Infrared Spectroscopic Study of Acidity of Amorphous Silica...

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Indian Journal of Chemistry Vol. 28A. March 19R9. pp. 222-225 Infrared Spectroscopic Study of Acidity of Amorphous Silica-Alumina A K BANDOPADHYAY* SISIR Kr ROY & G S MURTY Central Fuel Research Institute, P.O. FRI-828 108, Dhanbad Received 8 February 1988; revised and accepted 20 April 1988 Surface acid sites of amorphous silica-alumina catalysts have been characterized by infrared spectroscopy using three different probe molecules: pyridine, ammonia and carbon monoxide. The study reveals that quantitative es- timate of acidity depends on the method of preparation of the catalyst as well as on the probe molecule used. ; Acidic sites are generally regarded as the active centres on certain oxides, which catalyse many reactions involving hydrocarbons, t.g., polymeriza- tion, isomerization, alkylation and cracking. These processes generally involve carbonium ion me- chanism and silica-alumina catalysts are quite ac- tive for these processes due to the acidic nature of the mixed oxide, where a trivalent aluminium has replaced a tetravalent silicon in the silica lattice. The acid sites on oxide surfaces are of two types: Lewis and Btonsted 1. Lewis acid sites are capable of accepting electrons froin the adsorbate molecules and Bronsted acid sites can donate a proton to the adsorbate molecule. These two sites are generally considered to be interdependent. On dehydrated surface, the major contribution to acidity is from Lewis sites only. Addition of a small amount of water to such a surface will con- vert the Lewis sites to Bronsted sites, which don- ate protons to adsorbate molecules resulting in the formation of carbonium ions '. Infrared spectroscopy can be used to distinguish between Bronsted and Lewis sites:' when adsorb- ing basic molecules like ammonia, pyridine, piperi- dine, 2,6-lutidine, etc., which may act as both pro- ton acceptors and electron donors, are used as probes. These molecules have characteristic ab- sorptions in the infrared spectra, both in the coordinately bonded and in protonated forms. Po- sitions of the IR bands together with integrated molar extinction coefficients of the respective bands available in existing literature, lead to deter- mination of concentration of both types of acid sites. The object of this paper is to report the ef- fect of different probe molecules on measurement of acidity. Further, the change in acidity of silica- alumina as a result of prejiaration with different pore regulating agents is also highlighted. 222 Materials and Methods Preparation of catalysts All tHe four silica-alumina samples used in the present work were prepared with the same com- position, i.e., with silica: alumina ratio = 88:12. Sample A was prepared without using any pore regulating agent by gelation technique from sodi- um silicate and aluminium sulphate solutions. Samples B, C and D were prepared by coprecipi- tation method from the same solutions in presence of the pore regulating agents tetramethylammoni- um hydroxide, tetrapropylammonium hydroxide and tetrabutylammonium bromide, respectively. After gelation, the gels were aged, filtered and washed with distilled water. The gels were further base-exchanged with ammonium chloride solution. The base-exchanged gels were then slowly heated, first in the air-oven at 120°C and finally in the muffle furnace at 450°C. The surface area of the catalyst was determined by argon adsorption. IR spectral measurements The catalysts prepared as explained above were examined in the form of thin wafers (10-15 mg! ern") obtained by applying high pressure on the powders of the catalystst'. The spectra of the ad- sorbed species on the catalysts were recorded on an lR-75 Specord spectrophotometer using a spe- cial designed cell shown in Fig. I. The body of the cell was made of molybdenum glass with heating device at one end (A) and IR transmitting win- dows (CaF 2 ) on the other end (B). Passage (C) was meant for evacuation, injection of gas and inser- tion of wafers. At first, each wafer was calcined at 500°C for 2 hr inside the cell, kept under evacuation. After calcination the cell was cooled to room tempera- ture and the spectrum of the catalyst was re-

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Indian Journal of ChemistryVol. 28A. March 19R9. pp. 222-225

Infrared Spectroscopic Study of Acidity of Amorphous Silica-Alumina

A K BANDOPADHYAY* SISIR Kr ROY & G S MURTY

Central Fuel Research Institute, P.O. FRI-828 108, Dhanbad

Received 8 February 1988; revised and accepted 20 April 1988

Surface acid sites of amorphous silica-alumina catalysts have been characterized by infrared spectroscopy usingthree different probe molecules: pyridine, ammonia and carbon monoxide. The study reveals that quantitative es-timate of acidity depends on the method of preparation of the catalyst as well as on the probe molecule used.

;

Acidic sites are generally regarded as the activecentres on certain oxides, which catalyse manyreactions involving hydrocarbons, t.g., polymeriza-tion, isomerization, alkylation and cracking. Theseprocesses generally involve carbonium ion me-chanism and silica-alumina catalysts are quite ac-tive for these processes due to the acidic nature ofthe mixed oxide, where a trivalent aluminium hasreplaced a tetravalent silicon in the silica lattice.

The acid sites on oxide surfaces are of twotypes: Lewis and Btonsted 1. Lewis acid sites arecapable of accepting electrons froin the adsorbatemolecules and Bronsted acid sites can donate aproton to the adsorbate molecule. These two sitesare generally considered to be interdependent. Ondehydrated surface, the major contribution toacidity is from Lewis sites only. Addition of asmall amount of water to such a surface will con-vert the Lewis sites to Bronsted sites, which don-ate protons to adsorbate molecules resulting in theformation of carbonium ions '.

Infrared spectroscopy can be used to distinguishbetween Bronsted and Lewis sites:' when adsorb-ing basic molecules like ammonia, pyridine, piperi-dine, 2,6-lutidine, etc., which may act as both pro-ton acceptors and electron donors, are used asprobes. These molecules have characteristic ab-sorptions in the infrared spectra, both in thecoordinately bonded and in protonated forms. Po-sitions of the IR bands together with integratedmolar extinction coefficients of the respectivebands available in existing literature, lead to deter-mination of concentration of both types of acidsites. The object of this paper is to report the ef-fect of different probe molecules on measurementof acidity. Further, the change in acidity of silica-alumina as a result of prejiaration with differentpore regulating agents is also highlighted.

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Materials and MethodsPreparation of catalysts

All tHe four silica-alumina samples used in thepresent work were prepared with the same com-position, i.e., with silica: alumina ratio = 88:12.Sample A was prepared without using any poreregulating agent by gelation technique from sodi-um silicate and aluminium sulphate solutions.Samples B, C and D were prepared by coprecipi-tation method from the same solutions in presenceof the pore regulating agents tetramethylammoni-um hydroxide, tetrapropylammonium hydroxideand tetrabutylammonium bromide, respectively.After gelation, the gels were aged, filtered andwashed with distilled water. The gels were furtherbase-exchanged with ammonium chloride solution.The base-exchanged gels were then slowly heated,first in the air-oven at 120°C and finally in themuffle furnace at 450°C. The surface area of thecatalyst was determined by argon adsorption.

IR spectral measurementsThe catalysts prepared as explained above were

examined in the form of thin wafers (10-15 mg!ern") obtained by applying high pressure on thepowders of the catalystst'. The spectra of the ad-sorbed species on the catalysts were recorded onan lR-75 Specord spectrophotometer using a spe-cial designed cell shown in Fig. I. The body of thecell was made of molybdenum glass with heatingdevice at one end (A) and IR transmitting win-dows (CaF2) on the other end (B). Passage (C) wasmeant for evacuation, injection of gas and inser-tion of wafers.

At first, each wafer was calcined at 500°C for 2hr inside the cell, kept under evacuation. Aftercalcination the cell was cooled to room tempera-ture and the spectrum of the catalyst was re-

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BANDOPADHYAY et al.: ACIDITY OF AMORPHOUS SILICA-ALUMINA

Table 1- Lewis and Bronsted Acid Sites Measured Using Three Types of Probe MoleculesSample Surface Cone. of Lewis sites (L) Cone. of Bronsted Conc. of Lewis Cone, of Bronsted

sitesarea (I-tMol/g) sites (B) sites (I-tMol/m2) sites (I-tMol/m2)

(Wig) (I-tMol/g)

C6H5N NH) CO C6H5N NH) C6H5N NH) CO C6H5N NH)A. 326 110 420 (I) 86 26 200 0.34 1.29 (I) 0.26 0.08 0.61

(2) 51 (2) 0.16B. 148 47 200 (1)"19 15 130 0.32 1.35 (1) 0.13 0.10 0.88

(2) 18 (2)0.12C. 168 85 305 (1) 38 22 190 0.51 1.82 (I) 0.23 0.13 1.13

(2) 32 (2) 0.19D. 143 30 163 (1) 18 17 160 0.21 1.14 (1)0.13 0.12 1.12

(2) 12 (2)0.08

corded. Pyridine, adsorbed on zeolite, was then al-lowed to be adsorbed by the catalyst for some-time. After this, desorption was carried out underevacuation at 150aC. The spectrum of the chemi-sorbed species formed on the catalyst surface wasthen recorded at room temperature. Similar proce-dures were adopted for ammonia except that de-sorption was carried out at 100°C. For CO, theportion B of the cell was modified in such a wayas to record spectra at liquid nitrogen tempera-ture. CO was adsorbed as well as desorbed at li-quid nitrogen temperature. The spectra were alsotaken at liquid nitrogen temperature for all the ca-talysts.

Beer-Lambert law in integrated form was appli-ed for the determination of concentration ofsites":". The integral intensity of an absorptionband is related to concentration of adsorbed spe-cies through the following relation,

J (To) -1B= log T v dv= Ao"C"p"lO .

where Tlb T are the transmittances at frequency vof the catalyst before and after adsorption, C theconcentration in umol/g, p the thickness of thewafer in mg/cm ' and All the integral adsorptioncoefficient in cm. umol " I. The values of AI forpyridine, ammonia and CO were taken from liter-ature"::".

Results and DiscussionProbe molecules interact with acid sites and

form complexes. Pyridine forms complexes PyH +

with Bronsted sites and AI:Py with Lewis centres;the frequencies of absorption of the adsorbed spe-cies formed are 1540 and 1450 cm - I respectively.The Lewis (L) and Bronsted (B) centres for catal-yst C are shown in Fig. 2(3). Similarly, ammoniaforms complexes NHt with Bronsted and Al:NH3with Lewis centres which absorb at 1620 and1460 cm-1 respectively [Fig. 2(4)]. The concentr-ations of sites measured from pyridine and ammo-nia adsorption are given in Table 1.

Lewis and Bronsted sites determined from pyri-dine and ammonia are found to be different for

g-e:

~~~A~~~~T~,R_MO_C_OU_PL_E ~~B~~l~!~~__HEATiNGDEVICE

~TAL THREAD

GLASS.fBMlf.

HOLDER CONTAININGTH[ WAFER OFCATALYST

Fig. l=-Infrared cell for studying acidity of catalysts [A: cup shaped heating device covered with asbestos, B: part of the glasscell containing IR transmitting CaF, windows. C: passage for evacuation. gas inlet and insertion of water]

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INDIAN J CHEM. SEe. A. MARCH 19H9

:~ NH]1.0

5 l :z: 0.5

l0........a:••..

'I' 4 t~

2200 2100)(

CfCL>- 3l-v;z L•••I-3!: B(ll Py

2

3700 3600

---L....o!~_ ' ...L-__ ~ ..l.-\Ir--+-- __ -'- __ -'- __ ----L _

3500 3400 1700 1600 1500 1400

FREG.UENCY ( Cm -1 IFig. 2-Characterisation of Bronsted and Lewis sites by IR spectra

each of the four catalysts studied here. In fact, theconcentrations of both the sites (L and B) as mea-sured from ammonia adsorption are much higherthan those measured from pyridine adsorption.This can be explained on the basis of the fact thatammonia, being a strong base and relatively asmall molecule (kinetic diameter 2.62 A). has ac-cess to practically all the acid sites. This points tothe fact that in porous materials the quantitativeestimate of the acidity depends on the size of theprobe molecules used. The selection of probemolecule is important while correlating catalyticactivity with acidity.

Zaki et al" have demonstrated the usefulness ofCO as a surface probe for the characterization ofaprotonic sites on metal oxide surfaces. The ad-sorption bond to Lewis acid centres is a a-donorbond which leads to positive frequency shifts ofthe carbonyl stretching mode relative to the gasphase frequency of the free molecule (2143 ern - I).Fig. 2(5) shows that there are two types of Lewissites L( 1) and L( 2) respectively, the concentrationsof which are included in Table 1. Fig. 2(2) showsthree different types of Bronsted sites, B( 1), B( 2)and B(3) at 3660, 3560 and 3480 em - I respect-ively. Thus, this selective surface probe is able tothrow more light on the nature of surface sitesthan pyridine and ammonia can. The characteriza-tion of surfaces by a number of probes has the ad-

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vantage of knowing surface sites in a more de-tailed way. Fig. 2( 1) indicates two types of OHgroups on dehydrated surface of silica-alumina,one at 3750 ern - I due to isolated surface SiOHgroups and the other, a bonded one, at a lowerfrequency - 3600 em - I.

It can be seen from Table 1 that surface area aswell as acidity of silica-alumina has changed whentern plating agents like tetramethylammonium hy-droxide, etc., are used. This is due to the action ofstrongly basic quaternary ammonium compoundson the acid centres of silica-alumina. It is observedthat the acidity is maximum for the sample Cwhich has been prepared with tetrapropylammoni-urn hydroxide as pore regulating agent. It is inter-esting to note that tetrapropylammonium cation isthe preferred templating agent for the preparationof ZSM-5, the new generation shape-selective mo-lecular sieve 10. Table 1 also shows that althoughcatalysts Band D have practically the same sur-face area, the acidities determined by pyridine andammonia are different. This indicates that ternplat-ing agents affect the distribution of surface acidsites. It may be presumed that the variation inacidity is due to the difference in charge densitiesof tetramethyl and tetrabutyl groups.

AcknowledgementThe authors are thankful to Dr. E A Paukshtis,

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BANDOPADHYAY et al.: ACIDITY OF AMORPHOUS SILICA-ALUMINA

Institute of Catalysis, Novosibirsk, USSR for hiskind help and providing of infrastructure for car-rying out investigations and to Mr. 1. Das for helpin preparation of samples. Thanks are also due toDr. R. Haque, Director, CFRI for his kind permis-sion to publish this paper.

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