Proceedings of the Iowa Academy of Science Volume 86 Number Article 9 1979 A Kinetic Study of Propylene Metathesis over WO3-SiO2 Gel Russell Maatman Dordt College Craig Friesema Dordt College Let us know how access to this document benefits ouy Copyright ©1979 Iowa Academy of Science, Inc. Follow this and additional works at: https://scholarworks.uni.edu/pias Recommended Citation Maatman, Russell and Friesema, Craig (1979) "A Kinetic Study of Propylene Metathesis over WO3-SiO2 Gel," Proceedings of the Iowa Academy of Science, 86(1), 26-31. Available at: https://scholarworks.uni.edu/pias/vol86/iss1/9 This Research is brought to you for free and open access by the Iowa Academy of Science at UNI ScholarWorks. It has been accepted for inclusion in Proceedings of the Iowa Academy of Science by an authorized editor of UNI ScholarWorks. For more information, please contact [email protected]. Maatman and Friesema: A Kinetic Study of Propylene Metathesis over WO3-SiO2 Gel Proc. Iowa Acad. Sci. 86(1): 26-31. 1979 A Kinetic Study of Propylene Metathesis over WOa-Si02 Gel RUSSELL MAATMAN and CRAIG FRIESEMA Department of Chemistry, Dordt College, Sioux Center, Iowa 51250 Recent workers have reported anomalous results for the metathesis of propylene over WOa-Si02 gel in a flow system. One anomaly, confirmed in the present work, is an increase in catalytic activity with an increase in flow rate. The reaction was studied at a propylene partial pressure of0.37-1 atm between 435 and 493"C. Two reaction mechanisms have recently been proposed; one includes a bimolecular surface step and the other is a carbene chain mechanism. The rate law for the carbene mechanism has been developed. Attempts were made to fit the catalytic activities to the rate laws for these two mechanisms. An attempt was also made to determine whether or not the activities meet the site density criterion. This criterion states for a postulated slow step that the calculated density of active sites on the surface, determined by analyzing kinetic data using transition state theory, must be a physically possible site density. Our results are consistent with the bimolecular, but not with the carbene, rate law. However, our data are not consistent with the site density criterion for the bimolecular rate law. The theoretical methods used enable us to detect anomalies in our kinetic data, in this case the flow rate anomaly. Applying the same tests to systems reported in the literature indicates that inconsistency with a rate law or a physically possible site density is not found when there is no flow rate anomaly. INDEX DESCRIPTORS: metathesis mechanism, propylene metathesis, WOaSi02 gel, metathesis kinetics. The metathesis of propylene to ethylene and cis-2-butene occurs over copper and subsequently dried in a molecular sieve drying tube. many solid catalysts. Many of the catalysts for this reaction are mixed A conventional, all-Pyrex differential flow reactor was used. The oxides, such as the oxides of molybdenum and aluminum; cobalt, product was analyzed in an A-90-P Varian Gas Chromatograph, using a molybdenum, and aluminum; tungsten and aluminum; and tungsten and Porapak Q column and a tungsten filament thermal conductivity detec­ silicon (J). tor. In representative determinations, the ethylene:butene mole ratio We are interested here in the behavior of the WOa-Si02 gel catalyst. averaged 1.05. The true value was taken to be 1; results are reported on Moffat, Johnson, and Clark observed that this catalyst became more the basis of ethylene analysis. The catalyst was normally pretreated active with increased flow rate, and suggested that a very unusual mass with 02 for 5 hrs. at 500°. During a run the activity of the catalyst transfer problem existed (2, 3). Moffat later suggested that the anomaly increased for a period of about 2 hrs. and was then constant for at least could be explained if there were very few, but very active, sites (4). 12 hrs. This "break-in" period has been observed by others. Westhoff This explanation was offered partly because of our conclusion that and Moulijn concluded from gravimetric studies that the activity in­ some catalysts have extremely low site densities (5, 6). crease was due to a slight reduction ofW03 by propylene, with no more Luckner, McConchie, and Wills confirmed the anomalous results than 1 oxygen atom in 60 removed during the induction period (19). We obtained with the W03-Si02 gel catalyst (7). They suggested that the report only constant activities. problem arose because of a non-uniform impregnation of the silica gel It was possible during a run to change the temperature, the partial support. They did not need to invoke a low site density to interpret their pressure of propylene (P) by diluting with N., and the flow rate and, kinetic data and they were able to study the reaction under conditions after restoration of the original conditions, obtain the original activity. such that there was no flow rate anomaly. The thermal conversion was 0.2-0.3% at a flow rate of 10 ml/min and Several mechanisms for olefin metathesis have been proposed. negligible at 40 ml/min. Activities reported were calculated after the These include various forms of a mechanism which includes a bimolec­ thermal conversion was subtracted from the total conversion; the latter ular surface step; one form is the so-called four-center mechanism, in quantity was in the range 0.5-3.5%. Our error in measurement of which there is reaction between two olefin molecules adsorbed on activity, as determined from the average deviation in 7 paired determi­ adjacent pairs of sites. Recently, however, there has been considerable nations, was 3.4%. interest in a carbene mechanism, in which there is a chain reaction but not a bimolecular surface step. This mechanism has been suggested for RESULTS AND DISCUSSION heterogeneous and homogeneous liquid phase reactions and heterogen­ eous gas reactions (8-18), and is described in more detail in the Results Activity as a function of flow rate for three different catalyst weights and Discussion section. is given in Fig. 1. In Fig. 2, activity as a function of P is shown. In this paper we analyze the activity of a W03-Si02 catalyst for Arrhenius plots of the activities are given in Fig. 3 for 4 flow rate­ propylene metathesis. We have looked at the flow rate problem and catalyst weight-pretreatment combinations. have attempted to determine whether or not the results are consistent Our results agree with others in that our activities increased with with any of the proposed mechanisms. increasing flow rate. Pennella recently suggested that these anomalous results are not easily explained by an analysis of mass transfer (20). MATERIALS AND METHODS Pennella concluded for his system that reversible poison was intro­ duced into the feed stream ahead of the catalyst bed at a rate indepen­ Matheson C. P. propylene was used. No ethylene impurity was dent of the flow rate. Thus, the partial pressure of the poison decreases, detected; the non-Ca impurity was 0.09%. Both molecular sieve and with an increase in activity, as the flow rate increases at constant total Drierite drying tubes were used in the feed line. The catalyst, 6. 9 wt % pressure. If poisoning is indeed a complicating factor, we might antici­ WOa on silica gel, of 200 m2/g surface area, was provided by pate that the kinetic results for such a system cannot be reliably used in the Phillips Petroleum Company, and was used as 100-200 mesh the elucidation of the reaction mechanism. powder. Nitrogen (used to flush the reactor and to dilute the propylene) To pursue this matter further, we examine the two mechanisms for was freed of oxygen impurity by by-passing it through a tube of hot the reaction most recently proposed. We shall then examine rate data to Published by UNI ScholarWorks, 1979 1 Proceedings of the Iowa Academy of Science, Vol. 86 [1979], No. 1, Art. 9 KINETIC STUDY OF PROPYLENE METATHESIS 27 10.0 r----------------.,.------1 • 2.0 ~~.::------------------0-- ~ ,,\ ' ', \ \ ' ~/ \ \ ' .... ~ \ ', \ \ ' \ \ ' \ \ ' 8.0 1.s \ \ " \ \ ' I \ ' \ \ ' 1ii \ \ ' a... '& I' \ ', u \2 \I '-~ \ \ ' \ '~ \ ' ' -- 1 ~ \ \ ->-"' ~ \ \ i 1.0 ~ \ \ 0 6.0 :I u ~ \ \ 0 \ \ - ~• \ \ 1 E '• \ \ u \ \ 0 ~ \ \ • ·~ \ \ .!! . ~ \ \ -;: . \ \ -"' \ \ \ • ' \ \ u=' ~ \ I \ \ I ~ 4.0 \ \ I E ' • \ I \ • \ I ' \ I \ \ I 0 '- \ I ~ ' \ I >­ ' \ I .:!:: '', ____\ \1 I :;:> o.-=----__...,. ____...._ ___...._ ___~-------- g 1,• 1. o o.8 0.6 0.4 0.2 0.0 2.0 , , P, ATM. // // It II II Fig. 2. Activity vs. partial pressure of propylene; temperature, 475°C; ///'A flow rate JO ml/min; catalyst weight (normal pre-treat), 1.04 g. Circles Ill and solid line, experimental; 0, rate calculated if reaction is zero order; II/ , 1h, rate calculated if rate is proportional to p1h; 1, if proportional to P; 2, if rate is given by Eq. I. '• 1h•' and ''I'' curves fitted to P = 0 .P = I o~'----0 .......50-----+------'------'---' 100 150 200 points; "2" curve fitted to P = 0.67. P = I points. FLOW RATE, cm~mln Fig. I. Activity vs. flow rate. Square, 0.050. g catalyst. 475°; circle, 0.100 g catalyst, 466°; triangle, 1.00 g catalyst 477°. steps are conceivable: k1a l;I M + CaHs - H - C = M + C2H4 (2a) I determine whether either or both of the postulated mechanisms are possible. k1b l;I l;I Mz + CaHs - H - C = M + CHa - C = M (2b) The four-center mechanism I II The kinetic scheme for this mechanism is that which applies to a bimolecular surface reaction; it is not necessary that there be four Either Mor Mz can be considered a site.