Quaternary ammonium salts for hydrotalcite-type catalysts synthesis

1University of Bucharest, Faculty of Chemistry, Department of Organic Chemistry, Biochemistry and Catalysis, 4-12 Regina Elisabeta Av., S3, 030018, Romania,

email: octavian.pavel@chimie.unibuc.ro 2University of Bucharest, Faculty of Chemistry, Department of Physical Chemistry, 4-12 Regina Elisabeta Av., S3, 030018, Romania 3National Institute for Lasers, Plasma and Radiation Physics, 409 Atomistilor Street, PO Box MG-16, 077125, Măgurele, Romania

Octavian-Dumitru Pavel1*, Bogdan Cojocaru1, Bogdan Jurca2, Rodica Zăvoianu1, Ruxandra Bîrjega3, Vasile I. Pârvulescu1*

Introduction Experimental There is a growing interest focused on the synthesis and properties of layered double

hydroxides (LDH) as part of the anionic clays materials.

The main method of preparation of LDH is still the co-precipitation by contacting an

aqueous solution of the salts containing the target cations with an inorganic alkaline

solution.

However, it involves some disadvantages: multiple synthesis steps, high energy

consumption, use of specific vessels in each stage of the process, etc.

The mechano-chemical method is as an alternative to co-precipitation involving only

one step mixing in a mortar/mill of all the reactants followed by washing and drying.

The aim of this study was to perform a comparative analysis of the physico-

chemical properties and catalytic activity of hydrotalcites prepared by co-

precipitation and mechano-chemical methods in presence of organic alkalis. These

catalysts were investigated in cyanoethylation reaction of ethanol with acrylonitrile

toward 3-ethoxypropionitrile.

Acknowledgements

This work was supported by a grant of

the Romanian Ministery of Research

and Innovation, CCCDI – UEFISCDI,

project number PN-III-P1-1.2-PCCDI-

2017-0387/ 80PCCDI, within PNCDI

III.

The hydrotalcites Mg0,75Al0,25 were obtained at pH 10 by co-precipitation of magnesium

and aluminium nitrates and base solution (Tetra Methyl Ammonium Hydroxide and Tetra

n-Butyl Ammonium Hydroxides) under low supersaturation (HT-MgAl-TMAH-CP; HT-

MgAl-TBAH-CP).

In the mechanochemical method, all precursors were mechanically mixed in a Mortar

Grinder RM 200 (HT-MgAl-TMAH-MC; HT-MgAl-TBAH-MC).

All dryied samples were calcined in air atmosphere at 460°C (cHT-MgAl-TMAH-CP;

cHT-MgAl-TMAH-MC; cHT-MgAl-TBAH-CP; cHT-MgAl-TBAH-MC).

The resulted mixed oxides were then rehydrated in order to reconstruct the layered

structure (hyHT-MgAl-TMAH-CP; hyHT-MgAl-TMAH-MC; hyHT-MgAl-TBAH-

CP; hyHT-MgAl-TBAH-MC).

The characterisation of samples has been carried out by XRD, DRIFT, BET, DTA-TG,

irreversible adsorption of organic acids of different pKa values.

The cyanoethylation reaction were carried out 5h, reflux, ethanol/acrilonitrile = 3/1.

Results

Conclusion

Figure 1. The XRD patterns of hydrotalcite,

mixed oxides and reconstructed samples for:

(A) TMAH as hydrolysis agent and co-

precipitation method; (B) TMAH as hydrolysis

agent and mechano-chemical method; (C)

TBAH as hydrolysis agent and co-precipitation

method; (D) TBAH as hydrolysis agent and

mechano-chemical method

10 20 30 40 50 60 70 80

22

0

20

0

11

1

01

801

5

00

9;0

12

00

6

00

3

11

311

0

01

8

01

5

00

9;0

12

00

6

Inte

nsity (

a.u

.)

2q CuKa

HT-MgAl-TMAH-CP cHT-MgAl-TMAH-CP

hyHT-MgAl-TMAH-CP

00

3

11

311

0

(A)

10 20 30 40 50 60 70 80

01

801

50

09

;01

2

00

6

00

3

22

0

20

0

11

1

11

31

10

01

801

5

00

9;0

12

00

6

00

3

Inte

nsity (

a.u

.)

2q CuKa

HT-MgAl-TMAH-MC cHT-MgAl-TMAH-MC

hyHT-MgAl-TMAH-MC

11

31

10

(B)

10 20 30 40 50 60 70 80

01

801

5

00

9;0

12

00

6

00

3

22

0

20

0

11

1

11

31

10

01

801

5

00

9;0

12

00

6

00

3

Inte

nsity (

a.u

.)

2q CuKa

HT-MgAl-TBAH-CP cHT-MgAl-TBAH-CP

hyHT-MgAl-TBAH-CP

11

311

0

(C)

10 20 30 40 50 60 70 80

01

801

5

00

9;0

12

00

6

00

3

22

0

20

0

11

1

11

31

10

01

8

01

5

00

9;0

12

00

600

3

In

ten

sity (

a.u

.)

2q CuKa

HT-MgAl-TBAH-MC cHT-MgAl-TBAH-MC

hyHT-MgAl-TBAH-MC

11

31

10

(D)

4000 3500 3000 2500 2000 1500 1000 500

64

077

010

45

95

48

54

96

0

14

86

17

00

22

76

15

00

14

20

16

36

20

40

30

003

68

53

60

0

Ab

so

rba

nce

(a.u

.)

Wavenumber (cm-1)

TMAH HT-MgAl-TMAH-CP

cHT-MgAl-TMAH-CP hyHT-MgAl-TMAH-CP

CO2

(A)

4000 3500 3000 2500 2000 1500 1000 500

64

07

7010

45

95

48

54

96

0

14

86

17

00

22

76

15

00

14

20

16

36

20

40

30

0036

85

36

00

Ab

so

rba

nce

(a

.u.)

Wavenumber (cm-1)

TMAH HT-MgAl-TMAH-MC

cHT-MgAl-TMAH-MC hyHT-MgAl-TMAH-MC

CO2

(B)

100 200 300 400 500 600 7000

10

20

30

40

50

60

70

80

90

100

485 o

C

395 o

C

200

oC

117 o

C

TG

A (

%)

Temperature (oC)

HT-MgAl-TMAH-CP(A)

-150

-125

-100

-75

-50

-25

0

25

50

DT

A (u

V)

100 200 300 400 500 600 7000

10

20

30

40

50

60

70

80

90

100

484 o

C

397 o

C

204 o

C

120 o

C

TG

A (

%)

Temperature (oC)

HT-MgAl-TMAH-MC(B)

-150

-125

-100

-75

-50

-25

0

25

50

DT

A (u

V)

Figure 2. The DRIFT spectra of hydrotalcite,

mixed oxides and reconstructed sample for: (A)

TMAH as hydrolysis agent and co-precipitation

method; (B) TMAH as hydrolysis agent and

mechano-chemical method

Figure 3. The TGA-DTA profiles of hydrotalcite,

mixed oxides and reconstructed sample for: (A)

TMAH as hydrolysis agent and co-precipitation

method; (B) TMAH as hydrolysis agent and

mechano-chemical method

Scheme 1. The cyanoethylation reaction of ethanol with acrylonitrile (reaction

mechanism)

Samples Surface area

(m2·g-1)

Pore volume

(cm3·g-1)

Average pore

width

(Å)

Total number of

base sites

(mmol·g-1)*

Distribution of base sites

Strong base sites

(mmol·g-1)**

Weak and

medium base sites

(mmol·g-1)***

HT-MgAl-TMAH-CP 1.7 0.0054 126.65 6.83 0.30 6.53

cHT-MgAl-TMAH-CP 189.1 0.3585 75.84 8.39 0.39 8.00

hyHT-MgAl-TMAH-CP 0.5 0.0039 324.28 7.23 0.35 6.88

HT-MgAl-TMAH-MC 4.8 0.0118 99.04 6.68 0.25 6.43

c-HTMgAl-TMAH-MC 197.8 0.3185 64.41 8.20 0.35 7.85

hyHT-MgAl-TMAH-MC 1.1 0.0038 142.77 7.05 0.28 6.77

HT-MgAl-TBAH-CP 3.2 0.0077 97.89 7.03 0.32 6.71

cHT-MgAl-TBAH-CP 191.2 0.3466 74.33 8.40 0.39 8.01

hyHT-MgAl-TBAH-CP 1.4 00.4601 163.54 7.26 0.37 6.89

HT-MgAl-TBAH-MC 9.9 0.0189 76.09 6.70 0.27 6.43

cHT-MgAl-TBAH-MC 199.5 0.3022 63.87 8.30 0.37 7.93

hyHT-MgAl-TBAH-MC 5.3 0.0124 93.89 7.19 0.37 6.82 *AA=acrylic acid; **PhOH=phenol;

***equal to the difference: Total number of base sites - Strong base sites

Table 1. The surface area and basicity of materials

6,4 6,6 6,8 7,0 7,2 7,4 7,6 7,8 8,0 8,2

0

10

20

30

40

50

60

70

80

90

100

HT-MgAl-TBAH-CP

cHT-MgAl-TBAH-CP

hyHT-MgAl-TBAH-CP

HT-MgAl-TBAH-MC

cHT-MgAl-TBAH-MC

hyHT-MgAl-TBAH-MC

HT-MgAl-TMAH-CP

cHT-MgAl-TMAH-CP

hyHT-MgAl-TMAH-CP

HT-MgAl-TMAH-MC

cHT-MgAl-TMAH-MC

hyHT-MgAl-TMAH-MC

Acry

lon

itrile

co

nve

rsio

n (

%)

Weak and medium base sites (mmol•g-1)

Figure 4. The variation of the

acrylonitrile conversion after

5h reaction time vs. weak and

medium basicity of the

investigated catalysts M M OO

M O

TMAH and TBAH represent a viable alternative to traditional inorganic alkalis for hydrotalcite Mg/Al synthesis.

The economy of distilled water used to wash the gel obtained as well as the total absence of alkaline cations.

The catalytic activity followed the trend: mixed oxides > reconstructed samples > dried samples depending on the

variation of weak and medium base sites.

The selectivity towards 3-ethoxyproprionitrile is 100%.