Preparation and characterization of nonmetal promoter modified CuZnAl catalysts for higher alcohol from synthesis gas through complete liquid phase method

Article abstract of DOI:10.3906/kim-1212-67

A complete liquid phase technology and a function regulator were applied to prepare CuZnAl catalysts for higher alcohol synthesis. Characterizations showed that the introduction of the function regulator can change the reduction ability of copper oxides and the surface basicity of catalysts. Activity tests indicated that the selectivity of higher alcohol is high when considerable medium-strong basicity and the synergistic eects of copper ion and metal copper exist on the catalytic surface. The optimized modified CuZnAl catalyst without any metal additives provides a CO conversion of 28.9%, C₂₊OH selectivity of up to 42.8%, and hydrocarbon selectivity of 2.5%, with a total alcohol selectivity of 67.4% under the reaction conditions of 5.0 MPa, 250 C, H₂/CO = 1, and a gas hourly space velocity of 360 mL/g _cat h. TUeBITAK.

Full text of DOI:10.3906/kim-1212-67

Turk J Chem
Turkish Journal of Chemistry
(2014) 38: 381 – 387
¨
http://journals.tubitak.gov.tr/chem/
˙
c
⃝ TUBITAK
doi:10.3906/kim-1212-67
Research Article
Preparation and characterization of nonmetal promoter modified CuZnAl
catalysts for higher alcohol from synthesis gas through complete liquid phase
method
Shi-rui YU1,2, Xiao-dong WANG1, Wei HUANG1,∗
1Key Laboratory of Coal Science and Technology of Education Ministry and Shanxi Province,
Taiyuan University of Technology, Taiyuan, Shanxi, P.R. China
2School of Chemistry and Chemical Engineering, Guizhou University, Guiyang, Guizhou, P.R. China
Received: 27.12.2012
•
Accepted: 05.09.2013
•
Published Online: 14.04.2014
•
Printed: 12.05.2014
Abstract: A complete liquid phase technology and a function regulator were applied to prepare CuZnAl catalysts
for higher alcohol synthesis. Characterizations showed that the introduction of the function regulator can change the
reduction ability of copper oxides and the surface basicity of catalysts. Activity tests indicated that the selectivity of
higher alcohol is high when considerable medium-strong basicity and the synergistic effects of copper ion and metal
copper exist on the catalytic surface. The optimized modified CuZnAl catalyst without any metal additives provides a
CO conversion of 28.9%, C2+ OH selectivity of up to 42.8%, and hydrocarbon selectivity of 2.5%, with a total alcohol
selectivity of 67.4% under the reaction conditions of 5.0 MPa, 250 ◦ C, H2 /CO = 1, and a gas hourly space velocity of
360 mL/gcat h.
Key words: Complete liquid phase method, CuZnAl catalyst, higher alcohols, nonmetal promoter, syngas
1. Introduction
Higher alcohols can be used as pure unleaded fuels or as fuel additives in unleaded fuels and as sources of chemical
products,1,2 and have attracted interest in C1 chemistry. Heterogeneous catalysts have been employed for the
synthesis of higher alcohols. Alkali-promoted Cu-ZnO/Al2 O3 catalysts have been extensively studied for this
purpose, since Cu-ZnO/Al2 O3 catalysts have proved very efficient and cheap for methanol synthesis.3,4 Alkali
promoters neutralize the surface acidity, suppressing various side reactions such as hydrocarbon and dimethyl
ether formation, dehydration, and coke deposition.5−8 In addition, they have been found to increase the rate of
carbon chain growth and to enhance the yield and selectivity of higher alcohol with increasing basicity. However,
excess alkali loading might block the active sites on the catalyst surface and reduce the Brunauer–Emmett–Teller
surface area, thereby leading to activity loss.4 Alkali-promoted Cu-ZnO/Al2 O3 catalysts produce a mixture of
linear and branched alcohols that includes a large proportion of methanol and a large amount of hydrocarbons
together with a small amount of other oxygenates. As a result, alkali-promoted Cu-ZnO/Al2 O3 catalysts suffer
from low selectivity for C2 –C6 alcohols, though substantial efforts have been devoted in this respect.3 We found
that the coexistence of copper ions and copper metal is in favor of carbon chain growth in previous experimental
work.9 Thereby, organic bases such as triethanolamine instead of alkali metal promoters were introduced during
the preparation process of catalysts. Chelating agents such as N-methyl pyrrolidone were also introduced in
∗Correspondence: huangwei@tyut.edu.cn
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order to improve resistance to the reducibility of copper oxides in our work. Catalysis is controlled not only by
the chemical composition and size of the catalysts used, but also by the character of surface sites available on the
catalyst surface.10−12 These characteristics are associated with the catalyst preparation methods. Currently,
heterogeneous catalysts are generally prepared by traditional methods, such as coprecipitation, impregnation,
and sol-gel techniques.13−18 A novel method invented by us, named the complete liquid-phase method, has
been applied to prepare slurry catalysts. The main innovation is the preparation of slurry catalysts from the
raw material solution in a liquid-phase environment.19−21
Here, we report the preparation, characterization, and performances of nonmetal promoter modified
CuZnAl catalysts prepared by complete liquid-phase technology for higher alcohol from syngas in a slurry
reactor.
2. Experimental
2.1. Catalyst synthesis
2.1.1. Materials
Cu(NO3)2·3H2 O, Zn(NO3)2·6H2 O, and (C3 H7 O)3 Al were purchased from Tianjin Kermel Chemical Reagent
Co., Ltd. Polyvinyl-pyrrolidone (PVP) was purchased from Tianjin Damao Chemical Reagent Factory. Tri-
ethanolamine (TEA) was obtained from Tianjin Hongyan Reagent Factory and 1-methyl-2-pyrrolidone (NMP)
was purchase from Tianjin Hengxing Chemical Preparation Co., Ltd. All chemicals were of analytical reagent
grade and were used without further purification. Deionized double-distilled water was used to make the solu-
tions.
2.1.2. Catalyst preparation method
First, 0.1 mol aluminum isopropylate ((C3 H7 O)3 Al) was dissolved in a mixture of ethanol, surfactant PVP,
and deionized water, and kept at 80 ◦ C for 1.5 h in a water bath. After that, a certain amount of nitric acid
was added to the solution with vigorous stirring for 1 h at 95 ◦ C; then another solution was added, which was
prepared by dissolving 0.24 mol Cu(NO3)2·3H2 O and 0.12 mol Zn(NO3)2·6H2 O in an appropriate amount of
ethanol. Certain amounts of TEA and NMP were also added to the above solution. The mixture obtained was
stirred under reflux at 95 ◦ C for 10 h and then kept in a beaker at 30 ◦ C for 10 days to obtain a gel. The gel
was then heated in liquid paraffin from room temperature to 280 ◦ C at a heating rate of 10 ◦ C/min and held for
8 h at this temperature. A flow of 60 mL/min N2 was maintained throughout, and the CuZnAl slurry catalyst
resulted from this process. The catalysts prepared with different TEA/NMP ratios are denoted as xTyN, where
x and y refer to the added volumes (in mL) of TEA and NMP during the preparation, respectively.
2.2. Catalyst characterization
2.2.1. X-ray diffraction
Powder X-ray diffraction (XRD) analysis was performed with a Rigaku D/max-2500 powder diffractometer
(using CuKα radiation with a tube voltage of 40 kV, a tube current of 100 mA, and scan rate of 4◦ /min). The
phase identification was carried out by using the Joint Committee on Powder Diffraction Standards (JCPDS)
files.
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2.2.2. Temperature-program desorption
NH3 -temperature-program-desorption (NH3 -TPD) and CO2 -temperature-program-desorption (CO2 -TPD) were
performed on a TP-5000 instrument. Samples (100 mg) were first reduced by 5% H2 at 280 ◦ C for 60 min
in a quartz reactor and purged by He at 280 ◦ C for 30 min. The catalyst was cooled down to 50 ◦ C and
adsorbed NH3 (or CO2) at 50 ◦ C until saturation and was purged with high-purity He for 30 min to remove
the physisorbed NH3 (or CO2). The TPD data were collected in a flow of He from 50 to 800 ◦ C at a heating
rate of 10 ◦ C/min. The desorbed molecules were detected with a mass spectrometer (MS) by monitoring the
signal at m/z = 17 (NH3) or 44 (CO2).
2.2.3. Temperature programmed reduction
The reducibility of the catalysts was studied using temperature programmed reduction (TPR). H2 -TPR was
carried out in a laboratory-made microreactor. Prior to each TPR run, the 50.0 mg catalysts were heated to
150 ◦ C for 60 min under a He flow of 30 mL/min. After the sample cooled down to 50 ◦ C, the TPR analysis
was then carried out by ramping the temperature to 500 ◦ C under 5.0% H2 /N2 (30 mL/min), using a thermal
conductivity detector to record the reduction peaks.
2.3. Catalyst activity measurements
2.3.1. Reaction conditions
The higher alcohol synthesis reaction was carried out in a 500-mL slurry-phase continuously stirred tank reactor
equipped with a mechanical magnetic agitator. The reactant gas mixture of H2 and CO (1:1) was introduced
into the reactor at a pressure of 5.0 MPa, a gas hourly space velocity of 360 mL/gcat h, and a temperature of
250 ◦ C. Each run was maintained over 120 h, and the liquid sample was sampled every 12 h. It was judged to
have attained a steady state of reaction when the quantity of the sample of liquid product every 12 h was the
same in 2 consecutive readings and the material balance calculation showed that the syngas consumption was
equivalent to the yield of the product.
2.3.2. Product analysis
The products were analyzed using a gas chromatograph equipped with flame ionization and thermal conductivity
detectors, using GDX-502 and TDX-01 columns, respectively. The gaseous products were analyzed by online gas
chromatography, while the liquid products were collected in the trap and analyzed offline by gas chromatography.
3. Results and discussion
3.1. Catalyst characterization
3.1.1. XRD analysis
The powder XRD patterns are shown in Figure 1. For all the fresh catalysts, the diffraction peaks located at
2θ = 36.5◦ and 61.5◦ were assignable to Cu2 O (JCPDS: 65-3288), and those at 2θ = 43.3◦ , 50.4◦ , and 74.1◦
were assignable to metallic Cu (JCPDS: 04-0836). In addition, the diffraction peaks at 2θ = 31.7◦ , 34.4◦ ,
36.3◦ , 56.6◦ , 62.8◦ , 67.9◦ , and 69◦ corresponding to ZnO (JCPDS: 65-3411) were observed with the 5N10T
catalysts. Note that the peaks of Cu metal on the 5N10T shift to the left compared with the others, which may
indicate the formation of Cu–Zn alloy.
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•
♣−−Cu O
•−−Cu
2
∆−−ZnO
•
•
♣
♣
∆
1
∆
∆
♣
∆
∆
∆
∆
2
3
4
20
30
40
50
60
70
80
2 Theta (degree)
Figure 1. X-ray diffraction patterns for fresh 1. 0N0T; 2. 5N10T; 3. 5N15T; 4. 10N20T catalysts.
For the fresh catalysts, the presence of Cu metal and Cu2 O can be due to the decomposition of the
paraffin by heat to produce a reductive compound to cause CuO reduction, which has been proved by our
previous work.19,20 The XRD patterns show that the intensity of the diffraction peak corresponding to Cu
metal varies with different catalysts. The peak for 5N15T is lower in intensity than that for the others, while
those of 10N20T are the strongest. This may be due to the effect of several factors, such as the size of the active
metals, the concentration of organic bases, dispersion of Cu metal, and complexing agent dosage. In general,
XRD profiles of fresh catalysts by traditional preparation method have commonly CuO, ZnO, and Al2 O3
diffraction peaks,3,4 but Al2 O3 is amorphous in the catalysts prepared by complete liquid-phase technology.
This is because the former is heated and decomposed statically in gas phase at 320–400 ◦ C, while the latter is
in liquid phase with continuously stirring at 280 ◦ C.
3.1.2. Temperature-program desorption measurements
Both NH3 -TPD and CO2 -TPD were carried out on all catalyst samples. NH3 and CO2 thermodesorption
were monitored by MS detectors (Figures 2 and 3, respectively).
35
12
384
426.6
443.7
148
443
30
25
20
15
10
5
10
1
1
3
8
159
473
2
3
212.6
6
4
2
374
4
409
2
179.8
200
4
0
-5
100
300
400
500
600
700
800
100
200
300
400
500
600
700
800
Temperature (°C)
Temperature (°C)
Figure 2. NH3 -TPD spectra for fresh 1. 0N0T; 2.
Figure 3. CO2 -TPD spectra for fresh 1. 0N0T; 2.
5N10T; 3. 5N15T; 4. 10N20T catalysts.
5N10T; 3. 5N15T; 4. 10N20T catalysts.
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NH3 -TPD results are shown in Figure 2. For all the catalysts, the MS spectra of m/z = 17 exhibited
2 NH3 desorption peaks, corresponding to weak adsorption and medium-strong adsorption of NH3 on the
catalyst surface, respectively, but the relative amounts of weak acid and medium-strong acid are different. For
0N0T, the sites of medium-strong acid are more than those of weak acid. The opposite is true for 5N10T,
5N15T, and 10N20T. There are studies in the literature19,20 reporting that the weak acid favors the formation
of dimethyl ether (DME), and this is in agreement with the activity evaluation results. The wider desorption
peaks are indicative of a wide distribution of strength of acid sites, varying from weak to strong. These acid
sites are related to the contribution of different metal oxides, although the contribution of Al2 O3 is probably
predominant.22 Generally, the weak adsorption peak in the low temperature region is due to weak acid sites
or hydrogen bonding to the hydroxyl groups of the surface, while the strong adsorption peak in the higher
temperature region is due to the acid site resulting from the interaction between Cu, Zn, and Al compounds.23
As depicted in the NH3 -TPD spectra, the peak areas provide information on the concentrations of acid
sites on the catalysts. For the catalysts, the amounts of weak acid sites show little difference; however, the
medium-strong acid sites decreased with the increase in TEA dosage, which is because TEA is a strong base
and neutralizes part of the surface acid. This indicates that TEA dosage can change the medium acidity on the
catalyst surface.
CO2 -TPD results are reported in Figure 3. There are 2 CO2 -TPD desorption peaks for all catalysts
from 300 to 500 ◦ C, corresponding to medium-strong basic sites on the catalyst surface, which is indicative of a
wide strength distribution of basic sites, varying from medium to medium-high strength. Obviously, the amount
of medium-strong basic sites on the 5N10T is much more than that of the others. Meanwhile, the amount of
medium-strong basic sites is more than their own medium-strong acid sites in 5N10T by quantitative analysis.
3.1.3. Temperature programmed reduction of the catalysts
TPR was used to determine the reducibility of the CuZnAl catalysts. The TPR profiles of the catalysts are
represented in Figure 4. The H2 -TPR profiles of 0N0T and 10N20T display a broad reduction peak at 251 and
256 ◦ C, respectively. Moreover, there is more than one peak on 5N10T and 5N15T. The 4 reduction peaks at
241, 274, 353, and 397 ◦ C exist on 5N10T, with 3 peaks at 280, 353, and 391 ◦ C on 5N15T. Although the peak
at 241 ◦ C is absent for 5N15T, the peak at 280 ◦ C, which is asymmetric with a tail towards lower temperatures,
reveals a complex overlapping arising from reduction processes of different copper oxide species. The higher
temperature reduction peaks at around 353 and 391–397 ◦ C can be attributed to the reduction of smaller
copper oxide particles in zinc oxide or the partial reduction of zinc oxide. The process of reduction of smaller
copper oxide particles in zinc oxide into metallic Cu is more difficult and requires higher temperatures.24 and,
in addition, the gas solid reaction suffers from particles’ internal mass transfer resistance. Although ZnO is not
generally reduced under our experimental conditions, partial reduction of surface ZnO cannot be ruled out25,26
because the reduction of ZnO can occur via surface copper oxide by spilled hydrogen at a lower temperature.10
The result indicates the rationality of the existence of Cu–Zn alloy on 5N10T.
3.2. Catalytic activity measurements
The selectivities and conversions of CO hydrogenation over 0N0T, 5N10T, 5N15T, and 10N20T catalysts are
shown in Figures 5a and b. CO conversion is 22.6%, 28.9%, 21.2%, and 25.8% for 0N0T, 5N10T, 5N15T, and
10N20T, respectively. As for 0N0T, the hydrocarbon and methanol were the major products, accounting for
34.9% and 45.7%, respectively, and the selectivity of C2+ OH is 13.4%. As for 5N10T, C2+ OH is the dominant
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251
280
1
2
353
274
241
397
391
3
4
256
100
200
300
400
Temperature (°C)
Figure 4. TPR profiles for fresh 1. 0N0T; 2. 5N10T; 3. 5N15T; 4. 10N20T catalysts.
product, whose selectivity is up to 42.8%, and the hydrocarbon selectivity is only 2.5%. As for 5N15T and
10N20T, the hydrocarbon is the main product, reaching 39.2% and 39.5%, and the selectivity of C2+ OH is
20.7% and 7.7%. From the characterization of the surface acid and base of the catalysts, we can find that the
larger the ratio of base and acid is, the higher is the selectivity of C2+ OH. From the TPR profile of 5N10T,
it can be concluded that the existence of copper oxide and the partial reduction of ZnO favor the formation
of C2+ alcohols. It has been reported that copper oxides can stabilize the intermediate acyl species,27 and 2
formaldehyde molecules can form ethanol via aldol condensation, thereby causing growth of the carbon chain.
Furthermore, the basic environment can catalyze the reaction of aldol condensation, and suppress generation of
hydrocarbon and DME.
40
35
30
25
20
15
10
5
100
90
80
70
60
50
40
30
20
10
0
a
CH
DME
MeOH
EtOH
Mix alcohol
The others
b
5N10T
10N20T
0N0T
5N15T
0
0N0T
5N10T
5N15T
10N20T
Catalyst Samples
Catalyst Samples
Figure 5. a) Catalytic carbon-based selectivity towards hydrocarbon, DME, C+2 OH, methanol, the other oxygenates;
b) CO conversion at GHSV = 360 mL/h gcat , H2 /CO = 1, 250 ◦ C, and 5.0 MPa.
4. Conclusion
CuZnAl catalysts were prepared using the complete liquid phase method invented by us with TEA and NMP
as the function regulators. Characterizations clearly demonstrate that the ratio and amounts of TEA and NMP
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YU et al./Turk J Chem
have a great influence on the adjustment of the acidity and basicity of the catalyst surface and can alter the
reducibility of copper and zinc oxides. Catalytic testing results show that the existence of a large amount of
medium-strong basicity of catalytic surface favors the formation of C2+ alcohols and suppress the generation
of hydrocarbon and DME. The C2+ alcohol selectivity of 42.8%, which in ethanol is 45.6%, can be achieved
over CuZnAl catalyst without any alkali metal promoter. The present work exhibits a prospect for ethanol
production from CO hydrogenation over CuZnAl catalysts without alkali metals.
Acknowledgments
This work was supported by the National Science Foundation of China (21176167), the National Basic Research
Program of China (2011CB211709), Science and Technology Foundation of Guizhou Province ([2012]2154), the
Science and Technology Research Key Project of the Ministry of Education (212021), and the Key Project of
the National Nature Science Foundation (21336006).
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