Alumina-Supported Molybdenum Phosphide Hydroprocessing Catalysts

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Journal of Catalysis 218 (2003) 78–87www.elsevier.com/locate/jca

Alumina-supported molybdenum phosphide hydroprocessing catal

Paul A. Clark and S. Ted Oyama∗

Environmental Catalysis and Materials Laboratory, Department of Chemical Engineering (0211), Virginia Polytechnic Institute and State University,Blacksburg, VA 24061, USA

Received 1 October 2002; revised 27 February 2003; accepted 27 February 2003

Abstract

Alumina-supported molybdenum phosphide hydroprocessing catalysts, MoP/γ -Al2O3, with weight loadings from 3.5 to 39 wt% werprepared by temperature-programmed reduction of alumina-supported molybdenum phosphate precursors. The precursors weby incipient wetness impregnation of the support with aqueous molar solutions of ammonium paramolybdate and ammonium(Mo/P= 1/1). Effects of loading, reduction temperature, and heating rate on the catalysts were studied, and the samples were chby CO chemisorption, BET surface area, and X-ray diffraction (XRD) measurements. Compared to the bulk MoP and MoP/SiO2 systems(which are similar to one another), the MoP/γ -Al2O3 material showed different behavior. Whereas calcined bulk and silica-suppmolybdenum phosphates form Mo–P amorphous glasses which reduce directly to MoP, for the alumina-supported phosphates at hi(e.g., 13 wt% MoP/γ -Al2O3), a MoO3 component is visible which reduces sequentially with temperature to MoO2 and Mo metal, and thetransforms to MoP. The difference in behavior between these systems is attributed to the formation of a strongly bound amorphoussurface layer on the alumina support. Alumina has a strong affinity for phosphates, and appears to initially abstract a substantialphosphorus from the stoichiometric molybdenum phosphate mixture. The alumina then releases the phosphorus at high temperatuit to recombine with Mo metal to form MoP and generating active sites. The MoP/γ -Al2O3 catalysts performed well in hydroprocessing osimulated distillate containing dibenzothiophene and quinoline at conditions representative of industrial hydroprocessing (643 K,For the 13 wt% sample the hydrodesulfurization conversion was 57% and hydrodenitrogenation conversion was 62%. Catalytbased on equal chemisorption sites loaded in the reactor (70 µmol), was generally independent of the amount of MoP deposalumina surface, independent of the presence of X-ray visible molybdenum phases, and was associated with a relatively high treduction peak found at all loading levels of MoP/Al2O3 but not in bulk MoP. We conclude that the high-temperature peak gives rihighly dispersed MoP which is responsible for the bulk of the CO titration sites and the catalytic activity, and that the large MoPvisible by XRD make smaller contributions. 2003 Elsevier Inc. All rights reserved.

Keywords: Molybdenum; Phosphide; Phosphorus; Alumina; Hydrodesulfurization; Hydrodenitrogenation; Dibenzothiophene; Quinoline

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1. Introduction

Catalytic hydrotreating is an important refining procused to remove sulfur, nitrogen, and oxygen from hydrobon fuel liquids. Research in our laboratory has identimolybdenum phosphide, MoP, as an active and stabledroprocessing catalyst [1–3]. Additional work by the groof Prins [4] has shown that MoP has six times highertivity in the HDN of orthopropylaniline than MoS2/Al2O3.Studies in the group of Bussell have shown that MoP/SiO2has four times higher activity in the HDS of thiophene thMoS2/Al2O3 [5]. In this paper we report the synthesis a

* Corresponding author.E-mail address: [email protected] (S.T. Oyama).

0021-9517/03/$ – see front matter 2003 Elsevier Inc. All rights reserved.doi:10.1016/S0021-9517(03)00086-1

catalytic properties of MoP deposited on aγ -alumina sup-port.

MoP has a hexagonal WC-type structure, space grou=P6m̄2, with lattice parametersao = 322 pm andco =319 pm [6]. MoP is a metallic conductor with properties siilar to ordinary intermetallic compounds [7]. Fig. 1 shothe WC structure type adopted by MoP. The WC structype, along with the NiAs and WP structures, charactethe majority of transition metal monophosphides. Thestructure type is similar to NiAs, but distortions in the (00plane form a rectangular net, resulting in an orthorhobic structure [8]. A recent review summarizes the structpreparation, and catalytic activity of phosphides [9].

Removal of heteroatoms from fuel liquids is importabecause sulfur and nitrogen compounds corrode engine

http://www.elsevier.com/locate/jcat

P.A. Clark, S.T. Oyama / Journal of Catalysis 218 (2003) 78–87 79

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ponents, pollute the air as SOx and NOx , cause gelling ofuntreated liquids, and foul downstream reforming catalyThe problem is becoming increasingly important as gernment regulations become stricter [10] and the qualitpetroleum feedstocks declines [11]. Although hydroprocing catalysts have been widely studied, their basic comptions have changed little for many decades. They areically alumina-supported molybdenum sulfides with nicor cobalt promoters, NiMoS/Al2O3 or CoMoS/Al2O3 [12].The catalysts described in this work are completely difent, consisting of a compound of Mo and P with metarather than insulating or semiconducting properties.

In recent years, phosphorus has come to be usedsecondary promoter in commercial catalysts [13]. Theof phosphorus in hydroprocessing catalyst formulationsintroduced in a series of patents dating back to the 19[14–16]. The phosphorus in these catalysts is dispersethe catalyst surface in oxidized form, as shown by studwith X-ray photoelectron spectroscopy [17], nuclear mnetic resonance [18], and infrared spectroscopy [19].effect of phosphorus depends on its concentration and pical form, but in general it has a larger positive effect onHDN performance of Ni–Mo sulfides than in the HDS pformance of Co–Mo sulfide catalysts [20].

2. Experimental

Catalysts were prepared by impregnation and reducDesired quantities of ammonium paramolybdate tetradrate, (NH4)6Mo7O24 ·4H2O (Aldrich, 99%), and ammonium phosphate, (NH4)2HPO4 (Aldrich, 99%), were dis-solved in 1:1 stoichiometric proportions of metal to phphorus to form clear solutions in distilled water. Theselutions were used to fill the pores of aγ -alumina suppor(Degussa, Aluminumoxid C) by incipient wetness imprnation. The moist paste was calcined in air at 773 K for 6ground with a mortar and pestle, pressed, broken, and sito particles between 650 and 1180 µm in diameter for

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Table 1Compositional characteristics of MoP/Al2O3 samples

wt% MoP mmol MoP g−1 Al2O3 Molecules MoP nm−2 Al2O3

3.5 0.29 1.96.8 0.58 3.813 1.16 7.723 2.3 1539 3.8 25

(mesh size 14/20). Samples were prepared with loadingstween 0 and 39 wt% MoP (0 and 3.8 mmol g−1 Al2O3). Spe-cific compositional characteristics for the series of catalare reported in Table 1. The highest loading sample waspared by double impregnation, due to solubility limitatioof the metal and phosphate salts in distilled water. Muchthis study used the 13 wt% (1.16 mmol MoP g−1 Al2O3)MoP/Al2O3 material. Reference samples containing o1.16 mmol Mo g−1 Al2O3 (10.0 wt% Mo) or 1.16 mmoP g−1 Al2O3 (3.5 wt% P) were prepared in a similar maner to the catalysts, while aluminum phosphate, AlPO4, waspurchased from a commercial source (Alfa-Aesar Divisof Johnson-Matthey, 99.999%).

Temperature-programmed reductions were carried oupelletized catalyst samples (typically 0.3 g) placed in quu-tube reactors. The samples were heated with linear temature ramps in flowing hydrogen (Airco, 99.999%) to reduthe metal phosphate to metal phosphide. Unless othernoted, the space velocity was set at 650 µmol H2 s−1 g−1

of sample loaded (1000 cm3 (NTP) min−1 g−1), and theheating rate was 0.0833 K s−1 (5 K min−1). The effluentwas monitored continuously by a mass spectrometer (Atek/Dycor, MA100). All samples were flushed in hydrogflow for 2 h before starting the temperature programdry the sample and to provide a starting baseline inmass spectrometer. After reduction, the product was coto room temperature (RT) under helium (Airco, 99.999flow, and passivated progressively with 0.1% O2/He, 0.5%O2/He (Airco), and slow diffusion of air to the sampfor 24 h.

The total dynamic CO uptake (Air Products, 99.999was measured with a mass spectrometer by passing 5.6pulses over the sample in 65 µmol s−1 helium flow at RT un-til constant area peaks were obtained. Measurements mon freshly reduced samples are referred to here as inCO uptakes while measurements taken on passivatedples rereduced in H2 for 2 h at 723 K are referred to hereex situ CO uptakes. BET specific surface areas (Sg) of pas-sivated samples were measured with a Micromeritics 2sorption unit.

X-ray diffraction (XRD) patterns were obtained at a scrate of 0.035◦ 2θ s−1 using a Scintag XDS-2000 diffractometer with a Ni-filtered Cu-Kα (λ = 0.1541 nm) radiationsource. Crystallite sizes were estimated using the equatiScherrer,D = Kλ/β cosθ . The constantK was taken hereto be 0.9,λ is the wavelength of radiation,β is the peakwidth in radians, andθ is the angle of the reflection. Th

80 P.A. Clark, S.T. Oyama / Journal of Catalysis 218 (2003) 78–87

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average of the first three reflections in MoP was used toculate an average particle diameter [21]. Activation enerfor reduction reactions were estimated by the heatingvariation method of Redhead [22].

Hydrotreating activity was evaluated with a model dislate containing 2000 wppm N as quinoline (Aldrich, 99%500 wppm O as benzofuran (Aldrich, 99%), 3000 wppm Sdibenzothiophene (Aldrich, 99%), 20 wt% tetralin (Aldric99%), and balance tetradecane (Fisher, 99%). The catatest was carried out in an upflow, three-phase, fixed-beactor as described earlier [23]. An amount of catalystresponding to 70 µmol of CO sites (measured ex situ)loaded for each reaction. To start the reaction, catalystspretreated in flowing hydrogen (100 µmol s−1) at 723 K and1 atm pressure for 2 h. The pressure was then increas3.1 MPa (450 psig), the temperature was dropped to 64(370◦C), and a liquid feed rate of 5 cm3 min−1 (delivering9.7 µmol min−1 of quinoline and 6.3 µmol min−1 of diben-zothiophene) was initiated. Analysis of hydroprocessinguids was performed with a gas chromatograph (HewPackard, 5890A) on samples collected at 2- to 3-h interfor a time period of 60–80 h, until a stable steady statereached. In this report, hydrodesulfurization (HDS) isfined as the percentage disappearance of dibenzothiop(as no sulfur-containing intermediates were observed),drogenation (HYD) is defined as the percentage of origquinoline converted to hydrogenated quinolines, and hydenitrogenation (HDN) is defined as the percentage of ngen removed from the quinoline delivered to the catalys

3. Results and discussion

3.1. Synthesis of MoP/γ -Al2O3

Calcined catalyst precursors were reduced to MoP/Al2O3catalysts by temperature-programmed reduction in hygen at 0.0833 K s−1 (5 K min−1). Corresponding reductioprofiles for 0–23 wt% MoP/Al2O3 samples are presentein Fig. 2. Five water peaks evolved through the reducprocess, corresponding to reduction of different materpresent in the precursor and to different stages in theduction process. All of the samples, including the alumsupport, exhibited a peak at 370 K (with a shoulder at 4450 K) arising from physically adsorbed water. The alumsupport alone exhibited no significant features above 50while the molybdenum phosphate-loaded samples hadtures corresponding to reduction of the deposited speThese features were labeledα, β , γ , andδ in order of theirappearance with temperature. Theα peak occurred at thlowest temperatures, was relatively small, and was prein all of the samples. Theδ peak was also present in allthe samples, occurred at the highest temperatures, angenerally large and diffuse. In the lowest loading materi3.5 and 6.8 wt%, theα andδ peaks constituted the majoriof the reduction profile, and are attributed to well-disper

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Fig. 2. Temperature programmed reduction profiles for various MoPx/

Al2O3 samples reduced to 1223 K at 0.083 K s−1 (5 K min−1).

Fig. 3. Evolution of MoP during the reduction of MoPOx/Al2O3 (13 wt%MoP). Samples were reduced to intermediate temperatures, quenchedlium, and characterized by in situ CO uptake before passivation. (◦) MoO3;(y) MoO2; (+) Mo◦; (×) MoP, and (∗) Al2O3.

species on the alumina surface. Theγ peak first appeareas a small feature in the 6.8 wt% material, and was prlent at higher loadings along with theβ peak. Theβ andγ features slightly overlapped theα andδ features. But theβ andγ peak temperatures were in the range typicallyserved for the reduction of bulk molybdenum compounand will presently be shown to correspond to X-ray visiphase changes.

The reduction of the 13 wt% MoP/Al2O3 material wasfollowed by XRD of quenched and passivated intermeates. These spectra are shown in Fig. 3. Comparison of Fwith Fig. 3 allows association of the reduction peaksα, β ,γ , and δ with phase changes observed by X-ray diffrtion. The alumina-supported MoP precursor differs fromX-ray amorphous precursors for unsupported MoP [1silica-supported MoP [3] because of the visibility of MoO3.The formation of MoO3 is attributed to abstraction of th


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phosphate component by the alumina surface. The takof phosphate by alumina in standard phosphorus-promcatalysts is well established [24,25].

The α peak near 675 K resulted in no new featuresthe XRD spectrum, and was therefore attributed to dedroxylation of well-dispersed molybdate species. Thetemperature of this feature is thought to arise becausestrength of the alumina–molybdate bonds weakens themaining hydroxyl bonds, which are subsequently easieremove.

The β peak andγ peaks correspond to the transformtion of MoO3 to Mo metal through the intermediate MoO2.This behavior is typical of the reduction of bulk MoO3 in hy-drogen. This X-ray observable process must occur becthe phosphorus species are unavailable to the free molynum species. This demonstrates that the alumina fully bthe phosphate at this concentration level (13 wt% MoAt higher surface loadings, the phosphate groups saturthe alumina surface and become available to associatethe free MoO3. Here, theβ andγ peaks resulted in XRDchanges similar to substoichiometric MoPxOy (x < 1) ox-ides, where the value ofx depends on the relative amountP and Mo. Fortunately for the context of this report, catalyprepared at 1123 K contained only MoP in their X-ray pterns (regenerated in theδ peak at higher temperatures), athe catalytic activity was attributed to well-dispersed phanot observed by XRD. Therefore, detailed characterizaof the phase behavior of the samples with loading 2338 wt% at intermediate temperatures was unnecessary.

Fig. 3 shows that theδ peak is accompanied by the diappearance of molybdenum metal and the appearancmolybdenum phosphide above 1123 K. This demonstrthat theδ peak is associated with the freeing of phosprus within the sample, and its return to the molybdenspecies. Indirectly, this shows that theδ peak is associatewith reduction of alumina-bound phosphate groups, anddicates that phosphorus has a relatively fast diffusivitymolybdenum—because the freed phosphorus can penethe molybdenum metal completely to form pure MoP.

Fig. 4 shows the development of the in situ CO uptain intermediates from the reduction of 13 wt% MoP/Al2O3superimposed on its reduction profile. The CO uptakerelates reasonably well with the degree of completion ofreduction process, in which the dip at the end is likely dusintering/grain growth of particles of the active phase duthe high temperature at that stage.

Table 2 summarizes the CO uptakes obtained for13 wt% sample at the different stages of reduction markepeaksα, β , γ , andδ. It can be seen that the majority of thuptake is generated following theδ peak. Since theδ peakis also the major reduction peak in the low loading samwhich does not show an X-ray visible MoP phase, this resuggests that the measured CO uptake in all the sampassociated with well-dispersed molybdenum phosphidetightly on the alumina surface, rather than the X-ray visiphases.

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Fig. 4. Evolution of in situ CO uptake sites of quenched intermediatesing the preparation of 13 wt% MoP/Al2O3.

Table 2Evolution of CO uptake with the reduction peaksα, β, γ , andδ

Peak Temperature CO uptake, X-ray visibleevent interval, K µmol/g, % phase change

α 298–673 4 Noneβ 673–763 15 MoO3 → MoO2γ 763–863 10 MoO2 → Mo metalδ 863–1173 71 Mo metal→ MoP

Fig. 5. Reduction profiles for “blank” alumina samples reduced to 122at 0.083 K s−1 (5 K min−1). (a) 1.16 mmol g−1 POx/Al2O3; (b) AlPO4;(c) 1.16 mmol g−1 MoPOx/Al2O3; and (d) 1.16 mmol g−1 MoOx/Al2O3.

Fig. 5 shows reduction profiles of reference materincluding commercial AlPO4, the Al2O3 support contain-ing only ammonium phosphate, and the Al2O3 supportcontaining only ammonium molybdate. The supportederence materials were prepared with procedures (impnation, calcination, and pelletization) and molar load(1.16 mmol g−1 Al2O3) identical to those used for generating the 13 wt% MoP/Al2O3 catalyst. Samples containing the alumina support exhibited a large dehydrapeak near 373 K. This low temperature behavior is s


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ilar to that of the alumina blank and the supported clysts shown in Fig. 2, and is attributed to the releasephysically adsorbed water condensed in fine pores. Rence samples consisting of only P and Al2O3 had smoothbaselines between 600 and 900 K, and developed pabove 1100 K. A yellow-brown deposit formed downstreon the interior walls of the quartz reactor tube duringhigh-temperature peaks of the P and Al2O3 samples. Thedeposit is probably elemental phosphorus transportevolatile reduced species such as P2, P4, or PH3 [26,27], butcould be oxidized phosphorus transported as volatile P2O5.Vapor-phase transport of phosphorus has been reportelier for phosphorus-containing materials undergoing hitemperature TPR [19]. The molybdenum-containing mrials, Mo/Al2O3 and MoP/Al2O3, both had identifiableα,β , and γ peaks. These peaks are respectively attributo recombination of hydroxyl ligands from well-dispersmolybdates, MoO3 to MoO2 reduction, and MoO2 to Mometal reduction. The Mo/Al2O3 had a tail above 900 Kattributed to the reduction of strongly held molybdatescontrast to the reference samples, the MoP/Al2O3 samplehad a large feature, theδ peak, between 900 and 1200This feature in this temperature range only occurred inpresence of both phosphorus and molybdenum. X-rayfraction indicated a phase change from Mo to MoP accomnying theδ peak, demonstrating influx of phosphorus fromsource outside the molybdenum grains. Theδ peak in theMoP/Al2O3 samples included the reduction of aluminuphosphate above 1100 K, but with the reduction tempture of phosphorus decreased because of facilitation oreduction process by molybdenum.

X-ray diffraction spectra showing the presence of Min samples of different loading reduced to 1223 K0.0833 K s−1 (5 K min−1) are presented in Fig. 6. MoP wthe only crystalline product observed, becoming visible6.8 wt%, and increasing in intensity with loading. The XRspectra of the catalysts reduced to 1123 K at 0.0833 K−1

Fig. 6. X-ray diffraction results showing increased amounts of MoP wloading for samples reduced to 1223 K at 0.083 K s−1 (5 K min−1).(×) MoP, and (∗) Al2O3.

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(5 K min−1) and held for 2 h were similar to those shownFig. 6. The lattice constants of the supported MoP cataare reported in Table 3. The lattice parameters of the Mothe catalysts agree well with the literature values, indicathat supported MoP of good purity is formed.

The effect of final reduction temperature (heating rβ = 5 K min−1) on catalytic performance was evaluatedthe 13 wt% MoP/Al2O3 material. Of four temperatures investigated, successful samples were produced only atand 1123 K. Here, the primary criterion for success waspresence of sufficient and stable CO uptake for catalyticing. A sample reduced at 873 K for 2 h had insufficientsitu CO uptake (= 12 µmol g−1) to load 70 µmol of sites, ana sample reduced at 1223 K for 0.5 h exhibited significdaily decrease in its CO uptake (noticed because of theaging procedure used for catalyst samples) and was theralso not used. Although the sample prepared at 1023 Kadequate CO uptake, its XRD spectrum revealed the pence of both Mo metal and MoP, while the sample reduat 1123 K contained only MoP. Because of this, most ofsupported samples used for the characterization and reity studies were reduced at 1123 K.

Table 3 summarizes crystallite sizes of the molybnum phosphide calculated using the Scherrer equaThese values correspond to an average over the low i(1000), (1010), and (0001) planes. The sizes of the alumsupported MoP crystallites increased slightly with the loing amount. More dramatic, however, was their large s(about double) compared to the bulk material. This istributed to grain growth (Ostwald ripening or sintering)the crystallites at the 200 K higher temperature of 112used in the preparation of the supported catalysts compto the bulk material, which was prepared with a maximtemperature of 923 K.

Table 3 also reports site densities of surface metal atcalculated from the crystallite sizes of the X-ray visible Mand the wt% loading of the MoP in the sample. The calctions were performed assuming spherical or cubic partihaving surface site densities equal to 9.71 atoms nm−2. It isunderstood that the crystallite size as given by the Sch

Table 3X-ray diffraction results for MoP/Al2O3 catalysts

Sample Lattice Lattice Crystallite Total metaparameter, parameter, size,a site density,b

ao (pm) co (pm) nm µmol g−1

13% MoP/Al2O3 321 318 39 4323% MoP/Al2O3 322 319 40 7539% MoP/Al2O3 322 319 51 98MoP 322 318 19 680MoP referencec 322 319 – –

a Average size calculated from line broadening in the〈1000〉, 〈1010〉,and〈0001〉 directions.

b Site density= surface metal atoms per gram of sample. Calculateding the surface metal areal density of MoP (9.7 × 1018 surface Mo m−2),the MoP fractional weight loading, and the average MoP crystallite siz

c JCPDS-ICDD Powder Diffraction File 24-771.


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Fig. 7. Development of in situ CO uptake as a function of loading.samples were reduced to 1223 K at 5 K min−1 in hydrogen flow at670 µmol s−1 g−1 (1000 cm3 min−1 g−1).

Table 4Results of BET surface area and CO chemisorption measuremenMoP/Al2O3 samples

Sample Surface area In situa Ex situb Ex situb

fresh,c CO uptake, CO uptake CO uptakm2 g−1 µmol g−1 fresh,c spent,d

µmol g−1 µmol g−1

γ -Al2O3 91a 1 0 –6.8 wt% MoP 75 70 26 2013 wt% MoP 71 81 36 3123 wt% MoP 60 76 28 2939 wt% MoP 49 60 36 26

a Samples reduced to 1223 K (β = 5 K min−1, H2 flow = 650 µmolg−1) and then quenched.

b Pretreated in H2 at 723 K for 2 h (β = 5 K min−1, H2 flow = 650µmol g−1).

c Catalysts prepared by reduction to 1123 K for 2 h (β = 5 K min−1, H2flow = 650 µmol g−1).

d Spent catalysts washed in hexane and dried in air.

equation is only an approximation and ignores the preseof a distribution of sizes. The surface site density wasculated from the crystal structure of MoP [4] assumingposure of equal numbers of low index planes. It is assuin these calculations that the surface area to volume ratthe MoP crystallites was proportional to 6/d (spherical orcube shaped particles), and that the particles had thesity of bulk MoP [8] (7.50 g cm−3). The site density of theunsupported MoP was much larger than the experimendetermined CO uptake of 18 µmol g−1 [1]. This reveals thathe surface sites on bulk MoP are blocked in a ratio n38:1, similar to results found for WP [28]. The calculatsite densities were lower for the supported materials thanbulk due to a combination of larger MoP particles sizeslower MoP weight fraction in the mixture.

Table 4 summarizes measurements of surface area, inCO uptake, and ex situ CO uptake for MoP/Al2O3 samples.The in situ CO uptakes were collected on the series of s

-

Fig. 8. Effect of heating rate on the temperature-programmed reduforming 23 wt% MoP/Al2O3.

Table 5Apparent first-order activation energies of reduction events in the preption of 23 wt% MoP/Al2O3

Peak Activation energykJ mol−1

α 152β 157γ 130*

δ 160

* Nonlinear. Estimated using heating rates of 2 K and 5 K min−1. Seetext.

ples whose XRD spectra were presented in Fig. 6 andplotted in Fig. 7. The in situ CO uptakes rose from 181 µmol per gram of sample as loading increased froto 13 wt% MoP and then tapered off to 60 µmol per gras loading was further increased to 39 wt%. The decreahigh loadings is likely due to blocking of pores by phosphparticles and consequent loss of surface area. Becausesitu CO uptake of the fresh catalyst was used as the basloading the hydroprocessing reactor, these values wereaged over three measurements. The ex situ measuremeall of the catalyst samples were similar, regardless of loing, and the results on the fresh and spent catalysts showthe catalytic treatment led to only minor, if any, reductioin the ex situ CO uptake. Comparison of in situ and exCO uptakes reveals that only a portion of the uptake sitesrecovered after air exposure and subsequent rereductio

Heating-rate variation between 0.5 and 5 K min−1 wasinvestigated on the sample containing 2.3 mmol g−1 Al2O3(23 wt% MoP/Al2O3). The reduction profiles are displayein Fig. 8, and Table 5 reports the first-order activationergies calculated for each type of peak. As heating ratecreased, the relative intensity of theβ peak increased, anthe γ peak resolved into three smaller peaks. Furthermthe γ peak maxima did not conform to a linear activatienergy relationship. This behavior is consistent with sat


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tion of phosphorus on the alumina surface between 1323 wt%, and the overlap of MoO2 → Mo and MoPxOy →MoPx processes in theγ peak. The origin of increased intesity of theβ peak is not clear, but could be related to deereduction of the crystallites in the slower process.

3.2. Catalytic activity of MoP/γ -Al2O3 samples

The catalysts were evaluated for activity in hydrotreatof a simulated distillate containing quinoline (2000 wppN) and dibenzothiophene (3000 wppm S) at 643 K3.1 MPa. Importantly, catalysts were loaded into the retor using a constant 70 µmol ex situ CO uptake basis.choice of ex situ CO uptake as the basis for comparassumes catalytic activity to be due to surface sites geated through reduction at 723 K, and will be discussed laThe use of ex situ uptakes collected of air-exposed samensures that the sites are regenerated during catalytictreatment. Table 4 summarizes CO uptakes obtainedpretreatment in hydrogen at 723 K for 2 h.

Reference materials were also investigated for cataperformance. These standards included the reduced sualone, a calcined phosphate blank containing 1.16 mPOx g−1 Al2O3, and the phosphate blank exposed toreduction procedure used for the majority of MoP/Al2O3samples (1123 K for 2 h). An amount of 3.0 g of each blawas loaded, as their CO uptakes were negligible.

The hydrotreating performances of the samples areported in Table 6. The blank reference materials hadlow HDN and HDS conversions, as expected. ForMoP/Al2O3 samples there was only a minor effect of loaing on the HDN and HDS performances, which averagedand 52%, respectively. The amount of hydrogenated quline products was stable near 30% (except the phosphcontaining blanks), consistent with equilibration betwequinoline and 1,2,3,4-tetrahydroquinoline. Overall, thesults indicate that a comparison of activity based onuptakes is reasonable. The single low HDS conversio44% for the 39 wt% sample is likely due to partial poblockage for this high-loading sample. This would affthe larger dibenzothiophene more than the quinoline.

Table 6Hydroprocessing conversions of MoP/Al2O3 catalystsa

Sample % HDN % HYD % HDS

Al2O3 blank 2.4 32 1.1POx/Al2O3 3.3 18 4.7P/Al2O3 3.8 19 1.06.8 wt% MoP 54 31 5113 wt% MoP 52 33 5713 wt% MoPb 62 29 5723 wt% MoP 58 28 5339 wt% MoP 54 33 44

a Reduced to 1123 K (β = 5 K min−1, H2 flow = 650 µmol g−1) for2 h.

b Reduced to 1023 K (β = 5 K min−1, H2 flow = 650 µmol g−1) for1.5 h.

-

-r

rt

-

activity of the supported MoP/Al2O3 samples was modeately high, especially in HDN, and can be compared toof a commercial Ni–Mo–S/Al2O3, which at the same hydroprocessing conditions had HDN conversion of 38%HDS conversion of 79% [29].

Table 7 reports turnover frequencies (moles conveper CO uptake site per second) calculated for quinoHDN and dibenzothiophene HDS for MoP supported onmina compared with bulk MoP [1] and MoP on silica [3The silica-supported material shows somewhat lower aity when compared on this basis, and the alumina-suppomaterial is similar to the bulk. This suggests that the Mactive sites formed on the alumina support are similar tobulk material. Still the differences are small, and again incate that CO uptakes are a reasonable method for compdifferent samples.

Table 4 also reports the ex situ CO uptake values forspent catalysts. For these measurements the samplesremoved from the catalytic reactor, washed with hexaand dried in air. Subsequently, they were reduced in hygen under the same conditions used for the fresh cataThe ex situ CO adsorption sites persisted following catalreaction conditions and exposure to air. This remarkablebility in CO uptake values before and after reaction is csistent with the stability in conversion levels noted for thcatalysts.

X-ray diffraction traces for fresh and spent 13 wMoP/Al2O3 catalyst reduced to 1123 K are presentedFig. 9. MoP and Al2O3 were the only crystalline phases oserved, and the catalyst XRD signature was not significaaffected by the catalytic reaction. Similar results were fofor catalysts of other loadings, whose XRD patterns walso stable toward the catalytic test, and which resemthe spectra shown in Fig. 6.

For the 13 wt% MoP/Al2O3 catalyst reduced at 1023(i.e., 100 K lower than the “standard” temperature1123 K), the X-ray diffraction patterns for the fresh aspent catalysts contained features of both Mo and MoPshown in Fig. 10. Nevertheless, this material was alsotive and stable toward the catalytic reaction conditions. Tcatalyst was prepared at 1023 K using a 1.5-h soak. Cpared to the intermediate sample quenched at 1023 K, wcontained only Mo metal in its XRD spectrum (Fig. 3), tdiffraction trace of the catalyst sample reduced at 102contained mostly MoP. Thus, the molybdenum phosphidthe catalyst sample appeared during the 1.5-h soak tim

Table 7Comparison of hydroprocessing turnover rates of 13% MoP/Al2O3 with13% MoP/SiO2 and bulk MoP

Sample HDN, HDS,mol quinoline mol dibenzothiophenmol site−1 s−1 mol site−1 s−1

13 wt% MoP/Al2O3 1.2× 10−3 8.5× 10−4

13 wt% MoP/SiO2 7.2× 10−4 3.0× 10−4

Bulk MoP 1.5× 10−3 5.2× 10−4


e

par-ulksupd thac-

tionor-adema-sup-hilefor-tive.11

anN,s ofor-ax-

edalysttiond onthe

is

-ma-eofumFS

uced.5oP

ons ofinofat

f

lyb--hatese

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lescleswern byen-ityakethely aals.the

asese-ibed.

or-sors aste,

Fig. 9. X-ray diffraction of fresh and spent 13 wt% MoP/Al2O3 catalystprepared at 1123 K.

Fig. 10. X-ray diffraction of fresh and spent 13 wt% MoP/Al2O3 catalystprepared at 1023 K.

tirely by the isothermal progress of the process giving thδ

peak.

3.3. Nature of active sites

The choice of ex situ CO uptake as the basis of comison of catalysts deserves commentary. Ordinarily for bmaterials, a surface area basis is chosen. However, inported systems the surface area of the active phase ansupport material combines and the relative surface of thetive material is masked. By choosing a surface site titratechnique (CO uptake) that is known to be roughly proptional to the total surface area [28], an estimate can be mof the amount of active surface available on a supportedterial. The CO uptake basis then allows comparisons ofported materials on the basis of equal active surface. Wthe correlation between CO uptake and catalytic permance is subject to experimental uncertainties, the relacorrelation between CO uptake and catalytic activity (0

-e

HDN, 0.25 HDS) in this study is substantially better ththat found between catalytic activity and weight (0.32 HD0.45 HDS), surface area (0.51 HDN, 0.33 HDS), or molemolybdenum (1.33 HDN, 1.35 HDS). Here, the relative crelations represent variabilities, and are defined as the mimum span in values (maximum minus minimum) dividby the average value. For a previously undescribed catsystem exhibiting a wide range of loadings and preparaconditions, we feel that the consistency of results baseCO uptakes is very good. Furthermore, the similarity ofturnover frequencies on CO uptake basis with bulk MoPnotable.

The majority of CO uptake in the MoP/Al2O3 systemwas generated by theδ reduction peak at much higher temperatures, about 200 K higher, than required for the fortion of bulk MoP. Theδ peak was observed in all of thMoP/Al2O3 samples, even at the lowest loading level3.2 wt%, and is attributed to highly dispersed molybdenand phosphorus on the alumina surface. Our earlier EXAmeasurements [2] identified this dispersed phase in red6.8 wt% MoP/Al2O3 to be clusters of MoP between 0and 1.2 nm in size. The surface density (Table 1) of Mon the 6.8 wt% sample is 3.8 MoP per nm2, and appearsto be slightly above the saturation limit of molybdenumthe alumina in this system (note the presence of traceX-ray visible MoP in Fig. 6, and a slight gamma peakFig. 2). This saturation limit is slightly less than thatMoO3, which forms a saturated monolayer on aluminaaround 4.5 molecules/nm2 [30]. Higher surface loadings oMo and P up to 25 MoP/nm2 (39 wt%) extend well beyondmonolayer formation on the surface, and the excess modenum forms X-ray visible MoO3 until phosphorus saturates as well, and substoichiometric molybdenum phosp(MoPxOy , x < 1) species form at higher loadings. TheX-ray visible excess molybdenum species undergo retion in the temperature range of 650–850 K (at 5 K min−1),as found in ordinary bulk Mo compounds, but the particare generally very large. The relative sizes of the parti(Table 3) are much larger than bulk MoP produced at lotemperatures. The sizes of the particles and their dilutioalumina lead to much lower values of surface metallic dsities than those found in bulk MoP. The relative similarof turnover frequencies (Table 7) on the basis of CO uptsites in alumina and bulk MoP materials indicates thatcrystalline phases in the supported catalysts provide onminimal contribution to the active surface in these materi

Fig. 11 depicts a diagram of the proposed structure of6.8 wt% MoP/Al2O3 and 13 wt% MoP/Al2O3 materials.Here, the alumina particles and the associated oxide phare highly simplified for the purpose of clarity. Neverthless, the salient points of the process are clearly descrIn the 6.8 wt% loading material, well-dispersed MoPOx re-duces directly into MoP particles with diameters on theder of 1.2 nm. In the 13 wt% MoP material the precurhas distributed the molybdenum and phosphorus oxidewell dispersed MoPOx , well dispersed aluminum phospha


n

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Fig. 11. Proposed structural model for 7 and 13 wt % MoP/Al2O3 catalysts.

and crystalline MoO3. During the initial stages of reductio(i.e., theβ andγ peaks), the crystalline MoO3 reduces di-rectly into metal, just as ordinary crystalline MoO3 will doin the presence of hydrogen. During the latter stages oduction (i.e., theδ peak), the dispersed aluminum phosphis reduced to release phosphorus which is taken up bymolybdenum metal, and the well-dispersed MoPOx reducesdirectly into MoP particles with diameters on the order1.2 nm, as was found for the low loading sample.

3.4. Relationship to previous results in the literature

Although the nomenclature “MoP/Al2O3” has appearedbefore in the literature [31], the materials studied were inphosphate-promotedmolybdenum sulfides. Such phosppromoted sulfidic hydrotreating catalysts have been widinvestigated, as described in an extensive review by Iwamand Grimblot [13], but are not the same materials invegated in the present study. In our catalysts, the phosphis in a reduced state and the active molybdenum phospis metallic. Aside from the presence of Ni or Co promotecomparison of the preparative conditions reveals that theative amount of P to Mo in the promoted system is smwhereas in the present system it has exactly 1:1 molarportions. For example, optimal activity for thiophene HDin an alumina-supported sulfide catalyst containing 3.0 wNiO and 15 wt% MoO3 was achieved with a phosphorcontent of 1 wt% (0.24 mol P: mol Mo) [32]. Furthermothe activation procedure for the promoted system typicinvolved low-temperature (ca. 723 K) reduction in a sulfiatmosphere, whereas in the present study the reductiat high temperature (1123 K) in pure hydrogen. No defitive evidence for phosphide formation has been given forpromoted system, whereas this study clearly demonstthe presence of hexagonal MoP by XRD. Related work uEXAFS shows small crystallites containing Mo–P bonds

There are also similarities between the phosphidephosphorus-promoted sulfide systems. Both materialshibit a strong interaction between phosphate and alum[24,25]. Additionally, the presence of phosphate resultmoderate reduction in the surface area of the catalys

-

s

s

s

has been found that the interaction of alumina and phospincreases the dispersion of the metals in the Ni–Mo–Stem [33]. Molybdena also interacts strongly with alumisuggesting that the phosphate group alters the interactimolybdenum with the surface [33].

Because the catalytic activity is roughly independenloading from 6.8 to 39%, while the XRD of MoP increasdramatically in intensity, it is reasonable to conclude tthe majority of active sites are associated with highly dpersed MoP clusters in close contact with Al2O3, as notedearlier [2]. The nature of phosphorus in MoP/Al2O3 cat-alysts reported here is quite different from its oxidizstate in commercial phosphorus-promoted sulfides.MoP/Al2O3 catalysts reported in this paper are reducesubstantially higher temperatures than those employestandard sulfiding, and result in the reduction and tranof phosphorus from alumina to the metallic phase. Thattemperature of preparation is 200 K higher than for ordinbulk MoP is also remarkable, because it reveals that themina support stabilizes very small MoP particles, protecthem from sintering and maintaining their surface sites.

4. Conclusions

A series of MoP catalysts supported onγ -Al2O3 wasprepared and tested for hydroprocessing activity underulated industrial conditions. MoP/Al2O3 catalysts were effective at HDN and HDS in our tests, averaging 57 a52%, respectively. Interestingly, dispersion of the molybnum phosphate precursor on alumina revealed substandifferent phase behavior than found in bulk MoP and sisupported MoP. At loadings between 0 and∼ 6 wt%, boththe molybdenum and phosphorus species were distribon the alumina support with a strong interaction. At ab6 wt%, the molybdenum species saturated on the alumsupport and increases in surface loading led to free MoO3 inthe oxidized system. Above 13 wt%, the phosphorussaturates on the alumina surface and further increassurface loading produced combinations of MoO3 and sub-stoichiometric MoPxOy (x < 1) species. Regardless of tX-ray visible phases, catalytic activity was independenloading and was attributed to well-dispersed MoP formfrom the reduction of the first 6 wt% of molybdenum phophate deposited to the surface.

Higher loadings also resulted in strong interactiontween alumina and phosphate. Evidence for this was foin the presence of MoO3 in the oxidic precursor form othe 13 wt% MoP/Al2O3 catalyst, and by new peaksthe TPR profile. As reduction proceeded, MoO2 occurredat 763 K, molybdenum metal was seen between 8631073 K, and MoP was formed above 1173 K. The catalwere active for hydroprocessing of both dibenzothiophand quinoline, and were stable under hydroprocessingditions. Analysis suggests that the active sites are assoc


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with small (∼ 1.2 nm) phosphide crystallites on the alumicarrier surface.

Acknowledgments

This work was carried out with support from the UnitStates Department of Energy, Office of Basic Energy Sences, Grant DE-FG02-96ER14669. The authors thankHwan Lee for the surface area measurements.

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Alumina-Supported Molybdenum Phosphide Hydroprocessing Catalysts