carbon monoxide chemisorption· characterization …

CARBON MONOXIDE CHEMISORPTION·CHARACTERIZATION AND

TESTING OF PREPARED NICKEL CATALYSTS FORAMINATION OF ETHANOL

Achmad Hanafi Setiawan

Research and Development Centre for Applied Chemistry-L1PIKompleks PUSPITEK, Serpong, Tangerang 15310.

ABSTRACT contoh katalis yang dibuat dengan cara impregnasi sangatlahrendah(2%), tetapi yang dihasilkan oleh katalis yang dibuatdengan pertukaran ion secara amonia cukup tinggi(20%). Kata-lis yang dibuat dengan penukar ion cara natrium hidroksldamempunyai dispersi yang masih rendah(6%). Hal ini diperkira-kan karena adanya pembentukan natrium nikel silikat. Aminasialkohol menggunakan katalis logam nikel telab teramati ber-langsung pada suhu 503 "K dengan gas metan sebagai hasilsampingnya. Katalis yang dibuat dengan cara penukar ionsecara amonia memberikan hasil amina yang paling tinggi(57%)meskipun aktifitas tiap m2 logam nikel(4 x 10-7) sama dengancontoh yang dibuat dengan cara impregnasi. Logam natriumyang masih tersisa pada katalis yang dibuat melalui penukaranion secara natrium hidroksida diperkirakan meracuni keaktifankatalis pada reaksi aminasi dari etanol.

Three types of nickel metal catalyst supported on silica gelhave been prepared using impregnation, ion exchanged sodiumhydroxide and ion exchanged ammonia methods, in order toinvestigate the influence of preparation methods on metaldispersion and their activities for amination of ethanol. All thecatalyst samples had a nominal nickel loading of 5 % (wlw). Afterpreparation, the catalysts were activated by a drying stagefollowed by calcination and reduction. The result of transferringthe nickel salt to the support phase has shown that theimpregnation method was the most efficient, and the least efficientbeing the ion exchanged sodium hydroxide method. The resultsfrom the carbon monoxide chemisorption studies showed thatthe nickel is poorly dispersed(2%) in the impregnated sample,and highly dispersed(20%) in the ion exchanged sample preparedby the ammonia method. The dispersion was lower(6%) for thecatalyst prepared by the sodium hydroxide method due to theformation of sodium nickel silicates which were difficult toreduce. The amination of ethanol over these catalysts was foundto take place at 503 OK with methane being formed as a by-product. The ion exchanged sample prepared by the ammoniamethod gave the highest yield(57%) of ethylamine although itsspecific activitylm/ nickel metal(4 x 10-7) was similar to thesample prepared by the impregnation method. The retainedsodium in the ion exchanged sodium hydroxide catalyst poisonedmetal sites for ethanol aminatlon.

INTRODUCTION

It has been well documented that the ability of asubstance to act as a catalyst in a specific system dependson its chemical nature i.e. the specific chemical propertiesof the surface of the catalyst(l) where its structure islargely determined by the technique of preparation(2). Thepreparation techniques involve contact of the support witha solution containing a soluble saIt of the metal beingadded. The impregnation technique involves filling thepores of the support with the metal salt solution (3,4) andthe ion exchange technique involves exchange of the ionsin the salt solution with acidic protons on the supportsurface(3,5).

The surface structure and chemical state at the surfaceis determined from the dispersion of the active componentand its coordination geometry (6). Chemisorption of carbonmonoxide has been commonly used to probe the surfacetransition metal ion. It has been found that carbonmonoxideadsorbed onto the surface of nickel metal in a linear andbridge forms. The former is generally more intense in the'spectrum than the latter (6,7).

For catalysis to proceed, there must be some surfaceinteraction between the catalyst and the reactant but thisinteraction must not permanently change the chemical

INTISARI

Tiga jenis katalis logam nikel dalam silika gel telab dibuatmenggunakan metoda impregnasi, pertukaran ion secara nat-rium hidroksida dan secara amonia untuk meneliti pengaruhperbedaan cara pembuatan pada dispersi logam dan aktifitasnyadidalam reaksi aminasi dari etanol. Semua contoh katalismempunyai kadar nominal nikelS % berat. Setelah pembuatan,katalis tersebut kemudlan diaktifkan dengan pemanasan padasuhu rendah dan diikuti dengan proses kalsinasi dan prosesreduksi. Hasil proses pemindahan garam nikel kedalam suportmenunjukan bahwa cara impregnasi merupakan cara yangpaling efisien sedangkan cara pertukaran ion - natrium hidrok-sida kurang efisien. Dari hasil percobaan penyerapan gascarbon monoksida terlihat bahwa dispersi logam nikel pada

JKTI, VOL. 4 - No.2, Desember, 19946

nature of the catalyst/S), Therefore, in catalytic testing foramination of ethanol, the catalytic activity of the preparedcatalysts is determined for a model reaction to obtaininformation on catalyst performance for the amine pro-ducts. Aliphatic amines may be prepared by aminatingalcohols, aldehydes and ketones in the presence of ammo-nia and hydrogen over a suitable catalyst at elevatedtemperatures and pressures (9). Silver, copper and nickelsupported catalysts have resulted in a good selectivity foramines (10) forming primary, secondary and tertiaryamines preferentially and hydrocarbons, nitriles and ethersas by products. Optimum conditions for the dehydro-amination reaction depend on the type of reactor used andthe most important parameters which influence the opti-mum condition are type of catalyst used, reaction tempe-rature, molar ratio and total pressure (2). Supported coppercatalysts containing 10-65 % weight copper showed a highyield for the dehydroamination of aliphatic alcohols (11).Silica supported materials have been used in this reaction.The support itself showed no dehydroamination activity.The reaction mechanisms proposed by many workers (9,12, 13) are specific to a particular catalyst. Sharratt (14)has proposed the amination mechanisms of ethanol using a10 % nickel supported on silica catalyst prepared byimpregnation. He suggested that CH3-CH=* was the inter-mediate species in the reaction which was formed by thedehydration of ethanol.

The aim of this work is to obtain information of therelationship among the three preparation routes (impreg-nation, ion exchanged sodium hydroxide and ion ex-changed ammonia methods) of supported nickel catalyst,their dispersion, nickel metal surface area and catalysisperformance for the amination of ethanol.

EXPERIMENTAL

Materials

Nickel(II)nitrate hexahydrate, (Jansen Chimica), andsilica gel, Gasil 35, (CrosfieJd Chemicals), were used asthe starting materials. The impurity levels of Gasil 35 are1000-2000 ppm Al; less than 100 ppm of Mg,Ca and K;10-100ppm of Fe,Cu, Zn, Sr, Ba, P, S, Cl and F. The silicahas a surface area of 300 m2 s'. with mean pore diameterof 16 nm and mean pore volume of 1.55 cm-g! (15).

Helium(BOC Ltd) was used as the carrier gas for gaschromatography; Hydrogen(BOC Ltd) was used to reducethe catalyst and as the gas supply for the flame ionisationdetector(FID); Air(BOC Ltd) was also used for FlDanalysis; 5 % ammonia in hydrogen (G&E, Union car-bide Ltd) and ethanol (BDH) were used as reactants.

Catalyst preparationNickel supported on silica catalyst precursors were

prepared using either impregnation or ion exchange


method. Ion exchanged samples were prepared using eithersodium hydroxide or ammonium hydroxide asjae base.For the impregnation, silica gel (10 g) was mixed withdistilled water to make a homogeneous slurry and 30 mlof 0.28 M nickel salt solution in water was added to theslurry. The mixture was stirred for 30 minutes and wasthen left standing for about 120 minutes to allow bothmaterials to reach equilibrium. Excess water was thenremoved on a rotary evaporator and the catalyst precursorwas dried in an oven at 38~ OK Finally, the precursorwas calcined at 673 OK for four hours in a furnace. In theion exchange sodium hydroxide method, the ion exchangecolumn was prepared first by pouring a slurry (a mixtureof one part silica gel and two parts water) along the wallof the column allowing the particles to settle forming theion exchange bed. During this stage water was constantlypassed through the column with the excess solution beingdrained off at 1-2 ml per minute. The column was thentreated with dilute sodium hydroxide solution (pH about9) about 30 minutes and washed with distilled water untilthe filtrate neared neutrality. Afterwards, the ion exchangecolumn was eluted with 30 ml of a 0.28 M nickel saltsolution in water as used in the impregnation method andthe excess solution was drained off at a flow rate of 1- 2 mlper minute. The column was then washed with distilledwater to remove all the free metal salt. The solid obtainedwas dried at 383 OK overnight. Finally, the dried samplewas calcined at 673 OK in a furnace for four hours. In theion exchange ammonia method, 0.28 M nickel hexaaminesolution was prepared by dissolving nickel(lI)nitratehexahydrate in 30 ml of 3 M ammonia solution. This nickelsalt solution was then passed through the ion exchange bedwhich was prepared as in the sodium hydroxide techniqueand the excess solution was again drained off at 1-2 ml perminute. Next, the column was washed with distilled waterto remove all free metal salt. The solid precursor wasdried overnight at 383 OK and was then calcined at 673 OKin a furnace for four hours.

Chemical analysis of the precursors

The bulk composition of the catalyst was investigatedusing Perkin-Elmer M 1100, atomic absorption spectro-photometer. The sample preparation and calculation des-cribed by Mile et.al.(15) were used.

The percentage of sodium retained in the ion ex-changed sample using the sodium hydroxide method wasdetermined by flame photometry using a Corning 410flame photometer.

Carbon monoxide chemisorption

The apparatus used is shown schematically in Figure 1.The operation procedure and the results calculation des-cribed by Brooks and Christopher(16) were used.

7

Wt£,lTS10NEBRIDGE

OU1ln PIPE

RECORDER

turned on to the circulating mode. The gaseous mixture wascirculated at 15ml/min for 15 minutes, whilst the gaschromatograph was prepared for use and the catalyst-bedheated to the reaction temperature of 503 OK in helium.The reaction mixture was then passed over the catalyst for20 minutes. 100~ samples were withdrawn and ana lysedby gas chromatograph.

Figure 1: Schematic diagram of chemisorption apparatus.

The amination of ethanolThe initial part of this study was concerned with the

design, construction and calibration of a microreactoroperating at a temperature of 383 - 573 "K.; pressure ofapproximately 20 psi and a contact time of 20 minutes. Amicrocatalytic reactor system coupled to a gas chroma-tograph which its design was explained by Sharratt(14),was used throughout this study as illustrated in Figure 2.

RESULTS AND DISCUSSION

The results of the nickel and sodium content of thecatalyst precursors are given in Table 1.

Table 1: The nickel and sodium content (% w/w) in solidcatalyst precursors measured by atomicabsorption spectroscopy and flame photometryrespectively.

Preparation Method % Nickel % Sodium

Furnace/

REAcr,OR-- ----- --;;'PASS, ', I: :!...___ _ I

Impregnation 5.29Ion exchangedsodium hydroxide 3.37Ion exchanged ammonia 4.78

0.09

_ PERISTALTIC PUMP

• INJECTION PORT(!) PRESSURE Gt,UGE••••• 2-WA'11AP

'r ,·\lA"A.

,. NEEOU VALVE

MANUAL GAS-~SV ~".LE VALVE

VENl

It can be seen from the table that the impregnationtechnique was more efficient than the ion exchange intransferring the nickel salt to the support phase. This mightbe due to the step of washing process which present onlyin the ion exchange technique where the free nickel saltwas removed from the precursor. The sodium hydroxideion exchanged technique was the least efficient where 0.09% of the sodium was left in the precursor.

The results of estimating of the total nickel metalsurface area and dispersion for each catalyst are shown inTables 2 and 3.

Figure 2 : Schematic diagram of the apparatus for aminationreaction.

Preparation of samples: 0.2 g of the press-calcinedimpregnated and ion exchanged catalysts were loadedinto the 6 mm reactor used for catalyst testing. The catalystwas initially purged with He gas at 25 ml/min for onehour prior to reduction in hydrogen at 25 ml/min, pro-grammed from room temperature to its reduction tem-perature. The temperature was maintained for three hoursand then the catalyst was cooled in flowing hydrogen.The reactor was filled with 20 psi of hydrogen before itwas by-passed. .

Test reaction: The line was heated to 3830 K and sweptwith pure hydrogen several times. It was then filled with5 % of ammonia in hydrogen to 20 psi. 100 ul, pureethanol was then injected into the septum and the line

8

Table 2: Total nickel surface area of impregnated and ionexchanged samples based on the chemisorptionof carbon monoxide.

Samples Total nickel metal surface aream2/g

Impregnated

Ion exchangedsodium hydroxide method

Ion exchangedammonia method

14.32

38.01

131.24

JKTI, VOL 4 - No.2, Desember, 1994

Table 3: The percentage nickel dispersion of impregnatedand ion exchanged samples based on thechemisorption of carbon monoxide.

Samples % DispersionXm=1 Xm=2

Impregnated 2.15 1.07

Ion exchangedsodium hydroxide method 5.71 2.85

Ion exchangedammonia method 19.70 9.85

Note: Xm is the average number of surface nickel metal sitesassociated with the adsorption of one or two carbonmonoxide molecules.

The ion exchanged samples have both higher totalnickel metal surface areas and dispersion percentage thanthe impregnated sample. With ion exchange technique, theions of the metal precursor were strongly bound to thecarrier and could be expected to be atomically dispersed onthe surface. Little migration would then be expected tooccur during drying and calcination. In the case of impreg-nation technique, the nickel(lI) ion adsorbed is weakly heldby the carrier, so that migration and aggregate formationmay readily take place during drying and calcination. Thepoor dispersion of the ion exchanged sample prepared bythe sodium hydroxide method may be due to sodiumpoisoning of the metal sites (17).

The results of blank experiment in which ethanol,hydrogen and ammonia were circulated through the peris-taltic pump in the absence of the catalyst, are shown inFigure 3.

8.---------------------------------~.

oD6~

)(

••eo'0E

4s5ia:•...z1&1 2

~

o+-----.-----~----._----~----~----~o 10 20 50

TIME (minutK)

Figure 3: Ethanol concentration against the circulated times.


The highest ethanol concentration was obtained fromthe system when the ethanol was circulated for between 10and 30 minutes. As a result of this study, the reactantswere circulated for 10 minutes before being admitted to thecatalyst and 20 minutes circulating over the catalyst. Thisis in agreement with Sharatt's work who found that thesilicon tubing absorbed both some ammonia and ethanol ata certain time, although the circulating reactant mixturesfor 26 minutes over a 1 % silica supported nickel catalystprepared by impregnation method gave a 61 % conversionof ethanol to ethyl amine (14).

The studies of ethanol amination in the temperaturerange of 423 to 573 OK over the reduced catalyst preparedby impregnation method (at 16 x 10-4 moles of ethanol asthe initial concentration) are shown in Figure 4.

I Figure 4: Concentration of methane ( ~ ) and ethylamine ( ss )produced against reaction temperature of the amina-tion of ethanol over the catalyst prepared by impreg-nation method (with reaction mixtures containing 16 x10-4moles of ethanol).

60

At 503 OK the ethyl amine was formed, however itdecreased as the temperature was further increased: Thisobservation was also reported by Baiker et.al. (2) whocalculated that the dehydroamination of octanol withdimethylamine in the gas phase was an exothermic re-action. On the basis of these results, the ethanol aminationreaction was carried out at 503 OK and the experiment wasrepeated several times to check for consistency.

The reduced ion exchanged catalysts did not form anyethylamine after the standard reduction in hydrogen gas at673 OK for three hours. These catalysts were thereforereduced at the higher temperature of 773 "K,

A further blank experiment was carried out in whichethanol in a hydrogen atmosphere was passed over theimpregnated catalyst at 503 OK in the absence of ammonia.

9

The results in Figure 5 shows that the catalyst formed onlymethane presumably by the cracking of ethanol.

<5c0£;•.

,;.;

~z~ •.II! e

0

~s:0;

u E'"•...'"00u:

o 25RETENTICtI mE (minutes)

Figure 5: Chromatograms of the ethanol blank experiment(ethanol in hydrogen atmosphere over the catalystprepared by impregnation method) measured by usingGas Chromatograph PE 8500 Series.

The results for the ethanol amination reaction over thethree catalysts prepared by either impregnation or ionexchange are shown in Table 4 while Table 5 and 6show the number of moles of ethylamine and methaneformed, yield percentage of ethylamine and yieldpercentage of methane respectively, calculated per gcatalyst, over catalysts prepared by impregnation and ionexchange methods. Table 7 shows the ratio of molesmethane formed in the ethanol/hydrogen blank experimentand ethanol amination reaction over the different catalysts.

Table 4: Moles of ethylamine and methane per g catalystformed in the ethanol amination reaction' overcatalyst prepared by impregnation and ion ex-change methods.

Preparation method Moles of ethylami- Moles of methanene per g catalyst per g catalyst

Impregnation 6.10 x 10-6 0.4 X 10-8

Ion exchangedsodium hydroxide 5.80 x 10-6 1.8 x 10-8

Ion exchanged ammonia 59.25 x 10-6 4.0 X 10-8

10

Table 5: Moles of ethylamine formed and ethanol consumedper g catalyst in the ethanol ami nation reaction overthe catalysts prepared by impregnation and ion ex-change methods.

"Moles ofethylamineproduced perg catalyst

Moles ofethanolconsumed perg catalyst

% yield ofethylamine

Preparation method

Impregnation 0.22 x 10-4 6.10 x 10-6 27.73

Ion exchangedsodium hydroxide 0.23 x 10-4 5.80 X 10-6 25.22

Ion exchangedammonia 1.04 x 10-4 59.25 x 10-6 56.99

Table 6: Moles of ethanol consumed and methane producedper g catalyst in the ethanol ami nation reaction overcatalysts prepared by impregnation and ion exchangemethods.

Preparationmethod

Moles of ethanolconsumed per gcatalyst

Moles of methaneproduced per gcatalyst

% yieldformethane

Impregnation 0.22 x 10-4 0.4 x 10-8 0.22

Ion exchangedsodiumhydroxide 0.23 x 10-4 1.8 x 10-8 0.08

Ion exchangedammonia 1.04 x 10-4 4.0 x 10-8 0.04

Table 7: The mole ratio of methane per g catalyst formed inthe ethanol/hydrogen blank experiment and ethanolami nation reaction over catalyst prepared by impreg-nation and ion exchange methods.

Moles of methane Moles of methanePreparation per g catalyst per g catalystmethod formed in the formed in the Ratio

ethanol blank ethanol aminationexperiment reaction

Impregnation 0.2 x 10-8 0.4 X 10-8 2.0

Ion exchangedsodiumhydroxide 0.5 x 10-8 1.8 x 10-8 3.6

Ion exchangedammonia 0.6 x 10-8 4.0 X 10-8 6.7


Table 4,6 and 7 indicate that the amination reactionproduced more methane than the blank experimentTherefore, methane was the sole by-product of theamination reaction. The results in Table 4 and 5 demon-strate that the ion exchanged sample prepared by theammonia method gave the highest yield of ethylamine.The ion exchanged sample prepared by the sodiumhydroxide method only produced ethylamine, after thesame catalyst sample had been tested twice with freshethanol in circulating ammonia/hydrogen mixture. Theyield of ethylamine was similar to that obtained using thecatalyst prepared by the impregnation technique. Carbonmonoxide chemisorption studies showed that the nickelmetal dispersion was only 5.7 % and this can be correlatedwith retained sodium and its effect on the reduction of thecatalyst The percentage yield of ethyl amine obtained withthis catalyst after treatment with two aliquots of reactantmay be due to: i) restructuring of the surface after thefirst exposure to the reactants resulting in the exposure offresh active sites for the reaction; ii) increased effectiveresidence time of the reactants on the active sites; iii) thereaction taking place on an adsorbed carbonaceous over-layer (18,19). This catalyst had a nickel content of only 3.4% compared with 5.3 % for the impregnated and 4.8 % forthe ion exchanged ammonia sample. It also contained 0.09% by weight of sodium which could behave as a poison inthis reaction.

Table 8: Moles of ethylamine and methane per m2 nickel basedon the nickel metal areas estimated by carbonmonoxide chemisorption for the catalyst prepared byimpregnation and ion exchange methods

Table 9: The yield of ethylamine for each of the preparedcatalysts compare with their percent dispersion

Preparationmethod

% Dispersion % Yieldto ethylamine

Impregnation

Ion exchangedsodium hydroxide

Ion exchangedammonia

2.15 27.7

5.71 25.2

19.70 57.0

Table 9 shows the dispersion percentage based on thecarbon monoxide chemisorption, and the yield percentagecalculajed assuming that the ammonia and hydrogenconcentrations remained effectively constant during thereaction. Comparison of the conversion percentage toethylamine for the catalysts prepared by impregnation andion exchange methods, clearly shows that the ion ex-changed catalyst prepared by the ammonia method is themost selective for ethylamine formation.

CONCLUSIONS

The following conclusions could be drawn from thepresent investigation:

1. The dispersion of Ni(II), total nickel metal surface areaPreparation Moles of ethylamine Moles of methane and catalysis performance for the amination of ethanolmethod per m2 nickel met.al per m2 nickel metal are all affected by the preparation route.

Impregnation 4.26 x 10.7 2.79 x 10-10 2. The yield of ethylamine in the amination reaction isrelated to the number of nickel sites available in each

Ion exchanged catalyst.sodium hydroxide 1.53 x 10.7 14.74 x 10-10

3. The ion exchanged catalyst prepared by the ammoniaIon exchanged method is the most selective for the amination ofammonia 4.44 x 10-7 3.01 x 10-10 ethanol.

When the product yields are calculated on a metalsurface area basis(Table 8), it can be seen that the ionexchanged sample prepared by the sodium hydroxidemethod was less selective for ethylamine formation andproduced more methane per m? compared to the ionexchanged ammonia and impregnated samples. The tablealso shows that the impregnated and ion exchangedammonia method catalysts produce virtually equal amountsof ethylamine per m2 suggests that the selectivity to ethy-lamine in the amination reaction is related to the numberof nickel sites available.


4. The retained sodium in the ion exchanged sodiumhydroxide catalyst poisoned nickel metal sites.

ACKNOWLEDGEMENT

The author expresses his appreciation to Prof.G.Webband Dr.D.Stirling of Department of Chemistry, GlasgowUniversity, U.K. for helpful discussion and to the BritishCouncil and P3KT-LIPI for opportunity to pursue thehigher degree at the university.

11

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15.B.Mile, D.Stirling, M.Zammit, ALovell and M.Webb,The location of nickel oxide and nickel in silicasupported catalyst, J.Catai., 114, 217-25 (1988).

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Sambungan dari haL 5

ACKNOWLEDGEMENT

This investigation was supported, in part, by grant CA-33047 from the National Cancer Institute, Public HealthService, Bethesda, Maryland. LBSK wishes to acknow-ledge scholarship from Overseas Training Office, GeneralParticipant Training Project II (1986-1988), and fromOverseas Fellowship Program, World Bank, through theIndonesian State Ministry of Research and Technology(1988-1990), arid Teaching & Research Assistantship fromUniversity of Dlinois, Chicago (1990-1992).

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Isoflavonoid Constituents from Dalbergia monetaria,Phytochemistry, 24: 1071-1076 (1985).

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carbon monoxide chemisorption· characterization …

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