SiO2 Catalyst for The Condensation of Isobutene and Formaldehyde to Isopropene

159

B. Delman and G.F. Froment (Editors), Catalyst Deactivation 1987 1987 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

DEACTIVATION OF Ag Sb 0 !Si0 CATALYST FOR THE x y z 2 FORMALDEHYDE TO ISOPROPENE

CONDENSATION

OF

ISOBUTENE

AND

AN Li-Dun, JIANG Zhi-Cheng, YIN Yuan-Gen* Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, P.O. Box 97, Lanzhou, P.R. China >:< Present Address: Beijing Technical Institute for Chemical Fiber, Beijing, P.R. China

ABSTRACT The chemical composition, surface distribution of the elements, and crystal structure of fresh and deactivated Ag Sb 0 !Si0 catalysts were investigated by 2 Auger Microprobe (SAM), the X-ray Photo-Electron Spectroscopy (XPS),YS~anning Electron Microdiffraction (EMD) technique in Analytical Electron Microscopy (AEM), Atomic Absorption Spectroscopy (AAS) and the minireaction technique. The active phase of this catalyst and the causes of its deactivation were analyzed in terms of the differences in the composition and distribution of surface elements and the change in chemical composition and crystallographic structure of the active components of fresh and deactivated catalysts as well as by evaluating their activity. INTRODUCTION Ag xSby 0z !Si0 catalysts (1) appears to have higher activity and selectivity 2 for one-step gaseous condensation of isobutene and formaldehyde to isopropene than CrxPy 0z!Si0 catalyst (2); but its stability is not satisfactory. Yu (3) 2 has investigated the surface acidity of Ag xSby 0z !Si0 2 catalyst and proposed that the active sites appear to be proton acid and the cause of the deactivation of the catalyst is the irreversible adsorption of water molecules on them replacing the proton acid sites. Chen (4) di~ not find any Bronsted acid sit~s in this catalyst by in--situ IR using the pyridine adsorption method and attributed the deactivation mainly to the decrease in their electron acceptor sites. In this work, fresh and deactivated Ag xSby 0z !Si0 catalysts were characterized, mainly 2 by means of the XPS, SAM and EMD techniques in AEM, and AAS, to investigate the composition and structure of the active phase in this catalyst. We will discuss the reasons for its deactivation, which could provide useful information for improving the stability of this catalyst system. EXPERIMENTAL METHODS Preparation of the samples (1) Preparation of the catalyst. Silica gel

spheres

(20-50

mesh,

specific

160

surface area ca. 165 m2/g) dried at 1200C for 2 hrs were impregnated with a solution of silver nitrate and antimony chloride (until just wet) dried and washed with warm de-ionized water free from chloride ions, and then dried again and calcined at 5000C in air for 4 hrs. (2) Preparation of Sb2Q4' Sb203 (A.R., Fourth Factory of Chemicals, Shanghai) was calcined at 800°C in air for 3 hrs. XRD measurement confirmed its crystal phase as a-Sb 204. (3) Preparation of AgSb0 (A.R. Beijing Factory of Chemical Reagents) A9 3. 20 and Sb203 (as mentioned above) were thoroughly mixed in an equimolecular ratio and calcined in air at 960°C for 4 hrs. XRD measurement confirmed the crystal phase as cubic AgSb0 3. Evaluation of the activity of the catalysts The catalytic behavior of the catalysts was evaluated with the help of fixed-bed minireactor system which had a loading capacity of 14 m1 catalyst (ca. 7 g). The molar ratio of isobutene to formaldehyde was 6.6 - 6.8. The reaction was carried out at 300 - 3100C in periods of 50 min. with a contact time of 0.6 - 0.8 sec. Air was used as the regenerating gas flowing countercurrent1y against the reactant fluid; the regeneration was carried out at 5000C in periods of 50 min. Electron spectroscopy methods A PHI-550 multiple electron

spectrometer

(Physical

Electron

Co.,

Perkin

Elmer) was used to make XPS, AES and SAM measurements. A MgK (1253.6 eV) target was taken as XPS source. The C (284.6 eV) of contamination carbon and Si 1s 2p (103.4 eV) of the support Si0 were designated as the reference standards of 2 binding energy in the pure compounds and the catalyst samples respectively. The charge effect on the sample surface during XPS measurements was neutralized with a flooding electron gun. Because the Sb3d5/ 2 and 0ls peaks overlap, the quantitative analysis of the surface elements is carried out according to the following procedure: first. the peak area of the related peaks is measured (i.e. Si 2p' Sb3d3/2, Ag 3d5/2 etc.); secondly, Sb3d5/ 2 intensity is calculated with the photoelectron sections given by Scofield (5) and confirmed experimentally (6):

a I

Sb3d5/ 2

I

Sb3d3/ 2

Sb3d5/ 2

a

Sb3d3/ 2

then the related atomic ratio is computed using Wagner's atomic sensitivity factor (7). Owing to the charge effect, it is difficult to do AES (and SAM) analysis for the isolated catalyst samples. We controlled the energy of the

161

primary electron beam carefully, thereby controlling the yield electrons, and the AES spectra and SAM images of the isolated obtained successfully with a lateral resolution of 5 wm. Transmission electron microscopy (TEM) and the electron

of secondary samples were

microdiffraction

(EMD)

technique Both the topographic pictures and electron diffraction patterns were obtained by means of a Hithachi H-600 electron microscope. The support, Si0 2, of the catalysts was dissolved and removed (9), and the residue was transferred into the machine to take the topographic and EMD pictures. RESULTS AND DISCUSSION XPS Resu lts The measurements showing the chemical states of silver and antimony in some typical Ag- and/or Sb-containing compounds and Ag x Sby z /Si0 2 catalysts are shown in Table 1. It is clear that the difference in Sb3d3/ 2 binding energy between Sb(III) and Sb(V) is only 0.7 eV, so XPS is not an ideal method for judging the chemi ca 1 states of the Sb element (10). Nevertheless, the antimony on the surface of real catalysts (NA-142 and NA-128) can be considered as Sb(V) when we compare the binding energy of their Sb3d3/ 2 peaks with that in typical Sb-containing compounds.

°

TABLE 1 XPS results of Ag- and/or Sb-containing compounds and Ag xSbyO z/Si0 catalysts 2 Name of compounds or catalysts

Binding energy (eV) experim. value

A9 20

&Sb203

(equ tmoLmi x, )

Sb 204 AgSb0 3 NA-142(F)** NA-142(W)*** NA-128(F) NA-128 (~I)

* **

539.7 539.3 540.4 540.7 540.5 540.2 540.2

Sb3d3/ 2* literat. value

539.4(11) 539.7(12) 540.3(6,10)

Ag 3d5/ 2 experim. value

368.1

368.5 368.4 368.5 368.3

Owing to the overlap of Sb3d5/ 2 and 0ls peaks, the Eb value of the Sb 3d3/ 2 peak and its chemical shift were used to determine the Sb-chemical state. IIF'· means fresh catalyst.

*** "l~"

means "waste" cata lyst deactivated in stabil ity experiment.

162

A g,O-->2Ag+

to, ZAg + Sb,O, + 0, --> Ag,Sb,O.

2SbzO,l+ 0 2-)o 2Sb20~

200

400

6C0

800 T (°0

Fig. 1. TG spectrum of the equimolar mixture of Sb and A9 203 20. Because of the fact that the chemical shift of Ag peaks in different 3d5/ 2 compounds is too small for its valences to be distinguished and the AgM 4VV signal is too weak to define the peak position owing to the low Ag-content in real catalyst, it is difficult to determine the chemical state of silver with XPS-Auger parameters. We determined it only in individual fresh catalyst as Ag(l) with the latter method. From the results of thermal analysis of a pure compound system, Sb203 mixed with equimolar A9 20 (Fig. 1), it can be seen that Ag(O) and Sb are the major forms at 500-520 oC (13), the temperature at which 203 the catalysts are calcined. As a result of that, the summed weight of Ag and Sb in the catalyst is only 2.0% of the total weight. and the highly dispersed active components can transfer electrons to the support Si0 2, so they exist in higher oxidation s:ates. Table 2 compares the composition of the bulk and the surface of the catalysts, from results obtained by AAS and XPS respectively. The chemical analysis data are close to the ratio of the raw materials used, but in the surfac~ layers silver is always enriched. This surface segregation becomes more striking after about 1750 hrs of the reaction and is accompanied by a serious decrease in catalytic activity. TABLE 2 Comparison between catalysts

the

Catalyst number

Ag : Sb (atomic ratio) AAS analysis

XPS analysis

NA-142(F) NA-142(W)

0.24 0.23

1.00 3.00

bulk

1.00 1.00

and the

surface

composition

1.00 1.00

of

°

Ag xSby z /Si0 2

163

NA-14 2(new)

o

200

400

6CV

Kinetic energy (ev)

Fig. 2. AES Spectrum of fresh Ag Sb 0 /Si0 catalyst. x y z 2 AES and SAM analytical results The results of repeated measurements for various samples are shown in Figs. 2, 3, 4, 5. Figure 2 presents an AES spectrum for point analysis of the fresh catalyst. Figures 3, 4, and 5 show SAM images of the fresh catalyst, the used catalyst after 10 hrs of the reaction, and the regenerated "waste" catalyst, respectively. It is obvious that the surface distribution of Ag and Sb atoms is "symbiotic" for the first two samples (i.e. where Ag atoms exist, Sb atoms exist tc'o). But for the waste catalyst, which had undergone the reaction-regeneration cycle many times during an acummulated reaction time of 175 hrs and was markedly deactivated, the surface Ag and Sb atoms are non-symbiotic, although the A9 mvv and Sbmvv peaks are significant. When the waste catalyst was not regenerated, a carbon deposit (ca. 2 wt %) would covered the catalyst's surface. In· this state the Sbmvv peak was too weak to be detected, but the A9 mvv peak was still sufficiently intense to be measured. This proved further that the surface enrichment of silver is greater in waste catalyst than in fresh catalyst. TEM and EMD results From the TEM micrograph in Fig. 6a it can be seen that the active component particles, as concentrated clusters, are spheroidal and of quite a uniform size (0.10 - 0.15)Jm o.d.). The EMD pattern (Fig. 6b) is also very clear. Bright points in the pattern show some mono-crystals, which can be seen clearly on the fluorescent TEM screen. By measuring the distance R between the transmission poi nt and diffract i on poi nt (cyc 1e), the spaci ng d between two adjacent crystalline faces can be calculated from the equation LA

Rd

where L is the distance (mm) between the crystal and the photographic plate

and

164

Fig. 3. SAM Image of NA-142(F) catalyst. (a) Ag . mvv

Fig. 3. SAM Image of NA-142(F) catalyst. (b) Sb

mvv

165

Fig.

d

SAM Image of NA-142 worked for 10 hrs. (a) A9mvv'

Fi~.

4. SAM Image of NA-142 worked for 10 hrs. (b) Sb

mvv

166

Fig. 5.

SAM Image of NA-142(W) catalyst. (a) A9mvv '

Fig. 5. SAM Image of NA-142(W) catalyst. mvv '

(b) Sb

167

Fig. 6. (a) TEM Micrograph of NA-142(F) catalyst. (b) EMD Picture of NA-142(F) catalyst. A is the wavelength (A) of the electron beam (for this machine LA=29.96 mm.A). From the d values we can then infer the corresponding crystal phases (Table 3) on the basis of ASTM data. It is evident that the crystalline phases of fresh catalyst consist mainly of AgSb0 3 (cubic system) and Sb204 (perpendicular system). But for the waste catalyst, both.the topography (Fig. 7a) and the crystalline phases (Fig. 7b and Table 4) of the active comoonents varied greatly: the particle sizes are non-uniform, the large ones being 0.15 - 0.50 ~m, and the small ones only 25 60 A; the crystalline phases are changed to Sb and Ag metal. It is well worth 203 noticing that the immersion of the metal silver phase will make Ag-surface-segregation easy, because the Ag atom in silver metal possesses the smallest specific surface free energy (14). Exploration of the active phase in A9xSbyQz/Si02 catalyst and analysis of the causes of its deactivation As mentioned above, the results of XPS and SAM show that the surface enrichment in silver is much more striking in the waste catalyst than in the fresh one. Why is this so? According to basic thermodynamics, the formation of a new surface requires work accompanied by a positive change of free energy. In order to minimize the positive surface free energy, the component which has the

168

TABLE 3 The results of crystal phase analysis of catalyst by EMD. Spacing, d, ($.)

Relative i ntens i ty':'

5.789 3.034 2.902 2.296 2.045

s s

vw

w w

1.770 1. 678 1.569 1.513 1. 397 1.300

vs vw vw

*

vs = very

AgSb0

v v

v

v

v v v

s

vw

Fig. 7. TEM ~icrograph

NA-142(F)

3

v v v v

w

strong; s

components in

Sb204

w w

1.170 1.148

active

Corresponding crystal phases

w

1.924

the

strong;

w =

weak;

WI

very weak.

(a) and EMD pattern (b) of NA-142(W) after regeneration.

169

TABLE 4 The crystalline structure of active components in the waste catalyst, NA-142(W)* Spacing, d. 0

(A) 4.069 3.981 3.179 2.807 2.359 1.993 1.667 1. 613 1.359 1.256 1.180 1.123 1.048 0.953

Relative intensity':":' w w fs fs s s s w w w w w

w vw

Corresponding crystal phases Sb203 v v v v v v v v v

Ag(O)

v (v)

v v v v

* In order to avoid the interference of carbon deposit, the waste catalyst was regenerated ** s = strong; fs = fairly strong; w = weak; vw = very weak.

lowest free energy tends to migrate toward the external surface. In a Sb203 Ag(O) - Sb204 - AgSb0 - Si0 system. the lowest surface free energy is that of 3 2 silver, which then has the largest probability of migrating to the surface. Since, in order to migrate from the interior of a solid to its external surface, an element must overcome a barrier of activation energy, the migration process can occur only at higher temperatures. As a rule of thumb, the Tammann temperature, approximately 1/2 the melting-point temperature (Kelvin scale) of the metal (or compound) (15), is sufficiently high for the atoms (or ions) in the bulk to migrate to the external surface. As the activation temperature (520 oC) for the catalyst preparation and its regeneration temperature (500 oC) are both higher than the Tammann temperature of silver. the silver element in the catalyst has not only the potential (i.e. lowest surface free energy), but also finds itself in suitable conditions (500 - 520oC) to migrate towards the uppermost surface layer, leading to surface enrichment in silver. This is coincident with the fact that the decrease in numbers of electron acceptor sites could be clearly measured by ESR. The spin number per gram of the waste catalyst was reduced by 43% in comparison with the fresh one; furthermore. the topography of the waste catalyst changed and silver and antimony on the surface layers of the waste became non-symbiotic. Either the surface enrichment in silver or the non-symbiotic distribution bring about the separation of silver and antimony.

170

As mentioned above, the main phases of active components in the fresh catalyst are AgSb0 and Sb We have prepared Sb-containing (no silver) 3 204• in which the active phase is perpendicular catalyst, Sb Sb204 204/Si02, established by XRD. Its catalytic activity is very low (formaldehyde conversion is 39%). Xu and Ding (16) have established experimentally that if in Ag- and/or Sb-containing catalyst systems supported on Si0 2, the catalyst has the appropriate surface acid strength distribution and the right amount of electron acceptors, they show better catalytic behavior only when silver and antimony co-exist in the catalysts. Summing up the different results for Ag x y /Si0 2,

° Sb ° /Si0 2 and Ag Sb ° /Si0 2 cata l ys s , is reasonable to infer that a silver-antimony compound functions in the catalytic behavior of Ag Sb ° /Si0 2 x y

x y z

t

it

x y z catalyst. and the crystalline structure of the Ag-Sb compound appears to be whose crystal size is too small to be measured by XRD. cubic AgSb0 3• The catalyst was reduced in the course of the reaction due to the reactive properties of the reactants, especially formaldehyde. but during regeneration of the catalyst was partly reoxidized. After the repeated reaction-regeneration cycle the active phase would be gradually reduced. The silver element in the catalyst is able. as mentioned above. to migrate to the uppermost surface layer. which makes Ag-Sb separate and destroys the crystal structure of active components, thus varying the electron acceptor number on the catalyst surface.

In this way the catalyst is gradually deactivated by the Prins reaction (17). We also looked at carbon deposition and possible variation of the surface area of the catalyst. The carbon content was nearly constant. ca. 2 wt% of the total weight. and the surface area did not change at all (ca. 180 m2/g). These results imply that silver is an unstable factor in the catalyst. Replacing silver by another element whilst retaining the catalytic activity of the Ag-Sb catalyst should be the way to improve the stability of the catalyst system. There remains one question to clarify: why are conditions of AgSb0 formation for pure compounds 3 catalyst?

there differences in the and for highly dispersed

ACKNm~LEDGE~1ENTS

The authors wish to express their appreciation to Prof. Ding Shixin for supporting this work. and to Dr. Xu Xianlun, Mrs. Yu Ling, Gu Jinfang, Mr. Yang Dexin and Chen Gexin for supplying some of the samples and catalytic reaction data." We would also like to thank Prof. Zang Jingling (Dalian Institute of Chemical Physics) who performed the TEM and EMD measurements.

171

REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

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