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I N V E S T I G A T I O N O F T H E K I N E T I C S O F T H E I S O M E R I Z A T I O NO F n - P E N T A N E O N T H E H Y D R O G E N F O R M O F M O R D E N I T EA T L O W P A R T I A L P R E S S U R E S O F H Y D R O G E N

V . V . K h a r l a m o v , V . I . G a r a n i n ,a n d K h . M . M i n a c h e v

UDC 541 127: 542 952 1: 547 215:661 183 6

It was shown ear l ie r [1] that the iso mer izat ion of n-pentane on the H- fo rm of morden ite withoutmet als of group VIII is inh ibited by H 2 in the ran ge of pr es su re s 7-30 atm. However, it is known [2] thatwithout H 2 the reac t ion on thi s ca ta lys t prac t ica l l y does not occur . Therefo re , to de termine the mecha-nism of the iso mer izat ion on the H-f orm of morden ite, i t is of inter est to invest ig ate the kinet ics of thereac t ion a t low par t ia l press ure s of hydrogen (from 0 to 7-10 a tm). This was the purpose of the presentwork. To consid er the influence of deact ivat ion of the cataly st under these cond it ions on the rate of thepro cess we developed a spec ia l proced ure for conducting the experim ents and t rea t ing the experimen ta ldata.

M E T H O DThe iso meri zat i on of n-pentane was studied in a flow-through rea cto r, into which 10 cm 3 of mo r-denite in the H-f or m with a degr ee of exchange of Na + ions for H + of 95 eq. , was load ed. The ca tal ys t

was p re pa re d by the method of ion exchange of Na + cat ions in the ini t ial Na- for m for NH4 +, by five tre at-ments of mord enite with a 10 solut ion of NH4NO 3. Before the exper imen ts the catalyst was treated withair at 520 ~ for 5 h. Regen erat i on of the catalys t after each experime nt was conducted under the samecondit ions. Since the catalyst works unstab ly at low par t ial pr es su re s of hydrogen {Fig. 1) and lower s i tsact ivi ty to some constant level , which depends on the exp erim ental condit ions, we used the fol lowing pr o-ced ur e. Is om er iza tio n under the con trol c ondit ions at Ptot = 30 arm; pH2/PC 5 = 3.2, vc5 = 1-3 h -1 (depe nd-ing on the tempera t ure) was conducted on a f resh, regener a ted ca ta lys t . After thi s, the pre ssu re waslowered to the set value (Ptot = 4-8 arm), and we operated in this sys tem for 60-90 rain. In this case theact ivi t y of the cataly st , as has alrea dy been noted, was lowered (see Fig. 1). Then the contro l experimentwas rep eate d again. The degr ee of deact ivat ion of the cataly st (7) during work at low tem per atu re wasca lcula ted according to the formula

= ~nit/rfin (I)where r ini t i s the ra te of i somer iza t io n under the cont rol condi t ions before the bas ic experiment was con -ducted; r f l n i s the ra te of the i so meriza t ion reac t ion under the cont rol condi tions a f te r the bas ic expe ri -ment was conducted a t low pre ssu re . Consider ing the ac tiva t ion of the ca ta lys t , we ca lcula ted the correc t edreac t ion rat e (rco rr) in the basic ex periment, which would be obse rved in the absence of poisoning of theca ta lys t , according to the formula

~21 ,f l i

7 J O ~ O 9 0min

Fig. 1. Dependen ce of the y ield ofisopentane on the t ime of work ofthe c ataly st at 200 ~ and a pa rt i alpressure of hydrogen (PH2) 6 arm(1) and 230 ~ and pH 2 2 a tm (2).

N. D. Zelins kii Inst i tute of Organic Chemistr y, Academ y of Sciences of the USSR. Tr ans lat ed fromIzves t iya Akademii Nauk SSSR, Seriy a Khimich eskaya , No. 5, pp. 1006'1012, May, 1973. Origin al art icl esub mitt ed May 17, 1972.

9 1973 Con su l tan t s Bureau , a d iv i s ion o[ P lenum Pu bl i sh in g Corpora t ion , 227 Wes t 17 th S t r ee t , N ew York ,N . Y . 10011 . A l l r igh t s r eserved . Th i s ar t i c l e canno t be r eproduced [or any purpose wha t soev er w i thoutperm is s ion o f the publ i sher . A cop y o[ t /~ i s ar t i c l e i s a va i lab le [ rom the publ i sher [or $15 . 00 .

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0

r~(D

~

0

,-~

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o o

6 8 8 I

o

e , o z o ; o J oPNz, arm pxz~ armFig. 2 Fig. 3

Fig. 2. Depe nden ce of the rea ct ion rat e (r) on pH 2 at 200 ~line belongs to the observable reaction rate.Fig. 3. Dependence of the reac tion rate (r) on pH 2 at 210 ~line belongs to the observable reaction rate.

T he dotted

T he dotted

Figure 5 pres ents the dependence of the reactio n rate on the ratio of the partial pre ss ure s of n-pentaneand H 2 at variou s te mpe rat ure s. As can be seen fro m Fig. 5, E qs. (IV) and (V) are actua lly fulfilled underthe correspon ding conditions. The dependence of the rate constant of isome rizat ion of the carboniu m ion[Eq. (V)] on the temper ature corre spon ds to an activation energy of this step equal to 20 kca l/m ole . Sincethe activation e nergy of the reac tion was earli er [1] found equal to 31 kc al /mo le for the region in which thereactio n is de scrib ed by Eq. (IV), the thermal effect of the f irst step, calculated a ccording to the differenceof these values, is - 1t kc al /mo le , i .e . , the formation of a carboni um ion is an endothermic proc ess . Ifthe first step is consi der ed as ads orptio n of a n-penta ne molecul e on the surf ace of the catalyst, with theformation of a carbon ium ion, then i ts endo thermicity is unexpected, since usually adsorption is an exo-thermic p rocess .

Since Eqs. (IV) and (V) perm it a calculation of the equil ibrium constant a, then, knowing the heat ofthe proc ess Q, i t is possible to find the entropy change AS, correspo nding to the formation of a carboniumcat ion. The calculation gives AS =-2 0 ca l /m ole , deg.

To conf irm the postulated mechani sm it is of interest to det ermine the values of Q and AS by someindependent method and to compa re the values obtained with the experime ntal v alues . The firs t step of theproces s can be represented as follows

n-C.sHz~ -k H- -O -- M ~ n-Cs Hlx- -O--M+H2where M is the aluminosil icas framew ork of mordenite. In this reactio n the C-H bonds in the moleculesof the init ial hydrocarb on and the O -H bonds in the acid hydroxyl groups of mordenite are broken, whileH- H and C- O bonds are formed . The heat of this proce ss can be approximately estimated aecording to thediffe rence s of the e nerg ies of the bonds formed and broken, with the aid of the equation

25r f5r

.2A

i Ilo 21 3pNz, atmFig. 4

0 12 i ~6P. - cs/ P z

Fig. 5

3

2t o o

1,0

Fig. 4. De pendence of the react ion rate (r )o n pH 2 at 230 ~ The dottedline pertains to the observable reaction rate.Fig. 5. Depen dence of the rea ction rate (r) on the ratio of the partialpre ss ure s of n-pentane and hydrogen (Pn_CJPH2).

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TABLE 2. Data on Bond Energies , kca l /mo leond

C - H (sec-ondary inparaffins)O H

LiO I:I NaOH H~O

94 ]1 27 1tt5Average I 11509

Litera-C - O (in ture re-H Halcohols) ferenee90 103 [31

S, entropy un it s 1~ ' ~ Ys

20 ~ / . . . - ~ - -i _ iSol Liq Gas

Fig. 6. Dependence of the e ntropyon the aggr egat ed state of the sub-stance (sol - solid phase; l i q-liquid; gas .- ga se ou s substance): 1)water; 2) methanol; 3) cyclohexane;4) n-amyl cation.

Q = DH-H + Qc-o -- DC-H -- DO-H (VI)where D rep rese nts the energies of cleavage of the bonds. Theener gies of cleav age of the C- H and H- H bonds are known [3]. Asfor the ene rgy of cleavage of the O -H bond in the hydroxyl group ofmordenite, i t can be approxima tely conside red that i t is close to theenergy of this bond in metal hydroxides or in water. The ener gy ofthe C -O bond can be assu med equal to the e nergy of this bond inalcohol ates, whieh in turn evidently does not differ grea tly fro m theclea vage en erg y of C- O bond in alcoho ls (Table 2). The value of Q,caMulated aeeo rding to Eq. (VI) on the ba sis of the data of Tabl e 2,is equal to -1 6 kca l/m ole . The coincidence of this value with theexperime ntally found value is satisfac tory, if we consider the lowacc ura cy of the expe rimental data and the approximate nature of thecalculation.

A calculation of the entropy of the r eaction accordin g to theentropies of the bonds participating in the rea ction leads to poor

coincidence with the experi mentally found value. The cause of this may be the unconsidered contributionto the e ntrop y of the rea cti on of the change in the entr opies of the bond in the hydr oca rbon mo lecule that donot participa te in the reaction (as a result of the disappearanc e of translatio nal and rotational degre es of 'freed om during fo rmation of a carbo nium ion on the surfa ce of the catalyst from a molecule of n-pentanein the gas phase). There fore, a more corre ct result should be obtained in calculation accordin g to theformula

AS = S m + Sn-C,H,,+ -- S~-C,H,, -- So-~ (VII)where SH2 and Sn_CsHI2 are the entro pies of H 2 and n-pen tane in the gaseo us state; Sn_CfHII+ is the entro pyof the n -am yl ca tion on the surface of the catalyst; S O_H is the entropy of the O -H bond inthe acid hydroxylgroups of the catalyst.

Let us constr uct the dependen ce of the ent ropy of H20 on the aggre gate d state (Fig. 6). The di stanc esbetween the. solid, liquid, and gase ous state s we re selecte d along the X-axi s in such a way that the pointwould fit or~ a strai ght line. The ent rop ies of HzO in the liquid and gas eou s sta tes are known [4], while forthe solid stat e we took an ave rag e value, obtained by calc ulati on on the basis of the known entropies of ca l-cium and copper sulfate (Ssalts) and the cryst al hydrates (Scrh) acc ording to the formula

crh -- Ssalts(Sm~ nwhere n is :he numb er of I-I20 mole cule s per salt mole cule in the cry sta l hydra te. The en tropy of the O- Hbond in the acid hydroxyl group can be estim ated approximat ely ac cording to the formula SO_ H = 1/2(SHoO)so1. Plottin g the data for other su bsta nces on the graph (see Fig. 6)shows that the straight lines con-Estruct ed through the points correspo nding to the liquid and gaseous sta tes are approximately parallel , andther efor e, ':he entr opy for the solid state can be found by extrap olatio n of the straight line to inte rsec tionwith the Y-axis. The slopes of the straight l ines correspo nding to substances of different class es differsomewhat fr om one another, which is evidently associat ed with the different distances between the aggr e-gated states of substances of different cla sses (for example, alcohols and hydrocarb ons). On the basis ofthis , the graph of the entro py of the adso rbed amy l cation can be estimated by constructing a straight l inethrough a point corresp onding to the entropy of the gaseous amyl cation, to intersectio n with the Y-axis.To obtain the entropy of the amyl catio n in the gas phase, it is nec es sa ry to deduct the entropy of the C -H

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b o n d , w h i c h i s e q u a l t o 1 / 4 S C H 4 , f r o m t h e e n t r o p y o f n - p e n t a n e . S u b s t i tu t i o n o f t h e v a l u e s th u s o b t a i n e di n to E q . ( V II ) g i v e s A S = - 1 2 . 2 c a l / d e g . m o l e , w h i c h , i f w e c o n s i d e r t h e a p p r o x i m a t e n a t u r e o f t h e c a l c u l a -t io n , s a t i s f a c t o r i l y c o r r e s p o n d s t o t h e e x p e r i m e n t a l l y f o un d v a l u e.

T h u s , a n in v e s t i g a t i o n o f t h e k i n e t i c s o f t h e i s o m e r i z a t i o n o f n - p e n t a n e o n t h e H - f o r m o f m o r d e n i t ea t lo w p a r t i a l p r e s s u r e s o f h y d r o g e n g a v e a d d it io n a l e v i d e n c e in s u p p o r t o f t h e m e c h a n i s m o f t h e p r o c e s sp r o p o s e d e a r l i e r .

C O N C L U S I O N S1 . T h e k i n e t ic s o f t h e i s o m e r i z a t i o n o f n - p e n t a n e o n t h e H - f o r m o f m o r d e n i t e w a s i n v e s t ig a t e d a t

p a r t i a l p r e s s u r e s o f h y d r o g e n o f 2 - 6 a t m .2 . T h e d e g r e e o f d e a c t i v a t i o n o f t h e c a t a l y s t a n d t h e c o r r e c t e d r e a c t i o n r a t e u n d e r t h e s e c o n d i t i o n s

w e r e d e t e r m i n e d .3 . T h e h e a t a n d e n t r o p y o f t h e f o r m a t i o n o f t h e n - a m y l c a t io n w e r e d e t e r m i n e d , a n d a n i n d e p e n d e n t

a p p r o x i m a t e c a l c ul a ti o n o f t h e s e p a r a m e t e r s w a s p e r f o r m e d .L I T E R A T U R E C I T E D

1 . K h . M . M i n a c h e v , V . I . G a r a n i n , a n d V . V . K h a r l a m o v , I z v . A k a d . N a u k S S SR , S e r . K h i m . , 8 35( 1 9 7 0 ) .

2 . K h . M . M i n a c h e v , V . I. G a r a n i n , V . V . K h a r l a m o v , T . A . I s a k o v a , a n d E . l~ . S e n d e r o v , I z v . A k a d .N a u k S S S R , S e r . K h i m . , 1 7 3 7 ( 1 9 69 ) .

3 . V . I . V e d e n e e v , L . V . G u r v i c h , V . N . K o n d r a t e v , V . A . M e d v e d e v , a n d E . L . F r a n k e v i c h , C l e a -v a g e E n e r g i e s o f C h e m i c a l B o n d s . I o n i z a t io n P o t e n t i a l s a n d E l e c t r o n A f f i n it y , H a n d b o o k [i n R u s s i a n ] ,I z d - v o A N SS S R ( 1 9 6 2 ) .

4 . Y a . I . G e r a s i m o v , V . P . D r e v i n g , E . N . E r e m i n , A . V . K i s e l e v , V . P . L e b e d e v , G . M . P a n c h e n -k o v , a n d A . I . S h ly g i n, C o u r s e i n P h y s i c a l C h e m i s t r y [ in R u s s i a n ] , V o l . 1 , K h i m i y a ( 1 9 69 ) .

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