Chapter-l

Synthesis, Characterization and Catalytic Hydroxylation of Phenol over CuNiAI Ternary Hydrotalcites

1.1 INTRODUCTION

1.2 EXPERIMENTAL

1.3 PHYSIOCHEMICAL CHARACTERIZATION OF FRESH SAMPLES

1.3.1 Elemental Chemical Analysis

1.3.2 Powder X-ray Diffraction (PXRD)

1.3.3 Fourier-Transformed Infrared Absorption (FT-IR) Studies

1.3.4 Visible-Ultraviolet (Vis-UV) Spectral Studies

1.3.5 Thermal studies (Thermogravimetry (TG) and Differential Thermal

Analysis (DTA))

1.3.6 Temperature Programmed Reduction (TPR)

1.3.7 Specific Surface Area and Porosity Assessment

1.4 PHYSICOCHEMICAL CHARACTERISATION OF CALCINED

SAMPLES

1.4.1 Powder X-ray Diffraction (PXRD)

1.4.2 Specific Surface Area and Porosity'Assessment

1.4.3 Temperature Programmed Reduction (TPR)

1.5 CATALYTIC STUDIES

1.5.1 Effect of Different Metal Ion Concentration

1.5.2 Effect of Substrate: Catalyst Ratio

1.5.3 Effect of Reaction Temperature

1.5.4 Effect of the Reaction Medium and Oxidant

36

1.5.5 Effect of pH of the Medium

1.5.6 Effect of Reaction Time

1.5.7 Effect ofSubstrate:Oxidant Ratio

1.5.8 Effect of Calcination Temperature

1.6 CONCLUSIONS

1.7 REFERENCES

37

1.1 INTRODUCTION

It is well known in the chemistry of hydrotalcites that it is difficult to synthesize

pure binary OrAl hydrotalcites (I). This deviant behavior of ci+ in comparison with

other bivalent transition metal ions can be attributed to the nature of cation, which

exhibits a cooperative John-Teller distortion in octahedral coordination. John Teller

theorem states that, for a non-linear molecule in an electronically degenerate state,

distortion must occur to lower the symmetry, remove the degeneracy and lower the

energy. The distortion arises due to the unsymmetrical filling of the d-orbitals in an

octahedral field of transition metals. Cu(II), a d' configuration system, in octahedral

field wherein the ninth electron has an option of entering in d I- or d x? -I orbital and if

they are asymmetrically filled, then the degeneracy is destroyed i.e. the two orbitals are

no longer equal in energy. However, Cu2+ . containing hydrotalcite-like (HT-like)

compounds can be synthesized by co-existing them with other bivalent cations, which

by themselves can form lIT-like network. Our preliminary results on catalytic activity

studies for the hydroxylation of phenol showed no conversion of phenol over pure

NiAl-HT and around 13% conversion of phenol to dihydroxybenzenes over CuAl-HT

(a mixed phase of HT and malachite). [n the present chapter, emphasis is made to

understand the influence of nickel as co-cation on the physiochemical properties as well

on the catalytic properties of the resulting ternary CuNiAl system, in particular for

hydroxylation of phenol. The influence of various reaction parameters, namely,

substrate:catalyst ratio, reaction temperature, substrate:oxidant ratio, nature of oxidant,

solvent, pH, time-on-stream, and calcination temperature on the activity and selectivity

for the "sought for" reaction 'MlS studied.

1.2 EXPERIMENTAL

The samples were prepared by coprecipitation under low supersaturation. A

38

solution (A) containing the desired amount of metal (Ni, Cu, AI) nitrates and a solution

(B) containing precipitating agents (i.e., NaOH and Na2C03) were added slowly

(-I mllmin) and simultaneously, while maintaining the pH around 9-10 under vigorous

stirring at room temperature. The addition took ca. 90 min and the final pH was

adjusted to 10. The samples were aged in the mother liquor at 65°C for 18 h, filtered,

washed (until total absence of nitrates and Na in the washing liquids) and dried in an air

oven at 110°C for 12h. The samples were named as CuNiAIX-Y, where X and Y

correspond to atomic compositions of (Cu+Ni)/AI and CulNi respectively. The color of

the samples was bluish green, except for sample CuNiAI3 -5, which was gray.

1.3 PHYSIOCHEMICAL CHARACTERIZATION OF FRESH SAMPLES

1.3.1 Elemental Chemical Analysis

The results obtained are summarized in Table 1.1. The formulae calculated for

Table 1.1 Elemental chemical analysis of the samples synthesized

Sample Nia Cu' AI' M(II)/AI" NifCu" Solution Solid Solution Solid

CuNiAII-1 13.50 13.24 11.83 1.0 1.00 1.0 1.10 CuNiAI2-1 15.98 22.00 7.68 2.0 2.17 1.0 0.79 CuNiAI3-1 20.63 20.40 6.13 3.0 2.96 1.0 1.09 CuNiAI4-1 19.85 25.63 4.51 4.0 4.44 1.0 0.84 CuNiAI3-0.2 34.03 6.69 5.87 3.0 3.15 5.0 5.51 CuNiA13-5 7.53 35.83 6.14 3.0 3.04 0.25 0.23

'weight (%) batomic ratio

the samples are included in Table 1.2. The content of carbonate has been calculated

from M(II)! AI ratio, assuming that carbonate is the only interlayer anion balancing the

positive charge in the layers because of the presence of aluminum, in agreement with

results from other experimental techniques mentioned below; the interlayer water

content has been calculated from the TG curves. The M(II)/ AI and Ni/Cu ratios in the

solids are in fairly good agreement with the ratios existing in the starting solutions,

39

deviation being in most of the cases lower than 10%. The lack of coincidence between

the initial ratio of cations in the solutions, and the ratio in the solids isolated, is,

however, rather common in the literature, and has been usually ascribed to a

preferential precipitation of one or another cation as hydroxide (2).

Table 1.2 Chemical formulae and the lattice parameters of the samples

Sample Formula'

CuNiAI2-1 [NiO.30CUO.38AIO.32(OH)21 (C03)0.16 . 1.23 H2O 22.97 3.054

CuNiA13-1 [NiO.39Cu0.36AIO.25(OH)21 (C03)0.13 . 1.27 H2O 22.97 3.058

CuNiAI4-1 [NiO.37Cu0.44AIO.I 9(OH)21 (C03)0.10· 1.13 H2O 23.11 3.064

CuNiA13-0.2 [NiO.64CuO.12AIO.24(OH)z1 (C03)0.12 . 1.03 H2O 23.22 3.048

CuNiA13-5 [NiO.14CuO.61 AIO.25(OH)z1 (C03)0.13 . 0.96 H2O 22.88 3.076

'values have been rounded to two figures.

1.3.2 Powder X-ray Diffraction

The powder X-ray diffraction (PXRD) is a diagnostic tool for the phase

identification of these compounds. The PXRD patterns of all clay materials possessing

layered structure generally show sharp and symmetric peaks at lower angles (28) but

broad and asymmetric peaks at higher diffraction angles. The diffraction patterns for all

six samples are shown in Fig. 1.1. The patterns for samples CuNiAI2-1 to CuNIA13-5

(Fig.1.1 (b-t)) are typical of a hydrotalcite-like material with intercalated carbonate

anions, showing harmonics close to 28 = 11°, 24°, and 35°, corresponding to basal

spacing close to 7.6, 3.8, and 2.6 A, respectively. These peaks are ascribed to

diffraction by planes (003), (006), and (009) assuming a 3R packing of the layers (3,4).

The positions of the remaining peaks are in agreement with such an ascription.

However, the diffraction pattern of Cu-rich sample (CuNiA13-5) is relatively broad,

especially for higher order reflections, indicating the incompatibility of copper in a

however are broad and ill-defined and additional peaks around 18.3 and 20.5° (28)

were noted, can be ascribed to Al(OHh, gibbsite (JCPDS ; 12-0460). It is known in

40

chemistry of hydrotalcites, that a composition with M(II)/ Al atomic ratio less than 2.0

leads to a preferential segregation of aluminum as Al(OH)3. Hence, in the subsequent

discussion of this chapter, sample CuNiAll-l will not be considered. The thickness of

7.8

3.8 2.6

o 10 20 30 40 50 60 70 80

2 Theta (degrees)

Fig. 1.1 Powder X-ray diffraction patterns of (a) CuNiAll-l, (b) CuNiAI2-1, (c) CuNiAI3-1, (d) CuNiAI4-1, (e) CuNiA13-0.2, (I) CuNiA13-5 Bottom (a), Top (I)

the brucite-like layers (4.8 A) (5) and the interlayer space close to 2.8 A, suggesting

location of the carbonate anion with its molecular plane parallel to the brucite-like

layers. Lattice parameters 'a' is the distance between the neighboring cations in the

brucite-like layers, which can be estimated from the ionic radii of the cations in the

brucite-like lattice (6) and their molar fractions in the samples while the parameter 'c'

is three times the distance between the adjacent brucite-type layer, controlled mostly by

the size (and orientation) of the interlayer anion and the electrostatic: forces operating

between the interlayer anion and the layers. It is clear from Table 1.2 that the lattice

parameter 'a' increases with an increase in copper content while lattice parameter 'c'

41

decreases with an increase in Al content (i.e. with an increase on carbonate content).

The increase in lattice parameter 'a' can be accounted for the higher octahedral radius

of copper (0.73A in comparison with nickel, 0.69A), while the decrease in parameter

'c' can be accounted for a higher electrostatic interaction between the layer and the

interlayer.

1.3.3 Fourier-Trallsformed Infrared Absorption (FT-IR) Studies

Although infrared (IR) analysis is not a primary tool for the characterization of

hydrota1cites, yet it has been routinely used especially for the identification of the

foreign anions in the interlayer space and its interaction with the brucite-like sheets.

Besides that, information about the type of bonds formed by the anions and about their

orientation can also be obtained.

Generally, in all hydrotalcites, an absorption around 3500-3600cm·1 occur and

is attributed to the hydrogen bonding stretching vibrations of the OH group in the

brucite-like layer. The main absorption bands of the anions are observed between 1000

and l800cm· l. The carbonate anion (in our case) in the symmetric environment is

characterized by a DJh planner symmetry, with three IR active absorption bands,

observed at 1350- 1380cm- l (uJ), 850-870 cm- l (u2) and 670-690 cm- l (u4). However, in

some cases the presence of a shoulder around 1400 cm- I, or of a double band in the

region 1350-l400cm-I(7) has been attributed to the lowering of symmetry of the

carbonate (e2v symmetry) and to the disordered nature of the interlayer (7) which

causes the removal of the degeneracy of the UJ and u4modes.

The FT-IR spectra of the samples are shown in Fig. 1.2. All samples showed a

broad and an intense band between 4000 and 3000 cm-! due to the OH stretching mode

of layer hydroxyl groups and of interlayer water molecules (8). Although the position

of this band should be dependent on the nature of the layer cation, as its

42

electronegativity will modifY the electron density on the O-H bond (M-OH) but the

extreme broadness of this band because of hydrogen bonding does not make any sense

and relationship in the present case. A weak shoulder recorded in some cases (specially

in samples CuNiAI3-1 and CuNiAI3-5 (Fig. 1.2 (b and e» around 3000 cm- I has been

ascribed to the OH stretching mode of interlayer water molecules hydrogen-bonded to

inter layer carbonate anions (9, I 0). The bending mode of water molecules is responsible

for the weak band recorded at 1640-1620 cm- I. A rather sharp, intense band at 1375-

1365 cm- I is due to u3 anti symmetric stretching ofinterlayer carbonate, shifted from its

\054

4DOD 3~OO 3000 2500 2000 DOD IDOO 500 0

Wavenumber (cm- I)

Fig.I.2 FT-IR spectra of (a) CuNiAI2-1, (b) CuNiA13-I, (c) CuNiAI4-1, (d) CuNiA13-0.2, (e) CuNiA13-5 Bottom (a), Top (e)

position in free C032- species (-1450cm- l) because of strong hydrogen bonding with

hydroxyl sheets and H20 molecules in interlayer (8). However for sample CuNiAI4-1

(Fig.1.2 (c)), a split in u3 vibration is observed at 1372 cm·1 and 1471 cm- I with

simultaneous detection of a very weak shoulder at 1054 cm- I . This shoulder can be

ascribed to u I mode of carbonate; although this mode is IR-inactive in the D3h

symmetry of free carbonate, it becomes activated due to lowering of symmetry of

43

carbonate in the interlayer (probably to Clv or C2v), which is also responsible for

splitting of the v3 band. Displacement and splitting of this band has been observed

similarly in calcite and aragonite, where the local symmetry of carbonate is D3 and Cs,

respectively (11,12). The bands recorded below 1000 cm- I can be ascribed to v2 mode

of carbonate (ca. 850 cm- I), and to M-OH modes. These results confirm the presence

of carbonate, as well as the absence of nitrate, in the interlayer space of the

hydrotalcites synthesized. Simultaneous formation of an OH-containing phase

(mixenerite) cannot be discarded, although it is very unlikely because of the known

preferential intercalation of carbonate.

1.3.4 Visible-Ultraviolet (Vis-UV) Spectral Studies

The Vis-UV/DR spectra for all samples are given in Fig. 1.3. It is clear from

this figure that the copper rich sample has an absorption maximum around 770 nm and

, -- C I'.' '.

----~ , 0.'

\~ e

"Kb \', \. , \ d \ '.\

/\ ... ------

\~ . . , .......

'. a

o

'" Wavelength (em,l)

Fig. 1.3 UV -vis diffuse reflectance spectra of (a) CuNiAI2-1, (b) CuNiA13-I, (c) CuNiAI4-1, (d) CuNiA13-0.2, (e) CuNiA13-5

the Ni-rich sample around 660 nm, while for the other samples the absorption

maximum are in between these two values. If one looks at the UV region also, the Ni-

rich sample shows a maximum around 300 nm while the Cu-rich sample shows at 240

44

run. Further, CuNiA13-5, shows an additional peak around 390run. Only one spin-

allowed, Laporte-forbidden band is expected for Cu2+ species in octahedral sites, while

three are expected for Ni2+ species (13):

Taking into account of the closeness between water and hydroxyl in the

2 2 spectrochemical series (6), the broad band close to 770 run is due to Eg (D) .... T2g

(D) transition of octahedral copper (cf: [Cu(H20)6f+), broadness being due to distortion

of its co-ordination from octahedral symmetry, a Jahn-Teller effect, for energy

l minimization. For nickel containing samples, the peak corresponding to transition A2g

(F) .... IT 2g (F) is recorded outside the spectral range of the instrument used. The band

corresponding to transition l A2g (F) .... lT1g (F) splits because of spin-orbit coupling,

and usually extends from 71 0-645 run; finally, transition l A2g (F) .... IT Ig (P) gives rise

to a band at 390nm (13). The bands observed in the UV region (200-300 run) are

ascribed to charge transfer processes. These results indicate that the coordination of

2+ • Ni and Cu2

+ ions should be close to that existing in the hexa-aquo complexes,

confirming their location in the octahedral holes of the brucite-like layers.

1.3.5 Thermal Studies (Thermogravimetry (TG) and Differential Thermal Analysis

(DTA))

The thermal behavior of Hydrotalcites IS generally characterized by two

transitions:

45

J

The first one being reversible, endothermic, at low temperature corresponds to the loss

of interlayer water, without collapsing the structure and the second one, endothermic, at

higher temperature is due to the loss of hydroxyl group from the brucite like layer as

well as of the anions.

The nature of these two transitions depend on many factors such as:

M(II)/M(III) ratio, type of anions, low temperature treatment (hydration, drying etc.)

6,-------~~~------,

~ 5

= ~ 0

"C 4 ... ·8 ... 3 <II .c -0 "C 2 = <II ~

U 1 ... ,...

<I 0

5 -1

0 200 400 600 800 1000

Temperature ("C)

Fig. 1.4 Differential thermal analysis (DTA) curves of (a) CuNiAI2-1, (b) CuNiA13-I, (c) CuNiAI4-1, (d) CuNiA13-0.2, (e) CuNiA13-S Top (a), Bottom (e)

heat treatment (in case of oxidizable elements such as Cr(Ill)). For AI-containing

hydrotalcites, the first transition occurs in the temperature range ISO to 2S0°C (typical

for MgAICOJ-HT) while the second from 300 to 4S0°C.

The DTA curves recorded for samples shown in Fig. 1.4 are typical of

hydrotalcite-like materials (10, 14,IS). The curve for sample CuNiAI3-1 (Fig.I.4(b))

shows a rather sharp endothermic effect at I 77°C, followed by a broader endothermic

46

effect centered around 303°C. Previous studies with MgAl hydrotalcites have shown

~

~ • ~ ::: <l

100 ... , 90 o

80

-0.2 70

"-~ .......... _.

60 . " -,,-. ··"-1_ ..... , __ . _______ -0.4

50

40 -0.6

too ....

'" -.. --. -- ..... -- -..... -.... --_ ......

-.. --... ""

30 .................... _-_ ...

20 -0.8

o 275 550 825 1100

Temperature ("C)

Fig. 1.5 Thermogravimetric analysis (TG, dotted lines) and differential thermogravimetric analysis (DTG, solid lines) curves of (a) CuNiAI2-1, (b) CuNiA13·1, (c) CuNiA14-1, (d) CuNiA13-0.2, (e) CuNiA13-5 Top (a), Bottom (e)

~ • ~ -~ ::: <l ~

(7, 16),that the first peak corresponds to removal of interlayer water molecules, while

the second effect combines removal of hydroxyl groups from the brucite layers as water

molecules together with removal of inter layer carbonate anions as CO2• The broadness

of the second effect at 303°C would account for this double process (water and carbon

dioxide removal). The positions of these peaks shift slightly when passing from one

47

sample to another. Correspondingly, the TG curves (Fig.1.5) show two overlapped

weight losses whose inflection points, as determined from the DTG curves roughly

coincide with those of the DTA minima. The thermal behavior shown by sample

CuNiAI3-5 (Fig. 1.5 (e» differs from that usually shown by hydrotalcites.

It is noticeable the high temperature endothermic effect, recorded at 234°C,

whose intensity is significantly lower than that of the first endothermic peak, at 157°C.

Further, an additional endothermic peak is recorded at 592°C. The TG curve for this

sample shows a continuous weight loss and an additional weight loss fairly close to

600°C (undoubtedly detected in the DTG curve), a position coincident, within

experimental error, with that of the "unexpected" endothermic effect. This could

probably be due to the presence of some carbonate ions strongly held to the brucite-like

sheets. This behavior has been previously reported for other copper containing

hydrotalcite-like materials (17-19) and probably is related to the large amount of copper

present in this sample. In fact, to our knowledge, this high temperature thermal feature

is unique for copper containing hydrotalcites. Velu and Swamy (17), have observed this

high temperature endothermic peak for CuMnAl hydrotalcite and using EGA analysis,

found only CO2 at that temperature. They have attributed this feature to some type of

reaction occurring between brucite sheets and carbonate anion in the interlayer to form

some sort of oxycarbonate having chemical composition [M(II)M(IIJ),Oy(C0),]

wherein the meta~oxygen bond remains intact and hence carbonate ions are

substantially retained. Later recently, Alejandre et a!., (18) have attempted to

synthesize pure Cu-Al hydrotalcite and found similar thermal features and attributed

them to differences in the co-ordination behavior of carbonate anion within the brucite

lattice. The results observed here are in accordance with the proposal of Velu and

Swamy, as no significant split in the V3 mode of vibration of carbonate in the FT-JR

48

________________________________________________ J

spectra was noted in contrast to that results of Alejandre et aI., (18). An interesting

point of observation is that, for pure copper containing hydrotalcites irrespective of its

associated bivalent metal ion, this thermal transformation occurs at 590-600°C. In other

words, this amorphous metastable phase obtained during thermal treatment may be a

pure copper containing oxycarbonate where the possibility of influence/presence of

other metal ions is rather remote. Further, the temperature of this unusual endothermic

peak is influenced significantly by the presence of impurity phases and copper content

of the samples.

1.3.6 Temperature Programmed Reduction (TPR)

Temperature programmed reduction is a relatively new technique for

determining the reducibility of both bulk and supported catalysts. It is a highly sensitive

method for discriminating the reducibility of different species, thus providing the

information about its chemical state as well as its dispersion. Applicability of TPR for

characterization of hydrotalcites has been reported elsewhere (20,21). The analysis is

not easy because of the destruction of the layered structure when the layer cations are

reduced, as deduced from the DT AlTG results. The TPR profiles are shown in Fig. 1.6.

In all cases, a sharp reduction maximum is recorded, followed by a broader, underlying,

much weaker maximum. In the case of sample CuNiA13-5, the sharp maximum shows

a low temperature shoulder. The precise positions of these maxima for all samples, as

well as other quantitative data from the TPR study, are .summarized in Table 1.3. The

total amount of hydrogen consumed (Table 1.3) coincides rather well, within

experimental error, with the expected value, assuming the reduction processes (AI3+ is

not reduced under the experimental conditions used) (21).

Nf+ + ~ = Nf + 2 H+

Cu2+ + ~ = Cuo + 2 It

49

Deviations between expected and measured H2 consumption are not larger than 10% in

any case. Further, the ratio between the integrated areas of the peaks roughly coincides

~

= oj ~

" ~ = 0 Q. ~

" ... ;; = ..

Cii

10

9 l-

8 I-

7 l- e

6 r-

r- -l ......., :;

4 , I I I

o 200 400 600 800 1000

Temperature (0C)

Fig. 1.6 Temperature programmed reduction (TPR) curves of (a) CuNiAI2-1, (b) CuNiA13-I, (e) CuNiAI4-1, (d) CuNiAI3-0.2, (e) CuNiA13-S Bottom (a), Top (e)

with the Illtio between the molar fractions of both cations. As in sample CuNiA13-S

(Fig 1.6 (e» (that with the largest Cu2+ content) the first maximum is extremely

intense, one can conclude that the sharp maximum is due to reduction of Cu2+ cations,

while the second maximum arises from Nf+ reduction. Experimentally, to confinn this,

TPR curves for some samples were recorded up to the temperature just immediately

after the first reduction peak, and subsequently recorded the PXRD profile of the

residue. It shows exc lusively the diffraction lines of Cu (JCPDS ; 4-0836)

substantiating the above conclusion. Moreover, while reduction of cJ+ has been

completed at ca. 300°C, it is necessary to reduce the samples above 4S0-S00°C to

achieve complete reduction ofN!'+ species. A closer look at the reduction

50

Table 1.3 Quantitative results from TPR experiments

Sample CuNiAI2-1 CuNiA13-1 CuNiA14-1

6550 7396 7457

(Ni+Cu) 6

6184 6724 7415

CuNiA13-0.2 7576 6849 CuNiA13-5 7599 6922

'Ilmol H2 consumed Ig sample

bllmol (Cu+Ni)/g sample 'temperatures of the maxima d±lOoC

270 266 268

T2C,d

293 280 271

275 327 265 315

1.43 1.07 1.38 0.24 4.00

'ratio between integrated areas of the sharp and the broad peak (see text)

temperature for nickel, h (Table 1.3) for samp les CuNiAI2-1 to CuNiAI4-1 indicated a

decrease in its value with a decrease in aluminum content. This could probably be due

to dissolution of more aluminum in "in situ" generated NiO lattice (rock-salt structure)

to form some solid solution thereby hindering the reducibility of nickel. Similar

observation was brought out earlier by Rebours et aI., (22), for Ni-AI hydrotalcites by

perceiving a "decorated" three-phase model. Further TI (reduction temperature of

copper) was not affected much irrespective of the variation in nickel or aluminum

content. In other words, there is a facile thermodynamic nucleation and growth of "in

situ" generated CuO islands preventing the diffusion of At+ thereby avoiding the

possibility of formation of such solid solutions. A weak shoulder observed for

CuNiA13-5 corroborates the John-Teller effect of copper wherein copper may possibly

present in two different symmetric environments.

1.3.7 Specific SurJace Area and Porosity Assessment

Nitrogen adsorption-desorption isotherms for all five samples are plotted in

Fig.1.7. The curves belong to Type II in the IUPAC's classification (23), and show a

hysteresis loop closing at ca. 0.45 (P/Po). These curves indicate that all samples are

mesoporous, without micropores, i.e., the nitrogen molecules are unable to penetrate

51

the interlayer space of the hydrotalcites. The V-t plots (24) lead in all cases to straight

.. OJ) ~

f-<

'" Z ~ e ..... '0 • ~

300

2'0

200

uo

100

50

0

0 0.2 0.4 0.6 0.8 1

PlPo

Fig. 1.7 Nitrogen adsorption-desorption isotherms (-196°C) of (a) CuNiAI2-1, (b) CuNiA13-I, (c) CuNiAI4-1, (d) CuNiA13-0.2, (e)

CuNiA13-5 Bottom (a), Top(e) (0) adsorption; (e) desorption.

lines passing through the OrIgm when extrapolated, confirming the absence of

micropores

Table 1.4 Specific surface area and pore volume of the samples

Sample SSETa Veb CuNiAI2-1 105 0.29 CuNiA13-1 77 0.24 CuNiA14-1 50 0.17 CuNiA13-0.2 87 0.26 CuNiA13-5 89 0.23

'm2/g

b mlliquid N2/g

measurable by 1'2 adsorption. The pore size distribution curves show in all cases a

bimodal curve, with maximum contribution by pores with a diameter close to 35 A, and

an underlying contribution by pores with a diameter between 40-60 A. The values for

52

the specific surface areas, as calculated following the BET method (SBET), are given

in Table 1.4 together with total pore volume.

1.4 PHYSICOCHEMICAL CHARACTERIZATION OF CALCINED

SAMPLES

In hydrotalcite, the cations are homogeneously distributed and, upon their

calcination, result in well-dispersed mixed oxides with better physicochemical

properties than by starting from the pure mixed oxides using conventional solid state

route. In this study, the samples are calcined in air at 500°C for 2 h (heating rate up to

500°C was 10°C/min), i.e., just after the main endothermic effects. In addition, samples

CuNiA14-l and CuNiAI3-5 have been also calcined at 850°C in order to identity the

origin of the thermal effects recorded at high temperature. The samples are named as

CuNiAlX-Y-T, where T stands for the calcination temperature, in 0c.

1.4.1 Powder X-ray Diffraction

The PXRD patterns for the calcined samples are shown in Fig. 1.8. It has

already been reported for other hydrotalcites (lO,25,26) that upon calcination at this

"intermediate" temperature gives rise to solids mostly amorphous, with rather weak and

broad diffraction peaks. In our case, three peaks are recorded close to 2.42, 2.07, and

1.47 A, positions coincident with the main diffraction maxima of bunsenite (NiO,

JCPDS; 4-0835). However, for sample CuNiA13-5, in addition, sharper maxima is also

recorded, whose positions coincide with those of tenorite (CuO, JCPDS ; 5-0661). It

should be noted that sample CuNiA13-5, is that with the highest copper content, while

for the other samples the Cu content slightly exceeds that of nickel. In the other

samples the maxima due to CuO (if existing) should be included in the broad tails of

the main peaks of NiO. The change in the relative intensities of the peaks recorded

close to 2.42 and 2.07A (36 and 43°, 28, respectively) is undoubtedly due to the change

53

in the relative content of Ni and Cu. Jobbagy et aI., (27), have reported the presence (as

concluded from ~ray diffraction) of bunsenite as a single crystalline phase, where

Cu2+ ions should be dissolved up to a copper concentration ofx = 0.35, upon

'" '" c o Co

'" ~ ... £ <.J

'" -.. Q

o 20

2.42 2.07 1.47

40 60 80

2 Theta (degrees)

Fig. 1.8 Powder :X-ray diffraction pattern of (a) CuNiAI2-1, (b) CuNiAI3-1, (c) CuNiAI4-1, (d) CuNiA13-0.2, (e) CuNiA13-5 calcined at 500°C in air for 2h. Bottom (a), Top (e)

calcination of a N il _xCux(OH)2 precursor, without forming any discrete Cu-containing

phase. This is in contrast to our findings, where CuO segregate as a discrete phase,

probably due to the variation in the preparation methodology employed by them. In our

case, no peak was recorded which could be ascribed to Al-containing phases. This is

not unexpected, as usually at 400-500°C only the diffraction peaks of the divalent metal

oxide are recorded, although the positions do not exactly match with those reported for

the pure oxides, and it is generally assumed that At'+ cations are dissolved in the

M(II)O matrix; when the calcination temperature is increased (850-1000°C),

crystallization of the M(II)AI20 4 spinel is usually observed.

54

The PXRD patterns for samples calcined at 850°C are shown in Fig. 1.9. In this

case, crystallization of well-defined phases can be observed. All peaks recorded can be

ascribed to the presence of tenorite (CuO), bunsenite (NiO) or NiAI20 4 spinel (JCPDS;

10-0339). No difference between the natures of the crystalline phases can be seen for

different samples, although differences exist in the relative intensities of the

corresponding maxima, in agreement with the relative content of the different metal

cations. For sample CuNiA13-5-850, the main diffraction maxima recorded correspond

to CuO while for sample CuNiAI4-1-850 the main peaks are due to both CuO and NiO.

~

= ,; ~

" '" = o c.. '" " .. .. .s C,I

~ " ~

2 Theta (degrees)

Fig. 1.9 Powder X-ray diffraction pattern of Bottom CuNiAI4-1, Top CuNiA13-5 calcined at 850°C.

1.4.2 Specific Suiface Area and Porosity Assessment

Specific surface areas and total pore volumes for all six samples calcined at

500°C are included in Table 1.5 For all samples, except for CuNiA13-5-500, an

increase in both variables is observed upon calcination, compared to the values for the

uncalcined samples. This can be explained (9) assuming that removal of water and CO2

during calcination leads to fonnation of channels and pores (chimneys), thus

55

accounting for an increase in specific surface area. On the contrary, the value fir

sample CuNiAI3-5-500 coincides, within experimental error, with the value for the

uncalcined sample, despite both display

Table 1.5 Specific surface area and pore volume of calcined samples

Sample SBETa VEb CuNiAI2-1-500 123 0.32 CuNiAI3-1-500 122 0.36 CuNiAI4-1-500 105 0.33 CuNiA13-0.2-500 167 0.46 CuNiAI3-5-500 90 0.31

a m2 g-I

b ml liquid Nz g-I

different PXRD patterns. Both samples possessing the same specific surface area is

simply a coincidence; the rather "low" value for sample CuNiA13-5-500 can be

originated by the incipient crystallization of CuO, as shown by the PXRD patterns.

1.4.3 Temperature Programmed Reduction

The TPR curves for the samples calcined at 500°C are shown in Fig. 1.10.

16

14 I-

12 I-

10

o 200 400 600 800 1000

Temperature ("C)

Fig. 1.10 Temperature programmed reduction (TPR) curves of (a) CuNiAI2-1, (b) CuNiAI3-1, (c) CuNiAI4-1, (d) CuNiA13-0.2, (e) CuNiA13-5, calcined at 500°C in air for 2h Bottom (a), Top (e)

56

They are rather similar to those obtained for the corresponding uncalcined samples, but

a detailed analysis of the curves indicates that, despite the reduction maximum assigned

to reduction of copper is recorded still in the same position, the broad maximum

ascribed to reduction of Ni2+ shifts towards high temperatures in all cases. In other

words, Ni2+ species are more difficult to reduce in the calcined samples. This is

probably due to slow diffusion of AIl+ cations into "in situ" generated NiO matrix

(kinetic control) with an increasing temperature during TPR of fresh sample, while the

same process is facilitated through prior calcination resulting in a well defined Ni-AI

oxide solid solution, thereby preventing the accessibility of the reducing molecules to

nickel oxide surface (20).

1.5 CATALYTIC STUDIES

Hydroxylation of phenol was carried out in a two-neck glass reactor (50ml)

fitted with a condenser and a septum. Hydrogen peroxide (30% w/v) was added through

septum at once to the magnetically stirred solution of phenol containing catalyst kept at

the desired reaction temperature. The course of the reaction was monitored by taking

periodically small samples (0.05cm\ and analyzed by gas chromatography (GC) fitted

with OV-17 packed column (2m x 4mm i.d.), using flame ionization detector. To avoid

the possible reaction of H20 2 in the injection port of the GC, H20 2 was chemically

decomposed, assuming its presence in the system. Quantification was done after

considering the response factors of reactants and products, whose retention times were

determined using authentic samples, derived using standard mixtures containing both of

them. The conversion of phenol was also measured for the reactions carried out with

high substrate:catalyst ratios, by adding an external standard, here o-cresol, which

showed a variation of 5-10% (than obtained through direct analysis) probably due to tar

formation, depending on the nature of catalysts.

57

1.5.1 Effect of Different Metal Ion Concentration

Table 1.6 summarizes the activity of various catalysts studied for phenol

hydroxylation. On all catalysts, catechol and hydroquinone were formed as the major

products. It is clear that among the catalysts srudied, CuNiAll-l showed poor activity

indicating the influence of phase purity. Among the catalysts studied, CuNiA13-5 and

CuNiAI2-1 showed the maximum conversion, with nearly 50% aOz selectivity, and

were selected for further investigations. This high activity could be due to the large

concentration of copper in the former, while a larger specific surface area of the later

catalysts. This is further substantiated taking into account that a decrease in the copper

concentration of the catalyst having a similar (Cu+Ni)/AI atomic composition

decreased the activity. Further, variation in the copper concentrati:ll1 affected the

product distribution: hydroquinone is more

Table 1.6

Catalyst'

Specific surface area and phenol hydroxylation activity of various catalysts studied

Cony. (%)

Product distribution (wt. %) HzOzSelcc. (% )'

Cat HQ CatIHQ CuNiAll-l 14.8 9.5 5.3 1.8 29.6 NAd

CuNiAI2-1 22.6 14.6 8.0 1.8 45.2 105 CuNiA13-1 17.3 11.2 6.1 1.8 34.6 77 CuNiAI4-1 17.7 11.1 6.6 1.7 35.4 50 CuNiA13-S 23.7 14.1 9.6 1.5 47.4 89 CuNiA13-0.2 10.7 8.2 2.5 3.3 21.4 87

a --> Phenol - LOg; PhenoVHzOz (mole) = 2.0; Catalyst = lOmg ; Solvent = Water (IOml) ; Temp. = 65°C; Time = 2h; pH = 5.0 b --> Specific surface area

c --> Selectivity of HzOz calculated on the basis of dihydroxybenzenes formed d --> Not applicable as it has impurity phase along with HT-like phase

favored for the catalyst having higher copper concentration, while catechol is favored

for the catalyst having lesser copper concentration. Comparison of CuNiAI2-1 and

CuNiA13-1 indicated that, although they have nearly similar copper concentrations, the

activity of the former is higher owing to its larger specific surface area (Table 1.6).

58

Phenol hydroxylation over pure NiAI hydrotalcite did not result in measurable

conversion corroborating the necessity of copper in driving the reaction. These results

substantiate that copper is the active center involved in the hydroxylation reaction,

whose surface concentration probably controls the overall activity and selectivity of the

reaction.

1.5.2 EfIect of Substrate:Catalyst Ratio

An interesting observation was noted wherein the conversion of phenol

decreased with an increase in the weight of the catalyst, as illustrated in Fig.I.11 for

CuNiAI3-5. This could presumably be due to the spontaneous formation of coke

(reaction mixture turned dark black), a high catalyst concentration, obtained through

consecutive reactions of the primary products (which also consumes hydrogen

peroxide). To verify, coke formed was estimated for the reaction (substrate:catalyst

ratio of 10:1) and observed around 0.2-0.3g of coke per gram of catalyst (calculated

after taking due consideration of weight loss due to catalyst upon calcination in air).

When once coke is formed in the reaction, it masks the active centers of the catalyst

and thereby inhibiting the access of the reactant molecule to further undergo the

reaction. This is similar to the observation by Figueras and coworkers (28) for

faujastes, who reported that the yield of dihydroxybenzenes was unaffected by the mass

of the catalyst for FAU IS (SiiAI = 15), while it continuously decreased with an

increase in the mass for FAU 2.5 (SiiAI = 2.5). They claimed that dealumination results

in an enhancement of both acidity and mesoporosity facilitating the diffusion of

primary products and in tum preventing consecutive reactions. However, such an

Increase in the activity, in our case, with an increase in substrate:catalyst ratio was

noted to a certain extent and decreased with a further increase in the ratio (more than

100). This could probably be due to the decrease in the tumber of available active

59

centers for the reaction. A similar observation was noted for the other catalysts as well.

An optimum substrate: catalyst ratio of 100 was selected for further studies.

25

20 ~

~ 0 ~

= 15 .~ ~ ... ..

10 > = 0 U

5

0

4 10 20 40 100 200

Substrate:Catalyst ratio

Fig. 1.11 Variation of conversion of phenol with substrate:catalyst ratio

1.5.3 Effect of Reaction Temperature

Phenol hydroxylation reaction was carried out over these two catalysts

(CuNiA13-5 and CuNiAI2-1) with substrate: catalyst ratio of 100 in the temperature

range 30-90°C (Table 1.7). The conversion increased markedly with an increase in the

reaction temperature up to 65°C, with a stronger influence for CuNiA13-5. Furthermore,

the catechollhydroquinone (CatlHQ) ratio decreased for CuNiAI3-5 with an increase in

reaction temperature, in other words, facilitating the formation of hydroquinone. A

further rise in the reaction temperature decreased the conversion of phenol and could be

ascribed to the competitive thennal decomposition of H,O, at higher temperature

without being involved in the desired reaction. The decrease in the conversion was also

more evident for CuNiA13-5 than for CuNiAll-2. The presence of an optimum

concentration of nickel may render stability to the catalyst in sustaining the activity

even with subtle variations in the reaction conditions. It is well known that oxidation

60

activity of nickel catalysts is relatively poorer than that of copper containing catalysts,

but they possess a longer catalyst life (29). These results suggest that although copper is

the active center involved in the hydroxylation reaction, the possible variation of the

Table 1.7 Effect of the reaction temperature for phenol hydroxylation over CuNiA13-5 and CuNiAI2-1 (conditions as given in Table 1.6)

Catalyst Temp. Conv. Prod uct distribution H,O, C'C) (%) (wI. %) Selec. (%)

Cat. HQ Cat/HQ CuNiAI3-5 30 4.3 2.7 1.6 1.6 8.6

50 16.0 9.7 6.3 1.5 32 65 23.7 14.1 9.6 1.4 47.4 80 21.2 12.6 8.6 1.5 42.4 90 17.4 10.5 6.9 1.5 34.8

CuNiA12-1 30 7.4 4.3 3.1 1.4 14.6 50 21.9 14.2 7.7 1.8 43.6 65 22.6 14.6 8.0 1.8 45.2 80 20.4 12.4 8.0 1.5 40.8 90 20.6 12.3 8.3 1.5 41.0

geometric andlor electronic environment around copper in these catalysts differentiates

the overall course of the reaction.

1.5.4 Effect of the Reaction Medium and Oxidant

In order to study the influence of the reaction medium on phenol hydroxylation,

the reaction was perfonned in various solvents (other than water), namely acetone,

acetonitrile, ~butanol, THF and DMF. On both catalysts, none of the solvents other

than water showed measurable conversion. This fact (water behaves as a good solvent

for this reaction over these catalysts) is beneficial in both industrial and environmental

perspectives. In other words, the proximity of the hydroxylating agent and of the

substrate molecule on or near the active catalyst site is essential for driving the reaction.

In water, both phenol and H,O, dissolve simultaneously and approach the active center,

thereby generating hydroxy radicals, thought to be the active species involved in the

hydroxylation reaction. Moreover, such an electrophile is easier produced and

61

stabilized in water than in organic solvents. Possibly, the lack of a hydroxylated nature

for the other organic solvents may be responsible for the non-occurrence of this

reaction. Furthermore, the reaction was carried out using different oxidants other than

H20~ namely oxygen, air and t-bulylhydroperoxide. None of these oxidants

hydroxylated phenol to a significant extent under similar reaction conditions, possibly

due to the lack of generation of the active oxidant species and the solubility problems

associated with the reaclant and the oxidant.

1.5.5 Effect afpH of the Medium

Table 1.8 summarizes the effect of pH on the hydroxylation reaction using

water as solvent. No significant difference in the activity was observed between pH =

5.0 (which is the pH of the medium without any adjustment) and pH = 7.0. This is in

contrast to the results reported by Zhu el aL, (30) for copper containing hydrotalcites

who claimed a maximum conversion at pH = 7.0, while at other pH a decrease or a lack

of activity was observed. The higher activity at pH = 5.0 could possibly be due to a

better stabilization of hydroxy radical. Further, when the pH was measured after the

reaction, a decrease by one unit was observed (pH = 4.0), possibly due to ~neration

Table 1.8

Catalyst

CuNiA13·5

CuNiAI2-1

Influence of pH of the reaction medium for phenol hydroxylation over CuNiA13-5 and CuNiA12-l (conditions as given in Table 1.6)

Temp. PH Cony. Product distribution H,O, (0C) (%) (wt. %) Selec. (%)

Cat. HQ CatIHQ 65 5 23.7 14.1 9.6 1.4 47.4 65 7 19.2 12.1 7.1 1.7 38.4 65 9 0 0 0 0

80 5 20.4 12.4 8.0 1.5 40.6 80 7 20.2 12.3 7.9 1.5 40.5 80 9 0 0 0 0

of organic acids, which might have been formed through consecutive oxidation of

dihydroxybenzene. However, a careful IH NMR study of the reaction mixture was

62

done, which did not reveal proton corresponding to organic acids. Further, for our

samples also at pH = 9.0, no activity was observed. This could be due to generation of

HO' species, which would approach the active center thereby inhibiting the adsorption

of hydrogen peroxide, and in tum affecting the generation of hydroxy radicals.

1.5.6 Effect of Reaction Time

Fig.1.l2 shows the variation in the conversion of phenol over CllNiA13-5 at

different time intervals. It is clear from the figure that around 90% of the conversion

was achieved in the first 10 min, while a further increase in the reaction time did not

significantly enhance the conversion. This is in contrast to the bemvior of many

zeolite-based catalysts, which either showed a progressive growth in the conversion of

phenol or exhibited an induction time (31). No significant variation in the Cat/HQ ratio

was observed with the reaction time, suggesting the lack of inter-conversion

25,-----------------------------------,

§ 15 "f ~10 = 8 5

o 20

-+-%Conv.

-'-Cat/HQ

40 60 80 100 120 Time (min)

Fig. 1.12 Effect of the reaction time on phenol conversion and CatIHQ ratio

and/or consecutive reactions. In one of our experiments, catalyst was filtered after

30min of reaction and monitored the progress of the reaction in the absence of catalyst.

This was done to verifY whether leaching of metal ions had occurred by H,02 into the

aqueous phase during the reaction, which can by themselves act as active homogenous

63

centers. However, no change in the conversion of phenol with time was observed after

filtering the catalyst, confirming the non-leaching of metal ions from the HT-like

lattice.

1.5.7 Effect of Substrate:Oxidant Ratio

Variation of the substrate:oxidant ratio indicated that the conversion increased

with a decrease in the ratio for both the catalysts, whose values are summarized in

Table 1.9. Further, the increase in the conversion is nearly proportional to the increase

in H202 concentration for both catalysts, maintaining similar selectivity for

dihydroxybenzenes, corroborating the fact that the added ~02 is essentially used for

the desired hydroxylation reaction and not for the undesired consecutive reactions. In

addition, a continuous increase in the CatlHQ ratio was observed for these

Table 1.9 Influence of substrate:oxidant ratio for phenol hydroxylation over CuNiAI3-5 and CuNiA12-1 (conditions as given in Table 1.6)

Catalyst Sub:Oxi Cony. Product distribution (wI. %) H2 0 2 Ratio (%) Selec. (%)

Cat. HQ CatlHQ CuNiAI3-5 3: I 13.8 7.5 6.3 1.2 41.2

2:1 21.2 12.6 8.6 1.5 42.1 I: I 41.4 25.6 15.8 1.6 41.0 1:2 60.9 38.8 22.1 1.7 30.5

CuNiAI2-1 3:1 11.4 6.7 4.7 1.4 36.5 2:1 20.2 12.3 7.9 1.5 40.3 l:l 40.3 25.0 15.3 1.6 39.8 1:2 55.8 35.6 20.2 1.7 27.9

catalysts with a decrease in this ratio, indicating the preferential formation of catechol

at higher ~02 concentration. CuNiAI3-5 exhibited a maximum activity yielding 61%

conversion of phenol with a CatlHQ ratio of 1.8 at 65°C employing 1:2

substrate:oxidant ratio. The activity of this catalyst is comparable to that of some

titanium based zeolite catalysts reported in literature (32) indicating a promising role of

HT-based catalysts for commercial exploitation.

64

1.5.8 Effect of Calcination Temperature

Calcination of these materials results in non-stoichiometric mixed metal oxides

whose phase and crystallinity are influenced by their precursor chemical composition

and the calcination temperature. Fig. I. 13 shows the PXRD patterns of CuNiAI2-1

calcined at different temperatures. It is clear that calcination at 400°C leads to the

destruction of the HT-like network, giving rise to a diffuse amorphous pattern with

three peaks recorded close to 2.42, 2.07 and 1.47 A, positions coincident with the main

diffraction maxima ofbunsenite (NiO, JCPDS ; 4-0835).

2000 I

Fresh

I,.

'-_--'_,---:'-----'''--- 400

"'---'. ~- 600

800

o w w ~ • ~ w m ~

2 Theta (degrees)

Fig. 1.13 PXRD pattern ofCuNiAl2-1 calcined at different temperallres

An increase in the calcination temperature up to 8l0DC enhanced the crystallinity of

M(II}-O phases, as evidenced by the sharpening of these peaks. When the calcination

temperature is further increased, say at 800DC, crystallization of the M(II)AbO, spinel

occurred along with NiO. In addition, sharper maxima whose positions were coincident

with those of tenorite (CuO, JCPDS; 5-0661) were also observed. However, the

concentration of these phases varied, as evidenced from the relative intensities of the

corresponding maxima, in relation with the content of the different metal ions in the

65

parent HT -like lattice. Accordingly, for CuNiAI2-1, the main diffraction maxIma

recorded corresponded to the spinel with a weak CuO phase, while the reverse was

observed for CuNiA13-5. Hydroxylation of phenol was carried out over hydrotalcites

calcined at different temperatures yielding the results summarized in Table 1.10.

CuNiAI3-5 calcined at 150,400 and 600°C showed a similar conversion (-12%) while

the sample calcined at SOO°C exhibited a higher conversion (IS%). In the case of

CuNiAII-2, a continuous increase in the conversion with an increase in the calcination

temperature was observed. The higher activity of the high temperature calcined

catalysts (SOO°C) could be due to the inherent activity of spinel phase, which

crystallizes around this temperature, in mediating the reaction. Further, no significant

difference in the activity of these high temperature calcined catalysts was observed

(irrespective of the composition), confirming the stable activity of the spinel phase.

Furthermore, the specific surface areas of these catalysts were nearly the same,

validating the above observation. Fig.1.14 shows the variation in the BET specific

surface area of these samples with the calcination temperature. An increase in the

Table 1.10 Influence of calcination temperature for phenol hydroxylation over CuNiA13-5 and CuNiAI2-1 (conditions as given in Table 1.6)

Catalyst Calc.temp. Cony. Product distribution H20 2

(C) (%) (wt. %) Selee. (%) Cat. HQ CatlHQ

CuNiAI3-5 ISO 13.3 6.6 6.7 1.0 26.S 400 12.0 5.1 6.9 0.7 24.0 600 lOA 4.3 6.1 0.7 20.8 800 1804 10.9 7.5 1.5 36.0

CuNiAI2-1 150 lOA 5.9 4.5 1.3 20.8 400 12.8 7.2 5.6 1.3 25.5 600 15.6 804 7.2 1.2 31.2 800 18.9 11.0 7.9 104 36.6

specific surface area was observed with an increase in the calcination temperature up to

400°C for the CuNiAI2-1, while nearly similar specific surface areas were observed for

66

CuNiAI3-5 up to 600°C. The observed activity trends exhibited by these catalysts

could possibly be correlated to the variation in their specific surface areas. The

influence of the specific surface area of the mixed metal oxides as well the stability of

the spinel phase in the oxidation of aqueous solution of phenol has been emphasized

earlier (29). HO\\ever, the conversion observed for the calcined catalysts is lower than

that observed for fresh (uncalcined) hydrotalcites. This could possibly be due to the

crystal phase transformation coupled with the loss in the specific surface area.

If one were to ronsider the mechanism of hydroxylation reaction over these

materials, similar to Fenton's reaction, hydroxy radicals could participate as an

electrophile in this reaction. The hydroxy radical may be generated through oxidation

of Cu'+ ion in the HT-Iattice by ~02, which subsequently attack at ortho and para

positions of phenol resulting in the desired dihydroxybenzenes, respectively, catechol

and hydroquinone. To verify the participation of the hydroxy

150

~ N

E 120 ~ .. .. .. .. 90

" '" .;:! .. 60 " VJ

'" f;: .;:; 30 " Q.

VJ

0 0 200 400 600

Temperature ( 'C)

__ CuNiAI2-1

~CuNiAI3-5

800 1000

Fig. J.I4 Variation of BET surface area with calcination temperature

radical, effect of the added alcohol, which is a well-known scavenger for the hydroxy

radical (33), on the conversion of phenol was monitored for CuNiA13-5, as elucidated

67

in Fig.l.l5. A nearly linear plot showing a decreasing conversion with an increase in

ethanol concentration was observed confinning the mediation of hydroxy radical. A

25 ~

~ • 20 ~

c 15 .:: ~ ...

10 " .. c 0 5 U

0

-0.01 0.01 0.03 0.05 0.07 0.09

EtOH:H,O Mole ratio

Fig. 1.15 Variation of conversion of phenol with EtOH: H.?O mole ratio for CuNiA13·5 (conditions as given in Table 1.6)

further increase in the concentration of ethanol leads to a complete drop in the activity.

Possible reaction pathways for the hydroxylation reaction over these catalysts are

summarized in Scheme 1.1. It can be assumed that, a structural arrangement of the

substrate molecule and of an active oxidant species on the catalyst surface along with

residence time of the hydroxylated products control the overall activity and selectivity

of this reaction.

68

Scheme 1.1

Generation of electrophile

OH

Ho,,---I /OH Cu2+ + H202 •

HO/ I "---OH

OH

(I)

Attack of electrophile

~)i--~" •

(00H •

Regeneration of active site

H202 + ,OH

(2) + H02' •

OH

Ho,,---I ~H Cu3+

He! I "---OH

OH

(2)

OH &0" Catechol

OH

¢ OH

Hydroquinone

H02' + H20

(1) + 02 +H+

+ HO- + HO·

Attack at o-positior

Attack at p-position

69

1.6 CONCLUSIONS

1. CuNiA~ ternary hydro tal cites with different atomic compositions of

(Cu+Ni)/AI and CulNi were synthesized by coprecipitation method. PXRD

pattern of all the samples (where M(II)/M(IIIJ::: 2) shov-ed the presence of

hydrotalcite phase. However, the diffraction pattern of the sample CuNiAII­

I (M(II)/M(III) = I) showed a broad and il~defined HT-like phase along

with impurity phases.

2. Calcination of these materials at 500°C leads to mostly amorphous s:Jlids

containing predominantly CuO (tenorite) for samples containing larger

concentration of copper and NiO (bunsenite) for samples with larger

concentration of nickel. Crystallization of well defined phases (NiO, CuO

and NiAb04 (spinel) were observed when the calcination temperature

increased to 850"C.

3. Phenol hydroxylation was carried out over these catalysts using water and

Hz02 as solvent and oxidant respectively. On all the catalysts, catechol and

hydro quinone were formed as major products.

4. An increase in the conversion was observed with an increase in the copper

concentration and between the samples having similar copper concentration,

that with higher specific surface area yielded better activity.

5. Among the samples calcined at different temperatures those calcined at

800°C showed maximum conversion. However the conversion of calcined

samples were less than the fresh samples (uncalcined).

6. Electrophilic attack of the hydroxy radical is proposed to be the reaction

pathway for the formation of dihydroxybenzenes over these catalysts.

70

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