catalytic activity of layered a- (tin or zirconium) phosphates and

Applied Catalysis A: General, 92 (1992) 81-92 Elsevier Science Publishers B.V., Amsterdam

81

APCAT A2382

Catalytic activity of layered a- (tin or zirconium) phosphates and chromia-pillared derivatives for isopropyl alcohol decomposition

A. Guerrero-Ruiz

Dpto. de Quimica Inorgainica y Tknica, Facultad de Ciencias, UNED, Senda de1 Rey s/n, 28040 Madrid (Spain)

I. Rodriguez-Ramos and J.L.G. Fierro

Institute de Cat&is y Petroleoquimica, CSIC, Campus Universidad Authoma, 28049 Madrid (Spain)

and

A. Jimenez Lhpez, P. Olivera Pastor and P. Maireles Torres

Dpto. Quimica Znorghnica, Cristalografia y Mineralogia, Universidad de Malaga, Apartado 59, 29071 Malaga (Spain)

(Received 25 May 1992, revised manuscript received 11 September 1992)

Abstract

Layered cr-zirconium and cu-tin phosphates (a-ZrP and c&nP) and their chrcmia-pillared homologues have been evaluated from the point of view of surface acidity. The FT-IR of pyridine adsorption and the dehydration of 2-propanol at 180°C were used as chemical probes to measure acid sites. The studied materials were characterized by X-ray diffraction, surface area measurements and X-ray photoelectron spectroscopy. The latter technique was also used to reveal the chemical state of both fresh and used on-stream catalysts. Under the experimental conditions used, a-SnP presents a moderate Brensted and Lewis acidity, which is absent in cr-ZrP. In contrast, all the thermally stable porous materials, obtained by pillaring a-SnP and a-ZrP with chromium(LI1) oxide, show a marked enhancement of the acidity with respect to the original phosphates. These acid sites are strong enough to still retain a small proportion of pyridine upon outgassing at 500°C but insufficient to perform methanol decomposition into hydrocarbons. Interestingly, the 2-propanol conversion in the presence of air led to both dehydration and dehydrogenation, this latter functionality being activated in an alcohol-helium stream.

Keywords: Breneted acidity, chromia pillaring, Lewis acidity, 2-propanol decomposition, tin, zirconium.

Correspondence to: Dr. A. Guerrero-Ruiz, Dpto. de Qufmica Inor&ica y Thcnica, Fact&ad de Ciencias, UNED, Senda de1 Rey s/n, 28940 Madrid, Spain.

0926-860X/92/$05.00 0 1992 Elsevier Science Publishers B.V. All rights reserved.

a2 A. Guerrero-Ruiz et aL/AppL CataL A 92 (1992) 81-92

INTRODUCTION

Over the last few years, layered acid salts of tetravalent metals (with the formula Mw (HXO, ) 2 l H,O; M = Ti, Zr, Sn, Ge; X = P, As) have received considerable attention because they behave as ion exchangers [ 1 ] and are able to catalyze reactions such as dehydration, isomerixation, polymerization and al- kylation [2-111. Their catalytic properties are related to the presence of Brernsted and Lewis acid sites on their surfaces [ 12,131. In particular, c&r- conium phosphate is an effective dehydrogenation agent of alcohols at T> 200°C. Its catalytic activity is attributed to the presence of acid sites on the external surface [2,3], but, the majority of acid sites are located on the intracrystalline surface, which remains inaccessible to reactants at typical working temperatures. The inaccessibility of acid sites in chromium-exchanged zirconium phosphate impedes, for instance, dehydrogenation of ethane to ethylene [ 111.

Recently, it has been shown that the pillaring of layered compounds by or- ganic phosphates and phosphonates [ 141 facilitates the accessibility of the reactants to the internal surface area, but the materials so formed do not have sufficient thermal stability to be used in catalytic reactions at high temperatures [ 61. An effective way to maintain a permanent opening of the layer struc- ture is by intercalation of the inorganic polyoxocations, which, after calcina-’ tion, gives rise to interlayer metal oxides which act as pillars, so making the internal surface accessible to adsorption and catalysis. Many of these pillared compounds, specially pillared clays, have been successfully used in catalysis, showing high activity and selectivity, comparable to those of zeolites [ 15,161. Pillared zirconium or tin phosphates containing alumina and chromia as pillaring oxides have recently been prepared from colloidal suspensions of the respective phosphates [ 17-191. These materials exhibit high thermal stability, surface areas and porosity.

As is well known, the decomposition of isopropanol can be used as a test reaction to titrate acid-basic and redox properties of catalysts. The acid sites lead essentially to the dehydration product, propene (reaction (1) ) and acetone is obtained by dehydrogenation of isopropanol on basic or redox sites (reaction (2) ) [ 201, according to the following scheme:

(CH&CH=CH2+Hz0 (1) /1

(CH&-CH,-OH >

(CH&C=O+Hz (2)

With this background, a study of the isopropanol conversion was undertaken with the aim of investigating the parameters which govern both activity and selectivity, and also the nature of the sites involved in these reactions. Accord-

A. Guerrero-Ruiz et al./Appl. Catal. A 92 (1992) 81-92 83

ingly, this work deals with the results concerning cu-SnP, a-ZrP and their chromia-pillared homologues. The catalytic behaviour of these catalysts for reactions (1) and/or (2) are explained in terms of their surface and structural characteristics.

EXPERIMENTAL

Materials

cr-Zirconium phosphate (a-ZrP) was prepared by precipitation from a solution 0.13 M of ZrOClz*8Hz0, 0.71 M of HF and 5.31 M of H3P04 and warmed at 80°C for nine days, in order to accelerate precipitation by decomplexing the zirconium fluorocomplex [ 211. The solid was separated by filtration and washed with water up to a pH in access of four. Then it was dried over Pz05 and put into a dessecator with controlled humidity. a-Tin phosphate (a-SnP) was regenerated from a colloidal suspension of freshly prepared tetramethylam- monium-exchanged tin phosphate by addition of HCl low2 M. A product with a higher surface area than that prepared directly [ 221 is obtained by this pro- cedure. Tin and zirconium phosphates pillared by chromia to different extents (CrZrP and CrSnP ), were prepared as reported elsewhere [ 18,191. These precursors were calcined at 400’ C, under nitrogen, for 12 h and washed with hot water in order to remove possible segregated CrOg.

An aliquot of a-SnP was treated with 0.05 M CsCl solution, in excess of the cationic exchange capacity (c.e.c . = 6.08 mequiv. g-l) in order to exchange the protons on the external surface [ 231. The solid was filtered, washed and dried at 120°C.

Catalyst characterization

X-ray diffraction (XBD), on cast films, was recorded on a Siemens D501 diffractometer using Cu Ka! radiation and a graphite monochromator. Chro- mium was colourimetrically analyzed in alkaline chromate solutions obtained by treatment of samples in a NaOH/H202 mixture. Adsorption spectra were then recorded with a Kontron Uvicon 810 spectrophotometer.

The BET surface areas of the catalysts were determined by nitrogen adsorption at 77 K in an automatic Micromeritics ASAP 2000, using 0.162 nm2 as the cross-sectional area of the nitrogen-adsorbed molecule.

The infrared spectra were recorded on a Nicolet ZDX Fourier transformed spectrophotometer with a resolution of 4 cm-‘. Self-supporting wafers of the samples with weight-to-surface ratios of about 10 mg cmB2 were placed in a vacuum cell assembled with greaseless stopcocks and KBr windows. Pretreat- ments were carried out with an in-situ furnace. The adsorption of pyridine was effected after evacuation of materials at 200’ C for 1 h. Subsequently, the sam-

84 A. Guerrero-Ruiz et al./Appl. Catal. A 92 (1992) 81-92

plea were exposed to 2 Torr (1 Torr = 133.3 Pa) of pyridine vapour at room temperature for 5 min, and then outgassed at 200°C for 1 h. The IR spectra were recorded at room temperature.

The X-ray photoelectron spectra (XPS) were recorded with a Fisons ES- CALAB 200R spectrometer equipped with a Mg Ka! X-ray excitation source (h v = 1253.6 eV) and hemispherical electron analizer. The samples were tur- bopumped to ca. 10m3 N rnv2 before they were moved into the analysis chamber. The residual pressure in this ion-pumped chamber was maintained below 2 x 10m7 N mm2 during data acquisition. A 20 eV energy region of the photo- electrons of interest were signal-averaged for a number of scans in order to obtain good signal-to-noise ratios at reasonable acquisition times. Although surface charging was observed in all samples, accurate ( 20.2 eV) binding energies (BE) could be determined by charge referencing with a C 1s peak at 284.9 eV or a Au 4f7,, at 83.8 eV.

Activity measurements

Catalysts were tested in a 0.9 O.D. fixed-bed tubular glass reactor working at atmospheric pressure. The temperature was measured by a thermocouple placed in the outer wall of the catalyst bed. A catalyst charge of 0.15 g without dilution was used in each experiment. The isopropanol was fed into the reactor by bubbling a flow of helium (25 cm3 min-‘) through a saturator-condenser maintained at 20’ C, which allowed a constant isopropanol flow of 2.8 vol.-%. The reaction products were analyzed by an on-line gas chromatograph pro- vided with a thermal conductivity detector and a Vinicol column for the separation of products. The helium carrier was passed through a molecular sieve before it was saturated with isopropanol. A series of experiments were carried out by using a mixture of isopropanol-saturated helium and synthetic air (10 cm3 min-’ ). Prior to the reaction, the catalysts were pretreated at different temperatures in a helium flow for 12 h. In some cases the reaction of methanol dehydration was studied. Methanol was fed into the reactor by a mixture of helium with 11.6 vol.-% of methanol. A Chromosorb 101 column was used for the separation of products. The samples were pretreated in the helium flow at 350°C before use in the methanol test, and the reaction was studied at the same temperature.

RESULTS AND DISCUSSION

Structural and IR characterization

Table 1 lists chromium contents, basal spacings and surface areas of the catalysts studied. Precursors of chromia-pillared phosphates were calcined at 400 ’ C under nitrogen, in order to avoid oxidation of CJ”. Under these condi-

A. Guerrero-Ruiz et aL/Appl. Catal. A 92 (1992) 81-92

TABLE 1

85

Basal spacing, surface areas and chromium contents of pillared and non-pillared phosphates

Catalyst Temperature Cr (% ) d,,,n (A) (“C)

BET (m2 g-l)

lx-ZrP 180/He 7.33 1 CrZrPl 400/Nz 25.7 13.04 106 CrZrP2 400/Nz 31.6 sh 26.70 335 CrZrP3 4WN2 32.6 sh 17.60 313 CrZrP3/600 600/He 32.9 274

a-SnP 180/He - 7.55 35 CrSnP 4OO/Nz 28.3 13.90 339

A

xl 1600 1500 140( Wavenumber (cm-‘)

I I I 1 I

1700 1600 -1500 140(

Wavenumber (cd)

Fig. 1. IR spectra of pyridine adsorbed on: (A) zirconium phosphates: (a) aZrP, (b) CrZrP3, (c) CrZrP3 evacuated at 500” C after pyridine adsorption; (B ) tin phosphates: (a) a-SnP, (b) CrSnP.

tions, the obtained materials are still crystalline with a narrow pore-size dis- tribution [ 171. The presence of metal oxide pillars between layers makes the interlayer region accessible to different adsorptives, having a high surface area available with respect to non-pillared phosphates.

Studies by IR of pyridine adsorbed on the catalysts were performed in order to determine the nature (Brensted or Lewis) and the strength of the acidic centers present [ 8,131. Fig. 1 shows the IR spectra of catalysts after adsorption of pyridine. Under the working conditions, pyridine is only adsorbed on the external surface of cx-SnP and a-ZrP, where the catalytic active sites of these

86 A. Guerrero-Ruiz et al./Appl. Catal. A 92 (1992) 81-92

two materials are located. These two phosphates show a very different behaviour. In the case of cu-SnP (Fig. lB), Brensted (shoulder at 1550 cm-l) and Lewis (1457 and 1610 cm-‘) acid sites are present, while both sites, not sufficiently strong to interact with pyridine, are absent in cw-ZrP (Fig. 1A). The presence of Lewis acid sites, which are probably associated to deficiently co- ordinated metal ions on the external surface, agree well with the weak resist- ence to hydrolysis (broken bonds) of cr-SnP. This latter hydrolyzes more readily than a-ZrP.

In the chromia-pillared phosphates two types of acid sites are present, as shown in the IR spectra (Fig. 1A and B) by the appearance of bands at 1445, 1612 cm-’ (CrZrP) and 1451, 1614 cm-’ (CrSnP), characteristic of Lewis sites, and at 1478,1558 cm-’ (CrZrP) and 1491,1557 cm-’ (CrSnP), characteristic of Brensted sites. Comparatively chromia-pillared tin phosphate shows a higher proportion of Lewis sites than the zirconium phosphate homologues. Interestingly, the formation of chromia pillars between layers of zirconium phosphate leads to the appearance of acid sites on this material, associated to both the chromia pillar and the accessible internal surface. The presence of Brnrnsted acid sites in pillared phosphate is related to cation exchange properties exhibited by such materials (l-3 mequiv. g-l) [24]. This characteristic may provide these materials with a wide versatility as catalysts. Fig. 1A also shows that acid sites on chromia-pillared phosphates are very strong as the corresponding bands in the IR spectra remain intense after evacuation at 500°C.

Catalytic activity

Activity and selectivity of the different catalysts in the reaction of the decomposition of isopropanol at 180’ C are listed in Table 2 and displayed in Figs. 2 and 3, respectively. It can be remarked that the catalytic activity of pillared and non-pillared phosphates hardly changes with the time on-stream once the stationary regime is reached after an interval of 2 h. Comparison between the two non-pillared phosphates shows that, as expected, their behaviour is quite different. In the presence of a feed free of oxygen, zirconium phosphate is prac- tically inactive, as a consequence of the absence of redox and acid sites which are sufficiently strong on the external surface (Fig. 1) . Under the same conditions, tin phosphate produces exclusively propene, which is related to the presence of acid centers (Fig. 1) on a surface 30 times higher than that of zirconium phosphate. If the surface hydroxyl groups were solely responsible for the dehydration activity, replacement of the protons by caesium should eliminate this activity. Cs+-exchanged tin phosphate shows an activity 10 times lower than the pure phosphate. Since the Cs+ ion is too large to exchange into the interlayer space of the tin phosphate, the activity is associated with the protons of the surface [ 231.

A. Gzmrero-Ruiz et al./Appl. Catal. A 92 (1992) 81-92

TABLE 2

87

Activity of pillared and non-piked phosphates in the decomposition of isopropanol at 180°C

Catalyst F (“Cl Activity Conversion

@moI/g 8 1 (So)

Inert Air Inert Air

cx-ZrP 180 0.04 2.14 1 61 CrZrPl 300 0.31 1.51 9 43 CrZrP2 300 1.44 1.73 41 50 CrZrP3 300 1.65 2.11 47 61 CrZrP3/600 300 2.07 3.04 59 60

wSnP 180 1.24 1.40 35 40 ff-SnP/Cs 180 0.12 - 3 - CrSnP 300 2.66 2.95 76 34

“Pretreatment temperature in helium.

60

40

20

0 7

ZrP CrZrPl CrZrP2 CrZrP3CrZrP31600 SnP SnP-Cs CrSnP

El propene acetone

Fig. 2. Selectivities to acetone and propene of pillared and non-pillared phosphates in the reaction of isopropanol decomposition at 180°C in the absence of air.

When the reaction is carried out in the presence of air, the activity of the zirconium phosphate increases drastically (Table 2), acetone being the only product obtained via a oxidative dehydrogenation (Fig. 3). This high activity and selectivity toward acetone, which is indicative of redox sites, has not been mentioned so far in the literature. Under the same conditions, the catalytic activity of tin phosphate remains nearly equal to that in the absence of air. However, selectivity changes and acetone and propene, in a ratio close to 1: 1, are produced.

60 - I I

40 j

20 I!

oL

A. Guerrero-Ruiz et al./Appl. Catal. A 92 (1992) 81-92

I 1

J ZrP CrZrPi CrZrPP CrZrP3 CrZrP3/600 SnP CrSnP

Cl propene acetone

Fig. 3. Selectivities to acetone and propene of pillared and non-pillared phosphates in the reaction of isopropanol decomposition at 180 ’ C in the presence of air.

Pretreatment of zirconium phosphate in helium at 400°C leads to a 21% decrease in dehydrogenation activity in air, and an increase in dehydration activity, as a result of an increase in the number of strong acid sites. According to Hattori et al. [ 131 the acidity of zirconium phosphates increases with increasing thermal treatment temperature, attaining a maximum at 400’ C.

In short, at 18O”C, tin phosphate is much more acid than zirconium phosphate, but the latter presents redox sites, which may tentatively be associated to pairs M4+/M(4-s)+ (M=Z r on the external surface of the phosphates. ) M(4-s)+ species are regenerated to M4+ instantaneously in air, prolonging the activity of the catalyst for a considerable time.

Table 2 and Figs. 2 and 3 also show the activity and selectivity in the isopro- pan01 decomposition for the chromia-pillared phosphates. The samples were treated in helium at 300’ C to remove zeolitic water adsorbed in internal pores. Comparison with non-pillared phosphates shows a differential catalytic behaviour. Chromia-pillared tin phosphate presents a strongly increased activity with respect to cx-tin phosphate in air and inert atmospheres. Moreover, it has a very high selectivity for the dehydration of isopropanol, 100% in an inert atmosphere and x 90% in air (Figs. 2 and 3). This confirms that, as revealed by the adsorption of pyridine, the majority of the active centers are acid sites (Fig. 1).

In contrast with zirconium phosphate, chromia-pillared zirconium phosphates present catalytic activity for the dehydration of isopropanol. This behaviour is a consequence of the appearance of acid sites not only on the accessible internal surface but also on chromia pillars (see Fig. 1) . The effect of the

A. Guerrero-Ruiz et al./Appl. Catal. A 92 (1992) 81-92 a9

presence of chromia between zirconium phosphate layers can be followed along the series of chromia-pillared zirconium phosphate. It can be seen that the activity of chromia-pillared zirconium phosphates increase gradually with the content of chromium both in an inert and in an air atmosphere (Table 2). Note that increasing amounts of chromium provide higher pillar heights. The activity of these materials is higher in the presence than in the absence of air. This is due to the dual character of the catalysts which have acid and redox centers, the latter only working in oxidant atmosphere where they are regenerated.

Owing to the appearance of active sites in chromia-pillared zirconium phosphate, the selectivities in the isopropanol reaction with respect to a-zirconium phosphate are inversed in almost all the cases. In an inert atmosphere (Fig. 2)) pillared materials produce exclusively propene; and in air (Fig. 3)) the selectivity for the dehydration of isopropylalcohol increases with the chromium content, from 9 to 85%; in contrast with zirconium phosphate which produces exclusively acetone. The pillared material treated in helium at 600°C (Cr- ZrP3/600) presents the highest activity and selectivity for the dehydration of isopropanol, suggesting that the number of active acid sites increases with temperature.

In order to determine the strength of the acid centres present in these pillared materials, the methanol reaction was carried out over CrZrP2 and CrSnP catalysts at 350°C. A significant methanol conversion ( > 35% ) was achieved and dimethyl ether was the major product in both cases, with a selectivity higher than 90%. As only very small amounts of light hydrocarbons (C, and C,) were detected, the dehydration of methanol was the predominant reaction. The results imply that only acid sites with moderate strength are present in these catalysts.

Churacterisation by XPS

Photoelectron spectra of WM phosphates (M = Zr, Sn) and of chromia-pillared homologues were recorded for both fresh and used catalysts in the isopropanol conversion. The binding energies (BE) of P 2p, Sn 3d,,,, Zr Sd,,, and Cr 2~~~~ core levels are listed in Table 3. It can be noted that the BE of the P 2p peak for cr-SnP is approximately 0.7 eV higher than that for the a-ZrP catalyst, in good agreement with the expected tendency of a higher electrone- gativity of Sn*+ ion. As a consequence of this, the surface P-OH groups are more acid on tin phosphate, as already confirmed by the infrared spectra of chemisorbed pyridine. From an inspection of the data of Table 3 it is apparent that the BE of the P 2p peak is the same for the a-SnP and its chromia-pillared homologue, however, it increases by ca. 0.9 eV in chromia-pillared zirconium phosphates with respect to its non-pillared counterpart, suggesting that the surface P-O groups interact with the chromia of the pillar more readily and strongly that in the case of the cw-SnP homologue.

so A. Guerrero-Ruiz et al./Appl. CataL A 92 (1992) 81-92

TABLE 3

Binding Energies (eV) of core levels in tin and zirconium phosphates

Catalyst” P 2p Sn3&,z Cr2p,,, Zr 3d

cr-ZrP, 133.0 - 182.5 a-ZrP,o 133.1 - 182.6 cr-ZrP, 133.1 - 182.6

CrZrPBr 133.8 - 577.6 183.3 CrZrPuo 133.9 - 577.6 183.3 CrZrP3, 134.0 - 577.5 183.5

ff-SnP,o 133.7 407.3 - ff-SnP, 133.8 487.4 -

CrSnPr 133.6 487.1 577.7 - CrSnP, 133.8 487.3 577.6 - CrSnP, 133.9 187.4 577.6 -

“f: fresh. u0: after reaction in the presence of air. u: after reaction in the absence of air.

Comparison of BE values for the Sn 3&,, peak in both cu-SnP and its chromia-pillared analogue does not reveal any differences, however, important discrepancies appear by comparing the BE of the Zr 3&/z peak in a-ZrP and its chromia-pillared counterpart, this increasing by more than 0.7 in the latter. Moreover, as can be seen in Fig. 4 the Zr 3d doublet for the pillared-chromia zirconium phosphate not only lost resolution but also increased the full width at half maximum by ca. 0.30 eV with respect to the non-pillared homologue. These findings indicate that zirconium possesses two different environments, i.e., presence of PO-H and PO-Cr groups. The whole XPS data indicate that chromia pillars and zirconium phosphate are indeed cross-linked Finally, as the BE of the Cr 2p,,, peak at ca. 577.6 eV, which closely corresponds to Cr3+ [25], does not change after on-stream, at least at the detection level of the XPS technique, it can be inferred that the redox property if any cannot be associated to chromium ions.

CONCLUSIONS

Layered cr-Zr and cY-Sn phosphates show catalytic behaviour when com- pared for the reaction of 2-propanol decomposition at 180” C. At this temperature a-SnP shows moderate acidity, including Brensted and Lewis sites, whereas these are absent in a-ZrP, as revealed by FT-IR spectra of adsorbed pyridine. Thus, cvZrP is inactive in an inert atmosphere and cu-SnP produces only propene. Moreover, redox sites, associated with tin or zirconium, were detected when oxygen was added to the feed; so that, in air, a-ZrP produces exclusively acetone, whereas a-SnP produces both acetone and propene.

A. Guerrero-Ruiz et al./Appl. Catal. A 92 (1992) 81-92 91

Zr 3d

I I I I I

188 185 182 17’ BE (eV)

Fig. 4. Zr 3d core level spectra of zirconium phosphates. cr-ZrP: (a) fresh; (a’) after reaction in the absence of air. CrZrP3: (b) fresh, (b’ ) after reaction in the absence of air.

The incorporation of chromia pillars into a-ZrP and a-SnP substantially modifies the catalytic properties of such phosphates, increasing Brensted and Lewis acidity, which is due in part to the accessible internal surface and in part to the chromia pillar itself.

By means of XPS it has been observed that upon chromia pillaring the chemical environment of the tetravalent cation and phosphate is altered to a greater extent in a-ZrP than in the cu-SnP homologue, suggesting the possible exis- tence of a true cross-linking between the interlayer chromia pillars and the phosphate layer in the former.

From these results it can be inferred that these chromia-pillared phosphates possess active sites, acid and redox, which can be exploited for catalytic pur- poses. Additionally, their relatively high thermal stability, specific surface area and the possibility to enlarge interlamellar distances between the phosphates layers by appropriate growth of chromia pillars enable them to be tailored as catalysts for specific reactions.

ACKNOWLEDGEMENTS

This research was supported by the CICYT of Spain (Grants MAT89-0565 and MAT90-0298).

92

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catalytic activity of layered a- (tin or zirconium) phosphates and

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