Studies on sepiolite supported hydrotreating catalysts

T.S.R. Prasada Rao and G. Murali Dhar (Editors) Recent Advances in Basic and Applied Aspects of Industrial Catalysis Studies in Surface Science and Catalysis, Vol. 113 9 1998 Elsevier Science B.V, All rights reserved

579

STUDIES ON SEPIOLITE SUPPORTED H Y D R O T R E A T I N G CATALYSTS S.K.Maity, B.N.Srinivas, V.V.D.N.Prasad, Anand Singh, G.Murali Dhar & T.S.R.Prasada Rao Catalysis Division, Indian Institute of Petroleum, Dehradun ABSTRACT Sepiolite is a hydrous magnesium silicate, receiving considerable attention in catalysis as support as well as catalyst. In this investigation a sepiolite was used as support for MoO3 and the sulfided form used for hydrodesulfurization and hydrogenation reaction using thiophene and cyclohexene as model compounds respectively. The catalysts were characterized by surface area measurement, X-ray diffraction, IR and temperature programmed reduction methods. These results indicated that molybdenum is present as a monolayer upto 6 wt.% Mo loading and beyond this loading MoO3 crystallite growth is observed. The low temperature oxygen chemisorption results indicated that there is no crystallite growth upto the monolayer region and beyond this there is rapid growth of MoS2 crystallites. It was proposed that molybdenum interacts with basic hydroxyl groups attached to support. Linear correlations passing through origin are obtained between catalytic activities for thiophene hydrodesulfurization or cyclohexene hydrogenation and oxygen uptakes indicating that the anion vacancies created on reductive sulfiding are the seat of catalytic activity. Since correlations are obtained in the case of both HDS and HYD it is clear that oxygen chemisorption is not specific to any one of the functionalities but measures anion vacancies which are related to the state of dispersion of molybdenum. Keywords :

1.

Sepiolite, Molybdenum Sulfide Oxygen Chemisorption, Hydordesulfurization, Hydrogenation

INTRODUCTION

During the last few years there has been increasing interest in synthetic and semisynthetic silicates like clays as catalysts and catalyst supports. This is based on their low cost and favorable characteristics for the particular reaction in question. The ion exchange capacity, acidity, higher surface areas make them eminently suitable for use as catalyst and as catalytic supports. Recently a review [1] appeared on clays in hydrotreating catalysis, demonstrates the role of clays in such reactions. However, it is interesting to note that sepiolite is basic in nature due to the presence of magnesium and also has only a little ion exchange capacity. Notwithstanding these attributes its high surface area even after heating at high temperatures

580 and zeolitic micropores prompted many people to use it as a support for metals like Mn, V, Pd, Pt, Ni. Melo et al. [2] studied in detail NiMo supported on sepiolite for hydrotreating reactions at 400 ~ C and 20 Kg pressure. An optimum for HDS and hydrogenation was found on 12% MoO3 and 5 wt % NiO prepared by simultaneous impregnation. The optimum conditions for better conversion were found to be 400~ and 2.5 hrs of sulfidation. Increase in acidity of the support produces decrease in HDS and hydrogenation. A good correlation with a concentration of X NiO MoO3 phase and HDS activity was observed. They have studied untreated sepiolite, acid treated sepiolite and aluminated sepiolite as supports for Ni-Mo active phases. Among the three systems studied untreated sepiolite supported catalysts gave best performance indicating that basic sites are preferred for MoO3 anchoring compared to acidic sites. However the HDS or HYD activity with varying molybdenum concentration and dispersion of active phase etc. was not reported in literature. In this investigation a detailed study was made on the catalytic functionalities of MoO3 supported on sepiolite. 2.

EXPERIMENTAL

The catalysts were prepared by incipient wetting of commercial sepiolite support (clay repository,University of Mossourie,USA, S.A.=147mZ/g)with a solution of appropriate concentration of ammonium heptamolybdate.(at pH~ 8.5) The impregnated samples were dried in air at 120oc for overnight and then calcined in air at 450~

for 6 hours.

The oxygen chemisorption was measured at --77oc in a high-vacuum glass unit on catalyst sulfided at 400oc for 2 h using a CS2/H2 mixture (at a flow rate 40 cc/min) following the double isotherm procedure of Parekh and Weller for reduced molybdenum catalyst(3). The same system was also used for BET surface area measurements. X-ray diffractograms were recorded on a Rigaku model D-Max 11 lB. IR and TPR experiments were also carried out on molybdenum oxide samples and the experimental detailed was given elsewhere(4). Thiophene hydrodesulfurization and cyclohexene hydrogenation activities were evaluated at 400~ in a fixed bed reactor operating at atmospheric pressure and the products were analyzed by on line G.C. and the details are given elsewhere(5). 3. RESULTS 3.1.Catalyst Characterization a) BET Surface area and Pore volume The measurement of BET surface areas and pore volumes were carried out on pure sepiolite support and also on MoO3/sepiolite catalysts. The surface area decreases as a function of Mo-loading on sepiolite see in figure l(a) and the corresponding data is presented in table 1

581 Table.1 BETSA and LTOC data of Mo/Sepiolite Catalysts 0 EMSA Mo BETSA(m2g-1) O2-uptake Mo (m2g "1) Wt % (IX mol g" 1 cat) dispersion catatyst support 0 147 147 0 0 2 128 131 12.5 0.12 7.0 4 129 134 23 0.11 13.0 6 103 110 37 0.12 20.9 8 78 85 28 0.07 15.8 10 74 82 20 0.04 11.3 12 66 75 11 0.02 6.2

Surface coversage %0

0 5.5 10.0 20.3 20.3 15.3 9.4

Crystallite Size(A ~

29.4 31.9 29.8 52.5 91.9 200.7

I

b

0-8

3"

d ~r

0-6

_o---.

0-4

0-?.. 0

CATALYST SUPPORT

160

;,= Izo E

80

40

t

t

t

2

4

6

t 8

t IO

,

1 12

W t % Mo

Fig.-1 V a r i a t i o n of Surface area(a) a n d p o r e volume(b) with M o - l o a d i n g

582 It can be observed from the data presented that the surface area decreases sharply with the addition of 2 wt % Mo and then decreases slowly upto 4% and then decrease steadily with further Mo-loading. The surface area per gram of catalyst as well as per gram of support both show similar trends. However, surface area per gram of support is expected to be constant upto completion of monolayer (6,7) but is not in this case. This should not be taken as evidence for lack of monolayer formation since sepiolites are well known for the microporosity. This micropores can get blocked during preparation and if the blockage is non uniform the expected invariance of surface area per gram of support upto monolayer coverage is not observed. It will be further clarified latter during discussion of oxygen uptake results and hydrogenolysis reaction etc. Figure l(b) indicates that the pore volume decreases with the molybdenum loading and this reveals that some part of the pores are being blocked with Mo addition

b) X-ray Diffraction(XRD) The X-ray diffractograms of MoO3/sepiolite catalysts obtained by C u K a radiation are presented in figure 2. It can be seen that the XRD pattern of pure sepiolite resembles with spectra reported in literature (8,9). It is clear from XRD that the used support is a-sepiolite. From the X-ray diffractograms as a function of Mo-loading it can be noted that no line other than that of sepiolite can be seen. This can not be taken as that no molybdenum oxide lines are present, since the region 20-30, the 20 values between which molybdenum oxide lines of maximum intensity occur is overlapping with strong sepiolite lines. Therefore, X-ray diffraction results can not give any evidence about the presence of crystalline molybdenum oxide in the molybdenum catalysts supported on sepiolite.

4-00

1000

20.00

~).00

40"00

48-Gr

28

Fig.2-X-ray diffractograms of MoO3/sepiolite (a;pure, b;4, c;6, d;10, and e;14.)

583 c) F T I R The FTIR spectra of sepiolite supported molybdenum catalysts was obtained by KBr pellet technique and presented in figure 3. IR spectral region in the range of 1030-800 cm -1 is important for molybdates. The region 1050-900 cm -1 is represented by bonds due to terminal -1 Mo - O region and 850-700 cm represented antisymmetric Mo-O-Mo or O-Mo-O stretching vibrations or both. Sepiolite has strong bands in 400-600 cm -1 and 850 to 1200 cm -1 regions. There are also IR bands due to sepiolite in the region 600-850 cm -1. Addition of Mo upto 6 wt % level did not show appreciable changes in the spectra of sepiolite. A shoulder at 900 cm -1 becomes prominent at 8 wt % level of Mo. The intensity of this shoulder increases further upto 14 wt % Mo-loadings and also show a shift towards higher loading regions. A shoulder also develops at 995 cm -1 which increases with loading and forms into prominent band. The pure MoO3 show IR bands in 400-1050 cm -1 region.

There are prominent broad band between

450-750 cm -1, another band at 750-950 cm -1 and a sharp peak between 950-1050 cm -1. We found a band at 650 cm -1 typical of MoO3 and it becomes prominent between 10 and 14 wt %. Thus IR spectra clearly provides evidence that upto 6 wt % there is no indication for the formation of MoO3 but at higher Mo-loadings crystalline MoO3 is formed.

1200

I

I

I

I

I100

I(XX)

900

800

I 700

,

i 600

,

i ,500

,-!

cm~

Fig.3-FTIR spectra of MOO3/sepiolite

584

d) Temperature Programmed Reduction Temperature programmed reduction was conducted in the range 0~176 at a heating rate of 10 K/min using 5% H2/N2 mixture for both on pure sepiolite as well as sepiolite supported Mo catalysts and are shown in the fig 4. Pure sepiolite showed reduction feature characterized by two small peaks in the temperature range 600-740~ Addition of 2 wt % Mo on the support produces a sharp single reduction profile. This peak becomes broad with increase of Mo loading and a second peak appears as a shoulder on the low temperature side and then grows at higher molybdenum loading. The higher temperature peak obviously corresponds to difficultly reducible species and low temperature peak corresponds to relatively more easily reducible species. The Tmax of high temperature peak shifts to high

720 I

TEMPERATURE(* C )

745 " I

900

Fig.-4 TPR patterns of MOO3/sepiolite (a;pure, b;2, c;4, d;6, e;8, f;12, and g;14.) temperature side with the increasing Mo-loading whereas the Tmax of low temperature peak which, is difficult to identify clearly, hovers around 725~ The shift of the Tmax to higher temperature as a function of molybdenum loading was also noted by other workers on SiO2

585 supported Mo catalysts [10,11]. According to Regalbuto et al. (10) at lower loading the main peak was assigned as the reduction of polymolybdate MoO3 to MOO2. But Rajagopal et al. (11) reported that the low temperature peak is due to reduction of (M +6 -~ Mo 4+) amorphous, highly defective, multilayered Mo-oxides. The total amount of reduction understandably increases with the increase of molybdenum loading (sample weight taken constant). The reduction behavior can be understood if we analyze various oxomolybdenum species that can be present on surface of support as a function of molybdenum loading. Most of the data available on ~'-A1203, SiO2 supported catalysts and these data may be safely extrapolated to other supports. It is the basic hydroxyl groups with which the molybdenum oxide interacts on all type of supports. The type of such groups and number, and strength are likely to vary from support to support but the fundamental nature of interaction is expected to be similar. On A1203, molybdenum oxide is present as Mo(T) and M(O) polymeric at moderate loading and at much higher loading a small amount of molybdenum is present as crystalline MOO3. All the three types of species can be assumed to be present on other supports also but their concentration as a function of molybdenum is likely to vary considerably. The molybdenum tetrahedral species which is present at very low loadings is difficult to reduce and molybdenum octahedral polymeric species which increases with increasing of Mo-loading and crystalline molybdenum oxide which is present at higher Mo-loadings are comparatively easy to reduce. At low loadings Mo(T) as well as Mo(O) species coexist and both of the them get reduced to various degrees and the reduction envelop has contributions from both of them but it is difficult to distinguish individual contributions. At higher Mo-loading polymeric species and crystalline molybdenum oxide gets reduced. There is a single peak upto 6 wt % Mo-loading and above this Mo-loading two peak pattern is found. The two peaks may be ascribed to molybdenum tetrahedral species which is difficult to reduce and relatively more easily reducible polymeric octahedral species. At much higher temperatures a shoulder peak is observed at lower temperature region and which can be assigned as crystalline MoO3 which is not interacting with the support. Another possible explanation is that the first peak corresponds to MoO3-MoO2 reduction and the second peak corresponds to reduction of MoO2 -~ Mo. In our opinion later explanation is difficult to justify and it is more likely that the former explanation suits here better. However the results presented cannot give further clarification without further detailed experimentation.

e) Oxygen Chemisorption Oxygen chemisorption was carried out on various sepiolite supported Mo catalysts and the oxygen uptakes values as a function of molybdenum loading is presented in figure 5 and table 1. It can be seen that oxygen uptake increases with the molybdenum loading upto 6 wt % Mo on sepiolite and then starts decreasing. From the oxygen uptakes it is possible to calculate equivalent molybdenum sulfide area (EMSA). It is possible to calculate O/Mo which is a measure of dispersion and surface coverage by molybdenum. It is also possible to calculate crystallite size from EMSA. All these parameters are given in table 1. The corresponding values of O/Mo as a function of loading is shown in figure 6. It can be seen that the percentage of dispersion of molybdenum did not change appreciably upto 6 wt % and beyond this point there is sharp decrease upto the highest loading studied. The surface coverage shown in

586 table 1 also indicates that the surface coverage increases upto 6 wt % and then starts decreasing. It can also be noted that only a small part of surface is covered by molybdenum, indicating that molybdenum occupies selective sites on the surface. The crystallite sizes (table 1) calculated using EMSA did not change appreciably upto 6 wt % (around 20-29 A ~ but beyond this point the rapid crystal growth can be noted.

30 ~

[ 4030

] ----HDS I

20

~'m

20 ~ ~'~o

lo

lo

"6 0

E ~"

0 0 0

2

4

6

8

10

:=.

12

Wt % Mo

Fig.-5 Variation of O2-uptake and activities with Mo-loading. 3.2.a) Catalytic Activity Catalytic functionalities of sepiolite supported Mo-sulfided catalysts were evaluated using thiophene hydrodesulfurization and cyclohexene hydrogenation reactions at 400~

The

presulfidation was carried out at 400~ for 2 hrs with mixture of CS2 and H2. In both the cases first order rate constants are evaluated. The results of such experiments as a function of Mo-loading is given in table 2 and the activity of HDS and HYD reaction as a function of Moloading is shown in figure 5. Table.2 Activity parameters of MoS2/sepiolite catalysts KHYD Mo Wt %

HDS Rate*

HYD Rate*

0 2 4 6 8 10 12

0 7.2 13.6 21.2 14.7 10.8 6.2

0 3.2 6.4 10.0 7.5 4.9 2.5

QTOF#

IntrinsicActivity **

.,.,.,.

.

.

.

.

..

KHDS HDS 0 0.36 0.34 0.35 0.18 O.11 0.05

HYD 0 0.16 0.16 0.166 0.09 0.05 0.02

HDS 5.76 5.91 5.73 5.25 5.40 5.64

HYD 2.56 2.78 2.70 2.68 2.45 2.27

0.444 0.470 0.47 0.51 0.45 0.40

*( mol h ' l g "1 Cat) x 103 , #(mol h ' l g "1 Mo) x 103,**(mol h "1 ,pmol'lO2)xl04

587 It can be noted that the both catalytic activities increase in a linear manner upto 6 wt % Mo on sepiolite and then start decreasing. It is interesting to see that the linear part of the lines pass through the origin indicating that pure support by itself is inactive for this reaction. This fact was independently verified using pure support also. The maximum activity obtained at 6 wt % Mo and it is important to recall that at this Mo loading oxygen chemisorption also reaches maximum and with further increase of Mo-loading both activity and oxygen chemisorption start decreasing. The quasi turnover frequency (QTOF) (rate per gram of molybdenum) was calculated and presented in the fig.6 It can be seen that both tends behave in a similar manner. Initially QTOF did not change upto 6 wt % Mo on sepiolite and then started decreasing and it is similar to dispersion

~o 0.4 ~ "x 0.3 ~

-=-HDS/0.16 -"

HYD-l- 0.12

0.2

o.o8

o.1

0.04

.

o

0

2

4

6

8

10

12

Wt %Mo Fig.-6 Variation of QTOF and O/Mo with Mo loading

b) Oxygen Chemisorption and Activities Correlations The oxygen uptakes obtained were plotted against the rate of HDS of thiophene and the rate of HYD of cyclohexene. It can be seen that a good linear relationship passing through the origin is obtained for both HDS and HYD reactions (Fig 7.) and this means that the activity for both the functionalities varies in proportion to the increase in oxygen chemisorption

588

30 0

qp-, A

(1) 91 ~

X

20

0 v-

n'7

10

,,,m

0

E

0 0

11

12.5 20 23 28 02-uptake (l~mol gr1 cat)

37

40

Fig.-7 Correlation between activity and oxygen chemisorption

4.

DISCUSSION

Sepiolite support has high surface area and can accommodate a monolayer of molybdenum with the available surface area. However, surface area per gram of support did not give any evidence for the formation of monolayer. X-ray diffraction results are inconclusive due to severe over lapping of support lines. IR spectroscopy results indicate that above 6 wt % Mo loading there is an evidence for formation of crystalline molybdenum oxide. Oxygen chemisorption increases upto 6 wt % and then decreases. The surface coverage by molybdenum is low and O/Mo values are also low indicating that the molybdenum is covering a small part of the surface and out of the total molybdenum only a small fraction of it is titrated the by oxygen chemisorption. The crystallite size is more or less constant upto 6 wt % and increases rapidly growing to ~ 200 A ~ size at about 12 wt %. These results definitely indicate that 6 wt % Mo is a turning point. Catalytic activities for both thiophene hydrogenolysis and cyclohexene hydrogenation followed similar trends Important thing to note is that at 6 wt % molybdenum loading there is a transition for activities, oxygen chemisorption and other related parameters. The crystallite size is very small -- 30 A~ 6wt%) as determined from oxygen uptake values. The smaller size of crystallites and X-ray amorphous nature indicates that upto 6 wt % Mo level the supported molybdenum oxide is in the form of small crystallites (below X-ray detection limit) and can be termed as dispersed as monolayer. The region beyond 6 wt % is called post monolayer region. In the post monolayer region there is growth of crystallites of molybdenum oxide or sulfide The surface coverage of support is only about 20% at the highest coverage. This indicates that the molybdenum selectively interacts with part of the sepiolite surface. This is understandable since in the oxide state of molybdenum interacts with basic hydroxyl groups of particular strength present on the support surface. There is documented evidence in literature of this fact (12). Based on this we may regard that the molybdenum species are interacting with such basic hydroxyl groups connected with magnesium in the sepiolite. When all such groups

589 exhaust the coverage on the silicious part of the sepiolite takes place in the post monolayer region. In the sulfided state the monolayer should remain intact provided molybdenum interacts with surface strongly. However, there is no data available on strength of interaction of molybdenum with sepiolite support surface. The oxygen chemisorption which is conducted on catalyst sulfided at 400~ for 2 hrs, indicates that even after such treatment at that temperature the crystallite size is small upto 6 wt % indicating the strength of interaction is considerable, otherwise we would have observed big crystals. Sulfiding in reductive environment creates anion vacancies on the catalysts. These anion vacancies at edges and comers are supposed to be sites for catalyst activity. As the Mo-loading increases the monolayer patches increases and the anion vacancies at the edges and comers also increase and this increase is continuous upto monolayer coverage. Beyond the monolayer region big crystallites of MoS2 are formed and this decreases surface area and also very big crystallites only get sulfided superficially therefore the number of anion vacancies decrease. It is well known that the oxygen chemisorbs on anion vacancies(13,14). Therefore, the increase in oxygen chemisorption represents the growth of anion vacancies. It is also well known that coordinately unsaturated sites i.e. the sites containing these anion vacancies at edges and comers with various degrees of unsaturation are the seat of hydrodesulfurization and hydrogenation activities. However, both the reactions may take place on sites of different degrees of unsaturation. Therefore, it is not surprising that the activities followed similar trends as oxygen chemisorption and there is a nice correlation between oxygen chemisorption and activities. Since correlations are obtained with both HDS and HYD it is clear that oxygen chemisorption is not specific to any one of the functionalities. Since HDS and HYD take place on sites of different degrees of coordination, oxygen chemisorption fails to distinguish such sites. It only measures anion vacancies. We have observed that the QTOF which measures dispersion and oxygen chemisorption run in similar manner. Zmierczak et al (15) also concluded that the oxygen chemisorption measures general state of dispersion. It is also clear that anion vacancy creation is proportional to dispersion at least in the monolayer region. 5.

CONCLUSION

9

Molybdenum is better dispersed on sepiolite support compared to 3'-A1203 support.

9

The monolayer of molybdenum oxide/sulfide occurs at 6 wt.% Mo. Sepiolite supported molybdenum catalyst is more active for HDS reaction than Mo/AI203 catalyst.

9

Oxygen chemisorption is not specific to any one of the functionalities.

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590

.

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