Chemical modification of H-ZSM-5 by adsorption of rhodium and phosphorus complexes

Applied Catalysis, 50 (1989) 131-147 Elsevier Science Publishers B.V., Amsterdam -

131 Printed in The Netherlands

Chemical Modification of H-ZSM-5 by Adsorption of Rhodium and Phosphorus Complexes A. RAHMAN, A. ADNOT, G. LEMAY and S. KALIAGUINE* Dkpartement de GPnie Chimique et GRAPS, UniversitB Laval, QuSbec (Canada) and G. JEAN CANMET Energy Research Laboratories, Energy, Mines and Resources Canada, Ottawa, Ontario (Canada) (Received 14 April 1988, revised manuscript received 20 December 1988)

ABSTRACT ZSM-5 catalysts were modified with rhodium and phosphorus using rhodium acetylacetonate, rhodium chlorocarbonyl triphenylphosphine and triphenylphosphine by a gas-phase adsorption technique. IR spectroscopy of adsorbedpyridine indicateda strongpoisoningeffect ofphosphorus, regardless of the presence or absence of rhodium. Quantitative estimates of extra-particle and intra-pore lattice rhodium particles sizes and loadings were obtained using a model of ESCA intensity ratios. This analysis was performed on catalysts subjected to methanol-to-gasoline (MTG)tests, before and after grinding, and confirmed by XLBA. The catalytic activity of the samples in MTG and n-pentane conversion test was examined. The specific surface area of rhodium determined using X-ray photoelectron spectroscopic intensities and XLBA, and also the Brensted acidity determined from IR spectroscopy of adsorbed pyridine, are correlated with the catalytic activities in MTG and n-pentane cracking tests.

INTRODUCTION

This work originated from a previous study [ 11 in which metal-loaded ZSM5 catalysts were compared for their deoxygenation activity in the presence of carbon monoxide, In the course of this study, a methanol-to-gasoline (MTG) test was performed in order to establish the effect of the metal deposition on the acidic catalytic properties of the ZSM-5 support. In the MTG test an Rh/ ZSM-5 catalyst was found to behave peculiarly, showing a higher olefinic content of the products than would be expected from the concentration of Brensted acid sites measured after calcination (see Fig. 1 in ref. 1). However, this rhodium catalyst was prepared by gas-phase adsorption (GPA) of rhodium chlorocarbonyl triphenylphosphine, so that poisoning of strong acid sites by phos-

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132

phorus was probable even though the triphenylphosphine ligand is too bulky to penetrate the ZSM-5 pore lattice. This study was therefore undertaken in order to establish the respective roles of rhodium and phosphorus during an MTG test conducted on the calcined catalyst. The cracking of n-pentane on the same catalysts was also examined in an additional attempt to characterize the acidic properties of the Rh-P/ZSM-5 catalysts. Rhodium-loaded zeolites have been investigated as catalysts for the hydrogenation of carbon monoxide [ 2,3], hydrogenolysis of hydrocarbons [ 4,5], dehydrogenation of alkanes [ 6,7], hydrogenation of aromatics [ 8,9] and olefins [lo] and isomerization of alkanes [ 71. Rhodium-loaded zeolites have also been employed in attempts to heterogenize the rhodium complexes used in several homogeneous processes [ 111. Such catalysts have been prepared for hydroformylation of olefins [ 12-141 and carbonylation of methanol [ 15-201 and ethanol

[211.

Phosphorus is known to act as a poison for strong Bronsted acid sites in ZSM-5 catalysts [ 22-271 and it was shown recently [ 281 that such a poisoning effect can be obtained by the high-temperature gas-phase adsorption of triphenylphosphine. The object of this work was to compare rhodium catalysts prepared by the gas-phase adsorption of two rhodium complexes, one having organic ligands, Rh(C5H702)3, and the other with phosphine ligands, RhCl(C0) (Ph,P),. The catalytic performances of these two catalysts in MTG and n-pentane cracking reactions were also compared with those of both the H-ZSM-5 support and a ZSM-5 catalyst highly poisoned by gas-phase adsorption of triphenylphosphine. Structural characterization of the Rh-P/ZSM-5 catalysts was performed using X-ray diffraction (XRD), IR spectroscopy of adsorbed pyridine and X-ray photoelectron spectroscopy (XPS ) . EXPERIMENTAL

Catalyst preparation

The H-ZSM-5 support zeolites were prepared according to the procedure described by Gabelica et al. [ 291. Activation of the samples was accomplished by repeated ion exchange with ammonium nitrate solution, heating in dry nitrogen to 500°C and calcination in air at 500°C. The crystallinity of the samples was determined by XRD and elemental analysis (Si/Al ratios) was performed by atomic absorption spectrometry. Rhodium acetylacetonate, Rh ( C5H702)3, rhodium chlorocarbonyl triphenylphosphine, RhCl (CO) (Ph,P) 2t and triphenylphosphine, ( CsH5)3P, were purchased from Strems Chemicals. They were deposited on H-ZSM-5 zeolite samples by the gas-phase adsorption technique [ 281. The samples were then

133 TABLE 1 Compositions of catalysts Sample

Catalyst

H-ZSM-5 (H-ZA, Si/AI=41) H-ZSM-5 (H-ZB, Si/Al=38) Rh/H-ZA Rh/H-ZB Rh-P/H-ZA Rh-P/H-ZB P/H-ZA “RAA = rhodium acetyl acetonate; TPP = triphenylphosphine.

Complex used in preparation”

Loading (wt.-%) Rh

P

4.27 4.42 4.3 4.03 _

_

_ RAA RAA RCTPP RCTPP TPP RCTPP = rhodium

chlorocarbonyl

2.4 2.25 2.44 triphenylphosphine;

calcined in air at 5OO”C, except for sample F, which was calcined at 550°C. Analyses were performed after the calcination. Catalyst B is the zeolite labelled HZ-2 in ref. 1. Catalyst F is the same as that designated 4.0% Rh/HZ-2 in ref. 1. In this work, the latter catalyst was again submitted to full characterization and to an MTG test. The catalytic results were identical with those obtained previously but some differences were found in the XRD and IR of adsorbed pyridine, as discussed below. Two types of H-ZSM-5 samples, with Si/A1=38 and 41, were prepared, henceforth designated H-ZA and H-ZB, respectively. The rhodium loading in the samples was kept almost constant (4.4 wt.-% ). Another catalyst was designed to contain about the same amount of phosphorus as those prepared with rhodium chlorocarbonyl triphenylphosphine, with a loading of 2.4 wt.-% of phosphorus; this is designated sample G. The weight percentages of rhodium and phosphorus in the calcined samples, determined by atomic absorption spectrometry, are reported in Table 1. Catalyst characterization XRD X-ray diffraction patterns were obtained on a Philips spectrometer using Cu Ka, excitation. Crystallite sizes were calculated from the half-width of the rhodium line, using the Debye-Sherrer equation [ 301. IR of adsorbed pyridine Thin self-supporting wafers of the samples of ca. 6 mg were outgassed overnight at 400°C under vacuum (lo-” Torr). Pyridine vapour was passed over

134

these wafers, cooled at room temperature, for 5 min. Physisorbed pyridine was eliminated completely by degassing under vacuum at 200°C for 16 h. Spectra were recorded both before and after pyridine adsorption, at room temperature, in the spectral range 4000-1000 cm-l with 2 cm-l resolution, using a Digilab FTS-60 instrument. Details of the experimental method were given previously

[281. ESCA

The XPS measurements of all samples were carried out using a VG ESCALAB Mark II electron spectrometer. Mg Ka X-ray radiation (1253.6 eV) was used. The Si,, line at 103.4 eV was used as an internal standard and a measure of a charging correction factor. All binding energies were obtained with a precision of 2 0.1 eV. XPS intensities were calculated by integrating peak areas after subtraction of the baseline. Catalytic tests

MTG and n-pentane cracking reactions were carried out at atmospheric pressure and 400°C in a microcatalytic fixed-bed reactor. Methanol or n-pentane (99.99% pure, Fisher) were injected for 2 h into the reactor and a constant WHSV of 1.4 h-l was maintained for all runs (an incorrect value of 1.7 h-l was reported in ref. 1; it should also have been 1.4 h-l). The reactor assembly utilized in this work is similar to that described previously [ 281 except for the sample collecting device. Here all products leaving the reactor were collected at 15min intervals for 2 in a heated Valco automatic sampling valve controlled by a digital sequence programmer. After a catalytic test, the products stored in the sampling loops were analysed using a Perkin-Elmer Sigma 115 gas chromatograph equipped with two Porapak Q columns. RESULTS

FTIR Spectroscopy

Infrared spectra of adsorbed pyridine for all catalysts show bands due to chemisorbed pyridine at approximately 1546 cm-l (Brensted acid sites) and 1457 cm-l (Lewis acid sites). A third band at wavenumber 1491 cm-l is due to contributions of both Lewis and Bronsted acid sites. An estimate of the density ratio of Bronsted acid sites to Lewis acid sites was obtained using an adapted version of the method originally proposed by Rhee et al. [ 311 and described previously [ 281:

135

In calculating the Bronsted and Lewis acid site concentrations of the support (B, and L,, respectively), it is assumed that the Lewis acid sites observed in the IR spectrum of pyridine adsorbed on ZSM-5 are formed during calcination by dehydroxylation. For phosphorus-containing samples it is assumed that one phosphorus atom is exchanged for one Bronsted acid site, and for those with rhodium that one Rh2+ cation is exchanged for two Bronsted acid sites. Interestingly, only a very small amount of rhodium exchanged with the Bronsted acid sites, as seen from the data in Table 2 for catalysts C and D, whereas a heavy acid-poisoning effect of phosphorus (sample G) was observed. For the purpose of the calculations, it was therefore assumed that only phosphorus is exchanged with the Brsnsted acid sites when both rhodium and phosphorus are introduced simultaneously. The number of unexchanged Bronsted acid sites and of exchanged phosphorus and rhodium per unit cell calculated from IR absorbance ratios for all calcined samples are given in Table 2. IR spectra were also recorded before pyridine adsorption. In the OH region, the H-ZSM-5 supports (A and B) show bands at 3740 and 3610 cm-’ which correspond to OH vibrations in Si-OH and Al-OH, respectively. The same bands are also observed, with weaker intensity, in the spectra of samples containing only rhodium (samples C and D ) . When rhodium and phosphorus were both present in the sample (samples E and F), the OH bands are extremely weak and when only phosphorus is present (sample G) a new line is observed at about 3685 cm-’ (see ref. 28). After pyridine adsorption this new band is not altered, indicating that the new OH is not acidic. The Al-OH band representing Bronsted acid sites disappears completely after pyridine adsorption, whereas the SiOH band is not affected. The value of 0.6 B/u.c. reported for sample F in Table 2 is different from TABLE 2 Quantitative IR analysis of adsorbed pyridine Catalyst A B C D E F G

vBr.nsted

VLews

vB+L

(cm-‘)

(cm-‘)

(cm-‘)

1546 1545 1547 1546 1546 1540 1546

1457 1443 1454 1457 1457 1450 1445

1491 1491 1491 1491 1491 1491 1491

AR/AL'

Rh2+/u.cb

4.6

_

2.5 1.19 3.1 0.73 0.93 0.69

_ 0.15 0.05 _

P/U.C.b

_

1.52 1.00 1.53

B/u.c.~ (residual ) 1.8 1.6 1.3 1.7 0.28 0.6 0.27

“Ratio of absorbance6 at vBrensted (AB) and vLewis (AL). *u.c., Unit cell of the ZSM-5 support, A12.27Si93.i30192 (H-ZA) or Al2 46Si93.540192 (H-ZB).

136

that given previously because of the stoichiometry hypothesis involved in taking phosphorus poisoning into consideration. It is important to stress that the values for Brcansted (B/u.c.) and Lewis (Rh* + /u.c. ) acid site concentrations reported in Table 2 were calculated with various assumptions, and the validity of some of these hypotheses cannot be directly verified. For example, the aluminium Lewis acid sites formed on dehydroxylation may have different concentrations in the support and in the catalyst and this could introduce some systematic errors into the calculated data. Photoelectron spectroscopy The binding energies (BE) for Ols, A12p,Rhsd and Pzp lines and the P/Si and Rh/Si atomic ratios calculated from the intensities of these lines are reported in Table 3 for calcined samples. Table 4 shows similar results obtained for samples submitted to a 2-h catalytic MTG test. Ratios of atomic concentrations in the upper layers of a sample were calculated from the corresponding XPS peak areas using the following relationship: TABLE

3

XPS data for calcined samples Catalyst

E”

%

Al,,

Rhsb,*

Rhsti12

Ppp

(eV)

(ev)

(eV)

(ev)

(eV)

(ev)

(Rh/Si)*103

(P/Si)*103

BLllk

XPS

Bulk

XPS

C

4.3

533.2

75.6

309.9

314.5

-

26.7

36.3

-

_

E

5.0

533.0

75.4

310.1

314.8

135.0

F G

5.4 4.7

532.5 533.8

75.4 75.0

309.7 -

314.4 _

134.8 134.9

26.8 25.1 -

63.5 28.8 -

49.5 46.4

148.6 66.5

48.0

36.2

“Calculated

by adjusting BE scale at Si,, = 103.4 eV.

TABLE 4 XPS data for samples after MTG tests Catalyst E” (eV)

C E F

5.4 4.3 4.8

01, (eV)

Al,, (eV)

533.6 75.5 533.7 75.4 533.0 75.0

Rh,,,, (eV)

307.3 306.7 307.1

Rh/Si*103

P/Si.103

C/Si. lo3

Bulk

XPS

Bulk

XPS

Bulk

XPS

-

285.0 26.7 135.7 284.2 26.8

25.1 47.4

49.5

41.8

-

9.8 141.7

135.7 284.4 25.1

22.9

46.4

37.5

-

239.7

Rhsti,,

Pap

C,,

(eV)

(eV)

(eV)

312.1 311.3 311.9

“Calculated by adjusting BE scale at Si,,= 103.4 eV.

137 160

E

AP A Rh I’

,’

,’

,’

,’

I’

,’

,’

,’

,’

I’

,’

20

,’

(Rh

60 /Si)

BULK

,

,’

,I’

,,’

I 40

,’

-1’

I

‘0”’

,’

,’

80

100

IP/Sil

BULK

120

140

1’ 0

x lo3

Fig. l.(Rh/Si)ESCA, (P/Si)ESCA atomic ratio calculated from I&CA intensity ratio as a function of bulk atomic ratio (Rh/Si)bulk, ( P/Si)bulk for calcined unground catalysts. A, P; A ,Rh.

M

(->

nM ----.-.-.-

Si XPS-Tl+-Asi

AM

%i

ABi

0,

r?.MDM

Dsi

(1)

M=P or Rh, o is the cross-section of photoelectron emission, 3, is the escape depth, nM/nsi is the ratio of the atomic densities of element M and silicon and DM/Dsi is the electron detection efficiency ratio, which equals (E& Eksi) -‘D for the VG ESCALAB II, where Ek represents the kinetic energy of the emitted electron. Fig. 1 shows the P/Si and Rh/Si atomic ratios calculated from XPS intensity ratios using eqn. (1) as a function of the bulk values of P/Si for samples C, E, F and G. Rhodium 3d XPS lines obtained for calcined samples (C, E and F) and similar spectra for the same catalysts after they have been submitted to a catalytic MTG test are shown in Fig. 2. A model was recently derived by Kaliaguine et al. [ 321 to quantify the ESCA intensity ratios for supported catalysts. This model represents the support by a sheet stacking model and is therefore similar to that proposed earlier by Kerkhof and Moulijn [ 331. The new model, however, considers the supported phase to be divided into two parts: one part (the surface segregated fraction) is located on the external surface of the support particle (wt.-% x2, crystallite size C,) and the other part is well dispersed within the support pore lattice (wt.-% x1, crystallite size C,). All four adjustable parameters are not independent as x1+x2 = xn is the bulk value which can be measured independently. The procedure proposed previously [ 321 involves the experimental determination of ESCA intensity ratios for both the unground and the “perfectly ground” catalyst, so that two of the independent adjustable parameters can be where

C (AFTER MTGl

!

307

312

317

322

BINDING ENERGY

327

3 2

(eV)

Fig. 2. Comparison of Rhsd XPS lines of samples C, E and F before and after MTG tests.

calculated provided that the third can be given a reasonably assumed value. As the ZSM-5 pore diameter is between 5.1 and 5.8 A, it is reasonable to assume a value of C, between 2 and 6 A. Table 5 gives the calculated values for these parameters using the Rhad lines in the spectra of samples C, E and F obtained after their use in an MTG test. From these data, the metallic surface areas S, and S, can be calculated as

X-ray diffraction XRD patterns were recorded for samples A, B, C, E and F in the calcined state and after an MTG test. No new line corresponding to a phosphoruscontaining phase was observed, implying that the phosphorus oxidic phase is

139

TABLE 5 Results of quantitative treatment of XPS data for samples after MTG tests Catalyst (after MTG)

3~~ (wt.-%)

(lRhx+/&np

)XPS

Calculated from two XPS intensity ratios

Unground Ground xl catalysts catalysts (wt.%)

C

E

F

4.27

4.3

4.03

0.399

0.752

0.363

x2

(wt.%‘)

CL

S, (m*/g)

S, (m*/g)

Cg

(A)

(A)

0.291

3.62 3.72 3.83 3.93 4.04

0.65 0.55 0.44 0.34 0.23

2 3 4 5 6

159.6 134.6 108.9 82.5 54.3

74.8 51.2 39.6 32.5 27.8

0.17 0.17 0.17 0.17 0.18

0.258

2.95 3.03 3.11 3.20 3.28

1.35 1.27 1.19 1.10 1.02

2 3 4 r z

74.2 69.3 64.3 59.1 53.8

60.9 41.7 32.1 26.4 22.6

0.74 0.76 0.76 0.77 0.78

2.82 2.90 2.98 3.03 3.14

1.21 1.13 1.05 0.97 0.89

2 3 4 5 6

236.9 221.6 205.9 189.9 173.7

58.3 39.9 30.8 25.3 21.6

0.21 0.21 0.21 0.21 0.21

0.228

either finely dispersed or amorphous. For calcined catalysts C, E and F a new broad line at 28= 34.1 i 0.1’ was observed, corresponding to an RhzO, phase. The crystallite sizes calculated from the line broadening were 182, 90 and 80 A, respectively. For the same catalysts submitted to an MTG test, the metallic rhodium line at 20=40.8i 0.1’ was observed. The crystallite size calculated from the broadening of this line were 134,239 and 197 A, respectively. Well calcined rhodium catalysts show no significant metallic rhodium XRD line. Catalytic tests

The results of MTG tests are reported in Table 6 as the weight percent distribution of the products. The conversion of methanol is complete in all instances and, as all tests were conducted using the same inlet flow-rate and the same reaction time, the MTG activities of the various catalysts can be compared on the basis of the concentrations of intermediate products such as C,C, olefins and the overall production of end products as such as C&-C,,, aromatics, according to the MTG reaction pathway [ 341:

140 TABLE 6 Results of MTG tests Operating conditions: temperature, 400’ C; pressure, 1 atm; WHSV, 1.4 h-l; helium flow-rate, 30 ml (STP)/min. Catalyst

Parameter

Product distribution (wt.-%): (CG+H,),,l, HJ&,t.n+ H,Gwosb hydrocarbons %WGS Hydrocarbon distribution (wt.-% ): CH, C-C, paraffins C,-C, olefins Olefins in C,-C, (% ) C,+ Aromatics Aromatics in Cc+ (% ) H in hydrocarbons ( % ) Production of aromatics (gper2 h)

A

B

C

E

F

G

0.6 55.1 44.3 0.1

1.1 55.6 43.4 0

15.4 48.3 38.6 1.8

4.6 51.2 43.3 0.6

0 53.8 46.2 0

0.7 52.9 46.4 0.06

0.8 51.4 2.7 4.9 45.1 30.3 66.5 51.6 0.37

0.8 49.8 2.1 5.8 47.3 31.2 65.9 50.5 0.36

3.8 35.5 0.2 0.63 60.5 35.1 58.1 48.6 0.34

1.9 10.4 47.1 81.8 40.3 11.9 28.0 48.2 0.13

0.7 26.3 22.0 45.0 51.0 26.3 53.7 54.2 0.34

0.7 14.0 45.1 76.3 40.1 11.2 28.0 52.9 0.14

“Amount of water measured in the reaction products. *Amount of water converted by WGS (calculated from CO* production). ‘%WGS = lOOH,Owos/ (H,O,,,, + H,Owcs).

BCH,OH%CH,OCH,-

C,-C, olefins-

paraffins cycloparaffins aromatics

(2)

In addition to the hydrocarbon distribution in the products, Table 6 reports the percentage of water consumption through the water gas shift reaction: CO + H,O-

C02+H2

(3)

as measured from the production of carbon dioxide. In this instance the carbon monoxide reactant originates from some methanol decomposition: CH,OH-

CO+2Hz

(4)

so that the weight percent of (CO+H,),,,c, obtained by this reaction can be calculated from the production of both carbon monoxide and carbon dioxide.

141

1.8%

Fe/H-Z1

BRBNSTED

SITES

/ U.C

Fig. 3. C&-C, olefin production (g/g CH,OH) as a function of the number of Brensted acid sites per unit cell. The broken line represents data from refs. 1 and 35.

5%

4.6%

Fe/H-Z1

1.8%

Fe/H-Z1

Fe/H-Z1

BRBNSTEO

I

1

I

I

I

1.0 SITES

1.2

1.4

1.6

1.8

/ u.c

:

2

Fig. 4.Production of C,-C,, aromatics (g/g CH,OH) as a function of the number of Brensted acid sites per unit cell. The broken line represents data from refs. 1 and 35.

l.

.

xx’ -.

0

0’ ‘-?a__ /

--__

I’ I’

(al --__

l

-...

-----___

0”

G 01

,h, 0.2

E

F

I 0.4

0.6

,

0.8

BRQNSTED

I

I

1.0

1.2

SITES

,C

I

1.4

@IAd -*-_

,B

I

1.6

_.

,A

I

1.8

I

2.0

/ U.C.

Fig. 5. Primary production of C&C,,, paraffins ig/g CH,OH) Brensted acid sites ner unit cell.

as a function of the number of

142

0

I

0.2

I

0.4

I

0.6

1

1

1

0.8

1.0

1.2

BRQNSTED

SITES

I

I

1.6

1.4

I

1.8

I

J

2.0

/ U.C.

Fig. 6. Primary production of C5-CIO paraffins (g/g CH,OH) as a function of the number of Brensted acid sites per unit cell. The broken line represents data from refs. 1 and 35. Solid line, redrawn from Fig. 5 on the basis of methanol not decomposed. ---

18 -

*

A

-nc

L 3 8

16-

E

14-

2J

12-

e

lo-

--oE -

CARBON

.G

NUMBER

Fig. 7. Carbon numbers distribution of the hydrocarbon products of MTG tests.

Some of these results are reported as functions of the Brransted acid site concentration in Fig. 3 (production of C&-C, olefins, g/g CH,OH), Fig. 4 (production of C!&& aromatics, g/g CH,OH) and Fig. 5 (primary production of C,-C,, paraffins, g/g CH,OH). The upper curve in Fig. 5 was recalculated on the basis of the mass of methanol not decomposed and plotted in Fig. 6. Fig. 7 shows the carbon number distribution in the hydrocarbon products. In order to assess the cracking properties of the catalysts under study, a test of n-pentane cracking was performed under conditions similar to those used in the MTG tests. The results for n-pentane conversion and product distri-

143 TABLE 7 Results of n-pentane cracking tests Operating conditions as in Table 6. Parameter

Catalyst

Conversion (% ) Product distribution (wt.-% ): CH, C&-C, paraffins C-C, olefins Olefins in C&-C, (% ) Aiimatics C Aromatics in C,, (% ) Production of aromatics (gper 2 h)

CARBON

A

D

82.5

87

0.4 65.6 3.5 5.1 4.8 6.3 75 0.1

E

F

G

6.0

7.4

5.7

0.6 74.4 3.0 3.8 2.0 1.5

0.12 0.4 0.9 68 Trace 0.03

0.09 0.3 0.4 60 0.11 0.12

0.2 0.5 1.3 72 0.07 0.1

73 0.04

Trace Trace

91 0.003

70 0.002

NUMBER

Fig. 8. Carbon number distribution of the products of n-pentane conversion tests.

bution are given in Table 7. The carbon number distribution of the products is shown in Fig. 8. DISCUSSION

As is obvious from the comparison of the Rhsd binding energies shown in Fig. 2 and from XRD, rhodium is reduced to the metallic state under the con-

144

ditions of the MTG tests. This result was not entirely expected as the > 50% water content in the MTG reaction products does not make this atmosphere especiallyreducing. Catalysts, C, E and F are thus bifunctional catalysts containing both Brnrnsted acid sites and metallic rhodium sites. A comparison of the data for ( CO+Hz)calc. in the MTG tests reported in Table 6 suggests that the methanol decomposition reaction (4 ) occurs on metallic rhodium. Assuming a decomposition rate proportional to the metal surface area, these results suggest that the rhodium surface areas are in the proportions 1:0.33:0 for catalysts C, E and F. Interestingly, the % WGS values are also in the same proportions for these three catalysts, but this cannot be considered as proof that the WGS site is on the metal surface as in the absence of rhodium almost no carbon monoxide is present in the gas phase and therefore no WGS activity can be observed. Also interesting is the fact that if one subtracts a blank methane production of 0.7-0.8% (catalysts A, B and G), the activity for methane production over rhodium catalysts is increased by 3.1,1.2 and 0 for catalysts C, E and F, respectively, suggesting again a very small or very inactive metal surface for catalyst F. The production of C&-C, olefins reported in Fig. 3 does not seem to be affected much by the presence of metallic rhodium, as a smooth curve can represent all the data in Table 6. In particular, the points corresponding to catalysts E and G show very well that the C,-C, olefin conversion rate is entirely associated with the poisoning effect of phosphorus demonstrated previously [ 281, and not to the presence of rhodium. Figs. 4 and 5 show the production of aromatics and the primary production of C,-C,, paraffins, respectively. In this last instance the curve is obtained by assuming that the decreasing curve observed for ( Cg+ ) - (Ar) as a function of the number of Bronsted acid sites per unit cell (curve a in Fig. 5) is due to the increased cracking of C&-C,, paraffins and cycloparaffins yielding C,-C, alkanes (curveb inFig.5). Thusbyadding (C,,) - (Ar) + (C,-C,) oneobtains the curve reported in Fig. 5 as the primary production of C,-C,, paraffins (curve a+ b in Fig. 5). Both this curve and that for aromatics production reported in Fig. 4 show a continuous increase complementary to the decrease in C,-C, olefin production shown in Fig. 3. In Figs. 4 and 5, both points corresponding to catalysts C and E are slightly below the curves. These decreased productions can be explained by the decomposition of methanol over the rhodium surface, yielding less methanol converted by MTG as demonstrated by the smooth curve obtained when the production of aliphatics is expressed on the basis of nondecomposed methanol (Fig. 6). The fact that the points for catalyst F both lie on the curve is another independent indication of a very low methanol decomposition activity for this catalyst and possibly of a very small metal surface area. In Fig. 3 the data for C&-C, olefin production over P/ZSM-5 and Rh-P/ ZSM-5 catalysts are compared with similar data obtained in previous work

145

under the same conditions for H/ZSM-5, Fe/ZSM-5 and Ru/ZSM-5 catalysts. The iron and ruthenium data were on the same line as that for non-poisoned catalysts, showing that only the decrease in Brransted sites by ion exchange was responsible for the decrease in the olefin conversion rate. Fig. 3 shows that this is not the case with phosphorus-poisoned catalysts as the C2-Cd olefin yield is higher at high phosphorus loadings. As shown by the corresponding curves in Figs. 4 and 6, this increased olefin production is indeed accompanied by a decrease in both aromatics and paraffins. These differences are observed at high phosphorus loadings when a large fraction of the pore volume is plugged with phosphorus oxide. They may therefore be associated with a lower diffusivity of methanol in the ZSM-5 pores and C,-C, olefins produced in the outer shell of each ZSM-5 particle, secondary conversion to aromatics and paraffins thus being less significant. The comparison of n-pentane conversions (Table 7) shows high conversions for catalysts A and D and very low conversions for those catalysts containing phosphorus, irrespective of the presence or absence of rhodium. However, the comparison between catalysts A and D indicates that in addition to n-pentane cracking over Bronsted acid sites some hydrogenolysis must also take place over metallic sites, yielding C,-C, paraffms; not only is the overall conversion higher with catalyst D, but the total C&-C4paraffin production is also noticeably higher. Simultaneously, the comparison between catalysts E and G shows no significant difference between conversion and product distribution. It must therefore be concluded that the action of rhodium on n-pentane is only significant when cracking on acid sites is already proceeding and some hydrogen is present owing to aromatization of intermediate olefins. All these effects are illustrated in Fig. 8, which shows both that important conversions are obtained with catalysts A and D and that the conversion to propane, for example, is higher with the rhodium-containing catalyst D than with the bare zeolite catalyst A. Interestingly, the average rhodium particle size calculated from XLBA for catalyst C (134 A) is within the range of C, values calculated from XPS data and reported in Table 5. This would suggest a C, value of 2-3 A and a metal surface area (S, + S,) of 50-70 m2/g. With catalyst F, the agreement between the XLBA-calculated size (197 A) and the XPS values is also satisfactory, suggesting a C, value of about 5 A and a metal surface area close to 25 m’/g. Unfortunately, the agreement is not so good for catalyst E, which shows an XLBA particle size of 239 A. In this instance we suspect that the XLBA value is incorrect as the only difference in the method of preparation of catalysts E and F is a slightly higher calcination temperature for the latter. One would therefore expect more Rh203 agglomeration in calcined F sample. The higher calculated value of C, for this sample compared with sample E, corresponding to a lower XPS intensity ratio (Inh/lsi, see Fig. 1) for unground samples, seems logical. Therefore, accepting a value of 4-5 A for C, would yield a C, value of

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ca. 60 A and a metal surface area close to 30 m’/g. With these hypotheses, the rhodium surface areas in catalysts C, E and F would be in the proportions 60:30:25. As the methanol decomposition rates were in the proportions 1:0.33:0 it must be concluded that the rhodium surfaces prepared in the presence of phosphorus are less active in this reaction, especially when the calcination temperature is higher as in sample F. CONCLUSION

This work allowed the respective roles of phosphorus and rhodium in the modifications by the gas-phase adsorption technique of the catalytic properties of ZSM-5 in MTG reactions to be established. Phosphorus was shown to be responsible for very efficient poisoning of Bronsted acid sites irrespective of the presence or absence of rhodium. Only a minor exchange of protons for rhodium cations (presumably as Rh2+ ) was observed. This is in contrast with previous observations with iron catalysts prepared in a similar fashion and in which exchange with Fe’+ ions had a major effect on the catalytic properties

111.

The Rh203 phase present in the calcined catalyst is rapidly reduced to metallic rhodium in MTG tests. The metallic surface was found to be associated with the reactions of methanol decomposition and possibly methanation of carbon monoxide by hydrogen, these two gases being produced by methanol decomposition. The structure and activity of the metallic surface were shown to be affected by the preparation procedure. In particular, it seems that the rhodium surface prepared in the presence of phosphorus is less active in methanol decomposition. Experiments on n-pentane cracking over these catalysts confirmed the poisoning of acid sites by phosphorus and indicate that some hydrogenolysis of npentane over the rhodium may take place whenever hydrogen is formed. This work also illustrates the use of the XPS quantitation procedure described previously [ 28,321. The detailed information that it yields on the structure of supported rhodium stresses the usefulness of this technique and the need for a more thorough investigation of its limitations.

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