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Acid-activated organoclays: preparation, characterisation and catalytic activity of polycation-treated bentonites
Applied Clay Science 12 Ž1998. 479–494
Acid-activated organoclays: preparation, characterisation and catalytic activity of polycation-treated bentonites Christopher Breen ) , Ruth Watson Materials Research Institute, Sheffield Hallam UniÕersity, Sheffield S1 1WB, UK Received 9 July 1997; revised 20 November 1997; accepted 20 November 1997
Abstract Samples of SWy-2 and SAz-1 loaded with increasing amounts of the polycation magnafloc 206, wŽMe 2 NCH 2 CHOHCH 2 . n x nq Cl n , were acid-treated using 6 M HCl at 958C for 30, 90 and 180 min. The activity of these acid-activated polycation-exchanged clays for the conversion of a-pinene to camphene and limonene was determined and compared with that from clay samples Žwithout polycation. acid-treated in the same manner. Acid treatment of polycation-exchanged bentonites produced hybrid catalysts which enhanced the activity of the clays for the isomerisation of a-pinene to camphene and limonene. The presence of the polycation had a more marked influence on the activity of samples derived from SAz-1 increasing the yield from 25% for acid-activated SAz-1 with no added polycation to 50% camphene for acid-activated polycation-exchanged SAz-1. The increase in yield for corresponding samples derived from SWy-2 was only from 42 to 52%. This enhancement in yield for samples derived from SAz-1 was attributed to the increased hydrophobicity of the polycation loaded clay whilst the comparable yields for SWy-2 in the absence and presence of polycation may suggest that SWy-2 disperses well in the non-polar a-pinene. The total yields Žbased on a-pinene. for the most active catalysts was between 80 and 90%. These yields are directly comparable with those obtained by others using zeolites and pillared clays although the acid-activated polycation-treated clays were marginally less selective towards camphene. q 1998 Elsevier Science B.V. Keywords: organoclays; acid-catalysis; isomerisation; smectite
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Corresponding author. Tel.: q44-0114-253-3008; fax: q44-0114-253-3501; e-mail: [email protected]. 0169-1317r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 1 6 9 - 1 3 1 7 Ž 9 8 . 0 0 0 0 6 - 4
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1. Introduction In addition to their use as bleaching earths for the clarification of oils ŽMorgan et al., 1985. and in the carbonless copying paper Ž Fahn, 1973. , commercial acid-treated clays have been used as an effective solid source of protons for a considerable period and have found use in industrial processes such as the alkylation of phenols ŽKaplan, 1966. and the dimerisation and polymerisation of unsaturated hydrocarbons Ž Hojabri, 1971. . Moreover, acidactivated clays have been the focus of renewed interest in their role as high-surface-area supports for environmentally friendly catalysts in Friedel Crafts alkylation and acylation reactions ŽBrown, 1994; Clark et al., 1994. . Commercial acid-treated clays are normally prepared using a fixed quantity of acid chosen to remove the desired proportion of cations from the octahedral sheet ŽRhodes and Brown, 1995. and few investigations have addressed how the catalytic activity varies with octahedral sheet depletion Ž Rhodes and Brown, 1994; Breen et al., 1997a.. During acid-activation of clays the octahedral ions are leached out, the nitrogen surface area increases, the number Ž and type. of acid sites changes as do the sorptive and catalytic properties. These properties generally maximise under intermediate activation conditions and subsequently, decrease as the product takes on the properties of amorphous silica, which has been identified as the final product of acid leaching Ž Fahn, 1973; Komadel et al., 1990; Tkacˇ et al., 1994; Breen et al., 1995a,b. . It is now widely accepted that clays with a high octahedral Mg content leach more readily than those which have a high ˇ ˇ and octahedral Al population Ž Osthaus, 1956; Novak and Gregor, 1969; Cicel ˇ ˇ 1978; Breen et al., 1995a,b; Komadel et al., Novak, 1977; Novak and Cicel, 1996. and recent reports have established that clays with a high octahedral Fe content also leach rapidly Ž Janek and Komadel, 1993; Breen et al., 1997a.. An extensive investigation into the acid-leaching of a range of smectites Ž selected for their differing octahedal ion content. has shown that the elemental composition of the starting material did not make a significant contribution to the catalytic activity for the chosen test reaction but did play a key role in determining the severity of the activation conditions required for the optimisation of the catalytic activity ŽBreen et al., 1997a. . It is important to note that the extent of acid dissolution can significantly influence the activity of the catalyst for a particular reaction whether it is employed as a support ŽRhodes and Brown, 1992. or as a catalyst in its own right ŽRhodes and Brown, 1994; Breen et al., 1997a.. Using the formation of tetrahydropyranyl ether from dihydropyran and methanol as a test reaction Rhodes and Brown Ž1994. established that the catalytic activity of acid-activated clay for reactions in polar media was optimised when the acidity and swelling ability of the catalyst was at a maximum. This occurs at short acid-treatment times. In contrast the yield for the acid catalysed isomerisation of a-pinene to
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camphene, which represents reaction in a non-polar medium, was optimised when the external surface area reached a maximum, which occurs when leaching is extensive. They concluded that the reaction in polar media was optimised at short acid-treatment times because the hydrophilic clay attracts the polar reagents to the surface where the catalytic protons reside. This contrasts with the behaviour in non-polar media where the reaction yield was optimised when the clay was substantially leached because the catalyst presented an essentially hydrophobic surface which served to attract the non-polar reagents. This data is, however, limited to the study of a single clay with a high octahedral Al content. It is long established that the hydrophilic aluminosilicate surface of swelling clays can be rendered hydrophobic by exchanging the naturally occurring inorganic cations Ž Ca2q, Naq, Kq . with organocations such as tetramethylammonium ŽLee et al., 1990. hexadecyltrimethylammonium Ž Mortland et al., 1986; Boyd et al., 1988; Jaynes and Boyd, 1991a. and phenyltrimethylammonium ŽJaynes and Boyd, 1990, 1991b. to select a few from an extensive list. Indeed, Akelah et al. Ž1994. used organophilic polystyrene-montmorillonite supported onium salts to accelerate the reaction of thiocyanate and nitrite with alkyl and benzyl bromides. This knowledge coupled with the recent enhancement of the catalytic activity of pillared clays, via pillaring acid-activated clays, Ž Mokaya et al., 1994: Bovey et al., 1996. led us to consider acid treated organoclays as catalysts for reactions in non-polar media. The conversion of a-pinene to camphene, our chosen test reaction, has industrial significance because camphene is an intermediate in the synthesis of camphor, which has value due to its aroma and pharmaceutical properties ŽAlbert et al., 1989. . Acidified titanium oxide is usually employed in the transformation of a-pinene to camphene Ž Severino et al., 1993. , although recent studies have considered the use of zeolites Ž Yu et al., 1995; De Stefanis et al., 1995., pillared interlayer clays ŽPILCs. ŽDe Stefanis et al., 1995. and Žboth crystalline and amorphous. zirconium and tin phosphates Ž Cruz-Costa et al., 1996.. Here we report the use of acid-activated polycation-exchanged smectite for the conversion of a-pinene to camphene, via ring expansion, and limonene, via ring opening ŽWilliams and Whittaker, 1971.. Our ultimate goal is to produce cost-effective, hydrophobic catalysts of relatively high acidity in a short time using simple exchange procedures. This approach explores the resistance of organoclays to octahedral ion leaching and has the potential to reduce the concentration of Al, Mg and Fe in effluent flows from the leaching process. Moreover, the ability of acid-activated polycation-exchanged clays to swell in non-polar solvents should provide the opportunity to access a considerable portion of the potential surface area which has been estimated near 800 m2 gy1. Cognisant of the importance of the octahedral sheet composition on acid leaching we have used two starting clays of different octahedral composition, which have been the focus of acid leaching studies previously Ž Tkacˇ et al., 1994; Breen et al., 1995a,b., to evaluate the properties of the resulting catalysts. The
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results are compared with the yield from acid-activated alkyltrimethylammonium-exchanged clays which have been reported elsewhere Ž Breen et al., 1997b.. 2. Experimental method 2.1. Materials Both montmorillonites SAz-1 Ž Cheto, AZ, USA. and SWy-2 Ž Crook County, WY. were obtained from The Clay Mineral Repository of the Clay Minerals Society, Columbia, MO, USA. The elemental compositions given in Table 1 emphasise the different octahedral compositions of these two dioctahedral smectites. SAz-1 is richer in magnesium, but contains less octahedral iron, than SWy-2. These clays were used without further purification. The polycation used was Magnafloc 206, wŽMe 2 NCH 2 CHOHCH 2 . n x nq Cl n , which has a nominal relative molecular mass of 100 000 and the distance ˚ and the between charge centres, derived from a molecular model, was 4.8 A ˚ Magnafloc 206 was provided as a 50% active approximate length was 3000 A. solution and Kjeldahl nitrogen analysis confirmed that it contained 9.0% nitrogen. The adsorption of this polycation onto Csq-, Naq- and Kq-exchanged Texas bentonite has been described elsewhere Ž Breen et al., 1996. . 2.2. Adsorption isotherms Samples were prepared by introducing 10 cm3 of 50 g dmy3 SAz-1 or SWy-2 Ž0.5 g clay. into new polypropylene bottles together with a quantity of magTable 1 Elemental compositions of selected samples derived from SAz-1 and SWy-2 Sample
SiO 2 Žwt.%.
Al 2 O 3 Žwt.%.
MgO Žwt.%.
Fe 2 O 3 Žwt.%.
CaO Žwt.%.
Na 2 O Žwt.%.
SA-00 SA-30L SA-180L SA-30H SA-90H SA-180H SA-32-90H SW-00 SW-30H SW-90H SW-180H SW-13-90H
68.20 72.35 72.75 73.78 76.78 81.74 71.20 68.48 72.55 72.51 73.53 71.78
19.74 19.62 19.37 18.43 16.41 13.04 19.94 20.16 21.07 20.75 20.29 21.07
7.24 6.29 6.15 6.18 5.43 4.18 6.94 2.77 2.45 2.41 2.35 2.49
1.59 1.66 1.62 1.58 1.36 1.02 1.81 4.13 3.75 3.70 3.61 4.37
3.16 0.05 0.08 0.03 0.02 0.02 0.04 1.76 0.06 0.05 0.05 0.05
0.07 0.03 0.03 0.00 0.00 0.00 0.07 1.70 0.14 0.14 0.16 0.24
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nafloc 206 and sufficient deionised water to give a total volume of 20 cm3. The clay-adsorbate suspensions were then shaken at 258C for 2 h. The samples were centrifuged and the supernatant decanted before washing the sample once with water to remove excess polymer. The samples were dried at 1208C, ground and then stored. Nitrogen content was determined using a Gerhardt Vapodest Kjeldahl Autoanalyser. Samples for powder XRD and TG studies were air dried after centrifugation and ground to - 75 m m. 2.3. Sample preparation Using the adsorption isotherm data ŽFig. 1. as a guide, 2-g portions of each smectite were added to aqueous solutions of containing 300, 1500 and 3000 m l of polycation. These volumes of polycation were chosen to provide samples which had approximately 0.25, 1.0 and 1.5 times the CEC of polycation adsorbed. The resulting suspensions were agitated for 2 h in a rotary shaker operating at 250 rpm. The supernatant was removed and the solids were washed once with deionised water. This approach provided samples with loadings of 32, 108 and 173 mg Žg clay.y1 for SAz-1 and 17, 128 and 135 mg Žg clay.y1 for SWy-2. These non acid-treated organoclays are designated SA-28, SA-108 and SA-173 for SAz-1 and SW-17, SW-128 and SW-135 for SWy-2, where the number represents the polycation loading in mg Ž g clay.y1. One-gram portions of the variously exchanged clays were then activated using 6 M HCl for 30, 90 or 180 min. The polycation-treated clays were only treated at 958C, whereas SAz-1 and SWy-2 were treated at either 258C or at 958C for the times stated. Thus SA-30L is SAz-1 treated with 6 M HCl for 30 min at low temperature Ž L, 258C., whereas SW-180H has been treated with 6 M HCl for 180 min at high temperature ŽH, 958C.. Finally, the acid-activated organoclays use a combination of figures to identify the polycation loading and the acid-activation conditions. Hence, SA-32-90H is SAz-1 with 32 mg polyca-
Fig. 1. The adsorption of Magnafloc 206 on SWy-2 Ž`. and SAz-1 ŽB..
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tion Žg clay.y1 which has been treated with 6 M HCl for 90 min at 958C. The intention was to evaluate the ability of the variously acid-activated samples to catalyse the isoemerisation of a-pinene. In particular we were interested in the effect of different acid treatments on the organoclays formed from clays of different initial composition and to evaluate the resistance of the organoclays to acid leaching. 2.4. Sample characterisation X-ray diffraction profiles of pressed powder samples were obtained using a ˚. Philips PW1830 X-ray diffractometer using a copper tube Ž l s 1.5418 A y1 Ž . operating at 40 kV and 35 mA. Profiles were recorded at 28 2Q min . Samples for XRF analysis were prepared using the Li 2 B 4O 7 fusion method. The resulting beads were analysed on a Philips PW2400 XRF spectrometer using calibration graphs prepared from certified reference materials. Thermogravimetric traces were obtained using a Mettler TG50 thermobalance equipped with a TC10A processor. Samples Ž 7 mg. were heated from 358 to 8008C at 208C miny1 under a flow of 20 cm3 miny1 dry nitrogen carrier gas. 2.5. Catalytic actiÕity A 50-mg smectite was dried at 1208C for 16 h to remove physisorbed water, stoppered and placed in a desiccator to cool. The dried samples were added to 8.174 g Ž 0.06 mol. of a-pinene pre-heated to 808C and maintained at 808C for 2
Scheme 1.
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h. The solution was then separated from the catalyst using a syringe filter and stored in a dry sample vial. Control experiments confirmed that the filter exerted no influence on the reaction products and that no further reaction occurred even after five days at room temperature. The products Žsee Scheme 1. were identified as camphene Ž3., limonene Ž4., i-terpinolene Ž5. , g-terpinene Ž 6. , a-terpinene Ž7. and a-phellandrene Ž8. by GC-MS Ž Unicam Automass. . The amount of camphene, limonene and other minor products was determined using capillary GC with FID detection. A 3% OV-225 column was used and was held isothermally at 708C for 10 min. The injection port and detector were held at 1508C and the N2 flow rate was 12 cm3 miny1.
3. Results 3.1. Adsorption isotherms and elemental analysis The uptake curves in Fig. 1 are similar to those reported previously for the adsorption of this polycation on a low iron Texas bentonite Ž Breen et al., 1996. and show that both SWy-2 and SAz-1 display a high affinity for Magnafloc 206. The amount of sorbed polycation exceeded that required to satisfy the exchange capacities of SWy-2 and SAz-1 Ž 90 and 122 mg gy1, respectively. which is common behaviour in clay polycation systems ŽBreen et al., 1996; Denoyel et al., 1990.. The data in Table 1 shows that acid treatment at 258C generally had little effect on the elemental composition of SAz-1 and SWy-2 Žvalues not shown. as anticipated from the results of Breen et al. Ž 1995a. . Treatment with acid at room temperature simply replaces the resident Ca and Na exchange ions with protons. In contrast, treatment with hot acid for only 30 min Ž SA-30H. altered the octahedral composition of SAz-1 reducing the MgO content from 7.24% to 6.18%. After 3 h Ž SA-180H. , 34% of the Al 2 O 3 , 42% of the MgO and 35% of the Fe 2 O 3 content had been removed. These reductions in octahedral population are in line with other published results using similar activation conditions ŽBreen et al., 1995b, 1997a. and emphasises the reduced stability of clays rich in octahedral magnesium towards acid-leaching. The elemental composition of acid-activated, polycation-exchanged samples SA-32-90H and SW17-90H were almost identical to that of the starting materials which suggests that the presence of even small amounts of polycation protected the clay layer from acid attack. Note, however, that the data in Table 2 shows that the amount of adsorbed polycation decreased as the time of acid treatment was increased and that the effect was more marked with SAz-1 which contains more Ca2q-ions on the exchange sites when polycation is initially added. Sorption of polycations to Naq-exchanged montmorillonite in aqueous suspension results in the formation
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Table 2 Polycation loadings on samples derived from SAz-1 and SWy-2 Sample
Polycation loading Žmg Žg clay.y1 .
Sample
Polycation loading Žmg Žg clay.y1 .
SA-32 SA-32-30H SA-32-90H SA-32-180H SA-108 SA-108-30H SA-108-90H SA-108-180H SA-173 SA-173-30H SA-173-90H SA-173-180H
31.7 21.2 Ž67%. 19.7 Ž62%. 10.15 Ž32%. 108.3 98.0 Ž91%. 95.7 Ž88%. 80.5 Ž75%. 173.2 148.4 Ž86%. 131.8 Ž76%. 106.1 Ž61%.
SW-13 SW-13-30H SW-13-90H SW-13-180H SW-128 SW-128-30H SW-128-90H SW-128-180H SW-135 SW-135-30H SW-135-90H SW-135-180H
12.9 12.9 10.5 Ž81%. 128.3 102.3 Ž79%. 98.3 Ž77%. 135.3 114.9 Ž85%. 110.8 Ž82%. 106.9 Ž79%.
of large, low density flocs with the polycation contained in the interior. In contrast, sorption to Csq-montmorillonite Ž which forms tactoids prior to polycation addition as does Ca2q . produces smaller flocs with a greater proportion of polycation on the external surfaces of the flocs Ž Billingham et al., 1997b. particularly at low loadings of polycation. The increased removal of polycation from samples derived from SAz-1 probably reflects the higher proportion of ‘available’ polycation molecules bound to the external faces of Ca-SAz-1 tactoids, whereas in samples derived from the Naq-rich SWy-2, the polycation is bound inside the flocs and is thus less accessible to acid attack. 3.2. Catalytic actiÕity Fig. 2 illustrates how the catalyst preparation method affected the total conversion of a-pinene. In general the catalysts derived from SWy-2 were more active than those derived from SAz-1 and hot acid treatment of both SWy-2 and SAz-1 increased the total conversion more than mild acid treatment. The highest yields were achieved using acid-activated clays with the lowest polycation loading and the presence of the polycation exerted more influence on the samples derived from SAz-1 than those obtained from SWy-2. Catalysts prepared from samples with intermediate and high polycation loadings were no more effective than the mild acid-treated clays. This is particularly striking for the samples derived from SAz-1 and is best illustrated by comparing the total conversion of samples obtained by treating SAz-1 for 90 min. Cold acid treatment of SAz-1 increased the activity from 0 to 12%, hot acid treatment increases this 12% yield to 55% and finally acid-treatment of SA-32 enhanced this a further 26% to 81%. A similar commentary could also be applied to the
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Fig. 2. Total conversion of a-pinene over acid-activated samples derived from Ža. SAz-1 and Žb. SWy-2.
samples derived from SWy-2. The comparatively low isomerisation yield for SA-32-180H is currently attributed to the much reduced polycation loading on this sample compared to SA-32-30H and SA-32-90H ŽTable 2.. The distribution of products which make up the total conversion reported in Fig. 2 are given in Fig. 3. In all cases, camphene Ž 3. was the major and limonene Ž4. the minor product except for catalysts derived from SW-13 ŽSW-13-90H, SW-13-180H. where the yield of other products exceeded that of limonene. It is often difficult to discern small changes in the selectivity based on a product distribution plot such as Fig. 3. Consequently, the actual yield of
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Fig. 3. Product distributions for samples derived from Ža. SAz-1 and Žb. SWy-2.
camphene, limonene and other products Ž from Fig. 3. was calculated as a percentage of the total yield Žfrom Fig. 2., i.e., the selectivity. The results of these selectivity calculations, which are not listed here, showed that the various catalysts exhibited some variation in the product selectivity and that the selectivity was more erratic for samples derived from SAz-1 than for those derived from SWy-2. The selectivity data can best be summarised by saying that the selectivity over samples derived from SAz-1 was 54 " 5% towards camphene, 30 " 5% towards limonene and 18 " 9% for the other compounds. For samples derived from SWy-2 the values were very similar at 56 " 6% for camphene, 27 " 5% for limonene and 21 " 6% for the minor products.
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4. Discussion 4.1. Acid-actiÕated SAz-1 and SWy-2 Rhodes and Brown Ž1994. showed that the activity of acid-activated Texas bentonite for the isomerisation of a-pinene increased as the extent of acid leaching increased reaching a maximum as the available surface area maximised and then diminished to zero. They convincingly argued that the clay surface became more like hydrophobic silica than hydrophilic aluminosilicate as the leaching progressed and thus the surface became more attractive to the non-polar a-pinene. In the main, the data presented here for the clays without adsorbed polycation conform to that interpretation. The total conversion of a-pinene by hot acid-treated SAz-1 samples was more than three times that for cold acid treated samples, whilst treatment of SWy-2 at 958C doubled the yield obtained from the samples treated at 258C. Nonetheless, the samples derived from SWy-2 were very effective catalysts for the a-pinene conversion even when the XRF ŽTable 1. and XRD data Ž not illustrated. suggested that the clay maintained a considerable amount of lamellar character. MAS-NMR studies of SAz-1 Ž Breen et al., 1995b. and SWy-1 Ž Tkacˇ et al., 1994. leached in hot 6 M HCl support the view that SAz-1, which has a higher octahedral magnesium population than SWy-2, is leached more rapidly. Therefore, samples derived SAz-1 should attain the necessary hydrophobicity at shorter activation times. This is clearly not the case so the extent of acid activation cannot make the sole significant contribution to the activity for the isomerisation of a-pinene. We have recently investigated the use of acid-activated, tetramethylammonium-, dodecyltrimethylammonium- and hexadecyltrimethylammonium-exchanged smectites as catalysts for the isomerisation of a-pinene ŽBreen et al., 1997b.. As part of that study the activity of acid-treated clays which did not contain alkyltrimethylammonium ions was also investigated for comparative purposes. However, the experimental strategy required different amounts of acids for the different clays and cross comparison was not possible. The dioctahedral smectites used included Jelsovy-Potok, which is rich in octahedral aluminium Ž Breen et al., 1995a, 1997a. , Stebno, an iron-rich beidellite with a ˇ ˇ and Novak, 1977., and SWa-1, a ferruginous high octahedral Fe content ŽCicel Ž smectite Madejova´ et al., 1994. . Jelsovy-Potok was very mildly acid treated Ž0.1 M HCl at 258C. and activity increased very little over that of the untreated material Ž12% total conversion. . There was some evidence that the amount of octahedral iron plays a role in the activity insofar as Stebno exhibited a high initial activity for a-pinene conversion and was very active Ž 90% total conversion. after 60 min leaching in 6 M HCl at 258C. Therefore, the amount of octahedral iron in the clay may influence the catalytic ability. Another, as yet unproven possibility, is that the activity of the catalyst also reflects its ability to swell in the organic solvent. SWy-2 contains a large proportion of Naq-ions on
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its exchange sites and is known to swell more in aqueous solution than SAz-1 which is rich in Ca2q-exchange ions. We shall see as the discussion progresses that the ability of organoclays to swell in organic media could contribute to the activity of acid-activated organoclays hence the possibility exists that the activity of SWy-2 reflects the greater surface area it potentially offers in organic solvents. 4.2. Acid-actiÕated polycation-exchanged clays In general the total conversions of the most active catalysts prepared in this study were between 60 and 90%, with catalysts derived from SW-17 and SA-32 being the most active ŽFig. 2.. These conversions are considerably higher than those reported for extensively acid-leached Texas bentonite Ž Rhodes and Brown, 1994. but are similar to the values of 80 and 90% reported for permanently-porous materials including ultra stable zeolite Y ŽUSY., and Al- and Al,Fe-Pillared Clays Ždenoted Al-PILC and FAZA. Ž De Stefanis et al., 1995. . In our recent study of activity of acid-activated alkyltrimethylammonium-exchanged clays for the isomerisation of a-pinene Ž Breen et al., 1997b., we established that tetramethylammonium-exchanged clays provided the best precursors and only required activation at room temperature in 0.1 M HCl to effect total conversions Žbased on a-pinene. of 60 to 80%. Clays exchanged with the larger dodecyltrimethylammonium and octadecyltrimethylammonium ions were only effective when these cations occupied only 30% of the exchange sites. Indeed the yields and selectivities of the polycation treated clays used here are similar to those obtained using long-chain alkyltrimethylammonium-exchanged clays insofar as the activity decreases markedly with loading. The yields for the acid-activated alkyltrimethylammonium exchanged clays were obtained using identical conditions to those described herein whereas the samples of USY, Al-PILC, and FAZA had been extensively dehydrated Ž 4508C, 4 h. and the authors stated that they were thus operating as Lewis acid catalysts. Moreover, the yields were obtained after a reaction time of 5 h at 1008C in a sealed glass reactor at autogenous pressure, conditions which are more severe than the conditions reported herein. Furthermore, the absolute yields of camphene and limonene obtained using USY and PILCs were similar to those reported here at ca. 45 " 5% and 15 " 5%, respectively, but were slightly lower than those obtained from acid-activated alkyltrimethylammonium-exchanged clays ŽBreen et al., 1997b. . The selectivity of USY and the pillared clays were similar to eachother with a camphene:limonene ratio near three whereas the acid-activated organoclays Žwhether derived from alkyltrimethylammonium or polycation-exchanged clays. were less selective towards camphene with a ratio near 2.2:1. The fixed, porous nature of USY and the PILCs means that they are unable to swell in the reaction liquor and therefore may select against the size of the limonene molecule. This
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is less likely with the catalysts derived from alkyltrimethylammonium-clays because the charge balancing organocations do not bind the layers together as the alumina pillars do in PILCs and thus these organoclays can avail of their ability to expand in the reaction liquor. Billingham et al. Ž 1997a. showed that the ability of polycation-exchanged SWy-2 to swell in solvents depends upon the polycation loading. At low loadings the X-ray diffraction trace of polycation-exchanged SWy-2 Žafter immersion in an aqueous dispersion containing 3% polyethylene glycol. contained peaks which could be attributed to polycation ˚ . and layers expanded by a bilayer of polyethylene expanded layers Ž 15.2 A ˚ ˚ spacing Ž . glycol 18.4 A . At polycation loadings of ) 0.6 CEC, only the 15.2 A was obtained indicating that the polycation, with its multiple exchange sites, was effectively binding the clay layers together. Kullaj Ž 1989. reported that when pinene is heated with bentonite treated with 10% HCl the products are camphene and tricyclene which led De Stefanis et al. Ž1995. to suggest that the absence of tricyclene amongst the products derived from USY and PILCs indicated that the reactions were taking place within the pore network. Tricyclene was not identified in any of the product mixtures from the acid-activated polycation treated clays nor was its presence mentioned by Rhodes and Brown Ž 1994. Ž who used extensively acid-treated samples of Texas bentonite. or Breen et al. Ž1997b.. Moreover, no oxidation products of pinene were found amongst the products from the either the alkyltrimethylammonium or polycation-exchanged clays even though the relatively high structural iron content in SWy-2 provides potential redox-active centres.
5. Conclusions Acid treatment of polycation-exchanged bentonites produced hybrid catalysts which enhanced the activity of the clays for the isomerisation of a-pinene to camphene and limonene. The presence of the polycation had a more marked influence on the activity of samples derived from SAz-1 increasing the yield from 25% for acid-activated SAz-1 with no added polycation to 50% camphene for acid-activated polycation-exchanged SAz-1. The increase in yield for corresponding samples derived from SWy-2 was only from 42 to 52%. This enhancement in yield for samples derived from SAz-1 was attributed to the increased hydrophobicity of the polycation loaded clay whilst the comparable yields for SWy-2 in the absence and presence of polycation may suggest that SWy-2 disperses well in the non-polar a-pinene. The total yields Žbased on a-pinene. for the most active catalysts was 80–90%. These yields are directly comparable with those obtained by others using zeolites and pillared clays although the acid-activated polycation-treated clays were marginally less selective towards camphene.
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Acknowledgements We gratefully acknowledge the help of Bob Burton and Margaret West of the XRF unit at Sheffield Hallam University along with Dr. P. Komadel for critically reading the manuscript.
References Akelah, A., Rehab, A., Selim, A., Agag, T., 1994. Synthesis and application of organophilic polystyrene-montmorillonite supported onium salts in organic reactions. J. Mol. Catal. 94, 311–322. Albert, R.M., Traynor, S.G., Webb, R.L., 1989. Fragrance and flavour chemicals, in: Zinkel, J., Russell, D.F. ŽEds.., Naval Stores—Production, Chemistry, Utilisation. Pulp Chemicals Association, New York. Billingham, J., Breen, C., Yarwood, J., 1997a. Adsorption of polyamine, polyacrylic acid and polyethylene glycol on montmorillonite: An in situ study using ATR-FTIR. Vib. Spectrosc. 14, 19–24. Billingham, J., Breen, C., Rawson, J.O., Yarwood, J., Mann, B.E., 1997b. The adsorption of polycations on clays. A comparative ‘in-situ’ study using 133Cs and 23Na solution phase NMR. J. Colloid Interf. Sci. 193, 183–189. Bovey, J., Kooli, F., Jones, W., 1996. Preparation and characterisation of Ti-pillared acid-activated clay catalysts. Clay Miner. 31, 501–506. Boyd, S.A., Mortland, M.M., Chiou, C.T., 1988. Sorption characteristics of organic compounds on hexadecyltrimethylammonium-smectite. Soil Sci. Soc. Am. J. 52, 652–657. Breen, C., Madejova, ´ J., Komadel, P., 1995a. Characterisation of moderately acid-treated, size-fractionated montmorillonites using IR and MAS NMR spectroscopy and thermal analysis. J. Mater. Chem. 5, 469–474. Breen, C., Madejova, ´ J., Komadel, P., 1995b. Correlation of catalytic activity with infra-red, 29 Si MAS NMR and acidity data for HCl-treated fine fractions of montmorillonite. Appl. Clay Sci. 10, 219–230. Breen, C., Rawson, J.O., Mann, B.E., 1996. Adsorption of polycations on clays: an in situ study using 133 Cs solution phase NMR. J. Mater. Chem. 6, 253. Breen, C., Zahoor, F.D., Madejova, ´ J., Komadel, P., 1997a. Characterisation and catalytic activity of acid-treated, size-fractionated smectites. J. Phys. Chem., B 101, 5324–5331. Breen, C., Watson, R., Madejova, ´ J., Komadel, P., Klapyta, Z., 1997b. Acid-activated organoclays: preparation, characterisation and catalytic activity of acid-treated tetra-alkylammonium exchanged smectites. Langmuir 13, 6473–6479. Brown, D.R., 1994. Clays as catalysts and reagent supports. Geol. Carpath. Ser. Clays 45, 45–56. ˇ ˇ B., Novak, I., 1977. Dissolution of smectites in hydrochloric acid: I. Half-time of Cicel, dissolution as a measure of reaction rate. In: Konta, J. ŽEd.., Proc. 7th Conf. Clay Miner. Petrol. Karlovy Vary 1976. Charles University, Prague, pp. 163–171. Clark, J.H., Cullen, S.R., Barlow, S.J., Bastok, T.W., 1994. Environmentally friendly chemistry using supported reagent catalysts: structure–property relationships for Clayzic. J. Chem. Soc. Perkin Trans. 2, 1117–1130. Cruz-Costa, M.C., Johnstone, R.A.W., Whittaker, D.J., 1996. Catalysis of gas and liquid phase ionic and radical rearrangements of a- and b-pinene by metal ŽIV. phosphate polymers. Mol. Catal. A 104, 251–259.
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Denoyel, R., Durand, G., Lafuma, F., Audebert, R., 1990. Adsorption of cationic polyelectrolytes onto montmorillonite and silica: microcalorimetric study of their conformation. J. Colloid Interf. Sci. 139, 281–290. De Stefanis, A., Perez, G., Ursini, O., Tomlinson, A.A.G., 1995. PLS versus zeolites as sorbents and catalysts: II. Terpene conversions in alumina-pillared clays and phosphates and medium pore zeolites. Appl. Catal. A 132, 353–365. Fahn, R., 1973. Influence of the structure and morphology of bleaching earths and their bleaching action on oils and fats. Fette, Seifen, Anstrichm. 75, 77–82. Hojabri, F., 1971. Dimerisation of propylene and its uses in isoprene manufacture: I. Appl. Chem. Biotech. 21, 87–89. Janek, M., Komadel, P., 1993. Autotransformation of H-smectites in aqueous solution. The effect of octahedral iron content. Geol. Carpath. Ser. Clays 44, 59–64. Jaynes, W.J., Boyd, S.A., 1990. Trimethylphenylammonium-smectite as an effective adsorbent of water soluble aromatic compounds. J. Air Waste Manage. Assoc. 40, 1649–1653. Jaynes, W.F., Boyd, S.A., 1991a. Clay mineral type and organic compound sorption by hexadecyltrimethylammonium-exchanged clays. Soil Sci. Soc. Am. J. 55, 43–48. Jaynes, W.J., Boyd, S.A., 1991b. Hydrophobicity of the siloxane surfaces in smectites as revealed by aromatic hydrocarbon adsorption from water. Clays Clay Miner. 39, 428. Kaplan, H., 1966. One step process of acid activating mineral clays and alkylating phenolic compounds with an alkene hydrocarbon. U.S. Patent 3,287,422: 4 pp. ˇ ˇ B., 1990. Alteration of smectites by treatments Komadel, P., Schmidt, D., Madejova, ´ J., Cicel, with hydrochloric acid and sodium carbonate solutions. Appl. Clay Sci. 5, 113–122. Komadel, P., Madejova, ´ J., Janek, M., Gates, W.P., Kirkpatrick, R.J., Stucki, J.W., 1996. Dissolution of hectorite in inorganic acids. Clays Clay Miner. 44, 228–236. Kullaj, S., 1989. Bull. Shkencave Nat. 43, 81, ŽC.A. 112, 21154z.. Lee, J.F., Mortland, M.M., Chiou, C.T., Kile, D.E., Boyd, S.A., 1990. Adsorption of benzene, toluene and xylene by two tetramethylammonium-smectites having different charge densities. Clays Clay Miner. 38, 113–120. ˇ ˇ B., 1994. Infrared study of the octahedral site populations in Madejova, ´ J., Komadel, P., Cicel, smectites. Clay Miner. 29, 319–326. Mokaya, R., Jones, W., Davies, M.E., Whittle, M.E., 1994. The mechanism of chlorophyll adsorption on acid-activated clays. J. Solid State Chem. 111, 157–165. Morgan, D.A., Shaw, D.B., Sidebottom, M.J., Soon, T.C., Taylor, R.S., 1985. The function of bleaching earths in the processing of palm kernel, and coconut oils. J. Am. Oil Soc. 62, 292–299. Mortland, M.M., Shaobai, S., Boyd, S.A., 1986. Clay-organic complexes as adsorbents for phenol and chlorophenol. Clays Clay Miner. 34, 581–585. ˇ ˇ B., 1978. Dissolution of smectites in hydrochloric acid: II. Dissolution rate as a Novak, I., Cicel, function of crystallochemical decomposition. Clays Clay Miner. 26, 341–344. Novak, I., Gregor, M., 1969. Surface area and decolourising ability of some acid-treated montmorillonites. Proc. Int. Clay Conf. Tokyo, pp. 851–857. Osthaus, B.B., 1956. Kinetic studies on montmorillonites and nontronite by acid dissolution technique. Clays Clay Miner. 4, 301–321. Rhodes, C.N., Brown, D.R., 1992. Structural characterisation and optimisation of acid-treated montmorillonite and high-porosity silica supports for ZnCl 2 alkylation catalysts. J. Chem. Soc. Faraday Trans. 88, 2269–2274. Rhodes, C.N., Brown, D.R., 1994. Catalytic activity of acid-treated montmorillonite in polar and non-polar reaction media. Catal. Lett. 24, 285–291. Rhodes, C., Brown, D.R., 1995. Autotransformation and aging of acid-treated montmorillonite catalysts: a solid state 27Al NMR study. J. Chem. Soc. Faraday Trans. 91, 1031–1035.
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Severino, A., Vital, J., Lobo, L.S., 1993. Isomerisation of a-pinene over TiO 2 : kinetics and catalyst optimisation. In: Guisnet, M., et al. ŽEds.., Heterogeneous Catalysis and Fine Chemicals: III. Elsevier, Amsterdam, pp. 685–692. Tkac, ˇ I., Komadel, P., Muller, D., 1994. Acid-treated montmorillonites—a study by 29 Si and 27Al MAS NMR. Clay Miner. 29, 11–19. Williams, C.M., Whittaker, D., 1971. Rearrangements of pinene derivatives: Part I. Products of acid catalysed hydration of a-pinene and b-pinene. J. Chem. Soc. B, pp. 668–672. Yu, J.-Q., Zhou, P., Xiao, S.-D., 1995. Highly selective hydration reaction of a-pinene over H-mordenites pretreated with quaternary ammonium salts. Chin. J. Chem. 13, 280–283.
Acid-activated organoclays: preparation, characterisation and catalytic activity of polycation-treated bentonites Christopher Breen ) , Ruth Watson Materials Research Institute, Sheffield Hallam UniÕersity, Sheffield S1 1WB, UK Received 9 July 1997; revised 20 November 1997; accepted 20 November 1997
Abstract Samples of SWy-2 and SAz-1 loaded with increasing amounts of the polycation magnafloc 206, wŽMe 2 NCH 2 CHOHCH 2 . n x nq Cl n , were acid-treated using 6 M HCl at 958C for 30, 90 and 180 min. The activity of these acid-activated polycation-exchanged clays for the conversion of a-pinene to camphene and limonene was determined and compared with that from clay samples Žwithout polycation. acid-treated in the same manner. Acid treatment of polycation-exchanged bentonites produced hybrid catalysts which enhanced the activity of the clays for the isomerisation of a-pinene to camphene and limonene. The presence of the polycation had a more marked influence on the activity of samples derived from SAz-1 increasing the yield from 25% for acid-activated SAz-1 with no added polycation to 50% camphene for acid-activated polycation-exchanged SAz-1. The increase in yield for corresponding samples derived from SWy-2 was only from 42 to 52%. This enhancement in yield for samples derived from SAz-1 was attributed to the increased hydrophobicity of the polycation loaded clay whilst the comparable yields for SWy-2 in the absence and presence of polycation may suggest that SWy-2 disperses well in the non-polar a-pinene. The total yields Žbased on a-pinene. for the most active catalysts was between 80 and 90%. These yields are directly comparable with those obtained by others using zeolites and pillared clays although the acid-activated polycation-treated clays were marginally less selective towards camphene. q 1998 Elsevier Science B.V. Keywords: organoclays; acid-catalysis; isomerisation; smectite
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Corresponding author. Tel.: q44-0114-253-3008; fax: q44-0114-253-3501; e-mail: [email protected]. 0169-1317r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 1 6 9 - 1 3 1 7 Ž 9 8 . 0 0 0 0 6 - 4
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1. Introduction In addition to their use as bleaching earths for the clarification of oils ŽMorgan et al., 1985. and in the carbonless copying paper Ž Fahn, 1973. , commercial acid-treated clays have been used as an effective solid source of protons for a considerable period and have found use in industrial processes such as the alkylation of phenols ŽKaplan, 1966. and the dimerisation and polymerisation of unsaturated hydrocarbons Ž Hojabri, 1971. . Moreover, acidactivated clays have been the focus of renewed interest in their role as high-surface-area supports for environmentally friendly catalysts in Friedel Crafts alkylation and acylation reactions ŽBrown, 1994; Clark et al., 1994. . Commercial acid-treated clays are normally prepared using a fixed quantity of acid chosen to remove the desired proportion of cations from the octahedral sheet ŽRhodes and Brown, 1995. and few investigations have addressed how the catalytic activity varies with octahedral sheet depletion Ž Rhodes and Brown, 1994; Breen et al., 1997a.. During acid-activation of clays the octahedral ions are leached out, the nitrogen surface area increases, the number Ž and type. of acid sites changes as do the sorptive and catalytic properties. These properties generally maximise under intermediate activation conditions and subsequently, decrease as the product takes on the properties of amorphous silica, which has been identified as the final product of acid leaching Ž Fahn, 1973; Komadel et al., 1990; Tkacˇ et al., 1994; Breen et al., 1995a,b. . It is now widely accepted that clays with a high octahedral Mg content leach more readily than those which have a high ˇ ˇ and octahedral Al population Ž Osthaus, 1956; Novak and Gregor, 1969; Cicel ˇ ˇ 1978; Breen et al., 1995a,b; Komadel et al., Novak, 1977; Novak and Cicel, 1996. and recent reports have established that clays with a high octahedral Fe content also leach rapidly Ž Janek and Komadel, 1993; Breen et al., 1997a.. An extensive investigation into the acid-leaching of a range of smectites Ž selected for their differing octahedal ion content. has shown that the elemental composition of the starting material did not make a significant contribution to the catalytic activity for the chosen test reaction but did play a key role in determining the severity of the activation conditions required for the optimisation of the catalytic activity ŽBreen et al., 1997a. . It is important to note that the extent of acid dissolution can significantly influence the activity of the catalyst for a particular reaction whether it is employed as a support ŽRhodes and Brown, 1992. or as a catalyst in its own right ŽRhodes and Brown, 1994; Breen et al., 1997a.. Using the formation of tetrahydropyranyl ether from dihydropyran and methanol as a test reaction Rhodes and Brown Ž1994. established that the catalytic activity of acid-activated clay for reactions in polar media was optimised when the acidity and swelling ability of the catalyst was at a maximum. This occurs at short acid-treatment times. In contrast the yield for the acid catalysed isomerisation of a-pinene to
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camphene, which represents reaction in a non-polar medium, was optimised when the external surface area reached a maximum, which occurs when leaching is extensive. They concluded that the reaction in polar media was optimised at short acid-treatment times because the hydrophilic clay attracts the polar reagents to the surface where the catalytic protons reside. This contrasts with the behaviour in non-polar media where the reaction yield was optimised when the clay was substantially leached because the catalyst presented an essentially hydrophobic surface which served to attract the non-polar reagents. This data is, however, limited to the study of a single clay with a high octahedral Al content. It is long established that the hydrophilic aluminosilicate surface of swelling clays can be rendered hydrophobic by exchanging the naturally occurring inorganic cations Ž Ca2q, Naq, Kq . with organocations such as tetramethylammonium ŽLee et al., 1990. hexadecyltrimethylammonium Ž Mortland et al., 1986; Boyd et al., 1988; Jaynes and Boyd, 1991a. and phenyltrimethylammonium ŽJaynes and Boyd, 1990, 1991b. to select a few from an extensive list. Indeed, Akelah et al. Ž1994. used organophilic polystyrene-montmorillonite supported onium salts to accelerate the reaction of thiocyanate and nitrite with alkyl and benzyl bromides. This knowledge coupled with the recent enhancement of the catalytic activity of pillared clays, via pillaring acid-activated clays, Ž Mokaya et al., 1994: Bovey et al., 1996. led us to consider acid treated organoclays as catalysts for reactions in non-polar media. The conversion of a-pinene to camphene, our chosen test reaction, has industrial significance because camphene is an intermediate in the synthesis of camphor, which has value due to its aroma and pharmaceutical properties ŽAlbert et al., 1989. . Acidified titanium oxide is usually employed in the transformation of a-pinene to camphene Ž Severino et al., 1993. , although recent studies have considered the use of zeolites Ž Yu et al., 1995; De Stefanis et al., 1995., pillared interlayer clays ŽPILCs. ŽDe Stefanis et al., 1995. and Žboth crystalline and amorphous. zirconium and tin phosphates Ž Cruz-Costa et al., 1996.. Here we report the use of acid-activated polycation-exchanged smectite for the conversion of a-pinene to camphene, via ring expansion, and limonene, via ring opening ŽWilliams and Whittaker, 1971.. Our ultimate goal is to produce cost-effective, hydrophobic catalysts of relatively high acidity in a short time using simple exchange procedures. This approach explores the resistance of organoclays to octahedral ion leaching and has the potential to reduce the concentration of Al, Mg and Fe in effluent flows from the leaching process. Moreover, the ability of acid-activated polycation-exchanged clays to swell in non-polar solvents should provide the opportunity to access a considerable portion of the potential surface area which has been estimated near 800 m2 gy1. Cognisant of the importance of the octahedral sheet composition on acid leaching we have used two starting clays of different octahedral composition, which have been the focus of acid leaching studies previously Ž Tkacˇ et al., 1994; Breen et al., 1995a,b., to evaluate the properties of the resulting catalysts. The
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results are compared with the yield from acid-activated alkyltrimethylammonium-exchanged clays which have been reported elsewhere Ž Breen et al., 1997b.. 2. Experimental method 2.1. Materials Both montmorillonites SAz-1 Ž Cheto, AZ, USA. and SWy-2 Ž Crook County, WY. were obtained from The Clay Mineral Repository of the Clay Minerals Society, Columbia, MO, USA. The elemental compositions given in Table 1 emphasise the different octahedral compositions of these two dioctahedral smectites. SAz-1 is richer in magnesium, but contains less octahedral iron, than SWy-2. These clays were used without further purification. The polycation used was Magnafloc 206, wŽMe 2 NCH 2 CHOHCH 2 . n x nq Cl n , which has a nominal relative molecular mass of 100 000 and the distance ˚ and the between charge centres, derived from a molecular model, was 4.8 A ˚ Magnafloc 206 was provided as a 50% active approximate length was 3000 A. solution and Kjeldahl nitrogen analysis confirmed that it contained 9.0% nitrogen. The adsorption of this polycation onto Csq-, Naq- and Kq-exchanged Texas bentonite has been described elsewhere Ž Breen et al., 1996. . 2.2. Adsorption isotherms Samples were prepared by introducing 10 cm3 of 50 g dmy3 SAz-1 or SWy-2 Ž0.5 g clay. into new polypropylene bottles together with a quantity of magTable 1 Elemental compositions of selected samples derived from SAz-1 and SWy-2 Sample
SiO 2 Žwt.%.
Al 2 O 3 Žwt.%.
MgO Žwt.%.
Fe 2 O 3 Žwt.%.
CaO Žwt.%.
Na 2 O Žwt.%.
SA-00 SA-30L SA-180L SA-30H SA-90H SA-180H SA-32-90H SW-00 SW-30H SW-90H SW-180H SW-13-90H
68.20 72.35 72.75 73.78 76.78 81.74 71.20 68.48 72.55 72.51 73.53 71.78
19.74 19.62 19.37 18.43 16.41 13.04 19.94 20.16 21.07 20.75 20.29 21.07
7.24 6.29 6.15 6.18 5.43 4.18 6.94 2.77 2.45 2.41 2.35 2.49
1.59 1.66 1.62 1.58 1.36 1.02 1.81 4.13 3.75 3.70 3.61 4.37
3.16 0.05 0.08 0.03 0.02 0.02 0.04 1.76 0.06 0.05 0.05 0.05
0.07 0.03 0.03 0.00 0.00 0.00 0.07 1.70 0.14 0.14 0.16 0.24
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nafloc 206 and sufficient deionised water to give a total volume of 20 cm3. The clay-adsorbate suspensions were then shaken at 258C for 2 h. The samples were centrifuged and the supernatant decanted before washing the sample once with water to remove excess polymer. The samples were dried at 1208C, ground and then stored. Nitrogen content was determined using a Gerhardt Vapodest Kjeldahl Autoanalyser. Samples for powder XRD and TG studies were air dried after centrifugation and ground to - 75 m m. 2.3. Sample preparation Using the adsorption isotherm data ŽFig. 1. as a guide, 2-g portions of each smectite were added to aqueous solutions of containing 300, 1500 and 3000 m l of polycation. These volumes of polycation were chosen to provide samples which had approximately 0.25, 1.0 and 1.5 times the CEC of polycation adsorbed. The resulting suspensions were agitated for 2 h in a rotary shaker operating at 250 rpm. The supernatant was removed and the solids were washed once with deionised water. This approach provided samples with loadings of 32, 108 and 173 mg Žg clay.y1 for SAz-1 and 17, 128 and 135 mg Žg clay.y1 for SWy-2. These non acid-treated organoclays are designated SA-28, SA-108 and SA-173 for SAz-1 and SW-17, SW-128 and SW-135 for SWy-2, where the number represents the polycation loading in mg Ž g clay.y1. One-gram portions of the variously exchanged clays were then activated using 6 M HCl for 30, 90 or 180 min. The polycation-treated clays were only treated at 958C, whereas SAz-1 and SWy-2 were treated at either 258C or at 958C for the times stated. Thus SA-30L is SAz-1 treated with 6 M HCl for 30 min at low temperature Ž L, 258C., whereas SW-180H has been treated with 6 M HCl for 180 min at high temperature ŽH, 958C.. Finally, the acid-activated organoclays use a combination of figures to identify the polycation loading and the acid-activation conditions. Hence, SA-32-90H is SAz-1 with 32 mg polyca-
Fig. 1. The adsorption of Magnafloc 206 on SWy-2 Ž`. and SAz-1 ŽB..
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tion Žg clay.y1 which has been treated with 6 M HCl for 90 min at 958C. The intention was to evaluate the ability of the variously acid-activated samples to catalyse the isoemerisation of a-pinene. In particular we were interested in the effect of different acid treatments on the organoclays formed from clays of different initial composition and to evaluate the resistance of the organoclays to acid leaching. 2.4. Sample characterisation X-ray diffraction profiles of pressed powder samples were obtained using a ˚. Philips PW1830 X-ray diffractometer using a copper tube Ž l s 1.5418 A y1 Ž . operating at 40 kV and 35 mA. Profiles were recorded at 28 2Q min . Samples for XRF analysis were prepared using the Li 2 B 4O 7 fusion method. The resulting beads were analysed on a Philips PW2400 XRF spectrometer using calibration graphs prepared from certified reference materials. Thermogravimetric traces were obtained using a Mettler TG50 thermobalance equipped with a TC10A processor. Samples Ž 7 mg. were heated from 358 to 8008C at 208C miny1 under a flow of 20 cm3 miny1 dry nitrogen carrier gas. 2.5. Catalytic actiÕity A 50-mg smectite was dried at 1208C for 16 h to remove physisorbed water, stoppered and placed in a desiccator to cool. The dried samples were added to 8.174 g Ž 0.06 mol. of a-pinene pre-heated to 808C and maintained at 808C for 2
Scheme 1.
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h. The solution was then separated from the catalyst using a syringe filter and stored in a dry sample vial. Control experiments confirmed that the filter exerted no influence on the reaction products and that no further reaction occurred even after five days at room temperature. The products Žsee Scheme 1. were identified as camphene Ž3., limonene Ž4., i-terpinolene Ž5. , g-terpinene Ž 6. , a-terpinene Ž7. and a-phellandrene Ž8. by GC-MS Ž Unicam Automass. . The amount of camphene, limonene and other minor products was determined using capillary GC with FID detection. A 3% OV-225 column was used and was held isothermally at 708C for 10 min. The injection port and detector were held at 1508C and the N2 flow rate was 12 cm3 miny1.
3. Results 3.1. Adsorption isotherms and elemental analysis The uptake curves in Fig. 1 are similar to those reported previously for the adsorption of this polycation on a low iron Texas bentonite Ž Breen et al., 1996. and show that both SWy-2 and SAz-1 display a high affinity for Magnafloc 206. The amount of sorbed polycation exceeded that required to satisfy the exchange capacities of SWy-2 and SAz-1 Ž 90 and 122 mg gy1, respectively. which is common behaviour in clay polycation systems ŽBreen et al., 1996; Denoyel et al., 1990.. The data in Table 1 shows that acid treatment at 258C generally had little effect on the elemental composition of SAz-1 and SWy-2 Žvalues not shown. as anticipated from the results of Breen et al. Ž 1995a. . Treatment with acid at room temperature simply replaces the resident Ca and Na exchange ions with protons. In contrast, treatment with hot acid for only 30 min Ž SA-30H. altered the octahedral composition of SAz-1 reducing the MgO content from 7.24% to 6.18%. After 3 h Ž SA-180H. , 34% of the Al 2 O 3 , 42% of the MgO and 35% of the Fe 2 O 3 content had been removed. These reductions in octahedral population are in line with other published results using similar activation conditions ŽBreen et al., 1995b, 1997a. and emphasises the reduced stability of clays rich in octahedral magnesium towards acid-leaching. The elemental composition of acid-activated, polycation-exchanged samples SA-32-90H and SW17-90H were almost identical to that of the starting materials which suggests that the presence of even small amounts of polycation protected the clay layer from acid attack. Note, however, that the data in Table 2 shows that the amount of adsorbed polycation decreased as the time of acid treatment was increased and that the effect was more marked with SAz-1 which contains more Ca2q-ions on the exchange sites when polycation is initially added. Sorption of polycations to Naq-exchanged montmorillonite in aqueous suspension results in the formation
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Table 2 Polycation loadings on samples derived from SAz-1 and SWy-2 Sample
Polycation loading Žmg Žg clay.y1 .
Sample
Polycation loading Žmg Žg clay.y1 .
SA-32 SA-32-30H SA-32-90H SA-32-180H SA-108 SA-108-30H SA-108-90H SA-108-180H SA-173 SA-173-30H SA-173-90H SA-173-180H
31.7 21.2 Ž67%. 19.7 Ž62%. 10.15 Ž32%. 108.3 98.0 Ž91%. 95.7 Ž88%. 80.5 Ž75%. 173.2 148.4 Ž86%. 131.8 Ž76%. 106.1 Ž61%.
SW-13 SW-13-30H SW-13-90H SW-13-180H SW-128 SW-128-30H SW-128-90H SW-128-180H SW-135 SW-135-30H SW-135-90H SW-135-180H
12.9 12.9 10.5 Ž81%. 128.3 102.3 Ž79%. 98.3 Ž77%. 135.3 114.9 Ž85%. 110.8 Ž82%. 106.9 Ž79%.
of large, low density flocs with the polycation contained in the interior. In contrast, sorption to Csq-montmorillonite Ž which forms tactoids prior to polycation addition as does Ca2q . produces smaller flocs with a greater proportion of polycation on the external surfaces of the flocs Ž Billingham et al., 1997b. particularly at low loadings of polycation. The increased removal of polycation from samples derived from SAz-1 probably reflects the higher proportion of ‘available’ polycation molecules bound to the external faces of Ca-SAz-1 tactoids, whereas in samples derived from the Naq-rich SWy-2, the polycation is bound inside the flocs and is thus less accessible to acid attack. 3.2. Catalytic actiÕity Fig. 2 illustrates how the catalyst preparation method affected the total conversion of a-pinene. In general the catalysts derived from SWy-2 were more active than those derived from SAz-1 and hot acid treatment of both SWy-2 and SAz-1 increased the total conversion more than mild acid treatment. The highest yields were achieved using acid-activated clays with the lowest polycation loading and the presence of the polycation exerted more influence on the samples derived from SAz-1 than those obtained from SWy-2. Catalysts prepared from samples with intermediate and high polycation loadings were no more effective than the mild acid-treated clays. This is particularly striking for the samples derived from SAz-1 and is best illustrated by comparing the total conversion of samples obtained by treating SAz-1 for 90 min. Cold acid treatment of SAz-1 increased the activity from 0 to 12%, hot acid treatment increases this 12% yield to 55% and finally acid-treatment of SA-32 enhanced this a further 26% to 81%. A similar commentary could also be applied to the
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Fig. 2. Total conversion of a-pinene over acid-activated samples derived from Ža. SAz-1 and Žb. SWy-2.
samples derived from SWy-2. The comparatively low isomerisation yield for SA-32-180H is currently attributed to the much reduced polycation loading on this sample compared to SA-32-30H and SA-32-90H ŽTable 2.. The distribution of products which make up the total conversion reported in Fig. 2 are given in Fig. 3. In all cases, camphene Ž 3. was the major and limonene Ž4. the minor product except for catalysts derived from SW-13 ŽSW-13-90H, SW-13-180H. where the yield of other products exceeded that of limonene. It is often difficult to discern small changes in the selectivity based on a product distribution plot such as Fig. 3. Consequently, the actual yield of
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Fig. 3. Product distributions for samples derived from Ža. SAz-1 and Žb. SWy-2.
camphene, limonene and other products Ž from Fig. 3. was calculated as a percentage of the total yield Žfrom Fig. 2., i.e., the selectivity. The results of these selectivity calculations, which are not listed here, showed that the various catalysts exhibited some variation in the product selectivity and that the selectivity was more erratic for samples derived from SAz-1 than for those derived from SWy-2. The selectivity data can best be summarised by saying that the selectivity over samples derived from SAz-1 was 54 " 5% towards camphene, 30 " 5% towards limonene and 18 " 9% for the other compounds. For samples derived from SWy-2 the values were very similar at 56 " 6% for camphene, 27 " 5% for limonene and 21 " 6% for the minor products.
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4. Discussion 4.1. Acid-actiÕated SAz-1 and SWy-2 Rhodes and Brown Ž1994. showed that the activity of acid-activated Texas bentonite for the isomerisation of a-pinene increased as the extent of acid leaching increased reaching a maximum as the available surface area maximised and then diminished to zero. They convincingly argued that the clay surface became more like hydrophobic silica than hydrophilic aluminosilicate as the leaching progressed and thus the surface became more attractive to the non-polar a-pinene. In the main, the data presented here for the clays without adsorbed polycation conform to that interpretation. The total conversion of a-pinene by hot acid-treated SAz-1 samples was more than three times that for cold acid treated samples, whilst treatment of SWy-2 at 958C doubled the yield obtained from the samples treated at 258C. Nonetheless, the samples derived from SWy-2 were very effective catalysts for the a-pinene conversion even when the XRF ŽTable 1. and XRD data Ž not illustrated. suggested that the clay maintained a considerable amount of lamellar character. MAS-NMR studies of SAz-1 Ž Breen et al., 1995b. and SWy-1 Ž Tkacˇ et al., 1994. leached in hot 6 M HCl support the view that SAz-1, which has a higher octahedral magnesium population than SWy-2, is leached more rapidly. Therefore, samples derived SAz-1 should attain the necessary hydrophobicity at shorter activation times. This is clearly not the case so the extent of acid activation cannot make the sole significant contribution to the activity for the isomerisation of a-pinene. We have recently investigated the use of acid-activated, tetramethylammonium-, dodecyltrimethylammonium- and hexadecyltrimethylammonium-exchanged smectites as catalysts for the isomerisation of a-pinene ŽBreen et al., 1997b.. As part of that study the activity of acid-treated clays which did not contain alkyltrimethylammonium ions was also investigated for comparative purposes. However, the experimental strategy required different amounts of acids for the different clays and cross comparison was not possible. The dioctahedral smectites used included Jelsovy-Potok, which is rich in octahedral aluminium Ž Breen et al., 1995a, 1997a. , Stebno, an iron-rich beidellite with a ˇ ˇ and Novak, 1977., and SWa-1, a ferruginous high octahedral Fe content ŽCicel Ž smectite Madejova´ et al., 1994. . Jelsovy-Potok was very mildly acid treated Ž0.1 M HCl at 258C. and activity increased very little over that of the untreated material Ž12% total conversion. . There was some evidence that the amount of octahedral iron plays a role in the activity insofar as Stebno exhibited a high initial activity for a-pinene conversion and was very active Ž 90% total conversion. after 60 min leaching in 6 M HCl at 258C. Therefore, the amount of octahedral iron in the clay may influence the catalytic ability. Another, as yet unproven possibility, is that the activity of the catalyst also reflects its ability to swell in the organic solvent. SWy-2 contains a large proportion of Naq-ions on
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its exchange sites and is known to swell more in aqueous solution than SAz-1 which is rich in Ca2q-exchange ions. We shall see as the discussion progresses that the ability of organoclays to swell in organic media could contribute to the activity of acid-activated organoclays hence the possibility exists that the activity of SWy-2 reflects the greater surface area it potentially offers in organic solvents. 4.2. Acid-actiÕated polycation-exchanged clays In general the total conversions of the most active catalysts prepared in this study were between 60 and 90%, with catalysts derived from SW-17 and SA-32 being the most active ŽFig. 2.. These conversions are considerably higher than those reported for extensively acid-leached Texas bentonite Ž Rhodes and Brown, 1994. but are similar to the values of 80 and 90% reported for permanently-porous materials including ultra stable zeolite Y ŽUSY., and Al- and Al,Fe-Pillared Clays Ždenoted Al-PILC and FAZA. Ž De Stefanis et al., 1995. . In our recent study of activity of acid-activated alkyltrimethylammonium-exchanged clays for the isomerisation of a-pinene Ž Breen et al., 1997b., we established that tetramethylammonium-exchanged clays provided the best precursors and only required activation at room temperature in 0.1 M HCl to effect total conversions Žbased on a-pinene. of 60 to 80%. Clays exchanged with the larger dodecyltrimethylammonium and octadecyltrimethylammonium ions were only effective when these cations occupied only 30% of the exchange sites. Indeed the yields and selectivities of the polycation treated clays used here are similar to those obtained using long-chain alkyltrimethylammonium-exchanged clays insofar as the activity decreases markedly with loading. The yields for the acid-activated alkyltrimethylammonium exchanged clays were obtained using identical conditions to those described herein whereas the samples of USY, Al-PILC, and FAZA had been extensively dehydrated Ž 4508C, 4 h. and the authors stated that they were thus operating as Lewis acid catalysts. Moreover, the yields were obtained after a reaction time of 5 h at 1008C in a sealed glass reactor at autogenous pressure, conditions which are more severe than the conditions reported herein. Furthermore, the absolute yields of camphene and limonene obtained using USY and PILCs were similar to those reported here at ca. 45 " 5% and 15 " 5%, respectively, but were slightly lower than those obtained from acid-activated alkyltrimethylammonium-exchanged clays ŽBreen et al., 1997b. . The selectivity of USY and the pillared clays were similar to eachother with a camphene:limonene ratio near three whereas the acid-activated organoclays Žwhether derived from alkyltrimethylammonium or polycation-exchanged clays. were less selective towards camphene with a ratio near 2.2:1. The fixed, porous nature of USY and the PILCs means that they are unable to swell in the reaction liquor and therefore may select against the size of the limonene molecule. This
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is less likely with the catalysts derived from alkyltrimethylammonium-clays because the charge balancing organocations do not bind the layers together as the alumina pillars do in PILCs and thus these organoclays can avail of their ability to expand in the reaction liquor. Billingham et al. Ž 1997a. showed that the ability of polycation-exchanged SWy-2 to swell in solvents depends upon the polycation loading. At low loadings the X-ray diffraction trace of polycation-exchanged SWy-2 Žafter immersion in an aqueous dispersion containing 3% polyethylene glycol. contained peaks which could be attributed to polycation ˚ . and layers expanded by a bilayer of polyethylene expanded layers Ž 15.2 A ˚ ˚ spacing Ž . glycol 18.4 A . At polycation loadings of ) 0.6 CEC, only the 15.2 A was obtained indicating that the polycation, with its multiple exchange sites, was effectively binding the clay layers together. Kullaj Ž 1989. reported that when pinene is heated with bentonite treated with 10% HCl the products are camphene and tricyclene which led De Stefanis et al. Ž1995. to suggest that the absence of tricyclene amongst the products derived from USY and PILCs indicated that the reactions were taking place within the pore network. Tricyclene was not identified in any of the product mixtures from the acid-activated polycation treated clays nor was its presence mentioned by Rhodes and Brown Ž 1994. Ž who used extensively acid-treated samples of Texas bentonite. or Breen et al. Ž1997b.. Moreover, no oxidation products of pinene were found amongst the products from the either the alkyltrimethylammonium or polycation-exchanged clays even though the relatively high structural iron content in SWy-2 provides potential redox-active centres.
5. Conclusions Acid treatment of polycation-exchanged bentonites produced hybrid catalysts which enhanced the activity of the clays for the isomerisation of a-pinene to camphene and limonene. The presence of the polycation had a more marked influence on the activity of samples derived from SAz-1 increasing the yield from 25% for acid-activated SAz-1 with no added polycation to 50% camphene for acid-activated polycation-exchanged SAz-1. The increase in yield for corresponding samples derived from SWy-2 was only from 42 to 52%. This enhancement in yield for samples derived from SAz-1 was attributed to the increased hydrophobicity of the polycation loaded clay whilst the comparable yields for SWy-2 in the absence and presence of polycation may suggest that SWy-2 disperses well in the non-polar a-pinene. The total yields Žbased on a-pinene. for the most active catalysts was 80–90%. These yields are directly comparable with those obtained by others using zeolites and pillared clays although the acid-activated polycation-treated clays were marginally less selective towards camphene.
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Acknowledgements We gratefully acknowledge the help of Bob Burton and Margaret West of the XRF unit at Sheffield Hallam University along with Dr. P. Komadel for critically reading the manuscript.
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