Ind. Eng. Chem. Res. 1993,32, 2512-2515
2512
Hydration and Etherification of Propene over H-ZSM-5. 2. Deposition of Carbonaceous Compounds on the Catalysts Max H.W . Sonnemans KoninklijkelSheIl-Laboratorium,Amsterdam (Shell Research B. V.),P.O. Box 3003, 1003 AA Amsterdam, The Netherlands
The hydration and etherification of propene over H-ZSM-5 are accompanied by the deposition of carbonaceous compounds on the catalyst. Analysis of spent samples indicates that the interior of the zeolite becomes inaccessible due to the deposition of carbonaceous compounds resulting from the oligomerization of propene. The propene hydration and etherification reactions are relatively unaffected by this process, however, from which i t is concluded that these reactions proceed mainly on the external surface of the zeolite crystallites. Support for this concept is provided by the findings that the catalytic activity of H-ZSM-5 in these reactions (i) decreases when the crystallite size increases and (ii) is virtually zero when the external surface is passivated by the deposition of a Si02 layer. Introduction Although the acid-catalyzedhydration and etherification of alkenes are well-established processes, alternative catalysts or improvements of the current catalysts (acidic resins) are of interest, since augmented catalytic activities may lead to favorable lower reaction temperatures. In the preceding paper (Sonnemans, 1993), a study of the hydration and etherification (using methanol) of propene over H-ZSM-5 zeolites is reported. This study was performed in order to assess the potential of zeolites as catalysts for alkene hydration and etherification. The kinetics and catalysis of these reactions over H-ZSM-5, significantly influenced by reactant adsorption, were discussed in detail. The reaction rates were described in terms of a Langmuir-Hinshelwood formalism. During the hydration and etherification of propene, carbonaceous compounds are deposited on the H-ZSM-5 catalysts. These compounds result from the oligomerization of propene, a feature which has often been observed with acid-catalyzed reactions involving alkenes and seems to be inherent in these types of reactions (van den Berg et al., 1983; Quann et al., 1988). In this paper, the deposition of carbonaceous compounds on H-ZSM-5 during propene hydration and etherification is discussed in detail. Experimental Section The preparation and characterization of the H-ZSM-5 samples and the experimental details of the hydration and etherification of propene, leading to isopropyl alcohol (IPA) and methyl isopropyl ether (MIPE), respectively, have been described in the preceding paper. H-ZSM-5 samples with different crystallite sizes were obtained by varying the concentration of the template (tetrapropylammonium hydroxide) in the synthesis mixture. The crystallite sizes were estimated from the broadening of the Debije-Scherrer (X-ray diffraction) lines. Chemical vapor deposition (CVD)was performed in an Aixtron CVD apparatus in order to deposit thin layers of Si02 on the external surface of H-ZSM-5. Zeolite particles of 30-80mesh size were placed on a graphite susceptor heated by infrared radiation. Tetraethoxysilane vapor carried by a nitrogen flow was introduced into the reactor, and an appropriate deposition of Si02 was achieved at a pressure of 20 mbar and a substrate temperature of 450 OC. Spent H-ZSM-5 samples were analyzed for carbon (C), and occasionally also for nitrogen (N), by X-ray fluoresQass-5885/93/2632-2512$04.00/0
cence (XRF)and X-ray photoelectron spectroscopy (XPS). In other experiments, dry and water-saturated H-ZSM-5 zeolites were treated with propene (CSH6:He = 1:3 nL/h; GHSV = 2670 nL.h-lLmt-') at 155 "C and 2 bar for about 20 h, and analyzed for carbonaceous deposits by combustion mass spectrometric elemental analysis (CME) and XPS. Results and Discussion Carbonaceous Deposits on the H-ZSM-5 Catalysts. Preliminary experiments had been carried out to establish the optimum reaction conditions for propene hydration and etherification and to ensure that the reactants remained in the gas phase in which they behaved more or less ideally. Upon studying the hydration and etherification of propene over H-ZSM-5 as a function of time on stream (Figure l),it was consistently found that: (1)The yields remained constant (MIPE) or decreased slowly (IPA). Irrespective of this, the initial yields, determined by extrapolation to t = 0 h, were taken as a proper measure of the catalytic activity of the zeolite. (2) During the first few hours on stream, compounds of higher molecular weight were detected. Their concentration, estimated from the total area of their GC peaks, declined strongly with time on stream and became negligible after about 10-15 h. The formation of such higher molecular weight compounds in the early stage of the run is most likely due to the oligomerization of propene. Formation of such compounds from methanol, as in the methanol-to-gasoline (MTG) process, normally occurs at higher temperatures (Chang, 1983). The observed higher molecular compounds can be seen as intermediates formed during the deposition of carbonaceous compounds. Remarkably, the oligomerization of propene itself is strongly affected during the formation of the carbonaceous deposits, whereas propene hydration and etherification are less affected and continue for at least 24 h. In order to elucidate this differencethe H-ZSM-5 sample (1.01 mmol of Al/g), which had been used in propene etherification, was further analyzed. The spent sample, like all the others, had a brown color, indicating the deposition of carbonaceous compounds. Analysis of the sample for Bronsted acid site concentration, amount of carbonaceous deposits and micropore volume after 5 and 42 h on stream revealed that the deposition of carbonaceous 0 1993 American Chemical Society
Ind. Eng. Chem. Res., Vol. 32, No. 11, 1993 2513 lo
7
2
0
0
10
20
Time on stream
30
(h)
Figure 1. YieldofIPA(0) andMIPE(0)aswellas (theconcentration of) concomitant higher molecular weight compounds (+) versus time on stream for H-ZSM-5 (1.01 mmol of AVg). Table 1. Analysis of Fresh and Spent H-ZSM-SSamples Used in the Etherification of Propene H-ZSM-5: 1.01 mmol of Al/g
C/Si XPS MPV XRF [H+l C timeon stream (h) (mequiv/g) (w/w %) (mol/mol) (mol/mol) (mL/g) 0 0.72 0 0.17 0 0 8.4 0.49 0.54 0 5 8.9 0.51 0.59 42 0 H-ZSM-6 0.60 mmol of Al/@ time on C/Si N/Si N/A1 stream (h) (mol/mol) (mol/mol) (mol/mol) 0 0 0 0.63b 24 XRF 0.77 0.006 0.17 XPS: 2.38 0.015 a This sample was treated with propene at 156 O C for 7 h, before it was used in the etherification of propene. After 24 h i t was treated b Assuming that N/Al equals H+/A1of the fresh H-ZSM-5 with "3. sample.
compounds took place principally within the first 5 h (Table I). Moreover, the deposition of the carbonaceous compounds occurred nearly homogeneously throughout the zeolite crystallites, as indicated by a comparison of the C/Si molar ratios of the bulk (XRF) and the external surface (XPS). No acid sites could be determined by conductometric titration. After 5 h on stream the micropore volume (MPV the volume of the pores smaller than 20A) was virtually zero. These observations indicate that during the first few hours on stream carbonaceous compounds are deposited throughout the zeolite crystallites, with the result that the interior of the zeolite eventually becomes inaccessible. Despite this fact, propene hydration and etherification still continue (Figure 1). The oligomerizationof propene itself, followed through the detection of higher molecular weight compounds in the effluent gas stream, is directly affected,which suggests that this reaction took place in the interior of the zeolite catalyst. The effective blocking of the interior of the zeolite was demonstrated in two other ways. First, on passing only propene over an H-ZSM-5 sample (0.60 mmol of Al/g), the compounds of higher molecular weight only were detected. Their total concentration, estimated on the basis of the total area of their GC peaks (arbitrary units), decreased strongly with time, due to the deposition of carbonaceous compounds (Figure 2). However, the pres-
0
10
20
Time on stream
30
(h)
Figure 2. Yield of MIPE ( 0 )and (the concentration of) concomitant higher molecular weight compounds (+) versus time on stream for H-ZSM-5 (0.60 mmol of Al/g) which was treated with C3Hs during the first 7 h. After 24 h on stream the sample was treated with "3.
ence of carbonaceous deposits did not affect the etherification of propene (the yield of MIPE), which occurred after adding methanol to the propene feed. The fractional yield of MIPE of 0.028 (measured after 20 h on stream) H 0.076 m ~ l - m - ~ d , corresponds to a rate constant ~ L of which is just slightly lower than the rate constant of 0.096 m ~ l - m - ~obtained d, after running the propene/methanol feed directly (see the preceding paper). Although it can be assumed that the micropore volume is virtually zero, as for the foregoing H-ZSM-5 sample, propene etherification continued and decreased only slightly with time on stream. Brernsted acid sites were evidently still available under the reaction conditions, as indicated by the finding that a treatment of the spent H-ZSM-5 catalyst with NH3 resulted in the cessation of propene etherification. At the end of the run, the sample was analyzed for carbon (C) and nitrogen (N) (Table I). By comparing the C/Si molar ratios of the bulk (XRF)with those of the external surface (XPS), it can be concluded that carbonaceous compounds were deposited mainly on the external surface. This finding seems to be in contrast with the observation that when the normal propene/methanol feed was used (from the beginning of the run), the deposition occurred throughout the whole of the zeolite crystallites. However, the initial exposure of the zeolite catalyst to exclusively propene leads to a rapid oligomerization of propene, producing carbonaceous compounds which are mainly deposited in the outer shell of the zeolite crystallites. In the presence of methanol this oligomerization of propene extends further into the interior of the zeolite crystallites. A stronger adsorption of methanol on acid sites forces the propene to penetrate further into the zeolitepore structure. Furthermore, from the N/Si molar ratios of the bulk and the external surface, it can be concluded that the Brernsted acid sites (which were still available since they reacted were also located mainly on the external surface. with "3) The concentration of these acid sites (expressed by the N/A1 (= NH,+/Al) molar ratio) was much lower in comparison with the fresh catalyst (H+/Al = 0.63). Second, in other experiments, propene was passed over H-ZSM-5 zeolites with different aluminum contents at 155 OC for 20 h. Additionally, other batches of the same zeolites were saturated with water at 155 OC before the propene treatment was carried out. The amount of carbonaceous deposits was determined by CME (Table 11). The typical amount of carbonaceous deposits obtained (about 8 w/w 5% ) corresponds to 0.18 mL/g of oligomerized
2514 Ind. Eng. Chem. Res., Vol. 32, No. 11, 1993 Table 11. Analysis of Some Spent H-ZSM-5 Samples after Treatment with Propene C/Si (mol/mol) C
[All (mmol/g) 0.09 0.60 1.01 1.20
CME (w/w %) 8.0 (7.9)" 24.9 (22.9) 9.9 (9.8) 7.2 (6.9)
CME/XRF 0.4 (0.4) 1.4 (1.3) 0.6 (0.6) 0.4 (0.4)
XPS 0.6 (0.6) 3.1 (2.6) 4.5 (4.4) 0.7 (0.7)
5 In parentheses: the values obtained when the H-ZSM-5 samples had been presaturated with water (7 nL/h) at 155 O C for 4 h.
Table 111. Propene Etherification Rate Constants ~ H-ZSM-5 with Different Crystallite Sizes crystallite sizerange (fim)
specific surfacearea (fim2/fim3)
t-area (m2/g)
MPV (mL/g)
0.2-0.3 1.0-2.5 3-5
20-30 2.4-6.0 1.2-2.0
27 15
0.18 0.18 0.17
0'5 0.4
I
r -
la
0.
'5 -0
-
0.3
E
3
0.2
Y
C L of H
kui (mol.m4-s-1) 0.31 0.15 0.10
propene (calculated by assuming that the density of propene inside the zeolite is equal to the density of liquid propene: 0.52 g/mL). Such a volume of oligomerized propene is about equal to the micropore volume of ZSM-5 (Olsonet al., 1980). This indicates that the total micropore volume of the zeolite becomes clogged by propene oligomers, even when these pores were already filled with water. Again, it is observed that oligomerization of propene proceeds to a large extent in the outer shell of the zeolite crystallites, since the C/Si molar ratio of the external surface is generally higher than the C/Si molar ratio of the bulk. On the basis of the characterization of some spent H-ZSM-5 samples and the courses of propene hydration and etherification versus oligomerization over these catalysts, it is concluded that the interior of the zeolite crystallites becomes clogged by carbonaceous deposits, formed by oligomerization of propene. The observed activities of these catalysts in propene hydration and propene etherification must therefore be attributed to Bransted acid sites located in the outer shell of the zeolite crystallites. Similar observations of alkene oligomerization occuring inside zeolites (the alkene being generated by alcohol dehydration) have been reported recently (Williams et d., 1991). The external surface appears to suffer much less from propene oligomerization, since propene hydration and etherification remain nearly unaffected. Catalytic Activity of the External Surface. As discussed in the foregoing section, it is suggested that propene hydration and etherification reactions over H-ZSM-5 occur mainly in an outer shell, or even exclusively on the external surface of the zeolite crystallites. In order to gain more evidence, propene etherification was performed over three H-ZSM-6 samples with different crystallite sizes. The crystallite sizes had been estimated from the broadening of the Debije-Scherrer lines (Table 111). The values thus obtained were in satisfactory agreement with scanning electron microscopy observations. Specific surface areas (pm2/pm3)were calculated from the crystallite diameters. The aluminum contents of the samples are identical (ca. 0.02 mmol of AUg). Assuming the absence of aluminum gradients throughout the crystallites, the bulk concentrations of Bransted acid sites should also be equal, whereas the number of surface acid sites should be proportional to the specific surface area (Gilaon and Derouane, 1984). All samples have the same micropore volume (ca.0.18 mL/g) and do not contain extraframework aluminum species. For two samples, a clear difference in external surface area, as derived from the
H-ZSM-5 [All (mmol/g) [ef.Alla (mmol/g) k u i (mo1.m4-r1) original 1.20 0 4.1 X 1k2 SiOrcoated 1.12 0.12 0.7 X 1k2 [ef.All = concentration of extra-framework aluminum.
t-plot method (Voogd et al., 19911, was found (Table 111). The largest %weaWwas indeed found for the sample containing smaller particles, although the difference between these areas is somewhat less than expected on the basis of the specific surface areas. The reaction rate H propene etherification depend strongly constants ~ L for on the crystallite sizes (Table 111). Clearly, as is shown in Figure 3, the etherification activity increases with increasing specific surface area, which strongly supporta our earlier hypothesis that the acid sites on the external surface contribute significantly to the catalytic activity. Furthermore, it is possible to passivate the acid sites on the external surface of a H-ZSM-5 sample by chemical vapor deposition (CVD), a method described in the literature (Vansant, 1987; Hibino et al., 1991). By means of CVD, thin layers of Si02can be deposited on the external surface of zeolite particles via the decomposition of tetraethoxysilane, a compound which cannot enter the zeolite pore structure. The extent of deposition of Si02, and hence the thickness of the layer, was controlled by selecting appropriate experimental conditions (see the Experimental Section). Analysis of the external surface (XPS)of the SiO2-coated H-ZSM-5 did not reveal any aluminum, and the O/Si molar ratio was found to be 1.94. These data suggest that the thickness of the nonzeolitic Si02 layer is at least 40 A. No significant decrease in crystallinity was observed, but some extra-framework aluminum species had been formed (Table IV). The Si02coated H-ZSM-5 catalyst proved to be as active in n-hexane cracking as, and even more active in methanol dehydration than, the original H-ZSM-5. Therefore, it is concluded that molecules such as methanol and n-hexane are able to penetrate into the channels of the coated zeolite, showing that during the deposition of Si02 no blocking of the pore openingshas occurred. The reaction rate constant for propene etherification ( ~ L His ) significantly lower over the SiO2-coatedH-ZSM-5 than over the originalH-ZSM-5 (Table IV). For the SiO2-coated sample, only acid sites in the interior of the zeolite can contribute to the catalytic activity. Hence, the difference between the two rate constanta is related to the contribution of the acid sites
Ind. Eng. Chem. Res., Vol. 32, No. 11, 1993 2515 on the external surface to the catalytic activity. Normally, the activity of propene etherification over H-ZSM-5 decreases only slightly with time on stream; however, for the SiO2-coated H-ZSM-5 sample,the decrease in propene etherification activity was so marked that after as little as 3 h the yield of MIPE was already virtually zero. At that moment, the interior of the zeolite is inaccessible due to the deposition of carbonaceous compounds, and sincethere are no acid sites available on the external surface, no activity is observed. By determining the yield of IPA or MIPE as described previously (Figure l),it is evident that the calculatedvalues of the rate constants must be related to the concentration of acid sites on the external surface. Since there is a linear relationship between the surface and the bulk concentrations of acid sites, provided that the crystallite sizes of the various H-ZSM-5 samples are the same, the “bell” H the shape of the curves of the rate constants ~ L versus aluminum content or Bransted acid site concentration remains the same. The previously calculated turnover frequencies, defined as kl [H+l, must have higher values since now the concentration of surface acid sites must be taken into account. Furthermore, it is apparent that reactant adsorption, which significantly influences the catalytic activity as described in the preceding paper, is actually related to surface adsorption rather than to bulk adsorption.
Conclusions From this study it is concluded that during propene hydration and etherification over H-ZSM-5 catalysts, the interior of these zeolites becomes inaccessible due to the deposition of carbonaceous compounds resulting from the oligomerization of propene. The observed propene hydration and etherification reactions are relatively unaffected by this process; hence, these reactions proceed mainly on the external surface of the zeolite crystallites. Further support is provided by the findings that the catalytic activity of H-ZSM-5 in these reactions (i) decreases when the crystallite size increases and (ii) is
virtually zero when the external surface is passivated by the deposition of a Si02 layer.
Acknowledgment The author wishes to thank J. Nanne for supplying the ZSM-5 samples of different crystallite sizes,J. van Amstel for the N2 sorption measurements, and C. Megiris and J. Glezer for preparing the SiOz-coated H-ZSM-5. Literature Cited Chang, C. D. Hydrocarbons from methanol. Catal. Rev.-Sci.Eng. 1983,25,1. Gilson, J. P.; Derouane, E. G. On the external and intracrystalline surface catalytic activity of pentasil zeolites. J. Catal. 1984,88, 538. Hibino, T.; Niwa, M.; Murakami, Y. Shape-selectivity over H-ZSMd modiiedby chemicalvapor depositionof siliconalkoxide. J.Catal. 1991,128,551. Olson, D. H.; Haag, W. 0.; Lago, R. M. Chemical and physical properties of the ZSMd substitutional series. J. Catal. 1980,61, 390. Quann, R.J.; Green, L. A.; Tabak, S. A.; Krambeck, F. J. Chemistry of olefin oligomerisation over ZSM-5 catalyast. Ind. Eng. Chem. Res. 1988,27,565. Sonnemans, M. H. W. Hydration and etherification of propene over H-ZSM-5. 1. A kinetic study. Znd. Eng. Chem. Res. 1993, preceding paper in this issue. 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-ZSMd. J. Catal. 1983,80,130. Vansant, E. F. Pore Size engineering in zeolites. Stud. Surf. Sci. Catal. 1987,37,143. Voogd, P.; Scholten, J. J. F.; vanBekkum, H. Use of the t-plot-De Boer method in Dore volumedeterminationsof ZSM-5 twe - _ zeolites. COU. Surf. mi, 55, 163. Williams, C.; Makarova, M. A.; Malyaheva, L.; Paukshtis, E. A.; Talsi, E. P.: Thomas, J. M.: Zamaraev, K. I. Kinetic studies of catalvtic dehydration oftert-butanol on zeolite NaH-ZSM-5. J.Catal. 1991, 127,377.
Received for review February 3, 1993 Revised manuscript receiued June 21, 1993 Accepted July 25, 1993. Abstract published in Advance ACS Abstracts, October 1, 1993. @
