Catalysis Under Transient Conditions - American Chemical Society

10 Development of a Pulse Reactor with On-Line M S Analysis to Study the Oxidation of Methanol

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C. J . M A C H I E L S Ε. I. du Pont de Nemours & Co., Inc., Central Research and Development Department, Experimental Station, Wilmington, DE 19898

Whereas steady state kinetic experiments are usually more easy to perform and certainly more easy to analyze, information is normally only obtained pertaining to the rate limiting step in the reaction sequence. Experiments of a dynamic type, such as pulse experiments or other transients, can give additional information about reaction mechanisms that is often difficult to obtain by other techniques. Additional advantages of pulse experiments are that they often take less time and require just small amounts of materials, both catalyst and reactants. The major problems of the pulse and other transient techniques seem to be related to the lack of good and rapid transient analytical methods and the fact that data analysis is not straightforward. Many good reviews are available concerning the theory and applications of pulse and other transient experiments (1,2,3). In a typical pulse experiment, a pulse of known size, shape and composition is introduced to a reactor, preferably one with a simple flow pattern, either plug flow or well mixed. The response to the perturbation is then measured behind the reactor. A thermal conductivity detector can be used to compare the shape of the peaks before and after the reactor. This is usually done in the case of non-reacting systems, and moment analysis of the response curve can give information on diffusivities, mass transfer coefficients and adsorption constants. The typical pulse experiment in a reacting system traditionally uses GC analysis by leading the effluent from the reactor directly into a gas chromatographic column. This method yields conversions and selectivities for the total pulse, the time coordinate is lost. It is desirable to be able to continuously monitor all the reactants and products so that a time resolved response curve is obtained for the separate components in the pulse. The sequence in which the products appear and the shape of each individual peak can, in principle, give information on the reaction sequence and on. adsorption properties that cannot easily be obtained in any other way. Infrared spectroscopy has been used with good results, but the number of systems suitable is limited and at best only a small number of components can be analyzed for, Figure 1. The 0097-6156/82/0178-0239$05.00/0 © 1982 American Chemical Society In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.


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A nalytical methods for pulse experiments.

In Catalysis Under Transient Conditions; Bell, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

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most general method i s a mass spectrometer w i t h continuous i n l e t system, and s e v e r a l r e p o r t s have appeared r e c e n t l y d e s c r i b i n g such systems ( 4 - 8 ) . Many of these systems lacked s u f f i c i e n t speed f o r pulse experiments, o f t e n an experiment had t o be repeated s e v e r a l t i m e s , l o o k i n g at a d i f f e r e n t component each time.


Methanol O x i d a t i o n I n i t i a l t e s t s using the pulse reactor d e s c r i b e d i n t h i s paper have been done on the s e l e c t i v e o x i d a t i o n o f methanol t o formaldehyde using molybdate c a t a l y s t s . A conmercial c a t a l y s t f r o n Harshaw was used, a 3:1 m i x t u r e o f molybdenum t r i o x i d e and f e r r i c molybdate, as w e l l as the two separate phases. K i n e t i c experiments were done p r e v i o u s l y i n a d i f f e r e n t i a l reactor w i t h e x t e r n a l r e c y c l e using these same c a t a l y s t s as w e l l as s e v e r a l other p r e p a r a t i o n s o f molybdenun t r i o x i d e , i n c l u d i n g supported samples. Ihe steady s t a t e k i n e t i c experiments were done i n the temperature range 180-300 C, and besides formaldehyde, the f o l l o w i n g products were observed, d i m e t h y l e t h e r , dimethoxymethane, methyl formate, and carbonmonoxide. U s u a l l y v e r y l i t t l e carbon d i o x i d e was o b t a i n e d , and under c e r t a i n c o n d i t i o n s , hydrogen and methane can be produced. The steady s t a t e experiments shoved t h a t the two separate phases and the m i x t u r e are not v e r y d i f f e r e n t i n a c t i v i t y , g i v e approximately the same product d i s t r i b u t i o n s , and have s i m i l a r k i n e t i c parameters. The r e a c t i o n i s about .5 order i n methanol, n e a r l y zero order i n oxygen, and has an apparent a c t i v a t i o n energy of 18-20 k c a l / m o l . These k i n e t i c parameters are s i m i l a r t o those p r e v i o u s l y reported (9,10), but o f t e n f e r r i c molybdate was regcirded to be the major c a t a l y t i c a l l y a c t i v e phase, w i t h the excess molybdenum t r i o x i d e serving f o r mechanical p r o p e r t i e s and increased surface area (10*11/12). At l e a s t four d i f f e r e n t steps have been reported to be the r a t e determining step i n t h i s r e a c t i o n , ranging frcm the a d s o r p t i o n o f methanol to the d e s o r p t i o n o f the products and the r e o x i d a t i o n o f the s u r f a c e . S t u d i e s i n the r e c y c l e r e a c t o r using deuterium l a b e l e d methanol showed t h a t over a l i m i t e d temperature range, the r a t e l i m i t i n g step on the commercial c a t a l y s t i s the breaking o f a carbon-hydrogen bond (JL3). The r a t i o s o f r a t e constants i n Table I show t h a t a deuterium i n the h y d r o x y l group has l i t t l e e f f e c t on a c t i v i t y ; s e l e c t i v i t i e s are not a f f e c t e d . A primary k i n e t i c isotope e f f e c t i s observed, however, when completely deuterated methanol i s used, and the product d i s t r i b u t i o n a l s o changes s i g n i f i c a n t l y ; much more d i m e t h y l e t h e r i s formed and l e s s formaldehyde. For the formation o f the e t h e r , no carbon-hydrogen bonds need t o be broken.


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Table I K i n e t i c isotope e f f e c t on F e ( toO^^/NbO^ 2

k

CH OD


3

220 230 260

0.93 0.92 0.94

k

CD OD 3

0.26 0.37

Experimental A f l o w schematic o f the pulse r e a c t o r i s showi i n Figure 2. The f l e x i b l e design a l l o w s dynamic experiments o f v a r i o u s types. Helium c a r r i e r gas flows through the two channels o f a Gow-Mac thermal c o n d u c t i v i t y c e l l before and a f t e r passing through the r e a c t o r . Pulses o f methanol i n argon or an argon-oxygen mixture can be introduced using a s i x - p o r t Valco v a l v e w i t h a 1 c c sample l o o p . Pulses o f methanol can be a l t e r n a t e d with oxygen p u l s e s . A f o u r - p o r t Valco v a l v e a l l o w s a p p l i c a t i o n o f step changes. For t h i s purpose, the helium stream can be replaced by another methanol-containing stream. This s e t up can a l s o be used f o r pulses o f i s o t o p i c a l l y l a b e l e d species onto a steady s t a t e o f unlabeled m a t e r i a l s . Methanol streams are obtained using a s a t u r a t o r method and c o n c e n t r a t i o n s are determined by gas chromatographic a n a l y s i s . Back pressure r e g u l a t o r s i n the r e a c t o r stream and the sample gas stream maintain an equal pressure o f 1-2 p s i g i n both streams. Pulses can be d i s p e r s e d i n a c o l mm before the r e a c t o r i f d e s i r e d . The r e a c t o r i t s e l f Figure 3 i s made o f Pyrex, using 1mm c a p i l l a r y tubing wherever p o s s i b l e . The c a t a l y s t (0.2-0.5 g) i s on a f r i t t e d d i s k , 14 nm i n diameter. The c a t a l y s t bed i s s e v e r a l mm h i g h . A continuous sample t o the mass spectrometer i s taken from j u s t above the c a t a l y s t bed. A short s e c t i o n o f 1/16" s t a i n l e s s s t e e l tubing i s coupled t o a 1-m s t a i n l e s s s t e e l c a p i l l a r y , 0.15 nm i.d./2 nm o.d. P a r t o f the r e a c t o r e f f l u e n t i s pumped through t h i s c a p i l l a r y and through the i n l e t v a l v e o f the mass spectrometer. I f t h i s v a l v e i s opened, a s m a l l p o r t i o n o f the gas l e a k s through an o r i f i c e d i r e c t l y i n t o the i o n formation r e g i o n o f the ion source. The pressure r e d u c t i o n i n two stages _ from atmospheric t o approximately 1 mbar i n the v a l v e t o 10 -10 mbar i n the spectrometer ensures v i s c o u s f l o w i n the c a p i l l a r y and minimizes mass d i s c r i m i n a t i o n e f f e c t s . Good r e s u l t s were a l s o obtained when the s t a i n l e s s c a p i l l a r y was replaced by a s e c t i o n o f fused s i l i c a c a p i l l a r y .


10.

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MACHiELS

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T.C.

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STEP PULSE


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M.S. REACTOR

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lt

2

3

3


2


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The o n l y products observed i n the p u l s e experiments were d i m e t h y l e t h e r , formaldehyde and water. This i s a d i f f e r e n t product d i s t r i b u t i o n than i n the continuous f l o w experiments and i n d i c a t e s t h a t dimethoxyme thane and methyl formate are not primary p r o d u c t s . In the a n a l y s i s , peaks 18, 30, 31 and 45 are used f o r water, formaldehyde, methanol and d i m e t h y l e t h e r , r e s p e c t i v e l y . Fragmentation p a t t e r n s were a l s o determined f o r the deuterated s p e c i e s ; they are n e a r l y equal t o t h a t o f C H ~ O H . S n a i l amounts o f carbon monoxide cannot be determined with g r e a t accuracy. The f i g u r e shows c l e a r l y that a l l peaks have been delayed compared t o argon by s e v e r a l seconds. The peak f o r oxygen c o i n c i d e s w i t h t h a t for argon, dimethylether c o i n c i d e s w i t h methanol, and i s somewhat f a s t e r than formaldehyde. Water i s adsorbed most s t r o n g l y on the c a t a l y s t . In blank experiments, the d e l a y between peaks i s not more t h a t 0.2 s. Figure 6 shows the d e l a y s o f the tops o f the peaks f o r methanol, formaldehyde, and water r e l a t i v e t o argon as a f u n c t i o n o f temperature. As expected, there i s a decrease w i t h i n c r e a s i n g temperature, except f o r water which shows a minimum i n a l l cases. Figures 7-9 show the f r a c t i o n a l c o n v e r s i o n o f methanol i n the p u l s e as a f u n c t i o n o f temperature f o r the three c a t a l y s t s and the three methanol feeds. E v i d e n t l y the k i n e t i c isotope e f f e c t i s present on a l l three c a t a l y s t s and over the complete temperature range, i n d i c a t i n g t h a t the r a t e l i m i t i n g step i s the breaking o f a carbon-hydrogen bond under a l l c o n d i t i o n s . From these experiments, the e f f e c t cannot be determined q u a n t i t a t i v e l y as i n the case o f the continuous f l o w experiments, but t o o b t a i n the same c o n v e r s i o n of CD-CD, the temperature needs to be 50-60° h i g h e r . This corresponds t o a f a c t o r o f about three i n r e a c t i o n r a t e . The d i f f e r e n c e i n a c t i v i t y between M D C L and F e ( M D O J i s l a r g e r i n the pulse experiments compared t o the steady s t a t e results. 2

3

The c o n c l u s i o n s on the r a t e l i m i t i n g s t e p are again supported by the d i f f e r e n c e s i n product s e l e c t i v i t y i f completely deuterated methanol i s used; the s e l e c t i v i t y to dimethylether r e l a t i v e t o formaldehyde i s much l a r g e r . This i s shown for the three c a t a l y s t s i n Figures 10-12, i n which the r a t i o o f the amounts o f dimethylether and formaldehyde formed i s p l o t t e d as a f u n c t i o n o f temperature. In the case o f C H - Q D , the water observed i s a m i x t u r e o f H 0 , H D O , and D 0 , most o f i t being H D O . 2

2

A suggested r e a c t i o n mechanism based on k i n e t i c and isotope experiments, product d i s t r i b u t i o n s , and the e f f e c t on these o f a d d i t i o n o f water to the feed or l e a v i n g oxygen out i s shown i n Figure 13.


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Catalysis Under Transient Conditions - American Chemical Society

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