Branched Diol Monomers from the Sequential Hydrogenation of Renewable Carboxylic Acids

Article abstract of DOI:10.1002/cctc.201600710

A prominent challenge in replacing petrochemical polymers with bioderived alternatives is the efficient transformation of biomass into useful monomers. In this work, we demonstrate a practical process for the synthesis of multifunctional alcohols from five- and six-carbon acids using heterogeneous catalysts in aqueous media. Design of this process was guided by thermodynamic calculations, which indicate the need for two sequential high-pressure hydrogenations: one, reduction of the acid to a lactone at high temperature; two, further reduction of the lactone to the corresponding diol or triol at low temperature. For example, the conversion of mesaconic acid into (α or β)-methyl-γ-butyrolactone was achieved with 95 % selectivity at a turnover frequency of 1.2 min^?1 over Pd/C at 240 °C. Subsequent conversion of (α or β)-methyl-γ-butyrolactone into 2-methyl-1,4-butanediol was achieved with a yield of 80 % with Ru/C at 100 °C. This process is an efficient method for the production of lactones, diols, and triols, all valuable monomers for the synthesis of bioderived branched polyesters.

Full text of DOI:10.1002/cctc.201600710

DOI: 10.1002/cctc.201600710
Communications
Branched Diol Monomers from the Sequential
Hydrogenation of Renewable Carboxylic Acids
Charles S. Spanjers,[a] Deborah K. Schneiderman,[b] Jay Z. Wang,[a] Jingyu Wang,[a]
Marc A. Hillmyer,[b] Kechun Zhang,[a] and Paul J. Dauenhauer*[a]
A prominent challenge in replacing petrochemical polymers
with bioderived alternatives is the efficient transformation of
biomass into useful monomers. In this work, we demonstrate
a practical process for the synthesis of multifunctional alcohols
from five- and six-carbon acids using heterogeneous catalysts
in aqueous media. Design of this process was guided by ther-
modynamic calculations, which indicate the need for two se-
quential high-pressure hydrogenations: one, reduction of the
acid to a lactone at high temperature; two, further reduction
of the lactone to the corresponding diol or triol at low temper-
ature. For example, the conversion of mesaconic acid into (a
or b)-methyl-g-butyrolactone was achieved with 95% selectivi-
ty at a turnover frequency of 1.2 minꢀ1 over Pd/C at 2408C.
Subsequent conversion of (a or b)-methyl-g-butyrolactone into
2-methyl-1,4-butanediol was achieved with a yield of 80% with
Ru/C at 1008C. This process is an efficient method for the pro-
duction of lactones, diols, and triols, all valuable monomers for
the synthesis of bioderived branched polyesters.
tial value as a precursor for C5 chemicals [e.g., 2-methly-1,4-
butanediol and (a or b)-methyl-g-butyrolactone].[1] Globally,
80000 tons of IA are produced per year by using Aspergillus
terreus in a high-yielding fermentation process (0.72 ggꢀ1 from
glucose, with titers of up to 86 gLꢀ1).[2,3] Mesaconic acid (MA),
an isomer of IA, has similar potential as a precursor and was re-
cently produced by using E. coli.[4] The low cost of these acids
(ꢁ$2 kgꢀ1) enables their use in synthetic resins, plastics, rub-
bers, surfactants, and oil additives.[5,6] Mevalonic acid (MLA) is
a third organic acid that can also be produced efficiently from
the fermentation of carbohydrates.[7] MLA is a potentially val-
uable precursor for the synthesis of C6 alcohols, ethers, and
lactones. However, there are a few reports that outline effi-
cient, aqueous processes for the production of diols from five-
and six-carbon organic acids, including MA, IA, and MLA.
As depicted in Scheme 1, we propose an efficient two-step
catalytic method for the aqueous reduction of organic acids
that proceeds through a lactone intermediate. In a specific ex-
ample, MA is first reduced to (a or b)-methyl-g-butyrolactone
(MGBL), which is then used to produce 2-methyl-1,4-butane-
diol (MBDO); the same method can be used to reduce both IA
and MLA to the corresponding diol and triol, respectively. Such
The current petrochemical industry is based on the inexpen-
sive and efficient transformation of a few platform chemicals
into a wide array of products, including solvents, fine chemi-
cals, and monomers for modern plastics. Concerns over sus-
tainability have motivated a return to natural feedstocks; this
trend is particularly evident in the polymer industry, for which
consumer awareness of the adverse environmental impacts of
nondegradable plastics has driven the commercialization of
biodegradable polymers (e.g., the polyesters polylactide and
polyhydroxyalkanoates). However, there exist only a few effi-
cient and environmentally benign processes for monomer syn-
thesis from biomass.
Organic acids obtained by fermentation of glucose are at-
tractive feedstocks for the synthesis of branched multifunction-
al alcohols from biomass. Itaconic acid (IA) was identified as
one of the top-12 building-block chemicals owing to its poten-
[a] Dr. C. S. Spanjers, J. Z. Wang, Dr. J. Wang, Prof. K. Zhang,
Prof. P. J. Dauenhauer
Department of Chemical Engineering and Materials Science
University of Minnesota
421 Washington Ave SE Minneapolis, MN 55455 (USA)
E-mail: hauer@umn.edu
[b] Dr. D. K. Schneiderman, Prof. M. A. Hillmyer
Department of Chemistry
University of Minnesota
207 Pleasant St. SE Minneapolis, MN 55455 (USA)
Supporting Information for this article can be found under http://
dx.doi.org/10.1002/cctc.201600710.
Scheme 1. Hybrid process for the production of bioderived lactones, diols,
and branched polymers from glucose.
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a method would enable the efficient synthesis of branched C5
and C6 diols, which are useful for the synthesis of polymers
such as polyesters, among numerous other applications.
Hydrogenation of multifunctional carboxylic acids was evalu-
ated for a related four-carbon molecule, succinic acid (SA),
which proceeds through 4-hydroxybutyric acid and g-butyro-
lactone as intermediates to 1,4-butanediol (BDO). SA is used in
combination with diols to prepare polyesters or as an inter-
mediate for the synthesis of tetrahydrofuran (THF). BDO may
be used as a chain extender in polyurethane synthesis, for ex-
ample, whereas THF is useful both as a solvent and as a mono-
mer for polyether synthesis. Specific to the reduction of SA,
heterogeneous catalysts, including Re/C,[8] Pd-Re/C,[9] Pd-Re/
TiO2,[10,11] rhenium–copper–carbon composites,[12] Pd/TiO2,[13]
and Ru/C[14] have been utilized for aqueous BDO production at
temperatures ranging from 160 to 2408C and H2 pressures
ranging from 80 to 150 bar (10 bar=1 MPa). Examples of high
selectivity (over 50%) for BDO were reported only at low tem-
peratures (below 1708C) and high H2 pressures (over 140 bar).
Presumably, these conditions are necessary to overcome the
entropically unfavorable ring opening of g-butyrolactone with
hydrogen to form BDO.
ate temperature and pressure produced a low yield (45% at
1508C and 100 bar H2).[18] Several catalysts, including Ru/Star-
bon,[19] Ru/C,[20] Ru/TiO2,[21] and Pd/C,[22] were previously proven
ineffective for the reduction of IA to MBDO in a one-pot ap-
proach. Recently, Pd-Re/C was reported to be an effective cata-
lyst under conditions of relatively high temperature and low
H2 pressure (>80% yield of MBDO at 1808C and 40 bar),[23] in
conflict with the thermodynamics of the ring opening of the
lactone to the diol, which should prevent higher yields of
MBDO under these conditions (see Figure S5 in the Supporting
Information).
Figure 1a shows the detailed pathway for the hydrogenation
of IA and MA to MBDO. The enthalpy and Gibbs energy of
each reaction intermediate were calculated by DFT (M062X/6-
311+ +G[3df,3pd] theory level, implicit solvent, additional de-
tails in the Supporting Information) to determine the condi-
tions necessary to produce MBDO with high selectivity (Fig-
ure 1b). As expected, isomers IA and MA exhibit similar ther-
modynamic properties with a difference in Gibbs energy (DG)
of only 0.2 kcalmolꢀ1; hydrogenation to form methylsuccinic
acid (MSA) is highly favorable under all conditions (50<P<
200 bar, 80<T<2208C). Subsequent reduction of the carbonyl
group to make (2 or 3)-methyl-4-hydroxybutyric acid (MHBA)
followed by intramolecular esterification to form MGBL is also
downhill in free energy, which suggests that MGBL is the ther-
modynamically preferred product for all high-temperature,
low-H2-pressure conditions (Figures S1–S3).
Comparatively fewer examples of IA or MA hydrogenation
to MBDO exist in the literature. Whereas MBDO has been pro-
duced in high yield (>90%) from either IA or MGBL by using
homogenous Ru–triphos catalysts[15,16] [triphos=bis(diphenyl-
phosphinoethyl)phenylphosphine] and Ru complexes with tet-
radentate bipyridine ligands,[17] heterogeneous catalysts have
enjoyed less success. The synthesis of MBDO from IA at moder-
In contrast, the conversion of MGBL into MBDO is unfavora-
ble at high temperature and low pressure (DGrxn =3.5 kcal
Figure 1. a) Reaction pathway for the hydrogenation of itaconic acid (IA) and mesaconic acid (MA) to methylsuccinic acid (MSA), (2 or 3)-methyl-4-hydroxybu-
tyric acid (MHBA), (a or b)-methyl-g-butyrolactone (MGBL), and 2-methyl-1,4-butanediol (MBDO). Side products include 3-methyl-tetrahydrofuran (MTHF) and
(2 or 3)-methylbutyric acid (MBA). Note that the regioisomers of MHBA, MGBL, and MBA with alternate methyl positions are omitted from the drawing for
clarity. b) Calculated reaction energy diagram for the hydrogenation of IA or MA to MBDO for three different sets of temperatures and pressures, demonstrat-
ing that high pressures and low temperatures promote MBDO formation. Inset shows theoretical temperature dependence (dashed line) and experimental
overlay (markers) for the equilibrium ratio [MBDO]/([MBDO]+[MGBL]) at H2 pressure of 140 bar. Conditions for experimental data are in Table S10.
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molꢀ1 at 2208C and 50 bar) but is favorable at lower tempera-
tures and higher pressures (DGrxn =ꢀ3.5 kcalmolꢀ1 at 808C and
200 bar) (Tables S1–S9). The theoretical temperature depend-
ence of the ratio [MBDO]/([MBDO]+[MGBL]) at 140 bar of H2 is
displayed in the inset of Figure 1b (dashed line) (see the Sup-
porting Information for complete details). Experimental results
obtained from 10 hydrogenation trials at long reaction times
close to thermodynamic equilibrium between MGBL and
MBDO generally agree with these theoretical predictions
(markers, see the Supporting Information for complete experi-
mental information). Both theory and experiment show that
low temperatures (T<1408C) are required to produce MBDO
in high yield at 140 bar H2 pressure.
namic calculations indicate that the formation of the predomi-
nant byproduct, (2 or 3)-methylbutyric acid (MBA), from MGBL
is a highly favorable reaction (DG=ꢀ17.6 kcalmolꢀ1 at 2408C
and 140 bar). However, this product was observed with selec-
tivities less than 10%. The targeted diol, MBDO, was not ob-
served in these experiments owing to both thermodynamic
limitations and the low selectivity of the selected catalyst, Pd/
C.
Subsequent conversion of MGBL into MBDO was tested with
Ru/C, Pd/C, and Pt/C at temperatures ranging from 80 to
2008C (Figure 3). Pd and Pt produced low yields of MBDO, in
contrast with Ru/C at all considered temperatures. Because the
Experimental results for the aqueous hydrogenation of IA or
MA to MGBL using heterogeneous catalysts are shown in
Figure 2. For the tested conditions, hydrogenation of IA or MA
to form MSA was faster than the reduction and cyclization of
MSA to MGBL. As indicated by the turnover frequency (TOF) re-
ported in Figure 2, the rate of MGBL formation was dependent
on catalyst and temperature. Pd catalysts exhibited higher
TOFs than Pt, Ru, Cu2Cr2O5, and Rh. At 1608C and 70 bar H2,
Pd/TiO2 and Pd/C demonstrated similar activities.
Figure 3. Temperature dependence of the hydrogenation of a-methyl-g-bu-
tyrolactone to 2-methyl-1,4-butanediol. Reaction conditions: 2 wt% MGBL in
60 mL H2O, 1.0 g 5 wt% Ru/C, 140 bar. Data reported at time of maximum
MBDO selectivity: 808C (71 h—did not reach maximum), 1008C (6.3 h),
1208C (3.5 h), 1408C (1.8 h), 1608C (0.3 h), and 2008C (0.3 h). Pd/C (1.5 h),
Pt/C (1.5 h). TOF data is not presented for this reaction owing to the equilib-
rium that exists between MGBL and MBDO and the hydrogenolysis of MBDO
that occurs readily at temperatures greater than 1008C.
conversion of MBDO into isobutyl alcohol, n-butanol, and ulti-
mately propane can occur in series with the formation of
MBDO from MGBL, the data in Figure 3 are presented at the
time of maximum MBDO yield. With Ru/C at long reaction
times, hydrogenolysis of the MBDO CꢀO bonds occurs, which
results in the formation of methane and propane. The ob-
served mass-balance errors can presumably be attributed to
the formation of these gaseous products, as only products in
the aqueous phase were quantified. To maximize MBDO pro-
duction, it was necessary to conduct the MGBL reduction reac-
tions at low temperatures, for which both the rate of MBDO
hydrogenolysis was lower and the production of MBDO from
MGBL was thermodynamically favorable. For example, the ex-
perimentally observed values for the ratio of [MBDO]/
([MBDO]+[MGBL]) were 0.98 and 0.10 at 100 and 2008C, re-
spectively, with Ru/C.
Figure 2. Temperature and pressure dependence of MA or IA hydrogenation
to MGBL. Reaction conditions indicated by superscript a) 2 wt% MA in H2O,
b) 10 wt% IA in H2O, and c) 10 wt% MA in H2O. Total volume: 60 mL aque-
ous solution. Complete reaction details provided in the Supporting Informa-
tion. Selectivity is only reported for reaction conditions b and c. MGBL is
a mixture of a-methyl-g-butyrolactone and b-methyl-g-butyrolactone
[(38ꢂ2) and (62ꢂ2)%, respectively]. Error bars represent a 95% confidence
interval.
Further optimization was performed with the commercial
catalyst Pd/C. With this catalyst, increasing the reaction tem-
perature and pressure from 1608C and 70 bar to 2408C and
140 bar resulted in a 40-fold increase in the TOF. The inter-
mediate hydroxyacid, MHBA, was observed in small quantities
(<5%) in reactions achieving high conversion into MGBL. Se-
lectivity to MGBL was greater than 80% in all reactions with
the use of Pd/C. The maximum yield of MGBL at complete con-
version of IA was 82% at 2008C and 140 bar H2. Thermody-
Encouraged by these results, we investigated the reduction
of three C6 compounds: mevalonolactone (MLL) (more precise-
ly a mixture of MLA and MLL, as the two are in equilibrium in
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water), anhydromevalonolactone (AMLL), and b-methyl-d-valer-
olactone (BMVL). Under conditions similar to those used for
the reduction of MGBL (1008C, 140 bar H2), we found that the
reduction of MLL resulted in 3-methyl-1,3,5-pentanetriol
(MPTO) with high selectivity (91%). Additionally, the reduction
of BMVL and AMLL both gave 3-methyl-1,5-pentanediol
(MPDO) with similarly high selectivities (86 to 91%). A more
detailed description of this work is available in the Supporting
Information.
that stop at MGBL formation. If this first reaction was conduct-
ed with bimetallic catalysts that contained Re or Ru, then only
a small amount of ring opening of MGBL would occur, as the
thermodynamic equilibrium ratio of [MBDO]/([MBDO]+
[MGBL]) approaches zero at this temperature. The high tem-
perature exacerbates the entropic penalty of combining MGBL
and two hydrogen molecules to produce MBDO (Figure S4).
Moreover, the high reaction temperature would lead to unde-
sirable hydrogenolysis reactions of the little amount of MBDO
that was produced. Alternatively, a single low temperature for
the entire process (MA to MGBL to MBDO) would lead to negli-
gibly slow overall rates that are limited by the conversion of
MA into MGBL.
Pd/C was previously demonstrated to be active for the con-
version of IA into MGBL by Li et al., who examined the effects
of active carbon pretreatment on selectivity for MGBL forma-
tion.[22] Here, we show the effect of temperature and pressure
on the TOF for MGBL formation, demonstrating TOFs of
1.2 minꢀ1 at 2408C and 140 bar. The ability of palladium to in-
corporate hydrogen into its crystal lattice is possibly the
reason why its performance is superior to that of other tested
catalysts. Pd was highly active for both hydrogenation of the
double bond in IA or MA and the reduction of the free carbox-
ylic acid present in MSA, yet Pd/C did not catalyze the facile
conversion of MGBL into MBDO. Studies of succinic acid hydro-
genation by using Pd-based catalysts led to similar results, for
which a low concentration of the diol was formed by using
Pd-only catalysts.[13]
To demonstrate the potential of these branched alcohols as
monomers, we prepared a variety of aliphatic polyesters by
using condensation polymerization with tin(II) 2-ethylhexa-
noate as a catalyst. As described in detail in the Supporting In-
formation, BDO, MBDO, and MPD were each treated with
either SA or MSA to prepare each of the six possible polyes-
ters. As summarized in Figure 4, the addition of a methyl sub-
In the final step, thermodynamic calculations (Figure S5) in-
dicated that at 140 bar, the conversion of MGBL into MBDO is
only favorable at temperatures below approximately 1408C (at
higher H2 pressures, the threshold temperature is higher); thus,
a low-temperature-active catalyst was required for the final
process step. Bimetallic catalysts composed of Pd-Re have
been proposed for the conversion of closely related g-butyro-
lactone into BDO.[9–11] Here, we found that monometallic Ru/C
effectively promoted the conversion of MGBL into MBDO in
approximately 80% yield, as shown in Figure 3. The rate of Ru
leaching from Ru/C was sufficiently low, demonstrating process
viability (i.e., the rate of leaching was three to four orders of
magnitude lower than a viable rate[24]) and that the reaction
was catalyzed heterogeneously (complete details in the Sup-
porting Information).
Figure 4. Differential scanning calorimetry traces comparing the effect of
methyl substitution on glass-transition temperature of methyl-substituted
poly(butylene succinate) with non-methyl-substituted poly(butylene succi-
nate); the molar masses of these four samples are similar (each is ꢁ10 to
15 kgmolꢀ1). Neither methylsuccinic acid nor 2-methyl-1,4-butanediol is sym-
metric, and the two monomers may be oriented head-to-tail, head-to-head,
tail-to-tail, or tail-to-head within the polymer backbone.
Previous work addressing the conversion of g-valerolactone
into 1,5-pentanediol with homogeneous Ru-based catalysts
suggests that the intermediates for lactone hydrogenation to
diols are the lactol and open-chain hydroxy aldehyde; the re-
maining carbonyl group is then hydrogenated to yield the
diol.[15] It is apparent from the experiments performed here
that Ru/C promotes the CꢀO bond breaking that is necessary
for lactone ring opening. Re could also promote the ring-open-
ing mechanism given the results obtained with Re for succinic
acid and itaconic acid hydrogenation.[8,23] The undesired hydro-
genolysis of MBDO to isobutyl alcohol and n-butanol over Ru/
C catalysts is known; generically, RCH2OH+2H2!RH+CH4 +
H2O.[14] For MBDO, the end products of hydrogenolysis are
C3H8 +2CH4 +2H2O.
stituent to either the diacid or diol had a significant impact on
the thermal properties of the resulting polyester.[25,26] Whereas
polybutylene succinate, the polymer resulting from the poly-
merization of succinic acid and 1,4-butanediol, is semicrystal-
line with a melting point of Tm =114 8C, the methyl-substituted
polyesters were all fully amorphous with low glass-transition
temperature values (Tg below ꢀ358C for each sample). These
soft, rubbery polyesters may be appropriate for numerous ap-
plications, including use in coatings, adhesives, sealants, elasto-
mers, and foams.[27,28]
Splitting the overall process into two sequential steps with
monometallic catalysts enabled efficient production of MBDO
from IA or MA in high yield. The initial hydrogenation of IA or
MA at high temperatures (2408C) produces high TOFs on Pd/C
As illustrated here as well as in previous examples, biomass-
derived feedstocks can be utilized in hybrid fermentation/ther-
mocatalytic processes to produce drop-in replacements or en-
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tirely new chemicals that extend into new applications.[29] For
example, our method efficiently produces MBDO, a branched
C5 diol that imparts new physical properties to ordinarily
straight-chain polyesters. Anhydromevalonolactone can be
converted into MPDO, a drop-in replacement monomer in the
synthesis of specialty resins. Similarly, we also demonstrated
the reduction of mevalonolactone to produce MPTO, a C6 triol
qualitatively similar to glycerol. Although both MBDO and
MPTO were previously too expensive for most commercial ap-
plications, the development of an efficient reduction method
that utilizes inexpensive and abundant precursors (i.e., IA, MA,
and MLA) will enable their low-cost production and use.
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Published online on && &&, 0000
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C. S. Spanjers, D. K. Schneiderman,
J. Z. Wang, J. Wang, M. A. Hillmyer,
K. Zhang, P. J. Dauenhauer*
&& – &&
Branched Diol Monomers from the
Sequential Hydrogenation of
Renewable Carboxylic Acids
Glucose to rubbery polyesters: Fer-
mentation of glucose to mesaconic acid
(MA) is combined with sequential cata-
lytic reduction to methyl-g-butyrolac-
tone (MGBL) and 2-methyl-1,4-butane-
diol (MBDO) for the synthesis of rubbery
polyester polymers.
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ÝÝ These are not the final page numbers!