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 NH4NaM samples 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
Ind. Eng. Chem. Res., Vol. 27, No. 2, 1988 249 Table 11. Dependence of Maximum Conversion Levels on the Duration of Calcination in Flowing Air catalyst NH4(52)NaM NH4(97)NaM calc. temp/K calc. time/ h conv. to liquid/mass % feed conv./mass %
573 8 18 53
573 24 28 74
673 4" 38 81
673 8 61 85
673 24 66 87
673 7 45 84
673 25 83 86
Calcination in nitrogen.
Table 111. Liquid Product Distribution catalyst calc. temp/K calc. time/h time onstream/h conv. to liquid/mass % dimer/mass % trimer/mass % tetramer mass %
HM
NH4(97)NaM 573 673 773 873 573 673 773 873 10.5 11 25 25 8 12 24 24 1.5 1 0.9 2.3 0.6 1.9 1.4 1.3
NH4(85)NaM 773 13.5 0.9
NH4(64)NaM 773 10.5 1.5
NH4(52)NaM 573 673 773 873 24 24 4 8 3.7 0.8 1 1
NHd(33)NaM 773 12 0.9
NHd(l1)NaM 773 27 1
60
55
45
30
24
82
74
50
75
70
20
61
69
36
31
9
71
68 25 6
76 21 3
79 18 3
77 20 3
59 29 10
62 29 10
71 22 7
60 29 11
62 28 10
71 23 5
69 23 6
61 74 28 21 1 1 5
79 18 3
73 23 4
23 6
calculation in (ii). In other words, (ii) gives an estimation of the total activity of the catalyst and includes olefin isomerization. Analyses of the effluent gases indicated that considerable isomerization to cis-2- and trans-2-butene occurred. The latter two components constituted 8-10% of the olefinic feed. Hence, if all the other olefins are either oligomerized or isomerized such that the mass flow rate of cis- and trans-2-butene in the exit stream exceeds that at the inlet, the feed conversion, as defined in (ii) above, will be approximately 90%. Thus, a feed conversion approaching 90% can be taken as an indication that essentially all the other olefins have reacted, either by oligomerization or isomerization. Ammonia Titration. According to IR data all the samples calcined in vacuum were deammoniated by 600 K. However, low conversions obtained after calcination for 4 h at 573 K in the reactor indicated that the samples might not have been fully deammoniated. The effect of calcination time length on the extent of deammoniation in the reactor setup was determined by calcining a sample in a stream of high-purity nitrogen and analyzing the evolved ammonia by using a boric acid method (Vogel, 1966).
Results Effect of Duration of Calcination. The results of titrating ammonia evolving at 573 K are shown in Figure 1. As a check on the analytical procedure, NH4(97)NaM was calcined at 773 K for 23 h. After 8 and 23 h, 90% and 9570,respectively, of the computed total amount of NH3 initially present on the catalyst was detected. The remaining 5% may have been removed when the sample was dried overnight at 357-373 K in a static environment subsequent to ion exchange. Indeed, a small amount of Brernsted acidity was detected by IR analysis after evacuating NH4NaM at 313 K for 2 h, indicating that slight deammoniation had occurred.(Kojima et al., 1987b). The effect of duration of calcination in flowing air on butene oligomerization was investigated by calcining NH4(52)NaMat 573 and 673 K and NH4(97)NaMat 673 K for varying time lengths prior to reaction. The maximum oligomerization and feed conversions are given in Table 11. In the case of NH4(52)NaM,serious deactivation, both with respect to oligomerization and isomerization, was observed on the samples calcined at 573 K within a few hours on stream. A t a calcination temperature of 673 K, although the oligomerization conversions after 4, 8, and 24 h of calcination fell to 1670, 2970, and 4470,
I
1
1
\
s
.. 0 0
0 00
0'0
12'0
1'0
('0
TIYE,
,e 0 HRS
28 0
2,
28 0
0
Figure 1. Extent of deammoniation as a function of time. Nitrogen flow rate, 4300 h-' SGHSV. '00 0
1
I
I
I
c o
211
I
D
J u
G O
TIYE,
IO
u
I2
L
ti95
Figure 2. Effect of calcination temperature on conversion levels over HM.
respectively, after 10 h on stream, the total feed conversion remained independent of time on stream. In the case of NH4(97)NaM,the oligomerization conversion nearly doubled as the calcination time length at 673 K was increased from 7 to 25 h. The feed conversions were however essentially the same and nearly complete. Effect of Calcination Temperature. The effect of calcination temperature on butene oligomerization was investigated over HM, NM4(97)NaM,and NH4(52)NaM at four calcination temperatures: 573, 673, 773, and 873 K. As seen in Figure 2, the conversion to liquid product obtained over HM was at its maximum after calcination at 573 K and decreased with increasing calcination temperature. In contrast, NH4(97)NaM and NH4(52)NaM
250 Ind. Eng. Chem. Res., Vol. 27, No. 2, 1988
I
i
'.
I L
-
___
L
' 3
-~_ -_ 4
S r
u
t----+----i 6 d 1 3 12 c
T!Mk, iris
Figure 3. Effect of calcination temperature on conversion levels oyer NH4(97)NaM.
I 3 3
2 0
4 0
6 0
6 0
l!ME.
IO 0
I ?0
PPj
Figure 5. Effect of ammonium ion content on conversion to liquid product following calcination at 773 K.
conversion over NH4(52)NaMwas 77% and after 11h on stream it fell to 46%, while the figures for NH4(97)NaM were 82% and 62%, respectively.
_____
,
7--
L C
1 L
-__ 6 0
7 d U I, 0
; ' o
I V L . r9C
Figure 4. Effect of calcination temperature on conversion levels over NH4(52)NaM.
gave the lowest conversions to liquid product after calcination at 573 K, as shown in Figures 3 and 4. The highest conversions were obtained in the range 673-773 K. Representative product distributions are given in Table 111. In general there was a shift to lower chain-length hydrocarbons with decreasing conversion. NH4(64)NaMcalcined at 923 K and HM calcined at 993 K were tested in the large integral reactor. They both deactivated much more rapidly than the respective samples calcined at 673-773 K. Effect of Varying Ammonium Ion Content. The effect of varying the degree of exchange was investigated by performing butene oligomerization over samples with 11%, 33%, 52%, 64%, 85%, and 97% ammonium ion content, and over HM, after calcination at 773K. The results are plotted in Figure 5. It appears that above 52% exchange there is no proportional increase in butene oligomerization activity with increasing degree of exchange. Feed conversion levels were also comparable above 52 % exchange. Over NH4(11)NaM,however, the feed conversion dropped from 70% to 25% in 2 h on stream. Similar overall results were obtained with NH4(85)NaM, NH4(64)NaM, NH4(55)NaM, NH4(11)NaM, and HM in the large integral reactor. Because the oligomerization conversion levels were high (in the vicinity of 75%), butene oligomerization was carried out over NH4(97)NaM and NH4(52)NaMin the small reactor at a WHSV of 11 h-' in order to ensure that the absence of increase in activity with increasing degree of exchange was not due to conversion levels being too high. The conversion to liquid product over NH4(52)NaM decreased from 38% to 7% in 10 h on stream, while the corresponding figures for NH4(97)NaM were 47% and lo%, respectively. The maximum feed
Discussion Calcination under flowing nitrogen as opposed to vacuum calcination yielded a large difference in the extent of deammoniation at 573 K. While IR data indicated that deammoniation was virtually complete in the vicinity of 600 K (Kojima et al., 1987b) under vacuum, in the reactor configuration a higher temperature was required to achieve a comparable level of deammoniation. Similar results have been reported, for example, by Karge (1971). Titrating ammonia evolved during calcination showed that the bulk of deammoniation occurred during the first few hours. Since the presence of oxygen in the calcining medium typically facilitates deammoniation (Shikunov et al., 1973; Tsitsishvili et al., 1982), the extent of deammoniation in calcination prior to butene oligomerization was most probably greater than that obtained by ammonia titration which is given in Figure 1. At a calcination temperature of 673 K and an exchange level of 52%, 4 h was shown to yield a catalyst which could achieve an alkene conversion, through oligomerization and isomerization, exceeding 90%, thereby indicating accessibility of feed molecules to essentially all acid sites. However, the conversion to higher oligomers rose markedly as the duration of calcination was increased from 4 to 8 or 24 h. In particular, the lowest rate of deactivation was observed after 24 h of calcination. Thus it appears that the last sites to be deammoniated are catalytically important for butene oligomerization. At a calcination temperature of 573 K, however, the maximum total feed conversion rose from 53% to 74% and conversion to liquid product rose from 18% to 28% as the duration of calcination was increased from 8 to 24 h over NHJ52)NaM. In this case the number and nature of sites created by deammoniation for 8 h was evidently insufficient for significant alkene isomerization, and moreover the generated sites were subject to rapid deactivation. The amount of ammonia evolved after 24 h of calcination at 573 K from NH4(97)NaMwas 35%. greater than that from NH4(52)NaM,but in spite of this increase the conversion to liquid product was virtually identical over these samples. Figure 5 shows that maximum oligomerization conversion levels after calcination at 773 K increased from 9.5% to 31% and to 70% as ammonium ion content was increased from 11%to 33% and 52%, respectively. The increase in conversion from 31% to 70% is much higher than the increase in ion-exchange level. There was no significant increase in conversion to liquid product above
Ind. Eng. Chem. Res., Vol. 27, No. 2, 1988 251 a degree of exchange of 52%. These observations are in sharp contrast to the data obtained by Minachev et al. (1971), Gray and Cobb (1975), Ratnasamy et al. (1981),and Montes et al. (1984). All the above researchers reported an increase in activity with increasing ammonium ion content in mordenite. Minachev et al. (1971) studied isomerization of cyclohexane to methylcyclopentane and hydrogenation of benzene over NH4NaM with ammonium ion content ranging from 0 to 95%. After calcining NH4NaM at 793 K in flowing air, they found that the rate of isomerization increased and that of hydrogenation decreased linearly with increasing exchange level. Ratnasamy et al. (1981) prepared five samples of NH4NaM, ranging in ammonium ion content from 0 to 100% and calcined in static air at 823 K for 6 h. When o-xylene was passed over the samples, the conversion increased faster with increasing ammonium ion content above 61% than below this level, and moreover disproportionation occurred in addition to isomerization. Gray and Cobb (1975) examined hydroisomerization and hydrocracking of n-pentane over NH4NaM with ammonium ion content varying from 67% to 99%. A t optimum calcination temperature of 773 to 798 K, the rates of the above reactions increased significantly above an ammonium ion content of 85%. In agreement with their results, Montes et al. (1984) found that above a 90% ammonium ion content n-hexane hydrocracking activity over NH4NaM increased rapidly. One important difference between their work and the present study is that, with the exception of o-xylene disproportionation, the reactions studied by the above researchers, viz., isomerization, hydroisomerization, and hydrocracking, involve conversion of reactants to products of similar or smaller dimensions. In contrast, butene oligomerization involves formation of bulky molecules. As a result, the presence of highly branched trimers of butene, for example, should significantly reduce the “packing density” of molecules inside the main channel and inhibit further formation of large oligomers. That nearly all the alkenes except cis- and trans-2-butene were converted following calcination at 673 K -773 K but only 60430% of them were oligomerized appears to indicate that while there was no mass-transfer limitation to the extent that all the reactant molecules were able to access acid sites, the formation of butene oligomers was sterically hindered and there was no advantage in increasing the number of acid sites by increasing the percentage exchange above 50%. In particular, while Ratnasamy et al. (1981) have given evidence using measurements of isosteric heats of ammonia adsorption that the last 50% of the Na+ ions to be exchanged produce significantly stronger acid sites, this study shows that the strong sites accessed by ammonia cannot enhance the percentage of alkenes which are oligomerized. The effect of steric hindrance is even more pronounced at WHSV = 11h-l. As before, nearly all the alkenes were converted initially, but less than half were oligomerized. Although there was some increase in conversion to liquid product as percent exchange was raised from 52% to 97 % , the increase was nowhere near comparable to the increase in the number of potential acid sites. As noted earlier, following calcination at 773 K the effect of increasing percent exchange is evident below approximately 50% exchange, and the increase in activity appears to be nonlinear. In the vicinity of 50% exchange, the number of active sites generated which are active for oligomerizationare probably sufficient to allow the product molecules to fill the main channel. Any further increase in activity as a result of increasing percent exchange is thus not possible because near pore filling does not permit
I hlLNSl T I lNlfHSllY
I Z R I I O U N I OF ’I C E S O R B I N O > 5 5 0 U I T P O I +=RnOUNl OF PI OESOPBINO > 700 K I l P O I x = i IO-CONI w i t n I n
O . O O - - O:lIO-COHV +.RliOUNl
RFltR 6 n Of PI R O S . C R L C F R O M B P I I L ’ Y
R85.
1EMPERPTURE
.
K
Figure 6, Effect of calcination temperature on acidity and activity of NH4(97)NaM. Data normalized with respect to their maximum values; LPy band absorbance normalized with respect to maximum BPy band absorbance.
formation of branched oligomers. As in TPD and IR experiments (Kojima et al., 1987a,b), HM behaved differently from NH,NaM. At a calcination temperature of 573 K, HM gave an oligomerizationactivity higher than any other sample. At 673 and 773 K, however, the oligomerization activity over HM was much lower than that over mordenite, with percent exchange exceeding 50%. Moreover, the maximum conversion to liquid product following calcination at 573 K was still lower than that over NH4(52-97)NaM calcined at 673-773 K. Since the feed conversion over HM was approximately 85% at the above calcination temperatures, essentially all the olefins were able to access acid sites. From IR findings (Kojima et al., 198713) one difference resulting from calcination at different temperatures is the amount of Lewis acidity. The drop in conversion to liquid product with increasing calcination temperature is probably due to the increased presence of Lewis sites which enhance steric hindrance in the mordenite pores. The observation that the maximum conversion over HM calcined at 573 K is lower than that over NH,NaM calcined at 773 K may indicate that, even at a calcination temperature as low as 573 K, there is already considerably greater Lewis acidity on HM than on NH4NaM calcined at 773 K. At a calcination temperature of 873 K, the magnitude of. steric hindrance seems to increase significantly for all catalyst samples, and oligomerization conversion levels over NH4NaM fell to that over HM. The feed conversions, although much higher, also fell slightly. One of the aims of this study was to compare butene oligomerizationactivity with the number of sites adsorbing pyridine during TPD and the type of acidity measured by infrared using pyridine as a probe (Kojima et al., 1987a,b). These data are plotted as a function of calcination temperature for NH4(97)NaMin Figure 6. This figure shows the intensities of pyridinium ion (BPy) and coordinately bound pyridine (LPy) absorbances and the amount of pyridine adsorbed onto IR wafers calculated from integrated BPy and LPy absorbances and extinction coefficients of Hughes and White (1967). Also shown are the amounts of pyridine adsorbed during TPD experiments and desorbing above 550 and 700 K and conversions to liquid product after 1and 6 h on stream. All the data are normalized with respect to their maxima except the BPy and LPy band absorbances which are normalized with respect to the higher of the two maxima. Since at the pretreatment temperature of 573 K deammoniation was essentially complete under vacuum (TPD and IR) but incomplete in the reactor setup, the conversion to liquid
252
Ind. Eng. Chem. Res. 1988,27, 252-256
product was increased to a value corresponding to complete deammoniation for comparison with acidity characterization data. It is evident from this figure that neither the total number of Brernsted nor of Lewis sites as measured by pyridine IR analysis determine the oligomerization activity of mordenite. Similar trends were obtained for NH,(52,55)NaM. If butene oligomerization depends, inter alia, on the strength of sites, this is to be expected since adsorbing pyridine after successively higher calcination temperatures and taking IR spectra gives little indication of the strength of sites. Thermodesorption studies of Ghosh and Curthoys (1983) and Karge and Klose (1973) have established that most Brernsted sites are much weaker than Lewis sites. Thus although there is a large number of Br~nstedsites at low calcination temperatures, most of them are probably too weak to catalyze oligomerization. It should be recalled that the strength of sites cannot easily be inferred from pyridine TPD because new, stronger sites are formed during temperature programming and pyridine readsorbs onto these sites. The fact that increasing the calcination temperature from 673 to 773 K had little effect on oligomerization activity indicates either that the sites formed at 673 K are strong enough to oligomerize butene or that at 773 K the extra acidity strength generated by the formation of Lewis sites is countered by an increase in steric hindrance. A similar graph plotted against ammonium ion content for calcination at 773 K indicated that oligomerization activity most closely paralleled the total amount of acidity as measured by TPD or IR. Conclusions Calcination at 673-773 K was found to be optimal for butene oligomerization over NH,NaM. The activity increased nonlinearly with increasing ammonium ion content up to 52 % , the rate of increase in activity being greater than that in percent exchange in this region. Above 52%, no further increase in activity was observed at a conversion of 70-80%. A t a WHSV of 11 h-l, the maximum activity increased by 24% as the exchange level was increased from
52% to 97%. The absence of proportional increase in activity above ca. 50% exchange despite the generation of stronger acid sites as detected by ammonia adsorption may be explained by essentially complete filling of the pore channel by product oligomers, thus preventing the formation of additional product molecules. Acknowledgment The authors thank the Council for Scientific and Industrial Research, SASOL, and the University of Cape Town for financial assistance. Registry No. Poly(1-butene), 9003-28-5; polyisobutene, 9003-27-4; trans-poly(2-butene), 25656-69-3; cis-poly(2-butene), 25989-99-5.
Literature Cited Ghosh, A. K.; Curthoys, G. J. Chem. SOC., Faraday Trans. 1 1983, 79, 147. Gray, J. A.; Cobb, J. T., Jr. J. Catal. 1975, 36, 125. Hughes, T. R.; White, H. M. J.Phys. Chem. 1967, 71, 2192. Karge, H. 2. Phys. Chem. Neue Folge 1971, 76, 133. Karge, H.; Klose, K. 2. Phys. Chem. Neue Folge 1973, 83, 100. Kojima, M.; Rautenbach, M. W.; O’Connor, C. T., submitted for publication in J. Catal. 1987a. Kojima, M.; Rautenbach, M. W.; O’Connor, C. T., submitted for publication in J. Catal. 198713. Minachev, K. H.; Garanin, V.; Isakova, T.; Kharlamov, V.; Bogomolov, V. Adu. Chem. Ser. 1971, 2, 441. Montes, A.; Perot, G.; Guisnet, M. IX Ibero-American Symposium on Catalysis, 1984, 1651. Ratnasamy, P.; Sivasankar, S.; Vishnoi, S. J. Catal. 1981, 69, 428. Shikunov, B. I.; Mishin, I. V.; Piloyan, G. A.; Klyachko-Gurvich, A. L.; Lafer, L. I.; Yakerson, V. I.; Rubinshtein, A. M. Izu. Akad. Nauk SSSR, Ser. Khim. 1973,4, 767. Tsitsishvili, G. V.; Piloyan, G. 0.;Kvantaliani, L. K.; Chipashvili, D. S. Thermdyn. Anal. 1982, 1, 565. Vogel, A. I. A Text Book of Quantitative Inorganic Analysis Including Elementary Instrumental Analysis, 3rd ed.; Longmans Green: London, 1966; p 257. Received for review December 2, 1986 Revised manuscript received June 3, 1987 Accepted June 29, 1987
Direct Conversion of Methane to Methanol in a Flow Reactor Prasad S. Yarlagadda, Lawrence A. Morton, N o r m a n R. Hunter,* and H y m a n D. Gesser* Chemistry Department, University of Manitoba, Winnipeg, Manitoba, Canada R3T 2N2
The gas-phase homogeneous partial oxidation of methane with oxygen was studied over a range of operating conditions in a high-pressure tubular reactor. The experimental data indicated that methanol selectivities of 70-80% could be achieved a t methane conversions of 8-10% by operating at suitable reaction conditions. The higher methanol selectivities obtained in this study were probably due to the use of a Pyrex glass liner in the reactor, which may have minimized surface reactions. The partial oxidation of methane to methanol is a process of immense economic interest. The existing process for the manufacture of methanol involves the steam reforming of methane to synthesis gas, followed by the high-pressure catalytic conversion of synthesis gas to methanol. The process suffers from high cost and thermal inefficiencies in the steam reforming step. Futhermore, the methanol synthesis reactor is operated at relatively low conversions per pass due to the high exothermic nature of the reaction. The partial oxidation route offers the advantage of directly converting methane to methanol in a single-step reactor. However, the challenge lies in the 0SSS-5SS5/SS/2627-0252$01.50/0
selective oxidation of methane to methanol at a reasonable conversion level of about 10 mol % per pass. The potential for the partial oxidation route together with an economic evaluation has been reported by Edwards and Foster (1986). Their analysis showed that provided the selectivity for methanol formation is above 77 % ,the partial oxidation route has an economic advantage over the conventional synthesis route. Over the past 80 years, extensive research has been conducted on the oxidation of alkanes in an attempt to understand the chemistry of combustion, free-radical mechanisms, and oscillatory phenomena. Interestingly, Q 1988 American Chemical Society
