Alkylation of phenol and m-cresol over zeolites

Article abstract of DOI:10.1135/cccc20031949

The alkylation of phenols and phenol derivatives such as m-cresol are important reactions in a sequence of syntheses for the production of many important fine chemicals. Specifically it is of great importance to develop catalysts and processes, which are able to selectively produce one or other particular isomer or derivative. This paper presents results of an investigation into the alkylation of phenol and m-cresol, respectively, using methanol in the former case and propene in the latter. The catalysts of choice were H-ZSM-5 and H-MCM-22. In the case of phenol methylation it was found that controlling diffusivity by increasing crystal size was most conducive to the formation of p-cresol. H-MCM-22 showed a high selectivity to p-cresol. In the case of m-cresol propylation the selectivity over H-ZSM-5 to thymol was greater than 90% at conversions of around 50%, and 80% at conversions of around 85%. This high selectivity to thymol was considerably greater than reported in previous papers. In both systems mild reaction temperature and pressure conditions were desirable so as to minimize the formation of oligomeric products of methanol (via dimethyl ether) and propene, which could cause deactivation of the zeolite.

Full text of DOI:10.1135/cccc20031949

Alkylation Over Zeolites
1949
ALKYLATION OF PHENOL AND m -CRESOL OVER ZEOLITES
1
,
2
3
Cyril T. O’CONNOR *, Gillian MOON , Walter BÖHRINGER and
Jack C. Q. FLETCHER4
Catalysis Research Unit, Department of Chemical Engineering, University of Cape Town,
1
Private Bag, Rondebosch, Cape Town 7701, South Africa; e-mail: ctoc@chemeng.uct.ac.za,
2
3
4
gmoon@chemeng.uct.ac.za, wboe@chemeng.uct.ac.za, jcqf@chemeng.uct.ac.za
Received July 1, 2003
Accepted August 18, 2003
Th e alkylation of ph en ols an d ph en ol derivatives such as m-cresol are im portan t reaction s in
a sequen ce of syn th eses for th e production of m an y im portan t fin e ch em icals. Specifically it
is of great im portan ce to develop catalysts an d processes, wh ich are able to selectively pro-
duce on e or oth er particular isom er or derivative. Th is paper presen ts results of an in vestiga-
tion in to th e alkylation of ph en ol an d m-cresol, respectively, usin g m eth an ol in th e form er
case an d propen e in th e latter. Th e catalysts of ch oice were H-ZSM-5 an d H-MCM-22. In th e
case of ph en ol m eth ylation it was foun d th at con trollin g diffusivity by in creasin g crystal
size was m ost con ducive to th e form ation of p-cresol. H-MCM-22 sh owed a h igh selectivity
to p-cresol. In th e case of m-cresol propylation th e selectivity over H-ZSM-5 to th ym ol was
greater th an 90% at con version s of aroun d 50%, an d 80% at con version s of aroun d 85%.
Th is h igh selectivity to th ym ol was con siderably greater th an reported in previous papers. In
both system s m ild reaction tem perature an d pressure con dition s were desirable so as to m in -
im ize th e form ation of oligom eric products of m eth an ol (via dim eth yl eth er) an d propen e,
wh ich could cause deactivation of th e zeolite.
Keyw ord s: Electroph ilic arom atic substitution s; Alkylation ; H-ZSM-5; H-MCM-22; Th ym ol;
Cresol; Ph en ols; Zeolites; Heterogen eous catalysis.
Ph en ol an d its various derivatives are im portan t in dustrial base an d fin e
ch em icals with a wide ran ge of in dustrial application s ran gin g from use as
an tiseptics, disin fectan ts, perfum e additives, preservin g agen ts, h erbicides,
etc., as well as bein g used m ore widely in th e m an ufacture of en gin eerin g
plastics. Som e of th e m ain com poun ds of in terest are ph en ol itself, its de-
rivative cresols (m eth ylph en ols) obtain ed by th e m eth ylation of ph en ol,
an d th ym ol (2-isopropyl-5-m eth ylph en ol) obtain ed by th e propylation of
m-cresol. In th e m eth ylation of ph en ol it is h igh ly desirable to prom ote th e
selective form ation of in dividual cresol isom ers an d usually to m in im ize
th e form ation of an isole. In a m ixture of an isole, p-cresol an d o-cresol it is
possible to separate th e in dividual products by distillation because th eir
Collect. Czech. Chem. Commun. (Vol. 68) (2003)
doi:10.1135/cccc20031949

1
950
O’Connor, Moon, Böhringer, Fletcher:
boilin g poin ts, viz. 155, 201.9 an d 191 °C are sufficien tly dissim ilar. How-
ever, m-cresol with a boilin g poin t of 202.2 °C is difficult to separate from
p-cresol. Hen ce a process in wh ich on ly p-, o-cresol an d an isole are form ed
is desirable.
An im portan t secon d derivative of ph en ol alkylation is th ym ol. Natural
th ym ol is extracted from th e oil of th ym e an d m in t. It h as an tiseptic prop-
erties an d an tim icrobial activity on bacteria in volved in upper respiratory
1
tract in fection s . It is also an im portan t in term ediate used in th e syn th esis
of perfum es an d h ydrogen ation of th ym ol leads to m en th ol, a com pon en t
of fragran ces with a pepperm in t odour. Th ym ol is curren tly prepared in dus-
trially usin g th e Bayer process by th e liquid ph ase propylation of m-cresol
2
at 350–365 °C an d 5 MPa over activated alum in a .
It is clear from th e m olecular structure of both p-cresol an d th ym ol th at it
is possible th at sh ape-selective zeolites m ay be ideal acid catalysts for th e
syn th esis of both th ese products sin ce th ey are both th e slim m est of th e
various possible isom ers of each fam ily an d th erefore m ay be favoured
th rough product-sh ape selectivity.
Phenol Alkylation with Methanol
Cresols can be form ed via th e alkylation of ph en ol with m eth an ol. Typical
catalysts used for th e alkylation of ph en ols are H SO , BF , MgO, Al O an d
2
4
3
2
3
3
,4
SiO -supported Fe/V (refs ). Th e m eth yl group can alkylate in two ways on
2
th e ph en ol; first, on th e side ch ain OH group (O-alkylation ) an d secon d, on
th e ben zen e rin g (C-alkylation ). C-Alkylation leads to th e form ation of
cresol isom ers an d O-alkylation to th e form ation of an isole. An isole in turn
can react with itself or with m eth an ol to form m eth ylan isoles. Cresols can
also be form ed via a secon dary reaction of an isole with ph en ol or via in ter-
n al rearran gem en t of an isole. Cresols can react furth er with m eth an ol to
form xylen ols or m eth ylan isoles. Ph en ol itself can un dergo self-
con den sation to produce diph en yl eth er wh ich , it is worth n otin g, if subse-
quen tly m eth ylated in th e 4 position can lead via subsequen t cleavage of
th e O-ph en yl bon d to th e form ation of p-cresol an d ph en ol. Prelim in ary
studies of th is route by th e auth ors will be briefly referred to in th e results
section of th is paper. Meth an ol of course is well kn own to con vert readily
to dim eth yl eth er particularly over acidic zeolite catalysts such as H-ZSM-5
an d dim eth yl eth er can as well be con sidered to be a reactan t in th e case of
H-ZSM-5. Sim ilar proposition s h ave been m ade in th e case of th e alkylation
5
6
of, for exam ple, toluen e with m eth an ol or of olefin s with m eth an ol .
Collect. Czech. Chem. Commun. (Vol. 68) (2003)

Alkylation Over Zeolites
1951
With respect to cresols, th erm odyn am ically m-cresol is th e dom in an t of
7
th e th ree cresol isom ers. Stull et al. h ave reported th e equilibrium ratios of
p- to o-cresol in th e tem perature ran ge 200 to 350 °C to be in th e ran ge of
7
8
0
.25–0.3 (ref. ). However, recen tly Im bert et al. h ave reported th at th e
equilibrium p-:o-cresol ratio is h igh er an d for exam ple h ave reported a value
of 0.43 at 350 °C. Th e auth ors are in agreem en t with th ese latter values.
Zeolites are well kn own to h ave sh ape-selective properties, wh ich en able
th e th erm odyn am ic lim itation s to be superseded. In th e presen t con text
5
th e volum in ous literature on toluen e alkylation with m eth an ol is relevan t .
Th us it would be reason able to expect th at in th e m eth ylation of ph en ol
th e use of sh ape-selective zeolites m ay prom ote th e form ation of p-cresol,
th e slim m est of th e cresol isom ers an d th erefore th e m ost easily capable of
diffusin g from th e porous structure of th e sh ape-selective zeolite. An isole,
h owever, will also probably diffuse relatively easily out of th ese zeolites.
Th e O-:C-alkylation ratios obtain ed in th e m eth ylation of ph en ol over
9
various catalysts is of in terest. Lan dau et al. h ave sh own in studies usin g
H-Y, H-ZSM-5, H-Beta an d H-Morden ite, carried out at 180 °C, th at th ese ra-
1
0
tios were 2.3, 2.7, 2.1 an d 1.3, respectively. For H-Y, Garcia et al. an d
1
1
Marczewski et al. observed ratios of 3 an d 3.2 at 200 °C. Marczewski et
1
1
9
al. also foun d h igh er ratios th an Lan dau et al. for H-Morden ite (2.3) an d
H-ZSM-5 (9.0). San tacesaria et al.12 observed O-:C-alkylation ratios at 200 °C
over am orph ous silica-alum in a, γ-alum in a, Nafion -H an d ph osph oric acid
of 2, 5, 9 an d 9, respectively. It is clear th at if th e results of th e zeolites are
com pared with th ose of th e am orph ous acid catalysts, th e zeolites gen erally
appear to favour rin g as opposed to side ch ain alkylation (alth ough th e ra-
tio of O-:C-alkylation is still greater th an 1). Th e reports on th e activity of
H-ZSM-5, h owever, give con tradictory results.
Th e form ation of p-cresol an d th e ratio of p-:o-cresol are of particular in -
terest in th e presen t study. Th ere are m an y reports on th e m eth ylation of
ph en ol over a ran ge of zeolite catalysts such as H-USY, H-ZSM-5, H-Beta
9
–15
16
an d H-Morden ite
. In an an alysis by on e of th e auth ors of 47 differen t
reports on th e use of zeolites for ph en ol m eth ylation , th e experim en tally
observed ratio of p- to o-cresol in th e tem perature ran ge 180 to about 300 °C
was gen erally in th e region of 0.5 wh ich sh ould be com pared with th e th er-
m odyn am ically expected equilibrium ratio of aroun d 0.43 an d th e statisti-
cally expected ratio of 0.5, based on th e probability of th e electroph ilic
attack takin g place at eith er of two orth o-sites as opposed to on ly on e
para-site. However, o-cresol can also form as a secon dary product via
in tram olecular rearran gem en t of an isole or, m ore likely, as a product of th e
reaction of an isole with ph en ol.
Collect. Czech. Chem. Commun. (Vol. 68) (2003)

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O’Connor, Moon, Böhringer, Fletcher:
In th e case of H-USY, in particular, som e studies h ave in vestigated th e use
of partially alkali-exch an ged sam ples1
5,17
. In all cases of H/alkali-USY, h ow-
ever, n otwith stan din g th e quoted h igh selectivity to p-cresol th e lifetim es
are very sh ort an d yields are very low. Th e p- to o-cresol ratios observed are
n ot a stron g fun ction of th e ph en ol con version .
Alth ough m-cresol is th e m ost favoured isom er th erm odyn am ically, it is
n ot kin etically favoured in term s of th e electroph ilic attack of th e arom atic
rin g, th e OH-group of th e ph en ol bein g very stron gly orth o- an d para-
directin g in electroph ilic arom atic substitution . It is worth n otin g in th e
presen t con text th at th e orth o- an d para-directin g effect is in th e order CH <
3
OCH < OH. Kin etically th erefore o- an d p-cresol will dom in ate over m-cresol
3
in th e m eth ylation of ph en ol an d th is is in deed wh at is observed experi-
m en tally.
With respect to th e relative form ation of cresols an d an isole, th erm ody-
n am ically, cresol form ation is alm ost en tirely favoured over an isole. Th is is
because th e C–C bon d in th e case of cresols is m ore stable th an th e O–C
an isole bon d. However, an isole is th e kin etically preferred product un der
m ild reaction con dition s.
Previous studies h ave in dicated th at an in crease in reaction tem pera-
ture in creases th e ph en ol con version an d results in a reduction of th e
an isole:cresol ratio possibly due to th e system m ovin g towards th e th erm o-
dyn am ic equilibrium con cen tration s9
,12,14
. Moreover, th e activation en ergy
1
1
of cresols form ation is twice th at of an isole form ation an d, th erefore, an
in crease in tem perature will also in crease th e rate of form ation of cresols,
m uch m ore th an th at of an isole. An in crease in reaction tem perature also
results in a greater degree of form ation of m-cresol an d a decrease in th e
p-:o-cresol ratio.
In creasin g th e m eth an ol-to-ph en ol ratio caused a sligh t in crease in th e
p-:o-cresol ratio wh ile at th e sam e tim e results in an in crease in th e ratio of
an isole to cresols, i.e., lower m eth an ol:ph en ol ratios in creased th e selectiv-
9
ity to cresols . In creasin g th e reaction pressure also resulted in an in crease
1
8
in th e ratio of an isole to cresols .
Sin ce th e prim ary objective of th is study was th e form ation of p-cresol, it
was proposed th at th ere m ay be greater ben efit in usin g a ten m em bered
rin g zeolite. Jacobs et al.19 studied th e use of H-ZSM-22 an d H-Ferrierite for
th e con version of an isole but did n ot observe an y particular in crease in th e
form ation of p-cresol com pared with H-ZSM-5. Th ey con cluded th at th e re-
action occurred dom in an tly on th e extern al surface of th e zeolite crystal-
lites. In th e presen t study, it was n everth eless proposed th at th e use of
Collect. Czech. Chem. Commun. (Vol. 68) (2003)

Alkylation Over Zeolites
1953
H-MCM-22 m ay prom ote th e form ation of p-cresol, th is zeolite h avin g its
un ique dual twelve- an d ten -m em bered rin g structure.
m-Cresol Alkylation with Propene
As m en tion ed above, th ym ol (2-isopropyl-5-m eth ylph en ol) is curren tly pre-
2
pared in dustrially usin g th e Bayer process . In th e gas ph ase, supported
2
0
21
20,22
m etal sulfates , wide- an d m edium -pore zeolites , an d γ-Al O (refs
)
2
3
h ave all been reported as suitable catalysts for propylation of m-cresol usin g
propen e. Isopropyl alcoh ol can also be used as an alkylatin g agen t as sh own
over Mg-Al-h ydrotalcites23 an d Zn -alum in ate .
1
Of th e two alkyl substituen ts presen t, th e isopropyl group is m uch m ore
reactive th an th e m eth yl group so th at isom erisation of th e latter is in sig-
n ifican t. Th is is because in term ediates an d tran sition states for th e sh ift of a
m eth yl group in volve com parably un favourable m eth yl carben ium ion s or
related structures. Th e selectivity towards th ym ol is rath er h igh , as th ym ol
is th e kin etically preferred isom er in th is reaction . Th e m ajor by-products
are 3-isopropyl-5-m eth ylph en ol an d 4-isopropyl-5-m eth ylph en ol, wh ere
th e isopropyl group is in differen t position . In addition , th e 2-isopropyl-
3
-m eth ylph en ol isom er is sterically h in dered so th at it does n ot form in
substan tial am oun ts.
Th e aim of th e presen t paper is to presen t selected results from a recen t
1
6
com preh en sive study of th e m eth ylation of ph en ol an d th e propen e
propylation of m-cresol. Th e paper focuses on th e activity an d selectivity of
zeolite H-ZSM-5 an d, in th e case of ph en ol m eth ylation , zeolite H-MCM-22,
over a ran ge of reaction con dition s. Th e m ain purpose of th is work was to
in vestigate th e exten t to wh ich th ese zeolites are able, via product sh ape se-
lectivity, to prom ote th e production of th e m ost readily diffusible isom ers,
viz. p-cresol an d th ym ol.
EXPERIMENTAL
In th e case of th e alkylation of ph en ol with m eth an ol, th e zeolites used were H-MCM-22
an d H-ZSM-5. Th e Si/Al ratios were between 10–30 for th e form er an d 45–90 for th e latter.
2
Am orph ous silica-alum in a (Si/Al = 6, BET = 508m /g) used as a ben ch m ark exam ple of a
n on -sh ape-selective acid catalyst. H-MCM-22 h as a pore structure th at con sists of two in de-
2
4
pen den t ch an n el system s . Th e first, a two-dim en sion al ch an n el system con sists of
supercages defin ed by twelve-m em bered rin gs h avin g a diam eter of 7.1 Å an d a len gth of
1
8.2 Å. Th ese supercages are in tercon n ected th rough ten -m em bered rin g open in gs with di-
m en sion s of 4.0 x 5.4 Å form in g a two-dim en sion al ch an n el system . Th e secon d ch an n el is
a two-dim en sion al sin usoidal ch an n el defin ed by ten -m em bered rin gs (4.0 × 5.9 Å). Both
ch an n el system s exten d in th e sam e two dim en sion s but th ere is n o direct access between
Collect. Czech. Chem. Commun. (Vol. 68) (2003)

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O’Connor, Moon, Böhringer, Fletcher:
th ese two system s. Th e extern al surface of H-MCM-22 h as twelve-m em bered rin g pockets,
wh ich h ave a depth of 7.1 Å m ade up by h alf supercages. H-ZSM-5 h as a th ree-dim en sion al
pore structure con sistin g of two in tercon n ectin g ten -m em bered rin g ch an n els, on e of wh ich
is sin usoidal (5.3 × 5.6 Å) an d th e oth er straigh t (5.1 × 5.5 Å).
Four sam ples of H-MCM-22 were syn th esized by th e auth ors in th eir own laboratories
1
6
(
UCT sam ple) as well as by on e of th e auth ors in th e laboratory of Prof. R. Lobo at th e
Un iversity of Delaware (DE sam ples). Th e crystal diam eters observed from electron m icro-
graph s were variously 0.2 µm (UCT), 0.8 µm (DE-1) an d 1 µm (DE-2 an d DE-3). Th e m or-
ph ology was observed to be platelet in n ature with th e ch an n el system s exten din g in th e
direction of th e platelet diam eter.
Prior to use all catalysts were calcin ed at 500 °C in a syn th etic air m ixture (79 vol.% pure
n itrogen m ixed with 21 vol.% pure oxygen ) for 2 h . In th e case of th e m eth ylation of
ph en ol, experim en ts were carried out in th e gas ph ase an d tem peratures between 250 an d
4
00 °C. Th e feed m ixture, of equim olar am oun ts of m eth an ol an d ph en ol, un less oth erwise
stated, was pum ped via an evaporator to a fixed-bed plug flow reactor. Nitrogen was added
as th e d ilu en t gas. Th e feed p artial p ressu re was 20 kPa, total p ressu re 100 kPa an d
–
1
WHSVtotal reactan ts 14 h . Th e products were cooled after leavin g th e reactor an d liquid sam -
ples were an alysed on -lin e usin g a Ch rom pack CP Cresols capillary colum n . For determ in in g
m ass an d rin g balan ces, toluen e was used as an in tern al stan dard in th e reactor system .
In th e case of m-cresol propylation th e zeolites used were H-ZSM-5 an d H-Beta. Th e Si/Al
ratios were 45 an d 110 for H-ZSM-5 an d 75 for H-Beta. H-Beta h as a th ree-dim en sion al pore
structure con sistin g of th ree in tercon n ectin g twelve-m em bered rin g ch an n els, perpen dicular
to each oth er. Two of th e ch an n els are lin ear (6.6 × 6.7 Å). Th e th ird ch an n el is n on -lin ear
(
5.6 × 5.6 Å).
H-ZSM-5 was used in extruded form an d H-Beta as crystal powder. Prior to use th e cata-
lysts were calcin ed un der n itrogen at 250 °C. Gaseous propen e was fed via a th erm al m ass
flow con troller. Th e liquid m-cresol was fed via a HPLC pum p to an evaporator wh ere it was
m ixed with th e gaseous propen e. Th e m ixture was fed to a fixed-bed plug flow reactor. Th e
products were con den sed after leavin g th e reactor, an d liquid sam ples were taken . Th e total
reaction pressure was usually m ain tain ed at atm osph eric pressure but in som e in stan ces var-
ied up to 400 kPa. Th e reaction tem perature was usually h eld at 250 °C but also varied be-
–
1
tween 180 an d 350 °C. Th e WHSV (m-cresol) was gen erally h eld at 1 h but varied in som e
cases between 0.21–1.44 h –1. Th e propen e:cresol m olar ratio was h eld at 1 but varied in
som e cases. Product sam ples were an alyzed by capillary gas ch rom atograph y usin g a
Stabilwax DA colum n .
Am m on ia TPD experim en ts were carried out with a view to determ in in g th e tem perature
at wh ich th e m axim um desorption occurred, th is bein g in dicative of th e relative stren gth of
th e acid sites. In th ese experim en ts th e am m on ia, wh ich was fed as a 4% NH3 in He m ix-
ture, was adsorbed on 0.25 g sam ple of th e catalyst at 150 °C. Th e sam ple was th en flush ed
at th at sam e tem perature in flowin g h elium for 8 h before ram pin g began . Th e tem perature
was ram ped at 10 °C/m in from 150 to 600 °C.
Collect. Czech. Chem. Commun. (Vol. 68) (2003)

Alkylation Over Zeolites
1955
RESULTS AND DISCUSSION
Phenol Alkylation with Methanol
Th e system was deliberately run at differen tial con version s th rough out in
order to en sure th at it was possible to com pare th e perform an ce of cata-
lysts. Th e catalysts in itially deactivated rapidly over th e first two h ours an d
th en reach ed essen tially a steady state with respect to ph en ol con version .
Table I sh ows th e quasi-steady state ph en ol con version , O-:C-alkylation ra-
tio an d p-:o-cresol ratio over H-MCM-22, H-ZSM-5 an d am orph ous silica-
alum in a, at 300 °C. In term s of ph en ol con version , H-ZSM-5 was th e m ost
active catalyst studied. Alth ough th is zeolite h ad a relatively low Al con -
ten t, it is un likely th at th is differen ce in con version was due to differen ces
in stren gth of acid sites. Th e tem perature at wh ich th e m axim um
desorption peak occurred in am m on ia TPD experim en ts was used as an esti-
m ate of acid site stren gth . In th e case of H-ZSM-5 th is occurred at 372 °C
an d for H-MCM-22 at 282 °C. No correlation was observed between th ese
tem peratures an d eith er th e O-:C-alkylation ratio or th e p-:o-cresol ratio.
1
0
However, it h as been suggested th at weaker acidity favours O-alkylation
an d th ere are sligh t in dication s h ere to support th is proposal as th e am or-
ph ous silica-alum in a catalyst sh owed sligh tly greater degree of
a
O-alkylation . All th e catalysts sh owed very sim ilar product distribution s
with an isole an d cresol isom ers dom in an t. Sm all am oun ts (<4% selectivity)
of secon dary products, viz. m eth ylan isoles an d xylen ols, were presen t. Th e
zeolites sh owed sligh tly en h an ced selectivity to p-cresol com pared to silica-
alum in a. In th e latter case th ere are Lewis acid sites an d th e h igh est in ci-
den ce of o-cresol (Table I) is con sisten t with th is, given th e reports in th e
TABLE I
Ph en ol m eth ylation over H-ZSM-5, H-MCM-22 an d am orph ous silica-alum in a (gas ph ase,
3
00 °C, 20 kPa feed partial pressure, W/F = 0.07 gcat h /gfeed)
Ph en ol con version
%
O-:C-Alkylation
Catalyst
p-:o-Cresol
m-Cresol
ratio
H-ZSM-5
10.2
3.8
1.7
1.7
1.8
0.55
0.50
0.37
4.2
5.8
3.1
H-MCM-22
SiO -Al O
2.5
2
2
3
Collect. Czech. Chem. Commun. (Vol. 68) (2003)

1
956
O’Connor, Moon, Böhringer, Fletcher:
literature of th e relation sh ip between Lewis acidity an d preferen tial o-cresol
1
2
form ation . As sh own in Table I th e m-cresol con ten t was always less th an
5
% of th e cresol fraction .
Alth ough th erm odyn am ic equilibrium calculation s predict th at at 300 °C
th e products sh ould be alm ost en tirely cresols, at th is tem perature th e
O-:C-alkylation ratio was approxim ately 1.7 over all th e catalysts studied. At
th is tem perature, th e p-:o-cresol ratio was about 0.5, on ly sligh tly h igh er
th an th e expected equilibrium value of about 0.43 an d alm ost iden tical to
th e statistically expected value. Th ese product distribution s in dicate th at
th e reaction appears to be kin etically or diffusion -con trolled an d n ot th er-
m odyn am ically con trolled un der th e reaction con dition s used. Calculation s
were m ade of apparen t activation en ergies. Th e values were in th e region of
5
0 kJ/m ol, wh ich is in dicative of a diffusion -con trolled situation .
In order to sh ed ligh t on th e reaction path ways followed in th e m eth yl-
ation of ph en ol, a study was m ade of th e reaction s over H-MCM-22 at 300 °C
of each of th e m ajor products form ed durin g ph en ol m eth ylation , viz.
an isole, o-cresol, p-cresol an d m-cresol. Th ese results are sh own in Tables II
an d III. An isole form ed m ain ly m eth ylan isoles, ph en ol an d cresols. Th ese
1
9
results are sim ilar to th ose observed by Jacobs et al. . Th is is due to a m eth yl
transfer m echanism , which initially form s phenol and m ethylanisoles. Further
m eth yl tran sfer th en occurs from an isole to th e ph en ol product to form cre-
sols an d m ore ph en ol. Th is is con firm ed in th e reaction wh ere an equi-
TABLE II
Ph en ol m eth ylation m ajor products con verted over H-MCM-22 (gas ph ase, 300 °C, 20 kPa
feed partial pressure, W/F = 0.07 gcat h /gfeed
)
Selectivity of products form ed, m ole %
Con version
Reactan ts
%
an isole cresols
m eth ylan isoles xylen ols
ph en ol
Ph en ol + m eth an ol
An isole
3.8
60.4
37.4
19.2
68.7
1.2
37.8
12.7
1.0
2.1
1.6
–
6.5
3.7a
–
–
40.9
17.0
An isole:ph en ol
(
1:1 m olar ratio)
o-Cresol
p-Cresol
m-Cresol
3.9
7.3
3.6
0.2
0.7
0.0
68.0
88.2
97.7
0.0
0.0
0.0
15.9
5.7
15.9
5.4
1.2
1.1
a
Con version of an isole.
Collect. Czech. Chem. Commun. (Vol. 68) (2003)

Alkylation Over Zeolites
1957
m olar am oun t of ph en ol was co-fed with th e an isole. Th e con version of
an isole decreased as expected from its reduced partial pressure. Cresols were
th e m ajor product. Th e decrease in th e am oun t of o-cresol was alm ost
quan titatively equivalen t to th e in crease in p-cresol. Th is im plies th at, in
th e absen ce of ph en ol, m on om olecular m eth yl rearran gem en t occurs pro-
ducin g o-cresol from an isole. In th e presen ce of ph en ol as reactan t, h ow-
ever, a bim olecular tran sfer from th e an isole to th e ph en ol para-position is
n ow possible. Each cresol isom er was con verted over H-MCM-22. In th e
case of o-cresol an d p-cresol, m-cresol was th e m ajor product as expected.
Sin ce in th e m eth ylation of ph en ol over all th e catalysts studied, on ly a
sm all am oun t of m eth ylan isoles, xylen ols (<4% selectivity) an d m-cresol
(
<5% of th e cresols fraction ) were form ed, it can be deduced th at secon dary
reaction s of an isole an d th e cresols occur on ly to a sm all exten t at 300 °C.
Th e effect of tem perature on th e product distribution from ph en ol
m eth ylation over H-MCM-22 was in vestigated in th e ran ge 250–400 °C.
Con sisten t with previous fin din gs14 in creasin g th e tem perature caused th e
O-:C-alkylation ratio to decrease from 1.89 (250 °C) to 0.73 (400 °C). Th e
p-:o-cresol ratio decreased from 0.54 (250 °C) to 0.41 (400 °C). In creasin g
th e reaction tem perature also in creased th e form ation of m-cresol from 4 to
1
6% in th e cresol fraction . Sin ce th e cresols distribution at 300 °C is h ardly
in fluen ced by con version an d extrapolation to zero con version still sh ows
som e m-cresol, th ese observation s in dicate th at at h igh er tem peratures th e
prim ary form ation of m-cresol is en h an ced.
TABLE III
Cresol distribution form ed wh en con vertin g th e ph en ol m eth ylation m ajor products over
H-MCM-22 (gas ph ase, 300 °C, 20 kPa feed partial pressure, W/F = 0.07 gcat h /gfeed
)
Cresol distribution , %
Reactan ts
o-cresol
p-cresol
m-cresol
Ph en ol + m eth an ol
61.9
69
53
–
32.3
11
28
15
–
5.8
20
19
85
91
–
An isole
An isole:ph en ol (1:1 m olar ratio)
o-Cresol
p-Cresol
m-Cresol
9
26
74
Th erm odyn am ic equilibrium 8
37
16
47
Collect. Czech. Chem. Commun. (Vol. 68) (2003)

1
958
O’Connor, Moon, Böhringer, Fletcher:
As m en tion ed above, th e apparen t activation en ergies in dicated th at dif-
fusion m ay be th e con trollin g m ech an ism . Th is was observed to be th e case
even wh en th e crystallite size of H-MCM-22 was as sm all as th at of th e
H-ZSM-5 sam ple, viz. 0.2 µm . It would th us be expected th at th is effect
would be even greater in th e case of th e oth er H-MCM-22 sam ples wh ich
h ad crystallite sizes in th e ran ge 0.8–1 µm , th us h avin g a m uch lon ger
diffusion al path len gth . It was th us decided to in vestigate in greater depth
th e effect of th e crystallite size of th e zeolites studied on th e p-:o-cresol ra-
tio. Th e H-ZSM-5 sam ple an d four sam ples of H-MCM-22 were selected as
described in Experim en tal, viz. UCT, DE-1, -2, -3. As it is well kn own , th e
effect of diffusion on sh ape selectivity is m ost appropriately typified by th e
use of th e Th iele m odulus, φ. For exam ple, for an n-th order reaction :
n –1
φ = ξ L√[(n + 1)k CA ]/[2D ] ,
eff
wh ere ξ is geom etric factor for th e particular m orph ology, L is ch aracteristic
len gth of th e particle, k is rate con stan t, n is overall reaction order of th e ir-
reversible reaction , C is exit con cen tration an d D is effective diffusivity.
A
eff
For a platelet H-MCM-22 crystal with its ch an n els exten din g in th e direc-
tion of th e platelet diam eter, th e effective geom etry is th at of a cylin der
an d th erefore th e radius R of th e platelets is taken as th e ch aracteristic
len gth . In th e presen t study th e sh apes an d geom etric factors for each cata-
lyst are given in Table IV.
For a differen tially operatin g plug flow reactor an d irrespective of th e re-
action order, th e m ass balan ce equation becom es:
X = k · con stan t ,
TABLE IV
Sh apes an d geom etric factors for H-MCM-22 an d H-ZSM-5 catalysts
Catalyst
Sh ape
Geom etric factor ξ
Ch aracteristic len gth
H-MCM-22
H-ZSM-5
cylin der
sph ere
0.5
radius of platelets
radius of sph ere
0.33
Collect. Czech. Chem. Commun. (Vol. 68) (2003)

Alkylation Over Zeolites
1959
wh ere X is con version . Th us th e Th iele m odulus an d a m odified Th iele
m odulus can be expressed as follows:
φ/(1/D )0 = (con stan t′) ξ L X0.5
.5
eff
φm od/(1/D )0 = ξ L X0.5
.5
.
eff
Figure 1 sh ows th e relation sh ip between th e p-:o-cresol ratios obtain ed an d
a m odified Th iele m odulus, viz. for DE an d UCT sam ples of H-MCM-22 an d
on e sam ple of H-ZSM-5 wh ich h ad a crystallite size of 0.2 µm . It is clear
th at th e p-:o-cresol ratio is a stron g fun ction of crystal size. From th is figure
it becom es clear th at arguably th e m ost dom in an t variable in fluen cin g th e
p-:o-cresol ratio is th e diffusion al path len gth an d th at it is possible to in -
crease th is ratio by usin g larger crystallite sizes. At crystal sizes of H-MCM-22
up to about 0.8 µm it was n ot possible to observe th is effect. It is worth n ot-
in g th at previous studies h ave focused gen erally on th e role of th e stren gth
1
1
.2
.0
H-MCM-22
(
DE-2)
H-MCM-22
DE-3)
(
0
0
.8
.6
H-ZSM-5
Statistical ratio
H-MCM-22
UCT)
0
.4
H-MCM-22
DE-1)
(
(
0
0
.2
.0
0
.00
0.05
0.10
0.15
Modified Thiele modulus
0.20
FIG. 1
Th e relation sh ip between p-:o-cresol ratio an d a m odified Th iele m odulus φm od/(1/Deff)0.5
=
0
.5
ξ RX (assum ed rate con stan t k can be approxim ated by con version X for differen tial opera-
tion ; assum ed ch aracteristic len gth is radius of crystallites, R). Sh apes an d radii R (in µm ) of
crystallites were as follows: H-ZSM-5 (sph eres) 0.2, H-MCM-22 (UCT) (platelets) 0.2,
H-MCM-22 (DE-1) (platelets) 0.8, H-MCM-22 (DE-2) (platelets) 1.0, H-MCM-22 (DE-3) (plate-
lets) 1.0
Collect. Czech. Chem. Commun. (Vol. 68) (2003)

1
960
O’Connor, Moon, Böhringer, Fletcher:
an d n ature of acid sites in explain in g th e p-:o-cresol ratios but th e im por-
tan ce of th e crystallite size h as been largely ign ored.
Fin ally m en tion sh ould be m ade h ere of prelim in ary experim en ts carried
out to syn th esize selectively th e eth er 4-ph en oxytoluen e. Th ese m olecules
are obtain ed by m eth ylatin g diph en yl eth er in th e 4 position . Recen t pre-
lim in ary in vestigation s carried out in th e auth ors’ laboratory h ave sh own
th at zeolite Morden ite is capable of rath er selectively producin g 4-ph en oxy-
toluen e at h igh selectivity com pared to eith er of its oth er isom ers, viz. 3- or
2
-ph en oxytoluen e. Subsequen t cleavage of th e ph en oxy bon d will th en
lead to h igh ly selective form ation of p-cresol an d ph en ol. Th e results of th is
in vestigation will be publish ed sh ortly.
m-Cresol Alkylation with Propene
In th e presen t study of th e alkylation of m-cresol with propen e, th e cata-
lysts were zeolites H-ZSM-5 an d H-Beta, both h avin g sim ilar, h igh Si/Al ra-
tios of 110 an d 75, respectively. Figure 2 sh ows h ow th e selectivity to
th ym ol over H-ZSM-5 varies as a fun ction of con version for various tim es-
on -stream . Th is figure represen ts th e case for a sam ple with a low Si/Al
ratio. Th e con version in creased in itially for th e first th ree h ours an d th en
decreased slowly from 55 to 20% over 50 h . Steady state was attain ed after
about 100 h . At th e sam e tim e th e selectivity to th ym ol in creases steadily to
about 90% over th e sam e tim e-on -stream . In itially th ere are in dication s
1
00
5
0 h
2
4 h
8
6
0
0
3
h
1
h
4
2
0
Time-on-stream
0
0
0
20
40
60
80
100
m-Cresol conversion, %
FIG. 2
Typical course of selectivity to th ym ol as a fun ction of con version for various tim es-on -stream
–
1
(
H-ZSM-5, Si/Al = 45, 250 °C, 100 kPa, WHSVm-cresol = 1 h , propen e:m-cresol m olar ratio = 1)
Collect. Czech. Chem. Commun. (Vol. 68) (2003)

Alkylation Over Zeolites
1961
from th e product spectrum th at con siderable propen e oligom erization is oc-
currin g, wh ich probably results in th e form ation of soft coke an d con se-
quen t deactivation . Th is is in dicated by a m ultiplicity of peaks in th e low
carbon n um ber ran ge at th e begin n in g of th e ch rom atogram of th e prod-
ucts typical of a propen e oligom erization spectrum .
Figures 3 an d 4 sh ow th e results of an exten sive set of experim en ts in
wh ich th e effects of varyin g reaction tem perature, space velocity an d
1
00
8
0
WHSV = 0.21
WHSV = 0.41
WHSV = 0.62
6
4
0
0
WHSV = 0.62
propene/m-cresol = 2.34
2
20
00
WHSV = 0.62
propene/m-cresol = 0.43
2
2
0
0
3
50
1
80
WHSV = 0.21
350
WHSV = 1.44
propene/m-cresol = 0.43
3
00
0
50
100
150
200
250
Time-on-stream, h
FIG. 3
Con version of m-cresol with propen e over H-ZSM-5 (Si/Al
=
110) as
a
fun ction of
–
1
tim e-on -stream for differen t WHSVm-cresol (0.21, 0.41, 0.62, 1.44 h ), reaction tem peratures
180, 200, 220, 250, 300 an d 350 °C) an d propen e:m-cresol m olar ratios (0.43, 1, 2.34). Re-
(
action pressure 100 kPa. Black sym bols: stan dard con dition s (250 °C, WHSV
= 1,
propen e:m-cresol m olar ratio = 1)
1
00
1
80
2
20
WHSV = 1.44 propene/m-cresol = 0.43
2
00
8
6
4
2
0
0
0
WHSV = 0.41
WHSV = 0.62
3
00
WHSV = 0.62
propene/m-cresol = 2.34
WHSV = 0.62
propene/m-cresol = 0.43
3
50
3
50
WHSV = 0.21
0
0
WHSV = 0.21
0
50
100
150
200
250
Time-on-stream, h
FIG. 4
Selectivity to th ym ol as a fun ction of tim e-on -stream over H-ZSM-5 (con dition s as for Fig. 3).
Black sym bols: stan dard con dition s (as for Fig. 3)
Collect. Czech. Chem. Commun. (Vol. 68) (2003)

1
962
O’Connor, Moon, Böhringer, Fletcher:
propen e:m-cresol ratio on con version an d th ym ol selectivity over H-ZSM-5
were in vestigated. Measurem en ts m ade at n on -steady state con dition s were
foun d to be com parable an d, as sh own in Fig. 6, for exam ple, selectivities
did n ot ch an ge sign ifican tly with con version . Not surprisin gly con version
is seen to decrease with reaction tem perature durin g th e first 50 h . Th ere-
after con version in creased as tem perature in creased but th e deactivation of
th e first period on stream was n ot fully overcom e. Th e origin al con version
was recovered wh en th e tem perature in creased to 350 °C an d th e space ve-
locity was reduced fivefold. However it is clear th at it was ultim ately n ot
possible to recover th is origin al con cersion by ch an gin g tem perature or
space velocity or ph en ol:m-cresol feed ratios. A steady-state con version was
reach ed after 150 h on stream . Figure 4 sh ows im portan tly th at at th is
steady state con dition th e selectivity to th ym ol also attain ed a steady state
of about 90% wh ich was n ot sign ifican tly differen t to th at observed in th e
first 50 h on stream . Figure 5 sh ows th at th ere were two optim a with re-
spect to reaction tem perature wh en all th e oth er con dition s are equal. On e
at a reaction tem perature in th e region of 200 °C wh ich correspon ds to
m axim um selectivity to th ym ol an d th e oth er at a reaction tem perature of
about 250 °C wh ich in dicates m axim um con version . At h igh er reaction
tem perature, con version declin es again due to equilibrium lim itation s. In
th e tem perature ran ge 180–250 °C, h owever, th e selectivity to th ym ol was
again n ot sign ifican tly altered wh en con version decreased.
1
00
8
0
6
4
0
0
2
0
0
0
50
100
150
200
250
Reaction temperature, °C
FIG. 5
Variation in m-cresol con version an d selectivity to th ym ol at differen t reaction tem peratures
–
1
over H-ZSM-5 (Si/Al = 110, WHSVm-cresol = 1 h , propen e:m-cresol m olar ratio = 1, reaction
pressure 100 kPa). ᮀ Th ym ol selectivity, ᭹ m-cresol con version
Collect. Czech. Chem. Commun. (Vol. 68) (2003)

Alkylation Over Zeolites
1963
Figure 6 sh ows th e th ym ol selectivity as a fun ction of m-cresol con ver-
sion over H-ZSM-5 (Si/Al = 110) at 200, 250 an d 250 °C. Con version was
varied by varyin g space velocity, reaction pressure an d propen e:m-cresol
ratio. Th e followin g con clusion s can be drawn :
–
Con version h as a sign ifican t in fluen ce on selectivity, wh ich declin es
with in creasin g con version .
Reaction tem perature h as a stron g in fluen ce on selectivity, wh ich de-
clin es with in creasin g tem perature at a given con version .
Reaction tem perature is th e on ly param eter h avin g direct in fluen ce on
selectivity.
–
–
–
Th e in fluen ce of all oth er reaction param eters (space velocity, reaction
pressure, feed ratio) is essen tially of in direct n ature, i.e. via con version .
Th e equilibrium distribution of th e th ree m ajor th ym ol isom ers were ob-
tain ed by isom erizin g bin ary m ixtures of th ym ol an d th e th ym ol isom ers
over H-ZSM-5. At 250 °C th e distribution was foun d to be 20% th ym ol, 75%
3
-isopropyl-5-m eth ylph en ol an d 5% 4-isopropyl-5-m eth ylph en ol. It sh ould
be n oted th at 6-isopropyl-5-m eth ylph en ol h ardly form s due to severe steric
con strain ts. With sufficien tly in creased space tim e, each an d every startin g
m ixture reach ed th e sam e poin t of isom er distribution , wh ich on ly con -
tain ed ca 20% th ym ol in th e total th ym ol isom ers fraction . Th is is an oth er
in dication of th e kin etic con trol of th e th ym ol syn th esis from propen e an d
m-cresol on H-ZSM-5.
1
00
8
6
4
2
0
0
0
0
0
0
20
40
60
80
100
m-Cresol conversion, %
FIG. 6
Th ym ol selectivity as a fun ction of m-cresol con version over H-ZSM-5 (Si/Al = 110) at 200 (᭿),
50 (᭺) an d 250 (᭛) °C. Con dition s addition ally varied at th e differen t tem peratures were
2
space velocity, reaction pressure an d propen e:m-cresol m olar ratio. In itial sam ples, <4 h on (᭪)
Collect. Czech. Chem. Commun. (Vol. 68) (2003)

1
964
O’Connor, Moon, Böhringer, Fletcher:
Figure 7 sh ows th e variation in th ym ol selectivity as a fun ction of m-cresol
con version over H-ZSM-5 (Si/Al = 110) an d H-Beta (Si/Al = 75) at 250 °C
an d 100 kPa an d, in th e case of H-ZSM-5, at differen t space velocities an d
propen e:m-cresol ratios. Com pared to Fig. 6, wh ere selectivity versus con -
version is sh own for H-ZSM-5 at th e sam e reaction tem perature, two m ajor
differen ces attract atten tion , viz. a lower th ym ol selectivity at zero con ver-
sion (extrapolated) of less th an 90% over H-Beta com pared to alm ost 95%
over H-ZSM-5 an d a steeper declin e of selectivity with in creasin g con ver-
sion over H-Beta (full data poin ts). Apparen tly, th e m ore sh ape-selective
on e of th e two differen t zeolites used, H-ZSM-5, prom otes th e form ation of
th e slim m er isom er beyon d ch em ical kin etics, bein g in its prim ary product
(
h igh er zero con version selectivity) or in its secon dary product (h igh er se-
lectivity at h igh con version ). Th e latter in dicates th at sh ape-selective prop-
erties play a role in h ibitin g secon dary isom erization .
Table V sh ows selected results obtain ed in th e presen t study. It sh ould be
n oted th at th e selectivity to th ym ol was always in excess of 75%. Sm all
am oun ts of th e 3-isopropyl-5-m eth ylph en ol an d 4-isopropyl-5-m eth yl-
ph en ol isom ers form in approxim ately equal am oun ts, wh ich accoun t for
m ost of th e rest of th e total product. It is also useful to com pare th e above
results with th ose publish ed to date usin g a variety of catalysts. Th ese are
briefly sum m arized in Table VI.
1
00
8
6
4
2
0
0
0
0
0
0
20
40
60
80
100
m-Cresol conversion, %
FIG. 7
Th ym ol selectivity as a fun ction of m-cresol con version over H-ZSM-5 (Si/Al = 110, ᭺) an d
H-Beta (Si/Al = 75, ᮀ) at 250 °C an d 100 kPa (also differen t space velocities an d propen e:m-cresol
m olar ratios over H-ZSM-5). In itial sam ples, <4 h on (᭪ an d Ճ)
Collect. Czech. Chem. Commun. (Vol. 68) (2003)

Alkylation Over Zeolites
1965
As can be seen from th e results above, th e use of H-ZSM-5 is con ducive to
th e form ation of h igh yields of th ym ol an d especially to m in im ize th e for-
m ation of oligom eric an d polyisopropylated or polyarom atic com poun ds.
Th is will n ot on ly facilitate th e separation of th e desirable th ym ol but also
result in a less rapid deactivation of the catalyst. As can be seen from Tables V
an d VI, H-ZSM-5 allows operation at m uch m ilder tem perature an d lower
pressure con dition s th an th e presen t com m ercially used γ-Al O catalyst,
2
3
wh ich is h igh ly desirable.
TABLE V
Sum m ary of results in th e reaction of m-cresol with propen e usin g H-ZSM-5
Th ym ol
selectivity
m ole %
Th ym ol
yield
m ole %
Con version Tem perature Pressure WHSV Propen e:cresol
–
1
%
°C
kPa
h
m olar ratio
5
8
8
5
200
250
250
400
100
400
0.4
0.2
1.0
1
1
8
90
75
80
49.5
60
0
5
68
TABLE VI
Sum m ary of som e key fin din gs on th e form ation of th ym ol from th e alkylation of m-cresol
usin g propen e over various catalysts
m-Cresol
con version
%
Th ym ol
selectivity
%
Tem perature
WHSV
h
Propen e:m-cresol
m olar ratio
Referen ce
Catalyst
–1
°
C
2
a
Fiege
activated
alum in a
75
350–365
0.25
0.7
80
Biederm an n
a
γ-Al O
75
63
78
34
360
0.25
ca 0.15
0.5
0.83
75
80
70
75
2
1
2
3
3
et al.
Nitta et al.
Grabowska
1
9
γ-Al O
250
155
250
1
5
1
2
1
,b
Zn -alum in ate
H-ZSM-5
et al.
Wim m er
3.5
2
0
et al.
a
MPa reaction pressure. b Isopropan ol feed.
5
Collect. Czech. Chem. Commun. (Vol. 68) (2003)

1
966
O’Connor, Moon, Böhringer, Fletcher:
CONCLUSIONS
Th is paper h as reported on th e use of zeolites for two im portan t alkylation
reaction s of a sim ilar type, viz. th e m eth ylation of ph en ol an d th e
propylation of m-cresol. In both of th ese reaction s th e role of th e OH group
is im portan t sin ce it can in fluen ce th e position of electroph ilic attack by
virtue of its stron g orth o- an d para-directin g in fluen ce. In th e case of ph e-
n ol m eth ylation , two parallel prim ary reaction s take place, viz. side ch ain
alkylation to form an isole an d rin g alkylation to form cresols. Th e dom i-
n an t product at low tem perature is th e an isole. Cresols, h owever, can also
be form ed via secon dary reaction s of an isole with ph en ol or an isole with it-
self, th e latter reaction occurrin g, h owever, h alf as fast as th e reaction of
1
6
an isole with ph en ol . On e of th e m ain objectives was to in vestigate th e ex-
ten t to wh ich sh ape-selective zeolites, H-ZSM-5 an d H-MCM-22, would be
able to prom ote th e form ation of p-cresol. In th at regard th e ratio of p- to
o-cresol was closely m on itored. It was sh own th at th e crystallite size of th e
zeolites was m ost in fluen tial in in creasin g th e p-:o-cresol ratio. A m odified
Th iele m odulus was used to illustrate th is effect quan titatively.
In a m an n er an alogous to th e case of m eth ylation of ph en ol, th e alkyl-
ation of m-cresol is essen tially in fluen ced by th e presen ce of th e ph en olic
OH group in term s of structure directin g properties. However, th is reaction
differs in two im portan t respects. Th e presen ce of th e m eth yl group m eta to
th e OH group will con strain th e possibility of alkylation in th e orth o-
position between th e OH group an d th e CH group. In th is case th e isom ers
3
m ost likely to form are th e 2-isopropyl-5-m eth ylph en ol an d 4-isopropyl-
5
2
-m eth ylph en ol. It was observed th at th e selectivity to th e slim m er
-isom er, viz. th ym ol, was dom in an t an d th is is probably due to th e fact
th at it is already stron gly favoured from th e poin t of view of ch em ical ki-
n etics. Th e sh ape-selective properties of th e catalyst appear to play th e role
of in creasin g its prim ary selectivity as well as in h ibitin g th e possibility of
isom erization of th ym ol in secon dary reaction s.
Fin ally, in both reaction system s, side reaction s prom oted by th e acid cat-
alyst play a role. In th e case of th e m eth ylation of ph en ol, m eth an ol con -
verts readily to dim eth yl eth er, wh ich in turn is able to un dergo con version
to olefin s an d h igh -boilin g-poin t oligom ers th us resultin g in th e form ation
of “soft” coke in th e pore system . Th is was in dicated by th e deactivation ,
wh ich occurred at h igh reaction tem peratures. Sim ilarly, propen e in th e lat-
ter reaction can oligom erize an d th is was again in dicated by th e deactiva-
tion at h igh er reaction tem peratures. In both cases h igh er reaction
Collect. Czech. Chem. Commun. (Vol. 68) (2003)

Alkylation Over Zeolites
1967
tem peratures also resulted in less form ation of th e slim m olecule, viz. lower
p-:o-cresol ratios an d less th ym ol in th e th ym ol isom ers fraction .
Selectivity to th e slim isom er decreased m oderately with in creasin g con -
version . In creasin g th e space tim e, th e reaction pressure or th e con cen tra-
tion of th e alkylatin g reactan t resulted com m on ly in an in crease in th e
con version of th e ph en ol an d m-cresol, respectively, but with n o or very lit-
tle direct effect on th e selectivity. Con version an d reaction tem perature ap-
pear to be th e on ly param eters in fluen cin g selectivity over a given catalyst.
In sum m ary it h as been sh own th at zeolites such as H-ZSM-5 an d
H-MCM-22 are good catalysts for th e alkylation of ph en ol m olecules. A m a-
jor advan tage is th at m uch m ilder con dition s can be applied th an in th e
curren t tech n ical processes. However, it appears th at th e structure-directin g
role of th e OH group already plays a greater role in th e selectivity to various
products but th e sh ape-selectivity property of th e zeolites con tributes par-
ticularly in th e case of large zeolite crystals or m ore bulky product m ole-
cules, both of wh ich prom ote tran sport con trol, i.e. product sh ape selectiv-
ity. Moreover, th e relative sorptive properties of th e reactan ts will also play
a role in th is regard.
Th e expected reaction m ech an ism is an Eley–Rideal m ech an ism wh ere, in
th e case of ph en ol alkylation , m eth an ol is preferen tially th e adsorbed spe-
cies. Th e structure-directin g property of th e OH group will th en favour
alkylation in th e orth o-position . In th e case of th e th ym ol syn th esis,
propen e will be th e preferen tially adsorbed species an d, due to th e geom et-
ric con strain t of th e CH group, th e preferen tial attack by propen e will n ot
3
take place at th e orth o-position on th e rin g, wh ich is adjacen t to th e m eth -
yl group but at th e opposite position resultin g in th e very h igh selectivity
to th ym ol.
The authors wish to acknowledge support from the National Research Foundation, THRIP, CSIR
(
BioChemtek). Sued Chemie are thanked for supplying the catalyst samples.
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