Photocatalytic oxidation of cyclohexene on titanium(IV)oxide

Article abstract of DOI:10.1135/cccc20031985

Photocatalytic oxidation of cyclohexene was carried out over a series of titanium(IV) oxide catalysts. These species acting together with molecular oxygen significantly differed in their surface and structural properties. Attention was paid to elimination of the total decomposition of the alkene molecules to water and carbon dioxide and to favour formation of reaction intermediates, comprising epoxide units. Experiments were performed in liquid phase at atmospheric pressure, room temperature and under UV illumination. A quartz reactor was used and the reaction mixture was agitated by oxygen. Significant variations in the reaction selectivity were detected as a function of catalyst structural properties.

Full text of DOI:10.1135/cccc20031985

Photocatalytic Oxidation of Cyclohexene
1985
PHOTOCATALYTIC OXIDATION OF CYCLOHEXENE
ON TITANIUM(IV) OXIDE
2,
3
Jana KLISÁKOVÁ1,+, Petr KLUSOŇ * and Libor ČERVENÝ
Department of Organic Technology, Institute of Chemical Technology, Prague, Technická 5,
1
166 28 Prague 6, Czech Republic; e-mail: jana.klisakova@jh-inst.cas.cz,
2
3
p.kluson@seznam.cz, cervenyl@vscht.cz
Received Jun e 9, 2003
Accepted August 8, 2003
Ph otocatalytic oxidation of cycloh exen e was carried out over a series of titan ium (IV) oxide
catalysts. Th ese species actin g togeth er with m olecular oxygen sign ifican tly differed in th eir
surface an d structural properties. Atten tion was paid to elim in ation of th e total decom posi-
tion of th e alken e m olecules to water an d carbon dioxide an d to favour form ation of reac-
tion in term ediates, com prisin g epoxide un its. Experim en ts were perform ed in liquid ph ase
at atm osph eric pressure, room tem perature an d un der UV illum in ation . A quartz reactor was
used an d th e reaction m ixture was agitated by oxygen . Sign ifican t variation s in th e reaction
selectivity were detected as a fun ction of catalyst structural properties.
Keyw ord s: Ph otoch em ical oxidation ; Titan ia; Partial oxidation ; Cycloh exen e; Alken es;
Epoxidation ; Heterogen eous catalysis.
Oxidation processes play an im portan t role in th e production of a wide
ran ge of ch em icals. Man y of th em , such as keton es, aldeh ydes, epoxides,
etc., are typical precursors for drugs, vitam in s, fragran ces an d th ey are also
im portan t in term ediates for m an y com plex syn th eses1,2. However, n ot
m an y in dustrial oxidation processes could be referred to as green tech n olo-
gies1,3,4. Tradition al oxidan ts, such as m in eral acids, ch rom ates, m an gan ese
com plexes, perm an gan ates, h ydrogen peroxide, organ ic peroxides, etc., are
rath er question able in th e con text of th eir toxicity, corrosiven ess an d n ec-
essary strin gen t reaction con dition s. Un like th ese, m olecular oxygen repre-
sen ts1–5 a h igh ly reactive agen t, wh ich in cooperation with a suitably
ch osen h eterogen eous catalytic system (supported n oble m etals, m etal ox-
ides) could be expected to m eet required en viron m en tal stan dards.
+ Presen t address: J. Heyrovský In stitute of Ph ysical Ch em istry, Academ y of Scien ces of th e
Czech Republic; Dolejškova 3, 18223 Prague 8, Czech Republic.
Collect. Czech. Chem. Commun. (Vol. 68) (2003)
doi:10.1135/cccc20031985

1986
Klisáková, Klusoň, Červený:
Ph otocatalytic oxidation with titan ium (IV) oxide (titan ia) actin g togeth er
with m olecular oxygen as th e ph otoactive species is an option . It h as been
reported th at active species, wh ich m ay in clude h ydroxyl radical (OH• ),
superoxide an ion (O2• –), h ydrogen peroxide (H2O2) or sin glet oxygen (O2),
are gen erated on th e TiO2 surface. Th eir existen ce results from th e sem icon -
ductin g n ature of titan ia, its very stron g surface h ydroxylation an d th e
presen ce of oxygen an d UV ligh t. Active oxygen species m ay diffuse to th e
titan ia surface an d act th ere as surface sites. Th ese effects h ave been dis-
cussed elsewh ere in detail6–13. Ph otocatalytic oxidation with titan ia h as
been exten sively used in en viron m en tal6,11,13–20 ph otoch em istry for com -
plete degradation of various pollutan ts eith er in liquid or gas ph ase.
Typical processes of th is kin d are water decon tam in ation from ph en ol,
ch lorin ated ph en ols, ch lorin ated h ydrocarbon s, urea an d triazin e deriva-
tives, various arom atic com poun ds, arom atic am in es, carboxylic acids or
m an y VOC decom position reaction s usin g titan ia th in film s.
On th e oth er h an d, available in form ation on ph otooxidation with TiO2
for syn th etic purposes is rath er lim ited. Non eth eless, th is gen erally om itted
reaction (partial ph otooxidation ) h as a poten tial to provide a wide ran ge of
valuable in term ediates for fin e ch em istry. Epoxidation of olefin s, in cludin g
cyclic alken es, is an im portan t tran sform ation in syn th etic ch em istry an d
th e ph otooxidation com plies ideally with th e gen eral econ om y/ecology
requirem en ts.
In th is paper we report on partial oxidation 1,3,4 of pure cycloh exen e21–26
(n ot solved in a solven t, in itially th e system also does n ot con tain an y wa-
ter) on various types of titan ia ph otoactive species. Th e process selectivity
an d th e product distribution profiles h ave been discussed togeth er in term s
of th e structural properties of th e used catalysts. It seem s th at th e perfor-
m an ce of titan ia as a ph otocatalyst stron gly depen ds on th e surface an d
structural properties wh ich are often fun ction s of th e preparation m eth od.
Th us th e m ajor aim of th is study was to determ in e con dition s un der wh ich
com plete decom position of th e in itial alken e com poun d was lim ited an d
th e form ation of oxygen in term ediates favoured. Com m ercially available
catalysts were com pared with th ose produced by th e sol-gel m eth od. Th ese
were gen erated by usin g a stan dard sol-gel route an d also with in tem plates
of assem blies of reverse m icelles of th e n on -ion ic surfactan t to obtain h igh -
surface-area un iform particles27.
Collect. Czech. Chem. Commun. (Vol. 68) (2003)

Photocatalytic Oxidation of Cyclohexene
1987
EXPERIMENTAL
Catalysts an d Ch em icals
Com m ercial catalysts used in th is work were P-25, D-7701, D-7702 obtain ed from Degussa
an d S-7001 supplied by Eurosupport. Catalysts SG1, SG2 an d SG3 were prepared for th is
study by th e sol-gel process from titan ium tetraisopropoxide (TIOP) (Aldrich , 99.99%). Th e
first two (SG1 an d SG2) were obtain ed by followin g a stan dard sol-gel route, with acetic acid
an d am m on ia as reaction m edia. SG1 preparation con dition s differed from th ose for SG2 in
pH values durin g th e h ydrolysis an d con den sation steps (pH 3 an d 6). Th e process was
started by m ixin g TIOP an d acetic acid, followed by addition of water dropwise for 1 h . Th e
m ixture was vigorously agitated with bubbles of n itrogen an d kept at T = 343 K. Th e pH
value was adjusted by addition of am m on ia. Th e volum etric ratio for th e com pon en ts was
TIOP:acetic acid:water = 1:2:6. Th e evolved gel was subjected to th e agin g for 40 h , th en
dried un der vacuum (373 K) an d fin ally calcin ed at 493 K for 2 h .
SG3 was gen erated in reverse m icelles of a n on -ion ic surfactan t C33H60O10.5 (Triton
X-100, Aldrich , 99.99%) in cycloh exan e (Aldrich , HPLC grade, water con ten t less th an
0.01%). Th e m olar ratio of water to surfactan t was kept at R = 1 an d th e ratio of TX-100 to
cycloh exan e was 0.85 m ol kg–1. Th e organ ic con ten t in a raw gel was sign ifican tly reduced
by supercritical fluid extraction with carbon dioxide an d th en th e m aterial was th erm ally
treated to adjust its crystallograph ic ph ase structure. Th is m eth od an d ch aracteristic features
of th e obtain ed catalyst were described previously27
Oth er used ch em icals were cykloh exen e (Sigm a Aldrich , 99%) as
.
a
substrate an d
7-oxabicyclo[4.1.0]h eptan e (Sigm a Aldrich , 96%), cycloh exan ol, cycloh exan on e (Pen ta,
99.9%), cycloh ex-2-en -1-on e, cycloh ex-2-en -1-ol (both Sigm a Aldrich , 98%) as ch rom ato-
graph ic stan dards.
Typical Experim en t
Reaction s were con ducted at 293 K (isoth erm ally) an d atm osph eric pressure in liquid ph ase
usin g a coolin g-jacket quartz ph otoreactor (300 m l) fitted with a vacuum Hg lam p (400 W,
λm ax = 320–390 n m ). Th e source of th e UV ligh t used was n ot polych rom atic an d it con -
sisted of several sin gle irradiation wavelen gth s, wh ich is a typical feature of th is kin d of a
lam p. Th e reactor was equipped with a con den ser, a colum n with con cen trated sulfuric acid
(water trap) an d a trap com posed of a Dewar flask filled with a m ixture of aceton e an d solid
carbon dioxide (T = 238 K). Fin ally, in som e experim en ts, th e effluen t stream was ven ted
th rough a cartridge with ascarite (Aldrich ) for gravim etric evaluation of th e am oun t of car-
bon dioxide evolved. Oxygen , also used for an effective agitation of th e reaction m ixture,
en tered th e reactor un it at th e bottom by m ean s of
a ceram ic distributor. It was
pre-saturated with cycloh exen e at 293 K an d th e flow rate was digitally adjusted in th e
ran ge 100–1200 m l m in –1. Separately it was proved th at th e upper lim it value was n ecessary
to carry out th e experim en ts with out th e effect of th e extern al m ass tran sfer. Catalysts were
used as very fin e powders to lim it in tern al m ass tran sport effects. Th e with drawn sam ples
were an alysed on a gas spectrom eter HP 5890 with a HP10M colum n (50 m × 0.5 m m ×
0.5 µm ). Molecular structures of som e reaction in term ediates were determ in ed with GC-MS
Saturn 2000 (Varian ) an alytical system usin g an ion trap m ass an alyser.
Collect. Czech. Chem. Commun. (Vol. 68) (2003)

1988
Klisáková, Klusoň, Červený:
Ch aracterisation Meth ods
Specific surface areas (SBET) of th e catalytic sam ples were determ in ed on a Pulse Ch em isorb
2700 apparatus (Microm eritics) usin g a BET m eth od with n itrogen at 77 K. Th e cross-
section al area 0.162 n m 2 for a m olecule of n itrogen was con sidered.
High -resolution adsorption isoth erm s of n itrogen were m easured at 77 K up to a relative
pressure of P/P0 = 1. Th e volum etric m easurem en ts were carried out in a static regim e on
Om n isorp 100 (Coulter). Adsorption experim en ts with carbon dioxide (for SG3) at 298 K
were perform ed usin g a Sorptom atic 1800 (Carlo Erba).
Powder X-ray diffraction (XRD) was used to evaluate crystallograph ic ph ase structures of
TiO2 sam ples on RS XRD equipm en t (R. Seifert Co.) usin g Bragg–Bren tan focusin g geom -
etry, CoKα radiation an d a graph ite m on och rom ator. Particle size was estim ated from th e
Sch errer equation .
Surface im ages of th e used m aterials were taken on SEM m icroscope S-520 (Hitach i).
Sam ples were first sputtered with gold in plasm a.
Som e of th e catalysts were, after th e reaction , studied by IR spectroscopy on a Nicolet 740
spectrom eter (Nicolet). Th e sam e catalysts were also studied on a th erm ogravim etric appara-
tus TGA 750 (Stan ton & Redcroft) at a h eatin g rate of 20 °C m in –1 in air (10 m l m in –1) with
sam ples in a platin um h older.
RESULTS AND DISCUSSION
Catalysts
A good startin g poin t for th e discussion on ch aracteristic features of th e
used catalysts is Table I. It reflects basic differen ces between th e various
TiO2 in th is group. Th e th ree com m ercial products supplied by Degussa
(D-7701, D-7702, P-25) could be classified as low-surface-area sam ples obvi-
ously of m acroporous n ature. Variation s in SBET values are n ot too sign ifi-
TABLE I
Ch aracteristic features of th e used catalysts
Catalyst
S
BET, m 2 g–1
Crystallograph ic ph ase structure
Particle size, n m
D-7701
D-7702
P-25
25 ± 2
44 ± 3
63% an atase 37% rutile
40% an atase 60% rutile
80% an atase 20% rutile
98% an atase
30–50
30–50
30–50
10–20
9.5
50 ± 3
S-7001
SG1
209 ± 5
350 ± 7
400 ± 7
780 ± 9
>99% an atase
SG2
>99% an atase
8.5
SG3
>99% an atase
3.8
Collect. Czech. Chem. Commun. (Vol. 68) (2003)

Photocatalytic Oxidation of Cyclohexene
1989
can t (from 25 to 50 m 2 g–1), despite th e value for P-25 is twice as h igh as for
D-7701. Surface areas of th e catalysts S-7001, SG1 an d SG2 are h igh er of th e
order of m agn itude, likely reflectin g a con tribution of th e m esoporous re-
gion . Th e exception al SBET of n early 800 m 2 g–1 was foun d for th e SG3, gen -
erated in th e reverse m icelle en viron m en t.
Full adsorption isoth erm s of n itrogen at 77 K were also m easured for th e
used TiO2 sam ples. A typical adsorption curve well represen tin g th e group
of th e Degussa catalysts is given in Fig. 1 for P-25. Even at very low relative
pressures, n o sign ifican t disorders in surface-en ergetic h om ogen eity were
observed.
Mesopore size distribution s of th e S-7001, SG1 an d SG2 catalysts were
evaluated from th e desorption bran ch of th e n itrogen isoth erm an d calcu-
lated from th e m odified Kelvin equation (BJH m eth od). Th e distribution
curves were sim ilar an d covered th e ran ge from ≈1.8 to 12 n m , peakin g at
r = 4.1–4.8 n m . Th e m esopore size distribution pattern of SG3 (again th e
BJH m eth od) is sh own in Fig. 2. Un im odal distribution covers th e area from
1.3 up to 14.3 n m peakin g at r ≈ 3.8 n m . Owin g to th e steep in itial in crease
in th e dV/dr value, th e m ost sign ifican t in terval is located between r ≈ 1.5
an d 4 n m . Wh ereas S-7001, SG1 an d SG2 were typically m esoporous, SG3
structure accoun ts also for a sign ifican t con tribution of m icropores. Th e
con tribution of th e m icroporous region to th e overall volum e was deter-
m in ed from th e CO2 isoth erm by th e DR (DRS) m eth od (also Fig. 2). Th e
distribution curve starts just below r ≈ 0.6 n m , sh owin g quite a sh arp m axi-
m um at r ≈ 0.8 n m an d th en decayin g with th e in creasin g pore size.
30
25
20
15
10
5
0
1.00E-06
1.00E-05
1.00E-04
1.00E-03
1.00E-02
1.00E-01
P/P
1.00E+00
0
FIG. 1
Adsorption isoth erm of n itrogen at 77 K on th e surface of P-25
Collect. Czech. Chem. Commun. (Vol. 68) (2003)

1990
Klisáková, Klusoň, Červený:
Ph otoactivity of titan ia sign ifican tly differs with its crystallograph ic
ph ase structure. Rutile is, un like an atase, usually referred to as th e low-
active form . XRD revealed sh arp diffraction lin es of m ajor plan es of rutile
an d an atase for all th e Degussa sam ples. P-25 is m ostly an atase-type titan ia
with 80% of th is structure. Com position s of D-7701 an d D-7702 are n early
opposite with prevailin g an atase for th e form er (Table I). From th e width of
th e diffraction peaks an d th e Sch errer equation th e particle size was evalu-
ated. For P-25, D-7701 an d D-7702 sh arp ban ds correspon ded to th e calcu-
lated particle size of 30–50 n m . Th is in terval is con sisten t with th e
appearan ce of sph erical clusters (30–60 n m ) observed on SEM im ages of
th ese com m ercial catalysts. A typical picture of such a surface (P-25) is
given in Fig. 3.
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
0
2
4
6
8
10
12
14
r, nm
FIG. 2
Pore size distribution pattern s of SG3
FIG. 3
SEM im age of th e P-25 surface
Collect. Czech. Chem. Commun. (Vol. 68) (2003)

Photocatalytic Oxidation of Cyclohexene
1991
Th e rem ain in g four catalysts (S-7001, SG1, SG2, SG3) were pure an atase
sam ples. Particle size an alysis of SG1 an d SG2 (XRD) revealed crystallites
just below 10 n m , for S-7001 about 16 n m . Th e value calculated for S-7001
is in a good agreem en t with th e size of well visible particles in corporated in
a flat h om ogen eous surface observable on an im age of th is m aterial (Fig. 4).
Notable properties of SG3 in volved sm all crystallites for wh ich D ≈ 3.8 n m
was calculated.
Reaction
Partial oxidation of cycloh exen e over titan ia ph otoactive species yielded an
im pressive collection of in term ediates un der stan dard reaction con dition s
(mcat = 1 g, 200 m l of cycloh exen e, oxygen flow rate 1200 m l m in –1). Th e
distribution of in term ediates could be assum ed as a sen sitive in dicator of
th e existen ce of various sites on catalyst surfaces (structural variation s)
wh ich can catalyse oxidation an d epoxidation . We con sidered form ation of
cycloh exen ylh ydroperoxide as a com m on in itial in term ediate even with out
a catalyst, on ly in th e presen ce of oxygen , accelerated by UV ligh t.
Typically, five in term ediates were repeatedly proved in reaction m ixtures.
Th eir distribution was a fun ction of th e catalyst used. Due to th eir in terest-
in g structures, atten tion was prim arily paid to m olecules bearin g an
epoxide groupin g. Th ese were 7-oxabicyclo[4.1.0]h eptan e, 7-oxabicyclo-
[4.1.0]h eptan -2-on e an d 7-oxabicyclo[4.1.0]h eptan -2-ol. Two oth er m ajor
products were cycloh ex-2-en -1-on e an d cykloh ex-2-en -1-ol. We m ay h ypo-
th etically con sider m an y form al reaction sch em es in volvin g relation s of th e
in dividual com pon en ts of a reaction m ixture. Experim en tally th ere h as
been som e eviden ce for th e preferen tial in itial form ation of 7-oxabicyclo-
FIG. 4
SEM im age of th e S-7001 surface
Collect. Czech. Chem. Commun. (Vol. 68) (2003)

1992
Klisáková, Klusoň, Červený:
[4.1.0]h eptan e an d cycloh ex-2-en -1-on e. Th ese com poun ds were furth er
tran sform ed.
Kinetic Parameters
Th e kin etic param eters used for th e discussion of th e course of th e ph oto-
oxidation on differen t types of titan ia catalysts are given in Table II. Besides
th e con version of cycloh exen e (x, con version ach ieved) an d th e in itial reac-
tion rate r0, two types of selectivities an d yields were in troduced. Se1 an d Se2
were defin ed at x = 50% as th e am oun t of cycloh ex-2-en -1-on e relative to
th e overall am oun t of oth er in term ediates an d th at of 7-oxabicyclo-
[4.1.0]h eptan e also relative to th e total am oun t of oth er in term ediates, re-
spectively. Yields Y1 an d Y2 reflect th e fin al com position wh en th e reaction
was term in ated (zero reaction rate, typically at t = 400 m in ) for th e sam e
com poun ds. Th is tim e, h owever, all products were in cluded (reaction in ter-
m ediates, carbon dioxide an d water). Th e un reacted cycloh exen e was n ot
in cluded to th e calculation of th ese param eters.
From th e series of th e tested catalysts, on ly SG3 afforded to ach ieve a to-
tal con version of cycloh exen e. Oth er catalysts were deactivated, wh ich is
discussed later. In itial activity (r0 values) of all th e catalysts prepared by th e
sol-gel process was gen erally very h igh (SG1, SG2, SG3). In con trast, th e
Degussa catalysts (P-25, D-7701, D-7702) were, n ot surprisin gly, due to
th eir low surface area, m uch less active. Particularly in th is group, it is evi-
TABLE II
Kin etic param eters
1
2
x
%
Se
Se
Y1
Y2
r0–1
Catalyst
wt.%a
wt.%b
wt.%a
wt.%b
m m ol gcat m in –1
D-7701
D-7702
P-25
74
70
24
60
72
83
99
37
39
–
13
22
–
26
28
10
32
14
12
9
10
19
9
2.3
1.7
2.9
S-7001
SG1
42
22
17
12
13
33
45
59
7
1.6
24
34
45
14.2
19.9
32.8
SG2
SG3
a
b
Superscript 1 for cycloh ex-2-en -1-on e. Superscript 2 for 7-oxabicyclo[4.1.0]h eptan e.
Collect. Czech. Chem. Commun. (Vol. 68) (2003)

Photocatalytic Oxidation of Cyclohexene
1993
den t th at th e ph otoactivity m ay also depen d on th e con ten t of th e an atase
form . Th e two an atase-rich sam ples (P-25, D-7701) were m ore active th an
D-7702 with 60% of rutile. A typical course of th e reaction is dem on strated
in Fig. 5 for D-7702. Th e catalyst S-7001 did n ot follow th is tren d. Am az-
in gly, th is an atase-rich sam ple (S-7001), with a relatively h igh surface area,
was th e least active catalyst. On th e oth er h an d, all th e SG catalysts (also
pure an atase form s) revealed a n otable ph otoactivity.
Th e crystallograph ically m ixed sam ples (rutile/an atase) were m ore selec-
tive for cycloh ex-2-en -1-on e. In th is con text th e h igh est yield was ach ieved
on th e rutile-rich est catalyst D-7702 for th e group of Degussa TiO2. Se val-
ues were n ot evaluated for th e P-25 catalyst sin ce x = 50% was n ot ach ieved.
It was th at on e sufferin g m ost seriously from th e deactivation . An extraor-
din ary position for th e low-active, pure-an atase S-7001 appeared again . It
was iden tified as th e m ost selective catalyst for th is in term ediate ever ob-
served (cycloh ex-2-en -1-on e). Th e selectivity Se1 of 42% was recorded, de-
spite its low activity. Th e SGs were n ot too selective for th is in term ediate.
On th e oth er h an d, a sign ifican tly preferen tial form ation of 7-oxabicyclo-
[4.1.0]h eptan e was apparen t on all of th em . Th e Se2 value for SG3 was
n early 60% an d th e correspon din g yield 45%. Also th e two rem ain in g SGs
could be referred to as catalysts directin g th e partial oxidation reaction
th rough th e in itial epoxide stage.
100
80
60
40
20
0
0
100
200
300
400
500
t, min
FIG. 5
Partial ph otooxidation of cycloh exen e on D-7702. ᭜ Cycloh exen e, × cycloh ex-2-en -1-on e, ᭿
7-oxabicyclo[4.1.0]heptan-2-one, ꢀ 7-oxabicyclo[4.1.0]heptane, ᭹ 7-oxabicyclo[4.1.0]heptan-2-ol
Collect. Czech. Chem. Commun. (Vol. 68) (2003)

1994
Deactivation
Klisáková, Klusoň, Červený:
As m en tion ed, all catalysts, except SG3, suffered from deactivation . Elim i-
n ation of th is ph en om en on , causin g term in ation of th e reaction , was
rath er com plicated an d sustain ed even a doubled load of a catalyst (Fig. 6).
In frared spectroscopy revealed a stron g an d in creasin g surface h ydro-
xylation of titan ia with drawn from th e reaction . Th is is ch aracteristic fea-
ture of an atase, wh ich is, to a certain exten t, also respon sible for its
en h an ced ph otoactivity. Neverth eless, th e surface water over-saturation
could m an ifest a com pletely opposite effect – deactivation . Water m ole-
cules form ed as products of th e total oxidation are selectively trapped on
th e an atase surface, due to its stron g affin ity to water, an d th en act th ere in
active species blockin g. Th e an atase-rich est P-25 from th e group of
low-surface-area catalysts was affected m ost seriously. IR spectra also sup-
ported th e n otion th at cycloh ex-2-en -1-on e is a very stron gly adsorbed sur-
face species an d th us also poten tially respon sible for th e loose of activity.
As seen in Table II, all reaction s selective for th is in term ediate suffer m ore
from th e deactivation .
Attem pts were m ade to reactivate th erm ally th e used catalysts. TGA
curves of two sam ples of P-25 (fresh an d used) eviden ced th at above th e
773 K n o th erm al desorption appears. Also, an in itial “bum p” correspon d-
in g to th e deh ydroxylation is well apparen t. Th e used P-25 was treated at
773 K in air for th ree h ours an d th en used in th e stan dard ph otooxidation
experim en t. Th e ach ieved con version of cycloh exen e was n early iden tical
100
80
60
40
20
0
0
50
100
150
200
250
300
350
t, min
400
FIG. 6
Effect of th e in creased loadin g of a catalyst. ᭜ 2 g of D-7702, ᭹ 1 g of D-7702
Collect. Czech. Chem. Commun. (Vol. 68) (2003)

Photocatalytic Oxidation of Cyclohexene
1995
with th at of th e fresh catalyst. A little lower in itial activity was caused by
an in sign ifican t ch an ge in th e rutile con ten t durin g th e th erm al treatm en t.
Other Factors
For th e sake of com pleten ess, th e exigen cy of th e presen ce of oxygen , cata-
lyst an d UV ligh t were tested. Stan dard experim en ts for th e group of
Degussa catalysts were carried out first in n itrogen in stead of oxygen . A lim -
ited form ation of som e of th e oxidation in term ediates was foun d, arisin g
probably from TiO2 lattice oxygen (O2–). Wh en th is source was expen ded,
th e reaction stopped. Th e h igh est con version ach ieved was x = 9.8% for
D-7701. Accordin g to very low values of Y an d th e m ass balan ce calcula-
tion s, th e reaction in such an arran gem en t proceeded sign ifican tly to car-
bon dioxide an d water. It was in terestin g th at am on g th e partially oxidised
in term ediates th ose with th e epoxide groupin g were com pletely m issin g.
Som e effort h as also been spen t to describe th e progress of th e reaction
with out an y catalyst, on ly with oxygen , UV an d cyclic alken e. Cycloh exen e
was n ot foun d com pletely ph oto-in active; h owever, its attain ed con version
was on ly 3%. Besides som e water an d CO2, on ly 7-oxabicyclo[4.1.0]h eptan e
was form ed. Syn th etically, th ere is n o practical reason to utilise th is ar-
ran gem en t. Due to low tem perature of th e experim en ts (293 K) n o th erm al
decom position of cycloh exen e an d reaction in term ediates was observed.
Fin ally th e UV source was excluded from th e system an d th e experim en ts
were carried out as stan dard catalytic oxidation s (n ot ph otoxidation ) on
P-25 an d SG3. Negligible tran sform ation of cycloh exen e for both catalysts
was observed, obviously suggestin g th e in dispen sable role of UV in form a-
tion of active surface species on titan ia.
CONCLUSION
Partial UV ph otooxidation of cycloh exen e un der am bien t con dition s on
various types of titan ia was iden tified as a suitable system for syn th esis of
som e valuable in term ediates. Special atten tion was paid to cycloh ex-2-en -
1-on e an d to 7-oxabicyclo[4.1.0]h eptan e. Form al reaction sch em es were
suggested, reflectin g differen t reaction path ways. Th e reaction selectivity
profiles were depen den t, to a certain exten t, on structural properties of th e
used catalysts. On th e basis of a series of experim en ts, th e existen ce of sur-
face-active species on titan ia could be en visaged. Th eir form ation is sub-
stan tially depen den t on th e presen ce of oxygen , UV illum in ation an d also
surface h ydroxylation . On th e oth er h an d, exten sive surface coverage with
Collect. Czech. Chem. Commun. (Vol. 68) (2003)

1996
Klisáková, Klusoň, Červený:
water an d very stron g adsorptivity of cycloh ex-2-en -1-on e m ay cause th e
gen erally observed deactivation . Th e foun d differen ces of th e proportion
between th e yields of ph otoproducts (cycloh ex-2-en -1-on e to 7-oxabicyclo-
[4.1.0.]h eptan e) ascribed to th e differen t surface properties of particular
ph otocatalysts, could also actually reflect th e proportion between reaction
rates of two parallel run n in g ph otoprocesses, i.e., th e ph otocatalytic oxida-
tive degradation (leadin g to cycloh ex-2-en -1-on e an d oth er in term ediates)
an d th e direct ph otooxidation (producin g m ain ly 7-oxabicyclo[4.1.0.]h ep-
tan e).
The authors thank the Ministry of Education, Youth and Sports of the Czech Republic for funding
this research as a part of a general research project “Catalytic Processes in Chemistry and Chemical
Technology” (No. CEZ MSM223100001).
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