comparison of the methods for the determination of surface acidity of solid catalysts

This article was downloaded by: [McGill University Library] On: 15 December 2012, At: 07:35 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK Catalysis Reviews: Science and Engineering Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/lctr20 Comparison of the Methods for the Determination of Surface Acidity of Solid Catalysts Lucio Forni a a Istituto di Chimica Fisica Universita di Milano, Milano, Italy Version of record first published: 13 Dec 2006. To cite this article: Lucio Forni (1974): Comparison of the Methods for the Determination of Surface Acidity of Solid Catalysts, Catalysis Reviews: Science and Engineering, 8:1, 65-115 To link to this article: http://dx.doi.org/10.1080/01614947408071857 PLEASE SCROLL DOWN FOR ARTICLE Full terms and conditions of use: http://www.tandfonline.com/page/ terms-and-conditions This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. The publisher does not give any warranty express or implied or make any representation that the contents will be complete or accurate or up to date. The accuracy of any instructions, formulae, and drug doses should be independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims, proceedings, demand, or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with or arising out of the use of this material. Downloaded by [McGill University Library] at 07:35 15 December 2012 Comparison of the Methods for the Determination of Surface Acidity of Solid Catalysts LUCIO FORNI Istituto di Chimica Fisica Universitii di Milano Milano. Italy I . INTRODUCTION 66 I1 . DETERMINATION OF THE ACID STRENGTH OF THE CENTERS 67 A . Method of Adsorption of Colored Indicators 67 B . Spectrophotometric Method 69 C . Adsorption of Gaseous Basic Substances 71 D . Calorimetric Methods 73 E . OtherMethods 78 I11 . DETERMINATION OF THE SURFACE DENSITY OF ACID CENTERS 80 A . 80 B . Titration after Ionic Exchange 81 C . Titration with Bases in Nonaqueous Solvents 82 D . Calorimetric Titration 87 E . Adsorption and Desorption of Gaseous Bases 88 91 94 96 Direct Titration of Aqueous Suspensions F . Method of the Poisoning of Specific Surface Reactions G . Hydrogen-Deuterium Exchange Reactions H . Indicator Reactions Method I . Spectroscopic Methods 98 J . Reaction with Hydrides 100 K . OtherMethods 102 65 Copyright 0 1973 by Marcel Dekker. Inc . All Rights Reserved . Neither this work nor any part may he reproduced or transmitted in any form or by any means . electronic or mechanical. includ- ing phococopying. microfilming . and recording. or by any information storage and retrieval sys- tem. without permission in writing from the publisher . Downloaded by [McGill University Library] at 07:35 15 December 2012 66 L. FORNI IV. DETERMINATION OF THE NATURE OF ACID SITES: BRBNSTED TYPE AND LEWIS TYPE 103 A. Determination of Brhsted Sites Alone 103 B. Determination of Lewis Sites Alone 104 V. CONCLUSIONS 108 REFERENCES 111 I. INTRODUCTION The concept of surface acidity was originally introduced with the aim of justifying the presence of some substances formed in catalytic chemical reactions, not as a consequence of suppositions about the nature of surface-active sites of solid catalysts. The formation of such substances in some reactions (e.g., cracking, isomerization, or polymerization) can be better explained by admitting the forma- tion of reaction intermediates having the structure of a carbonium ion, which can be formed by interaction between the reacting sub- stance (hydrocarbon) and an acid center. As an example, in the cracking of alkylaromatics catalyzed by decationated zeolites, the following reaction mechanism is generally accepted: where the first stage can be interpreted as an electrophilic substi- tution of the proton onto the alkyl group. A complete description of the surface acid properties of a solid must involve the determination of the acid strength of the sites, their density (number of acid centers per unit surface area of the solid), and their nature (Bransted or Lewis type). Such a description is not easy to make, since the strength and the density of the sites are generally strictly connected to each other and, besides, the distribu- tion of the acid strength is usually heterogeneous. Furthermore, most of experimental methods can distinguish the centers only on the grounds of their strength. They cannot distinguish between Bransted and Lewis centers, but simply give a measure of total acidity of both types. lished in the literature [I-31. Nevertheless, none of them gives a complete list of the available methods. The scope of the present Some excellent reviews on the argument have recently been pub- Downloaded by [McGill University Library] at 07:35 15 December 2012 SURFACE ACIDITY OF SOLID CATALYSTS 67 paper is then a collection and a critical comparison of all the methods actually employed for the determination of surface acidity of solids. Each of them, in fact, taken by itself, allows some useful informa- tions to be collected, but can give rise to some criticisms. The com- bination of the information obtainable from two or more of them can often be the only way to give a complete picture of the surface acid properties of the solid under examination. 11. DETERMINATION OF THE ACID STRENGTH OF THE CENTERS According to Walling [4], the acid strength of a solid can be de- fined as its ability to convert a neutral base, adsorbed on its sur- face, into the corresponding conjugated acid. If the reaction takes place through the transfer of a proton from the solid surface to the adsorbed molecule (Brdnsted acidity) or of an electron pair from the adsorbed molecule to the solid surface (Lewis acidity), the acid strength can be expressed, respectively, by means of the Hammett function H, in the following way [5-71: H, = pK, + log ([Bl/[BH+l) (1) or where K, is the equilibrium constant of the dissociation of the acid, and [B], [BH'], and [AB] are the concentrations of neutral base, its conjugated acid, and the addition product formed during the adsorp- tion of the base on the Lewis center, respectively. A. Method of Adsorotion of Colored Indicators An immediate application of Walling's analysis, originally adopted by Walling, Weil-Malherbe, and Weiss [a, Ikebe et al. [9], and many others, is the observation of the color shown by suitable indicators adsorbed on the solid surface, If the adsorbed indicator assumes the color of its acid form, the value of H, of the surface is lower or equal to the pK, of the indicator. The lower the value of H, (and the lower the pKJ, the higher is the acid strength of the solid. Benzene, isooctane, decalin, or cyclohexane may be employed as solvents. In Table 1 the most important indicators are reported. In the last column of the table the wt% of H,SO, in sulfuric acid solution, which has the acid strength corresponding to the respective pK,, is given for some of the indicators. In Table 2 the acid strength, obtained by such a method by various authors [lo-131, is given. Downloaded by [McGill University Library] at 07:35 15 December 2012 68 L. FORNI TABLE 1 Basic Indicators Used for the Measurement of Acid Strength of Solids Indicator Neutral red p-Ethoxychrysoidin Methyl red Phenylazonaphth ylamine Aminoazox ylene p-Dimethylaminoazobenzene (dimethyl yellow or butter yellow) 2-Amino-5-azotoluene 1,4-Diisopropylaminoan- Benzeneazodiphenylamine 4-Dimethylaminoazo-1-naph- Crystal Violet p-Nitro benzeneazo-#( p’-nitro)- diphenylamine Dicinnamalacetone Benzalacetophenone Anthraquinone thraquinone thalene Color Base form Acid form pK, H,SO,, 5% Yellow Red +6.8 8 X Yellow Red +5.0 Yellow Red +4.8 Yellow Red +3.5 - - Yellow Red +4.0 5 x 10-5 - Yellow Red +3.3 3 x 10-4 Yellow Red +2.0 5 x 10-3 - Blue Red +1.7 Yellow Purple +1.5 2 X lo-’ t1.2 3 x lo-’ Yellow Red Blue 0.1 Yellow +0.8 Orange Purple +0.43 - Yellow Red -3.0 48 Colorless Yellow -5.6 71 Colorless Yellow -8.2 90 TABLE 2 Acid Strength of Some Solids Solid acids ~ Hll Refs. Original kaolinite Hydrogen kaolinite Original montmorillonite Hydrogen montmorillonite Silica-alumina Silica-magnesia 1.0 mmole/g H3B03/Si0, 1.0 mmole/g H3P04/Si02 1.0 mmole/g H2SO4/Si0, NiSO, . xH,O heat-treated (350°C) NiSO,. xH,O heat-treated (460°C) ZnS heat-treated (300°C) ZnS heat-treated (500°C) ZnO heat-treated (300°C) TiO, heat-treated (400°C) AI,O,-B,O3 -3.0 - -5.6 -5.6 - -8.2 +1.5 - -3.0 -5.6 - -8.2 <-8.2 <-8.2 +1.5 - -3.0 t1.5 - -3.0 -5.6 - -8.2 <-8.2 +6.8 - -3.0 +6.8 - +1.5 +6.8 - +4.0 +6.8 - +3.3 +6.8 - C3.3 +6.8 - +1.5 10 10 10 10 10 10 10 10 10 10 11 11 12 12 13 13 Downloaded by [McGill University Library] at 07:35 15 December 2012 SURFACE ACIDITY OF SOLID CATALYSTS 69 In the case of black or dark-colored solids, when the observation of the indicators color is impossible or very difficult, a small amount of a white solid of known acidity may be added to the sample and the acidity value obtained corrected for such an addition. reproducibility to be obtained, although some difficulty may arise either in the determination of the exact end point of titration or due to moisture contamination. Also, the acidity values obtained are not absolute because they are not related to energetic factors, but simply to the pK, values of the indicators employed. Other disadvantages of the method are the impossibility of making acidity determinations in the real working conditions of the catalyst and sometimes the long period required for the equilibrium between adsorbed and free base to be reached. The adsorption method is generally quite accurate and allows good B. Spectrophotometric Method Since the visual judgment of the color shown by the indicator in the preceding method can be uncertain at times, some absorption spectra of dyeing materials, adsorbed on various solids, have been determined [13,14]. For example, Leftin and Hobson [13] recorded the absorption spectra of phenylazonaphthylamine (pK, = +4.0) on a 12% alumina silica-alumina catalyst for both the basic and acid form of the indicator. Such spectra were recorded in pure isooctane and in an ethanolic solution, acidified with HC1, respectively. The re- sults are reported in Fig. 1. One can observe that the spectrum of the adsorbate reveals that it is adsorbed exclusively in its acid form. In a similar way Dzisko and co-workers [15] determined the sur- face acid strength of mixtures of oxides. They employed the indica- tors reported in Table 3 and established the following qualitative scale of acid strength: SiO, . A1,0, > ZrO, . SiO, - GqO, . SiO, > Be0 . SiO, - MgO . SiO, > Y,O, . SiO, > L+O,. SiO, > SnO . SiO, - PbO . SiO,. Finally, Kobayashi [16-191 recorded the absorption spectra of dimethyl yellow, dimethyl red, and bromophenol blue adsorbed over partially n-butylamine covered silica-alumina in a nonpolar solvent. He determined not only the acid strength, but also the total number of acid centers present on the catalyst surface. Apart from some discrepancies due to the change in the activity of the adsorbed base when surface coverage became higher and higher, he confirmed that the values of H,, obtained by this method, are independent on the nature of the indicator employed. The spectrophotometric method gives good qualitative informa- tions on the form in which the dyeing substance is adsorbed onto the solid surface, but does not eliminate the main disadvantages con- Downloaded by [McGill University Library] at 07:35 15 December 2012 70 L. FORNI 0.8 - 0, u C 0 0.6 - + 0, 2 0.4 - 0.2 - C 4500 A 6000 FIG. 1. Absorption spectra for phenylazonaphthylamine. (a) In isooctane solution, (b) In ethanolic HCI, and (c) adsorbed on silica-alumina [ 131. TABLE 3 Basic Indicators for Spectrophotometric Determination of Acid Strength Indicator PK, Phenylazonaphthylamine pDimethylaminoazobenzene Aminoazobenzene Benzeneazodiphenylamine pNitroaniline o-Nitroaniline p-Nitrodiphenylamine 2,4-Dichloro-6-nitroaniline p-Nitroazobenzene 2,4-Dinitroaniline Benzalacetophenone p-Benzoyldiphenyl Anthraquinone 2,4,6-Trinitroaniline 3-Chloro-2,4,6-trinitroaniline p-Nitrotoluene Nitrobenzene 2,4-Dinitrotoluene +4.0 +3.3 t2.8 t1.5 +1.1 -0.2 -2.4 -3.2 -3.3 -4.4 -5.6 -6.2 -8.1 -9.3 -9.7 -10.5 -11.4 -12.8 Downloaded by [McGill University Library] at 07:35 15 December 2012 SURFACE ACIDITY OF SOLID CATALYSTS 71 nected with the adsorption method, e.g., nonabsolute acidity value determinations and nonreal working conditions of the catalyst. C. Adsorption of Gaseous Basic Substances The strength with which a base adsorbs on the surface acid cen- ters of a solid is directly proportional to the acid strength of the centers. If, after the adsorption, the solid is heated at growing tem- peratures and the quantity of desorbed base is recorded, a measure of the acid strength of the centers can be obtained. Before the exper- iment the solid must be pretreated in order to obtain reproducible results. Such a treatment usually consists in the elimination of the volatile impurities by evacuation and/or heating and flushing in an inert gas flow. The less volatile impurities can often be eliminated by converting them in more volatile compounds by reaction with oxygen or hydrogen. The adsorption equilibrium of the base can often be reached at relatively high temperatures and low pressures. A real chemical reaction of the base with the surface may also occur. Peri [20] and Wilmot [21], for example, showed that an exchange re- action between ammonia and OH surface groups, with the formation of water and NH, surface groups, may take place together with the adsorption of ammonia. Such a side reaction, on the other hand, usually wastes but a small fraction of the adsorbed ammonia, so that it is difficult to determine the amount of ammonia consumed in this way by simply analyzing the gaseous phase, particularly if the volume of the gaseous phase is large. A method for reducing the error due to such a side reaction is to make a series of cyclic ad- sorptions and desorptions on the sample by varying the temperature or the pressure of the system. In this way it is likely that only the reversibly adsorbed ammonia takes part in such cycles. The measures of acid strength can be performed by determining the amount of desorbed ammonia obtained by heating in vacuo [22,23], or in a closed system [24], or by flash desorption in an inert gas flow [25-271. Webb [23] worked with HF-A1,0, samples at various HF percentages. After outgassing and dehydrating at 500°C and’ lo* Torr for 16 hr, the solid was exposed for 30 min at 175°C to 10 Torrs pressure of gaseous ammonia. The desorption was made by evacuating the sample up to 500°C and collecting the desorbed ammonia in a liquid-nitrogen cooled trap. From the difference in weights of adsorbed and desorbed base, the amount of base remained on the solid surface could be determined. The results are reported in Fig. 2. One can observe that the higher the HF percentage in the solid, the higher the amount of adsorbed ammonia. This means that the acid strength of the solid is directly proportional to the HF frac- tion. Downloaded by [McGill University Library] at 07:35 15 December 2012 72 L. FORNI 0.8 0.6 0.4 0.2 0.0 200 3 00 400 0~500 FIG. 2. Fraction of ammonia retained on catalyst surface vs evacuation temperature. (A) 0.0% HF content, (0) 3.23%, (A) 0.65%, (0) 6.46% [ 231. The amount of base desorbed by heating a previously covered solid may also be determined by differential thermal analysis (DTA). In such a case, by combining DTA with thermogravimetric analysis (TGA), it is possible to make a simultaneous determination of both the acid strength and the distribution of the centers as a function of their acid strength. One of the best examples of such a procedure is given in a paper by Shirasaki et al. [28]. They worked on silica- alumina covered with pyridine, n-butylamine, or acetone. By plotting both the changes of solid temperature [with respect to the reference sample (DTA)] and of solid weight (TGA) vs temperature (see Fig. 31, they simultaneously determined the amount of adsorbed base (x) and adsorption heat (S). By plotting S vs x one can get (dS/dx), which is directly proportional to the acid strength of the centers. By plotting x vs (dS/dx), the number of centers of given acid strength can be ob- tained. By means of the DTA technique Bremer and Steinberg [29] also observed an inverse dependence of the amount of adsorbed base (pyridine) on the pretreating temperature of the solid (MgO . SiO,). In fact, preheating at high temperature gave a pyridine desorption peak at a lower temperature, with progressively higher pyridine de- sorption peak temperatures as the preheating temperature was lowered. employed, particularly with ammonia. The main advantage of this method is that the acidity measurements can be made in the real working conditions of the catalyst. Another advantage, particularly The gaseous bases adsorption-desorption method has been widely Downloaded by [McGill University Library] at 07:35 15 December 2012 [...]... amount of a base is dissolved After equilibration an indicator, which adsorbs on the solid surface, is added and assumes the color of its acidic or basic form The quantity of base needed to impart to the solid the color 83 Downloaded by [McGill University Library] at 07:35 15 December 2012 SURFACE ACIDITY OF SOLID CATALYSTS of the basic form of the indicator represents the measure of the quantity of acid... thus explaining the large interaction heats shown for 8 > 0.1 Hsieh also discusses the effect of acid strength of the centers on the catalytic activity of the solid 7t 5 0 f 1 I I I 1 I I 1 I l 2 3 4 5 6 7 8 6 FIG 5 Differential heat of adsorption for NH, on silica-alumina vs surface coverage t341 The methods based on the determination of the immersion heat of the solid have the advantage of allowing... close to the total number of sites present on the solid surface If the solid surface is colored, the method can still be employed by adding to the mixture a given quantity of a white acid solid The end point of titration is determined by observing the change in color of the white solid surface A correction factor must obviously be introduced for the quantity of base consumed in the neutralization of such... the distribution of the acid strength of the centers by means of where B is the quantity of adsorbed pyridine and qo the heat of adsorption of benzene Neglecting the small temperature dependence of the term A(T,), i.e., assuming it as a constant, the value of g, could be calculated from the slope of the curve obtained by plotting the first member of Eq (6) vs l/Tc Most of the data were calculated for. .. present, each of them characterized by a different value of activation energy, then more than one peak appear in the chromatogram The total amount of hydrogen initially present may be determined both from the quantity of exchanged deuterium and from the quantity of water formed during the reaction The fraction of the various forms of hydrogen can be determined from the ratios of the areas of the corresponding... performed the experiments by poisoning them with quinoline The results a r e reported in Fig 21 The measure of the specific catalytic activity of a single site is given by the slope of the linear portion of the curves of the type reported in Fig 20, while each point of the curve of Fig 21 corresponds to the specific activity of the various partially decationated catalysts It may be observed that catalysts. .. ketone at growing temperatures Other experiments were also made with various alcohols in CC1, [45,46] A comparison was made of frequency shifts of the OH group band at various concentrations of the alcohol in the solvent, in the presence of the solid, with the known acidity constants of the alcohols From such a comparison the authors determined the following pK, values for the solids examined: magnesia,... plotted vs the volume of added titre, a curve is obtained (Fig 17) which flattens in correspondence of the base volume needed for the titration of total acid amount of the solid sample The progressive flattening of the Fig 17 curve with the increase in the volume of added base may be due either to the heterogeneity of the centers, as suggested by Topchieva et al [71], o r to the decrease in the diffusion... particle size They also outlined the influence of sample quantity on the results of acidity determination Their results clearly indicate that the reaction reaches an equilibrium in which the specific quantity of acid transferred from the solid surface to the solution increases with an increase in the amount of employed sample Since below 0 1 g of sample the increase in acidity became negligible, the authors... -0.4 111 DETERMINATION OF THE SURFACE DENSITY OF ACID CENTERS The number of acid centers present on a solid surface is usually expressed as surface density, e.g., as the number of centers, o r millimoles, per unit weight o r unit surface area A Direct Titration of Aqueous Suspensions When an acid solid is suspended in water, it often lowers the pH of the aqueous phase A direct titration of the aqueous . description of the surface acid properties of a solid must involve the determination of the acid strength of the sites, their density (number of acid centers per unit surface area of the solid) ,. out of the use of this material. Downloaded by [McGill University Library] at 07:35 15 December 2012 Comparison of the Methods for the Determination of Surface Acidity of Solid Catalysts. instructions for authors and subscription information: http://www.tandfonline.com/loi/lctr20 Comparison of the Methods for the Determination of Surface Acidity of Solid Catalysts Lucio Forni a a