Modification of zsm-5 zeolite with trimethyl phosphite part 1. structure and acidity

MICROPOROUS AND MESOPOROUS MATERIALS Microporous

Modification

and

Mesoporous

Materials

20!

1998)

363

369

of ZSM-5 zeolite with trimethyl phosphite Part 1. structure and acidity Pekka Tynjiilii, Tuula T. Pakkanen *

Drprrrtment

of’C/rrtnist~~~,

Received 1 I September

bh~iwrsitl~

of’Jocmt4zr,

FIN-K0101

1997; received in revised form 6 November

1997;

Jo1wsurr.

Fidrml

accepted19November1997

Abstract In the present work we have studied the structure and the acidic properties of HZSM-5 zeolite modified with trimethyl phosphite. The modifications were carried out by using a chemical vapour deposition method as well as impregnation from n-octane solution. Both modification techniquesresultedin materialswith similaracidic properties. Trimethyl phosphite reacted with both Br(insted acid sites and terminal SiOH groups. The modifications resulted in the total elimination of strong Br(insted acid sites accompanied by the formation of new Briinsted type sites with decreased acid strength. However, the total acidity of the modified zeolites stayed relatively high. A possible structural transformation resulting from the phosphite modifications was proposed to be an insertion of phosphorus species between the structural aluminiumand silicon--oxygen tetrahedra. However. on the basis of 3’P MAS NMR studies no evidence of the isomorphous substitution of structural T-atoms by phosphorus was found. 0 1998 Elsevler Science B.V. Kq~o&:

FTIR:

MAS NMR

spectroscopy;

TPD: Trimethyl

1. Introduction Zeolites with a pentasile structure, especially ZSM-5, are widely used as a shape selective additive in modern composite catalysts for the industrial processing of hydrocarbons. The catalytic importance of zeolites is generally related to the presence of tetrahedrally coordinated aluminium ions resulting in a negative charge into the framework balanced by protons thus giving rise to the BrGnsted type acidity [ 1,2]. The strength of the BrGnsted acid sites is mainly affected by the con-

* Corresponding author. Fax: 358 13 2513344.

Tel: 358

1387-181 1/98,‘$19.00 ~LJ 1998 Sl387-181 I197)00050-4

PI/

Elsevirr

13 2513340:

Science

B.V.

All

rights

reserved

phosphite;

ZSM-5

centration of aluminium in the first coordination sphere of the acid site and the SILO--Al bond angle [3,4]. The purpose of the post-synthesis modifications is usually to tailor the concentration or the strength of the acidic centers in order to enhance the formation of desirable reaction products. The modifications of zeolites with phosphorus compounds such as phosphoric acid [5], trimethyl phosphite [6] and trimethyl phosphine [7] have been observed to produce materials with favorable catalytic properties. However, in the literature there have been different views of the effect of phosphorus modifications on the acidity of ZSM-5 zeolite. It has been reported that trimethyl phosphite reacts primarily with the acid sites located at

364

P. ~~njiilii,

T. T. Pukkaner~

/ Mic~roporou.s

the pore openings of the catalyst crystals and thus the intracrystalline acid sites remain unmodified [6]. According to another study trimethyl phosphite eliminates all Briinsted acid sites and decreases notably the concentration of weak acidic centers as well [8]. However, it has also been reported that the modification of ZSM-5 zeolite with trimethyl phosphite results in the loss of strong Briinsted acidity accompanied by the formation of new active centers with a weaker acid strength [9, lo]. The purpose of the present work was to study the effect of trimethyl phosphite modification on the structure and acidity of ZSM-5 zeolite. The modifications were carried out by using both CVD and impregnation techniques. In the second part of the work (P. Tynjala, T.T. Pakkanen, S. Mustamaki, unpublished results) the catalytic properties of the phosphorus modified zeolites were characterized in the conversion reaction of C,-C, alcohols to hydrocarbons.

2. Experimental ZSM-5 zeolite with a silicon to aluminium ratio of 54.4 and a crystallinity of 99% was prepared according to the literature methods [ 111. NH,ZSM-5 was converted to the corresponding hydrogen form, HZSM-5, by calcination in air at 813 K for 2 h. Catalyst P,ZSM-5 was prepared by the chemical vapour deposition of trimethyl phosphite on zeolite HZSM-5. The modification was carried out in a quartz tube reactor at ambient pressure. HZSM-5 was dehydrated at 673 K for 2 h in a nitrogen flow. Vapourized trimethyl phosphite was introduced to the reactor at 388 K by using nitrogen as a carrier gas. The volume of trimethyl phosphite was calculated to exceed at least three times the total number of Bronsted acid sites which was estimated from the aluminium content ( 1.O wt.%) of the zeolite. The modified zeolite sample was calcined in air at 8 13 K for 2 h. Sample PzZSM-5 was prepared by heating HZSM-5 zeolite (5.0 g) with the solution of trimethyl phosphite (2 ml ) in n-octane (25 ml ) under reflux for 20 h. After the reaction the zeolite was

und Me.wporou.s

Mutrriuis

20 [ IYYH) 363

36Y

filtered and washed with dichloromethane and npentane. The catalyst was dried at 373 K for 1 h and calcined in air at 773 K for 5 h. For the chemical analysis the modified zeolites were transferred into the liquid phase by using a HF/HNO, method. The phosphorus content of the samples was determined spectrophotometritally by using a colourimetric vanadomolybdophospheric acid method [ 121. The effect of the phosphite modifications on the acidic properties of the zeolite was studied by temperature programmed desorption of ammonia (NH,-TPD, determined with a Altamira TPDanalyzer). Zeolite samples were dehydrated under a helium stream (30 cmjimin) at 823 K for 1 h. Ammonia ( 10% in helium) was adsorbed at 373 K for 1 h and the physisorbed ammonia was removed (at 373 K for 1 h). The system was heated up to 823 K with a heating rate of 20 K/min under a helium stream, and the amount of desorbed ammonia was detected with a TCD detector. The phosphorus modified zeolites were characterized with a Nicolet Magna 750 FTIR spectrometer by using a diffuse reflectance method. The sample chamber was equipped with both nitrogen and vacuum line connections. The effect of phosphorus modifications on the acidity of the zeolite ZSM-5 was studied by chemisorption of pyridine. The zeolite with a 50 wt.% diamond powder matrix was dehydrated under nitrogen and vacuum conditions (ca. 300 Pa) at 673 K for 2 h. Pyridine was introduced to the chamber at 388 K for 15 min at a pressure of about 300 Pa. Physisorbed pyridine was removed from the zeolite by heating at 423 K for 2 h under vacuum. The IR spectra were measured at ambient temperature. NMR measurements were carried out with a Bruker AMX-400 FT NMR spectrometer equipped with a magic angle spinning probehead. For ‘H MAS NMR measurements (400.131 MHz, pulse length 2.0 ps, recycle delay 2.0 s) the samples were dehydrated under vacuum at 673 K for 8 h. Before the “7A1 MAS NMR measurements (65.177 MHz, pulse length 0.6 us, recycle delay 0.5 s) the samples were contacted with the vapour of 3 M NH,NO, solution for three days in vacuum. For 29Si MAS NMR (49.695 MHz, pulse length 5.0 us, recycle delay 10s) and 31P MAS NMR

measurements ( 161.976 MHz, pulse length 7.75 us, recycle delay 20 s, reference compound: hydroxy apatite resonating at 2.8 ppm relative to phosphoric acid) no sample pretreatments were required. The spinning rate for the 7 mm spinner was 5 kHz during the acquisition of data. The catalytic cracking of n-hexane over the unmodified HZSM-5 and the phosphorus modified ZSM-5 zeolite was carried out in a continuous flow reactor at 643 K. Reaction products were analyzed with a Hewlett-Packard 5890 Series 11 gas chromatography (on-line system) equipped with a flame ionization detector.

3. Results and discussion Zeolite HZSM-5 (Si/Al = 54.4) was modified with trimethyl phosphite by using a chemical vapour deposition technique (P,ZSM-5) as well as impregnation from n-octane solution (P,ZSM-5 ) resulting in catalysts with the phosphorus contents of 2.2 and 2.7 wt.%, respectively. The crystallinity and the silicon to aluminium ratio of the phosphite impregnated zeolite sample (P,ZSM-5) was 87% and 57%, respectively, thus indicating no significant dealumination of the framework. The characterization of the materials is summarized in Table 1. The effect of the phosphite modifications on the zeolite structure was characterized by 29Si, 27A1 and 31P MAS NMR spectroscopy. The acidity of the samples was characterized by temperature programmed desorption of ammonia as well as with FTIR and ‘H MAS NMR spectroscopy.

Table

3.1. FTIR studies The catalytic activity of zeolite materials is mainly due to the presence of structural bridging hydroxyl groups, Si(OH)Al, in which the protons are balancing the negative charge of aluminiurn-oxygen tetrahedra. These strong Briinsted acid sites are observed in the IR spectrum of ZSM-5 zeolite at wavenumber of 3614cm-’ [Fig. l(a)]. IR bands at 3740 and 3662 cm-’ have been assigned to silanol groups terminating the lattice and extra-lattice aluminium hydroxide species, respectively. A broad band at ca. 3500 cm-’ is due to hydrogen bonded silanol groups. Modifications with trimethyl phosphite resulted in drastic changesin the OH region of IR spectra [Fig. 1(a)]. In the spectra of P,ZSM-5 and P,ZSM-5 the band of terminal silanol groups at 3740 cm-’ disappeared accompanied by the formation of two weak bands at 3746 and 3727 cm-‘. The band at 3727 cm-’ has been assigned to a lateral interaction of a silanol group oxygen with an electron pair acceptor site [9]. Another effect of the modihcations was the disappearance of the Briinsted acid site signal at 3614 cm-’ thus indicating the loss of strong acidity in the samples.It has been reported that trimethyl phosphite modification results in the replacement of strong Brijnsted acid sites with H,PO, species [6], and the structure of these new weak acid sites has been proposed to be as presented in Fig. 1. The band due to P-OH vibrations appeared at a wavenumber of 3668 cm-’ and it was observed as a weak shoulder in the IR spectra of the phosphorus modified zeolites. A higher wavenumber compared with that

1

Characterization

of the trimethyl Modification

HZSM-5 P,ZSM-5 P,ZSM-5 a Not determined. b Determined from

phosphite method

CVD Impreg.

the aluminium

conetent

modified

ZSM-5

zeolites

Si:AI

Crystallinity

54.4 a 57

YY a 87

of unmodified

HZSM-5

(%)

P (wt.%)

41, P

Acidity

2.2 2.1

0.P ll.3fl

1060 930 935

(pmol:g)

366

Fig. I. (a) FTIR zeilite HZSM-5

spectra of zeolite HZSM-5 and phosphorus moditied

and phosphorus ZSM-5.

moditied

of Al (OH )Si groups indicates a lower acid strength of the formed sites. A broad band at ca. 3600 cm-’ was not assigned to strong Bronsted acidity since the intensity of the band was not atfected by the chemisorption of pyridine. The calculated Al to P ratios of P,ZSM-5 and P,ZSM-5 were 0.52 and 0.30. respectively, thus indicating a reaction capacity of trimethyl phosphite in excess of the structural aluminium content. The result can be explained by the reaction of trimethyl phosphite not only with the Bronsted acid sites but also with the terminal SiOH groups (Fig. 1). Basic pyridine chemisorbed on Bronsted acid sites (as a pyridinium ion) resulted in IR bands at the wave numbers of 1638 and I547 cm ~’ and the bands due to pyridine coordinated with Lewis acid sites appeared at 1625, 1455 and 1450 cm ‘. A strong band at 1491 cm-’ has been assigned to

HO

y /\/

0

7 ,0

‘*,/

kSi

\/

H ,

Rmm3

\

CalclMtlon

ZSM-5

in the OH

region.

(b)

Chemisorption

of pyridine

on

pyridine on both Briinsted and Lewis acid sites [Fig. l(b)]. Although the elimination of strong acid sites was concluded on the basis of the IR characterization of phosphite modified ZSM-5, a clear evidence of the presence of Briinsted type acidity was observed via the chemisorption of pyridine [Fig. 1 (b)]. The characteristic bands due to the chemisorption of pyridine on Briinsted acid sites were clearly detectable in both phosphorus modified samples. The broadening of the pyridinium ion signals could be due to the interaction of pyridine with new weak acid sites (Al--OH UP) formed during the modifications. Phosphite modifications decreased significantly the number of Lewis acid sites and also seemed to decrease the intensities of the pyridinium ion signals. This could be due to the decreasedpore dimensions resulting in a reduced diffusion rate of the quite bulky pyridine molecules. More information about the effect of the phosphorus modifications on the acidic properties was obtained by the TPD of ammonia. 3.2. TPD qf’anmonitr

Scheme trimethyl

I, Proposed phosphite.

structure

of ZSM-5

reolite

moditied

with

The temperature programmed desorption of ammonia over HZSM-5 and the phosphite modi-

fied catalysts is presented in Fig. 2. In the TPD curve of HZSM-5 two desorption maxima at temperatures of 528 and 763 K were detected indicating the presence of weak and strong acid sites, respectively. The peak at 763 K is due to the desorption of ammonia chemisorbed at strong Briinsted acid sites. The desorption peak at lower temperature has been assigned to weak acid sites as well as ammonia molecules adsorbed in the vicinity of the strong Bronsted sites. In the spectra of the phosphite-modified samples only one peak due to weak acid sites was observed. The asymmetric shape of the curves may be due to the slightly different acid strength of the new active centers compared with the weak acid sites of the unmodified zeolite. In spite of the total elimination of strong acid sites, phosphorus modifications resulted in only a slight reduction of the total acidity. The obtained result is in agreement with results reported earlier [9] thus confirming the replacement of the strong bridging hydroxyl groups by new Briinsted type sites with decreased acid strength. 3.3. Crucking

alkanes (C,-C,) was dominant although the formation of longer saturated C, hydrocarbons took place as well. The formation of alkenes and aromatics was not significant under the present reaction conditions. ZSM-5 modified with trimethyl phosphite showed no catalytic activity in the cracking of n-hexane. Due to the loss of strong Briinsted sites and to the incapability of the new acidic centers formed in the modifications to protonate n-hexane molecules, no conversion was observed. The result is in good agreement with the FTIR and TPD characterizations described above.

3.4. MAS NMR studies The iH MAS NMR spectrum of zeolite HZSM-5 is composed of two main signals with the chemical shifts of 4.1 and 1.7 ppm assigned to Briinsted acid sites and terminal silanol groups, The phosphite respectively. modifications decreased drastically the intensifies of both bands indicating the total loss of strong Bronsted acidity as well as of terminal SiOH groups. The result confirms the results obtained by other characterization methods. However, the presence of the new acidic centers (POH ) observed in the FTIR spectra was not clearly detectable in the proton MAS

of’n-hexane

In the catalytic cracking of n-hexane over unmodified HZSM-5 the production of small

P 2 ZSM-5.

Y35

t 100

150

200

250

300

350

400

450

SO0

550

600

Temperature (O C) Fig. 2. Temperature

programmed

desorption

of ammonia

over

HZSM-5

and trimethyl

phosphite

modified

ZSM-5

zeolites.

NMR spectra of the phosphite modified samples. The ‘H MAS NMR signal of P-OH groups in SAP0 materials appears at 1.5 (+0.3)ppm [ 131. Thus, a very weak resonance observed at ca. 2 ppm in the phosphorus modified zeolites could be due to the POH groups of the new acid sites. The shift of the OH resonance from 4.1 to 2 ppm indicates a decreased acid strength of the Al(OH)P sites compared with that of Al(OH)Si sites. The 29Si MAS NMR spectrum of HZSM-5 is composed of three signals at the chemical shift of -106, -112 and -115ppm. The band at - 106 ppm has been assigned to silicon tetrahedra surrounded by three other silicon- oxygen tetrahedra and by one aluminium tetrahedron and the signals at - 112 as well as a shoulder at 115 ppm to the silicon tetrahedra surrounded by four other SiO, groups (Fig. 3). The decreased intensity of the signal at - 106 ppm is proposed to be due to the insertion of the phosphorus group into the framework thus resulting in the lack of adjacent silicon and aluminium tetrahedra. Another explanation for the decreased intensity is the reaction of trimethyl phosphite with SiOH groups (Fig. 1). The 31P MAS NMR spectrum of the phosphite modified ZSM-5 (Fig. 3) was composed of two main signals at -2.6 and - 12.6 ppm which can be assigned probably to terminal --P--OH and -P=O species. respectively [14], thus being in accordance with the proposed structure of the new acidic centers (Fig. 1). The higher intensity of the band at -2.6 ppm indicates the higher number of POH groups compared with the number of P-O species. Two broad signals with weak intensities at 23 and 15 ppm were also detected. From to the lack of the signal at -30 ppm due to structural PO, tetrahedra it was concluded that no isomorphous substitution of structural T-atoms by phosphorous had occurred. It was furthermore concluded from the “Al MAS NMR spectra (Fig. 3) that a transformation of the zeolite framework had taken place because the signal at 55 ppm due to tetrahedrally coordinated aluminium decreased significantly after the modifications accompanied by the appearance of a new band at - 8 ppm. The decrease of the band at 55 ppm may be a consequence of the delamination of the framework as well as the distortion of

-2.6

‘i‘:-12.6

-106 ppm

-80

-112

-100

-120

29Si MAS

HZSM-5

i40

NMR

A’L

P, ZSM-5 Pz ZSM-5

g 55

ppm

80

*‘Al

Fig. 3. MAS NMR modified ZSM-5.

-8

60

40 MAS

spectra of HZSM-5

i0

0

NMR

and trimethyl

phosphite

the symmetry of the tetrahedral Al atoms, i.e. quadrupolar broadening. The band at - 8 ppm may be assigned to octahedrally coordinated aluminium atoms.

4. Conclusions In the present work we have studied the effect of trimethyl phosphite modifications on the acidity and the structure of zeolite HZSM-5. The modifications were carried out by the chemical vapour

369

deposition technique as well as by the impregnation from n-octane solution. The acidic properties of the phosphorus modilied zeolites were studied by FTIR and ‘H MAS NMR spectroscopy as well as the temperature programmed desorption of ammonia. It became obvious that the phosphorus modifications resulted in the total elimination of strong Briinsted acidity accompanied by the formation of new Bronsted type sites with reduced acid strength. The total number of acid sites stayed relatively high during the modifications. Major structural transformations in the phosphorus modified zeolites were concluded on the basis of 24Si and “‘Al MAS NMR studies. According to chemical analysis the Al/P ratios of the modified materials indicated the reaction of trimethyl phosphite in excess of the tetrahedral aluminium content due to the reaction of the phosphite with both Bronsted acid and terminal silanol sites. However, on the basis of MAS NMR studies no evidence for the isomorphous substitution of structural T-atoms with phosphorus was found.

Acknowledgement Financial support from the Development Center (TEKES) acknowledged.

Technology is warmly

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