The role of non-stoichiometric spinel for iso-butanol formation from biomass syngas over Zn-Cr based catalysts

Article abstract of DOI:10.1039/c7ra02627g

A series of Zn-Cr based catalysts modified by K promoter has been prepared by different methods and their performances for iso-butanol formation from biomass syngas were investigated in a fixed bed reactor. The ZnCr-c catalyst which was prepared by co-precipitation method showed the best catalytic performance for iso-butanol formation, over which ca. 30% of CO conversion and ca. 20% of iso-butanol selectivity were achieved. Multi-characterization studies were then conducted to reveal the internal causes for different performances for iso-butanol formation over different catalysts including high resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS), temperature programmed reduction (TPR) and X-ray photoelectron spectroscopy (XPS). The results reveal that ZnCr-c catalyst contains the maximum amount of non-stoichiometric spinel among all the catalysts. This fact implies that the non-stoichiometric spinel is the active phase for iso-butanol synthesis from syngas. The reducibility, texture parameters and basic property of catalysts are further important factors for the formation of iso-butanol over Zn-Cr based catalysts from biomass syngas.

Full text of DOI:10.1039/c7ra02627g

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PAPER
The role of non-stoichiometric spinel for iso-
butanol formation from biomass syngas over Zn–Cr
based catalysts†
Cite this: RSC Adv., 2017, 7, 20135
*
Siyi Ding, Qianqian Yang, Huaping Ren, Qiang Ma, Yunzhen Zhao
Shaopeng Tian,
*
and Zongcheng Miao
A series of Zn–Cr based catalysts modified by K promoter has been prepared by different methods and their
performances for iso-butanol formation from biomass syngas were investigated in a fixed bed reactor. The
ZnCr-c catalyst which was prepared by co-precipitation method showed the best catalytic performance for
iso-butanol formation, over which ca. 30% of CO conversion and ca. 20% of iso-butanol selectivity were
achieved. Multi-characterization studies were then conducted to reveal the internal causes for different
performances for iso-butanol formation over different catalysts including high resolution transmission
electron microscopy (HRTEM), X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS), temperature
programmed reduction (TPR) and X-ray photoelectron spectroscopy (XPS). The results reveal that ZnCr-
c catalyst contains the maximum amount of non-stoichiometric spinel among all the catalysts. This fact
implies that the non-stoichiometric spinel is the active phase for iso-butanol synthesis from syngas. The
reducibility, texture parameters and basic property of catalysts are further important factors for the
formation of iso-butanol over Zn–Cr based catalysts from biomass syngas.
Received 3rd March 2017
Accepted 17th March 2017
DOI: 10.1039/c7ra02627g
rsc.li/rsc-advances
Higher alcohols (HA), typically dened as alcohols possess-
ing two or more carbon atoms, have attracted considerable
1. Introduction
recent interest owing to their broad range of applications. In the
chemical and polymer industries, they are widely employed as
feedstocks and intermediates for the synthesis of commodity
and specialty products. Among the higher alcohols from
biomass syngas, iso-butanol is one of the most important clean
synthetic fuels and chemical intermediates. Iso-butanol is
considered to be an effective and clean fuel for automobiles
with a high octane rating. Iso-butanol is also a general inter-
mediate that is widely utilized for producing various chemicals,
such as plasticizers and pharmaceuticals, etc. Until now, iso-
butanol is a petroleum-derived product; it is mainly produced
through the reaction of propylene carbonylation with high cost
and low yield rate. The rapid increase in the consumption of iso-
butanol as well as the petroleum crisis means that a low-cost
and petroleum-free method of manufacturing iso-butanol is
required.7,14,15
A promising non-noble metal family of catalysts, Zn–Cr
based catalysts modied with alkali-metal promoter, has been
widely studied for producing iso-butanol from syngas because
of its longer time for retaining high activity and less severe coke
deposition.16–18 They have shown rather high activity (10–30%
CO conversion) and selectivity to alcohols, especially branched
products, with methanol and isobutanol comprising up to 94%
of the alcoholic fraction. That will simplify the process of
separation of iso-butanol production in the alcohol phase and
greatly contribute to the industrialization of iso-butanol.
Energy is the foundation for survival and development of
human society, and limitation of energy resources restrict the
sustainable development of the economy and society.1–3
Unfortunately, energy limitations are becoming increasingly
prominent in recent decades especially in developing countries
such as China and India. Therefore, the exploration and
development of safe and sustainable alternatives to fossil fuels
is one of the most important global priorities today.4–7
Biomass, which is generated from natural resources, shows
huge potential as an alternative to fossil fuels as it is carbon
neutral and easily cultivated in many different environ-
ments.8–10 Firstly, biomass can be converted directly into liquid
fuels, called “biofuels”, to reduce the dependence on fossil fuels
and cut down greenhouse gas emissions. Besides, gasication
of biomass to syngas (a gaseous mixture predominantly
composed of CO, H2, CO2 and CH4) and its subsequent
conversion to higher alcohols were also identied as a prom-
ising process for the production of “green”, high quality trans-
portation fuels.11–13
College of Science, Xijing University, Xi’an, Shaanxi 710000, China. E-mail:
miaozongcheng@xijing.edu.cn; zyz19870226@163.com; Tel: +86 18991150632; +86
18792791568
† Electronic supplementary information (ESI) available: The typical catalytic
performance of ZnCr-c catalyst under different reaction conditions and some
XAS spectra for Zn and Cr. See DOI: 10.1039/c7ra02627g
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Additionally, the origin of this abnormally high selectivity of
From what has been discussed above, we may safely draw the
iso-butanol is also the motivation for understanding this cata- conclusion that there is still debate about the real active sites for
lytic process and the real active sites for iso-butanol in detail. iso-butanol formation on Zn–Cr oxide catalysts. In this paper,
Oxides based on Zn and Cr have been studied extensively, a series of Zn–Cr based catalysts were prepared by different
and thorough reviews of the work have been published.16–19 methods and their performances for iso-butanol synthesis from
Recently, Tan7 investigated the effect of different alkali metals biomass syngas were investigated in a xed reactor. These
over Cr/ZnO catalysts for iso-butanol synthesis from syngas, catalysts also were investigated by multi-characterization tech-
they found that K modied catalyst Cr/ZnO-K exhibited excel- niques such as resolution transmission electron microscopy
lent catalytic ability for iso-butanol synthesis. Wu6,20 systemi- (HRTEM), X-ray diffraction (XRD), temperature programmed
cally studied the mechanism of iso-butanol formation over Cu– reduction (TPR) and X-ray photoelectron spectroscopy (XPS) to
Zr based catalyst and discovered that the mechanism of iso- explore the real active phase for iso-butanol synthesis over Zn–
butanol formation consists of complex consecutive reactions; Cr based catalysts.
methanol is the initial product in the rst step of the reaction
and also the reactant for the next step of reaction to produce C2+
alcohol and iso-butanol. Ethanol is formed subsequently via the
2. Experimental section
2.1 Preparation of catalysts
addition of a C1 (formyl) intermediate to the a-carbon of
methanol (a-addition), and then aldol condensation occurs
rapidly between methanol and ethanol to produce propanol (b-
addition), which further reacts with methanol to produce iso-
butanol (b-addition). In general, chain growth rates are low
for iso-butanol because of its steric hindrance and the lack of
two b-hydrogens required for aldol condensation reactions.
Therefore, iso-butanol becomes a preferred end product of
alcohol chain growth reactions. From the discussion above, we
know that some consensus have been reached about the
mechanism of iso-butanol formation among the world’s scien-
tists, in terms of the three steps to form iso-butanol, namely the
formation of methanol, methanol to ethanol, and ethanol to
iso-butanol. It must be stressed here that, methanol could be
generated easily, while the rst chain growth step, to produce
ethanol, is a rate-determining step. Once ethanol is generated,
iso-butanol will easily form through aldol condensation (b-
addition).
Though extensive studies on modied Zn–Cr based catalysts
have been conducted in the mechanism of iso-butanol forma-
tion, there is still no denite answer to the question “what
might the active phase actually be”. In recent decades, many
studies have aimed at revealing the nature of the active sites in
order to maximise the isobutanol productivity, but their identity
remains controversial. ZnO has been generally identied as the
active species and the promotional role of chromium was
attributed to the formation of spinel, which prevents the sin-
tering of small ZnO crystallites, or acting as a high surface area
support.21,22 Bertoldi37–39 held a different point of view that it is
a non-stoichiometric spinel phase which leads to the formation
of higher alcohols.37–39
2.1.1 Catalyst preparation by different methods
Mechanical mixture method. Powders of ZnO and Cr2O3 were
prepared by a typical co-precipitation method respectively, and
then mixed mechanically (the detailed processing of the co-
precipitation method is described in the “Co-precipitation
method” section below). The Zn/Cr molar ratio was limited to
0.8 and the amount of impregnated K2O was limited to 3.0 wt%,
respectively. The obtained catalysts were nally granulated into
30 to 40 meshes for catalytic performance evaluation. In this
paper, catalysts were denoted as “ZnCr-a” for Zn–Cr catalysts
prepared by a mechanical mixture method.
Fractional precipitation method. Appropriate amounts of
chromium nitrate (Cr(NO3)3$9H2O) with (NH4)2CO3$CH3NO2 as
precipitant were combined. Then, zinc nitrate (Zn(NO3)2$6H2O)
was introduced in the dark green suspension aer it was aged
for 1 h. The following processing was the same as described in
the “Co-precipitation method” section. The molar ratio of Zn to
Cr was limited to 0.8 and the amount of impregnated K2O was
limited to 3.0 wt%, respectively. In this paper, catalysts were
denoted as “ZnCr-b” for Zn–Cr catalysts prepared by fractional
precipitation method.
Co-precipitation method. The Zn–Cr based catalysts were
prepared by co-precipitation of a solution (1 mol lꢀ1) of
Zn(NO3)2$6H2O mixed with Cr(NO3)3$9H2O with (NH4)2-
CO3$CH3NO2 as precipitant at 60 ꢁC, pH ¼ 11, in a well-stirred
and thermo-stated container. The molar ratio of Zn to Cr was set
to 0.8. The precipitate was settled at room temperature for 15 h,
then washed to pH ¼ 7 with deionized water. Aer drying at
120 ꢁC for 12 h, the precursor was the calcined at 350 ꢁC.
Calcined catalyst was powdered and impregnated with 3.0 wt%
K2O as a promoter using K2CO3 with incipient wetness method,
then it was dried at 120 ꢁC for 12 h and calcined at 400 ꢁC.
Finally, the promoted catalyst was pressed and crushed in 30–40
mesh. In this paper, catalysts were denoted as “ZnCr-c” for Zn–
Cr catalysts prepared by co-precipitation method.
Before starting discussion of such a non-stoichiometric
phase, we would like to remind the reader about the basic
notions regarding spinels, specically referring to those con-
taining Zn and Cr. These materials follow the general formula
ZnCr2O4. The ideal normal spinel, where Zn2+ and Cr3+ cations
occupy tetrahedral and octahedral sites, respectively, is ther-
modynamically more stable than the ideal inverse spinel, where
the position of Zn2+ and half of the Cr3+ species are reversed. In
2.2 Characterization of catalysts
reality, the distribution of cations is seldom at these two The morphology and microstructure of samples were investi-
extremes but lies in between the two structures; such solids are gated using a eld emission transmission electron microscope
dened as non-stoichiometric spinels.
(HRTEM, JEM-2100F) operated at 200 kV.
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The XPS spectra were measured by a Vg Escalab MK-2 spec-
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2.3 Anderson–Schultz–Flory (A–S–F) distribution analysis
trophotometer with a monochromatic Al-Ka (1486.6 eV) source
and an accelerating voltage of 12.5 kV as well as power of 250 W.
The sample was pressed into the xed sample holder (10 mm
diameter and 1 mm thick wafer), then it was transferred to
a high vacuum chamber for further analysis with a vacuum
pressure of 2 ꢂ 10ꢀ8 Pa. Charge referencing was done against
adventitious carbon (C 1s, 284.8 eV).
The X-ray powder diffraction (XRD) patterns of the prepared
catalysts were obtained by a Bruker D8 powder diffractometer
equipped with a Ni-monochromator and Cu-K (40 kV, 100 mA)
radiation.
Nitrogen adsorption–desorption isotherms were carried out
at ꢀ195.8 ꢁC using a Micromeritics ASAP 2020 analyzer. Before
adsorption, the samples were out-gassed at 350 ꢁC for 10 h. The
specic surface area (SBET) was evaluated using the BET
method, while the pore size distribution was calculated
according to the Barrett–Joyner–Halenda (BJH) formula applied
to the desorption branch.
In order to investigate the role of non-stoichiometric spinel
ZnxCr2/3(1ꢀx)O for iso-butanol synthesis for biomass syngas on
Zn–Cr based catalysts, an Anderson–Schultz–Flory (A–S–F)
distribution is adopted,
"
#
ꢀ
ꢁ
2
Wn
n
ð1 ꢀ aÞ
log
¼ n log þ log
a
where Wn is the weight fraction of an alcohol (containing n
carbon atoms) in total alcohols in the products. The parameter
a is dened as the chain growth probability that a molecule
continues reacting to form a longer chain. The parameter a is
greater than zero but is less than one because a part of inter-
mediates desorb on the catalyst surface in each step of the
growth of carbon chain.
2.4 Activity measurements of catalysts
The catalytic reactions were carried out by using a xed-bed
reactor made of stainless steel with 5 ml catalysts. The ow
rate of the feed gas was controlled by a mass ow controller, and
the exit gas was measured by a wet test meter. The catalyst was
reduced according to the designed temperature program, i.e.
from room temperature to 360 ꢁC, by using a mixture of H2/N2 ¼
10/90 for over 2 h. The reactions were run by using a feed
biomass syngas at a space velocity of 3000 hꢀ1. The biomass
syngas mainly includes CO, H2, CO2 and CH4. The molecular
ratio of H2/CO is ca. 2.6. The reactions were implemented at
400 ꢁC, 10 MPa. The product stream was analysed by using a GC
4000 gas chromatograph. Syngas and exit gas were analysed by
using a column of carbon sieves and a thermal conductivity
detector TCD to monitor H2, CH4, CO and CO2. CHx mixtures
were analysed by a GDX-403 column and a ame ionization
detector. Water and methanol were detected by a GDX-401
column and a thermal conductivity detector. The mixtures of
alcohol products were analysed by another GC-7A with a chro-
mosorb 101 column and a ame ionization detector.
Temperature-programmed reduction (TPR) measurements
was used to test the redox properties of different prepared
catalysts. H2-TPR was recorded on a TP-5050 automatic chem-
ical adsorption instrument in the range from 50 to 600 ꢁC with
a 10% H2/Arꢀm1 ixture owing at 30 ml minꢀ1 and heated at a rate
ꢁ
ꢁ
of 5 C min . A 100 mg sample was treated at 300 C in pure
argon ow for 2 h to eliminate the adsorption of physically
ꢁ
adsorbed water and impurities, and then cooled to 50 C. The
argon was then replaced and stabilized by the reductive gas. The
effluent gas was analyzed by a thermal conductivity detector
(TCD).
The temperature programmed desorption of carbon dioxide
(CO2-TPD) was recorded on a TP-5050 automatic chemical
ꢁ
adsorption instrument in the range from 50 to 550 C. For the
sake of accuracy, pure standard mixtures [zeolite + activated
carbon] were used during preliminary TPD measurements. The
ꢁ
catalysts were saturated with CO2 ow at 50 C aer pretreat-
ment at 400 ꢁC for 180 min in reducing gas which contains 10%
H2 in Ar. Then pure Ar was owed for 120 min at the same
temperature to eliminate the adsorption of physically adsorbed
water and impurities. Finallꢀy1, the temperature was increased at
3. Results and discussion
3.1 Morphology and microstructure of catalysts
ꢁ
a constant rate of 5 C min under an appropriate amount of
Ar ow. The desorbed species were analyzed by thermal The TEM and HRTEM images of catalysts prepared by different
conductivity measurements.
methods are shown in Fig. 1 and 1-1 to 1-3, respectively. In our
The X-ray absorption spectrum (XAS) data of Zn and Cr were previous experiments, we have found that the catalysts with
obtained at the beamline BL14W1 of the Shanghai Synchrotron non-stoichiometric Zn–Cr spinel structure all have particles of
Radiation Facility (SSRF). The storage ring was operated at 3.5 around 6 nm in diameter while the catalysts with zinc oxide
GeV with a constant current of 250 mA. A Si (111) mono- phase structure have particles of larger than 20 nm in diameter.
chromator was used for the tests. The catalysts were homoge- The microstructures of the catalysts, shown in Fig. 1-1 to 1-3,
neously smeared on the tapes to reach the optimum thickness further conrm this conclusion. It is seen that the ZnCr-
(Dm ꢂ d z 1, Dm is the absorption edge jump and d is the a catalysts, prepared by mechanical mixture method, had
physical thickness of catalysts).
a wurtzite crystal structure of zinc oxide phase and chromiu-
XAS data were analyzed using the specialized soware m(III) oxide phase. There was no non-stoichiometric spinel
ATHENA. The absorption curves were obtained aer removing phase in ZnCr-a catalysts. However, the non-stoichiometric
the pre-edge and post-edge background. All of the curves were spinel phase appeared both in ZnCr-b catalysts (prepared by
rstly normalized to unity before comparison. The Fourier fractional precipitation method) and in ZnCr-c catalysts
transform (FT) spectra were obtained in a Hanning window in (prepared by co-precipitation method). It was worth noting that
ꢀ1
the range of 3–14.5 A for Zn atom and 3–13.5 Aꢀ1 for Cr atom. the zinc oxide phase and chromium oxide phase also were
˚
˚
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Fig. 1 TEM images of the catalysts prepared by different methods. (1-1) HRTEM image of the catalyst prepared by mechanical mixture method
(ZnCr-a). (1-2) HRTEM image of the catalyst prepared by fractional precipitation method (ZnCr-b). (1-3) HRTEM image of the catalyst prepared by
co-precipitation method (ZnCr-c).
found in ZnCr-b catalysts while these two phases could not be peaks for potassium species were found in any of the XRD
found in ZnCr-c catalysts which is consistent with XRD analysis patterns. No K2O (JCPDS card 47-1701) or K2CO3 (JCPDS card 49-
in Fig. 2. Hence, a conclusion could be obtained cautiously that 1093) peaks were found, possibly because the K2O phases over
none of Zn2+ and Cr3+ ions formed non-stoichiometric spinel the catalyst surfaces were highly dispersed. The particle size
phase in ZnCr-a catalyst, while part of Zn2+ and Cr3+ ions calculated by the Scherrer equation from XRD signals is 27.8 nm
formed the non-stoichiometric spinel phase in ZnCr-b catalyst for ZnO, 15.8 nm for Cr2O3 and 5.9 nm for non-stoichiometric
and all of the Zn2+ and Cr3+ ions formed the non-stoichiometric spinel particles. These values are consistent with TEM images
spinel phase in ZnCr-c catalyst. The conclusion also could be analysis. The main reections of ZnCr-a XRD signals were
further conrmed by the results of TPR and XPS spectra.
consistent with the standard pattern of zinc oxide and chro-
mium(III) oxide phase while the XRD signals of non-
stoichiometric spinel could not be found.7 Concerning the
ZnCr-b and ZnCr-c XRD signals, the main reections were
consistent with the standard pattern of non-stoichiometric
spinel (ZnxCr2/3(1ꢀx)O).23–25 All the results of XRD patterns are
3.2 Characterization of the catalysts using powder X-ray
diffractometry
Fig. 2 shows the powder X-ray diffractometry (XRD) patterns of
Zn–Cr catalysts prepared by different methods. No diffraction
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are similar to the curves of Zn–Cr spinel structure. Comparing
the curves of ZnCr-b and ZnCr-c catalysts, it is obvious that the
intensity of peak located at 9668 eV became stronger in the
sequence of ZnCr-b to ZnCr-c catalyst. In a similar report,
Stewart et al.42 have shown that the intensity of the peak located
at 9668 eV become stronger when the cation disorder distribu-
tion in the spinel structure of Zn–Fe spinel become more
extensive. It is highly probable that these changes in our
experiment have the same origin for Zn–Cr spinel. Chen et al.43
also studied the cation distribution in normal and non-
stoichiometric Zn–Cr spinel structure and the XANES spectra
showed a similar change as we observed here. Therefore, the
intensity change of the peak located at 9668 eV shows a local
structural change in Zn–Cr spinel which results from consid-
erable zinc cation transference from tetrahedral to octahedral
sites.
Fig. 4 shows the normalized XANES spectra for Cr atoms in
catalysts. The huge difference between the curves of ZnCr-
a catalyst and ZnCr-b (ZnCr-c) catalyst also indicates that the
structure of ZnCr-a catalyst is very different from the others.
Comparing carefully the spectra of ZnCr-b and ZnCr-c catalyst,
it is obvious that the intensity of pre-edge peak (see inset in
Fig. 4, upper le corner) becomes stronger in the sequence of
ZnCr-b to ZnCr-c catalyst. In theory, the pre-edge peak is due to
the quadrupole transitions from the 1s to the 3d electronic
orbital, which is forbidden because of the electric dipole. The
increase of the pre-edge peak is due to the enhancement of the
orbital p–d mixing, allowed in tetrahedral symmetry but
forbidden in octahedral symmetry. Thus, the increase of the
pre-edge peak reects the increase of the occupation number of
Cr atoms in tetrahedral sites. It is worth noting that Cr atoms
should locate at octahedral sites rather than tetrahedral sites in
a normal Zn–Cr spinel. In other words, the cation disorder
distribution becomes more extensive in ZnCr-c catalyst than
that of ZnCr-b catalyst. Non-stoichiometric spinel has formed in
both ZnCr-b and ZnCr-c catalysts. All data are well consistent
with the results of the XANES spectra for Zn atoms.
Fig. 2 XRD patterns of the catalysts prepared by different methods.
consistent with TEM images analysis. It was worth noting that
neither zinc nor chromium oxide phase diffraction was found in
ZnCr-b catalyst. Combining with the results of TEM images, the
small part of zinc or chromium oxide may be highly dispersed
on the catalysts surface in an amorphous state.
3.3 XAS analysis of catalysts
Synchrotron radiation technique is a powerful tool to investi-
gate the chemical environment around central metal atoms.
Both XANES and EXAFS provide valuable information about the
coordination numbers and distances of central metal atoms.
Recently, they have been used to investigate the cation distri-
bution in spinels such as CuFe2O4, ZnFe2O4, MnFe2O4, etc.44–48
Fig. 3 shows the normalized XANES spectra for Zn atoms in
catalysts. The curve of ZnCr-a catalyst shows a huge difference
from the curves of ZnCr-b and ZnCr-c catalysts, indicating that
the structure of ZnCr-a catalyst is very different from the others.
All the peaks of ZnCr-a catalyst are similar to the curves of
standard ZnO foil while the peaks of ZnCr-b and ZnCr-c catalyst
Fig. 3 The normalized XANES spectra for Zn atoms in catalysts.
Fig. 4 The normalized XANES spectra for Cr atoms in catalysts.
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Table 1 Texture parameters of catalysts
The corresponding Fourier transforms EXAFS signals for Zn
atoms in catalysts are compared in Fig. 5 In theory, the rst and
second shell distance from the central Zn atom in Zn–Cr spinel
Catalyst
SBET (m2 gꢀ1
)
Vp (cm3 gꢀ1
)
Rp (nm)
˚
structure is 1.60 and 3.01 A respectively (the instrumental error
ZnCr-a
ZnCr-b
ZnCr-c
57.3
74.0
95.6
0.124
0.207
0.287
5.8
11.9
13.4
has been considered). As shown in Fig. 5, both of these
distances are observed in ZnCr-b and ZnCr-c catalysts. On
taking a close look at the signals, it is obvious that besides the
˚
two normal peaks at 1.60 and 3.01 A, an additional peak also
˚
appears at 2.60 A which clearly results from the cation disorder volumes and radii of catalysts are also inuenced by the particle
distribution. Stewart et al.42 have found a similar change in size of catalysts. Bigger pore radii and pore volumes more
ZnFe2O4 spinel. For catalysts in our experiment, when Zn atoms readily allow bigger molecules (e.g. iso-butanol) to diffuse from
˚
substitute to octahedral sites, a new shell would appear at 2.60 A the active sites inside the pore, which may be a signicant factor
and result in a new peak. Not surprisingly, the location of this for better catalytic performance for the formation of iso-
additional peak corresponds to the second coordination butanol.
distance of Cr atom (seen in Fig. S3†). It further clearly
conrmed the conclusion that some part of Zn cations migrate
into octahedral sites, signifying cation disorder distribution
and appearance of the non-stoichiometric spinel.
3.5 Characterization of the catalysts using X-ray
photoelectron spectra
X-Ray photoelectron spectra (XPS) studies of the samples were
carried out in order to investigate the chemical state and
average chemical valences of the elements over the catalysts
From the analysis of XANES and EXAFS, one conclusion
could be stated that the preparation method could affect the
cation distribution in the Zn–Cr spinel structure. Specically,
the catalyst would only contain ZnO and Cr2O3 phase if the
catalyst was prepared by a mechanical mixture method while
the non-stoichiometric spinel would be formed if the catalyst
was prepared by fractional precipitation or co-precipitation
method. The amount of non-stoichiometric spinel (the degree
of cation disorder distribution) reaches a maximum when the
catalyst was prepared by the co-precipitation method (ZnCr-c
catalyst).
3.4 Texture parameters of catalysts
The texture parameters for the catalysts prepared by different
methods are shown in Table 1. The Brunauer–Emmett–Teller
surface areas (SBET) of the catalysts increased linearly from 57.3
to 95.6 m2 gꢀ1 in the sequence of ZnCr-a, ZnCr-b and ZnCr-c
catalysts. The changing trend of catalysts SBET could be
explained by the different particle sizes of catalysts which has
been conrmed by TEM images and XRD patterns. The pore
Fig. 6 (a) XPS analysis: Zn 2p spectra of catalysts prepared by different
methods. (b) XPS analysis: Cr 2p spectra of catalysts prepared by
different methods.
Fig. 5 Fourier transform EXAFS signals for Zn atoms in catalysts.
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surface. The results are shown in Fig. 6a and b. Features caused hydrogen consumption peaks appearing around 375 and
by the presence of Zn and Cr elements were all quite prom- 412 ꢁC. The peak at lower temperature can be attributed to the
inent.22,26 In Fig. 6a, the binding energy value of Zn in ZnCr- reduction of Cr6+ species to Cr3+ species, and the peak at the
a catalyst was located at an average of 1021.9 eV, while the higher temperature should be due to the reduction of Cr3+
binding energy value of Zn in ZnCr-b and ZnCr-c catalyst located species to Cr2+ species. The ZnO phase over ZnCr-a catalyst
at around 1022.1 and 1022.4 eV, respectively. This indicates that could not be reduced in the temperature range of the experi-
the valency of Zn over the ZnCr-c catalyst surface is slightly ments. Each catalyst of ZnCr-b and ZnCr-c had only one main
larger than that of ZnCr-a and ZnCr-b catalysts. Fig. 6b shows hydrogen consumption peak in the temperature range from 328
the typical XPS spectra of Cr 2p3/2. The binding energy changing to 335 ꢁC which could be attributed to the reduction of non-
trend of Cr is very similar to that of Zn over the catalysts. The stoichiometric Zn–Cr spinel. The hydrogen consumption
binding energy value of Cr in ZnCr-c catalyst which is located at values for the samples of ZnCr-b and ZnCr-c catalysts were
around 577.6 eV is also slightly larger than that of ZnCr-a and about 1.32 and 1.71 mmol gꢀ1, respectively. It has previously
ZnCr-b catalysts which is located at around 576.7 and 577.1 eV, been found that Zn2+ and Cr3+ in Zn–Cr normal spinel are very
respectively.22,26 The spectra of all catalysts contained a typical difficult to be reduced. However, each of our catalysts was in
Cr6+ peak at 578.1 eV, indicating that each of catalysts had partly
a “metastable state” because of the presence of non-
been over-oxidized.27 The difference of XPS peak intensities may stoichiometric spinel which contain a large number of defects
be the result from the different amount of samples for each and oxygen vacancies, and that should make the catalysts much
measurement.
more reducible than normal Zn–Cr spinel. Our XPS analyses of
In a previous study, Battistoni28 found that all of the Zn 2p the samples showed that the Zn and Cr valences were higher in
and Cr 2p binding energy values for the non-stoichiometric non-stoichiometric spinel samples than in normal spinel
spinel compounds lie, within the experimental errors, at samples which also meant that our catalysts could be reduced
slightly larger values than those for ZnO and Cr2O3 oxide. The much more easily than normal spinel.
difference could be attributed to a different Madelung potential
Aer H2 adsorption, zinc oxide has become free state Zn2+
.
contribution in the spinels and in the oxide. The Zn 2p and Cr Tan7 recently discovered that the active species in the studied
2p binding energy values increased in the order of ZnCr-a, ZnCr- reaction are free state Zn2+ located in octahedral site (disor-
b and ZnCr-c catalysts in our experiments, most likely resulting dered distribution) inside the non-stoichiometric spinel struc-
from the similar reason: the formation of non-stoichiometric ture. If the reducibility of non-stoichiometric spinel is higher,
spinel. Catalyst ZnCr-c has the highest valence suggesting that there will be more free state Zn2+ formed from zinc oxide, as
it has the highest amount of the non-stoichiometric spinel a result the catalyst activity will be better.7
structure. This is consistent with the results of TEM and XRD
patterns.
3.7 Temperature programmed desorption of carbon dioxide
(CO2-TPD) analysis of the catalysts
3.6 Temperature program reduction of hydrogen (H2-TPR)
analysis of the catalysts
The basicity property of the samples prepared by different
methods are estimated by temperature programmed desorption
of carbon dioxide (CO2-TPD) analysis and the results are shown
in Fig. 8. These TPD patterns differ between samples in their
positions, heights and widths, indicating changes not only in
The redox properties of the different catalysts were evaluated by
performing H2-TPR analyses of the catalysts, and the results are
shown in Fig. 7. For the ZnCr-a catalyst, there were two
Fig. 7 H2-TPR profiles of catalysts prepared by different methods.
Fig. 8 CO2-TPD profiles of catalysts prepared by different methods.
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the total basicity, but also in the distribution of the basicity based on our experimental results and other researchers’
strength.
When the specic uptakes are compared, it is obvious that
calculations.40,41
The catalysts performance for the formation of iso-butanol
the CO2 uptakes are increased in the sequence of ZnCr-a, ZnCr- and MeOH from biomass syngas was measured under the
b and ZnCr-c catalysts. Both weak and strong basicity sites are optimal reaction conditions and the typical results are shown in
few when the catalysts are prepared by mechanical mixture Table 2. As a comparison, the catalytic performance of ZnO and
method and are both enhanced markedly when the catalysts are Cr2O3 catalysts for the formation of iso-butanol are also shown
prepared by fractional precipitation and co-precipitation in Table 2.
methods. An accurate evaluation of the basic properties of the
In Table 2, the catalyst prepared by mechanical mixture
catalysts required quantitative analyses to be performed. The method showed the worst catalytic performance among the Zn–
values of CO2 desorption for the ZnCr-a, ZnCr-b and ZnCr-c Cr based catalysts, only 17.3% of the CO was converted, and the
samples were about 0.21, 1.95 and 2.54 mmol gꢀ1, respec- selectivity for alcohol and iso-butanol was extremely poor, with
tively. This further proved that both the weak and strong selectivity for alcohol at a minimum of 37.7% and the iso-
basicity sites increased in the sequence of ZnCr-a ZnCr-b and butanol amounted to just 3.13 wt% of alcohol production.
ZnCr-c catalysts and reached the maximum value when the The catalyst prepared by fractional precipitation method (ZnCr-
catalyst was prepared by the co-precipitation method.
b catalyst) showed much better catalytic performance for iso-
In general, the basicity property is closely related to the butanol synthesis from biomass syngas. The CO conversion of
intrinsic nature of catalysts, especially surface properties. As ZnCr-b catalyst increased to 26.5%; the alcohol selectivity was
has been discovered, the amount of non-stoichiometric spinel improved to 42.2%; and the iso-butanol selectivity was
appeared and increased for catalysts prepared by fractional enhanced to 13.6%. The catalyst prepared by co-precipitation
precipitation and co-precipitation method, respectively. The method (ZnCr-c catalyst) presented the best catalytic perfor-
catalysts with non-stoichiometric spinel phase showed more mance for the formation of iso-butanol from biomass syngas.
surface defects and oxygen vacancies. That should strongly The CO conversion was enhanced to 30.4%; the total alcohol
effect on the distribution of the basicity property and enhance rate was the highest at 0.091 g mlꢀ1 hꢀ1; the alcohol and iso-
the basicity sites markedly.
butanol selectivity also reached maximum values of 49.6 and
As discussed above, iso-butanol is formed through a-addi- 20.2%, respectively. It is worth noting that the selectivity value
tion and b-addition over Zn–Cr based catalyst. Previous of methanol plus iso-butanol is around 94% in alcohol
researchers have proved that the basicity property affects production which is a typical characteristic of optimal Zn–Cr
strongly the productivity and selectivity of iso-butanol. They catalysts. This should simplify the process of products separa-
found that basicity affects dramatically the b-addition (aldol tion and then contribute greatly to the industrialization of iso-
condensation) process which is a necessary step to form iso- butanol.
butanol over Zn–Cr based catalyst, indicating basicity property
The present catalysts prepared by different methods pre-
is one of the key factors for iso-butanol formation. The non- sented a lower selectivity for iso-butanol than that of the Zn–Cr
stoichiometric spinel could offer more basicity sites that samples reported by Epling and co-workers,21,22 possibly due to
should be an important factor for high productivity and selec- the differences in the reaction conditions and/or sample
tivity of iso-butanol.
composition and texture. It must be stressed that the point of
our present study is not to improve on the highest reported
selectivity for iso-butanol, but to study the role of non-
stoichiometric spinel for iso-butanol synthesis over Zn–Cr
3.8 Catalytic performance of the catalysts
The optimal reaction conditions were rstly investigated over based catalysts.
the ZnCr-c catalyst for the formation of iso-butanol including
As will be noticed, the catalytic performance, especially the
different reaction temperature, different reaction pressure and alcohol distribution over ZnCr-a catalyst displays a huge
different gas hourly space velocity (GHSV). From the thermo- difference from the ZnCr-b and ZnCr-c catalysts. From the
dynamic results of other research,40,41 increasing the reaction results of alcohol distribution, it can be readily seen that the
pressure is always benecial for the formation of alcohols while selectivity for alcohol with higher carbon number decreased
high reaction temperature is detrimental to this reaction and dramatically over ZnCr-a catalyst while the alcohol products
results in lower yields of higher alcohols. What must be stressed mainly contain methanol and iso-butanol over ZnCr-b and
here is that not only the catalysts performance but also the ZnCr-c catalysts. In order to nd the causes in depth for this
kinetic factors and energy consumption need to be considered huge difference and investigate the role of non-stoichiometric
while identifying the optimal reaction conditions. The results of spinel for iso-butanol synthesis from biomass syngas on Zn–
typical performance under different conditions over ZnCr-c Cr based catalysts, an Anderson–Schultz–Flory (A–S–F) distri-
catalyst are shown in Tables S1–S4.† From Tables S1–S4,† it bution was adopted for the catalytic performance of ZnCr-
can be found easily that the ZnCr-c catalyst shows the best a catalyst and the result is shown in Fig. 9.
performance for iso-butanol synthesis from biomass syngas
From the result of Fig. 9, it is obvious that the alcohol
under the optimised reaction conditions, with temperature of distribution over ZnCr-a catalyst obeys the A–S–F distribution
400 ꢁC, pressure of 10 MPa and GHSV of 3000 hꢀ1. In our which means the mechanism for higher alcohol formation is via
reaction conditions, equilibrium limitation has not yet reached a CO insertion mechanism.29–32 In the CO insertion mechanism,
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Table 2 Catalytic performance of the catalysts prepared by different methodsa
Alcohol distribution (wt%)
CO conversion
(%)
Alcohol selectivity
(%)
Total alcohol
Catalyst
rate (g mlꢀ1 hꢀ1
)
Methanol
Ethanol
Propanol
Iso-butanol
C5+ alcohol
ZnCr-a
ZnCr-b
ZnCr-c
ZnO
17.3
26.5
30.4
15.5
5.23
37.7
42.2
49.7
31.4
15.3
0.033
0.075
0.091
0.035
0.011
72.6
75.4
74.3
54.6
67.1
15.3
4.71
1.47
21.8
16.5
8.29
4.08
2.91
13.5
9.16
3.13
13.6
20.2
4.77
4.16
0.69
0.95
1.22
0.52
0.85
Cr2O3
a
Reaction conditions: temperature ¼ 400 ꢁC, pressure ¼ 10 MPa, GHSV ¼ 3000 hꢀ1
.
growth reactions. The a-addition is a rate-determining step with
high energy barriers while the b-addition is a rapid reaction
step. According to this theory, methanol and iso-butanol are the
main production in alcohol phase as shown in Table 2.
The question arises as to why the alcohol distribution
displays a huge difference between ZnCr-a and ZnCr-c (ZnCr-b)
catalysts. Based on the preceding analysis of multi-
characterization results, the main components are ZnO and
Cr2O3 phase in ZnCr-a catalyst while the main component is
non-stoichiometric spinel ZnxCr2/3(1ꢀx)O phase in ZnCr-b and
ZnCr-c catalysts. It can be inferred that, with the change of the
preparation methods, the change of catalyst components may
be the main reason for the differences of alcohols distribution.
Specically, the mechanism for higher alcohol formation is the
CO insertion mechanism over ZnCr-a catalyst which is
comprised of ZnO and Cr2O3 phases without non-
stoichiometric spinel phase. However, once the non-
stoichiometric spinel phase is formed, as for ZnCr-b and
ZnCr-c catalysts, the mechanism of the higher alcohol forma-
tion will be changed to b-addition because it is a rapid reaction
step with lower energy barrier. The alcohol distribution will
then change dramatically from the A–S–F distribution to
methanol plus iso-butanol distribution. Hence, the non-
stoichiometric spinel phase should be the active phase for b-
addition to form iso-butanol. The ZnCr-c catalyst shows the
better catalytic performance owing to the higher amount of the
non-stoichiometric spinel phase, further conrming this
conclusion.
Fig. 9 A–S–F distribution of alcohols over the catalyst prepared by
mechanical mixture method.
the chain initiation and chain propagation proceed similarly to
form surface alkyl species (CnHz*) and termination by CO
insertion gives alcohols through surface acyl species (CnHzCO*)
followed by hydrogenation. The CO insertion mechanism
rationalizes why the catalysts mainly produce linear primary
alcohols and why the alcohol distribution obeys the A–S–F rule.
According to this theory, the selectivity for alcohols with higher
carbon number will decrease dramatically as shown in Fig. 9.
In Table 2, from the results of the alcohol distribution over
ZnCr-b and ZnCr-c catalysts, the alcohol products mainly
contain methanol and iso-butanol which obviously do not obey
the A–S–F distribution rule. That means the mechanism for
higher alcohol formation, mainly forming iso-butanol, is not by
CO insertion but instead by b-addition.33–36 In the b-addition
mechanism, iso-butanol is formed through the addition of a C1
(formyl) intermediate to the a-carbon of methanol (a-addition)
to produce ethanol, then the aldol condensation process (b-
addition) between methanol and ethanol to produce propanol,
and the aldol condensation process (b-addition) between
methanol and propanol to produce iso-butanol. Chain growth
rates are low for iso-butanol (2-methyl-1-propanol) because of
its steric hindrance and the lack of the two b-hydrogens
required for aldol condensation reactions. Therefore, iso-
butanol becomes a preferred end product of alcohol chain
The non-stoichiometric Zn–Cr spinel is a metastable state
that leads to much more defects and oxygen vacancies. These
defects and oxygen vacancies could make the catalyst more
readily adsorb and activate reactant molecules during the
reactions and benet the catalytic performance. The lager BET
surface area (SBET), easier reduction, and higher number of
basic sites over the ZnCr-c catalyst also make a substantial
contribution to the better catalytic performance for the forma-
tion of iso-butanol.
4. Conclusions
In this study, we prepared a series of Zn–Cr based catalysts
using different methods and then investigated the catalysts
performance for iso-butanol synthesis from biomass syngas.
Among the tested catalysts, the catalysts prepared by co-
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The authors thank the Shanghai Synchrotron Radiation Facility
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