Journal of Catalysis 197, 344 –349 (2001) doi:10.1006/jcat.2000.3095, available online at http://www.idealibrary.com on New Methods for Quantitative Determination of Brønsted Acid Sites on Solid Acids: Applicability and Limits for Al 2 O 3 -Promoted SO 4 2− /ZrO 2 Catalysts Roberta Olindo, ∗ Alain Goeppert,† David Habermacher,† Jean Sommer,† ,1 and Francesco Pinna ∗ ∗ Dipartimento di Chimica, Universit` a Ca’ Foscari di Venezia, Dorsoduro 2137, 30123 Venice, Italy; and †L aboratoire de Physico-Chimie des Hydrocarbures, Facult´ e de Chimie, Universit´ e L ouis Pasteur, 4 rue B laise Pascal, F-67070 Strasbourg Cedex, France Received August 4, 2000; revised October 11, 2000; accepted October 19, 2000; published online December 21, 2000 Two quantitative methods, both based on H/D exchange, have been developed for the Brønsted acid sites titration on solid acids. The agreement between the results obtained with the two meth- ods on unpromoted sulfated zirconia (SZ) is remarkable:0.061 and 0.064 mmol of H/g of catalyst were respectively found with the first (anhydride method) and the second (isobutane method) method. Furthermore, the use of a small alkane as the probe molecule with the isobutane method gives us information about the capacity of the solid to activate an alkane. Nevertheless, this last method is hard to apply to very active catalysts such as 3%alumina-promoted sul- fated zirconia (SZA3). Indeed, it was not possible in this case to find optimal temperature conditions, low enough to minimize sec- ondary reactions such as isomerization orcracking and, at the same time, sufficiently high for an accurate quantitative H/D exchange measure. On the other hand, the anhydride method was always applicable to SZAn catalysts and the obtained results were very reproducible. c  2001 Academic Press Key Words: sulfated zirconia;alumina-promoted sulfated zirco- nia; acid sites titration; H/D exchange; alkane activation. 1. INTRODUCTION Solid acids have found widespread applications in many catalyticreactionsconcerninghydrocarbons,suchasalkane or alkene isomerization, oligomerization, alkylation, and cracking. Regarding the economic value of solid acid catalysts in the oil industry (1, 2), methods giving information about their acid sites(number,type,and strength) are ofthe great- est importance for understanding their catalytic activity. However,asrecentlyunderlined in severalreviews(3,4), the reliability and the applicability of various suggested methods are questionable. The most frequently used tech- niques are based on the adsorption of basic compounds. 1 To whom correspondence should be addressed. Fax: 33 (0)3 88 45 46 47. E-mail: sommer@chimie.u-strasbg.fr. Calorimetric titrations with Hammett indicators suffer from a large number oflimitations:the equilibrium israrely achieved; non-acid sites may also produce the change of color;this method gives the sum of the amount of Brønsted and Lewis acid sites (3). TPD ofbasicmoleculesmaybe misleadingbecause reac- tions different from acid–base interactions often occur be- tween the base probe and the solid surface. Furthermore, TPD of benzene or pyridine cannot be used to measure the acidity of sulfated zirconia or composite materials based on it (5). Calorimetry has uncertainties due to thermal effects in- duced byinteractionsother than hydron transfer (6).In ad- dition, acid–base interactions may involve more than one hydron transfer; thus, some molecules, treated as simple bases in calorimetric studies, were later proved by NMR to be, at least in part, diprotonated (6, 7). Moreover, these methods give the total number of acid sites(Lewisand Brønsted) but do not differentiate between them. In those cases when only one of the two types of acid sites is catalytically active, the results from these methods do not correlate with catalytic activity. IR spectroscopy of absorbed basic probes such as pyri- dine allows us to easily distinguish the two types, but only semiquantitative measurements are possible because the exact value of the extinction coefficient of these species is very seldom known (8). Besides, as recently reported by Morterra, this method has led to at least an overestimation of the Lewis acidity of SZ systems (9). Therefore, in relation to our recent work on H/D ex- change occuring between small alkanes and solid acids, we have developed two complementary methods of chem- ical titration of sites in solid acids. In our previous work (10–13) we used the regioselective H/D exchange to achieve qualitativecomparisonsbetweendifferentsolidacidsunder various conditions. We monitor also the exchange taking place between the deuterated acid catalyst and water, describe the precise 0021-9517/01 $35.00 Copyright c  2001 by Academic Press All rights of reproduction in any form reserved. 344