Op ti mum Synthesis of Mesoporous Silica Ma te rials from Acidic Condition

Transcription

1 Jour nal of the Chi nese Chem i cal So ci ety, 2002, 49, Op ti mum Synthesis of Mesoporous Silica Ma te rials from Acidic Condition Hong-Ping Lin a ( ), Chin-Lung Kuo b ( ), Ben-Zu Wan b ( ) and Chung-Yuan Mou c * ( ) a De part ment of Chem is try, Na tional Cheng Kung Uni ver sity, Tainan 701, Taiwan, R.O.C. b De part ment of Chem i cal En gi neering, Na tional Tai wan Uni ver sity, Tai pei 106, Taiwan, R.O.C. c De part ment of Chem is try and Cen ter for Con densed Mat ters Sci ences, Na tional Tai wan Univer sity, Taipei 106, Taiwan, R.O.C. On the ba sis of the mix tures com posed of C ntmab/ac ids/teos/wa ter, the mesoporous ma te ri als with well-ordered hex ag o nal struc tures can be syn the sized un der acidic con di tion (ph < 1.0) with a care ful con trol of the sil ica con den sa tion. How ever, the in duc tion pe riod time for the for ma tion of surfactant-silica pre cip i ta - tion var ied with the em ployed acid source and de creased in the se ries: HNO 3 < HBr < HCl < H 2SO 4 at the same acid con cen tra tion. In creasing the acid con cen tra tion not only in duced a fast pre cip i ta tion rate, but also pro - voked more nu clei seeds for ma tion and re duced the par ti cle size of the mesoporous sil ica prod ucts. A proper ag ing time is also sig nif i cant for get ting the well-ordered mesoporous ma te ri als via acid route, and a lon ger ag ing time would in duce a grad ual de com po si tion of the mesostructures. For mor phol ogy, in acid syn thetic me dium, var i ous in ter est ing shell-like curved pat terns on the sur face of mesoporous sil ica par ti cles were formed. The phys i cal prop er ties (such as d-spacing, pore size, BET sur face area and po ros ity) of the cal cined mesoporous sil ica are al most not af fected by the na ture of the acid source, but the pore size of the mesoporous ma te ri als can be tai lored by vary ing the chain length of the surfactant. Adding the proper amount of am mo nia so lu tion for ad just ing the ph value be tween 8-10 can im prove the hex ag o nal mesostructures or der of the mesoporous ma te ri als while main tain ing the orig i nal mor phol ogy. IN TRO DUC TION The mesoporous ma te rial based on sil ica has been in - ten sively in ves ti gated for sci en tific in ter ests and prac ti cal ap - pli ca tions. 1,2 In par tic u lar, the dis cov ery of a new fam ily of mesoporous ma te ri als M41S by re search ers of Mobil Oil Corp. has at tracted great at ten tion by sci en tists and en gi - neers. 3,4 These M41S fam ily s mesoporous ma te ri als with uni formly sized nanochannels, tun able pore size ( nm) and high sur face area (~1000 m 2 /g) have been a fo cus of re cent ap pli ca tions for cat a lysts, cat a lytic sup ports, mo lec u - lar sep a ra tion, chem i cal sen sors, elec tronic and op ti cal de - vices, and ad vanced ma te ri als Typically, the mesoporous ma te ri als are syn the sized in al ka line con di tion, where the cationic qua ter nary am mo nium sur fac tants (S + ) com bine with the neg a tive-charged sil i cate spe cies (I - ) through strong elec tro static in ter ac tions (S + I - ). 12, 13 Due to the strong elec tro - static in ter ac tions, the sur fac tants and sil i cate oli go mers pre - fer to com bine to gether to form the mesostructural par ti cles, rather than ag gre gate to gether to form a hi er ar chi cal struc ture or grow on the sur face of other sub strates. This be hav ior ham pers the al ka line-made mesoporous ma te ri als in mor - phologies tai lor ing such as in low di elec tric con stant ma te - rial. For the ap pli ca tions of the al ka line-made mesoporous sil ica in semi con duc tors, re mov ing the high-concentration so dium ions, ex ist ing in so dium sil i cate as sil ica source, is still a se ri ous prob lem. How ever, Huo et al. in 1994, 14 has of - fered an other syn the sis method for man u fac tur ing meso - porous ma te rial in an acidic con di tion (ph < 1), where the sil - ica source is tet ra ethyl orthosilicate, and the acidic source is hy dro chlo ride acid. In syn thetic com po si tion there are no al - ka line metal ions. Un der such a strong acid con di tion, the sil - ica spe cies are cationic (I + ) and com bine with the cationic sur fac tants (S + ) via a bridged counterion (X - ). Ow ing to this weaker hy dro gen-bonding in ter ac tion (S + X - I + or S + X - I 0 ) in the mesoporous ma te ri als pre pared from the acid route, the surfactant/counterion/sil ica mesostructures can grad u ally grow into macro-scaled (i.e. mi cro- or millim-metered) mor - phologies and ho mo ge neously coat on the sur face of var i ous sub strates (such as sil i con wa fer and mica) or in ter face be - tween sil ica wa ter/air and wa ter/oil. Thus, var i ous de sired hi - er ar chi cal struc tures (i.e. gyroidal spheres, sil ica fi ber and Ded i cated to the cel e bra tion of the sev en ti eth an ni ver sary of Chem i cal So ci ety the lo cated in Tai pei. * Cor re sponding au thor. Tel : ; fax: ; cymou@ccms.ntu.edu.tw

2 900 J. Chin. Chem. Soc., Vol. 49, No. 5, 2002 Lin et al. ropes) and the thin mesoporous sil ica film has been suc cess - fully syn the sized by us ing TEOS as sil ica pre cur sor and cationic surfactant (qua ter nary am mo nium halides) as the struc ture-directing agent at room tem per a ture To the best of our knowl edge, the acidic route is thus the most pop u lar way to get in ter est ing and ver sa tile morphologies. At the same time, a mild ex trac tion in po lar sol vent can eas ily re - move the tem plate S + X -, and leave a pure sil ica frame work with out a trace of al ka line metal ionic spe cies. The syn the sis pro ce dures and com po si tions for pre par - ing the acid-made mesoporous ma te ri als have been pre sented in pre vi ous stud ies, 14 and some ki netic ex per i ments of the micellar sur face ca tal y sis pro cess and some con trol ling fac - tors have also been ex plored in one of our pre vi ous pa pers. 15 How ever, the con sid er able con trol ling fac tors that sen si - tively af fect the sil ica con den sa tion rate and de gree have not been dis cussed in de tail yet. Thus, in this pa per, we con tinue to thor oughly in ves ti gate a num ber of var i ous chem i cal and phys i cal fac tors (in clud ing con cen tra tion of acid, acid source, age ing time and NH 4OH ad di tion), which have great in flu ences on the par ti cle size, sur face pat terns, mor phol ogy and orderness of the acid-made mesostructures. A care ful con trol on the sil ica con den sa tion rate, con den sa tion de gree and surfactant-silica in ter ac tion strength could pro vide one es sen tial idea to de sign the par ti cle size, prod uct qual ity con - trol and orderness of the acid-made mesoporous silicas for fit ting the de sired ap pli ca tions. EX PER I MEN TAL Ma te rials The or ganic tem plate, alkyltrimethylammonium bro - mide, de noted as C ntmab (n = 14-18), was ob tained from Acrôs. The sil ica source was tet ra ethyl orthosilicate (TEOS, Acrôs, 98%) from Acrôs. Sul fu ric acid (H 2 SO 4 ), hy dro chlo - ric acid (HCl), hydrobromic acid (HBr), ni tric acid (HNO 3) and am mo nia (NH 4OH, 30%) were pur chased from Merck, Acrôs or Janssen Chimica. All chem i cals were used with out fur ther pu ri fi ca tion. Syn the sis A given amount of the surfactant C ntmab was dis - solved into wa ter with stir ring to form a clear so lu tion and then the proper amount of acid source was added. Af ter stir - ring 10 min utes at syn thetic tem per a ture of C, TEOS was added into that aque ous so lu tion with stir ring at rpm. The gel com po si tion is (in moles): 1 surfactant: ( ) TEOS: ( ) HX: ( ) H 2O. The de - tailed syn the sis pro ce dure and chem i cal com po nents can be found else where. 25 The in duc tion time was mea sured by ob - serv ing a white pre cip i ta tion for ma tion. For ex plor ing the re - la tion ship of ag ing time and meso-structural orderness of the sil ica prod uct, the gel mix ture was aged for dif fer ent ag i ta - tion times. For im prov ing the orderness of the mesoporous sil ica, a suit able amount of 1.0 M NH 4OH aque ous so lu tion was added to ad just the ph value to 8-9. The re sult ing solid prod uct was re cov ered by fil tra tion, washed with deionized wa ter, dried at 100 C or room tem per a ture. To re move the or ganic spe cies oc cluded in the nanochannels of the meso - porous ma te ri als, the as-synthesized sam ple was cal cined in air at 560 C for 6 hr. Char ac ter iza tion Pow der X-ray dif frac tion (XRD) pat terns were col - lected on a Scintag X1 diffractometer us ing Cu K ra di a tion ( = nm). The N 2 ad sorp tion-desorption iso therms were ob tained at 77 K on a Micrometric ASAP 2010 ap pa ra - tus. The sam ples were de gassed at 250 C un der 10-3 torr for 6 hr be fore anal y sis. The scan ning elec tron mi cros copy (SEM) was taken on an Hitachi S-800 op er ated at an ac cel er at ing volt age of 20 kev. El e men tary anal y sis data were taken from the Perkin-Elmer RE SULTS AND DIS CUS SION Surfactant-Silica Mesostructures For ma tion Rate are De pend ent on Acid Con cen tra tion and Source Pre vi ously, when we in ves ti gated the ki net ics of for ma - tion of the mesoporous ma te ri als, which are de pend ent on [H + ], [X - ] and the bind ing abil ity of X -, we al ways had to con - trol the ionic strength by add ing a huge amount of so dium salts to those re ac tion com pos ites. 25 How ever in real syn the - sis com pos ites, the so dium salts are not nec es sary for syn the - siz ing the acid-made mesoporous ma te ri als and are very un - de sir able for low-dielectric films prep a ra tion. It is, thus, nec - es sary to es tab lish a re la tion be tween the mesoporous sil ica ma te ri als for ma tion and acid con cen tra tion and acid sources as a cri te rion for pre par ing the acid-made mesoporous silicas. Fig. 1 shows that the in duc tion time of the sur fac - tants-silica pre cip i ta tion de creases with the con cen tra tion of ac ids. This re sult in di cates that the H + can cat a lyze the poly - mer iza tion of the sil ica spe cies at ph < 1, and is par al lel to pre vi ous re ports. 28 Be sides the [H + ] ef fect, the in duc tion time is also de pend ent on the acid sources. 25 At the same [H + ], the in duc tion time de creases fol lows the se quence: HNO 3 < HBr < HCl < H 2SO 4. To ex plain this, let s ex am ine the for ma tion

3 Op ti mum Syn the sis of Mesoporous Sil ica J. Chin. Chem. Soc., Vol. 49, No. 5, pro cess of the mesoporous sam ples from the acid route: Si-OEt + H 2 O + H + Si-OH EtOH (1) X - + Si-OH 2 + X - + H 2O-Si (2) X - + H 2O-Si X - HO-Si (3) Where = C ntmab; X - = NO 3 -, Cl -, Br - or S 1/2O 2 - Step (1) is the hy dro ly sis pro cess of the sil ane TEOS. This step is rap idly ac com plished in high-acidity con di tion (ph < 1) at the pres ence of the C ntmax surfactant. Then the silanol group of the hy drated TESO is protonated in the acid con di tion. The cationic sil ica spe cies then com bines with the S + X - mi celles through an elec tro static in ter ac tion at the in ter - me di ate state. (Step (2)) Then, the S + X - sites cat a lyze con den - sa tion re ac tion of the pos i tive-charged sil ica spe cies (I + ) in the acidic route. Thus, the counterion from acid source with the larger bind ing af fin ity can gen er ate more S + X - ac tive sites, which ac cel er ate the sil ica con den sa tion. This be hav ior should fol low the or der in the bind ing af fin ity (or hydropho - bicity) of the counterion in the well-known Hofmeister se ries: NO 3 - > Br - > Cl - > SO ,30 The counterion with less bind ing strength has less ef fi ciency to cat a lyze the sil ica spe cies poly - Fig. 1. A plot of the in duc tion time of the sil ica-sur - factant pre cip i ta tion for ma tion from 1.0 C 16 TMAB: ( ) HX : 4.76 TEOS : 762 H 2O com po si tions; HX = HNO 3 ; HBr; HCl; H 2 SO 4. mer iza tion. In the fi nal step (3), the sil ica spe cies fur ther con - denses on the mi celle sur face and forms the mesoporous prod uct (S + X - I 0 ), where the in ter ac tions be tween the sur - factant and sil ica frame work are the hy dro gen bond ing. Mesostructural Order and Mor phol ogy Con trol As shown in the last sec tion, the rate of for ma tion of the mesoporous sil ica can be con trolled by vary ing counterion spe cies and their con cen tra tions. It would be in ter est ing then to in ves ti gate the con se quence of vary ing the rate. The most ob vi ous thing to look for will be the mor phol ogy of the prod - uct be cause it is de ter mined by rates of nu cle ation, growth and ag gre ga tion. In this sec tion, we would like to ex am ine the ef fects on par ti cle size and shape, and sur face pat tern. In ad - di tion, the op ti mum ag ing time for get ting well or dered meso-structural silicas are also de pend ent on the acid con - cen tra tion. Par ti cle-size con trol Based on the con cept of clas si cal crys tal growth the ory, it is ex pected that in creas ing the num ber of the nu clei seeds will then re duce the crys tal size of the prod uct. This should also ap ply as well to the acid-made mesoporous com pos ites. Fig. 2 shows the scan ning elec tronic mi cros copy (SEM) fig - ures of the sphere-like par ti cles ob tained from the C 16 TMAB- TEOS-HCl-H 2O com po nents at dif fer ent acid i ties. It is ob vi - ous that the par ti cle size de creases with an in crease of the HCl/surfactant ra tio (de noted as r). The size can be tuned in the range from 15 m to 3 m at the r in the range of The acid ity-dependence of the par ti cle size can be ex - plained based on the sil ica spe cies for ma tion rate. In acid me - dia, the sil ica spe cies oligomerization is cat a lyzed by the H +. When the sil ica oli go mers grow to a large enough size, they are en er get i cally fa vored to com bine with the sur fac tants and form the sub mi cro-sized nu clei seeds for the for ma tion of mesoporous prod ucts dur ing the in duc tion pe riod. 30 With the in crease of the acid con cen tra tion, the sil ica spe cies ag gre ga - tion rate in creases and pro duces more nu clei seeds in the so - lu tion. More seeds make the av er age par ti cle size smaller. In con trast, less nu clei num ber and larger par ti cles form at lower acid ity. So, this is a con ve nient way to con trol the par ti cle size of the mesoporous silicas by ad just ing the acid ity. This re sult is one ex am ple of nu clei num ber con trol ling that is in par al lel to a quench ing pro ce dure used by Mann s group for pro duc ing ex tremely fine particulates of mesoporous sil ica in the range of tens of nanometers. 31 Sur face patterns for ma tion When mak ing a close ex am i na tion, one can eas ily find that the larger par ti cles have a smooth outer shell with out any sur face pat terns (Fig. 2A, B). On the other hand, the smaller

4 902 J. Chin. Chem. Soc., Vol. 49, No. 5, 2002 Lin et al. par ti cles syn the sized at higher acid ity (Fig. 2C, D) are in gyroidal spheres pos sess ing lots of curved stripes or pat terns on their sur faces as pre vi ously ob served by Ozin s group. 32 In ad di tion to an HCl acid source, the mesoporous silicas with ob vi ous sur face pat terns also can be syn the sized by us ing HNO 3. Fig. 3 dis plays many rep re sen ta tive sur face pat terns on the mesoporous sil ica par ti cles. Ac cord ingly, the growth of the high-curvature sur face pat terns may be a ki netic con - trol ling pro cess. This the o ret i cally in ter est ing phe nom e non needs fur ther ki netic stud ies and the cor re spond ing the o ret i - cal stud ies to ob tain the cor rect ex pla na tion. Un der stand ing the for ma tion mech a nism could pro vide better un der stand ing of the shape vari a tions on the nat u ral shells and di a toms. Aging time effect Fig. 4A shows the XRD pat terns of un cal cined meso - porous prod ucts syn the sized from the same re ac tion com po - si tions of C 16TAMB-HCl-TEOS-H 2O at T = 30 C for dif fer - ent ag ing times. We clearly found that the XRD peak widths of the mesoporous sam ple grad u ally be came broader, and the peak num ber de creased as well with ag ing time. In this com - po si tion, the ag ing time of 1 h is just long enough to get well-ordered mesoporous silicas. Aging for too long will in - duce a grad ual col lapse of the hex ag o nal mesostructures un - der highly acidic so lu tion. Based on these re sults, we pro pose that a mesostructure col lapse re ac tion takes place in the acid-made mesostructures as fol lows: X - OH-Si X - + Amor phous SiO 2 (4) Weak hy dro gen bond Ow ing to the weak hy dro gen bond ing in ter ac tions (S + X - I 0 ) be tween the sil ica wall and the sur fac tants, the acidmade mesoporous sil ica prod ucts show a stron ger ten dency A B 30 m 20 m C D 10 m 5 m Fig. 2. SEM mi cro graphs of the mesoporous ma te ri als ob tained from 1.0 C 16 TMAB: r HCl: 8.30 TEOS: 1083 H 2O com po si tions at 30 C. A. r = 12.0; B. r = 34.2; C. r = 52.3; D. r = Fig. 3. SEM mi cro graphs of var i ous sur face curved pat terns on the mesoporous sil ica par ti cles syn - the sized from 1.0 C 16TMAB: 20.5 HNO 3: 8.30 TEOS: 1083 H 2 O com po si tions at 32 C for 12 h.

5 Op ti mum Syn the sis of Mesoporous Sil ica J. Chin. Chem. Soc., Vol. 49, No. 5, to wards the more ther mo dy nam i cally fa vored sep a ra tion of surfactant and sil ica upon fur ther sil ica con den sa tion. This en er get i cally fa vor able pro cess there fore step by step in duces the for ma tion of amor phous sil ica. 33 Com paring the SEM mi cro graphs of the two sam ples aged for 1 h and 24 hr (Fig. 4 B, C), it is ob served that the par - ti cle of the 1 hr-aging sam ple ap par ently re mained as in the in di vid ual gyroidal spheres. In con trast, the 24 hr-aged sam - ple ap peared as a shape less surfactant-silica ag gre ga tion with a lot of tiny par ti cles that can be as signed as amor phous silicas. Thus a suit able ag ing time is also an im por tant con trol - ling fac tor for syn the siz ing the well-ordered mesostructural silicas un der acid con di tion. More over, the op ti mum ag ing time to ob tained the sta ble and well-ordered meso-structural silicas are also de pend ent on the acid con cen tra tion and acid source. In gen eral, the acid source with the stron ger bind ing counterion and the higher acid con cen tra tion needs less ag ing time to get the well-ordered hex ag o nal struc tures. To the best of our knowl edge, the in ves ti ga tions of ag ing time on the mesoporous prod ucts have not been pre vi ously re ported. With out care fully ob serv ing the ag ing time ef fect, one can not have a good con trol on the reproducibility and qual ity of the acid-synthesized mesoporous silicas. Of ten one may make the wrong ex pla na tion and pre dic tions on the ex per i - men tal re sults. Ef fects of Acid Source on the Phys i cal Properties of the Acid-Made Esoporous Silicas As in the afore men tioned sec tions, the sil ica spe cies ag - gre ga tion and con den sa tion re ac tions are rel a tive to the acid con cen tra tion and the sources. For mak ing ra tio nal com par i - A A (110) (200) IV 24 hr III II 6 hr I (110) (200) 1 hr /degree B /degree C B IV III II 800 I m 3 m Fig. 4. XRD pat terns and SEM and mi cro graphs of the mesoporous ma te ri als pre pared from 1.0 C 16 TMAB: 77.0 HCl: 8.30 TEOS: 1083 H 2O com - po si tions for dif fer ent ag ing time at 30 C. A. XRD; B. SEM of 2-h aged sam ple; C. SEM of 16-h aged sam ple p/p 0 Fig. 5. XRD pat terns (A) and N 2adsoption-desorption iso therms (B) of the cal cined ma te ri als ob tained from 1.0 C 16TMAB: 20.5 HX: 4.76 TEOS: 762 H 2 O sys tems by us ing dif fer ent acid sources at 30 C. I. HNO 3 (ag ing time = 12 h); II. HBr (ag - ing time = 1 day); III. HCl (ag ing time = 2 day); IV. H 2SO 4 (ag ing time = 2 day).

6 904 J. Chin. Chem. Soc., Vol. 49, No. 5, 2002 Lin et al. Table 1. Physical Properties of the Acid-Made and Calcined Mesoporous Silica Materials Synthesized from C n TMAX-TEOS-HX-H 2 O Components Surfactant a Acid source d 100 (as-made)/nm d 100 (calcined)/nm Pore size b /nm BET surface area/m 2 /g C 16 TMAB HCl C 16 TMAB HBr C 16 TMAB H 2 SO C 16 TMAB HNO C 14 TMAB HNO C 18 TMACl HCl a C 18 TMACl systems were synthesized at 40 C, C 16 TMAB and C 14 TMAB at 30 C. b The values were gotten from the adsorption portion of the N 2 adsorption-desorption isotherm by using the Barrett- Joyner-Halenda calculation model. sons of the phys i cal prop er ties of the acid-made mesoporous silicas us ing dif fer ent acid sources, we care fully ad justed the ag ing time to have sim i lar sil ica con den sa tion in all of the sam ples and avoid the for ma tion of side prod ucts for ma tion (i.e. amor phous silicas). Fig. 5 re veals the rep re sen ta tive X-ray dif frac tion pat terns (XRD) and N 2 ad sorp tion-des - orption iso therms of the cal cined mesoporous ma te ri als syn - the sized from C 16 TMAB-TEOS-HX-H 2 O at 30 C by us ing dif fer ent acid sources (HX = HCl, HBr, HNO 3, H 2SO 4). One can clearly find that all of these sam ples pos sess the 2 to 3 XRD peaks char ac ter is tic of hex ag o nal struc ture (Fig. 5A). The XRD pat terns are al most flat be tween 2 = 20 and 30 de - grees, in di cat ing that no amor phous sil ica par ti cles were formed. 18 More over, the N 2 ad sorp tion-desorption iso therms of the cal cined sam ples have lit tle dif fer ence in their pat terns and shape (Fig. 5B). Ba sically, all acid-synthesized sam ples pos sess nar row pore size dis tri bu tions. This re sult re veals that all of these in or ganic ac ids are suit able for pre par ing high-quality mesoporous ma te ri als un der acid con di tion. The na ture of the acid source does not have ap par ent ef fects on the nano-structure or der of the mesoporous ma te ri als. In brief, with a care ful con trol on ag ing time, the mesoporous ma te ri - als with well-ordered hex ag o nal struc ture can be readily syn - the sized from var i ous in or ganic ac ids. 25 that the size of the me di ated counterion X - is rel a tively less im - por tant in de ter min ing the pore size than the chain length of the surfactant. Over all, the d-spacing and pore size is tun able, which in creases with the in crease of the chain length of the surfactant. 34 These re sults are in par al lel to those of MCM-41 silicas syn the sized from ba sic me dia (ph > 8). 35,36 Com paring the shrink age of the d 100 value af ter cal ci na - tion at 560 C, one can find that the shrink age per cent age of the acid-synthesized mesoporous ma te ri als is about 10-15% which is larger than that of ba sic-made MCM-41 ma te ri als (about 5%). The larger lat tice shrink age in di cates that the mesostructures of the acid-made mesoporous silicas are less con densed. Re ducing the lat tice shrink age per cent age is pos - si ble by fine con trol ling the sil ica con den sa tion de gree at suit able com pos ites and ag ing time. The mesoporous silicas with high sil ica con den sa tion can have larger ther mal and hy - (110) (200) B (210) (300) Pore Size Con trol We have fab ri cated the mesoporous ma te ri als with dif - fer ent C ntmax (n 14; X= Cl or Br) sur fac tants. The phys i - cal prop er ties (in clud ing the d 100 of the as-synthesized and cal - cined sam ples, pore size and BET sur face area) of these ma te - ri als from dif fer ent acid sources and chain-length sur fac tants are listed in Table 1. Ba sically, all these cal cined mesoporous ma te ri als ex hibit large sur face area (~1000 m 2 /g) and uni form pore size. The prod ucts pre pared from var i ous surfactant sources with the same car bon chain length but by us ing dif fer - ent acid sources, have al most the same pore size. It is sup posed (110) (200) A /degree Fig. 6. XRD pat terns of the cal cined mesoporous sam - ples syn the sized from C 16 TMAB-HCl-TEOS- H 2O com po si tions with and with out am mo nia so lu tion ad di tion. A. with out am mo nia ad di - tion; B. with am mo nia ad di tion.

7 Op ti mum Syn the sis of Mesoporous Sil ica J. Chin. Chem. Soc., Vol. 49, No. 5, dro ther mal sta bil ity. Ad di tion of Am mo nia So lu tion To in ves ti gate the ef fect of NH 4OH ad di tion on the hex - ag o nal struc ture orderness of the mesoporous ma te ri als, the acid-made gel so lu tion af ter 1 hr ag ing time was neu tral ized by add ing NH 4OH aque ous so lu tion to ad just the ph value to about , and then fur ther aged for 2 hr. Fig. 6 shows the com par i son of the XRD pat terns of cal cined prod ucts pre - pared with and with out ph value ad just ment. It was found that there are ex tra char ac ter is tic peaks of (210) and (300) in am mo nia-added sam ples. The ex is tence of higher dif frac tion peaks in di cates higher orderness. The im prove ment of meso - structural or der of the mesoporous ma te ri als can be as cribed to that the NH 4 OH ad di tion trans forms the in ter ac tions of the sil ica wall and surfactant from weak hy dro gen bond ing to strong elec tro static in ter ac tion as fol lows: S + X - I 0 + NH 4OH S + I - + NH 4X + H 2O (5) Weak hydrogen bond ing Strong electrostatic interaction The stron ger elec tro static in ter ac tions in duce the sil ica wall and surfactant to re ar range and re struc ture into a well-ordered mesostructure like that pre pared from ba sic route at am bi ent tem per a ture. 37 Using the SEM and op ti cal mi cro scope to ex am ine the morphologies of the am mo nia-neutralized mesoporous silicas, we found that the morphologies are the same as the orig i nal acid-synthesized ones. Thus, this am mo nia neu tral iza tion pro cess can pro vide a con ve nient way to pre pare the hi er ar - chi cal struc tures un der acidic con di tion with well- ordered mesostructure at am bi ent tem per a ture, while the al ka line treat ment later con verts it into al ka line form. 34 CON CLU SIONS Mesoporous ma te ri als with hex ag o nal struc ture have been syn the sized from var i ous acid sources. The be hav ior of the mesoporous ma te ri als for ma tion in acid con di tion is re - ally rel a tive to that of an acid ity-controlled sil ica-con dens - ation re ac tion. Thus, we have pre sented a new pro ce dure to have a fine abil ity to syn thet i cally con trol the size and shape of the mesoporous ma te ri als ac cord ing to the sil ica chem is try and nu cle ation the ory Many worth while re sults on the syn the sis of mesoporous silicas have been laid out; one could stand on these bases and ex tend im por tant ap pli ca tions in ca - tal y sis, sep a ra tion tech nol ogy and opticoelec tronics. AC KNOWL EDG MENT We thank Mr. Chung-Yuan Tang and Ching-Yuan Lin for help ing with the SEM and TEM mi cro graphs prep a ra tion. This re search was sup ported by the China Pe tro leum Co. and the Na tional Sci ence Coun cil of Tai wan (NSC M ). Re ceived July 1, Key Words Mesoporous; Surfactant; Acid con di tion; MCM-41. REF ER ENCES 1. Yanagisawa, T.; Shimzu, T.; Kuroda, K.; Kato, C. Bull. Chem. Soc., Jpn. 1990, 63, Zhao, D.; Yang, P.; Hou, Q.; Chmelka, B. F.; Stucky, G. D. Cur rent Opin ion in Solid State and Ma te rials Sci ence 1998, 3, Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Na ture 1992, 359, Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T.-W.; Olson, D. H.; Sheppard, E. W.; Hig gins, S. B.; Schlenker, J. L. J. Am. Chem. Soc. 1992, 114, Corma, A. Chem. Rev. 1997, 97, Lee, T., Yao, N.; Aksay, I. A. Langmuir 1997, 13, Aksay, I. A.; Trau, M.; Manne, S.; Honma, I.; Yao, N.; Zhou, L.; Fenter, P.; Eisenberger, P. M.; Gruner, S. M. Sci ence 1996, 273, Trau, M.; Yao, N.; Kim, E.; Xia, Y.; Whitesides, G. M.; Aksay, I. A. Na ture 1997, Antonelli, D. M.; Ying, J. Y. Curr. Opin. Colloid In ter face Sci. 1996, 1, Sayari, A. Chem. Ma ter. 1996, 8, (a). Wu, C. G.; Bein, T. Sci ence 1994, 264, (b). Wu, C. G.; Bein, T. Sci ence 1994, 266, Monnier, A.; Schüth, F.; Huo, Q.; Kumar, D.; Margolese, D.; R. Maxwell, S.; Stucky, G. Krishnamurty, D.; M.; Petroff, P.; Firouzi, A.; Janicke, M.; Chmelka, B. F. Sci ence 1993, 261, Firouzi, A.; Kumar, D.; Besier, T.; Sieger, P.; Hou, Q.; Walker, S. A.; Zasadzinski, J. A.; Glinla, C.; Nicol, J.; Margolese, D.; Stucky, G. D.; Chemalka, B. F. Sci ence 1995, 267, Huo, Q.; Margolese, D. I.; Ciesla, U.; Feng, P.; Gier, T. E.; Sieger, P.; Leon, R.; Petroff,. P. M.; Schüth F.; Stucky, G. D. Na ture 1994, 368, 317.

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