Interaction of H2O and CD3OH with sulfuric acid studied by 1H-NMR: broad-line at 4 K and high resolution at 300 K

Colloids and Surfaces A: Physicochemical and Engineering Aspects 158 (1999) 207 – 210 www.elsevier.nl/locate/colsurfa

Interaction of H2O and CD3OH with sulfuric acid studied by 1H-NMR: broad-line at 4 K and high resolution at 300 K Patrice Batamack *, Jacques Fraissard Laboratoire de Chimie des Surfaces, associe´ au CNRS-ESA 7069, Uni6ersite´ Pierre et Marie Curie, Tour 55, 4 Place Jussieu, 75252 Paris Cedex 05, France

Abstract The interaction of limited amounts of H2O and CD3OH with H2SO4 has been quantitatively studied by broad-line H NMR at 4 K. Simulation of broad-line spectra shows that in aqueous sulfuric acid solutions, H2SO4·xH2O with x 5 4, the first ionisation yields the hydroxonium ion whereas HSO− 4 is such a weak acid that no interaction with water molecules is detected. The results for CD3OH/H2SO4 = 0.52 and 1.02 show that CD3OH is protonated and hydronium ion are formed. © 1999 Elsevier Science B.V. All rights reserved.

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Keywords: H2O; CD3OH; Sulfuric acid; 1H-NMR

1. Introduction Sulfuric acid is used in many industrial processes, particularly in the alkylation of isobutane by butenes for the production of branched highoctane-number hydrocarbons [1]. The dehydration of alcohols by sulfuric acid to form ethers is known to proceed via the protonation of the alcohol [2]. The reaction of alcohols with excess sulfuric acid leads to hydronium ion formation [3]. In aqueous solution, sulfuric acid produces the hydroxonium ion, which is believed to be solvated [4,5]. Broad-line proton NMR under ‘rigid lattice’ conditions is a powerful and a convenient tech* Corresponding author.

nique for studying short-range proton–proton interactions [6,7]. It is used here to elucidate the species formed and their geometry when relatively small amounts of H2O and CD3OH interact with H2SO4. 2. Experimental Reagent grade, greater than 98% H2SO4 from Prolabo was used for CD3OH/H2SO4 mixtures. Proton broad-line NMR at 4 K gave 0.06 H2O/ H2SO4 for this product, that is ca. 99% H2SO4. Methanol-d3 (99.6% d) was supplied by the Euriso-top company (France). Solutions 0.52 CD3OH/H2SO4 and 1.02 CD3OH/H2SO4 were prepared in a ‘glove-box’ by gravimetry and sealed in NMR tubes.

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Reagent grade, 95 – 98% H2SO4 from Prolabo was used for H2O/H2SO4 solutions. Proton broadline NMR at 4 K gave H2SO4·0.23H2O for this solution, that is ca. 96% H2SO4. Acid solutions, xH2O/H2SO4, with x = 0.5, 1.0, 2.0, 3.0 and 4.0 were prepared by dilution of the original reagent with distilled water and sealed in NMR tubes [8]. 1 H NMR spectra at room temperature were recorded on a Bruker MSL-400 spectrometer with a home-made 5-mm probe; the repetition time was 5 s. Chemical shifts are expressed relative to TMS as external reference using the usual conventions. For broad-line experiments, the samples were quenched in liquid helium, and the 1H NMR spectra recorded at 4 K on a homemade continuous wave 60 MHz spectrometer with phase detection and signal accumulation. The spectra are absorption derivatives. They are theoretically symmetrical with respect to the centre and, in practice, the two parts of the experimental spectrum are averaged; for this reason we show only half of each spectrum.

3. Analysis of broad-line 1H spectra The basic physical effect is a dipolar magnetic interaction between any pair of protons, proportional to r − 3, where r is the distance between the protons [9]. The broad-line spectra were simulated by means of a program written in Fortran. They correspond to the weighted sum of the contributions of the various species for which the following magnetic configurations were calculated (Fig. 1): (i) H2O, r-distant two-spin configuration [10] + also used for CD3OH+ 2 ; (ii) H2O…HO and H3O considered as deformed: a magnetic configuration

Fig. 1. Models used for simulations of the broad-line spectra.

with three-spins at the vertices of an isosceles triangle [11,12], where r is the base and r% the equal sides; (iii) H3O+ assumed to have C3v symmetry, a magnetic configuration with three r-distant spins at the vertices of an equilateral triangle [13,14]; (iv) OH, either a two-spin configuration [15] (if some OH groups are paired) or an absorption with a pure Gaussian or Lorentzian shape located at the centrum of the absorption spectrum, because the corresponding R distances are large relative to r and r%; a Gaussian function corresponds to protons ‘statistically’ distributed [16] and a Lorentzian function to ‘diluted’ spins [16]. A linear combination of these two functions can be used. Each of the corresponding functions (except for the Gaussian and the Lorentzian) is convoluted by a Gaussian which allows for the interactions between the hydrogen nuclei of the configuration and: (i) those belonging to neighbouring configurations; (ii) the other nuclei with non-zero spin in the vicinity. When the nuclear interaction between these nuclei and H atoms is small relative to the interconfiguration interaction between protons, the parameter of each Gaussian is related to a distance X which is close to the shortest distance between a proton of the configuration considered and a proton outside it. When a magnetic configuration with three-protons at the vertices of an isosceles triangle is used, the r value is always found typical of a water molecule (between 145 and 165 pm, usually between 155 and 165 pm). When r%/r is less than or equal to 1.1, deformed hydroxonium ions are detected. For larger r%/r, the internal O O distance of H2O…HO is calculated assuming C2v symmetry for the groups and OH distances of 100 pm; the presence of hydrogen-bonded complexes is indicated by usual O O distances. Larger O O distances are sometimes obtained. In such cases the total concentrations of OH and H2O, either using a three H configuration or only distinct configurations for these last species, are identical. The r%/r values corresponding to the simulations always lead to clear conclusions. The half-derivative spectra of H2O have a maximum about 6×10 − 4 Tesla and a minimum around 2× 10 − 4 Tesla; those of H3O+ have two

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Table 1 Number of oxygen-protonated species per H2SO4 (denoted n) after adsorption of the stated number of molecules per H2SO4 (denoted x), distances (in pm) used for simulations of the spectra xCD3OH/H2SO4 x9 3%

H3O+ ions

CD3OH+ 2 ions

OH groups

n9 10%

r 92

X 95

n 910%

r9 2

X 95

n 9 10%

r92

X 95

0.52 1.02

0.37 0.53

169 170

260 260

0.13 0.49

170 172

260 251

1.08 0.25 0.16

246 245 295

302 255 303

xH2O/H2SO4 x9 3%

H3O+ ions

0.23 0.50 1.00 2.0 3.0 4.0

OH groups

H2O molecules

n9 10%

r92

X95

n 910%

r9 2

X 95

n 910%

r9 2

X 95

0.23 0.50 0.96 1.00 1.05 1.00

166 167 167 167 166 165

267 265 255 253 248 247

1.77 1.50 1.04 1.00 0.95 1.0

238 236 225 217 204 202

291 270 285 269 243 241

0.0 0.0 0.0 0.92 1.75 3.00

147 152 151

198 198 196

maxima, especially a characteristic one at about 10–12× 10 − 4 Tesla, the other being near the origin, where are also those of OH groups. The minimum in well-resolved spectra is observed at 6×10 − 4 Tesla. The spectra of H2O…HO usually show two maxima, near the origin and from 4 to 6× 10 − 4 Tesla, and a minimum at 4 × 10 − 4 Tesla, but the relative intensities of these signals do not correspond to that of the same concentration of magnetically independent H2O and OH [12]. The procedure for simulating the NMR broad-line spectra is described in detail in Ref. [8]. The numerical base takes into account the total number of protons in the sample, which is equal to the number of protons in the reagent (sulfuric acid and its water content) plus the number of adsorbed methanols or plus twice the number of adsorbed water molecules. Acceptable values of X must be greater than (or at least equal to) those of r and r% when the influence of the interactions with non-zero spin nuclei is negligible. In the usual range of 910% accuracy on the species concentrations and with the above constraints, the simulation is unique due to the particular shape of the spectrum of each magnetic configuration and the abscissa values of the maxima or minima.

4. Results and discussion The proton NMR spectra of aqueous sulfuric acid solutions show a single Lorentzian signal whose chemical shift increases with the acid concentration from 8.7 ppm for 4 H2O/H2SO4 to 11.2 ppm for 0.52 H2O/H2SO4 and then decreases. The broad-line spectra of 0.52 and 1.02 H2O/H2SO4 mixtures show the presence of hydroxonium ions and ‘free’ hydroxyl groups. For xH2O/H2SO4 with x]2, in addition to the above contributions, ‘free’ water is observed (Table 1). For 35 x 54, water molecules preferentially form ‘clusters’ [8]. We conclude that the second Brønsted site of sulfuric acid is weak, in agreement with the literature, and that the hydroxonium ion is not hydrated at these levels of water loading although interaction with neighbouring species increases with dilution (X decreases with dilution). The proton chemical shifts of 0.52 CD3OH/ H2SO4 and 1.02 CD3OH/H2SO4 are 11.4 and 11.1 ppm, respectively, greater than those observed in superacid media (9.3–9.5 ppm) using sulfur dioxide as solvent [17]. The broad-line spectra of 0.52 CD3OH/H2SO4 and 1.02 CD3OH/H2SO4 samples are more complex. Hydroxonium ions are iden-

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tified. Ab initio calculations show that the H–H distance in CD3OH+ 2 is 158 pm. The broad-line spectrum of the superacid CF3SO3H with CD3OH (1:1) reveals the formation of methoxonium ions with a H–H distance of 1709 2 pm [18]. This value, about 10 pm greater than the calculated value (15892 pm), is probably due to hydrogen bonds. A two-spin species with an intraproton distance of 17092 pm is found in 0.52 CD3OH/ H2SO4 and 1.02 CD3OH/H2SO4 samples (Table 1). This species, by analogy to that found in 1 CD3OH/CF3SO3H, is the methoxonium ion. This is in agreement with the literature [2]. In 1.02 CD3OH/H2SO4 (Fig. 2) the number of H3O+ ions (0.53/H2SO4) is comparable to that of CD3OH+ 2 species (0.49/H2SO4), and the sum of the number of OH groups (two different environments) present is 0.41. The proton balance in this sample shows that 1.55 Brønsted sites of sulfuric acid are involved in the formation of the ionic species. It is can attack known that the nucleophile HSO− 4 to give CD OSO OH, which in turn CD3OH+ 2 3 2 may be attacked by CD3OH to yield CD3OCD3 [2].

Fig. 2. Half-derivative 1H NMR broad-line spectrum of 1.02 CD3OH/H2SO4. dF(h)/d(h) in a.u. is the derivative of the absorption signal, F(h), where h is the variation of the applied magnetic field relative to the centre of the spectrum. The weighted contributions of the oxygen-protonated species are shown.

5. Conclusion 1

H broad-line under rigid lattice conditions shows that in addition to the methoxonium ions, hydroxonium ions are already formed when H2SO4 interacts with 0.5 CD3OH. In 1.0 CD3OH/ H2SO4 the results show that the second Brønsted site of sulfuric acid is involved in the transformation of the alcohol. In contrast, the interaction of water molecules with the first Brønsted site of sulfuric acid yields H3O+ ions which seem not to be hydrated at low hydration levels ( 5 4 H2O/ H2SO4) though there is long-range interaction with neighbouring oxy-protonated species. The second Brønsted site of sulfuric has no affinity for water molecules. References [1] A. Corma, A. Martinez, Catal. Rev.-Sci. Eng. 35 (1993) 483. [2] J. March, Advanced Organic Chemistry: Reactions, Mechanisms, and Structure, 4th ed., Wiley, New York, 1992, pp. 389 and 1012, and references therein. [3] R.J. Gillespie, J.A. Leisten, Quart. Rev. Chem. Soc. 8 (1954) 40. [4] T. Kja¨llman, I. Olovsson, Acta Cryst. B28 (1972) 1692. [5] M. Liler, Reaction Mechanisms in Sulphuric Acid and Other Strong Acid Solutions, Organic Chemistry Series 23, Academic Press, London, 1971, p. 26 and references therein. [6] V. Semmer, P. Batamack, C. Dore´mieux-Morin, R. Vincent, J. Fraissard, J. Catal. 161 (1996) 186. [7] P. Batamack, C. Dore´mieux-Morin, R. Vincent, J. Fraissard, J. Phys. Chem. 97 (1993) 9779, and references therein. [8] P. Batamack, J. Fraissard, Catal. Lett. 49 (1997) 129. [9] D. Freude, Stud. Surf. Sci. Catal. 52 (1989) 169, and references therein. [10] G.E. Pake, J. Chem. Phys. 16 (1948) 327. [11] E.R. Andrew, N.D. Finch, Proc. Phys. Soc. 70B (1957) 980. [12] C. Dore´mieux-Morin, J. Magn. Res. 21 (1976) 419 and 33 (1997) 505. [13] E.R. Andrew, R.J. Bersohn, J. Chem. Phys. 18 (1950) 159. [14] R.E. Richards, J.A.S. Smith, Trans. Faraday Soc. 48 (1952) 675. [15] A.L. Porte, H.S. Gutowsky, J.E. Boggs, J. Chem. Phys. 36 (1962) 1695. [16] A. Abragam, Les Principes du Magne´tisme Nucle´aire, Presses Universitaires de France, Paris, 1961. [17] G.A. Olah, G.K.S. Prakash, J. Sommer, Superacids, Wiley, New York, 1985, p. 178, and references therein. [18] P. Batamack, J. Fraissard, Colloids and Surfaces A: Physicochemical and Eng. Aspects (1999) in press.