2628
J . Phys. Chem. 1993, 97, 2628-2639
Thermodynamics of Adsorbed Molecules for a New Acid-Base Topchemistry of Alumina Antonella Cervasinit Eniricerche Sp. A, via Maritano 26, 20097 San Donato Milanese, Italy
Aline Auroux' Institut de Recherches sur la Catalyse, Laboratoire Propre du CNRS Associh h I'UCB, Lyon I , 2 avenue Albert Einstein, 69626 Villeurbanne. Chdex, France Received: February 19, 1992; In Final Form: December 15, 1992
In the present study, two methods have been applied for characterizing the surface acidity and basicity of an amphoteric y-alumlna: a titration method using Hammett indicators and a gas phase adsorption method using basic (i.e. piperidine, diethylamine, n-butylamine, ammonia, pyridine, aniline), amphoteric (Le. pyrrole, water, methylalcohol terr-butyl alcohol, acetonitrile), and acidic (i.e. m-cresol, carbon dioxide, acetic acid, sulfur dioxide) probe molecules by means of combined volumetry-calorimetry technique. The scales of acid-base strength obtained by the two methods and their reliability have been compared. Both the strength and number of acidic sites were related to the pKa of the probes. When using weaker probes (PKa I0) only the very strong acid sites are titrated. At pKa 1 5 a plateau is reached. Moreover, in order to evaluate more precisely the gas-solid interactions, the thermodynamic behaviour of the adsorbed probes has been studied, including the isotherm, the differential heat, the integral heat and the differential molar entropies of adsorption as a function of the coverage. Integral heat corresponding to the chemisorbedvolume has been taken as a criterion to compare the acid strength of the sites. Besides, integral heat curves describe a parabola which curvature has been related to the strength of the adsorbate-adsorbent system. The values of the differential molar entropies of adsorbate confirm an immobile layer in the range 0 4 . 4 degree of coverage.
Introduction Determination of thestrength oftheacidicor basic sites exposed
on the solid surface as well as their distribution is a necessary requirement to understand the catalytic properties of acidic solids. A number of definitions of the acid-base concept have been proposed to describe quantitatively the acidity and the basicity of molecules or ions in solutions. However acidity and basicity functions for the solid are not properly defined thermodynamically. According to Bransted's definition, an acid is a proton donor and a base a proton acceptor (a base must have available a pair of electrons to share with the proton) and an acid-base reaction is simply the trarisfer of a proton from an acid to a base. A base in the Lewis theory is the same as in the Bransted one, namely, a compound with an available pair of electrons, either unshared or in a A orbital. A Lewis acid, however, is any species which has a vacant orbital. In a Lewis acid-base reaction the unshered pair of the base forms a covalent bond with the vacant orbital of the acid.! A variety of methods has been suggested to determine the aciditylbasicity of solid surfaces.',2 They differ from each other in the chemical and physical principles on which they are based, and none of them can be regarded as universal in its use. However, for the determination of strength and amount of a solid acid/ base sites there are two main methods which provide a good approach of standardization of acid-base properties: a titration method using indicators and a base or an acid in apolar solvents as titrant and a gaseous base or acid adsorption method. In previous studie+ we have used adsorption microcalorimetry to meaure the differential heat of adsorption of NH3 and COz on a series of 20 metal oxides in order to predict the thermodynamic behavior of new catalysts. * To whom correspondence should be addressed. Dipartimento di Chimica-Fisica ed Elettrochimica, Universita di Milano, Via Golgi 19, 20133 Milano, Italy.
' P r e m t address:
In the present study we are seeking a thermodynamic scale of surface acidity for solids related to the pKa scale. For this study, we have chosen a typical amphoteric solid such as alumina in its y form which is characterized by the presence of both acid and base sites on its surface and presents a broad distribution of site strength. Aluminas are used extensively as adsorbents and active catalysts and catalyst supports and despite the widespread interest in catalytic aluminas there is still only a limited understanding about the real nature of the surface of alumina. So in this article we also briefly review the existing surface models of alumina and give an approach in the interpretation of reported chemisorption. To evaluate the surface acidity and basicity of alumina we have used both adsorption microcalorimetry and adsorption isotherms of acid-base probe molecules. Heat flow microcalorimetry is one of the best methods known for measuring accurately the differential heats of adsorption and so of characterizing a catalyst by the energy distribution of its surface sites. It allows us to determine the heat evolved during adsorption of appropriate probe molecules from the gas phase. The various thermodynamic parameters of adsorption such as the isotherm, the differential heat, the integral heat of adsorption and the entropy of adsorbate were determined as a function of the coverage for a series of fifteen probe molecules with pKa varying over the full range of the pK, scale. To complete this study the adsorbed volume data were compared with those obtained from a titration method carried out with Hammett indicators, Several adsorption studies of various alumina6-I9concerning one or another probe (mainly NH3,6.11s15 H z O , pyridine,8,10,15-18 ~ n-butylamine,l3S02,14and CO2I9)and various techniques (mainly IR") have been reported in the literature but to our knowledge, there has been no extended work involving a comparative study of so many different probes.
0022-3654/93/2091-2628~04.Q0/00 1993 American Chemical Society
A New Acid-Base Tophemistry of Alumina
Adsorbent Surface
The Journal of Physical Chemistry, Vol. 97, No. 11, 1993 2629
TABLE I: Properties of Alumina sample
origin
surfacea (m2/g)
pore volume (cmj/g)
y-AI?Oj
UOP
183.50
0.577
The overall reaction at a gas/solid interface is
impurities
Si02 = 455 ppm CI = 732 ppm Br- 5 50 ppm I- 5 50 ppm
This formulation emphasizes the need to characterize both the starting material and the adsorption complex. The alumina surface is certainly an extremely complicated Determined after thermal treatment as reported in the experimental surface. Among the different surface models and site configurations reported in the literature, Knozinger and R a t n a ~ a m y ~ ~ part. proposed five possible OH configurations: There are some following questions arising: What is the structure of the surface layer and what do H H H I chemisorption and catalytically active sites look like? Are all possible lattice sites (cus anions and cations) active sites or are certain specific site configurations required? Lewis acid and base sites produced during the regular dehydroxylation process can hardly be all involved in catalytic Ia IIa IIb reactions as active sites. It has already been that only H defect sites can be considered as active sites because of their low H I site density. The configuration of such defect sites can hardly be predicted from idealizing model considerations. The use of suitable specific probe molecules, however, can provide us with information about the number of such sites. Ill
(Ia) a terminal OH group is coordinated to a single tetrahedral A13+cation; (Ha) a bridging OH group links a tetrahedral and an octahedral cation (thrice as frequent as type Ia); (IIb) the OH group links two cations in octahedral positions (thrice as frequent as type 111); (111) the OH group is coordinated to three cations in octahedral interstices; (Ib) the OH group is coordinated to a single cation in an octahedral interstice if possible vacant cation positions are existing. The most important result of the above considerations is that we should expect a minimum of five different OH configurations on the surface of y-alumina. The OH groups in these various configurations bear slightly differing net charges. As a consequence, they should possess different properties. The protonic acidity of the O H groups will decrease as the net chargeon them becomes more negative. Of course, their basicity will increase at the same time. This should explain the amphoteric character of alumina. The pretreatment of the solid is probably the most important experimental parameter. During the pretreatment, the regular dehydroxylation process leads to the formation of cus oxygens (Lewis base site) and of anion vacancies (Lewis acid site) which expose cus aluminum ions. Consequently the oxygen bears a small negative charge, the vacancy a small positive charge, or in other words, a weak Lewis base site and a neighboring weak Lewis acid site have been created by the regular dehydroxylation process. Since catalytic activity develops during the surface dehydroxylation, we can now ask whether these regularly formed acidic and basic sites could be identified with catalytically active sites. We may get an answer to this question from a comparison of the number of lattice sites formed by the dehydroxylation process and the number of catalytically active or chemisorption sites. During dehydroxylation, an anion vacancy (Lewis acid site) and a cus oxygen atom (Lewis base site) are formed for every two hydroxyls which leave the surface as water. Knozinger and R a t n a ~ a m y 2estimated ~ the total number of OH groups on alumina, i.e. 14.5 X l O I 4 cm-*, as well as the number of OH groups retained at 773 K, i.e. 4 X lOI4cm-2. From these values one can evaluate the numbers of Lewis acid and Lewis base sites formed by the surface dehydroxylation process, i.e. 5.25 X 10'4 cm-2.
Experimental Section Sample and Pretreatment. The properties and the source of alumina studied have been collected in Table I. The purity of the y phase was verified by means of XRD analysis. y-Alumina was subjected to a standard pretreatment before any measurement like surface area and specific pore volume determinations, acidity-basicity measurements by calorimetric and volumetric adsorptions of probe molecules or by titration with Hammett indicators. In this pretreatment, 0.08-0.1 g of alumina in powder form was introduced in silica or pyrex cells and calcined in dry oxygen atmosphere as the temperature was raised from room temperature to 773 K (2 K/min) before being maintained for 16 h at the same temperature. Afterwards the cell was pumped out to 1.33 mPa for 2 h before cooling to room temperature. y-Alumina BET surface area and pore volume were determined from N2 adsorption and desorption isotherms collected until PIP0 = 1 at the temperature of N2 condensation. A 1800 series Sorptomatic apparatus, from Carlo Erba was used. Consequently all the numbers of acid and base sites determined further have been expressed per unit surface area. Acid and Base Strength Distribution Measurements. Calorimetric and Volumetric Gas-Solid Titrations. The 15 probe molecules used to titrate the acid-base sites of alumina display acid, basic, or amphoteric characteristics. The pK, in aqueous solutions and the nature of acid/base couples are reported in Table 11. All the probe molecules used as titrants were analytical grade of purity > 99.5 wt %. Aniline and m-cresol were distilled on freshly activated molecular sieves; ammonia was dried on sodium wires. Before any use they were all purified by successive freeze-thaw cycles. Some general physicochemical charact e r i s t i c ~ of ~ ~these - ~ ~ probe molecules which will be required in further calculations are given in Table 111. This table reports the density of liquid molecules at 296 K, the boiling and critical temperatures, thesaturatedvapor pressure at 296 K, the calculated section area, the corresponding monolayer volume, the entropy in the gas phase at 353 K, the entropy of the liquid phase at 353 K and the heat of liquefaction of all the probes at 353 K. The heats of adsorption of the probe molecules were measured in heat-flow microcalorimeters of the Tian-Calvet type, C80 and HT from SETARAM, linked to a glass volumetric line to permit the introduction of successive small doses of gases or vapors. The equilibrium pressure relative to each adsorbed amount was measured by means of a differential pressure gauge (Datametrics).
Gervasini and Auroux
2630 The Journal of Physical Chemistry, Vol. 97, No. 1 1 , 1993
TABLE II: Dissociation Coastants of the Acid and Base Probe Molecules in Aqueous Solutions at 298 K DK
I
~~
probe molecule
basic
basic piperidine 11.12 11.00 diethylamine 10.77 n-butylamine 9.24 ammonia 5.19 pyridine aniline 4.63 amphoteric 0.40 py rr oIe -1.74 water -2.00 methyl alcohol fer!-butyl alcohol -5.00 -10.00 acetonitrile acidic m-cresol carbon dioxide acetic acid sulfur dioxide
acidic
acid species
basic species
CcHioNHi+ (C:H
