Ind. Eng. Chem. Res. 1993,32, 1579-1587
1579
Kinetics of the Liquid-Phase Hydrogenation of (-) -a-Pinene over Electrolessly-Deposited Ni-P/y-A1203 Catalyst Sun-Hua KO and Tse-Chuan Chou' Department of Chemical Engineering, National Cheng Kung University, Tainan, Taiwan, Republic of China 701 The kinetics of hydrogenation of (-)-a-pinene in the liquid phase using electrolessly-deposited Ni-Ply-AlzOa as catalyst was studied. The results showed that the products were extremely selective with respect to the formation of cis-pinane. A reaction mechanism with dissociative adsorption of both hydrogen and a-pinene was proposed. It was found that the hydrogenation of a-pinene was characterized by cis-addition. On the surface of the catalyst, two distinct types of active sites resulting from the presence of phosphorus were introduced. The apparent activation energy of hydrogenation of (-)-a-pinene in the temperature range 110-150 "C was 68.0 kJ/mol of pinene. The reaction orders were 1.24 and 1.14 for pinene and hydrogen, respectively. The experimental results correlated well with the theoretical analysis.
Introduction Hydrogenation of a-pinene is an important process in the perfumery, food, pharmaceutical, and adhesive industries (Casbas et al., 1989). In general, both cis- and trans-pinane are the products of hydrogenation of a-pinene. However, trans-pinane is not easily oxidized to pinane hydroperoxide which can be further hydrogenated and pyrolyzed to synthesize linalool (Canova, 1977;Pavlin, 1982). The selectivity of cis-pinane in the hydrogenation may play an important role in the syntheses of linalool. In our previous studies (Chou and Lee, 1985; Hwang and Chou, 1987; Chou et al., 1990),kinetics of some oxidation systems were experimentally and theoretically studied. Recently, the hydrogenation of (-)-a-pinene over electrolessly-deposited catalyst was explored in our previous reports (Chou and KO,1989; KOand Chou, 1993). In this study, the kinetics of hydrogenation of a-pinene over the novel electrolessly-deposited catalyst is considered. In the preparation of the electrolessly-depositedcatalyst, nickel ion was reduced by the reducing agent, sodium hypophosphite. However, the deposition of nickel was accompanied by the reduction of the reducing agent itself, and a small amount of phosphorus was incorporated into the nickel deposit. The presence of phosphorus resulted in the novel characteristics of electrolessly-depositednickel catalyst. First, the presence of phosphorus is thought to have converted nickel deposit to an amorphous structure of higher activity than the crystalline state. Second, the phosphorus also offers better resistance to corrosive attack. Third, the incorporated phosphorus could make the electron density of nickel atom lower, as reported by Okamoto et al. (1980). It is expected that the active sites of the electrolesslydeposited catalyst are divided into two types: one is a bulk nickel phase which has higher electron density and the other is a dispersed phase consisting of nickel atom clusters in the neighborhood of phosphorus with lower electron density. The latter phase is more suitable for adsorption of an alkene molecule, which has ?r electrons. In this study, a reaction mechanism for hydrogenation of a-pinene over the electroless Ni catalyst is proposed and systematically investigated. Both the theoretical and
* Author to whom correspondence should be addressed. 0888-5885/93/2632-1579$04.00/0
experimental analysis of the kinetics of hydrogenation of (-)-a-pinene are explored in this paper.
Experimental Section Preparation of Catalyst. All chemicals used in the preparation of catalyst were reagent-grade. The support material was y-aluminum oxide (Merck) powder, and its purity is specified as follows: chloride (Cl) < 0.02 % ,sulfate (SO4) < 0.1 % ,arsenic (As) < 0.0005 % ,iron (Fe) 0.02 % . The aluminum oxides, in this study, were sized and rinsed with distilled water under ultrasonic and used subsequently without further purification. The powder was sensitized and activated in acidic stannous and palladium(11) aqueous solution, respectively. The pretreated aluminum oxide was rinsed and dried. The deposition solution contained 0.076 M nickel sulfate, 0.059 M sodium succinate, and 0.255 M sodium hypophosphite. The initial pH of the solution was 5.50. Deposition was carried out in this solution at 88 "C for 20 min with a ratio of 2 g of pretreated aluminum oxide/100 mL of deposition solution as described in our previous reports (Chou and KO,1989; KOand Chou, 1993). The catalyst, denoted 72 in previous papers (Chou and KO,1989; KO and Chou, 19931, was prepared under a rigorous condition of pretreatment and deposition. In this study, the catalysts in stock were covered with acetone. The total nickel loading of catalyst was determined by the dimethylglyoxime method in which nickel is stripped off completely with nitric acid, separated by dimethylglyoxime, and titrated by EDTA. The phosphorus content of catalyst was determined by a colorimetric method with the molybdenum blue reaction (Snell and Ettre, 1973). The prepared catalyst was also analyzed by ICP (inductively coupled plasma). The results of ICP indicated that the catalyst is a composition of nickel, phosphorus, and aluminum oxide. Traces of palladium and stannic hydroxide and/or oxide are also found, and their amounts are less than 10 ppm. Apparatus and Procedures of Hydrogenation. (-1a-Pinene (TCI) without solvent was chosen as a model compound for hydrogenation, which was carried out in a Parr-type autoclave equipped with a speed-controllable agitator. The reactants were directly used without further purification. Both catalyst and a-pinene were loaded into the reactor. The system was purged with nitrogen and 0 1993 American Chemical Society
1580 Ind. Eng. Chem. Res., Vol. 32, No. 8,1993
preheated to attain the desired temperature within f 2 "C. Then purified hydrogen was introduced to initiate reaction at the desired pressure within &3% . The kinetic data were obtained by the initial rate method with sampling per minute of hydrogenation. The consumption of a-pinene was analyzed by GC with a 9-mcolumn packed with 5% OV-1. The initial reaction rate was calculated for conversions of a-pinene less than 5%. The reaction order of pinene was obtained under various initial concentrations of pinenewith the resulting products as solvent. A preliminary experiment was carried out to make sure that the system was studied under a reaction-controlled region. In this preliminary experiment, a model compound was hydrogenated over prepared catalyst with various speeds of stirring at 150 OC and 400 psig until the reaction rate was not affected by the increase of stirring speed up to 500 rpm. All subsequent experiments were carried out at 550 rpm agitation in order to decrease the effect of mass transfer on the kinetic data.
hydrogen with dissociation and the bonding is not too strong. The adsorption of alkene may occur by several methods which are dependent on whether hydrogen exists or not (Bart6k et al., 1985). As reported previously (Chou and KO,1989; KO and Chou, 1993), the adsorption of (-)-apinene significantly depends on the steric factors. It was suggested that the exo rule would apply to this bridged bicyclo system as stated by Bart6k et al. (1985). The exo rule for addition reactions to the olefinic bond states that the addition occurs predominantly from the exo direction which is more weakly hindered than the endo direction, although the resulting endo product is thermodynamically unstable. The mechanism proposed in this study could consider the following adsorption of pinene as the determining step of isomeric product:
Theoretical Analysis
*
In general, the kinetics of hydrogenation of alkenes is well described by the Langmuil-Hinshelwood model which was popularized by Hougen and Watson (1943). In this study, a mechanism will be proposed to describe the hydrogenation catalyzed by electrolessly-deposited catalysts. The phosphorus which exists in the nickel layer of catalyst could decrease the electronic density of the neighboring nickel atom as discussed previously (Chou and KO,1989; KOand Chou, 1993). Therefore, the nickel atoms of catalyst are classified into type A and type B nickel which are designated as the nickel atoms with deficient and sufficient electronic density, respectively. The catalyst surface can be described as shown in Scheme I.
*
*
A
B
B
B
or simplified expressions (2a) and (2b) to be (2c) and (2d), respectively:
A
B
Scheme I Ni Ni
P Ni Ni Ni Ni Ni Ni Ni
Ni
Ni
P Ni Ni
Moreover, the distribution profile of phosphorus on the surface layer of nickel of the catalyst is assumed to be coincident with that of phosphorus in the bulk layer of nickel as confirmed by EPMA (electron probe microanalysis) reported previously (Chou and KO,1989; KO and Chou, 1993). Mechanism I. In our previous studies (Chou and KO, 1989; KO and Chou, 19931, the amorphous structure of nickel on an electrolessly-deposited Ni catalyst was examined by XRD. Although the catalytic behavior of amorphous nickel may be quite different from that of crystalline nickel, the adsorption of hydrogen on the amorphous nickel prepared by electroless deposition method was assumed to be dissociative:
where equilibrium of reversible adsorption was assumed because the elements in Group VIII, in general, adsorb
Because the double bond of a-pinene molecule locates at the six-membered ring with some rigidity, the dissociative adsorption of C--C on both type A sites may twist the ring part unstably if the distance between these two A sites is too short (when both sites locate with respect to the same phosphorus atom) or if the distance is too long (when both sites locate with respect to different phosphorus atom). Therefore, the formation of adsorbed species
8-8 i
i
A
A
is assumed to be neglected. It is expected that pinene can be adsorbed on nickel via formation of T - and u-complex as described by Mitsui et al. (1975). In the hydrogenation of (-)-a-pinene in this study, the catalyst was preheated with pinene. I t is assumed that pinene molecule is dissociatively adsorbed on both type A and type B of Ni atom as shown in eqs 2a and 2b. The pinene molecule is adsorbed in the exo direction, which resulta in the endo product, Le., cis-pinane. In this study, a small amount of tram-pinane was observed in the reaction system. This result suggests that both isomerization of cis-pinane and hydrogenation of a-pinene with trans-addition might occur to a limited extent.
Ind. Eng. Chem. Res., Vol. 32, No. 8, 1993 1581 Therefore, the following intermediates of half-hydrogenated pinene was considered: +
*
*
A
1
+
*A
+
*0
(34
A
B
In view of the steric factors, it can be assumed that K 1 (or K’1) is larger than K 3 , K‘3, and K”3. Furthermore, KZ is larger than K’:! because the nickel with deficient electronic density may more easily adsorb the electronrich alkene. The eqs 5a to 7c can be rearranged as follows:
The last step, the hydrogen addition of half-hydrogenated pinene, was assumed to be the rate-determining step:
(g L w
+ ** +
+
B
H
+! B
*B
*0
(4b)
H
Lw H
+
(W
Zt8
H
The rate law of hydrogenation with respect to the mechanism proposed as eqs l a to 4c could be derived as follows. For eqs l a and lb, respectively, klCH(8A)’ k’,cH(8B)2
= k-1(H8A)2 or = k-l(H8B)’
or
(K1CH)’/2(8A) (K’1cH)’/2(8B)
= (He,)
(Sa)
= (HOB) (5b)
where H 8 A and Hog are the fraction of type A and the fraction of type B active sites occupied by a hydrogen atom, respectively. Hydrogen is easily and dissociatively adsorbed on nickel atoms. Pinene, having a highly steric structure with both sides of bicyclo bridges, is expected to be adsorbed on nickel with comparative difficulty. It is possible,therefore, that the values of K 1 and K’1 are not smaller than K:!and K’:!,which can be derived from eqs 2a and 2b as follows:
Now, a constant is introduced to describe the ratio of number of type A sites to number of type B sites: 7 =NTA/N~
(11)
where NTAand NTBare the total number of type A and type B active sites, respectively. In our previous reports (Chou and KO,1989; KO and Chou, 19931,the distribution profile of phosphorus was as uniform as that of nickel on the cross section of catalyst powder. It is reasonable to assume that the surface concentration of phosphorus is the same as the bulk concentration of phosphorus in a particle which can be determined by ICP or colorimetric method with the molybdenum blue reaction. The results indicate that the mole ratio of phosphorus to nickel in bulk was about 1:5 to 1:lO. Therefore, q is assumed to be about 0.1-0.2 in this case. According to the definition of fractional coverage, 0, the total coverage of species A and B can be written as follows: z 6 A E 1 and zo~=1
k2C)iJ(eA)(eB)
= k-2(EeAeB)
or
KZcE(eA)(eB)
= (Ee,&)
or
(6a) k’zcE(8B)2
= k’4(EeB>
O r K’ZCE(8B)’
= (E8B>
(6b)
In eqs 2a and 2b, only ero-addition (cis-addition) was considered. Nevertheless, the derivation of eqs 6a and 6b involved both exo- and endo-addition. It is more suitable to consider dB as OBcis and egtrans as reported by Siege1and Dmuchovsky (1962). Similarly, derivation for half-hydrogenation is K,(EeAeB)
(He,) = (EHeA)(6),
(6,)
These equations can be rewritten in terms of the adsorption fraction of hydrogen:
(7a)
where
1582 Ind. Eng. Chem. Res., Vol. 32, No. 8, 1993 a1
And the last steps, eqs 4a to 4c, may be rewritten as
(K2/(KlK’l)1’2)(CE/CH)
= (K’Z/Kl)(CE/CH)
For simplicity, the difference in equilibrium constants that results from the different electron densities of the active sites may be neglected when both adsorptions of reactants are assumed to be weak. Therefore, a1= tl and p1= y1 Rearranging eqs 13a and 13b yields
(14)
(a&?)(HBA)3 + [a1(1- 7) - 8121(H6A)2 + 3 p 1 ( ~ e A-)2 = o (15) Assuming ( H ~ A
