Ind. Eng. Chem. Res. 1993,32, 2506-2511
2506
Hydration and Etherification of Propene over H-ZSM-5. 1. A Kinetic Study M a x H. W. Sonnemans KoninklijkelSheIl-Laboratorium,Amsterdam (Shell Research B. V.),P.O. Box 3003, 1003 AA Amsterdam, The Netherlands
A study of the hydration and etherification (with methanol) of propene over H-ZSM-5 reveals that the rates of these bimolecular reactions are significantly influenced by reactant adsorption. The catalytic activity of aluminum-rich H-ZSM-5 for both reactions decreases with increasing Brcansted acid site concentration (aluminum content) because the polar reactants become too strongly adsorbed. A kinetic and catalytic study led to a rate equation based on a Langmuir-Hinshelwood mechanism. Applying this model, the observed increase of the apparent activation energies of both reactions with increasing aluminum content of H-ZSM-5 is explained by an increase of the heats of adsorption of the polar reactants.
Introduction The acid-catalyzed hydration and etherification of alkenes to alcohols and ethers is a well-established technology and is of significant commercial importance. The chemistry of these types of reactions is fairly simple, e.g., propene is converted to isopropyl alcohol (IPA) or methyl isopropyl ether (MIPE) by a reaction with water (R = H) or methanol (R = CHd: CH,CH=CH,
+ ROH
-
(CH,),HCOR
Similarly, methyl tert-butyl ether (MTBE) is formed by a reaction between isobutene and methanol. Low molecular weight ethers are in the gasoline boiling range and are known to have a high blending octane number. The need to meet stringent environmental standards has caused a remarkable growth in the demand for these ethers (Chang and Leiby, 1992). Hence, there is an incentive for improving the processes for their preparation. The catalysts currently used on an industrial scale in these processes are acidic (sulfonated) cation-exchange resins, such as Amberlyst 15. Alkene hydration and etherification processes using other acidic resins, clays, and zeolites are disclosed in patents and in the literature (Hutchings et al., 1992). Zeolite-based catalysts, to date mainly exploited in industrial hydrocarbon conversions, could be of benefit in the hydration and etherification processesof lower alkenes. In the literature, the number of reports on the application of acidic zeolites as catalysts for these reactions is fairly limited. Alkene hydration employing zeolite catalysts may provide alcohols that are essentially free of ether and hydrocarbon byproducts, but for such high selectivities to be obtained, the alkene conversions must be kept low (Kikuchi et al., 1986). The zeolites yield better results in alkene etherification processes. From the preliminary work of Chu and Kuhl(1987) on MTBE synthesis, it was concluded that medium-pore zeolites such as ZSM-5 and ZSM-11gave the highest isobutene conversionsand MTBE selectivities. In comparison with the Amberlyst 15 catalyst, these zeolites have a higher thermostability, while the high MTBE selectivity obtained is less dependent on the methanolbobutene ratio. Other zeolitesare less active (ferrierite) or show a lower selectivity to MTBE (mordenite, zeolite 8). The activity of zeolites in alkene hydration and etherification reactions has often been expressed in terms of alkene conversion and selectivity to or space time yield of 0888-5885/93/2632-2506$04.00/0
the corresponding alcohol or ether. Studies of the kinetics of alkene hydration and etherification over acidic zeolites are sparse, whereas the kinetics of hydrocarbon conversions, e.g. cracking reactions, over these catalysts have been studied extensively (Wielers et al., 1991). Furthermore, the kinetics of alkene etherification over acidic resins are now also well-known, and in many cases LangmuirHinshelwood (LH) type rate expressions have been derived (Subramaniam and Bhatia, 1987; Rehfinger and Hoffmann, 1990). The LH rate equations for the synthesis of MTBE giving the best fit of experimental data were the ones derived from a mechanism in which the ratedetermining step was assumed to be the reaction between methanol adsorbed on one acid site and isobutene adsorbed on two acid sites (Tejero et al., 1987). Clearly, the studies of the kinetics of alkane cracking reactions over zeolites and alkene etherification reactions over acidic resins are directly related to the use of these catalysts in industrial processes. Despite the fact that zeolites such as ZSM-5 may offer a number of advantages over acidic resins, studies of the kinetics of alkene hydration and etherification over zeolites are hard to find. In a previous paper (Sonnemans, 1993) it was established that the kinetics of propene hydration over H-ZSM-5 are of a Langmuir-Hinshelwood type. This is not surprising in view of the facta that (i) propene and water differ strongly in polarity and (ii) similar alkene hydration kinetics were established over acidic ionexchange resins. In this paper other results of our propene hydration study as well as results of a study of propene etherification (using methanol) over H-ZSM-5 zeolites are reported. The zeolites used varied in aluminum content and hence in concentration of Brcansted acid sites. The aim of this study was to establish a relationship between zeolite properties (acidity,reactant adsorption) on the one hand and catalytic propene hydration or etherification activity on the other. The results are in agreement with results of previously studied alkene hydration and etherification reactions over acidic resins in that for both reactions over H-ZSM-5 reactant adsorption dominates the catalytic activity.
Experimental Section H-ZSM-5. ZSM-5 zeolites were exchanged with excess NH4+ (1M aqueous NH4N03 solution) at 90 "C overnight. Subsequently, the zeolites were calcined in air at 500 "C overnight. The aluminum content was determined by X-ray fluorescence (XRF) and the crystallinity by X-ray 0 1993 American Chemical Society
Ind. Eng. Chem. Res., Vol. 32, No. 11,1993 2507 powder diffraction (XRD). The amount of extra-framework aluminum species was calculated from the 27Al MAS NMR peak areas of octahedral (ascribed to extra-framework) and tetrahedral (framework)aluminum and the total aluminum content. Micropore volumes and external surface areas were determined by N2 sorption (de Boer et al., 1966). The concentrations of Br~rnstedacid sites in the calcined ZSM-5 samples were determined by conductometric titration and infrared (IR) spectroscopy (Crocker et al., 1993). Propene Hydration and Etherification. Hydration and etherification (with methanol) of propene were performed in a fixed-bed microflow reactor using 2 mL of the zeolite (particles of 30-80-mesh size) diluted with 6 mL of Sic. Water or methanol was added to a flowing CaHs/He gas mixture by means of an HPLC pump at a rate of about 0.03 mLK,/min (gasphase flows of evaporated water and methanol being 2.2 and 0.9 nL/h, respectively). The feed lines to the reactor and to the gas chromatograph were maintained at 160 “C in order to evaporate the water or methanol added and to heat the feed and effluent gas stream. The ratio of the feed components for the hydration and etherification of propene were C3Hs:H20:He = 2.2: 2.2:3 nL/h (GHSV = 3700 nL-h-l.Lat-9 and C3Hs:CHr 0H:He = 1.4:0.9:3 nL/h (GHSV = 2650 nL.h-l-Lmt-l), respectively, unless otherwise stated. Reaction temperature and total pressure were 155“C and 2 bar (abs), unless otherwise stated. Samples of the effluent gas stream were taken every hour and analyzed on-line by gas chromatography.
Results and Discussion Zeolite Materials and Reaction Conditions. All H-ZSM-5 samples were highly crystalline, and the experimentally determined micropore volumes (0.15-0.18 mL/g) corresponded well with values reported in the literature (Olsen et al., 1980). Generally, no extraframework aluminum species were detected by solid-state 27Al NMR, except in one single H-ZSM-5 sample (containing 1.01 mmol of Al/g). The experimentally determined concentrations of Bransted acid sites were found to be consistently lower than the theoretical maximum concentration calculated on the basis of the aluminum content, assuming an H+/A1molar ratio of 1 (Crocker et al., 1993). Preliminary experiments were carried out to establish the optimum reaction conditions. At 155 OC the activities of propene hydration and etherification were measured in the kinetic regime. The reaction rates were not affected by external mass transfer, e.g., the yield of IPA did not change upon avariation of the gas velocity, while the space time 7 was kept constant by varying the volume of the catalyst simultaneously. By performing the experiments at a total pressure of 2 bar, it was ensured that the reactants remained in the gas phase and hence that no liquid phases were formed inside the catalyst bed. Compressibility factors, derived from the reduced temperatures and reduced pressures of the various components, were found to be (nearly) equal to 1,indicating that the gas mixture behaved more or less ideally (Coulson et al., 1983). In control experiments, no activity was observed when the feed was passed through an empty reactor, over Sic, or over a nonacidic zeolite. Neither “rehydroxylation” (the conversion of Lewis acid sites to Bransted acid sites) nor dealumination occurred in H-ZSM-5 under the reaction conditions, as was indicated by IR and 27Al MAS NMR measurements on some spent samples. In addition to MIPE resulting from the etherification of propene with methanol, dimethyl ether (DME) was
Table I. Fractional Yield of IPA or MIPE (Y) as a Function of the Concentration of Propene in the Feed ([CsH& over H-ZSM-5 (0.60 mmol of Al/g)
rcSHs10 2.2 13.4 13.4 10.7 12.5 19.2 28.1
T4
(8)
Mb
Propene Hydration to IPA 1.0 6.7 1.0 1.0 0.9 1.0 Propene Etherification to MIPE 1.5 0.5 1.5 0.5 1.3 0.3 1.2 0.2
yield 0.0115 0.0123 0.0115 0.0275 0.0238 0.0210 0.0191
Space time T = V/F,in which Vis the volume of catalyst and F the total volumetric gas flow. M = [ROHId[Cs&lo. (I
Table 11. Fractional Yields of IPA and MIPE and the Rate Constants k1 (First-Order Kinetics) and ~ L H (Langmuir-Hinshelwood Kinetics) of H-ZSM-5 Zeolites with Different Aluminum Contents
0.09 0.22 0.38 0.60 0.69 1.01 1.20 0.09 0.22 0.38 0.60 0.69 1.01 1.20 4
Propene Hydration to IPA4 0.4 X 10-9 0.04 0.04X 3.1 X 10” 0.20 0.29 X 0.46 X 1CF2 5.6 X 10-9 0.22 1.23 X 1t2 17.0 X 10-9 0.37 0.60 X 6.3X 10-9 0.43 4.6 X 10-9 0.72 0.44X 2.4 X 10-9 0.84 0.22 X Propene Etherification to MIPE4 0.5 X 1O-s 0.04 0.07 X 3.6 X 10-9 0.20 0.50 X 6.8 X 10-9 0.22 0.42 X 2.20 X 18.2 X 10-9 0.37 10.4 X 10-9 0.43 1.47 X 0.72 1.20 X 1k2 8.4 X 10-9 0.70 X 5.3 X 10-9 0.84
0.5 X 3.9 X 6.7 X le2 16.8 X 1t2 8.1 X le2 5.5 X le2 2.9 X
0.3 X 2.7 X 3.2 X 10-2 9.6 X le2 4.7 X 10-2 5.8 X 4.1 X
Reaction conditions: see the Experimental Section.
formed through dimerization (and dehydration) of methanol. Furthermore, the yields of IPA, MIPE, and DME often decreased slowly with time on stream, which forced us to take the initial yield as an appropriate measure of the catalytic activity. In the course of this paper, the yield always refers to the initial yield of the process. Changes in the volume of the gas stream were neglected since the yields of IPA or MIPE were quite low. Kinetics of Propene Hydration and Etherification. In order to establish the kinetics of propene hydration and propene etherification over H-ZSM-5, the concentration (i.e. flow) of propene at the inlet of the reactor ( [ C ~ H ~ Iwas O ) varied (Table I). The GHSV (1/7) was maintained constant by adjusting the flow of helium. For testing purposes, the H-ZSM-5 sample containing 0.60 mmol of Al/g was used since it showed the highest activity (Table 11). The fractional yields of IPA or MIPE (y)are well below their equilibrium values (Ye),which under our reaction conditions are about 0.025-0.030 and about 0.100.15, respectively. In Figure 1A a plot of -Ye ln(1 (Y/Ye))versus 7 is shown in order to check reversible firstorder kinetics. The data are rather scattered, and straightline fits (lines through the origin) are poor. In fact, the yields of IPA or MIPE appear to be dependent on the concentration of propene at the inlet of the reactor (Table I), which weakens a reversible first-order (in propene) rate equation as a valid description of the kinetics of these reactions. A more valid kinetic description can be obtained by applying Langmuir-Hinshelwood kinetics, for reasons to be discussed in the next section. In the following Lang-
2508 Ind. Eng. Chem. Res., Vol. 32, No. 11, 1993
A
s 7 r = kLH(1- V/(M- Y) e e I
I
A substitution of this rate equation in the equation of space time T for an integral plugflow reactor (Lee, 1985)
I
I.
9' T
I I
= [C3H610Sb((dY/r)
leads to I
4
y - ( M -1)ln(1- Y) e ~LH(T/[C&]O) (4) The validity of the LH rate equation (eq 4) has been checked by plotting Y- (M-1)ln(1- Y) versus T/[C~&IO, as shown in Figure 1B. Clearly, the data fit straight lines (through the origin), indicating that the LangmuirHinshelwood model is more appropriate for describing the rates of propene hydration and propene etherification over H-ZSM-5 than reversible fit-order kinetics. Equation 4 was also valid for these reactions over aluminumrich H-ZSM-5 (1.01 mmol of Al/g; Sonnemans, 1993) and aluminum-poorH-ZSM-5 (0.09 mmol of Al/g). For small fractional yields of IPA or MIPE, ln(1- Y) equals -Y, and eq 4 simplifies to
I
1
Y
0 0
P I I
/
0
MY = kLH(T/ [c.&lO) (5) Since M and [C3&10 are the ROH/CsHe molar ratio and the concentration of propene a t the inlet of the reactor, respectively, one finally obtains Y = ~LHT/[ROH]~
(6) Hence, for given values of ~ L and H space time T , the yield of IPA or MIPE decreases as the concentration of the polar reactant in the feed ([ROHIo) increases, i.e., the polar reactant actually inhibits the reaction.
0.6
0.3
I, (m3 a * moP) Figure 1. (A) -Ye ln(1- (Y/Ye)), with Y being the fractional yield T / [ c ~ H,
of IPA (0) or MIPE (0)as a function of space time T for H-ZSM-5 containing 0.80 mmol of Al/g. (B)Y - (M-1) ln(1- r),with Y being or MIPE ( 0 )as a function of T / [Cfilo the fractional yield of IPA (0) for H-ZSM-5 containing 0.60 mmol of AUg.
muir-Hinshelwood (LH) rate equation (Lee, 19851,
r=
k[s12K&~[C3H~l[ROHl (1+ Kp[C,H,] -I-KR[ROH])~
(1)
Kp and K R are the adsorption constants of propene (subscript P) and the polar compound ROH, viz. water or methanol (subscript R), respectively; k (m3.mol-'d) is the rate constant of the rate-determining step between the two adsorbed species; and [SI (mol/m3) is the total adsorption site concentration. The adsorption of the products is neglected because of their small concentrations (yields less than 3%). Furthermore, we assume that KR[ROHl >> (1+ Kp[C&I) which can be justified by the fairly high heats of adsorption of water and methanol in protonic zeolites (Ison and Gorte, 1984). The simplified LH rate equation is then
r = ~LH[C&I/[ROHI
(2)
with k,, = k[SI2(K,/KR) (3) Knowing that [C&] = [C&lo(l Y) and [ROHI = [C&]o(M- Y) with M = [ROH10/[C~H~lo, the following expression of the rate equation is obtained:
-
Propene Hydration and Etherification over H-ZSM-6 Varying in Aluminum Content. The invalidity of reversible first-order kinetics appears also in a catalysis study. For both propene hydration and propene etherification over H-ZSM-5 samples with different aluminum contents, reversible first-order rate constants (kl)have been determined by dividing -Ye ln(1- ( Y/Ye)) by T (Table 11). Similarly, Langmuir-Hinshelwood rate constants ( ~ L H )were determined by dividing { Y - (M- 1). ln(1- Y)] by T/[C3H&. The first-order rate constants kl for hydration and etherification both attain a maximum at an intermediate aluminum content. Such a maximum in the rate constant cannot be explained on the basis of the acidity of H-ZSM-5, since a first-order rate constant of an acid-catalyzedreaction increases only with an increase of the acidity of the catalyst. For example, first-order rate constants derived from activities of acidic ZSM-5 and stabilized faujasites in the cracking of n-hexane or cumene dealkylation were found to be proportional to the aluminumcontent (Haaget al., 1984;Sohnetal., 1986;Wielers et al., 1991). With increasing aluminum content the Bransted acid site concentration increases (Table 11: [H+l). Hence, one expects a proportional increase in the propene hydration and propene etherification rate constants, especially since the H-ZSMd samples used in this work must all have the same (intrinsic) acid strength, concluded on the basis of their aluminum content (Barthomeuf, 1987). Such a proportional increase of the rate constants has not been found. In Figure 2 the'LH reaction rate constants" for propene hydration and propene etherification of the various H-ZSM-5 samples have been plotted against their aluminum content (the data of H-ZSM-5 containing 0.60 mmol of Al/g are not shown since this sample has a larger external surface area than the others; Sonnemans, 1993). The LH rate constants also attain a maximum at an
Ind. Eng. Chem. Res., Vol. 32, No. 11, 1993 2509 10 I
1
*O 10 re’
= IE 2
-P
6
1
4
A Y
1
Y
0 0
-
2
0
0.5
[All
1 .o
1.5
(mmollp)
0.5
0
[H’l
1.o
(meq/g)
Figure 2. Langmuir-Hinshelwood rate constant kLH for propene hydration (0)and propene etherification ( 0 ) of H-ZSM-5 as a function of the aluminum content.
Figure 3. Rate constant k (calculatedby eq 8) for propene hydration (0) and propene etherification( 0 )of H-ZSM-6as a function of the Brernsted acid site concentration.
intermediate aluminum content, but in contrast to kl, these rate constants are a function of k , [SI, and the adsorption constants of both reactants according to eq 3. Because k represents the rate constant of the acid-catalyzed reaction of two adsorbed species (and therefore depends only on the acidity of the catalyst), the “bell” shape of the curves H be explained on the basis of [SI and KPIKR. of ~ L can This proves that sorption properties (i.e., the concentration of adsorption sites [SI and the ratio of the adsorption constants Kp and KR)of H-ZSM-5 are dependent on the aluminum content and significantly influence the rate of propene hydration and etherification. With increasing aluminum content (acid site concentration), k and [SI will increase, and thus it has to be assumed that KPIKRis inverselydependent on the aluminum content. If the value of this ratio is strongly declining with increasing aluminum content through a preferential adsorption of ROH, it is again concluded that the polar reactant actually inhibits the rate of the reaction. This effect of inhibition is more pronounced for the aluminum-rich H-ZSM-5 samples. The Langmuir-Hinshelwood rate equation found is not unique since it has been proposed before as the correct rate equation for describing the kinetics of etherification reactions over acidic resins (Subramaniam and Bhatia, 1987; Rehfinger and Hoffmann, 1990) and Ti silicalite (Changet al., 1992). Thus, H-ZSM-5 does not differ from other acidic catalysts used for the bimolecular alkene hydration and etherification reactions. However, it differs significantly from rate equations of bimolecular hydrocarbon conversions over H-ZSM-5. The origin of this difference is attributed to the composition of the feed. Whereas the compounds in hydrocarbon conversions are all quite apolar, the feed used for alkene hydration and etherification consists of the apolar alkene and polar water or methanol. Therefore, preferential adsorption (of the polar compound) on these acidic catalysts leads to a reaction rate dominated by adsorption. Catalytic Activity Influenced by t h e Sorption Properties of H-ZSM-5. Except for the fact that the amount of absorbed water or methanol in a zeolite is proportional to the aluminum content (Nakamoto and Takahashi, 1982; Ison and Gorte, 1984), no data of concentration ranges for which propene and water or methanol can be simultaneously present inside a zeolite have been reported. This is most likely due to the inability of measuring the adsorption of alkenes over protonic zeolites since these species oligomerize very rapidly (van den Berg et al., 1983).
Recognizingthat there is no evidence as to whether Kp/ KR indeed decreases with increasing aluminum content, the following empirical relationship has been derived:
K ~ I =K e+[All) ~ (7) According to this relationship, K ~ I K Rdecreases exponentially with increasing aluminum content and the expression satisfies the condition that the rate constant k (calculated by dividing kLH by [S12(KP/KR);eq 3) increases linearly with the Brransted acid site concentration. This condition is based on the fact that the rate constant k (of the rate-determining reaction between two adsorbed species) depends only on the acidity of the catalyst as has been shown many times for hydrocarbon conversions over zeolites. Moreover, it is assumed that the concentration of adsorption sites [SI is equal to the aluminum content, i.e., [SI = [All (Nakamoto and Takahashi, 1982). By trial and error, the values of u for propene hydration ( u = 5.5) and propene etherification ( u = 4.3) have been determined. The rate constants k of H-ZSM-5 in the hydration and etherification of propene have thus been calculated as follows: k = kLHe(u[AI1)/ [All2 (8) (obtained by combining eqs 3 and 7) and are shown in Figure 3 as a function of the Brrzlnsted acid site concentration. “Turnover frequencies”, defined as kl [H+l, for both reactions can be determined. For propene hydration this turnover frequency is equal to 17.9 (correlation coefficient: 0.988) and for propene etherification the turnover frequency is equal to 5.8 (correlation coefficient: 0.966). The exponential decrease of KPIKR with increasing aluminum content has been determined empirically, and at present no additional data are available which would further support the validity of eq 7. However, more information concerning the influence of reactant adsorption on the reaction rate can be obtained by measuring the temperature dependency of the LH rate constant kLH. Starting from eq 3, the temperature dependency of kLH is determined by an “apparent activation energy” (E:), which is related to the activation energy of the ratedetermining reaction (E,) and the heats of adsorption the reactants ( Q p and QR) as follows: (9) The heat of adsorption is defined as the negative value of
2510 Ind. Eng. Chem. Res., Vol. 32, No. 11, 1993
I
I
4
Table IV. (Apparent) Activation Energies (E.') of Alkene Hydration and Etherification over Various Acidic Catalysts product IPA
I 4
-5
I\\ I
-7 1 2.2
2.4 1OOOlT
2.6
(t?)
Figure 4. Arrhenius plot of the rate constants kLH for propene hydration of H-ZSM-5 containing 0.09 (+), 0.60 (O),and 1.01 mmol of Al/g (A).
"
I 0
0.5
[All
1 .o
1.5
(mmollg)
Figure 5. Apparent activation energies (El)for propene hydration ( 0 )and propene etherification (0)of H-ZSM-5 as a function of the aluminum content. Table 111. Apparent Activation Energies (E.') of H-ZSM-5 with Different Aluminum Contents, in the Hydration and Etherification of Promne to IPA and MIPE [All (mmol/g) 0.09 0.60
Ed (kJ/mol) IPA 50.7 87.5
MIPE 113.3 135.0
[All (mmol/g) 1.01 1.20
E,' (kJ/mol) IPA MIPE 114.0 121.1 198.9 137.2
the adsorption enthalpy. Rate constants kLH have been determined as a function of the temperature for both propene hydration and propene etherification. The reaction temperature was varied in the range of 120-170 "C. The resulting Arrhenius plots afford straight lines for both propene hydration (Figure 4) and etherification, enabling the apparent activation energy E,' to be calculated (Table 111). In Figure 5,the apparent activation energies,obtained for some H-ZSM-5 zeolites with different aluminum contents, are plotted against the aluminum content of these zeolites. As can be seen clearly in Figure 5, the apparent activation energies of propene hydration and propene etherification increase more or less linearly with increasing aluminum content. The only exception, viz. the rather low E,' obtained for the H-ZSM-5 containing 1.01 mmol of Al/g in the etherification of propene, is ascribed to extraframework aluminum species in this sample. The presence of extra-framework aluminum species has a large effect
MIPE MTBE
acid catalyst acidic resins H-ZSM-5 H-ZSM-5 Amberlsst 15
MTBE MTBE
methylsulfuric acid p-toluenesulfonic acid
El
(kJ/mol) 90-150 50-140 113-198 71-92 87-91 103
reference Petrus, 1982 this work this work Rehfinger, 1990, Giccuel, 1983 Gicquel, 1983 Gicquel, 1983
on the formation of MIPE (and DME). This has been demonstrated by steaming the H-ZSM-5 sample (1.20 mmol of Al/g), the presence of extra-framework aluminum species being confirmed by 27Al MAS NMR, and by testing this sample in the etherification of propene. The LH rate constant for the steamed sample is 0.165 moLm-%-l, an increase by a factor of 4 compared to the original (extraframework aluminum-free) H-ZSM-5. Micropore volume and external surface area of this H-ZSM-5 sample were not changed upon steaming, and a possible increase of the concentration of B r ~ n s t e dacid sites is limited by the maximum concentration (calculated on the basis of aluminum content) of 1.2 mequiv/g. Hence, the increase H be attributed to an enhancement of intrinsic of ~ L must properties, e.g., the acid strength or the adsorption constants of the compounds. The increase of the apparent activation energy cannot be related directly to an increase of the difference in heats of adsorption (QR-Qp) basedoneq 9 since the dependence of E, on the aluminum content itself is unknown. Nevertheless, if it is assumed that E, is independent of the aluminum content (acid site concentration), based on the fact that the intrinsic acid strength of H-ZSM-5 is independent of the aluminum content in the range of 0-1.2 mmol of Al/g (Barthomeuf, 1987)) then it follows that differences in (QR- Qp) are the origin of different values of the apparent activation energies. Hence, knowing then that (QR- Qp) is proportional to the aluminum content, it can be concluded that the ratio of the two adsorption must decrease exponentially with the constants, KP/KR, aluminum content according to eq 7. In the literature, activation energies of alkene hydration and etherification have only been reported for reactions catalyzed by liquid acids and acidic resins. Values found for E, range from 90 to 150 kJ/mol for propene hydration (Petrus, 1982) and from 70 to 105 kJ/mol for MTBE synthesis (Gicquel and Torck, 19831, depending on the type of liquid acid and acidic resin (Table IV). The apparent activation energies for the formation of IPA and MIPE over H-ZSM-5 are comparable with these literature data. For a comparison of the formation of MIPE and MTBE, the difference in stability between secondary and tertiary carbenium ions (about 45 kJ/mol) has to be taken into account. By applying eq 9 to propene hydration and to propene etherification, and assuming that the activation energies (E,) of these two reactions are approximately equal (due to a close similarity between the transition states of the oxonium ions (CH&HCO+HR, R = H, CH3), it is found that the difference between the apparent activation energies is directly related to the difference between the heats of adsorption of methanol and water. This difference is about 55 kJ/mol (Figure 51, which is in reasonable agreement with reported differences in proton affinity between methanol and water in the gas phase, 65 kJ/mol (Lias et al., 19841, and in H-ZSM-5, 50 kJ/mol (Aronson et al., 1986).
Ind. Eng. Chem. Res., Vol. 32, No. 11,1993 2511
Conclusions Summarizing, this study of the hydration and etherification (with methanol) of propene over H-ZSM-5 warrants the conclusion that the catalytic activity of this type of zeolite for these reactions is significantly influenced by reactant adsorption. The reaction rates can be described in terms of a Langmuir-Hinshelwood formalism on the basis of the following observations: (1) the yields of IPA or MIPE decrease with increasing concentration of the polar reactant, (2) the catalytic activities of aluminum-rich H-ZSM-5 samples decrease with increasing Bransted acid site concentration (aluminum content), and (3) the apparent activation energies increase with increasing Bransted acid site concentration. These features cannot be explained by a first-order kinetic model since a first-order rate constant only increases with increasing zeolite acidity. By applying the Langmuir-Hinshelwood kinetic model, the increase in the apparent activation energies for both reactions with increasing Bransted acid site concentration of H-ZSM-5 is explained by an increase of the heats of adsorption of the polar reactants.
Nomenclature E, = activation energy (kJ/mol) E,' = apparent activation energy (kJ/mol) Ki = adsorption constant of compound i (m3/m0l or bar') k = 2nd order rate constant (m3.mol-1d) kl = (reversible)first-order rate constant (8-1) ~ L =H Langmuir-Hinshelwoodrate constant (mol.m-3.s-1) M = [ROHld[CsHslo Q = heat of adsorption (kJ/mol) r = reaction rate (mol.m-3.s-1) Y = fractional yield, [productl/[C~H~l~ a = parameter defined according to eq 7 T = space time ( 8 ) [il = concentration of compound i (mol/m3 or bar) [SI = total adsorption site concentration (m0l/m3) Subscripts
LH = Langmuir-Hinshelwood P = propene R = polar compound ROH 0 = reference to the inlet of the reactor 1 = reference to first-order kinetics
Literature Cited Aronson,M. T.;Gorte, R. J.; Farneth, W. E. The influence of oxonium ion and carbenium ion stabilities on the alcohoUZSM-5interaction. J. C Q t d 1986, 98, 434. Barthomeuf, D. Zeolite acidity dependence on structure and chemical environment. Correlations with catalvsis. Mater. Chem. Phvs. 1987, 17, 49. Chang, E. J.;Leiby, S. M. Ethers help gasoline quality. Hydrocarbon Process. 1992, 71 (2), 41. Chang,K.-H.; Kim, G.-J.;Ahn, W.-S. Methyl tert-butylether synthesis over titanium-silicalite I catalysts. Znd. Eng. Chem. Res. 1992,31, 125.
Chu, P.; Kiihl, G. H. Preparation of methyl tert-butylether (MTBE) over zeolite catalysts. Znd. Eng. Chem. Res. 1987,26,365. Coulson, J. M.; Richardson, J. F.; Sinnott, R. K. Chemical Engineering; Pergamon Press: New York, 1983; Vol. 6. Crocker, M.;Herold, R. H. M.;Sonnemans, M. H. W.; Emeis, C. A,; Wilson, A. E.; van der Moolen, J. N. Studies on the acidity of mordenite and ZSM-5. I. Determination of Brensted acid site concentrations in mordenite and ZSM-5 by conductometric titration. J. Phys. Chem. 1993, 97, 432. de Boer, J. H.; Lippens, B. C.; Linsen, B. G., Broekhoff, J. C. P.; van den Heuvel, A.; Osinga, Th. J. The t-curve of multimolecular Nzadsorption. J. Colloid. Interface. Sci. 1966,21, 405. Gicquel, A.; Torck, B. Synthesis of methyl tertiary butyl ether catalyaed by ion-exchange resin. Influence of methanol concentration and temperature. J. Catal. 1983,83, 9. Haag, W. 0.; Lago, R. M.; Weisz, P. B. The active site of acidic aluminosilicate catalysts. Nature 1984,309,589. Hutchings, G. J.; Nicolaides, C. P.; Scurrell, M. S.Developments in the production of methyl tert-butyl ether. Catal. Today 1992,15, 23. Ison, A.; Go&, R. J. The adsorption of methanol and water on H-ZSM-5. J. CQtd. 1984,89, 150. Kikuchi, E.; Matsuda, T.; Shimomura, K.; Kawahara, K.; Morita, Y. Catalytic vapor-phase hydration of lower olefins over protonic zeolite catalysts. Stud. surf. sci. &tal. 1986,28,771. Lee, H. H. Heterogeneous Reactor Design; Butterworth Publiehere: Stoneham, MA, 1985. Lias, S. G.; Liebman, J. F.; Levin, R. D. Evaluatedgas phase basicities and proton affiiities of molecules;Heats of formation of protonated molecules. J. Phys. Chem. Ref. Data 1984,13,695. Nakamoto, H.; Takahashi, H. Hydrophobic natures of zeolite ZSM5. Zeolites 1982, 2, 67. Olson, D. H.; Haag, W. 0.; Lago, R. M. Chemical and physical properties of the ZSM-5substitutional series. J. Catal. 1980,61, 390. Petrus, L. Hydration of lower alkenes catalyaed by strong acid ion exchangers. Thesis, University of Groningen, 1982. Rehfinger, A,; Hoffmann, U. Kinetics of methyl tertiary butyl ether liquid phase synthesis catalysed by ion exchange resin 1. Intrinsic rate expression in liquid phase activities. Chem. Eng. Sci. 1990, 45,1605. Sohn,J. R.; DeCanio, S. J.; Fritz, P. 0.;Lunsford, J. H. Acid catalyeis by dealuminated zeolite Y. 2. The role of aluminium. J. Phys. Chem. 1986,90,4&17. Sonnemans, M. H. W. The hydration of propene over acidic zeolites. Appl. &tal. A: -General 1993, 94, 215. Subramaniam, C.; Bhatia, S. Liquid phase synthesis of methyl tertbutyl ether catalysed by ion exchange resin. Can. J. Chem. Eng. 1987, 65, 613. Tejero, J.; Cunill, F.; Iborra, M. Molecular mechaniems of MTBE synthesis on a sulphonic acid ion exchange reein. J. Mol. Catal. 1987, 42, 257. van den Berg, J. P.; Wolthuizen, J. P.; Clague, A. D. H.; Hays, G. R.; Huis, R.; van Hooff, J. H. C. Low-temperature oligomerisation of small olefins on zeolite H-ZSM-5. J. Catal. 1983,80, 130. Wielers, A. F. H.; Vaarkamp, M.; Post, M. F. M. Relation between properties and performance of zeolites in paraffin cracking. J. Catal. 1991,127, 51. Received for review February 3, 1993 Revised manuscript received June 21, 1993 Accepted July 25, 1993. Abstract published in Advance ACS Abstracts, October 1, 1993. @
