hydrocarbon from methanol

8/8/2019 Hydrocarbon From Methanol

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C A T A L . R E V . - S C I . E N G . , 2 5 ( 1 ) . 1-118 ( 1 9 8 3 )

Hydrocarbons from MethanolCLARENCE D . CHANGMobil R e s e a r c h and D e v e l o p m e n t C o r p o r a t i o nC e n t r a l R e s e a r c h D iv is io nP r i n c e t o n . N ew Jersey 08540

I . I N T R O D U C T I O N .................................... 2I1 . G E N E R A L C O N S I D E R A T I O N S ....................... 2

A . Stoichiometric C o n s t r a i n t s ...................... 2B . Thermochemistry ............................... 3I11 . C A T A L Y S T S ....................................... 4A . E a r l y O b s e r v a t i o n s ( p r e -1 9 6 0 ) .................. 4

B . Z e o l i t e C a t a l y s t s ............................... 8C . N o n z e o l i t i c C a t a l y s t s ........................... 35IV . R E A C T I O N K I N E T I C S .............................. 36V . REACTION MECHANISM ............................ 44A . E t h e r Formation ................................ 44B . Hydrocarbon F o r m a t i o n ......................... 4 8

V I . THE MOBIL METHANOL-TO-GASOLINE( M T G ) P R O C E S S ................................... 75A . F i x e d - B e d P r o c e s s ............................. 75B . F l u i d - B e d P r o c e s s ............................. 84C . Product C h a r a c t e r i s t i c s ........................ 99D . O l e f i n P r o d u c t i o n .............................. 100VII . D U A L - F U N C T I O N A L C A T A L Y S I S ................... 1 0 3

V I I I . P O S T S C R I P T ...................................... 110R E F E R E N C E S ...................................... 111

1Copyright 0 983 by Marcel Dekker. Inc. 0022-2348/83/250 1-0001%3.50/0

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2 C H A N G

I. INTRODUCTIONThe conversion of methanol to hydrocarbons is a remarkablereaction. The mechanism involves C-C bond formation from C ,fragments generated in the presence of certain acidic catalysts

and reagents. The precise na ture of these reactive C1 speciesis unknown at present and is the subject of lively debate. Theconsiderable diversity of c urrent opinion will become apparentfrom the following account.Occasional reports of hydrocarbon formation from methanol,often as a minor side-reaction accompanying ether formation, canbe found in the literature dating back as far a s 1880. However,it was not until the early 1970s that the industrial potential ofthe reaction emerged, due mainly to the confluence of two events:the discovery by workers at Mobil of the selective catalytic con-version of methanol to high octane gasoline over zeolite ca ta lysts[l, 21, and the 1973 Arab oil embargo. The embargo instigateda worldwide reassessment of alternatives to petroleum. Sincemethanol can be made from coal (or virtually any carbon source)by proven technology, the Mobil development offered a new al-ternative to classical synfuel processes such as the Fischer-Tropsch and Bergius.applied aspects of hydrocarbon formation from methanol hasgrown over the last decade and continues to expand.

An attempt is made in this survey to cover the journal litera-ture comprehensively, while the patent literature is, of necessity,treated with greater selectivity.

A sizable body of l iterature dealing with th e theoretical and

11. GENERAL CONSIDERATIONSA . Stoichiometric Constraints

Hydrocarbon formation from methanol requires the eliminationof oxygen, which can occur via th ree modes: elimination a s H ,O ,CO, or CO, (elimination as 0, s prohibited by thermodynamics).Reactions involving the coproduction of oxygenates such as for-mates can also be written. However, with th e possible exceptionof methane formation, as discussed later, no clear-cut experimen-tal examples of thi s reaction are known. For th e presen t, t he re -fore, the discussion will be limited to hydrocarbons as the soleorganic product.may also be produced. In the simplest case, where H 2 is the onlycoproduct , the stoichiometry can be written as follows:

Depending on the nature of hydrocarbons formed, H 2 or coke

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HYDROCARBONS FROM METHANOL 3CHSOH * [C H ZI + H2O ( 1)Here [CH, ] represents mono-olefins, cycloparaffins, or theaverage composition of a mixture of paraffins and aromatics.

Acetylenic compounds or polyenes have not been observed insignificant amounts in studies reported to date.reactions can be written.

When paraffins ar e the sole hydrocarbon pro ducts , severalThe principal reactions are

( n + l ) C H 3 0 H * CnH2n+2 + C + ( n + 1)HZO ( 2)(2n + 1 ) C H 3 0 H + 2CnHzn+z i O + H 2 0 (3)

n = 1 , , 2 , 3, ... ( 4 )Equation ( 4 ) is derivable from (3) by allowing the water-gasIn forming aromatics from methanol, 3 mol of H 2 (exclusive ofshift reaction to occur and go to completion.

H 2 in HzO) are necessarily eliminated or transferred for each aro-matic ring generated. The general equation for mononuclear aro-matics formation is

nCHJOH + C n H z n - 6 + 3H, + nH,O, n = 6, 7 , . . . ( 5)Although pure examples of ( 5 ) have not been observed to date,

the enhancement of aromatics in the presence of certain metal/zeo-lite combinations, with concomitant H2 production, is known [ 31 .

B . ThermochemistryHydrocarbon formation from methanol is an exothermic reaction.

By way of illustration. reaction enthalpies 141 associated with E q s .(11, ( 2 1 , and (5) are plotted in Fig. 1 for n-paraffin. linear 1-olefin , and methylbenzenes formation at 316OC. The olefins curv e,which may also be cons idered rep resentat ive of a balanced paraf -fin-aromatic distribution (i.e., with no H, or coke make) is seento level off at -AHR = 300-400 ca l/ g. This is in accord with theenthalpy data of Chang and Silvestri [21 fo r hydrocarbon forma-tion from methanol at 3 7 l o C , which is shown in Fig. 2 as a func-tion of conversion.will vary depending on product distribution.large reaction heat is a major constraint in reactor design.

It is also apparent from Fig. 1 that the degree of exothermicityFrom a practical standpoint, the control and removal of this

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4 C H A N G

r400 PARAFFINS + C + H 2 0-600b 4 6 8 10 12CARBON NUMBERFLG. 1. Hydrocarbon formation from methanol. Heats of re-action for selected hydrocarbon products at 600 K .

111. CATALYSTS

A . Early Observations (pre- 1960)The earliest report of hydrocarbon formation from methanol

is that of LeBel and Greene [ 5 ] , who described in 1880 the

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HYDROCARBONS FROM METHANOL 5lo3 I I 1 1 1 1 1 1 I I I l r l l l I I I I 1 1 1 1 1 1 I 1 I l l

210

AT 100% CONVERSION:1 OH R -400 ca l I g ( - I674 k J / k g 1I I I I I I 1 l l I I I I I I l l 1 I I I I 1 l I l I I 1 1 1 1 1 1lo-' 10' 3 10-2 10- I 1

L H S V - '

FIG. 2. Methanol conversion to hydrocarbons. Heat of reac-tion vs space-time at 371% [ 2 ] .

decomposition of methanol in molten ZnC1,. Hexamethylbenzene(HMB) nd light gases, mostly CH,,, were identified as the mainproducts, A possible equation for this extraordinary reactionis

C15 CH30H FUSED ZnC& '@ + 3CH4+ I 5 y OC

C

AG~"" = -265.2 kcal/rnolA s written, the reaction has AG = -261 kcalfmol of HMB at 283OC.the melting point of ZnC1,.

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6 CHANG

This large driving force for HMB formation has mechanistic aswell as process implications which are later discussed.

LeBel and Creene interpreted their result, perhaps propheti-cally, in terms of V H , " condensing to benzene, followed by ex-haustive ring methylation by a Friedel-Crafts reaction with CH,Clgenerated in situ.In 1914 Sernagiotto [ 61 reported the decomposition of methanolby P,O, in a paper titled "On the Chemistry of Nascent Methylene."The vigorous reaction yielded a mixture of hydrocarbons with thegeneral formula ( C H , ) n . Propene and butene were identified viabromination. Hexene and hexane, along with unidentified solidproducts, were also produced. For this example, P,O, would prob-ably be more accurately described a s a reactant ra ther than a ca t-alyst in the true sense.A patent was later granted to Grosse and Snyder 171 for theconversion of methanol and DME to hydrocarbons over ZnC1, at375-675OC and superatmospheric pres su res. Table 1 containstypical results.The heavier fractions contained significant HM B .Al,O,.DME, and above 350C, CH,, C z H 4 as well as C O , CO,, and H,were detected.Topchieva and Ballod [ 91 compared the activity of silica gel,Al$, and silica-alumina (Al,O, 30% Sio,70%), for methanol con-version to DME . The aluminosilicate catalyst, after adsorbingmethanol (0.58 mmol/g) at 20C was heated to 4 O O O C and gaveCO,, C2H4, C O . and C,H, in addition to D M E , C , and unreactedmethanol,Cullinane et al. [ 101 found that methanol contacted with acti-vated Al,O, or Type-C Al,O, at 45OOC yielded small quantities ofhexamethylbenzene.Gorin [ 111 reacted a mixture of dimethyl ether (18.2 mol%)andisobutane over amorphous silica-alumina at 37OoC, 150 psig, and6 .8 h- l GHSV. The conversion of dimethyl ether to hydrocar-bons was 94.58, while no net change in isobutane occurred. Theyield distribution on an isobutane-free basis is shown in Table 2.In the absence of isobutane, at higher temperature, lower pres-sure, and higher space velocity, considerable methane, CO , andcarbon were produced, while th e yield of C,+ hydrocarbon wasless than 15%. Although no explanation of the role of isobutanewas offered, one can surmise that it served to moderate the largereaction exotherm . According to Gorin, methanol reacts moreslowly and produces les s liquid hydrocarbons, although no datawere given.

The light hydrocarbons are mostly isobutane.Adkins and Perkins [8] studied the reactions of methanol overAt 300-350C methanol was quantitatively converted to

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HYDROCARBONS FROM METHANOL 7TABLE 1

Conversion of Methanol to Hydrocarbons ov er Fused ZnC1, [ 71(425OC, 166 atm)

Mole percent MeOHconverted to

C H 4 1 .0C2H6 0 . 4

1.34 .01 .0

1 9 . 61 . 5

C4H8 0 .7C, to 85OC (1.5 tom) 33.1Higher boiling produc ts 30. Oa

7.4Other -100.0

18%hexarnethylben zene based on MeOH converted .

A patent is sued to Fawcett and Howk [12] claims th e di rectconversion of methanol to hydrocarbon wax in the presence ofa metal rnolybdite ca ta ly st , e . g . , CoMoO,, modified with nickelchromite at 100-350C at 70-1000 atm. Here the question ari sesas to whether methanol is not simply a sou rce of synthesis gasvia dissociation, and therefore whether the actual hydrocar-bon-forming reaction is a Fischer-Tropsch reaction.the case in an earlier study by Eidus [13] where methanol wasreacted over a cobalt-thoria-kieselguhr catalyst under Fischer-Tropsch conditions and found to give the s a m e hydrocarbonproducts as with CO + 2H2, albeit in considerably loweryield.

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8 C H A N G

TABLE 2Dimethyl Ether Conversion over Si02-A1,03 [ 111(37OoC, 10 atm, 6 . 8 h-l QHSV)

Products %CHI4 5 . 7C2H4 1 0 . 2C3H6 + C3H8n-C4H,, + C,H8i -Ci -Csc7c +Carbon

8 . 15 . 2

2 6 . 41 1 . 0

3 . 02 1 . 6

8 . 69 9 . 8

B. Zeolite Catalysts1. Introduction

Zeolites are porous, crystalline aluminosilicates composed ofA104 and S O 4 tetrahedra, interconnected through shared oxy-gen atoms, forming a three-dimensional framework. Since everyoxygen in such a structure (viewed as an infinite lattice) isshared by two tetrahedra, the framework will possess a netnegative charge.Mn+ in the structure, leading to the general representationThis is balanced by (exchangeable) cations

where n is the charge on the cation and z is the water of hydra-tion. In general, y / x ? 1 , and in some exceptional cases, e.g.,the synthetic zeolites ZSM-5 114, 151 and ZSM-11 [ 1 6 , 1 7 1 , 5


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HYDROCARBONS FROM METHANOL 9

Erionite 8 Ring ZSM-5 10 Ring(Stroight Channel1 Y Zeolite 12 Ring

FIG. 3. Typical zeolite pore geometries.

When M is a proton, th e zeolite becomes a s tron g Bronstedacid. A s catalysts, zeolites are unique in their ability to dis-criminate between reactant molecules and to control product se-lectiv ity, depending on molecular size and shape , This phenom-enon, known as "shape-selective catalysis [ 18-22], is a con-sequence of th e well-defined geometry of zeolite po res, chan-nels , and cages. These are of molecular dimensions.

The catalytically most significant zeolites are those havingpore openings characterized by 8-, l o - , and 12-rings of oxy-gen atoms. Some typical pore geometries are shown in Fig. 3.For comprehensive treatments of zeolite st ru ct ure and chem-istr y, the reader is referred to the classic monographs of Breck1231 and Rabo 1241.2 . ZSM-5 and Related Zeolites

The most extensive work on hydrocarbons from methanol hasThere is little questioneen done with ZSM-5 zeolite catalysts.that these are the most effective catalysts yet discovered forth is reaction.

a . Structu re of ZSM-5 an d ZSM-11The unique catalytic properties of ZSM- 5 zeolites have beenattributed to their crystal stru ctu re [ 1, 1621.Zeolite ZSM-5 [ 14 , 15, 251 has orthorhombic symmetry Pnmawith cell parameters a = 20 .07 , b = 19.92, c = 13.42 A. The

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10 C H A N G

FIG. 4 . ZSM-Kchannel system.

channel system, represented in F i g . 4 , consists of straight chan -nels running parallel to 10101 and intersecting sinusoidal chan-nels parallel to [ 1001. The channels are ellipsoidal with 10-ringopenings, having the approximate free dimensions 5 . 4 X 5 . 6 w(stra ight channels) and 5 . 1 X 5 . 4 1 sinusoidal channels) basedon oxygen radii of 1 . 3 5 A. Figure 5 shows a stereo-pair drawingof the ZSM-5 framework viewed along [ 0101.116 , 17 , 251. ZSM-11 has tetragonal symmetry I h 2 , with cellparameters a = 2 0 . 1 , c = 1 3 . 4 1. The channel structure, shownin Fig. 6 , consists of intersecting straight channels, with 5 . 1 X5 . 5 free dimension. The framework is shown in Fig. 7.

Closely related to ZSM-5 in structure and properties is ZSM-11

b . General Reaction PathThe conversion of methanol over ZSM-5 zeolites was investi-

gated by Chang and Silvestri [ 21 who found the reaction to pro-ceed according to the following general reaction path:paraffins

2CH3OH r H,OCHS -C2'--C5'-- aromaticscycloparaffinsC,+ olefins-H20 -H20+H20

Scheme AThis was established by monitoring changes in product distribu-tion as a function of varying contact time, shown in Fig. 8. Anessentially identical reaction path was obtained with DME as feed,Fig. 9 , confirming its intermediacy in the reaction sequence.

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HYDROCARBONS FROMMETHANOL 11

ZSM-5 Pnma 8 T,-,*P1viewed a long (010J

Secondary buildlng units: com plex 5-1Framework density: 1 7 . 9 T/1000 A3Channels:Fault planes: (100)

{ [OIO] 5. 4 x 5.6 - l o o ] -0 5 . 1 x 5 . 5 r . t -Type sp ec ie s: Mobil synthetic zeolite ZSM-5Na,Al,Si,~,O,,, - 16 H,O with n c 7 andtypically about 3

orthorhombic, Pnma, a= 20 . 1 b=19.9 c = 1 3 . 4 AFIGURE 5 1251.

F I G . 6 . ZSM-11 hannel s y s t e m .

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12 CHANG

viewed along [ l o o ]Secondary building units: complex 5 -1Framework density: 17.7 T/1000 A3Channels: < 100, 5.1 x 5 . 5 * * *Fault planes: (100)Type species: Mobil synthetic zeolite ZSM-11

Na,Al,Si,.,O,, - 16 H,O with n < 16 andtypically about 3

tetragonal, Iam2, a=20.1 c=13.4 AFIGURE 7 [ 2 5 ] .

This reaction path has been confirmed in general by many work-A variation has been proposed by Voltz and Wise [301 based oners [26-291

resu lts from a fluid-bed kinetic st ud y. The modified scheme,Scheme B , contains a direct path to olefins from methanol:

2CHjOH CH,OCH, + H2OJolefins\

Scheme B

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7

/

\

DMHEH

1

K

lo-

I

I0

I

STM(m)

FG.8RaonphfmhnconontohdoabnoHZSM-5371OC)2

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14 CHANG

- -d IMETHYL ETHER -a\

5mYK+5

,,----4-

30

20

10

010' lo-' 11SPACE TIME (FLG. 9. Reaction path for dimethyl ether conversion to hydro -

carbons over HZSM-5 (371OC) 1 2 1 .

Anderson et al. [27 ] and Ceckiewicz I431 also recognized thispossibility, This is formally identical to the mechanism ofBalandin et al. [31] for ethanol dehydration over alumina, whichwas elucidated by the kinetic isotope method. The relative ki-netic significance of this auxiliary step is unknown.

c. Product and DistributionTable 3 [2 2 ] contains typical hydrocarbon selectivities at 371OCfor ZSM-5 and ZSM-11, showing the general similarity between thetwo zeolites. The hydrocarbons span a relative narrow range ofmolecular weights, terminating at about C l 0 . At this stage of re-action, conversion of the intermediate olefins is essentially com-

plete and the main products are isoparaffins and aromatics.The aromatics are mostly methyl-substituted. as seen in Table4 . Alm shown i s a comparison of the observed isomer dis tr ibu-tion with their thermodynamic equilibrium values. It is evident

that the xylene distribution is close to equilibrium, while among

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TABLE 3 [22]Methanol Conversion to Hydrocarbons over Various Zeolites(37OoC, 1 atm, 1 LHSV)

Hydrocarbon distribution ( w t % ) inErionite ZSM-5 ZSM-11 ZSM-4 Mordenite

c, 5.5 1.0 0. 1 8.5 4.5c 2 2 - 36.3 0.5 0.4 1 1 . 2 11.0c3 1.8 16.2 6 .0 19.1 5.9

c2 0.4 0.6 0 .1 1.8 0.3

c,*- 39.1 1.0 2 .4 8.7 15.7c, 5.7 2 4 . 2 25.0 8.8 13.8c,2- 9 .0 1. 3 5.0 3.2 9.8C5+ aliphatic 2 . 2 14.0 32.7 4.8 18.6A, 1 .7 0.8 0.1 0.44 7 10.5 5.3 0.5 0.9A8 18.0 1 2 . 4 1.3 1 .0A 3A10

7.5 8.4 2 .2 1.03.3 1.5 3.2 2 . 0

A 11+ 0 . 2 26.6 15.1

the higher aromatics, 1,3,5-trimethylbenzene, 1,2 ,3, 4-, an d1,2,3,5-tetramethylbenzenes all significantly short of theirequilibrium values. This was att ribut ed to zeolite "shape-selec-t ivi ty," as was the sharp cut-off in product molecular weight.

d. Effect of Tempera tureThe effect of temperature on ZSM-5 selec tivity a t low w a c e

velocity (LHSV = 0 . 6 - 0 . 7 h - l ) is shown in Fig. i0 121. Below3OOOC the main reaction is dehydrati on to DME. With increasingtemperature th e sequ ence of Scheme A is followed, while above45OOC light olefins and CHI, become significant a s a result ofsecondary crac king . Above about 5OOOC the dissociat ion ofmethanol to H2 nd CO becomes measurable.

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16 CHANGTABLE 4 [2]

Aromatics Distribution from MethanolConversion over HZSM-5

Normalized Normalized Equilibriumdistribution isomer distributions

(wt%) distributions (371OCBen zeneTolueneEthylben zeneXylenes:

mP

0

Trimethylbenzenes:123124135

Ethyltoluenes:0

m + PIsopropylben zeneTetramethylbenzenes:1234

12351245

Other Al0aA l l +

4 . 125.6

1 .99 . 0

22.81 0 .0

0 . 911.1

2 . 1

0 . 74 . 10 . 2

[2 3 .9

[[. 41 .9

2 .02 . 70 .4

["::I2 3 . 5

[:3[::::I3 3 . 4

Diethylbenzenes + dimethylethylben zenes .

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HYDROCARBONS FROM METHANOL 1 7I 0 0

90

80

I-WVWcI'3y 5012 40J3I30 30

7 0a

60

w

20

10

0

WATER

C2 - C 4 HYDROCARBONS

AROMATICS -

300 400 500T E M P E R A T U R E 'C

FIG. 10. Effect of temperature on methanol conversion overHZSM-5 ( 0 . 6 - 0 . 7 h-' LHSV, 1 atm) [ 2 ] .

e. Effect of PressureThe main effect of varying reactant partia l press ure on ZSM-5selectivity is to change the relative ra tes of th e olefin-forming andaromatization steps [ 321 . Decreasing pressure tends to decouple

the two reactions, while increasing pressure enhances the overlapof the two reactions. This is illustrated in Figs. 11 and 1 2 forPMeOH = 0.04 and 50 atm. These plots may be compared withFig. 8 depicting the normal atmospheric reaction path. The sig-nificance of the shaded regions will b e discussed la te r.

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18 C H A N G

8 5 0 -5z0 40-25g 20-

30300

10

0

60"I - -0- WATER-.

- DIMETHYLETHER o/,,

-- AROMATICS- v

FIG. 11. Methanol reaction path at low pres sure (HZSM-5 cat-alyst, 371OC) [ 3 2 1 .

Thus at subatmospheric partial pressures, high selectivity toolefins is possible at complete conversion of methanol (Table 5 ) .The most striking effect of raising pressur e above atmosphericis an increase in the degree of aromatic subst itu tion, especiallyin durene selectivity. A s shown in Table 4 , durene is not thethermodynamically favored tetramethylbenzene isomer. It s pref -erential formation is a consequence of zeolite shape-selectivity .Among the tetramethylbenzenes , durene possesses a critical di-

ameter most compatible with the zeolite channel dimensions, i .e . ,it is the bulkiest isomer able to move with relative ease throughthe zeolite channel system, and is therefore more easily formedthan the other C l o aromatics within the zeolite.that in large pore zeolites, C lo equilibrium is attained.

It is shown later

f . Modifications and IsotypesKaeding and Bu tte r [331 reported that ZSM-5 modified by treat-ment with phosphorus compounds showed high (70%) selectivity for

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H Y D R O C A R B O N S FROM METHANOL 19

E 501 //

I/

I

I-V300K 20-n

10-

0L

/

2-0 I5 40 I /mKI-5!?0 30I-V 0- PARAFFINS300K 20n

E 50 //

,

0- AROMATICS10

0

0- AROMATICS

10-2 10-1LHSV-' , hr.

1

FIG. 1 2 . Methanol reaction path at high pressure (HZSM-5 cat-alyst, 371OC) [32].

C2-CI , olefin formation from methanol. The following w as proposedas to the nature of trimethyl phosphite modification :

HI

O\ /o \* l /o \ s i /o0 /si\o o /+ , \o \o

10

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20 CHANG

TABLE 5Methanol Conversion at Low Pre ssures [ 1611(427OC, 1 LHSV, 1 atm total pressurea)

Methanol partial pressure, atm: 1 . 0 0 0 . 2 5 0 . 1 7 0 . 0 7Conversion, 8: 99+ 99+ 99+ 99+Hydrocarbon distribution, wt %:

Ethylene 3 . 2 1 2 . 4 17 .4 2 1 . 0Propylene 4 .8 1 8 .2 2 6 . 5 3 8 . 7Butenes 2 . 2 6 . 3 7 .6 1 8 . 5Pent enes 0 . 4 0 . 3 0 . 7 2 . 4

(Total C,-C, olefins) ( 1 0 .6 ) ( 3 7 .2 ) ( 5 2 .2 ) ( 8 0 .6 )MethaneC2-C5paraffinsC,+ nonaromaticAromatics

1 . 5 0.8 0 .6 0 . 54 3 . 0 3 9 . 4 2 4 . 2 1 5 . 63 . 9 2 . 3 2 . 7 1 . 34 1 .0 2 0 .3 2 0 .3 2 .0

aHelium diluent at subatmospheric MeOH part ial pressures.

CH3 0 \ P/OCH

+CH ,OH

Other than to reduce th e activity of th e ZSM-5 cid sites, thefunction of P-modification is unknown.The conversion of ethanol ov er "Ultrasil" zeolite, which isZSM-8 [ 3 4 , 3 5 1 , has been described by Russian workers 128 , 36 ,

371. Representative data in Table 6 show that this catalyst be-haves similarly to ZSM-5.

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HYDROCARBONS FROM METHANOL 2 1TABLE 6

Methanol Conversion over "Ultrasil" Zeolite [ 361Reaction conditions:

Temperature, O CPressure, torrTime on stream, min

Hydrocarbons, w t %:C H 4

C,HeC 2 H 4 + C2H6

C 3 H 6i-C 4H l on-C4H10C 4Hsi-C,H,,n-C,H,, and higher

300 300 450 4 50 4 5030 8 0 3 0 8 0 8 060 6 0 60 3 60

7 . 9 9 5 . 5 2 1 1 . 9 44 . 9 7 2 1 . 8 4 1 5 . 7 43 . 0 3 2 . 0 9 5 . 245 . 5 2 1 5 . 3 9 3 1 . 8 3

22 .25 10 .12 7 .691 . 5 7 0 . 3 3 1 . 2 27 . 08 3 . 9 8 1 4 . 4 9

1 6 . 7 8 3 . 2 9 2 . 6 23 0 . 8 1 3 7 . 4 4 9 . 2 3

9 . 4 88 . 1 01 . 4 7

29 .884 .980 . 5 1

2 0 . 9 83 . 5 421.06

1 . 7 46 . 3 8

1 0 . 3 514 .9823.06

4 . 8 610 .661 0 . 0 817 .80

Spencer and Whittam 1551 have described a high silica zeoliteof unknown structure called Nu-1, which behaves similarly toZSM- 5 in methanol conversion, but deactivates quickly.3. Other Zeolites

a. Small Pore ZeolitesZeolites which sorb linear hydrocarbons and exclude branchedones have received attention as catalysts for methanol conversionto light hydrocarbons such as ethene and propene. Str uc tur es ofsome of these small pore zeolites are illustrated in Figs. 13-16 .The erionite structure, Fig. 1 3 , is hexagonal, containing"supercages" supported by columns of cancrinite units linkedthrough double-6-rings ( D 6 R ) . Access to, and between, thesupercages is gained through 8-rings.of 6-rings is AABAAB [231 as compared to AABAAC in erionite.Offretite is closely related to erionite except that the sequence

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22 CHANG

Secondary building units: single 6-rings in AABMC sequence(single 4-rings)

Framework density: 15.6 T/l000 A 3

Channels: - [ O O l ] - 3.6 x 5.2 ***Fault planes: (001)Type species: Erionite (Na2,Ca ..),,, Al,Si,,O,, 27 H 2 0

hexagonal, P63/mmc, a=13.3 c=15.1 AFIG. 13. Erionite.

Intergrowths of erionite and offre tite, e . g . , zeolite T , occur fre-quently.is A B C A B C , and the D6R units are linked together through tiltedIn the chabazite framework, Fig. 1 4 . the D6R layer sequence

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CHABAZITE

Secondary building units:

Framework density:Chamels:Fault planes:Type species:

R i m 36 T[1]viewed along [OOl] hex

single 6-rings in AABBCC sequence(double 6- or single 4-rings)14.6 T/1000 A3- [OOl] - 3.6 x 3 . 7 * * +(0011Chabazite Ca,Al,,Si,,O,, a 40 HzOtrigonal, Rj m, a=13.2 c=15.1 A(Tr ue symmetry is lower, possibly triclinic)FIGURE 14 1251.

4-rings. The framework conta ins large ellipsoidal cavit ies (Fig.15), each entered through six 8-rings. These cavities ar e joinedtogether via their 8-rings, forming a 3-dimensional channel s y s -tem.Zeolite ZK-5, Fig. 16, consists of trunca ted cuboctahedrajoined through D6R units in a body-centered str uct ure . Themain channel system is defined by 8-rings.

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24 C H A N G

FIG. 15. Chabazite cavity,

Methanol conversion over a variety of small pore zeolites hasbeen reported by Chang et 81. 1381. Representative data areshown in Table 7. At 339-538OC the produc ts , restricted largelyto the C 2 - C , range, were mainly olefins. The presence of signi-ficant amounts of methane in some experiments is symptomatic ofcoke laydown.exchanged chabazites has been investigated by Cobb et al. [391and Singh et al. (40, 411. Both teams of investigators observedshort-term catalyst deactivation due to coking, but found the cat-alyst to be regenerable. Cobb et al. observed long-term irrevers-ible deactivation, which was att ributed to structu ral degradation,based on drastic changes observed in the x- ray pattern . Thiswas disputed by Singh et al., who saw no such deactivation in athree-month study comprising 21 regenerative cycles. It wasspeculated that the discrepancy may have been due to differ-ences in ion-exchange procedures. Singh et al. contended thattheir method of preparation led to an ffultrastab le orm of cha-bazite . f

Ceckiewicz [42, 431 studied methanol over the hydrogen formof zeolite T , Coking was rapid and was attributed to the forma-tion of nondesorbable cyclic hydrocarbons in the zeolite su per -cages.

The conversion of methanol to light olefins over various cation-

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ZK-5 lm3m 96 T[1]viewed along [ l o o ]

Secondary building units: double 6-rings

Framework density: 14.7 T/1000 A3(single 4-, 6- o r 8-rings)

< l o o > 8 3 .9* * *1 (100, S 3.9*+*Channels: -Fault planes: ----Type species: synthe t i c zeolite ZK-5 Na3,A1,Si,,O,,, * 98 H,O

cubic, ImJm, a=18.7 AFIGURE 16 [25].

Wunder and Leupold [44] report that selectivity to ethene andpropene is significantly enhanced over a mixture of chabazite anderionite when these zeolites are Mn-exchanged. A methanol/watermixture ( 3 0 ~ 0 1 %2 0 ) was reacted at 4OO0C, and gave the resultshown in Table 8. The product was 66.3% C2-C4 lefins.an x- ray pat tern similar to herschelite (isost ruc tural with cha -bazite) but with line broadening due to small crystal size, is ac-tive for converting methanol to C,-C hydrocarbons at 30O-55O0C.Typical results are shown in Table 9. The product consistsmainly of ethene (16.6 vol%),propene (41.2%), and propene(22.4%).

Whittam and Spencer [45] report that "zeolite M C H , " which hasDownl

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26 C H A N GTABLE 7

Methanol Conversion over Small Pore Zeolites [ 381Erionitea Zeolite T Chabazite Z K - 5

Reaction conditions:Temperature, OCPressure, atmLHSV (WHSV) , h-Conversion, %

Hydrocarbons, wt % :CH 4CH2H6

C3HaCzH4

C3H6

C4H10C4HBc 5+

3 70 3 4 1 -3 7 81 11 ( 3 . 8 )

9 . 6 1 1 . 1

5 . 50 . 4

3 6 . 31 . 8

3 9 . 15 . 79 . 02 . 2

3 . 60 . 7

4 5 . 70

3 0 . 0

1 0 . 03 . 1

5381b

100

3 . 34 . 4

2 5 . 43 3 , 32 1 . 21 0 . 4

2 .0

5 381b

100

3 . 20

2 1 . 43 1 . 81 3 . 52 2 . 67.5

abPulse microreactor, 1 pL MeOH in He, 500 h-l GHSV.De-aluminized ; Si02/A1,03 = 1 6 .

Inui et al. [ 4 6 , 4 7 1 synthesized an erionite-offretite catalyst byrapid crystallization in the presence of tetramethylammonium hy-droxide. Methanol (128 in N 1 ) was reacted over the calcined(54OOC) material at 4OO0C, 1000 h - l GHSV and gave (m18) ethane( 2 5 ) , propene ( 3 2 . 7 ) , butene ( 1 9 . 2 ) , methane ( 4 . 6 ) , propane( 6 . 4 1 , butane ( 4 . 7 1 , and C,+ ( 7 . 5 ) at complete conversion.

b . Large Pore ZeolitesFaujasite-type zeolites, e . g . , X and Y . mordenite, and mazzite( Z S M - 4 ) , are large pore zeolites which have been found to catalyzemethanol conversion to hydrocarbons.

The faujasite st ru ctur e, Fig. 1 7 , is built up of truncated octa-hedra interconnected via D6R units. Faujasites contain extremelylarge supercages ( ~ 1 3 iameter) e ntered into through 12-rings.

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H Y D R O C A R B O N S FROM METHANOL 2 7

TABLE 8Methanol Conversion over Mn-Exchanged Chabazite-Erionite I441

(40O0C. 90 %conversion)Products Vol %

C2H43H6

C4HBC H 4C2H6C3H8

C sf nonaromaticAromaticsco

C4H10

H2H20Dimethyl ether


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28 C H A N G

TABLE 9Methanol Conversion over Zeolite H -MCH [ 451(45OoC, pulse microreactor)

Products Vol %~~~ ~

CH4C 2 H 6C2H4

c 3H8C 3 H 6i -C4Hlon - C 4 H 1 0l-C4H8i - C 4 H 82-C4H8

3.60 . 2

16.62 2 . 441.2

04 . 92.72.04.9

little success. The effect of increasing hydrogenation activityof the metal component was mainly to enhance methanol dissocia-tion [51].The influence of acid st re ng th of the protonic si tes in zeoliteY on dimethyl ether conversion a t 35OOC was investigated byCormerais et al. [ 521. A series of zeolites containing varyingconcentrations of Na and K ions was prep ared . The acid str ength swere determined by pyridine adsorption-desorption at differenttemperatures. I t was found that dimethyl ether reacted only whenthe total number of Na and K ions per unit cell was less than 1 6 . 4 ,and that the active sites must be of sufficient str ength to retainpyridine at temperatures at least as high as 450OC.The catalytic dehydration of methanol by synthetic H-mor-denite was studied by Swabb and Gates [ 531 a t 99.5-240OC.Olefins were detected at 24OOC ; however, deactivation was rapid.Natural mordenite, exchanged with various cations, was foundby Zatorski and Krzyzanowski [ 541 to be highly active for meth-anol conversion to C1-C hydrocarbons at 350-500C, but to havea short life-time.

Mordenite and ZSM-4 were investigated by Chang et al. [ 221for methanol conversion. Table 3, a s shown previously, sum-marizes the reaction conditions and hydrocarbon distributions

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FAUJASITE Fd3m 192 T[1]viewed along (1111

Secondary building units: double &rings(single 4- or 6-rings)

Framework density: 1 2 . 7 T/1000 A 3Channels: g 7 . 4 * * *Fault planes: (1111Type species: Faujasite (Na,, Ca, Mg),,Al,,Si,,,O,,, 240 H,O

cubic, Fd3m, a=24.7 AFIGURE 17 [ 25 ] .

obtained.erionite and intermediate pore ZSM-5 and ZS M-1 1 .Included for comparison are results from small pore

4. Zeolite Shape-SelectivityTable 3 provides a concise illustration of zeolite shape-selec;

tivity in methanol conversion to hydrocarbons, Small pore (4 .3 A )erionite, as already shown, produces only low molecular weight hy-drocarbons. It does not sorb benzene and therefore cannot pro-duce aromatics. The intermediate pore ( < 6 (A> ZSM-5 and ZSM-11truncate the hydrocarbon distribution at C,, , while the large

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30 C H A N G

1MORDENITE Cmcm

viewed along

Secondary building units: complex 5 -1nFramework density: 17.2 T/1000 A

Channels: [OOlI 12 6 .7 x 7.0* tj 0101 - 2.9 x 5.7*

Type species: Mordenite Na,Al,Si,O,, * 24 H,Oorthorhombic, Cmcm, a=18.1 b=20. 5 c=7. 5 A

FLGURE 18 [ 2 5 ] .

pore ( < 8 8 ) ZSM-4 and mordenite, which can accommodate mole-cules as bulky a s hexamethylbenzene, produce this compoundalong with other aromatics.The normalized aromatics distribution is shown in Fig. 20 andserves to highlight the influence of pore size. These results areconsistent with the critical dimensions of the methyl-substitutedbenzenes, some of which are listed in Table 11.dimensions cannot generally be used to gauge precisely whatIt should be pointed out, however, that crystallographic pore

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HYDR OCAR BONS FROM METHANOL 31

MAZZITE PGJmmc 24 T,[1] . 12 T,[m]viewed along [OOl]

Secondary building units: complex 5 -1Framework density: 1 6 .1 T/1000 A 3

(or s ingle 4-rings)

Channels: [ O O l ] 12 7 . 4 *Fault planes: (100)Type species: Mazzite Mg,(Ca, K2)3Al,oSiz60,2 28 H,O

hexagonal, P63/mmc, a = 18 .4 c = 7 .6 AFIGURE 19 [ 2 5 1 .

molecules will be sorbed and what molecules excluded. Moleculeslarger than the calculated aperture size of zeolites are sometimesfreely sorbed.cules, the kinetic diameter from the Lennard-Jones potential pro-vides a more satisfactory, though still not an absolute, matchwith zeolite pore dimensions [ 2 3 1 .

It has been found that for certain sorbate mole-

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32 C H A N G

TABLE 10 1481Methanol Decomposition over MetalCat ion -Exchanged Pau jasitesa

Temperature ( O C ) 330-380 360-390% conversion of CH,OH toGaseous product composition

gaseous pro ducts 51 30

(m l ) :CH2CH4C 2 H 4C 2H6C3H6C3HsC4Hsi -C4Hloi -CsHlzHexanesOther

Trace22.524.5

2 .710.8

4 . 48.0

13.96 .2

10.343.01 7 . 0

2 . 99.2

7 .84.0

4.3 2.02 .7 3.8

100.0 100.0- -

a

b9 . 46 w t % Zn.CLiquid product mainly unreacted CH,OH , H,O, and ( C H , ) *O.

Continuous-flow; 1 atm; 1.5 LHSV; samples analyzed at 2-3 hon stream.

The nonequilibrium distribution of tetramethylbenzenes fromZSM-5 has already been noted.tween 1,2,4,5-tetramethylbenzene durene) distributions in theAl, fractions among the intermediate and large pore zeolites,Table 1 2 . This demonstrates that the large pore zeolites per-m i t the attainment of equilibrium.

Comparison is here made be-

It will be recalled that durene

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HYDROCARBONS FROM METHANOL 3 3

75

50

ZSM-5lo-vo:L5

A 6 A?0

I S M - 4

75YO50 -

25 -

ISM - I I

0 A 6 A7 A8 A 9 AIO All

100

75

50

25

MORDENITE

F I G . 20. Aromatics distribution from methanol conversion overvarious zeolites [ 221.

is the bulkiest molecule that can diffuse readily through ZSM-5and ZSM-11.We have shown earlier that there is a large driving force for

methanol to form hexamethylbenzene in the presence of acid cat-alysts , Thermodynamic calculations [ 1721 reveal, however, that

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34 C H A N G

TABLE 1 1Critical Diameters of Selected Polymethylbenzenes [ 221

6 . 1

6 . 46 . 7

6 . 9

7 . 1

aEquilibrium diameters (rmin) estimated from Courtauld models.Kinetic diameter % 2 - 1 6r,h.

TABLE 12 [22]A ,, Shape- Selectivity of Various Zeolites

Catalyst %1245TMB inhydrocarbon %1 2 4 5TMB inA10ZSM - 5ZSM- 1 1

ZSM-4Mordenite

1 . 61 . 2

1 . 10 . 7

5 0 . 18 4 . 0

3 4 . 63 7 . 2

Equilibrium (37OOC) 33.4

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TABLE 13 [ 1651Coke Formation and Deactivation in DimethylEther Conversion, 35OOC

Residual Coke lhy dro -Catalyst activity (8) Wt%coke carbons X l o 3

HZSM-5 90 1.5 0 .3H Y 8 9 . 3 40H -erionite lchabazite 0.15 8.6 90H-mordenite 0 . 1 5 6.8 200

polymethylbenzenes formation must be a kinetically controlled phe-nomenon. Shape-selective zeolites such a s ZSM-5 preven t th e com-pletion of th is reaction as a consequence of st er ic hindrance .

Catalyst deactivation due to coke deposition is fairly rapid insmall pore and large pore zeolites. The intermediate pore zeo-lites such as ZSM-5, on the other hand, have a high coke toler-ance. Rollmann and Walsh [ 1641 have presented evidence thatcarbon formation in zeolites is a shape-selective reaction con-trolled by pore geometry.et al. [ 1651. Table 1 3 summarizes the res ult s obtained by thes eworkers on the conversion of dimethyl eth er over H-erionitelchabazite, HZSM-5, H Y , and H-mordenite. An inverse correla-tion is evident between deactivation r at e and coke selec tivity .

This has been confirmed by Cormerais

C . Nonzeolitic CatalystsAccording to Dolgov [ 5 6 1 , heating methanol in the presence

of 86%H,SO, at 135-140C gives 2 - 4 % ethene, with the balancedimethyl ethe r.ethene by decomposition over a catalyst composed of 39.1-43%A1 2 0 , + T i 0 2 , 0 . 8 3 - 0 . 9 3 % Fe203,0 . 1 5 - 0 . 3 % Mg, and a tr ace ofCuO deposited on clay pellets and calcined at 700-750OC. A t100-15O0C, 1-1.5 atm, and ~ 0 . 0 3 - W H S V , conversion was 85%and only ethene was detected in th e reactor effluent. The cat -alyst required air regeneration after 1 5 - 2 0 h .

Pearson [58] reported that when methanol or trimethyl phos-phate was heated with phosphorus pentoxide [ 61 andlor polyphos-phoric acid at 190C, a 36-39%yield of hydrocarbons was obtained

Matyushenskii and Freidlin [ 571 converted dimethyl ether toDownl

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36 CHANG

with the average formula C H2. over a range CnH2n+z to CnHZn-10.Apart from dimethyl et her , t he products contained alkanes, cyclo-alkanes, alkenes, substi tuted monoaromatics (maximal a t C 1 2 ) , andother compounds,dimethyl ether conversion to hydrocarbons in fused ZnC1, (283OC)is suppressed by cofeeding hydrogen. Methyl chloride was s i m i -larly converted to hydrocarbons in the fused salt.zinc iodide at 2 O O O C to hydrocarbons. The gasoline range ( C 5 -C15) fraction consisted mainly of branched paraffins, and con-tained a high percentage ( 49 . 7% ) of tr ip tane . Only small amountsof C,- hydrocarbons were formed and about 56% based on C feed)gas oil range (230-270OC) hydrocarbons were produced. Heavierfractions amounted to 2-3% based on C feed, with little solid resi-due.Supported aluminum sulfate was found by Hargis and Kehoe[611 to catalyze methanol and dimethyl ether conversion to hy-drocarbons.Kikkawa et al. [ 621 used supported aluminum dihydrogenphos-phate as catalyst to convert methanol into a predominantly olefinicproduct (20-608 yield of C,-C, olefins at 375-425OC).

Heteropolyacids and salts of heteropolyacids have been usedby On0 et al. 163, 641 to convert methanol. Typical da ta , ob-tained at 300C, 2 . 1 2 Xpressure are shown in Table 14 . The hydrocarbons were mostlyin the C 2 - C 5 range. Very little aromatics were formed.The resu lt s of Cormerais et al. [ 651 using silica-alumina cat-alyst will be discussed later in connection with reaction mechan-i s m s .

Bell and Chang [ 591 found that coke formation during methanol/

K i m et al. [SO] converted methanol ( > 9 9 %conversion) over bulk

mol/h feedrate, and atmospheric

IV. REACTION KINETICSRelatively little has been published to date on the kinetics ofhydrocarbon formation from methanol. Global kinetic treatments

are mainly found in the accessible li terature . However, in viewof the great complexity of the reaction, rigorous kinetic treat-ments may neither be practicable nor have much practical justi-fication. Further , t he large heat effects requ ire special consid-eration. This has usually been neglected in the published ex-perimental work,The kinetic scheme is simplified somewhat by th e observation[30, 66, 671 that over a wide range of conversions th e initialstep of ethe r formation is much more rapid than the subsequent

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TABLE 14 [63]Product Distribution of the Conversion ofMethanol into Hydrocarbons

CatalystHTPa CuTP AgTP HTSa CUTS AgTS

Product distribution, %:bMeOH 1 .3 2 . 1 0 3.9 4.7 0MeOMe 38.6 37.3 2.0 57.5 35.5 20.1Hydrocarbons 60.1 61.5 98.0 38.6 50.8 79.9

Hydrocarbon distribution, %:bCH4 3.7 5.2 9.0 1.6 7.3 3.2C2H4 11.3 9.5 9.0 10.3 1 1 . 2 10.3C 2 H 6 0.9 0.8 5.2 0.5 0. 5 1 . 2

8 6 8.3 8 .5 3.8 8.3 8.7 8.3C3H8 16.1 13.4 34.0 9.8 14.5 21.9c4 39.3 39.2 26.1 41.8 35.1 36.4c5 12.5 13.7 7.2 15.8 15.1 13.3c 6 7.9 9.7 5.7 11.9 7.6 5.4

a

bCalculated on a carbon-number basis.TP and TS indicate dodecatungstophosphate and dodecatung-

stosilicate, respectively.

olefin-forming step, and is essentially at equilibrium.trating this fact appear in Fig. 21.genate mixture can be conveniently treated as a single kineticspecies or "lump" [ 681 .anol over Z SM-5 [SO] indicate that the rate of oxygenate dis-appearance is first-order in oxygenates.scribed in some detail later.

Data illus-Thus the equilibrium oxy-

Data from a pilot plant study of gasoline synthesis from meth-This work will be de-

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0

H

F

CN

FOW

$MH

F

P

R

O

DMF

C

N

FOW

-}MDMEQLBA

DME

n

Q

FG.2CnonomhnandmhehtohdoabnoHZM-5

nmzd[

s

3O

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Chen and Reagan [ 691 discovered that the reaction is autocat -alytic over ZSM-5. They proposed the following scheme:

where A = oxygenates, B = olefins, and C = aromatics + paraffins.Neglecting k, at low conversions, experimental data were fittedaccording to the expression

giving characteristic sigmoidal cu rv es .alyst loadings and reactor configurations, confirming reactor iso-thermicity, and demonstrating that the result s were not du e toautothermal effects. Figure 23 shows data obtained with catalystsof different activity. A linear correlation between k, and intrinsicacid activity is indicated.Autocatalysis was confirmed by Ono et al. [70, 711. Theseworkers, however, proposed a sequence of two bimolecular re-actions :

Figure 22 shows the fit with data obtained using various cat-

where A and B ar e oxygenates and olefins, respectively.x = conversion of A and w = weight of catalysts, these result in

With

where c1 = k1/k2 and f3 = [Bl,/[A],. Figure 24 shows the fit ob-tained at three temperatures.[ 681 to include an additional bimolecular term due to homologationof olefins :

The kinetic scheme of Chen and Reagan was modified by Chang

k2A + B - C

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40 CHAN G

1o 1kl lk2=.02 k2-55

0.8 - 0 CATALYST (a ) -A CATALYST (b )0 CATALYST (c)VCAT AL YST (d)

0.6 - -0 4 - -

0.2 - -0

0 I I

(a) Reactor: &in.-o.d. copper; catalyst:10% zeolite diluted with 7 -alumina.(b) Reactor: Q-in.-0.d. copper; catalyst :1.4% zeolite diluted with M/50 copperpowder and y-alumina.(c) Reactor :A-in .-0.d . copper ;catalyst :3% zeolite diluted with 50/50 copperpowder and y-alumina.(d) Reactor: +in.-0.d. stainless steel ;catalyst : 10% zeolite diluted with?-alumina.

FIG, 22. Autocatalysis in methanol conversion (HZSM-5 at-alyst, 37OOC) 1691.

B + C k3 w CC A D

where A = oxygenate, B = (:CH,), C = olefins, and D = paraffins +aromatics.

The species B was assumed to have carbene-like character,attacking both oxygenates and olefins. Invoking the steady-

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HYDROCARBONS FROM METHANOL 4 17 . - I 7 I 1 1

0 CATALYST AA CATALYST B0 CATALYST C

FIG. 23 . Correlation between methanol rate constant k, andintrinsic catalyst acid activity [691 . For catalyst identifications,see Fig. 22.

state assumption on B and eliminating time, the following ex-pression resulted:

where u = C I A , K 1 = k, /kz , and k, = k+/k, . Data at th ree pre s-sures were correlated with this model and are shown in Fig. 25 .

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42 C H A N G

reaction timeh

FIG. 24. Hydrocarbon yield vs reaction time. Solid lines aretheoretical [ 7 11.Doelle and co-workers 1721 studied both sorption and reactionkinetics of methanol an d dimethyl eth er conversion over ZSM-5.

A t 115-2OO0C he kinetics of methanol conversion obeyed the ra telaw

k l P ~,OHr =H 01 + k2P

with

k, = 6 X l o 5 exp (-80 y p ) mol gesebarFor dimethyl ether, no effect of crystal size on product distribu-tion was observed, and the intracrystalline Thiele modulus wasless than unity for 0 . 5 pm crystals.

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HYDROCARBONS FROM METHANOL 4 3

oxygenate

F I G . 2 5 . Methanol reaction path. Solid lines are theoretical[681 .

The conversion of methanol over chabazite was investigated byAnthony et al. [ 731. Using curve-fitting techniques for modeldiscrimination, it was found that the data could be representedby

kloa3PAr =A ( 1 + k,oclPA)3where

1a = 1 + bt/SL

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44 C H A N G

accounts for catalyst deactivation and Sk is a selectivity functiondefined byS.. = a. + b.x. +11 1 1 1 c x.2i l

wheremoles of i produced for the j-th data point

moles of methanol reacted for the j- th data point.. =11xj = conversion of methanol fo r the j-th data pointThe kinetics of dimethyl ether conversion over HZSM-5 was

studied over the temperature range 227-300C by van den Berg1741. These results are discussed in the following section.

V. REACTION MECHANISMThe reaction path of acid-catalyzed hydrocarbon formation frommethanol may be viewed essentially as composed of th ree key steps:ether formation, initial C-C bond formation, and aromatization with

ti-transfer.The mechanism of the first two steps is discussed in this sec-

tion. The final stag es , comprising olefin condensation, cyclization ,and H-transfer over acidic catalysts, have been well studied andproceed via classical carbenium mechanisms. This topic will there-fore not be covered in detail here since it has been comprehensivelytrea ted in a number of excellent reviews [ 75-77]. In addition,papers by Galwey [781 on carbenium pathways in reactions of C 2 +alcohols on montmorillonite, and Dejaifve et al. [ 791 on methanoland olefins conversion over ZSM-5 are pertinent.be discussed since these compounds are peculiar to the methanoltransformation reaction.

The mechanism of polymethylbenzenes formation will, however,

A . Ether FormationThe mechanism of ether formation from alcohols over oxide cat-alysts, particularly Al2O3,has been extensively investigated. Thebulk of work has concentrated on alcohols having B-hydrogens.

Several comprehensive surveys have been published [8 0 , 81, 841 .In contrast , the lite rature on methanol dehydration is relativelysparse . Methanol etherification is similar in many respects to thatof the higher alcohols; however, since methanol lacks a parentolefin, sufficient differences may be found a s to warrant i ts dis -cussion here.

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HYDROCARBONS FROMMETHANOL 4 5~~ ~~~

The dehydration of methanol on alumina and amorphous silica-alumina was studied by Parera and Figoli 1821. The effect of aseries of nitrogenous poisons (various amines and N -heterocycles)or dimethyl ethe r formation was monitored. Silica-alumina was ir-reversibly poisoned, while alumina was reversibly poisoned. Thiswas taken as evidence that the reaction over alumina involved sur-face methoxy groups:

/A1\ /A1\ /A1\ P'\0 0This interpretation was basedas diethylamine dissociativelymina :

0on the assumption that bases suchbut reversibly chemisorb on alu-

EtI E t HI I

Because of the greater nucleophilicity of N vs 0 , the nitrogenousbase competes with methanol for the surface oxygen, thereby in-hibiting methoxylation. On the other hand, the reaction oversilica-alumina involves Bronsted sites, which are strongly poi-soned by nitrogen base, forming stable quate rnary ions. Ac-cording to these investigators ether formation over silica-alu -mina may be represented by the following scheme:

CH,OH + H + + CH,OH,+C H30H; + C H 3 0 H -+ (CH,),O + H 2 0 + H+Figueras et al. [83] studied t he kinetics of methanol dehydra -

tion over silica-alumina at 160-200C and derived th e rate exp re s-sion

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46 C H A N Gwhere Pa is the alcohol partial p res sure.the rate-limiting step cannot be the interaction of two surfacealcoholate groups, which would lead to a rate equation of theform

It was concluded that

r = ke2 = kaP/ ( l + aPI2At low pre ss ures th e reaction orde r would be uni ty , which is notin agreement with the observed half-ord er. Figueras et al. pro-posed two modes of chemisorption, both of which assumed theconcerted action of acidic and basic centers [ 8 5 - 8 8 1 . Nucleo-philic attack on carbon by the basic site would generate C H , + ,which then interac ts with an acid-generated surface methoxygroup [89] to give ethe r. The observed half-order was ration-alized on the ground that one of th e species, CH,O or C H , , mustbe reversibly adsorbed. Primary carbonium ions ar e much lessstable than alkoxide structures, and therefore equilibrium withthe undissociated species would be readily established :

H HHC-0-H/ \ .. -! * H I /////?-/$/,/////?/-, I ////

The thermal decomposition of methanol adsorbed on aluminawas investigated by Matsushima and White [go ] using deuteriumlabeling. Desorption of methanol sta rt ed at 77OC. At T > 77OC,significant concentrations of CH,O, H20, and CH,OCH, were ob-served, and above 427OC, CO was predominant while CH, wassignificant. Coadsorption of CD,OD and CH,OH was carried out .Under desorption at T < 237OC, the ether contained only C H , O C H , ,C H , O C D , , and CD,OCD,, while at higher temperatures deuteriumdistribution became random. Thermal decomposition of CH,OHadin the presence of gaseous C D , O C D , gave only C H , O C H , , indi-cating that ether is formed by a bimolecular reaction between ad-jacent surface methoxides.

Schmitz [ 911 studied th e dehydration of methanol over silica-alumina at 289-418OC and found that the reaction becomes first-order in methanol at the higher temperatures.Schmitz observed an induction period, possibly the resul t ofth e initial formation of surface carboxylate groups [ 8 4 ] , which,though not ether intermediates, could nevertheless influence theinitial ra te of dehydration by occupying active sites . Another

Interestingly,

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HYDROCARBONS FROM METHANOL 4 7explanation might involve competitive sorption of product waterprior to establishment of steady-state with respect to surface hy-droxyl concentration.Detrekoy and Kallo [ 921 investigated th e dehydration of meth-anol over clinoptilolite by infrared spectroscopy. Methanol ( 6torr) was adsorbed on H -clinoptilolite at various temperatures.At 25OC the 3620 cm- ' band (acidic OH on clinoptilolite) disap-pears and two bands at 2950 and 2840 cm-' (CH stretching) ap-pe ar . Upon evacuation, th e CH bande decreased significantly ;however, the OH band did not reappear. This indicated tha tonly weakly adsorbed methanol is removed. Absorption at 16OOCfollowed by evacuation caused only a partial disappearance of t he3620 cm-l band and a lower intensity of the 2950 and 2840 cm-Ibands, At 4OO0C, methanol can be completely removed and theCH bands are shifted to 2860 and 2960 cm-l, indicating the p res -ence of surface methoxyls.Detrekoy and Kallo found that methanol dehydration also oc-curs on dehydroxylated (at T > 400OC) clinoptilolite. They a t -tribute this to the formation of Bronsted sites from Lewis sitesby hydration dur ing reaction with methanol. The following mech-anism was proposed :

/ CH OH/Si, /O \ ,A /O\ / ,A\ 3

Further reaction would be catalyzed by the acidic OH groups thusgenerated.The dehydration kinetics was also investigated in this study.An Rideal-Eley mechanism was proposed.In a study of methanol reaction on synthetic germanic near-faujasite using I 3 C - N M R , Derouane et al. [931 observed partialmethoxylation of the sur face at 3OO0C, with ether formation.

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48 C H A N G

Surface methoxyls could be hydrolyzed with water back to meth-anol at 25OC. Additional small amounts of dimethyl ether and s u r -face formate were also found after hydrolysis.In summary, the weight of evidence favors th e intermediacy ofsurface alkoxyls in ether formation from alcohols. Beranek andKraus [ 941 conclude that the mechanism involves essentially anucleophilic substitution whereby the surface alkoxide is attackedby another molecule, either from the gas phase or from a weaklyadsorbed s ta te . Arguments in suppor t of th is mechanism weresummarized a s follows:1.

2 .3.

4 .

5 .

"Similar products are obtained b y th e decomposition of metalalkoxides containing no B-hydrogens and by the reaction ofcorresponding alcohols on alumina at lower temperatures" [ 951."Acetic acid and pyridine are poisons for the formation ofethers" [ 961 ."The different degrees of water inhibition on the eth er andolefin formation from ethanol on alumina, and the agreementof ether /ethylene selectivity ratios found experimentally withthose calculated by the Monte Carlo simulation of t he hyd ra tedsurface of alumina" [ 971 .ttCorrelation between the ra te of ether formation from ethanoland the surface concentration of ethoxide species. determinedby IR spectroscopy" [ 1601.'%The ositive value of th e Taft reaction parameter for th e for -mation of ether in contrast to negative values for the olefinformation on the same catalyst" [ 943 .

The, second pa rt of Statement 5 applies only to C 2 + alcohols.B . Hydrocarbon Formation

The mechanism of initial C-C bond formation from methanol isan unresolved question at present.abound in the literature, and run the gamut from carbene to freeradical schemes. A s of this writing, however, little supportingexperimental evidence has appeared. It seems appropriate, nev er-thele ss, to survey and discuss th e diverse en tri es in this t'mech-anism sweepstakes.acknowledged.

Hypothetical mechanisms

The speculation of the earliest workers [ 5, 61 has already been

1. Via Surface Alkoxyls

question were Topchieva and collaborators [ 91 in connection withAmong contemporary investigators, the first to consider this

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TABLE 15Decomposition of Methyl Derivatives [ 951

C H 3 0 H over ( C H 3 ) z O overA l ( O C H 3 ) s at Al,03 at 45OoC, A1203at 45OoC,Product 385OC (mol8) 0 .5 LHSV 0 .5 LHSVCH4 22 . 5H, 35.2

C2H4 1.3co 31.1C3H6 2 . 5(cH 3 ) 2 0 7.1Other 0 . 3

MajorMajorMajorMinorMinorMajorMinor

MajorMajorMajorMinorMinorMajorMinor

a study of the adsorption of methanol vapor on SiO,, SiO2-Al2O3,and Al,03 surfaces. It was found that a portion of th e methanolwas irreversibly adsorbed on SiO,-Al,O, and A1,0,, which uponheating and pumping to 4 O O O C evolved C,H,, C,H,, CO. and CO,.The formation of surface methoxy groups was regarde d as the pr i -mary step. Hydrocarbon formation was considered to occur bycondensation of methoxy groups, accompanied by dehydration andH- transfer. The mechanistic details of his condensation were notspecified. Subsequently, Heiba and Landis [951 showed that thethermolysis products of aluminum alkoxides ar e virtually identicalto the prod uc ts of alumina-cataly zed decomposition of alcohols orethers, as shown in Table 15. It is seen that the main productsare CH,, H,, CO, (CH,),O. and smaller amounts of C2H4 andC,H,. Based on the observation that Al(OCH,Q), decomposedmore readily than Al(OCH,),, it was concluded that cleavage ofthe C - 0 bond is a heterolytic process, with the flow of electronsin the direction of Al, leaving a positively charged carbon moietya s the reactive intermediate. A negative activation entropy wasobserved, suggesting a cyclic transition state. A fre e radicalmechanism was rejected on the basis of negligible reactivity overnonacidic solids, e.g., quartz, at temperatures up to 45OOC.Mg, and A1 and the compound Na[Al(OMe),] was rep or ted byPfeifer and Flora [981. Ethene was th e only hydrocarbon prod-u c ob served .

In a later stud y th e thermal decomposition of methoxides of N a ,

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50 C H A N G

2. Carbenes and CarbenoidsVenuto and Landis [ 751 proposed an a-elimination mechanismto account for olefins formed during methanol dehydration to di-

methyl ether over NaX at 26OoC, a s reported by Mattox 1991, an dfrom methanol reaction over Rex and ZnX at 330-390C, as ob-served by Schwart z and C i r i c [48]. According to th is view,methanol adsorbed on the zeolite surface loses water to form adivalent carbenoid species, which then polymerize to form ole-fins :

/3H-CH,-OH -+ H20 :CH,u

n:CH, + (CH,),, n = 2 , 3, 4, 5Swabb and Gates [ 531 studied the dehydra tion of methanol overH-mordenite at 155-240OC. Traces of olefin were de tected at240OC. It was speculated that the olefins were formed by ana-elimination mechanism, where bond scission is facilitated bycooperative action of acidic and basic sites in the zeolite lattice:

Basic Bronstedsite \

Salvador and Kladnig [ 1001 investigated the surface reactionsof methanol on HY and NaY at 20-350C using IR, GLC , adsorp-tion isotherm, and TGA techniques. A t room temperature, physi-cal adsorption occ urs on both zeolites. With HY, methoxylation ofsurface hydroxyl groups begins a t 2OoC, reaching a maximum atQ 130OC. A t 12OoC, dimethyl e th er formation begins and reaches amaximum a t 21OoC, an d around 25OOC secondary cracking react ionsoccu r forming predominantly butane and propene. This was ac -companied by darken ing of the catalys t due to coking which wasenhanced with a further temperature increase.Kladnig favored an a-elimination mechanism to explain olefin for-mation. They differed with th e acid-base mechanism of Swabband Gates 1531, proposing that the generation of carbenoid spe-cies occurs by decomposition of the methoxylated surface :

Salvador and

\JSiOCH, -+ - S O H + :C H,/ /

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HYDROCARBONS FROMMETHANOL 51Condensation of the carbene would give olefin.sumed to arise via H -transfer reactions.

An a-elimination mechanism involving a carbenoid intermediatewas also proposed by Chang and Silvestri 1 2 1 for methanol reac-tion over ZSM-5. However, it was considered unlikely th e olefinswere formed by polymerization of the diradical intermediate. Inview of the high reactivity of carbe ne s, the probability of such anevent would be low, as demonstrated in studies on ketene photoly-sis [ 1013. By the same token, the presence of free carbenes wouldbe unlikely. Rathe r, a concerted reaction between methylene donorand acceptor was proposed involving simultaneous a-elimination andsp3 insertion into methanol o r dimethyl ether a s the primary step .

Alkanes were as-

-+ CH,CH,OR' + R O H , R , R ' = H or alkylH

C H z

An ionic mechanism involving methyl cations was rejected sincethese species would be expected to form methane readily via hy-dride abstraction from methanol, dimethyl ether, or hydrocar-bons. The reaction

C H, + + CH,OH + &H,OH + CH,,is extremely rapid in the gas phase [102]. However, methanenormally accou t s for < l%f the hydrocarbons formed over ZSM-5.not observed over ZSM-5.Chang and Silvestri suggested that bond dissociation can beinduced by, or facilitated in the presence of the str ong electro-static field in the zeolite interior [ 103-1051 , the zeolite acting asa potent ionizing ''solvent . I 1 This model prompted a theoreticalstudy by Beran and Jiru [ 1061 of the influence of electrostaticfield s tr en gth on the reactivity of methanol in the zeolite cavity.The small size of th e methanol molecule permitted approximationof the zeolite field by a homogenous field. Computations weredone by the INDO method. Orientation of the methanol moleculewith respect to the external field is shown below:

Note also that 2 ,OH could deprotonate to formaldehyde, which is

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52 C H A N GUpon indreasing the field strength from 0-8 V/& the 0 - H , bondlength increases from 1.01 to 1.31 A, the C-0 bond is weakenedby stretching from 1.33 to 1.38-1.40 8 , while one of the hydro-gens moves toward 0 ( e .g . , 0 - H , decreases from 2.04 to 1.79 1).Further, with increasing field strength H, becomes more nega-tively charged while the methyl hydrogens become more positive.A similar calculation was performed on a pair of colliding methanolmolecules to simulate ethe r formation. Beran and J iru concludedthat at 2-4 V / 8 he formation of dimethyl eth er becomes possiblewhile at 4-8 V / 8 pecies such as CH,O and C H , can exist.

The electrostatic field gradient has been estimated to be 1-3V I A at 2.5-3.0 1 rom surface cations, and in zeolites such asZSM-5 can be much higher because of structural influences 1741.Pickert et al. [lo71 calculated a field strength of 6.3 VIA forpoints 2 1 rom an occupied S,, site in Cay, with SiIA1 = 2.The presence of small amounts of methyl ethyl et her in th eproducts of methanol conversion over ZSM-5 was reported byChang and Silvestri [21. Cormerais et al. [65] found MeOEtamong the products of dimethyl ethe r decomposition over silica-alumina at 423 K . This compound could either be a key reactionintermediate or simply a secondary product of the methanol-to-ethene reaction. From kinetic evidence, Cormerais et al. deducedthat this compound was not formed via reaction of Me,O withethene. It was determined that in the presence of excess ethene ,amounting to 30X that of th e products from Me,O, the rate ofMeOEt formation from Me,O was increased by only a factor of 5.More significantly, the formation of propene from MeOEt was morethan 10 times faster than from Me,O. In view of the observed un-reactivity of e thene, it was concluded that propene is formed bytwo successive C H , insertions into Me,O, leading to MeOPr, whichcleaves to propene. Another set of experiments gave analogous re-sults for C, hydrocarbons. Cormerais et al. [65, 1281 proposed achain growth "rake mechanism," Fig. 26, to explain their results.In this scheme, chain growth occurs by carbene insertion intosurface alkoxy species, which are transformed into olefins viacarbenium ions.Chang and Chu 11081 reported that when methanol is reactedover ZSM-5 in the presence of propane, t he usual high iso- to-nor-ma1 ratio of product butanes [ 21 is significantly lowered. This isshown in Table 16 where the butane i ln is seen to be 3.8 for thecontrol experiment and 1.1 when propane is added. The thermo-dynamic equilibrium i/n is 0 . 7 5 at the reaction conditions. However,under the same conditions propane and isobutane were virtually in-er t in the absence of methanol.l 3 C . 10% " C ) , it was found that the selectivity to singly-labeledbutanes was 30-45 imes higher than that expected from random

In the presence of l 3 C H 3 O H ( 9 0 %

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Me

PHASEPHASE

Me2

8O MeOEt C2H4 MeOPr C,H, M e O B u C4H,...4 1 4 1 A I

. . I t - I t I t - I t I t I t

F I C . 26. "Rake" mechanism for dimethyl ether conversion tohydrocarbons [ 651.

TABLE 16Effect of Propane on Methanol Conversion ov er HZSM-5a [ 1081

(37OoC, 1 atm, 0.4 h- ' L H S V ( C H , O H ) )Hydrocarbon product

( w t%P CH ,OH /He C H ,OH /C H8CH, 0.86 0.90C2O 0.93 0.90

(C,O)C,2- 3.63 4.53

c z 2 - 2.35 2.36-c -d

i-Cbo 35.17 23.14n-C,O 9.20 20.24C,2- 1.88 2.34C 5 + aliphatic 21.32 20.98Aromatics 24.66 24.62

100.00 100.00i /n c,' 3.8 1.1'Three moles CH 30H/1 mole diluent .bNormalized on a propane-free basis.CActual C = 25.36% of hydrocarbon.dNet conversion of C 3 % 5%.

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54 C H A N G

3 ti /n

0n = O I 2 3 4

I 3C"

FIG. 2 7 . Isolnormal ratio of -labeled bu tanes [ 1081 .

distribution. It was concluded that propane methylation had oc-curre d. Fu rth er , the butane il n increased with increasing I3 Csubstitution, as shown in Fig. 27 . The iln of the 13C,H, is %2 .8 ,and reflects the fact that they arise primarily from self-reactionof I 3 C H 3 0 H . The singly-substituted butanes, mostly the productof methylation, have iln % l , lose to the equilibrium value. Fromthese considerations it was deduced that the reactive C , interme-diate is carbene-like, and the mode of attack is insertion into ansp3 C-H bond. Such carbene insertions are indiscriminate in liq-uid phase, all C-H bonds of the su bs trate being subject to attackwith equal probability [ 1091 :

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( 7 5 % ) ( 2 5 % )although for a homogeneous gas-pha se reaction th er e is some evi-dence of discrimination in favor of secondary ove r primary C - Hbonds [ 1101. In the CH,OH /C ,H e reaction, carbene insertion willlead statistically to higher concentrations of n -bu tane relative toi-butane.oxonium ion according to an Olah-type mechanism (vide infra) , onthe oth er han d, will yield high i /n butane ra tios due to stabilityof ter tia ry carbenium ions in the transition sta te. In the methyla-tion of propene with the CH,F-SbF, complex in S0 ,ClF [ 1111, forexample, CH , is formed via H -transfer, and i-butane is the majoralkylation prod uct. The following pathway has been proposed toaccount for i-butane formation :

Attack by a cationic species such as C H 3 + or methyl

H

""'\f H - ?,C H 3 C H ,

SbF 6

The alternative to sp3 C -H insertion, namely C , addition to thedouble bond of propene genera ted from propane via dehydrogena-tion, also received considerat ion, Although propane itself islargely unreactive, it was believed possible tha t th e unfavorabledehydrogenation equilibrium could be displaced by a "drain-off"reaction involving addition of a highly reactive species across thedouble bond of propene . Carbene addi tion would yie ld, classically,methylcyclopropane [ 1091 . However, subse quen t protonation andrearrangement gives preferentially the tertiary butyl cation, whichwil l either yield 2-methylpropene via loss of a proton, or i-butanevia H-transfer. If the attacking ~ p e c ie swere cationic, branchedproducts would similarly result :

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56 C H A N G

C/ \C

RH I- -C-C + R ++Another pathway involving propylium ion a s an electrophile can

also be written:

but leads to the same conclusion. This alternative was thereforerejected as an explanation of th e observed resu lts. However, inthe case of reaction of methanol alone, these mechanisms, withpropene as an intermediate, can be invoked to explain the char-acteristic high i/n butane selectivity.In another attempt at "trapping" the reactive C , intermediate,Chang and Lang [1151 reacted methanol over HZSM-5 n the pres-ence of acetonitrile. I t was reasoned that a carbene would insertmainly into the C-H bonds and to a lesser exten t, add to theC S N group. A cationic intermediate, on the other hand, wouldattack the C Z N electron system exclusively, forming a nitriliumion [ 173- 751 :

++CH3 + C H , C = N -t CH3C=N-CH,Upon hydrolysis, N-substituted acetamides would resu lt . This

is a Ritter-type sequence [ 1761 . Carbene addition to the C G Ngroup would result in a highly reactive azirine intermediate t 1771,which would yield the same nitrilium ion in the presence of acidiccatalysts. Thus the sole presence in the final product of N-sub-stituted acetamides would be evidence of a cationic intermediacy(although the alternative of acetamide formation via reaction ofamines with acetic acid from acetonitrile hydrolysis cannot beruled ou t) . On the other hand , the presence of higher nitriles ,with or without the acetamides, must be taken a s a strong indi-cation of carbene involvement, It was also found that acetonitrile,

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itself stable over the temperature range of interest, served tomoderate the reaction by competing strongly for the acid sites.At 454OC an equimolar mixture of methanol and acetonitrilegave mostly acetic acid and methylamines (36%methanol conver-

sion). This is the result of acetonitrile hydrolysis and methanolamination. At 496OC ( 79% methanol conv ersion), hydroc arbo ns,CH,CH,CN, and N-substituted acetamides were in evidence, aswell as CH,COOH + CH,COOCH,, CH,CH,COOH, and methylamines.Upon raising the temperature to 538OC, the reactor effluent con-tained 16.3%hydrocarbons and 7 . 0% 0- and N-compounds (exclu-sive of unreacted acetonitrile ). These 0 - and N-compounds con-sisted of 12.8%CH,COOH + C H , C O O C H , , 4 6 . 6 % CH,CH,CN, 3 7 . 2 %N-methyl-w-aminonitriles, and 3 . 4 % higher esters, pyrroles, etc.The CH,CH,CN is taken a s evidence of carbene insert ion ,while the N -methyl-w-aminonitries may result from an analogousnitrenium insertion [ 1781 where the nitrenium ion is generatedfrom a methylamine precursor:

+R,R,R,N * R1R2N: + :R ,H

+ PI -H+RIRZN: + CH3 CN * R,R,N :CH,CN - lR ,NCH*CNThe reaction of methanol in the presence of zinc iodide at 2 O O O C

was reported by K im et al. [60] to yield highly branche d hydro-carbons. Particularly high selectivity to tripta ne was observed.They postulated that the intermediate is a carbene complexed withth e s al t, similar to th e Simmons-Smith reagent (CH,I, + Zn(Cu)) ,which reacts with alkenes to form cyclopropane derivatives viacarbene addition to the double bond.3. Oxonium Ions and Ylids

In their mechanism Chang and Silvestri left open the questionof stabilization of th e intermediate carbene in the transition state.A plausible resolution of this question may be found in th e mech-anism of van den Berg et al. [ 1121. In their view, dimethyl etherfrom methanol dehydration reacts with a Bronsted acid site to forma dimethyloxonium ion I , which reac ts with another molecule of di-methyl ether to form, via 11. and a fter elimination of methanol, atrimethyloxonium ion 111:

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58 C H A N G

H+

I

- C H 3 0 H/O \ /O\-,O\ -i A1

I1 I11

The critical step in their proposal is a Stevens-type intramolecu-lar rearrangement of the trimethyloxonium ion 111 to a methylethyloxonium ion V :

4 0

111 IV VN

VI V II

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HYDROCARBONS FROM METHANOL 59~-~~~ ~Structure V I , which is equivalent to the oxonium ylid (CH,),O+CH,-,is seen to contain the stabilized C , carbenoid species.tion of the carbenoid into the adjacent C - 0 leads to C -C bond for -mation. The formation of ylid depends on th e assumption tha t theconjugate basic sit es in ZSM-5 a re sufficiently st ro ng to induce po-larization of a C - H bond on a methyl group. A s indicated pre-viously, the ab initio field calculations of Beran and Jiru [ 1061lend support to this assumption.considered by van den Berg "741.onium ion is generated by hydride abstraction.presented the following scheme comparing the steps in the alkox-onium Route ( A ) and th e carboxonium Route ( B ) .

Cis-inser-

An alternate cationic mechanism involving carboxonium ions wasIn this variation the carbox-

Van den Berg

1H,-O-CH,+

+C H 3 0 C H z C H 3 + CHBOH,~~lCH,OH,+ + C,H, + CH,OH

Route A

H f

A 2 /

"i +C H 3 - 0 - C H , + H ,B21C H , CHOHB3 i- H , O C H ,4 +

CH,CH,OH + CHSOCH,B41 +CZH, + HZO + C H , O C H ,Route B

The overall gas phase entha lpies of reaction of the va rious st ep swere calculated and are shown in Table 1 7 .

How-ever, ab initio calculations [ 1131 indicate that although the 1-hydroxyethyl cation is more stable than the methoxymethyl cat -ion, the energy barrier separating the two is on the order of

A major difference is found between Steps A 2 and B2.

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60 CHANGTABLE 17 [32]

The Conversion of Methanol and Ethanol to Hydrocarbonsover ZSM-5 Class Zeolitesa

Methanol EthanolPercent age conversion 99+ 99Hydrocarbon distribution (wt8) :C 1 1 . 2 0.1c2 0.8 1.8

6.0 7 . 61 2 . 4 17.3

C 5+ nonaromatic 4 4 . 8 4 0 . 2Aromatics 34.8 33.0

Wt%durene in aromatics 58.8 0.9Wt%ethylbenzenesb in aromatics 0 . 6 4.1

T = 370 k l 0 C ; P = 50 atrn; LHSV = 1 . 2 h- l .bEt benzene, MeEt benzenes, diMeEt benzenes, and diEt ben-zenes.

260 kJJmo1,- assuming that t he rearrangement proceeds through th e0-protonated oxirane :

This is comparable to the expected energy ba rr ie r of Step A 2 .Step A 3 is highly exothermic and, in view of th e behavior of t heN-analogue [ 1141, is expected to have a low activation energy.Figure 28 is an energy diagram comparing Routes A and B. Itwas concluded that Route A, involving the trimethyloxonium ion,is favored.

According to van den Berg [ 741, kinetics of dimethyl etherreaction over HZSM-5 is zero-order at 227-300OC. Chang and

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