Methylamine synthesis over solid ..

JWRNAL OF

MOLECULAR YSIS YS IS Journal of Molecular Catalysis 93

1994) 337-355

Methylamine synthesis over solid acid catalysts: reaction kinetic measurements D.T. Chen 1*a,L. L. Zhang a, J.M. Kobe a, Chen Yi b, J.A. Dumesic a7* Department

Chemical Engmeenng, b Chemtshy

Department,

Unrversrty

W~scon.sm, Madwon, WI 53706

USA

h%wlJmg Unrverslty, NanJmg, Chma

Received 28 January 1994, accepted 3 June 1994

Abstract

Kmetlc studies were performed performed over slhca-alumma and acldlc zeohtes zeohtes to determme the effects of temperature (from 600 to 700 K) and reactant pressures ( < 80 Torr) on the rates of methylamme synthesis from methanol and ammonia Related kmehc studies were conducted of methylamme dlsproportlonahon dlsproportlonahon reactions, methanol dehydration, and reactions of methanol and dlmethyl ether with methylammes The results suggest that the reactive species for methylamme synthesis are adsorbed nitrogenous bases, while methoxy methoxy groups are reactive species for dlmethyl ether production Dlsproportlonahon reactions may be important pathways for the production of the higher-substituted ammes durmg methylamme synthesis The combination of rmcrocalonmetnc measurements, reported elsewhere, and kmetlc studies suggests that acid sites sites are required m these reactions for the strong adsorption of ammoma and methylammes, and weak adsorption sites are required to facilitate desorptlon of adsorbed amme species from the acid sites acid, Zeolttes Keywords Keywords Acid catalysts, Alumna, Kmetlcs, Methylamme, Slhca, Sohd acid,

1. Introduction

The synthesis of methylammes from methanol and ammoma over acid catalysts IS an important mdustnal mdust nal process This reaction may also be a useful probe of catalyst acldlty For example, m contrast to reactions such as hydrocarbon crackmg, the products of methylamme synthesis are limited to three substituted methylammes, water, wat er, and dlmethyl ether Furthermore, the small number of reactants and products allows for detailed studies studie s of of adsorption processes l] Methylamme Correspondmg author Current address 3M Corporation, St Paul, MN 55144. USA 0304-5102/94/$07 00 0 1994 Elsevler Scxence B V All nghts reserved SSDI0304-5102(94)00118-F

338

D T Chen et al /Journal

of Mo lecular Cat aly st s 93 (1994) 337-355

synthesis also proceeds without catalyst deacttvatton over a variety of sohd actd catalysts Most of the literature on methylamme synthesis relates to the goal of achieving high selectivity towards the lower-substituted ammes, dimethylamme (DMA) particular Mochida, et al studied a variety of metal oxide and zeohte catalysts and found that Lewis acid sites alone are not active for the reaction [ 21 They suggested that the highest selectivity to DMA was achieved over acid sites of medium strength In contrast, Weigert found that Bronsted acid sites associated with H-mordemte were not selective for the lower substituted ammes and that Na-mordemte was more selective, though less active [ 31 Shannon, et al 41 proposed that a combmation of strong and weak sues is necessary for high activity and selectivity, with weaker sites allowing for weaker adsorption of the methylammes and more facile methanol adsorption. These researchers suggested that Lewis acid sites may be associated with low DMA selectivity, and they performed studies directed towards the formulation of shape-selective catalysts for production of DMA 5-101 Small-pore catalysts such as ZK-5 61, rho 51, and enomte and chabazite [ 81 were found to have low selectivity towards trrmethylamme ( 50%) Similar work was conducted by Herrmann 1 l] and Segawa and Tachibana 121 The latter workers formulated dealummated H-mordemte catalyst with high DMA selectivity The primary focus of the aforementioned literature has been the development of highly selective catalysts, and the correspondmg kmetic data were collected at high conversions The present study was undertaken to probe the surface chemistry by collectmg kinetic data at low conversions, m the form of turnover frequencies, activation energies, and kmettc reaction orders In addition, we report catalytic activities for the followmg related reactions methylamme disproportronation reactions, reactions of methanol and dimethyl ether with arnmes, and methanol dehydration Finally, we suggest catalytic cycles that are consistent with adsorption spectroscopic, and kmetic data For our work we have chosen to study acidic silicon-alummum oxides, which have proven to be effective catalysts for methylamme synthesis and which are used commercially It is important to note that we have specifically avoided the study of shape-selective, small-pore zeohtes to msure that our kmetic data are not influenced by transport hmttattons In this respect, we have limited our studies to silica-alumma and the relatively large-pore zeohtes Y, mordemte, and ZSM-5

2. Experimental A flow-through apparatus operating at 1 atm pressure was used for reactio kmetics measurements The reactor was made from a Calon VCR bulkhead union with an inner diameter of 0 7 cm Gaseous reactants were fed to the reactor through mass-flow controllers (Hastings HFC) Typical flowrates were 580 cm3/mm for

D T Chen et al / oum al of Mol ecular Cat aly m 93 (1994) 337-355

339

Table 1 Catalyst acldx propemes Sample NaH-Y-33 NaH-Y-65 NaH-Y-82 H-mordemte H-ZSM5-13 H-ZSM5-35 H-ZSM5-225 sihca-alumma

Acid site density (pmol/g) 360 1000 1450 710 670 360 74 90 Brensted/200 total

’ Value estimated from .%/Al ratlo of zeohte

He and 7 cm3/mm for NH,. Methanol was fed to the system with a syrmge pump (Harvard Apparatus Model 11) , typically at 0 4 cm3 /h For methylamme synthesis reactlon studies, methanol inlet pressures were typically held between 1 5 and Torr, while ammonia feed pressures were typically m the range of 4 to 6 Torr Feed pressures were vaned as necessary to measure reaction orders or to conduct kmetlc studies of related reactlons (I methanol dehydration, methylamme dlsproportlonatlon reactions, and reactions of methanol and dlmethyl ether with methylamines) For these latter studies, methylammes were fed to the system through Nupro needle valve The entire system was made of stainless steel and heated to 423 IS Analyses of the feed and reactor effluent streams were performed using a gas chromatograph (Hewlett-Packard 5890A) with a packed column of 0 05 M/8% KOH washed Carbowax on Carbopack B Samples (20-75 mg) were first treated under vacuum for 1 h at 300,473,573, and 723 K, followed by calcmatlon under flowing oxygen (100 cm3/mm) at 723 The catalysts used m these kinetic studies are listed m Table 1 Three catalysts were prepared from NaH-Y zeohte (Lmde), with 33, 65, and 82% of the Na exchange cations replaced by protons, and these catalysts are designated as NaHY-33, NaH-Y-65, and NaH-Y-82, respectively H-mordemte was obtamed from Conteka Corporation. Two H-ZSM-5 catalysts were obtamed from T Degnan of Mob11 Corporation (Paulsboro) with &/Al ratios of 13 and 225, and these samples are designated as H-ZSM5-13 and H-ZSM5-225, respectively Another H-ZSM-5 catalyst, designated as H-ZSM5-35, had a G/Al ratio of 35 and was provided by W Haag of Mob11 Corporation (Prmceton) . The amorphous srhca-alummacatalyst was obtamed from Davison Corporation and had an alummum loading of 13 wt% The slhca catalyst was a fumed slllca obtamed from Cabot Corporation (grade EH5)

The acidities of the catalysts were assessed by infrared spectroscopic and mlcrocalonmetnc measurements of pyndme adsorption The detads of these measurements for the NaH-Y zeohtes [ 131, H-mordemte [ 13,141, and silica-alumina [ 151

340

D T Chen et al / oum al of Mo lecular Cat aly st s 93 (1994) 337-3S5

Table 2 Average enthalpy Base

changes of adsorptlon

on H-ZSM5-35

Proton

H-ZSM5-35

aftimty

AKlm.~

and H-mordemte

(all values are m kJ/mol) H-mordemte

AH,,,

AH,,,,

NH,

857 7

- 151

-69

- 158

-72

MMA DMA TMA

895 8 922 6 938 5

-204 - 245 - 155

-98 - 142 -71

-219 - 207 - 140

-91 -134 -66

CH,OH DME

773 6 807 9

-91

-42 -66

-87 -99

Hz0

723 8

-61

-60 -71

-75

have been described elsewhere We report m Table 1 the number of Bronsted acid sites and the total number of acid sites for those catalysts that also display Lewis acidity Further mlcrocalonmetnc measurements of the adsorptlon of ammonia, methylammes, methanol, dlmethyl ether, and water on H-ZSM-5 and H-mordemte catalysts [l] have been previously reported We summmze the latter data Table 2

3. Results 3

Methylamzne syrztheszs reactzons

Catalytic actlvltles and apparent activation energies for methylamme synthesis were determined for all catalysts, and reactlon orders m ammonia and methanol were measured for selected samples Turnover frequencies were calculated by normahzmg the rate measurements to the number of Brgnsted acid sites for each catalyst presented m Table 1 Slhca did not show measurable activity for methylamme synthesis or dlmethyl ether (DME) formation at the temperatures of this study Typical ammoma conversions were less than 1 O%, while methanol conversions were usually less than 20% For the actlvatlon-energy studies, ammonia conversions were as large as 6 3% for slhca-alumma and 2 4% for NaH-Y-65, while methanol conversions were 63% and 27% for the same catalysts Apparent actlvatlon energies were measured for production of each of the methylammes and DME, typically over a temperature range from 550 to 720 K Fig shows Arrhemus plots for rates of total methylamme synthesis for selected catalysts The rate of DME production was generally of the same order of magnitude as the rate of total methylamme production over all catalysts Examples of Arrhemus plots for DME production are presented m Fig Table 3 summanzes actlvatlon energies for production of the mdlvldual products of methylamme synthesis for the different catalysts The apparent actlvatlon energy

D T Chen et al /Journal

0’0013

Rg

1 Arrhemus

H-ZSM5-225

(

0014

I

plots for total methylamme ), and sdlca-alumma

of Molecular

..I

0 0015

production

Catalysrs 93 (1994) 337-355

..I

00016

on NaH-Y-33

.

00017 . .

341

00016

CO), NaH-Y-65

(V). H-ZSM5-35

( A ),

(+

for total methylamme formation varies from 11 to 26 kcal/mol, with an average value of 17 kcal/mol The average apparent actlvatlon energies for mono-, dl-, and tnmethylamme formation were 13, 20, and 33 5 kcal/mol, respectively DME formation proceeds with an apparent actlvatlon energy m the range from 9 to 25 kcal/mol, with an average value of 14 5 kcal/mol The dependence of total methylamme formation at 603 K on ammonia partial pressure IS shown m Fig 3 Amme production was first-order m ammonia over NaH-Y-65, H-ZSM5-13, and H-mordemte catalysts The ammonia kmetlc order was also equal to unity over H-ZSM5-35 at 623,673, and 723 K DME production over H-ZSM5-35, NaH-Y-65, and H-mordemte catalysts was inhibited by mcreasmg ammonia partial pressures, as shown m Fig

00013

0.0014

00016

0.0016

00017

l/r (K-‘) Rg 2 Arrhemus plots for total methylamme and slhca-alumma (+

producbon

on NaH-Y-33

(0).

00016

NaH-Y-65

) H-ZSM5-35

(

)

342

of Mo lecular Cata ly st s 93 (1994) 337-355

D T Chen et al /Journal

Table 3 Apparent activation ylamme formation

energtes

Sample

The total amme actwahon

energy IS the apparent activation energy for total meth-

Achvatlon energies (kcalimol) Total ammes

MMA

DMA

NaH-Y-33

123

109

B

NaH-Y-65 NaH-Y-82

26 3 177

119 17 I

24 0

H-ZSM5-13 H-ZSMS-35 H-ZSM5-225

109 124 167

78 12 0 159

176 173 a

sihca to alumma average

204 167

134 12 8

204 19 9

a For both of these catalysts,

DMA was produced only at the lughest temperature

an apparent activation energy for DMA productlon of total ammes IS thus larger than for MMA

TMA

DME 18

396

96 167 116 96 24

214 33 5 studled, I

109 145

,723 K

Therefore,

could not be calculated, and the acttvatron energy for productlon

The vartatron of total methylamme formatron on methanol pressure is dependent on the catalyst Methylamme formatron at 603 K over the H-mordemte catalyst displayed a postttve-order dependence on methanol at low pressures (below 6 Torr) but became zero order at higher pressures The sthca-alumma catalyst, at temperatures of 573,623, and 673 K, also exhibited a posmve reaction order ove the range of pressures studted These results are shown m Fig 5 The H-ZSM-5 catalysts demonstrated negative reaction orders m methanol at high pressures, as shown m Fig 6 for H-ZSM5-13 and H-ZSM5-35 The NaH-Y-65 catalyst also displayed a negative reaction order DME productron was generally first order

5000;...:...:...:...:...:...;

NH, Pressure (Torr) Rg 3 Dependence of methylammes production H-ZSM5-13 (0). and H-mordemte (0)

rate on ammonia partial pressure at 603 K for NaH-Y-65

(V),

D T Chen et al /Journal

800~-, 700-- ‘I\

20

NH, F?~ssur:~(Torr) Rg

Dependence

of dlmethyl ether producbon

and at 603 K for NaH-Y-65

343

. . . : . . . : . . . : . . . : . . .

900

Q) G, CTi;j

of Mo lecular Cat aly st s 93 (1994) 337-355

(V),

80

100

rate on ammonia partial pressure at 673 K for H-ZSM5-35

and H-mordemte

(A

(0)

methanol for all catalysts For example, Fig 7 shows DME production rate versus methanol partial pressure for NaH-Y-65, H-mordemte, H-ZSM5-13, and slhcaalumma Turnover frequencies for total methylamme formation durmg methylamme synthesis were calculated at methanol and ammoma inlet pressures of 1 8 and 5 4 Ton-, respectively These results are shown m Table 4 The turnover frequencies appear to vary by a factor of - 50 between zeohtes, while slhca-alumma shows slightly higher activity. 3

thy lamne dmproportlonat lon reactions

Kinetic studies were made of methylamme dlsproportlonatlon reactions over HZSM5-35, because these reactions may be important durmg methylamme synthesis . . . . ; . . . . 1 _ . .

350

300--

200 250:;

150

100

/

!-

. :j;+/-:d .‘“;“.”

.

.

.

I . . . . -

15

20

CH,OH Pressure (Torr) Rg 5 Dependence of methylammes and at 623 K for slhca-ahunma (+

production

rate on methanol patt~al pressure at 603 K for H-mordemte

(0)

344

D T Chen et al /Journal

g!

of Molecular

Caralysls 93 (1994) 337-355

2oo-

150

,,. 0

L_ “‘:‘“:“‘:‘“:“‘:..‘:...:...0 2 4 6

6

10

12

14

16

CH,OH Pressure (Torr) Fig 6 Dependence of methylammes and H-ZSM5-35 ( A

productlon

rate on methanol partml pressure at 603 K for H-ZSM5-13

(0)

from methanol and ammonia When monomethylamme (MMA) was passed over the catalyst at a pressure of 2 8 Torr, the pnmary products were NH3 and DMA The results also showed that DMA can dlsproportlonate further to tnmethylamme (TMA) and MMA at higher temperatures For example, 8% of the methylammes produced were TMA at 592 K, and the TMA selectlvlty increased to 17% at 700 K The apparent activation energy for dlsproportlonatlon 1s low, equal to 9 4 kcal/ mol for MMA consumption Importantly, the dlsproportlonatlon of MMA 1s rapid compared to the reaction between ammonia and methanol to form methylammes For example, the turnover frequency of dlsproportlonatlon 1s 2 3 and 7 1 ks- at temperatures of 592 and 699 K, respectively, and these rates are faster than the

20

15

&,OH

Pr&ure

(Torr)

Fig 7 Dependence of DME production rate on methanol partial pressure mordemte (0). H-ZSM5-13 (O), and sdlca-alurmnaat 623 K ( 4)

at 603 K for NaH-Y-65

(0).

H-

D T Chen et al /Journal

of Molecular Catalysis 93 (1994) 337- 355

345

Table 4 Methylamme respectively Sample

synthesis

reactton rates at 667 K Inlet pressures of ammoma

Turnover frequency

and methanol are 5 4 and 1 8 Torr,

Reactlon rate (pm01 g-’ ks-‘)

(ks-‘) NaH-Y-33 NaH-Y-65

0 089 049

32 490

NaH-Y-82 H-mordemte H-ZSM5-13

0 022 12 0 66

32 850 440

H-ZSM5-35 H-ZSM5-225

061 0 040

220 29

slhca-alumma

56

500

turnover frequency for methylamme synthesis on this catalyst at 667 K (0 61 ks- ‘) The dlsproportlonatlon of DMA was also studied At a partial pressure of DMA equal to 1 2 Ton; MMA and TMA were the predominant products, with small amounts of NH3 formed via secondary dlsproportlonatlon of MMA The rate of DMA dlsproportlonatlon 1s approximately twice as fast as MMA dlsproportlonatlon (the turnover frequency 1s equal to 4 4 ks- at 592 K and 12 4 ks- at 699 K) This reaction proceeds with an activation energy of 9 2 kcal/mol In these studies, the DMA conversion vaned from 13 to 59% 3

Reactions of methylammes

with methanol

Reactions of methanol with MMA and DMA were studied to evaluate the relative rates of dlsproportlonatlon and senes methylatlon processes Reaction condltlons were identical to those used for methylamme dlsproportlonatlon studies, except fo the addition of 2 9 Torr of methanol for the MMA reaction and 3 2 Torr of methanol for the DMA reaction It can be seen m Fig 8 that the addition of methanol to DMA over H-ZSM5-35 leads to a decrease m the amount of MMA formed from DMA, most hkely due to reaction between MMA and CH,OH to form DMA The rate of TMA production remains essentially unchanged with methanol addition Slmllar behavior was observed for the addition of methanol to MMA Speclfitally, addition of methanol decreased the rate of NH, formation, the rate of DMA production remained essentially unchanged, and there was a slight increase m the rate of TMA formation 34

Reactions of methylammes

and ammoma with DME

The reactlvltles of DME with NH3, MMA, and DMA were measured ZSM5-35 for comparison with the correspondmg reactlvltles of methanol

over HAmmo-

346

Chen et al /Journal

D

Q) = zo”

of Molecular Catalysrs 93 (1994) 337-355

2000--

:x00-00 ‘3 u) gS lOOO-U=

,OE ns

.

500-0

“‘:“‘:“‘:‘“:“‘:“’

840

880

880

700

Temperature (K) Fig 8 Influence of methanol on DMA disproportlonation (0) MMA and ( 0) TMA produced Sohd characters represent DMA dlsproportlonatlon results m the absence of methanol

ma and DME were passed over H-ZSM5-35 at pressures of 4 7 and 6 3 Tort-, respectively, to produce MMA, DMA, and CH,OH The results m Ftg 9 indicate that the apparent actrvatron energy for methanol formatton IS 16 6 kcal/mol, while the acttvatton energy for total methylamme formatron IS 21 7 kcal/mol, about 9 kcal/mol higher than for the correspondmg reaction between ammonia and methanol Addtttonal studies suggest that while methylamme formatron may have posmve-order dependence on DME at pressures below 15 Torr, the reaction becomes zero order m DME at higher pressure The effect of adding 3 Torr of DME to 3 Torr of MMA over H-ZSM5-35 1s negligible However, as Fig 10 shows, the addition of 3 4 Torr of DME to 1 2 Torr of DMA leads to a srgmficant reduction m the rate of MMA formatton and a smaller reduction m TMA formatton at higher temperatures This effect IS smaller than the effect of methanol addition to DMA looo’.........:.........:.........:.........; OS 32 K m

=o OQ, ‘3 ttj SE mz 2E a&

100-r \ 10-r k-my -

rMeOH

4-r

0’00140 ~~.-......:......“.:.... 00145

MMA

+

0 00150

hA

““:‘“’ 0 00155

0 00160

l/T (K-l) Fig 9 Arrhemus plots for the production of (0) methanol, ( A ) MMA, and ( + ) DMA from the reactlon of ammoma and dlmethyl ether over the H-ZSM5-35 catalyst

Chen et al /Journal

0

. .

of Mol ecular Cat aly st s 93 (1994) 337-355

: . . . ? . . . : . . . : . . . : . . 600

620

640

660

Temperature (K) Rg

10 Influence of dlmethyl

characters

ether on DMA dlsproportlonatlon

represent DMA dlsproportlonatlon

(0)

MMA and (0)

TMA produced

Sohd

results m the absence of DME

15 CH,& Fig 11 Dependence

of DME production

Pressure Eorr)

rate on methanol partial pressure, m the absence of ammes, at 603 K for

H-ZSM5-35

3 5 Methanol dehydration The dehydration

of methanol to DME was studled at 603 K by flowing CH30H at 1-15 Torr over H-ZSM5-35 The results of Fig 11 show that the reactlon IS first order with respect to methanol pressure The turnover frequency for methanol dehydration IS 2 6 ks- at a methanol pressure of 1 TOIT Using the data shown Fig 4, extrapolated to 1 Torr ammoma and 1 Torr methanol, the turnover frequency for methanol dehydration durmg methylamme synthesis over H-ZSM5-35 IS estlmated to be 0 22 ks- ’ This latter rate IS only 8 5% as fast as the rate of methanol dehydration m the absence of ammonia Methanol dehydration to DME also occurs slowly m the presence of ammoma on the NaH-Y-65 and H-mordemte catalysts, which exhlblt turnover frequencies of 0 074 and 0 75 ks- ‘, respectively

348

D T Chen et al /Journal

of Molecular Catalysts 93 (1994) 337-355

4. Discussion 4

Factors affectmg dmethyl

ether production

The rate of DME formation is inversely proportional to ammonia partial pressure for all catalysts studted This behavior mdicates that ammoma blocks surface species Involved m DME formatton Infrared spectroscopy reveals only methoxy (CH,O) species on the surface under conditions of methanol dehydration 11 therefore, methoxy species are the probable surface mtermediates involved m methanol dehydration to DME, and these species are blocked by adsorbed ammonia or ammes The rate of DME formation from methanol m the absence of ammoma is first order m methanol for all catalysts at 603 K Because the acid sites m the H-ZSM5 catalysts are widely separated, reactions between methoxy species on different acid sites are not probable Thus, methanol dehydration mvolves reaction of methoxy species with another form of methanol that 1s not associated specifically with an acid site, e physisorbed methanol Earlier calorimetric studies have suggested the presence of two sites, one strong and acidic, the other bemg a weaker adsorption site The first-order dependence of the rate of methanol dehydration on the methanol pressures can be explained if the acid sites are saturated with methoxy species (1 the surface concentration is independent of the methanol pressure) while the concentration of physisorbed methanol is linearly dependent on the methanol pressure In situ IR studies indicate that the surface coverage by methoxy species is high under reaction conditions for methanol dehydration The rate of methanol dehydration to form DME m the presence of ammonia is also first order m methanol Infrared spectroscopic studies show that the surface coverage by methoxy species is low under methylamme synthesis reaction conditions We speculate that the surface coverage by methoxy species under these conditions is primarily controlled by the pressure of ammonia or methylammes, and thus the fraction of sites populated by methoxy species is a weak function of methanol pressure The rate of methanol conversion to dimethyl ether is sigmficantly slower m the presence of ammoma or other ammes Accordmgly, this reaction pathway is suppressed under methylamme synthesis reaction condrtions This behavior is further evidence that methanol dehydration takes place through an mtermediate (e methoxy species) that is blocked by adsorbed ammonia and methylammes 4

ESfect of reactant pressures on reaction kmetlcs

Prior to descnbmg the factors controllmg the observed reaction kinetics, it is important to examme the possibihty of mass-transport hmitations The extent of mtraparticle limitations can be assessed by the dimensionless Damkohler number for diffusive transport, Rr C, D,, where R IS the rate of reaction per umt volume,

D T Chen et al /Joum al

of Mo lecular Cat aly m 93 (1994) 337-355

349

rP 1s the radms of the particle, C, 1s the reactant concentration at the surface of the particle, and D, 1s the effective dlffuslvlty [ 161 Mass-transport hmltatlons are negligible when the value of the Damkohler number IS less than unity Under methylammes synthesis condltlons, the value of this dlmenslonless group 1s approximately 0 06, suggestmg that the reaction kmetlcs observed m this study are controlled pnmardy by the surface chemistry Kinetic studies of methylamme synthesis show that the methylamme formation rate 1s first order with respect to ammoma pressure for reaction temperatures from 603 to 723 K This result indicates that the surface coverage by ammonia 1s low under methylamme synthesis condltlons This behavior 1s consistent with the results of m situ IR studies that show the surface to be predommantly covered with methylammes under methylamme synthesis condltlons The ongm for this behavior 1s that methylammes adsorb more strongly than ammoma (see Table 2) For example, the pressure of methylammes at the reactor outlet 1s typically on the order of 0 2 Torr, about 500 times lower than that of ammoma at the highest pressures studied However, the heat of adsorptlon of MMA 1s approximately 55 kJ/mol higher than that of ammonia, and the surface coverage of ammonia 1s expected to be 1% of the surface coverage of MMA at 623 K A result of this study 1s the understanding that the dependence of the methylamme synthesis rate on methanol pressure 1s rather complex H-ZSMJ and NaH-Y-65 catalysts show negative orders with respect to methanol, while H-mordemte and slhca-alumma show zero and posltlve methanol orders This kmetlc behavior 1s surpnsmg because methanol adsorption 1s relatively weak compared to the adsorption of ammonia and methylammes 11 (see Table 2)) and the surface coverage of adsorbed methanol on the acid sites 1s expected to be low compared to the strongly basic methylammes The observed kmetlc behavior can be explamed m the followmg manner When methanol reacts with adsorbed ammonia to form methylammes, a first-order dependence on methanol pressure would be expected d the acid sites are covered pnmardy by ammoma, this behavior 1s observed for several catalysts at low methanol pressures As the methanol pressure 1s mcreased and the rate of formation of methylammes increases, the reactlon products displace ammonia from the active sites (because methylammes adsorb more strongly than ammonia) The rate of methylamme synthesis decreases and approaches zero order m methanol The ongm for the negative reactlon orders with respect to methanol 1s not clear at present We suggest that this behavior may be related to the mhlbltlon by weakly adsorbed methanol of methylamme desorptlon from the acid sites, as dlscussed later m this paper 4

Formulation

of catalytic cycles

The results of mlcrocalonmetnc adsorption studies provide important mformatlon about the energetic stabilities of different molecules on the surface It can

350

D

Chen et aI /Joum aI

of Mo lecular Cat aly sis 93 (1994) 337-355

be seen m Table 2 that the heats of adsorption of the ammes on acid sites increase m the order NH, < MMA < DMA, m accord with the proton affinities of these molecules The heat of adsorption of TMA on acid sites 1s similar to that of NI-&, despite the fact that TMA has the highest gas-phase proton affinity; this behavior may be due to stenc factors Methanol adsorbs on acid sites with a heat of adsorption lower than that of any of the ammes Thus, as noted above, ammonia and methylamme coverages should be much higher than that of methanol The mlcrocalonmetnc studies also show that all catalysts possess sites on which the reactants and products of methylamme synthesis weakly adsorb with strengths ranging from about 50-140 kJ/mol These sites may be associated with hydrogen bonding or van der Waals mteractxons wlthm the catalyst mrcropores A cntlcal aspect m the formulation of catalytic cycles for methylamme synthesis reactions mvolves the possible role of surface methoxy species As noted above, we suggest that methoxy species are reactive mtermedlates m the dehydration of methanol to DME It IS less clear whether these species are mvolved m the synthesis of methylammes from methanol and ammonia We now present a general sequence of steps that descrrbes the essential aspects of methylamme synthesis and related catalytic processes The first steps m the sequence are adsorptlon and desorptlon of all reactants and products onto the weak, non-acidic sites of the catalyst, represented by # These steps are followed by migration of weakly adsorbed species to acid sites, represented as Bronsted acid sites, H , where * represents the zeohtlc lattice oxygen The dehydration of methanol to DME can be represented by the followmg two reaction steps mvolvmg methoxy species (CH; CH3 OH# + H * +CH;

+H,O#

(1)

CH,OH#+CH;

+DME#

(2)

+H*

The catalytic cycle for methanol dehydration thus consists of Steps 1 and 2. This reaction can exhibit first-order kinetics with respect to methanol pressure If the surface 1s saturated with methoxy species Also, this reaction will be suppressed the presence of ammonia or ammes, because these bases Interact more strongly with acid sites than do methoxy species Under methylamme synthesis reaction condltlons, the surface ~111 be saturated with DMA and MMA, and only a small fraction of the acid sites ~111 contam ammoma and methanol molecules Accordmgly, the probablllty of havmg adjacent adsorbed methanol and ammonia molecules 1s low, and, the rate of methylamme synthesis vta reaction between these adsorbed species would be negligible Therefore, we suggest that methylamme synthesis takes place by reaction of a strongly adsorbed species on an acid site with a second species that 1s weakly adsorbed on the catalyst Methylamme synthesis and dlsproportlonatlon reactions can be wntten with or without partlclpatlon of methoxy species Consider, for example, the following steps

D T Chen et al /Journal

NH,#+CH;

*HMMA*

+#

MMA# + CH; =HDMA* DMA# + CH; +HTMA*

of Molecular Catalysrs 93 (1994) 337-355

351

(3a)

+#

(3b)

+#

(3c)

A serious difficulty with these steps for MMA dlsproportlonatlon 1s that they cannot predict the observed effect of adding methanol to MMA over the acid sites, 1 e , the suppression of ammonia formation In particular, the NH3 produced m the reverse of Step 3a cannot readsorb on acid sites because these sites are covered by strongly adsorbed MMA species Accordmgly, adding methanol would not suppress the formatlon of ammonia durmg MMA dlsproportlonatron, instead, methanol would react with methoxy species m Step 2 to form DME In contrast to this predicted behavior, the expenmental data show that DME 1s not produced when methanol and MMA are passed over H-ZSM-5, and the pnmary effect of methanol 1s to suppress ammonia formation from MMA We suggest that the followmg types of steps are mvolved m methylamme synthesis and amme dlsproportlonatlon reactions MMA# + HMMA * =NH,#+HDMA*

(4a)

MMA# + HMMA **NH;

+DMA#

(4b)

DMA# + HDMA * =MMA#+HTMA*

(5a)

DMA# + HDMA * +HMMA’+TMA#

(5b)

NH, # + HTMA * = MMA# + HDMA

Pa)

NH, # + HTMA * +HMMA*

(6b)

+DMA#

TMA# + NH,* + MMA# + HDMA

(7a)

TMA# + NH; +HMMA*

(7b)

+DMA#

The advantages of these steps for explaining the expenmental data are that two acid sites need not be m close proximity for chemical reaction, and more weakly adsorbed species such as NH,* are placed directly on acid sites The physical basis for steps such as 4a and 4b 1s that the quaternary amme species associated with the acid sites may rotate on the surface such that dlsproportlonatlon of these species with weakly adsorbed species may lead to either of the reaction products becommg associated with the acid site The reaction scheme represented by Steps 4-7 1s also consistent with the expenmental result that the rates of MMA and DMA dlsproportlonatlon are an order of magnitude higher than the rates of methylamme synthesis from methanol and ammonia In particular, the pressures of MMA and DMA under the methylamme synthesis condltlons of this study are at least an order of magnitude lower than the pressures of MMA and DMA employed for studies of dlsproportlonatlon reactions The surface coverages by weakly adsorbed MMA and

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D T Chen et al /Journal

of

olecular Cat aly st s 93 (1994) 337-355

DMA are directly proportional to pressure, and the reactlon scheme represented by Steps 4-7 gives the proper dependence of the dlsproportlonatlon rate on pressure The reaction steps leading to methylamme synthesis and to the effects of adding methanol to MMA and DMA may be wntten as follow CH, OH# + NH;

=HMMA*

+H,O#

CH, OH# + HMMA * +HDMA* CH, OH# + HDMA * +HTMA*

+H,O# +H,O#

(8) (9) (10)

Finally, reaction steps must be included that allow for the reaction of dlmethyl ether with ammonia and the ammes We suggest the followmg analogous steps DME# + NH,’ = HMMA * + CH30H#

(11)

DME# + HMMA * +HDMA*

+CH,OH#

(12)

DME# + HDMA * *HTMA*

+CH30H#

(13)

The above reaction steps form catalytic cycles for the reactions of methanol, DME, and ammes, and these cycles are depicted graphically m Fig 12 Major points that must be satisfied by this scheme are that methylamme synthesis involves the formation and partlclpatlon of strongly adsorbed species, and the rate of methylamme synthesis 1s a rather complex function of the methanol pressure Furthermore, these kinetics must be achieved despite the fact that the surface coverage of the acid sites by methanol species must be low to be consistent with the mlcrocalonmetnc and IR spectroscopic results In the absence of weak sites, methylammes must desorb directly from the acid sites to the gas phase Because the heats of adsorption of MMA and DMA are at least 200 kJ/mol, the maximum rates of desorptlon are equal to 10h4 s-l at 623 K, assuming a normal value of the desorptlon preexponentlal factor equal to 1013 -’ This maximum rate 1s several orders of magnitude lower than the observed rate of methylamme dlsproportlonatton Furthermore, the activation energies of methylamme synthesis and related reactions are slgmficantly lower than the heats of MMA or DMA adsorption on the acid sites. However, the desorptlon of strongly adsorbed MMA and DMA onto weaker sites is much faster (e lo4 s- ‘>, and high turnover frequencies for MMA and DMA dlsproportlonatlon can be achieved Accordmgly, we suggest that acid sites combined with weak adsorption sites are critical for methylamme synthesis and related reactions, 1.e acid sites are required for the cntlcal reaction steps and weak sites facilitate desorptlon of strongly adsorbed ammes from the acid sites The complex reaction kinetics of methylamme synthesis with respect to the methanol pressure can be explamed by the proposed catalytic cycle m terms of the surface coverages by CH,OH# and CH; As noted above, vacant weak sites, #, are required to facilitate desorptlon of strongly adsorbed amme species Because the range of heats of adsorption for species on the weak sites 1s much narrower (ca 90

D T Chen et al /Journal

353

Molecular Catalysis 9 3 (1994) 337-355

DME#

MeOH#

Rg 12 Catalytrc cycles for methylamme synthesis and related reactlons Each box represents a reactive surface mtermedlate Verhcal hnes represent rcactlon paths, and mtersectmg honzontal bars separate reactants and products Arrows and shading refer to reaction rates under typical methylamme synthesis reactlon condmons dark, sohd lines refer to pnmary production pathways, hght, solid lines refer to a secondary pathways, and light, dashed lines refer to mmor pathways under reaction condltlons Arrows associated with honzontai bars indicate reversiblhtles of the steps

kJ/mol) than the range for heats of adsorption on the acid sites (ca 170 kJ/mol), the surface coverage of these weak sites by CH30H# species can become slgmficant at high methanol pressures [ l] Accordingly, these species can suppress amme desorptlon A direct effect of methanol on methylamme synthesis kmetlcs IS the formation of methoxy species m the presence of methanol For example, DME formation under methylamme synthesis reaction condltlons IS not mhlblted by methanol, and this observed behavior IS consistent with the suggestlon that methoxy species are reactive intermediates for this reaction However, while these species are involved m DME formation from methanol, surface methoxy species are not apparently involved m methylamme synthesis or dlsproportlonatlon steps Therefore, higher surface coverages by methoxy species lead to lower coverages by reactive amme species on the acid sites and to lower rates of methylamme synthesis

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T Chen et al /Journal

of Molecular Catalysts 93 (1994) 337-355

5. Conclusions

Mlcrocalonmetrrc and infrared spectroscopic studies, reported elsewhere, have been used m conJunctlon with the present kmetlc studies to probe the catalytic cycles involved m methylamme synthesis and related reactlons over acid catalysts The results of these studies have suggested the importance of both strong and weak adsorption sites m methylamme synthesis and related reactions The reaction orders with respect to ammonia pressure suggest that one of the reactive species IS strongly adsorbed, I adsorbed mtrogenous bases for the productlon of methylammes, or methoxy groups for the production of dlmethyl ether The reaction orders with respect to methanol pressure demonstrate more comphcated behavior, suggesting that other reactive species may be weakly adsorbed Amme species can react with both methanol and dlmethyl ether, although reactions with methanol are more favorable Dlsproportlonatlon reactions are fast, suggestmg that these may be important pathways for the production of the higher-substituted ammes durmg methylamme synthesis The combmatlon of mlcrocalonmetnc measurements and kinetic studies suggests that acid sites are required m these reactions for the strong adsorption of ammoma and methylammes, and weak adsorptton sites are required to facilitate desorptlon of adsorbed amme species from the acid sites

Acknowledgements

This work was supported by the Office of Basic Energy Sciences of the Department of Energy and through a Joint China-US Cooperative Research Grant admmlstered by the National Science Foundation Two of us (DTC and JMK) would like to thank the National Science Foundation for graduate fellowships We also wish to thank Randy Cortrrght at the Umverslty of Wlsconsm for valuable insight durmg the latter stages of this proJect

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