deuterium exchange reactions of alcohols over oxidic catalysts

Journal of Molecular Catalysis, 68 (1991) 33-43

33

Adsorption structures and mechanisms of hydrogen/ deuterium exchange reactions of alcohols over oxidic catalysts W. Baumaun, S. Lippert and K. Thomlce Inst. fiir Physikalische Chemiq 1040 Vienna (Austria)

Technische Universitzit W&m, f3etreidemurkt 9,

(Received October 15, 1990; accepted November 15, 1990)

Abstract Hydrogen-deuterium exchange reactions of the carbon chain of alcohols using DaOpretreated MgO, LaaO,, SmaO, and BPO, catalysts were examined. The surfaces of the catalysts were saturated with Da0 and afterwards the reactions were carried out in the pulse mode. MgO, LaaO,, SmaO, are catalysts with strong basic and weak acidic sites on their surfaces (El&-catalyst). BPO, is a catalyst with stronger acidic and weaker basic sites (El-catalyst). Hydrogen-deuterium exchange takes place at the hydrowl group and at the &position to the hydroxyl group of the alcohols when MgO, LaaOa and SmaOa were used. On carrying out the reaction with BPO,, no deuterium uptake was detected in the chain of carbon atoms, but rather only at the hydroxyl group. The dependence of deuterium uptake on temperature was examined using gas chromatographic and mass spectrometric methods. We selected a temperature range beginning with a deuterhrm uptake of 10% into the carbon chain and ending with a conversion of 5% to reaction products (such as alkenes, aldehydes or ketones by dehydration or dehydrogenation reactions) as a ‘deuteration window’. The temperature limits of a window are a compromise between the loss of reactant and an appreciable deuterium uptake. The ‘windows’ appeared between 170 and 290 “C for the alcohols used. The limits of deuterium uptake and onset of secondary reactions can be correlated with the potential curves of elimination reactions.

Introduction The demand for isotopically labelled compounds in chemical science is increasing with the growing application of physical methods of analysis, but their actual use is often restricted by high prices. In chemical science deuteriumlabelled compounds play an important role in examining reaction mechanisms, for kinetic studies and, last but not least, as solvents. Our studies of heterogeneously-catalysed hydrogen-deuterium exchange reactions in the gas phase were aimed to develop a method for a cheaper production of at least some deuterim-labelled compounds. Therefore as a deuterium source we use deuterium oxide, which is known as a cheap and harmless reagent compared with Da or other deuterated compounds. The selection of catalysts was based on the previous work of Thomke [l-5], who examined the catalytic dehydration of the CY-and @disubstituted 0304-5102/91/$3.50

CQ1991 - Elsevier Sequoia, Lausanne

34

ethanols R’CHOHCH2R2 (R’ = H, CHa, CH#Ha; R2=H, CH,, CH,CH,) by means of hydrogen-deuterium exchange reactions. Using BPO,, deuterium was detected only in the hydroxyl group and in the formed alkenes. Using Sm,O,, deuterium was found in the hydroxyl group, the formed alkenes and in the carbon chain of the alcohol at /3-and P’-positions. Using Ca3(P04)2, deuterium appeared in the hydroxyl group, but not in the carbon chain of the alcohol nor in the alkenes. The formation of alkenes by /3-eliminationreactions of alcohols in homogeneous solution has been well studied and can proceed in three different ways (see Pig. 1): (1) Prom the alcohol OH- is ilrst split off. Ln a second step, a proton is removed from the carbenium ion and an alkene is formed (El-mechanism). (2) A proton is removed 6.rst and a carbanion is formed. In a second step OH- is split off (ElcB-mechanism). (3) Both the proton and the hydroxyl group are removed at nearly the same moment; there is no ionic intermediate state (E2-mechanism). E 1 and E ZcB are two-step mechanisms.The proton split-off is a reversible reaction in both cases. This means that H/D-exchange in El is only possible if OH- is already removed. The carbenium ion cannot revert to the alcohol, hence deuterium exchange occurs during the second step, causing deuterium uptake only into the alkene. Thor&e investigated eliiation reactions by the use of various deuterated 2-butanols [5]. Catalysts favoring the El mechanism of dehydration of 2-butanol will be called ‘El-catalysts’ (such as BP04, Taa06, MoOa and ahuninosilicates [6]). Under the conditions of an El cB-mechanism, H/D exchange can take place during the 6rst step of the reaction. It is possible that the carbanion stabilizes by integration of D+ , whereby a deuterated alcohol is formed. If the alcohol first exchanges more than one deuterium atom and then undergoes the dehydration reaction, deuterium can also be detected in the alkene. Catalysts supporting this reaction pathway during the catalytic dehydration of 2-butanol will be called ‘ElcB-catalysts’ (such as Laa03, Sm203,Th02 or alkaline earth oxides [5, 61). If both proton and OH- are removed simultaneously, an E2-mechanism can be invoked. There is no possibility for H/D exchange, and deuterhrm is

El E2 EI~B F’ig. 1. Three possible routes of Selimination reactions.

35

Fig. 2. (Abscissa) potential energy vwsus (ordinate) reaction coordinate. e and 8 indicate the intermediate state of a carbenium ion and a carbanion in El and ElcB respectively. Fig. 3. Model of interaction of alcohols with EPAs and EPDs on the surface. 8 and (B represent EPD (Lewis base) and EPA (Lewis acid) sites of the catalyst surface.

detected neither in the carbon chain of the alcohol nor in the alkene (E&catalysts: some amphoteric oxides, in some cases Ca3(PO& [5, 61). It should be pointed out that the mechanism of a dehydration reaction depends on the reactants, catalysts and temperature. Moreover, it is impossible to determine pure mechanisms based on product distribution. But we can consider an E 1cB mechanism coupled with a small amount of E 1, for example. Halbz et al. [6] have described the mechanisms by their potential curves (see Pig. 2). By investigating /3-eliminationreactions using different catalysts, conclusions can be drawn about the strength of the active sites on the surface of catalysts. Dehydration of alcohols depends upon the (relative) strength of EPA-(electron-poor-) and EPD-(electron-rich-) sites on the surface [6, 71. (1) If acidic sites (EPA) are very strong compared with the basic sites on the surface, it is easy to abstract OH- from the alcohol, causing carbenium ion formation (El mechanism). (2) On the contrary, if acidic sites are very weak in relation to basic ones, proton abstraction will proceed quite easily by formation of a carbanion (ElcB mechanism). (3) Finally, if acidic and basic sites are of equal strength, the proton and the hydroxyl group will be removed simultaneously (E2 mechanism). Alcohols interact with the surface via B-protons and hydroxyl groups f7] (see Pig. 3). For the synthesis of deuterated alcohols, El cB catalysts seemed to be the best choice. In the literature they are also described as solid bases [SJ, for example LaaO,, Sm,Oa, MgO, CaO, SrO, BaO, Zr203, ThOa or CeOa. As a representative El catalyst we used BP04.

Experimental A simple microcatalytic pulse reactor can be constructed by replacing the glass liner of a GC split/splitlessinjection port for capillary columns by a silica tubing containing a sintered quartz plug as a catalyst support. By variation of temperature and preliminary pressure, reaction conditions can be changed easily. The GC was connected with a F’innigan MAT ion trap

36

detector ITD 700. As separation column we used a Durabond Wax fused silica capillary column 30 mx 0.32 mm, film thickness 50 run, from J&W Scientific. Reactants: As deuterium source DZO 99.9% from Merck was used. All reactants and catalysts used were of the highest available purity. Boron phosphate BP& (EGA-Chemie, Ste~e~~buch) was calcined 10 h at 500 “C (BET surface: 7.4 m2 g-l). Magnesium oxide was prepared by 10 h decarboxylation of MgCOs at 1080 “C 111.As BET surface 14.9 m2 g- ’ was measured. After the decarboxylation no more MgCOa was detectable in the formed MgO by X-ray diffraction. After treating the MgO with 3 pulses of DzO, it was totally transformed into Mg(OD)2. Thus, the active catalyst for deuteration is not MgO but its deutero-oxide! ~~~~ oxide LaaO, (Merck PA.) was calcined 2 h at 500 “C, placed in the reactor and flushed with carrier gas (He 5.0). The analysis of D20treated LaaOa indicated a conversion to La(OD),. No hexagonal LaaO, was detectable. It is possible that the catalyst contained small amounts of La20&0, (BET surface: 7.3 m2 g-*)_ Samarium oxide SmaOa (Research Chemicals, 99.9%) was prepared in a similar manner. X-ray diiTraction indicated a high amount of Sm(OD),. Only a small amotmt of cubic Tl,O,-like Sm,Os and small amounts of basic samarium carbonate were detected (BET surface: 7.2 m2 g- ‘). At lower temperatures the catalysts also contained adsorbed D20, because when the weight increase after Da0 treatment was measured, the oxides had taken up more than the equivalent amormt of D20. Specific surfaces of catalysts were measured by an areameter (StrohIein) X-ray diffraction measurements were carried out with a Phillips powder di@ractometer PW 1710. After Cal&nation (30 min at 300 “C) the reactions were carried out in the pulse mode. The catalysts were treated with two to four pulses of deuterium oxide followed by one pulse of the reactant (one pulse= I EiLl)-

Calculation

of deuterium

content iu products

A common method to determine the deuteriumcontent of a pure substance (pure in the chemical sense, not in the sense of containing isotopes) is to measure the height ratio of the molecule peaks (Mi+) in the mass chromatogram. Mass c~orna~~ of compounds including oxygen often show a significant peak at MC + 1 caused by chemical ionisation (oxonium ion formation by Hf addition). This can be interpreted as a higher deuterium content. We gauged the system by injection of different volumesof undeuterated reactant. Thus we could correct the mass c~orna~~ of deute~~d compounds by subtracting the peaks due to chemical ion&&ion.

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The percentage of molecules including i deuteriumatoms can be calculated by eqn. (l)*.

Equation 2* is used to calculate the maximum degree of deuteration (&& d_=

A x

0

xl00

For example, in the carbon chain of l-propanol it was possible to substitute protons by deuterons only in the Z-position. Ignoring the proton of the hydroxyl group only two (=A) of seven (=X) protons in the molecule are exchangeable; d,, would be (2/7) X 100 = 28.6%! The third equation provides the percentage of protons exchanged (%D) in relation to the exchangeable ones. For example, if all exchuweable protons of the carbon chain are replaced by deuterons,the degree of deuteration is not 28.6%, as calculated above, but 100%. This can be calculated by eqn. (3)*.

(3)

Results

The acidity of the hydroxyl proton is high enough to exchange in an excess of Da0 without a catalyst. Therefore it was only necessary to look at deuterium exchange in the carbon chain of alcohols. In Figs. 4(a, b) to 1l(a, b) below, the deuterium uptake (%D) in the carbon chain of alcohols is shown on the left side and the conversions (%C) to secondary products are shown on the right side, both as a function of temperature (in “C). The temperature ranges, beginning at 10% deuterium uptake (left-hand figures) and ending at 5% of conversion to secondary products (right-hand figures) will be called ‘deuteration windows’. In methanol we could not detect any deuterium at the carbon atom. Ethanol includes three, while all other primary alcohols include two exchangeable pprotons. The results are shown in Figs. 4(a, b) to 7(a, b). *i = Number of deuterium atoms taken up. Note that 0 sisA! hi= Height of the peak belonging to a molecule that has taken up i deuterium atoms. X=Number of all protons in the molecule. A = Number of exchangeable protons in the molecule.

u

:

:

190

01

170

10..

lea

_.

.-.

:

230

:

250

:

270

240

280

--

: ’

310

oxide

uptake

(a) and conversion

U Samariumoxide

of deuterium

280

Lanthanum

(a) and conversion

-X Magnesiumoxide

*

uptake

C- Samariumoxide

.X. Magnesiumoxide

of deuterium

290

_ __-.

dependence

TEMFEFlAN!4E

220

dependence

TEMPERANRE

210

Fig. 5. Temperature

:

200

01

5

25

10.: n

‘.

20 ..

%D 15

(a)

‘m

a!A

20

30

Fig. 4. Temperature

(a>

%D

50-

(b) of 1-propanol.

(b)

170

7oC

170

507

70 ..

80 ..

(b) of ethanol.

@I

%G

so -.

100 -

Isa

190

m

230

250 TEMPERANRE

210

270

250270290

TEMPERATURE

210

2gO

0 Swnwii

.X Wpeeium

1

Sameriwn

oxide

axida

oxide

Megneeiunoxide

*-oxide

U

x

l Lenthenumoxide

39

40

41

8

42 TABLE

1

Deuteration windows and window width of catalysts causing highest deuterium uptake Alcohol

ethanol 1-propanol Z-propanol 1-butanol 2-butanol 1-pentanoi 2-pentanol 3-pentanol

KP (“C>

Window

Widow

(“0

(“Cl

79 97 82 117 100 138 119 116

170-240 190-280 180-260 200-260 180-270 200-290 200-260 1SO-260

70 90 80 60 90 90 60 80

width

Catalyst

Sm203 Sm203 Sm203 Sm203 Sm203 Sm203

MgO w@s

Secondary alcohols: In 2-propanol six, in 2-butanol and 2-pentanol five and in 3-pentanol four protons could be exchanged. Deuterium uptake (%D) and conversion (%C) are shown in Pigs. 8(a, b) to ll(a, b). The deuteration windows for the catalysts with highest deuterium uptake in the carbon chain are shown in Table 1.

Discussion

It was shown that alcohols undergo the exchange reaction at /3-and /3’-positions when MgO, SmaOa and L&O3 (ElcB catalysts) were used. Moreover, the proton of the hydroxyl group was also exchanged. Methanol does not contain P-protons, therefore we could not find deuterium at the carbon atom. BP04 did not catalyse any deuterium uptake into the carbon chain of alcohols. BPOl (El catalyst) influences the alcohols such that hydroxyl abstraction proceeds first. It is not possible to stop the reaction at a certain temperature to obtain deuterated reactants, because the energy level of the molecules is high enough to form alkenes. With increasing temperature the deuterium uptake goes through a maximum. The lower deuterium content at high temperatures is caused by the lower Da0 content of the catalyst at the upper limit of the deuteration windows. The deuteration windows described above can be compared with the principal path an ElcB reaction follows. The first step (removal of a proton) is possible at relatively low temperatures because the activation energy for this process is low compared with the activation energy for the abstraction of the hydroxyl group. The low activation energy for H+ abstraction is on the other hand caused by strong interactions with the basic sites (EPD) of the surface. Thus we can consider that the lower temperature limit of the deuteration window can be compared with the first step of the potential curve of the ElcB reaction (H+ split off). At higher temperature ranges

43

(upper limit of the window), most of the molecules also have enough energy to undergo the second step of the reaction (OH- split off). Oxidic. catalysts (MgO, Sm203 and La,O,) were totally transformed to their deutero-oxides by D20 treatment. We consider that the catalysts also contained a certain amount of adsorbed DzO.

References 1 2 3 4 5 6 7 8

K. Thomke, Z. Phys. Chem., N. F., 106 (1977) 225. K. Thomke and H. Noller, 5th 1n.t. Congr. CataL, Miami, 1972, Paper 84, pp. 1183-1191. K. Thomke, Z. Phys. Chem., N. F., 107 (1977) 99. K. Thor&e, Z. Phys. Chem., N. F., IO6 (1977) 295. K. Thomke, Z. Phys. Chem., N. F., IO5 (1977) 87. I. Hal&z, H. Vinek, K. Thomke and H. Noller, Z. Phys. Chem., N. F., I44 (1985) 157. H. Noller and W. Kladnig, Cutal. Rev. Sci. Eng., 13 (1976) 149. G. Zhang, H. Hattori and K. Tanabe, Appl. Catal., 36 (1988) 189.