Materials derived from synthetic organo-clay complexes as novel hydrodesulfurization catalyst supports

MICRO~ROUS AND MESOPOROUS MATERIALS

Materials derived from synthetic organo-clay complexes as novel hydrodesulfurization catalyst supports’

-~ Abstract A scrims of mesoporous synthetic organo-clay complcxc~ has been prepared by hydrothermal crystallization of gels containing silica. magnesium hydroxide. lithium fluoride, and an organic of choice, followed by calcination to remove the organics. The organic serves to impart structural order to the inorganic network that dots not disappear upon its removal. The choice of organic modifier can be used to control the pore structure of the resulting mesoporous materials. Pore size distributions appear in some zascs to he related to the type of polymer packing upon clay forIll~tti~)~~ in situ. Thcsc materials al-e being explored as Co Mo llydrojeslllfurization ( HDS) catalyst supports. Preliminary HDS results show performance c~)~illnei~stir~~tc with commercial catalysts for the mcsoporous materials when a model heavy oil feed is used ( I wt% S 21s dibenzothio(lhene in hexadecane). Temperature programmed reduction experiments of used catalysts suggoct a relationship hetwcen HDS activity ;md cast of reduction of the CoClo:cl;~y catalysts. Reactivity of the Cohlo ilay also corl-elates with the percentage of mrsoporc volume remaining after reaction. Losses in mcsopore volume are largely ~-ccoup~~i by rec:dcination. suggesting that reversible coke is formed inside rhe port structure of clays l;istol- than iwide conventional alumina. !(’ 19% Etswicr Science B.V. &!~ti-~&,s;

Hectorite

clay: Clay-organic

compic’i:

li~clritdcsulfurizution;

1. Introduction f.~~dro~~rocessin~ representsa crucial component of current petroleum refining operations in terms of both environmental and economic considera tions. The literature in this area is extensive and many reviews are available for an in-depth look at the field [I-X]. A focus of intensive efYort has been the development of adequate catalysts particularly for l~~drodesulfuriz~tion ( HDS 1 [3- X].

Polymer-clays;

HDS supports:

~~~CSO~OIW

Regulations concerning maximum sulfur contents of oil and subsequentSO, emissionsupon combustion zre becoming increasingly more stringent. At the same time, cr-ude oils XI-C becoming heavier :md contain increasing amounts of sulfur. HDS catalysts with higher performance are therefore being sought in order to meet tighter environmental restrictions. Typical industrial HDS catalysts are comprised of Co( Nil-Mo sulfides on an alumina support. As refiners have begun to process heavier crude oil feedstocks. l~~~~~iil~c~lti~l~ of the pore structure oi’the alumina support has attracted increasing attention 19--l61. Specifically, several groups have investigated the effects of incorporat-

ing varying percentages of macroporosity into the alumina pore structure [ 13. 15.161. In its resid upgrading plant, Texaco has recently successfully implemented CO-MO catalysts with some macroporosity [ 171. We describe here the development of a new class of support materials with pore sizes primarily in the mesoporous range. in addition to some macroporosity. Our approach is to create synthetic organo-clay complexes called SOCCs. This entails a clay mineral synthesis from a silicate sol -gel that contains an organic species. The pore size arises when the SOCCs are calcined to remove the organ& occluded in the pore network. Clay minerals are layered aluminum or magnesium silicates of significant cation exchange capacity and both external and internal surface area [ 181. Because natural clays are abundant and inexpensive. the!, have ;I rich history of use in the hydroprocessing industry [ 19.20]l The SOCCs are unique for several reasons. including: (1) the pore size and distribution can be controlled via synthetic melhods. and (7) the purity is high compared to natural minerals which contain many dopants and impurity phaws. The mineral of choice for this work is hecrorite. :I magnesium silicate smectite clay, because of its ease of crystallization at relatively low tempcratures and because of its lower surface acidity (and therefore low cracking activity) compared to aluminosilicate clays such as montI7io~illonite. Systematic investigations of the synthesis structure relationships for the SOCC materials 11a~c been performed [21 -241. The port sire distributions and information concerning the extended microstructure of some calcined SOCCs ha\e also been reported [25]. This work represents the first application of these materials to catalysis, in partic-

ular HDS. Fig. I provides a schematic representation of what the SOCCs are postulated to look like before and after calcination. The evidence indicates that small clay orystallites with polymerintercalated layers arc themselves somewhat imbedded in a matrix whose pores and voids are also filled with polymer. .4fter removal of the polymer upon calcination, the open network remains behind to accommodate the CO--MO-S catalytic phase and subsequent catalytic reaction.

2. Experimental The details for the hydrothermal crystallization of these hectorites are provided elsewhere [35], although a general description follows. Hydroxypropylmethyl cellulose (HPMC) was from Dow Chemical Co., Midland, MI (Methocel 240s). hvdroxvethyl cellulose (HEC) was from i Aqualon Co.,- Wilmington, DE (Natrosol 250 HHXR), and poly(dimethyl diallyl ammonium chloride) (PDDA, Agefloc WT40) was obtained from CPS Chemical Co.. Inc.. Old Bridge. NJ. Tetraethyl ammonium chloride (TEA. Aldrich) was also used in some clay preparations. Reactants in molar ratios of LiF:MgO:SiO, 0.266:1.00:1.52 are refluxcd for 2 days as a 2 wt% aqueous slurry. This yields the ideal hectorite composition Ex,,.,,[I-i,,.,,,MgS.33Si,0,,(OH LL where Es =exchangeablc monocation. The amount of polymer added is chosen such that, if all of the polymer is incorporated into the clay and assuming complete reaction, 20 ~1% of the SOCC would be organic. Lithium( I ) ions occur in the lattice as isomorphous substitutions Ihr Mg( II ). leaving a negative charge that is carried by the basal oxygen

surface and compensated for by the presence 01 exchangeable cations and water molecules within the interlayer or gallery. In the inorganic synthetic hectorite, Li( I ) is also the exchangeable cation. A certain amount of this Li( I ) can be replaced by organic or organometallic cations directly from a precursor organic-containing gel. When the neutral polymers are used, Li( I ) ions arc also present in the interlayer in the cation exchongc sites. The SCllll-ce of MgO was a freshly synthesized Mg(OH)> slurry. The source of silica was Ludox I-IS-30. To isolate the clay, the slurry was centrifuged and washed three times or until the decant was clear, aliowed to air dry, then ground 10 a powder. Calcination of the SOCCs was carried out in a tube furnace using quart7 boats at 400 C under air or oxygen flow for at least 4 h and often longer. For brevity. the synthetic ctrgano-hectnritss are referred to as synHPMC. synHEC’. synPDDA. and synTEA. The purely inorganic hectoritc is synLi.

The catalyst pretreatment process for both the clay-supported and the reference catalysts consisted of loading into the HDS reactor under N,, purging in N2 at 20 C for 30 min at 1000cm.‘,:min, drying in N, at 1.50C for 60 min and at 400 C for 60 min. and finally sul~~~ill~in ;I 5% H,S:‘H, mixture at 400 C for 2 h prior to use as catalysts, The laboratory scaleliquid-phase continuous-flow HDS unit is shown schematically in Fig. 2. The reactor consists of a thick-walled 0.375 in ID 316 SS tube, \vith I g catalyst diluted with 5 g tabular ahmlina ( LaRochc T-i 061, 10 m’,g) sitting between plugs of quartz wool. Beneath the lower plug was a 0.125 in ID, 0.375 in OD deadman used to l~liliiiliize volume between the reactor and the liquid receiver. The liquid test feed consistsof’1 .O wt”i, sulfuras dibenzothiophcnc (DBT ). dissolved in hexadeca~~e and is representati\;e of a middle distillate oil. All liquid-filled lines were heated to 50 C. ‘Typical c~3nditiorlsfor cat;ilytic testing arc also provided in Fig. 3. The products were diluted with hexane ( I mg product,“200 ml hexane). separated using H DBS-MS column, and analyTcd using a HP 5890 GC-MS Series I I PILIS. Random errors associated with GC-MS concentration measurements were lessthan Y%, and the reproducibility of conversion measurementswas ir: 15”; ,c of the reported values. Sclcctivity has been delincd as the percentage of biphenyl (the selective H DS prodnct from diben7othiophcne) divided by the percentage of dibcnzothiophenc converted times 100.

After drying the clay supports at 400 C in Nl (< 3 ppm H,O) overnight. the clays wcrc calcined at 400 C for 5 h in air which had been passed through a moisture trap consisting of Drieritc and molecular sieves. Aqueous ammonium hcptamolybdate ( AIfa, ( NH~)~,~~o?O~~. 1H20, 99.999%) solutions were prepared so that a metal loading ot 6 wt% MO would fill 80% of the available pore volume of the clays. Following Mo impregnation and recalcination using the aforementioned conditions, the pore volumes were measured again ~tsing an established LN, physisorption protocol. Aqueous cobalt nitrate ( Alfa. c’o( NO,q), hH,O. The hydrodesulfurization activities of a wide 90.999%) was impregnated onto the calcined range of transition tnetal sultides correlate with Mo clay materials so as to provide ;I 2 wt% C‘o the caseof reduction of surf;lce-bound sulfur [36]. loading spread out over 80-8% of the r~~~~~~i~~i~~~ One way to determine the t.e~iuc~bilit~of a material pore volume. For comparison purposes. conimc‘ris to heat the material at a fixed heating rate in cially available oxidized Co Mo (Crosfield 365) the presenceof H, and monitor the or-gases using and Ni MO (Crosfield 504) were also tested. The either a thermal conductivity detector (used here) Crosfield 465 and Crnsfield 504 reference catalysts or a mass spectrometer. Such an ~~~3ei-ilneIitis were first calcined at 600 C for 1 h in 1% O2 Ho. hereafter referred to as ten7i7erattit-e-programmcci cooled to room temperature under N,, and stored reduction (TPR). Approximately 0. IO g of spent in a X,-purged glovebox for later use. catalyst was placed into a 0.25 in 1I1 stainlesssteel

U-tube in between plugs of quartz c\c~l and attached lo an Altamira AMI-I dynamic chemisorption apparatus. The pretreatment protocol consisted of purging in Ar at 35 c‘ for I h, drying in Ar at 150 C for I h, heating in Ar to 500 c’ fot I h, cooling in Ar to 450 C. reducing in t-I, at 450 C for I h. purging in Ar at 500 C, and cooling to 35°C in Ar. The TPR experiments for each catalyst consisted of Hz chetnisorption at 35 C and 60 seem for I h, followed by TPR at 5 C min. N2 adsorption and desorption isotherms were collected on an .4utosorb-6. Approximately 0. IO g of material was weighed int(> ;I Pyrex sample tube and evacuated to 80 mTorr overnight at room temperature, then backfilled with He. The static physisorption experiments measured the amount of nitrogen adsorbed or desorbed as ;I function of pressure(PIP,, = 0.025~0.999, increments of 0.025). Pore size distributions were calculated using the Bare&Joyner-Halenda ( BJH ) method. Normally. the desorption isotherm is used as ;I basis for the calculation of pore size distributions. However, an nrlifact at 19.5 A radius occurs for all layered materials [27,2X], and indeed was observed for all of our clay samples(seeFig. 3 ). Hence. the adsot-p-

tion pore volumes and pore size distributions are reported in all tables.

3. Resultsand discussion

Table I shows the physical properties of the weight per cent organic incorporated (determined by CHN microanalysis) and the subsequent basal spacing (&,,) from the X-ray diffraction (XRD) pattern. It also shows the effect that calcination has upon the surface area and pore volume. The behavior can be divided between two groups of organ&: cationic 1’s. neutral. For the cationic organics (Li, TEA. PDDA). the weight per cent organic uptake is < I()?~. the r/-spacing is low ( I4-- 16 A). and neither the surface areas nor pore volumes change appreciably after calcination. All of these properties can bc explained by the cation exchange behavior of the clay. which sets a limit on the amount of organic that can be sorbed (up to the cation exchange capacity of the clay). Therefore. the amount is a low but typical value

(4 Li lo-

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-

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100

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.“”

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Radius, w

1 lo"

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-.--

100

/ 1000

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A

of < 10%. Because this vttlue is low the &spacing is not ~ppreci~tbly s~~~ollen-both Li and TEA arc fairly small cations, and the PDDA polymer chains are apparently uncoiled. Reca~se not much organic is present in these SOCCs, their removal b> calcination is not significantly afrccting either the surface area or the pore volume. The situation is much ditferent for the neutral cellulose and pal> (vinyl

10

The

per-

centagc ot‘ organic upt;tke is much higher ( L NLC).

resulting in a much higher t/-spacing in the XRD of‘ 18--Z1 A. The basal spacing swellsto incorporate this large ~~~l~ou~lt of organic. In addition. both the surface ;ire:t :tnd pore volume values increase :ippreciably when pores and voids previously occluded by organic are opened up after calcination. Figs. 3 Lrnd 4 show the desorption and adsorplion pore size distributions. respectively. of’ the c4cincd SOCCs. In Fig. 3. the control minoral

8103

100

Radius, A

Radius, A

(dj ‘PtiDA 20.3.h

__ Radius, A

with no organic iyLi) displays only the sharp peak at about 19 A that is always observed as an artifxt for lamellar materials [ 37.281. The other three samples also display this artifilct but there are also real contributions to porosity. especially when cationic orgtnics are used. For ewmplr. pe;~ks at 31 .A and 120 .A radii OCCL~I- fbr synTEA ;ind synPDDA, respectively. An additional. quite interesting i’cature occurs for synPDDA in the adsorption-based data [Fig. l(d)]. This s;implc displays a reproducible and distinguishable series of sh:trp spikes upon the overall background. These spikes correspond to certain pore radii that :~re n well-defined within the ovrrnll matrix. at I? A, 20 A. 32 A. 65 .A. and 123 A. It is proposed that these arise I’rom individual polymer strands nt threads arranging themselves in concentric circles (‘onion skins’) ;IS fibrils of dit&ering rxiii in the initial clay sol --gel. The result is ;t ~t~~ne\~,ll~~tperi-

1000

]I,

i

‘--”

odic distribution of pores resulting from clay layers arI-anging themselves around such fibrils. A visual representation of such ;4 hcheme is given in Fig. 5. It is well-known that the best solvents for a particular polymer will greatly lessen the degree of coiling and enhance the tendew); to form linear chains [29]. Which may be the factor of ~inportance here. Previous small angle neutron scattering d;ita reveal that synPDDA materials ~~lwys display a diKerent scattering profile from the other materials [25]. This further supports the xgument that synPDDA has ;a si~rlif~c~~ntl~ different structure compared to the other materials. Before c~lcinatioii, synPDDA has :i different profile at low-q values. indicating that the packing densities of polymer and clnj ilre quite diff‘erent from the rest. After c&inution, the profile is different at high-q values. :tnd is closest to a natural clay ;it these smaller length SC&%

The reactivities for the synli- and synHPMCsupported catalysts are almost the same, and arc lower than the others. Not coincidentally, these materials are composed of much narrower pores ( Figs. 3 and 4) that become blocked by carbonaceous by-products from the HDS reaction. This is evidenced by the sharp decrease in pore volume after I-IDS (see Table 3). In fact. all of the materials lost pore volume during the sulfiding and HDS processes. Fig. 6 displays a strong correlation between mesopore volume remaining after HDS and the HDS conversions at various temperatures. The synTEA-supported material retains the most mesopore volume among the SOCC-derived materials. and possesses a HDS activity commensurate with the commercial alumina-supported materials. .4lmost all of the pore volume lost during HDS is recouped following calcination (Table 3). suggesting that this loss is due to coke formation.

3.2. HDS ,.c:wlt\. Table 2 provides HDS results from a model hcaky oil feed (1 wt% S as dibenzothiophene in hexadecane). Reactivity at three different temperatures was measured: 400 C, 350 C, and 300 C. The total percentage conversion is presented and followed in parentheses by the biphcnyl selectivity. Results for Co-Mo catalysts derived from SOCCs are compared to two commercial catalysts: Crosfield’s 465 (CO-MO- alumina) and 504 (Ni M+alumina).

C.ltnty\t

JO0c

is0 (‘

i(N) (’

TPR has been proven as an effective tool for studying the structure of sulfided, supported transition metals [26]. Provided a high hydrogen partial pressure and a high reaction temperature are used. a complete picture of all sulfidable species can bc obtained. In fact. four diKerent sulfur species occ~~r: ( 1 ) stoichiometric sulfur species in agreement with bulk thermodynamics: (2) nonstoichiometric sulfur species. S,: (3) S--H groups; and (4) adsorbed H,S. A correlation has been found between thiophenc HDS activity and tho S, reduction temperature [71,,( Ssurf)] when many different transition metals are compared [26]. Co MO and Ni- MO alumina catalysts display by far the best behavior in this respect [26] ~the TIcd of S, is only 127 C and the thiophene HDS reaction rate c0nstan1 is verv_ high at 91 x 10 -’ m-‘/kg s. In this work c\e have ranked relative percentage conversions of DBT with respect to the T&S,,,,-) of various supported Co MO HDS catalysts. Fig. 7 displays the TPR data for the CO-MO-~ synTEA catalyst as an example, which has a Trcd( Ssnrf) ot‘ 1SO C. Fig. 8 shoM,s the Trcd(Ssurr) and %DBT conversion data for all the catalysts studied. which displays a clear correla-

% Remaining Mesopore

tion between HDS activity surfilce sulfur species.

Volume

and stnbility

of the

4. Conclusions A new class of potential supports for Co- Mo HDS catalysts has been prepared from synthetic organo-clay complexes, specifically from hectorite clays. When the pores are too large (approaching i~~~cr~?p~~res) the activity goes down. It is the

mesoporous range which is of most interest for increasingly dominant heavy oils. however. The choice of 1 wt?G S as DBT in hexadecane (rather than a smaller, more typical solvent) was intention:~lly made to reflect this interest in heavier oils. The most promising size range occurs for the catalyst derived from synTEA with an average pore radius of about 30 A. In general, a lower activity and selectivity to biphenyl is observed for the SOCc‘s than for commercial catalysts, although it is elicou~~gilig that synTEA is close. Keeping in

3 & i7j

8 t-

tration, etc.). Of most interest in this respect are the cationic polymers. including molecular modeiing and small angle neutron scattering of thei] solution structures. Future work will also involve a thorough evaluation of the contribution of surface acidity to catalyst performance; one irnport~~l~t \;ariable to examine in this respect is the amount of Li in the catalyst. The HDS catalytic application of these mesoporous materials is currently being evaluated with an actual crude oil with its il~liel-ently more complex lni~t~lr~ of heaviei components.

150

100

The technical expertise of N. Tomczyk (CHM .4NL) for GC analysis is gratefully acknowledged. J.S. Gregar (CHM ANL) fabricated the speciality &ssWare used throu~llout. WC thank Dr. Charles 22 Hoppin of Amoco Polymers for useful discussions regarding polymers. This work was performed under the auspices of the L’S Department ol Energy. Oticc of Basic Energy Sciences, Division of Chemical Sciences. under contract number W-3 I- I(%ENG-38 (funding for KAC and KS ). and the Oflice of Fossil Encrgq Bartlesville (funding for C‘LM and JRB).

L------... 140

160 T,(Ssurt)>

.-

180

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200

References 220

‘C

mind that this system is not yet tuned, future work is warranted. Some cracking activity is observed. which is not surprising considering that a calcined clay is being used. The fact that the cracking activity is low is consistent with the relatively lou surface acidity of the hectoritc precursors. In future. it would be desirable to tune the pore hizc of the catalyst resulting from a particular SO<‘<‘ throu& synthetic control (polymer type. conc~n-

121 Z.S. Ying, B. Gewrt. Chem. Res. 34 (1995)

J.E. Ottcrstedt. 2.566.

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K.4.

[3] [.?4]

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[XI]

cdil..

[2i]

P.J. hlangnw. 4 Rwebo\. A.D. van L;rnpcld. J.A. Moulijn. J. Catal. 151 I 1995) 178. D.H. Evcrctl. in: Grew2LL’ Sing, Stocckli I Edr.), Cliar,lcterization 01‘ Porous Solid\. sot. (‘l1c111. Ind. London. 1979. pp. 23. 299 A. Clcarticld. R C;rhill. penal comlnunlcntinl?. A.X. Schmidt. C.A. Marhc\. Principles 01‘ Clish-Polymer

75

I. I62 Clay

[2X] 1291

Theor\

and

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McGraw-I

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1111. NY.

l9JS.

p. 7X.