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Surface properties of hydrotalcite based Zn(Mg)Al oxides and their catalytic activity in aldol condensation of furfural with acetone Lucie Smoláková, Karel Frolich, Jaroslav Kocík, Oleg Kikhtyanin, and Libor Capek Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b04927 • Publication Date (Web): 31 Mar 2017 Downloaded from http://pubs.acs.org on April 1, 2017

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Industrial & Engineering Chemistry Research

Surface properties of hydrotalcite based Zn(Mg)Al oxides and their catalytic activity in aldol condensation of furfural with acetone Lucie Smoláková1, Karel Frolich*1, Jaroslav Kocík1, Oleg Kikhtyanin2, Libor Čapek1 1

Department of Physical Chemistry, Faculty of Chemical Technology, University of

Pardubice, Studentská 573, 532 10 Pardubice, Czech Republic 2

Research Institute of Inorganic Chemistry, RENTECH-UniCRE, Chempark Litvínov, Záluží

– Litvínov, 436 70, Czech Republic

1 ACS Paragon Plus Environment


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ABSTRACT Basic mixed oxides MgAl, ZnMgAl and ZnAl were successfully prepared from hydrotalcite precursors synthesized by urea method. Materials with the same molar ratio (M2+/Al3+) = 2 were studied to describe the influence of Mg/Zn ratio on their physicochemical properties. Materials were tested as catalysts of the aldol condensation of furfural with acetone. For samples with similar particle sizes and surface BET areas, the varying catalytic activity was related to the different acidobasic properties. Higher furfural conversion and selectivity to longer carbon chain F2Ac product was observed for samples with higher total amount of basic sites. More specifically, it correlated with the population of Me2+-O2- pairs that represented dominant type of basic sites in all studied catalysts. At the same Al loading, Mg2Al mixed oxide exhibited higher specific surface area, higher total amount of basic sites and higher amount of acid sites than Zn2Al oxide.

KEYWORDS: Zn(Mg)Al mixed oxides; hydrotalcite-like precursors; surface; acidobasicity; condensation of furfural with acetone


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1. INTRODUCTION Nowadays, the demand of energy is one of the main problems of the world. In general, the energy can be divided into two groups – renewable and non-renewable. Due to the constantly increasing consumption of non-renewable sources, there is a great effort to replace it by renewable one. Solar, wind, hydro, geothermal and biomass sources of energy are available in nature. Non-renewable sources of energy are often denoted as fossil energy, which includes oil, coal and natural gas. Renewable fuels can be divided on biofuel of first generation and second generation. Biofuel of first generation are prepared from sources for food industry (biodiesel, bioethanol) and biofuel of second generation is produced from the plant biomass which includes lignocellulosic materials. Main components of lignocellulosic feedstock are cellulose and hemicellulose. Hemicellulose represents starting material for production of furfural that contains a heteroaromatic furan ring and an aldehyde functional group

1, 2

. Furfural can be used for the production of chemicals and fuels 3. This can be

achieved by direct hydrogenation, but by this way we obtain a linear five to six carbon hydrocarbons, which are unsuitable as fuel. In contrast, the aldol condensation of furfural and acetone provides the formation of hydrocarbons of chain length to 13 carbons that can subsequently be transformed after hydrogenation and thorough hydrodeoxygenation to highquality diesel fuel. Target products from the condensation of furfural with acetone are shown in Scheme 1. Furfural reacts with acetone to form furfural acetone monomer 4-(2-furyl)-3buten-2-one (FAc), followed by condensation with another furfural to form furfural acetone dimer 1,4-pentadien-3-one, 1,5-di-2-furanyl (F2Ac) 4, 5. Aldol condensation of furfural and acetone is commonly performed using homogenous catalysts, mostly NaOH

6, 7

. It brings several problems as difficult catalyst regeneration and

the corrosion of equipment. Recently, there has been a shift towards heterogeneous base catalysts, such as MgO/NaY 8, TiO2 9 and dolomite catalyst 10. Susceptibility to CO2 and poor regenerability belong to disadvantages of basic oxide catalysts. Another group of tested materials are zeolites

11, 12

. When protonic zeolites are used as catalysts for this reaction, the

formation of an additional product, (FAc)2, occurs as a result of FAc dimerization over Brønsted acid sites 11. The limitation of protonic zeolites is generally low furfural conversion (around 30 %) and the formation of heavy carbonaceous deposits in their micropores which lowers the catalytic activity. Better catalytic results and stability are observed for impregranted zeolites with KNO3 which bear Lewis basic sites after calcination, plausibly extraframework K2O species 12. 3 ACS Paragon Plus Environment


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Basic oxide catalysts prepared from hydrotalcite (HT) precursors show very good catalytic performance

5, 13

. Advantage of mixed oxides prepared by thermal decomposition of HTs is

variable basicity (different amount and strength of basic sites), a high surface area and a poorly crystallized structure. Faba et al.

14

studied activity and selectivity being correlated

with physico-chemical properties of different Mg–Zr, Mg–Al and Ca–Zr mixed-oxide catalysts which contained different amount of basic sites in aldol condensation furfural and acetone. They found that catalysts with the highest concentration of basic sites (especially medium-strength basic sites) were the most active and selective for the C13 fraction. In this work, we focused on the analysis of acido-basic properties of ZnAl, MgAl and ZnMgAl mixed oxides and the comparison their activity and selectivity in aldol condensation of furfural with acetone. MgAl, ZnAl and ZnMgAl mixed oxides were prepared from HTprecursors with the same M2+/M3+ molar ratio in order to see the role of Me2+ content (Zn and Mg) on their physicochemical properties. Additionally, the synthesis setup of HT-precursor was examined. Basic sites were studied by combination of (i) CO2 adsorption calorimetry allowing the analysis of the strength of basic sites and the distinguishing of reversibly and irreversibly adsorbed CO2, and (ii) FTIR-CO2 reflecting the interaction of CO2 with the basic sites and their structure. The combination of TPD-NH3 and FTIR-CO was used in order to analyse the acidic properties of mixed oxide catalysts. The condensation of furfural with acetone was carried out in a stirred batch reactor.

Scheme 1. Aldol condensation of furfural with acetone 2. EXPERIMENTAL 2.1 Catalyst preparation ZnAl, MgAl and ZnMgAl hydrotalcite-like precursors (HT) were synthesized by urea method with the same M2+/Al3+ molar ratio of 2 (M2+ represents Mg and Zn cations). Hydrolysis was carried out in 1000 ml 2-neck batch glass reactor equipped by shaft stirrer. The appropriate amount of Zn(NO3)2.6H2O, Mg(NO3)2.6H2O and Al(NO3)3.9H2O [total 4 ACS Paragon Plus Environment

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concentration of M2+ and Al cations was 0.15M (type A) or 1M (type B)] was dissolved in 500 ml of distilled water and the solution was inserted to the reactor placed in the glycerol -

batch pre-heated to 100 °C. Urea (molar ratio of urea/NO3 = 3) was dissolved in the above described solution and the reaction mixture was stirred (500 rpm) at 100 °C for 24 h. Resulting product was filtered off, washed thoroughly with the distilled water, dried at 60 °C for 24 h and finally calcined at 450 °C for 4 h in muffle oven. The mixed oxides (type A) were designated as Zn2Al, Zn1Mg1Al and Mg2Al, where the numbers indicate the Zn/Al or Mg/Al molar ratios. Mg2Al(B) represents mixed oxide prepared by using of 1M concentration of Mg2+ and Al3+cations (type B). 2.2 Catalyst characterization X-ray diffractograms (XRD) were recorded with Bruker AXS D8-Advance diffractometer using Cu Kα radiation (λ =0.154056 nm) with a secondary graphite monochromator. Thermogravimetric analysis (TGA) of hydrotalcites was performed using a TA Instruments TGA Discovery series equipment. Approximately 20 mg of sample was heated in an open alumina crucible. It operated at heating ramp of 10 °C.min-1 from room temperature to 900 °C in the flow of nitrogen (20 ml.min-1, Linde 3.0). Specific surface area of the hydrotalcites and related mixed oxides was measured at the boiling point of the liquid nitrogen (77 K). It was determined by the fitting of experimental data to the BET isotherm. Chemical composition of mixed oxides was determined using XRF analysis (recorded with X-ray fluorescence spectrometer Philips PW1404). The calorimetric/volumetric experiments were carried out using an isothermal TianCalvet type microcalorimeter (BT 2.15, SETARAM) combined with a homemade volumetric/manometric device (Pfeiffer Vacuum gauges). 400 mg of mixed oxide was outgassed by slowly increasing temperature with simultaneous careful evacuation up to the residual pressure 10-4 Pa at 450 °C. The adsorption isotherms and heats of adsorption were measured at 34 °C by step-by-step introduction of CO2 (99.9993 %) into the cell. Details of experiment setup and evaluation were described elsewhere 15. Infrared spectra (32 scans; resolution of 1 cm-1) were collected on a Nicolet 6700 FTIR spectrometer equipped with MCT/A detector. Samples were pressed into the self-supporting wafers with density of ca. 10 mg.cm-2 and placed into the home-made low temperature IR cell for transmission measurement. Samples were in situ outgassed in a dynamic vacuum at 450 5 ACS Paragon Plus Environment


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°C up to the residual pressure 10-4 Pa. The CO (99.997 %) adsorption was performed at the equilibrium pressure at around 4 mbar (sample saturation) at nominal temperature of liquid nitrogen. The system was equilibrated for 1h. After that, CO pressure was reduced and the evacuation was carried out until the removing of vibrational bands of adsorbed CO (approximately 1 h). IR spectra of the surface CO2 (99.9993 %) complexes were collected at equilibrium pressures of the order of ten mbar at room temperature (RT) and after the desorption in dynamic vacuum at RT followed by elevated temperature 100 °C. The spectrum of dehydrated sample at temperature of liquid nitrogen recorded before probe adsorption was subtracted from each spectrum shown in this work. The temperature-programmed desorption of ammonia (NH3-TPD) was performed on a Micromeritics AutoChem II 2920 (Micromeritics Instrument Corp., USA). Desorption signals were detected by a thermal conductivity detector (TCD). Prior to the test, 100 mg of sample was placed in a quartz reactor, heated (10 °C.min-1) to 450 °C and maintained for 1 h in a flow of helium (25 ml.min-1). Subsequently, the sample was cooled down to 70 °C and saturated in a flow of gas mixture containing 5 vol. % of NH3 in helium (15 ml.min-1) for 30 min. Then, the sample was purged in the flow of helium for 30 min in order to remove the physically absorbed NH3. Finally, the TPD experiment was carried out with a linear heating rate of 10 °C.min-1 in a flow of He (25 ml.min-1). 2.3 Catalytic test Aldol condensation of furfural with acetone was carried out in a 100 ml stirred batch reactor (a glass flask reactor) at temperature of 50 °C. Prior to the catalytic tests, the mixture of 39.5 g of acetone (dried with molecular sieve 3A) and 6.5 g of furfural (acetone to furfural molar ratio 10/1) was pre-heated to the reaction temperature of 50 °C. After that, 2 g of catalyst (grain of 0.25-0.5 mm) was added and the reaction proceeds at 50 °C for 6 h. It has been previously established that the reaction is not limited neither by external nor internal mass transfer under the chosen reaction conditions (the test with changing stirring rate and catalyst particle size) 5. Samples were withdrawn from the reaction mixture during the experiment at 5, 10, 20, 40, 60, 90, 120, 180, 240, 300 and 360 min. Catalyst was separated from the reaction mixture by filtration and the products were analysed by Agilent 7890A gas chromatograph equipped with a flame ionization detector and HP 5 capillary column (30 m/0.32 mm ID/0.25 µm). 3. RESULTS AND DISCUSSION 6 ACS Paragon Plus Environment

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3.1 Structural and textural properties of Zn(Mg)Al hydrotalcites and related mixed oxides The XRD patterns of Zn2Al-HT, Zn1Mg1Al-HT, Mg2Al-HT and Mg2Al(B)-HT are depicted in Fig. 1A. Zn2Al-HT, Zn1Mg1Al-HT and Mg2Al-HT exhibited sharp, intensive and symmetric diffraction peaks at 2θ ≈ 11.6°, 23.3°, 34.5°, 39.1°, 46.6°, 60.1° and 61.5°, which are typical for double-layered structure of hydrotalcites (Fig. 1A);

16-19

. It shows to the well-

crystalline structure of these hydrotalcites. In contrast to the XRD pattern of Mg2Al-HT, the XRD pattern of Mg2Al(B)-HT (prepared from more concentrated solution; total concentration of Mg2+ and Al3+ cations was 1M instead of 0.15M used for the synthesis of Mg2Al-HT) exhibited less intensive and broader symmetric diffraction peaks corresponding to the hydrotalcite phase. A

250

Zn2Al-HT

Zn2Al

500

B

d(110)

d(006)

Intensity (a.u.)

d(003)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Zn1Mg1Al

Zn1Mg1Al-HT Mg2Al

Mg2Al-HT

Mg2Al(B)

Mg2Al(B)-HT

10

20

30

40

50

60

70

80

10

20

30

40

50

60

70

2 theta (°)

2 theta (°)

Figure 1. Diffractograms of (A) Zn(Mg)Al hydrotalcite-like precursors and (B) Zn(Mg)Al mixed oxides. Table 1 gives the values of unit cell parameters a (a = 2 d110) and c (c = 3 d003), which were calculated based on d003 and d110 corresponding to the diffraction lines at 2θ ≈ 11.6° and 60.1°. It is clearly seen that the a parameter was slightly higher for Zn-Al hydrotalcites compared to Mg-Al hydrotalcites. This difference may be explained by the increase in layer charge density, because the electronegativity of zinc is larger than that of magnesium

20, 21

.

Generally, the atom radius of Zn2+ is bigger than that of Mg2+. Results suggest that the Mg2+ was isomorphically substituted by Zn2+ in the layer of Zn1Mg1Al-HT that is composed of 49.8 % of ZnAl-HT and 50.2 % of MgAl-HT phase. It is evident that Mg2Al-HT had higher value



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of specific surface area than Zn2Al-HT and Zn1Mg1Al-HT and also had three times higher value of specific surface area than Mg2Al(B)-HT. Table 1 The unit cell parameters and specific surface area for Zn(Mg)Al based hydrotalcites.

Zn2Al-HT

a (nm) 0.3072

c (nm) 2.2674

SBET (m2.g-1) 13

Zn1Mg1Al-HT

0.3075

2.2733

5

Mg2Al-HT

0.3045

2.2611

30

Mg2Al(B)-HT

0.3041

2.2857

10

Zn(Mg)Al hydrotalcites

c = 3d003, a = 2d110

Fig. 1B shows the XRD patterns of Zn2Al, Zn1Mg1Al, Mg2Al and Mg2Al(B) mixed oxides. In all the cases, the thermal decomposition of hydrotalcite precursors led to the disappearance of the (003) and (006) reflections characteristic to layered double hydroxide structure and the formation of new reflections characteristic for the presence of appropriate mixed oxides. In the XRD patterns of Mg2Al and Mg2Al(B) mixed oxides, two intensive diffraction peaks were observed at 2θ ≈ 43.0° and 62.5° that are typical for MgO periclasetype structure 22, 23. The diffraction peaks at 2θ ≈ 32.1°, 34.3°, 36.4° and 57.1° were observed in the XRD pattern of Zn2Al mixed oxide. These diffraction lines are characteristic to the presence of ZnO phase

18

. The XRD pattern of Zn1Mg1Al mixed oxide showed the

characteristic diffraction peaks (at 2θ ≈ 32.1°, 34.3°, 36.4°, 43.0°, 57.1° and 62.5°) of a well crystallized Mg(Al,Zn)O phase. Table 2 Chemical composition, the crystallite size and the specific surface area determined for Zn(Mg)Al mixed oxides. XRF Zn(Mg)Al mixed oxides

XRD

N2-BET

ZnO

MgO

Al2O3

(wt. %)

(wt. %)

(wt. %)

Zn2Al

74.5

-

25.1

Zn/Mg/Al molar ratio 1.9/0/1

Zn1Mg1Al

36.0

22.7

36.2

0.5/0.7/1

4.1

2.9

155

Mg2Al

-

59.2

40.7

0/1.9/1

-

4.1

183

Mg2Al(B)

-

60.1

39.6

0/1.9/1

-

4.6

155

a

Determined from the diffraction line at 2θ = 36.4°


DZnOa

DMgOb

(nm)

(nm)

SBET (m2.g-1)

3.5

-

92

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b

Determined from the diffraction line at 2θ = 43.0°

The mean particle size (crystallite size) was determined using the half width of the MgO reflection at 2θ = 43.0° or of the ZnO reflection at 2θ = 36.4° (Table 2). While the crystallite size of MgO was slightly lower in the Zn1Mg1Al mixed oxide (2.9 nm) than in the Mg2Al mixed oxide (4.1 nm), the crystallite size of ZnO was slightly higher in the Zn1Mg1Al mixed oxide (4.1 nm) than in the Zn2Al mixed oxide (3.5 nm). Mg2Al mixed oxide prepared from less concentrated solution exhibited slightly lower crystallite size (4.1 nm) than in the case of Mg2Al(B) sample (4.6 nm). The experimentally determined M2+/Al molar ratio was close to the theoretical (from synthesis) one for studied materials (Table 2). Some deviation was observed for Zn1Mg1Al where higher content of aluminium was experimentally detected. The specific surface area of Zn1Mg1Al mixed oxide (155 m2.g-1) was between the values of appropriate Zn2Al (92 m2.g-1) and Mg2Al (183 m2.g-1) mixed oxides prepared by the same procedure (Table 2). Mg2Al(B) mixed oxide exhibited slightly lower value of specific surface area than Mg2Al mixed oxide, but the same as Zn1Mg1Al mixed oxide. Figure 2 depicts TGA (A) and DTG (B) curves of as-synthesized hydrotalcite-like materials. A total weight loss showed a clear dependence on the chemical composition of the samples: it was 41.9 and 44.8 % for Mg2Al and Mg2Al(B), respectively, but it decreased to 36.8 % for Zn1Mg1Al and to 32.2 % for Zn2Al. For both Mg2Al hydrotalcites (Fig. 2B), the first weight loss occurred between 20 and 250 ºС due to the removal of physically adsorbed and interlayer water molecules as well as the decomposition of residual urea. The second weight loss was observed in DTG curves at temperatures above 250 °C and it originated from H2O, CO2, NO and NH3 removal due to the dehydroxylation of brucite-like sheets and the decomposition of carbonates and compounds derived from urea in the interlayer. In this temperature range, DTG curves for both Mg2Al hydrotalcites indicated several different peaks, with the most intense ones at 320-330 and 420-430 °C. Their intensity (especially of that observed at T≈320 °C) decreased in the DTG curve of Zn1Mg1Al. Simultaneously, two new peaks at T=130 and 230 °C and less pronounced peak at T=260 °C appeared. Finally, the DTG curve of Zn2Al exhibited a very strong peak at T=210 °C, several peaks in the range of T=240-290 °C and a distinct peak with a maximum at 815 °C. An observed difference in the shape of DTG curves suggests that the chemical composition of hydrotalcite-like materials determines both the nature and the number of constituent functional groups.



100

A

Zn2Al-HT Mg2Al-HT

90

Mg2Al(B)-HT Zn2Al-HT

80 Zn1Mg1Al-HT 70 Mg2Al-HT

Der.weight (%/°C)

Weight loss (%)

B

0.05

Zn1Mg1Al-HT

60 Mg2Al-HT (B) 50 200

400

600

200

800

Temperature (°C)

400

600

800

Temperature (°C)

Figure 2. TGA (A) and DTA (B) curves of Mg2Al, Mg2Al(B), Zn1Mg1Al and Zn2Al hydrotalcite-like precursors. 3.2 Basicity of Zn(Mg)Al mixed oxides Amount and strength of basic sites in studied materials was analysed by CO2 adsorption calorimetry. Calorimetric curves of CO2 adsorption obtained for Zn2Al, Zn1Mg1Al, Mg2Al and Mg2Al(B) mixed oxides at 34 °C are depicted in Fig. 3. The differential heat of adsorption is depicted as a function of sample coverage. In terms of energy, the decrease of adsorption heat with increasing coverage reflects significant adsorption heterogeneity for all studied oxides.

150

150

Mg2Al Zn1Mg1Al Mg2Al(B) Zn2Al 100

125

Qdiff (kJ.mol-1)

125

Qdiff (kJ.mol-1)

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100

75

50

75

25

0.0

0.1

0.2

0.3

0.4

0.5

0.6

nads (mmol.g-1)

50

25

0 0.0

0.1

0.2

0.3

0.4

0.5

nads (mmol.g-1)


0.6

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Figure 3. Calorimetric curves of the CO2 adsorption at 34 °C on Mg2Al, Mg2Al (B), Zn1Mg1Al and Zn2Al mixed oxides. In inset: the first vs. second run of the adsorption for Mg2Al mixed oxide. First CO2 adsorption step was related to heats in the range of 150 - 140 kJ.mol-1 with near zero CO2 equilibrium pressure indicating strong chemical CO2 bounds. Next doses were followed by the gradual decrease of heats being related to the less stable CO2 species. Heats dropped to 25 kJ.mol-1 (at equilibrium pressure ca. 77 mbar) being generally considered as the lowest value for the specific CO2 adsorption 23, 24. Based on that the amount of all basic sites was established (Table 3). The end of the run was reflected by the energy plateau of 17 kJ.mol-1 (being the condensation heat of CO2) and the equilibrium pressure between 200 – 250 mbar (isotherms shown in SI 1). Previously, we compared the calorimetric data together with adsorption isotherms and we showed that the specific adsorption of CO2 on the basic sites of an oxide (160 – 25 kJ.mol-1) cover the equilibrium pressure region ca. 0-100 mbar 15. The amount of irreversibly and reversibly adsorbed CO2 could be distinguished based on heats of adsorption. The portion of basic sites with the irreversibly adsorbed CO2 was determined by the second run of CO2 adsorption. This process involved CO2 adsorption on such samples where reversibly adsorbed CO2 was removed from the material in vacuum for 1h. The starting heats within the second run were 60 - 50 kJ.mol-1 and the shape of curve followed the first step one (inset of Fig. 3). The amount of irreversibly bound CO2 was calculated as the difference in the amount of basic sites (amount of adsorbed CO2) obtained in the first and in the second run with the end of specific CO2 adsorption at 25 kJ.mol-1. It should be stressed that this analysis provides the selective information about the interaction of CO2 with the basic sites, and other physically adsorbed CO2 on the surface of mixed oxides was omitted (the physically adsorbed CO2 is important in the process of reversible CO2 adsorption/desorption process applied when the mixed oxides are used as adsorbents). It is also noteworthy that irreversibly and reversibly adsorbed CO2 are distinguished in this work for temperature 34 °C that is near ambient conditions being used in many TPD and FTIR-CO2 experiments 15. Table 3 The basicity and acidity of Zn(Mg)Al mixed oxides. Basic sites (CO2 calorimetry)

Acid sites (TPD-NH3)

Zn(Mg)Al mixed

µmol.g-1

oxides

-1

> 50 kJ.mol *

> 25 kJ.mol

> 50 kJ.mol *

> 25 kJ.mol

Zn2Al

18 (31)

58

0.20 (31)

0.63

µmol.m-2 -1

-1

-1

µmol.g-1

µmol.m-2

125

1.36


P**

0.46


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Zn1Mg1Al

107 (52)

205

0.69 (52)

1.32

138

0.89

Mg2Al

179 (45)

395

0.98 (45)

2.16

175

0.96

2.25

1.19

1.06

Mg2Al (B)

83 (42)

195

0.54 (42)

1.26

184

1.48

* The irreversible chemisorbed CO2; in parenthesis: the percentage from all CO2 ** Ratio of amount of basic sites (CO2 calorimetry) to acid sites (TPD-NH3)

The total amount of basic sites per square meter of a catalyst increased in order Zn2Al < Mg2Al(B) ≈ Mg1Zn1Al < Mg2Al (Table 3). It is clearly seen that there was not observed any synergetic effect for Mg1Zn1Al mixed oxide, as its total amount of basic sites was approximately in the middle between the total amount of basic sites of Mg2Al and Zn2Al mixed oxides. This observation is in agreement with

25-27

where the presence of transition

metal Zn in MgAl mixed oxide caused the reduction of basicity, which moreover was more pronounced with the increasing Zn concentration. However, some changes are seen in the distribution of irreversibly and reversibly adsorbed CO2. While Mg1Zn1Al mixed oxide exhibited actually the highest relative population of irreversibly chemisorbed CO2 (52 %), both Mg2Al (45 %) and Zn2Al (31 %) mixed oxides (type A samples with the same preparation procedure) exhibited lower amount of irreversibly chemisorbed CO2. In agreement with that Pavel et al.

25

observed higher relative population of stronger basic sites

in ZnMgAl mixed oxide than in MgAl mixed oxide (Mg/Al = 3). Nevertheless, no clear correlation was observed between the population of irreversibly adsorbed CO2 and the composition of mixed oxide. This is in line with the study of Bezen et al. 26. Comparing the basicity of two Mg2Al and Mg2Al(B) mixed oxides with the same Mg/Al molar ratio, Mg2Al mixed oxide prepared from less concentrated solution had significantly higher total amount of basic sites than Mg2Al(B) mixed oxide, but the relative population of irreversibly adsorbed CO2 was approximately the same for both materials. The structure of CO2 adsorption complexes was determined by FTIR. FTIR spectra of adsorbed CO2 are depicted in Fig. 4. Spectra were taken after the admission of about 100 mbar CO2 equilibrium pressure to the sample cell. Subsequent evacuation at RT and elevated temperature 100 °C was realized to reveal relative stability of present species.


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0.8

Mg2Al Absorbance (a.u.)

Absorbance (a.u.)

0.8

0.6

0.4

0.2

0.0

Mg2Al(B)

0.6

0.4

0.2

0.0 1800

1600

1400

1200

1800

-1

1600

1400

1200

Wavenumber (cm-1)

Wavenumber (cm ) 0.5

0.25

Zn2Al

Zn1Mg1Al 0.4

Absorbance (a.u.)

Absorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.3

0.2

0.1

0.20

0.15

0.10

0.05

0.0

0.00 1800

1600

1400

1200

1800

-1

1600

1400

1200

Wavenumber (cm-1)

Wavenumber (cm )

Figure 4. FTIR spectra of the CO2 interaction with Mg2Al, Mg2Al (B), Zn1Mg1Al and Zn2Al mixed oxides. Spectra obtained at 100 mbar CO2 equilibrium pressure and subsequent evacuation at RT (in black) followed by evacuation at 100 °C for 5 min (in red). Detail on the spectral region of carbonates. CO2 interaction with the basic sites resulted in the formation of several types of (pseudo)carbonate species. Carbonates were represented by overlapping bands in the region 1150 – 1950 cm-1. Decrease of the whole spectral intensity was observed with the evacuation already at RT and markedly at 100 °C. All features were still present in the desorption spectra at 100 °C, but their population significantly changed (Fig. 4, red colour). The bands with relatively lower stability were observed in the regions 1650 – 1800 cm-1 and 1150 – 1350 cm-1 being ascribed to bidentate carbonate (chelating and bridged) and bicarbonate species, respectively. Relatively more stable bands (obviously dominant in the desorption spectra at 100 °C) were detected in the range 1350 – 1600 cm-1, mirroring the presence of unidentate carbonate species. Bidentate and unidentate carbonates were formed on the basic sites represented by Men+-O2- pairs and O2-, respectively. Bicarbonate formation involved surface hydroxyl group

23, 28, 29

. The presence of bicarbonates is connected with residual hydroxyls

observed on samples treated at 450 °C (spectra shown in SI 2). FTIR results are in a good correlation with calorimetric data. The highest adsorption energies were related to unidentate carbonate species, followed by bidentate and bicarbonate 13 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

species. It is important to stress that the amount of irreversibly chemisorbed CO2 detected by the calorimeter with the energies over 50 – 60 kJ.mol-1 (see Table 3) involved all unidentate and part of bidentate carbonates. It is evident from the presence of corresponding bands after evacuation of material at RT that was done by the same way as in the case of CO2 adsorption calorimetry experiment (see above). There is significant portion of bidentate carbonates and all bicarbonates with lower stability and interaction energies below 50 kJ.mol-1. Decrease of the whole CO2 spectral intensity (already at RT) together with calorimetric adsorption curves without marked energy steps/plateaus indicated that the bidentate carbonates are energetically heterogeneous. This is caused very probably by the varying deformation energy during the formation of adsorption complexes. For the formation of bidentate carbonates slight change in the (equilibrium) position of surface ions is expected. The formed complexes have similar geometry but their stability varies. From the comparison of spectra at near saturation (100 mbar), the bidentate carbonates involving both Men+ and O2- are the most populated species for all observed samples. It gives evidence of high concentration of unsaturated Men+ cations on the surface of oxides being connected with their moderate basicity. The difference among studied oxide samples was obviously in the total intensity of carbonate spectra which was reflected in the total basicity of samples. Samples containing Zn were significantly less basic than samples with Mg in the structure (compare type A samples with the same preparation procedure) being in parallel with calorimetric data. Additionally, similar envelope IR curves showed similar distribution among basic sites being again in well correlation with calorimetric data. Some smaller deviation was observed for the sample Zn2Al with the lowest population of irreversibly chemisorbed CO2. It should be noted that the additional bands in the region 2250 – 2450 cm-1 were also observed after CO2 adsorption (spectra shown in SI 3). These bands were readily decomposed during the evacuation of gas from IR cell at RT and corresponded to the least stable species of molecularly bound CO2 to Lewis acid sites represented by Men+. Taking to account very high absorption coefficients, population of such complexes on studied oxide samples was expected to be low 30. Complexes are related to the lowest energies on calorimetric curve. 3.3 Acidity of Zn(Mg)Al mixed oxides The amount of acid sites was determined by temperature programmed desorption of NH3 (NH3-TPD). The NH3-TPD curves are depicted in Fig. 5. The sample saturation with NH3 was realized before TPD at temperature 70 °C. With this saturation at 70 °C, the amount of 14 ACS Paragon Plus Environment

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physisorbed NH3 on the studied mixed oxides is suppressed. After that, the determined amount of desorbed ammonia corresponds in majority to the chemisorbed NH3.

0.016

Mg2Al Zn1Mg1Al Zn2Al

0.012

TCD signal (mV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Mg2Al(B)

0.008

0.004

0.000 50

100

150

200

250

300

350

400

450

Temperature (°C)

Figure 5. TPD-NH3 profiles for Mg2Al, Mg2Al(B), Zn1Mg1Al and Zn2Al. According to

31

, chemisorbed NH3 interacts via nitrogen lone pair with Lewis acid sites

represented by unsaturated metal cations and, where favourable, simultaneously interacts via hydrogen bond to nearby basic oxygen or hydroxyl group. Some portion of NH3 can also -

undergo heterolytic dissociation to NH2 on acid-base pairs involving partially basic oxygen. Due to the several possible binding conditions, the ammonium complexes vary in its stability. All the observed TPD curves in this work were tailed at the high temperature side mirroring the presence of ammonia complexes with the different structure. Considerable higher temperature tail was observed for Zn1Mg1Al and Zn2Al samples giving evidence for larger heterogeneity of adsorption sites for oxides with Zn in the structure . The amount of acid sites per gram of catalyst increased in the order Zn2Al < Zn1Mg1Al < Mg2Al ≈ Mg2Al(B) mixed oxides. The particular data are presented in Table 3. This acidity relation with oxide composition is in accordance with other authors, who observed higher acidity of MgAl mixed oxides than ZnAl mixed oxides contrast to MgAl mixed oxide

25-27

and lower total acidity of ZnMgAl mixed oxide in

19

. It is interesting that Mg2Al(B) mixed oxide exhibited

approximately the same total amount of acid sites as Mg2Al mixed oxide, but significantly lower amount of basic sites (Table 3). The structure of Lewis acid sites was characterized by CO probe molecule and FTIR technique. The FTIR spectra of CO adsorbed on the studied Mg2Al, Mg2Al(B), Zn1Mg1Al and 15 ACS Paragon Plus Environment


Zn2Al mixed oxides at nominal temperature of liquid nitrogen are depicted in Fig. 6. The CO admission to the sample resulted in the formation of bands in the region 2100 – 2200 cm-1 being evidence for the non-reactive adsorption and corresponding to carbonyl species where CO is molecularly adsorbed. 0.6

Mg2Al 2152

0.4 0.3 0.2 0.1

2152 0.4 0.3 0.2 0.1 2179

2179 0.0 2250

Mg2Al(B)

2166 0.5

Absorbance (a.u.)

Absorbance (a.u.)

0.6

2166

0.5

2200

2150

2100

0.0 2250

2050

2200

2150

2100

2050

Wavenumber (cm-1)

Wavenumber (cm-1) 0.20

0.4

Zn2Al

Zn1Mg1Al

2152

2152

Absorbance (a.u.)

Absorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.3

0.2 2166 0.1

0.15

0.10

0.05

2177 2196

2196+2176 0.0 2250

2200

2150

2100

2050

0.00 2250

2200

2150

2100

2050

Wavenumber (cm-1)

Wavenumber (cm-1)

Figure 6. FTIR spectra of the CO interaction with Mg2Al, Mg2Al(B), Zn1Mg1Al and Zn2Al mixed oxides at the nominal temperature of liquid nitrogen. At the lowest coverage degree, the symmetrical band with maximum at 2179 cm-1was observed in the spectra of Mg2Al and Mg2Al(B) mixed oxides. The symmetrical band with a maximum at 2196 cm-1 was observed in the spectra of Zn2Al mixed oxide. FTIR spectra of Zn1Mg1Al mixed oxide containing both Mg and Zn revealed in the presence of two bands at 2196 and 2176 cm-1 (at the lowest coverage degree). The maximum of the band at 2179 cm-1 shifted to 2166 cm-1 and the maximum of the band at 2196 cm-1 shifted to the 2177 cm-1 with the increasing amount of CO on the sample. This phenomenon was ascribed to the static (chemical) shift on the flat oxide surface, i.e. acidity decreased as a result of donating electron density from the adsorbate to the solid

32

.

Static shift is partially compensated with the opposite dynamic shift being a result of a dipoledipole interaction between neighbouring CO oscillators. Dynamic shift for MgAl mixed oxides accounts for +3.5 cm-1 33. At the highest coverage the low frequency shoulder of the 16 ACS Paragon Plus Environment

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main band at around 2152 cm-1 appeared in the spectra of Mg2Al and Mg2Al(B) being ascribed to H-bonded CO. In contrast, spectra of Zn1Mg1Al and Zn2Al mixed oxides were characterized at high coverage stages by the dominant band of H-bonded CO with separated maximum at 2152 cm-1. From the above observations, band at 2176 (shifted to 2166) cm-1 was ascribed to the carbonyl species with coordinatively unsaturated Mg2+ ions and band at 2196 (shifted to 2177) cm-1 to carbonyls with Zn2+ ions. Based on the missing bands at 2245 – 2200 cm-1 and variation of the intensity of bands at 2166 and 2177 cm-1 with Mg and Zn content, resp., no interaction of CO with unsaturated Al3+ is supposed CO adsorption in the region 1485 – 1135 cm

34-36

. Additionally, bands of the reactive

-1 37, 38

, mirroring presence of carbonite-like

anions (CO)2n− , were not observed. The absence of bands from stable carbonite-like structures, characteristic for MgO and CaO with the highly basic O2-, supports the carbonyl band assignment above. The CO/FTIR experiments gave important evidence about the presence of Mg2+ and Zn2+ on the surface of studied mixed oxides. Unsaturated Mg2+ and Zn2+ ions affect their acid-base properties. Considering previously derived basicity by CO2 adsorption (see above), it stands behind major population of medium strength basic sites involving both Me2+O2- ions. 3.4 Aldol condensation of furfural with acetone The aldol condensation of furfural with acetone was selected as an attractive reaction to produce C8 and C13 compounds which can be used as additives to fossil hydrocarbon fractions. On the other hand, from scientific point of view, aldol condensation is a useful and commonly accepted tool to investigate the basic properties of solid materials. In this case, acetone is considered not only as a reactant, but also as a solvent. This minimizes the formation of compounds with higher molecular weight

39

that formation could lead to the

deactivation of catalyst 40. As compounds with higher molecular weight are possessing varied solubility

41

, its formation could lead to the heterogeneity of reaction mixture. Recently we

have shown that the interaction of basic catalyst with furfural during aldol condensation of furfural and acetone provokes the occurrence of Cannizzaro reaction 42, which results in the deactivation of basic active sites with formed furoic acid. To limit the effect of the by-reaction and to decrease the catalyst deactivation high acetone/furfural ratio is more preferred. Under identical reaction conditions, studied MgAl, ZnAl and ZnMgAl mixed oxides exhibited different values of the conversion of furfural and the selectivity to the individual products (Fig. 7). The dependence of the furfural conversion on the time is depicted in Fig. 17 ACS Paragon Plus Environment


7A. The catalytic results showed that the highest conversion of furfural was achieved on Mg2Al mixed oxide (Fig. 7A). This catalyst achieved conversion of furfural 100 % after 4h of running reaction. It is evident from Fig. 7 that the conversion of furfural decreased in order Mg2Al > Zn1Mg1Al > Mg2Al(B) > Zn2Al.

100

100

B

A

Selectivity FAc-OH (%)

Furfural Conversion (%)

80

60

40

Mg2Al Zn1Mg1Al

20

Mg2Al Zn1Mg1Al

80

Zn2Al Mg2Al(B)

60

40

20

Zn2Al Mg2Al(B) 0

0 0

100

200

300

400

0

Time (min)

100

20

40

60

Mg2Al

100

Mg2Al

D

Zn1Mg1Al

Zn1Mg1Al Zn2Al

80

80

Conversion (%)

50

C

Zn2Al

40

Selectivity F2Ac (%)

Mg2Al(B)

Selectivity FAc (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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60

40

20

Mg2Al(B) 30

20

10

0

0 0

20

40

60

80

100

0

20

Conversion (%)

40

60

80

100

Conversion (%)

Figure 7. The activity of the Mg2Al, Mg2Al (B), Zn1Mg1Al and Zn2Al mixed oxides in the aldol condensation of furfural with acetone. Reaction conditions: 2 g of the catalyst, reaction temperature 50 °C, acetone to furfural molar ratio = 10 Dependence of the selectivity to the individual products on the furfural conversion is depicted in Figs. 7B, C, D. It is evident that the selectivity to the products formed in subsequent reaction systems depend on the value of furfural conversion. The selectivity to FAc-OH decreased with increasing conversion of furfural (Fig. 7B) while the selectivity to subsequent dehydrated products FAc and F2Ac, correspondingly, increased (Figs. 7C and 7D). In order to discuss the different selectivity’s of studied catalysts, the selectivity to the individual products was compared at the same value of furfural conversion of 40 % (isoconversion data). At this value of the conversion of furfural, all studied catalysts exhibited the same selectivity to FAc-OH (ca. 50 %, Table 4). The difference was evident in the formation of subsequent dehydrated products FAc and F2Ac (Table 4). While the selectivity to FAc


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increased in order Mg2Al < Mg2Al(B) < Zn1Mg1Al < Zn2Al, the selectivity to F2Ac increased in opposite order, i.e. in order Zn2Al < Zn1Mg1Al < Mg2Al(B) < Mg2Al. Table 4 Selectivity of Zn(Mg)Al mixed oxides achieved at 39.9% conversion of furfural. Reaction conditions: molar ratio of Ac/F = 10, 2 g of catalyst, T = 50 °C, t = 6 h. Zn(Mg)Al mixed oxides

Selectivity at 39.9% conversion of furfural (%) FAc-OH

FAc

F2Ac

Zn2Al

52.3

36.9

8.1

Zn1Mg1Al

51.8

30.5

16.1

Mg2Al

50.1

25.2

22.8

Mg2Al(B)

49.8

27.9

18.0

In general, the conversion of furfural and the selectivity to the products depend on the complex contribution of acido-basic, textural and structural properties of a particular oxide. In detail we focused on the role of acido-basic properties of Zn2Al, Mg2Al, Mg2Al(B) and Zn1Mg1Al mixed oxides. We related the amount of basic/acid sites to the BET surface area (Table 3) and we used oxides with similar crystallite size (Table 2) for the aldol condensation reaction to clearly correlate the conversion/selectivity and the basicity/acidity separately on other variables. Firstly, the conversion of furfural increased with increasing specific surface area (Table 2), increasing total amount of basic sites per square meter of a catalyst (Fig. 8A) and decreasing amount of acid sites per square meter of a catalyst (Fig. 8A). In principle, all oxide surface basic sites (Lewis O2-, Men+-O2- pairs and Brønsted OH- sites) can catalyse the aldol condensation. In our case, it is evident that all studied catalysts contained dominant relative population of Men+-O2- pairs (high relative population of bidentate carbonates; Fig. 4). The dominant role of Men+-O2- pairs could explain apparently linear correlation of furfural conversion on the total amount of basic sites per square meter of a catalyst (Fig. 8A). It is also clear that the catalyst characterized by a lower amount of basic sites (Zn2Al) formed in more extend intermediate FAc, while the catalyst with a higher total amount of basic sites (Mg2Al) formed higher amount of subsequent adduct F2Ac (Fig. 9). Fig. 5 shows that the TPD curves of both Mg1Zn1Al and Zn2Al had a right-side tail which evidences that these samples could contain small amount of stronger acid sites. Dehydration of Fac-OH to FAc could be catalysed by acid sites (dehydration), so the presence of stronger acid sites in both Zncontaining samples could favour this stage. In contrary, F2Ac formation is concerned with the 19 ACS Paragon Plus Environment


presence of basic sites. The obtained data allowed supposing the kinetically dependent character of the reaction which proceeded on surface basic sites of an oxide. Secondly, Mg2Al, Mg1Zn1Al and Zn2Al mixed oxides were prepared by the same way and had very similar M2+/Al3+ molar ratio. Comparing the properties of these three materials, the specific surface area, the amount of basic and the amount of acid sites of Mg1Zn1Al mixed oxide was approximately in the middle between the values of appropriate Mg2Al and Zn2Al mixed oxides. Thus, these properties of Mg1Zn1Al mixed oxide were a result of the combination of Mg2Al and Zn2Al mixed oxides. However, it should be stressed that Mg1Zn1Al contained the highest amount of irreversibly bound CO2 on basic sites together with the lowest absolute amount of acid sites per square meter of a catalyst. As we discussed above, the term irreversibly bound CO2 on basic sites includes all O2- species and part of Men+-O2- pairs. Thus, it could be suggested that all energetically heterogeneous Men+-O2- pairs possess approximately the same activity in aldol condensation of furfural carried on the calcined mixed oxide catalysts. It is hard to distinguish the contribution of Lewis O2- and Brønsted OH- sites due to its significantly less population in the studied catalysts. In addition, it should be stressed that after the condensation of furfural with acetone the water molecule is subtracted in following reaction step forming dehydrated products FAc and F2Ac. This water is expected to form additional Brønsted basic OH- species, which promotes the reaction rate. That is the reason the conversion of furfural is increased and stabilized after several hours of the running reaction. Nevertheless, the presence basic sites as strong as possible would be preferable, e.g., in transesterification reaction as it is mentioned by Antunes et al. observed slightly higher ester yield for ZnMgAl mixed oxide than for ZnAl. A

Mg2Al

B

Mg2Al

80

100

80

Zn1Mg1Al

Zn1Mg1Al

60

Mg2Al(B)

40

Mg2Al(B)

40

Zn2Al

Zn2Al

20

20

0 0.0

0.5

1.0

60

1.5

2.0

2.5

Amount of basic and acid (red) sites -2 (µmol.m )

0.0

0.5

1.0

1.5

2.0

0 2.5

Ratio of amount of basic to acid sites


Conversion of furfural (%)

100

Conversion of furfural (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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20

who

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Figure 8. The dependence of the furfural conversion on the amount of basic or acid sites per square meter of catalyst (A) and on the ratio of amount of basic to acid sites (B). Reaction conditions: 2 g of the catalyst, reaction temperature 50 °C, acetone to furfural molar ratio = 10, t = 6 h. 50

50

40

Zn2Al Zn1Mg1Al

30

30

Mg2Al(B)

20

Mg2Al Mg2Al

Mg2Al(B)

20

Zn1Mg1Al 10

Zn2Al

Selectivity to F2Ac (%)

40

Selectivity to FAc (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


10

0

0 0.5

1.0

1.5

2.0

2.5

-2

Amount of basic sites (µmol.m )

Figure 9. The dependence of FAc and F2Ac selectivity achieved at 39.9% conversion of furfural on the amount of basic sites. Reaction conditions: 2 g of the catalyst, reaction temperature 50 °C, acetone to furfural molar ratio = 10 Thirdly, Mg2Al and Mg2Al(B) mixed oxides (prepared by the using of different total concentration of M2+ and Al3+ cations) contained the same amount of acid sites as well as the irreversible bound CO2 (Table 3). Comparing the catalytic behaviour of these mixed oxides, the principal role of basic sites in the condensation reaction is evident. Both of these samples had approximately the same amount of Lewis acid sites (see Table 3), However, Mg2Al mixed oxide (lower concentration of cations in parent solution) exhibited higher values of the conversion of furfural (Fig. 8) and the selectivity to F2Ac (Fig. 9 ) in comparison to Mg2Al (B). This shows to the critical role of the experimental conditions of the synthesis of hydrotalcites (and subsequently the synthesis of mixed oxide catalysts) to their final acidobasic properties and subsequently to the catalytic activity. Finally, it is interesting to compare the catalytic behaviour of Mg2Al(B) mixed oxide (exhibiting worse structural and textural properties in comparison to Mg2Al) with all other studied catalysts that possessed the same M2+/Al3+ molar ratio. It is observed that the conversion of furfural obtained for MgAl(B) is outside the apparently linear correlation of furfural conversion on the total amount of basic and acid sites per square meter of a catalyst (Fig. 8A). Nevertheless, these differences are minimized when the furfural conversion is expressed on the ratio of total amount of basic to acid sites per square meter of a catalyst. It 21 ACS Paragon Plus Environment


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shows to the probable contribution of both basic and acid sites on the aldol condensation of furfural with acetone. 4. CONCLUSIONS Zn1Mg1Al mixed oxide exhibited the value of specific surface area, the amount of acid sites and the amount of basic sites between the values corresponding to the appropriate Zn2Al and Mg2Al mixed oxides. However, the difference was found in the amount of reversibly and irreversibly bound CO2. At the same M2+/Al3+ molar ratio, Mg1Zn1Al mixed oxide exhibited the highest relative population of irreversibly bound CO2 on basic sites (52 %), both Mg2Al (45 %) and Zn2Al (31 %) mixed oxides exhibited lower amount of irreversibly bound CO2. For all studied oxides, major population of basic sites was represented by the medium strength Me2+-O2- pairs with large heterogeneity. The basicity of Mg2Al mixed oxides was significantly dependent on the preparation of parent hydrotalcite precursor. The hydrotalcite originated from less concentrated solution within co-precipitation led to the Mg2Al mixed oxide with a higher amount of basic sites, as it was shown on two different Mg2Al samples (Mg2Al and Mg2Al(B)). This shows to the critical role of the experimental conditions of the synthesis of hydrotalcites (and subsequently the synthesis of mixed oxide catalysts) to their final acido-basic properties and subsequently to the catalytic activity. In other words, detail description of the preparation procedure is necessary for relevant comparison of even chemically same catalysts. The activity of studied catalysts in the aldol condensation of furfural with acetone was directly correlated with the total amount of basic sites on the surface. In this line, Mg2Al with the highest concentration of basic sites showed the best performance in the reaction. In more detail, all energetically heterogeneous Men+-O2- pairs (dominant type of basic sites in all studied catalysts) possess approximately the same activity in aldol condensation of furfural carried on the calcined mixed oxide catalysts. All presented results would help to customize synthesis of target catalyst in the future. ASSOCIATED CONTENT Supporting information Adsorption isotherms of CO2, spectra of OH stretching vibrations, spectra of molecularly adsorbed CO2.


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AUTHOR INFORMATION Corresponding author * Tel: +420-466037261; E-mail: [email protected] ACKNOWLEDGEMENT The authors thank the Czech Science Foundation for the support (Project No. GA15-21817S).



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