Oxidation of methanol on the surfaces of copper-silver alloys

Materials Chemistry 7 (1982) 331 - 346

OXIDATION O F METHANOL ON THE S U R F A C E S

O F COPPER-SILVER AL-

LOYS

D LAZAROV*, St MANEV*, K IVANOV** * Faculty o/ Chemtstry- Unzverstty o f SOFIA -Bulgarta ** Htgher Pedagog!cal Instttute- SHOUMEN - Bulgaria Received 2 November 1981, revised 23 March 1982 - The catalytic oxidation of methanol on copper, silver and copper salver alloys has been studied m a large range of temperature changes, alloy compositions, reaction gas maxture compos~tlons and flow rates A reaction mechamsm is proposed and the reaction conditions providing for h~hest catalytic activity and selectav~ty are outlined It is possible using the scheme, to explain why copper is not so statable as a catalyst as silver A correlation has been estabhshed between the phase dmgram of the blmetalhc copper-stlver system and its catalytic achwty Some copper-silver alloys prove statable to be used as industrial catalysts for the oxidation of methanol to formaldehyde Summary

INTRODUCTION It is well aknowledged that sliver and copper are the best metal catalysts for the partial oxidation o f methanol to formaldehyde 1- 2 The present paper commumcates some results o f a more elaborate investigation, which aims at the eluc~datlon o f the posslbd~tles for modlficatton and tmprovement o f the actw~ty and the selectivity o f sdver catalysts, or to the replacement o f sdver with other metals or metal oxides Data are received, concerning the activity o f bimetallic catalysts and the role o f the modifying agents At the same tune we try to study 0390-6035/82/030331-1652 0 0 / 0 Copyrlsht © 1982 by CENFOR S R L All rtghts of reproduction m any form reserved

332 the possl~ofl~hes for the production of catalysts with high activity and selectivity The oxldahon of methanol is studied on the surfaces of salver-copper catalysts m a large range of temperature changes, gas flow rates and reaction mixture composlhons Both metals belong to IB group of the periodic system and have stmdar electron configurations, hence certain simtlantles m thetr catalytic behav1our as well The two metals differ sBmfi¢,antly m their crystallograpluc chaxactenstlcs, leading to the exlstance of two regions of hmlted mlsclbdlty m the phase dmgram at small admixture contents, and an eutectlcum at 40% copper content Thus the better part of the catalyst samples are two phase systems The data wall allow to establish whether a correlation exists between the catalyhc behawour of this system and its phase diagram There are only two commumcat~ons a'4 studying the catalytic actlwty of copper-salver alloys for the oxidation of methanol, deahng however with catalysts w~th less than 5 % copper

EXPERIMENTAL The pure metals axe melted under argon atmosphere at 1500 K and mechanically ground to produce catalyst samples with particles of 1 5-2 mm diameter The catalytic activity Is measured m a flow equipment, d~splayed m Fig 1 High purity mtrogen 6 flows through drymg columns 2 and manostat 3, its flow

''' °

Ftg 1 - Scheme o f the experzmental apparatus

333 rate being controlled by rheometer 4 and valve 5 The gas is enriched m methanol vapours m the evaporating column 8 and achieves a definite composition at a fixed temperature m condensor 9 An air flow, passing through drying colunms 2', manostat 3', rheometer 4', and valve 5', is mtxed with the above gas flow, producing the reachon mixture of a known composition The reaction gas mxxture passes then through reaction cell 10, equipped with a heater and a therrnocouple, situated m the center of the catalyst sample The exxt gases are analyzed for the content of formaldehyde (the sulptude method s ), carbon monoxide and carbon dloxade (chromatographically) Methanol m the mltlal gas mtx ture Is quantitatively determined after ad~rphon on sdlcagel surface

RESULTS The effect of temperature, feedflow rate, oxygen to methanol ratio and al loy composition on the catalytic actw~ty is stud~ed. The expertments allow to determine the total degree of methanol oxidation, the partial degrees of oxidation to formaldehyde, CO and CO2, as well as the select~wty of the catalyst samples, eshmated as the ratlo between the forrqaldehyde obtained and the oxld_,zed methanol m per cents

Influence of the temperature on the catalytic activity Figures 2, 3 and 4 display the role of temperature changes on the cataly-tlc actwltles of copper, Sliver and a copper-salver alloy, containing 60 at % Cu The volume flow rate being one and the same, it is evident that the yields of the &fferent products are representative of the rates of their production The temperature dependence of the reachon rate confirms that the reaction is earned out in the transitional kmetlc-dlffus~on controlled range A comparison between the three dmgrams shows that silver enters the dfffusmn regime at a lowest temperature, dlsplaymg highest catalytic activity Copper has the lowest achvlty, the alloy exlubltmg an intermediate activity The results for the actw~ty of copper, plotted m F~g 5, are at a considerably lower volume flow rate Under these condlt~ons the copper catalyst allows diffusion controlled kinetics This is the cause for the pecuharitles m the changes of the rates of production of the different products Formaldehyde and CO2 y~elds pass through maximum values and the CO content rises exponentially w~th tem-

334

Cu

o-

o

w-- 9104~'

40 30 2O 10 0

6OO

?~

8OO

9OO T,K

Fzg 2 - Changes m the catalytzc activity o f copper as a f u n c t w n o f the temperature Volume flow r a t e - 9 1 0 -4 , h "1, 02 C H 3 O H = 0 . 3

6O

A 9 4-1 W= ~lO, h

c~ 50

40

2C 10

0

CO2

do

'

~o

'

do

'

T.K

~o

Fig 3 - Changes m thecatalytic actzvtty o f sdver as a function o f the temperature Volume flow r a t e - 9 104, h "1, 02 CH3OH=O 3

335

44

W=910h

~S

o--

50 4O 3O 2O 10 0

. 600

, 700

.

r-CO-

,-,

.

800

900

TK

Fig 4 - Changes zn the catalytzc actwzty o f 60% Cu-40% Ag alloy as a funct,on o f the temperature Volume flow rate - 9 104, h "l, 02 CH30H = 0.3

41 2 1~ '°[ W=10, Cu

~

/

l

7O 6O

4O 3C 20 10

T.K F~g 5 - Changes m the catalytw actwffy o f copper as a function o f the temperature. Volume flow r a t e - 2 1 0 4 , h "l, 02 C H 3 0 H = 0 3

336 Table 1 %

T=723K W= 2 104,h "t

T=823K W=2 104,h "1

T=873K W=2 104 ,h "1

T?%

Cu

Cu60Ag40

Cu

Ag

Cu6oAg4o

Cu

Ag

Cu6oA~4o

CH20 CO CO2 Total

31 9 49 1 32 0.9 200 82 55 1 58 2

47 3 18 66 55 7

532 84 191 807

550 52 45 647

539 19 45 605

440 162 229 831

611 27 46 684

503 34 63 600

Ag

T=873K W=2104 ,h "1 V 10 s , mol/s ml CH20 CO CO2 Total

~

52 110 34 196

167 07 13 18.7

T= 873K W=5 104 ,h "1

C~.160~40

Cu

Ag

Cu60Ag40

132 09 17 158

242 09 61 312

378 00 22 400

321 08 39 368

T= 873K W = 11.104,.h"I

Cu

Ag

Cu60Ag40

432 641 1.8 00 98 61 548 702

688 14 78 78.0 104 ,h-1

T=873K W = 5 104,h"i 02

T= 873K W= 5.104 , h"1 03

T=873KW=5 05

n%

Cu

Ag

Cu60A~40

Cu

Ag

Cu60Ag40

Cu

Ag

Cu6oAg4o

CH~O CO CO2 Total

381 94 69 544

463 02 19 484

423 03 17 443

412 88 138 638

574 04 43 621

5223 138 63 599

420 52 285 75.1

60 9 01 10 1 71 1

64 3 04 13 8 78.5

O2/CH3OH

perature, the total degree of converston reaching 100% A smular but less strongly expressed behavlour is observed with the other two catalyst samples (salver and salver-copper alloy) as well. This is ewdenced by the data in Table 1. Influence of the oxygen to methanol ratio on the catalytic activity Copper shows highest sensltmty towards this reaction parameter The total

337

Tei?3K

411

1@ 0'

Fig 6 - Dependence o f the catalytzc actzv~ty o f copper on the oxygen to meth anol ratzo Volume flow r a t e - 8 , 7 1 0 4 , h "1, T : 8 7 3 K

90 f Cu 8O 7O 6O ~0

401 :IC 2C 10 i

I

(~4

I

F~g 7 - Dependence o f the catalytw actlvzty o f copper on the oxygen to methanol ratzo Volume flow rate - 1 5104, h "l, T = 8 7 3 K

338 and the partial degrees of conversion are plotted m Figs 6 and 7 as a funchon of the oxygen-methanol ratio for two different values of the volume rate. In both cases the yields of formaldehyde depend shghtly on the oxygen to methanol ratio The yield of CO2 increases and the increase is higher at lower volume flow rates The CO yield is constant and low at high flow rates, stdl decreasing on raasmg the oxygen to methanol raho at lower flow rates On these grounds the value of O 2 CH3OH = 0 3 Is chosen for the study of the role of the other reaction parameters, since R prowdes constant formaldehyde yields and low reaction rates of the side rea'ctlons A smaflar behavlour is observed with pure silver and salver-copper alloy samples, the formaldehyde yields and selectlwty being shghtly tugher (Table 1)

Influence of the reaction mixture flow rate on the catalytic activity The methanol volume flow rate (the gas volume taken at 273 K) influences most s~gmficantly the results for the copper catalyst. The data, concerning the rates of production of CH20, CO and CO2 and of CH3OH oxidation are displayed m Figs 8 and 9 for two different temperatures At lower temperature the rates

4O

C

~

. . . . 20

30

40

~

IN)

70

K)

CO. gO

~

100

Fig 8 - Rate o [ CH20, CO and C02 production and o f C H 3 0 H ox~datzon {ks) on copper as a function o f the volume f l o w rate. T = 773 K, 02 . C H 3 0 H = 0 3

339

5O

//

40

3C

20

10

0

20

~

40

5O

60

7O

80

90.__100W~10

Fzg 9 - Rate o f CH20, CO and CO 2 productzon and o f CHaOHoxldatton (VsJ on copper as a f u n c t w n o f the volume flow rate T = 973 K, 02 CH3OH= 0 3

of CH20 and CO 2 production increase on rising the flow rate, the relative increase being the same for both products At higher temperature however the rate of CH20 praduct~on reaches a constant value and the rate of CO2 production retains its low temperature values The content of CO is however notably high, especlally at low values of the gas flow rate Pure salver and the alloy behave m a sunflar way, dlsplaymg higher actwltles and selectw~t~es (Table 1) It is noteworthy that the copper-stlver alloy samples investigated, show analogous behavmur to t t of sdver, rather than copper, with respect to the above reactmn parameters

Influence of the catalyst composition on the catalytic activity The degree of methanol oxidation and the selectivity are ploted as a function of the alloy composition m Fig. 10 The different curves refer to different temperature or flow rate values A certain correlation with the phase dmgram of the system can be estabhshed The eutectlcum composition displays the lowest catalytic activity throughout the whole temperature range" studied

340

~t,S,%

--9--

"

7O

l'° 50 40 30 20 10 O

,•, 10

20

4' • 30

6 40

,~** 50

7b

i

90

100

Cu.ot% Ftg 10 - Total degree o f ox~datzon ( ~ ) - curve:~ 1 ', 2', 3', 4' and selecttvtty (s) - curves 1, 2, 3, 4 o f the salver-copper alloys as a f u n c t i o n o f the copper content Curves 1 and 1' - T = 8 7 3 K, I4/=9 104 , h "1, curves 2 and 2 ' - T = 8 7 3 K, I¢= = 4 . 8 . 1 0 4 , h "l, curves 3 and 3 ' - T = 773 K, W = 5 104 , h "1, curves 4 and 4 ' T = 5 9 3 K, I ¢ = 5 104 , h "1

DISCUSSION The discussion of the present results is rendered difficult by the fact that the reaction is earned out m an intermediate region when the rates of gas diffusion and of the chemical reactions are commensurable This statement Is based on the observations tlmt the rate and degree of methanol oxldanon are influenced by the gasodynamics (F~gs 8 and 9 and Table l) of the process on one side On the other side the different catalysts vary in their activity, which is miprobable, assuming a totally diffusion controlled reaction kinetics. Carrying out the process under purely reaction kinetics controlled regmie is hampered by the extremely low reaction rate at low temperature. A totally diffusion controlled regime at h~h temperature is however unfavourable due to the low select~vity Working m the mtermedmte region renders certain advantages, allowing to study the effect of the reaction parameters on the different processes and to elucidate the mechantsm of the catalytic action

341 The oxidation of methanol ~s known to pass through many parallel and consecutlve reactions 1 2 3 4 5 6 7 8 9 10 11

12

CHaOH > CH20 + H 2 CHaOH + 0 502 > CH20 + H20 CHaOH + 0 502 > CO2 + 2H2 CHaOH + 02 - - ~ CO2 + H2 + H20 CH30H + 1 502 ----~ C02 + 2H20 CH20 + 0 502 > CO 2 + H 2 CH20+O 2 ~ CO 2 + H 2 0 CH20 + 0 502 ' CO + H 2 0 CH20 -----~ CO + H2 CHaOH ~ CO + 2H2 CO+0502 ' CO2 CHaOH + 0 502 -~ CO + H2 + H20

The contribution of any of these reactions to the total oxidation-reduction process is different and depends upon the m~tial reaction mtxture, the temperature, the gasodynamics of the system and the nature of the catalyst The reactmns can be combined m the following plot (F~g 11) Methanol participates m reactions of

v,

Ftg 11 - Reactzon scheme o f m e t h a n o l oxzdatton

342 two types-oxldatmn and dehydrogenation, leading to CH20 (1 and 2) and the side products CO (10 and 12) and CO2 (3, 4 and 5) The remaining reactions proceed consecutively, consuming the produced CH20 As a result, formaldehyde is either oxidized or dehydrogenated leading to CO and CO2 (6, 7, 8 and 9) The processes of diffusion participate m the total kinetics of the reaction as well - dlffusaon of the m~tml substances towards the catalyst surface with velocity V ° , the reaction product CH20 to the gas volume with velocxty V', the side products w~th velooty V" for CO and V"' for CO2 The present results wtll be discussed, assuming the scheme m Fig 11 and keepmg into consideration the fact that the rates of the chemical processes and that of diffusion are of the same order of magmtude We consider the model scheme d~splayed m Fig. 12 relevant to the correlation between the rate of methanol oxadat~on and the temperature Three separate kinetic regions can be dlscrmamated The chemical processes are rate ltmltmg m the first region (I). Tlus Is the temperature range where the s~de reactions 3, 4 and 5 proceed parallel to the mare processes 1 and 2 The ratio between the different products depends upon the relat~onslup between the rates of these processes The reaction rates on the other s~de are strongly dependent upon temperature and reaction n-axture composition

5/

"[.K I

Fzg 12 - Scheme o f the temperature dependence o f the rate o f methanol oxzdatzon I - r e a c t t o n kmetzcs controlled regzon, II - dzffuslon o f the mztzal compounds towards the catalyst surface ts rate hmttmg, I I I - dzffuszon o f the reaction products towards the gas volume zs rate hmztzng

343 The material balance proves m all cases that the oxygen consumed is less m quantity than what is needed, were the process proceeding along reactions 2 and 5 Tlus is an evidence for the considerable contribution o f the dehydrogenahon mechamsm accordmg to reaction 1 The absence o f noticeable CO quantities indicates that processes 8, 9, 10 and 12 can be neglected, the mare reactton products being formaldehyde, CO2 and H 2 The dlffusxon processes are rate-limiting m region II It should be noted that the parallel reactions enter this region at different temperatures, the differences bemg however comparatively small The rates of the processes are determmated by dlffusxon o f the reacting substances towards the catalyst surface This statement is based on the observation that the gasodynamlcs o f the system influences to an equal extent the quantlt~es of CH20 and CO 2 produced I e the mare reachon 1 and 2 and the s~de processes 3, 4 and 5 are equally affected by changes m the reaction mtxture flow rate (Fig 8) This is possible only m case that the processes are diffusion - hm~ted and the ratio between the mare and the side products remains equal to the value typical for the temperature and the oxygen to methanol ratio chosen Region III is also dfffusaon controlled, the diffusion of the reactaon products from the catalyst surface towards the gas volume being rate determining This concluslon is justified by the data m Fig 5 and Table 1 The yield o f formaldehyde suffers sharp decrease from a def'mlte value of the temperature on m the cases of lower gas flow rates The decrease can be attributed to an enhanced parhclpatlon o P C H 2 0 m the consecuttve reachons 6, 7, 8 and 9, this being posstble due to longer hfetmae o f C H 2 0 molecules on the catalyst surface This renders an mdtrect support to the suppos~tlon that C H 2 0 desorphon has turned the rate - hm~tmg step o f the process We have grounds to rule out the posslbdlty of CHzO yield decrease due to consecutlve reactions m the gas phase, ~ e a heterogeneous-homogeneous mechamsm of the process The data m Table 1 at 873 K and W = 2 104 h "1 showing the differences m product yields for the three catalysts, can be used as evidence These observations maplay that processes on the catalyst surface are dominating, the gas phase reactions having prachcally no consequence. In the case of copper, reaction 6, 7, 8 and 9 proceed more quickly than m the other cases, m this way the stay o f the main product proving most fatal on copper surfaces Ttus model 1sjustified by the data plot~:l m Fig 5, showmg a nearly exponentml rise m the produchon of CO w~th temperature, complementary to a decrease m the C H 2 0 and CO2 yields Th~s is the reason for the increase m the CH20 and CO2 yields m region I and 1I, as expected according to Fig 12, followed by a decrease m region III. The decrease m Fig 12 is not presented since it is assumed that the mare

344 product does not participate m consecutwe reaction Otherwise a decrease is to be expected, as shown m Fig 5 These data refer to a comparatively low volume flow rate when all diffusion processes are slow In addition this model agrees with the fact that practically the same level of oxtdatlon (about 62%) is achieved with all catalyst samples for the same values of the gas flow rate and a methanol to oxygen ratxo, wluch ts an evidence for one and the same rate lmamng step m all cases - dlffus~on (region II) The model proposed agrees with the experimental data It maphes the extstence of an mtermedmte region, charactertzed by a smooth transition from a region where the total rate of methanol oxidation ~s determined by the rates of the mare and the side parallel reactions to a diffusion controlled region, which can be provmonally divided rote two subreglons In the ftrst one the diffusion of the reagents towards the catalyst surface is rate determining step, m the second - the diffusion of the products to the gas volume The transition between the regions I and II requires a considerably large temperature interval, depending upon the nature of the catalyst and the reactton gas nuxture flow rate The more active the catalyst, the lower the temperature at crosspomt A The transttion from region II to III does not depend upon the catalyst but stall it ~s function of the gas flow tare Point B xs shifted to lower temperatures for lower gas flow rates However the nature of the catalyst can affect slgmficantly the temperature dependance of CH20 and CO2 yields m region III. According to the proposed model region II ~s appropriate to carry out the selective oxidation of methanol to CH20 It ~s a region where the rate of the mare process is high enough, the stde consecutive reactions proceeding sttll with a neghg~ble rate For an optimal technological regime, requtrmg a large region II, a catalyst of lugh act~wty should be selected This is an especially tmportant condition the satisfactory catalyst for th~s reaction should comply with The present mvest~gatlon shows that the principal deficiency of copper compared to stlver as catalysts lies not m the fact that copper displays lower selecttvlty towards CH20, but rather in ~ts lower actlwty, i e. crosspomt A is situated to the right m the model scheme, rendering region II very narrow Thus when a sufficiently lugh catalytic act~vzty of copper is reached, ensuring a diffusion controlled reaction, the system enters region III. The present invemgatlon of the catalytic, achvzty of copper-salver alloys shows It to be a function of the alloy composition Thas fact was rendered observable only by carrying out the reaction m the intermediate region. The data plotted m F~g 10 show a lower catalytic aet~wty of the 60% Ag composltton, compared to the other alloy composmons Tbas correlation can be observed throughout a large tern-

345 perature range We are reclined to ascribe th~s behavlour to the fine grams typical for the structure of an eutect~cum It as aknowledged that the smaller particles are characterized by different thermodynamxc properties and different mobility of their electrons compared to the bulk phase This result however is an unambiguous ev'~dence that the different phases influence each other and penetrate one into other, affecting thus their catalytic act~vaty The data for the temperature dependence of the catalytac actlwty (Fags 2, 3 and 4) and its dependence upon the alloy composltaon (Fig 10) show a prmclpal possabillty for a partial substitution of silver with copper The 60-80% Cu compositions are of mterest, allowmg a diffusion controlled reaction at temperatures close to those typical for the silver catalyst and providing selectwlty commensurable with that of silver

CONCLUSION The present investigation shows, The oxidation of methanol on Sliver, copper and copper-silver alloys proceeds in a transltaonal kmetac - dfffusaon controlled region for temperatures ranging from 600 to 980 K, gas flow rates from 1,5 104 to 12 104 h "I and oxygen to methanol ratio from 0 2 to 0 6 Two regions can be distinguished m the dfffusaon kmetacs In the first one a rate 1Lrmtmg step ~s the dfffusaon of the imtlal substances, m the second the diffusion of the reaction products The first region is favourable with respect to formaldehyde production, while the second allows for the side consecutwe processes to proceed to a measurable degree The products of the total oxidation are obtamed both as a result of the parallel reactions and the addataonal oxidataon of formaldehyde. Copper ~smore unapproprmte as a catalyst for the selective ox~dataon of methanol compared to silver not only because of ats lower activity for the mare reaction The prmc~pal reason lies m the fact that copper is characterized by a narrow temperature range for the first dfffusaon regaon A more stabdlzed working regtme is typical for the copper-silver alloys, thetr characteristics bemg closer to those of pure silver Thas amphes that copper catalysts, modified by comparatively small quantlt~es of silver, can display satisfactory actavitaes, selectwltles and stabdltles

346 A correlation is observed between the catalyst actlvatles and the phase diagram o f the copper-sliver system The eutectlcum composmon dxsplays the lowest activity

REFERENCES G J kova G C. MN

GOLODEZ - Heter Catal Oxtdatton o f Organtc Compounds., ed NauDumka, Klev, 1978 BOND - Catalysts by Metals, Acad Press, London, 1962 SHASHALEVITCH, A E OBRASZOW - Advances m Techmques and

Chemtcal Industrial Sciences J D ULINDER, R L TOLEFSON - Conn J Chem Eng, 52, 4, 518, 1974 I M KOLSHGOP, V A SINGER - Volumetrtc analys~s, v 1, p 272, Gostchmalzdat, Moskow, 1952