Mechanistic study on dehydrogenation of methanol with [RuCl2(PR3)3]-type catalyst in homogeneous solutions

Journal of Molecular Catalysis A: Chemical 108 (1996) 87-93

Institute

oflndustrial

Science,

The University

Received 7 September

of Tokyo.

22-1

Roppongi

1995; revised 1 I December

7 Chome. Minato-ku.

1995; accepted 14 December

Tokyo 106. Japan

1995

Abstract Catalytic dehydrogenation of methanol has been investigated in homogeneous solutions with a series of Ru(lI) complexes, [RuCl,(P(p-C,H,X),),] (X = H (I), Me (21, F (31, Ohl[e (4)) and [RuCI,(PMePh,),] (g>. In the gas phase, hydrogen was formed selectively (> 99.5%), and formaldehyde, methylal (formaldehyde dimethyl acetal) and methyl formate were found in the liquid phase with satisfactory stoichiometry to the formed hydrogen. The reaction was retarded by the extra addition of free phosphine, suggesting the presence of pre-equilibrium dissociation of phosphine ligand. Kinetic analyses from this viewpoint explained well the dependence of rate on the concentration of catalyst (saturation curve). The order of evaluated pre-equilibrium constant (I= 2 > 5) is in accord with the general idea that the dissociation of phosphine ligand is controlled principally by steric buik of ligands. The order of rate (3 > 1 > 2 > 4) for I-4, possessing the same cone angle of phosphine ligand, correlated clearly with hasicity of phosphines. The results are interpreted in terms of the mechanism of rate-determining P-hydrogen abstracuorl ,,_ the Ru-OCH, intermediate. Keywords:

Ruthenium; Methanol; Dehydrogenation; Phosphiae-derivative complexes; Ligand effect

It is well known that Ru(I1) complexes are particuhtrly catalytically active for dehydrogenation of methanol to give H, and dehydrogenated product(s) (HCHO, H,C(OMe),, HCO,Me) [l-4]. The catalysts include a very common [RuC12(PPh&] [l], and some complexes of other metal species are reported to be catalytically-active thetmally [4] or under pho-

toirradiation [5-71. In this reaction, formation of coordinatively unsaturated species is considered to be important. For example, [RuH,(PPhJ),] and [RuH,(N,)(PPh,),] are efficient catalysts, which have good leaving groups (H, and N,, respectively) [4]. For [Ru(OCOMe)Cl(PR,),], formation of a vacant site by bidentate to monodentate rearrangement of acetate ligand was suggested [3]. When S&I) ligand is introduced into R&l) methyl acetate) complex, acetic aci (and/or

’ Corresponding author. ’ Present address: Yokkaichi Research Laboratories, Japan Synthetic Rubber Co., Ltd., Kawajiri-cho 510. Japan.

100, Yokkaichi-shi.

We-ken

1381-I 169/96/$15.00 0 1996 Elsevier Science B.V. All rights reserved Sl381-I 169(96)00003-9

PI!

[12]. In this unique

reaction,

the first step to

L.-C. Yart~et ul. / Journal oj’Molertclur CutulysisA: Chemicul 108 (1996) 87-93

88

tial turnover rates were obtained fro linear slopes below a catalyst turnover number of 3.0. In the liquid phase, formaldehyde, methylal (formaldehyde dimethyl acetal) and methyl fox-mate were found, which would be formed according the following reactions [9,13]. ,0H --) HCHO + H,

(1)

HCHO + 2CH@H -+ CH,(OCH,),

+ I-I,0 (2)

time I h Fig. I. Time course for the hydrogen evolution with 1 used as

2HCHO

4

HCO,CH 3

(3)

catalyst ([II = 0.045 n&f).

dehydrogenate methanol to reduce formaldensidered to be slowest in the overall Thus, in order to elucidate the factors affecting the methanol dehydrogenation in more detail, we studied here the kinetics and ligand series of Ru(II) complexes, effect using 4)03)31 (X = W (I), Me (2), F [Rue1 ,(P( p-C )) and i&Cl,(P

presentative time course for t as monitored by z evolution. The evolved gas was exclusively H,, and t amounts of other gaseous products such as C were negligibly small (Co < 0.2% and CH, < 0.004%). Turnover numbers are calculated from the amount (mole) of H, formed divi charged amount (mole) of R&I) c

Table

Table 1 shows t e amounts of hy dehydrogenated products in the liquid phase, which were formed until catalyst deactivation became noticeable. Herein the concentrations of catalysts were chosen to be as high as possible to obtain greater amounts of products. It is obvious from Table 1 that in every case there is satisfactory stoichiometry between them. 2.1. Dependence of the rate on the concentration of added tertiary phosphines d that the reactions with 5).1).7l (I), ERuCl,(P( p> and [RuC1,(PMePh,),] (5) by extra addition of P(C,H,),, Me), and PitiePh,, respectively. hen the reciprocal of the initial rate (R) was plotted as a function of the concentration of added phosphine, linear dependence was observed with a slight deviation in the region of low concentration (Fig. 2a, b and c). Rate retardation by the extra addition of free ligand suggests that the catalytically active

I

Amounts of hydrogen and liquid-phase products formed in the dehydrogenation of methanol with catalysts 1-5 Catalyst

Concentration

Reaction time

Hz

Liquid-phase product/mm01

/mM

/h

/mm01

HCHO (1)

CH,(OCH,)*

0 + If + 2 x III) (III

HCO,CH

s (III)

/mm01

1

0.80

30

1.39

0.54

0.92

trace

2

0.65

99

I .42

0.48

0.40

0.22

1.32

3

0.35

72

0.72

trace

0.05

0.35

0.75

4

0.43

I65

I so

0.00

0.86

0.40

1.66

5

0.80

23:

2.60

0.00

0.77

0.91

2.59

1.46

-m

L.-C. Yang ct al./Jcxmu~l

r~Molecuhr

Cutu1y.G A: Chemical 108 11996) 87-93

89

where [Ru], is the concentratio as charged ([RuCl,(PR,), Since the reactant methanol is (also acting as a solvent), a pseu ion uld result for [NleOH] = k). ence Eq. (5) can be rearranged to give Eq. (6). 1

1

[PR,I (6)

z=k[Rulo+

This equation accounts for the observed linear relationships between 1/R and [PR j],,,,, in -concentration region. e upward deviations in the low-concentration region would be due to the inherent contribution of the ligand dissociation to the total concentration of free ligand, which should become appreciable in this region. The values of k and K obtained by coupled use of intercepts and slopes in Fig. 2 are summarized in Table 2. 2.2. Dependence

I

0.0

0.5

1.0

1.5

[PMePh21rddI mM Fig. 2. Effect of the addition of free PR3 on the mitia: rate with (a) 1, (b) 2 and (cl 5 used as catalysts. [l] = 0.80 mM. [2] = 0.15 mM, [51= 0.17 mM.

species is formed by pre-equilibrium sociation (Eq. (4)).

ligand dis-

If the rate of dehydrogenation is expressed as a second-order rate equation with respect to the [RuCl,(PR,),] formed and methanol, Eq. (5) is deduced. R = k,[RuCl,(PRJ),I

[Me0

on the catalyst concentration

With 1, 2 and 5 were used as catalyst, the catalyst concentration was varied between 0.022-0.285 m 0.045-0.80 mM 1, respectively. 0.045-o. 169 m served initial rate plotted as a function of the COPCDT.I .“::on of catalyst is given as circles in Fig. 3a, b and c. The observed tendency of saturation in the high concentration region seems to be in accord with the presence of equilibrium degree of dissociation in Eq. (41, because t l..Vouldbe lowered in The postulation above was verified quantitatively as follows. When no extra P the relation [[Ruc~,(PR,),]] = [PR,] results in

Table 2 Evaluated values of equilibrium

constant (#)

and rate constant

(Ii) Catalyst

k/h_’

K/M

8.5

1

2.9X IO-J

2

2.8x

IOmJ

4.0

s

2.0x

10-4

7.1

L.-C. Yung Ed ~1. /Journal

c$Moleculur Catalysis A: Chenaid

IO8 t 1996) 87-93

theoretical values is evident, which supports the above postulation. A similar saturation 1:urve was already pointed out for a different type of with alcohols catalyzed by reaction [RuCl,(PPh,),], but was not analyzed quantitatively [ 141. 0.0

0.0

0.2

0.4

[catalyst]

0.6

chanistic considerations

0.8

I mM

has been recognized that an equilibrium for the dissociation of phosphine ligand from strongly depends on [M L?I] Taking into consideraste lk of tion this general tren it seems reasonable that and 2 are close to each the values of K for other (Table 2), because cone angle is common 145”) for P( P-C~I--I~)~ and ?( pMe),. The smaller cone angle of PMePh, ( 8 = 136’“)would thus give a smaller value of K for 5. contain the phosphine ligand with e angle ( 145”), the difference ia the overall dehydrogenation rate may be caused the process after the pre-equilibrium of Eq. ( ne of the plausible mechanisms for methanol rogenation consists of three kinds of suce processes [ I6- 191: ( 1) coordination of methanol to the metal complex resulting in the ation of M-QCH, intermediate, (2) formation of a hydride complex from the M-OCH, intermediate via P-hydrogen abstraction, which liberates formaldehyde, and (3) !ecomposition of the hydride complex with which regenerates the catalyst. A the second process seems to be rate-determining, because a lot of methoxy complexes are known [XI] and stoichiometric formation of H, by the reaction of hydride complex with protonic compound is common [21]. This view is he deuterium isotope effect on the drogenation of 2-propanol-2-d, 1,),]4- 1221, where the RJR, claimed to be c~:n:;istent with ing P-hydrogen abstraction. Fig. 4 shows the on logarithm of initial rate vs. pK, It

[catalyst]

! mM

0.80

Fig.

3. Dependence

catalyst.

of

the initial

rak

on the concentration

of

(a) I, (b) 2. (c) 5.

(4).

ence,

the concentration of KRuCI,(PR,J,] is expressed as Eq. (7), and the rate as Eq. ($1.

Eq.

(7)

k and function of [Ru],. En Pig. 3, the relationships based on this theoretical analysis are also give as solid lines. Coincidence of experimental an

L.-C. Yang et al./Journal

of Molecular Catalysis A: Chemical 108 (1996187-93

91

ost the electron density on Ru, philic) interaction of Ru with strength of thi crease in the order of ). It is to br: noted that the same order is ected from the vi interactions, since electron-w are suggested to enhance the interaction between the vacant metal d orbital and the C-H p

P&3 Fig. 4. fnitial rate plotted as a function of pK, of HP(&,H,X);

for ~RuCI,(P(~-C,H,X~~)~I

catalysts.

value of conjugated acid of phosphine ligand, HPR;. Initial rates were dete ined here in the region of low catalyst concentration (0.045 m in which all the rates are approximately fustorder with respect to the catalyst concentrations. It is obvious in Fig. 4 that the correlation is ar and the order (H) > 2 (Me) > the reverse of that for p K, measure for the strength of ine [23]. It is of interest t a similar plot against the ammett a value also gave a linear relations (Fig. 5) fi4,24]. te that the The results in Figs. 4 and 5 i originates difference in the rate among from the electronic effect of phosphine ligand, and that the rate-determining step could be analyzed in terms of the basicity (electron-donating

-3.6 -3.8 -4.0

Fig. 5. Effect of substituent X in [RuCI&P( p-C,H.,X),), lyst on the initial rate.

3.

Preparations were made un er an argon atmosphere using standard vacuum-manifold and Schlenk techniques. All reagents and solvents were of reagent grade. Light petroleum refers

-3.2

3

possible to propose an alternative echanism including a rate-determining oxidative addition of methanol 1201. However, this process would become more advantageous for center with higher electron density For example, the second-order rate constant for oxidative addition of [IrCl(CO)(P( p-C,H,X),),] (X = was reported to increase in the order of < OMe i29], which is apparent1 e order observed here. In concl of methanol dehydrogenation would be accelerated by introducing (1) bulky ligandts) to enhance the ligand dissociation, and (2) less electron-donating ligand(s) to promote the P-hydrogen abstraction.

cata-

L.-C. Yurrg er ul./Jnurnul

92

of Moletulur Curulysis A: Clwmicul108

standards. Elemental analyses were conducted using a Yanaco MT-3 element analyzer. 3.1. Synthesis of [RuCl,(Pfp-C, H4 Me), )_3I

11996187-93

yield), “P NMR: S(P) = 27.7 ppm. ‘“F NP.IR: 6(F) = - 108.4 ppm. Analyses found: C 57.83, H 3.59; calcd for C,,H,6F,P,Cl,Ru: C 57.87 and H 3.24%.

RuCl, - 3H, (0.50 g, 1.91 mmol) was dissolved in methanol (100 ml), and heated under reflux for 5 min. After cooling to room temperature, P( p-C,H,Me), (3.54 g, 11.6 mmol) was added to the homogeneous solution, and then refluxed for 2 h. Upon cooling to room temperature, reddish brown crystals were formed, which were filtered, washed with diethyl. ether (5 ml X 2), and dried under vacuum ( 1.5 9 NMR: 6(P) = 39.9 ppm. A 5.95; calcd for C,, 5.85%.

3.4. Synthesis of [RuCl,(PMePh, i31

3.2. Synthesis of

3.5. Catalytic reaction

RuCl 3 - 3H,O (0.50 g, 1.9 1 mmol) and PMePh, it.40 g, 12.0 mmol) were dissolved in methanol (100 ml), and heated under reflux for 14 h. Upon cooling to room temperature yellow crystals were formed, which were filtered, washed with diethyl ether (5 ml X 21, and dried under vacuum (1.07 g, 72% yield). “P NMR: 6(P) = 18.7 ppm. Analyses found: C 60.46 H 5.09; calcd for C,,H,9P,Cl,Ru: C 60.62 and H 5.09%.

(0.12 g, 0.46 mmol) was dis-

The reaction solution was prepared by dis-

01 (30 ml), and heated under reflux for 5 min. After cooling to room tempera( 1.OOg, 2.84 mmol) was ture, P( p-C,W,0Me), ded to the homogeneous solution, and then perarefluxed for 3 h. Upon cooling to room were ture, brown crystals were formed, wh filtered, washed with diethyl et dried under vacuum (0.32 NMR: 6(P) = 39.7 ppm. 16, H 5.13; calcd for C,, 61.56 and H 5.17%.

solving calculated amount of the Ru(I1) come reacplex in degassed metharul (200 ml). tion was performed at 64°C (refluxing) under a nitrogen atmosphere (1 atm) in a thermostated room (within + 1°C). The reactor was equipped 2O’C) to which a gas volumetric measureburet was attached. ment was started after refluxing became stationary. Products were analyzed by GC (TCEP, Porapak T and active carbon columns). For the sake of higher precision for analyzing formaldehyde, CO and CO, (Porapak T), a methane convertor was used with a FI detector.

3.3. Synthesis of [RuCI,VQK,

H,

F), j31

uC1, * 3H 2O

(0.15 g, 0.57 mmol) was dissolved in methanol (30 ml), and heated under reflux for 5 min. After cooling to room temperature, P( p-C 4F)3 (1.09 g, 3.45 mmol) was added to the homogeneous solution, and then refluxed for 21 h. Upon cooiing to room te perature and reducing the solution volume in V~CUO,brown crystals were formed, which were filtered, washed with light petroleum (5 ml x 2) etroleum-diethyl ether (1: 1 v/v, 5 ml X 2), and dried under vacuum (0.11 g, 17%

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