I n d . E n g . Chem. Res. 1988,27, 243-248
243
Gas-Phase Hydrogenation of Butyronitrile over Supported Platinum Cata1ysts Scott T. McMillant and Pradeep K. Agrawal* School of Chemical Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332
The gas-phase hydrogenation of butyronitrile over supported Pt catalysts was studied in a differential flow reactor a t atmospheric pressure and a t 473 K. T o investigate the influence of metal-support interactions, three supports (yA1203,Si02, and MgO) were employed. The reaction experiments using supported and unsupported Pt catalysts showed a dramatic influence of the pretreatment on catalytic behavior. For the supported Pt catalysts, the activity for butyronitrile conversion was a t least an order of magnitude higher for uncalcined samples than that for calcined samples. In addition, the uncalcined samples formed an increased fraction of higher amines as the hydrogenation products. The hydrogenation vs cracking selectivity of supported catalysts was greatly influenced by the nature of support. The catalytic behavior of Pt/MgO was least distinguishable from that of Pt black; the contrarv was true for Pt/Al,O,. The metal-support interactions appear to follow the order -pAl,O, > Si02"> MgO. Aliphatic polyamines are employed in a wide spectrum of industrial applications. For example, dimethylamine (a secondary amine) is used as a solvent in the manufacture of acrylic resins. Other industrial applications of amines include fungicides, chelating agents, and surfactants. In addition, all of the following pharmaceutical products require amines in their manufacture: caffeine, analgesics, antihistamines, and tranquilizers. Polyamines have been conventionally produced either by reacting ethylene dichloride with ammonia at elevated temperatures or by catalytic ammination of the corresponding alcohol. Within the past decade, a considerable effort has been expended in the development of new catalytic processes for the synthesis of aliphatic amines. One method of preparation which has emerged from this effort is the liquid-phase hydrogenation of nitriles. Reactions of this type are typically carried out at high pressures and moderate temperatures. The reaction mechanism, shown in Figure 1, for the hydrogenation of nitriles was first proposed over 60 years ago by Von Braun et al. (1923) and has been generally accepted with only minor modifications. Von Braun et al. postulated that the reaction proceeds stepwise by the formation of an intermediate imine, which may react further or be hydrogenated to the primary amine. Once formed, the primary amine may react with the intermediate imine to give a secondary intermediate which after hydrogenolysis forms the secondary amine and ammonia. A similar sequence is proposed for the formation of the tertiary amine. A great deal of effort has been made to minimize the coupling reactions (production of secondary and tertiary amines), hence favoring primary amine formation. The major product or the selectivity of the reaction has been shown to depend upon a number of factors. Three of the more significant factors are the following: (i) the reaction conditions, (ii) the physical and chemical nature of the reactant nitrile, and (iii) the type of catalyst. Rylander et al. (1973) reported that as the reactor pressure was increased from 50 to 100 psig during the hydrogenation of valeronitrile over Rhz03,the selectivity toward pentylamine increased from 23% to 100%. In another study, Rylander (1979) observed that secondary and teritary amines were formed during the hydrogenation of aliphatic nitriles, whereas only primray amines were formed during the hydrogenation of aromatic nitriles. This Present address: Department of Chemical Engineering, Northeastern University, Boston, MA 02115. 0888-5885/88/2627-0243$01.50/0
observation was attributed to steric hindrance effects. Greenfield (1967) noted the effect of catalytic metal on the selectivity: Ni and Co yield primary amine as the major reaction product, whereas Pt, Pd, and Rh are shown to be more selective in the formation of secondary and tertiary amines. Yet another way to enhance the formation of primary amine, which has seen a great deal of industrial application, is the use of ammonia as a solvent (Freifelder, 1960). I t is postulated that ammonia prevents or minimizes the formation of secondary or tertitary amine by removing the imine from the reaction mixture by the mechanism shown in Figure 2. In summary, the intermediatepimine reacts with ammonia to form an intermediate which can be further hydrogenated to form primary amine. Apparently, this ammonia-imine complex prevents the coupling of imine with primary amine and, thus, minimizes the formation of higher amines. Qualitatively similar, although less dramatic, results have been obtained with other solvents. There are few indications which suggest that the primary amine may undergo further hydrogenolysis to yield a hydrocarbon and ammonia. Such an important aspect of the selectivity has been almost ignored in the literature related to the liquid-phase hydrogenation of nitriles. It is possible that at high pressure and low temperatures, liquid-phase reaction conditions, the hydrocarbon is not formed or it is formed to a negligible extent. It is desirable to develop a gas-phase process which operates near atmospheric pressure, at moderate temperatures, and in the absence of a solvent. Therefore, the present study Wac undertaken to investigate the gas-phase hydrogenation of butyronitrile. Reported below are the results obtained from reaction studies as well as from catalyst characterization experiments. The catalysts were characterized by using Hi chemisorption and temperature-programmed reduction techniques.
Experimental Section A. Reaction Studies. The liquid reactant (butyronitrile) and all of the amine products have their normal boiling points below 473 K; thus a lower limit of 473 K on the reactor operation was considered to be satisfactory to ensure gas-phase hydrogenation. Figure 3 shows a schematic of the reactor setup. A fixed-bed shallow reactor was employed to permit differential reactor operation. The reactor was made of 0.006-m-o.d., 0.38-m-long Pyrex tube which was expanded in the center (0.d. 0.013 m) to house 0 1988 American Chemical Society
244 Ind. Eng. Chem. Res., Vol. 27, No. 2, 1988 (HYDROGENATIOI REACTIONS:)
---I--RCH = NH
RC I N + H2
(Nitrile)
RCH
:
(Imine)
--?--*
NH + H2
RCH2NH2 ( P r i m a r y Amine)
(HYDROCEWLYSIS REACTIOIS: ) RCH
I
--?--*
NH + RCH2NH2
- NH - CH2R
RCH
I
t
NH2
-
RCH
-
NH
CH2R + H2
--!!--*
(RCH2)2NH + NH3
I
(Secondary Amine)
NH2 RCH
z
NH + (RCH2)2NH
--?--*
RCH
I
-N-
(CH2R)2
NH2
-
RCH
I
N
-
(CH2R12 + H2
--$--*
(RCH2)3N + NH3 ( T e r t i a r y Amine)
NH2
(HYDROCRACKIWG REACTION:)
--I--*
RCH2NH2 + H2
RCH + NH3 (Hyirocarbon 1
Figure 1. Reaction mechanism for the hydrogenation of an aliphatic nitrile proposed by Von Braun et al. (1923). (REACTION WITH n " I A ) RCN
+
RCHNH
H2
+
-----+
NH3
RCHNH
-----e
RCHNHz
I
RCHNH2
+
HZ
-----+
RCH2NH2
+
NH3
I NH2
Figure 2. Proposed reaction pathway for the use of ammonia as a solvent.
the catalyst bed in a shallow bed mode. The catalyst is supported on a plug of glass wool, with the reactor held vertically inside a Thermolyne tube furnace. The reactor temperature is controlled by a Honeywell temperature controller (Model AV-726 HB114) using a chromel-alumel thermocouple placed in the annular region between the reactor tube and the furnace; this configuration of thermocouple location reduces the oscillations, and accurate temperature control within f l K can be achieved. Accurate temperature measurements of the-catalyst bed are made using another chromel-alumel thermocouple embedded in the center of the catalyst bed; the temperature is read directly using a digital temperature readout (Omega Engineering, Model 199). The reactor flow system also includes a reactor bypass line which is used to measure the feed composition and is used to establish a steady-state feed composition before starting the reaction. The gases flow downward in the reactor; the effluent is fed directly to the sampling system (heated at 473 K) of a gas chromatograph. The stainless steel tube (0.006-m diameter) leading to the GC is also maintained a t 473 K to prevent any condensation of reactant or products. The gas manifold for the reactor feed consists of three inlets: (1) 4 mol % O2 in He for oxidation and calcination, (2) ultrahigh-purity He, and (3) ultrahigh-purity H,. All the gases pass through appropriate guard beds to remove
Figure 3. Schematic diagram of reactor flow apparatus.
trace contaminants such as HzO, 02, etc., from H2and He. Each gas passes through an on/off valve, Brooks Model B744 flow controller, Supelco Model 2-2920 rotameter, and fiially the gas manifold. The single gas line from manifold goes to the reactor through a 0.0125-m-diameter stainless steel tubing (-0.15 m long). The liquid reactant, butyronitrile, is pumped by using a Sage Instruments Model 341A syringe pump through 0.0016-m capillary tubing into the 0.0125-m stainless steel tubing where it undergoes flash vaporization and mixes with the upcoming H2stream. The expansion and mixing chamber (0.0125-m stainless steel tubing) is maintained a t 473 K to prevent any condensation. Subsequently, the reactant inlet line to the reactor is also heated to 473 K, and no part of the reactor flow system after the mixing chamber experiences temperatures below 473 K. One-way check valves are installed to restrict the flow of H2 and any other gas into the syringe pump and to restrict the flow of butyronitrile into the gas manifold. A thermal conductivity detector in a gas chromatograph (Hewlett-Packard, Model 5730) is used to analyze the reactor effluent composition. Separation of nitrile, amines, and butane is obtained on a 0.006-m x 1.83-m column packing of Carbopak B/4% Carbowax 26M/0.8% KOH. The separation is achieved by temperature programming from 413 to 483 K at a linear rate of 4 K/min. The light gases (ammonia, propane, and butane) are separated and analyzed on the same column by operating it a t room temperature. All these reactants/products can be separated over a 15-min interval, and the lower limit of detectability is approximately 50 ppm. B. Reactor Operation. After placing catalyst in the reactor between glass wool plugs, the catalysts were reduced at 623 K in flowing H2(m3/min) for nearly 16 h. After reduction, while maintaining Hz flow, the bed temperature was reduced to the desired reaction temperature (generallybetween 473 and 523 K). Then the reactor was bypassed, and the syringe pump for butyronitrile was turned on. A steady-state feed composition, as monitored by GC analysis, was established in approximately 15 min. The feed was then directed to the reactor to start the reaction, and the reactor effluent was analyzed continuously by using on-line gas chromatography. The reaction rate for each product species could be obtained as a function of on-stream time. C. Catalyst Preparation. The Pt catalysts studied in the reactor were prepared using pore volume saturation (impregnation) of the support with chloroplatinic acid. Three supports, y-alumina, silica, and magnesia, with surface areas of 190,300, and 50 m2/g, respectively, were employed. For each support, chloroplatinic acid was diluted with enough distilled water to yield a solution which would just fill the pores. The amount of chloroplatinic acid for each support was constant, however, to yield 1 wt YO
Ind. Eng. Chem. Res., Vol. 27, No. 2, 1988 245 Table 1. Hydrogen Chemisorption Measurements for the Impregnated A1,08-, SiOz-, and MgO-Supported Catalysts" surface particle area, size, d, samDle m2/e dimersion. D nm 1. PtfA120 calcined 0.91 0.35 2.6 reduced 1.54 0.55 1.6 2. Pt/Si02 calcined 0.66 0.24 3.8 reduced 1.69 0.61 1.5 3. Pt/MgO calcined 0.51 0.18 5.0 reduced 1.06 0.38 2.4 "All catalysts were reduced at 623 K prior t o chemisorption measurements.
platinum. The impregnating solution was slowly added to the support, while stirring vigorously, to ensure homogeneity of the catalyst. After impregnation, the catalyst was dried at room temperature for 24 h. The method of further pretreatment at this point varied. For the first series, the catalysts were calcined in air at 623 K for 16 h and then loaded into the reactor where they were reduced at 623 K before starting the reaction. For the second series, the calcination step was omitted, and the catalysts were loaded into the reactor after room-temperature drying and were subsequently reduced a t 623 K. D. Hydrogen Chemisorption, The Pt surfaces area measurements were conducted by using H2 chemisorption in a standard pore volume surface area analyzer. The chemisorption measurements were made at room temperature and at H2 pressures in the range of 1-25 torr. The monolayer coverage was determined by extrapolation of a linear fit to zero pressure. To calculate the number of surface Pt atoms, a stoichiometry of one hydrogen atom per surface Pt atom was assumed (Dorling et al., 1971). The surface area was obtained by using the value 0.09 nm2 per surface Pt atom; this corresponds to the average surface area for a Pt atom in the low-index planes (Kittel, 1971).
Results and Discussion As discussed in the previous section, Pt catalysts supported on A1203,SOz, an MgO were studied. For each support, two different methods of catalyst pretreatment were used: (i) after impregnation the catalyst was dried, calcined, and reduced, and (ii) the same pretreatment sequence as in (i) was followed except that the calcination step was omitted. A discussion of the results obtained on these catalysts is presented below. A. Hydrogen Chemisorption, Table I summarizes the results obtained from hydrogen chemisorption measurements. For each catalyst, the Pt crystallite size was calculated by assuming hemispherical geometry. In all cases, the Pt is moderately dispersed, and the crystallite size varies between 1.5 and 5.0 nm. The following observations
can be made: (i) for each support, the crystallite size is larger on catalysts which were calcined than those in which no calcination step was used; (ii) for calcined samples, the Pt crystallite size increased in the order Al,03 < Si02< MgO; and (iii) for uncalcined samples, the Pt crystallite size varied in the order A&03 SiOz < MgO. The sintering of Pt metal during the calcination step has been previously reported (Dorling et al., 1971); in this work the crystallite size is increased roughly by a factor of 2 when the calcination step is used. The fact that even for identical preparation and pretretament methods the crystallite size is influenced by the support is indicative of the role of metal-support interactions. We have examined the possibility that the Pt crystallite size is higher on MgO due to its lower surface area or larger average pore size. However, that variable alone cannot account for the differences observed in the reaction studies (selectivity). Supported by the reaction studies results, we conclude that metal-support interactions are stronger between Pt and A1203 (or SOz)than those between Pt and MgO and that these interactions lead to different crystallite sizes on various supports. B. Reaction Studies. The results reported represent steady-state activity/ selectivity measurements. Steady state was achieved in a 1-2-h period, and subsequent changes in reaction conditions (e.g., flow rate, reactant composition, and reaction temperature) resulted in a rapid establishment of a new steady-state activity. The reaction products which were monitored included butylamine, dibutylamine, tributylamine, butane, and propane. In the results presented below, the hydrogenation activity is defined in terms of the rate of nitrile conversion to butylamine (BA), dibutylamine (DBA), and tributylamine (TBA), whereas the cracking activity is defined in terms of the rate of nitrile conversion to the hydrocarbon products, propane, and butane. The selectivities were defined as
-
Y(0) = Y(BA) = Y(HA) =
ratehydrogenation ratehydrogenation + ratecracking rateBA latehydrogenetion + ratecracking ~~~~DBA+TBA latehydrogenation
-k ratecracking
Tables I1 and I11 summarize the reaction rates for each product and the selectivities for the different catalyst samples. The rates for propane and tributylamine were generally small and are not shown individually in Table 11, but they have been accounted for in the total rates of cracking and hydrogenation, respectively. A number of observations can be made based on the results shown in Tables I1 and 111. The total rate of hydrogenation for the calcined samples (run1,3, and 5) varies by less than a factor of 2 and appears to be independent
Table 11. Summary of the Reaction Rates for Each Product for Supported Catalysts" rates of butyronitrile conversion, mol/ (g of Pt-h) total support pretreatment butane butylamine dibutylamine hydrogenation 1. Mgo calcined 0.23 0.83 0.83 2. Mgo not calcined 2.07 7.43 3.28 10.71 3. A1203 calcined 0.45 0.07 0.68 0.75 4. A1203 not calcined 0.90 0.25 6.70 9.65 5. SiOB calcined 0.61 0.43 0.43 6. Si02 not calcined 6.15 3.07 7.44 12.73
cracking 0.23 2.07 0.47 1.15 0.61 8.20
"All catalysts were reduced a t 623 K prior to reaction. Reaction conditions: 473 K; 1 atm; 10 mol % butyronitrile in hydrogen.
246
Ind. Eng. Chem. Res., Vol. 27, No. 2, 1988
Table 111. Summary of the Selectivites for the Supported Catalysts" higher butylamine overall amine support pretreatment VHA) Y(0) UBA) 0.78 0.78 0.00 1. Mgo calcined 0.84 0.58 0.26 not calcined 2. Mgo 0.61 0.06 0.56 3. A1203 calcined 0.87 4. A1203 not calcined 0.89 0.02 0.41 0.41 0.00 calcined 5. SiOz 0.61 0.15 0.46 not calcined 6. SiOz "Reaction conditions: 473 K; 1 atm; 10 mol % butyronitrile in hydrogen.
of the support. A similar observation can be made for the total rate of cracking. The overall selectivity (Table 111) for hydrogenation is highest at 78% for the MgO-supported catalyst and is lowest at 41% for the silica-supported catalyst. It is noteworthy, however, that for the calcined catalysts, the hydrogenation products comprise 100% primary amine or butylamine over MgO- and Si02-supported catalysts. On the other hand, for the AlZO3-supportedcatalyst, only 9.3% of the butyronitrile hydrogenated goes to produce primary amine; the blance results in the formation of secondary amine. When one looks at the uncalcined samples (run 2,4, and 6), the total rates of hydrogenation are again constant (within 20%), independent of the support. The total rates of cracking vary, however, depending on the support. Several observations are important here: (i) for each support, higher selectivity for hydrogenation is observed over the uncalcined sample than that over the calcined sample; (ii) the total rates of hydrogenation and cracking are approximately an order of magnitude higher on the uncalcined samples than those on calcined samples; and (iii) the higher amines form a greater fraction of the total hydrogenation products over the uncalcined samples than those over the calcined catalysts. Although the Pt surface areas (and degree of Pt dispersion) are higher for uncalcined catalysts than those for calcined samples (Table I), the surface area enhancement, by a factor of approximately 2, cannot account for more than an order of magnitude increase in the total rates of hydrogenation and cracking. Also, the fact, that increased amounts of dibutylamine and tributylamine are formed over uncalcined samples cannot be interpreted in terms of a Pt crystallite size effect alone. This observation is most dramatic on the silica-supported catalyst. Here, for the calcined sample (run 5), 100% of the butyronitrile hydrogenated forms primary amine, whereas on the uncalcined sample (run 6) only 24% of the butyronitrile hydrogenated yields primary amine. Qualitatively similar, although less dramatic, results are obtained for MgO- and A1203-supportedcatalysts. A closer inspection of the product distribution (Table 111) indicates that the major cause of higher selectivity over uncalcined samples is the preferential increase in the formation of higher amines. In an attempt to separate hydrogenation from hydrogenolysis, the selectivity toward primary amine was determined. As shown in Table 111, this selectivity is markedly
dependent on the support and decreases for both the calcined and uncalcined samples in the following order: MgO > SiOz > A1203. The changes in the activity and selectivity behavior shown are quite complex and cannot be interpreted in terms of a single measurable variable. Several variables may be considered here: (i) Pt crystallite size, (ii) metal-support interactions, (iii) oxidation state of the metal (ease of reducibility), and (iv) restructuring of the Pt surface by various treatment methods. Within the range of variables studied, the Pt crystallite size cannot be the sole variable responsible for the observed behavior. A comparison of runs 4 and 6 shows that although the Pt crystallite sizes are nearly similar for these runs (Table I), the activity and selectivity behavior is markedly different. Similar conclusions are apparent by a comparison of the runs 2 and 3. The role of metal-support interactions is rather difficult to quantify but it is obvious that such a role would affect the metal crystallite size and the ease of metal reducibility, as well as the restructuring of the Pt surface (or the nature of surface sites. The fact that identical catalyst preparation and pretreatment procedures resulted in the formation of Pt crystallites that are different in sizes over different supports (Table I) shows that the support may modify the catalytic behavior. However, the nature and extent of this modification in not clear. In order to eliminate the support effects, Pt black powder (Alfa Research Chemicals, Stock 40039) was studied in the reactor. By use of scanning electron microscopy, the Pt particle size was observed to be approximately 0.2 km (200 nm). Again, two different pretreattments were used: (i) the metal powder was treated in Hzat 623 K for 12 h before starting the reaction, and (ii) the metal was treated with 4% O2(balance He) at 623 K for 12 h. The run with the first sample showed transient behavior typical of the supported metal catalysts, and steady state was attained within a 2-h period. With the oxidized sample, however, the activity and selectivity behavior changed markedly during the reaction course, presumably due to the slow reduction of PtO to a metallic state. The steady-state behavior of Pt black and the initial behavior of oxidized Pt sample are summarized in Tables IV and V. On the Pt black (reduced) sample, the highest selectivity (99%) of any catalyst studied was observed. Only trace quantities of butane were formed. In addition, two-thirds of the butyronitrile hydrogenated yielded primary amine; the balance was dibutylamine. Over the oxidized sample, on the other hand, a quite different behavior was observed. First, primary amine was the only hydrogenation product; the rate of hydrogenation did not change significantly from that over the reduced sample. Second, the rate of cracking (butane formation) was 70-fold greater on the oxidized sample as compared to that on the reduced sample. This resulted in a rather poor hydrogenation selectivity (56%) over the oxidized Pt catalyst. Based on these results, it is possible to make the following conclusions: (i) Pt sites on reduced metal catalysts have activity for the formation of dibutylamine, and (ii) Pt sites on oxidized metal catalysts do not have activity for the formation of higher
Table IV. Summary of the Reaction Rates for Each Product for the Pt Black Reduced and Oxidized Samples" rates of butvronitrile conversion. moll(e of Pt-h) total catalyst pretreatment butane butylamine dibutylamine hydrogenation cracking 0.13 0.07 0.20 0.002 1. Pt black reduced at 623 K 0.002 0.19 0.15 2. Pt black oxidized at 623 K 0.15 0.19 "Reaction conditions: 473 K; 1 atm; 10 mol % butyronitrile in hydrogen.
Ind. Eng. Chem. Res., Vol. 27, No. 2, 1988 247 Table V. Summary of the Selectivities for the Reduced and Oxidized Pt Black Samples' butylhigher overall amine amine catalyst pretreatment Y(0) Y(BA) Y(HA) 1. Pt black reduced at 623 K 0.99 0.64 0.35 2. Pt black oxidized at 623 K 0.56 0.56 0.00
2,
I
"Reaction conditions: 473 K; 1 atm; 10 mol % butyronitrile in hydrogen.
200.0
360.0 520.0 680.0 840.0 REACTOR T E M P E R A T U R E (IC
1000.0
Figure 5. Temperature-programmedreduction spectra obtained for the alumina-supportedcatalyst, calcined in 10% Oz/He at 623 K for 12 h.
'
\
x 200.0
360.0
520.0 680.0 840.0 REACTOR T E M P E R A T U R E (IO
1000.0
Figure 4. Temperature-programmedreduction spectra obtained for the MgO-supportedcatalyst, calcined in 10% Oz/He at 623 K for 12 h.
amines but they have rather high activity for butane formation. These observations over unsupported Pt catalysts are of great importance in helping to explain the behavior observed over supported Pt catalysts. First, one observes a total lack of higher amine formation over Pt/MgO and Pt/Si02 catalysts when the pretreatment included calcination. However, higher amines were observed over uncalcined catalysts. This suggests the possibility that the calcined samples may not have been completely reduced by H2 treatment at 623 K for 1 2 h. It is not clear based on our results, however, as to what the degree of reduction might have been. Second, for the calcined Pt/A1203 catalysts (run 3), the major hydrogenation product was dibutylamine. The fraction of primary amine within the hydrogenation products over Alz03-supportedcatalysts decreased from 9.3% for the calcined sample to 2.6% for the uncalcined sample. This trend is consistent with those observed for the SiOz- and MgO-supported catalysts. It is possible, however, that a specific surface structure of Pt on Al,O, permitted the formation of dibutylamine, but this structure was not present on the MgO- and Si02-supported catalysts. Some catalyst samples were investigated by using temperature-programmed reduction (TPR). Prior to the TPR experiment, the catalyst was treated with 10% O2 (balance He) at 623 K for 10 h. The TPR experiments were performed by flowing 1% H2 (in argon) over the oxidized sample at room temperature. The bed temperature was increased linearly at a rate of 20 K/min, and the consumption of Hzwas monitored by using a thermal conductivity detector. Figures 4 and 5 show the results obtained on MgO- and A1203-supported catalysts, respectively. In both cases, three peaks in the TPR chromatograms are observed. It is clear that the MgO-supported catalyst is reduced more readily than the A1203-supported catalyst. Of greater interest is the possibility that the H2 treatment at 623 K may not have reduced these catalysts
completely. Such a possibility appears to be particularly pronounced for the A1203-supportedPt. The hydrogen chemisorption measurements (Table I) were made after reducing the catalysts at 623 K. For one calcined Pt/A120, catalyst, hydrogen chemisorption measurements were made after reducing it at 623 K and at 743 K. Identical chemisorption isotherms were obtained for both cases, but different reaction rates and selectivity behavior were observed for the two catalysts. This suggests that the degree of reduction was close to unity for the purpose of Pt surface area (and crystallite size) determination. However, still significant differences in selectivity behavior remained. It is thus evident that any modification in the structure of catalytic sites which may be caused by metal-support interactions were not identified by dispersion measurements. Finally, the overall Pt dispersions were high enough that one can conclude that the activity enhancement for uncalcined samples over calcined samples involves some restructuring of the Pt surface sites. For the supported Pt catalysts, the total rates of hydrogenation and cracking were each about an order of magnitude higher on the uncalcined samples than on the calcined samples. On the other hand, for the unsupported Pt catalyst, only the rate of cracking was observed to increase significantly after the oxidation step. Therefore, a decrease in the selectivity of calcined, supported catalysts cannot be solely explained in terms of an increase in the activity of surface sites caused by the presence of oxygen. It appears that the support might play a more direct role in the activity/selectivity behavior. The role of acidic supports (e.g., yA1203)has been suggested in the hydrodenitrogenation of coal-derived liquids because of their affinity for the basic compounds such as amines. However, basic supports (e.g., MgO) are not expected to play a direct role in our reaction system. It will be shown in a later communication that the selectivity is dependent on Pt crystallite size. However, the changes in selectivity which are presented above are different from and beyond those one would attribute to Pt crystallite size effect. Conclusions The reaction system studied is very complex. A number of factors which influence the catalytic behavior seem to be important. These might be (i) Pt crystallite size, (ii) structure of active catalytic sites, and (iii) metal reducibility. These factors are difficult to separate, because they all are manifestations of metal-support interactions. On
Ind. Eng. Chem. Res. 1988, 27, 248-252
248
the basis of the reaction studies over supported catalysts, the behavior of Pt/MgO is least distinguished from the behavior of Pt black; the contrary is true for the Pt/A120, system. Hence, the metal-support interactions are strongest for the Pt/A1,0, and weakest for the Pt/MgO systems. Acknowledgment We acknowledge partial support of this work from a grant from the Dow Chemical Company. One of us ( S . T.M.) was the recipient of an Exxon Fellowship Award during part of this study. Discussions with Ed Vrieland were helpful. Registry No. H3C(CH2)&N,109-74-0;Pt, 7440-06-4; H3C(CHZ)2CH3,106-97-8;H&(CHZ),NHz, 109-73-9;H&(CHZ)SNH-
(CH?)&H,, 111-92-2; MgO, 1309-48-4; Hz, 1333-74-0. Literature Cited Dorling, T. A.; Lynch, B. W. J.; Moss, R. L. J . Catal. 1971,20, 190. Freifelder, M. J. Am. Chem. SOC.1960, 82, 2386. Greenfield, H. Znd. Eng. Chem. Prod. Res. Dev. 1967, 6(2), 142. Kittel, C. Introduction to Solid State Physics, 4th ed.; Wiley: New York, 1971. Rylander, P. Catalytic Hydrogenation in Organic Synthesis; Academic: New York, 1979; p 138. Rylander, P. N.; Hasbrouck, L.; Karpenko, I. Ann. N. Y. Acad. Sci. 1973, 214, 100. Von Braun, J.; Blessing, G.; Zobel, F. Ber. Detsch. Chem. Ges. 1923, B56, 1888.
Received for review August 25, 1986 Accepted August 27, 1987
Butene Oligomerization over Ion-Exchanged Mordenite Masami Kojima,* Marc W. Rautenbach, and Cyril T. O’Connor Department of Chemical Engineering, University of Cape Town, Rondebosch 7700, Republic of South Africa
The effect of varying calcination temperature and Na+-NH4+ exchange levels in synthetic mordenite on the oligomerization of butenes was studied a t 51 atm and 473 K. The conversion over NH,NaM did not increase significantly above an ammonium ion content of ca. 50%. Selectivity did not depend on percent exchange. The highest activity was observed after calcination a t 673-773 K. At a calcination temperature of 873 K, conversion levels of all the catalysts fell markedly. The data obtained suggest that steric factors play a greater role than the effect of increasing acidity in butene oligomerization over mordenite. In previous studies (Kojima et al., 1987a,b), the acidity of sodium mordenite (NaM) ion-exchanged with ammonium chloride to varying degrees has been characterized by temperature-programmed desorption (TPD) and infrared spectroscopy using pyridine as a probe molecule. Pyridine, and not ammonia, was chosen because its molecular dimensions are much closer to those of product molecules of butene oligomerization than ammonia. One of the main findings was that at degrees of exchange above 50% there was essentially no increase in the total number of sites adsorbing pyridine. In this study, butene was ohgomerized over the previously studied NH4NaMsamples to investigate the effect of catalyst pretreatment temperatures and degrees of exchange on the activity of mordenite and to correlate TPD and IR data with mordenite activity. Experimental Section Catalysts. Hydrogen mordenite (HM, &/A1 = 5.8, Z900H)and sodium mordenite (Si/Al= 6.0, ZSOONa) were supplied by Norton Co. in lll6-in. binderless extrudates. NaM was ion-exchanged repeatedly with NH4Cl to give percent exchanges equal to 11%, 33%, 52%, 64%, 86%, and 97% following which the samples were dried in an oven overnight at 353-373 K. The percentage of sodium ions replaced by ammonium is indicated in parentheses, e.g., NH4(52)NaM. Procedures. Six grams of catalyst crushed and sieved to a 212-1000-pm-size fraction, and 75 g of 1/16-in.extrudates diluted with 60 mL of 2-mm glass beads were packed in an 18-mm-i.d. small reactor and 25.4-mm-i.d. large integral reactor, respectively. Mixtures of butaneslbutenes obtained from Sasol (typical composition given in Table I) were dried over 3A molecular sieves and pumped by 0S88-5885/88/2627-0248~01.50/0
Table I. Typical Feed Composition component mass % component propane 2 1-butene propene 2 isobutene isobutane 3 trans-2-butene n-butane 11 cis-2-butene
mass % 65 9 4 4
using a high-pressure diaphragm pump. The results obtained in the large and small reactors were comparable, and hence mostly the data obtained in the small reactor are reported. The description of run procedures will be limited to the small reactor hereafter. Calcination was achieved in flowing medical air (4000-4500 h-l SGHSV, purified by ethanol/C02 and 3A molecular sieves) unless stated otherwise. All the runs were performed at 51 bar and 473 K. The WHSV based on the total inlet stream was approximately 3.5 h-l. The effluent stream had a liquid condensing unit maintained at ca. 323 K. The gas was analyzed by FID using a 6-mm X 5.7-m column packed with n-octane/Poracil C and the liquid product using a 6-mm X 3.8-m column packed with 3% OV-101 on Chromosorb W-HP. The mass balances obtained were within the range 95-100% with an average value of 98%. The products were grouped into polymers of butenes as dimers, trimers, up to pentamers. Conversion Data. Two conversions, both based on mass percent, were defined as follows: (i) conversion to liquid products (denoted in the figures as liquid conversion) = 100 [(mass flow rate of liquid products, excluding monomers)/(mass flow rate of olefinic feed)]; and (ii) feed conversion = 100 [I - (C,exit mass flow rate of olefin i)/(Z,inlet mass flow rate of olefin i)]. If the exit mass flow rate of an olefin is greater than its inlet flow rate, then the former is set equal to its inlet rate for the purpose of 0 1988 American Chemical Society
