Catalytic activity and product distribution in the cracking of C6-C8 alkenes on the ammonium salt of 12-tungstophosphoric acid

Applied Catalysis, 77 (1991) 261-266 Eisevier Science Publishers B.V., Amsterdam

251

Catalytic activity and product distribution in the cracking of CG-Csalkenes on the ammonium salt of 12-tungstophosphoric acid Vikram S. Nayak’ and John B. Moffat* Department of Chemistry and Gwiph- Waterloo Centre for Graduate Work in Chemistry University of Waterloo, Waterloo, Ontario, N2L 3Gl (Canada), tel. (+ l-519)885121 1 ex. 2502, fax. (+ I -519)74fx435 (Received 22 March 1991, revissd manuscript received 26 June 1991)

Abatract Ths cracking of C&s aikenes has been studied on the ammonium salt of 12-tungstqhosphoric acid inamountaup (NH,PW). Theproducta range from Cg hydmcarbonsto aromatics,the latter appearing to 36% of the products. The primary producta from the alkenea provide strong evidence that condensation pmcesses are strongiy favoured on them catalysts. The cmcking abiiity of the catalyst decreaw rapidly with time on stream but attains a steady stats after approximately three hours. The activity of the catalyst is shown to be dependent on the wnce of the ammonium ion employed in the preparation of the salt with the activity of NHQW for hexene cracking decreasing in the order NH,C1>NI&NOI> (NH,),COs> (IUI&)sSO,.Thecrackingactivityof NI&PWfortheC&.&aikenes decreases in the order: octenes > heptenea z=hexenes. Keywords: aIkenes, ammonium 12-tungstophosphate, cracking, heteropoly acids, microporosity, tungstophosphoric acid

INTRODUCTION

Heteropoly oxometalates offer a number of interestingpossibilities as heterogeneouscatalysts [ 1,2]. Their isostructuralproperties,that is, the ability of these solids to alter their catalytic functionality with a change in the elemental composition of the anion, while retainingthe originalanion structure, provide an opportunity to investigatecompositional effects in catalysis [ 31. Their high acidic strengthinvites comparison with other acidic catalysts [4]. Not the least, the possibility of preparingcertain of these substanceswith microporous structuresadds yet another positive aspect to their application as catalysts [ 51. Heteropoly oxometalates may be considered as metal-oxygencluster compounds [ 61 since the anions are high molecularweightcagelikestructures.The ‘Present address: GueIph Chemical Labs Ltd., 246 Silvercreek Parkway N., GueIph, Ontario, Canada, NlH lE7.

0166-9634/91/$03.56

0 1991 Blsevier Science Publishers B.V. AU rights reserved.

252

Fig. 1. Heteropoly oxometalate anion (PW&& ) of Keggin structure; large circles: central atom (P) and peripheral metal atoms (W); small circles: oxygen atoms.

anions of Keggin structure (Fig. 1) have a central atom, such as phosphorus, as in the present work, bonded to four oxygen atoms arranged tetrahedrally. Twelve octahedra with oxygen atoms at their vertices and a peripheral metal atom, such as tungsten as in the present case, at each of their centres, surround the central tetrahedron and share oxygen atoms between themselves and the latter unit. Thus, the stoichiometry of the Keggin anion may be represented as (PO,) ( W12036)3- or PW,,O&-, for example. There are 12 terminal oxygen atoms, each bonded only to the peripheral metal atom of an octahedron, the remaining oxygen atoms bridge either two peripheral metal atoms or the central atom and one of the former. The parent acid in the present work, 12-tungstophosphoric acid (H,PW,,O, abbreviated to HPW), has a cubic Pn3m structure [ 71 (Fig. 2). Four water molecules surround the proton in an approximately coplanar configuration but as a result of a two-fold thermal disorder only two of these molecules are hydrogen-bonded to the proton at a given time. The hydrogen atoms of the water molecules are in turn hydrogen-bonded to the terminal oxygen atoms of the anions. Although the surface area of HPW is only approximately 6 m2/g [8] and is evidently nonporous, photoacoustic Fourier transform infrared (PAS FT-IR ) studies have shown that polar molecules such as ammonia [ 91, pyridine [ lo] and methanol [ 111areable to penetrate into the bulk of the solid, apparently between the cations and anions, the latter being packed and unable to accept

253

Fig. 2. Anion-cation configuration in HSPW120m*nHz0

[ 71.

additional molecules. Earlier work had suggested that while alkenes are sorbed into the secondary structures of these solids, alkanes are unable to penetrate into the bulk structure [ 121. Recent diffusion and sorption measurements show that the sorption capacity of HPW for 1-hexene at 20°C is approximately five times that for hexane [ 131. Work in this laboratory has shown that heteropoly oxometalates with microporous structure can be prepared by replacement of the proton in the parent acid by certain monovalent cations, in particular the ammonium ion, in precipitation reactions [ 5,141. Ammonium 12-tungstophosphate is found to have a surface area of approximately 128 m2/g and pore sixes of 7-12 A [5,14]. More recent work has shown that the cations can be exchanged through an ion exchange process, while retaining the micropore structure in expanded or contracted form depending on the size of the monovalent cation [ 15,161. The present work focusses on one of these microporous heteropoly oxometalates, namely ammonium 12-tungstophosphate ( (NH1)3PW12040, abbreviated to NHPW), which has been shown to be active and selective in the conversion of methanol to hydrocarbons [ 121, the isomerixation and transalkylation of methylethylbenzene [17], the alkylation of toluene with methanol [ 181 and the cracking of hexane [ 191. Earlier work on the conver-

254

sion of methanol to hydrocarbonson the heteropoly oxometalateshas shown that NHPW is a more active catalyst for this process than its parent acid (HPW) [12]. Furthermore,while with HPW the predominantproducts from methanol are alkenes, in contrast, with NHPW the products are primarily saturated species. PAS FT-IR studies have shown that Brsnsted sites (i.e. protons) are primarilyresponsiblefor the activity of both HPW and NHPW. The differencein activityand selectivityof these two catalystsin the methanol conversion process thus must be attributed to the differences in their acidic strengthsand distributionsof these. Measurementsby titration methods support such contentions [ 201. Recent work has also shown that both the relative amountsand sourcesof a particularcation influencethe microporousstructure [ 211. The interstitialvacancieswhich exist in the parent acids are apparently separatedfrom one another by the terminal oxygen atoms of the anions [ 51. Introductionof largermonovalentcations are believedto induce both a translation and rotation of the anions so that the interstitialvoids become interconnected and channels are formed. The presence of residualprotons may thus cause some of the channels to become blocked and the micropore volume reduced. The present work is concerned with the activity and product selectivity of NHPW for the cracking of C&-C, alkenes. Alkenes are of particular interest since such molecules should be capable both of diffusing into the micropores of NHPW and penetrating into the bulk structureof this solid The dependence of the catalyticactivityon the method of preparationof the catalystand time-on-streamas well as the interdependenceof product selectivityand conversion are explored. EXPEIUIMENTAL

The ammoniumsalts of 12-tungstophosphoricacid were preparedby treating aqueoussolutions of 12-tungstophosphoricacid (BDH-Analar) with stoichiometricamounts of aqueous solutions of various ammoniumsalts. The resultingtwo-phase systemswere evaporatedto drynessand heatedin air at 383 K for 4 h. The detailedprocedure for the preparationof the ammonium salts of 12tungstophosphoric acid (NHPW) from different sources of ammonium ions has been reportedelsewhere [ 12,141. The reactions were performed in a fixed bed tubular reactor of 1 cm I.D., employed in previous work [19]. Hex-1-ene, hept-1-ene, or act-1-ene (Aldrich) used as receivedwas introducedinto the helium ( > 99.99%) carriergas by two variabletemperaturesaturatorsconnected in series.Both the reactant and product lines were maintainedat a sufficientlyhigh temperatureto avoid condensationof the hydrocarbons.The reactor feed and product streamswere analyzed by a Hewlett Packard gas chromatograph (HP 5390) fitted with a flame ionization detector and a HP-1 (cross linked methyl silicone gum) col-

255 umn (0.2 mm x 50 m ) . Samplesfrom both the reactorfeed and product streams were drawnthrough heated gas samplingvalves. RESULTS

The conversion of ( Ci) of reactant i and selectivity (Sj) to a productj were defined as Ci= moles of i consumed/moles of i introduced;and Sj = moles of j produced/moles of all products formed. In the present work steady state is taken to have been attained when the (A)

2

4 W/F (gwodii-‘1

6 do-’

Fig. 3. ln [l/ (1 -x) ] vs. W/F plob for [A] ammonium-124amgsbphosphate in the conversion of hexeneaat 646 (A ), 673 (0 ) and 698 K ( 0 ) and [B] ammonium12-tamgstophoaphate preparedfromdifferentsourcesof NH;c ions in the convereionof hexenes at 673 K; (0 ) NH&l, (0) NfzNOs, (0) W-b)&Os, W) @J%)zSO,.

256

conversion changed by less than 1% over an on-stream time of one hour. In the present work such a steady state was generally achieved after approximately 3 h on stream. The conversions measured at steady state were found to be well fitted with the residence times through the usual integrated first order relation (Figs. 3 and 4).

(1) where x is the conversion in a plug flow reactor, k, is the rate constant (dm3 g- ’ s- ’ ), W is the weight of the catalyst (g) and F is the volumetric gas flowrate through the reactor ( dm3 s-’ ). Since the C&C& alkenes rapidly isomerize

0.

OX

T

,: 4 I

0.

0.l

200

100 W/F

Igwc*

300

Iltre-’ 1

Fig. 4. ln [ l/ ( 1- le) ] vs. W/F plotsfor ammonium 12-tungbphoephate heptmesand [B] octenesat646 (A),673 (0) and698K (0).

in the conversion of [A]

257 TABLE 1 Activity of NHPW in the cracking of hexenee at 673 K NH: source

BET surface area” (m*lg)

Activity, k,,,x 10’

162 143 112 65

2.45 1.96 1.46 0.95

NH&l NHINOs U’JH,)&Os (NH&W,

(dmsg-’ s-1)

“After calcination at 523 K for 2 h.

40

20

I20

i6o2002402203203e

TIME 011 STnEAu

(mln)

Fig. 5. Variation of catalytic activity of ammonium 12-tungstophosphate prepared from different NH: sources with time on stream in the cracking of hexenes at 673 K, ( 0 ) NH&l, ( 0 ) NH,N03, (A) W-L)&Os, (VI (NH&SO,.

258

at the temperatures employed in the present work, it is assumed that an equilibrium mixture of isomers of a particular alkene exists prior to the commencement of the cracking process. Consequently, x represents the overall conversion of the equilibrium mixture of isomers of the reactant alkene. The source of the ammonium ion is found to have a significant influence on the activity of the catalyst (Fig. 3B). The first order rate constants decrease

50

“s! _f 40 f f

E

Z

3

30

a E 5 i= Y 20

lo

a

40

60

I20

I60

200

240

TIME ON STREAM fmid Fig. 6. Variation of cat&tic activity of ammonium 12-tu4pt0phoephat.eprepared from (NH&CO3 with time-on-stream in the cracking of hexenes (0 ) , heptenea (A ) and octanes ( 0 ) at 673 K.

259 TABLE 2 Activity of NHPW in the cracking of C&s Reactant

Activity, k, (dm3 g-’ 8-l) 648 K

Hexenes Heptenes Octenes

alkenes at three different temperatures

673 K

698 K

1.16.10-’

1.48-10-5

2.0*10-5

4.10.10-5 4.4-10-4

5.0.10-6 4.90.10-4

6.4.10-’ 6.0.10-4

with the ammonium salt used in the preparation in the order NH&l> NH,NO,> (NH&CO,> (NH,),SO, (Table 1). Regardless of the alkene examined or the source of the ammonium cation, the activity of the catalyst decreases rapidly during the first three hours of time-on-stream (Figs. 5 and 6). The data shown in Figs. 5 and 6 were obtained by assuming the validity of eqn. ( 1) over the entire range of conversion from 0 to 100%. The initial rates of decrease in the activity are, however, dependent on the source of the ammonium cation and on the initial activities. The initial activities and the decrease in these with time-on-stream follow the order (NH&CO, > NH&l > NH4N03 > (NH&SO,. The activities at 673 K under steady-state conditions are summarized in Table 1 for the various ammonium cation sources in the cracking of hexene. The rates of attainment of steady-state conditions are also dependent on the reactant alkene studied (Fig. 6)) as are the steady-state values of activity (Table 2). The steady state activities at a given temperature decrease in the order octenes > heptenes > hexenes. The principal products formed from the cracking of the three alkenes over NHPW between 648 and 698 K are Cz, CB,Cd, C$,,C6 alkanes and alkenes, C,, aliphatics and C&-C8 aromatics. The term “aliphatic” is employed for convenience here to refer to the open chain hydrocarbons, both saturated and unsaturated as well as their cyclic analogues but not including the aromatic forms. Although the products are similar with all three alkenes, the selectivities are strongly dependent upon the reactant alkene. With hexene as the reactant, as the conversion increases from a low value the selectivities to C, hydrocarbons, butenes and butanes, pentanes and aromatics increase while those to C, hydrocarbons, pentenes and C,, alkenes decrease (Fig. 7). The selectivities to Cz hydrocarbons, the butenes and the aromatics each pass through a maximum at approximately 20-40% conversion while a minimum is observed for propane and the pentenes at approximately 10% conversion. At low conversions the highest selectivities are observed for propane, pentenes and the C,, aliphatics which are evidently the primary products. C2 hydrocarbons and the pentanes do not exceed selectivities of 5%

260

w2

6

z i E Y 3

1

30

4

20

2

IO

C

0

40

4

20

2

0

0 '6

20

'2

'I

la

4

C

0

C$+ALIPlmcS

30

'30

'20

20

-10

la

(

I

20

40

60

60

20

40

60

0

60

HEXENES CONVERSION (WI Fig. 7. Dependence of selectivity for different hydrocarbons on the conversion of hexenes at 646 ( A ), 673 (l )and 698 K (0 ) on ammonium 12-tungstophosphate.

at any conversions. It is also evident from Fig. 7 that the selectivities for C, hydrocarbons, C, hydrocarbons and the aromatics increase with increasing temperature while those for C, alkanes and C, alkenes and the C,, aliphatics

261

z * c

TRANS-2-WfENE

CIS-P-BIJTENE

1

Fig. 8. Dependence of selectivities for C, hydrocarbons on the conversion of hexenes at 648 (A ), 673 ( 0 ) and 698 K ( 0 ) on ammonium 12-tungstophosphate.

decreasewith increasingtemperature.In contrast little or no dependenceon the reaction temperatureis displayedby the selectivityto pentanes. Among the variousbutenes produced, the selectivitiesto the cis- and transbut-2-ene do not exceed 4% at any conversion (Fig. 8). The selectivityto lbutene and isobutene increases to approximately 10% at conversions of lo20% and decreaseswith further increase in conversion. The predominant C, hydrocarbonproduced is evidentlyisobutanewhose selectivityreaches40% at high conversion. The selectivityto isobutane increaseswith decreasingreaction temperaturewhile those to the C4alkenes decrease. With the heptenes the selectivitiesto the C6+ aliphaticsand the aromatics are approximately40 and 20%, respectively,at low conversions and decrease with increasingconversion (Fig. 9). These appearto be the primaryproducts. C3hydrocarbons,the butenes, the butanes,the pentenes and the C6aliphatics increase with increasingconversion with the C6 aliphaticspassing through a maximumat approximately10% conversion. The selectivitiesto C3hydrocarbons and the butenesbecome essentiallyconstant and approximatelyequalfor conversionsgreaterthan 20%. With the octenes the C6aliphaticsare the dominant product at low conversions but the quantities produced decrease precipitously as conversion increasesto 10% (Fig. 10). Concomitantlythe selectivityto aromaticsincreases sharply, reaches a maximum at approximately 15% conversion and falls off sharply.Meanwhile,the butenes have been increasingand continue to do so until a maximumis reachedin the vicinity of 80% conversion.

262

The selectivities to C3 hydrocarbons and to the pentenes are relatively high (approximately 15 and 20%, respectively) at low conversions and increase relatively little over the entire range of conversions. The selectivity to butanes is negligibly small at low conversions but increases to approximately 15% for high conversions. DISCUSSION

The primary products from the hexenes appear to be propanejpropene, pentenes and Cg+ aliphatics, each with selectivities, at low conversions, of approximately 30%. With the heptenes, the C,, aliphatics and aromatics are evidently primary products, with selectivities of approximately 40 and 20%, respectively, at low conversions. The primary products with the octenes are C, aliphatics. With the hexenes the dominant products at high conversions are C3 hydrocarbons, butanes, and to a lesser extent the pentenes. Under the same conditions but from the heptenes, C3 hydrocarbons, butenes and butanes are the major products, while with the octenes, the butenes, pentenes and C, hydrocarbons dominate. It is of interest to compare these observations with those obtained with HZSM-5 where the silicon/aluminum ratio is 36 [ 191. With the hexenes and this latter catalyst the primary products are the butenes, the pentenes and the propenes, in order of decreasing amount. In contrast with the observations on NHPW with which the butenes are not primary products and amount to less than lo%, with H-ZSM-5 the butenes are present in quantities as large as 35%. The amounts of the C, hydrocarbons and pentenes are similar in both catalysts. With H-ZSM-5 at high conversions the dominant products are the butenes, C, hydrocarbons and to a lesser extent the pentenes, although both the aromatics and butanes have each increased above 10%. The most notable distinction between the two types of catalysts for high conversions is the presence of quantities of butanes approaching 40% with NHPW. With H-ZSM-5 and the heptenes, the C3 hydrocarbons and butenes are primary products and dominate at high conversions as well. This is in sharp contrast with the observations for NHPW for which the aromatics and C,+ aliphatics are the primary products. However, while the selectivity to aromatics decreases with increasing conversion on the latter catalyst there is a small increase on H-ZSM-5. As observed with the hexenes the amounts of butanes in the products from NHPW increase with conversion to selectivities close to 20% at high conversions but are virtually absent with H-ZSM-5. With the octenes the primary products are the butenes, pentenes and C, hydrocarbons with H-ZSM-5 but the hexenes with NHPW. With the former catalyst the same products dominate at high conversions although some de-

SO

o- cp, c; A-C c’ 3’ 3

@-C .

-

C;ALIPHATICS

A - t&&lPtU~~cs .-

AROMATICS

40

2

30

c % ii

20

IO

0

20

30 HEPTENES

40

50

CWVERSlOW

60 (%I

Fig. 9. Dependence of eelectivitiee for different hjdrocarbone on the conversion of heptenee at 673 K over ammonium l%-tungetophosphate.

crease and increase in the amounts of the pentenes and C3 hydrocarbons,respectively,have occurred. The most striking difference between the behaviour of the two catalysts is undoubtedlythe major changes in selectivitiesto the various products as the conversion increases. While the quantities of the products from the ZSM-5 catalystare remarkablyconstant over a wide rangeof conversionvalues,those with NHPW vary considerably. Perhaps most notable is the maximum observedfor the selectivityto aromaticsfrom both the hexenes and the octenes, found with NHPW but not with H-ZSM-5. It is important in this regard to

0 - C6 ALlPtiATlCS

-C&

-%

1, - C@, ALPHATICS

c:

. - AROMATICS

-C4

lo

20

30

40

70

60

90

Fig. 10. Dependence of &ectivitiee for different hydrocarbons on the conversion of octenee at 673 K over emmonium 12-tungetophoephete.

note that the activity of the latter catalysts is approximatelyone thousand times largerthan that of the former. The steady-stateactivity of NHPW is evidentlyconsiderablylowerthan the initial activity and is stronglydependenton the source of the ammoniumion. Since the observed order of activities of NHPW prepared from the various anions correlateswith that of the surfaceareas (Table I ) it can he concluded

265

that the activity is proportional to the surface area of the prepared catalyst, althoughother dependencesrelatedto structuraldifferencesresultingfrom the use of differentpreparativeanions and/or residualquantitiesof these cannot be excluded. It is important to note that, while the averagemicropore radius remainsrelativelyconstant at 10 & 1 A for the NHPW preparedfrom different sourcesof ammoniumion, the microporevolumesdecreaseas the BET surface area decreases [21]. Thus, the change in surface area with the source of the ammoniumion can be directlyrelated to loss of microporousstructurepresumably due to blocking of pores by either residualprotons and/or trapped anions from the ammoniumsalts.Unfortunately,however,the Brenstedacid strengths and their distributionalso vary with the sourceof the ammoniumion [ 221 and consequently,it is difficult to assessthe relativeimportanceof the two factors. Assuminglinearity,extrapolationof an activity versus surface area plot suggeststhat, in the absence of the micropores,little or no crackingactivitywould be expected.Consequently,it maybe concludedthat the relativelylargealkene moleculesemployedin the presentwork areunableto penetratethe bulk structure of the solid in spite of the presence of a polar bond. This inabilitymay be attributed,at least tentatively,to the presence of the relativelylarge number of saturated C-C bond units in each of these molecules. Comparison of the activity of NHPW for the crackingof hexane [4] with that for hexene reveals valuesthat are very similar,althoughnot identical,providing furthersupport for this conclusion. One of the strikingobservationsin the presentwork is the appearance,with relativelylargeselectivities,of primaryproducts not expected from the simple cracking through/?-scission of carbenium ions, which are usually considered as intermediatesin the cracking process. For example, while Cz, C, and C, hydrocarbonsareexpected from the crackingof hexenes,the pentenesand C6+ aliphatics observed as primary products in this work are not anticipated. It appears that relatively fast condensation processes’are able to occur with NHPW to produce larger carbenium ions which rapidly crack to yield these primaryproducts. It is evident that, in addition to the aforementionedcracking and condensation processes, cyclixation is also occurring. Aromatics are formed at relatively low conversions but diminish in quantity at higher conversions. With hexene and octene the C6+ and C6 aliphaticswhich are primaryproducts, respectively,are reduced in concentration as the aromatics form at higherconversions suggesting,not surprisingly,that the latter form from the former. However,with heptene,with which both C6+ aliphaticsand the aromaticsare primary products, the concentrations of both such types of species diminish with increasingconversion.The reductionin amountsof aromaticsappearsto be approximatelycoincident with the increased production of butenes while decreasesin the latter are accompanied by increases in the quantitiesof butanes, the later particularlyevident at high conversion. Dejaifve et al. [23]

266

have studied the conversion of a number of alkenes including 1-butene on HZSM-5 zeolite. A mechanistic scheme proposed by these authors to account for the formation of aromatics involved a number of competitive processes including (1) ionic polymerization; (2) condensation of an olefin and a carbenium ion; and (3) hydrogen-transfer between cyclic hydrocarbons and light alkenes producing aromatics and saturated C&C& hydrocarbons. Although similar reactions may lead to the formation of aromatics on ammonium 12-tungstophosphate, the advantages of a narrow pore size distribution and the presence of pores restricted to the 5-6 A range are not available with NHPW. Although there is evidence from the present work that the entry of the reactant alkenes into the micropores of NHPW may be more significant than their penetration into the secondary structure of the heteropoly oxometalates, nevertheless, the latter cannot be dismissed entirely. Further, polar species formed from the reactant alkenes undoubtedly can enter the bulk structure. Thus, two transport processes which may be subject to diffusional effects are extant in the present system. Haag et al. [24] have shown that the effectiveness factor for n-hexene is 1 and 0.86 for crystallite diameter 0.05 and 2.70 p, respectively. While the average crystal diameters for NHPW prepared from the four sources of ammonium ion fall in the range from 0.7 to 1.2 pm [22] it may be prudent to conclude no more than that the effectiveness factor for hexene is greater than 0.86. In addition, Haag et al. [24] have found that the total length of the molecular chain has a minor effect. Consequently the effectiveness factor for heptene and octene should not differ greatly from that expected for hexene. Finally, it may be noted that the present work has been done at 700 K or less in contrast with the temperature of 811 K employed by Haag et al. In spite of the aforementioned it must be concluded that diffusional effects undoubtedly are operative in the processes reported here. Finally, the importance of recent publications from Anderson et al. [ 25-271 should be noted. In this work the retained and desorbed products from the reaction of 1-hexene on H-ZSM-5 [ 251 and SAPO-34 [ 261 and 1-octene on HZSM-5 [ 271 at 353-593 K have been studied. These authors note that retained products may not only affect the size/shape selectivity of catalysts such as HZSM-5 but they may also function as possible precursors to the formation of coke and the deactivation of the catalyst. It has frequently been reiterated by various authors that coke is formed in all reactions of hydrocarbons on acid catalysts. The present catalyst is not an exception. However, in the case of NHPW two possibilities apparently exist. The precursors to coke may not only be retained in the microporous structure but may also enter the secondary structure of the solid to be trapped there. Unfortunately, no reports of the results of studies of the deactivation of heteropoly oxometalates in the presence of alkenes are presently available in the literature.

267

CONCLUSIONS

Ammonium 124ungstophosphate, a metal-oxygen cluster compound, is an active catalyst for the conversion of hex-1-ene, hept-1-ene and act-1-ene, although rapid deactivation is observed in the first 2-3 h on-stream. At low conversion the process follows first-order kinetics. However the rates are found to depend on the source of the ammonium ion, an observation which is attributed to differences in surface areas and micropore volumes. Although polar species are expected to be capable of entering the secondary (bulk) structure of the solid evidence from the present work suggests that the C6-Cs alkenes do not do so to any significant extent. The primary products formed from the three alkenes are not those expected from the usual cracking process involving /3scission of carbenium ions but rather those expected from condensation processes and consequently the latter processes appear to be highly favoured on this catalyst. ACKNOWLEDGEMENT

The financial support of the Natural Sciences and Engineering Research Council of Canada is gratefully acknowledged.

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