Ind. Eng. Chem. Res. 1995,34, 457-467
457
Selective Hydrogenation of (-)-a-Pinene over Nickel Catalysts Prepared by Electroless Deposition Sun-Hua KO and Tse-Chuan Chou* Department of Chemical Engineering, National Cheng Kung University, Tainan, Taiwan, Republic of China 701
TsongJen Yang Department of Material Science, Feng Chia University, Taichung, Taiwan, Republic of China 407
The selective hydrogenation of (-)-a-pinene i n the liquid phase over several supported and skeletal nickel catalysts prepared by electroless deposition was studied. The (-)-a-pinene, which had bridged bicyclic structure, was chemisorbed on the catalysts with minimum steric hindrance, and the hydrogens were transferred to the proximal face of the double bond to yield the main product. The results showed that the selectivity of cis-pinane increased with decreasing acidity of support, temperature of hydrogenation, and electron density of nickel. On the other hand, the selectivity of cis-pinane increased with the pressure of hydrogen. A reaction mechanism with isomerization as well a s half-hydrogenation was proposed. On the basis of both the steric and electronic effects, three types of reaction conditions were included. Steric effect favors the direct hydrogenation, which results in cis addition. Electronic effect, however, induces indirect hydrogenation, which produces more stable trans-pinanes. In the hydrogenation of (-1-a-pinene, which was a nucleophilic reagent, the role of boron and phosphorus in nickel catalysts was described as a n electron donor and acceptor, respectively. The experimental results correlated well with the theoretical analysis.
Introduction Hydrogenation of (-)-a-pinene over Ni-Ply-AlzOscatalysts, which were prepared by electroless deposition, was studied in our previous papers (KOand Chou, 1992, 1993a,b). The experimental results were correlated to the theoretical model derived by the modified Horiuti and Polanyi mechanism (Horiuti and Polanyi, 1934; KO and Chou, 1993a). Owing t o the apparent factor of steric hindrance of (-)-a-pinene, the cis product of hydrogenation was predominant. In general hydrogenation of alkene over supported catalysts, the selectivity is affected by the structure of alkene (Siegel, 19911, the type of active metal and support (Satterfield, 19911,the interaction between metal and support (Tauster, 1986), the poisoning of catalyst, and the operating conditions such as temperature (Cocker et al., 1966) and pressure (Siegel et al., 19661, etc. Furthermore, several studies (Okamoto et al., 1980a,b, 1982a,b; Karpinski, 1990) indicated that the electron density of active metal, which was modified by the promoter and/or the second component in alloy catalysts, also affected the adsorption of species as well as the catalytic selectivity. For example, addition of phosphorus, which is thought to withdraw electron from the nickel atom, results in the decrease of electron density of nickel in nickel-phosphorus catalysts (Okamot0 et al., 1980a,b, 1982a,b). On the other hand, boron, as an electron donor, increases the electron density of nickel in nickel-boron catalysts (Okamoto et al., 1980a,b, 1982a,b). The electronic transference between nickel and phosphorus (or boron) was confirmed by the results of X-ray photoelectron spectroscopy (Okamoto et al., 1979, 1980b). The correlation of electron density of nickel to the selectivity of CO and butadiene hydrogenation has also been proposed (Phillipson et al., 1969; Okamoto et al., 1980a, 1982b). ~
~
~~
~~
~
~
* Author t o whom correspondence should be addressed. 0888-5885/95/2634-0457$09.0~IO
Hydrogenation over nickel catalysts prepared by electroless deposition has been studied recently (Yang and Chen, 1988; KOand Chou, 1992,1993a-c, 1994a,b). However, the selectivity of hydrogenation was seldom concerned. The details of the role of boron or phosphorus were not discussed and described, either. In our previous papers (KOand Chou, 1992, 1993a,b), phosphorus was found to quantitatively affect the kinetics of hydrogenation of (-)-a-pinene. The hydrogenation accompanied by isomerization was only qualitatively discussed by means of the half-hydrogenation mechanism. The effect of electron density of nickel on the selectivity of hydrogenation of (-1-a-pinene is interesting. In this study, the selectivity of hydrogenation of (-1a-pinene over various nickel catalysts prepared by electroless deposition is systematically investigated. The effect of electron density of nickel, which is resulting from the presence of boron or phosphorus, on the selectivity of hydrogenation is also explored.
Experimental Section All chemicals used in the preparation of catalysts were reagent-grade. The prepared catalysts are divided into two types. One type is the supported catalysts with aluminum oxide or kieselguhr as the support. The procedure of preparing the supported catalysts was described in our previous papers (KOand Chou, 1993a, 1994a,b). The deposition conditions for preparing these catalysts are summarized in Table 1. The other type is the skeletal catalysts such as NiP/Al, NiB/Al, or NVAl catalysts. These skeletal catalysts are prepared as follows: The fine powder of aluminum was first deposited with Nip, NiB, or Ni as shown in Table 1. Aqueous solution of sodium hydroxide (2.8 M, 50 m u g of aluminum) was then used to leach the aluminum in these aluminum-supported catalysts at 70 "C for 1h.
0 1995 American Chemical Society
458 Ind. Eng. Chem. Res., Vol. 34, No. 2, 1995 Table 1. Deposition Conditions in Preparation of Catalysts NiPlAl203 or Nikieselguhr NiPkieselguhr NiPlAl NiBkieselauhr NiBIAl or NUN sodium hypophosphite, sodium hypophosphite, sodium borohydride, dimethylamineborane, hydrazine, 260 95 79 119 2740 sodium succinate, sodium citrate, ethylenediamine, sodium acetate, EDTA-2Na, 67; 59 340 1200 162 Na-hydroxyacetate, 612 stabilizing agent, ppm thiourea, 1.33 Pb2+,6 nickel salt, nickel chloride, nickel sulfate, nickel chloride, nickel chloride, nickel acetate, mmol/L 76 126 168 303 482 temperature, "C 88 88 95 70 90 5.5 9.W ca. 14b 10 5.5 PH time, min 30 30 20 60 60
-
condition reducing agent, mmol/L complexing agent, mmol/L
Ammonium chloride buffer, 0.93 mom. Sodium hydroxide aqueous solution, 1.0 mom.
Scheme 1
Scheme 2 (8)
(9)
3. (9)
The apparatus and procedures of hydrogenation of (-1-a-pinene were described in our previous papers (KO and Chou, 1993a, 1994a,b).
Theoretical Analysis Although the mechanism of hydrogenation of (-)-apinene was proposed in our previous papers (KOand Chou, l992,1993a,b)which qualitatively mentioned the occurrence of isomerization, the effect of hydrogenation conditions on the selectivity of cis-pinane was not analyzed. It was suggested that the double-bond migration of alkene during the course of hydrogenation might result in isomerization of alkene (Eigenmann and Arnold, 1959; Siegel et al., 1966; Clarke and Rooney, 1976; Gault, 1981; Siegel, 1991). The selectivity of products via direct hydrogenation may be different from that via isomerization followed by further hydrogenation (Siegel, 1991). The mechanism of hydrogenation of (-)-a-pinene, accompanied by isomerization, is described in detail in the Appendix and summarized in Scheme 1, where (1) is a-pinene; (2) and (3) are associatively adsorbed a-pinenes which can be half-hydrogenated to (4)-(7). These half-hydrogenated species can be either further hydrogenated to pinane, cis-pinane (81,or trans-pinane (9), or isomerized to associatively adsorptive species, (10)-(13), or desorptive species, (14) and (15). Similarly, these isomers can be half-hydrogenated to (4)(7) or (16)-(19), and further hydrogenated to produce cis- and trans-pinane. Because the steric hindrance of structure of both isomers (14) and (15) is less than that of a-pinene during the associative adsorption, the cis-pinane selectivity in hydrogenation of (1) is larger than that in
hydrogenation of both (14) and (15) which are the double-bond-migrationproducts of a-pinene via similar isomerization. Thus, only P-pinene (14) is considered to be the product of isomerization of a-pinene and only (4) and (7) are considered to be the half-hydrogenated species from em and endo direction, respectively. The simplified reaction path is shown in Scheme 2. All the concentrations of species and the rate constants of reactions are summarized as follows.
[ ch ] kCt
I
[ ham ] where [E,] and [Epl are concentrations of a- and P-pinene, respectively. The subscripts a and /3 involve the species resulting from a- and j3-pinene, respectively. The superscripts c and t involve the species adsorbed from e m and endo direction, respectively. The halfhydrogenated species with concentrations of [RCland [Rt] can be hydrogenated to the cis- and trans-pinane, respectively. Furthermore, the chemisorption of hydrogen on two active sites of the catalyst is assumed to be dissociative:
*
where equilibrium of reversible adsorption was assumed.
Ind. Eng. Chem. Res., Vol. 34,No. 2, 1995 469 At steady state, the psuedo-reaction rate of all species with respect to the simplified mechanism proposed as Scheme 2 could be derived as follows. C&H'
OW,"
= (k-,"
OW,"
= (k-,"
=k -,'
f
k&'&)
+ kp,"eH) + k-p~'+ k C K ~ [ H 2 Y 6 ~
Similarly, eqs 6, 8, and 10 for trans species can also be rearranged to obtain the o**at7o q t , and O * H ~as follows.
where
d[trunsYdt = kt6,[Rt]
(12)
Equations 5, 7,and 9 can be rearranged as follows: [E,"] = (k,"[Eal
+ k-,"[RcI)B2/(k-," + k&"8H)
(13)
460 Ind. Eng. Chem. Res., Vol. 34, No. 2, 1995
Furthermore, all the experimental results revealed that the concentration of P-pinene was much lower than that of a-pinene. Therefore, eq 23 becomes
or u**,C/o**, t endo direction for ,&pinene > ex0 direction for ,&pinene > exo direction for a-pinene (30) Furthermore, assuming In spite of both low concentration and less stability of @-pineneduring hydrogenation, this does not mean that isomerization never occurs. It is possible that the isomer can be easily adsorbed and/or hydrogenated. Apparently, increasing of o**aCand u**pC,which concern the disappearance rate of associatively adsorbed a- and ,+pinene, respectively, from ex0 direction, decreases the stability of associative adsorption of a- and P-pinene, respectively, from e m direction. Similarly, increasing of and a**$decreases the stability of associative adsorption of a- and P-pinene, respectively, from endo direction. Moreover, increasing of U*H' and U * H ~decreases the stability of half-hydrogenated intermediates, (4) and ( 7 ) in Scheme 2, respectively. Therefore, &, is regarded as a term concerning the direct hydrogenation of a-pinene and Xp is a term concerning the hydrogenation of P-pinene resulting from isomerization of a-pinene (i.e., indirect hydrogenation of a-pinene). The term &-p involves the hydrogenation of half-hydrogenated species. Therefore, eq 24b can be analyzed as follows. Case I. Steric Effect is Dominant. The steric hindrance of the isopropylidene bridge is larger than that of the methylene bridge. The steric factor significantly retards the associative adsorption of a-pinene from endo direction. It gives
However, during associative adsorption of pinene, the steric hindrance of ,&pinene is expected to be less than that of a-pinene. The reason is either the double bond of "/3-pinene" (the species (14) shown in the Appendix) does not locate at the bridged bicyclic ring or the double bond of ''B-pinene" (the species (15))locates at the bridged bicyclic ring without linking to the methyl group. Therefore, both bridges of P-pinene, methylene and isopropylidene, only slightly affect whether the P-pinene is adsorbed from e m or endo direction. This gives
kC/kt5 1
(31)
gives
The term & in eq 24b is much larger than unity because ka' >> kat in eq 25. On the other hand, both the terms &-p and Xp in eq 24b approach unity. That is, direct hydrogenation of a-pinene produces more cispinanes than that of indirect hydrogenation of a-pinene, i.e., isomerization of a-pinene following hydrogenation. Conclusively, when the steric effect is dominant, the term &-&& >> 1. Thus, the formation of cis-pinane dominates in hydrogenation of a-pinene when the factor of steric hindrance is significant. The summary is shown in Table 2. Case 11. Both Steric Effect and Electronic Effect Are Important. If the reaction conditions favor the isomerization of a-pinene to P-pinene, the selectivity of cis-pinane will decrease. This means that the selectivity of cis- or trans-pinane in direct hydrogenation is controlled by the direction of adsorption of a-pinene. Owing to the steric hindrance of bicyclic bridges, the main product is cis-pinane even if the trans isomer is more thermodynamically stable than the cis isomer. On the other hand, if a-pinene is first isomerized to P-pinene and further hydrogenated to pinane, the selectivity is mainly affected by the stability of adsorbed P-pinene. That is, the adsorbed ,&pinene may be either hydrogenated instead of desorbed to form B-pinene, resulting in cis-pinane dominates, or desorbed to form P-pinene followed by readsorption and subsequently hydrogenated to pinane, resulting in the decrease of the selectivity of cis-pinane. In this case, both the electronic effect and the steric effect sigmficantly influence the selectivity of cis-pinane. Although the latter, steric effect, favors the cis-addition, the increase of electron density of nickel increases the electronic interaction between the nucleophilic pinene and nickel active sites, i.e., increases trans addition. Simultaneous consideration of the both electronic and steric effects gives eq 26 and
Ind. Eng. Chem. Res., Vol. 34, No. 2, 1995 461 Table 2. Summary of Theoretical Analysis of Eq 24b and the Selectivity of cis-Pinane electronic case steric effect effect &-&q4 I significant insignificant =-1 I1 moderate moierate =-1 I11 insignificant significant ca. 1
k;/kd
>
selectivity of cis-pinane high moderate to high moderate
1, kaRC/kuRt= 1, k-,C/k-d < 1, k-,c/k-,t = 1, kc/kt 4 1 ( 3 3 )
or
(34) Conclusively, Za approaches unity and the selectivity of cis-pinane decreases. Case 111. Electronic Effect is Dominant. Besides the steric effect of the structure of a-pinene, several factors may affect the selectivity of the product as described in the Introduction. Moreover, the reaction mechanism may also shift from Scheme 2 t o another one. If the electronic effect dominates in the reaction path of Scheme 2, it makes the rate constants resulting from the ex0 direction be similar to the ones from the endo direction. This means that the electronic interaction decreases the difference between the adsorption from em and endo direction; i.e., all the k's with superscript c are similar to the k's with superscript t. This gives eq 26 and
k,CIk:
=
1, kac/kdt
1, k-;/k-; = 1, k-,c/k-,t = 1, kc/kt + 1 ( 3 5 )
=
or
o**;/o**;= 1, o**;/o**;+ 1
(36)
Therefore, the terms Za-B, Za, and Zp approach unity and selectivity of cis-pinane decreases. The analysis of cases 1-111 is summarized in Table 2.
Results and Discussion Effect of Acidity of Support. The selectivity of cispinane product in hydrogenation of a-pinene over catalysts prepared by electroless deposition is evaluated by the term [cisy[trans], which is the concentration ratio of cis- to trans-pinane in the hydrogenation of a-pinene. The supported catalysts are NiP catalysts supported by a-alumina, y-alumina, and kieselguhr. Furthermore, a skeletal catalyst, NiP/Al, is also studied. The results, as shown in Figure 1indicate that the selectivity of cispinane decreases with the acidity of support for the supported catalysts. For example, the [cisy[trans] ratio over less acidic a-alumina supported catalyst is about 24 at 80% conversion. However, the [cisY[trunsl ratio over acidic y-alumina supported catalyst decreases to about 10 at the same conversion. It is well-known that the acidity of support favors the isomerization which includes both skeletal isomerization and double-bond-migration isomerization. For the supports such as alumina or kieselguhr, the acidity of these catalysts can only induce the isomerization via doublebond migration because of less acidity. It is reasonable t o assume that only double-bond migration is involved in the isomerization reaction path of a-pinene as shown in Scheme 2.
/
n
/
Y
4
1 20
1
1
40
I
I
60
I
I 80
I
1 0
pinane at 550 rpm, 150 "C and 400 psig: (0)supported NiP/aAzo3 catalyst. ( 0 )supported NiPly-AlzOs catalyst, (0) supported NiPheselguhr catalyst, (m)skeletal NiP/Al catalyst.
Therefore, acidity of support induces the hydrogenation via isomerization and the selectivity of cis-pinane decreases as shown in Figure 1. Moreover, the selectivity of cis-pinane over skeletal catalyst is also smaller than that of a-alumina supported catalyst. The results indicate that the interaction between Al and Ni atoms also affects the reaction path of hydrogenation, and will be discussed in later. When the less acidic support is used, the term Za is much larger than unity because kaC>> kat in eq 25. The reaction conditions correspond to case I. The selectivity of cis-pinane is very high as shown in the curve Nip/ a-AlzO3 of Figure 1 and Table 2. On the other hand, more acidic support induces the isomerization of a-pinene. The increasing electronic interaction between the half-hydrogenated species and support favors the occurrence of double-bond migration. Accordingly, k - p c , k&, k-&, and k a t increase and k-&, k-&, k&, and kpRt decrease in eqs 25 and 26. This results in the increase of o**aC(and a**d)of Za, the decrease of a**pc (and a*$) of ED,and the insignificant change of o*HC(and o*H~) but large decrease of k - a C (and Lat)of L.The isomerization makes more unstable adsorbed a-pinene and more stable adsorbed @-pinene. Therefore, the term & in eq 24b decreases with a slight change of Za-b and Zb and the selectivity of cis-pinane decreases. This case of reaction conditions corresponds to case 11. The interconversion of cases resulting from the variation of acidity of supports is summarized in Table 3. The
462 Ind. Eng. Chem. Res., Vol. 34, No. 2,1995 Table 3. Interconversion among Cases I, 11, and 111 Resulting from Variation of Reaction Conditions factor acidity low to high
increasing terms
decreasing terms kic/kjt k-&, k - a t , k p C , eqs 25,26, 31 k p t , U**pc, U*$ k-pRt, ~7**~',a**,t pressure high to low 0 maC, u r e a t , m f , a*$, eqs 25, 26,31 mHC, uq+, en temperature low to high all k's and ds eqs 25,26, 31 to eqs 26,35 electron density of kat, k a t , k-,C, k-&C kaC, k&, k - d , k - d t eqs 25,26, 31 to eqs 26,35 nickel low to high
k&, k d t , k-&,
L-B 51 to 5 1
& % L - B L Z B case to > 1 =l to 21 to > 1 1to 11
5 1 to Cl >>1 to -
=l to -
>>1 to
>1
I to I1
cl to =l >>l to =l =1 to *l >>1 to "-1 I to I11 51 to =1
>>1 to =l
=l to =l >>l to =l I to I11
Table 4. Comparison of Experimental Results and Theoretical Analysis of Selectivity of cis-Pinane exptl result," theoretical analysis affecting factor [cisy[trunsl classification cis selectivity low 24 case I high acidity of support 10 case I1 moderate to high high high 4.3 case I high pressure of hydrogen 2.3 case I1 moderate to high low case I high temperature of hydrogenation low 12 5 case I11 moderate high case I high electron density of nickel low 3.8 moderate 2.5 case I1 moderate to high high 1.4 case I11 moderate a
The condition of hydrogenation corresponding to the value of [cisY[trunsl is described under Results and Discussion.
experimental results correlate the theoretical analysis ones well as shown in Table 4. Effect of Hydrogen Pressure. The results reveal that the selectivity of cis-pinane increases with hydrogen pressure as shown in Figure 2. For example, the cisltrans ratio over NYAl catalyst at 80% conversion decreases from 4.3 t o 2.3 when the pressure decreases from 400 to 25 psig as shown in Figure 2b. All the results are similar for both NiP and NiB catalysts and are independent of the types of catalysts in this work, i.e., whether the catalysts are supported catalysts or skeletal catalysts as shown in Figure 2. When the pressure of hydrogen is high, the catalyst is saturated with hydrogen and the reaction should be rapid. Halfhydrogenated a-pinene is more easily further hydrogenated to pinane. In other words, the concentration of available hydrogen decreases when the hydrogen pressure decreases. The half-hydrogenated a-pinene can not only be further hydrogenated but can also be isomerized to ,&pinene. Therefore, the selectivity of cis-pinane decreases when the pressure decreases. When the pressure is high, the half-hydrogenated a-pinene is easily hydrogenated. The term & is much larger than unity because kac >> kat in eq 25. The reaction conditions correspond to case I. The selectivity of cis-pinane is very high as shown in Figure 2 and Table 2. On the other hand, low pressure increases the possibility of isomerization of a-pinene. Thus, all K's remain constant but 8 H decreases and 8 increases. The decrease of several terms is shown in Table 3. Although the variation of Za and Zp is not so clear, the effect of pressure on the selectivity can be evaluated by eq 24a. When the pressure is high, 8 H approaches unity so that U**ac and u**f approach kaCk,& and k - f kpRC, respectively. The term 8 is smaller and makes D * H ~ approach ke/O2. This gives
+
+
On the other hand, low pressure makes 8 H approach 0, maC approach k-ac, a$ approach k-Dc and U*H~ approach k-& + k-&. This gives
Apparently, the k's in eqs 25 and 26 give
Therefore, the term &-p&Zp in eq 24b decreases and the selectivity of cis-pinane decreases. These reaction conditions correspond to case 11. The experimental results confirms the theoretical analysis ones as shown in Table 4. This result suggests that the increase of hydrogen pressure shifts indirect hydrogenation to direct hydrogenation, i.e., the reaction involving mainly direct hydrogenation of a-pinene (as shown in Scheme 2). A similar conclusion was also made by Siegel et al. (1966) and Siegel (1991). Effect of Temperature. The results as shown in Figure 3 indicate that the cis-isomer product decreases with increasing temperature. For example,the cisltruns ratio over NiB/Al catalyst at 50%conversion decreases from 12 to 5 when the temperature increases from 110 to 150 "C as shown in Figure 3a. The effect of temperature on the selectivity may be attributed t o several thermodynamic factors such as the isomerization between cis- and trans-pinane as well as the thermal deformation of a-pinene which allows the shift of the equilibrium of the adsorbed species from ex0 to endo direction. However, the isomerization between cisand trans-pinane in the temperature range of this study may be negligible (Ipatieff et al., 1953). The thermodynamic analysis about the adsorption of species is considered. When the temperature is high, the associative adsorptions of both a- and P-pinene are insignificantly affected by the steric hindrance of bicyclic bridges. This implies that the rate constants of ex0 direction are similar to the ones of endo direction. In other words, the electronic interaction is large enough to overcome the steric hindrance during adsorption of species. Therefore, the terms &-p, &, and 2 p approach unity and the selectivity of cis-pinane decreases. This reaction condition corresponds to case 111. The results
Ind. Eng. Chem. Res., Vol. 34, No. 2, 1995 463
6 s 0
I
20
I
I 40
I
1 60
I
I
I
80
Conversion of Pinene,
1 1
%
4
'
I
20
I
I 40
1 60
I
L----
OO
Conversion of Pinene,
2
1
I
80
Conversion of P i n e n e ,
20
40
1
2
8
0
I
I
60
80
Conversion of P i n e n e ,
1
2
0
Figure 2. (a) Effect of pressure of hydrogen on the selectivity of cis-pinane over skeletal NiP/Al catalyst at 550 rpm and 150 "C: (0) 400 ( 0 )100, (0)50, and (B)25 psig. (b)Effect of pressure of hydrogen on the selectivity of cis-pinane over skeletal NVAl catalyst at 550 rpm and 150 "C: (0) 400, ( 0 )50, and (0)25 psig. (c) Effect of pressure of hydrogen on the selectivity of cis-pinane over supported Nikieselguhr 400, ( 0 )200, (0)100, and (B)50 psig. (d) Effect of pressure of hydrogen on the selectivity of ciscatalyst at 550 rpm and 150 "C: (0) pinane over supported NiBkieselguhr catalyst at 550 rpm and 150 "C: (0)400, ( 0 )100, and (0) 25 psig.
are similar to the reports that more trans isomers are produced a t higher temperature (Cocker et al., 1966). Effect of Electron Density of Nickel. It is wellknown that nickel catalysts modified by boron (nickel boride), phosphorus (nickelphosphide), aluminum ( k e y nickel) and zinc (Urushibara nickel) show peculiar activity and selectivity for hydrogenation reactions.
Many XPS studies of nickel boride (Ni-B), nickel phosphide (Ni-P), Raney nickel (R-Nil, Urushibara nickel (U-Ni), and decomposed nickel (D-Ni) catalysts have been carried out to characterize the surface states of the catalysts and to explain the differences in the selectivities and activities of the catalysts for hydrogenation reactions. It is also well-known that the electron
464 Ind. Eng. Chem. Res., Vol. 34, No. 2, 1995 a 5
0 I
n10
r"e
c
e
o
4-
0 n
2t3
3-
11.' L
L u
u
U
U
\ n
\ n (I,
'c, 0
0
1
L u d L d 40
20
OO
00
80
Conversion of P i n e n e ,
1
2
od
'
I
1
20
I
40
I
I
00
I
I
1
80
Conversion of Pinene,
1 0
2
5
b e
4
f
0 n
r"8
L u -4
\ cr, 'c,
n
0
U
2
0
I
I
I
20
I 40
I
I
00
I
1
1
BO
Conversion of P i n e n e ,
1
%
Figure 3. (a) Effect of temperature on the selectivity of cis-pinane over skeletal NiB/Al catalyst a t 550 rpm and 400 psig: (0)150, (0)130, and (0)110 "C. (b)Effect of temperature on the selectiviQ of cis-pinane over supported Nikieselguhr catalyst at 550 rpm and 400 psig: (0)150, ( 0 )130, and (0) 110 "C.
transfers between the nickel atoms and the component element in Ni-B and Ni-P catalysts were Ni-B;
Ni-B
(40)
Ni-P;
Ni-P
(41)
The extent of the electron transfer is evaluated in terms
40
00
BO
Conversion of P i n e n e ,
1 0
2
Figure 4. (a)Effect of electron density of nickel on the selectivity of cis-pinane at 550 rpm, 150 "C and 50 psig: (0)supported Nip/ kieselguhr catalyst, ( 0 )supported Nikieselguhr catalyst, and (0) supported NiBkeselguhr catalyst. (b)Effect of electron density of nickel on the selectivity of cis-pinane at 550 rpm, 150 "C and 25 psig: (0)skeletal NiP/Al catalyst, ( 0 )skeletal NVAl catalyst, skeletal NiB/Al catalyst. and (0)
of a parameter Aq in X P S analysis, which is defined by Okamoto et al. (1980a). In general, the Aq is equal to 0 for D-Ni catalyst. The electron-rich nickel donating by boron in Ni-B catalysts corresponds to Aq < 0. On the other hand, Aq 0 indicates the electron-deficient nickel resulting from the transfer of electrons from
Ind. Eng. Chem. Res., Vol. 34, No. 2, 1995 466 nickel to phosphorus in Ni-P catalysts. The existence of component Al in R-Ni also shows Aq -= 0. The selectivity of hydrogenation correlated with Aq in some other reaction systems was reported (Phillipson et al., 1969; Okamoto et al., 1980a, 1982b). In o u r studies, both the Ni-P and Ni-B catalysts are prepared by electroless deposition method which is different from the method reported by Okamoto et al. (1979). However, the role of boron and phosphorus is assumed to be similar to that described by Okamoto et al. (1979), i.e., the nickel as the electron acceptor and donor in Ni-B and Ni-P catalysts, respectively. Furthermore, the Nikieselguhr and Ni/Al catalysts prepared by electroless deposition are also assumed to make Aq = 0. Owing to the nucleophilic properties of a-pinene, the interaction between the electron-rich nickel and a-pinene is weaker than that between electron-deficient nickel and a-pinene. This interaction implies the stability of associatively adsorbed pinene on nickel. Therefore, Ni-B catalysts show weaker interaction and less stable adsorption resulting in insignificant steric effect. Therefore, eq 40 makes Kat, K d Cdecrease and Kat, K a t increase resulting in that the term Za approaches unity. The reaction conditions correspond to case 111. The theoretical analysis of electron density distribution of nickel, which was induced by boron and phosphorus, was confirmed by the experimental results as shown in Figure 4. Boron and phosphorus play a contrary role in affecting the selectivity of hydrogenation. The results show that the aluminum in skeletal catalysts promotes the roles of boron and phosphorus for influencing the selectivity of cis-pinane as shown in Figure 4b. For example, the cisltrans ratio over Nip/ AI catalyst, about 3.8, a t 40% conversion is higher than that over Ni/Al and NiB/Al catalysts, about 2.5 and 1.4, respectively. Both NiP and NiB catalysts favor the selectivity of cis-pinane, and the effects of boron and phosphorus on the selectivity of cis-pinane are insignificant when kieselguhr is used as the support as shown in Figure 4a. The results show that the inductive change of electron density is also affected by the residue component and the nature of support in skeletal and supported catalysts, respectively. The experimental results correlate the theoretical analysis ones well as shown in Table 4.
Conclusions The selective hydrogenation of (-)-a-pinene over both types of nickel catalysts, supported and skeletal catalysts prepared by electroless deposition, is obtained. High selectivity of cis-pinane is ascribed to the steric hindrance of the structure of (-)-a-pinene during the adsorption. However, significant electronic effect may diminish the steric effect. The electronic interaction between adsorbed pinene and the active site of catalyst increases with the acidity of support, the temperature of hydrogenation, the electron density of nickel, and with decreasing pressure of hydrogen. A reaction mechanism with isomerization as well as half-hydrogenation is proposed to describe the hydrogenation of (-)-a-pinene. The experimental results correlate with the theoretical analysis ones well. Acknowledgment The support of the National Science Council of the Republic of China (NSC-83-0416-E006-007) and National Cheng Kung University is gratefully acknowledged.
Nomenclature [E,], [Epl = concentration of a- and P-pinene, respectively, M [Eac],[Ef] = concentration of associativelyadsorbed a-and ,&pinene, respectively, from ex0 direction, M [Eat],[Est]= concentration of associativelyadsorbed a- and p-pinene, respectively, from endo direction, M [Rc],[Rt]= concentration of half-hydrogenatedpinene from ex0 and endo direction, respectively, M [cis], [trans] = concentration of cis- and trans-pinane, M [Hz] = concentration of hydrogen in liquid phase, M 6 = fraction of vacant active sites, dimensionless 6 H = fraction of active sites adsorbing hydrogen atom, dimensionless kat, k-ac, kpc, k-pc, k d C ,k - d C 9kpRc, k-pRC,kat, k-at, kpt, k - t , k&, k-&, kpRt, k-$, kc, kt, kH, k-H = reaction rate constant as defined in Scheme 2 KH,Kac,K&, Kat,K& = equilibrium constant for reversible adsorption Hz = hydrogen molecule Ni-B = nickel boride catalysts Ni-P = nickel phosphide catalysts R-Ni = Raney nickel catalysts U-Ni = Urushibara nickel catalysts D-Ni = decomposed nickel catalysts Aq = a parameter in XPS analysis defined by Okamoto et al. (1980a) vic, vzc, v3', 74', u**ac,u**pC, (T*H', u**,~,u**$,(T*H~= terms defined in eqs 16 and 17 elc, E$, E$, ~ 4 dC ~ = , terms defined in eqs 18 and 19 &-p, &, Zp = terms defined in eq 24b * = active site of catalyst Subscripts H = hydrogen atom, adsorption involvinghydrogen atom, half-hydrogenation R = half-hydrogenation or its reverse reaction * = adsorption at one active site a = species involving direct hydrogenation ,8 = species involving indirect hydrogenation Superscripts c = cis addition, ex0 direction, endo product t = trans addition, endo direction, ex0 product Appendix. Adsorption of Pinene via Hydrogenation or Isomerization (-)-a-Pinene (1) can be associatively chemisorbed on the active sites of nickel catalyst, as described by the Horiuti and Polanyi mechanism (Horiuti and Polanyi, 19341, from ex0 (2) and endo (3) direction. 9
8
10
i
Both adsorbed pinene (2) and (3) can be further halfhydrogenated as follows. ?
S
466 Ind. Eng. Chem. Res., Vol. 34, No. 2, 1995
(7)
(6)
In fact, the relative position between C2 and C7 in (41, (51, (6), or (7) can be further distinguished into the chair form and the boat form. In hydrogenation of a-pinene, hydrogen is then added to the half-hydrogenated a-pinene, (4)-(7), to give the product, cis-pinane ( 8 ) or trans-pinane (9). Furthermore, both ( 8 ) and ( 9 ) can be also distinguished into
Further desorption of isomers (10)-(13)gives 8
8
II
i
(15)
(14)
Both (14)and (15)are the products of isomerization of a-pinene. It is known as the results of double-bond migration of alkene. However, it is noted that the double-bond migration to cIc '6 in (4) or (7) is considered to be of difficult because of great strain. Furthermore, isomers (10)-(13) can also be halfhydrogenated and further hydrogenated as follows.
I
?
(Allb) (9)
the chair form (+) and the boat form (-1. Moreover, half-hydrogenated a-pinene, (4)-(7), can either reverse to the adsorbed a-pinene, (2)and (3),or be isomerized t o the adsorbed stereoisomers of a-pinene, (10)-(13).
'q
(12)
-H
P;
t
i
8
9
(8)
(A12a)
\
I
10
i' 4
Literature Cited Clarke, J. K. A,; Rooney, J. J. Stereochemical Approaches to Mechanisms of Hydrocarbon Reactions on Metal Catalysts. MU. Catul. 1976,25,125-183. Cocker, W.; Shannon, P. V. R.; Staniland, P. A. The Chemistry of Terpenes. Part I. Hydrogenation of the Pinenes and the Carenes. J . Chem. SOC.C 1966,(l),41-47. Eigenmann, G . W.; Arnold, R. T. Stereospecific Hydrogenation of a-Pinene Derivatives. J . Am. Chem. SOC.1969,81,3440-3442.
Ind. Eng. Chem. Res., Vol. 34, No. 2, 1995 467 Gault, F. G. Mechanisms of Skeletal Isomerization of Hydrocarbons on Metals. Adv. Catal. 1981,30, 1-95. Horiuti, I.; Polanyi, M. Exchange Reactions of Hydrogenation on Metallic Catalysts. Trans.Faraday SOC. 1934,30,1164-1172. Ipatieff, V. N.; Huntsman, W. D.; Pines, H. Studies in the Terpene Series. XVII. The Thermal Isomerization of Pinane at High Pressure. J.Am. Chem. SOC.1983, 75, 6222-6225. Karpinski, Z. Catalysis by Supported, Unsupported, and ElectronDeficient Palladium. Adv. Catal. 1990,37, 45-100. KO, S. H.; Chou, T. C. Preparation and Characteristics of Electroless Deposited Metal Catalysts. In The 7th Seminar on Science and Technology ROC-Japan Joint Workshopon Catalysis: New Frontiers in Catalysis; National Central University: Chungli, Taiwan, ROC, Dec 10-11,1992; pp 164-186. KO,S. H.; Chou, T. C. Kinetics of the Liquid-Phase Hydrogenation of (-)-a-Pinene over Electrolessly-Deposited Ni-Ply-AlZOs Catalyst. Znd. Eng. Chem. Res. 1993a, 32, 1579-1587. KO,S. H.; Chou, T. C. The Chemisorptive Behavior and Temperature-Programmed Desorption of Nickel-Phosphorus/Alumina Catalyst Prepared by Electroless Deposition. J. Chin. Znst. Chem. Eng. 1993b, 24, 393-400. KO, S. H.; Chou, T. C. Selective Hydrogenation of (-)-a-Pinene. In 1993International Symposium on Organic Reactions Tainan; National Cheng Kung University: Tainan, Taiwan, ROC, December 11-13, 1993c; pp 253-254. KO,S. H.; Chou, T. C. Hydrogenation of (-)-a-Pinene over Ni-P/ Nylon6,6 Catalysts Prepared by Electroless Deposition. J.Chin. Inst. Chem. Eng. 1994a, 25, 155-161. KO,S. H.; Chou, T. C. Hydrogenation of (-)-a-Pinene over NickelPhosphorus/Aluminum Oxide Catalysts Prepared by Electroless Deposition. Can. J. Chem. Eng. 199423, 72, 862-873. Okamoto, Y.; Nitta, Y.; Imanaka, T.; Teranishi, S. Surface Characterization of Nickel Boride and Nickel Phosphide Catalysts by X-Ray Photoelectron Spectroscopy. J. Chem. SOC., Faraday Trans. 1 1979, 75,2027-2039. Okamoto, Y.; Nitta, Y.; Imanaka, T.; Teranishi, S. Surface State and Catalytic Activity and Selectivity of Nickel Catalysts in Hydrogenation Reactions 111. Electronic and Catalytic Properties of Nickel Catalysts. J . Catal. 1980a, 64, 397-404. Okamoto, Y.; Nitta, Y.; Imanaka, T.; Teranishi, S. Surface State, Catalytic Activity and Selectivity of Nickel Catalysts in Hydrogenation Reactions. Part 2. Surface Characterization of Raney Nickel and Urushibara Nickel Catalysts by X-Ray Photoelectron Spectroscopy. J . Chem. SOC.,Faraday Trans. 1 1980b, 76,9981007.
Okamoto, Y.; Fukino, K.; Imanaka, T.; Teranishi, S. Surface State and Catalytic Activity and Selectivity of Nickel Catalysts in Hydrogenation Reactions. IV.Electronic Effects on the Selectivity in the Hydrogenation of 1,3-Butadiene. J . Catal. 1982a, 74, 173-182. Okamoto, Y.; Matsunaga, E.; Imanaka, T.; Teranishi, S. Surface State and Catalytic Activity and Selectivity of Nickel Catalysts in Hydrogenation Reactions. V. Electronic Effects on Methanation of CO and C02. J . Catal. 1982b, 74, 183-187. Phillipson, J. J.; Wells, P. B.; Wilson, G. R. The Hydrogenation of Alkadienes. Part 111. The Hydrogenation of Buta-1,3-diene Catalysed by Iron, Cobalt, Nickel, and Copper. J . Chem. SOC. A 1969, 1351-1363. Sattedield, C. N. Heterogeneous Catalysis in Industrial Practice, 2nd ed.; McGraw-Hill New York, 1991; pp 10-12. Siegel, S. Heterogeneous Catalytic Hydrogenation of C=C and CIC. In Comprehensive Orgunic Synthesis; Trost, B. M., Editor-in-chief; Pergamon Press: Oxford, 1991; Vol. 8, pp 417442. Siegel, S.; Dunkel, M.; Smith, G. V.; Halpern, W.; Cozort, J. Stereochemistry and the Mechanism of Catalytic Hydrogenation of Cycloalkenes. VII. Interaction Mechanisms Which Control the Ratio of Stereoisomers. J.Org. Chem. 1966,31,2802-2806. Tauster, S. J. Strong Metal-Support Interactions: Facts and Uncertainties. In Strong Metal-Support Interaction; Baker, R. T. K., Tauster, S. J., Dumesic, J. A., Eds.; ACS Symposium Series 298; American Chemical Society: Washington, DC, 1986; pp 1-9. Yang, T. J.; Chen, C. Y. Hydrogenation of a-Pinene over Electroless Nickel Catalyst Supported on Kieselguhr. In International Symposium on Catalysis-The Use of Metal Complexes in the Preparation of Catalysts; Lava1 University: Quebec, Canada, July 1988.
Received for review April 15, 1994 Revised manuscript received September 16, 1994 Accepted September 30, 1994@ IE940250M
@
Abstract published in Advance ACS Abstracts, December
15, 1994.
