38

CHAPTER 2

EXPERIMENTAL

2.1 CHEMICALS

The following chemicals were purchased and used as received for

the synthesis of support, catalysts and for sulfidation purpose. Tri-block

copolymer (Pluronic P123, EO20PO70EO20, molecular weight = 5800,

Aldrich), aluminum isopropoxide (Aldrich, 98%), and tetraethyl orthosilicate

(TEOS, Merck, 99%), hydrochloric acid (Merck, 35%), sodium metasilicate

(Qualigens, AR Grade), aluminum sulfate (E-Merck, AR Grade, India),

cetyltrimethylammonium bromide (CTAB, OTTO Chemie), ammonium

paramolybdate (Finar, AR, 98%), nickel nitrate (Aldrich, 98%), ammonium

fluoride (Spectrochem, 99%), ortho-phosphoric acid (Finar, 85%),

isopropanol (SRL, India Ltd, 99%), carbon disulfide (Spectrochem, 99.5%),

dimethyl sulphide (Spectrochem, 99%), dimethyl disulfide (Aldrich, 99%),

di-tert-butyl polysulfide (Aldrich).

Methylcyclohexylamine (MCHA, Aldrich, 98%) was purchased

and purified by vacuum distillation method prior to use. Hydrogen sulfide was

generated in a Kipp’s apparatus by the reaction between ferrous sulfide and

diluted hydrochloric acid (1:1). H2S gas generated was bubbled through water

and then introduced into the system. High pure nitrogen and hydrogen gas

was obtained from Indo gas limited; the gases were passed through silica gel

to absorb the moisture during the reaction and pretreatment of the catalysts.

The commercial -Al2O3 support was purchased from Sasol.

39

2.2 SYNTHESIS OF SUPPORT

2.2.1 Synthesis of Si-SBA-15 and Al-SBA-15 Support

The Si-SBA-15 material was synthesized using Pluronic P123

block copolymer (EO20PO70EO20) as the structure directing agent and

tetraethyl orthosilicate (TEOS) as the silica source. In a typical synthesis, 12 g

of Pluronic P123 was dispersed in calculated amount of distilled water and the

resultant solution is mixed with 2 M HCl at 313 K. Then 25.5 g of TEOS was

added to the above solution. The mixture was stirred at 313 K for 24 h and

then aged at 373 K for 48 h without stirring. The obtained solid product was

recovered by filtration, washed with deionized water and dried at 353 K

overnight. Calcination was carried out in static air at 823 K for 6 h

(Zhao et al 1998b). This SBA-15 silica was used as a parent material to

prepare aluminium containing SBA-15 via post-synthetic grafting of

aluminium (Baca et al 2008). In the preparation of aluminium containing

SBA-15 with different Si/Al (10, 20, 30 and 40) ratios, the Si-SBA-15

material was treated with calculated amount of aluminium isopropoxide

dissolved in isopropanol; the obtained product was filtered and washed with

isopropanol, dried and calcined at 773 K for 5 h.

2.2.2 Synthesis of AlMCM-41 Support

AlMCM-41 was synthesized hydrothermally. Sodium metasilicate

was used as the silica source. 10.6 g of sodium metasilicate and 0.157 g of

aluminium sulfate were dissolved in 60 g of water and then thoroughly stirred

until a clear solution was obtained. Cetyltrimethylammonium bromide (3.36 g)

was dissolved in 20 g of water. To this solution, the mixture of sodium

metasilicate and aluminium sulfate was added drop wise. The final mixture

was stirred for 1 h, and then the pH of the resulting gel was adjusted to 10.5 to

11 using 4 N sulfuric acid followed by stirring for 3 h. The resulting

40

homogenous solution was transferred into an autoclave and heated to 413 K

for 20 h. The resulting precipitate was recovered by filtration, washed with

deionized water, dried in air at ambient temperature and finally calcined at

823 K for 1 h in a nitrogen flow and for 12 h in air flow. The sodium form of

AlMCM-41 material was converted to protonated form by reflexing with 1M

ammonium chloride at 353 K for 12 h followed by calcination at

773 K for 6 h (Sardhar Basha et al 2006).

2.2.3 Synthesis of Fluorine Modified SBA-15 Support

1 g of the parent SBA-15 was treated with 250 ml of different

molar concentrations (0.5, 1, 2 and 4 M) of an aqueous solution of ammonium

fluoride at room temperature for 30 h for the synthesis of fluorine modified

SBA-15. The resultant solution was filtered, washed with distilled water and

dried at 353 K overnight and calcined under air at 773 K for 6 h

(Luque et al 2005).

2.2.4 Synthesis of Phosphorus Modified SBA-15 Support

Phosphorus modified SBA-15 supports was synthesized by the

direct addition of various concentrations (0.5 – 4 M) of ortho phosphoric acid

to the solution containing P123 and tetra ethyl orthosilicate at 313 K. The

mixture of the solution was subsequently treated at 313 K for 24h under

constant stirring. The resulting mixture was transferred in to Teflon bottle and

hydrothermally treated at 373 K for 24 h without stirring. The solid product

obtained was then filtered, washed with distilled water and dried at 363 K for

12 h in air atmosphere. Template was removed by calcination in air. First the

dried solid product was heated at 523 K for 3 h and then the temperature was

raised to 823 K and further calcined for 4 h (Colilla et al 2007).

41

2.3 PREPARATION OF CATALYSTS

2.3.1 Preparation of NiMo/Al-SBA-15 Catalyst

Ni-Mo impregnated catalysts were prepared by successive wet

impregnation of Al-SBA-15 with aqueous solution of ammonium

paramolybdate and nickel nitrate as source of MoO3 and NiO. In the present

study, three series of NiMo/Al-SBA-15 catalysts have been synthesized. For

the first series, Al-SBA-15(10) was first impregnated with a constant amount

(3 wt.%) of NiO followed by various amount of MoO3. These catalyst are

designated as reverse order catalyst (x wt.% MoO3-3 wt.% NiO/Al-SBA-

15(10) where x= 8, 12, 18 and 24) in the text. After each impregnation, the

catalyst was dried at 383 K for overnight and calcined at 823 K for 6 h.

Second series of catalyst was prepared by reversing the order of

impregnation. Al-SBA-15(10) was first impregnated with MoO3 followed by

constant amount of NiO. This catalyst is designated as normal order catalyst

[3 wt.% NiO-x MoO3/Al-SBA-15(10) where x = optimized amount of MoO3].

Another catalyst was prepared with 3 wt. % NiO and optimum loading of

MoO3 by co-impregnation method. For this the precursor solutions of Mo and

Ni are impregnated simultaneously and represented as (3 wt.% NiO. x wt.%

MoO3)/Al-SBA-15(10) where x = optimized amount of MoO3.

The third series of catalyst were prepared by the impregnation of

constant amount of NiO (3 wt. %), followed by optimum amount of MoO3

over the supports with various Si/Al ratios (Si/Al = 20, 30, 40) and pure

Si-SBA-15.

2.3.2 Preparation of AlMCM-41 and -Al2O3 Supported NiMo Catalyst

High surface area AlMCM-41 supported Ni-Mo catalyst was

prepared by wet impregnation of 7 wt. % NiO (nickel nitrates) and 24 wt. %

42

MoO3 (ammonium paramolybdate) was impregnated. After each

impregnation, the catalyst was calcined at 773 K for 6 h. For comparison

-Al2O3 supported NiMo catalyst was also prepared by wet sequential

impregnation procedure using 3 wt. % of NiO and by 12 wt. % of MoO3.

2.3.3 Synthesis of F-SBA-15 and P-SBA-15 Supported NiMo

Catalysts

The NiMo supported on fluorine and phosphorous modified

SBA-15 catalyst was prepared by the sequential wet impregnation of 3 wt.%

NiO followed by 12 wt.% MoO3 over fluorine and phosphorous containing

SBA-15.

2.4 SULFIDATION OF THE CATALYSTS

All the catalysts were sulfided prior to the reaction. For the

sulfidation, the catalysts was packed inside the reactor between the Pyrex

glass wool and activated for 4-5 h at 773 K with CO2 free air in order to

remove the moisture present inside the catalytic system, and then the catalyst

was flushed with high pure nitrogen for 20-30 minutes to remove the air

inside the reactor tube. Then the catalyst was sulfided in the flow of

di-tertiary butyl polysulfide (DBPS) and hydrogen (1:20 mmol of DBPS:H2)

at 673 K for 3 h.

2.5 CHARACTERIZATION OF THE CATALYSTS

The physico-chemical characteristics of the synthesized supports

and catalysts were analyzed by various techniques. The specification of the

instruments used for the analysis and the analysis conditions are described

below.

43

2.5.1 X-ray Diffraction (XRD)

The mesoporosity and crystalline nature of the SBA-15,

Al-SBA-15(10-40), fluorine and phosphorous modified SBA-15 and NiO,

MoO3 impregnated catalysts were confirmed by low and high angle X-ray

diffraction (XRD) techniques. The XRD patterns were recorded on

BRUCKER D8 diffractometer using Cu K radiation ( = 1.54 Å). The

diffractograms were recorded in the 2 range 0.5 to 60 with a step size of

0.05 . The count time was 4 s over each step. The peak height gives the

intensity of the corresponding peak.

2.5.2 Textural Analysis

N2 adsorption-desorption isotherms were obtained with the help of

Belsorp mini instrument (BEL Japan Inc.) using nitrogen as an adsorbent at

liquid nitrogen temperature. All the catalysts were degassed at 573 K under

the flow of nitrogen gas for 3 h. Helium was used as the carrier gas. From

the monolayer volume of the adsorbed nitrogen, BET surface area was

calculated. The specific surface area was calculated using Brunauer- Emmett-

Teller (Equation (2.1))

1/n [P0/P]-1 = 1/nmc + (c-1)/nmc (P/P0) (2.1)

where ‘n’ is the number of N2 adsorbed at a relative pressure P0 and nm is the

amount adsorbed constituting a monolayer volume surface coverage. The

BET constant ‘c’ is related to the energy of adsorption in the first layer and

consequently its value mirrors the adsorbent-adsorbate interactions. The total

surface area (St) of the sample can be obtained using the following

Equations (2.2) to (2.4)

St = nmAcsN (2.2)

44

nm = Wm/M (2.3)

St = WmNAcs/M (2.4)

where ‘N’ is Avogadro number (6.023x1023 molecules/mol), ‘M’ is the

molecular weight of the adsorbate, Wm is the weight of adsorbate constituting

a monolayer surface coverage, nm is the amount adsorbed constituting a

monolayer surface coverage and Acs is the molecular cross sectional area of

the adsorbate molecule.

The specific surface area (S) of the solid can be calculated from the

total surface area (St) and the sample weight (m) after degassing

S = St /m (2.5)

2.5.3 Thermal Analysis

Thermal analysis is widely used to study the structural stability of

as-synthesised form of molecular sieves. It provides information about the

temperature required for the removal of adsorbed water, decomposition of the

occluded organic templates in the pores and channels of molecular sieves.

Data obtained from TG, DTA and DTG study are useful in evaluating the

thermal properties of molecular sieves. The shape and splitting of the

endotherms (low temperature) helps to identify the location of water

molecules. The temperature at which an exotherm appears in the DTA after

the loss of water molecules, gives helpful information about the temperature

required to remove the template molecules from the pores of the molecular

sieves during calcinations. Phase transformations (if any) can also be

understood from the exotherms obtained at higher temperatures. TGA

samples were performed on Mettler TA 3000 system.

45

2.5.4 Inductively Coupled Plasma-Optical Emission Spectroscopy

(ICP-OES)

Inductively coupled plasma (ICP) optical emission spectroscopy

was used for the determination of the aluminium, molybdenum, nickel and

phosphorous content in each above synthesized samples. The measurements

were performed with a Perkin-Elmer OPTIMA 5300 DV by dissolving the

sample in a mixture of HF and HNO3.

2.5.5 Fourier Transform-Infrared Spectroscopy (FT-IR)

Fourier- Transform infrared spectroscopy has been extensively used

for identifying the various functional groups that are present on the support

and active component of the catalysts. FT-IR spectra were recorded with a

Nicolet Magna 550 IR spectrometer equipped with a MCT detector using 256

scans at a resolution of 4 cm-1. 2 mg of the catalyst in powder form is made as

a thin pellet by mixing 200 mg of KBr in order to be transparent to the

infrared beam. The catalyst powder was pressed into a self-supported wafer

(~15 mg) and placed into Spectra tech insitu IR cell, equipped with CaF2

windows and double walls with a space for cooling agent.

2.5.6 Temperature Programmed Desorption of Ammonia (TPD)

Temperature programmed desorption (TPD) is an important

characterization technique used to determine the acidity of the material. The

TPD experiments were performed on a Micromeritics instrument.

About 50 mg of the sample is pretreated at 500 °C in helium flow (20 cc/min)

for 1 hour, cooled to 100 °C in helium flow. The saturation of the sample was

done by passing 10% NH3 in helium gas for 30 min and the baseline

correction was made by flushing with helium at 100 °C. The sample was

cooled to room temperature in helium flow and the TPD measurements were

46

started from room temperature to 550 °C at a heating rate of 10 °C per

minutes. Desorption of ammonia at different temperature gives information

about the strength of acid sites.

2.5.7 Temperature Programmed Reduction (TPR)

H2-Temperature programmed reduction is a valuable technique to

determine the type and reducibility of the Ni and Mo species present in the

oxidic precursors of the catalysts. The temperature programmed reduction

experiments were performed on a Micromeritics instrument equipped with a

thermal conductivity detector. About 75 mg of the sample was pre-treated at

115 °C in high pure helium gas atmosphere (25 cc/min) for 1h and then

cooled to room temperature in helium atmosphere. Prior to the analysis base

line correction was set to stable at room temperature using 5% H2/Ar

(25 cc/min), and TPR profiles were recorded from room temperature to

850 °C at a heating rate of 10 °C /min.

2.5.8 Nuclear Magnetic Resonance (NMR)

Solid state MAS-NMR spectra of 27Al and 31P can prove the

incorporation of aluminium and phosphorous into the silica matrix of

SBA-15 support. The tetrahedral and octahedral aluminium was clearly

identified using these solid state NMR studies. The spectra were recorded on

a BRUKER DSX 300, NMR Spectrometer with a magnetic field strength of

7.04 Tesla and Al(H2O)63+ and ortho phosphoric acid was used as a reference

compound for aluminium and phosphorous respectively.

2.5.9 Scanning Electron Microscope (SEM)

The morphology of the synthesized samples was investigated using

scanning electron microscopy. The samples were suspended in methanol and

the specimen stub was dipped into the liquid and removed. The catalyst

47

powder was deposited onto the surface of the stub evenly when methanol got

evaporated. This specimen was then coated with gold for 2 min using an ion

sputter coater and viewed under SEM model ESEM Quanta 200 FEI scanning

electron microscopy and Jeol-JSM 5800 instruments operated at 15-25 kV

with TMP + EDAX scanning electron microscope.

2.5.10 FT-Raman Spectroscopy

The FT-Raman spectroscopy is a very powerful characterization

technique for obtaining detailed information about the molecular structure of

the supported catalysts. The FT-Raman spectra of the catalysts were recorded

on Alpha-300 witech using Nd: YAG 532 nm laser with a resolution of about

1 cm-1. The laser power used was 10 mW and thermally cooled CCD is used

as a detector. The spectra were recorded in the range of 100-1200 cm-1.

2.5.11 Diffuse Reflectance Spectroscopy (DRS)

The coordination environments of Ni and Mo in oxide catalysts can

be analyzed UV–Vis DRS spectrophotometer. In the present study, the diffuse

reflectance spectra of the Ni-Mo catalysts were investigated by Shimadzu

2450 over the range of 190–800 nm using BaSO4 as reference. In general, Mo

and Ni are present in two different environments namely octahedral and

tetrahedral. Presence of such environments can be determined from the

positions of the absorption maxima.

2.5.12 Transmission Electron Microscopy (TEM)

Transmission electron microscopic images were traced for the

calcined catalysts on JEOL model 3010 electron microscopes operating at

200 kV. Catalyst sample powder was dispersed on to ‘holy carbon’ coated grids,

which was then introduced to the microscope column, and evacuated to less than

1 10-6 Torr. Specimens were enlarged using thin photographic paper.

48

2.6 ANALYSIS OF SULFUR

The sulfur removed during the course of the reaction from ‘W’ g of

the catalyst is converted to H2S and the exit gas from the reactor was bubbled

through lead acetate solution.

Pb (OAC)2 + H2S PbS +2AcOH (2.6)

The lead sulfide formed was filtered through a previously weighed

G-4 sintered crucible, washed with water and dried. The crucible with the

sulfide was weighed again and the weight of the sulfide was calculated from

the difference in weight.

239.29 g PbS 32.1 g sulfur (2.7)

32.1X g PbS X = Y g sulfur (2.8)

239.28

The sulfur removed per unit weight of the catalyst =Y/W = Z (a) g sulfur (2.9)

The sulfur left behind on the used catalyst was oxidized to

SO3/SO2 by passing carbon dioxide free air at 773 K. The exit gas from the

reactor is a mixture of oxides of sulfur and carbon (carbon deposited during

the reaction). The mixture of gases was bubbled through an aqueous solution

of hydrogen peroxide (30 Vol. %) cooled in ice to convert the oxides of sulfur

to sulfuric acid. Carbon dioxide does not dissolve in H2O2. The sulfuric acid

formed was titrated with sodium hydroxide using methyl red as the indicator.

From the titre value, the amount of sulfur was calculated as follows;

Volume of NaOH of strengthN required to neutralize = V ml of Sulfuric acid formed (2.10)

1000 ml of 1N Sulfuric acid = 1 equivalent of H2SO4 (2.11)

49

Weight of Sulfuric acid = V NaOH x N NaOH x 49.05 = X g (2.12)

1000

98.1 g of Sulfuric acid contains = 32.1 g of sulfur (2.13)

X g of sulfuric acid = 32.1 X =Y g of sulfur (2.14)

98.1

Weight of the sulfur per = YUnit weight of the catalyst = Z (b) g sulfur (2.15)

W

2.7 FLOW UNIT FOR CATALYST ACTIVITY MEASUREMENT

Catalytic reactions were carried out in a differential flow reactor

operating at atmospheric pressure; infusion pump being used to displace the

reactants from the feeder into the reactor. The flow unit is shown in Figure 2.1.

The reactor was made of Pyrex glass tube with a spiral S, wound

coaxially so that the reactants while passing through the spiral can be

preheated to the same temperature as that of the catalyst before entering the

reactor. The reactor was provided with a thermocouple well. The electrically

heated furnace was well insulated with asbestos packing to minimize the heat

loss by radiation. The temperature measured using a thermocouple was

controlled within ± 2 ºC using a sunvic regulator. The reaction temperature

was measured using a previously calibrated thermocouple. The catalyst was

packed inside the reactor between beds of Pyrex glass wool (GW) as shown in

Figure 2.1. Glass beads (GB) were loaded on either side of glass wool.

The reaction products were passed through a water-cooled double

surface condenser (DSC) attached to the end of the reactor. The liquid

products were collected in a trap (CT) cooled in ice and gaseous products

were collected after bubbling through 0.01N HCl.

50

Figure 2.1 Flow unit for catalytic reaction at atmospheric pressure

51

Procedure: After bringing the reactor to the reaction temperature, the

apparatus was flushed with nitrogen for 10 minutes at the beginning of every

run; known quantities of reactants were displaced at the desired rate into the

reactor, which was maintained at the reaction temperature. Pure hydrogen gas

(50 cm3/min) was also passed along with the reactant for all the catalytic runs.

The time for which the reactants were in contact with the catalyst was

expressed as the time taken for the flow, the volume of reactant equivalent to

the volume of the catalyst at a particular temperature. The contact time is

given by

60 X Vc

Contact time = h (2.16) VR

1 LHSV = h-1 (2.17)

Contact time

where ‘Vc’ is the volume of the catalyst and ‘VR’ is the volume of the reactant

flowing per hour at standard temperature pressure. The vapors leaving the

reactor were first passed through the trap cooled in ice and uncondensed gases

were collected in the burette after bubbling through 0.01N HCl

(Shanthi et al 1989). About 30 minutes were allowed for the catalyst to attain

a steady state activity as shown by the constant rate of gas evolution. The

liquid products collected after one hour was preserved for analysis. The

volume of the gas evolved was measured at regular intervals of time.

After each run, the reactor was flushed with nitrogen to remove the

remaining liquid from the catalyst chamber while the temperature was raised

to 773 K. The catalyst was regenerated by maintaining it in a current of

carbon dioxide free air for 6 h. This procedure for regeneration was found to

be satisfactory to restore the catalyst activity to its original level.

52

2.8 PRODUCT ANALYSIS

The reactant methylcyclohexylamine and the corresponding

reaction product (methylcyclohexane) were estimated by gas chromatography

using a Shimadzu gas chromatograph (GC-17A) fitted with a thermal

conductivity detector using hydrogen as the carrier gas and 30 m x 0.32 mm

Rtx-5 (crossbond 5% diphenyl 95% dimethyl polysiloxane) column. The

output of the detector was fed into an automatic recorder after suitable

amplification the constituent products in the reaction mixture were identified

by comparison with the authentic samples.

In the case of amine, the catalytic activity was measured by

estimating the ammonia formed from the amine. Activity is defined as the

number of moles of amine converted to ammonia per unit time per unit

weight of the catalyst. It was calculated as follows:

Weight of the catalyst sample : W g (2.18)

Amine feed ratio : A mol h-1 (2.19)

Ammonia formed : B mol h-1 (2.20)

% Conversion of Amine : B X 100 = C (2.21)

A

Amine feed (mol h-1) X % Ammonia formedActivity = (2.22)

Weight of the catalyst X 100

A X C= = X mol h-1g-1 (2.23) W X 100