcohol groups to produce two monoalcohols. Methyl-branched monoalcohols are produced through a methyl shift in a secondary diol followed by dehydration. Esters are produced by dehydrogenative coupling between an adsorbed methoxy and a primary alcohol. Both dehydrogenation to a ketone and esterification to a methyl ester are in equilibrium with the corresponding alcohol and were reversible. Dehydration of diols is the slowest observed reaction and not a main pathway to monoalcohols. SCM-DHDO of glucose, dihydroxyacetone, and cellulose all produced similar high molecular weight species indicating that condensation of intermediates can produce undesired side products.

Full text of DOI:10.1002/cssc.201801816

Accepted Article
Title: Investigation of the Reaction Pathways of Biomass Derived
Oxygenate Conversion into Mono-alcohols in Supercritical
Methanol with CuMgAl Mixed Metal Oxide.
Authors: Peter H. Galebach, Sean Thompson, Ashley M Wittrig, Scott
J. Buchanan, and George W. Huber
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To be cited as: ChemSusChem 10.1002/cssc.201801816
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A Journal of
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ChemSusChem
10.1002/cssc.201801816
Investigation of the Reaction Pathways of Biomass Derived Oxygenate Conversion into
Mono-alcohols in Supercritical Methanol with CuMgAl Mixed Metal Oxide.
a
a
b
b
Authors: Peter H. Galebach , Sean Thompson , Ashley M. Wittrig , J. Scott Buchanan , Prof.
George W. Huber .
a*
[
a] Department of Chemical and Biological Engineering, University of Wisconsin, 1415
Engineering Drive, Madison, Wisconsin 53706, United States.
b] ExxonMobil Research and Engineering, 3545 Route 22 East Clinton Township, Annandale,
NJ 08801, United States.
[
*
Corresponding author: George W. Huber
Email: gwhuber@wisc.edu
Table of Contents:
Cellulose is converted to alcohols over CuMgAlO catalyst in supercritical methanol. The
x
reaction proceeds through retro-aldol condensation of cellulose to oxygenates, condensation with
methanol, and C-C scission reactions to mono-alcohols.
Keywords: Biomass, C-C coupling, Fuels, Hydrodeoxygenation, Supercritical Fluids.
Abstract:
The reaction pathways for conversion of cellulose into C
2
-C mono-alcohols by supercritical
6
methanol depolymerization and hydrodeoxygenation (SCM-DHDO) over a CuMgAl oxide
catalyst have been elucidated using a range of model compounds. SCM-DHDO of intermediate
oxygenates including glycerol, methyl lactate, and 1,2-ethanediol produces similar products as
SCM-DHDO of cellulose. The pathway to C
2
-C mono-alcohols occurs through rapid C-C
6
coupling reactions between methanol and diols followed by C-C scission between vicinal alcohol
groups to produce two mono-alcohols. Methyl branched mono-alcohols are produced through a
methyl shift in a secondary diol followed by dehydration. Esters are produced by
dehydrogenative coupling between an adsorbed methoxy and a primary alcohol. Both
dehydrogenation to a ketone and esterification to a methyl ester are in equilibrium with the
corresponding alcohol and were reversible. Dehydration of diols is the slowest observed reaction
and not a main pathway to mono-alcohols. SCM-DHDO of glucose, dihydroxyacetone, and
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ChemSusChem
10.1002/cssc.201801816
cellulose all produced similar high molecular weight species indicating that condensation of
intermediates can produce undesired side products.
Keywords: Cellulose, hydrodeoxygenation, supercritical methanol, alcohols, C-C coupling
Introduction:
In recent years several biomass conversion technologies have been studied as a method to
produce renewable fuels and chemicals.[
1-3]
A recent approach first proposed by Ford and co-
workers involves the supercritical methanol depolymerization and hydrodeoxygenation (SCM-
[
4-8]
DHDO) of biomass into mono-alcohols and cyclohexyl alcohols with a CuMgAl catalyst.
This process has a number of advantages including very high yields (up to 121 wt% due to
methanol incorporation and complete conversion of biomass to products) of fuel range alcohols,
all reactions occur in a single reactor, and it utilizes all fractions of biomass including the energy
rich lignin fraction. The alcohol products could either potentially be used directly as a fuel or
upgraded via condensation or oligomerization.[
9-11]
Other biomass conversion processes like
enzymatic hydrolysis, supercritical water hydrolysis, or aqueous phase hydrodeoxygenation
require separate depolymerization and sugar conversion steps that leads to lower yields and
[
12-14]
higher processing costs.
While single stage processes like pyrolysis have the benefit of low
processing costs, they suffer from poor fuel quality and often require additional downstream fuel
[
15, 16]
upgrading.
solvents, and feedstocks.
MO/Al , where M was a promoting metal either Ce, Mg, Mn, Ni, or Zn, and produced C
Subsequent studies have shown that SCM-DHDO works with alternate catalysts,
[17-19]
Wu et al. examined SCM-DHDO of cellulose using CuO-
-C
alcohols. Yin et al. demonstrated that SCM-DHDO of sugars from pyrolysis oil produced a
2
O
3
4
7
[
17]
[
18]
similar alcohol mixture to cellulose.
isolate monomers from soda lignin. Palkovits and co-workers showed that cellulose produces
Huang used supercritical ethanol to depolymerize and
[
19]
O
catalyst.[ Much of the work on
20]
C
1
-C
3
mono-alcohols and diols in water with CuO-ZnO/Al
2
3
elucidating the chemistry occurring in SCM-DHDO has focused on the lignin fraction while the
reaction pathway of cellulose is largely unknown, but is thought to involve hydrodeoxygenation
and methanol incorporation reactions.[
8, 17, 19, 21, 22]
We have previously used isotopic labeling and model compounds to show that SCM-DHDO of
cellulose occurs through retro-aldol condensation of soluble sugars followed by reactions with
methanol to form a mixture of C
showed that methanol is incorporated into all the liquid products. The reaction also produced
high molecular weight species that varied between C to C25 with 2 to 4 oxygen atoms and 4 to
2 double bond equivalents. Although our research showed that retro-aldol condensation is an
2
-C mono-alcohols, diols, esters, and ethers. Isotopic studies
6
7
[23]
1
intermediate step during conversion of cellulose, we do not yet understand the subsequent
reactions that produce the large number of products from the smaller oxygenates. Retro-aldol
condensation of glucose and fructose produces C
2
-C oxygenates like dihydroxyacetone and
4
glyceraldehyde. In this study we used glycerol and other polyols as model compounds to help
elucidate the reactions that occur in SCM-DHDO. Glycerol conversion to 1,2-propanediol
[
24-32]
conversion has been studied extensively with copper and noble metal catalysts.
Glycerol
conversion occurs primarily through dehydration of glycerol to hydroxyacetone over acid sites
[
31]
followed by hydrogenation over metal sites to 1,2-propanediol.
Copper catalysts have
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ChemSusChem
10.1002/cssc.201801816
improved selectivity to 1,2-propanediol due to lower C-C hydrogenolysis activity while noble
metal catalysts have high hydrogenation activity after dehydration, but suffer from high
[
26, 31]
selectivity to undesired C-C scission reactions to produce 1,2-ethanediol, CO, or CH .
4
Although this is a widely studied reaction, conversion of glycerol to 1,2-propanediol with copper
catalysts has mainly been done with water which eliminates the possible reactions between
methanol and reactants that may be occurring during SCM-DHDO. Copper catalysts have also
been studied for Guerbet coupling of alcohols and conversion of diols to aldehydes and ketones
which includes many of the same reactions likely occurring in SCM-DHDO including C-C
[
9, 33-36]
coupling, esterification, and dehydrogenation reactions.
Despite the abundance of
literature on glycerol conversion in water and on coupling of alcohols, to our knowledge no
study has examined conversion of polyols in an alcohol solvent. The objective of this paper is to
elucidate the chemistry that occurs for SCM-DHDO of cellulose and cellulose derived chemicals
by studying the SCM-DHDO of C
2
-C oxygenates.
4
Experimental Setup:
Reactions were run with high pressure batch reactors made from Swagelok unions with a bleed
[
23]
valve for gas analysis.
These reactors are used in similar high pressure biomass conversion
processes in the literature and were used in this study to maintain consistency between reaction
[
8, 37]
systems.
During a typical reaction, a reactor is put in a sand bath preheated to 300°C. The
reaction time is measured from when the sand bath reheats to 300°C to when the reactor is
removed. Liquid samples were taken from opened reactors and filtered using a 0.22 μm syringe
filter. Reactions with polyols were run with 10 mg of catalyst for every 100 mg of model
compound to keep conversion low. Reactions with mono-alcohols were run with 50 mg of
catalyst. Reactions with triol and diol model compounds were run from 0 minutes (just after
reheating to 300°C) to 49 minutes depending on the reactivity of the model compound. Catalyst
amount and reaction time were varied to achieve approximately 10-50% conversion of each
model compound. All reactions with mono-alcohols were run for 1 hour. Model compound
reactions to make high MW products were run for 4 hours with 100 mg of catalyst.
Product Analysis:
Liquid products were analyzed by gas chromatography using a Shimadzu GC-2010 equipped
with a RTX-VMS column and a flame ionization detector and by mass spectroscopy using a
Shimadzu GCMS-QP2010 mass spectrometer equipped with a RTX-VMS column. The GC oven
-1
was held at 40°C for 5 minutes then ramped to 240°C at 7.5°C min and held for 15 minutes.
Products were quantified using calibrations of known standards. Dodecane was used as an
internal standard to measure methanol conversion.
Mass spectrometric data were acquired with a Bruker solariX XR Fourier-transform ion
cyclotron resonance (FT-ICR) mass spectrometer with a 15 T actively shielded superconducting
magnet. Instrument control, data acquisition and preliminary processing were performed on
Bruker Daltonics ftmsControl 2.1.0 and Bruker Compass DataAnalysis 4.5 software.
Atmospheric pressure chemical ionization (APCI) was used to investigate each sample. Samples
were taken directly from the liquid after reaction. All samples were diluted as needed in LCMS
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ChemSusChem
10.1002/cssc.201801816
grade methanol prior to analysis. Flow rates were controlled by the instrument syringe pump at
-
1
1
80-300 µL h . The conditions were set to an APCI temperature of 380-400°C, corona needle
-1
current of 1200-1250 nA, dry gas flow of 3.5-4.0 L min , and dry gas temperature of 200°C.
Nitrogen was used as the drying gas. The data was processed by using the sine-squared
apodization method in absorption mode. Each mass spectrum is an average of at least 80 scans.
[
38]
After acquisition, elemental formulas for each peak were assigned by PetroOrg software.
The selectivity and carbon yield for reactions run for 4 h and with complete conversion is
defined by carbon % shown in equations 1 and 2. Our previous study showed that SCM-DHDO
of cellulose produces high MW species that are not detectable by GC.[ In this study we are
only reporting the yield and selectivity of GC detectable products.
23]
(
1)
2)
(
The reactions with model compounds involve many C-C addition and C-C scission steps which
complicate selectivity and yield calculations based on carbon amounts in the products. Instead,
the selectivity for low conversion model compound studies is defined as the moles of reactant
that were converted to each product as shown in equation 3. In this way incorporation of
methanol should not affect the selectivity and if a reaction results in two moles of a product, e.g.
2
,3-butanediol to 2 moles of ethanol, the selectivity to the C-C scission product is not double
counted. We have also defined the yield in equation 4 by mole % rather than C % to avoid
greater than 100% yields due to methanol incorporation.
(
3)
4)
(
The rates of different reactions were also compared using space time yield which is defined here
as the moles of product from reactant divided by the moles of reactant, the mass of catalyst, and
the reaction time, shown in equation 5.
(5)
Space time yield also allows comparison between reactions in which the catalyst mass, feed type,
and reaction time are not held constant. Since these reactions were performed in batch reactors
and heat up times could not be completely eliminated, the calculated space times in this paper
should only be examined qualitatively. Space time yields were only calculated at ~30%
conversion as some conversion occurred during the initial heating, preventing an accurate
measure of reaction time for low conversion points, and at higher conversions the product
mixture becomes too complex to attribute products to specific reaction pathways.
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ChemSusChem
10.1002/cssc.201801816
The thermodynamics of each reaction were calculated using values taken from Aspen
[
39]
APVpure32 database which is based on the most recent thermodynamic data from DIPPR. In
cases where the thermodynamics of each reaction were not available in a database we calculated
values using Gaussian 09 software (geometry optimization and frequency calculations with
[
40]
B3LYP/6-311+G(2d,p)).
Catalyst Preparation:
Catalyst was prepared using co-precipitation according to literature.[7, 23, 41-43] In a typical
synthesis a 150 mL solution of DI water containing 0.06 moles of Mg(NO ·6(H O), 0.025
moles of Al(NO ·9(H O), and 0.015 moles of Cu(NO ·3(H O) was added to a 187.5 mL
solution of DI water containing 0.025 moles of Na CO over the course of 1 hour at 60°C. The
3
)
2
2
3
)
3
2
3
)
2
2
2
3
pH was kept at 10 by addition of a 250 mL 1 M NaOH solution. The combined solution was
aged at 60°C for 24 h then washed and filtered. The filtered catalyst was dried in a 110°C oven
-1
overnight. The dried catalyst was calcined by ramping to 460°C at 5°C min then holding at
60°C for 12 hours in an air atmosphere.
4
The calcined catalyst contains CuO sites rather metallic Cu which is required for hydrogenation
[
44]
reactions. In typical SCM-DHDO the catalyst is reduced in-situ from hydrogen reformed from
methanol. However, in-situ reduction introduces an induction period during which the catalyst
will act as a solid base/acid that can catalyze aldol and retro-aldol condensation reactions, but is
inactive for hydrogenating alkenes, ketones, or aldehydes. To remove this induction period we
-1
reduced the calcined catalyst at 350°C in a flow through cell for 4 hours with a 1°C min
temperature ramp using 100 mL min-1 of H
. The reduced catalyst was cooled to room
temperature and then passivated by alternating 100 mL min flows of Ar and 1% O
over the catalyst 7 times. 1% O /balance Ar was then flowed over the catalyst for 1 hour before
2
-1
2
/balance Ar
2
removing the catalyst from the cell. Reduced and passivated catalysts were used immediately
after preparation to reduce oxidation from the atmosphere over time. X-ray diffraction of the
0
reduced and passivated catalyst confirmed that the surface contained well dispersed Cu sites.
XRD was performed with a Bruker D8 Discover diffractometer using a Cu Kα source
(λ=1.54184 Å). Scans were collected via an area detector with 300 s acquisition time to obtain
data from 5 to 80° 2θ in 3 steps of 25°. Unless noted otherwise, all catalysts in this study were
reduced and passivated to keep the Cu oxidation state consistent between reactions.
Results and Discussion:
Conversion of cellulose with pre-reduced catalyst
Table 1 shows the SCM-DHDO of cellulose with a pre-reduced catalyst and a calcined (non-
reduced) catalyst. The pre-reduced catalyst had higher overall carbon yield and selectivity to
mono-alcohols than the calcined catalyst and better selectivity to GC-FID identifiable products.
The carbon yield is defined as moles of carbon in GC detected products divided by the moles of
carbon in the cellulose. The carbon yields are above 100% because of methanol incorporation
[
23]
into the products.
The carbon yield to total detectable products increased from 90% to 119%
and the selectivity to mono-alcohols increased from 28% to 48% after pre-reducing the catalyst.
The selectivity to diols decreases after pre-reduction from 24% to 14% and selectivity to esters
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ChemSusChem
10.1002/cssc.201801816
and ethers remain essentially constant. Reactions with calcined catalyst after 8 hours reaction
time had lower carbon yield and product selectivity than the pre-reduced catalyst.
Table 1. Total carbon yield and selectivity for SCM-DHDO of cellulose with calcined catalyst
and pre-reduced catalyst.
Reduced and
Calcined[a]
passivated
Total carbon yield (%C)[b]
90 (0.5)
119 (1.7)
Selectivity-%C by carbon number of product (standard error)
C
C
C
C
C
2
3
4
5
6
oxygenates
oxygenates
oxygenates
oxygenates
oxygenates
5 (0.3)
10 (0.0)
26 (0.3)
15 (0.1)
7 (0.1)
9 (0.1)
12 (0.0)
28 (0.1)
14 (0.1)
10 (0.0)
Unspecified C
esters, and ethers
Selectivity-%C by product type (standard error)
2
-C
6
+ alcohols,
3
8 (0.7)
27 (0.0)
3
Mono-alcohols
Diols
Ethers
Esters
28 (0.9)
24 (0.3)
6 (0.1)
4 (0.0)
48 (0.1)
14 (1.0)
6 (0.0)
5 (0.0)
Unspecified C -C6+ alcohols,
2
3
8 (0.7)
27 (0.0)
[
c]
esters, and ethers
[
[
a]
calcined data is from 4 h time point during previous study23
Total carbon yield = moles C in all detected product/moles C in feed
b]
molecule. Total carbon yield is above 100% because methanol is
incorporated into the products.
[
c]Unspecified products are observed in the GC-FID, but cannot be
identified due to other similar mass spectra or a lack of reference spectra
Higher MW products, which are not detectable by GC, can form by condensation of ketones and
[
45]
aldehydes such as dihydroxyacetone. We analyzed the liquid products using Fourier-transform
ion cyclotron resonance mass spectrometry (FT-ICR MS) to evaluate the higher MW products.
Products below 100 Da are not seen due to a low MW cutoff in the FT-ICR MS. Because of this,
most liquid products observed in the GC are not observed by FT-ICR MS and vice versa. Based
on the retention time of products around the low MW cutoff, approximately 13 to 24% of the GC
observable products may be observed in the FT-ICR MS while C10+ polyoxygenates seen in the
FT-ICR MS are likely not seen in the GC. The carbon, oxygen, and double bond equivalents of
the high MW products from cellulose using calcined and pre-reduced catalyst are shown in
Figure 1. The liquid products from the reaction with pre-reduced catalyst (c,d) have much lower
oxygen content, carbon number, and double bond equivalents (DBE) than the products with
calcined catalyst (a,b). The spectra from the pre-reduced catalyst has a large peak at 7C, 2O, and
1
DBE, but only smaller peaks in the rest of the spectra. The overall intensity of spectra with pre-
reduced catalyst was very low due to a lack of observed species. Many of these peaks may be
due to background noise. Conversely the spectra from the calcined catalyst (a, b) has two main
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ChemSusChem
10.1002/cssc.201801816
peaks centered at 12C, 2O, 4 DBE and 26C, 3O, 9 DBE with a broad distribution of similarly
sized products. The DBE increases linearly with increasing carbon size at a ratio of
approximately 0.4:1. The high number of DBE and low oxygen content (1-4 oxygen) suggests
that these high MW products are primarily 1 to 2 ring aromatics with a small amount of 3+ ring
aromatics. The lack of these species in the products using the pre-reduced catalyst suggests that
the initial hydrogenation activity of the catalyst is necessary to decrease selectivity to this side
reaction.
Figure 1. FT-ICR MS results from 4 hour SCM-DHDO reaction with cellulose using calcined
CuMgAlO
x
catalyst (a,b) and reduced and passivated CuMgAlO catalyst (c,d). Carbon number
x
vs. oxygen (a,c) double bond equivalents (b,d).
Conversion of dihydroxyacetone and glucose with calcined catalyst
SCM-DHDO with dihydroxyacetone and glucose using the calcined catalysts were done to test
whether high MW products are produced. The FT-ICR MS spectra of products from SCM-
DHDO of dihydroxyacetone and glucose with the unreduced catalyst are shown in Figure 2 a-d.
Glucose was used to examine the possibility of oligomerization of soluble sugars after
depolymerization. Dihydroxyacetone was used to examine the possibility of condensation of
ketones after retro-aldol condensation of glucose. Oligomerization of glucose would be expected
to produce distinct peaks centered at C , C12, or C18 while condensation of dihydroxyacetone
6
should produce a complicated mixture of aromatics.[45, 46] The high molecular weight products
from glucose and dihydroxyacetone appear to be similar in composition with carbon numbers
from 8 to 36, oxygen numbers from 0 to 6, and double bond equivalents (DBE) from 2 to 14.
Most products from glucose and dihydroxyacetone are concentrated around 24C, 2O, and 9DBE,
although glucose appears to have slightly lower oxygen content and fewer DBE than
dihydroxyacetone. The products from dihydroxyacetone and glucose are slightly different than
the products from cellulose using the calcined catalyst in Figure 1 which has the high MW
products centered at 12C, 2O, and 4DBE. However, the DBE of the products from all 3 feeds
appear to increase with carbon number with a similar slope (~0.4 to 0.47 DBE:C) which suggests
that the products are similar in functionality. The ratio of DBE to carbon and the low amount of
oxygen at high carbon numbers suggest these products are linked aromatic or furanic units.
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ChemSusChem
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Glucose is known to produce humins via a reaction pathway through hydroxymethylfurfural
[
47-49]
(
HMF).
tetrahydrofuran which make it doubtful that this is the pathway to high MW products.
Another possible pathway is condensation of ketones which can produce high carbon number
However, SCM-DHDO of HMF is selective to hexanols and methyl branched
[
23, 50]
[
51-54]
aromatic compounds.
This pathway is the most likely as it is supported by our observation
of high MW products from conversion of hydroxyacetone and is in agreement with previous
[
45, 46]
work on conversion of sugars to oxygenated aromatics.
Figure 2. FT-ICR MS results (Carbon number vs. oxygen and double bond equivalents) from 4
hour SCM-DHDO reaction with dihydroxyacetone (a-b) and glucose (c-d).
Reaction Conditions: 300°C, 100 mg model compound, 100 unreduced CuMgAlO , 2.4 g
x
methanol, dodecane tracer.
SCM-DHDO of mono-alcohols
The rate of C-C coupling of mono-alcohols with methanol and C-C scission of mono-alcohols is
shown in Figure 3. Products from these reactions were produced by four reactions: 1) C-C
coupling between a reactant alcohol and methanol, 2) C-C scission between an alcohol group and
a neighboring carbon of the reactant alcohol, 3) dehydrogenation of a secondary alcohol to a
ketone, and 4) esterification of a reactant alcohol to a methyl ester. The rates of dehydrogenation
and esterification were verified to be near equilibrium by calculating the Qreaction (the mole
fraction of products divided by mole fraction of reactants) and dividing this value by the
equilibrium constant (Kequil). A value of Qreaction/Kequil=1 indicates the reaction is at equilibrium.
The effluent composition from esterification of ethanol with methanol was found to have a
Qreaction/Kequil of 0.66 indicating the reaction was near equilibrium. The Gibbs free energy (at
5
73.15 K and 385 bar) for ethanol esterification to methyl acetate and propanol esterification to
-1
-1
methyl propanoate are 29.5 kJ mol and 35.1 kJ mol respectively. While the standard Gibbs
free energy for dehydrogenation of isopropanol to acetone and 2-butanol to butanone are 25.7 kJ
mol and 20.2 kJ mol respectively, indicating the equilibrium conversion for dehydrogenation
at these conditions is low.
-1
-1
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ChemSusChem
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C-C coupling reactions were observed between the mono-alcohols and methanol. The chemistry
that is occurring during C-C coupling appears to be similar to Guerbet chemistry of ethanol
[
55-57]
conversion into higher alcohols.
the rate of Guerbet coupling of ethanol with copper catalysts in the literature.
involves dehydrogenation of the reactant alcohol to an aldehyde or ketone followed by keto-enol
The rate of C-C coupling to the mono-alcohols is similar to
[
33]
The first step
[58]
tautomerization to the enol. Since methanol cannot form an enol, formaldehyde is always the
electrophile during condensation. C-C coupling is also only observed at the α carbon which is
consistent with Guerbet chemistry. C-C coupling of methanol with primary alcohols appears to
be slower than C-C coupling of methanol with secondary alcohols (0.34-0.71 STY for primary
alcohols vs. 0.77-1.81 STY of secondary alcohols). The difference in rates may be due to the
relative concentration of the ketone compared to the aldehyde. The ketone is always present in
thermodynamic equilibrium with the alcohol, but aldehydes are not observed. The absence of
aldehydes appears to be due to a more favorable dehydrogenative coupling pathway methanol
with the feed alcohol to form a methyl ester rather than the aldehyde. Dehydrogenative coupling
of alcohols to esters is catalyzed by Cu and is reversible through hydrogenation of the ester over
[59, 60]
Cu.
The absence of the aldehyde in solution could account for the lower rate of C-C
coupling with primary alcohols. The Gibbs free energy for C-C coupling of ethanol with
methanol to form propanol and water is -60.6 kJ mol-1 indicating that C-C coupling is
thermodynamically favorable.
We observed C-C scission of the secondary alcohols to an alcohol and an alkane. C-C scission of
the primary alcohols also likely takes place. However, scission of the primary alcohols would
produce methanol which cannot be quantified and an alkane which is in the gas phase which was
not analyzed. C-C scission appears to take place through C-C hydrogenolysis although the
specific reaction mechanism is not known. C-C scission always takes place between the hydroxyl
carbon and either of the α carbons. C-C scission is never observed between C-C bonds two bonds
away from the C-O. For example, C-C scission of 2-butanol only produces 1-propanol and
ethanol and never produces isopropanol. The Gibbs free energy of C-C scission of ethanol to
-1
methanol and methane is -51.4 kJ mol indicating that C-C scission is also thermodynamically
favorable. Dehydration of the mono-alcohols was not quantified as the gas phase was not
analyzed. The Gibbs free energy of dehydration and hydrogenation of ethanol to ethane is -102.7
-1
kJ mol which is also thermodynamically favorable.
We also performed reactions with select aldehydes, ketones, and methyl esters to demonstrate
that dehydrogenation to aldehydes and ketones and dehydrogenative coupling to esters are
reversible. The results from these reactions are included in the supplemental information in Table
S1 and S2. The results show that aldehydes are rapidly converted to alcohols and methyl esters
or condense with methanol to form higher alcohols. Ketones are converted to secondary alcohols
or condense with methanol to form higher alcohols. Methyl esters are hydrogenated to alcohols
although this reaction is slower than hydrogenation of aldehydes and ketones.
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ChemSusChem
10.1002/cssc.201801816
Figure 3. Space time yields for SCM-DHDO of mono-alcohols. Space time yields are limited to
C-C addition and C-C scission products since dehydration cannot be accurately quantified and
dehydrogenative reactions are in equilibrium. Reaction Conditions: 300°C, 1 hour, 100 mg
model compound, 50 mg reduced and passivated CuMgAlO , 2.4 g methanol, dodecane tracer.
x
SCM-DHDO of polyols
Our previous study of SCM-DHDO of dihydroxyacetone showed that C oxygenates produce a
3
[23]
similar product distribution as SCM-DHDO of cellulose. In this paper we started with glycerol
as a model compound for cellulose rather than dihydroxyacetone. Glycerol was found to produce
a similar product distribution as dihydroxyacetone and cellulose. During conversion of glycerol
we observed a number of intermediates including 1,2-ethanediol and methyl lactate that when
converted on their own produced a similar product distribution to glycerol as shown in Table 2.
The primary products are C alcohols including isobutanol (6.9% to 19.3% carbon yield), 2-
4
butanol (6.1% to 8.1% carbon yield), and 2,3-butanediol (3.3% to 6.5% carbon yield). Mono-
alcohols are the most abundant product type (57% to 97% carbon yield) and contain a mixture of
linear and methyl branched primary and secondary alcohols. The highest carbon yield is
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ChemSusChem
10.1002/cssc.201801816
obtained with 1,2-ethanediol as the reactant demonstrating that this feed has the largest amount
of methanol incorporation. SCM-DHDO of these four reactants produce a similar product
distribution suggesting that there exists a common reaction pathway between them. A possible
common intermediate is 1,2-propanediol. This molecule can be produced from dehydration and
hydrogenation of glycerol, dehydrogenation of 1,2-ethanediol followed by aldol condensation
with methanol, or hydrogenation of methyl lactate.[60] In order for these reactants to give similar
products, the dehydration of glycerol, C-C coupling with 1,2-ethanediol, and hydrogenation of
methyl lactate must be rapid.
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ChemSusChem
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Table 2. SCM-DHDO carbon yield from cellulose, glycerol, 1,2-ethanediol, and methyl lactate.
Reaction conditions. 300°C, 4 hours, 100 mg feed compound, 100 mg CuMgAlO pre-reduced,
.4 g methanol, dodecane tracer.
x
2
Feed
Cellulose
119
Selectivity-%C by carbon number of product (standard error)
Glycerol
1,2-ethanediol Methyl lactate
[
a]
Total carbon yield (%C)
139
155 140
C
C
C
C
C
2
3
4
5
6
9.2
12.3
27.8
13.7
10.4
10.7
13.1
41.0
16.7
3.2
16.3
18.1
38.2
13.2
2.2
12.4
15.6
38.9
13.6
3.5
Unspecified C -C6+ alcohols,
2
esters, and ethers
26.6
15.3
12.2
16.0
Selectivity-%C by product type (standard error)
Mono-alcohols (%C)
Diols (%C)
Esters (%C)
Ethers (%C)
Ketones (%C)
48.2
13.7
6.1
5.0
0.5
61.4
18.1
2.7
2.2
0.4
59.6
17.2
4.8
6.3
0.0
63.4
8.4
2.0
8.7
1.6
Most abundant products
Primary alcohols
Ethanol (%C)
9.2
4.6
3.3
2.1
2.7
10.2
6.3
1.7
0.8
0.3
16.3
6.2
2.5
1.4
0.2
12.1
7.0
2.7
0.7
0.3
1
1
1
1
-propanol (%C)
-butanol (%C)
-pentanol (%C)
-hexanol (%C)
Secondary alcohols
2
2
3
2
-butanol (%C)
-pentanol (%C)
-hexanol (%C)
-hexanol (%C)
6.1
3.0
2.3
1.8
7.4
4.0
1.1
0.4
8.0
3.1
0.8
0.4
8.1
3.8
1.1
0.4
Methyl branched alcohols
Isobutanol (%C)
6.9
2.7
1.7
19.3
6.3
1.4
14.2
4.1
0.8
17.5
6.3
1.7
2
2
-methyl-butanol (%C)
-methyl-pentanol (%C)
Diols
2
2
1
2
-methyl-1,2-propanediol (%C)
,3-butanediol (%C)
,2-butanediol (%C)
1.2
6.1
1.8
4.0
6.5
0.7
4.4
6.4
0.9
1.7
3.3
0.3
,3-pentanediol (%C)
3.3
5.5
3.8
2.8
Sum of abundant products (%C)
Total carbon yield = moles C in all detected product/moles C in feed molecule. Total carbon yield is above 100%
59.0
76.1
73.4
69.9
[
a]
because methanol is incorporated into the products.
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10.1002/cssc.201801816
Identification of Reaction Pathway for SCM-DHDO of diols
We measured the product selectivity for SCM-DHDO of 1,2-ethanediol (1,2-EDO), 1,2-
propanediol (1,2-PDO), 1,2-butanediol (1,2-BDO), 2,3-butanediol (2,3-BDO), and glycerol (Gly)
at 10%, 30%, and 50% conversion to further investigate the reaction pathway. The products from
the reaction at 30% conversion are shown in Table S3 and the yield of products to the different
reaction pathways at 10%-50% conversion are shown in Figure S1. The main products from the
diols are formed from C-C coupling of the diol with methanol resulting in products such as 2,3-
butanediol or 2,3 pentanediol. Large amounts of ketones (hydroxyacetone) and esters (methyl
lactate) are also produced from the diols by dehydrogenation and esterification pathways
respectively. The yield of products towards the dehydrogenation pathway decreases as
conversion increases from 10% to 50% showing that dehydrogenation is reversible as more
methanol is reformed. The yield of products towards esterification increases from 10 to 30%
conversion, but either remains constant or decreases from 10% to 50% indicating esterification
of the feed alcohols is near equilibrium. The major products from glycerol include
hydroxyacetone > methyl-lactate >1,2-propanediol. Mono-alcohols are produced in small
quantities from C-C scission or dehydration. The reactions observed with diols are the same as
the mono-alcohols. We only studied vicinal diols since these were the only diols observed during
SCM-DHDO of cellulose. Since the diols are vicinal, C-C scission cannot take place via retro-
aldol condensation which requires a β-hydroxy ketone. Thermodynamic properties of large
oxygenates such as 2,3-pentanediol, 3-methyl-1,2-butanediol, 1-hydroxy-butanone, and 2-
hydroxy-methyl butyrate are not available in the Aspen database so the thermodynamics of the
reactions with these compounds were calculated using Gaussian 09 software. Calculations of
compound energies in Gaussian assume a vacuum so the contribution of pressure was not
accounted for. The thermodynamics of all studied reactions are included in Table S4 in the
supplemental information.
Using the data from Table S1 we have put together a reaction pathway for SCM-DHDO of
glycerol shown in Figure 4. Figure 4 assumes that the catalyst has been reduced before reaction.
Without pre-treatment the conversion of glycerol would still occur through the same pathway,
but reactions that occur over Cu sites would not be catalyzed the copper oxide had been reduced
to Cu metal by hydrogen generated from methanol reforming. Glycerol undergoes two possible
-1
reactions, either dehydration to hydroxyacetone (∆G=-65.4 kJ mol ) or C-C scission to 1,2-
-1
ethanediol (∆G=-18.2 kJ mol ). 1,2-ethanediol is dehydrated and hydrogenated to ethanol (∆G=-
-
1
1
06.1 kJ mol ) or undergoes C-C coupling with methanol to produce 1,2-propanediol.
-1
Hydroxyacetone also produces 1,2-propanediol via hydrogenation (∆G=-19.5 kJ mol ). 1,2-
propanediol can dehydrogenate back to hydroxyacetone or esterify to methyl lactate via
dehydrogenative coupling although the ΔG for both of these reactions is greater than 0 (∆G=19.5
and 20.8 kJ mol respectively). 1,2-propanediol dehydrates and hydrogenates to form 1-propanol
∆G=-100.0 kJ mol ) and undergoes C-C scission to form ethanol and methanol (∆G=-39.4 kJ
-
1
-1
(
-1
-1
mol ) or 1,2-ethanediol and methane (∆G=-57.4 kJ mol ). 1,2-propanediol produces 2,3-
butanediol through C-C coupling at the first position (∆G=-103.9 kJ mol ) or 1,2-butanediol
-1
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ChemSusChem
10.1002/cssc.201801816
-
1
(
∆G=-74.3 kJ mol ) by C-C coupling at the third position. No C-C coupling is observed at the
[
61]
tertiary carbon due to the lack of the hydrogen atoms necessary for facile coupling.
butanediol and 2,3-butanediol undergo dehydrogenation (∆G=-38.2 kJ mol and ∆G=51.4 kJ
mol respectively) to produce 1-hydroxy-butanone or 3-hydroxy-butanone. 1,2-butanediol
undergoes esterification (∆G=-29.9 kJ mol ) to produce 2-hydroxy-methyl butyrate. 1,2-
butanediol forms 1-butanol via dehydration (∆G=-83.6 kJ mol ) or 1-propanol and methanol via
C-C scission between the first and second positions (∆G=-25.7 kJ mol ). 1,2-butanediol also
1,2-
-1
-1
-1
-1
-1
produces 1,2-ethanediol and ethane from C-C scission between the second and third positions
-1
(
∆G=-34.5 kJ mol ). No products from scission between the third and fourth positions of 1,2-
butanediol are observed which is consistent with the observation from the mono-alcohols that the
mechanism of C-C scission requires scission adjacent to a C-O. 1,2-butanediol also undergoes C-
-1
C coupling at the first position to produce 2,3-pentanediol (∆G=-50.8 kJ mol ) or C-C coupling
-1
at the third position to 3-methyl-1,2-butanediol (∆G=-28.7 kJ mol ). 2,3-butanediol undergoes
-1
C-C coupling at the first or fourth positions to produce 2,3-pentanediol (∆G=-37.1 kJ mol ).
Diols larger than 2,3-pentanediol cannot be differentiated using the GCMS method used in this
paper so the products after C-C coupling to 2,3-pentanediol and 3-methyl-1,2-butanediol are not
known. 2,3-butanediol is converted to 2-methyl-1,2-propanediol via a methyl shift (∆G=28.8 kJ
-1
mol ). No methyl shift was observed in the other model compounds. 2-methyl-1,2-propanediol
-1
is dehydrated to isobutanol (∆G=-54.0 kJ mol ) and 2,3-butanediol is dehydrated to 2-butanol
-
1
(
∆G=-68.1 kJ mol ). Ethanol is produced from C-C scission of 2,3-butanediol between the
-1
second and third positions (∆G=-8.3 kJ mol ) while 1,2-propanediol is produced from C-C
-1
scission of 2,3-butanediol between the first and second positions (∆G=-20.3 kJ mol ). As larger
alcohols are produced more C-C scission products are possible. 2,3-pentanediol can undergo C-C
scission at 3 positions to produce either 1,2-butanediol and methane, 1,2-propanediol and ethane,
or 1-propanol and ethanol. Figure 4 is not comprehensive as the mono-alcohols can undergo C-C
coupling, C-C scission, dehydrogenation, esterification, and dehydration reactions.
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ChemSusChem
10.1002/cssc.201801816
Figure 4. Reaction pathway from glycerol to mixture of mono-alcohols, diols, and esters with
the Gibbs free energy of reaction at standard conditions shown.
In Figure 5 we report the measured space time yields (STY) of each reaction pathway (measured
at 30% conversion). The STY of dehydrogenation and esterification are not shown due to the
reactions being near equilibrium. Glycerol can undergo either dehydration (251 STY) or C-C
scission (27 STY). Dehydration of glycerol is the fastest observed reaction. C-C coupling of 1,2-
ethanediol with methanol to produce 1,2-propanediol (58 STY) is fast compared to
dehydration/hydrogenation of 1,2-ethanediol to ethanol (4.4 STY). 1,2-propanediol undergoes
fast C-C coupling at the first position to 2,3-butanediol (69 STY) or slower C-C coupling at the
third position to produce 1,2-butanediol (15 STY). C-C scission of 1,2-propanediol occurs
between either the first and second positions to produce ethanol (23 STY) or between the second
and third positions to produce 1,2-ethanediol (8.8 STY). Dehydration/hydrogenation of 1,2-
propanediol to 1-propanol (5.3 STY) is the slowest observed reaction from 1,2-propanediol. 1,2-
butanediol also undergoes fast C-C coupling at the first position to 2,3-pentanediol (98 STY) or
slower C-C coupling at the third position to 3-methyl-1,2-butanediol (18 STY). C-C scission of
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ChemSusChem
10.1002/cssc.201801816
1
,2-butanediol occurs either between the first and second positions to produce 1-propanol (27
STY) or between the second and third positions to produce 1,2-ethanediol (11 STY). 1,2-
butanediol can also dehydrate to 1-butanol (3.3 STY). 2,3-butanediol undergoes C-C coupling at
either the first or fourth positions to produce 2,3-pentanediol (14 STY). 2,3-butanediol undergoes
C-C scission between the first and second positions to produce 1,2-propanediol (13 STY) or
between the second and third positions to produce ethanol (22 STY). 2,3-butanediol undergoes a
methyl shift to produce 2-methyl-1,2-propanediol (89 STY). 2,3-butanediol can also dehydrate to
produce 2-butanol (5.5 STY).
The analysis of STY of the model compounds shows that SCM-DHDO of a poly-ol occurs
through multiple reactions. Dehydration of glycerol is the fastest observed reaction, but
dehydration of the resulting diols is slow (3.3-5.5 STY). C-C coupling is the next fastest reaction
and occurs primarily at the first position of a primary diol (58-98 STY) or secondarily at a
methyl carbon at the α position of a primary or secondary diol (14-18 STY). C-C coupling at the
first position of a primary diol is consistently 4-5 times faster than C-C coupling at the methyl
carbon. The difference in rate could be due to an electronic effect where the carbanion that
performs nucleophilic attack of the formaldehyde during C-C coupling is stabilized by the
[
41, 58]
hydroxyl group.
By comparing the rate of the methyl shift to the rate of C-C coupling to 1-propanol in Figure 3
0.71 STY) it is clear that methyl branching is primarily due to a methyl shift in the secondary
The methyl shift on 2,3-butanediol is the next fastest reaction (89 STY).
(
diol rather than condensation of methanol and a mono-alcohol. C-C scission only occurs between
carbons adjacent to an alcohol group and is faster between vicinal alcohol groups (22-27 STY)
than a methyl and a hydroxyl carbon (8.8-13 STY). Analysis of STY shows that C-C scission of
diols is the primary route to mono-alcohols rather than dehydration. Comparing the STY in
Figure 5 to the STY in Figure 3 shows that the rate of C-C coupling and C-C scission of diols is
1
to 2 orders of magnitude faster than to mono-alcohols.
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ChemSusChem
10.1002/cssc.201801816
Figure 5. Reaction network and space time yields (mmolproduct mmolreactant-1
SCM-DHDO of C -C
reaction. ΔG° is in kJ mol . Reaction Conditions: 300°C, 100 mg model compound, 10 mg
reduced and passivated CuMgAlO , 2.4 g methanol, dodecane tracer.
g
catalyst h-1) for
-1
2
4
oxygenates. The width of each arrow is proportional to the rate of
-1
x
The results from measuring the rates of each model compound elucidate many interesting
insights about the SCM-DHDO chemistry. A downside of many hydrodeoxygenation reactions is
that the reactions produce lower value alkanes rather than alcohols.[62-64] In SCM-DHDO
dehydration/hydrogenation is the slowest observed reaction and mono-alcohols are primarily
produced from C-C scission of a diol rather than dehydration which makes mono-alcohols
relatively stable end products. The reaction rates explain why conversion in SCM-DHDO is so
rapid at early reaction times but the products are stable at long reaction times. Initially the
products are more highly oxygenated which leads to the rapid dehydration observed in glycerol.
The resulting primary diols such as 1,2-ethanediol, 1,2-propanediol, and 1,2-butanediol undergo
C-C coupling which produces the less reactive secondary diols which eventually undergo C-C
scission to mono-alcohols which are the least reactive species.
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ChemSusChem
10.1002/cssc.201801816
Conclusion
This study elucidated some of the key chemistry occurring during SCM-DHDO of biomass. Pre-
reducing the catalyst increases carbon yield to identifiable products by approximately 35% and
decreases selectivity to side reactions that produce high MW products by increasing the
hydrogenation activity of the catalyst during early reactions times. High MW products are likely
produced from rapid condensation of intermediate oxygenates at short reaction times to produce
recalcitrant cyclic aromatics. The reaction pathway for SCM-DHDO of cellulose occurs through
C-C cleaving retro-aldol condensation reactions to produce intermediate C oxygenates like
3
dihydroxyacetone and glycerol followed by either condensation of these intermediates to high
MW species or dehydration to diols. These diols then undergo C-C coupling with methanol into
C
4
-C diols which undergo C-C scission reactions to mono-alcohols. The high amount of
6
methanol incorporation observed during conversion of biomass is due to C-C coupling of
methanol with diols. The rate of condensation of methanol with diols is approximately 10-100
times faster than methanol coupling with other mono-alcohols. Mono-alcohols are primarily
produced via C-C scission between vicinal diol groups and not by dehydration. The rate of C-C
scission of diols is approximately 10 times faster than dehydration/hydrogenation of the diols.
Methyl branched alcohols are produced from a methyl shift in a secondary diol followed by
dehydration. Thermodynamic and kinetic data show that both dehydrogenation and esterification
of diols and mono-alcohols are reversible under the reaction conditions. The rate of conversion
of mono-alcohols is 10-100 times slower than the other steps indicating that mono-alcohols can
be produced in high selectivities with SCM-DHDO reactions.
Acknowledgements
This work was supported by ExxonMobil. We would like to acknowledge Michael P. Lanci from
ExxonMobil for his helpful discussion during this study. Calculations using Gaussian were
performed by Nathaniel Eagan in the George Huber group. The authors gratefully acknowledge
use of facilities and instrumentation supported by NSF through the University of Wisconsin
Materials Research Science and Engineering Center (DMR-1121288).
Supporting Information
Product selectivities and conversions from model compound studies of C
Selectivity to reaction pathways at 10-50% conversion. Thermodynamics of reactions with C
oxygenates and single alcohols.
3
oxygenates.
-C
2
4
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10.1002/cssc.201801816
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