THE VAPOUR-PHASE HYDROGENATION OF CHLOROBENZENE ON A RHODIUM-ALUMINA CATALYST*

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1 THE VAPOUR-PHASE HYDROGENATION OF CHLOROBENZENE ON A RHODIUM-ALUMINA CATALYST* Katsuyuki NAKANO,Masamitsu MOROFUJI**, Shinighiro GONDOand Koighiro KUSUNOKI Department of Chemical Engineering, Fukuoka, Japan In order to clarify the relationship between operating conditions and selectivity of a consecutive reaction system, the vapour-phasehydrogenationof chlorobenzeneon a rhodium-alumina catalyst was studied and the reaction mechanismsin the first and the second stage reaction processes were expressed by a Langmuir-Hinshelwood type of rate law. Hydrogen is considered to be adsorbed atomically without strong competition with organic components. The selectivity of benzene increased at high reaction temperatures and low partial pressures of hydrogen. In the external diffusion-controlled region, the selectivity of benzene was reduced compared with that obtained in the reaction-controlled region. Kyushu University, Introduction In order to clarify the relationship between operating conditions and the selectivity of a consecutive reaction system, we have tried to study the kinetics of the hydrogenation of chlorobenzene to cyclohexane via benzene on a rhodium-alumina catalyst. The reaction mechanisms of this reaction system have not been investigated in the gaseous phase, although there has been some research1) done in the liquid phase. During this investigation, wefound that the rate of this reaction was expressed by a Langmuir-Hinshelwood form of rate law. On the basis of this rate e- quation, the relationship between operating conditions and the selectivity of this consecutive reaction system was explained. In addition, we studied the influence of external diffusion on the selectivity of the consecutive gas-solid catalytic reaction. Kusunoki et al.b) obtained the computational result of the effect of external diffusion on the consecutive gas-solid reaction. This result is in general accord with the experimental result in this paper. Since Wheeler6) noted the effect of internal diffusion in the porous catalyst, it has been reported by many investigators that internal diffusion reduces the selectivity of intermediate in the consecutive catalytic reaction. This effect was not investigated and will not be discussed here. When considering the dimensions of the catalyst used in this experiment, the rhodium layer deposited on the alumina pellet is so thin that this effect is assumed to be negligible, although such an effect does exist. 1. Experimental 1. 1 Apparatus and procedure A schematic diagram of experimental apparatus is shown in Fig. 1. A tubular flow reactor, which is made of stainless steel and is 16 mmin inner diameter and 600 mmin length, was used and the temperature was controlled with an oil bath. The reactions were carried out at 110, 130 and 150 G under atmospheric pressure. Hydrogen from a gas cylinder was allowed to flow through a soap-film flow meter and then mixed with chlorobenzene in an evaporator, where chlorobenzene was injected by an accurate micro-pump. A burette was also used to measure moreaccurately the flow rate of chlorobenzene. The mixed gas was introduced into the reactor with a fixed bed of catalysts. For the differential reactor, 2 to 4g of catalyst pellets were packed, and 10 to 40g for the integral reactor. The catalyst pellets were diluted with quartz particles when they VOL6 NO

2 (2) (3) ^14ip^1 r3i9 nil lall & ]0\ fá"fr 1 is r^ 2U3 I19 1 hydrogen gascylinder 2 nitrogen gascylinder 3 needlevalve 4 rotamete 5 soapfilmflowmeter 6 manometer 7silicagel dryer 8 head tank 9 burette 10 micropump ll evaporator 12 agitator 13 oil bath 14 reactor 15 thermocouple 16heater 17 condenser 18 trap 19 cold trap Fig. 1 Experimental apparatus reduction conditions T =220 c ph= g#5cc/sec 2 hrs 2nddaY ^ A 3rd day ^ reaction conditions Tr=150 c -5 FM=4.03x 10 mol/sec dh=10 0I 1 -J t Fig. 2 Changes in catalytic activity with process time (hr) /T?20i 1 i 1 -i- 1 i 1- t Tr=150 c 15_ Pnf0.9atm å 5 PMr -1 atm ri E o 0 _ 01^ » å "I0*" ao ao 7.0 Fj^x 1O5 ( mol/sec] Fig. 3 Mass transfer effect on initial reaction rates were packed. Then the reaction temperature differences along the length of the reactor were less than 2.5 G, even at the highest conversion. The gas leaving the reactor was sampled with a cold trap made of Pyrex glass, which was inserted in a mixture of trichloroethylene and solid carbon dioxide to keep the temperature at -70 C. The composition of the sampled solution was analyzed by gas chromatography, using a column packed with Bentone Catalysts used The catalysts used were nominally 3x3 mm cylindrical pellets made by Engelhard Co. in Japan. These catalysts, consisting of 0.5% by weight of rhodium, are prepared by depositing rhodium on the surface ofalumina pellets to a thickness of ca mm. Before each run, they were reduced by a stream of hydrogen at a flow rate of9.5 cc/ sec at 220 C for two hours Materials used Commercially available hydrogen and nitrogen gases were used without further purification. Organic reactants were guaranteed reagent chlorobenzene and benzene. Somepapers have been published on the effect of a small amount of oxygen contained as an impurity either in hydrogen or nitrogen, and they indicate an increase in catalytic activity. This phenomenon was examined in this experiment, but no such effect could be found Reaction scheme The hydrogenation of chlorobenzene is expressed in the following reaction scheme. O I I (1) This is an exothermic consecutive reaction for the organic reactant within the temperature range of 1 10 to 150 C under atmospheric pressure. From the thermodynamic point of view, the reverse reaction rates were considered to be negligible in both stages under these reaction conditions. 2. Experimental Results 2. 1 Calculation of the initial rate of reaction In the differential reactor, rates of reactions are calculated by use of the following equations. rl r2=w j/p-(/mo~/m) (fc) The conversion of the reaction was about 10%for the differential reactor Changes in catalytic activity with time Generally speaking, the activity of the catalyst decreases during the early stage ofa run and then reaches a steady value after a certain amountof the reactants has been injected into the reactor. Fig. 2 shows the changes in catalytic activity with time. To obtain accurate data, experimental runs must be carried out during the period of stable catalyst activity Mass transfer effects The reaction rates were measuredat various flow rates with a constant gas composition and are plotted in Fig. 3. The constant reaction rates were obtained at flow rates above 5.0X 10~5 mole/sec of chlorobenzene where olh *s 9.0, hence it may be concluded that the mass transfer resistance outside the catalyst pellet is negligible. The effect of internal diffusion was not investigated. But the rhodium layer deposited on the alumina pellet 260 JOURNALOFCHEMICALENGINEERING OFJAPAN

3 calculated curves / model A2 XsO'c! 10- / ro / V,0 1 / ^^ 5-/ ^A^13O C - o / X ^ /a/ PMo= 0-1 atm // Pbo^ 0 ov Pmo= 0.1 atm SjT Pb.=0 ^' 0 \å t^xt^,-; Fig. 4 Effect of the partial pressure of hydrogen on initial reaction rates is so thin that this effect is assumed to be negligible Analysis of the performance of the differential reactor The effects of the partial pressure of each component on the reaction rates in the first and the second stages of the reaction are expressed in Figs. 4 and 5. To change the partial pressure of hydrogen, hydrogen was diluted with nitrogen and the total flow rate was kept constant. From the logarithmic plots of the reaction rates versus the partial pressures of hydrogen and chlorobenzene, the power-type rate equations were obtained as follows : riootwflg* at 150 C (4) h^p^pvr at 130 C (5) ^ oph " at!30 and 150 G (6) Fig, 6 shows the experimental results on the effect of the partial pressure of benzene, where the partial pressures of chlorobenzene and hydrogen were kept constant Performance of the integral reactor The relation of the product distribution vs. time factor is shown in Fig. 7. The weights of the catalyst pellets were 10 to 40 g in this experiment. 3. Selection and Determination of the Reaction Mechanis m If the adsorption step of hydrogen controls the reaction rate, the rate of the reaction must be proportional to the first power of the partial pressure of hydrogen. If the desorption step of the reaction products controls the reaction rate, the rate cannot depend upon the partial pressures ofchlorobenzene and hydrogen. From s^ g / calculated curves en / model A2 \ / " / / ^ A ^ 5 7 a^"a 1'30 c // PHo=0.5atm C" / Pbo=0 o'l ~ PH(r 0.5atm PBo= 0 o-_o_ 0150 c it // ^ 130 C ^z====:==^-^ Pm0 [atm] Fig. 5 Effect of the partial pressure of chlorobenzene on initial reaction rates n v io\ Pho== ^ \Rw-o.i Q. "0 5-/^\n -5"b C * ^ 0 ' ' ' '-'0 0 G PB0 (atm) Fig. 6 Effect of the partial pressure of benzene initially added on initial reaction rates 1.0 ^-.. 1 \e\jvi calculated curves W/F [g-cat-hr/mol] OOA 110 C «<>^ 130 à"æf! 150 Fig. 7 Product distribution curves VOL6 NO

4 K^KbPhPb KhKbPhPb K%p%pB KsHKBp%pB KhKbPhPb KHpHpBKspHpB {I +JKhPh+KhcPhc)* {X ^'KhTh^KhcPhg)2 {\ (1 +KmPm+KbPb) (\+<JKhPh)q (I +KhPh+KmPm+KbPb+KhcPhc)* (1 {1 +KhPh+KhcPhc)3 {\ +KmPm+KbPb) (1 +KmPm+KbPb) A2 Al Reaction stage Reaction mechanism Driving force First stage KhPhPm KhPhPm KmPhPm A3 Bl B2 B3 B4 (1 ^KhPh+KmPm +KbPb-j-KhcPhc) 3 (1 ^KhPh+KhcPhc)2 (1 +KmPm+KbPb) {X +JKhPh+KbPb+KhcPhc)2 (1 +KhPh+KmPm+KbPb+KhcPhc)2 (1 +KhPh+KhcPhc) (1 +KmPm+KbPb) (1 +KhPh+KbPb+KhcPhc) (I +KmPm+KbPb+KhcPhc) Adsorption term Table 1 Supposed reaction mechanisms A2 A2' A3 Second stage Bl B2 B2r B3 B4 Note Reaction rate equation : r=k (Driving force)/(adsorption term) A: Hydrogen is adsorbed in atomic state. B: Hydrogen is adsorbed in molecular state. 1 : The surface reaction occurs between the organic reactant and hydrogen on the same site of the catalyst. 2: The surface reaction occurs between the organic reactant and hydrogen on a different site of the catalyst. 3: The reaction occurs between the organic reactant in the gas phase and hydrogen adsorbed on the catalyst. 4: The reaction occurs between the organic reactant adsorbed on the catalyst and hydrogen in the gas phase. In the second-stage reaction, it is supposed that the rate-determining step is the simultaneous addition of three benzene, except in the mechanisms A2' and B2'. " ' " indicates that the addition of one hydrogenmolecule to benzene controls the reaction rate. hydrogen molecules to j r c ^O-^ 130*^4. ^- w Ot7- OAO$h=7 surfacereaction ^h=9 surface reactioncontrol control I I I I ' à" 0^h= 9 external diffusion control,ft=1.1*10*mol/sec 5I 1 0 ' x 1-] Fig. 8 Conversion-selectivity curves the relations of Eqs.(4) to (6), these mechanisms are rejected. Weconcluded, therefore, that the surface reaction on the catalyst determines the reaction rate. The rate equations for the first and second stages of the reaction were determined by using Hougen- Watson's technique3), and the parameters such as rate constants and adsorption equilibrium constants were estimated by Marquardt's method4) using the experimental results obtained with an integral reactor. Thedetails of this estimation methodwere mentioned in the original paper by Hashimoto et al.2), so that we will not discuss these methods further here. All the reaction mechanisms checked in the calculation are summarized in Table 1. The rate equations based on A2 and B2 mechanisms fitted the experimental results well. But the data, as plotted, deviate from the calculated line more in B2 than in A2. In addition, many researchers have reported that transition metals adsorb hydrogen atomically. From these facts, we conclude that the rate equations of the first and second stage of the reaction can be expressed as follows: k\ KHKMpHpM (l +jkhph +KHCpHcY(\ +KupM+KBpB) (?) K K*MKBpBpB (l +jkmph +KhcPhc)V +KmPm+KbPb) (8) The values of the constant which appear in the rate equations are expressed as the following Arrheniustype equations : ^^^ X lo8*"19'094^27 2= 1.07 x l08*-17'230/i^ KH= l.75 x l0-6e10>*26/et KM=3.38 x l0-5e1*>1a0/rt KB=3.87 x 10-V5*558^ KHC=0.238e3>1!i«'RT 4. Discussion of the Selectivity of Benzene The relationship between the selectivity of benzene and the conversion ofchlorobenzene is shown in Fig JOURNALOF CHEMICALENGINEERING OFJAPAN

5 When the reaction was carried out in the higher temperature region covered in this experiment, the selectivity of benzene was increased because of the difference in activation energies between the first and second reaction stages. In the external diffusion control region, the selectivity of benzene was lowered as compared with that in the reaction control region. This fact seems to require the following explanation : A large amount of benzene is adsorbed on the catalyst. Therefore, when the external diffusion resistance becomes large, the desorption rate of benzene will decrease to increase its surface concentration, and the hydrogenation of benzene will be accelerated to decrease its selectivity. The selectivity of benzene, at a gas flow rate as low as one sixth of the critical rate (above which the reaction rate governs the overall rate), was less than that obtained at the critical flow rate by only 2%. This slight decrease in the selectivity is explained qualitatively from the rate equations. First, the selectivity of benzene can be expressed by the use ofeqs.(7) and (8) as in the following. r2 _ i _ KK-lKBp\pB 5=1-^=1- KKmPu^ ^KhPh -\-KHcPhcY (15) The fact that the increase of external diffusion resistance is accompanied by a decrease in surface concentrations of the reactants and an increase in those of the products is applied to this equation. Then such changes of both pmand pb bring about a decrease in the selectivity. On the other hand, the term of hydrogen chloride, being one of the products, plays an important role in the selectivity because of the forthpowerterm, and that works foward an increase in the selectivity. Therefore, the overall change in the selectivity maybe muchless than that without the adsorption of hydrogen chloride in the rate process. Fig. 9 shows that the selectivity of benzene is increased when the reaction is carried out at a low partial pressure of hydrogen. Wecan see easily, when referring to the rate equations, that the dependency of the reaction rate on the hydrogen partial pressure is different in the two steps of the reaction. It seems reasonable to consider that this difference in the dependency of the two reaction rates on the hydrogen partial pressure causes the increase of selectivity of benzene at a low partial pressure of hydrogen. Conclusion 1) The kinetics of the hydrogenation of chlorobenzene was studied and the mechanisms of this consecutive reaction system were found to be expressed as a Langmuir-Hinshelwood type of rate law. Consider- a Pivi=0.1 atm u>9 ~o PH=0.1 0PH=0.4^ A PH=0.2 OR) I ,5 X (-) Fig. 9 Effect of the partial pressure of hydrogen on the selectivity of benzene ing the reaction mechanisms in detail, it is concluded that hydrogen is adsorbed atomically without strong competition with organic components. 2) The selectivity of benzene increased at high reaction temperatures and low partial pressures of hydrogen. In the mass transfer-controlled region, the selectivity of benzene was reduced compared with that obtained in the reaction-controlled region. Nomenclature F = flow rate of reactant [mole/sec] [cc/sec] f = mole fraction of reaction component [-] k± = reaction rate constant of the first-stage reaction [mole/g-cat. hr] k%= reaction rate constant of the second-stage reaction [mole/g-cat. hr] p = partial pressure of reaction component [atm] R - gas constant [cal/mole- K] r-i = reaction rate of the first-stage reaction [mole/g-cat. hr] r% = reaction rate of the second-stage reaction [mole/g-cat. hr] S T = selectivity of = temperature benzene [=/b/(/b+/<?)] [ G] [-1 [ K] Tr = reaction temperature [ G] t = process time [hr] W = weight of catalyst X = conversion pellets [g] [-] an = initial molar ratio of hydrogen to chlorobenzene [-] < Subscript> B = benzene C = cyclohexane H = hydrogen HC = hydrogen chloride M = chlorobenzene T = total 0 = initla state Literature Cited 1) Brown,J. EL, H. W. Durand Soc., 58, 1594 (1936) and C. S. Marvel: J. Am. Chem. 2) Hashimoto, K., K. Tsuto, K. Miyamoto, N. Hashimoto, N. -Goto, T. Tada and S. Nagata: /. Chem. Eng. Japan, 2, 158 (1969) 3) Hougen, O. A. and K. M. Watson: "Chemical Process Principles, Part III", Wiley, New York (1947) 4) Marquardt, D. W.: J. Soc. Indust. Appl. Math., ll, 431 (1963) 5) Nishitani, H., M. Morofuji, S. Gondo and K. Kusunoki: Preprint for the 8th General Symposiumof the Soc. of Chem. Engrs., Japan, 2-6, Nagoya (1969) 6) Wheeler, A.: Advances in Catalysis, 3, 249 (1951) VOL6 NO

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