monoclinic transition in single crystals of zeolite ZSM-5

The orthorhombic/monoclinic transition in single crystals of zeolite ZSM-5 H. van Koningsv,e, ld

Labomto O, ,JAppli~ rl Ph)'.sir'.,. l)el[7 t',i',,,'i:,it)' ~![ 7"echmdog3', l.mr',tzwr'q I. 2628 (~] I)H[I, The Netherlands and J. C. Jansen and H. van Bekkum Laboralo O, rJ O~ga,ic ChrmL~tO,. l)el[/ ~ r,ivr'r,i/)' ,[ 7"rclm,,Ir~g~' . ~ , l i t , t h i n , BL De(l?, The NelheHa,ds (Received 10 March 19,~'7: reviwd 21 ,~lm' 19,77)

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X-ray photographs of single crystals of zeolite H-ZSM-5 (Si~.9~AIo.o,Ho,o,Oz~) at different temperatures are presented. The reversible orthorhombic/monoclinic transition, previously observed with XRD and 29Si m.a.s.n.m.r, on powder samples of H-ZSM-5, is confirmed. Upon cooling, the orthorhombic H-ZSM-5 single crystal changes into an aggregate of monoclinic twin domains. The directions of a and c in the orthorhombic single crystal and in the monoclinic twinned crystal (ct ~ 90.5 °) are identical. The deviation of c~ from 90 ° can be ascribed to a mutual shift of successive (010) pentasil layers along c, induced by a distortion or rotation of the T, and 1-6 rings interconnecting the layers. The space group of the monoclinic phase most probably is P~ln11. Keywords: H-ZSM-5; X-ray diffraction; temperature-dependent symmetry change; (twinned) single crystal

INTRODUCTION In a recent paper, it detailed ci-yslal structure analysis of zeolite ZSM-5 was cornpared with earlier pul)lisl'led resuhs on the same con~l)ound. ~ T h e slruciure can be successfully described in the ovtht~rhoml)ic space g r o u p Pnnta. Conflicting resuhs on the physical properties of calcined powder santples of ZSM-5 have been reported. "-'':~ T o avoid these contradictions, a detailed pretreatnlent o1 the calcined powder silnil)les has been suggested. ~ "Flae resuhing ZSM-5 modification, H-ZSM-5, shows a reversible phase Iransili~n ill a b o u t 330 K with no a p p a r e n l hysteresis (the t e n i p e r a t u r e being d e p e n d e n t on Ihe AI ctmlenl) mid exhibits monoclinic s y m m e t r y below and orthorhotnbic s y m m e t r y above the transilion t e m p e r a t u r e . :~-~ AI r o o m temperatt, re, the s y m m e t r y change can I)e reversibly induced by loading/unloading H-ZSM-5 powcler with N H f and with various organic molecules, i>'7 The sorbate-loaded and -unhladed H-ZSM-5 show o r i h o r h o n i b i c and nlonoclhlic Syllilllell), respectively. T h e reports citecf-'-7 present studies on d~e symnaetry change in p o w d e r samples of H-ZSM-5 using X-ray diffi'action and 2USi nt.a.s.n.m.r, spectrt~scopy. This p a p e r describes the examination of the reversible orthorhoml~ic/naonoclinic s y m m e t r y change in single crystals o f H-ZSM-5 using X-ray iJhomgraphs taken at different t e m p e r a t u r e s .

EXPERIMENTAL ZSM-5 single crystals, with Si/AI ratios o f about 300 (van Koningsveld el al. I), were growrl according to il 0144-2449/87/060564-05 $03.00 (~) 1987 Butterworth Publishers 564

ZEOLITES, 1987, Vol 7, November

p r o c e d u r e described recently by L e r m e r el al. '~ Severill as-synthesized single cr)suds of ZSM-5 were calcined m 811 K. T h e hyclrclgen forln o[" Ihe cr)si',fls (H-ZSM-5) was obtained I)y ion-exchange tff the calcined crystals with NH ICI l'¢)llowed by a second calcimttion at 811 K. T h r e e H-ZSM-5 crysutls were oriented, with a, b and c parallel to the i-olation axis, on a universal h i g h - l e m p e r a m r e device For single crystal dif|'raction. ~ Rotation i~ht~tcJgral~hs were hlken ill l e m p e r a l u r e s rallgillg [r(ml rt~tml ten~peratttre elI) to 400 K and. subsequelltly, ch>wnwmd tc~ rt~om t e m p e r a t u r e with intervals c>[' l I) K. All i)hotc~gral~hs were m a d e with a rotmitm angle o[ 30 ° . T h e t)rthorhollll)ic/nlonoclhlic (
RESULTS "Fhe rolation pholographs of Ihe H-ZSM-5 cryslals al 400 K e×hibi! ord~orht)nll)ic symmelry (Figure 1 [Iop]). Upon cooling m 295 K. the diagrams sill] show mirror symmelry with respect to Ihe equalor. [n addition, splitting of the diffraction spots is clbserved (FiEure 1 [boltom]). T h e i~laotographs at 295 K can be interpreted assuming the presence t)f (nearly) equally

Orthorhombic/monoclinic transition in single crystals of zeolite ZSM-5: H. van Koningsveld et al. S

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Figure 1 Details of rotation photographs of H-ZSM-5 crystals around [100], [010] and [001] (left to right) at 400 K {top) and 295 K (bottom), Rotation axis vertical

Pol)ulated orielll.ations of Illollocjilljc twin domains, illdllcing nlhl()r synlnlelry around Ihe e(ltl;llol-, l)if-

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t'raclion] b r o a d e n i n g is not observed, so the twin chmlains llltlsl have a size of a! least 0.1 p.m. T h e splitting o[ the la),er lines ht the rotation diagram a r o u n d b at ~95 K implies thai the b axis has a different direction in b,)th domains. T h e diagrams a r o u n d a and c al 295 K show diffraction spots to bc split up in the layer lines. ThereFore, the mon<)clinic

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reac[justment of" the crystals was necessary ch,Fiug the lemperature change, it is concluded d~at die clirection o [ a avid c is not si~nilicanll), ;.d'fecled l)y tile olin

Iransitloll+ The hkl} and 0kl Weissenberg })h~m>gra})hs of the H-ZSM-5 crystals at 295 K (Fig, re 2) show Ihal lhe twin domains have COlllnlOl! a* and b* axes. T h e angle between the c :'~ axes in both domains is about I.()°. T h e change in die direction ot'b d u r i n g the o/m

(b) Figure 2 Weissenberg photographs of an H-ZSM-5 crys'(al at 295 K. (a) hkO projection; (b) Okl projection

ZEOLITES, 1987, Vol 7, November

565

Orthorhombic/monoclinic transition in single crystals of zeolite ZSM-5: H. van Koningsveld et al. s

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transition must occur in the (100) plane, ht conclusion, the orthorhombic H-ZSM-5 single crystal changes into an aggregate of monoclinic twin ch~mains when the temperature is lowered. The domains have cotnmon a and e axes. The axial angle ~ is about 90.5 °. The direction of a and c in the crystal is not affected by the symmetry change. The deviation of 0~fl'om 90 ~ is caused by a change it] tile direct ion ~t" b in the (I 00) plane. The orienltatio,1 of die direct and

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reciprocal axes in the twin domains is given in Fig.,,, 3. The reversible character o f tile synmtetl-), change is

illustrated hy tile diagrams shown ill Figure 4. If~. The transition does not show apparent hysteresis as tile temperature is cycled. DISCUSSION.

The orthorhombic ZSM-5 framework cant be buih i~t the following way. A building unit, containing 12 'I" sites (T = Si, AI) and shown in Figure 5n, fi)rms chains along e by applying a twofi)ld screw axis parallel to c. More chains are generated by inversion centers, giving the (010) pentasil layer shown in Figure 5a. The three-dimensional fiamework is co,npleted hy linking neighboring (010) htyers via mirror planes. Therehy, a second type of" pentasil layers I3erpendicular to (100) is generated. A (100) cross section is shown in Figure 5(b). It shows that neighboring (010) pcntasil sheets (,-elated by ,n) are interconnected across m 13)' T-O-T bridges forming fl)ur- and six-naembered rings (T, and T(i rings). The lnirror phme contains only O atoms. The twin formation during tile o/m transition can be ascribed to a mutual shift ~)t successive (010) layers along +e or - c . These two opposite shear de[brmations have equal prol)ahility leading to a twin domain structure given in Fig, re 6. After the deformation, m is lost. The Ilk() Weissenberg photogrhph given in Figure 3a shows the presence of several hk0 reflections with h = odd, confirming the absence of the a glide plane perpenldicular to c. The space group of the monoclinic phase )',1 most hkely ts 1 ~/n I I. Boxhoorn et al.n° ascribe the structural changes to a possible final T., ring closure of adjacent silanol groups. However, this mechanism does not explain a •

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*The same structural changes can be induced by sorbed molecules. X-ray photographs of NHz loaded/unloaded crystals, taken at 295 K, are available on request.

566

ZEOLITES, 1987, Vol 7, November

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reversible process without hysteresis. The shifting of the (010) layers can be realized by distortion of tile T i and T6 rings, inducing translocation of the (010) pentasil layers. If, to a first approximation, the TO~ tetrahedra are considered as rigid bodies, the mutual shift of successive (010) layers will affect the T-O-T angles in the '1",, and T 6 rings, connecting the layers

Orthorhombic/rnonoclinic transition in single crystals of zeolite ZSM-5: H. van Koningsveld et al.

across m (Figure 5b), most prominently. However, these foul" independent T - O - T angles, ahhough all relatively small, do not have exceptionally small values (Table 1). T h e shearing might as well be realized by a rotation of T.j (and Ttl) rings around the stippled axis (Figure 5b), nearly conserving the T - O - T angles in the rings but affecting the T - O - T angles of nearly 180° in a region more distant f i ' o m m. egSi m.a.s.n.m.r, spectra at room temperature have heen interpreted on the basis of a quantitative relationship between the average T - O - T angle and the chemical shift for tile Si(OSi)I signals (i.e., signals from silicon atoms tetrahedrally joined, via oxygen bridges, to four other silicons).it Analogous spectra of monoclinic silicalite at 297 K correspond to average T - O - T angles ranging between 146 ° and 157 °, according to Hay et al.," or between 146.8 ° and 162.2 °, according to Klinowski et al.:-' T h e spectrum of the orthorhoml)ic phase at 353 K corresponds to average T - O - T angles between 153 ° and 163°) T h e corresponding angles in as-synthesized orthorlaombic ZSM-5, determined hy X-ray analysis, I range fi'om 150.5 ° to 162.8 ° (Table I), in good agreement with the n.m.r, resuhs. Unfortunately, the question as to which individual T - O - T angles are affected mostly by the shear deformation cannot be answered from these data. T o study these aspects in more detail, a structural analysis of a single crystal of H-ZSM-5 is necessary. In the present twinned na()noclinic crystal, the orientation of the twin domains is to(J close together to p e r m i t single-domain intensity m e a s u r e m e n t s . Measurement of the reciprocal lattice points together inevitably leads to orthorhombic symmetry because hoth twin domains have eqtml probalfility. Efff)rts to produce monoclinic single crystals of ZSM-5 are continued. T o that end, mechanical strain will be applied to a heated calcined crystal.

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atoms (not drawn) are about midway between T atoms. (a) (010) layer with T~2 building unit indicated. (b) I100) layer with T4 and Ts

Table 1 Si-O-Si angles in an as-synthesized ZSM-5 single cwstal (van Koningsveld et al. 1)

rings indicated by bolded lines

Si-O-Si angles (°) Within the (010) pentasil layer

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Si(1)-O(1)-Si(2) Si(2)-O(2)-Si(3) Si(3)-O(3)-Si(4) Si(4)-O(4)-Si(5) Si(5)-O(5)-Si(6) Si(2)-O(6)-Si(6) Si(7)-O(7)-Si(8) Si(8)-O(8)-Si(9) Si(9)-O(9)-Si(10) Si(10)-O(10)-Si(11} Si(11)-O(11)-Si(12} Si(8)-O(t 2)-Si(12) Si(2)-O(13)-Si(8) Si(5)-O(14}-Si(11) Si(1)-O(15}-Si(10) Si(1)-O(16}-Si(4) Si(7)-O(17)-Si(4) Si(6)-O(18)-Si(9) Si(6)-O(19)-Si(3) Si(12}-O(20)-Si(3) Si(5)-O(21)-Si(1) Si(11 }-O(22)-Si(7}

153.1(3) 149.8(3) 175.8(4) 156.1(3) 147.9(2) 158.2(3) 156.2(3) 154.4(3) 154.6(3) 164.4(4) 153.5(3) 164.6(3) 175.9(4) 169.1(3) 148.2(3) 164.8(3) 149.4(3) 145.1(2) 162.9(3) 147.7(3) 145.4(2) 150.4(3)

Between (010) pentasil layers

T6-ring Si(7)-O-(23)-Si(7) Si(12)-O(24)-Si(12)

153.3(4) 146.3(3)

Si(9)-O(25)-Si(9) Si(10)-O(26)-Si(10)

148.0(4) 144.9(3)

T4-ring

Average SilOSi)4 angles (°) Si(1) Si(2) Si(3) Si(4) Si(5) Si(6) Si(7) Si(8) Si(9) Si(10) Si(11) Si(12)

152.9 159.3 159.1 161.5 154.6 153.5 152.3 162.8 150.5 153.0 159.4 153.0

Orthorhombic/monoclinic transition in single crystals of zeolite ZSM-5: H. van Koningsveld et al.

ACKNOWLEDGEMENTS The authors thank Profs. F muinstra and P. M. de Wolff, Laboratory of Applied Ph),sics, fi)r helpful remarks. REFERENCES 1 van Koningsveld, H. etaL Acta Crystallogr. 1987, B43, 127 2 Wu, E. L. et al. J. Phys. Chem. 1979, 83, 2777

3 Hay, D. G. and Jaeger, H. J. Chem. Soc., Chem. Commun. 1984, 1433 4 Hay, D. G. et al. J. Phys. Chem. 1985, 89, 1070 5 Klinowski, J. et al. Zeolites 1987, 7, 73 6 Fyfe, C. A. etal. J. Chem. Sot., Chem. Commun. 1984, 541 7 Kokotailo, G. T. et al. Pure Appl. Chem. 1996, 58, 1367 8 Lermer, H. et al. Zeofites 1985, 5, 131 9 Tuinstra, F. and Fraase Storm, G. M. J. AppL Crystallogr. 1978, 11,257 10 Boxhoorn, G. et al. Zeolites 1984, 4, 15 11 Smith, J. V. and Blackwell, C. S. Nature 1983, 303, 223